Hydrogen Oxidation and Evolution Reaction on Platinum in Alkaline Electrolyte: Fixed Reference vs. Overpotential and the Effect of H2 Concentration

Abstract

Abstract Understanding the kinetics of the hydrogen oxidation and evolution reaction (HOR/HER) in alkaline media is of great interest, but the underlying mechanism and the dependency on the hydrogen pressure are still debated. Herein, we investigated the HOR/HER on polycrystalline platinum in alkaline electrolyte using a rotating disk electrode (RDE). Unlike typically done, we performed the data analysis on a fixed potential scale, which has implications for the assessment of RDE mass-transport corrections and the definition of reaction orders. Analyzing the kinetic currents on a fixed potential, we find hydrogen reaction orders of essentially one and zero for the HOR and HER, respectively, at larger overpotentials. In contrast, fitting the kinetic currents in a potential around equilibrium with a Butler-Volmer model yields differing kinetic parameters and fractional reaction orders. Our findings can be explained with a potential-dependent transition in the HOR mechanism from Tafel–Volmer at small overpotentials to Heyrovsky–Volmer at higher overpotentials, with a concomitant change in the H2 reaction order from fractional to unity, thus reconciling previous propositions on the alkaline HOR/HER mechanism. Our results demonstrate the strength of using a fixed potential scale, and we expect this approach to be beneficial for studies of other electrocatalytic reactions.