van der Merwe, M.; Lee, Y.; Wibowo, R.E.; Kokumai, T.; Efimenko, A.; Arce, M.D.; Jimenez, C.E.; Howchen, B.; Suarez Anzorena, R.; Lucentini, I.; Escudero, C.; Schuck, G.; Kochovski, Z.; Favaro, M.; Starr, D.E.; Reuter, K.; Scheurer, C.; Bär, M.; Garcia-Diez, R.: Unravelling the mechanistic complexity of the oxygen evolution reaction and Ir dissolution in highly dimensional amorphous hydrous iridium oxides. Energy & Environmental Science 18 (2025), p. 1214-1231
10.1039/d4ee02839b
Open Access Version
Abstract:
Understanding the oxygen evolution reaction (OER) and Ir dissolution mechanisms in amorphous, hydrous iridium oxides (am-hydr-IrOx) 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-IrOx 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-IrOx 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-IrOx, 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-IrOx 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-IrOx, providing an alternative perspective within a dual-mechanistic framework.