Application of electrochemical techniques in pyrochemical processes – Electrochemical behaviour of rare earths at W, Cd, Bi and Al electrodes

The electrochemical behaviour of some rare earths ions (REs) – from the light to heavy lanthanides (i.e. Ce, La, Pr, Gd, Er, Ho) and Y – were investigated in the eutectic LiCl–KCl at different substrates: (i) liquid metals Cd and Bi, (ii) aluminium, and (iii) tungsten. The electrode reaction of the RE(III)/RE couples at the Cd and Bi pool electrodes was elucidated by cyclic voltammetry. The differences between the equilibrium potential adopted by a RE electrode and the E1/2r observed with the same RE(III) solution at the liquid electrodes were consistent with the activity coefficients of RE in the liquid metal phase. The relative partial molar Gibbs energies and activities of RE in the RE–Cd and RE–Bi intermetallic compounds could be estimated by the analysis of the open circuit chronopotentiograms using Cd and Bi coated tungsten electrodes. The Gibbs energies of formation of different intermetallic compounds, as well as their molar entropies and enthalpies of formation were also calculated from the temperature dependence of the emf. The redox potential of the RE(III)/RE couples at the Al electrode was observed at more positive potentials than that at the inert electrode (W). This potential shift was explained by a lowering of the activity of the REs in the Al phases due to the formation of intermetallic compounds. Electromotive force measurements for various intermetallic compounds in two-phase coexisting states were carried out. The activities and relative partial molar Gibbs energies of REs were also obtained. Moreover, the molar entropies and enthalpies of the aluminium-rich alloys were also calculated from the temperature dependence of the emf measurements.