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

Abstract

We examined the kinetic and transport processes involved in Mg production from MgO via electrolysis at ca 1250K 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±4Acm−2 and 0.5±0.12 respectively. The kinetics of the anode are described by two Tafel equations: at an overvoltage below 0.4V, the exchange current density is 0.81±0.2mAcm−2 with an anodic transfer coefficient of 0.5±0.1; above 0.4V overvoltage the values are 0.14±0.05mAcm−2 and 0.7±0.2 respectively. The diffusion coefficients of the electroactive species are D(Mg2+)=5.2±0.6E−5cm2s−1 and D(Mg2OF42-)=7.2±0.2E−6cm2s−1. The ionic conductivity of the electrolyte is ca 2.6Scm−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 0.5Acm−2, enabling a reduction in operating cost if the thermal energy is substituted for valuable electric work.