Discovery of aza-aromatic anolytes for aqueous redox flow batteries via high-throughput screening

Aza-aromatics have recently emerged as a propitious class of electroactive compounds for energy storage in aqueous redox flow batteries (ARFBs). Here, using high-throughput virtual screening (HTVS), we explored a focused chemical subspace of aza-aromatics to determine the top performing candidates as anolytes in ARFBs. First, we designed a virtual chemical library that contains 13,406 aza-aromatic redox pairs, which was populated through the chemical functionalization of alloxazine, phenazine, and indigo backbones with five different R-groups that are known to affect the key battery properties. Then, we predicted the redox potential, aqueous solubility, and the likelihood of decomposition due to the undesirable hydration and tautomerization reactions of the compounds. An analysis of the decomposition thermodynamics of the aza-aromatic subclasses revealed differing correlations between the redox properties and the chemical stability of the compounds, where the latter is found to strongly depend on the type and quantity of the functional groups. Consequently, a total of 516 anolyte candidates that have lower redox potential and higher solubility than a typical anolyte compound, alloxazine 7-carboxylic acid (ACA), have been identified. Additionally, an automated vendor search for the HTVS-predicted top-performing compounds yielded two molecules that are readily purchasable for experimental validation. Finally, an analysis of the quantitative structure-property relationships showed that the mid-sized aza-aromatics, which are not well-explored in experiments, achieved the largest property tunability windows. Based on the new findings, we also propose a molecular engineering strategy in a way to balance the inherent trade-offs among the redox, solubility and chemical stability features of the aza-aromatic anolytes for ARFBs.