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

Abstract

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 TiO 2 and ZrO 2 in molten calcium chloride at 900°C. The in situ resistance measurements carried out by the IR drop method conclusively proved that TiO 2 electrode remains highly conducting throughout the electro-deoxidation process and hence is amenable for reduction. The ZrO 2 electrode, on the other hand, developed very high resistance midway in the electrodeoxidation, 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 TiO 2 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 Ca 2? 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 Ca 2? /Ca electrode in stage 2.

Difference in the Mechanism of Electrochemical Deoxidation of Conducting and Non-conducting Solid Oxide Preforms: An Experimental Demonstration with TiO2 and SiO2 Pellet Electrodes in CaCl2 Melt

“…The assembly of the reactor vessel with all the electrodes in place was carried out in an argon atmosphere glove box. The details of the cell-assembly are given [5] . After assembly, the cell was brought outside and heated to 1173 K with the help of a resistance heating furnace.…”

“…A solid metal part will be in close physical contact with the oxide preform through which the electrons from an external DC power source will be supplied to it. The thermodynamic stability and electrical characteristics of an oxide electrode are known to have strong influence on its electro-reduction behaviour [3,5,6] in addition to many other parameters such as applied voltage, duration of electrolysis, sintering temperature of the oxide performs etc. [3,7,8] .…”

“…[3,7,8] . Several studies have been reported on the electrochemical reduction of oxides such as TiO 2 [1,3,5,[8][9][10][11] , Nb 2 O 5 [7,12,13] , UO 2 [14][15][16] , SiO 2 [17][18][19][20][21][22][23][24][25][26] etc. The process is being pursued in view of production of solar grade silicon as well [27] .…”

Corrosion of high density graphite anodes during direct electrochemical de-oxidation of solid oxides in molten CaCl2 medium

“…[13,34] during the electro-deoxidation process. Many reports indicate that the unusually low current and energy efficiencies of the FFC process can be attributed to such secondary reactions relating to the use of graphite anode [13,[31][32][33][34][35]. However, a systematic study of the graphite corrosion in an electro-deoxidation cell has not been published.…”

“…The problem was studied by using graphite as anode in the electrochemical de-oxidation of powder compacted and sintered Nb 2 O 5 pellet electrodes in molten CaCl 2 at 1173 K and at a constant voltage of 3.1 V. The degradation of the graphite anodes, recovered from the electro-de-oxidation cells at predetermined intervals of time (2,9,22,26,35, 44 and 56 h) during the long-duration electrolysis, was determined by mass loss measurements and the morphological changes by SEM imaging. The graphite rods were found corroded differently along the lengths, with the maximum corrosion being at the meltimmersed surface of the electrode close to the melt-gas interface.…”