Abstract: The sluggish kinetics of the Volmer step (H₂O + e^- → H* + OH^-) in alkaline hydrogen evolution reaction (HER) and the corrosive chloride ions in seawater pose dual challenges for efficient seawater electrolysis. Here a MoNi₄ alloy/amorphous NiMoO_x (MoNi₄/NiMoO_x) catalyst is synthesized by one-step electrodeposition, delivering an overpotential of only 28 mV at 10 mA cm^-2 and stably running at 500 mA cm^-2 in alkaline natural seawater for 800 h. The electrode can be scaled up to 15 cm×15 cm, and a cut piece (110 cm²) works stably in an industrial electrolyzer at 2 A cm^-2 for 750 h. X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure (XAFS) reveal that MoNi₄/NiMoO_x generates electron-rich low-valent Ni^d+ (0 < d < 1) species through electron transfer from MoNi₄ to NiMoO_x. In situ Raman spectroscopy, bode phase analysis, and local pH measurements demonstrate that this interfacial electronic effect in MoNi₄/NiMoO_x accelerates the Volmer rate-determining step by facilitating interfacial water dissociation, and maintains a lower local pH that is thermodynamically more favorable for HER. Theoretical calculations show that MoNi₄/NiMoO_x possesses a d-band center shifted closer to the Fermi level, which provides stronger H* adsorption, a nearly thermoneutral Volmer dissociation step, and intrinsic Cl^- repulsion, leading to the good stability in seawater. This strategy provides a simple and scalable route to high-performance non-noble metal catalysts for direct seawater electrolysis.

Key words: MoNi₄/NiMoO_x, Alkaline seawater electrolysis, Hydrogen evolution reaction, Volmer step