Electrochemical Surface Composition of Iridium Oxide IrO ₂ (110): Implications for the Oxygen Evolution Reaction Mechanism

The mechanism of the Oxygen Evolution Reaction (OER) on iridium oxide IrO₂(110) highly depends on the stable structure of the surface under various electrochemical conditions. Through a first-principles-based Pourbaix diagram, we identified the stable di- and trioxygen surface terminations of IrO₂(110) at electrode potentials relevant to the OER. Thermodynamics calculations revealed that the OER proceeds at an electrode potential where the surface is terminated with a hydrotrioxide species (-IrOOOH), which electrochemically decomposes to produce an oxygen gas. Our results point to a cyclic OER mechanism that is electrode potential-limited by the electrochemical dissociation of water to regenerate the hydrotrioxide-terminated surface. These findings provide new insights into the role of the initial surface composition of the catalyst in the reaction mechanism of the OER.