Intervalence Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Chloride Melts: Experiment and Quantum-Chemical Calculations
Authors: Popova Anna V. , Kremenetsky Vyacheslav G. , Kuznetsov Sergey A. 
Published: February 2020

The standard rate constants of charge-transfer (k
s) for the Nb(V)/Nb(IV) redox couple were determined by cyclic voltammetry in KCl – NbCl5 and CsCl – NbCl5 chloride melts at glassy carbon and platinum electrodes. The Nb(V) electrochemical reduction process in the NaCl–KCl (equimolar mixture) – NbCl5 melt was shown being reversible even at high polarization scan rate (v = 2.0 Vs-1); hence, the determination of k
s appeared impossible. Values of k
s are larger in KCl–NbCl5 melt than in CsCl–NbCl5 one over the entire studied temperature range; the values of k
s obtained at platinum electrode are larger than those for glassy carbon electrode1.
Quantum-chemical calculations were carried out at the DFT/B3LYP theory level with use of the quasi-relativistic basis set Stuttgart. For Na-system under consideration the ECP basis Crenbl was used too.
Previously,2-7 in quantum-chemical calculations of nM+·[NbF7], and nM+·[NbCl6], model systems (M = Na, K, Cs; n = 1÷n
lim), containing a complex anion with a cation outer-sphere (OS) shell, it was found that compositions with an intermediate number of OS cations m <n
lim, where n
lim is the limiting number of OS cations bound to a given complex, are thermodynamically most stable. Typically, the change in the values calculated activation energies Eact
of charge transfer on the Nb complex consistent with the change of the experimental standard rate constants of charge transfer in a series of molten salts type (Na–K)Cleq–KCl–CsCl.
However, in chloride melts containing [NbCl6] complex, there is an abnormally large computed Eact
value in the Na-system. This indicates the difference in the mechanisms of charge transfer in the Na-system, on the one hand, and the K-, Cs-systems on the other.
In this work the calculated data for extended model systems such as 2M+∙[NbCl6] + 18MCl (M – Na, K, Cs) are analyzed in spirit of works.8-9 In this study, temperature dependences of parameters analyzed are follow-up calculated.
It was found that at temperatures (T ≥873K) compositions of the most stable particles in these systems differ. Namely, in the Na-system it corresponds to electroneutral particle Na+∙[NbCl6]-,and in the K-, Cs-systems the anion [NbCl6]- is the most stable. Contribution in Eact
of the work of particle approach to the cathode surface for these cases is essentially different.
This may explain the anomalous value of the Na-system Eact
compared with K-, Cs-systems. Model systems of the present approximation level do not allow to consider such effects, their analysis is possible only in model systems of the higher level of approach.
Acknowledgments
The work was financially supported by Russian Foundation for Basic Research (15-03-02290_a).

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