Graphene zinc oxide g zno nanocomposite for electrochemical supercapacitor applications

Journal of Science: Advanced Materials and Devices (2016) 454e460 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Graphene-zinc oxide (G-ZnO) nanocomposite for electrochemical supercapacitor applications Murugan Saranya a, *, Rajendran Ramachandran b, **, Fei Wang b, *** a b Platinum Retail Ltd, Chorleywood Road, Rickmansworth, United Kingdom Department of Electronic and Electrical Engineering, Southern University of Science and Technology, Shenzhen 518005, China a r t i c l e i n f o a b s t r a c t Article history: Received 12 September 2016 Accepted October 2016 Available online 13 October 2016 Graphene-ZnO nanocomposites (G-ZnO) were prepared by a facile solvothermal approach Well, crystalline ZnO nanoparticles with size in the range of 30e70 nm are uniformly deposited on the graphene sheets, as evidenced by different techniques The electrochemical properties of the prepared nanocomposites were examined by measuring the specific capacitance in M KOH solution using cyclic voltammetry and galvanostatic chargeedischarge techniques G-ZnO nanocomposites showed a good capacitive behavior with a specific capacitance of 122.4 F/g as compared to graphene oxide (2.13 F/g) and rGO (102.5 F/g) at mV/s scan rate Results demonstrated that such hybrid materials are promising electrode materials for high-performance supercapacitor applications Crown Copyright © 2016 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/) Keywords: Solvothermal Graphene-ZnO Graphene oxide Cyclic voltammetry Specific capacitance Introduction Supercapacitors are charge storage devices of tremendous interest in view of its high power density, fast charging/discharging rate, long cycle life, a wide operating temperature range and environmentally benign Still, the low energy density of these supercapacitors has forced huge difficulties in utilizing them as essential energy sources to replace batteries [1] Hence continuous effort has been embraced to use nanostructured materials with enhanced specific capacitance Most research is focused on the development of different electrode materials like carbon, conducting polymers, metal oxides and out of which carbon-based materials like activated carbon, carbon nanotube, and carbon aerogels is paid more attention for energy storage devices [2] Activated carbon and carbon nanotube shows good electrical double layer capacitance because of their excellent conductivity and high surface area, where the storage process is non-Faradaic and the storage of energy is electrostatic The key to achieving high capacitance increases the surface area and electrical conductivity of the material As of late graphene * Corresponding author ** Corresponding author *** Corresponding author E-mail addresses: drmsaran19@gmail.com (M Saranya), ramnano2009@gmail com (R Ramachandran), wangf@sustc.edu.cn (F Wang) Peer review under responsibility of Vietnam National University, Hanoi has been the most encouraging material for energy storage applications due to its high electrical conductivity, thermal conductivity, superior chemical stability, unique mechanical strength and a large surface to volume ratio than other carbon materials [3] Disorder in the atomic configuration of graphene can have dramatic effects on its electronic, thermal, magnetic and other properties [4,5] Supercapacitors are categorized into two types based on the charge storage mechanism viz electric double layer capacitors (EDLCs) and pseudocapacitor The latter stores charge faradically, which allows them to achieve higher capacitance properties and enhanced energy densities than EDLCs Polymers and metal oxides like NiO [6], RuO2 [7], MnO2 [8], Co3O4 [9], and V2O5 [10] exhibit this type of capacitance called pseudocapacitance which involves redox reactions and often the pseudocapacitance of such polymers and metal oxides show higher specific capacitance than EDLCs However, the relatively low conductivity and poor stability of such materials usually require the addition of conductive phases e.g carbon-based to enhance the charge transfer In this way, these two could be merged together for the fabrication of a hybrid capacitor, where both faradaic and non-faradaic processes can be used for charge storage and enhanced electrochemical properties It has been accounted that the combination of carbon material with polymer/metal oxides or both exhibit higher specific capacitance due to the combination of the redox reaction of metal oxide and high surface area/conductivity of graphene than their individual form due to a positive synergistic effect [11] In the most recent http://dx.doi.org/10.1016/j.jsamd.2016.10.001 2468-2179/Crown Copyright © 2016 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) M Saranya et al / Journal of Science: Advanced Materials and Devices (2016) 454e460 455 Fig Schematic digram of G-ZnO nanocomposite synthesis couple of years, graphene-based composites are being examined for supercapacitor applications In general, the specific capacitance of graphene is lesser than the expected value due to restacking of the graphene sheets which could be improved by making it as a composite with other materials Wu et al have reported a maximum specific capacitance of 210 F/g for grapheneepolyaniline composite [12] Among the metal oxides, RuO2 exhibits higher specific capacitance yet its usage is limited due to its high cost and toxicity Thus, the fabrication of supercapacitor electrode materials with low-cost production is challenging in the field of energy storage devices and hence it is imperative to explore more desirable materials for supercapacitor applications Zinc oxide (ZnO) is a potential semiconductor material with amazing optical and electrical property Because of the superior electrical conductivity of ZnO, it is widely used in many applications ranging from optoelectronics [13], gas sensors [14], energy storage and solar cells [15] Lu et al reported the maximum specific capacitance of 61.7 F/g for grapheneezinc oxide thin film which prepared by ultrasonic spray pyrolysis method [16] and another report showed the maximum specific capacitance of 146 F/g for G-ZnO nanocomposite [17] Here, we report a facile approach to synthesize ZnO/Graphene nanocomposites by the solvothermal process These composites were utilized to fabricate supercapacitor electrodes to probe their electrochemical properties and results revealed that the nanocomposite materials had a good electrochemical performance as electrode materials with a specific capacitance of 122.4 F/g Fig XRD pattern of G-ZnO composite and graphene oxide (Inset picture) 456 M Saranya et al / Journal of Science: Advanced Materials and Devices (2016) 454e460 Fig (a) DRS UV spectrum of G-ZnO composite and (b) optical band gap diagram of G-ZnO was centrifuged (Eppendorf centrifuge 5430, 460W, Germany) at 7500 rpm with deionized water and dried at 60  C for 24 h 2.3 Synthesis of G-ZnO nanocomposites Graphene-ZnO nanocomposites were prepared by our previously reported solvothermal method as described in the literature [19] In brief, GO (5 mg) was dissolved in 20 ml of ethylene glycol and sonicated for h Then 20 mg of zinc acetate dissolved in 20 ml of ethylene glycol was added to the above GO dispersed solution under continuous stirring Later, 0.1 M NaOH in ml of distilled water was added to the mixture After 30 of stirring, the mixture was transferred into the Teflon-lined stainless steel autoclave and maintained at 160  C for 48 h The obtained final mixture was centrifuged and washed with de-ionized water and ethanol several times, and dried at 60  C for 24 h The weight of graphene in the G-ZnO composite is weight% Fig FTIR spectrum (a) G-ZnO composite (b) Graphene Oxide Materials and experimental 2.1 Materials Graphite powder, sodium nitrate, potassium permanganate, hydrogen peroxide, sodium hydroxide, potassium hydroxide, ethylene glycol, and zinc acetate were purchased from SigmaeAldrich All the materials were used as received without any further purification 2.2 Preparation of graphene oxide Graphene oxide was prepared through modified Hummer's method as described in the literature [18] Graphite powder (1 g) and sodium nitrate (1 g) were mixed with 46 ml of concentrated sulfuric acid The mixture was kept in an ice bath for four hours under continuous stirring To it, potassium permanganate (6 g) was added and the reaction was then allowed at 35  C for two hours and diluted with 92 ml de-ionized water Then the temperature was raised to 98  C and maintained for two hours The solution was further diluted with 200 ml warm water and 20 ml hydrogen peroxide and further stirred for another one hour After the reaction, the color of the solution turned into yellowish brown, which 2.4 Electrochemical measurements Cyclic Voltammetry and chronopotentiometry experiments were investigated by using a CHI 600C electrochemical workstation in a three electrode system Glassy carbon electrode, Ag/AgCl, and platinum wire electrode were used as working, reference and counter electrodes respectively Experiments were carried out at room temperature in M KOH electrolyte in the potential range from to À0.8 V 2.5 Electrode preparation The fabrication of the modified working electrode, glassy carbon was polished with 0.03 mm alumina powder, rinsed thoroughly with de-ionized water, sonicated with ethanol and deionized water for After a few minutes, 0.5 mg of the active material (G-ZnO, GO, rGO) in mL Nafion solution was coated on the GC electrode (geometric surface area of the electrode is 0.0706 cm2) and allowed to dry at room temperature for few hours 2.6 Characterization The X-ray diffraction system (BRUKER, D8 Advance, Germany) was used for the X-ray analysis with Cu-Ka radiation (l ¼ 540 Å) Step scanning was used with 2q intervals from 8 to 60 Scanning Electron Microscope images were taken from the system (FEI Quanta FEG 200) The electrochemical performance was done by the system CHI 600C work station, Version 5.01 M Saranya et al / Journal of Science: Advanced Materials and Devices (2016) 454e460 457 Fig HRTEM images of G-ZnO nanocomposite at various magnifications and EDS spectrum of G-ZnO nanocomposite (Inset picture of b) Results and discussion 3.1 Structural analysis The schematic diagram of G-ZnO nanocomposites synthesis is given in Fig The XRD pattern of graphene oxide and G-ZnO composites is shown in Fig The inset picture showed the XRD pattern of GO and the peak was observed at 2q ¼ 10 suggesting that the perfect oxidation and the interlayer distance of graphene are 8.8 Å [20] The increased interlayer distance is due to the intercalation of oxygen functional groups during the oxidation process The major diffraction peaks in G-ZnO composite were observed at 2q value 31.8 , 34.4 , 36.2 , 47.5 , 56.5 , 62.7, 66.3 , 68.0 , 69.1 and 76.9 which correspond to (100), (002), (101), (102) (110), (103), (200), (112), (201) and (202) crystalline plane of ZnO respectively These crystalline planes are indexed to the wurtzite structure of ZnO particles matched with the JCPDS No 36-1451 [21] The average ZnO crystalline size as calculated from Scherrer formula was 14 nm It could be seen that there is no diffraction peak of carbon in the composite Due to the good crystallinity of ZnO Fig Cyclic voltammetry performance (a) Graphene Oxide (b) G-ZnO composite at different scan rates 458 M Saranya et al / Journal of Science: Advanced Materials and Devices (2016) 454e460 peak at 398 nm due to ZnO particles in graphene sheets [17] The optical band gap is obtained by the following equation, with the help of absorption spectra ahnịn ẳ A hn Eg Fig Comparison Cyclic voltammetry response of GO, rGO and G-ZnO composite at 100 mV/s scan rate where ‘a’ is the absorption coefficient, ‘h’ is the Planck constant, ‘y’ is the light frequency, ‘Eg’ is the band gap, ‘n’ is either ½ for an indirect transition or for a direct transition and A is a constant According to the above equation, the band gap of the as-obtained ZnO-Graphene nanocomposites is 3.12 eV, which is shown in Fig 3(b) The FTIR spectrum of GO and G-ZnO composite is shown in Fig It is observed that the oxygen functional groups of GO are revealed by the peaks at 1726, 1217 and 1055 cmÀ1 corresponding to C]O stretching, CeO is stretching and CeO bending [24] respectively These oxygen functional groups are generated during the oxidation process of the graphite by Hummer's method [25] In the case of G-ZnO composite, it could be observed that the oxygen functional groups were almost reduced, which is indicating the reduction of GO during the hydrothermal process The absorbance peak at 1581 and 450 cmÀ1 indicated the skeletal vibration of graphene sheets and stretching vibration of ZneO [26] Thus, these results indicate, the formation of ZnO on graphene matrix To further know the morphology of these nanostructures, HRTEM analysis was recorded and the corresponding images are given in Fig It can be seen that the wrinkled structure of graphene sheets is well decorated with ZnO nanoparticles with particle size in the range from 30 to 70 nm The uniform distribution of ZnO nanoparticles on the graphene sheets which contributes to an excellent electrical conductivity of the composite The presence of carbon, zinc and oxygen were further confirmed from EDS spectrum given in the inset picture of Fig 5(b) 3.2 Electrochemical measurements Fig Plot of scan rate verse specific capacitance for GO, rGO, and G-ZnO nanocomposite nanoparticles in composite, the diffraction of carbon atoms in graphene is weakened The ZnO nanoparticles covering the graphene sheets in the composite and gave strong diffraction peaks Hence, the carbon diffraction peak is disappeared in the composite [22] The electronic structure of the G-ZnO nanocomposites was measured by UV diffusive reflectance spectroscopy, as shown in Fig 3(a) It is observed that the graphene-ZnO composite spectrum showed low absorption in the visible and infrared region; however, high absorption in the ultraviolet region, which is in good agreement with the literature by other workers [23] Also, it exhibited a The capacitive performances of the materials were further evaluated by cyclic voltammetry (CV) and galvanostatic charge/ discharge techniques in M KOH electrolyte The CV was run at different scan rates in the potential ranging from to À0.8 V in M KOH electrolyte (Fig 6) The shapes of the CV loop in our experiment exhibit quasi-rectangular shape indicating the capacitive behavior of the capacitor [27] As seen from the Fig, though the curve is partially rectangular, it shows deviation from an ideal rectangular shape with some redox peaks due to Faradaic reaction of ZnO, which indicates the pseudocapacitive nature of the material Also, could be observed from this figure is that as compared to GO, the addition of ZnO to the graphene gives a better and a symmetric curve From the Fig 7, it can be seen that the current level and area of CV curves is higher for G-ZnO nanocomposite than rGO and GO, which indicates more capacitive nature of electrode The possible faradic process can be explained as follows: ZnO ỵ OH 4ZnOOH ỵ e Table ZnO based electrode materials with specific capacitance values Electrode material [SC] (F/g) [SR] (mV/s) [EL] [R] Electrode preparation Carbon aerogel/ZnO Functionalized CNT/ZnO 25 59 10 M KOH 0.1 M TBAPC/DMF [29] [30] Activated graphene/ZnO Graphene/ZnO Graphene/ZnO 84 61.7 122.4 e 50 M KOH M KCl M KOH [31] [16] This work Active material coated on Ni foam electrode Two symmetric electrodes separated by thin polypropylene separator Active material coated on stainless steel electrode ZnO deposited on graphene by USP method Active material coated on carbon paper electrode [SC] e Specific capacitance, [SR] e Scan Rate, [EL] e Electrolyte, [R] e Reference M Saranya et al / Journal of Science: Advanced Materials and Devices (2016) 454e460 Fig Charge/Discharge behavior of GO and G-ZnO electrodes at current density of 0.1 A/g As the scan rate increases, the total peak current also increases demonstrating the good rate properties and capacitance behavior The specific capacitance of the composite is calculated from the following equation [28] Cẳ Iỵ I ị mn where Iỵ is maximum current in the positive scan (A), IÀ is maximum current in the negative scan (A), m is the mass of the active material (g) and y is the scan rate (V/s) The specific capacitance is proportional to the area under the CV curve, which is larger for G-ZnO than GO and rGO A maximum specific capacitance of 122.4 F/g was obtained at a scan rate mV/s for G-ZnO which is higher than that of rGO (102.5 F/g) and GO (2.13 F/g) respectively The high specific capacitance achieved in the G-ZnO may be due to effective electrical and ionic conductivity Fig shows the plot of scan rate versus specific capacitance and from the graph, it is evident that the specific capacitance decreases at higher scan rates, which could be due to the presence of inner active sites that cannot sustain the redox transitions Also, this decrement indicates that the parts of the surface of the electrode are inaccessible at high charging/discharging rates It shows that the G-ZnO composites could be used for high-performance supercapacitor applications A comparison has been given in Table on the specific capacitance of the present work with other reported materials 459 The galvanostatic charge/discharge is a reliable technique to evaluate the electrochemical capacitance of materials under controlled current conditions Fig demonstrates the chargeedischarge behavior of GO and G-ZnO composite at a constant current of 0.1 A/g Both curves exhibited a nearly triangular shape and typical features of potential charge/discharge viz linear response to time with asymmetric capacitive behavior during the discharge process The sudden drop in current at the starting of discharge is due to the internal resistance of electrode material [32] It can be seen that the IR drop of GO is much higher than GZnO, which indicates the larger electrode/electrolyte interfacial resistance in the former Low internal resistances are important for energy storage devices and less energy will be wasted to produce unwanted heat during charge/discharge processes A discharge time of 130 s has been noticed for G-ZnO electrode which is higher than GO discharge time 7.5 s A high discharge time is a clear evidence for the high specific capacitance of G-ZnO electrode These results are in accordance with the CV measurements Electrochemical impedance analysis is an informative technique to evaluate the properties of conductivity and charge transport in the electrode/electrolyte interface Fig 10 shows the Nyquist plot and equivalent fitting circuit of GO and G-ZnO electrodes In the low-frequency region, the impedance plot is increased sharply and tends to become vertical which is due to the capacitive nature of electrode The 45 straight line can be observed in Fig 9(b) shows the pure capacitance behavior of G-ZnO The intercept of higher frequency on X axis yields the electrolyte resistance (Rs) and the diameter of semicircle yields the charge transfer resistance (Rct) [32] A small electrolyte resistance of 12.2 U was observed for GZnO which is due to the excellent conductivity of graphene sheets in ZnO The Warburg impedance can be observed in G-ZnO electrode related to the diffusional impedance of the electrochemical systems Thus, the EIS analysis clearly demonstrated that the graphene-ZnO composite provides easier access to charge between electrode and electrolyte and hence a maximum specific capacitance have been achieved These results suggested that the G-ZnO electrode could be used for high-performance supercapacitor applications Conclusion In summary, XRD evolution demonstrates wurtzite structure of ZnO and a particle size of ~150 nm as observed from SEM investigation The electrochemical behavior of composite was studied by cyclic voltammetry and electrochemical impedance spectroscopy Cyclic voltammetry results show a maximum specific capacitance of 122.4 F/g for G-ZnO composite electrode at mV scan rate The Fig 10 Nyquist plot and an equivalent circuit of GO (a) and G-ZnO (b) 460 M Saranya et al / Journal of Science: Advanced Materials and Devices (2016) 454e460 improved specific capacitance is due to the synergistic effect of ZnO and graphene in the composite materials EIS analysis of composite shows the low resistance and hence easy access of ions towards the maximum specific capacitance This suggested that the G-ZnO electrode is a promising material for supercapacitor applications Acknowledgment This work was completed with financial support from the Platinum Retails Ltd., UK References [1] H Wang, J Lin, Z.X Shen, Polyaniline (PANi) based electrode materials for energy storage and conversion, J Sci Adv Mater Devices (2016) 225e255 [2] Y Zhang, X Sun, L Pan, H Li, Z Sun, C Sun, B KangTay, Carbon nanotubeeZnO nanocomposite electrodes for supercapacitors, Solid State Ionics 180 (2009) 1525e1528 [3] W.T Song, J Xie, S.Y Liu, Y.X Zheng, G.S Cao, T.J Zhu, X.B Zhao, Graphene decorated with ZnO nanocrystals with improved electrochemical properties prepared by a facile in situ hydrothermal route, Int J Electochem Sci (2012) 2164e2174 [4] T Zhu, E Ertekin, Phonons, localization and thermal conductivity of diamond nano threads and amorphous graphene, Nano Lett 16 (2016) 4763e4772 [5] T Zhu, E Ertekin, Generalized Debye-Peierla/Allen-Feldman model for the lattice thermal conductivity of low-dimensional and disordered materials, Phys Rev 93 (2016) 155414e155425 [6] D.W Wang, F Li, F.H Cheng, Hierarchical porous nickel oxide and carbon as electrode materials for asymmetric supercapacitor, J Power Sources 185 (2008) 1563e1568 [7] J Shen, T Li, W Huang, Y Long, N Li, M Ye, One-pot polyelectrolyte assisted hydrothermal synthesis of RuO2-reduced graphene oxide nanocomposites, Electrochim Acta 95 (2013) 155e161 [8] Y.F Yuan, Y.B Pei, S.Y Guo, J Fang, J.L Yang, Sparse MnO2 nanowires clusters for high-performance supercapacitors, Mat Lett 73 (2012) 194e197 [9] S.K Meher, G.R Rao, Ultralayered Co3O4 for high-performance supercapacitor applications, J Phys Chem C 115 (2011) 15646e15654 [10] Z.J Lao, K Konstantinov, Y Tournaire, S.K Ng, G.X Wang, H.K Liu, Synthesis of vanadium pentoxide powders with enhanced surface area for electrochemical capacitors, J Power sources 162 (2006) 1451e1454 [11] G Yu, X Xie, L Pan, Z Bao, Y Cui, Hybrid nanostructured materials for highperformance electrochemical capacitors, Nano Energy (2012) 213e234 [12] Q Wu, Y Xu, Z Yao, Y Liu, G Shi, Supercapacitors based on flexible graphene/ polyaniline nanofiber composite films, ACS Nano (2010) 1963e1970 [13] A.B Djurisic, A.M.C Ng, X.Y Chen, ZnO nanostructures for optoelectronics: materials properties and device applications, Prog Quan Electr 34 (2010) 191e259 [14] G Singh, A Chodhary, D Harananth, A.G Joshi, N Singh, S Singh, R Pasricha, ZnO decorated luminescent graphene as a potential gas sensor at room temperature, Carbon 50 (2012) 385e394 [15] K.K Purushothaman, V Suba Priya, S Nagamuthu, S Vijayakumar, G Muralidharan, Synthesising of ZnO nanopetals for supercapacitor applications, Micro Nano Lett (2011) 668e670 [16] T Lu, Y Zhang, H Li, L Pan, Y Li, Z Sun, Electrochemical behaviors of grapheneeZnO and grapheneeSnO2 composite films for supercapacitor, Electrochim Acta 55 (2010) 4170e4173 [17] T Lu, L Pan, H Li, G Zhu, T Lv, X Liu, Z Sun, T Chen, D.H.C Chua, Microwaveassisted synthesis of graphene-ZnO nanocomposite for electrochemical supercapacitors, J Alloys Compds 509 (2011) 5488e5492 [18] R Ramachandran, S Felix, G.M Joshi, B.P.C Ragupathy, S.K Jeong, A.N Grace, Synthesis of Graphene platelets by chemical and electrochemical route, Mater Res Bull 48 (2013) 3834e3842 [19] M Saranya, G Srishti, I Singh, R Ramachandran, C Santhosh, C Harish, T.M Vanchinathan, M BhanuChandra, A.N Grace, Solvothermal preparation of ZnO/graphene nanocomposites and its photocatalytic properties, Nanosci Nanotechnol Lett (2013) 349e355 [20] A.K Mishra, S Ramprabhu, Functionalized graphene sheets for arsenic removal and desalination of sea water, Desalination 228 (2011) 39e45 [21] H Zeng, Y Cao, J Yang, Z Tang, X Wang, L Sun, Synthesis, optical and electrochemical properties of ZnO nanowires/graphene oxide heterostructures, Nanoscale Res Lett (2013) 133e139 [22] R Ramachandran, M Saranya, P Kollu, B.P.C Raghupathy, S.K Jeong, A.N Grace, Solvothermal synthesis of zinc sulfide decorated graphene (ZnS/G) nanocomposites for novel supercapacitor electrodes, Electrochim Acta 178 (2015) 647e657 [23] R.K Jamal, M.T Hussein, A.M Suhail, Three-photon absorption in ZnO film using short pulse laser, J Mod Phys (2012) 21668e21677 [24] C Xu, X Wang, L Yang, Y Wu, Fabrication of a grapheneecuprous oxide composite, J Solid State Chem 182 (2009) 2486e2490 [25] J Wu, X Shen, L Jiang, K Wang, K Chen, Solvothermal synthesis and characterization of sandwich-like graphene/ZnO nanocomposites, Appl Surf Sci 256 (2009) 2826e2830 [26] K SeokKim, K.Y Rhee, S JinPark, Influence of multi-walled carbon nanotubes on the electrochemical performance of transparent graphene electrodes, Mater Res Bull 46 (2011) 1301e1306 [27] D Zhang, X Zhang, Y Chen, P Yu, C Wang, Y Ma, Enhanced capacitance and rate capability of graphene/polypyrrole composite as electrode material for supercapacitors, J Power Sources 196 (2011) 5990e5996 [28] C Harish, V Sreeharsha, C Santhosh, R Ramachandran, M Saranya, T.M Vanchinathan, K Govardhan, A.N Grace, Synthesis of polyaniline/graphene nanocomposites and its optical, electrical and electrochemical properties, Adv Sci Eng Med (2013) 140e148 [29] D Kalpana, K.S Omkumar, S SureshKumar, N.G Renganathan, A novel high power symmetric ZnO/carbon aerogel composite electrode for electrochemical supercapacitor, Electrochim Acta 52 (2006) 1309e1316 [30] L.S Aravinda, K.K Nagaraja, Nagaraja, K UdayaBhat, B.R Bhat, ZnO/carbon nanotube nanocomposite for high energy density supercapacitors, Electrochim Acta 95 (2013) 119e125 [31] M Selvakumar, D KrishnaBhat, A ManishAggarwal, S PrahladhIyer, G Sravani, Nano ZnO-activated carbon composite electrodes for supercapacitors, Phys B 405 (2010) 2286e2289 [32] G.S Gund, D.P Dubal, B.H Patil, S.S Shinde, C.D Lokhande, Enhanced activity of chemically synthesized hybrid graphene oxide/Mn3O4 composite for highperformance supercapacitors, Electrochim Acta 92 (2013) 205e215  ... which is larger for G- ZnO than GO and rGO A maximum specific capacitance of 122.4 F /g was obtained at a scan rate mV/s for G- ZnO which is higher than that of rGO (102.5 F /g) and GO (2.13 F /g) respectively... band gap diagram of G- ZnO was centrifuged (Eppendorf centrifuge 5430, 460W, Germany) at 7500 rpm with deionized water and dried at 60  C for 24 h 2.3 Synthesis of G- ZnO nanocomposites Graphene- ZnO. .. dried at 60  C for 24 h The weight of graphene in the G- ZnO composite is weight% Fig FTIR spectrum (a) G- ZnO composite (b) Graphene Oxide Materials and experimental 2.1 Materials Graphite powder,