The Charge Transfer for Nb(V)/Nb(IV) and Ta(V)/Ta(IV) Redox Couple in Electrode Surface: Experimental and Calculation Methods

Abstract

The diffusion coefficients and Standard rate constants ks of charge transfer for Nb(V)/Nb(IV) and Ta(V)/Ta(IV) in NaCl–KCl (the equimolar mixture)–K2Nb(Ta)F7, KCl–K2Nb(Ta)F7, and CsCl–K2Nb(Ta)F7 melts are determined by using cyclic voltammetry. An unconventional series of the standard rate constants is found: ks (KCl) ˂ ks (CsCl) ˂ ks (NaCl–KCl). Ab initio calculations carried out by using a PC Gamess/Firefly quantum-chemical program showed that the charge transfer activation energy can change not monotonically in the Na–K–Cs series, in compliance with the reorganization energy relationship. This leads, in its turn, to nonmonotonic change in the charge transfer rate constants. Тhe diffusion coefficients decrease when the composition of the second coordination sphere changes from sodium to cesium. Logarithms of the diffusion coefficients of the com plexes Nb(V) and Nb(IV) or Ta(V)/Ta(IV) are the linear functions of the alkali metal cation radius. Such changes of D values are well known. They are due to a weakening of the counterpolarizing effect when passing from Na to Cs, which, in its turn, leads to the shortening of the metal–ligand bond in the complexes. The diffusion coefficients decrease as the oxidation degree increases, while the activation energy of the diffusion process increases. Thus, with the increasing of the ionic potential of the diffusing particles the values of D decrease, while ΔU increases. The obtained results agree with numerous data on the effect of the central atom oxidation degree on the diffusion coefficients and activation energy of diffusion processes. Quantum-chemical calculations showed that the 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.