A highly soluble and readily accessible viologen negolyte for pH-neutral aqueous organic redox flow batteries

Aqueous organic redox flow batteries (AORFBs) operated at neutral pH are considered a promising energy storage technology for massive-scale renewable energy storage. The development of suitable electrolytes is of particular significance because they determine the performances and cost of AORFBs. Viologen based redox-active materials are typical negative electrolytes for pH-neutral AORFBs. However, the widespread adoption of most reported viologen derivatives is limited by their solubility in water and the complicated synthesis. Herein, we propose a densely hydroxylated viologen, which can be obtained readily through a one-step synthesis with a yield of 70%, promising an ideal negative electrolyte for pH-neutral AORFBs. When operated in a 0.1 M practical pH-neutral AORFB against N Me -TEMPO, the battery displays an open-circuit voltage of 1.32 V, and delivers a capacity retention rate of 99.979% per cycle (temporal capacity fade rate of 0.18% per hour) over 600 consecutive cycles. An energy-dense pH-neutral AORFB based on this densely hydroxylated viologen is demonstrated and the mechanism leading to battery capacity fade is analyzed.