Ternary MoWNi Alloy as a Bifunctional Catalyst for Alkaline Hydrogen Oxidation and Evolution Reactions

Abstract

The hydrogen economy, as an emerging paradigm for sustainable energy, relies on efficient hydrogen oxidation (HOR) and hydrogen evolution reactions (HER). These reactions require effective catalysts to enhance reaction kinetics and reduce costs. Platinum (Pt) is widely used but faces issues such as high cost and CO poisoning. Non-precious metal catalysts, particularly Ni-based alloys, are being explored as viable alternatives. This study introduces a ternary MoWNi alloy catalyst synthesized via microwave-assisted methods and annealing. The MoWNi alloy catalyst achieves a current density of 3.5 mA·cm⁻² at an overpotential of 100 mV in HOR and requires only 25 mV overpotential to reach a current density of 10 mA·cm⁻² in HER, making it comparable to commercial 20% Pt/C catalysts. Notably, the catalyst also exhibits superior stability and resistance to CO toxicity. These findings underscore the potential of MoWNi alloy catalysts in advancing hydrogen-based energy systems.

1. Introduction

Hydrogen (H₂), a critical raw material for hydrogenation, petroleum refining, and fertilizer production, has also been regarded as an ideal carbon-free energy carrier to replace fossil fuels due to its zero emissions during use, high energy density, potential for renewable production, versatile applications, and ability to facilitate energy storage [1]. The hydrogen economy, an emerging economic paradigm centered on the use of hydrogen as a primary energy carrier for storage, transportation, and conversion, stands as a viable alternative to alleviate the pressures exerted by traditional petroleum-based economic systems [2,3]. Central to this concept are the Hydrogen Oxidation Reaction (HOR) and its reverse counterpart, Hydrogen Evolution Reaction (HER), which together facilitate the efficient utilization of hydrogen, thereby supporting the realization of a hydrogen-based economy [4,5,6]. To fully harness the potential of the hydrogen economy, electrocatalysts that efficiently mediate both HOR and HER are essential. Traditional catalysts have typically been optimized for one reaction over the other. However, bifunctional catalysts integrate the capabilities of both reactions. Their ability to catalyze both reactions within the same material framework enhances energy efficiency and simplifies system architecture, thereby potentially lowering costs and making these technologies more viable for widespread adoption [7]. Moreover, In a closed-loop system for hydrogen production, storage, and utilization, bifunctional catalysts capable of promoting both the HOR and HER facilitate seamless hydrogen fuel cycling, enabling the construction of distributed energy storage and regional energy networks [8]. Currently, bifunctional electrocatalysts for HOR and HER rely on precious metals such as Pt to lower reaction potentials and enhance kinetics [9]. However, the uneven geographical distribution of these metals leads to high costs [10,11], and issues like CO poisoning [12,13,14] significantly impede their widespread application. Moreover, with the rapid advancement of alkaline polymer electrolyte fuel cells and alkaline water electrolyzers, Pt-based catalysts exhibit inferior reaction kinetics in alkaline media compared to acidic conditions [15,16,17]. This underscores the urgent need for cost-effective, high-performance bifunctional hydrogen electrocatalysts suitable for alkaline environments.

Non-precious metal catalysts, particularly those based on nickel (Ni), have emerged as promising alternatives to Pt-based catalysts [18,19,20,21,22]. Research has shown that strategies such as alloying [23,24,25] can not only address the issue of scarcity but also mitigate the limitations associated with CO poisoning [26,27,28]. The introduction of various non-precious metal elements can modulate the hydrogen binding energy (HBE) and hydroxyl binding energy (OHBE) of Ni, effectively tailoring it to meet the requirements of both HOR and HER [29,30,31]. For instance, a NiMo alloy bi-functional catalyst was prepared by the Chen group and applied to aqueous nickel-metal hydride gas batteries [32]. It showed a high HOR mass specific dynamic current of 28.8 mA mg⁻¹ at 50 mV and a low HER overpotential of 45 mV at 10 mA cm⁻² current density, superior to most non-precious metal catalysts. Similarly, Cao et al. [5] report a bifunctional electrocatalyst, namely, Ni₄Mo nanoparticles grown on reduced oxide graphite, showing a current density of 1.63 mA cm⁻² at an overpotential of 100 mV for the HOR and an overpotential of 51 mV at 10 mA cm⁻² for the HER in an alkaline electrolyte, surpassing those of the commercial 20 wt% Pt/C catalyst. In their works, the DFT calculation data show that the incorporation of Mo element enhances the adsorption of H^* and OH^* on Ni4Mo alloy, thereby improving the activities of HER and HOR. Since the alkaline HOR activity of the binary alloy still cannot meet the industrial requirements, the Huang group [33] further grew the NiMoW ternary alloy on the Ni net by hydrothermal annealing process and H₂ atmosphere. It showed excellent HOR catalytic activity (current density of 6.6 mA cm⁻² at 100 mV RHE), which was twice that of Pt/C electrocatalysts, and also exhibited Pt-like overpotentials of almost zero for HER. These studies suggest that charge transfer between alloy components may regulate the electronic structure of the alloy, affect the HBE and OHBE of the catalyst, and ultimately promote HOR/HER activity.

Herein, we construct MoWNi alloy nanoparticles through microwave-assisted methods followed by annealing. The incorporation of molybdenum (Mo) and tungsten (W) into the Ni lattice modifies the electronic structure surrounding Ni atoms, striking a balance in its hydrogen binding capability. This synergistic interaction among Mo, W, and Ni fosters enhanced catalytic activity for both HOR and HER. Electrochemical assessments reveal that at 0.1 V versus RHE, the MoWNi alloy catalyst achieves a current density of 3.5 mA·cm⁻², with a kinetic current density of 13.01 mA·cm⁻² and an exchange current density of 3.55 mA·cm⁻², outperforming commercial 20% Pt/C under identical conditions. Remarkably, under a 60 mV overpotential, the catalyst was tested continuously for 5 h with minimal current damping and, upon exposure to 1000 ppm CO for 1000 s, retained 81% of its original current density, demonstrating robust HOR activity and CO tolerance. In HER, the overpotential at 10 mA·cm⁻² is merely 25 mV, with a Tafel slope of 31.6 mV·dec⁻¹, both figures surpassing those of commercial 20% Pt/C. These findings highlight the potential of MoWNi alloy catalysts in advancing the practicality and efficiency of hydrogen-based energy systems.

2. Results and Discussion

2.1. Structural and Compositional Analyses

The synthesis of MoWNi alloy nanoparticles is depicted in Figure 1a. Initially, elemental sources of Ni, Mo, and W were combined and subjected to microwave heating at 200 °C, resulting in a flower-like precursor characterized by green lamellar intercalations (Figure 1b). Energy-dispersive X-ray spectroscopy (EDX) analysis confirms the homogeneous distribution of all metal elements within the lamellar layers (Figure S1). Subsequently, the black MoWNi powder was obtained by annealing in a mixture of hydrogen and argon at 600 °C (Figure 1a). Scanning electron microscopy (SEM) reveals a transformation from the initial flower-like morphology to a granular structure (Figure 1c). Transmission electron microscopy (TEM) further elucidates that the MoWNi ternary alloy adopts an interconnected nanosheet configuration composed of nanoparticles with a porous surface (Figure 1d). The BET surface area of the alloy, determined from nitrogen adsorption and desorption isotherms, is 65.4 m²·g⁻¹ (Figure S2). High-resolution transmission electron microscopy (HRTEM) images display crystal lattice fringes with distinct outlines; a spacing of 0.201 nm corresponds to the (111) crystal face of Ni, and 0.175 nm matches the (200) crystal face of Ni, with an interfacial angle of 56° between the two sets of crystal fringes (Figure 1e). X-ray diffraction (XRD) analysis corroborates the face-centered cubic structure of the MoWNi ternary alloy (Figure S3). Additionally, EDX mapping demonstrates uniform dispersion of Ni, Mo, and W throughout the sample (Figure 1f).

X-ray photoelectron spectroscopy (XPS) analysis was conducted on the NiMoW catalyst to examine the valence states of its constituent elements. The XPS spectra of the precursor reveal that the peaks at binding energies of 881.0 eV and 863.4 eV in the Ni 2p spectrum correspond to the satellite peak of Ni, while the peaks at 875.1 eV and 857.5 eV are attributed to Ni²⁺ 2p_1/2 and Ni²⁺ 2p_3/2, respectively (Figure 2a). In the Mo 3d spectrum, binding energies of 235.5 eV and 232.1 eV are consistent with Mo⁶⁺ (Figure 2b). The W 4f spectrum exhibits binding energies that can be assigned to W⁶⁺ (35.5 eV and 37.6 eV). Additionally, a peak at 41.2 eV in the spectrum is identified as W 4p_1/2 (Figure 2c). Ni, Mo, and W in the precursor exist in high oxidation states. In the XPS profile of the alloy, the Ni 2p spectrum shows binding energies that can be deconvoluted into contributions from Ni⁰ (852.6 eV and 869.7 eV) and Ni²⁺ (855.7 eV and 873.0 eV). Furthermore, peaks at 876.0 eV and 859.0 eV are associated with vibrational excitation of high-spin Ni ions (Figure 2d) [34]. The Mo 3d spectrum can be deconvoluted to reveal Mo⁰ (227.9 eV and 231.1 eV), Mo⁴⁺ (228.7 eV and 232.3 eV), and residual Mo⁶⁺ (235.1 eV) (Figure 2e). Similarly, the W 4f spectrum can be deconvoluted to identify W⁰ (31.4 eV and 33.5 eV) and W⁶⁺ (35.3 eV and 37.5 eV) (Figure 2f). Additionally, a binding energy of 40.52 eV corresponds to W 4p1/2 (Figure 2f) [35]. We also conducted XPS testing and analysis on the oxygen element on the material surface (Figure S4) and found that the XPS peaks of the oxygen elements on the material surface are mainly attributed to metal oxides, which may be beneficial for enhancing the HER performance [36].

2.2. Electrocatalytic Activities of MoWNi for HOR and HER

In a standard three-electrode configuration, the HOR electrocatalytic activity of the MoWNi alloy catalyst was assessed in a hydrogen-saturated 0.1 M KOH solution. For comparison, binary MoNi and WNi alloy catalysts, synthesized via the same methodology, were evaluated alongside commercial 20% Pt/C. A scan rate of 5 mV/s was employed to mitigate the impact of double-layer capacitance and ensure measurements under steady-state conditions. Polarization curves for various alloy catalysts prepared using the identical procedure are presented in Figure 3a. Notably, several catalysts initiate HOR current at onset potentials as low as 0 V versus RHE, indicative of their substantial catalytic efficacy in basic media. In contrast, the current densities of 1.5 mA/cm² and 2.4 mA/cm² observed for WNi alloy (doped with W) and MoNi alloy (doped with Mo), respectively, at 0.1 V versus RHE, are surpassed by the 3.5 mA/cm² achieved by the MoWNi alloy catalyst, highlighting the synergistic enhancement of HOR catalysis through combined Mo and W doping. Furthermore, Figure 3a illustrates that the ternary MoWNi alloy outperforms the commercial 20% Pt/C catalyst across the entire voltage spectrum, with the latter exhibiting a current density of 2.7 mA/cm² at 0.1 V, which is inferior to that of the MoWNi alloy catalyst.

Subsequently, we examined the HOR characteristics of the ternary MoWNi alloy catalyst in an Ar-saturated 0.1 M KOH solution, as depicted in Figure 3b. It revealed that within the potential window of 0 to 0.15 V versus RHE, the catalyst exhibited negligible current density, approaching zero, thereby suggesting an absence of significant HOR catalytic activity under these conditions. This observation confirms that the observed current in hydrogen-saturated solutions is indeed attributable to the catalyst’s role in promoting the hydrogen reaction, effectively discounting any contributions from the oxidation of the alloy catalyst itself or other contaminants. We then proceeded to investigate the relationship between the HOR polarization curve of the ternary MoWNi alloy catalyst and the rotational speed of the electrode. Considering the impact of mass transport limitations, it was anticipated that increasing the rotational speed would enhance the current density due to improved reactant accessibility. To this end, a Koutecky–Levich (K–L) plot was constructed at 50 mV, as illustrated in Figure S5. A linear correlation was observed between the reciprocal of the total current density and the square root of the rotational speed, yielding a slope value of 4.63 cm²·mA⁻¹·s^−1/2. This experimental slope closely aligns with the theoretical value predicted by the two-electron transfer theory (4.87 cm²·mA⁻¹·s^−1/2), affirming that the HOR predominantly takes place on the surface of the catalyst. Then, the kinetic current density (j_k) was calculated using the K–L equation. Utilizing the K–L equation, the j_k was further extrapolated for the ternary MoWNi alloy catalyst, which stood at 13.01 mA·cm⁻² at a potential of 50 mV, as shown in Figure 3c. Notably, this value represents a substantial increase compared to those obtained for binary MoNi and WNi alloys, as well as commercial 20% Pt/C catalysts, reflecting enhancements of approximately 9.7-fold, 2.0-fold, and 1.1-fold, respectively. To delve deeper into the catalyst’s electrochemical properties, we employed the Butler–Volmer (B–V) equation to determine the exchange current density (j₀) within the micro-polarization region (−10 mV to 10 mV). The calculated j₀ for the MoWNi alloy catalyst amounted to 3.55 mA·cm⁻², as presented in Figure 3c,d, significantly surpassing the values recorded for binary MoNi (1.31 mA·cm⁻²), WNi (0.44 mA·cm⁻²), and commercial 20% Pt/C (1.9 mA·cm⁻²) catalysts. These outcomes are consistent with the fitting results derived from the B–V equation in the Tafel region, as demonstrated in Figure 3e, and the MoWNi exhibits high j_k and j₀ values that are comparable to those of state-of-the-art non-precious metal electrocatalysts reported in the literature (Table S1), reinforcing the superior catalytic efficiency of the ternary MoWNi alloy catalyst.

Electrochemical impedance spectroscopy (EIS) was employed to examine the reaction kinetics associated with hydrogen oxidation over the catalysts. The charge transfer resistance (R_ct), a parameter directly linked to the catalytic reaction kinetics, serves as an indicator wherein a reduced R_ct value corresponds to accelerated reaction rates. This magnitude is typically extracted from the Nyquist plot [37]. Analysis of the Nyquist plot reveals that the charge transfer resistance for the ternary MoWNi alloy catalyst stands at merely 4.2 Ω, notably lower than the 6.3 Ω observed for the binary MoNi alloy, the 6.5 Ω recorded for the WNi alloy, and the 4.7 Ω measured for the commercial 20% Pt/C catalyst (Figure 3f). These findings suggest that the synergistic interplay facilitated by the co-doping of molybdenum (Mo) and tungsten (W) in the ternary MoWNi alloy significantly enhances charge transfer processes during hydrogen oxidation, thereby augmenting the overall reaction kinetics.

The electrochemically active surface area (ECSA) of the catalysts is directly proportional to their double-layer capacitance (C_dl), with a higher double-layer capacitance indicative of a larger ECSA. The double-layer capacitance at various scan rates was assessed (Figure S6a–c), and the relationship between the ECSAs of different catalysts was established (Figure S6d). It was found that the specific C_dl for MoWNi was 122.7 mF·cm⁻², compared to 42.8 mF·cm⁻² for MoNi and 20.1 mF·cm⁻² for WNi. The ternary MoWNi alloy catalyst exhibited the highest double-layer capacitance, corresponding to the largest electrochemically active surface area. This suggests that a greater number of catalytically active sites are involved in the electrochemical reaction, leading to more active sites facilitating the HOR, thereby enhancing the overall catalytic efficiency.

Stability is a critical parameter in the practical deployment of catalysts. The long-term stability was assessed by comparing linear sweep voltammetry (LSV) curves before and after the experiment (Figure 4a). After 2000 cyclic voltammetry (CV) cycles, at a potential of 0.1 V vs. RHE, the current density of the ternary MoWNi alloy catalyst under HOR conditions decreased by only 0.35 mA·cm⁻², from 3.5 mA·cm⁻² to 3.15 mA·cm⁻². Moreover, chronopotentiometry tests were conducted to evaluate the sustained performance of the MoWNi catalyst under constant voltage in comparison with commercial catalysts. Under a 60 mV overpotential, the catalyst was tested continuously for 5 h with minimal current damping. The XRD spectrum and XPS spectrum show that MoWNi alloy still remains the crystal lattice of Ni, and the surface W element has undergone slight oxidation, which has not affected the material performance (Figures S7 and S8). Conversely, the commercial 20% Pt/C catalyst exhibited substantial attenuation, retaining only 53.4% of its original current density after 5 h. This significant decline is attributed to the gradual agglomeration of Pt nanoparticles during prolonged stability tests [38] (Figure 4b). In fuel cell applications, platinum-based precious metals at the anode are susceptible to poisoning by raw hydrogen-containing carbon monoxide (CO) [39,40]. Poisoning occurs due to the preferential binding of CO to Pt, which inhibits hydrogen adsorption and deactivates the catalyst [41]. CO poisoning tests were performed to assess the CO tolerance of the catalysts (Figure 4c). During the initial 200 s of the test, high-purity hydrogen was introduced to stabilize the current density. At the 200th second, the gas was switched to hydrogen-containing 1000 ppm CO. The results are shown in Figure 4c. The ternary MoWNi alloy catalyst maintained a stable current density of 2.5 mA·cm⁻² in the first 200 s but experienced a rapid decrease upon the introduction of 1000 ppm CO, dropping to 76% (1.9 mA·cm⁻²) of its original value after 200 s of CO exposure. With continued CO introduction, some active sites were regenerated. The current density slightly increased, returning to 81% (2.0 mA·cm⁻²) after 600 s of CO exposure and remained stable thereafter (Figure 4d). In contrast, the current density of the commercial 20% Pt/C catalyst (2.4 mA·cm⁻²) continued to decline following CO introduction, and after 1000 s, it dropped to 54% (1.3 mA·cm⁻²) of its original value (Figure 4d). Our prepared ternary MoWNi alloy catalyst demonstrates superior CO tolerance compared to the commercial 20% Pt/C catalyst.

The HER performance of Ni was also evaluated using a standard Figure 5a, which presents the HER polarization curves of MoWNi and comparison samples in H₂-saturated 1 M KOH solution. The ternary MoWNi alloy catalyst exhibits the highest catalytic activity, achieving a high current density at a very low overpotential, surpassing that of commercial 20% Pt/C. At a current density of 10 mA·cm⁻², the overpotential of the ternary MoWNi alloy catalyst is only 25 mV, which is lower than 32 mV for commercial 20% Pt/C, 51 mV for the binary MoNi alloy, and 160 mV for the WNi alloy. The reaction rate of the catalyst’s HER process can be determined by fitting the Tafel equation, with its magnitude dictated by the rate-controlling step of HER. As shown in Figure 5b, the Tafel slope of the ternary MoWNi alloy catalyst is 31.6 mV dec⁻¹, which is lower than 32.7 mV dec⁻¹ for commercial Pt/C, indicating that both the ternary MoWNi alloy catalyst and commercial Pt/C undergo the Volmer-Tafel mechanism during HER. The Tafel reaction serves as the rate-controlling step for the catalyst, and the ternary MoWNi alloy catalyst demonstrates a faster hydrogen evolution efficiency compared to commercial 20% Pt/C. A stability analysis of the HER was conducted, and the polarization curve of the catalyst before and after 2000 CV cycles in 1.0 M KOH solution was examined, as shown in Figure 5c. It was observed that after 2000 CV acceleration cycles, the overpotential of the ternary MoWNi alloy catalyst increased by only 9 mV, from 25 mV to 34 mV, at a current density of 10 mA·cm⁻². Additionally, we assessed its operational stability through multi-step chronopotentiometry. Under constant currents of 10, 20, and 40 mA·cm⁻², the overpotential curve over time for the ternary MoWNi alloy catalyst was tested, as shown in Figure 5d. The overpotential remained relatively stable for up to 30,000 s. The minimal increase in overpotential after 2000 CV cycles and the stability observed under constant current testing indicates that the prepared ternary MoWNi alloy catalyst possesses good HER stability, which is advantageous for practical applications in materials science.

To elucidate the underlying operational mechanisms of the MoWNi bifunctional catalyst, we conducted X-ray Photoelectron Spectroscopy (XPS) analyses on both the binary MoNi alloy, WNi alloy, and the MoWNi alloy system. As shown in Figure 6, the binding energy corresponding to Ni⁰ in the Ni 2p spectrum of MoWNi alloy exhibits shifts to lower energies by 0.2 eV and 0.3 eV, respectively, compared with the binary MoNi alloy and WNi alloy. This observation indicates an electron transfer from the Mo, W site to the Ni site, leading to electron enrichment on the Ni surface. Consequently, these electronic structure modifications facilitate the preferential adsorption of H on the Ni and OH on the Mo, W site [42].

3. Materials and Methods

3.1. Materials and Chemicals

Ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O) (AR), ammonium tungstate hydrate ((NH₄)₁₀W₁₂O₄₁·xH₂O) (AR), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) (AR), ammonia (AR), ethylene glycol (AR), ethanol (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Argon, 5% hydrogen argon mixture and nitrogen (99.99%) were purchased from Qingdao Xinkeyuan Technology Co., Ltd. (Qingdao, China). Nafion solution (5%) was purchased from Dupont(US), Wilmington, DE, USA. Commercial Pt/C (20 wt%) was purchased from Sigma-Aldrich (Shanghai, China). Deionized water was self-made.

3.2. Synthesis of MoWNi

First, Ni(NO₃)₂·6H₂O 1.744 g, (NH₄)₆Mo₇O₂₄·4H₂O 96 mg, and (NH₄)₁₀W₁₂O₄₁·xH₂O 190 mg were weighed and dissolved in 3 mL deionized water, followed by the addition of 1.2 mL ammonia and 15 mL glycol. The solution was then stirred on a magnetic stirrer for 30 min and mixed thoroughly. The solution was transferred to a 100 mL polytetrafluoroethylene autoclave and maintained in a microwave heated to 200 °C for 6 min and then cooled naturally to room temperature, washed with deionized water three times, and baked in an oven at 80 °C overnight to obtain the precursor.

Weigh 0.5 g of the precursor on a porcelain boat, place the porcelain boat in the center of the tube furnace, first replace the air in the tube furnace with argon gas, then heat up to 600 °C in 5% hydrogen argon atmosphere for 1 h, the gas flow rate is maintained at 75 mL·min⁻¹, and then cool down to room temperature. The ternary MoWNi alloy catalyst was obtained.

3.3. Synthesis of MoNi, WNi

In the preparation process of precursor, no (NH₄)₁₀W₁₂O₄₁·xH₂O is added; other preparation processes are the same, that is, catalyst MoNi is obtained. In the preparation process of the precursor system, no (NH₄)₆Mo₇O₂₄·4H₂O is added, and other preparation processes are the same; that is, the catalyst WNi is obtained.

3.4. Physical Characterization

Powder X-ray diffraction (XRD) was used to characterize the phase and crystal structure of the material by the X’pert PRO X-ray diffractometer of PANalytical company in Almelo, the Netherlands. Scanning electron microscopy (SEM) was used to characterize the microscopic morphology of the materials using the JSM-7500F scanning electron microscope of Nippon Electronics Co., Ltd. (Huizhou, China). Transmission electron microscope (TEM) was used to characterize the microstructure, crystal structure, and crystal composition of materials by Tecni G20 transmission electron microscope of FEI Company in Hillsboro, OH, USA. X-ray photoelectron spectroscopy (XPS) used the K-alpha 250xi X-ray photoelectron spectroscopy of Thermo Fisher Scientific company in Waltham, MA, USA to characterize the composition, element content, and valence state of the material. Raman spectroscopy uses the HR Evolution Raman spectrometer from HORIBA Scientific in Palaiseau, France to characterize the degree of graphitization of materials and the defects of carbon materials.

The catalysts were tested using inductively coupled plasma (ICP, US-Thermo Fisher-ICAP-RQ/6000, OES/MS, Waltham, MA, USA) instrument to obtain data on the atomic ratios of the catalysts.

The samples were activated under vacuum at 120 °C for 12 h to remove solvent molecules and guest molecules from the samples, after which the samples were tested for N₂ adsorption and desorption using a physical adsorption apparatus at 77 K. The pore structure, specific surface area, and pore size of the materials were calculated using the density flooding theory (NLDFT) and specific surface area test (BET).

3.5. Electrochemical Characterizations

All electrochemical tests in this thesis were carried out at room temperature with a typical three-electrode system using a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) and RRDE-1A rotating disc electrode (4 mm diameter, Jiangsu Taizhou Deyi Analytical Instrument Co., Ltd., Taizhou, China). The three electrodes used were the Ag/AgCl electrode (reference electrode), stone grinding rod (counter electrode), and rotating disk electrode (RDE) (working electrode). The RDE needs to be polished and activated before use. The specific method was as follows: drawing a figure-of-eight pattern on chamois paper with 1.0 μm, 100 nm, and 50 nm polishing powder (alumina powder) for 3 min, then ultrasonic with water for 5 s, and repeating three times, and after that, the electrode was activated in 0.5 M H₂SO₄, and the electrodes were used to remove the sample residues by cycling the electrodes with Ag/AgCl as the reference electrode at a sweep rate of 100 mV·s⁻¹ for four circles in a voltage interval from −1~1 V. The electrodes were also used as reference electrodes to remove the sample residue on the electrode. The electrolyte was 0.1 M KOH during the HOR test and 1.0 M KOH during the HER test, and the Ag/AgCl reference electrode needed to be calibrated before use by using two Pt wires as the working electrode and counter electrode, respectively, and Ag/AgCl as the reference electrode, and the polarization curves were carried out in the 0.1 M KOH and 1.0 M KOH electrolytes. of the electrolyte, and the value of X when Y = 0 is the corrected voltage, (E_{RHE 0.1 M KOH} = E_Ag/AgCl + 0.961 V, E_{RHE 1.0 M KOH} = E_Ag/AgCl + 1.028 V). All potentials in this paper are corrected for reversible hydrogen electrodes. iR is corrected by testing out the impedance Ω in the same environment using a three-electrode system, and the corrected voltage is the measured voltage-current × resistance. That is, E_correction = E_measurement − iR.

3.6. Preparation of Working Electrode

5 mg of catalyst was weighed and added into 460 μL of ethanol, and then 4 μL of Nafion was added and sonicated for 1 h to make the catalyst slurry uniformly dispersed. 2.5 μL of the catalyst slurry was drop-coated onto the glassy carbon electrode three times and dried at room temperature, and the loading amount of the catalyst was about 0.6 mg·cm⁻².

3.7. Electrochemical Measurements for HOR

The HOR test was performed in a 0.1 M KOH electrolyte. Prior to the HOR test, the 0.1 M KOH solution was saturated by passing high-purity hydrogen for 30 min, while high-purity hydrogen was passed all the time during the test to ensure that the solution was in a saturated state. After the solution was saturated, the catalyst was first activated, and the CV cycle was carried out for 20 revolutions until stabilization in the voltage range of −1.2~0.4 V vs. Ag/AgCl, with a sweep rate of 50 mV·s⁻¹. The LSV polarization curves were determined at a sweep rate of 5 mV·s⁻¹ and a voltage range of −0.1~0.15 V vs. RHE. Electrochemical impedance (EIS) tests were performed at an overpotential of 30 mV, an AC amplitude of 5 mV, and a frequency range of 10 kHz~10 mHz. The ECSA was tested with the parameters of voltage range of 0.1~0.2 V vs. RHE and scanning rates of 10, 20, 40, 60, 80, and 100 mV·s⁻¹. The electrochemical stability of the catalysts was tested by two methods: the accelerated degradation method (ADT) and the timed current method. The accelerated aging method was performed by CV cycling the catalyst for 2000 cycles in 0.1 M KOH solution with a scan rate of 50 mV·s⁻¹ and a voltage range of −1.2~0.4 V vs. Ag/AgCl. The LSV curves were compared before and after the cycling to understand the stability of the catalyst during the reaction. The timed current method was used to test the current density of the catalysts with time at a constant voltage of 0.1 V. The anti-CO activity of the catalyst was illustrated by the percentage decrease in current when high-purity hydrogen was exchanged for hydrogen-containing 2000 ppm CO at 200 s during the timed current test.

3.8. Electrochemical Measurements for HER

The HER test was performed in 1.0 M KOH solution. Before the HER test, the 1.0 M KOH solution was saturated by passing nitrogen for 30 min, and nitrogen was passed all the way through the test to ensure that the solution was maintained in the N₂ saturated state. After saturating the solution, the activation was performed first, and the CV cycle was performed for 20 revolutions until stabilized at a voltage range of −1.5~0.9 V vs. Ag/AgCl with a scan rate of 50 mV·s⁻¹. LSV tests were carried out at a scan rate of 5 mV·s⁻¹ and a voltage range of −0.15~0.9 V vs. RHE. The electrochemical stability of the catalysts was tested using two methods: an accelerated aging method and a chronopotential method. The accelerated aging method was performed by CV cycling the catalyst for 2000 cycles in 1.0 M KOH solution with a scan rate of 50 mV·s⁻¹ and a voltage range of −1.5~0.9 V vs. Ag/AgCl, and the LSV curves before and after were compared to understand the stability of the catalyst during the reaction. The chronopotentiometric method was used to test the voltage of the catalyst over time at constant current densities of 10, 20, and 40 mA·cm⁻², respectively.

3.9. Calculation of j_k, j₀, and ECSA

The Koutecky–Levich equation was used to calculate the kinetic current density. According to the Koutecky–Levich equation, the measured overall HOR current density j can be divided into kinetic current density j_k and diffusion current density j_d as follows:

$${\displaystyle \frac{1}{j}}={\displaystyle \frac{1}{j_d}}+{\displaystyle \frac{1}{j_k}}$$

(1)

where j_d of the rotating disk electrode can be expressed by the Levich equation as follows:

$$j_d=0.62nF{D}^{3/2}{v}^{-1/6}{C}_0{w}^{1/2}=B{C}_0{w}^{1/2}$$

(2)

where n is the number of electrons involved in the oxidation reaction; F is Faraday’s constant; D is the diffusion coefficient of the reactants; v is the viscosity of the electrolyte; C₀ is the solubility of H₂ in the electrolyte; B is Levitch’s constant; and ω is the rotational speed.

The j₀ can also be obtained by fitting the linear part of the Tafel plot, where the Butler–Volmer equation can be converted to the Tafel equation as follows:

$$\eta =Log\left(j_0\right)+b\times Log\left(j\right)$$

(3)

The ECSA of the material is often concerned as an important index reflecting the catalytic performance of the material, which is more responsive to the intrinsic activity of the catalytic material. The ECSA is directly proportional to the capacitance of the catalyst’s double layer (C_dl), and the ECSA can be obtained by measuring the C_dl and calculating the C_dl. ECSA is obtained. The method of measuring C_dl in the electrochemical workstation is as follows: choose a 0.1 V electric window in a non-Faraday interval, test the cyclic voltammogram curves with different scanning rates, and plot the current density of the intermediate potential against the scanning rate, and then a straight line is obtained, and the slope of the straight line is the double electric layer capacitance.

$$i_c=v{C}_{dl}$$

(4)

where $i_c$ is the charging and discharging current of the double-layer capacitor; v is the scan rate.

$$ECSA={\displaystyle \frac{C_{dl}}{C_s}}$$

(5)

4. Conclusions

This study mainly investigates the application of MoWNi alloy nanoparticles as efficient bifunctional catalysts in hydrogen evolution and oxidation reactions under alkaline conditions. The ternary MoWNi alloy catalyst, synthesized via microwave-assisted methods followed by annealing, exhibits superior electrocatalytic performance. It achieves a current density of 10 mA·cm⁻² in the HER with only 25 millivolts overpotential, which is lower than 32 mV for commercial 20% Pt/C, and a current density of 3.5 mA·cm⁻² in the HOR with only 100 millivolts overpotential, which is higher compared to commercial 20% Pt/C catalysts (2.7 mA·cm⁻² at the overpotential of 100 mV). Additionally, the catalyst demonstrates excellent stability (the catalyst was tested continuously at 60 mV for 5 h with minimal current damping) and resistance to CO poisoning, making it potentially applicable in practical applications such as fuel cells and water electrolysis technologies. These findings highlight the potential of MoWNi alloy catalysts in advancing the practicality and efficiency of hydrogen-based energy systems.