(528a) Studying Stability: An Investigation of Metal to Insulator Transition Induction in Strontium Iridate Perovskites for Electrochemical Water Splitting

Hydrogen gas is a critical industrial chemical and energy-dense fuel that is expected to play a vital role in the decarbonization of our economy as society tackles the growing threat of climate change. Water electrolysis is an attractive, carbon-free process to split water into hydrogen and oxygen gas when coupled with renewably produced electricity. The greatest technological challenge facing the feasibility of water electrolysis lies with improving the catalysis of the oxygen evolution reaction (OER), which has intrinsically sluggish kinetics given its complex 4-step reaction scheme. Moreover, there is a specific need for active and stable OER catalysts in acidic electrolyte for use in proton exchange membrane (PEM) electrolyzers. However, only iridium-based materials have been identified as both sufficiently active and stable catalysts for the OER in acidic media. Given the expensive and scarce nature of iridium, we aim to improve the feasibility of catalysts by maximizing the activity and stability of catalysts while simultaneously minimizing their iridium content. This work seeks to understand and improve the stability of catalysts with reduced iridium content for the OER in acidic conditions.

To achieve this goal, we study a strontium iridate perovskite material system and employ B-site substitution (SrIr_xM_1-xO₃) with first row transition metals (M = Zn, Ni, and Co) to tune various geometric and electronic structure properties. Each catalyst within this group of materials experiences surface rearrangement when performing OER catalysis in harsh oxidative and acidic environments, which initially improve the catalysts’ intrinsic OER activities. However, we have identified that with extensive electrochemical activity cycling this rearrangement process eventually causes a critical material instability. By using a variety of electrochemical and spectroscopic characterization techniques we have recently made the novel discovery that SrIr_0.8M_0.2O₃ catalysts undergo a detrimental metal to insulator transition (MIT) with prolonged OER performance. Through EXAFS analysis, we provide evidence that such a transition is induced by geometric changes to the perovskite crystal structure caused by the OER. We additionally show through various B-site substitutions the rate at which a MIT is induced is effectively tuned. Lastly, we offer a fundamental understanding of the nature of MIT induction by relating total faradaic charge to number of active sites throughout a catalyst’s lifetime. By uncovering this fundamental relationship, we present a novel material stability metric to assess the stability against OER-induced MIT for materials subject to the transition. As conductivity of catalysts is crucial to their electrochemical performance, it is imperative to understand the nature of the MIT to enable more informed catalyst development to ultimately improve the economics and efficiency of electrochemical water splitting for carbon-free hydrogen fuel production.