Dual-pathway kinetic equation for the hydrogen oxidation reaction on Pt electrodes

In the past 2 years, several experimental observations and theoretical studies indicated that the commonly used Butler-Volmer equation was inappropriate for describing the kinetics of the hydrogen oxidation reaction (HOR) on a Pt electrode. Motivated by these works, we developed an HOR kinetic equation from a dual-pathway model to describe the complex kinetic behavior over the whole relevant potential region. A simple expression was found for the reaction intermediate's coverage as a function of overpotential. Three intrinsic kinetic parameters were determined by analyzing published polarization curves measured with microelectrodes (high mass transport) and rotating disk electrodes (relatively low mass transport, high accuracy). The results show a fast, inversed exponential rising of kinetic current at small overpotentials through the Tafel-Volmer pathway, and a much more gradual rise at eta > 50 mV through the Heyrovsky-Volmer pathway. This behavior is dramatically different from the single-exponential increase of the Butler-Volmer equation. The puzzling dependence of anode overpotential on Pt loading in H-2-fed proton-exchange membrane fuel cells can now be explained by the dual-pathway kinetic equation, providing a sound basis for fuel cell modeling, optimization, and diagnosis. The principle and approach used in this study can be applied to other electrocatalytic reactions.