One-step electrodeposition of W, Mo-Ni₃S₂/NF catalyst: an efficient hydrogen evolution electrode for alkaline media

Abstract

A self-supported tungsten (W), molybdenum (Mo)-Ni₃S₂/nickel foam (NF) hydrogen evolution reaction (HER) electrode was successfully fabricated on NF via constant current electrodeposition. The morphology, elemental composition, and electrocatalytic HER performance of the electrodes were systematically characterized via scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and an electrochemical workstation. Results indicate that the surface of the W, Mo-Ni₃S₂/NF electrode consists of rough and refined nano-spherical particles with certain amorphous characteristics. In 1 M KOH, the W, Mo-Ni₃S₂/NF electrode demonstrates superior catalytic HER activity and stability. In particular, it achieves an overpotential of only 76 mV at a current density of 10 mA cm⁻². After undergoing 2000 cyclic voltammetry cycles and 12 h of continuous electrolysis, the electrode retains its high HER activity. The nano-spherical morphology and coexistence of amorphous/crystalline structures significantly enhance the electrochemical active surface area and expose more catalytic active sites. Moreover, the incorporation of W and Mo effectively modulates the electronic structure of Ni₃S₂, reducing charge transfer resistance, and consequently, enhancing the overall HER catalytic performance of the electrode.

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1. Introduction

As one of the clean energy sources, hydrogen (H) gas (H₂) has garnered significant attention in the transition away from traditional fossil fuels due to its high energy density, renewability, and zero emissions.¹ Among various H₂ production technologies, water electrolysis stands out because of its abundant raw material (H₂O), high conversion efficiency, and environmental friendliness, leading to rapid advancements and extensive research in the green H sector. However, owing to cathode polarization, solution resistance, and contact resistance, a higher applied potential is still required to overcome the reaction energy barrier during practical H₂ production via water electrolysis.² Although platinum (Pt)-based catalysts exhibit superior performance in H evolution reaction (HER), their scarcity and high cost necessitate the development of non-noble metal catalysts that exhibit high intrinsic activity and long-term durability.³

Nickel (Ni)-based sulfide electrocatalysts are highly promising for alkaline HER in water electrolysis. Ni atoms possess unpaired 3d electrons in their outer shell, which can readily hybridize with 1s orbital electrons of H atoms to form metal–H bonds, promoting the adsorption of the reaction intermediate H_ads.⁴ In addition, the relatively high electronegativity of sulfur (S) atoms, upon forming a composite with the transition metal Ni, effectively reduces the electron density of Ni atoms. This phenomenon weakens the binding strength of metal–H_ads, facilitating the desorption of H and ultimately enhancing electrocatalytic performance. For example, Mahanthappa et al.⁵ developed NiS–NiS₂ electrodes supported on layered porous S-doped graphitic carbon (C) nitride (SGCN) nanosheets as bifunctional catalysts. These electrodes achieved an overall water splitting current density of 50 mA cm⁻² at a low cell voltage of 1.66 V. Their study revealed that Ni²⁺/Ni³⁺ acted as redox active centers, and the interface between NiS–NiS₂ and SGCN nanosheets featured abundant S vacancies and strong electronic coupling. Doping other elements into Ni-based sulfide catalysts is an effective strategy for further enhancing their electrocatalytic activity. For example, Fathollahi et al.⁶ fabricated porous Ni–Fe–S nanosheets on Ni foam (NF) substrates via the dynamic H bubble template method. These nanosheets exhibited an overpotential of 85 mV and 173 mV at a current density of 10 mA cm⁻² and 100 mA cm⁻², respectively. The incorporation of iron (Fe) and S played a critical role in boosting HER catalytic performance. This optimization arose from the chemical interactions and structural reconfigurations of metal–S bonds, wherein the electronegativity of S dominates the bonding properties, while Fe doping modulates the electronic structure.

As non-noble metal alkaline electrolytic water catalysts, Ni–Mo-based electrodes exhibit significant potential. Bau et al.⁷ conducted a combined experimental and theoretical study, revealing the dominant role of Mo³⁺ ions in enhancing HER performance. Their theoretical simulations showed that upon the incorporation of Mo³⁺ active centers onto Ni(111) surface, the system exhibited excellent thermodynamic stability and markedly improved catalytic activity within the HER potential range. Moreover, electron transfer occurred from the Ni surface to the molybdenum (Mo) surface upon alloy formation due to the higher electronegativity of Mo compared with that of Ni. This redistribution of electrons modified the electronic structure, leading to an optimal proton binding energy that facilitated efficient HER performance.^8,9 Yang et al.¹⁰ employed in situ variable-temperature near ambient pressure X-ray photoelectron spectroscopy (XPS) technology to systematically investigate the formation mechanism of the WNi₄@WO₂ heterostructure. They found that the electronic interaction between tungsten (W) and Ni optimized H* adsorption energy. In addition, the alloy structure offered more active sites, providing a new idea for doping other elements for a Ni-based sulfide electrode. Based on these research outcomes, the current study introduced W and Mo into the Ni₃S₂/NF electrode to fabricate a W, Mo co-doped Ni₃S₂/NF electrocatalyst and then comprehensively evaluated its HER catalytic performance and mechanisms. Compared with chemical synthesis methods, such as hydrothermal¹¹ or high-temperature solid-phase synthesis,¹² which involve preparing catalytically active materials and subsequently coating them onto substrates, electrodeposition directly deposits catalytically active materials onto the substrate to prepare self-supporting electrodes.¹³ This method not only simplifies the operational process and enhances controllability but also effectively reduces contact resistance between the catalytically active material and the substrate, improving the electrical conductivity of the electrode and enhancing its long-term stability.

In the current study, the self-supported W, Mo-Ni₃S₂/NF electrode was successfully synthesized on NF substrate via a one-step constant current electrodeposition method. The incorporation of W and Mo not only optimized the morphology of the electrode but also modulated electron distribution around Ni and S atoms, enhancing its catalytic performance. The W, Mo-Ni₃S₂/NF electrocatalyst exhibited an overpotential of 76 mV at a current density of 10 mA cm⁻² and a Tafel slope of 125.7 mV dec⁻¹. In addition, the W, Mo-Ni₃S₂/NF electrode demonstrated excellent mechanical robustness and long-term electrochemical stability. The current study presents a novel and efficient strategy for fabricating non-noble metal catalysts under alkaline conditions by using a simple electrodeposition method.

2. Experimental

2.1 Reagents

The NF substrate was purchased from Kunshan Desco Electronics Co., Ltd (Jiangsu, China), and the high-purity N was obtained from Changsha Ruichong Gas Co., Ltd (Hunan, China). The 20% Pt/C and 5 wt% Nafion used to prepare the Pt/C electrode were purchased from Sigma-Aldrich Reagent Company, USA. All the other reagents, including Ni sulfate hydrate (NiSO₄·6H₂O), sodium (Na) molybdate (Na₂MoO₄·2H₂O), Na tungstate (Na₂WO₄·2H₂O), thiourea (CH₄N₂S), Na citrate (Na₃C₆H₅O₇·2H₂O), Na chloride (NaCl), boric acid (H₃BO₃), potassium (K) hydroxide (KOH), and hydrochloric acid (HCl), were procured from Shanghai Sinopharm Chemical Co., Ltd (Shanghai, China). All the reagents were of analytical grade and could be used directly.

2.2 Preparation method

2.2.1 Preprocessing of NF. Before electrodeposition, a NF cutting that measured 1 cm × 1 cm × 0.03 cm (with reserved tabs) was sequentially ultrasonicated in acetone and ethanol for 10 min each to remove surface organic contaminants. Subsequently, it was washed with deionized water and further ultrasonicated in 10 wt% HCl for 20 min to eliminate surface oxides and activate the NF surface. The foam was then repeatedly rinsed with deionized water until the pH of the rinse solution reached 7. Finally, the foam was vacuum-dried at 50 °C for 8 h.

2.2.2 W, Mo-Ni₃S₂/NF electrocatalyst. Electrodeposition was performed on a CHI660E electrochemical workstation. The electrolyte solution used to prepare the Ni₃S₂/NF electrode consisted of 100 g L⁻¹ of NiSO₄·6H₂O, 100 g L⁻¹ of CH₄N₂S, 70 g L⁻¹ of Na₃C₆H₅O₇·2H₂O, 20 g L⁻¹ of NaCl, and 40 g L⁻¹ of H₃BO₃. The pH of the solution was adjusted to 4.0 by using 10 wt% HCl. Electrodeposition was conducted at 40 °C in a water bath under a constant current density of 30 mA cm⁻² for 60 min. After deposition, the samples were sequentially washed with acetone, anhydrous ethanol, and deionized water to remove any residual electrolyte, followed by vacuum drying at 50 °C for 6 h.

The preparation process of W, Mo-Ni₃S₂/NF was basically the same as that of Ni₃S₂/NF, with the only difference being the addition of Na₂WO₄·2H₂O and Na₂MoO₄·2H₂O to the plating solution. The W-Ni₃S₂/NF and Mo-Ni₃S₂/NF used as control electrodes were prepared by adding Na₂WO₄·2H₂O and Na₂MoO₄·2H₂O to the electrolyte, respectively. Table 1 summarizes the composition of the electrolyte and the electrodeposition conditions employed in the preparation of the aforementioned electrocatalysts. Fig. 1 illustrates the synthesized process of the W, Mo-Ni₃S₂/NF electrocatalyst. The mass loading of the W-Ni₃S₂/NF, Mo-Ni₃S₂/NF and W, Mo-Ni₃S₂/NF electrocatalyst are approximately 4.1, 3.7 and 3.8 mg cm⁻², respectively.

Table 1 Electrodeposition parameters and electrolyte composition of Ni₃S₂/NF, W-Ni₃S₂/NF, Mo-Ni₃S₂/NF and W, Mo-Ni₃S₂/NF electrodes

	Ni3S2/NF	W-Ni3S2/NF	Mo-Ni3S2/NF	W, Mo-Ni3S2/NF
NiSO4·6H2O (g L^−1)	100	100	100	100
CH4N2S (g L^−1)	100	100	100	100
Na3C6H5O7·2H2O (g L^−1)	70	70	70	70
Na2WO4·2H2O (g L^−1)	—	10	—	10
Na2MoO4·2H2O (g L^−1)	—	—	30	30
NaCl (g L^−1)	20	20	20	20
H3BO3 (g L^−1)	40	40	40	40
pH	4.0	4.0	4.0	4.0
Current density (mA cm^−2)	30	30	30	30
Time (min)	60	60	60	60
Temperature (°C)	40	40	40	40

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Fig. 1 Schematic illustration of the preparing process of W, Mo-Ni₃S₂/NF electrode.

2.2.3 Pt/C electrode. The ink was prepared by mixing 8 mg of 20% Pt/C powder with 80 μL of 5 wt% Nafion, 460 μL of anhydrous ethanol, and 460 μL of deionized water. The ink was then sonicated in an ice bath for 1 h to ensure uniform dispersion. Subsequently, 35 μL of the ink was evenly cast onto the surface of a glassy C electrode (GCE), which was then placed horizontally under an infrared lamp and dried for 3–5 min prior to measurement. The resulting catalyst loading on the GCE surface was 1 mg cm⁻².

2.3 Characterization

The microstructure and elemental composition distribution of the W, Mo-Ni₃S₂/NF electrocatalyst were characterized via transmission electron microscopy (TEM) with a JEM-F200 multipurpose electron microscope (JEOL Ltd, Japan), scanning electron microscopy (SEM) with a MIRA4 LMH field-emission scanning electron microscope (TESCAN Ltd, CZ), and energy-dispersive X-ray spectroscopy (EDS) with an Ultim Max 65 EDS detector (Oxford Instruments, UK). The crystal structure of the samples was analyzed via X-ray diffraction (XRD) by using a SmartLab SE multipurpose X-ray diffractometer (Rigaku, Japan) with a Cu K_α radiation source (wavelength: 0.1541 nm), operating at tube voltage of 40 kV and tube current of 40 mA, with a scanning rate of 5° min⁻¹. The elemental composition and chemical states on the surface of the electrocatalyst were analyzed via X-ray photoelectron spectroscopy (XPS) by using a K-Alpha X-ray photoelectron spectrometer (Thermo Fisher Scientific Inc., USA) with an aluminum target (h_v = 1486.6 eV) employed during the measurements.

2.4 Electrochemical methods

Electrochemical measurements were performed using a CHI660E electrochemical workstation with a standard three-electrode system. A 100 mL glass electrolytic cell was employed for the tests. The H evolution performance of the electrodes was evaluated at room temperature (25 °C, 1 M KOH). Prior to testing, high-purity N₂ was bubbled through the electrolyte for 10 min to remove dissolved O₂, and the entire three-electrode system was immersed in the electrolyte for 3 min to ensure stabilization. The prepared electrode, graphite electrode, and saturated calomel electrode (SCE) served as the working electrode, counter electrode, and reference electrode, respectively. To facilitate potential comparison, all potentials were converted into the reversible H electrode (RHE; E_vs.RHE) potential by using the Nernst equation:¹⁴

E_vs.RHE = E_vs.SCE + 0.242 + 0.059 × pH − 0.000791 × (T − 298.15) − iR. (1)

In this study, the polarization curve was recorded within the potential range of E_vs.SCE from 0 V to −1.6 V at a scan rate of 2 mV s⁻¹. The double-layer capacitance (C_dl) of the electrocatalyst was determined via cyclic voltammetry (CV) with scan rates that ranged from 10 mV s⁻¹ to 100 mV s⁻¹ in the non-faradaic region. To evaluate the cycling stability of the samples, 2000 CV cycles were performed within the potential window of E_vs.SCE from −0.2 V to −0.6 V. The charge transfer efficiency of the electrode was assessed via electrochemical impedance spectroscopy. Chronopotentiometry (CP) curves were employed to investigate the electrochemical stability of the electrocatalyst under various current density conditions.

3. Results and discussion

The XRD patterns are presented in Fig. 2. As illustrated in Fig. 2(a), the Ni₃S₂/NF electrode exhibits distinct diffraction peaks at 2θ = 44.4°, 51.8°, and 76.3°, which correspond to the (111), (200), and (220) crystal planes of Ni, respectively (JCPDS 04-0850).¹⁵ This phenomenon may be attributed to the relatively thin catalytic layer formed during deposition. In addition, the diffraction peaks located at 21.7°, 31.1°, 37.7°, and 55.1° are associated with the (101), (110), (003), and (122) crystal planes of Ni₃S₂, respectively (JCPDS 44-1418).¹⁶ The presence of the Ni₃S₂ phase, which is an excellent H evolution catalyst, significantly enhances the H evolution catalytic activity of the electrode.¹⁷ Moreover, the broadened diffraction peaks suggest a certain degree of amorphization in the electrode structure. In addition to the three main peaks of Ni, the XRD patterns of W-Ni₃S₂/NF and Mo-Ni₃S₂/NF exhibit a diffraction peak that corresponds to the (110) crystal plane of Ni₃S₂ at 31.7°. For the W, Mo-Ni₃S₂/NF electrode, the degree of peak broadening is enhanced, indicating more pronounced amorphous characteristics. No phases that contain W or Mo are found in the pattern, because W and Mo atoms have substituted some Ni atoms into the lattice, forming a substitutional solid solution,¹⁸ which is difficult to characterize via XRD. The atomic radius of W or Mo are larger than that of Ni, as shown in the magnified view of Fig. 2(b). This substitution induces lattice distortion and expansion, leading to a shift of the diffraction peaks toward smaller angles.¹⁹ The atomic radius of W is slightly larger than that of Mo, and thus, the negative angle shift of the diffraction peak in the W-Ni₃S₂/NF pattern is the most significant, while those of Mo-Ni₃S₂/NF and W, Mo-Ni₃S₂/NF fall between W-Ni₃S₂/NF and Ni₃S₂/NF. Fig. S1 also illustrates the crystal structure of Ni₃S₂, where Ni atoms occupy two distinct crystallographic positions and form octahedral and tetrahedral coordination with S atoms, while S atoms connect to Ni atoms via bridging or terminal bonding to form layered or chain-like structural units.²⁰

Fig. 2 (a) XRD patterns of Ni₃S₂/NF, W-Ni₃S₂/NF, Mo-Ni₃S₂/NF and W, Mo-Ni₃S₂/NF electrodes; (b) partial magnification of XRD patterns.

Fig. S2 presents the SEM image of NF, clearly illustrating a smooth foam-like structure. As shown in Fig. 3(a and b), the surface of Ni₃S₂/NF exhibits a cellular morphology with minimal undulation. Upon introducing W, however, the electrode surface transitions from a relatively flat planar interface into a rough surface characterized by island-like protrusions, with more pronounced gaps between nanoparticles. This phenomenon arises because the valence electrons of transition elements possess empty orbitals, enabling Ni and W metal ions to form complexes in the presence of chelating agents (e.g., Cit). Consequently, the initially disparate deposition potentials become closer, leading to induced co-deposition. The deposition process is completed within a relatively short period and followed by concurrent growth. Consequently, a cellular structure forms on the electrode surface, leading to increased roughness.²¹ Fig. 3(c) presents the SEM image of the Mo-Ni₃S₂/NF electrode. Upon introducing Mo, a dense nanoparticle structure forms on the surface of the electrocatalyst. This phenomenon may be attributed to the relatively high electronegativity of Mo, which enables an appropriate concentration of Mo ions in the electrolyte to accelerate the reduction process. Consequently, the nucleation rate of crystal nuclei exceeds their growth rate, leading to grain refinement.²² Consistent with the aforementioned discussion, the surface of the W, Mo-Ni₃S₂/NF electrocatalyst in Fig. 3(d) is relatively rough and features a layer of nano-spherical particle deposits, which increased the area of exposed nanostructures. Such a surface morphology offers abundant active sites for the HER process, facilitates direct contact between the electrolyte and the electrode material, promotes faster ion transport and exchange along the diffusion path, and thus, enhances the H evolution catalytic activity of the electrocatalyst.²³ In addition, the elemental mapping in Fig. 3(e) confirms the uniform distribution of Ni, S, W, and Mo on the surface of the electrocatalyst, verifying the successful synthesis of W, Mo-Ni₃S₂/NF. The corresponding elemental composition and content are presented in Fig. S3.

Fig. 3 SEM images of (a) Ni₃S₂/NF, (b) W-Ni₃S₂/NF, (c) Mo-Ni₃S₂/NF, and (d) W, Mo-Ni₃S₂/NF electrodes; (e) the corresponding elemental mappings of W, Mo-Ni₃S₂/NF electrode.

To investigate microscopic morphology and structural characteristics, the W, Mo-Ni₃S₂/NF electrode was analyzed via TEM, as illustrated in Fig. 4. W, Mo-Ni₃S₂/NF consists of numerous stacked nanospheres, with rough edges that enhance the contact area with the electrolyte [Fig. 4(a)], which is consistent with the SEM results. In the high-resolution TEM image [Fig. 4(b)], the lattice fringes are relatively indistinct, and the fringe spacing of d = 0.29 nm corresponds to the (110) crystal plane of Ni₃S₂.²⁴ This phenomenon indicates the presence of a nanocrystalline Ni₃S₂ phase within the electrode, along with a relatively high degree of amorphization. Fig. 4(c) further corroborates this result. The presence of nanocrystals leads to relatively sharp diffraction rings in the selected area electron diffraction pattern, while the typical amorphous halo ring pattern remains evident. In addition, the mapping results in Fig. 4(d) reveal the uniform distribution of those elements, which aligns well with the aforementioned SEM-based elemental mapping EDS results, providing additional confirmation of the primary chemical composition of the W, Mo-Ni₃S₂/NF electrode.

Fig. 4 (a) TEM image, (b) HRTEM image, (c) SAED result and (d) the corresponding elemental mappings of W, Mo-Ni₃S₂/NF electrode.

Furthermore, the XPS technique was adopted to analyze the elemental composition and bonding state of the electrode surface, and the analysis results are presented in Fig. 5. In the full spectrum of Fig. S4, the presence of Ni, S, W, Mo, C, and oxygen (O) elements in the electrode can be clearly observed. The observed C and O peaks may arise from the organic compounds or atmospheric O₂ and CO₂ on the electrode surface. Fig. 5(a) displays the Ni 2p spectrum, revealing two Ni²⁺ valence states, with peaks at about 855.3 eV and 873.4 eV correspond to Ni 2p_3/2 and Ni 2p_1/2,²⁵ respectively. In addition, the peak at 852.5 eV is indexed to the metal Ni state.²⁶ The S 2p spectrum shown in Fig. 5(b) exhibits two peaks at 162.7 eV and 168.5 eV, which are attributed to S 2p_1/2 and SO_x,²⁷ and corresponding to the Ni–S compound and surface oxidation of the S element, respectively. The spectrum of W 4f in Fig. 5(c) presents two peaks at 35.1 eV and 37.2 eV, which belong to W 4f_7/2 and W 4f_5/2, respectively. In accordance with the pertinent literature,^28,29 W⁶⁺ was present in the W, Mo-Ni₃S₂/NF electrocatalyst and combined to form WS₂. Fig. 5(d) shows the high-resolution of the Mo 3d spectrum, where the binding energy peaks at 226.4 eV and 232.4 eV can be attributed to Mo 3d_5/2 and Mo 3d_3/2, respectively, suggesting the existence of Mo⁴⁺. The other peak located at 235.3 eV was assigned to the oxidation of Mo (MoO_x).³⁰ Notably, the binding energy of Ni 2p and Mo 3d exhibits slight positive shifts relative to their standard peaks (852.6 eV and 231.1 eV), while the binding energy of S 2p demonstrate a negative shift (164.0 eV). This phenomenon suggests an enhanced electron transfer capability of the electrode, enabling easier electron transfer from Mo and Ni to S. Such behavior facilitates H adsorption and desorption during HER.²⁶ In the W, Mo-Ni₃S₂/NF electrode, the highly electronegative S atoms extract electrons from the metal sites, serving as active sites for stabilizing reaction intermediates. In addition, the active material Ni₃S₂ forms an amorphous structure that is conducive to proton binding and electron transfer, accelerating charge transfer within the electrode and promoting electron accumulation on S atoms, enhancing the H evolution catalytic activity of the electrocatalyst.

Fig. 5 The high-resolution XPS spectra of (a) Ni 2p, (b) S 2p, (c) W 4f and (d) Mo 3d of W, Mo-Ni₃S₂/NF electrode.

The polarization curves of the Ni₃S₂/NF, W-Ni₃S₂/NF, Mo-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes are presented in Fig. 6(a). For comparison, the Pt/C electrode and NF substrate were also evaluated under identical conditions. Typically, a smaller HER overpotential correlates with lower energy consumption during water electrolysis and better catalytic performance. The overpotential at 10 mA cm⁻² (η₁₀) was employed as the evaluation criterion.³¹ The Pt/C electrode exhibits exceptional H evolution performance, with an overpotential of only 36 mV. The W, Mo-Ni₃S₂/NF electrode also demonstrates excellent H evolution performance, achieving an overpotential of 76 mV at 10 mA cm⁻². This value is 33.3%, 43.3%, and 58.5% lower than those of Ni₃S₂/NF (114 mV), W-Ni₃S₂/NF (134 mV), and Mo-Ni₃S₂/NF (183 mV), respectively. Table 2 and Fig. 6(g) summarize the overpotential of other similar non-noble metal catalysts at a current density of 10 mA cm⁻². Evidently, the W, Mo-Ni₃S₂/NF electrocatalyst exhibits superior HER performance compared with other analogous non-noble metal catalysts. This enhanced performance can primarily be attributed to the synergistic catalytic effect between W and Mo doping and Ni₃S₂. In addition, the overpotential of Ni₃S₂/NF is lower than those of W-Ni₃S₂/NF and Mo-Ni₃S₂/NF. This phenomenon arises because W and Mo are co-deposited with Ni, reducing Ni content in the electrode, and thus, decreasing the amount of the active material Ni₃S₂, and consequently, reducing its activity.

Fig. 6 Electrochemical properties of W, Mo-Ni₃S₂/NF electrodes. (a) Polarization curves; (b) Tafel fitting curves; (c) CV curves in PBS solution; (d) TOF vs. potential curves; (e) the linear plots of Δj vs. scanning rate and C_dl values; (f) normalized curves of j_ECSA vs. potential and (g) overpotential of W, Mo-Ni₃S₂/NF electrocatalyst compared with other non-noble catalysts reported in recent years.

Table 2 Comparison of HER activity of W, Mo-Ni₃S₂/NF and other non-noble catalysts

Electrode	Electrolyte	Current density (mA cm^−2)	μ (mV)	Ref.
Ni3S2/NF	1 M KOH	10	114	This work
W, Mo-Ni3S2/NF	1 M KOH	10	76	This work
Ni/CNTs-SnO2	1 M KOH	10	259	32
2D Ni/C-m	1 M KOH	10	110	33
NiCo-LDH/Ni/NF	1 M KOH	10	93	34
Ni–W alloys	1 M KOH	10	166	35
P–Ni/WN	1 M KOH	10	45	36
Ni(OH)2-CQDs/NF	1 M KOH	10	90	37
Ni/VN/Ni-NC	1 M KOH	10	84	38
NiFe2O4/Ni-Fe	1 M KOH	10	66	39
Ni@NiO/CFP	1 M KOH	10	172	40
Ni-MoO2	1 M KOH	10	75	41
F-CoPS	1 M KOH	10	74	42
Fe3O4/NiO/rGO-BN	1 M KOH	10	117	43
Ni–Cu	1 M KOH	10	70	44
NiS@MoS2	1 M KOH	10	189	45
Co-doped WS2	1 M KOH	10	127	46

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Based on the Tafel equation:⁴⁷ η = a + b log j (where η and j represent overpotential and current density, respectively, b is the Tafel slope, and a is a constant associated with temperature and pressure), the strong polarization regions of the polarization curves were linearly fitted, as depicted in Fig. 6(b). The resulting Tafel slopes for the Ni₃S₂/NF, W-Ni₃S₂/NF, Mo-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes were 156.2, 122.2, 117.1, and 125.7 mV dec⁻¹, respectively. Compared with NF's Tafel slope of 254.3 mV dec⁻¹, the electrodes prepared in this study exhibited significantly reduced Tafel slopes, indicating faster HER kinetics. Given the relatively inferior performance of the Mo-Ni₃S₂/NF electrode, its subsequent electrochemical data were excluded from further analysis for a more intuitive understanding of the data and analysis.

The intrinsic catalytic activity of the electrodes was evaluated using turnover frequency (TOF). As shown in Fig. 6(c), the CV curves of different electrodes were measured in a phosphate buffered solution at pH = 7. In accordance with eqn (2):⁴⁸

The calculated n values for the Ni₃S₂/NF, W-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes were 1.210 × 10⁻⁴, 1.508 × 10⁻⁴, and 2.428 × 10⁻⁴ mol, respectively. Based on these results, the TOF curves were derived by applying eqn (3) [see details in Fig. 6(d)]. At an overpotential of 150 mV, the TOF values of the Ni₃S₂/NF, W-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes were 0.47, 0.51, and 0.77 s⁻¹, respectively. These results indicate that the W, Mo-Ni₃S₂/NF electrode exhibits superior intrinsic catalytic activity and possesses more active sites for H evolution per unit area.

In addition to intrinsic catalytic activity, the overall catalytic performance of an electrode is also closely associated with its electrochemical active surface area (ECSA), where HERs occur on the electrode surface.⁴⁹ The CV curves of the electrodes in the non-faradaic region were measured at scan rates that ranged from 10 mV s⁻¹ to 100 mV s⁻¹ (Fig. S5). A linear relationship was fitted between the current density difference Δj and scan rate. The slope of the resulting straight line corresponded to two times the C_dl value. As shown in Fig. 6(e), the C_dl values for the Ni₃S₂/NF, W-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes are 4.72, 5.36, and 7.13 mF cm⁻², respectively. The cathodic polarization curves were normalized via ECSA, as depicted in Fig. 6(f). Under the same current density, the W, Mo-Ni₃S₂/NF electrode exhibited the lowest overpotential, indicating a significantly larger electrochemical active surface area. This finding corroborates the results obtained from the previous SEM analysis. By combining the above analyses, the superior catalytic activity of the W, Mo-Ni₃S₂/NF electrode can be attributed to its larger electrochemical active surface area and enhanced intrinsic catalytic activity.

Fig. 7(a) presents the Nyquist curves of the Ni₃S₂/NF, W-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes measured at −100 mV in 1 M KOH. The inset shows the equivalent circuit used for fitting, which includes resistance components, such as solution resistance (R_s) and charge transfer resistance (R_ct).⁵⁰ The detailed fitting results are summarized in Table 3. As indicated in the table, the R_ct values of the Ni₃S₂/NF, W-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes are 0.97, 1.00, and 0.71 Ω, respectively. These results indicate that the W, Mo-Ni₃S₂/NF electrode exhibits better electrical conductivity and faster charge transfer rate due to faster charge migration during the HER process.

Fig. 7 (a) Nyquist plots of Ni₃S₂/NF, W-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes; (b) polarization curves before and after 2000 CV cycles, (c) multi-step chronopotentiometry curve, (d) chronopotentiometry curves at constant 10 and 50 mA cm⁻² and (e) 72 h chronopotentiometry curves at a 10 mA cm⁻² of W, Mo-Ni₃S₂/NF electrode.

Table 3 Fitting results for Ni₃S₂/NF, W-Ni₃S₂/NF, and W, Mo-Ni₃S₂/NF electrodes

Electrode	Rs/(Ω)	Rct/(Ω)	CPE/(mF)
Ni3S2/NF	1.26	0.97	0.30
W-Ni3S2/NF	1.21	1.00	0.58
W, Mo-Ni3S2/NF	1.26	0.71	0.72

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The stability of the H evolution electrode serves as a critical parameter for assessing its performance. Fig. 7(b) illustrates the polarization curves of the W, Mo-Ni₃S₂/NF electrode before and after 2000 CV cycles (scan range: 0 mV to −300 mV versus RHE, scan rate: 100 mV s⁻¹). The results indicate that the H evolution activity of the W, Mo-Ni₃S₂/NF electrode exhibits only a minor reduction. As shown in Fig. 7(c), the multistep electrolysis curves under current density that ranges from 10 mA cm⁻² to 100 mA cm⁻² demonstrate a steady and incremental increase in potential with increasing current density, highlighting the excellent mass transfer capability and mechanical robustness of the W, Mo-Ni₃S₂/NF electrode. Furthermore, Fig. 7(d) presents the CP curves of the W, Mo-Ni₃S₂/NF electrode in 1.0 M KOH. Under a constant current density of 10 mA cm⁻² and 50 mA cm⁻², the electrode maintains stable operation for 12 h, with minimal fluctuations in the potential. Furthermore, the 72 h long-term constant current electrolysis test in Fig. 7(e) demonstrated that the potential decay was merely approximately 3.3%, which not only corroborates the data presented in Fig. 7(d) but also suggests that the electrode possesses promising potential for practical applications. These findings confirm that the W, Mo-Ni₃S₂/NF electrocatalyst developed in the current study possesses remarkable stability and durability. The SEM images and elemental composition analysis of the W, Mo-Ni₃S₂/NF electrode before and after 2000 CV cycles are displayed in Fig. 8 and Table 4, respectively. After electrolysis, cracks are observed in certain regions of the electrode surface, accompanied by a reduction in the number of cellular structures and smoother interconnections. The data presented in the table indicate a decrease in the contents of both W and Mo. This suggests that the observed morphological changes may be associated with the dissolution of W and Mo elements. Nevertheless, the electrode largely retained its original morphology after 2000 CV cycles, indicating its good electrochemical stability. Fig. 9 displays the XPS spectra of the W, Mo-Ni₃S₂/NF electrocatalyst before and after 2000 CV cycles to examine changes in elemental states. In the Ni 2p spectrum, the characteristic binding energy peak associated with metallic Ni vanishes after cycling, indicating that Ni⁰ is oxidized and dissolved during the HER process in electrolyte environment. Concurrently, a notable increase in the binding energy of the S 2p_1/2 peak is observed, suggesting that sulfur species may have undergone electron gain, potentially forming reduced sulfur intermediates or stabilizing higher oxidation states through charge redistribution. For W 4f, an increased relative intensity of the W 4f_5/2 component is evident, reflecting a rise in the proportion of W⁶⁺,⁵¹ likely due to surface oxidation under HER processes. Similarly, for Mo 3d, the disappearance of the Mo–O related peak (MoOx) accompanied by an enhanced Mo 3d_5/2 signal indicates a transformation of molybdenum into the Mo⁶⁺ state, consistent with oxidative conversion during prolonged electrochemical cycling.⁵²

Fig. 8 SEM images of W, Mo-Ni₃S₂/NF electrode; (a and b) initial and (c and d) after 2000 cycles.

Table 4 Elemental composition of the W, Mo-Ni₃S₂/NF electrode before and after 2000 CV cycles

Electrode	Ni/(wt%)	S/(wt%)	W/(wt%)	Mo/(wt%)
Initial	69.15	14.98	6.40	9.47
After 2000CV	70.88	14.55	5.68	8.89

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Fig. 9 XPS characterization of W, Mo-Ni₃S₂/NF before and after 2000 CV test. (a) Ni 2p, (b) S 2p, (c) W 4f and (d) Mo 3d.

4. Conclusion

The self-supported W, Mo-Ni₃S₂/NF electrode prepared via one-step constant current electrodeposition on NF substrate demonstrates remarkable H evolution activity, exhibiting an overpotential of 76 mV at a current density of 10 mA cm⁻² and a Tafel slope of 125.7 mV dec⁻¹. This phenomenon can be attributed to the following factors: (1) morphologically, the refined spherical particles and roughened surface enhance the contact area with the electrolyte, providing a significantly larger active surface area for the reaction. (2) In terms of phase and elemental composition, the coexistence of crystalline and amorphous structures introduces abundant structural defects, exposing more active centers and unsaturated sites that facilitate catalytic activity. (3) From the perspective of electron distribution, the incorporation of W and Mo modifies the electronic structure, enabling the highly electronegative S atoms to more effectively absorb electrons from metal sites, forming stable reaction intermediates, which not only accelerate the charge transfer rate but also promote the accumulation of electrons on S atoms, significantly enhancing the H evolution catalytic performance of the electrocatalyst.

Author contributions

Wenyu Tan: writing – review & editing, writing – original draft, formal analysis, data curation, conceptualization. Hanwei He: validation, supervision, project administration, funding acquisition.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The data that support the findings of this study are available from the corresponding author HW. He, upon reasonable request.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5ra07318a.

Acknowledgements

Authors very thanks for the help from the friends and the authoritative opinion and suggestions from the reviews.

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