Significance of electrochemical Brønsted factors. Kinetic studies over a wide range of temperatures

Abstract

The fact that for simple discharge processes, the temperature dependence of Tafel's b coefficient (=“RT/βF”) is smaller than that indicated by the usual defining relation has been referred to in earlier publications but its significance has received little attention partly owing to an insufficiency of experimental data. The temperature dependence of Tafel slopes for hydrogen evolution has been examined at several metals including Hg within the temperature range +65 to –125°C in methanolic and ethanolic acid solutions. Bromide discharge was also studied in acetonitrile. In some cases, b decreases with T but not with the required slope R/βF or R/αF; in fact, b is better represented by a relation such as b=(RT/βF)+ C with β > 0.5, i.e., b has a finite intercept value at T= 0 K. At Ni, two slopes are observed, one of which increases with decreasing T, an effect which is probably caused by specific adsorption of anions. In all cases, b eventually increases with decreasing T at low temperatures (< –75°C) after an initial decrease. The form of the experimental relation involving C is examined in terms of proton tunnelling and in relation to effects of potential on the entropy of activation in electrochemical reactions and solvent dipole orientation at the interface.