Graphene

Graphene oxide-polyaniline nanocomposites for high performance supercapacitor and their optical,electrical and electrochemical propertiesSurajit Konwer 1Received:4October 2015/Accepted:31December 2015/Published online:13January 2016ÓSpringer Science+Business Media New York 2016Abstract Polyaniline/Graphene oxide (PA/GO)compos-ites were prepared by chemical polymerization of aniline with different wt%of GO under acid conditions.The synthesized samples were characterized by using Fourier transform infra red spectroscopy,ultraviolet–visible absorption,X-ray diffraction,scanning electron micro-scopy,transmission electron microscopy and thermo-gravimetric analysis.It is found that the dc electrical conductivity dramatically increased to 84S/m for PA/GO (5wt%)composite at 150°C compared to pure PA (0.075S/m).The PA/GO composites showed a reversible electrochemical response up to 150th repeated cycles as revealed by the cyclic voltammetry study.High specific capacitance of PA/GO composite of 543.75F/g was obtained in the potential range from 0to 0.50V at 2mA compared with 266.66F/g for pure PA by galvanostatic charge–discharge analysis.Incorporation of GO into the polymer matrix has a pronounced effect on the electrical conductivity and electrochemical capacitance performance of PA/GO nanocomposites.1IntroductionThe pi-conjugated polymer composites with various fillers have become the subject of increased research interest in last few decades due to their various applications in dif-ferent fields such as electrode in rechargeable batteries,light emitting diodes,photoluminescence,sensors,super-capacitors etc.[1–5].In the family of various pi-conjugated polymers polyaniline (PA)is one of the most promising conducting polymers due to its low cost,easy synthesis,tremendous environmental and chemical stability,rela-tively high conductivity,and unique redox behavior [6–10].In recent times,a two-dimensional nanosheets of cova-lently bonded carbon atoms,graphene oxide (GO)bearing various oxygen functional groups on their basal planes and edges,has received a rapidly growing research interest [11–17].These oxygen-containing groups impart GO sheets with the function of strong interaction with polar small molecules or polymers to form GO intercalated or exfoliated composites [18–25].The thermal stability [18–21]and electrical properties [24,25]of polymers could be greatly improved by the incorporation of GO nanosheets.In addition,some polymer-GO or clay nanocomposites have become accessible in the form of end-functionalized derivatives because of small particle sizes and intercalation properties [24,25].Therefore,the potential of using gra-phene oxide—based materials for various applications such as supercapacitor,sensor etc.have attracted much attention very recently [26–30].Conducting polymer-based supercapacitors can be classified into three types [31,32].Type I is a symmetric system where same p -dopable conducting polymers are used at both the electrodes of the capacitor.Type II is an asymmetric system based on two different p-dopable con-ducting polymers to be used as electrode materials.Type III is a symmetric system based on a conducting polymer,which can be used both as p-and n-doped electrode materials.In this work we have attempted to build a Type I supercapacitor using two identical electrodes with same conducting polymer composite materials.&Surajit Konwerksurajit27@1Laboratory for Advanced Polymer Materials and Nanotechnology,Department of Chemistry,Gargaon College,Sivasagar,Assam 785686,IndiaJ Mater Sci:Mater Electron (2016)27:4139–4146DOI 10.1007/s10854-016-4273-3In the present work,effort has been made to prepare graphene oxidefilled polyaniline nanocomposites,eluci-date their optical,electrical and electrochemical properties and also to build a supercapacitor electrode using two identical electrodes with the same conducting polymer composite material.The thermal behaviour,surface mor-phology,the effect of GO on the electrical and electro-chemical capacitance performance of the PA/GO composite for supercapacitor applications has been studied thoroughly.2ExperimentalAniline was obtained from Aldrich Co.and used without further purification.The natural graphiteflake of size (crystalline,300mesh,Alfa Aesar)from Shanker Graphites and Chemical,New Delhi,India,hydrochloric acid(HCl), sulphuric acid(H2SO4),nitric acid(HNO3),sodium nitrate (NaNO3),potassium permanganate(KMnO4),potassium persulphate(K2S2O8)were of analytical reagent grade chemicals(Merck)and used as received.Acetonitrile was obtained from Merck and purified by standard methods. For all purposes double distilled water was used.2.1Preparation of graphene oxideGraphene oxide(GO)was synthesized from natural gra-phite by a modified Hummers method.Graphite(5g)and NaNO3(2.5g)were mixed with120mL of H2SO4(95%) in a500mLflask.The mixture was stirred for30min within an ice bath.While maintaining vigorous stirring, KMnO4(15g)was added to the suspension.The rate of addition was carefully controlled to keep the reaction temperature lower than20°C.The ice bath was then removed,and the mixture was stirred at room temperature overnight.As the reaction progressed,the mixture gradu-ally became pasty,and the colour turned into light brownish.At the end,150mL of H2O was slowly added to the pasty with vigorous agitation.The reaction temperature was rapidly increased to98°C with effervescence,and the colour changed to yellow.The diluted suspension was stirred at98°C for1day.Then,50mL of H2O was added to the mixture.For purification,the mixture was washed by rinsing and centrifugation with5%HCl and then deion-ized water for several times.Afterfiltration and dry under vacuum,the GO was obtained as a gray powder.2.2Preparation of polyaniline–graphene oxidecompositesPolyaniline/graphene oxide(PA/GO)composites were prepared by in situ polymerization of aniline in a suspension of graphene oxide in acidic solution.The weight%of graphene oxide to aniline was varied as1,3 and5%;the resulting composites were named as PA/GO (1%),PA/GO(3%),and PA/GO(5%),respectively. Typically,aniline was dissolved in1M HCl and GO was dispersed in the resulting solution by bath-sonicating for 1h.While maintaining vigorous stirring at room temper-ature,another solution of K2S2O8with a mole ratio to aniline of1:4in1M HCl was rapidly poured to the mix-ture.Polymerization of aniline started after about5min, while the colour of the mixture changed into green.The mixture was allowed to stir at room temperature overnight and then diluted by100mL of water.The composites were collected byfiltration and repetitively washed with water and ethanol until thefiltrate become colourless.Pristine polyaniline was also synthesized by adopting the procedure as described above in the absence of GO in the reaction mixture.2.3Electrochemical measurementsUsing a compression-molding machine,pellets of com-posite electrodes were made.High pressure was applied (1.5–2ton)to the sample to get hard round shaped pellet (1.5cm diameter,2mm breadth).The two electrode capacitor cells were constructed with electrolyte PEO as separator using a sandwich type construction(electrode/ separator/electrode)with a current-collector silver paste. The electrodes were pre-wetted with electrolyte before use. The capacitor performance was characterized by means of galvanostatic charge–discharge tests using on an Autolab PGSTAT302N at room temperature.Out of the three-electrodes cell,the working electrode was the polymer composite,platinum wire was the counter electrode,and standard calomel electrode(SCE)was used as reference electrode.3CharacterizationFourier transform infrared spectroscopy(FTIR)was used to record FTIR spectra by Impact410,Nicolet,USA,using KBr pellets.The ultraviolet–visible(UV–Vis)absorption spectroscopy of the samples in1-Methyl-2-pyrrolidon solvent was recorded using Shimadzu UV-2550UV–Vis spectrophotometer in the range of300–800nm.The sur-face morphology of the composites was observed by scanning electron microscope(SEM)of model JSM-6390LV,JEOL,Japan.The surface of the sample was coated with platinum before SEM analysis.Transmission electron microscope(TEM)measurements were conducted on a PHILIPS CM200microscope at200kv.The TEM samples were prepared by dispensing a small amount ofdry powder in ethanol.Then,one drop of the suspension was dropped on 300mesh copper TEM grids covered with thin amorphous carbon films.To study the thermal degra-dation of the samples,thermogravimetric analysis (TGA)was performed using TG 50,Shimatzu thermogravimetric analyser,Japan from temperature range 298to 973K with a heating rate of 10°C min -1under the nitrogen flow rate of 30mL/min.The X-ray diffraction (XRD)study was carried out at room temperature (ca.298K)on Rigaku X-ray diffractometer with Cu K a radiation (k =0.15418nm)at 30kV and 15mA using a scanning rate of 0.050°/s in the range of 2h =(10–70)°.Using a compression-moulding machine,pellets of composite samples were made.High pressure was applied (1.5–2ton)to the sample to get hard round shaped pellet (1.5cm diameter,2mm breadth),which was used to measure the electrical conductivity.4Results and discussionThe possible interaction of PA/GO composite is schemat-ically depicted in Fig.1.The oxygen containing functional groups viz.epoxy,hydroxyl etc.existed on the surface and pores of the GO promote the hydrogen bonds between the GO and the benzenoid and quinoid moieties of the polymer chain.The p –p stacking between the polymer backbone and the GO sheets contribute in increase of conductivity in PA/GO composite.In addition the polymers are absorbed in the pores and gallery of GO,which gives the composite without distinguishing the individual phase.So,there is a possibility for aniline monomer to polymerize on the sur-face and also in the pores of GO sheets.The FTIR spectra of GO,PA and PA/GO composite is revealed in Fig.2.The absorption peak at 3430cm -1is attributable to the N–H stretching vibrations of theleucoemeraldine component of the PA powder sample [33].The weak peak at 2924cm -1corresponds to aromatic sp 2CH stretching.The C=C stretching deformation of the quinoid ring in the emeraldine salt and benzenoid rings in leucoemeraldine is depicted at 1631and 1461cm -1respectively [34].The peaks at 1283and 1152cm -1cor-respond to C–N stretching of the secondary aromatic amine and C=N stretching,respectively [33].In GO,the charac-teristic broad and intense O–H peak at 3434cm -1,strong C=O peak in carboxylic acid and carbonyl moieties at 1730cm -1,C–OH peak at 1406cm -1,C–O–C peak at 1222cm -1,C–O stretching peak at 1077cm -1was observed.By comparison,the spectrum of the PA/GO composites,the absorption peaks are similar to PA except that the characteristic peak of C=O group at 1728cm -1was observed from PA/GO composite.The absorption peaks at *1640and 1450cm -1represent the quinoid and benzenoid structures of the PA in unexposed PA/GO composite.The presence of PA characteristic vibrations,suggesting PA can be successfully deposited on the GO surface.Figure 3gives the UV–Vis absorption spectra of the PA and PA/GO composite.The PA/GO composite is in the doped state as reflected by the presence of the polaron band transition at about 330–340nm.In addition,the p –p *transition was observed at about 310–320nm for the benzenoid rings.The absorption band at 602–639nm is attributed to the excitation from the HOMO (highest occupied molecular orbital)of the three-ring benzenoid part of the system to the LUMO (lowest unoccupied molecular orbital)of the localized quinoid ring and the two surrounding imine nitrogens in thePA.Fig.1Schematic presentation depicting interaction of GO sheet and PAchainFig.2FTIR spectra of a GO,b PA and c PA/GO compositeIn Fig.4,the XRD pattern of pure PA,GO and PA/GO composite was observed.A characteristic broad peak of amorphous PA was observed at about 2h =25.8°.The corresponding peaks for the PA/GO composite at 26.9°and 55.2°match the (111)and (222)planes of rhombohedral system with rhomb-centred lattice [ref No.—PCPDFWIN 75-2078and calculated from ISCD using POWD-12??(1997)].The crystalline peaks observed in the composite due to the dominant crystalline nature of GO.Thermogravimetric profiles of PA and PA/GO com-posites are given in Fig.5.Initially at temperature range 90–115°C the weight loss for PA reveals the loss of moisture from the polymer matrix.In case of PA/GO composite nearly a 7–8%weight loss has occurred at 100–120°C,due to removal of water from the galleryspace of the GO framework.In PA/GO composite the weight loss near 210°C is probably due to removal of the oxygen-containing functional groups.After 250°C,major weight loss has occurred due to decomposition of the PA from the composite.Finally at 450°C,almost 24%weight loss for GO-based PA composite has been observed.The introduction of GO exhibits a beneficial effect on the thermal stability of polymer composites.Scanning electron microscope (SEM)images of the pure PA,GO and PA/GO are shown in Fig.6.The GO inherits the layer-by-layer and network structure but in a denser stacking compared with the randomly aggregate structure having rough surface for the pure PA is observed.In the SEM micrographs of PA/GO composite,it clearly shows the polymers are grown in the pores and galleries of GO.Thereby it is difficult to distinguish the individual phase,i.e.,GO and PA in PA/GO composite.This imparts the high in-plane conductivity between adjacent GO layers of thecomposites.Fig.3UV–Vis spectra of a PA,b PA/GO (5%),c PA/GO (3%),and d PA/GO (1%)compositeFig.4XRD analysis of a pure PA,b PA/GOcompositeFig.5TGA of a PA and b PA/GO compositeTypical morphology of GO is shown in the TEM image of Fig.7.The GO prepared here has a typically curved,layer like structure with the size of tens of nanometers.For PA/GO composite,TEM image shows that all the GO sheets are homogeneously surrounded with PA and mainly grown on the surface or intercalate between the GO sheets.SEM and TEM images reveal that the chemically modified GO and the PA formed a uniform composite with the PA absorbed on the GO surface and/or filled between the GO nanosheets.5Electrical properties5.1Current–voltage relationshipFigure 8shows the I–V characteristics of PA and PA/GO composites having different GO content.For all the sam-ples,the potential difference (in Volt)increases linearly with the applied current (in mA)which clearly indicates the semiconducting behaviour.The conducting behaviour of the PA/GO composites gradually increases with respect to the applied current as well as increasing percentage of GO incorporates into the polymer matrices.5.2DC electrical conductivityThe electrical conductivities of the PA,GO and the com-posite material was determined and the GO sample shows alow conductivity of 0.5S/m similar to that reported in the earlier literature [35].The PA sample shows conducting behaviour with the maximum conductivity of 0.075S/m.However the dc electrical conductivity of the PA/GO (5%)composite is found to be increased dramatically (47–84S/m)in comparison to that of PA and GO alone.The electrical conductivity of PA and PA/GO composite was measured in the temperature range of 25°C C T C 150°C.Such enormous enhancement of the conductivity of PA/GO composite might be attributed to the extended H-bonding between the PA and GO allowing extended p -conjugation in polymer chain.The electrical conductivity increases with temperature showing semi conducting behaviour and the maximum conductivity is found to be 84S/m for PA/GO (5wt%)at 110°C and after that no deviation in conductivity was observed (Fig.9).At high temperature,the mobility of the charge carrier increases with the increase in inter-chain and intra-chain hoping.An increase in interchain and intrachain hopping results in a high charge carrier mobility within the com-posite,which leads to an increase in the conductivity at appropriate high temperatures [36,37].Fig.6SEM images of a PA,b GO and c PA/GOcompositeFig.7TEM images of a GO and b PA/GOcompositeFig.8I–V curves of a PA/GO (1%),b PA/GO (3%),c PA/GO (5%),and d PA,investigated by two probes Keithley Source meter at room temperature6Electrochemical propertiesPA/GO films (1,3and 5%)deposit on indium tin oxide coated glass is used as working electrodes.The Ag/AgCl and Pt wire are used as reference electrode and counter electrode respectively.KCl (0.1M)solution as supporting electrolyte prepared in 10ml acetonitrile.The cyclic voltammogram (CV)of PA and PA/GO composites are analyzed at a scan rate of 50mVs -1(Fig.10).The electrochemical band gap of the samples is calcu-lated by using the following formulae [38].HOMO ¼Àu ox onsetþ4:71ÂÃeV ðÞ;ð1ÞLUMO ¼Àu red onsetþ4:71ÂÃeV ðÞ;ð2ÞE g ec ¼u ox onset Àu red onset ÀÁeV ;ð3Þwhere,u onset ox and u onset redare oxidation onset potential and reduction onset potential respectively.The electrochemical band gap for PA (calculated from the voltammogram)is 3.2eV which decreases to 2.17eV for PA/GO (5%)composite.The band gaps of the composites decreases with increase in the amount of incorporated GO into the PA matrices (Table 1).The incorporation of GO changes the electronic band structure of PA/GO composites which manifest a new mid-gap state.This results the decreasing of band gap.The optical band gap is calculated by using the fol-lowing equation [39]E opt g eV ðÞ¼1240=k edge nm ðÞð4Þwhere E g opt is the optical band gap and k edge is the absorption edge.The comparison of the optical band gaps with elec-trochemical band gaps of PA and PA/GO composites was also observed.The electrochemical band gaps actually leads to the formation of charge carriers.On the contrary,optical transition does not reveal the formation of freechargeFig.9Temperature dependence of electrical conductivity of a PAand b PA/GOcompositeFig.10Cyclic voltammogram for PA and PA/GO composite in 1.0M KCl solutioncarriers,as the excited state in conjugated polymers may be viewed as a bound exciton[36].The optical band gap cannot be directly compared to the electrochemical band gap.The band gaps of polymer composites were found to be 1.94–2.06eV(optically)and2.17–2.94eV(electrochemi-cally).The electrochemical band gaps give higher values than the optical band gaps.However both the method establishes the same trend of band gap.6.1Charge capacityThe area under the CV could be integrated to give us the information about the charge capacity as well as electro activity of the polymer composites.The cyclic voltam-metric study of PA/GO composites up to150th repeated cycle reveals that cathodic and anodic peaks are nearly symmetrical above each other with minimum separation (Fig.11).This property emphasizes that PA/GO compos-ites may be useful as a prominent material to be used in rechargeable batteries or any other electrical devices.6.2Charge–discharge studiesThe Galvanostatic charge/discharge curves of PA and PA/ GO composites are shown in Fig.12.The average specific capacitance values,Cg(F/g)of the samples were estimated from the discharge process according to the following equation[40]Cg¼I D tD VÂmð5Þwhere,I is the current loaded(A),D t is the discharge time (s),D V is the potential change during discharge process, and m is the ma ss(g)of active material in a single electrode.The mass of active material used for PA,PA/GO(3%) and PA/GO(5%)electrode is4.5,3.5and4mg,respec-tively.The specific capacitance of PA is266.66F/g while for PA/GO(3%)and PA/GO(5%)are507.14and 543.75F/g respectively at2mA current in the range from 0.2to0.60V.The incorporation of GO into the PA matrix enhanced the capacitance behaviour of PA/GO composites. In GO,some functional groups such as–OH,–COOH, epoxy etc.present on the surface and pores of the GO sheets and at high temperature after acid treatment they could promote the adsorption of molecular chains and monomers onto the pores.Moreover the p–p stacking between the polymer backbone and the GO sheets may also contribute to extend the conjugation length of the polymer composite which allows the counter ions to readily pene-trate into the polymer matrix and access their internal surface.Therefore the specific capacitance increases with increase the GO content in PA/GO composite.It is note-worthy that PA/GO(5%)shows a better electrochemical capacitance compared with PA/GO(3%).Table1Electrochemical data of PA and PA/GO composites Sample u onsetox/EHOMOu onsetred/ELUMOE g ec(eV)E g opt(eV) PA 2.10/-6.81-1.10/-3.61 3.20 3.40PA/GO composites(1%) 1.60/-6.31-1.34/-3.37 2.94 2.06 (3%) 1.04/-5.75-1.36/-3.35 2.40 1.98 (5%) 1.32/-6.03-0.85/-3.86 2.171.94 Fig.11Successive electrochemical cycles for PA/GO composite upto150thcyclesFig.12Galvanostatic discharge curve at2mA of a pure PA,b PA/GO(3%),and c PA/GO(5%)pellet electrode7ConclusionHigh-performance electrode material based on polyaniline doped with graphene oxide sheets have been fabricated via oxidative in situ polymerization successfully.FTIR and XRD revealed the incorporation of GO into the polymer matrix.UV–visible study showed the characteristic red shift of PA/GO nanocomposites due to the extended con-jugation of polymer chains.The optical band gap of PA/ GO composites has been decreased from2.06to1.94eV and the electrochemical band gap has been decreased from 2.94to2.17eV on addition of GO(1,3and5%).The electrical conductivity of the composites increased from47 to84S/m with increasing GO and temperature compared with the conductivity of pure PA(0.075S/m).The com-posite shows gratifying reversible electrochemical response to intaking charge capacity almost unchanged even up to 150th cycles.The specific capacitance of the PA/GO(5%) composite electrodes reached543.75F/g.With remarkably enhanced specific capacitance compared with polymer alone,the PA/GO composite electrode can be considered a promising candidate for future development of safe and cost effective electrochemical supercapacitors. 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