The thermodynamic activities of NdCl3 and GdCl3 in molten LiCl-KCl eutectic were measured across a broad concentration range using novel solute- and solvent-based electrodes. These electrodes use stable binary metal alloys of known activity to eliminate measurement errors related to electrochemical reversibility and electrode material instability or reactivity, meaning that the activity of molten salt species can be accurately determined by measuring the electromotive force between combinations of the solute- and solvent-based electrodes. In this method, the relative activity of each species was defined first as a function of concentration. Next, the electromotive force between a NdCl3- or GdCl3-based electrode and a solvent-based electrode with a known electromotive force was measured, which allowed the activity of NdCl3 and GdCl3 to be calculated for the concentrations measured in the first electrochemical cell. This two-step method stabilizes the electroactive species so that electromotive force measurements can be used to accurately determine the activity of an electroactive species over a wide concentration range and provides a simple approach to measuring thermochemical properties of molten salts. The approach will be described in detail with pertinent examples.

Measuring activity values in molten salts using solute- and solvent-based electrochemical systems

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