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We used a special membrane electrode assembly to measure operando X-ray absorption spectra and resonant photoemission spectra of mesoporous templated iridium oxide films. These films were calcined to different temperatures to mediate the catalyst activity. By combining operando resonant photoemission measurements of different films with ab initio simulations we are able to unambiguously distinguish μ2-O (bridging oxygen) and μ1-O (terminal oxygen) in the near surface regions of the catalysts. We find the intrinsic activity of iridium oxide scales with the formation of μ1-O (terminal oxygen). Importantly, we show that peroxo species do not accumulate under reaction conditions. Rather, the formation of μ1-O species, which are active in O-O bond formation during the OER, are the most oxidized oxygen species observed, which is consistent with an O-O rate limiting step. Thus, the oxygen species taking part in the electrochemical oxidation of water on iridium electrodes are more involved and complex than previously stated. This result highlights the importance of employing theory together with true and complementary operando measurements capable of probing different aspects of catalysts surfaces during operation.
