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.
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