Aqueous redox flow batteries (RFBs) are attractive candidates for low-cost, grid-scale storage of energy from renewable sources. Quinoxaline derivatives represent a promising but underexplored class of charge-storing materials on account of poor chemical stability in prior studies (with capacity fade rates >20%/day). Here, we establish that 2,3-dimethylquinoxaline-6-carboxylic acid (DMeQUIC) is vulnerable to tautomerization in its reduced form under alkaline conditions. We obtain kinetic rate constants for tautomerization by applying Bayesian inference to ultraviolet–visible spectroscopic data from operating flow cells and show that these rate constants quantitatively account for capacity fade measured in cycled cells. We use density functional theory (DFT) modeling to identify structural and chemical predictors of tautomerization resistance and demonstrate that they qualitatively explain stability trends for several commercially available and synthesized derivatives. Among these, quinoxaline-2-carboxylic acid shows a dramatic increase in stability over DMeQUIC and does not exhibit capacity fade in mixed symmetric cell cycling. The molecular design principles identified in this work set the stage for further development of quinoxalines in practical, aqueous organic RFBs.

中文翻译：

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氧化还原液流电池中取代基对喹喔啉性能和降解的影响

水性氧化还原液流电池（RFB）是低成本、电网规模可再生能源存储的有吸引力的候选者。由于先前研究中化学稳定性差（容量衰减率> 20％/天），喹喔啉衍生物代表了一类有前途但尚未充分开发的电荷存储材料。在这里，我们确定 2,3-二甲基喹喔啉-6-羧酸 (DMeQUIC) 在碱性条件下很容易以还原形式发生互变异构。我们通过将贝叶斯推理应用于来自操作流动池的紫外-可见光谱数据，获得了互变异构的动力学速率常数，并表明这些速率常数定量地解释了循环电池中测量的容量衰减。我们使用密度泛函理论 (DFT) 建模来识别互变异构抗性的结构和化学预测因子，并证明它们定性地解释了几种市售和合成衍生物的稳定性趋势。其中，喹喔啉-2-羧酸的稳定性比 DMeQUIC 显着提高，并且在混合对称电池循环中没有表现出容量衰减。这项工作中确定的分子设计原理为进一步开发实用的水性有机 RFB 中的喹喔啉奠定了基础。