An extremely stable, energy-dense (53.6 Ah L −1 , 2 m transferrable electrons), low crossover (permeability of <1 × 10 −13 cm 2 s −1 using Nafion 212 (Nafion is a trademark polymer from DuPont)), and potentially inexpensive anthraquinone with 2-2-propionate ether anthraquinone structure (abbreviated 2-2PEAQ) is synthesized and extensively evaluated under practically relevant conditions for use in the negolyte of an aqueous redox flow battery. 2-2PEAQ shows a high stability with a fade rate of 0.03-0.05% per day at different applied current densities, cut-off voltage windows, and concentrations (0.1 and 1.0 m) in both a full cell paired with a ferro/ferricyanide posolyte as well as a symmetric cell. 2-2PEAQ is further shown to have extreme long-term stability, losing only ≈0.01% per day when an electrochemical rejuvenation strategy is employed. From post-mortem analysis (nuclear magnetic resonance (NMR), liquid chromatography-mass spectrometry (LC-MS), and cyclic voltammetry (CV)) two degradation mechanisms are deduced: side chain loss and anthrone formation. 2-2PEAQ with the ether linkages attached on carbons non-adjacent to the central ring is found to have three times lower fade rate compared to its isomer with ether linkages on the carbon adjacent to the central quinone ring. The present study introduces a viable negolyte candidate for grid-scale aqueous organic redox flow batteries.

Redox flow batteries (RFBs) can achieve long lifetimes and high performance when employing highly selective and conductive membranes. Neutral and alkaline RFBs suffer from higher resistances due to lower cation conductivity, compared to acidic RFBs utilizing proton transport. We report the use of a sulfonated Diels-Alder poly(phenylene) membrane that exhibits low and stable potassium area specific resistance and high efficiency RFB cycling relative to Nafion, as well as undetectable ferricyanide crossover. An alkaline (pH 12) organic anthraquinone derivative RFB using this membrane demonstrates over 10 days of cycling without capacity loss from crossover. A neutral chelated chromium complex RFB using this membrane demonstrates a peak discharge power of 1.23 W cm−2, and 80% energy efficiency cycling at an average discharge power density of 446.3 mW cm−2. Finally, the membrane exhibits similar favorable conductivity for many monovalent cations, opening the opportunity to improve the cycling and crossover performance of other acidic, neutral, and alkaline RFBs.

Organic reactants are promising candidates for long-lifetime redox flow batteries, and synthetic chemistry unlocks a wide design space for new molecules. Minimizing crossover of these molecules through ion exchange membranes is one important design consideration, but the ways in which the crossover rate depends on the structure of the crossing species remain unclear. Here, we contribute a systematic evaluation of size and charge-based effects on dilute-solution small molecule permeability through the Nafion NR212 cation exchange membrane. We found that increasing the magnitude of charge number z with the same sign as membrane fixed charges, achieved here by successive sulfonation of quinone redox cores, results in more than an order of magnitude permeability reduction per sulfonate. Size-based effects, understood by comparing the Stokes radii of the quinones studied, also reduces permeability with increasing effective molecule size, but doubling the effective size of the redox reactants resulted in a permeability decrease of less than a factor of three.

An iron complex, tris(4,4′‐bis(hydroxymethyl)‐2,2′‐bipyridine) iron dichloride is reported, which operates at near‐neutral pH with a redox potential of 0.985 V versus SHE. This high potential compound is employed in the posolyte of an aqueous flow battery, paired with bis(3‐trimethylammonio)propyl viologen tetrachloride in the negolyte, exhibiting an open‐circuit voltage of 1.3 V at near‐neutral pH. It demonstrates excellent cycling performance with a low temporal capacity fade rate of 0.07% per day over 35 days of cycling. The extended cycling lifetime is the result of low permeability and improved structural stability of the newly developed iron complex compared to that of the iron tris(bipyridine) complex. The combination of high redox potential and low capacity fade rate compares favorably with those of all previously demonstrated organic and organometallic aqueous posolytes. Extensive investigation into the possible degradation mechanisms, including post‐mortem chemical and electrochemical analyses, indicates that stepwise ligand dissociations of the iron complex are responsible for the reported capacity loss during cell cycling. This investigation provides unprecedented insight to guide further improvements of such metalorganic compounds for energy storage and conversion applications.

Aqueous organic redox flow batteries are promising candidates for large‐scale energy storage. However, the design of stable and inexpensive electrolytes is challenging. Here, we report a highly stable, low redox potential, and potentially inexpensive negolyte species, sodium 3,3′,3′′,3′′′‐((9,10‐anthraquinone‐2,6‐diyl)bis(azanetriyl))tetrakis(propane‐1‐sulfonate) (2,6‐N‐TSAQ), which is synthesized in a single step from inexpensive precursors. Pairing 2,6‐N‐TSAQ with potassium ferrocyanide at pH=14 yielded a battery with the highest open‐circuit voltage, 1.14 V, of any anthraquinone‐based cell with a capacity fade rate <10 %/yr. When 2,6‐N‐TSAQ was cycled at neutral pH, it exhibited two orders of magnitude higher capacity fade rate. The great difference in anthraquinone cycling stability at different pH is interpreted in terms of the thermodynamics of the anthrone formation reaction. This work shows the great potential of organic synthetic chemistry for the development of viable flow battery electrolytes and demonstrates the remarkable performance improvements achievable with an understanding of decomposition mechanisms.