A Surface Engineering Strategy to Suppress Cathodic Precipitation in Seawater Electrolysis

Electrochemical seawater splitting powered by renewable energy offers a pathway to green hydrogen, but cathodic precipitation of Mg/Ca hydroxides severely impairs kinetics and deactivates catalysts. Here, we develop an ammonia‐modified Cu ₃ P/MoP (A‐Cu ₃ P/MoP) heterostructure that delivers efficient and precipitation‐resistant hydrogen evolution in natural and alkaline seawater. Density functional theory calculations reveal that surface NH ₄ ⁺ species dramatically reduce the overall free energy of reaction intermediates and lower the potential‐determining step to 1.24 eV at the *HOH transition, thereby facilitating the Volmer step. Benefiting from this optimized interfacial chemistry, A‐Cu ₃ P/MoP achieves 0.50 A cm ⁻² at 574 mV in natural seawater and 1.0 A cm ⁻² at 416 mV in alkaline seawater, outperforming Pt/C. The catalyst maintains stable operation for over 500 h at 100 mA cm ⁻² , whereas Pt/C rapidly degrades within 50 h due to severe Mg/Ca hydroxide deposition. In a 25 cm ² zero‐gap electrolyzer paired with a lab‐designed anode, A‐Cu ₃ P/MoP sustains ~420 mA cm ⁻² at 1.23 V for more than 50 h in seawater with negligible performance loss. Mechanistic studies indicate that interfacial NH ₄ ⁺ forms a positively charged layer that electrostatically repels Mg ²⁺ /Ca ²⁺ , suppressing hydroxide precipitation and enabling durable seawater electrolysis.