| Summary: | Abstract We investigated the oxygen evolution reaction (OER) activity changes of cobalt oxide (CoOx) thin films on Ir(111) and Pt(111) substrates by repeated OER measurements in 0.1 M KOH. Atomic force microscopy and X‐ray photoelectron spectroscopy analysis of the as‐prepared CoOx/Ir(111) and CoOx/Pt(111) showed similar surface morphologies of the CoOx thin films and almost the same OER overpotentials, which were estimated to be around 430 mV. However, after three OER measurements, the overpotential of CoOx/Ir(111) decreased by 70 mV, whereas that of CoOx/Pt(111) increased slightly. Structural analysis showed that CoOx/Ir(111) revealed the island‐like nanostructures of CoOx dispersed on Ir(111) surface, accompanied by the generation of CoOOH. In contrast, for CoOx/Pt(111), the Pt(111) substrate remains covered by the CoOx thin film. The results suggest that the interaface at CoOx (CoOOH) nano‐islands and Ir(111) substrate are responsible for reducing the OER overpotential.
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