An immitance study of the mechanism of hydrogen reactions on a tungsten carbide electrode: Part II. Analysis in terms of a non-linear model

Abstract

The results of an ac immitance study of a smooth tungsten carbide electrode in H 2-saturated 0.5 M H 2SO 4 in a wide range of hydrogen overpotentials were analysed in terms of a non-linear model which assumed that: (1) hydrogen reactions are coupled in parallel with an additional electrochemical adsorption reaction which blocks a portion of the electrode surface area for the hydrogen adsorption; (2) adsorption of the electroactive species is of the Langmuirian type; and (3) the kinetics of each elementary step of the overall faradaic process can be described by an appropriate Butler-Volmer equation. The model agreed with the experimental results if the Heyrovský-Volmer mechanism for hydrogen reactions was assumed, while the Tafel-Volmer mechanism was not supported. This analysis gave estimates for several thermodynamic and kinetic parameters of the system and for the two faradaic processes. According to these estimates, the additional adsorption reaction involved a partial charge transfer. This suggested that the additional adsorption could not be of the Langmuirian type. Further support for the model was obtained from the dc results for the electrode system under study.