Electrochemical Separation of Barium Ions from LiCl-KCl-BaCl2 Electrolyte Using Liquid Bi and Sb Electrodes
Authors: Kim Hojong , Lichtenstein Timothy , Smith Nathan Douglas 
Published: February 2020

Barium is one of the most reactive alkaline-earth metals, thus, its halide compounds are highly stable and are often employed as components of supporting electrolytes. For this reason, barium ions dissolved in halide electrolytes are one of the most challenging species to separate electrochemically. According to the standard reduction potentials of alkali/alkaline-earth elements in the chloride system at 600 °C, barium metal is expected to deposit last from LiCl-KCl-BaCl2 solution because BaCl­2 has the highest stability in the electrolyte, i.e., the lowest redox potential (E0
Ba2+/Ba = -3.74 V; E0
K+/K = -3.66 V; E0
Li+/Li = -3.49 V, vs. Cl–/Cl2(g)). However, by using liquid metal electrodes of bismuth and antimony, we will demonstrate that electrochemical separation of barium is possible in LiCl-KCl-BaCl2 electrolyte at 500‒700 °C.

The standard reduction potential analysis in LiCl-KCl-BaCl2 system suggests Li is the first species to deposit, followed by K and Ba; however, barium was found to be the first species to deposit into these liquid metal electrodes. During electrolysis in LiCl-KCl-BaCl2 (54-36-10 mol%) under a constant cathodic current density of 50 mA/cm2, barium ions were selectively deposited into the bismuth and antimony electrodes at 510 °C and 650 °C, respectively. The exceptional deposition behavior of barium ions was ascribed to the extremely low thermodynamic activity of barium in these liquid metals, implying strong chemical interactions between barium and the liquid metals.

In efforts to elucidate the deposition behavior of barium in LiCl-KCl-BaCl2, the thermodynamic properties of Ba-Bi and Ba-Sb alloys were determined by electromotive force (emf) measurements. Emf values were measured at 450–800 °C using a binary solid-state electrolyte (CaF2-BaF2, 97-3 mol%) at mole fractions x
Ba = 0.05–0.80 with the following electrochemical cells of [Ba(s) | CaF2-BaF2 | Ba-Bi] and [Ba(s) | CaF2-BaF2 | Ba-Sb]. Reproducible emf values within ±5 mV were obtained during cooling-heating cycle at dilute alloy compositions (x
Ba < 0.35), however, increased hysteresis was apparent for higher alloy compositions (x
Ba > 0.35) possibly due to the formation of meta-stable phases and/or unstable electrical contacts between the electrodes and the electrolyte during thermal cycling. For each alloy composition, the emf values were analyzed to determine the activity values (a
Ba), the excess partial molar Gibbs energy, and the temperature-independent partial molar entropy and enthalpy as well as the transition temperatures. The emf measurements were further corroborated using powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC) to determine the relevant crystal structures and phase transition temperatures for each alloy composition.

Based upon the emf measurements, the activity values of barium were determined to be as low as 6.6´10–
15 in liquid bismuth at 500 °C and 6.2´10–
15 in liquid antimony at 650 °C, both at barium mole fraction of x
Ba = 0.05. Such a low activity value can shift the redox potential of barium to the most positive potentials among constituent cations (Ba2+, K+, and Li+), resulting in the selective deposition of barium ions into liquid metals over lithium or potassium ions. By exploiting the differential interactions of constituent ions with liquid metals as well as the high liquid-state solubility of Ba in liquid metals, it was possible to separate conventionally non-separable barium species from the electrolyte solution.

This study suggests that liquid metals of Bi and Sb can potentially be employed in separating out alkali/alkaline-earth fission products such as Ba, Sr, and Cs from molten salt electrolytes in electrochemical processing for used nuclear fuel recycling, which may enable direct purification and recycling of molten salt electrolytes (e.g., LiCl-KCl eutectic) and reduce the volume of nuclear waste.