Two-Phase Rare-Earth Alloys as Reference Electrodes in Molten Chlorides for Reliable Electrochemical Measurements

Electromotive force measurements were used to examine pure rare-earth reference electrodes (Nd, Gd) in molten chloride salts (LiCl-KCl-RECl3) over a temperature range of T = 400-800 degrees C to determine the effects of disproportionation reactions (i.e., Nd + NdCl3 -> NdCl2) and active metal dissolution on their longevity and stability. Several methods for developing and calibrating a stable two-phase Gd-Bi electrode were investigated including the use of a pure Gd metal reference electrode continuously submerged in the electrolyte, a pure Gd metal reference electrode intermittently dipped into the electrolyte, and a transient technique that deposited pure Gd onto an inert W wire. Electromotive force measurements were also employed to examine the lifetime of the two-phase alloy as the reference electrode in comparison to a pure rare-earth metal as the reference electrode. Pure rare-earth metal did not exhibit stability for more than 15 h, whereas two-phase Gd-Bi electrodes were found to be stable for over 24 days at T = 500-700 degrees C.