Reactive Electrophilic O Species Evidenced in High-Performance Iridium Oxohydroxide Water Oxidation Electrocatalysts.

Although quasi-amorphous iridium oxohydroxides have been identified repeatedly as superior electrocatalysts for the oxygen evolution reaction (OER), an exact description of the performance-relevant species has remained a challenge. In this context, we report the characterization of hydrothermally prepared iridium(III/IV) oxohydroxides that exhibit exceptional OER performances. Holes in the O 2p states of the iridium(III/IV) oxohydroxides result in reactive O species, which are identified by characteristic near-edge X-ray absorption fine structure (NEXAFS) features. A prototypical titration reaction with CO as a probe molecule shows that these O species are highly susceptible to nucleophilic attack at room temperature. Similarly to the preactivated oxygen involved in the biological OER in photosystem II, the electrophilic O species evidenced in the iridium(III/IV) oxohydroxides are suggested to be precursors to species involved in the O-O bond formation during the electrocatalytic OER. The CO titration also highlights a link between the OER performance and the surface/subsurface mobility of the O species. Thus, the superior electrocatalytic properties of the iridium (III/IV) oxohydroxides are explained by their ability to accommodate preactivated electrophilic O species that can migrate within the lattice.