Chemical Synthesis and Characterization of Nickel Sulphide Thin Film Electrode for Supercapacitor Performances

All Nickel Sulphide thin films were deposited onto the stainless steel substrate by modified chemical bath deposition method. The structural, surface morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) respectively. The electrochemical capacitor performances were examined by using cyclic voltammetry and galvanostatic charge-discharge method. The NiS electrode exhibits a good cycling performance. The specific capacitance of 353 Fgm -1 has been obtained in 2 M KOH solution at a scan rate 50 mVs -1 within the potential range 0 to 0.8 V Vs Ag/AgCl. In charge-discharge behaviors, the maximum energy density (E) of 11.7 Whkg -1 and power density (P) of 4.3 kWkg -1 was obtained at a current density 1 mA/cm 2 . Impedance spectroscopic analysis revealed that the ESR is 5 Ω in KOH electrolyte. Keywords— Nickel Sulphide (NiS), Thin films, Cyclic voltammetry, Supercapacitor, Charge-discharge


INTRODUCTION
In recent years, the synthesis and characterization of metal chalcogenide thin films have attracted great attention due to their various applications. A large number of techniques successfully utilized to prepare chalcogenide thin films. Nickel sulphide (NiS) is exists in a number of metal chalcogenide phases with the most attracting application in various fields such as IR detectors, solar storage devices, etc [1]. In this work, NiS thin films were prepared by modified bath deposition (MCBD) method. The modified bath deposition is also known as successive ionic layer adsorption and reaction (SILAR) method. In this method, thin films are obtained by immersing substrate into separately placed cationic and anionic precursors and rinsing between every immersion with distilled water. Thus, precipitation formation is avoided [2,3]. The structural and surface morphology of thin film was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively.
In the energy storage applications field, electrochemical supercapacitors are most studied because it offers wide applications such as, power hybrid electric vehicles, camera flash bulbs, pulsed lasers or any other device that requires a large amount of charge delivered in a very short time [4]. Electrochemical supercapacitors can fill the energy and power gaps between the fuel cell and batteries. The major properties investigated for the electrochemical supercapacitors are; rapid rates of charge-discharge rate, increase the energy density without affecting the high power performance. In present work, electrochemical capacitive properties of the NiS thin film electrodes were investigated for improve supercapacitor performance. Many articles related to studies of NiS electrode for capacitive performance have been appeared in the literature [5][6][7][8][9].
In this paper, section I contains introduction to preparation method and electrochemical characterization techniques for NiS electrodes. The section II contains the experimental details and also discuss on optimizes condition for NiS thin film by MCBD method. In section III describes results and discussion, all possible characterizations of NiS thin film electrode is discussed in this section. Section IV contains concludes research work with future scope.

II. METHODOLOGY
The NiS thin films electrode was prepared by modified bath deposition (MCBD) method. Analytical reagent Nickel Sulphate (NiSO 4 ) and sodium sulfide [Na 2 S·H 2 O] were used in the deposition of NiS thin films. The cation precursor was 0.1M Nickel Sulphate solution. The pH was adjusted to ~9 by adding liquid ammonia. The source of sulfide ions was 0.5M sodium sulfide (pH~12). Prepared solutions were taken into beakers and for rinsing purpose distilled water was used. The deposition was carried out at room temperature (27 o C) using unstirred conditions. When the substrate (stainless steel) is immersed in cationic precursor solution for 65 second, nickel ions get adsorbed on the substrate surface.
The substrate is rinsed in flowing distilled water for 45 second to remove the loosely bound or excess nickel ions. Then, the substrate is immersed in anionic precursor solution for 65 second. The sulphide ions react with preadsorbed nickel ions to form a layer of NiS over substrate. Rinsing the substrate again in the flowing water for 45 second separated out the unadsorbed sulphide ions. By making several trial experiments, NiS thin film deposition conditions were optimized [2,10,11]. Film thickness was determined by weighing method, it found that ~0.3 μm for 55 cycles.

Structural and Morphological Analysis:
Structural analysis of as-deposited NiS thin film on glass substrate was carried out by X-ray diffraction technique is shown in Figure 1. XRD pattern exhibits film was nanocrystalline and three peaks of NiS as (300), (220) and (161) were observed. There is a good agreement with the JCPDS file 86-2281 [12][13][14].   Figure 2 shows the SEM image of the chemically asdeposited NiS thin film on glass substrate. The substrate is well covered with irregular round-shaped grains in the asdeposited state. Surface also covered with agglomerates of different sizes. It is difficult to find approximately size of particles. The porous space between the particles can also be seen [11]. Such type of morphology leads to high surface area, which provides the structural foundation for the high specific capacitance.

Electrochemical properties of the NiS thin film electrode:
Chemically deposited NiS electrodes performances were tested using CV. The capacitance can be estimated by the following equation [14,15], Where, C is the total capacitance, I the current density (A/cm 2 ), v the sweep rate (V/s), v i the initial and v f the final voltages (V). The integral on the right hand side of equation (1.1) is the area under the CV. Thus, the total surface charge, (or total capacitance) of the deposit material can be estimated by evaluating the area under the capacitive current-voltage curve portion of a CV. The specific capacitance (Fgm -1 ) of the electrode was obtained by dividing the capacitance to weight dipped in the electrolyte. The interfacial capacitance (Fcm -2 ) was obtained by dividing the capacitance to area dipped in the electrolyte.
Cyclic voltamogram of the NiS electrode of thickness 0.00016 gm/cm 2 , in aqueous electrolyte 2 M solution of KOH were studied in the voltage range of 0 to +0.8 V Vs Ag/AgCl. The KOH electrolyte gave the largest current, which was greater than the other electrolytes. The area of working electrode was 1cm 2 . The voltammetric responses of NiS electrode at different scan rates are shown in figure 3. It was found that the current under curve is slowly increased with scan rate [5][6][7][8][9]. Maximum capacitance obtained for NiS at 50 mVs -1 scan rate is 353 Fgm -1 . of potential variation with time of cycling are shown in Figure 4, it can be seen that, the nonsymmetric behavior of voltage-time curve was seen, that is IR drop was observed [16]. The maximum energy density of 11.7 Whkg -1 and power density of 4.3 kWkg -1 was obtained at a current density 1 mAcm -2 .  Figure 5 shows Nyquist plots obtained for as deposited NiS electrodes. It displays a semicircle in the high frequency and a linear curve in the low-frequency region. It is observe that, initial non-zero intercept in high frequency regime at the beginning of the semicircle and is due to the electrical resistance of the electrolyte (R ele ) [17]. The values for ESR for as-deposited NiS thin films are 5 Ω.

IV. CONCLUSION AND FUTURE SCOPE
In conclusion, the modified chemical bath deposition method is low cost and promisingly used for the preparation of porous electrode for supercapacitor applications. The XRD analysis showed the NiS thin films are amorphous. The SEM of NiS film showed porous structure and the film surface covered with agglomerates of different sizes. The electrochemical study revealed that, the as-deposited NiS thin film electrode had a specific capacitance of 353 Fgm -1 at the scan rate 50 mVs -1 . Charge-discharge curves confirmed that the capacitance consisted from EDLC and psedudocapacitance. The energy density (E) of 11.7 Whkg -1 and power density (P) of 4.3 kWkg -1 was obtained at a current density 1 mA/cm 2 . Impedance spectroscopic analysis revealed that the ESR is 5 Ω in KOH electrolyte.
Although, the performance of these electrodes is poor as compared to conventional expensive electrodes, the further research to overcome technical aspects will definitely bring them to raise the efficiency of the supercapacitor. .