Electrochemistry and Raman spectroscopy of niobium reduction in basic and acidic pyrrolidinium based ionic liquids. Part I: 1-Butyl-1-methylpyrrolidinium chloride with NbCl5

The electrochemical behaviour of basic and acidic ionic liquids (x)1-butyl-1-methyl-pyrrolidinium chloride-(1 − x)NbCl5 at x = 0.80, 0.60, and 0.40 was investigated over the temperature range 90–160 °C in respect to the electrochemical deposition of niobium. This is discussed in connection with in situ Raman spectroscopic data. The mechanism of electrochemical reduction of niobium (V) depends strongly on the structure and composition of the electrochemical active species of niobium (V) defined by the molar composition of the ionic liquids. In basic mixtures at x = 0.80, the self-reduction of Nb(V) → Nb(IV) was confirmed by Raman spectroscopy and [NbCl6]2− (75%) and [NbCl6]− (25%) octahedral complexes are the electrochemical active species at 140 °C. At x = 0.60, the dominant species are NbCl6− octahedra. In acidic ionic liquids at x = 0.40, the electrochemical reduction of two species occurs, NbCl6− and Nb2Cl11−, and it is limited by one electron transfer for NbCl6− at −0.45 V and by two electrons transfer for Nb2Cl11− at −1.12 V and −1.27 V, respectively. In basic melts the electrodeposition of metallic niobium is presumably possible at potentials negative of – 2.1 V versus Pt-quasi reference at a temperature of 140 °C.