We introduce a novel Organic Redox Flow Battery (ORBAT), for meeting the demanding requirements of cost, eco-friendliness, and durability for large-scale energy storage. ORBAT employs two different water-soluble organic redox couples on the positive and negative side of a flow battery. Redox couples such as quinones are particularly attractive for this application. No precious metal catalyst is needed because of the fast proton-coupled electron transfer processes. Furthermore, in acid media, the quinones exhibit good chemical stability. These properties render quinone-based redox couples very attractive for high-efficiency metal-free rechargeable batteries. We demonstrate the rechargeability of ORBAT with anthraquinone-2-sulfonic acid or anthraquinone-2,6-disulfonic acid on the negative side, and 1,2-dihydrobenzoquinone-3,5-disulfonic acid on the positive side. The ORBAT cell uses a membrane-electrode assembly configuration similar to that used in polymer electrolyte fuel cells. Such a battery can be charged and discharged multiple times at high faradaic efficiency without any noticeable degradation of performance. We show that solubility and mass transport properties of the reactants and products are paramount to achieving high current densities and high efficiency. The ORBAT configuration presents a unique opportunity for developing an inexpensive and sustainable metal-free rechargeable battery for large-scale electrical energy storage. The integration of electrical energy generated from solar and wind power into the grid is faced with the challenge of intermittent electricity output from these sources. This challenge can be effectively met by storing the electricity during times of excess production and releasing the electrical energy to the grid during times of peak demand. Rechargeable batteries are very attractive for energy storage because of their high energy efficiency and scalability.1-3 Since grid-scale electrical energy storage requires hundreds of gigawatt-hours to be stored, 4 the batteries for this application must be inexpensive, robust, safe and sustainable. None of today's mature battery technologies meet all of these requirements. The vanadium redox flow battery is one such battery technology that has reached an advanced level of development for grid-scale applications.5 However, the limited resources of vanadium, the high expense associated with the cell materials, and the toxicity hazard of using large quantities of soluble vanadium, have been the major challenges to the widespread adoption of the vanadium redox flow battery. 2,6,7 Aiming to overcome these disadvantages, we have demonstrated for the first time an aqueous redox flow battery that uses water-soluble organic redox couples at both electrodes that are metal-free. Such a battery has the potential to meet the demanding cost, durability, eco-friendliness, and sustainability requirements for grid-scale electrical energy storage. We have termed this battery an Organic Redox Flow Battery (ORBAT).In ORBAT, two different aqueous solutions...

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 ...

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.

Covalent and non-covalent strategies are utilized for the functionalization of carbon nanotubes with positively charged groups and subsequently negatively charged, inorganic catalysts, leading nanomaterials for electrocatalytic water splitting. Both multi- and single-walled carbon nanotubes react with under microwave irradiation and solvent-free conditions, whereas the formation of non-covalent nanoconjugates is accomplished by reaction of carbon nanotubes with trimethylammonium acetyl pyrene