Hydrothermal Synthesis of Polyhedral Nickel Sulfide by Dual Sulfur Source for Highly-Efficient Hydrogen Evolution Catalysis

Transition metal sulfides are cheap and efficient catalysts for water splitting to produce hydrogen; these compounds have attracted wide attention. Nickel sulfide (NiS2) has been studied in depth because of its simple preparation process, excellent performance and good stability. Here, we propose a modification to the hydrothermal synthesis method for the fabrication of a highly efficient and stable NiS2 electrocatalyst prepared by two different sulfur sources, i.e., sulfur powder and C3H7NaO3S2 (MPS), for application in hydrogen evolution reactions. The obtained NiS2 demonstrated excellent HER performance with an overpotential of 131 mV to drive -10 mA cm−1 in 0.5 M H2SO4 solution with 5mV performance change after 1000 cycles of stability testing. We believe that this discovery will promote the industrial development of nonprecious metal catalysts.


Introduction
Fossil fuels have been extremely important over the past hundred years, at the same time causing very serious damage to the environment [1][2][3]. In order to alleviate the pressure on the ecological environment, people have begun to look for more green, efficient and low-cost methods of energy generation [4][5][6]. Hydrogen is a clean energy source; it has a calorific value higher than fossil fuels and may be produced from water without pollution to the environment [7]. Therefore, the usage of catalysts to optimize the decomposition of water for hydrogen production has attracted great attention from researchers [8][9][10][11][12]. At the present, there are a variety of extremely stable and high-efficiency catalysts, such as platinum and palladium; however, their application is limited by their high cost [13,14]. Thus, there is a need to find a low-cost and stable catalyst.
Nonprecious transition metals are abundant and exhibit a good performance for hydrogen evolution reactions [15]. Nonprecious transition metal derivatives such as oxides, sulfides, carbides, nitrides, and alloys have been studied, and all demonstrated good catalytic performance. As a transition metal, nickel-based materials have been thoroughly studied in terms of catalysis [1,[16][17][18][19]. Nickel-based dichalcogenides, specifically, NiS 2 , are narrow-bandwidth semiconductors performing well in acidic environments with a relatively small Tafel slope and a low overpotential [17,[20][21][22][23]. Some NiS 2 crystals have irregular morphologies and complicated structures, causing instability in the performance of the catalyst. If there is further improvement in the catalytic performance and stability of NiS 2 , it is expected to be widely used in industry.
In this work, we propose a hydrothermal synthesis method of polyhedral NiS 2 (MS) from C 3 H 7 O 3 NaS 2 (MPS) and sulfur powder used as the dual sulfur source. The obtained NiS 2 was used for hydrogen evolution reactions (HER), performed in an acidic environment of 0.5M H 2 SO 4 . The catalyst showed excellent performance: the turn-on voltage at a current density of 10 mA cm −2 was 131 mV, and the Tafel slope was 50 mV dec −1 . In order to ensure the relevance of this technique for practical applications, we also conducted a stability test of 1000 circles; the resulting performance change was only 5 mV. For the renewable energy industry, we believe that this method of preparation of NiS 2 electrodes will promote the development of electrocatalysts.

Synthesis of NiS 2 (MS) with Dual Sulfur Source
Cleaned carbon fiber paper (CFP), washed using deionized water and absolute ethanol, was prepared. Then, 1.2 mM of Ni(NO 3 ) 2 ·6H 2 O and 1.6 mM of MPS were mixed into the Teflon-lined autoclave with 25 mL deionized water and stirred for 15 min. After the above step, 0.8 mM of sulfur powder was added to the solution and stirred for 15 min slowly. After finishing this step, the CFP was placed into a container and was heated to 180 • C for 8 h. After this, the CFP was taken out and washed.

Synthesis of NiS 2 with MPS
Cleaned carbon fiber paper (CFP), washed by deionized water and absolute ethanol, was prepared. Then, 1.2 mM of Ni(NO 3 ) 2 ·6H 2 O and 1.6 mM of MPS were added to the Teflon-lined autoclave with 25 mL deionized water and stirred for 15 min. After finishing this step, the CFP was placed into a container and heated to 180 • C for 8 h. After this, the CFP was taken out and washed.

Synthesis of NiS 2 with S
Cleaned carbon fiber paper (CFP), washed by deionized water and absolute ethanol, was prepared. Then, 1.2 mM of Ni(NO 3 ) 2 ·6H 2 O was added to the Teflon-lined autoclave with 25 mL deionized water and stirred for 15 min. After the above step, 0.8 mM of sulfur powder was added to the solution and stirred for 15 min slowly. After finishing this step, put CFP was placed into a container and heated to 180 • C for 8 h. Finally, the CFP was taken out and washed.

Electrochemical Measurements
A CHI760E electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China) was used to analyze the performance of samples in 0.5 M H 2 SO 4 . A three-electrode system was used to test samples. The measured potentials were converted to a reversible hydrogen electrode (E(RHE) = E Hg/Hg 2 Cl 2 + 0.241 + 0.0591 pH). Prior to testing, nitrogen had to be bubbled into H 2 SO 4 solution to remove oxygen from the solution. The range of LSV test was from −0.8 to 0 V with the scan rate of 2 mV s −1 . Tafel data could be calculated from the LSV test. Different scan rates of the CV test (5 to 90 mV s −1 ) were carried out to reveal the ECSA of samples. EIS measurements were carried out in a frequency range from 10 5 to 0.1 Hz with an AC voltage of 5 mV. Figure 1a,b present scanning electron microscopy (SEM) images of NiS 2 (MS) prepared by sulfur powder and MPS, forming a regular polyhedral block structure on the carbon fiber which was from the carbon fiber paper (CFP). Transmission electron microscopy image (TEM) captured from the edge of the sample is shown on Figure 1c. Figure 1d represents the HRTEM image of NiS 2 (MS), with regular crystal plane spacing of 0.283 nm, corresponding to the (200) plane according to the following XRD card (JCPDS#65-3325) [24,25]. In order to compare the proposed NiS 2 (MS) synthesis method, i.e., from dual sulfur sources, with other techniques, we also studied the morphology (Figures S1 and S2) of the synthesized material by two other techniques. As can be seen from Figure S1a,b, the carbon fiber surface also has some regularly shaped products stacked together. However, only a small amount of product adhered to the surface compared with the material synthesized by the dual sulfur source, which had a certain degree of negative influence on the conductivity between the substances. Figure S2 is SEM patterns of NiS 2 synthesized by sulfur powder as the sole sulfur source. The resulting product was relatively small and scattered on the surface of the carbon fiber, which was in sharp contrast to the products synthesized by the previous two schemes. During the HER process, the other two samples easily detached from the carbon fiber, resulting in poor performance and stability. In comparison, the samples prepared by the dual sulfur source interlaced with each other provided a larger coverage of the electrode, and thus, improved resistance and catalytic performance.

Electrochemical Measurements
A CHI760E electrochemical analyzer (CH Instruments, Chenhua Co., Shanghai, China) was used to analyze the performance of samples in 0.5 M H2SO4. A three-electrode system was used to test samples. The measured potentials were converted to a reversible hydrogen electrode (E(RHE) = E Hg/Hg2Cl2 + 0.241 + 0.0591 pH). Prior to testing, nitrogen had to be bubbled into H2SO4 solution to remove oxygen from the solution. The range of LSV test was from −0.8 to 0 V with the scan rate of 2 mV s −1 . Tafel data could be calculated from the LSV test. Different scan rates of the CV test (5 to 90 mV s −1 ) were carried out to reveal the ECSA of samples. EIS measurements were carried out in a frequency range from 10 5 to 0.1 Hz with an AC voltage of 5 mV. Figure 1a,b present scanning electron microscopy (SEM) images of NiS2 (MS) prepared by sulfur powder and MPS, forming a regular polyhedral block structure on the carbon fiber which was from the carbon fiber paper (CFP). Transmission electron microscopy image (TEM) captured from the edge of the sample is shown on Figure 1c. Figure 1d represents the HRTEM image of NiS2 (MS), with regular crystal plane spacing of 0.283 nm, corresponding to the (200) plane according to the following XRD card (JCPDS#65-3325) [24,25]. In order to compare the proposed NiS2 (MS) synthesis method, i.e., from dual sulfur sources, with other techniques, we also studied the morphology (Figures S1 and S2) of the synthesized material by two other techniques. As can be seen from Figure S1a, b, the carbon fiber surface also has some regularly shaped products stacked together. However, only a small amount of product adhered to the surface compared with the material synthesized by the dual sulfur source, which had a certain degree of negative influence on the conductivity between the substances. Figure S2 is SEM patterns of NiS2 synthesized by sulfur powder as the sole sulfur source. The resulting product was relatively small and scattered on the surface of the carbon fiber, which was in sharp contrast to the products synthesized by the previous two schemes. During the HER process, the other two samples easily detached from the carbon fiber, resulting in poor performance and stability. In comparison, the samples prepared by the dual sulfur source interlaced with each other provided a larger coverage of the electrode, and thus, improved resistance and catalytic performance. X-ray diffraction (XRD) analysis was performed in order to investigate the structure and composition of the obtained NiS2 [25]. Analyzing the shape of the peaks in the XRD picture, it can be X-ray diffraction (XRD) analysis was performed in order to investigate the structure and composition of the obtained NiS 2 [25]. Analyzing the shape of the peaks in the XRD picture, it can be seen that the obtained substance had good crystallinity. The diffraction peaks of 31.7 • , 36.7 • , 41.3 • , 45.4 • , 52.,9 • , 56.4 • and 62.9 • corresponded to NiS 2 planes according to JCPDS#65-3325 of (111), (200), (210), (211), (220), (221) and (311), respectively. The diffraction peak at 27 • was attributed to the presence of carbon fiber [24]. The peak at 65 • may be have been contributed to the presence of sulfur. Figure 2b represents the full x-ray photoelectron spectroscopy (XPS) spectra of NiS 2 (MS) used for confirmation of its elemental composition [17,26]. As shown in Figure 2c, two spin-orbit doublets and two satellites can be deconvoluted from the Ni 2p spectrum, namely 853.5 eV and 856.1 eV corresponded to 2p3/2 of Ni 2+ and Ni 3+ , and 871.1 eV and 874.5 eV were attributed to 2p1/2 of Ni 2+ and Ni 3+ respectively [17,[26][27][28][29]. Figure 2d shows the deconvolution of S 2p peaks: the peak at 161.52 eV belonged to S 2p3/2, while that at 162.87 eV corresponded to S 2p1/2 of Ni-S bondings [30,31]. Due to the surface oxidation of NiS 2 , the peak at 166.7 eV could be attributed to S-O bonding [19].

Results and Discussion
, (211), (220), (221) and (311), respectively. The diffraction peak at 27° was attributed to the presence of carbon fiber [24]. The peak at 65° may be have been contributed to the presence of sulfur. Figure 2b represents the full x-ray photoelectron spectroscopy (XPS) spectra of NiS2 (MS) used for confirmation of its elemental composition [17,26]. As shown in Figure 2c, two spin-orbit doublets and two satellites can be deconvoluted from the Ni 2p spectrum, namely 853.5 eV and 856.1 eV corresponded to 2p3/2 of Ni 2+ and Ni 3+ , and 871.1 eV and 874.5 eV were attributed to 2p1/2 of Ni 2+ and Ni 3+ respectively [17,[26][27][28][29]. Figure 2d shows the deconvolution of S 2p peaks: the peak at 161.52 eV belonged to S 2p3/2, while that at 162.87 eV corresponded to S 2p1/2 of Ni-S bondings [30,31]. Due to the surface oxidation of NiS2, the peak at 166.7 eV could be attributed to S-O bonding [19]. In order to further prove the applicability of the synthesized NiS2, electrochemical analysis was carried out to investigate the HER performance of the electrocatalyst in 0.5 M H2SO4 aqueous solution. Linear scan voltammetry (LSV) curves were measured to analyze the HER activity of NiS2, as shown in Figure 3a. The electrode based on NiS2 and grown on carbon paper from MPS and sulfur powder showed excellent HER performance, with an overpotential of 131 mV to drive −10 mA cm −1 . Compared with the performance of the NiS2 synthesized using MPS or sulfur powder as the sulfur source, the overpotential was 197 mV and 261 mV, respectively. There may be two reasons for switching on the voltage gap. First, the increase in the loading of the substrate was obvious; provided more active sites for hydrogen adsorption and desorption. From the SEM image, it can be seen that there were many substrates on the surface that is synthesized by the dual sulfur source, which may have caused more defects in the boundary crystal planes, thereby improving the catalytic performance to a certain extent. It can be seen from the XRD pattern that the crystallinity of the sample prepared using the dual sulfur source was better. Combined with the TEM image of the sample, the high crystal plane (200) of the sample can be seen. This sample showed excellent In order to further prove the applicability of the synthesized NiS 2 , electrochemical analysis was carried out to investigate the HER performance of the electrocatalyst in 0.5 M H 2 SO 4 aqueous solution. Linear scan voltammetry (LSV) curves were measured to analyze the HER activity of NiS 2 , as shown in Figure 3a. The electrode based on NiS 2 and grown on carbon paper from MPS and sulfur powder showed excellent HER performance, with an overpotential of 131 mV to drive −10 mA cm −1 . Compared with the performance of the NiS 2 synthesized using MPS or sulfur powder as the sulfur source, the overpotential was 197 mV and 261 mV, respectively. There may be two reasons for switching on the voltage gap. First, the increase in the loading of the substrate was obvious; provided more active sites for hydrogen adsorption and desorption. From the SEM image, it can be seen that there were many substrates on the surface that is synthesized by the dual sulfur source, which may have caused more defects in the boundary crystal planes, thereby improving the catalytic performance to a certain extent. It can be seen from the XRD pattern that the crystallinity of the sample prepared using the dual sulfur source was better. Combined with the TEM image of the sample, the high crystal plane (200) of the sample can be seen. This sample showed excellent performance in the process of electrocatalytic water decomposition. We summarized the overpotential of the HER of similar samples that have been published so far; Figure S3. Figure 3b shows the Tafel slope of 50 mV dec −1 for NiS 2 with MPS and sulfur powder and the other two Tafel slopes of 53 mV dec −1 and 74 mV dec −1 for NiS 2 with MPS or sulfur powder, respectively. The lowest overpotential and Tafel slope for NiS 2 fabricated by MPS and sulfur powder indicated the best performance of hydrogen evolution reaction and high intrinsic catalytic activity supported by its superior exchange current density. To assess the stability of the catalytic performance of NiS 2 , cycling voltammetry (CV) was conducted. Thus, after 1000 cycles of stability testing at −10 mA cm −1 , the performance loss was only 5 mV. performance in the process of electrocatalytic water decomposition. We summarized the overpotential of the HER of similar samples that have been published so far; Figure S3. Figure 3b shows the Tafel slope of 50 mV dec −1 for NiS2 with MPS and sulfur powder and the other two Tafel slopes of 53 mV dec −1 and 74 mV dec −1 for NiS2 with MPS or sulfur powder, respectively. The lowest overpotential and Tafel slope for NiS2 fabricated by MPS and sulfur powder indicated the best performance of hydrogen evolution reaction and high intrinsic catalytic activity supported by its superior exchange current density. To assess the stability of the catalytic performance of NiS2, cycling voltammetry (CV) was conducted. Thus, after 1000 cycles of stability testing at −10 mA cm −1 , the performance loss was only 5 mV. Electrochemical impendence spectroscopy (EIS) was used to assess the charge transfer performance of the synthesized NiS2, which also determined the double layer capacitance [1,16]. The Nyquist plots (Figure 3d) confirmed that the NiS2 fabricated from MPS and sulfur powder showed excellently smaller charge transfer resistance (Rct), compared to other two materials, in the experiment and simulation in 0.5 M H2SO4 solution. The experimental data was fitted according to the electrical model presented in the inset of Figure 3d. Cad and Rad represent adsorption capacitance and adsorption resistance, respectively. The lowest charge transfer resistance of 53 Ω for NiS2 prepared by MPS and sulfur powder was observed from the fitting experiment data in Figure 3d. The charge transfer resistances of NiS2 with only MPS and only sulfur powder were 178 Ω and 258 Ω, respectively. Contrasting the value of Rct of the samples, it can be observed that the NiS2 fabricated by MPS and sulfur powder had improved electrical conductivity, allowing electrochemical processes to occur on the surface. Nonfaradaic double-layer capacitance (Cdl) was calculated for further analysis of the NiS2 activity. The Cdl of NiS2 fabricated by the dual sulfur source was 10.3 mF cm −2 , and the Cdl of the material prepared by MPS and sulfur powder were 0.44 mF cm −2 and 1.67 mF cm −2 , respectively. Different scan rates of cyclic voltammetry measurement were carried out in order to determine the electrochemical surface area (ECSA) of NiS2, as shown in Figure 4a. The catalytic performance of the working electrode was normalized to 1 cm 2 . Figure 4a-d show cyclic voltammograms of NiS2 prepared by different experimental schemes which were measured in the nonfaradaic capacitance current range with different scan rates. Electrochemical impendence spectroscopy (EIS) was used to assess the charge transfer performance of the synthesized NiS 2 , which also determined the double layer capacitance [1,16]. The Nyquist plots (Figure 3d) confirmed that the NiS 2 fabricated from MPS and sulfur powder showed excellently smaller charge transfer resistance (R ct ), compared to other two materials, in the experiment and simulation in 0.5 M H 2 SO 4 solution. The experimental data was fitted according to the electrical model presented in the inset of Figure 3d. C ad and R ad represent adsorption capacitance and adsorption resistance, respectively. The lowest charge transfer resistance of 53 Ω for NiS 2 prepared by MPS and sulfur powder was observed from the fitting experiment data in Figure 3d. The charge transfer resistances of NiS 2 with only MPS and only sulfur powder were 178 Ω and 258 Ω, respectively. Contrasting the value of R ct of the samples, it can be observed that the NiS 2 fabricated by MPS and sulfur powder had improved electrical conductivity, allowing electrochemical processes to occur on the surface. Nonfaradaic double-layer capacitance (C dl ) was calculated for further analysis of the NiS 2 activity. The C dl of NiS 2 fabricated by the dual sulfur source was 10.3 mF cm −2 , and the C dl of the material prepared by MPS and sulfur powder were 0.44 mF cm −2 and 1.67 mF cm −2 , respectively. Different scan rates of cyclic voltammetry measurement were carried out in order to determine the electrochemical surface area (ECSA) of NiS 2 , as shown in Figure 4a. The catalytic performance of the working electrode was normalized to 1 cm 2 .

Conclusions
In this work, we proposed a one-step hydrothermal method for NiS2 (MS) synthesis from dual sulfur sources and applied the obtained material as a catalyst for HER. The resulting material had a regular morphology with more specific surface area and active sites. The electrochemical performance and stability of NiS2 prepared by the hydrothermal method improved significantly. We believe that our findings will have a positive impact on the industrial application of nonprecious metal catalysts.

Conclusions
In this work, we proposed a one-step hydrothermal method for NiS 2 (MS) synthesis from dual sulfur sources and applied the obtained material as a catalyst for HER. The resulting material had a regular morphology with more specific surface area and active sites. The electrochemical performance and stability of NiS 2 prepared by the hydrothermal method improved significantly. We believe that our findings will have a positive impact on the industrial application of nonprecious metal catalysts.
Supplementary Materials: The following are available online at http://www.mdpi.com/2079-4991/10/11/2115/s1, Figure S1. (a) and (b) are SEM images of NiS 2 fabricated by MPS as the only sulfur source. Figure S2. (a) and (b) are SEM patterns of the product which prepared by sulfur powder only. Figure S3. Contrast of HER activity of electrocatalysts of similar materials.