Cation defects enable synergistic coupling of dual pathways in a hierarchical heterojunction for efficient alkaline water oxidation

The slow oxygen evolution reaction (OER) dynamics limit electrochemical water electrolysis efficiency. Coupling the adsorbate evolution mechanism (AEM) with the lattice oxygen mechanism (LOM) enhances performance, but achieving their synergistic operation remains challenging. Here, we report a cation defect engineering strategy to address this challenge. Controlled in-situ sulfidation transforms a NiFe 2 O 4 /Ni(OH) 2 heterojunction into a multiphase Ni 1-x Fe 2 O 4 /Ni 3 S 4 /Ni 1-x (OH) 2 catalyst featuring tailored cation vacancies and stabilized high-valence Ni 3+ species. Advanced testing and density functional theory (DFT) calculations demonstrate that this unique architecture enables concurrent AEM and LOM pathways: cation vacancies facilitate the AEM on Ni 1-x Fe 2 O 4 while activating the LOM in Ni 1-x (OH) 2 . The in-situ formed Ni 3 S 4 phase promotes water dissociation, supplying OH − to dynamically replenish the lattice oxygen consumed by LOM, thus establishing a self-sustaining catalytic cycle that ensures durability. The resulting catalyst affords small overpotentials of 224 mV in alkaline freshwater and 208 mV in alkaline seawater at 100 mA cm −2 , while maintaining durability beyond 200 h and 100 h, respectively. Under anion-exchange membrane water electrolysis conditions, it attains 1 A cm −2 at merely 1.70 V, sustaining steady operation surpassing 210 h. This study demonstrates a defect-mediated principle for synchronizing dual OER pathways, offering a framework for fabricating efficient and robust electrocatalysts.