Site‐Specific Ir Single Atoms in Spinel Induce 5 <i>d</i> Spin Polarization for Enhanced Oxygen Evolution

ABSTRACT Precise control of the coordination environment of single‐atom centers is essential to reveal how local symmetry governs electronic structure and catalytic behavior. Here, we develop a Li‐assisted vacancy‐engineering strategy to selectively embed Ir single atoms into tetrahedral and octahedral sites of spinel ZnCo 2 O 4 , producing well‐defined single‐atom catalysts with precisely controlled coordination environments. Theoretical calculations and experiments reveal that octahedral incorporation induces 5 d electronic reconfiguration and spin polarization in Ir, driven by strengthened hybridization between Ir 5 d states and the surrounding Co‐O framework. This coordination‐controlled electronic state reshapes oxygen‐intermediate binding energetics, thereby enhancing intrinsic reactivity. Consequently, octahedral‐site Ir exhibits an exceptional oxygen evolution reaction (OER) mass activity of 5520 A/g Ir at 300 mV, which is 920 times higher than that of IrO 2 , and maintains stability for over 200 h in an anion‐exchange membrane electrolyzer. These findings highlight the key role of crystallographic site selection in tuning 5 d single‐atom electronic structure and offer mechanistic insight into coordination‐controlled OER reactivity.