Electrochemical study on the electrodeoxidation of Nb2O5 in equimolar CaCl2 and NaCl melt

The electrodeoxidation of Nb2O5 in equimolar CaCl2 and NaCl was studied by cyclic voltammetry, chronoamperometry, AC impedance and constant voltage electrolysis techniques. Three cathodic current peaks in the cyclic voltammogram, three current rates in the transient current–voltage curves and three response semicircles in the AC impedance spectra were observed and support a three-step electrodeoxidation mechanism of Nb2O5. Intermediates NbO2 and NbO were observed by XRD and confirms the following electrodeoxidation sequence: Nb2O5 → NbO2 → NbO → Nb. The charge transfer resistances and the activation energies of the electrodeoxidation steps of Nb2O5 were obtained by simulating the AC impedance spectra with equivalent circuits. The electrodeoxidation reaction of Nb2O5 is controlled by the charge transfer process at the low voltage range and by the diffusion process at the high voltage range. Free oxygen ions were released in the electrodeoxidation steps, and CaO is a catalyst of the process.