Transient B─O─M Bonding Stabilizes Metastable Iron Sites in Oxyhydroxide Anode for a Stable AEM Water Electrolyzer

ABSTRACT The Fe‐containing oxyhydroxide anodes for AEM water electrolyzers (AEMWE) suffer from high‐valent Fe dissolution and structural instability at high current densities. Herein, a facile hydrolytic NaBH 4 pretreatment is reported to engineer an ultrastable CoNiFe oxyhydroxide (CNFOOH) anode. The distinct reduction potentials (E θ (M 3+ /M 2+ )) of constituent metals are leveraged to thermodynamically drive the selective Fe 3+ reduction, facilitating in situ formation of transient B‐O‐Fe motifs and a corrosion‐resistant mixed phase on the CNFOB. Comprehensive characterizations and theoretical calculations revealed that, since the B─O─Fe sacrificial cleavage during the initial OER stage, the unfavorable deep reconstruction is prevented by the suppressed Fe over‐oxidation and dissolution. Consequently, the CNFOB electrode achieves exceptional activity, only requiring 287 and 384 mV overpotentials to deliver 1000 mA cm −2 in 1 m KOH and alkaline seawater, respectively. Remarkable durability is emphasized, with stable operation maintained for over 1000 h at both 500 and 1000 mA cm −2 . When configured as a self‐supported anode in a practical AEMWE, the cell requires only 1.75 V to reach 1 A cm −2 and maintains stable operation for 1000 h. This work provides fundamental insight into stabilizing metastable sites via selective transient motifs and establishes a scalable route for durable and sustainable green hydrogen production.