A model of the potential-dependent adsorption of charged redox-active species at the electrode surface

A model of a non-polarizable interface is developed and considered in terms of the specific adsorption of charged redox species and the nonspecific adsorption of the supporting electrolyte. The adsorption coefficients of charged redox species depend on their charges and the potential at the inner Helmholtz plane. The adsorbed amounts of the redox species and supporting electrolyte are estimated using the Frumkin isotherm and Gouy-Chapman theory, respectively. The potential dependence of the adsorbed amount of either one or both of the oxidized and reduced forms of the redox species is maximized near its standard potential. The electrocapillary equation at a non-polarizable liquid metal-solution interface under an externally controlled potential is revisited. The derived electrocapillary equation facilitates calculating the electrocapillary curve of the non-polarizable interface based on the quantity of adsorbed ions. The electrocapillary curve calculated based on the model may have a concave part, which would indicate that the interface is thermodynamically forbidden, near the standard redox potential of the redox couple.