Redox flow batteries (RFBs) are of interest for large-scale energy storage, but implementation has been challenged by their low energy density, high complexity, high cost, and insufficient lifetime due to several types of irreversible losses in the electrolyte. To address the issue of system complexity and irreversible losses, we have undertaken a detailed analysis of the concept of the symmetric redox flow battery, or SRFB, which relies on a single parent molecule as the charge storage species in both the positive and negative electrode reactions. Herein we elaborate on the operating principles and advantages of a SRFB and report the salient electrochemical properties of a class of organic molecules, diaminoanthraquinones (DAAQs) as promising candidates for use in SRFB redox electrolytes. Modeling of various modes of operation for SRFBs are presented along with an example of an operational, lab-scale SRFB based on a DAAQ system. © The Author Renewable energy sources are rapidly growing in both implementation and research interest due to rising fossil fuel prices and concerns over pollution and climate change.1 However, a major challenge for implementing renewables, such as wind and solar, on a large scale, is their inherent temporal intermittency. For continued growth of renewables, while maintaining the expected level of stability/reliability in the energy grid, significant advances are needed in the area of large-scale energy storage. 2-7Electrochemical energy storage (EES) systems have been broadly identified as promising for short-term grid leveling and long-term energy storage.8 Of the available technologies, redox flow batteries (RFBs) are gaining increasing levels of attention as a viable approach for grid-scale EES.9-13 RFBs differ from conventional batteries in that charge is stored in a pair of liquid-phase redox couples with disparate electrochemical potentials, unlike the solid-phase anodes and cathodes used in conventional batteries (e.g. Li ion, Ni-Cd, and Pb-acid).14 Thus, these liquid species can be stored in tanks of arbitrary size and flowed through separate electrode stacks to store and recover electrical energy. These properties lend well to modular, flexible designs that decouple energy density and power. RFBs also enjoy comparatively high stability due to the lack of phase changes upon cycling.Most of the existing RFB technologies rely on aqueous transition metal redox couples. [9][10][11][12][13] The most common systems use transition metal cations in various oxidation states such as the Fe-Cr system 15 or the extensively studied aqueous vanadium flow battery (VFB). 16Hybrid systems have also been developed that make use of a solidliquid phase change at one electrode, such as the Zn-bromide flow cell.12 Implementations of RFBs are still in their early stages, and several significant research challenges have precluded their scalable implementation. Some of the major challenges include (1) low energy density, (2) high system cost and (3) limited lifetime. 9,13,17 The VFB is the most widely ...

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Electrical energy storage devices will play a key role in the development of sustainable energy production and usage, and for integrating intermittent and renewable sources into the energy landscape. One strategy for developing improved energy storage materials and devices is to take advantage of capacitive and pseudocapacitive electrodes such as activated carbons or conducting polymers. However, these materials generally suffer from low energy densities. Functionalization of these materials with pendant redox units has been proposed as a method to improve the energy densities while maintaining the high rate capability. In this report, we present the synthesis and thorough characterization of one such candidate pendant molecule, 2,5-bis(methylthio)thieno [3,2-b]thiophene, and assess its potential use as a cathode material. Electrochemical, spectroelectrochemical and computational data suggest that bis(methylthio)thieno [3,2-b]thiophene is a lightweight molecule, capable of undergoing multiple reversible redox processes, and a good candidate for improving the energy density of cathode materials while still offering high rate (power) capability.

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Organosulfur compounds with multiple thiol groups are promising for high gravimetric energy density electrochemical energy storage. We have synthesized a poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT)/poly(3,4-ethylenedioxythiophene) (PEDOT) composite cathode for lithium-ion batteries with a new method and investigated its electrochemical behavior by charge/discharge cycles and cyclic voltammetry (CV) in an ether-based electrolyte. Based on a comparison of the electrochemical performance with a carbonate-based electrolyte, we found a much higher discharge capacity, but also a very attractive cycling performance of PDMcT by using a tetra(ethylene glycol) dimethyl ether (TEGDME)-based electrolyte. The first discharge capacity of the as-synthesized PDMcT/PEDOT composite approached 210 mAh g(-1) in the TEGDME-based electrolyte. CV results clearly show that the redox reactions of PDMcT are highly reversible in this TEGDME-based electrolyte. The reversible capacity remained around 120 mAh g(-1) after 20 charge/discharge cycles. With improved cycling performance and very low cost, PDMcT could become a very promising cathode material when combined with a TEGDME-based electrolyte. The poor capacity in the carbonate-based electrolyte is a consequence of the irreversible reaction of the DMcT monomer and dimer with the solvent, emphasizing the importance of electrolyte chemistry when studying molecular-based battery materials.