Electrocatalytic water splitting is a promising method for clean hydrogen production, but typically relies on scarce and expensive noble metal catalysts like Pt/Pd and RuO₂/IrO₂ for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. These catalysts suffer from stability issues and often function optimally only for a single half-cell reaction. Bifunctional catalysts capable of catalyzing both HER and OER are desirable for improved efficiency and system simplification. However, the sluggish kinetics of OER, due to the O-O bond formation, remain a challenge. Heterostructured catalysts have shown promise in improving water electrolysis efficiency by modulating the electronic structure through charge transfer. Most existing heterostructures utilize unidirectional charge transfer, limiting performance optimization. This study aims to develop a highly efficient and robust bifunctional water electrolysis catalyst that overcomes these limitations by employing a novel multidirectional charge transfer concept within a heterostructured design. The catalyst's stability at high current densities is also a key focus, critical for practical industrial applications.

Previous research has extensively explored noble-metal-based electrocatalysts for water electrolysis, highlighting their excellent performance but poor stability and scarcity. Studies have also focused on developing bifunctional catalysts to improve overall water electrolysis performance and simplify system design. Heterostructured catalysts, particularly those employing transition metals or noble metals, have been investigated for morphology control, compositional optimization, and elemental doping to enhance electrocatalytic activity. Charge transfer within the heterostructure is critical for modulating the electronic structure and improving the Faradaic efficiency. However, most studies focus on unidirectional charge transfer, potentially limiting the optimization of electronic structure for both HER and OER. This research builds upon previous work exploring perovskite oxide and transition-metal dichalcogenide heterostructures with unidirectional charge transfer but innovates by introducing a multidirectional charge transfer mechanism for improved performance and stability.

The study synthesized a heterostructured catalyst comprising perovskite oxide (La0.5Sr0.5CoO3-δ, LSC) and potassium ion-bonded molybdenum diselenide (K-MoSe₂). K-MoSe₂ was synthesized via a molten-metal-assisted intercalation method, converting the semiconducting 2H-phase MoSe₂ to metallic 1T-MoSe₂. LSC was synthesized via the sol-gel method. LSC and K-MoSe₂ were then mixed using ball milling at various weight ratios to optimize the heterostructure. The resulting LSC/K-MoSe₂ heterostructure was characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-angle annular dark-field (HAADF) imaging, energy-dispersive spectroscopy (EDS), high-resolution TEM (HR-TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Brunauer-Emmett-Teller (BET) analysis, Raman spectroscopy, ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and UV-Vis-NIR spectroscopy. Electrochemical characterization included linear sweep voltammetry (LSV) for HER and OER, cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and chronopotentiometry. Density functional theory (DFT) calculations were performed to analyze charge transfer and catalytic activity. The optimal LSC/K-MoSe₂ ratio (5:4) was determined based on electrochemical performance. Overall water electrolysis was tested using a two-electrode configuration with LSC/K-MoSe₂ as both cathode and anode.

The LSC/K-MoSe₂ heterostructure exhibited significantly enhanced HER and OER activity compared to individual components. The optimized 5:4 LSC/K-MoSe₂ catalyst showed an overpotential of 128 mV at 10 mA cm⁻² for HER and 230 mV at 10 mA cm⁻² for OER in 1 M KOH, surpassing the performance of IrO₂ for OER. The improved HER and OER kinetics were attributed to the increased electrical conductivity of MoSe₂ and enhanced oxygen intermediate adsorption on the LSC surface, respectively. The complementary charge transfer from LSC and K to MoSe₂ resulted in a high purity 1T-MoSe₂ phase (over 90%), leading to an electron-rich surface and enhanced conductivity. DFT calculations confirmed the complementary charge transfer and its influence on reducing the energy barrier for phase transition and improving HER and OER kinetics. The overall water electrolysis performance of the LSC/K-MoSe₂||LSC/K-MoSe₂ couple outperformed the state-of-the-art Pt/C||IrO₂ couple, requiring lower cell voltages at 10 and 100 mA cm⁻². The LSC/K-MoSe₂||LSC/K-MoSe₂ couple demonstrated exceptional stability, operating for over 2,500 h at 100 mA cm⁻² without significant performance degradation. Characterization after the long-term stability test confirmed the structural and chemical integrity of the catalyst. The energy efficiency of the overall water electrolysis using LSC/K-MoSe₂||LSC/K-MoSe₂ was calculated to be 75.4% at 100 mA cm⁻².

The findings demonstrate the effectiveness of a novel multidirectional charge transfer strategy for designing high-performance and robust bifunctional water electrolysis catalysts. The superior performance of LSC/K-MoSe₂ over noble-metal catalysts stems from the synergistic combination of enhanced HER and OER kinetics. The multidirectional charge transfer within the heterostructure optimizes the electronic structure for both half-reactions. The exceptional long-term stability at high current densities showcases the practical potential of this catalyst for industrial applications. This study provides valuable insights into catalyst design, demonstrating that complementary charge transfer can lead to significant improvements in water electrolysis efficiency and stability.

This work successfully developed a highly efficient and stable noble-metal-free bifunctional catalyst for overall water electrolysis. The multidirectional charge transfer strategy significantly enhances both HER and OER kinetics, surpassing the performance of state-of-the-art Pt/C||IrO₂. The exceptional long-term stability of this catalyst opens new avenues for developing cost-effective and sustainable hydrogen production technologies. Future research could explore further optimization of the catalyst composition and structure, and investigate its performance under different operating conditions and electrolytes.

While the LSC/K-MoSe₂ catalyst demonstrates exceptional performance and stability, future studies could address potential scalability challenges associated with the synthesis methods. The study primarily focused on alkaline conditions; further research is needed to investigate its performance in acidic or neutral electrolytes. Further investigation into the long-term degradation mechanisms could also be explored to further improve the catalyst's durability.