The thermal electrolytic production of Mg from MgO: A discussion of the electrochemical reaction kinetics and requisite mass transport processes

We examined the kinetic and transport processes involved in Mg production from MgO via electrolysis at ca 1250 K with in a eutectic mixture of MgF₂-CaF₂, using a Mo cathode, and carbon anode. Exchange current densities, transfer coefficients, and diffusion coefficients of the electroactive species were established using a combination of cyclic and linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The cathode kinetics are described by a concentration dependent Butler-Volmer equation. The exchange current density and cathodic transfer coefficient are 11±4 A cm^-2 and 0.5±0.12 respectively. The kinetics of the anode are described by two Tafel equations: at an overvoltage below 0.4 V, the exchange current density is 0.81±0.2 mA cm^-2 with an anodic transfer coefficient of 0.5±0.1; above 0.4 V overvoltage the values are 0.14±0.05 mA cm^-2 and 0.7±0.2 respectively. The diffusion coefficients of the electroactive species are D(Mg²⁺)=5.2±0.6E-5 cm² s^-1 and D(Mg2OF42-)=7.2±0.2E-6 cm² s^-1. The ionic conductivity of the electrolyte is ca 2.6 S cm^-1. A 3D finite element model of a simple cell geometry incorporating these kinetic and transport parameters suggest that up to 27% of the energy required to drive the electrolysis reaction can be supplied thermally for a current density of 0.5 A cm^-2, enabling a reduction in operating cost if the thermal energy is substituted for valuable electric work.