The Composition of the Second Coordination Sphere and Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Halide Melts

Abstract

The effect of the second coordination sphere and the working electrode nature on the standard rate constants of charge transfer (k s) for the Nb(V)/Nb(IV) redox couple in chloride-fluoride and fluoride melts was studied.The standard rate constants for the Nb(V)/Nb(IV) redox couple were calculated based on cyclic voltammetry data and the Nicholson’s equation.The following unusual series of the standard rate constants of charge transfer is experimentally determined in chloride-fluoride melts: k s (KCl) < k s (CsCl) < k s (NaCl-KCl).Ab initio calculations carried out by using PC Gamess/Firefly quantum chemical program showed that charge transfer activation energies can actually change in nonmonotonic manner in the series sodium-potassium-cesium, which leads to nonmonotonic change of the charge transfer rate constants in chloride-fluoride melts.In pure fluoride melts, the following dependence of the standard rate constants was obtained using cyclic voltammetry: ks (CsF) < ks (KF) < ks (NaF-KF).It has been established that the standard rate constants of charge transfer increase with increasing temperature, as well as with replacement of the glassy carbon electrode with a platinum one.AcknowledgmentsThe work was financially supported by the Russian Foundation for Basic Research (project 12-08-01178-а).