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

Abstract

The large-scale deployment of polymer electrolyte water electrolysis (PEWE) is largely limited by the use of O2 evolution reaction (OER) catalysts based on scarce and expensive iridium in PEWE anodes. The ensuing need for better-performing, Ir-based OER catalysts requires an 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 determine the oxidation state of surface Ir in a range of Ir oxides with different surface compositions, crystal structures, and OER activities. Our results 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 to correlate with the onset of O2 evolution, thus strongly hinting at the involvement of Ir in oxidation states ≥+5 in the OER and indirectly discarding those mechanisms that do not consider such states as a part of the reaction sequence.