Self-Standing Mo/MoO<sub>2</sub> Porous Flake Arrays\nfor Efficient Hydrogen Evolution Reaction in High-pH Media

The alkaline hydrogen evolution reaction (HER) is limited\nby scarce\nproton availability, resulting in slower reaction kinetics compared\nto those under acidic conditions. Enhancing the local chemical environment\nof protons on the catalyst surface can improve the intrinsic reaction\nkinetics. Here, we design a Mo/MoO<sub>2</sub> metallic heterojunction\nthat creates an acidic-like environment with a proton-rich surface,\nsignificantly enhancing HER performance in alkaline electrolytes,\nas confirmed by in situ spectroscopy and electrochemical analysis.\nA self-standing Mo/MoO<sub>2</sub> catalytic electrode is fabricated\nvia a controlled pyrolysis-reduction strategy. This electrode exhibits\nexceptional HER activity, with low overpotentials of 65 mV at 10 mA\ncm<sup>–2</sup> and 315 mV at 500 mA cm<sup>–2</sup>, a Tafel slope of 38.2 mV dec<sup>–1</sup>, and stability\nexceeding 60 h at −300 mA cm<sup>–2</sup> in alkaline\nsolution. The porous flake array structure of the Mo/MoO<sub>2</sub> heterojunctions enhances the adjacent hydronium (H<sub>3</sub>O<sup>+</sup>) concentration, resulting in a Δ<i>G</i><sub>H*</sub> value of 0.15 eV and a water dissociation energy barrier\nof 0.37 eV in an alkaline medium. The successful preparation of a\nlarge-area electrode (2 cm × 2 cm) demonstrates the scalability\nof this approach for fabricating molybdenum-based catalytic electrodes\nwith enhanced HER activity in alkaline environments.