06 Carbon Carbon Supercap J Energy Chem

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Journal of Energy Chemistry Vol. 22 No. 2 2013

CONTENTS

151 A perspective on carbon materials for future energy

application

Dang Sheng Su, Gabriele Centi

174 Role of carbon matrix heteroatoms at synthesis of car-

bons for catalysis and energy applications

Volodymyr V. Strelko

183 Important roles of graphene edges in carbon-based

energy storage devices

Yoong Ahm Kim, Takuya Hayashi, Jin Hee Kim, Mori-

nobu Endo

195 Synthesis and functionalization of carbon xerogels to

be used as supports for fuel cell catalysts

Jose L. Figueiredo, Manuel F. R. Pereira

202 Electrocatalytic conversion of CO2 to liquid fuels us-

ing nanocarbon-based electrodes

Chiara Genovese, Claudio Ampelli, Siglinda Perathoner,

Gabriele Centi

214 Functional porous carbon-based composite electrode

materials for lithium secondary batteries

Kai Zhang, Zhe Hu, Jun Chen

226 Carbon/carbon supercapacitors

Elzbieta Frackowiak, Qamar Abbas, Francois Beguin

241 Efficient conversion of fructose to 5-

hydroxymethylfurfural over sulfated porous carbon

catalyst

Liang Wang, Jian Zhang, Longfeng Zhu, Xiangju Meng,

Feng-Shou Xiao

245 Synthesis of SAPO-34/graphite composites for low

temperature heat adsorption pumps

L. Bonaccorsi, L. Calabrese, E. Proverbio, A. Frazzica,

A. Freni, G. Restuccia, E. Piperopoulos, C. Milone

251 Facile filling of metal particles in small carbon nan-

otubes for catalysis

Hongbo Zhang, Xiulian Pan, Xinhe Bao

257 Stability and activity of carbon nanofiber-supported

catalysts in the aqueous phase reforming of ethylene

glycol

T. van Haasterecht, C. C. I. Ludding, K. P. de Jong,

J. H. Bitter

270 A correlation between structural changes in a Ni-Cu

catalyst during decomposition of ethylene/ammonia

mixture and properties of nitrogen-doped carbon

nanofibers

O. Yu. Podyacheva, A. N. Shmakov, A. I. Boronin,

L. S. Kibis, S. V. Koscheev, E. Yu. Gerasimov, Z. R. Is-

magilov

279 Carbon nanotubes decorated -Al2O3 containing

cobalt nanoparticles for Fischer-Tropsch reaction

Yuefeng Liu, Thierry Dintzer, Ovidiu Ersen,

Cuong Pham-Huu

290 Simultaneous formation of sorbitol and gluconic acid

from cellobiose using carbon-supported ruthenium

catalysts

Tasuku Komanoya, Hirokazu Kobayashi, Kenji Hara,

Wang-Jae Chun, Atsushi Fukuoka

296 Synergistic effect between few layer graphene and

carbon nanotube supports for palladium catalyzing

electrochemical oxidation of alcohols

Bruno F. Machado, Andrea Marchionni, Re-

vathi R. Bacsa, Marco Bellini, Julien Beausoleil,

Werner Oberhauser, Francesco Vizza, Philippe Serp

305 Phosphorylated mesoporous carbon as effective cata-

lyst for the selective fructose dehydration to HMF

A. Villa, M. Schiavoni, P. F. Fulvio, S. M. Mahurin,

S. Dai, R. T. Mayes, G. M. Veith, L. Prati

312 Purified oxygen- and nitrogen-modified multi-walled

carbon nanotubes as metal-free catalysts for selective

olefin hydrogenation

Peirong Chen, Ly May Chew, Aleksander Kostka, Kun-peng Xie, Martin Muhler, Wei Xia

321 Ru particle size effect in Ru/CNT-catalyzed Fischer-

Tropsch synthesis

Jincan Kang, Weiping Deng, Qinghong Zhang, Ye Wang

329 Ammonia-treatment assisted fully encapsulation of

Fe2O3 nanoparticles in mesoporous carbons as stable

anodes for lithium ion batteries

Fei Han, Wen-Cui Li, Duo Li, An-Hui Lu


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Journal of Energy Chemistry Vol. 22 No. 2 2013

336 Enhanced reversible capacity of Li-S battery cathode

based on graphene oxide

Jin Won Kim, Joey D. Ocon, Dong-Won Park, Jaey-

oung Lee

341 Hierarchical nanostructured composite cathode with

carbon nanotubes as conductive scaffold for lithium-

sulfur batteries

Xiaofei Liu, Qiang Zhang, Jiaqi Huang, Shumao Zhang,

Hongjie Peng, Fei Wei

347 Porous V2O5-SnO2/CNTs composites as high perfor-

mance cathode materials for lithium-ion batteries

Qi Guo, Zhenhua Sun, Man Gao, Zhi Tan,

Bingsen Zhang, Dang Sheng Su

Http://www.jenergchem.org

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CONTENTS

Porous V2O5-SnO2/CNTs composites have been stepwise syn-

thesized by a hydrothermal treatment and a subsequent heat

treatment in air. The cyclic capacity and rate capability of thecomposite cathode have been greatly improved via decreasing

the particle size and coating with more conductive material, as

compared to the commercial V2O5. See the article on Pages

347355.

151

A perspective on carbon materials for future energy application

Dang Sheng Su, Gabriele Centi

Carbon materials play a critical role for the development of new or

improved technologies and devices for a sustainable production and

use of renewable energy.

174

Role of carbon matrix heteroatoms at synthesis of carbons forcatalysis and energy applications

Volodymyr V. Strelko

The effect of heteroatoms on the reactivity of carbons in gasification

processes, their catalytic activity and electrochemical behaviour in

supercapacitors was studied experimentally and by quantum chemi-

cal calculations.


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183

Important roles of graphene edges in carbon-based energy stor-

age devices

Yoong Ahm Kim, Takuya Hayashi, Jin Hee Kim, Morinobu Endo

Edge-Controlled Nanocarbons: Controlling the number (or type) of

edges relative to the basal planes is critical for maximizing the elec-

trochemical performance of carbon-based energy storage devices.

195

Synthesis and functionalization of carbon xerogels to be used

as supports for fuel cell catalysts

Jose L. Figueiredo, Manuel F. R. Pereira

Tuning the surface chemistry of carbon xerogels enhances the per-

formance of PEMFC catalysts.

202

Electrocatalytic conversion of CO2 to liquid fuels using

nanocarbon-based electrodes

Chiara Genovese, Claudio Ampelli, Siglinda Perathoner,

Gabriele Centi

A novel approach to recycle CO2 to high energy density liquid fu-

els in a gas phase photo-electrocatalytic (PEC) device using low-

cost nanocarbon materials doped with suitable metals as electrocat-

alysts.

214

Functional porous carbon-based composite electrode materials

for lithium secondary batteries

Kai Zhang, Zhe Hu, Jun Chen

Functional porous carbon-based composite electrode materials have

been reviewed for electrochemical devices with energy storage andconversion.


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226

Carbon/carbon supercapacitors


The capacitance of nanoporous carbons is enhanced when pores fit

with the size of desolvated ions. Pseudo-faradaic reactions involv-

ing surface groups, hydrogen electrosorption and the carbon/redox

couple interface might be source of an additional contribution.

241

Efficient conversion of fructose to 5-hydroxymethylfurfural over

sulfated porous carbon catalyst

Liang Wang, Jian Zhang, Longfeng Zhu, Xiangju Meng, Feng-

Shou Xiao

The carbon-based solid acid catalyst shows excellent catalytic per-

formances in the dehydration of fructose to HMF.

245

Synthesis of SAPO-34/graphite composites for low temperature

heat adsorption pumps

L. Bonaccorsi, L. Calabrese, E. Proverbio, A. Frazzica, A. Freni,

G. Restuccia, E. Piperopoulos, C. Milone

Novel composite material was made by growing SAPO-34 on com-

mercial graphite fibres by in-situ hydrothermal synthesis and used

as a new thermal conductive adsorbent material for low temperature

heat adsorption pumps.

251

Facile filling of metal particles in small carbon nanotubes for

catalysis

Hongbo Zhang, Xiulian Pan, Xinhe Bao

A versatile method is developed for introduction of metal particles

in carbon nanotubes with a diameter


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257

Stability and activity of carbon nanofiber-supported catalysts in

the aqueous phase reforming of ethylene glycol

T. van Haasterecht, C. C. I. Ludding, K. P. de Jong, J. H. Bitter

Carbon nanofiber supported nickel, cobalt, and platinum catalystshowed comparable activity in the aqueous phase reforming of ethy-

lene glycol. Rapid deactivation due to oxidation and leaching was

observed for cobalt while sintering was observed for nickel and plat-

inum.

270

A correlation between structural changes in a Ni-Cu catalyst

during decomposition of ethylene/ammonia mixture and prop-

erties of nitrogen-doped carbon nanofibers

O. Yu. Podyacheva, A. N. Shmakov, A. I. Boronin, L. S. Kibis,

S. V. Koscheev, E. Yu. Gerasimov, Z. R. Ismagilov

The proposed mechanism of N-CNF growth on a Ni-Cu catalyst dur-

ing ethylene-ammonia decomposition.

279

Carbon nanotubes decorated -Al2O3 containing cobalt

nanoparticles for Fischer-Tropsch reaction

Yuefeng Liu, Thierry Dintzer, Ovidiu Ersen, Cuong Pham-Huu

The hierarchically structured CNTs on -Al2O3 was synthesized and

used as support for Co-based catalysts in Fischer-Tropsch synthesis.

290

Simultaneous formation of sorbitol and gluconic acid from cel-

lobiose using carbon-supported ruthenium catalysts

Tasuku Komanoya, Hirokazu Kobayashi, Kenji Hara, Wang-

Jae Chun, Atsushi Fukuoka

A green and energy-saving process was developed for the hydrolytic

disproportionation of cellobiose to sorbitol and gluconic acid in water

under Ar. Carbon-supported ruthenium catalyzed this reaction viathe hydrolysis and hydrogen transfer.


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296

Synergistic effect between few layer graphene and carbon nan-

otube supports for palladium catalyzing electrochemical oxida-

tion of alcohols

Bruno F. Machado, Andrea Marchionni, Revathi R. Bacsa,

Marco Bellini, Julien Beausoleil, Werner Oberhauser,

Francesco Vizza, Philippe Serp

This paper reports the high electrocatalytic oxidation of ethanol, ethy-

lene glycol and glycerol in half cells on anode catalysts made of Pd

nanoparticles supported on few layer graphene, carbon nanotubes

and a nanotube-graphene composite.

305

Phosphorylated mesoporous carbon as effective catalyst for the

selective fructose dehydration to HMF

A. Villa, M. Schiavoni, P. F. Fulvio, S. M. Mahurin, S. Dai, R. T. Mayes,

G. M. Veith, L. Prati

Phosphorylated mesoporous carbon showed a good activity and se-lectivity for the dehydration of fructose to HMF in water, making good

candidate for large scale production of HMF with the advantage of

easy recyclability and separations.

312

Purified oxygen- and nitrogen-modified multi-walled carbon

nanotubes as metal-free catalysts for selective olefin hydro-

genation

Peirong Chen, Ly May Chew, Aleksander Kostka, Kunpeng Xie, Mar-

tin Muhler, Wei Xia

Nitrogen-functionalized carbon nanotubes used as metal-free cata-

lysts were more active than oxygen-functionalized nanotubes in se-

lective olefin hydrogenation reactions. The catalytic activity can beascribed to nitrogen-containing groups and surface defects related to

nitrogen species.

321

Ru particle size effect in Ru/CNT-catalyzed Fischer-Tropsch syn-

thesis

Jincan Kang, Weiping Deng, Qinghong Zhang, Ye Wang

Ru/CNT is an efficient catalyst for diesel fuel production from syngas,

and the TOF and C10-C20 selectivity increases with the size of Ruparticles from 2.3 to 6.3 nm.


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329

Ammonia-treatment assisted fully encapsulation of Fe2O3nanoparticles in mesoporous carbons as stable anodes for

lithium ion batteries

Fei Han, Wen-Cui Li, Duo Li, An-Hui Lu

Ultrafine Fe2O3nanoparticles with 45 nm size and rationally tailored

loading of 47 wt% were fully encapsulated into tubular mesoporous

carbon matrix, which were designed as high capacity and excellent

stability anode materials.

336

Enhanced reversible capacity of Li-S battery cathode based on

graphene oxide

Jin Won Kim, Joey D. Ocon, Dong-Won Park, Jaeyoung Lee

Graphene oxides were used to enhance the reversibility of Li-S bat-tery. Oxygen groups of graphene oxide sheets can anchor the sulfur

of lithium polysulfides,which can effectively enhance the utilization of

sulfur and reversibility of Li-S battery.

341

Hierarchical nanostructured composite cathode with carbon

nanotubes as conductive scaffold for lithium-sulfur batteries

Xiaofei Liu, Qiang Zhang, Jiaqi Huang, Shumao Zhang,

Hongjie Peng, Fei Wei

A hierarchical composite cathode containing commercial agglomer-ated multi-walled carbon nanotube and sulfur for Li-S battery exhib-

ited excellent Li storage performance.

347

Porous V2O5-SnO2/CNTs composites as high performance cath-

ode materials for lithium-ion batteries

Qi Guo, Zhenhua Sun, Man Gao, Zhi Tan, Bingsen Zhang,

Dang Sheng Su

V2O5-SnO2/CNTs composites with reduced particle size and porous

structure were synthesized by a facile hydrothermal method. The

composites exhibited improved rate capability and specific capacity

compared with commercial V2O5 when used as cathode electrodesfor lithium ion batteries.


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Preface to Special Issue on Carbon Materials for Energy Application

The rising cost and limited availability of fossil fuels, and the increasing concerns related to their role on global pollution

and greenhouse effect have pushed considerably the need to accelerate the transition to a more sustainable use of energy based

largely on renewable energy sources. Nanocarbon materials play a critical role in this transition, as they are the key materials for

components of different devices necessary in enabling this transition (batteries, fuel cells, solar cells, etc.).

This issue collects 22 contributions, including one perspective and six review papers on the topic of carbon materials for

energy applications, written by well-known experts in this field. It is really an exciting special issue that gives a very updated

view of this topic, as well as trends and outlooks in this breakthrough research area. The initial perspective paper introduces the

different possibilities offered from the growing level of knowledge in this area, testified from the exponentially rising number of

publications. It also discusses the basie concepts for a rational design of these nanomaterials.

The following six reviews address different specific aspects of synthesis, characterization and use of carbon nanomaterials,

from fuel cells to composite electrodes, supercapacitors and photoelectrochemical devices for CO 2 conversion. These reviews

represent an unique opportunity for the readers to be updated on the latest developments of new carbon families such as fullerene,

graphene, and carbon nanotube, and their derived nanocarbon materials (from carbon quantum dots to nanohorn, nanofiber, nano

ribbon, etc.). Second generation nanocarbons, including modification of these nanocarbons by surface functionalization or doping

with heteroatoms to create specific tailored properties, and nanoarchitectured supramolecular hybrids, are also discussed.

Finally, 1 communication and 14 full articles discuss several aspects of the use of these nanocarbon materials to develop new

catalysts for a range of applications (from biomass conversion to Fisher-Tropsch reaction and electrochemical devices) and new

materials for energy storage and conversion (adsorption pumps, Li-ion and Li-S batteries, electrodes for electrochemical uses).

We thus believe that this special issue dedicated to the use and development of carbon materials for energy applications

represents a unique occasion for young and experienced researchers as well as for managers in the field of sustainable energy

to have an updated view on this enabling topic for the future of our society. We thus invite all to have this special issue as a

privileged component of your bookshelf.

Dang Sheng Su and Gabriele Centi

Professor Dang Sheng Su

Shenyang National Laboratory for Materials Science

Institute of Metal Research

Chinese Academy of Sciences

Shenyang 110006, Liaoning

China

E-mail: [email protected]

Professor Gabriele Centi

Dipartimento di Ingegneria Elettronica

Chimica ed Ingegneria Industriale

University of Messina and INSTM/CASPE

V. le F. Stagno DAlcontres 31, 98166, Messina

Italy

E-mail: [email protected]


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Journal of Energy Chemistry 22(2013)226240

Review

Carbon/carbon supercapacitors


Institute of Chemistry and Technical Electrochemistry, Poznan University of Technology, Piotrowo 3, 60-965 Poznan, Poland

[Manuscript received December 26, 2012; revised February 18, 2013 ]

Abstract

Supercapacitors, or electrochemical capacitors, are a power storage system applied for harvesting energy and delivering pulses during short

periods of time. The commercially available technology is based on charging an electrical double-layer (EDL), and using high surface area

carbon electrodes in an organic electrolyte. This review first presents the state-of-the-art on EDL capacitors, with the objective to betterunderstand their operating principles and to improve their performance. In particular, it is shown that capacitance might be enhanced for

carbons having subnanometric pores where ions of the electrolyte are distorted and partly desolvated. Then, strategies for using environment

friendly aqueous electrolytes are presented. In this case, the capacitance can be enhanced through pseudo-faradaic contributions involving i)

surface functional groups on carbons, ii) hydrogen electrosorption, and iii) redox reactions at the electrode/electrolyte interface. The most

promising system is based on the use of aqueous alkali sulfate as electrolyte allowing voltages as high as 2 V to be reached, due to the high

overpotential for di-hydrogen evolution at the negative electrode.

Key words

supercapacitors; electrochemical capacitors; porous carbons; electrolytes; pore size; pseudocapacitance

Elzbieta Frackowiak is a Professor in the Insti-

tute of Chemistry and Technical Electrochemistry

at Poznan University of Technology, Poland. Her

research interests are especially devoted to stor-age/conversion of energy in electrochemical ca-

pacitors, Li-ion batteries, fuel cells. Main top-

ics: application of activated carbon materials

for supercapacitors and hydrogen storage, use of

composite electrodes from nanotubes, conducting

polymers, doped carbons and transition metal ox-

ides for supercapacitors. She serves as Chair of Division 3 Electrochemical

Energy Conversion and Storage of the International Society of Electrochem-

istry (20092014). She was the winner of the Foundation for Polish Science

Prize (2011). She is author of 150 publications, a few chapters and tens

of patents and patent applications. Number of citations ca. 6370, Hirsch

index 37.

Qamar Abbas is post-doctoral fellow at Insti-

tute of Chemistry and Technical Electrochem-

istry in Poznan University of Technology, Poz-

nan (Poland). He received his PhD in Techni-

cal Sciences from Institute of Inorganic Chem-

istry at Graz University of Technology, Graz (Aus-

tria). His research focuses on enhancing the per-

formance of microporous carbon based superca-

pacitors in environmental friendly electrolytes. A

part of his work is related to the corrosion investi-

gations of current collectors in supercapacitorsunder testing conditions.

Corresponding author. Tel: +48-61-6653632; Fax: +48-61-6652571; E -mail: [email protected]

The Foundation for Polish Science is acknowledged for supporting the ECOLCAP Project realized within the WELCOME program, co-financed from

European Union Regional Development Fund.

Copyright2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.


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Journal of Energy Chemistry Vol. 22 No. 2 2013 227

Francois B eguin is a Professor in Poznan Univer-

sity of Technology (Poland), where he has been

recently awarded the WELCOME stipend from

the Foundation for Polish Science. His research

activities are devoted to chemical and electro-

chemical applications of carbon materials, with

a special attention to the development of nano-

carbons with controlled porosity and surface func-

tionality for applications to lithium batteries, su-percapacitors, electrochemical hydrogen storage,

reversible electrosorption of water contaminants.He published over 250 publications in high rank international journals and

his works are cited in 8300 papers, with Hirsch index 46. He is also involved

in several books dealing with carbon materials and energy storage. He is a

member of the International Advisory Board of the Carbon Conferences and

he launched the international conferences on Carbon for Energy Storage and

Environment Protection (CESEP). He is a member of the editorial board of

the journal Carbon. He was a Professor of materials science in Orleans Uni-

versity (France) until 2012, and he was Director of national programmes on

Energy Storage (Stock-E), Hydrogen and Fuel Cells (H-PAC) and electricity

management (PROGELEC) in the French Agency for Research (ANR).

1. Introduction

Supercapacitors (or ultracapacitors, or electrochemical

capacitors) based on activated carbon electrodes are an energy

storage device which has been the object of important research

in the last decade [1,2]. They provide higher energy density

than dielectric capacitors, while demonstrating higher power

density than batteries [3,4]. Therefore, they are particularly

adapted for applications which require energy in bursts dur-

ing short period of time, e.g., automobiles, tramways, buses,

cranes, forklifts, wind turbines and in opening emergency

doors of airplanes. Since the basic operating principle of su-

percapacitors is the electrostatic attraction of ions on the elec-trode/electrolyte interface, the commercially available super-

capacitors demonstrate a high degree of reversibility, being

able to withstand a high number of charge/discharge cycles,

ca. 1000000 cycles.

Because of high electrical conductivity, low cost and

availability at ease, porous carbons are used as electrode mate-

rials in supercapacitors. Activated carbons (AC) provide high

surface area and their porosity can be tailored to the desired

pore size distribution by varying the activation process or type

of precursor. The correlation of ion size of the electrolytic

system to the pore size of carbons has opened new research

horizons in the field of supercapacitors [5,6]. Besides the

pure electrostatic attraction of ions (electrical double-layer)which plays in all kinds of electrochemical capacitors, the

performance of capacitors can be enhanced by pseudocapac-

itive contributions. The later might be related with the pres-

ence of surface oxygenated and nitrogenated functionalities,

electrochemical hydrogen storage, carbon interface with re-

dox species [7,8].

The energy density of supercapacitors depends on the

square of the operating voltage, which is controlled by the

stability window of the electrolyte [7]. Aqueous electrolytes

have a limited stability window up to 0.70.8 V in acidic

and alkaline pH value [9] and up to 1.81.9 V in neutral pH

value [10], while non-aqueous electrolytes have a stability

window up to 2.72.8 V [11]. The voltage window in or-

ganic electrolytes is limited mainly due to the presence of im-

purities, like traces of water, and active sites on the surface

of microporous carbons [12]. However, aqueous electrolytes

give much higher capacitance values in comparison to organic

electrolytes.

This article reviews the basic role played by carbon mate-

rials in energy density enhancement of supercapacitors, tak-

ing into account the influence of porous texture on elec-

trical double-layer capacitance and of surface functional-

ity on pseudo-capacitance. A part of the discussion is also

dedicated to the contributions of hydrogen storage and car-

bon/electrolyte interface to the overall capacitance. Finally,

the importance of the electrolytic systems on the voltage, and

consequently energy density, is also considered.

2. General properties of electrical double-layer capacitors

(EDLCs)

The main energy storage mechanism in AC/AC superca-

pacitors arises from the reversible electrostatic accumulation

of ions on the surface of activated carbon. Upon polarization,

the charge at the electrode surface is neutralized by a layer of

counter ions at a distance d(Figure 1a), resulting in a capaci-

tanceCas described by Helmholtz [13] in Equation (1):

C=

r0A

d

or

C

A =

r0d

(1)

where, r and 0 are the dielectric constants of the electrolyte

and vacuum, respectively, and A is the surface area of the

interface. Gouy and Chapman [1416] proposed a diffuse

model of the electrical double-layer, in which the potential de-creases exponentially away from the surface to the fluid bulk

(Figure 1b). In order to resolve the failure of Gouy-Chapman

model for highly charged double-layers, Stern [17] suggested

a model combining Helmholtz and Gouy-Chapman models,

and taking account of the hydrodynamic motion of the ionic

species in the diffuse layer and the accumulation of ions close

to the electrode surface, as presented in Figure 1(c).

Based on EDL formation, the most known supercapaci-

tor (electrical double-layer capacitor-EDLC) is the symmet-

ric one, i.e., with two identical electrodes immersed in an

aqueous or an organic electrolyte (Figure 2). In the in-

dustrial capacitors, the electrode material is a high surface

area (>1500 m2g1) activated carbon which coats a current

collector (aluminum in organic electrolyte, stainless steel in

aqueous KOH). Considering the small value ofdin Formula

(1), the capacitance of each electrode is very high. The

two electrodes are separated by a porous membrane (paper,

glass fibre, polymer) named separator. A binder (polyvinyli-

dene fluoride-PVdF, carboxymethylcellulose-CMC, polyte-

trafluoroethylene-PTFE) agglomerates and links the grains of

active materials with the current collector. A percolator (car-

bon black, carbon nanotubes) is added for improving the elec-

trodes conductivity.


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228 Elzbieta Frackowiak et al./ Journal of Energy Chemistry Vol. 22 No. 2 2013

Figure 1. Helmholtz (a), Gouy-Chapman (b) and Stern models (c) of the electrical double-layer formed at a positively charged electrode in aqueous electrolyte.

IHP refers to the distance of the closest ion sheath, and OHP to the non-specifically adsorbed ions. The diffuse layer begins from OHP and can have a thickness

in the range of 10100 nm. After the diffuse layer, the bulk electrolyte starts (from Ref. [18])

Figure 2. Typical electrical double-layer capacitor in its charged state

According to Figure 2, in its charged state, a supercapac-

itor is equivalent to two capacitors of capacitance C+and Cin series. The capacitance of the total system is given by For-

mula (2):

1

C =

1

C++

1

C(2)

where,C is the cell capacitance, C+ andC are the respec-

tive capacitances of the positive and negative electrodes. As

the capacitance of the two electrodes is different, even in a

symmetric capacitor, Formula (2) indicates that the value of

Cis determined by the electrode with the smallest capacitancevalue.

Recent investigations show that classical models of the

double-layer do not apply when microporous carbonsare used

as electrode materials. It has been demonstrated that the elec-

trosorption of ions is favored in subnanometric pores which

are smaller than the solvation sphere, suggesting that ions are

at least partially desolvated [5,6,19,20]. Electrochemical stud-

ies carried out in pure ionic liquid electrolytes have shown

that the highest capacitances are obtained when the pore size

matches the diameter of the ionic species [21].

The molecular mechanisms which play in carbon elec-

trodes remain unclear, especially the large capacitance val-

ues achieved seem to demand a much higher level of chargeseparation at the interface under the influence of the applied

potential. Whether the capacitance enhancement depends on

pore structure and/or other factors is difficult to be described

through experiments alone. Moving from the conventional

Helmholtz model to a situation where the ionic species en-

ter the pores partially desolvated and arrange in lines within

entire pore length, various factors come into play which might

result in a capacitance increase. Concepts based on cylin-

drical mesopores and cylindrical micropores, both shown in

Figure 3, have been considered in literature [22,23]. In the

mesopore regime (2 to 50 nm), solvated counter ions approach

the pore wall and form an electrical double-cylinder capacitor

(EDCC) of capacitance given in Equations (3a) and (3b):

C= 2r0L

ln

b

a

(3a)

C= r0

b ln

b

bd

A (3b)

where,L is the pore length,b and a are the radii of the outer

and inner cylinders, respectively. In such case, the effect of

pore size and pore curvature becomes prominent as compared

with the distance d.

In case of micropores (


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Figure 3. Schematic diagram (vertical axis) of (a) a negatively charged meso-

pore with cations approaching the pore wall to form an electrical double-

cylinder capacitor (EDCC) with radii b and a for the outer and inner cylin-

ders, respectively, separated by a distanced, and (b) a negatively charged

micropore of radiusbwith cations of radiusa0 lining up to form an electrical

wire-in-cylinder capacitor (EWCC) (from Refs. [22,23])

Macropores (>50 nm) are large enough so that pore cur-

vature is no longer significant, so the classical Equation (1)

can be applied. Equations (3b) and (4) have been used to fit

the experimental data from Ref. [5] for supercapacitors built

with nanoporous carbons of diverse pore size. Taking Equa-

tion (4) into account, the anomalous increase in capacitance

with decreasing pore size [5] can be rationalized.

A further discussion which stressed that Gouy-Chapman-

Stern theory cannot apply in dense ionic systems came from

Kornyshev [24]. Taking the ion packing constraints in ionic

liquids (ILs) into account, the so-called lattice saturation

effect, an alternative mean field theory (MFT), was suggested.

The multi-layered structure contains layers of ionic species

close to the planar electrode surface; the charge of the closest

layer is larger than the charge of the electrode and is counter-

balanced in the following layers; this is called the overscreen-

ing effect [2527]. Further, a molecular dynamics (MD) sim-

ulation approach was adopted for carbon nanotube microp-

ores of various sizes in ionic liquids [28,29]. This approach

predicts that overscreening at small voltages is high. How-

ever, the calculated capacitance values differ from the typi-

cal experimental ones by an order of magnitude. The Monte

Carlo simulation of a model ionic liquid in slit-like metallic

nanopores was presented by Kondrat et al. [30]. They de-

scribed that the superionic state of ions inside a nanosized

pore is responsible for the anomalous increase in capacitance

with decreasing pore width, assuming that the pore is not

empty at zero voltage. They also observed that for narrow

pores, the capacitance as a function of voltage exhibits a peak

before dropping down to zero at higher voltages. This drop of

capacitance at high voltage, attributed to saturation of poros-

ity, has been observed experimentally by Mysyk et al. [31]

and will be further discussed in Paragraph 4.

A different approach to MD simulation was adopted by

Merlet et al. [32] for an EDLC constituted of microporous

carbon electrodes with an ionic liquid as electrolyte. The

EDLC simulation cell is shown in Figure 4, where the top

panel is a snapshot extracted from a simulation, and the bot-

tom panel illustrates the electrification of an electrode held at

various potentials. The two key features taken into account

were a realistic atomistic structure of the carbide-derived car-

bon (CDC) electrode [33] and the polarization of the electrode

atoms by the ionic charges. Such approach allows simulations

of conducting electrodes of arbitrary geometry under constant

applied potentials to be performed, i.e. in the same way as ex-

periments are performed [34,35]. Through this simulation, ca-

pacitance values of 87 and 125 Fg1 were obtained for CDC-

1200 and CDC-950, respectively, far higher than the values

Figure 4. EDLC simulation cell. Upper panel: the simulation cell consists of a BMI-PF6 ionic liquid electrolyte surrounded by two porous electrodes (CDC-

1200) held at constant electrical potentials (blue: C atoms, red: the three sites of BMI +and green: PF6 ions; a coarse-grained model is used to describe these

ions). Lower panel: structure of the electrode for various voltages. For each value, the same snapshot is shown twice: the ionic distribution is shown on the left.

The degree of charging of the electrode atoms is shown on the right, where carbon atoms are colored according to the charge q they carry (green: q0 and yellow: q=0). The charging mechanism involves the exchange of ions between the bulk and the electrode (from Ref. [32])


15/25


reported in previous simulations [28,29] of ionic liquids ad-

sorbed in carbon nanotubes; these simulations did not con-

sider that the electrode is wetted by the ionic liquid even at

null potential.

The main parameters which characterize the performance

of supercapacitors include: i) the power density essentially

greater than for batteries, ii) an excellent cyclability (up to 100

times higher than batteries), iii) fast charge/discharge process,

and iv) low equivalent series resistance (ESR).

The maximum power Pof a supercapacitor is calculated

according to Equation (5):

P = U2

4R (5)

where, Uis the maximum cell voltage (V), R is the equiva-

lent series resistance (), andPis the maximum power (W).

The factors which mainly contribute to the overall series re-

sistance of supercapacitors are the electronic resistance of the

electrode material, the contact resistance between electrode

material and current collector, the electrolyte resistance, the

ionic diffusion resistance due to the movements in microp-

ores and the ionic resistance caused by the separator. Thanksto the electrostatic charge storage mechanism, the series re-

sistance does not include any charge transfer resistance con-

tribution associated with electron exchange, as observed for

redox reactions. Thus, the series resistance is lower than that

of batteries at cell level, explaining the higher power density

of supercapacitors compared with batteries.

The maximum energy Eis given by Equation (6):

E= 1

2CU2 (6)

where, C is capacitance (F), U is the maximum cell voltage

(V), and Eis the energy (J). The charge storage is achieved on

the surface of the active material, at the difference of accumu-

lators where the charge is stored in the bulk of the material,

and the energy density of EDLCs is less than that of Li-ion

cells. However, this storage mechanism also allows a very fast

delivery of the stored charge. Thus, EC devices can deliver all

the stored energy in a short time, about 5 s; this process is

fully reversible and energy update can be achieved within the

same time period.

In general, the energy is expressed as per mass (Wh/kg)

or volume (Wh/m3) of the device; in case of capacitance, the

values are in F/kg and F/m3, respectively. Most industrial ap-

plications require small size systems, for which the volumet-

ric parameters are more relevant. Since scientific publications

rather concern the optimization of the electrode material, inthis case, the capacitance is expressed in F/g or F/cm 3 for one

electrode; the energy is then in Wh/g or Wh/cm3. Definitely,

high density materials are more adapted for enhancing the vol-

umetric energy, requiring strictly microporous carbons with a

very low amount of mesopores.

3. Electrolytes for supercapacitors

Both Equations (5) and (6) show that power density

and energy density of supercapacitors are proportional to the

square of voltage. The cell voltage is mainly limited by the

electrolyte stability. The advantage of aqueous electrolytes

like acids (H2SO4) and alkalis (KOH) is a higher conductivity

(up to 1 Scm1) as compared with other electrolytic sys-

tems. The major disadvantage of aqueous solutions is their

restricted stability window, about 0.70.8 V. Most of the com-

mercial devices use organic electrolytes, i.e., N(C2H5)+4BF

4

dissolved in acetonitrile (CH3CN) or propylene carbonate

(PC), so that the operating voltage reaches 2.72.8 V. Non-

aqueous electrolytes with good conductivity and higher oper-

ating voltage (up to 3.54 V) are highly desirable. Aprotic

ionic liquids seem to be promising, although the published re-

sults are still the object of high controversy.

The properties of an electrolytic system for an electro-

chemical capacitor include: i) a good conductance which de-

termines the power output capability, ii) a good ionic ad-

sorption which determines the specific double-layer capaci-

tance, and iii) the dielectric constant which also determines

the double-layer capacitance value and its dependence on

electrode potential as well as the extent of ionization or ion

pairing of the solute salt, which influences the conductance.

In order to achieve a high power supercapacitor system, the

internal electrolyte resistance and the structural resistance of

the porous carbon electrode material should be minimized [1].

This can be achievedby an electrochemically compatible elec-

trolyte salt or an acid or alkali which is strongly soluble in the

solvent to be used. Minimum ion pairing and maximum free

mobility of dissociated ions should be achieved in dissolved

state. Equation (7) characterizes the dissociation of any salt

MA at concentrationc, into its free ions:

M AKc M+ +A (7)

(1)c c c

where, is the dissociation degree of the salt molecules at

concentration c.

Non-aqueous electrolytic solutions are significantly

weak, so that the value of is appreciably less than its value

in aqueous solutions, which is near 1. This leads to higher

ESR values for non-aqueous solution based devices than for

aqueous one using the same electrode materials and cell ge-

ometries. Solvents like water provide strong solvation and a

tendency for complete dissociation or minimum ion pairing.

Such solvents are usually those which have high dielectric

constants, often with hydrogen bonded structures with large

dipolar moments. Moreover, in the case of tetraalkylammo-

nium salts, which are used commonly for non-aqueous elec-

trolytes, different principles apply. The extent of ion pairingis usually less than that for inorganic salts owing to their large

ionic radii and their alkyl groups tending to interact well with

organic solvents.

The two major classes of electrolytic media extensively

used in supercapacitors include aqueous and non-aqueous

ones in recent years.

3.1. Aqueous media

Based on the knowledge from accumulators, the obvious


16/25


choice for supercapacitor electrolytes has initially been sul-

phuric acid (H2SO4) and KOH. Highly concentrated solutions

are used in order to overcome the ESR factor and to maximize

the power capability. However, the acid solutions are highly

corrosive in nature as compared with concentrated KOH, es-

pecially for current collectors. Most of the fundamental stud-

ies in KOH and H2SO4 have been performed using gold cur-

rent collectors; the operating voltage window in these media

is less than 1 V [9]. Recent investigations by Khomenko et al.

[36] have shown that it is possible to enhance the operating

voltage of carbon based supercapacitors in aqueous H2SO4up

to 1.6 V, by different optimized carbons as positive and nega-

tive electrodes and/or by balancing the mass of electrodes.

However, due to the limitations in both acidic and al-

kaline media, a quest of neutral pH electrolytes has started

in recent years. Activated carbons demonstrate a stability

window of 2 V in Na2SO4 aqueous electrolyte, and a sym-

metric carbon/carbon cell can operate up to 1.6 V with good

charge/discharge cycle life [37]. Electrochemical character-

ization of seaweed carbons in Na2SO4 has shown that the

nature of the electrode material and electrolyte pH influenceboth the capacitance values and the stability window [38].

The migration of hydrated alkali ions in the bulk electrolyte

and within the inner pores of activated carbon increases in the

order of Li+


17/25


rapidly tends to a plateau for SSA higher than 1500 m 2g1.

It has been suggested that, due to the decrease of average pore

wall thickness in highly activated carbons, the electric field

(and the corresponding charge density) no longer decays to

zero within the pore walls [63]. In fact, it has been observed

that the average pore size increases together with the specific

surface area when the activation degree increases. It suggests

that the interaction of ions with pore walls is weaker in larger

pores, and according to Equation (1) the effect of porosity de-

velopment on capacitance is counterbalanced by the increase

of ion-wall distance [6]. Figure 5 shows that the normalized

capacitance in F/cm2 (specific capacitance from Ref. [6] di-

vided by BET SSA) increases dramatically as the average mi-

cropore size L0decreases.

Similar increase of normalized capacitance in pores

smaller than 1 nm has been observed with carbide derived car-

bons in acetonitrile-based electrolyte containing 1.5 molL1

Et4NBF4 [5]. Considering the diameters of solvated ions in

this electrolytic medium, e.g., 1.30 nm for Et4N+ and 1.16 nm

for BF4, and the diameters of bare ions, e.g., 0.68 nm for

Et4

N+and 0.48 nm for BF

4 [64], it shows that the pores below

1 nm are smaller than the size of solvated ions. The capaci-

tance increase for pore size


18/25


the charge determined by integration of the respective voltam-

mograms,Qexp, up to a voltage of 3 V. For PC the theoreti-

cal and experimental values are almost identical, confirming

that the narrowing of the voltammetry curve is due to poros-

ity saturation by NEt+4 ions. By contrast, in the case of VC,

the theoretical value of charge is larger than the experimen-

tal one, demonstrating that for this carbon the porosity is not

saturated, at least for the maximum voltage of 3 V reached in

this experiment.

During galvanostatic cycling, the porosity saturation is

reflected for PC by the non-linear shape of the voltage-time

curve, whatever the current density (Figure 8). At 960 mA/g,

the drop of capacitive current at ca. 1.52 V (Figure 8b)

shows the difficulty to store more energy by further increasing

voltage up to the electrolyte stability limits [31].

Figure 7. Cyclic voltammograms for EDLCs based on PC carbon (left-hand

side Y-axis for current) and VC carbon (right-hand side Y-axis for current).

Adapted from Ref. [31]

Figure 8. Galvanostatic charge-discharge of EDL capacitors based on nanoporous carbon PC at current density of 80 mA

g

1

(a) and 960 mA/g (b). The straightpart of the discharging line is extrapolated in order to discriminate the point of porosity saturation. This corresponds to a voltage of about 12 V, depending on

the current density. From Ref. [31]

Summarily, the porous texture strongly influences the

electrochemical properties of carbons. The capacitance with

Et4NBF4-based electrolyte is optimal in subnanometric pores,

suggesting the distortion of solvation shell. If the porous vol-

ume is not sufficiently developed, pores may be saturated by

ions although being in the range 0.70.8 nm, leading to a limit

of the maximum voltage and consequently of energy and de-

liverable power. In some cases, the local structure of carbon

may also be responsible for electrochemical intercalation dur-

ing charging [66,67].

5. Pseudo-capacitive contributions involving carbon elec-

trodes

Considering the electrochemically available surface area

of activated carbons and the chargeamount which could be ac-

cumulated in the electrical double-layer, the capacitance val-

ues reported do not exceed 150 F/g. Apart from typical elec-

trostatic interactions in the electrical double-layer, redox re-

actions with electron transfer on the electrode/electrolyte in-

terface can greatly contribute in enhancing the charge storage

process and the energy. However, due to their typical faradic

origin, these processes exhibit a slow kinetic of the heteroge-

neous reaction (limited mainly by the diffusion of the involved

electrochemical species) and a moderate cycle life (connected

with changes of the material structure undergoing oxidation

or reduction process).

5.1. Pseudo-capacitance originating from heteroatom doped

carbons

Capacitance of nanoporous carbons can be enhanced

through quick faradaic reactions or local modification of the

electronic structure, both originating from the presence of

oxygenated and nitrogenated functionalities in the carbon net-

work [68]. Since functional groups are generally present

in small amount in activated carbons, enrichment techniques

have been developed. The general ways to obtain heteroatom


19/25


enriched carbons are: i) the carbonization of a suitable het-

eroatom rich precursor, ii) the carbon post-treatment in oxy-

gen or nitrogen containing atmosphere, and iii) grafting of

molecules containing suitable functional groups.

5.1.1. Oxygen enriched carbons

Interesting carbons were obtained by one-step carboniza-tion at 600 C of a seaweed biopolymer, e.g., sodium al-

ginate, or seaweeds themselves, without any further activa-

tion [69,70]. The material resulting from alginate carboniza-

tion is slightly microporous (SBET= 273 m2/g) and it contains

a high amount of oxygen (15 at%) in the form of phenol

and ether groups (COR, 7.1 at%), keto and quinone groups

(C = O, 3.5 at%) and carboxylic groups (COOR, 3.4 at%).

Three-electrode cyclic voltammograms in 1 molL1 H2SO4show cathodic and anodic humps at around 0.1 V and

0 V vs. Hg/Hg2SO4, respectively, which can be attributed

to quinone/hydroquinone pair [71] or pyrone-like structures

[72]. Despite low BET specific surface area of this carbon, the

capacitance in 1 molL1 H2SO4mediumreaches 200 F/g, i.e.a value comparable to the best activated carbons available on

the market. Some additional performance improvement was

obtained by incorporating carbon nanotubes in the seaweeds

before thermal treatment [73].

5.1.2. Nitrogen enriched carbons

Nitrogen can be substituted to carbon (lattice nitrogen)

or in the form of functional groups (chemical nitrogen)

at the periphery of polyaromatic structural units [74,75], as

shown in Figure 9.

Figure 9. Nitrogenated functional groups in carbon network of (a) pyridinic

(N-6), (b) pyrrolic, (c) pyridonic (N-5), (d) quaternary (N-Q), and (e) oxidized

nitrogen (N-X)

Nitrogen enriched carbons were obtained by ammoxida-

tion of nanoporouscarbons [76] or by activation of carbonized

nitrogen rich polymers [77,78]. A linear correlation has been

found between capacitance in H2SO4 medium and the nitro-

gen content for a series of nitrogen enriched carbons of com-

parable porous characteristics (SBET800 m2/g) [7,78]. This

enhancement of capacitance is interpreted by pseudo-faradaic

charge transfers involving nitrogenated functionalities, such

as in Figure 10 [68]:

Figure 10. A possible pseudo-faradic charge transfer involving pyridinic ni-

trogen [68]

Self-standing C/C composite electrodes presenting

pseudo-capacitive properties and high electrical conductiv-

ity have been obtained by one-step pyrolysis of carbon nan-

otube/polyacrylonitrile blends at 700 C [79]. Whereas the

specific surface area of polyacrylonitrile (PAN) carbonized

at 700 C is negligible (SBET= 6 m2g1), the C/C compos-

ite formed by pyrolysis of a CNT/PAN (30/70 wt%) blend at

700 C has a more developed porosity (SBET= 157 m2g1,

Vmeso= 0.117 cm3g1), with mesopores due to the templat-

ing effect of CNTs. The nitrogen content measured on this

composite by XPS is 7.3 at%. The capacitance determined

for the C/C composite is in the order of 100 F/g in 1 molL

1

H2SO4, whereas under the same conditions the pristine CNTs

give 18 Fg1 and carbonized PAN a negligible value. The re-

markable capacitive behavior of this kind of composite is due

to a synergy between CNTs and the nitrogenated functionality

of carbonized PAN.

The beneficial effect of nitrogen in composites with an

incorporated nanotubular backbone has been also demon-

strated using melamine as nitrogen-rich carbon precursor

[80]. Polymerized melamine/formaldehyde blends formed

with different proportions of melamine in the presence of mul-

tiwalled carbon nanotubes are carbonised at 750 C. The ni-

trogen content in the carbons varies from 7.4 to 14 wt%. Thematerials are typically mesoporous with a BET specific sur-

face area ranging from 329 to 403 m2/g. In 1 mol/L H2SO4,

the composites demonstrate high charge propagation with ca-

pacitance as high as 126 F/g at 5 A/g current load. The pres-

ence of nitrogenated functionalities has a profitable effect on

the capacitance values by modifying the electronic properties

as well as wettability.

Nitrogenated carbons prepared in different conditions us-

ing N-rich precursors have been also investigated in super-

capacitors. Melamine polymerized in mica [81] and fur-

ther treated by ammonia gave capacitance values as high as

280 F/mL in KOH medium [82]. A very high capacitance of

340 F/g has been reached in 1 mol/L H2SO4 using templated

carbons obtained by pyrolysis of acrylonitrile in NaY zeolite

as scaffold [83]. This high value results from the synergy be-

tween the highly developed surface area of the material, the

pseudo-faradaic reactions related to the presence of the nitro-

genated functionalities and their high accessibility provided

by the straight channels inherited from the zeolite substrate.

5.2. Electrografted carbons


20/25


Figure 11. Grafting mechanism of a diazonium cation on a conducting surface (carbon, semiconductor or metal)

Surface modification of carbon materials implying an

electron transfer when the substrate is connected to a cur-

rent generator has been coined as electrografting. Such kind

of modification leads to the binding of desired functional

groups onto conductive surfaces. The reagents used for the

modification of activated carbon surfaces by electrografting

include aryl diazonium salts, amines, carboxylates, alcohols,

Grignard reagents and halides [84,85]. As far as supercapaci-

tors are concerned, only grafting through diazonium salts has

been used until now. As schematized in Figure 11, this pro-

cess is a concerted mechanism in which the diazonium cation

is reduced and one nitrogen molecule is eliminated [86].

By an appropriate choice of the functional group Rpresent on the aryl diazonium salt, nanoporous carbons may

exhibit pseudo-capacitive properties. A carbon electrode

functionalized by 8.4 wt% catechol demonstrates a capaci-

tance of 250 F/g over a potential range from 0.4 to 0.75 V

in 1 mol/L H2SO4, as compared with 150 F/g for the pristine

carbon [87]. By attaching anthraquinone (AQ) to a carbon

surface, the capacitance could be enhanced up to 40% [88],

although the BET specific surface area significantly decreases

from 1500 to 1185 m2/g [89]. Figure 12 compares the voltam-

mograms of the Black Pearl carbon modified with 11 wt% AQ

and the unmodified carbon, and the reversible redox wave of

AQ giving the pseudocapacitive effect can be seen at about0.2 V vs. Ag/AgCl [90]. A supercapacitor based on this

system was tested for 10000 charge/discharge cycles and 14%

loss of faradaic capacitance was observed for 11 wt% loading

as compared with 17% capacitance loss for the unmodified

carbon material.

Weissmann et al. observed that AQ concentration on the

carbon surface depends on pH, hence affecting the superca-

pacitor performance [91]. At pH = 14, the surface concen-

tration was found to be close to 91010 mol/cm2 for the

modified electrode, while at pH = 7 the value decreased to

61010 mol/cm2 and at pH = 0.5 the concentration further

decreased to 5.6

10

10

mol/cm

2

. Cyclic voltammetric mea-surements showed that the shape of the redox peaks is greatly

affected in acidic pH, giving a poorly resolved cathodic wave,

while in alkaline region a good reversibility of the redox cou-

ple was observed.

Figure 13 shows the reversible redox waves of a dihy-

droxybenzene (DHB)-grafted carbon cloth at 0.41 V and 0.65

V; the average specific capacitance increases from 141 F/g

for the unmodified carbon cloth (C) to 201 F/g for the DHB-

modified carbon (C-DHB), between 0.2 V and 0.8 V vs.

Ag/AgCl [92]. Grafting the same cloth with anthraquinone

(C-AQ) also produced an appreciable enhancement in average

specific capacitance, giving 367 F/g over a potential range of

0.35 V.

Figure 12. Cyclic voltammograms in 0.1 mol/L H2SO4 of unmodified

(solid line) and modified (dashed line) Black Pearl carbon with 11 wt% an-

thaquinone (AQ) (from Ref. [90])

Figure 13. Cyclic voltammograms of C-AQ (solid line), C-DHB (dotted line)

and unmodified-C (dashed line) in 1 mol/L H2SO4 (from Ref. [92])

As seen in Figure 13, the anthraquinone (AQ) and di-

hydroxybenzene (DHB)-modified carbon cloths are electro-


21/25


chemically active in different potential ranges. Therefore, C-

AQ and C-DHB carbons have been used respectively for the

negative and positive electrodes of an asymmetric superca-

pacitor in 1 mol/L H2SO4 [92]. The galvanostatic discharge

characteristics of the asymmetric capacitor are shown in Fig-

ure 14, where they are compared with the performance of a

symmetric C/C capacitor built from the unmodified carbon.

In the voltage range between 0.7 and 0.2 V, the slope for C-

AQ/C-DHB system is much lower than that for C/C system,

indicating a contribution of the AQ and DHB redox couples.

The capacitance increases from 30 F/g1 for C/C supercapac-

itor to 65 F/g for C-AQ/C-DHB one between 0.2 and 0.7 V.

However, higher ESR values in case of the modified C-AQ/C-

DHB supercapacitor as compared with unmodified C/C one

suggest that the former device requires further optimization in

order to get better performance.

Figure 14. Galvanostatic discharge curves at 0.2 A/cm2 for the asymmetric

(thick line) C-AQ/C-DHB and symmetric (dotted line) C/C supercapacitors

in 1 mol/L H2SO4 (aq) (from Ref. [92])

5.3. Pseudo-capacitance related with electrochemical hydro-

gen storage in aqueous neutral medium

According to the thermodynamic stability of water, the

maximum theoretical voltage of electrochemical capacitors in

aqueous electrolyte is 1.23 V. In practice, for systems operat-

ing in KOHor H2SO4media, the voltage is limited to less than

1 V. Recently, twice larger potential window than in KOH and

H2SO4 has been claimed for the carbon/alkali sulfate system

[10,37,38]. Figure 15 shows cyclic voltammograms of an acti-vated carbon (AC) in 2 mol/L Li2SO4recorded with a gradual

decrease of negative potential cut-off. The rectangular-shaped

voltammograms at potentials higher than the reduction poten-

tial of water (0.35 V vs. NHE in this electrolyte) are typical

of the double-layer charging. Below 0.35 V vs. NHE, wa-

ter is reduced, and a pseudo-capacitive contribution related to

reversible sorption of nascent hydrogen takes place together

with the double-layer formation; during the anodic sweep,

the electro-oxidation of stored hydrogen appears as a hump

around 0.4 V vs. NHE [10]. The sharp negative current leap

from potentials below 1.0 V vs. NHE, indicates H2gas evo-

lution, and the overpotential for H2 evolution is evaluated to

ca. 0.6 V.

Figure 16 shows the maximum and minimum poten-

tials of the positive (E+) and negative (E) electrodes of an

AC/AC capacitor vs. a reference electrode, as a function of

the maximum voltage applied [10]. The E0Vvalues represent

the electrode potential when the voltage is set to 0 V between

two successive cycles at different values of maximum volt-

age. For a maximum voltage of 1.8 V, the negative electrode

potential reaches 0.81 V vs. NHE, which is lower than the

thermodynamic limit for water reduction (0.35 V vs. NHE),

but still higher than the practical negative potential limit of H2evolution evaluated in Figure 15, ca. 1 V vs. NHE.

Figure 15. Three-electrode cyclic voltammograms of activated carbon in

2 mol/L Li2SO4. The loops are obtained by stepwise shifting the negative

potential limit to more negative values. The vertical line at 0.35 V vs NHE

corresponds to the thermodynamic potential for water reduction (from Ref.

[10])

Figure 16. Potential limits of positive (E+) and negative (E) electrodes

during the galvanostatic (200 mA/g) cycling of a symmetric AC/AC superca-

pacitor in 2 mol/L Li2 SO4 up to different values of maximum voltage. The

E0Vvalues correspond to the electrodes potential when the working voltage is

shifted to 0 V before each change of maximum voltage. The lower horizontal

line represents the negative potential limit related with a noticeable H2evolu-

tion estimated in three-electrode cell. The upper horizontal one corresponds

to the thermodynamic limit for water oxidation (from Ref. [10])

As a consequence of these properties, voltage values as


22/25


high as 2 V have been reported for AC/AC capacitors operat-

ing in Li2SO4, Na2SO4 and K2SO4 [10,37,38,40]. Figure 17

exemplifies the cyclic voltammograms of an AC/AC capacitor

in 2 mol/L Li2SO4[10]. Up to 1.21.4 V, the curves exhibit a

rectangular shape, typical for charging the electrical double-

layer. Above 1.4 V, one can observe a positive peak which

is attributed to water reduction and hydrogen storage in the

negative electrode. Correspondingly, during the voltage de-

crease, a pseudo-capacitive contribution appears below ca. 0.8

V; according to Figure 16 this value corresponds to a negative

electrode potential higher than 0.2 V vs. NHE, allowing the

oxidative desorption of hydrogen from the negative electrode.

The voltage extension in aqueous alkali sulfates, by com-

parison with basic or acidic electrolytes, is attributed either to

the important overpotential for di-hydrogen evolution at the

negative electrode [10] or to the strong solvation of cations

and anions [40]. Hydrogen storage in the negative electrode at

the highest voltage values provides capacitance enhancement.

Enhancing both capacitance and voltage in these electrolytic

media gives rise to very promising systems in terms of energy

density and environment compatibility.

Figure 17. Cyclic voltammograms recorded at different values of maximum

voltage for an AC/AC capacitor in 2 mol/L Li 2SO4(from Ref. [10])

5.4. The carbon/redox couples interface as a source of pseu-

docapacitance

The previously mentioned strategies to enhance capaci-

tance are closely related with the electrode material. A new

concept has been presented recently, where the iodide/iodine

redox couple from the electrolyte solution is at the origin of

pseudo-capacitance [93]. The electrochemical activity of the

electrolyte is based on Reactions (8) to (11) which occur on

the electrode/electrolyte interface of the positive electrode of

an AC/AC capacitor:

2I1 I2+2e (8)

3I1 I3+2e (9)

2I13 3I2+2e (10)

I2+6H2O 2IO13 +12H

++10e (11)

The capacitance of the carbon electrode has been evalu-

ated by cyclic voltammetry in three-electrode cell for different

types of alkali counter-ions (Figure 18). It increases with the

van der Waals radius of the alkali ion as follows: 300 F/g forLiI, 492 F/g for NaI, 1078 F/g for KI and 2272 F/g for RbI.

However, for caesium ion, which has the biggest radius, the

capacitance decreases to 373 F/g [94]. An analysis of the

alkali cation properties indicates that this phenomenon is in

perfect accordance with the ion-solvent and solvent-solvent

interactions measured in the form of potential energy as well

as cation mobility values and diffusion coefficients tenden-

cies [95]. However, the discharge capacitance of real AC/AC

systems does not exceed 280 F/g at 1 A/g, which is due to

the relatively low capacitance value of the negative electrode,

contributing to lowering the capacitance of the two-electrode

cell by Equation (2).

Figure 18. Three-electrode cyclic voltammograms of a carbon electrode for

1 mol/L alkali iodide solution (from Ref. [94])

Given the fact that only the positive electrode exhibits an

exceptional capacitance in the iodide-based systems, the vana-

dium/vanadyl redox couple has been employed for the nega-

tive electrode in an AC/AC system. The electrolytic aque-

ous solutions were 1 mol/L KI for the positive electrode and1 mol/L1 VOSO4 for the negative one; the two electrolytic

compartments were separated by a Nafion membrane. The re-

ported capacitance values are about 1200 F/g and 670 F/g for

the positive and negative electrodes, respectively. The rela-

tively high capacitance of the negative electrode could be ex-

plained considering the multi-electron Reactions (12) to (16)

[96]:

VOH2++H++ e V2++H2O (12)

[H2V10O28]4+54H++30e 10V2++28H2O (13)


23/25


[H2V10O28]4+44H++ 20e 10VOH2++18H2O

(14)

HV2O37 +13H

++ 10e 2V+7H2O (15)

HV2O37 +9H

++6e 2VO+5H2O (16)

The cyclic voltammogramsof the AC/AC device recordedat various scan rates are presented in Figure 19. The energy

density at 1.0 V voltage range was reported to be at the level

of 19 Wh/kg, which is an exceptional value for an aqueous

electrolyte system [96].

The quinone/hydroquinone couple has also been used

as redox-active additive for an AC/AC capacitor in 1 mol/L

aqueous H2SO4 solution [97,98]. A battery-like behaviour

has been observed at the positive electrode and a pseudo-

capacitive hydrogen electrosorption process at the negative

one. The authors suggest that it is the consequence of an

asymmetric voltage splitting between the electrodes after the

incorporation of hydroquinone. A tremendous capacitance

value of 5017 F/g was recorded by cyclic voltammetry at1 mV/s for the positive electrode, probably due to the devel-

opment of the quinoid redox reactions on the activated carbon

surface. Meanwhile, the capacitance of the negative electrode

also increases significantly when compared with the value ob-

tained for the electrode operated in the electrolyte without

hydroquinone (from 290 to 477 F/g). Even if the values of

capacitance are slightly doubtful due to different capacitance

values recorded from different methods, the idea of exploiting

the quinone/hydroquinone redox couple from the electrolyte

is reasonable and needs to be investigated more deeply.

Figure 19. Cyclic voltammograms at various scan rates of an AC/AC capac-

itor operating in iodide/vanadium conjugated redox couples as electrolytic

solutions (from Ref. [96])

6. Conclusions

The present researches on carbon/carbon supercapacitors

are essentially dedicated to improving the specific energy,

which can be achieved either by enlarging the voltage range or

by enhancing the capacitance. Different strategies have been

presented in this review, by taking account of that these im-

provements should not be realized at the expense of power,

and that cost and environment issues are priorities if one re-

ally intends the commercialization of the systems.

Presently, only AC/AC capacitors in organic electrolyte

are commercially available. Owing to the research efforts dur-

ing the last years, their operation behavior is better understood

and optimizations of materials can be suggested. It is now

well-demonstrated that capacitance is optimal in subnanomet-

ric pores and that, under the effect of polarization, the ions

solvation sphere is distorted, meaning that at least they loose

partly some of their solvating molecules. Designing carbons

containing essential pores in nanometer range is an objective,

providing that the pore volume is sufficiently developed. Oth-

erwise, during charging the capacitor, the porosity might be

saturated at a voltage smaller than the maximum possible one

for the considered electrolyte, leading to a energy limitation

of the system.

The voltage window is essentially controlled by the elec-

trochemical stability of the electrolyte in the presence of acti-

vated carbons. Traditional organic electrolytes are able to op-

erate up to 2.72.8 V. Some recent works using different sol-

vents or mixtures show only the possibility of incremental im-

provements. In recent years, ionic liquids have been suggested

as alternative to the organic media. Unfortunately, their elec-

trical conductivity at room temperature is very low and they

are not appropriate for power systems. Although demonstrat-

ing smaller voltage window than organic electrolytes, neutral

aqueous electrolytes are interesting as far as low cost, safe and

environment friendly devices are expected.

Besides the later attractive properties, aqueous elec-

trolytes are able to promote pseudo-faradic reactions with car-bon electrodes: i) by the presence of functional groups, ii)

through hydrogen electrosorption, and iii) by redox reactions

at the electrode/electrolyte interface.

Overall, one can see that carbon based systems offer a

wide range of possibilities depending on the nanoporous tex-

ture and surface functionality of carbons, and on the kind of

electrolyte. The future trend should not be in a unique kind

of system, but in the development of various options, using a

specific combination of components allowing the desired per-

formance to be reached.

Acknowledgements

The Foundation for Polish Science is acknowledged for support-ing the ECOLCAP Project realized within the WELCOME Program,

co-financed from European Union Regional Development Fund.

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