Argonne compared iridium crystal faces: Ir(111) leads in hydrogen reactions, Ir(110) in oxygen reduction

Researchers at Argonne National Laboratory, led by electrochemist Pietro Papa Lopes, compared the electrochemical reactivity of three single-crystal iridium surfaces, Ir(111), Ir(100) and Ir(110), in acidic (HClO4) and alkaline (KOH) electrolytes. The work was published in ACS Catalysis. Using rotating disk and rotating ring-disk electrode methods, the team benchmarked four reactions: hydrogen evolution and oxidation, oxygen reduction (ORR) and hydrogen peroxide reduction (HPRR) acs.orgACS Catalysisacs.orgOpen source ↗.

According to the authors, iridium is important for energy conversion technologies, yet in its metallic surface state it remains largely underexplored as an electrocatalyst. The new data are intended to complement existing results on platinum and gold surfaces.

Hydrogen reactions and oxygen reduction

All reactions depended strongly on surface orientation, but not monotonically on surface defects or on a lower coordination number of surface sites. For hydrogen evolution and oxidation, Ir(111) was the most active face in both electrolytes. It lowered the kinetic barrier of water dissociation more effectively than Ir(110). Ir(110), in contrast, was the most active face for ORR at both pH values. In the laboratory's account, hydrogen reactions slowed considerably in alkaline media, where the surface first has to split water molecules.

Hydrogen peroxide selectivity

ORR selectivity depended strongly on pH and potential. Hydrogen peroxide yields were below 1% in acid. In alkaline electrolytes they rose dramatically and exceeded 50% near open-circuit conditions acs.orgACS Catalysisacs.orgOpen source ↗. The authors found a strong correlation between co-adsorption of Oad and OHad species and H2O2 production in alkaline media. They suggest that water bond-breaking kinetics is an important parameter directing ORR selectivity.

The HPRR data show a pH-dependent picture. In acid, low HPRR activity was not accompanied by H2O2 accumulation during ORR at potentials above 0.4 V, so ORR does not necessarily proceed through a peroxide intermediate. In alkaline media, H2O2 accumulation correlated with reduced HPRR activity.

The authors explain the effects of surface sites and pH through adsorbate speciation, lateral interactions and interfacial water dynamics. Papa Lopes said the aim is to give the community benchmark reactivity data for well-defined iridium surfaces, so that iridium can be compared with more widely used catalyst materials such as platinum.