Oxidized-State Accumulation Controls Water Oxidation Kinetics on a Model Iridium Atomic Array

Water oxidation is vital for renewable energy conversion but remains kinetically complex due to the strong coupling of electron transfer with proton-involved chemical processes of bond formation and rupture, which are not fully captured by phenomenological Tafel analyses. Here, we design CeO₂ nanorod-supported iridium atomic arrays (Ir/CeO₂) as model catalyst to elucidate the molecular-scale information about the kinetics mechanism. We reveal that the applied bias does not directly act on the reaction coordinate but regulates electrocatalytically generated current through oxidative charge accumulation. This build-up of oxidized states significantly reduces the activation energy for *OOH formation by facilitating the O─O coupling step. Meanwhile, the electron-buffering capacity of CeO₂ support prevents Ir over-oxidation and dissolution during charge accumulation, thereby enhancing catalyst stability. As a result, the Ir/CeO₂ catalyst delivers superior activity and durability in proton exchange membrane water electrolyzers, achieving industrial-level current densities at low cell voltages. These findings provide molecular insights into charge-controlled water oxidation kinetics and highlight the essential role of purely chemical steps in describing the kinetics of multi-electron reactions.