Capacitive behavior of graphene–ZnO composite film for supercapacitors

Short Communication

Capacitive behavior of graphene–ZnO composite ?lm for supercapacitors

Yanping Zhang,Haibo Li,Likun Pan *,Ting Lu,Zhuo Sun

Engineering Research Center for Nanophotonics and Advanced Instrument,Ministry of Education,Department of Physics,East China Normal University,Shanghai 20062,China

a r t i c l e i n f o Article history:

Received 22March 2009

Received in revised form 11June 2009Accepted 13July 2009

Available online 18July 2009Keywords:Graphene ZnO

Supercapacitors Composite Electrodes

a b s t r a c t

Graphene–ZnO composite ?lm was synthesized for its potential application in supercapacitors.Graphene was prepared by a modi?ed Hummers method and hydrazine reduction process,and ZnO was deposited on graphene by ultrasonic spray pyrolysis.The electrochemical characteristics of the ?lm were investi-gated through electrochemical impedance spectrometry,cyclic voltammetry and chronopotentiometry tests.The results showed that graphene–ZnO composite ?lm exhibited an enhanced capacitive behavior with better reversible charging/discharging ability and higher capacitance values,by comparison to pure graphene or ZnO electrode.

ó2009Elsevier B.V.All rights reserved.

1.Introduction

Supercapacitors characterized by high power density and long cycle life have attracted much attention [1,2].Recently increasing interest has been focused on their development in hybrid electric vehicle systems,backup energy sources,small-scale consumer electronics,etc.due to the power requirement of wide applications [3–6].

Carbon materials such as activated carbon,carbon aerogels,and carbon nanotubes (CNTs)are usually utilized as electrodes in sup-ercapacitors,and exhibit good electric double-layer capacitive per-formance because of their excellent conductivity,high surface area and stable chemical property [7–9].Recently graphene has been recognized for high mobility,chemical and mechanical stability due to its unique structure of two-dimensional layered hexagonal lattice of carbon atoms [10,11].By now graphene can be obtained in bulk quantity by chemical reduction of graphene oxide in solu-tion.Therefore,the potential applications of graphene in various microelectrical devices,such as ?eld-effect transistors,ultrasensi-tive sensors and electromechanical resonators,are in prospect [12,13].Moreover,the electrochemical characteristics of graphene have been evaluated by Meryl D.Stoller et al.as the electrode of supercapacitors and a speci?c capacitance of 135F/g in aqueous electrolyte has been achieved [14].It is known that metal oxide such as RuO 2,IrO 2,MnO 2and NiO x can improve the capacitance of carbon-based supercapacitors,as they can contribute pseudo-capacitance to the total capacitance apart from the double-layer

capacitance from carbon materials [1,15,16].Unfortunately,most of metal oxides suffer the low abundance and high cost [1,17,18].Zinc oxide (ZnO)has already been evaluated in the application of optoelectronic devices such as solar cells,gas sensors and light emitting diodes,due to its promising electrical and optical proper-ties [19,20].Researchers have begun to study its capacitive applica-tion.Kalpana et al.fabricated ZnO/carbon aerogel composite electrodes,reaching very high speci?c capacitance of 500F/g in 6M KOH solution [21].However,as a novel electrode material for supercapacitors,ZnO has not been extensively investigated so far.

In our previous work,we investigated supercapacitor based on CNT–ZnO composite ?lm electrode and gel polymer poly(vinyl alcohol)(PVA)–polyacid phosphomolybdic acid (PMA)solid state electrolyte.Such a capacitor structure exhibits a good capacitive behavior with a speci?c capacitance of 126.3F/g [22].In this work,we present graphene–ZnO composite ?lm as electrode of superca-pacitors.The graphene was fabricated by a modi?ed Hummers method and hydrazine reduction process,and the ZnO was depos-ited on graphene by ultrasonic spray pyrolysis (USP).The electro-chemical measurements show that graphene–ZnO composite ?lm enhances capacitive properties with better reversible charging/dis-charging ability and higher capacitance values than those of pure graphene or ZnO electrode.

2.Experimental

Graphite oxide (GO)was synthesized by a modi?ed Hummers method [12,23].Graphite powder with a mixture of HNO 3and H 2SO 4(2:1)were stirred in an oil bath at 80°C.The solid residue

0022-0728/$-see front matter ó2009Elsevier B.V.All rights reserved.doi:10.1016/j.jelechem.2009.07.010

*Corresponding author.Tel.:+862162234132;fax:+862162234321.E-mail address:lkpan@https://www.360docs.net/doc/d35644156.html, (L.Pan).Journal of Electroanalytical Chemistry 634(2009)

68–71

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m

after?ltration together with KMnO4was then added into H2SO4in ice-bath.GO dispersion was obtained after H2O2was added.The graphene was synthesized by reducing the mixture using hydra-zine monohydrate.Then the graphene was pasted on the indium tin oxide(ITO)glass substrates.

ZnO was deposited onto the graphene?lm and ITO glass by USP at a frequency of1.65MHz,respectively.The precursor solution was zinc acetate aqueous solution and the?ow rate of air as a car-rier gas was2ml/min.The substrates’temperature and deposition time were set at420°C and5min,respectively.The as-made graphene,pure ZnO?lm and graphene–ZnO composite?lm were named as ITO–G,ITO–ZnO and ITO–G–ZnO for study.

The surface morphology and structure of the electrodes were observed by?eld-emission scanning electron microscopy(FESEM, Hitachi4700)and atomic force microscopy(AFM,SPA-400).To investigate the capacitive properties of the electrodes,electro-chemical impedance spectrometry(EIS),cyclic voltammetry(CV) and chronopotentiometry experiments were performed using CHI 660C electrochemical workstation in a three-electrode mode, including a platinum foil as counter electrode and a standard calo-mel electrode as reference electrode.The experiments were oper-ated at room temperature with1M KCl solution as electrolyte. 3.Results and discussion

Fig.1a shows the FESEM image of graphene sheets.As shown, the transparency of the graphene suggests a thin thickness of?lm over the entire substrate.The wrinkles observed were probably caused by the oxygen functionalization and the resultant defects during the preparation of GO[24].Generally,graphene can attain large surface area owing to such unique two-dimensional struc-ture,which allows the sheets to adjust themselves physically to adapt the different types of electrolytes[14].From AFM image in Fig.2,it can be seen that the thickness of the graphene is about 3.89nm.It can be ascribed to the overlapping of graphene layers and the presence of functional groups derived during the fabrica-tion process[25].Signi?cant changes in the surface morphology of ITO–G–ZnO sample can be observed from Fig.1b.The graphene sheets were covered by densely packed and irregularly shaped

ZnO Fig.1.FESEM images of(a)as-synthesized graphene and(b)graphene–ZnO composite

?lm.

Fig.2.AFM image of the as-synthesized graphene.

Y.Zhang et al./Journal of Electroanalytical Chemistry634(2009)68–7169

grains,spreading in a large https://www.360docs.net/doc/d35644156.html,ing ZnO particles to decorate graphene sheets can combine double-layer capacitance from graphene and pseudo-capacitance from ZnO,which will bene?t the total capacitance.

EIS were conducted in 1M KCl solution at the frequency range of 10kHz to 10mHz.Fig.3shows the Nyquist diagrams of different electrodes.A semi-circle arc and a straight line have been ob-served.The high-frequency arc is ascribed to the double-layer capacitance (C dl )in parallel with the charge transfer resistance (R ct )at the contact interface between electrode and electrolyte solution [26].At lower frequencies,the impedance plot should the-oretically be a vertical line,which is parallel to the imaginary axis.However,the low frequency straight line departures from that ex-pected,with a slope angle smaller than p /2for these electrodes,which is explained by the electrode surface inhomogeneity and the existence of ‘‘constant phase element”[27,28].The resistances R ct ,calculated from the diameter of the high-frequency arc,are about 17X and 3X for ITO–G and ITO–G–ZnO electrodes,respec-tively,which indicates that the incorporation of ZnO can improve the charge transfer performance of graphene electrode.

Fig.4a shows the CV curves of the electrodes measured at a scan rate of 100mV/s.For ITO–ZnO electrode,two peaks at 0.07V and à0.2V can be obviously seen,caused by redox reactions of ZnO.This redox process is mainly governed by the intercalation and deintercalation of K +from electrolyte into ZnO [29]:ZnO +K ++e àM ZnOK.Due to the internal resistance of ITO–G electrode,its curve shape is slightly distorted from rectangle and close to par-allelogram,revealing a good double-layer capacitive behavior of graphene.The CV curve of ITO–G–ZnO electrode exhibits nearly rectangle shape with two redox peaks from ZnO,indicating that graphene–ZnO composite ?lm has better reversible charging/dis-charging processes than pure graphene electrode.The redox reac-tion from ZnO contributes pseudo-capacitance to the total

capacitance of ITO–G–ZnO electrode apart from the double-layer capacitance from graphene.

The speci?c capacitance (C sp )can be calculated from the vol-tammograms by

C sp ?

2I s ?m

e1T

where I ,s and m are the charge current,scan rate and mass of the single electrode,respectively.Fig.4b shows the plot of scan rates verse speci?c capacitance of the electrodes.The ITO–G–ZnO elec-trode achieves the highest speci?c capacitance of 11.3F/g among all.The speci?c capacitance tends to be stable with the increase in scan rates,accounting for its good discharge ef?ciency and elec-trodynamic property.

To further investigate the capacitive behavior of ITO–G–ZnO electrode,chronopotentiometry was performed at a constant cur-rent of 0.3,0.5,0.7,1and 1.2mA,respectively,within the

potential

Fig.3.Nyquist impedance plots for ITO–G,ITO–ZnO and ITO–G–ZnO

electrodes.

Fig.4.(a)CV curves at a scan rate of 100mV/s and (b)plots of scan rates verse speci?c capacitance of ITO–G,ITO–ZnO and ITO–G–ZnO

electrodes.

Fig.5.Charge–discharge curves of ITO–G–ZnO electrode.

70Y.Zhang et al./Journal of Electroanalytical Chemistry 634(2009)68–71

range of0–1V.As seen from Fig.5,each charge–discharge curve exhibits almost linear line,indicating typical behavior of superca-pacitors.Furthermore,the IR drop due to the internal resistance of the electrode was hardly observed due to the well-formed elec-trode/electrolyte interface.

4.Conclusions

Graphene synthesized via a modi?ed Hummers method and hydrazine reduction process showed sheet-structure.The graph-ene–ZnO composite?lm was prepared by depositing ZnO on graphene.The results of electrochemical experiments indicated that compared to pure graphene and ZnO electrodes,the composite electrode achieved:(i)promising reversible charging/discharging ability with typical supercapacitor behavior and(ii)higher speci?c capacitance,which was ascribed to the contribution of both dou-ble-layer capacitance from graphene and pseudo-capacitance from ZnO.

Acknowledgements

This work was supported by Special Project for Shanghai R&D Public Service Platform(No.07DZ22944),Special Project for Nano-technology of Shanghai(No.0752nm011)and Key Project for Industrial Innovation of Shanghai(No.07XI-025).

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