AQUEOUS ELECTROLYTE, REDOX FLOW BATTERY AND USE THEREOF

Abstract

Disclosed is an aqueous electrolyte solution having a temperature of at least 30° C. comprising a compound with at least one redox-active residue of formula (I)

(X—C.sub.5H.sub.4)Fe(Y—C.sub.5H.sub.3—Z)  (I), wherein X is a residue of formula —(C.sub.nH.sub.2n)-FG or of formula —(C.sub.nH.sub.2n)-Sp-(C.sub.nH.sub.2n)-FG or of formula —(C.sub.nH.sub.2n)-Brgp-, Y is hydrogen or a residue of formula —(C.sub.nH.sub.2n)-FG or of formula —(C.sub.nH.sub.2n)-Sp-(C.sub.nH.sub.2n)-FG, Z is hydrogen or a covalent bond, which links the residue of formula (I) with the remainder of the molecule, FG is a functional group selected from —OH, —SH, —NO.sub.3, —NO.sub.2, —CN, —OR.sub.1, —SR.sub.1, —(O—CH.sub.2—CH.sub.2).sub.o—OR.sub.2, —(O—CH.sub.2—CH.sub.2).sub.o—NR.sub.3R.sub.4R.sub.5.sup.+ (An.sup.m−).sub.1/m, —COR.sub.2, —COO.sup.− (Kat.sup.m+).sub.1/m, —COOR.sub.2, —SO.sub.3.sup.− (Kat.sup.m+).sub.1/m, —SO.sub.3R.sub.2, —SO.sub.4.sup.− (Kat.sup.m+).sub.1/m, —SO.sub.4R.sub.2, —PO.sub.4.sup.2− (Kat.sup.m+).sub.2/m, —PO.sub.4(R.sub.2).sub.2, —PO.sub.3.sup.2− (Kat.sup.m+).sub.2/m, —PO.sub.3(R.sub.2).sub.2, —NR.sub.3R.sub.4R.sub.5.sup.+ (An.sup.m−).sub.1/m —N.sup.+R.sub.3R.sub.4—C.sub.tH.sub.2t—SO.sub.3.sup.− or —NR.sub.2—SO.sub.2—R.sub.3, Brgp is a divalent bridging group which links the residue of formula (I) with the remainder of the molecule, Sp is —O—, —S—, —SO— or —SO.sub.2—, R.sub.1 is C.sub.1-C.sub.4 alkyl, R.sub.2 is hydrogen or C.sub.1-C.sub.4 alkyl, R.sub.3, R.sub.4 and R.sub.5 independently of one another represent hydrogen or alkyl, Kat is an m-valent inorganic or organic cation, An is an m-valent inorganic or organic anion, m is an integer between 1 and 4, n represents an integer between 2 and 4, t is an integer between 2 and 5, and is an integer from 1 to 50, preferably from 3 to 20.

The electrolyte may be used in redox flow batteries and is characterized by high stability of the redox active compounds at elevated temperatures.

Claims

1. An aqueous electrolyte solution having a temperature of at least 30° C. comprising a compound with at least one redox-active residue of formula (I)
(X—C.sub.5H.sub.4)Fe(Y—C.sub.5H.sub.3—Z)  (I), wherein X is a residue of formula —(C.sub.nH.sub.2n)-FG or of formula —(C.sub.nH.sub.2n)-Sp-(C.sub.nH.sub.2n)-FG or of formula —(C.sub.nH.sub.2n)-Brgp-, Y is hydrogen or a residue of formula —(C.sub.nH.sub.2n)-FG or of formula —(C.sub.nH.sub.2n)-Sp-(C.sub.nH.sub.2n)-FG, Z is hydrogen or a covalent bond, which links the residue of formula (I) with a remainder of the compound, FG is a functional group selected from —OH, —SH, —NO.sub.3, —NO.sub.2, —CN, —OR.sub.1, —SR.sub.1, —(O—CH.sub.2—CH.sub.2).sub.o—OR.sub.2, —(O—CH.sub.2—CH.sub.2).sub.o—NR.sub.3R.sub.4R.sub.5.sup.+ (An.sup.m+).sub.1/m, —COR.sub.2, —COO.sup.− (Kat.sup.m+).sub.1/m, —COOR.sub.2, —SO.sub.3.sup.− (Kat.sup.m+).sub.1/m, —SO.sub.3R.sub.2, —SO.sub.4.sup.− (Kat.sup.m+).sub.1/m, —SO.sub.4R.sub.2, —PO.sub.4.sup.2− (Kat.sup.m+).sub.2/m, —PO.sub.4(R.sub.2).sub.2, —PO.sub.3.sup.2− (Kat.sup.m+).sub.2/m, —PO.sub.3(R.sub.2).sub.2, —NR.sub.3R.sub.4R.sub.5.sup.+ (An.sup.m−).sub.1/m —N.sup.+R.sub.3R.sub.4—C.sub.tH.sub.2t—SO.sub.3.sup.− or —NR.sub.2—SO.sub.2—R.sub.3, Brgp is a divalent bridging group which links the residue of formula (I) with the remainder of the compound, Sp is —O—, —S—, —SO— or —SO.sub.2—, R.sub.1 is C1-C.sub.4 alkyl, R.sub.2 is hydrogen or C.sub.1-C.sub.4 alkyl, R.sub.3, R.sub.4 and R.sub.5 independently of one another represent hydrogen or alkyl, Kat is an m-valent inorganic or an organic cation, An is an m-valent inorganic or an organic anion, m is an integer between 1 and 4, n represents an integer between 2 and 4, t is an integer between 2 and 5, and o is an integer from 1 to 50.

2. The aqueous electrolyte solution according to claim 1, wherein the aqueous electrolyte solution has a temperature from 30 to 90° C.

3. The aqueous electrolyte solution according to claim 1, wherein the aqueous electrolyte solution comprises at least one conducting salt.

4. The aqueous electrolyte solution according to claim 1, wherein the aqueous electrolyte solution has a charge state of less than 90%.

5. The aqueous electrolyte solution according to claim 1, wherein Y is hydrogen and m is 1 or 2.

6. The aqueous electrolyte solution according to claim 1, wherein FG is a functional group selected from —(O—CH.sub.2—CH.sub.2).sub.o—OR.sub.2, —COR.sub.2, —COO.sup.− (Kat.sup.m+).sub.1/m, —SO.sub.3.sup.− (Kat.sup.m+).sub.1/m, —SO.sub.4.sup.− (Kat.sup.m+).sub.1/m, —PO.sub.4.sup.2− (Kat.sup.m+).sub.2/m, —PO.sub.3.sup.2− (Kat.sup.m+).sub.2/m or —NR.sub.3R.sub.4R.sub.5.sup.+ (An.sup.m−).sub.1/m.

7. The aqueous electrolyte solution according to claim 1, wherein Kat is selected from a hydrogen cation, an alkali metal cation an alkaline earth metal cation, an ammonium cation, or a quaternary ammonium cation.

8. The aqueous electrolyte solution according to claim 1, wherein An is selected from a halide ion, a hydroxide ion, a phosphate ion, a sulfate ion, a perchlorate ion, a hexafluorophosphate ion or a tetrafluoroborate ion.

9. The aqueous electrolyte solution according to claim 1, wherein Z is a covalent bond which connects the at least one redox-active residue of the formula (I) to a polymer backbone selected from the group of polymethacrylates, polyacrylates, polystyrenes, polyalkylene glycols, polyalkylene imines or polyvinyl ethers, and wherein the polymer backbone includes 5 to 100 residues of the formula (I).

10. The aqueous electrolyte solution according to claim 1, wherein the aqueous electrolyte solution comprises oligomers or polymers with recurring structural units of the formula (II) and optionally further structural units derived from solubility-facilitating comonomers ##STR00011## wherein ME is a recurring structural unit derived from a polymerizable monomer, BG is a covalent bond or a bridging group, FC represents a residue of formula (X—C.sub.5H.sub.4) Fe (Y—C.sub.5H.sub.3—Z), X, Y and Z have the meanings defined in claim 1, and r is an integer from 2 to 150.

11. The aqueous electrolyte solution according to claim 1, wherein the redox-active compound is a compound of formula (II)
(X—C.sub.5H.sub.4)Fe(Y—C.sub.5H.sub.4)  (II), wherein X and Y have the meaning defined in claim 1.

12. The aqueous electrolyte solution according to claim 1, wherein the redox-active compound is a compound of formula (III)
[(X—C.sub.5H.sub.4)Fe(Y—C.sub.5H.sub.3—)]—R.sub.7—[(—C.sub.5H.sub.3—Y)Fe(C.sub.5H.sub.4—X)].sub.p  (III), wherein X and Y have the meaning defined in claim 1, R.sub.7 is a two- to four-valent organic group, and p represents an integer from 1 to 4.

13. The aqueous electrolyte solution according to claim 1, wherein n is 2.

14. A redox flow battery for storing electrical energy comprising a reaction cell with two electrode chambers for a catholyte and an anolyte, which are each connected to at least one liquid reservoir, the two electrode chambers being separated by a membrane, being equipped with electrodes, and each being filled with electrolyte solutions which contain redox-active components in liquid form dissolved or dispersed in an aqueous electrolyte solvent, as well as, optionally comprising conducting salts dissolved therein, and wherein the redox flow battery is characterized in that the anolyte contains a water-soluble redox-active component and that the catholyte in the electrode chamber contains an aqueous electrolyte solution according to claim 1.

15. The redox flow battery according to claim 14, wherein the anolyte contains a compound comprising one or more bipyridiyl groups in the molecule as redox-active component.

16. The redox flow battery according to claim 14, wherein the electrode chambers for catholyte and anolyte are separated by a semipermeable membrane impermeable to the redox couple in the catholyte, and the anolyte contains a zinc salt as a redox-active component.

17. A redox flow battery for storing electrical energy, comprising a reaction cell with an electrode chamber for an electrolyte solution, which is connected to at least one liquid reservoir, the electrode chamber being equipped with a cathode and an anode, and being filled with electrolyte solution which contains redox-active components in liquid form dissolved or dispersed in an aqueous electrolyte solvent, as well as, optionally conductive salts dissolved therein, wherein the electrolyte solution contains an aqueous electrolyte solution as defined in claim 1 and a zinc salt as a further redox-active component.

18. The redox flow battery according to claim 14, wherein the redox flow battery comprises a zinc solid anode with a redox pair zinc(II)/zinc(0).

19. The redox flow battery according to claim 14, wherein a charge state of the catholyte or of the catholyte and the anolyte is less than 90%.

20. The redox flow battery according to claim 14, wherein the redox flow battery is adapted for a storage of electrical energy for mobile and stationary applications, wherein the storage includes stationary storage for emergency power supply, for peak load balancing, or for intermediate storage of electrical energy from renewable energy sources.

Description

EXAMPLE 1: SYNTHESIS OF FERROCENE-BIS-SULFONIC ACID

[0134] ##STR00005##

[0135] The synthesis of ferrocene-bis-sulfonic acid followed a modified literature procedure (K. Chanawanno, C. Holstrom, L. A. Crandall, H. Dodge, V. N. Nemykin, R. S. Herrick, C. J. Ziegler, Dalton Trans. 2016, 45, 14320) and is described below.

[0136] Ferrocene (8.00 g, 44 mmol) was dissolved in acetic anhydride (100 mL). Chlorosulfonic acid (5.7 mL, 86 mmol) was added slowly. The reaction mixture was stirred in an argon atmosphere for 20 hours at room temperature. The suspension was then filtered in an argon atmosphere. The filtrate was washed with acetic anhydride. Then the product was dried. One part (3.46 g, 8.5 mmol) of the dried product was dissolved in ethanol (10 mL). Then, a 1 M NaOH solution (10 mL) freshly prepared from NaOH (4.0 g, 100 mmol) in ethanol (100 mL) was slowly added. The solution was stirred for 30 min at room temperature. The precipitate was collected on a filter and then washed with ethanol and then dried in vacuo. Thus, the sodium salt of ferrocene-bis-sulfonic acid was obtained as a yellow solid (2.1 g, 4.7 mmol).

EXAMPLE 2: STABILITY TEST OF FERROCENE-BIS-SULFONIC ACID (COMPARATIVE)

[0137] The ferrocene from Example 1 was oxidized using Oxone® and the stability of the “charged” molecule was investigated. The charged electrolytes showed an irreversible precipitate after storage at 60° C. (also in the absence of air) after three days.

EXAMPLE 3: INVESTIGATION OF THE TEMPERATURE STABILITY OF ELECTROLYTE SOLUTIONS

[0138] ##STR00006##

Preparation of Electrolytes:

[0139] Catholyte: 400 mM solution of ferrocene derivative (1.77 g, 4 mmol) in deionized water (10 mL [0140] Anolyte: 600 mM solution of the viologen (3.00 g, 6 mmol) in deionized water (10 mL).

[0141] Battery test: Electrolyte solutions were separated using a Fumasep FAA-3-50 anion exchange membrane with 5 cm.sup.2 surface area. Graphite plates with a graphite fleece were used as electrodes. A persistaltic pump pumped the electrolyte solutions through tubing into reservoirs located in a heatable sand bath and through the half cell.

[0142] In the first three cycles, 101.1, 100.3 and 100.1 mAh could be charged.

[0143] Correspondingly, 100.6, 100.5 and 100.4 mAh were discharged.

[0144] After approximately 12 days of constant cycling, 99.4, 99.3 and 99.3 mAh were charged and 99.6, 99.6 and 99.5 mAh were discharged, respectively.

[0145] The experiment was also carried out with a sand bath heated to 60° C., in which both storage tanks of the electrolytes were located. The solutions, which had been cycled for almost 19 days in the meantime, were used for this purpose. In the process, 99.0 mAh could be charged and 98.5 mAh discharged after 24 hours.

[0146] Ferrocene from Example 3 can be purchased commercially from TCI Chemicals, for example. The synthesis of this compound is described in ACS Energy Lett. 2017, 2, 639-644.

[0147] The synthesis of the viologen derivative of Example 3 is also described in ACS Energy Lett. 2017, 2, 639-644.

EXAMPLE 4: SYNTHESIS OF A FUNCTIONALISED FERROCENE

[0148] ##STR00007##

[0149] Vinylferrocene (100 mg, 0.47 mmol) was dissolved in tetrahydrofuran (2 mL) in a microwave glass (5 mL). Then, 2,2-dimethoxy-2-phenylacetophenone (DMPA, 3 mg, 0.01 mmol) was added. After a magnetic stir bar was added to the microwave tube, it was sealed with a Teflon septum and argon was passed through the clear orange solution for 15 minutes. Then 2-mercaptopropanoic acid (50 μL, 0.56 mmol) was added using a microsyringe. This solution was then placed in a beaker on a stir plate in an UV chamber (365 nm) and irradiated for 20 minutes. The solution was then given a period of 10 minutes to cool. A total of 120 minutes was irradiated. The solvent was removed and the crude product was dissolved in dichloromethane and purified by column chromatography (silica, EtOAc/hexane 2/8). After the solvent was removed by distillation, the crude product was dried under high vacuum. The product was obtained as a highly viscous orange oil.

EXAMPLE 5: SYNTHESIS OF A FERROCENE POLYMER

[0150] ##STR00008##

[0151] By means of a free-radical copolymerization of a ferrocene monomer and a sulfobetaine methacrylate, a ferrocene-containing copolymer could be prepared. The ferrocene monomer was obtained by reacting the hydroxyethyl ferrocene with methacrylic acid chloride.

EXAMPLE 6: SYNTHESIS OF HYDROXYETHYL FERROCENE

[0152] ##STR00009##

[0153] Ferrocene was dissolved in methyl-THF and reacted with tert-butyllithium. Subsequently, ethylene oxide was added. The product was subsequently obtained after column chromatographic separation.

EXAMPLE 7: SYNTHESIS OF A FERROCENE POLYMER

[0154] ##STR00010##

[0155] A copolymer of METAC ([2-(methacryloyloxy)ethyl]trimethylammonium chloride) and a ferrocene-containing methacrylamide with a composition of 10 to 30% of the ferrocene monomer can be prepared with molar masses around 10,000 g/mol (M.sub.n) by free radical polymerization. Aqueous electrolytes based on this polymer were temperature stable at 60° C. and showed no degradation (i.e., capacity drop) over 100 cycles.