Insights into an air-stable methylene blue catholyte towards kW-scale practical aqueous organic flow batteries

Aqueous organic flow batteries (AOFBs) possess unique advantages, including element abundance and tailorability, compared to the traditional flow batteries (FBs). However, achieving an air-stable and high-performance electrolyte is still one of the main challenges for their practical applications. Generally, the stability of an aqueous organic electrolyte is associated with the structure of the organic redox-species and electrolyte environment. By the virtue of electrolyte optimization and in situ/ex situ NMR and EPR techniques, we found in the methylene blue (MB) electrolyte that both oxygen-resistant MB radicals generated by the comproportionation reaction and reduced MB states in the acidic electrolyte displayed much more stable molecular structures. This definitely plays a vital role in the high reversibility of MB molecules under ambient air conditions. Taking advantage of the air-stable MB electrolyte, a kW-scale AOFB stack was assembled for the first time, which exhibited stable capacity at 80 mA cm(-2) for over 500 cycles. Moreover, the stack based on 0.5 M MB electrolyte still achieved a stable long-life cycle performance for similar to 32 days with a capacity of similar to 510 A h. The impressive stack performance enables the MB catholyte to be a promising candidate for large-scale energy storage.