Preprint of "Cobalt Oxidation State Controls Ir–CoOₓ Catalysts for Acidic Oxygen Evolution: From Model Surfaces to PEM Water Electrolyzers"

Water electrolysis is a key technology for sustainable hydrogen production, yet its efficiency in proton exchange membrane (PEM) electrolyzers is limited by the sluggish oxygen evolution reaction (OER) and the high cost of iridium-based catalysts. Combining Ir with Co and its oxides has emerged as a promising strategy to reduce catalyst cost. Nevertheless, the nature of the Ir–CoOₓ interaction, the identity of the active phase, and the specific role of Ir under acidic OER conditions remain poorly understood. To address this issue, we use model epitaxial Co3O4 (111) and CoO(111) thin films supporting Ir nanoparticles to elucidate the role of the oxide environment in Ir-CoOx OER catalysts. Using synchrotron radiation photoelectron spectroscopy coupled with electrochemistry, we reveal distinct, oxide-dependent transformation pathways. Our results show that Ir/Co3O4 undergoes controlled oxidation into mixed Ir 3+ /Ir 4+ states and forms a stable, surface-confined (oxy)hydroxide layer, whereas Ir/CoO exhibits pronounced structural instability and phase transitions. These differences directly govern the activity–stability balance of the catalysts. Crucially, these mechanistic insights are validated in technologically relevant systems, such as magnetron-sputtered, mixed IrCoOx catalyst layers with a Co3O4 -like environment; these demonstrate enhanced activity and durability, outperforming pure Ir and IrO2 in a single-cell PEM water electrolyzer while operating at very low Ir loading even under prolonged operation. This work underscores the pivotal role of oxide support in stabilizing active iridium species and paves the way for designing efficient, low-iridium content catalysts for acidic OER.