1 Energy and Materials, Department of Chemistry, Technical University of Denmark2 Department of Chemistry, Technical University of Denmark3 Department of Chemistry, Technical University of Denmark4 Department of Energy Conversion and Storage, Technical University of Denmark5 unknown
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.
Electrochemical Society. Journal, 1994, Vol 141, Issue 5, p. 1212-1220