Scalable Synthesis of Sulfonated TEMPO Catholyte for Stable AORFB

The development of chemically robust and synthetically accessible redox-active organics is paramount for the advancement of sustainable aqueous organic redox flow batteries (AORFBs). While TEMPO derivatives are premier catholyte candidates, their practical utility is frequently hampered by complex, multistep synthesis and the susceptibility of the charged oxoammonium state to irreversible ring-opening degradation. We novelly provide HOTEMPO-SO₃Na, an anionic TEMPO derivative at high yield and purity, through simple Corey-Chaykovsky epoxidation of readily available triacetonamine followed with ring opening by NaHSO₃ and H₂O₂ oxidation. The new strategy started from economic raw materials, eliminated halide ion-exchange protocols, and complex column purification requirement, offering a highly scalable and halide-free alternative for large-scale production. In a pH-neutral AORFB, the system demonstrates outstanding durability with a capacity fade rate of 0.0066% per cycle over 600 cycles in 0.1 M, though it deteriorates much faster in high concentration. This work establishes a scalable framework for engineering high-performance anionic redoxmers through simplified, atom-economic epoxidation-sulfonation strategy.