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 c arbon and tungsten electrodes in molten KCl-LiCl eutectic was studied by li near sweep voltammetry (LSV), convolutive potential sweep voltammetry (CPSV ), chronopotentiometry and chronoamperometry. In the case of lead depositio n, the initial nucleation stage was found to influence the shape of the vol tammetric current peak. CPSV was consequently believed to be a more reliabl e method than LSV for the calculation of the diffusion coefficient of the P b(II) ion. Similar complications were not observed for zinc and magnesium s ince the nucleation overpotentials for these metals were significantly lowe r than for lead. On the other hand, lithium codeposition made it difficult to interpret the zinc and especially the magnesium convolution voltammogram s. Chronopotentiometry yielded practically identical results for D-Pb(II) a s the voltammetric techniques. However, due to a substantial residual curre nt, the Sand equation was not obeyed for the Zn(II) and Mg(II) ions. Determ ination of diffusion coefficients from single potentiostatic current transi ents 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 degreesC were calculated for all three ions.