A Redox‐Buffering System for Stabilizing the Lattice Oxygen Mechanism in <scp>C</scp> e <scp>O</scp> ₂ / <scp>F</scp> e <scp>OOH</scp> Heterostructure Electrocatalysts for Highly Stable Anion Exchange Membrane Water Electrolyzers

Lattice oxygen participation is crucial for oxygen‐evolution reaction (OER) performance, but stabilizing the active high‐valence cation remains a major challenge. This study focuses on iron oxyhydroxide (FeOOH), which exhibits a delicate balance between high‐valence states and stability. A heterostructure (CeO 2 /FeOOH) with an electron‐rich, high‐valence‐state interface was synthesized via a simple co‐precipitation method. Due to the work‐function disparity between CeO 2 and FeOOH, electron accumulation occurs in CeO 2 , while FeOOH attains a high‐valence state. This enhanced valence state strengthens Fe–O covalency, facilitating lattice oxygen participation in oxygen‐evolution reaction. Furthermore, electron‐abundant CeO 2 functions as a redox buffer, where the electron‐reservable Ce 3+ /Ce 4+ redox couple stores excessive oxygen and donates electrons to stabilize high‐valence FeOOH. By incorporating this “redox‐buffering system,” Fe dissolution was minimized, significantly improving catalyst stability under harsh oxidizing conditions. The anion exchange membrane electrolyzer exhibited outstanding performance, delivering a current density of 500 mA cm −2 at 1.69 V, with remarkable stability over 100 h at 1 A cm −2 . These findings provide a new strategy for stabilizing high‐valence‐state oxygen‐evolution reaction catalysts, offering valuable insights for designing efficient and durable electrochemical systems.