Electrochemical impedance investigations of redox mechanisms of refractorymetal compounds in molten salts. I. Niobium chloride and oxychloride in CsCl-NaCl eutectic melt

Electrochemical studies of the redox mechanisms of niobium chloride and oxy chloride in CsCl-NaCl eutectic melts at 550 degrees C are reported. Cyclic voltammograms and electrochemical impedance spectra have been measured in s itu over a wide potential range between the limits of chlorine evolution an d metal deposition at various oxide concentrations 0 less than or equal to Ox/Nb less than or equal to 3. In analyzing the impedance spectra we have f ocused on the determination of the Warburg impedance and coefficients, resp ectively, which reflects the variation of the concentration of redox specie s as a function of electrode potentials, Taking into account the relevant r edox equilibria these Warburg coefficient versus potential curves can be qu antitatively described by simple model calculations. This is presented for the first time in detail for a high temperature impedance investigation. In comparison with the corresponding voltammograms this analysis yields the f ollowing main results for the redox mechanisms. For pure NbCl5 in CsCl-NaCl eutectic melts evidence is found for a three step reduction mechanism acco rding to Nb(V)-Nb(IV)-Nb(III)-Nb. With oxide addition to a Nb(IV) containin g melt oxocomplexes form which are reduced in two steps from Nb(V) to Nb(IV ) to Nb-metal or metal oxide deposits. Clear indications for dioxocomplexes are derived from the impedance analysis which form in melts with Ox/Nb = 2 . Comparison with the corresponding melts containing tantalum chloride or o xychloride is discussed where dioxocomplexes are not found. (C) 1999 Elsevi er Science Ltd. All rights reserved.