Large-scale energy storage is becoming increasingly critical to balance the intermittency between renewable energy production and consumption 1. Organic redox flow batteries (RFBs), based on inexpensive and sustainable redox-active materials, are promising storage technologies that are cheaper and have fewer environmental hazards than the more mature vanadium-based batteries (typically < 15 Wh/dm 3 , vs. 20-35 Wh/dm 3 , respectively) 2,3. Unfortunately, they have shorter calendar lifetimes and lower energy-densities and fundamental insight at the molecular level is thus required to improve performance 4,5. Here we report two in situ NMR methods to study flow batteries, which are applied on two separate anthraquinones, 2,6-dihydroxyanthraquinone, DHAQ and 4,4'-((9,10-anthraquinone-2,6diyl)dioxy) dibutyrate, DBEAQ as redox-active electrolytes. In one method we follow the changes of the liquids as they flow out of the electrochemical cell, while in the second, we observe the changes that occur in both the positive and negative electrodes in the full electrochemical cell. Making use of the bulk magnetisation changes, observed via the 1 H NMR shift of the water resonance, and the linebroadening of the 1 H shifts of the quinone resonances as a function of state of charge, we determine the potential differences of the two one-electron couples, identify and quantify the rate of electron transfer between reduced and oxidised species and the extent of electron delocalization of the unpaired spins over the radical anions. The method allows electrolyte decomposition and battery self-discharge to be explored in real time, showing that DHAQ is decomposed electrochemically via a reaction which can be minimized by limiting the voltage used on charging. Applications of the new NMR metrologies to understand a wide range of redox processes in flow and other battery systems are readily foreseen. The two in situ NMR setups Ex situ characterization of RFBs can be challenging due to the high reactivity, sensitivity to sample preparation and short lifetimes of some of the oxidised and/or reduced redox-active molecules and ions within the electrolytes. However, one of the distinct features of RFBs is the decoupling of energy storage and power generation, providing different opportunities for in situ monitoring. To date, methods such as in situ optical spectrophotometry 6 and Electron Paramagnetic Resonance (EPR) 7 have been used to study, for example, crossover of quinones and vanadyl ions, but considerable opportunities remain to improve characterization methods to address limitations inherent to each method and to probe different phenomena. Nuclear Magnetic Resonance (NMR) spectroscopy was used to study benzoquinone and polyoxometalate redox reactions in an in situ
