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Electrochimica Acta 88 (2013) 526– 529

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al h om epa ge: www.elsev ier .com/ locate /e lec tac ta

reparation and electrochemical properties of the ternary nanocomposite ofolyaniline/activated carbon/TiO2 nanowires for supercapacitors

iangqiang Tan ∗, Yuxing Xu, Jun Yang, Linlin Qiu, Yun Chen, Xiaoxiao Chentate Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

r t i c l e i n f o

rticle history:eceived 9 October 2012eceived in revised form 26 October 2012ccepted 26 October 2012vailable online 2 November 2012

eywords:anocompositeolyaniline

a b s t r a c t

We herein report the synthesis of ternary nanocomposites consisting of polyaniline (PANI), activatedcarbon, and TiO2(B) components, which involves the preparation of activated carbon/TiO2(B) nanowires(ACTB) using sonochemical–hydrothermal method, and their subsequent composites with PANI via in situpolymerization. The morphology and structure of ACTB/PANI ternary nanocomposites are character-ized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transforminfrared spectra (FTIR) and X-ray diffraction (XRD). Morphology analysis shows that the porous networklayer of PANI homogeneously coated on the outer surface of ACTB support. The electrochemical proper-ties of the ternary nanocomposite as the electrode material for electrochemical capacitors are examined

iO2(B) nanowiresolymerizationupercapacitor

by cyclic voltammetry and galvanostatic charge/discharge test in an organic electrolyte (1.0 M LiClO4 inpropylene carbonate). The results show that the ternary nanocomposites have a specific capacitance aslarge as 286 F g−1 in the potential range from −3 to 3 V (vs. SCE) at a charge–discharge current densityof 1.0 A g−1, which is a significant improvement compared to those of the three separate components,demonstrating that the ACTB/PANI nanocomposites are promising materials for supercapacitor electrode.

© 2012 Elsevier Ltd. All rights reserved.

. Introduction

Supercapacitors have attracted considerable attention over theast decades because they can combine the advantages of the highower density of conventional capacitors with the high energyensity of rechargeable batteries [1–3]. The performances of super-apacitors are primarily determined by the electrode materials [4].he active materials of electric double-layer capacitors (EDLCs) andseudo-capacitors including conducting polymers, carbon mate-ials and transition metal oxides have their own advantages andisadvantages. Carbon materials have high rate capability and goodlectrical conductivity, long life-cycles, but low specific capacitance5–9]. On the other hand, conducting polymers are cost-effective,exible, but poor in cyclability [10,11]. Among the conductingolymers, polyaniline (PANI) is considered as one of the mostromising candidates for the application of supercapacitor elec-rode due to its ease of synthesis, electrochemical reversibility and

igh capacitance [12–15]. The composite materials of PANI andarbon materials have attracted dramatic attention as they can

∗ Corresponding author. Tel.: +86 10 82545008; fax: +86 10 82545008.E-mail address: qtan@mail.ipe.ac.cn (Q. Tan).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.10.126

integrate the advantages of their separate components and show ahigher capacitance and better rate capability [16,17].

Nanostructured electrode materials have garnered sustainedresearch interest due to their more favourable rates and capac-itances than those of traditional materials. The inorganic 1Dnanomaterials have been studied widely as the electrode mate-rials for energy-storage devices owing to their size/dimensionaldependent properties. Among those inorganic nanomaterials, TiO2nanowires are seemed to be extremely attractive for hybrid-supercapacitor applications because of their excellent physicaland chemical properties [18,19]. In addition, the modification ofactivated carbon by TiO2 nanowires could decrease the ion con-centration in the double-layer of activated carbon since the chargeson the surface of TiO2 are higher than those of the other regions,resulting in the reduction of the polarization of activated carbon[20].

Herein, we reported a facile process for the design and fabri-cation of the activated carbon/TiO2(B) nanowires (ACTB), whichwas used subsequently as the supporting materials for the prepa-ration of ternary nanocomposites of ACTB/PANI using an in situ

polymerization method. The introduction of TiO2(B) nanowirescould greatly improve the long-term cycle stability of ACTB/PANIcomposite. The morphologies, microstructures and electrochemi-cal performances of the resulting products were investigated and

mica Acta 88 (2013) 526– 529 527

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coordination compound with the nitrogen atom in PANI [26].Fig. 3 shows the TEM and SEM images of as-prepared sam-

ples. As seen in Fig. 3b, TiO2(B) nanowires were well distributed

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he effect of TiO2(B) nanowires on the electrochemical performanceas also studied.

. Experimental

.1. Preparation of ACTB nanowires

The ACTB nanowires were prepared byonochemical–hydrothermal method. Firstly, the granular tita-ium dioxide powders were mixed with activated carbon with anverage surface area of 1690 m2/g in mass ratio of 8:100 in 70 mL of0 M aqueous NaOH solution, followed by vigorous ultrasonicationor 2 h. Then, the hydrothermal reaction was carried out in a sealedeflon-lined autoclave at 160 ◦C for 24 h. After being cooled downo room temperature, the suspension was neutralized with 0.2 Mitric acid to neutral or weak acidic state and then filtered toollect the products. The products thus obtained were washedith deionized water and anhydrous ethanol several times,

espectively, and then dried at 80 ◦C under vacuum. The productsbtained above were heated in air at 400 ◦C for another 2 h to giveise to ACTB nanowires. Subsequently, 4.2 g of ACTB and 4.5 mL ofniline were added into 100 mL of distilled water, and the mixtureas then stirred for sufficient dispersion. After adequately mixing

nd stirring the solution, 50 mL of 0.25 M aqueous ammonium per-xydisulfate solution (APS) was added dropwise as an oxidant toolymerize the aniline monomer. The polymerization process wasarried out at 0–5 ◦C for 16 h under stirring. Finally, the obtainedowders were collected by filtration and washed with deionizedater and anhydrous ethanol several times and then dried in a

acuum oven at 60 ◦C for 24 h. The morphologies and structures ofhe resulting nanocomposites were characterized by X-ray diffrac-ion (XRD, D/Max-RB, Cu K�), transmission electron microscopyTEM, JEOL JEM-2100CX) and scanning electron microscopy (SEM,EOL JSM-6301F). Fourier transformation infrared (FTIR) spectra ofhe samples were recorded on a Bruker EQUINOX55 spectrometern the transmission mode.

.2. Fabrication of the working electrodes

The fabrication of the working electrodes was carried outs follows: Briefly, the electroactive materials, carbon black andolyvinylidene fluoride (PVDF) were mixed in a mass ratio of5:10:5 and dispersed in N-methylpyrrolidone (NMP). The result-

ng composite was stirred mechanically for 6 h to form a stickylurry. The resulting slurry was pasted onto a clean Al foil, fol-owed by drying at 80 ◦C under vacuum for 24 h. The constantrea of the electrodes used in this study was 1 cm2 (1 cm × 1 cm)nd the electrolyte was organic electrolyte (1.0 M LiClO4 in propyl-ne carbonate). The capacitance properties were studied by cyclicoltammetry (CV) and galvanostatic charge/discharge tests. Cyclicoltammetry measurement was performed between −3 and 3.0 Vsing CHI-604B model electrochemical working station (Shanghai,hina). The galvanostatic charge/discharge performance of thelectrodes was evaluated using a Land-2001A battery workstationystem (Wuhan Jinnuo Company) within the potential range of −3o 3.0 V.

. Results and discussion

The XRD patterns of TiO2(B) nanowires, ACTB and ACTB/PANIanocomposites are shown in Fig. 1. The phase structure of the

roduct in Fig. 1a could be identified as TiO2(B) (JCPDS 74-1940)ince all the diffraction peaks are consistent with TiO2(B) reportedn the literature [19,21]. In Fig. 1c, the diffraction peaks of the sam-le could be assigned to ACTB and PANI, confirming the formation of

Fig. 1. XRD patterns of TiO2(B) (a), ACTB (b), and ACTB/PANI (c), respectively.

ACTB/PANI in the ternary nanocomposite. The crystalline peaks at2� = 20.5◦ and 25◦ respectively correspond to the (0 2 0) and (2 0 0)planes of PANI in its emeraldine salt form [22]. In addition, thediffraction peaks of TiO2(B) are observed both in Fig. 1b and c, indi-cating the presence of TiO2(B) in ACTB nanowires and ACTB/PANInanocomposites.

Fig. 2a and b shows the FTIR spectra of PANI and ACTB/PANInanocomposites, respectively. The characteristic peaks of PANI(Fig. 2a) were assigned as follows: the characteristic peaks at 1562and 1486 cm−1 can be ascribed to C N and C C stretching mode forthe quinoid and benzenoid rings, which indicate the oxidized stateof the emaraline salt of PANI [23]. The peaks at 1300 and 1245 cm−1

are corresponding to C N stretching mode for the benzenoid ring.While the peak at 1131 cm−1, which is the characteristic of PANIconductivity and the degree of delocalization of electrons, wasattributed to the inplane bending vibration of C H [24,25]. Fig. 2bindicates that the main characteristic peaks of PANI are visible inFTIR spectra of ACTB/PANI composite. The presence of ACTB parti-cles leads to the shift of some peaks and changes of relative intensityin Fig. 2a. In particular, the peak at 1562 cm−1 shifted to higherwavenumber of 1574 cm−1, could be assigned to the stretchingmode of C N. In addition, the peak associating with the dopingof PANI also shifts from 1131 to 1139 cm−1. These obvious changessuggested the existence of an interaction between ACTB nanowiresand PANI since the transition metal titanium has a tendency to form

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Fig. 2. FTIR spectra of PANI (a) and ACTB/PANI nanocomposite (b), respectively.

528 Q. Tan et al. / Electrochimica Acta 88 (2013) 526– 529

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trode (Fig. 5b), the initial specific capacitance is 292 F g−1 and the

Fig. 3. TEM image of TiO2(B) nanowire (a); SEM images of

mong the activated carbon particles. The length and the diameterf TiO2(B) nanowires were approximate 1–3 �m and 100–200 nm,espectively, which were confirmed by TEM observation in Fig. 3a.n Fig. 3c and d, the morphology of the ternary nanocompositeeemed to be a nanometer-scale, porous network layer homoge-eously coating on the outer surface of the ACTB due to the highhemical activity and surface area of ACTB. The activated carbonas used as the main supporting material for the deposition of PANIarticles and TiO2(B) nanowires provided enhanced wires intercon-ected between activated carbon and PANI particles, which woulde beneficial to further improve the mechanical strength of theomposite.

The CVs (within the potential window from −3 to 3 V vs. a satu-

ated calomel electrode (SCE) at scan rate of 20 mV s−1) of ACTB andCTB/PANI composites are shown in Fig. 4. Fig. 4a shows that theV of ACTB nanowires is similar to that of activated carbon, whichroduces a curve close to the ideal rectangular shape. The shape

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(b) and ACTB/PANI nanocomposites (c and d), respectively.

of CV curves in Fig. 4b indicates that the capacitance characteristicof ACTB/PANI composites is distinct from the electric double-layercapacitance of ACTB. The two pairs of redox and reduction peaksobserved in Fig. 4b, which would be caused by the chemical state ofPANI, resulted in the redox capacitance and indicated the pseudo-capacitance besides the electric double layer capacitance.

The variation of specific capacitance with cycle number for acti-vated carbon/PANI(AC/PANI) and ACTB/PANI capacitors is given inFig. 5. The result indicates that ACTB/PANI composite electrode(Fig. 5a) has the specific capacitance at approximate 286 F g−1 at thefirst cycle, which could be remained at approximate 230 F g−1 after2000 cycles (above 80% of the original value). As for the ACTB elec-

capacitance declined dramatically to 124 F g−1 after 2000 cycles.This result indicated that the ACTB/PANI nanocomposite electrodepossesses long-term cycle stability over 2000 charge–discharge

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ests. This is mainly because of the presence of TiO2(B) nanowiresith good mechanical intensity, which are very advantageous to

educe the electrochemical degradation of AC/PANI composite andmprove its cycle stability as electrode material [27]. Furthermore,he network structure layer of PANI with nano-sized pores cre-tes electrochemical accessibility for electrolyte ions during theharging or discharging process, which is a significant effect innhancement of capacitance. In addition, the activated carbon withigh surface area has been widely used as a support for superca-acitor and battery electrode materials [28–33].

. Conclusion

In this paper, the ternary ACTB/PANI nanocomposites were syn-hesized via in situ polymerization. The initial specific capacitancef ACTB/PANI composite electrode was 286 F g−1, which could beemained at approximate 230 F g−1 after 2000 cycles (above 80% ofhe original value compared to 42% for AC/PANI electrode), demon-trating that ACTB/PANI electrode had excellent cycle stability. Theernary ACTB/PANI nanocomposites, which integrated the mechan-cal strength of TiO2(B) nanowires, highly porous network structuref PANI, and high surface area of activated carbon, displayedemarkably improved electrochemical performances. Therefore,e believe that the ACTB/PANI composites can integrate the advan-

ages of its three separate components and will be a potentialow-cost candidate for supercapacitor.

cknowledgment

This work supported by the Knowledge Innovation Program ofhe Chinese Academy of Sciences (Grant No. KGCX2-YW-341)

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