Insights on Tafel Constant in the Analysis of Hydrogen Evolution Reaction

Tafel-equation-based electrochemical analysis is widely employed in hydrogen evolution reaction (HER) to evaluate and characterize electrocatalysts. Tafel slope and exchange current density are the only two parameters that are invariably obtained and discussed with respect to Tafel equation in the literature. Herein, insights on Tafel constant in the analysis of HER are discussed, and its practical advantage is indicated. It is proposed that Tafel constant can be considered as the onset potential of HER. Tafel constant becomes the defining parameter between two electrocatalysts when other parameters such as Tafel slope or exchange current density become same. The significance of the same Tafel constants is illustrated using CoSe2 and NiSe2. Variation of the Tafel constant within a series of Co(1-x)NixSe2 is examined. The occurrence of the same Tafel constants between two electrocatalysts leads to one electrocatalyst becoming the better catalyst in the lower current regime, whereas the other exhibits higher activity in the higher current regime. Furthermore, concepts developed here are applied to several literature examples, and the significance of Tafel constant in the analysis of HER is established.