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

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:

k_s (CsF) < k_s (KF) < k_s(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.

Acknowledgments

The work was financially supported by the Russian Foundation for Basic Research (project 12-08-01178-а).