The theory of charge transfer in electrode kinetics, especially for coupled atom/electron transfer processes such as the hydrogen evolution reaction, is still not well understood, particularly with respect to the temperature dependence of the kinetics and the nature of the effects of potential on the energy and entropy of activation.

By means of steady-state polarization measurements the kinetics of the hydrogen evolution reaction at Hg from several proton sources (H₃O⁺, ROH₂⁺, DMFH⁺, EtNH₃⁺, Et3NH⁺, H₂O, CH₃COOH, CF₃COOH) in aqueous and nonaqueous media (CH₃OH, 1 propanol, 2-propanol, isobutano1. HCONH2, DMF and AN) were investigated over a wide range of temperature. In all cases proton discharge was from well defined states and was the rate-controlling step at Hg at which coverage by the H intermediate is negligible (<0.03%). Kinetic activation parameters were determined at a constant metal-solutIon potential rather than at constant overpotential (giving only the apparent activation parameters), by means of isothermal and non- isothermal reference electrode cell measurements which allowed corrections to be made for variation of the reference electrode potential with temperature and solution composition.

Comparison of these chemically significant activation parameters allows medium and proton source effects to be distinguished in the activation process Current theories about electron and proton transfer in the electrochemical hydrogen evolution reaction place little emphasis on the specific nature of the solvated proton source in solution but only emphasize long-range fluctuations in solvent dielectric polarization in the activation process. However, the present results indicate that specific as well as general medium effects have to be included in any interpretation of the mechanism of proton discharge, and probably in other electrode processes too. Also, a linear free energy relationship, i.e., Br^nsted relation, between the rate of electrochemical proton transfer and the acid/base strength of proton donors in several solvents in clearly demonstrated in the present work, indicating the importance of specific bond effects in the activation process.

It is shown that the entropy of activation or the corresponding frequency factor for proton discharge is substantially dependent on electrode potential and not constant as usually assumed. These results show that the effect of electrode potential on electrochemical rates for a simple discharge process must now be considered as a combination of two effects:​ i) an "inner" effect on the Fermi level of electrons in the metal relative to their energy in vacuum, and ii) an "outer" effect on the solvent structure and state of reactant ion and solvated transition state in the double-layer at the electrode interphase. Previous theories of electron and atom transfer at electrodes have not treated this important entropy effect, but have been concerned mainly with quantal and solvation reorganization aspects of the energy of activation with little reference to the structural aspects of the process in the interphase.

Consideration of the effect of electrode potential on both the enthalpy and entropy of activation results in a symmetry factor of the form, β = β_H + TH_S where β_H and Tβ_S are the enthalpic and entropic components of the overall symmetry factor β. Contrary to conventional assumptions, the Tafel slope b is not given by the usual relation b = ±2. 3RT//3F but rather by b = ±2 3RT/(β_H+Tβ_S)F which can now account for the often observed "anomalous" temperature dependence of the Tafel slope.

A variety of behaviour is found from cases where 0 is dominated by the β_H term giving rise to conventional behaviour of b as a f(T), to cases where 0 is determined mainly by the Tβ_S term, giving b almost independent of T. An important compensation effect between values of β_H and Tβ_S is demonstrated that makes 0 approximately constant and near 0.5 at 298 K, even though β_H and Tβ_S vary considerably The enthalpy and entropy of activation also vary together in a systematic and compensatory way.