Sulfonated tryptanthrin anolyte increases performance in pH neutral aqueous redox flow batteries

Aqueous organic redox flow batteries (AORFBs) hold great promise as low-cost, environmentally friendly and safe alternative energy storage media. Here we present aqueous organometallic and all-organic active materials for RFBs with a water-soluble active material, sulfonated tryptanthrin (TRYP-SO3H), working at a neutral pH and showing long-term stability. Electrochemical measurements show that TRYP-SO3H displays reversible peaks at neutral pH values, allowing its use as an anolyte combined with potassium ferrocyanide or 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt monohydrate as catholytes. Single cell tests show reproducible charge-discharge cycles for both catholytes, with significantly improved results for the aqueous all-organic RFB reaching high cell voltage (0.94 V) and high energy efficiencies, stabilized during at least 50 working cycles. Redox flow batteries working at a neutral pH combine high stability and environmental safety, but their power output is still limited. Here, the authors present an aqueous, all-organic redox flow battery, with sulfonated tryptanthrin as an anolyte solution, reaching a cell voltage of 0.94 V.