RevisedManuscript 15July2015 Title: Alkaline Quinone Flow Battery Authors: Kaixiang Lin1, Qing Chen2, Michael R. Gerhardt2, Liuchuan Tong1,Sang Bok Kim1,Louise Eisenach3, Alvaro W. Valle3, David Hardee1, Roy G. Gordon1,2*, Michael J. Aziz2*, & Michael P. Marshak1,2* Affiliations: 1 Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, USA. 2 Harvard John A. Paulson School of Engineering and Applied Sciences, 29 Oxford Street, Cambridge, Massachusetts 02138, USA. 3 Harvard College, Cambridge, Massachusetts 02138, USA. *Correspondence and requests for materials should be addressed to MJA ([email protected]), RGG ([email protected]), or MPM ([email protected]). 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 analkaline flow battery based on redox-active organic molecules that are composed entirely of earth-abundant elements and are non-toxic, non-flammable, and safefor use in residential and commercial environments. The batteryoperates 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 storingthe electro-active species outside the battery container itself (3–5). 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 pumpedinto the cell wherethey can be charged and discharged, before being returnedto storage in an external storage tank. Scaling the amount of energy to be stored thus involves simply making larger tanks (Fig 1A).Existingflow batteries arebased on metal ions in acidicsolutionbut there are challenges with corrosivity, hydrogen evolution, kinetics, materials cost and abundance, and efficiencythat thus far have prevented large-scale commercialization.The use of anthraquinones in an acidic aqueous flow battery can dramatically reducebattery costs(6,7); however, the use of bromine in theother half of the system precludes deployment in residential communities due to toxicity concerns. Wedemonstrate that quinone-based flow batteries can be adapted to alkaline solutions, where hydroxylated anthraquinones are highly soluble andbromine 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 tolower the reduction potential and expand the battery voltage (6). In alkaline solution, these OH groups are deprotonated toprovide 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(11,12), we initially targeted commercially available 2,6-dihydroxyanthraquinone (2,6-DHAQ), which we find exhibits a room temperature solubility >0.6 M in 1 M KOH. This system can achieve power densities in excess of 0.45 W cm(cid:237)2 at room temperature and 0.7 W cm(cid:237)2 at 45 °C. The use of alkaline electrolyte exploits pH as a parameter to shift the thermodynamic potentials of proton-dependent reactions to more negative values. In acid solutions, AQ undergoes a two-electron two-proton reduction at a single potential, which shifts to more negative values as the pH increases (6). When the pH exceeds 12 the reduction potential of 2,6- DHAQ becomes pH-independent because the reduced species is generated in its fully- deprotonated form (Fig. S1). In contrast with the pH-dependent electrochemical behavior of quinones (negative terminal), the ferro/ferricyanide redox couple (positive terminal) has a pH- independent redox potential. This contrasting pH-dependence can be exploited through the development oflow reduction potential quinones at high pH. The cyclic voltammograms (CVs) of 2,6-DHAQ and ferro/ferricyanide predict an equilibrium cell potential of 1.2 V upon combination of these two half-reactions (Fig. 1B). A quantitative analysis of the CV of 2,6- DHAQ at pH 14 (Fig. S2) revealed redox behavior consistent with two one-electron reductions at potentials separated by only 0.06 V with rapid kinetic rate similar toquinones in acid (6). This behavior raises interesting questions about the relationship between quinone redox and hydrogen bonding(13). Cell testing was performed at 20 °C using solutions of 0.5 M 2,6-DHAQdipotassium salt and 0.4 M K Fe(CN) both dissolvedin 1 M KOH.These solutions were pumped through a flow cell 4 6 constructed from graphite flow plates and carbon paper electrodes, which were separated by a Nafion membrane. A charging current of 0.1 Acm(cid:237)(cid:21) was applied to charge the cell, and polarization curves were measured at 10%, 50%, and 100% states of quinone charge (SOC). The open-circuit voltage (OCV)is 1.2Vat 50% SOC; its dependence onSOC is shown in Fig. 2A. The polarization curves (Fig. 2B) showno sign of redox kinetic limitations and exhibit apeak galvanic power densityexceeding 0.4W cm(cid:237)(cid:21). The cell was cycled at a constant current density of ±0.1 A cm(cid:237)(cid:21) for 100 cycles (Fig. 3A). The current efficiency exceeded99%, with a stable round-trip energy efficiency of 84%. A 0.1% loss in capacity per cycle was observed during cycling, which appears to be a continuous loss of electrolyteover the 100 cycles. Three possible loss mechanisms were explored: chemical decomposition, electrolyte crossover through the membrane, and leakage from the pumping system. Chemical and electrochemical stability studies showed that the negative electrolyte is stable. 10 mM of 2,6-DHAQ was heated in 5 M KOH solution at 100°C for 30 days and was characterized by proton NMR. Cyclednegative electrolyte was also collected and characterized by the samemethod; both studies showed no degradation product at the sensitivitylevel of 1% (Fig S3).Membrane crossover contamination has beena common challenge in acid-based redox flow batteries where most electro-active molecules are either neutral or positive and tend to migrate through proton-conductive membranes (5). In this alkaline system, however, all the electro-active molecules remain negatively charged in all charge states, leading to a dramatic decrease inthe degree of crossover during cell cycling. Cyclic voltammetry of the ferro/ferricyanide electrolyte collected at the end of cycling showedno evidence of the presence of 2,6-DHAQ. This observation places an upper limit on crossoverof0.8% of the DHAQ, implyingacrossover current densityless than2.5 μAcm(cid:237)(cid:21) (Fig. S4). Finally, hydraulic leakage was investigated because an apparent but unquantifiable small decrease in fluid levels was observedin the reservoirs. After cell cycling, the cell was washedwith DI water until no coloration of eluent could be observed. The cell was then dissembled; coloration was found on the gaskets indicatingthe likely siteofelectrolyte leakage (Fig. S5).This source of capacity loss—the equivalent to roughly 8 drops in our system—is expected to become negligible as system size is scaled up. By increasing the temperature to 45°C, the peak galvanic power density increases from 0.45W cm(cid:237)(cid:21)to approximately0.7W cm(cid:237)(cid:21) (Fig. 2C),as the cell area-specific resistance (ASR) decreases from about 0.878 to 0.560(cid:525)(cid:3)(cid:70)(cid:80)2, estimated from the linear parts of the polarization curves in Fig. 2. The majority of this ASR decrease comes from a change in the high frequency area-specific resistance (r )measured by electrochemical impedance spectroscopy (Fig. S6). In hf both cases, the r contributes more than70% of the ASR and is indeed the limiting factor to the hf cell current and power outputs. Ther is dominated by the resistance of the membrane, which is hf an order of magnitude higherthan the resistanceof the same membrane in a pH 0 acid solution (14). The sluggish kinetics of the hydrogen evolution reaction in alkaline solutionon carbon electrodes results in a largerpractical stability window in base rather than inacid (Fig. S7). Consequently, quinones with substantially more negative reduction potentials are feasible as negative electrolyte materials. Preliminary investigations into the synthesis of different hydroxy- substituted anthraquinones suggest further increases in cell potential are possible.Self- condensation reactions of substituted benzene yield 2,3,6,7-tetrahydroxy-AQ (THAQ)(15) and 1,5-dimethyl-2,6-DHAQ (15-DMAQ)(Fig. S8 and S9). The CV’s of these species in 1 M KOH suggest cell potentials vs. ferri/ferrocyanide approaching 1.35 V (Fig. 4A and B), which exceeds that of many aqueous rechargeable batteries (Fig. 4C). The cyanide ions in both ferro-and ferricyanide arebound too tightly to be released under most conditions. Consequently, it is non-toxic in both oxidized and reduced forms, and is even permitted for use as a food additive(10). The use of ferrocyanide offers significant advantages over bromine because it is non-volatile and non-corrosive, allowing simpler and less expensive materials of construction. In addition, these tri- and tetra-anionic organometallic molecules exhibit low crossover rates through cation-exchange membranes. The results reported herein highlight the ability of hydroxy-substituted anthraquinone and ferrocyanide to function as stable flow battery electrolytes in alkaline solution. The use of organic and organometallic coordination complexes in base, rather than aqueous metal ions in acid, resolves serious cost, corrosion, and safety concerns of previous flow battery chemistries. Alkaline flow batteries can compensate for higher membrane resistance with higher voltage, leading to performance similar totheir acidic counterparts. In addition, quinone-ferrocyanide alkaline chemistry avoids the membrane crossover, corrosivity, toxicity and regulations associated with bromine. 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Schroeder, Method for producing 3-hydroxy-2- methylbenzoic acid (2004), WO2003080542A3. Acknowledgments: This work was funded by the U.S. DOE ARPA-E award # DE-AR0000348. Methods, along with any additional Extended Data display items and Source Data, are available in the online version of the paper; references unique to these sections appear only in the online paper. M.P.M., K.L., R.G.G. and M.J.A. formulated the project. K.L., S.B.K., and D.H. synthesized, analyzed and purified the compounds. K.L., L.T. and S.B.K. collected and analysed the NMR and MS data. K.L. A.V. L.E. measured solubility. K.L., Q.C. L.E. and M.R.G. collected and analysed the electrochemical data. K.L., Q.C., M.R.G., M.P.M., R.G.G. and M.J.A. wrote the paper, and all authors contributed to revising the paper. Supplementary Materials: Materials and Methods Supplementary Text Fig. S1 to S9 References (17-22) Fig.1 Cyclic Voltammetry of Electrolyte and Cell Schematic. (A)Cyclic voltammogram of 2 mM 2,6-DHAQ (dark cyan curve) and ferrocyanide (gold curve) scanned at 100 mV/s on glassy carbon electrode; arrows indicate scan direction. Dotted line represents cyclic voltammogram of 1 M KOH background scanned at 100 mV/s on graphite foil electrode. (B) Schematic of cell in charge mode. Cartoon on top of the cell represents sources of electrical energy from wind and solar. Curved arrows indicate direction of electron flow and white arrows indicate electrolyte solution flow. Blue arrow indicates migration of cations across the membrane. Essential components of electrochemical cells are labeled with color-coded lines and text. The molecular structure of oxidized and reduced species are shown on corresponding reservoirs. Fig. 2Cell Performance. (A) Cell OCP vs. SOC. All potentials were taken when cell voltage stabilized to within ± 1mV. 100% SOC was reached by potentiostatic holding at 1.5 V until the current decreases to below 20mA/cm2. (B) and (C), Cell voltage & power density vs. current density at 20 oC and 45 oC, respectively, at 10%, 50%, and ~100% SOC. Electrolyte composition: At 20 °C, 0.5 M 2,6-DHAQ and 0.4 M ferrocyanide were used in negative electrolyte and positive electrolyte, respectively. At 45 °C, both concentrations were doubled. In both cases, potassium hydroxide content started at 1 M for both sides in the fully discharged state. Fig. 3Cell Cycling Performance. (A)Representative voltage vs. time curves during 100 charge-discharge cycles at 0.1 A/cm2, recorded between the 10th and 19th cycles. (B) Capacity retention, current efficiency and energy efficiency values of 100 cycles. Normalized capacity is evaluated based on the capacity of the first charge and discharge cycle. Fig.4 Molecular Structure and Cyclic Voltammetry of 2,6-DHAQ Derivatives.(A) and (B) Molecular structures and cyclic voltammogram of 2,3,6,7-THAQ (pink) and 1,5-DMAQ (olive), respectively, plotted along with ferrocyanide (orange curve) scanned at 100 mV/s on glassy carbon electrode. Both 2,6-DHAQ derivatives/ferrocyanide couples showed higher equilibrium potential than 2,6-DHAQ/ferrocyanide. (C), Selected aqueous secondary batteries showing voltage and flow status. Literature data from (3) and (16). A Negative Electrolyte - - Positive Electrolyte Reservoir 2e 2e Reservoir NN O NN CC NN O CC CC FFee33++ O NN CC CC CC NN O Cations NN N O Pump Pump N C C C N O Fe2+ C C N C N O O N Ion-selective Membrane Porous Carbon Electrodes Current Collector & Flow Field B E = 1.20 V -2m) O O c A O Fe(CN)3- m 2,6-DOHAQ 6 y ( sit n e D Fe(CN)4- nt 6 re O r O u C O O 2,6-reDHAQ Potential (V vs. SHE)