Surface constrained electron-hole rich species active in the electrocatalytic water splitting

Iridium and its oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotential and the high corrosion resistance observed for Ir-based anodes. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nano-fabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrOx centers (i.e. oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally IrV species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (µ1-O and µ1-OH) that are responsible for water activation and oxidation, due to the bound oxygen’s transformation into an oxyl susceptible to nucleophilic attack water. Oxygen bound to three iridium centers (µ3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a µ1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface to bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.