Effects of Sulfuric Acid Molarity on Iridium Oxides

Iridium oxide is by far the only viable catalyst for the oxygen evolution reaction (OER) in industrial PEM water electrolysis. Gaining a fundamental understanding of how Ir catalyzes OER under industrial conditions is crucial to advancing catalysts and electrodes for the green energy transition. However, there are discrepancies in results between fundamental benchtop experiments and electrolyzer stacks deployed in the field. Benchtop experiments in literature are widely performed at low molarities (e.g., 0.1 M) for convenience, whereas data taken from stack tests display trends reminiscent of higher molarities (e.g., 5 M). To examine how these discrepancies may affect catalyst performance, four different species of iridium oxide (Ir black, hydrous IrO_x, rutile IrO₂, and amorphous IrO_x) were evaluated in different molarities of sulfuric acid using a benchtop rotating disk electrode (RDE) setup. Cyclic voltammetry (CV) cycles taken at each of these conditions generally revealed narrower cycles in 5 M than in 0.1 M, reflecting the fact that iridium dissolved into the solution at the higher molarity. Additionally, OER performance for each of the catalysts yielded lower OER activity in 5 M. This can be attributed to the greater potential requirement at lower pH’s to generate oxide species capable of OER, as illustrated in the Pourbaix diagram for iridium. Shifts in CV peaks at different conditions support the theory that different oxide species form in different molarities, which has great implications for studying iridium oxide performances in industrial PEM water electrolyzers. Performing benchtop studies at high molarities may better simulate real-world conditions in a membrane electrode assembly, helping researchers understand the nature of iridium oxides in devices and how they may differ from prior fundamental study results.