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A special membrane electrode assembly to measure operando X‐ray absorption spectra and resonant photoemission spectra of mesoporous templated iridium oxide films is used. These films are calcined to different temperatures to mediate the catalyst activity. By combining operando resonant photoemission measurements of different films with ab initio simulations these are able to unambiguously distinguish µ2‐O (bridging oxygen) and µ1‐O (terminal oxygen) in the near‐surface regions of the catalysts. The intrinsic activity of iridium oxide scales with the formation of µ1‐O (terminal oxygen) is found. Importantly, it is shown that the 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, is 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. [ABSTRACT FROM AUTHOR]

• Novel synthesis approach for PEO-PB-PEO templated mesoporous Ir/TiO x. • Characterization of structural and electrocatalytical properties. • Ir/TiO x films need minimum loading of 30 wt% Ir for sufficient conductivity. • Calcination at 400 °C leads to highest electrocatalytic activity. The depletion of fossil resources coupled with an increasing demand in both sustainable and renewable energy sources further strengthen the development of highly efficient electrocatalytic water splitting catalysts. Iridium oxide is well known for its excellent activity and stability during acidic oxygen evolution reaction (OER). However, high prices and rare abundance demand the most efficient utilization. We report a new synthesis approach for iridium/titanium-based mixed oxides with a tailored mesopore structure employing iridium acetate and titanium chloride as precursors. The new Ir/TiO x system outperforms OER catalysts based on other titanium precursors by a factor of up to two. The material is used as a model system to investigate the impact of synthesis and structural parameters on OER performance. We find that calcination temperature and iridium content are the main synthesis parameters to achieve tailored structures and superior electrochemical activity. The highest surface area and OER-activity are attained for samples with a fully developed mesoporous network and small crystallite sizes obtained by a moderate calcination around 400 °C. A minimum content of iridium is required to achieve sufficient electrical conductivity, whereas at higher iridium content the OER performance scales approximately linearly. [ABSTRACT FROM AUTHOR]