Aqueous electrolyte, redox flow battery and uses thereof

Abstract

The present invention provides an aqueous electrolyte solution having a temperature of at least 30° C., comprising a compound having at least one redox-active moiety of formula (I), (X-C5H4)Fe(Y-C5H3-Z) (I) X is a group of the formula -(C n H 2n )-FG or formula-(C n H 2n )-Sp-(C n H 2n )-FG or formula-(C n H 2n )-Brgp- part, Y is hydrogen or a group of formula -(C n H 2n )-FG or formula-(C n H 2n )-Sp-(C n H 2n )-FG part, Z is hydrogen or a covalent bond connecting the moiety of formula (I) to the rest of the molecule; FG is -OH, -SH, -NO3, -NO2, -CN, -OR1, -SR1, -(O-CH2-CH2) o -OR2, -(O-CH2-CH2) o -NR3R4R5 + (An m- ) 1 / m , -COR2, -COO - (Kat m+ ) 1 / m , -COOR2, -SO3 - (Kat m+ ) 1 / m , -SO3R2, -SO4 - (Kat m+ ) 1 / m , -SO4R2, -PO4 2- (Kat m+ ) 2 / m , -PO4(R2)2, -PO3 2- (Kat m+ ) 2 / m , -PO3(R2)2, -NR3R4R5 + (An m- ) 1 / m , -N + R3R4-C t H2t -SO3 - or a functional group selected from -NR2-SO2-R3; Brgp is a divalent linking bridging group connecting the moiety of formula (I) to the remainder of the molecule; Sp is -O-, -S-, -SO- or -SO2-; R1 is C1-C4 alkyl, R2 is hydrogen or C1-C4 alkyl; R3, R4 and R5 independently represent hydrogen or alkyl; Kat is an m-valent cation, An is an m-valent anion, m is an integer from 1 to 4, n is an integer from 2 to 4, t is an integer from 2 to 5, and o is an integer of 1 to 50, preferably 3 to 20; Regarding the aqueous electrolyte solution. The electrolyte can be used in redox flow batteries and is characterized by high stability of the redox active compounds at high temperatures.

Description

[Technical field]

[0001] The present invention relates to an aqueous electrolyte and a redox flow battery comprising said aqueous electrolyte. Redox flow batteries, also known as Redox Flow Batteries (hereinafter RFBs), are used to store electrical energy based on electrochemical redox reactions. Redox flow batteries contain one or two polarity specific chambers (half cells) separated by a membrane, filled with a liquid and fed by a pump from one or more separate tanks of any size. Each liquid contains water in which redox active substances and inorganic or organic salts (conductive additives) are dissolved. The conductive additives themselves may also be redox active. [Background technology]

[0002] Due to their great scalability, RFBs are particularly suitable as stationary energy storage devices to meet various performance and capacity demands, for example as buffer systems for renewable energies, both in the civil (e.g. single-family and multi-family buildings) and industrial sectors (e.g. wind and solar power plants). They therefore have great potential to ensure the stability of the power grid and for distributed energy supply. But mobile applications (electric cars, trucks, ships) are also conceivable.

[0003] Existing RFBs are electrochemical energy storage. The substances required to set the potential at the electrodes are liquid, dissolved or particulate redox active species that occur in particulate form and are converted to the other redox levels during charging or discharging in an electrochemical reactor. To do this, electrolyte solutions (catholyte (catholyte), anolyte (anolyte)) are taken from a cylinder and actively pumped to the electrodes. The positive (anode) and negative (cathode) compartments are separated in the reactor by a semipermeable membrane, usually highly selective for protons. While the electrolyte is pumped, a current can be passed. And the charging process is simply the reverse of that process. This means that the amount of energy that can be stored in an RFB is directly proportional to the size of the storage tank. However, the power that can be extracted is a function of the size of the electrochemical reactor.

[0004] RFBs have a complex system technology roughly equivalent to that of fuel cells. Typical sizes of individual reactors are in the approximate range of 2-50kW. The reactors are modular and very easy to combine, and the tank size can be adapted to almost any size.

[0005] A wide variety of redox-active chemical compounds have already been proposed for use in accumulators or batteries such as RFBs. A review of the prior art can be found, for example, in the article by Feng Pan and Quing Wang, Molecules 2015, 20, 20499-20517 "Redox Species of Redox Flow Batteries: A Review" or in the article by Winsberg et al., Angew. Chem. 2017, 36, 686-711, where it is already known that RFBs contain ferrocene as catholyte.

[0006] Ferrocene, bis(η 5 -cyclopentadienyl)iron(II), [Fe(η 5-C5H5)2] is a yellow-orange compound, soluble in organic solvents and sparingly soluble in water. Ferrocene is the parent compound and one of the most important representatives of metallocenes, prepared, for example, by reacting iron(II) chloride with cyclopentadienylsodium. Due to its aromatic nature, ferrocene is prone to electrophilic substitution reactions and can be functionalized in various ways. For use in aqueous electrolytes, ferrocene needs to be functionalized by incorporating polar groups. The functionalization of ferrocene is shown in the paper by A. Salmon “Synthese und Elektrochemie funktioneller Ferrocenyl- und Multiferrocenyl-Verbindungen”, Bielefeld (2001).

[0007] RFBs containing functionalized ferrocene in the electrolyte are also known from WO2018 / 032003A1 and US2018 / 0072669A1.

[0008] So far in the literature, RFBs have been used mainly at room temperature. There are few references in the literature to tests at higher temperatures, e.g., 40-50 °C, and there are no systematic studies on the behavior of aqueous electrolytes containing ferrocene (derivatives) at high temperatures. Commercially available vanadium RFBs (hereafter "V-RFBs") are known to cause problems at temperatures above 40 °C, since the electrolyte is irreversibly damaged.

[0009] Therefore, V-RFBs have limited temperature range or require complex cooling. Certain applications, such as replacing diesel generators on mobile phone base stations in India, thus preclude the use of V-RFBs. Special electrolytes containing HCl have been developed for V-RFBs. V-RFBs have high temperature stability and can therefore withstand temperatures above 40°C, sometimes up to about 50-60°C (see US2012 / 0077067A1). However, these electrolytes are highly corrosive. No other RFB system is disclosed for these relatively high temperatures.

[0010] In particular, RFBs containing organic redox-active compounds have not been used at these high temperatures so far. A further complicating factor is that many electrolytes containing organic redox-active compounds are not stable at high temperatures. In this context, it should also be noted that the stability of many organic redox-active substances has been incorrectly evaluated so far even at room temperature (M.-A. Goulet, M. J. Aziz, J. Electrochem. Soc. 2018, 165, A1466-A1477). No information on the aging of electrolytes has been obtained from charge / discharge tests carried out so far. RFBs containing N,N,N-2,2,6,6-heptamethylpiperidinyloxy-4-ammonium chloride (TEMPTMA) as an organic redox-active compound are also affected by these decomposition processes. At high temperatures, side reactions are accelerated and the capacity of the RFB is rapidly lost. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] WO2018 / 032003A1 [Patent Document 2] US2018 / 0072669A1 [Patent Document 3] US2012 / 0077067A1 [Non-patent literature]

[0012] [Non-Patent Document 1] Molecules 2015, 20, 20499-20517 “Redox Species of Redox Flow Batteries: A Review” [Non-Patent Document 2] Angew. Chem. 2017, 36, 686-711 [Non-Patent Document 3] MJ Aziz, J. Electrochem. Soc. 2018, 165, A1466-A1477 Summary of the Invention [Problem to be solved by the invention]

[0013] It has now been surprisingly found that functionalized ferrocene derivatives have high stability at high temperatures. These redox-active compounds allow the production of aqueous electrolyte solutions (aqueous electrolyte solutions) that are stable at high temperatures. Furthermore, these redox-active compounds allow the production of RFBs that can be operated at high temperatures. Hot electrolytes can occur when operating RFBs in high temperature environments, for example when operating in tropical or subtropical regions and / or when storing electrolyte solutions at high temperatures and / or when handling high powers during charge / discharge processes in the electrode chambers.

[0014] It has been found that ferrocenes functionalized with solubilizing groups attached to the cyclopentadienyl ring via spacers having 2-4 carbon atoms can achieve the desired combination of water solubility with suitable temperature stability, and it has been shown that ferrocenes directly substituted on the cyclopentadienyl ring with solubilizing groups or C1 spacers do not exhibit the desired stability.

[0015] The use of the electrolyte according to the invention allows RFBs with organic redox active compounds to be used at "high temperatures". In this way, new fields of application for RFBs can be developed without the need for additional cooling.

[0016] An object of the present invention is to provide an aqueous electrolyte containing a ferrocene derivative having sufficient stability for the operation of a battery or storage device even at high temperatures and having redox activity.

[0017] Another object of the present invention is to provide a redox flow battery that can be operated at high temperatures without significant decomposition of the electrolyte used. [Means for solving the problem]

[0018] This object is achieved by providing an aqueous electrolyte solution as claimed in claim 1.

[0019] The present invention relates to an aqueous electrolyte solution having a temperature of at least 30° C., comprising a compound having at least one redox active moiety of formula (I), (X-C5H4)Fe(Y-C5H3-Z) (I) X is a group of the formula -(C n H 2n )-FG or formula -(C n H 2n )-Sp-(C n H 2n )-FG or formula -(C n H 2n )-Brgp-, Y is hydrogen or a group of the formula -(C n H 2n )-FG or formula -(C n H 2n )-Sp-(C n H 2n )-FG portion, Z is hydrogen or a covalent bond connecting the moiety of formula (I) to the remainder of the molecule; FG is -OH, -SH, -NO3, -NO2, -CN, -OR1, -SR1, -(O-CH2-CH2) o -OR2, -(O-CH2-CH2) o -NR3R4R5 + (An m- ) 1 / m , -COR2, -COO - (Kat m+ ) 1 / m , -COOR2, -SO3 - (Kat m+ ) 1 / m , -SO3R2, -SO4 - (Kat m+ ) 1 / m , -SO4R2, -PO4 2- (Kat m+ ) 2 / m , -PO4(R2)2, -PO3 2- (Kat m+ ) 2 / m , -PO3(R2)2, -NR3R4R5 + (An m- ) 1 / m , -N+ R3R4-C t H 2t -SO3 - or a group selected from -NR2-SO2-R3 Brgp is a divalent linking bridging group connecting the moiety of formula (I) to the remainder of the molecule; Sp is -O-, -S-, -SO- or -SO2-; R1 is C1-C4 alkyl, preferably methyl; R2 is hydrogen or C1-C4 alkyl, preferably hydrogen or methyl, in particular hydrogen; R3, R4 and R5 are independently hydrogen or alkyl, preferably C1-C4 alkyl, methyl, propyl or alkyl, preferably C1-C4 alkyl; Kat is an m-valent inorganic or organic cation, An is an inorganic or organic anion of valence m; m is an integer from 1 to 4; n is an integer from 2 to 4, t is an integer from 2 to 5, and o is an integer of 1 to 50, preferably 3 to 20. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The redox-active compounds used in the electrolyte solution according to the invention can be low molecular weight organic molecules, oligomers or polymers. These molecules can be present as particles dissolved in the electrolyte solution or dispersed or suspended, for example as dispersions, microgels or nanogels. The redox-active compounds comprise at least one moiety of formula (I), where Z represents hydrogen or is linked to the rest of the molecule via a covalent bond Z.

[0021] Preferably, the redox-active compounds containing a moiety of formula (I) used in the electrolyte solutions according to the invention are water-soluble, but may also be compounds that are dispersible in water.

[0022] As used herein, water solubility of a compound means a solubility of at least 1 g of compound in 1 liter of water at 25°C.

[0023] In the present specification, low molecular weight molecules are to be understood as meaning compounds which have no repeating structural units derived from monomers and which have at least one, preferably 1 to 6, particularly preferably 1 to 4, in particular 1 to 3 and very particularly preferably one or two moieties of formula (I) contained therein.

[0024] In this specification, an oligomeric molecule should be understood to mean a compound derived from a monomer and having 2 to 10 repeating structural units, each having a moiety of formula (I). The moiety of formula (I) can also be attached to the oligomeric structure via a spacer.

[0025] In the present specification, a polymer molecule is understood to mean a compound having 10 or more, preferably 11 to 50 or more, repeating structural units derived from monomers and each having a moiety of formula (I). The moiety of formula (I) can also be attached to the polymer backbone structure via a spacer.

[0026] In a preferred embodiment, the electrolyte solution according to the invention comprises a redox-active component having 1 to 6, preferably 1 to 4, in particular 1 to 3 and very preferably 1 to 2 moieties of formula (I) in the molecule.

[0027] Examples of preferably used low molecular weight, oligomeric and polymeric redox-active compounds are given below.

[0028] The molar mass of the redox-active compound containing at least one moiety of formula (I) used in the electrolyte according to the invention can vary within wide limits. Redox-active compounds containing a moiety of formula (I) with a molar mass in the range of 150 to 80,000 g / mol, preferably in the range of 250 to 50,000 g / mol, particularly preferably in the range of 500 to 20,000 g / mol.

[0029] The viscosity of the electrolyte used according to the present invention is typically between 1 mPas and 10 3 mPas, particularly preferably 1 to 10 2 mPas, very preferably from 1 to 20 mPas (measured at 25° C. using a rotational viscometer, plate / plate).

[0030] The redox-active compounds used according to the invention are characterised in that at least one group X is present on at least one cyclopentadienyl ring. The group X consists of an alkylene group having 2 to 4 carbon atoms and a solubility-imparting group FG as spacer, or an alkylene-oxide-alkylene group having 2 to 4 carbon atoms and a solubility-imparting group FG as spacer, or an alkylene-sulfide-alkylene group having 2 to 4 carbon atoms and a solubility-imparting group FG as spacer, or an alkylene spacer carrying a divalent linking bridging group Brgp linking the moiety of formula (I) to the remainder of the molecule.

[0031] The bridging group Sp is oxygen, sulfur or -SO- or -SO2-, in particular -O-.

[0032] The alkylene spacer may be a moiety of the formula -CH2-CH(CH3)-, -(CH2)3-, -(CH2)4- or, in particular, -(CH2)2-.

[0033] The alkylene-oxide-alkylene spacer can be a moiety of the formula -CH2-CH(CH3)-O-CH2-CH(CH3)-, -(CH2)3-O-(CH2)3-, -(CH2)4-O-(CH2)4- or, in particular, -(CH2)2-O-(CH2)2-.

[0034] The alkylene-sulfide-alkylene spacer can be a moiety of the formula -CH2-CH(CH3)-S-CH2-CH(CH3)-, -(CH2)3-S-(CH2)3-, -(CH2)4-S-(CH2)4- or, in particular, -(CH2)2-S-(CH2)2-.

[0035] The FG groups are described in detail above.

[0036] The divalent bridging group Brgp can be any divalent residue that connects the moiety of formula (I) to the rest of the molecule, such as the polymer backbone (main chain). Examples of bridging groups Brgp are alkylene, -O-, -S-, -SO-, -SO2-, -CO-, -CO-O-, -NR2-, -CO-NR2- and -SON2NR2-.

[0037] The moiety of formula (I) in the redox-active compound according to the invention may, in addition to the X group, have on the cyclopentadienyl ring a group of the formula -(C n H 2n )-FG or formula -(C n H 2n )-Sp-(C n H 2n )-FG (moiety Y). However, Y is preferably hydrogen.

[0038] The electrolyte solution according to the invention comprises water or water and an organic solvent, as well as said redox active compound and optionally further substances dissolved therein, which are used to balance the charge during charging and discharging of the battery or which have a positive effect on the stability or performance parameters of the battery. Substances responsible for the charge balance are called conductive additives or conductive salts, and substances which have a positive effect on the stability or performance parameters are called auxiliary additives.

[0039] Examples of electrolyte solvents are alcohols (e.g., ethanol), carbonates (e.g., propylene carbonate), nitriles (e.g., acetonitrile), amides (e.g., dimethylformamide, dimethylacetamide), sulfoxides (e.g., dimethylsulfoxide), ketones (e.g., acetone), lactones (e.g., γ-butyrolactone), lactams (e.g., N-methyl-2-pyrrolidone), nitro compounds (e.g., nitromethane), ethers (e.g., tetrahydrofuran), chlorinated hydrocarbons (e.g., dichloromethane), carboxylic acids (e.g., formic acid, acetic acid), inorganic acids (e.g., sulfuric acid, hydrogen halides, or hydrohalic acids). Preferably, a mixture of water and a carbonate (e.g., propylene carbonate) or a nitrile (e.g., acetonitrile) is preferred. Water is particularly preferred.

[0040] Important additives are usually organic or inorganic salts. Examples of salts containing anions are selected from the group consisting of halides (e.g., fluoride, chloride, bromide, iodide), hydroxide, anions of inorganic acids (e.g., phosphate, sulfate, nitrate, hexafluorophosphate, tetrafluoroborate, perchlorate, chlorate, hexafluoroantimony, hexafluoroarsenate, cyanide, etc.), or anions of organic acids (e.g., acetate, formate, trifluoroacetate, trifluoromethanesulfonate, pentafluoroethanesulfonate, nonafluorobutanesulfonate, butyrate, citrate, fumarate, glutarate, lactate, malate, malonate, oxalate, pyruvate, tartrate, etc.). Particularly preferred are chloride ions, fluoride ions, hydroxide ions, phosphate ions, sulfate ions, perchlorate ions, hexafluorophosphate ions, and tetrafluoroborate ions; and furthermore hydrogen ions (H +), cations of alkali metals or alkaline earth metals (e.g. lithium, sodium, potassium, magnesium, calcium), zinc, iron, as well as substituted or unsubstituted ammonium cations (e.g. tetrabutylammonium, tetramethylammonium, tetraethylammonium), the substituents generally being alkyl groups. Particularly preferred are hydrogen ions, lithium ions, sodium ions, potassium ions, tetrabutylammonium ions and mixtures thereof. Particularly conductive salts are: NaCl, KCl, LiPF6, LiBF4, NaBF4, NaPF6, NaClO4, NaOH, KOH, Na3PO4, K3PO4, Na2SO4, NaSO3CF3, LiSO3CF3, (CH3)4NOH, n-Bu4NOH, (CH3)4NCl, n-Bu4NCl, (CH3)4NBr, n-Bu4NBr, n-Bu4NPF6, n-Bu4NBF 4、 n-Bu4NClO 4、 NH4Cl and mixtures thereof, where n-Bu represents an n-butyl group.

[0041] Particularly preferred electrolyte solutions according to the invention contain a conductive additive comprising an anion selected from the group of halide, hydroxide, phosphate, sulfate, perchlorate, hexafluorophosphate or tetrafluoroborate ions, in particular these anions and a cation selected from hydrogen ions, alkali or alkaline earth metal cations, and substituted or unsubstituted ammonium cations.

[0042] Examples of auxiliary additives are surfactants, viscosity modifiers, biocides, buffers, stabilizers, catalysts, conductivity additives, antifreeze agents, temperature stabilizers and / or foam breakers.

[0043] The surfactant may be nonionic, anionic, cationic or amphoteric. Nonionic surfactants (e.g., polyalkylene glycol ethers, fatty alcohol propoxylates, alkyl glucosides, alkyl polyglucosides, octylphenol ethoxylates, nonylphenol ethoxylates, saponins, phospholipids) are particularly preferred.

[0044] Examples of buffer solutions are carbonate-bicarbonate buffer, carbonate-silicate buffer, acetic acid-acetate buffer, phosphate buffer, ammonia buffer, citric acid or citrate buffer, tris(hydroxymethyl)aminomethane, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, 4-(2-hydroxyethyl)-piperazine-1-propanesulfonic acid, 2-(N-morpholino)ethanesulfonic acid, barbital acetate buffer.

[0045] The electrolyte solution according to the present invention is at high temperatures, which may occur in certain areas of the battery or electrical storage device, for example during the charging and / or discharging process and / or only if the electrolyte solution is stored in an external storage device, if an electrolyte solution is used.

[0046] An aqueous electrolyte solution having a temperature of 30 to 90°C, preferably 30 to 50°C, particularly 40 to 50°C, is preferred.

[0047] Also preferred are aqueous electrolyte solutions that include at least one conductive salt.

[0048] Aqueous electrolyte solutions containing at least one conductive salt are preferred, especially salts contained in highly concentrated salt solutions (brines).

[0049] The concentration of the conductive salt in the aqueous electrolyte solution according to the present invention is preferably from 14% by mass to the saturation limit. Saturated salt solutions are often used, and some of the salt is precipitated.

[0050] Particularly preferred electrolyte solutions according to the invention have a state of charge of less than 90%, in particular up to 80%, very preferably 70-80%. These electrolyte solutions are characterized by a particularly good temperature stability of the redox-active compound which comprises at least one moiety of formula (I) in the electrolyte. Of course, electrolyte solutions with a state of charge of 100% are also possible.

[0051] SOC (State of Charge) is a characteristic value for the state of charge of an electrolyte solution or a battery containing this electrolyte solution. The value of the state of charge indicates the still available capacity of the electrolyte solution or the battery in relation to its nominal value. The state of charge is given as a percentage of the fully charged state.

[0052] The state of charge can be determined, for example, by the open circuit voltage (VOC), which depends on the state of charge and which increases as the state of charge increases. Cell voltage is measured "open circuit", i.e., the cell voltage resulting from a given state of charge without an external load.

[0053] Also preferred is an aqueous electrolyte solution comprising at least one compound having a redox-active moiety of formula (I) where Y is hydrogen and m is 1 or 2.

[0054] More preferably, FG is -(O-CH2-CH2) o -OR 2, -COR2, -COO - (Kat m+ ) 1 / m ,-SO3 - (Kat m+ ) 1 / m , -SO4 - (Kat m+ ) 1 / m , -PO4 2- (Kat m+ ) 2 / m ,-PO3 2- (Kat m+ ) 2 / m or -NR3R4R5 + (An m- ) 1 / mThe aqueous electrolyte solution contains a compound having at least one redox-active moiety of formula (I), wherein the functional group is selected from the group consisting of:

[0055] Preferred cations Kat are the hydrogen cation, an alkali metal cation, an alkaline earth metal cation or an ammonium cation, in particular the hydrogen cation, the sodium, potassium, magnesium or calcium cation, or a quaternary ammonium cation.

[0056] Preferred anions An are selected from the group of halides, hydroxides, phosphates, sulfates, perchlorates, hexafluorophosphates or tetrafluoroborates.

[0057] Particularly preferred aqueous electrolyte solutions comprise compounds having at least one redox active moiety of formula (I), where Z is a covalent linking group, and the moiety of formula (I) is attached to a polymer backbone selected from the group of polymethacrylates, polyacrylates, polystyrenes, polyalkylene glycols, polyalkyleneimines or polyvinyl ethers, preferably the polymer backbone having from 5 to 100 groups of formula (I).

[0058] Highly preferred aqueous electrolyte solutions include oligomers or polymers having repeating structural units of formula (II) and, optionally, further structural units derived from solubilizing comonomers. [ka] Where: ME is a repeating structural unit derived from a polymerizable monomer, BG is a covalent bond or a cross-linking group, FC is a moiety of the formula (X-C5H4)Fe(Y-C5H3-Z), X, Y and Z have the meanings defined above, r is an integer of 2 to 150, preferably an integer of 2 to 80, and most preferably an integer of 8 to 40.

[0059] The repeating structural units ME and BG form the backbone of an oligomer or polymer comprising multiple units of the redox-active unit of formula (I) above.

[0060] Examples of classes of substances which can form the backbone of the oligomer or polymer are polymers derived from ethylenically unsaturated carboxylic acids or their esters or amides, such as polymethacrylates, polyacrylates, polymethacrylamides or polyacrylamides, polymers derived from ethylenically unsaturated aryl compounds, such as polystyrene, polymers obtained from vinyl esters of saturated carboxylic acids and their derivatives, such as polyvinyl acetates or polyvinyl alcohols, polymers derived from olefins or bicyclic or polycyclic olefins, such as polyethylene, polypropylene or polynorbornene, polyimides derived from imide-forming tetracarboxylic acids and diamines, polymers derived from naturally occurring polymers and their chemically modified derivatives, such as cellulose or cellulose ethers, as well as polyurethanes, polyvinyl ethers, polythiophenes, polyacetylenes, polyalkylene glycols and their derivatives, such as their ethers, such as polyethylene glycol or polyethylene glycol methyl ether, poly-7-oxanorbornene, polysiloxanes, or polyalkyleneimines and their derivatives, such as their amines, such as polyethyleneimine or N,N,N',N'-tetramethyl-polyethyleneimine.

[0061] Below are examples of combinations of structural units ME and bridging groups BG for some of the above classes of materials. [ka]

[0062] Particularly preferred classes of materials forming the backbone (main chain) of the oligomer or polymer are polymethacrylates, polyacrylates, polymethacrylamides, polyacrylamides, polystyrenes, polyethyleneimines and polyvinyl ethers.

[0063] The redox-active units of formula (I) are covalently attached to the polymer backbone.

[0064] The polymers containing the redox-active moieties can take the form of linear polymers, comb and star polymers, dendrimers, ladder polymers, cyclic polymers, polycatenanes and polyrotaxanes.

[0065] The use of comb and star polymers, dendrimers, ladder polymers, cyclic polymers, polycatenanes and polyrotaxanes is preferred, as these types are characterized by high solubility and the viscosity of the resulting solutions is generally lower than that of the corresponding linear polymers.

[0066] The solubility of the polymers containing the redox-active components used according to the invention can be further improved by copolymerization or functionalization with, for example, ethylene glycol, methacrylic acid, acrylic acid or styrene sulfonic acid.

[0067] Preferred solubilizing comonomers are vinyl alcohol, vinyl acetate, methyl vinyl ether, methacrylic acid, acrylic acid, methacrylic acid alkyl esters, acrylic acid ethyl ester, methacrylic acid amides, acrylic acid amides, vinyl sulfonates, vinyl phosphonic acid or styrene sulfonate.

[0068] The redox-active compounds used according to the invention can be prepared by conventional methods. The oligomers and polymers can be prepared using conventional polymerization processes. Examples include bulk polymerization, solution polymerization or emulsion or suspension polymerization. These procedures are well known to those skilled in the art.

[0069] Low molecular weight compounds containing one or more moieties of formula (I) typically have the structure (X-C5H4)Fe(Y-C5H3-)-R where X and Y have the meanings defined above. R is hydrogen or a single- or multiply-bonded organic moiety, which may optionally contain additional (X-C5H4)Fe(Y-C5H3-) moieties.

[0070] Highly preferably, the aqueous electrolyte solution comprises a compound of formula (II) as redox active compound. (X-C5H4)Fe(Y-C5H4) (II) where X and Y have the meanings defined above.

[0071] Furthermore, particularly preferred aqueous electrolyte solutions contain as redox-active compound a compound of formula (III) [(X-C5H4)Fe(Y-C5H3-)]-R7-[(-C5H3-Y)Fe(C5H4-X)] p (III) where X and Y are as defined above, R7 is a divalent to tetravalent organic group; p is an integer from 1 to 4.

[0072] R7 as a divalent to tetravalent organic group is understood to mean an organic moiety which is linked to the remainder of the molecule via 2, 3 or 4 covalent bonds.

[0073] Examples of divalent organic moieties include alkylene, alkyleneoxy, poly(alkylene-oxy), alkylene sulfide, poly(alkylene sulfide), alkyleneamino, poly(alkyleneamino), cycloalkylene, arylene, aralkylene, or heterocyclylene.

[0074] The alkylene group can be either branched or unbranched. The alkylene group typically contains 1 to 20 carbon atoms, preferably 2 to 4 carbon atoms. Examples of alkylene groups are methylene, ethylene, propylene and butylene. The alkylene group can be optionally substituted, for example, with a carboxyl or sulfonic acid group, a carboxyl ester or sulfonic acid ester group, a carboxyl amide or sulfonic acid amide group, a hydroxyl or amino group, or a halogen atom.

[0075] Alkyleneoxy and poly(alkyleneoxy) groups can include both branched and unbranched alkylene groups. The alkylene groups occurring in the alkyleneoxy or poly(alkyleneoxy) groups typically contain 2 to 4 carbon atoms, preferably 2 to 3 carbon atoms. The number of repeating structural units in the poly(alkyleneoxy) group can vary within a wide range. A typical number of repeating structural units is 2 to 50. Examples of alkyleneoxy groups are ethyleneoxy, propyleneoxy and butyleneoxy. Examples of poly(alkyleneoxy) groups are poly(ethyleneoxy), poly(propyleneoxy) and poly(butyleneoxy).

[0076] The alkyleneamino and poly(alkyleneamino) groups can contain both branched and unbranched alkylene groups. The alkylene groups occurring in the alkyleneamino or poly(alkyleneamino) groups typically contain 2 to 4 carbon atoms, preferably 2 to 3 carbon atoms. The number of repeating structural units in the poly(alkyleneamino) group can vary within a wide range. A typical number of repeating structural units is 2 to 50. A typical number of repeating structural units is 2 to 50. Examples of alkyleneamino groups are ethyleneamino, propyleneamino and butyleneamino. Examples of poly(alkyleneamino) groups are poly(ethylene-amino), poly(propyleneamino) and poly(butyleneamino).

[0077] Cycloalkylene groups typically contain 5, 6 or 7 ring carbon atoms, each of which can be substituted independently of the other. An example of a substituent is an alkyl group or two alkyl groups, which can further form a ring with the ring carbon to which they are attached. An example of a cycloalkylene group is cyclohexylene. Cycloalkylene groups can be optionally substituted, for example, with carboxyl or sulfonic acid groups, carboxyl ester or sulfonic acid ester groups, carboxyl amide or sulfonic acid amide groups, hydroxyl or amino groups, or halogen atoms.

[0078] The arylene group is typically a cyclic aromatic group containing 5 to 14 carbon atoms, each of which may be substituted independently of the other. Examples of arylene groups are o-phenylene, m-phenylene, p-phenyl, o-biphenylyl, m-biphenylyl, p-biphenylyl, 1-naphthyl, 2-naphthyl, 1-anthryl, 2-anthryl, 9-anthryl, 1-phenanthryl, 2-phenanthryl, 3-phenanthryl, 4-phenanthryl, or 9-phenanthryl. The arylene group may be optionally substituted, for example, with a carboxyl or sulfonic acid group, a carboxyl ester or sulfonic acid ester group, a carboxyl amide or sulfonic acid amide group, a hydroxyl or amino group, or a halogen atom. Further examples of substituents are an alkyl group or two alkyl groups, which together with the ring carbon atoms to which they are attached may form a further ring.

[0079] Heterocyclylene groups are typically cyclic groups having 4 to 10 ring carbon atoms and at least one ring heteroatom, each of which may be independently substituted. Examples of heteroatoms include oxygen, nitrogen, phosphorus, boron, selenium, or sulfur. Examples of heterocyclylene groups are furandiyl, thiophenediyl, pyrroldiyl, or imidazoldiyl. Preferably, the heterocyclylene group is aromatic. The heterocyclyl group may be optionally substituted with a carboxyl or sulfonic acid group, a carboxyl ester or sulfonic acid ester group, a carboxyl amide or sulfonic acid amide group, a hydroxyl or amino group, or a halogen atom. Further examples of substituents include alkyl groups, i.e., two alkyl groups, which together with the ring carbons to which they are attached may form a further ring.

[0080] An aralkylene group is typically an aryl group to which one or two alkyl groups are covalently bonded. The aralkyl group can be covalently bonded to the rest of the molecule through the aryl moiety and its alkyl moiety, or through two alkyl moieties. The aralkylene group can be substituted on the aromatic ring, for example with alkyl groups or halogen atoms. Examples of aralkylene groups are benzylene or dimethylphenylene (xylylene).

[0081] Examples of R7 as a trivalent organic moiety are alkyltriyl, alkoxytriyl, tris-poly(alkylenoxy), tris-poly(alkyleneamino), cycloalkyltriyl, aryltriyl, aralkyltriyl or heterosyltriyl. These moieties correspond to the double bond moieties already detailed, with the difference that they are attached to the rest of the molecule by three covalent bonds instead of two.

[0082] Examples of R7 as a tetravalent organic moiety are alkylquatanyl, alkoxyquatanyl, quatapoly(alkyleneoxy), quatapoly(alkyleneamino), cycloalkylquatanyl, arylquatanyl, aralkylquatanyl, or heterocyclylquatanyl. These groups correspond to the double bond moieties already detailed, with the difference that this moiety is connected to the rest of the molecule by four covalent bonds instead of two.

[0083] Very particular preference is given to aqueous electrolyte solutions containing a compound with at least one redox-active moiety of formula (I) in which n is 2.

[0084] These compounds have a particularly good combination of water solubility and temperature stability, and are particularly easy to synthesize by treating the cyclopentadienyl ring of ferrocene with butyllithium followed by reaction with ethylene oxide or by reacting vinylferrocene with thiol derivatives.

[0085] In a further embodiment, the present invention relates to a first type of redox flow battery for storing electrical energy, comprising a reaction cell having two electrode chambers for catholyte and anolyte, each of which is connected to at least one liquid reservoir, the electrode chambers being separated by a membrane and equipped with electrodes, each filled with an electrolyte solution comprising a redox active component in liquid form dissolved or dispersed in an aqueous electrolyte solvent and, optionally, a conductive salt dissolved therein, and, optionally, other additives. This redox flow battery is characterized in that the anolyte comprises a water-soluble redox active component and that the catholyte in the electrode chambers comprises an aqueous electrolyte solution according to the above definition.

[0086] The electrolyte solution in the electrode chambers can be coated with paraffin oil to minimize oxygen diffusion.

[0087] A preferred redox flow battery of this first type also comprises an aqueous electrolyte solution according to the above definition in the liquid reservoir(s).

[0088] A preferred redox flow battery of this first type comprises an anolyte that contains, as the redox active component, a compound that contains one or more bipyridyl groups in the molecule.

[0089] These are typically compounds which contain at least one redox-active moiety of formula (IV) or formula (V) in the molecule, preferably 1 to 6, very particularly preferably 1 to 3 groups and very particularly preferably 1 to 2 moieties of formula (IV) or formula (V) in the molecule. [ka] wherein the line extending from the nitrogen atom in the structures of formulas IV and V represents a covalent bond connecting the structures of formulas IV and V to the rest of the molecule; R8 and R9 are, independently of each other, alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, heterocyclyl, halogen, hydroxy, amino, nitro or cyano; An represents an inorganic or organic anion having a valence of m; b and c are each independently an integer from 0 to 4, preferably 0, 1 or 2, in particular 0; m is an integer from 1 to 4; a is a number having the value 2 / m.

[0090] Compounds containing one or more bipyridyl groups in the molecule as redox-active moieties can be low molecular weight organic molecules, oligomers or polymers, which preferably contain at least one moiety of formula (IV) or formula (V) linked to the remainder of the molecule via a covalent bond.

[0091] The redox-active compounds comprising a moiety of formula (IV) or formula (V) are preferably water-soluble, however, they may also be compounds that are dispersible in water.

[0092] The redox-active components preferentially used in the anolytes are compounds of formula IVa or Va [ka] Where: R8, R9, An, a, b, c and m have the meanings defined above; R 10 and R 11 are each independently hydrogen, alkyl optionally substituted with a carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino group, for example a substituted 2-trialkylammoniumethyl group, Cycloalkyl optionally substituted with a carboxylic acid ester, a carboxylic acid amide, a carboxylic acid, a sulfonic acid or an amino group; or Aryl optionally substituted with a carboxylic acid ester, a carboxylic acid amide, a carboxylic acid, a sulfonic acid or an amino group; or means an aralkyl optionally substituted with a carboxylic acid ester, a carboxylic acid amide, a carboxylic acid, a sulfonic acid or an amino group, In particular, C1-C6 alkyl, C1-C6 alkyl substituted with a carboxylic acid ester group, a carboxylic acid amide group, a carboxylic acid group, a sulfonic acid group, or an amino group, very preferably propionate, isobutionate, ethyl or methyl; R 12 and R 13 are independently, independently of each other, hydrogen, alkyl optionally substituted with a carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino group, for example a substituted 2-trialkylammoniumethyl group, Cycloalkyl optionally substituted with a carboxylic acid ester, a carboxylic acid amide, a carboxylic acid, a sulfonic acid or an amino group; or Aryl optionally substituted with a carboxylic acid ester, a carboxylic acid amide, a carboxylic acid, a sulfonic acid or an amino group; or means an aralkyl optionally substituted with a carboxylic acid ester, a carboxylic acid amide, a carboxylic acid, a sulfonic acid or an amino group, In particular, C1-C6 alkyl, or two parts R 12 and R 13 taken together form a C1-C3 alkylene group, In particular, C1-C6 alkyl, C1-C6 alkyl substituted with a carboxylic acid ester group, a C1-C6 alkyl substituted with a carboxylic acid amide group, a C1-C6 alkyl substituted with a carboxylic acid group, a C1-C6 alkyl substituted with a sulfonic acid group, or a C1-C6 alkyl substituted with an amino group; or taken together represent ethylene, very preferably propionate, isobutionate, ethyl or methyl, or taken together represent ethylene.

[0093] Examples of particularly preferred compounds are any anion for charge balancing such as N,N'-di-C1-C4-alkylbipyridyl cations, N,N'-dimethylbipyridyl halides, for example N,N'-dimethylbipyridyl chloride, and the like.

[0094] A preferred redox flow battery of the first type comprises an aqueous electrolyte solution having a temperature of at least 30°C, preferably 30 to 90°C, and especially 30 to 50°C, which comprises a redox active compound comprising at least one moiety of formula (IV) or formula (V).

[0095] In a more preferred redox flow battery of this first type, the electrode chambers for the catholyte and the anolyte are separated by a semi-permeable membrane that is impermeable to the redox couple in the catholyte, and the anolyte contains a zinc salt as the redox active component.

[0096] In a further embodiment, the present invention also relates to a second type of redox flow battery for storing electrical energy comprising a reaction cell having electrode chambers for an electrolyte solution, connected to at least one liquid storage device, the electrode chambers being equipped with a cathode and an anode and filled with an electrolyte solution comprising a redox active component in liquid form dissolved or dispersed in an aqueous electrolyte solvent and optionally a conductive salt dissolved therein and possibly further additives. This redox flow battery is characterised in that the electrolyte solution comprises an aqueous electrolyte solution according to the above definition and that a zinc salt is the further redox active component.

[0097] This second type of redox flow cell is characterized by having only one liquid reservoir and by not needing to use a membrane to separate the reaction cell into an anode and a cathode.

[0098] A preferred redox flow battery of this second type also comprises an aqueous electrolyte solution according to the above definition in a liquid reservoir.

[0099] Preferably, this second type of redox flow cell uses a zinc solid anode having the redox couple zinc(II) / zinc(0).

[0100] Examples of zinc salts for the first and second type of redox flow batteries are zinc chloride, zinc fluoride, zinc bromide, zinc iodide, zinc nitrate, zinc nitrite, zinc bicarbonate, zinc sulfate, zinc perchlorate, zinc tetrafluoroborate and zinc hexafluorophosphate. In addition to zinc salts with inorganic anions, zinc salts with organic anions, such as zinc acetate, zinc oxalate or zinc formate, can also be used.

[0101] In preferred embodiments of the second type and the first type of redox flow battery according to the invention, zinc is used as the redox active anode material. Of particular note are the different physical states that zinc can assume within the battery.

[0102] During the charging process, zinc(II) cations dissolved in the electrolyte are reduced to elemental zinc(0) on the anode surface. For this to happen, two electrons must be absorbed. The active substance zinc is found in the battery in both dissolved and solid form.

[0103] The zinc solid anode may exist permanently as a metallic electrode or may be formed in situ during the charging process of the battery on an electrically conductive surface within the chamber via reduction of zinc cations, which may function primarily as the active material, but may also function secondarily as a conductive additive or as part of a conductive additive mixture.

[0104] During the charging process, zinc(II) cations dissolved in the electrolyte are reduced to elemental zinc(0) on the anode surface. The anode can consist of any electrically conductive material, preferably a metal, in particular zinc or an alloy of zinc. Metallic zinc is deposited on the electrode surface by absorbing two electrons. In the opposite case, metallic zinc is transferred from the electrode surface by releasing two electrons to the zinc ions, which accumulate in the anolyte.

[0105] A particularly preferred redox flow battery according to the invention has a zinc solid anode with the redox couple zinc(II) / zinc(0).

[0106] Compared to conventional organic redox flow batteries, this is due to the reduced production costs. The zinc anode is much more cost-effective to manufacture than known organic anodes. Meanwhile, the redox couple, zinc(lI) / zinc(0), is characterized by a very good stability against external environmental influences, such as sensitivity to oxygen. Conventional systems must be kept in the absence of oxygen, which significantly complicates the construction of the battery and increases its operating costs. By using a zinc anode, this can be completely eliminated.

[0107] Zinc also has a very high overpotential in aqueous media, thus allowing for an extremely high potential window. The potential window should be understood as the voltage (potential range) that can be reached between the cathode and the anode, in the region of the electrochemical voltage series, due to the position of the redox couple, without undesirable side reactions or decomposition of the redox active species, the conductive additives, the electrolyte, or any other components of the whole battery. Conventional aqueous flux batteries are limited to a potential window of around 1.2 V. Beyond this, hydrogen gas would be generated. By using a zinc anode, the potential window can be extended to 2 V or more. This considerably increases the electrical output (power) per battery (cell). Since zinc has a very high overpotential compared to hydrogen, the battery can be operated safely, despite a high voltage of 2 V, without hydrogen at the anode.

[0108] The term "battery" is used in the context of this description in its broadest sense: it may be a single rechargeable electrochemical cell (battery) or a combination of several such electrochemical cells (batteries).

[0109] Particularly preferred redox flow batteries of the invention have a state of charge of the catholyte or catholyte and anolyte of less than 90%, in particular up to 80%, very preferably 70-80%. These redox flow batteries are characterized by a particularly good temperature stability of the electrolyte, in particular of the redox active compound comprising at least one moiety of formula (I) in the electrolyte. The state of charge is measured as described above. Of course, the state of charge of the catholyte or catholyte and anolyte in the RFB according to the invention can also be 100%.

[0110] In addition to the components described above, a redox flow battery according to the present invention can also include other elements or components conventional in such batteries.

[0111] In a redox flow battery according to the present invention, selected redox active components are present in the chamber(s) and are present in the chamber(s) in dissolved, liquid or dispersed form.

[0112] The redox potential of the redox-active component can be measured, for example, by cyclic voltammetry, a method known to those skilled in the art (see Allen J. Bard, Larry R. Faulkner, "Electrochemical Methods: Fundamentals and Applications", 2001, 2nd edition, John Wiley & Sons; Richard G. Compton, Craig E. Banks, "Understanding Voltammetry", 2010, 2nd edition, Imperial College Press).

[0113] A first type of redox flow battery according to the present invention includes a semipermeable or microporous membrane, which serves the following functions: Separation of anode and cathode compartments Retention of redox active components in the catholyte and anolyte, i.e. retention of cathode and anode active materials · The permeability of the conductive salt of the charge-balancing electrolyte, i.e. the anions and / or cations of the conductive salt or the charge carriers in the electrolyte in general.

[0114] The membrane may be a size-exclusion membrane, e.g., a dialysis membrane, but may also be an ion-selective membrane. The membrane prevents the passage of redox-active ferrocene compounds to the anode compartment and prevents the passage of redox-active components of the anolyte to the cathode compartment. The passage of dissolved zinc(II) cations need not, but may, be inhibited by the membrane.

[0115] The material of the membrane can be made of plastic, ceramic, glass, metal or fiber sheet depending on the application. Examples of materials are organic polymers, cellulose or modified cellulose, such as cellulose ethers or cellulose esters, polyethersulfone, polysulfone, polyvinylidene fluoride, polyester, polyurethane, polyamide, polypropylene, polyvinyl chloride, polyacrylonitrile, polystyrene, polyvinyl alcohol, polyphenylene oxide, polyimide, polytetrafluoroethylene and their derivatives, or even ceramics, glass or felt. Membranes made of multiple materials (composite) are also possible.

[0116] The membrane and the resulting redox flow battery can be used in various configurations. Examples of this are flat membranes, pocket filter designs and wide modules. These embodiments are known to those skilled in the art. Flat membranes are preferred.

[0117] The thickness of the membranes used according to the invention can vary within wide limits: typical thicknesses are in the range of 0.1 μm to 5 mm, particularly preferably between 10 μm and 200 μm.

[0118] In addition to the above redox active components, electrolyte and optionally a membrane, the redox flow battery according to the present invention preferably comprises the following further components: · Means of transport, e.g. pumps etc., as well as tanks and tubes for the transport and storage of redox-active components an electrode, preferably composed of graphite, graphite fleece, graphite paper, carbon nanotube carpet, activated carbon, carbon black or graphene; Optionally a current conductor, for example a current conductor made of graphite or metal

[0119] The negative electrode preferably comprises zinc and may well comprise, for example, the following materials: Stainless steel, Hastelloy or alloys containing iron-chromium-nickel, graphite, graphite fleece, graphite paper, carbon nanotube carpet, activated carbon, soot or graphene.

[0120] The redox flow battery according to the present invention comprises, as further optional but preferred components, current conductors, which will establish the best possible electrical contact between the electrode materials and an external current source or sink.

[0121] As current conductors in the redox flow battery according to the invention, aluminum, alumina alloys, alloys, copper, stainless steel, Hastelloy, iron-chromium-nickel alloys, precious metal-coated titanium or tantalum, in particular platinum and / or iridium and / or ruthenium oxide-coated titanium, niobium, tantalum, hafnium or zirconium can be used.

[0122] Current conductors are all components that allow the transmission of electrical current to the electrodes. The redox reactions take place on the electrodes (cathode and anode) that are in direct contact with the electrolyte.

[0123] The redox flow battery according to the invention can be used in a wide variety of fields. In the broadest sense, it can be the storage of electrical energy for stationary and mobile applications. The invention also relates to the use of the redox flow battery for these purposes.

[0124] Examples of its applications include stationary storage for emergency power supply, peak load compensation, and use with electrical energy from renewable energy sources, particularly solar, hydroelectric, wind, or gas, coal, biomass, tidal, and marine power.

[0125] The redox flow batteries according to the present invention can themselves be connected together in series or in parallel in known manner. The present invention also includes the following items. [Item 1] An aqueous electrolyte solution having a temperature of at least 30° C., comprising a compound having at least one redox active moiety of formula (I), (XC 5 H 4 )Fe(YC 5 H 3 -Z) (I) X is a group of the formula -(C n H 2n )-FG or formula -(C n H 2n )-Sp-(C n H 2n )-FG or formula -(C n H 2n )-Brgp-, Y is hydrogen or a group of the formula -(C n H 2n )-FG or formula -(C n H 2n )-Sp-(C n H 2n )-FG portion, Z is hydrogen or a covalent bond connecting the moiety of formula (I) to the remainder of the molecule; FG is -OH, -SH, -NO 3 , -NO 2 , -CN, -OR 1 , -SR 1 , -(O-CH 2 -CH 2 ) o -OR 2 , -(O-CH 2 -CH 2 )o -NR 3 R 4 R 5 + (An m- ) 1 / m , -COR 2 , -COO - (Kat m+ ) 1 / m , -COOR 2 , -SO 3 - (Kat m+ ) 1 / m , -SO 3 R 2 , -SO 4 - (Kat m+ ) 1 / m , -SO 4 R 2 , -PO 4 2- (Kat m+ ) 2 / m , -PO 4 (R 2 ) 2 , -PO 3 2- (Kat m+ ) 2 / m , -PO 3 (R 2 ) 2 , -NR 3 R 4 R 5 + (An m- ) 1 / m 、-N + R 3 R 4 -C t H 2t -SO 3 - Or, -NR 2 -SO 2 -R 3 and Brgp is a divalent linking bridging group connecting the moiety of formula (I) to the remainder of the molecule; Sp is -O-, -S-, -SO- or -SO2 - and R 1 is C 1 -C 4 Alkyl, R 2 is hydrogen or C 1 -C 4 is alkyl, R 3 、R 4 and R 5 are each independently hydrogen or alkyl, Kat is a cation with a valence of m, An is an anion with a valence of m, m is an integer from 1 to 4; n is an integer from 2 to 4, t is an integer from 2 to 5, and o is an integer of 1 to 50, preferably 3 to 20; The aqueous electrolyte solution. [Item 2] 2. The aqueous electrolyte solution according to item 1, characterized in that it has a temperature of 30 to 90°C, preferably 30 to 50°C. [Item 3] 3. The aqueous electrolyte solution according to claim 1, further comprising at least one conductive salt. [Item 4] 4. The aqueous electrolyte solution according to at least one of items 1 to 3, characterized in that the state of charge is less than 90%. [Item 5] 5. The aqueous electrolyte solution according to at least one of items 1 to 4, characterized in that Y is hydrogen and m is 1 or 2. [Item 6] FG is -(O-CH 2 -CH 2 ) o -OR 2, -COR 2 , -COO - (Kat m+ ) 1 / m ,-SO 3 - (Kat m+ ) 1 / m , -SO 4 - (Kat m+ ) 1 / m , -PO 42- (Kat m+ ) 2 / m ,-PO 3 2- (Kat m+ ) 2 / m or -NR 3 R 4 R 5 + (An m- ) 1 / m The aqueous electrolyte solution according to at least one of items 1 to 5, characterized in that the functional group is selected from [Item 7] 7. The aqueous electrolyte solution according to at least one of items 1 to 6, characterized in that Kat is a hydrogen cation, an alkali metal cation, an alkaline earth metal cation or an ammonium cation, in particular a hydrogen cation, a sodium cation, a potassium cation, a magnesium cation or a calcium cation, or a quaternary ammonium cation. [Item 8] 8. The aqueous electrolyte solution according to at least one of items 1 to 7, characterized in that An is selected from the group consisting of halide ions, hydroxide ions, phosphate ions, sulfate ions, perchlorate ions, hexafluorophosphate ions and tetrafluoroborate ions. [Item 9] 9. The aqueous electrolyte solution according to at least one of items 1 to 8, characterized in that Z is a covalent linking group, and the moiety of formula (I) is attached to a polymer backbone selected from the group of polymethacrylates, polyacrylates, polystyrenes, polyalkylene glycols, polyalkyleneimines or polyvinyl ethers, preferably the polymer backbone having 5 to 100 groups of formula (I). [Item 10] characterised in that it comprises an oligomer or polymer having repeating structural units of formula (II) and, optionally, further structural units derived from solubilising comonomers,

change

[0126] [example] The following examples illustrate the invention without limiting it.

[0127] Example 1: Synthesis of ferrocene-bis-sulfonic acid [ka] The synthesis of ferrocene-bis-sulfonic acid was performed according to a modified literature procedure (K. Chanawanno, C. Holstrom, LA Crandall, H. Dodge, VN Nemykin, RS Herrick, CJ Ziegler, Dalton Trans. 2016, 45, 14320) as follows.

[0128] Ferrocene (8.00 g, 44 mmol) was dissolved in acetic anhydride (100 mL). Chlorosulfonic acid (5.7 mL, 86 mmol) was slowly added. The reaction mixture was stirred at room temperature for 20 h under argon atmosphere. The suspension was then filtered under argon atmosphere. The filtrate was washed with acetic anhydride. The product was then dried. A portion of the dried product (3.46 g, 8.5 mmol) was dissolved in ethanol (10 ml). A freshly prepared 1 M NaOH solution (10 mL) from NaOH (4.0 g, 100 mmol) in ethanol (100 mL) was then slowly added. The solution was stirred at room temperature for 30 min. The precipitate was collected on a filter, washed with ethanol and then dried under reduced pressure. Thus, the sodium salt of ferrocene-bis-sulfonic acid was obtained as a yellow solid (2.1 g, 4.7 mmol).

[0129] Example 2 Stability test of ferrocene-bis-sulfonic acid (comparison) Ferrocene from Example 1 was oxidized with Oxone® to investigate the stability of the "charged" molecule. The charged electrolyte shows irreversible precipitation after 3 days of storage at 60° C. (even with the exclusion of air).

[0130] Example 3: Examination of temperature stability of electrolyte solutions [ka]

[0131] Preparation of electrolyte: Catholyte (catholyte): 400 mM solution of ferrocene derivative (1.77 g, 4 mmol) in deionized water (10 mL) Anolyte (anolyte): 600 mM solution of viologen (3.00 g, 6 mmol) in deionized water (10 mL)

[0132] Battery test: Electrolyte solution is applied to a battery with a surface area of ​​5 cm 2 The separation was achieved by a Fumasep FAA-3-50 anion exchange membrane from Sigma-Aldrich. Graphite plates with graphite fleece served as electrodes. A continuous pump pumped the electrolyte solution through a tube into a reservoir tank placed in a heated sand bath and then through the half-cell.

[0133] The first three cycles allowed charging to 101.1, 100.3 and 100.1 mAh, with corresponding discharges of 100.6, 100.5 and 100.4 mAh.

[0134] After constant cycling for about 12 days, 99.4, 99.3 and 99.3 mAh were charged and 99.6, 99.6 and 99.5 mAh were discharged, respectively.

[0135] Experiments were also carried out with a sand bath heated to 60 ° C, in which both storage tanks of electrolyte were placed. For this purpose, a solution was used that was cycled for almost 19 days: 99.0 mAh was charged and 98.5 mAh was discharged after 24 hours.

[0136] Ferrocene from Example 3 can be obtained commercially, for example, from TCI Chemicals. The synthesis of this compound is shown in ACS Energy Lett. 2017, 2, 639-644.

[0137] The synthesis of viologen derivatives from Example 3 is also shown in ACS Energy Lett. 2017, 2, 639-644.

[0138] Example 4: Synthesis of functionalized ferrocenes [ka] Vinylferrocene (100 mg, 0.47 mmol) was dissolved in tetrahydrofuran (2 mL) in a microwave glass (5 mL). 2,2-Dimethoxy-2-phenylacetophenone (DMPA, 3 mg, 0.01 mmol) was then added. The microwave glass was then fitted with a stir bar, sealed with a Teflon septum, and the clear orange solution was flushed with argon for 15 min. 2-Mercaptopropanoic acid (50 μL, 0.56 mmol) was then added via microsyringe. The solution was then placed in a beaker on a stir plate in a UV chamber (365 nm) and irradiated for 20 min. Afterwards, the solution was allowed to cool for 10 min. Irradiation was continued for a total of 120 min. The solvent was removed and the crude product was dissolved in dichloromethane and purified by column chromatography (silica, 2 / 8 EtOAc / hexane). After removal of the solvent by distillation, the crude product was dried under high vacuum. The product was a viscous orange oil.

[0139] Example 5: Synthesis of ferrocene polymers [ka] Ferrocene-containing copolymers were produced by free radical polymerization of ferrocene monomer and sulfobetaine methacrylate. Ferrocene monomer was obtained by reacting hydroxyethylferrocene with methacrylic acid chloride.

[0140] Example 6: Synthesis of hydroxyethylferrocene [ka] Ferrocene was dissolved in methyl THF and reacted with tert-butyl lithium. Ethylene oxide was then added. The product could then be obtained after separation by column chromatography.

[0141] Example 7: Synthesis of ferrocene polymers [ka] Copolymers of METAC ([2-(methacryloyloxy)ethyl]trimethylammonium chloride) and ferrocene-containing methacrylamide with ferrocene monomer compositions of 10-30% can be produced by free radical polymerization with molecular weights of around 10,000 g / mol (Mn). Aqueous electrolytes based on this polymer were temperature stable at 60°C and showed no degradation (i.e., loss of capacity) over 100 cycles.

Claims

1. 1. An aqueous electrolyte solution having a temperature of at least 30° C. comprising a redox-active compound of formula (I) or (II) or an oligomeric or polymeric redox-active compound having repeating structural units of formula (III) and optionally further structural units derived from solubilizing comonomers, (X-C ５ H ４ )Fe(Y-C ５ H ４ ) (I) [(X-C ５ H ４ )Fe(Y-C ５ H ３ -)]-R ７ -[(-C ５ H ３ -Y)Fe(C ５ H ４ -X)] ｐ (II) 【Chemistry 1】 Where: X is a group represented by the formula -(C ｎ H ２ｎ )-FG or formula -(C ｎ H ２ｎ )-Sp-(C ｎ H ２ｎ )-FG group, Y is hydrogen or a group of the formula -(C ｎ H ２ｎ )-FG or formula -(C ｎ H ２ｎ )-Sp-(C ｎ H ２ｎ )-FG group, FG is -OH, -SH, -OR １ , -SR １ , -(O-CH ２ -CH ２ ) ｏ -OR ２ , -(O-CH ２ -CH ２ ) ｏ -NR ３ R ４ R ５ ＋ (An ｍ－ ) １／ｍ , -COR ２ , -COO － (Kat ｍ＋ ) １／ｍ , -SO ３ － (Kat ｍ＋ ) １／ｍ , -SO ４ － (Kat ｍ＋ ) １／ｍ , -SO ４ R ２ , -PO ４ ２－ (Kat ｍ＋ ) ２／ｍ , -PO ４ (R ２ ) ２ , -PO ３ ２－ (Kat ｍ＋ ) ２／ｍ , -PO ３ (R ２ ) ２ , -NR ３ R ４ R ５ ＋ (An ｍ－ ) １／ｍ , -N ＋ R ３ R ４ -C ｔ H ２ｔ -SO ３ － Or, -NR ２ -SO ２ -R ３ and Brgp is a divalent linking bridging group connecting the moiety of formula (I) to the remainder of the molecule; Sp is -O-, -S-, -SO- or -SO ２ - and R １ is C １ -C ４ is alkyl, R ２ is hydrogen or C １ -C ４ is alkyl, R ３ , R ４ and R ５ are each independently hydrogen or alkyl, Kat is a cation having a valence of m; An is an m-valent anion, m is an integer from 1 to 4, n is an integer from 2 to 4, t is an integer from 2 to 5, and o is an integer from 1 to 50, preferably from 3 to 20; R ７ is a divalent to tetravalent organic group, p is an integer from 1 to 4; ME is a repeating structural unit derived from a polymerizable monomer, BG is a covalent bond or a cross-linking group; FC is a compound represented by the formula (X'-C ５ H ４ )Fe(Y-C ５ H ３ -Z) group, Z is hydrogen; X′ is a group represented by the formula -(C ｎ H ２ｎ )-Brgp- group, Brgp is a divalent linking bridging group connecting the moiety of formula (I) to the remainder of the molecule; r is an integer from 2 to 150; The aqueous electrolyte solution.

2. 2. The aqueous electrolyte solution according to claim 1, characterized in that it has a temperature of 30 to 90°C, preferably 30 to 50°C.

3. 3. Aqueous electrolyte solution according to claim 1, characterized in that it contains at least one conductive salt.

4. Aqueous electrolyte solution according to at least one of claims 1 to 3, characterized in that the state of charge is less than 90%.

5. 5. Aqueous electrolyte solution according to claim 1, characterized in that Y is hydrogen and m is 1 or 2.

6. FG is -(O-CH ２ -CH ２ ) ｏ -OR ２， -COR ２ , -COO － (Kat ｍ＋ ) １／ｍ , -SO ３ － (Kat ｍ＋ ) １／ｍ , -SO ４ － (Kat ｍ＋ ) １／ｍ , -PO ４ ２－ (Kat ｍ＋ ) ２／ｍ , or -PO ３ ２－ (Kat ｍ＋ 6. The aqueous electrolyte solution according to claim 1, wherein the functional groups are selected from the group consisting of

7. 7. The aqueous electrolyte solution according to claim 1, wherein Kat is a hydrogen, an alkali metal, an alkaline earth metal or an ammonium cation, in particular a hydrogen, a sodium, a potassium, a magnesium or a calcium cation, or a quaternary ammonium cation.

8. 8. Aqueous electrolyte solution according to claim 1, characterized in that An is selected from the group of halides, hydroxides, phosphates, sulfates, perchlorates, hexafluorophosphates or tetrafluoroborates.

9. An aqueous electrolyte solution according to claim 1, characterized in that the ME is a polymer backbone selected from the group of polymethacrylates, polyacrylates, polystyrenes, polyalkylene glycols, polyalkyleneimines or polyvinyl ethers, preferably the polymer backbone having 5 to 100 groups of formula (I).

10. 10. Aqueous electrolyte solution according to claim 1, characterized in that n is 2.

11. The redox-active compound is a compound of formula (I), X is a group of formula -(C n H 2n )-OH or -(C n H 2n )-S-(C n H 2n )-FG; Y is hydrogen; and, 11. The aqueous electrolyte solution according to claim 1, wherein FG and n are defined as defined in claim 1.

12. 1. A redox flow battery for storing electrical energy comprising a reaction cell having two electrode chambers for catholyte and anolyte, each of which is connected to at least one liquid reservoir, the electrode chambers being separated by a membrane and equipped with electrodes, each filled with an electrolyte solution comprising a redox active component in liquid form dissolved or dispersed in an aqueous electrolyte solvent and optionally a conductive salt dissolved therein, and optionally further additives, 12. The redox flow battery, characterized in that the anolyte comprises a water-soluble redox active component and the catholyte in the electrode chamber comprises an aqueous electrolyte solution according to any one of claims 1 to 11.

13. 13. The redox flow battery according to claim 12, wherein the anolyte contains a compound having one or more bipyridyl groups in the molecule as a redox active component.

14. 14. The redox flow battery of claim 13, wherein the electrode chambers for the catholyte and the anolyte are separated by a semi-permeable membrane impermeable to the redox couple in the catholyte, the anolyte comprising a zinc salt as the redox active component.

15. A redox flow battery for storing electrical energy, comprising a reaction cell having electrode chambers for an electrolyte solution, connected to at least one liquid storage device, the electrode chambers being equipped with a cathode and an anode and filled with an electrolyte solution comprising a redox active component in liquid form dissolved or dispersed in an aqueous electrolyte solvent and, optionally, a conductive salt dissolved therein, and optionally further additives, 12. The redox flow battery, characterized in that the electrolyte solution comprises an aqueous electrolyte solution according to at least one of claims 1 to 11 and, as further redox active component, a zinc salt.

16. 16. The redox flow battery of claim 15, comprising a zinc solid anode having the redox couple zinc(II) / zinc(0).

17. 17. Redox flow battery according to at least one of claims 12 to 16, characterized in that the state of charge of the catholyte or of the catholyte and the anolyte is less than 90%.

18. 18. Use of the redox flow battery according to at least one of claims 12 to 17 for storing stationary and mobile electrical energy, in particular for stationary storage for emergency power supplies, peak load compensation, intermediate storage of electrical energy from renewable energy sources, in particular photovoltaic, hydroelectric, wind or gas, coal, biomass, tidal and marine power.

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