Unlocking Simultaneously the Temperature and Electrochemical

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Unlocking Simultaneously the Temperature and Electrochemical Windows of Aqueous Phthalocyanine Electrolytes Zhifeng Huang, Peng Zhang, Xinpei Gao, Dirk Henkensmeier, Stefano Passerini, and Ruiyong Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00467 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 15, 2019

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ACS Applied Energy Materials

Unlocking Simultaneously the Temperature and Electrochemical Windows of Aqueous Phthalocyanine Electrolytes Zhifeng Huang, † Peng Zhang,‡,§ Xinpei Gao,#,‖ Dirk Henkensmeier,⊥,Δ,+ Stefano Passerini,#,‖ Ruiyong Chen*,† †

Transfercenter Sustainable Electrochemistry, Saarland University and KIST Europe, 66125 Saarbrücken, Germany ‡

§

INM-Leibniz Institute for New Materials, 66123 Saarbrücken, Germany

School of Materials Science and Engineering, PCFM Lab, Sun Yat-sen University, Guangzhou 510275, China #

‖ ⊥

Helmholtz Institute Ulm, Helmholtzstrasse 11, 89081 Ulm, Germany

Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany

Fuel Cell Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea Δ

Division of Energy and Environment Technology, University of Science and Technology, Seoul 02792, Republic of Korea +

Green School, Korea University, Seoul 02841, Republic of Korea

AUTHOR INFORMATION Z. Huang and P. Zhang contributed equally. Corresponding Author * E-mail: [email protected], Tel.: +49 681 302 58350

ACS Paragon Plus Environment

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Abstract The change from organic solvents to aqueous solvents for safe and robust battery electrolytes is desirable for electrochemical energy storage. Thermodynamically, water has an electrochemical stability window of 1.23 V and pure water freezes at 0 °C. Such properties clearly restrict the highvoltage applications and temperature adaptability of aqueous electrolytes. Herein, we report an aqueous supporting electrolyte containing imidazolium chloride, showing unprecedented large temperature and electrochemical windows. Thermal analysis over -80 to 80 °C shows such an aqueous electrolyte to be free of transition events of icing and phase changes. X-ray scattering results of these aqueous solutions in the presence of active materials reveals the pivotal role of imidazolium chloride to preserve the liquid phase at rather low temperatures. Metal phthalocyanines with electroactive organic ligand rings and multi-electron transfer reactions at low negative potentials (-0.2 to -1.6 V vs. Ag) are demonstrated in water-based anolytes for redox flow batteries for the first time over a broad temperature range.

Keywords: redox flow batteries, supporting electrolyte, imidazolium chloride, temperature stability, electrochemical window TOC


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1. INTRODUCTION State-of-the-art battery technologies are challenged by the ever-increasing energy storage demands in terms of cost and performance.[1-5] Compared to the flammable organic solvent-based electrolytes for rechargeable Li-ion batteries with high operating voltage (typically above 3 V),[6] aqueous electrolytes have been widely used in lead-acid and redox flow batteries (RFBs) with intrinsic safety and fast ion transport rates.[7] However, applications of aqueous electrolytes are typically limited to low-voltage systems (< 2 V). Commonly, aqueous H2SO4, KOH and NaCl solutions are used as supporting electrolytes to dissolve redox-active species, and are the media for transport of charge-carrier ions and diffusion of reactants. The application of aqueous electrolytes for rechargeable batteries is restricted by the hydrogen and oxygen evolution reactions. To break such limits, efforts have been recently made to increase the operating voltage of aqueous electrolytes.[8-15] These reports are based on strategies to suppress water activity under electrolysis conditions by using concentrated supporting salts,[8,12,13] by adjusting the pH values of the electrolytes,[11] or by forming protective interphases on electrode surfaces.[8,16] Another restriction to use water in electrolytes is its high freezing point. At sub-zero temperatures, aqueous electrolytes typically exhibit undesirable bulk water behaviour such as icing.[17] The practical applications of rechargeable batteries in low temperature environments will often recall the organic solvents with typically low freezing points.[5,18,19] In nature, supercooled liquid water under atmospheric pressure can exist in the absence of ice nuclei or in small droplet size.[20] The use of co-solvents and anti-freezing agents will, however, reduce the solubility for supporting salts.[21,22] In the presence of foreign solutes, the freezing-point can be depressed, termed as the colligative properties of dilute solutions. It has been observed that the freezing points can be largely depressed for the common acidic, basic and neutral aqueous electrolytes with high


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concentrations. For a NaCl/H2O mixture at the point of saturation, a freezing point of -21.2 °C has been determined.[23] An aqueous electrolyte containing 9 M (mol L-1solution) NaClO4 enables a supercapacitor operating at -30 °C.[24] Nevertheless, addition of surplus supporting salts often leads to a general decrease in the dissolving capability of water for the redox-active solutes.[25-28] Aqueous alkali electrolytes (6 to 10 M KOH) can be operated at -25 °C or even at -40 °C.[29-31] Although 2 M H2SO4 has a freezing point of about -13 °C,[32] it can hardly inhibit the instability of vanadium species below 10 °C in vanadium RFBs.[1] Despite the individual demonstrations of aqueous electrolytes either with high voltages or operation under low temperature conditions as aforementioned, a simultaneous realization of both functions has not been achieved yet. So far, most studies concern the development of novel redox couples,[33] rather than the optimization of the supporting electrolytes. The chemical features, solubility, coordination states and electrochemical reversibility of electroactive materials, however, can be largely affected by the supporting ions.[28,34,35] Herein, a supporting electrolyte with concurrently widened temperature and electrochemical stability is demonstrated by using a mixture of water and 1-butyl-3-methylimidazolium chloride (BMImCl). First of all, the temperature stability window can be largely extended to cover a range from -80 °C to a temperature approaching the boiling point of water. The critical role of BMImCl on the inhibition of water icing and the phase separation of the electroactive species at low temperatures has been revealed by means of X-ray scattering. Secondly, the simultaneous broadening of the electrochemical stability window of the supporting electrolyte enables the study of metal complexes with multi-electron transfer reactions at low negative potentials in aqueous systems over a broad temperature range. Until now, only organic solvents have been used to study their


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reduction reactions.[18,36-39] We show that these merits permit operating a phthalocyanine/Fe RFB under conditions that were never accessible previously in conventional aqueous electrolytes.

2. EXPERIMENTAL SECTION Nickel (II) phthalocyanine tetrasulfonic acid tetrasodium salt (NiTsPc, ≥98.0%), copper (II) phthalocyanine tetrasulfonic acid tetrasodium salt (CuTsPc, ≥98.0%) and BMImCl (≥98.0%) were purchased from Sigma-Aldrich. FeCl2∙4H2O (≥99%) was supplied from Fluka. All chemicals were used as received without further purification. Small-angle X-ray scattering (SAXS) experiments were performed with a Xenocs XEUSS 2.0 set-up, equipped with a Cu Kα source (wavelength, λ = 1.54 Å) and a Pilatus 1M detector (Dectris, Switzerland). The samples were sealed in the thin-wall (wall thickness 0.01 mm), quartz capillaries (Hilgenberg GmbH, Malsfeld, Germany) with epoxy resin. All samples were measured in the vacuum chamber attached to the X-ray set-up. A heating stage with the XEUSS 2.0 set-up was used to characterize structural changes as a function of temperature in the range of -20 to 20 °C (with a stability of ±1°C) at an interval of 2 °C per step. The sample to detector distance was calibrated with silver behenate. The radial integration of the primary data was undertaken with the Foxtrot software (version 3.2.7, SOLEIL, France). Conductivities were determined by an automated conductimeter equipped with a frequency analyzer and a thermostatic bath (MMates Italia). The samples were housed in sealed glass conductivity cells (mounted in an argon-filled glove box) equipped with two platinized platinum electrodes. The cell constants were determined using a 0.01 M KCl standard solution. The measurements were run in the temperature range from -40 to 70 °C, the equilibration time at each temperature is 1 h.


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Differential scanning calorimetry (DSC) was carried out by using a TA Instruments Q2000 with liquid N2 cooling. The samples were hermetically sealed in Al pans. The thermal treatment included cycling from -80 to 80 °C at a rate of 5 °C min-1 with a sub-ambient annealing procedure to promote any possible crystallization. Cyclovoltammetric (CV) measurements were performed between -32 and 65 °C using a BioLogic SP150 potentiostat/galvanostat via EC-Lab software. A typical three-electrode system was conducted using a glassy carbon working electrode (1 mm in diameter), a Pt foil counter electrode, and a Ag wire quasi-reference electrode. Prior to each measurement, the solutions were bubbled with Ar gas for 10 min to eliminate the dissolved oxygen. For flow cell tests, a home-made cell with an active surface area of 4 cm-2, acid and thermally activated graphite felts (GFD4.6 EA, SGL), and a cross-linked methylated polybenzimidazole (PBI) anion exchange membrane (the weight ratio of cross-linker to PBI is 10%, 45 μm in thickness) were used, as described elsewhere.[40,41] Different concentrations (from 7.5 mM to 0.2 M) of NiTsPc or CuTsPc in neat water and in 10 m BMImCl/H2O supporting electrolyte were used as anolytes. Accordingly, FeCl2 solutions with a four-fold concentration (from 30 mM to 0.8 M) were used as catholytes. The electrolytes (8 mL on each side) were circulated into the cell by using a peristaltic pump (Easy pump, China) with flow rates of 30 mL min-1. The flow cell performance was evaluated at variable temperatures (from -32 to 65°C) between 0.5–1.4 V or 0.5–1.85 V using a BioLogic SP150 potentiostat. Prior to the galvanostatic cycling, the electrolytes were bubbled with Ar gas to eliminate the dissolved oxygen and protected afterwards using paraffin oil (Roth, Germany). The theoretical energy density of the full cell is calculated based on a two-electron reaction of the metal phthalocyanines, and single volume of positive or negative electrolyte. To determine the internal cell resistance, electrochemical impedance spectroscopy (EIS) was


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performed by applying a sine voltage waveform with an amplitude of 10 mV at the open circuit voltage after assembling the cell. Low temperature experiments were carried out in a freezer. High temperature experiments were performed in a temperature chamber.

Figure 1. (a) Chemical structure of Na4[MeTsPc]. (b) Photos show 1 mL H2O and a solution of 10 m BMImCl/H2O in glass vials. (c) Typical 2D [2θ, Ѱ] polar patterns of aqueous NiTsPc and NiTsPc/BMImCl samples collected at 20 and -20 °C; the grey area indicates positions of the beam stop; Pilatus detector modular gaps and flight tube shadows were masked off for data integration. (d,e) Evolution of the SAXS patterns during the in situ cooling/heating experiments between 20 and -20°C for aqueous NiTsPc sample, and aqueous NiTsPc/BMImCl sample, respectively. Concentration of NiTsPc is 0.05 M.


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3. RESULTS AND DISCUSSION Water-soluble MeTsPc (Me = Ni, Cu, Figure 1a) with electroactive organic ligand rings[42] were studied as redox-active materials. SAXS, as a sensitive technique to characterize the bulk crystallization of water, phase separation and formation of agglomerate structures in a solution,[43] was recorded in a cooling/heating loop between 20 and -20 °C for the NiTsPc solutions (0.05 M) in neat water and in a 10 molality (m, mol kg-1water) BMImCl/H2O (Figure 1b). The 2D scattering images are shown in Figure 1c in [2θ, Ѱ] polar coordinates. At 20 °C, no scattering ring was observed for both samples. However, at -20 °C a new scattering ring can be seen for the neat aqueous NiTsPc sample (top-right in Figure 1c), indicating the presence of crystallized species at such a low temperature. In contrast, there is no scattering ring for the sample containing BMImCl, implying that the NiTsPc material is well dispersed in water and the absence of bulk water behavior at -20 °C. To trace the structural changes with temperature, in situ SAXS experiments of the two samples were carried out by first cooling the samples from 20 to -20 °C (left panels in Figure 1d,e), and then heating to 20 °C (right panels in Figure 1d,e). The patterns were recorded at an interval of 2 °C per step. In Figure 1d, upon cooling a peak at q = 0.32 Å-1, corresponding to a real space of 19.6 Å calculated by 2π/q, appeared at temperature below -12 °C. This peak can be attributed to the (100) reflection of the NiTsPc crystals.[44] Meanwhile, peaks in the high q values of 1.6 and 1.72 Å-1 were observed below -12 °C in Figure 1d, which can be attributed to (100) and (002) reflections of ice.[45] During the reverse heating process, these crystallization peaks disappeared at above 2 °C. In a strong contrast, the aqueous sample containing BMImCl showed a distinct X-ray scattering profile without any crystalline feature corresponding to ice and NiTsPc crystals over the whole temperature loop (Figure 1e). The scattering features are characterized by weak and broad


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humps centred at around q = 0.85, 1.65 Å-1, corresponding to the charge alternation of the ionic domains and adjacency order, respectively.[46] These results suggest that the BMImCl can stabilize water molecules at low temperatures against their crystallization, and can impose a strong interaction on the metal phthalocyanine solute.

Figure 2. (a) DSC thermograms of the 10 m BMImCl/H2O. (b) Ionic conductivity of the 10 m BMImCl/H2O solution and in the presence of active materials. (c) CV curves of the 10 m BMImCl/H2O, and in the presence of 10 mM NiTsPc, CuTsPc and FeCl2, measured at room temperature and 100 mV s-1. (d) Electrochemical stability window of the 10 m BMImCl/H2O tested from -32 to 65 °C at 20 mV s-1. Inset in (d) shows the electrochemical stability window as a function of temperature. (e) Temperature-dependent CV curves of the 10 mM active CuTsPc and FeCl2 in the 10 m BMImCl/H2O measured at 100 mV s-1. Inset in (e) shows ΔE as a function of temperature. By using DSC (Figure 2a), the temperature stability window of the 10 m BMImCl/H2O supporting electrolyte was further studied. Remarkably, no resolvable endothermal and exothermal peaks can be seen from -80 to 80 °C. Such a broad temperature stability window of aqueous electrolytes has not been reported before. This is superior to that from a recent observation of aqueous solutions,[47] and also superior to those of many organic solvents.[6] In addition, high temperature operation of aqueous electrolytes is potentially safer than that of electrolytes based on


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organic solvents with low boiling points and high vapour pressures. Such broad temperature adaptability is favourable for practical applications with low risk of operation failure under extreme temperature conditions. The ionic conductivity of the BMImCl/H2O mixture decreases with decreasing temperature from 114.5 mS cm-1 at 70 °C to 40 mS cm-1 at room temperature and then to 0.7 mS cm-1 at -40 °C (Figure 2b). These values are much higher than those of carbonatebased electrolytes (about 10 mS cm-1 at room temperature, and 0.01 mS cm-1 at -40 °C).[6] After adding the active species into the 10 m BMImCl/H2O mixture, there was a slight decrease in the ionic conductivity (Figure 2b). At -40°C, ionic conductivities of about 0.5 and 0.34 mS cm-1 can be still maintained for the 0.2 M CuTsPc and the 0.8 M FeCl2, respectively. Figure 2c shows the CV curves recorded at room temperature for the 10 m BMImCl/H2O, also in the presence of the active species NiTsPc, CuTsPc and FeCl2. The electrochemical activity of the phthalocyanines is shown by a series of multiple reduction/oxidation pairs from 0 to -1.8 V vs. Ag (Figure 2c, Figure S1), corresponding to the four-electron transfer reactions. As can be seen, these reactions occur within the electrochemical stability window of the supporting electrolyte. However, such electrochemical redox peaks cannot be observed in common aqueous solutions of H2SO4, KOH and NaCl (Figure S2). With decreasing temperature, the electrochemical stability window of the 10 m BMImCl/H2O increased up to 3.2 V at -32 °C (inset in Figure 2d). Especially, the low voltage boundary is more sensitive to the temperature decrease with a shift of about 0.3 V towards lower potentials, arising from the improved kinetic inhibition of hydrogen evolution reaction or the reduction of the BMIm+ cations. Interestingly, the supporting electrolyte can still ensure the redox reactions of the Fe-based and CuTsPc couples at various temperatures (Figure 2e). Remarkably, the peak separations (ΔE) for the Fe-based redox couple and for the marked redox reactions of O1/R1 and O2/R2 for the CuTsPc are less sensitive to the temperature at a fast


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potential sweep rate of 100 mV s-1 (inset in Figure 2e), implying little dependence of intrinsic reaction kinetics on temperature. As shown in Figure 2c,2e, the first two-electron transfer reactions for NiTsPc and CuTsPc at around -0.45/-0.39 V and -0.77/-0.67 V are highly reversible, corresponding to the MeIITsPc4- ⇄ MeIITsPc5- ⇄ MeIITsPc6-. Nevertheless, further reduction of these compounds to -1.2 and -1.48 V showed poor reversibility, independent of the temperature.

Figure 3. Electrochemical performance of flow cells using 10 m BMImCl/H2O supporting electrolyte. (a) Voltage profiles of a CuTsPc (7.5 mM)/FeCl2 (0.03M) flow cell measured at room temperature and 2.5 mA cm-2 with different cutoff charge voltages, and (b) the corresponding cycling stability over 100 cycles tested between 0.5 and 1.4 V. (c) Cycling performance of a NiTsPc (50 mM)/FeCl2 (0.2 M) flow cell tested between 0.5–1.4 V at 5 mA cm-2; after 40 cycles, the catholyte was replaced with fresh electrolyte containing 0.2 M HCl. (d) Cycling performance of a CuTsPc (0.2 M)/FeCl2 (0.8 M) flow cell at room temperature, Inset shows the voltage profile at 6.25 mA cm-2. Such phthalocyanines enabling multiple electron transfer reactions are promising to be used as redox-active materials for RFBs. Taking the MeTsPc in 10 m BMImCl/H2O as anolyte, flow cell tests were performed by using FeCl2 (with a 200% excess based on the two-electron transfer


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reaction of the MeTsPc) in 10 m BMImCl/H2O as catholyte, and a cross-linked methylated PBI based Cl--exchange membrane.[40,41] With a cutoff charge voltage of 1.85 V, four charge/discharge plateaus were observed (Figure 3a), in agreement with the CV data. However, fast capacity fade and low Coulombic efficiency (CE, about 85%) were observed (Figure S3). By setting the cutoff to 1.4 V, thus utilizing the two-electron reaction of the anolyte, the flow cell showed stable capacity retention over 100 cycles (with a fading rate of 0.09% per cycle) at room temperature and 2.5 mA cm-2. We found that the low operating current density is limited by the ion conductivity performance of the membrane at neutral pH value.[41] The Coulombic, voltage (VE) and energy efficiencies (EE) were about 96%, 88% and 84%, respectively (Figure 3b). The dependence of the CE on the cutoff voltages indicates that a poor electrochemical reversibility at deep reduction of the MeTsPc species. In addition, it was observed that the capacity decays over the first 40 cycles (0.27% per cycle) with a development of turbidity in the catholyte arising from parasitic side reactions of the iron species,[48] which can be however recovered upon replacing the catholyte with a fresh solution containing 0.2 M HCl (Figure 3c). Nevertheless, we have found that the iron species can be stabilized more effectively in the 10 m BMImCl/H2O (Figure S4) than in neat water. For the flow cell with a relative high concentration of 0.2 M CuTsPc, an initial discharge capacity of about 7.2 Ah L-1 (i.e., 67% of the theoretical capacity) and a high CE of about 96% have been observed (Figure 3d). Such an easily accessible high-capacity of metal complexes in aqueous electrolytes is superior to that in typical non-aqueous systems.[49]


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Figure 4. (a) Normalized temperature-dependent voltage profiles. Cycling performance of a NiTsPc (0.05 M)/FeCl2 (0.2 M) flow cell: (b) at -20 and -32 °C, 2.5 mA cm-2, and (c) at 65°C and increased current densities. After 10 cycles, 0.2 M HCl was added into the catholyte in (c).

Figure 4a shows the normalized voltage profiles of the MeTsPc anolytes against the iron cathodic couple tested from -32 to 65 °C. As the temperature decreased, the two-step slopes in the


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charge/discharge curves are less resolved with an increase in polarization, which may arise from the reduced ion conductivity of the membrane. The internal cell resistance, determined from electrochemical impedance spectroscopy, increased from about 2.5 Ω at room temperature to 8.1 Ω at -20 °C. This is similar to a previous observation using saturated aqueous LiCl,[47] but less severe compared to organic solvent-based electrolytes.[50] At -20 and -32 °C, the flow cell showed steady CE of about 95% (Figure 4b). Meanwhile, VE of about 73.0% and 72.1% can be still observed at -20 and -32 °C, respectively, compared to that of about 92% at room temperature. In addition, the electrolyte utilization at -20 and -32 °C remains about 59% and 51%, respectively, in comparison to about 82% at room temperature. Such a temperature dependence of capacity retention is much better than that for non-aqueous electrolytes-based RFBs.[18] At -32°C, a relatively fast capacity fade of about 0.8% per cycle was observed (Figure 4b). Note that when operating at sub-zero temperatures, a change in the ion channels, swelling and mechanical properties of the employed membrane could occur, and eventually causes an increase in crosscontamination level. When tested at 65 °C, a high CE of 99.6% was observed (Figure 4c). Nevertheless, these steady operating efficiencies observed at both low and high temperatures verify the excellent temperature adaptability of the studied system.

4. CONCLUSIONS In summary, our proposed aqueous electrolyte can circumvent water crystallization at temperatures down to -80 °C and hydrogen evolution under rather low negative potentials. Meanwhile, the occurrence of a phase separation of the electroactive material is suppressed in the BMImCl/water supporting electrolytes. Furthermore, it was found that the electrochemical stability potential can be extended when decreasing temperature and reached -1.8 V vs. Ag at -


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32 °C. By choosing inexpensive metal phthalocyanines as electroactive materials, the redox reactions at low negative potentials are surprisingly accessible in our water-based electrolyte. The unprecedent temperature stability of the electrolytes was successfully demonstrated between -32 and 65 °C in flow cells. Other complexes with better electrochemical reversibility of four-electron transfer reactions should be surveyed in the future. ASSOCIATED CONTENT Supporting Information. Figure S1-Figure S4. Further CV and flow cell cycling data, and visual observations of Fe2+ electrolytes. AUTHOR INFORMATION E-mail: [email protected], Tel.: +49 681 302 58350 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the KIST Europe basic research funding, the German-Korean joint SME R&D projects of ZIM-AIF and MOTIE/KIAT, and the support of the Helmholtz Association. Z.H. is grateful for a China CSC abroad studying fellowship. REFERENCES (1) Soloveichik, G. L. Flow batteries: Current Status and Trends. Chem. Rev. 2015, 115, 1153311558. (2) Darling, R. M.; Gallagher, K. G.; Kowalski, J. A.; Ha, S.; Brushett, F. R. Pathways to LowCost Electrochemical Energy Storage: a Comparison of Aqueous and Nonaqueous Flow Batteries. Energy Environ. Sci. 2014, 7, 3459-3477.


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(3) Huskinson, B.; Marshak, M. P.; Suh, C.; Er, S.; Gerhardt, M. R.; Galvin, C. J.; Chen, X.; Aspuru-Guzik, A.; Gordon, R. G.; Aziz, M. J. A Metal-Free Organic-Inorganic Aqueous Flow Battery. Nature 2014, 505, 195-198. (4) Wei, X.; Nie, Z.; Luo, Q.; Li, B.; Chen, B.; Simmons, K.; Sprenkle, V.; Wang, W. Nanoporous Polytetrafluoroethylene/Silica Composite Separator as a High-Performance All-Vanadium Redox Flow Battery Membrane. Adv. Energy Mater. 2013, 3, 1215-1220. (5) Chen, R.; Ren, S.; Knapp, M.; Wang, D.; Witter, R.; Fichtner, M.; Hahn, H. Disordered Lithium-Rich Oxyfluoride as a Stable Host for Enhanced Li+ Intercalation Storage. Adv. Energy Mater. 2015, 5, 1401814. (6) Gong, K.; Fang, Q.; Gu, S.; Li, S. F. Y.; Yan, Y. Nonaqueous Redox-Flow Batteries: Organic Solvents, Supporting Electrolytes, and Redox Pairs. Energy Environ. Sci. 2015, 8, 3515-3530. (7) Dunn, B.; Kamath, H.; Tarascon, J. M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928-935. (8) Suo, L.; Borodin, O.; Gao, T.; Olguin, M.; Ho, J.; Fan, X.; Luo, C.; Wang, C.; Xu, K. “Waterin-Salt” Electrolyte Enables High-Voltage Aqueous Lithium-Ion Chemistries. Science 2015, 350, 938-943. (9) Shan, X.; Charles, D. S.; Lei, Y.; Qiao, R.; Wang, G.; Yang, W.; Feygenson, M.; Su, D.; Teng, X. Bivalence Mn5O8 with Hydroxylated Interphase for High-Voltage Aqueous Sodium-Ion Storage. Nat. Commun. 2016, 7, 13370. (10) Chen, L.; Shao, H.; Zhou, X.; Liu, G.; Jiang, J.; Liu, Z. Water-Mediated Cation Intercalation of Open-Framework Indium Hexacyanoferrate with High Voltage and Fast Kinetics. Nat. Commun. 2016, 7, 11982. (11) Gu, S.; Gong, K.; Yan, E. Z.; Yan, Y. A Multiple Ion-Exchange Membrane Design for Redox Flow Batteries. Energy Environ. Sci. 2014, 7, 2986-2998. (12) Chen, R.; Hempelmann, R. Ionic Liquid-Mediated Aqueous Redox Flow Batteries for High Voltage Applications. Electrochem. Commun. 2016, 70, 56-59. (13) Zhang, Y.; Ye, R.; Henkensmeier, D.; Hempelmann, R.; Chen, R. “Water-in-Ionic Liquid” Solutions towards Wide Electrochemical Stability Windows for Aqueous Rechargeable Batteries. Electrochim. Acta 2018, 263, 47-52. (14) Bichat, M. P.; Raymundo-Piñero, E.; Béguin, F. High Voltage Supercapacitor Built with Seaweed Carbons in Neutral Aqueous Electrolyte. Carbon 2010, 48, 4351-4361.


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(15) Tatlisu, A.; Huang, Z.; Chen, R.

High‐Voltage and Low‐Temperature Aqueous

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