Film electrode for the kinetic study of irreversible solid-to-solid reactions

Abstract

In the last twenty years, direct electro-reduction of solid oxides in molten salt has been investigated extensively. Compared with thermodynamics, kinetics of this solid-to-solid reaction is far less concerned. In this work, Butler-Volmer model was adapted for cyclic voltammetric study of this reaction. Film electrode with three kinds of geometry was proposed to obtain kinetic parameters from the current-overpotential equations. AgCl reduction in 0.5 M KCl–0.5 M NaNO3 was chosen as a model system. The main predictions of planar electrode and cylindrical electrode (inward) were well demonstrated by the electrodes constructed in this work, and the characteristics of cylindrical electrode (outward) were revealed by experimental results from literature. With the obtained relations, kinetics of AgCl/Ag reaction was carefully examined. By in-situ formation of UO2 film on a planar surface in LiCl-KCl-UO2Cl2, the charge transfer coefficient and exchange current density for UO2/U reaction were obtained as 0.19 and 20 mA cm−2, respectively. With the model, electron transfer in an irreversible solid state reaction can now be quantitatively described.