Mechanism of Hydrogen Evolution at Tellurium Cathodes in Hydrochloric Acid

Abstract

Hydrogen overpotential at tellurium cathodes was studied in the concentration range 0.005N‐5N aqueous hydrochloric acid at 30°C. The results indicated absence of specific adsorption. In dilute solutions a simple electrochemical mechanism, with two Tafel line slopes of 0.04 v and 0.12 v at 30°C, was established as the rate‐determining mechanism. In concentrated solutions, a rate‐determining dual electrochemical catalytic mechanism, with a slope of 0.06 v at 30°C, was suggested to explain the results in the low current density range. This dual mechanism changed to a simple one at high cathodic polarization.Theoretical calculations of the Tafel line slopes for various dual mechanisms were also carried out. It was found that, for a dual slow discharge electrochemical mechanism or a dual slow discharge catahtic mechanism, the slope was exactly the same as that for the simple slow discharge, i.e., 0.12 v at 30°C.The effect of neutral salts on hydrogen overpotential, μ, at constant pH was studied. Addition of lanthanum chloride increased the numerical value of μ at constant pH and current density. From the salt effect onμ in 0.05N hydrochloric acid, the zeta potential was calculated and was found to have a value of ca. 0.05 v.