Designing and fabricating a stable thermodynamic reference electrode (TRE) for use in molten fluoride salts has proven challenging for several reasons. An ideal TRE will not allow mass diffusion of molten salt across its system boundary while it will allow ionic contact across the system boundary. Appropriate material candidates for the fabrication of a TRE need to remain inert in molten salt environments and be electrically insulating, all while remaining stable at temperatures greater than 700C. UC Berkeley has been working with Ultramet on the Department of Energy SBIR DE-FOA-0002359 to develop such a TRE for 2LiF-Be2F (FLiBe) which is resilient to degradation after long term FLiBe exposure, produces repeatable electrochemical data, and can be incorporated into molten salt systems modularly.

This first testing campaign consisted of two main experimental thrusts which characterized the materials degradation and the electrochemical response of the TRE. The first experimental thrust concerns the degradation of a boron-nitride (BN) coating on a porous graphite substrate under long term exposure to FLiBe at high temperatures. Six (6) unique BN coated graphite foam coupons were exposed to FLiBe held at 700C for 96 hours. The cross sections of each coupon were imaged by macro lens photography for initial qualitative results which help determine the presence and efficacy of the BN coating after the FLiBe exposure. The next phase of research in this thrust requires SEM/EDS imaging of the coupons for quantitative results that will determine the approximate BN thickness remaining on the coupons and indicate if any materials migrated into the graphite foam pores during the exposure. The second experimental thrust characterizes the electrochemical response of the novel TRE in FLiBe. The Ni/NiF2 redox couple was used as a reference reaction for this study since it has been previously studied in FLiBe. Ultramet provided UC Berkeley with open-ended cylinder TRE bodies for electrochemical characterization. Each TRE body consists of graphite foam coated in BN and contained lanthanum-fluoride (LaF3) coated on the inside diameter which acts as an ionic membrane and a mass barrier. Ultramet’s TRE was loaded with NiF2 and assembled with a Ni wire electrode. Open circuit potential (OCP) and polarizability of the TRE were measured using a Gamry Ref 600 potentiostat, where the novel TRE was connected to the working lead, a second Ni wire was connected to the counter lead, and a third Ni wire was connected to the reference lead. Initial polarizability testing confirmed there was ionic contact between the interior TRE body and the exterior bulk salt. Next steps include characterizing the degradation to the LaF3 membrane to ensure it still functions as a mass barrier. Long term OCP drift can be studied after confirmation the LaF3 membrane still functions. These results indicate the novel TRE shows promise for accurately monitoring salt chemistry under high temperature environments for timescales up to 96 hours and beyond.