Unravelling the mechanistic complexity of the oxygen evolution reaction and Ir dissolution in highly dimensional amorphous hydrous iridium oxides

Abstract

Understanding the oxygen evolution reaction (OER) and Ir dissolution mechanisms in amorphous, hydrous iridium oxides (am-hydr-IrO_x) is hindered by the reliance on crystalline iridium oxide theoretical models to interpret its behaviour. This study presents a comprehensive investigation of hydrous iridium oxide thin films (HIROFs) as a model for am-hydr-IrO_x to elucidate electronic and structural transformations under OER conditions of proton exchange membrane water electrolyzers (PEM-WE). Employing in situ and operando Ir L₃-edge X-ray absorption spectroscopy supported by density functional theory calculations, we introduce a novel surface H-terminated nanosheet model that better characterizes the short-range structure of am-hydr-IrO_x compared to previous crystalline models, which exhibits elongated Ir–O bond lengths compared to rutile-IrO₂. This atomic model unveils the electronic and structural transformations of am-hydr-IrO_x, progressing from H-terminated nanosheets to structures with multiple Ir vacancies and shorter bond-lengths at OER potentials. Notably, Ir dissolution emerges as a spontaneous, thermodynamically driven process, initiated at potentials lower than OER activation, which requires a parallel mechanistic framework describing Ir dissolution by Ir defect formation. Moreover, our results provide mechanistic insights into the activity-stability relationship of am-hydr-IrO_x by systematically screening the DFT-calculated OER activity of diverse Ir and O chemical environments. This work challenges conventional perceptions of iridium dissolution and OER mechanisms in am-hydr-IrO_x, providing an alternative perspective within a dual-mechanistic framework.