Redox flow batteries (RFBs) are a viable technology to store renewable energy in the form of electricity that can be supplied to electricity grids. However, widespread implementation of traditional RFBs, such as vanadium and Zn-Br RFBs, is limited due to a number of challenges related to materials, including low abundance and high costs of redox-active metals, expensive separators, active material crossover, and corrosive and hazardous electrolytes. To address these challenges, we demonstrate a neutral aqueous organic redox flow battery (AORFB) technology utilizing a newly designed cathode electrolyte containing a highly water-soluble ferrocene molecule. Specifically, water-soluble (ferrocenylmethyl)trimethylammonium chloride (FcNCl, 4.0 M in HO, 107.2 Ah/L, and 3.0 M in 2.0 NaCl, 80.4 Ah/L) and N-ferrocenylmethyl-N,N,N,N,N-pentamethylpropane-1,2-diaminium dibromide, (FcNBr, 3.1 M in HO, 83.1 Ah/L, and 2.0 M in 2.0 M NaCl, 53.5 Ah/L) were synthesized through structural decoration of hydrophobic ferrocene with synergetic hydrophilic functionalities including an ammonium cation group and a halide anion. When paired with methyl viologen (MV) as an anolyte, resulting FcNCl/MV and FcNBr/MV AORFBs were operated in noncorrosive neutral NaCl supporting electrolytes using a low-cost anion-exchange membrane. These ferrocene/MV AORFBs are characterized as having high theoretical energy density (45.5 Wh/L) and excellent cycling performance from 40 to 100 mA/cm. Notably, the FcNCl/MV AORFBs (demonstrated at 7.0 and 9.9 Wh/L) exhibited unprecedented long cycling performance, 700 cycles at 60 mA/cm with 99.99% capacity retention per cycle, and delivered power density up to 125 mW/cm. These AORFBs are built from earth-abundant elements and are environmentally benign, thus representing a promising choice for sustainable and safe energy storage.

Liu and co-workers reported a series of rationally designed two-electron storage viologen molecules as anolytes for high-voltage and high-power pH-neutral aqueous organic redox flow batteries. The synthetic and computational chemistry presented has opened a new avenue for designing energy-dense redox-active organic molecules for building neutral AORFBs with high power density and high energy density, and it promises economical, benign, and widespread uses of redox flow batteries in large-scale energy storage. HIGHLIGHTSTwo-electron storage viologens were designed for energy-storage applications Neutral aqueous organic redox flow batteries up to 1.38 V and 130 mW/cm 2 An integrated approach of synthesis, electrochemistry, and computational modelling Molecular engineering is a powerful strategy for developing redox-active molecules DeBruler et al., Chem 3, 961-978 December 14, 2017 ª SUMMARYAqueous organic redox flow batteries (AORFBs) are highly attractive for largescale energy storage because redox-active organic molecules are synthetically tunable, sustainable, and potentially low cost. Here, we show that rational molecular engineering yielded a series of two-electron storage viologen molecules as anolyte materials for AORFBs. In neutral NaCl solutions, these viologen anolytes have a theoretical capacity of up to 96.5 Ah/L in H 2 O and exhibit a reduction potential as low as À0.78 V versus normal hydrogen electrode. The neutral aqueous flow batteries with two two-electron storage viologen molecules delivered a cell voltage of up to 1.38 V and outstanding battery performance, including a power density of up to 130 mW/cm 2 , capacity retention of up to 99.99% per cycle, and energy efficiency of up to 65% at 60 mA/cm 2 . Density functional theory calculations revealed that the 1e À and 2e À reduced redox states of these molecules were stabilized by the high charge delocalization of their frontier SOMO or HOMO.

Liu and co-workers report a molecular engineering study of ferricyanide and ferrocyanide catholytes for pH neutral aqueous organic redox flow battery applications using a strategy of cation modulation. Compared with traditional potassium or sodium ferricyanide and ferrocyanide catholytes, the newly designed (NH 4 ) 3 [Fe(CN) 6 ] and (NH 4 ) 4 [Fe(CN) 6 ] catholytes manifest unprecedented electron storage capacities and robust battery performance, which promise sustainable and economical energy storage. Meanwhile, this work presents an efficient pathway to the development of high-performance redox active electrolytes for redox flow batteries.

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Redox flow batteries using synthetically tunable and resource abundant organic molecules have gained increasing attention for large-scale energy storage. Herein we report a sulfonatefunctionalized viologen molecule, 1,1′-bis(3-sulfonatopropyl)-4,4′bipyridinium, (SPr) 2 V, as an anolyte in neutral aqueous organic redox flow batteries (AORFBs) functioning through a cation chargetransfer mechanism. Demonstrated (SPr) 2 V/KI AORFBs manifested high current performance from 40 to 100 mA/cm 2 with up to 71% energy efficiency. In extended cycling studies, the (SPr) 2 V/KI redox flow battery delivered stable cycling performance at 60 mA/cm 2 , up to 67% energy efficiency, and 99.99% capacity retention per cycle. Density functional theory modeling of the electrostatic charge surface of (SPr) 2 V and its charged state, [(SPr) 2 V] −1 , suggests charge repulsion and size exclusion enable their compatibility with a cation exchange membrane. The present findings expand the battery design of neutral viologen AORFBs and represent an attractive RFB technology for sustainable and benign renewable energy storage.