Anodic Overpotentials in the Electrolysis of Alumina

Abstract

Overpotentials for the anodic reaetion in the electrolysis of alumina dissolved in molten fluoride electrolytes have been measured by a steady state technique using C, CO₂ + CO/Al₂O₃(l) reference electrode. Overpotentials for the discharge of oxygen-containing anions in the systems Na₂AlF₆ + CaF₂ + Al₂O₃ and Na₃AlF₆ + Li₃₋AlF₆ + Al₂O₃ are reported as a function of current density, temperature, and solvent composition. The linear portion of the Tafel curves extends over approximately 1.5 powers of ten in current density. The slopes are independent of solvent. The heat of activation evaluated from the temperature dependence is 13 ± 4 kcal/mole, which is in excellent agreement with the value determined by potentiostatic means. A theoretical kinetic analysis of possible anodic reactions leading to the evolution of, CO₂ is presented. In each of the possible routes it appears that the rate-determining; step is a two-electron transfer reaction in which oxide ions or oxygen-containing anions are discharged.