Conjugate-Driven Electron Density Delocalization of Piperidine Nitroxyl Radical for Stable Aqueous Zinc Hybrid Flow Batteries

Stable and soluble redox-active nitroxyl radicals are highly desired for high-capacity and long-life aqueous zinc hybrid flow batteries (AZHFBs). Here we report a pi-pi conjugated imidazolium and p-pi conjugated acetylamino co-functionalized 2,2,6,6-tetramethylpiperidine-N-oxyl (MIAcNH-TEMPO) as stable catholyte for AZHFBs. The incorporation of double-conjugate substituents could delocalize the electron density of the N-O head and thus remarkably stabilize the radical and oxoammonium forms of TEMPO, avoiding the side reaction of ring-opening. Consequently, the applied MIAcNH-TEMPO/Zn AZHFB demonstrates the hardly time-dependent stability with a constant capacity retention of 99.95 % per day over 16.7 days at a high concentration catholyte of 1.5 M and high current density of 50 mA cm(-2). This proposed molecular engineering strategy based on electron density regulation of redox-active structures displays an attractive efficacy and thus represents a remarkable advance in high-performance AZHFBs.

Automated summary

We report a stable catholyte MIAcNH-TEMPO for aqueous zinc hybrid flow batteries (AZHFBs) with high stability and long-lasting capacity. By incorporating double-conjugate substituents, the electron density of TEMPO can be delocalized, resulting in the stabilization of its radical and oxoammonium forms and avoiding side reactions. The MIAcNH-TEMPO/Zn AZHFB exhibits excellent stability with a constant capacity retention of 99.95% per day over 16.7 days at high concentration catholyte and high current density.