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

A study of the effect of the second coordination sphere on the standard rate constants of charge transfer ( k s ) 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: k s (CsCl) < k s (KCl). The redox process in the NaCl–KCl (equimolar mixture) – NbCl 5 melt was reversible. The standard rate constants of charge transfer were experimentally determined for chloride-fluoride melts: k s (KCl) < k s (CsCl) < k s (NaCl–KCl). The dependence of the standard rate constants was obtained for pure fluoride melts: k s (CsF) < k s (KF) < k s (NaF–KF). The influence of the first coordination sphere composition on the charge transfer standard rate constants was found experimentally: k s (fluoride melts) < k s (chloride-fluoride melts) < k s (chloride melts), both at glassy carbon and platinum electrodes. It was found that the k s increases with the temperature increase and substitution of a glassy-carbon electrode for a platinum one.