On the growth and water oxidation stability of hydrous iridium oxide†

Abstract

Hydrous iridium oxide (HIROF) is a highly active catalyst for the oxygen evolution reaction (OER) with broad application in pH sensing and charge storage devices. However, the mechanisms driving its growth, as well as the associated iridium dissolution, remain incompletely understood. To address this knowledge gap, we employ online inductively coupled plasma mass spectrometry (ICP-MS) to monitor iridium dissolution from sputtered thin films of varying thicknesses during electrochemical cycling. Complementary techniques, including atom probe tomography (APT), ellipsometry, and X-ray photoelectron spectroscopy (XPS), are used to study oxidation states and interface composition. Our findings reveal a tri-phase interface consisting of metallic iridium, compact anhydrous oxide, and hydrous oxide, where dissolution predominantly occurs at the metal–compact oxide interface, driven by transient processes during cycling. HIROF growth strongly depends on iridium grain size, with smaller grains inhibiting growth due to the accumulation of an inner compact IrO₂ layer. This effect is linked to increased oxophilicity, which lowers the reducibility of compact oxide. These insights advance understanding of HIROF growth mechanisms, offering strategies to optimize its performance and stability, particularly in proton exchange membrane water electrolyzers (PEMWEs), where iridium scarcity is critical. Broader implications extend to hydrous oxide formation on other noble and non-noble metals, potentially further advancing other electrochemical applications.