Metal deposition from chloride melts: I. Rates of diffusion in solvent melt

Electrochemical reduction of metal ions at glassy carbon and tungsten electrodes in molten KCl–LiCl eutectic was studied by linear sweep voltammetry (LSV), convolutive potential sweep voltammetry (CPSV), chronopotentiometry and chronoamperometry. The metal–metal ion systems studied consisted of Pb/Pb(II), Zn/Zn(II) and Mg/Mg(II). In the case of lead deposition, an initial nucleation stage was found to influence the shape of the voltammetric current peak. Because of this, CPSV was believed to be a more reliable method than LSV for the calculation of the diffusion coefficient of the Pb(II) ion. Such types of complications were not observed for zinc and magnesium, since the nucleation overpotentials for these metals were significantly lower than for lead. The convolution voltammograms for magnesium were difficult to interpret because of lithium co-deposition at these negative potentials. Chronopotentiometry and voltammetry yielded practically identical results for DPb(II). Chronopotentiometric results for reduction of Zn(II)-ions and Mg(II)-ions did not obey the Sand equation very well due to residual currents. Neither was determination of diffusion coefficients from single potentiostatic current transients and the Cottrell equation found to be very reliable for such determinations. LSV seemed to be the most accurate method for determining diffusion coefficients for Zn(II) and Mg(II) ions. Empirical expressions for the temperature dependency of the diffusion coefficient in the temperature range 400–500 °C were calculated for all three ions.