Aqueous redox flow batteries (RFBs) are deployable, high-performance electrochemical energy storage systems for incorporating renewable energy resources into the electrical power grid. The decoupled architecture of RFBs entails storing redox-active materials in external reservoirs for reversible energy conversion, thereby enabling the scalability of battery capacity and energy density. The energy density of current RFBs is still limited by the low water solubility and charge storage capacity of redox-active materials. The molecular engineering of organic molecules to achieve high water solubility with multi-electron transfer kinetics holds excellent promise in realizing energy-dense aqueous RFBs. Inspired by this, we have synthesized 1',1''',1'''''-(benzene-1,3,5-triyltris(methylene))tris(1-(3-(trimethyl ammonio) propyl)-[4'',4'''-bipyridine]-1,1'-diium) nonachloride (BTTMPB) as a 3 e- storage anolyte for neutral pH aqueous RFBs. The molecule has a surprisingly high-water solubility of 4.0 M in H2O and 2.4 M in 2.0 M NaCl due to the enhanced intermolecular interactions revealed by the mechanistic solvation study. The molecule has a rate constant (k0) of 1.6 x 10-2 cm s-1 and a diffusion coefficient of 3.1 x 10-6 cm2 s-1 for the first reduction process. The synthesized anolyte delivered an aqueous RFB with an open-circuit potential of 0.92 V and a theoretical energy density of 125.9 Wh L-1. The aqueous RFB has excellent cycling performance from 10 to 30 mA cm-2, the energy efficiency of up to 80%, capacity retention of 99.96% per cycle at 20 mA cm-2, and demonstrated energy density of 29.1 Wh L-1. This work provides insights into the rational molecular design and functionalization approach for developing high-capacity redox-active compounds for next-generation aqueous RFBs.

High-capacity viologen-based anolyte for high energy density neutral pH aqueous redox-flow batteries

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