Temperature-dependent 51V nuclear magnetic resonance spectroscopy for the positive electrolyte of vanadium redox flow batteries

Abstract

Temperature-dependent ⁵¹V nuclear magnetic resonance (NMR) spectroscopy is used to study the high temperature stability of the VO₂⁺ positive electrolyte of vanadium redox flow batteries (VRFBs). The NMR spectra at high temperatures feature significant line broadening of the VO₂⁺ signal and a narrow line from VO(OH)₃. The temperature, acid concentration, and VO₂⁺ concentration dependencies of the line broadening collectively indicate the formation of paramagnetic VO²⁺ with increasing temperature and consequent paramagnetic dipolar broadening. In order to more clearly monitor the signal from VO(OH)₃, which is indicative of the thermal instability of the VO₂⁺ electrolyte, paramagnetic dipolar broadening of the VO₂⁺ signal is intentionally induced by adding an appropriate amount of VO²⁺. This new analysis shows that, contrary to the previous perception, VO(OH)₃ exists even at room temperature. The induced paramagnetic dipolar broadening can be utilized to assess approaches to improve temperature stability of the vanadium electrolyte.