Electrochemical reduction mechanism of NbF5 and NbCl5 in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate

The electrochemical reduction mechanism of niobium was studied in the ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate ([BMP][OTf]) containing niobium halides (NbCl5 and NbF5). The influence of the electrolyte composition, temperature, and nature of the substrate were systematically investigated. In situ electrochemical techniques, such as cyclic voltammetry (CV), differential pulse voltammetry (DPV), electrochemical quartz crystal microbalance (EQCM) and rotating ring disc electrode (RRDE), were used, besides the ex situ characterization techniques scanning electron microscopy (SEM) and vibrational spectroscopy. The number of transferred electrons was estimated from the half width of the cathodic peaks in the DPV measurements. Furthermore, the product of the diffusion coefficient and the number of transferred electrons at the power of 3/2 were evaluated from RRDE measurements to be approx. 3 × 10−10 m2/s. Thicker and better adherent niobium-based deposits were obtained from NbCl5 in [BMP][OTf] than from NbF5 in [BMP][OTf]. EQCM measurements combined with DPV indicated the potential region where niobium-based deposits could be obtained as well as their stoichiometry. Vibrational spectroscopy reveals that NbF5 and NbCl5 interact differently with [BMP][OTf]. The spectra show that in the case of NbF5 – [BMP][OTf] only niobium(V) anionic species [NbF6A]2- and [NbF5A]- (where A = [OTf]-) are formed, while in NbCl5 – [BMP][OTf] both niobium(V) and niobium(IV) anionic species are present. At low NbCl5 concentrations, [NbCl4A2]2- and [NbCl4A2]- are obtained, while at high NbCl5 concentration, [NbCl5A]2- and [NbCl5A]- are found. The electrochemical niobium reduction mechanism depends on temperature and on the anionic species that are involved in the reduction process. In the case of NbCl5 – [BMP][OTf], at high cathodic potentials the formation of subvalent Nb-clusters (possibly Nb6Cl14) was observed.