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

N. Leonard, M. Korenko, C. Larson, K. Blood, L. J. Venstrom, S. Nudehi, S. Duncan, R. Diver, F. Simko, J. Priscak, J. Schoer, P. T. Kissinger, R. Palumbo

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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 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.

Original languageEnglish (US)
Pages (from-to)155-169
Number of pages15
JournalChemical Engineering Science
Volume148
DOIs
StatePublished - 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

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