Journal of Energy Chemistry Vol. 22 No. 2 2013 of Energy Chemistry Vol. 22 No. 2 2013 CONTENTS 151 A...

Journal of Energy Chemistry Vol. 22 No. 2 2013

CONTENTS

151 A perspective on carbon materials for future energyapplication

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-basedenergy storage devices

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

195 Synthesis and functionalization of carbon xerogels tobe 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 electrodematerials 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 carboncatalyst

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

245 Synthesis of SAPO-34/graphite composites for lowtemperature 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-supportedcatalysts in the aqueous phase reforming of ethyleneglycol

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

270 A correlation between structural changes in a Ni-Cucatalyst during decomposition of ethylene/ammoniamixture and properties of nitrogen-doped carbonnanofibers

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 decoratedααα-Al2O3 containingcobalt nanoparticles for Fischer-Tropsch reaction

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

290 Simultaneous formation of sorbitol and gluconic acidfrom cellobiose using carbon-supported rutheniumcatalysts

Tasuku Komanoya, Hirokazu Kobayashi, Kenji Hara,Wang-Jae Chun, Atsushi Fukuoka

296 Synergistic effect between few layer graphene andcarbon nanotube supports for palladium catalyzingelectrochemical 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-walledcarbon nanotubes as metal-free catalysts for selectiveolefin 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 ofFe2O3 nanoparticles in mesoporous carbons as stableanodes for lithium ion batteries

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

Journal of Energy Chemistry Vol. 22 No. 2 2013

336 Enhanced reversible capacity of Li-S battery cathodebased on graphene oxide

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

341 Hierarchical nanostructured composite cathode withcarbon 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

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CONTENTS

Porous V2O5-SnO2/CNTs composites have been stepwise syn-thesized by a hydrothermal treatment and a subsequent heattreatment in air. The cyclic capacity and rate capability of thecomposite cathode have been greatly improved via decreasingthe particle size and coating with more conductive material, ascompared to the commercial V2O5. See the article on Pages347–355.

151A perspective on carbon materials for future energy applica tion

Dang Sheng Su, Gabriele Centi

Carbon materials play a critical role for the development of new orimproved technologies and devices for a sustainable production anduse of renewable energy.

174Role of carbon matrix heteroatoms at synthesis of carbons fo rcatalysis and energy applications

Volodymyr V. Strelko

The effect of heteroatoms on the reactivity of carbons in gasificationprocesses, their catalytic activity and electrochemical behaviour insupercapacitors was studied experimentally and by quantum chemi-cal calculations.

183Important roles of graphene edges in carbon-based energy st or-age devices

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

Edge-Controlled Nanocarbons: Controlling the number (or type) ofedges relative to the basal planes is critical for maximizing the elec-trochemical performance of carbon-based energy storage devices.

195Synthesis and functionalization of carbon xerogels to be us edas 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.

202Electrocatalytic conversion of CO 2 to liquid fuels usingnanocarbon-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.

214Functional porous carbon-based composite electrode mater ialsfor lithium secondary batteries

Kai Zhang, Zhe Hu, Jun Chen

Functional porous carbon-based composite electrode materials havebeen reviewed for electrochemical devices with energy storage andconversion.

226Carbon/carbon supercapacitors

Elzbieta Frackowiak, Qamar Abbas, Francois Beguin

The capacitance of nanoporous carbons is enhanced when pores fitwith the size of desolvated ions. Pseudo-faradaic reactions involv-ing surface groups, hydrogen electrosorption and the carbon/redoxcouple interface might be source of an additional contribution.

241Efficient conversion of fructose to 5-hydroxymethylfurfur al oversulfated 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.

245Synthesis of SAPO-34/graphite composites for low temperat ureheat 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 usedas a new thermal conductive adsorbent material for low temperatureheat adsorption pumps.

251Facile filling of metal particles in small carbon nanotubes f orcatalysis

Hongbo Zhang, Xiulian Pan, Xinhe Bao

A versatile method is developed for introduction of metal particlesin carbon nanotubes with a diameter <1.5 nm, which may enableunderstanding of catalysis over subnanometer sized particles.

257Stability and activity of carbon nanofiber-supported catal ysts inthe 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 wasobserved for cobalt while sintering was observed for nickel and plat-inum.

270A correlation between structural changes in a Ni-Cu catalys tduring 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.

279Carbon nanotubes decorated ααα-Al2O3 containing cobaltnanoparticles for Fischer-Tropsch reaction

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

The hierarchically structured CNTs on ααα-Al2O3 was synthesized andused as support for Co-based catalysts in Fischer-Tropsch synthesis.

290Simultaneous formation of sorbitol and gluconic acid from c el-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 hydrolyticdisproportionation of cellobiose to sorbitol and gluconic acid in waterunder Ar. Carbon-supported ruthenium catalyzed this reaction viathe hydrolysis and hydrogen transfer.

296Synergistic effect between few layer graphene and carbon na n-otube supports for palladium catalyzing electrochemical o xida-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 Pdnanoparticles supported on few layer graphene, carbon nanotubesand a nanotube-graphene composite.

305Phosphorylated mesoporous carbon as effective catalyst fo r theselective 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 goodcandidate for large scale production of HMF with the advantage ofeasy recyclability and separations.

312Purified oxygen- and nitrogen-modified multi-walled carbonnanotubes as metal-free catalysts for selective olefin hydr o-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 tonitrogen species.

321Ru particle size effect in Ru/CNT-catalyzed Fischer-Trops ch 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.

329Ammonia-treatment assisted fully encapsulation of Fe 2O3nanoparticles in mesoporous carbons as stable anodes forlithium ion batteries

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

Ultrafine Fe2O3 nanoparticles with 4–5 nm size and rationally tailoredloading of 47 wt% were fully encapsulated into tubular mesoporouscarbon matrix, which were designed as high capacity and excellentstability anode materials.

336Enhanced reversible capacity of Li-S battery cathode based ongraphene 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 sulfurof lithium polysulfides,which can effectively enhance the utilization ofsulfur and reversibility of Li-S battery.

341Hierarchical nanostructured composite cathode with carbo nnanotubes as conductive scaffold for lithium-sulfur batte ries

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.

347Porous V 2O5-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 porousstructure were synthesized by a facile hydrothermal method. Thecomposites exhibited improved rate capability and specific capacitycompared with commercial V2O5 when used as cathode electrodesfor lithium ion batteries.

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 pollutionand greenhouse effect have pushed considerably the need to accelerate the transition to a more sustainable use of energybasedlargely on renewable energy sources. Nanocarbon materialsplay a critical role in this transition, as they are the key materials forcomponents of different devices necessary in enabling thistransition (batteries, fuel cells, solar cells, etc.).

This issue collects 22 contributions, including one perspective and six review papers on the topic of carbon materials forenergy applications, written by well-known experts in thisfield. It is really an exciting special issue that gives a veryupdatedview of this topic, as well as trends and outlooks in this breakthrough research area. The initial perspective paper introduces thedifferent possibilities offered from the growing level of knowledge in this area, testified from the exponentially rising number ofpublications. 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 CO2 conversion. These reviewsrepresent 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, nanoribbon, etc.). Second generation nanocarbons, including modification of these nanocarbons by surface functionalization or dopingwith 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 newcatalysts for a range of applications (from biomass conversion to Fisher-Tropsch reaction and electrochemical devices) and newmaterials 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 applicationsrepresents a unique occasion for young and experienced researchers as well as for managers in the field of sustainable energyto have an updated view on this enabling topic for the future of our society. We thus invite all to have this special issue asaprivileged component of your bookshelf.

Dang Sheng Su and Gabriele Centi

Professor Dang Sheng SuShenyang National Laboratory for Materials ScienceInstitute of Metal ResearchChinese Academy of SciencesShenyang 110006, LiaoningChinaE-mail: [email protected]

Professor Gabriele CentiDipartimento di Ingegneria ElettronicaChimica ed Ingegneria IndustrialeUniversity of Messina and INSTM/CASPEV. le F. Stagno D’Alcontres 31, 98166, MessinaItalyE-mail: [email protected]

Journal of Energy Chemistry 22(2013)226–240

Review

Carbon/carbon supercapacitors

Elzbieta Frackowiak, Qamar Abbas, Francois Beguin∗

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]

AbstractSupercapacitors, or electrochemical capacitors, are a power storage system applied for harvesting energy and delivering pulses during shortperiods of time. The commercially available technology is based on charging an electrical double-layer (EDL), and using high surface areacarbon 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 theirperformance. In particular, it is shown that capacitance might be enhanced forcarbons having subnanometric pores where ions of the electrolyte are distorted and partly desolvated. Then, strategies for using environmentfriendly 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 mostpromising 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 highoverpotential for di-hydrogen evolution at the negative electrode.

Key wordssupercapacitors; electrochemical capacitors; porous carbons; electrolytes; pore size; pseudocapacitance

Elzbieta Frackowiak is a Professor in the Insti-tute of Chemistry and Technical Electrochemistryat 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 materialsfor supercapacitors and hydrogen storage, use ofcomposite electrodes from nanotubes, conducting

polymers, doped carbons and transition metal ox-

ides for supercapacitors. She serves as Chair of Division 3 “ElectrochemicalEnergy Conversion and Storage” of the International Society of Electrochem-istry (2009–2014). She was the winner of the Foundation for Polish Science

Prize (2011). She is author of 150 publications, a few chapters and tensof patents and patent applications. Number of citations ca.6370, Hirschindex 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. Apart of his work is related to the corrosion investi-gations of current collectors in supercapacitors

under testing conditions.

∗Corresponding author. Tel: +48-61-6653632; Fax: +48-61-6652571; E-mail: [email protected] Foundation for Polish Science is acknowledged for supporting the ECOLCAP Project realized within the WELCOME program, co-financed from

European Union Regional Development Fund.

Copyright©2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved.

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

Francois Beguin is a Professor in Poznan Univer-sity of Technology (Poland), where he has been

recently awarded the WELCOME stipend fromthe Foundation for Polish Science. His researchactivities 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 andhis works are cited in 8300 papers, with Hirsch index 46. He isalso involvedin several books dealing with carbon materials and energy storage. He is a

member of the International Advisory Board of the Carbon Conferences andhe launched the international conferences on Carbon for Energy Storage andEnvironment 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 nationalprogrammes onEnergy 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 electrochemicalcapacitors) based on activated carbon electrodes are an energystorage device which has been the object of important researchin the last decade [1,2]. They provide higher energy densitythan dielectric capacitors, while demonstrating higher powerdensity than batteries [3,4]. Therefore, they are particularlyadapted 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 emergencydoors of airplanes. Since the basic operating principle of su-percapacitors is the electrostatic attraction of ions on the elec-trode/electrolyte interface, the commercially availablesuper-capacitors demonstrate a high degree of reversibility, beingable to withstand a high number of charge/discharge cycles,ca. 1000000 cycles.

Because of high electrical conductivity, low cost andavailability at ease, porous carbons are used as electrode mate-rials in supercapacitors. Activated carbons (AC) provide highsurface area and their porosity can be tailored to the desiredpore size distribution by varying the activation process ortypeof precursor. The correlation of ion size of the electrolyticsystem to the pore size of carbons has opened new researchhorizons in the field of supercapacitors [5,6]. Besides thepure electrostatic attraction of ions (electrical double-layer)which plays in all kinds of electrochemical capacitors, theperformance 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 thesquare of the operating voltage, which is controlled by thestability window of the electrolyte [7]. Aqueous electrolyteshave a limited stability window up to 0.7–0.8 V in acidic

and alkaline pH value [9] and up to 1.8–1.9 V in neutral pHvalue [10], while non-aqueous electrolytes have a stabilitywindow up to 2.7–2.8 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 surfaceof microporous carbons [12]. However, aqueous electrolytesgive much higher capacitance values in comparison to organicelectrolytes.

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 alsodedicated 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, andconsequently 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 accumulationof ions on the surface of activated carbon. Upon polarization,the charge at the electrode surface is neutralized by a layerofcounter ions at a distanced (Figure 1a), resulting in a capaci-tanceC as described by Helmholtz [13] in Equation (1):

C =

(

εrε0A

d

)

orC

A=

( εrε0

d

)

(1)

where,εr andε0 are the dielectric constants of the electrolyteand vacuum, respectively, andA is the surface area of theinterface. Gouy and Chapman [14−16] proposed a diffusemodel 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-Chapmanmodel for highly charged double-layers, Stern [17] suggesteda model combining Helmholtz and Gouy-Chapman models,and taking account of the hydrodynamic motion of the ionicspecies in the diffuse layer and the accumulation of ions closeto 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 anaqueous or an organic electrolyte (Figure 2). In the in-dustrial capacitors, the electrode material is a high surfacearea (>1500 m2·g−1) activated carbon which coats a currentcollector (aluminum in organic electrolyte, stainless steel inaqueous KOH). Considering the small value ofd in Formula(1), the capacitance of each electrode is very high. Thetwo 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 ofactive materials with the current collector. A percolator (car-bon black, carbon nanotubes) is added for improving the elec-trodes conductivity.

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 thicknessin the range of 10−100 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 capacitanceC+ andC−in series. The capacitance of the total system is given by For-mula (2):

1C

=1

C++

1C−

(2)

where,C is the cell capacitance,C+ andC− are the respec-tive capacitances of the positive and negative electrodes.Asthe capacitance of the two electrodes is different, even in asymmetric capacitor, Formula (2) indicates that the value ofC is determined by the electrode with the smallest capacitancevalue.

Recent investigations show that classical models of thedouble-layer do not apply when microporous carbons are usedas electrode materials. It has been demonstrated that the elec-trosorption of ions is favored in subnanometric pores whichare smaller than the solvation sphere, suggesting that ionsareat least partially desolvated [5,6,19,20]. Electrochemical stud-ies carried out in pure ionic liquid electrolytes have shownthat the highest capacitances are obtained when the pore sizematches 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 appliedpotential. Whether the capacitance enhancement depends onpore structure and/or other factors is difficult to be describedthrough experiments alone. Moving from the conventionalHelmholtz model to a situation where the ionic species en-ter the pores partially desolvated and arrange in lines withinentire pore length, various factors come into play which mightresult in a capacitance increase. Concepts based on cylin-drical mesopores and cylindrical micropores, both shown inFigure 3, have been considered in literature [22,23]. In themesopore regime (2 to 50 nm), solvated counter ions approachthe pore wall and form an electrical double-cylinder capacitor(EDCC) of capacitance given in Equations (3a) and (3b):

C =2πεrε0L

ln

(

b

a

) (3a)

C =εrε0

b ln

[

b

b−d

] A (3b)

where,L is the pore length,b anda are the radii of the outerand inner cylinders, respectively. In such case, the effectofpore size and pore curvature becomes prominent as comparedwith the distanced.

In case of micropores (<2 nm), desolvated or partiallydesolvated counter ions line-up to form an electrical wire-in-cylinder capacitor (EWCC), which capacitance is given byEquation (4):

C

A=

εrε0

b ln

[

b

a0

] (4)

where,a0 is the radius of the inner cylinder.


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 radiib anda for the outer and inner cylin-ders, respectively, separated by a distanced, and (b) a negatively chargedmicropore of radiusb with cations of radiusa0 lining up to form an electricalwire-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 fitthe experimental data from Ref. [5] for supercapacitors builtwith nanoporous carbons of diverse pore size. Taking Equa-tion (4) into account, the anomalous increase in capacitancewith decreasing pore size [5] can be rationalized.

A further discussion which stressed that Gouy-Chapman-Stern theory cannot apply in dense ionic systems came fromKornyshev [24]. Taking the ion packing constraints in ionicliquids (ILs) into account, the so-called lattice saturationeffect, an alternative mean field theory (MFT), was suggested.The multi-layered structure contains layers of ionic speciesclose 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 [25−27]. Further, a molecular dynamics (MD) sim-ulation approach was adopted for carbon nanotube microp-ores of various sizes in ionic liquids [28,29]. This approachpredicts 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 MonteCarlo simulation of a model ionic liquid in slit-like metallicnanopores was presented by Kondrat et al. [30]. They de-scribed that the superionic state of ions inside a nanosizedpore is responsible for the anomalous increase in capacitancewith decreasing pore width, assuming that the pore is notempty at zero voltage. They also observed that for narrowpores, the capacitance as a function of voltage exhibits a peakbefore dropping down to zero at higher voltages. This drop ofcapacitance 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 byMerlet et al. [32] for an EDLC constituted of microporouscarbon electrodes with an ionic liquid as electrolyte. TheEDLC simulation cell is shown in Figure 4, where the toppanel is a snapshot extracted from a simulation, and the bot-tom panel illustrates the electrification of an electrode held atvarious potentials. The two key features taken into accountwere a realistic atomistic structure of the carbide-derived car-bon (CDC) electrode [33] and the polarization of the electrodeatoms by the ionic charges. Such approach allows simulationsof conducting electrodes of arbitrary geometry under constantapplied 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 F·g−1 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: PF−6 ions; a coarse-grained model is used to describe theseions). 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 chargeq they carry (green:q<0, red:q=>0 and yellow:q=0). The charging mechanism involves the exchange of ions between the bulk and the electrode (from Ref. [32])


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 atnull potential.

The main parameters which characterize the performanceof supercapacitors include: i) the power density essentiallygreater than for batteries, ii) an excellent cyclability (up to 100times higher than batteries), iii) fast charge/discharge process,and iv) low equivalent series resistance (ESR).

The maximum powerP of a supercapacitor is calculatedaccording to Equation (5):

P =U2

4R(5)

where,U is the maximum cell voltage (V),R is the equiva-lent series resistance (Ω), andP is the maximum power (W).The factors which mainly contribute to the overall series re-sistance of supercapacitors are the electronic resistanceof theelectrode material, the contact resistance between electrodematerial and current collector, the electrolyte resistance, theionic 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 forredox reactions. Thus, the series resistance is lower than thatof batteries at cell level, explaining the higher power densityof supercapacitors compared with batteries.

The maximum energyE is given by Equation (6):

E =12

CU2 (6)

where,C is capacitance (F),U is the maximum cell voltage(V), andE is the energy (J). The charge storage is achieved onthe 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-ioncells. However, this storage mechanism also allows a very fastdelivery of the stored charge. Thus, EC devices can deliver allthe stored energy in a short time, about 5 s; this process isfully reversible and energy update can be achieved within thesame time period.

In general, the energy is expressed as per mass (Wh/kg)or volume (Wh/m3) of the device; in case of capacitance, thevalues 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 publicationsrather concern the optimization of the electrode material,inthis case, the capacitance is expressed in F/g or F/cm3 for oneelectrode; 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 avery low amount of mesopores.

3. Electrolytes for supercapacitors

Both Equations (5) and (6) show that power densityand energy density of supercapacitors are proportional to the

square of voltage. The cell voltage is mainly limited by theelectrolyte stability. The advantage of aqueous electrolyteslike acids (H2SO4) and alkalis (KOH) is a higher conductivity(up to∼ 1 S·cm−1) as compared with other electrolytic sys-tems. The major disadvantage of aqueous solutions is theirrestricted stability window, about 0.7–0.8 V. Most of the com-mercial devices use organic electrolytes, i.e., N(C2H5)+4 BF−4dissolved in acetonitrile (CH3CN) or propylene carbonate(PC), so that the operating voltage reaches 2.7–2.8 V. Non-aqueous electrolytes with good conductivity and higher oper-ating voltage (up to 3.5–4 V) are highly desirable. Aproticionic 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 determinesthe double-layer capacitance value and its dependence onelectrode potential as well as the extent of ionization or ionpairing of the solute salt, which influences the conductance.In order to achieve a high power supercapacitor system, theinternal electrolyte resistance and the structural resistance ofthe porous carbon electrode material should be minimized [1].This can be achieved by an electrochemically compatible elec-trolyte salt or an acid or alkali which is strongly soluble inthesolvent to be used. Minimum ion pairing and maximum freemobility of dissociated ions should be achieved in dissolvedstate. Equation (7) characterizes the dissociation of any saltMA at concentrationc, into its free ions:

MA←Kc−−→M+ +A− (7)

(1−α)c αc αc

where,α is the dissociation degree of the salt molecules atconcentrationc.

Non-aqueous electrolytic solutions are significantlyweak, so that the value ofα is appreciably less than its valuein aqueous solutions, which is near 1. This leads to higherESR values for non-aqueous solution based devices than foraqueous one using the same electrode materials and cell ge-ometries. Solvents like water provide strong solvation andatendency for complete dissociation or minimum ion pairing.Such solvents are usually those which have high dielectricconstants, often with hydrogen bonded structures with largedipolar 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 largeionic radii and their alkyl groups tending to interact well withorganic solvents.

The two major classes of electrolytic media extensivelyused in supercapacitors include aqueous and non-aqueousones in recent years.

3.1. Aqueous media

Based on the knowledge from accumulators, the obvious


choice for supercapacitor electrolytes has initially beensul-phuric acid (H2SO4) and KOH. Highly concentrated solutionsare used in order to overcome the ESR factor and to maximizethe power capability. However, the acid solutions are highlycorrosive 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 mediais less than 1 V [9]. Recent investigations by Khomenko et al.[36] have shown that it is possible to enhance the operatingvoltage of carbon based supercapacitors in aqueous H2SO4 upto 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 startedin recent years. Activated carbons demonstrate a stabilitywindow of 2 V in Na2SO4 aqueous electrolyte, and a sym-metric carbon/carbon cell can operate up to 1.6 V with goodcharge/discharge cycle life [37]. Electrochemical character-ization of seaweed carbons in Na2SO4 has shown that thenature 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 electrolyteand within the inner pores of activated carbon increases in theorder of Li+ <Na+ <K+, and the suitability of electrolytesfor capacitors should vary as Li2SO4 <Na2SO4 <K2SO4

[39]. However, the highest operating voltage is displayed inLi2SO4. Fic et al. [40] have suggested that the stronger hydra-tion of Li+ compared with Na+ and K+ ions is responsible forlarger voltage in Li2SO4 solution. According to Gaos’ results[10], the maximum cell voltage is essentially limited by thepositive electrode; when the maximum potential of the later(E+) is too high, it leads to irreversible electro-oxidation onthe active sites of the carbon electrode. A controlled chemicaloxidation of carbon with hydrogen peroxide can help to pushthe maximum potential of the positive electrode to slightlylower values and by this way allows the maximum voltageto be slightly increased, while keeping good cycle life.

The use of neutral aqueous electrolytes, like lithium sul-fate, not only eliminates the disadvantages related to corro-sion, but also gives an opportunity to realize high voltage andenergy density supercapacitors with environmental friendly,cost effective and safe materials. Further research worksshould be carried out in these media to realize supercapaci-tors with optimized performance.

3.2. Non-aqueous media

Non-aqueous electrolytes are preferred for supercapac-itors due to the fact that they allow higher operating volt-ages, and higher energy density according to Equation (6)than aqueous ones. Tetraalkylammonium (R4N+) salts havebeen used for many years due to their good solubility in non-aqueous solvents and moderately good conductivity. How-ever, this has to be balanced with some disadvantages, suchas being expensive and sensitive to moisture contents and

subsequent recombination shuttle reactions leading to self-discharge. The best suitable candidate in this category appearsto be 1 mol·L−1 Et4NBF4 in propylene carbonate (PC) or ace-tonitrile. A comparison between aqueous and organic elec-trolytes shows that the later allows high voltages desirable forhigh energy and power density while the former allows highcapacitance and lowerRs values [41].

Some groups have proposed to use mixtures of low vis-cosity linear carbonates and PC for the realization of EDLCswith an operative voltage as high as 3 V which have high per-formance and high cycling stability [42,43]. Recently, an op-erating voltage of 3.75 V has been reported for EDLCs us-ing 0.7 mol·L−1 Et4NBF4 in adiponitrile at room temper-ature [44]. However, the conductivity of such medium ismuch lower than that of conventional solutions in acetonitrile.Consequently, the ohmic loss is relatively important and thereal voltage window between the electrodes is not as high asclaimed. Intensive cyclability tests should be realized inthismedium in order to correctly evaluate its potentialities versusother organic electrolytes.

Neat ionic liquids (ILs) have been proposed as environ-ment friendly and safe electrolytes. EDLCs based on ILsseem to be able to operate at voltage as high as 3.5–3.7 V withhigh cycling stability [45−50]. However, because of relativelyhigh viscosity and low conductivity of these electrolytes,ESRvalue at room temperature of ILs-based EDLCs is consider-ably higher than that of conventional electrolytes. The highESR is buffered by the increase of the working voltage of thecell, and following Equation (5) the power density remains atan acceptable level [51]. Further investigations in this fieldreveal that, when operating in the temperature range requiredfor HEV applications, the use of ILs as electrolytes in super-capacitors instead of aprotic organic solvents allows the maxi-mum cell voltage and consequently maximum specific energyto be improved.

4. Carbons for EDLCs

A large variety of carbons can be considered as active ma-terials in EDLC electrodes [2,52]. The main materials uti-lized for research purposes are high surface area activatedcarbons [53], carbon aerogels [54], carbon nanotubes (CNTs)[55−57], template porous carbons [19,58−60], activated car-bon nanofibers (CNFs) [61] and activated carbon fabrics [62].The capacitance largely depends on the electrode material,es-pecially on its specific surface area, pore size distribution,pore structure, electrical conductivity and surface wettabil-ity. Activated carbons (ACs) are the most widely used activematerials for EDLC applications, because of their high BETspecific surface area (in the range of 1000–2500 m2·g−1),commercial availability and relatively low cost. In order toobtain electrodes, mixtures of activated carbon powder withconductive carbon black and an organic binder in solution areused to coat metallic current collectors.

In general, the specific capacitance of carbons is propor-tional to the specific surface area (SSA) for low values, butit


rapidly tends to a plateau for SSA higher than 1500 m2·g−1.It has been suggested that, due to the decrease of average porewall thickness in highly activated carbons, the electric field(and the corresponding charge density) no longer decays tozero within the pore walls [63]. In fact, it has been observedthat the average pore size increases together with the specificsurface area when the activation degree increases. It suggeststhat the interaction of ions with pore walls is weaker in largerpores, and according to Equation (1) the effect of porosity de-velopment on capacitance is counterbalanced by the increaseof ion-wall distance [6]. Figure 5 shows that the normalizedcapacitance in F/cm2 (specific capacitance from Ref. [6] di-vided by BET SSA) increases dramatically as the average mi-cropore sizeL0 decreases.

Similar increase of normalized capacitance in poressmaller than 1 nm has been observed with carbide derived car-bons in acetonitrile-based electrolyte containing 1.5 mol·L−1

Et4NBF4 [5]. Considering the diameters of solvated ions inthis electrolytic medium, e.g., 1.30 nm for Et4N+ and 1.16 nmfor BF−4 , and the diameters of bare ions, e.g., 0.68 nm forEt4N+and 0.48 nm for BF−4 [64], it shows that the pores below1 nm are smaller than the size of solvated ions. The capaci-tance increase for pore size<1 nm is explained by the distor-tion of the ion solvation shell [65], leading to a smaller dis-tanced of the ions to the carbon surface, leading to improvedcapacitance according to Equation (1). The linear dependenceof capacitance of template carbons with their ultramicropore(<0.7–0.8 nm) volume is another proof that ions are partiallydesolvated in these pores [19].

Figure 5. Normalized capacitance vs. average pore size for bituminous coalderived nanoporous carbons in various electrolytes (1 mol/L H2SO4, 6 mol/LKOH and 1 mol/L TEABF4 in acetonitrile) (adapted from data of Ref. [6])

A 3-electrode set-up has been used to determine the elec-trochemical characteristics of CDC-based positive and nega-tive electrodes in acetonitrile (ACN) containing 1.5 mol·L−1

Et4NBF4, using a silver quasi-reference electrode [20]. Fig-ure 6 shows the discharge capacitance values versus the av-erage pore size of the carbons for the cell, the positive andnegative electrodes [20]. Both the capacitance of the posi-tive and negative electrodes, as well as the cell capacitance,pass through a maximum at a different carbon pore size. Theaverage pore size value where the positive electrode capaci-tance is maximum (anion adsorption) is 0.7 nm. This value isbetween the bare anion diameter (0.48 nm) and the solvated

ion diameter in acetonitrile (1.16 nm). This confirms that theanions need to be partially desolvated to enter these microp-ores. The same concept can apply to cation adsorption at thenegative electrode, since the average pore size (0.76 nm) cor-responding to the capacitance maximum is between the barecation diameter (0.68 nm) and the solvated cation diameter(1.30 nm). Thus, it appears that there is an optimum pore sizeto maximize the capacitance for carbons in ACN+1 mol·L−1

Et4NBF4 electrolyte; this optimum pore size depends on theion size. According to Formula (2), the overall capacitanceCof the cell is mostly influenced by the lowest value betweenC+ andC−. Because the tetraethylammonium cation is largerthan the tetrafluoroborate, the cell capacitance is controlled bythe capacitance of the negative electrode.

Figure 6. Specific capacitances calculated from a constant current discharge(inset, colors as for main plot) for the anion/positive electrode (C−) andcation/negative electrode (C+) show similar behavior until a critical averagepore size of∼0.8 nm is reached. At pore sizes below this value, the capaci-tance values of positive and negative electrodes diverge (from Ref. [20])

Although using carbons with pores in the optimal range0.7–0.8 nm, porosity may be saturated by NEt+

4 cations be-fore reaching the maximum possible voltage for the con-sidered electrolyte, e.g., 2.7–2.8 V for NEt4BF4 in ACN[31]. Figure 7 shows the cyclic voltammogram of a two-electrode cell built in ACN+1.5 mol·L−1 NEt4BF4 electrolytefrom a carbon PC (SDFT = 1434 m2·g−1) of average pore sizeL0 = 0.7 nm (optimal value mentioned above). For compari-son, the voltammogram of a carbon VC (SDFT = 2160 m2·g−1)of average pore sizeL0 = 1.4 nm is also represented. WhileVC shows a perfectly rectangular voltammogram in the wholevoltage range, for PC the capacitive current dramatically de-creases from 1.5 V. According to the pore size distributionobtained from nitrogen adsorption, all pores of PC are nar-rower than 1 nm and consequently able to optimally interactwith ions. However, if one considers the pores larger thanthe diameter of desolvated NEt+

4 cations (0.68 nm), the cor-respondingSDFT values are 198 m2·g−1 and 964 m2·g−1 forPC and VC, respectively [31]. From these values, the max-imum theoretical charge,Qmax, able to be accommodated inthe pores has been calculated for both carbons, by consideringthe projected surface area of NEt+

4 ions, and compared with


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, confirmingthat 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 notsaturated, at least for the maximum voltage of 3 V reached inthis experiment.

During galvanostatic cycling, the porosity saturation isreflected 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. 1.5−2 V (Figure 8b)shows the difficulty to store more energy by further increasingvoltage up to the electrolyte stability limits [31].

Figure 7. Cyclic voltammograms for EDLCs based on PC carbon (left-handside 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 onnanoporous 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 1−2 V, depending onthe current density. From Ref. [31]

Summarily, the porous texture strongly influences theelectrochemical properties of carbons. The capacitance withEt4NBF4-based electrolyte is optimal in subnanometric pores,suggesting the distortion of solvation shell. If the porousvol-ume is not sufficiently developed, pores may be saturated byions although being in the range 0.7–0.8 nm, leading to a limitof the maximum voltage and consequently of energy and de-liverable power. In some cases, the local structure of carbonmay also be responsible for electrochemical intercalationdur-ing charging [66,67].

5. Pseudo-capacitive contributions involving carbon elec-trodes

Considering the electrochemically available surface areaof activated carbons and the charge amount which could be ac-cumulated in the electrical double-layer, the capacitanceval-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 storageprocess and the energy. However, due to their typical faradicorigin, these processes exhibit a slow kinetic of the heteroge-neous reaction (limited mainly by the diffusion of the involvedelectrochemical species) and a moderate cycle life (connectedwith changes of the material structure undergoing oxidationor reduction process).

5.1. Pseudo-capacitance originating from heteroatom dopedcarbons

Capacitance of nanoporous carbons can be enhancedthrough quick faradaic reactions or local modification of theelectronic structure, both originating from the presence ofoxygenated and nitrogenated functionalities in the carbonnet-work [68]. Since functional groups are generally presentin small amount in activated carbons, enrichment techniqueshave been developed. The general ways to obtain heteroatom


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 ofmolecules containing suitable functional groups.

5.1.1. Oxygen enriched carbons

Interesting carbons were obtained by one-step carboniza-tion at 600C 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 containsa high amount of oxygen (15 at%) in the form of phenoland ether groups (C–OR, 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 mol·L−1 H2SO4

show cathodic and anodic humps at around –0.1 V and0 V vs. Hg/Hg2SO4, respectively, which can be attributedto quinone/hydroquinone pair [71] or pyrone-like structures[72]. Despite low BET specific surface area of this carbon, thecapacitance in 1 mol·L−1 H2SO4 medium reaches 200 F/g, i.e.a value comparable to the best activated carbons available onthe market. Some additional performance improvement wasobtained by incorporating carbon nanotubes in the seaweedsbefore 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],asshown 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) oxidizednitrogen (N-X)

Nitrogen enriched carbons were obtained by ammoxida-tion of nanoporous carbons [76] or by activation of carbonizednitrogen rich polymers [77,78]. A linear correlation has beenfound between capacitance in H2SO4 medium and the nitro-gen content for a series of nitrogen enriched carbons of com-parable porous characteristics (SBET≈800 m2/g) [7,78]. Thisenhancement of capacitance is interpreted by pseudo-faradaiccharge transfers involving nitrogenated functionalities, suchas in Figure 10 [68]:

Figure 10. A possible pseudo-faradic charge transfer involving pyridinic ni-trogen [68]

Self-standing C/C composite electrodes presentingpseudo-capacitive properties and high electrical conductiv-ity have been obtained by one-step pyrolysis of carbon nan-otube/polyacrylonitrile blends at 700C [79]. Whereas thespecific surface area of polyacrylonitrile (PAN) carbonizedat 700C is negligible (SBET = 6 m2·g−1), the C/C compos-ite formed by pyrolysis of a CNT/PAN (30/70 wt%) blend at700C has a more developed porosity (SBET = 157 m2·g−1,Vmeso= 0.117 cm3·g−1), with mesopores due to the templat-ing effect of CNTs. The nitrogen content measured on thiscomposite by XPS is 7.3 at%. The capacitance determinedfor the C/C composite is in the order of 100 F/g in 1 mol·L−1

H2SO4, whereas under the same conditions the pristine CNTsgive 18 F·g−1 and carbonized PAN a negligible value. The re-markable capacitive behavior of this kind of composite is dueto a synergy between CNTs and the nitrogenated functionalityof carbonized PAN.

The beneficial effect of nitrogen in composites with anincorporated nanotubular backbone has been also demon-strated using melamine as nitrogen-rich carbon precursor[80]. Polymerized melamine/formaldehyde blends formedwith different proportions of melamine in the presence of mul-tiwalled carbon nanotubes are carbonised at 750C. 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 onthe capacitance values by modifying the electronic propertiesas 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 as280 F/mL in KOH medium [82]. A very high capacitance of340 F/g has been reached in 1 mol/L H2SO4 using templatedcarbons obtained by pyrolysis of acrylonitrile in NaY zeoliteas scaffold [83]. This high value results from the synergy be-tween the highly developed surface area of the material, thepseudo-faradaic reactions related to the presence of the nitro-genated functionalities and their high accessibility providedby the straight channels inherited from the zeolite substrate.

5.2. Electrografted carbons


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

Surface modification of carbon materials implying anelectron transfer when the substrate is connected to a cur-rent generator has been coined as electrografting. Such kindof modification leads to the binding of desired functionalgroups onto conductive surfaces. The reagents used for themodification of activated carbon surfaces by electrograftinginclude aryl diazonium salts, amines, carboxylates, alcohols,Grignard reagents and halides [84,85]. As far as supercapaci-tors are concerned, only grafting through diazonium salts hasbeen used until now. As schematized in Figure 11, this pro-cess is a concerted mechanism in which the diazonium cationis reduced and one nitrogen molecule is eliminated [86].

By an appropriate choice of the functional group Rpresent on the aryl diazonium salt, nanoporous carbons mayexhibit pseudo-capacitive properties. A carbon electrodefunctionalized by 8.4 wt% catechol demonstrates a capaci-tance of 250 F/g over a potential range from−0.4 to 0.75 Vin 1 mol/L H2SO4, as compared with 150 F/g for the pristinecarbon [87]. By attaching anthraquinone (AQ) to a carbonsurface, the capacitance could be enhanced up to 40% [88],although the BET specific surface area significantly decreasesfrom 1500 to 1185 m2/g [89]. Figure 12 compares the voltam-mograms of the Black Pearl carbon modified with 11 wt% AQand the unmodified carbon, and the reversible redox wave ofAQ giving the pseudocapacitive effect can be seen at about−0.2 V vs. Ag/AgCl [90]. A supercapacitor based on thissystem was tested for 10000 charge/discharge cycles and 14%loss of faradaic capacitance was observed for 11 wt% loadingas compared with 17% capacitance loss for the unmodifiedcarbon material.

Weissmann et al. observed that AQ concentration on thecarbon surface depends on pH, hence affecting the superca-pacitor performance [91]. At pH = 14, the surface concen-tration was found to be close to 9×10−10 mol/cm2 for themodified electrode, while at pH = 7 the value decreased to6×10−10 mol/cm2 and at pH = 0.5 the concentration furtherdecreased to 5.6×10−10 mol/cm2. Cyclic voltammetric mea-surements showed that the shape of the redox peaks is greatlyaffected 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 0.65V; the average specific capacitance increases from 141 F/gfor 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 averagespecific capacitance, giving 367 F/g over a potential rangeof0.35 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-


chemically active in different potential ranges. Therefore, C-AQ and C-DHB carbons have been used respectively for thenegative and positive electrodes of an asymmetric superca-pacitor in 1 mol/L H2SO4 [92]. The galvanostatic dischargecharacteristics of the asymmetric capacitor are shown in Fig-ure 14, where they are compared with the performance of asymmetric 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/g−1 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 onesuggest that the former device requires further optimization inorder 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 supercapacitorsin 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, themaximum theoretical voltage of electrochemical capacitors inaqueous electrolyte is 1.23 V. In practice, for systems operat-ing in KOH or H2SO4 media, the voltage is limited to less than1 V. Recently, twice larger potential window than in KOH andH2SO4 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 Li2SO4 recorded with a gradualdecrease of negative potential cut-off. The rectangular-shapedvoltammograms at potentials higher than the reduction poten-tial of water (−0.35 V vs. NHE in this electrolyte) are typicalof the double-layer charging. Below−0.35 V vs. NHE, wa-ter is reduced, and a pseudo-capacitive contribution related toreversible sorption of nascent hydrogen takes place togetherwith the double-layer formation; during the anodic sweep,the electro-oxidation of stored hydrogen appears as a humparound 0.4 V vs. NHE [10]. The sharp negative current leap

from potentials below−1.0 V vs. NHE, indicates H2 gas evo-lution, and the overpotential for H2 evolution is evaluated toca. 0.6 V.

Figure 16 shows the maximum and minimum poten-tials of the positive (E+) and negative (E−) electrodes of anAC/AC capacitor vs. a reference electrode, as a function ofthe maximum voltage applied [10]. TheE0V values representthe electrode potential when the voltage is set to 0 V betweentwo successive cycles at different values of maximum volt-age. For a maximum voltage of 1.8 V, the negative electrodepotential reaches−0.81 V vs. NHE, which is lower than thethermodynamic limit for water reduction (−0.35 V vs. NHE),but still higher than the practical negative potential limit of H2

evolution evaluated in Figure 15, ca.−1 V vs. NHE.

Figure 15. Three-electrode cyclic voltammograms of activated carbonin2 mol/L Li2SO4. The loops are obtained by stepwise shifting the negativepotential limit to more negative values. The vertical line at −0.35 V vs NHEcorresponds to the thermodynamic potential for water reduction (from Ref.[10])

Figure 16. Potential limits of positive (E+) and negative (E−) electrodesduring the galvanostatic (200 mA/g) cycling of a symmetric AC/AC superca-pacitor in 2 mol/L Li2SO4 up to different values of maximum voltage. TheE0V values correspond to the electrodes potential when the working voltage isshifted to 0 V before each change of maximum voltage. The lower horizontalline represents the negative potential limit related with anoticeable H2 evolu-tion estimated in three-electrode cell. The upper horizontal one correspondsto the thermodynamic limit for water oxidation (from Ref. [10])

As a consequence of these properties, voltage values as


high as 2 V have been reported for AC/AC capacitors operat-ing in Li2SO4, Na2SO4 and K2SO4 [10,37,38,40]. Figure 17exemplifies the cyclic voltammograms of an AC/AC capacitorin 2 mol/L Li2SO4 [10]. Up to 1.2–1.4 V, the curves exhibit arectangular shape, typical for charging the electrical double-layer. Above 1.4 V, one can observe a positive peak whichis attributed to water reduction and hydrogen storage in thenegative electrode. Correspondingly, during the voltage de-crease, a pseudo-capacitive contribution appears below ca. 0.8V; according to Figure 16 this value corresponds to a negativeelectrode potential higher than−0.2 V vs. NHE, allowing theoxidative 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 tothe important overpotential for di-hydrogen evolution at thenegative electrode [10] or to the strong solvation of cationsand anions [40]. Hydrogen storage in the negative electrodeatthe highest voltage values provides capacitance enhancement.Enhancing both capacitance and voltage in these electrolyticmedia gives rise to very promising systems in terms of energydensity and environment compatibility.

Figure 17. Cyclic voltammograms recorded at different values of maximumvoltage for an AC/AC capacitor in 2 mol/L Li2SO4 (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 newconcept has been presented recently, where the iodide/iodineredox couple from the electrolyte solution is at the origin ofpseudo-capacitance [93]. The electrochemical activity oftheelectrolyte is based on Reactions (8) to (11) which occur onthe electrode/electrolyte interface of the positive electrode ofan AC/AC capacitor:

2I−1←→ I2 +2e− (8)

3I−1←→ I3 +2e− (9)

2I−13 ←→ 3I2+2e− (10)

I2 +6H2O←→ 2IO−13 +12H+ +10e− (11)

The capacitance of the carbon electrode has been evalu-ated by cyclic voltammetry in three-electrode cell for differenttypes of alkali counter-ions (Figure 18). It increases withthevan 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, thecapacitance decreases to 373 F/g [94]. An analysis of thealkali cation properties indicates that this phenomenon isinperfect accordance with the ion-solvent and solvent-solventinteractions measured in the form of potential energy as wellas cation mobility values and diffusion coefficients tenden-cies [95]. However, the discharge capacitance of real AC/ACsystems does not exceed 280 F/g at 1 A/g, which is due tothe relatively low capacitance value of the negative electrode,contributing to lowering the capacitance of the two-electrodecell by Equation (2).

Figure 18. Three-electrode cyclic voltammograms of a carbon electrode for1 mol/L alkali iodide solution (from Ref. [94])

Given the fact that only the positive electrode exhibits anexceptional 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/L−1 VOSO4 for the negative one; the two electrolyticcompartments were separated by a Nafion membrane. The re-ported capacitance values are about 1200 F/g and 670 F/g forthe 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)


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

(14)

HV2O3−7 +13H+ +10e−←→ 2V+7H2O (15)

HV2O3−7 +9H+ +6e−←→ 2VO+5H2O (16)

The cyclic voltammograms of the AC/AC device recordedat various scan rates are presented in Figure 19. The energydensity at 1.0 V voltage range was reported to be at the levelof 19 Wh/kg, which is an exceptional value for an aqueouselectrolyte system [96].

The quinone/hydroquinone couple has also been usedas redox-active additive for an AC/AC capacitor in 1 mol/Laqueous H2SO4 solution [97,98]. A battery-like behaviourhas been observed at the positive electrode and a pseudo-capacitive hydrogen electrosorption process at the negativeone. The authors suggest that it is the consequence of anasymmetric voltage splitting between the electrodes aftertheincorporation of hydroquinone. A tremendous capacitancevalue 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 carbonsurface. Meanwhile, the capacitance of the negative electrodealso increases significantly when compared with the value ob-tained for the electrode operated in the electrolyte withouthydroquinone (from 290 to 477 F/g). Even if the values ofcapacitance are slightly doubtful due to different capacitancevalues recorded from different methods, the idea of exploitingthe quinone/hydroquinone redox couple from the electrolyteis 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 electrolyticsolutions (from Ref. [96])

6. Conclusions

The present researches on carbon/carbon supercapacitorsare essentially dedicated to improving the specific energy,

which can be achieved either by enlarging the voltage range orby enhancing the capacitance. Different strategies have beenpresented 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 electrolyteare commercially available. Owing to the research efforts dur-ing the last years, their operation behavior is better understoodand optimizations of materials can be suggested. It is nowwell-demonstrated that capacitance is optimal in subnanomet-ric pores and that, under the effect of polarization, the ionssolvation sphere is distorted, meaning that at least they loosepartly some of their solvating molecules. Designing carbonscontaining 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 besaturated at a voltage smaller than the maximum possible onefor the considered electrolyte, leading to a energy limitationof the system.

The voltage window is essentially controlled by the elec-trochemical stability of the electrolyte in the presence ofacti-vated carbons. Traditional organic electrolytes are able to op-erate up to 2.7–2.8 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 suggestedas alternative to the organic media. Unfortunately, their elec-trical conductivity at room temperature is very low and theyare not appropriate for power systems. Although demonstrat-ing smaller voltage window than organic electrolytes, neutralaqueous electrolytes are interesting as far as low cost, safe andenvironment friendly devices are expected.

Besides the later attractive properties, aqueous elec-trolytes are able to promote pseudo-faradic reactions withcar-bon electrodes: i) by the presence of functional groups, ii)through hydrogen electrosorption, and iii) by redox reactionsat the electrode/electrolyte interface.

Overall, one can see that carbon based systems offer awide range of possibilities depending on the nanoporous tex-ture and surface functionality of carbons, and on the kind ofelectrolyte. The future trend should not be in a unique kindof system, but in the development of various options, using aspecific combination of components allowing the desired per-formance to be reached.

AcknowledgementsThe 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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