ELECTROCHEMICAL-BEHAVIOR OF SOLUTIONS OF NIOBIUM CHLORIDES IN FUSED ALKALI CHLORIDES

The electrochemical properties of solutions of niobium chlorides in the fused eutectic LiCl-KCl were studied by transient electrochemical techniques. The fundamental role of temperature was determined. At temperatures ranging from 380-600-degrees-C, two reversible redox reactions were detected : Nb(V) + e <-- --> Nb(IV), and Nb(IV) + e <-- --> Nb(III). Reduction of Nb(III) led to metallic niobium, however the deposition was perturbed by the formation of insoluble non-stoichiometric niobium subhalides (mean oxidation state between 2 and 3) at the electrode surface. At higher temperatures (620-degrees-C) these perturbing compounds became unstable, and coherent metal deposit were obtained. The presence of oxide ions induced a rapid decrease of the wave corresponding to the redox reaction Nb(V)/Nb(IV). Addition of LiF to the electrolyte stabilized the higher oxidation state, and produced a negative shift of about 0.3 V in the standard potential of the couple Nb(V)/Nb(IV). Spontaneous formation of niobium carbide occurred at the surface of carbon immersed in a solution containing Nb(III) and Nb(IV). This compound gives rise to redox reactions such as, Nb(III) + C + 3e <-- --> NbC, which must be considered when a vitreous carbon electrode is used as working electrode.