We have demonstrated the repeated cycling of a redox flow cell based on water-soluble organic redox couples (ORBAT) at high voltage efficiency, coulombic efficiency and power density. These cells were successfully operated with 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) at the positive electrode and anthraquinone-2,6-disulfonic acid (AQDS) at the negative electrode. Reduction of the voltage losses arising from mass transport limitations, and understanding of the chemical transformations of BQDS during charging have led to these improvements in performance. The specific advances reported here include the use of organic redox couples in the free-acid form, improvements to the flow field configuration, and novel high-surface-area graphite-felt electrode structures. We have identified various steps in the chemical and electrochemical transformations of BQDS during the first few cycles. We have also confirmed that the crossover of the reactants through the membrane was not significant. The performance improvements and new understanding presented here will hasten the development of ORBAT as an inexpensive and sustainable solution for large-scale electrical energy storage. With the increasing penetration of solar photovoltaic and windbased electricity generation, the variable and intermittent output of these energy generation systems is a grave concern for stable operation of the electricity grid. To buffer the inevitable surges in electricity supply and demand, large-scale energy storage systems are needed. Such energy storage systems must be capable of storing thousands of giga-watt hours of electricity per day. Rechargeable batteries are particularly attractive for electrical energy storage because of their high energy efficiency and scalability.1-3 However, for such a largescale application, these batteries must be inexpensive, robust, safe, and sustainable. None of today's commercially-available batteries can meet all the performance and cost targets at this scale of deployment of energy storage. This situation has led to a global search for a transformational solution.In 2013, we described an organic redox flow battery -also known as ORBAT -that uses water-soluble organic redox couples as a safe, scalable, and efficient energy storage system with the potential to meet the United States Department of Energy (DoE) cost target of $100/kWh for large-scale energy storage. 4 In such a battery, aqueous solutions of two different water-soluble organic redox couples -quinones and anthraquinones or their derivatives -were circulated past carbon electrodes in an electrochemical cell. In our current system, the positive electrode is supplied with a solution of 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) and the negative electrode uses a solution of anthraquinone-2,6-disulfonic acid (AQDS). The positive and negative electrode compartments are separated by a proton-conducting polymer electrolyte membrane (Figure 1). During charge and discharge, the redox couples undergo rapid proton-coupled electron transfer to store ...

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We report here the synthesis, characterization and properties of 3,6-dihydroxy-2,4-dimethylbenzenesulfonic acid (DHDMBS) as a new positive side electrolyte material for aqueous organic redox flow batteries (ORBAT). We have synthesized this material in pure form in high yield and confirmed its structure. We have determined that the standard reduction potential, the rate constant of the redox reaction, and the diffusion coefficient are ideally suited for use in ORBAT. Specifically, DHDMBS overcomes the major issue of Michael reaction with water faced with 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) and similar unsubstituted benzoquinones in the selection of positive electrolyte materials. DHDMBS can be synthesized relatively inexpensively. We have demonstrated the chemical stability of DHDMBS to repeated electrochemical cycling through NMR and electrochemical studies proving the absence of products of the Michael reaction. A flow cell with DHDMBS and anthraquinone-2,7-disulfonic acid has now been shown to operate close to 100% coulombic efficiency for over 25 cycles when continuously cycled at 100 mA/cm 2 , and can sustain current densities as high as 500 mA/cm 2 without noticeable chemical degradation. However, there was a slow decrease in the capacity of the flow cell attributable to the crossover of DHDMBS from the positive side of the cell. Thus, the present study has shown DHDMBS as a promising candidate for the positive side material for an all-organic aqueous redox flow battery in acidic media, and our future efforts will focus on understanding the crossover of DHDMBS and the effects of long-term cycling. As intermittent renewable energy sources based on solar photovoltaics and wind turbines are installed worldwide, electrical energy storage facilities will be of paramount importance to maintain the stability of the electricity grid and to meet the growing demand for clean energy. Rechargeable batteries have shown promise in meeting this large energy storage demand, because of their high efficiency and scalability.1-3 Redox flow batteries (RFBs) are especially attractive for stationary applications due to their scalability and their inherent ability to address the power and energy requirements independently.4,5 However, none of today's battery technologies can meet the demands of robustness, low-cost, and environmental-friendliness simultaneously. 6 Recently, flow batteries based on aqueous solutions of simple watersoluble organic redox couples, or ORBAT, have become the subject of great interest because of their prospect of offering such a gridscale energy storage solution. Thus, the investigation into simple organic molecules that can be readily synthesized or procured for use in ORBAT has grown significantly. 7-12Specifically, we and others have found that quinone-and anthraquinone-based molecules have the essential electrochemical characteristics for the design of ORBAT. [13][14][15][16][17][18][19] We first reported the properties of an "all-quinone" organic redox flow battery in 2014 in which the ...

The promising attributes of 3,6-dihydroxy-2,4-dimethylbenzenesulfonic acid (DHDMBS) as a positive side material for aqueous organic redox flow batteries have been reported previously by our group. In the present study, we focus on understanding and mitigating the crossover of DHDMBS from the positive side of the cell to the negative side and the possible degradation pathways that could lead to capacity fade. We also uncover a slow process of "protodesulfonation" of DHDMBS that results in capacity fade during long-term cycling under strongly acidic conditions. We demonstrate the benefit of low-permeability membranes, mixed electrolytes in a symmetric cell configuration, use of electrolyte solutions with reduced acidity, and operating protocols involving polarity switching that reduce the rate of capacity fade substantially. These insights were used towards demonstrating long-term cycling of a symmetric cell with a capacity fade rate <0.02% per hour. The understanding and methods presented here are aimed at advancing the development of Organic Redox Flow Batteries (ORBAT) as an inexpensive and sustainable solution for large-scale electrical energy storage.

A new redox flow battery system based on iron sulfate and anthraquinone disulfonic acid (AQDS) is shown here to have excellent electrical performance, capacity retention, and chemical durability. While these redox couples, iron(II)/iron(III) and AQDS are well known individually, their combination in a redox flow battery is shown here for the first time to provide unique benefits for large-scale energy storage. Based on iron sulfate, a waste product of the steel industry, the active materials cost for this battery is anticipated to be $66/kWh. Cycling studies of over 500 cycles in the symmetric cell configuration show a negligibly low capacity fade rate of 7.6 × 10−5% per cycle. This symmetric cell also shows a notably high average coulombic efficiency of 99.63%. Using a graphite felt electrode modified with multi-walled carbon nanotubes (MWCNTs), we could achieve a peak power density of 194 mW cm−2. The major voltage losses are ascribed to the ohmic resistance of the electrode and electrolyte. Despite the lower cell voltage of the system relative to the vanadium flow battery, the iron–AQDS flow battery system presents a good prospect for simultaneously meeting the demanding requirements of cost, durability and scalability for large-scale sustainable energy storage.

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The use of water-soluble organic redox couples is a new and attractive pathway to a sustainable electrical energy storage system with the potential to be inexpensive and environmentally-friendly.[1][2][3] Further, several modifications of this type of redox flow battery arrangement using inorganic materials in combination with organic redox couples have also become the subject of research. [4][5] We have demonstrated the repeated cycling of a redox flow cell based on water-soluble organic redox couples (ORBAT) at high voltage efficiency, coulombic efficiency and power density. Recently, we presented for the first time the synthesis, characterization and properties of 3,6-dihydroxy-2,4-dimethylbenzenesulfonic acid (DHDMBS) as a new positive side electrolyte material for aqueous organic redox flow batteries (ORBAT). [6] DHDMBS overcame the major issue of the Michael reaction with water faced with previously reported positive electrolyte materials such as 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) and other unsubstituted benzoquinones. The present study focuses on the crossover of DHDMBS from the positive side of the cell to the negative side. Specifically, we have explored various approaches to mitigate the effects of crossover. These approaches include low-permeability membranes, a new symmetric cell configuration using mixed electrolytes, and operating protocols that involve polarity switching. These approaches were found to reduce fade rates by 70% - 85%. We also uncover mechanistic pathways that lead to slow chemical modification that cause capacity to decrease under strongly acidic conditions during long-term cycling. The new understanding and methods presented here will contribute towards the development of ORBAT as an inexpensive and sustainable solution for large-scale electrical energy storage. Acknowledgement The Authors acknowledge the financial support for this research from ARPA-E Open-FOA program (DE-AR0000337), the University of Southern California, and the Loker Hydrocarbon Research [1] B. Yang, L. Hoober-Burkhardt, F. Wang, G. K. Surya Prakash, and S. R. Narayanan, “An Inexpensive Aqueous Flow Battery for Large-Scale Electrical Energy Storage Based on Water-Soluble Organic Redox Couples,” J. Electrochem. Soc., vol. 161, no. 9, pp. A1371–A1380, 2014. [2] B. Yang, L. Hoober-Burkhardt, S. Krishnamoorthy, A. Murali, G. K. S. Prakash, and S. R. Narayanan, “High-Performance Aqueous Organic Flow Battery with Quinone-Based Redox Couples at Both Electrodes,” J. Electrochem. Soc., vol. 163, no. 7, pp. A1442–A1449, 2016. [3] B. Huskinson, M. P. Marshak, C. Suh, S. Er, M. R. Gerhardt, C. J. Galvin, X. Chen, A. Aspuru-Guzik, R. G. Gordon, and M. J. Aziz, “A metal-free organic–inorganic aqueous flow battery,” Nature, vol. 505, no. 7482, pp. 195–198, 2014. [4] E. S. Beh, D. De Porcellinis, R. L. Gracia, K. T. Xia, R. G. Gordon, and M. J. Aziz, “A Neutral pH Aqueous Organic–Organometallic Redox Flow Battery with Extremely High Capacity Retention,” ACS Energy Lett., vol. 2, no. 3, pp. 639–644, 2017. [5] B. Hu, C. DeBruler, Z. Rhodes, and T. L. Liu, “Long-Cycling Aqueous Organic Redox Flow Battery (AORFB) toward Sustainable and Safe Energy Storage,” J. Am. Chem. Soc., vol. 139, no. 3, pp. 1207–1214, 2017. [6] L. Hoober-Burkhardt, S. Krishnamoorthy, B. Yang, A. Murali, A. Nirmalchandar, G. K. S. Prakash, and S. R. Narayanan, “A New Michael-Reaction-Resistant Benzoquinone for Aqueous Organic Redox Flow Batteries,” J. Electrochem. Soc., vol. 164, no. 4, pp. A600–A607, 2017. Figure 1

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