Characterization of a Thermodynamic Reference Electrode for Molten LiF-BeF2 (FLiBe): Part II. Materials Analysis

A previous publication reported the electrochemical characterization of a thermodynamic reference electrode (TRE) for 2LiF-BeF2 (FLiBe) that used the Ni/Ni(II) couple, a BN body and a LaF3 ionic membrane [F. Carotti, H.Wu and R.O. Scarlat, J. Electrochem. Soc. 164, 12 (2017)]. This paper presents the post-operation characterization of the two TREs, operated in the previous publication for 100 hours and 15 hours: ICP-MS of salt samples and SEM/EDS and XRD of BN samples. The results confirm Ni(II) depletion from FLiBe by reaction of NiF2 with the BN electrode body. NiF2 concentration decreases to 0.090 ± 0.011 mol%, measured by ICP-MS. Metallic Ni powders on the BN tube inner surface are identified by XRD and SEM-EDS. The LaF3 membrane maintains its structural integrity after LaF3 reaches its solubility limit in FLiBe, determined to be 0.14 ± 0.03 mol% LaF3. The effective chemical separation between NiF2-doped FLiBe and the main FLiBe bath is confirmed by no increase in Ni content in the main salt bath. LaF3 mass transport to the main salt bath is limited by the 0.3 mm diameter hole in the BN tube that houses the TRE. No significant interaction between FLiBe and BN is detected, as demonstrated by no variation in the B content in the main salt bath. These results point to the feasibility of this design as a stable TRE, as long as NiF2 depletion is addressed and LaF3 is allowed to equilibrate to its solubility limit.