Determination of diffusion coefficients of depositing ions in molten chlorides by transient electrochemical techniques

The electrochemical reduction of Pb(II), Zn(II) and Mg(II) 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. In the case of lead deposition, the initial nucleation stage was found to influence the shape of the voltammetric current peak. CPSV was consequently believed to be a more reliable method than LSV for the calculation of the diffusion coefficient of the Pb(II) ion. Similar complications were not observed for zinc and magnesium since the nucleation overpotentials for these metals were significantly lower than for lead. On the other hand, lithium codeposition made it difficult to interpret the zinc and especially the magnesium convolution voltammograms. Chronopotentiometry yielded practically identical results for D_Pb(II) as the voltammetric techniques. However, due to a substantial residual current, the Sand equation was not obeyed for the Zn(II) and Mg(II) ions. Determination of diffusion coefficients from single potentiostatic current transients and the Cottrell equation was not found to be a very reliable method. Empirical expressions for the temperature dependence of D in the range 400–500 °C were calculated for all three ions.