Reactive Electrophilic O(I−) 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(I−) 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(I−) 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(I−) 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(I−) species. Thus, the superior electrocatalytic properties of the iridium (III/IV) oxohydroxides are explained by their ability to accommodate preactivated electrophilic O(I−) species that can migrate within the lattice.