excellent stability can provide a good choice to meet this demand. [2][3][4][5] However, the extensively studied all-vanadium RFBs suffer some long-standing problems, such as the high costs of cation exchange membranes and vanadium-based active materials and the electrolyte crossover and corrosion issues in strong acidic conditions. [6] Aqueous organic redox flow batteries (AORFBs), after being reported, [7,8] received particular attention among the existing flow battery technologies, primarily because organic redox-active materials possess numerous advantages including great natural abundance, facile structural tailorability, tunable electrochemical properties, and potentially low costs. The solutions of a number of organic active molecules, such as viologen, anthraquinone, phenazine, and azobenzene derivatives, have been investigated as the anolytes for AORFBs. And a few organic molecules are available for the catholytes, typically, 2,2,6,6-tetramethyl piperidin-1-yl)oxyl (TEMPO) and metallocene derivatives. [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Considering the high redox potential of TEMPO and its derivatives (ranged from 0.8 to 1.0 V versus standard hydrogen electrode, SHE), the introduction of highly hydrophilic groups, such as the quaternary ammonium group, is a promising approach to further improve the energy density. [20,23] However, up to date, the capacity degradation mechanism on the TEMPO/viologen systems remains unclear, and a lack of appropriate spectroscopic methods was established to study the electrochemical reversibility of redox-active organic species, especially in radical involved systems.Given that the chemically stable pyrrolidinium group has an ultrawide electrochemical window and strong polarity, it has been frequently utilized as a cationic moiety of ionic liquids. [33,34] When coupled with Clion, the pyrrolidinium functionalized organic species also appear to be highly watersoluble. Herein, we report the successful synthesis of pyrrolidinium cation functionalized TEMPO and extended viologen (Pyr-TEMPO and [PyrPV]Cl 4 ), which can serve as a highlysoluble organic redox pair with high cell voltage and excellent redox reversibility in AORFBs (Figure 1). The introduction Aqueous organic redox flow batteries (AORFBs) are regarded as a promising candidate for grid-scale, low-cost and sustainable energy storage. However, their performance is restricted by low aqueous solubility and the narrow potential gap of the organic redox-active species. Herein, a highly-soluble organic redox pair based on pyrrolidinium cation functionalized TEMPO and extended viologen, namely Pyr-TEMPO and [PyrPV]Cl 4 , which exhibits high cell voltage (1.57 V) and long cycling life (over 1000 cycles) in AORFBs is reported. The intrinsic hydrophilic nature of the pyrrolidinium group enables high aqueous solubilities (over 3.35 m for Pyr-TEMPO and 1.13 m for [PyrPV]Cl 4 ). Furthermore, the interaction of nitroxyl radicals with water is observed, which may be helpful to prevent collision-induced...

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Aqueous organic redox flow batteries (AORFBs) are regarded as a promising solution for grid-scale and sustainable energy storage, but some long-standing problems such as low energy density and cycling stability should be resolved. Herein, a highly soluble bipyridine modified with a bridging phenylene group and two quaternary ammonium terminals, namely, [(NPr)2PV]·4Cl, was synthesized and used as an ultralow-potential and two-electron storage anolyte for AORFBs. The phenylene group, which is linked but not coplanar with the two pyridinium redox centers, can thus prevent their communication and result in an exceptionally low redox potential (−0.77 V vs standard hydrogen electrode, 2e–). Moreover, the introduction of a phenylene group can warrant a certain degree of large π-conjugation effects and mitigate the intramolecular Coulombic repulsion between the two positively charged pyridinium centers, thus helping to enhance the electrochemical stability. When paired with 4-trimethylammonium-TEMPO as the catholyte, [(NPr)2PV]·4Cl enabled an exceptionally high cell voltage up to 1.71 V. The AORFB delivers outstanding battery performances, specifically, ∼89% energy efficiency, ∼100% Coulombic efficiency, and ∼99.94% capacity retention per cycle during a long-term cycling process. The two overlapped single-electron reductions of [(NPr)2PV]·4Cl from the initial cationic form to the monoradical form and then to the quinoid form during the charging process were clearly verified by a series of spectroscopic techniques, including no-deuterium nuclear magnetic resonance and electron paramagnetic resonance. This work presents a significant improvement for the construction of high-voltage AORFBs by virtue of the designability, diversity, and tunability of multiredox organic molecules.

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