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Electrochimica Acta 55 (2010) 2311–2318

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

reparation and electrochemical properties of polyaniline doped withenzenesulfonic functionalized multi-walled carbon nanotubes

o Gao, Qinbing Fu, Linghao Su, Changzhou Yuan, Xiaogang Zhang ∗

ollege of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China

r t i c l e i n f o

rticle history:eceived 23 June 2009eceived in revised form8 November 2009ccepted 21 November 2009

a b s t r a c t

Nanocomposite of benzenesulfonic functionalized multi-walled carbon nanotubes doped polyaniline(PANi/f-MWCNTs) was synthesized via a low-temperature in situ polymerization method. The PANi/f-MWCNTs composite has a thin film of PANi coating uniformly on the surface of the f-MWCNTs. Theelectrochemical results show that PANi/f-MWCNTs nanocomposite possesses good rate response, whichcould ascribe to the uniform structure and the better conductivity of composite as well as the in situ

vailable online 26 November 2009

eywords:upercapacitorolyanilinen situ doping/de-doping process

doping/de-doping process between the benzenesulfonic acid groups of f-MWCNTs and PANi chain. Inaddition, the composite also has better capacity and cyclability than PANi/p-MWCNTs composite. It couldattribute to the presence of f-MWCNTs, which makes more electrolyte contact with PANi to participatein faradaic redox reactions and dopes with the PANi polymer chain through the benzenesulfonic acidgroups to form stable polyemeraldine salts.

enzenesulfonic functionalizedulti-walled carbon nanotubes

. Introduction

As energy storage devices, electrochemical capacitors (ECs)ridge the gap between batteries and conventional dielectricapacitors due to the virtues of high capacitance storage, fastharge/discharge ability and long cycle-lives [1]. ECs store energyhrough either ion adsorption (electrical double layer capacitors,DLCs) or fast and reversible surface redox reactions (pseudocapac-tors). It is well known that the pseudocapacitor shows a superiornergy density to the EDLCs, but at the same time it has a relativelymaller power density, because however fast its redox kinetics, thearadaic process is slower than the nonfaradaic process. In addi-ion, the high cost of pseudocapacitive materials is also a hurdlehat must be overcome in practical applications. Hence, nanocom-osites combining pseudocapacitors and EDLCs with high surfacerea have been studied to achieve both high power and energyensity. Because the pseudocapacitive reaction is known to be an

nterfacial phenomenon, most studies have focused on the elec-

roactive materials deposited on the carbon nanotubes (CNTs) dueo its unique properties such as uniform diameters, high chem-cal stability, favorable electronic conductivity and large surfacerea [2,3]. When CNTs serve as the substrate for pseudocapacitive

∗ Corresponding author at: Nanjing University of Aeronautics and Astronautics,ollege of Material Science and Technology, Applied Chemistry Department, Yudaotreet 29, Nanjing, JIangsu 210016, PR China. Tel.: +86 025 52112902;ax: +86 025 52112626.

E-mail addresses: azhangxg@163.com, azhangxg@nuaa.edu.cn (X. Zhang).

013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2009.11.068

© 2009 Elsevier Ltd. All rights reserved.

materials coating, electrochemical performance is reinforced sincethey facilitate exposure of the increased surface area to electrolyteand improve electric conductivity due to contact with a conduc-tive substrate [4–6]. However, owing to the inherent strong vander Waals interactions and poor interface compatibility within thematrices, the CNTs almost form as large bundles and present intrin-sic chemical stability [7], which makes phase separation betweenthe active material and CNTs [8]. Therefore, it is necessary toprocess surface modification of the CNTs before practical applica-tion.

On the other hand, as a typical electrically conducting polymer(ECP), polyaniline (PANi) has attracted great interests in pseudoca-pacitors [9–13]. Generally, combination of PANi with CNTs basedon chemical interaction or grafting of polymer chains onto the sur-face of CNTs by covalent bonding [14] and formation of PANi inthe presence of CNTs (in situ polymerization) are two importantapproaches to preparing PANi/CNTs composites [15]. Recent stud-ies show that the latter method (in situ polymerization) is a goodapproach for synthesizing homogeneous polymer/CNTs nanocom-posites [16,17]. In this case, besides that functionalized groupshelp to disperse CNTs homogeneously in the reaction medium, themonomers can be adsorbed on the surface of f-MWNTs based onthe strong electron and hydrogen bonding interactions between

functionalized groups and the amino groups of monomer. Further-more, the covalent functionalized group on the surface of CNTscan dope into the polyaniline chain, which would avoid potentialmicroscopic phase separation in the nanocomposite and ensure thehigh electrochemical properties.

2 ica Ac

tbffht

2

2

frTsaMmwc(3nfwfwFP

2

utn(tUU0d(iTshao1cpm

2

8rsutfs

(the carbon–hydrogen bonds and the carbon–carbon bonds in thearomatic ring), respectively [25]. Therefore, all above results con-firm that benzenesulfonic group is successfully modified on thesurface of f-MWCNTs.

312 B. Gao et al. / Electrochim

Hence in this paper, a novel and facile method was reportedo prepare nanocomposite of PANi/f-MWCNTs. F-MWCNTs here isifunctional both for the dopant and self-assembly template. It isound that a thin film of PANi coated uniformly on the surface of the-MWCNTs. The electrochemical results show that PANi/f-MWCNTsave better rate response and cyclability as well as high capacityhan PANi/p-MWCNTs composite.

. Experimental

.1. Synthesis of PANi/f-MWCNTs composite

Multi-walled carbon nanotubes (MWCNTs) were purchasedrom Nanotech Port Co., Ltd. (Shenzhen China) and purified byefluxing them in nitric acid (HNO3) at 25 ◦C for 6 h before use.he purified MWCNTs were denoted as p-MWCNTs. The benzene-ulfonic functionalized MWCNTs (f-MWCNTs) were synthesizedccording to the literatures [18–20]. To prepare the PANi/f-WCNTs composite, a low-temperature in situ polymerizationethod was used. Briefly, 200 mg f-MWCNTs and 100 mg anilineere suspended by ultrasonication for 10 min in 200 ml ethanol

ontaining 0.2 M H2SO4, then 50 ml 0.1 M ammonium persulfate(NH4)2S2O8) solution (volume ratio of water versus ethanol is0:70) was added to above mixture solution with vigorous mag-etic stirring at 0 ◦C. Finally, after being static placed in refrigerator

or 24 h at 0 ◦C, the precipitate was centrifugal filtered and washedith deionized water and ethanol, followed dried under vacuum

or 24 h at 50 ◦C. The 25 wt.% of mass load of PANi in compositeas evaluated by calculating the weight difference of MWCNTs.

or comparison, PANi/p-MWCNTs composite with same content ofANi and pure PANi are synthesized via the same route.

.2. Material characterizations

The morphology of as-prepared samples was investigated bysing images which were taken on FEI TECNAI G2 20S-TWINransmission electron microscope (TEM) and LEO1430VP scan-ing electron microscope (SEM), respectively. Infrared spectrumIR) was recorded with a model 360 Nicolet AVATAR FT-IR spec-rophotometer. The spectra resolution of the IR system is 4 cm−1.ltraviolet–visible (UV–vis) spectra were performed on a Hitachi-3010 with a scanning speed at 200 nm min−1 and a bandwidth.1 nm. The samples were ground into a fine powder and thenissolved in N-methyl-2-pyrrolidone (NMP) to form a solution0.005 mg ml−1). Raman spectra were recorded with a RENISHAWnVia Raman system using an Argon ion laser operating at 514.5 nm.he laser beam was focused to a 0.10 mm diameter spot on theample with a laser power of 1 mW to avoid laser-induced localeating. The samples were pressed into a depression at the end of3 mm diameter stainless steel rod, held at a 30◦ angle in the pathf the laser beam. The spectra resolution of the Raman system iscm−1. The electric conductivity measurements are performed onompressed powder pellets of samples by the conventional four-robe DC method (SDY-5 Four-Point probe meter, China). All theeasurements were carried out at room temperature (RT).

.3. Preparation of electrode

The sample electrodes were fabricated by mixing and grinding0 wt.% sample with 15 wt.% carbon black and 5 wt.% polytetrafluo-oethylene (PTFE), then pressed onto a graphite substrate (graphite

upport was first abraded with ultrafine SiC paper, rinsed in anltrasonic bath of water for 15 min, then etched in 1 M H2SO4 solu-ion at RT for 10 min, and finally rinsed in an ultrasonic bath of wateror 30 min. The exposed geometric area of the treated graphiteupport is equal to 1 cm × 1 cm). The active material mass on an

ta 55 (2010) 2311–2318

electrode was 5 mg. All electrochemical measurements were doneby a three-electrode system, in which PANi/f-MWCNTs electrode,platinum foil and standard calomel electrode (SCE) were used asworking, counter and reference electrode, respectively. The elec-trolyte was 1 M H2SO4 solution. The cyclic voltammetry (CV) andgalvanostatic charge/discharge (GCD) test were performed by usinga CHI660B electrochemical workstation (Chenhua, China).

3. Results and discussion

First, we used Raman and IR analysis to testify the possibility offunctionalized MWCNTs with benzenesulfonic group. Fig. 1 showsthe Raman spectra of f-MWCNTs and p-MWCNTs. The typical fea-tures in the Raman spectra are the G band at 1580 cm−1 and theD band at 1350 cm−1. The G band is usually assigned to the E2gphonon of C sp2 atoms, while the D band is a breathing mode of �-point phonons of A1g symmetry [21]. The intensity ratio betweenthe D band and G band (ID/IG) is sensitive to the surface character ofMWCNTs, which is the index of graphitization degree of MWCNTs.For p-MWCNTs, the ID/IG ratio was calculated as 0.771. However,the value of ID/IG increased to 0.901 for f-MWCNTs, which sug-gests that a significant increase in the disordered sp3 state carbonafter the functionalized process, is consistent with a high degree offunctionalization [18]. This could be attributed to the presence of areactive aryl-SO3H radical derived from diazonium functionaliza-tion reaction, which reacts with the surface carbon of MWCNTs toform a MWCNTs-arene bond [18,22].

Fig. 2 shows IR spectra of f-MWCNTs and p-MWCNTs. In contrastwith f-MWCNTs, a weak absorption in the whole range shown bythe p-MWCNTs suggests the low functionalization degree. But for f-MWCNTs, a broad band at 3425 cm−1 is corresponding to the O–Hgroup’s stretch motion and retortion absorption [23]. The strongpeak at 1108 cm−1 are the character absorption peaks of sulfonategroup, which is assigned to the O S O group’s symmetric and anti-symmetric stretching motions [24]. The peaks at 585, 729, 811,1635 and 2923 cm−1 are attributed to in-plane, out-plane bendingvibration and stretching motion of aromatic C C and C–H groups

Fig. 1. Raman spectra of f-MWCNTs and p-MWCNTs.

B. Gao et al. / Electrochimica Acta 55 (2010) 2311–2318 2313

F–

arpmNPbsMwbobtii00

F

ig. 2. IR spectra of f-MWCNTs and p-MWCNTs (note: str – stretching motions; benbending motions).

Fig. 3 shows IR spectra of PANi/f-MWCNTs, PANi/p-MWCNTsnd PANi. The characteristic peaks around 1585 and 1498 cm−1 cor-espond to the quinoid ring and the benzene ring, respectively. Theeak located at 1296 cm−1 is the C–N stretching band of an aro-atic amine, but the characteristic band of polyaniline base is theQ N stretching band at 1141 cm−1 [26,27]. These indicate that

ANi was prepared in both composites. In addition, note that theands at 1108 cm−1 in Fig. 2, characteristic of the O S O group’stretching motion originated from the benzenesulfonic group of f-WCNTs, is blue-shifted to 1112 cm−1. This could ascribe to theithdrawing induction effect of quinoid units, which suggest the

enzenesulfonic groups from f-MWCNTs are linked on the nitrogenf PANi backbone via the same doping process into the PANi back-one, just as the normal dopant of Cl−. Furthermore, it is known thathe intensity ratio of the bands at 1585 and 1489 cm−1 can provide

nformation on the degree of oxidation of the polymer [28,29], thats, the fraction of quinoid and benzenoid units. For PANi this ratio is.624, which apparently change to 0.631 for PANi/f-MWCNTs and.645 for PANi/p-MWCNTs. This result could be attributed to the

ig. 3. IR spectra of PANi/f-MWCNTs, PANi/p-MWCNTs composites and PANi.

Fig. 4. UV–vis spectra of PANi/f-MWCNTs, PANi/p-MWCNTs composites and PANiin NMP.

site-selective interaction between the quinoid ring of the PANi andMWCNTs [30].

Fig. 4 reveals UV–vis spectra of PANi/f-MWCNTs, PANi/p-MWCNTs and PANi in NMP. For both PANi/f-MWCNTs andPANi/p-MWCNTs composites, two broad bands are seen with max-ima at 326 and 623 nm, which is essentially the same with thatfor PANi. These correspond to the �–�* transition centered on thebenzenoid unit of PANi and to the quinoid exciton band, respec-tively [28]. Furthermore, they show a new band with a maximum at262 nm. This can be assigned to the �–�* transition centered on thequinoid unit [27], which points to an increased number of quinoidring units and its existence is apparently related to the presenceof MWCNTs. However, this new band is already being seen to beweakly present in PANi/f-MWCNTs and consistent with the resultfrom IR analysis (see the intensity ratio of quinoid and benzenoidunits), which suggests a decreased number of quinoid ring unitsin polymer chain compared with that in PANi/p-MWCNTs. It couldbe attributed to the high degree of benzenesulfonic functionaliza-tion, which disrupts the graphene sheet structure. In this way, theplanar quinoid-imine ring units of the PANi backbone cannot comesufficiently close to the hexagonal lattice of the MWCNTs surfaceand therefore the efficient �–�* interaction cannot take place [27].

Fig. 5 depicts TEM and SEM images of the PANi/f-MWCNTs com-posite. It is easy to observe that a thin and uniform layer was coatedon the sidewall of MWCNTs. Taking into account the IR analysis, itcan be inferred that the active materials of coating layer is PANi.Furthermore, as shown in Fig. 5d, the PANi/f-MWCNTs compositemainly reveals as nanotubular morphology, reflecting a homoge-neous characteristic. For comparison, we also present TEM imageof the PANi/p-MWCNTs composite with the same PANi contentin Fig. 6. As expected, a drastic change in dispersity is observed.The aggregations of PANi and the bare p-MWCNTs randomly co-exist in a mixture when the same procedure is employed just usingp-MWCNTs instead of f-MWCNTs.

The formation mechanism of PANi/f-MWCNTs nanocompositesis believed to involve strong interaction between aniline monomerand f-MWCNTs, shown in Scheme 1. Wu et al. described the �–�*

electron and strong hydrogen bonding interactions between ani-line monomers and MWCNTs in the preparation of PANi/MWCNTscomposite [31]. In this case, it should be noted that the function-alization process of the acids mixture on MWCNTs occurs mainlyin open ends and defect sites with relatively small amount (such

2314 B. Gao et al. / Electrochimica Acta 55 (2010) 2311–2318

ages

aiiTaalet

Fig. 5. TEM (a–c) and SEM (d) im

s the inclusion of five- or seven-membered rings stem from thenitial formation of the tubes and local damaged framework dur-ng the treatment of purification in the carbon network [32–34]).herefore, the sidewalls of MWCNTs still presented intrinsic inertnd most of the MWCNTs remain randomly configured as bundles

lthough the solubility has been improved, which would make ani-ine monomer not being concentrated at the surface of p-MWCNTsffectively. Hence, PANi still mainly polymerized and grown in solu-ion, so that phase separation between PANi and p-MWCNTs is

Scheme 1. Illustration for the synthesis pro

of PANi/f-MWCNTs composite.

obtained, as observed in Fig. 6. However, as mentioned in Raman,IR and UV–vis spectra in our work, diazonium method affords ahigh degree of benzenesulfonic functionalization on the surfaceof MWCNTs, which suggests that the interactions between PANiand f-MWCNTs different from that in PANi/p-MWCNTs. In Fig. 3,

the characteristic peaks of the quinoid ring and the benzene ringof PANi in PANi/f-MWCNTs are downshifted to 1568 cm−1 evencompared with the peaks of PANi/p-MWCNTs (1578 cm−1); thepeaks of C–N and N Q N stretching bands are also red-shifted

cedure of PANi/f-MWCNTs composite.

B. Gao et al. / Electrochimica Acta 55 (2010) 2311–2318 2315

ciawPstsggadPCo

pPibtaaoaattacPtfpwiwto

Fig. 6. TEM images of PANi/p-MWCNTs composite.

learly. These spectral bathochromic shift phenomena of chem-cally synthesized nanocomposite can be put down to the largemount of benzenesulfonic groups on the surface of f-MWCNTs,hich interacts with the nitrogen atoms in the –NH– group of

ANi via –O–H· · ·N– and –O· · ·H–N– hydrogen bonds [31,35]. Suchtrong interaction ensures that the aniline monomer is preferen-ially adsorbed and aligned on the surface of f-MWNTs before the initu polymerization. It has been reported that benzenesulfonic acidroup-rich template can promote the order and extended conju-ation of the resulting PANi chains with limited branching [36]. Inddition, it is also reported that low temperature and sulfonic acidoping are conducive to the aniline monomer polymerizing intoANi chain with high molecular weight and low defects [37,38].onsequently, a uniform PANi thin layer is grown and finally coatedn the surface of f-MWCNTs during the polymerization.

The electrochemical performance of PANi/f-MWCNTs com-osites is measured by cyclic voltammetry. For comparison,ANi/p-MWCNTs synthesized with the same PANi content are alsonvestigated. As shown in Fig. 7, the CV characteristic curves ofoth electrodes, different from ideal rectangular shape of the elec-ric double layer capacitance, consist of two redox couples (C1/A1nd C2/A2) and capacitive current produced from the phase ofctive materials. Peak C1/A1 is attributed to the redox reactionf PANi between a semiconducting state (leucoemeraldine form)nd a conducting sate (polaronic emeraldine form). Peak C2/A2 isssigned to the redox reaction of degradation products [39]. In addi-ion, as one can see, PANi/f-MWCNTs show better rate responseshan PANi/p-MWCNTs. We believe that this improvement could bettributed to the uniform morphology and better electrochemicalonductivity (9 S cm−1 for PANi/f-MWCNTs is higher than those ofANi/p-MWCNTs (5 S cm−1) and PANi (2 S cm−1)). It is also reportedhat the benzenesulfonic groups decorated onto f-MWCNTs sur-aces would be helpful to form better channel-like network forroton transportation [40]. Hence the PANi/f-MWCNTs compositeill be randomly entangled and cross-linked when it is palletized

n a paste electrode, forming a three-dimensional porous networkith good proton and electron conductivity, which would facili-

ate electrolyte ion (H+) and electron transport into the inner spacef electrode. Furthermore, the possible rapid in situ doping/de-

Fig. 7. Cyclic voltammetry of PANi/f-MWCNTs (a) and PANi/p-MWCNTs (b) elec-trodes within the potential window −0.2 to 0.8 V (vs. SCE) at different scan rates in1 M H2SO4.

doping process between the benzenesulfonic groups on the surfaceof f-MWCNTs and PANi (displayed in Scheme 2) may also make cer-tain contribution to the rate response improvement. In this way, aportion of the required acid-doped proton can be directly derivedfrom sulfonic acid group rather than diffused from the electrolytesolution. However, for the PANi/p-MWCNTs composite, due to thephase separation between PANi and p-MWCNTs [8,41], the aggre-gations of PANi (see Fig. 6) would block some pores that can beserving as ion diffusion path, thus it is rather difficult for solutionion to diffuse within inner space of PANi/p-MWCNTs [42], whichwould cause a large equivalent series resistance (ESR). Therefore,the rate response of PANi/f-MWCNTs is better than that of PANi/p-MWCNTs.

Fig. 8 shows the galvanostatic charge/discharge curves of thePANi/f-MWCNTs and PANi/p-MWCNTs electrodes with differentcurrent densities. The curves shape of the two composites does not

present the characteristic of ideal capacitor (enantiomorphous tri-angular shape), but in agreement with the result of the CV curves.In the experiment, we found that f-MWCNTs and p-MWCNTs onlyshow 20 and 14 Fg−1, which contribute to no more than 10% of the

2316 B. Gao et al. / Electrochimica Acta 55 (2010) 2311–2318

Sf

tPfaa

C

C

w�i(tvMTcet2Pactnst

cheme 2. The possible in situ doping/de-doping process between the benzenesul-onic acid groups of f-MWCNTs and PANi chain.

otal capacitance of the composite, indicating that the capacity ofANi/f-MWCNTs and PANi/p-MWCNTs composites mainly resultsrom PANi matrix. The specific capacitance of the composites (Cm)nd the utilization of PANi in the composites (Cm,PANi) are calculateds Eqs. (1) and (2) [42], respectively:

m =(

I × �t

�V × m

)(1)

m,RuO2 = (Cm − (1 − ωPANi)Cm,MWCNTs)ωPANi

(2)

here I is the discharge current, m is the mass of the composite,V is the potential drop, �t is the total discharge time, Cm,MWCNTs

s the specific capacitance of f-MWCNTs (20 F g−1) or p-MWCNTs14 F g−1), and ωPANi are the wt.% of PANi in composites, respec-ively. Based on Eqs. (1) and (2), the calculated specific capacitancealues and the utilization of PANi in PANi/p-MWCNTs and PANi/f-WCNTs composites at different current densities are listed in

able 1. It is notable that, as the introducing of f-MWCNTs, the spe-ific capacitances of PANi/f-MWCNTs and utilizations of PANi arenhanced compared with PANi/p-MWCNTs electrode. For example,he capacity of PANi/f-MWCNTs electrode at 0.25 and 1.0 A g−1 is38.1 and 192.4 F g−1, respectively, which is twice as much as that ofANi/p-MWCNTs electrode (114.1 F g−1 at 0.25 A g−1 and 71.3 F g−1

t 1.0 A g−1), indicating the higher utilization of PANi and better

apacity of the composite. The enhanced capacity could attributeo the uniform morphology of PANi/p-MWCNTs composite withanometer size, which maintains the three-dimensional poroustructure for the electrolyte ions to diffuse and contact the elec-roactive PANi within the inner space of the electrode. Thus more

Fig. 8. The charge/discharge curves of PANi/f-MWCNTs (a) and PANi/p-MWCNTs (b)electrodes within the potential window −0.2 to 0.8 V (vs. SCE) at different currentdensities in 1 M H2SO4.

PANi will participate in the electrochemical reaction, resulting intoa higher capacity.

Fig. 9 shows dependence of specific capacitance on dischargecurrent density for PANi/p-MWCNTs and PANi/f-MWCNTs elec-trodes. PANi/f-MWCNTs electrode shows a better rate performancethan PANi/p-MWCNTs, although the specific capacitances decreasegradually as current density increases. For PANi/f-MWCNTs elec-trode, the specific capacitance at a current density of 2.0 A g−1 dropsabout 33% compare with that at 0.25 A g−1, whereas the capacitanceof PANi/p-MWCNTs electrode drops 52% under similar conditions.Possible reasons can be attributed to the uniform structure and thebetter proton and electron conductivity of composite as well as therapid in situ doping/de-doping process mentioned above, whichimprove the ESR of PANi/f-MWCNTs, and therefore diminish theohmic drop at high discharging current density [43].

The cyclability of PANi/f-MWCNTs and PANi/p-MWCNTs elec-trodes was monitored by a chronopotentiometry measured at0.25 A g−1, as shown in Fig. 10. Both electrodes show an obvi-

ous decrease in the specific capacitance during first 100 cycles.Subsequently the PANi/p-MWCNTs electrode still exhibits a slightfading, whereas PANi/f-MWCNTs electrode reveals a stable spe-cific capacitance. The decrease in capacity could be mainly ascribedto the degradation of PANi chain during doping/de-doping pro-

B. Gao et al. / Electrochimica Acta 55 (2010) 2311–2318 2317

Table 1The specific capacitances (Cm) of PANi/f-MWCNTs and PANi/p-MWCNTs composites and utilization (Cm,PANi) of PANi in composites.

Sample 0.25 A g−1 0.5 A g−1 0.75 A g−1 1.0 A g−1

Cm (F g−1) Cm,PANi (F g−1) Cm (F g−1) Cm,PANi (

PANi/f-MWCNTs 238.1 892.4 217.9 811.6PANi/p-MWCNTs 114.1 414.4 94.8 337.2

FP

coFw(ttMawsrif

Fc

ig. 9. Specific capacitance and capacitance retention ratio of PANi/f-MWCNTs andANi/p-MWCNTs electrodes as a function of current density.

esses, which would break polymer chains and generate solubleligomers that results in a significant loss of polymer mass [44–46].urthermore, it is also reported that O2 existing in electrolyteould transform the semiconducting state into an oxidized one

pernigraniline) [26,29,47]. This would also lead to the attenua-ion capacity of PANi. However, in PANi/f-MWCNTs, as a result ofhe existence of a large number of benzenesulfonic acid groups, f-

WCNTs can serve as a large molecular dopant which is integratednd essentially locked to the PANi chains. This polyemeraldine saltith the polyanion is extremely stable and once formed ensures

tability of the desired electrochemical properties. In addition,eports indicate that the functionalized groups also contribute tonhibit the diffusion of O2 through the polymer chain [26]. There-ore, the PANi/f-MWCNTs electrode has better cyclability.

ig. 10. Cycle-life of PANi/f-MWCNTs and PANi/p-MWCNTs electrodes at a constanturrent density of 0.25 A g−1.

[[[[[[

[[[[[[[[[[

[[

[

[

F g−1) Cm (F g−1) Cm,PANi (F g−1) Cm (F g−1) Cm,PAN (F g−1)

204.9 759.6 192.4 709.680.4 279.6 71.3 243.2

4. Conclusions

In summary, a low-temperature in situ polymerization methodto synthesize the nanocomposite of PANi/f-MWCNTs is described,which is simple and facile. The TEM, SEM and IR results show thatPANi/f-MWCNTs have a thin film of PANi coating uniformly on thesurface of the f-MWCNTs. The electrochemical results show thatPANi/f-MWCNTs have good rate response. Such good rate capabil-ity could be ascribed to the uniform morphology and the betterelectron and proton conductivity of composite as well as the insitu doping/de-doping process between the benzenesulfonic acidgroups of f-MWCNTs and PANi chain. In addition, the compositealso has better capacity and cyclability. It could be attributed to thef-MWCNTs, which makes more PANi contact with the electrolyteto participate in faradaic redox reactions and dopes with the PANipolymer chain through the benzenesulfonic acid groups to formstable polyemeraldine salts.

Acknowledgements

This work was supported by National Basic Research Programof China (973 Program) (No. 2007CB209703), National NaturalScience Foundation of China (No. 20403014, No. 20633040, No.20873064) and Graduate Innovation Plan of Nanjing University ofAeronautics and Astronautics (BCXJ08-08).

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