Redox flow battery electrolytes

Abstract

The present invention relates to novel combinations of redox active compounds for use as redox flow battery electrolytes. The invention further provides kits comprising these combinations, redox flow batteries, and method using the combinations, kits and redox flow batteries of the invention.

Claims

1. A redox flow battery comprising: a positive electrode; a first redox active composition comprising a first redox active compound corresponding in structure to any one of General Formulas (1)-(3), or a mixture thereof as a positive electrode electrolyte, the positive electrode electrolyte contacting the positive electrode; a negative electrode; a second redox active composition comprising (i) a second redox active compound corresponding in structure to any one of General Formulas (1)-(3), or a mixture thereof; and/or (ii) another second redox active compound selected from the group consisting of a metal, a metal oxide, a metal-ligand coordination compound, bromine, chlorine, iodine, oxygen, an organic dye, a salt thereof, and a mixture thereof as a negative electrode electrolyte, the negative electrode electrolyte contacting the negative electrode; and a separator interposed between the positive electrode and the negative electrode; wherein ##STR00057## wherein R.sup.1-R.sup.18 is each independently selected from hydrogen; hydroxyl; carboxy; linear or branched, optionally substituted C.sub.1-6 alkyl optionally comprising at least one heteroatom selected from N, O and S; a carboxylic acid; an ester; a halogen; optionally substituted C.sub.1-6 alkoxy; optionally substituted amino; nitro; carbonyl; phosphoryl; phosphonyl; cyanide; and sulfonyl; and wherein at least one of R.sup.1-R.sup.4 in General Formula (1), at least one of R.sup.5-R.sup.10 in General Formula (2) and/or at least one of R.sup.11-R.sup.18 in General Formula (3) of the first redox active compound is a substituted amine selected from —NHR/NH.sub.2R+, —NR.sub.2/NHR.sub.2.sup.+ and —NR.sub.3.sup.+, where R is selected from the group consisting of —C.sub.nH.sub.2nOH, —C.sub.nH.sub.2nNH.sub.2, —C.sub.nH.sub.2nNR.sub.2, —C.sub.nH.sub.2nCO.sub.2H and —C.sub.nH.sub.2nSO.sub.3H, wherein n is an integer selected from 1, 2, 3, 4, 5 or 6, where R is H or C.sub.1-C.sub.6 alkyl or substituted C.sub.1-C.sub.6 alkyl comprising at least one heteroatom selected from N, O and S.

2. The redox flow battery according to claim 1, further comprising: a positive electrode reservoir comprising the positive electrode immersed within the positive electrode electrolyte, said positive electrode reservoir forming the first redox flow battery half-cell; and a negative electrode reservoir comprising the negative electrode immersed within the negative electrode electrolyte, said negative electrode reservoir forming the second redox flow battery half-cell.

3. The redox flow battery according to claim 1, further comprising: a first circulation loop comprising a storage tank containing the positive electrode electrolyte, piping for transporting the positive electrode electrolyte, a chamber in which the first electrode is in contact with the positive electrode electrolyte, and a pump to circulate the positive electrode electrolyte through the circulation loop; optionally a second circulation loop comprising a storage tank containing the negative electrode electrolyte, piping for transporting the negative electrode electrolyte, a chamber in which the second electrode is in contact with the negative electrode electrolyte, and a pump to circulate the negative electrode electrolyte through the circulation loop; and optionally control hardware and software.

4. A redox flow battery cell stack comprising at least two redox flow batteries according to claim 1.

5. An energy storage system comprising a redox flow battery according to claim 1 or a redox flow battery cell stack comprising the redox flow battery; connected to an electrical grid.

6. A method of storing electrical energy, the method comprising applying a potential difference across the first and second electrode of a redox flow battery according to claim 1, wherein the first redox active compound is oxidized or wherein the second redox active compound comprised by the second redox active composition is reduced.

7. A method of providing electrical energy, the method comprising applying a potential difference across the first and second electrode of a redox flow battery according to claim 1, wherein the first redox active compound is reduced or wherein the second redox active compound is oxidized.

8. The redox flow battery according to claim 1, wherein the first and second redox active compound correspond in structure to different General Formulas (1)-(3).

9. The redox flow battery according to claim 1, wherein said first and second redox active compound are water-soluble.

10. The redox flow battery according to claim 1, wherein said first and second redox active composition are liquid.

11. The redox flow battery according to claim 1, wherein the first and second redox active composition are provided in separate compartments.

12. The redox flow battery according to claim 11, wherein the first and second redox active composition are each provided in a half-cell of a redox flow battery.

13. The redox flow battery according to claim 1, wherein the first redox active composition comprises at least one first redox active compound corresponding in structure to any one of General Formulas (1), (2) or (3), or a mixture thereof; optionally including at least one reduction and/or oxidation product thereof corresponding in structure to General Formula (1)(a) or (b), (2)(a) or (b), or (3)(a) or (b); or a mixture thereof.

14. The redox flow battery according to claim 1, wherein the second redox active composition comprises at least one second redox active compound corresponding in structure to any one of General Formulas (1), (2) or (3), or a mixture thereof; optionally including at least one reduction and/or oxidation product thereof corresponding in structure to General Formula (1)(a) or (b), (2)(a) or (b), or (3)(a) or (b); or a mixture thereof.

15. The redox flow battery according to claim 1, wherein the first redox active composition comprises as a first redox active compound at least one benzohydroquinone corresponding in structure to General Formula (1), optionally including at least one reduction and/or oxidation product thereof corresponding in structure to General Formula (1)(a) or (b); or a mixture thereof.

16. The redox flow battery according to claim 1, wherein the second redox active composition comprises as the second redox active compound at least one anthraquinone corresponding in structure to General formula (3), optionally including at least one reduction and/or oxidation product thereof corresponding in structure to General formula (3) (a) or (b); or a mixture thereof; or as the second redox active compound at least one benzohydroquinone corresponding in structure to General Formula (1), optionally including at least one reduction and/or oxidation product thereof corresponding in structure to General Formula (1)(a) or (b); or a mixture thereof; or as the second redox active compound at least one naphthoquinone corresponding in structure to General formula (2), optionally including at least one reduction and/or oxidation product thereof corresponding in structure to General formula (2)(a) or (b); or a mixture thereof.

17. The redox flow battery according to claim 1, wherein in General Formula (1): R.sup.1 is selected from —H, —SO.sub.3H, optionally substituted C.sub.1-6 alkyl, and optionally substituted amine; R.sup.2 is selected from —H, —OH, —SO.sub.3H, C.sub.1-6 alkoxy, and optionally substituted amine; R.sup.3 is selected from —H, —OH and C.sub.1-6 alkoxy; and R.sup.4 is selected from —H, —SO.sub.3H, optionally substituted C.sub.1-6 alkyl, optionally substituted amine, and halogen.

18. The redox flow battery according to claim 17, wherein R.sup.1 and/or R.sup.4 are independently selected from substituted C.sub.1-6 alkyl selected from R.sup.5—SO.sub.3H and R.sup.5—CO.sub.2H, wherein R.sup.5 is a C.sub.1-6 alkyl optionally comprising at least one heteroatom selected from N, O and S.

19. The redox flow battery according to claim 18, comprising (1) a first redox active compound selected from at least one of the following benzohydroquinones: ##STR00058## a mixture thereof, and optionally an oxidation product thereof; and (2) a second redox active compound selected from the following at least one anthraquinone: ##STR00059## and optionally a reduction product thereof; or at least one of the following benzohydroquinones: ##STR00060## a mixture thereof, and optionally an oxidation product thereof.

20. The redox flow battery according to claim 17, wherein R.sup.1, R.sup.2 and/or R.sup.4 are independently selected from —NH.sub.2/NH.sub.3.sup.+, —NHR/NH.sub.2R.sup.+, —NR.sub.2/NHR.sub.2.sup.+ and —NR.sub.3.sup.+, where R is H or optionally substituted C.sub.1-6 alkyl, optionally comprising at least one heteroatom selected from N, O and S.

21. The redox flow battery according to claim 20, wherein the compound of General Formula (1) is one of the following compounds: ##STR00061## ##STR00062## or a quinone form thereof.

22. The redox flow battery according to claim 1, wherein in General Formula (2): R.sup.5 and R.sup.6 are independently selected from —H, —OH and C.sub.1-6 alkoxy; and R.sup.7-R.sup.10 are independently selected from —H and —SO.sub.3H.

23. The redox flow battery according to claim 1, wherein in General Formula (3): R.sup.11, R.sup.12 and R.sup.14 are independently selected from —H, —OH and C.sub.1-6 alkoxy; and R.sup.13 and R.sup.15-R.sup.18 are independently selected from —H and —SO.sub.3H.

24. The redox flow battery according to claim 1, wherein in General Formula (3): R.sup.11 is —SO.sub.3H; R.sup.12 is —SO.sub.3H, R.sup.11, R.sup.13 and R.sup.14 are —OH; R.sup.16 is —SO.sub.3H, R.sup.11 and R.sup.14 or R.sup.11, R.sup.12 and R.sup.14 are —OH; R.sup.12 and R.sup.16 are —SO.sub.3H, R.sup.11 and R.sup.14 or R.sup.11, R.sup.13 and R.sup.14 are —OH; R.sup.13 and R.sup.16 are —SO.sub.3H, R.sup.11, R.sup.12 and R.sup.14 are —OH; R.sup.12 and R.sup.17 are —SO.sub.3H; or R.sup.11 and R.sup.14 are —SO.sub.3H; wherein each of the others of R.sup.11-R.sup.18 is/are C.sub.1-6 alkoxy or —H.

25. The redox flow battery according to claim 24, wherein the compound of General Formula (3) is the compound below or a hydroquinone form thereof: ##STR00063##

26. The redox flow battery according to claim 1, wherein the first redox active composition and/or the second redox active composition is a liquid.

27. The redox flow battery according to claim 1, wherein the first and/or the second redox active composition further comprises a solvent, optionally selected from water, an ionic liquid, methanol, ethanol, propanol, isopropanol, acetonitrile, acetone, dimethylsulfoxide, glycol, a carbonate, a polyether, tetrahydrofuran, dioxolane, sulfolane, dimethylformamide, diethylformamide, CO.sub.2, supercritical CO.sub.2, and a mixture thereof.

28. The redox flow battery according to claim 27, wherein the solvent comprises at least about 40 wt %, at least about 50 wt %, at least about 60 wt %, at least about 70 wt %, at least about 75 wt %, at least about 80%, at least about 85 wt %, at least about 90 wt %, at least about 95 wt %, or at least about 98 wt % water, relative to the total solvent.

29. The redox flow battery according to claim 1, wherein the first and/or the second redox active composition has a pH between <zero and about 14, between about zero and about 14, between about 7 and about 14, between about 9 to about 14, between about 10 and about 12, or between about 12 and about 14.

30. The redox flow battery according to claim 1, wherein the first and/or the second redox active compound are present in a concentration of between about 0.3 M and about 12 M, between about 0.5 M and about 2 M, between about 2 M and about 4 M, between about 4 M and about 6 M, or between about 6 M and about 10 M.

31. The redox flow battery according to claim 1, wherein the first redox active compound has a standard reduction potential that is at least 0.3 volts higher than the standard reduction potential of the second redox active compound.

32. The redox flow battery according to claim 1, wherein the first redox active compound has a standard reduction potential of at least about 0.0 volts, at least about +0.5 V, at least about +0.6 V, at least about +0.7 V or more against a standard hydrogen electrode; and/or wherein the second redox active compound has a standard electrode potential of about +0.3 V or less, about +0.1 V or less, about 0.0 V or less, about −0.5 V or less, about −0.6V or less, about −1.0V or less or about −1.2 V or less against a standard hydrogen electrode.

Description

9. DESCRIPTION OF THE FIGURES

(1) FIG. 1 schematically illustrates a redox flow battery comprising the inventive combination of redox active compositions and its function.

(2) FIG. 2 A-K shows charge/discharge curves of selected redox active quinone compounds on graphite electrodes.

(3) FIG. 3 shows further preferred compounds according to General Formulas (1), (2) and (3).

10. EXAMPLES

(4) In the following, particular examples illustrating various embodiments and aspects of the invention are presented. However, the present invention shall not to be limited in scope by the specific embodiments described herein. The following preparations and examples are given to enable those skilled in the art to more clearly understand and to practice the present invention. The present invention, however, is not limited in scope by the exemplified embodiments, which are intended as illustrations of single aspects of the invention only, and methods which are functionally equivalent are within the scope of the invention. Indeed, various modifications of the invention in addition to those described herein will become readily apparent to those skilled in the art from the foregoing description, accompanying figures and the examples below. All such modifications fall within the scope of the appended claims.

Example 1: Preparation of Low Molecular Weight Aromatic Lignin-Derived Compounds by Cracking and Reduction by a Nickel Catalyst

(5) Reductive cracking of a modified lignin-derived component was carried out by means of a catalyst comprising nickel, e.g. supported on activated carbon (Ni/C). The catalysts are typically prepared by an incipient-wetness impregnation method and further treated by a carbothermal reduction method known in the art.

(6) Herein, nickel nitrate(II) hexahydrate [Ni(NO.sub.3).sub.2 6H.sub.2O] is used and optionally added into water in a beaker known in the art. The solution is then stirred, e.g. for at least 30 min, to prepare an impregnation stock solution. Activated carbon having a water absorption capacity of typically above 1.8 mL g.sup.−1 is added into the solution and the beaker may then covered by a culture dish to keep the sample wet for a prescribed time, preferably more than 12 h, more preferably 24 h. The sample is then dried at a temperature above 80° C., e.g. 120° C. overnight. The actual reduction is carried out in a container such as a preferably horizontal furnace in a flow of inert gas such as N.sub.2. The flow is, e.g., 10 mL min.sup.−1 or more, preferably 30 mL min.sup.−1 or more. The reduction temperature preferably reaches at least 400° C., preferably 450° C., e.g. over set time period such as at least 30 min, preferably at least 60 min. The temperature for conducting the reduction is maintained at 450° C. for at least 1 h, more preferably for at least 2 h. The Ni/SBA-15 catalysts are reduced at 550° C. for 2 h. The Ni/Al.sub.2O.sub.3 catalyst is reduced at 700° C. for 2 h. The metal loading for each nickel- and copper-based catalyst is 10% (w/w) relative to the support. Herein, birch sawdust serves as lignocellulosic material and is treated with the ethanol-benzene mixture (v/v ratio 1:2) for 12 h. The treated birch sawdust, solvent (m/v 1:20), and catalyst (w/w 20:1) are placed in an autoclave reactor. The reactor is sealed and purged with Ar 4 to 6 times to expel air. Then, the reducing reaction is conducted at 200° C. at a stirring speed of at least 300 rpm, preferably 500 rpm. When the desired reaction time (usually 2 to 10 h) is reached, the reactor is cooled to ambient temperature before sampling.

(7) Typically, the reaction generates 4-propylguaiacol and 4-propylsyringol as major products, together with minor alkene-substituted 4-propylguaiacol and 4-propylsyringol, as determined by standard gas chromatography. The compounds are isolated according to step (F), preferably by extraction.

Example 2: Preparation of Monomeric Aromatic Lignin-Derived Molecules from Lignosulfonate of a Sulfite Process by Electrooxidation

(8) A 1 M aqueous NaOH solution of lignosulfonate is prepared, comprising 1% (W/W) lignosulfonate. Said solution is subjected to an electrooxidation. Therein, the solution is employed as anolyte. A 1 M aqueous solution is employed as catalyte. A flow cell with a flow rate of 250 ml/min is used. Electrolysis is allowed to take place galvanostatically for 8 h applying current of 1 mA/cm.sup.2. A typical resulting voltage is 1.4 V. The voltage curve typically is asymptotic and the solution changes preferably color from brown to dark brown.

(9) Samples of the solution are taken every hour over a time span of 8 h and subsequently examined photometrically. Thereof, an absorption profile typical for ortho-benzoquinone is determined. Hence, a lower molecular weight aromatic lignin-derived compound, quinone compound, is prepared by said method.

(10) Said compound is then isolated. Therefore, said compound is extracted by dichloromethane and subsequently subjected to cycles of charging and discharging processes in a flow cell. The voltage curve shows that the compound is redox active, which may be reversibly electrolyzed.

Example 3: Preparation of an Annulated Quinone Compound by a Friedel-Crafts Acylation

(11) Vanillin as a low molecular weight aromatic lignin-derived compound is provided and further annulated and oxidized in five steps as follows:

(i) Synthesis of 4-(benzyloxy)-3-methoxybenzaldehyde (2)

(12) ##STR00016##

(13) Vanillin (1) (1.0 eq.) and benzyl chloride (1.2 eq.) are dissolved in N,N-dimethylformamide and potassium iodine (0.5 mol %) is added. Afterwards potassium carbonate is added and the reaction is stirred above 60° C., preferably between 60 to 120° C. for at least 1 h, preferably 1 to 8 h. After completion of the reaction, the solution is diluted with distilled water and extracted with an appropriate solvent. The organic phase is washed with brine and the product is then isolated from the organic phase.

(ii) Synthesis of 4-(benzyloxy)-3-methoxybenzoic Acid (3)

(14) ##STR00017##

(15) A mixture of 1,2-dimethoxyethane and potassium hydroxide (5 to 20 eq.) is purged with oxygen and the calculated amount of isolated product 2 (1.0 eq.) is added. After the absorption of oxygen ceases, the mixture is diluted with distilled water and neutral organic products are extracted with an appropriate solvent. The aqueous layer is acidified and the acidic organic products are extracted with an appropriate solvent. Product 3 is isolated from the organic layer.

(iii) Synthesis of 4-(benzyloxy)-3-methoxybenzoyl Chloride (4)

(16) ##STR00018##

(17) Isolated product 3 (1.0 eq.) is dissolved in thionyl chloride (5-20 eq.) and the mixture is stirred at 60 to 120° C. for 1 to 8 h. After completion of the reaction excess thionyl chloride is evaporated to yield desired acyl chloride 4.

(iv) Synthesis of Anthraquinones (5-7)

(18) ##STR00019##

(19) Aluminiumtrichloride (0.1 eq.) is added to the crude acyl chloride 4 and the mixture is stirred for 30 to 300 min at −20 to 60° C. After completion of the reaction the mixture is carefully quenched with bicarb solution. The product is extracted with an appropriate solvent and the organic layer is washed with brine. The product is then isolated from the organic phase.

(v) Synthesis of 2,6-dihydroxy-3,7-dimethoxyanthracene-9,10-dione 8 and 2,6-dihydroxy-1,7-dimethoxyanthracene-9,10-dione 9

(20) ##STR00020##

(21) Anthraquinone 5 or 6 are dissolved in ethyl acetate, methanol or ethanol and palladium on charcoal (1 to 30 weight %) is added. The mixture is stirred at room temperature under hydrogen atmosphere (1-10 bar). The catalyst is filtered off and the product (9) is isolated from the mixture.

(22) The product is then characterized by spectrographic means, and provided as redox active compound according to the present invention.

Example 4: Derivatization of (Hydro-)Quinones

(23) Substituents are introduced into the low molecular weight lignin-derived components.

Example 4.1 Reduction of Dimethoxy Benzoquinone

(24) ##STR00021##

(25) 23.2 g of sodium dithionite (0.134 mol, 1.32 eq.) was added to the suspension of 17.0 g (0.101 mol, 1.0 eq.) 2,6-dimethoxycyclohexa-2,5-diene-1,4-dione in 100 mL H.sub.2O. After 2 h stirring at room temperature the precipitate was filtered off and dried in the air to give 15.85 g (0.093 mol, 92% yield) of 2,6-dimethoxybenzene-1,4-diol as a white solid.

Example 4.2: Oxidation of Methoxy Benzohydroquinone

(26) ##STR00022##

(27) 1.4 g of catalyst Cu/AlO(OH) was added to a solution of 8.2 g (0.059 mol) 2-methoxy-1,4-dihydroxybenzene in 250 ml ethyl acetate, and the reaction mixture was stirred at room temperature for 147 h under an O.sub.2 atmosphere. After the conversion determined by HPLC reached 99%, the reaction mixture was filtered, and the recovered catalyst was washed with ethyl acetate (100 mL×3). The filtrate was collected and solvent was removed in vacuo to give 7.66 g (0.055 mol, 95% yield) of 2-methoxycyclohexa-2,5-diene-1,4-dione as a yellow-brownish solid.

Example 4.3: Acetylation of Methoxy Benzohydroquinone

(28) 8.24 g (0.059 mol, 1.0 eq.) of 2-methoxybenzene-1,4-diol was weighed into a 250 mL reaction flask equipped with a reflux condenser. 60 mL of dichloroethane and 15 mL (0.159 mol, 2.7 eq.) of acetic anhydride were added. 12 mL (0.096 mol, 1.63 eq.) of boron trifluoride ether solution was then slowly added at room temperature with stirring. The reaction mixture was heated to 90° C. for 20 hours. The mixture was cooled to 60° C., 30 mL H.sub.2O was added followed by 10 mL HCl (6 M). The resulting mixture was heated to 100° C. for 30 min, cooled down and extracted with ethyl acetate (150 mL×3). The combined extracts were washed sequentially with H.sub.2O (100 mL), saturated sodium bicarbonate (100 mL) and H2O (100 mL) and then dried with anhydrous sodium sulfate. The solvent was removed in vacuo to give a brown solid residue, which was washed with methanol to give 7.49 g (0.041 mol, 70% yield) of 1-(2,5-dihydroxy-4-methoxyphenyl)ethan-1-one as a beige solid.

Example 4.4 Addition of Isonicotinic Acid to Benzoquinone

(29) 2.16 g (0.02 mol, 1.0 eq.) of p-benzoquinone was suspended in 6.4 mL of acetic acid. 2.46 g (0.02 mol, 1.0 eq.) of nicotinic acid was added and the mixture was stirred for 2 h at rt. The resulting dark mixture was diluted with 3 mL of water and treated with 6.6 mL of HCl (6 M). On cooling, solid precipitated which was filtered off and dried overnight at 60° C. to give 3.13 g (0.012 mol, 59% yield) of 3-carboxy-1-(2,5-dihydroxyphenyl)pyridin-1-ium chloride as an yellow solid.

Example 4.5 Sulfonation of Anthraquinone

(30) ##STR00023##

(31) A solution of anthraquinone was heated (180° C.) in a solution of 20%-40% SO.sub.3 in concentrated sulfuric acid (oleum), resulting in a mixture of sulfonated anthraquinones. The crude mixture was poured onto ice and partially neutralized with calcium hydroxide. Subsequently, the mixture was filtrated and concentrated to yield the final product.

Example 4.6: Sulfonation of Hydroquinone (1,4-Dihydroxybenzene)

(32) ##STR00024##

(33) A solution of hydroquinone was heated (80° C.) in a solution of 20%-40% SO.sub.3 in concentrated sulfuric acid (oleum), resulting in a mixture of sulfonated hydroquinones. The crude mixture was poured onto ice and partially neutralized with calcium hydroxide. Subsequently, the mixture was filtrated and concentrated to yield the final product.

Example 4.7: Sulfonation of 1,4-Dihydroxy-2,6-dimethoxybenzene

(34) A solution of hydroquinone was heated (80° C.) in a solution of 20%-35% SO.sub.3 in concentrated

(35) ##STR00025##
sulfuric acid (oleum), resulting in a mixture of sulfonated 1,4-dihydroxy-2,6-dimethoxybenzenes. The crude mixture was poured onto ice and partially neutralized with calcium hydroxide. Subsequently, the mixture was filtrated and concentrated to yield the final product.

Example 4.8: Sulfonation of 2-Methoxyhydroquinone

(36) ##STR00026##

(37) A solution of 2-methoxyhydroquinone was heated (80° C.) in a solution of 20%-40% SO.sub.3 in concentrated sulfuric acid (oleum), resulting in a mixture of sulfonated 2-methoxyhydroquinones. The crude mixture was poured onto ice and partially neutralized with calcium hydroxide. Subsequently, the mixture was filtrated and concentrated to yield the final product.

Example 4.9: Synthesis of 2,5-bis{[(2-hydroxyethyl)(methyl)amino]methyl}benzene-1,4-diol

(38) ##STR00027##

(39) In a round-bottom flask 40.0 g hydroquinone (0.36 mol, 1 eq) and 24.0 g paraformaldehyde (0.80 mol, 2.2 eq) were dissolved in toluene (200 mL). 64 mL 2-(methylamino)ethanol (0.80 mol, 2.2 eq) was added and the reaction mixture was heated under reflux for 20 h. After cooling to room temperature the solvent was removed in vacuum and the residue was recrystallized from acetone to yield 65.2 g of product (63% yield) as an off-white solid.

Example 4.10: Synthesis of 2,6-bis[(dimethylamino)methyl]-3,5-dimethoxybenzene-1,4-diol

(40) ##STR00028##

(41) 8.51 g 2,6-dimethoxyhydroquinone (50 mmol, 1 eq) and 3.30 g paraformaldehyde (110 mmol, 2.2 eq) were dissolved in ethanol (130 mL). 19 mL of dimethylamine solution in ethanol (5.6 M, 110 mmol, 2.2 eq) was added and the reaction mixture was stirred at room temperature for 20 h. After completion of the reaction, the solvent was removed in vacuum to obtain 12.2 g of product (86% yield). Analytically pure sample was obtained by recrystallization from acetone.

Example 5: Cycling Tests of Different Combinations of Redox Active Quinone Compounds

(42) Cycling Tests

(43) Selected redox active compositions were subjected to electrochemical measurements. Therefore, a small laboratory cell employing selected quinones/hydroquinones in different combinations as positive and negative redox active compounds were evaluated with constant-current charge-discharge experiments and open-circuit voltage measurement with a BaSyTec battery test system (BaSyTec GmbH, 89176 Asselfingen, Germany). This cell consists of four main parts: a graphite felt (with an area of 6 cm.sup.2, 6 mm in thickness) was employed as the positive and negative electrode, and a cation exchange membrane was used to separate the positive and negative electrolytes. Positive and negative electrolytes evaluated in each cycling test are specified in table 5 below. The electrolytes were provided in an aqueous solution of 20% sulfuric acid in water. The pH of the electrolyte solutions was <0. No additives were used. The electrolytes were pumped by peristaltic pumps to the corresponding electrodes, respectively. In the charge-discharge cycles, the cell was charged at a current density of 10 mA cm.sup.2 up to 1.2 V and discharged at the same current density down to −0.4 V cut-off.

(44) TABLE-US-00005 TABLE 5 Evaluated electrolyte combinations Open Circuit Coulombic Voltage efficiency # Posilyte Negolyte (OCV) (CE) Figure 1 0 0.69 V 97% 2A 2 0.79 V 93% 2B 3 0.70 V 97% 2C 4 0.64 V 98% 2D 5 0.72 V 97% 2E 6 0 0.66 V 99% 2F 7 0.71 V 91% 2G 8 0.68 V 90% 2H 9 0.75 V 99.5%  21 10 0 0.73 V 96% 2J 11 0.68 V 96% 2K

(45) FIG. 2A-K show the charge/discharge curves of selected redox active quinone compounds on graphite electrodes.