Solution of electrochemical flux equations with variable diffusion coefficient and transference number

Abstract

Finite difference solution of the nonlinear partial differential equation of diffusion and migration for constant current electrochemical processes in binary mixtures of molten salts has been carried out for several important cases. Concentrations at the electrode surface as a function of time, and concentration--distance profiles between the electrode surface and the bulk electrolyte, have been computed for the following model cases, the first two of which have analytical solutions that could be used to test the computation (D is the interdiffusion coefficient, and M is a migrational term whose form depends on the mobilities of the ions and on the electrode reaction): (I) constant D, M = 1; (II) constant D, constant M not equal to 1; (III) D varying exponentially with composition, M = 1; (IV) D constant, varying M; (V) D and M varying. The results demonstrate that chronopotentiometric measurements can be applied to the evaluation of diffusion coefficients of concentrated electrolytes, in cases where the Sand equation does not apply, by analysis of the early portion of the chronopotentiogram.