Benchmarking Hydrogen and Oxygen Electrocatalytic Reactions in Acidic and Alkaline Media on Iridium Single-Crystal Surfaces

Abstract Material composition, surface structure, adsorbate coverage, electrolyte composition, and pH are all factors that play a critical role in the reactivity of the electrochemical interface. Iridium, despite its importance for energy conversion technologies, remains largely underexplored as an electrocatalyst in its metallic surface state. Here, we benchmark the hydrogen evolution and oxidation reactions (HER/HOR), oxygen reduction reaction (ORR), and hydrogen peroxide reduction reaction (HPRR) on Ir(111), Ir(100), and Ir(110) single crystals in acidic (HClO4) and alkaline (KOH) media using RDE/RRDE methods. Across both pH conditions, all reactions exhibit strong dependency on surface orientation but are not monotonically dependent on surface defects or lower surface site coordination number. For HER/HOR, Ir(111) is the most active facet in both electrolyte media, lowering the water dissociation kinetic barrier more effectively than on Ir(110). In contrast, Ir(110) is the most active for ORR in both pH conditions, whereas selectivity is strongly pH- and potential-dependent. Hydrogen peroxide yields are below 1% in acidic media but rise dramatically in alkaline electrolytes, reaching very high values (>50%) near open-circuit conditions. The impact of co-adsorption between Oad and OHad species shows a strong correlation to H2O2 production in alkaline media, suggesting water bond-breaking kinetics is an important parameter directing ORR selectivity. Additionally, HPRR trends indicate that in acidic media, ORR does not necessarily proceed through H2O2 intermediates, as low HPRR activity is not accompanied by H2O2 accumulation during ORR at E > 0.4 V. In contrast, in alkaline media, H2O2 accumulation correlates with reduced HPRR activity. The influence of surface sites and pH can be rationalized in terms of adsorbate speciation, lateral interactions, and interfacial water dynamics. Our results offer benchmark reactivity data on Ir(hkl) surfaces and across pH that complements literature results on Pt and Au surfaces while solidifying the role of interfacial water processes in determining the functional properties of electrochemical interfaces in aqueous environments.