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

Temperature-dependent 51 V nuclear magnetic resonance (NMR) spectroscopy is used to study the high temperature stability of the VO 2 + positive electrolyte of vanadium redox flow batteries (VRFBs). The NMR spectra at high temperatures feature significant line broadening of the VO 2 + signal and a narrow line from VO(OH) 3 . The temperature, acid concentration, and VO 2 + concentration dependencies of the line broadening collectively indicate the formation of paramagnetic VO 2+ 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 VO 2 + electrolyte, paramagnetic dipolar broadening of the VO 2 + signal is intentionally induced by adding an appropriate amount of VO 2+ . 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.