Deciphering the high overpotential of the oxygen reduction reaction via comprehensively elucidating the open circuit potential

Abstract

The onset potential of the oxygen reduction reaction (ORR) maintains an activation overpotential exceeding ∼0.20 V, even with the most efficient catalysts. Despite decades of devotion, the origin of such high overpotential has been a long-standing topic of controversy. As the critical point for generating an apparent current in the ORR below the open circuit potential (OCP), understanding the reasons behind the OCP contributes to decrypting the inherent essence of the overpotential. Unfortunately, the implications represented by time-dependent OCP have not been clarified. In this work, the time-dependent OCP has been categorized into three stages: an initial rapid transition stage, a quasi-steady state stage, and a steady-state stage, wherein the quasi-steady state stage has been substantiated as the mixed potential arising from the oxidation of Pt and the ORR, accompanied with the dissolution of Pt. The quasi-steady OCP persists for an extended period due to the sluggish kinetics of the ORR on the oxidized Pt surface, the specific value of which was predominantly determined by the kinetics of Pt oxidation. It has been verified that the concept of the quasi-steady OCP is rooted in kinetics rather than equilibrium thermodynamics. Therefore, elevating the energy barrier of Pt oxidation through the substitution of H₂O by D₂O reduced the overpotential of the ORR significantly, evidencing that the bottle-neck of reducing the overpotential of the ORR is the inhibition of the oxidation of electrocatalysts kinetically. Hence, the reduction peak potential of Pt-based catalysts during the cathodic scan in linear sweeping voltammetry can be selected as a novel and straightforward indicator for predicting ORR performance and expediting the screening of highly efficient catalysts.