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 “π–π” conjugated imidazolium and “p–π” 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.