Fabrication of rGO/SrSeO4 nanocomposite as an electrode material with enhanced specific power for supercapacitor applications

Fossil fuels are the major source of energy in the world, which are being depleted day by day. Fossil fuels are also polluting the environment by releasing greenhouse gases such as carbon dioxide. To overcome environmental pollution as well as energy crisis, exploration of new ways of storing energy is the need of the time. In the presented work, reduced graphene oxide/Strontium selenate nanocomposite has been synthesized as an electrode material for energy storage devices. The structure of rGO/ SrSeO4 nanocomposite was confirmed by x-ray diffraction while the morphology was investigated by scanning electron microscopy. The electrochemical measurements, such as cyclic voltammetry, galvanostatic charge/discharge, electrochemical impedance spectroscopy and cyclic stability, were also carried out. The synthesized nanocomposite showed excellent specific power of 684.16 W/kg with 3.67 Wh/kg specific energy, which is significantly much better than already reported metal selenide materials.


Introduction
The ever-increasing population and industrialization have led, on one hand, to the depletion of natural energy resources and on the other, to global warming and climatic changes [1][2][3][4]. These energy crises and environmental pollution has induced a demand to discover new and eco-friendly energy storage and conversion technologies [5,6]. After lithium-ion batteries, supercapacitors (SC) have emerged as new energy storage devices owing to their high efficiency, unique power density and long cycling ability [7][8][9][10][11][12].
SCs can be subdivided into electric double-layer capacitors (EDLCs) and pseudocapacitors based on their charge storing mechanism [13,14]. Pseudocapacitors can store energy more efficiently than EDLCs, however, they are expensive and possess less stability as compared to EDLCs. By the use of redox-active electrode materials, the capacity to store charge can be increased for pseudocapacitors [15][16][17]. Over the decades, various attempts have been made to prepare electrode materials with improved properties because electrode material is the crucial element of SC to boost their performance [18]. Several metal oxides (cobalt oxide, iron oxide, manganese oxide, ruthenium oxide and nickel oxide, etc.) selenides (germanium selenide, tin selenide, etc.) as well as sulphides (cobalt sulfide, molybdenum sulfide, etc.) have been investigated as electrodes for SCs owing to their theoretical high capacitance and redox behaviour [17,[19][20][21][22][23][24][25][26]. However, the high volume expansion and poor electrical conductivity values of metal oxides prevent such material to be employed in practical applications [27].
Selenium as an electroactive material is prominently used for energy storing devices such as batteries, fuel cells and supercapacitors [28,29] because electrical properties of selenium-based materials are higher as compared to other members of its group, i.e. metal sulphides and oxides [30,31]. For example, GeS 2 reported by Wang et al. exhibits high specific capacitance of 300 F/g at the current density of 1 A/g [32]. Similarly, SnSe reported by Zhang et al. demonstrated a specific capacitance of 228 F/g at current density of 0.5 A/g [33].
Carbonaceous materials such as carbon nanotubes [34], activated carbon [35] and carbon aerogels [36] are gaining more attention owing to their less cost, easy processing and microstructure. The most common material is activated carbon for EDLCs due to its high specific surface area as reported 500-2000 m 2 /g [37,38]. However, non-uniform pore size (from micro to macro pores) reduces its specific capacitance [39]. This may be attributed to the fact that electrolyte ions cannot be accessible to micropores [40] while the surface to volume ratio is very less for macropores that results in poor capacitance [9]. Carbon nanotubes show somewhat better capacitance due to its improved surface area as well as conductivity as compared to activated carbon [41]. The improved surface area, along with excellent conductivity, is required to achieve good specific energy.
Graphene has overcome the shortcomings of other carbon materials due to its fine surface area, improved lattice structure and excellent electrical properties [42,43]. The specific capacitance of 550 F/g with 2675 m 2 /g surface area can be achieved due to carbon atoms that are sp 2 hybridized [44]. Thus the incorporation of rGO in SrSeO 4 can be expected to enhance the stability as well as overall electrical performance of the fabricated nanocomposite [45].
Herein, we report an efficient synergetic approach by preparing binder-free SrSeO 4 / rGO nanocomposite to achieve high-energy electrodes. The higher conductivity of rGO as well as better electrical properties of selenium-based materials make them suitable for use in energy storage devices [46]. The rGO was prepared by Hummer's method [47] while SrSeO 4 as well as nanocomposites were synthesized via hydrothermal route. The higher value of specific power as exhibited by the nanocomposite (i.e. 684.16 W/kg) makes it an excellent candidate for supercapacitor applications as most of the small devices of common use (vehicles and aeronautics) require high specific power [48].

Synthesis of reduced graphene oxide
The reduced graphene oxide was synthesized by using the graphite powder via Hummer's method. For this purpose, 50 mL of H 2 SO 4 was poured into a beaker that was already set on the ice bath with a magnetic stirrer. The weighed amount of graphite powder (2 g) was added instantly after the sulphuric acid was poured into the beaker with the temperature of the solution maintained below 20°C throughout. A 6 g of KMnO 4 was added slowly with continuous stirring for 3 h. After the stirring of 3 h, 100 mL of deionized water was poured dropwise in the solution mixture. The temperature of the reaction mixture was kept below 50°C to initiate the oxidation reaction. The dark brown colour of the solution indicated that the graphene oxide was synthesized. Lastly, the reaction was stopped by adding 10 mL of H 2 O 2 to the reaction mixture. Afterwards, the solution mixture was centrifuged with HCl and washed with deionized water for several time so as to maintain the pH of reduced graphene oxide solution to neutral. In the end, it was dried in an oven to obtain a powder of reduced graphene oxide.

Synthesis of strontium selenate (SrSeO 4 )
The SrSeO 4 was synthesized by using the hydrothermal method. The 0.1 M solution of strontium nitrate was prepared and stirred for 10 min. Afterwards, 0.22 g of 0.1 M SeO 2 was added to the above solution and stirred for a while so that the selenium dioxide gets dissolved in strontium nitrate. The 2 mL of hydrazine monohydrate was poured in the solution, which act as a reducing agent after which a change in colour from white to light orange color was observed. After being stirred for more 10 min, the solution mixture was transferred to stainless steel autoclave of 100 mL capacity and put in an electric oven at 200°C for 24 h. The autoclave was cooled naturally and the grayish-black crystals of strontium selenate were obtained. The product was centrifuged and washed several times with deionized water to remove the impurities. The product was dried in an oven at 60°C and grounded in agate mortar and pestle and stored for further use.

Synthesis of composite (rGO/SrSeO 4 )
The nanocomposite of SrSeO 4 with rGO was prepared by adding the weighed amount (0.5 g) of already prepared reduced graphene oxide in the strontium nitrate solution with continuous stirring for 15 min. Subsequently, 0.22 g of SeO 2 was added to the abovementioned mixture. After the complete dissolution of selenium dioxide, 2 mL of hydrazine monohydrate was poured dropwise. The solution thus obtained was then transferred to 100 mL of stainless steel autoclave and given heat treatment of 200°C in an electric oven for 24 h. The resulting solution was centrifuged to remove the impurities by washing with deionized water several times. The blackish needle-like crystals were obtained which were dried in an oven and used as electrode material.

Characterization
The phase and purity of materials investigated in the present study were carried out by powder X-Ray diffractometer (PXRD) analysis by using X-ray diffractometer Bruker D8. The nickel filter was used for the absorption of CuK β radiation and used CuK α radiations. The surface morphology was investigated by scanning electron microscopy (SEM) using scanning electron microscope TESCON MIRA 3. The electrochemical investigations were done by three-electrode system using Autolab PGSTAT 204 workstation at room temperature. The working electrodes (rGO, SrSeO 4 and their composite) were prepared by mingling the synthesized materials into deionized water to form a slurry. The slurry was then pasted onto nickel foam substrate and dried at 60°C in an electric oven. The electrochemical performance was observed by employing cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) as well as the electrochemical impedance spectroscopy (EIS) measurements. The electrolyte used in this study was 2M KOH solution. The Ag/AgCl, Pt wire and the prepared materials (synthesized materials @Ni-foam) were utilized as reference, counter and the working electrode, respectively (Figure 1).

Structural analysis
The synthesized SrSeO 4 was verified by powder x-ray diffraction (PXRD) analysis and the respective diffractogram is presented in Figure 2 The XRD pattern of reduced graphene oxide (rGO) is also shown in Figure 2 The pattern showed two peaks (marked as green with steriks sign) that correspond to (002) and (100) planes, which indicate the successful synthesis of rGO. The XRD pattern of composite (SrSeO 4 /rGO) showed the peaks for both the materials. The presence of peaks for both the materials indicated the successful formation of a composite as no other peak of impurity was observed. The crystallite size as calculated by the Scherrer formula comes out to be 28.7 nm.
The SEM images of all the synthesized materials are shown in Figure 3(a-c). The reduced graphene oxide has sheet-like morphology, as shown in Figure 3(a). The SEM image of strontium selenite showed rod-shaped morphology with a diameter of 40-50 nm range ( Figure  3b). The strontium selenate rods are distributed on the surface of the reduced graphene oxide sheet in the composite as shown in Figure 3(c). The functional groups of graphene oxide, i.e. -OH and -COOH groups, may probably serve as active sites for the adsorption of Sr 2+, thus enabling the uniform distribution of SrSeO 4 on the GO sheets [49].

Electrochemical studies
The supercapacitive behaviour of SrSeO 4 and its composite with rGO was tested through CV, GCD and EIS measurements. Figure 4(a-c) shows the cyclic voltammograms of fabricated nanostructures at various scan rates (5-50 mVs −1 ) in a potential range of 0-0.5 V. The presence of a pair of redox peaks and non-rectangular shape is indicative of the fact that the prepared materials exhibit pseudocapacitive behavior and charge is stored through Faradic mechanisms [35]. Similar results of cyclic voltammetry curves have also been reported by Kavyashree et al. [50]. According to the previous reports, the material exhibiting pseudocapacitive behaviour may be employed as an electrode material for pseudocapacitor applications [51]. It can also be noticed that with increasing the scan rates (from 5 to 50 mVs −1 ), the shape of voltammograms does not vary much. Only a minimal shift in peak position is observed, which suggests that rapid electron transfer occurs between electrodes and the electrolyte [52].
The specific capacity of SrSeO 4 and its composite with rGO nanostructures was calculated from the following equation 1 [35] and the values are tabulated in Table 1.
where I denotes the voltammetric current, ν represents the scan rate, m is mass of the used electrode and V is the potential window respectively during Faradic reaction employed. An insight into Table 1 tells that specific capacitance (C sp ) declines with the increment in scan rate. The lower potential of redox peaks (i.e. higher C sp ) at lower scan rate attributes to the efficient diffusion of ionic species to the electrode from the electrolyte [53]. It is also evident from the voltammograms that redox peaks appear even at higher scan rates, i.e. 50 mVs −1 which affirms the good electrochemical reversibility of rGO/ SrSeO 4 composite in aqueous KOH electrolyte [54].
GCD tests of the studied electrodes (SrSeO 4 and rGO /SrSeO 4 ) at various current densities in the range of 0.12-1.6 Ag −1 were carried out and the results are shown in Figure 4(a and b). The well-defined plateau regions in GDC graphs are coincident with the redox peaks as obtained in cyclic voltammograms thus confirming the pseudocapacitive nature of bare as well as rGO/ SrSeO 4 electrodes. The galvanostatic discharge time as observed from the graph comes out to be 70, 41, 11 and 5 s and 87, 57, 12 and 6 s at a current density of 0.12, 0.2, 0.8 and 1.6 Ag −1 for SrSeO 4 and rGO/ SrSeO 4 respectively. Clearly discharge time is reduced with an increment of current density. This is probably due to the reason that at lower current density the electrolyte ions acquire adequate time to reach the active material and possess small resistance owing to lower excitation of ions as explained by Faisal et al. [35]. However, at higher current densities the time restraint as well as resistance of electrolyte ions become significant and result in diffusion of inner of the active electrode.
The discharge C sp with varying current densities was calculated using Equation (2) [46] and exhibited in Figure 5 and Table 2.
where I represents the discharging current, t is the time and V denotes the potential difference respectively. Interestingly rGO/ SrSeO 4 exhibited high C sp value of 30Fg −1 at a current density of 0.12 Ag −1 as compared to bare SrSeO 4 (23 Fg −1 ) at the same current density. This may be ascribed to the fact that larger surface area and higher conductivity of GO favour the good rate transfer of electrons at the electrode-electrolyte interface thus ensuring the prepared electrodes suitable for supercapacitors applications. Moreover, the binder-free preparations of studied electrode material make it more fascinating for use as a supercapacitor because it possesses the ability to reduce the contact resistance which is found in the electrolyte [55].
Some essential parameters for supercapacitors, e.g. specific energy and specific power, were also deduced from GCD profile as indicated in the Ragone plot ( Figure  5d) by using Equations (3) and (4) respectively [56,57].
where C represents specific capacitance, V denotes the potential window while t is the discharge time respectively.   The measured specific energy and power values are provided in Table 2. As indicated from the Ragone plot-specific energy declines while an increment in specific power is observed due to an increase in current density. This may be attributed to the fact that at higher current densities some hindrance in flow of charge carriers occurs which results in increased polarization of electrode material [51]. Moreover, specific power is observed to be greater as compared to the previously reported value for various chalcogenides [58,59]. The larger value of specific power might be ascribed to the enhanced surface area as well as electrical conductivity of rGO/ SrSeO 4 [49,60]. The higher value of specific power as exhibited by the nanocomposite (i.e. 684.16 W/Kg) makes it an excellent candidate for supercapacitor applications as most of the small devices of common use (vehicles and aeronautics) require high specific power [48].
The cyclic life of supercapacitor is an important parameter for its practical application. The electrochemical stability of nanocomposite was explained by the capacitance retention curve. For this purpose, the active material electrode undergone CV measurements in 2M  KOH at 100 mVs −1 for 1000 cycles. The cyclic plot ( Figure  6a) depicts that initially capacitance is increased with the number of cycles which then becomes stable. This initial increase may be observed owing to the reason that the active materials are not consumed initially [61]. The constant capacitance reveals the robust stability of our prepared composite electrode material and makes it a potential candidate to be used in energy storage devices.

EIS measurements
To confirm the electrical conductivity, electrochemical impedance spectroscopic studies of SrSeO 4 and rGO /SrSeO 4 were carried out in the frequency range of 100 kHz to 0.01 Hz with an AC voltage at 0.5 mV amplitude. The corresponding Nyquist plots are revealed in Figure 6(b) with an inset of equivalent circuit model. Generally, four types of electrochemical resistances are found in electrode materials: R ct (charge transfer resistance) R s (contributions of ionic resistance of the electrolyte intrinsic resistance and contact resistance between the active material and the current collector), Z w (the Warburg impedance) and C F (the electrochemical capacitance) [62].
The more vertical slope of the composite electrode as compared to SrSeO 4 suggested that the composite possesses lower ion diffusion resistance. The semicircle region is related to the faradaic redox reactions and its diameter exhibits R ct [63]. Here the semicircle is smaller in rGO/ SrSeO 4 nanocomposite (12 ohm) than SrSeO 4 (14 ohm) indicating lower value of the semicircle region is related to the faradaic redox reactions and its diameter exhibits the R ct. in the nanocomposite as compared to bare SrSeO 4 . The Warburg section for rGO-based SrSeO 4 composite is less than bare SrSeO 4 thus illustrating that the diffusion of ions through pores becomes swift in our rGO-based nanocomposite. This enhanced electrical conductivity of GO-based SrSeO 4 nanocomposite may be a consequence of the increased surface area of the nanocomposite due to the incorporation of graphene in it as well as complete coverage of rGO-based SrSeO 4 on nickel foam.

Conclusion
Strontium selenite composite with reduced graphene oxide has been successfully synthesized by hydrothermal synthesis on nickel foam substrate. The synthesized rGO/SrSeO 4 composite indicated the C sp value of 30 F/g at 0.12 A/g current density as tested by galvanostatic charge/discharge measurements. The high specific power 684.16 W/kg with 3.67 Wh/kg specific energy is observed by this composite which is significantly better than some already reported metallic selenides. The stability test performed at 0.1 V/s for 1000 cycles exhibited that initially C sp increases gradually with the increase in several cycles and then becomes almost constant until 1000 cycles. This observed trend may be ascribed to the slow activation of electrode material as well as good access of electrolytic ions to the electrode surface with the increment in number of cycles. All these results make our fabricated rGO based composite a suitable material for supercapacitors applications.

Disclosure statement
No potential conflict of interest was reported by the author(s).