Tianhui Xu, Yufeng Liu, Xianzhi Yuan, Mingbao Huang*, Zhipeng Xiang, Kai Wan, Zhiyong Fu*, Zhenxing Liang
HO-TEMPO and its derivatives have been considered promising positive materials due to their favourable electrochemical reversibility and stability, while they still suffer either insufficient solubility or low redox potential for the application in aqueous redox flow batteries (ARFBs). Herein, a novel class of TEMPO-based compounds modified by annular quaternary ammonium (AQA) groups, viz., N-methylpyrrolidinium-2,2,6,6-tetramethylpiperidine-1-oxyl chloride (P-TEMPO) and N-methylmorpholidinium-2,2,6,6-tetramethylpiperidine-1-oxyl chloride (Mor-TEMPO), are designed to address the above issues. Benefiting from strong electron-withdrawing and highly hydrophilic personalities of the grafted AQA substituents, both P-TEMPO and Mor-TEMPO compounds exhibit high redox potential (0.95 V and 0.98 V vs. SHE) and aqueous solubility (>3.0 M). For demonstration, the flow battery fed with 2.0 M Mor-TEMPO as catholyte shows a competitive cell voltage of 1.32 V and a high capacity of 43.5 Ah L-1, pairing with bis(3-trimethylammonio)propyl viologen tetrachloride as anolyte. This work provides an annular quaternary ammonium-functionalized strategy to improve solubility and redox potential of the TEMPO, demonstrating the feasibility to increase capacity and voltage of the aqueous redox flow battery by molecular engineering.