(509aq) Degradation Mechanisms of Calcium Iridium Oxides for Oxygen Evolution Reaction in Acid

The development of active and acid-stable water-splitting catalysts is crucial to meet the requirements of proton exchange membrane (PEM) technologies for the sustainable production of hydrogen via water electrolysis. Ir-based metal oxides are promising for the oxygen evolution reaction (OER) in acidic environments, however, long-term stability remains a critical challenge. Degradation can be caused by oxidation or dissolution in harsh acidic conditions, as well as other processes; improved understanding of these mechanisms is necessary to improve viability of these technologies.

In this work, we focus on Ca₂IrO₄ to develop a holistic picture of catalyst electronic and geometric structure evolution under applied potentials by probing electrochemically active surface area, metal dissolution, Ir valence, and surface morphology. We observe an initial activity increase in parallel with increasing capacitance and minor iridium dissolution. Extensive chronoamperometry tests at oxidizing potentials lead to significant activity loss that occurs simultaneously with a dramatic drop in capacitance and a change in impedance. These results indicate that there may be surface reconstruction or change in catalyst conductivity that contribute to degradation. Characterization techniques including X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), grazing incidence X-ray diffraction (GI-XRD), and X-ray absorption spectroscopy (XAS) were used after various electrochemical stability testing protocols to understand how materials respond to different reaction conditions. XPS reveals a slight change in iridium peak positions and a shift in relative intensity of iridium chemical species with extended electrochemical testing. SEM/EDS and XPS show that calcium leaches from the surface but is retained in the bulk. Similarly, GI-XRD confirms retention of bulk material crystallinity after OER testing protocols. Using a combination of electrochemical and spectroscopic tools, we aim to provide an understanding of the material degradation for future catalyst design with balanced activity and long-term stability.