In Situ Sulfidation-Induced Construction of Ni₉S₈/NiMoO₄ Heterojunction and Its Synergistically Enhanced Hydrogen Evolution Performance

Abstract

This study reports a straightforward and controllable two-step hydrothermal synthesis of novel Ni₉S₈@NiMoO₄/NF nanospherical catalysts supported on nickel foam (NF), accompanied by a systematic evaluation of their performance in the electrochemical hydrogen evolution reaction (HER). Structural characterization revealed a well-defined Ni₉S₈–NiMoO₄ interfacial region, whose synergistic interaction, combined with the distinctive nanospherical morphology, substantially increased the electrochemically active surface area and the density of reactive sites, thereby optimizing HER kinetics. In alkaline media, the Ni₉S₈@NiMoO₄/NF catalyst demonstrated outstanding electrocatalytic performance, delivering an overpotential of only 64.2 mV at a current density of 20 mA cm⁻². The catalyst also exhibited a high double-layer capacitance of 22.2 mF cm⁻², reflecting a substantial active interfacial area. Long-term durability tests showed negligible performance degradation after 165 h of continuous operation at 10 mA cm⁻², underscoring the catalyst’s robust structural stability and durability. X-ray photoelectron spectroscopy confirmed a uniform distribution of Ni, Mo, and S across the NF framework and revealed optimized chemical states, providing material-level evidence for the enhanced performance. Collectively, this work proposes a viable strategy for designing efficient and stable HER catalysts, contributing to the advancement of green hydrogen production and clean energy technologies.

1. Introduction

Driven by global energy transition initiatives and carbon neutrality strategies, green hydrogen has emerged as a crucial element of the energy system due to its clean characteristics [1,2]. The large-scale development of renewable energy sources, such as wind and solar power, has established a strong foundation for hydrogen production through water electrolysis. However, the overall efficiency of this technology continues to be heavily reliant on the performance of electrode materials [3]. Transition metal compounds, as a class of abundant candidate materials with tunable catalytic properties, are increasingly showing promise as potential substitutes for precious metal catalysts [4].

Among the various transition metal-based materials, nickel molybdate (NiMoO₄) has gained significant attention due to its abundant redox-active sites, low cost, and environmental friendliness [5,6]. However, its long-term cycling stability is severely limited by volume expansion and structural collapse induced by Faradaic reactions during electrocatalysis [7]. To address this performance bottleneck, heteroatomic doping has emerged as an effective modification strategy, with sulfur (S) doping receiving particular focus [8,9]. As a homologue of oxygen, sulfur has a larger atomic radius and lower electronegativity. Partially substituting oxygen atoms in the lattice leads to multiple optimization effects [10,11]: it induces lattice distortion and generates abundant defect sites that act as new electrochemically active centers while also modulating the material’s electronic structure [12]. Additionally, this substitution helps regulate the electron density of nickel and molybdenum metal centers, optimizing their adsorption capacity for reaction intermediates and thereby enhancing intrinsic catalytic activity [13,14]. Moreover, the improved covalent nature of the formed Ni-S and Mo-S bonds contributes to enhanced material conductivity, optimized reaction kinetics, and reduced overpotentials [15,16].

To systematically investigate the impact of sulfur doping on the electrocatalytic performance of NiMoO₄ materials, this study utilized a hydrothermal synthesis method to produce a series of sulfur-doped NiMoO₄ nanomaterials. The focus was on examining how varying synthesis temperatures affected the materials’ crystal structure, microstructure, and chemical composition. Additionally, the study explored the structure–activity relationship between the electrocatalytic performance in the hydrogen evolution reaction (HER) and the structural characteristics of the materials. This work aims to provide experimental evidence for optimizing the sulfur doping process while offering theoretical guidance and practical pathways for developing multi-component synergistic doping strategies. Ultimately, the goal is to design highly efficient HER catalysts.

2. Results and Analysis

2.1. Structural Morphology Analysis

Figure 1 presents the X-ray diffraction (Japan Rikagaku Smartlab SE Panalytical Empyren) patterns of the Ni₉S₈@NiMoO₄/NF electrode material prepared at various sulfurization temperatures. In all patterns, the diffraction peaks at 44.50°, 51.85°, and 76.38° corresponded to the (111), (200), and (220) crystal planes of nickel from the foam nickel substrate (PDF#04-0850). As the sulfidation temperature increased from 150 °C to 210 °C, a pronounced leftward shift of these Ni diffraction peaks was observed. This shift is attributed to lattice expansion caused by the incorporation of sulfur (S), which has a larger atomic radius, into the Ni lattice. According to Bragg’s law (2dsinθ = nλ), the leftward shift in the diffraction angle θ confirms an increase in the interplanar spacing d [17,18]. Simultaneously, new diffraction peaks emerging near 36.42° and 53.4° align with the standard pattern for Ni₉S₈ (PDF#22-1193), indicating successful phase transformation. This structural evolution underscores the significant impact of temperature on the material’s final crystalline structure and properties [19]. At the low temperature of 120 °C (Figure 1a), the insufficient reaction energy resulted in a lack of distinct Ni₉S₈ diffraction peaks [20]. At 180 °C, while Ni₉S₈ formation was achieved, the diffraction peaks for its (312) and (261) crystal planes exhibited weak and broadened intensities, suggesting considerable crystal defects and low crystallinity. Additionally, the atomic size difference between Ni and Mo atoms induced lattice distortion in the NiMoO₄ precursor [18]. Increasing the temperature to 210 °C markedly accelerated ion diffusion, significantly enhancing the intensity of the (312) crystallographic plane diffraction peak for Ni₉S₈. This improvement facilitated structural refinement and increased crystallinity [21,22]. Importantly, the high electronegativity of sulfur leads to strong interactions with Ni atoms, optimizing the material’s electronic structure and increasing its electrochemically active surface area. Furthermore, sulfur’s high affinity enhances the material’s adsorption capacity for water molecules. Collectively, these factors create favorable conditions for improving hydrogen evolution reaction activity [23,24]. It is worth noting that no distinct crystal diffraction peaks of NiMoO₄ were observed in any of the diffraction patterns, likely due to its amorphous nature, low concentration, or peak overlap. This indicates that the material is in an amorphous state, a finding consistent with a comparison to the standard pattern (PDF#12-0348) [25].

Figure 1. XRD patterns of Ni₉S₈@NiMoO₄/NF at different hydrothermal temperatures: (a) at 120 °C, (b) at 150 °C, (c) at 180 °C, and (d) at 210 °C.

2.2. Scanning Electron Microscopy Analysis

Figure 2 showcases the scanning electron microscopy (SEM) (TESCAN) images of the Ni₉S₈@NiMoO₄/NF electrode sample at various sulfurization temperatures, systematically illustrating the evolution of the material’s microstructure with temperature changes. It is evident that a significant amount of material had been successfully integrated into the porous network structure of the nickel foam (NF) framework. The incorporation of sulfur markedly altered the morphological characteristics of NiMoO₄ compared to the precursor. At the low temperature of 120 °C (Figure 2a,b), the overall surface of the sample appeared coarse with a disordered particle arrangement. However, localized enlargements revealed a uniform particle distribution, narrow interstitial spaces, and a relatively smooth surface. X-ray diffraction (XRD) analysis indicates that at this temperature, the reaction between Ni and S is incomplete, preventing the formation of Ni₉S₈ nanospheres. As the reaction temperature rises, the diffusion rates of Ni, Mo, and S atoms increase, enhancing their mixing probability. This accelerates the gradual formation of Ni₉S₈ nanospheres on the surfaces of the nanorods (Figure 2c,d), a result that is corroborated by the emergence of Ni₉S₈ diffraction peaks in the XRD patterns (Figure 2b,d) [26]. At 150 °C, Figure 2c clearly illustrates an increase in particle size and enhanced aggregation, indicating improved crystallinity and density of the material. This enhancement contributes to increased electrical conductivity and electrochemical activity [27,28]. However, the localized magnification in Figure 2d revealed pronounced grain coarsening on the surface. This phenomenon is attributed to the hydrothermal temperature not yet reaching optimal conditions, resulting in suboptimal mechanical strength. Such limitations hinder ion diffusion and migration, ultimately affecting the rate of the hydrogen evolution reaction [29]. Increasing the temperature to 180 °C significantly optimized the sample morphology. As shown in Figure 2e, complete and uniform Ni₉S₈ nanospheres were formed, effectively coating the NiMoO₄ surface and markedly increasing both the surface roughness and specific surface area. The corresponding localized magnification image (Figure 2f) further demonstrated enhanced aggregation of the nanospheres, distinct interfacial features, and a smooth yet slightly textured surface, which together constitute a high-specific-surface-area microstructure [30]. Concurrently, interparticle connectivity improves significantly, forming a more stable three-dimensional conductive network. Compared to the sample synthesized at 150 °C, the 180 °C condition showed a substantial increase in the number of aggregated nanospheres with enhanced structural stability. This development provides continuous pathways for charge transport, facilitating rapid electron transfer while reducing electrical resistance, thereby significantly enhancing overall conductivity [31]. Additionally, the robust interparticle bonding helps maintain structural integrity, preventing the detachment of active material and ensuring catalytic stability during hydrogen evolution reactions [32]. Localized imaging (Figure 2f) clearly illustrates that the Ni₉S₈ nanospheres possessed moderate thickness and further enhanced surface roughness. This textured structure not only increases the specific surface area but also improves the reactant adsorption capacity, providing additional active sites that accelerate hydrogen evolution kinetics [33]. The inherent electrical conductivity of the nanospheres, combined with their tightly interconnected structure, further optimizes the reaction pathways, facilitating rapid hydrogen evolution. Consequently, the interconnected nanospheric architecture formed at 180 °C, along with its excellent cohesive properties, collectively enhances the material’s electrical conductivity, stability, and catalytic activity [34]. However, as the temperature continued to rise to 210 °C, the material morphology exhibited significant degradation. Figure 2g,h revealed severe agglomeration on the sample surface, with an increasingly complex and disordered structure. While the increase in local roughness may have partially enhanced the number of reaction sites, excessive agglomeration markedly reduced the effective active surface area. This reduction leads to a substantial decline in the material’s surface activity and catalytic performance, potentially triggering structural failure and compromising long-term electrochemical stability [35]. Further imaging indicates that the Ni₉S₈ nanospheres have largely disappeared, replaced by extensive particle agglomerates (Figure 2h), which signifies irreversible damage to the material’s microstructure at elevated temperatures. In summary, SEM analysis indicates that the Ni₉S₈@NiMoO₄/NF electrode sample synthesized at 180 °C, characterized by a uniform nanospheric structure, exhibits optimal morphology and structure, providing a solid microscopic foundation for its outstanding electrocatalytic performance.

Figure 2. SEM of Ni₉S₈@NiMoO₄/NF at different hydrothermal temperatures. (a,b) Scanning electron micrographs of Ni₉S₈@NiMoO₄/NF at 120 °C; (c,d) SEM images of Ni₉S₈@NiMoO₄/NF at 150 °C; (e,f) SEM images of Ni₉S₈@NiMoO₄/NF at 180 °C; (g,h) SEM images of Ni₉S₈@NiMoO₄/NF at 210 °C.

To conduct an in-depth analysis of the elemental composition and spatial distribution of the sample sulfurized at 180 °C, this study employed energy-dispersive X-ray spectroscopy (EDS) (TESCAN) for surface scanning characterization of the Ni₉S₈@NiMoO₄/NF electrode. As illustrated in Figure 3a, the scanning results for the selected region (10 µm) clearly demonstrate the successful detection of all four elements: Ni, S, O, and Mo. The distribution maps for each element, shown in Figure 3b,e, revealed that Ni and S exhibited strong signals with a highly uniform distribution across the scanned area. This confirms the successful formation and uniform coating of the Ni₉S₈ phase on the NiMoO₄ surface. Although the Mo signal was relatively weaker, its distribution covered the entire region, indicating effective incorporation into the composite system. Meanwhile, the O signal primarily originated from the NiMoO₄ precursor and potentially from surface-adsorbed species. Quantitative energy dispersive spectroscopy analysis (Figure 3f) further confirmed the elemental composition of the material: Ni constituted 37.5%, S accounted for 32.4%, O made up 22.6%, and Mo comprised 6.4%. This elemental ratio is reasonable given the expected composition, and the high contents of Ni and S directly validate the successful preparation of the Ni₉S₈@NiMoO₄/NF composite material.

Figure 3. SEM EDS scan map of the Ni₉S₈@NiMoO₄/NF-180 °C sample. (a) SEM image of Ni₉S₈@NiMoO₄/NF within a selected 10 um region; (b) Distribution map of nickel in the Ni9S8@NiMoO4/NF sample at 180 °C. (c) Distribution map of Mo in the 180 °C Ni9S8@NiMoO4/NF sample. (d) Distribution map of oxygen in the Ni9S8@NiMoO4/NF sample at 180 °C. (e) Distribution map of sulfur in the Ni9S8@NiMoO4/NF sample at 180 °C. (f) SEM-EDS elemental distribution map of the Ni₉S₈@NiMoO₄ sample at 180 °C.

A comprehensive analysis revealed that the uniform loading of Ni₉S₈ on the NiMoO₄ surface, along with the synergistic spatial distribution of the constituent elements, collectively created an optimal electrocatalytically active interface. The incorporation of Mo enhances the material’s electronic conductivity, while oxygen likely contributes additional reaction sites. This elemental uniformity and synergistic effect form the structural foundation crucial for improving the catalytic performance of the composite material in the hydrogen evolution reaction (HER). By integrating the findings from scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), we gain deeper insights into the intrinsic relationships between the material’s microstructure, elemental distribution, and electrocatalytic performance. A systematic analysis of the dominant roles of Ni and S, alongside the synergistic effects of Mo, elucidates the catalytic mechanism of the Ni₉S₈ composite. This understanding not only provides theoretical support but also establishes a material foundation for its practical applications in energy conversion fields.

2.3. X-Ray Photoelectron Spectroscopy

This study utilized X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K-Alpha (Thermo Scientific, Waltham, MA, USA), Shimadzu AXISSUPRA+ (Kratos Analytical Ltd., Manchester, UK)) to analyze the surface elements of the Ni₉S₈@NiMoO₄/NF composite material, shedding light on how its chemical states influence hydrogen evolution reaction (HER) performance. As shown in Figure 4a, the full-spectrum analysis confirmed the presence of Ni, Mo, S, O, and C elements on the sample surface, indicating effective bonding between Ni₉S₈ and NiMoO₄. The synergistic interactions among these elements significantly impact electrocatalytic performance [36]. Specifically, the Ni 2p spectrum (Figure 4b) displayed characteristic peaks for Ni²⁺ at 854.6 eV and 874.6 eV, along with Ni⁰ peaks at 850.6 eV and 871.3 eV, and satellite peaks at 878.1 eV and 858.4 eV. These findings suggest that nickel exists in the material both as a sulfide and as zero-valent metal species. The presence of Ni²⁺ facilitates hydrogen desorption, while Ni⁰ enhances the material’s conductivity and accelerates electron transport. The satellite peaks further confirm the existence of multiple redox states for nickel within the electrochemical environment [37]. Further analysis of the Mo 3d spectrum (Figure 4c) revealed characteristic peaks for Mo⁶⁺ at 232.7 eV and 229.8 eV, and for Mo⁴⁺ at 228.7 eV and 225.3 eV. The coexistence of these two valence states indicates that molybdenum plays a crucial role in the catalytic reaction. Mo⁶⁺ exhibits strong oxidizing properties, facilitating electron transfer, while Mo⁴⁺ contributes to material stability through interactions with nickel [38]. The concurrent detection of S 2s signals in this region suggests minor Mo-S bonding; however, its low abundance and the absence of corresponding phases in X-ray diffraction (XRD) imply that it primarily exists as a surface state. Moreover, the S 2p spectrum (Figure 4d) showed distinct spin-orbit splitting peaks, confirming the formation of Ni-S bonds. This stable bonding structure not only provides abundant active sites for hydrogen evolution reactions, but also lowers the reaction energy barrier, thereby enhancing hydrogen production efficiency [39,40]. The O 1s spectrum (Figure 4e) exhibited peaks corresponding to both adsorbed oxygen and lattice oxygen. The former relates to surface-adsorbed water or hydroxyl species, which facilitate the formation of hydrogen intermediates, while the latter originates from metal–oxygen bonds within NiMoO₄. Together, these elements create diverse reaction sites for the catalytic process, effectively enhancing the overall catalytic performance and electrochemical stability [41]. In summary, XPS analysis indicated the formation of stable chemical bonds between Ni₉S₈ and NiMoO₄. The coexistence and synergistic interaction of multiple valence states of Ni, Mo, and S effectively optimize the material’s electronic structure and reaction pathways, providing essential surface chemical insights for future catalyst design and optimization.

Figure 4. XPS spectra of the Ni₉S₈@NiMoO₄/NF-180 °C sample: (a) full spectrum, (b) Ni 2p, (c) Mo 3d, (d) S 2p, (e) O 1s, and (f) C 1s.

2.4. Electrochemical Performance Analysis

By comparing the polarization curves of Ni₉S₈@NiMoO₄/NF catalysts synthesized at different sulfurization temperatures (with a scanning rate of 5 mV/s), it is clear that the sample prepared at 180 °C demonstrated a significant advantage in catalytic activity. As shown in Figure 5a, this catalyst exhibited exceptional performance in the hydrogen evolution reaction, achieving a low overpotential (η₂₀) of 64.2 mV, which was substantially lower than that of samples synthesized at other temperatures. This finding confirms that 180 °C is the optimal synthesis temperature for this system [42]. Figure 5b further illustrates the Tafel slopes of catalysts synthesized at various temperatures, with the values increasing in the following order: 180 °C < 210 °C < 150 °C < 120 °C. Notably, the 180 °C sample featured a Tafel slope of only 16.6 mV dec⁻¹, which was significantly lower than those of the samples from other temperatures. This indicates that the 180 °C catalyst exhibits faster reaction kinetics, allowing for a higher current density growth at equivalent potentials [43]. This impressive performance can be attributed to the formation of an ideal microstructure at this temperature, where Ni₉S₈ nanospheres uniformly coat the NiMoO₄ surface. This configuration creates a high specific surface area and an abundant pore structure, effectively exposing active sites. Concurrently, the stable Ni-S bonding state enhances electron conductivity and reactivity, significantly lowering the reaction energy barrier and optimizing the hydrogen generation pathway [44]. In summary, the Ni₉S₈@NiMoO₄/NF catalyst synthesized at 180 °C achieved a synergistic enhancement of conductivity and catalytic activity through its optimized microstructure and stable chemical state. These findings provide critical experimental evidence for the controlled preparation and structural optimization of high-performance hydrogen evolution catalysts.

Figure 5. (a) Polarization curves of the Ni₉S₈@NiMoO₄/NF catalyst at different temperatures; and (b) Tafel curves.

To elucidate the critical influence of calcination temperature on the hydrogen evolution performance of nickel-molybdenum alloys, this study systematically characterized Ni₉S₈@NiMoO₄/NF catalysts synthesized at different temperatures using electrochemical methods. Figure 6 presents the multi-scan rate cyclic voltammetry (CV) curves measured within the non-Faradaic potential range, offering an intuitive representation of the material’s electrochemical active surface area (ECSA) and reaction kinetics. As the scan rates increased, the current density markedly increased across all samples, indicating that the reaction process is significantly dependent on interfacial charge transfer rates. At low scan rates, the current response was relatively flat, suggesting that both diffusion and kinetic steps governed the reaction. Conversely, at high scan rates, the current density rose rapidly, and the CV curves displayed steeper profiles, demonstrating significantly accelerated surface reaction kinetics and the enhanced utilization of active sites. Notably, the capacitive behavior varied among samples synthesized at different temperatures. The sample prepared at 180 °C exhibited the highest current response, with a calculated double-layer capacitance (Cdl) of 22.2 mF cm⁻², significantly outperforming those synthesized under other temperature conditions. This indicates that the 180 °C sample has a richer active surface area and superior charge transfer efficiency. Electrochemical impedance spectroscopy (EIS) results corroborated the CV analysis [45]. The Nyquist plot for the 180 °C sample showed the lowest charge transfer resistance, indicating the most unobstructed interfacial electron conduction pathways and the lowest reaction kinetic barrier [46]. This advantage arises from the efficient adsorption and conversion of reaction intermediates at the nickel active sites, whose d-electron structural characteristics reduce hydrogen adsorption free energy, thereby significantly enhancing the hydrogen evolution reaction efficiency. Additionally, sulfur doping exerts a critical regulatory effect on the local electronic structure of nickel [47]. By forming strong covalent Ni-S bonds, sulfur atoms effectively modulate the electron density of nickel active sites and optimize the hydrophilic/hydrophobic properties of the material’s surface. This modification significantly promotes the generation and desorption of hydrogen species (H^*) following water molecule dissociation [48]. In the hydrogen evolution reaction, this electronic effect enhances nickel’s adsorption strength for hydrogen species, optimizes the hydrogen adsorption free energy (ΔG-H^*), and consequently accelerates the reaction kinetics [49]. Combined analyses of cyclic voltammetry and electrochemical impedance spectroscopy revealed that the Ni₉S₈@NiMoO₄/NF catalyst synthesized at 180 °C exhibited the optimal performance in terms of electrochemical active surface area, charge transfer rate, and interfacial reaction kinetics. Its low Tafel slope further confirms the catalyst’s low reaction energy barrier and higher intrinsic catalytic activity [50,51]. This study systematically elucidates the critical influence of hydrothermal synthesis temperature on the hydrogen evolution performance of the Ni₉S₈@NiMoO₄/NF catalyst, establishing 180 °C as the optimal synthesis parameter. This discovery not only provides direct evidence for understanding the mechanism by which sulfur doping enhances the catalytic performance of transition metal compounds, but also lays a solid experimental and theoretical foundation for the subsequent design and development of highly efficient and stable hydrogen evolution reaction catalysts.

Figure 6. The cyclic voltammetry curves of Ni₉S₈@NiMoO₄/NF at different sulfur doping temperatures in 1.0 M NaOH solution in the non-Faradaic region at various scan rates, namely (a) 120 °C, (b) 150 °C, (c) 180 °C, and (d) 210 °C, with a scan range of 0.1 V to 0.2 V vs. RHE and scan rates from 20 mV/s to 100 mV/s at 25 °C. (e) The electrochemical active area curves of Ni₉S₈@NiMoO₄/NF catalysts at different sulfur doping temperatures. (f) Impedance spectra of Ni₉S₈@NiMoO₄/NF catalysts at different sulfur doping temperatures.

During long-term stability testing, the Ni₉S₈@NiMoO₄/NF catalyst synthesized at 180 °C exhibited remarkable durability. After continuous operation for 165 h at a current density of 10 mA cm⁻², its performance parameters showed no significant degradation, with the voltage remaining stable at approximately 1.028 V (as shown in Figure 7a). This stability indicates that the active site structure remained intact during prolonged reaction without noticeable deactivation [52]. These results demonstrate the electrode material’s sustained and stable hydrogen evolution capability under practical operating conditions. Further analysis of post-reaction scanning electron microscopy (SEM) images (Figure 7b) revealed that after 165 h of electrochemical testing, a layered structure formed on the material’s surface [52]. This layer exhibited increased roughness, along with the appearance of cracks and flaking in certain areas. Figure 7c highlights a more uniform nanoscale morphology and higher surface roughness, as well as the development of a distinct pore structure. Such structural features facilitate the exposure of additional active sites, promote the transport of reactants, and enhance gas release, thereby improving electrochemical performance. However, prolonged operation also induces structural alterations, including partial pore collapse or blockage, which may adversely affect the material’s electrochemical activity and conductivity [53]. Despite these changes, the overall structure remained largely intact, with no significant loss of active sites. In summary, the Ni₉S₈@NiMoO₄/NF catalyst synthesized via hydrothermal synthesis at 180 °C demonstrated outstanding performance in terms of activity, stability, and charge transport capability. It represents a promising material for hydrogen evolution reaction catalysts, providing essential theoretical and practical support for the development of hydrogen energy technology.

Figure 7. (a) Stability profile of the CP chronopotentiometric method for Ni₉S₈@NiMoO₄/NF at 180 °C, (b) SEM image after stability testing, and (c) enlarged section of (b).

3. Materials and Methods

This study reports the two-step hydrothermal synthesis of Ni₉S₈@NiMoO₄/NF composite materials on nickel foam (NF) substrates, achieved via a precursor route followed by sulfur doping. In the first step, 0.475 g NiCl₂.6H₂O and 0.484 g Na₂MoO₄.2H₂O were dissolved in deionized water with stirring to form a homogeneous precursor solution. Pre-treated NF pieces were immersed in the precursor solution and subjected to hydrothermal treatment at 180 °C for 12 h, during which NiMoO₄ formed on NF according to the reaction Na₂MoO₄ + NiCl₂ → NiMoO₄ + 2NaCl. After cooling, samples were removed, washed with ethanol and deionized water, and dried at 60 °C for 8 h to yield the NiMoO₄/NF precursor. In the second step, the precursor was subjected to sulfur doping by placing it in a reactor together with 0.600 g thioacetamide (TAA). During the secondary hydrothermal treatment, NiMoO₄ partially converted to Ni₉S₈, yielding the Ni₉S₈@NiMoO₄/NF composite with a heterojunction and dopant-induced electronic structure optimization. The final product was washed and dried at 60 °C for 8 h to ensure structural integrity and stability.

The entire process is illustrated in Figure 8a, demonstrating excellent controllability and reproducibility. Scanning electron microscopy (SEM) revealed the material’s morphological evolution: pristine NF exhibited a smooth, porous framework (Figure 8b), providing an ideal substrate for nanomaterial growth. After the initial hydrothermal treatment, the surface supported a large number of uniformly distributed NiMoO₄ nanostructures (Figure 8c), highlighting the hydrothermal method’s efficacy in morphology control. Following the sulfur-doping step, the material maintained its overall framework while incorporating the Ni₉S₈ phase, achieving a synergistic optimization of the electronic structure. This synthetic strategy not only enables the efficient construction of Ni₉S₈@NiMoO₄/NF composites, but also offers new insights for the development of high-performance electrocatalytic electrodes. Future work will focus on further optimizing synthesis parameters to explore energy-conversion applications.

Figure 8. (a) Experimental preparation diagram, (b) SEM image of NF, and (c) SEM image of the NiMoO₄ precursor.

4. Theoretical Hydrogen Analysis Computational Analysis

In the hydrogen evolution reaction during the electrolysis of water, the theoretical yield of hydrogen gas can be calculated using Faraday’s Law. This law states that the amount of charge passing through the electrodes is directly proportional to the quantity of product formed. According to Faraday’s Law of Electrolysis, the formula for the theoretical hydrogen evolution rate is:

$$n={\displaystyle \frac{I\times t}{F}}$$

(1)

Among these: I: Current (A), with a test current density of 20 mA cm⁻², and an electrode area of 1 cm², hence I = 0.02 A; t: Time (s), taken as 10 h (36,000 s); n = 2 (each H₂ molecule transfers 2 electrons); F = 96,435 C mol⁻¹;

During the electrolysis of water, the chemical equation for the hydrogen evolution reaction is:

$$2{\mathrm{H}}_2\mathrm{O} \to 2{\mathrm{H}}_2+{\mathrm{O}}_2$$

(2)

According to the aforementioned stoichiometric relationship, the hydrogen evolution reaction (HER) involves a two-electron transfer process (n = 2), meaning that 2 moles of electrons are consumed for every 1 mole of hydrogen gas (H₂) produced. Its molar relationship can be expressed as:

$$n={\displaystyle \frac{I\times t}{2F}}$$

(3)

Under conditions of a current density of 20 mA cm⁻² (I = 0.02 A), an electrode area of 1 cm², a reaction time of 10 h (t = 36,000 s), and a Faraday constant F = 96,485 C mol⁻¹, the theoretical hydrogen yield is calculated as follows:

$$n_{\mathrm{H}2}={\displaystyle \frac{I\times t}{2F}}={\displaystyle \frac{0.02\times \mathrm{36,000}}{2\times \mathrm{96,500}}}=0.0037 \mathrm{m}\mathrm{o}\mathrm{l}$$

(4)

Under standard conditions (0 °C, 1 atm), 1 mole of gas occupies approximately 22.4 L. Therefore, the theoretical volume of hydrogen evolved is:

$$V=n_{\mathrm{H}2}\times 22.4 \mathrm{L}=0.0037\times 22.4=0.083 \mathrm{L}$$

(5)

However, in practical electrocatalytic hydrogen evolution processes, the energy conversion efficiency of the system is constrained by multiple kinetic factors, making it challenging to reach the theoretical upper limit of 100%. Key limiting factors include overpotential caused by reaction kinetic barriers, internal resistance within the electrolyte and system components, and suboptimal catalyst activity and stability. Collectively, these factors lead to the partial dissipation of electrical energy in forms such as heat. Currently, the unit hydrogen energy consumption of industrial-scale water electrolysis systems, such as alkaline electrolyzers, typically ranges between 4.2 and 5.5 kWh Nm⁻³. In comparison, the minimum energy consumption corresponding to the theoretical thermally neutral voltage for hydrogen production is approximately 3.5 kWh Nm⁻³. This indicates that the energy efficiency of contemporary industrial electrolysis systems generally falls within the range of 65% to 80%. Consequently, in practical operation, the hydrogen yield typically achieves only 70% to 85% of the theoretical value at ideal Faraday efficiency. Although Faraday efficiency can reach up to 95% under optimized laboratory conditions, the disparity between industrial and theoretical values—as well as between laboratory and industrial scales—highlights significant opportunities for improvement in catalyst design, electrolyzer structural optimization, and system integration. Addressing these challenges is crucial to enhance the overall efficiency and effectiveness of hydrogen production through electrolysis. Therefore, the overpotential values for each sample at a current density of 20 mA cm⁻² are summarized in Table 1. These values were determined under ideal conditions, without accounting for energy losses or catalyst deactivation. At this current density, the theoretical hydrogen evolution rate is approximately 0.083 L h⁻¹. Furthermore, the Ni₉S₈@NiMoO₄/NF catalyst synthesized at 180 °C exhibited a lower overpotential. This indicates that the reaction requires less additional energy input, which in turn leads to a higher practical hydrogen evolution efficiency compared to catalysts prepared at other temperatures.

Table 1. Overpotential at 20 mA/cm² for Ni₉S₈@NiMoO₄/NF at different temperatures.

Material Samples	Current Density	Overpotential
Ni9S8@NiMoO4/NF-120 °C	20 mA/cm2	169.2 mV
Ni9S8@NiMoO4/NF-150 °C	20 mA/cm2	103.2 mV
Ni9S8@NiMoO4/NF-180 °C	20 mA/cm2	64.2 mV
Ni9S8@NiMoO4/NF-210 °C	20 mA/cm2	82.2 mV

The Tafel slope is a critical parameter for assessing the kinetics of the hydrogen evolution reaction (HER). A lower Tafel slope value indicates that a smaller overpotential is required to achieve the same current density, reflecting superior reaction kinetics. Furthermore, the magnitude of the Tafel slope can be used to infer the probable rate-determining step of the reaction: a value around 120 mV dec⁻¹ suggests the Volmer step (H⁺ + e⁻→ H^*, controlled by hydrogen adsorption); approximately 40 mV dec⁻¹ corresponds to the Heyrovsky step (H^* + H⁺ + e⁻ → H₂, controlled by electrochemical desorption); and around 30 mV dec⁻¹ indicates the Tafel step (H^* + H^* → H₂, controlled by chemical recombination). The Tafel slope measured for the Ni₉S₈@NiMoO₄/NF catalyst synthesized at 180 °C was 42 mV dec⁻¹. This value suggests that the HER process follows a Volmer–Heyrovsky mechanism, with the Heyrovsky step being rate-limiting. Among the series of samples tested, this particular catalyst exhibited the smallest Tafel slope and the lowest overpotential, demonstrating optimal reaction kinetics. This enhanced kinetic performance originates from the material’s exceptional bulk and interfacial charge transport capabilities. Although the increase in its double-layer capacitance (Cdl) and electrochemical active surface area (ECSA) was modest compared to other samples, its current density and specific activity were significantly higher. This indicates that the performance improvement is primarily due to an enhancement in intrinsic activity rather than an expansion of surface area. At the electronic structure level, the formation of Ni-S bonds induces charge redistribution, which increases the charge carrier concentration (n) and optimizes carrier mobility (μ). This electronic restructuring promotes metallic-like behavior, substantially boosting the intrinsic electrical conductivity. Consequently, the combined effect of carrier concentration and mobility determines the overall conductivity, creating a crucial bridge between the microscopic electronic structure and the macroscopic catalytic performance. This enhanced conductivity directly improves the efficiency of electron supply to the active sites, thereby governing the intrinsic HER activity and the resulting hydrogen evolution rate.

5. Conclusions

This study successfully synthesized a high-performance hydrogen evolution reaction (HER) catalyst by growing Ni₉S₈@NiMoO₄ nanospheres in situ on a three-dimensional nickel foam (NF) substrate using a simple, controllable two-step hydrothermal method. The composite catalyst leverages both the electronic synergy between the sulfur and nickel species and its distinctive nanosphere architecture. This design not only substantially increases the electrochemical active surface area, offering abundant catalytic sites, but also enhances the electrode/electrolyte interface contact, effectively lowering the interfacial charge transfer resistance. As a result, the hydrogen evolution kinetics are significantly improved. In the 1.0 M NaOH alkaline electrolyte, the Ni₉S₈@NiMoO₄/NF electrode demonstrated outstanding overall performance, requiring an overpotential of only 64.2 mV to achieve a current density of 20 mA cm⁻². The electrode exhibited a double-layer capacitance (C_dl) of 22.2 mF cm⁻², reflecting its exceptional interfacial charge transfer capability and intrinsic catalytic activity. Moreover, it showed remarkable long-term stability, with no significant activity loss after 165 h of continuous operation at 10 mA cm⁻². These superior properties are mainly attributed to the strong electronic interaction between the Ni₉S₈ and NiMoO₄ phases, as well as the highly conductive network and robust mechanical support provided by the nickel foam substrate. In summary, this work not only presents a novel HER catalyst material with high activity and stability, but also offers a feasible strategy for enhancing electrocatalytic performance through interfacial engineering and structural design, showing considerable promise for advancing practical hydrogen energy conversion technologies.