Regulation of Ni₃S₂@NiS Heterostructure Grown on Industrial Nickel Net for Improved Electrocatalytic Hydrogen Evolution

Abstract

A novel all-in-one catalytic electrode containing a Ni₃S₂@NiS heterostructure (Ni₃S₂@NiS/Ni-Net) was in situ synthesized on an industrial nickel net (Ni-Net) using a one-step solvothermal method, in which ethanol was the solvent and thioacetamide was the sulfur source, respectively. The effects of the addition amount of the sulfur source on the composition, morphology, and electronic structure of the Ni₃S₂@NiS heterostructures and their electrocatalytic hydrogen evolution reaction (HER) activities were investigated. When 2 mmol of sulfur source was introduced, the prepared Ni₃S₂@NiS/Ni-Net electrode with a nanorod-like structure required overpotentials of 207 and 322 mV to drive the current densities of 100 and 500 mA/cm², respectively, in 1 M KOH solution, and only needed the overpotential of 429 mV to deliver 1000 mA/cm². Meanwhile, the Ni₃S₂@NiS/Ni-Net electrode can operate stably at a high current density of 90 mA/cm² under harsh alkaline conditions for at least 100 h. The results show that the Ni₃S₂@NiS/Ni-Net electrode has high activity and stable HER performance at a high current density, which provides a new idea for the development of high-efficiency electrodes for industrial alkaline hydrogen production.

1. Introduction

With the exhaustion of fossil energy and the intensification of environmental problems, finding a green, clean, and sustainable renewable energy source has become an irresistible research direction. Hydrogen is a new energy that has attracted much attention from researchers because of its zero carbon emissions and high calorific value [1,2,3,4]. Electrocatalytic water splitting is widely regarded as one of the most promising technologies for hydrogen production [5]. However, both the hydrogen evolution reaction (HER) on cathodes and the oxygen evolution reaction (OER) on anodes have limitations due to the need for high overpotential. Therefore, efficient hydrogen evolution reactions can be achieved by using suitable catalysts as electrode materials. Nowadays, the most high-performance electrocatalysts for hydrogen production are precious-metal-based materials (e.g., Pt, Ru, etc.), but they are difficult to popularize and apply due to the disadvantages of low earth abundance and high price. Therefore, it is crucial to develop a new, low-cost and high-efficiency non-precious-metal-based catalytic electrode material for hydrogen production, which is the key to the development of water electrolysis technology [6].

In recent decades, transition-metal-based materials, emerging as a class of low-cost and easy-to-prepare catalysts, have shown great potential in the realm of electrocatalytic water splitting for green hydrogen production [7,8,9,10,11,12,13,14,15,16]. Among them, Ni-based sulfides have been documented to exhibit desirable electrocatalytic HER performance [12,13,14,15,16]. In particular, metallic nickel sulfide, as a typical representative of sulfides, is considered to be a more effective alternative to precious-metal-based electrocatalysts due to the presence of unique abundant metal bonds (M–M), which endows it with good conductivity, facilitating electron transfer during electrocatalysis process [14,16]. The reason is that nickel sulfide (Ni₃S₂), a promising electrocatalyst, has consistently shown good catalytic HER activity. In order to further enhance the catalytic activity of Ni₃S₂, constructing a Ni₃S₂-based heterostructure has proven to be an effective route [17,18,19,20,21]. Based on this concept, a unique Ni₃S₂@NiS heterostructure has been constructed through the introduction of NiS to increase the phase boundaries of the different components, thus generating more active sites, which simultaneously enhance the electrocatalytic hydrogen evolution performance. Most recently, a number of researchers have devoted great efforts to exploit all-in-one catalytic electrodes containing Ni₃S₂ prepared in situ using nickel foam as the substrate (Ni₃S₂/NF), and their performances have been optimized by adopting some corresponding modulation strategies. However, it is difficult for industrial applications of nickel foam to be realized due to its soft texture and the easily collapsed skeleton structure during the preparation process of catalysts. Therefore, the use of a hard industrial nickel net (Ni-Net) instead of nickel foam as the new nickel substrate is of great significance to realize the industrial application of nickel-based electrocatalysts.

In this work, a novel all-in-one catalytic HER electrode containing a Ni₃S₂@NiS heterostructure was synthesized in situ on nickel net (Ni₃S₂@NiS/Ni-Net) using the one-step solvothermal method. The as-obtained Ni₃S₂@NiS nanorod array electrode showed remarkable electrocatalytic performance, with overpotentials of 207, 322 and 429 mV to drive current densities of 100, 500 and 1000 mA/cm², respectively, in 1 M KOH solution. Moreover, it can operate stably at a high current density of 90 mA/cm² under harsh alkaline conditions for at least 100 h. This work shown here provides some new insights into the development of high-efficiency electrodes for industrial alkaline hydrogen production.

2. Results and Discussion

2.1. Synthesis and Microstructural Characterization of Ni₃S₂@NiS/Ni-Net Materials

In the solvothermal synthesis system, the industrial nickel net (Ni-Net) was first subjected to a series of treatments comprising acetone, 3 M HCl, ethanol and deionized water. After, ethanol as the solvent, thioacetamide (TAA) as the sulfur source, and the treated Ni-Net as the nickel source and substrate were placed into a 50 mL Teflon-lined stainless steel autoclave. Subsequently, the autoclave was heated in an oven at 160 °C for 12 h. The different Ni₃S₂@NiS heterostructures grown on clean Ni-Net (Ni₃S₂@NiS/Ni-Net) were prepared by modulating the content of the sulfur source. As illustrated in Figure 1, when 2 mmol of sulfur source was introduced into the synthetic system, a nanorod-like Ni₃S₂@NiS heterostructure was obtained. Importantly, the effect of the different amounts of sulfur source (0.5, 1, 2 and 3 mmol) on the phase, composition and microstructure of the Ni₃S₂@NiS heterostructure were investigated in detail, as below.

X-ray diffraction (XRD) characterization was first employed to disclose the crystalline phase structure. XRD patterns of Ni-Net substrate, Ni₃S₂@NiS/Ni-Net-0.5, Ni₃S₂@NiS/Ni-Net-1, Ni₃S₂@NiS/Ni-Net-2 and Ni₃S₂@NiS/Ni-Net-3 are shown in Figure 2, respectively. The peaks located at 44.48°, 51.83° and 76.35° corresponded to the (111), (200) and (220) planes of Ni (PDF#70-1849), respectively, and the peaks at 18.43°, 31.31°, 32.21°, 35.71°, 40.47°, 48.84°, 50.14°, 52.64°, 57.43° and 59.70° can be assigned to the (110), (101), (300), (021), (211), (131), (410), (401), (330) and (012) planes of NiS (PDF#86-2281). In addition, the peaks situated at 21.77°, 31.13°, 37.78° and 49.73° and 55.73°, 21.77°, 31.13°, 37.78°, 49.73°, 55.2° and 55.4° can be attributed to the (101), (110), (003), (113), (122) and (300) crystal planes of Ni₃S₂ (PDF#76-1870), indicating the successful generation of Ni₃S₂. The results show that a self-supported catalytic electrode containing a Ni₃S₂@NiS composite structure was successfully prepared on Ni-Net using a one-step solvothermal method.

Most notably, the diffraction peaks of Ni₃S₂ are stronger than those of NiS in Ni₃S₂@NiS/Ni-Net-0.5. With the increase in TAA, the intensity of diffraction peaks of NiS in Ni₃S₂@NiS/Ni-Net-1 was significantly enhanced, which was ascribed to the fact that the resulting Ni₃S₂ continued to react with a sulfur source and convert into NiS [22]. When the sulfur content continued to increase, the diffraction peaks of both NiS and Ni₃S₂ in Ni₃S₂@NiS/Ni-Net-2 were stronger than the former two samples because the Ni-net was further vulcanized by the enriched sulfur source to produce Ni₃S₂. This indicates that the obtained Ni₃S₂@NiS/Ni-Net-2 has more of a Ni₃S₂@NiS heterostructure compared to Ni₃S₂@NiS/Ni-Net-0.5 and Ni₃S₂@NiS/Ni-Net-1. Meanwhile, it can be observed from the XRD patterns that the diffraction peaks of Ni₃S₂@NiS/Ni-Net-3 and Ni₃S₂@NiS/Ni-Net-2 basically remained the same. It is not difficult to see that the increase from 2 mmol to 3 mmol in the sulfur source content does not change the phase composition of Ni₃S₂@NiS significantly. In order to obtain the phase composition of Ni₃S₂@NiS formed at the surface of Ni₃S₂@NiS/Ni-Net-2, the mass proportions of Ni₃S₂ and NiS in this material were determined to be about 29.8% and 28.6%, respectively, using the fitting and quantitative analysis of the XRD pattern (Table S1).

To further verify the formed NiS, Ni₃S₂, or mixed phases of Ni₃S₂@NiS/Ni-Net electrodes, the Raman spectra of these materials were presented in Figure S1. Ni₃S₂@NiS/Ni-Net-1, Ni₃S₂@NiS/Ni-Net-2 and Ni₃S₂@NiS/Ni-Net-3 displayed a series of Raman peaks at ~205, 304 and 352 cm⁻¹, which could be assigned to Ni₃S₂ [23]. In addition, the peaks at ~174, 222 and 350 cm⁻¹ corresponded to the NiS [24]. The results indicated the existence of the Ni₃S₂/NiS compound in the three materials. However, there were no Raman peaks observed in the Ni₃S₂@NiS/Ni-Net-0.5 material, which could be due to the low content of the Ni₃S₂/NiS compound generated by the slight sulfuration of S sources.

The SEM images of Ni₃S₂@NiS/Ni-Net at different magnifications are exhibited in Figure 3 and Figure S2. In Figure 3A,B, SEM images of Ni₃S₂@NiS/Ni-Net-0.5 present scattered short micrometer rods on the Ni-Net, which results from a severe lack of sulfur source reacting with the Ni-net. Figure 3C,D show the SEM images of the Ni₃S₂@NiS/Ni-Net-1, and it can be seen that amounts of a hill-like structure are formed on the Ni-Net substrate. The SEM images of Ni₃S₂@NiS/Ni-Net-2 in Figure 3E,F and Figure S2C demonstrate that nanorod array structures with a diameter of about 300–400 nm were uniformly grown on the surface of the Ni-Net substrate, contributing to a significant increase in the specific surface area of the sample, which might lead to the full exposure of catalytically active sites for electrocatalytic HER. As displayed in Figure 3G,H, the Ni₃S₂@NiS/Ni-Net-3 exhibits almost the same nanorod array structure as the Ni₃S₂@NiS/Ni-Net-2. Moreover, the elemental mapping of Ni₃S₂@NiS/Ni-Net-2 confirms that Ni and S elements are homogeneously distributed over the whole sulfide layer of Ni₃S₂@NiS (Figure 3K,L). In addition, Figure S3 revealed that the thickness of the sulfide layer in the Ni₃S₂@NiS/Ni-Net-2 sample was determined to be around 1.41 μm. As demonstrated in the EDS pattern of Ni₃S₂@NiS/Ni-Net-2, the atomic ratio of Ni to S is approximately 17:20 (Figure S4).

A TEM test was then conducted to further investigate the microstructure of the Ni₃S₂@NiS/Ni-Net-2 electrode. As revealed in Figure 4A,C, a dominant rod−like structure decorated with a certain number of nanoparticles can be clearly observed. Figure 4B shows the HRTEM image of the sample’s nanoparticles; two sets of lattice fringes with spacing distances of 0.287 nm and 0.186 nm can be found, corresponding to the (110) crystal plane of Ni₃S₂ and the (131) crystal facet of NiS, respectively. As illustrated in Figure 4D, the HRTEM image of the nanorods in the sample reveals the presence of two sets of lattice stripes, associated with the (110) crystallographic plane of Ni₃S₂ and the (131) crystallographic plane of NiS, respectively. This indicates that the sample contains Ni₃S₂ and NiS on both nanorods and nanoparticles, which is consistent with the XRD results.

2.2. XPS Analysis of the Resulting Samples

The chemical element compositions and valence states of Ni₃S₂@NiS/Ni-Net-0.5, Ni₃S₂@NiS/Ni-Net-1 and Ni₃S₂@NiS/Ni-Net-2 samples were analyzed by XPS. The XPS peaks of Ni 2p and S 2p were calibrated according to the position of the C 1s peak at 284.8 eV (Figure S5). From the survey XPS spectra in Figure 5A, the four elements, including C, O, Ni and S, can be detected in the Ni₃S₂@NiS/Ni-Net-0.5, Ni₃S₂@NiS/Ni-Net-1 and Ni₃S₂@NiS/Ni-Net-2 samples [25]. This indicates that the sulfide source successfully reacted with the Ni-Net substrate during the one-step solvothermal method to produce the nickel sulfide phase.

Subsequently, the high-resolution XPS spectra of Ni 2p and S 2p for the three samples are shown in Figure 5B,C, respectively. From the Ni 2p spectra of these three samples in Figure 5B, the peaks at 852.90 eV, 853.08 eV and 852.90 eV correspond with Ni⁰ of Ni 2p_3/2, while the peaks at 870.29 eV, 870.30 eV and 870.29 eV identify with Ni⁰ of Ni 2p_1/2 [25,26,27]. The Ni²⁺ 2p_3/2 signal peaks and their satellite peaks are clearly shown at 853.70 eV, 853.80 eV and 853.70 eV, and the Ni²⁺ 2p_1/2 signal peak and its satellite peaks are clearly shown at 871.20 eV, 871.24 eV and, 871.20 eV. The peaks located at 855.90 eV, 855.95 eV and 855.93 eV are assigned to Ni³⁺ 2p_3/2, and the peaks at 873.50 eV, 873.52 eV and 873.51 eV correspond to Ni³⁺ 2p_{1/2, respectively} [23,28]. It is shown that the Ni element in the Ni₃S₂@NiS/Ni-Net catalytic electrode exists in the form of Ni⁰, Ni²⁺ and Ni³⁺. It is worth noting that the Ni⁰ signal peaks originate from Ni-Net and Ni₃S₂, respectively. In particular, compared with the other two samples, the peak intensities of Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2 increased significantly in Ni₃S₂@NiS/Ni-Net-1, owing to the higher content of NiS compared to that of Ni₃S₂ in this material (see XRD patterns in Figure 2).

Importantly, compared with Ni₃S₂@NiS/Ni-Net-0.5, the Ni²⁺ 2p_3/2 and Ni³⁺ 2p_3/2 peaks of Ni₃S₂@NiS/Ni-Net-1 underwent positive shifts of ~0.1 eV and ~0.05 eV, respectively (note that the NiS content of Ni₃S₂@NiS/Ni-Net-1 is higher than that of Ni₃S₂@NiS/Ni-Net-0.5). Meanwhile, the Ni²⁺ 2p_3/2 and Ni³⁺ 2p_3/2 peaks of Ni₃S₂@NiS/Ni-Net-2 showed slight negative shifts of ~0.1 eV and ~0.02 eV compared to those of Ni₃S₂@NiS/Ni-Net-1, respectively (note that the Ni₃S₂ content of Ni₃S₂@NiS/Ni-Net-2 is higher than that of Ni₃S₂@NiS/Ni-Net-1) [29,30,31]. This suggests that the Ni sites from Ni₃S₂ of Ni₃S₂@NiS/Ni-Net-2 could be mainly catalytically active sites for the absorption of H₂O to generate H* during HER electrocatalysis [25].

The S 2p high-resolution spectra of Ni₃S₂@NiS/Ni-Net-0.5, Ni₃S₂@NiS/Ni-Net-1 and Ni₃S₂@NiS/Ni-Net-2 are shown in Figure 5C. We can see that one peak at the binding energy of 163 eV of Ni₃S₂@NiS/Ni-Net-0.5 can be assigned to S²⁻. A slight negative shift of approximately 0.01 eV was observed when the concentration of NiS increased, as evidenced by a comparison of Ni₃S₂@NiS/Ni-Net-0.5 and Ni₃S₂@NiS/Ni-Net-1. Apparently, a significant negative shift of around 0.11 eV of the S²⁻ peak in Ni₃S₂@NiS/Ni-Net-2 was detected by the comparison with Ni₃S₂@NiS/Ni-Net-1, indicating that the increase in the Ni₃S₂/NiS heterostructure facilitates electron capture by S sites and thus accelerates the water splitting process [32,33]. The signal peak located at approximately 164 eV indicates the presence of the signal peak of thiol S, which may be due to the presence of the remaining S-containing organic matter resulting from an incomplete reaction of a portion of TAA, because of its microsolubility in the ethanol solvent.

2.3. Electrochemical Evaluation of Ni₃S₂@NiS/Ni-Net for the HER

The HER electrocatalytic performances of Ni₃S₂@NiS/Ni-Net samples, Pt/C and blank Ni-Net were evaluated using a conventional three-electrode system. The electrolyte for HER measurements was a 1.0 M KOH solution. Figure 6A shows the iR-corrected LSV polarization curves for Ni₃S₂@NiS/Ni-Net electrodes, Pt/C and Ni-Net. It is clear that Ni₃S₂@NiS/Ni-Net-2 had the best HER performance, with low overpotentials of 207 and 322 mV to deliver 100 mA/cm² and 500 mA/cm², respectively, indicating that the increase in Ni₃S₂@NiS heterostructure had a promoting effect on the HER performance. Notably, Ni₃S₂@NiS/Ni-Net-2 exhibited an outstanding HER activity at large current densities, only needing quite a low overpotential of 429 mV at 1000 mA cm⁻². Tafel plots of these materials were obtained from the HER polarization curves in Figure 6B. Although Pt/C had the lowest Tafel slope (101.2 mV dec⁻¹), the Tafel slope of the Ni₃S₂@NiS/Ni-Net-2 catalyst was 116.3 mV dec⁻¹, which was smaller than those of the other three Ni₃S₂@NiS/Ni-Net samples and Ni-Net, suggesting that it had fastest HER kinetics [34,35].

The electrochemically active surface area (EASA) is a pivotal parameter for assessing the electrocatalytic activity, directly proportional to the double-layer capacitance (C_dl) value of the catalyst [26,27,28]. As shown in Figure 7A–D, the cyclic voltammetry (CV) curves of every prepared sample with different scan rates were measured in a non-faradaic region (−1.01 V~−0.89 V vs. SCE). A linear relationship was gained by plotting the difference in the anodic and cathodic current densities relative to varying sweep speeds at −0.95 V. The double-layer capacitance (C_dl) values of Ni₃S₂@NiS/Ni-Net-0.5, Ni₃S₂@NiS/Ni-Net-1, Ni₃S₂@NiS/Ni-Net-2 and Ni₃S₂@NiS/Ni-Net-3 were calculated to be 27.66, 49.31, 89.92 and 8.63 mF cm⁻², respectively (Figure 7E). As illustrated in Figure 7F, the EASA value is based on the following formula: EASA = C_dl/C_s (C_s is the specific capacitance of the electrocatalyst, usually C_s = 0.04 mF cm⁻² in an alkaline electrolyte) [36]. The EASA of Ni₃S₂@NiS/Ni-Net-2 was measured to be 562 cm², much higher than those of 172.9 cm² for Ni₃S₂@NiS/Ni-Net-0.5, 308.2 cm² for Ni₃S₂@NiS/Ni-Net-1 and 53.3 cm² for Ni₃S₂@NiS/Ni-Net-3. This indicates that the Ni₃S₂@NiS/Ni-Net-2 catalytic electrode is capable of exposing a greater number of active sites during the HER process, which is more conducive to the electrochemical hydrogen evolution reaction [37,38,39].

Furthermore, electrochemical impedance spectroscopy (EIS) was measured to further explore the electron transfer kinetics of the catalyst. In the Nyquist plot of Figure 8A, the electron transfer resistance (R_ct) value of Ni₃S₂@NiS/Ni-Net-2 was 1.42 Ω, lower than that of Ni₃S₂@NiS/Ni-Net-0.5 (1.81 Ω), Ni₃S₂@NiS/Ni-Net-1 (1.80 Ω) and Ni₃S₂@NiS/Ni-Net-3 (2.12 Ω), suggesting its faster charge transfer kinetics under HER operating conditions. Therefore, the Ni₃S₂@NiS/Ni-Net-2 material exhibited a superior charge transfer rate and commendable reaction kinetics.

In Figure 8B, a chronoamperometric current (I-t) curve of the Ni₃S₂@NiS/Ni-Net-2 was displayed at a constant current density of 90 mA/cm². The Ni₃S₂@NiS/Ni-Net-2 sample basically retained its catalytic activity over 100 h, indicating that this electrode has a good HER stability at high current density. Figure 8C illustrates the multi-step chronoamperometric curve of the Ni₃S₂@NiS/Ni-Net-2 catalytic electrode at varying current densities. It can be observed that Ni₃S₂@NiS/Ni-Net-2 is capable of maintaining basically stable current densities within an overpotential range of 0 to 550 mV, with a resolution of 50 mV, demonstrating that Ni₃S₂@NiS/Ni-Net-2 was quite stable under an electrocatalytic HER process at a wide current density range (0–250 mA/cm²). Therefore, the Ni₃S₂@NiS/Ni-Net-2 displays superb HER performance for alkaline water electrolysis (Table S2).

To verify the structural stability of the Ni₃S₂@NiS/Ni-Net-2 catalytic electrode after the HER test, the high-resolution XPS spectra of the Ni₃S₂@NiS/Ni-Net-2 sample before and after the HER test were subjected to analysis. Figure 9A revealed the high-resolution Ni 2p XPS spectra, showing that the Ni³⁺ content of Ni 2p_1/2 and Ni 2p_3/2 is significantly reduced while the Ni²⁺ content is increased, suggesting the partial transformation of Ni³⁺ to Ni²⁺. This indicates that Ni₃S₂@NiS is reshaped and transformed into NiO or Ni(OH)₂ during the HER test [27,40,41,42,43]. From the S 2p high-resolution XPS spectra in Figure 9B, it can be observed that the S²⁻ peak is significantly reduced after the HER test due to the etching of S on the electrode surface, which indicates that the Ni₃S₂@NiS heterostructure on the electrode surface has been reduced. Due to the reconfiguration of the catalyst, the resulting nickel oxide hybridizing catalyst exhibits a favorable HER performance [40,41,42,43].

3. Materials and Methods

3.1. Preparation of Materials

3.1.1. Chemicals and Reagents

Thioacetamide (CH₃CSNH₂, A.R.), ethanol (CH₃CH₂OH, A.R.), potassium hydroxide (KOH, A.R.), acetone (CH₃COCH₃, A.R.) and hydrochloric acid (HCl, A.R.) were purchased from Sinopharm Chemical Reagent (Beijing, China) with no further purification.

3.1.2. Preparation of Clean Nickel Net

To start with, several pieces of Ni-Net with a size of 1 cm × 5 cm were sonicated sequentially with acetone, 3 M HCl, alcohol and deionized water, after which the treated Ni-Net was placed in a Petri dish to dry naturally and set aside.

3.1.3. Synthesis of Ni₃S₂@NiS/Ni-Net

Next, 2 mmol of thioacetamide was weighed and 15 mL of anhydrous ethanol was measured with a measuring cylinder and then mixed in a 50 mL Teflon-lined stainless steel autoclave. The pretreated Ni-Net substrate was placed into the liner to fully submerge it. The liner was then placed in the metal reaction kettle with the lid fastened, and the kettle was screwed to ensure a tight seal. The autoclave was placed in an oven and heated at 160 °C for 12 h. After the autoclave was cooled to room temperature, the reacted Ni-Net substrate was taken out and cleaned using ultrapure water and ethanol several times, and then put it into a Petri dish to dry naturally. The sample prepared according to the above method was denoted Ni₃S₂@NiS/Ni-Net-2. To further investigate the effect of TAA content on the Ni₃S₂@NiS sample, some control experiments with different TAA contents of 0.5 mmol, 1 mmol and 3 mmol were carried out according to the above steps, and the resulting samples were named Ni₃S₂@NiS/Ni-Net-0.5, Ni₃S₂@NiS/Ni-Net-1 and Ni₃S₂@NiS/Ni-Net-3, respectively.

3.2. Material Characterization

The as-obtained samples were subjected to a physical phase analysis using an X-ray diffraction (XRD) diffractometer (Rigaku Corporation, D/MAX-2200PC, Tokyo, Japan) The morphology and microstructure of samples were characterized by using a scanning electron microscope (SEM, Hitachi, S4800, Tokyo, Japan) and transmission electron microscope (TEM, Tecnai G2 F20 S-TWIN, Hillsboro, OH, USA). To determine the elemental composition and surface chemical state, an X-ray photoelectron spectrometer (XPS, XIS supra, Kratos analytical, Manchester, UK) was employed.

3.3. Electrochemical Performance Test Characterization

All HER performance characterizations of samples were investigated using a CHI 660E (CHI660E, Chenhua Instruments Inc., Shanghai, China) electrochemical workstation. Electrochemical tests were conducted in a standard three-electrode system, in which a saturated calomel electrode (SCE) serves as a reference electrode, a graphite-carbon rod acts as a counter electrode (CE) and the Ni₃S₂@NiS/Ni-Net electrode functions as a working electrode (WE). The potentials are all calibrated according to the following formulae: ${\mathrm{E}}_{\mathrm{VS}.\mathrm{RHE}}={\mathrm{E}}_{\mathrm{VS}.\mathrm{SCE}}+0.24+0.059\mathrm{pH}$.

All samples were subjected to testing in an alkaline environment with a solution of 1.0 M KOH. The linear scanning voltammetry (LSV) curve was scanned at a rate of 5 mV/s, and the Tafel slope was obtained from the polarization curve of the hydrogen evolution reaction (HER). The frequency range of electrochemical impedance spectroscopy (EIS) was 0.01 to 10,000 Hz. Cyclic voltammetry (CV) curves were initiated at a potential of −0.9 V, a high potential of −0.89 V, a low potential of −1.01 V and a termination voltage of −0.9 V. The chemical stability tests were performed using a multistep timed current (step) test as well as a timed current (I-t) test.

4. Conclusions

In conclusion, a novel all-in-one catalytic HER electrode comprising a Ni₃S₂@NiS heterostructure grown on the industrial nickel net (Ni-Net) was successfully synthesized through a solvothermal route. The characterization of the physical phase, morphology and electrochemical properties demonstrate that the content of the sulfur source can modulate the Ni₃S₂@NiS heterostructure. The optimized Ni₃S₂@NiS-Ni-Net electrode can achieve high current densities of 100 mA/cm², 500 mA/cm² and 1000 mA/cm² at overpotentials of 207 mV, 322 mV and 429 mV, respectively, under alkaline conditions. The electrode also exhibits excellent HER stability, as evidenced by the maintenance of a large current density of 90 mA/cm² for up to 100 h in a KOH solution. This study presents a novel approach to the preparation of non-precious metal catalytic electrodes for electrocatalytic green hydrogen production, and thus promotes the application of metal sulfide electrodes in practical industry.