A Six-Electron Energy Storage Material for Ultra-Stable Aqueous Organic Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) offer sustainable, large-scale energy storage using tunable, earth-abundant organic molecules, avoiding resource limitations. While most aqueous redox flow battery materials utilize single- (n = 1) or dual-electron (n = 2) transfer mechanisms, stable multi-electron (n > 4) redox systems remain largely unexplored. A six-electron (n = 6) phenazine-based negative electrolyte, 2,2',2''-(diquinoxalino[2,3-a:2',3'-c]phenazine-2,8,14-triyltris(oxy))tripropionic acid (PPA) is engineered with a π-extended fused-ring core and branched hydrophilic side chains. As supported by molecular dynamics simulation, the C─O linked tri-substituted propanoic acid groups in PPA disrupt intermolecular π-π stacking, achieving an unprecedented aqueous solubility of 1.2 m with six electron storage (7.2 m electron concentration, 193.0 Ah L^-1 theoretical capacity). When paired with a ferrocyanide, the PPA-based AORFB gives a cell voltage of 1.31 V at 3.0 m electron concentration, along with a Coulombic efficiency exceeding 99.8% and a 85% utilization of its six-electron capacity. The system demonstrates exceptional cycling stability, sustaining a capacity decay rate of 0.032% per cycle, 0.095% per day over 80 operational days. This work provides a scalable approach for stable multi-electron energy storage materials applicable in AORFBs.