One‐Step Cationic Grafting of 4‐Hydroxy‐TEMPO and its Application in a Hybrid Redox Flow Battery with a Crosslinked PBI Membrane

Abstract By using a one‐step epoxide ring‐opening reaction between 4‐hydroxy‐2,2,6,6‐tetramethylpiperidine 1‐oxyl (4‐hydroxy‐TEMPO) and glycidyltrimethylammonium cation (GTMA + ), we synthesized a cation‐grafted TEMPO (g + ‐TEMPO) and studied its electrochemical performance against a Zn 2+ /Zn anode in a hybrid redox flow battery. To conduct Cl − counter anions, a crosslinked methylated polybenzimidazole (PBI) membrane was prepared and placed between the catholyte and anolyte. Compared to 4‐hydroxy‐TEMPO, the positively charged g + ‐ TEMPO exhibits enhanced reaction kinetics. Moreover, flow battery tests with g + ‐TEMPO show improved Coulombic, voltage, and energy efficiencies and cycling stability over 140 cycles. Crossover of active species through the membrane was not detected.