Probing Surface Oxide Formation and Dissolution on/of Ir Single Crystals via X-ray Photoelectron Spectroscopy and Inductively-Coupled Plasma Mass Spectrometry

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.