Molecular Engineering of Azobenzene-Based Anolytes Towards High-Capacity Aqueous Redox Flow Batteries

Aqueous redox flow batteries (RFBs) are promising alternatives for large-scale energy storage. However, new organic redox-active molecules with good chemical stability and high solubility are still desired for high-performance aqueous RFBs due to their low crossover capability and high abundance. We report azobenzene-based molecules with hydrophilic groups as new active materials for aqueous RFBs by utilizing the reversible redox activity of azo groups. By rationally tailoring the molecular structure of azobenzene, the solubility is favorably improved from near zero to 2 M due to the highly charged asymmetric structure formed in alkaline environment. DFT simulations suggest that the concentrated solution stability can be enhanced by adding hydrotropic agent to form intermolecular hydrogen bonds. The demonstrated RFB exhibits long cycling stability with a capacity retention of 99.95 % per cycle over 500 cycles. It presents a viable chemical design route towards advanced aqueous RFBs.