Metal Coordination Determines the Catalytic Activity of IrO2 Nanoparticles for the Oxygen Evolution Reaction

H2 production through water electrolysis is a promising strategy for storing sunlight energy. For the oxygen evolution reaction, iridium oxide containing materials are state-of-the-art due to their stability in acidic conditions. Moreover, precious metal content can be reduced by using small nanoparticles that show high catalytic activities. We performed DFT calculations on a 1.2 nm large IrO2 Wulff-like stoichiometric nanoparticle model (IrO2) with the aim of determining the factors controlling the catalytic activity of IrO2 nanoparticles. Results show that at reaction conditions tetra- and tricoordinated iridium centers are not fully oxidized, the major species being IrO(OH) and IrO(OH)2, respectively. Moreover, the computed overpotentials show that low coordinated iridium centers are more active than the pentacoordinates sites of the well-defined facets. These low coordination sites are likely more abundant on amorphous nanoparticles, which could be one of the factors explaining the higher catalytic activity observed for non-crystalline materials.