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  1. Abstract

    Ferrocyanide, such as K4[Fe(CN)6], is one of the most popular cathode electrolyte (catholyte) materials in redox flow batteries. However, its chemical stability in alkaline redox flow batteries is debated. Mechanistic understandings at the molecular level are necessary to elucidate the cycling stability of K4[Fe(CN)6] and its oxidized state (K3[Fe(CN)6]) based electrolytes and guide their proper use in flow batteries for energy storage. Herein, a suite of battery tests and spectroscopic studies are presented to understand the chemical stability of K4[Fe(CN)6] and its charged state, K3[Fe(CN)6], at a variety of conditions. In a strong alkaline solution (pH 14), it is found that the balanced K4[Fe(CN)6]/K3[Fe(CN)6] half‐cell experiences a fast capacity decay under dark conditions. The studies reveal that the chemical reduction of K3[Fe(CN)6] by a graphite electrode leads to the charge imbalance in the half‐cell cycling and is the major cause of the observed capacity decay. In addition, at pH 14, K3[Fe(CN)6] undergoes a slow CN/OHexchange reaction. The dissociated CNligand can chemically reduce K3[Fe(CN)6] to K4[Fe(CN)6] and it is converted to cyanate (OCN) and further, decomposes into CO32‐and NH3. Ultimately, the irreversible chemical conversion of CNto OCNleads to the irreversible decomposition of K4/K3[Fe(CN)6] at pH 14.

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  2. Abstract

    Aqueous organic redox flow batteries (AORFBs) have received increasing attention as an emergent battery technology for grid‐scale renewable energy storage. However, physicochemical properties of redox‐active organic electrolytes remain fine refinement to maximize their performance in RFBs. Herein, we report a carboxylate functionalized viologen derivative, N,N′‐dibutyrate‐4,4′‐bipyridinium,(CBu)2V, as a highly stable, high capacity anolyte material under near pH neutral conditions.(CBu)2Vcan achieve solubility of 2.1 M and display a reversible, kinetically fast reduction at −0.43 V vs NHE at pH 9. DFT studies revealed that the high solubility of(CBu)2Vis attributed to its high molecular polarity while its negative reduction potential is benefitted from electron‐donating carboxylate groups. A 0.89 V (CBu)2V/(NH)4Fe(CN)6AORFB demonstrated exceptional energy storage performance, specifically, 100 % capacity retention with a discharge energy density of 9.5 Wh L−1for 1000 cycles, power densities of up to 85 mW cm−2, and an energy efficiency of 70 % at 60 mA cm−2.(CBu)2Vnot only represents the most capacity dense viologen with pendant ionic groups and also exhibits the longest (1200 hours or 50 days) and the most stable flow battery performance to date.

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  3. Abstract

    Aqueous organic redox flow batteries (AORFBs) have been recognized as a promising technology for large‐scale, long‐duration energy storage of renewables (e.g., solar and wind) by overcoming their intermittence and fluctuation. However, simultaneous demonstration of high energy densities and stable cycling are still challenging for AORFBs. Herein, asymmetrically substituted sulfonate viologen molecular designs, e.g. (1‐[3‐sulfonatopropyl]‐1′‐[4‐sulfonatobutane]‐4,4′‐bipyridinium (3,4‐S2V), as capacity dense, chemically stable anolytes for cation exchange AORFBs are presented. The robust cycling performance of 3,4‐S2V is confirmed using half‐cell and full‐cell flow battery studies at pH neutral conditions. The 3,4‐S2V based AORFB is demonstrated with a discharge capacity of 23.2 Ah L−1for 1700 cycles or 100 days without observing chemical degradation. Furthermore, a 3,4‐S2V/(NH4)4[Fe(CN)6] AORFB with a discharge capacity of 259.9 mAh is demonstrated for 50 days of authentic energy storage for the first time with a total capacity retention of 97.77% or a temporal capacity retention rate of 99.955% per day, representing the most stable, longest cycled AORFB to date.

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  4. Abstract

    Aqueous organic redox flow batteries (AORFBs) are highly attractive for large‐scale energy storage because of their nonflammability, low cost, and sustainability. (2,2,6,6‐Tetramethylpiperidin‐1‐yl)oxyl (TEMPO) derivatives, a class of redox active molecules bearing air‐stable free nitroxyl radicals and high redox potential (>0.8 V vs NHE), has been identified as promising catholytes for AORFBs. However, reported TEMPO based molecules are either permeable through ion exchange membranes or not chemically stable enough for long‐term energy storage. Herein, a new TEMPO derivative functionalized with a dual‐ammonium dicationic group,N1, N1, N1, N3, N3, 2, 2, 6, 6‐nonamethyl‐N3‐(piperidinyloxy)propane‐1,3‐bis(ammonium) dichloride (N2‐TEMPO) as a stable, low permeable catholyte for AORFBs is reported. Ultraviolet–visible (UV–vis) and proton nuclear magnetic resonance (1H‐NMR) spectroscopic studies reveal its exceptional stability and ultra‐low permeability (1.49 × 10−12 cm2 s−1). Coupled with 1,1′‐bis[3‐(trimethylammonio)propyl]‐4,4′‐bipyridinium tetrachloride ((NPr)2V) as an anolyte, a 1.35 VN2‐TEMPO/(NPr)2V AORFB with 0.5 melectrolytes (9.05 Wh L−1) delivers a high power density of 114 mW cm−2and 100% capacity retention for 400 cycles at 60 mA cm−2. At 1.0 melectrolyte concentrations, theN2‐TEMPO/(NPr)2V AORFB achieves an energy density of 18.1 Wh L−1and capacity retention of 90% for 400 cycles at 60 mA cm−2.

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  5. Abstract

    Redox‐active anthraquinone molecules represent promising anolyte materials in aqueous organic redox flow batteries (AORFBs). However, the chemical stability issue and corrosion nature of anthraquinone‐based anolytes in reported acidic and alkaline AORFBs constitute a roadblock for their practical applications in energy storage. A feasible strategy to overcome these issues is migrating to pH‐neutral conditions and employing soluble AQDS salts. Herein, we report the 9,10‐anthraquinone‐2,7‐disulfonic diammonium saltAQDS(NH4)2, as an anolyte material for pH‐neutral AORFBs with solubility of 1.9 min water, which is more than 3 times that of the corresponding sodium salt. Paired with an NH4I catholyte, the resulting pH‐neutral AORFB with an energy density of 12.5 Wh L−1displayed outstanding cycling stability over 300 cycles. Even at the pH‐neutral condition, theAQDS(NH4)2 /NH4I AORFB delivered an impressive energy efficiency of 70.6 % at 60 mA cm−2and a high power density of 91.5 mW cm−2at 100 % SOC. The present AQDS(NH4)2flow battery chemistry opens a new avenue to apply anthraquinone molecules in developing low‐cost and benign pH‐neutral flow batteries for scalable energy storage.

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  6. Water soluble ferrocene (Fc) derivatives are promising cathode materials for aqueous organic redox flow batteries (AORFBs) towards scalable energy storage. However, their structure–performance relationship and degradation mechanism in aqueous electrolytes remain unclear. Herein, physicochemical and electrochemical properties, battery performance, and degradation mechanisms of three Fc catholytes, (ferrocenylmethyl)trimethylammonium chloride (C1-FcNCl), (2-ferrocenyl-ethyl)trimethylammonium chloride (C2-FcNCl), and (3-ferrocenyl-propyl)trimethylammonium chloride (C3-FcNCl) in pH neutral aqueous electrolytes were systemically investigated. UV-Vis and gas chromatography (GC) studies confirmed the thermal and photolytic C x -Cp − ligand dissociation decomposition pathways of both discharged and charged states of C1-FcNCl and C2-FcNCl catholytes. In contrast, in the case of the C3-FcNCl catholyte, the electron-donating 3-(trimethylammonium)propyl group strengthens the coordination between the C 3 -Cp − ligand and the Fe 3+ or Fe 2+ center and thus mitigates the ligand-dissociation degradation. Consistently, the Fc electrolytes displayed cycling stability in both half-cell and full-cell flow batteries in the order of C1-FcNCl < C2-FcNCl < C3-FcNCl. 
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  7. Aqueous redox flow batteries could provide viable grid-scale electrochemical energy storage for renewable energy because of their high-power performance, scalability, and safe operation ( 1 , 2 ). Redox-active organic molecules serve as the energy storage materials ( 2 , 3 ), but only very few organic molecules, such as viologen ( 4 , 5 ) and anthraquinone molecules ( 6 ), have demonstrated promising energy storage performance ( 2 ). Efforts continue to develop other families of organic molecules for flow battery applications that would have dense charge capacities and be chemically robust. On page 836 of this issue, Feng et al. ( 7 ) report a class of ingeniously designed 9-fluorenone (FL) molecules as high-performance, potentially low-cost organic anode electrolytes (anolytes) in aqueous organic redox flow batteries (see the figure, top). These FL anolytes not only display exceptional energy storage performance but also exhibit an unprecedented two-electron storage mechanism. 
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