The Effect of the Second Coordination Sphere on Electrochemistry of Niobium Complexes in Alkali Halide Melts

Abstract

A study of the effect of the second coordination sphere on the standard rate constants of charge transfer (ks) for the Nb(V)/Nb(IV) redox couple in chloride, chloride-fluoride and fluoride melts has been presented. By using cyclic voltammetry, the standard rate constants of charge transfer were determined for chloride melts: ks (CsCl) < ks (KCl). The redox process in the NaCl–KCl (equimolar mixture) – NbCl5 melt was reversible. The standard rate constants of charge transfer were experimentally determined for chloride-fluoride melts: ks (KCl) < ks (CsCl) < ks (NaCl–KCl). The dependence of the standard rate constants was obtained for pure fluoride melts: ks (CsF) < ks (KF) < ks (NaF–KF). The influence of the first coordination sphere composition on the charge transfer standard rate constants was found experimentally: ks (fluoride melts) < ks (chloride-fluoride melts) < ks (chloride melts), both at glassy carbon and platinum electrodes. It was found that the ks increases with the temperature increase and substitution of a glassy-carbon electrode for a platinum one.