Dynamic Cobalt Leaching‐Redeposition Equilibrium in Co ₉ S ₈ /Co‐Ni ₃ S ₂ Electrocatalysts Boosts Oxygen Evolution Activity

ABSTRACT Developing catalysts that provide high catalytic activity and durability is critical for practical water electrolysis. However, most current studies focus on activity origins, overlooking the role of metal ion leaching and redeposition in catalytic stability under long‐term operation. Herein, we synthesized cobalt‐nickel sulfide composites (Co 9 S 8 /Co‐Ni 3 S 2 ) and investigated the dynamic changes in their reaction sites, particularly the leaching‐redeposition processes in the oxygen evolution reaction (OER). Characterization analysis indicates that Co and Ni ions leach from the surface of Co 9 S 8 /Co‐Ni 3 S 2 , combine with OH − in the electrolyte, and subsequently redeposit onto the catalyst surface, where they are further electrooxidized to CoOOH and NiOOH, thus boosting performance. Specifically, the leaching‐redeposition of Co exerts a dominant effect on boosting catalytic activity and stability. Additionally, dynamic equilibrium roughens the surface, improving electroactive area and mechanical stability. Density functional theory (DFT) calculations confirm a lattice oxygen‐mediated mechanism, and the composite exhibits an optimized Co d‐band center with strong Ni‐Co 3d orbital overlap, enabling optimal intermediate adsorption and electron transfer. When assembled into an anion exchange membrane water electrolyzer (AEMWE), Co 9 S 8 /Co‐Ni 3 S 2 operates for 500 h at 100 mA cm −2 under an applied potential of 1.59 V, outperforming most reported electrocatalysts. These findings offer design insights for durable OER electrocatalysts.