Unprecedented Aqueous Solubility of TEMPO and its Application as High Capacity Catholyte for Aqueous Organic Redox Flow Batteries

Despite the excellent electrochemical properties of non-functionalized 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), its use in aqueous organic redox flow battery (AORFB) is hindered to date due to its insolubility in water. However, in this study, an unprecedented solubility of 5.6 M is demonstrated in an aqueous solution of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), which is 80 times higher than in water (0.07 M). A computational study reveals that the unique interaction between TEMPO and TFSI is essential to achieve this record solubility. TEMPO catholytes are tested in symmetric flow cells, demonstrating high capacity (23.85 AhL^-1), high material utilization (89%), and robust reversible performance with long-term stability (low capacity fading of 0.082%/day). When paired with sulfonated viologen anolyte ((SPr2)V), an AORFB with low capacity fading over cycling (0.60%/day, 0.048%/cycle) is achieved, constituting the first example of a non-functionalized TEMPO catholyte for AORFB. Notably, this solubilization strategy could be applied to other unexplored chemistries in aqueous electrolytes, leading to the development of new AORFBs.