Novel electrochemical measurements on direct electro-deoxidation of solid TiO2 and ZrO2 in molten calcium chloride medium

A solid metal oxide cathode undergoes significant chemical changes during the molten salt electro-deoxidation process. The changes in the chemical composition lead to changes in the electrical resistivity and potential of the electrode. Two novel electrochemical techniques, based on these two parameters, have been employed to study the electro-deoxidation of solid TiO₂ and ZrO₂ in molten calcium chloride at 900 °C. The in situ resistance measurements carried out by the IR drop method conclusively proved that TiO₂ electrode remains highly conducting throughout the electro-deoxidation process and hence is amenable for reduction. The ZrO₂ electrode, on the other hand, developed very high resistance midway in the electro-deoxidation, and could not be reduced completely. The resistance measurements give strong indication that the electron-transfer reactions taking place at the cathode determine the rate and efficiency of the electro-deoxidation process to a great extent. The low-current galvanostatic electro-deoxidation of TiO₂ electrodes, in conjunction with a graphite pseudo reference electrode to monitor the half cell potentials, showed that the metal oxide passes through two stages during the electrolysis; a high current, low resistant stage 1, where Ca²⁺ ions are inserted to the metal oxide cathode to produce different intermediate compounds and stage 2 where electro-deoxidation of the cathode take place continuously. Removal of oxygen, from the cathode, in stage 1 of the electro-deoxidation is considered to be insignificant. The anodic and cathodic voltages in this stage remained more or less stable at ~1.4 V and ~−1 V, respectively. When the oxygen ions in the melt were depleted at the end of this stage, both the anode and cathode potentials were increased in the anodic direction and this behaviour suggested that the graphite pseudo reference electrode was changed from a C/CO electrode in stage 1 to a Ca²⁺/Ca electrode in stage 2.