SYNTHESIS OF COBALT SULPHIDE THIN FILM ELECTRODE BY MODIFIED CHEMICAL BATH DEPOSITION ROUTE AND ITS SUPERCAPACITORS PERFORMANCE

* M. S. Sonawane 1 , P. K. Baviskar 2 and R. S. Patil 3 . 1. Department of Physics, R. C. Patel Institute of Technology, Shirpur425405(M. S.) India. 2. Department of Physics, School of Physical Sciences, NMU, Jalgaon425001(M. S.) India. 3. Department of Physics, P. S. G. V. P. M’S Arts, Science & Commerce College, Shahada425409 (M. S.) India. ...................................................................................................................... Manuscript Info Abstract ......................... ........................................................................ Manuscript History


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Metal sulfides have recently emerged as new promising electrode materials for supercapacitor due to their excellent redox reversibility and relatively high capacitance. We have chosen Cobalt Sulphide as a electrode for supercapacitor because of its higher stability, high abundance, low cost, better safety, and environmental friendliness in comparison with other metal sulphide. Many techniques have been currently used in order to produce CoS thin films electrode. Feng Tao et al studied the CoS x was prepared by a very simply chemical precipitation method for high capacitance. The CoS x has excellent electrochemical capacitive characteristic with potential range -0.3 to 0.35 V Vs SCE. CoS x shows the deliver specific capacitance up to 474 Fg -1 at 5 mA and 369 Fg -1 at 50 mA chargedischarge current in 6 M KOH solution [4]. P. Justin and G. Ranga Rao, prepared sphere-like CoS nanostructures are synthesized in water/ethylene glycol mixed solvents by simple hydrothermal technique using L-cysteine amino acid. The bimodal porous sphere-like CoS shows high-rate capacitance from 363 Fg -1 at 5 mV s -1 to 314 Fg -1 at 50 mV s -1 . [5]. Shu-Juan Bao et al investigated a biomolecule-assisted hydrothermal process is developed to synthesize cobalt sulfide (CoS). Researchers found that the CoS nanowire assembled by nanoflowers provides a very high specific capacitance (508 Fg −1 ) with high specific energy, excellent discharge rate, and good stability [6]. Zusing Yang et al prepared a cobalt sulfide (CoS) hexagonal nanosheets (HNSs) onto transparent fluorine-doped tin oxide (FTO) substrates through a drop-dry process to prepare two types of supercapacitors (SCs); high rate and large perarea capacitance.The CoS HNSs provided high values of capacitance 326.4 Fg -1 . Researchers found that, the CoS HNSs electrodes exhibit excellent capacity properties, including high energy density (13.2 Whkg −1 ), power density (17.5 kWkg −1 ), energy deliverable efficiency (81.3-85.3%) [7]. Fulian Luo et al studied the pseudo-capacitive properties of the 3D flower-like CoS in 6 M KOH solution. A specific capacitance of 586 F g -1 is obtained at a charge-discharge current density of 1 Ag -1 [8]. J. Wang et al, prepared the nanosize cobalt sulfides were synthesized by one step chemical reaction method and it shows the best electrochemical properties. Cobalt sulfide nanopowders show promise as cathode active materials for lithium-rechargeable batteries [9].
In the present work, we report the synthesis of CoS thin films by modified chemical bath deposition (MCBD) method onto stainless steel (SS) substrate. The supercapacitive behavior of CoS film electrodes in KOH electrolyte was studied in terms of with respect to various parameters such as, scan rates, stability cycles, charging-discharging, electrochemical impedance spectroscopy (EIS) study and efficiency.

Experimental details:-
The stainless steel and glass substrates were used for the preparation of CoS thin film electrode. The stainless steel substrate was polished with polish paper to a rough finish, and ultrasonicated before use. In the present work, the MCBD method was used to deposit the CoS thin films on SS substrates which had been treated as described above.
The aqueous (0.02 M) CoSO 4 solution placed in a 50 ml beaker used as cationic precursors. The pH of the solution was ~8. The pH of cationic precursors was raised with adding of NH 3 .The anionic precursors was (0.05 M) Na 2 S with pH ~12 was placed in another 50 ml beaker. For the deposition of CoS thin films a well cleaned glass or SS substrate was immersed in cationic precursors solution of Cobalt Sulphate for 60 second in which Co 2+ ions were adsorbed on the surface of substrate. The substrate was rinsed with distilled water for 65 second to remove unabsorbed Co 2+ ions. Then the substrate was immersed in anionic precursors of sodium disulphate solution for 60 second in which S 2ions are reacted with adsorbed Co 2+ ions on glass substrate. This was followed by rinsing again in distilled water for 65 second to remove unreacted S 2ions. This completes one deposition cycle for deposition of CoS thin film. The CoS thin film deposited on glass or SS substrate by repeating such deposition cycle for 20, 25, 30, 35, 40, 45 and 50 times. The deposition carried out at ~27 o C. The deposited CoS films were black, homogeneous with a good adherence to the substrate [10][11][12][13][14]. The thickness of the films was measured by weight difference method using sensitive microbalance.
The pseudocapactive properties of CoS thin films electrodes were studied using cyclic voltammetry (CV), galvanostatic charge/discharge test and EIS in 2M KOH aqueous solution. These experiments were carried out with potentiostat/galvanostat (Princeton Applied Research, PARSTAT 4000). A typical three electrode cell was employed; CoS as a working electrode platinum wire as the counter electrode, and Ag/AgCl used as the reference electrode. All electrochemical experiments were carried out at room temperature.

Structural and Morphological Analysis:-
The CoS thin films were grown onto glass substrates under optimized conditions using MCBD technique. The morphology and crystal structure of nanocrystalline CoS thin films were investigated by scanning electron microscopy and X-ray diffraction techniques respectively. Structural analysis of CoS thin films was carried out by X-ray diffraction technique.  The substrate is well covered with fine elongated particles having different sizes. Surface also covered with small agglomerates of different sizes. It is difficult to find approximately size of particles. Some micro porous space between the fine particles can also be seen. The porosity of films can enhance the redox [15][16].

Supercapacitive properties of the CoS thin film electrode:-
Chemically deposited CoS electrodes were used in the electrochemical capacitor and their performances were tested using CV. To perform cyclic voltammetry tests a series of changing voltages at a constant sweep rate (dV/dt) is applied and the response current is recorded. The supercapacitive studies carried out by means of effect of scan rate and stability studies. The capacitance can be estimated by the following equation [17][18][19][20][21][22][23], 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 CoS electrode of thickness 0.00017 gm/cm 2 , in aqueous electrolyte 2M solution of KOH were studied in the voltage range of 0 to + 0.7 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 CoS electrode at different scan rates are shown in figure 1.3. It was found that the current under curve is slowly increased with scan rate. This showed that voltammetric currents are directly proportional to the scan rate of CV, indicating an ideally capacitive behavior [4-9, 24, 25].  respectively, as the scan rate was increased from 10 to 100 mVs -1 . It is found that, with increase in scan rate, there is a slight decrease in the specific capacitance that is comparable to previously reported data. Maximum capacitance obtained for CoS at lower scan rate (10 mVs -1 ) is 531 Fgm -1 .

Effect of Concentration of Electrolyte:-
The effect of concentration of KOH electrolyte was studied by keeping the constant scan rate and potential. The concentration of the KOH electrolyte was varied from 1 M to 3 M.   684

Galvanostatic Charge-Discharge Studies:-
The CoS electrode was subjected to galvanostatic charge-discharge cycling between 0 and +0.7 V in 2 M KOH solution at a current density of 1 mAcm −2 . Typical curves of potential variation with time of cycling are shown in figure 1.7, it can be seen that, the nonsymmetric behavior of voltage-time curve was seen, that is IR drop was observed. The discharge profile showed two parts; a resistive component arising from the sudden voltage drop (linear portion parallel to y-axis) representing the voltage change due to the internal resistance and a capacitive component (curved portion) related to the voltage change due to the change in energy within the capacitor. The specific energy (E), and specific power (P) and coulomb efficiency (η%) are calculated using following equations [7,22], E =

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The capacitance, energy density and power density of the CoS electrode were calculated using equation (1.1), (1.2) and (1.3). The specific capacitance value is 168, 118 and 76 Fgm -1 for CoS thin film electrode at current density 1, 2 and 3 mAcm -2 , respectively. The maximum energy density of 11.4 Whkg -1 and power density of 7.4 kWkg -1 was obtained at a current density 1mAcm -2 . This increase in specific energy and power may be due to increased time for charging and discharging of capacitor. The energy efficiency, 31% was obtained. Variation of energy density with power density values of CoS electrode at different current densities is shown in figure 1.8.

Electrochemical impedance analysis (EIS studies):-
Electrochemical impedance spectroscopy, a powerful technique for the investigation of the capacitive behavior of electrochemical cells, has been also used to characterize our materials. Figure 1.9 presents Nyquist plots obtained for as deposited CoS electrodes. It displays a semicircle in the high frequency and a linear curve in the low-frequency region. The semicircle in the high-frequency region is related to the reaction kinetics at the electrode and electrolyte interfaces [4,8,26,27]. The linear curve at the low-frequency region can be attributed to the diffusion controlled process in the electrolyte. The 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 ). The values for equivalent series resistance (ESR) for as deposited CoS thin films are 1.8 Ω.

Conclusion:-
In conclusion, CoS films have been prepared successfully at room temperature by MCBD method and used in supercapacitors. The XRD measurements confirmed that the CoS was deposited in the form of polycrystalline. The SEM analysis showed that the substrate is well covered with different sizes fine elongated particles. The as-prepared CoS thin film electrode has excellent electrochemical capacitive characteristic with potential range 0 to +0.7 V Vs Ag/AgCl at constant concentration 2 M KOH solution. The electrochemical study revealed that the Cobalt Sulphide electrode showed high specific capacitance of 531 Fgm -1 . The specific energy (E), specific power (P) and coulomb efficiency (η%) was 11.4 Whkg -1 and 7.4 kWkg -1 , and 31%, respectively. Impedance spectroscopic analysis revealed that the ESR is 1.8 Ω at the imaginary axis.
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.