Fundamental understanding of the active sites mediating the hydrogen oxidation/evolution reactions (HOR/HER) is critical to the design of an efficient HOR/HER electrocatalysts for affordable hydrogen exchange membrane fuel cells and electrolyzers. HOR/HER activities in terms of exchange current density were systematically investigated on carbon supported Ir nanoparticles with size from 3 to 12 nm in alkaline electrolyte. 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 remaining constant when normalized to the surface area of the sites with an average HBE of – 0.33 eV, indicating that those sites, accounting for only about 15 to 30 % of the total sites, shoulder the great majority of the HOR/HER activity.
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