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

摘要

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-fluoride and fluoride melts is presented. By using cyclic voltammetry, the following series of the standard rate constants of charge transfer has been experimentally determined: k_s (KCl) < k_s (CsCl) < k_s (NaCl–KCl). Quantum-chemical calculations showed that charge transfer activation energies could actually change in a nonmonotonic manner in the sodium-potassium-cesium series, resulting in a nonmonotonic change of charge transfer of rate constants in chloride-fluoride melts. In pure fluoride melts, the following dependence of the standard rate constants has been obtained: k_s (CsF) < k_s (KF) < k_s (NaF–KF). It has been established that k_s tend to increase in response to both temperature increase and substitution of a glassy-carbon electrode for a platinum one.