Electrochemistry of the Nb(V)/Nb(IV) Redox Couple in the KCl-K2NbF7 Melt

The electrochemistry of the Nb(V)/Nb(IV) redox couple in the KCl-K2NbF7 melt has been studied at 1073-1173 K on glassy carbon and platinum electrodes by transient electrochemical techniques. The diffusion coefficients of Nb(V) and Nb(IV) were determined by linear sweep voltammetry, chronopotentiometry and chronoamperometry. The values obtained by these methods at the different substrates are also in a good agreement. Diffusion coefficients decreased when niobium oxidation state increased, while the activation energies for diffusion increased. The standard rate constants of charge transfer (k(s)) on platinum and glassy carbon electrodes for Nb(V)/Nb(IV) redox couple were calculated on the basis of cyclic voltammetry data at a sweep rate 0.75 < v <= 2.0 V s(-1), where a mixed diffusion and electron-transfer control was observed. It was found that k(s) increased with increasing temperature and values of k(s) obtained at platinum electrode were higher than at a glassy carbon electrode. The values of the standard rate constants testify that the redox process Nb(V) + e(-) <-> Nb(IV) proceeds quasi-reversibly, mostly under diffusion control.