Funcionalización química de materiales
carbonosos para almacenamiento y generación
de energía
María José Mostazo López
Departamento de Química Inorgánica
Departamento de Química Física
Instituto Universitario de Materiales
Facultad de Ciencias
FUNCIONALIZACIÓN QUÍMICA DE MATERIALES CARBONOSOS
PARA ALMACENAMIENTO Y GENERACIÓN DE ENERGÍA
María José Mostazo López
Tesis presentada para aspirar al grado de
DOCTORA POR LA UNIVERSIDAD DE ALICANTE
MENCIÓN DE DOCTORA INTERNACIONAL
Doctorado en Ciencia de Materiales
Dirigida por:
Diego Cazorla Amorós Emilia Morallón Núñez
Catedrático de Química Inorgánica Catedrática de Química Física
Alicante, marzo de 2019
Este trabajo ha sido financiado por la Generalitat Valenciana a través de un contrato de
formación de personal investigador de carácter predoctoral (Programa VALi+d,
ACIF/2015/374).
Diego Cazorla Amorós Emilia Morallón Núñez
Catedrático de
Química Inorgánica
Catedrática de
Química Física
Diego Cazorla Amorós, Catedrático de Química Inorgánica, y Emilia
Morallón Núñez, Catedrática de Química Física, ambos de la
Universidad de Alicante,
CERTIFICAN QUE:
Dña. María José Mostazo López, Licenciada en Química, ha realizado
en el Departamento de Química Inorgánica y en el Instituto
Universitario de Materiales de la Universidad de Alicante, bajo nuestra
dirección, el trabajo que lleva por título: Funcionalización química de
materiales carbonosos para almacenamiento y generación de
energía, que constituye su memoria para aspirar al grado de Doctora,
reuniendo, a nuestro juicio, las condiciones necesarias para ser
presentada y defendida ante el tribunal correspondiente.
Y para que conste a efectos oportunos, en cumplimiento de la
legislación vigente, firmamos el presente certificado en Alicante, a 26
de febrero del 2019.
Capítulo 0
Agradecimientos
En primer lugar, quisiera agradecer a mis directores de Tesis, los
profesores Diego Cazorla Amorós y Emilia Morallón Núñez, por la
oportunidad de realizar este trabajo en sus grupos de investigación, por su
paciencia y su dedicación.
A Ramiro Ruiz Rosas, por todas sus enseñanzas, por estar siempre
dispuesto a ayudar y por los consejos que me ha dado durante estos años.
A la Generalitat Valenciana, por la concesión de un contrato de
formación predoctoral para financiar esta Tesis (ACIF/2015/374), así
como las becas para la realización de estancias en otros centros
(BEFPI/2016/006 y BEFPI/2017/036).
I wish to thank Shiraishi Sensei for the warm welcome I received in
his laboratory at Gunma University. To all my labmates in Shiraishi’s lab,
for their kind help and support. I also want to thank all the friends I met
in Kiryu for the nice moments we shared.
I would like to thank Prof. Andrea Balducci for his kind attention
during my stay in Jena. To my labmates in Jena, for their nice welcome
and kindness.
A mis compañeros del GEPE y del MCMA, a los que estuvieron y a
los que han llegado a la largo de estos años, por los almuerzos, los cafés,
las discusiones científicas y todos los buenos momentos que hemos
pasado juntos. Gracias a todos, porque ha sido un placer compartir con
vosotros esta etapa.
A mis amigos, tanto a los de dentro como a los de fuera de la
universidad, por ayudarme a desconectar y por acompañarme durante
estos años.
A mi familia, y en especial, a mis padres; por creer en mí y por
apoyarme siempre. Todo lo que he conseguido os lo debo a vosotros.
A Juan, por acompañarme desde el principio de esta andadura, por
ayudarme a sacar siempre lo mejor de mí misma, por los kilómetros
recorridos para visitarme y por su apoyo incondicional.
A mis padres
A Juan
Índice
ÍNDICE
Objetivos y estructura general de la Tesis Doctoral
1. Introducción …………………………………………………….. 3
2. Objetivos……………………………………………………........ 3
3. Estructura de la Tesis Doctoral…………………………….......... 7
Capítulo 1. Introducción general.
1.1. Materiales carbonosos ……………………………………..... 15
1.1.1. Carbón activado …………………………………….. 16
1.1.2. Materiales carbonosos nanomoldeados……………... 19
1.2. Química superficial de los materiales carbonosos ………….. 21
1.2.1. Oxígeno ……………………………………………… 21
1.2.2. Nitrógeno …………………………………………….. 22
1.3. Síntesis de materiales carbonosos dopados con nitrógeno …. 24
1.3.1. Síntesis directa a partir de un precursor nitrogenado ... 25
1.3.2 Post-tratamiento de materiales carbonosos con
reactivos nitrogenados ……………………………………… 26
1.3.2.1 Funcionalización con nitrógeno mediante
tratamientos térmicos ………………………………….. 26
1.3.2.2 Funcionalización química basada en reacciones
orgánicas a baja temperatura …………………………... 29
1.3.2.3. Funcionalización electroquímica ……………... 34
1.4. Aplicaciones en almacenamiento y generación de energía … 35
1.4.1. Condensadores electroquímicos ……………………... 35
4
1.4.1.1. Modelos de doble capa eléctrica ………………. 37
1.4.1.2. Características de los condensadores
electroquímicos ………………………………………… 41
1.4.1.3 Configuraciones de condensador electroquímico 45
1.4.1.4. Tipos de electrolitos …………………………… 48
1.4.1.5. Materiales carbonosos como electrodos de
supercondensadores ……………………………………. 52
1.4.2. Pilas de combustible …………………………………. 58
1.4.2.1. Reacción de reducción de oxígeno ……………. 60
1.4.2.2. Electrocatalizadores para la ORR ……………... 62
1.4.2.3. Materiales carbonosos funcionalizados con
nitrógeno como electrocatalizadores de la ORR ………. 63
1.5. Referencias …………………………………………………. 65
Capítulo 2. Materiales, métodos y técnicas experimentales.
2.1. Introducción ………………………………………………... 85
2.2. Materiales ………………………………………………….. 85
2.2.1 Carbones activados …………………………………... 85
2.2.2. Materiales carbonosos nanomoldeados ……………... 86
2.3. Modificación de la química superficial de los materiales
carbonosos ……………………………………………………… 86
2.3.1. Funcionalización química mediante métodos basados
en reacciones orgánicas…………………………………….. 87
2.3.1.1 Oxidación química ……………………………. 88
2.3.1.2 Introducción de grupos funcionales amida…….. 88
Índice
2.3.1.3 Introducción de grupos funcionales amina…….. 89
2.3.2. Polimerización de anilina……………………………. 90
2.3.3. Tratamientos térmicos……………………………….. 90
2.4. Técnicas de caracterización………………………………… 91
2.4.1. Adsorción física de gases……………………………. 91
2.4.1.1 Teoría de Brunauer-Emmet-Teller…………….. 93
2.4.1.2. Ecuación de Dubinin-Radushkevich………….. 95
2.4.1.3 Teoría del funcional de densidad no localizada
bidimensional………………………………………….. 96
2.4.2. Espectroscopia fotoelectrónica de rayos X………….. 98
2.4.3. Desorción a temperatura programada……………….. 100
2.4.4. Caracterización electroquímica……………………… 102
2.4.4.1. Configuraciones de celda electroquímica…….. 103
2.4.4.2. Técnicas electroquímicas……………………... 105
2.5. Referencias…………………………………………………. 122
Chapter 3. N-functionalization of activated carbon.
3.1. Introduction………………………………………………… 129
3.2. Materials and methods……………………………………… 132
3.2.1 Activated carbon……………………………………… 132
3.2.1.1. Chemical oxidation with HNO3………………. 132
3.2.1.2. Generation of amide functionalities…………... 133
3.2.1.3. Generation of amine functionalities…………… 134
3.2.2. Electrochemical characterization……………………. 134
6
3.2.3. Porous texture and surface chemistry characterization 135
3.3. Results and discusión………………………………………. 136
3.3.1. Porous texture characterization……………………… 139
3.3.2. Surface chemistry characterization………………….. 141
3.3.2.1. XPS…………………………………………… 139
3.3.2.2. Temperature Programmed Desorption………... 147
3.3.3. Electrochemical characterization……………………. 154
3.3.3.1. Cyclic Voltammetry…………………………... 154
3.3.3.2. Galvanostatic charge-discharge cycles………... 158
3.4. Conclusions………………………………………………… 163
164 3.5.
References…………………………………………………..
Annex to Chapter 3. Nitrogen functionalization of zeolite
templated carbon by electrochemical and chemical methods…... 173
Chapter 4. Electrochemical performance of N-doped activated
carbons in aqueous electrolyte
4.1. Introduction………………………………………………… 183
4.2. Materials and methods……………………………………… 185
4.2.1 Activated carbon……………………………………… 185
4.2.2. Chemical functionalization of activaded carbon…….. 186
4.2.2.1. Synthesis of KUA-CONH2……………………. 186
4.2.2.2. Synthesis of KUA-N………………………….. 186
4.2.3. Porous texture and surface chemistry characterization 187
4.2.4. Electrochemical characterization……………………. 188
4.2.4.1. Three electrode cell configuration…………….. 188
Índice
4.2.4.2. Two electrode cell configuration……………… 189
4.4. Results and discusión………………………………………. 191
4.2.1 Surface chemistry and porous texture characterization. 191
4.3.2 Electrochemical characterization…………………….. 195
4.3.2.1 Characterization of carbon materials…………... 195
4.3.2.2 Characterization of symmetric supercapacitors. 199
4.3.2.3 Characterization of asymmetric supercapacitors 204
4.4. Conclusions………………………………………………… 210
4.5. References………………………………………………….. 212
Chapter 5. Electrochemical performance of N-doped
zeolite templated carbon in aqueous electrolyte
5.1. Introduction………………………………………………… 217
5.2. Materials and methods……………………………………… 219
5.2.1 Zeolite templated carbons……………………………. 219
5.2.2. Physicochemical characterization…………………… 220
5.2.3. Electrochemical characterization…………………… 221
5.2.3.1 Three electrode cell configuration……………... 221
5.2.3.2 Two electrode cell configuration………………. 222
5.3. Results and discusión………………………………………. 223
5.3.1. Physicochemical characterization…………………… 223
5.3.2. Electrochemical characterization……………………. 225
5.3.2.1. Electrochemical characterization before electro-
oxidation……………………………………………….. 225
8
5.3.2.2. Electrochemical characterization after electro-
oxidation………………………………………………... 228
5.3.2.3. ZTC and N-ZTC supercapacitors using acid
electrolyte………………………………………………. 235
5.4. Conclusions………………………………………………… 241
5.5. References………………………………………………….. 243
5.6. Annex to Chapter 5..………………………………………... 250
Chapter 6. Electrochemical performance of N-doped activated
carbons in organic electrolyte
6.1. Introduction………………………………………………… 255
6.2. Materials and methods……………………………………… 257
6.2.1 Synthesis of carbon materials………………………… 261
6.2.1.1. Pristine carbon material……………………...... 257
6.2.1.2 N-functionalization of carbon materials at mild
conditions………………………………………………. 257
6.2.1.3 Heat treatments………………………………… 258
6.2.2. Physicochemical characterization……………………. 258
6.2.3. Electrochemical characterization…………………….. 259
6.3. Results and discusión………………………………………. 260
6.3.1. Physicochemical characterization of carbon materials 260
6.3.1.1 Porous texture………………………………….. 260
6.3.1.2 Surface chemistry characterization…………….. 263
6.3.2. Electrochemical characterization…………………….. 271
Índice
6.3.2.1. Effect of surface chemistry modification at
mild conditions………………………………………… 275
6.3.2.2. Effect of surface chemistry modification by
heat treatments…………………………………………. 278
6.4. Conclusions………………………………………………… 281
282 6.5.
References…………………………………………………..
Annex to Chapter 6. Chemical functionalization of a
commercial activated carbon with nitrogen groups……………...
293
Chapter 7. Electrochemical performance of N-doped activated
carbons in non-conventional electrolytes
7.1. Introduction…………………………………………………... 297
7.2. Materials and methods……………………………………….. 300
7.2.1 Activated carbon……………………………………….. 300
7.2.2. Chemical functionalization of activated carbon………. 300
7.2.3. Porous texture and surface chemistry characterization.. 301
7.2.4. Electrolyte preparation………………………………… 305
7.2.5 Electrochemical characterization………………………. 302
7.2.5.1 Three electrode cell configuration………………. 302
7.2.5.2 EDLC investigation……………………………… 303
7.3. Results and discussion……………………………………….. 304
7.3.1. Surface chemistry and porous texture characterization.. 304
7.3.2. Electrochemical characterization……………………… 307
7.3.2.1 Three-electrode cell configuration………………. 307
10
7.3.2.2. EDLC investigation……………………………... 313
7.4. Conclusions ………………………………………………….. 319
7.5. References……………………………………………………. 323
Chapter 8. Electrochemical performance of N-doped activated
carbons as electrocatalysts for the ORR
8.1. Introduction………………………………………………… 327
8.2. Materials and methods……………………………………… 330
8.2.1 Synthesis of activated carbons……………………….. 330
8.2.1.1 Pristine activated carbon………………………. 330
8.2.1.2 Chemical functionalization of activated carbon
at mild conditions……………………………………… 330
8.2.1.3 Preparation of polyaniline/activated carbon
composite………………………………………………
331
8.2.1.4 Post-thermal treatments……………………….. 331
8.2.2 Porous texture and surface chemistry characterization 331
8.2.3. Electrochemical activity towards ORR……………… 332
8.3. Results and Discussion……………………………………... 333
8.3.1 Surface chemistry characterization…………………... 333
8.3.2. Porous texture characterization……………………… 340
8.3.3 Electroactivity towards ORR…………………………. 342
8.3.3.1 Pristine activated carbon………………………. 343
8.3.3.2 N-doped activated carbons at mild conditions… 343
8.3.3.3 Polyaniline/activated carbon composite……….. 344
Índice
8.3.2.4 Heat-treated activated carbons………………… 346
8.3.2.5 Selectivity to water formation…………………. 347
8.4. Conclusions ………………………………………………... 349
8.5. References …………………………………………………. 351
Annex to Chapter 8……………………………………………… 357
Chapter 9. General conclusions………………………………. 361
Summary/Resumen…………………………………………….. 377
Objetivos y estructura general
de la Tesis Doctoral
Objetivos y estructura general de la Tesis Doctoral
3
1. Introducción
La investigación en el área de almacenamiento y producción de
energía eléctrica es un tema de interés creciente debido a la demanda
energética de la sociedad actual. Uno de los retos principales de la
comunidad científica reside en el desarrollo de sistemas más eficientes y
sostenibles medioambientalmente. En este contexto, el empleo de
sistemas electroquímicos, como los supercondensadores y las pilas de
combustible, constituye una alternativa muy prometedora para avanzar
hacia una producción energética sostenible.
En este ámbito, los materiales carbonosos son ampliamente utilizados
debido a sus propiedades únicas, que les confiere una gran versatilidad
para ser utilizados como componentes de diversos dispositivos. No
obstante, es necesario continuar investigando para diseñar materiales que
permitan mejorar las prestaciones de estos sistemas de almacenamiento y
generación de energía. Por estos motivos, la presente Tesis Doctoral se
enmarca en el estudio y desarrollo de materiales carbonosos para su
aplicación en dispositivos electroquímicos de almacenamiento y
producción de energía.
2. Objetivos
La presente Tesis Doctoral tiene como objetivo general el desarrollo
de materiales carbonosos por medio de nuevas metodologías de
funcionalización con nitrógeno para su aplicación como electrodos en
condensadores electroquímicos y electrocatalizadores en pilas de
4
combustible. A continuación, se describen los objetivos específicos de
este trabajo:
(i) Funcionalización química de materiales carbonosos de
porosidad elevada con grupos funcionales nitrogenados por
medio de estrategias basadas en reacciones orgánicas en
condiciones suaves.
(ii) Caracterización fisicoquímica y electroquímica de los
materiales carbonosos funcionalizados con nitrógeno, así
como materiales carbonosos obtenidos por nanomoldeo.
(iii) Evaluación del comportamiento electroquímico de los
materiales como electrodos de supercondensadores en
electrolitos acuosos, orgánicos y líquidos iónicos.
(iv) Estudio del efecto de grupos funcionales nitrogenados en el
comportamiento electroquímico de carbones activados de
elevada porosidad como electrocatalizadores de la reacción de
reducción de oxígeno.
3. Estructura de la Tesis Doctoral
La presente Tesis Doctoral ha sido realizada en el grupo Materiales
Carbonosos y Medio Ambiente (MCMA), perteneciente al Departamento
de Química Inorgánica, y en el Grupo de Electrocatálisis y Electroquímica
de Polímeros (GEPE) del Departamento de Química Física. Ambos
grupos de investigación pertenecen a su vez al Instituto Universitario de
Materiales de la Universidad de Alicante. El desarrollo del trabajo ha sido
complementado por medio de dos estancias de investigación en centros
Objetivos y estructura general de la Tesis Doctoral
5
extranjeros. La primera estancia se realizó en el Graduate School of
Science and Technology, de la Universidad de Gunma (Japón), bajo la
supervisión del Profesor Soshi Shiraishi. La segunda estancia se llevó a
cabo bajo la supervisión del Profesor Andrea Balducci, en el Center for
Energy and Environmental Chemistry, perteneciente a la Universidad
Friedrich Schiller de Jena (Alemania).
Dado que la memoria se presenta para aspirar al grado de Doctora por
la Universidad de Alicante con mención de Doctora Internacional, los
capítulos correspondientes a los resultados obtenidos y las conclusiones
generales se han redactado en inglés para cumplir con la normativa
vigente.
La Tesis Doctoral se encuentra dividida en nueve capítulos. A
continuación, se describe el contenido de cada uno de ellos.
Capítulo 1. Introducción general
En este capítulo, se describen las propiedades fisicoquímicas de los
materiales carbonosos estudiados en la Tesis Doctoral, así como diversas
metodologías para sintetizar materiales carbonosos de elevada porosidad
dopados con nitrógeno, con especial hincapié en los métodos basados en
post-tratamientos por medio de reacciones orgánicas. Además, se detallan
los fundamentos teóricos de las aplicaciones energéticas en las que se
utilizan estos materiales: condensadores electroquímicos y pilas de
combustible, prestando especial atención al efecto de la textura porosa y
la química superficial de los materiales en estos sistemas.
6
Capítulo 2. Materiales, métodos de síntesis y técnicas
experimentales
Este capítulo resume las metodologías de síntesis y funcionalización
química de materiales carbonosos utilizados en la presente Tesis Doctoral.
Además, se describen los fundamentos téoricos y experimentales de las
técnicas utilizadas para caracterizar los materiales y evaluar su
comportamiento electroquímico.
Chapter 3. N-functionalization of activated carbon at mild
conditions
En este capítulo se realiza la funcionalización química de un carbón
activado de microporosidad elevada con grupos funcionales nitrogenados
por medio de reacciones orgánicas a temperatura baja. El proceso de
funcionalización engloba reacciones de amidación (en tres etapas) y
aminación (por medio de un reordenamiento de Hoffman). El efecto de la
modificación química en la química superficial y la textura porosa se
describe de forma detallada por medio de distintas técnicas de
caracterización fisicoquímica. Además, se evalua el efecto de los grupos
funcionales nitrogenados en el comportamiento electroquímico de los
materiales en medio acuoso. Por último, se describe en el anexo del
capítulo la aplicación de la metodología de funcionalización química a
materiales nanomoldeados obtenidos empleando zeolitas como plantilla.
Los resultados derivados de este estudio han dado lugar a la
publicación de un artículo en la revista Carbon:
Objetivos y estructura general de la Tesis Doctoral
7
Mostazo-López MJ, Ruiz-Rosas R, Morallón E, Cazorla-Amorós D.
Generation of nitrogen functionalities on activated carbons by amidation
reactions and Hofmann rearrangement: Chemical and electrochemical
characterization. Carbon 2015; 91: 252-265
Chapter 4. Electrochemical performance of N-doped activated
carbons in aqueous electrolyte.
En este capítulo, se describen y comparan dos metodologías de
funcionalización química de carbones activados: el protocolo de
amidación (descrito en el capítulo 3) y una reacción de funcionalización
directa (en una única etapa). Posteriormente, se evalua la influencia de
ambas estrategias de funcionalización en las propiedades fisicoquímicas
y electroquímicas de los materiales carbonosos. Estos materiales se
emplean como electrodos de condensadores simétricos y asimétricos (en
masa). Los dispositivos se caracterizan empleando distintas técnicas
electroquímicas para determinar su capacidad, energía y potencia. La
combinación de las técnicas de caracterización permite deducir el efecto
de la funcionalización química en la estabilidad electroquímica de los
materiales como electrodos de supercondensadores en medio acuoso.
Los resultados obtenidos en este capítulo se han publicado en la
revista International Journal of Hydrogen Energy:
Mostazo-López MJ, Ruiz-Rosas R, Morallón E, Cazorla-Amorós D.
Nitrogen doped activated carbon prepared by a mild method.
Enhancement of the supercapacitor performance. International Journal of
Hydrogen Energy 2016; 41: 19691-19701
8
Chapter 5. Electrochemical performance of N-ZTC in aqueous
electrolyte
En este capítulo, se emplean dos materiales carbonosos
nanomoldeados para evaluar el papel de los grupos funcionales
nitrogenados en su comportamiento como electrodos de
supercondensadores en medio acuoso. Para esto, se utilizan dos materiales
carbonosos nanomoldeados: uno dopado con nitrógeno y el otro sin dopar.
La caracterización fisicoquímica y electroquímica de los materiales
permite esclarecer qué grupos funcionales nitrogenados tienen una mayor
influencia en la aplicación de los materiales carbonosos como electrodos
de supercondensadores en medio ácido.
Los resultados de este trabajo han permitido la elaboración de un
artículo científico, que ha sido publicado en la revista Carbon:
Mostazo-López MJ, Ruiz-Rosas R, Castro-Muñiz A, Nishihara H,
Kyotani T, Morallón E, Cazorla-Amorós D. Ultraporous nitrogen-doped
zeolite-templated carbon for high power density aqueous-based
supercapacitors. Carbon 2018; 129: 510-519
Chapter 6. Electrochemical performance of N-doped activated
carbons in organic electrolyte
En este capítulo, se describe el efecto de los grupos funcionales
nitrogenados en el comportamiento electroquímico de carbones activados
como electrodos de condensadores en medio orgánico. Para esto, se
emplean distintas estrategias para sintetizar una serie de carbones
Objetivos y estructura general de la Tesis Doctoral
9
activados de elevada porosidad con distintos grupos funcionales
nitrogenados. Para esto, se combinan las metodologías de
funcionalización química en condiciones suaves y post-tratamientos
térmicos para sintetizar una serie de carbones activados de elevada
porosidad con distintos grupos funcionales nitrogenados. La evaluación
de condensadores simétricos basados en estos materiales permite deducir
el efecto del nitrógeno en las características de los condensadores
(capacidad, energía, etc.) en medio orgánico. Para profundizar en el efecto
de la funcionalización en la estabilidad de los materiales carbonosos, los
condensadores se evaluan por medio de un test de durabilidad a voltaje
constante y temperatura elevada para acelerar la degradación de los
mismos. Finalmente, en el anexo del capítulo se describe el empleo de la
funcionalización química en condiciones suaves en un carbón activado
comercial para evaluar el efecto en su comportamiento como electrodo de
condensadores en medio orgánico.
Chapter 7. Electrochemical performance of N-doped activated
carbons in non-conventional electrolytes
En este capítulo, se presenta el estudio del comportamiento
electroquímico de carbones activados obtenidos por funcionalización
química en condiciones suaves en electrolitos no convencionales: líquidos
iónicos y sales innovadoras en electrolito orgánico. Dado que estos
electrolitos permiten trabajar a voltajes de operación muy elevados, se
realizó un estudio de las ventanas de potencial de todos los materiales en
ambos electrolitos con el fin de elucidar el papel de los grupos funcionales
nitrogenados en estos electrolitos. Los materiales son utilizados para
10
preparar condensadores asimétricos en líquido iónico, para evaluar las
mejoras producidas por los electrodos funcionalizados con nitrógeno,
especialmente en lo que concierne a la durabilidad del dispositivo.
Los resultados obtenidos en este capítulo han permitido la
elaboración de un artículo cientifico, que se encuentra actualmente en fase
de revisión:
Mostazo-López MJ, Krummacher J, Balducci A, Morallón E, Cazorla-
Amorós D. Electrochemical performance of N-doped superporous
activated carbons in non-conventional electrolytes. Energy Storage
Materials 2019; en revisión.
Chapter 8. Electrochemical performance of N-doped activated
carbons as electrocatalysts for the ORR
En este capítulo, se emplean distintas estrategias para sintetizar una
serie de carbones activados de elevada porosidad con distintos grupos
funcionales nitrogenados. Para esto, se combinan las metodologías de
funcionalización química en condiciones suaves, polimerización de
anilina y post-tratamientos térmicos. Estos materiales se emplean para
evaluar el efecto de los grupos funcionales nitrogenados en el
comportamiento electroquímico de materiales microporosos como
electrocatalizadores de la reacción de reducción de oxígeno, prestando
atención tanto a la actividad catalítica como a la selectividad a la
formación de agua.
Objetivos y estructura general de la Tesis Doctoral
11
Chapter 9. General conclusions.
En este capítulo, se presentan las conclusiones generales de la Tesis
Doctoral.
Capítulo 1
Introdución general
Introducción general
15
1.1. Materiales carbonosos
Los materiales carbonosos son utilizados en una amplia variedad de
aplicaciones debido a sus propiedades únicas. Los carbones activados y
fibras de carbón activadas son los materiales más empleados como
adsorbentes y catalizadores. No obstante, se han desarrollado una serie de
materiales con propiedades muy prometedoras en distintos campos, como
la industria electrónica, electroquímica, metalurgia, adsorción, catálisis,
etc. Algunos ejemplos de estos materiales son: fulerenos, nanotubos de
carbono, materiales carbonosos nanomoldeados, etc. Todos estos
materiales presentan láminas grafénicas en los que la coordinación y
disposición de los átomos de carbono definen el tipo de nanoestructura
[1]. En las aplicaciones energéticas, el empleo de distintos materiales
carbonosos es muy habitual. No obstante, para alcanzar la elevada
demanda energética y extender el uso de los distintos dispositivos a
aplicaciones nuevas, es necesario desarrollar nuevos materiales con
propiedades óptimas. En ese sentido, la química superficial tiene gran
relevancia, ya que permite adaptar las propiedades fisicoquímicas del
material a la aplicación deseada. En catálisis, la presencia de grupos
funcionales condiciona la interacción del material con adsorbatos o
reactivos, ya sea a través de la modificación de las propiedades texturales,
impidiendo el acceso de estas moléculas a la porosidad o por las
interacciones electrostáticas o químicas que puedan producirse entre la
superficie y la molécula [2]. Del mismo modo y en referencia a las
aplicaciones electroquímicas, la química superficial puede afectar a la
capacidad de almacenamiento de energía [3].
Capítulo 1
16
Por estos motivos, en la actualidad hay un gran interés en la
modificación de la química superficial y en la preparación de materiales
carbonosos dopados con heteroátomos [4,5]. Esta Tesis Doctoral se centra
en el efecto de los grupos funcionales nitrogenados en materiales
carbonosos de elevada porosidad, como son los carbones activados y los
materiales carbonosos nanomoldeados. A continuación, se presenta una
descripción detallada de las características principales de estos materiales.
1.1.1. Carbón activado
Los carbones activados son materiales utilizados en una amplia
variedad de aplicaciones, debido principalmente a su elevada superficie
específica, su estructura porosa y a la presencia de distintos grupos
funcionales en su superficie [6]. Estos materiales están constituidos por
láminas irregulares de carbono, de manera que los espacios interlaminares
conforman la porosidad de los mismos [7].
Estos materiales se utilizan en la industria como adsorbentes,
permitiendo separar compuestos en fase gas y líquida y purificar efluentes
contaminados. También tienen aplicación como catalizadores o soporte
de catalizadores [2]. En el campo de la electroquímica, se utilizan
principalmente como electrodos en dispositivos de almacenamiento y
generación de energía [8,9].
Los carbones activados se sintetizan mediante un proceso
conocido como activación, utilizando como precursor un material rico en
carbono. Los precursores utilizados son muy variados: breas, alquitranes,
carbones minerales, madera, etc. La química superficial y las propiedades
Introducción general
17
texturales del material dependen del precursor, del agente activante y de
la temperatura del tratamiento. La Figura 1.1 resume las etapas principales
de los dos métodos de activación: física y química.
El proceso de activación física se lleva a cabo habitualmente en dos
etapas: (i) proceso de carbonización del precursor (normalmente, entre
400 y 850 ºC); (ii) tratamiento térmico del material carbonizado a 800-
1000 ºC en atmósfera oxidante (vapor de agua, CO2 o mezcla de ambos)
[7,10]. Este proceso de activación permite obtener materiales con un
grado elevado de microporosidad. No obstante, también se forman poros
de mayor tamaño (mesoporos), de manera que la distribución de tamaños
de poros del material resultante es ancha [11].
En el proceso de activación química, se realiza la mezcla del
precursor carbonoso con el agente activante (KOH, H3PO4, ZnCl2, etc.) y
se trata térmicamente a elevada temperatura (entre 400 y 900 ºC) en
atmósfera inerte [7,10,12]. Posteriormente, es necesario realizar una etapa
de lavado para eliminar los productos de reacción y el agente activante
que no haya reaccionado [12]. En comparación con la activación física, la
activación química permite obtener materiales con un mayor desarrollo de
microporosidad. Además, los rendimientos suelen ser superiores [11–14].
En lo que concierne a la textura porosa, el parámetro fundamental (además
de la temperatura) para generar materiales de una porosidad determinada
es la relación entre la cantidad de precursor y el agente activante; sin
embargo, es difícil controlar la química superficial durante el proceso de
activación.
Capítulo 1
18
Figura 1.1. Esquema de los procesos de activación física y activación química [7].
El uso de distintos agentes activantes químicos condiciona la
microporosidad del carbón activado sintetizado. De este modo, la
activación con H3PO4 y ZnCl2 permite obtener materiales microporosos,
pero con una distribución de tamaños de microporos ancha [10,13,15,16].
En consecuencia, no es posible obtener carbones activados con elevada
superficie específica utilizando estos agentes activantes. Sin embargo, el
uso de hidróxidos metálicos como agentes activantes permite obtener
carbones activados esencialmente microporosos con una superficie
específica muy elevada [12,14,17]. Concretamente, la activación química
con KOH proporciona desarrollos mayores de microporosidad estrecha.
La optimización de distintas variables (tiempo de activación, temperatura
de activación, velocidad de calentamiento, etc.) permite obtener carbones
activados con áreas superficiales aparentes muy elevadas y una
distribución de tamaños de poros estrecha, alcanzando valores superiores
a 3000 m2/g con un tamaño medio de poro de 1.4 nm [17].
Precursor
Material
carbonizado
Carbón
activado
Carbón
activado
Activación
física
Activación
química
Carbonización Gasificación
CO2, H2O, etc.
Impregnación/Carbonización/Lavado
KOH, NaOH, H3PO4, etc.
Introducción general
19
1.1.2 Materiales carbonosos nanomoldeados.
Los materiales carbonosos nanomoldeados son una variante de los
materiales carbonosos nanoestructurados cuya característica principal es
su porosidad ordenada. Estos materiales están constituidos por láminas de
átomos de carbono (hibridación sp2) separadas entre sí, de manera que el
espacio resultante entre dichas láminas puede ser considerado como un
poro. A diferencia de los carbones activados, estos materiales presentan
una estructura porosa muy ordenada en el espacio.
Figura 1.2. Estructura de ZTC [18].
Estos materiales se sintetizan utilizando métodos de nanomoldeo, en
los que se emplea una plantilla, de manera que la estructura del material
obtenido depende de la plantilla utilizada. Las plantillas se pueden
clasificar como blandas (“soft-templates”), cuando emplean surfactantes
como moldes, o bien duras (hard-templates), como sólidos inorgánicos.
Capítulo 1
20
Algunos ejemplos de materiales carbonosos obtenidos mediante estas
técnicas son: carbones mesoporosos ordenados (OMC, del inglés ordered
mesoporous carbon), materiales de porosidad jerárquica (HPC,
hierarchical porous carbon) y materiales nanomoldeados obtenidos
utilizando zeolitas como plantilla (ZTC, zeolite templated carbon) [18].
Concretamente, los ZTCs se caracterizan por presentar una porosidad
muy elevada. Están formadas por láminas de grafeno curvadas, cuya
estructura es una réplica inversa de la zeolita empleada [19–21]. En
consecuencia, estos materiales presentan un tamaño de poro uniforme y
definido, que depende del sólido plantilla utilizado en la síntesis. En
particular, los ZTCs obtenidos a partir de zeolitas Y presentan propiedades
especialmente interesantes, debido a la elevada interconectividad de su
microporosidad (tamaño de poro: 1.2 nm), que le confiere una superficie
específica aparente muy elevada (de incluso 4000 m2/g). La Figura 1.2
presenta un modelo de estructura de este tipo de materiales. Además, su
red porosa presenta baja tortuosidad, de manera que facilita la difusión de
moléculas e iones a través de la misma [18]. Debido a la singularidad de
su estructura, estos materiales presentan una concentración muy elevada
de sitios borde accesibles (diez veces superior a los carbones activados
convencionales). Además, estos sitios presentan una reactividad superior
a la habitual [22]. Como consecuencia, estos materiales presentan un
elevado contenido en oxígeno cuando se exponen al aire. Además, su
química superficial se puede modificar por medio de diversas estrategias
[23–25]. Estas propiedades hacen que sea un material de interés en una
Introducción general
21
gran variedad de aplicaciones: supercondensadores, adsorción de gases,
pilas de combustible, baterías, soporte de catalizadores, etc. [18,20].
1.2. Química superficial de los materiales carbonosos
Como se ha indicado, los materiales carbonosos presentan una
química superficial muy heterogénea. Esto se debe a la presencia de sitios
activos en la superficie. Los más comunes son los átomos de carbono
insaturados en los extremos de las láminas grafénicas (sitios “borde”),
que, del mismo modo que las imperfecciones en los planos basales,
presentan una elevada densidad electrónica y, en consecuencia, pueden
quimisorber heteroátomos como el hidrógeno, oxígeno, nitrógeno, azufre,
etc.
1.2.1. Oxígeno
El oxígeno es el heteroátomo más abundante en los materiales de
carbón, debido a que reaccionan espontáneamente con el aire a bajas
temperaturas. Se encuentra en la superficie de los materiales carbonosos
en forma de grupos funcionales identificables por la química orgánica.
Las principales formas químicas del oxígeno en un material carbonoso se
resumen en la Figura 1.3 [6,26].
Se pueden generar grupos oxigenados mediante tratamientos de
oxidación en fase líquida o gaseosa [6]. Algunos de los reactivos más
utilizados son el O2 en fase gas y el HNO3, el (NH4)2S2O8 y H2O2 en fase
líquida. El tipo y cantidad de grupos generados depende del tratamiento
utilizado. Por ejemplo, tanto la oxidación mediante HNO3 como con aire,
genera grupos oxigenados en toda la superficie; sin embargo, el HNO3
Capítulo 1
22
produce una mayor cantidad grupos carboxílicos, mientras que el aire da
lugar a fenoles y carbonilos en mayor medida.
Figura 1.3. Grupos funcionales oxigenados presentes en la superficie de los materiales
carbonosos: (a) ácido carboxílico, (b) lactona, (c) hidroxilo, (d) carbonilo, (e) quinona,
(f) pirona, (g) éter, (h) anhídrido carboxílico, (i) cromeno y (j) lactol.
En general, estas especies se pueden clasificar atendiendo a su
carácter ácido-base. Se consideran grupos ácidos los ácidos carboxílicos,
anhídridos, hidroxilos, lactonas y lactoles. Sin embargo, existe una mayor
controversia en lo que concierne a los grupos funcionales básicos.
Algunos trabajos identifican como básicos a los grupos cromenos, pironas
y quinonas [26], aunque los electrones de los planes basales de las
láminas de grafeno también han sido identificados como sitios básicos
[27].
1.2.2. Nitrógeno
Los carbones activados no presentan nitrógeno de forma espontánea,
como sucede con el oxígeno. Los grupos funcionales nitrogenados
Introducción general
23
presentes en los carbones activados se pueden generar, de forma general,
mediante la carbonización/activación de un precursor que contenga
nitrógeno, o bien, mediante un post-tratamiento del material con un
reactivo nitrogenado, como NH3 o HCN [28].
La Figura 1.4 resume los grupos nitrogenados principales que se
pueden encontrar en la superficie de un material de carbón [6,26,28].
Figura 1.4. Grupos funcionales nitrogenados presentes en la superficie de los materiales
carbonosos: (a) piridina, (b) piridona, (c) piridina N-oxidada, (d) nitro, (e) nitroso, (f)
amida, (g) lactama, (h) nitrilo, (i) amina primaria, (j) amina secundaria, (k) amina
terciaria, (l) imina, (m) nitrógeno cuaternario en el borde de la lámina, (n) nitrógeno
cuaternario en el interior de la lámina y (ñ) pirrol.
Los grupos funcionales nitrogenados presentan, en general,
propiedades básicas que pueden facilitar la interacción entre moléculas
ácidas y la superficie del material carbonoso, mediante enlaces covalentes,
enlaces de hidrógeno y/o interacciones dipolo-dipolo. Esta característica
permite que los carbones activados nitrogenados se puedan utilizar en
distintas aplicaciones, tanto en fase gaseosa como líquida. En lo que
Capítulo 1
24
concierne a interacciones sólido-gas, estos materiales se pueden utilizar
para la eliminación de H2S [29,30], control de SO2 [31] y captura de CO2
[32–34]. En fase líquida, presentan una elevada capacidad de adsorción
de aniones y metales pesados. Además, también se ha investigado su uso
en la reacción de reducción de oxígeno en pilas de combustible y en
supercondensadores [9].
1.3. Síntesis de materiales carbonosos dopados con nitrógeno
Como se ha descrito, la química superficial es un parámetro
fundamental que condiciona gran parte de las aplicaciones de los
materiales carbonosos. En la actualidad, existen numerosos estudios
centrados en la introducción de distintos heteroátomos en este tipo de
materiales [28,35–37].
Se pueden introducir grupos funcionales nitrogenados en un material
carbonoso principalmente mediante dos métodos: (I) empleando en el
proceso de síntesis un precursor carbonoso que contenga nitrógeno; y (II)
el post-tratamiento de un material carbonoso con un reactivo nitrogenado
[10]. Ambas metodologías suelen emplear elevadas temperaturas en el
proceso de síntesis o modificación. La química superficial del material
resultante depende del tratamiento utilizado, que incluye el
precursor/reactivo nitrogenado y la temperatura del tratamiento. Esto se
debe a que no todos los compuestos nitrogenados son estables
térmicamente; los tratamientos a elevada temperatura (superior a 600 ºC)
generan especies aromáticas, como nitrógeno cuaternario, piridina y
pirrol; mientras que los tratamientos a baja temperatura (inferior a 600
ºC), producen especies como aminas, amidas y lactamas, que por
Introducción general
25
tratamiento térmico a temperaturas superiores pueden transformarse en las
especies aromáticas citadas [6].
1.3.1. Síntesis directa a partir de un precursor nitrogenado
Los métodos más comunes para obtener materiales carbonosos de
elevada porosidad funcionalizados con nitrógeno se pueden resumir del
siguiente modo [28,36]:
(i) Carbonización y activación física (con vapor de H2O o
CO2) [38] o química (mediante KOH, H3PO4, etc.) [39] de
un precursor nitrogenado, como poliacrilonitrilo,
polianilina o melamina.
(ii) Depósito químico en fase vapor de un precursor que
contenga nitrógeno (acetonitrilo) empleando distintas
plantillas (zeolitas, sílices, etc.) seguido de un tratamiento
térmico [24,25,40].
(iii) Carbonización hidrotermal de precursores biomásicos que
contengan nitrógeno (glucosamina, ácido cianúrico, etc.).
De forma general, estos materiales presentan mayor contenido y una
distribución homogénea del heteroátomo en la matriz del material
carbonoso (no exclusivamente en la superficie). Además, como la
temperatura habitual en estos procesos de síntesis es elevada (> 600 ºC),
estos materiales presentan una química superficial compuesta
principalmente por especies aromáticas, como pirrol y piridina. También
es habitual la presencia de nitrógeno cuaternario distribuido en las láminas
Capítulo 1
26
grafénicas de los materiales. Un ejemplo es el ZTC dopado con nitrógeno,
que presenta un elevado contenido de nitrógeno cuaternario [25].
1.3.2. Post-tratamiento de materiales carbonosos con reactivos
nitrogenados
Una de las metodologías más utilizadas para incorporar nitrógeno
en los materiales carbonosos se basa en post-tratamientos aplicados a un
material que ha sido sintetizado previamente. Los métodos más habituales
se basan en la reacción de un material carbonoso con un reactivo que
contenga nitrógeno por medio de un tratamiento térmico. No obstante,
también se han utilizado ampliamente estrategias basadas en reacciones
orgánicas para incorporar grupos funcionales en materiales
nanoestructurados, o inmovilizar metales en carbones activados. Por
último, las técnicas electroquímicas son de gran interés debido a su
elevada selectividad. A continuación, se detallan las características
principales de estos métodos de funcionalización.
1.3.2.1. Funcionalización con nitrógeno mediante tratamientos térmicos
Los materiales carbonosos pueden tratarse con reactivos que
contengan nitrógeno, en fase líquida o gaseosa, para introducir grupos
nitrogenados en la superficie. Se han utilizado reactivos muy variados,
como NH3 o HCN en fase gaseosa, o N,N-dimetilformamida, urea o
melamina en fase líquida [31,41]. El reactivo más empleado es el NH3,
que puede utilizarse solo (aminación) o en presencia de aire
(amoxidación). La química superficial y las propiedades texturales del
material dependen del material carbonoso de partida, del reactivo
nitrogenado y de la temperatura del tratamiento [42].
Introducción general
27
De este modo, la química superficial de los carbones nitrogenados
obtenidos mediante post-tratamiento puede ser más heterogénea que la de
los obtenidos a partir de un precursor carbonoso, o proporcionar grupos
funcionales distintos (como amidas, aminas, etc.), ya que pueden
presentar especies de baja estabilidad térmica si la temperatura del
tratamiento no es elevada. Además, el nitrógeno suele anclarse a la
superficie mediante una reacción química con los sitios reactivos de la
superficie del material carbonoso. En consecuencia, el contenido en
nitrógeno suele ser menor que en aquellos materiales obtenidos a partir de
un precursor carbonoso, donde el heteroátomo se encuentra inicialmente
en la estructura del precursor; sin embargo, este método puede ser más
selectivo hacia determinados grupos funcionales. Puesto que esta Tesis
Doctoral, se centra en la introducción de grupos funcionales nitrogenados
por este método, se va a explicar con más detalle la bibliografía
relacionada.
En este tipo de tratamientos, la textura porosa está fuertemente
influenciada por la temperatura. En el caso de tratamientos con NH3,
normalmente tiene lugar un aumento de la microporosidad y de la
superficie específica [43], y sólo en algunos casos disminuye [44] o se
mantiene constante [42]. Cuando se realizan tratamientos de amoxidación,
la estructura porosa y la superficie específica pueden no variar o bien
aumentar o decrecer. En el caso de emplear otros reactivos, como aminas
o HNO3, la superficie específica y la porosidad suele disminuir [45].
En los procesos de post-tratamiento, es frecuente realizar una etapa
previa de preoxidación en fase líquida, puesto que la presencia de grupos
Capítulo 1
28
oxigenados en los materiales carbonosos facilita la introducción de otros
heteroátomos mediante reacciones de sustitución, condensación y
similares, funcionando el grupo oxigenado como punto de unión con la
superficie a funcionalizar. Un ejemplo es la impregnación de un carbón
oxidado en HNO3 con urea, seguida de un tratamiento térmico, en el que
se observó que la adsorción de urea dependía de la cantidad y tipo de
especies oxigenadas presentes [46]. Este tipo de funcionalización se basa
en la interacción ácido-base entre una amina y un ácido carboxílico, en la
que este último es el compuesto que se encuentra sobre la superficie del
material. El ión amonio generado queda anclado a la superficie por un
enlace iónico, y mediante un tratamiento térmico da lugar a la formación
de amidas, imidas y nitrilos.
Otra estrategia para obtener materiales carbonosos de elevada
porosidad con grupos funcionales nitrogenados consiste en realizar
tratamientos térmicos en atmósfera inerte de materiales compuestos
carbón/polímero. Un ejemplo son los carbones activados y fibras de
carbón activadas obtenidas a partir de materiales compuestos
carbón/polianilina y carbón/polipirrol [47–49]. Los materiales
compuestos se pueden preparar por polimerización química o
electroquímica [47]. En estos materiales, el depósito de polímero sobre el
material carbonoso se produce mediante enlaces no covalentes, aunque es
posible la interacción covalente entre grupos funcionales del material
carbonoso y el polímero [47]. Esta metodología permite la generación de
una película de polímero conductor sobre la microporosidad del material
carbonosos [50], de manera que, cuando son tratados térmicamente,
Introducción general
29
conducen a la generación de materiales carbonosos avanzados en los que
se generan residuos del material carbonizado en la microporosidad del
material [48]. Los grupos funcionales generados dependen de la
temperatura a la que se realice el tratamiento térmico. En el caso de
materiales obtenidos a partir de polianilina, a elevada temperatura (800
ºC), se da la formación de heterociclos nitrogenados, con una mayor
selectividad de pirroles/piridonas. Esta estrategia permite obtener
materiales con propiedades electroquímicas interesantes para su
aplicación en condensadores electroquímicos [47,48] y pilas de
combustible [49].
1.3.2.2. Funcionalización química basada en reacciones orgánicas a baja
temperatura
Este procedimiento se ha empleado en diversos materiales
carbonosos para distintas aplicaciones. En carbones activados, se han
realizado estudios para introducir aminas, mediante un enlace amida, para
la adsorción de plomo [51] y para la captura de CO2 [34]; también para
anclar complejos de metales de aplicación catalítica, mediante reacciones
de amidación [52–54]. Las estrategias más habituales emplean un proceso
de oxidación previo para aumentar la concentración de sitios reactivos en
superficie. La generación de ácidos carboxílicos permite que reaccionen
con las aminas sin necesidad de utilizar un tratamiento térmico, que pueda
disminuir la porosidad. El método más utilizado para funcionalizar
materiales carbonosos es el tratamiento con SOCl2 en un disolvente
orgánico. Este tratamiento produce cloruros de ácido en la superficie, los
derivados de ácido son más reactivos, de manera que la sustitución
Capítulo 1
30
nucleofílica en este grupo funcional por aminas, alcoholes, etc., queda
facilitada. Por tanto, se trata de una ruta sintética adecuada para la
funcionalización de materiales carbonosos y, en concreto, para la
introducción de grupos nitrogenados (aminas y amidas) en la superficie.
Sin embargo, estas modificaciones pueden cambiar la textura porosa
del material. Tamai y col. [55] observaron que la microporosidad de los
carbones activados disminuye cuando se introduce etilendiamina y
hexametilendiamina, mientras que la mesoporosidad no resulta afectada.
Esto supone un inconveniente para la aplicación de estos materiales. En
lo que concierne a la captura de CO2, los grupos amina son deseables
porque pueden experimentar quimisorción de CO2, pero si la cantidad de
microporos, muy necesarios para que el material posea una elevada
capacidad de adsorción del mismo, se ve reducida, toda mejora causada
por la introducción de aminas puede verse disminuida y hasta anulada por
completo.
Por otro lado, los grupos nitrogenados pueden mejorar el
comportamiento de los electrodos de supercondensadores, ya que pueden
aumentar la capacidad del material mediante reacciones faradaicas
superficiales (pseudocapacidad), la conductividad eléctrica y la
mojabilidad, entre otros aspectos [9]. La capacidad del material es
directamente proporcional a la superficie del mismo, por lo que, como en
el caso de la captura de CO2, la disminución de su superficie como
consecuencia de la introducción de grupos funcionales es desfavorable.
Introducción general
31
Esta pérdida de superficie específica está relacionada con el tamaño
del grupo funcional anclado a la superficie, ya que los grupos muy
voluminosos pueden bloquear el acceso a los microporos. En este sentido,
existen estudios previos en los que la formación selectiva de aminas
superficiales (directamente enlazadas al anillo aromático del carbón
activado) mediante una ruta distinta de reacciones orgánicas (reducción
de grupos nitro generados por tratamiento en HNO3 concentrado), no
disminuye la porosidad de la muestra de carbón activado [56]. Se debe
tener en cuenta que la diferencia fundamental entre ambos métodos
[55,56] es que el primero introduce las aminas generando un grupo amida,
de tal manera que la superficie resultante presentará, por cada sitio
reactivo, un grupo amida y la amina enlazada al mismo, además de la
cadena alifática correspondiente; mientras que, en el segundo método, las
aminas están directamente ancladas a la superficie del material, por lo que
el volumen de la especie generada es mucho menor.
Por tanto, la introducción de grupos nitrogenados mediante
reacciones orgánicas permite obtener grupos funcionales de forma más
selectiva, sin necesidad de emplear un tratamiento térmico que pueda
influir en las propiedades texturales del material y que modifique de forma
no deseada su química superficial. Las propiedades texturales del material
resultante están relacionadas con el tamaño de los grupos funcionales
generados en la superficie que, si no es elevado, no debe disminuir
significativamente la superficie específica.
Algunos investigadores han realizado funcionalizaciones similares a
las descritas anteriormente con el fin de introducir amidas primarias [57]
Capítulo 1
32
y aminas aromáticas [58] en la superficie de nanotubos de carbono. En
ambos casos, los nanotubos de carbono se tratan previamente para generar
ácidos carboxílicos en la superficie. Posteriormente, reaccionan con
SOCl2 para generar cloruros de ácido y, a continuación, ambas muestras
se tratan con reactivos nitrogenados (disolución acuosa de NH3 y
(NH4)2CO3/Piridina/DMF, respectivamente) para generar amidas
primarias.
En el segundo caso [58], los investigadores realizan una reacción más
para generar aminas primarias a partir de amidas primarias. Para esto,
utilizan la transposición de Hofmann, una reacción ampliamente
estudiada en química orgánica que consiste en tratar una amida primaria
con bromo en medio básico. En estas condiciones, el bromo reacciona con
el nitrógeno de la amida y genera un intermedio de reacción (bromoamida)
que, en medio básico, se desprotona dando lugar a la transposición del
sustituyente de la amida y la pérdida del anión bromuro, de tal manera que
se forma un isocianato. Esta especie reacciona con la base presente en el
medio. En el caso de NaOH, se forma un ácido carbámico, que en medio
básico se desprotona y disocia en forma de CO2, dando lugar a una amina
primaria. Si la base presente es NaOCH3, la especie que se genera es un
carbamato estable. En este caso, la hidrólisis de esta especie genera la
amina primaria [59].
Esta última ruta de modificación es de especial interés si el material
en el que se aplica es un sólido microporoso, ya que, como se ha indicado
anteriormente, permite introducir grupos funcionales nitrogenados en
condiciones suaves que no alteran en gran medida la porosidad, que
Introducción general
33
resulta fundamental en aquellas aplicaciones en las que los carbones
nitrogenados son interesantes. Además, permite determinar la influencia
de grupos superficiales de naturaleza muy diferente en un material
carbonoso concreto y, por tanto, estimar cómo modifica la química
superficial las propiedades de estos materiales y su efecto en distintas
aplicaciones, como captura de CO2, supercondensadores o reducción de
oxígeno.
Por estos motivos, en esta Tesis Doctoral se plantea la modificación
de la química superficial de un carbón activado mediante reacciones
orgánicas y estudiar el efecto de la química superficial en su
comportamiento como electrodos de supercondensadores. La ruta de
modificación consiste en una oxidación química con HNO3 para generar
preferentemente ácidos carboxílicos, la posterior activación de estos
grupos mediante SOCl2, que facilite la creación de grupos funcionales
amida y, consecutivamente, la obtención de aminas a partir de estos. La
ruta de modificación se ilustra en la Figura 1.5.
Figura 1.5. Modificación del carbón activado KUA mediante: (a) oxidación química
con HNO3 (KUA-COOH), (b) tratamiento con SOCl2 (KUA-COCl), (c) amidación
(KUA-CONH2) y (d) aminación (KUA-NH2).
Capítulo 1
34
1.3.2.3. Funcionalización electroquímica
El uso de técnicas electroquímicas ha suscitado gran interés para
modificar la química superficial de materiales carbonosos. Esto se debe a
que estas técnicas son sencillas de aplicar y controlar, se dan a temperatura
ambiente y presión atmosférica, y proporcionan elevada reproducibilidad,
sensibilidad y selectividad [60].
Las técnicas electroquímicas se han utilizado ampliamente para
introducir grupos funcionales nitrogenados en los materiales carbonosos.
El proceso de funcionalización se basa en la aplicación de una corriente
para anclar una molécula disuelta, un ión del elecrolito o incluso una
molécula del disolvente en la superficie del material. La técnica más
conocida es el uso de sales de diazonio con distintos heteroátomos para
modificar el material con grupos amina o grupos nitro, aunque también es
posible introducir otros heteroátomos (Cl, Br, etc.) [61–63]. Otros
ejemplos se basan en la oxidación de aminas o ácidos aminobenzoicos
para anclar diversos grupos funcionales a la superficie [64][65]. Estas
estrategias se han empleado con materiales carbonosos de elevada
porosidad, permitiendo conservar las propiedades derivadas de la textura
porosa del material sin funcionalizar [23].
Introducción general
35
1.4. Aplicaciones en almacenamiento y generación de energía
eléctrica.
1.4.1. Condensadores electroquímicos
Los condensadores electroquímicos o supercondensadores son
sistemas de almacenamiento de energía de elevada potencia compuestos
por dos electrodos en contacto con un electrolito y separados por una
membrana. Estos dispositivos presentan un amplio abanico de
aplicaciones: vehículos eléctricos, dispositivos digitales, etc. En
comparación con los condensadores convencionales, estos dispositivos
proporcionan densidades de energía superiores, debido al uso de
electrodos que permiten un almacenamiento superior de carga. La Figura
1.6 presenta el diagrama de Ragone con los principales dispositivos de
almacenamiento y generación de energía. En comparación con las baterías
y las pilas de combustible, los supercondensadores presentan una potencia
muy superior, pero una densidad de energía limitada. Por estos motivos,
es necesario continuar investigando para poder alcanzar densidades de
energía que permitan extender su uso a aplicaciones propias de las baterías
[9,66–68].
Capítulo 1
36
Figura 1.6. Diagrama de Ragone [69].
Dependiendo del mecanismo de almacenamiento de carga, se pueden
clasificar como condensadores de doble capa eléctrica (EDLC, electric
double-layer capacitor) o pseudocondensadores [8,9]. Los EDLC están
basados en interacciones electrostáticas debidas a la formación de una
doble capa eléctrica en la interfase electrodo-disolución. Este mecanismo
es el predominante cuando se utilizan materiales carbonosos de elevada
porosidad como electrodos. En el caso de los pseudocondensadores, el
almacenamiento de energía viene dado por reacciones farádicas
reversibles que se dan en la superficie del electrodo. Este proceso es el
que rige el mecanismo de almacenamiento de carga cuando se utilizan
óxidos metálicos o polímeros conductores como electrodos del
condensador electroquímico. En esta Tesis Doctoral, los condensadores
electroquímicos empleados son de tipo doble capa eléctrica. La figura 1.7
muestra los componentes de un condensador electroquímico basado en
materiales carbonosos porosos.
Energía específica (Wh/kg)
Po
ten
cia
es
pe
cífic
a (W
/kg
)
Introducción general
37
Figura 1.7. Esquema de un condensador electroquímico o supercondensador basado en
carbones activados.
Debido a la naturaleza electrostática del fenómeno de energía, los
EDLC se pueden cargar y descargar en segundos, lo que les confiere una
potencia muy elevada. Además, el mecanismo es idealmente reversible,
por lo que presentan una elevada eficiencia coulómbica y tiempos de vida
media muy superiores a los de las baterías. Concretamente, los EDLC
pueden cargarse y descargarse durante miles de ciclos (>500000 ciclos),
sin perder la capacidad original del dispositivo.
1.4.1.1. Modelos de doble capa eléctrica
Para comprender en mayor medida los procesos de almacenamiento
de energía que suceden en los EDLC es necesario describir las
características de la doble capa eléctrica que se forma en la interfase
electrodo/disolución. La doble capa eléctrica es una consecuencia de la
separación de cargas que se produce entre dos fases debido a la diferencia
de potencial causada por su distinta composición química.
---------
Carbón activadoMembrana porosaColector de corriente
+++++++++
Capítulo 1
38
Se han propuesto distintos modelos para describir el proceso de
formación de la doble capa eléctrica [70,71]. La Figura 1.8 muestra el
esquema propuesto por Helmholtz (1874) para describir la doble capa
eléctrica. Este modelo se basa en la formación de dos láminas paralelas
cargadas de signo opuesto. Una de ellas se encuentra localizada en la
superficie del electrodo y la otra en la disolución (capa rígida). La
separación entre ambas es del orden del Angstroms. La carga
correspondiente a los iones en disolución neutraliza la carga superficial
del electrodo. Según este modelo, el potencial eléctrico disminuye
linealmente con el aumento de la distancia al electrodo, hasta hacerse nulo
en el plano de la capa rígida.
Figura 1.8. (a) Esquema de la doble capa eléctrica y (b) variación del potencial con la
distancia al electrodo según el modelo de Helmholtz.
Gouy y Chapman propusieron en 1913 un modelo de capa difusa de
la doble capa eléctrica. En este modelo, no tiene lugar la formación de una
capa rígida de los iones en la disolución, sino una capa difusa como
consecuencia de la agitación térmica que neutraliza globalmente la carga
del electrodo (Figura 1.9a). En este modelo, el potencial eléctrico varía de
forma exponencial con la distancia al electrodo (Figura 1.9b).
(a) (b)
-
+++++++++
-
-
-
-
-
-
-
-
-
-
-
- -
+
++
++
+
+
-
-
--
-
-
-
-
-
-
++
++
+
+ +
+
+
+
+
+
--
-
-
Introducción general
39
Figura 1.9. (a) Esquema de la doble capa eléctrica y (b) variación del potencial con la
distancia al electrodo según el modelo de Gouy-Chapman.
Posteriormente, Stern propuso en 1924 un modelo que combina los
modelos propuestos por Helmholtz y Gouy-Chapman, con la formación
de una capa rígida cerca del electrodo y una difusa. Este modelo tiene en
cuenta el movimiento hidrodinámico de los iones en la capa difusa y la
acumulación de los mismos en la superficie del electrodo. De esta forma,
la carga del electrodo se compensa tanto por los iones que se encuentran
cerca del mismo como por los que se encuentran en la capa difusa (Figura
1.10a). En este modelo, el potencial eléctrico varía de forma lineal cerca
del electrodo y disminuye de forma exponencial cuando aumenta la
distancia al mismo (Figura 1.10b) y la capacidad total de la interfase se
puede considerar como dos condensadores en serie.
(a) (b)+++++++++
-
--
- -
---
-
- -
--
-
--
-
- -+
++
+ +
+
+
-
-
-
-
-
-
-
-
-
--
Capítulo 1
40
Figura 1.10. (a) Esquema de la doble capa eléctrica y (b) variación del potencial con la
distancia al electrodo según el modelo de Stern.
Finalmente, Grahame propuso en 1947 un nuevo modelo teniendo en
cuenta la solvatación de los iones en disolución y el carácter dipolar del
agua. Teniendo en cuenta estas variables, se propone la existencia de
distintos planos de acercamiento a la superficie del electrodo,
dependiendo de si el ión se encuentra solvatado o interacciona de forma
directa. Este modelo propone la existencia de diversas capas (Figura
1.11): (i) la capa real en la superficie del electrodo; (ii) una segunda capa
correspondiente al plano interno de Helmholtz en la disolución (IHP), en
la que los iones se encuentran adsorbidos en la superficie del electrodo y
han perdido su esfera de solvatación; (iii) una tercera capa, denominada
plano externo de Helmholtz (OHP), donde se encuentran los iones
solvatados. Las cargas se distribuyen en la zona compacta, de espesor 0.5
nm aprox. (debido a las moléculas de disolvente e iones), y una capa
difusa, de 1 a 100 nm, debido a los iones distribuidos térmicamente.
(a) (b)+++++++++
-
-
---
-
-
-
--
-
--
-
-
-
-
-
-
+
+
+
+
+
+
+
+
++
-
--
-
-
-
-
-
Introducción general
41
Figura 1.11. Esquema de la doble capa eléctrica según el modelo de Grahame.
1.4.1.2. Características de los condensadores electroquímicos
1.4.1.2.1. Energía y potencia
Los dos parámetros fundamentales para caracterizar un sistema de
almacenamiento son la energía y la potencia.
El trabajo realizado cuando se aplica un voltaje en un
supercondensador para trasladar una carga a la interfase
electrodo/electrolito, viene dado por la ecuación (1.2).
𝑑𝑤 = 𝑉 𝑑𝑄 (1.1)
Donde dW es el diferencial de trabajo, V es el voltaje y dQ el
diferencial de carga.
El trabajo realizado es equivalente a la energía almacenada
(considerando despreciable las pérdidas por disipación de calor), y el
voltaje está relacionado con la carga y la capacidad, por lo que se puede
integrar la ecuación (1.2) como se indica a continuación:
+++++++++
+
+
+
+
+
+
+
-
-
-
--
Catión
solvatado-
+
Anión adsorbido
específicamente
IHP
OHP
Capítulo 1
42
𝑊 = 𝐸 = ∫ 𝑉 𝑑𝑄𝑄
0= ∫
𝑄
𝐶
𝑄
0 𝑑𝑄 =
1
2 𝐶𝑉2 (1.2)
Donde W es el trabajo realizado, E es la energía y C es la capacidad.
Por tanto, la energía de un condensador depende fundamentalmente de la
capacidad y el voltaje aplicado. Ambas variables vienen dadas por los
componentes utilizados en su fabricación (electrodos y electrolitos).
La potencia máxima suministrable del condensador electroquímico
viene dada por la ecuación (1.3).
𝑃 = 𝑉2
4 𝐸𝑆𝑅 (1.3)
Donde P es la potencia máxima y ESR es la resistencia equivalente
en serie del dispositivo.
De las expresiones de energía y potencia características de un
condensador electroquímico, se puede deducir que las características
principales de estos dispositivos son: resistencia equivalente en serie,
voltaje y capacidad. A continuación, se detallan los factores principales
que afectan estas variables.
1.4.1.2.2. Resistencia equivalente en serie
La resistencia equivalente en serie de un condensador electroquímico
depende de los componentes de la celda electroquímica: (i) la resistencia
del electrodo (que incluye la resistencia interpartícula e intrapartícula); (ii)
la resistencia entre el electrodo y el colector de corriente; (iii) la resistencia
del electrolito; (iv) la resistencia debida a la difusión de los iones en la
Introducción general
43
estructura porosa del material; (v) resistencia del separador; (vi)
resistencias de contacto externas [72].
La pérdida de potencia generada por el aumento de ESR produce un
aumento significativo del calor del sistema que puede limitar su
comportamiento. Cuando el dispositivo se degrada (debido a su operación
a elevada potencia, elevada temperatura o a la generación de gases),
aumenta la ESR y conduce a un empeoramiento del comportamiento del
dispositivo y reducción del tiempo de vida [72].
Por estos motivos, se debe prestar especial atención a minimizar la
ESR de los electrodos carbonosos. Tradicionalmente, los electrodos de
carbones activados se preparan utilizando un aglomerante que permite
conformar el material [73]. Algunos ejemplos de los aglomerantes son:
teflón (PTFE, del inglés politetrafluoroetilene), fluoruro de polivinilideno
(PVDF, del inglés polyvinylidene) y carboximetilcelulosa (CMC). Este
componente aumenta el contacto interpartícula; sin embargo, los
aglomerantes habituales disminuyen la conductividad global del electrodo
(al tener una conductividad menor que los carbones activados). Por este
motivo, la elaboración de los electrodos incluye el uso de promotores de
conductividad. El más empleado es el negro de carbón, aunque también
se ha propuesto el uso de nanotubos de carbono [74].
Algunas estrategias novedosas para minimizar la resistencia de los
electrodos son: (i) empleo de materiales avanzados de elevada
conductividad como fase activa, como los nanotubos de carbono [74]; (ii)
reducir la tortuosidad de la red porosa de los electrodos, mediante el
Capítulo 1
44
empleo de materiales carbonosos nanoestructurados de porosidad
ordenada [74,75]; (iii) sintetizando el material carbonoso directamente
sobre el colector de corriente [76,77] ; (iv) preparación de monolitos de
materiales carbonosos, con una estructura continua que mejore el contacto
interpartícula en comparación con el carbón activado en polvo [78].
La ESR se puede determinar a partir del diagrama de Nyquist (por
medio de EIS) o bien a partir de la caída óhmica cuando se descarga el
condensador a corriente constante. Estas técnicas se discuten en mayor
medida en el Capítulo 2.
1.4.1.2.3. Voltaje
En las aplicaciones prácticas de los condensadores electroquímicos,
es necesario utilizar ventanas de potencial (o voltajes) elevados para
maximizar el almacenamiento de carga. Sin embargo, el voltaje depende
del electrodo y del electrolito. En el caso del electrolito, la estabilidad
depende intrínsecamente del disolvente (acuoso, orgánico y líquido
iónico) y de la sal conductora utilizada. En medio acuoso, el potencial
termodinámico de descomposición del agua limita la operación de los
supercondensadores a voltajes en torno a 1V. En el caso de electrolitos
orgánicos, se pueden alcanzar valores de 2.5 – 3V. Finalmente, el uso de
líquidos iónicos como electrolitos (libres de disolvente) permiten alcanzar
voltajes de incluso 5V.
Los materiales carbonosos presentan una superficie casi idealmente
polarizable en disoluciones electrolíticas. No obstante, se deben emplear
materiales con elevada estabilidad electroquímica (elevado sobrepotencial
Introducción general
45
para las reacciones de descomposición del electrolito) para obtener una
durabilidad superior.
1.4.1.2.4. Capacidad
Los mecanismos de almacenamiento de energía en un condensador
electroquímico son: (i) formación de la doble capa eléctrica debido a la
adsorción de los iones en la superficie del material carbonoso (proceso
capacitivo) y (ii) reacciones redox rápidas y reversibles, que se dan en la
superficie de los electrodos (proceso pseudocapacitivo). En los
condensadores de doble capa eléctrica (basados en materiales
carbonosos), el mecanismo principal es la electroadsorción de iones en la
porosidad del material. No obstante, la presencia de grupos funcionales en
la superficie puede dar lugar a reacciones redox rápidas que contribuyen
al almacenamiento de energía mediante pseudocapacidad.
Los métodos de determinación de la capacidad de un condensador y
de un electrodo se detallan en el Capítulo 2.
1.4.1.3. Configuraciones de condensador electroquímico
1.4.1.3.1. Condensador simétrico
La configuración simétrica es la más sencilla y común de los
condensadores electroquímicos. Consiste en utilizar dos electrodos del
mismo material con la misma masa. Se trata de la configuración más
frecuente y más sencilla de utilizar en la industria. Sin embargo, en esta
configuración no es posible utilizar totalmente las prestaciones de los
electrodos y electrolitos para maximizar el almacenamiento de carga.
Capítulo 1
46
1.4.1.3.2. Condensador asimétrico
Los materiales utilizados como electrodos en condensadores pueden
no mostrar la misma capacidad en el intervalo de potenciales en los que
cada electrodo trabaja. El caso más destacable es cuando se emplean
materiales esencialmente pseudocapacitivos (polímeros conductores y
óxidos metálicos), en los que las reacciones redox se producen a
potenciales específicos, y por tanto no se da una variación constante de la
capacidad con el voltaje. En estos casos, la capacidad depende del
potencial aplicado. En el caso de los condensadores basados en materiales
carbonosos, este efecto se puede dar cuando se utilizan electrolitos con
tamaños de catión y anión diferentes, y el tamaño de poro efectivo para la
electroadsorción es similar al de uno de los iones [79]. En estos casos, la
adsorción de uno de los iones será superior a la del otro, y presentará
mayor capacidad en el intervalo de potencial en el que su adsorción sea
favorable. En estos casos, se puede optimizar la masa de los electrodos
para maximizar la energía almacenada en el condensador. La relación
óptima de masas fue propuesta por Snook y col. [80]:
𝑚+
𝑚−= √
𝐶−
𝐶+ (1.4)
Donde 𝑚+ y 𝑚− son las masas de los electrodos positivo y negativo,
respectivamente, y 𝐶+ y 𝐶− son las capacidades de los electrodos positivo
y negativo, respectivamente.
Otro de los motivos de desaprovechamiento de la energía máxima de
un condensador es que los materiales sean estables en ventanas de
Introducción general
47
potenciales significativamente diferentes. En un condensador en
funcionamiento, el electrodo positivo se polariza desde el potencial a
circuito abierto hasta potenciales positivos, que están limitados por la
descomposición del electrolito. El electrodo negativo funciona de forma
análoga, pero a potenciales negativos. Si el electrodo positivo trabaja en
una menor ventana de potenciales como consecuencia de que el potencial
a circuito abierto es alto y se alcanza pronto la descomposición del
electrolito, el electrodo negativo no podrá emplear toda su ventana de
potenciales de estabilidad completa; por tanto, no se podrá utilizar el
voltaje teórico completo que proporcionan ambos electrodos en dicho
electrolito. En este caso, se puede determinar la relación óptima de masas
conociendo la ventana de estabilidad y la capacidad de los electrodos en
dichas ventanas, estableciendo una relación igual de cargas entre el
electrodo positivo y negativo [81]:
𝑄 = 𝑄+ = 𝑄− (1.5)
𝑚+ · 𝐶+ · 𝛥𝐸+ = 𝑚− · 𝐶− · 𝛥𝐸− (1.6)
𝑚+
𝑚−=
𝐶−·𝛥𝐸−
𝐶+· 𝛥𝐸+ (1.7)
Donde Q es la carga del condensador electroquímico, 𝑄+ es la carga
del electrodo positivo, 𝑄− es la carga del electrodo negativo, 𝛥𝐸+ y 𝛥𝐸−
son las ventanas de potenciales de estabilidad de los electrodos positivo y
negativo, respectivamente.
Capítulo 1
48
1.4.1.3.3. Condensador híbrido
Los condensadores híbridos son aquellos que emplean distintos
materiales como electrodo positivo y negativo. Normalmente, uno de los
electrodos presenta comportamiento propio de condensador y el otro
presenta un comportamiento característico de batería [8]. En estos
sistemas, se pueden alcanzar voltajes más elevados.
1.4.1.4. Tipos de electrolitos
Los electrolitos utilizados en los condensadores electroquímicos
pueden ser líquidos, sólidos o semi-sólidos o “activos-redox” [82]. En esta
Tesis Doctoral, los electrolitos empleados son líquidos.
Los electrolitos líquidos se clasifican como acuosos, orgánicos o
líquidos iónicos. Las propiedades principales a tener en cuenta para
seleccionar un electrolito son su ventana de potenciales de estabilidad y
su conductividad iónica. La ventana de potenciales de estabilidad es un
parámetro clave en lo que concierne a la energía específica, mientras que
la conductividad iónica tiene una influencia muy notable en la potencia
del dispositivo [83]. A continuación, se describen las propiedades
principales de los distintos tipos de electrolitos.
1.4.1.4.1. Electrolitos acuosos
Los electrolitos acuosos presentan diversas ventajas para su uso en
supercondensadores. En comparación con los electrolitos orgánicos y
líquidos iónicos, tienen mayor conductividad (~ 1 S/cm) y proporcionan
valores superiores de capacidad específica (de hasta 300 F/g por medio de
Introducción general
49
carbones activados de elevada microporosidad [3,84]) Además, presentan
menor coste, son menos perjudiciales para el medio ambiente y facilitan
la construcción del dispositivo, dado que no requieren el uso de cámaras
de atmósfera inerte, como sucede con los electrolitos orgánicos y líquidos
iónicos. Sin embargo, el potencial termodinámico de descomposición del
agua (1.23 V) limita los voltajes de estos supercondensadores a
aproximadamente 1V. Por tanto, los condensadores electroquímicos
basados en electrolitos acuosos proporcionan valores de energía inferiores
a los característicos en medio orgánico o líquido iónico [9,82,83].
Los electrolitos acuosos se pueden clasificar como: ácidos, básicos y
neutros. Los más representativos de cada uno de estos grupos son H2SO4,
KOH y Na2SO4. En medio ácido, el voltaje suele estar limitado a 1-1.3 V
[82]. Sin embargo, dado que los materiales carbonosos pueden presentar
distintos sobrepotenciales para la descomposición del agua, se ha
conseguido ampliar el voltaje a 1.6 V utilizando materiales distintos como
electrodos positivo y negativos y balanceando la masa de los electrodos
[85]. En el caso de los electrolitos neutros, la capacidad específica y la
conductividad iónica es inferior a la de H2SO4 y KOH, por lo que los
dispositivos presentan mayor ESR. Sin embargo, estos electrolitos son
menos corrosivos y presentan mayor ventana de estabilidad, por lo que
permiten alcanzar voltajes superiores. Se han determinado valores de 1.9-
2.2V empleando configuraciones simétricas en Li2SO4 [86,87]. En
Na2SO4, se han obtenidos voltajes de incluso 2.2 V por medio de
configuraciones asimétricas [77].
Capítulo 1
50
1.4.1.4.2. Electrolitos orgánicos
Los electrolitos orgánicos más comunes están basados en sales de
amonio cuaternarias disueltas en un disolvente orgánico, como el
carbonato de propileno (PC) o el acetonitrilo (ACN). Las sales más
empleadas son tetrafluoroborato de tetraetilamonio (TEABF4) y
tetrafluoroborato de tetraetilmetilamonio (TEMABF4). Estos electrolitos
tienen menor conductividad (~ 0.02 S/cm) y mayor coste que los acuosos,
y proporcionan capacidades específicas inferiores (entre 150 y 200 F/g)
[9,88]. Sin embargo, permiten alcanzar valores mayores de energía que
los electrolitos acuosos, debido a la elevada estabilidad electroquímica,
que permite trabajar a voltajes de 2.5 – 2.8 V en electrolitos orgánicos
convencionales. Por estos motivos, son los electrolitos más empleados en
dispositivos comerciales [82,83]. No obstante, los condensadores basados
en electrolitos orgánicos se degradan fácilmente en condiciones de
operación severas (elevada temperatura y elevado voltaje), que son
necesarias en diversas aplicaciones de estos dispositivos. Además, la
presencia de trazas de agua puede disminuir notablemente el voltaje del
dispositivo [9]. La degradación se debe principalmente a la disminución
de la estabilidad electroquímica a elevada temperatura, así como a la
oxidación del material y descomposición del electrolito en condiciones de
elevado voltaje [89,90]. Por estos motivos, se está realizando un gran
esfuerzo investigador para desarrollar electrolitos orgánicos novedosos,
basados en sales conductoras y disolventes orgánicos nuevos, que
permitan mejorar las propiedades fisicoquímicas y aumentar la ventana de
estabilidad de los electrolitos orgánicos, así como optimizar su interacción
Introducción general
51
con los materiales carbonosos. Concretamente, el uso de sales basadas en
el catión pirrolidinio (Pyr14BF4) ha permitido alcanzar voltajes superiores
a 3V en PC. También se ha encontrado que el uso de disolventes orgánicos
como adiponitrilo (ADN) y 2,3 carbonato de butileno (2,3 BC), en
combinación con sales conductoras convencionales, permite trabajar a
voltajes superiores al PC empleando sales conductoras convencionales a
temperatura ambiente [91–93].
1.4.1.4.3. Líquidos iónicos
Los líquidos iónicos se definen como sales que se encuentran en
estado líquido a temperaturas inferiores a los 100 ºC [94,95]. Están
compuestos por un catión orgánico asimétrico y una anión orgánico o
inorgánico [82]. Debido a sus propiedades únicas, esta clase de
compuestos está alcanzando un gran impacto en diversas aplicaciones en
el campo de la energía (supercondensadores, baterías, pilas de
combustible, etc.) debido a que presentan una serie de propiedades que se
pueden optimizar según la aplicación requerida [96].
Las propiedades de los líquidos iónicos varían en función de la
naturaleza química de los cationes y aniones que los componen [96]. En
general, presentan bajas presiones de vapor y baja inflamabilidad, por lo
que se consideran más seguros que los disolventes orgánicos [97].
Además, presentan elevada estabilidad química y electroquímica, y dada
su conductividad intrínseca, se pueden emplear con electrolitos sin
necesidad de añadir un disolvente. Una ventaja adicional es su elevada
Capítulo 1
52
estabilidad térmica, que permite su aplicación en un intervalo amplio de
temperaturas (entre -30 y +60 ºC) [68].
Los condensadores basados en líquidos iónicos proporcionan
capacidades específicas similares a las proporcionadas en electrolitos
orgánicos [98]. No obstante, permiten alcanzar voltajes superiores a 3V,
por lo que pueden alcanzar valores de energía mayores que los basados en
otros electrolitos [94,97]. Sin embargo, los líquidos iónicos presentan
elevada viscosidad y menor conductividad que los electrolitos orgánicos
(< 0.01 S/cm), por lo que limitan la ESR del condensador y, por tanto, su
potencia [92,97]. Para superar está limitación, se ha investigado el uso de
electrolitos basados en mezclas de líquidos iónicos y disolventes
orgánicos. Estos electrolitos permiten aumentar la conductividad y
disminuir la viscosidad, pero mantienen en gran medida el voltaje de
operación, ya que pueden alcanzar valores de incluso 3.5 V [92,99].
1.4.1.5. Materiales carbonosos como electrodos de supercondensadores
Como se ha descrito en las secciones anteriores, los materiales
carbonosos presentan una combinación de propiedades fisicoquímicas
únicas para su aplicación en condensadores electroquímicos. Estos
materiales presentan una conductividad eléctrica relativamente elevada y
una superficie específica elevada, parámetros indispensables para
proporcionar potencia y capacidad elevadas, respectivamente. Además, su
relativamente bajo coste es un parámetro clave para aplicaciones
industriales. Se pueden encontrar en una variedad elevada de morfologías
(polvo, fibras, telas, monolitos, etc.), que permite adecuarlos a las
características necesarias en distintas aplicaciones (dispositivos flexibles,
Introducción general
53
móviles o sistemas estacionarios). Sin embargo, la propiedad más
interesante es la gran diversidad que presentan en términos de textura
porosa y química superficial. Ambas propiedades se pueden modificar por
medio de distintas estrategias de síntesis o post-tratamientos para adecuar
su uso a aplicaciones muy diversas. En condensadores electroquímicos,
se ha demostrado que estos parámetros alteran las propiedades
electroquímicas de los materiales carbonosos, por lo que es posible
adaptar su uso a distintos electrolitos y sistemas [83].
A continuación, se describe la influencia de las características de los
materiales carbonosos en su aplicación como electrodos de
supercondensadores.
1.4.1.5.1. Influencia de la textura porosa
La textura porosa de los materiales carbonosos (definida por
superficie específica, volumen de poros y distribución de tamaños de
poro) es un parámetro clave en su comportamiento como electrodos de
supercondensadores [9,84].
En general, la capacidad específica de los materiales carbonosos es
directamente proporcional a la superficie específica a valores bajos, pero
alcanza un valor constante a superficies específica superiores a 1200-1500
m2/g [100]. Una explicación plausible para este efecto es el decrecimiento
del espesor medio de las paredes de los poros en carbones muy activados,
que produce que el campo eléctrico (y la densidad de carga
correspondiente) no decaiga a cero entre las paredes del poro. Por otro
lado, se ha observado que el tamaño medio de poro aumenta con la
Capítulo 1
54
superficie específica cuando el grado de activación aumenta. Esto sugiere
que la interacción de los iones con las paredes de los poros es más débil
en poros de mayor tamaño y el efecto del desarrollo de la porosidad es
contrarrestado por el aumento de la distancia ión-pared [101], de manera
que la capacidad normalizada (por la superficie específica) aumenta
conforme disminuye el tamaño medio de los microporos. Sin embargo, el
mecanismo de almacenamiento de carga no se puede relacionar
exclusivamente con efectos superficiales. Los tamaños de los iones, de los
poros y la conexión entre estos y con la superficie externa de las partículas
del electrodo tienen una gran influencia en los valores de capacidad,
especialmente en condiciones de elevada potencia [9].
Se ha demostrado que la formación de la doble capa eléctrica está
determinada por el tamaño de los iones y de los poros. La presencia de
microporosidad permite alcanzar valores elevados de capacidad.
Raymundo-Piñero y col. [101] estudiaron el efecto del tamaño de poro en
electrolito orgánico y acuoso. En ambos electrolitos, la capacidad
normalizada aumentaba conforme disminuye el tamaño medio de los
microporos. Dado que el tamaño de los iones es mayor en los electrolitos
orgánicos, el tamaño medio de poro óptimo para la formación de la doble
capa es superior en este medio. Teniendo en cuenta que el tamaño del
catión (no solvatado) es más cercano al tamaño medio de poro que el
catión solvatado, es posible concluir que los iones penetran los poros sin
su esfera de solvatación. No obstante, los materiales carbonosos suelen
presentar distribuciones de tamaños de poros suficientemente anchas que
sugieren una contribución de los poros de tamaño superior al medio. De
Introducción general
55
hecho, la presencia microporos de mayor tamaño (> 0.7 nm) y mesoporos
es beneficiosa en materiales de porosidad jerárquica para promover una
difusión rápida de los iones hacia la entrada de lo microporos estrechos
[102]. Sin embargo, un exceso de mesoporosidad puede ser perjudicial
para la aplicación del material, dado que disminuye la capacidad
volumétrica del dispositivo.
Además, se pueden dar efectos de tamiz molecular, en los que no se
da electroadsorción de cationes comunes (Mg2+, Li+) cuando el tamaño de
poro es menor que el de los cationes hidratados, por lo que no contribuyen
a la formación de la doble capa eléctrica [79,103,104]. Este efecto se
observó también empleando líquidos iónicos como electrolitos, en los que
se puede descartar el efecto de la solvatación del disolvente [105].
Finalmente, la conectividad de los poros con la superficie de la
partícula también juega un papel fundamental en la formación de la doble
capa eléctrica. Bleda-Martínez y col. [84] demostraron la importancia de
la disposición de la microporosidad en la superficie externa del material.
En las fibras de carbón, la microporosidad se encuentra dispuesta
principalmente de forma perpendicular al eje de la fibra, por lo que
presenta baja tortuosidad. En el caso de los carbones activados, se
caracterizan por una red porosa muy desordenada, que le confiere un
grado elevado de tortuosidad. En consecuencia, el carbón activado mostró
una pérdida de capacidad completa en condiciones de medida de elevada
frecuencia (elevada potencia) en 0.5 M Na2SO4, mientras que la fibra de
carbón retuvo el 25% de capacidad. Este se debe a la mayor tortuosidad
en el camino difusional del electrolito en el carbón activado.
Capítulo 1
56
Figura 1.12. Esquema de los problemas difusionales experimentados por los iones para
acceder a la superficie interna de carbones activados y fibras de carbón activadas.
Por tanto, los materiales con una porosidad accesible son deseables
en aplicaciones que requieran elevada potencia. Por estos motivos, los
materiales carbonosos de porosidad jerárquica, donde los macroporos,
mesoporos y microporosos se encuentran ordenados, de manera que
disminuye el camino difusional del electrolito, son de especial interés para
estas aplicaciones [106]. Del mismo modo, los carbones nanomoldeados
con porosidad ordenada y distribución de tamaños de poros homogénea
[18] son de gran utilidad para ser empleados en dispositivos de elevada
potencia.
1.4.1.5.2. Influencia de la química superficial
Como se ha descrito a lo largo de la introducción, la química
superficial es un parámetro característico de los materiales carbonosos que
tiene una gran influencia en diversas aplicaciones. Además, se puede
modificar para mejorar propiedades dependiendo de la aplicación
deseada. Por estos motivos, existen numerosas publicaciones centradas en
el estudio de materiales carbonosos dopados con heteroátomos como
Fibra de carbón activada
+
+
+
+
++
+
+
+
+
+
+
++
+ +
++
Carbón activado
Introducción general
57
electrodos de supercondensadores. La química superficial de los
materiales carbonosos afecta a diversas propiedades fisicoquímicas y
electroquímicas de los materiales, como son: mojabilidad, conductividad
eléctrica, estabilidad electroquímica y contribución a la capacidad por
medio de procesos pseudocapacitivos. No obstante, el efecto de los grupos
funcionales se debe analizar de forma detallada.
En el caso de los grupos funcionales oxigenados, se ha encontrado
una correlación positiva entre la capacidad y la cantidad de grupos que
desorben como CO [3]. Esto se debe principalmente a la contribución a la
pseudocapacidad de los grupos quinona [3,22,107,108] y al carácter
hidrofílico de los hidroxilos, que aumentan la mojabilidad del electrodo.
El efecto de la pseudocapacidad depende del pH del electrolito, debido a
que implica el intercambio de protones. En medio básico, la contribución
farádica de los grupos CO es significativamente menor, aunque se ha
detectado cierta contribución [109]. Además, también se han detectado
procesos pseudocapacitivos en medio orgánico [110].
Los grupos que desorben como CO2, en cambio, son perjudiciales
para los materiales carbonosos, debido a su carácter electrón-aceptor, que
produce una menor deslocalización de la carga y, por tanto, una
disminución de la conductividad eléctrica [107,111]. Por último, se ha
observado que los grupos funcionales oxigenados disminuyen la
estabilidad electroquímica de los materiales carbonosos en electrolitos
acuosos [112] y orgánicos [89].
Capítulo 1
58
En cambio, la naturaleza de la influencia de los grupos nitrogenados
requiere un mayor esfuerzo investigador, debido principalmente a su
coexistencia inevitable con grupos oxigenados. Se ha observado que los
grupos funcionales nitrogenados pueden favorecer distintas propiedades
fisicoquímicas que mejoran su comportamiento como electrodos en
supercondensadores. Concretamente, se han detectado contribuciones
pseudocapacitivas por medio de grupos piridina [113], piridona [114] y
aminas (en medio orgánico) [48]. Además, el nitrógeno cuaternario
permite aumentar la mojabilidad del electrodo [113,115,116]. Finalmente,
se ha observado que el nitrógeno cuaternario aumenta la estabilidad
electroquímica de los materiales cuando se utilizan como electrodos de
supercondensadores en medio orgánico, mientras que los grupos amina
tienen un efecto perjudicial [48].
1.4.2. Pilas de combustible
Las pilas de combustible son dispositivos de generación de energía
eléctrica [117]. A diferencia de las baterías, en estos dispositivos los
reactivos se alimentan de forma continua durante el funcionamiento de la
pila. Estos dispositivos electroquímicos están compuestos por dos semi-
celdas (cátodo y ánodo) separadas por una membrana intercambio iónico.
Estos dispositivos permiten generar elevadas densidades energéticas
(ver diagrama de Ragone en Figura 1.6) de forma eficiente, lo que permite
su empleo en aplicaciones como: sistemas de emergencia, sistemas
estacionarios, dispositivos portátiles y locomoción. En comparación con
Introducción general
59
tecnologías energéticas convencionales, estos dispositivos proporcionan
mayor eficiencia y menores emisiones de productos contaminantes.
Entre todos los combustibles disponibles para estos dispositivos, el
hidrógeno es el más deseado debido a que el único producto de reacción
es agua. No obstante, se han estudiado una amplia variedad de
combustibles, como diversos alcoholes e hidrocarburos, entre los que
destaca el metanol y el etanol. En estos dispositivos, el combustible
(hidrógeno, metanol, etc) se alimenta al ánodo para su oxidación, mientras
que en el cátodo tiene lugar la reacción de reducción de oxigeno (ORR,
del inglés oxygen reduction reaction).
Existen diversos tipos de pilas de combustible. Se clasifican
principalmente atendiendo al electrolito empleado, que determina el tipo
de combustible, temperatura de operación y catalizador utilizado. La
Figura 1.6 resume el funcionamiento de los distintos tipos de pilas de
combustible.
En el desarrollo de esta tecnología, los materiales carbonosos juegan
un papel esencial. Estos se encuentran presentes en distintos componentes
de las pilas de combustible de baja temperatura, debido fundamentalmente
a su elevada conductividad eléctrica, estabilidad en condiciones de
operación de la pila y bajo coste. Concretamente, se utilizan como
material en placas bipolares, componente de los difusores de gases y
soporte del catalizador presente en el cátodo. En los últimos años, además,
han despertado un gran interés como alternativa a los catalizadores
basados en platino u otros metales empleados en la ORR, ya que presentan
Capítulo 1
60
actividad catalítica en ausencia de platino u otros metales nobles en su
composición.
Figura 1.13. Esquema de los tipos de pilas de combustible [118].
1.4.2.1. Reacción de reducción de oxígeno
La reacción de reducción del oxígeno se produce en el cátodo de la
pila de combustible. El potencial de equilibrio teórico para la ORR es
1.229 V (vs NHE) en condiciones estándar y en medio ácido. Sin
embargo, la reacción sucede a sobrepotenciales elevados, disminuyendo
su eficiencia para su aplicación práctica. Según el medio, la reacción
puede darse por medio de dos mecanismos [119]:
ELECTROLITO
CÁTODOÁNODOAire, O2Combustible
Carga
Gases sin reaccionar
(O2, N2)
Gases sin reaccionar
(H2, CO, etc.)
PEMFC (60-90 ºC)
DMFC (60-90 ºC)
PAFC (180-220 ºC)
MCFC (550-650 ºC)
SOFC (800-1000 ºC)
AFC (60-90 ºC)
Introducción general
61
Medio ácido Medio básico
𝑂2 + 4𝐻+ + 4𝑒− → 2𝐻2𝑂 𝑂2 + 2𝐻2𝑂 + 4𝑒− → 4𝑂𝐻− Ec. I
𝑂2 + 2𝐻+ + 2𝑒− → 𝐻2𝑂2 𝑂2 + 𝐻2𝑂 + 2𝑒− → 𝐻𝑂2− + 𝑂𝐻− Ec. II
𝐻2𝑂2 + 2𝐻+ + 2𝑒− → 2𝐻2𝑂 𝐻𝑂2− + 𝐻2𝑂 + 2𝑒− → 3𝑂𝐻− Ec. III
2𝐻2𝑂2 → 2𝐻2𝑂 + 𝑂2 2𝐻𝑂2− → 2𝑂𝐻− + 𝑂2 Ec. IV
En cada electrolito, la reacción se puede producir por un
mecanismo de 2 (ecuaciones II y III) o 4 electrones (ecuación I). La
reacción por medio de 4 electrones es preferible en pilas de combustible
debido a que permite una producción mayor de energía y, además, genera
exclusivamente agua como producto de reacción. En cambio, la reacción
vía de 2 electrones produce la mitad de energía y da lugar a la formación
de productos nocivos. El H2O2 producido por este mecanismo puede
reducirse de nuevo a H2O, o bien, puede producir agua (ecuación III) y
oxígeno debido a una reacción secundaria de desproporción (ecuación
IV). En este último caso, se regeneraría parte del O2 utilizado; parte del
mismo puede ser recirculado de nuevo en el sistema; sin embargo, es
inevitable que se pierda parte del oxígeno producido. Estas reacciones
suceden en paralelo dependiendo de las propiedades del catalizador
utilizado [119,120].
Capítulo 1
62
1.4.2.2. Electrocatalizadores para la ORR
El electrocatalizador más empleado en la ORR es el platino soportado
en materiales carbonosos [121–123]. Este material presenta un
sobrepotencial bajo para esta reacción y una elevada selectividad a la
formación de agua. No obstante, presenta una serie de inconvenientes,
como son: baja estabilidad en las condiciones de operación de la pila de
combustible, sensibilidad al envenenamiento por CO (por migración del
ánodo al cátodo) y elevado coste [121]. En la bibliografía, se pueden
encontrar diferentes estudios para aumentar la eficacia de los catalizadores
basados en platino. Las principales estrategias se centran en: producción
de monocapas de platino sobre otros metales de menor coste (paladio,
hierro, etc.), aleaciones de platino con otros metales (rutenio, paladio,
iridio, plata, etc.), desarrollo de catalizadores basados en nanopartículas
de platino con morfología y tamaño controlados [121,124].
Sin embargo, se ha planteado también el reemplazo del platino como
electrocatalizador de la ORR. En el estudio de catalizadores alternativos
para la ORR, los materiales carbonosos juegan un papel fundamental tanto
como soporte [125–129] como catalizadores en sí mismos (sin presencia
de metal) [130]. Como soporte de catalizadores, se han empleado diversos
materiales (negro de carbón, nanotubos de carbono, nanofibras de
carbono, grafeno, etc.) para soportar tanto platino como otros metales (Ag,
Au, Co, Fe, etc.) [131–134]. La presencia de heteroátomos ha resultado
ser beneficiosa para aumentar la actividad catalítica [125]. Como
electrocatalizadores, numerosos estudios han señalado una elevada
actividad catalítica asociada a la presencia de heteroátomos en superficie
Introducción general
63
(azufre, boro, nitrógeno, etc.) [37]. Concretamente, los materiales
carbonosos dopados con nitrógeno han mostrado excelentes propiedades
catalíticas en la ORR [130]. Además, la estructura de los materiales
carbonosos influye de forma determinante [130,135,136]. Se ha señalado
que la microporosidad puede producir un aumento del potencial de inicio
de reacción [130,137,138].
Dado que esta Tesis Doctoral se centra en el estudio de los grupos
funcionales nitrogenados en las propiedades electroquímicas de los
materiales carbonosos, en la siguiente sección se describirán las
propiedades de los materiales carbonosos funcionalizados con nitrógeno
como electrocatalizadores de la ORR.
1.4.2.3. Materiales carbonosos funcionalizados con nitrógeno como
electrocatalizadores de la ORR
Los materiales carbonosos dopados con nitrógeno son muy
prometedores para sustituir al platino como catalizador de la ORR [139].
La presencia de grupos funcionales nitrogenados se ha relacionado con
una mayor electroactividad. No obstante, la influencia de los distintos
grupos funcionales nitrogenados no ha sido esclarecida.
Algunos autores señalan los sitios N-C-N como activos para la ORR.
Indican, además, que la presencia de nitrógeno produce cambios
inherentes en la geometría que se traducen en un aumento de
electroactividad [140]. Otros estudios proponen que el carácter electrón-
dador de los grupos nitrogenados producen una redistribución de la
densidad electrónica alrededor de los átomos de nitrógeno conduciendo a
Capítulo 1
64
un aumento de la actividad catalítica [139,141,142]. De esta forma, el
aumento de densidad de carga positiva en los átomos de carbono
adyacentes al nitrógeno favorece la quimisorción del oxígeno y debilita el
enlace O-O [139]. Por último, se ha señalado que, además del papel de los
grupos nitrogenados como sitios activos, la presencia de nitrógeno
produce cambios estructurales que contribuyen de forma adicional a un
aumento de la electroactividad.
Dada la dificultad de producir e identificar grupos nitrogenados
específicos, resulta difícil esclarecer qué grupos funcionales son
responsables de la mejora de la respuesta catalítica [130]. Este aspecto es
clave para avanzar hacia el diseño de electrocatalizadores óptimos. Por
estos motivos, se está realizando un gran esfuerzo investigador,
empleando estrategias tanto experimentales como teóricas, para elucidar
los grupos funcionales responsables de la mejora de la actividad
electrocatalítica. Diversos estudios señalan que los materiales carbonosos
con elevada concentración de piridinas presentan mayor actividad
catalítica [143–145] Sin embargo, otros estudios señalan que la mejora de
la actividad catalítica viene dada por la presencia de varios grupos
funcionales: pirroles, nitrógeno cuaternario [146] y oxidados
[147].Recientemente, se han señalado los sitios N-C-O y los grupos N-Q
en posición zigzag como electroactivos para la ORR [148,149]. Además,
el aumento de la actividad electrocatalítica debido a los grupos N-Q
también ha sido señalado mediante estudios teóricos por Ikeda y col [150].
Estos autores también descartaron la contribución de los grupos
piridínicos a la actividad catalítica.
Introducción general
65
En cuando a la selectividad de la reacción, algunos estudios indican
que las piridinas presentan una elevada selectividad a la producción de
agua, mientras que el nitrógeno cuaternario es más selectivo a la
formación de peróxido de hidrógeno. Sin embargo, un estudio reciente
señala un aumento de la selectividad a la formación de agua asociado a la
presencia de nitrógeno cuaternario en posición zigzag en el borde de las
láminas de grafeno [149]. También se ha indicado un aumento de
selectividad asociado a la presencia sitios N-C-O (piridonas) [148].
1.5. Referencias
[1] T.J. Bandosz, Surface chemistry of carbon materials, in: Carbon
Mater. Catal., John Wiley & Sons, Inc, 2009: pp. 45–92.
[2] P. Serp, J.L. Figueiredo, Carbon Materials for Catalysis, John
Wiley & Sons, Inc., Hoboken, New Jersey, USA, 2008.
[3] M.J. Bleda-Martínez, J.A. Maciá-Agulló, D. Lozano-Castelló, E.
Morallón, D. Cazorla-Amorós, A. Linares-Solano, Role of surface
chemistry on electric double layer capacitance of carbon materials,
Carbon 43 (2005) 2677–2684.
[4] C. Yin, M. Aroua, W. Daud, Review of modifications of activated
carbon for enhancing contaminant uptakes from aqueous solutions,
Sep. Purif. Technol. 52 (2007) 403–415.
[5] M.S. Shafeeyan, W.M.A.W. Daud, A. Houshmand, A. Shamiri, A
review on surface modification of activated carbon for carbon
dioxide adsorption, J. Anal. Appl. Pyrolysis. 89 (2010) 143–151.
[6] T.J. Bandosz, C.O. Ania, Surface chemistry of activated carbons
and its characterization, in: T.J. Bandosz (Ed.), Act. Carbon
Surfaces Environ. Remediat., 1st ed., Elsevier, 2006: pp. 159–229.
[7] F. Rodriguez-Reinoso, Activated carbon: structure,
characterization, preparation and applications, in: H. Marsh, E.A.
Capítulo 1
66
Heinz, F. Rodriguez-Reinoso (Eds.), Introd. to Carbon Technol.,
Universidad de Alicante, 1997: pp. 35–102.
[8] B.E. Conway, Electrochemical supercapacitors: Scientific
Fundamentals and Technological Applications, Springer, New
York, 1999.
[9] F. Béguin, E. Frackowiak, eds., Carbons for Electrochemical
Energy Storage and Conversion Systems, 1st ed., CRC Press, 2009.
[10] O. Ioannidou, A. Zabaniotou, Agricultural residues as precursors
for activated carbon production-A review, Renew. Sustain. Energy
Rev. 11 (2007) 1966–2005.
[11] J.A. Maciá-Agulló, B.C. Moore, D. Cazorla-Amorós, A. Linares-
Solano, Activation of coal tar pitch carbon fibres: Physical
activation vs. chemical activation, Carbon 42 (2004) 1367–1370.
[12] A. Linares-Solano, D. Lozano-Castelló, M.A. Lillo-Ródenas, D.
Cazorla-Amorós, Carbon activation by alkaline hydroxides
preparation and reactions, porosity and performance, Chem. Phys.
Carbon 30 (2008) 1–62.
[13] A. Ahmadpour, D.D. Do, The preparation of active carbons from
coal by chemical and physical activation, Carbon 34 (1996) 471–
479.
[14] D. Lozano-Castelló, M.A. Lillo-Ródenas, D. Cazorla-Amorós, A.
Linares-Solano, Preparation of activated carbons from Spanish
anthracite: I. Activation by KOH, Carbon 39 (2001) 741–749.
[15] F. Caturla, M. Molina-Sabio, F. Rodríguez-Reinoso, Preparation of
activated carbon by chemical activation with ZnCl2, Carbon 29
(1991) 999–1007.
[16] M. Molina-Sabio, F. RodRíguez-Reinoso, F. Caturla, M.J. Sellés,
Porosity in granular carbons activated with phosphoric acid,
Carbon 33 (1995) 1105–1113.
Introducción general
67
[17] M. Lillo-Ródenas, D. Lozano-Castelló, D. Cazorla-Amorós, A.
Linares-Solano, Preparation of activated carbons from Spanish
anthracite: II. Activation by NaOH, Carbon. 39 (2001) 751–759.
[18] H. Nishihara, T. Kyotani, Templated nanocarbons for energy
storage, Adv. Mater. 24 (2012) 4473–4498.
[19] H. Itoi, H. Nishihara, T. Kogure, T. Kyotani, Three-Dimensionally
Arrayed and Mutually Connected 1.2-nm Nanopores for High-
Performance Electric Double Layer Capacitor, J. Am. Chem. Soc.
20 (2011) 1165–1167.
[20] H. Nishihara, T. Kyotani, Zeolite-templated carbons - three-
dimensional microporous graphene frameworks, Chem. Commun.
54 (2018) 5648–5673.
[21] H. Nishihara, Q. Yang, P. Hou, M. Unno, S. Yamauchi, R. Saito,
J.I. Paredes, A. Martínez-Alonso, J.M.D. Tascón, Y. Sato, M.
Terauchi, T. Kyotani, A possible buckybowl-like structure of
zeolite templated carbon, Carbon 47 (2009) 1220–1230.
[22] H. Itoi, H. Nishihara, T. Ishii, K. Nueangnoraj, R. Berenguer-
Betrián, T. Kyotani, Large Pseudocapacitance in Quinone-
Functionalized Zeolite-Templated Carbon, Bull. Chem. Soc. Jpn.
87 (2014) 250–257.
[23] C. González-Gaitán, R. Ruiz-Rosas, H. Nishihara, T. Kyotani, E.
Morallón, D. Cazorla-Amorós, Successful functionalization of
superporous zeolite templated carbon using aminobenzene acids
and electrochemical methods, Carbon 99 (2016) 157–166.
[24] H. Nishihara, P. Hou, L. Li, M. Ito, M. Uchiyama, T. Kaburagi, A.
Ikura, J. Katamura, T. Kawarada, K. Mizuuchi, T. Kyotani, High-
Pressure Hydrogen Storage in Zeolite-Templated Carbon, J. Phys.
Chem. C 113 (2009) 3189–3196.
[25] H. Itoi, H. Nishihara, T. Kyotani, Effect of Heteroatoms in Ordered
Microporous Carbons on Their Electrochemical Capacitance,
Langmuir 32 (2016) 11997–12004.
Capítulo 1
68
[26] M.A. Montes-Morán, D. Suárez, J.A. Menéndez, E. Fuente, The
Basicity of Carbons, in: Nov. Carbon Adsorbents, Elsevier Ltd,
2012: pp. 173–203.
[27] C.A. Leon y Leon, J.M. Solar, V. Calemma, L.R. Radovic,
Evidence for the protonation of basal plane sites on carbon, Carbon
30 (1992) 797–811.
[28] W. Shen, W. Fan, Nitrogen-containing porous carbons: synthesis
and application, J. Mater. Chem. A 1 (2013) 999–1013.
[29] M. Seredych, T.J. Bandosz, Role of Microporosity and Nitrogen
Functionality on the Surface of Activated Carbon in the Process of
Desulfurization of Digester Gas, J. Phys. Chem. C 112 (2008)
4704–4711.
[30] A. Bagreev, J. Angel Menendez, I. Dukhno, Y. Tarasenko, T.J.
Bandosz, Bituminous coal-based activated carbons modified with
nitrogen as adsorbents of hydrogen sulfide, Carbon 42 (2004) 469–
476.
[31] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, The
role of different nitrogen functional groups on the removal of SO2
from flue gases by N-doped activated carbon powders and fibres,
Carbon 41 (2003) 1925–1932.
[32] J. Yu, M. Guo, F. Muhammad, A. Wang, F. Zhang, Q. Li, G. Zhu,
One-pot synthesis of highly ordered nitrogen-containing
mesoporous carbon with resorcinol–urea–formaldehyde resin for
CO2 capture, Carbon 69 (2014) 502–514.
[33] A. Houshmand, M.S. Shafeeyan, A. Arami-Niya, W.M.A.W.
Daud, Anchoring a halogenated amine on the surface of a
microporous activated carbon for carbon dioxide capture, J. Taiwan
Inst. Chem. Eng. 44 (2013) 774–779.
[34] A. Houshmand, W.M.A.W. Daud, M.-G. Lee, M.S. Shafeeyan,
Carbon Dioxide Capture with Amine-Grafted Activated Carbon,
Water, Air, Soil Pollut. 223 (2011) 827–835.
Introducción general
69
[35] M.R. Benzigar, S.N. Talapaneni, S. Joseph, K. Ramadass, G.
Singh, J. Scaranto, U. Ravon, K. Al-Bahily, A. Vinu, Recent
advances in functionalized micro and mesoporous carbon
materials: Synthesis and applications, Chem. Soc. Rev. 47 (2018)
2680–2721.
[36] Y. Deng, Y. Xie, K. Zou, X. Ji, Review on recent advances in
nitrogen-doped carbons: Preparations and applications in
supercapacitors, J. Mater. Chem. A 4 (2015) 1144–1173.
[37] J.P. Paraknowitsch, A. Thomas, Doping carbons beyond nitrogen:
an overview of advanced heteroatom doped carbons with boron,
sulphur and phosphorus for energy applications, Energy Environ.
Sci. 6 (2013) 2839–2855.
[38] D. Mang, H.P. Boehm, K. Stanczyk, H. Marsh, Inhibiting effect of
incorporated nitrogen on the oxidation of microcrystalline carbons,
Carbon 30 (1992) 391–398.
[39] M. Zhou, F. Pu, Z. Wang, S. Guan, Nitrogen-doped porous carbons
through KOH activation with superior performance in
supercapacitors, Carbon 68 (2014) 185–194.
[40] C.O. Ania, V. Khomenko, E. Raymundo-Piñero, J.B. Parra, F.
Béguin, The Large Electrochemical Capacitance of Microporous
Doped Carbon Obtained by Using a Zeolite Template, Adv. Funct.
Mater. 17 (2007) 1828–1836.
[41] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, J.
Find, U. Wild, R. Schlögl, Structural characterization of N-
containing activated carbon fibers prepared from a low softening
point petroleum pitch and a melamine resin, Carbon 40 (2002) 597–
608.
[42] R.J.J. Jansen, H. van Bekkum, Amination and ammoxidation of
activated carbons, Carbon 32 (1994) 1507–1516.
[43] S. Biniak, G. Szymański, J. Siedlewski, A. ŚwiaTkowski, The
characterization of activated carbons with oxygen and nitrogen
surface groups, Carbon 35 (1997) 1799–1810.
Capítulo 1
70
[44] F. Xie, J. Phillips, I.F. Silva, M.C. Palma, J.A. Menéndez,
Microcalorimetric study of acid sites on ammonia- and acid-
pretreated activated carbon, Carbon 38 (2000) 691–700.
[45] M. Seredych, D. Hulicova-Jurcakova, G.Q. Lu, T.J. Bandosz,
Surface functional groups of carbons and the effects of their
chemical character, density and accessibility to ions on
electrochemical performance, Carbon 46 (2008) 1475–1488.
[46] Y. El-Sayed, T.J. Bandosz, Role of surface oxygen groups in
incorporation of nitrogen to activated carbons via
ethylmethylamine adsorption., Langmuir 21 (2005) 1282–9.
[47] A. Gabe, M.J. Mostazo-López, D. Salinas-Torres, E. Morallón, D.
Cazorla-Amorós, Synthesis of conducting polymer/carbon material
composites and their application in electrical energy storage, in:
Hybrid Polym. Compos. Mater. Process., 2017: pp. 173–209.
[48] D. Salinas-Torres, S. Shiraishi, E. Morallón, D. Cazorla-Amorós,
Improvement of carbon materials performance by nitrogen
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions, Carbon 82 (2015) 205–213.
[49] Ana Cristina Ramírez-Pérez, Javier Quílez-Bermejo, Juan Manuel
Sieben, Emilia Morallón, Diego Cazorla-Amorós, Effect of
Nitrogen-Functional Groups on the ORR Activity of Activated
Carbon Fiber-Polypyrrole-Based Electrodes, Electrocatalysis 9
(2018) 697–705.
[50] D. Salinas-Torres, J.M. Sieben, D. Lozano-Castello, E. Morallón,
M. Burghammer, C. Riekel, D. Cazorla-Amorós, Characterization
of activated carbon fiber/polyaniline materials by position-resolved
microbeam small-angle X-ray scattering, Carbon 50 (2012) 1051–
1056.
[51] J. Zhu, J. Yang, B. Deng, Ethylenediamine-modified activated
carbon for aqueous lead adsorption, Environ. Chem. Lett. 8 (2009)
277–
[52] A.R. Silva, M. Martins, M.M. Freitas, A. Valente, C. Freire, B. de
Castro, J.L. Figueiredo, Immobilisation of amine-functionalised
Introducción general
71
nickel(II) Schiff base complexes onto activated carbon treated with
thionyl chloride, Microporous Mesoporous Mater 55 (2002) 275–
284.
[53] A.R. Silva, V. Budarin, J.H. Clark, C. Freire, B. de Castro, Organo-
functionalized activated carbons as supports for the covalent
attachment of a chiral manganese(III) salen complex, Carbon 45
(2007) 1951–1964.
[54] J.A.C. Alves, C. Freire, B. de Castro, J.L. Figueiredo, Anchoring
of organic molecules onto activated carbon, Colloids Surfaces A
Physicochem. Eng. Asp. 189 (2001) 75–84.
[55] H. Tamai, K. Shiraki, T. Shiono, H. Yasuda, Surface
functionalization of mesoporous and microporous activated
carbons by immobilization of diamine., J. Colloid Interface Sci.
295 (2006) 299–302.
[56] M. Abe, K. Kawashima, K. Kozawa, H. Sakai, K. Kaneko,
Amination of Activated Carbon and Adsorption Characteristics of
Its Aminated Surface, Langmuir 16 (2000) 5059–5063.
[57] C.-T. Hsieh, H. Teng, W.-Y. Chen, Y.-S. Cheng, Synthesis,
characterization, and electrochemical capacitance of amino-
functionalized carbon nanotube/carbon paper electrodes, Carbon
48 (2010) 4219–4229.
[58] A. Gromov, S. Dittmer, J. Svensson, O.A. Nerushev, S.A. Perez-
García, L. Licea-Jiménez, R. Rychwalski, E.E.B. Campbell,
Covalent amino-functionalisation of single-wall carbon nanotubes,
J. Mater. Chem. 15 (2005) 3334–3339.
[59] M.B. Smith, J. March, Rearrangements, in: March’s Adv. Org.
Chem. React. Mech. Estructure, 6th ed., John Wiley & Sons, Inc.,
Hoboken, New Jersey, USA, 2007: pp. 1607–1608.
[60] C. González-Gaitán, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Electrochemical Methods to Functionalize Carbon
Materials, in: Chem. Funct. Carbon Nanomater., CRC Press, 2015:
pp. 230–261.
Capítulo 1
72
[61] D. Bélanger, J. Pinson, Electrografting: a powerful method for
surface modification, Chem. Soc. Rev. 40 (2011) 3995.
[62] M. Toupin, D. Bélanger, Spontaneous functionalization of carbon
black by reaction with 4-nitrophenyldiazonium cations, Langmuir
24 (2008) 1910–1917.
[63] T. Breton, D. Bélanger, Modification of carbon electrode with aryl
groups having an aliphatic amine by electrochemical reduction of
in situ generated diazonium cations, Langmuir 24 (2008) 8711–
8718.
[64] R.S. Deinhammer, M. Ho, J.W. Anderegg, M.D. Porter,
Electrochemical Oxidation of Amine-Containing Compounds: A
Route to the Surface Modification of Glassy Carbon Electrodes,
Langmuir 10 (1994) 1306–1313.
[65] X. Li, Y. Wan, C. Sun, Covalent modification of a glassy carbon
surface by electrochemical oxidation of r-aminobenzene sulfonic
acid in aqueous solution, J. Electroanal. Chem. 569 (2004) 79–87.
[66] E. Frackowiak, Q. Abbas, F. Béguin, Carbon/carbon
supercapacitors, J. Energy Chem. 22 (2013) 226–240.
[67] B.E. Conway, Transition from “supercapacitor” to “battery”
behavior in electrochemical energy storage, J. Electrochem. Soc.
138 (1991) 1539–1548.
[68] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors.,
Nat. Mater. 7 (2008) 845–54.
[69] R. Kötz, M. Carlen, Principles and applications of electrochemical
capacitors, Electrochim. Acta 45 (2000) 2483–2498.
[70] E. Frackowiak, F. Béguin, Carbon materials for the electrochemical
storage of energy in capacitors, Carbon 39 (2001) 937–950.
[71] A.J. Bard, L.R. Faulkner, Electrochemical Methods. Fundamentals
and Applications, 2nd ed., John Wiley & Sons, Inc, New York,
2001.
Introducción general
73
[72] Aiping Yu, V. Chabot, J. Zhang, Electrochemical supercapacitors
for energy storage and delivery. Fundamentals and applications,
CRC Press, Taylor & Francis Group, Boca Raton, 2013.
[73] S. Shiraishi, Electrochemical Performance, in: M. Inagaki (Ed.),
Mater. Sci. Eng. Carbon Charact., Tsinghua University Press
Limited. Elsevier Inc, 2016: pp. 205–226.
[74] Q.-L. Chen, K.-H. Xue, W. Shen, F.-F. Tao, S.-Y. Yin, W. Xu,
Fabrication and electrochemical properties of carbon nanotube
array electrode for supercapacitors, Electrochim. Acta 49 (2004)
4157–4161.
[75] H. Jiang, P.S. Lee, C. Li, 3D carbon based nanostructures for
advanced supercapacitors, Energy Environ. Sci. 6 (2013) 41–53.
[76] S. Leyva-García, D. Lozano-Castelló, E. Morallón, D. Cazorla-
Amorós, Silica-templated ordered mesoporous carbon thin films as
electrodes for micro-capacitors, J. Mater. Chem. A 4 (2016) 4570–
4579.
[77] R. Berenguer, F.J. García-Mateos, R. Ruiz-Rosas, D. Cazorla-
Amorós, E. Morallón, J. Rodríguez-Mirasol, T. Cordero, Biomass-
derived binderless fibrous carbon electrodes for ultrafast energy
storage, Green Chem. 18 (2016).
[78] S. Shiraishi, Highly-durable carbon electrode for electrochemical
capacitors, Bol. Grupo Español Carbón 28 (2013) 18–24.
[79] G. Salitra, A. Soffer, L. Eliad, Y. Cohen, D. Aurbach, Carbon
Electrodes for Double-Layer Capacitors I. Relations Between Ion
and Pore Dimensions, J. Electrochem. Soc. 147 (2000) 2486.
[80] G.A. Snook, G.J. Wilson, A.G. Pandolfo, Mathematical functions
for optimisation of conducting polymer/activated carbon
asymmetric supercapacitors, J. Power Sources 186 (2009) 216–
223.
[81] C. Peng, S. Zhang, X. Zhou, G.Z. Chen, Unequalisation of
electrode capacitances for enhanced energy capacity in
asymmetrical supercapacitors, Energy Environ. Sci. 3 (2010) 1499.
Capítulo 1
74
[82] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, A review
of electrolyte materials and compositions for electrochemical
supercapacitors, Chem. Soc. Rev. 44 (2015) 7484–7539.
[83] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Carbons and
electrolytes for advanced supercapacitors, Adv. Mater. 26 (2014).
[84] M.J. Bleda-Martinez, D. Lozano-Castelló, D. Cazorla-Amorós, E.
Morallón, Kinetics of Double-Layer Formation: Influence of
Porous Structure and Pore Size Distribution, Energy & Fuels 24
(2010) 3378–3384.
[85] V. Khomenko, E. Raymundo-Piñero, E. Frackowiak, F. Béguin,
High-voltage asymmetric supercapacitors operating in aqueous
electrolyte, Appl. Phys. A Mater. Sci. Process. 82 (2006) 567–573.
[86] K. Fic, G. Lota, M. Meller, E. Frackowiak, Novel insight into
neutral medium as electrolyte for high-voltage supercapacitors,
Energy Environ. Sci. 5 (2012) 5842–5850.
[87] Q. Gao, L. Demarconnay, E. Raymundo-Piñero, F. Béguin,
Exploring the large voltage range of carbon/carbon supercapacitors
in aqueous lithium sulfate electrolyte, Energy Environ. Sci. 5
(2012) 9611–9617.
[88] D. Lozano-Castelló, D. Cazorla-Amorós, A. Linares-Solano, S.
Shiraishi, H. Kurihara, A. Oya, Influence of pore structure and
surface chemistry on electric double layer capacitance in non-
aqueous electrolyte, Carbon 41 (2003) 1765–1775.
[89] D. Cazorla-Amorós, D. Lozano-Castelló, E. Morallón, M.J. Bleda-
Martínez, A. Linares-Solano, S. Shiraishi, Measuring cycle
efficiency and capacitance of chemically activated carbons in
propylene carbonate, Carbon 48 (2010) 1451–1456.
[90] P.W. Ruch, D. Cericola, A. Foelske-Schmitz, R. Kötz, A. Wokaun,
Aging of electrochemical double layer capacitors with acetonitrile-
based electrolyte at elevated voltages, Electrochim. Acta 55 (2010)
4412–4420.
Introducción general
75
[91] A. Brandt, P. Isken, A. Lex-Balducci, A. Balducci, Adiponitrile-
based electrochemical double layer capacitor, J. Power Sources 204
(2012) 213–219.
[92] A. Balducci, Electrolytes for high voltage electrochemical double
layer capacitors: A perspective article, J. Power Sources 326 (2016)
534–540.
[93] J. Krummacher, C. Schütter, L.H. Hess, A. Balducci, Non-aqueous
electrolytes for electrochemical capacitors, Curr. Opin.
Electrochem. 9 (2018) 64–69.
[94] M. Galiński, A. Lewandowski, I. Stepniak, Ionic liquids as
electrolytes, Electrochim. Acta 51 (2006) 5567–5580.
[95] R.D. Rogers, G.A. Voth, Ionic Liquids, Acc. Chem. Res. 40 (2007)
1077–1078.
[96] D.R. Macfarlane, N. Tachikawa, M. Forsyth, J.M. Pringle, P.C.
Howlett, G.D. Elliott, J.H. Davis, M. Watanabe, P. Simon, C.A.
Angell, Energy applications of ionic liquids, Energy Environ. Sci.
7 (2014) 232–250.
[97] A. Brandt, S. Pohlmann, A. Varzi, A. Balducci, S. Passerini, Ionic
liquids in supercapacitors, MRS Bull. 38 (2013) 554–559.
[98] S. Pohlmann, B. Lobato, T.A. Centeno, A. Balducci, The influence
of pore size and surface area of activated carbons on the
performance of ionic liquid based supercapacitors, Phys. Chem.
Chem. Phys. 15 (2013) 17287–17294.
[99] A. Krause, A. Balducci, High voltage electrochemical double layer
capacitor containing mixtures of ionic liquids and organic
carbonate as electrolytes, Electrochem. Commun. 13 (2011) 814–
817. doi:10.1016/j.elecom.2011.05.010.
[100] O. Barbieri, M. Hahn, A. Herzog, R. Kötz, Capacitance limits of
high surface area activated carbons for double layer capacitors,
Carbon 43 (2005) 1303–1310.
Capítulo 1
76
[101] E. Raymundo-Piñero, K. Kierzek, J. Machnikowski, F. Béguin,
Relationship between the nanoporous texture of activated carbons
and their capacitance properties in different electrolytes, Carbon 44
(2006) 2498–2507.
[102] C. Vix-guterl, E. Frackowiak, K. Jurewicz, M. Friebe, J.
Parmentier, F. Béguin, Electrochemical energy storage in ordered
porous carbon materials, Carbon 43 (2005) 1293–1302.
[103] L. Eliad, G. Salitra, A. Soffer, D. Aurbach, Ion Sieving Effects in
the Electrical Double Layer of Porous Carbon Electrodes:
Estimating Effective Ion Size in Electrolytic Solutions, J. Phys.
Chem. B 105 (2001) 6880–6887.
[104] D. Eliad, L., Pollak, E., Levy, N., Salitra, G., Soffer, A., Aurbach,
Assessing optimal pore-to-ion size relations in the design of porous
poly (vinylidene chloride) carbons for EDL capacitors, Appl. Phys.
A Mater. Sci. Process. 82 (2006) 607–613.
[105] C.O. Ania, J. Pernak, F. Stefaniak, F. Be, Solvent-free ionic liquids
as in situ probes for assessing the effect of ion size on the
performance of electrical double layer capacitors, Carbon 44
(2006) 3126–3130
[106] Y. Li, Z.-Y. Fu, B.-L. Su, Hierarchically Structured Porous
Materials for Energy Conversion and Storage, Adv. Funct. Mater.
22 (2012) 4634–4667.
[107] M.J. Bleda-Martínez, D. Lozano-Castelló, E. Morallón, D.
Cazorla-Amorós, A. Linares-Solano, Chemical and
electrochemical characterization of porous carbon materials,
Carbon 44 (2006) 2642–2651.
[108] B.E. Conway, V. Birss, J. Wojtowicz, The role and utilization of
pseudocapacitance for energy storage by supercapacitors, J. Power
Sources 66 (1997) 1–14.
[109] H.A. Andreas, B.E. Conway, Examination of the double-layer
capacitance of an high specific-area C-cloth electrode as titrated
from acidic to alkaline pHs, Electrochim. Acta 51 (2006) 6510–
6520.
Introducción general
77
[110] K. Nueangnoraj, H. Nishihara, T. Ishii, N. Yamamoto, H. Itoi, R.
Berenguer, R. Ruiz-Rosas, D. Cazorla-Amorós, E. Morallón, M.
Ito, T. Kyotani, Pseudocapacitance of zeolite-templated carbon in
organic electrolytes, Energy Storage Mater. 1 (2015) 35–41
[111] C.A. Leon y Leon, L.R. Radovic, Interfacial chemistry and
electrochemistry of carbon surfaces, in: P.A. Thrower (Ed.), Chem.
Phys. Carbon, Vol. 24, Marcel Dekker, Inc New York, 1994: pp.
213–310.
[112] S. Leyva-García, K. Nueangnoraj, D. Lozano-Castelló, H.
Nishihara, T. Kyotani, E. Morallón, D. Cazorla-Amorós,
Characterization of a zeolite-templated carbon by electrochemical
quartz crystal microbalance and in situ Raman spectroscopy,
Carbon 89 (2015) 63–73.
[113] O. Ornelas, J.M. Sieben, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, J. Geng, N. Soin, E. Siores, B.F.G. Johnson, On the origin
of the high capacitance of nitrogen-containing carbon nanotubes in
acidic and alkaline electrolytes, Chem. Commun. 50 (2014)
11343–11346.
[114] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H.
Zhu, G.Q. Lu, Nitrogen-Enriched Nonporous Carbon Electrodes
with Extraordinary Supercapacitance, Adv. Funct. Mater. 19
(2009) 1800–1809.
[115] S.L. Candelaria, B.B. Garcia, D. Liu, G. Cao, Nitrogen
modification of highly porous carbon for improved supercapacitor
performance, J. Mater. Chem. 22 (2012) 9884–9889.
[116] M. Kawaguchi, T. Yamanaka, Y. Hayashi, H. Oda, Preparation and
capacitive properties of a carbonaceous material containing
nitrogen, J. Electrochem. Soc. 157 (2010) A35–A40.
[117] O.Z. Sharaf, M.F. Orhan, An overview of fuel cell technology:
Fundamentals and applications, Renew. Sustain. Energy Rev. 32
(2014) 810–853.
[118] M. Winter, R.J. Brodd, What Are Batteries, Fuel Cells, and
Supercapacitors? Chem. Rev. 104 (2004) 4245–4270.
Capítulo 1
78
[119] J. Masa, W. Xia, M. Muhler, W. Schuhmann, On the Role of Metals
in Nitrogen-Doped Carbon Electrocatalysts for Oxygen Reduction,
Angew. Chemie - Int. Ed. 54 (2015) 10102–10120.
[120] H.-H. Yang, R.L. McCreery, Elucidation of the Mechanism of
Dioxygen Reduction on Metal-Free Carbon Electrodes, J.
Electrochem. Soc. 147 (2000) 3420.
[121] A. Morozan, B. Jousselme, S. Palacin, Low-platinum and
platinum-free catalysts for the oxygen reduction reaction at fuel cell
cathodes, Energy Environ. Sci. 4 (2011) 1238–1254.
[122] V.R. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C.A.
Lucas, G. Wang, P.N. Ross, N.M. Markovic, Trends in
electrocatalysis on extended and nanoscale Pt-bimetallic alloy
surfaces, Nat. Mater. 6 (2007) 241–247.
[123] G. Wu, P. Zelenay, Nanostructured nonprecious metal catalysts for
oxygen reduction reaction, Acc. Chem. Res. 46 (2013) 1878–1889.
[124] C. Wang, H. Daimon, T. Onodera, T. Koda, S. Sun, A general
approach to the size- and shape-controlled synthesis of platinum
nanoparticles and their catalytic reduction of oxygen, Angew.
Chemie - Int. Ed. 47 (2008) 3588–3591.
[125] C.W.B. Bezerra, L. Zhang, K. Lee, H. Liu, A.L.B. Marques, E.P.
Marques, H. Wang, J. Zhang, A review of Fe-N/C and Co-N/C
catalysts for the oxygen reduction reaction, Electrochim. Acta 53
(2008) 4937–4951.
[126] C.W.B. Bezerra, L. Zhang, H. Liu, K. Lee, A.L.B. Marques, E.P.
Marques, H. Wang, J. Zhang, A review of heat-treatment effects on
activity and stability of PEM fuel cell catalysts for oxygen
reduction reaction, J. Power Sources 173 (2007) 891–908.
[127] P. Trogadas, T.F. Fuller, P. Strasser, Carbon as catalyst and support
for electrochemical energy conversion, Carbon 75 (2014) 5–42.
[128] S. Shahgaldi, J. Hamelin, Improved carbon nanostructures as a
novel catalyst support in the cathode side of PEMFC: A critical
review, Carbon 94 (2015) 705–728.
Introducción general
79
[129] F. Zaragoza-Martín, D. Sopeña-Escario, E. Morallón, C.S.-M. de
Lecea, Pt/carbon nanofibers electrocatalysts for fuel cells, J. Power
Sources 171 (2007) 302–309.
[130] K.H. Wu, D.W. Wang, D.S. Su, I.R. Gentle, A Discussion on the
Activity Origin in Metal-Free Nitrogen-Doped Carbons for
Oxygen Reduction Reaction and their Mechanisms,
ChemSusChem. 8 (2015) 2772–2788
[131] L. Kuai, B. Geng, S. Wang, Y. Zhao, Y. Luo, H. Jiang, Silver and
Gold Icosahedra: One-Pot Water-Based Synthesis and Their
Superior Performance in the Electrocatalysis for Oxygen
Reduction Reactions in Alkaline Media, Chem. – A Eur. J. 17
(2011) 3482–3489
[132] H.W. Liang, W. Wei, Z.S. Wu, X. Feng, K. Müllen, Mesoporous
metal-nitrogen-doped carbon electrocatalysts for highly efficient
oxygen reduction reaction, J. Am. Chem. Soc. 135 (2013) 16002–
16005.
[133] A. Gabe, J. García-Aguilar, Á. Berenguer-Murcia, E. Morallón, D.
Cazorla-Amorós, Key factors improving oxygen reduction reaction
activity in cobalt nanoparticles modified carbon nanotubes, Appl.
Catal. B Environ. 217 (2017) 303–312
[134] Y. Liang, H. Wang, P. Diao, W. Chang, G. Hong, Y. Li, M. Gong,
L. Xie, J. Zhou, J. Wang, T.Z. Regier, F. Wei, H. Dai, Oxygen
Reduction Electrocatalyst Based on Strongly Coupled Cobalt
Oxide Nanocrystals and Carbon Nanotubes, J. Am. Chem. Soc. 134
(2012) 15849–15857
[135] J. Park, Y. Nabae, T. Hayakawa, M.A. Kakimoto, Highly selective
two-electron oxygen reduction catalyzed by mesoporous nitrogen-
doped carbon, ACS Catal. 4 (2014) 3749–3754.
[136] Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, A review on non-
precious metal electrocatalysts for PEM fuel cells, Energy Environ.
Sci. 4 (2011) 3167–3192.
[137] A. Gabe, R. Ruiz-Rosas, C. González-Gaitán, E. Morallón, D.
Cazorla-Amorós, Modeling of oxygen reduction reaction in porous
Capítulo 1
80
carbon materials in alkaline medium. Effect of microporosity, J.
Power Sources (2019) 451–464
[138] J.Y. Choi, R.S. Hsu, Z. Chen, Highly active porous carbon-
supported nonprecious metal-N electrocatalyst for oxygen
reduction reaction in PEM fuel cells, J. Phys. Chem. C 114 (2010)
8048–8053.
[139] S. Maldonado, K.J. Stevenson, Influence of nitrogen doping on
oxygen reduction electrocatalysis at carbon nanofiber electrodes, J.
Phys. Chem. B 109 (2005) 4707–4716.
[140] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-Doped
Carbon Nanotube Arrays with High Electrocatalytic Activity for
Oxygen Reduction, Science 323 (2009) 760–764.
[141] V. Strelko, V. Kuts, P. Thrower, On the mechanism of possible
influence of heteroatoms of nitrogen, boron and phosphorus in a
carbon matrix on the catalytic activity of carbons in electron
transfer reactions, Carbon 38 (2000) 1499–1503
[142] Q. Wei, X. Tong, G. Zhang, J. Qiao, Q. Gong, S. Sun, Nitrogen-
Doped Carbon Nanotube and Graphene Materials for Oxygen
Reduction Reactions, Catalysts 5 (2015) 1574–1602.
[143] N.P. Subramanian, X. Li, V. Nallathambi, S.P. Kumaraguru, H.
Colon-Mercado, G. Wu, J.W. Lee, B.N. Popov, Nitrogen-modified
carbon-based catalysts for oxygen reduction reaction in polymer
electrolyte membrane fuel cells, J. Power Sources 188 (2009) 38–
44.
[144] J.D. Wiggins-Camacho, K.J. Stevenson, Mechanistic discussion of
the oxygen reduction reaction at nitrogen-doped carbon nanotubes,
J. Phys. Chem. C 115 (2011) 20002–20010.
[145] P. Zhang, F. Sun, Z. Xiang, Z. Shen, J. Yun, D. Cao, ZIF-derived
in situ nitrogen-doped porous carbons as efficient metal-free
electrocatalysts for oxygen reduction reaction, Energy Environ.
Sci. 7 (2014) 442–450
Introducción general
81
[146] W. Xia, J. Masa, M. Bron, W. Schuhmann, M. Muhler, Highly
active metal-free nitrogen-containing carbon catalysts for oxygen
reduction synthesized by thermal treatment of polypyridine-carbon
black mixtures, Electrochem. Commun. 13 (2011) 593–596.
[147] C. González-Gaitán, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Functionalization of carbon nanotubes using
aminobenzene acids and electrochemical methods. Electroactivity
for the oxygen reduction reaction, Int. J. Hydrogen Energy 40
(2015) 11242–11253.
[148] J. Quilez-Bermejo, C. González-Gaitán, E. Morallón, D. Cazorla-
Amorós, Effect of carbonization conditions of polyaniline on its
catalytic activity towards ORR. Some insights about the nature of
the active sites, Carbon 119 (2017) 62–71.
[149] J. Quílez-Bermejo, E. Morallón, D. Cazorla-Amorós, Oxygen-
reduction catalysis of N-doped carbons prepared: Via heat
treatment of polyaniline at over 1100 °C, Chem. Commun. 54
(2018) 4441–4444.
[150] T. Ikeda, M. Boero, S.F. Huang, K. Terakura, M. Oshima, J.I.
Ozaki, S.F. Hang, K. Terakura, M. Oshima, J.I. Ozaki, Carbon
Alloy Catalysts: Active Sites for Oxygen Reduction Reaction, J.
Phys. Chem. C 112 (2008) 14706–14709.
Capítulo 2
Materiales, métodos y técnicas
experimentales
Materiales, métodos y técnicas experimentales
85
2.1. Introducción
En este capítulo, se presentan los materiales, métodos de síntesis y
técnicas experimentales utilizados en esta Tesis Doctoral. Además, se
describen de forma general los métodos de funcionalización de la
superficie utilizados para la preparación de nuevos materiales. No
obstante, las condiciones experimentales específicas se encuentran
detalladas en cada uno de los capítulos.
2.2 Materiales
2.2.1 Carbones activados.
En esta Tesis Doctoral, se ha empleado principalmente un carbón
activado sintetizado en los laboratorios del grupo de investigación
Materiales Carbonosos y Medio Ambiente (conocido como KUA) y un
carbón activado comercial (YP50F).
El carbón activado KUA se obtuvo mediante activación química de
una antracita española, cuyas condiciones de síntesis se estudiaron
previamente en el grupo de investigación Materiales Carbonosos y
Medio Ambiente [1]. De forma resumida, este método consiste en
mezclar el precursor carbonoso con hidróxido potásico para
posteriormente realizar una etapa de pirólisis a una temperatura
determinada. Finalmente, se realiza un lavado en medio ácido para
eliminar los restos de agente activante y productos inorgánicos
resultantes de la reacción. De este modo, se obtiene un material de
elevada microporosidad cuyas propiedades fisicoquímicas resultan de
Capítulo 2
86
especial interés en el campo del almacenamiento electroquímico de
energía.
Adicionalmente, se han realizado ensayos de funcionalización
química empleando un carbón activado comercial con aplicación en
supercondensadores (YP50F, Kuraray Chemical, Japón). Las
propiedades de este material se detallan en el anexo del capítulo 6.
2.2.2 Materiales carbonosos nanomoldeados.
Se han empleado dos materiales carbonosos nanomoldeados (ZTC y
N-ZTC), obtenidos a partir de distintos precursores [2-4]. La síntesis de
estos materiales se realizó en el laboratorio del profesor Kyotani en la
Universidad de Tohoku (Japón), como resultado de una colaboración.
Estos materiales se sintetizaron mediante depósito químico en fase vapor
(CVD, del inglés chemical vapor deposition) utilizando una zeolita Y
como plantilla. Se utilizó propileno como precursor del ZTC y
acetonitrilo en el caso del N-ZTC. La información detallada acerca de la
síntesis y caracterización de estos materiales se encuentra en el capítulo
5.
2.3. Modificación de la química superficial de los materiales
carbonosos.
Los materiales KUA, YP50F y ZTC se han utilizado como
materiales de partida para introducir grupos funcionales (principalmente,
nitrogenados) en su superficie. A continuación, se describen de forma
detallada los métodos empleados.
Materiales, métodos y técnicas experimentales
87
2.3.1 Funcionalización química mediante métodos basados en
reacciones orgánicas.
Se han realizado distintos tratamientos de funcionalización química
para introducir grupos funcionales nitrogenados en materiales
carbonosos. Principalmente, se han utilizado dos protocolos basados en
reacciones orgánicas.
El primero consiste en la introducción de grupos funcionales amida
por medio de tres etapas de reacción: (i) oxidación (química o
electroquímica); (ii) tratamiento con SOCl2 para producir cloruros de
ácido y (iii) reacción con un reactivo nitrogenado. Este tratamiento se
ha utilizado para introducir grupos nitrogenados sobre los materiales
KUA y ZTC. Además, este tratamiento también se ha realizado en los
carbones activados KUA e YP50F sin llevar a cabo los procesos previos
de oxidación química y tratamiento con SOCl2.
El segundo tratamiento de funcionalización permite convertir las
amidas generadas por medio del primer protocolo en grupos funcionales
aminas. Este tratamiento se ha realizado sobre el carbón activado KUA.
A continuación, se describe de forma detallada el procedimiento
experimental utilizado para modificar la química superficial de los
materiales.
Capítulo 2
88
2.3.1.1. Oxidación química
La oxidación química se ha realizado utilizando HNO3 como agente
oxidante. Este tratamiento ha sido adaptado a partir de estudios previos
realizados en nuestro grupo de investigación [5].
En un vaso de precipitados se pone en contacto 1g de material con
40 mL de disolución de HNO3 65 % durante 3 horas con agitación
magnética a temperatura ambiente. Posteriormente, el carbón activado se
filtra y se lava con agua destilada hasta que el pH del agua de lavado sea
constante. Por último, la muestra se seca a 100 ºC.
2.3.1.2. Introducción de grupos funcionales amida
Este proceso requiere el uso de dos etapas consecutivas: en primer
lugar, tratamiento para formar cloruros de ácido a partir de los ácidos
carboxílicos y, en segundo lugar, reacción con un reactivo nitrogenado
para anclar el nitrógeno por sustitución nucleofílica sobre el cloruro de
ácido.
Formación de cloruros de ácido
Este procedimiento experimental se ha adaptado a partir de la
referencia [6]. Las reacciones se llevaron a cabo dentro de un sistema
Schlenk. Se introduce 1g de material carbonoso (resultante tras la
oxidación) en 50 mL de tolueno en un matraz de fondo redondo. A
continuación, se añaden 5 mL de SOCl2. La mezcla se mantiene a reflujo
a 120 ºC durante 5 horas en atmósfera de Ar. Posteriormente, se filtra y
Materiales, métodos y técnicas experimentales
89
el polvo obtenido se lava con tolueno y se seca a vacío a 120 ºC durante
14 horas.
Reacción con un reactivo nitrogenado
Las reacciones de amidación y aminación se han adaptado de la
funcionalización propuesta por Gromov y colaboradores para nanotubos
de carbono [7].
1g de KUA-COCl se añade a una disolución 2M de NH4NO3 en
DMF (300 mL) en un matraz de fondo redondo. La mezcla se agita para
obtener una dispersión homogénea. A continuación, se añade lentamente
y con agitación continua 300 mL de piridina. La mezcla resultante se
agita a 70ºC durante 65 horas en atmósfera de Ar. La muestra obtenida
se filtra y se lava con agua y etanol.
Este tratamiento se ha realizado utilizando como material de partida
un material carbonoso prístino y ese mismo material modificado
mediante oxidación química y tratamiento con SOCl2.
2.3.1.3. Introducción de grupos funcionales amina
Se añaden 10 mL de Br2 a 650 mL de disolución 3% de NaOCH3
en CH3OH en un matraz de fondo redondo. A continuación, se añaden
500 mg de material carbonoso a la disolución y la dispersión resultante
se agita a 70ºC durante 5 horas. Posteriormente, se añaden 4 mL de Br2
adicionales y se mantiene en agitación durante 20 horas a 70ºC. El
producto se filtra y se lava con Na2CO3 saturado, agua y etanol. Para
continuar, el material carbonoso se somete a hidrólisis en 500 mL de
Capítulo 2
90
NaOH 0.1M con agitación magnética durante 24 horas. Finalmente, la
muestra se filtra, se lava con agua y se seca a 100ºC en una estufa.
2.3.2. Polimerización de anilina
Este método consiste en la generación de un polímero conductor
(polianilina) en la superficie del material carbonoso, de manera que se
obtiene un material compuesto carbón activado/polianilina. Para esto, se
realiza la síntesis química de polianilina a partir de anilina previamente
adsorbida en el material carbonoso (en este caso, el carbón activado
KUA) [8]. Este método consiste en la oxidación de anilina en medio
ácido en presencia de un oxidante [9]. El proceso se lleva a cabo en un
reactor (que contiene una disolución 1M de HCl a 0 ºC) en el que se
adiciona el material carbonoso en el que se ha realizado la adsorción de
anilina. Posteriormente, se añade el agente oxidante (NH4)2S2O8 disuelto
en 1M HCl (a 0ºC) con una relación monómero:oxidante 1:1. Una vez
obtenido el material compuesto (KUA/PANI), se lava con 1M HCl y se
trata con NH4OH con el fin de obtener la polianilina en su forma
desdopada. Previamente al proceso de polimerización, se estudió la
adsorción del monómero de anilina en el material carbonoso, con el fin
de determinar la cantidad de anilina adsorbida y, de este modo, conocer
la cantidad de oxidante que es necesario adicionar al medio de reacción,
así como obtener diferentes cantidades de anilina adsorbida.
Materiales, métodos y técnicas experimentales
91
2.3.3 Tratamientos térmicos
Los materiales carbonosos obtenidos en las secciones 2.3.1 y 2.3.2,
se han utilizado como materiales de partida para realizar tratamientos
térmicos que modifiquen los grupos funcionales en condiciones que
permitan preservar la textura porosa del material original. Estos
tratamientos se han llevado a cabo en un horno tubular en atmósfera
inerte, utilizando una velocidad de calentamiento lenta (5ºC/min), hasta
alcanzar la temperatura final (500, 600 y 800 ºC), que se mantiene
durante una hora. En estas condiciones, el tratamiento afecta
principalmente a la química superficial, de manera que produce la
descomposición de los grupos funcionales y su conversión en otras
especies de elevada estabilidad térmica. Los detalles específicos de cada
tratamiento se pueden consultar en los capítulos 6 y 8.
2.4 Técnicas de caracterización
2.4.1 Adsorción física de gases
La adsorción física de gases es una técnica muy utilizada para
caracterizar propiedades texturales de los sólidos porosos [10]. El
proceso de adsorción consiste en el fenómeno que experimenta un sólido
(adsorbente) cuando entra en contacto con un gas (adsortivo) en un
recipiente a volumen constante y a una temperatura dada. En estas
condiciones, las fuerzas de interacción (de tipo Van der Waals) entre el
adsorbente y el adsortivo producen la acumulación de las moléculas de
gas en la interfase, que da lugar a una variación de la presión del
Capítulo 2
92
recipiente. Una vez alcanzado el equilibrio, la fisisorción se completa y,
por tanto, la presión alcanza un valor constante.
La cantidad de gas adsorbido se puede calcular de forma
volumétrica (a partir de la variación de presión que experimenta el
recipiente) o de forma gravimétrica (por medio de la determinación del
aumento de masa del sólido). Este cálculo se lleva a cabo a distintas
presiones relativas (P/P0), de manera que la representación de la cantidad
de gas adsorbido en función de las distintas presiones relativas se conoce
como isoterma de adsorción. A partir de estos experimentos, se pueden
determinar parámetros de la textura porosa de los materiales, como el
volumen de microporos, el área superficial, distribución de tamaños de
poros, etc.
Se pueden utilizar distintos adsorbatos (N2, He, Ar, etc.) para
obtener isotermas de adsorción. El adsortivo más común es el N2 a 77K.
A esta temperatura, el gas se adsorbe en un intervalo amplio de
presiones relativas (de 10-8 a 1), lo que permite la adsorción en todo el
intervalo de porosidad. No obstante, este adsorbato no permite
caracterizar aquellos sólidos que presenten una microporosidad muy
estrecha (<0.7 nm) debido a problemas cinéticos derivados de problemas
difusionales del N2 a baja temperatura y el momento cuadrupolar de la
molécula. Por tanto, es necesario un tiempo de equilibrio
extremadamente largo. En estos casos, se utiliza la adsorción de CO2 a
273K, que permite la caracterización de la microporosidad más estrecha,
para completar el estudio de las propiedades texturales de este tipo de
materiales [10–12].
Materiales, métodos y técnicas experimentales
93
En este trabajo, las isotermas de adsorción se han realizado en un
equipo volumétrico (Autosorb-6-Quantachrome). El equipo consta,
además, de una unidad de desgasificación independiente. Las muestras
utilizadas en esta Tesis Doctoral se han desgasificado a 200 ºC durante 4
horas. El intervalo de presiones relativas utilizado para las isotermas de
N2 fue de 0 a 1 y, en el caso de las isotermas de CO2, de 0 a 0.03.
Figura 2.1. Equipos de adsorción física de gases Autosorb-6-Quantachrome.
El análisis de las isotermas de adsorción requiere la aplicación de
varias teorías. Las más comunes son la teoría Brunauer, Emmet y Teller
y el modelo de Dubinin-Radushkevich.
2.4.1.1 Teoría de Brunauer-Emmet-Teller
La teoría de Brunauer-Emmet-Teller (BET) es uno de los modelos
de fisisorción más empleados para la determinación de la superficie
específica de materiales porosos [13]. La teoría BET se basa en el
modelo cinético de adsorción de Langmuir. Esta teoría parte del
concepto de equilibrio de adsorción, en el que la velocidad de adsorción
Capítulo 2
94
y desorción del adsorbato son iguales. Además, la teoría considera los
siguientes supuestos: (i) sólo se puede adsorber una molécula en cada
sitio libre de la superficie del adsorbente; (ii) la superficie del sólido está
compuesta por sitios homogéneos en términos de energía; (iii) la
saturación se alcanza por completo en una monocapa; (iv) las fuerzas (de
repulsión o atracción) entre moléculas adsorbidas son despreciables en
comparación con las interacciones adsorbente-adsortivo.
La teoría de BET consiste en una extensión de la teoría Langmuir a
adsorción en multicapa y establece algunos supuestos adicionales: (i) la
primera capa adsorbida es similar a la propuesta en el modelo de
Langmuir; (ii) el calor de adsorción es igual al calor molar de
licuefacción en todas las capas, excepto en la primera; (iii) las
condiciones de evaporación-condensación son idénticas, excepto en
todas las capas; (iv) cuando P/P0 = 1, el adsortivo condensa en la
superficie del adsorbente (es decir, el número de capas se considera
infinito).
La isoterma de adsorción según la teoría BET se puede expresar
mediante la ecuación (2.1).
𝑃
𝑃𝑜
𝑛(1−𝑃
𝑃𝑜)
=1
𝑛𝑚𝐶+
𝐶−1
𝑛𝑚𝐶
𝑃
𝑃𝑜 (2.1)
donde P y Po son la presión y la presión de saturación, respectivamente
(en Pa o mm Hg); n y 𝑛𝑚 son los moles adsorbidos por gramo de
adsorbente a una determinada presión (P) y en la monocapa superficial,
Materiales, métodos y técnicas experimentales
95
respectivamente; y C es un parámetro relacionado con el calor de
adsorción.
La representación gráfica 𝑃 𝑃𝑜⁄ /(𝑛𝑚(1 − 𝑃 𝑃𝑜⁄ )) en función de
𝑃 𝑃𝑜⁄ da lugar a una recta, de cuya pendiente y ordenada en el origen se
puede obtener 𝑛𝑚 y C. El valor de 𝑛𝑚 permite determinar la superficie
específica utilizando la ecuación 2.2.
𝑆 = 𝑛𝑚 ∙ 𝑎𝑚 ∙ 𝑁𝐴 ∙ 10−18 (2.2)
donde S es la superficie específica aparente del sólido (m2/g), NA es
el número de Avogadro (6.023·1023 moléculas/mol) y am es el área que
ocupa una molécula de adsorbato (en el caso del N2 a -196 ºC, el valor es
0.162 nm2).
La ecuación BET se debe aplicar en un intervalo de presiones
relativas determinado: 0.05P/P00.3. A presiones relativas inferiores a
0.05, la superficie no es energéticamente homogénea, debido a la
presencia de microporos estrechos que producen el aumento del
potencial de adsorción. A presiones relativas superiores a 0.3, se pueden
dar fenómenos de condensación capilar.
2.4.1.2. Ecuación de Dubinin-Radushkevich
La ecuación de Dubinin-Radushkevich (DR) se utiliza para analizar
la textura porosa de materiales microporosos [14]. Esta ecuación está
basada en la teoría del potencial de Polanyi, que se basa en el supuesto
de que las moléculas de gas se condensan en la superficie del sólido
debido a fuerzas de atracción existentes entre la superficie y las
Capítulo 2
96
moléculas. La fuerza de atracción a un punto dado de la superficie se
mide mediante un potencial de adsorción. La teoría considera que el
espacio de adsorción está compuesto por una serie de superficies
equipotenciales.
Esta teoría permite determinar el volumen de poros de material a
partir de las isotermas de adsorción. Esta ecuación supone la
condensación del adsorbato en los microporos en capas de potenciales
iguales. La ecuación es la siguiente (2.3):
𝑉
𝑉𝑜= exp (−
1
(𝐸0𝛽)2 (𝑅𝑇𝐿𝑛𝑃
𝑃0)
2
) (2.3)
donde V es el volumen adsorbido a una presión P, V0 es el volumen de
microporos del sólido, E0 es la energía característica dependiente de la
estructura porosa, es el coeficiente de afinidad característico del
adsorbato y P0 es la presión de saturación del adsorbato a la temperatura
de trabajo. La presentación de LnV en función de (Ln(P0/P))2 permite
obtener el valor de V0 a partir de la ordenada en el origen.
La aplicación de esta ecuación a isotermas de N2 y CO2 permite
determinar el volumen de microporos total (< 2 nm) y el volumen de
microporos más estrechos (< 0.7 nm), respectivamente.
2.4.1.3. Teoría del funcional de densidad no localizada bidimensional
La teoría del funcional de densidad no localizada (NLDFT, del
inglés non local density functional theory) es muy utilizada para
determinar las propiedades texturales de los sólidos microporosos
(superficie específica y distribución de tamaños de poros) [10]. En esta
Materiales, métodos y técnicas experimentales
97
Tesis Doctoral, este modelo se ha utilizado para la determinación de la
distribución de tamaños de poros. Este parámetro gobierna muchas
propiedades de los materiales carbonosos en diversas aplicaciones, como
el almacenamiento de hidrógeno [15] o en condensadores
electroquímicos [16].
Los cálculos NLDFT se basan en la obtención de isotermas de
adsorción en función de distintos tamaños de poros y geometrías [17-
19]. La distribución de tamaños de poros se puede obtener resolviendo la
ecuación integral de adsorción (ecuación 2.4).
𝑁(𝑝) = ∫ 𝑓(𝑤)𝐾(𝑃. 𝑤)𝑑𝑤𝑏
𝑎 (2.4)
Donde N(p) es isoterma de adsorción experimental (en función
de la presión P, w es el tamaño de poro, (a,b) es el intervalo de tamaños
de poros existente, f(w) representa la distribución de tamaños de poros a
determinar y K es el conjunto de isotermas teóricas expresadas en
función de las densidades de los fluidos en los poros (en función de P y
w). La función K se calcula basándose en un modelo de poro y se conoce
como la función “kernel” de la ecuación integral. El cálculo del “kernel”
que representa un modelo de adsorción dado involucra dos procesos: (i)
el modelado del espacio del poro y la evaluación del potencial sólido-
fluido en los poros; y (ii) cálculo de las densidades de equilibrio de un
fluido confinado en los poros (de acuerdo con el modelo del mismo).
Este modelo considera los poros como hendiduras
unidimensionales en las que el espacio del poro está confinado entre
paredes grafíticas semi-infinitas y uniformes energéticamente. En
Capítulo 2
98
consecuencia, las isotermas calculadas mediante NLDFT no coinciden
de forma exacta con las medidas experimentales obtenidas para carbones
activados. El modelo bidimensional (2D-NLDFT) propuesto por Jagiello
[17] proporciona una mejor aproximación a las propiedades texturales de
los sólidos microporosos. Este modelo considera los carbones activados
como una red estructural basada en grafeno e introduce el efecto de las
heterogeneidades energéticas y las curvaturas superficiales en las
paredes de los poros.
2.4.2. Espectroscopia fotoelectrónica de rayos X
La espectroscopía fotoelectrónica de rayos X (XPS, del inglés X-ray
photoelectron spectroscopy) es una técnica espectroscópica muy
utilizada en la caracterización superficial de sólidos [20], debido a que
permite obtener información de las capas más superficiales (de 1 a 10
nm). Consiste en determinar la energía cinética de los electrones que
emite el material cuando este se irradia con un haz de rayos X. La
energía cinética de los electrones fotoemitidos viene dada por la
ecuación (2.5).
𝐸𝐶 = 𝐸ℎ − 𝐸𝑒𝑒 − (2.5)
donde Ec es la energía cinética del electrón emitido, Eh es la energía de
la fuente de rayos X utilizada, Eee es la energía de ligadura del electrón
excitado relativo al nivel de Fermi y es la función de trabajo
combinación del espectrómetro y la muestra.
Materiales, métodos y técnicas experimentales
99
Para un determinado átomo se pueden determinar distintas energías
para los electrones procedentes de las capas internas o de la capa de
valencia. Por tanto, las energías de ligadura dependen tanto del átomo de
procedencia como de su carga, por lo que mediante esta técnica se puede
obtener información de la naturaleza de los átomos y del estado químico
de los mismos en la superficie. En general, la energía de ligadura
aumenta con el estado de oxidación del átomo. Además, cambia por el
entorno químico del átomo en cuestión, por lo que cuanto mayor sea el
carácter electronegativo de los átomos adyacentes, mayor será la energía
de ligadura de este.
El sistema experimental consta de una fuente de rayos X (ánodo de
Al o Mg), un detector de electrones y un analizador de energía de
electrones. Además, presenta una cámara para trabajar en condiciones de
ultra alto vacío (5·10-7 Pa).
El equipo utilizado en este trabajo es un espectrómetro VG-
Microtech Multilab 3000, perteneciente a los servicios técnicos de
investigación de la Universidad de Alicante. El equipo presenta un
analizador de electrones semiesférico (con energía de paso de 2-200 eV)
y una fuente de radiación de rayos X con ánodos de Mg y Al.
En este trabajo, se ha empleado la técnica XPS para determinar la
composición superficial de los materiales carbonosos. Para este
propósito, se han detectado las transiciones electrónicas C(1s), O(1s) y
N(1s). El análisis de estas transiciones permite cuantificar e identificar
los grupos funcionales superficiales de los materiales estudiados. Las
Capítulo 2
100
condiciones utilizadas para deconvolucionar los espectros se detallan en
cada capítulo. En todos los casos, se tomó como referencia la transición
C1s (284.7 eV). Los valores de energía de ligadura tienen una precisión
de ± 0.2 eV.
2.4.3. Desorción a temperatura programada
La técnica de desorción a temperatura programada (TPD, del inglés
temperature programmed desorption) tiene gran importancia en el
estudio de materiales carbonosos, debido a que permite caracterizar su
química superficial. La técnica consiste en someter a la muestra a un
programa de temperaturas controlado (rampa lineal) en atmósfera
controlada (gas inerte). Los gases procedentes de la descomposición de
los grupos superficiales se analizan en un detector, que se encuentra
acoplado a la salida de la termobalanza. El detector más utilizado es el
espectrómetro de masas. El análisis de los gases resultantes de la
desorción permite obtener información acerca de la composición
química y la estabilidad de los grupos funcionales superficiales.
Los gases desorbidos por la muestra son, principalmente, CO, CO2
(Figura 2.2) y H2O [21–23], y pueden proceder de la descomposición de
los grupos funcionales oxigenados superficiales o descomposición de
compuestos inorgánicos presentes y de la desorción del agua adsorbida
en el material. Estos gases desorben a distintas temperaturas,
relacionadas con la distinta energía de descomposición de cada grupo
oxigenado. Los grupos carboxílicos, lactonas y anhídridos descomponen
como CO2 a distintas temperaturas según la estabilidad de los mismos,
Materiales, métodos y técnicas experimentales
101
aunque suelen ser relativamente bajas (300-500ºC), mientras que los
grupos carbonilos, quinona, fenoles o éter son más estables y
descomponen a elevadas temperaturas como CO (600-1200ºC).
Figura 2.2. Grupos funcionales oxigenados presentes en materiales carbonosos y gases
emitidos por su descomposición.
La cantidad total de oxígeno superficial en las muestras se
determina a partir de las cantidades de CO y CO2 desorbidos al someter
las muestras hasta una temperatura de 950 ºC, atendiendo a la ecuación
(2.6).
[O] = 2[CO2] + [CO] (2.6)
En este trabajo, se ha utilizado un equipo DSC-TGA (TA
Instruments, SDT 2960 Simultenaous) acoplado a un espectrómetro de
masas (Thermostar, Balzers, GSD 300 T3). En los experimentos
Capítulo 2
102
realizados, se utilizaron 10 mg de muestra y el análisis se realizó usando
flujo de Helio como gas de arrastre (100 ml/min) y desde temperatura
ambiente hasta 950ºC. La velocidad de calentamiento fue de 20ºC/min.
2.4.4. Caracterización electroquímica
La caracterización electroquímica de los materiales se ha llevado a
cabo utilizado diversas técnicas y dispositivos electroquímicos. En los
siguientes apartados, se describen las configuraciones de celda y los
fundamentos de las técnicas electroquímicas utilizadas en este trabajo.
2.4.4.1. Configuraciones de celda electroquímica
En esta Tesis Doctoral, se han utilizado celdas de dos y tres
electrodos. Las celdas de dos electrodos se han empleado para
caracterizar condensadores electroquímicos, mientras que las celdas de
tres electrodos permiten realizar un estudio fundamental de las
propiedades electroquímicas de los materiales.
Las celdas electroquímicas de dos electrodos son las unidades
elementales a partir de las que se construyen las baterías y los
supercondensadores [24]. Están compuestas por dos materiales activos
(electrodos positivo y negativo), dos colectores de corriente, un
separador y un electrolito. Los colectores de corriente son conductores
eléctricos que garantizan la transferencia de electrones hacia o desde los
materiales. El separador debe ser un conductor iónico (que permita
mantener el flujo de corriente en la célula) y aislante eléctrico para evitar
el cortocircuito del dispositivo. El separador y los materiales activos
Materiales, métodos y técnicas experimentales
103
deben encontrarse en un medio conductor, como un electrolito líquido,
sólido o polimérico.
Figura 2.3. Configuraciones de celda de (a) dos y (b, c) tres electrodos.
La Figura 2.3a muestra el esquema de los componentes de una celda
de dos electrodos utilizada para caracterizar un supercondensador. En
este sistema, la corriente o el voltaje entre los electrodos positivo y
negativo se registran o controlan para evaluar el comportamiento del
condensador [25]. Los métodos galvanostáticos (cronopotenciometría),
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
e-
e-
e-
e-
e-
e-
e-
e-
(+) (-)
Electrodo
positivoElectrodo
negativo
SeparadorElectrolito Electrodo de
trabajo
Electrolito
Electrodo de
referencia Contraelectrodo
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
e-
e-
e-
e-
e-
e-
e-
e-Electrodo de
trabajoContraelectrodo
Electrolito
Electrodo de
referencia(c)
(a) (b)
Capítulo 2
104
potenciostáticos (cronoamperometría) o potenciodinámicos (voltametría
cíclica) son los más utilizados para caracterizar estos dispositivos. Sin
embargo, en una celda de dos electrodos, no se pueden caracterizar las
propiedades electroquímicas de los materiales de forma individual. Para
obtener información fundamental acerca de las propiedades de cada
electrodo, es necesario utilizar una configuración de tres electrodos
mediante el uso de un potenciostato.
La configuración de tres electrodos consta de: (i) un electrodo de
trabajo; (ii) electrodo de referencia y (iii) contraelectrodo. Los tres
componentes se encuentran inmersos en el electrolito de trabajo. Las
Figuras 2.3b y c muestran distintos sistemas experimentales utilizados
para construir una celda de tres electrodos. El sistema experimental
propuesto en la Figura 2.3b es el más empleado para estudiar las
propiedades electroquímicas de los materiales. El electrodo de trabajo se
compone del material a caracterizar, que se deposita o aglomera sobre un
colector de corriente. El electrodo de referencia (Ag/AgCl, electrodo
reversible de hidrógeno (ERH), etc.) y contraelectrodo (Pt, Au, etc.)
utilizados depende del medio electrolítico [26]. Este sistema
experimental se ha utilizado en esta Tesis Doctoral para realizar el
estudio electrocatalítico de los materiales carbonosos. Para esto, se ha
utilizado un electrodo rotatorio de disco-anillo (apartado 2.4.4.2.6),
sobre el que se deposita el material carbonoso a caracterizar.
Para caracterizar materiales para su aplicación posterior en
condensadores electroquímicos, se ha utilizado el sistema experimental
propuesto en la Figura 2.3c. En este caso, la celda de tres electrodos se
Materiales, métodos y técnicas experimentales
105
construye simplemente al introducir un electrodo de referencia en la
celda de dos electrodos. Por medio de esta configuración, se pueden
estudiar las propiedades electroquímicas de uno de los dos electrodos,
que constituye el electrodo de trabajo. El otro material se utiliza como
contraelectrodo, cuya masa se sobredimensiona para evitar interferencias
con el electrodo de trabajo debido a que se produzca un aumento de la
densidad de corriente en ese electrodo. Esta configuración permite
caracterizar los electrodos empleando unas condiciones cercanas a las
del dispositivo final y ha sido la empleada en esta Tesis Doctoral. Dado
que en este trabajo se han utilizado distintos tipos de celdas, en cada
capítulo se especifican los detalles referentes a: (i) configuración de
celda; (ii) construcción del electrodo de trabajo; (iii) tipo de electrodo de
referencia y contraelectrodo.
2.4.4.2 Técnicas electroquímicas
2.4.4.2.1. Voltametría cíclica
La voltametría cíclica es una de las técnicas más versátiles para
estudiar procesos electroquímicos [27]. En esta técnica, se produce la
variación del potencial con el tiempo a una velocidad de barrido dada (v)
desde un potencial inicial hasta alcanzar un potencial final (Figura 2.4).
Una vez alcanzado el potencial final, se lleva a cabo el barrido inverso
(de Eb a Ea). La figura 2.4 representa la variación del potencial con el
tiempo que se debe aplicar para obtener un voltagrama cíclico. Durante
el barrio de potencial se registra la corriente que circula entre el
electrodo de trabajo y el contraelectrodo en configuración de 3
electrodos, o la corriente que circula entre ambos electrodos en la
Capítulo 2
106
configuración de 2 electrodos. La representación de la corriente
resultante en función del potencial proporciona un voltagrama cíclico.
Esta técnica permite evaluar el comportamiento de sistemas muy
diferentes: procesos capacitivos, reacciones en disolución, reacciones en
superficie, etc [27]. En esta Tesis Doctoral, se ha empleado
fundamentalmente para evaluar las propiedades electroquímicas de los
materiales carbonosos y la del supercondensador: capacidad, ventana de
estabilidad electroquímica, etc.
Figura 2.4. Variación del potencial (E) con el tiempo (t) durante un voltagrama
cíclico.
La voltamperometría cíclica permite determinar la capacidad de un
electrodo a partir de la ecuación (2.6.) [28].
𝐶 = 𝑞
𝐸=
∫ 𝐼𝑑𝑡
𝐸 (2.6)
Donde I es la corriente obtenida durante el voltamperograma cíclico y E
es el potencial aplicado.
t
E
Ea
Eb
E0
Materiales, métodos y técnicas experimentales
107
Figura 2.5. Voltagrama cíclico característico de (a) un material que presenta un
comportamiento puramente capacitivo y (b) un material con grupos funcionales
electroactivos en su superficie.
La Figura 2.5 muestra los voltagramas característicos de los
procesos electroquímicos más comunes en los materiales carbonosos
estudiados en este trabajo. En el caso de un material carbonoso en el que
la respuesta electroquímica fundamental se deba a procesos de doble
capa eléctrica (es decir, que no presente grupos electroactivos en su
superficie), el voltagrama cíclico correspondiente presenta una forma
idealmente rectangular (Figura 2.5a) [28]. Si el voltagrama se registra en
un intervalo de potenciales en el que el material no es estable, se
observan corrientes farádicas correspondientes a la oxidación del
material y del electrolito.
En el segundo caso (Figura 2.5b), se presenta el comportamiento de
un material carbonoso que, además de la contribución de la doble capa
eléctrica, presenta un comportamiento con corrientes farádicas como
consecuencia de la presencia de grupos funcionales electroactivos, en las
condiciones de estudio, en su superficie (por ejemplo, quinonas).
(a) (b)
Capítulo 2
108
2.4.4.2.2. Voltametría de barrido lineal
La voltametría de barrido lineal es muy similar a la
voltamperometría cíclica. Del mismo modo, se realiza un barrido de
potencial a una velocidad de barrido dada desde un potencial inicial a un
potencial final [29]. En este caso, no se lleva a cabo el barrido inverso y
el experimento finaliza cuando se alcanza Eb. Ambas técnicas permiten
determinar el potencial al que tiene lugar una reacción electroquímica,
por lo que son muy útiles para estudiar procesos electrocatalíticos. No
obstante, la voltametría de barrido lineal es de especial interés en
procesos irreversibles, en los que el barrido inverso de potencial no
proporciona información adicional.
2.4.4.2.3. Cronopotenciometría
La cronopotenciometría es una técnica galvanostática que se basa en
la medida del potencial en función del tiempo cuando se realiza un salto
de corriente desde un valor inicial a un valor final y se mantiene
constante con el tiempo [27]. El uso de esta técnica permite registrar
ciclos de carga-descarga galvanostáticos, puesto que tras alcanzar un
potencial se invierte el signo de la corriente hasta alcanzar potencial
cero. A partir de los mismos, se pueden determinar parámetros
característicos de los supercondensadores, como: la capacidad
específica, reversibilidad, durabilidad, caída óhmica, energía, potencia,
etc [24].
La Figura 2.6a muestra el salto de la corriente con el tiempo durante
un ciclo de carga-descarga galvanostático. Esta técnica consta de dos
Materiales, métodos y técnicas experimentales
109
etapas: (i) un proceso de carga, en el que se aplica una corriente
constante hasta alcanzar un potencial límite; (ii) un proceso de descarga,
en el que se aplica una intensidad de corriente de polaridad opuesta hasta
alcanzar el potencial inicial. En el caso de un proceso capacitivo, los
procesos de carga y descarga se ajustan a una recta [28].
Figura 2.6. (a) Variación de la corriente con el tiempo durante un ciclo de carga-
descarga galvanostático. (b) Ciclo de carga-descarga galvanostático.
Se puede determinar la capacidad específica de un electrodo o un
condensador por medio de la ecuación (2.7) [28].
𝐶 =𝐼𝛥𝑡
𝑚𝛥𝐸 (2.7)
Donde I es la intensidad de corriente eléctrica aplicada (A), Δt es el
tiempo de descarga (s), ΔE es la ventana de potencial cuando se
caracteriza el material o el voltaje de celda cuando se caracteriza al
condensador (V) y m es la masa del electrodo o la de ambos electrodos
cuando se trata del supercondensador.
Los ciclos de carga-descarga galvanostáticos permiten determinar la
caída óhmica, que está relacionada con la resistencia del material y del
t
I
Δt
0 ΔE
t
IR
E
Δt
(b) (a)
Capítulo 2
110
dispositivo. En numerosas publicaciones, se resta el valor de la caída
óhmica al voltaje o potencial aplicado cuando se determina la capacidad.
En estos casos, los valores de capacidad facilitados son mayores que los
obtenidos cuando se incluye la caída óhmica en el cálculo. Ambos
procedimientos son válidos, pero se debe especificar de forma precisa el
método de cálculo [25]. En este trabajo, todos los cálculos de capacidad
específica se han realizado incluyendo la caída óhmica en el valor de la
diferencia de potencial aplicado o el voltaje para determinar la capacidad
del electrodo o condensador, respectivamente.
Se pueden determinar las características principales de los
condensadores electroquímicos empleando ciclos de carga-descarga en
distintas condiciones. Los estudios principales que se utilizan para
caracterizar estos dispositivos son: (i) obtención de ciclos a distintas
densidades de corriente (A/g), para determinar la energía en función de
la potencia (diagrama de Ragone); (ii) obtención de numerosos ciclos de
carga-descarga a una densidad de corriente y voltaje determinados, para
evaluar la durabilidad del dispositivo [28].
2.4.4.2.4. Cronoamperometría.
En esta técnica, se mide la corriente eléctrica resultante en función
del tiempo al aplicar un potencial constante. Se trata de una técnica muy
empleada para evaluar la durabilidad de los condensadores
electroquímicos, y en este caso se realiza un salto de voltaje. Como se ha
indicado en el apartado 2.4.4.2.3, la forma más habitual de caracterizar
los ciclos de vida de un dispositivo de almacenamiento de energía es la
Materiales, métodos y técnicas experimentales
111
aplicación de un número elevado de ciclos; sin embargo, los
condensadores presentan mayor durabilidad que las baterías, por lo que
es necesario aplicar numerosos ciclos para observar una degradación
significativa. Una alternativa para evaluar la durabilidad en
supercondensadores es la aplicación de condiciones de temperatura
elevada y elevado voltaje de carga para acelerar el proceso de
degradación. Utilizando esta técnica, se puede determinar el tiempo de
vida de un condensador electroquímico de forma más rápida que por
medio de ciclos de carga-descarga, lo que supone una ventaja desde el
punto de vista industrial [25, 30].
Estos estudios se llevan a cabo en celdas de dos electrodos en
condiciones de voltaje constante, que se encuentra habitualmente por
encima del límite de estabilidad [30]. Las características del condensador
(capacidad, resistencia, etc.) se determinan antes y después de la etapa
de cronoamperometría. La Figura 2.7 muestra la evolución de la
corriente (conocida como corriente de fuga) durante el pulso de voltaje.
Una vez el condensador está completamente cargado, la corriente de
fuga decrece rápidamente y alcanza un valor constante conforme
aumenta el tiempo. Esta corriente presenta componentes relacionados
con la doble capa eléctrica y con procesos farádicos (debido a procesos
de degradación) [30, 31]. La integral de la curva I-t proporciona la carga
que fluye por la interfase electrodo/electrolito como consecuencia de la
aplicación del pulso de voltaje [28].
Capítulo 2
112
Figura 2.7. Variación de la corriente de fuga durante la aplicación de un voltaje
constante.
2.4.4.2.5. Espectroscopia de impedancia electroquímica.
La espectroscopia de impedancia electroquímica es una técnica muy
utilizada en diversas aplicaciones electroquímicas, como pilas de
combustible, corrosión, caracterización de capas finas y recubrimientos,
sensores, procesos biológicos, etc [32].
Su característica principal es que puede emplearse para estudiar
procesos con escalas de tiempo de distintos órdenes de magnitud. Esta
técnica se basa principalmente en la perturbación del potencial del
electrodo o voltaje de la celda electroquímica con una señal alterna de
amplitud pequeña, que permite realizar medidas en equilibrio o estado
estacionario [33].
Las perturbaciones que se suele emplear en sistemas
electroquímicos son sinusoidales. En este caso, se han utilizado
perturbaciones sinusoidales del potencial o voltaje aplicado:
𝑉 (𝑡) = 𝑉0𝑠𝑒𝑛2𝜋𝑓𝑡 (2.8)
V
t
IVoltaje constante
Corriente de fuga
Materiales, métodos y técnicas experimentales
113
Donde V(t) es el potencial o voltaje aplicado al tiempo t, V0 es la
amplitud del potencial o el voltaje y f es la frecuencia en Hz.
La corriente de respuesta correspondiente es una función sinusoidal
a la misma frecuencia pero con un desplazamiento de fase:
𝐼 (𝑡) = 𝐼0𝑠𝑒𝑛(2𝜋𝑓𝑡 + Ф) (2.9)
Donde I(t) es la corriente al tiempo t, I0 es la amplitud de la corriente, y
Ф es el desplazamiento de fase.
De forma análoga a la ley de Ohm, se define la impedancia como la
relación entre el voltaje y la corriente:
𝑍 = 𝑉 (𝑡)
𝐼 (𝑡) (2.10)
La impedancia presenta una magnitud (𝑍0 = 𝑉0 𝐼0⁄ ) y una fase (Ф) y
por tanto se puede expresar como un vector con la siguiente notación
compleja:
𝑍 = 𝑍0(𝑐𝑜𝑠Ф + 𝑗𝑠𝑒𝑛Ф) = 𝑍′ + 𝑗𝑍′′ (2.11)
Donde j = √(−1), Z’ es la parte real y Z’’ la parte imaginaria de la
impedancia.
Las representaciones más comunes de la impedancia son: el
diagrama de Nyquist (o plano complejo) y el diagrama de Bode [33]. El
diagrama de Nyquist se obtiene al representar la parte imaginaria en
función de la parte real de la impedancia. Estas representaciones son
muy útiles para obtener parámetros de impedancia a partir de los
Capítulo 2
114
espectros. No obstante, no proporcionan una información completa, ya
que no aparecen los valores de frecuencia a los que se llevó a cabo el
experimento. Para representar esta información, se utiliza el diagrama de
Bode, en el que se representa el ángulo de fase (Ф) y el logaritmo de la
magnitud (logZ0) en función del logaritmo de la frecuencia (log f) y, por
tanto, proporcionan toda la información obtenida en los experimentos de
impedancia.
Para modelar los procesos electroquímicos que tienen lugar durante
un experimento de impedancia, se utilizan combinaciones de elementos
de circuitos eléctricos (condensadores, resistencias, etc.), que se conocen
como circuitos equivalentes. La Figura 2.8a muestra un circuito
equivalente básico para representar un condensador electroquímico y su
diagrama de Nyquist correspondiente, que representa un condensador
ideal en serie con una resistencia [24]. La doble capa eléctrica se
representa por medio de la capacidad (C) y la resistencia eléctrica en
serie (ESR). Esta resistencia en serie representa el comportamiento no
ideal del sistema, y es consecuencia de caídas óhmicas diversas en la
celda electroquímica: la resistencia del electrolito (contribución iónica),
la resistencia de contacto (entre las partículas de carbón y en la interfase
del colector y el material carbonoso) y la resistencia intrínseca de los
componentes (colectores de corriente y material carbonoso). El diagrama
de Nyquist correspondiente a este sistema es una recta vertical en
paralelo al eje imaginario. Sin embargo, los condensadores
electroquímicos presentan un diagrama de Nyquist distinto al
correspondiente a un condensador RC simple (figuras 2.8b y 2.9),
Materiales, métodos y técnicas experimentales
115
debido fundamentalmente a la porosidad de los electrodos de materiales
carbonosos. El comportamiento electroquímico de los electrodos porosos
fue propuesto por De Levie [34,35].
Figura 2.8. (a) Condensador C con una resistencia eléctrica en serie R. (b) Diagrama
de Nyquist para el circuito equivalente presentado en (a).
Este modelo sostiene que el circuito equivalente debe presentar una
serie de componentes RC en serie o en paralelo, que comienzan en la
superficie externa del material y engloban la distribución interna de
poros y superficie porosa. Esta serie de componentes RC presentan
distintas constantes de tiempo RC que se pueden observar como la
respuesta eléctrica de la doble capa cargándose en la superficie interna
del electrodo. La Figura 2.9 muestra el diagrama de Nyquist
correspondiente a un condensador electroquímico compuesto de
electrodos porosos. Estos se pueden dividir en tres partes: (i) a
frecuencias muy altas, solo la parte externa del electrodo es accesible a
los iones y, por tanto, el valor de la impedancia es el correspondiente a la
resistencia eléctrica en serie (ESR); (ii) a frecuencias menores, se
observa un aumento de la parte real e imaginaria de la impedancia con
Z
R C
-Z
R
f 0
f ∞
(a) (b)
Capítulo 2
116
un ángulo de 45º (región de Warburg), como consecuencia de la
distribución de resistencias/capacidades en la porosidad del material, y
que se corresponde con la EDR (del inglés, equivalent distributed
resistance); (iii) a las frecuencias más bajas, se alcanza la capacidad
máxima del material.
Figura 2.9. Diagrama de Nyquist de un electrodo poroso compuesto de poros
uniformes. Adaptado de [28].
El modelo de De Levie ha sido actualizado posteriormente para
tener en cuenta otras características como, por ejemplo, la forma de poro
[36] y la distribución de tamaños de poros [37]. Recientemente, Fletcher
y col. [38] propusieron un modelo de circuito equivalente universal que
consiste en una red de RC paralelas que se encuentran en serie con un
componente RC en paralelo (Figura 2.10), que describe los electrodos
f 0
f ∞
ESR + EDRESR
Z
-Z
Materiales, métodos y técnicas experimentales
117
porosos no ramificados. Este modelo permite explicar algunos de los
defectos de los dispositivos actuales, como: pérdida de capacidad a
elevada frecuencia, disminución del voltaje a circuito abierto, etc.
Figura 2.10. Circuito equivalente universal propuesto por Fletcher y col. [38] para un
condensador simétrico basado materiales carbonosos porosos.
2.4.4.2.6. Electrodo rotatorio de disco-anillo
Para estudiar la cinética y el mecanismo de reacción sobre un
electrodo, y concretamente en el caso de la reacción de reducción de
oxígeno (ORR), es necesario utilizar herramientas que permitan
controlar y determinar el transporte del reactivo a la superficie del
electrodo y su efecto en la cinética de transferencia electrónica. El
método más empleado para este propósito es el electrodo rotatorio de
disco (RDE, del inglés rotating disk electrode). Este consiste en un disco
de un material conductor (carbón vítreo, oro, etc.) incorporado en un
material aislante (teflón, etc.).
Capítulo 2
118
En una disolución electrolítica que contenga un exceso de electrolito
soporte, se puede despreciar el efecto de la migración, de manera que
solo existen dos procesos mayoritarios relacionados con el transporte de
masa: la difusión y la convección. En el caso de una disolución sin
convección, el espesor de la capa de difusión en la superficie del
electrodo aumenta progresivamente conforme aumenta el tiempo de
reacción, dando lugar a una densidad de corriente no estacionaria. Sin
embargo, en una disolución con convección (como agitación o rotación
del electrodo), el espesor de la capa de difusión (y, por tanto, la densidad
de corriente) permanece constante. Por lo tanto, la convección controla
el espesor de la capa de difusión y la difusión controla el transporte del
reactivo a través de esta. El uso de un equipo de RDE que controle de
forma precisa la velocidad de rotación, permite analizar
cuantitativamente la cinética de la reacción que se está produciendo en el
electrodo.
La teoría más utilizada para analizar los datos obtenidos por medio
de RDE para el estudio de la catálisis de ORR se conoce como la teoría
de Koutechy-Levich [39], que proporciona la relación existente entre el
número de electrones involucrado en la reacción, el coeficiente de
difusión de oxígeno, la concentración de oxígeno, viscosidad del
electrolito y velocidad de rotación del electrodo. Mediante el estudio de
estos parámetros, se puede determinar la cinética de la reacción y su
mecanismo, a partir de los cuales se pueden seleccionar y diseñar
catalizadores para su uso en aplicaciones que involucren la ORR [40].
La ecuación de Koutecky-Levich expresa la corriente global del disco
Materiales, métodos y técnicas experimentales
119
(iD), que engloba la contribución cinética relacionada con la velocidad de
la reacción de transferencia electrónica (iK) y la velocidad de difusión del
reactivo (iL):
1
𝑖𝐷=
1
𝑖𝐾+
1
𝑖𝐿 (2.12)
En esta ecuación, iD es la corriente medida e iL es la corriente límite
de difusión. El término iL viene dado por la ecuación de Levich:
𝑖𝐿 = 0.62𝑛𝐹𝐴𝐶0(𝐷0)2 3⁄ 𝜈−1 6⁄ 𝜔1 2⁄ (2.13)
Donde n es el número de electrones involucrados en la reacción, F es la
constante de Faraday (96485 C/mol), C0 es la concentración de la
especie electroactiva en el seno de la disolución, D0 es la constante de
difusión de la especie electroactiva, 𝜈 es la viscosidad cinemática del
electrolito, A es el área del electrodo (cm) y 𝜔 es la velocidad angular de
rotación del RDE.
De esta forma, el uso del RDE permite obtener información acerca
de la cinética de la reacción. Sin embargo, para profundizar en el
mecanismo de reacción es necesario utilizar un electrodo rotatorio disco-
anillo (RRDE, del inglés rotating ring-disk electrode), ya que permite
estudiar la formación de intermedios y productos de reacción.
El RRDE consiste en un electrodo de disco concéntrico con un
electrodo en forma de anillo. Ambos electrodos se encuentran
incorporados y separados en un material aislante (Figura 2.11). Dado
que ambos se encuentran aislados eléctricamente, ambos electrodos
Capítulo 2
120
funcionan como dos electrodos hidrodinámicos separados que, durante
la medida, experimentan rotación a la misma velocidad [40]. En este
sistema, la presencia del anillo no afecta a la corriente y el potencial del
electrodo de disco [39].
Figura 2.11. (a) RRDE utilizado en este trabajo (Pine Research Instrumentations,
EE.UU.) [41]. (b) Componentes del RRDE.
El RRDE se utiliza para realizar experimentos muy diversos que
permiten deducir la presencia de productos intermedios de reacción y el
número de electrones involucrados en la reacción.
Un ejemplo típico del uso del RRDE es el estudio de la ORR. Esta
reacción tiene lugar en el electrodo de disco, de manera que puede
producir dos productos de reacción: agua y peróxido de hidrógeno. Una
vez tiene lugar la reacción en el electrodo de disco, los productos de
reacción se alejan de la superficie del mismo y llegan al electrodo anillo
debido a la convección. En el electrodo anillo se pueden oxidar si el
potencial se fija para oxidar selectivamente uno de los productos: el
peróxido de hidrógeno. De este modo, la medida de la corriente del
anillo permite cuantificar la cantidad de peróxido de hidrógeno que se ha
producido durante la ORR.
Electrodo de disco
(carbón vítreo)
Material aislante
(teflón)
Electrodo anillo
(platino)
(a) (b)
Materiales, métodos y técnicas experimentales
121
Dado que el uso de RRDE requiere examinar dos potenciales (el del
anillo y el del disco), es necesario utilizar un bipotenciostato (que
permite controlar el potencial de los dos electrodos por separado) o bien,
un potenciostato y una fuente externa que permita mantener un potencial
constante en el anillo [39].
EL RRDE presenta una eficiencia de colección característica, N, que
depende exclusivamente de las dimensiones geométricas de los
componentes y es independiente de la velocidad de rotación, coeficiente
de difusión, etc [39]. Este parámetro establece la cantidad de productos
generados en el disco que posteriormente alcanzan el anillo. La
eficiencia de colección viene dada por la ecuación (2.14).
𝑁 = −𝑖𝑅
𝑖𝐷 (2.14)
Este parámetro para un electrodo dado se puede determinar a partir
de las dimensiones geométricas del mismo, o bien, experimentalmente
midiendo la corriente en el disco y en el anillo en un sistema en el que el
producto de reacción sea estable [39][40]. En esta tesis, el RRDE
utilizado (E7R9 Thin Gap RRDE, Pine Research Instrumentation, EE.
UU.), presenta una eficiencia de colección de 0.37.
El uso del RRDE permite determinar el número de electrones
involucrado en la ORR y la cantidad de peróxido de hidrógeno generado
durante la reacción. Estos valores vienen dados por las ecuaciones (2.15)
y (2.16) [40].
𝑛 = 4𝑖𝐷
𝑖𝐷+ 𝑖𝑅 𝑁⁄ (2.15)
Capítulo 2
122
% 𝐻2𝑂2 = 1002𝐼𝑅
𝐼𝐷𝑁+ 𝐼𝑅= 100
4−𝑛
2 (2.16)
Por tanto, una vez conocido N, la determinación de iD e iR permite
calcular el número de electrones involucrados en la reacción y el
porcentaje de peróxido de hidrógeno producido y, de esta manera,
evaluar la actividad catalítica del material estudiado para su aplicación
en la ORR.
2.5 Referencias
[1] D. Lozano-Castelló, M.A. Lillo-Ródenas, D. Cazorla-Amorós, A.
Linares-Solano, Preparation of activated carbons from Spanish
anthracite: I. Activation by KOH, Carbon 39 (2001) 741–749.
[2] H. Itoi, H. Nishihara, T. Ishii, K. Nueangnoraj, R. Berenguer-
Betrián, T. Kyotani, Large Pseudocapacitance in Quinone-
Functionalized Zeolite-Templated Carbon, Bull. Chem. Soc. Jpn.
87 (2014) 250–257.
[3] H. Nishihara, P. Hou, L. Li, M. Ito, M. Uchiyama, T. Kaburagi,
A. Ikura, J. Katamura, T. Kawarada, K. Mizuuchi, T. Kyotani,
High-Pressure Hydrogen Storage in Zeolite-Templated Carbon, J.
Phys. Chem. C 113 (2009) 3189–3196.
[4] R. Berenguer, H. Nishihara, H. Itoi, T. Ishii, E. Morallón, D.
Cazorla-Amorós, T. Kyotani, Electrochemical generation of
oxygen-containing groups in an ordered microporous zeolite-
templated carbon, Carbon 54 (2013) 94–104.
[5] M.J. Bleda-Martínez, J.A. Maciá-Agulló, D. Lozano-Castelló, E.
Morallón, D. Cazorla-Amorós, A. Linares-Solano, Role of surface
chemistry on electric double layer capacitance of carbon
materials, Carbon 43 (2005) 2677–2684.
[6] C. Willocq, S. Hermans, M. Devillers, Active carbon
functionalized with chelating phosphine groups for the grafting of
Materiales, métodos y técnicas experimentales
123
model Ru and Pd coordination compounds, J. Phys. Chem. C
(2008) 5533–5541.
[7] A. Gromov, S. Dittmer, J. Svensson, O.A. Nerushev, S.A. Perez-
García, L. Licea-Jiménez, R. Rychwalski, E.E.B. Campbell,
Covalent amino-functionalisation of single-wall carbon
nanotubes, J. Mater. Chem. 15 (2005) 3334–3339.
[8] D. Salinas-Torres, J.M. Sieben, D. Lozano-Castello, E. Morallón,
M. Burghammer, C. Riekel, D. Cazorla-Amorós, Characterization
of activated carbon fiber/polyaniline materials by position-
resolved microbeam small-angle X-ray scattering, Carbon 50
(2012) 1051–1056.
[9] X. Zhang, S.K. Manohar, Polyaniline nanofibers: chemical
synthesis using surfactants, Chem. Commun. (2004) 2360–2361.
[10] D. Lozano-Castelló, F. Suárez-García, D. Cazorla-Amorós, Á.
Linares-Solano, Porous Texture of Carbons, in: F. Béguin, E.
Frackowiak (Eds.), Carbons Electrochem. Energy Storage
Convers. Syst., 1st ed., CRC Press, 2010: pp. 115–162.
[11] D. Cazorla-Amorós, J. Alcañiz-Monge, M.A. De La Casa-Lillo,
A. Linares-Solano, CO2 as an adsorptive to characterize carbon
molecular sieves and activated carbons, Langmuir 14 (1998)
4589–4596.
[12] D. Lozano-Castelló, D. Cazorla-Amorós, A. Linares-Solano,
Usefulness of CO2 adsorption at 273 K for the characterization of
porous carbons, Carbon 42 (2004) 1231–1236.
[13] S. Brunauer, P.H. Emmett, E. Teller, Adsorption of gases in
multimolecular layers, J. Am. Chem. Soc. 60 (1938) 309–319.
[14] M.M. Dubinin, The potential theory of adsorption of gases and
vapors for adsorbents with energetically nonuniform surfaces,
Chem. Rev. 60 (1960) 235–241.
[15] M. Jordá-Beneyto, F. Suárez-García, D. Lozano-Castelló, D.
Cazorla-Amorós, A. Linares-Solano, Hydrogen storage on
Capítulo 2
124
chemically activated carbons and carbon nanomaterials at high
pressures, Carbon 45 (2007) 293–303.
[16] Y. Li, Z.-Y. Fu, B.-L. Su, Hierarchically Structured Porous
Materials for Energy Conversion and Storage, Adv. Funct. Mater.
22 (2012) 4634–4667.
[17] J. Jagiello, J.P. Olivier, 2D-NLDFT adsorption models for carbon
slit-shaped pores with surface energetical heterogeneity and
geometrical corrugation, Carbon 55 (2013) 70–80.
[18] P.I. Ravikovitch, G.L. Haller, A. V Neimark, Density functional
theory model for calculating pore size distributions: pore structure
of nanoporous catalysts, Adv. Colloid. Interface Sci. 76–77
(1998) 203–226.
[19] P.I. Ravikovitch, A. Vishnyakov, R. Russo, A. V. Neimark,
Unified Approach to Pore Size Characterization of Microporous
Carbonaceous Materials from N2, Ar, and CO2 Adsorption
Isotherms, Langmuir 16 (2000) 2311–2320.
[20] S. Tougaard, Surface Analysis: (b) X-Ray Photoelectronic
Spectroscopy, in: P. Worsfold, A. Townshend, C. Pool (Eds.),
Encycl. Anal. Sci., 2nd ed., Elsevier, Oxford, 2005: pp. 445–456.
[21] M.C. Román-Martínez, D. Cazorla-Amorós, A. Linares-Solano,
C.S.M. de Lecea, TPD and TPR characterization of carbonaceous
supports and Pt/C catalysts, Carbon 31 (1993) 895–902.
[22] H. Boehm, Surface oxides on carbon and their analysis: a critical
assessment, Carbon 40 (2002) 145–149.
[23] J. Figueiredo, M.F. Pereira, M.M. Freitas, J.J. Órfão, Modification
of the surface chemistry of activated carbons, Carbon 37 (1999)
1379–1389.
[24] J.-F. Fauvarque, P. Simon, Principles of Electrochemistry and
Electrochemical Methods, in: F. Béguin, E. Frackowiak (Eds.),
Carbons Electrochem. Energy Storage Convers. Syst., 1st ed.,
CRC Press, Taylor & Francis Group, Boca Raton, 2009: pp. 1–36.
Materiales, métodos y técnicas experimentales
125
[25] S. Shiraishi, Electrochemical Performance, in: M. Inagaki (Ed.),
Mater. Sci. Eng. Carbon Charact., Tsinghua University Press
Limited. Elsevier Inc, 2016: pp. 205–226.
[26] M. Inagaki, H. Konno, O. Tanaike, Carbon materials for
electrochemical capacitors, J. Power Sources 195 (2010) 7880–
7903.
[27] A.J. Bard, L.R. Faulkner, Electrochemical Methods.
Fundamentals and Applications, 2nd ed., John Wiley & Sons, Inc,
New York, 2001.
[28] B.E. Conway, Electrochemical supercapacitors: Scientific
Fundamentals and Technological Applications, Springer, New
York, 1999.
[29] V.D. Parker, Linear Sweep and Cyclic Voltammetry, Compr.
Chem. Kinet. 26 (1986) 145–202.
[30] D. Salinas-Torres, S. Shiraishi, E. Morallón, D. Cazorla-Amorós,
Improvement of carbon materials performance by nitrogen
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions, Carbon 82 (2015) 205–213.
[31] P. Ratajczak, K. Jurewicz, F. Béguin, Factors contributing to
ageing of high voltage carbon/carbon supercapacitors in salt
aqueous electrolyte, J. Appl. Electrochem. 44 (2014) 475–480.
[32] N. Bonanos, B.C.H. Steele, E.P. Butler, Applications of
Impedance Spectroscopy, in: E. Barsoukov, J.R. Macdonald
(Eds.), Impedance Spectrosc. Theory, Exp. Appl., 2nd ed., 2005:
pp. 205–537.
[33] S. Krause, Impedance Methods, Encycl. Electrochem. (2007)
196–229.
[34] R. de Levie, On porous electrodes in electrolyte solutions: I.
Capacitance effects, Electrochim. Acta 8 (1963) 751–780.
[35] R. de Levie, On porous electrodes in electrolyte solutions—IV,
Electrochim. Acta 9 (1964) 1231–1245.
Capítulo 2
126
[36] H. Keiser, K.D. Beccu, M.A. Gutjahr, Abschätzung der
porenstruktur poröser elektroden aus impedanzmessungen,
Electrochim. Acta 21 (1976) 539–543.
[37] H.-K. Song, Y.-H. Jung, K.-H. Lee, L.H. Dao, Electrochemical
impedance spectroscopy of porous electrodes: the effect of pore
size distribution, Electrochim. Acta 44 (1999) 3513–3519.
[38] S. Fletcher, V.J. Black, I. Kirkpatrick, A universal equivalent
circuit for carbon-based supercapacitors, J. Solid State
Electrochem. 18 (2014) 1377–1387. 2328-4.
[39] A.J. Bard, L.R. Faulkner, Methods involving forced convection-
hydrodynamic methods, in: Electrochem. Methods. Fundam.
Appl., 2nd ed., John Wiley & Sons, Inc, 2001: pp. 331–367.
[40] Z. Jia, G. Yin, J. Zhang, Rotating Ring-Disk Electrode Method,
Elsevier B.V., 2014.
[41] https://www.pineresearch.com.
Chapter 3
N-functionalization of
activated carbon at mild
conditions
N-functionalization of activated carbon
129
3.1. Introduction
Surface chemistry plays a major role in determining the
physicochemical properties of carbon materials. It highly relies on the
presence of functionalities from different heteroatoms (i.e. oxygen and
nitrogen, and some others in lesser amounts), either linked to the carbon
surface or introduced inside the carbon atom framework. Along with
their structural and textural properties, surface chemistry dictates the
potential use of carbon materials in a wide variety of applications in the
fields of catalysis, energy storage, environmental protection and
biomedicine [1-3]. More particularly, nitrogen functionalities in porous
carbon materials modifies their electronic properties and the
electrochemical reactivity and confer basic character to the carbon
surface, enhancing the interaction with acid molecules [4].
Consequently, the production of nitrogen-containing porous carbon
materials have raised a great interest due to their promising performance
as electrocatalysts for the oxygen reduction reaction in fuel cells [5], as
electrodes for supercapacitors [6-8] and as adsorbents for the capture of
acid gases [9,10].
Nitrogen-containing porous carbons are usually synthesized by two
methodologies: reaction of the material with a nitrogen-containing
reagent (NH3, HCN, etc) or carbonization/activation of nitrogen-rich
carbon precursors (urea, melamine, polyaniline, etc.) [4,6,7]. The surface
chemistry of the corresponding material depends on the treatment
conditions, which includes the nature of the selected precursor/reagent
and the treatment temperature. Hence, direct synthesis requires the use
Chapter 3
130
of high temperatures (over 600º C) which does not allow to control the
type of functional groups that are formed on the carbon material.
Similarly, post-treatments with gaseous reagents are usually conducted
at high temperatures, though other approaches at low temperatures
(under 600º C) are found to be interesting for attaining different
functional groups, such as amides [11] or imides [6].
Post-treatment through organic reactions can be a quite interesting
approach because they permit the use of soft treatment conditions that
may preserve the structural characteristics of the carbon materials, and
provide different reaction pathways for finely tuning the surface
chemistry with a wide variety of surface groups [12-19]. Nitrogen
functionalization is usually achieved through the oxidation of the carbon
material with HNO3 in order to generate carboxylic acids on its surface,
which act as the anchorage points for the subsequent reactions [6, 18].
They can directly react with nitrogen-containing reagents to attach this
heteroatom onto the surface, but when this material is previously treated
with SOCl2 the amount of nitrogen introduced in the carbon material is
higher [16]. When this reaction is conducted using amines as the
nitrogen-containing reagent, it produces an amide group substituted with
an aliphatic chain, which can block the microporosity depending on the
size of the hydrocarbon chain of the amine used as reagent [16].
However, when the amine is directly attached to the surface [19] or a
primary amide is produced instead of a secondary one, the blocking of
porosity might be lower. Also, primary amides can be converted to
N-functionalization of activated carbon
131
amines directly anchored to the carbon surface by Hofmann
rearrangement that has been proven to work with SWCNTs [20].
Figure 3.1. Chemical route used for the modification of the activated carbon KUA:
chemical oxidation with HNO3 (KUA-COOH), SOCl2 treatment and amidation (KUA-
CONH2) and amination (KUA-NH2).
Interestingly, the selectivity of the synthetic route towards the
desired amides or amines is not usually very high due to the complex
surface chemistry of the carbon materials. For these reasons, in this
chapter, the modification of the surface chemistry of a superporous
activated carbon with the introduction of N functional groups through
organic reactions schematized in Figure 3.1, is studied. The modification
approach consists in: (i) chemical oxidation with HNO3 for the
generation of carboxylic acids (KUA-COOH); (ii) amidation treatment
by introduction of acyl chloride functionalities and conversion of these
groups into amide groups (KUA-CONH2); (iii) transformation of amides
into amine groups by Hofmann rearrangement (KUA-NH2). The porous
texture, surface chemistry and electrochemical properties in aqueous
electrolyte of all the obtained materials will be analyzed, trying to
understand the reactions occurring that may explain the structural
changes of the functional groups introduced on the carbon material
Chapter 3
132
surface. This methodology was also applied to functionalize a
microporous zeolite templated carbon (Annex to Chapter 3).
3.2. Materials and methods
3.2.1. Activated carbon
A highly microporous activated carbon prepared in our laboratory
has been used as the starting material for nitrogen incorporation via
organic chemical modification. The pristine material, henceforth named
KUA, has been obtained by chemical activation of an anthracite with
KOH using an impregnation ratio of activating agent to raw material of
4:1 and an activation temperature of 750º C under inert atmosphere,
which was held for 1 hour. More details about the preparation process
are available elsewhere [21].
3.2.1.1. Chemical oxidation with HNO3
The chemical oxidation of the activated carbon KUA was done
using HNO3 as oxidant. Nitric acid is known as strong oxidizing agent
that yields an important amount of carboxylic acids when used for
treating carbon materials, and it has been widely used in literature for
oxygen surface functionalization [22-24].
The oxidation treatment has been adopted from previous studies of
our research group [24]. Briefly, 1g of activated carbon (KUA) was
contacted with 40 mL of HNO3 65 wt% under stirring during 3 hours at
room temperature. After that, KUA-COOH was filtrated and washed
N-functionalization of activated carbon
133
with Milli-Q water until the pH of elution was neutral. Finally, the
sample (named KUA-COOH) was dried at 100º C.
3.2.1.2. Generation of amide functionalities
The amidation process consists in two sequential steps: first, acyl
chlorides are generated from carboxylic functionalities and, secondly, a
nitrogen-containing compound is anchored to the carbonyl via
nucleophilic substitution over the acyl chloride. These reactions were
carried out inside a Schlenk system. Although the use of less toxic
compounds is desirable and is considered in the literature [25], this
approach contributes interestingly to the development of mild conditions
for the modification of carbon materials. The acyl chloride formation is
accomplished as follows [26]: Dry KUA-COOH (1 g) was introduced in
a round bottom flask with 50 mL of toluene and 5 mL of SOCl2 was
added to the flask. The mixture was refluxed at 120ºC for 5 hours under
Ar atmosphere. After that time, the sample was washed with toluene and
dried under vacuum for 14h.
The reaction for the generation of the amide group has been adapted
from that proposed by Gromov and coworkers for CNTs [20]. KUA-
COCl was added into a 2 M NH4NO3/DMF solution (activated carbon to
solution ratio of 1g/300mL) in a round bottom flask. Then, 300 mL of
pyridine were added slowly to the round bottom flask under continuous
stirring at room temperature. The mixture was stirred at 70 ºC for 65
hours under Ar atmosphere. The obtained amidated sample (KUA-
Chapter 3
134
CONH2) was washed with abundant water and ethanol, filtered and dried
at 100º C overnight.
3.2.1.3. Generation of amine functionalities
Amine functionalization was carried out using a Hofmann
rearrangement in the previously attached amides [20]: 10 mL of Br2 was
added into a solution of 3% wt. NaOCH3 in CH3OH. KUA-CONH2 (0.5
g) were added to that solution, and the mixture was stirred at 70º C for 4
hours. Then, additional 4 mL Br2 were added, and the mixture was
stirred for 20 hours at 70º C. The product was isolated by filtration,
washed with saturated Na2CO3, water and ethanol. The aminated
activated carbon was obtained after hydrolyzing the recovered sample in
500 mL of 0.1M NaOH for 24 hours, washing with water, and finally
filtering and drying the sample at 100º C overnight. This activated
carbon is named KUA-NH2.
3.2.2. Electrochemical characterization
Carbon electrodes for electrochemical characterization were
prepared by mixing the activated carbon with acetylene black and
polytetrafluoroethylene (PTFE) as binder in a ratio of 90:5:5 (w/w). The
total weight of the electrode was 9 mg (dry basis). For shaping the
electrodes, a sample sheet was cut into a circular shape with an area of
1.2 cm2 and pressed for 5 min at 2 tons to guarantee a homogeneous
thickness. After that, the electrode was placed on a gold disk used as a
current collector. The electrodes were impregnated for 2 days into 1M
H2SO4 previously to electrochemical measurements.
N-functionalization of activated carbon
135
The electrochemical characterization was performed by using an
Autolab PGSTAT302 for cyclic voltammetry (CV) and Arbin SCTS for
galvanostatic charge-discharge cycles. The electrochemical
characterization of the different materials synthesized was performed by
using a three-electrode configuration. As counter electrode, more than
20 mg of a KUA electrode was used. Both electrodes were placed
against each other and separated by a nylon membrane (pore size: 320
nm). Ag/AgCl/KCl (3M) was used as reference electrode in all cases.
1M H2SO4 was used as aqueous electrolyte. The electrochemical
performance of all samples was tested by CV at sweep rates between 1
and 50 mV/s and galvanostatic charge-discharge (GCD) cycles at current
densities of 50-50000 mA/g. The potential range used for all the
measurements was 0.2-0.8 V vs Ag/AgCl/KCl (3M). The capacitance in
the CV was calculated from the area of the voltammogram, while in the
GCD experiments, the capacitance was obtained using the next equation:
𝐶𝑔 =𝑗·𝑡
∆𝑉 (3.1)
Where j is the current density (A/g), t is the discharge time and ∆𝑉
is the difference of potential (0.6 V in all experiments). The results are
expressed in F/g, taking into account the weight of the active material of
the electrode.
3.2.3. Porous texture and surface chemistry characterization
The porous texture characterization was carried out by N2
adsorption-desorption isotherms at -196º C and by CO2 adsorption at 0º
C by using an Autosorb-6-Quantachrome apparatus. The samples were
Chapter 3
136
outgassed at 200º C for 4 hours before the experiments. The apparent
surface area was obtained from N2 adsorption-desorption isotherms by
using the BET equation. The micropore volume was determined by
Dubinin-Radushkevich method applied to the CO2 and N2 adsorption
isotherms.
The surface chemistry of the samples was analyzed by TPD and
XPS. TPD experiments were performed by heating the samples (10
mg) to 950º C (at a heating rate of 20º C/min) under a helium flow rate
of 100 mL/min. The analyses were carried out by using a TGA-DSC
instrument (TA Instruments, SDT Q600 Simultaneous) coupled to a
mass spectrometer (Thermostar, Balzers, BSC 200). X-ray Photoelectron
Spectroscopy (XPS) analyses were carried out using a VG-Microtech
Multilab 3000 spectrometer, equipped with an Al anode. The
deconvolution of N1s spectra was carried out by using Gaussian
functions with 20% of Lorentzian component. FWHM of the peaks was
kept between 1.3 and 1.5 eV and a Shirley line was used for estimating
the background signal.
3.3. Results and discussion
3.3.1. Porous texture characterization
Figure 3.2 shows the N2 adsorption-desorption isotherms for the
original activated carbon and the activated carbon at different stages of
the proposed surface chemistry modification protocol. It can be seen that
nitrogen uptake in KUA mainly occurs at low relative pressures,
showing type I isotherm, characteristic of a microporous material. The
N-functionalization of activated carbon
137
wide knee observed in the isotherm at low relative pressures indicates
that the material has a wide micropore size distribution. Both
characteristics of the nitrogen isotherm are also observed for all the
modified activated carbons, although the nitrogen uptake seems to be
affected differently by each chemical treatment.
Figure 3.2. N2 adsorption-desorption isotherms of the activated carbons KUA, KUA-
COOH, KUA-CONH2 and KUA-NH2.
Table 3.1 summarizes the porous texture for all the activated carbon
samples. The pristine activated carbon (KUA) presents a large apparent
surface area. The micropore volume determined by CO2 adsorption at 0º
C (VDRCO2) corresponds to the volume of the narrowest micropores (<0.7
nm), while the obtained by N2 adsorption at -196º C (VDRN2) corresponds
to the whole microporosity (<2nm) [27]. The higher micropore volume
measured by nitrogen adsorption indicates that the activated carbon
presents a wide micropore size distribution.
0
200
400
600
800
1000
1200
0 0.2 0.4 0.6 0.8 1
Ad
sorb
ed V
olu
me
(cm
3/g
ST
P)
P/Po
KUA
KUA-COOH
KUA-CONH₂
KUA-NH₂
Chapter 3
138
Table 3.1. Textural properties and XPS elemental surface composition for the pristine
and modified activated carbons.
Sample SBET
(m2/g)
VDRN2
(cm3/g)
VDRCO2
(cm3/g)
CXPS
(at.%)
OXPS
(at.%)
NXPS
(at.%)
KUA 3140 1.10 0.56 90.9 8.8 0.3
KUA-COOH 2680 0.96 0.45 81.8 18.2 -
KUA-CONH2 2080 0.74 0.44 84.2 12.0 3.8
KUA-NH2 2450 0.91 0.42 79.1 17.9 3.0
KUA-
CONH2_T950
- - - 94.5 4.4 1.1
KUA-
NH2_T950
- - - 89.9 8.8 1.3
The sample KUA-COOH showed a decrease in the nitrogen uptake,
in the apparent surface area and in the pore volume (Figure 3.2 and
Table 3.1). This effect is related to the generation of surface groups that
can occupy the entrance and some volume of the microporosity [23, 24].
In the case of KUA-CONH2, the amidation treatment produced a further
decrease of VDRN2, whereas VDR
CO2 remains invariable. This decrease
suggests that the modification of the surface chemistry involves the
largest micropores, while the narrowest microporosity is probably not
accessible to the reagents.
The amination treatment leaded to an increase of the specific surface
area and VDRN2 of KUA-NH2 when compared to KUA-CONH2 sample
(Table 3.1 and Figure 3.2). The narrowest microporosity remains again
practically invariable. If we assume that the desired reaction has been
carried out, the size of the functional groups that occupy or block the
microporosity in KUA-NH2 should be smaller than those in KUA-
CONH2, because of the loss of a CO molecule in the production of
N-functionalization of activated carbon
139
amines from amides. This means that the change in the microporosity is
in agreement with the intended functionalization of the activated carbon
sample.
3.3.2. Surface chemistry characterization
3.3.2.1. XPS
Table 3.1 compiles the composition of the surface chemistry of the
samples obtained by XPS. The pristine activated carbon (KUA) is
composed mainly by carbon and a relatively high amount of oxygen.
XPS analysis of KUA-COOH confirms that the oxidation of this
material has been produced during the treatment with HNO3. The same
technique also corroborates that nitrogen is attached onto the surface of
the activated carbon KUA-CONH2, remaining on it after completing the
treatment for amination (KUA-NH2). It is important to highlight that
more than 3 at. % nitrogen content has been successfully loaded onto the
carbon surface while producing only a minor change of the porous
texture of the activated carbon (less than 5% when BET surface area of
KUA-COOH and KUA-NH2 are compared, Table 3.1). This is a
remarkable result since maintaining the porosity of the material is of
huge importance for all the surface-dependent applications of nitrogen-
doped porous carbons. In comparison, other amidation reactions over
activated carbon produced a loss of specific surface area of 42% and
90%, depending on the amine used as reagent in the amidation reaction
[16]. The results indicate that the amidation procedure proposed in this
PhD Thesis is less detrimental to the porous texture than other
Chapter 3
140
approaches involving larger amines. To prove the viability of the method
in other highly microporous carbon materials, the amidation reaction
was successfully applied over a zeolite-templated carbon (ZTC),
producing an attachment of nitrogen of 1.4 at. %. More details about the
N-funtionalization of ZTC are available in Annex 3.
The oxygen amount measured in each step (Table 3.1) can also give
some information about the process responsible for the nitrogen
functionalization. After the amidation step, the introduction of nitrogen
is accompanied by a consumption of oxygen, which evidences that the
attachment of nitrogen to the surface has been produced mainly through
oxygen functional groups. On the other hand, the sample KUA-NH2
presents an oxygen content similar to KUA-COOH (18%,
approximately), and it has also been confirmed by TPD measurements,
see section 3.3.2.2. This result is initially unexpected, because this step
might lead to the loss of CO of the amide group through a rearrangement
and decarboxylation, in order to obtain amine functionality, so that some
loss of oxygen should happen. This increase is probably related to a
collateral oxidation during the amination treatment. This will be
explained later in more detail. The amount of residual bromine
containing species has been determined to be 0.1% at., and therefore are
not considered to be relevant in the electrochemical performance of
KUA-NH2.
N-functionalization of activated carbon
141
Figure 3.3. N1s XPS deconvolution of (a) KUA-CONH2, (b) KUA-NH2.
Figures 3.3.a and 3.3.b show the N1s XPS spectra of the samples
KUA-CONH2 and KUA-NH2. They have been deconvoluted according
to the literature [9,14,28-32]. Table 3.2 summarizes the results of the
XPS analysis. The spectra of both samples revealed the existence of at
least three species with different binding energies. In the case of KUA-
CONH2, the spectrum can be separated into three peaks located at 400.7,
399.8 and 398.8 eV. The peak at 399.8 eV is assigned to amides, amines,
lactams and imides on the surface [30]. The formation of lactams and
imides during amidation is plausible from already formed amides
395396397398399400401402403404
I (a
.u.)
Binding Energy (eV)
(a)
395396397398399400401402403404
I (a
.u.)
Binding Energy (eV)
(b)
Chapter 3
142
groups, since they can condense with adjacent hydroxyls and carboxylic
acids that were already over the carbon surface or that can be formed
during the initial stages of the wet acid oxidation of the surface. In this
sense, the generation of these species could take place from anhydrides
[33] and lactones [34], as is showed in Figures 3.4a and 3.4b. Similar
paths have been proposed for the treatment of non-porous carbon
materials with NH3 gas [6]. At this stage of the modification protocol,
the presence of amines should be discarded because their formation
could not be explained from the expected reactions; however, it is not
possible to discard its presence with the information obtained by XPS.
Table 3.2. Assignment of N1s deconvoluted curves to nitrogen functional groups.
Sample Binding
Energy (eV)
Functional
Group
N
(at. %)
Relative
%
KUA-CONH2 400.7 ± 0.2 Pyrrole,
pyridone
0.72 19
399.8 ± 0.2 Amide, Lactam,
Amine, Imide
1.89 50
398.8 ± 0.2 Pyridine, Imine 1.17 31
KUA-NH2 400.5 ± 0.2 Pyrrole,
Pyridone
0.72 27
399.6 ± 0.2 Amide, Lactam,
Amine, Imide
1.25 48
398.5 ± 0.2 Pyridine, Imine 0.65 25
KUA-CONH2
after TPD
398.4 ± 0.2 Pyridine 0.55 49
400.6 ± 0.2 Pyrrole,
Pyridone
0.57 51
KUA-NH2
after TPD
398.3 ± 0.2 Pyridine 0.42 31
400.2 ± 0.2 Pyrrole,
Pyridone
0.92 69
N-functionalization of activated carbon
143
The peak at 400.7 eV in the N1s spectra of KUA-CONH2 (400.7
eV) is assigned to pyrrole and pyridone functional groups, while that
found at 398.8 eV is associated to the presence of pyridines and imines
[9,28,30,32]. These signals are not related to the presence of adsorbed
pyridine or DMF, since their desorption has not been detected by
following the 52 and 42 m/e- lines during the TPD experiments,
respectively. The formation of nitrogen heterocycles at such low
temperatures is an unexpected outcome of the proposed surface
modification protocol. This striking result can be explained taking into
account the heterogeneity of the activated carbon surface and the
availability of different surface oxygen groups. Pyridone can be formed
from amide groups when they condense with adjacent hydroxyls in order
to produce lactams, which are tautomers of pyridone (Figure 4c) [31,35].
As for the pyiridines/imines, the imines can be generated by reaction
between a carbonyl group (aldehyde or ketone) and a nitrogen reactant,
such as secondary/tertiary amines and ammonia according to the
literature [36,37]. The imines obtained from the last one are more stable
when they are linked to two carbon atoms forming an aromatic ring (i.e
pyrroles) [37] rather than in form of terminal imines; nevertheless both
of them could coexist in the carbon surface. Possible reaction pathways
for the formation of imines and pyrroles are shown in Figures 3.5a and
3.5b.
Chapter 3
144
Figure 3.4. (a) Formation of lactams from lactones; (b) formation of imides from
anhydrides; (c) tautomerization equilibrium of the functional groups lactam (left) and
pyridone (right); (d) formation of pyridones from pyridines.
When the imines are formed, condensation reactions with aldehydes
and ketones can take place, and nitrogen aromatic heterocycles would be
produced [38], which structure will depend on the distance between the
carbonyls. Hence, two carbonyls separated by two carbon atoms allow
the formation of cycles of five members (pyrrole), Figure 3.5b, whilst a
distance of three carbon atoms, Figure 3.5c, entails the production of six
member rings (pyridine). The condensation required to form a
heterocycle implies the loss of two H2O molecules. It should be pointed
out that the formation of aromatic rings requires the formation of an
intermediate (enamines), as illustrated in Figure 3.5. The atomic
N-functionalization of activated carbon
145
composition measured by XPS seems to validate this hypothesis. The
oxygen amount diminishes in 6.2 at. % after the amidation treatment,
whereas the oxygen consumption expected from the amount of freshly
formed amides and heterocycles is 5.7 at.%, according to the reactions
proposed in Figures 3.4 and 3.5.
The analysis of the N1s spectra of KUA-NH2 shows significant
changes in comparison with KUA-CONH2. Both samples present peaks
with similar binding energies, as can be confirmed in Table 3.2, but their
distribution is different. The peak at 399.6 eV should be associated to the
presence of amines on the surface [30,32]. Nevertheless, this assignment
is not enough to confirm the formation of amines from amides, since
both functional groups appear in the same range of binding energy,
according to the literature. On the other hand, the peaks at 400.4 and
398.5 eV can be assigned to the functional groups formed following the
reactions showed in Figure 3.5, as in the case of KUA-CONH2.
However, in the sample KUA-NH2 there exists a larger relative amount
of species of high binding energy (pyrroles and/or pyridones), Table 3.2.
Chapter 3
146
Figure 3.5. Formation of (a) imines, (b) pyrroles and (c) pyridones from aldehydes and
ketones; (d) thermal decomposition of pyridone to form pyrrole through loss of a CO
group.
The increase in the oxygen amount detected by XPS and TPD after
amination indicates that other reactions have taken place in addition to
the conversion of amides to amines. This is confirmed when the step c in
Figure 3.1 is directly applied to KUA-COOH. These results proved that
the attachment of methoxide and/or hydroxyl groups occurs onto the
surface of KUA-NH2 and that it takes place through substitution
reactions without any further oxidation. This can also be related to the
larger relative amount of N-functional groups at 400.4 0.2 eV in this
sample in comparison with KUA-CONH2. A possible explanation is the
nucleophilic substitution during the reaction in the adjacent position of
the nitrogen on the pyridine rings (Figure 3.4.d). Under this hypothesis,
the methoxide present in the media during the first step of the reaction,
N-functionalization of activated carbon
147
or the hydroxide ions present in the second step, could be attached to the
adjacent carbon of the nitrogen group due to the influence of the latter,
which confers -deficient character to the ring [35]. This process could
also happen sequentially: first, attachment of methoxide into the pyridine
ring and secondly, the substitution of methoxide by hydroxide ions, as is
shown in Figure 3.4.d. Hence, the substitution in 2 position of pyridine
would generate pyridones, so that the amount of groups at the binding
energy of 400.4 eV would increase as well as the oxygen content.
Finally, the decrease of nitrogen content in KUA-NH2 (Table 3.1)
can be associated to the hydrolysis of imines to carbonyls (explaining
the decrease of the peak at 398.5 eV, Table 3.2), which is favored in acid
media [37], but that can also happen easily in water [39]. As for the
decrease detected at 399.5 eV, it can be related to the loss of part of the
amides and their derivatives of the KUA-CONH2, since amines are
stable in the preparation conditions.
3.3.2.2. Temperature Programmed Desorption
Table 3.3 reports the amount of CO and CO2 desorbed, as well as
the total oxygen amount (calculated as CO + 2CO2) during the TPD
experiments for all the studied samples. It can be appreciated that the
tendencies in the amount of total oxygen for each preparation step
coincide with those obtained by XPS (Table 3.1). Differences in the
obtained amounts are due to the techniques being not quantitatively
comparable. XPS gives information about the most external surface of
the sample; while the oxygen amount obtained from TPD analyses is
Chapter 3
148
usually underestimated, since some oxygen functional groups can
thermally decompose forming water, which has not been quantified, or
can decompose at temperatures higher than those reached at the end of
the TPD experiment.
Table 3.3. Amount of evolved CO, CO2 and total O obtained from the TPD
experiments.
Sample CO
(μmol/g)
OCO
(wt. %)
CO2
(μmol/g)
OCO2
(wt. %)
O
(μmol/g)
O
(wt. %)
KUA 2100 3.4 500 1.6 3100 5.0
KUA-COOH 4030 6.5 1680 5.4 7390 11.8
KUA-CONH2 2490 4.0 1030 3.3 4550 7.3
KUA-NH2 2800 4.5 1480 4.7 5760 9.2
Figure 3.6 shows the CO and CO2 TPD profiles for the pristine and
surface-modified activated carbons obtained after each synthesis step.
The pristine sample (KUA) presents a remarkable variety of oxygen
groups on its surface, all of them generated during the activation
process. Its aimed surface functionalization with carboxylic acids seems
to be accomplished after the nitric acid treatment, as pointed out by the
large amount of CO2 desorption observed between 200 and 400ºC as
consequence of the decomposition of carboxylic acids [22, 40-42].
Collaterally, the amount of desorbed CO, which in pristine KUA sample
results from the decomposition of carbonyls and quinones (CO at
temperatures over 700ºC) and as phenols (related to CO evolution at
600-700ºC) in a lower extent [40], have also increased after the nitric
acid oxidation. Some CO desorption at temperatures lower than 600ºC is
observed because of the presence of anhydride groups, being their
amount also higher for the KUA-COOH sample. These groups thermally
N-functionalization of activated carbon
149
decompose as CO and CO2, thus the CO desorption is accompanied by
CO2 evolution in similar amounts at temperatures between 400 and
600ºC. The results from the nitric acid treatment are in line with
previous studies from our research group [24,43] and the abundant
literature regarding wet oxidation of porous carbons [23, 44].
Figure 3.6. Comparison between (a) CO2 and (b) CO TPD profiles of KUA-COOH,
KUA-CONH2 and KUA-NH2.
Furthermore, the comparison between the profiles from the oxidized
and the amidated and aminated samples in Figure 3.6 allows us to clarify
both the changes in the surface oxygen groups produced during the
0
0.5
1
1.5
2
0 200 400 600 800 1000
CO
2(μ
mol/
g)
Temperature (ºC)
KUA
KUA-COOH
KUA-CONH₂
KUA-NH₂
(a)
0
0.5
1
1.5
2
2.5
3
3.5
0 200 400 600 800 1000
CO
(μ
mo
l/g
s)
Temperature (ºC)
KUA
KUA-COOH
KUA-CONH₂
KUA-NH₂
(b)
Chapter 3
150
different steps of the functionalization and the most probable mechanism
for the formation of the nitrogen groups. The functionalization with
nitrogen groups during amidation is produced through substitution and
condensation reactions onto surface oxygen groups, as pointed out by
the lower CO2 and CO (at high temperatures) evolutions measured for
the KUA-CONH2 sample if compared to that of KUA-COOH. More
concretely, the comparison of the CO2 profiles evidences consumption
of carboxylic acids (300ºC) and lactones (350-600ºC), as can be
observed in Figure 3.6a (total consumed amount being 650 μmol/g).
This decrease is expected according to the desired reaction, since the
formation of amide groups and its derivatives requires the consumption
of CO2-desorbing groups (see Figure 3.1). On the other hand, the KUA-
CONH2 sample shows a decrease in the evolution of CO groups at high
temperatures, Figure 3.6.b, that is quantified to be 1500 μmol/g. This
consumption of CO groups is related to the formation of another type of
nitrogen functional groups (Figures 3.5a, 3.5b and 3.5c) that, taking into
account the results obtained by XPS, might be imines, pyrroles,
pyridones and pyridines.
Further evidences of the attained nitrogen functionalization have
been obtained during the TPD experiments. Indeed, Figure 3.7.a, which
presents the m/z=14 signal (arbitrary units) as a function of temperature
for the TPD of KUA-CONH2, shows a peak at 380º C that is related to
the evolution of a nitrogen-containing molecule. It also coincides with a
CO desorption shoulder (Figure 3.6.b), providing enough evidences of
the decomposition of the amide group. Furthermore, the continuous N
N-functionalization of activated carbon
151
desorption along the whole temperature range is probably due to lactams
decomposition, which can either desorb at higher temperatures or react
forming pyridines, and the degradation of aromatic rings (pyrroles,
pyridones and pyridines) at even higher temperatures [30]. This is
confirmed by the XPS experiments obtained for the samples thermally
treated until 950 ºC, Table 3.2, where an important decrease in the N
content is detected and only contributions of pyrrole/pyridone and
pyridines are observed.
Additional comparison can be drawn between the TPD profiles
of the KUA-CONH2 and KUA-NH2 samples, Figure 3.6. As was
observed by XPS, the amination reaction increases the amount of
oxygen on the surface in the form of CO and CO2-evolving groups. The
CO2 profile suggests the formation of carboxylic acids during the
amination step, although direct comparison between the profiles of
KUA-COOH and KUA-NH2 (Figure 3.6.a) confirms the overall
consumption of carboxylic acids for the whole synthesis route. A
possible explanation for the increase of CO2-evolving groups in KUA-
NH2 is the esterification of lactams (that have been formed in the
amidation step, Figure 3.4.a), which can react with the methoxide
present in the media and form esters that would subsequently be
hydrolyzed to form again lactone moieties (reverted reaction of Figure
3.4.a), evidencing that the Hofmann rearrangement to produce amines
does not take place over lactams. This is supported by the loss of
nitrogen detected by XPS after amination step (Table 3.2). There have
also been important changes in the CO evolution profile after the
Chapter 3
152
amination of KUA-CONH2 sample, Figure 3.6.b. If the low temperature
region of both desorption profiles are compared, a decrease in the CO
desorption related to amide decomposition (region between 200 and
400ºC) is observed for KUA-NH2. This decrease is probably the
consequence of a consumption of amides and lactams, which evidences
the conversion of these groups into amines and lactones, respectively.
This seems to be supported by the differences in the desorbed amount of
nitrogen species in this region of temperatures in KUA-NH2 (Figure
3.7.b). Moreover, the sample KUA-NH2 presents higher amount of
phenols (CO desorption at 600-700ºC) and a higher CO desorption at the
end of TPD run (temperatures higher than 900ºC). The last one is
produced simultaneously to the increase of the N-related signal recorded
at high temperatures for KUA-NH2, as shown in Figure 3.7.b. This
increase could be a consequence of a favored rearrangement or
formation of N-containing aromatic rings (Figures 3.5.b and 3.5.c)
during amination [30]. As for the CO increase, a favored insertion of
hydroxyl groups during the amination treatment is possible due to the -
deficient behavior of pyridines, which facilitates nucleophilic
substitution reactions on the adjacent carbon (Figure 3.4.d). The
resulting pyridones can be transformed into pyrroles by thermolysis,
which produces the loss of a CO molecule, as is shown in Figure 3.5d
[45]. This is supported by the XPS results obtained for the samples
KUA-CONH2 and KUA-NH2 after the TPD treatment up to 950º C
(Table 3.2), where the higher amount of pyrroles measured after TPD of
KUA-NH2 seems to be connected to the presence of a higher amount of
pyridones in the original KUA-NH2 sample.
N-functionalization of activated carbon
153
Figure 3.7. I14 line during TPD experiment for (a) KUA-CONH2 and (b) KUA-NH2
samples.
0 200 400 600 800 1000
I 14
(a.u
.)
Temperature (ºC)
(a)
0 200 400 600 800 1000
I 14
(a.u
.)
Temperature (ºC)
(b)
Chapter 3
154
3.3.3. Electrochemical characterization.
3.3.3.1. Cyclic Voltammetry
The pristine and modified materials have been characterized by
cyclic voltammetry in the 0.2-0.8V potential range in 1M H2SO4 using
different scan rates. Figure 3.8 shows the voltammograms for all
activated carbon electrodes at low scan rate (1 mV/s). Gravimetric and
surface capacitances (gravimetric capacitance divided by the BET
surface area) are reported in Table 3.4. The formation of the electrical
double layer is manifested in all of them, evidenced by the rectangular
shape of their voltammograms between 0.2V and 0.65V. At high
positive potentials an oxidation current is observed for all the samples
and the oxidation current depends on the electrode material. The low
scan rate used for the CV measurements provides enough time for ion
diffusion into the pore network, so all the surface wetted by the
electrolyte will be available for the formation of the double layer. The
calculated capacitance is different for each material due to the
differences in surface area and surface chemistry. However, it can be
seen that the order in the values of the gravimetric capacitance (Table
3.4) does not follow in all the cases the trends observed in the BET
surface area in Table 3.1. Thus, the activated carbon with the highest
capacitance is the pristine carbon (KUA), but the highly oxidized KUA-
COOH shows the lowest capacitance in spite of having higher surface
area than KUA-CONH2 and KUA-NH2.
N-functionalization of activated carbon
155
Figure 3.8. 2nd cyclic voltammograms in the potential range 0.2-0.8 V for KUA, KUA-
COOH, KUA-CONH2 and KUA-NH2 electrodes.1M H2SO4. v=1 mV/s.
When capacitance is divided by BET surface area (i.e. surface
capacitance, second column in Table 3.4), values concordant with those
found in the literature for porous carbons are obtained [46], except for
the one obtained for KUA-COOH. These results are an example of the
influence of the surface chemistry in the electrochemical behavior of
carbon materials. The effect of the surface oxygen groups in capacitance
has been studied in previous works of our research group [24]. In
general, oxygen functionalities improve the behavior of the electrodes
mainly due to two effects: increase of wettability of the surface, which
facilitates interactions with the electrolyte, and introduction of functional
groups capable of experimenting redox reactions, generating
pseudocapacitance [24,47]. These advantages are related to CO-
desorbing groups, while the presence of CO2-desorbing groups is
unfavorable because of their electron-withdrawing properties that
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
j (A
/g)
E (V vs Ag/AgCl/KCl)
KUA
KUA-COOH
KUA-CONH₂
KUA-NH₂
Chapter 3
156
diminish the delocalization of the charge (and thereby the electrical
conductivity). This effect explains the lower specific capacitance in
KUA-COOH. However, the specific capacitance for KUA-CONH2 is
similar or even higher than for the other materials. In this sample, a
significant part of the CO2-desorbing groups are subsequently
transformed into amides and derivatives, thus partially deactivating their
negative influence, while keeping the positive influence coming for the
presence of CO-desorbing [24] and other N-containing [48] groups.
Table 3.4. Gravimetric (Cg) and surface capacitances (Cg/SBET) and Ohmic drop
(IRdrop) for all the samples measured at different electrochemical conditions. 1 M
H2SO4.
Sample Cg
[1mV/s]
(F/g)
Cg/SBET
[1mV/s]
(mF/m2)
Cg
[0.5 A/g]
(F/g)
Cg
[20 A/g]
(F/g)
Cg
[50 A/g]
(F/g)
IRdrop
[20 A/g]
(mV)
KUA 299 95 268 82 33 106
KUA-COOH 171 64 138 6 3 380
KUA-CONH2 217 105 206 125 82 66
KUA-NH2 249 102 209 17 2 272
The impact of the changes in the surface chemistry in the
capacitance retention was analyzed by recording CV at different scan
rates, Figure 3.9. The pristine activated carbon is able to keep the CV
characteristics up to 20 mV/s, while KUA-COOH and KUA-NH2
yielded tilted voltammograms at scan rates of 10 mV/s and higher. In
terms of capacitance retention, the three samples seem to behave
similarly. Interestingly, the best results are obtained for the amidated
sample (KUA-CONH2) that is able to retain the CV shape up to 50
mV/s. Since the micropore size distribution seems to be similar for all
the samples, as already discussed in section 3.3.1, these differences in
N-functionalization of activated carbon
157
behavior must be related to the changes in the surface chemistry of the
samples.
Figure 3.9. Gravimetric capacitance at different scan rates for KUA, KUA-COOH,
KUA-CONH2 and KUA-NH2 electrodes.1M H2SO4. v= 1, 10, 20 and 50 mV/s.
The negative effect of surface oxygen groups and especially CO2-
desorbing groups in electrical conductivity can explain the low
capacitance retention in KUA-COOH. KUA-NH2 also has similar
capacitance retention, but higher specific capacitance at all scan rates. In
the case of KUA-CONH2, the capacitance experiences the smaller
decrease with scan rate, as proved in Figure 3.9. Consequently, this
electrode has the highest specific capacitance at the highest tested scan
rate. The capacitance retention capability is undoubtedly related to their
surface chemistry, since it presents basically the same pore size
distribution than the parent sample and the lowest specific surface area
of the all studied materials. This effect has been studied in more detail
using galvanostatic charge-discharge analyses.
0
50
100
150
200
250
300
350
0 20 40 60
Cap
acit
an
ce (
F/g
)
Scan rate (mV/s)
KUAKUA-COOH
KUA-CONH₂KUA-NH₂
Chapter 3
158
3.3.3.2. Galvanostatic charge-discharge cycles
The rate performance of the original and the modified samples has
been characterized by chronopotentiometry at different specific currents
(from 0.05 to 50 A/g). Figure 3.10 shows the 4th galvanostatic charge-
discharge cycle at 0.5 A/g, 20 A/g and 50 A/g, and the obtained
capacitance and ohmic drop at 20 A/g are compiled in Table 3.4. The
discharge time, which is directly related to the capacitance reported in
Table 3.4, follows the order KUA>KUA-NH2~KUA-CONH2>KUA-
COOH at 0.5 A/g. However, this tendency changes at higher specific
currents (>10 A/g), being then KUA-CONH2>>KUA>>KUA-
NH2>KUA-COOH. This result is in agreement with the capacitance
values and capacitance retention observed in the cyclic voltammetry
study, section 3.3.3.1. An outstanding capacitance value of 82 F/g was
registered for the KUA-CONH2 at 50 A/g, being high considering that it
is achieved with a surface loading of 8 mg/cm2, a value in the range of
those used in the commercial formulation of supercapacitors [49].
The ohmic drop, reflected by the sudden drop in potential when
moving from charge to discharge at 0.8V in Figures 3.10.b and 3.10.c, is
also affected by the surface chemistry of the materials. This ohmic drop
is associated to the electrical series resistance of the electrode, which
includes the inherent resistance of the materials used in the different
components of the cell (current collector, membrane, etc.), the
interparticle electrical resistance (being connected to the electrode
preparation), the intraparticle electrical resistance (which is related to the
inherent conductivity of the sample) and the ion diffusional resistance
N-functionalization of activated carbon
159
(which depends on the pore size distribution and connectivity). Since the
cell and the electrode preparation is identical in all the cases, and ion
diffusional resistance must be similar for the analyzed materials, the
important differences in the ohmic drop values must be related to the
surface chemistry. As discussed before, the higher amount of CO2-
evolving groups results in a worse rate performance in KUA-COOH,
while the presence of N functionalities in KUA-NH2, which improve the
wettability and electrical conductivity [6,8,48], seems to be responsible
of its slightly better performance, although the behavior as
supercapacitor electrode is still worse than the starting material, KUA.
This is probably due to the lower amount of functional groups in KUA
that facilitate the charge delocalization on its surface with respect to that
of KUA-NH2. Very interestingly, the capacitance retention is greatly
improved for the KUA-CONH2 electrode, being much higher than that
of the oxidized carbon KUA-COOH and even higher than that of the
pristine activated carbon. Thus, the gravimetric capacitance of KUA-
CONH2 is higher than that found for KUA at specific currents higher
than 10 A/g, thanks to the lower ohmic drop found for this material
(Table 3.4). This material seems to behave similar in terms of
capacitance retention than other N-containing porous carbon materials
prepared from N-containing precursors reported in literature [4,6,50-54].
Other N-doping techniques, such as ammoxidation or urea treatment
[55,56], do not seem to produce such a good capacitance retention as the
amidation route proposed in this work.
Chapter 3
160
Figure 10. Galvanostatic charge-discharge curves at current densities of (a) 0.5 A/g,
(b) 20 A/g and (c) 50 A/g. Potential range: 0.2-0.8V.
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 200 400 600
E (
V v
s A
g/A
gC
l/K
Cl)
Time (s)
KUA
KUA-COOH
KUA-CONH₂
KUA-NH₂
(a)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 2 4 6 8
E (
V v
s A
g/A
gC
l/K
Cl)
Time (s)
KUA
KUA-COOH
KUA-CONH₂
KUA-NH₂
(b)
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.4 0.8 1.2 1.6 2
E (
V v
s A
g/A
gC
l/K
Cl)
Time (s)
KUA
KUA-CONH₂(c)
N-functionalization of activated carbon
161
The mechanism behind the capacitance and the electrical
conductivity improvement in N-containing carbon materials is still under
debate in the literature [6-8,11,57-59]. Theoretical studies [59] have
proven that the nitrogen of pyrrole groups improves the electron
mobility of the carbon matrix through introduction of electron-donor
properties and increases the catalytic activity of the carbon in electron
transfer reactions. Also, the nitrogen atoms present in a six member ring
in the edge of a graphene layer can be considered as pyrrole-like
functionalities due to the conjugation of its two p electrons with the
system of the graphene layer. Hulicova-Hurkacova and coworkers
indicated that lactams and imides can be considered as pyrrole-like
groups, since they present both characteristics [6]; also, they proposed
the participation of nitrogen and oxygen of imide, lactam and pyridone
groups in pseudocapacitance processes. This explains the high
capacitance retention of the electrode KUA-CONH2, which presents a
higher amount of pyrrole-like groups on its surface. On the other hand,
they rejected the participation of amine and amide groups in the
improvement of electro-donating properties. However, we consider that
the presence of amides can also improve the behavior of the sample
KUA-CONH2, because part of the carboxylic acids that were present on
the surface of KUA-COOH has been replaced and, although amides are
also electron-withdrawing groups, when they are attached to the
aromatic ring through the carbonyl group, this character is lower than
that of carboxylic acids. Also, Hsieh and coworkers [11] indicated the
possible contribution of amide groups in pseudocapacitive processes in
supercapacitors of CNTs modified by a synthetic path quite similar to
Chapter 3
162
our approach. Finally, the benefits of pyridines in carbon material with
respect to electrical conductivity is unclear, whereas its positive
influence in capacitance seems to be associated with enhanced
wettability [57], enhanced interaction with ions [58] and the occurrence
of pseudofaradaic processes [57], which is reported to take place through
the following reaction [60]:
HC=N + H2O ↔ CHN=O + 2e- + 2H+
In a different work about the effect of nitrogen functionalities in the
electrochemical behavior of carbon materials [8], the contribution of
pyridines in the surface of nitrogen-containing CNTs to the enhancement
of capacitance was found to be higher in basic electrolyte, so its
presence is not expected to be critical in the behavior of the materials
studied in this work at acidic conditions.
Hence, the improvements in the capacitance retention found in
KUA-CONH2 can be explained by the lower amount of carboxylic acids
in the surface when compared to KUA-NH2 (Figure 3.6) and by the
nitrogen functional groups on its surface. Since the main differences in
the chemical composition in terms of nitrogen surface groups between
KUA-CONH2 and KUA-NH2 is the presence of amide groups and
derivatives in the former that have been replaced by amines and
carboxylic acids in the latter, and considering that amines have a higher
electron-donating character than amides, the enhanced performance of
the KUA-CONH2 is attributable to the cyclic amides (lactams and
imides) and their pyrrol-like effect on the electrochemical performance
of carbon electrodes [6].
N-functionalization of activated carbon
163
3.4. Conclusions
Functionalization of activated carbon using an organic chemistry
protocol has been carried out by amidation treatment and Hofmann
rearrangement. Nitrogen content of about 3 at% is achieved through this
process under mild conditions. As consequence of the heterogeneous
surface chemistry of activated carbons, a wide range of functional
groups have been produced onto the surface of the modified samples,
showing different nitrogen and oxygen functional groups on KUA-
CONH2 and KUA-NH2. The amidation treatment produces amides (and
cyclic derivatives, such as imides and lactams) on KUA-CONH2, but the
presence of CO-desorbing groups on KUA-COOH yields the generation
of nitrogen aromatic heterocycles (pyridine, pyridone and pyrrole).
Consequently, KUA-CONH2 presents a complex surface chemistry with
different nitrogen functional groups, some of them also stable at high
temperatures (pyrrole and pyridine). The post-treatment by Hofmann
rearrangement produces the conversion of amides into amines, obtaining
an activated carbon (KUA-NH2) with different nitrogen functionalities
(amines, pyridines and pyrroles) and a high amount of surface oxygen
groups. Different reaction pathways are proposed to explain the
observed changes.
The different surface chemistry of all activated carbons allows us to
study its effect on their electrochemical performance. At low current
densities, the capacitance is mainly governed by the apparent surface
area while the specific capacitance is greater for the N-containing
samples, evidencing the influence of nitrogen functional groups.
Chapter 3
164
Interestingly, KUA-CONH2 showed the highest capacitance retention,
keeping an outstanding value of 83 F/g at 50 A/g, a value especially high
considering the electrode thickness. This capacitance retention was not
registered for any of the other electrodes and, therefore, it might be due
to the lower amount of electron-withdrawing groups, as carboxylic
acids, and to the nitrogen functional groups existing on KUA-CONH2,
such as pyridines, cyclic amides (lactams and imides) and pyrroles,
which improve the charge delocalization and thereby increase the
electrical conductivity.
3.5. References
[1] Bandosz TJ, Ania CO. Surface chemistry of activated carbons and
its characterization. In: Bandosz TJ, editor. Activated Carbon
Surfaces in Environmental Remediation, New York:1st ed,
Elsevier; 2006, p. 159–229.
[2] Bandosz TJ. Surface Chemistry of Carbon Materials. In: Serp P,
Figueiredo JL, editors. Carbon Materials for Catalysis, John
Wiley & Sons, Inc; 2009, p. 45–92.
[3] Radovic LR. Surface Chemical and Electrochemical Properties of
Carbons. In: Beguin F, Frackowiak E, editors. Carbons for
Electrochemical Energy Storage and Conversion Systems, Boca
Raton, FL: Taylor & Francis (CRC Press); 2010, p. 163–219.
[4] Shen W, Fan W. Nitrogen-containing porous carbons: synthesis
and application. J Mater Chem A 2013;1(4): 999-1013.
N-functionalization of activated carbon
165
[5] Gong KP, Du F, Xia ZH, Durstock M, Dai LM. Nitrogen-doped
carbon nanotube arrays with high electrocatalytic activity for
oxygen reduction. Science 2009;323:760–4.
[6] Hulicova-Jurcakova D, Kodama M, Shiraishi S, Hatori H, Zhu
ZH, Lu GQ. Nitrogen-enriched nonporous carbon electrodes with
extraordinary supercapacitance. Adv Funct Mater
2009;19(11):1800–9.
[7] Salinas-Torres D, Shiraishi S, Morallón E, Cazorla-Amorós D.
Improvement of carbon materials performance by nitrogen
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions. Carbon 2015;82:205-13.
[8] Ornelas O, Sieben JM, Ruiz-Rosas R , Morallón E, Cazorla-
Amorós D, Geng J, et al. On the origin of the high capacitance of
nitrogen-containing carbon nanotubes in acidic and alkaline
electrolytes. Chem Commun 2014;50:11343-6.
[9] Raymundo-Piñero E, Cazorla-Amorós D, Linares-Solano A. The
role of different nitrogen functional groups on the removal of SO2
from flue gases by N-doped activated carbon powders and fibres.
Carbon 2003;41(10):1925–32.
[10] Sevilla M, Valle-Vigón P, Fuertes AB. N-Doped polypyrrole-
based porous carbons for CO2 capture. Adv Func Mater
2011;21(14):2781–7.
[11] Hsieh C-T, Teng H, Chen W-Y, Cheng Y-S. Synthesis,
characterization, and electrochemical capacitance of amino-
functionalized carbon nanotube/carbon paper electrodes. Carbon
2010;48(15):4219–29.
Chapter 3
166
[12] Silva AR, Martins M, Freitas MM, Valente A, Freire C, de Castro
B, et al. Immobilisation of amine-functionalised nickel(II) Schiff
base complexes onto activated carbon treated with thionyl
chloride. Microporous Mesoporous Mater 2002;55(3):275–84.
[13] Silva AR, Martins M, Freitas MMA, Figueiredo JL, Freire C,
de Castro B. Anchoring of copper(II) acetylacetonate onto an
activated carbon functionalised with a triamine. Eur J Inorg Chem
2004;10:2027–35.
[14] Silva AR, Budarin V, Clark JH, Freire C, de Castro B. Organo-
functionalized activated carbons as supports for the covalent
attachment of a chiral manganese(III) salen complex. Carbon
2007;45(10):1951–64.
[15] Alves JAC, Freire C, de Castro B, Figueiredo JL. Anchoring of
organic molecules onto activated carbon. Colloids Surf A
2001;189(1-3):75–84.
[16] Tamai H, Shiraki K, Shiono T, Yasuda H. Surface
functionalization of mesoporous and microporous activated
carbons by immobilization of diamine. J Colloid Interface Sci
2006;295(1):299–
[17] Peñas-Sanjuán A, López-Garzón R, Domingo-García M, López-
Garzón FJ, Melguizo M, Pérez-Mendoza M. An efficient
procedure to bond nanostructured nitrogen functionalities to
carbon surfaces. Carbon 2012;50(11):3977–86.
[18] El-Sayed Y, Bandosz TJ. Role of surface oxygen groups in
incorporation of nitrogen to activated carbons via
ethylmethylamine adsorption. Langmuir 2005;21(4):1282–9.
N-functionalization of activated carbon
167
[19] Abe M, Kawashima K, Kozawa K, Sakai H, Kaneko K.
Amination of activated carbon and adsorption characteristics of its
aminated surface. Langmuir 2000;16(11):5059–63.
[20] Gromov A, Dittmer S, Svensson J, Nerushev OA, Perez-García
SA, Licea-Jiménez L, et al. Covalent amino-functionalisation of
single-wall carbon nanotubes. J Mater Chem 2005;15:3334–9.
[21] Lozano-Castelló D, Lillo-Ródenas MA, Cazorla-Amorós D,
Linares-Solano A. Preparation of activated carbons from Spanish
anthracite. I Activation by KOH. Carbon 2001;39(5):741–9.
[22] Boehm HP. Surface oxides on carbon and their analysis: a critical
assessment. Carbon 2002;40(2):145–9.
[23] Moreno-Castilla C, López-Ramón MV, Carrasco-Marın F.
Changes in surface chemistry of activated carbons by wet
oxidation. Carbon 2000;38(14):1995–2001.
[24] Bleda-Martínez MJ, Lozano-Castelló D, Morallón E, Cazorla-
Amorós D, Linares-Solano A. Chemical and electrochemical
characterization of porous carbon materials. Carbon
2006;44(13):2642–51.
[25] Jeong YN, Choi MY, Choi HC. Preparation of Pt- and Pd-
decorated CNTs by DCC-activated amidation and investigation of
their electrocatalytic activities. Electrochim Acta 2012; 60: 78-84.
[26] Willocq C, Hermans S, Devillers M. Active carbon functionalized
with chelating phosphine groups for the grafting of model Ru and
Pd coordination compounds. J Phys Chem C 2008;112(14):5533–
41.
Chapter 3
168
[27] Cazorla-Amorós D, Alcañiz-Monge J, de la Casa-Lillo MA,
Linares-Solano A. CO2 as an adsorptive to characterize carbon
molecular sieves and activated carbons. Langmuir
1998;14(16):4589–96.
[28] Raymundo-Piñero E, Cazorla-Amorós D, Linares-Solano A, Find
J, Wild U, Schlögl R. Structural characterization of N-containing
activated carbon fibers prepared from a low softening point
petroleum pitch and a melamine resin. Carbon 2002;40(4):597–
608.
[29] Biniak S, Szymański G, Siedlewski J, ŚwiaTkowski A. The
characterization of activated carbons with oxygen and nitrogen
surface groups. Carbon 1997;35(12):1799–810.
[30] Jansen RJJ, van Bekkum H. XPS of nitrogen-containing
functional groups on activated carbon. Carbon 1995;33(8):1021–
7.
[31] Kapteijn F, Moulijn JA., Matzner S, Boehm HP. The development
of nitrogen functionality in model chars during gasification in
CO2 and O2. Carbon 1999;37(7):1143–50.
[32] Yamada Y, Kim J, Matsuo S, Sato S. Nitrogen-containing
graphene analyzed by X-ray photoelectron spectroscopy. Carbon
2014;70:59–74.
[33] Smith MB, March J. March’s Advanced Organic Chemistry:
Reaction, Mechanisms and Estructure. 6th ed. Hoboken, New
Jersey, USA: John Wiley & Sons, Inc. 2007: 1429–30.
N-functionalization of activated carbon
169
[34] Smith MB, March J. March’s Advanced Organic Chemistry:
Reaction, Mechanisms and Estructure. 6th ed. Hoboken, New
Jersey, USA: John Wiley & Sons, Inc. 2007: 1436.
[35] Clayden J, Greeves N, Warren S, Wothers P. Organic Chemistry.
1st ed. Oxford University Press. 2001: 1150–1.
[36] Smith MB, March J. March’s Advanced Organic Chemistry:
Reaction, Mechanisms and Estructure. 6th ed. Hoboken, New
Jersey, USA: John Wiley & Sons, Inc. 2007: 1281–4.
[37] Clayden J, Greeves N, Warren S, Wothers P. Organic Chemistry.
1st ed. Oxford University Press. 2001: 349–51.
[38] Clayden J, Greeves N, Warren S, Wothers P. Organic Chemistry.
1st ed. Oxford University Press. 2001: 1186–7.
[39] Smith MB, March J. March’s Advanced Organic Chemistry:
Reaction, Mechanisms and Estructure. 6th ed., Hoboken, New
Jersey, USA: John Wiley & Sons, Inc. 2007: 1263–7.
[40] Román-Martínez MC, Cazorla-Amorós D, Linares-Solano A,
Salinas-Martínez de Lecea C. Tpd and TPR characterization of
carbonaceous supports and Pt/C catalysts. Carbon
1993;31(6):895–902.
[41] Lopez-Ramon MV, Stoeckli F, Moreno-Castilla C, Carrasco-
Marin F. On the characterization of acidic and basic surface sites
on carbons by various techniques. Carbon 1999;37(8):1215–21.
[42] Figueiredo JL, Pereira MFR, Freitas MMA, Órfão JJ.
Modification of the surface chemistry of activated carbons.
Carbon 1999;37(9):1379–89.
Chapter 3
170
[43] Bleda-Martínez MJ, Maciá-Agulló JA, Lozano-Castelló D,
Morallón E, Cazorla-Amorós D, Linares-Solano A. Role of
surface chemistry on electric double layer capacitance of carbon
materials. Carbon 2005;43(13):2677–84.
[44] Moreno-Castilla C, Ferro-García MA, Joly JP, Bautista-Toledo I,
Carrasco-Marín F, Rivera-Utrilla J. Activated carbon surface
modifications by nitric acid, hydrogen peroxide, and ammonium
peroxydisulfate treatments. Langmuir 1995;11(11):4386–92.
[45] Schmiers H, Friebel J, Streubel P, Hesse R, Kopsel R. Change of
chemical bonding of nitrogen of polymeric N - heterocyclic
compounds during pyrolysis. Carbon 1999;37(12):1965–78.
[46] Qu D. Studies of the activated carbons used in double-layer
supercapacitors. J Power Sources 2002;109(2):403–11.
[47] Conway BE, Birss V, Wojtowicz J. The role and utilization of
pseudocapacitance for energy storage by supercapacitors. J Power
Sources 1997;66(1-2):1–14.
[48] Lota G, Grzyb B, Machnikowska H, Machnikowski J, Frackowiak
E. Effect of nitrogen in carbon electrode on the supercapacitor
performance. Chem Phys Lett 2005;404(1-3):53–8.
[49] Stoller MD, Ruoff RS. Best practice methods for determining an
electrode material's performance for ultracapacitors. Energy
Environ Sci 2010;3:1294-301.
[50] Alabadi A, Yang X, Dong Z, Tan B. Nitrogen-doped activated
carbons derived from a co-polymer for high supercapacitor
performance. J Mater Chem A 2014;2:11697–705.
N-functionalization of activated carbon
171
[51] Paraknowitsch JP, Zhang J, Su D, Thomas A, Antonietti M. Ionic
liquids as precursors for nitrogen-doped graphitic carbon. Adv
Mater 2010;22(1):87–92.
[52] Kim K-S, Park S-J. Synthesis and high electrochemical
capacitance of N-doped microporous carbon/carbon nanotubes for
supercapacitor. J Electroanal Chem 2012;673:58–64.
[53] Wu C, Wang X, Ju B, Jiang L, Wu H, Zhao Q, et al.
Supercapacitive performance of nitrogen-enriched carbons from
carbonization of polyaniline/activated mesocarbon microbeads. J
Power Sources 2013;227:1–7.
[54] Wickramaratne NP, Xu J, Wang M, Zhu L, Dai L, Jaroniec M.
Nitrogen enriched porous carbon spheres: attractive materials for
supercapacitor electrodes and CO2 adsorption. Chem Mater
2014;26(9):2820–8.
[55] Pietrzak R, Jurewicz K, Nowicki P, Babeł K, Wachowska H.
Microporous activated carbons from ammoxidised anthracite and
their capacitance behaviours. Fuel 2007;86(7-8):1086–92.
[56] Jurewicz K, Pietrzak R, Nowicki P, Wachowska H. Capacitance
behaviour of brown coal based active carbon modified through
chemical reaction with urea. Electrochim Acta 2008;53(16):5469–
75.
[57] Seredych M, Hulicova-Jurcakova D, Lu GQ, Bandosz TJ. Surface
functional groups of carbons and the effects of their chemical
character, density and accessibility to ions on electrochemical
performance. Carbon 2008;46(11):1475–88.
Chapter 3
172
[58] Jeong HM, Lee JW, Shin WH, Choi YJ, Shin HJ, Kang JK, et al.
Nitrogen-doped graphene for high-performance ultracapacitors
and the importance of nitrogen-doped sites at basal planes. Nano
Lett 2011;11(2):2472–7.
[59] Strelko VV, Kuts VS, Thrower PA. On the mechanism of possible
influence of heteroatoms of nitrogen, boron and phosphorus in a
carbon matrix on the catalytic activity of carbons in electron
transfer reactions. Carbon 2000;38(10):1499–503.
[60] Frackowiak E, Lota G, Machnikowski J, Vix-Guterl C, Béguin F.
Optimisation of supercapacitors using carbons with controlled
nanotexture and nitrogen content. Electrochim Acta
2006;51(11):2209–14.
N-functionalization of activated carbon
173
ANNEX TO CHAPTER 3.
Nitrogen functionalization of zeolite templated carbon by
electrochemical and chemical methods.
In Chapter 3, the applicability of nitrogen doping through organic
chemistry pathways to an activated carbon was demonstrated. However,
it is interesting to expand the use of this methodology to other carbon
forms. As seen in Chapter 1, zeolite templated carbons (ZTCs) are
materials obtained as a replica of a zeolite that have unique structure and
properties [1,2]. Nevertheless, as consequence of their high reactivity,
their structure is very fragile and hinders their potential use in a wide
range of applications. For this reason, nitrogen doping appears as a
promising strategy for improving the properties of these materials [3].
More information related to this topic is provided in Chapter 5.
Here we propose the nitrogen doping of ZTC through the amidation
protocol developed in Chapter 3. The scheme of the modification
protocol is shown in Figure A3.1. This nitrogen functionalization
method was slightly modified in order to achieve a successful doping of
ZTC. First, all surface modification methods (oxidation and amidation)
were applied over the carbon-zeolite composite prior to the removal of
the template by HF washing (named as cZTC). In this way, the
preservation of the structure of ZTC is guaranteed. The second important
modification of the applied protocol is that the oxidation treatment was
carried out via electrochemical methods, to avoid the damage of the
carbon material.
Chapter 3
174
Figure A3.1. Chemical route used for the modification of the activated carbon KUA:
(a) electrochemical oxidation in 0.5M H2SO4 electrolyte (cZTC-COOH), (b) SOCl2
treatment and amidation (cZTC-CONH2) and (c) removal of the zeolite (ZTC-CONH2).
Table A3.1 shows the textural parameters obtained for cZTC and
ZTC. The results evidence that N2 is not accesible to the microporosity
of the composite. However, the narrow microporosity was found to be
0.11 cm3/g for cZTC. If this value is expressed per gram of carbon
material in cZTC ( 20 % of the composite), the narrow micropore
volume estimated is 0.55 cm3/g of ZTC, what corresponds to a BET
surface area of around 1500 m2/g [4]. Interestingly, this value
corresponds to around 42 % of the BET surface area obtained for ZTC,
evidencing that almost one side of the curved graphene layers of ZTC is
exposed to the CO2 adsorbate in cZTC.
Table A3.1. Textural properties obtained for cZTC and ZTC.
Sample SBET
(m2/g)
VDRN2
(cm3/g)
aVDRCO2
(cm3/g)
bVDRCO
2
(cm3/g)
cZTC 11 0.004 0.11 0.55
ZTC 3600 1.55 - - a m (g) cZTC; b m (g) ZTC
The electro-oxidation of cZTC was carried out by CV in 0.5 M
H2SO4 using the experimental procedure developed by Nueangnoraj and
coworkers for ZTC [5]. Figure A3.2 shows the CVs obtained for ZTC
N-functionalization of activated carbon
175
and cZTC upon positive polarization. Under these conditions, ZTC
experiences oxidation reactions with the electrolyte that lead to an
increase of oxygen functionalities [1]. The presence of the zeolite in
cZTC should avoid the electro-oxidation process; however, the materials
show electrochemical response as evidenced by the increase of oxidation
current at 0.8 V. Also, the electrical double layer of cZTC at 0.7 V
(Figure A3.2a, red line) is 60% of that found for the ZTC (Figure
A3.2b, black line), suggesting that only one side of the graphene layer is
exposed to the electrolyte in the carbon-zeolite composite, what is in
agreement with the CO2 adsorption data. The increase of oxygen
functional groups after the electrochemical treatment was confirmed by
XPS (samples cZTC and cZTC_ox, Table A3.2). Nevertheless, the
characteristic redox pair of the quinone-hydroquinone groups is not
clearly observed in cZTC.
Figure A3.2. Cyclic voltammograms for the electro-oxidation of (a) cZTC and (b)
ZTC. v = 1 mV/s. 0.5M H2SO4.
The electro-oxidized carbon-zeolite composite (cZTC_ox sample)
was further functionalized to introduce nitrogen groups on the carbon
surface. Table A3.2 summarizes the chemical composition of the sample
-1000
0
1000
2000
3000
-0.3-0.10.1 0.3 0.5 0.7 0.9 1.1
C (
F/g
)
E (V vs Ag/AgCl/KCl)
(b)
-1000
0
1000
2000
3000
-0.3-0.10.1 0.3 0.5 0.7 0.9 1.1
C (
F/g
)
E (V vs Ag/AgCl/KCl)
(a)
Chapter 3
176
obtained after N-doping treatment (cZTC-CONH2). The amidation
reaction produces an attachment of 1.4 % at. N that is accompanied by a
large decrease of oxygen functional groups. The amount of nitrogen is
retained on the carbon material (ZTC-CONH2) after the removal of the
zeolite by HF washing as confirmed by XPS (1.4 at. %, Table A3.2).
Figure A3.3 shows the XPS spectra obtained for cZTC-CONH2 and
ZTC-CONH2, evidencing that nitrogen is anchored mainly in form of
amides, pyridines and pyrroles [6–8].
Table A3.2. Chemical composition of the samples obtained by XPS.
Sample CXPS
(% at.)
OXPS
(% at.)
NXPS
(% at.)
SiXPS
(% at.)
AlXPS
(% at.)
cZTC 60.5 16.7 - 15.9 6.9
cZTC_ox 59.5 31.5 - 8.6 1.3
cZTC-CONH2 70.0 13.3 1.3 12.2 3.6
ZTC-CONH2 91.4 6.9 1.4 0.3 -
Figure A3.3. N1s XPS spectra obtained for cZTC-CONH2 and ZTC-CONH2 samples.
395396397398399400401402403404405
I (a
.u.)
Binding Energy (eV)
cZTC-CONH₂
ZTC-CONH₂
N-functionalization of activated carbon
177
The N-doped ZTC was characterized by CV in 1M H2SO4. Figure
A3.4a shows the CVs obtained for ZTC and ZTC-CONH2 in a potential
window where the materials are stable. The N-doped ZTC evidences a
similar curve than the pristine carbon material, although the oxidation
current is much lower, what is a characteristic of the N-doped carbons.
When these carbons are exposed to positive polarization (at 1 V), the
electro-oxidation process takes place and the characteristic redox pair of
quinone-hydroquinone is observed for both carbons (Figure A3.3b). This
electrochemical process is strongly dependent on the structure of ZTC
[9] and, consequently, confirms that the porous structure of the pristine
ZTC is retained after the N functionalization treatment.
Figure A3.4. Cyclic voltammograms obtained for ZTC (black curve) and ZTC-CONH2
(red curve) samples (a) before and (b) after electro-oxidation at 1.0 V vs
Ag/AgCl/KCl.
In conclusion, a zeolite-templated carbon was successfully
functionalized by electrochemical and chemical methods based in
electro-oxidation treatments and amidation reactions. To preserve the
porous structure of the pristine ZTC, the doping methods were applied
-500
-250
0
250
500
-0.3 0 0.3 0.6
C (
F/g
)
E (V vs Ag/AgCl/KCl)
(a)
-800
-400
0
400
800
-0.3 0 0.3 0.6
C (
F/g
)
E (V vs Ag/AgCl/KCl)
(b)
Chapter 3
178
keeping the zeolite template in the composite material. Oxygen
functional groups were attached to the composite and converted
afterwards into nitrogen functionalities, that are retained in the porous
carbon after the removal of the zeolite. The electrochemical
characterization of the N-doped ZTC evidenced that the unique
properties of ZTC remained after the functionalization treatments.
References
[1] H. Itoi, H. Nishihara, T. Ishii, K. Nueangnoraj, R. Berenguer-
Betrián, T. Kyotani, Large Pseudocapacitance in Quinone-
Functionalized Zeolite-Templated Carbon, Bull. Chem. Soc. Jpn.
87 (2014) 250–257.
[2] R. Berenguer, H. Nishihara, H. Itoi, T. Ishii, E. Morallón, D.
Cazorla-Amorós, T. Kyotani, Electrochemical generation of
oxygen-containing groups in an ordered microporous zeolite-
templated carbon, Carbon 54 (2013) 94–104.
[3] D. Salinas-Torres, S. Shiraishi, E. Morallón, D. Cazorla-Amorós,
Improvement of carbon materials performance by nitrogen
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions, Carbon 82 (2015) 205–213.
[4] T.A. Centeno, F. Stoeckli, The assessment of surface areas in
porous carbons by two model-independent techniques, the DR
equation and DFT, Carbon 48 (2010) 2478–2486.
[5] K. Nueangnoraj, R. Ruiz-Rosas, H. Nishihara, S. Shiraishi, E.
Morallón, D. Cazorla-Amorós, T. Kyotani, Carbon–carbon
asymmetric aqueous capacitor by pseudocapacitive positive and
stable negative electrodes, Carbon 67 (2014) 792–794.
[6] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano,
The role of different nitrogen functional groups on the removal of
SO2 from flue gases by N-doped activated carbon powders and
fibres, Carbon 41 (2003) 1925–1932.
N-functionalization of activated carbon
179
[7] R.J.J. Jansen, H. van Bekkum, XPS of nitrogen-containing
functional groups on activated carbon, Carbon 33 (1995) 1021–
1027.
[8] Y. Yamada, J. Kim, S. Matsuo, S. Sato, Nitrogen-containing
graphene analyzed by X-ray photoelectron spectroscopy, Carbon
70 (2014) 59–74.
[9] S. Leyva-García, K. Nueangnoraj, D. Lozano-Castelló, H.
Nishihara, T. Kyotani, E. Morallón, D. Cazorla-Amorós,
Characterization of a zeolite-templated carbon by electrochemical
quartz crystal microbalance and in situ Raman spectroscopy,
Carbon 89 (2015) 63–73.
Chapter 4
Electrochemical performance of
N-doped activated carbons in
aqueous electrolyte
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
183
4.1. Introduction
Supercapacitors have attracted considerable interest as energy
storage devices thanks to their high power density, a key missing feature
of fuel cells and electrical batteries. They are based on the formation of
an electrical double layer on the extensive surface of porous carbon
materials, which is profited to store energy that can be delivered in few
seconds. This energy storage mechanism is often complemented with
pseudocapacitive processes produced by fast redox reactions occurring
in electroactive surface functional groups. The development of electrode
materials with large surface area, electroactive functional groups and
high electrochemical stability is crucial for the improvement of the
energy density and durability of these devices [1,2].
Activated carbons are the most employed materials as electrodes for
supercapacitors, mainly due to their large apparent surface area, proper
electrical conductivity, high electrochemical stability and competitive
production cost [2]. Also, their surface chemistry can be conveniently
modified to introduce surface functionalities that could improve the
performance of these materials by an increase of electrochemical
stability, conductivity, wettability or pseudocapacitance [3]. For
instance, oxygen groups and in aqueous electrolytes can increase the
wettability of the surface, improving the electrolyte-electrode interaction
and rendering a larger amount of the surface accessible for the formation
of the electrical double layer, and can also participate in reversible
faradaic reactions that contribute to the energy storage through
pseudocapacitance [4]. Likewise, the presence of nitrogen on the surface
Chapter 4
184
of activated carbons improves the performance of activated carbon as
electrodes of supercapacitors by increasing the wettability of the surface,
the electrochemical stability, the conductivity or the contribution of
pseudocapacitive processes [5-8]. Inversely to the ubiquity of surface
oxygen groups, nitrogen functionalities are not as easily introduced in
the structure of carbon materials, and the development of new
procedures for obtaining nitrogen-containing porous carbons has
therefore attracted a great interest.
Nitrogen-doped porous carbon materials can mainly be obtained by
two pathways: reaction of the carbon material with a nitrogen-containing
reagent (NH3, HCN, etc) or carbonization/activation of nitrogen-rich
carbon precursors (urea, polyaniline, etc.) [6,9]. However, these
approaches are usually carried out at high temperatures and, in the case
of post-treatments, these conditions strongly modify the porosity of the
pristine carbon material [10,11]. Also, the control of the nitrogen
functionalities that are formed during post-treatments is a challenge.
Therefore, post-modification treatments at low temperature are highly
desirable for obtaining different nitrogen functional groups while
preserving the characteristic porous texture of the original carbon
material.
In Chapter 3, the modification of the surface chemistry of a highly
porous activated carbon using organic chemistry reactions was proposed
[11]. Briefly, this treatment consisted in the oxidation of the carbon
material to introduce oxygen functionalities that, afterwards, were
converted into nitrogen functionalities through an amidation treatment.
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
185
In a last step, the post-conversion of the formed amides into amine
functional groups was achieved by a Hofmann rearrangement [12]. The
modification protocol allowed the incorporation of different nitrogen
functionalities. Interestingly, this method also leads to the formation of
pyridines, pyridones and pyrroles, which have a positive impact in the
electrochemical behavior of carbon materials [6,8], but are only
introduced using high temperature treatments. However, the oxidation
treatment carried out during the first step of the functionalization
protocol produced a remarkable decrease of the microporosity along
with the generation of non-desirable functional groups (CO2-evolving
groups) from the point of view of the capacitor performance. In this
chapter, a modification of this approach that allows the introduction of
nitrogen on carbon materials but preventing the previous incorporation
of oxygen functional groups is proposed. The electrochemical
performance of the pristine and the nitrogen-functionalized activated
carbons has been assessed as electrodes for supercapacitors.
4.2 Materials and methods
4.2.1. Activated carbon
A highly microporous activated carbon prepared in our laboratory
has been used as the starting material for nitrogen incorporation via
organic chemical modification. The pristine material, henceforth named
KUA, has been obtained by chemical activation of a Spanish anthracite
(11 wt% of ash content) with KOH using an impregnation ratio of
activating agent to raw material of 4:1 and an activation temperature of
Chapter 4
186
750º C under inert atmosphere, which was held for 1 hour. More details
about the preparation process are available elsewhere [13].
4.2.2. Chemical functionalization of activated carbon
4.2.2.1. Synthesis of KUA-CONH2
In Chapter 3, an approach for the incorporation of amides over
activated carbon was satisfactorily developed [11]. Also, other nitrogen
functional groups were introduced on the surface of the carbon material
via secondary reactions. The amidation treatment was carried out as
follows:
(i) Chemical oxidation with HNO3 in order to introduce oxygen
functional groups on the surface of KUA [14].
(ii) Generation of acyl chloride functionalities by reaction of the
activated carbon obtained on step (i) with SOCl2 in toluene
under Ar atmosphere.
(iii) Reaction of the activated carbon obtained in step (ii) with 2M
NH4NO3/DMF solution and pyridine.
The obtained activated carbon was named KUA-CONH2. More
details about the preparation process are available in Chapter 3 [11].
4.2.2.2. Synthesis of KUA-N
In this case, step (iii) is directly carried out over activated carbon
KUA, in order to obtain nitrogen functional groups by using a single
treatment via reaction with the existing CO2 and CO-evolving groups in
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
187
the pristine material. In this step (iii), 400mg of KUA were added into
140 mL of 2M NH4NO3/DMF solution (activated carbon to solution
ratio of 1g/300mL) in a round bottom flask. Then, 140 mL of pyridine
were added slowly to the round bottom flask under continuous stirring at
room temperature. The mixture was stirred at 70 ºC for 65 hours. The
obtained sample (KUA-N) was washed with abundant water and ethanol,
filtered and dried at 100º C overnight.
4.2.3. Porous texture and surface chemistry characterization
The porous texture characterization was carried out by N2
adsorption-desorption isotherms at -196º C and by CO2 adsorption at 0º
C by using an Autosorb-6-Quantachrome apparatus. The samples were
outgassed at 200º C for 4 hours before the experiments. The apparent
surface area was obtained from N2 adsorption isotherms by using the
BET equation in the 0.05-0.20 range of relative pressures. The total
micropore volume was determined by Dubinin-Radushkevich (DR)
method applied to N2 (relative pressures from 0.01 to 0.05) adsorption
isotherms. The volume of the narrow microporosity (i.e., pore sizes
below 0.7 nm) was calculated from the DR method applied to the CO2
adsorption isotherms (relative pressures from 0.0001 to 0.25) [15]. The
pore size distribution for both samples has been calculated from the N2
adsorption isotherms using the 2D-NLDFT Heterogeneous surface
model [16] and by applying the Solution of Adsorption Integral Equation
Using Splines (SAIEUS, available online at http://www.nldft.com/)
Software.
Chapter 4
188
The surface chemistry of the samples was analyzed by X Ray
Photoelectron Spectroscopy (XPS) and Temperature Programmed
Desorption (TPD). XPS measurements were performed by using a VG-
Microtech Multilab 3000 spectrometer, equipped with an Al anode. The
deconvolution of N1s spectrum was carried out by using Gaussian
functions with 20% of Lorentzian component. FWHM of the peaks was
kept between 1.4 and 1.7 eV and a Shirley line was used for estimating
the background signal. TPD experiments were performed by heating the
samples (10 mg) to 950º C (at a heating rate of 20º C/min) under a
helium flow rate of 100 mL/min. The analyses were carried out by using
a TGA-DSC instrument (TA Instruments, SDT Q600 Simultaneous)
coupled to a mass spectrometer (Thermostar, Balzers, BSC 200).
4.2.4. Electrochemical characterization
4.2.4.1. Three electrode cell configuration
Carbon electrodes for electrochemical characterization were
prepared by mixing the activated carbon with acetylene black and
polytetrafluoroethylene (PTFE) as binder in a ratio of 90:5:5 (w/w). The
total weight of the electrode was 9 mg (dry basis). For shaping the
electrodes, a sample sheet was cut into a circular shape with an area of
1.2 cm2 and pressed for 5 min at 2 tons to guarantee a homogeneous
thickness. After that, the electrode was attached to a gold disk used as a
current collector by using a conducting adhesive (colloidal graphite
suspension, Hitasol GA-715, Hitachi Chemical Co., Ltd.). The
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
189
electrodes were impregnated for 2 days into 1M H2SO4 previously to
electrochemical measurements.
The electrochemical characterization of the electrodes was
performed by cyclic voltammetry in a Biologic VSP multichannel
potentiostat and using a T-type Swagelock cell in a three-electrode
configuration. 1M H2SO4 was used as aqueous electrolyte. As counter
electrode, more than 20 mg of KUA was used. Both electrodes were
tightly pressed against each other and separated by a nylon membrane
(Teknokroma membrane filters, pore size: 320 nm). Ag/AgCl/KCl (3M)
was used as reference electrode in all cases. The electrochemical
characterization of all samples was tested by CV at sweep rates of 1 and
2 mV/s. CV capacitance was calculated from the area of the
voltammogram. The results are expressed in F/g, taking into account the
weight of the active material of the working electrode.
4.2.4.1. Two electrode cell configuration
Symmetric and asymmetric in mass capacitors were assembled for
all carbon materials. For symmetric capacitors, two electrodes (surface
area: 0.196 cm2) were prepared with a weight of ~1.3 mg (active phase)
each. The thickness of the electrodes was close to 0.2 mm in all cases. In
case of asymmetric in mass configuration, the mass of the electrodes was
determined following the procedures detailed by Peng et al [17], which
allows to charge the supercapacitor cells in expanded voltage with
respect to the symmetric configuration, taking full profit of the stability
potentials of each electrode. The electrodes were attached to a stainless
Chapter 4
190
steel collector by using a conducting adhesive. Supercapacitors were
constructed by pressing both electrodes against each other and
separating them by a nylon membrane filter (pore size: 320 nm). These
devices were characterized by CV at different scan rates, galvanostatic
charge-discharge (GCD) cycles at current densities from 0.25 to 32 A/g
and Electrochemical Impedance Spectroscopy (EIS) in 1M H2SO4
solution. Impedance spectra were measured at 0.05 V in the frequency
range of 10 mHz to 100 kHz with an amplitude voltage of 10 mV.
Autolab PGSTAT302 potentiostat was employed for EIS and CV
measurements and Arbin SCTS potentiostat for galvanostatic charge-
discharge cycles. A durability test for symmetric and asymmetric
capacitors was performed by 5000 galvanostatic charge-discharge cycles
at a current density of 1 A/g and a voltage of 1.2 V in case of symmetric
configuration and 1.4V for asymmetric cells. Current density and
specific capacitance is defined based on the total active weight of the
carbon material included in both electrodes.
The energy density and power density of symmetric and asymmetric
supercapacitors was calculated in order to obtain all relevant information
about their performance. Energy density was obtained during the
discharge cycle by the following equation (4.1):
E = ∫ 𝑉𝑑𝑄𝑄
0 (4.1)
Where V1 is the cell voltage of charge (1.2V for symmetric
configuration and 1.4V for asymmetric configuration) and V2 the voltage
of discharge (0 V in both cases).
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
191
Power density was calculated according to equation (4.2):
𝑃𝑚𝑎𝑥 =𝑉𝑚𝑎𝑥
2
4 𝐸𝑆𝑅 𝑚 (4.2)
Where ESR is the equivalent series resistance (determined from
ohmic drop in the charge–discharge cycles), m is the total active mass of
the electrodes and Vmax is the operating voltage.
4.4. Results and discussion
4.2.1. Surface chemistry and porous texture characterization.
Table 4.1 shows the surface properties of the parent activated carbon
and those obtained after the functionalization treatments. The results of
the XPS analyses evidence that nitrogen atoms have been successfully
anchored through the proposed protocols on the surface of KUA-CONH2
and KUA-N. It should be noted that both functionalization treatments
have leaded to a similar nitrogen content (3.8 and 4.1 at.% for KUA-
CONH2 and KUA-N, respectively), while oxygen content increases
significantly for KUA-CONH2 sample. The changes in surface chemistry
on the sample KUA-CONH2 have been previously studied [11], and we
found that this increase in oxygen was related to the chemical oxidation
treatment carried out prior to amidation step. In case of KUA-N, TPD
measurements show a relevant decrease in the CO-evolving groups, with
a net total diminution (490 µmol /g) that is especially relevant in the
region of thermal decomposition of phenol and quinone-like groups.
However, XPS results evidence an increase of oxygen in both
functionalized activated carbons. It should be considered that this
Chapter 4
192
technique only measures the atomic composition of the most external
region of carbon particles, whereas TPD reflects the CO and CO2
evolving from the thermal decomposition of surface oxygen groups in
both the inner and the outer regions of carbon particles. Hence, TPD
provides more precise information about the overall changes of oxygen
functional groups during the functionalization treatment. The decrease in
oxygen surface groups determined by TPD indicates that part of the
introduced nitrogen groups has been attached to the surface by
consumption of these kind functional groups. These reactions may lead
to the formation of imines, pyridines and pyrroles [11]. The formation of
these functional groups is confirmed by N1s XPS deconvolution
obtained for KUA-N (Figure 4.1), which reveals the presence of
pyrroles/pyridones (400.4 ± 0.2 eV), imines and pyridines (398.8 ± 0.2
eV) [18-21]. Also, other functionalities, such as quaternary nitrogen
(401.8 ± 0.2 eV) [22], amines, amides or cyclic amides (399.6 ± 0.2 eV)
[20], are formed on the surface of this carbon material. The absence of
chemisorbed pyridine after both functionalization treatments was
confirmed by immersing the pristine carbon in pyridine, and following
this step by analogous washing and drying processes carried out for the
synthesis of KUA-N and KUA-CONH2. XPS analysis revealed nitrogen
content of 0.4 at. %, that is negligible in comparison with the nitrogen
content of the parent activated carbon.
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
193
Table 4.1. Textural properties and elemental surface composition (XPS and TPD) for
the pristine and modified activated carbons.
Sample NXPS
(at.%)
OXPS
(at.%)
CO2 TPD
(µmol/g)
COTPD
(µmol/g)
SBET
(m2/g)
VDRN2
(cm3/g)
VDRCO2
(cm3/g)
KUA 0.3 8.8 500 2100 3460 1.19 0.56
KUA-N 4.1 11.1 700 1610 3450 1.19 0.55
KUA-CONH2 3.8 12.0 1030 2490 2160 0.74 0.44
Figure 4.1. N1s XPS deconvolution of KUA-N.
Figure 4.2 shows the nitrogen adsorption-desorption isotherms of
the pristine and the N-containing activated carbons and the calculated
PSD. It can be seen that the generation of functional groups on KUA-
CONH2 produces a large decrease of the nitrogen uptake, which means
that a noticeable part of the microporosity is blocked (Figure 2b),
producing a large drop of ca. 35% of the surface area and micropore
volume, Table 4.1. The blockage of the porosity has been related to the
damaging effect produced by the oxidation treatment on the
microporosity [11]. However, when the proposed functionalization
396397398399400401402403404405
I (a
.u.)
Binding Energy (eV)
Chapter 4
194
treatment is directly applied over the pristine material (obtaining KUA-
N), the nitrogen adsorption isotherm superimposes with that from the
pristine sample, showing that the porosity of the KUA sample does not
change after the amidation treatment (KUA-N sample) due to the mild
conditions used (Figure 4.2a).
Figure 4.2. (a) N2 adsorption-desorption isotherms of the activated carbons KUA,
KUA-N and KUA-CONH2. (b) Pore-size distributions of KUA, KUA-N and KUA-
CONH2 by DFT calculations.
0
200
400
600
800
1000
1200
0.0 0.2 0.4 0.6 0.8 1.0
Ad
sorb
ed
vo
lum
e (
cm
3/g
)
P/P0
KUA
KUA-N
KUA-CONH₂
(a)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 1 2 3 4 5
Po
re v
olu
me (
cm
3/g
nm
)
Pore width (nm)
KUA
KUA-N
KUA-CONH₂
(b)
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
195
The minor differences found in the PSD (Figure 4.2b) are due to
small discrepancies within the NL-DFT fitting of both isotherms. Thus,
this pathway seems to be appropriate for functionalization of carbon
materials with a high microporosity development, since it allows the
incorporation of a high amount of nitrogen functionalities (4.1 at.%
XPS) along with the preservation of porous texture.
4.3.2 Electrochemical characterization
4.3.2.1 Characterization of carbon materials
The electrochemical performance of all carbon materials has been
studied in a three electrode cell configuration by cyclic voltammetry.
Figure 4.3 shows the second cyclic voltammograms for all electrodes at
a potential range where they are stable (in the absence of degradation
reactions). The gravimetric capacitance for all the carbon materials has
been determined from CV measurements and is shown in Table 2. It can
be observed that the gravimetric capacitance is mainly related to the
specific area of the sample, with KUA and KUA-N showing the largest
capacitance due to its largest apparent surface area, while KUA-CONH2
displays the lowest capacitance due to the decrease in porosity after the
functionalization treatment.
Although sample KUA-N has a capacitance quite close to sample
KUA, there are interesting differences in the shape of the
voltammogram. Taking into account that this functionalization treatment
does not change the apparent surface area of the pristine activated
carbon, these differences are related to the changes on the surface
Chapter 4
196
chemistry produced by the modification protocol. Thus, KUA shows two
redox processes at around 0.2V and 0.4V, that can be related to the
presence of electroactive surface oxygen groups [14], but KUA-N does
not present these redox processes, and the electrochemical behavior
shown by this electrode seems to be exclusively related to the formation
of electrical double layer on the inner surface of this carbon material
(Figure 4.3). This is in agreement with the characterization of the
materials, since both have similar porous texture (Table 4.1 and Figure
4.1), but sample KUA-N has a lower content of CO-groups (Table 4.1)
which are the electroactive oxygen functional groups in acid medium
[4].
Figure 4.3. 2nd cyclic voltammograms in the potential range between 0V and 0.6V for
KUA, KUA-CONH2 and KUA-N electrodes. 1M H2SO4. v=1 mV/s.
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
-0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
j (A
/g)
E (V vs Ag/AgCl/KCl)
KUA KUA-N KUA-CONH₂
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
197
Table 4.2. Gravimetric capacitance (Cg) determined for KUA, KUA-CONH2 and
KUA-N electrodes in the potential range between 0V and 0.6V by cyclic voltammetry.
1M H2SO4. v=1 mV/s.
Sample Cg (F/g)
KUA 256
KUA-N 229
KUA-CONH2 200
These changes related to the absence of electroactive oxygen
functional groups in acid medium may affect the stability of the
electrode. Oxygen functional groups can often be beneficial for the
electrochemical performance of carbon materials since they can provide
pseudocapacitance [4]; however, when these materials reach positive
potentials (>0.8V), some prejudicial functionalities, as the CO2-type
groups, which are the starting point in degradation mechanism of carbon
electrodes [23], can be formed. For these reasons, the potential range of
stability of KUA and KUA-N have been determined in order to assess
the effect of electroactive oxygen groups on the electrochemical stability
of these carbon materials. Figure 4.4a shows the cyclic voltammograms
for KUA and KUA-N in potential range between -0.5V and 0.6V. It is
clearly seen that, beyond the differences already found in Figure 4.3,
both electrodes present a quite similar electrochemical behavior down to
-0.2 V. However, KUA presents a broad peak at -0.35V in the negative
scan that can be related to the reduction of oxygen functionalities. This
peak does not appear in the case of KUA-N, evidencing the conversion
of electroactive oxygen groups into different species. Figure 4.4b shows
the third cyclic voltammograms for KUA and KUA-N in the potential
Chapter 4
198
range between 0V to 1.0V. Again, both electrodes reveal similar
performance at positive potentials evidenced by the oxidation current
happening at 0.9V. However, the effect of the electroxidation on the
materials is different; in case of KUA, the redox processes at around
0.4V resulting from electrooxidation are more noticeable, mainly in the
reduction one, that indicates an increase of surface oxygen groups during
the positive scan, being this broad reduction peak lower in the case
KUA-N activated carbon. Given that the electrochemical degradation of
porous carbon materials in aqueous electrolytes seems to be related to
the formation of oxidized species over the surface of the electrodes when
submitted to positive potentials [24], this is a promising result in terms
of a possible increase in the durability of porous carbon electrodes to
positive potentials.
Figure 4.4. 3rd cyclic voltammograms for KUA and KUA-N electrodes in the potential
range (a) -0.5-0.6V and (b) 0-1.0V. 1M H2SO4. v=2 mV/s.
-3
-2
-1
0
1
2
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
j (A
/g)
E (V vs Ag/AgCl/KCl)
KUA
KUA-N
(a)
-1.5
-0.5
0.5
1.5
2.5
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j (A
/g)
E (V vs Ag/AgCl/KCl)
KUA
KUA-N
(b)
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
199
4.3.2.2. Characterization of symmetric supercapacitors
In order to study in detail the effect of surface chemistry on the
performance of these materials from an application point of view,
symmetric supercapacitors for the samples KUA, KUA-N and KUA-
CONH2 have been studied.
Figure 4.5a shows the Nyquist plot obtained for all symmetric
capacitors. At very high frequencies (points closer to the Y-axis), the
capacitors behave like a resistance, while at low frequencies, the
imaginary part of the resistance sharply increases and a curve close to a
vertical line is seen, indicative of a pure capacitive behavior [25]. In the
middle frequency domain, the influence of the electrode porosity and
conductivity can be seen. The resistance of the cell can also be obtained
from the X-axis intercept of the straight line observed at low frequencies
of the diagram (the region where capacitive behavior of the capacitors is
observed [26]) for all cases. The cell resistance is contributed by the
resistance of charge transfer through the carbon particles and grain-
boundaries at the electrode-electrolyte surface (represented by a larger
diameter of the semicircle with increasing the resistance [27]) and by
diffusive problems of ions within the pore system (indicated by the
appearance of a 45º impedance line, also known as the Warburg region,
after the end of the semicircle and prior to the formation of the vertical
line characteristic of capacitive behavior [25]). As can be seen, KUA-
CONH2 evidences the largest resistance (~3.8 Ω). This can be explained
by the presence of a large amount of carboxylic acids and their cyclic
species (Table 4.1), produced prior to the generation of nitrogen
Chapter 4
200
functional groups, that have an electron-withdrawing character and
produce an increase of the resistance of the carbon material. The clear
Warburg region observed for this sample is a consequence of the
changes in porosity and the increased concentration in surface oxygen
groups that hinder the mobility of ions within this pore network. KUA-N
shows an even lower internal resistance than KUA. Since both samples
have similar porous texture and close surface oxygen concentration
(Table 4.1) this difference cannot be explained by diffusional problems
along the porosity. Then, the improvement found for KUA-N should be
related to the generation of electron-donor nitrogen functionalities [8] or
to the decrease of detrimental electron-withdrawing oxygen groups
improving the conductivity of the material. This positive effect of the
conversion of oxygen groups into nitrogen functionalities is not
observed in KUA-CONH2 due to the previous generation of a large
amount of carboxylic acids during the oxidation treatment with nitric
acid.
Table 4.3. Gravimetric capacitance (Cg), energy density (E), maximum power density
(P) and energy efficiency determined for KUA, KUA-CONH2 and KUA-N symmetric
supercapacitors at the voltage 1.2V by galvanostatic charge-discharge cycles. 1M
H2SO4. j= 1A/g.
Sample Cg
(F/g)
E
(Wh/kg)
Pmax
(kW/kg)
Energy
efficiency (%)
KUA 63 10.3 43.3 74
KUA-N 59 10.5 72.4 79
KUA-CONH2 53 8.7 20.0 75
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
201
Figure 4.5. (a) Nyquist plot for the symmetric supercapacitors KUA (black), KUA-N
(blue) and KUA-CONH2 (red). (b) Ragone plot at 1.2V for all symmetric
supercapacitors. Galvanostatic charge-discharge cycles for the samples at (c) 1A/g and
(d) 16 A/g. 1 M H2SO4 solution.
These differences on the resistance of the carbon material play a
major role in their behavior as electrodes for supercapacitors, since the
performance of supercapacitors at high power density depends on the
rate of charge and discharge, which is faster when the overall resistance
is lower. This explains the best performance observed for KUA-N in the
Ragone plot (measured using GCD experiments at 1.2V under different
specific currents) shown in Figure 4.5b. This supercapacitor keeps larger
energy density at high power density due to its lower resistance, and
provides a maximum power density of 72.4 kW/kg (Table 4.3). As
0
1
2
3
4
5
0 1 2 3 4 5
-Z''
(Ω
)
Z' (Ω)
(a)
1
10
0.1 1 10 100
E (
Wh
/Kg
)
P (kW/Kg)
(b)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 50 100 150
Volt
age (
V)
Time (s)
(c)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 2 4 6
Volt
age (
V)
Time (s)
(d)
Chapter 4
202
expected, KUA shows similar energy density and capacitance at low
power density (10.3 Wh/kg for KUA and 10.5 Wh/kg for KUA-N at 0.5
kW/kg, and gravimetric capacitances of 63 F/g vs 59 F/g, respectively)
but is not able to keep these values when power demand is high due to
its larger internal resistance. This is confirmed in Figure 4.5c and Figure
4.5d, where galvanostatic charge-discharge profiles are shown at
different current densities. As can be observed in Figure 4.5c, at 1 A/g,
KUA and KUA-N based symmetric capacitors show similar capacitance
(Table 4.3) (which is derived from the discharge time of the profiles)
due to its similar apparent surface area as was pointed out in section
4.3.2.1. Although KUA shows a slightly higher capacitance, KUA-N
shows better performance as supercapacitor due to the lower presence of
electroactive species, which make more efficient the charge-discharge
process. This can be noticed in the shape of the profile, which is closer
to the ideal triangular shape in the case of KUA-N, and also in the
energy efficiency (i.e. the ratio between the energy employed in
charging the cell and the energy recovered upon discharge of the cell) of
the KUA-N capacitor (Table 4.3). Also, at high current density (16 A/g),
Figure 4.5d, the best performance of KUA-N is pointed out by the lower
ohmic drop produced during the transition of charge to discharge in the
GCD profile, which provides a larger capacitance retention at high
current. KUA-CONH2 based capacitor evidences lower capacitance and
energy density than the other supercapacitors as consequence of its
lower porosity. Also, its maximum power density is the lowest (Table
4.3) due to the detrimental effect of CO2-evolving groups on the
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
203
resistance of the carbon material (Figure 4.1a) that produces a larger
ohmic drop and, consequently, a loss of power density.
The effect of nitrogen functionalities upon the durability of these
capacitors has been assessed by carrying out 5000 cycles of
galvanostatic charge-discharge at 1 A/g and 1.2V of cut-off voltage.
Figure 4.6 shows the evolution of gravimetric capacitance along the
whole experiment. All the capacitor cells behaved adequately upon the
test, showing a capacitance retention higher than 92%. The better
electrochemical stability of a nitrogen-containing activated carbon
obtained by carbonization of PANI-functionalized KUA in organic
media has been recently demonstrated [6], and seems to be also found in
aqueous electrolyte. Given the possibility of achieving a larger stability
in these materials, asymmetric in mass capacitors have been constructed
from these carbon materials.
Figure 4.6. Cyclability test for KUA, KUA-N and KUA-CONH2 supercapacitors at
1.2V. 1A/g. 5000 cycles. 1 M H2SO4 solution.
0
10
20
30
40
50
60
70
80
90
100
0 1000 2000 3000 4000 5000
Cg (
F/g
)
Nº Cycles
KUA
KUA-N
KUA-CONH₂
Chapter 4
204
4.3.2.3. Characterization of asymmetric supercapacitors
Asymmetric in mass supercapacitors, where the cells are set up with
the same carbon material as positive and negative electrode, but with
different weights, have been obtained using the method proposed by
Peng et al. [17]. Briefly, this method consists in (i) determining the
working potential windows (∆𝑽+ and ∆𝑽−) of electrochemical stability
for the positive and negative electrodes from the open circuit potential
(EOCP), (ii) determine the capacitances (Cg+ and Cg-) in the established
potential ranges, (iii) determine the optimum weight ratio (𝒘+ 𝒘−⁄ ) of
the electrodes by equaling the charge stored in each electrode while
fixing the values of the potential windows to those determined in (i). The
expression used in the last step reads as follows:
𝒘+ 𝒘−⁄ = 𝑪𝒈−· |∆𝑽−| (𝑪𝒈+
· ∆𝑽+)⁄ (4.3)
Then, by using the capacitance of each electrode in Eqn. (4.3), the
mass ratio for the asymmetric supercapacitors was calculated to be 1.31,
1.12 and 1.02 for KUA, KUA-N and KUA-CONH2, respectively. Table
4.4 shows the capacitances and potential window values, the weight of
the electrodes employed for the construction of each capacitor cell and
the open circuit potential of each carbon material. It is important to note
that the sum of the potential windows of each electrode corresponds to
the working voltage of the capacitor, and that the three cells will be able
to operate up to 1.4 V. Such a high voltage is expected to increase the
energy density of the capacitors, but will also expose the capacitor to
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
205
harsh conditions, where the possible influence of nitrogen functionalities
could be observed.
Table 4.4. Parameters employed for the design of asymmetric capacitors, and open
circuit potential for each carbon material. 1M H2SO4.
Supercapacitor 𝑪𝒈+
(F/g)
𝑪𝒈−
(F/g)
∆𝑽+
(V)
∆𝑽−
(V)
𝒘+
(mg)
𝒘−
(mg)
EOCP
(V)
KUA 263 298 0.65 0.75 1.30 1.05 0.25
KUA-N 252 283 0.70 0.70 1.22 1.08 0.20
KUA-CONH2 213 199 0.67 0.73 1.18 1.14 0.23
Figure 4.7a shows galvanostatic charge-discharge cycles obtained
for the asymmetric supercapacitors at 1 A/g. In this case, the capacitance
obtained for all asymmetric supercapacitors follows the same order as
for the symmetric capacitors: KUA > KUA-N>> KUA-CONH2.
However, KUA-N based capacitor provides the lower ohmic drop due to
the lower electrical resistance of this carbon material, which provides the
highest power density. Again, a quasi-triangular shape profile is
observed in the case of KUA-N based capacitor. Figure 4.7.b shows the
Ragone plot obtained for all asymmetric supercapacitors working at
1.4V. As in the case of symmetric devices (section 3.2.2), KUA and
KUA-N based capacitors evidence largest energy density than KUA-
CONH2 (Table 4.5) due to their larger apparent surface area. The huge
potential of the asymmetric in mass design for enhancing energy storage
can be observed when the energy densities achieved in this configuration
and those obtained in the symmetric design are compared (Table 4.3). It
is important to note that KUA-N based capacitor shows a slightly higher
energy density than KUA in spite of having lower capacitance, what
Chapter 4
206
indicates again the better performance of this material as electrode for
supercapacitor. In addition, KUA-N based asymmetric capacitor is able
to maintain the superior power density that has already shown in
symmetric configuration.
Figure 4.7. (a) Galvanostatic charge-discharge cycles for KUA, KUA-N and KUA-
CONH2 asymmetric supercapacitors at 1.4V and 1 A/g. (b) Ragone plot at 1.4V for all
asymmetric supercapacitors.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
0 50 100 150 200 250
Vo
lta
ge (
V)
Time (s)
KUA
KUA-N
KUA-CONH₂
(a)
1
10
100
0.1 1 10
E (
Wh
/Kg
)
P (KW/Kg)
KUA
KUA-N
KUA-CONH₂
(b)
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
207
Table 4.5. Gravimetric capacitance (Cg), energy density (E), power density (P),
coulombic efficiency and energy efficiency determined for KUA, KUA-CONH2 and
KUA-N asymmetric supercapacitors at the voltage 1.4V by galvanostatic charge-
discharge cycles. 1M H2SO4. j=1A/g.
Sample Cg
(F/g)
E
(Wh/Kg)
Pmax
(kW/kg)
Coulombic
efficiency
(%)
Energy
efficiency
(%)
KUA 72 14.1 52.5 90 55
KUA-N 67 14.5 61.2 99 66
KUA-CONH2 41 8.3 31.6 98 62
The most remarkable result is the improvement in coulombic and
energy efficiency observed in the case of those supercapacitors
constructed with nitrogen-functionalized activated carbon as electrodes
(KUA-N and KUA-CONH2), Table 4.5. Coulombic efficiency is under
the allowable limit in the case of KUA, and points out that either
massive electrolyte decomposition or carbon degradation is occurring in
this capacitor. On the other hand, the coulombic efficiency is close to
100% in the case of the N-containing carbon capacitors. This fact clearly
evidences that the presence on the surface of nitrogen functional groups
can avoid the irreversible faradaic reactions that occur at the surface of
carbon materials in the positive electrode and that are connected to the
malfunctioning of aqueous capacitor cells. More striking results can be
seen on the durability test conducted over these capacitors, Figure 4.8a.
The profile of capacitance for KUA-N and KUA-CONH2 based
capacitors is similar during the GCD test at 1 A/g for 5000 cycles, and
when compared to that of the corresponding symmetric capacitors
(Figure 4.6), a slightly higher capacitance drop can be seen in this case,
providing a capacitance retention of 83 and 85%, respectively. This is an
Chapter 4
208
expected result that can be explained by the higher cut-off voltage of
these capacitors, which forces both electrodes to work closer to its
electrochemical stability limits. On the other hand, the performance of
the cell assembled using KUA shows some oscillations during the
duration of the test, rendering unreliable the KUA asymmetric capacitor.
This behavior is probably connected to the evolution of gases in this cell
during the test. The absence of such problems in the capacitors
constructed using N-containing carbons undoubtedly speaks about the
impact of nitrogen groups upon the electrochemical stability of activated
carbons in aqueous media. However, after around 3000 cycles, the
capacitance stabilizes.
Ragone plot of asymmetric capacitors was measured again after
durability test, Figure 4.8b. These results evidence that KUA-N and
KUA-CONH2 based asymmetric supercapacitors keep most of their
original performance, since they provide high energy densities at high
power density. However, KUA based asymmetric capacitor shows an
overall decrease of energy density, which is especially large at high
power density. It should be noted that, during the last 2000 cycles of
durability test, the gravimetric capacitance determined for KUA is
slightly larger than that of KUA-N. However, KUA-N provides higher
energy density (Figure 4.8b) evidencing again the importance of the
energy efficiency achieved during the charge-discharge processes (Table
4.6). The generation of detrimental functional groups on KUA during
cycles along with irreversible reactions occurring on the electrodes in
this supercapacitor decreases coulombic and energy efficiencies,
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
209
explaining why KUA has a lower energy density than KUA-N even
when its gravimetric capacitance is higher. Also, a decrease of maximum
power of 62% is attained for KUA capacitor, which is a much severe
drop than those obtained for KUA-N and KUA-CONH2 based
asymmetric capacitors (37 and 14%, respectively, Tables 4.5 and 4.6).
This loss of power density confirms the improved stability of nitrogen-
containing activated carbons as electrodes for asymmetric
supercapacitors in aqueous electrolyte.
Figure 4.8. (a) Durability test for KUA, KUA-N and KUA-CONH2 based asymmetric
capacitors at 1.4V, specific current of 1A/g and 5000 cycles. (b) Ragone plot obtained
after durability test for KUA, KUA-N and KUA-CONH2 based supercapacitors.
0
20
40
60
80
100
0 1000 2000 3000 4000 5000
Cg
(F
/g)
Nº Cycles
KUA
KUA-N
KUA-CONH₂
(a)
1
10
100
0.1 1 10
E (
Wh
/kg
)
P (kW/kg)
KUAKUA-N
KUA-CONH₂
(b)
Chapter 4
210
Table 4.6. Energy density (E), power density (P), and energy efficiency determined for
KUA, KUA-CONH2 and KUA-N asymmetric supercapacitors after durability test at
the voltage 1.4V by galvanostatic charge-discharge cycles. 1M H2SO4. j=4A/g.
Sample Cg
(F/g)
E
(Wh/k
g)
Pmax
(kW/kg
)
Coulombic
efficiency
(%)
Energy
efficiency
(%)
KUA 32.2 5.0 19.7 98 45
KUA-N 44.3 8.4 38.9 99 60
KUA-CONH2 23.9 3.9 27.1 98 49
4.4. Conclusions
Chemical functionalization of an activated carbon with a high
surface area was carried out by following two different pathways: (i)
amidation of an activated carbon previously oxidized (KUA-CONH2)
and (ii) direct treatment of the pristine activated carbon with the same
nitrogen reagents (KUA-N). Both treatments lead to a similar content of
nitrogen species (~4 at. % XPS). The amidation treatment for generation
of KUA-CONH2 produces a decrease of 30% of microporosity due to
the detrimental effect of the oxidation step. In case of KUA-N, this step
is not carried out and consequently the obtained carbon material
preserves 100% of the porous texture of the pristine carbon material,
making this single-step treatment more appropriate for functionalization
of highly microporous carbon materials, which is applied at mild
conditions.
The surface chemistry and porous texture of these carbon materials
clearly influence the electrochemical performance of these carbon
materials. KUA-CONH2 showed the lower capacitance due to its
decrease in porosity. In addition, KUA-N evidences a slight decrease of
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
211
capacitance due to the removal of electroactive oxygen groups. On the
other hand, impedance spectroscopy showed that the presence of
nitrogen functionalities in KUA-N promoted the conductivity of the
electrode, facilitate ion diffusion and electrolyte polarization. The effect
of these changes on the electrochemical performance of these carbon
materials was thoroughly analyzed by studying their performance as
electrodes of symmetric and asymmetric supercapacitors. KUA-CONH2
based capacitors showed the lower energy density along with a lower
power density as consequence of the presence of a larger amount of
CO2-evolving groups, which strongly affect the conductivity of the
carbon material and the whole device. However, KUA-N based
capacitors showed similar energy density, better capacitance retention
and higher power density than the pristine activated carbon as
consequence of the beneficial effect of the generation of electron-donor
nitrogen groups and the removal of detrimental electroactive oxygen
functionalities, producing a supercapacitor with higher energy and
coulombic efficiency. Also, both nitrogen-functionalized activated
carbons revealed higher stability than the pristine activated carbon,
which is not able to safely operate at 1.4V voltage in aqueous medium.
Since no changes have been made in the textural properties of the parent
carbon, the improvements found for KUA-N based capacitors are
undoubtedly related to the presence of stable nitrogen functionalities in
the activated carbon.
Chapter 4
212
4.5. References
[1] P. Simon, Y. Gogotsi. Materials for electrochemical capacitors,
Nat. Mater. 7 (2008) 845–854.
[2] F. Béguin, E. Frackowiak, editors. Carbons for Electrochemical
Energy Storage and Conversion Systems. 1st ed. CRC Press;
2009.
[3] T.J. Bandosz, C.O. Ania. Surface chemistry of activated carbons
and its characterization. In: T.J. Bandosz, editor. Act. carbon
surfaces Environ. Remediat. 1st ed., Elsevier; 2006, p. 159–229.
[4] M.J. Bleda-Martínez, J.A. Maciá-Agulló, D. Lozano-Castelló, E.
Morallón, D. Cazorla-Amorós, A. Linares-Solano. Role of surface
chemistry on electric double layer capacitance of carbon
materials, Carbon 43 (2005) 2677–2684.
[5] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H.
Zhu, G.Q. Lu. Nitrogen-Enriched Nonporous Carbon Electrodes
with Extraordinary Supercapacitance, Adv. Funct. Mater. 19
(2009) 1800–9.
[6] D. Salinas-Torres, S. Shiraish, E. Morallón, D. Cazorla-Amorós.
Improvement of carbon materials performance by nitrogen
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions, Carbon 82 (2015) 205–213.
[7] C.-T. Hsieh, H. Teng, W.-Y. Chen, Y.-S. Cheng. Synthesis,
characterization, and electrochemical capacitance of amino-
functionalized carbon nanotube/carbon paper electrodes. Carbon
48 (2010) 4219–4229.
[8] O. Ornelas, J.M. Sieben, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, J. Geng, et al. On the origin of the high capacitance of
nitrogen-containing carbon nanotubes in acidic and alkaline
electrolytes, Chem. Commun. 50 (2014) 11343–11346.
[9] W. Shen, W. Fan. Nitrogen-containing porous carbons: synthesis
and application, J Mater. Chem. A 1 (2013) 999-1013.
Electrochemical performance of N-doped activated carbons in aqueous electrolyte
213
[10] H. Tamai, K. Shiraki, T. Shiono, H. Yasuda. Surface
functionalization of mesoporous and microporous activated
carbons by immobilization of diamine. J Colloid Interface Sci.
292 (2006) 299–302.
[11] M.J. Mostazo-López, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós. Generation of nitrogen functionalities on activated
carbons by amidation reactions and Hofmann rearrangement:
Chemical and electrochemical characterization. Carbon 91 (2015)
252–65.
[12] A. Gromov, S. Dittmer, J. Svensson, O.A. Nerushev, S.A. Perez-
García, L. Licea-Jiménez, et al. Covalent amino-functionalisation
of single-wall carbon nanotubes. J Mater. Chem. 15 (2005) 3334–
3339.
[13] D. Lozano-Castelló, M.A. Lillo-Ródenas, D. Cazorla-Amorós, A.
Linares-Solano. Preparation of activated carbons from Spanish
anthracite, Carbon 39 (2001) 741-749.
[14] M.J. Bleda-Martínez, D. Lozano-Castelló, E. Morallón, D.
Cazorla-Amorós, A. Linares-Solano. Chemical and
electrochemical characterization of porous carbon materials,
Carbon 44 (2006) 2642–2651.
[15] D. Cazorla-Amorós, J. Alcañiz-Monge, M.A. De La Casa-Lillo
A.Linares-Solano. CO2 as an adsorptive to characterize carbon
molecular sieves and activated carbons. Langmuir 14 (1998)
4589–4596.
[16] J. Jagiello, J.P. Olivier. 2D-NLDFT adsorption models for carbon
slit-shaped pores with surface energetical heterogeneity and
geometrical corrugation. Carbon 55 (2013) 70–80.
[17] C. Peng, S. Zhang, X. Zhou, G.Z. Chen. Unequalisation of
electrode capacitances for enhanced energy capacity in
asymmetrical supercapacitors. Energy Environ. Sci. 3 (2010)
1499-1502.
[18] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano.
The role of different nitrogen functional groups on the removal of
Chapter 4
214
SO2 from flue gases by N-doped activated carbon powders and
fibres. Carbon 41 (2003) 1925–1932.
[19] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, J.
Find, U. Wild, R. Schlögl. Structural characterization of N-
containing activated carbon fibers prepared from a low softening
point petroleum pitch and a melamine resin, Carbon 40 (2002)
597–608.
[20] R.J.J. Jansen, H. van Bekkum. XPS of nitrogen-containing
functional groups on activated carbon, Carbon 33 (1995) 1021–
1027.
[21] Y. Yamada, J. Kim, S. Matsuo, S. Sato. Nitrogen-containing
graphene analyzed by X-ray photoelectron spectroscopy, Carbon
70 (2014) 59–74.
[22] F. Kapteijn, J.A. Moulijn, S. Matzner, H.-P. Boehm. The
development of nitrogen functionality in model chars during
gasification in CO2 and O2, Carbon 37 (1999) 1143–1150.
[23] R. Berenguer, R. Ruiz-Rosas, A. Gallardo, D. Cazorla-Amorós, E.
Morallón, H. Nishihara, et al. Enhanced electro-oxidation
resistance of carbon electrodes induced by phosphorus surface
groups, Carbon 95 (2015) 681–689.
[24] P. Su-Il, L. Eung-Jo, K. Tae-Young, L. Seo-Jae, R. Young-
Gyoon, K. Chang-Soo. Role of surface oxides in corrosion of
carbon black in phosphoric acid solution at elevated temperature,
Carbon 32 (1994) 155–9.
[25] R. Kötz, M. Carlen. Principles and applications of electrochemical
capacitors. Electrochim. Acta 45 (2000) 2483–98.
[26] B.E. Conway. Electrochemical supercapacitors: Scientific
Fundamentals and Technological Applications. New York:
Springer; 1999.
[27] S. Fletcher, V.J. Black, I. Kirkpatrick. A universal equivalent
circuit for carbon-based supercapacitors. J. Solid State
Electrochem 18 (2014) 1377–1387.
Chapter 5
Electrochemical performance
of N-doped zeolite templated
carbon in aqueous electrolyte
Electrochemical performance of N-ZTC in aqueous electrolyte
217
5.1. Introduction
Supercapacitors are one of the most relevant energy storage devices
due to their extraordinary power density, which is much higher than that
provided by other systems, such as batteries and fuel cells. These
devices can be composed by different electrode materials (carbon
materials, conducting polymers and metal oxides) and electrolytes
(aqueous, organic and ionic liquids). However, the development of the
greener, safer and cheaper aqueous-based supercapacitors is severely
limited by their low operation voltage. In consequence, their energy
density is lower than that showed by those supercapacitors based on
organic or ionic liquid based electrolytes [1-3].
Most of the strategies for overcoming this limitation are focused on
the development of electrode materials with high electrochemical
stability and capacitance. Among all the possibilities, the development
of ultraporous carbon materials with well-developed microporosity and
ordered structure is highly desirable. In this sense, zeolite-templated
carbons (ZTC) evidence a unique 3D-ordered microporous structure that
shows extremely profitable properties for this application [4,5]. First,
they have an outstanding apparent surface area and an ordered structure
with uniform pore size distribution (1.2 nm) that maximizes the
formation of the electrical double layer and improves the mobility of
ions within the porosity, making them especially interesting as electrode
materials for supercapacitors. Moreover, a large amount of highly
reactive edge sites are present in their structure, which eases the
incorporation of electroactive oxygen functionalities that produce an
Chapter 5
218
extraordinary pseudocapacitance boost in acid media [6]. However,
these materials can be easily overoxidized, leading to the degradation of
the structure and the loss of their unique properties.
The electrochemical performance of carbon materials can be tuned
by the introduction of heteroatoms (such as O, N, P or B) [7-16].
Concretely, nitrogen functional groups have been found to modify the
electrochemical stability, electrical conductivity, the wettability and
pseudocapacitance of carbon materials [8, 17-20]. Nevertheless, the role
of the different N functionalities still is not fully understood. Nitrogen-
doped carbons can be synthesized following two different strategies: (i)
by using a nitrogen-containing precursor as source or (ii) through post-
treatments of a carbon material with nitrogen reagents [21,22]. However,
these approaches usually require the use of high temperatures that would
damage the fragile porous network of ZTCs. Hence, a plausible strategy
for obtaining N-doped ZTC while preserving its unique structure would
rely on the use of a nitrogen-containing gas as a raw precursor.
Following this premise, Kyotani et al [7, 23, 24] synthesized N-ZTC by
using acetonitrile as CVD precursor. By careful combination of the CVD
conditions and the template, they were able to obtain a nitrogen-rich
microporous ZTC with a similar structure of non-doped ZTC, while
showing a larger microporosity than that found for other N-doped ZTCs
in the literature [25,26]. The advantageous ordered structure of the
resulting material was profited for achieving a better understanding of
the effect of N-doping in the electrochemical performance of carbon
materials in organic electrolyte [27], allowing to use them as model
Electrochemical performance of N-ZTC in aqueous electrolyte
219
materials for understanding the effect of surface chemistry in
electrochemical properties and supercapacitor performance. However,
the effect of nitrogen doping on the electrochemical performance of
highly microporous N-doped ZTC in aqueous electrolyte and in
supercapacitors application, has not been assessed before in the
literature.
In this chapter, the differences on the performance of N-doped and
non-doped zeolite templated carbons through physicochemical and
electrochemical characterization are shown. The effect of nitrogen
functional groups on the electrochemical behavior of these carbons in
two different electrolytes (1 M H2SO4 and 0.5 M KOH) is thoroughly
analyzed by different techniques. Also, their performance as electrodes
for supercapacitors is studied and related to the functional groups formed
on each zeolite template carbon.
5.2. Materials and methods
5.2.1. Zeolite templated carbons
Zeolite templated carbons were synthesized by chemical vapor
deposition of different precursors and using zeolite Y as a template (Na-
form, SiO2/Al2O3 = 5.6, obtained from Tosoh Co. Ltd.) by following the
method reported elsewhere [6, 28, 29]. As a result, two different ZTCs
were obtained: non-doped zeolite templated carbon (ZTC) and N-doped
zeolite templated carbon (N-ZTC) [23].
Chapter 5
220
5.2.2 Physicochemical characterization
The structure of the samples was analyzed by X-ray diffraction
(XRD) recorded on Shimadzu XRD-6100 instrument with Cu-K
radiation. The porous texture was characterized by N2 physisorption
technique carried out at -196 °C, by using a volumetric sorption analyzer
(BEL Japan, BELSORP-max). The apparent specific surface area was
calculated by the Brunauer–Emmett–Teller method (SBET) using the N2
adsorption data in the relative pressure (P/P0) range of 0.01–0.05. The
total micropore volume was calculated from the Dubinin–Radushkevich
equation (VDRN2).
The surface chemistry of the samples was analyzed by X Ray
Photoelectron Spectroscopy (XPS) and Temperature Programmed
Desorption (TPD). XPS analyses were performed by using a VG-
Microtech Multilab 3000 spectrometer with an Al anode. N1s spectra
were deconvoluted by using Gaussian functions with 20% of Lorentzian
component. A Shirley line was used as background and the FWHM of
the peaks was kept between 1.4 and 1.7 eV. TPD experiments were
carried out by using a TGA-DSC instrument (TA Instruments, SDT
Q600 Simultaneous) coupled to a mass spectrometer (Thermostar,
Balzers, BSC 200), by heating the samples (10 mg) up to 950 ºC
(heating rate: 20 ºC/min) under helium atmosphere (flow rate: 100
mL/min).
Electrochemical performance of N-ZTC in aqueous electrolyte
221
5.2.3 Electrochemical characterization
5.2.3.1 Three electrode cell configuration
Carbon electrodes for electrochemical characterization were
prepared by mixing the carbon material with acetylene black and
polytetrafluoroethylene (PTFE) as binder in a ratio of 90:5:5 (w/w). The
total weight of the electrode was 9 mg (dry basis). For shaping the
electrodes, a sample sheet was cut into a circular shape with an area of
1.2 cm2 and pressed for 5 min at 2 tons to guarantee a homogeneous
thickness. After that, the electrode was attached to a gold current
collector by means of conducting adhesive (colloidal graphite
suspension, Hitasol GA-715, Hitachi Chemical Co., Ltd.). The
electrodes were soaked for 2 days into the electrolyte (1M H2SO4 or
0.5M KOH) previously to electrochemical measurements.
The electrochemical characterization of the electrodes was
performed by cyclic voltammetry in a Biologic VSP multichannel
potentiostat and using a T-type Swagelock cell in a three-electrode
configuration. 1M H2SO4 was used as aqueous electrolyte. A capacitive
electrode with more than twice the capacitance (electrode weight per
gravimetric capacitance) of the working electrode was prepared from a
commercial activated carbon and used as counter electrode. Both
electrodes were tightly pressed against each other and separated by a
nylon membrane (Teknokroma membrane filters, pore size: 450 nm).
Ag/AgCl/KCl (3M) was used as reference electrode in all cases. The
electrochemical characterization of all samples was studied by cyclic
Chapter 5
222
voltammetry (CV) at sweep rate of 2 mV/s. CV capacitance was
calculated from the area of the voltammogram. The results are expressed
in F/g, taking into account the weight of the active material of the
working electrode. The materials were also characterized by
Electrochemical Impedance Spectroscopy (EIS). Impedance spectra
were measured at 0.3 V vs Ag/AgCl/KCl in the frequency range of 10
mHz to 100 kHz with an amplitude voltage of 10 mV.
5.2.3.2 Two electrode cell configuration
Symmetric capacitors (in mass) were assembled for both carbon
materials. The electrodes were prepared by using the method described
above (section 5.2.3.1) with a weight of ~1.3 mg (active phase) per
electrode and a geometrical area of 0.196 cm2 (thickness: 0.2 mm). The
electrodes were attached to a stainless-steel collector following the
previously mentioned protocol and using the same separator as described
in section 5.2.3.1. These devices were characterized by CV at different
scan rates, galvanostatic charge-discharge (GCD) cycles at current
densities from 1 to 20 A/g and EIS in 1M H2SO4 solution. Impedance
spectra were measured at 0.05 V in the frequency range of 10 mHz to
100 kHz with an amplitude voltage of 10 mV. The measurements were
repeated in freshly prepared cells for their verification. Autolab
PGSTAT302 potentiostat was employed for EIS and CV measurements
and Arbin SCTS potentiostat for galvanostatic charge-discharge cycles.
A durability test was performed by 50000 GCD cycles at a current
density of 5 A/g and a voltage of 1.2 V. Current density and specific
capacitance is defined based on the total active weight of the carbon
Electrochemical performance of N-ZTC in aqueous electrolyte
223
material in the cell (two electrodes). The energy density and power
density were calculated as described elsewhere [17].
5.3 Results and discussion
5.3.1 Physicochemical characterization
Table 5.1 summarizes the porous texture and chemical composition
of the samples. Although both samples are obtained by following the
same procedure and the replica of the zeolite is successfully obtained
providing an ordered microporous structure (see XRD patterns, Figure
A5.1, and N2 adsorption-desorption isotherms, Figure A5.2, in annex of
Chapter 5), N-ZTC shows ca. 70 % of the apparent surface area and
micropore volume of ZTC.
The surface chemistry of both materials was characterized by XPS
and TPD. The amount of nitrogen and oxygen detected using XPS and
the evolved CO and CO2 quantities during TPD are compiled in Table
5.1. N-ZTC shows nitrogen content of 3.7 at. % detected by XPS.
According to the N1s XPS spectrum (Figure 5.1.b), this nitrogen is
attached to the surface as the following functional groups: quaternary
nitrogen (401.2 ± 0.2), pyrrole/pyridone (400.5 ± 0.2) and pyridines
(398.5 ± 0.2). The percentage of each kind of functional group is 63 %,
17% and 20%, respectively. Hence, quaternary nitrogen is the most
abundant nitrogen functionalities.
TPD experiments are used to study in detail the oxygen
functionalities that exist on the surface of both carbons [30-33]. Both
materials have a large amount of oxygen functional groups, ca. 3.5
Chapter 5
224
mmol/g or 5.5% wt of oxygen. More specifically, N-ZTC presents a
larger amount of CO2-evolving groups in the 200-400 ºC temperature
range (Figure A5.3 in annex of Chapter 5), which points out the presence
of more carboxylic moieties in the nitrogen-doped sample. Interestingly,
both samples show a similar amount of CO-evolving groups, but the
TPD profile for the evolution of CO, Figure 1a, evidences that the
amount of phenolic and ether groups (related to the CO evolution at 600-
700 ºC) is predominant in ZTC, whereas evolution of CO at higher
temperatures is greater in N-ZTC. Since the CO evolution is still high at
the end of the maximum temperature used in the TPD run, it can be
considered that the total amount of oxygen functionalities is larger in
this sample. Hence, surface oxygen groups of high thermal stability
(such as carbonyls and/or even pyrenes) are found on its surface. In
accordance to these findings, the surface oxygen functionalities present
in ZTC has been previously studied by TPD and Fourier transform
infrared spectroscopy and they have been assigned as: acid anhydride
(15%), ether (66%), hydroxyl (15%), and carbonyl (4%), whereas N-
ZTC presents a lower amount of ethers and hydroxyl functional groups
along with a larger amount of carbonyls [7, 34].
Table 5.1. Textural properties and surface chemical composition (XPS and TPD)
obtained for zeolite templated carbons.
Sample SBET
(m2/g)
VDRN2
(cm3/g)
NXPS
(at.%)
CO2 TPD
(µmol/g)
CO TPD
(µmol/g)
O TPD
(µmol/g)
ZTC 3600 1.55 - 470 2330 3280
N-ZTC 2760 1.05 3.7 510 2730 3750
Electrochemical performance of N-ZTC in aqueous electrolyte
225
Figure 5.1. (a) Comparison between CO TPD profiles of ZTC (black line) and N-ZTC
(blue line) samples. (b) N1s XPS deconvolution of N-ZTC sample.
5.3.2 Electrochemical characterization.
5.3.2.1. Electrochemical characterization before electro-oxidation
The carbon materials were characterized in a three-electrode cell in
acid and alkaline media in order to assess their electrochemical behavior.
It should be noted that ZTC can be easily oxidized at low potentials in
1M H2SO4, producing a very large amount of quinone functional groups
that are responsible of a large boost in pseudocapacitance [6,35]. In
order to avoid such feature, Figure 5.2.a shows the CVs obtained for
ZTC and N-ZTC in a potential range where no important oxidation
occurs and the performance of both pristine carbon materials can be
analyzed. The CVs obtained for both carbons have a broad oxidation
peak during the positive sweep that reaches a maximum at 0.26 V for N-
ZTC and at 0.32 V for ZTC. At the negative sweep, the corresponding
reduction peaks at potentials close to 0.16 and 0.23V are observed
respectively for N-ZTC and ZTC. The redox processes observed in the
ZTC electrode are undoubtedly related to electroactive functional groups
0
0.5
1
1.5
2
2.5
3
0 200 400 600 800
CO
(µ
mo
l/g
s)
Temperature (ºC)
(a)
396398400402404
I (a
.u.)
Binding Energy (eV)
(b)
Chapter 5
226
that, according to previous studies, correspond with the pseudocapacitive
contribution of the quinone/hydroquinone redox pair [6]. However, N-
ZTC shows larger pseudocapacitance in a wider potential range,
delivering a capacitance 38 F/g higher than that of ZTC (see Cg values
on Table 5.2). Since gravimetric capacitance is to some extent
proportional to the surface area of porous carbon materials [36,37], this
result must be a consequence of the presence of a higher amount of
electroactive surface oxygen functionalities contained in this material
together with nitrogen groups on N-ZTC.
Figure 5.2. 3rd cyclic voltammograms for ZTC (black curve) and N-ZTC (blue curve)
electrodes in (a) 1M H2SO4 and (b) 0.5 M KOH. v = 2 mV/s.
Table 5.2. Gravimetric and normalized capacitances of the carbon materials.
Sample Cg
[H2SO4] (F/g)
Cg
[KOH] (F/g)
Cg/SBET
[H2SO4] (µF/cm2)
Cg/SBET
[KOH] (µF/cm2)
ZTC 235 232 6.0 5.9
N-ZTC 273 224 9.9 8.1
In order to achieve a better understanding of the origin of the higher
capacitance of N-ZTC, an analogous CV was recorded in basic medium
(Figure 5.2.b) for both materials. In this case, the current of the redox
-1
-0.5
0
0.5
1
-0.3 -0.1 0.1 0.3 0.5 0.7
j (A
/g)
E (V vs Ag/AgCl/KCl)
(a)
-1
-0.5
0
0.5
1
-1.1 -0.9 -0.7 -0.5 -0.3 -0.1
j (A
/g)
E (V vs Ag/AgCl/KCl)
(b)
Electrochemical performance of N-ZTC in aqueous electrolyte
227
peaks previously seen in acid medium is notoriously decreased. Hence,
the pseudocapacitance observed in acid medium is a consequence of
functionalities that are not electroactive in basic medium. In this sense, it
has been reported that pyridine groups can contribute to largest
capacitance in basic media than in acid media via 1 electron process that
is impeded in acid electrolyte due to its protonation in this medium [8,
38]. This is the opposite trend observed for N-ZTC. However, it must be
considered that, since the presence of pyridinic nitrogen in N-ZTC is
low, it should not be expected that they have a high contribution to
capacitance in alkaline electrolyte.
Thus, the highest pseudocapacitive response of pristine N-ZTC in
acid electrolyte should be a consequence of its oxygen functionalities.
As seen in section 5.3.1, N-ZTC has larger amount of carbonyl
functional groups along with other CO-evolving oxygen functionalities
of higher thermal stability (Figure 5.1.a). These functional groups could
take part in redox reactions involving protons [11], explaining why the
pristine N-ZTC shows both a higher capacitance than ZTC in acid
medium and a capacitance decrease in alkaline electrolyte.
Also, it should be noted that the surface capacitance (Cg/SBET in
Table 5.2) is larger for N-ZTC in both media. Since the
pseudocapacitance contribution is mostly neglected in alkaline media,
the difference in surface capacitance in 0.5 M KOH can be related to an
improvement of wettability as consequence of the different surface
chemistry. This improvement can be a consequence of oxygen or
nitrogen functional groups. It is well-known that CO-evolving groups
Chapter 5
228
increase the wettability of carbon materials [11]. Also, the contribution
of N-Q to the wettability of carbons was previously reported [8, 39, 40].
Since both moieties exist on larger amounts on N-ZTC, the improvement
in surface capacitance can be related to an enhanced wettability of this
carbon material. Moreover, the distribution of the functional groups is
expected to be different for N-ZTC and ZTC, with N-Q being found
inside the pore network, increasing the wettability of the whole surface
of N-ZTC, whereas the CO-evolving groups of ZTC are expected to be
preferentially located at edge sites at the entrance of pores, delivering a
lower wetting of the inner porosity.
5.3.2.2 Electrochemical characterization after electro-oxidation
This section reports the performance of zeolite templated carbons
after exposing them to electro-oxidation conditions in acid and alkaline
media. Both carbons were electro-oxidized in 1M H2SO4 by performing
three CVs between -0.2 and 1.0 V at 2 mV/s. Figure 5.3 shows these
CVs for both carbon materials. On the first anodic sweep, both
electrodes show a large irreversible oxidation current when the potential
is shifted to values more positive than 0.6 V. It is well stablished that
this oxidation treatment produces electro-active oxygen functional
groups in zeolite templated carbons [6, 28, 41], and also some
degradation of the structure and electro-gasification [32]. The oxidation
current is larger for the ZTC electrode (see the higher current of the
black CV at 1.0 V in Figure 5.3.a compared to that of Figure 5.3.b),
evidencing that more intensive oxidation occurs in this sample. The net
increase of the currents of the reversible peaks at ca. 0.3V, which are
Electrochemical performance of N-ZTC in aqueous electrolyte
229
related to electroactivity of the quinone/hydroquinone functionalities,
observed after the electro-oxidation treatments corroborates this
phenomenon (see the differences between red -initial- and purple -final-
CVs in Figure 5.3.a).
Figure 5.3. Cyclic voltammograms for the electro-oxidation of ZTC (a) and N-ZTC
(b). v = 2 mV/s. 1M H2SO4.
Figure 5.4. 3rd cyclic voltammograms for ZTC and N-ZTC after electro-oxidation. v =
2 mV/s. 1M H2SO4.
-2
0
2
4
6
-0.3-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j (A
/g)
E (V vs Ag/AgCl/KCl)
ZTC1st cycle3rd cycleZTC ox
(a)
-2
0
2
4
6
-0.3-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j (A
/g)
E (V vs Ag/AgCl/KCl)
N-ZTC1st cycle3rd cycleN-ZTC ox
(b)
-2
-1
0
1
2
-0.3 -0.1 0.1 0.3 0.5 0.7 0.9
j (A
/g)
E (V vs Ag/AgCl/KCl)
ZTC ox
N-ZTC ox
Chapter 5
230
Table 5.3. Gravimetric and equivalent series resistance of the oxidized carbon
materials.
Sample Cgox
[H2SO4] (F/g)
Cgox
[KOH] (F/g)
ESR+EDR
[H2SO4] (Ω)
ZTC 445 246 1.8
N-ZTC 384 230 0.3
N-ZTC also shows the same features (Figure 5.3.b). However, the
observed increase of these redox processes is lower. Consequently, the
capacitance recorded for ZTC after electro-oxidation (Cgox, Table 5.3) is
larger than that found for N-ZTC. It is interesting to note that the
voltammograms after the oxidation show different redox processes for
both carbon materials. Then, in ZTC an increase in the voltammetric
charge is observed in a wide potential window, from -0.1 up to 0.6 V,
while in the case of N-ZTC, the increase in voltammetric charge is only
observed from 0.2 V to 0.7 V. More significant differences upon the
electro-oxidation between these materials are highlighted by overlapping
their CVs (Figure 5.4). Although both CVs show similar features, ZTC
does show a larger pseudocapacitance from 0.05 to 0.4 V. Since the
structure of both materials is essentially identical [23] (see XRD
patterns, Figure S2 in annex of chapter 5), their different performance
under electro-oxidative conditions should be related to their different
surface chemistry. The electro-oxidized electrodes were analyzed by
TPD and it was found that approximately the same amount of oxygen
functionalities (6690 µmol/g for ZTC and 6740 µmol/g for N-ZTC) are
generated in the surface of both materials after the treatment. Itoi et al.
Electrochemical performance of N-ZTC in aqueous electrolyte
231
[6] proposed that the incorporation of oxygen groups on ZTC upon the
electro-oxidation happens because of the large amount of reactive edge
sites that exist on these carbons. Since N-ZTC incorporates the same
amount of oxygen functional groups, it can be inferred that larger
irreversible oxidation currents experimented by ZTC may be connected
to gasification reactions, where certain oxygen functionalities may
evolve as CO and CO2 (and therefore, their presence is not detected by
TPD over the oxidized electrode) during the CV sweep. Moreover, the
electroactivity of certain functional groups showed by electro-oxidized
ZTC seem to be hindered or even impeded in N-ZTC, since both carbons
have developed a similar amount and type of oxygen functionalities.
Figure 5.5. Electrochemical impedance spectra for ZTC and N-ZTC electrodes after
electro-oxidation in 1M H2SO4. E = 0.3V vs Ag/AgCl.
0
1
2
3
4
5
0 1 2 3 4 5
-Z''
(Ω
)
Z' (Ω)
ZTC oxN-ZTC ox
Chapter 5
232
EIS measurements were carried out for the electro-oxidized samples
in order to deepen into the changes produced by the electrochemical
oxidation. This technique allows one to distinguish the different
resistance contributions affecting the electrodes (diffusive problems,
electrode-electrolyte interface, etc.) [42]. Figure 5.5 shows the Nyquist
plots obtained in 1 M H2SO4 for both samples after the electro-oxidation
in the acid electrolyte. The main differences between both profiles are
observed at low frequencies, where an almost vertical line, related to
capacitive behavior, is observed. The onset frequency for reaching
capacitive behavior is higher in the case of N-ZTC. The equivalent series
resistance (ESR) plus the equivalent distributed resistance (EDR) of the
cells was calculated from the x-intercept of this line. The values obtained
for both electrodes are summarized in Table 5.3. It can be observed that
ZTC electrode shows a higher resistance than N-ZTC and a remarkably
Warburg region, related to diffusion processes through the pore network
[43]. In this work, the electrodes and the electrochemical cells were
prepared by following the same procedure and, consequently, the
differences observed are related to the electrode materials. As discussed
before (section 5.3.1), the zeolite templated carbons in this study initially
have a similar ordered structure and the same pore size of 1.2 nm. The
main differences of both carbons are their surface chemistry and the
larger apparent surface area of non-doped ZTC. In this sense, N-ZTC
has an important content in N-functionalities that improve both electrical
conductivity and wettability [20, 44-46], which can explain a decrease in
the electrolyte diffusion resistance.
Electrochemical performance of N-ZTC in aqueous electrolyte
233
The differences in electrochemical stability when exposed to high
oxidation potentials were further studied in alkaline electrolyte. Figure
5.6 shows the cyclic voltammograms obtained for N-ZTC and ZTC in
basic medium under the oxidative conditions. Again, the current density
increases when the potential is shifted to more positive values and an
irreversible faradaic current related to oxidation processes is observed
(black curve, Figure 5.6.a and 5.6.b). As happened in acid medium, the
current density obtained for ZTC is higher than for N-ZTC electrode,
pointing out the higher electrochemical stability of the N-doped carbon.
Figure 5.6. Cyclic voltammograms for the electro-oxidation of ZTC (a) and N-ZTC
(b). v = 2 mV/s. 0.5M KOH.
Furthermore, the CV obtained for N-ZTC remains practically
invariable after positive polarization, while, in case of ZTC, there is a
small increase of capacitance (Table 5.2 and 5.3). This increase of
capacitance can be explained by different phenomena: (i) increase of
wettability and (ii) appearance of pseudocapacitance. In general, carbon
materials have an increase of wettability when oxygen functional groups
(mainly CO-evolving groups) are attached to their surface [47].
Consequently, the improvement of capacitance of ZTC is mainly
-1
-0.5
0
0.5
1
1.5
2
-1.1-0.9-0.7-0.5-0.3-0.1 0.1 0.3
j (A
/g)
E (V vs Ag/AgCl/KCl)
ZTC1st cycle3rd cycleZTC ox
(a)
-1
-0.5
0
0.5
1
1.5
2
-1.1-0.9-0.7-0.5-0.3-0.1 0.1 0.3
j (A
/g)
E (V vs Ag/AgCl/KCl)
ZTC1st cycle3rd cycleZTC ox
(b)
Chapter 5
234
attributed to the generation of oxygen functionalities on this carbon after
electro-oxidation. As expected, this effect is not observed in N-ZTC
electrode owing to the contribution of N-Q functionalities to the
wettability of this material. Regarding pseudocapacitance, quinones are
reported to be electroactive at lower pHs. However, some small
pseudocapacitive behavior is observed in alkaline electrolyte for both N-
ZTC and ZTC electrodes (Figure 5.7). Previous research reported the
evidence of pseudocapacitance in alkaline medium due to the
contribution of oxygen functionalities in carbon cloths when the pH is
above 11 due the previous activation in acid medium of pyrone
derivatives and another unidentified species [48]. Thus, different CO-
evolving functional groups could be responsible of the electroactivity
showed in KOH electrolyte by both carbon materials, although this is an
issue that needs further research.
Figure 5.7. 3rd cyclic voltammograms for ZTC (a) and N-ZTC (b) after electro-
oxidation. v = 2 mV/s. 0.5M KOH.
-1
-0.5
0
0.5
1
-1.1 -0.9 -0.7 -0.5 -0.3 -0.1 0.1
j (A
/g)
E (V vs Ag/AgCl/KCl)
ZTC ox
N-ZTC ox
Electrochemical performance of N-ZTC in aqueous electrolyte
235
5.3.2.3. ZTC and N-ZTC supercapacitors using acid electrolyte
The performance of the materials as electrodes for supercapacitors
was assessed by using a symmetric configuration (in mass) in 1M
H2SO4. Prior to assembling the two-electrode cell of ZTC, both positive
and negative electrodes were electro-oxidized (as described in section
5.3.2.2) in order to stabilize their response and avoid any problem
related with the large irreversible current shown by ZTC during the first
loading cycles [49]. Since the electro-oxidation current shown by N-
ZTC in the CV studies was lower, the stability of the negative electrode
is not affected (i.e. the negative electrode does not reach potentials
where hydrogen evolution occurs) and, consequently, this pre-treatment
was not needed for N-ZTC electrodes.
Figure 5.8. Steady state cyclic voltammograms for ZTC and N-ZTC based
supercapacitors. v = 10 mV/s. V = 1.2 V. 1M H2SO4.
-2
-1.5
-1
-0.5
0
0.5
1
1.5
-0.1 0.2 0.5 0.8 1.1
j (A
/g)
Voltage (V)
ZTCN-ZTC
Chapter 5
236
Figure 5.8 shows cyclic voltammograms obtained for ZTC and N-
ZTC based capacitors operating at 1.2V and Table 5.4 compiles the
gravimetric capacitance obtained from these CVs. The main contribution
to the gravimetric capacitance is given at low voltage, since the
electrodes in the capacitors are working around the open circuit potential
(OCP, 0.34 and 0.41 V for ZTC and N-ZTC electrodes, respectively),
where the pseudocapacitance contribution is maximized. As the voltage
increases, the current decreases sharply, until reaching approximately a
constant value when the voltage is beyond 0.7 V. At these voltages, the
potentials of positive and negative electrodes are expected to be shifted
at least 0.35 V from the OCP (since the voltage of the cell results from
the difference of potential between both electrodes). Thus, they have
reached potentials where these materials mainly have the contribution of
electrical double layer (Figures 5.3 and 5.4), and therefore a much lower
capacitance is obtained from each electrode and, accordingly, in the
supercapacitor.
In the case of N-ZTC supercapacitor, the charge is lower than for
ZTC at voltages close to zero due to the lower pseudocapacitance
contribution on this material (Figure 5.4), while the charge drop of the
supercapacitor at medium and high voltages in N-ZTC is mitigated due
to the higher charge of this material as the potential of the electrodes is
shifted from OCP.
Electrochemical performance of N-ZTC in aqueous electrolyte
237
Table 5.4. Parameters obtained for ZTC and N-ZTC based capacitors in 1M H2SO4.
Cell CCV (F/g)
CGCDa
(F/g)
CGCDb
(F/g)
R (Ω)
E
(Wh/kg)
Pmax
(kW/kg)
Cf/Co
(%)
ZTC 60 61 19 3.50 5.9 23 88
N-ZTC 56 57 46 0.85 7.5 98 91 a Capacitance at 1 A/g. b Capacitance at 20 A/g.
Figure 5.9. GCD cycles for ZTC and N-ZTC based supercapacitors at (a) 1 A/g and (b)
20 A/g. V = 1.2 V. (c) Ragone plot obtained for ZTC and N-ZTC based supercapacitors
at 1, 2.5, 5, 10 and 20 A/g. V = 1.2 V. (d) Electrochemical impedance spectra for ZTC
and N-ZTC based supercapacitors. V = 0.05 V. 1M H2SO4.
Figure 5.9.a shows the galvanostatic charge-discharge (GCD) cycles
obtained for ZTC and N-ZTC based capacitors at 1 A/g. The shape of
the cycles differs from the ideal triangle characteristic of carbon
0
0.5
1
1.5
0 50 100 150
Vo
lta
ge (
V)
Time (s)
ZTCN-ZTC
(a)
0
0.5
1
1.5
0 1 2 3 4 5 6
Vo
lta
ge (
V)
Time (s)
ZTC
N-ZTC
(b)
0.1
1
10
0.1 10
E (
Wh
/kg)
P (kW/kg)
ZTCN-ZTC
(c)
0
1
2
3
4
5
0 1 2 3 4 5
-Z''
(Ω
)
Z' (Ω)
ZTCN-ZTC
(d)
Chapter 5
238
materials with pure contribution of electrical double layer formation due
to the strong influence of pseudocapacitive processes and other
phenomena previously discussed (see section 5.3.2.1). Under these
conditions, both cells evidence similar gravimetric capacitance (Table
5.4). However, the increase of current density up to 20 A/g produces a
decrease in capacitance more accused in the case of ZTC, due to a higher
ohmic drop (Figure 5.9b). This plays an important role on the retention
of energy at high power density, as can be seen in the Ragone plot
(Figure 5.9c). Thus, when energy is considered, N-ZTC based
supercapacitor shows only a small improvement compared to ZTC one
at 1 A/g. However, the difference in energy becomes larger at high
current density, since N-ZTC based capacitor only experiences a
decrease of 28% of the initial energy at 20 A/g, while in case of ZTC
cell, a loss of 78% is observed. These differences on the energy retention
at high power density are usually related to certain properties of the
electrodes, such as electrical conductivity and ion mobility within the
porosity. These properties can be better understood by EIS.
Figure 5.9d shows the Nyquist plot obtained for ZTC and N-ZTC
based capacitors at V = 0.05 V recorded after finishing the determination
of the Ragone plot. The profiles shown in Figure 9.d are characteristic of
carbon materials with a mainly capacitive behavior [43]. At high
frequencies, a semicircle is observed in both supercapacitors. This
semicircle is produced by the parallel combination of the bulk
capacitance of the electrolyte and the electrical resistances to charge
propagation in the electrode [50], and it includes contact resistance
Electrochemical performance of N-ZTC in aqueous electrolyte
239
between particles. At medium frequencies, the Warburg region
(evidenced by a 45º line in the Nyquist plot) is observed, while at low
frequencies, the shape of the curve becomes close to a vertical line,
characteristic of the capacitive behavior [42]. Table 5.4 summarizes the
cell resistance obtained for both capacitors. As happened in three
electrode cell configuration (section 5.3.2.2.), N-ZTC capacitor shows
much lower resistance. These cell resistances are governed by: (i) the
diffusive problems of the ions through the porosity (Warburg region
[43]) and (ii) the electrical resistances to charge propagation in the
electrode (indicated by the diameter of the semicircle) [50]. Both the
Warburg and the semicircle resistances are lower in N-ZTC, although
the latter resistance is much lower. For understanding the difference in
this resistance, it should be noted that both ZTC and N-ZTC
supercapacitors have been assembled using the same electrolyte, the
same electrode composition and by following the same procedure.
Therefore, we propose that the main differences between the cell
resistances of these supercapacitors are mainly related to the inherent
electrical conductivity of the electrode materials that decrease the
contact resistance between particles. As discussed before (section
5.3.2.2), the main differences of both carbons are related to the presence
of nitrogen heteroatoms in N-ZTC. The resistance value obtained for this
capacitor is four times lower than that calculated for ZTC cell (Table
5.4), and it evidences the better electrical conductivity and charge
propagation of N-ZTC electrodes. In accordance to this finding, the
maximum power obtained for N-ZTC based capacitor (calculated from
the resistance measured from the ohmic drop of GCD cycles, which is in
Chapter 5
240
good agreement to that obtained from the EIS analyses) is four times
larger than the value obtained for ZTC cell. This value outperforms
those found in the literature for other carbon electrodes in
supercapacitors [5,51,52], such as activated carbons (61.2 kW/kg in 1M
H2SO4) [17], activated carbon nanofibers (20 kW/kg in 6M KOH) [53],
hierarchical porous carbons (52.7 kW/kg in 1M H2SO4) [54], carbon
nanotubes (43.3 kW/kg in 1M Et4NBF4/propylene carbonate) [55] and
other templated carbons (28kW/kg in 1M H2SO4) [56]. This outstanding
improvement of N-ZTC based capacitor is probably a consequence of its
connected nanopore structure, which facilitates the accessibility of the
electrolyte [4], and the large quantity of N-Q functionalities and pyrrole
on the surface of this material, that are able to increase both wettability
and electrical conductivity [18, 20, 44, 45].
Figure 5.10. Durability test for ZTC and N-ZTC based supercapacitors at 1.2V. 5 A/g.
50000 cycles. 1 M H2SO4 electrolyte.
0
20
40
60
80
100
120
0 10000 20000 30000 40000 50000
C/C
o (%
)
Nº Cycles
ZTCN-ZTC
Electrochemical performance of N-ZTC in aqueous electrolyte
241
The presence of nitrogen functionalities can also modify the stability
of porous carbon materials [17,19,22]. In order to explore this option,
the durability of both cells was evaluated by 50000 GCD cycles at 1.2 V.
Figure 5.10 shows the evolution of the capacitance retention (C/Co)
during the test. Both capacitors evidence a similar capacitance retention
after the durability test (Table 5.4), although it is somewhat higher in
case of N-ZTC cell. However, it can be seen in Figure 5.10 that ZTC
based capacitor shows larger changes of capacitance along cycles,
evidencing a less stable performance. The improvement of stability in
this capacitor is probably a consequence of the N functionalities formed
on N-ZTC electrodes, since they are able to increase the stability by
preventing the incorporation of detrimental oxygen functionalities [17].
5.4 Conclusions
In this chapter, the electrochemical behavior of non-doped and N-
doped zeolite templated carbons were studied as electrodes for
supercapacitors in different aqueous electrolytes. The materials were
synthesized by CVD of different precursors and they evidenced a
practically identical structure but different surface chemistry. The role of
the different N-functionalities in their electrochemical performance was
elucidated by different techniques. The study carried out by cyclic
voltammetry evidenced a higher resistance to electro-oxidation and
degradation in case of N-ZTC in acid and alkaline media. Also, the
different electroactivity in 1M H2SO4 of the functional groups generated
at positive potentials was demonstrated. The CV study suggested that N-
ZTC has better wettability than ZTC as consequence of the large amount
Chapter 5
242
of N-Q functionalities at its surface. Also, it was shown that N-Q
functionalities provide better conductivity to this carbon.
The effect of the different performance of these carbons as
electrodes for supercapacitors was assessed in acid electrolyte using a
two-electrode cell configuration. The results showed that both capacitors
provide similar capacitance, but larger energy in case of N-ZTC. This
result is the outcome of the large dependence of capacitance on potential
in the highly oxidized ZTC. This behavior produces a large capacitance
at low voltages, but a much smaller capacitance at high voltages, where
most energy would be stored. In consequence, the more capacitive
behavior of N-ZTC renders an improved energy density.
More interestingly, N-ZTC based supercapacitor provided a
maximum power that is four times larger than that showed by ZTC
based supercapacitor (98 and 23 kW/kg), as consequence of the
improvement of electrical conductivity produced by N-Q functionalities
in N-ZTC electrodes. The durability of the capacitors was evaluated by
50000 GCD cycles at 5 A/g and 1.2 V. Both capacitors evidenced high
capacitance retention after the durability test, but N-ZTC based capacitor
showed a most stable performance along cycles, pointing out the
stabilizing effect of N functional groups. These results provide clear
evidences of the advantages of doping advanced porous carbon materials
with nitrogen functionalities for the improvement of the performance of
aqueous based supercapacitors.
Electrochemical performance of N-ZTC in aqueous electrolyte
243
5.5 References
[1] F. Béguin, E. Frackowiak, Carbons for Electrochemical Energy
Storageand Conversion Systems,1st ed., Boca Raton, FL: Taylor
& Francis (CRC Press) 2010.
[2] A.G. Pandolfo, A.F. Hollenkamp, Carbon properties and their role
in supercapacitors, Journal of Power Sources 157 (2006) 11–27.
[3] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Carbons and
Electrolytes for Advanced Supercapacitors, Adv. Mater. 26
(2014) 2219–2251.
[4] H. Itoi, H. Nishihara, T. Kogure, T. Kyotani, Three-
Dimensionally Arrayed and Mutually Connected 1.2 nm
Nanopores for High-Performance Electric Double Layer
Capacitor, J. Am. Chem. Soc. 133 (2011) 1165–1167.
[5] H. Nishihara, T. Kyotani, Templated Nanocarbons for Energy
Storage, Advanced Materials. 24 (2012) 4473–4498.
[6] H. Itoi, H. Nishihara, T. Ishii, K. Nueangnoraj, R. Berenguer-
Betrián, T. Kyotani, Large Pseudocapacitance in Quinone-
Functionalized Zeolite-Templated Carbon, Bull Chem Soc Jpn. 87
(2014) 250–257.
[7] H. Itoi, H. Nishihara, T. Kyotani, Effect of Heteroatoms in
Ordered Microporous Carbons on Their Electrochemical
Capacitance, Langmuir 32 (2016) 11997–12004.
[8] O. Ornelas, J.M. Sieben, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, J. Geng, N. Soin, E. Siores, B.F.G. Johnson, On the
origin of the high capacitance of nitrogen-containing carbon
nanotubes in acidic and alkaline electrolytes, Chem. Commun. 50
(2014) 11343–11346.
[9] M. Seredych, D. Hulicova-Jurcakova, G.Q. Lu, T.J. Bandosz,
Surface functional groups of carbons and the effects of their
chemical character, density and accessibility to ions on
electrochemical performance, Carbon 46 (2008) 1475–1488.
Chapter 5
244
[10] D. Hulicova-Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz,
Combined Effect of Nitrogen- and Oxygen-Containing Functional
Groups of Microporous Activated Carbon on its Electrochemical
Performance in Supercapacitors, Adv. Funct. Mater. 19 (2009)
438–447.
[11] M.J. Bleda-Martínez, D. Lozano-Castelló, E. Morallón, D.
Cazorla-Amorós, A. Linares-Solano, Chemical and
electrochemical characterization of porous carbon materials,
Carbon 44 (2006) 2642–2651.
[12] R. Berenguer, R. Ruiz-Rosas, A. Gallardo, D. Cazorla-Amorós, E.
Morallón, H. Nishihara, T. Kyotani, J. Rodríguez-Mirasol, T.
Cordero, Enhanced electro-oxidation resistance of carbon
electrodes induced by phosphorus surface groups, Carbon 95
(2015) 681–689.
[13] D. Hulicova-jurcakova, A.M. Puziy, O.I. Poddubnaya, F. Suárez-
García, J. M. D. Tascón, G. Quing Lu, Highly Stable Performance
of Supercapacitors from Phosphorus-Enriched Carbons, J Am
Chem Soc 2009; 131(4):5026–7.
[14] D.-W. Wang, F. Li, Z.-G. Chen, G.Q. Lu, H.-M. Cheng, Synthesis
and Electrochemical Property of Boron-Doped Mesoporous
Carbon in Supercapacitor, Chem. Mater. 20 (2008) 7195–7200.
[15] J.P. Paraknowitsch, A. Thomas, Doping carbons beyond nitrogen:
an overview of advanced heteroatom doped carbons with boron,
sulphur and phosphorus for energy applications, Energy Environ.
Sci. 6 (2013) 2839–2855.
[16] M. Enterría, M.F.R. Pereira, J.I. Martins, J.L. Figueiredo,
Hydrothermal functionalization of ordered mesoporous carbons:
The effect of boron on supercapacitor performance, Carbon 95
(2015) 72–83.
[17] M.J. Mostazo-López, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Nitrogen doped superporous carbon prepared by a mild
method. Enhancement of supercapacitor performance, Int. J.
Hydrogen Energy 41 (2016) 19691–19701.
Electrochemical performance of N-ZTC in aqueous electrolyte
245
[18] M.J. Mostazo-López, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Generation of nitrogen functionalities on activated
carbons by amidation reactions and Hofmann rearrangement:
Chemical and electrochemical characterization, Carbon 91 (2015)
252–265.
[19] D. Salinas-Torres, S. Shiraishi, E. Morallón, D. Cazorla-Amorós,
Improvement of carbon materials performance by nitrogen
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions, Carbon 82 (2015) 205–213.
[20] T. Kwon, H. Nishihara, H. Itoi, Q.-H. Yang, T. Kyotani,
Enhancement Mechanism of Electrochemical Capacitance in
Nitrogen-/Boron-Doped Carbons with Uniform Straight
Nanochannels, Langmuir 25 (2009) 11961–11968.
[21] González-Gaitán C, Ruiz-Rosas R, Morallón E, Cazorla-Amorós
D. Electrochemical Methods to Functionalize Carbon Materials.
In: V.K. Thakur, M.K. Thakur, editors. Chemical
functionalization of carbon nanomaterials, Boca Raton, FL:
Taylor & Francis (CRC Press); 2015: p. 230–261.
[22] C. González-Gaitán, R. Ruiz-Rosas, H. Nishihara, T. Kyotani, E.
Morallón, D. Cazorla-Amorós, Successful functionalization of
superporous zeolite templated carbon using aminobenzene acids
and electrochemical methods, Carbon 99 (2016) 157–166.
[23] H. Nishihara, P. Hou, L. Li, M. Ito, M. Uchiyama, T. Kaburagi, et
al., High-Pressure Hydrogen Storage in Zeolite-Templated
Carbon, J. Phys. Chem. C 113 (2009) 3189–3196.
[24] P.-X. Hou, H. Orikasa, T. Yamazaki, K. Matsuoka, A. Tomita, N.
Setoyama, et al., Synthesis of nitrogen-nontaining microporous
carbon with a highly ordered structure and effect of nitrogen
doping on H2O adsorption, Chem. Mater. 17 (2005) 5187–5193.
[25] Y. Kwon, K. Kim, R. Ryoo, N-doped zeolite-templated carbon as
a metal-free electrocatalyst for oxygen reduction, RSC Adv. 6
(2016) 43091–43097.
Chapter 5
246
[26] C.O. Ania, V. Khomenko, E. Raymundo-Piñero, J.B. Parra, F.
Béguin, The large electrochemical capacitance of microporous
doped carbon obtained by using a zeolite template, Adv. Funct.
Mater. 17 (2007) 1828–1836.
[27] K. Nueangnoraj, H. Nishihara, T. Ishii, N. Yamamoto, H. Itoi, R.
Berenguer, et al., Pseudocapacitance of zeolite-templated carbon
in organic electrolytes, Energy Storage Mater. 1 (2015) 35–41.
[28] R. Berenguer, H. Nishihara, H. Itoi, T. Ishii, E. Morallón, D.
Cazorla-Amorós, et al., Electrochemical generation of oxygen-
containing groups in an ordered microporous zeolite-templated
carbon, Carbon 54 (2013) 94–104.
[29] Z. Ma, T. Kyotani, Z. Liu, O. Terasaki, A. Tomita, Very high
surface area microporous carbon with a three-dimensional nano-
array structure: Synthesis and its molecular structure, Chem.
Mater. 13 (2001) 4413–4415.
[30] H. Boehm, Some aspects of the surface chemistry of carbon
blacks and other carbons, Carbon 32 (1994) 759–769.
[31] Y. Otake, R.G. Jenkins, Characterization of oxygen-containing
surface complexes created on a microporous carbon by air and
nitric acid treatment, Carbon 31 (1993) 109-121.
[32] M.C. Román-Martínez, D. Cazorla-Amorós, A. Linares-Solano,
C.S.M. de Lecea, TPD and TPR characterization of carbonaceous
supports and Pt/C catalysts, Carbon 31 (1993) 895–902.
[33] P. Burg, D. Cagniant, Characterization of carbon surface
chemistry, in: L.R. Radovic, editor. Chemistry and Physics of
carbons, vol. 30, Boca Raton, FL: Taylor & Francis (CRC Press);
2008: p. 129-175.
[34] H. Nishihara, Q. Yang, P. Hou, M. Unno, S. Yamauchi, R. Saito,
et al., A possible buckybowl-like structure of zeolite templated
carbon, Carbon 47 (2009) 1220–1230.
[35] S. Leyva-García, K. Nueangnoraj, D. Lozano-Castelló, H.
Nishihara, T. Kyotani, E. Morallón, et al., Characterization of a
Electrochemical performance of N-ZTC in aqueous electrolyte
247
zeolite-templated carbon by electrochemical quartz crystal
microbalance and in situ Raman spectroscopy, Carbon 89 (2015)
63–73.
[36] D. Cazorla-Amorós, D. Lozano-Castelló, E. Morallón, M.J.
Bleda-Martínez, A. Linares-Solano, S. Shiraishi, Measuring cycle
efficiency and capacitance of chemically activated carbons in
propylene carbonate, Carbon 48 (2010) 1451–1456.
[37] O. Barbieri, M. Hahn, A. Herzog, R. Kötz, Capacitance limits of
high surface area activated carbons for double layer capacitors,
Carbon 43 (2005) 1303–1310.
[38] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H.
Zhu, G.Q. Lu, Nitrogen-enriched nonporous carbon electrodes
with extraordinary supercapacitance, Adv. Funct. Mater. 19
(2009) 1800–1809.
[39] S.L. Candelaria, B.B. Garcia, D. Liu, G. Cao, Nitrogen
modification of highly porous carbon for improved supercapacitor
performance, J. Mater. Chem. 22 (2012) 9884-9889.
[40] M. Kawaguchi, T. Yamanaka, Y. Hayashi, H. Oda, Preparation
and capacitive properties of a carbonaceous material containing
nitrogen, J. Electrochem. Soc. 157 (2010) A35-A40.
[41] R. Ruiz-Rosas, M.J. Valero-Romero, D. Salinas-Torres, J.
Rodríguez-Mirasol, T. Cordero, E. Morallón, et al.,
Electrochemical performance of hierarchical porous carbon
materials obtained from the infiltration of lignin into zeolite
templates., ChemSusChem 7 (2014) 1458–1467.
[42] B.E. Conway, Electrochemical supercapacitors: Scientific
Fundamentals and Technological Applications. New York:
Springer; 1999.
[43] R. Kötz, M. Carlen, Principles and applications of electrochemical
capacitors, Electrochim. Acta 45 (2000) 2483–2498.
Chapter 5
248
[44] G. Lota, K. Lota, E. Frackowiak, Nanotubes based composites
rich in nitrogen for supercapacitor application, Electrochem.
Commun. 9 (2007) 1828–1832.
[45] Y. Zhou, X. Xu, B. Shan, Y. Wen, T. Jiang, J. Lu, et al., Tuning
and understanding the supercapacitance of heteroatom-doped
graphene, Energy Storage Mater. 1 (2015) 103–111.
[46] V. Strelko, V. Kuts, P. Thrower, On the mechanism of possible
influence of heteroatoms of nitrogen, boron and phosphorus in a
carbon matrix on the catalytic activity of carbons in electron
transfer reactions, Carbon 38 (2000) 1499–1503.
[47] M.J. Bleda-Martínez, J.A. Maciá-Agulló, D. Lozano-Castelló, E.
Morallón, D. Cazorla-Amorós, A. Linares-Solano, Role of surface
chemistry on electric double layer capacitance of carbon
materials, Carbon 43 (2005) 2677–2684.
[48] H.A. Andreas, B.E. Conway, Examination of the double-layer
capacitance of an high specific-area C-cloth electrode as titrated
from acidic to alkaline pHs, Electrochim. Acta 51 (2006) 6510–
6520.
[49] K. Nueangnoraj, R. Ruiz-Rosas, H. Nishihara, S. Shiraishi, E.
Morallón, D. Cazorla-Amorós, et al., Carbon–carbon asymmetric
aqueous capacitor by pseudocapacitive positive and stable
negative electrodes, Carbon 67 (2014) 792–794.
[50] S. Fletcher, V.J. Black, I. Kirkpatrick, A universal equivalent
circuit for carbon-based supercapacitors, J. Solid State
Electrochem. 18 (2014) 1377–1387.
[51] S.L. Candelaria, Y. Shao, W. Zhou, X. Li, J. Xiao, J. Zhang, et al.,
Nanostructured carbon for energy storage and conversion, Nano
Energy 1 (2012) 195–220.
[52] Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai,
Carbon materials for chemical capacitive energy storage, Adv.
Mater. 23 (2011) 4828–4850.
Electrochemical performance of N-ZTC in aqueous electrolyte
249
[53] T. Le, Y. Yang, Z. Huang, F. Kang, Preparation of microporous
carbon nanofibers from polyimide by using polyvinyl pyrrolidone
as template and their capacitive performance, J. Power Sources
278 (2015) 683–692.
[54] D. Salinas-Torres, R. Ruiz-Rosas, M.J. Valero-Romero, J.
Rodríguez-Mirasol, T. Cordero, E. Morallón, et al., Asymmetric
capacitors using lignin-based hierarchical porous carbons, J.
Power Sources 326 (2016) 641–651.
[55] D.N. Futaba, K. Hata, T. Yamada, T. Hiraoka, Y. Hayamizu, Y.
Kakudate, et al., Shape-engineerable and highly densely packed
single-walled carbon nanotubes and their application as super-
capacitor electrodes, Nat. Mater. 5 (2006) 987–994.
[56] G.A. Ferrero, A.B. Fuertes, M. Sevilla, N-doped porous carbon
capsules with tunable porosity for high-performance
supercapacitors, J. Mater. Chem. A 3 (2015) 2914–2923.
Chapter 5
250
ANNEX TO CHAPTER 5
Figure A5.1. XRD patterns obtained for ZTC and N-ZTC carbon materials.
Figure A5.2. N2 adsorption desorption isotherms measured at -196 ºC obtained for
ZTC and N-ZTC samples.
0 10 20 30 40 50
I (a
.u.)
2Ɵ (degrees, Cu-Kα)
0
200
400
600
800
1000
1200
1400
0 0.2 0.4 0.6 0.8 1
Ad
sorb
ed
Volu
me (
Cm
3/g
ST
P)
P/P0
Electrochemical performance of N-ZTC in aqueous electrolyte
251
Figure A5.3. Comparison between CO2 TPD profiles of ZTC and N-ZTC samples.
0
0.2
0.4
0.6
0.8
1
100 200 300 400 500 600 700 800 900
CO
2(µ
mo
l/g
s)
Temperature (ºC)
ZTC
N-ZTC
Chapter 6
Electrochemical performance of
N-doped activated carbons in
organic electrolyte
Electrochemical performance of N-doped activated carbons in organic electrolyte
255
6.1. Introduction
Electrochemical capacitors are energy storage devices of great
interest mainly due to their high power density, durability and broad
operation temperature range [1,2]. However, it is necessary to increase
their energy density for enabling their use in a wider range of
applications. This requires the use of electrode materials and electrolytes
able to increase both the capacitance and the operation voltage of the
electrochemical capacitor. Regarding the electrode materials, carbon
materials are the most commonly used since they have a well-developed
porosity, electrochemical stability and tunable surface chemistry [3]. In
case of the electrolyte, the use of non-aqueous electrolytes (organic
solvents and ionic liquids) has attracted great interest as consequence of
their possible use at high operation voltage [4,5]. Nonetheless, the use of
high operation voltage dramatically affects the performance of the
electrodes, since carbon materials undergo reactions with the electrolyte
that decrease their electrochemical stability and, consequently, diminish
the cycle life of the device [6-8]. Hence, the development of carbon
materials with improved electrochemical properties is highly needed for
their use in electrochemical capacitors.
The performance of carbon materials can be enhanced by following
different strategies [9]. One of the main approaches consists in
synthesizing materials doped with different heteroatoms [10,11], such as
sulfur, phosphorus or nitrogen, since they modify the electrochemical
behavior of the material [6, 12-14]. In case of nitrogen doping, it has
been proven that can increase the wettability, conductivity and
Chapter 6
256
electrochemical stability [6, 7, 15]. However, the role of the different
nitrogen groups is not still well understood.
Nitrogen doped carbon materials can be prepared by several
methods [16-20], that can be classified as follows: (i) by using a nitrogen
compound as nitrogen and carbon source in different synthesis methods,
such as carbonization (followed by activation), templating methods or
hydrothermal carbonization; (ii) through post-treatments of a material
previously synthesized with a nitrogen-containing reactant in gas or
liquid phase. The second method usually involves the use of high
temperatures to favor the incorporation of nitrogen, wether in gas or
liquid pase. However, this can affect the porosity of the pristine carbon
material and makes difficult to control the type of functionalities that are
formed on the surface. In this sense, we have developed nitrogen
functionalization methods at mild conditions that allow the incorporation
of a wide range of nitrogen functionalities and allows the preservation of
the structure of the pristine carbon material, even when using carbon
materials with extraordinary microporosity (see chapters 3 and 4 and
references [8, 21]).
The aim of this work is to elucidate the role of different nitrogen
functionalities on the performance of activated carbons as electrodes for
electrochemical capacitors in organic electrolyte, mainly focusing on the
effect on the electrochemical stability and durability of the device. We
propose the synthesis of nitrogen-doped activated carbons with high
apparent surface area by combining functionalization methods based on
wet methods at low temperatures and post-thermal treatments at
Electrochemical performance of N-doped activated carbons in organic electrolyte
257
different temperatures, whose combination allows a selective
modification of the surface chemistry. The effect of different nitrogen
functionalities and doping methods on the performance of these
materials as electrodes for electrochemical capacitors is tested in organic
electrolyte and severe conditions of temperature and operation voltage.
6.2. Materials and methods
6.2.1. Synthesis of carbon materials
6.2.1.1. Pristine carbon material
An activated carbon with high microporosity prepared in our
laboratory has been used as the starting material for nitrogen
incorporation via post-modification treatments. The pristine material
(named as KUA) has been synthethized by chemical activation of a
Spanish anthracite with KOH using an impregnation ratio of activating
agent to raw material of 4:1 and an activation temperature of 750º C
under inert atmosphere, which was held for 1 hour. More details about
the preparation process are available elsewhere [22] (chapter 2).
6.2.1.2. N-functionalization of carbon materials at mild conditions
KUA was further functionalized with nitrogen functional groups by
two different approaches based on the organic chemistry reactions
described in chapters 3 and 4. Briefly, the first approach consisted in a
three-step protocol: (i) chemical oxidation with HNO3, (ii) treatment
with SOCl2 and (iii) amidation reaction with NH4NO3/DMF and
pyridine. The obtained sample was named as KUA-CONH2. In the
Chapter 6
258
second approach, the third step of the first method is directly applied
over pristine sample (KUA). This sample was named as KUA-N. More
details about the preparation methods are found in chapters 3 and 4.
6.2.1.3. Heat treatments
The carbon materials (KUA, KUA-CONH2 and KUA-N samples)
were heat treated at two different temperatures for 1 hour under N2
atmosphere using a flow of 200mL/min and a heating rate of 5 Cº/min.
The obtained samples are named as KUA_T, KUA-CONH2_T and
KUA-N_T, where T is the final heating temperature (500 and 800 ºC).
6.2.2. Physicochemical characterization
The surface chemistry of the samples was analyzed by X Ray
Photoelectron Spectroscopy (XPS) and Temperature Programmed
Desorption (TPD). XPS analyses were performed using a VG-Microtech
Multilab 3000 spectrometer with an Al anode. N1s spectra were
deconvoluted using Gaussian functions with 20% of Lorentzian
component. A Shirley line was used as background and the FWHM of
the peaks was kept between 1.4 and 1.7 eV. TPD experiments were
carried out using a TGA-DSC instrument (TA Instruments, SDT Q600
Simultaneous) coupled to a mass spectrometer (Thermostar, Balzers,
BSC 200), by heating the samples up to 950 ºC (heating rate: 20 ºC/min)
under helium atmosphere (flow rate: 100 mL/min).
The porous texture characterization was carried out by N2
adsorption-desorption isotherms at -196º C and by CO2 adsorption at 0º
Electrochemical performance of N-doped activated carbons in organic electrolyte
259
C using an Autosorb-6-Quantachrome apparatus. The samples were
outgassed at 200º C for 4 hours before the experiments. The apparent
surface area was obtained from N2 adsorption isotherms by using the
BET equation in the 0.05-0.20 range of relative pressures. The total
micropore volume was determined by Dubinin-Radushkevich (DR)
method applied to N2 (relative pressures from 0.01 to 0.05) adsorption
isotherms. The volume of the narrow microporosity (i.e., pore sizes
below 0.7 nm) was calculated from the DR method applied to the CO2
adsorption isotherms (relative pressures from 0.0001 to 0.25) [23].
6.2.3. Electrochemical characterization
The carbon materials were used for the preparation of two-electrode
cells (Al/Mg-body cell, Honsen Corporation, Japan). The electrodes
were prepared by mixing the carbon material with acetylene black and
PTFE in a ratio of 85:10:5 wt %. The electrodes were pressed to shape
disks with a diameter of 13 mm and a thickness of ~0.5 mm (net weight:
35 mg). Aluminium paper was used as current collector and 1M
triethylmethylamonium tetrafluoroborate diluted in propylene carbonate
(TEMA-BF4/PC) as electrolyte. The cells were assembled using an
Argon glovebox.
The cells were tested by galvanostatic charge-discharge (GCD)
cycles at different current densities (40, 100, 200, 400, 600, 1000 mA/g)
using an operation voltage of 2.5 V and 40ºC. The capacitance was
calculated from the discharge curve of the GCD cycles as reported
previously in the literature [24]. The gravimetric capacitance is referred
Chapter 6
260
to the active mass of the carbon in the full cell. Afterwards, a durability
test (based on GCD cycles and floating test) was performed by applying
the following procedure: (i) 5 GCD cycles at 2.5 V and 40 ºC; (ii) 5
GCD cycles at 2.5 V and 70 ºC; (iii) 5 GCD cycles at 3.2 V and 70 ºC;
(iv) voltage is kept at 3.2 V for 100 hours (floating test). After finishing
the floating test (iv), steps (iii), (ii) and (i) were subsequently repeated.
The capacitance, resistance and integrated leakage current were
calculated. The retention of capacitance is given at 2.5 V and 40 ºC. The
current density was kept as 40 mA/g during the experiment.
6.3. Results and discussion
6.3.1. Physicochemical characterization of carbon materials
6.3.1.1 Porous texture
Figure 6.1 illustrates the N2 adsorption-desorption isotherms
obtained for all activated carbons. The profiles evidence that all carbon
materials show an adsorption-desorption curve characteristic of
microporous materials. Table 6.1 summarizes the porous texture
parameters obtained from N2 and CO2 isotherms for all the activated
carbons and the pristine KUA, the treated at 800ºC (KUA_800) and the
oxidized in nitric acid (KUA_COOH) has been included for comparison
pourpouse. The changes in the porous texture of the samples obtained by
oxidation (KUA-COOH) and chemical functionalization (KUA-CONH2
and KUA-N) were previously discussed in Chapters 3 and 4. The most
significant change is observed for KUA-COOH and KUA-CONH2
Electrochemical performance of N-doped activated carbons in organic electrolyte
261
samples, as consequence of the generation of oxygen functionalities in
KUA-COOH and their conversion into nitrogen functional groups that
occupy or block part of the microporosity [21] (Table 6.1, Figure 6.2b).
However, when the chemical method is directly applied over the pristine
sample (sample KUA-N), the microporosity of the material is fully
preserved (as evidenced in Figure 6.1a., Table 6.1).
Regarding the heat-treated samples, some loss of the microporosity
is detected when the heat treatment is carried out over KUA and KUA-N
samples (Figure 6.1a and c, Table 6.1). This change could be produced
by some structural rearrangement in the materials [25]. However, the
heat treatments over KUA-CONH2 produce an increase of the micropore
volume (see samples KUA-CONH2_500 and KUA-CONH2_800). This
increase of microporosity is related to the removal of functional groups
that occupies or block part of the microporosity in the pristine sample.
Interestingly, the thermal treatments at different temperatures (500 ºC
and 800 ºC) produce identical modifications on the porous texture of the
pristine sample (KUA-N and KUA-CONH2). As consequence, the N2
adsorption isotherms obtained for samples KUA-N_500 and KUA-
N_800 overlaps (Figure 6.1a.). The same is observed for KUA-
CONH2_500 and KUA-CONH2_800 samples (Figure 6.1b).
Chapter 6
262
0
200
400
600
800
1000
0 0.2 0.4 0.6 0.8 1
Ad
sorb
ed
Vo
lum
e (
cm
3/g
)
P/P0
KUAKUA-NKUA-N_500KUA-N_800
(a)
0
200
400
600
800
1000
0 0.2 0.4 0.6 0.8 1
Ad
sorb
ed
Vo
lum
e (
cm
3/g
)
P/P0
KUAKUA-CONH₂KUA-CONH₂_500KUA-CONH₂_800
(b)
0
200
400
600
800
1000
0 0.2 0.4 0.6 0.8 1
Ad
sorb
ed
Vo
lum
e (
cm
3/g
)
P/P0
KUA
KUA_800
(c)
Figure 6.1. N2 adsorption-desorption isotherms obtained for all activated carbons: (a)
KUA, KUA-N and related heat-treated samples, (b) KUA, KUA-CONH2and related
heat-treated samples, (c) KUA and related heat-treated sample.
Electrochemical performance of N-doped activated carbons in organic electrolyte
263
Table 6.1. Porous texture of the activated carbons.
Sample SBET
(m2/g)
VDRN2
(cm3/g)
VDRCO2
(cm3/g)
KUA 3080 1.19 0.57
KUA_800 2720 1.05 0.49
KUA-N 2960 1.18 0.52
KUA-N_500 2800 1.11 0.49
KUA-N_800 2770 1.09 0.48
KUA-COOH 2770 1.06 0.49
KUA-CONH2 2390 0.97 0.45
KUA-CONH2_500 2630 1.02 0.41
KUA-CONH2_800 2630 1.0 0.43
The CO2 micropore volumes (collected in Table 6.1) provide
information about the narrow microporosity of the samples (< 0.7 nm)
[23]. It is observed that the chemical functionalization and the heat
treatments produce a small decrease of the narrow microporosity. The
largest micropore volume was found for the pristine sample KUA,
followed by the KUA-N activated carbon and its related heat-treated
samples (KUA-N_500 and KUA-N_800).
6.3.1.2 Surface chemistry characterization
The surface chemistry of the activated carbons was characterized by
XPS and TPD. Table 6.2 collects the data related to the chemical
composition of the samples. Figures 6.2 and 6.3 collect the TPD profiles
obtained for all activated carbons. The pristine carbon material KUA
possesses a large amount of oxygen functionalities that decompose as
Chapter 6
264
CO and CO2 (Figure 6.2). As discussed in chapters 3 and 4, the chemical
modification methods produce the attachment of nitrogen to the surface
of the pristine activated carbon by consumption of oxygen functional
groups, as confirmed by XPS (Table 6.2). Figure 6.4 shows the
deconvoluted N1s spectra obtained for all N-doped activated carbons
and Table 6.3 summarizes the relative amount of the detected N
functional groups. In case of KUA-N sample, the attachment of nitrogen
occurs directly over the pristine carbon material via reaction mainly with
CO-evolving groups. Thus, this treatment produces nitrogen groups
derived from these oxygen functionalities, such as imines, amines and N
heterocycles (pyrroles, pyridones, and pyridines) [18,26-28] (Figure
6.2a). In case of KUA-CONH2 sample, an oxidation treatment is carried
out, previously to the incroporation of nitrogen, to increase the amount
of oxygen functional groups that can react during the amidation
treatment. The oxidation treatment with HNO3 produces an increase of
both CO and CO2 evolving-groups, and consequently the nitrogen
moieties produced after the nitrogen doping treatment are mainly in form
of amides (and cyclic amides), pyridines and pyrroles/pyridones [18,26-
28], as was described in chapter 4. Hence, both N-doped carbons
obtained at mild conditions evidence the presence of different N
functional groups, being KUA-CONH2 richer in those derived from
CO2-evolving groups (amides, lactams and imides).
Electrochemical performance of N-doped activated carbons in organic electrolyte
265
Table 6.2. Surface composition of the activated carbons obtained by XPS and TPD.
Sample CO2
(µmol/g)
CO
(µmol/g)
O
(µmol/g)
O
(at. %)
N
(at. %)
KUA 450 1970 2870 8.8 0.3
KUA_800 170 620 960 2.3 -
KUA-N 450 1750 2640 7.5 3.7
KUA-N_500 250 1620 2120 5.1 2.3
KUA-N_800 130 505 720 3.0 1.7
KUA-COOH 1790 3770 7360 15.8 -
KUA-CONH2 1140 2370 4650 10.2 4.2
KUA-CONH2_500 250 1650 2140 8.0 3.4
KUA-CONH2_800 140 570 830 4.5 2.1
The surface chemistry of the carbon materials was subsequently
modified by heat treatments under inert atmosphere to produce activated
carbons with different composition while keeping the porous texture of
the pristine materials. These treatments produce a decrease of oxygen
and nitrogen groups (Table 6.2). TPD profiles allow to get information
about the changes produced in the oxygen functionalities (Figures 6.2
and 6.3). When the treatment at 500 ºC is carried out, some oxygen
functionalities are removed from their surface; the main loss comes from
CO2-evolving groups, since most part of these functionalities are not
thermally stable at temperatures higher than 600 ºC [29]. Thus, no
significant changes are produced on KUA-N at this temperature, as a
result of the small amount of CO2-evolving groups in this sample.
Consequently, KUA-N_500 shows a similar composition than that of
KUA-N, with slightly lower oxygen content. More important differences
are found for KUA-CONH2_500 due to the larger amount of oxygen
(specially, CO2-evolving groups) of the parent carbon (KUA-CONH2).
Chapter 6
266
After the treatment, most of the CO2 groups are removed from this
sample (80 % decrease, Table 6.2). Moreover, an important decrease of
the of CO-evolving groups is also observed (30 % decrease, Table 6.2).
Interestingly, both carbons heat-treated at 500 ºC show almost identical
content of oxygen groups (Table 6.2, Figures 6.2 and 6.3).
0
0.2
0.4
0.6
0.8
1
0 200 400 600 800 1000
CO
2 (µ
mo
l/g
s)
Temperature (ºC)
KUA
KUA_800
KUA-N
KUA-N_500
KUA-N_800
(a)
0
0.5
1
1.5
2
0 200 400 600 800 1000
CO
(µ
mo
l/g
s)
Temperature (ºC)
KUA
KUA_800
KUA-N
KUA-N_500
KUA-N_800
(b)
Figure 6.2. Comparison between (a) CO2 and (b) CO TPD profiles of KUA,
KUA_800, KUA-N, KUA-N_500 and KUA-N_800.
Electrochemical performance of N-doped activated carbons in organic electrolyte
267
0
0.2
0.4
0.6
0.8
1
0 200 400 600 800 1000
CO
2(µ
mo
l/gs)
Temperature (ºC)
KUA
KUA-CONH₂
KUA-CONH₂_500
KUA-CONH₂_800
(a)
0
0.4
0.8
1.2
1.6
2
0 200 400 600 800 1000
CO
(µ
mo
l/g
s)
Temperature (ºC)
KUAKUA-CONH₂KUA-CONH₂_500KUA-CONH₂_800
(b)
Figure 6.3. Comparison between (a) CO2 and (b) CO TPD profiles of KUA, KUA-
CONH2, KUA-CONH2_500 and KUA-CONH2_800.
The heat treatment at 800 ºC removes most of oxygen. Around 70-
80 % of oxygen functionalities are removed from KUA, KUA-N and
KUA-CONH2 when they are heat treated at 800 ºC. N functionalities
also decrease upon heat treatment and around 50% of N remains after
the treatment. Hence, the obtained samples show similar oxygen content
(mainly in form of carbonyles [29]) and almost identical porous texture
(section 6.3.2.1).
Chapter 6
268
396398400402404
I (a
.u.)
Binding Energy (eV)
KUA-N_500
KUA-N_800
KUA-N(a)
396398400402404
I (a
.u.)
Binding Energy (eV)
KUA-CONH2_500
KUA-CONH2_800
KUA-CONH2(b)
Figure 6.4. N1s XPS spectra deconvoluted for the N-doped activated carbons.
The deconvolution of N1s XPS spectra obtained for all N-doped
activated carbons allows to assess the modifications produced on the
nitrogen moieties during the thermal treatments (Figure 6.4). Table 6.3
collects the percentage of each nitrogen group obtained from the N1s
XPS spectra. As previously described, the N-doped carbons at mild
conditions (KUA-N and KUA-CONH2) have similar nitrogen contents
but, according to the modification pathway, some of the generated
groups are different. The peak at 399.8 ± 0.2 eV can be assigned to
amides (and cyclic amides) and amines, being more likely the formation
Electrochemical performance of N-doped activated carbons in organic electrolyte
269
of amides in KUA-CONH2 (due to the previous oxidation process) and
amines in KUA-N [30].
Table 6.3. Assignment of N1s deconvoluted curves to nitrogen functional groups.
Sample Binding
Energy
(eV)
Functional
Group
N
(at. %)
Percentage of
N species
KUA-N 401.9 ± 0.2 Quaternary 0.4 10
400.7 ± 0.2 Pyrrole,
Pyridone
0.9 25
399.8 ± 0.2 Amide,
Lactam,
Amine, Imide
1.3 35
398.7 ± 0.2 Pyridine, Imine 1.1 30
KUA-N_500 401.5 ± 0.2 Quaternary 0.4
17
400.2 ± 0.2 Pyrrole,
pyridone
1.0 42
399.0 ± 0.2 Pyridine, Imine 0.9 41
KUA-N_800
402.7 ± 0.2 Oxidized N 0.2 14
400.8 ± 0.2 Pyrrole,
Pyridone
0.9 51
398.7 ± 0.2 Pyridine 0.6 35
KUA-CONH2 400.7 ± 0.2 Pyrrole,
pyrdidone
0.72 19
399.8 ± 0.2 Amide, lactam,
amine, imide
1.89 50
398.8 ± 0.2 Pyridine, imine 1.17 31
KUA-
CONH2_500
400.9 ± 0.2 Pyrrole,
pyridone
1.16 34
399.6 ± 0.2 Amide, lactam,
amine, imide
1.28 38
398.5 ± 0.2 Pyridine, imine 0.95 28
KUA-
CONH2_800
402.5 ± 0.2 Oxidized N 0.19 11
400.8 ± 0.2 Pyrrole,
pyridone
0.88 52
398.7 ± 0.2 Pyridine, imine 0.63 37
Chapter 6
270
When the samples are heat treated at 500 ºC, the nitrogen moieties
of lower thermal stability are removed, i.e., those with a single chemical
bond, such as amines and amides [31]. Indeed, the heat treatment of
KUA-N produces a loss of ⁓ 1.4 at. % N (Table 6.2), that is in agreement
with the amount of amines detected in KUA-N, and that does not appear
in the spectrum of KUA-N_500 (Figure 6.4a), evidencing the removal of
mainly this functional group during the treatment. In case of KUA-
CONH2, a lower loss of nitrogen is detected (0.8 at % XPS, Table 6.2).
Likewise, the main decrease is associated to the functional groups at
399.8 ± 0.2 eV, i.e., amide-like functionalities (Table 6.3, Figure 6.4b),
but they are still detected on KUA-CONH2_500. Moreover, this sample
evidences an increase of pyrrole/pyridones contribution (400.7 eV) that
can be related to the rearrangements of amides to generate pyrroles upon
heating [27], explaining the higher retention of nitrogen in this sample.
The heat treatments at 800 ºC strongly decrease the nitrogen groups
of the N-doped samples (Table 6.2, Figure 6.4). The samples KUA-
N_800 and KUA-CONH2_800 show similar nitrogen content (1.7 at. %
and 2.1 at. %, respectively, Table 6.2) and distribution of functionalities,
with a main contribution of pyrroles/pyridones and a lower presence of
pyridines and quaternary nitrogen (Table 6.3). These modifications are
undoubtedly related to the removal of nitrogen groups of lower thermal
stability as well as their conversion into more stable groups, such as
nitrogen heterocycles [27].
Thus, the combination of nitrogen doping at mild conditions with
post-thermal treatments allow the production of activated carbon with
Electrochemical performance of N-doped activated carbons in organic electrolyte
271
similar microporosity and different surface functionalities. Hence, their
electrochemical characterization in organic electrolyte can be useful to
get information about the role of the different N functional groups in the
electrochemical performance of the materials as electrodes for
supercapacitors.
6.3.2. Electrochemical characterization
The electrochemical performance of the samples was evaluated as
electrodes in symmetric electrochemical capacitors in organic electrolyte
(1M TEMABF4/PC) by using a two-electrode cell configuration. The
cells were tested by GCD cycles at different current densities. Table 6.4
collects the capacitance and energy obtained from the galvanostatic test
and Figure 6.5 shows, as an example, some of the chronopotentiograms
obtained for the capacitors at 40 mA/g. The curves evidence an ideal
capacitive behaviour, characterized by the triangular shape, for all the
activated carbon-based capacitors. All of them provide large capacitance
(36-41 F/g) and energy values (32-37 Wh/kg) as consequence of their
well-developed microporosity. The small differences found between the
samples are related to their slightly different apparent surface area, as
confirmed by the similar surface capacitance values obtained for these
materials (13-15 µF/cm2). Interestingly, the lowest values were obtained
for the non-doped samples with higher oxygen content (KUA and KUA-
COOH), evindencing a detrimental effect derived from these functional
groups [32].
Chapter 6
272
0
0.5
1
1.5
2
2.5
3
0 2000 4000 6000
Volt
age (
V)
Time (s)
KUA
KUA-N
KUA-CONH₂
(a)
0
0.5
1
1.5
2
2.5
3
0 2000 4000 6000
Vo
lta
ge (
V)
Time (s)
KUA-N_800
KUA-CONH₂_500
KUA-CONH₂_800
(b)(b)
Figure 6.5. GCD cycles obtained for activated carbons-based symmetric capacitors.
1M TEMABF4/PC. j = 40 mA/g. V = 2.5 V.
Figure 6.6 shows the evolution of the capacitance when increasing
the current density during the galvanostatic cycling test. As observed in
aqueous electrolyte (Chapter 3), the oxidation process produces a
detrimental effect on the rate performance of KUA-COOH when used as
electrode for supercapacitors, and again, when the amidation treatment is
carried out, the replacement of oxygen groups by nitrogen functionalities
with higher conductivity (cyclic amides, pyrroles, etc.) [30, 33]
Electrochemical performance of N-doped activated carbons in organic electrolyte
273
improves the behaviour, and consequently, a higher capacitance is
retained when the current density is increased (Figure 6.6a). The heat
treatments of the activated carbons (Figure 6.6b) produce an
improvement of the rate performance, but no significant benefits are
observed for the N-doped samples, evidencing that the increase of the
charge-discharge rate is mainly a consequence of an increase of
conductivity due to the removal of electron-withdrawing oxygen
functionalities.
Table 6.4. Gravimetric capacitance, surface capacitance, energy density, retention of
capacitance at 1000 mA/g (C1/C0), retention of capacitance after floating test (C0/Cf),
increase of resistance and integrated leakage current obtained for all activated carbon-
based capacitors. V = 2.5 V. j = 40 mA/g.
Capacitor C0
(F/g)
C0/SBET
(mF/cm2)
E
(Wh/kg)
C1/C0
(%)
Cf/C0
(%)
ΔR
(Ω)
IL
(mA
h)
KUA 41 13 37 45 29 33 37
KUA_800 38 14 33 48 43 30 29
KUA-N 41 14 36 42 39 20 32
KUA-N_500 41 15 35 45 39 32 36
KUA-N_800 40 14 35 49 44 28 31
KUA-COOH 36 13 31 30 20 50 26
KUA-CONH2 37 15 32 49 55 13 22
KUA-CONH2_500 39 15 33 37 22 86 33
KUA-CONH2_800 38 15 32 72 33 34 41
Chapter 6
274
0
10
20
30
40
50
0 200 400 600 800 1000
Cap
aci
tan
ce (F
/g)
j(mA/g)
KUA
KUA_COOH
KUA-CONH₂
(a)
0
10
20
30
40
50
0 200 400 600 800 1000
Cap
acit
an
ce (
F/g
)
j (mA/g)
KUA_800
KUA-N_800
KUA-CONH₂_800
(b)
Figure 6.6. Gravimetric capacitance at different current densities for the symmetric
capacitors. 1M TEMABF4/PC. j = 40, 100, 200, 500, 750 and 1000 mA/g. V = 2.5 V.
The activated carbon-based capacitors were submitted to a
durability test to assess the effect of surface chemistry in the
electrochemical stability of the carbon materials. The most common
method for the assessment of the durability of energy storage devices
consists in applying thousands of GCD cycles at the operative voltage of
the devices [9]. Since capacitors are based in electrostatic processes and
consequently provide a much larger cycle life than batteries, this
methodology is considerably more time demanding for the former than
Electrochemical performance of N-doped activated carbons in organic electrolyte
275
for the latter [1]. An alternative to this procedure is the use of a floating
test, consisting in keeping the cell charged during long time in order to
accelerate the degradation of the device [34-38]. Moreover, it is well-
known that electrochemical capacitors experience degradation at high
temperature [5,32,39,40]. Hence, the combination of high voltage
holding with the use of high temperature constitutes an adequate
procedure to accelerate the degradation of electrochemical capacitors.
Figure 6.7 shows, as an example, the evolution of capacitance and
coulombic efficiency during the steps of the durability test (described in
section 6.2.3) for some of the capacitors. Table 6.4 compiles the
parameters (retention of capacitance (Cf/C0), increase of resistance (ΔR)
and integrated leakage current (IL)) related to this test determined for all
capacitors. The analysis of these data allows to throroughly discuss the
effect of surface chemistry modification on the performance of carbon
materials as electrodes for supercapacitors in organic electrolyte.
6.3.2.1. Effect of surface chemistry modification at mild conditions
Figure 6.7a shows the performance of the pristine carbon material
during the durability test. Before the floating step, galvanostatic cycling
is performed at different temperatures and voltage conditions, being
these parameters subsequently increased with the evolution of cycles.
The increase of temperature and voltage does not affect the gravimetric
capacitance, but decreases the coulombic efficiency evidencing the
ocurrence of faradaic processes under these conditions. The harmful
effect of the floating conditions is clearly observed in the capacitance
Chapter 6
276
values determined after the durability test. For the pristine sample, only
a 29% of capacitance is retained after the ageing test.
0
20
40
60
80
100
120
0
20
40
60
80
100
0 5 10 15 20 25 30 35
Eff
icie
ncy (
%)
Cap
acit
an
ce (
F/g
)
Nº Cycles
KUA_800
KUA-N_800
KUA-CONH₂_800
40ºC0-2.5V
70ºC0-3.2V
70ºC0-3.2V
70ºC3.2V
100h
70ºC0-3.2V
70ºC0-2.5V
40ºC0-2.5V
(b)
Figure 6.7. Evolution of capacitance and coulombic efficiency during the durability
test. 1M TEMABF4/PC. j = 40 mA/g.
In general, the modified samples show a similar trend than the
pristine samples. Thus, a remarkable loss of capacitance (50-70 %) is
Electrochemical performance of N-doped activated carbons in organic electrolyte
277
detected after the floating test for all supercapacitors (Table 6.4). The
differences in the performance are consequence of their different surface
chemistry. First, it is confirmed that the oxidation process strongly
affects the behavior of the capacitor, since a retention of capacitance of
20 % and an increase of resistance of 50 Ω is detected for KUA-COOH
based capacitor. Thus, the ageing protocol produces an enormous
degradation of the electrodes due to reaction between the electrolyte and
the oxygen groups present on the surface of the electrodes [32].
Otherwise, the N-doping treatments at mild conditions improve the
performance of the carbon materials. Specially, KUA-CONH2 based
capacitor shows the best performance of the all tested cells (Table 6.4).
The retention of capacitance obtained is 55%, which means an increase
of 26 % in comparison with the pristine carbon based capacitor (KUA).
Moreover, KUA-CONH2 based capacitor shows the lowest integrated
leakage current, that is associated to the ocurrence of faradaic processes
during the charging of the device (oxidation, gasification, etc.) [6, 38].
As discussed in section 6.3.1.2, KUA-CONH2 shows predominancy of
amide-like functional groups as well as a lower amount of nitrogen
heterocycles (pyridines and pyrroles/pyridones). Also, the sample has a
larger content of oxygen functional groups than the pristine sample as
consequence of the oxidation process carried out prior to the amidation.
However, the increase in stability cannot be related to the oxygen
functionalities since they damage the electrochemical stability, as
evidenced by KUA-COOH based capacitor. Actually, the increase of
retention of capacitance detected for KUA-CONH2 is even higher when
compared to the former oxidized sample KUA-COOH (36%). Thus, the
Chapter 6
278
improvement on the performance is undoubtely related to the generation
of stable nitrogen surface functional groups.
Furthermore, KUA-N based capacitor also has better performance
than the pristine carbon based capacitor (Figure 6.7a, Table 6.4),
evidencing an increase of capacitance retention of 11%. Since this
sample is synthesized by a direct replacement of oxygen groups by
nitrogen functionalities of KUA, the improvement might be a result of
the combined effect caused by the removal of detrimental oxygen groups
and generation of nitrogen moities with higher electrochemical stability
(pyrroles, pyridines, etc.). Moreover, the comparison of the N-doped
samples (KUA-N and KUA-CONH2) allows to distinguish the effect of
the different nitrogen functional groups and evidence a remarkable
stabilizing effect related to amides and cycles amides, since these
functional groups only exist on the surface of KUA-CONH2 and the
corresponding capacitor shows a higher retention of capacitance and a
lower increase of resistance than KUA-N based capacitor.
In order to deepen into the role of the different functionalities, the
effect of heat treatments on the performance of activated carbons as
electrodes for supercapacitors is throroughly discussed.
6.3.2.2. Effect of surface chemistry modification by heat treatments
The durability of the capacitors built with the heat treated samples
was also analyzed and the related parameters are collected in Table 6.4.
Some meaningful differences are found when comparing the capacitor
performance of the samples heat treated at 500 ºC. KUA-N_500 based
Electrochemical performance of N-doped activated carbons in organic electrolyte
279
capacitor has better performance than the pristine carbon material KUA,
evidenced by an increase of retention of capacitance of 10 %. This
sample has a similar surface chemistry than the parent KUA-N, with
slighly lower content of oxygen and nitrogen groups (section 6.2.1.2).
Thus, the improvement is related, again, to the removal of detrimental
oxygen functional groups as well as the generation of more stable
nitrogen groups. Nevertheless, the analogous KUA-CONH2_500 based
capacitor shows lower stability through the durability test (22 % of
capacitance retention, Table 6.4) than the related carbon based
capacitors. Interestingly, the heat treated N-doped carbons (KUA-
CONH2_500 and KUA-N_500) show different behavior upon durability
even though their surface chemistry is almost identical in terms of
amount of oxygen and nitrogen heteroatoms (section 6.2.1.2). The main
difference arises from their parent carbons (KUA-CONH2 and KUA-N).
KUA-CONH2 has a larger amount of surface functionalities, that upon
heating may decompose and generate a larger amount of reactive sites.
Consequenly, the capacitor based on the activated carbon prepared by
the heat treatment of KUA-CONH2 might experience more interactions
with the electrolyte, leading to a higher degradation process.
The heat treatment of the samples at 800 ºC also affect the durability
of the devices. It is clearly observed that the treatment under these
conditions improve the performance as consequence of the high decrease
of oxygen functionalities. Thus, KUA_800 and KUA-N_800 evidence a
similar behaviour upon the floating test, with an increase of retention of
capacitance of 14 % for both capacitors (in comparison with KUA-
Chapter 6
280
based capacitor). Hence, no further improvement results from the
nitrogen groups present on KUA-N_800. In case of KUA-CONH2_800
based capacitor, the values of retention of capacitance are higher than
the heat treated at 500 ºC due to the generation of surface functionalities
with higher electrochemical stability at this temperature
(pyrroles/pyridones). However, the performance is still poorer than that
found for the capacitors based on heat treated carbons at 800 ºC
(KUA_800 and KUA-N_800), since the surface reactivity of this carbon
material is higher than the found in the related materials.
These results evidence that heat treatments improve the performance
of carbon electrodes upon durability mostly due to the removal of
detrimental oxygen functionalities. However, the performance is further
improved when the doping strategies are carried out by wet methods at
low temperature. KUA-N and KUA-CONH2 show better performance
than the related heat treated samples not only in terms of retention of
capacitance but also in the increase of resistance. Indeed, both N-doped
carbons based capacitors evidence the lowest increase of resistance of
the whole series. This is a consequence of combining the beneficial
effect of the decrease in oxygen content with the production of nitrogen
functional groups with remarkable stabilizing effect, specially in the case
of amide-like functionalities. Also, the generation of reactive sites
during the thermal treatment is avoided under these conditions. Thus, the
nitrogen doping method at mild conditions is a promising strategy to
improve the performance of any carbon material as electrode for
supercapacitors.
Electrochemical performance of N-doped activated carbons in organic electrolyte
281
As a proof of concept, a commercial activated carbon used for
supercapacitor application was succesfully functionalized with nitrogen
and tested upon durability in organic electrolyte (Annex 6.1). After the
durability test, an increase of retention of capacitance of 7 % was
determined, evidencing the viability of this methodology to improve the
electrochemical stability of carbon materials.
6.4. Conclusions
Activated carbons with similar porosity but different surface
chemistry have been prepared by combining chemical functionalization
methods at mild conditions and post-thermal treatments. The doping
methods at low temperature produce the incroporation of 4% at. of N in
form of different functionalities by consumption of oxygen moieties.
The heat treatments diminish the content of surface functionalities and
produce rearrangements of the nitrogen groups. The surface composition
and porosity of the heat-treated samples is almost identical for the whole
series of carbons.
The surface chemistry of these carbon materials clearly influences
the electrochemical performance when tested as electrodes for
electrochemical capacitors in organic electrolyte. They show large
capacitance values with no significant differences as consequence of
their similar porous texture. The presence of oxygen functional groups
affects the rate performance of the capacitor due to the decrease of
conductivity, while nitrogen functional groups (cyclic amides, pyridines
and pyrroles) slightly increase the conductivity of the carbon material.
Chapter 6
282
The effect of surface functionalities upon durability was thoroughly
studied. The oxygen functionalities strongly damage the performance of
the activated carbons. Thus, the heat treatments of the samples produce
an improvement of the electrochemical stability due to the decrease of
detrimental oxygen groups. However, the performance is further
increased by nitrogen doping at mild conditions, since the treatment
combines the positive effect of removing oxygen groups with their
replacement by nitrogen groups with high electrochemical stability,
which is specialy beneficial in case of the generation of amide-like
functional groups.
6.5 References
[1] B.E. Conway, Electrochemical supercapacitors: Scientific
Fundamentals and Technological Applications, Springer, New
York, 1999.
[2] W. Raza, F. Ali, N. Raza, Y. Luo, K.-H. Kim, J. Yang, S. Kumar,
A. Mehmood, E.E. Kwon, Recent advancements in supercapacitor
technology, Nano Energy 52 (2018) 441–473.
[3] F. Béguin, E. Frackowiak, eds., Carbons for Electrochemical
Energy Storage and Conversion Systems, 1st ed., Boca Raton, Fl:
Taylor & Francis (CRC Press), 2010.
[4] A. Balducci, Electrolytes for high voltage electrochemical double
layer capacitors: A perspective article, J. Power Sources 326
(2016) 534–540.
[5] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Carbons and
electrolytes for advanced supercapacitors, Adv. Mater. 26 (2014).
[6] D. Salinas-Torres, S. Shiraishi, E. Morallón, D. Cazorla-Amorós,
Improvement of carbon materials performance by nitrogen
Electrochemical performance of N-doped activated carbons in organic electrolyte
283
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions, Carbon 82 (2015) 205–213.
[7] S. Shiraishi, Heat-Treatment and Nitrogen-Doping of Activated
Carbons for High Voltage Operation of Electric Double Layer
Capacitor, Key Eng. Mater. 497 (2011) 80–86.
[8] M.J. Mostazo-López, R. Ruiz-rosas, E. Morallón, D. Cazorla-
Amorós, Nitrogen doped superporous carbon prepared by a mild
method. Enhancement of supercapacitor performance, Int. J.
Hydrogen Energy 41 (2016) I9691–I9701.
[9] E. Frackowiak, F. Béguin, Carbon materials for the
electrochemical storage of energy in capacitors, Carbon 39 (2001)
937–950.
[10] J.P. Paraknowitsch, A. Thomas, Doping carbons beyond nitrogen:
an overview of advanced heteroatom doped carbons with boron,
sulphur and phosphorus for energy applications, Energy Environ.
Sci. 6 (2013) 2839–2855.
[11] Z. Yang, H. Nie, X. Chen, X. Chen, S. Huang, Recent progress in
doped carbon nanomaterials as effective cathode catalysts for fuel
cell oxygen reduction reaction, J. Power Sources 236 (2013) 238–
249.
[12] R. Berenguer, R. Ruiz-Rosas, A. Gallardo, D. Cazorla-Amorós, E.
Morallón, H. Nishihara, T. Kyotani, J. Rodríguez-Mirasol, T.
Cordero, Enhanced electro-oxidation resistance of carbon
electrodes induced by phosphorus surface groups, Carbon 95
(2015) 681–689.
[13] C. González-Gaitán, R. Ruiz-Rosas, H. Nishihara, T. Kyotani, E.
Morallón, D. Cazorla-Amorós, Successful functionalization of
superporous zeolite templated carbon using aminobenzene acids
and electrochemical methods, Carbon 99 (2016) 157–166.
[14] O. Ornelas, J.M. Sieben, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, J. Geng, N. Soin, E. Siores, B.F.G. Johnson, On the
origin of the high capacitance of nitrogen-containing carbon
nanotubes in acidic and alkaline electrolytes, Chem. Commun. 50
Chapter 6
284
(2014) 11343–11346.
[15] M.J. Mostazo-López, R. Ruiz-Rosas, A. Castro-Muñiz, H.
Nishihara, T. Kyotani, E. Morallón, D. Cazorla-Amorós,
Ultraporous nitrogen-doped zeolite-templated carbon for high
power density aqueous-based supercapacitors, Carbon 129 (2018)
510–519.
[16] W. Shen, W. Fan, Nitrogen-containing porous carbons: synthesis
and application, J. Mater. Chem. A 1 (2013) 999–1013.
[17] Y. Deng, Y. Xie, K. Zou, X. Ji, Review on recent advances in
nitrogen-doped carbons: Preparations and applications in
supercapacitors, J. Mater. Chem. A 4 (2015) 1144–1173.
[18] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano,
The role of different nitrogen functional groups on the removal of
SO2 from flue gases by N-doped activated carbon powders and
fibres, Carbon 41 (2003) 1925–1932.
[19] H. Itoi, H. Nishihara, T. Kyotani, Effect of Heteroatoms in
Ordered Microporous Carbons on Their Electrochemical
Capacitance, Langmuir 32 (2016) 11997–12004.
[20] H. Nishihara, P. Hou, L. Li, M. Ito, M. Uchiyama, T. Kaburagi,
A. Ikura, J. Katamura, T. Kawarada, K. Mizuuchi, T. Kyotani,
High-Pressure Hydrogen Storage in Zeolite-Templated Carbon, J.
Phys. Chem. C 113 (2009) 3189–3196.
[21] M.J. Mostazo-López, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Generation of nitrogen functionalities on activated
carbons by amidation reactions and Hofmann rearrangement:
Chemical and electrochemical characterization, Carbon 91 (2015)
252–265.
[22] D. Lozano-Castelló, M.A. Lillo-Ródenas, D. Cazorla-Amorós, A.
Linares-Solano, Preparation of activated carbons from Spanish
anthracite: I. Activation by KOH, Carbon 39 (2001) 741–749.
[23] D. Cazorla-Amorós, J. Alcañiz-Monge, M.A. De La Casa-Lillo,
A. Linares-Solano, CO2 as an adsorptive to characterize carbon
Electrochemical performance of N-doped activated carbons in organic electrolyte
285
molecular sieves and activated carbons, Langmuir 14 (1998)
4589–4596.
[24] D. Lozano-Castelló, D. Cazorla-Amorós, A. Linares-Solano, S.
Shiraishi, H. Kurihara, A. Oya, Influence of pore structure and
surface chemistry on electric double layer capacitance in non-
aqueous electrolyte, Carbon 41 (2003) 1765–1775.
[25] M.J. Bleda-Martínez, D. Lozano-Castelló, E. Morallón, D.
Cazorla-Amorós, A. Linares-Solano, Chemical and
electrochemical characterization of porous carbon materials,
Carbon 44 (2006) 2642–2651.
[26] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, J.
Find, U. Wild, R. Schlögl, Structural characterization of N-
containing activated carbon fibers prepared from a low softening
point petroleum pitch and a melamine resin, Carbon 40 (2002)
597–608.
[27] R.J.J. Jansen, H. van Bekkum, XPS of nitrogen-containing
functional groups on activated carbon, Carbon 33 (1995) 1021–
1027.
[28] Y. Yamada, J. Kim, S. Matsuo, S. Sato, Nitrogen-containing
graphene analyzed by X-ray photoelectron spectroscopy, Carbon.
70 (2014) 59–74.
[29] J.. Figueiredo, M.F.. Pereira, M.M.. Freitas, J.J.. Órfão,
Modification of the surface chemistry of activated carbons,
Carbon 37 (1999) 1379–1389.
[30] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H.
Zhu, G.Q. Lu, Nitrogen-Enriched Nonporous Carbon Electrodes
with Extraordinary Supercapacitance, Adv. Funct. Mater. 19
(2009) 1800–1809.
[31] R. Arrigo, M. Hävecker, S. Wrabetz, R. Blume, M. Lerch, J.
McGregor, E.P.J. Parrott, J.A. Zeitler, L.F. Gladden, A. Knop-
Gericke, R. Schlögl, D.S. Su, Tuning the acid/base properties of
nanocarbons by functionalization via amination, J. Am. Chem.
Soc. 132 (2010) 9616–9630.
Chapter 6
286
[32] D. Cazorla-Amorós, D. Lozano-Castelló, E. Morallón, M.J.
Bleda-Martínez, A. Linares-Solano, S. Shiraishi, Measuring cycle
efficiency and capacitance of chemically activated carbons in
propylene carbonate, Carbon 48 (2010) 1451–1456.
[33] V.. Strelko, V.. Kuts, P.. Thrower, On the mechanism of possible
influence of heteroatoms of nitrogen, boron and phosphorus in a
carbon matrix on the catalytic activity of carbons in electron
transfer reactions, Carbon 38 (2000) 1499–1503.
[34] S. Shiraishi, Electrochemical Performance, in: M. Inagaki (Ed.),
Mater. Sci. Eng. Carbon Charact., Tsinghua University Press
Limited. Elsevier Inc, 2016: pp. 205–226.
[35] D. Weingarth, H. Noh, a Wokaun, R. Kötz, A reliable
determination method of stability limits for electrochemical
double layer capacitors, Electrochim. Acta 103 (2013) 119–124.
[36] D. Weingarth, A. Foelske-Schmitz, R. Kötz, Cycle versus voltage
hold - Which is the better stability test for electrochemical double
layer capacitors?, J. Power Sources 225 (2013) 84–88.
[37] P.W. Ruch, D. Cericola, A. Foelske-Schmitz, R. Kötz, A.
Wokaun, Aging of electrochemical double layer capacitors with
acetonitrile-based electrolyte at elevated voltages, Electrochim.
Acta 55 (2010) 4412–4420.
[38] P. Ratajczak, K. Jurewicz, F. Béguin, Factors contributing to
ageing of high voltage carbon/carbon supercapacitors in salt
aqueous electrolyte, J. Appl. Electrochem. 44 (2014) 475–480.
[39] R. Kötz, M. Hahn, R. Gallay, Temperature behavior and
impedance fundamentals of supercapacitors, J. Power Sources154
(2006) 550–555.
[40] O. Bohlen, J. Kowal, D.U. Sauer, Ageing behaviour of
electrochemical double layer capacitors: Part I. Experimental
study and ageing model, J. Power Sources 172 (2007) 468–475.
Electrochemical performance of N-doped activated carbons in organic electrolyte
287
ANNEX TO CHAPTER 6.
Chemical functionalization of a commercial activated
carbon with nitrogen groups
In Chapter 6, the effect of nitrogen doping on the performance of
activated carbons as electrodes for supercapacitors has been studied. It
was found that nitrogen doping at mild conditions can improve the
electrochemical stability and consequently increase the durability of the
devices. This is mainly explained by the replacement of oxygen
functional groups (that degrade the carbon materials at extreme
potentials) by nitrogen groups with high electrochemical stability. Thus,
it appears as a promising method for improving the performance of any
carbon electrode, independently of the synthesis method.
In this annex, the nitrogen doping strategy developed in Chapter 4 is
used to functionalize a commercial activated carbon (YP50F, Kuraray
Chemical, Japan) that is used as electrode in supercapacitors. The
nitrogen doping method described in section 6.2.2.2 was applied to
YP50F and the obtained sample was named as YP50F-N.
Table A6.1. Textural properties and elemental surface composition (XPS and TPD) for
YP50F and YP50F-N.
Sample NXPS
(at.%)
OXPS
(at.%)
CO2 TPD
(µmol/g)
COTPD
(µmol/g)
SBET
(m2/g)
VDRN2
(cm3/g)
YP50F - 7.0 160 520 1790 0.71
YP50F-N 2.3 5.8 100 560 1740 0.69
Chapter 6
288
Table A6.1 summarizes the textural properties and surface
composition of the pristine and modified carbons. The nitrogen
functionalization protocol (avoiding the oxidation process) over YP50F
produces the attachment of nitrogen functional groups (2.3 at. % XPS,
Table A6.1) as well as a decrease of oxygen of 1.2 at. % XPS,
evidencing that nitrogen is anchored to the surface by substitution of
oxygen moieties. Figure A6.1 shows the N1s XPS spectrum recorded for
YP50F-N, that evidences that nitrogen is attached to the surface in form
of pyridines, amines/amides and pyrroles/pyridones [1–4].
396398400402404
I (a
.u.)
Binding Energy (eV)
Figure A6.1. N1s XPS spectrum obtained for YP50F-N activated carbon.
Electrochemical performance of N-doped activated carbons in organic electrolyte
289
0
200
400
600
800
1000
0 0.2 0.4 0.6 0.8 1
Ad
sorb
ed
vo
lum
e (
cm
3/g
)
P/Po
YP50F YP50F-N
Figure A6.2. N2 adsorption-desorption isotherms of the activated carbons YP50F and
YP50F-N.
The activated carbon YP50F provides large apparent surface area
with well-developed microporosity (Figure A6.2). As happened when
using the activated carbon KUA (Chapters 4 and 6), the chemical
functionalization does not reduce or block the microporosity of the
pristine carbon material and allows the preservation of the whole
apparent surface area. Thus, these carbons have identical porous texture
but different surface chemistry.
The carbon materials were electrochemically characterized in a
three-electrode cell in aqueous electrolyte. Figure A6.3 shows the CVs
obtained in a potential window where the materials are stable. Both
evidence a rectangular shape characteristic of porous carbons and
provide large capacitance values (156 and and 131 F/g for non-doped
and N-doped activated carbons, respectively). However, the CVs display
Chapter 6
290
some differences in the electrochemical performance of the materials.
YP50F exhibits the characteristic redox pair associated with the
quinone-hydroquinone redox process [5], while this response is not
observed in the case of the N-functionalized carbon. As seen in Chapter
4, these electroactive oxygen groups can be responsible of degradation
processes under oxidation conditions, and consequently, the lower
occurrence of this process may lead to an improvement of the durability
of supercapacitors based on this N-doped carbon. To deepen into the
performance of this material, the durability of the corresponding
capacitors was evaluated in organic medium under severe conditions of
temperature and voltage, as described in Chapter 6. Figure A6.4 shows
the evolution of capacitance and coulombic efficiency during the
durability test performed for YP50F and YP50F-N. As happened with
N-doped KUA, the nitrogen doping does not affect the capacitance in
organic electrolyte due to preservation of the microporosity. Also, the
nitrogen functionalities do not increase the capacitance. However, there
is an improvement of retention of capacitance upon durability of 7 %
that is related to the generation of nitrogen groups with high
electrochemical stability produced by the nitrogen doping at mild
conditions.
Electrochemical performance of N-doped activated carbons in organic electrolyte
291
-250
-200
-150
-100
-50
0
50
100
150
200
-0.2 0 0.2 0.4 0.6 0.8
C (
F/g
)
E (V vs Ag/AgCl/KCl)
Figure A6.3. Cyclic voltammograms in the potential range between 0V and 0.6V for
YP50F and YP50F-N electrodes. 1M H2SO4. V=1 mV/s.
0
20
40
60
80
100
120
0
10
20
30
40
50
60
70
0 5 10 15 20 25 30 35
Eff
icie
ncy (
%)
Cap
acit
an
ce (
F/g
)
Nº Cycles
YP50F
YP50F-N
70ºC0-2.5V
70ºC0-3.2V
70ºC3.2V
100h
70ºC0-3.2V
70ºC0-2.5V
40ºC0-2.5V
40ºC0-2.5V
Figure 6.4. Evolution of capacitance and coulombic efficiency during the durability
test for YP50F and YP50F-N based capacitors. 1M TEMABF4. j = 40 mA/g.
Chapter 6
292
In conclusion, a microporous commercial activated carbon used for
supercapacitor application has been functionalized with nitrogen groups
by following organic chemistry protocols. The chemical method allowed
the incorporation of 2 N at. % in form of different groups, by
consumption of oxygen functionalities. The microporosity of the pristine
carbon is fully retained after the chemical treatment. The nitrogen
doping affects the electrochemical response of the carbon in acid
medium, by removing the electroactivity associated to the quinone-
hydroquinone redox pair. Consequently, this leads to an improvement of
the electrochemical stability of the N-doped carbon which is
demonstrated when tested as electrode for electrochemical capacitor in
organic electrolyte.
References.
[1] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano,
The role of different nitrogen functional groups on the removal of
SO2 from flue gases by N-doped activated carbon powders and
fibres, Carbon 41 (2003) 1925–1932.
[2] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, J.
Find, U. Wild, R. Schlögl, Structural characterization of N-
containing activated carbon fibers prepared from a low softening
point petroleum pitch and a melamine resin, Carbon 40 (2002)
597–608.
[3] R.J.J. Jansen, H. van Bekkum, XPS of nitrogen-containing
functional groups on activated carbon, Carbon 33 (1995) 1021–
1027.
[4] Y. Yamada, J. Kim, S. Matsuo, S. Sato, Nitrogen-containing
graphene analyzed by X-ray photoelectron spectroscopy, Carbon
70 (2014) 59–74.
Electrochemical performance of N-doped activated carbons in organic electrolyte
293
[5] M.J. Bleda-Martínez, D. Lozano-Castelló, E. Morallón, D.
Cazorla-Amorós, A. Linares-Solano, Chemical and
electrochemical characterization of porous carbon materials,
Carbon 44 (2006) 2642–2651.
Chapter 7
Electrochemical performance of
N-doped activated carbons in
non-conventionalelectrolytes
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
297
7.1. Introduction
Electrochemical double-layer capacitors (EDLC), or
supercapacitors, are currently the most suitable energy storage devices
for a wide range of high-power applications. These devices are based on
the physical electrosorption of ions at the interface of an electrode and
an electrolyte [1–3]. Consequently, EDLC can be charged and
discharged in seconds, providing high specific power (10 kW/kg) and
long-cycle life (> 500000 cycles). Nevertheless, the energy density
needs to be increased to introduce this technology into new applications,
such as electric vehicles. This parameter depends on the capacitance and
the cut-off voltage employed during the operation of the device, which
are mostly governed by the two main components of supercapacitors: the
electrolyte and the electrode material. The current commercially
available supercapacitors are based on activated carbons, whose large
surface area produces high capacitance values, and organic electrolytes,
based on solutions of an ammonium salt in propylene carbonate (PC) or
acetonitrile, which can operate at 2.5-2.8 V [1,4]. One of the main
strategies for improving their performance is the use of innovative
electrolytes, such as those based on ionic liquids (ILs) or new
conducting salts (in organic solvents), since they can expand the
operation voltage to values larger than 3V [5,6].
The development of new electrolytes based on ILs has attracted
consiberable attention due to their wide electrochemical stability
window (>3V), high thermal stability, low volatility and non-
flammability [5,7–10]. However, their high viscosity and low
Chapter 7
298
conductivity (in comparison with conventional organic electrolytes)
hinders the capacitance and power provided by EDLC based in solvent-
free ILs. Recently, the use of mixtures of ILs with organic solvents (PC
or acetonitrile), in which IL acts as a conducting salt, has appeared as a
promising alternative because it allows the production of electrolytes
with low viscosity and higher conductivity. However, the operative
voltage oof EDLCs containing these electrolytes is often limited in
comparison with that of EDLC based on solvent-free ILs, but still
overcomes the values of conventional organic electrolytes.
An alternative to the use of ILs is the investigation of new
conducting salts, dissolved in an organic solvent (PC or acetonitrile), for
the preparation of advanced electrolytes [5]. The use of these salts
allows the preparation of electrolytes with high conductivity and low
viscosity. Furthermore, the EDLCs based on these electrolytes perform
at voltages larger than 3V. Several studies have been dedicated to
different cations (phosphonium and quaternary ammonium-based
cations) combined mainly with tetrafluoroborate (BF4-) as anion.
Recently, pyrrolidinium-based cations have appeared as promising
candidates for the generation of advanced electrolytes [1,4]. Since the
ions of the conducting salts are involved on the formation of electrical
double layer, the selection of a conducting salt with adequate ion sizes,
chemical and electrochemical stability is crucial for the development of
advanced electrolytes [5].
Moreover, the appropriate selection of ions has a strong influence on
the carbon-electrolyte interface [11–13]. To take full benefit from the
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
299
advantages of innovative electrolytes, the physicochemical properties of
carbon materials must be also carefully considered. In this sense, carbon
materials with well-developed microporosity and adequate pore size
distribution (with optimized ion size/pore size ratios) are highly
desirable since they provide high specific capacitance [11–14] However,
their electrochemical stability can be compromised when using high
voltage operation conditions. We have proven previously that nitrogen-
doped carbon materials can improve several properties, such as electrical
conductivity or electrochemical stability (in aqueous and organic
electrolytes) [15–17], avoiding detrimental reactions with the electrolyte
and increasing the durability of the device. However, the role of surface
chemistry, and specifically nitrogen functionalities, has not been
assessed yet in non-conventional electrolytes. As discussed above, the
performance of EDLC strongly depends on the interation at the
electrode-electrolyte interface. Thus, surface chemistry appears as a key
parameter to be investigated for the development of EDLCs based in
non-conventional electrolytes.
In this chapter, the effect of nitrogen functionalization treatments on
the electrochemical properties of the activated carbons with high surface
area was analyzed using non-conventional electrolytes. For this, nitrogen
functionalization is carried out throught chemical post-treatments at mild
conditions over a superporous activated carbon with tailored porous
structure. The effect of nitrogen functional groups on the
electrochemical performance of the carbon materials is assessed in two
PC-based electrolytes: (i) a non-conventional conducting salt (1-butyl-1-
Chapter 7
300
methylpyrrolidium tetrafluoroborate (Pyr14BF4)) in PC and (ii) a mixture
of ionic liquid (1-butyl-1-methylpyrrolidium bis-(tri-
fluoromethylsulfonyl)imide (Pyr14TFSI)) with PC.
7.2. Materials and Methods
7.2.1. Activated carbon
A superporous activated carbon prepared in our laboratory has been
used as the starting material for nitrogen incorporation via post-
modification treatments based in organic chemistry reactions. The
pristine material (named as KUA), has been obtained by chemical
activation of a Spanish anthracite with KOH using an impregnation ratio
of activating agent to raw material of 4:1 and an activation temperature
of 750º C under inert atmosphere, which was held for 1 hour. More
details about the preparation process are available elsewhere [18].
7.2.2. Chemical functionalization of activated carbon
KUA was further functionalized with nitrogen functional groups by
two different strategies based on the organic chemistry protocols. This
methodology was accurately described in chapters 3 and 4. Briefly, the
first approach consisted in a three-step protocol: (i) chemical oxidation
with HNO3, (ii) treatment with SOCl2 and (iii) amidation reaction with
NH4NO3/DMF and pyridine. The obtained sample was named as KUA-
CONH2. In the second method, the third step of the first protocol is
directly applied over pristine sample (KUA). This sample was named as
KUA-N.
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
301
7.2.3. Porous texture and surface chemistry characterization
The porous texture characterization was carried out by N2
adsorption-desorption isotherms at -196º C and by CO2 adsorption at 0º
C by using an Autosorb-6-Quantachrome apparatus. The samples were
outgassed at 200º C for 4 hours before the experiments. The apparent
surface area was obtained from N2 adsorption isotherms by using the
BET equation in the 0.05-0.20 range of relative pressures. The total
micropore volume was determined by Dubinin-Radushkevich (DR)
method applied to N2 (relative pressures from 0.01 to 0.05) adsorption
isotherms. The volume of the narrow microporosity (i.e., pore sizes
below 0.7 nm) was calculated from the DR method applied to the CO2
adsorption isotherms (relative pressures from 0.0001 to 0.25) [19].
The surface chemistry of the samples was analyzed by X Ray
Photoelectron Spectroscopy (XPS) and Temperature Programmed
Desorption (TPD). XPS measurements were performed by using a VG-
Microtech Multilab 3000 spectrometer, equipped with an Al anode. TPD
experiments were performed by heating the samples (10 mg) to 950º C
(at a heating rate of 20º C/min) under a helium flow rate of 100 mL/min.
The analyses were carried out by using a TGA-DSC instrument (TA
Instruments, SDT Q600 Simultaneous) coupled to a mass spectrometer
(Thermostar, Balzers, BSC 200).
7.2.4. Electrolyte preparation
Two different electrolytes were employed in this chapter: 1M
Pyr14TFSI/PC and 1M Pyr14BF4/PC. The IL Pyr14TFSI (IoLiTec, 99.5%)
Chapter 7
302
was dried by using a molecular sieve until the water content was lower
than 20 ppm. The conducting salt Pyr14BF4 was purified as described
elsewhere [20]. PC (Sigma Aldrich, 99.7 %) was used as organic
solvent. The water content of the electrolytes was determined to be
lower than 30 ppm, as measured by Karl-Fisher technique.
7.2.5 Electrochemical characterization
7.2.5.1. Three electrode cell configuration
The electrochemical characterization of the carbon materials was
performed by using a Swagelock-type cell in a three-electrode
configuration. The working electrodes were prepared by mixing the
activated carbon with acetylene black and polytetrafluoroethylene
(PTFE) as binder in a ratio of 85:10:5 (w/w). The total weight of the
electrode was 4-5 mg (dry basis). For shaping the electrodes, a sample
sheet was cut into a circular shape with an area of 0.95 cm2. The counter
electrodes were prepared using a commercial activated carbon as active
material (Norit DLC Super30, SBET = 1618 m2/g) with a mass loading
larger than 30 mg/cm2, as described elsewhere [14]. The electrodes were
dried overnight at 120 ºC under vacuum prior to assembling the cell.
The cells were prepared inside an Ar-filled glovebox with water and
oxygen contents < 1 ppm. The working and counter electrodes were
tightly pressed against each other and separated by a glass microfiber
membrane (Whatman GF/D, thickness: 675 µm). Prior to assembling the
cell, the separator and the electrodes were soaked with 140 µL of
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
303
electrolyte and kept under vaccum for 5 minutes. Ag was used as
reference electrode in all cases.
The electrochemical characterization of all samples was tested by
CV at sweep rate of 1 mV/s. The capacitance was calculated from the
electric charge of the CV. The results are expressed in F/g, considering
the weight of the active material of the working electrode.
7.2.5.2. EDLC investigation
Asymmetric in mass capacitors were assembled for all carbon
materials in an Argon glovebox in 1M PYR14TFSI/PC electrolyte. For
this, two electrodes (surface area: 0.283 cm2) were prepared with a
weight of ~1.1 mg (active phase) each. Supercapacitors were constructed
by pressing both electrodes against each other and separating them by a
filter glass microfiber membrane (Whatman GF/D, thickness: 675 µm).
These devices were characterized by CV at different scan rates and
galvanostatic charge-discharge (GCD) cycles at current densities from 1
to 20 A/g in 1M PYR14TFSI/PC. CV measurements were performed in a
Biologic VSP 300 and the galvanostatic tests in a Arbin SCTS
galvanostat. A durability test for asymmetric capacitors was performed
by 2000 GCD cycles at a current density of 1 A/g and a voltage of 3.0 V.
Current density and specific capacitance is defined based on the total
active weight of the carbon material included in both electrodes.
The energy density and power density of asymmetric
supercapacitors was calculated in order to obtain all relevant information
Chapter 7
304
about their performance. Energy density was obtained during the
discharge cycle by the following equation (1):
E = (7.1)
Where V1 is the cell voltage of charge (3V) and V2 the voltage of
discharge (0 V).
Power density was calculated according to equation (2):
(7.2)
Where td is the time of discharge in the GCD cycle.
7.3 Results and Discussion
7.3.1 Surface chemistry and porous texture characterization
Table 7.1 summarizes the main parameters related to the
physicochemical properties of the pristine and N-functionalized
activated carbons. These properties and the effect of nitrogen doping on
the surface chemistry and porous texture of the samples has been
previously discussed (in chapters 3, 4 and 6) and are summarized below.
As evidenced by XPS analyses, nitrogen doping was satisfactorily
achieved for KUA-N and KUA-CONH2 samples through consumption
of oxygen functionalities. Both N-doped carbons show similar nitrogen
spectra and content (⁓ 4 at. % XPS), but the functionalities generated on
their surfaces are different. This is a consequence of the different oxygen
functional groups involved in the reaction. For the production of KUA-N
sample, the nitrogen doping is carried out directly over the pristine
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
305
carbon material; as a result, the nitrogen functionalities generated on this
carbon are derived mainly from CO-evolving groups. Thus, the main
functional groups found over this carbon material are amines,
pyridines/imines and pyrroles/pyridones (chapters 4 and 6) [16].
Nevertheless, the nitrogen functional groups produced on KUA-CONH2
involve the consumption of CO2 and CO evolving-groups, since the
pristine carbon was previously oxidized to increase the oxygen content
prior to the nitrogen doping step. Hence, there is a contribution of groups
derived from CO2 (amide-like functionalities) and CO groups (pyridines,
pyrroles, etc.), as previously discussed in chapter 3 [21]. Consequently,
the N-doped carbons show similar nitrogen content, but different surface
functionalities. Also, KUA-CONH2 has a larger amount of oxygen
functional groups than the other carbons.
Table 7.1. Surface composition determined for all activated carbons by XPS and TPD.
Sample CXPS
(at. %)
NXPS
(at.%)
OXPS
(at.%)
CO2 TPD
(µmol/g)
CO TPD
(µmol/g)
O TPD
(µmol/g)
KUA 90.9 0.3 8.8 500 2100 3100
KUA-N 87.6 3.6 8.8 450 1750 2640
KUA-CONH2 88.8 4.1 12.0 1140 2370 4650
The porous texture of the samples was assessed by N2 and CO2
adsorption isotherms. The pristine carbon material is a microporous
material with high apparent surface area (chapter 4) [16]. Moreover,
most of the microporosity of the pristine material remains in the N-
doped samples due to the mild conditions of the treatment. When the
nitrogen doping is carried out directly over the pristine carbon (KUA-N
Chapter 7
306
sample), the microporosity is fully preserved since only chemical
reactions involving CO groups take place to attach nitrogen moieties.
However, KUA-CONH2 evidences some loss of the microporosity (30
%) mainly due to the oxidation pre-treatment done prior to the nitrogen
functionalization treatment [21].
Table 7.2. Porous texture parameters of the activated carbons.
Sample SBET
(m2/g)
SNLDFT
(m2/g)
VDRN2
(cm3/g)
VDRCO2
(cm3/g)
KUA 3080 2230 1.19 0.57
KUA-N 2960 2270 1.18 0.52
KUA-CONH2 2390 1800 0.97 0.45
Figure 7.1 illustrates the pore size distributions obtained for the
pristine and modified activated carbons. The sample KUA shows a wide
pore size distribution, with an average pore size of 1.4 nm. Moreover,
the profile shows a contribution of micropores with size lower than 1
nm. The nitrogen doping does not significantly modify the pore size of
the samples. The small differences in the profile are a consequence of
discrepancies in the fitting performed by NL-DFT. However, the
nitrogen functionalization treatment seems to decrease the size of the
micropores in the whole range of porosity. Thus, the nitrogen doping at
mild conditions allows the generation of carbon materials with similar
porous texture but different functionalities.
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
307
0
0.2
0.4
0.6
0.8
1
0 1 2 3 4
Pore
volu
me (
cm
3/g
nm
)
Pore width (nm)
KUA
KUA-N
KUA-CONH₂
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3
Pore
volu
me (
cm
3/g
)
Pore width (nm)
KUAKUA-N
KUA-CONH₂
(b)
Figure 7.1. (a) Differential and (b) cumulative pore size distributions of KUA, KUA-N
and KUA-CONH2 by DFT calculations.
7.3.2 Electrochemical characterization
7.3.2.1 Three-electrode cell configuration
The electrochemical performance of the samples was tested in three-
electrode cells in 1M Pyr14TFSI/PC and 1M Pyr14BF4/PC. Figure 7.2
shows the CVs recorded for the carbon materials from the open circuit
potential (EOCP) upon negative (Figure 7.2a) and positive polarization
(Figure 7.2b) in 1M Pyr14BF4/PC. The pristine activated carbon (KUA)
Chapter 7
308
displays faradaic processes under conditions of negative polarization. A
cathodic current is recorded starting at -1.70 V, with the corresponding
reverse anodic peak at -0.30 V. These faradaic charge transfer reactions
can be associated to oxygen functionalities and decomposition of PC
[14,22–24]. At positive polarization, a redox processes are observed at
1.0 V, and an anodic current starts at 1.2 V, evidencing the occurrence of
oxidation processes.
The CVs clearly evidence than nitrogen doping affects the
electrochemical performance of the activated carbons. First, the N-doped
carbons (KUA-N and KUA-CONH2) display lower EOCP than the parent
non-doped carbon, indicating that the generation of nitrogen
functionalities modifies the carbon-ion interaction during non-charging
regime [25]. Moreover, the nitrogen doping affects the electrochemical
stability of the carbon materials. At negative polarization from the open
circuit potential, both samples present a faradaic process at -1.70 V.
However, the current values reached during the reverse scan are much
lower than those of the non-doped carbon, highlighting the lower
ocurrence of degradation processes in the N-doped carbons.
Furthermore, the N-doped carbons show a CV with a square shape
among the whole potential range, evidencing an ideal EDLC behavior
even at potentials close to the limit value of stability.
Similar features are observed at the positive potential range (Figure
7.2a). The lower EOCP detected for the N-doped carbons provides a wider
potential window upon positive polarization. Thus, the symmetric
configuration in supercapacitor cells using these materials might provide
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
309
higher durability, since a better balance of the charge is expected during
the operation of the device. Moreover, at high positive potentials (> 1.2
V), the faradaic current detected for the N-doped carbon materials is
considerably lower than the one exhibited by KUA, demostrating the
improvement of electrochemical stability under oxidative conditions due
to the formation of nitrogen groups.
-400
-200
0
200
400
600
-0.2 0.2 0.6 1 1.4 1.8
Ca
pa
cit
an
ce (
F/g
)
E vs Ag (V)
(b)
-400
-200
0
200
400
600
-2 -1.4 -0.8 -0.2 0.4
Ca
pa
cit
an
ce (
F/g
)
E vs Ag (V)
(a)
Figure 7.2. Cyclic voltammograms obtained for KUA (black), KUA-N (blue) and
KUA-CONH2 (red) electrodes in (a) negative and (b) positive potential ranges. 1M
Pyr14BF4/PC. v = 1 mV/s.
-400
-200
0
200
400
600
-0.2 0.3 0.8 1.3 1.8
Ca
pa
cit
an
ce (
F/g
)
E vs Ag (V)
(b)
-400
-200
0
200
400
600
-2.5 -1.9 -1.3 -0.7 -0.1
Ca
pa
cit
an
ce (
F/g
)
E vs Ag (V)
(a)
Figure 7.3. Cyclic voltammograms obtained for KUA (black), KUA-N (blue) and
KUA-CONH2 (red) electrodes in (a) negative and (b) positive potential ranges. 1M
Pyr14TFSI/PC. v = 1 mV/s.
Chapter 7
310
The potential window of the carbon materials was also analyzed by
CV in 1M Pyr14TFSI/PC. As detected in 1M Pyr14BF4/PC, the carbon
electrodes exhibit a cathodic current related to decomposition processes.
However, in this case the faradaic current is detected at more negative
potentials (- 2.0 V), evidencing a larger electrochemical stability
window in case of the IL mixture. Under conditions of positive
polarization, KUA exhibits larger faradaic currents when increasing the
potential, evindencing lower electrochemical stability also in this
electrolyte. Furthermore, KUA-CONH2 electrode displays an increase of
faradaic cathodic current at 1.5 V, that is not observed in case of KUA-N
electrode, evidencing lower electrochemical stability for the former than
for the latter.
Even though surface chemistry influences the electrochemical
stability of the carbon materials in the electrolytes, all of them display
larger electrochemical stability window in IL-based electrolyte than in
the other electrolyte. This result is in agreement with the performance of
other carbons in the literature [11,13,26,27].
The gravimetric capacitance was determined using a potential
window in absence of faradaic contributions. Table 7.3 collects the
values obtained under positive (C+) and negative (C-) polarization in
both electrolytes. However, it should be remarked that C+ and C- cannot
be exclusively associated to adsorption of anions and cations,
respectively. Several studies have demonstrated that the charge storage
in supercapacitors is not produced by single electrosorption of ions on
the surface of the charged electrode. The formation of the EDL can be
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
311
produced by different mechanisms depending on the electrolyte and the
electrode, as well as the electrode polarization [28,29]. Indepently of
mechanism governed in the formation of the EDL, the relative pore/ion
size is a key parameter to be considered [13,14,30,31], since the ions
might diffuse across the pore network involved in the charging process.
The pristine activated carbon displays extraordinary capacitance
values in both electrolytes because its porosity has been tailored to be
used in non-conventional electrolytes [14]. Even though 1M
Pyr14TFSI/PC shows higher viscosity than 1M Pyr14BF4/PC (Table 7.4),
both electrolytes provide large capacitance values when using KUA as
electrode material. The main differences in terms of capacitance are a
consequence of the different sizes of the ions: 0.46 nm (BF4-), 0.79 nm
(TFSI-) and 1.1 nm (Pyr14+) [11,32]. On the other hand, KUA has a large
micropore volume and an average pore size of 1.4 nm. Hence, KUA
provides an adequate pore size distribution to allow the diffusion of the
electrolyte-containing ions to its inner surface.
The N-doped activated carbons show slightly lower capacitance
values in both electrolytes. This is related to the small loss of
microporosity produced during the functionalization treatment. KUA-N
evidenced an almost insignificant modification of porous texture.
However, the ions of the electrolytes have large sizes, and consequently
small changes on the pore width might affect the formation of the EDL.
Thus, the differences in capacitance might arise from the small
modification of the pore sizes in these samples, as well as the slightly
lower available surface area. More interesting features are observed for
Chapter 7
312
KUA-CONH2 activated carbon. This sample displays similar
capacitance values in 1M Pyr14TFSI/PC than KUA-N, but slightly lower
values in 1M Pyr14BF4/PC. Since KUA-CONH2 shows lower micropore
volume than KUA-N, the formation of EDL in the IL-based electrolyte
might not involve the whole microporosity of both carbons, but only a
range of micropores that contain both carbon materials. Since BF4- anion
has lower size than TFSI-, it can access smaller micropores. Hence, the
significantly higher capacitance values detected in 1M Pyr14BF4/PC for
KUA-N, mainly in the positive potential window, can be a consequence
of the significant increase of micropores lower than 1nm observed after
functionalization in this sample.
Table 7.3. Gravimetric capacitance calculated for the carbon electrodes in the
electrolytes in different potential ranges.
Sample 1 M Pyr14BF4/PC 1M Pyr14TFSI/PC
C+ (F/g) C-(F/g) C+ (F/g) C-(F/g)
KUA 187 185 168 193
KUA-N 165 161 153 163
KUA-CONH2 147 153 157 165
Table 7.4. Viscosity and conductivity of the electrolytes at 20 ºC [11].
Electrolyte Viscosity
(mPa s)
Conductivity
(mS/cm)
1M Pyr14BF4/PC 4.2 9.8
1M Pyr14TFSI/PC 5.1 7.3
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
313
7.3.2.2 EDLC investigation
Asymmetric capacitors (in mass) were constructed to evaluate the
performance of the carbon electrodes at high operation voltage (3 V) in
1M Pyr14TFSI/PC. The mass loading of the carbon materials was
balanced by following the methodology proposed by Peng et al. [33]. By
following this strategy, the performance of the EDLC can be enhanced
and the premature ageing of cells should be avoided. For this, the
capacitance values were determined in the electrochemical stability
windows determined in section 7.3.2.1. The mass ratio was calculated by
using the equation (3):
(7.3)
Where w+/w- is the mass ratio of the electrodes, C+ and C- are the
capacitance values under positive and negative polarization, and ΔV+
and ΔV- are the positive and negative windows, respectively.
Figure 7.4a shows the CV obtained for the three activated carbon-
based capacitors at 20 mV/s. The capacitors display an ideal rectangular
shape, characteristic of electrical double layer, and provide large
capacitance values (37-40 F/g). The lowest value was determined for
KUA-CONH2 as consequence of its lower porosity. Figure 7.4b
illustrates the evolution of capacitance when increasing the scan rate.
The capacitors provide high retention of capacitance at large scan rates,
demostrating values of ⁓ 20 F/g at 500 mV/s. The capacitors based on
KUA-N and KUA-CONH2 supply slighly higher retention of
capacitance, as happened in aqueous (chapters 3 and 4) [16,21] and
Chapter 7
314
organic electrolytes (chapter 6). The improvement of capacitance
retention in all media might be undoubtely related to an enhancement of
electrical conductivity due to the substitution of electron-withdrawing
oxygen functional groups with electron-donor nitrogen functionalities
[34].
-60
-30
0
30
60
0 0.5 1 1.5 2 2.5 3
Cap
acit
an
ce (
F/g
)
Voltage (V)
(a)
0
10
20
30
40
50
0 200 400 600
Cap
acit
an
ce (
F/g
)
Scan rate (mV/s)
(b)
Figure 7.4. (a) Cyclic voltammograms obtained for KUA (black), KUA-N (blue) and
KUA-CONH2 (red) based capacitors. v = 20 mV/s. (b) Evolution of capacitance with
the scan rate (20, 50, 100, 200 and 500 mV/s). 1M Pyr14TFSI/PC.
The performance of the capacitors was analyzed using a cycling test at
different current densities. Figure 7.5 illustrates the GCD cycles and the
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
315
Ragone plot obtained for the capacitors. Table 7.5 collects the
parameters of the devices obtained from the cycling test. The
gravimetric capacitance values are among the largest reported in the
literature in IL-based electrolytes [35–39]. Furthermore, the devices
show outstanding performance in terms of volumetric capacitance since
the carbon electrodes contain almost negligible mesoporosity, which is
commonly used to enhance the performance in ILs [35,36,38,39] (not
reported in these references). Hence, the combination of highly
microporous activated carbons with IL-based electrolytes allows the
production of devices with excellent energy densities in gravimetric and
volumetric basis (44-48 Wh/kg and 16-17 Wh/L, respectively, Table
7.5).
Table 7.5. Gravimetric capacitance (Cg), energy density (E), coulombic efficiency and
energy efficiency determined for KUA, KUA-CONH2 and KUA-N asymmetric
supercapacitors at the voltage 3V by GCD cycles. 1M Pyr14TFSI/PC. j=1 A/g.
Sample Cg
(F/
g)
Cv
(F/cm3)
Eg
(Wh/Kg)
Ev
(Wh/L)
Coulombic
efficiency
(%)
Energy
efficiency
(%)
KUA 40 14 46 16 99 84
KUA-N 40 14 48 17 100 89
KUA-
CONH2
37 14 44 16 100 87
Moreover, the N-doped carbon-based capacitors evidence larger
coulombic and energy efficiency than the pristine KUA-based capacitor,
providing better reversibility of the N-doped samples upon cycling. This
must be related to the lower occurrence of faradaic processes, as
discussed in section 7.3.2.1. Thus, the generation of stable nitrogen
Chapter 7
316
functionalities, as well as the decrease of detrimental oxygen groups,
enhance the electrochemical performance of carbon electrodes, as
happened in organic and aqueous electrolytes.
0
0.5
1
1.5
2
2.5
3
3.5
0 50 100 150 200 250
Volt
age (
V)
Time (s)
KUA
KUA-N
KUA-CONH₂
(a)
1
10
100
1 10
E (
Wh
/Kg)
P (kW/Kg)
KUA
KUA-N
KUA-CONH₂
(b)
Figure 7.5. (a) Galvanostatic charge-discharge cycles for KUA, KUA-N and KUA-
CONH2 asymmetric supercapacitors at 3V and 1A/g. (b) Ragone plot at 3V for all
asymmetric supercapacitors. j = 1-10 A/g. 1M Pyr14TFSI/PC.
Figure 7.5 illustrates the Ragone plot obtained for all activated
carbon-based capacitors in 1M Pyr14TFSI/PC. At low power density (1.5
kW/kg), the cells provide similar energy density values (44-48 Wh/kg,
Table 7.5). However, the N-doped carbon-based capacitors can keep
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
317
higher values when increasing the power density up to 9 kW/kg
(14Wh/kg for KUA, and 15-18 Wh/kg for the N-doped activated carbon-
based capacitors). This is related to the improvement of conductivity
produced by the nitrogen functional groups [15,16,21,34].
Preliminary investigation about the durability of the capacitors have
been made carrying out GCD cycles at 3 V. Figure 7.6 shows the
evolution of capacitance along cycles during the cyclability test for
capacitors built with KUA and KUA-N since both have similar
porosities but different electrochemical properties. The non-doped
carbon-based capacitor experiences a remarkable loss of capacitance
during the first thousand cycles, and afterwards it retains around 60 % of
the initial capacitance. Interestingly, the capacitor based on KUA-N
carbon exhibits a stable performance along cycles, providing a
capacitance retention of 90 % after the durability test. This improvement
might be undoubtedly related to the lower ocurrence of faradaic
reactions during the charge-discharge process, that lead to the
degradation of the electrodes and electrolytes [22,24]. Since the
materials employed in the construction of the cells have very similar
porous texture, the differences on the performance might be related to
the modifications on the surface chemistry produced by the nitrogen
doping treatment. The reduction of detrimental oxygen functional groups
[22] as well as the generation of nitrogen groups with higher
electrochemical stability [16,17] are responsible of the improvement of
the performance of N-doped activated carbon-based supercapacitors.
Chapter 7
318
0
0.2
0.4
0.6
0.8
1
1.2
0 500 1000 1500 2000
C/C
0
Nº Cycles
KUA KUA-N
Figure 7.6. Cyclability test for KUA, KUA-N and KUA-CONH2 based supercapacitors
at 3V. j = 1A/g. 2000 cycles. 1M Pyr14TFSI/PC.
7.4. Conclusions
In this chapter, the effect of nitrogen functionalization at mild
conditions on the electrochemical performance of activated carbons in
non-conventional electrolytes was studied. The generation of nitrogen
functionalities at low temperature allows the preservation of most of the
tailored porous texture of the pristine carbon material, which is crucial
for the development of EDLC based in innovative electrolytes.
The electrochemical characterization of the samples in two different
electrolytes, with the same cation but different anion, allows to elucidate
the different parameters affecting the electrode-electrolyte interface. It
was found that surface chemistry does not significantly modify the
capacitance, but has a strong effect on the degradation of both electrodes
and electrolytes. The nitrogen doping decreases the degradation
processes occurring in the carbon electrodes under positive and negative
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
319
polarization, when approaching the limits of the electrochemical stability
window.
Supercapacitors based on these activated carbons in IL-based
electrolyte displayed very high capacitance and energy densities, with
improved rate performance as consequence of the generation of nitrogen
functional groups. The effect of nitrogen doping on the durability of the
devices was analysed by a cycling test, evidencing an enhancement of
the performance of N-doped activated carbons due to the beneficial
effect of nitrogen doping at mild conditions.
7.5 References
[1] F. Béguin, V. Presser, A. Balducci, E. Frackowiak, Carbons and
electrolytes for advanced supercapacitors, Adv. Mater. 26 (2014).
[2] W. Raza, F. Ali, N. Raza, Y. Luo, K.H. Kim, J. Yang, S. Kumar,
A. Mehmood, E.E. Kwon, Recent advancements in supercapacitor
technology, Nano Energy 52 (2018) 441–473..
[3] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors.,
Nat. Mater. 7 (2008) 845–54.
[4] C. Zhong, Y. Deng, W. Hu, J. Qiao, L. Zhang, J. Zhang, A review
of electrolyte materials and compositions for electrochemical
supercapacitors, Chem. Soc. Rev. 44 (2015) 7484–7539.
[5] A. Balducci, Electrolytes for high voltage electrochemical double
layer capacitors : A perspective article, J. Power Sources 326
(2016) 534–540.
[6] J. Krummacher, C. Schütter, L.H. Hess, A. Balducci, Non-
aqueous electrolytes for electrochemical capacitors, Curr. Opin.
Electrochem. 9 (2018) 64–69.
[7] A. Brandt, S. Pohlmann, A. Varzi, A. Balducci, S. Passerini, Ionic
liquids in supercapacitors, MRS Bull. 38 (2013) 554–559.
Chapter 7
320
[8] D.R. Macfarlane, N. Tachikawa, M. Forsyth, J.M. Pringle, P.C.
Howlett, G.D. Elliott, J.H. Davis, M. Watanabe, P. Simon, C.A.
Angell, Energy applications of ionic liquids, Energy Environ. Sci.
7 (2014) 232–250.
[9] W.Y. Tsai, R. Lin, S. Murali, L. Li Zhang, J.K. McDonough, R.S.
Ruoff, P.L. Taberna, Y. Gogotsi, P. Simon, Outstanding
performance of activated graphene based supercapacitors in ionic
liquid electrolyte from -50 to 80°C, Nano Energy 2 (2013) 403–
411.
[10] V.L. Martins, R.M. Torresi, Ionic liquids in electrochemical
energy storage, Curr. Opin. Electrochem. 9 (2018) 26–32.
[11] S. Pohlmann, R.S. Kühnel, T.A. Centeno, A. Balducci, The
Influence of Anion-Cation Combinations on the Physicochemical
Properties of Advanced Electrolytes for Supercapacitors and the
Capacitance of Activated Carbons, ChemElectroChem 1 (2014)
1301–1311.
[12] S. Pohlmann, C. Ramirez-Castro, A. Balducci, The Influence of
Conductive Salt Ion Selection on EDLC Electrolyte
Characteristics and Carbon-Electrolyte Interaction, J.
Electrochem. Soc. 162 (2015) A5020–A5030.
[13] S. Pohlmann, B. Lobato, T.A. Centeno, A. Balducci, The
influence of pore size and surface area of activated carbons on the
performance of ionic liquid based supercapacitors, Phys. Chem.
Chem. Phys. 15 (2013) 17287–17294.
[14] S. Leyva-García, D. Lozano-Castelló, E. Morallón, T. Vogl, C.
Schütter, S. Passerini, A. Balducci, D. Cazorla-Amorós,
Electrochemical performance of a superporous activated carbon in
ionic liquid-based electrolytes, J. Power Sources 336 (2016) 419–
426.
[15] M.J. Mostazo-López, R. Ruiz-Rosas, A. Castro-Muñiz, H.
Nishihara, T. Kyotani, E. Morallón, D. Cazorla-Amorós,
Ultraporous nitrogen-doped zeolite-templated carbon for high
power density aqueous-based supercapacitors, Carbon 129 (2018)
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
321
510–519.
[16] M.J. Mostazo-López, R. Ruiz-rosas, E. Morallón, D. Cazorla-
Amorós, Nitrogen doped superporous carbon prepared by a mild
method. Enhancement of supercapacitor performance, Int. J.
Hydrogen Energy 41 (2016) I9691–I9701.
[17] D. Salinas-Torres, S. Shiraishi, E. Morallón, D. Cazorla-Amorós,
Improvement of carbon materials performance by nitrogen
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions, Carbon 82 (2015) 205–213.
[18] D. Lozano-Castelló, M.A. Lillo-Ródenas, D. Cazorla-Amorós, A.
Linares-Solano, Preparation of activated carbons from Spanish
anthracite: I. Activation by KOH, Carbon 39 (2001) 741–749.
[19] D. Cazorla-Amorós, J. Alcañiz-Monge, M.A. De La Casa-Lillo,
A. Linares-Solano, CO2 as an adsorptive to characterize carbon
molecular sieves and activated carbons, Langmuir 14 (1998)
4589–4596.
[20] C. Schütter, C. Ramirez-Castro, M. Oljaca, S. Passerini, M.
Winter, A. Balducci, Activated carbon, carbon blacks and
graphene based nanoplatelets as active materials for
electrochemical double layer capacitors: A comparative study, J.
Electrochem. Soc. 162 (2015).
[21] M.J. Mostazo-López, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Generation of nitrogen functionalities on activated
carbons by amidation reactions and Hofmann rearrangement:
Chemical and electrochemical characterization, Carbon 91 (2015)
252–265.
[22] D. Cazorla-Amorós, D. Lozano-Castelló, E. Morallón, M.J.
Bleda-Martínez, A. Linares-Solano, S. Shiraishi, Measuring cycle
efficiency and capacitance of chemically activated carbons in
propylene carbonate, Carbon 48 (2010) 1451–1456.
[23] M. Hahn, A. Würsig, R. Gallay, P. Novák, R. Kötz, Gas evolution
in activated carbon/propylene carbonate based double-layer
capacitors, Electrochem. Commun. 7 (2005) 925–930.
Chapter 7
322
[24] P.W. Ruch, D. Cericola, A. Foelske-Schmitz, R. Kötz, A.
Wokaun, Aging of electrochemical double layer capacitors with
acetonitrile-based electrolyte at elevated voltages, Electrochim.
Acta 55 (2010) 4412–4420.
[25] C.A. Leon y Leon, L.R. Radovic, Interfacial chemistry and
electrochemistry of carbon surfaces, in: P.A. Thrower (Ed.),
Chem. Phys. Carbon, Vol. 24, Marcel Dekker, Inc New York,
1994: pp. 213–310.
[26] A. Brandt, A. Balducci, Theoretical and practical energy
limitations of organic and ionic liquid-based electrolytes for high
voltage electrochemical double layer capacitors, J. Power Sources
250 (2014) 343–351.
[27] G. Moreno-Fernández, C. Schütter, J.M. Rojo, S. Passerini, A.
Balducci, T.A. Centeno, On the interaction of carbon electrodes
and non conventional electrolytes in high-voltage electrochemical
capacitors, J. Solid State Electrochem. 22 (2018) 717–725.
[28] A.C. Forse, C. Merlet, J.M. Griffin, C.P. Grey, New perspectives
on the charging mechanisms of supercapacitors, J. Am. Chem.
Soc. 138 (2016) 5731–5744.
[29] M. Salanne, B. Rotenberg, K. Naoi, K. Kaneko, P.-L. Taberna,
C.P. Grey, B. Dunn, P. Simon, Efficient storage mechanisms for
building better supercapacitors, Nat. Energy 1 (2016) 16070.
[30] C. Largeot, C. Portet, J. Chmiola, P.-L. Taberna, Y. Gogotsi, P.
Simon, Relation between the ion size and pore size for an electric
double-layer capacitor, J. Am. Chem. Soc. 130 (2008) 2730–
2731.
[31] D. Lozano-Castelló, D. Cazorla-Amorós, A. Linares-Solano, S.
Shiraishi, H. Kurihara, A. Oya, Influence of pore structure and
surface chemistry on electric double layer capacitance in non-
aqueous electrolyte, Carbon. 41 (2003) 1765–1775.
[32] A. Balducci, R. Dugas, P.L. Taberna, P. Simon, D. Plée, M.
Mastragostino, S. Passerini, High temperature carbon – carbon
supercapacitor using ionic liquid as electrolyte, J. Power Sources
Electrochemical performance of N-doped activated carbons in non-conventional electrolytes
323
165 (2007) 922–927.
[33] C. Peng, S. Zhang, X. Zhou, G.Z. Chen, Unequalisation of
electrode capacitances for enhanced energy capacity in
asymmetrical supercapacitors, Energy Environ. Sci. 3 (2010)
1499.
[34] V. Strelko, V.. Kuts, P.. Thrower, On the mechanism of possible
influence of heteroatoms of nitrogen, boron and phosphorus in a
carbon matrix on the catalytic activity of carbons in electron
transfer reactions, Carbon 38 (2000) 1499–1503.
[35] M. Sevilla, G.A. Ferrero, N. Diez, A.B. Fuertes, One-step
synthesis of ultra-high surface area nanoporous carbons and their
application for electrochemical energy storage, Carbon 131
(2018) 193–200.
[36] A.B. Fuertes, M. Sevilla, High-surface area carbons from
renewable sources with a bimodal micro-mesoporosity for high-
performance ionic liquid-based supercapacitors, Carbon 94 (2015)
41–52.
[37] E. Redondo, W.-Y. Tsai, B. Daffos, P.-L. Taberna, P. Simon, E.
Goikolea, R. Mysyk, Outstanding room-temperature capacitance
of biomass-derived microporous carbons in ionic liquid
electrolyte, Electrochem. Commun. 79 (2017) 5–8.
[38] S.W. Xu, Y.Q. Zhao, Y.X. Xu, Q.H. Chen, G.Q. Zhang, Q.Q. Xu,
D.D. Zhao, X. Zhang, C.L. Xu, Heteroatom doped porous carbon
sheets derived from protein-rich wheat gluten for supercapacitors:
The synergistic effect of pore properties and heteroatom on the
electrochemical performance in different electrolytes, J. Power
Sources 401 (2018) 375–385.
[39] Y. Zhu, M. Chen, Y. zhang, W. Zhao, C. Wang, A biomass-
derived nitrogen-doped porous carbon for high-energy
supercapacitor, Carbon 140 (2018) 404–412.
Chapter 8
Electrochemicalperformance of
N-doped activated carbons as
electrocatalysts for the ORR
N-doped activated carbons as electrocatalysts for the ORR
327
8.1. Introduction
Fuel cells have attracted considerably attention as energy conversion
devices for stationary and mobile applications mainly due to their high
efficiency and low emisions. However, the development of new catalysts
for the oxygen reduction reaction that takes place in the cathode is still a
key challenge to expand this technology. The most common materials are
based on platinum and other noble metals supported on carbon materials
[1–3]. These catalysts provide the highest electroactivities. However, they
experience several drawbacks that hinders their large-scale application,
such as their high cost, low availability and poor durability [4]. Hence, an
extensive research is been carried out to develop new catalysts capable to
increase the use of fuel cells in industry.
One of the most promising alternatives to replace electrocatalysts
based in noble metals are metal-free carbon materials doped with different
heteroatoms (nitrogen, phosphorous, sulfur, etc.) [5,6]. These materials
have low cost, high surface area, good mechanical and electrical
properties and high stability. In particular, nitrogen-doped carbon
materials have demonstrated outstanding electrocatalytic properties [7].
The presence of nitrogen can modify the electron-donor properties of the
carbon material and provide a redistribution of electronic density,
producing an increase of the electrocatalytic activity for the ORR.
However, the role of the different nitrogen functional groups is not still
well understood [8]. Moreover, several works have pointed out the
enhancement of electrocatalytic performance when increasing the surface
area of the carbon material, either working as catalyst support or catalyst
Chapter 8
328
itself [9]. Thus, the synthesis of new carbon materials with large surface
area and selective nitrogen functionalities appears as a promising
approach for the development of new electrocatalysts for ORR.
Nitrogen-containing porous carbon materials can be synthesized by
following different methods [10]. They are summarized as follows:
(i) Carbonization of a nitrogen-containing precursor (such as
pyridine, melamine, polyaniline, etc), which can be followed
by an activation process (either physical or chemical).
(ii) Hydrothermal carbonization of nitrogen containing-
compounds (glucosamine, cyanuric acid, etc.).
(iii) Templating approaches using a nitrogen-containing precursor
followed by a thermal treatment.
(iv) Post-thermal treatments of a material previously synthesized
with a nitrogen-containing reactant in gas or liquid phase.
The first method has been extensively employed to produce N-doped
carbon materials derived from polyaniline with large nitrogen content
[11]. These carbons display outstanding electroactivity as catalyst
supports [12–14] and good response when working themselves as
electrocatalysts towards ORR [15–21]. The well-defined structure of
polyaniline leads to the formation of carbon materials with predominancy
of certain types of functionalities depending on the treatment temperature
[16]. Moreover, polyaniline has been extensively used to prepare hybrid
material composites by chemical or electrochemical polymerization of
aniline over a carbon support [11]. These composites can be converted
N-doped activated carbons as electrocatalysts for the ORR
329
into advanced N-doped carbons by thermal post-treatments [11,22]. The
main advantage of this strategy is the generation of porous carbons with
surface functionalities characteristic of polyaniline-derived carbon
materials while remaining most part of the porous structure of the pristine
carbon support.
Another promising methodology to generate N-doped carbon
materials with high surface area is based on chemical post-treatments
through organic chemistry reactions [23,24]. In this procedure, nitrogen
doping is carried out at mild conditions and only depends on the reactivity
between the carbon surface (i.e. oxygen functional groups) and the
nitrogen-containing reaction medium. This approach leads to the
anchoring of a wide range of nitrogen functionalities. Due to the mild
conditions of the treatment, nitrogen groups with low thermal stability (i.e.
amides, amines) are generated on these carbon materials. Thus, the surface
chemistry of these carbons can also be easily tuned by selective post-
thermal treatments under inert atmosphere.
In this chapter, we report the performance of N-doped superporous
activated carbons as electrocatalysts for the ORR. An activated carbon
with very well developed microporosity (SBET > 3000 m2/g) was used as
starting material for further functionalization. The samples are obtained
by different N-doping pathways: (i) chemical polymerization of aniline;
(ii) nitrogen functionalization by organic reactions; (iii) thermal post-
treatments of the samples obtained with the pathways (i) and (ii). The
combination of these strategies allows to preserve most part of the porous
texture of the pristine carbon material while the surface chemistry is
Chapter 8
330
severely modified, as described in chapter 6 and ref. [22]. Thus, these
activated carbons can be used to assess the role of different nitrogen
functionalities in the electroactivity for the ORR.
8.2. Materials and methods
8.2.1. Synthesis of activated carbons
8.2.1.1 Pristine activated carbon
A superporous activated carbon synthesized in our laboratory has
been used as the starting material for nitrogen functionalization by
different methods. The pristine material (named as KUA), has been
obtained by chemical activation of a Spanish anthracite with KOH using
an impregnation ratio of activating agent to raw material of 4:1 and an
activation temperature of 750º C under inert atmosphere, which was held
for 1 hour. More details about the preparation process are available
elsewhere [25].
8.2.1.2. Chemical functionalization of activated carbon at mild conditions
KUA was further functionalized with nitrogen functional groups by
two different strategies based on the organic chemistry protocols. This
methodology was accurately described in chapters 3 and 4. Briefly, the
first approach consisted in a three-step protocol: (i) chemical oxidation
with HNO3, (ii) treatment with SOCl2 and (iii) amidation reaction with
NH4NO3/DMF and pyridine. The obtained sample was named as KUA-
CONH2. In the second method, the third step of the first protocol is
N-doped activated carbons as electrocatalysts for the ORR
331
directly applied over pristine sample (KUA). This sample was named as
KUA-N.
8.2.1.3. Preparation of polyaniline/activated carbon composite
PANI/KUA composite was prepared by chemical polymerisation of
aniline as described elsewhere [26]. The aniline (concentration: 70mM)
was adsorbed on the carbon material during 24 h at 30 ºC. Afterwards, the
sample was treated with ammonium persulfate solution in 1M HCl for one
hour in a reactor at 0 ºC. The aniline monomer: ammonium persulfate
molar ratio was 1:1. The composite was washed with 1M HCl, 1M
NH4OH and distilled water, and finally dried under vacuum for 24 h.
8.2.1.4. Post-thermal treatments
The carbon materials (KUA, KUA-CONH2 and KUA-N samples) and
PANI/KUA composite were heat treated at different temperatures under
N2 atmosphere using (200mL/min, 1h) and a heating rate of 5 Cº/min. The
obtained samples are named as S_T, where S es is the name of the pristine
sample (KUA, KUA-N, KUA-CONH2 and KUA/PANI) and T is the final
heating temperature (600 and 800 ºC).
8.2.2. Porous texture and surface chemistry characterization
The porous texture characterization was carried out by N2 adsorption-
desorption isotherms at -196º C and by CO2 adsorption at 0º C by using
an Autosorb-6-Quantachrome apparatus. The samples were outgassed at
200º C for 4 hours before the experiments. The apparent surface area was
obtained from N2 adsorption isotherms by using the BET equation in the
Chapter 8
332
0.05-0.20 range of relative pressures. The total micropore volume was
determined by Dubinin-Radushkevich (DR) method applied to N2
(relative pressures from 0.01 to 0.05) adsorption isotherms. The volume
of the narrow microporosity (i.e., pore sizes below 0.7 nm) was calculated
from the DR method applied to the CO2 adsorption isotherms (relative
pressures from 0.0001 to 0.25) [27].
The surface chemistry of the samples was analyzed by X Ray
Photoelectron Spectroscopy (XPS) and Temperature Programmed
Desorption (TPD). XPS measurements were performed by using a VG-
Microtech Multilab 3000 spectrometer, equipped with an Al anode. TPD
experiments were performed by heating the samples (10 mg) to 950º C
(at a heating rate of 20º C/min) under a helium flow rate of 100 mL/min.
The analyses were carried out by using a TGA-DSC instrument (TA
Instruments, SDT Q600 Simultaneous) coupled to a mass spectrometer
(Thermostar, Balzers, BSC 200).
8.2.3. Electrochemical activity towards ORR
The evaluation of electroactivity towards ORR was carried out in a
three-electrode cell using an Autolab PGSTAT302 bipotentiostat
(Metrohm, Netherlands). A rotating ring-disk electrode (RRDE, Pine
Research Instruments, USA) equipped with a glassy carbon disk (5.61 mm
diameter) and a platinum ring was used as the working electrode. A
platinum wire was used as the counter electrode and RHE electrode as
reference electrode. The carbon samples were deposited onto the glassy
carbon disk. The amount of catalyst was optimized to reach the highest
N-doped activated carbons as electrocatalysts for the ORR
333
limiting current. The optimum value was determined to be 240 µg. The
catalysts were deposited by droping 120 µL of a 2 mg/mL dispersion made
of each sample in ethanol (1.5 % Nafion® perfluorinated resin solution,
Sigma Aldrich), obtaining a catalyst charge of 0.96 mg/cm2.
The electrochemical tests were carried out by Cyclic voltammetry
(CV) and linear sweep voltammetry (LSV) in the potential range 0.0 - 1.0
V vs. RHE. CVs were performed in a N2-saturated and O2-saturated
atmosphere at 5 mV/s. LSV experiments were conducted in an O2-
saturated atmosphere at different rotation rates (between 400 and 2025
rpm) at 5 mV/s on the GC rotating disk electrode. The temperature was
kept at 25 ºC during the experiment. The potential of the Pt ring electrode
was held at 1.5 V vs. RHE during LSV experiments. The electron transfer
number was determined from the RRDE measurements:
𝑛 = 4 𝐼𝑑
𝐼𝑑+ 𝐼𝑟 𝑁⁄ (8.1)
Where Ir and Id are the intensities measured at the ring and the disk,
respectively, and N is the collection efficiency of the ring (0.37).
8.3. Results and Discussion
8.3.1. Surface chemistry characterization
The effect of the different nitrogen functionalization treatments was
analysed by XPS and TPD. Table 8.1 summarizes the surface composition
of the carbon materials. The effect of nitrogen doping at mild conditions
(for the generation of KUA-N and KUA-CONH2) was already discussed
in chapters 3 and 4, and refs. [23] and [28]. For comparison purposes, the
Chapter 8
334
data related to these samples (and their derived heat-treated samples,
KUA-CONH2_800 and KUA-N_800) are included in Table 8.1. Briefly,
the post-functionalization treatment produces the decrease of oxygen
functionalities of the pristine carbon material to attach different nitrogen
moieties. Thus, KUA-N sample has lower oxygen content than the pristine
carbon material and it contains nitrogen functionalities derived from CO-
evolving groups, such as amines and nitrogen heterocycles (chapter 4)
[28]. The synthesis of KUA-CONH2 involves a pre-oxidation treatment to
increase the amount of oxygen functional groups (CO2 and CO-evolving
groups), that afterwards react to produce nitrogen functionalities.
Consequently, this sample has contributions of nitrogen groups derived
from CO2 evolving groups (amide-like functionalities) and from CO
evolving groups (amines, imines, nitrogen heterocycles) (chapter 3) [23].
Moreover, KUA-CONH2 has higher oxygen content than the pristine
carbon material.
Table 8.1. Surface composition of the activated carbons obtained by XPS and TPD.
Sample CO2
(µmol/g)
CO
(µmol/g)
O
(µmol/g)
O
(at. %)
N
(at. %)
KUA 450 1970 2870 8.8 0.3
KUA_800 170 620 960 2.3 -
KUA/PANI 1320 3560 6200 11.4 6.0
KUA/PANI_600 380 1860 2620 4.5 4.7
KUA/PANI_800 210 740 1050 6.7 2.0
KUA-CONH2 1140 2370 4650 10.2 4.2
KUA-CONH2_800 140 570 830 4.5 2.1
KUA-N 450 1750 2640 7.5 3.7
KUA-N_800 130 505 720 3.0 1.7
N-doped activated carbons as electrocatalysts for the ORR
335
The polymerization of aniline over KUA produces the generation of
a polyaniline film inside the microporosity of the carbon material [26, 29].
The attachment of nitrogen was confirmed by XPS (6.0 at. % N, Table
8.1). KUA/PANI composite also evidences an increase of oxygen content,
as confirmed by XPS and TPD. Figure 8.1 illustrates the TPD profiles
obtained for KUA, KUA/PANI composite and the derived heat treated-
samples (KUA/PANI_600 and KUA/PANI_800). KUA/PANI composite
displays larger amount of CO2 (carboxylic acids, lactones and anhydrides)
and CO evolving groups (phenols, quinones, etc) [30]. This is related to
the use of an oxidizing agent on the polymerization of aniline in presence
of the carbon material that may react with the carbon surface to produce
oxygen functional groups.
The effect of heat treatments over the pristine and N-doped
functionalized activated carbons was previously discussed in chapter 6.
The heat treatment diminishes most of the oxygen and nitrongen
functional groups in KUA-CONH2 and KUA-N materials. Similarly, the
treatments over the composite strongly decrease the amount of oxygen
functional groups. At 600 ºC, ⁓71 % of CO2 evolving groups are removed
from KUA/PANI composite. The amount of CO evolving groups is
severely decreased. These modifications lead to the generation of a N-
doped activated carbon with similar oxygen content than the pristine
sample (2620 and 2870 µmol/g for KUA/PANI_800 and KUA,
respectively, Table 8.1). Nevertheless, the type of functionalities is
different, being remarkably lower the concentration of groups that
thermally desorb at temperatures lower than around 600 ºC (i.e., phenols)
Chapter 8
336
[30]. It is also worth noting that the concentration of carbonyl type groups
(above 600 ºC) is larger in KUA/PANI_600 than in the composite,
indicating that the heat treatment produces the conversion of phenols into
carbonyl groups upon heating at 800 ºC.
Figure 8.1. Comparison between (a) CO2 and (b) CO TPD profiles of KUA, KUA/PANI,
KUA/PANI_600 and KUA/PANI_800.
The heat treatments at 800 ºC diminishes the oxygen content in all
samples. The largest oxygen content was detected for KUA/PANI_800
(1050 µmol/g, Table 8.1). This can be explained by the presence of higher
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 200 400 600 800 1000
CO
2(µ
mo
l/g
s)
Temperature (ºC)
KUA
KUA/PANI
KUA/PANI_600
KUA/PANI_800
(a)
0
0.6
1.2
1.8
2.4
3
3.6
0 200 400 600 800 1000
CO
(μ
mo
l/g
s)
Temperature (ºC)
KUA
KUA/PANI
KUA/PANI_600
KUA/PANI_800
(b)
N-doped activated carbons as electrocatalysts for the ORR
337
concentration of moieties in the parent composite (KUA/PANI sample),
that may lead to higher reactivity between the carbon surface and the
PANI coating upon heating.
The nitrogen functionalities are also modified as consequence of the
heat treatments. Figure 8.3 shows the devonvoluted N1s XPS spectra
obtained for KUA/PANI composite and the related heat-treated samples.
The assignement of the different nitrogen functionalities is collected in
Table 8.3. The values obtained for the other N-doped samples are included
for comparison purposes. KUA/PANI composite exhibits a main peak
associated to the presence of amines at 399.7 ± 0.2 eV [31–34]. Moreover,
there is also a contribution of pyrroles/pyridones (400.8 ± 0.2 eV) that can
be explained by reactions with oxygen functionalities during the
polymerization of aniline [29]. The carbonization at 600 ºC produces the
decrease of nitrogen groups as well as their conversion into different
surface functionalities. Thus, KUA/PANI after treatment at 600ºC shows
two different peaks related to pyrroles/pyridones (400.6 ± 0.2 eV) and
pyridines/imines (398.6 ± 0.2 eV) [31–34]. After heat treatment at 800 ºC,
the main contribution of N-functional groups is detected at 400.8 ± 0.2
eV, related to pyrroles/pyridones (70 % N, Table 8.3), and a lower
presence of pyridines/imines (398.3 ± 0.2 eV, 30 % N, Table 8.3). These
results are in agreement with the mechanism proposed for carbonization
of PANI [35,36]. During the heat treatment the polymer chain experiences
bond breaking that, upon heating at higher temperatures, react again to
form N-containing heterocycles (pyrroles/pyridones and pyridines)
through cross-linking reactions. Thus, the amines present on KUA/PANI
Chapter 8
338
composite may experience degradation upon heating and react to generate
nitrogen groups with higher thermal stability, such as nitrogen
heterocycles.
Figure 8.2. N1s XPS spectra deconvoluted for the KUA/PANI composite and the N-
doped activated carbons.
As discussed in chapter 6, KUA-N and KUA-CONH2 also experience
loss and conversion of their nitrogen functionalities into groups with
higher thermal stability [37]. At 800 ºC, there are two main contributions
of pyridine (398.7 ± 0.2 eV, Table 8.2) and pyrrole/pyridone (400.7 ± 0.2
eV, Table 8.2). However, the concentration of pyrrole/pyridone groups is
lower than that found for KUA/PANI_800. Hence, the use of different
396398400402404
I (a
.u.)
Binding Energy (eV)
KUA/PANI_600
KUA/PANI_800
KUA/PANI
N-doped activated carbons as electrocatalysts for the ORR
339
surface modification methods and post-thermal treatments allows to
obtain activated carbons with very different surface chemistry.
Table 8.2. Assignment of N1s deconvoluted curves to nitrogen functional groups.
Sample Binding
Energy
(eV)
Functional
Group
N
(at. %)
Percentage
of N species
KUA/PANI 400.8 ± 0.2 Pyrrole,
pyridone
2.2 36
399.7 ± 0.2 Amines 3.8 64
KUA/PANI_600 400.6 ± 0.2 Pyrrole,
pyridone
2.9
61
398.6 ± 0.2 Pyridine, Imine 1.8 39
KUA/PANI_800
400.8 ± 0.2 Pyrrole,
pyridone
1.4 70
398.3 ± 0.2 Pyridine, imine 0.5 30
KUA-N 401.9 ± 0.2 Quaternary 0.4 10
400.7 ± 0.2 Pyrrole,
pyridone
0.9 25
399.8 ± 0.2 Amide, Lactam,
Amine, Imide
1.3 35
398.7 ± 0.2 Pyridine, Imine 1.1 30
KUA-N_800 402.7 ± 0.2 Oxidized N 0.2 14
400.8 ± 0.2 Pyrrole,
pyridone
0.9 51
398.7 ± 0.2 Pyridine, imine 0.6 35
KUA-CONH2 400.7 ± 0.2 Pyrrole,
pyridone
0.7 19
399.8 ± 0.2 Amide, lactam,
amine, imide
1.9 50
398.8 ± 0.2 Pyridine, imine 1.2 31
KUA-CONH2_800 402.5 ± 0.2 Oxidized N 0.2 11
400.8 ± 0.2 Pyrrole,
pyridone
0.9 52
398.7 ± 0.2 Pyridine, imine 0.6 37
Chapter 8
340
8.3.2. Porous texture characterization
The porous texture of the samples was analysed by N2 and CO2
adsorption isotherms. Figure 8.3 illustrates the N2 adsorption isotherms
obtained for the pristine activated carbon and the PANI-derived carbon
materials. For comparison purposes, the isotherm of KUA-CONH2 was
also included. All samples evidence a type I isotherm characteristic of
microporous materials.
Figure 8.3. N2 adsorption isotherms obtained for KUA (pristine sample), KUA/PANI
composite and N-doped activated carbons.
Table 8.3 compiles data of the porous texture of the samples. The
effects of nitrogen doping at mild conditions were thoroughly discussed
in chapters 3, 4 and 6. The functionalization at mild conditions produces
some minor modifications on the porous texture of the samples. When
using a single step modification protocol (for the production of KUA-N),
the microporosity of the pristine carbon materials is fully retained [24].
0
200
400
600
800
1000
0 0.2 0.4 0.6 0.8 1
Ad
sorb
ed
vo
lum
e (
cm
3/g
)
P/Po
KUA
KUA/PANI
KUA/PANI_600
KUA/PANI_800
KUA-CONH₂
N-doped activated carbons as electrocatalysts for the ORR
341
However, the combination of oxidation process and amidation reactions
leads to some loss of the apparent surface area, due to the generation of
surface functionalities that occupies or block some part of the
microporosity (see chapter 3 and ref. [23]).
Regarding KUA/PANI composite, the formation of polyaniline layer
over the pristine carbon material KUA increases the amount of of nitrogen
and oxygen functionalities (see section 8.3.1) but produces the most
important decrease of the apparent surface area and micropore volume
(Table 8.3). After the heat treatments, the obtained activated carbons
(KUA/PANI_600 and KUA/PANI_800) recover part of the porous
texture, due to the decomposition of PANI layer. Hence, all heat-treated
samples display similar apparent surface area (2400-2800 m2/g, Table
8.3). However, PANI-derived samples have around ⁓25 % lower
micropore volume than N-doped heat-treated carbons (KUA-N and KUA-
CONH2). This is a consequence of the carbon layer formed in the heat-
treated substrate from PANI decomposition.
Chapter 8
342
Table 8.3. Porous texture of the samples.
Sample SBET
(m2/g)
VDRN2
(cm3/g)
VDRCO2
(cm3/g)
KUA 3080 1.19 0.57
KUA_800 2680 1.05 0.49
KUA/PANI 1590 0.54 0.37
KUA/PANI_600 2420 0.81 0.49
KUA/PANI_800 2470 0.84 0.56
KUA-COOH 2770 1.06 0.49
KUA-CONH2 2390 0.97 0.45
KUA-CONH2_500 2630 1.02 0.41
KUA-CONH2_800 2630 1.0 0.43
KUA-N 2960 1.18 0.52
KUA-N_500 2800 1.11 0.49
KUA-N_800 2770 1.09 0.48
8.3.3. Electroactivity towards ORR
The electroactivity of the carbons towards the ORR was analysed in
alkaline electrolyte (0.1 M KOH) by using a RRDE. LSV curves were
measured at different rotating rates for all samples. The contribution of
electrical double layer was substracted by recording CV in N2-saturated
0.1 M KOH. Figure 8.4 shows the LSV curves obtained for some samples
at 1600 rpm. The LSV for commercial 20% Pt/Vulcan was included for
comparison purposes. Table 8.4 collects the main properties related to the
performance of the samples as electrocatalysts for the ORR derived from
the LSV curves.
N-doped activated carbons as electrocatalysts for the ORR
343
8.3.3.1. Pristine activated carbon
The pristine carbon material evidences a two-wave curve
characteristic of microporous carbon materials [9]. The high onset
potential (0.81 V, Table 8.4) demonstrated by this electrocatalyst can be
explained by the high concentration of edge sites provided by its well-
developed microporosity [9], which have been proposed as
electrocatalytic sites [38–43].
8.3.3.2. N-doped activated carbons at mild conditions
The effect of nitrogen functionalities was thoroughly analysed. The
catalysts obtained by chemical methods (KUA-N and KUA-CONH2,
Table 8.1) have not improved electrocatalytic properties in comparison
with the pristine carbon material (KUA). The onset potential was slightly
decreased in both samples. Moreover, the limiting current was severely
diminished for KUA-CONH2 sample (Figure A1). This is mainly related
to the surface chemistry of the carbon material. This sample has large
amount of nitrogen species in form of different functionalities (see section
8.3.1). It has been previously pointed out in the literature that only certain
nitrogen functional groups work as active sites for the ORR [44]. This
sample has N-heterocycles (pyridines, pyrroles/pyridones), which have
been proposed as electrocatalytic sites towards ORR [15,19–21,45,46].
Thus, an improvement of the electrocatalytic response would be expected
on this sample. The decrease on the performance cannot be associated
with a decrease of conductivity since we have previously shown that it is
enhanced in different electrolytes (see chapters 3, 6 and 7). Hence, the
Chapter 8
344
poor electrocatalytic behaviour might be a consequence of the surface
modification protocol used to functionalize these samples. For instance,
the oxidation process involves the generation of oxygen functionalities,
which are attached to the carbon edge sites. Thus, if the functionalization
method does not produce exclusively nitrogen groups with higher
electroactivity (i.e., pyridones, edge type N-Q), the N-doped sample might
result as a worse electrocatalyst, since a noticeable amount of edge sites
is lost during the surface modification reaction. Hence, the sample
experiences a deactivation during the functionalization treatment.
A similar effect is expected for KUA-N sample, even though the
oxidation process is avoided. It is expected that nitrogen functional groups
are anchored to the carbon surface by consumption of oxygen
functionalities. However, the mechanism might affect or involve the
reactivity of the adjacent carbons. This effect was also proposed by Tuci
et al [47]. The electrografting of pyridine functional groups to N-doped
carbon nanotubes resulted in a decrease of inherent electrochemical
properties and “switched them off” towards ORR. Thus, functionalization
at mild conditions might produce also a deactivation of the carbons as
electrocatalysts.
8.3.3.3. Polyaniline/activated carbon composite
The electroactivity towards ORR of KUA/PANI composite was also
evaluated. Table 8.4 compiles the onset potential recorded for this sample.
This sample displays lower onset potential and limiting current than
pristine activated carbon (see Figure A1). This poor performance is
N-doped activated carbons as electrocatalysts for the ORR
345
explained by its lower microporosity (Table 8.3, section 8.3.2) as well as
its different surface chemistry (section 8.3.1). This sample has a large
concentration of amine functionalities, which are not electroactive
towards ORR [46]. Moreover, the formation of the polyaniline film over
the microporosity of the carbon surface might also decrease the
concentration of accessible edge sites, resulting in an overall decrease of
the catalytic electroactivity of the composite.
Figure 8.4. (a) LSV curves for the catalysts in O2-saturated 0.1 M KOH at 1600 rpm. (b)
LSV curves at different rotating rates for KUA/PANI_800. v = 5 mV/s.
-6
-5
-4
-3
-2
-1
0
0 0.2 0.4 0.6 0.8 1
j (m
A/c
m2)
E (V vs RHE)
Pt/Vulcan
KUA
KUA-N_800
KUA_800
KUA-CONH₂_800KUA/PANI_800
(a)
-6
-5
-4
-3
-2
-1
0
0 0.2 0.4 0.6 0.8 1
j (m
A c
m2)
E (V vs RHE)
400 rpm
2025 rpm
(b)
Chapter 8
346
8.3.2.4. Heat-treated activated carbons
The electrocatalytic performance of the carbon materials is improved
for the N-doped samples after heat treatment at 800 ºC (KUA-N_800,
KUA-CONH2_800 and KUA/PANI_800). All the samples display higher
onset potential, being the largest value for KUA/PANI_800 (0.88 V vs
RHE). Moreover, these carbons provide higher limiting currents than the
other samples, including the pristine KUA sample the heat treated
KUA_800 (Figure 8.4).
Table 8.4. Electrochemical parameters of the electrocatalysts calculated from the RRDE
experiments in O2-saturated 0.1M KOH at 5 mV/s and 1600 rpm.
Sample Eonset
(V vs RHE)
n
(at 0.5 V)
KUA 0.82 2.5
KUA_800 0.82 2.7
KUA/PANI 0.80 2.4
KUA/PANI_600 0.82 2.7
KUA/PANI_800 0.88 3.4
KUA-CONH2 0.79 2.6
KUA-CONH2_800 0.85 3.4
KUA-N 0.81 2.8
KUA-N_800 0.84 3.1
Pt/Vulcan 0.91 3.9
Since the parent non-doped carbon (KUA_800) does not show any
enhancement of the catalytic response (Figure 8.4), the increase of the
performance observed for the N-containing samples after treatment at
800ºC is not only related to the effect of heat treatment. This enhancement
of the electrocatalytic response for the ORR is undoubtedly related to the
existence of nitrogen functional groups. The N-doped carbons have
N-doped activated carbons as electrocatalysts for the ORR
347
similar content of pyridines (0.5-0.6 at. % XPS, Table 8.1) and
pyrroles/pyridones (0.9 – 1.4 at. % XPS, Table 8.1). The main difference
on the nitrogen functional groups arises for KUA/PANI_800, whose
content of pyrroles/pyridones is larger (1.4 at. % XPS). Moreover, this
carbon has higher content of CO-evolving groups (Table 8.1), being more
likely the presence of pyridones. The improvement of the ORR through
pyridones (N-C-O sites) was already proposed in the literature [15,17].
The enhancement of ORR by pyridine-containing carbon catalysts
has been also proposed [19–21,45], but there is controversy about its
positive effect [44]. Nevertheless, the three samples display similar
pyridine content, and consequently this functional group cannot be
responsible of the large improvement observed in KUA/PANI_800. Its
enhanced electrocatalytic properties might be related to the pyridone
functional groups.
It is also worth noting that in spite of the fact that KUA/PANI_600
has higher amount of N species and similar porous texture in comparison
with KUA/PANI_800, its performance towards ORR is significantly
poorer (Figure A1). This is related to the lower conductivity of the sample
when heat-treated at this temperature (600 ºC) [16].
8.3.2.5. Selectivity to water formation
The electron transfer number involved in the ORR was determined
by monitoring the current registered in the Pt ring electrode during the
experiments. Table 8.4 collects the values at 0.5 V registered for all
samples. Figure 8.5 illustrates the evolution of the number of electrons
Chapter 8
348
with the potential for a selection of samples. The pristine carbon material
(KUA) catalyzes the ORR through a mechanism in two steps [9]. The first
one occurs at high potentials and involves the reduction of oxygen to
hydrogen peroxide. Afterwards, the hydrogen peroxide that remains in the
microporosity of the activated carbon is subsequently reduced to
hydroxile ions. Both steps happen through a 2e- process. Hence, the
increase of number of electrons observed for this sample when shifting
the potential to more negative values is explained by the sum of the
electrons involved in both reaction steps (2 + 2 e- pathway) [9].
Figure 8.5. Electron transfer number calculated from RRDE experiments of activated
carbon electrocatalysts in O2-saturated 0.1 M KOH at 5 mV/s and 1600 rpm.
The N-doped carbons heat-treated at 800ºC involve a larger number
of electrons for the ORR. Specifically, the number measured at 0.5 V is
3.4, 3.4 and 3.1 for KUA/PANI_800, KUA-CONH2_800 and KUA-
N_800, respectively. This points out a higher selectivity for the formation
of water than for the non-doped carbons. For comparison purposes, the
1.5
2
2.5
3
3.5
4
0 0.2 0.4 0.6 0.8
n
E (V vs RHE)
KUA
KUA_800
KUA-N_800
KUA-CONH₂_800KUA/PANI_800
N-doped activated carbons as electrocatalysts for the ORR
349
measurement of KUA_800 (non-doped and heat-treated pristine sample)
was also included in Figure 8.5. This sample shows an enhancement of
the number of electrons along the whole range of potentials in comparison
with the pristine sample KUA. However, its selectivity to water formation
is lower than that demonstrated by the N-doped ones heat treated at 800ºC.
Hence, the enhancement manifested by these samples is related to the
formation of N-functional groups with higher electrocatalytic activity for
the ORR. These samples have large content of pyridones (N-C-O sites),
which have been proposed to enhance the selectivity to water formation
[15,17]. Moreover, the N-doped carbons does not show the important
increase of number of electrons (to 2 +2 e- pathway) displayed by the non-
doped samples at low potentials. The result is especially interesting in case
of KUA/PANI_800, since the number of transfered electrons decreases
with decreasing the potential and the values at the highest potentials are
closer to 4 (i.e., at around 0.7V). Thus, this sample shows the highest
selectivity to water formation compared to the other N-doped samples.
8.4. Conclusions
In this chapter, several N-doped superporous activated carbons (1-4
at. % XPS) were prepared by following different post-functionalization
treatments, including methods involving organic chemistry pathways,
polymerization of aniline and post-thermal treatments. The combination
of these strategies leads to the production of activated carbons with large
apparent surface area and different surface chemistry. The obtained
samples have different functional groups depending on the post-
modification treatment, such as moieties with low thermal stability
Chapter 8
350
(amines, amides) and groups with high thermal stability (pyrroles,
pyridines, etc.). Thus, they have been used to monitor the effect of
nitrogen functional groups on the performance of highly microporous
activated carbons as electrocatalysts for ORR.
The pristine sample displays remarkable electroactivity towards ORR
due to its well-developed microporosity. The N-doped activated carbons
evidence different electrocatalytic performance depending on the
functionalization strategy. The doping methods at low temperature
produce a decrease of the catalytic response due to the generation of non-
catalytic active sites (amines, amides) and the decrease of catalytic edge
sites. The performance is improved for N-doped activated carbons after
heat treatment at 800ºC as consequence of the generation of
electrocatalytic nitrogen-containing functional groups. Polyaniline-
derived activated carbon (heat-treated at 800 ºC) provided the highest
electroactivity (onset potential of 0.88 V) and improved selectivity to
water formation. This enhanced behaviour is explained by its highest
concentration of N-C-O sites.
N-doped activated carbons as electrocatalysts for the ORR
351
8.5. References.
[1] C.W.B. Bezerra, L. Zhang, H. Liu, K. Lee, A.L.B. Marques, E.P.
Marques, H.Wang, J. Zhang, A review of heat-treatment effects on
activity and stability of PEM fuel cell catalysts for oxygen
reduction reaction, J. Power Sources 173 (2007) 891–908.
[2] C.W.B. Bezerra, L. Zhang, K. Lee, H. Liu, A.L.B. Marques, E.P.
Marques, H. Wang, J. Zhang, A review of Fe-N/C and Co-N/C
catalysts for the oxygen reduction reaction, Electrochim. Acta 53
(2008) 4937–4951.
[3] X. Yu, S. Ye, Recent advances in activity and durability
enhancement of Pt/C catalytic cathode in PEMFC. Part II:
Degradation mechanism and durability enhancement of carbon
supported platinum catalyst, J. Power Sources 172 (2007) 145–154.
[4] R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N.
Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay,
K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M.
Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A.
Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K.I. Kimijima, N.
Iwashita, Scientific aspects of polymer electrolyte fuel cell
durability and degradation, Chem. Rev. 107 (2007) 3904–3951.
[5] X. Yan, Y. Jia, X. Yao, Defects on carbons for electrocatalytic
oxygen reduction, Chem. Soc. Rev. 47 (2018) 7628–7658.
[6] A. Morozan, B. Jousselme, S. Palacin, Low-platinum and
platinum-free catalysts for the oxygen reduction reaction at fuel cell
cathodes, Energy Environ. Sci. 4 (2011) 1238–1254.
[7] G. Wu, A. Santandreu, W. Kellogg, S. Gupta, O. Ogoke, H. Zhang,
H.L. Wang, L. Dai, Carbon nanocomposite catalysts for oxygen
reduction and evolution reactions: From nitrogen doping to
transition-metal addition, Nano Energy 29 (2016) 83–110.
[8] K.H. Wu, D.W. Wang, D.S. Su, I.R. Gentle, A Discussion on the
Activity Origin in Metal-Free Nitrogen-Doped Carbons for
Oxygen Reduction Reaction and their Mechanisms,
Chapter 8
352
ChemSusChem 8 (2015) 2772–2788.
[9] A. Gabe, R. Ruiz-Rosas, C. González-Gaitán, E. Morallón, D.
Cazorla-Amorós, Modeling of oxygen reduction reaction in porous
carbon materials in alkaline medium. Effect of microporosity, J.
Power Sources (2019) 451–464.
[10] Y. Deng, Y. Xie, K. Zou, X. Ji, Review on recent advances in
nitrogen-doped carbons: Preparations and applications in
supercapacitors, J. Mater. Chem. A 4 (2015) 1144–1173.
[11] A. Gabe, M.J. Mostazo-López, D. Salinas-Torres, E. Morallón, D.
Cazorla-Amorós, Synthesis of conducting polymer/carbon material
composites and their application in electrical energy storage, in:
Hybrid Polym. Compos. Mater. Process., 2017: pp. 173–209.
[12] S.O. Material, S. Web, N. York, A. Nw, High-Performance
Electrocatalysts for Oxygen Reduction Derived from Polyaniline,
Iron, and Cobalt, Science (80) 332 (2012) 443–447.
[13] H.W. Liang, W. Wei, Z.S. Wu, X. Feng, K. Müllen, Mesoporous
metal-nitrogen-doped carbon electrocatalysts for highly efficient
oxygen reduction reaction, J. Am. Chem. Soc. 135 (2013) 16002–
16005.
[14] N. Gavrilov, I.A. Pašti, M. Mitrić, J. Travas-Sejdić, G. Ćirić-
Marjanović, S. V. Mentus, Electrocatalysis of oxygen reduction
reaction on polyaniline-derived nitrogen-doped carbon
nanoparticle surfaces in alkaline media, J. Power Sources 220
(2012) 306–316.
[15] J. Quilez-Bermejo, C. González-Gaitán, E. Morallón, D. Cazorla-
Amorós, Effect of carbonization conditions of polyaniline on its
catalytic activity towards ORR. Some insights about the nature of
the active sites, Carbon 119 (2017) 62–71..
[16] J. Quílez-Bermejo, E. Morallón, D. Cazorla-Amorós, Oxygen-
reduction catalysis of N-doped carbons prepared: Via heat
treatment of polyaniline at over 1100 °c, Chem. Commun. 54
(2018) 4441–4444.
N-doped activated carbons as electrocatalysts for the ORR
353
[17] R. Silva, D. Voiry, M. Chhowalla, T. Asefa, Efficient metal-free
electrocatalysts for oxygen reduction: Polyaniline-derived N- and
O-doped mesoporous carbons, J. Am. Chem. Soc. 135 (2013)
7823–7826.
[18] J. Zhang, Z. Zhao, Z. Xia, L. Dai, A metal-free bifunctional
electrocatalyst for oxygen reduction and oxygen evolution
reactions, Nat. Nanotechnol. 10 (2015) 444.
[19] F. Zhou, G. Wang, F. Huang, Y. Zhang, M. Pan, Polyaniline
derived N- and O-enriched high surface area hierarchical porous
carbons as an efficient metal-free electrocatalyst for oxygen
reduction, Electrochim. Acta 257 (2017) 73–81.
[20] A. Zhao, J. Masa, M. Muhler, W. Schuhmann, W. Xia, N-doped
carbon synthesized from N-containing polymers as metal-free
catalysts for the oxygen reduction under alkaline conditions,
Electrochim. Acta 98 (2013) 139–145.
[21] L. Lai, J.R. Potts, D. Zhan, L. Wang, C.K. Poh, C. Tang, H. Gong,
Z. Shen, J. Lin, R.S. Ruoff, Exploration of the active center
structure of nitrogen-doped graphene-based catalysts for oxygen
reduction reaction, Energy Environ. Sci. 5 (2012) 7936–7942.
[22] D. Salinas-Torres, S. Shiraishi, E. Morallón, D. Cazorla-Amorós,
Improvement of carbon materials performance by nitrogen
functional groups in electrochemical capacitors in organic
electrolyte at severe conditions, Carbon 82 (2015) 205-213.
[23] M.J. Mostazo-López, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Generation of nitrogen functionalities on activated
carbons by amidation reactions and Hofmann rearrangement:
Chemical and electrochemical characterization, Carbon 91 (2015)
252–265.
[24] M.J. Mostazo-López, R. Ruiz-Rosas, S. Shiraishi, E. Morallón, D.
Cazorla-Amorós, Nitrogen doped activated carbons prepared at
mild conditions as electrodes for supercapacitors in organic
electrolyte, in: 7th Int. Conf. Carbon Energy Storage Environ.
Prot., 2017: p. 23.
Chapter 8
354
[25] D. Lozano-Castelló, M.A. Lillo-Ródenas, D. Cazorla-Amorós, A.
Linares-Solano, Preparation of activated carbons from Spanish
anthracite: I. Activation by KOH, Carbon 39 (2001) 741–749.
[26] D. Salinas-Torres, J.M. Sieben, D. Lozano-Castello, E. Morallón,
M. Burghammer, C. Riekel, D. Cazorla-Amorós, Characterization
of activated carbon fiber/polyaniline materials by position-resolved
microbeam small-angle X-ray scattering, Carbon 50 (2012) 1051–
1056.
[27] D. Cazorla-Amorós, J. Alcañiz-Monge, M.A. De La Casa-Lillo, A.
Linares-Solano, CO2 as an adsorptive to characterize carbon
molecular sieves and activated carbons, Langmuir 14 (1998) 4589–
4596.
[28] M.J. Mostazo-López, R. Ruiz-rosas, E. Morallón, D. Cazorla-
Amorós, Nitrogen doped superporous carbon prepared by a mild
method. Enhancement of supercapacitor performance, Int. J.
Hydrogen Energy 41 (2016) I9691–I9701.
[29] M.J. Bleda-Martínez, E. Morallón, D. Cazorla-Amorós,
Polyaniline/porous carbon electrodes by chemical polymerisation:
Effect of carbon surface chemistry, Electrochim. Acta 52 (2007)
4962–4968.
[30] J.. Figueiredo, M.F.. Pereira, M.M.. Freitas, J.J.. Órfão,
Modification of the surface chemistry of activated carbons, Carbon
37 (1999) 1379–1389.
[31] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, J.
Find, U. Wild, R. Schlögl, Structural characterization of N-
containing activated carbon fibers prepared from a low softening
point petroleum pitch and a melamine resin, Carbon 40 (2002) 597–
608.
[32] E. Raymundo-Piñero, D. Cazorla-Amorós, A. Linares-Solano, The
role of different nitrogen functional groups on the removal of SO2
from flue gases by N-doped activated carbon powders and fibres,
Carbon 41 (2003) 1925–1932.
[33] R.J.J. Jansen, H. van Bekkum, XPS of nitrogen-containing
N-doped activated carbons as electrocatalysts for the ORR
355
functional groups on activated carbon, Carbon 33 (1995) 1021–
1027.
[34] Y. Yamada, J. Kim, S. Matsuo, S. Sato, Nitrogen-containing
graphene analyzed by X-ray photoelectron spectroscopy, Carbon
70 (2014) 59–74.
[35] S. Kuroki, Y. Hosaka, C. Yamauchi, A solid-state NMR study of
the carbonization of polyaniline, Carbon 55 (2013) 160–167.
[36] Z. Rozlívková, M. Trchová, M. Exnerová, J. Stejskal, The
carbonization of granular polyaniline to produce nitrogen-
containing carbon, Synth. Met. 161 (2011) 1122–1129.
[37] K.G. Latham, W.M. Dose, J.A. Allen, S.W. Donne, Nitrogen doped
heat treated and activated hydrothermal carbon: NEXAFS
examination of the carbon surface at different temperatures,
Carbon 128 (2018) 179–190.
[38] M. Gara, R.G. Compton, Activity of carbon electrodes towards
oxygen reduction in acid: A comparative study, New J. Chem. 35
(2011) 2647–2652.
[39] K. Waki, R.A. Wong, H.S. Oktaviano, T. Fujio, T. Nagai, K.
Kimoto, K. Yamada, Non-nitrogen doped and non-metal oxygen
reduction electrocatalysts based on carbon nanotubes: Mechanism
and origin of ORR activity, Energy Environ. Sci. 7 (2014) 1950–
1958.
[40] N.P. Subramanian, X. Li, V. Nallathambi, S.P. Kumaraguru, H.
Colon-Mercado, G. Wu, J.W. Lee, B.N. Popov, Nitrogen-modified
carbon-based catalysts for oxygen reduction reaction in polymer
electrolyte membrane fuel cells, J. Power Sources 188 (2009) 38–
44.
[41] P.H. Matter, U.S. Ozkan, Non-metal catalysts for dioxygen
reduction in an acidic electrolyte, Catal. Letters. 109 (2006) 115–
123.
[42] P.H. Matter, L. Zhang, U.S. Ozkan, The role of nanostructure in
nitrogen-containing carbon catalysts for the oxygen reduction
Chapter 8
356
reaction, J. Catal. 239 (2006) 83–96.
[43] X. Chu, K. Kinoshita, Surface modification of carbons for
enhanced electrochemical activity, Mater. Sci. Eng. B. 49 (1997)
53–60.
[44] T. Ikeda, M. Boero, S.F. Huang, K. Terakura, M. Oshima, J.I.
Ozaki, S.F. Hang, K. Terakura, M. Oshima, J.I. Ozaki, Carbon
Alloy Catalysts: Active Sites for Oxygen Reduction Reaction, J.
Phys. Chem. C. 112 (2008) 14706–14709.
[45] C.V. Rao, C.R. Cabrera, Y. Ishikawa, In search of the active site in
nitrogen-doped carbon nanotube electrodes for the oxygen
reduction reaction, J. Phys. Chem. Lett. 1 (2010) 2622–2627.
[46] C. González-Gaitán, R. Ruiz-Rosas, E. Morallón, D. Cazorla-
Amorós, Functionalization of carbon nanotubes using
aminobenzene acids and electrochemical methods. Electroactivity
for the oxygen reduction reaction, Int. J. Hydrogen Energy 40
(2015) 11242–11253.
[47] G. Tuci, C. Zafferoni, A. Rossin, L. Luconi, A. Milella, M.
Ceppatelli, M. Innocenti, Y. Liu, C. Pham-Huu, G. Giambastiani,
Chemical functionalization of N-doped carbon nanotubes: A
powerful approach to cast light on the electrochemical role of
specific N-functionalities in the oxygen reduction reaction, Catal.
Sci. Technol. 6 (2016) 6226–6236.
N-doped activated carbons as electrocatalysts for the ORR
357
ANNEX TO CHAPTER 8
Figure A8.1. LSV curves for the catalysts in O2-saturated 0.1 M KOH at 1600 rpm. v =
5 mV/s.
-5
-4
-3
-2
-1
0
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
j (m
A/c
m2)
E (V vs RHE)
KUA
KUA/PANI
KUA/PANI_600
KUA-CONH₂KUA-N
Chapter 9
General conclusions
General conclusions
361
In this PhD Thesis, several nitrogen functionalization methodologies
were employed to produce carbon materials with similar porosity but
different surface chemistry. The effect of nitrogen functionalization on
their porous texture and surface chemistry was thoroughly analyzed.
Moreover, the effect of surface chemistry in their electrochemical
performance was extensively studied in different electrolytes, mainly
dealing with two applications: as electrodes for supercapacitors and
electrocatalysts for the oxygen reduction reaction. Herein, the main
conclusions derived from this work are summarized.
Nitrogen functionalization of activated carbons at mild conditions
• Nitrogen-functionalization of activated carbon using an organic
chemistry protocol at mild conditions was achieved by three
different approaches, which are summarized as follows: (i)
amidation treatment, through a combination of chemical
oxidation, acyl chloride formation and reaction with nitrogen
reagents; (ii) amination treatment by Hofmann rearrangement;
and (iii) direct reaction with nitrogen reagents. These methods
allowed the attachment of ⁓ 4 at. % N.
• The modification pathways leaded to the formation of a wide
range of surface functionalities. The amidation treatment
produced amides (and cyclic derivatives, such as imides and
lactams) derived from CO2-desorbing groups and nitrogen
aromatic heterocycles (pyridine, pyridone and pyrrole) due to
the consumption of CO-evolving groups. The post-treatment by
Hofmann rearrangement produced the conversion of amides into
Chapter 9
362
amines, obtaining an activated carbon with different nitrogen
functionalities (amines, pyridines and pyrroles) and a high
amount of surface oxygen groups. The direct reaction with
nitrogen reactant produced mainly amines and nitrogen
heterocycles.
• The functionalization treatments slightly decreased de
microporosity of the pristine activated carbon. The direct
incorporation of nitrogen via reaction with nitrogen reagents
fully preserved the porous texture of the pristine carbon
material, making this single-step treatment more appropriate for
functionalization of highly microporous carbon materials at
mild conditions.
Electrochemical performance of N-doped activated carbons in aqueous
electrolyte
• The electrochemical characterization of N-doped activated
carbons in aqueous electrolytes showed that, at low current
densities, the capacitance is mainly governed by the apparent
surface area while the surface capacitance is greater for the N-
containing samples due to the influence of nitrogen functional
groups.
• The N-doped activated carbons obtained by amidation and direct
functionalization displayed higher surface capacitance and
improved conductivity. They showed higher capacitance
retention due to the removal of detrimental oxygen
General conclusions
363
functionalities and generation of electron-donating nitrogen
functional groups, such as cyclic amides, pyridines and pyrroles.
• The capacitor based on the N-doped activated carbon obtained
by direct functionalization showed similar energy density, better
capacitance retention and higher power density than the pristine
activated carbon due to the presence of beneficial N groups and
the removal of detrimental electroactive oxygen functionalities.
Also, the nitrogen-functionalized activated carbons used as
electrodes for supercapacitors revealed higher stability than the
pristine activated carbon when working at high voltage
conditions in aqueous medium.
Electrochemical performance of N-doped zeolite-templated carbons in
aqueous electrolyte
• The physicochemical characterization of non-doped and N-
doped zeolite templated carbons evidenced a practically
identical structure but different surface chemistry.
• The N-doped ZTC displayed higher resistance to electro-
oxidation and degradation in acid and alkaline media. Nitrogen-
doping provided higher wettability and improved conductivity
due to presence of quaternary nitrogen.
• Symmetric supercapacitors based on these carbons provided
similar capacitance, but larger energy in case of N-ZTC due to
its higher capacitive behaviour. N-ZTC based supercapacitor
displayed an outstanding maximum power, that is four times
Chapter 9
364
larger than that showed by non-doped ZTC based supercapacitor
(98 and 23 kW/kg), due to the improvement of electrical
conductivity produced by N-Q functionalities in N-ZTC
electrodes. N-ZTC based capacitor evidenced better
performance of ZTC upon cycling due to the stabilizing effect
of N functional groups.
Electrochemical performance of N-doped activated carbons in organic
electrolytes.
• Activated carbons with similar porosity but different surface
chemistry were prepared by combining chemical
functionalization methods at mild conditions and post-thermal
treatments. The heat treatments diminish the content of surface
functionalities and produce rearrangements of the nitrogen
groups. The porosity of the heat-treated samples is almost
identical for the whole series of carbons.
• The surface chemistry of these carbon materials did not
significantly affect the capacitance due to their similar porous
texture.
• The presence of oxygen functional groups affected the rate
performance of the capacitor due to the decrease of conductivity,
while nitrogen functional groups (cyclic amides, pyridines and
pyrroles) slightly increased the conductivity of the carbon
material and improved the performance.
General conclusions
365
• The effect of surface functionalities upon durability was
thoroughly studied. The oxygen functionalities strongly
damaged the performance of the activated carbons. The heat
treatments of the samples produce an improvement of the
electrochemical stability due to the decrease of detrimental
oxygen groups. The performance was further increased by
nitrogen doping at mild conditions, since the treatment
combines the positive effect of removing oxygen groups with
their replacement by nitrogen groups with high electrochemical
stability, which is specialy beneficial in case of the generation
of amide-like functional groups.
Electrochemical performance of N-doped activated carbons in non-
conventional electrolytes.
• The electrochemical characterization of N-doped activated
carbons obtained at mild conditions in two different non-
conventional electrolytes, based in ionic liquids and new
conducting salts (in organic medium), evidenced that surface
chemistry does not significantly modify the capacitance, but
strongly decreases the degradation processes occurring in the
carbon electrodes under positive and negative polarization,
when approaching the limits of the electrochemical stability
window.
• Supercapacitors based on these activated carbons in ionic liquid-
based electrolyte displayed very high capacitance and energy
densities, with improved rate performance due to the presence
Chapter 9
366
of nitrogen functional groups. The durability of the devices
evidenced an enhancement of the performance of N-doped
activated carbons due to the beneficial effect of nitrogen doping
at mild conditions.
Electrochemical performance of N-doped activated carbons as
electrocatalysts for the oxygen reduction reaction
• The effect of nitrogen functional groups on the performance of
highly microporous activated carbons as electrocatalysts for
ORR was assessed by using samples obtained by different post-
functionalization treatments, including methods involving
organic chemistry pathways, polymerization of aniline and post-
thermal treatments.
• These samples have different functional groups according to the
post-modification treatment, such as moieties with low thermal
stability (amines, amides) and groups with high thermal stability
(pyrroles, pyridines, etc.).
• The pristine sample displays remarkable electroactivity towards
ORR due to its well-developed microporosity. The N-doped
activated carbons evidence different electrocatalytic
performance depending on the functionalization strategy.
Polyaniline-derived activated carbon (heat-treated at 800 ºC)
provided the highest electroactivity (onset potential of 0.88 V)
and improved selectivity to water formation. This enhanced
behaviour is explained by its highest concentration of N-C-O
sites.
Conclusiones generales
367
En esta Tesis Doctoral, se han utilizado diversas metodologías de
funcionalización con nitrógeno para producir materiales carbonosos con
textura porosa similar pero distinta química superficial. Se analizó
detalladamente el efecto de la funcionalización con nitrógeno en la textura
porosa y la química superficial. Además, se estudió el efecto de la química
superficial en el comportamiento electroquímico de los materiales en
distintos electrolitos, con el fin de utilizarlos en dos aplicaciones
principales: como electrodos de supercondensadores y como
electrocatalizadores de la reacción de reducción de oxígeno. A
continuación, se presentan las conclusiones principales derivadas de este
trabajo.
Funcionalización con nitrógeno de carbones activados en condiciones
suaves.
• Se ha realizado la funcionalización química con grupos
funcionales nitrogenados de un carbón activado por medio de un
protocolo basado en reacciones orgánicas en condiciones suaves,
que se puede resumir del siguiente modo: (i) tratamiento de
amidación, mediante reacciones de oxidación química, formación
de cloruro de ácido y reacción con reactivos nitrogenados; (ii)
tratamiento de aminación mediante reordenamiento de Hofmann,
y (iii) reacción directa con reactivos nitrogenados. Estos métodos
permiten la incorporación de ⁓ 4 at. % N.
• Estas rutas de modificación química produjeron la formación de
una amplia variedad de grupos funcionales superficiales. El
tratamiento de amidación produjo amidas (y amidas cíclicas, como
Capítulo 9
368
imidas y lactamas), derivados de grupos funcionales que desorben
como CO2, y heterociclos aromáticos (piridinas, piridonas y
pirroles), debido al consumo de grupos que desorben como CO.
Los post-tratamientos mediante el reordenamiento de Hofmann
produjeron la conversión de amidas en aminas, dando lugar a un
carbón activado con distintos grupos funcionales nitrogenados
(aminas, piridinas y pirroles) y una cantidad elevada de grupos
funcionales oxigenados. La reacción directa con los reactivos
nitrogenados produjo principalmente la formación de aminas y
heterociclos nitrogenados.
• Los tratamientos de funcionalización decrecieron levemente la
microporosidad del carbón activado prístino. La incorporación
directa de nitrógeno mediante reacción con reactivos
nitrogenados, permitió conservar la porosidad del material
original, de manera que este tratamiento en una etapa única resultó
el más apropiado para la funcionalización de materiales
carbonosos de elevada microporosidad en condiciones suaves.
Comportamiento electroquímico de carbones activados dopados con
nitrógeno en electrolito acuoso.
• La caracterización electroquímica de carbones activados dopados
con nitrógeno en condiciones suaves mostró que, a bajas
densidades de corriente, la capacidad está gobernada
principalmente por la superficie específica aparente, mientras que
la capacidad superficial es mayor para las muestras que contienen
Conclusiones generales
369
nitrógeno debido a la influencia de los grupos funcionales
nitrogenados.
• Los carbones activados obtenidos por amidación y
funcionalización directa mostraron capacidades superficiales y
conductividades superiores. Estas muestras presentan mayor
retención de capacidad debido a la eliminación de grupos
funcionales oxigenados perjudiciales y a la generación de grupos
nitrogenados electrón-dadores, como amidas cíclicas, piridinas y
pirroles.
• El condensador basado en el carbón activado dopado con
nitrógeno obtenido mediante funcionalización directa mostró, en
comparación con el condensador basado en el material prístino,
energía similar, retención de capacidad mayor y potencia superior,
debido a la presencia de grupos funcionales nitrogenados
beneficiosos y a la eliminación de grupos oxigenados
perjudiciales. Los carbones activados dopados con nitrógeno
utilizados como electrodos de condensadores mostraron mayor
estabilidad que el material original cuando se emplearon en
condiciones de voltaje elevado en medio acuoso.
Comportamiento electroquímico de materiales carbonosos
nanomoldeados dopados con nitrógeno en electrolito acuoso.
• La caracterización fisicoquímica de los materiales estudiados
(materiales carbonosos nanomoldeados dopados con nitrógeno, N-
ZTC, y sin dopar, ZTC) mostró que estos materiales tienen una
estructura prácticamente idéntica y distinta química superficial.
Capítulo 9
370
• El material N-ZTC mostró mayor resistencia a la electro-
oxidación y a la degradación en medio ácido y alcalino. El dopado
con nitrógeno proporcionó mayor mojabilidad y un aumento de la
conductividad debido a la presencia de nitrógeno cuaternario.
• Los supercondensadores simétricos basados en estos materiales
produjeron valores de capacidad similares, pero mayor energía en
el caso de N-ZTC debido a su mayor carácter capacitivo. El
supercondensador basado en N-ZTC mostró una potencia máxima
extraordinaria, que es cuatro veces superior a la proporcionada por
el condensador basado en ZTC no dopado (98 y 23 Wh/kg,
respectivamente), debido a la mejora de la conductividad eléctrica
producida por el nitrógeno cuaternario presente en los electrodos
de N-ZTC. El condensador basado en N-ZTC mostró mejor
comportamiento durante el ciclado debido al efecto estabilizante
de los grupos funcionales nitrogenados.
Comportamiento electroquímico de carbones activados dopados con
nitrógeno en electrolitos orgánicos.
• Se han preparado carbones activados con porosidad similar pero
química superficial distinta mediante la combinación métodos
basados en funcionalización química en condiciones suaves y
post-tratamientos térmicos. Los tratamientos térmicos produjeron
la disminución del contenido de grupos funcionales superficiales
y generaron un reordenamiento de grupos funcionales
nitrogenados. La porosidad de las muestras tratadas térmicamente
es casi idéntica para todos los materiales.
Conclusiones generales
371
• La química superficial de estos materiales no afectó
significativamente a la capacidad debido a la textura porosa
similar de los mismos.
• La presencia de grupos funcionales oxigenados afectó a la
retención de capacidad a elevadas densidades de corriente
debido a la pérdida de conductividad, mientras que los grupos
funcionales nitrogenados (amidas cíclicas, piridinas y pirroles)
producen un aumento leve de la conductividad de los materiales
y mejoran el comportamiento a estas densidades de corriente.
• El efecto de los grupos funcionales nitrogenados en la
durabilidad del condensador se estudió en detalle. Los grupos
oxigenados empeoraron significativamente el comportamiento
de los carbones activados. Los tratamientos térmicos condujeron
a un aumento de la estabilidad electroquímica debido a la
eliminación de grupos funcionales oxigenados. La mejora fue
superior en el caso de la funcionalización con nitrógeno en
condiciones suaves, ya que el tratamiento combina el efecto
positivo de la eliminación de grupos oxigenados con su
sustitución por grupos nitrogenados con elevada estabilidad
electroquímica, especialmente en el caso de la generación de
grupos tipo amida.
Comportamiento electroquímico de carbones activados dopados con
nitrógeno en electrolitos no convencionales.
• La caracterización electroquímica de los carbones activados
dopados con nitrógeno en condiciones suaves en dos electrolitos
Capítulo 9
372
no convencionales distintos, basados en líquidos iónicos y sales
conductoras nuevas (en medio orgánico), mostró que la química
superficial no modifica significativamente la capacidad, pero
decrece severamente los procesos de degradación que suceden en
los electrodos en condiciones de polarización a potenciales
negativos y positivos, cuando se aproximan a los límites de
estabilidad electroquímica.
• Los supercondensadores basados en estos carbones activados y
electrolitos basados en líquido iónico mostraron capacidades y
energías muy elevadas, además de una mejora de la retención de
capacidad a elevadas densidades de corriente debido a la presencia
de grupos funcionales nitrogenados. La durabilidad de los
dispositivos mostró una mejora del comportamiento de los
condensadores basados en carbones activados dopados con
nitrógeno debido al efecto beneficioso de la funcionalización con
nitrógeno en condiciones suaves.
Comportamiento electroquímico de carbones activados dopados con
nitrógeno como electrocatalizadores de la reacción de reducción de
oxígeno
• Se estudió el efecto de los grupos funcionales nitrogenados en
el comportamiento de carbones activados de elevada
microporosidad como electrocatalizadores de la reacción de
reducción de oxígeno, utilizando distintas muestras sintetizadas
por medio de diversas estrategias de funcionalización, que
Conclusiones generales
373
incluyen métodos basadas en reacciones orgánicas,
polimerización de anilina y post-tratamientos térmicos.
• Estas muestras presentan grupos funcionales diferentes, que
dependen de la temperatura de post-modificación, como grupos
con estabilidad térmica baja (aminas, amidas) y grupos de
elevada estabilidad térmica (pirroles, piridinas, etc.).
• La muestra original presenta una electroactividad destacable en
la reacción de reducción de oxígeno debido a su elevada
microporosidad. Los carbones activados dopados con nitrógeno
mostraron electroactividades distintas dependiendo de la
estrategia de funcionalización. Los carbones activados
derivados de polianilina (tratados térmicamente a 800 ºC)
proporcionaron la mayor electroactividad, con un potencial de
inicio de 0.88 V y mayor selectividad a la formación de agua.
Esta mejora del comportamiento se ha relacionado con la mayor
concentración de sitios N-C-O.
Summary
Summary
377
This PhD Thesis focuses on the functionalization of carbon materials
with high microporosity content with nitrogen functional groups and on
their use as electrodes for supercapacitors and electrocatalysts for the
oxygen reduction reaction. Hence, this work describes the incorporation
of nitrogen functionalities by different methodologies on highly
microporous carbon materials (activated carbons and zeolite templated
carbons), their chemical and electrochemical characterization in different
electrolytes and their performance in the proposed applications.
Nitrogen functionalization of a highly microporous activated carbon
(BET surface area higher than 3000 m2/g) has been achieved using several
post-modification treatments based on organic pathways at mild
conditions. These reaction pathways produced the attachment of 4 at. %
in form of different nitrogen functional groups (amides, amines, pyrroles,
etc.) while preserving most of the microporosity of the pristine activated
carbon. The controlled step-by-step modification of the surface chemistry
allowed to assess the influence of the different nitrogen surface groups in
the electrochemical performance of the carbon materials in different
electrolytes: aqueous (1M H2SO4 and 0.1M KOH), organic (1M
TEMAB4/propylene carbonate (PC) and 1M Pyr14BF4/PC) and ionic
liquid-based electrolytes (1M Pyr14TFSI/PC).
The electrochemical performance of N-doped activated carbons
obtained at mild conditions as electrodes for supercapacitors was assessed
in 1M H2SO4. The N-doped activated carbons showed improved rate
performance, larger stability and energy efficiency than the pristine
carbon material when working at high voltages in aqueous electrolyte.
378
These improvements are related to the presence of surface nitrogen
functionalities that provide higher electrochemical stability, avoiding the
formation of detrimental oxygen groups during the operation of the
supercapacitor.
The effect of nitrogen groups in the performance of carbon materials
as electrodes for supercapacitors is also assessed using two zeolite
templated carbons (ZTC) with comparable structure and different surface
chemistry. These materials were synthesized by chemical vapor
deposition of different precursors, producing a non-doped and a N-doped
carbon material (4 at. % XPS) in which most of the functionalities are
quaternary N. N-doped ZTC evidenced larger surface capacitance in acid
electrolyte, increased electrical conductivity and larger resistance to
electro-oxidation, providing higher electrochemical stability than ZTC.
ZTC and N-ZTC capacitors were constructed using 1M H2SO4. N-ZTC
based capacitor evidenced higher energy density and a power density four
times higher than ZTC-based capacitor, producing an outstanding
maximum power of 98 kW/kg due to the enhancement of electrical
conductivity produced by quaternary nitrogen groups. These results
provide clear evidences of the advantages of doping advanced porous
carbon materials with nitrogen functionalities.
The effect of surface chemistry on the performance N-doped and non-
doped activated carbons as electrodes for symmetric supercapacitors was
analyzed in organic electrolyte (1M TEMABF4/propylene carbonate). For
this purpose, several nitrogen-doped activated carbons were synthesized
by different post-modification methods based on organic chemistry
Summary
379
protocols and selective thermal post-treatments under inert atmosphere.
The combination of both methods allowed the production of carbon
materials with very similar surface area (2400-3000 m2/g) and different
surface chemistry. The capacitors based on these carbons showed high
specific capacitance (37-40 F/g) and energy density (31-37 Wh/kg). The
electrochemical stability of the supercapacitors was evaluated by a
floating test under severe conditions of voltage and temperature. The
results evidence an improvement of the durability of nitrogen-doped
activated carbons modified by chemical treatments at mild conditions due
to the decrease of detrimental oxygen functionalities and the generation
of nitrogen groups with higher electrochemical stability.
The electrochemical performance of nitrogen-doped and non-doped
superporous activated carbons as electrodes for supercapacitors was
assessed in non-conventional electrolytes. The devices provide large
capacitance values (up to 150-180 F/g) in both 1M Pyr14TFSI/PC and 1M
Pyr14BF4/PC due to their tailored porous texture (well-developed
microporosity and low mesopore volume). The nitrogen-doped activated
carbons evidence higher electrochemical stability when exposed to
degradation potentials at positive and negative polarization. The activated
carbon-based capacitors displayed outstanding capacitance (37-40 F/g
and 14 F/cm3) and energy values (44-48 Wh/kg and 16-17 Wh/L) with
promising durability due to the stabilizing effect of nitrogen doping at
mild conditions.
The performance of N-doped activated carbons as electrocatalysts for
the oxygen reduction reaction has been analysed in alkaline electrolyte.
380
The activated carbons were prepared via post-functionalization using
polimerization of aniline, organic chemistry reactions and post-thermal
treatments under inert atmosphere. These treatments leaded to the
formation of different nitrogen functionalities with different content. The
evaluation of the electroactivity of the carbon materials towards ORR
evidenced that nitrogen groups generated at high temperatures were
highly selective towards water formation. Among the investigated
samples, polyaniline-derived activated carbon carbonized at 800 ºC
displayed the best performance (onset potential of 0.88V and an electron
transfer number of 3.4), which was attributed to the highest concentration
of N-C-O sites.
Resumen
381
Esta Tesis Doctoral trata sobre la funcionalización de materiales
carbonosos con elevado contenido de microporosidad con grupos
funcionales nitrogenados y su uso como electrodos de
supercondensadores y electrocatalizadores de la reacción de reducción de
oxígeno. De este modo, este trabajo describe la incorporación de grupos
funcionales nitrogenados mediante distintas metodologías en materiales
carbonosos de elevada microporosidad (carbones activados y materiales
carbonosos nanomoldeados), su caracterización química y electroquímica
en distintos electrolitos y su comportamiento en las aplicaciones
propuestas.
Se ha llevado a cabo la funcionalización con nitrógeno de un carbón
activado de elevada microporosidad (área superficial BET mayor que
3000 m2/g) por medio de distintos métodos de post-modificación basados
en reacciones orgánicas en condiciones suaves. Estas reaccions
produjeron el anclaje de 4 at. % de nitrógeno en forma de distintos grupos
funcionales (amidas, aminas, pirroles, etc.) conservando la mayor parte de
la microporosidad del material original. Esta estrategia de modificación
controlada de la química superficial permitió evaluar la influencia de los
distintos grupos funcionales nitrogenados en el comportamiento
electroquímico de los materiales carbonosos en electrolitos diferentes:
acuoso (1M H2SO4 y 0.1M KOH), orgánico (1M TEMAB4/carbonato de
propileno (PC) y 1M Pyr14BF4/PC) y basados en líquidos iónicos (1M
Pyr14TFSI/PC).
Los carbones activados dopados con nitrógeno en condiciones suaves
se emplearon como electrodos de supercondensadores con el fin de
382
evaluar su comportamiento electroquímico en 1M H2SO4. Los carbones
activados dopados con nitrógeno mostraron mayor retención de capacidad
a elevada densidad de corriente, mayor estabilidad electroquímica y
mayor eficiencia energética que el condensador basado en el material
original a voltajes de operación elevados en electrolito acuoso. Esta
mejora está relacionada con la presencia grupos funcionales nitrogenados
que proporcionan mayor estabilidad electroquímica, ya que evitan la
formación de grupos funcionales oxigenados perjudiciales durante el
funcionamiento del supercondensador.
El efecto de los grupos funcionales nitrogenados en el
comportamiento de los materiales carbonosos como electrodos de
supercondensadores se estudió también empleando dos materiales
carbonosos nanomoldeados (ZTC) con estructura comparable y química
superficial diferente. Estos materiales se sintetizaron mediante depósito
químico en fase vapor utilizando distintos precursores, que producen un
material carbonoso dopado con nitrógeno (N-ZTC) y uno no dopado
(ZTC). N-ZTC presenta un 4 at. de nitrógeno, fundamentalmente en forma
de nitrógeno cuaternario. N-ZTC mostró mayor capacidad superficial en
electrolito ácido, mayor conductividad eléctrica y mayor resistencia a la
electro-oxidación, por lo que proporciona una mayor estabilidad
electroquímica que el material no dopado. Los dos materiales se utilizaron
para construir supercondensadores en 1M H2SO4. El condensador basado
en N-ZTC mostró mayor densidad de energía y una densidad de potencia
cuatro veces superior a la proporcionada por el condensador basado en
ZTC, con una potencia máxima extraordinaria de 98 kW/kg debido al
Resumen
383
efecto beneficioso de la mejora de conductividad eléctrica producida por
el nitrógeno cuaternario. Estos resultados proporcionan evidencias claras
acerca de las ventajas de dopar materiales carbonosos porosos avanzados
con grupos funcionales nitrogenados.
El efecto de la química superficial en el comportamiento
electroquímico de carbones activados dopados con nitrógeno como
electrodos de supercondensadores se estudió también en electrolito
orgánico (1M TEMABF4/carbonato de propileno) empleando una
configuración de celda simétrica. Para este propósito, se prepararon
materiales carbonosos dopados con nitrógeno mediante distintas rutas de
post-modificación basadas en reacciones orgánicas y post-tratamientos
térmicos selectivos en atmósfera inerte. La combinación de ambos
métodos premitió la producción de materiales carbonosos con área
superficial aparente muy similar (2400-3000 m2/g) y química superficial
distinta. Los condensadores basados en estos materiales mostraron
elevada capacidad (37-40 F/g) y densidad de energía (31-37 Wh/kg). La
estabilidad electroquímica de los condensadores se evaluó por medio de
un test de durabilidad en condiciones severas de voltaje y temperatura.
Los resultados evidencian una mejora de la durabilidad de los carbones
activados dopados con nitrógeno mediante tratamientos químicos en
condiciones suaves debido a la eliminación de grupos funcionales
oxigenados y a la generación de grupos funcionales nitrogenados con
mayor estabilidad electroquímica.
El comportamiento electroquímico de carbones activados dopados
con nitrógeno y sin dopar como electrodos de supercondensadores se
384
evaluó también en electrolitos no convencionales. Los dispositivos
presentan elevada capacidad (de 150-180 F/g) en 1M Pyr14TFSI/PC y 1M
Pyr14BF4/PC debido a su adecuada textura porosa (elevada
microporosidad y bajo volumen de microporos). Los carbones activados
dopados con nitrógeno presentan elevada estabilidad electroquímica
cuando se exponen a procesos de degradación a potenciales de
polarización positivos y negativos. Los condensadores basados en estos
carbones activados presentan extraordinarias capacidades (37-40 F/g y 14
F/cm3) y densidades de energía (44-48 Wh/kg y 16-17 Wh/L) con una
durabilidad prometedora debido al efecto estabilizador del dopado con
nitrógeno en condiciones suaves.
El comportamiento de los carbones activados dopados con nitrógeno
como electrocatalizadores de la reacción de reducción de oxígeno ha sido
analizado en electrolito alcalino. Los materiales se prepararon por medio
de tratamientos de post-funcionalización empleando polimerización de
anilina, protocolos basados en reacciones orgánicas y post-tratamientos
térmicos en atmósfera inerte. Estos tratamientos conducen a la formación
de grupos funcionales nitrogenados con distinto contenido. El estudio de
la electroactividad de los materiales carbonosos en la reacción de
reducción de oxígeno mostró que los grupos nitrogenados a elevada
temperatura son muy selectivos a la formación de agua. Entre todas las
muestras investigadas, los carbones activados derivados de polianilina
carbonizados a 800ºC presentan el mejor comportamiento (potencial de
inicio de 0.88 V y número de transferencia de electrones de 3.4), que ha
sido atribuido a la mayor concentración de sitios N-C-O.
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