Self-Standing Mo/MoO₂ Porous Flake Arrays for Efficient Hydrogen Evolution Reaction in High-pH Media

The alkaline hydrogen evolution reaction (HER) is limited by scarce proton availability, resulting in slower reaction kinetics compared to those under acidic conditions. Enhancing the local chemical environment of protons on the catalyst surface can improve the intrinsic reaction kinetics. Here, we design a Mo/MoO₂ metallic heterojunction that creates an acidic-like environment with a proton-rich surface, significantly enhancing HER performance in alkaline electrolytes, as confirmed by in situ spectroscopy and electrochemical analysis. A self-standing Mo/MoO₂ catalytic electrode is fabricated via a controlled pyrolysis-reduction strategy. This electrode exhibits exceptional HER activity, with low overpotentials of 65 mV at 10 mA cm^-2 and 315 mV at 500 mA cm^-2, a Tafel slope of 38.2 mV dec^-1, and stability exceeding 60 h at -300 mA cm^-2 in alkaline solution. The porous flake array structure of the Mo/MoO₂ heterojunctions enhances the adjacent hydronium (H₃O⁺) concentration, resulting in a ΔG_H* value of 0.15 eV and a water dissociation energy barrier of 0.37 eV in an alkaline medium. The successful preparation of a large-area electrode (2 cm × 2 cm) demonstrates the scalability of this approach for fabricating molybdenum-based catalytic electrodes with enhanced HER activity in alkaline environments.