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...

We report the performance of a hydrogen-chlorine electrochemical cell with a chlorine electrode employing a low precious metal content alloy oxide electrocatalyst for the chlorine electrode: (Ru 0.09 Co 0.91 ) 3 O 4 . The cell employs a commercial hydrogen fuel cell electrode and transports protons through a Nafion membrane in both galvanic and electrolytic mode. The peak galvanic power density exceeds 1 W cm −2 , which is twice previous literature values. The precious metal loading of the chlorine electrode is below 0.15 mg Ru cm −2 . Virtually no activation losses are observed, allowing the cell to run at nearly 0.4 W cm −2 at 90% voltage efficiency. We report the effects of fluid pressure, electrolyte acid concentration, and hydrogen-side humidification on overall cell performance and efficiency. A comparison of our results to the model of Rugolo et al. [Rugolo et al., J. Electrochem. Soc., 2012, 159, B133] points out directions for further performance enhancement. The performance reported here gives these devices promise for applications in carbon sequestration and grid-scale electrical energy storage. arXiv:1206.2883v1 [cond-mat.mtrl-sci]

We have demonstrated the performance of an aqueous redox flow battery composed of a negative electrode consisting of a redox couple between anthraquinone di-sulfonate and its corresponding hydroquinone, and a positive electrode consisting of a redox couple between hydrobromic acid and bromine. The peak power density is approximately 0.6 W/cm2. After 750 deep cycles, the average discharge capacity retention is 99.84% per cycle and the average current efficiency is 98.35%.

Flow batteries are of interest for low-cost grid-scale electrical energy storage in the face of rising electricity production from intermittent renewables like wind and solar. We report on investigations of redox couples based on the reversible protonation of small organic molecules called quinones. These molecules can be very inexpensive and may therefore offer a low cost per kWh of electrical energy storage. Furthermore they are known to rapidly undergo oxidation and reduction with high reversibility under some conditions, suggesting the possibility of high current density operation, which could lead to low cost per kW. We report cyclic voltammetry measurements for 1,4-parabenzoquinone in neutral pH aqueous solution using a three-electrode setup. We report full fuel cell measurements as well, utilizing p-benzoquinone in an acidic solution as a positive electrode material and a hydrogen negative electrode, where current densities in excess of 240 mA cm -2 have been achieved to date. These initial results indicate that the quinone/hydroquinone redox couple is a promising candidate for use in redox flow batteries.

We develop a model for a regenerative hydrogen-chlorine fuel cell (rHCFC) including four voltage loss mechanisms: hydrogen electrode activation, chlorine electrode activation, chlorine electrode mass transport, and ohmic loss through the membrane. The dependences of each of these losses as a function of two "operating parameters," acid concentration and temperature; and five "engineering parameters," exchange current densities at both electrodes, membrane thickness, acid diffusion layer thickness, and cell pressure, are explored. By examining this large parameter space, we predict the design target and ultimate limitations to the performance characteristics of this cell. We identify chlorine electrode activation as the dominant contribution to the loss for low current density, high-efficiency operation and membrane resistance as the dominant contribution to the loss at maximum galvanic power density. We conclude that, with further research, a more optimal cell could be developed that operates at greater than 90% voltage efficiency at current densities >1 A cm 2 in both electrolytic and galvanic modes.

At present, there is an ongoing search for approaches toward the storage of energy from intermittent renewable sources like wind and solar. Flow batteries have gained attention due to their potential viability for inexpensive storage of large amounts of energy. While the quinone/hydroquinone redox couple is a widely studied redox pair, its application in energy storage has not been widely explored. Because of its high reversibility, low toxicity, and low component costs, we propose the quinone/hydroquinone redox couple as a viable candidate for use in a grid-scale storage device. We have performed single-electrode tests on several quinone/hydroquinone redox couples, achieving current densities exceeding 500 mA/cm 2 , which is acceptable for use in energy applications. We fabricated a full cell using para-benzoquinone at the positive electrode against a commercial fuel cell hydrogen electrode separated by a Nafion membrane. We evaluated its performance in galvanic mode, where it reached current densities as high as 150 mA/cm 2 . The results from these studies indicate that the quinone/hydroquinone redox couple is a promising candidate for use in redox flow batteries.

We develop a model for a regenerative hydrogen-chlorine fuel cell including four voltage loss mechanisms: hydrogen electrode activation, chlorine electrode activation, chlorine electrode mass transport, and ohmic loss through the membrane. The dependencies of each of these losses as a function of two "operating parameters", acid concentration and temperature; and five ``engineering parameters", exchange current densities at both electrodes, membrane thickness, acid diffusion layer thickness, and cell pressure, are explored. By examining this large parameter space, we predict the design target and ultimate limitations to the performance characteristics of this cell. We identify chlorine electrode activation as the dominant contribution to the loss for low current density, high-efficiency operation and membrane resistance as the dominant contribution to the loss at maximum galvanic power density. We conclude that a "dream" cell should be attainable with further research that operates at greater than 90% voltage efficiency at current densities >1A/cm2.

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