Hydrothermal Synthesis of CuWO4-Reduced Graphene Oxide Hybrids and Supercapacitor Application

Corresponding Author: Chandra Sekhar Rout School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar751013, India Email: csrout@iitbbs.ac.in, csrout@gmail.com Abstract: This study reports a facile hydrothermal synthesis of Copper tungsten oxide (CuWO4) and CuWO4-reduced graphene oxide hybrid nanoparticles and its application as an electrode for supercapacitor application. The morphology and composition has been characterized using X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and EDAX. The supercapacitive behavior has been studied from cyclic voltammetry and galvanostatic charge-discharge tests. The CuWO4-reduced graphene oxide hybrid nanoparticles show highest specific capacitance of 35.71 F/g at a current density of 0.25 A/g with excellent cycling stability.


Introduction
The rising consumption of energy and the rapid depletion of global energy resources have made researchers (Dresselhaus and Thomas, 2001;Eisenberg and Nocera, 2005) to develop alternate energy sources. The need for clean and sustainable energy sources has always attracted the scientific community to develop (Liu et al., 2010a;Pushparaj et al., 2007;Rakhi et al., 2012) highly efficient energy storing devices. In the past decade the rapid growth in human population, advancement of hybrid electronic devices and exhaustion of energy sources have given many highly performing (Yang et al., 2011;Aricol et al., 2005) energy storage devices. Among various energy storage systems, supercapacitor also known as ultracapacitor or electrochemical capacitor has acquired the electronics market due to its fast charge-discharge rate, long cycle life, high power density and high reliability. Compared to battery and fuel cells the supercapacitor (Gao, 2005) has successfully fulfilled the need for (Khaligh and Li, 2010) energy backup systems, consumer portable devices and electrical hybrid automobiles. To design high performing supercapacitor (Yan et al., 2014) many criteria are taken into consideration such as high specific capacitance, large rate capability, high cycle stability, toxicity and the cost of the material.
Supercapacitors are categorized into two classes (Halper and Ellenbogen, 2006;Cultura II and Salameh, 2015) based on their energy storing mechanisms (i) Electrochemical Double Layer Capacitors (EDLCs) (ii) pseudocapacitors. EDLC (Yoon et al., 2004) stores charge physically through electrostatic mechanism or non faradic process, while pseudocapacitor (Conway et al., 1997) stores charge chemically through redox reactions known as faradic process. For high performing EDLC (Kim et al., 2006) specific surface area, electrical conductivity, pore size and distribution are taken into consideration. For EDLCs, capacitance values are directly proportional to electrode surface area. Hence, increase of surface area (Lota et al., 2008) is of primary importance for obtaining increased content of the energy stored. For pseudocapacitors (Lu and Zhou, 2011;Nithya et al., 2013;Wu et al., 2011) increase of active material usage in a redox reaction is of primary importance for increasing pseudo-capacitance values. So, activated carbon, CNTs, mesoporous carbon or generally porous carbon materials are used as EDLC while transition metal oxides, conducting polymers are used as pseudocapacitors.
Metaloxides (Wang et al., 2015;Yuan et al., 2014) are highly used to develop high performing supercapacitor. RuO 2 shows specific capacitance of 1585 F/g (Ponrouch et al., 2013) but due to its toxicity and high cost synthesis, the investigation for novel, low cost, environment friendly material is still going on. Nanostructuring (Chen et al., 2013;Wua et al., 2015;Yu et al., 2015) the materials is a reasonable method to improve the supercapacitance of the material. Among many nanomaterials CuWO 4 (Kumar and Karuppuchamy, 2014;Kumar et al., 2015) is a promising material for supercapacitor. It is also used for water splitting (Tang et al., 2016;Gaillard et al., 2013) and photocatalyst (Yourey, 2014). The preparation of CuWO 4 plays vital role in determining the supercapacitance. Hydrothermal synthesis (Yang et al., 2013) of metal oxides is a promising method for large scale and low-cost production with high-quality crystals. Recently many works have been reported on graphene based metal oxides (Liu et al., 2010b;Wang et al., 2015;Dong et al., 2012;Huang et al., 2012) to enhance the performance of supercapacitor because graphene shows (Geim, 2009) large surface area, high electrical conductivity (Neto et al., 2009), thermal conductivity (Balandin, 2011), profuse interlayer structure and excellent mechanical stability.
In this study we have reported the facile hydrothermal synthesis of CuWO 4 and CuWO 4reduced graphene oxide hybrid nanostructures. The morphology and composition have been characterized using XRD, SEM and EDAX. The electrochemical measurements have been performed to study specific capacitance of the prepared materials.

Preparation of CuWO 4 and CuWO 4 -Reduced Graphene Oxide Hybrid Nanostructure
The synthesis of CuWO 4 and CuWO 4 -reduced graphene oxide hybrid nanostructures followed facile hydrothermal method. A mixture of 0.392 g CuO 4 S, 0.79 g Na 2 WO 4 .2N 2 O and 1.92 g urea was mixed in 40 mL of de-ionised water and stirred for 1 h at room temperature at 900 rpm. Then the solution was transferred to a 50 mL Teflon lined stainless steel autoclave and heated for 12 h at 200°C in hot air oven. After the synthesis, the precipitate was collected and rinsed several times with ethanol. Then the collected sample was dried at 70°C for and annealed at 450°C for 12 h.
For the synthesis of CuWO 4 -reduced graphene oxide hybrid nanostructures the same procedure has been followed as mentioned above but with addition graphene oxide to obtain different concentration of CuWO 4reduced graphene oxide hybrid nanostructures.

Material Characterization
The elemental composition and morphology of the prepared sample was investigated by Energy dispersive X-ray spectroscopy (EDAX) and Field Emission Scanning Electron Microscopy [(FESEM) (MERLIN Compact with GEMINI I electron column, Zeiss Pvt. Ltd., Germany)]. The powder X-ray diffraction spectra were measured by (Bruker D8 advanced diffractometer) method with Cu Kα radiation, λ = 0.154184 nm. The electrochemical performance was recorded by a three electrode electrochemical test using a Potentiostat (PG-16125, Techno science instrument, Bangalore, India).

Preparation of Electrode and Electrochemical Measurement
The working electrode for electrochemical measurement was prepared by mixing the obtained sample with ethanol and then coating it on nickel foam. The prepared sample and ethanol was sonicated for almost 20 min to make homogeneous mixture and then coated on the rectangular nickel foam strip for two times to get uniform coating. Then the coated nickel foam was dried at room temperature in a vacuum container for almost 24 h followed by pressing. The mass of the Ni foam was recorded before the coating and after the coating to know the amount of material deposited on it. The electrochemical performance of the CuWO 4 and CuWO 4 -reduced graphene oxide hybrid nanostructures was observed by a three electrode electrochemical test using a Potentiostat (PG-16125, Techno science instrument, Bangalore, India) in 3M aqueous KOH solution. The Cyclic voltammetry and Charge-discharge tests were carried out at room temperature within a potential range between -0.1 to 0.4 V Vs. Ag/AgCl electrode. The coated nickel strip was used as the working electrode and Pt coil as counter electrode in this three electrode test. Specific capacitance (Cs) of a material is obtained from the cyclic voltammetry curve at different scan rates using the Equation 1: where, the numerator gives the total area under the CV curve, [V f -V i ] is called the potential window (V f = Final potential value and V i = Initial potential value], 'm' is mass loaded on the Working electrode and 's' is the scan rate.
The specific capacitance can also be obtained from the charge-discharge curve using Equation 2: where, 'I' is the discharge current, 'm' is the mass of sample loaded on the working electrode and 'dV/dt' is the slope of the discharge curve.

Composition and Morphology Study
Figure 2(a) and (b) shows the FESEM picture of CuWO 4 -reduced graphene oxide hybrid nanostructures. From the image it can be clearly observed that the prepared sample consists of nanoparticles of avg. diameter 20-30 nm. The uniform growth of the 80 mg CuWO 4 -reduced graphene oxide hybrid nanostructures nanoparticle was examined from the Fig. 2(c) shows the EDAX spectrum and elemental composition data which demonstrates the presence of Cu, W, O and C that confirms the proper formation of CuWO4-reduced graphene oxide hybrid nanopartcles. For further understanding of chemical composition of the prepared sample the elemental mapping is studied which is shown in Fig. 3. Figure 1 shows the X-ray diffraction peaks of the CuWO 4 -reduced graphene oxide hybrid nanostructures and CuWO 4 . The peak matches completely with the characteristic peaks of CuWO 4 (Jcpds No 88-0269). In addition to the characteristic peaks of CuWO 4 , the peaks of graphene can be observed from the figure. The peak at 2θ = 25.95° can be interpreted as the merged peak of CuWO 4 [(110)] and graphene [(111)]. The characteristic peaks of graphene shows two peaks at 2θ = 26.5° (111) and 54.6° (222). Here, in figure two peaks were observed due to the presence of graphene one at 2θ = 25.95° and the other at 55°. The combined presence of both CuWO 4 and graphene confirms the formation of pure CuWO 4reduced graphene oxide hybrid nanostructures.

Electrochemical Measurement
The electrochemical measurements were carried out to study the supercapacitive behavior of the synthesized materials. The measurements followed three electrode cyclic voltammetry and charge discharge test in 3M aqueous KOH solution. The experiments were performed keeping the potential window in the range of -0.1 to 0.4 volt. Figure 4(a) shows the cyclic voltammetry measurement of CuWO 4 and 80 mg CuWO 4 -reduced graphene oxide hybrid nanostructures. From this figure it was calculated that CuWO 4 shows specific capacitance (Cs) of 11.8 F/g while 80 mg CuWO 4 -reduced graphene oxide hybrid nanostructures shows specific capacitance (Cs) of 16.6 F/g at 2 mv/s scan rate. The cyclic voltammetry curves of 80 mg CuWO 4 -reduced graphene oxide hybrid nanostructures at different scan rates can be examined from the Fig. 4(b). It can be observed that the decrease of specific capacitance and shifting of the reduction peak towards left takes place with increase in scan rate. The maximum obtained specific capacitance of the 80 mg CuWO 4 -reduced graphene oxide hybrid nanostructures was found to be 16.6 F/g at 2 mv/s. The galvanostatic charge discharge test was conducted to have better understanding of the supercapacitive performance of the material. Figure 4(c) shows the charge-discharge curves of the 80 mg CuWO 4 -reduced graphene oxide hybrid nanostructures at different current densities of 0.25, 0.5, 1 and 2 A/g. The maximum value of specific capacitance from charge discharge curves was found to be 35.71 F/g at current density of 0.25 A/g. Here the specific capacitance decreases with increase in current density which can be clearly observed from the figure. From the Fig. 4(d) the cycle stability of the 80 mg CuWO 4 -reduced graphene oxide hybrid nanostructures CuWO 4 can be clearly observed. The long Cycle stability is the most important criteria for a material to perform as supercapacitor. Here, from the figure it can be inferred that the CuWO 4 -reduced graphene oxide hybrid nanostructures material shows good cycle stability over 1000 cycles. The galvanostatic charge discharge test for cycle stability was performed at 10 A/g. The material has maintained the initially obtained specific capacitance value even at 1000 cycles. This ensures the material to be a promising candidate for supercapacitor application. Figure 4(e) shows a comparison curve of different synthesized materials on the basis of their specific capacitance value. The different synthesized materials are CuWO 4 , 40 mg CuWO 4 -reduced graphene oxide hybrid nanostructures, 80mg CuWO 4 -reduced graphene oxide hybrid nanostructures and 160 mg CuWO 4 -reduced graphene oxide hybrid nanostructures. From the charge discharge curve it was calculated that the synthesized material shows specific capacitance of 25, 31.2, 35.7 and 15.6 at 0.25 A/g respectively. Hence, the 80 mg CuWO 4reduced graphene oxide hybrid nanostructures shows highest specific capacitance of 35.7 F/g with excellent cycle stability and can be utilized as a promising electrode for supercapacitor application in future.

Conclusion
Here we report the successful synthesis of CuWO 4 and CuWO 4 -reduced graphene oxide hybrid nanostructures by hydrothermal route. The XRD, FESEM, EDAX illustrates the morphology and structure of the prepared sample in detail. The supercapacitive behavior has been studied using cyclic voltammetry and galvanostatic charge discharge tests. Among all prepared samples the CuWO 4 -reduced graphene oxide hybrid nanostructures shows maximum specific capacitance of 35.71 at 0.25 A/g with excellent cycle stability. Considering all the obtained data, the CuWO 4 -reduced graphene oxide (with 80 mg GO) hybrid nanostructures can be utilized as an electrode for supercapacitor application.

Funding Information
Dr. C.S. Rout would like to thank DST (Government of India) for the Ramanujan fellowship. This work was supported by the DST-SERB Fast-track Young scientist