Surface Ir⁺⁵ Formation as a Universal Prerequisite for O₂ Evolution on Ir Oxides

The large-scale deployment of polymer electrolyte waterelectrolysis(PEWE) is largely limited by the use of O-2 evolution reaction(OER) catalysts based on scarce and expensive iridium in PEWE anodes.The ensuing need for better-performing, Ir-based OER catalysts requiresan improved understanding of the relation between these materials'activity and their physicochemical operando properties.To shed light on this matter, here, we employed operando modulation excitation X-ray absorption spectroscopy to determinethe oxidation state of surface Ir in a range of Ir oxides with differentsurface compositions, crystal structures, and OER activities. Ourresults reveal that, irrespectively of these diverging properties,the surface Ir in all catalysts systematically undergoes a linear,potential-driven oxidation that stabilizes once a +5 state is reached.The completion of this surface oxidation process is then showed tocorrelate with the onset of O-2 evolution, thus stronglyhinting at the involvement of Ir in oxidation states & GE;+5 inthe OER and indirectly discarding those mechanisms that do not considersuch states as a part of the reaction sequence.