Solute- and solvent-based electromotive force cells for activity measurements of electroactive species in molten salts

Abstract

A novel combination of solute- and solvent-based electrodes was used to determine the activities of NdCl3 and GdCl3 in molten LiCl-KCl eutectic using electromotive force measurements across a broad concentration range of dissolved electroactive species. First, an electrochemical cell using solute-based electrodes was used to define the relative activity of each species as a function of concentration and the concentration region in which Henry's law was applicable. Next, the electromotive force between a solute-based electrode with a known NdCl3 or GdCl3 concentration and a solvent-based electrode with a known electromotive force was measured. The solvent-based electrode potential was converted to the Lithium Eutectic Electrode (LEE) potential, which allowed the activity of dissolved NdCl3 and GdCl3 to be calculated at the concentrations measured using the solute-based electrodes. This two-cell method uses electromotive force measurements to accurately determine the activity of electroactive species over a wide concentration range and provides a simple approach to closing the knowledge gap of thermochemical properties of molten salts.