Electrochemical Behavior of Niobium Complexes in Chloride-Fluoride Melts with Addition of Alkaline Cations

Abstract

The standard rate constants of charge transfer (k s) for the redox couple Nb(V)/Nb(IV) in NaCl-KCl(equimol.)-NaF(10 wt.%)-K2NbF7 (I) and NaCl-KCl(equimol.)-K2NbF7 (II) melts with addition of alkaline earth metal cations were determined. It was found that addition of alkaline earth metal cations resulted in increasing of k s to the certain ratio of Me2+/Nb(V) for the all alkaline earth metal cations due to substitution of Na+ and K+ cations by Me2+ in the second coordination sphere of niobium complexes that leads to decreasing of niobium fluoride complexes strength. Further addition brought to some decrease of the standard rate constants because the viscosity of melts increasing which brings to decrease of the diffusion coefficients. It was determined that in the melt (II) k s a little bit increased with increasing scan rate probably due to the secondary electrode process.