Vacancy-Directed Phytate Coordination Couples Ni-Site Polarization with Interfacial Hydration for Durable Photoelectrochemical Seawater Oxidation

Photoelectrochemical seawater oxidation is limited by a coupled interfacial challenge: low-coordinated metal sites that favor hole utilization and oxygen evolution are also susceptible to chloride attack, whereas protective surface modification can compromise water access. Here, oxygen vacancies in amorphous NiCr2O4–x on α-Fe2O3 are repurposed to direct multidentate phytate coordination, constructing a molecular interface that couples Ni-site polarization with interfacial hydration. Catalyst-facing M–O–P coordination withdraws electron density from vacancy-activated Ni sites, strengthens interfacial polarization, accelerates hole delivery, and reconstructs oxygenated-intermediate bonding, lowering the free-energy requirement of the *O → *OOH step from 0.66 to 0.45 eV. Meanwhile, electrolyte-facing phosphate groups remain exposed as persistent hydrogen-bonding nodes, preferentially stabilize O-down interfacial water, and organize a continuous hydration network that preserves H2O/OH– accessibility while suppressing Cl– approach and adsorption. Operando infrared spectroscopy, in situ ATR-SEIRAS, molecular dynamics simulations, ion mapping, and dissolution analyses collectively connect this coupled solid–liquid interfacial regulation with accelerated intermediate turnover and reduced corrosion. The resulting photoanode delivers 4.43 mA cm–2 at 1.23 V versus RHE, operates for 300 h with negligible decay in alkaline simulated seawater, and retains 82% of its initial photocurrent after 200 h in alkaline real seawater. These results establish vacancy-directed molecular coordination as a strategy for coupling active-site polarization with hydration-mediated protection in durable seawater photooxidation.