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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. 

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.

 

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.

 

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.

 

Aqueous redox flow batteries using low-cost organic and inorganic active materials have received growing interest for sustainable energy storage. In this study, a low-cost, high redox potential (1.08 V vs. NHE) and high capacity ammonium bromide (NH 4 Br, 214.4 A h L −1 ) catholyte was coupled with an organic viologen anolyte to demonstrate 1.51 V high voltage (SPr) 2 V/Br − aqueous redox flow batteries under pH neutral conditions for the first time. Benefitting from the high water solubility of both the NH 4 Br catholyte and (SPr) 2 V anolyte, the newly designed (SPr) 2 V/Br − organic flow battery was operated at up to 1.5 M and an energy density of up to 30.4 W h L −1 . Using multiwall carbon nanotubes as an electrochemical additive for the Br 3 − /Br − redox couple, the highly energy dense (SPr) 2 V/Br − flow battery manifested outstanding current performance, up to 78% energy efficiency at 40 mA cm −2 current density and 227 mW cm −2 power density, the highest power density known for pH neutral organic flow batteries.