Conducting Polyaniline Nanowire Arrays for High Performance Supercapacitors

Kai Wang,†,‡ Jiyong Huang,† and Zhixiang Wei*,†

National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China, and GraduateSchool of the Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

ReceiVed: NoVember 29, 2009; ReVised Manuscript ReceiVed: February 11, 2010

Vertically aligned conducting polymer nanowire arrays have great potential applications in supercapacitorelectrode materials. In this paper, we report a facial one-step template-free approach to synthesize large arraysof vertically aligned polyaniline (PANI) nanowires on various conducting substrates by using a galvanostaticcurrent method. The as-prepared large arrays of PANI nanowires had very narrow diameters and were orientedperpendicular to the substrate, which was a benefit to the ion diffusion when being used as the supercapacitorelectrode. The highest specific capacitance of PANI nanowire arrays was measured as 950 F ·g-1 and kept ashigh as 780 F ·g-1 at a large charge-discharge current density (40 A ·g-1). Furthermore, the capacitances inseveral different electrolytes, including HClO4, lithium bis(trifluoromethane sulfonyl) (LiTFSI) aqueous solutionand nonsolvent electrolyte ionic liquid, were investigated. The results indicated that the orientation ofnanostructures could dramatically enhance the electrochemical performance of functional nanomaterials.

Introduction

Nowadays, supercapacitors, also called electrochemical ca-pacitors, possessing higher energy density than dielectriccapacitors and higher power density than batteries, have attractedgreat interest in energy storage because they fill the gapsbetween dielectric capacitors and batteries.1 According tocharge-discharge mechanisms, supercapacitors can be dividedinto electrical double-layer capacitors (EDLCs) and pseudoca-pacitors.2 EDLCs based on carbon materials can obtain a longcycle life (>105 cycles) but with relatively low specificcapacitances.3-10 Compared with EDLCs, pseudocapacitors havemuch higher specific capacitance due to their redox properties.11

Among pseudocapacitor electrode materials, conducting poly-mers have been considered as promising candidates because oftheir low cost, facile synthesis, flexibility, and high pseudocapaci-tance.12-18 In particular, polyaniline (PANI) is one of the mostuseful materials because on its high theoretical specific pseudoca-pacitance owing to multiple redox states.19-21

Recently, nanostructure materials were found to be particu-larly advantageous for a supercapacitor because they providehigh surface area leading to high specific capacitance.22,23 Inparticular, vertically aligned nanowires had been regarded asideal supercapacitor electrode material due to their large specificarea and optimized ion diffusion path.24-28 Several methods havebeen reported to prepare aligned PANI nanowires, includingtemplate synthesis,29-31 stepwise electrochemical deposition,32

and dilute chemical polymerization.33 As a result, aligned nano-tubes (or nanowires) of conducting polymers have shownenhanced performance as supercapacitor electrode materials.25-27

However, the studies on the large arrays of PANI nanowiresare still rarely reported for high performance supercapacitors.

Herein, we report a facile one-step approach to preparevertically aligned PANI nanowire arrays on various substrates.In this method, large area arrays of uniform PANI nanowireswith an average diameter of about 50 nm were prepared on a

variety of conducting substrates (Au, Pt, stainless steel, graphite,etc.) by a galvanostatic deposition process. Importantly, alignednanowire arrays exhibited high capacitance as an electrodematerial for supercapacitors even at very high charge-dischargecurrent densities.

Experimental Section

Materials. Aniline (Aldrich) was distilled under vacuumbefore using, and HClO4 (Sinopharm Chemical Reagent Co.,China) was used as purchased without any further purification.1-Methylimidazole and ethyl bromide (both obtained from J&KChemical Ltd.) were dried and distilled before use. Lithiumbis(trifluoromethane sulfonyl) (LiTFSI) salt was purchased from3M, U.S.A. Ultrapure water (18.2 MΩ · cm) was acquired byusing a Milli-Q water purification system from Millipore.

Synthesis. 1-Ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI) was synthesized by a reportedmethod34 and obtained as a transparent liquid.

Aniline was obtained by an electrochemical oxidation by agalvanostatic current procedure in a one-compartment cell witha three-electrode configuration. Typically, an Au plate (2 × 2cm) was used as the working electrode, and other electrodessuch as Pt and stainless steel were also used for comparison. Aplatinum plate and saturated calomel electrode (SCE) were usedas counter and reference electrodes. Electrolyte solution wascomposed of 0.1 M aniline and 1 M HClO4. Polymerizationwas carried out at the constant current, 0.01 mA · cm-2, for anhour. After polymerization, the working electrode was takenout from the electrolyte solution, washed with ultrapure water,ethanol, and ether, and dried under the air for further charac-terization.

Characterization. Prior to electrochemical testing, theelectrolytes were deoxygenated by bubbling with N2 for 20 min.The capacitance performance of PANI nanowire arrays wasevaluated by cyclic voltammetry (CV), galvanostatic charge-discharge, and electrochemical impedance spectra (EIS) in 1M HClO4, 1 M LiTFSI aqueous solution, and ionic liquidEMITFSI under N2 protection. The experiments were carriedout in the three-electrode cell as described in the polymerization

* To whom correspondence should be addressed. Tel.: (+86) 10-82545565. Fax: (+86) 10-62639373. E-mail: weizx@nanoctr.cn.

† National Center for Nanoscience and Technology.‡ Graduate School of the Chinese Academy of Sciences.

J. Phys. Chem. C 2010, 114, 8062–80678062

10.1021/jp9113255 2010 American Chemical SocietyPublished on Web 04/09/2010

procedure when measuring in the aqueous solution. While usingionic liquid as electrolyte, an Ag/AgCl electrode was used asthe reference electrode instead of saturated calomel electrodeto avoid introducing water into the hydrophobic ionic liquidEMITFSI. An Ag/AgCl electrode was prepared by an anodicoxidation method in hydrochloric acid solution from an Agstring. All the electrochemical experiments were performed byVMP3 Potentiostat/Galvanostat (EG&G, Princeton AppliedResearch). The morphologies of PANI nanowire arrays wereinvestigated by Hitachi S-4800 field emission scanning electronmicroscope (FE-SEM).

Results and Discussion

PANI was electropolymerized on conducting substrates (Pt,Au, stainless steel, etc.) by a galvanostatic deposition process.As-prepared PANI was intensive green and formed a uniformfilm on the electrode. The green color indicated that PANI wasin a doping state and possessed good conductivity. From SEMpictures it was revealed that the morphology of PANI appearedas white dots from the top view (Figure 1a). But when thesample was tilted by 30° (Figure 1b,c), it was clearly observedthat the PANI layer was composed of aligned nanowires, whichare all oriented perpendicular to the substrate. To get a cross-section view of PANI nanowire arrays, the PANI layer wasfreeze-dried and snicked using a blade (Figure 1d). SeparatedPANI nanowires can be clearly observed, which further affirmedthe structure and alignment of PANI nanowires. The lower leftof Figure 1d is a curved film of PANI underlying nanowires,indicating the nanowires were produced after the formation ofPANI films. The average diameter of PANI nanowires was about50 nm, and their average length is about 0.4 µm measured fromSEM images. Moreover, PANI nanowire arrays were uniformlydistributed on the whole electrodes, which made them suitablematerials for electrochemical performance investigations.

For the formation of PANI nanowire arrays, it could beexplained by a seedling growth process. Compact nuclei ofPANI were deposited to form a film by electropolymerizationon the conductive substrate at an initial stage when galvanostaticcurrent was carried out. And then the PANI would grow alongone dimension instead of forming new nuclei due to the anilinemonomer being incessantly consumed in the deposition process.

A further deposition resulted in an extended length along theinitial nuclei so that the nanowires of PANI formed. On thecondition of this experiment system, PANI nanowires possessmild growth steps because a dilute aniline monomer solutionand a low polymerization current density was selected. Inaddition, stereohindrance effect existed among the nanowires.So the aligned polyaniline nanowires array was finally produced.This formation mechanism of PANI nanowire arrays consistedof the models reported by Liu32 and Epstein.33 The nucleationand growth process have almost no relationship with thesubstrate for PANI, therefore, aligned nanowires could beproduced on large varieties of conducting substrates, includingAu, Pt, stainless steel, graphite, and so on.

The aligned PANI nanowire arrays are considered as excellentelectrode materials of supercapacitors due to their high specificarea and ordered nanostructures. Figure 2a was the CV curvesof nanowires with a potential range from -0.2 V to 0.85 V (vsSCE) at different scan rates in 1 M HClO4 aqueous solution.The CV curves exhibited two pairs of distinct redox peaks ofPANI. The first pair of redox peaks are ascribed to thetransformation between the leucoemeraldine base (LB) andemeraldine salt (ES) states of PANI, and the second pair ofredox peaks are ascribed to the transformation between ES andpernigraniline base (PB) states.35,36 From the CV curves, it wasobserved that the peak current increased with improving thescan rate, which indicated a good rate ability of PANI nanowirearrays.

Galvanostatic charge-discharge tests were then carried outto evaluate the capacitance of PANI nanowires at a series ofcurrent densities using a potential window 0-0.7 V versus SCE.PANI nanowire arrays showed typical capacitance characteristicsas displayed in the charge-discharge curves of Figure 2b. Thespecific capacitance can be calculated according to equation,C ) (I × t)/(m × 4V), where C is specific capacitance [F g-1],I and t are charge-discharge current and time, respectively,4V is 0.7 V in our measurement, and m is the mass of PANIfilm on the substrate electrode.37,38 Figure 2c presents the specificcapacitance corresponding to different current densities in 1 MHClO4 aqueous solutions calculated by equation upward. Atevery current density, the charge-discharge was performed infive consecutive cycles. And the specific capacitance finally was

Figure 1. Morphologies of PANI nanowire arrays obtained from (a) top view and (b, c) tilted 30°. (d) Cross-section of PANI nanowires arrays andseparated nanowires are clearly observed due to avoiding aggregation using a freeze-drying process.

Conducting Polyaniline Nanowire Arrays J. Phys. Chem. C, Vol. 114, No. 17, 2010 8063

a mean value of five cycles. The specific capacitance of PANInanowire arrays was about 950 F g-1 at a current density of 1A g-1, which is higher than the specific capacitance of the PANInanowire network (742 F ·g-1) reported by Gupta39 and thePANI nanowire arrays (700 F ·g-1) synthesized by templatemethod reported by Cao.29 Moreover, the specific capacitanceonly had a little decrease with the increase of current densities.Even at a very high current density of 40 A g-1, the specificcapacitance could still achieve to 780 F g-1, which remainedapproximate to 82% of the highest specific capacitance. TheCoulombic efficiencies (i.e., discharge capacitance divided bycharge capacitance) at different current densities (not shown)were almost kept constant at 100%, which indicated that theside reactions rarely occurred in our experiment voltage window.

The outstanding performance came from the novel morphol-ogy of aligned PANI nanowire arrays. As is commonly known,the pseudocapacitance of PANI coming from the redox reactioninvolving counterion influx and outflux from the polymer.40-42

The merit of vertically aligned nanowires is that they benefitthe ion diffusion from a bulky solution to the surface of PANInanowires, as illustrated by a cartoon (Figure 3). The counterionshereby can reach or leave the surface of PANI nanowires fast,even at a high charge-discharge rate. On the other hand, PANInanowires with narrow diameters (c.a. 50 nm) can shorten thecharge transport distance in the PANI materials. Thus, thecounterions easily penetrated the inner layer of PANI, whichmade nearly full use of the electrode materials. Optimized ionicdiffusion path and narrow diameters can reduce ionic diffuseresistance and charge transfer resistance, and therefore, the

supercapacitors can get a very high specific capacitance eventwith a large current density.

The above-mentioned electrochemical process was furtherproved by the EIS measurement. Figure 2d is a Nyquist plot ofthe EIS test in the same electrolyte with a frequency loop from20 kHz to 1 Hz using a perturbation amplitude of 5 mV at theopen circuit potential. The intersection of the plots at the X-axisrepresents solution resistance (Rs) or equivalent series resistance(ESR).43 The Rs is mainly contributed by the uncompensatedsolution resistance. Because a strong electrolyte (1 M HClO4)

Figure 2. Electrochemical capacitance behavior of PANI nanowire arrays in HClO4 aqueous solution: (a) cyclic voltammetry at different scan rate;(b) typical galvanostatic charge-discharge curves at several current densities; (c) specific capacitance in different current densities; (d) Nyquistplots at a frequency range from 20 kHz to 1 Hz. (the inset is an enlarged curve of the high frequency region).

Figure 3. Schematic of the optimized ion diffusion path in nanowirearrays.

8064 J. Phys. Chem. C, Vol. 114, No. 17, 2010 Wang et al.

was employed, the result exhibited that Rs was only about 0.6Ω. At the high-frequency region, the diameter of semicirclepresented the charge transfer resistance (Rct) in the electro-chemical system, which was approximated to 0.12 Ω judgingfrom the slope of the curve at low-frequency region (insert ofFigure 2d). The low value of Rct proved that PANI nanowireswith narrow diameters help the electrolyte ions penetrate intothe polymer and access the inner layer of the polymer easily.44

PANI nanostructure electrode can be described by using the“classical” finite-length transmission line model initially pro-posed by Macdonald45 and developed by other researchers.46

At the low frequency region, another x-intersection is equal tothe Rs+ 1/3RΣ, where RΣ stands for ion diffusion resistance. Theion diffusion resistance was only about 0.39 Ω, calculated fromFigure 2d, which showed that the counterions can quicklytransport from bulky solution to the PANI nanowires surface.In general, the rate of an electrode process depends on diffusionas well as charge transfer. The EIS testing illustrated that PANInanowire arrays possessed a reduced ion diffusion path andcharge transfer resistancethat redound to electrolyte ion diffusionto the polymer surface and reach the inner layer of the polymerphase. Therefore, PANI nanowire arrays attained a highcapacitance even at a high charged-discharged rate.

To further understand the capacitance behavior of PANInanowire arrays in different surroundings, LiTFSI and ionicliquid EMITFSI were also employed as electrolytes duringelectrochemical measurements. Figure 4a depicted the CVcurves of PANI nanowire arrays in three different electrolytesat a scan rate of 20 mV s-1. PANI showed various characteristics

in different electrolytes. Instead of two pairs of separated peaksin HClO4, only one pair of overlapped redox waves showed upin 1 M LiTFSI solution. It is elucidated that PANI experienceda different redox process in neutral solution, as reportedpreviously.47 In a neutral aqueous solution, protonic doping-dedoping of PANI almost could not happen due to a lowconcentration of H+. The LE state of PANI was oxidized tothe emeraldine base (EB) state at a higher potential than that inwater, and then EB was directly oxidized to a PE state. Thisprocess contains two consecutive oxidation steps and onlyexhibited one pair of overlapped redox waves.

As is commonly known, water has several shortages as asolvent in an electrochemical system, such as volatility and thenarrow potential window. To investigate the electrochemicalbehavior of PANI nanowire arrays at a larger potential range,ionic liquid EMITFSI was chosen as the electrolyte. WhenEMITFSI is being used, two pairs of distinguished redox peakswere observed for PANI nanowire arrays, as shown in Figure4a. However, the second pair of redox waves shifted positively,which might be ascribed to inserting/expulsing bulky EMI+

cations, which needs a higher overpotential than that of smallinorganic ions.48-54

Figure 4b shows the plots of specific capacitances to currentdensities of PANI nanowire arrays in different electrolytes.According to the plots, the specific capacitance of PANInanowire arrays was highest in HClO4 aqueous solution andwas lowest in ionic liquid EMITFSI. The difference of specificcapacitance in three electrolytes is ascribed to the differentproperties of electrolytes and varying redox mechanisms. There

Figure 4. Electrochemical capacitance behavior of PANI nanowire arrays with different electrolytes: (a) cyclic voltammetry curves (E vs SCE inHClO4, LiTFSI aqueous solution vs Ag/AgCl in EMITFSI); (b) specific capacitance plots with different current densities; (c) Nyquist plots at afrequency range from 20 kHz to 1 Hz; (d) Ragone plots of PANI nanowire arrays with different electrolyte.

Conducting Polyaniline Nanowire Arrays J. Phys. Chem. C, Vol. 114, No. 17, 2010 8065

was no doping-dedoping step but a direct redox process in the1 M LiTFSI aqueous solution. A high conductivity of PANI inHClO4 than that in LiTFSI lead to a more sufficient use ofelectrode materials, and therefore, the specific capacitance washigher. The ionic liquid EMITFSI has low ionic conductivityand high viscosity compared with aqueous electrolytes.55 As aresult, the specific capacitance in ionic liquid is the lowest.

It was further proved by the above-mentioned explanation,by Nyquist plots, in the above three electrolytes, as shown inFigure 4c. From the x-intersection in the high frequency regionof Nyquist plots, the values of Rs of 1 M HClO4, 1 M LiTFSI,and EMITFSI were obtained as 0.6, 3.6, and 8 Ω, respectively.The value of Rs reflected the difference in ionic conductivityand viscosity of a three-electrolyte system. Moreover, iondiffusion resistance (RΣ) of 1 M HClO4, 1 M LiTFSI aqueoussolution, and EMITFSI were, respectively, 0.39, 1.2, and 3 Ω,calculated from the x-intersection in the low frequency region.In other words, EMITFSI possessed the largest ionic diffusionresistance, and aqueous HClO4 exhibited the lowest ionicdiffusion resistance. On the other hand, the charge transferresistances (Rct) are 0.12, 0.3, 0.85 Ω, obtained from the slopeof the Nyquist plots. Therefore, ion transport in electrolyte andcharge-transfer in electrode material are different in threedifferent electrolytes, which led to their different specificcapacitances.

Figure 4d represented the Ragone plots of PANI nanowirearrays in the aforementioned electrolytes. The energy densitycan approach 130 W kg-1 at a power of 700 W kg-1 in HClO4

solution. Even at a high power density of 28000 W kg-1, theenergy density still can be kept at approximately 100 W kg-1,which exhibited a large power range that can be obtained whilemaintaining a relatively high energy density. The tendencies ofRagone plots in these different electrolytes are quite similar toeach other. Getting a high power density without the largescarifying energy density further indicated that the novelstructure of PANI nanowire arrays possess enhanced electro-chemical capacitance performance as electrodes.

The cyclic life of electrode materials is one of the mostimportant parameters for practical applications. The cyclic lifetests for PANI nanowires were carried out in foregoing threeelectrolytes at a constant current density 20 A g-1 as shown inFigure 4d. In the test of the first 100 cycles in HClO4 aqueoussolution, PANI nanowire arrays had an enormous loss (16%)in specific capacitance. The loss was ascribed to the mechanicaldegradation of the polymer.17,20,56 However, there was only aslight decrease of capacitance in the subsequent cycles and onlyabout 6% loss in the subsequent 400 cycles. The cyclic life wasimproved in LiTFSI electrolyte, and the specific capacitancekept about 88% of the original capacitance after 500 consecutivecharge-discharge cycles (Figure 5). That was because the stressdestroying the polymer was reduced thanks to the exclusion ofdoping-dedoping in the redox process.57 When using EMITFSIas an electrolyte, the capacitance almost had no decrease after500 cycles. This excellent cycle stability is ascribed to differentdoping-dedoping mechanisms of PANI and the intrinsicstability of the ionic liquid.49,55,57-60 On the other hand, ionicliquid EMITFSI owning a wider potential window where noside reaction happened can make the polymer more stable. It isindicated that one may find in the future that an electrolytesystem can get high capacitance and good stability simultaneously.

Conclusions

A facile strategy was reported to prepare large arrays ofaligned PANI nanowires by a galvanostatic current method.

PANI nanowires with about 50 nm diameters were uniformlydistributed on the whole substrate and oriented perpendicularto the substrate. The electrochemical measurement illustratedthat aligned nanowire arrays show higher capacitance valuesthan previously reported disordered nanowires39 and nanowirearrays prepared by using template method.29 Importantly, thespecific capacitance can keep high value even at the large currentdensity. EIS measurement proved that nanowire arrays possessa reduced ion diffusion resistance and charge transfer resistance,which all benefit the improvement of the electrochemicalcapacitance. The capacitance behavior of PANI nanowire arrayswere also investigated in several different electrolytes, includingHClO4, LiTFSI aqueous solution, and nonsolvent electrolyteEMITFSI ionic liquids. The results illustrate that PANI nano-wires show a quite stable capacitance in ionic liquids duringthe cyclic life test, which may guide to the finding of a suitableelectrolyte for their future applications.

Acknowledgment. The work was supported by the NationalNatural Science Foundation of China (Grant 20974029), Na-tional Basic Research Program of China (2006CB932100,2009CB930400), and Chinese Academy of Science (KJCX2-YW-M13).

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