This paper can be cited as: 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 organicinorganic aqueous flow battery", Nature 505, 195-198 (2014).The formatted version of this manuscript can be found at the following link: http://www.nature.com/nature/journal/v505/n7482/full/nature12909.html A metal-free organic-inorganic aqueous flow batteryBrian Huskinson 1 *, Michael P. Marshak 1,2 *, Changwon Suh 2 , Süleyman Er 2,3 , Michael R. *These authors contributed equally to this work.As the fraction of electricity generation from intermittent renewable sources-such as solar or wind-grows, the ability to store large amounts of electrical energy is of increasing importance. Solid-electrode batteries maintain discharge at peak power for far too short a time to fully regulate wind or solar power output 1,2 . In contrast, flow batteries can independently scale the power (electrode area) and energy (arbitrarily large storage volume) components of the system by maintaining all of the electro-active species in fluid form 3-5 . Wide-scale utilization of flow batteries is, however, limited by the abundance and cost of these materials, particularly those using redox-active metals and precious metal electrocatalysts 6,7 . Here we describe a class of energy storage materials that exploits the favourable chemical and electrochemical properties of a family of molecules known as quinones. The example we demonstrate is a metal-free flow battery based on the redox chemistry of 9,10-anthraquinone-2,7-disulphonic acid (AQDS). AQDS undergoes extremely rapid and reversible two-electron two-proton reduction on a glassy carbon electrode in sulphuric acid. An aqueous flow battery with inexpensive carbon electrodes, combining the quinone/hydroquinone couple with the Br 2 /Br  redox couple, yields a peak galvanic power density exceeding 0.6 W cm Solutions of AQDS in sulphuric acid (negative side) and Br 2 in HBr (positive side) were pumped through a flow cell as shown schematically in Fig. 1a. The quinone-bromide flow battery (QBFB) was constructed using a Nafion 212 membrane sandwiched between Toray carbon paper electrodes (six stacked on each side) with no catalysts; it is similar to a cell described elsewhere (see figure 2 in ref. 7). We report the potential-current response (Fig. 1b) and the potential-power relationship ( measured with respect to the quinone side of the cell). As the SOC increased from 10% to 90%, the open-circuit potential increased linearly from 0.69 V to 0.92 V. In the galvanic direction, peak power densities were 0.246 W cm 2 and 0.600 W cm 2 at these same SOCs, respectively ( Fig. 1c). In order to avoid significant water splitting in the electrolytic direction, we used a cutoff voltage of 1.5 V, at which point the current densities observed at 10% and 90% SOCs were −2.25 A cm −2 and −0.95 A cm −2 , respectively, with corresponding power densities of −3.342 W cm −2 and −1.414 W cm −2 .In Fig. 2 we report the results of initial cy...

Abstract:Storage of photovoltaic and wind electricity in batteries could solve the mismatch problem between the intermittent supply of these renewable resources and variable demand. Flow batteries permit more economical long-duration discharge than solid-electrode batteries by using liquid electrolytes stored outside of the battery. We report an alkaline flow battery based on redox-active organic molecules that are composed entirely of earth-abundant elements and are non-toxic, non-flammable, and safe for use in residential and commercial environments. The battery operates efficiently with high power density near room temperature. These results demonstrate the stability and performance of redox-active organic molecules in alkaline flow batteries, potentially enabling cost-effective stationary storage of renewable energy. Main Text:The cost of photovoltaic (PV) and wind electricity has dropped so much that one of the largest barriers to getting the vast majority of our electricity from these renewable sources is their intermittency (1-3). Batteries provide a means to store electrical energy; however, traditional, enclosed batteries maintain discharge at peak power for far too short a duration to adequately regulate wind or solar power output (1, 2). In contrast, flow batteries can independently scale the power and energy components of the system by storing the electro-active species outside the battery container itself (3)(4)(5). In a flow battery, the power is generated in a device resembling a fuel cell, which contains electrodes separated by an ion-permeable membrane. Liquid solutions of redox-active species are pumped into the cell where they can be charged and discharged, before being returned to storage in an external storage tank. Scaling the amount of energy to be stored thus involves simply making larger tanks ( Fig 1A). Existing flow batteries are based on metal ions in acidic solution but there are challenges with corrosivity, hydrogen evolution, kinetics, material cost and abundance, and efficiency that thus far have prevented large-scale commercialization. The use of anthraquinones in an acidic aqueous flow battery can dramatically reduce battery costs (6, 7); however, the use of bromine in the other half of the system precludes deployment in residential communities due to toxicity concerns.We demonstrate that quinone-based flow batteries can be adapted to alkaline solutions, where hydroxylated anthraquinones are highly soluble and bromine can be replaced with the non-toxic ferricyanide ion (8, 9) -a food additive (10). Functionalization of 9,10-anthraquinone (AQ) with electron-donating groups such as OH has been shown to lower the reduction potential and expand the battery voltage (6). In alkaline solution, these OH groups are deprotonated to provide solubility and greater electron donation capability, which results in an increase in the open circuit voltage of 47% over the previously reported system. Because functionalization away from the ketone group provides molecules with the highest solubility...

We demonstrate a successful high-throughput screening approach for the discovery of inexpensive, redox-active quinone molecules for organic-based aqueous flow batteries.