Systematic Tuning of Iridate Perovskite Catalysts to Probe Reaction and Degradation Mechanisms for Water Oxidation in Acidic Conditions

As our global energy supply expands to meet growing demands and evolves to incorporate a larger fraction of renewable sources, we must adapt our fuel and chemical production industry to effectively convert or store these vital resources. Harnessing renewable electricity in the form of chemical bonds via electrocatalytic processes that operate at room temperature and pressure is a promising sustainable alternative to conventional thermochemical routes. A wide range of fuel- and chemical-producing reduction reactions require an electron-donating counter reaction, ideally using an abundant resource such as water. Therefore, the oxidation of water to molecular oxygen, also known as the oxygen evolution reaction (OER), plays a critical role in these technologies.

Currently, iridium-based materials offer the best combination of OER activity and stability in acidic conditions that are necessary for polymer-electrolyte membrane (PEM) electrolyzers; however, they still have insufficient performance for scaling up towards widespread technology deployment. We use iridium-based perovskite oxide (ABO₃) structures as a versatile material framework to establish systematic control of material electronic and geometric structure and to investigate OER reaction mechanisms as well as material degradation mechanisms. Partial or complete substitution of A and B sites enables control over crystallite size, strain, and oxygen deficiency. Many iridate perovskites exhibit initial activity increase upon exposure to acid and oxidizing potentials, correlated with increase in iridium oxidation state, as revealed by in situ X-ray absorption spectroscopy (XAS). Ex situ XAS and X-ray diffraction highlight bulk structural changes and amorphization induced by extensive electrochemical cycling. Lastly, impedance spectroscopy reveals an OER-induced metal to insulator transition with extensive electrochemical cycling, which provides new insight to material degradation mechanisms upon exposure to acidic and oxidizing environments. We present our work towards improved fundamental understanding of these highly dynamic materials under controlled reaction conditions to inform more robust future catalyst design.