A Zwitterionic Azo Posolyte for Long-Lifetime Aqueous Redox Flow Batteries

A growing share of variable renewable generation requires low-cost, long-duration grid-level energy storage. Aqueous organic redox flow batteries (AORFBs) offer tunable molecular chemistry and scalable flow architecture; acidic systems enable high power and leverage mature vanadium-flow hardware. A central challenge is designing posolytes that combine high redox potential, solubility, capacity density, and stability. Here, we report a two-electron azo-based zwitterionic molecule 4,4'-azo-bis(1-pyridinium-3-propane-sulfonate) (ABPS) that addresses these constraints through intrinsic structural features. The zwitterionic character dramatically enhances water solubility (1.30 M in 2.0 M H₂SO₄) while maintaining overall electroneutrality, thereby intrinsically reducing molecule crossover and suppressing capacity decay during cycling. Symmetric cell testing confirms outstanding stability over 3800 cycles (∼100 days) with an average coulombic efficiency (CE) of 99.98% and nearly zero capacity loss (0.198% year^-1). In the full cell demonstration, ABPS delivers a high voltage of 1.14 V (paired with V^2+/3+). A capacity density of 48.5 Ah L^-1 and the corresponding posolyte energy density of 55.3 Wh L^-1 are achieved (1.0 M molecule concentration), and an ultralow capacity decay rate of 0.084% year^-1 over 1100 h of operation. The rational design of azo-based zwitterionic structure thus offers a promising universal route to durable, high-power acidic AORFB posolytes.