Comparison of entropic and enthalpic components of the barrier symmetry factor, β, for proton discharge at liquid and solid Hg in relation to the variation of Tafel slopes and β with temperature

Abstract

In order to provide information from a new direction on the temperature variation of the barrier symmetry factor, β, and the corresponding Tafel slope, b=RT/βF, for the step of proton discharge at the Hg electrode, the kinetics of the cathodic H₂ evolution reaction have been studied over a wide temperature range through the melting point of Hg from several proton donors, in a series of suitable non-aqueous solvents that enable low-temperature experiments to be performed. The potential dependence of both the heat and entropy of activation has been determined through the melting point of Hg, enabling the corresponding enthalpic (β_H) and entropic (β_S) components of β in the relation β=β_H+Tβ_S to be evaluated. Substantial differences in β_H and β_s for solid and liquid Hg surfaces are found including, in some cases, a difference of sign of the β_s factor. The temperature-dependence of β, which is associated with the non-conventional variation of b with temperature, now demonstrated for many systems, arises in a complex way which is depnendent on the state and structure of the electrode metal's surface, in relation probably to solvent adsorption and orientation at the interface. The usually assumed value of 0.5 for the barrier symmetry factor is entirely coincidental when this quantity is identified with β_H, derived for 298 K. Only when β_s= 0, is β_H near 0.5, otherwise there is a compensatory variation of β_H with Tβ_s at a given T.