A High‐Water‐Soluble TEMPO‐Based Polymer Catholyte Material Utilizing Polyvinylimidazole Backbone for Aqueous Redox‐Flow Batteries

Aqueous organic redox flow batteries (AORFBs) are promising for large-scale energy storage due to their low cost, high safety, and lightweight design. Here, this study designs a polymer material, P-T-N-4, based on a hydrophilic polyvinylimidazole backbone, incorporating 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and quaternary ammonium groups, achieving high-water-solubility (39 Ah L^-1 in 1.0 m NaCl aqueous solution (NaCl_aq)) and low viscosity (6.3 mPa s in 1.0 m NaCl_aq at 10 Ah L^-1). Using P-T-N-4 as the cathode, methyl viologen (MV) as the anode, and 1.0 m NaCl_aq as the electrolyte, a series of P-T-N-4/MV AORFBs is assembled and performed charge-discharge tests in air atmosphere. The P-T-N-4/MV AORFB at 20 Ah L^-1 demonstrates stable cycling for over 400 cycles at 30 mA cm^-2, exhibiting an average capacity retention of 99.88% per cycle (99.94% per hour), an average Coulombic efficiency of 99.18% (except the first cycle of 87.36%), and material utilization of 83.1%. Furthermore, the assembled P-T-N-4/MV AORFB achieves stable cycling for over 100 cycles even at a higher concentration of 30 Ah L^-1, with a capacity retention rate of 99.96% per cycle during the first 50 cycles.