Electrochemical impedance and cyclic voltammetry study of the influence of oxide on the redox chemistry of niobium and tantalum in chloride melts

The redox process of penta- and tetravalent niobium and tantalum species dissolved in CsClNaCl eutectic melts (65-35 mol%) have been investigated by electrochemical impedance spectroscopy and cyclic voltammetry. In particular, the influence of oxide has been studied. A strong correlation between the structures of the cyclic voltammograms and the potential dependence of the Warburg diffusion impedance is observed, which is determined by the concentrations of the redox species under consideration. Without any oxide added to melts containing TaCl5 or NbF72− the TavTaIV and NbVNbIV redox process, respectively, is observed in the cyclic voltammograms. This is also the case after equilibrating the melts with the respective metals. Plots of the Warburg coefficients as a function of the electrode potential exhibit minima near the equilibrium potentials. This confirms the presence of both redox species. Addition of oxide, added as Na2O, strongly influences the redox structures of TaVTaIV and NbVNbIV in the volammograms and the minima of the Warburg coefficient curves. When the structure is completely suppressed, for example, when TaV is titrated with an equivalent amount of oxide, the corresponding minima disappear. From the impedance spectra additional informations have been obtained: Indications of electronic conduction, most pronounced for tantalum, and an adsorption process at the electrode in NbIV systems.