Large-scale energy storage is becoming increasingly critical to balancing renewable energy production and consumption¹. Organic redox flow batteries, made from inexpensive and sustainable redox-active materials, are promising storage technologies that are cheaper and less environmentally hazardous than vanadium-based batteries, but they have shorter lifetimes and lower energy density^2,3. Thus, fundamental insight at the molecular level is required to improve performance^4,5. Here we report two in situ nuclear magnetic resonance (NMR) methods of studying redox flow batteries, which are applied to two redox-active electrolytes: 2,6-dihydroxyanthraquinone (DHAQ) and 4,4′-((9,10-anthraquinone-2,6-diyl)dioxy) dibutyrate (DBEAQ). In the first method, we monitor the changes in the ¹H NMR shift of the liquid electrolyte as it flows out of the electrochemical cell. In the second method, we observe the changes that occur simultaneously in the positive and negative electrodes in the full electrochemical cell. Using the bulk magnetization changes (observed via the ¹H NMR shift of the water resonance) and the line broadening of the ¹H shifts of the quinone resonances as a function of the state of charge, we measure the potential differences of the two single-electron couples, identify and quantify the rate of electron transfer between the reduced and oxidized species, and determine the extent of electron delocalization of the unpaired spins over the radical anions. These NMR techniques enable electrolyte decomposition and battery self-discharge to be explored in real time, and show that DHAQ is decomposed electrochemically via a reaction that can be minimized by limiting the voltage used on charging. We foresee applications of these NMR methods in understanding a wide range of redox processes in flow and other electrochemical systems.

大规模储能对于平衡可再生能源的生产和消费变得越来越重要¹。有机氧化还原液流电池由廉价且可持续的氧化还原活性材料制成，是一种很有前途的存储技术，比钒基电池更便宜、对环境的危害更小，但它们的寿命更短，能量密度更低^2,3。因此，需要分子水平的基本洞察力来提高性能^4,5. 在这里，我们报告了研究氧化还原液流电池的两种原位核磁共振 (NMR) 方法，它们适用于两种氧化还原活性电解质：2,6-二羟基蒽醌 (DHAQ) 和 4,4'-((9,10-蒽醌) -2,6-二基）二氧基）二丁酸酯（DBEAQ）。在第一种方法中，我们监测液体电解质流出电化学电池时¹ H NMR 位移的变化。在第二种方法中，我们观察了全电化学电池正负极同时发生的变化。使用体磁化强度变化（通过水共振的¹ H NMR 位移观察）和¹作为电荷状态函数的醌共振的 H 位移，我们测量两个单电子对的电位差，识别和量化还原物质和氧化物质之间的电子转移速率，并确定电子离域的程度自由基阴离子上的未配对自旋。这些 NMR 技术可以实时探索电解质分解和电池自放电，并表明 DHAQ 通过限制充电时使用的电压可以最小化的反应进行电化学分解。我们预见到这些 NMR 方法在理解流动和其他电化学系统中广泛的氧化还原过程中的应用。