nickel oxide composite for binder-free supercapacitors

nickel oxide composite for binder-free supercapacitors

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One-step fabrication of electrochemically reduced graphene oxide/nickel oxide composite for binder-free supercapacitors Saeed Shahrokhian a,b,*, Rahim Mohammadi a, Elham Asadian b a b

Department of Chemistry, Sharif University of Technology, Tehran 11155e9516, Iran Institute for Nanoscience and Technology, Sharif University of Technology, Tehran, Iran

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Article history:

A three-dimensional (3D) graphene/Nickel oxide (ERGO/NiO) composite electrode have

Received 24 April 2016

been fabricated directly on a Nickel foam substrate via a one-step electrochemical co-

Received in revised form

deposition in an aqueous solution containing Nickel nitrate and GO. By using this simple

11 July 2016

and one-step electrochemical deposition, it is possible to produce binder-free composite

Accepted 12 July 2016

electrodes with improved electrochemical properties using a low-cost, facile and scalable technique. It is observed from FE-SEM images that graphene oxide sheets affect the electrodeposition of nickel oxide. The optimized ErGO/NiO electrode developed in this work


exhibits high charge storage capacity with a specific capacitance of 1715.5 F g1 at current

Reduced graphene oxide

density 2 A g1 and hierarchical morphological structure which facilitates electrolyte

Nickel oxide

diffusion to the electrode surface. A good cycling stability was observed for the modified

Composite electrode

electrodes in alkaline media. EIS measurements showed low values of internal resistance


(Rs) and charge transfer resistance (Rct) for the modified electrodes, indicating that the

Electrochemical deposition

prepared nanocomposite is appropriate for supercapacitor applications in comparison to NiO/NF electrode. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The increasing of energy consumption and growing concerns about environmental pollution and global warming have stimulated researches on energy storage and conversion from alternative energy sources. In this case, Supercapacitors; which are also known as ultra-capacitors or electrochemical capacitors, have attracted great attention in recent years as promising energy storage devices due to their high power density, rapid charging/discharging rates, long cycle life and low maintenance cost [1e4]. Generally, supercapacitors can be

divided into electric double-layer capacitors (EDLCs) and pseudocapacitors based on their charge storage mechanisms. As EDLCs, involve electrochemical inactive materials with high surface area which used to store energy through charge accumulation at the electrode/electrolyte interface. EDLCs show ultrahigh power density and excellent cycle life; however, their specific capacitances and energy densities are usually restricted by the limited effective surface area of active materials. Pseudocapacitors on the other hand, are dominated by reversible and fast Faradaic reactions on the surface of electrode materials. Compared with EDLCs, pseudocapacitors exhibit better specific capacitances and higher

* Corresponding author. Department of Chemistry, Sharif University of Technology, Tehran 11155e9516, Iran. Fax: þ98 21 66002983. E-mail address: [email protected] (S. Shahrokhian). 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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energy densities, but usually suffer from poor electrical conductivity. Generally, the material components of supercapacitors can be classified into three major categories: (1) carbon materials such as activated carbon, carbon nanotubes, graphene and etc. [5e7], (2) conductive polymers, such as polyaniline and polypyrrole [8e10] and (3) transition metal oxides and hydroxides, such as RuO2, NiO, MnO2 and CuO [11e13]. Nanostructured transition metal oxides and hydroxides have attracted extensive attention in Electrochemical Capacitors (ECs) since they exhibit illustrious properties including high redox activity and capacity [11e13]. As an important family of metal oxides, cobalt and nickel oxide plays a significant role in ECs [14e18]. However, low electrical conductivity of these materials limits their application in ECs. In order to overcome this problem, synthesizing composites of them with different carbon based materials with high electrical conductivity is a significant strategy to improve their specific capacitance. Among various carbon materials for energy storage devices, graphene is considered as the most promising electrode material [19]. Recently, graphene nanosheets have been attracted a great deal of attention due to their unique electronic, mechanical and thermal properties such as ultra-high electrical conductivity (103e104 S m1), specific surface area (theoretical value ~2630 m2 g1) and good thermal conductivity (~5000 W m1 K1) [20e22]. As a result, tremendous researches have been devoted to discovering interesting characteristics of graphene-based materials over the past few years. For this reason, various graphene composites with transition metal oxide or hydroxide have been fabricated to be used as electrode materials for supercapacitors [14e18]. In this regard, different routes have been used for the fabrication of grapheneemetal oxides/hydroxide composite electrodes including chemical vapor deposition [23e25], chemical and hydrothermal precipitation [26,27], microwave assisted synthesis [28e30] and electrochemical deposition [7,31,32]. Among all the above mentioned methods, electrodeposition is a simple, rapid, green and low cost approach, which allows tailoring not only the metal oxide/hydroxide composition but also the porosity (pore size and morphology) of the final product to achieve optimized electrochemical behavior of the resulting electrodes. Moreover, through this method, the composite can be produced directly onto the current collector, avoiding the use of binders and additives that introduce additional resistances to the electrode [33]. Different electrochemical approaches including potentiostatic, galvanostatic, potentiodynamic and pulse methods can be used to deposit electroactive materials on the surface of the electrodes [34]. Among these various electrodeposition techniques, potentiodynamic route have been attracted extensive attention since it can lead to more controlled deposits compared to other methods due to the “discontinuous” deposition process associated with a break in the deposition conditions during each cycle [35,36]. One-step electrochemical co-deposition of graphene and metal oxide/hydroxides is of great importance due to the simplicity and time saving procedure. In this regard, Zhao et al., co-electrodeposited a MnO2/graphene oxide coating on carbon paper using cyclic voltammetry and the composite


used as electrode for SCs (Supercapacitors), which displayed increased electrochemical charge storage ability, attaining specific capacitances of 829 F g1 at current density of 1 A g1  mez and co-workers [38] elec[37]. More recently, Garcı´a-Go trodeposited CoOx/graphene foams, using a one-step electrodeposition process on stainless steel substrate. The optimized ERGO/CoOx developed in that work exhibits a specific capacitance of 608 F g1 at a current density of 1 A g1 and increased reversibility when compared to single CoOx. Herein, for the first time an ERGO/NiO composite was synthesized on nickel foam substrate by a one-step potentiodynamic procedure in the potential range of 0.5 to 1.5 V vs Ag/AgCl (3.5 M) in a solution containing 1 mg mL1 GO and 10 mM nickel nitrate as electrolyte. In order to obtain the optimum condition for the electrochemical deposition, various scan rates between 50 and 200 mV s1 were selected to deposit the composite material. On the other hand, to investigate the effect of GO nanosheets, NiO electrode was also prepared in the absence of GO in the electrolyte solution. The electrochemical results revealed that 3D nickel foam/ERGO/NiO architecture has superior electrochemical properties including high capacitance, good cycling stability and good rate capability performance.

Experimental All chemicals were of analytical reagent grade purchased from Sigma Aldrich and used as received without any further purification. Also, all aqueous solutions were prepared with DI water (Millipore Water Purification, 18 MU).

Synthesis of graphene oxide Graphene oxide (GO) nanosheets were synthesized by the modified Hummers method as reported by Marcano et al. [39]. In brief, concentrated H2SO4 was added to a mixture of graphite flakes and NaNO3, and the mixture was cooled to 0  C using an ice bath. KMnO4 was added slowly in portions to keep the reaction temperature below 20  C. The reaction was heated to 35  C while stirred for 7 h. Additional KMnO4 was added in one portion, and the reaction was stirred for 12 h at 35  C. The reaction mixture was cooled to room temperature and poured onto ice containing 30% H2O2. Finally, the GO suspension was filtered until the pH of supernatant become neutral.

Synthesis of ERGO/nickel oxide nanocomposite Graphene oxide with a concentration of 1 mg mL1 was dispersed in DI water and then to this dispersion, 0.1454 g Ni(NO3)2.6H2O and NaNO3 0.1 M was added under magnetic stirring. This suspension was sonicated for 1 h and used as a supporting electrolyte for electrodeposition of graphene/ nickel hydroxide composite. Electrochemical experiments were performed by an Autolab PGSTAT 101 (Metrohm, Netherlands) in a threeelectrode system with Nickel foam (1 cm  1 cm), Pt foil (1 cm  2 cm) and Ag/AgCl (KCl, 3.5 M) as working, counter and reference electrodes, respectively. Potentiodynamic route was used for


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the electrodeposition of composite in the potential range of 0.5 to 1.5 V in the GO/Nickel nitrate electrolyte suspension. It should be noted that prior to each experiment, the electrolyte solution was deaerated for 10 min to ensure the complete removal of any dissolved oxygen molecules. Potentiodynamic approach with fixed 20 cycles but under different scan rates (from 50 to 200 mV s1) was chosen for electrodeposition. Modified electrodes were dried at 60  C overnight and then heated in a furnace at 300  C for 2 h in air. For comparison, NiO/NF was also prepared in optimized conditions of ERGO/ NiO/NF electrode. The mass loading of ERGO/NiO on NF surface for 200, 150, 100 and 50 mV s1 at fix cycle number 20, were between 0.4 and 0.7 mg. For the NiO/NF electrode the mass loading was ~0.6 mg.

Characterization X-ray diffraction (XRD) patterns of GO, NiO/NF and hybrid NiO/ERGO/NF were recorded with a X-ray diffractometer (GBCMMA, Instrument) in the 2q range from 0 to 80 using Cu Ka radiation. The crystallographic structures of the electrodes were determined by X'Pert Pro MPD X-ray diffraction (XRD) software. The structure and surface morphology of NiO/NF and NiO/ERGO/NF hybrid were studied by a field-emission scanning electron microscope (FE-SEM MIRA 3 TESCAN, 15 kV, Czech) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector. FTIR spectra were obtained by using a Perkin Elmer FTIR spectrometer (spectrum 100).

Fig. 1 e Voltammograms of ERGO/NiO/NF composite deposited at various scan rates (50e200 mV s¡1 ) at fixed 20 cycles, A) 200 mV s¡1 , B) 150 mV s¡1, C) 100 mV s¡1, D) 50 mV s¡1 , E) rate capability of composite electrodes at various scan rates (5, 10, 20, 30, 40 and 50 mV s¡1).

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Electrochemical studies Electrochemical experiments were performed in 1 M NaOH solution with an Autolab PGSTAT 101 (Metrohm, Netherlands) in a threeeelectrode system with NF, Pt foil and Ag/AgCl, as the working, auxiliary and reference electrodes, respectively. All potentials are quoted versus Ag/AgCl. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a galvanostat/potentiostat EG&G model 273A (Princton Applied Research, USA) equipped with a frequency response detector, superimposed with a sinusoidal voltage with an amplitude of 5 mV in the frequency range of 100 kHz to 10 mHz. All EIS measurements were conducted at an open circuit potential. The voltammetric charge (q) integrated from cyclic voltammograms can be used as an effective criterion for determining the pseudocapacitance using the following equation; Z C¼

  i*dv 2*ðm*yDVÞ

Where, i is the current in A, m is the mass of active material (g), y is the scan rate (V/s) and DV is the investigated potential window. The specific capacitance of electrode was also derived through galvaonostat charge/discharges measurements using the following equations, Cs ¼ i  Dt=m  DV, where Cm is the specific capacitance in F/g, Dt is the discharge time (s), i is the discharge current (A), DV is the potential range (V), and m (g) is weight of the active material.

Results and discussions In order to evaluate the influence of the CV electrodeposition scan rate, the ERGO/NiO composites were fabricated at different scan rates (i.e. 50, 100, 150, and 200 mV s1) for 20 cycles between 0.5 to 1.5 V (vs. Ag/AgCl (3.5 M KCl)) in the


deposition electrolyte and after thermal annealing as the results are shown in Fig. 1. It is obviously distinct that the CV curves show a pair of nearly reversible redox peaks for the composites deposited at different scan rates, indicating their pseudocapacitive behaviors [40e42]. Nickel hydroxide (NiOOH) would be formed at the surface of the Ni-based electrodes from faradaic redox reactions between NiO and NiOOH in alkaline solution as follows [42,43]: NiO þ OH 4 NiO(OH) þ e (1). The larger of the area integrated within the currentepotential curves indicates the higher capacitance of ERGO/NiO/NF deposited at scan rate of 200 mV s1. This can be elucidated considering the more porous nature of the deposits formed in this case. In fact, for the composite deposited at higher scan rates, the residence time of the electrode at any potential is shorter, resulting in a declined total amount of material deposited in each cycle [35]. As a result, the deposited nanocomposite at scan rate of 200 mV s1 shows highest specific capacitance. For the hybrid electrode that deposited at scan rate 200 mV s1, by increasing the scan rate from 5 to 50 mV s1, the capacity of ERGO/NiO/NF decreases which can be attributed to the limited diffusion of ions from the electrolyte into the active materials at higher scan rates. In fact, higher scan rates is accompanied with the unsatisfactory time available for ion diffusion and adsorption of ions inside the active materials [44]. Moreover, with the increasing scan rates, the anodic and cathodic peaks shifted in a positive and negative direction, respectively. These phenomena could be attributed to the polarization of the electrodes at the higher scan rates [45]. Even at 50 mV s1, the CV curve still shows a pair of redox peaks, indicating that this diverse structure is beneficial for fast redox reactions [46]. Hence, the scan rate of 200 mV s1 is selected as the optimum potentiodynamic scan rate for investigation of capacitance performance. The as-prepared graphene based composite are also characterized by FT-IR and XRD techniques. FT-IR spectra provide proofs for the in situ electrodeposition of ERGO. As shown in

Fig. 2 e A) FT-IR spectra of GO (black) and ERGO/NiO/NF (red), B) XRD pattern of GO (blue), ERGO/NiO/NF composite (red), NiO/ NF (black). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article)


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Fig. 3 e FE-SEM images of Nickel foam at different magnifications (AeC), FE-SEM of ERGO/NiO/NF composite at different magnifications (D, E), F) EDS spectra of composite determined by square in C (inset shows atomic percent of C, O and Ni elements), FE-SEM of NiO/NF electrode at different magnifications (G, H), I) EDS spectra of NiO/NF electrode determined by square in H (inset shows atomic percent of O and Ni elements).

Fig. 2 A, it can be clearly seen that almost no absorbance peaks or very low intense ones are observed for oxygen functional groups in the case of ERGO/NiO/NF except for the hydroxyl groups (eOH) at around 3400 cm1, which can be attributed to the presence of water molecules in the composite material. While for GO, many overlapped and intense absorbance peaks are obtained. The absorption bands at 1640 and ~1745 cm1 were attributed to carboxyl and carbonyl (C]O) groups, respectively. The epoxy (CeO) band at around 1225 cm1 and hydroxyl groups (eOH) at about 3400 cm1 of ERGO obviously decreased in comparison to GO, indicating the effectiveness of in situ electrochemical reduction of GO in composite. Furthermore, the band which appeared at about 626 cm1 is attributed to NieO stretching vibration mode. The high reduction degree of GO nanosheets can provide high

conductivity and large-current discharge capacity [42]. The XRD pattern of GO, NF, NiO/NF and ERGO/NiO/NF are shown in Fig. 2. B. As can be seen, the XRD pattern for GO shows a clear diffraction peak at around 2Ɵ ¼ 10.85 with d spacing 0.808 nm as the characteristic peak for GO material which is attributed to the successful exfoliation of carbon sheets in graphite with a d spacing of 0.34 nm [47]. The peaks at 2Ɵ 44 , 52 and 76 is attributed to (111), (200) and (220) planes of nickel foam substrate. Two peaks are observed at 37.9 , 44.9 corresponding to (111) and (200) planes of NiO, respectively, which indicates that NiO is crystalline in nature [30]. The SEM images in Fig. 3 (AeC) show the morphology of the Ni foam substrate at different magnifications, which show the porous nature of nickel foam substrate that makes it a promising candidate for supercapacitor applications.

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200 R² = 0.9726


Current density (A/g)





-100 R² = 0.9622


-200 0







Scan rate (mV/s) Fig. 4 e Cathodic (black) and anodic (red) peak currents vs Scan rate of the redox peaks for ERGO/NiO/NF. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article)

However, it is clear that there is a coating layer formed on the Ni foam in the high magnification FE-SEM after applying potentiodynamic route and thermal annealing of composite (Fig. 3 D, E), indicating the successful deposition of ERGO/NiO composite on the surface of Ni foam. These nanosheets are growing uniformly and perpendicularly on the surface of Ni foam substrate. Moreover, Fig. 3F shows the elemental analysis of the composite prepared at the scan rate of 200 mV s1


in which, the presence of carbon, nickel and oxygen in close values to each other, proves that ERGO/NiO uniformly electrodeposited on the nickel foam substrate. To evaluate the electrochemical performance of the composite electrode, electrochemical measurements were conducted in a three-electrode cell using 1 M NaOH aqueous solution as the electrolyte. Fig. 4 shows variations of anodic and cathodic peaks at different scan rates (5e50 mV s1) for ERGO/NiO/NF electrode, which corresponding CV curves showed previously in Fig. 1A. As can be seen, both anodic and cathodic peaks increase linearly by increasing the scan rate, which demonstrates that the redox reaction is a surface controlled process. This observation can be attributed to the charge propagation in the layer of ERGO/NiO/NF composite and also indicates that diffusion of electrolyte ions to the surface of composite is fast. Fig. 5A shows galvanostatic discharge plots of ERGO/NiO/ NF electrode at different current densities from 2 to 40 A g1. The plateau that observed at potential around 0.35 V is attributed to the electrochemical redox reactions at the interface between Niþ2 and Niþ3 active electrode material and aqueous electrolyte as previously mentioned. However, at higher current densities, the electrolyte ions can only diffuse into the surface and interlayer spaces of the electrode so that a significant portion of the active material remains intact, resulting in a decrease of specific capacitance for the modified electrode. By increasing the current density from 2 to 40 A g1, the specific capacitance decreases from 1715.5 to 1066.7 F g1. Such a high retention of specific capacitance (more than 62% of its initial capacitance at very high current density 40 A g1) may be attributed to the synergetic effect between graphene nanosheets and nickel oxide nanostructures which shortens the electrolyte diffusion through composite electrode (Fig. 5B). In order to further investigate the effect of ERGO nanosheets on the electrochemical performance of the composite, nickel oxide electrode was also prepared in similar conditions except without adding graphene oxide to the electrolyte

Fig. 5 e Galvanostatic charge/discharges of ERGO/NiO/NF composite deposited at scan rate 200 mV s¡1 at different current densities (2e40 A g¡1 ), B) rate capability of composite electrodes at various current densities (2, 5, 10, 20 and 40 A g¡1 ).


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Fig. 6 e A) Cyclic voltammograms of NiO deposited on NF surface at scan rate of 200 mV s¡1 by 20 cycles at different scan rates (5, 10, 20, 30, 40 and 50 mV s¡1), B) rate capability at different scan rates, C) Galvanostatic charge/discharges of NiO/NF deposited at 200 mV s¡1 at different current densities (2, 5, 10, 20 and 40 A g¡1 ), D) rate capability of NiO/NF electrode at various current densities.

solution. Fig. 3G and H demonstrate the FESEM images of NiO/ NF electrode surface at different magnifications. It is obvious that a porous film of Nickel oxide is formed on a surface of electrode as evidenced by the cracks in the film structure. By comparison morphological images of NiO/NF and ERGO/NiO/ NF, It is obviously seen that graphene oxide sheets affects the electrodeposition of nickel oxide. Fig. 3F shows EDS spectra of NiO/NF electrode which shows Nickel and oxygen elements in close values to each other proves that as an earlier case for ERGO/NiO/NF, NiO structures uniformly covered the surface of nickel foam substrate. Electrochemical methods including cyclic voltammetry and chronopotentiometry were also applied to further study the NiO/NF electrode. Fig. 6A represents voltammograms of NiO/NF that are obtained at different

scan rates. Here, similar to hybrid electrode, a pair of redox peaks is clearly identified in each of the CV curve, revealing the pseudocapacitive reaction originated from redox mechanism of NiO and NiOOH pair. By increasing the scan rate from 5 to 50 mV s1, the specific capacitance decreased dramatically from 1046.8 to 471.6 F g1, which is lower than the composite electrode (Fig. 6B). The rate capability of NiO/NF electrode was also investigated and the results are shown in Fig. 6C. As can be seen, the capacitance of NiO/NF electrode varies by increasing the current density from 2 to 40 F g1 while retaining 25% of its initial value even at the current density as high as 40 F g1. The higher retention value of the composite electrode compared to NiO/NF electrode may be attributed to the presence of graphene nanosheets, which not

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Fig. 7 e A) Capacitance retention vs cycle number for ERGO/NiO/NF (black) and NiO/NF (red) at scan rate 30 mV s¡1, B) 1st and 2000th cycle of nanocomposite at scan rate 30 mV s¡1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article)

only enhances the electrical conductivity but also facilitates the electrolyte ions diffusion through the electrode/electrolyte interface. Long cycle stability is another important requirement for practical applications of supercapacitors [48,49]. The cycle life

Fig. 8 e EIS spectra of ERGO/NiO/NF before (black) and after 2000 successive cycles and NiO/NF (blue) at open circuit potential. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article)

was measured over 2000 cycles for the modified electrodes at a scan rate 30 mV s1 Fig. 7A exhibits excellent longeterm cycling stability of the ERGO/NiO/NF electrode which is higher than NiO/NF. In the case of both electrodes, increasing the specific capacitance at the initial cycles can be resulted in the activation of NiO in the alkaline medium, as previously reported [47]. As can be seen, even after 2000 cycles, the ERGO/ NiO/NF and NiO/NF electrodes still retained their specific capacitances at 78.8% and 70.7%, respectively. Fig. 7B shows 1st and 2000th cycles for ERGO/NiO/NF electrode. The excellent cycling stability of ERGO/NiO/NF electrode should originates from the good structure stability and good physical contact of graphene sheets and nickel oxide structures on the surface of Ni foam substrate [50,51]. EIS analysis was also used to study the capability of the electrode materials in supercapacitor applications (Fig. 8). The intercept of the Nyquist plot at high frequency region represents the equivalent series resistance (Rs) of the electrode which originates from the internal resistance of electrode, electrolyte resistance and resistances of all contacts in electrochemical system. Each curve consists of a semicircle arc in high frequencies and a straight line in the low frequency region. The intercept of the semicircle on the real axis (Z0 ) at high frequency is Rs which is lower for composite than NiO/NF electrode, confirming the good conductivity of the electrolyte and very low internal resistance of the composite electrode which may be attributed to the higher electrical conductivity of composite compared to NiO/NF electrode. Additionally, the high frequency semicircle can be attributed to the resistance against the charge transfer process (Rct) between the electrode surface (electronic conductivity) and the electrolyte (ionic conductivity). The small semicircle illustrates the more limited charge


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transfer resistance of composite than NiO/NF. The vertical line parallel to imaginary axis in low frequency region for both electrodes suggests a typical capacitive behavior of electrodes. As seen from inset of Fig. 8 after 2000 cycles, Rs and Rct remains approximately constant and the steeper slope of the plot at low frequency region obtained after cycling than that before cycling for hybrid electrode indicates the more obvious pseudo-capacitive behavior of the electrode after 2000 cycles [52,53].

Conclusions A simple and facile one-step electrochemical method was presented to fabricate the ERGO/NiO/NF electrode. Graphene nanosheets were also used not only to enhance the stability of NiO nanostructures but also to improve the electrical conductivity of the resulting composite electrode. The surface morphology of the nanocomposite and NiO/NF electrode and theirs capacitive behavior studied. Microstructure measurements indicated the deposition of perpendicular sheets of crystalline NiO on the surface of nickel foam. The composite electrode has a specific capacitance of 1715.5 F g1 at current density 2 A g1 which retains 62% of its initial value at very high current density 40 A g1. Such high capacitance retention may be attributed to the synergetic effect between graphene nanosheets and nickel oxide nanostructures which shortens the electrolyte diffusion through composite electrode as proved by microstructure analysis. This behavior can be resulted from the uniform distribution of nickel oxide coating on graphene nanosheets. This uniform coating and higher conductivity of the composite, facilitates electrolyte diffusion and electron transfer throughout the nanocomposite. Lower values of ESR and Rct observed for composite than NiO/NF. The results revealed that the composite electrode can be used as a new electrode material for supercapacitor applications.

Acknowledgments The authors gratefully acknowledge the support of this work by the Research Council and the Center of Excellence for Nanostructures of the Sharif University of Technology, Tehran, Iran. They are grateful to Institute of National Science Foundation (INSF, Iran) (Grant No.94/44025) for financial supports of this work.
















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