Redox-active organic materials have been considered as one of the most promising "green" candidates for aqueous redox flow batteries (RFBs) due to the natural abundance, structural diversity, and high tailorability. However, many reported organic molecules are employed in the anode, and molecules with highly reversible capacity for the cathode are limited. Here, a class of heteroaromatic phenothiazine derivatives is reported as promising positive materials for aqueous RFBs. Among these derivatives, methylene blue (MB) possesses high reversibility with extremely fast redox kinetics (electron-transfer rate constant of 0.32 cm s −1 ), excellent stability in both neutral and reduced states, and high solubility in an acetic-acid-water solvent, leading to a high reversible capacity of ≈71 Ah L −1 . Symmetric RFBs based on MB electrolyte demonstrate remarkable stability with no capacity decay over 1200 cycles. Even concentrated MB catholyte (1.5 m) is still able to deliver stable capacity over hundreds of cycles in a full cell system. The impressive cell performance validates the practicability of MB for large-scale electrical energy storage. Redox Flow BatteriesNowadays, the extensive exploitation of fossil fuels and the increasing demand for large-scale electrical energy storage call for sustainable and innovative technologies for generating and storing energy from renewable resources. [1] Compared with inorganic materials that have been applied in commercial batteries, redox-active organic materials are advantageous in structural diversity, resource sustainability, tunable properties, and potential material cost. [1b,2] Consequently, various kinds of organic compounds including carbonyl compounds, [1b,3] Adv. Mater. 2019, 31, 1901052 The authors declare no conflict of interest.

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Redox flow batteries (RFBs) are among the most promising grid‐scale energy storage technologies. However, the development of RFBs with high round‐trip efficiency, high rate capability, and long cycle life for practical applications is highly restricted by the lack of appropriate ion‐conducting membranes. Promising RFB membranes should separate positive and negative species completely and conduct balancing ions smoothly. Specific systems must meet additional requirements, such as high chemical stability in corrosive electrolytes, good resistance to organic solvents in nonaqueous systems, and excellent mechanical strength and flexibility. These rigorous requirements put high demands on the membrane design, essentially the chemistry and microstructure associated with ion transport channels. In this Review, we summarize the design rationale of recently reported RFB membranes at the molecular level, with an emphasis on new chemistry, novel microstructures, and innovative fabrication strategies. Future challenges and potential research opportunities within this field are also discussed.

Supramolecular chemistry is a central topic in modern chemistry.

A graphite intercalation compound (GIC) framework enables the use of Na–K liquid alloy in ultra-stable and high-capacity anodes.

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Nonaqueous redox flow batteries (RFBs) have received significant research interest, but the lack of promising separators with advanced performance seriously hinders the development of nonaqueous RFBs. Here, a robust yet flexible membrane with enhanced selectivity for nonaqueous RFBs is designed via in situ synthesis of metal–organic frameworks (MOFs) in a porous polymeric membrane (Celgard) with a gradient density. The crossover of active species is mitigated by the reduced effective pore size while high ionic conductivity is maintained, which is attributed to the 3D channel structure of MOFs and their gradient distribution in the membrane. A Li/ferrocene RFB with the MOF‐imbedded membrane delivers an excellent high‐rate capability and enhanced cycling stability. The discharge capacity reaches as high as ≈94% of theoretical value at a current density of 4 mA cm−2, and maintains 76% even at 12 mA cm−2. Moreover, a much slower capacity decay rate is achieved (0.09% per cycle over 300 cycles) by using the composite membrane compared with the pristine Celgard membrane (0.24% per cycle). The demonstrated strategy provides new insight into rational design and fabrication of size‐sieving separators for RFBs and can promote further research of MOFs' capability in energy storage.

In article number https://doi.org/10.1002/adma.201901052, Yu Zhao, Guihua Yu, and co‐workers explore a class of phenothiazine‐derived molecules as catholytes for aqueous redox flow batteries. The highly delocalized conjugation of redox intermediate endows these molecules with pronounced structural stability. Flow cells based on methylene blue, a phenothiazine derivative, demonstrate a high capacity and unprecedented stability, validating its practicability for large‐scale electrical energy storage.