ELECTROCHEMICAL IMPEDANCE AND CYCLIC VOLTAMMETRY STUDY OF THE INFLUENCE OF OXIDE ON THE REDOX CHEMISTRY OF NIOBIUM AND TANTALUM IN CHLORIDEMELTS

The redox processes of penta- and tetravalent niobium and tantalum spe cies dissolved in CsCl-NaCl eutectic melts (65-35 mol%) have been inve stigated by electrochemical impedance spectroscopy and cyclic voltamme try. 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 c onsideration. Without any oxide added to melts containing TaCl5 or NbF 72- the Ta-V-Ta-IV and Nb-V-Nb-IV redox process, respectively, is obse rved in the cyclic voltammograms. This is also the case after equilibr ating the melts with the respective metals. Plots of the Warburg coeff icients as a function of the electrode potential exhibit minima near t he equilibrium potentials. This confirms the presence of both redox sp ecies, Addition of oxide, added as Na2O, strongly influences the redox structures of Ta-V-Ta-IV and Nb-V-Nb-IV in the voltammograms and the minima of the Warburg coefficient curves. When the structure is comple tely suppressed, for example, when Ta-V is titrated with an equivalent amount of oxide, the corresponding minima disappear. From the impedan ce spectra additional informations have been obtained: Indications of electronic conduction, most pronounced for tantalum, and an adsorption process at the electrode in Nb-IV systems. (C) 1997 Elsevier Science Ltd.