Chemical and electrochemical behaviour of chromium in molten chlorides

The chemical and electrochemical behaviour of chromium ions in the equimolar CaCl2 + NaCl mixture at 550°C has been extensively studied by employing various electrochemical techniques including cyclic and square wave voltammetry, chronopotentiometry, chronoamperometry, and potentiometry, utilizing different substrates such as molybdenum, tungsten, and glassy carbon as working electrodes. This study also includes comparative analyses with results obtained from the eutectic LiCl + KCl mixture at 450°C. It was observed that the stable oxidation states of chromium species were consistent across both molten salts. Notably, in the CaCl2 + NaCl melt with tungsten substrates, a novel electrochemical wave was detected, attributed to the enhanced oxidation of trace amounts of O2− ions facilitated by the presence of chromium ions. Kinetic parameters related to mass and charge transfer during the electrochemical reduction and oxidation of Cr(II) ions, as well as the reduction of Cr(III), were derived. Additionally, chronoamperometric studies revealed no nucleation events in the CaCl2 + NaCl mixture across all substrates. However, in the LiCl + KCl melt, the nucleation of metallic chromium on glassy carbon substrates was found to dominate the electroreduction process. The study also examined the electrochemical behaviour of chromium oxo-anions, such as Cr2O72− and CrO42−, revealing that these complexes produce stable Cr(VI) species in the melt.