Structure Dependent Accessibility of Active Sites Governs Catalytic Activity and Stability of Iridium Oxides in the Acidic Oxygen Evolution Reaction

Deep understanding of the structural influence on the activity–stability relationship of iridium oxides in acidic oxygen evolution reaction (OER) is desired for better catalyst designs and, consequently, improved atom efficiency of the rare-earth element. However, identifying the variables governing the reaction mechanism remains a challenge in environments where multistep reactions and catalyst degradation occur simultaneously. Surface interrogation scanning electrochemical microscopy (SI-SECM) was employed to directly quantify the transient reactivity of OER intermediates on iridium oxide surfaces (amorphous IrOx and crystalline IrO2). Electrochemical measurements revealed a structure dependence of the electrolyte-accessible depth of active site layers, the variation of which controls the kinetics of charge storage and ultimately governs the OER catalysis. Contrary to the common models in the literature, effective charge storage exhibited little influence on the lowering of the OER kinetic barrier. Rather, the activity of iridium oxide catalysts was governed by the electrolyte permeability of the material, which controlled the number of active sites.