Electric Double Layer Engineering Induces O‐Down Interfacial Water Reorientation for Efficient and Long‐Term Seawater Electrolysis

Direct seawater electrolysis for hydrogen production is plagued by sluggish hydrogen evolution reaction (HER) kinetics and severe Mg/Ca hydroxide deposition, which restrict its large‐scale application. Herein, we address these two issues via a targeted electrolyte‐mediated electric double layer (EDL) engineering strategy by introducing a chelating agent, ethylene diamine tetra (methylene phosphonic acid) sodium (EDTMPS). EDTMPS adsorbed on the Pt surface effectively chelates Mg 2+ /Ca 2+ to inhibit hydroxide deposition. Meanwhile, it disrupts the hydrogen bond networks of interfacial H 2 O and increases the thickness of EDL, thereby reducing the free H 2 O reorganization energy barrier by weakening the interfacial electric field intensity. Electron transfer from Pt to EDTMPS generates a localized non‐uniform electric field, further facilitating the reorientation of H 2 O from the conventional H‐down to the more readily adsorbable O‐down configuration for HER participation inferred by second harmonic generation spectra and theoretical calculations. Simultaneously, the abundant ‐[P‐O] − groups in EDTMPS synergistically promote H 2 O dissociation with Pt active sites and accelerate the HER kinetics. The assembled natural seawater electrolyzer achieves 1.0 A cm −2 at 2.41 V and long‐term stability for over 2500 h at industrial‐level current densities. This work offers a sustainable pathway toward scalable green hydrogen production from natural seawater via electrolyte‐mediated EDL modification.