Binary Ni–W metal sulfides with polyhedral nanostructures towards efficient hydrogen evolution

Abstract

Designing multi-transition metal-based sulfides holds promise for alkaline water electrolysis, whereas the selection of suitable, cheap candidates and a facile building strategy remains challenging. Herein, based on the previous theory of combining a 3d-transition metal (Ni) with a non-3d-transition metal (W) to lower hydrogen adsorption energy barriers, we develop an indirect method to access Ni/W sulfides supported by nickel foam (NiWO-S/NF) with polyhedral nanostructures. The unique structure not only provides large surface areas for exposing abundant active sites, but also improves catalyst/interface contact and facilitates mass or charge transportation. In addition, the binary metals are supposed to generate a synergistic effect to boost the hydrogen evolution reaction (HER) properties of NiWO-S/NF via the sulfurization method. Both physical characterization and DFT calculations prove that the fine tuning of electron transport, water dissociation capability and hydrogen adsorption of NiWO-S/NF benefits from sulfurization, thus greatly improving the HER kinetics. Furthermore, NiWO-S/NF demonstrates high electrocatalytic performances with structural stability in a long-term HER process. Therefore, the two-step building of binary metal sulfide nanostructures may provide a new method for applications of various transition metal materials with unique architecture and high efficiency in the alkaline HER.

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1. Introduction

The rapid development of modern society has led to a series of issues such as the energy crisis and environmental pollution, making new green and sustainable fuels a hot topic of concern for researchers today.^1,2 Currently, hydrogen energy has become one of the most important strategic development directions for many countries around the world due to its advantages such as high energy density, zero pollution and zero carbon emissions.^3,4 As is well known, water electrolysis to produce hydrogen using renewable energies such as photovoltaic and wind power is the most effective source of green hydrogen; moreover, electrocatalytic water splitting is increasingly becoming a very mature technology for hydrogen production technology around the world, but it is constrained by factors such as slow kinetics and limited conversion rates. Therefore, even though the mechanism of the hydrogen evolution reaction (HER) has been extensively studied, the rapid development of efficient, stable, and low-cost catalysts is still a matter of concern.

Research shows that the reasons for the superior activity of electrocatalysts are usually the presence of a higher number of active sites on the surface, faster reaction kinetics, and modified electronic properties.⁵ In this regard, binary transition metal oxides may be promising candidates due to the fact that they have nanostructures that are relatively controllable in synthesis and they have been widely applied in magnetism, superconductivity, water electrolysis, hydrogen storage, and even in heterogeneous catalysts recently. High activity is usually associated with a large number of active sites generated by highly ordered structures with voids. In addition, these multi-element metal oxides have enhanced physical, chemical, and electronic properties with appropriate element specificity, making them ideal electrocatalysts for water splitting, which has aroused great interest. NiWO₄ is a well-known and widely used hydrogenation desulfurization catalyst, but further research has found that it can serve as a functional material for super-capacitors.⁶ Compared with single metal oxides, NiWO₄ has higher conductivity and it is expected to be used in the synthesis of electrocatalysts to enhance their performance. Srirapu and coworkers confirmed in their published article that NiWO₄ nanoparticles exhibit OER activity, and NiWO₄ loaded onto a nickel mesh requires an overpotential of 363 mV under alkaline conditions to achieve a current density of 10 mA cm⁻².⁷ However, in current research NiWO₄ and its derivatives have not been widely explored as HER catalysts. In addition, research has found that metal sulfides such as MoS₂, Ni₃S₂ and Ni–Mo have high electrocatalytic activity and low cost as HER catalysts, and have shown great potential for industrial applications.⁸ Therefore, considering that W and Mo belong to the same group, i.e., the VIB group of transition metals, and expecting that they have similar catalytic properties, studying the preparation of Ni–W composite oxide or sulfide nanomaterials and analyzing the relationship between electrocatalytic performance and the structure–activity relationship should have guiding significance for the development of high-performance and inexpensive HER catalysts.

Research has found that S–H_ads are easily generated on the surface of transition metal sulfide catalysts during the HER process, and the formation of S–H_ads is beneficial for the adsorption of H. However, S–H_ads bonds on the surface of metal sulfides are usually very strong, making it difficult to convert H_ads to H₂ under HER conditions.^9,10 As the hydrogen evolution reaction progresses, the coordination state on the surface of metal sulfides will become coordination saturated, making the adsorption of water molecules more difficult.¹¹ The above factors will significantly inhibit the HER catalytic activity of metal sulfides. Existing research has found that electronic interactions between metals and metal sulfides can transfer electrons from metals to metal sulfides, which will promote water adsorption and activation by changing the electron density distribution of electrocatalysts, and optimize H adsorption and desorption.¹² Therefore, the HER performance of electrocatalysts will be significantly improved. Consequently, studying the changes in the electronic structure of Ni–W composite oxide nanomaterials before and after vulcanization and their structure–activity relationship with HER properties is of great theoretical significance for revealing the HER catalytic mechanism of NiWS nanomaterials.

Based on the above analysis, this work studied the controlled synthesis of unique rhombic dodecahedral nanostructures of NiWO on an NF substrate to give NiWO/NF, which was further converted into a NiWO-S/NF catalyst through gas phase sulfurization strategy, and significantly improved HER performance in the alkaline system was achieved. The unique rhombic dodecahedral nanostructure inherited from the precursor ensures a highly dispersed catalytically active surface of the material, and optimizes the interfacial electronic interactions of Ni₃S₂ and WS₂/WO₃, thereby improving the conductivity and intrinsic activity of the catalyst and reducing the reaction energy barrier of the rate-controlled step. The influence of changes in electronic interactions within the catalyst on its HER performance was analyzed, providing a reference for the preparation and design of efficient binary transition metal HER catalysts.

2. Results and discussion

The schematic diagram in Fig. 1 shows that the NiWO-S/NF sample was synthesized through a two-step method. The NiWO/NF precursor with a unique rhombic dodecahedral nanostructure was obtained through the first step of the hydrothermal reaction, and partially converted into the corresponding metal sulfides in the second step. In the process of gas phase sulfurization, S replaces O and preferentially coordinates with Ni. With the extension of the reaction time, a layer of Ni₃S₂ and a small amount of WO₃ and WS₂ are formed on the surface of the dodecahedron. Thus, a large number of Ni–S bonds are provided as active sites to facilitate the adsorption of active *H and the acceleration of catalytic reaction kinetics. Therefore, we obtained a heterogeneous composite structure with a surface composed of Ni₃S₂/WO₃/WS₂ by this method, which is more conducive to maintaining low S–H_ads coverage to promote the HER than the sample with a completely vulcanized surface. The optical photographs of the prepared NiWO/NF, NiWO-S/NF and Ni₃S₂/NF samples are shown in Fig. S1.† After the blank foam nickel mesh with silver white metal color was hydrothermally prepared into NiWO/NF samples, the material surface turned dark gray. The NiWO-S/NF sample obtained after sulfurization of NiWO/NF turned black overall, similar in appearance to the Ni₃S₂/NF sample prepared under the same sulfurization conditions.

Fig. 1 Schematic illustration of the two-step access to NiWO-S/NF samples.

The crystal structures of the as-prepared samples are presented by XRD patterns in Fig. 2a and Fig. S2.† In Fig. S2a,† all NiWO/NF samples synthesized at different reaction times can detect metallic Ni from NF at 44.5°, 52.2°, and 76.8° (PDF no. 00-003-1051).¹³ In addition, diffraction peaks of NiWO₄ (positions at 30.9°, 36.6° and 52.3°, PDF no. 00-015-0755) and WO₃ (positions at 35.6° and 56.6°, PDF no. 01-089-4482) can also be detected, respectively.^14,15 Furthermore, two diffraction peaks that do not belong to the above three types appeared in the low diffraction region of the sample obtained at a shorter reaction time, and disappeared with the extension of the reaction time. Due to the very strong XRD diffraction intensity of the NF substrate, the other peaks belonging to catalysts are relatively weak. The surface of the corresponding sample was peeled off and characterized by XRD characterization to further determine the composition of the substance, as shown in Fig. S2b.† In addition to the diffraction peaks of metal Ni, NiWO₄ and WO₃, several characteristic peaks belonging to W₅O₁₄ (PDF no. 00-041-0745) and W₁₇O₄₇ (PDF no. 01-079-0171) can also be detected in the low diffraction region. As shown in Fig. 2a, the XRD spectrum shows a comparison of the NiWO/NF sample obtained from a 12 h reaction as a precursor with the corresponding product NiWO-S/NF after sulfurization and the reference sample Ni₃S₂/NF. The characterization results of the reference sample Ni₃S₂/NF (PDF no. 01-073-0698) indicate that the six crystal planes (100), (110), (111), (200), (210) and (211) can match well with the characteristic peaks at 21.8°, 31.2°, 37.9°, 44.5°, 50.2° and 55.3°, indicating that the surface of NF completely transforms into Ni₃S₂ after sulfurization treatment.¹⁶ For NiWO-S/NF samples, except for the characteristic peaks of Ni metal derived from the NF, only the diffraction peaks attributed to Ni₃S₂ can be detected from 21.8°, 31.2°, 37.9° and 44.5°, indicating a decrease in the crystallinity of Ni₃S₂ after sulfurization.¹⁷ It may be that the presence of W species (tungsten oxide or tungsten sulfide) hindered the growth of Ni₃S₂ grains. In the process of gas phase sulfurization, the WS₂ phase will inevitably be generated. However, WS₂ grows slowly at 350 °C, and the content on the surface of the material is relatively small, which is difficult to detect using XRD. The presence of partial tungsten oxide species reduces the number of Ni/W–S bonds on the surface of the material, which helps to reduce the coverage of S–H_ads, which may be more reactive than a fully vulcanized surface.

Fig. 2 (a) XRD patterns of NiWO/NF, NiWO-S/NF and Ni₃S₂/NF. (b) Illustration of the NiWO sample. SEM morphologies of (c and d) NiWO/NF, (e and f) NiWO-S/NF and (g and h) Ni₃S₂/NF.

Fig. 2c–h shows the SEM morphology images corresponding to the synthesized samples, while the corresponding morphologies of the remaining precursor samples and reference samples for exploring synthesis conditions are shown in Fig. S3–S5.† As shown in Fig. S3,† the NiWO/NF samples synthesized at different reaction times exhibit a relatively regular rhombic dodecahedral structure. With the extension of the reaction time, some of the originally existing strip-shaped impurities disappeared and agglomeration occurred. The morphology of the powdered samples peeled off from the corresponding samples is shown in Fig. S4.† In addition to the rhombic dodecahedron structured nanocrystals, large blocks of other crystals can also be observed as carriers. As shown in Fig. 2c and d, NiWO grains grow vertically on the NF substrate. In high-resolution SEM images, the structure of NiWO grains is observed to have an average size of about 1.5 μm and a relatively regular rhombic dodecahedron structure, which is consistent with the morphology of the NiWO₄ monoclinic system. After gas-phase sulfurization, as shown in Fig. 2e and f, it can be seen that the original nanostructure of the rhombic dodecahedron remains intact. Compared with the NiWO precursor, the surface of the NiWO-S rhombic dodecahedron is covered with a layer of smaller random crystals, which becomes rougher and the average size of the crystals increases to around 2 μm. The SEM morphology of Ni₃S₂/NF as a control group is shown in Fig. 2g and h, where NF has been completely covered by Ni₃S₂ crystals, and the originally smooth surface has become rough and uneven.

Based on the relevant research results of Srirapu et al., the formation process of the NiWO₄ precursor catalyst is inferred as follows (Fig. 2b).⁷ In a precursor solution with weak acidity (pH = 6.7), Na₂WO₄ preferentially hydrolyzes to form amorphous H₂WO₄. Based on its heteropoly acid characteristics, it is easy to generate poly-WO₄²⁻ ion clusters and amorphous polytungstic acid (such as H₂₄W₁₂O₄₈) precipitates. Subsequently, H₂WO₄ can be further dehydrated to generate amorphous WO₃. When the concentration of WO₄²⁻ ions in the solution drops to a certain range, Ni²⁺ ions migrate to the solid surface of the amorphous mixture and undergo surface reactions with H₂WO₄ and WO₄²⁻ ions dissociated from polytungstic acid to generate NiWO₄. Its crystallinity increases with the prolongation of the reaction time. The reason for the formation of NiWO, a special three-dimensional nanostructure, may be the influence of substrate dispersion. The loosely stacked crystal growth method provides a large open space, which is conducive to sufficient contact between the catalyst and electrolyte interface, and improves the mass transfer efficiency of the catalyst.

As shown in Fig. 3, the XPS characterization results demonstrate the chemical environment of the synthesized NiWO/NF and NiWO-S/NF surface elements. The XPS full spectrum analysis results shown in Fig. 3a indicate that both catalysts contain Ni, W, and O elements. The detection of S element in NiWO-S/NF shows that the precursor is partially converted from the corresponding metal oxide to sulfide under an H₂S atmosphere.^18,19 In Fig. 3b, the two characteristic peaks Ni 2p_1/2 (873.2 eV) and Ni 2p_3/2 (855.6 eV) in the Ni 2p region of NiWO/NF, as well as the two derived satellite peaks (879.3 eV and 861.9 eV), are attributed to the presence of NiWO₄ in the material. Compared with the former, the sulfided NiWO-S/NF shows two characteristic peaks attributed to Ni(0) at 870.1 eV and 852.7 eV, which are due to the reducibility of the H₂S atmosphere during the sulfidation process.¹⁹ As shown in Fig. 3c, there are two pairs of characteristic peaks in the W 4f region at binding energies of 35.2 eV and 37.4 eV, and 31.9 eV and 34.1 eV, respectively, attributed to W 4f_7/2 and W 4f_5/2 orbitals, corresponding to W⁶⁺ in NiWO₄ and W⁴⁺ in WS₂.¹⁴ As shown in Fig. 3d, in the S 2p region of NiWO-S/NF, the two characteristic peaks of S 2p_3/2 (161.7 eV) and S 2p_1/2 (162.9 eV) indicate the presence of binding energy between metal and sulfur, while the SO₄²⁻ characteristic peak detected at 168.8 eV indicates partial oxidation of sulfur in air.^20,21 In addition, the comparison of the binding energies of the characteristic peaks in Fig. 3b and c shows that the two characteristic peaks of Ni 2p_1/2 and Ni 2p_3/2 in the sulfided NiWO-S/NF have shifted by 0.5 eV compared with those in the NiWO/NF, while the two characteristic peaks of W 4f_7/2 and W 4f_5/2 have shifted by 0.3 eV, respectively. This indicates a change in the strong electronic interaction between Ni and W after sulfurization of NiWO/NF, which is beneficial for optimizing the electronic structure and improving the conductivity of the NiWO-S/NF material, thereby having a positive effect on the overall electrocatalytic performance.²² In short, XPS analysis shows that the average valence state of Ni on the surface of the vulcanized material decreases and the electronic structure of Ni changes significantly. The large number of Ni–S bonds on the surface provides the key active site, which is conducive to the adsorption of active *H and the acceleration of catalytic reaction kinetics. On the surface of NiWO-S/NF, the existing forms of W elements are more diverse, and the electronic structure also shows obvious changes. As a typical high-priced metallic element, W has more empty d orbitals to play a key electronic regulatory role. Through the electronic interaction between W and Ni, the S–H_ads bond is significantly optimized to alleviate the overstrong adsorption of the key intermediate *H on the surface of Ni–S, thus effectively accelerating the HER process.

Fig. 3 XPS spectra of NiWO/NF and NiWO-S/NF. (a) Survey, (b) Ni 2p, (c) W 4f, and (d) S 2p spectra and (e) growth process of NiWO-S. In the hydrothermal reaction process, NiWO₄ crystallizes in a monoclinic structure with a space group of P2/c. Subsequently, the surface layer was partially sulfurized into the corresponding Ni₃S₂ and WS₂ octahedra.

The crystal structure of NiWO-S/NF was further characterized by high-resolution transmission electron microscopy (HRTEM). As shown in Fig. 4a, lattice fringes of 0.288 nm can be observed in the enlarged region A, corresponding to the (111) crystal plane of NiWO₄, indicating that NiWO/NF and NiWO-S/NF nanocrystals are mainly composed of NiWO₄. In the enlarged B region, lattice fringes of 0.289 nm can be observed, corresponding to the (110) crystal plane of Ni₃S₂; and in the enlarged C region of Fig. 4b, lattice fringes at 0.610 nm can be observed, corresponding to the (002) crystal plane of WS₂.²³ Based on the above characterization results, it can be concluded that the process of converting NiWO/NF to NiWO-S/NF leads to partial sulfurization, and Ni is more prone to sulfurization than W. Therefore, the smaller sized random crystals covered on the surface of the NiWO-S material shown in Fig. 2e and f may be due to the higher content of Ni sulfide nanoparticles. According to the TEM mapping characterization results of the NiWO-S/NF sample in Fig. 4c, the four elements Ni, S, W, and O are uniformly distributed on the material surface.

Fig. 4 (a and b) HRTEM and (c) TEM mapping of the NiWO-S/NF sample. Small images of A, B and C show the enlargement of corresponding areas in the figure.

In order to explore the effect of polyphase sulfide structure on the hydrogen evolution performance of Ni-based catalysts, the electrochemical properties of the prepared series of catalysts were tested using a three electrode system at room temperature. As shown in Fig. 5, the HER electrocatalytic performance of all prepared samples was tested in 1.0 M KOH using Pt/C and Ni₃S₂/NF as references. The remaining data on precursor synthesis and optimal performance conditions are presented in Fig. S7 and S8.† The current density measured under an applied potential is one of the key parameters of electrocatalysts, which reflects the catalytic activity of the corresponding electrode. In the polarization curve shown in Fig. 5a, it can be seen that Pt/C is undoubtedly the catalyst with the best HER activity. The test results of Ni₃S₂/NF LSV obtained by direct sulfurization of NF indicate that its performance as a HER catalyst is not ideal, and the overpotential required for a current density of 10 mA cm⁻² is 206 mV. The NiWO/NF synthesized by the hydrothermal method can achieve the same current density at an overpotential of 184 mV. The reason for the improved catalyst performance may be ascribed to its unique 3D nanostructure and W modification of transition metal nickel based materials. The HER performance of the NiWO-S/NF catalyst after sulfidation is significantly improved, with an overpotential of only 52 mV required to achieve the same current density, which is superior to the values for many reported electrocatalysts (Table S1†). The reasons for this phenomenon are multifaceted. On the one hand, the unique nanostructure inherited from the precursor NiWO/NF material further improves the mass transfer efficiency between the electrolyte and the interface after sulfurization. On the other hand, the *H intermediate adsorption effect of S as a foreign non-metallic atom has been proved to excite the intrinsic activity of the catalyst. With the introduction of S, the enhanced electronic interaction between Ni and W also promotes the intrinsic activity of the material and charge transfer. In order to further explain the excellent HER performance of NiWO-S/NF in alkaline environments, the intrinsic activity of the above catalysts was evaluated using the Tafel slope. As shown in Fig. 5b, in the overpotential range between 0 V and 0.4 V, NiWO-S/NF has the lowest Tafel slope (124.9 mV dec⁻¹), which is significantly lower than the Tafel values for NiWO/NF (127.2 mV dec⁻¹) and Ni₃S₂/NF (126.7 mV dec⁻¹) catalysts. The lower Tafel slope indicates the excellent HER performance of the NiWO-S/NF catalyst, as well as optimization of the HER reaction kinetics rate limiting step based on the performance of the precursor NiWO/NF and the Volmer–Heyrovsky combination mechanism followed by its HER process in alkaline media.

Fig. 5 Electrocatalytic measurements for the HER in 1.0 M KOH. (a) Linear sweep voltammogram (LSV). (b) Tafel plots. (c) Electrochemical impedance spectroscopy (EIS). (d) Determined double-layer capacitance (C_dl). (e) Stability test of NiWO-S/NF LSV curves before and after cyclic voltammetry (CV) for 5000 cycles and (f) the 24-h chronoamperometry test.

Electrochemical impedance spectroscopy (EIS) is another important indicator for evaluating electrocatalysts, and the charge transfer resistance (R_ct) obtained from it can accurately reflect the electron transfer rate of electrode materials in liquid-phase reactions. The electrochemical impedance of alkaline HER kinetics was evaluated using the Nyquist plot shown in Fig. 5c, and the test data were fitted using the simple equivalent circuit diagram included in the figure. For this equivalent circuit, R_s is the solution resistance representing the electrical transmission characteristics, and CPE is the constant phase element, acting as different components according to the alpha index. R_ct, which stands for the charge transfer resistance, represents the electrocatalytic kinetics between the catalyst interface and the electrolyte interface. In Table S2,† the calculated values of R_s and R_ct are summarized. As shown in Fig. 5c, NiWO-S/NF has the lowest R_ct value of 0.74 Ω and the smallest arc radius, indicating rapid electron transfer of the HER on the 3D stacked NiWO-S nanocrystals dispersed on the rough surface of the NF conductive substrate. It can be considered that the rough surface of the NiWO-S/NF catalyst provides a larger surface area compared to other samples, resulting in more contact between the catalyst and electrolyte, which promotes substance and charge transfer in HERs. In addition, smaller R_ct values indicate expected catalytic kinetics and better conductivity, which is beneficial for improving HER catalytic activity.

In addition, the HER activity of all the prepared catalyst samples was evaluated using electrochemically active surface area (ECSA) testing. The electrochemical active surface area (ECSA) is considered one of the important parameters reflecting the number of active sites in a catalyst, and its value is directly proportional to the double-layer capacitance (C_dl). The C_dl can be obtained by fitting CV curves at different scan rates (Fig. S9†). As shown in Fig. 5d, the C_dl, C_s, and ECSA of the three catalysts were calculated based on the CV curves of the Faraday region, and the results are shown in Table S3.† As shown in Table S3,† the C_dl value of NiWO-S/NF is 106.5 mF, much higher than those of NiWO/NF (0.77 mF) and Ni₃S₂/NF (20.9 mF), and the ECSA value is 2662.5 cm², much higher than those of NiWO/NF (19.25 cm²) and Ni₃S₂/NF (522.5 cm²). The high electrochemical capacitance of NiWO-S/NF indicates that its unique 3D stacked rhombic dodecahedron structure has the largest ECSA, and the high electrochemical capacitance and large ECSA also establish its excellent HER activity. The above results indicate that NiWO-S/NF synthesized by simple gas-phase sulfurization of NiWO/NF not only has abundant active sites, but also has higher HER catalytic activity compared to other samples.

In practical applications, catalyst stability is one of the key parameters determining the application prospects of NiWO-S/NF materials, and the long-term electrolytic water test conducted in 1.0 M KOH is one of the commonly used methods to evaluate the stability of HER catalysts. As shown in Fig. 5e, the test results of the LSV curve indicate that after 5000 cycles of voltammetry testing, the potential difference of the NiWO-S/NF catalyst before and after the reaction to reach a current density of 400 mA cm⁻² does not exceed 10 mV. In order to further verify the stability of the electrode in the electrocatalytic process, a continuous 24-hour chronoamperometry test was conducted at a potential of −1.2 V (vs. SCE), and the corresponding potential time curve is shown in Fig. 5f. The test results indicate that when the current density of the NiWO-S/NF catalyst stabilizes, it only shows a slight decrease even after 24 hours of continuous reaction. All stability test results provide intuitive and reliable evidence, indicating that the NiWO-S/NF catalyst maintains its excellent performance during long-term alkaline water electrolysis and has good HER reaction stability. Fig. S10† shows the SEM morphology of the NiWO-S/NF catalyst after stability testing. In Fig. S10a,† it can be seen that the nanoparticles on the NiWO-S/NF catalyst still maintain good dispersibility, and the loaded nanocrystals do not exhibit significant detachment or agglomeration. As shown in Fig. S10b,† partial damage to the sulfide shell layer on the surface of the nanoparticles on the catalyst can be observed in the high-resolution SEM image, while the internal crystal structure remains intact, and the overall nanostructure of the catalyst is maintained. The partial damage to the shell layer may be related to the impact of the gas flow caused by the violent precipitation of H₂ bubbles on the surface of the material during the catalyst testing process, corresponding to a slight decrease in the performance of the material after 5000 cycles of cyclic voltammetry testing.

In addition, theoretical calculations were conducted based on density functional theory (DFT) to determine the source of high catalytic activity and the enhancing effect of foreign S for the HER in alkaline media. The NiWO/NF and NiWO-S/NF models were constructed to study the electron interaction, respectively. The density of states (DOS) was first investigated to directly identify the electronic entanglement in these computed molecules. As shown in Fig. 6a and b, both materials exhibit metallicity, indicating fast electron transfer. After the sulfurization process, the DOSs of NiWO-S/NF underwent significant changes, with more electrons appearing near the Fermi level. This means that the original electronic structure of NiWO/NF has been finely tuned by S atoms to favor further charge exchange. It is evident that the rearrangement of electron distribution results is closely related to the adsorption and desorption processes of reaction intermediates. Therefore, based on the combination of adsorbed active H (H*) and protons from adjacent H₂O to generate adsorbed H₂, as well as the final desorption process of H₂ from the surface, the HER pathway composed of double-layer adsorption of water molecules and subsequent cleavage to form adsorbed H* was simulated on modeled surfaces. The S–H_ads bond on the surface of metal sulfides is usually very strong, which seriously affects the catalytic surface coverage and desorption rate of *H.^24,25 The W atom itself has more empty d orbitals and more special electronic properties, which can significantly affect the electronic structure of the Ni–S bond and optimize the adsorption strength of S–H_ads. In Fig. 6d, compared to NiWO/NF, the lower water dissociation free energy of NiWO-S/NF represents faster water splitting kinetics and smoother formation of H*. As shown in Fig. 6e, compared with NiWO/NF (−2.61 eV), NiWO-S/NF exhibits a better level of H* adsorption (−2.29 eV). Due to the fact that the closer the H* adsorption energy (ΔG_H*) is to zero, the better the HER activity of the catalyst, NiWO-S/NF exhibits enhanced hydrogen evolution activity thermodynamically. According to the previous analysis results, the improvement of material adsorption performance is closely related to the internal electronic structure, which can be further confirmed by the differential charge density. As shown in Fig. 6c, for NiWO/NF, the introduction of S atoms will increase the electron density around W atoms. The presence of a small number of W–S bonds on the surface of the material, as a key electronic regulator, can significantly optimize the electronic structure of Ni–S to promote *H desorption. Overall, S plays the role of the key electronic “bridge”, while regulating the electronic structure of W and Ni, resulting in better ΔG_H2O* and ΔG_H*, ultimately leading to improved HER activity.^26–28

Fig. 6 (a) DOS of NiWO/NF and (b) DOS of NiWO-S/NF. (c) The difference charge density of NiWO-S/NF, where the yellow and cyan represent the depletion and accumulation of electron density, respectively. (d) The energetic pathway comparison of the HER. (e) ΔG_H* comparison on NiWO/NF and NiWO-S/NF.

In summary, the excellent HER performance of the NiWO-S/NF catalyst can be roughly summarized in the following three aspects: (i) the unique 3D stacked nanocrystal structure provides a large specific surface area and open space, which promotes mass and charge transfer between the catalyst and electrolyte interface while exposing abundant active sites; (ii) compared with the NiWO/NF precursor, the introduction of sulfides in NiWO-S/NF generates new active sites, which improve the intrinsic activity of the catalyst while enhancing the overall conductivity; and (iii) partial sulfurization promotes the adsorption and activation of water by positively charged oxides, while negatively charged sulfides weaken the S–H_ads bonds on the catalyst surface, enhancing the electronic interaction between cations and anions. This overall optimizes the adsorption and desorption of H on electrocatalysts, promoting the Volmer and Heyrovsky steps of the HER. A simple schematic diagram of the NiWO-S/NF interface under reduction potential is shown in Fig. 7.

Fig. 7 Schematic diagram of the NiWO-S/NF interface under reduction potential.

3. Conclusions

In summary, we have developed facile two-step access to Ni–W sulfides supported on NF (NiWO-S/NF) with polyhedral nanostructures. The delicate architectures of binary metallic sulfides can be indirectly realized by designing a metal oxide precursor framework in the first step. The large surface provided by unique nanostructures, the abundant active sites as well as synergistic effects derived from multiple metal oxides and sulfides may contribute to the excellent HER properties of NiWO-S/NF. Moreover, NiWO-S/NF can maintain high HER activity and structural stability in long-term water electrolysis. Physical characterization and DFT calculations demonstrate that the elevated HER performance is derived from the optimized adsorption/dissociation of water molecules and the adsorption and H* thanks to sulfurization. Such an indirect building strategy of multiple metal oxides and sulfide nanostructures may be valuable for wider application in other non-noble metals to achieve high efficiency in alkaline water electrolysis.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (52174283) and the Fundamental Research Funds for the Central Universities (24CX03012A).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi01806k

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