vie

duca

, D

nive

form

be pformed carbon nanotube electrodes, or by electrochemical co-deposition. The composites combine the large pseudocapacitance of theconducting polymers with the fast charging/discharging double-layer capacitance and excellent mechanical properties of the carbon

Supercapacitors store electrical energy through double- discovered that electrical charges could be stored on the

conductors, giving rise to the so-called double-layer. Anaccumulation of charges, Dq, of opposite sign takes placeacross such interfaces to an extent that is dependent on

* Corresponding author. Tel.: +44 115 951 4171; fax: +44 115 951 4115.E-mail address: george.chen@nottingham.ac.uk (G.Z. Chen).

Available online at www.sciencedirect.com

Progress in Natural Science 18layer charging, faradaic processes, or a combination ofboth. The amount of energy stored is usually small andcan be delivered instantaneously, making supercapacitordevices able to provide pulsed high power rather than highamount of energy. Various terms have emerged to denotethe device including double-layer capacitor, supercapaci-tor, ultracapacitor, power capacitor, gold capacitor,power cache or electrochemical double-layer capacitor.This plethora of names is confusing, and since the termdouble-layer capacitor strictly only refers to devices thatutilize the double-layer capacitance, but does not includethose that rely on the large pseudocapacitance, this term

plates of a so-called condenser, now referred to as a capac-itor. However, the rst patent only dates back to 1957where a capacitor based on high surface area carbon wasdescribed [2].

According to the intrinsic principles of charge storageand discharge in supercapacitors, there are two kinds ofcapacitance: double-layer and pseudocapacitance. The lat-ter involves a faradaic process while the former is non-faradaic.

A separation of ionic charges (or ionic charges and elec-tronic charges) arises at interfaces between solids and ionicsolutions, especially at colloids, metal electrodes, and semi-nanotubes. The electrochemically co-deposited composites are the most homogeneous and show an unusual interaction between thepolymer and nanotubes, giving rise to a strengthened electron delocalisation and conjugation along the polymer chains. As a result theyexhibit excellent electrochemical charge storage properties and fast charge/discharge switching, making them promising electrode mate-rials for high power supercapacitors. 2008 National Natural Science Foundation of China and Chinese Academy of Sciences. Published by Elsevier Limited and Science inChina Press. All rights reserved.

Keywords: Supercapacitor; Carbon nanotube; Conducting polymer

1. Introduction

1.1. Charge storage mechanisms in supercapacitors

cannot cover the whole range of devices. Therefore, in thispaper, supercapacitor is used to avoid confusion.

The rst double-layer supercapacitor was the Leyden jar[1] made in 1746 at Leyden in the Netherlands, where it wasRe

Carbon nanotube and confor super

Chuang Peng, Shengwen Zhang

School of Chemical and Environmental and Mining Engineering, U

Received 18 January 2008; received in revised

Abstract

Composites of carbon nanotubes and conducting polymers can1002-0071/$ - see front matter 2008 National Natural Science Foundation oand Science in China Press. All rights reserved.

doi:10.1016/j.pnsc.2008.03.002w

cting polymer compositespacitors

aniel Jewell, George Z. Chen *

rsity of Nottingham, University Park, Nottingham NG7 2RD, UK

25 February 2008; accepted 12 March 2008

repared via chemical synthesis, electrochemical deposition on pre-

www.elsevier.com/locate/pnsc

(2008) 777788f China and Chinese Academy of Sciences. Published by Elsevier Limited

the potential of the electrode (related to the potential dier-ence built up across such an interface, DV). This results in acapacitance C = Dq/DV or d(Dq)/d(DV) which is referredto as the double-layer capacitance [3]. Charge storage intrue double-layer capacitors is purely electrostatic in nature(separation of ion and electron charges) and no chemicalreaction is involved. Consequently, charging and discharg-ing is highly reversible and takes place practically instanta-neously. Charge storage in both the Leyden jar and the rstpatented supercapacitor is attributed to double-layercapacitance.

Apart from the double-layer capacitance, there is alsothe possibility to utilise the large pseudocapacitance that

layer capacitance.

materials may vary, e.g. 4.510 lF/cm2 [9,10] for carbonblack, 1015 lF/cm2 [11] for activated carbons andapproximately 2035 lF/cm2 for graphite powders [4].However, the theoretical capacitance calculated from theabove values multiplied by the specic surface area (m2/g) may not be attainable in practice since the capacitanceof the carbon material highly depends on the pore accessi-bility and pre-treatment [4].

Most carbon materials for electrochemical capacitorshave surface oxygen groups, e.g. carboxyl, carboxylic, phe-nolic, lactone, aldehyde, and ether groups, depending onthe source of the carbon and the pre-treatment methods.Alkaline treatment with aqueous NaOH or KOH solutioncan both increase the specic area and introduce oxygenfunctional groups to the carbon surface [12]. The presenceof such oxygen functional groups can enhance the carbon

778 C. Peng et al. / Progress in Naturais associated with electrosorption of ions or metal adatomsand especially some redox processes. Such pseudocapaci-tance arises when, for thermodynamic reasons, the charge,q, required for progression of an electrode process, e.g.electrosorption or redox conversion of an oxidized speciesto a reduced species in liquid or solid solutions, is a func-tion of potential, V. Then the derivative dq/dV correspondsto a faradaic capacitance (or pseudocapacitance) [3]. Thuspseudocapacitance can be associated with either a redoxreaction, for which potential is a logarithmic function ofthe ratio of activities of the oxidized species, or with a pro-cess of progressive occupation of surface sites on an elec-trode by an underpotential-deposited species [3]. Themaximum capacitance realisable in such systems, especiallyfor the redox case, is substantially larger (10100 times)than those for carbon double-layer systems [4]. Fig. 1 illus-trates how pseudocapacitance arises: when the conductingpolymer is being charged, it loses electrons and becomespolycations, causing the anions in the solution (Cl in thiscase) to intercalate into the conducting polymer in order tomaintain electro-neutrality.

Batteries and low temperature fuel cells are typical lowpower devices, whereas conventional capacitors may havea specic power of >106 watts per kg at very low specicenergy (Fig. 2). Supercapacitors ll the gap between batter-ies and conventional capacitors in terms of both specicenergy and power, i.e. they have a specic power as highas conventional capacitors and a specic energy close tothat of batteries. Therefore, supercapacitors can improveFig. 1. Illustration of pseudocapacitance in a conducting polymer.The double-layer capacitance of carbon materials is pro-portional to their specic area and specic capacitance interms of area. The specic capacitance of dierent carbonbattery performance in terms of power density when com-bined with batteries.

1.2. Materials for supercapacitors

Supercapacitors can be made from a variety of materialswhose selection depends largely on the type of capacitanceto be utilised, as shown in Fig. 3. Carbon in its dispersedand conducting form is the most widely used commercialmaterial for supercapacitors. These materials can beobtained by an activation pre-treatment of existing carbonmaterials made initially from thermal carbonization ofcoal, pitch, wood, coconut shells or polymers [4]. The mostcommonly used pre-treatments are hot nitrogen [6], hydro-gen [7], carbon dioxide or steam ux [8], through whichhigh surface area and pore accessibility are achieved. Dueto the good conductivity, large specic area, and cycle sta-bility, carbon materials exhibit a large and stable double-

Fig. 2. Sketch of ragone plot for various energy storage and conversiondevices. (Reprinted [5]).

l Science 18 (2008) 777788surface hydrophilicity in aqueous solutions and introduceredox processes that contribute pseudocapacitance to the

e supercapacitor materials.

tural Science 18 (2008) 777788 779overall capacitance [13,14]. However, it was found thatalthough the stability of the activated carbon increases withthe oxygen content when the carbon is used for the anode,it decreases when used for the cathode [15]. Additionally,both the stability and conductivity of the activated highsurface area carbon decrease with increasing surface area[16], which is quite understandable because the overall elec-trical conductivity and mechanical integrity decrease as thematerials become more porous. Consequently, it has beenfound that the increased capacitance from the surface oxy-gen groups can be oset by the increases in both internalresistance and leakage current [17].

As one of the conducting and porous carbons, carbonnanotubes (CNTs) also possess ultra high mechanicalstrength, good electrical properties, high specic area,and high dimensional ratios. Their application in electro-chemical double-layer capacitors has been studied in detail[18,19].

Metal oxides or hydroxides such as ruthenium, cobalt,nickel, and manganese oxides/hydroxides are a family ofmaterials used in supercapacitor applications. These mate-rials are conducting or semi-conducting and exhibit redoxactive behaviours, giving rise to pseudocapacitance. Ruthe-nium oxide (hydrous or anhydrous) as a capacitive materialhas been studied widely since ruthenium dioxide was rstreported as a new electrode material in 1971 [20]. Theapplication of RuO2 in supercapacitors is based on its

Fig. 3. Taxonomy of th

C. Peng et al. / Progress in Nametallic type conductivity and reversible redox reactionsin aqueous media (Ru2+, Ru3+, and Ru4+ couples). Ruthe-nium oxide gives relatively constant and appreciable capac-itance over a 1.4 V range [20] with specic capacitanceranging from 600 to 1000 F/g depending on the prepara-tion procedure, measurement conditions, use of support,etc. [21]. Another signicant advantage is the ultra high sta-bility or long cycle life. Thermally formed RuO2 lms on Tiwith TiO2 or Ti2O5 could be charged over 10

5 timesbetween 0.02 and 1.2 V, or even to as far as 1.4 V (vs.RHE), with little degradation. The only drawback of ruthe-nium oxide is the high cost of the material, thereby limitingits use as a supercapacitor material to aerospace and mili-tary applications [22] with commercial use far from eco-nomical. Other metal oxides with lower cost have alsobeen synthesized and tested for supercapacitor applica-tions, e.g. Co3O4 [23,24], NiO [25,26], and MnO2 [27,28].Although these metal oxides are much more cost eective,their specic capacitance is lower, normally between 20 and200 F/g; other properties like potential window and con-ductivity are also not comparable with ruthenium oxide.

Conducting polymers are the third group of candidatematerials for supercapacitors due to their good electricalconductivity [2931], large pseudocapacitance [3234],and relatively low cost. The most commonly used conduct-ing polymers include polyaniline (PANI), polypyrrole(PPy), and poly[3,4-ethylenedioxythiophene] (PEDOT).The electrochemical capacitance and charge storage prop-erties of conducting polymers have been studied by cyclicvoltammetry, electrochemical impedance spectroscopy,and chronopotentiometry. Conducting polymers have avery large specic capacitance that is close to Rutheniumoxides, e.g. 775 F/g for PANI [33], 480 F/g for PPy [35],and 210 F/g for PEDOT [36]. However, conducting poly-mers commonly have poor mechanical stability due torepeated intercalation and depletion of ions during charg-ing and discharging.

The specic capacitance and the potential range of thethree categories of materials are presented in Fig. 4. Car-bon normally exhibits specic capacitance under 200 F/gwhile RuO2 shows specic capacitance close to 1000 F/g.Conducting polymers also have high specic capacitance.Fig. 4 also illustrates the applicable potential ranges of

the three materials in aqueous solutions. Carbon has a very

Fig. 4. Illustration of specic capacitance of porous carbon, RuO2, andconducting polymers.

turawide potential and is suitable to be a cathode in a superca-pacitor. RuO2 and conducting polymers are more suitableto be anodes. A combination of conducting polymers andcarbon for positive and negative electrodes in supercapac-itors is both scientically and commercially applicabledue to the low cost of the two materials.

Aside from the three categories of pure materials, thereis a new tendency to synthesize composite materials com-bining two or more pure materials for supercapacitors.The great promise is to combine carbon nanotubes witheither metal oxides or conducting polymers. Compositesof CNT combined with RuO2 [37], NiO [38], and MnO2[39] have been prepared and exhibit good potential for sup-ercapacitor applications. However, composites of CNTand conducting polymer are even more interesting andpromising since they can combine two relatively cheapmaterials to gain the large pseudocapacitance of the con-ducting polymers coupled with the conductivity andmechanical strength of the CNT. The synthesis and chargestorage properties of such composites are the interest ofthis paper and will be discussed based on both the existingliterature and experimental ndings of the authors.

2. Conducting polymer and CNT composites by chemical

polymerisation

The technologies of chemical polymerisation of anilineand pyrrole have been known to industries for about a cen-tury. With the discovery of the conducting polymer in1963 [40], extensive research on the conducting polymerswas initiated by MacDiarmid in 1976 for their possibleapplications in sensors [2], energy storage and actuators[4], among others.

Conducting polymers prepared by chemical polymerisa-tion exhibit high conductivity, good stability, and negligi-ble solubility in aqueous solutions. Chemical synthesis ofconducting polymers is simply achieved by oxidation ofcorresponding monomers using an oxidizing agent andthus is preferred for mass-production in industry as it pro-vides a cheap and ecient route to achieve polymerisation[41,42]. For instance, aniline can be chemically oxidizedinto polymer form in aqueous solution by a variety of oxi-dants: (NH4)S2O8, K2Cr2O7, KIO3, FeCl3, KMnO4,KBrO3, KClO3; as long as the oxidant is capable of with-drawing a proton from an aniline molecule without form-ing a strong co-ordination bond (either with thesubstrate/intermediate or with the nal product) it can beused [43]. Similarly, pyrrole could be oxidized by a varietyof transition metal ions from metallic salts, such as FeCl3,Fe(NO3)3, Fe2(SO4)3, K3Fe(CN)6, and CuCl2, all of whichare common oxidizing agents for the synthesis of highlyconducting PPy [44]. Thiophene and its derivatives can bechemically oxidized to produce the corresponding conduct-ing polymers in organic solution. However, because of thegood solubility of monomers in aqueous solutions, chemi-

780 C. Peng et al. / Progress in Nacal synthesis of PANI and PPy is both more cost eectiveand environmentally friendly compared with synthesisfrom thiophene derivatives. However, the downside ofthe chemical polymerisation is that it cannot achieve thesame level of homogeneity and integrity in its polymerisedproduct as can be produced by electrochemical polymerisa-tion. An integrated electrode lm can be easily obtained viaelectrochemical synthesis while chemically prepared con-ducting polymers are agglomerates or pressed pellets ofsmall particles.

Fig. 5 gives the SEM images illustrating the morphologyof the polymer/CNT composites obtained by chemicalpolymerisation. As can be observed in Fig. 5(a) and (b),most of the CNTs were coated with a homogeneous layerof polymers and the nanocomposites formed a coralloidnetwork. The average diameters of the composites areassessed to be 50100 nm for PANI/CNT and 120200 nm for PPy/CNT. Note that the diameter of the pris-tine CNT is 2030 nm, as evidenced by a group of tangledpristine CNTs which happen to be captured in Fig. 5(b).Some agglomerates could be spotted in Fig. 5(a), whichhas been explained previously [45]. It can be concluded thatPANI tends to preferentially grow on itself rather than onthe CNTs after an initial period of CNT coating; this is oneof the inevitable shortcomings of chemical synthesis ofPANI/CNT composites.

The electrochemical properties of the polymer/carbonmaterial composite synthesized from chemical polymerisa-tion have been studied by cyclic voltammetry, as shown inFig. 5(c) and (d). From the cyclic voltammograms, thepresence of CNTs in the composites did not fundamentallychange the electrochemical properties of the polymers.Additionally, the CVs of the composites are quasi-rectan-gular in shape, which is in accordance with the behaviourof an ideal capacitor. This behaviour of rectangular CVshape represents the rapid response of current to thechange of potential, which is essential to ensure optimumenergy storage during fast charge and discharge processes.

Previously, a review has been carried out [46] on thechemical polymerisation of conducting polymers and theirderivatives on dierent materials, i.e. glass, polymer, silica,metal oxides, ber, and textile, using chemical deposition.It was found that almost all types of material can be coatedwith conducting polymers and the new composite materialsyielded can be used in various elds. Some of the currentresearch on chemical synthesis of conducting polymers inthe presence of fullerenes and CNTs, and the possibleapplications of these composites were also reviewed [47].However, this paper mainly focused on composites of con-ducting polymers and CNTs for application insupercapacitors.

After 20 years of intensive study of conducting polymersand their chemical, in-situ polymerisation, much work hasbeen carried out in order to optimize the processes.Attempts to synthesize conducting polymers of micro- ornano-scale in order to improve their electrochemical andmechanical performance have always been the most

l Science 18 (2008) 777788intense. At the early stages, a template synthesis methodwas employed in 1989 [49] by using membranes with linear,

e CNe mposi

turaFig. 5. Illustration of the morphological and electrochemical features of th(a) PANI/CNT composite with 10 wt% of CNT being added to the anilinadded to the pyrrole monomer dispersion; (c) cyclic voltammogram of comcomposite (b) in 1 M KCl at a scan rate of 50 mV/s.

C. Peng et al. / Progress in Nacylindrical pores. The nanoscopic polypyrrole and polyan-iline showed enhancements in electronic conductivity com-pared to analogous polymer bulk conductivity [50].

The template method was then followed by an attemptto coat the conducting polymers on carbon materials withsmall particle size, high surface area, good mechanicalstrength, and excellent conductivity. Various carbon mate-rials, such as carbon black [51], activated carbon, carbonber, single walled carbon nanotubes (SWNTs) [52], andmulti-walled carbon nanotubes (MWNTs), were dispersedinto solution prior to polymerisation to form a uniformsuspension; the polymerisation then occurs on the surfaceof the suspended carbon materials to nish the coating.From the XPS analysis of the CNTs, polypyrrole, andthe CNT/PPy composites [52], it was suggested that thereis no chemical reaction between the CNTs and polypyrrole,and the CNTs function as a template for the polymerisa-tion of conducting polypyrrole.

The chemical polymerisation has also been studied as afunction of a wide variety of synthesis parameters: pH, rel-ative concentration of reactants, polymerisation tempera-ture and time, and with a number of oxidizing agentsand dierent protonic acids [5355].

The reaction yield was found to be independent of mostvariables, while the molecular weight and the electrical con-ductivity of the polymerised polyaniline could be remark-ably aected. However, it was believed that larger initialaniline concentration leads to shorter induction time andthe formation of higher oxidation states of polyaniline,T/polymer composites synthesized through chemical polymerisation [48].onomer dispersion; (b) PPy/CNT composite with 20 wt% of CNT beingte (a) in 1 M H2SO4 at a scan rate of 50 mV/s; (d) cyclic voltammogram of

l Science 18 (2008) 777788 781hydrolysis of the polymer chain, and chlorine substitution[56].

Due to the low dispersibility of CNTs in water, ultra-sound has been applied to achieve a better dispersion ofCNTs in aqueous solution before and during chemicalpolymerisation. Consequently, some studies on the eectof ultrasound on the polymerisation products were carriedout. It was found that the crystallinity of polyaniline syn-thesized using the ultrasonic method is much higher thanthat synthesized by stirring. However, polyaniline withhigher crystallinity does not possess higher conductivity[57]. It was also reported that the polymerisation of 3,4-eth-ylenedioxythiophene (EDOT) by Iron(III) chloride in 1 Msulfuric acid was accelerated by the irradiation of ultra-sound, and thus resulted in higher yield; without changingthe molecular bonding patterns of PEDOT, the ultrasoundcaused emulsication by the rapid motion of the moleculeswhich resulted in ecient mixing [58]. Generally, the eectsof the ultrasound on the polymerisation are believed to berapid mixing in multiphase systems [59], the formation offree radicals, and mechanical shocks [58,60].

The benets of introducing carbon nanotubes into con-ducting polymers through chemical in-situ polymerisationhave been intensively studied, with many interesting prop-erties and behaviours of the composites having been dis-covered. It was suggested, with the aid of Raman studies,that the in-situ chemical polymerisation of PANI/MWNTcomposites leads to eective site-selective interactionsbetween the quinoid ring of the PANI and the MWNTs

tura[61]. This facilitated charge-transfer processes between thetwo components as the composite shows much smallerlow temperature resistance compared with both PANIand MWNT; additionally, a weaker temperature depen-dence of the resistivity was found with the composite com-pared with pure PANI. According to IR, elemental, andwide angle X-ray diraction analyses [62], it was concludedthat the increase in conductivity of the PANI/MWNTcomposite may be due to a doping eect of carbon nano-tubes where the nanotubes compete with chloride ions,residual from the sonication of CNTs in HCl solvent.

From a study of the PEDOT/CNTs composites [34], itwas found that, due to the presence of nanotubes, a syner-gistic eect of the two components was observed, with anecient energy extraction from PEDOT even though thecarbon nanotubes and chemically polymerised PEDOTwere only mixed through mechanical stirring. Here, thecomposite contained 15 wt% of PEDOT and showed agood capacitance value of 95 F/g in a three-electrode sys-tem, compared with the capacitance of pure CNTs, whichwas only 1015 F/g, and the capacitance of pure PEDOT,which ranges from 80 to 100 F/g. Moreover, other studiesfound that the polymer coated carbon materials show lar-ger charge/discharge rates and higher specic capacitancein application. Because the conducting polymers can assistthe ions from the electrolyte to quickly reach the internalsurfaces of the carbon electrode, they increase the accessi-bility of the carbon materials with high surface area duringhigh rates of charge/discharge [63]. This hypothesis wasproven by the high specic capacitance of the carbon/poly-mer composites obtained by several recent research works:synthesized activated carbon/polyaniline gave a speciccapacitance of 273 F/g [64] at a scan rate of 50 mV/s,and polypyrrole and carbon nano-ber composite showeda capacitance of 545 F/g at a scan rate of 200 mV/s [65].However, it has been pointed out that increasing the poly-mer content in the polymer/carbon composites couldincrease the capacitance, but it also increases the chargingtime constant. To optimize the performance of the capaci-tor, the geometry of the cell needs careful consideration[66].

The eect of solvent applied on the polymerisation pro-cess has also been studied [32]. Pyrrole monomers were dis-persed in three dierent solutions: water, diethyl ether, andacetonitrile, respectively, for the polymerisation. The poly-mers yielded from the three dierent solutions showed dif-ferent conductivity and capacitance, and the dependencyon solvent was declared to be directly related to the surfaceroughness of PPy powder.

The eect of electrolyte during the electrochemical char-acterisation has been studied [34]. It was found out that forthe PEDOT/CNTs composite, acidic medium (1.0 MH2SO4) was the preferable electrolyte resulting incapacitance of 120 F/g in a two-electrode system, whereasthe alkaline (6.0 M KOH) and organic solution (1.0 M

782 C. Peng et al. / Progress in NaTEABF4 in acetonitrile) produced only 80 and 60 F/g,respectively.The eect of parent carbon materials characteristics(porosity, void volume, and surface area) on the electronicconductivity of the resultant composite has been explored[51,67]. A study was conducted of four dierent types ofmulti-walled (MWNTs) and single-walled nanotubes(SWNTs) for their possible application as electrode materi-als for supercapacitors [67]; the high purity MWNTs werereported to be able to supply twice the capacitance of theSWNTs.

It is worth mentioning that in 2004 a general chemicalroute to synthesize polyaniline nanobers using interfacialpolymerisation at an aqueous/organic interface was devel-oped [68]. The organic solvent contains aniline and anaqueous phase contains the oxidant plus the doping acid.The polyaniline product, polymerised at the interface, isalmost exclusively composed of nanobers with relativelyuniform diameters. The diameters of the nanobers werefound to be controlled by the doping acid used in the poly-merisation. Other attempts to improve the performance ofthe CNT/polymer composites include acid treatment of thecarbon nanotubes before the polymerisation to create func-tional groups [69] and irradiation of the CNT/polymercomposites by a 60Co c-ray source under atmospheric pres-sure at ambient temperature [70].

Whilst much of this early work focused on the alterationof operating procedures during polymerisation, it is nota-ble that the current trend of the research in this area isnow focusing on the cell design towards the application.

A discussion on the calculation of the specic capaci-tance of the polymer/MWNTs composite electrodes withpolypyrrole and polyaniline has been presented [45]. It issuggested that the preferable way to test and calculatethe specic capacitance of the electrode material is in atwo-electrode cell rather than the standard three-electrodecell which is widely used in lab research and normally givesonly the specic capacitance of individual electrodes.

In 2001, an industrial prototype of a hybrid cell wasdesigned [41]. The cell was composed of poly(4-uoro-phenyl-3-thiophene) as the positive electrode and activatedcarbon as the negative electrode in the electrolyte of 1 MNEt4CF3SO3; the test cells were reported to reach48 Wh/kg of maximum energy associated with 9 kW/kgof maximum power. The same group later developed a sup-ercapacitor module of 3 V and 1.5 kF with another deriva-tive of thiophene (poly(3-methylthiophene)) using a similarcell conguration [42]. In 2002, a hybrid electrochemicalcapacitor was assembled, with PANI/activated carboncomposite as the positive electrode and activated carbonas the negative electrode in a 6 M KOH electrolyte [71].The hybrid cell showed an improved specic energy andspecic power. Similar results were reported later the sameyear with PPy/CNT composites, poly(3-methylthiophene)/CNT composites, and carbon nanotubes in 1 M LiClO4/acetonitrile solution as the electrolyte [72]. In 2005, dier-ent combinations of four materials (PANI/CNT compos-

l Science 18 (2008) 777788ite, PPy/CNT composite, PEDOT/CNT composite, andactivated carbon) were studied for their performance as

electrode materials in supercapacitors with 1M H2SO4 aselectrolyte [73]. The combination of PANI/CNT as thepositive electrode and activated carbon as the negative elec-trode were reported to be the best among all the couplings,with a specic capacitance of 330 F/g at a potential win-dow of 1 V.

As listed in Table 1, both the composites of PPY andPANI with CNT showed large specic capacitance (200700 F/g) at various scan rates or charge/discharge current

or Pt substrates [82]. Also in 2002 polypyrrole was depos-ited on CNTs through galvanostatic oxidation of mono-mers in sulfuric acid in order to nd an alternative andrelatively cheap method to enhance the capacitance of car-bon nanotubes [83]. The PPy modied electrodes had anelevated specic capacitance of 180 F/g compared with50 F/g for the unmodied CNT electrode. The charge lossof the PPy modied CNT electrode was less than 20% after2000 galvanostatic charging-discharging cycles [83].

Following these studies, much work has been done to

on

ren

C. Peng et al. / Progress in Natural Science 18 (2008) 777788 783densities in aqueous solutions. They have been proven tobe a powerful candidate for application in supercapacitorelectrodes.

3. Electrochemical deposition of conducting polymers on

CNT electrodes

Since the discovery of carbon nanotubes [75], theirapplications as electrodes have been reported for oxidationof dopamine [76], study of protein electrochemistry [77],detection of nitrides [78], Li ion intercalation studies [79],and electrochemical double-layer capacitors [18]. Due totheir exceptional electrical, chemical, and surface proper-ties, CNTs are becoming a powerful candidate in a widerange of applications [18,7679].

The rst attempt to electrochemically deposit conduct-ing polymers on carbon nanotubes was made in 1999 [80]where MWNT electrodes were used for the deposition ofPANI lms, and higher current density and more eectivepolymerisation were found compared with those depositedon Pt electrode. The possible applications in sensors andmagnetic shielding were suggested from this work. In2000, PANICNT coaxial nanowires were prepared elec-trochemically on aligned CNTs [81]. The thickness of thePANI layer was estimated to be 4050 nm by TEM images,with the CNT framework expected to oer a highermechanical strength compared with the bare coaxial nano-wires [81]. Separately, thin and uniform PPy lms, whichwere also coated on individual CNTs of well-alignedCNT arrays, were produced via potentiodynamic polymer-isation in aqueous solution [82]. The Faradaic current forPPy deposition on CNTs is much higher than that on Tior Pt substrate. The ion diusion and migration pathwaysare short due to the unique structure of the electrode mate-rial. PPy coated CNT array electrodes show signicantlyimproved capacitance compared with PPy coated on Ti

Table 1The performance of polymer/carbon composites by chemical polymerisati

Electrode Electrolyte Potentialwindow (V)

Scan rate/curdensity

PPy/MWNTs (20 wt%) 1 M H2SO4 0.6 to 0.2 2 mA/gPANI/MWNTs (20 wt%) 1 M H2SO4 0.4 to 0.2 2 mA/gPPy/VGCF 6 M KOH 0.9 to 0.2 30 mV/sPPy/MWNTs 1 M H2SO4 00.6 2 mV/sPANI/activated carbon 4 M KCl 0.2 to 0.5 50 mV/spowder (16 wt%)PPy/SWNT (50 wt%) 7.5 M KOH 0.9 to 0 10 mA/ginvestigate the properties and applications of conductingpolymers on carbon nanotube electrodes. Among others,PPy [8284] and PANI [80,81,8588] have been most suc-cessful in forming a coating on nanotubes via electrochem-ical polymerisation. Both MWNTs [80,82,83,86] andSWNTs [84,85,87,88] were studied as electrodes. The prep-aration varies in terms of both substrate and electrochem-ical methods: Well-aligned CNT arrays [82], pellets pressedwith binder [83,87,88], CNT buckypaper [84,85], and CNTlms by solvent casting [86] have been used as a substratefor electro-deposition of conducting polymers. PANI orPPy was coated on CNT by potentiodynamic [8082,85],galvanostatic [83,84,86] or potentiostatic [80,87,88] electro-polymerisation. Electrochemically grown conducting poly-mers decreased the contact resistance between CNTs [84]producing composites with improved electrical conductiv-ity. Raman [84,85] and FTIR [85] spectroscopy showed evi-dence of possible interaction between CNTs and polymers.The composites showed excellent response to nitrite detec-tion with high sensitivity [86]. As electrode materials forsupercapacitors, the composites also exhibited much higherspecic capacitance, specic energy and power than CNTor electron conducting polymers (ECP) alone [87]. A spe-cic capacitance of 463 F/g was achieved for 73 wt% PANIon SWNTs [88].

However, since ECPs were deposited on CNT preformsrather than individual tubes, it is impossible to obtainhomogeneous composites of ECPs and CNTs. SEM images[87,88] show that as the polymerisation charge increases,the ECP coating grows thicker and eventually forms a foul-ing that prevents ECP growth on inner CNTs, i.e. the elec-tro-deposition only takes place on CNTs that have goodcontact with the monomer solution while the CNTs insidehave little or no polymer coatings, resulting in a heteroge-neous structure. Therefore, as the polymer grows thicker,

t Mass speciccapacitance (F/g)

Electrode speciccapacitance (F/cm2)

Referenceelectrode

Reference

506 3.09 Hg/Hg2SO4 [45]670 2.30 Hg/Hg2SO4 [45]588 Ag/AgCl [65]192 Hg/Hg2SO4 [67]273 Ag/AgCl [64]265 Ag/AgCl [74]

turathe material will behave like pure PANI and the eects ofthe CNTs will be diminished. Electro-deposition of ECPson aligned CNT arrays [82] is likely to give a more homo-geneous structure and a higher utility of CNT surface fordeposition. However, due to the inecient charge transferin the interfacial region, partial polymer coverage on CNTwith a bias on the tips was observed, i.e. only a predeter-mined portion of CNT length was covered by polymer [81].

ECPs have also been deposited on porous carbon elec-trochemically [89,90]. The combination of double-layercapacitance and pseudocapacitance led to a greatlyimproved specic capacitance of 180 F/g compared with92 F/g for bare carbon electrodes [89]. Recently, high spe-cic capacitance of 1600 F/g and high current density of45 mA/cm2 were achieved for PANI coated porous carbon[90]. ECPs deposited on CNTs or other porous carbon pre-forms both utilise the good conductivity and high surfacearea of the substrate carbon materials. Intrinsically, thereis little dierence in the charge storage mechanismsbetween these methods.

4. Electro-co-deposition of conducting polymer/CNT

composites

Apart from chemical oxidation and electro-depositionof ECPs on CNTs, composites of ECPs and CNTs can alsobe prepared by electro-co-deposition from a solution con-taining monomers and dispersed CNTs. Dispersion ofCNTs in aqueous solution can be achieved through acidtreatment which can also introduce surface oxygen-con-taining groups such as carboxylic acids [91,92]. Electro-chemical co-deposition of composites of ECPs and CNTswas rst reported in a stable aqueous solution containingionized (anionic) CNTs and pyrrole monomers [93]. Here,the CNT acted as both a charge carrier during the electro-deposition and also a strong and conducting dopant in theas-prepared PPyCNT composite. The SEM imagesrevealed a uniform coating layer of PPy on the surface ofindividual CNTs. Another remarkable nding is that bothCV and AC impedance results showed good conductivityof the composite even at negative potentials at which purePPy would become non-conducting. Given the extensivework performed on chemically synthesized CNTECPcomposites for supercapacitors, there is comparatively littleresearch on electrochemical synthesis and capacitanceproperties. However, in-situ electrochemical co-depositionof CNTs with the three most important conducting poly-mers, namely, PPy [93,94], PANI [95], and PEDOT [96],has been achieved. Due to the low solubility of EDOTmonomers in aqueous solution, PEDOTCNT was depos-ited from a metastable emulsion of acetonitrile and watercontaining EDOT + 0.3 wt% CNTs pretreated with ultra-sound [96]. The composites all exhibited improved elec-trode kinetics and enhanced electrochemical capacitance,expressed as the electrode specic capacitance (F/cm2).

784 C. Peng et al. / Progress in NaThe redox pseudocapacitive behaviour, i.e. ion expulsionand uptake, of PPyCNT electrodes has been studied usingEQCM [97]. It was also found that increasing the concen-tration of CNTs in the polymerisation solution decreasesthe thickness of the polymer coating on each CNT [94].

In this paper, the electro-co-deposition of the PPyCNTcomposite lms was carried out in an aqueous solution of0.25 mol/L pyrrole + 0.3 wt% CNT [21,93]. PANICNTcomposite was prepared from a solution containing0.25 mol/L aniline + 1.0 mol/L HCl + 0.3 wt% CNT [95].The PEDOTCNT lm was deposited from a mixture of5 ml acetonitrile and 5 ml water containing 0.25 MEDOT + 0.3 wt% CNT. After 10 min ultrasonic treatment,this mixed solution formed a metastable emulsion whichcould last long enough for the electrochemical depositionof thin lms [96]. For comparison, pure PPy was electrode-posited from 0.25 M pyrrole + 0.5 M KCl in water; purePANI from a 0.25 M aniline + 1.0 M HCl aqueous solu-tion; and pure PEDOT from 0.25 M EDOT + 0.5 MKClO4 in acetonitrile. The electrochemical characterisationof the PPyCNT and PPy lms was carried out in a 0.5 MKCl aqueous solution. The same solution was applied forthe PEDOTCNT and PEDOT lms. The PANICNTand PANI lms were studied in 1.0 M HCl.

Fig. 6(a) shows the TEM image of two almost parallelnanotubes coated and joined by PPy, each showing the cen-tral cavity and the amorphous polymer layer. Just visiblebehind the coating are the graphitic fringes of the nano-tubes. The thickness of the PPy coating is ca. 10 nm. Theoverall SEM image of PEDOT/CNT (Fig. 6(b)) compositepresents similar and uniform microstructures, dieringfrom the conducting polymers deposited on a pre-madeCNT substrate. All composites show a three dimensional(3D) network composed of interconnected brils with sim-ilar diameters between 30 and 60 nm. The diameters of theas-received CNTs were in the range of 1030 nm. This dif-ference indicates that the conducting polymer formed auniform coating on the surface of individual CNTs. There-fore, CNTs serve as the backbone to form the 3D network,greatly enhancing the mechanical properties and electricconductivity of the composite lm. Furthermore, themicro- and nanometre pores in the composites provideenough pathways for the movement of ions and solventmolecules within the composite lms, resulting in improvedelectrochemical properties.

Fig. 7 shows the cyclic voltammograms of PANI, PPy,and PEDOT and their composites with CNTs at potentialranges selected so that no signicant faradaic currents,due to the conversion of the polymer between the reducedand the oxidized states, were observed. It can be seen thatat both negative and positive ends of the potential scan,all the CVs of the composites exhibited capacitive featureswith almost straight and vertical current variations at theend potentials, which suggest a fast charge/dischargeswitching resulting from high electronic and ionic conduc-tivity. For pure PPy and pure PEDOT, the fast chargedischarge switching was only seen at the positive end,

l Science 18 (2008) 777788but not at the negative end of the potential scan, asshown in both Fig. 7(b) and (c), apparently due to the

turaC. Peng et al. / Progress in Napolymers becoming more resistive at more negative poten-tials. Such behaviour of PPy and PEODT contrasts thatof the corresponding composites, conrming the anionicdopant role of the CNTs in the latter. This eect wasmost signicant on the CV of the PEDOT/CNT lm,showing much larger oxidation and reduction currentsat more negative potentials. The CV of PANI/CNT showslittle dierence from that of pure PANI (Fig. 7(a)) due tothe coexistence of Cl during the electro-deposition. How-ever, PPy/CNT and PEDOT/CNT lms exhibit signi-cantly larger CV currents than pure conductingpolymers, especially at potentials close to the negativelimit. This dierence in current between pure polymerand the composites can be explained by that the latterhas a signicantly more porous structure for ion trans-port, and higher and potential independent electronic con-ductivity through the embedded CNTs.

Fig. 6. (a) High-resolution transmission electron micrograph of PPy/CNTcomposite. The coating and joining was achieved by electrolyzing adegassed solution of 0.2% CNTs and 0.5 M pyrrole at 0.8 V for 2 minusing a bare copper grid suspended on a Pt wire as the working electrode.The total charge passed was about 9 mC (Reprinted with permission [93]);(b) electrochemically co-deposited PEDOT/CNT composite at a deposi-tion charge of 0.3 C/cm2 (reprinted with permission [96]).l Science 18 (2008) 777788 785It was suggested that the highly conjugated CNTs couldenhance the electron delocalisation along the polymerchains in the PANICNT composites [95], but this eectshould be more inuential on the oxidized or doped poly-mers, rather than the reduced polymers at the negativepotentials. Another eect, as discussed above, is the elec-trostatic repulsion of the anionic CNTs to the electronson the polymer chains. Unlike the small anions whichcan move to within close proximity of the positive chargeson the polymer chain, the immobility of the CNTs meansthat charge balancing to the polymer chain upon oxidationmay be more eective near the anionic groups on the CNTsthan at locations where no anionic group is present. In thelatter case, charge balancing has to be undertaken by thesmall anions from the electrolyte. Therefore, these two dif-ferent doping regions in the polymerCNT composite maybe responsible for the two observed charge transfer pro-cesses with that at the more negative potential being asso-ciated with the anionic CNTs.

Fig. 7. Cyclic voltammograms of PANI/CNT vs. PANI (a) 1000 mV/s in1.0 mol/L HCl; PPy/CNT vs. PPy (b) 500 mV/s in 0.5 mol/L KCl; andPEDOT/CNT vs. PEDOT (c) 200 mV/s in 0.5 mol/L KCl. Depositioncharge for each lm: 6.5 mC. Reprinted with permission [98].

Composites of conducting polymers and carbon nano-

turatubes show improved mechanical, electrical, and electro-chemical properties compared with conducting polymersalone, leading to a wide variety of applications includingsensors, catalysis, and energy storage. ECPCNT compos-ites combined the large pseudocapacitance of the polymersand the mechanical and structural properties of the nano-tubes and are thus highly promising in novel supercapaci-tors with ultra-high capacitance and power density. Threemethods have been developed to prepare ECPCNT com-posites. Chemical oxidation is a simple, low cost methodsuitable for mass production. Electrochemical depositionof ECPs on CNT preforms leads to an inhomogeneousstructure; as a result, the synergistic eect of the two com-ponents is reduced. Electrochemically co-deposited com-posites have a homogeneous network structurefacilitating both electron and ion movements. Therefore,they exhibit greatly improved electrochemical capacitancecompared with pure conducting polymers.

Acknowledgement

The authors thank the EPSRC for nancial support(GR/R68078/02).

References

[1] Leyden jar. Encyclopdia Britannica Online. http://www.britan-nica.com/ed/ article-9048045 [2008-02-29].

[2] Becker HI. US Patent 2 800 616, Low Voltage ElectrolyticCapacitor. 1957.

[3] Conway BE, Birss V, Wojtowicz J. The role and utilization ofpseudocapacitance for energy storage by supercapacitors. J PowerSources 1997;66(12):114.

[4] Conway BE. Electrochemical supercapacitors: scientic fundamen-tals and technological applications. New York: Kluwer Academic/Plenum Publishers; 1999.

[5] Kotz R, Carlen M. Principles and applications of electrochemicalcapacitors. Electrochim Acta 2000;45(1516):248398.

[6] Phillips J, Xia B, MeneAndez JA. Calorimetric study of oxygenadsorption on activated carbon. Thermochim Acta 1998;312(12):8793.Though not referring to the electrochemical capacitance,other properties and applications of co-deposited ECPCNT composites have been reported recently [99,100].The composites showed improved thermal stability [99]and enhanced charge transport properties [101] comparedwith pure conducting polymers. The composites havefound applications in biosensors with high sensitivity andfast response [100,102]. ECP/CNT composites have alsobeen synthesized electrochemically using surfactantassisted CNT dispersion containing monomers [103].SEM images suggested that the CNTs stood vertically onthe deposited lms, forming an aligned structure.

5. Conclusions

786 C. Peng et al. / Progress in Na[7] Oliveira LCA, Silva CN, Yoshida MI, et al. The eect of H2treatment on the activity of activated carbon for the oxidation oforganic contaminants in water and the H2O2 decomposition.Carbon 2004;42(11):227984.

[8] Pastor-Villegas J, Duran-Valle CJ. Pore structure of activatedcarbons prepared by carbon dioxide and steam activation atdierent temperatures from extracted rockrose. Carbon2002;40(3):397402.

[9] Bockris JOM, Kita H. Analysis of galvanostatic transients andapplication to the iron electrode reaction. J Electrochem Soc1961;108(7):67685.

[10] Engelsman K, Lorenz WJ, Schmidt E. Underpotential deposition oflead on polycrystalline and single-crystal gold surfaces: Part I.Thermodynamics. J Electroanal Chem 1980;114(1):110.

[11] Rudge A, Raistrick I, Gottesfeld S, et al. A study of theelectrochemical properties of conducting polymers for applicationin electrochemical capacitors. Electrochim Acta 1994;39(2):27387.

[12] Bleda-Martnez MJ, Lozano-Castello D, Morallon E, et al. Chem-ical and electrochemical characterization of porous carbon materi-als. Carbon 2006;44(13):264251.

[13] Li H, Xi H, Zhu S, et al. Preparation, structural characterization,and electrochemical properties of chemically modied mesoporouscarbon. Microporous Mesoporous Mater 2006;96(13):35762.

[14] Andreas HA, Conway BE. Examination of the double-layercapacitance of a high specic-area C-cloth electrode as titratedfrom acidic to alkaline pHs. Electrochim Acta2006;51(28):651020.

[15] Nakamura M, Nakanishi M, Yamamoto K. Inuence of physicalproperties of activated carbons on characteristics of electric double-layer capacitors. J Power Sources 1996;60(2):22531.

[16] Pandolfo AG, Vasallo AM, Paul GL. Practical aspects & hurdles inthe development of low-cost high-performance supercapacitors. In:Proceedings of the seventh international seminar on double layercapacitors and similar energy storage devices. Florida EducationalSeminar; 1997. Available from: http://www.cap-xx.com/resources/docs/ppr_1997_practical.pdf.

[17] Hsieh CT, Teng H. Inuence of oxygen treatment on electricdouble-layer capacitance of activated carbon fabrics. Carbon2002;40(5):66774.

[18] Chen JH, Li WZ, Wang DZ, et al. Electrochemical characterizationof carbon nanotubes as electrode in electrochemical double-layercapacitors. Carbon 2002;40(8):11937.

[19] Li C, Wang D, Liang T, et al. A study of activated carbonnanotubes as double-layer capacitors electrode materials. MaterLett 2004;58(29):37747.

[20] Trasatti S, Buzzanca G. Ruthenium dioxide: a new interestingelectrode material. Solid state structure and electrochemical behav-iour. J Electroanal Chem 1971;29(2):A15.

[21] Sugimoto W, Yokoshima K, Murakami Y, et al. Charge storagemechanism of nanostructured anhydrous and hydrous ruthenium-based oxides. Electrochim Acta 2006;52(4):17428.

[22] Trasatti S, Kurzweil P. Electrochemical supercapacitors as versatileenergy stores. Platinum Met Rev 1994;38(2):4656.

[23] Srinivasan V, Weidner JW. Capacitance studies of cobalt oxide lmsformed via electrochemical precipitation. J Power Sources2002;108(12):1520.

[24] Shinde VR, Mahadik SB, Gujar TP, et al. Supercapacitive cobaltoxide (Co3O4) thin lms by spray pyrolysis. Appl Surf Sci2006;252(20):748792.

[25] Ganesh V, Pitchumani S, Lakshminarayanan V. New symmetricand asymmetric supercapacitors based on high surface area porousnickel and activated carbon. J Power Sources 2006;158(2):152332.

[26] Zheng YZ, Zhang ML. Preparation and electrochemical propertiesof nickel oxide by molten-salt synthesis. Mater Lett2007;61(18):39679.

[27] Lee HY, Goodenough JB. Supercapacitor behaviour with KClelectrolyte. J Solid State Chem 1999;144(1):2203.

[28] Hu CC, Tsou TW. Ideal capacitive behaviour of hydrous manganese

l Science 18 (2008) 777788oxide prepared by anodic deposition. Electrochem Commun2002;4(2):1059.

C. Peng et al. / Progress in Natural Science 18 (2008) 777788 787[29] Boara G, Sparpaglione M. Synthesis of polyanilines with highelectrical conductivity. Synth Met 1995;72(2):13540.

[30] Hung S, Wen T, Gopalan A. Application of statistical designstrategies to optimize the conductivity of electrosynthesized poly-pyrrole. Mater Lett 2002;55(3):16570.

[31] Morvant MC, Reynolds JR. In-situ conductivity studies of poly(3,4-ethylenedioxythiophene). Synth Met 1998;92(1):5761.

[32] Noh KA, Kim DW, Jin CS, et al. Synthesis and pseudo-capacitanceof chemically-prepared polypyrrole powder. J Power Sources2003;124(2):5935.

[33] Gupta V, Miura N. High performance electrochemical supercapac-itor from electrochemically synthesized nanostructured polyaniline.Mater Lett 2006;60(12):14669.

[34] Lota K, Khomenko V, Frackowiak E. Capacitance properties ofpoly(3,4-ethylenedioxythiophene)/carbon nanotubes composites. JPhys Chem Solids 2004;65(23):295301.

[35] Fan LZ, Maier J. High-performance polypyrrole electrode materialsfor redox supercapacitors. Electrochem Commun 2006;8(6):93740.

[36] Xu Y, Wang J, Sun W, et al. Capacitance properties of poly(3,4-ethylenedioxythiophene)/polypyrrole composites. J Power Sources2006;159(1):3703.

[37] Qin X, Durbach S, Wu GT. Electrochemical characterization onRuO2xH2O/carbon nanotubes composite electrodes for high energydensity supercapacitors. Carbon 2004;42(2):4513.

[38] Lee JY, Liang K, An KH, et al. Nickel oxide/carbon nanotubesnanocomposite for electrochemical capacitance. Synth Met2005;150(2):1537.

[39] Wu M, Snook GA, Chen GZ, et al. Redox deposition of manganeseoxide on graphite for supercapacitors. Electrochem Commun2004;6(5):499504.

[40] Bolto BA, McNeill R, Weiss DE. Electronic conduction inpolymers. III. Electronic properties of polypyrrole. Aust J Chem1963;16(6):1090103.

[41] Laforgue A, Simon P, Fauvarque JF, et al. Hybrid supercapacitorsbased on activated carbons and conducting polymers. J ElectrochemSoc 2001;148(10):A11304.

[42] Laforgue A, Simon P, Fauvarque JF, et al. Activated carbon/conducting polymer hybrid supercapacitors. J Electrochem Soc2003;150(5):A64554.

[43] Feast WJ. Synthesis of conducting polymers. In: Handbook ofconducting polymers. New York: Marcel Dekker Inc.; 1986. p. 143.

[44] Nalawa HS, Dalton LR, Schmidt WF, et al. Conducting polyanilineand polypyrrole: studies of their catalytic properties. PolymerCommun 1985;27:2402.

[45] Khomenko V, Frackowiak E, Beguin F. Determination of thespecic capacitance of conducting polymer/nanotubes compositeelectrodes using dierent cell congurations. Electrochim Acta2005;50(12):2499506.

[46] Malinauskas A. Chemical deposition of conducting polymers.Polymer 2001;42(9):395772.

[47] Wang C, Guo ZX, Fu S, et al. Polymers containing fullerene orcarbon nanotube structures. Prog Polym Sci 2004;29(11):1079141.

[48] Zhang S, Chen GZ. Data from the lab work on the chemicalsynthesis of polymer/CNTs composite of the group. 2005.

[49] Cai Z, Martin CR. Electronically conductive polymer bers withmesoscopic diameters show enhanced electronic conductivities. JAm Chem Soc 1989;111(11):41389.

[50] Delvaux M, Duchet J, Stavaux PY, et al. Chemical and electro-chemical synthesis of polyaniline micro- and nano-tubules. SynthMet 2000;113(3):27580.

[51] Wampler WA, Rajeshwar K, Pethe RG, et al. Composites ofpolypyrrole and carbon black: Part III. Chemical synthesis andcharacterization. J Mater Res 1995;10(7):181122.

[52] Fan J, Wan M, Zhu D, et al. Synthesis, characterizations, andphysical properties of carbon nanotubes coated by conductingpolypyrrole. J Appl Polym Sci 1999;74(11):260510.[53] Cao Y, Andreatta A, Heeger AJ, et al. Inuence of chemicalpolymerization conditions on the properties of polyaniline. Polymer1989;30(12):230511.

[54] Armes SP, Miller JF. Optimum reaction conditions for thepolymerization of aniline in aqueous solution by ammoniumpersulphate. Synth Met 1988;22(4):38593.

[55] Ayad MM, Salahuddin N, Shenashin MA. The optimum HClconcentration for the in situ polyaniline lm formation. Synth Met2004;142(13):1016.

[56] Fu Y, Elsenbaumer RL. Thermochemistry and kinetics of chemicalpolymerization of aniline determined by solution calorimetry. J AmChem Soc 1994;6(5):6717.

[57] Liu H, Hu XB, Wang JY, et al. Structure, conductivity, andthermopower of crystalline polyaniline synthesized by the ultrasonicirradiation polymerization method. Macromolecules2002;35(25):94149.

[58] Li WK, Chen J, Zhao JJ, et al. Application of ultrasonic irradiationin preparing conducting polymer as active materials for superca-pacitor. Mater Lett 2005;59(7):8003.

[59] Bradley M, Grieser F. Emulsion polymerization synthesis ofcationic polymer latex in an ultrasonic eld. J Colloid InterfaceSci 2002;251(1):7884.

[60] Xia H, Wang Q. Ultrasonic irradiation: a novel approach to prepareconductive polyaniline/nanocrystalline titanium oxide composites.Chem Mater 2002;14(5):215865.

[61] Cochet M, Maser WK, Benito AM, et al. Synthesis of a newpolyaniline/nanotube composite: in-situ polymerisation andcharge transfer through site-selective interaction. Chem Commun2001;16:14501.

[62] Zengin H, Zhou W, Jin J, et al. Carbon nanotube doped polyaniline.Adv Mater 2002;14(20):14803.

[63] Ingram MD, Pappin AJ, Delalande F, et al. Development ofelectrochemical capacitors incorporating processable polymer gelelectrolytes. Electrochim Acta 1998;43(1011):16015.

[64] Ryu KS, Lee YG, Kim KM, et al. Electrochemical capacitor withchemically polymerized conducting polymer based on activatedcarbon as hybrid electrodes. Synth Met 2005;153(13):8992.

[65] Kim JH, Sharma AK, Lee YS. Synthesis of polypyrrole and carbonnano-ber composite for the electrode of electrochemical capacitors.Mater Lett 2006;60(1314):1697701.

[66] Izadi-Najafabadi A, Tan DTH, Madden JD. Towards high powerpolypyrrole/carbon capacitors. Synth Met 2005;152(13):12932.

[67] Frackowiak E, Jurewicz K, Delpeux S, et al. Nanotubular materialsfor supercapacitors. J Power Sources 2001;9798:8225.

[68] Huang J, Kaner RB. A general chemical route to polyanilinenanobers. J Am Chem Soc 2004;126(3):8515.

[69] Zhou C, Kumar S, Doyle CD, et al. Functionalized single wallcarbon nanotubes treated with pyrrole for electrochemical superca-pacitor membranes. Chem Mater 2005;17(8):19972002.

[70] Su SJ, Kuramoto N. Processable polyanilinetitanium dioxidenanocomposites: eect of titanium dioxide on the conductivity.Synth Met 2000;114(2):14753.

[71] Park JH, Park OO. Hybrid electrochemical capacitors based onpolyaniline and activated carbon electrodes. J Power Sources2002;111(1):18590.

[72] Xiao Q, Zhou X. The study of multiwalled carbon nanotubedeposited with conducting polymer for supercapacitor. ElectrochimActa 2003;48(5):57580.

[73] Frackowiak E, Khomenko V, Jurewicz K, et al. Supercapacitorsbased on conducting polymers/nanotubes composites. J PowerSources 2006;153(2):4138.

[74] An KH, Jeon KK, Heo JK, et al. High-capacitance supercapacitorusing a nanocomposite electrode of single-walled carbon nanotubeand polypyrrole. J Electrochem Soc 2002;149(8):A105862.

[75] Iijima S. Helical microtubules of graphitic carbon. Nature1991;354:568.

[76] Britto PJ, Santhanam KSV, Ajayan PM. Carbon nanotubeelectrode for oxidation of dopamine. Bioelectrochem Bioenerg1996;41(1):1215.

[77] Davis JJ, Coles RJ, Hill HAO. Protein electrochemistry at carbonnanotube electrodes. J Electroanal Chem Chem 1997;440(12):27982.

[78] Liu P, Hu J. Carbon nanotube powder microelectrodes for nitritedetection. Sensor Actuat B 2002;84(23):1949.

[79] Zhao J, Gao QY, Gu C, et al. Preparation of multi-walled carbonnanotube array electrodes and its electrochemical intercalationbehavior of Li ions. Chem Phys Lett 2002;358(12):7782.

[80] Downs C, Nugent J, Ajayan PM, et al. Ecient polymerization ofaniline at carbon nanotube electrodes. Adv Mater1999;11(12):102831.

[81] Gao M, Huang S, Dai L, et al. Aligned coaxial nanowires of carbonnanotubes sheathed with conducting polymer. Angew Chem2000;39(20):36647.

[82] Chen JH, Huang ZP, Wang DZ, et al. Electrochemical synthesis ofpolypyrrole lms over each of well-aligned carbon nanotubes. SynthMet 2002;125(3):28994.

[83] Frackowiak E, Jurewicz K, Szostak K, et al. Nanotubular materials

[91] Sandler J, Shaer M, Prasse T, et al. Development of a dispersionprocess for carbon nanotubes in an epoxy matrix and the resultingelectrical properties. Polymer 1999;40(21):596771.

[92] Shaer MSP, Fan X, Windle AH. Dispersion and packing of carbonnanotubes. Carbon 1998;36(11):160312.

[93] Chen GZ, Shaer MSP, Coleby D, et al. Carbon nanotube andpolypyrrole composites: coating and doping. Adv Mater2000;12(7):5226.

[94] Hughes M, Chen GZ, Shaer MSP, et al. Controlling thenanostructure of electrochemically grown nanoporous compositesof carbon nanotubes and conducting polymers. Compos Sci Technol2004;64(15):232531.

[95] Wu M, Snook GA, Gupta V, et al. Electrochemical fabrication andcapacitance of composite lms of carbon nanotubes and polyaniline.J Mater Chem 2005;15:2297303.

[96] Peng C, Snook GA, Fray DJ, et al. Carbon nanotube stabilisedemulsions for electrochemical synthesis of porous nanocompositecoatings of poly[3,4-ethylene-dioxythiophene]. Chem Commun2006:462931.

788 C. Peng et al. / Progress in Natural Science 18 (2008) 777788as electrodes for supercapacitors. Fuel Process Technol 2002;7778:2139.

[84] Nuria FA, Martti K, Viera S, et al. Synthesis and characterization ofcarbon nanotube-conducting polymer thin lms. Diamond RelatMater 2004(13):25660.

[85] Baibarac M, Baltog I, Godon C, et al. Covalent functionalization ofsingle-walled carbon nanotubes by aniline electrochemical polymer-ization. Carbon 2004;42(15):314352.

[86] Guo M, Chen J, Li J, et al. Fabrication of polyaniline/carbonnanotube composite modied electrode and its electrocatalyticproperty to the reduction of nitrite. Anal Chim Acta2005;532(1):717.

[87] Gupta V, Miura N. Polyaniline/single-wall carbon nanotube(PANI/SWCNT) composites for high performance supercapacitors.Electrochim Acta 2006;52(4):17216.

[88] Gupta V, Miura N. Inuence of the microstructure on thesupercapacitive behavior of polyaniline/single-wall carbon nanotubecomposites. J Power Sources 2006;157(1):61620.

[89] Chen WC, Wen TC, Teng H. Polyaniline-deposited porouscarbon electrode for supercapacitor. Electrochim Acta2003;48(6):6419.

[90] Mondal SK, Barai K, Munichandraiah N. High capacitanceproperties of polyaniline by electrochemical deposition on a porouscarbon substrate. Electrochim Acta 2007;52(9):325864.[97] Snook GA, Chen GZ, Fray DJ, et al. Studies of deposition of andcharge storage in polypyrrole-chloride and polypyrrole-carbonnanotube composites with an electrochemical quartz crystal micro-balance. J Electroanal Chem 2004;568:13542.

[98] Peng C, Jin J, Chen GZ. A comparative study on electrochemical co-deposition and capacitance of composite lms of conductingpolymers and carbon nanotubes. Electrochim Acta2007;53(2):52537.

[99] Han G, Yuan J, Shi G, et al. Electrodeposition of polypyrrole/multiwalled carbon nanotube composite lms. Thin Solid Films2005;474(12):649.

[100] Zeng J, Gao X, Wei W, et al. Fabrication of carbon nanotubes/poly(1,2-diaminobenzene) nanoporous composite via multipulsechronoamperometric electropolymerization process and its electro-catalytic property toward oxidation of NADH. Sensor Actuat B2007;120(2):595602.

[101] Tamburri E, Orlanducci S, Terranova ML, et al. Modulation ofelectrical properties in single-walled carbon nanotube/conductingpolymer composites. Carbon 2005;43(6):121321.

[102] Tsai YC, Li SC, Liao SW. Electrodeposition of polypyrrole-multiwalled carbon nanotubeglucose oxidase nanobiocompositelm for the detection of glucose. Biosens Bioelectron2006;22(4):495500.

[103] Zhang X, Zhang J, Liu Z. Conducting polymer/carbon nanotubecomposite lms made by in situ electropolymerization using an ionicsurfactant as the supporting electrolyte.Carbon2005;43(10):218691.

Carbon nanotube and conducting polymer composites for supercapacitorsIntroductionCharge storage mechanisms in supercapacitorsMaterials for supercapacitors

Conducting polymer and CNT composites by chemical polymerisationElectrochemical deposition of conducting polymers on CNT electrodesElectro-co-deposition of conducting polymer/CNT compositesConclusionsAcknowledgementReferences