Structural Influence of Spatially Separated Quaternary Ammonium on the Stability of TEMPO Catholytes

Balancing ionization-induced solubility and redox stability is critical for advancing TEMPO-based aqueous organic redox flow batteries (AORFBs). Current strategies mainly focus on changing ionic γ-substituent types (i.e., quaternary ammonium, sulfate, pyrrolidinium, etc.) or the spacer between the redox center and ionic groups. In this study, we examined the influence of spatially separated quaternary ammonium modifications on the stability of the TEMPO-based catholytes. Such modifications were synthesized via dimethylamine-induced epoxide ring-opening of 4-oxiranyl-TEMPO, followed by alkylation with a series of halogenated hydrocarbons. Though quaternary ammonium is separated by one carbon spacer, structural modification enhanced aqueous solubility (up to 2.54 M), modulated redox potential (E1/2 = 613–665 mV vs Ag/AgCl), and enhanced long-term stability of the TEMPO catholyte. The isopropyl functional derivative (C3i) shows optimal performance, enabling AORFBs with a capacity decay of 0.0019% per cycle over 1000 cycles at 0.5 M. DFT charge distribution analysis reveals suppressed reactivity at the β-H positions by isopropyl functionalization, providing a mechanistic basis for its superior durability. These findings underscore the critical role of spatially separated quaternary ammonium structures in enhancing the stability of TEMPO-based catholytes.