A First-Principles Surface Reaction Kinetic Model for Hydrogen Evolution under Cathodic and Anodic Conditions on Magnesium

Abstract

A surface kinetic model is developed using a proposed mechanism for the anodic and cathodic branches of the hydrogen evolution observed over magnesium (i.e. the negative difference effect). The key element to the model is that hydrogen evolution requires the removal of adsorbed OH from the surface. This step is achieved in the cathodic branch by reductive desorption of OH to form OH− from the surface, and in the anodic branch by the dissolution of both adsorbed OH along with the Mg atom to which it is attached (thus combined desorption of OH− alongside dissolution of Mg2+). Steady state theory is applied to derive expressions for the hydrogen evolution rate as a function of potential. First-principles parameters obtained from the literature are used to compute the rate constants for the individual mechanistic steps, where available, and reasonable values based on cohesive energies are used to estimate the remaining parameters where first-principles information is not available. These rate constants are then applied to the surface kinetic model to show that the model provides reasonable agreement with the observed phenomenology for hydrogen evolution over magnesium, both in terms of the shape and order of magnitude of hydrogen evolved, and the transition potential (i.e. the open circuit potential) between the anodic and cathodic hydrogen evolution branches.