Electrodeposit of niobium metal from K2NbF7-LiF-NaF-KF-Na2O melts at 700-degrees-C has been investigated. It was found that the equilibrium oxidation state of niobium was four for initial O2-/Nb(V) ratios of up to at least one. On the other hand when a niobium metal sheet was used for the reduction, average oxidation states close to five were obtained. Cyclic voltammetry showed that NbF72- is reduced in two steps. A mechanism Nb(V) --> Nb(IV) --> Nb(0) is proposed. When oxide is present, new waves due to reduction of niobium mono-oxofluoro and dioxofluoro complexes are observed at -0.6 and -0.74 V, respectively. In addition plating experiments were also performed. The substrates in our work were low-carbon steel, the anodes niobium metal, and the current density was around 90 mA/cm2. It was found that the presence of at least 1 mole percent of oxide was necessary to obtain current efficiencies higher than 30%. The highest current efficiencies obtained were around 95%. For oxide/Nb(V) molar ratios equal to or higher than one, partially nonmetallic surface layers were deposited.
Journal of the Electrochemical Society, 1994, Vol 141, Issue 5, p. 1212-1220