(667e) Catalytic Performance and Material Stability Trends of Iridium 5+ Materials for Acidic Water Oxidation

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. Proton exchange membrane water electrolysis (PEMWE) is an attractive, carbon-free process that uses renewable electricity to produce “green” hydrogen from water. To meet the growing global demand for green hydrogen, improvements to PEMWE costs and efficiencies are critical. An area of significant potential for reducing costs and improving energy efficiency is oxygen evolution reaction (OER) catalysis. Only iridium (Ir)-based materials have been accepted as both active and stable catalysts for the OER under the acidic conditions of a PEMWE. Given the expensive and scarce nature of Ir, our research aims to maximize the intrinsic activity and stability of catalysts while simultaneously minimizing their Ir content, to limit both the material and operating electricity costs of PEMWE. We report our investigation on a series of tunable crystalline M₃IrO₇ (M=Pr, Nd, Eu, Sm) materials to offer fundamental understanding and characterization of Ir 5+ catalysts for the OER. Leveraging X-ray spectroscopy and diffraction refinement, we characterize and compare the geometric properties and Ir environments across the series of M₃IrO₇ materials, with the ultimate goal of relating intrinsic OER performance with geometric and electronic properties of the catalysts to determine fundamental structure-function relationships. We additionally investigate dissolution behavior of these compounds under OER conditions, report S-numbers as an intrinsic stability metric, and offer context to other standard Ir-based OER catalyst materials. To elucidate fundamental relationships between stability and geometric and electronic structure properties, we relate material stability in acidic conditions to Ir-O bond covalency, lattice structure, and M-site cation species. Our work aims to inform future Ir-based catalyst development for ultimate high-efficiency hydrogen production via PEMWE.