Six‐Electron‐Storage Polynuclear Quinoxaline Derivatives with Multiple S ‐Propanoic Acid Side‐Groups for High‐Capacity Aqueous Redox Flow Batteries

ABSTRACT Aqueous organic redox flow batteries are promising for constructing large‐scale energy storage systems with inherent safety and eco‐friendliness, but restricted by limited capacity and cyclability. Herein, we design and synthesize two types of six‐electron‐storage triquinoxalinylene derivatives functionalized with six or three S ‐propionic acid side groups, namely HATN‐HSPA or HATN‐TSPA, both demonstrating a reversible three‐step six‐electron storage capability. The extended π ‐conjugation structure of triquinoxalinylene redox core and rational modification of S ‐propionic acid groups bring them high electrochemical reversibility and stability. The strong solubilizing effect of hexa‐substituted S ‐propanoic acid groups rendered HATN‐HSPA a maximum solubility of 1.26 m (i.e., a charge storage concentration of 7.56 m ) and a theoretical capacity (202.62 Ah L −1 ), which are over three times those of traditional organic active molecules with single‐ or two‐electron storage processes. The flow batteries grounded in nearly saturated HATN‐HSPA anolyte achieved a record‐breaking capacity of 150 Ah L −1 . The flow batteries grounded in 0.5 m HATN‐HSPA anolyte also released high‐capacity retention of 98.4% after 600 cycles (or 60 days), with a capacity decrement of 0.0027% cycle −1 (or 0.027% day −1 ). This work presents a multi‐electron molecular engineering methodology for developing high‐storage and durable multi‐electron‐storage organic species for scalable and sustainable grid‐scale energy storage.