DIFFUSION AND CHARGE-TRANSFER KINETICS OF THE Nb(V)/Nb(IV) REDOX COUPLE IN MOLTEN FLUORIDE SALTS

This study investigates the diffusion and charge-transfer kinetics of the Nb(V)/Nb(IV) redox couple in high-temperature fluoride melts with different cationic compositions. The aim was to determine the transport characteristics of electrochemically active niobium complexes and to establish how the composition of the molten electrolyte affects the kinetics of the electrode reaction. The study employed linear and cyclic voltammetry, chronoamperometry, and chronopotentiometry over the temperature range 1073–1173 K. The diffusion coefficients of Nb(V) and Nb(IV) complexes increased systematically with temperature, while Nb(IV) exhibited higher diffusional mobility than Nb(V) under comparable conditions. Moreover, the close agreement between diffusion coefficients obtained by different electrochemical techniques and at glassy carbon and platinum electrodes confirmed the reproducibility of the transport parameters. The standard charge-transfer rate constants were determined from cyclic voltammetric data using the Nicholson method and the experimentally measured diffusion coefficients of both redox forms. The results showed that the charge-transfer rate constant remained relatively insensitive to the potential scan rate within the investigated range but depended strongly on the cationic composition of the fluoride melt. For both glassy carbon and platinum electrodes, the same sequence of charge-transfer kinetics was observed: KF > CsF > NaF–KF. Consequently, the electrode kinetics cannot be attributed solely to the nature of the electrode material and is strongly influenced by the composition of the molten electrolyte and the structure of electrochemically active niobium complexes. Quantum-chemical calculations further revealed that interactions between [NbF₇]²⁻ and outer-sphere cations modify the electronic structure of the complex, providing a molecular basis for the experimentally observed changes in charge-transfer kinetics.