The electrochemical reactions of Tb(III) were investigated on a W electrode, Bi pool electrode and Bi film electrode in eutectic LiCl–KCl by transient electrochemical techniques. The exchange current densities of the Tb(III)/Tb(0) redox couple were determined on W and Bi film electrodes at different temperatures by the linear polarization method. On both Bi electrodes, the redox potential of Tb(III)/Tb couple was observed at less negative potential values than that on the W electrode, which indicated underpotential deposition of Tb occurring on the both Bi electrodes. The result of cyclic voltammetry performed on the Bi pool electrode suggested that the electrochemical reaction of Tb(III) to Tbin liquid Bi was a quasi-reversible and diffusion-controlled process. From the cyclic voltammogram and square wave voltammogram of Tb(III) obtained on the Bi film electrode, three reduction signals corresponded to the formation of Tb–Bi intermetallic compounds. The thermodynamic data, such as the activities of Tb in Tb–Bi alloys and the standard Gibbs free energies of formation for different Tb–Bi intermetallic compounds, were estimated using open circuit chronopotentiometry in the temperature range from 673 to 873 K. Moreover, the electrochemical preparation of Tb–Bi alloys was conducted in LiCl–KCl–TbCl3 melts on a liquid Bi electrode by galvanostatic and potentiostatic electrolysis, respectively. The Tb–Bi alloys were characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). XRD results showed that the Tb–Bi alloys were composed of the TbBi phase and TbBi, TbBi3/4 and TbBi3/5 phases, respectively.

The electrochemistry of praseodymium was studied on liquid Zn film electrode in LiCl-KCl eutectic melts by transient electrochemical techniques. Cyclic voltammogram and square wave voltammogram showed that eight reduction peaks, corresponding to the formation of eight Pr-Zn intermetallic compounds, were observed at less negative potential values, which indicated the underpotential deposition of praseodymium occurs on liquid Zn film electrode. Thermodynamic properties on the formation for Pr-Zn intermetallic compounds, such as activities and relative partial molar Gibbs free energies of Pr in two-phase coexisting states as well as the standard Gibbs free energies of formation for Pr-Zn intermetallic compounds, were estimated using electromotive force (emf) measurement in the temperature range from 723 to 873 K. Electrochemical preparation of Pr-Zn alloys was executed by potentiostatic and galvanostatic electrolysis on Zn film electrode and liquid Zn pool electrode, respectively. X-ray diffraction (XRD) indicated that Pr-Zn alloy were comprised of different Pr-Zn intermetallics. The surface microstructures and micro-zone chemical analyses of Pr-Zn alloys were also characterized by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The results of SEM-EDS displayed that the distribution of Pr is non-homogeneous in Pr-Zn alloys.

The electrochemical behavior of Yb(III) in LiCl–KCl–YbCl3 melt was studied on inert W and reactive Cu electrodes by cyclic voltammetry, square wave voltammetry and open circuit chronopotentiometry. The results indicated that the reduction of Yb(III) on W electrode takes place in only one step of Yb(III) to Yb(II). The system of Yb(II)/Yb(0) was not observed within the electrochemical windows, which inhibits the extraction of Yb from the melt on an inert electrode. In contrast, the electroreduction of Yb(II) occured at a more positive potential on Cu electrode than that on W electrode, due to the formation of various Yb–Cu intermetallic compounds. The extraction of ytterbium on Cu electrode was performed by galvanostatic and potentiostatic electrolysis to prepare Yb–Cu alloys, respectively. These samples were characterized by X–ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS). XRD results showed that the Yb-Cu compounds, YbCu, YbCu2, YbCu5, YbCu6.5 and Yb0.1Cu0.99, were obtained. The working electrode was replaced and characterized by SEM and XRD during electroextraction every 3 h. It was found that thermodynamic metastable phase was easily formed even if the concentration of Yb(III) decreases to 3.95 × 10−8 mol cm−3 in LiCl–KCl melt. The extraction efficiency was about 99.9% for Yb(III) after potentiostatic electrolysis at −2.3 V for 18 h.

Rare earth (RE) metals and their alloys have attracted considerable practical interests due to their functional properties. Because of their negative deposition potentials, RE metals cannot be electrochemically deposited from aqueous media. Using molten salt as medium provides a unique opportunity for the electrowinning and electrorefining of high-purity RE metals, as well as for the electrochemical formation of their alloys and intermetallic compounds. Certainly, the electrochemical behaviors of RE metals and their alloys have been investigated in a number of different molten salts comprising all-fluorides, all-chlorides and mixed chloride-fluoride media. Based on the results, RE and their alloys were produced by molten salt electrolysis. In this paper, the developments of preparation of RE metals and their alloys by electrolysis in molten salts in recent years were systematically summarized on both the local and international levels. Attention was paid mainly to the electrodeposition of RE metals and their alloys, including RE–Mg, RE–Al, RE–Ni, RE–Co, RE–Cu, RE–Fe and RE–Zn alloys.

The electrochemical behavior of Tb(III) in the LiCl-KCl eutectic melts was studied on Mo and Al electrodes by cyclic voltammetry, square wave voltammetry and open circuit chronopotentiometry in the temperature range of 773–873 K. On a Mo electrode, the reduction of Tb(III) was one-step electrochemical process. The diffusion coefficient of Tb(III) was determined by applying the Berzins and Delahay equation. The activation energy for diffusion of Tb(III) ions was found to be 30.5 kJ mol−1. The equilibrium potential of Tb(III)/Tb(0) redox couple was measured by open circuit chronopotentiometry, with subsequent calculation of the apparent standard potential, ETb(III)/Tb(0)*0, and the apparent Gibbs free energy of formation, ΔGf*0(TbCl3). The activity coefficients for Tb(III), γTb(III)γTb(III) was also determined from the difference of apparent and standard Gibbs free energies of formation, ΔGf*0(TbCl3)−ΔGf0(TbCl3,SC). On an Al electrode, the reduction potential of Tb(III)/Tb was observed at more positive potential values than that on Mo electrode, due to the formation of Al-Tb intermetallic compound when Tb(III) ions react with the Al substrate. The TbAl2 intermetallic compound characterized by XRD (X-ray diffraction) and SEM-EDS (scanning electron microscopy and energy dispersive spectrometer), was obtained in the LiCl-KCl melts containing Tb(III) by potentiostatic electrolysis at −1.8 V and −1.9 V (vs Ag/Ag+), respectively. The activity of Tb, in the Al phase, as well as the standard Gibbs free energy, enthalpy and entropy of formation for TbAl2 were estimated from the open circuit chronopotentiometric measurements.

The electrochemical formation of Mg-Al-Y alloys was studied in the LiCl-NaCl-MgCl2 melts by the addition of AlF3 and YCl3 on a molybdenum electrode at 973 K (700 °C). In order to reduce the volatilization of salt solvent in the electrolysis process, the volatile loss of LiCl-NaCl-MgCl2 and LiCl-KCl-MgCl2 melts was first measured in the temperature range from 873 K to 1023 K (600 °C to 750 °C). Then, the electrochemical behaviors of Mg(II), Al(III), Y(III) ions and alloy formation processes were investigated by cyclic voltammetry, chronopotentiometry, and open circuit chronopotentiometry. The cyclic voltammograms indicate that the under-potential deposition of magnesium and yttrium on pre-deposited Al leads to formation of Mg-Al and Al-Y intermetallic compounds. The Mg-Al-Y alloys were prepared by galvanostatic electrolysis in the LiCl-NaCl-MgCl2-AlF3-YCl3 melts and characterized by X-ray diffraction and scanning electron microscopy with energy dispersive spectrometry. Composition of the alloys was analyzed by inductively coupled plasma-atomic emission spectrometer, and current efficiency was also determined by the alloy composition.

The work presents an electrochemical study on the formation of Ni-Tb intermetallic compounds in the LiCl-KCl-TbCl3 melts on tungsten and nickel electrodes at 773 K (500 °C) by electrochemical techniques. For a tungsten electrode, cyclic voltammetry and square-wave voltammetry showed that the electrochemical reduction of Tb(III) proceeded in a one-step process involving three electrons at −2.06 V (vs Ag/AgCl). For a nickel electrode, the reduction potential of Tb(III)/Tb was observed at more positive values than those on W electrode by cyclic voltammetry, due to the formation of Ni-Tb intermetallic compounds. Square-wave voltammetry and open-circuit chronopotentiometry put into evidence the formation of intermetallic compounds at around −1.27, −1.63, and −1.88 V, respectively. Three alloy samples were obtained by potentiostatic electrolysis on a Ni electrode at various potentials and analyzed by X-ray diffraction, scanning electron micrograph, and energy-dispersive spectrometry. The analysis results confirmed the formation of Ni17Tb2, Ni5Tb, and Ni2Tb alloy compounds.

This work presents an electrochemical extraction of cerium and synthesization of Al-Ce alloy in LiCl-KCl melts on Mo and Al electrodes by chlorination of CeO2 using AlCl3 at 873 K. The cyclic voltammogram on Mo electrodes in LiCl-KCl-CeO2 melt showed no obvious reduction wave other than the reduction of Li(I). After the addition of AlCl3, the signals of the reaction of Ce(III)/Ce(0) and the synthesization of Al-Ce and Al-Li alloys were investigated by cyclic voltammetry, square-wave voltammetry, open-circuit chronopotentiometry and chronopotentiometry. These results indicated that AlCl3 can chloridize CeO2 and that it is possible to extract cerium and form Al-Ce and Al-Li-Ce alloys in LiCl-KCl-CeO2-AlCl3 melts. According to potentiostatic electrolysis, only the Al4Ce layer coated the Al electrodes. According to galvanostatic electrolysis, Al-Ce (Al4Ce, Al3Ce, and Al92Ce8), Al2Li3, and Al phases were formed on Mo electrodes, and the content of cerium in the Al-Li-Ce alloys was more than 17 wt%.

An electrochemical approach for the preparation of Mg-Li-Ce alloys by co-reduction of Mg, Li and Ce on a molybdenum electrode in KCl-LiCl-MgCl2-CeCl3 melts at 873 K was investigated. Cyclic voltammograms (CVs) and square wave voltammograms indicated that the underpotential deposition (UPD) of cerium on pre-deposited magnesium led to the formation of Mg-Ce alloys at electrode potentials around −1.87 V. The order of electrode reactions was as follows: discharge of Mg(II) to Mg-metal, UPD of Ce on the surface of pre-deposited Mg with formation of Mg-Ce alloys, discharge of Ce(III) to Ce-metal and after that the discharge of Li+ with the deposition of Mg-Li-Ce alloys, which was investigated by CVs, chronoamperometry, chronopotentiometry and open circuit chronopotentiometry. X-ray diffraction (XRD) illuminated that Mg-Li-Ce alloys with different phases were obtained via galvanostatic electrolysis by different current densities. The microstructures of Mg-Li-Ce alloys were characterized by optical microscopy (OM) and scanning electron microscopy (SEM), respectively. The analysis of energy dispersive spectrometry (EDS) showed that Ce existed at grain boundaries to restrain the grain growth. The compositions and the average grain sizes of Mg-Li-Ce alloys could be obtained controllably corresponding with the phase structures of the XRD patterns.XRD patterns of all samples obtained by galvanostatic electrolysis with different current densities on Mo electrodes in LiCl-KCl-6.0 wt.% MgCl2-3.3 wt.%CeCl3 melts at 873 K for 2 h (1) Sample 1, −3.11 A/cm2; (2) Sample 2, −4.66 A/cm2; (3) Sample 3, −6.21 A/cm2; (4) Sample 4, −7.76 A/cm2; (5) Sample 5, −9.32 A/cm2