Electrochemical analysis of diffusion behavior and nucleation mechanism for Dy(II) and Dy(III) in phosphonium-based ionic liquids

In this study, the solubility, diffusion coefficient, and nucleation behavior of rare-earth metal complexes in a phosphonium-based ionic liquid (IL) were investigated in order to analyze the electrodeposition mechanism of the Dy metal. For the solubility of Dy(TFSA)3 and Nd(TFSA)3 in an IL, the temperature dependence was also evaluated from 340 to 470 K using ultraviolet-visible spectroscopy. The solubility curves obtained for Dy(III) and Nd(III) have similar tendencies and show a relatively good fit with the modified Apelblat equation. A series of thermodynamic parameters (ΔsolG∞, ΔsolH∞, and ΔsolS∞) were also estimated, and the results reveal that the dissolution process of rare-earth metal salts in polar ILs is controlled by enthalpy. For electrochemical analyses, cyclic voltammetric measurements reveal that the reduction process of Dy(III) proceeds in two steps by way of Dy(II) [Dy(III) + e− → Dy(II), Dy(II) + 2e− → Dy(0)], when the IL contains a small amount of water (<100 ppm). The number of electrons transferred in the first cathodic reaction at −2.45 V was 0.99 ± 0.01, as evaluated by the EMF method, and a divalent dysprosium complex was found to exist in the IL using Raman and fluorescence spectroscopic analyses. The diffusion coefficients of Dy(II) and Dy(III) in an IL were measured from 318 to 378 K using a semi-integral analysis. The diffusion coefficients of Dy(II) were larger than those of Dy(III) in the entire range of temperatures measured, and the activation energies for diffusion of Dy(II) and Dy(III) were 28.0 and 53.4 kJ mol−1, respectively. In addition, the nucleation and growth processes of Dy(0) were evaluated from chronoamperometry. These results indicate that the nucleation mechanism of Dy(0) changes from instantaneous nucleation to progressive nucleation when the applied overpotential becomes more negative than the deposition peak potential of Dy(0), as estimated from a voltammogram. Furthermore, electrowinning of the Dy metal from 0.1 mol dm−3 Dy(III) in an IL was carried out using a three-electrode system. The surface morphology of the electrodeposits showed accumulated round particles with 0.8-0.9 μm diameter, as observed by a scanning electron microscope. Most of the Dy electrodeposits were in the metallic state, while a part of their top surface was in the oxidation state, as evaluated from the EDX, XPS, and XRD analyses.