Interfacial engineering of Mo-doped Ni₃S₂/FeNi₂S₄ heterostructures for durable industrial level-current-density AEM water electrolysis

Abstract

Developing efficient non-noble-metal-based electrocatalysts is vital for cost-effective energy conversion technologies. Anion exchange membrane water electrolyzers (AEMWEs) are emerging as a promising platform for green hydrogen production due to their ability to operate in alkaline media with low-cost catalyst materials. In this study, we designed and synthesized a Mo-doped Ni₃S₂/FeNi₂S₄ hybrid nanocomposite as a high-performance oxygen evolution reaction (OER) anode for AEMWE. Experimental and theoretical analyses reveal that Mo incorporation into the Ni₃S₂/FeNi₂S₄ hybrid triggers interfacial charge redistribution, optimizing hydroxide adsorption, modulating active sites, and enhancing catalytic kinetics. The Mo-doped Ni₃S₂/FeNi₂S₄ electrode delivers an overpotential of 220 mV at 50 mA cm⁻² in 1.0 M KOH (without iR compensation). It exhibits a low Tafel slope of 41.7 mV dec⁻¹ with excellent long-term stability over 50 h in half-cell OER testing. When implemented as the anode in a single-cell AEMWE with a Pt/CC cathode, it achieves cell voltages of 1.66, 1.85, 1.98, and 2.18 V at 1, 2, 3, and 5 A cm⁻², respectively, at 60 °C, corresponding to theoretical energy consumptions of 45.2–58.0 kWh kg⁻¹ H₂ and voltage efficiencies of 86.5–67.4% (assuming 100% H₂ selectivity). Over 200 hours of continuous operation at 0.5 A, the cell voltage increased gradually from ≈1.65 V to ≈1.80 V, with the electrode retaining ∼91.7% of its initial performance, underscoring its robust structural and interfacial stability under prolonged alkaline conditions. These results highlight the potential of Mo-doped Ni₃S₂/FeNi₂S₄ as a low-cost, high-performance anode for practical AEM water electrolysis, with further device-level optimization and direct hydrogen quantification planned for future studies.

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1. Introduction

Advancing efficient technologies for green hydrogen production is crucial for establishing a sustainable hydrogen economy.^1–4 Electrochemical water splitting, which encompasses the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode, is among the most promising and scalable methods for hydrogen generation using renewable energy sources.^5–9 However, the sluggish kinetics of the OER due to complex multi-electron transfer processes pose a significant challenge, often accounting for over 90% of the total energy input required during electrolysis.¹⁰ Anion exchange membrane (AEM) water electrolysis is gaining increasing attention as a next-generation technology for green hydrogen production, combining the advantages of traditional alkaline and proton exchange membrane (PEM) electrolyzers.^11–17 AEM systems enable the use of non-precious metal catalysts in a highly alkaline environment, thereby reducing the overall system cost.^18,19 Nonetheless, the development of efficient and durable bifunctional electrocatalysts that can operate under realistic AEM conditions and deliver high current densities remains a formidable challenge. Achieving long-term stability and low overpotentials for both HER and OER in alkaline environments is critical to the wide-scale deployment of AEM water electrolyzers.^20–22

Recent progress in developing advanced anode catalysts has significantly advanced the practical deployment of anion exchange membrane water electrolyzers (AEMWEs). For instance, Kaplan et al. reported Co₀.₆₆Fe₀.₃₄ layered double hydroxides (LDH) coupled with pristine and vacancy-engineered V₂CT_x MXenes, where vacancy engineering promoted enhanced charge redistribution and stabilized highly oxidized Co species during OER.²³ Their optimized composite demonstrated a low overpotential of 304 mV at 10 mA cm⁻² and improved cell voltage in AEM electrolyzer testing, underscoring the role of MXene-derived structures in boosting catalytic activity and stability. Similarly, Du et al. developed a corrosion-resistant NiFe LDH electrode that sustained industrial-level current density (1.0 A cm⁻²) with only 200–220 mV overpotential in simulated and natural seawater.²⁴ A dense interlayer structure prevented chloride penetration, enabling the catalyst to operate stably for over 9000 hours, marking a breakthrough in addressing Cl⁻-induced degradation for seawater electrolysis. More recently, Wang et al. integrated NiFe sulfides with Ti₃C₂ MXene to construct (Ni, Fe)S₂@Ti₃C₂, which leveraged strong interfacial bonding to suppress Fe dissolution, trigger lattice oxygen activity, and enhance Cl⁻ resistance.²⁵ This catalyst enabled AEM seawater electrolysis at industrially relevant current densities (0.5 A cm⁻²) with prolonged stability (500 h), achieving 70% efficiency and low energy consumption (48.4 kWh kg⁻¹ H₂). Collectively, these works highlight diverse strategies—vacancy engineering, corrosion shielding, and MXene–sulfide integration—that significantly improve the robustness and performance of AEM anodes.

In recent years, a wide range of materials, including borides, carbon-based materials, phosphides, nitrides, metal oxides/hydroxides, sulfides, and selenides, have been explored as electrocatalysts for the oxidation of various organic compounds and water splitting reactions.^26–28 Bimetallic and trimetallic catalysts, which often act as bifunctional precatalysts, outperform monometallic catalysts because of their thermodynamic instability and the ability to undergo self-reconstruction in alkaline electrolysis. Transition-metal sulfides, such as those based on Ni and Co, have shown significant potential for the HER, although their OER performance is often hindered by limited active sites. On the other hand, NiFe composites excel in OER activity but are less effective for the HER, with ongoing debates about the exact roles of Ni and Fe as active sites.²⁹ Recent research highlights NiFe sulfides as promising OER catalysts, with sulfur species playing a key role in their catalytic properties, although their precise contribution remains unclear. NiFe sulfides (e.g., Ni₃S₂, Ni₂S₄, NiS, FeS, FeS₂) stand out for their intrinsic electrochemical stability, high conductivity, and durability, but their HER activity is limited by weak adsorption of hydrogen intermediates due to insufficient active sites.^30–33 Strategies like electronic structure modification, surface morphology adjustments, and heteroatom doping (e.g., with Co, W, Fe, or Mo) have been employed to enhance catalytic performance.^29,34–36 Mo doping, in particular, improves the electronic structure and conductivity of materials, while the formation of heterostructures increases active surface sites, enhancing both HER and OER activities.³⁷ These advancements in designing bifunctional electrocatalysts by creating heterostructure interfaces and optimizing charge/electron transfer rates have opened new possibilities for developing efficient catalysts for water splitting.

In this study, interfacial engineering was employed to synthesize Mo-doped Ni₃S₂/FeNi₂S₄ nanosheets through a two-step hydrothermal route, yielding a structurally stable 2D hybrid nanocomposite. The incorporation of Mo into the Ni₃S₂/FeNi₂S₄ framework induces charge redistribution, which optimizes hydroxide adsorption, modulates active sites, and enhances electron transfer kinetics. As a result, the Mo-doped Ni₃S₂/FeNi₂S₄ catalyst exhibits excellent OER activity, requiring only 220 mV overpotential at 50 mA cm⁻² with a low Tafel slope of 41.7 mV dec⁻¹, surpassing the undoped counterparts. When integrated into an anion exchange membrane water electrolyzer (AEMWE) with a Pt/CC cathode, the electrode achieves cell voltages of 1.66, 1.85, 1.98, and 2.18 V at 1, 2, 3, and 5 A cm⁻², respectively, at 60 °C, corresponding to theoretical energy consumptions of 45.2–58.0 kWh kg⁻¹ H₂ and voltage efficiencies of 86.5–67.4%. Moreover, the cell retains ∼91.7% of its initial performance over 200 h, with only a gradual increase in voltage from ≈1.65 V to ≈1.80 V, confirming the robust structural and interfacial stability of the catalyst under prolonged alkaline operation. This work introduces an effective strategy to enhance the catalytic performance of Ni–Fe sulfide electrocatalysts through Mo doping, advancing their application as durable and high-performance anodes for practical AEM water electrolysis.

2. Experimental section

2.1. Preparation of Mo-doped NiFe-LDH

Initially, a Ni foam piece measuring 1 × 4 cm² was cleaned by ultrasonic treatment in hydrochloric acid, deionized (DI) water, and ethanol, each for 5 min, followed by drying at 45 °C in a hot air oven. For the preparation of Mo-doped NiFe-LDH, 3 mmol of nickel(II) nitrate hexahydrate, 3 mmol of iron(III) nitrate nonahydrate, 0.5 mmol of sodium molybdate dihydrate, 6 mmol of urea, and 2.4 mmol of ammonium fluoride were added to 30 mL of DI water and stirred for 30 minutes. Subsequently, the clear solution mixture was transferred into a 50 mL Teflon-lined stainless-steel autoclave. The cleaned Ni foam was then placed in the autoclave, which was sealed and heated to 120 °C for 6 h. After heating, the autoclave was allowed to cool to room temperature. The resulting Mo-doped NiFe-LDH grown on the Ni foam was thoroughly washed multiple times with DI water and methanol, then dried at 60 °C for 12 h. Finally, the sample was sealed and stored for further characterization. For comparison, NiFe-LDH was synthesized following the same procedure as that used for the Mo-doped NiFe-LDH, except that sodium molybdate dihydrate was not added. In addition, the remaining product in the vessel was collected using a centrifuge and washed with DI water and methanol. The powdered sample was dried and used for Brunauer–Emmett–Teller (BET) analysis.

2.2. Preparation of Mo-doped Ni₃S₂/FeNi₂S₄

0.09 g of sodium sulfide nonahydrate was dissolved in 30 mL of DI water and transferred to a Teflon-lined stainless-steel autoclave. Subsequently, the above-synthesized Mo-doped NiFe-LDH deposited on Ni foam (NF) was immersed in the autoclave containing the sulfur solution and the autoclave was sealed and heated to 90 °C for 9 h. After cooling down to room temperature, samples were taken out, washed with water and methanol, and dried at 60 °C for 12 h. Ni₃S₂/FeNi₂S₄ was prepared following the same procedure using a NiFe-LDH sample instead of Mo-doped NiFe-LDH. The synthesis parameters were optimized to achieve the best OER performance. Mo doping concentration was varied from 0.1 to 0.75 mmol Na₂MoO₄·2H₂O, and the optimal performance was obtained for 0.5 mmol, which provided a balance between active-site density and charge-transfer ability (Fig. S1). Lower Mo content resulted in insufficient electronic modulation, while higher doping led to partial site blockage and reduced conductivity. The sulfidation temperature and duration were also optimized based on preliminary trials and literature guidance, and a condition of 90 °C for 9 h yielded phase-pure sulfide without sulfate formation.^38,39 The catalyst loading was determined from the weight difference between the bare Ni foam and the foam after synthesis, following drying at 60 °C. The total loading mass of all samples ranged between 4.8 mg and 5.0 mg, ensuring consistent active material amounts across all electrodes for reliable electrochemical comparison.

2.3. Physical characterization

The crystal phase and nanostructure of the synthesized samples were analyzed using X-ray diffraction (XRD) with a Rigaku Ultima IV system equipped with a Cu Kα radiation source (D/MAX-2500). The morphology and microstructure were examined using scanning electron microscopy (SEM, TESCAN MIRA3 LM). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were acquired using a JEOL JEM-2100F instrument at an accelerating voltage of 300 kV. The oxidation states of the elements in the samples were investigated through X-ray photoelectron spectroscopy (XPS) performed with a Thermo Fisher Scientific NEXSA G2 system at the High-Tech Analysis Core Facility, GNU. Additionally, the Brunauer–Emmett–Teller (BET) surface area and pore-size distribution of all synthesized samples were measured using an Autosorb iQ₂ automated gas sorption analyzer (Quantachrome Instruments, USA). Nitrogen adsorption–desorption isotherms were recorded at 77 K, and all samples were degassed under vacuum at 100 °C for 10 h before analysis to remove surface-adsorbed species.

2.4. Electrochemical measurements

All electrochemical measurements for OER were conducted using a Zive SP1 electrochemical workstation (CH Instruments, Inc., USA) using a standard three-electrode setup. Binder-free Mo-doped NiFe-LDH, Mo-doped Ni₃S₂/FeNi₂S₄, and other comparison catalysts (with an active area of 1 cm²) were used as the working electrodes. An Ag/AgCl electrode and platinum wire served as the reference and counter electrodes, respectively. Before all electrochemical measurements, each electrode was activated by performing cyclic voltammetry (CV) scans in the potential range of 0.0–0.6 V (vs. Hg/HgO) at a scan rate of 10 mV s⁻¹ for 10 consecutive cycles in 1.0 M KOH. This activation step ensured surface stabilization and reproducible electrochemical performance during subsequent OER testing. The linear sweep voltammograms (LSV) were measured at a scan rate of 1 mV s⁻¹ in Ar-saturated 1 M KOH solution for OER. The electrolyte used for all electrochemical measurements was 1.0 M KOH aqueous solution prepared from ≥99% pure KOH pellets (Sigma-Aldrich) and deionized water (18.2 MΩ cm). The measured pH of this solution is approximately 13.8 ± 0.1 at 25 °C, consistent with the expected value for 1 M alkaline electrolyte. All recorded potentials were converted to the reversible hydrogen electrode (RHE) scale using the following equation:

E_RHE = E_Ag/Agcl + 0.197 + 0.059 × pH (1)

Electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range of 100 kHz to 0.1 Hz, using an excitation amplitude of 10 mV. Cyclic voltammetry (CV) measurements were conducted at different scan rates (4, 6, 8, 10, 12, and 14 mV s⁻¹) within the non-faradaic potential range (0.1 to 0.2 V vs. Ag/AgCl) on the electrode. The electrochemically active surface area (ECSA) of various electrode materials was evaluated by comparing their double-layer capacitance values (C_dl). To determine C_dl, the difference between the anodic and cathodic current densities was plotted against the scan rate, and the slope of the resulting linear fit was used to extract C_dl. The corresponding equation is given as follows:

where I is the current density (mA cm⁻²) and ν is the scan rate (mV s⁻¹).

In the AEM water electrolysis single-cell setup, a PiperION AEM membrane (Versogen, U.S.) was assembled between a Mo-doped Ni₃S₂/FeNi₂S₄ anode (5 cm²) and a Pt/C@CC cathode, both of which were used in a catalyst-coated substrate (CCS) configuration rather than a conventional membrane electrode assembly (MEA).

2.5. Computational details

Density functional theory (DFT) calculations were performed to investigate the effects of Mo doping on the properties of the Ni₃S₂ and FeNi₂S₄ structures. All the DFT calculations were performed using the Cambridge Serial Total Energy Package (CASTEP).⁴⁰ The generalized gradient approximation with the Perdew–Burke–Ernzerhof functional was applied for the exchange–correlation energy with spin polarization.⁴¹ For the cell and geometry optimizations, the on-the-fly generated norm-conserving and Tkatchenko-Scheffler methods were used for the pseudopotential and dispersion corrections, respectively.⁴² The two-point steepest descent algorithm was used for the cell optimizations of angle-constrained unit cells of trigonal phase Ni₃S₂ (space group: R32) and cubic phase FeNi₂S₄ (space group: Fd3̄m).⁴³ The energy cut-off value was 1252 eV. The convergence criteria for energy, force, stress, displacement, and self-consistent field were set to 2 × 10⁻⁵ eV per atom, 0.05 eV Å⁻¹, 0.1 GPa, 0.002 Å, and 2 × 10⁻⁶ eV per atom. Furthermore, 4 × 4 × 4 k-points and 8 × 8 × 6 k-points were utilized with a Monkhorst–Pack grid for the cell optimizations of the unit cell structures of Ni₃S₂ and FeNi₂S₄, respectively.⁴⁴ For the surface systems, a 1 × 1 × 1 k-point was used. Mulliken charge was used to analyze the atomic charges. The doping formation energies of Mo-doped Ni₃S₂ and FeNi₂S₄ were calculated using the following equations:

Doping formation E = E_Mo-doped − [E_undoped − µ_Fe (or µ_Ni) + µ_Mo]

where E_Mo-doped and E_undoped are the total energies of Ni₃S₂ or FeNi₂S₄ with and without Mo-doping, respectively. µ_Fe, µ_Ni and µ_Mo are the chemical potentials of Fe, Ni, and Mo atoms based on their unit cell structures.

3. Results and discussion

3.1. Synthesis and structural characterization

The synthesis of Mo-doped Ni₃S₂/FeNi₂S₄ composites is illustrated in Scheme 1, outlining a two-step hydrothermal process. First, Mo-doped NiFe-LDH nanosheet arrays are fabricated on NF. Second, sulfurization is performed using sodium sulfide nonahydrate, converting the Mo-doped NiFe-LDH to Mo-doped Ni₃S₂/FeNi₂S₄. The morphology and structure of a catalyst significantly affect its performance. Moreover, the design and architecture of nanomaterials with unique morphologies can significantly enhance their electrochemical performance. As shown in Fig. S2(a–c), the NiFe-LDH nanosheets are vertically aligned on the surface of the NF. Upon the introduction of sodium molybdate dihydrate, the size of the nanosheets decreased Fig. S2(d–f), and after sulfurization, the morphology did not change, as shown in Fig. S2(g–i). The SEM energy dispersive X-ray spectroscopy (EDS) spectral mapping of Mo-doped Ni₃S₂/FeNi₂S₄ (Fig. S3) confirms the incorporation of Mo into the Mo–Ni₃S₂/FeNi₂S₄ electrode. Additionally, smaller, thinner sheets increase the surface area, providing more active sites for interaction with the electrolyte, thereby enhancing the utilization of the active materials.

Scheme 1 Schematic illustration of the Mo-doped Ni₃S₂/FeNi₂S₄ hybrid nanocomposite electrode synthesis process.

To further investigate the nanostructures of the synthesized electrocatalysts, TEM and high-resolution TEM (HR-TEM) was used to examine Mo-doped NiFe-LDH and Mo-doped Ni₃S₂/FeNi₂S₄, as presented in Fig. 1. The ultrathin nanosheet structures of the Mo-doped NiFe-LDH and Mo-doped Ni₃S₂/FeNi₂S₄ are visible in Fig. 1a, b, i and j, respectively. The corresponding HR-TEM images reveal a fringe spacing of 0.243 nm, which corresponds to the (015) lattice planes of Mo-doped NiFe-LDH (Fig. 1c), while fringes of 0.20 nm and 0.23 nm correspond to the (110) and (511) lattice planes of Mo-doped Ni₃S₂/FeNi₂S₄ (Fig. 1k), respectively, as confirmed by the standard reference card (PDF#47-1740). Although the HR-TEM images appear slightly less sharp due to the ultrathin and beam-sensitive morphology, the observed lattice fringes and corresponding SAED patterns clearly confirm the crystalline nature of the Mo-doped NiFe-LDH and Mo-doped Ni₃S₂/FeNi₂S₄ nanostructures. Selected area electron diffraction (SAED) patterns are shown in Fig. 1d and l confirm the crystalline nature of both the Mo-doped NiFe-LDH and Mo-doped Ni₃S₂/FeNi₂S₄. In the EDS elemental mapping analysis of Mo-doped NiFe-LDH (Fig. 1e–h) and Mo-doped Ni₃S₂/FeNi₂S₄ (Fig. 1m–p), Ni, Fe, Mo, and O (for Mo-doped NiFe-LDH), and Ni, Fe, Mo, and S (for Mo-doped Ni₃S₂/FeNi₂S₄) were found to be evenly distributed across the samples. However, the percentage of Mo decreased in the Mo-doped Ni₃S₂/FeNi₂S₄ composite after sulfurization compared to that of the Mo-doped NiFe-LDH. This reduction in Mo content can be attributed to the volatilization or loss of Mo species during the sulfurization process.

Fig. 1 (a and b) TEM, (c) HRTEM, (d) SAED, and (e–h) EDS elemental mapping images of Ni, Fe, Mo, and O, respectively for the Mo-doped NiFe-LDH/NF electrode, and (i and j) TEM, (k) HRTEM, (l) SAED, and (m–p) EDS elemental mapping images of Ni, Fe, Mo, and S, respectively, for the Mo-doped Ni₃S₂/FeNi₂S₄ electrode.

The wettability of the synthesized samples was investigated using contact angle measurements (Fig. S4). The bare Ni foam exhibited a high contact angle of 99.04°, indicating its hydrophobic nature. In contrast, all modified samples showed nearly zero contact angles, demonstrating superhydrophilic behavior. This enhanced wettability is crucial for electrochemical applications, as it facilitates better electrolyte penetration and increases the effective contact area between the catalyst and electrolyte, thereby improving the catalytic performance.

The powder XRD patterns of the synthesized NiFe LDHs, Mo-doped NiFe LDHs, Ni₃S₂/FeNi₂S₄, and Mo-doped Ni₃S₂/FeNi₂S₄ electrocatalysts are presented in Fig. 2a. The main diffraction peaks observed can be assigned to NiFe LDHs (PDF#38-0715) and Ni (PDF#04-0850). Specifically, peaks at 11.4, 22.7, 33.4, 34.4, 38.7, 45.9, 60.0, and 61.2° correspond to the (003), (006), (101), (012), (015), (018), (110), and (113) planes of NiFe-LDH/NF and Mo-doped NiFe-LDH. The peaks at 44.9, 51.8, and 76.8° are attributed to the (111), (200), and (220) planes of the NF, respectively. Notably, the intensity of the peaks decreased upon Mo doping of the NiFe LDHs, likely owing to the introduction of lattice distortions and defects caused by the incorporation of Mo atoms.³⁷ The XRD pattern did not display any distinct peaks corresponding to Mo, likely because the highly intense peaks of Ni overshadowed the weaker Mo signals. After the sulfurization process, the peaks corresponding to the (101), (110), (003), (113), and (122) diffraction planes observed at 2θ values of 21.6, 31.2, 38.0, 49.8, and 54.8°, respectively, are characteristic of the Ni₃S₂ phase.^45–47 The additional characteristic peaks at 31.4, 39, 50.2, and 55.4°, corresponding to the (311), (400), (511), and (440) planes, respectively (Fig. 2a), align well with the cubic FeNi₂S₄ phase (JCPDS: 47-1740), confirming the formation of the FeNi₂S₄ phase in the composite.^33,48,49 Owing to the close d-spacings of Ni₃S₂ and FeNi₂S₄, several reflections (e.g., near 31–33°, 37–39°, 44–46°, and 55–56°) appear as partially resolved doublets; the zoomed insets with JCPDS markers (Fig. S5) confirm the coexistence of FeNi₂S₄ ((311), (400), (511), (440)) with neighboring Ni₃S₂ peaks. For clarity, the standard reference diffraction patterns of NiFe-LDH, Ni₃S₂, and FeNi₂S₄ (JCPDS #38-0715, #44-1418, and #47-1740, respectively) are displayed as stick plots beneath the XRD curves in Fig. 2a. Diffraction peaks corresponding to metal sulfides were not prominently visible in the XRD pattern, which can be attributed to the low sulfurization temperature, resulting in the formation of amorphous metal sulfides. In addition, all diffraction peaks of the hybrid nanocomposite electrode were in good agreement with those of the NiFe-LDH (JCPDS: 51-0463), Mo-doped Ni₃S₂ (JCPDS: 44-1418), and Mo-doped FeNi₂S₄ (JCPDS: 47-1704) phases. Furthermore, the evolution of the surface area and mean pore diameter during the synthesis and modification of NiFe-based materials was systematically investigated through nitrogen adsorption–desorption analysis (Fig. 2b and c and S6). The observed increase in surface area from NiFe-LDH to Ni₃S₂/FeNi₂S₄ (13.083 to 18.094 m² g⁻¹) can be attributed to the sulfurization process, which creates a porous structure by replacing O atoms with S. This modification disrupts the crystalline structure of the LDH, leading to enhanced porosity and a slight increase in the surface area. The introduction of Mo into the NiFe-LDH matrix significantly increased the surface area (from 13.083 to 119.13 m² g⁻¹). This increase is likely due to the doping-induced modification of the electronic structure and morphology, which promotes the formation of smaller particles with higher dispersion. Additionally, Mo atoms may induce defects and enhance the porosity within the layered structure. Further sulfurization of Mo-doped NiFe-LDH resulted in the formation of Mo-doped Ni₃S₂/FeNi₂S₄, which exhibited the highest surface area (175 m² g⁻¹). The combination of Mo doping and sulfurization synergistically improves the textural properties of the material by introducing additional structural voids and active sites, thereby maximizing the available surface for catalytic reactions.

Fig. 2 (a) XRD patterns of all synthesized electrodes (reference diffraction peaks of NiFe-LDH (JCPDS 38-0715), Ni₃S₂ (JCPDS 44-1418), and FeNi₂S₄ (JCPDS 47-1740) are shown as stick patterns for comparison), (b) N₂ adsorption–desorption isotherms of Mo-doped Ni₃S₂/FeNi₂S₄, and (c) pore-size distribution plot of Mo-doped Ni₃S₂/FeNi₂S₄.

3.2. DFT studies

DFT calculations were performed to investigate the effects of Mo doping on the catalytic properties of the Ni₃S₂ and FeNi₂S₄ structures. We first constructed model systems of Ni₃S₂ and FeNi₂S₄ surfaces based on the unit cells of trigonal phase Ni₃S₂ (space group: R32) and cubic phase FeNi₂S₄ (space group: Fd3̄m). Based on the dominant surface types inferred from the XRD results, Ni₃S₂ (110) and FeNi₂S₄ (311) surface systems were considered. For the doping of Mo atoms in the Ni₃S₂ and FeNi₂S₄ surface systems, we considered two types of doping regions, namely, surface and inside regions. Moreover, the Mo doping sites were the Ni sites in the Ni₃S₂ system and Fe and Ni sites in the FeNi₂S₄ system. In general, there are two types of spin states for Ni, Fe, and Mo atoms: high- and low-spin states. For high-spin states, the initial spin states of the Ni, Fe, and Mo atoms are +2, +4, and +2, respectively. In contrast, the initial value of the low-spin states of all metal atoms is zero. We considered the two types of spin states to construct model systems of Ni₃S₂ and FeNi₂S₄ surfaces. Based on these considerations, model systems of the Ni₃S₂ and FeNi₂S₄ surfaces were geometrically optimized (Fig. S7). Using these surface systems, we investigated the total atomic charges of each component (i.e., Ni, Fe, and S atoms) before and after doping with a Mo atom (Fig. S8). We found that for the Ni₃S₂ surface systems, the total charges of S and Ni atoms decreased due to Mo doping in the high- and low-spin states, respectively. For the FeNi₂S₄ surface systems, the total charges of Ni and Fe atoms significantly increased by Mo doping under low-spin-states conditions, while the charge of S atoms decreased. High-spin states resulted in only marginal changes in the total charges of Ni, Fe, and S atoms upon Mo doping. Based on these findings, we investigated the surface atomic charges in each system (Fig. 3). Interestingly, Mo doping enhanced the positive charges of Ni and Fe atoms and the negative charges of S atoms on the FeNi₂S₄ surface, unlike those on the Ni₃S₂ surface. Accordingly, we predicted that the catalytic performance of surface atoms of FeNi₂S₄ could be improved by doping with Mo atoms.

Fig. 3 Charges of surface atoms in Ni₃S₂ and FeNi₂S₄ surface systems based on Mo doping, doping sites (i.e., Ni and Fe sites), doping region (i.e., surface and inside regions), and spin states (i.e., high- and low-spin states).

Finally, to investigate the possibility of Mo doping of Ni₃S₂ and FeNi₂S₄ surface systems, we calculated the doping formation energy of each surface system (Fig. S9). We found that all cases showed thermodynamically favorable doping formation energies under conditions of high-spin states. This indicates that Mo atoms can be doped into the Ni₃S₂ and FeNi₂S₄ surface systems, regardless of the doping conditions (i.e., Ni and Fe sites and surface and inside regions). However, for low-spin states, the Mo atom can only be doped in the inside region of the FeNi₂S₄ surface system from the thermodynamic point of view. Overall, we predicted that doping with Mo atoms could regulate the atomic charges of the Ni₃S₂ and FeNi₂S₄ surface systems. In particular, it can enhance the surface atomic charges of FeNi₂S₄ to increase the number of active sites for catalytic reactions.

3.3. XPS studies

Fig. 4 provides insights into the changes in the surface chemical states and electronic interactions, as observed through XPS. The XPS survey spectrum shown in Fig. S10a aligns with the elemental mapping, indicating the presence of Mo, Ni, Fe, and O in Mo-doped NiFe-LDH. Similarly, the XPS survey spectrum confirms the presence of Ni, Fe, S, Mo, and O elements in the Mo-doped Ni₃S₂/FeNi₂S₄ catalyst (Fig. 4a and S10b). The atomic percentages estimated from quantitative XPS analysis are Ni (15.7 at%), Fe (1.74 at%), S (3.86 at%), O (51.6 at%), and Mo (0.58 at%). The small yet distinct Mo content verifies the successful incorporation of Mo species into the Ni₃S₂/FeNi₂S₄ lattice. The low doping ratio is consistent with the precursor concentration (0.5 mmol Na₂MoO₄·2H₂O) and confirms that Mo acts as an electronic promoter rather than a major compositional element. In Fig. 4a, the peaks at 853.9 and 871.5 eV for Mo-doped NiFe-LDH are assigned to Ni²⁺, while the peaks at 855 and 872.7 eV correspond to Ni³⁺. Satellite peaks associated with Ni²⁺ were observed at 860.2 and 878.2 eV. For Mo-doped Ni₃S₂/FeNi₂S₄, the Ni 2p XPS spectrum in Fig. 4a shows Ni²⁺ peaks at 854.4 and 872 eV, with additional peaks at 855.9 and 873.6 eV attributed to Ni³⁺, and satellite peaks located at 860.4 and 878.5 eV. The Ni 2p_3/2, Ni 2p_1/2, and satellite signals in the Mo-doped Ni₃S₂/FeNi₂S₄ sample are shifted to higher binding energies than those of the Mo-doped NiFe-LDH.^33,37,49 In Fig. 4b, the Fe 2p signals display peaks at 711.07 and 724.4 eV, corresponding to Fe³⁺. A pre-peak was observed at 705.1 eV, with satellite peaks from Fe³⁺ at approximately 720.2 and 735.7 eV. Compared to Mo-doped NiFe-LDH, the Fe 2p_3/2 and Fe 2p_1/2 peaks, along with the satellite peaks in Mo-doped Ni₃S₂/FeNi₂S₄, are shifted to higher binding energy.^31,33,48,49Fig. 4c presents the Mo 3d spectra for both Mo-doped NiFe-LDH and Mo-doped Ni₃S₂/FeNi₂S₄, showing peaks at 230.6, 233.8, 231.2, and 233.9 eV, which correspond to the Mo 3d_5/2 and Mo 3d_3/2 states in each material, respectively.^33,37 The Mo 3d peaks in Mo-doped Ni₃S₂/FeNi₂S₄ exhibit a shift to higher binding energies compared to those observed in Mo-doped NiFe-LDH. Additionally, the O 1s spectrum (Fig. 4d, upper) can be deconvoluted into three distinct peaks located at approximately 529, 530, and 530.6 eV, corresponding to the binding energies of O²⁻, OH⁻, and adsorbed water, respectively.^50,51 Notably, among the three peaks, the prominent OH⁻ peak indicates that hydroxide is the predominant form of O present in the Mo-doped NiFe-LDH sample. The relatively weak lattice-oxygen peak reflects the limited amount of metal–oxygen bonds in the LDH framework, where surface hydroxyls and adsorbed species are dominant. Additionally, the high-resolution S 2p spectrum in Fig. 4d (lower) shows peaks at 161 eV and 162.4 eV, which are identified as S²⁻.^33,35,48,49

Fig. 4 High-resolution XPS spectrum of (a) Ni 2p, (b) Fe 2p, (c) Mo 3d, (d) O 1s, and S 2p of Mo-doped NiFe-LDH and Mo-doped Ni₃S₂/FeNi₂S₄ electrodes.

3.4. Electrochemical performance analysis

3.4.1. OER. The oxygen evolution reaction (OER) performance of NiFe-LDH, Ni₃S₂/FeNi₂S₄, Mo-doped NiFe-LDH, and Mo-doped Ni₃S₂/FeNi₂S₄ electrodes was evaluated using a standard three-electrode configuration at a scan rate of 1 mV s⁻¹ in 1.0 M KOH electrolyte. As shown in Fig. 5a, the potentials required to reach a current density of 50 mA cm⁻² were 1.52, 1.50, 1.46, and 1.45 V (vs. RHE) for NiFe-LDH, Ni₃S₂/FeNi₂S₄, Mo-doped NiFe-LDH, and Mo-doped Ni₃S₂/FeNi₂S₄, respectively, corresponding to overpotentials of 290, 270, 230, and 220 mV. These results indicate that Mo doping significantly enhances the intrinsic catalytic activity of both NiFe-LDH and Ni₃S₂/FeNi₂S₄, with Mo-doped Ni₃S₂/FeNi₂S₄ showing the best performance. To gain insight into reaction kinetics, Tafel slope analysis was carried out (Fig. 5b). Mo-doped Ni₃S₂/FeNi₂S₄ demonstrated a Tafel slope of just 41.7 mV dec⁻¹, which is lower than those of Mo-doped NiFe-LDH (44.3 mV dec⁻¹), Ni₃S₂/FeNi₂S₄ (53.7 mV dec⁻¹), and NiFe-LDH (63 mV dec⁻¹). The lower Tafel slope suggests faster OER kinetics, likely due to a synergistic effect between Mo doping and the unique hybrid structure of the Ni₃S₂/FeNi₂S₄ composite. Compared to the catalysts summarized in Table S1, the present activity demonstrates superior performance. Electrochemical impedance spectroscopy (EIS) was also performed to assess the charge transfer properties at the electrode–electrolyte interface. As shown in the Nyquist plots (Fig. 5c), the Mo-doped Ni₃S₂/FeNi₂S₄ electrode exhibited the lowest charge transfer resistance (R_ct) of 0.128 Ω, compared to 0.255 Ω for Mo-doped NiFe-LDH, 0.271 Ω for Ni₃S₂/FeNi₂S₄, and 0.312 Ω for NiFe-LDH.^52,53 This suggests more efficient electron transport across the interface, facilitated by the nanosheet morphology, enhanced conductivity, and increased electrochemical surface area of the Mo-doped hybrid structure. The EIS was fitted using the equivalent circuit R_s − [(R1‖Q1)] − {(R2 − Q2)‖Q3}, as shown in Fig. S11. In this model, R_s corresponds to the solution resistance, R1‖Q1 represents the resistance and non-ideal capacitance of the surface film and double-layer interface, R2–Q2 describes the charge-transfer process at the electrode–electrolyte interface, and Q3 accounts for the low-frequency diffusion or adsorption of reaction intermediates. The fitted results revealed that the Mo-doped Ni₃S₂/FeNi₂S₄ electrode exhibited the lowest charge-transfer resistance (R₂) among all samples, confirming its superior electron-transfer kinetics and faster OER reaction rate. These results are consistent with the observed overpotential and Tafel slope trends, validating the role of Mo incorporation in enhancing interfacial conductivity and catalytic efficiency. To assess operational robustness, stepwise stability tests were conducted at increasing current densities from 60 to 150 mA cm⁻², with each step maintained for 1 h (Fig. 5d). The Mo-doped Ni₃S₂/FeNi₂S₄ electrode exhibited stable potential at each step, confirming excellent potential retention under varying operating loads. Long-term durability was further evaluated by chronopotentiometry at a constant current density of 50 mA cm⁻² over 50 hours (Fig. 5e). The Mo-doped Ni₃S₂/FeNi₂S₄ electrode showed outstanding stability with negligible degradation in performance as can be seen in Fig. 5f, underscoring its potential for practical implementation in AEM water electrolysis systems.

Fig. 5 (a) LSV curves, (b) Tafel plots, (c) Nyquist plots of all synthesized samples, (d) step stability, (e) long-term stability, and (f) before and after stability LSV curve of Mo-doped Ni₃S₂/FeNi₂S₄ electrode.

As shown in Fig. S12, the TEM results confirmed that the ultrathin nanosheet structure of Mo-doped Ni₃S₂/FeNi₂S₄ remained largely intact after continuous electrolysis for the OER. However, elemental mapping revealed a decrease in Mo content after continuous electrolysis. This reduction could be attributed to the loss of Mo species during electrochemical cycling, possibly due to the migration or dissolution of Mo into the electrolyte or its conversion into less stable phases under the reaction conditions. XPS measurements showed that while the chemical states of Ni and Fe remained largely unaffected, Mo and S were found in lower amounts in the final sample after electrolysis. As shown in Fig. S13, after the OER durability test, the characteristic Mo 3d and S 2p peaks observed in the pristine sample (Fig. 4) became notably weaker or disappeared. This indicates significant surface reconstruction during OER, where Mo–S species were oxidized to higher-valent Mo–O states or partially leached, and S²⁻ species were converted into sulfate/oxyhydroxide moieties. Such transformations are consistent with the formation of a Ni/Fe/Mo-oxyhydroxide surface layer, which serves as the true active phase during OER. These observations confirm that the Mo dopant not only tunes the initial electronic environment but also promotes a favorable reconstruction pathway, enhancing catalytic activity and durability.

The electrochemical surface area (ECSA) for the OER was estimated to be using the corresponding double-layer capacitance (C_dl) values obtained from cyclic voltammetry (CV) measurements (Fig. 6a and S14).^54,55 Among the studied samples, the Mo-doped Ni₃S₂/FeNi₂S₄ electrode exhibited the highest C_dl value of 4 mF cm⁻², which was notably higher than that of Mo-doped NiFe-LDH (2.26 mF cm⁻²), Ni₃S₂/FeNi₂S₄ (1.66 mF cm⁻²), and NiFe-LDH (1.18 mF cm⁻²). This suggests that the superior electrocatalytic performance of the Mo-doped Ni₃S₂/FeNi₂S₄ electrode is primarily due to the greater number of catalytically active sites. The mass activity of the synthesized catalysts for the OER was also evaluated to understand their intrinsic catalytic efficiency.^56,57 As shown in Fig. 6b, the Mo-doped Ni₃S₂/FeNi₂S₄ catalyst exhibited the highest mass activity of 9.126 mA mg⁻¹, outperforming Mo-doped NiFe-LDH (6.718 mA mg⁻¹), Ni₃S₂/FeNi₂S₄ (6.6 mA mg⁻¹), and NiFe-LDH (5.027 mA mg⁻¹). All electrodes possessed nearly identical mass loadings (≈4.8–5.0 mg), confirming that the superior performance of the Mo-doped Ni₃S₂/FeNi₂S₄ catalyst arises from its intrinsic activity rather than mass variation. Turnover frequency (TOF) is another important parameter used to evaluate catalytic performance.⁵⁸ TOF values were calculated across a range of potentials (1.35 V to 1.7 V vs. RHE) for the GOR. Among the studied catalysts, the Mo-doped Ni₃S₂/FeNi₂S₄ sample exhibited the highest TOF of 9.49 s⁻¹ at 1.7 V (vs. RHE). This superior TOF value highlights its enhanced catalytic activity compared to the other variants (Fig. 6c and d, and S15), thus demonstrating its potential for efficient electrochemical applications.

Fig. 6 (a) C_dl plots, (b) mass activity values, (c) reverse-sweep CV of Mo-doped Ni₃S₂/FeNi₂S₄ for charge integration and calculation of electrochemically accessible sites, (d) TOF values at different potentials for Mo-doped Ni₃S₂/FeNi₂S₄. The inset shows the TOF values of NiFe-LDH, Ni₃S₂/FeNi₂S₄/NF, Mo-doped NiFe-LDH, and Mo-doped Ni₃S₂/FeNi₂S₄ at 1.7 V vs. RHE.

3.4.2. AEMWE performance. A two-electrode electrolyzer using Mo-doped Ni₃S₂/FeNi₂S₄ anode and Pt/CC cathode was assembled to access the industrial performance at 60 °C in 1 m KOH electrolyte (Fig. 7a). The optimal Pt/CC‖Mo-doped Ni₃S₂/FeNi₂S₄ exhibits notable industrial activities in AEMWE, achieving a current density of 3 A at 1.98 V, which are superior to Pt/CC‖Ni₃S₂/FeNi₂S₄ (2.7 A), Pt/CC‖Mo-doped NiFe-LDH (2 A), Pt/CC‖NiFe-LDH (2 A), and Pt/CC‖RuO₂ (2.21 A). At 60 °C, the Mo-doped Ni₃S₂/FeNi₂S₄‖Pt/CC electrode exhibits cell voltages of 1.70, 1.85, 1.98 and 2.18 V at 1, 2, 3 and 5 A cm⁻², respectively (Fig. 7b). To confirm reproducibility, AEMWE measurements were performed on multiple independently synthesized samples, all of which exhibited nearly identical polarization behavior, confirming the reliability of the observed performance (Fig. S16). From the polarization data, we extracted cell voltages at industrially relevant current densities (1, 2, 3, and 5 A cm⁻²) and used them to calculate energy metrics (specific energy consumption, SEC) and efficiencies using the thermoneutral voltage at 60 °C (V_TN ≈ 1.47 V). Table S2 reports the extracted values. At 3 A cm⁻², the Mo-doped Ni₃S₂/FeNi₂S₄ electrode displays the lowest cell voltage among the tested materials (≈1.98 V), with corresponding SEC ≈ 52.8 kWh kg⁻¹ H₂ (≈4.74 kWh Nm⁻³) and a voltage efficiency of ≈74.2% (HHV energy efficiency ≈74%, LHV ≈ 63%). Across the tested range, the Mo-doped Ni₃S₂/FeNi₂S₄ electrode consistently required lower cell voltages than the undoped counterparts and commercial RuO₂, demonstrating the benefit of Mo doping and the Ni₃S₂/FeNi₂S₄ hybrid architecture for overall cell-level operation (full data in Table S2). In this work, FE(H₂) was not determined due to the absence of gas quantification equipment in our current setup. Given that the reaction conditions were optimized for OER in alkaline media with negligible side reactions expected, the measured current density can be considered a reasonable indicator of H₂ evolution. Nevertheless, direct FE(H₂) measurement is important for complete device-level evaluation and will be included in future work. From the polarization slopes in the 2–5 A cm⁻² range, we estimate the area-specific resistance (ASR) using ΔV/Δj. For Mo-doped Ni₃S₂/FeNi₂S₄ we find ASR ≈ 0.11 Ω cm², lower than the ASR estimated for the other electrodes (see Table S2). The low ASR indicates reduced ohmic losses in the cell and helps to explain the relatively low cell voltages at high current densities.

Fig. 7 (a) Schematic diagram and reaction flow chart of the AEMWE electrolyzer; (b) polarization curves of the Pt/CC‖Mo-doped Ni₃S₂/FeNi₂S₄ and other couples operating at 60 °C in 1.0 M KOH; (c) chronopotentiometry of Pt/CC‖Mo-doped Ni₃S₂/FeNi₂S₄, and Pt/CC‖RuO₂ at 0.5 A cm⁻² and 60 °C over 200 h and 63 h respectively.

The long-term potentiometric test indicates that the Mo-doped Ni₃S₂/FeNi₂S₄ electrode retained approximately 91.7% of its initial performance over 200 h, demonstrating excellent stability under prolonged alkaline operation (Fig. 7c). The gradual increase in cell voltage from ≈approximately 1.65 V to ≈approximately 1.80 V corresponds to an average rate of +0.75 mV h⁻¹, highlighting the robust structural and interfacial integrity of the catalyst. When the benchmark precious catalyst RuO₂ was employed, a rapid initial increase in cell potential was observed. Although the potential stabilized afterward, the stability test was terminated after 63 hours of operation due to the early rise in potential. As summarized in supplementary Table S3, the obtained activities and efficiencies are superior to those of currently reported electrolyzers.

4. Mechanism and origin of enhanced activity

The superior catalytic activity of the Mo-doped Ni₃S₂/FeNi₂S₄ hybrid can be attributed to both the nature of the active species and the favorable electrode properties that arise from its tailored structure. DFT calculations reveal that Mo doping induces charge redistribution within the Ni₃S₂ and FeNi₂S₄ lattices, particularly enhancing the positive charges on Ni/Fe atoms and negative charges on S atoms at the FeNi₂S₄ surface. This electronic restructuring optimizes the binding energies of reaction intermediates, enabling more favorable adsorption/desorption during the oxygen evolution reaction (OER). In other words, Mo acts as an electronic modulator, tuning the d-band states of Ni and Fe centers so that they can more effectively cycle between oxidation states. These tuned active species, stabilized under operating conditions, are thus primarily responsible for the improved catalytic turnover. Complementary characterization supports the origin of this enhanced performance. The superhydrophilic nature of the Mo-doped hybrid (near-zero contact angle) ensures efficient electrolyte penetration and intimate contact with catalytic sites. Electrochemical impedance spectroscopy shows a drastically reduced charge-transfer resistance, indicating rapid electron transport across the electrode–electrolyte interface. Furthermore, the enlarged electrochemical surface area, inferred from the high double-layer capacitance, confirms the presence of a dense population of accessible active sites. The outstanding mass activity indicates that these sites are not only abundant but also highly efficient on a per-mass basis, while the superior turnover frequency demonstrates the intrinsic capability of each site to accelerate the OER. Together, the electronic modulation provided by Mo doping (creation of optimized active species) and the favorable electrode characteristics (wettability, conductivity, high active-site density, and intrinsic activity) act synergistically. This dual contribution explains why the Mo-doped Ni₃S₂/FeNi₂S₄ hybrid achieves industrially relevant current densities and remarkable durability in AEM water electrolysis.

5. Conclusion

In this study, a Mo-doped Ni₃S₂/FeNi₂S₄ hybrid nanocomposite was successfully fabricated on Ni foam using a facile two-step hydrothermal approach and evaluated as an efficient oxygen evolution reaction (OER) catalyst for anion exchange membrane water electrolysis (AEMWE). DFT calculations revealed that Mo incorporation induces charge redistribution within the Ni₃S₂/FeNi₂S₄ framework, optimizing hydroxide adsorption and tuning the local electronic environment. This effect enhances the availability of active sites, conductivity, and overall catalytic performance in alkaline media. The Mo-doped Ni₃S₂/FeNi₂S₄ electrode exhibited excellent OER activity, requiring only 220 mV overpotential at 50 mA cm⁻² with a low Tafel slope of 41.7 mV dec⁻¹, outperforming the undoped counterparts. Electrochemical impedance analysis further confirmed reduced charge-transfer resistance, consistent with improved reaction kinetics. When integrated into a single-cell AEMWE with a Pt/CC cathode, the catalyst achieved cell voltages of 1.66, 1.85, 1.98, and 2.18 V at 1, 2, 3, and 5 A cm⁻², respectively, at 60 °C, corresponding to theoretical energy consumptions of 45.2–58.0 kWh kg⁻¹ H₂ and voltage efficiencies of 86.5–67.4%. Moreover, the long-term chronopotentiometric test at 0.5 A demonstrated a gradual voltage increase from ≈1.65 V to ≈1.80 V over 200 h, with the electrode retaining ∼91.7% of its initial performance, highlighting its excellent durability and interfacial stability under prolonged alkaline operation. These results underscore the effectiveness of Mo doping and hybrid heterostructure engineering in enhancing the catalytic performance of Ni–Fe sulfides, providing a promising pathway for the development of robust, non-noble-metal-based electrocatalysts for practical AEM water electrolysis and sustainable hydrogen production.

Author contributions

Komal. Patil: conceptualization, methodology, writing – original draft. Jiyoon. Lee: data curation, formal analysis. Seyeon. Cho: visualization. Dhanaji. Malavekar: writing – review & editing. Pravin. Babar: visualization. Ruturaj. Jadhav: formal analysis. Daim. Choi: data curation. Tae. Kyung Lee: formal analysis. Nochang. Park: supervision. Dong-Won. Kang: supervision. and Jongsung. Park: supervision, project administration, funding acquisition.

Conflicts of interest

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

Data availability

The author confirms that the data supporting the findings of this study are available in the article and its supplementary information (SI). Supplementary information: additional experimental details, synthesis optimization data, XRD reference matching, extended XPS spectra, EIS circuit fitting, replicate AEMWE performance curves, TEM/EDS mapping images, and supporting DFT results. See DOI: https://doi.org/10.1039/d5ta08510a.

Acknowledgements

J. P. acknowledges the support of the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (RS-2023-00257494). T. K. L. acknowledges the support of Human Resources Development of Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by Ministry of Trade, Industry and Energy of Korea (No. RS-2024-00398425). N. P. acknowledges the support of the Korea Electronics Technology Institute (KETI) grant. This work was also supported by the Glocal University 30 Project Fund of Gyeongsang National University in 2025.

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Footnote

† K. Patil and J. Lee contributed equally.

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