Proton-Resistant N-Heterocycle-Linked TEMPO Catholytes for Long-Lasting Neutral Aqueous Organic Redox Flow Batteries

TEMPO is a widely studied catholyte for aqueous organic redox flow batteries (AORFBs) but suffers from proton-induced ring-opening degradation when its solubility is enhanced via hydrophilic substitution at the 4-position, leading to structural failure and rapid capacity fade. To address this issue, five TEMPO derivatives were synthesized through N-acetylamino bridging and nitrogen-containing heterocycle grafting strategy. Combined analyses using atomic dipole moment-corrected Hirshfeld (ADCH) charges, Fukui functions, and linear ion trap mass spectrometry (LTQ-XL) reveal that aromatic heterocycle functionalization enables favorable charge redistribution during redox cycling, enhancing both redox kinetics and molecular stability. In particular, dimethylaminopyridine-functionalized TEMPO (DMA-TEMPO) exhibits enhanced π-conjugation and basicity, which suppresses proton-driven ring-opening and significantly improves structural resilience. 1 M DMA-TEMPO catholyte delivers exceptional cycling performance, retaining 99.98% of its capacity after 560 cycles, while 2 M system maintains 97% capacity over 100 cycles. Compared to its structural analog 1 M PA-TEMPO, the cycle life is improved 18-fold. This study offers a robust molecular design strategy for developing proton-resistant catholytes, advancing the practical deployment of long-lasting AORFBs for grid-scale energy storage.