A study of the reaction pathways during electrochemical reduction of dense Nb2O5 pellets in molten CaCl2 medium

• Layer-by-layer analysis of the pellets electro-deoxidised for different time periods under similar experimental conditions.
• Catholyte and electrolyte melt analysed and results given in terms of total solubilised basicity and total insolubilised basicity.
• A striking relationship found between the electro-deoxidation of HD pellets and changes in the composition of melt.
• CV with Nb2O5 pellet tip as WE and polarisation studies – the pellet is a conductor and electrochemically reactive.
• A new three step mechanism along with a model for the electrodeoxidation of HD Nb2O5 pellets proposed.

Electrochemical behaviour of powder compacted and sintered Nb2O5 pellet electrodes (~4 mm thick, open porosity ~10%) in molten CaCl2 medium at 1173 K was studied using cyclic voltammetry and polarisation measurements. Similar dense Nb2O5 pellets were subsequently electrolysed by using them as cathodes against graphite counter electrode in the molten CaCl2 electrolyte medium with an aim to gain insight into the electro-reduction mechanism. The chemical changes that are taking place in the cathode and also in the electrolyte melt at different stages of electrolysis were analysed. The electro-reduction experiments were carried out at an applied voltage of 3.1 V and terminated after 2, 9, 22, 26 and 35 h of electrolysis and the visibly distinct layers present in the partially reduced samples were analysed. The analysis showed presence of CaxNbyOz compounds, sub-oxides of Nb like NbO2, NbO, etc. and metallic Nb in the samples. A pellet, which was electrolysed for 44 h was found to be reduced completely to Nb metal. The CaO, Ca and CaCO3 produced and released to the melt during the cathodic polarisation was found to be associated with the electrochemical changes that are taking place on the electrode and this was ascertained by determination of the ‘total solubilised basicity (TSB)’ and ‘total insolubilised basicity (TIB)’ of the melt as a function of time during electrolysis. Based on the results, a three-stage mechanism comprising (i) cathodic insertion of Ca2+ into the oxide pellet, (ii) reduction of the oxide electrode via formation and decomposition of ternary intermediates and (iii) chemical reduction of the cathode with electro-generated calcium metal has been proposed for electro-reduction of dense Nb2O5 pellets.

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