The goal of the present research was the study of Mg²⁺, Ca²⁺, Sr²⁺ and Ва²⁺ (Me²⁺) strongly polarizing cations influence on the standard rate constants of charge transfer (k_s) for the redox couple Nb(V)/Nb(IV) in NaCl-KCl_(equimol.)-K₂NbF₇ (I) and NaCl-KCl_(equimol.)-NaF(10 wt.%)-K₂NbF₇ (II) melts. Electrochemical studies were carried out in the temperature range of 973-1173 K by cyclic voltammetry. Voltammetric curves were recorded at a glassy carbon electrode vs. a glassy carbon quasi-reference electrode. The sweep rate (v) varied from 0.4 up to 2.0 V s^-1. The electrochemical redox process:

Nb(V) + e^- ↔ Nb(IV) (1)

was classified as quasi-reversible at a sweep rate ν>0.5 V s^-1. The values of k_s were calculated by using the Nicholson’s method [1]. It was shown that k_s increase with increasing temperature. It is due to increase in the number of particles capable of overcoming the potential barrier for the electron transfer process.

Addition of alkaline earth metal cations resulted in increasing of k_s to the certain ratio of Me²⁺/Nb(V) for the all alkaline earth metal cations due to substitution of Na⁺ and K⁺ cations by Me²⁺ in the second coordination sphere of niobium complexes that leads to decreasing of niobium fluoride complexes strength. Further addition brought to some decrease of the standard rate constants because the viscosity of melts increasing which brings to decrease of the diffusion coefficients.

The temperature dependences of molten salt systems with addition of strongly polarizing cations were determined and the activation energies of charge transfer were calculated. Values of activation energy for systems with strongly polarizing cations are considerably less than activation energies of the initial systems.

The standard rate constants in molten salt system (I) were higher than in the melt (II) due to lower viscosity. It was determined that in the melt (I) k_s a little bit increased with increasing scan rate probably due to the secondary electrode process which accompanied redox process (1). In the system with sodium fluoride (II) the redox process (1) occurs at more negative potentials that allow eliminating the secondary process.

References:

[1] R.S.Nicholson, J. Anal. Chem., 1965. Vol. 37 (1965), 1351.

[2] B.B. Damaskin, O.A.Petriy, Introduction of electrochemical kinetics, Vysshaya Shkola Publ., Moscow, 1975 (in Russian).