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Title:
AQUEOUS COMPOSITION AS ELECTROLYTE COMPRISING IONIC LIQUIDS OR LITHIUM SALTS
Document Type and Number:
WIPO Patent Application WO/2019/174910
Kind Code:
A1
Abstract:
The present invention relates to aqueous solution containing at least one ionic liquid and/or at least one lithium salt as supporting components and at least one redox active species. It thereby allows the solution to be used as an electrolyte improving the performance and characteristics for redox active organics in batteries. Moreover, the present invention refers to the use of such solutions as electrolytes in batteries and to batteries containing such solutions.

Inventors:
CHEN, Ruiyong (Am Gehlenberg 1, Saarbrücken, 66125, DE)
YE, Ruijie (Prausestr. 24, Berlin, 12203, DE)
HEMPELMANN, Rolf (St. Herblainer Str. 11, St. Ingbert, 66386, DE)
KIM, Sangwon (Dudweilerstr. 2, Saarbrücken, 66111, DE)
MÖLLER, Alexander (Auheimer Sand 18, Hanau, 35394, DE)
HARTWIG, Jan (Friedhofstr. 10, Alzenau, 63755, DE)
KRAWCZYK, Nastaran (Am Berg 15, Fulda, 35394, DE)
GEIGLE, Peter (Industriestr. 19, Alzenau, 63755, DE)
Application Number:
EP2019/054763
Publication Date:
September 19, 2019
Filing Date:
February 26, 2019
Export Citation:
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Assignee:
KIST EUROPE FORSCHUNGSGESELLSCHAFT MBH (Saarland University, Campus E7 1, Saarbrücken, 66123, DE)
CMBLU PROJEKT AG (Industriestr. 19, Alzenau, 63755, DE)
International Classes:
H01M8/18
Domestic Patent References:
WO2017212179A12017-12-14
Foreign References:
JP2017117752A2017-06-29
US20160233536A12016-08-11
KR20160035338A2016-03-31
Other References:
K. TAKECHI ET AL: "A Highly Concentrated Catholyte Based on a Solvate Ionic Liquid for Rechargeable Flow Batteries", ADVANCED MATERIALS, vol. 27, no. 15, 10 March 2015 (2015-03-10), DE, pages 2501 - 2506, XP055590733, ISSN: 0935-9648, DOI: 10.1002/adma.201405840
G.L. SOLOVEICHIK, CHEM. REV., vol. 115, 2015, pages 11533
L. LI ET AL., ADV. ENERGY MATER., vol. 1, 2011, pages 394
R. CHEN ET AL.: "Redox: Principles and Advance Applications", vol. 103, 2017, INTECH, article "Redox flow batteries: fundamentals and applications"
W. WANG ET AL., ADV. FUNCT. MATER., vol. 23, 2013, pages 970
J. WINSBERG ET AL.: "Angew. Chem. Int.", vol. 56, 2017, pages: 686
Attorney, Agent or Firm:
VON STOSCH, Andreas et al. (Prinzregentenstr. 22, München, 80538, DE)
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Claims:
Claims:

1 . An aqueous solution suitable for use as an electrolyte in batteries comprising (i) as a supporting component at least one ionic liquid and/or at least one lithium salt and (ii) as a redox active component at least one redox active organic compound.

2. The aqueous solution according to clam 1 comprising at least one ionic liquid, which is composed of small anions and larger organic cations.

3. The aqueous solution according to claim 1 or 2 comprising at least one ionic liquid selected from the group consisting of imidazolium-based ionic liquids and quaternary ammonium salts.

4. The aqueous solution according to claim 3 comprising 1 -butyl-3-methylimidazolium chloride or 1 -ethyl-3-methylimidazolium chloride.

5. The aqueous solution according to claim 3 comprising tetrabutyl ammonium chloride or tetraethylammonium chloride.

6. The aqueous solution according to any of claims 1 to 5 comprising at least one lithium salt, which is composed of a lithium cation and a larger anion, the at least one lithium salt being preferably an bis(trifluoromethanesulfonyl)imide lithium salt.

7. The aqueous solution according to any of claims 1 to 6 having a molality of the support component in the range of 5 to 20 m.

8. The solution according to any of claims 1 to 7 comprising at least one unsubstituted or substituted quinone, preferably selected from the group consisting of hydroquinones, benzoquinones or anthraqui nones, as the redox active component.

9. The aqueous solution according to claim 8 comprising a hydroquinone selected from the group consisting of 2-methoxyhydroquinone, 2,6-dimethoxyhydroquinone, and a salt of 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium, preferably a chloride salt thereof.

10. The aqueous solution according to claim 8 comprising a salt of 2-methanaminium- N,N,N-triethyl-9,10-anthraquinone, preferably a bromide salt thereof.

1 1 . The aqueous solution according to any of claims 8 to 10 comprising at least one unsub stituted or substituted quinone having a concentration of more than 1 M, preferably more than 2 M in the aqueous solution as defined by any of claim 1 to 10.

12. The aqueous solution according to any of claims 1 to 1 1 comprising at least one addi tive, preferably selected from the group consisting of an inorganic acid and an organic base.

13. The aqueous solution according to claim 12 comprising at least one acid selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid and nitric ac id.

14. The aqueous solution according to claim 12 comprising triethanolamine.

1 5. Use of an aqueous solution according to any of claims 1 to 14 for use as an electrolyte in batteries, in particular redox flow batteries.

1 6. A battery comprising an aqueous solution according to any of claim 1 to 14 and, op tionally, an anion exchange membrane.

1 7. The battery according to claim 1 6 comprising an anion exchange membrane for con ducting chloride anions.

1 8. The battery according to claim 1 6 or 1 7 comprising a cation exchange membrane to conduct lithium cations.

19. The battery according to any of claims 1 6 to 1 8 comprising a polybenzimidazole membrane as an anion exchange membrane.

Description:
Aqueous composition as electrolyte comprising ionic liquids or lithium salts

Technical Field

The present invention is related to an aqueous composition or solution comprising ionic liquids and/or lithium salts suitable and a redox active species, in particular an organic re dox active species, suitable for use as an electrolyte, in particular for use as an electrolyte in redox flow batteries. Moreover the present invention relates to the use of such a composi tion or solution for use as an electrolyte in batteries, in particular redox flow batteries and to a redox flow battery comprising such compositions or solutions.

Background

High performance, low cost and safe energy storage systems are essential for sustainable energy strategies. Redox flow batteries offer a promising approach due to their economy and scalability, especially for large-scale stationary applications compared to other electro chemical energy storage systems (G.L. Soloveichik, Chem. Rev., 2015, 1 15, 1 1533). By storing energy in an electrolyte in external tanks, redox flow batteries offer the option to decouple the energy and power of the system, which creates design flexibility for practical applications. The performance of redox flow batteries generally depends e.g. on the overall cell voltage, the concentration of active species in electrolytes and the operation current density upon cycling.

Various disadvantages exist for conventional redox flow battery systems based on transition metal cations as redox species (L. Li, et al., Adv. Energy Mater., 201 1 , 1 , 394). For example, numerous prior art systems use electrolytes with low chemical and electrochemical stability. Thereby, precipitate and gas evolution may occur upon operation with varied temperatures or voltages. External devices to control the operating conditions, to manage heat generation and to monitor the degree of oxidation/reduction of active species typically significantly increase the complexity and the cost of the total system. In addition, conventional prior art aqueous-based redox flow batteries suffer from low operation voltage due to limits for the electrochemical stability window of water (R. Chen, et al., Chapter: "Redox flow batteries: fundamentals and applications", Redox: Principles and Advance Applications, M.A.A. Kha- lid (Ed.), InTech, 201 7, 103). Also, the energy density (typically below 30 Wh U 1 ) is limited due to low solubility of active species (for instance, about 1 .6 M for vanadium species for vanadium redox flow batteries). Although organic solvents may afford higher voltage opera tion (for instance, about 2 V for a V(acac) 3 system), they typically suffer from flammability (thereby raising safety concerns), evaporation loss (problems related to storage, transport, and pollution) and low or very low solubility (about 0.1 M) for electroactive species (W. Wang, et al., Adv. Funct. Mater., 2013, 23, 970). Therefore, there is a need for more potent, readily available and directly employable, safe and preferably low cost electrolytes for re dox flow battery systems.

Unlike other redox flow batteries systems utilizing the redox chemistry of transition metals, the newly emerging systems using organic molecules as redox active components became lateky more attractive as analternative 0. Winsberg, et al., Angew. Chem. Int. Ed., 2017, 56, 686). Organic materials are relatively inexpensive and structurally diverse. Synthesis and chemical structure of organic molecules can be designed on purpose. Organics can be ob tained from natural sources. However, some challenges remain, such as low solubility of organics in aqueous solutions and limited cell voltage due to the narrow electrochemical window of water.

Summary of the invention

It is an object of the present invention to provide an electrolyte suitable for being for a bat tery, in particular a redox flow battery, which allows to enhance the solubility and to im prove the redox properties of low-cost organic compounds as redox-active species and to provide a battery, in particular a redox flow battery, comprising such an electrolyte.

The present invention thereby solves the object by the provision of an aqueous solution or composition being suitable for being used as an electrolyte, redox flow battery systems comprising such aqueous solutions or compositions, and the use of such aqueous solutions or compositions according to the invention as electrolytes in redox flow batteries. ;>

Description of the invention

The aqueous solution or composition of the invention suitable for use as an electrolyte in redox flow batteries is characterized by comprising (i) at least one ionic liquid and/or at least one lithium salt as a supporting component of the inventive solution. As a second component, the inventive composition or solution comprises (ii) at least one redox active organic compound. The terms "composition" and "solution" are used interchangeably by the present application. The present invention thereby provides a solvent based system, whereby the solvent is water. The supporting component of the inventive solution or com position allows to support and enhance the solubility of the redox active species and pre vents its sublimation in an aqueous system, thereby improving the properties of the in ventive solution or composition as an electrolyte for batteries.

The system may comprise - by a first embodiment - at least one "ionic liquid", which is also termed "ionic salt". Such "ionic liquids" structurally typically reflect organic salts, which are liquid at a temperature of less than 100°C, e.g. at room temperature. They are a supporting component of the aqueous solution or composition according to the invention, e.g. they are dissolved in the water solvent. Preferably, the at least one ionic liquid is prefer ably hydrophilic, e.g. exhibiting AGsl < -1 13 mj/m2.

By a second embodiment according to the invention, at least one lithium salt is dissolved in the aqueous solvent. Typically, the inventive aqueous solution or composition comprises either hydrophilic ionic liquids or lithium salts as a component, but may also comprise both components. The aqueous solution according to the invention may comprise hydrophilic ionic liquids of the same type or a mixture of distinct hydrophilic ionic liquids, e.g. two, thhree or four disitinct ionic liquids or more. In analogy, the aqueous solution according to the invention may comprise more than one lithium salt dissolved therein, e.g. two, three, four or more.

The organic salt typically representing the at least one hydrophilic ionic liquid may prefera bly be composed of a small anion, e.g. selected from the class of halogenides, preferably chloride, and a larger organic cation. The larger cation may be selected from imidazolium, pyridinium, pyrrolidium, guanidinium or ammonium. The organic cation may be alkylated by an alkyl chain of 1 to 1 5 carbon atoms. It is preferred that the hydrophilic ionic liquid is an imidazolium-based ionic liquid or based on quaternary ammonium salts. More specifi cally, the hydrophilic ionic liquid may be chosen from an organic salt selected from 1 - butyl-3-methylimidazolium chloride or 1 -ethyl-3-methylimidazolium chloride. The aqueous solution may also comprise a salt selected from tetrabutyl ammonium chloride or tetrae- thylammonium chloride.

By its second embodiment, the aqueous solution or composition according to the invention comprises at least one lithium salt, which is preferably composed of a lithium cation and a larger anion, typically of organic nature. More preferably, the aqueous solution or composi tion may comprise a bis(trifluoromethanesulfonyl)imide (TFSI ) lithium salt.

The aqueous solution according to the invention may preferably exhibit a molality of the supporting component of at least 5 m, more preferably at least 9 m. It may also exhibit a molality of the suporting component in the range of from 5 m to 20 m.

In order to be suitable for beeing used as an electrolyte, the aqueous solution of the inven tion comprises at least one redox active species of typically organic nature. The redox- active species, e.g. the organic compound of redox active character, may be selected from an unsubstituted or substituted quinone. More preferably, the quinones being comprised by the inventive solution or composition are selected from the group consisting of hydroqui- nones, benzoquinones or anthraquinones. They may be chosen e.g. from a hydroquinone, more specifically from a hydroquinone selected from the group consisting of 2- methoxyhydroquinone, 2,6-dimethoxyhydroquinone, and a salt of 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -aminium, preferably a chloride salt thereof, or from a salt of 2-methanaminium-N,N,N-triethyl-9,10-anthraquinone, preferably a bromide salt thereof.

The aqueous solution according to the invention may comprise unsubstituted or substituted quinones, preferably as disclosed above, having a solubility of more than Ί M, preferably more than 2 M in the aqueous solution or composition as defined above.

The aqueous solution or composition according to the invention may comprise at least one additive, in particular an additive for enhancing the solubility of the redox active species. That additive may be selected from the group consisting of an inorganic acid, an organic acid and an organic base. When adding an inorganic acid, the inorganic acid may prefera bly be selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid and nitric acid or a micture of any of the above. The aqueous solution may also com prise a carbonic acid, e.g. formic acid. When adding an organic base to the inventive solu tion or composition triethanolamine may be preferred.

The aqueous solution according to the invention is used as an electrolyte, in particular as an electrolyte for use batteries, such as in redox flow batteries. The battery, in particular the redox flow battery, according to the invention comprises an aqueous solution according to the invention. In addition, it may optionally comprise an anion exchange membrane and/or a cation exchange membrane. The anion exchange membrane should preferably be suitable or adapted for conducting chloride anions. The cation exchange membrane should be suit able for or ahould be adapted to conduct lithium cations. A preferred anion exchange membrane to be provided according to the invention is a polybenzimidazole membrane.

Ionic liquids being mixed with the water solvent or dissolved therein for providing an aque ous solution according to the first embodiment of the present invention are known to be used as environmental friendly media for many synthetic and reaction processes. Many of the physical properties of ionic liquids such as viscosity, hydrophi I icity and ionic conductiv ity depend on the nature and size of their cation and anion constituents and thus can be adjusted by changing the molecular structure, such as by modifying the alkyl chain-length and side chains. The solubility of various redox active species in ionic liquids depends mainly on polarity and hydrogen bonding ability. Air and water stable, water soluble ionic liquids are promising for practical applications according to the present invention. The in ventive aqueous solution comprising at least one ionic liquid can also inhibit sublimation of organic redox active species (such as compounds of the class of (substituted) hydroquinones or anthraquinones, e.g. 1 ,4-benzoquinone), which have a higher vapor pressure (0.1 mmHg or more) at room temperature. Thereby, the present invention improves the stability of the aqueous solution or composition according to the invention in terms its application as an electrolyte for batteries. In the first embodiment directed to aqueous ionic liquid based solution or composition, the inventive solution or composition preferably comprises at least one (concentrated) ionic liquid as a supporting salt, wherein the ionic liquid preferably comprises a larger organic cation group, such as l -butyl-3-methylimidazolium (BMIm + ) and/or tetrabutylammonium (TBA + ), and an anion, preferably a small anion, such as Cl\ However, any anion of the hal ogen group or other anions, such as hydroxy or organic anions, such as carbonic acids, e.g. formic acid, may be used according to the invention as well. Such a solution or composi tion may preferably be concentrated (e.g. containing more than 1 M or more than 2 M or more than 3 M of the at least one ionic liquid). In order to provide a concentrated solution or composition according to the invention, the molality (mol kg '1 S0 [vem, m) of the at least ion ic liquid dissolved therein may typically be larger than 2 m, or larger than 3 m or larger than 5 m or, more specifically range from 5 to 20 m.

The inventive aqueous solution or composition may be used to dissolve redox active com pounds, e.g. of the hydroquinone, benzoquinone or anthraquinone class, e.g. methoxy- or ethoxy-substituted hydroquinones or benzoquinones substituted by one, two, three or four methoxy- or ethoxy-substituents, such as 2-methoxyhydroquinone or 2,6- dimethoxyhydroquinone, effectively. The concentration of the redox active species in the inventive solution or composition is preferably larger than 1 M or larger than 2M or larger than 3M. As an example, the solubility of 2-methoxyhydroquinone can be increased from 1 .8 M in pure water to 6 M (corresponding to a theoretical capacity of 1 60 Ah L 1 ) in a con centrated BMlmCI containing aqueous solution according to the invention.

To further enhance the solubility of the at least one ionic liquid component, it may be pre ferred to add an acidic additive, such as hydrochloric acid, to the concentrated electrolyte solution or composition. Its concentration may typically range from 0.2 to Ί M. It may also be preferred to employ C anions as charge carrier for a redox flow battery according to the invention, as they allow the use of low cost anion exchange membranes for the batteries. Moreover, Cl exhibits good mobility in aqueous solutions, i.e. for in the inventive aqueous solution or composition, thereby further contributing to the performance of the battery.

By the second embodiment, the redox-active species may be dissolved in an aqueous solu tion or composition containing at least one lithium salt, wherein the anion of the lithium salt is preferably a TFSL anion, which may improve the solvation process. As disclosed above, the redox active species may be an organic redox-active compound, e.g. 2- methoxyhydroquinone or other methoxy-substituted hydroquinones or benzoqui nones, may be dissolved in a water solvent system (to provide the inventive aqueous solution or compo sition) with a concentration of more than 2.0 M or more than 3M, e.g. by 4.2 M, thereby providing aqueous solution or composition according to the invention comprising at least one lithium salt, e.g. comprising bis(trifluoromethanesulfonyl)imide lithium salt (LiTFSI), preferably concentrated other lithium salts such as Lithium bis(fluorosulfonyl)imide, Lithium trifluoromethanesulfonate. The molality (mol kg 1 solvent , m) of the lithium salt for the inventive solution or composition may preferably be larger than 2 m or larger than 3 m or larger than 5 m or the molality may specifically range from 5 m to 20 m. An electrolyte comprising such at least one lithium salt as supporting salt for the redox active species allows the use of a cation exchange membrane to conduct Li + . By the addition of an organic or inorganic acid, e.g. HCI, both H + and Li + can be used as charge carriers for a redox flow battery, fur ther improving the battery's performance.

Accordingly, the present invention more specifically provides an aqueous ionic liquid con taining solution or an aqueous lithium salt containing solution suitable for being used as an electrolyte for a battery, in particular a redox flow battery, comprising a bi- or multivalent metal ion as a anolyte, preferably V3+ and Zn2+.

Also, 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -amonium chloride may be used as a catho- !yte. It may be dissolved in pure water with a concentration of 2 M. However, that redox active compound does not maintain stable and reversible electrochemical performance over cyclic voltammetry (CV) cycling. By using e.g. a aqueous ionic liquid (e.g. BMlmCl) con taining or lithium salt (e.g. LiTFSI) containing solution as supporting salt, enhanced electro chemical redox reversibility has been observed for 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan- 1 -amonium chloride from CV measurements and galvanostatic charge/discharge over a long-term cycling. Accordingly, the present invention may provide a redox flow battery comprising at least one ionic liquid containing aqueous solution or at least one lithium salt containing aqueous solution or containing bothat least one ionic liquid and at least one lithium salt, comprising 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -amonium chloride as a catholyte, preferably in a concentration of more than 0.05 M or more than 0.1 M or more than 0.5 M or, more specifically, ranging from 0.08 to 1 M.

Modification of the ionic liquid and/or lithium salt containing aqueous solution or composi tion of the invention by the addition of HCI does not only promote solubility of the compo nents of the inventive composition, but can also enhance the reaction kinetics, as observed from the CV measurements. Low polarization and fast reaction can be achieved according ly. These are important for a redox flow battery with high voltage efficiency and high energy efficiency.

A redox flow battery according to the present invention comprises the inventive solution or composition. It may preferably comprise an anolyte comprising V 3+ , a catholyte comprising a hydroquinone or benzoquinone derivative, preferably methoxy-substituted hydroquinone or benzoquinone (e.g. 2-methoxyhydroquinone) dissolved in an aqueous solution or com position according to the invention containing a ionic liquid cation, e.g. BMIm + , and ani ons, preferably Cl ions, and, optionally, an anion exchange membrane separating the anolyte and the catholyte. Accordingly, the 2-methoxyhydroquinone/V battery system may employed by the use of the inventive solution or composition. It may e.g. comprise a cross- linked methylated polybenzimidazole membrane to conduct anions. The 2- methoxyhydroquinone/V system according to the invention typically has an average dis charge voltage of about 0.8 V.

The present invention is also directed to high cell voltage redox flow battery systems. Ac cording to the invention a hydroquinone or benzoquinone based redox flow battery, e.g. a methoxy-substituted hydroquinone, e.g. 2-methoxyhydroquinone/Zn battery system may be employed with the redox active species being dissolved in the inventive aqueous solution or composition. Thereby, the Zn 2 7Zn redox couple may be employed as anolyte/anode. A discharge voltage of about 1 .25 V is e.g. observed for the 2-methoxyhydroquinone/Zn bat tery system. Free selection of the anolyte allows the system with high overall cell voltage.

The present invention also provides a redox flow battery comprising an aqueous solution or composition comprising V 3+ as an anolyte, and 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 - amonium chloride as a catholyte. The aqueous solution preferably contains at least one ionic liquid having cation, preferably containing BMInT as the cation, and an anion, prefer ably Cl ions, and, optionally, an anion exchange membrane separating the anolyte and the catholyte. The concentration of 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -amonium chlo ride as the catholyte is preferably larger than 0.08 M and more specifically ranges from 0.08 to 1 M. The operation current density preferably ranges from 10 to 100 mA cm 2 . The capacities and cycling efficiencies reach constant values after about initial 10 cycles. A re versible capacity of about 60 mAh (or 6 Ah L 1 ) has been obtained for 1 M 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride. As will be realized, the invention is capable of modification in various respects without de parting from the invention. Accordingly, the drawings and description of the preferred em bodiments set forth hereafter are to be regarded as illustrative in nature, and not as restric tive.

Description of drawings

Table 1 : Solubility of some organics in Ί 0 m water-ionic liquid or water-lithium salt mixtures without and with additives

Figs. 1 a and 1 b show the chemical structure of 2-methoxyhydroquinone and its solubility in pure water and other aqueous ionic liquids, aqueous lithium salt (molality: 10 m), with and without the addition of 1 M HCI.

Figs. 2a and 2b compare cyclic voltammetry curves of 0.1 M 2-methoxyhydroquinone in pure water and other aqueous ionic liquid, aqueous lithium salt (molality: 10 m), with and without the addition of 1 M HCI. Potential sweep rate: 50 mV s 1 . Working electrode: glassy carbon; Counter electrode: Pt foil; Reference electrode: Ag wire.

Figs. 3a and 3b compare the cell resistance measured from impedance and voltage profiles (50 th cycle) of redox flow batteries with a commercial Nafion 1 1 7 membrane and a cross- linked polybenzimidazole membrane. Catholyte: 10 mL 0.08 M 2-methoxyhydroquinone in 10 m BMlmCI with 1 M HCI; anolyte: 10 mL 0.16 M V 3+ with 1 M HCI and saturated NaCl. Fig. 3c shows the charge/discharge capacities, coulombic efficiency, voltage efficiency, and energy efficiency with the crosslinked polybenzimidazole anion exchange membrane. Flow rates: 35 mL min 1 . Current density: 10 mA cm 2 .

Fig. 4 Voltage profile and cycling stability of a 2-methoxyhydroquinone/Zn hybrid redox flow battery. Catholyte: 10 mL 0.3 M 2-methoxyhydroquinone and 0.5 M HCI; Anolyte: 10 mL 0.3 M ZnCh and 0.3 M NH 4 CI. Flow rates: 35 mL min 1 . Current density: 1 .25 mA cm 2 . Membrane: a crosslinked polybenzimidazole anion exchange membrane.

Fig. 5a depicts the chemical structure of 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride. Figs. 5b and 5c show the cyclic voltammetry curves of 0.1 M 2- [(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -amonium chloride in pure water and in 1 M HCI. Potential sweep rate: 20 mV s 1 . Working electrode: glassy carbon; Counter electrode: Pt foil; Ref erence electrode: Ag wire. Figs. 6a and 6b compare the cyclic voltammetry curves of 0.1 M 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -amonium chloride in concentrated LiTFSI and BMlmCl without and with the addition of 1 M HCI. Potential sweep rate: 20 mV s 1 . Working elec trode: glassy carbon; Counter electrode: Pt foil; Reference electrode: Ag wire.

Figs. 7 a and 7b show voltage profiles, cycling efficiencies and capacities of a redox flow battery with a crosslinked polybenzimidazole membrane. Catholyte: 10 mL 0.08 M 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -amonium chloride in 10 m BMlmCl with 1 M HCI; anolyte: 10 mL 0.1 6 M V 3 ' with 1 M HCI and saturated NaCI. Flow rates: 35 mL min ~1 . Cur- rent density: 10 mA cnr 2 .

Figs. 8a and 8b show voltage profiles, cycling efficiencies and capacities of a redox flow battery with a crosslinked polybenzimidazole membrane. Catholyte: 10 mL 0.5 M 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -amonium chloride in 10 m BMlmCl with 1 M HCI; anolyte: 10 mL 1 .6 M V 3+ with 1 M HCI and saturated NaCI. Flow rates: 35 mL min '. Cur rent density: 25 mA cnr 2 .

Figs. 9a and 9b show voltage profiles, cycling efficiencies and capacities of a redox flow battery with a crosslinked polybenzimidazole membrane. Catholyte: 10 mL 1 .0 M 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -amonium chloride in 10 m BMlmCl with 1 M HCI; anolyte: 20 mL 1 .6 M V 3+ with 1 M HCI and saturated NaCI. Flow rates: 35 mL min '. Cur rent density: 100 mA cnr 2 for the first 36 cycles, then 50 mA cnr 2 for the following cycles.

Table 1 : Solubility of some organics in 10 m water-ionic liquid or water-lithium salt mix tures without and with additives

a: without additive; b: with 1 M HC ; c: with 0.3 g triethanolamine per mL solvent Examples Example 1 :

The solubility of organic compounds at room temperature was measured by adding organics into 1 mL supporting electrolyte under stirring with an increment of 0.1 mmol until the or ganics cannot be dissolved any more. The last recorded amount was recognized as the sol ubility of the organics.

Vanillin is slightly soluble in water, but has high solubility in aqueous ionic liquids. An ex pansion in volume of the solution was observed during the dissolution. A maximal concen tration of 4 M was observed for vanillin in BMImCI-H 2 0, TBACl-H 2 0 (tetrabutylammonium chloride) and UTFSI-H 2 0 (Table 1 ). The solutions of vanillin in BMImCl-H 2 0 and TBACI- H 2 0 are colorless when fresh prepared, but turn yellow after two days. The solution of van illin in LiTFSI-H 2 0 is also colorless but only at low concentration. With the increase of con centration, it shows a pink color. This distinct color indicates a possible coordination be tween lithium and vanillin.

Vanillic acid is almost insoluble in water and can only slightly be dissolved in LiTFSI-H 2 0 and TBACl-H 2 0. Nevertheless, its solubility in 10 m BMImCI-H 2 0 reaches 1 M. The solu tion is in yellow and turns brown at higher concentration.

Example 2:

2-methoxyhydroquinone has high solubility in water (about 2 M) and its solubility in aque ous ionic liquids and the solubility in acid is better (Fig. 1 ) according to the invention. The low concentrated solutions show yellow to amber color, while the color turns darker with the increase of solute. High concentration of BMImCl-H 2 0 leads to high viscosity and the volume is expanded as much as about two times when 12 mmol MBHQ is dissolved in 1 mL BMImCI-H 2 0, which gives a final concentration of 6 M.

CV measurements were performed in a three-electrode cell consisting of a glassy carbon rod working electrode, a platinum foil counter electrode and a silver wire quasi-reference elec trode. CV measurements of 2-methoxyhydroquinone in water (Fig. 2a) and different aque ous ionic liquids and salt (Fig. 2b) were performed. For the CV measurement in water, KCI was added as supporting salt. 2- methoxyhydroquinone exhibits good electrochemical reactivity and reversibility. The aver age redox potential in water is 0.27 V vs. Ag but with large peak separation of 0.64 V (Fig. 2a).

The average redox potentials in aqueous ionic liquids and salt are generally higher than that in water, which is 0.38 vs. Ag in LiTFSI-FhO and 0.51 V vs. Ag in BMlmCI-FbO, respective ly (Fig. 2b). The highest potential occurs in TBACI-H 2 0 at 0.54V vs. Ag. The polarization in TBACI-H 2 0 is also the largest (0.87 V). In comparison, the kinetics in BMlmCI-FbO and LiTFSI-FbO are better as the peak separation is 0.53 V and 0.48 V, respectively.

The electrochemical behavior of 2-methoxyhydroquinone was also investigated under acid ic conditions (Fig. 2b). In the presence of protons, the reaction kinetics of 2- methoxyhydroquinone is significantly improved. The peak separations are reduced to 0.40 V in water, 0.30 V in BMImCI-H 2 0, 0.38 V in LiTFSl-H 2 0 and 0.54 V in TBACI-H 2 0. At the same time, protons also help enhancing the redox potential, which is raised to 0.35 V vs. Ag in water, to 0.58 V vs. Ag in BMImCl-H 2 0, 0.55 V vs. Ag in LiTFSI-H 2 0 and to 0.58 V vs. Ag in TBACl-H 2 0. In addition, acid may suppress the unwanted side reactions (demethoxy- lation and hydroxylation) of 2-methoxyhydroquinone.

The influence of acid on the electrochemical behavior of 2-methoxyhydroquinone was also investigated by using phosphoric acid (H3PO4). The acidity of phosphoric acid is relatively weak. Phosphoric acid is less corrosive compared to HCI for practical applications. With addition of H3PO4 from 1 M to 3 M in 10 m BMImCl-H 2 0, the average redox potential of 2- methoxyhydroquinone is 0.50 V vs. Ag. However, this value is lower than that with HCI. Although the peak separation is also reduced to 0.42 V in the electrolyte with 1 M H3PO4, this value is still larger than that with HCI.

Example 3:

By its two methoxy groups, 2,6-dimethoxyhydroquinone is insoluble in water. Its solubility in BMImCI-H 2 0 as well as in TBACl-H 2 0 is limited. Nevertheless, the solubility of 2,6- dimethoxyhydroquinone in neutral and acidic LiTFSI-H 2 0 reaches 0.7 M and 0.5 M, re spectively (Table 1 ).

Example 4:

2-methanaminium-/V,/V,/V-triethyl-9,10-anthraquinone bromide is an organic salt containing quaternary ammonium cation and a bromide anion. The ionic configuration does not lead to a good solubility in water, in BMImCl-H 2 0 and in TBACl-H 2 0. However, it can be well dissolved in 10 m LiTFSI-H 2 0 with a solubility of 0.5 M (Table 1 ). Pale yellow-brown pre cipitate was observed when the concentration of 2-methanaminium-N,N,N-triethyl-9, 10- anthraquinone bromide reaches 0.3 M in 10 m UTFSI-H 2 0 with the presence of triethano lamine.

2-methanaminium-N,N,N-triethyl-9,10-anthraquinone bromide is also soluble in 15 m UTFSI-H2O, while the solubility in 5 m LiTFSI-H 2 0 is poor.

With the addition of 0.3 g triethanolamine per milliliter 10 m LiTFSI supporting electrolyte, the solubility of 2-methanaminium-N,N,N-triethyl-9,10-anthraquinone bromide can reach 0.3 M.

Example 5:

2-methoxyhydroquinone was tested as active species for catholyte (0.08 M 2- methoxyhydroquinone in 10 m BMlmCI with 1 M HCI) with the combination of vanadium anolyte (Fig. 3, 0.1 6 M V 3+ with 1 M HCI and saturated NaCI). V 3+ electrolytes were ob tained by diluted commercial vanadium sulfate electrolytes. 1 M HCI and saturated NaCI were added and used as anolyte. Catholyte and anolyte, each with a volume of 10 mL, were stored in two sealed glass vials.

A flow cell with an active area of 4 cm 2 was used for galvanostatic charge/discharge meas urements. The graphite felts with uncompressed thickness of 5 mm and compression of 20% were pretreated in 3 M H 2 SO. t solution for 24 h and then thermally processed at 500°C for 12 h in static air. Two pieces of graphite felts were used for the cathode and anode. A commercial cation exchange membrane Nafion 1 1 7 and a crosslinked methylated polybenzimidazole membrane are compared for the cycling performance with vanadium anolyte (Fig. 3a, 3b). Polybenzimidazole membrane shows significant low resistance (0.3 W, Fig. 3a) from the impedance measurements, compared to the Nafion 1 1 7 membrane (6.6 W). Accordingly, only very low current density of 0.25 mA cm 2 can be applied for the flow battery with Nafion 1 1 7 membrane (Fig. 3b), which has large ohmic drop, low voltage effi ciency (54.5%) and low capacity (4 mAh).

In contrast, with the use of a polybenzimidazole membrane, the discharge voltage shifted up to 0.8 V with an increased voltage efficiency of 82% (Fig. 3b). In addition, the capacity increased to about 1 6 mAh. Over cycling (Fig. 3c), the capacity drops during the initial 10 cycles, then increase progressively to about 1 8 mAh after 60 cycles, then keep constant up to 1 00 cycles. A Coulombic efficiency of about 98% was observed.

Example 6:

2-methoxyhydroquinone was tested as active species for catholyte (0.3 M 2- methoxyhydroquinone in 10 m BMlmCI with 0.5 M HCI) with the combination of zinc an ode (Zn plate anode, anolyte consists of 0.3 M ZnCh and 0.3 M NH 4 CI). The Zn plate with flow channels (1 .6 mm in thickness, polished and then washed with 3 M H SQ and dis tilled water before being assembled into a cell) was sandwiched between a Cu current col lector and a gasket (with a free space of 3 mm in thickness, allowing the Zn 2 7Zn plating reactions on the surface of the Zn plate). Catholyte and anolyte, each with a volume of 10 mL, were stored in two sealed glass vials. A crosslinked methylated polybenzimidazole membrane was used.

With the use of Zn anode with low negative redox potential (-0.76 V, thermodynamically), a high cell voltage of 1 .25 V was obtained (Fig. 4). Over 200 cycles, steady cycling perfor mance was observed (Inset in Fig. 4).

Example 7:

2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride (Fig. 5a) is an organic salt and has high solubility in water (2 M). However, it was found chemically and electrochemical ly instable. CV measurements of 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride in pure water (Fig. 5b) and in 1 M HCI (Fig. 5c) showed poor redox reversibility. Only oxi dation peaks in pure water can be seen, which decreases significantly over cycling. With the presence of acid, the reactivity is slightly improved. However, the oxidation product is still instable under acidic conditions and can be only partially reduced, leading to the poor reversibility.

The solubility of 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride in neutral and acidic BMImCI-H 2 0 was 1 .2 M and 0.9 M, respectively. These values could be underesti mated, as the solutions are viscous and in dark color. It is difficult to recognize whether more solute could be dissolved. The solution of 0.6 M 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride in TBACI-H 2 0 reaches relative high viscosity.

In contrast, 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride shows excellent stability and reversibility in aqueous ionic liquids or lithium salt from the CV measurements. When 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride is tested in LiTFSI-H 2 0, reversible redox peak can be observed from the CV measurements (Fig. 6a). The oxidation peak appears at 0.70 V vs. Ag, whereas the corresponding reduction peak is located at 0.08 V vs. Ag. Enhanced current response has been observed by adding HCI. In addition, the polarization decreases, indicating enhanced reaction kinetics. As shown in Fig. 6b, in BMImCI-H 2 0 solution, a reversible redox reaction with an average oxidation and reduction potential of 0.50 V vs. Ag and a peak separation of 0.61 V was observed. With the addition of HCI, the average oxidation and reduction potential shifts to 0.61 V and the peak separa tion reduces by 0.07 V.

Example 8:

2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride in 10 m BMImCI-H 2 0 contain ing 1 M HCI was employed as catholyte for flow battery tests. Commercial vanadium sulfate electrolytes were diluted to specific concentrations containing 1 M HCI and saturated NaCI and used as anolyte. Both electrolytes were covered by paraffin oil during the battery cy cling. A crosslinked methylated polybenzimidazole membrane was used in the flow cell, allowing the transport of Cl . Different concentrations of 2-[(2,5-dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride catholytes were used ranging from 0.08 to 1 M. Accordingly, V 3+ with concentrations of 0.Ί 6 to 1 .6 M were used. Excess of V 3+ anolyte was used for 1 M 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride catholyte. Different current densities were applied from 10 to 100 mA cm 2 .

During initial 10 cycles, capacity drops were observed for all tests. Afterwards, the capacities reach steady values of about 3, 20 and 60 mAh for the battery with catholyte concentrations of 0.1 6 M (Fig. 7), 0.5 M (Fig. 8) and 1 M (Fig. 9), respectively. The voltage curves are relatively overlapped at the 50 th and the 100 th cycles, indicating 2-[(2,5- dihydroxyphenyl)sulfanyl]ethan-1 -aminium chloride is stable over long-term cycling. At 10 and 25 mA cm 2 , average discharge voltages are located at about 0.75 V (Fig. 7a, Fig. 8a), whereas the average voltage shifts down to about 0.5 V when higher current densities of 50 and 100 mA cnr 2 were applied (Fig. 9a). The voltage efficiencies reduce from 70% at 10 mA cm 2 (Fig. 7b) to 64% at 25 mA cnr 2 (Fig. 8b), then to 45% at 50 mA cnr 2 (Fig. 9b), re spectively. Independent on the applied current densities and the concentrations of the cath olytes, the Coulombic efficiencies remain about 97.5%.