Electrochemical Studies of the Molten System K[sub 2]NbF[sub 7]-Na[sub 2]O-Nb-(LiF-NaF-KF)[sub eut] at 700°C

Abstract

Various voltammetric methods have been used to study FLINAK (LiF‐NaF‐KF eutectic melt 46.5–11.5–42 mol %) melts containing about 1 mol % niobium‐fluoro and ‐oxofluoro complexes with Nb in oxidation states (V) and (IV) at 700°C and varying amounts of Na2O in the range 0<nOo/nnormalNbo<7 . A slow, spontaneous reduction of Nb(V)‐fluoro complexes to Nb(IV)‐fluoro complexes was observed in the absence of Nb metal. With a stoichiometric amount of Nb metal, Nb(V) is reduced rapidly to Nb(IV) according to the reaction 4NbF72−+normalNb⇌5NbFxfalse(x−4false)− . The solubilities of Nb2O5 and KNbO3 in FLINAK were measured and found to be 1.8 and 0.13 mol %, respectively. The following reactions probably occur when increasing amounts of oxide are added to a melt originally containing NbFxfalse(x−4false)−:NbFxfalse(x−4false)−+O2−⇌normalNbfalse(IVfalse)‐normalmono‐normaloxo‐normalfluoride+normalfluoride , 5normalNbfalse(IVfalse)‐normalmono‐normaloxo‐normalfluoride+O2−⇌4NbO2F43−+normalNb , and NbO2F43−+O2−+Alk+⇌AlkNbO3false(normalsfalse)+4F− false(Alk+=Li+ , Na+ , or K+false) . At oxide to niobium molar ratios higher than three, AlkNbO3false(normalsfalse) redissolves, and at nOo/nnormalNbo>4 , all Nb and O added are dissolved. Strong indications of the coexistence of the oxygen rich Nb(V)OF complexes and O2− ions in FLINAK at nOo/nnormalNbo>4 have been found. An equilibrium/sampling/analysis technique was also used to study this system without Nb metal added. The results mainly agree with the results of the voltammetric studies. However, no indications of spontaneous reduction of Nb(V) to Nb(IV) were observed, even after 24 h. © 2000 The Electrochemical Society. All rights reserved.

Electrochemical behavior of hexafluoroniobate, heptafluorotungstate, and oxotetrafluorovanadate anions in N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide room temperature ionic liquid

“…Although electrochemical data are available for some chloro-and bromo-metallate anions [17][18][19][20][21][22][23], there are not many reports on electrochemical data for fluorometallate and oxofluorometallate anions and, to our knowledge, no reports are available for the electrochemical behavior of Nb(V)F 6 À , W(VI)F 7 À , and V(V)OF 4 À at 298 K. In this paper, the electrochemical behavior of Nb(V)F 6 À , W(VI)F 7 À , and V(V)OF 4 À in BMPyrTFSA at 298 K is discussed in detail, where BMPyrTFSA was chosen as a medium because of its high electrochemical stability and easy handling. following relation holds [24]:…”

Influence of the Second Coordination Sphere on the Roughness of Niobium and Tantalum Coatings Obtained in Chloride-Fluoride Melts

“…3. It is characterized by two peaks R 1 and R 2 , corresponding to the following processes [25][26][27][28][29][30][31][32][33][34] :…”

Electrochemistry of the Nb(V)/Nb(IV) Redox Couple in the KCl-K2NbF7 Melt

“…, [12] 298 298 T T ψ ψ = [13] The values of the ψ T function, obtained by means of equations [12] and [13] and used in conjunction with expression [11] made it possible to calculate the standard rate constants of charge transfer for the Nb(V)/Nb(IV) redox couple.…”