Conducting Polyaniline Nanowire Arrays for High Performance Supercapacitors
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 Graduate School of the Chinese Academy of Sciences,Beijing 100190,People’s Republic of China Recei V ed:No V ember 29,2009;Re V ised Manuscript Recei V ed:February 11,2010
Vertically aligned conducting polymer nanowire arrays have great potential applications in supercapacitor electrode materials.In this paper,we report a facial one-step template-free approach to synthesize large arrays of vertically aligned polyaniline (PANI)nanowires on various conducting substrates by using a galvanostatic current method.The as-prepared large arrays of PANI nanowires had very narrow diameters and were oriented perpendicular to the substrate,which was a bene?t to the ion diffusion when being used as the supercapacitor electrode.The highest speci?c capacitance of PANI nanowire arrays was measured as 950F ·g -1and kept as high as 780F ·g -1at a large charge -discharge current density (40A ·g -1).Furthermore,the capacitances in several different electrolytes,including HClO 4,lithium bis(tri?uoromethane sulfonyl)(LiTFSI)aqueous solution and nonsolvent electrolyte ionic liquid,were investigated.The results indicated that the orientation of nanostructures could dramatically enhance the electrochemical performance of functional nanomaterials.
Introduction
Nowadays,supercapacitors,also called electrochemical ca-pacitors,possessing higher energy density than dielectric capacitors and higher power density than batteries,have attracted great interest in energy storage because they ?ll the gaps between dielectric capacitors and batteries.1According to charge -discharge mechanisms,supercapacitors can be divided into electrical double-layer capacitors (EDLCs)and pseudoca-pacitors.2EDLCs based on carbon materials can obtain a long cycle life (>105cycles)but with relatively low speci?c capacitances.3-10Compared with EDLCs,pseudocapacitors have much higher speci?c capacitance due to their redox properties.11Among pseudocapacitor electrode materials,conducting poly-mers have been considered as promising candidates because of their low cost,facile synthesis,?exibility,and high pseudocapaci-tance.12-18In particular,polyaniline (PANI)is one of the most useful materials because on its high theoretical speci?c pseudoca-pacitance owing to multiple redox states.19-21
Recently,nanostructure materials were found to be particu-larly advantageous for a supercapacitor because they provide high surface area leading to high speci?c capacitance.22,23In particular,vertically aligned nanowires had been regarded as ideal supercapacitor electrode material due to their large speci?c area and optimized ion diffusion path.24-28Several methods have been reported to prepare aligned PANI nanowires,including template synthesis,29-31stepwise electrochemical deposition,32and dilute chemical polymerization.33As a result,aligned nano-tubes (or nanowires)of conducting polymers have shown enhanced performance as supercapacitor electrode materials.25-27However,the studies on the large arrays of PANI nanowires are still rarely reported for high performance supercapacitors.Herein,we report a facile one-step approach to prepare vertically aligned PANI nanowire arrays on various substrates.In this method,large area arrays of uniform PANI nanowires with an average diameter of about 50nm were prepared on a
variety of conducting substrates (Au,Pt,stainless steel,graphite,etc.)by a galvanostatic deposition process.Importantly,aligned nanowire arrays exhibited high capacitance as an electrode material for supercapacitors even at very high charge -discharge current densities.Experimental Section
Materials.Aniline (Aldrich)was distilled under vacuum before using,and HClO 4(Sinopharm Chemical Reagent Co.,China)was used as purchased without any further puri?cation.1-Methylimidazole and ethyl bromide (both obtained from J&K Chemical Ltd.)were dried and distilled before use.Lithium bis(tri?uoromethane sulfonyl)(LiTFSI)salt was purchased from 3M,U.S.A.Ultrapure water (18.2M ?·cm)was acquired by using a Milli-Q water puri?cation system from Millipore.Synthesis.1-Ethyl-3-methyl imidazolium bis(tri?uoromethane sulfonyl)imide (EMITFSI)was synthesized by a reported method 34and obtained as a transparent liquid.
Aniline was obtained by an electrochemical oxidation by a galvanostatic current procedure in a one-compartment cell with a three-electrode con?guration.Typically,an Au plate (2×2cm)was used as the working electrode,and other electrodes such as Pt and stainless steel were also used for comparison.A platinum plate and saturated calomel electrode (SCE)were used as counter and reference electrodes.Electrolyte solution was composed of 0.1M aniline and 1M HClO 4.Polymerization was carried out at the constant current,0.01mA ·cm -2,for an hour.After polymerization,the working electrode was taken out 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,the electrolytes were deoxygenated by bubbling with N 2for 20min.The capacitance performance of PANI nanowire arrays was evaluated by cyclic voltammetry (CV),galvanostatic charge -discharge,and electrochemical impedance spectra (EIS)in 1M HClO 4,1M LiTFSI aqueous solution,and ionic liquid EMITFSI under N 2protection.The experiments were carried out 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@https://www.wendangku.net/doc/a65124517.html,.?
National Center for Nanoscience and Technology.?
Graduate School of the Chinese Academy of Sciences.
J.Phys.Chem.C 2010,114,8062–8067
806210.1021/jp9113255 2010American Chemical Society
Published on Web 04/09/2010
procedure when measuring in the aqueous solution.While using ionic liquid as electrolyte,an Ag/AgCl electrode was used as the reference electrode instead of saturated calomel electrode to avoid introducing water into the hydrophobic ionic liquid EMITFSI.An Ag/AgCl electrode was prepared by an anodic oxidation method in hydrochloric acid solution from an Ag string.All the electrochemical experiments were performed by VMP3Potentiostat/Galvanostat (EG&G,Princeton Applied Research).The morphologies of PANI nanowire arrays were investigated by Hitachi S-4800?eld emission scanning electron microscope (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 uniform ?lm on the electrode.The green color indicated that PANI was in a doping state and possessed good conductivity.From SEM pictures it was revealed that the morphology of PANI appeared as white dots from the top view (Figure 1a).But when the sample was tilted by 30°(Figure 1b,c),it was clearly observed that the PANI layer was composed of aligned nanowires,which are all oriented perpendicular to the substrate.To get a cross-section view of PANI nanowire arrays,the PANI layer was freeze-dried and snicked using a blade (Figure 1d).Separated PANI nanowires can be clearly observed,which further af?rmed the structure and alignment of PANI nanowires.The lower left of Figure 1d is a curved ?lm of PANI underlying nanowires,indicating the nanowires were produced after the formation of PANI ?lms.The average diameter of PANI nanowires was about 50nm,and their average length is about 0.4μm measured from SEM images.Moreover,PANI nanowire arrays were uniformly distributed on the whole electrodes,which made them suitable materials for electrochemical performance investigations.For the formation of PANI nanowire arrays,it could be explained by a seedling growth https://www.wendangku.net/doc/a65124517.html,pact nuclei of PANI were deposited to form a ?lm by electropolymerization on the conductive substrate at an initial stage when galvanostatic current was carried out.And then the PANI would grow along one dimension instead of forming new nuclei due to the aniline monomer being incessantly consumed in the deposition process.A further deposition resulted in an extended length along the initial nuclei so that the nanowires of PANI formed.On the condition of this experiment system,PANI nanowires possess mild growth steps because a dilute aniline monomer solution and a low polymerization current density was selected.In addition,stereohindrance effect existed among the nanowires.So the aligned polyaniline nanowires array was ?nally produced.This formation mechanism of PANI nanowire arrays consisted of the models reported by Liu 32and Epstein.33The nucleation and growth process have almost no relationship with the substrate for PANI,therefore,aligned nanowires could be produced on large varieties of conducting substrates,including Au,Pt,stainless steel,graphite,and so on.
The aligned PANI nanowire arrays are considered as excellent electrode materials of supercapacitors due to their high speci?c area and ordered nanostructures.Figure 2a was the CV curves of nanowires with a potential range from -0.2V to 0.85V (vs SCE)at different scan rates in 1M HClO 4aqueous solution.The CV curves exhibited two pairs of distinct redox peaks of PANI.The ?rst pair of redox peaks are ascribed to the transformation between the leucoemeraldine base (LB)and emeraldine salt (ES)states of PANI,and the second pair of redox peaks are ascribed to the transformation between ES and pernigraniline base (PB)states.35,36From the CV curves,it was observed that the peak current increased with improving the scan rate,which indicated a good rate ability of PANI nanowire arrays.
Galvanostatic charge -discharge tests were then carried out to evaluate the capacitance of PANI nanowires at a series of current densities using a potential window 0-0.7V versus SCE.PANI nanowire arrays showed typical capacitance characteristics as displayed in the charge -discharge curves of Figure 2b.The speci?c capacitance can be calculated according to equation,C )(I ×t )/(m ×4V ),where C is speci?c capacitance [F g -1],I and t are charge -discharge current and time,respectively,4V is 0.7V in our measurement,and m is the mass of PANI ?lm on the substrate electrode.37,38Figure 2c presents the speci?c capacitance corresponding to different current densities in 1M HClO 4aqueous solutions calculated by equation upward.At every current density,the charge -discharge was performed in ?ve consecutive cycles.And the speci?c capacitance ?nally
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 and separated nanowires are clearly observed due to avoiding aggregation using a freeze-drying process.
Conducting Polyaniline Nanowire Arrays J.Phys.Chem.C,Vol.114,No.17,20108063
a mean value of ?ve cycles.The speci?c capacitance of PANI nanowire arrays was about 950F g -1at a current density of 1A g -1,which is higher than the speci?c capacitance of the PANI nanowire network (742F ·g -1)reported by Gupta 39and the PANI nanowire arrays (700F ·g -1)synthesized by template method reported by Cao.29Moreover,the speci?c capacitance only had a little decrease with the increase of current densities.Even at a very high current density of 40A g -1,the speci?c capacitance could still achieve to 780F g -1,which remained approximate to 82%of the highest speci?c capacitance.The Coulombic ef?ciencies (i.e.,discharge capacitance divided by charge capacitance)at different current densities (not shown)were almost kept constant at 100%,which indicated that the side 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 reaction involving counterion in?ux and out?ux from the polymer.40-42The merit of vertically aligned nanowires is that they bene?t the ion diffusion from a bulky solution to the surface of PANI nanowires,as illustrated by a cartoon (Figure 3).The counterions hereby can reach or leave the surface of PANI nanowires fast,even at a high charge -discharge rate.On the other hand,PANI nanowires with narrow diameters (c.a.50nm)can shorten the charge transport distance in the PANI materials.Thus,the counterions easily penetrated the inner layer of PANI,which made nearly full use of the electrode materials.Optimized ionic diffusion path and narrow diameters can reduce ionic diffuse resistance and charge transfer resistance,and therefore,the
supercapacitors can get a very high speci?c capacitance event with a large current density.
The above-mentioned electrochemical process was further proved by the EIS measurement.Figure 2d is a Nyquist plot of the EIS test in the same electrolyte with a frequency loop from 20kHz to 1Hz using a perturbation amplitude of 5mV at the open circuit potential.The intersection of the plots at the X -axis represents solution resistance (R s )or equivalent series resistance (ESR).43The R s is mainly contributed by the uncompensated solution resistance.Because a strong electrolyte (1M HClO 4
)
Figure 2.Electrochemical capacitance behavior of PANI nanowire arrays in HClO 4aqueous solution:(a)cyclic voltammetry at different scan rate;(b)typical galvanostatic charge -discharge curves at several current densities;(c)speci?c capacitance in different current densities;(d)Nyquist plots at a frequency range from 20kHz to 1Hz.(the inset is an enlarged curve of the high frequency
region).
Figure 3.Schematic of the optimized ion diffusion path in nanowire arrays.
8064J.Phys.Chem.C,Vol.114,No.17,2010Wang et al.
was employed,the result exhibited that R s was only about 0.6?.At the high-frequency region,the diameter of semicircle presented the charge transfer resistance (R ct )in the electro-chemical system,which was approximated to 0.12?judging from the slope of the curve at low-frequency region (insert of Figure 2d).The low value of R ct proved that PANI nanowires with narrow diameters help the electrolyte ions penetrate into the polymer and access the inner layer of the polymer easily.44PANI nanostructure electrode can be described by using the “classical”?nite-length transmission line model initially pro-posed by Macdonald 45and developed by other researchers.46At the low frequency region,another x -intersection is equal to the R s +1/3R Σ,where R Σstands for ion diffusion resistance.The ion diffusion resistance was only about 0.39?,calculated from Figure 2d,which showed that the counterions can quickly transport from bulky solution to the PANI nanowires surface.In general,the rate of an electrode process depends on diffusion as well as charge transfer.The EIS testing illustrated that PANI nanowire arrays possessed a reduced ion diffusion path and charge transfer resistancethat redound to electrolyte ion diffusion to the polymer surface and reach the inner layer of the polymer phase.Therefore,PANI nanowire arrays attained a high capacitance even at a high charged -discharged rate.
To further understand the capacitance behavior of PANI nanowire arrays in different surroundings,LiTFSI and ionic liquid EMITFSI were also employed as electrolytes during electrochemical measurements.Figure 4a depicted the CV curves of PANI nanowire arrays in three different electrolytes at a scan rate of 20mV s -1.PANI showed various characteristics in different electrolytes.Instead of two pairs of separated peaks in HClO 4,only one pair of overlapped redox waves showed up in 1M LiTFSI solution.It is elucidated that PANI experienced a different redox process in neutral solution,as reported previously.47In a neutral aqueous solution,protonic doping -dedoping of PANI almost could not happen due to a low concentration of H +.The LE state of PANI was oxidized to the emeraldine base (EB)state at a higher potential than that in water,and then EB was directly oxidized to a PE state.This process contains two consecutive oxidation steps and only exhibited one pair of overlapped redox waves.
As is commonly known,water has several shortages as a solvent in an electrochemical system,such as volatility and the narrow potential window.To investigate the electrochemical behavior of PANI nanowire arrays at a larger potential range,ionic liquid EMITFSI was chosen as the electrolyte.When EMITFSI is being used,two pairs of distinguished redox peaks were observed for PANI nanowire arrays,as shown in Figure 4a.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 small inorganic ions.48-54
Figure 4b shows the plots of speci?c capacitances to current densities of PANI nanowire arrays in different electrolytes.According to the plots,the speci?c capacitance of PANI nanowire arrays was highest in HClO 4aqueous solution and was lowest in ionic liquid EMITFSI.The difference of speci?c capacitance in three electrolytes is ascribed to the different properties 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 in HClO 4,LiTFSI aqueous solution vs Ag/AgCl in EMITFSI);(b)speci?c capacitance plots with different current densities;(c)Nyquist plots at a frequency range from 20kHz to 1Hz;(d)Ragone plots of PANI nanowire arrays with different electrolyte.
Conducting Polyaniline Nanowire Arrays J.Phys.Chem.C,Vol.114,No.17,20108065
was no doping-dedoping step but a direct redox process in the 1M LiTFSI aqueous solution.A high conductivity of PANI in HClO4than that in LiTFSI lead to a more suf?cient use of electrode materials,and therefore,the speci?c capacitance was higher.The ionic liquid EMITFSI has low ionic conductivity and high viscosity compared with aqueous electrolytes.55As a result,the speci?c 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 in Figure4c.From the x-intersection in the high frequency region of Nyquist plots,the values of R s of1M HClO4,1M LiTFSI, and EMITFSI were obtained as0.6,3.6,and8?,respectively. The value of R s re?ected the difference in ionic conductivity and viscosity of a three-electrolyte system.Moreover,ion diffusion resistance(RΣ)of1M HClO4,1M LiTFSI aqueous solution,and EMITFSI were,respectively,0.39,1.2,and3?, calculated from the x-intersection in the low frequency region.
In other words,EMITFSI possessed the largest ionic diffusion resistance,and aqueous HClO4exhibited the lowest ionic diffusion resistance.On the other hand,the charge transfer resistances(R ct)are0.12,0.3,0.85?,obtained from the slope of the Nyquist plots.Therefore,ion transport in electrolyte and charge-transfer in electrode material are different in three different electrolytes,which led to their different speci?c capacitances.
Figure4d represented the Ragone plots of PANI nanowire arrays in the aforementioned electrolytes.The energy density can approach130W kg-1at a power of700W kg-1in HClO4 solution.Even at a high power density of28000W kg-1,the energy density still can be kept at approximately100W kg-1, which exhibited a large power range that can be obtained while maintaining a relatively high energy density.The tendencies of Ragone plots in these different electrolytes are quite similar to each other.Getting a high power density without the large scarifying energy density further indicated that the novel structure of PANI nanowire arrays possess enhanced electro-chemical capacitance performance as electrodes.
The cyclic life of electrode materials is one of the most important parameters for practical applications.The cyclic life tests for PANI nanowires were carried out in foregoing three electrolytes at a constant current density20A g-1as shown in Figure4d.In the test of the?rst100cycles in HClO4aqueous solution,PANI nanowire arrays had an enormous loss(16%) in speci?c capacitance.The loss was ascribed to the mechanical degradation of the polymer.17,20,56However,there was only a slight decrease of capacitance in the subsequent cycles and only about6%loss in the subsequent400cycles.The cyclic life was improved in LiTFSI electrolyte,and the speci?c capacitance kept about88%of the original capacitance after500consecutive charge-discharge cycles(Figure5).That was because the stress destroying the polymer was reduced thanks to the exclusion of doping-dedoping in the redox process.57When using EMITFSI as an electrolyte,the capacitance almost had no decrease after 500cycles.This excellent cycle stability is ascribed to different doping-dedoping mechanisms of PANI and the intrinsic stability of the ionic liquid.49,55,57-60On the other hand,ionic liquid EMITFSI owning a wider potential window where no side reaction happened can make the polymer more stable.It is indicated that one may?nd in the future that an electrolyte system can get high capacitance and good stability simultaneously. Conclusions
A facile strategy was reported to prepare large arrays of aligned PANI nanowires by a galvanostatic current method.PANI nanowires with about50nm diameters were uniformly distributed on the whole substrate and oriented perpendicular to the substrate.The electrochemical measurement illustrated that aligned nanowire arrays show higher capacitance values than previously reported disordered nanowires39and nanowire arrays prepared by using template method.29Importantly,the speci?c capacitance can keep high value even at the large current density.EIS measurement proved that nanowire arrays possess a reduced ion diffusion resistance and charge transfer resistance, which all bene?t the improvement of the electrochemical capacitance.The capacitance behavior of PANI nanowire arrays were also investigated in several different electrolytes,including HClO4,LiTFSI aqueous solution,and nonsolvent electrolyte EMITFSI ionic liquids.The results illustrate that PANI nano-wires show a quite stable capacitance in ionic liquids during the cyclic life test,which may guide to the?nding of a suitable electrolyte for their future applications. Acknowledgment.The work was supported by the National Natural Science Foundation of China(Grant20974029),Na-tional Basic Research Program of China(2006CB932100, 2009CB930400),and Chinese Academy of Science(KJCX2-YW-M13).
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