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

• Published in: Journal of solid state electrochemistry 2023-12, Vol.27 (12), p.3383-3391
• Main Authors: Osarinmwian, C., Roberts, E. P. L.
• Format: Article
• Language: English
• Subjects: Analytical Chemistry; Calcium chloride; Cathodes; Characterization and Evaluation of Materials; Chemistry; Chemistry and Materials Science; Condensed Matter Physics; Deoxidizing; Electrochemistry; Energy Storage; Ionization; Mathematical models; Metal oxides; Molten salts; Nonequilibrium thermodynamics; Ohmic drop; Original Paper; Oxygen; Physical Chemistry; Thermodynamic equilibrium; Titanium dioxide; Voltage drop
• ISSN: 1432-8488; 1433-0768
• DOI: 10.1007/s10008-023-05624-5

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 TiO 2 (s) + 4e −   → Ti(s) + 2O 2− at the 3PI. The model couples slow diffusive transport and fast oxygen ionisation while assuming a negligible ohmic potential drop in bulk molten CaCl 2 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.