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

Temperature-dependent V-51 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.