One-Step Synthesis of Self-Supported Ni3S2/NiS Composite Film on Ni Foam by Electrodeposition for High-Performance Supercapacitors

Herein, a facile one-step electrodeposition route was presented for preparing Ni3S2/NiS composite film on Ni foam substrate (denoted as NiSx/NF). The NiSx granular film is composed of mangy interconnected ultra-thin NiSx nanoflakes with porous structures. When applied as electrodes for supercapacitors, the ultra-thin nanoflakes can provide more active sites for redox reaction, and the interconnected porous structure has an advantage for electrolyte ions to penetrate into the inner space of active materials quickly. As expected, the obtained NiSx/NF sample exhibited high gravimetric capacitance of 1649.8 F·g−1 and areal capacitance of 2.63 F·cm−2. Furthermore, a gravimetric capacitance of 1120.1 F·g−1 can be maintained at a high current density of 20 mA·cm−2, suggesting a good rate capability. The influence of the different molar ratios of electrodeposition electrolyte (NiNO3 and thiourea) on the morphology and electrochemical properties of NiSx/NF sample was investigated to provide an optimum route for one-step electrodeposition of Ni3S2/NiS composite film. The outstanding performance indicated the Ni3S2/NiS composite film on Ni foam has great potential as an electrode material for supercapacitors.


Introduction
At present, the traditional non-renewable fossil energy represented by oil, coal, and natural gas is rapidly consumed, leading to vigorous development of new energy resources (such as solar, wind, and tidal energy). However, these new energy resources are subject to natural environmental conditions. For example, the efficiency of solar energy is limited in rainy and cloudy weather areas. Therefore, the electrochemical energy storage devices connected with them can solve these energy storage and conversion problems [1][2][3][4]. At present, new types of batteries such as lithium-ion batteries, potassium-ion batteries, and lithium-sulfur batteries have the advantage of high energy density, but their low power density and short life cycle limit their application, especially in energy storage systems, which require high-speed and high-power storage devices. Supercapacitors are a new type of energy storage device, which are complementary to batteries because of their high power density and long life

Preparation of Ni 3 S 2 /NiS Composite on Ni Foam
Ni 3 S 2 /NiS composite (denoted as NiS x /NF) was electrodeposited into the Ni foam by cyclic voltammetry(CV) method using Gamry electrochemical workstation (Reference 1000, Gamry Instruments). In a typical synthesis, the electrodeposition process was carried out at a scan rate of 5 mV·s −1 in the range of −1.2-0.2 V for 30 cycles in a three-electrode system with Ni foam as the working electrode, Ag/AgCl as the reference electrode, and Pt as the counter electrode. The electrodeposition solution was prepared by mixing Ni(NO 3 ) 2 ·6H 2 O and thiourea in 80 mL H 2 O. The Ni 3 S 2 /NiS composite on Ni foam was prepared using a different solution concentration of Ni(NO 3 ) 2 ·6H 2 O and thiourea. For better comparison, the molar concentration of thiourea was fixed as 0.5 mol/L −1 , and the molar concentration of Ni(NO 3 ) 2 ·6H 2 O was 1, 2.5, and 5 mmol/L −1 , respectively. The resulting Ni 3 S 2 /NiS composites were denoted as NiS x /NF-1, NiS x /NF-2.5, and NiS x /NF-5, respectively.

Electrochemical Measurement
The electrochemical measurements were carried out on a three-electrode system with NiS x /NF as the working electrode, Hg/HgO as the reference electrode, and Pt sheet as the counter electrode. An amount of 6 M KOH aqueous solution was used as electrolyte. The cyclic voltammetry (CV) was performed at scan rates of 2, 5, 10, 20, 30, 40, and 50 mV·s −1 in the range of 0-0.7 V (vs. Hg/HgO) on the Gamry electrochemical workstation (Reference 3000, Gamry Instruments Co., Ltd., Philadelphia, PA, USA), respectively. The galvanostatic charge-discharge (GCD) test was carried out at different current densities of 1, 2, 3, 4 5, 6, 8, 10, 12, 16, and 20 mA·cm −2 on the Arbin electrochemical workstation (Arbin Instruments Corp., College Station, Texas, USA), respectively. The electrochemical impedance spectroscopy (EIS) measurement was carried out within a frequency response in the range of 0.01-100 kHz and an AC amplitude of 5 mV on the Gamry electrochemical workstation (Reference 3000).
The galvanostatic capacitance (C s , F·g −1 ) and areal capacitance (C a , F·cm −2 ) were calculated using the following formulas: where I, t, V, m, and S are the discharge current (A), the discharge time (s), the potential voltage (V), the total mass of the active materials (g), and the geometric area of electrode, respectively.

Characteristics
The XRD patterns of the NiS x powder obtained from NiSx/NF electrode are shown in Figure 1. The diffraction peaks are well-matched with both Ni 3 S 2 planes (PDF#44-1418) and NiS phase (PDF# 12-0041), confirming that the mixed phase of Ni 3 S 2 and NiS was formatted in the as-prepared NiS x composite film. The impurity peaks at 51.8 • are attributed to the (200) crystal planes of residual nickel metal (PDF# 04-0850) from Ni foam.   Figure 2 shows the SEM images of NiSx composite on Ni foam prepared using different molar ratios of NiNO3 and thiourea electrolyte. In the low-magnification SEM images (Figure 2a, c, e), NiSx has uniformly deposited on the framework of the Ni foam. Further, NiSx composite thin films showed granular morphology, confirmed by the SEM images at high magnification (Figure 2b, d, f). As the molar concentration of Ni(NO3)2 increased, the thickness and the wrinkle degree of NiSx film also increased. Meanwhile, some microspheres on the surface of NiSx composite films were observed. The increase of microspheres can provide more active sites for redox reaction, resulting in high specific capacitance of NiSx composite. It is worth noting that large cracks gradually appear in the film when concentration of NiNO3 electrolyte increases. Therefore, the tightness of the film is reduced with the increase of Ni(NO3)2 molar concentration. The expanding transverse crack may cause the NiSx composite film to fall off, resulting in a rapid decline in the capacitance of NiSx composite electrode after repeating charge-discharge tests. More detailed morphology was further observed by field-emission scanning electron microscopy (FE-SEM) and a transmission electron microscope (TEM), as shown in Figure 2g-h. The FE-SEM image (Figure 2g) shows that the granular morphology was composed of mangy interconnected ultra-thin NiSx composite nanoflakes with porous structures, which is advantageous for electrolyte to penetrate into the inner space of active materials quickly. The reasons for the increase of wrinkle degree of NiSx film may be as follows. When the concentration of electrodeposition solution is relatively low, the as-prepared films should be relatively dense. As the arrays of supported NiSx nanoflakes gradually form, the film has porous characteristics, so the film will become more and more rough. The TEM image (Figure 2h) further confirms the ultra-thin NiSx composite nanoflakes were overlapping and interconnected. The energy-dispersive X-ray spectroscopy (EDX) mapping was further employed to investigate the elemental distribution of Ni and S. As shown in Figure 3, elements Ni and S were uniformly distributed, confirming the successful deposition of the NiSx nanosheets.  Figure 2 shows the SEM images of NiS x composite on Ni foam prepared using different molar ratios of NiNO 3 and thiourea electrolyte. In the low-magnification SEM images (Figure 2a,c,e), NiS x has uniformly deposited on the framework of the Ni foam. Further, NiS x composite thin films showed granular morphology, confirmed by the SEM images at high magnification ( Figure 2b,d,f). As the molar concentration of Ni(NO 3 ) 2 increased, the thickness and the wrinkle degree of NiS x film also increased. Meanwhile, some microspheres on the surface of NiS x composite films were observed. The increase of microspheres can provide more active sites for redox reaction, resulting in high specific capacitance of NiS x composite. It is worth noting that large cracks gradually appear in the film when concentration of NiNO 3 electrolyte increases. Therefore, the tightness of the film is reduced with the increase of Ni(NO 3 ) 2 molar concentration. The expanding transverse crack may cause the NiS x composite film to fall off, resulting in a rapid decline in the capacitance of NiS x composite electrode after repeating charge-discharge tests. More detailed morphology was further observed by field-emission scanning electron microscopy (FE-SEM) and a transmission electron microscope (TEM), as shown in Figure 2g-h. The FE-SEM image ( Figure 2g) shows that the granular morphology was composed of mangy interconnected ultra-thin NiS x composite nanoflakes with porous structures, which is advantageous for electrolyte to penetrate into the inner space of active materials quickly. The reasons for the increase of wrinkle degree of NiS x film may be as follows. When the concentration of electrodeposition solution is relatively low, the as-prepared films should be relatively dense. As the arrays of supported NiS x nanoflakes gradually form, the film has porous characteristics, so the film will become more and more rough. The TEM image (Figure 2h) further confirms the ultra-thin NiS x composite nanoflakes were overlapping and interconnected. The energy-dispersive X-ray spectroscopy (EDX) mapping was further employed to investigate the elemental distribution of Ni and S. As shown in Figure 3, elements Ni and S were uniformly distributed, confirming the successful deposition of the NiS x nanosheets.

Electrochemical Performance
The electrochemical performance of samples NiSx/NF-1, NiSx/NF-2.5, and NiSx/NF-5 were evaluated by a three-electrode system. Figure 4a-c shows the typical CV curves of samples. It can be seen that there are well-defined redox peaks at different scan rates from 2 to 50 mV s -1 , which indicates the Faraday reaction nature of the NiSx/NF electrode [29]. The redox reaction of nickel sulfides in alkaline electrolyte is expressed as shown below [30][31][32].
When the scan rate increased, the corresponding current increased, and the redox peaks moved to both sides due to an enhanced polarization at high scan rate. Meanwhile, active materials failed to fully contact with ions at high scan rate, resulting in a reduction in the number of active site for redox reactions, so it can be observed there were some changes in the shape of CV curves with the increased scan rates. Figure 4d shows the comparison of the CV curves of three samples (NiSx/NF-1,

Electrochemical Performance
The electrochemical performance of samples NiSx/NF-1, NiSx/NF-2.5, and NiSx/NF-5 were evaluated by a three-electrode system. Figure 4a-c shows the typical CV curves of samples. It can be seen that there are well-defined redox peaks at different scan rates from 2 to 50 mV s -1 , which indicates the Faraday reaction nature of the NiSx/NF electrode [29]. The redox reaction of nickel sulfides in alkaline electrolyte is expressed as shown below [30][31][32]. NiS When the scan rate increased, the corresponding current increased, and the redox peaks moved to both sides due to an enhanced polarization at high scan rate. Meanwhile, active materials failed to fully contact with ions at high scan rate, resulting in a reduction in the number of active site for redox reactions, so it can be observed there were some changes in the shape of CV curves with the increased scan rates. Figure 4d shows the comparison of the CV curves of three samples (NiSx/NF-1,

Electrochemical Performance
The electrochemical performance of samples NiS x /NF-1, NiS x /NF-2.5, and NiS x /NF-5 were evaluated by a three-electrode system. Figure 4a-c shows the typical CV curves of samples. It can be seen that there are well-defined redox peaks at different scan rates from 2 to 50 mV·s −1 , which indicates the Faraday reaction nature of the NiS x /NF electrode [29]. The redox reaction of nickel sulfides in alkaline electrolyte is expressed as shown below [30][31][32]. NiS When the scan rate increased, the corresponding current increased, and the redox peaks moved to both sides due to an enhanced polarization at high scan rate. Meanwhile, active materials failed to fully contact with ions at high scan rate, resulting in a reduction in the number of active site for redox reactions, so it can be observed there were some changes in the shape of CV curves with the increased scan rates. Figure 4d shows the comparison of the CV curves of three samples (NiS x /NF-1, NiS x /NF-2.5, and NiS x /NF-5) at the same scan rate of 10 mV·s −1 . Usually, the area enclosed by CV curves can reflect the specific capacitance (gravimetric capacitance or areal capacitance) of active materials. Herein, the sample NiS x /NF-5 shows the largest area enclosed by CV curves between three samples, suggesting that it has the highest areal specific capacitance. Nanomaterials 2019, 9, x FOR PEER REVIEW 6 of 11 NiSx/NF-2.5, and NiSx/NF-5) at the same scan rate of 10 mV s −1 . Usually, the area enclosed by CV curves can reflect the specific capacitance (gravimetric capacitance or areal capacitance) of active materials. Herein, the sample NiSx/NF-5 shows the largest area enclosed by CV curves between three samples, suggesting that it has the highest areal specific capacitance. The galvanostatic charge-discharge (GCD) tests were carried out at different current densities ranging from 1 to 20 mA cm −2 . Typical GCD curves at current densities of 1, 2, 4, 6, 8, and 10 mA cm −2 are shown in Figure 5. It was revealed that the GCD curves of those three samples were all non-linear, further indicating that the energy storage of the NiSx/NF electrode came from the Faraday reaction. Figure 5d exhibits the GCD curves at the same current density of 1 mA cm -2 for comparison. The corresponding mass loading, areal capacitance, and gravimetric capacitance of as-prepared NiSx/NF are provided in Figure 5a,b. Obviously, areal capacitance increases from 0.46 to 2.63 F cm −2 with the increase of molar concentration of NiNO3. The NiSx/NF-5 sample has the highest areal capacitance of 2.63 F cm −2 , while NiSx/NF-2.5 exhibits the highest gravimetric capacitance of 1649.8 F g -1 . The areal and gravimetric capacitance of NiSx/NF electrodes as a function of charge−discharge current densities are summarized in Figure 6c, d. Due to the influence of electrodeposition solution, the specific capacitance shows a great difference. The possible explanation is as follows. Under the same cycle number of electrodeposition, it is obvious that the mass loading and thickness of NiSx (Ni3S2/NiS) film coated on the Ni foam will increase when the molar concentration of Ni(NO3)2 increases, as shown in Figure 2a-f and Figure 6a. It can also promote the increase of areal capacitance. At the same time, by comparing the morphologies of NiSx/NF-1, NiSx/NF-2.5, and NiSx/NF-5, it found that the porous structure and the degree of wrinkle of the film increase with the increase of the concentration, which means that the specific surface area of the active electrode materials will increase, and more active points will be exposed to the electrolyte solution. In addition, the microspheres anchored on NiSx film increase gradually as the molar concentration of Ni(NO3)2 increases, which further increases the utilization of active materials. The galvanostatic charge-discharge (GCD) tests were carried out at different current densities ranging from 1 to 20 mA·cm −2 . Typical GCD curves at current densities of 1, 2, 4, 6, 8, and 10 mA·cm −2 are shown in Figure 5. It was revealed that the GCD curves of those three samples were all non-linear, further indicating that the energy storage of the NiS x /NF electrode came from the Faraday reaction. Figure 5d exhibits the GCD curves at the same current density of 1 mA·cm −2 for comparison. The corresponding mass loading, areal capacitance, and gravimetric capacitance of as-prepared NiS x /NF are provided in Figure 5a,b. Obviously, areal capacitance increases from 0.46 to 2.63 F·cm −2 with the increase of molar concentration of NiNO 3 . The NiS x /NF-5 sample has the highest areal capacitance of 2.63 F·cm −2 , while NiS x /NF-2.5 exhibits the highest gravimetric capacitance of 1649.8 F·g −1 . The areal and gravimetric capacitance of NiS x /NF electrodes as a function of charge−discharge current densities are summarized in Figure 6c,d. Due to the influence of electrodeposition solution, the specific capacitance shows a great difference. The possible explanation is as follows. Under the same cycle number of electrodeposition, it is obvious that the mass loading and thickness of NiS x (Ni 3 S 2 /NiS) film coated on the Ni foam will increase when the molar concentration of Ni(NO 3 ) 2 increases, as shown in Figures 2a-f and 6a. It can also promote the increase of areal capacitance. At the same time, by comparing the morphologies of NiS x /NF-1, NiS x /NF-2.5, and NiS x /NF-5, it found that the porous structure and the degree of wrinkle of the film increase with the increase of the concentration, which means that the specific surface area of the active electrode materials will increase, and more active points will be exposed to the electrolyte solution. In addition, the microspheres anchored on NiS x film increase gradually as the molar concentration of Ni(NO 3 ) 2 increases, which further increases the utilization of active materials. Therefore, the specific capacitance should be enhanced. However, when the thickness and mass loading of the deposited NiS x film are excessive, there is no doubt that it is difficult for electrolyte to penetrate into the inner area of the electrode materials, which leads to the low effective utilization of the active materials. Therefore, it is natural that the mass specific capacitance will be reduced. Therefore, the specific capacitance should be enhanced. However, when the thickness and mass loading of the deposited NiSx film are excessive, there is no doubt that it is difficult for electrolyte to penetrate into the inner area of the electrode materials, which leads to the low effective utilization of the active materials. Therefore, it is natural that the mass specific capacitance will be reduced.   Therefore, the specific capacitance should be enhanced. However, when the thickness and mass loading of the deposited NiSx film are excessive, there is no doubt that it is difficult for electrolyte to penetrate into the inner area of the electrode materials, which leads to the low effective utilization of the active materials. Therefore, it is natural that the mass specific capacitance will be reduced.   Compared with other reported materials, the material prepared by our work has obvious performance advantages. The specific capacitance value was higher than those of previous nickel sulfide-based materials as supercapacitor electrodes, for instance: nanosheet-based Ni 3 S 2 microspheres on Ni foam (981.8 F·g −1 ) [33], porous NiS nanoflake arrays (718 F·g −1 ) [34], Ni 3 S 2 on Ni foam with rGO (1462 F·g −1 ) [35], 3D graphene/Ni 3 S 2 (741 F·g −1 ) [36], 3D reduced graphene oxide wrapped Ni 3 S 2 nanoparticles on Ni Foam (816.8 F·g −1 ) [37], Ni 3 S 2 @β-NiS materials (1158 F·g −1 ) [32], and graphene-coupled flower-like Ni 3 S 2 (1315 F·g −1 ) [38]. The enhanced specific capacitance should be attributed to the porous structure formed by interconnected ultra-thin nanoflakes and synergistic effect between Ni 3 S 2 and NiS.
The electrochemical impedance spectroscopy (EIS) of the as-prepared NiS x /NF-1, NiS x /NF-2.5, and NiS x /NF-5 was used to study the intrinsic electrochemical behavior, and corresponding Nyquist curves are shown in Figure 7. In the low-frequency region, all curves exhibit a straight line, indicating capacitive behavior. In the high-frequency region, all curves exhibit a small semicircle. The semicircle is related to the electrode surface properties, and the corresponding diameter represents the value of charge-transfer resistance (R ct ). The diameter of the semicircle for the samples (NiS x /NF-1, NiS x /NF-2.5, and NiS x /NF-5) decreases gradually, meaning that charge transfer and ion transfer rate have been improved. Further, the intercept of Nyquist curve at the Z' axis represents equivalent series resistance (ESR). The ESR values of the NiS x /NF-1, NiS x /NF-2.5, and NiS x /NF-5 electrodes are 0.36, 0.34, and 0.33 Ω, respectively. It also indicates the contact resistance at the interface of the NiS x film and Ni foam was very low. Compared with other reported materials, the material prepared by our work has obvious performance advantages. The specific capacitance value was higher than those of previous nickel sulfide-based materials as supercapacitor electrodes, for instance: nanosheet-based Ni3S2 microspheres on Ni foam (981.8 F g −1 ) [33], porous NiS nanoflake arrays (718 F g −1 ) [34], Ni3S2 on Ni foam with rGO (1462 F g −1 ) [35], 3D graphene/Ni3S2 (741 F g −1 ) [36], 3D reduced graphene oxide wrapped Ni3S2 nanoparticles on Ni Foam (816.8 F g −1 ) [37], Ni3S2@β-NiS materials (1158 F g −1 ) [32], and graphene-coupled flower-like Ni3S2 (1315 F g −1 ) [38]. The enhanced specific capacitance should be attributed to the porous structure formed by interconnected ultra-thin nanoflakes and synergistic effect between Ni3S2 and NiS.
The electrochemical impedance spectroscopy (EIS) of the as-prepared NiSx/NF-1, NiSx/NF-2.5, and NiSx/NF-5 was used to study the intrinsic electrochemical behavior, and corresponding Nyquist curves are shown in Figure 7. In the low-frequency region, all curves exhibit a straight line, indicating capacitive behavior. In the high-frequency region, all curves exhibit a small semicircle. The semicircle is related to the electrode surface properties, and the corresponding diameter represents the value of charge-transfer resistance (Rct). The diameter of the semicircle for the samples (NiSx/NF-1, NiSx/NF-2.5, and NiSx/NF-5) decreases gradually, meaning that charge transfer and ion transfer rate have been improved. Further, the intercept of Nyquist curve at the Z' axis represents equivalent series resistance (ESR). The ESR values of the NiSx/NF-1, NiSx/NF-2.5, and NiSx/NF-5 electrodes are 0.36, 0.34, and 0.33 Ω, respectively. It also indicates the contact resistance at the interface of the NiSx film and Ni foam was very low. The cycle stability was evaluated by repeating the charge-discharge test for 500 cycles, as shown in Figure 8. In repeated charging and discharging cycles, the capacitance of the NiSx/NF electrode decreased gradually. After 500 cycles, the capacitance decreased to 50% of the original capacitance, suggesting that the cycle stability of NiSx/NF electrode is not good. Based on SEM images, the cracks in the NiSx film on Ni foam can be observed. The repeated charge and discharge process may cause the loose connection between NiSx and Ni foam, and then NiSx drops off from Ni foam, resulting in the decrease of the capacitance of the NiSx/NF electrode materials. The cycle stability was evaluated by repeating the charge-discharge test for 500 cycles, as shown in Figure 8. In repeated charging and discharging cycles, the capacitance of the NiS x /NF electrode decreased gradually. After 500 cycles, the capacitance decreased to 50% of the original capacitance, suggesting that the cycle stability of NiS x /NF electrode is not good. Based on SEM images, the cracks in the NiS x film on Ni foam can be observed. The repeated charge and discharge process may cause the loose connection between NiS x and Ni foam, and then NiS x drops off from Ni foam, resulting in the decrease of the capacitance of the NiS x /NF electrode materials.

Conclusions
In this paper, the mixed Ni3S2/NiS composite (NiSx/NF) was prepared by electrodeposition on the Ni foam by cyclic voltammetry. Then, the influence of the different molar ratios of NiNO3 and thiourea electrolyte on the morphology and electrochemical properties of NiSx/NF electrode was investigated. The granular morphology and interconnected ultra-thin nanoflakes presented in NiSx thin films can provide more active sites for redox reaction, resulting in high specific capacitance of NiSx/NF. As-obtained NiSx/NF exhibits a high gravimetric capacitance up to 1649.8 F g −1 at 1 mA cm −2 . Meanwhile, areal capacitance of 2.63 F cm −2 can be achieved, demonstrating great application potential of Ni3S2/NiS composite.