ELECTRODEPOSITION OF MAGNESIUM FROM HALIDE MELTS - CHARGE-TRANSFER AND DIFFUSION KINETICS

Electrodeposition of magnesium from molten MgCl2 and MgCl2-MgF2 (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 degrees C. Underpotential deposition of an adsorbed layer of re duced Mg(II)-species was observed during cyclic voltammetry and potent ial step measurements. The electrode capacitance was determined from g alvanostatic 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 diffusio n controlled process was identified. The determined concentration of t he 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 elect rolyte. (C) 1997 Elsevier Science Ltd.