Design of TEMPO-Based Polymer Cathode Materials for pH-Neutral Aqueous Organic Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) represent an advancing class of electrochemical energy storage systems showing considerable promise for large-scale grid integration due to their unique aqueous organic chemistry. However, the use of small-molecule active materials in AORFBs is significantly limited by the issue of stability and crossover. To address these challenges, we designed a high-water-solubility polymer cathode material, P-T-S, which features a polyvinylimidazole backbone functionalized with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and sulfonate groups. P-T-S exhibits a solubility of 34 Ah L^-1 in water and 31 Ah L^-1 in 1.0 M NaCl aqueous solution (NaCl_aq). When paired with methyl viologen to assemble a pH-neutral AORFB with a theoretical capacity of 15 Ah L^-1, the system exhibits a material utilization rate of 92.0%, an average capacity retention rate of 99.74% per cycle (99.74% per hour), and an average Coulombic efficiency of 98.69% over 300 consecutive cycles at 30 mA cm^-2. This work provides a new design strategy for polymer materials for high-performance AORFBs.