Synthesis of nickel sulfide as a promising electrode material for pseudocapacitor application

Abstract

In the present investigation, in addition to pseudocapacitor application, the growth of interconnected nanorods/nanoplates of nickel sulfide (NiS) on a titanium (Ti)-substrate has been explored. An additive-free synthesis is performed by using a simple chemical bath deposition method. Structure and morphology of as-prepared NiS films are characterized by various characterization techniques such as X-ray diffraction, field effect scanning electron microscopy, high resolution transmission electron microscopy, and selected area electron diffraction etc. Electrochemical properties of the NiS thin film electrodes are studies by means of cyclic voltammetry and galvanostatic charge–discharge spectra obtained in 1 M aqueous KOH electrolyte. The NiS electrode demonstrates the notable pseudocapacitive activities including high specific capacitance (788 F g⁻¹ at 1 mA cm⁻²), good rate capability (640 F g⁻¹ at 50 mA cm⁻²), excellent cycling stability (98% retention after 1000 cycles) and high energy density (27.4 W h kg⁻¹) as well as good power density (3.05 kW kg⁻¹). Such an empirical performance is mostly due to the interconnected-type surface of NiS, which provides fast electron and ion transport. The obtained results indicate that the NiS thin film is a capable candidate as an electrode material for supercapacitor application.

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1. Introduction

In the area of electrochemical technology, undoubtedly the most important challenges are new renewable energy sources and energy storage materials. In recent years, increase in the requirements of electrical vehicles, portable and other electronic devices that need excellent electrical energy at elevated power level in quite small pulses have encouraged substantial attention towards electrochemical supercapacitors because of their outstanding characteristics of extensive cycling life, high power density, rapid recharge capability, higher capacitance, light weight, and flexibility.^1–6 Also, electrochemical supercapacitors continue to attract intense research aimed at improving their performance for ever demanding above discussed energy storage applications and have an efficient potential to tie the performance gap between dielectric capacitors and batteries with better density values of power and energy.^1–3 In recent years along with the different supercapacitor electrode materials, metal sulfides such as cobalt sulfide (CoS), nickel sulfide (NiS), molybdenum sulfide (MoS), copper sulfide (CuS) etc., are being considered as a promising class of active electrode materials because of their reasonably high specific capacitance values and exceptional redox reversible reaction activities.^4–6 Amongst diverse metal sulfides, NiS is one of the crucial constituent of metal sulfides and has come forward interesting attention with the advantage of remarkable electronic conduction, slight toxicity, cost-effective, simplicity of fabrication and a variety of valance states.^7,8 Also it has been extensively investigated and found widespread applications in supercapacitors,^8,9 lithium ion batteries,¹⁰ dye-sensitized solar cells,^11,12 etc. Presently, various thin film deposition techniques use such as, physical vapor deposition, chemical vapor deposition, electrochemical deposition, chemical bath deposition (CBD), hydrothermal, chemical spray method and sol–gel etc., are frequently in use to synthesize variety of metal oxide/chalcogenide nanostructures. Among variety of deposition techniques, CBD is one of the unsophisticated and easy methods used for thin film deposition, which offers the possibility of preparing good quality adherent films on a large area at relatively low operating temperatures. In general, consecutively to evade spontaneous precipitation in aqueous bath, a pH regulating agent or a complexing agent is needed which are making the procedure more complicated. Therefore, in the present study, to overcome the above tribulation in aqueous solution, NiS thin film deposition reaction was conducted in an ethanol-based solution without considering any pH regulating or complexing agent. The resultant procedure is based on the dissolution of thioacetamide and metal salt in ethanol and heating the desired solutions @ 70 °C.^13,14 Herein, we report on additive-free synthesis of NiS thin film by CBD method which further is characterized for its structure and morphology studies. Electrochemical supercapacitive performance of NiO film deposited on titanium (Ti)-substrate, as an active electrode material, has been tested and reported.

2. Experimental details

Nickel nitrate (Ni(NO₃)₂·6H₂O), thioacetamide (CH₃CSNH₂) and ethanol (C₂H₅OH) were purchased from Sigma Aldrich, Korea. Doubly distilled water was used during all experiments. All chemicals were analytical grade and used as received without further purification.

2.1 Preparation of NiS thin film

In typical experiment, 0.1 M of Ni(NO₃)₂·6H₂O and 0.1 M of CH₃CSNH₂ were dissolved in C₂H₅OH in 50 mL capacity falcon tube. Then pieces of cleaned titanium (Ti) substrate were vertically dipped in the desired solution followed by closing the lid of the falcon tube compactly and performing the reaction in a water bath maintained at 70 °C for a time period of 4 h. Finally, after the deposition, the deposited films were successively washed with ethanol, dried under argon stream, and stored beneath dark before various measurements.

2.2 Characterization techniques used

X-ray diffraction (XRD) pattern measurement of chemically deposited NiS thin film was carried out on X-ray diffractometer (Model: Rigaku D/MAX 2500 V, Cu-Kα radiation at λ = 0.15418 nm) in the 2θ range of 20° to 70°. The field emission scanning electron microscopy (FE-SEM) analysis was performed on a JSM-6700F. Surface morphology of NiS thin films was also analyzed by high resolution transmission electron microscope (HRTEM) (TEM, Model: JEOL-2100F). Crystallinity of NiS film obtained from XRD measurement was supported by selected area electron diffraction (SAED) pattern.

2.3 Electrochemical measurements

Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements of as-prepared thin film electrodes were performed on a CHI 627B electrochemical analyzer by distinctive three electrode system with NiS thin film as the working electrode, Ag/AgCl as the reference electrode, and platinum plate as the counter electrode in 1 M KOH electrolyte at different scan rates (10–100 mV s⁻¹). CV plots were obtained to measure the capacitive performances of NiS thin film electrode. Furthermore, GCD studies were performed at the current densities of 1–50 A g⁻¹ in 1–1.5 V potential window.

3. Results and discussion

3.1 Structural analysis

The XRD pattern of CBD NiS thin film, on Ti-substrate, is shown in Fig. 1. From the XRD pattern of NiS, well-defined diffraction peaks are clearly evidenced; indicating polycrystalline nature of as-deposited NiS. XRD pattern of NiS thin film consists of prominent diffraction peaks at 29.90°, 34.22° and 45.3°, correspond to (100), (101), and (102) Bragg's reflections, respectively. Observed reflections are in good accordance with JCPDS file no. 02-1273 of NiS. The peaks designated by star (*) are of substrate ‘Ti’ foil. The average crystallite size calculated along highly preferred (102) plane using well-known Scherrer's relation [D = 0.9λ/β cos θ] is 19 nm.

Fig. 1 XRD pattern of NiS thin film.

3.2 Morphology confirmation

Surface morphology of as-deposited NiS thin film on Ti foil was analyzed by FESEM photoimage which is displayed in Fig. 2(a). The FESEM image of NiS film clearly shows a uniformly distributed network of nanorods over the substrate surface with several air voids. Under close inspection these nanorods contain bottom walls, immersed in, supporting for an existance of upright-standing nanoplates. Such an open vacant space between nanorods/nanoplates can be advantageous for diffusion process of electro-active species as during intercalation/de-intercalation process shrinkage of internal resistance of material takes place which improves the supercapacitive performance.¹⁵ Noticeably, these nanorods/nanoplates are well-interconnected to each other by forming a network-type architecture which eventually will support for an easy transportation of electrons in their profound locations.

Fig. 2 (a) FESEM, (b) TEM, (c) HRTEM, and (d) SAED photoimages of NiS thin film.

TEM image of NiS nanorod/nanoplate is shown in Fig. 2(b). From TEM image it is undoubtedly seen that the product has nanorod/nanoplate (average diameter ≈ 150 nm)-like surface morphology and it is in accordance with the FESEM study. The SAED pattern of NiS is shown in Fig. 2(d). From SAED pattern, occurrence of circular rings with some of the bright spots is supporting for the polycrystalline nature of synthesized NiS, which coincides with XRD study.

4. Electrochemical properties

4.1 Cyclic-voltammetry study

NiS is an extremely attractive candidate as electrode material for supercapacitors because of its higher electrical conductivity at room temperature. Commonly its capacitance is primarily gained from the pseudocapacitance based on the subsequent reversible redox reaction, which signifies the conversion between NiS and NiSOH:^15–17

NiS + OH⁻ ↔ NiSOH + e⁻ (1)

Electrochemical supercapacitive properties of the as-prepared NiS thin film electrodes were examined by CV and GCD experiments in three electrode configuration. The specific capacitance values (from the CV curves) were calculated from the following equation;

where ‘C_S’ is specific capacitance in F g⁻¹, ‘I’ is the current in Ampere, ‘m’ is the mass of the electro-active material in gram and ‘dV/dt’ is scan rate in mV s⁻¹.

CV measurements were employed to characterize the capacitive performances of as-deposited NiS thin film electrode. Fig. 3(a) illustrates the CV curves of the NiS electrode in 1 M KOH electrolyte at 10–100 mV s⁻¹ scan rates with a fixed potential window ranging from 1.0–1.5 V. It is observed that the shape of CV curves of NiS electrode demonstrates the pseudocapacitive behavior. It consists of two strong redox peaks, which indicate that the capacitive characteristics are mostly directed by the faradaic reactions.¹⁸ From CV curves it is also found that, with increase in scan rate anodic peak position moves positively while cathodic peak position moves negatively leading to an incessant increasing the potential distance between oxidation and reduction peaks. Observed shift in oxidation and reduction peaks from 10–100 mV s⁻¹ scan rates is very small indicating this reasonably little resistance of the electrode is because of the intimate contact between the working electrode i.e. NiS and substrate i.e. Ti.¹⁹

Fig. 3 (a) Cyclic voltammograms within a fixed potential window ranging from 1.0–1.5 V at different scan rates in 1 M KOH electrolyte, (b) relationship between the log of cathodic peak current and square root of scan rate, (c) CV cyclic stability @ 4 mV s⁻¹ scan rate, and (d) variation of capacity retention as a function of number of cycles of NiS thin film electrode.

The value of anodic and cathodic currents increases with increasing scan rate from 1–100 mV s⁻¹ however; there is no noticeable alteration in the CV's even at a high scan rate of 100 mV s⁻¹, indicating fast charge–discharge behavior. According to eqn (2), the specific capacitance of the NiS electrode based on the total mass of the device was calculated. NiS electrode exhibits as high as specific capacitance value of 786 F g⁻¹ @ 10 mV s⁻¹ and sustained up to 418 F g⁻¹ @ 100 mV s⁻¹. The variation of log of anodic peak current as a function of square root of scan rate (calculated from CV curves) is shown in Fig. 3(b). Linear relationship suggests a diffusion-controlled process for the redox reaction of NiS electrodes.²⁰

Excellent cycling stability is also a crucial factor when practical electrochemical operations are considered. The long term cyclic stability [Fig. 3(c)] of NiS thin film electrode was attempted out up to 1000 cycles at the scan rate of 4 mV s⁻¹ and the corresponding capacitance retention plot is presented in Fig. 3(d). The specific capacitance values of NiS electrode for 1^st and 1000^th cycles are 1015 and 967 F g⁻¹, respectively. From Fig. 3(d), it is observed that, primarily the value of specific capacitance is increased up to 400 cycles and finally after 1000 cycles ∼96% retention of specific capacitance with 4% loss in original specific capacitance value is achieved, indicating excellent cyclic stability of chemically deposited NiS thin film electrode.

4.2 Galvanostatic charge–discharge measurement

In order to catch the additional information regarding the prospective of chemically synthesized NiS thin film as electrode material, GCD measurements were conducted in 1 M KOH electrolyte at the different current densities (1–50 A g⁻¹) in the 1–1.5 V potential window. Fig. 4(a) shows the variation of potential as a function of discharging time at 1–50 A g⁻¹ current density. There is no significant I–R drop, confirming that that the NiS electrode is suitable for supercapacitor application. The specific capacitance of the electrode from GCD measurement was calculated by using following equation;where ‘I’ is the discharge current, ‘Δt’ is the discharge time, ‘m’ is the mass of the electro-active material, and ‘ΔV’ is the potential window. The calculated specific capacitance values of NiS electrode from GCD are 788, 728, 702 and 640 F g⁻¹ at 1, 5, 10 and 50 mA cm⁻² current densities, respectively. The variation of specific capacitance as function of current density is shown in Fig. 4(b). It is found that the specific capacitance value of NiS electrode gradually decreases with increase in the current density. Such a decrease in specific capacitance is mainly owing to the diffusion limit of the active electrolyte ions at the enormous discharge currents, which is because of the time limit and the fact that simply the outer active surface can be exploited for the storage of charge.²¹ The attained high value of specific capacitance (788 F g⁻¹) is primarily due to the interconnected-type network of nanorods/nanoplates that provides more accessible area for the diffusion of electro-active species throughout the surface. The specific capacitance value of NiS electrode is decreased from 788 F g⁻¹ (at 1 mA cm⁻²) to 640 F g⁻¹ (at 50 mA cm⁻²) with ensuing in the excellent retention rate of 82%, indicating the potential of NiS as an electrode material in the supercapacitor application. Power density (P) and energy density (E) are vital parameters while characterizing the electrochemical performance of any supercapacitor which were calculated using the following equations;

E = (0.5 × SC × ΔV²)/3.6 (4)

P = E/ΔT (5)

where ‘E’ denotes to the energy density in W h kg⁻¹, ‘SC’ denotes the specific capacitance in F g⁻¹, ‘P’ corresponds to the power density in W h kg⁻¹, ‘ΔV’ refers to window used in discharge process which is in volt, and ‘ΔT’ is the discharging time.

Fig. 4 (a) Galvanostatic charge–discharge measurements at current densities of 1, 5, 10, 50 mA cm⁻², (b) plot of effect of current density on SC value, and (c) plot of current density as a function of power density (Ragone plot) of NiS thin film electrode.

The variation of energy density with power density (Ragone plot) is displayed in Fig. 4(c). The NiS thin film electrode exhibits high energy density of 27.4 W h kg⁻¹ and high power density of 3.05 W h kg⁻¹ at 1 and 50 mA cm⁻² current densities, respectively. This result covers the way that, the NiS as an electrode material in the development of high performance electrochemical supercapacitors because of the high value of specific capacitance and excellent cyclability with good retention capability. Based on the observed results it is concluded that, the cost-effective NiS thin film prepared by additive-free non-aqueous chemical bath deposition method can be used as a potential candidate for constructing high performance supercapacitor electrodes.

5. Conclusions

In conclusion, NiS film, prepared on Ti-substrate by a simple and inexpensive chemical bath deposition method, shows considerable electrochemical supercapacitor performance. The XRD and SAED analyses confirm the formation of polycrystalline NiS without the trace of any impurity level. Interconnected-type nanorods/nanoplates morphology of NiS, permitting effortless diffusion of KOH electrolyte throughout the internal electro-active region of the NiS electrode, ensures decreased resistance and an enhanced supercapacitive performance. NiS thin film electrode exhibits as high as specific capacitance of 788 F g⁻¹ at 1 mA cm⁻². Furthermore, NiS film electrode composed of nanorods/nanoplates demonstrates energy density of 27.4 W h kg⁻¹ (at mA cm⁻²) and power density of 3.05 kW kg⁻¹ (50 mA cm⁻²) with excellent (96%) cyclic stability up to 1000 cycles. Thus, these considerable electrochemical properties make NiS thin film as potential electrode material while fabricating symmetric/asymmetric electrochemical supercapacitors commercially.

Acknowledgements

Authors extend their appreciation to the International Scientific Partnership Program ISPP at King Saud University for funding this research work through ISPP# 0032.

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