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.
Bibliographical noteFunding Information:
We are grateful to the US Department of Energy ARPA–E program for financial support for this work under the cooperative agreement DE–AR0000421 . The conductivity part of this work was financially supported by Slovak Grant Agency project VEGA–2/0116/14 . We are further grateful to Miroslav Boca (Slovak Academy of Sciences, Bratislava) for his contribution to the conductivity measurement experiments.
- Electrochemical impedance spectroscopy
- Magnesium electrolysis from MgO
- Molten salts
- Solar thermal electrochemistry