Thermodynamic Properties of Nd in Liquid Nd-Bi Alloys Using Electrochemical Methods

Preliminary cyclic voltammograms performed on LiCl-KCl-NdCl₃ electrolyte (1 mol% NdCl₃) at 700°C show that the reduction of Nd³⁺/Nd(s) takes place at approximately -1.7V, which is equivalent to approximately -2.9V vs. Cl^-/Cl₂(g) in close agreement with the standard electrode potential based on the free energy of formation. The measurement was performed using an inert tungsten working electrode, graphite counter-electrode and Ag/Ag⁺ reference electrode with a thin-walled quartz tube (see Figure 1 in image file). With an aim to separate Nd using liquid bismuth electrode in LiCl-KCl molten salt electrolyte, it is important to understand the equilibrium redox potential of Nd in the liquid Bi. Thus, our work focuses on establishing the thermodynamic properties of Nd in liquid Nd-Bi alloys using electrochemical methods-electromotive force (emf) technique.

We will present an electrochemical cell that measures the emf values of Nd in Bi at various mole fractions (0 < x_{Nd (in Bi)} < 0.3) using the binary solid-state electrolyte CaF₂-NdF₃. The choice of CaF₂-NdF₃ is due to the thermodynamic stability of CaF₂, which will minimize side reactions and electronic conduction providing accurate thermodynamic properties.

The electrochemical cell for the Nd-Bi alloys is:

Nd(s) |CaF₂-NdF₃| Nd (in Bi)

where solid CaF₂-NdF₃ serves as the electrolyte, pure Nd as the reference electrode, and Nd-Bi alloys as the working electrode (see Figure 2 in image file). The electrode reactions are written below: 

                                Reference Electrode:           Nd(s) + 3F^- → NdF₃ + 3e^-

                                Working Electrode:             NdF₃ + 3e^- → Nd (in Bi) + 3F^-

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                                 Cell Reaction:                      Nd(s) → Nd (in Bi)

From the measured emf values  for this cell, the thermodynamic activity of Nd  is obtained using the Nernst equation (see Equation 1 in image file), where ΔG_{Nd (in Bi)} is the partial molar free energy of Nd in Bi, n is the number of electrons exchanged in the reaction (n = 3), F is the Faraday constant, R is the ideal gas constant, and T is the temperature. Using the temperature dependence of the emf, the partial molar entropy and enthalpy of Nd can be estimated using the Gibbs-Helmholtz equation (see Equations 2 and 3 in image file).

The solid CaF₂-NdF₃ electrolyte will be prepared from high purity CaF₂ powder (99.9%) and NdF₃ powder (99.9%) uniaxially pressed at 30 MPa into a pellet 75 mm wide by 17 mm thick and sintered at 1000°C for 3 hrs. The Nd-Bi alloys (0.05 < x_Nd < 0.30) will be prepared by melting high purity Nd (>99.1%) and Bi into ~10 mm diameter ingots using an arc melter and induction heater under an argon environment in order to eliminate reactions with air and water. The pure Nd reference electrodes will be similarly fabricated. The electrodes will then be placed into holes drilled into the CaF₂-NdF₃ pellet and electrical connections will be made with tungsten wires (see Figure 2 in image file).

The cell will then be placed into an alumina crucible and loaded into a test chamber. The test chamber will be placed inside a crucible furnace, evacuated to ~1 Pa, heated to 80°C and 270°C, holding for 8 hrs at each temperature for removal of residual moisture, and then the chamber will be purged with argon gas flowing at ~10 mL/min and will be heated to 900°C to ensure thorough melting of the electrodes to establish the electrical contacts with the electrolyte and electrical leads. 

The emf values will be measured using an AUTOLAB PGSTAT-302N potentiostat-galvanostat at 450°C to 900°C in 30°C increments. At each temperature step, the emf E_cell will be recorded for each working electrode and the measured data will be analyzed to calculate the temperature dependence to obtain thermodynamic properties of Nd in Bi electrode.