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.

Redox-active 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives have recently been investigated to expand the choice of catholyte for aqueous flow batteries (AFBs). However, the effects of substituent R in 4position on redox potential and corresponding capacity fading mechanism are still unclear. Here, we conduct comparative studies of four R−TEMPO with R = −OH, −NH 2 , −COOH, and −NHCOCH 3 in zinc hybrid AFBs. Experimental and theoretical analyses reveal that low-radical head charge population sum and radical energy, depending on R in 4-position, play a critical role in enhancing redox potential and cycling life of R−TEMPO. The electronic effect brought along by N-acetyl could redistribute the charge and lower systematic energy, making the ring-opening joint sturdy and therefore suppress the side reactions. Accordingly, the 4-NHCOCH 3 −TEMPO/Zn battery achieves a high capacity retention of >99.65%/day and an open-circuit voltage of 1.71 V. Our findings on the effects of substituent are greatly anticipated to boost the high-energy density, long-life, and eco-friendly TEMPO-based AFBs.

Redox‐active organics are highly attractive in aqueous organic redox flow batteries (AORFBs). However, the lack of capacity dense, stable organic catholytes remains a challenge to develop energy‐dense, long cycle‐life AORFBs. Herein, a stable organic catholyte, 4‐[3‐(trimethylammonium)acetylamino]‐2,2,6,6‐tetramethylpiperidine‐1‐oxyl chloride (TMAAcNHTEMPO) is developed through rational molecular engineering using connective acetamido and trimethylammonium groups. Paired with bis‐(trimethylammonium) propyl viologen tetrachloride anolyte, stable AORFBs (up to 1500 cycles) with a low capacity fade rate of ca. 0.0144% h−1 are achieved. Experimental characterizations and theoretical simulations revealed that TMAAcNH‐TEMPO is largely stabilized by the reduced reactivity of the nitroxyl radical moiety that mitigates a ring‐opening side reaction.

The utilization of redox-active and stable cyclic nitroxide radicals (CNRs) holds a great promise in neutral aqueous organic redox flow batteries (AORFBs) for large-scale energy storage. Herein, a new class of CNRs with five-membered ring pyrrolidine and pyrroline motifs for AORFBs is reported. By rational molecular engineering of introducing CC double bond into the pyrrolidine-based molecule, 3-carbamoyl-2,2,5,5-tetramethylpyrroline-1-oxyl (CPL) with a high redox potential of 0.76 V (vs Ag/AgCl) is demonstrated, which is 160 mV higher than the common 2,2,6,6-tetramethylpiperidine 1-oxyl derivatives with a six-membered ring as the core structure. Density functional theory calculations reveal that the much enhanced redox potential for CPL is largely contributed by lowered standard free energy in reduction reaction and charge population sum of NO radical head. When paired with the BTMAP-viologen anolyte, the CPL-based AORFB delivers constant capacity retention of up to 99.96%/cycle over 500 cycles.

An organic dye Basic blue 3 (BB3) was firstly reported as a two-electron catholyte for aqueous redox flow battery. The exceptional stability of BB3 enabled the full battery to achieve...