Iron Self-Redox-Mediated Promotion of a Dittmarite-Type NH ₄ NiFePO ₄ ·H ₂ O Catalyst for the Alkaline Oxygen Evolution Reaction

Direct seawater electrolysis is an attractive strategy for sustainable hydrogen production, yet it suffers from sluggish oxygen evolution reaction (OER) kinetics and severe chloride-induced corrosion. Herein, we report the rational design of a layered ammonium nickel-iron phosphate (NH₄NiFePO₄·H₂O, NNFPH) with a dittmarite-type framework as a robust, high-performance, and low-temperature OER electrocatalyst. Experimental analyses reveal that the Fe species undergoes reversible redox transitions during the OER process, serving as an internal electron transfer mediator to improve charge transport and to optimize the electronic states of Ni active sites. Meanwhile, the strongly electronegative phosphate ligands construct a protective electrostatic field, effectively suppressing chloride corrosion. As a result, NNFPH delivers a low overpotential of 323 mV at 100 mA cm^-2 and remarkable durability over 120 h at 200 mA cm^-2 in alkaline natural seawater. Mechanistic studies reveal that the <i>in situ</i>-generated NiOOH species serve as dominant active sites while reversible Fe²⁺/Fe³⁺ redox cycling enhances the conductivity and structural stability. This work highlights the structural and electronic engineering of layered transition metal phosphates as a promising strategy for high-performance seawater electrolysis.