Electrochemical Performance of the Li-Bi Reference Couple in Molten LiCl-Li2O-Li
Authors: Phillips William , Merwin Augustus , Chidambaram Dev 
Published: February 2020

Introduction

Closing the nuclear fuel cycle is a key step in the development of a more secure and environmentally responsible nuclear energy sector. Reprocessing of used nuclear fuel (UNF) is essential for all conceivable closed fuel cycles. To accomplish this goal, while simultaneously reducing the risk of nuclear proliferation, a pyrometallurgical reprocessing operation has been developed (1). Electrolytic reduction of UO2 in a LiCl-Li2O electrolyte at 650°C is an important step in this operation (2). As the electrolytic reduction process proceeds, Li is reduced on the cathode surface and subsequently dissolves into the electrolyte (3, 4). Corrosion of materials exposed to the resulting LiCl-Li2O-Li system has not been extensively tested, and the reference electrodes typically used in molten salt systems are not reliable in the presence of dissolved metallic Li. Therefore, the Li-Bi reference couple has been investigated as a means of providing a stable reference voltage for electrochemical measurements in the molten LiCl-Li­2O-Li system (5, 6).

Experimental

Experiments were conducted in an Ar filled glovebox located at the University of Nevada, Reno. The reference electrode was constructed using a porous graphite tube to act as an ion bridge which contained various concentrations of Li-Bi. The stability of the Li-Bi reference electrode was tested in an electrolyte of LiCl with various concentrations of Li2O and Li at 650°C.

Results

Open circuit potential measurements for Cu and Ni measured independently against a Li-Bi reference and each other are shown in Figure 1. It can be observed that the Li-Bi reference provided a stable potential for the measurement of the Ni and Cu electrodes and the independent OCP measurements of Ni and Cu vs. Li-Bi were in good agreement with the direct measurement of the Cu and Ni OCP. Further, the reference electrode thus constructed was verified to behave in a Nernstian manner.

Figure 1: Open circuit potential of Cu and Ni measured independently against the Li-Bi reference electrode and each other in LiCl-2wt%Li­2O-0.3wt%Li at 650°C

Acknowledgements: This work was performed under the auspices of the Department of Energy (DOE) under contracts DE-NE0008262 and DE-NE0008236, and the US Nuclear Regulatory Commission (NRC) under contracts NRCHQ-11-G-38-0039, NRC-HQ-10-G-38-0027, NRC-HQ-13-G-38-0027. W.P. and A.M acknowledge the Fellowship Award from the USNRC. Dr. Kenny Osborne serves as the program manager for the DOE award and Ms. Nancy Hebron-Isreal serves as the grants program officer for the NRC awards.

References:

1. J. J. Laidler, J. E. Battles, W. E. Miller, J. P. Ackerman, E. L. Carls, Development of Pyroprocessing Technology. Progress in Nuclear Energy
31, 131-140 (1997).
2. S. D. Herrmann, S. X. Li, M. F. Simpson, S. Phongikaroon, Electrolytic Reduction of Spent Nuclear Oxide Fuel as Part of an Integral Process to Separate and Recover Actinides from Fission Products. Separation Science and Technology
41, 1965-1983 (2006).
3. T. Takenaka, K. Shigeta, H. Masuhama, K. Kubota, Influence of Some Factors upon Electrodeposition of Liquid Li and Mg. ECS Transactions
49, 441-448 (2009).
4. A. S. Dworkin, H. R. Bronstein, M. A. Bredig, Miscibility of Metals with Salts. VI. Lithium-Lithium Halide Systems. The Journal of Physical Chemistry
66, 572-573 (1962).
5. X. Ning et al., Self-healing Li-Bi liquid metal battery for grid-scale energy storage. Journal of Power Sciences
275, 370-376 (2015).
6. M. Iizuka, Y. Sakamura, T. Inoue, Electrochemical reduction of (U–40Pu–5Np)O2 in molten LiCl electrolyte. Journal of Nuclear Materials
359, 102-113 (2006).

Figure 1