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 Zn2+/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.