A Fused Quinone-Pyrazine-Based Aqueous Flow Battery Negolyte with Record Volumetric Capacity and Long Lifetime

Aqueous organic flow batteries hold great promise to store the massive electricity generated from intermittent renewables. Tremendous efforts have been made in tailoring existing redox cores to achieve the desired properties, including redox potentials, solubility, and stability. Here, we fuse quinone-pyrazine redox motifs to create a unique four-electron redox core, then decorate it with water-solubilizing groups to yield 2,2'-((6,11-dioxo-6,11-dihydrobenzo[b]phenazine-2,3-diyl)bis(oxy))dipropionic acid (DCNQBP), which achieves a record-high volumetric capacity of 121 Ah L^-1 and a temporal fade rate as low as 0.018% day^-1. Employing in situ pH and IR monitoring, cyclic voltammetry, NMR, and DFT calculations, we reveal that DCNQBP undergoes a four-electron transfer mechanism, with the C═N groups of the pyrazine ring and the C═O groups of the quinone ring alternately uptaking electrons, differing from parent quinone and pyrazine derivatives. Detailed experimental and theoretical analyses indicate that the minor decomposition results from hydrolysis-induced chain cleavage. Avoiding deep discharge effectively slows down the decomposition and prolongs the lifetime. The work establishes a strategy of fusing redox motifs to create new redox platforms with doubled electron-storing capacity and markedly improved structural stability.