Confirmation of the rate-determining step for ORR in the high potential region: adsorption process of oxygen molecules on the catalyst surface

Abstract

The oxygen reduction reaction kinetics at low overpotentials (<15 mV vs. OCP) were investigated using PtNi and PtPb catalysts, revealing near-zero H⁺ (0.05–0.12) and first O₂ (0.89–1.02) reaction orders, which conclusively identify O₂ adsorption as the rate-determining step. Notably, PtNi demonstrated the lowest activation energy (21.81 kJ mol⁻¹ vs. 22.48 for Pt/C and 24.73 for PtPb), which was directly correlated with its narrower HOMO–LUMO gap (−1.02 eV). This reduced orbital energy spacing facilitates electron flow during the chemical adsorption process, destabilizing transition states and lowering activation barriers.