Electrodeposition of magnesium from halide melts—charge transfer and diffusion kinetics

Electrodeposition of magnesium from molten MgCl 2 and MgCl 2-MgF 2 (78-22 mol%) mixtures was studied by electrochemical techniques. The process was found to be quasi-reversible with a cathodic electrochemical rate constant for the total charge transfer reaction of about 10 −3 cm/s at 780 °C. Underpotential deposition of an adsorbed layer of reduced Mg(II)-species was observed during cyclic voltammetry and potential step measurements. The electrode capacitance was determined from galvanostatic pulses and found to depend on the applied current density. This has been explained by a pseudo capacitance due to the formation of an adsorbed layer. At potentials positive to nucleation a diffusion controlled process was identified. The determined concentration of the diffusing species, as well as the fact that Mg(II) species are the only cations in the system, indicate that the process is diffusion of dissolved metal (the product) from the electrode interface to the electrolyte.