## Abstract

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 MgF_{2}-CaF_{2}, 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(Mg^{2+})=5.2±0.6E-5 cm^{2} s^{-1} and D(Mg2OF42-)=7.2±0.2E-6 cm^{2} 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.

Original language | English (US) |
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Pages (from-to) | 155-169 |

Number of pages | 15 |

Journal | Chemical Engineering Science |

Volume | 148 |

DOIs | |

State | Published - Jul 12 2016 |

### Bibliographical note

Funding 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.

## Keywords

- Chronoamperometry
- Electrochemical impedance spectroscopy
- Magnesium electrolysis from MgO
- Molten salts
- Solar thermal electrochemistry
- Voltammetry