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 redoxactive 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 NMe-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.