Simultaneous Acceleration of Water Dissociation and Hydrogen Desorption via Alloy and Size Effects for Enhanced Kinetics and Stability in Seawater Electrolysis

The rational design of low‐cost, efficient, and corrosion‐resistant electrocatalysts is crucial for hydrogen production via direct seawater electrolysis. Herein, a dual modulation strategy targeting composition and valence states is developed by synthesizing an ultrasmall RuRe alloy catalyst. Comprehensive characterization reveals that incorporating inexpensive Re as a secondary component facilitates the formation of ultrasmall alloys with tunable compositions. Moreover, the enhanced oxophilicity induced by the size effect promotes the spontaneous partial oxidation of the alloy surface. Density functional theory (DFT) calculations confirm that the introduction of Re accelerates the Volmer step, while the generated surface oxidation states not only promote the Tafel step but also suppress Cl − adsorption, thereby enhancing both catalytic activity and long‐term stability in seawater splitting. As a result, the ultrasmall RuRe alloy exhibits superior HER kinetics, achieving low Tafel slopes of 32.25 mV dec −1 in 1.0 M KOH, 42.68 mV dec −1 in alkaline seawater, and 163.17 mV dec −1 in real seawater, along with a stable operation exceeding 180 h. Furthermore, an on‐site photovoltaic‐driven direct seawater electrolysis system was constructed, confirming the practical feasibility of the ultrasmall RuRe catalyst. This study provides an effective strategy for designing highly efficient and durable electrocatalysts for hydrogen production.