Modelling diffusion controlled electro-deoxidation of metal oxide to metal in molten salt

Abstract

Diffusion is a fundamental irreversible process intervening in the evolution of many out-of-equilibrium systems and is successfully described by Fick’s law obtained from non-equilibrium thermodynamics. Despite this, numerical simulations of solid state electro-deoxidation in the diffusion-controlled regime in molten salt remain elusive. Here, a new model for diffusion controlled three-phase interline (3PI) penetration in a porous cathode during electro-deoxidation is validated against experimental observation. This penetrating 3PI model is applied at high overpotential and benchmarked using the oxygen ionisation TiO2(s) + 4e− → Ti(s) + 2O2− at the 3PI. The model couples slow diffusive transport and fast oxygen ionisation while assuming a negligible ohmic potential drop in bulk molten CaCl2 electrolyte. The 14 nm s−1 penetration rate of the 3PI and the order of magnitude of 3PI currents (derived from an exchange current density and cathodic transfer coefficient of 0.32 A cm−2 and 0.01, respectively) in the chronoamperometric data for porous cathodes are in good agreement with experimental observation.