Acord transformatiu CRUE-CSICThe oxygen evolution reaction catalyst optimization is hindered because in the desirable acidic conditions the sole active catalysts are RuO and IrO. Thus, the understanding of the factors controlling the reactivity of these materials is mandatory. In this contribution, DFT (PBE-D2) periodic calculations are performed to analyze the catalytic activities of the main ((1 1 0), (0 1 1), (1 0 0) and (0 0 1)) IrO surfaces. Results show that the reaction only occurs if the Ir=O species on the surfaces exhibit an oxyl character. The water nucleophilic attack mechanism is the most favorable pathway on the (1 1 0), (1 0 0) and (0 0 1) surfaces. In contrast, for the (0 1 1) facet the oxo-coupling is preferred. The required overpotentials for the four IrO surfaces depend on the feasibility to oxidize the Ir-OH to Ir-O species and this is tuned by the coordination of the unsaturated iridium sites: the (1 0 0) and (0 0 1) surfaces appear to be more active than the (1 1 0) and (0 1 1)
