Title

The Thermal Electrolytic Production of Mg from MgO: A Discussion of the Electrochemical Reaction Kinetics and Requisite Mass Transport Processes

Document Type

Article

Publication Date

7-12-2016

Journal Title

Chemical Engineering Science

Volume

148

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−2and 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-" role="presentation" style="box-sizing: border-box; display: inline-block; line-height: normal; font-size: 14.4px; word-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; color: rgb(80, 80, 80); font-family: Arial, Helvetica, "Lucida Sans Unicode", "Microsoft Sans Serif", "Segoe UI Symbol", STIXGeneral, "Cambria Math", "Arial Unicode MS", sans-serif; position: relative;">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.