The 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) is currently the most extensively studied catholyte material for neutral aqueous organic redox flow batteries (AORFBs).

However, modifying TEMPO by adding hydrophilic substituents at its C4 position to boost solubility often triggers a proton-induced ring-opening degradation reaction. This results in the destruction of the molecule's structure and the rapid decay of battery capacity, severely constraining the research and commercialization of AORFBs.

To address this issue, Professor He Gang's research group at the Frontier Institute of Xi'an Jiaotong University (XJTU) built upon previous work to design and synthesize five TEMPO derivatives using N-acetylamino bridging and nitrogen-containing heterocyclic grafting strategies.

Analysis using Atomically Defined Charge-Corrected Hirshfeld (ADCH) charges, Fukui functions, and Linear Ion Trap Mass Spectrometry (LTQ-XL) showed that functionalization with aromatic heterocycles achieves favorable charge redistribution during the redox cycle.

This improves both redox kinetics and molecular stability. Crucially, the dimethylaminopyridine-functionalized TEMPO (DMA-TEMPO) displayed stronger conjugation and alkalinity, which effectively suppressed the proton-driven ring-opening reaction and significantly enhanced its molecular structural stability.

Battery performance tests showed that the 1 M DMA-TEMPO catholyte exhibits excellent cycling performance, maintaining 99.98 percent capacity retention after 560 cycles. The 2 M system still had 97 percent capacity retention after 100 cycles, representing an 18-fold increase in cycle life compared to the structurally analogous 1 M PA-TEMPO.

The research results were published as a paper and featured on the inside cover of Angewandte Chemie International Edition, an authoritative international chemistry journal.