Electrochemical and Raman Spectroscopic Investigations on the Speciation and Behavior of Chromium Ions in Fluoride Doped Molten LiCl-KCl

The speciation and behavior of chromium ions in LiCl-KCl-CrF3 and LiCl-KCl-LiF-CrF3 melts were investigated by using cyclic voltammetry, square wave voltammetry, chronopotentiometry and Raman spectroscopy. Electrochemical reduction of Cr(III) to Cr(0) in LiCl-KCl-CrF3 is a two-step process mediated by the formation of Cr(II). Most Cr(III) ions exist in the form of CrCl63− while CrCl6-xFx3− (x ≤ 3) ions are minor in the molten salt medium. Based on the cyclic voltammetric results obtained at different potential scanning rates, the diffusion coefficients of Cr(III) and Cr(II) in LiCl-KCl-CrF3 melt were determined between 723 and 873 K, and their dependence on temperature can be described as lnDCr(III) = −4.90-4322.66/T and lnDCr(II) = −4.71-4007.09/T respectively. The diffusion coefficients of chromium ions in LiCl-KCl-CrF3 are higher than those in LiCl-KCl-CrCl3, which can be explained by the hopping mechanism. In addition, a series of apparent standard potentials of the Cr(III)/Cr(II) and Cr(II)/Cr(0) couples were obtained over the same temperature range using the open-circuit chronopotentiometry technique. With the addition of LiF into the LiCl-KCl-CrF3 mixture, the dominant chromium species change from CrCl63− to CrCl6-xFx3− (x ≤ 6), which is supported by the appearance of the new peaks in both square wave voltammograms and Raman spectra.