ARTICLE Electrochemical Behavior and Speci c Capacitance

CHINESE JOURNAL OF CHEMICAL PHYSICS VOLUME 27, NUMBER 6 DECEMBER 27, 2014

ARTICLE

Electrochemical Behavior and Specific Capacitance of Polyaniline/SilverNanoparticle/Multi-walled Carbon Nanotube Composites

Jia Li, Di Zhang, Jin-bao Guo, Jie Wei∗

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029,China

(Dated: Received on March 17, 2014; Accepted on October 31, 2014)

In this work, we fabricated the polyaniline/silver nanoparticle/multi-walled carbon nan-otube (PANI/Ag/MWCNT) composites by in situ polymerization of aniline on the wall ofAg/MWCNTs with different aniline to Ag/MWCNT mass ratios. The chemical structureof the ternary composites was characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Scanning electron microscope andhigh-resolution transmission electron microscopy were used to observe the morphology ofthe ternary composites. The results showed that the polyaniline PANI layer was preparedsuccessfully and it covered Ag/MWCNTs completely. In addition, Ag nanoparticles betweenthe MWCNT core and the PANI layer existed in the form of elemental crystal, which couldcontribute to the electrochemical performance of the composites. Then we prepared thecomposite electrodes and studied their electrochemical behaviors in 1 mol/L KOH. It wasfound that these composite electrodes had very low impedance, and exhibited lower resis-tance, higher electrochemical activity, and better cyclic stability compared with pure PANIelectrode. Particularly, when the mass ratio of aniline to Ag/MWCNTs was 5:5, the compos-ite electrode displayed a small equivalent series resistance (0.23 Ω) and low interfacial chargetransfer resistance (<0.25 Ω), as well as 160 F/g of the maximum specific capacitance at acurrent density of 0.25 A/g in KOH solution. We could conclude that the composite materialhad potential applications as cathode materials for lithium batteries and supercapacitors.

Key words: Carbon nanotubes, Silver nanoparticles, Polyaniline, Electrochemical behav-iors

I. INTRODUCTION

Electrochemical capacitor (EC), known as a superca-pacitor or a ultracapacitor, is considered to be one ofthe high-power systems because of its pulse high powersupply, long cycle life (>105 cycles), simple operationalmechanism, and high dynamics of charge propagation[1–3]. In the past decade, EC technology has experi-enced an impressive growth in terms of the increase inperformance owing to the discovery of new electrodematerials, including carbonaceous materials [4–6], con-ducting polymers (CPs) [7–11], etc.

Carbonaceous materials, given their favorable me-chanical strength, chemical stability, and electricalproperties, are of particular interest for ECs. Among allthe carbonaceous compounds, carbon nanotube (CNTs)have the highest frequency response with a “knee fre-quency” (where, on a Bode plot, there is a sharpchange in slope) greater than 100 Hz, as opposed tothe 1−10 Hz of most commercially available capacitors

∗Author to whom correspondence should be addressed. E-mail:[email protected], FAX: +86-10-64454598

[12]. Meanwhile, CPs with the π-conjugated structures,have been extensively investigated as active electrodematerials in energy storage systems in the past twodecades. Polyaniline (PANI) is one of the most promis-ing CP materials due to its simple preparation process,low cost, chemical stability, and high conductivity [13].Furthermore, it’s worth mentioning that the conduc-tivity of PANI can be reversibly controlled by simpledoping/dedoping with acids/bases [14].

Nowadays, composites of CNTs and PANI have beendiscussed thoroughly and systematically [15–17]. Thehybrid of CNTs and PANI is reported to show syner-gistic effects that combine the advantages of both mate-rials: PANI provides superior pseudocapacitance, whileCNTs act as a framework that helps PANI to sustainfrom the strains in charging/discharging cycling pro-cess [18]. To further increase the pathways for electrontransfer to achieve quick charging and discharging, onestrategy is decorating metal nanoparticles on the sur-faces of CNTs. The attempt was made by Kim et al.,that after Ag nanoparticles were introduced into thebinary carbonaceous materials and CP composites, theternary composites showed remarkably increased cur-rent, lower resistivity, quicker response, and better spe-

DOI:10.1063/1674-0068/27/06/718-724 718 c⃝2014 Chinese Physical Society

Chin. J. Chem. Phys., Vol. 27, No. 6 Polyaniline/Ag/Multi-walled Carbon Nanotube Composites 719

cific capacitance. This reason could be due to the bridgeeffect of Ag between carbonaceous materials and CPs[19].

In the present work, we firstly prepared the ternarycomposites of PANI, silver nanoparticles, and multi-walled carbon nanotubes (PANI/Ag/MWCNTs) withdifferent aniline to Ag/MWCNT mass ratios by in situpolymerization of aniline on the wall of Ag/MWCNTs,and then investigated the electrochemical performanceof the ternary composites in alkaline electrolyte solu-tion by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spec-troscopy (EIS). The composite electrodes exhibitedquite low resistance, high electrochemical activity, aswell as good cyclic stability, and were expected to showpotential applications as cathode materials for ECs.

II. EXPERIMENTS

A. Materials

MWCNTs (diameter of 60−100 nm, length of5−15 µm, purity of 95%), synthesized by the chemicalvapor deposition method, were purchased from Shen-zhen Nanotechnologies Co. Ltd. Aniline, ethylene gly-col (EG), polyvinyl pyrrolidone (PVP, K30, Mr=104),sodium lauryl benzenesulfate (SDBS), AgNO3, H2SO4,HNO3, NH3·H2O, ammonium persulphate (APS), KOHand Na2SO4 were all analytical grade and obtained fromSinopharm Chemical Reagent Beijing Co., Ltd. All thereagents were used as received.

B. Preparation of Ag/MWCNT composites

Ag/MWCNT composites were prepared according tothe previously reported procedure [20, 21]. MWC-NTs were firstly modified as follows: 0.25 g of MWC-NTs was dispersed in 40 mL strong acid mixture(H2SO4:HNO3=3:1, volume ratio) at room tempera-ture. After ultrasonication for 2−3 h, the mixture wasfiltered and washed with deionized water three timesthrough Millipore membrane, and then air-dried to gen-erate the modified MWCNTs. Next, 10 mg of the mod-ified MWCNTs were dispersed in 15 mL of EG andultrasonicated for 15 min. Simultaneously, into another15 mL of EG, 63 mg of AgNO3 were added, followed byPVP as stabilizer and SDBS as surfactant with the massratio of PVP:SDBS:Ag being 1:1:1. Afterwards, the twoprepared EG solutions were mixed together by ultrason-ication for another 3 h at room temperature, washedwith acetone and deionized water for three times, andthen filter through Millipore membrane to finally gen-erate the Ag/MWCNT composites.

C. Preparation of PANI/Ag/MWCNT composites

The homogenous PANI/Ag/MWCNT compositeswere prepared by in situ polymerization of aniline on

the wall of Ag/MWCNT composites. And the mass ra-tio of aniline to Ag/MWCNT as varied from 9:1, 8:2,7:3, 6:4 to 5:5. When the mass ratio was 6:4, the electro-chemical properties of PANI/Ag/MWCNT compositeselectrode were similar to that of the pure Ag/MWCNTcomposite electrodes. And when it dropped below 6:4,the Ag/MWCNTs could not be covered completely,that is why the minimum mass ratio we prepared was5:5. In a typical run, take the mass ratio being 6:4 forexample, the synthesis procedure was as follows: 60 mgof aniline monomers and 0.4 mL of HNO3 (68wt%) wereadded into 10 mL of deionized water. The 40 mg of theas-prepared Ag/MWCNT composites were added intoanother 10 mL of deionized water containing 0.1 mol/Lof SDBS, and then ultrasonicated for 30 min to sepa-rate the Ag/MWCNT composites. Following that, theaniline solution was added to the Ag/MWCNT solu-tion under vigorous magnetic stirring at 0 C. Subse-quently, 10 mL APS aqueous solution was added drop-wise with 1:1 mole ratio of APS to the aniline monomer.Then the mixture was kept to react for another 8 hunder constant stirring at 0 C. The resulting prod-uct was washed with deionized water and methanoluntil the filtrate was colorless. Then the residue wasdedoped by immersing in 3wt% NH3·H2O solution atroom temperature with stirring for 12 h. After fil-tration, the dedoped product was dried in vacuum for24 h to yield the PANI/Ag/MWCNT composites. Forcomparison, pure PANI specimen was also synthesizedthrough the above mentioned method without the pres-ence of Ag/MWCNTs.

D. Characterization

The morphology and surface chemistry were char-acterized by scanning electron microscopy (SEM, Hi-tachi S-4700, Japan), high-resolution transmission elec-tron microscopy (HRTEM, JEOL TEM-3010, Japan),and X-ray photoelectron spectroscopy (Thermo VG ES-CALAB XPS-250, UK). The structure was analyzed byFourier transform infrared (FT-IR, PerkinElmer Spec-trum RX1, America). And the crystal lattice was deter-mined by X-ray diffraction (XRD, Rigaku D/Max2500VB2+/pc, Japan).

E. Preparation of working electrode and electrochemicalmeasurements

Working electrodes were prepared by mixing 85wt%as-prepared composites with 10wt% acetylene black and5wt% poly(tetrafluoroethylene). The three constituentswere firstly dispersed in ethanol to form a black paste,then coated onto a nickel foam substrate (1 cm2), andfinally dried in vacuum oven at 80 C for 24 h. By doingthis, each working electrode could generally hold 4 mgof as-prepared materials and 1 cm2 of surface area.

All electrochemical experiments were conducted ina three-electrode setup: the as-prepared working elec-

DOI:10.1063/1674-0068/27/06/718-724 c⃝2014 Chinese Physical Society

720 Chin. J. Chem. Phys., Vol. 27, No. 6 Jia Li et al.

FIG. 1 FT-IR spectra of PANI/Ag/MWCNT compositeswith different aniline to Ag/MWCNT mass ratios and purePANI.

trode, a graphite electrode as a counter electrode, and aHg/HgO as a reference electrode. Meanwhile, the mea-surements were carried out in a 1 mol/L KOH aqueouselectrolyte at room temperature. CV curves were car-ried out in a potential range from −0.7 V to +0.3 V atdifferent scan rates of 2, 5, 10, 20, 50 mV/s with CS350 electrochemical workstation. EIS was performedwith the frequency ranging from 100 kHz to 0.01 Hz atthe open potential. GCD curves were measured in apotential range from −0.7 V to +0.3 V at different cur-rent densities of 0.25, 0.50, 1.25, 2.50 A/g by computercontrolled cycling equipment (Corrtest C350, WuhanChina).

III. RESULTS AND DISCUSSION

A. FT-IR characterization

The FT-IR spectra of PANI/Ag/MWCNT compos-ites with different mass ratios of aniline to Ag/MWCNTand pure PANI are shown in Fig.1. From the spec-trum of pure PANI, the peak at 1590 cm−1 was at-tributed to the stretching mode of quiniod ring, thepeak at 1497 cm−1 was from the stretching mode ofbenzenoid ring, the peak at 1307 cm−1 was associ-ated with C−N stretching vibration, and the peaksat 1165 and 830 cm−1 corresponded to the plane-bending and out-of-plane bending vibration of C−H,respectively. These characteristic peaks demonstratedthe successful polymerization of aniline. After intro-duction of the Ag/MWCNT composites, as it can beseen, the entire spectrum was similar to that of thepure PANI, although the relevant characteristic peaksweakened in varying degrees, indicating that anilinemonomers had also successfully polymerized in theternary PANI/Ag/MWCNT composite system.

B. XPS characterization

The surface chemistry of the PANI/Ag/MWCNTcomposites was further investigated using XPS spec-

tra. Take the composites with the mass ratio of ani-line to Ag/MWCNT at 5:5 for example, the wide-scanspectrum in Fig.2(a) indicated the presence of N, C,and Ag elements in the composites. And the highbinding energy peaks at 530 eV of O1s was mainlydue to the adsorption of oxygen in the air. Further-more, the XPS spectra of N, C, and Ag elements ex-hibited their specific existence forms. N1s spectrum(Fig.2(b)) showed that the most nitrogen centered at399.9 eV belonged to amine (-NH-) in benzenoid amineor amide groups, two small peaks centered at 398.3and 401.1 eV were attributed to imine groups (=NH-)and positively charged nitrogen groups (N+), respec-tively, suggesting successful polymerization of aniline[22]. Moreover, the area fractions were 63.97%(-NH-),24.86%(=NH-), 13.03%(N+), respectively, by calcula-tion. In C1s spectrum as shown in Fig.2(c), C1s couldbe deconvoluted to four peaks centered at 248.5 eV (sp2-hybridized carbon), 285.0 eV (sp3-hybridized carbon),285.8 eV (C−N), and 286.5 eV (C=N) with area frac-tions of 38.21%, 25.63%, 19.99%, and 16.16%, respec-tively. The area fraction ratio of -NH- to =NH- in N1sspectrum was 63.97/24.86, it was obviously larger thanthe area fraction ratio 19.99/16.16 of C−N to C=N inC1s spectrum. The difference could be explained byother surface functional groups on the wall of MWCNT,such as C−O (286.6 eV), which was so close to C=N(286.5 eV) [23]. The XPS spectra of N1s and C1s in-dicated successful polymerization of PANI on the sur-face of Ag/MWCNTs, which was consistent with theforegoing FT-IR analysis. In addition, Ag3d core-leverspectrum is exhibited in Fig.2(b), and the bind energypeaks at 374.2 and 368.2 eV were attributed to Ag3d3/2and Ag3d5/2, indicating that Ag element existed in thezero valent state in the PANI/Ag/MWCNT composites[24].

C. Morphology analysis

Figure 3(a) shows HRTEM image of Ag/MWCNTs.Obviously, a large number of Ag nanoparticles with di-ameters ranging from 5 nm to 30 nm successfully at-tached to the surface of MWCNTs and distributed uni-formly without agglomeration. After polymerization ofaniline on the wall of Ag/MWCNTs, it can be directlyobserved in Fig.3 (b)−(e) that there was PANI layercoated completely on the surface of Ag/MWCNT com-posites. Moreover, from Fig.3(d), clear lattice fringeswith an interval of approximately 0.24 nm were seeninside the PANI layer and on the surface of MWCNTs.The result corresponded to a typical lattice distanceof Ag cluster (111) plane, which confirmed that theAg nanoparticles between the MWCNT core and PANIlayer existed in the form of elemental crystal.

To further study the crystal lattice, XRD diffrac-tion patterns of pure PANI, Ag/MWCNTs, and thePANI/Ag/MWCNT composite with the mass ratio



FIG. 2 XPS spectra of PANI/Ag/MWCNT composites with aniline to Ag/MWCNT mass ratio of 5:5. (a) Surveys scan,(b) N1s spectrum, (c) C1s spectrum, and (d) Ag3d spectrum.

FIG. 3 HRTEM images of (a) Ag/MWCNTs and (b)−(c) PANI/Ag/MWCNT with aniline to Ag/MWCNTs mass ratioof 6:4. (d) The local amplification photographs of (c). The inset of (d) shows HRTEM image of Ag nanoparticle in thePANI/Ag/MWCNT composites. (e) SEM image of PANI/Ag/MWCNT composites with aniline to Ag/MWCNT mass ratioof 5:5. (f) XRD patterns of (i) pure PANI, (ii) Ag/MWCNTs, and (iii) PANI/Ag/MWCNT composites with aniline toAg/MWCNT mass ratio of 5:5.



FIG. 4 CV curves of pure PANI electrode andPANI/Ag/MWCNT composites electrodes with differentaniline to Ag/MWCNT mass ratios at 50 mV/s in the po-tential range from −0.7 V to 0.3 V.

of 5:5 are shown in Fig.3(f). The diffraction peaksat 2θ=38.0, 44.1, 64.3, 77.2, and 81.5 for bothAg/MWCNTs and the PANI/Ag/MWCNT compositescorresponded to Ag (111), (200), (220), (311), and (222)reflections (JCPDS card number 4-0783), confirmingthe existence of Ag0 in the binary and ternary com-posite again. And the characteristic peaks appeared at2θ=26.02, which can be assigned to the diffraction sig-natures of the distance between the walls of MWCNTsand their inter-planar spacing [25].

D. Electrochemical characterizations

The electrochemical properties of the ternaryPANI/Ag/MWCNT composite electrodes were stud-ied by CV, GCD and EIS analyses. Figure 4 showsthe CV curves of pure PANI and the ternary compos-ites with different aniline to Ag/MWCNT mass ratiosat 50 mV/s. Compared with pure PANI, the com-posite electrodes exhibited a gradually increased cur-rent density response as the mass ratio of aniline toAg/MWCNT dropped. What’s more, when the massratio fell to 7:3, 6:4, and finally 5:5, the ternary compos-ite electrodes showed better capacitance performance,since the current density response was larger. Be-sides, there was an interesting observation that a re-versible anodic peak occurred at around −0.125 V,which possibly corresponded to the conversion reactionof sliver nanoparticles in the alkaline KOH electrolytesolution, indicating pseudocapacitance characteristicsof the ternary composites [26].

From the analysis, the composite electrodes with themass ratios of aniline to Ag/MWCNT at 7:3, 6:4, and5:5 were found to show better capacitance performance.So taking the three composite electrodes for instance,we further studied other electrochemical performances.Figure 5(a) shows the CV curves of the ternary com-posite electrodes with the mass ratio of 5:5 at differentscan rates of 2, 5, 10, 20, and 50 mV/s. The composite

FIG. 5 (a) CV curves of PANI/Ag/MWCNT compositeelectrodes with aniline to Ag/MWCNTs mass ratios of 5:5at different scan rates in the potential range from −0.7 V to0.3 V. (b) Specific capacitance values of PANI/Ag/MWCNTcomposite electrodes with different aniline to Ag/MWCNTmass ratios as a function of scan rate.

electrodes exhibited both electric double layer capac-itance (EDLC) and pseudo capacitance response overthe whole potential range. Meanwhile, the shapes ofthe CV curves remained unchanged with the scan rateincreasing from 2 mV/s to 50 mV/s, suggesting goodrate capability. The result was the same when the massratios were 6:4 and 7:3 (Fig.S1 and Fig.S2 in supple-mentary material).

The specific capacitance value of an electrode can becalculated according to the following equation [27]:

C =

∫IdV

vmV(1)

where I is the response current density, V is the poten-tial, v is the potential scan rate, and m is the mass ofthe electroactive material in the electrode.

The specific capacitance values of the three compos-ite electrodes changed with the scan rate, as shownin Fig.5(b). For the ternary composites with differentmass ratios, it was found that with the scan rate increas-ing, the specific capacitance values decreased instead.This can be contributed to the greater charge mobiliza-tion per unit time leading to lower degree of orderlyarrangement. Obviously, the ternary composite with



FIG. 6 (a) Charge-discharge curves of PANI/Ag/MWCNTcomposite electrodes with aniline to Ag/MWCNT mass ra-tios of 5:5 at different current densities. (b) Specific capac-itance values of PANI/Ag/MWCNTs composite electrodeswith different aniline to Ag/MWCNTs mass ratios versusdifferent current densities.

the mass ratio of 5:5 showed the highest specific capaci-tance and exhibited maximum capacitance of 140.6 F/gat a potential scan rate of 2 mV/s for all the measuredpotential scan rates, which could be explained by thefact that the largest content of Ag/MWCNTs could leadto small resistance and improve charge transfer of com-posite electrode.

Figure 6(a) exhibited typical GCD analysis of theternary composites with the mass ratio at 5:5, when thecharge-discharge current densities rose from 0.25 A/g to2.5 A/g. The composite electrodes exhibited good ca-pacitive behaviors over the potential ranging from −0.7V to 0.3 V. The discharge time decreased as the cur-rent density increased. Meanwhile, the GCD curveswere symmetric and non-linear over the whole potentialrange revealing the typical faradaic capacitances fromPANI and Ag nanoparticles [28]. The ternary compos-ites with the mass ratio at 6:4 and 7:3 performed thesame behaviors (Fig.S3, Fig.S4 in supplementary mate-rial). And for the three composite electrodes, the elec-trode with the mass ratio at 5:5 exhibited the longestdischarge time at the same current density.

The average specific capacitances of the PANI/Ag/MWCNT composites were calculated from discharge

FIG. 7 Nyquist plots of Ag/MWCNT electrodes andPANI/Ag/MWCNT composite electrodes with different ani-line to Ag/MWCNT mass ratios.

process according to the following equation [29]:

C =I∆t

m∆V(2)

where ∆t is the charge time and ∆V was the potentialrange during charge process.

The capacitance performance of the PANI/Ag/MWCNT composite electrodes was evaluated bycharge/discharge at different current densities. Asshown in Fig.6(b), we calculated the specific capaci-tance values of all the PANI/Ag/MWCNT compositeelectrodes at the different current density. The com-posite electrode with the mass ratio of 5:5 performedhigher specific capacitance values than that of the othertwo at the same discharge current density.

EIS, which explained the frequency dependence ofelectrode/electrolyte system, was analyzed between thefrequencies of 0.01 Hz and 100 kHz in Fig.7. Inthe Nyquist plot, the intercept of the real compo-nent reflected the equivalent series resistance (ESR)of the electrode materialand ESR further determinedthe rate of charging/discharging of the electrodes; thediameter of semicircle in the high frequency regionrepresented the interfacial charge transfer resistance,and larger diameter indicated higher interfacial chargetransfer resistance, which further suggested poorer con-ductivity of the electrodes [30]. As shown in Fig.7,the PANI/Ag/MWCNT composite electrodes all exhib-



ited a smaller intercept (0.23 Ω) on the real compo-nent and smaller diameter of a semicircle (<0.25 Ω)compared with the Ag/MWCNT composite electrodes,which could be explained by the contribution of PANIlayer to the interfaces where the conduction mechanismconverted from ionic to electronic [31]. The Warburgdiffusion region of PANI/Ag/MWCNT composite elec-trodes, corresponding to a 45 straight line after semi-circular region in the medium frequency range, indi-cated that the ternary composite electrode with themass ratio of 5:5 exhibited shorter diffusion path lengthof the ion in the electrolyte. This is because the War-burg diffusion region of the ternary composite elec-trodes with the mass ratio of 5:5 was shorter than theother two.

IV. CONCLUSION

In this work, we prepared the PANI/Ag/MWCNTcomposites by in situ polymerization of aniline onthe wall of Ag/MWCNTs with different aniline toAg/MWCNTs mass ratios. According to the resultsof FT-IR, XPS, SEM and HR-TEM, it was clear thatAg/MWCNTs were covered completely by the PANIlayer, and Ag nanoparticles existed in the form of ele-mental crystal between the MWCNT core and the PANIlayer. Then we prepared the composite electrodes andstudied their electrochemical performance in 1 mol/LKOH by CV, GCD, and EIS. It was found that theelectrochemical performances of the composites wereremarkably enhanced compared with that of the purePANI. And the composite electrodes exhibited a quitelow resistance, which would facilitate electron transferin the electrode material to achieve quick charging anddischarging. Particularly, when the mass ratio of anilineto Ag/MWCNTs was 5:5, the composite electrode dis-played a small equivalent series resistance (0.23 Ω) andsmall interfacial charge transfer resistance (<0.25 Ω), aswell as 160 F/g of the maximum specific capacitance ata current density of 0.25 A/g. These results suggestedthat the composite material had potential applicationsas cathode materials for lithium batteries and superca-pacitors.

Supplementary material: CV and charge-discharge curves of PANI/Ag/MWCNT composite elec-trodes with aniline to Ag/MWCNT mass ratios at 6:4and 7:3.

V. ACKNOWLEDGEMENTS

This work was supported by the Doctoral Programof Higher Education of China (No.20110010110007)and the Beijing Municipal Natural Science Foundation(No.2102035).

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ARTICLE Electrochemical Behavior and Speci c Capacitance