Spectroscopic study of vanadium(V) precipitation in the vanadium redox cell electrolyte

Abstract

In this study, characterisation of vanadium electrolyte and vanadium(V) red precipitate formed in the positive half-cell electrolyte of a vanadium redox cell is present using different spectroscopic techniques. 51V solution NMR showed that the main peak at about −545 ppm in the spectra of the redox electrolyte could be attributed to the monomer species of VO 2 + ions and the intensity of the peak decreased with both temperature and aging time. These results confirmed the formation of a red precipitate in the redox electrolyte due to polymerisation of the monomer species of VO 2 + in the strong acidic media at elevated temperatures and with aging. Electron spin resonance (ESR) measurements showed that V(IV) ions were also present in the electrolyte. The presence of V(IV) ions may play an important role in the stability of the electrolyte. The static solid state 51V NMR spectrum of the thermal precipitate dried at room temperature showed a peak at −243 ppm which is characteristic of V(V) in distorted octahedral oxygen coordination similar to that formed for crystalline and gel forms of V 2O 5. Characteristics of V 2O 5 were also identified by X-ray diffraction (XRD) and had a fibrous morphology before heating. However, transition electron microscopy (TEM) showed that conversion of the fibrous morphology of V 2O 5 to the small crystalline morphology of V 2O 5 occurred after heating at 520°C.