Engineering Interfacial Hydroxyl Spillover for Ultrastable High‐Current‐Density Hydrogen Evolution

ABSTRACT Stability at industrially relevant current densities remains a central challenge for transition‐metal phosphide based hydrogen evolution electrocatalysts operating in alkaline condition. To combat catalyst degradation, here the interfacial hydroxyl chemistry is regulated by robustly constructing an amorphous CoP/CeO 2− x heterostructure through a facile two‐step electrodeposition strategy. Operando Raman spectroscopy combined with density functional theory calculations reveals that oxygen‐vacancy‐rich CeO 2− x selectively captures *OH intermediates generated on Co sites, enabling efficient hydroxyl spillover away from CoP and thereby suppressing P oxidation and dissolution. The amorphous CoP framework further facilitates interfacial coupling and hydroxyl migration by providing abundant unsaturated coordination sites and dense Co/Ce interfacial contact. Benefiting from this synergistic effect, CoP/CeO 2− x delivers enhanced activity together with remarkable durability, sustaining 1 A cm −2 for over 320 h in alkaline electrolyte. When implemented as the cathode in an anion exchange membrane water electrolyzer, the catalyst operates stably for 750 h at 1 A cm −2 and 60°C with a low degradation rate of ≈52 µV h −1 . This work demonstrates the effectiveness of interfacial hydroxyl‐spillover engineering for stabilizing CoP‐based electrocatalysts under practical operating conditions.