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

Understanding the kinetics of the electrochemical hydrogen oxidation and evolution reaction (HOR/HER) in alkaline media is of great interest, but the underlying reaction mechanism and the dependency on the hydrogen partial pressure are still debated. In this work, we investigated the HOR/HER kinetics on a polycrystalline platinum electrode in alkaline electrolyte using the rotating disk electrode (RDE) technique. Unlike typically done, we rigorously performed data treatment and analysis on a fixed potential scale with a fixed reference potential, which has implications for the use and assessment of RDE mass-transport corrections and the definition of reaction orders. Analyzing the kinetic current curves on a fixed potential scale, we find reaction orders with respect to the hydrogen partial pressure of essentially one and zero for the HOR and HER, respectively, at larger overpotentials. In contrast, fitting the kinetic current curves in a potential range around equilibrium with a generalized 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 with a rate-determining Volmer step at small overpotentials to Heyrovsky-Volmer with a rate-determining Heyrovsky step at more positive overpotentials, with a concomitant change in the H2 reaction order from fractional to unity, thus reconciling previous propositions in the debate on the alkaline HOR/HER mechanism. Our results demonstrate the strength and advantages of using a fixed potential scale rather than the commonly employed overpotential, and we expect this approach to be beneficial also for kinetic studies of other electrocatalytic reactions.