Electrochemical Behavior of Solutions of Niobium Chlorides in Fused Alkali Chlorides

The electrochemical properties of solutions of niobium chlorides in the fused eutectic were studied by transient electrochemical techniques. The fundamental role of temperature was determined. At temperatures ranging from 380–600°C, two reversible redox reactions were detected : , and . 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°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 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, , which must be considered when a vitreous carbon electrode is used as working electrode.