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.

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.

DFT calculations were conducted to provide unprecedented thermodynamic insights on tetrahydrofuran (THF) solvation, isomerization, and complexation of possible MgÀ Cl coordination species for popular MgÀ Cl electrolytes for magnesium batteries. Computational results using the M06-2x functional with the 6-31 + G(d) basis set indicate that trigonal bipyramidal e,e-cis-tbp-MgCl 2 (THF) 3 dichloride species and octahedral [MgCl (THF) 5 ] + monochloride species are the dominant mononuclear species. These two can combine to form the dinuclear species [(μ-Cl) 3 Mg 2 (THF) 6 ] + with a free energy À 6.30 kcal/mol, which is calculated to be the dominant MgÀ Cl species in solution. Two mono-cation species, [(μ-Cl) 3 Mg 2 (THF) 6 ] + and [MgCl(THF) 5 ] + , have comparable LUMO energies, thus both of them can act as active species for Mg deposition. However, the significant dominance of the dinuclear species in the electrolyte indicates that it is the primary species involved in reversible Mg deposition.[a] J.

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