Electrodeposition of niobium from fluoroniobate K2NbF7 solutions in fused NaCl-KCl
1992; Elsevier BV; Volume: 179; Issue: 1-2 Linguagem: Inglês
10.1016/0925-8388(92)90225-x
ISSN1873-4669
AutoresAbdeslam Barhoun, Y. Berghoute, F. Lantelme,
Tópico(s)Advancements in Battery Materials
ResumoThe mechanism of the electrodeposition of niobium from fluoroniobate solution in molten NaCl-KCl at 720 °C was studied by voltammetry and chronopotentiometry. It was shown that the reaction proceeds in two steps: first a reversible redox reaction, Nb(V)+e↔Nb(IV), followed by a quasi-reversible reduction, Nb(IV)+4e→Nb(0). The standard potentials of the redox couples Nb(V)/Nb(IV) and Nb(IV)/Nb(0) are E540=0.394 V and E400 = −0.178 Vvs. the Ni/Ni2+ reference electrode. The diffusion coefficients of the two electroactive species are DNb(V)=1.83×10−5 cm2 s−1 and DNb(IV)=2.34×10−5 cm2 s−1. On a nickel electrode the electrodeposition of well-crystallized metallic niobium occurred at a potential of −0.5 V vs. Ni/Ni2+. Better adherence and cohesion were achieved when pre-reduced solutions of niobium(IV) were used. At potentials lower than −0.7 V, a black powder was obtained. X-ray analysis indicated the presence of metallic niobium and of a new phase containing potassium and niobium. In solutions containing niobium(IV) a thin layer of niobium carbide formed spontaneously on carbon. Voltammetric investigations indicated that the formation of niobium carbide occurred before the metal deposition; the redox reaction corresponding to the formation of the niobium carbide was found to be effectively irreversible.
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