Brønsted relationships for heterogeneous proton transfer at electrode interfaces

Abstract

The proton discharge step of electrochemical cathodic hydrogen evolution at electrodes provides a situation analogous to proton transfer from an acid (proton source) to a base (proton acceptor, of which the negatively charged electrode surface is the analogue). Brønsted relations for this heterogeneous, proton-transfer process have been experimentally derived for H deposition at the Hg electrode from eight proton donors in three solvents: water, dimethylformamide and acetonitrile, by plotting the log of the current density for proton-discharge-controlled hydrogen evolution, evaluated at a fixed potential, against the log of the acid-dissociation constant of the proton donor as a measure of its donor strength. The resulting Brønsted coefficients, α, for heterogeneous proton transfer in the electrode kinetics of cathodic H₂ evolution at Hg are compared with the analogous symmetry coefficient, β which is a Brønsted-type coefficient for effects of changes of electrical energy of the reacting particles, characterizing the linear free-energy relation expressed by Tafel's equation for the polarization behaviour of the electrode reaction.