Gaining fundamental insights into the formation and the stability of surface oxides on iridium (Ir) surfaces is pivotal to oxygen evolution reaction (OER) electrocatalysis. Herein, we examined the potential-dependent structural and chemical changes occurring on planar Ir(111), Ir(210), and nanofaceted Ir(210) single-crystal surfaces using electrochemistry, scanning probe microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectrometry. We show that, after polarization in OER conditions, Ir surface atoms feature mixed oxidation states(0), (+III), and (+IV)and then enrich into Ir­(+IV) due to the dissolution of Ir­(+III) species. The rate of surface and near-surface layer enrichment in Ir­(+IV) species depends on the modulation mode of the potential (linear potential sweeps vs. potential steps) and is faster on opened surfaces. By combining fits derived from the XPS spectra and OER activity measurements, we found that the OER specific activity varies with the Ir oxidation state and is closely related to the fraction of Ir­(+III) species.

Structural defects are of significant importance in (electro)catalysis, as they provide sites of unusually high activity that find applications in many key electrochemical processes. However, tools to characterize surface defects remain scarce and complex, especially for nanocatalysts where classical methods such as transmission electron microscopy or X-ray scattering are limited in their ability to probe the structure and distribution of the active surface sites. Herein, we show that the ratio between the CO ads stripping charge (Q CO ) and the charge required to desorb underpotentially deposited H atoms (Q H ) in structurally disordered Ptbased nanocatalysts scales almost linearly with the surface distortion descriptor obtained via advanced physical methods. This trend is valid in both rotating disk electrode configuration and in a real fuel cell device, thus providing the scientific community with a powerful and versatile approach for semiquantitative estimation of the surface lattice distortion in Pt-based catalysts without the need for exhaustive structural characterization.