Correlating Hydrogen Oxidation/Evolution Reaction Activity with the Minority Weak Hydrogen-Binding Sites on Ir/C Catalysts

Fundamental understanding of the active sites mediating hydrogen oxidation/evolution reaction (HOR/HER) is critical to the design of an efficient HOR/HER electrocatalyst for affordable hydrogen exchange membrane fuel cells and electrolyzers. Here we report the existence of the most active sites on carbon supported iridium nanoparticles (Ir/C) for HOR/HER in alkaline electrolyte by investigating activities of Ir/C with varying particle sizes in the range of 3-12 nm. The distribution of surface sites is quantified by deconvoluting the H-upd desorption peak in cyclic voltammograms. The portion of the sites with the lowest hydrogen binding energy (HBE) increases with the increase of the particle size or the decrease of the total electrochemical active surface area (t-ECSA). The HOR/HER activity normalized to t-ECSA decreases as t-ECSA increases while it remains constant when normalized to the surface area of the sites with an average HBE of -0.33 eV, accounting for only about 15-30% of the total sites, shoulder the great majority of the HOR/HER activity.