Kinetics of the Hydrogen Electrode Reaction

It is well recognized that the standard Butler–Volmer equation is lacking in an adequate description of the kinetics of the hydrogen electrode reaction over the complete range of potentials for the alkaline as well as the acid electrolytes. Further, it is unable to explain the asymmetry in current vs potential observed in the hydrogen evolution reaction (HER) vs the hydrogen oxidation reaction (HOR). In fact, even kinetic descriptions via two-step mechanisms (Volmer–Heyrovsky, Volmer–Tafel, or Heyrovsky–Tafel) are individually applicable only in limited potential ranges. We present an approach that provides explicit rate expressions involving kinetics of all the three steps (Tafel–Volmer–Heyrovsky) simultaneously, as well as more limiting rate expressions based on two-step pathways. The analysis is based on our recently developed graph–theoretic approach that provides accurate rate laws by exploiting the electrical analogy of the reaction network. The accuracy of the resulting rate expressions, as well as their asymmetric potential dependence, for both HOR and HER is illustrated here based on step kinetics provided in the literature for Pt catalyst in 0.5 M NaOH solution.