Aqueous electrolyte, redox flow battery and use thereof

Abstract

An aqueous electrolyte solution having a temperature of at least 30° C. is disclosed, comprising a compound (X-C5H4)Fe(Y-C5H3-Z) (I) having at least one redox-active residue of formula (I), wherein X is a compound of 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- residue, Y is hydrogen or formula -(C n H 2n )-FG or formula-(C n H 2n )‑Sp‑(C n H 2n )-FG residue, Z is hydrogen or a covalent bond connecting the residue of formula (I) to the rest of the molecule, and FG is a functional group selected from the group consisting of: -OH, -NO3, -NO2, -CN, -OR1, -(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 + R3R4C t H 2t ‑SO3 ‑Or -NR2-SO2-R3, Brgp is a divalent bridging group connecting the residue of formula (I) to the rest 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 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 between 1 and 50, preferably 3 and 20. The electrolyte can be used in a redox flow battery and is characterized by a high stability of the redox-active compound at elevated temperatures.

Description

[0001] The present invention relates to aqueous electrolytes and redox flow batteries containing the same. Redox flow batteries, also known as redox flux batteries (hereinafter also referred to as "RFBs"), are used to store electrical energy based on electrochemical redox reactions. Redox flow batteries contain a chamber or two polarity-specific chambers (half-cells) separated by a membrane, which are filled with a liquid and fed by a pump from one or more separate tanks of any size. The corresponding liquid contains water in which a redox-active substance and an inorganic or organic salt (conductive additive) are dissolved. The conductive additive may also be redox-active itself.

[0002] Due to their excellent scalability, RFBs are particularly suitable as stationary energy storage systems for various performance and capacity requirements. For example, as buffer systems for renewable energy in the private (e.g. single-family or multi-family) and industrial sectors (e.g. wind and solar power plants). They therefore have great potential for ensuring grid stability and for decentralized energy supply. However, mobile applications (electric vehicles, trucks or ships) are also conceivable.

[0003] Existing RFBs are electrochemical energy storage devices. The substances required for setting the potential at the electrodes are liquid, dissolved or particulate redox-active substances, which are transferred to their respective other redox phases during the charging or discharging process in the electrochemical reactor. To this end, the electrolyte solution (catholyte, anolyte) is taken out of the tank and actively pumped to the electrodes. The anode chamber and the cathode chamber are separated in the reactor by a semipermeable membrane, which usually exhibits high selectivity for protons. As long as the electrolyte solution is pumped in, the current can be drawn. The charging process is a simple reversal of the process. Therefore, the amount of energy that can be stored in the RFB is proportional to the size of the storage tank. On the other hand, the extractable power is a function of the size of the electrochemical reactor.

[0004] RFBs have a complex system technology that roughly corresponds to that of fuel cells. The typical size range for a single reactor is about 2-50 kW. The reactors can be combined very easily in a modular manner and the tank size can also be adjusted almost arbitrarily.

[0005] Various redox-active compounds have been proposed for use in accumulators or batteries, such as RFBs. An overview of the prior art can be found, for example, in the article "Redox Species of Redox Flow Batteries: A Review" by Feng Pan and Quing Wang in Molecules 2015, 20, 20499-20517 or in the article by Winsberg et al. in Angew. Chem. 2017, 36, 686-711. RFBs containing ferrocene as cathode electrolyte are known.

[0006] Ferrocene (bis(η 5 -cyclopentadienyl)iron(II), [Fe(η 5 -C5H5)2]) is a yellow-orange compound that is easily soluble in organic solvents and poorly soluble in water. Ferrocene is one of the most important representatives of the parent compound and metallocene and can be prepared, for example, by reacting iron(II) chloride with sodium cyclopentadienyl. Due to its aromatic properties, ferrocene can undergo electrophilic substitution and can therefore be functionalized in a variety of ways. For use in aqueous electrolytes, ferrocene must be functionalized by introducing polar groups. An overview of the many possibilities for functionalizing ferrocene is given in the paper by A. Salmon entitled "Synthesis and electrochemistry of functionalferrocenyl and multiferrocenyl compounds" Bielefeld (2001).

[0007] RFBs containing functionalized ferrocene in the electrolyte are also known from WO 2018 / 032003 A1 and from US 2018 / 0072669 A1.

[0008] So far, in the literature RFBs have been used mainly at room temperature. In the scientific literature, there is little evidence of studies at elevated temperatures (e.g. 40-50°C), and there are no systematic studies of the behavior of aqueous electrolytes containing ferrocene (derivatives) at elevated temperatures. Commercially available vanadium RFBs (hereinafter also referred to as "V-RFBs") are known to cause problems at temperatures greater than 40°C, because the electrolyte precipitates irreversibly.

[0009] V-RFBs are therefore limited by their application temperatures or the cooling required. Special applications such as replacement of diesel generators on mobile phone masts in India therefore abandon V-RFBs. Special HCl-containing electrolytes have been developed for V-RFBs, which are more temperature-stable and therefore allow temperatures greater than 40° C., and in some cases up to about 50 to 60° C. (see US 2012 / 0077067 A1). However, these electrolytes are extremely corrosive. Other RFB systems have not yet been described at these relatively high temperatures.

[0010] In particular, RFBs containing organic redox-active compounds have not yet been used at these high temperatures. This is complicated by the fact that many electrolytes containing organic redox-active compounds are unstable at elevated temperatures. In this context, it should also be noted that the stability of many organic redox-active materials even at room temperature has been misjudged so far (M.-A. Goulet, M. A. Ziz, J. Electrochem. Soc. 2018, 165, A1466-A1477). Simple charge / discharge tests performed so far do not provide any information about the aging behavior of the electrolyte. Therefore, the RFB containing N, N, N-2, 2, 6, 6-heptamethylpiperidinyloxy-4-ammonium chloride (TEMPTMA), which is described as the most stable organic redox-active compound, is also affected by these decomposition processes. At elevated temperatures, side reactions are further promoted and lead to a rapid loss of RFB capacity.

[0011] It has now been found that functionalized ferrocene derivatives surprisingly show high stability at elevated temperatures. These redox-active compounds allow the preparation of aqueous electrolyte solutions that are stable at elevated temperatures. In addition, these redox-active compounds allow the preparation of RFBs that can operate at elevated temperatures. Higher temperatures of the electrolyte may occur during operation of the RFB in an environment with elevated temperatures, for example during operation in tropical or subtropical regions, and / or also during storage of the electrolyte solution at elevated temperatures, and / or also during high power tolerance during charge / discharge operations in the electrode compartment.

[0012] It has been found that ferrocenes functionalized with solubility-promoting groups attached to the cyclopentadienyl ring via a spacer having two to four carbon atoms can achieve the desired combination of water solubility and sufficient temperature stability. It has also been shown that ferrocenes in which the cyclopentadienyl ring is directly substituted with a solubility-promoting group or a C1 spacer do not exhibit the desired stability.

[0013] By using the electrolyte according to the invention, RFBs with organic redox-active compounds become usable for "high temperature" applications. In this way, new fields of application for RFBs can be opened up without the need for additional cooling.

[0014] The present invention is based on the object of providing aqueous electrolytes containing redox-active ferrocene derivatives which exhibit sufficient stability for battery or accumulator operation even at elevated temperatures.

[0015] It is a further object of the present invention to provide a redox flow battery that can be operated at elevated temperatures without the electrolyte used undergoing significant decomposition.

[0016] This object is solved by providing an aqueous electrolyte solution as described in claim 1 .

[0017] 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 residue of formula (I)

[0018] (X-C5H4)Fe(Y-C5H3-Z) (I),

[0019] Where X is 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- residues,

[0020] Y is hydrogen or of the formula -(C n H 2n )-FG or formula-(C n H 2n )-Sp-(C n H 2n )-FG residues,

[0021] Z is hydrogen or a covalent bond connecting the residue of formula (I) to the rest of the molecule,

[0022] FG is a functional group selected from the following: -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 -NR2-SO2-R3,

[0023] Brgp is a divalent bridging group connecting the residue of formula (I) to the rest of the molecule,

[0024] Sp is -O-, -S-, -SO- or -SO2-,

[0025] R1 is C1-C4 alkyl, preferably methyl,

[0026] R2 is hydrogen or C1-C4-alkyl, preferably hydrogen or methyl, and especially hydrogen,

[0027] R3, R4 and R5 independently of one another represent hydrogen or alkyl, preferably C1-C4-alkyl, and in particular methyl, ethyl, propyl or butyl,

[0028] Kat is an m-valent inorganic or organic cation,

[0029] An is an m-valent inorganic or organic anion,

[0030] m is an integer between 1 and 4,

[0031] n represents an integer between 2 and 4,

[0032] t is an integer between 2 and 5, and

[0033] o is an integer of 1-50, preferably 3-20.

[0034] The redox-active compounds used in the electrolyte solution according to the invention may be low molecular weight organic molecules, oligomers or polymers. These molecules may be dissolved in the electrolyte solution or may be dispersed or suspended as particles, for example as a dispersion, microgel or as a nanogel. The redox-active compound contains at least one residue of formula (I), wherein Z represents hydrogen or is linked to the rest of the molecule via a covalent bond Z.

[0035] Preferably, the redox-active compound containing a residue of the formula (I) used in the electrolyte solution according to the invention is soluble in water. However, it may also be a compound that is dispersible in water.

[0036] The water solubility of a compound is understood in the context of the present specification to mean the solubility of at least 1 g of the compound in 1 L of water at 25°C.

[0037] In the context of the present description, low molecular weight molecules are understood to mean compounds which have no repeating structural units derived from monomers and contain at least one, preferably one to six, particularly preferably one to four, in particular one to three and very particularly preferably one or two residues of the formula (I).

[0038] In the context of the present specification, an oligomer molecule is understood to be a compound having two to ten repeating structural units derived from monomers, each of which carries a residue of formula (I). The residue of formula (I) may also be linked to the oligomer backbone via a spacer.

[0039] In the context of this specification, a polymer molecule is a compound having more than ten, preferably eleven to fifty repeating structural units derived from monomers, each of which carries a residue of formula (I). The residue of formula (I) may also be linked to the polymer backbone via a spacer.

[0040] In a preferred embodiment, the electrolyte solution according to the invention contains redox-active components having one to six, preferably one to four, in particular one to three and very preferably one to two residues of the formula (I) in the molecule.

[0041] Examples of preferably used low molecular weight oligomeric and polymeric redox-active compounds are given below.

[0042] The molar mass of the redox-active compound containing at least one residue of the formula (I) used in the electrolyte according to the invention can vary over a wide range. Particularly preferred are redox-active compounds containing residues of the formula (I) having a molecular weight in the range of 150 to 80,000 g / mol, preferably in the range of 250 to 50,000 g / mol and very preferably in the range of 500 to 20,000 g / mol.

[0043] The viscosity of the electrolyte used according to the invention is generally in the range of from 1 mPas to 10 3 mPas, particularly preferably 1 to 10 2 mPas and most preferably in the range of 1 to 20 mPas (measured at 25° C. using a rotational viscometer (plate / plate)).

[0044] The redox-active compounds used according to the invention are characterized in that at least one group X is present on at least one cyclopentadienyl ring. The group X consists of an alkylene group having two to four carbon atoms as a spacer and a solubility-promoting group FG, or of an alkylene-oxy-alkylene group having two to four carbon atoms as a spacer and a solubility-promoting group FG, or of an alkylene-thio-alkylene group having two to four carbon atoms as a spacer and a solubility-promoting group FG, or of an alkylene spacer and a divalent bridging group Brgp which connects the residue of the formula (I) to the rest of the molecule.

[0045] The bridging group Sp is oxygen, sulfur or -SO- or -SO2-, in particular -O-.

[0046] The alkylene spacer may be a residue of the formula: -CH2-CH(CH3)-, -(CH2)3-, -(CH2)4- or in particular -(CH2)2-.

[0047] The alkylene-oxy-alkylene spacer may be a residue 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-.

[0048] The alkylene-thio-alkylene spacer may be a residue 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-.

[0049] The FG group is described in detail above.

[0050] The divalent bridging group Brgp can be any divalent residue that connects the residue of formula (I) to the rest of the molecule, for example to the polymer backbone. Examples of bridging groups Brgp are alkylene, -O-, -S-, -SO-, -SO2-, -CO-, -CO-O-, -NR2-, -CO-NR2- and SO2NR2.

[0051] In addition to the radical X, the residue of the formula (I) in the redox-active compounds to be used according to the invention may contain on the cyclopentadienyl ring a radical of the formula -(C n H 2n )-FG or formula-(C n H 2n )-Sp-(C n H 2n )-FG(residue Y). However, Y is preferably hydrogen.

[0052] The electrolyte solution according to the invention contains water or water and an organic solvent and the above-mentioned redox-active compound (one or more) and optionally other substances dissolved therein. These substances play a role in balancing the charge during the charging or discharging process of the battery or have a positive influence on the stability or performance parameters of the battery. The substances responsible for charge balancing are called conductive additives or conductive salts, and the substances that have a positive influence on the stability or performance parameters are called auxiliary additives.

[0053] Examples of electrolyte solvents are water or mixtures of water with alcohols (e.g., ethanol), carbonates (e.g., propylene carbonate), nitriles (e.g., acetonitrile), amides (e.g., dimethylformamide, dimethylacetamide), sulfoxides (e.g., dimethyl sulfoxide), 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), and inorganic acids (e.g., sulfuric acid, hydrogen halides, or hydrohalic acids). Preferred are water or mixtures of water with carbonates (e.g., propylene carbonate) or with nitriles (e.g., acetonitrile). Water is particularly preferred.

[0054] Conductive additives are usually organic or inorganic salts. Examples for this are salts containing anions selected from halides (fluoride, chloride, bromide, iodide), hydroxide, inorganic acid anions (e.g. phosphate, sulfate, nitrate, hexafluorophosphate, tetrafluoroborate, perchlorate, chlorate, hexafluoroantimonate, hexafluoroarsenate, cyanide) or anions of organic acids (e.g. acetate, formate, trifluoroacetate, trifluoromethanesulfonate, pentafluoroethanesulfonate, nonafluorobutanesulfonate, butyrate, citrate, fumarate, glutarate, lactate, malate, malonate, oxalate, pyruvate, tartrate). Particularly preferred are chloride and fluoride ions, hydroxide ions, phosphate ions, sulfate ions, perchlorate ions, hexafluorophosphate ions and tetrafluoroborate ions; and cations selected from the group consisting of hydrogen ions (H + ), alkali metal or alkaline earth metal cations (e.g. lithium, sodium, potassium, magnesium, calcium), zinc, iron, and substituted or unsubstituted ammonium cations (e.g. tetrabutylammonium, tetramethylammonium, tetraethylammonium), wherein the substituents may generally be alkyl groups. Hydrogen ions, lithium ions, sodium ions, potassium ions, tetrabutylammonium ions and mixtures thereof are particularly preferred. In particular, the 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-Bu4NBF4, n-Bu4NClO4, NH4Cl and mixtures thereof, wherein n-Bu represents an n-butyl group.

[0055] Particularly preferred electrolyte solutions according to the invention contain a conductive additive comprising anions selected from the group consisting of halide ions, hydroxide ions, phosphate ions, sulfate ions, perchlorate ions, hexafluorophosphate ions or tetrafluoroborate ions, in particular a conductive additive consisting of these anions and a cation selected from the group consisting of hydrogen ions, alkali metal or alkaline earth metal cations, and substituted or unsubstituted ammonium cations.

[0056] Examples of auxiliary additives are surfactants, viscosity modifiers, biocides, buffers, stabilizers, catalysts, conductive additives, antifreeze agents, temperature stabilizers and / or defoamers.

[0057] The surfactant may be nonionic, anionic, cationic or amphoteric. Nonionic surfactants (e.g. polyalkylene glycols, fatty alcohol propoxylates, alkyl glucosides, alkyl polyglucosides, octylphenol ethoxylates, nonylphenol ethoxylates, saponins, phospholipids) are particularly preferred.

[0058] Examples of buffers include carbonate-bicarbonate buffers, carbonate-silicate buffers, acetic acid-acetate buffers, phosphate buffers, ammonia buffers, citric acid buffers or citrate buffers, tris(hydroxymethyl)aminomethane, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, 4-(2-hydroxyethyl)piperazine-1-propanesulfonic acid, 2-(N-morpholino)ethanesulfonic acid, barbiturate-acetate buffer).

[0059] The electrolyte solution according to the invention has an elevated temperature. This can occur when the electrolyte solution is used only in certain areas of the cell or accumulator, for example during the charging and / or discharging process in the electrode compartment and / or when the electrolyte solution is stored in an external storage tank.

[0060] An aqueous electrolyte solution having a temperature of 30 to 90°C, preferably 30 to 50°C and in particular 40 to 50°C is preferred.

[0061] Furthermore, aqueous electrolyte solutions containing at least one conductive salt are preferred.

[0062] Particularly preferred are aqueous electrolyte solutions containing at least one conductive salt, in particular those salts which are contained in highly concentrated salt solutions (brine).

[0063] The particularly preferred concentration of the conductive salt in the aqueous electrolyte solution according to the invention is between 14% by weight and up to the saturation limit. Frequently, saturated salt solutions are used, in which a portion of the salt is present in the form of a precipitate.

[0064] Particularly preferred electrolyte solutions according to the invention have a state of charge of less than 90%, in particular up to 80% and very preferably 70 to 80%. These electrolyte solutions are characterized by a particularly good temperature stability of the redox-active compound containing at least one residue of the formula (I) in the electrolyte. Of course, electrolyte solutions with a state of charge of 100% are also possible.

[0065] The state of charge (SOC) is a characteristic value of the state of charge of an electrolyte solution or a battery containing such an electrolyte solution. The state of charge value indicates the still available capacity of the electrolyte solution or the battery relative to the nominal value. The state of charge is specified as a percentage of the fully charged state.

[0066] The state of charge can be determined, for example, via the open circuit voltage (VOC). The open circuit voltage depends on the state of charge; it increases as the state of charge increases. The battery voltage is measured in an "open circuit", i.e. it is the battery voltage that occurs at a given state of charge without an external load.

[0067] Also preferred are aqueous electrolyte solutions containing compounds having at least one redox-active residue of the formula (I) in which Y is hydrogen and m is 1 or 2.

[0068] Also preferred are aqueous electrolyte solutions containing compounds having at least one redox-active residue of formula (I), wherein FG is a functional group selected from: -(O-CH2-CH2) o -OR2, -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 / m .

[0069] Preferred cations Kat are hydrogen cations, alkali metal cations, alkaline earth metal cations or ammonium cations, in particular hydrogen cations, sodium, potassium, magnesium or calcium cations, or quaternary ammonium cations.

[0070] Preferred anions An are selected from the group consisting of halide, hydroxide, phosphate, sulfate, perchlorate, hexafluorophosphate or tetrafluoroborate.

[0071] Particularly preferred aqueous electrolyte solutions contain compounds having at least one redox-active residue of formula (I), wherein Z is a covalent bond connecting the residue of formula (I) to a polymer backbone selected from the group consisting of polymethacrylates, polyacrylates, polystyrenes, polyalkylene glycols, polyalkyleneimines or polyvinyl ethers, wherein the polymer backbone preferably has 5 to 100 groups of formula (I).

[0072] Very particularly preferred aqueous electrolyte solutions contain oligomers or polymers having repeating structural units of the formula (II) and optionally having further structural units derived from solubility-promoting comonomers

[0073]

[0074] in

[0075] ME is a repeating structural unit derived from a polymerizable monomer,

[0076] BG is a covalent bond or bridging group,

[0077] FC represents a residue of the formula (X-C5H4)Fe(Y-C5H3-Z),

[0078] X, Y and Z have the meanings defined above, and

[0079] r is an integer of 2-150, preferably 2-80 and particularly preferably 8-40.

[0080] The repeating units ME and BG form the backbone of an oligomer or polymer which contains several units of the redox active unit of formula (I) as defined above.

[0081] Examples of classes of substances which can form the oligomeric or polymeric backbone 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 aromatic compounds, such as polystyrene, polymers derived from vinyl esters of saturated carboxylic acids or their derivatives, such as polyvinyl acetate or polyvinyl alcohol, 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 chemically modified derivatives thereof, such as cellulose or cellulose ethers, and polyurethanes, polyethylene ethers, polythiophenes, polyacetylenes, polyalkylene glycols and their derivatives, such as their ethers, such as polyethylene glycol or polyethylene glycol methyl ether, poly-7-oxa-norbornene, polysiloxanes, or polyalkyleneimines and their derivatives, such as their amines, such as polyethyleneimine or N,N,N`,N`-tetramethyl-polyethyleneimine.

[0082] The following are examples of combinations of structural units ME and bridging groups BG for some of the above substance classes. These are

[0083]

[0084]

[0085] Particularly preferably used classes of substances which form the main chain of the oligomers or polymers are polymethacrylates, polyacrylates, polymethacrylamides, polyacrylamides, polystyrenes, polyethyleneimines or polyvinyl ethers.

[0086] The redox active unit of formula (I) is covalently bonded to the polymer backbone.

[0087] The polymers containing the redox-active component may be present as linear polymers or they may be comb- and star-shaped polymers, dendrimers, ladder polymers, cyclic polymers, polycathenes and polyrotaxanes.

[0088] Preferably, comb and star polymers, dendrimers, ladder polymers, cyclic polymers, polycathenes and polyrotaxanes are used. These types are characterized by increased solubility and the viscosity of the solutions obtained is generally lower than that of the corresponding linear polymers.

[0089] The solubility of the polymers containing redox-active components used according to the invention can be further improved by copolymerization or functionalization, for example with ethylene glycol, methacrylic acid, acrylic acid or styrene sulfonates.

[0090] Preferred solubility-promoting comonomers are vinyl alcohol, vinyl acetate, methyl vinyl ether, methacrylic acid, acrylic acid, alkyl methacrylates, ethyl acrylate, methacrylic acid amide, acrylic acid amide, vinyl sulfonates, vinyl phosphonic acid or styrene sulfonates.

[0091] The preparation of the redox-active compounds used according to the invention can be carried out by conventional methods. Oligomers and polymers can be prepared by conventional polymerization processes. Examples for this are bulk polymerization, polymerization in solution or emulsion or suspension polymerization. These processes are known to those skilled in the art.

[0092] Low molecular weight compounds containing one or more residues of formula (I) generally have the structure

[0093] (X-C5H4)Fe(Y-C5H3-)-R,

[0094] wherein X and Y have the meanings defined above, and R is hydrogen or a mono- or polyvalent organic residue which may optionally contain further (X-C5H4)Fe(Y-C5H3) residues.

[0095] Very particularly preferred aqueous electrolyte solutions contain as redox-active compound a compound of the formula (II)

[0096] (X-C5H4)Fe(Y-C5H4)(II),

[0097] wherein X and Y have the meanings as defined above,

[0098] Other very particularly preferred aqueous electrolyte solutions contain as redox-active compound a compound of the formula (III)

[0099] [(X-C5H4)Fe(Y-C5H3-)]-R7-[(-C5H3-Y)Fe(C5H4-X)] p (III)

[0100] wherein X and Y have the meanings as defined above,

[0101] R7 is a divalent to tetravalent organic group, and

[0102] p is an integer from 1 to 4.

[0103] R7 as a di- to tetravalent organic radical is understood to be an organic radical which is linked to the rest of the molecule via two, three or four covalent bonds.

[0104] Examples of divalent organic residues are alkylene, alkyleneoxy, poly(alkylene-oxy), alkylenesulfide, poly(alkylenesulfide), alkyleneamino, poly(alkyleneamino), cycloalkylene, arylene, aralkylene or heterocyclylene.

[0105] The alkylene group can be branched or unbranched. The alkylene group usually contains one to twenty carbon atoms, preferably two to four carbon atoms. Examples of alkylene groups are: methylene, ethylene, propylene and butylene. The alkylene group can be optionally substituted by, for example, a carboxyl or sulfonic acid group, by a carboxylate or sulfonate group, by a carboxylic acid amide or sulfonamide group, by a hydroxyl or amino group, or by a halogen atom.

[0106] Alkyleneoxy and poly(alkyleneoxy) groups may contain branched and unbranched alkylene groups. The alkylene groups present in the alkyleneoxy or poly(alkyleneoxy) groups typically contain two to four carbon atoms, preferably two to three carbon atoms. The number of repeating units in the poly(alkyleneoxy) group may vary over a wide range. The typical numerical range of repeating units is 2-50. Examples of alkyleneoxy groups are: ethyleneoxy, propyleneoxy and butyleneoxy. Examples of poly(alkyleneoxy) groups are: poly(ethyleneoxy), poly(propyleneoxy) and poly(butyleneoxy).

[0107] Alkyleneamino and poly(alkyleneamino) groups may contain branched and unbranched alkylene groups. The alkylene groups present in the alkyleneamino or poly(alkyleneamino) groups typically contain two to four carbon atoms, preferably two or three carbon atoms. The number of repeating units in the poly(alkyleneamino) group may vary over a wide range. Typical values ​​for repeating units range from 2 to 50. Examples of alkyleneamino groups include: ethyleneamino, propyleneamino and butyleneamino. Examples of poly(alkyleneamino) groups are: poly(ethyleneamino), poly(propyleneamino) and poly(butyleneamino).

[0108] Cycloalkylidene group usually contains five, six or seven ring carbon atoms, each of which can be substituted independently. The example of substituent includes alkyl group or two alkyl groups, which can form another ring together with the ring carbon they are connected. The example of cycloalkylidene group is cyclohexylene. Cycloalkylidene group can be optionally replaced by, for example, carboxyl or sulfonic acid group, by carboxylate or sulfonate group, by carboxylic acid amide or sulfonamide group, by hydroxyl or amino group, or by halogen atom.

[0109] Arylene groups are typically cyclic aromatic groups containing five to fourteen carbon atoms, and each of them can be substituted independently. The example of arylene groups includes o-phenylene, m-phenylene, p-phenylene, o-biphenylene, m-biphenylene, p-biphenylene, 1-naphthyl, 2-naphthyl, 1-anthryl, 2-anthryl, 9-anthryl, 1-phenanthryl (phenantolyl), 2-phenanthryl, 3-phenanthryl, 4-phenanthryl or 9-phenanthryl. Arylene groups can be optionally by, for example, carboxyl or sulfonic acid groups, by carboxylate or sulfonate groups, by carboxylic acid amide or sulfonamide groups, by hydroxyl or amino groups, or by halogen atoms. Other examples of substituents include alkyl groups or two alkyl groups, which can form another ring together with the annular carbon atoms connected with them.

[0110] Heterocyclyl groups are typically cyclic groups with four to ten ring carbon atoms and at least one ring heteroatom, each of which can be independently substituted. Examples of heteroatoms include oxygen, nitrogen, phosphorus, boron, selenium or sulfur. Examples of heterocyclyl groups include furandiyl, thiophenediyl, pyrrolediyl or imidazolediyl. Heterocyclyl groups are preferably aromatic. Heterocyclyl groups can be optionally substituted by, for example, carboxyl or sulfonic acid groups, by carboxylate or sulfonate groups, by carboxyamide or sulfonamide groups, by hydroxyl or amino groups, or by halogen atoms. Another example of a substituent is an alkyl group or two alkyl groups, which can form another ring together with the ring carbon to which they are connected.

[0111] Aralkylene groups are typically aryl groups covalently bound to one or two alkyl groups. Aralkylene groups may be covalently bound to the rest of the molecule via their aryl moiety and their alkyl moiety or via two alkyl moieties. Aralkylene groups may be substituted on the aromatic ring, for example by an alkyl group or by a halogen atom. Examples of aralkylene groups are benzylidene or dimethylphenylene (xylylene).

[0112] Examples of R7 as trivalent organic residues are alkyltriyl, alkoxytriyl, tri-poly(alkyleneoxy), tri-poly(alkyleneamino), cycloalkyltriyl, aryltriyl, aralkyltriyl or heterocyclyltriyl. These residues correspond to the divalent residues described in detail above, with the difference that they are linked to the rest of the molecule with three covalent bonds instead of two.

[0113] Examples of tetravalent organic residues for R are alkyltetrayl, alkoxytetrayl, tetra(alkyleneoxy), tetra(alkyleneamino), cycloalkyltetrayl, aryltetrayl, aralkyltetrayl or heterocyclyltetrayl. These residues correspond to the divalent residues already described in detail above, with the difference that they are linked to the rest of the molecule with four covalent bonds instead of two.

[0114] Particular preference is given to aqueous electrolyte solutions which contain compounds having at least one redox-active residue of the formula (I) in which n is 2.

[0115] These compounds have a particularly advantageous combination of water solubility and temperature stability. In addition, such compounds can be synthesized in particular by treating the cyclopentadienyl ring of ferrocene with butyllithium and subsequently reacting it with ethylene oxide, or reacting vinylferrocene with a thiol derivative.

[0116] In a further embodiment, the invention relates to a first type of redox flow battery for storing electrical energy, comprising a reaction cell and two electrode chambers of a cathode electrolyte and an anode electrolyte, each of which is connected to at least one liquid reservoir, the electrode chambers being separated by a membrane, being provided with electrodes, and each being filled with an electrolyte solution containing redox-active components dissolved or dispersed in liquid form in an aqueous electrolyte solvent and optionally conductive salts dissolved therein and possible further additives. This redox flow battery is characterized in that the anode electrolyte contains water-soluble redox-active components and the cathode electrolyte in the electrode chambers contains an aqueous electrolyte solution as defined above.

[0117] The electrolyte solution in the electrode compartment may be coated with paraffin oil to minimize oxidative diffusion.

[0118] Preferred redox flow batteries of this first type also contain an aqueous electrolyte solution as defined above in the liquid reservoir(s).

[0119] Preferred redox flow batteries of this first type are those in which the anode electrolyte contains a compound containing one or more bipyridyl groups in the molecule as redox active components.

[0120] These are generally compounds which contain at least one redox-active residue of the formula (IV) or (V) in the molecule, preferably one to six, in particular one to four, very preferably one to three and most preferably one to two residues of the formula (IV) or (V) in the molecule.

[0121]

[0122] in

[0123] The lines extending from the nitrogen atoms in the structures of Formula IV and V represent covalent bonds connecting the structures of Formula IV and V to the rest of the molecule,

[0124] R8 and R9 independently represent alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, heterocyclic, halogen, hydroxy, amino, nitro or cyano,

[0125] An represents an m-valent inorganic or organic anion,

[0126] b and c are independently of one another an integer from 0 to 4, preferably 0, 1 or 2, especially 0,

[0127] m is an integer from 1 to 4, and

[0128] a is a number having a value of 2 / m.

[0129] The compounds containing one or more bipyridyl groups as redox-active components in the molecule may be low molecular weight organic molecules, oligomers or polymers. Preferably, these contain at least one residue of formula (IV) or formula (V) which is covalently linked to the rest of the molecule.

[0130] Preferably, the redox-active compounds containing residues of formula (IV) or formula (V) are soluble in water. However, they may also be compounds that are dispersible in water.

[0131] The redox-active component preferably used in the anolyte is a compound of the formula IVa or Va

[0132]

[0133] in

[0134] R8, R9, An, a, b, c and m have the meanings as defined above,

[0135] R 10 and R 11 are independently of one another hydrogen, alkyl optionally substituted by carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino groups, for example 2-trialkylammonium-ethyl, cycloalkyl optionally substituted by carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino groups, aryl optionally substituted by carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino groups, or aralkyl optionally substituted by carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino groups, in particular C1-C6-alkyl, C1-C6-alkyl substituted by carboxylate groups, C1-C6-alkyl substituted by carboxamide groups, C1-C6-alkyl substituted by carboxylic acid groups, C1-C6-alkyl substituted by sulfonic acid groups, or C1-C6-alkyl substituted by amino groups, and very preferably propionate, isobutyrate, ethyl or methyl, and

[0136] R 12 and R 13are independently hydrogen, alkyl optionally substituted by carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino groups, for example 2-trialkylammonium-ethyl, cycloalkyl optionally substituted by carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino groups, aryl optionally substituted by carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino groups, or aralkyl optionally substituted by carboxylate, carboxamide, carboxylic acid, sulfonic acid or amino groups, or two residues R 12 and R 13 Together they form a C1-C3-alkylene group, in particular a C1-C6-alkyl group, a C1-C6-alkyl group substituted by a carboxylate group, a C1-C6-alkyl group substituted by a carboxamide group, a C1-C6-alkyl group substituted by a carboxylic acid group, a C1-C6-alkyl group substituted by a sulfonic acid group, or a C1-C6-alkyl group substituted by an amino group, or together they are ethylene, and very preferably propionate, isobutyrate, ethyl or methyl, or together they are ethylene.

[0137] Examples of particularly preferred compounds for use are N,N'-di-C1-C4-alkylbipyridyl cations, such as N,N'-dimethylbipyridyl halides, such as N,N'-di-methylbipyridyl chloride, which charge balances any anions.

[0138] A preferred redox flow battery of the first type contains as the anolyte an aqueous electrolyte solution having a temperature of at least 30° C., preferably 30-90° C., and in particular 30-50° C., containing a redox-active compound comprising at least one residue of formula (IV) or formula (V).

[0139] In a further preferred redox flow battery of this first type, the electrode compartments for the cathode electrolyte and the anolyte are separated by a semi-permeable membrane which is impermeable to the redox couples in the cathode electrolyte, and the anolyte contains a zinc salt as the redox active component.

[0140] In a further embodiment, the invention also relates to a redox flow battery of the second type for storing electrical energy, comprising a reaction cell having an electrode chamber for an electrolyte solution, the electrode chamber being connected to at least one liquid reservoir, wherein the electrode chamber is equipped with a cathode and an anode and is filled with an electrolyte solution containing a redox-active component dissolved or dispersed in liquid form in an aqueous electrolyte solvent and a conductive salt optionally dissolved therein and possibly further additives. The redox flow battery is characterized in that the electrolyte solution contains an aqueous electrolyte solution as defined above and a zinc salt as a further redox-active component.

[0141] This second type of redox flow batteries is characterized by the fact that they need only have one liquid reservoir and do not require the use of membranes to separate the reaction cell into anode and cathode compartments.

[0142] Preferred redox flow batteries of this second type also contain an aqueous electrolyte solution as defined above in the liquid reservoir.

[0143] A preferred redox flow battery of this second type uses a zinc solid anode with the redox couple zinc(II) / zinc(0).

[0144] Examples of zinc salts for the first and second types 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.

[0145] In a preferred embodiment of the redox flow battery of the second type and of the first type according to the invention, zinc is used as redox-active anode material. Particular emphasis should be placed on the different aggregation states that zinc can assume within the battery.

[0146] During the charging process, the zinc(II) cations dissolved in the electrolyte are reduced to elemental zinc(0) on the anode surface. Two electrons must be accepted for this purpose. The active material zinc is therefore present in the battery both in dissolved and solid form.

[0147] The zinc solid anode may be permanently present as a metallic electrode, or it may be formed in situ on a conductive surface inside the chamber by reduction of zinc cations during the battery charging process. The zinc cations may serve primarily as active material and also secondarily as a conductive additive or as part of a conductive additive mixture.

[0148] During the charging process, the zinc (II) cations dissolved in the electrolyte are reduced to elemental zinc (0) on the anode surface. The anode can be made of any conductive material, preferably a metal, in particular zinc or a zinc alloy. By receiving two electrons, metallic zinc is deposited on the electrode surface. In the reverse case, metallic zinc is converted from the electrode surface by releasing two electrons into zinc ions, which accumulate in the anode electrolyte.

[0149] Particularly preferred redox flow batteries according to the invention have a zinc solid anode and a redox couple zinc(II) / zinc(0).

[0150] The advantage over the fully organic redox flow batteries described above is, on the one hand, reduced production costs. Zinc anodes are much more cost-effective to produce than known organic anodes. On the other hand, the redox couple zinc(II) / zinc(0) is characterized by very good stability to external environmental influences, such as sensitivity to oxygen. Conventional systems must be kept oxygen-free; this significantly complicates the cell design and increases the operating costs. With the use of zinc anodes, this can be completely avoided.

[0151] Zinc also has a very high overvoltage in aqueous media and allows for an extremely high potential window. The potential window is understood to be the maximum voltage (potential range) that can be reached between the cathode and the anode due to the position of the redox pair within the range of the electrochemical voltage series, without undesirable side reactions or decomposition of redox active materials, conductive additives, electrolytes, or any other components of the entire battery. Conventional aqueous flux cells are limited to a potential window of about 1.2V. When exceeded, hydrogen will otherwise be produced. By using a zinc anode, the potential window can be expanded to greater than 2V. This results in a substantial increase in the electrical power of each cell. For hydrogen, zinc has a very high overvoltage, so despite the high voltage of 2V, hydrogen is not formed at the anode and the battery pack can be operated safely.

[0152] In the context of this specification the term "battery" is used in its broadest sense. It may be a single rechargeable electrochemical cell or a combination of several such electrochemical cells.

[0153] Particularly preferred redox flow batteries according to the invention have a state of charge of the cathode electrolyte or cathode electrolyte and anode electrolyte of less than 90%, more particularly up to 80% and most preferably from 70 to 80%. These redox flow batteries are characterized by particularly good temperature stability of the electrolyte and in particular the redox-active compound containing at least one residue of formula (I) in the electrolyte. The state of charge is determined as described above. Of course, the state of charge of the cathode electrolyte or cathode electrolyte and anode electrolyte in the RFB according to the invention can also be 100%.

[0154] In addition to the components described above, the redox flow battery according to the present invention may contain other elements or components commonly used in such batteries.

[0155] In the redox flow battery according to the invention, the selected redox-active component is used in one or more chambers, being present in the one or more chambers in dissolved liquid or dispersed form.

[0156] The redox potential of the redox-active component can be determined, for example, by means of cyclic voltammetry. This method is known to those skilled in the art (compare Allen J. Bard and 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).

[0157] A first type of redox flow battery according to the invention comprises a semipermeable membrane or a microporous membrane.

[0158] This satisfies the following function

[0159] Separate anode and cathode chambers

[0160] Maintaining redox active components in the cathode electrolyte and in the anode electrolyte, i.e. maintaining cathode and anode active materials

[0161] • Permeability to the conductive salt acting as the charge-balancing electrolyte, ie to the anions and / or cations of the conductive salt or to the charge carriers contained in the electrolyte in general.

[0162] The membrane may be a size exclusion membrane such as a dialysis membrane, but may also be an ion selective membrane. The membrane prevents the passage of the redox active ferrocene compound into the anode compartment and the passage of the redox active components of the anode electrolyte into the cathode compartment. The passage of dissolved zinc (II) cations need not but may also be inhibited by the membrane.

[0163] Depending on the application, the material of the membrane can consist of plastic, ceramic, glass, metal or textile web. Examples of materials are organic polymers such as cellulose or modified cellulose, for example 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 also ceramic, glass or felt. Membranes composed of several materials (composite materials) are also possible.

[0164] The membranes and the resulting redox flow batteries can be used in a variety of configurations. Examples include planar membranes, bag filter configurations, and wound modules. These embodiments are known to those skilled in the art. Preferably, planar membranes are used.

[0165] The thickness of the films used according to the invention can vary within wide ranges. Typically the thickness is between 0.1 μm and 5 mm, particularly preferably in the range between 10 μm and 200 μm.

[0166] In addition to the above-mentioned redox-active components, electrolyte and optional membrane, the redox flow battery according to the invention preferably contains further components. These are

[0167] Delivery mechanisms, such as pumps, as well as tanks and pipes for transporting and storing redox-active components

[0168] An electrode, preferably consisting of or comprising graphite, graphite nonwoven fabric, graphite paper, carbon nanotube felt, activated carbon, carbon black or graphene

[0169] Optionally a current collector, for example made of graphite or metal.

[0170] The negative electrode preferably contains zinc and may also contain, for example, the following materials:

[0171] Stainless steel, Hastelloy or an alloy containing iron-chromium-nickel, graphite, graphite non-woven, graphite paper, carbon nanotube felt, activated carbon, carbon black or graphene.

[0172] The redox flow battery according to the invention contains current collectors as further optional but preferred components. These have the task of establishing the best possible electrical contact between the electrode material and an external current source or sink.

[0173] Aluminum, aluminum alloys, copper, stainless steel, Hastelloy, iron-chromium-nickel alloys, titanium or tantalum coated with precious metals, in particular titanium, niobium, tantalum, hafnium or zirconium coated with platinum and / or iridium and / or ruthenium oxides can be used as current collectors in the redox flow battery according to the present invention.

[0174] All components capable of transmitting current to the electrodes are called current collectors. Redox reactions occur at the electrodes (cathode and anode) in direct contact with the electrolyte.

[0175] The redox flow battery according to the invention can be used in various fields. In the broadest sense, this can be the storage of electrical energy for mobile and stationary applications. The invention also relates to the use of a redox flow battery for these purposes.

[0176] Examples of applications include use as stationary storage for emergency power supply, peak load balancing, and for intermediate storage of electrical energy from renewable energy sources, in particular in the fields of photovoltaics, hydropower and wind power, from gas, coal, biomass, tidal and marine power plants.

[0177] The redox flow batteries according to the invention can be interconnected in series or in parallel in a manner known per se.

[0178] The following examples illustrate the invention without limiting the invention thereto.

[0179] Example 1: Synthesis of ferrocene-disulfonic acid

[0180]

[0181] The synthesis of ferrocene-disulfonic acid was carried out according to a modified literature process (K. Chanawanno, C. Holstrom, LA Crandall, H. Dodge, VN Nemykin, RS Herrick, CJ Ziegler, Dalton Trans. 2016, 45, 14320) and is described below.

[0182] Ferrocene (8.00g, 44mmol) is dissolved in acetic anhydride (100mL). Chlorosulfonic acid (5.7mL, 86mmol) is slowly added. The reaction mixture is stirred at room temperature for 20 hours in an argon atmosphere. The suspension is then filtered in an argon atmosphere. The filtrate is washed with acetic anhydride. The product is then dried. A portion (3.46g, 8.5mmol) of the dried product is dissolved in ethanol (10mL). Then, a 1M NaOH solution (10mL) newly prepared by NaOH (4.0g, 100mmol) in ethanol (100mL) is slowly added. The solution is stirred at room temperature for 30min. The precipitate is collected on the filter and then rinsed with ethanol and then dried in a vacuum. Therefore, the sodium salt (2.1g, 4.7mmol) of ferrocene-bissulfonic acid as a yellow solid is obtained.

[0183] Example 2: Stability test of ferrocene-disulfonic acid (comparative)

[0184] With the help of The ferrocene from Example 1 was oxidized and the stability of the "charged" molecule was investigated. The charged electrolyte showed irreversible precipitation after three days of storage at 60° C. (also in the absence of air).

[0185] Example 3: Temperature stability study of electrolyte solution

[0186]

[0187] Preparation of electrolyte:

[0188] Catholyte: 400 mM solution of ferrocene derivative (1.77 g, 4 mmol) in deionized water (10 mL)

[0189] Anolyte: 600 mM solution of viologen (3.00 g, 6 mmol) in deionized water (10 mL).

[0190] Battery test: The electrolyte solution was separated by a Fumasep FAA-3-50 anion exchange membrane with a surface area of ​​5 cm2. Graphite plates with graphite nonwoven fabric were used as electrodes. A peristaltic pump pumped the electrolyte solution through a pipe into a reservoir located in a heatable sand bath and through the half-cell.

[0191] In the first three cycles, 101.1, 100.3 and 100.1 mAh can be charged, and 100.6, 100.5 and 100.4 mAh can be discharged accordingly.

[0192] After about 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.

[0193] Experiments were also conducted using a sand bath heated to 60° C., in which two reservoirs of electrolyte were located. For this purpose, a solution was used that had been circulated for almost 19 days. During this process, 99.0 mAh could be charged and after 24 hours, 98.5 mAh was discharged.

[0194] Ferrocene from Example 3 is commercially available, for example, from TCI Chemicals. The synthesis of this compound is described in ACS Energy Lett. 2017, 2, 639-644.

[0195] The synthesis of the viologen derivative of Example 3 is also described in ACS Energy Lett. 2017, 2, 639-644.

[0196] Example 4: Synthesis of functionalized ferrocene

[0197]

[0198] Vinylferrocene (100mg, 0.47mmol) was dissolved in tetrahydrofuran (2mL) in a microwave glass (5mL). Then, 2,2-dimethoxy-2-phenylacetophenone (DMPA, 3mg, 0.01mmol) was added. After the magnetic stirring bar was added to the microwave glass, it was sealed with a Teflon diaphragm and argon was flowed through the transparent orange solution for 15 minutes. Then 2-mercaptopropionic acid (50 μL, 0.56mmol) was added using a micro syringe. Then the solution was placed in a beaker on a stirring plate in a UV chamber (365nm) and irradiated for 20 minutes. Then the solution was left to stand for 10 minutes to cool. Irradiation totaled 120 minutes. The solvent was removed and the crude product was dissolved in dichloromethane and purified by column chromatography (silicon dioxide, EtOAc / hexane 2 / 8). After the solvent was removed by distillation, the crude product was dried under high vacuum. The product as a high viscosity orange oil was obtained.

[0199] Example 5: Synthesis of ferrocene polymer

[0200]

[0201] Ferrocene-containing copolymers can be prepared by free radical copolymerization of ferrocene monomer and sulfobetaine methacrylate. Ferrocene monomer is obtained by reacting hydroxyethylferrocene with methacrylic acid chloride.

[0202] Example 6: Synthesis of Hydroxyethylferrocene

[0203]

[0204] Ferrocene is dissolved in methyl-THF and reacted with tert-butyllithium. Subsequently, ethylene oxide is added. The product can then be obtained after column chromatography separation.

[0205] Example 7: Synthesis of ferrocene polymer

[0206]

[0207] With the aid of free radical polymerization, it is possible to prepare n ) is a copolymer of METAC ([2-(methacryloyloxy)ethyl]trimethylammonium chloride) and methacrylamide containing ferrocene, wherein the composition is 10 to 30% of ferrocene monomer. Aqueous electrolytes based on this polymer are temperature stable at 60°C and show no degradation (i.e., capacity reduction) through 100 cycles.

Claims

1. An aqueous electrolyte solution having a temperature of at least 30° C., comprising at least one compound having a redox-active residue of formula (I) (X-C5H4)Fe(Y-C5H3-Z) (I), Where X is 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- residues, Y is hydrogen or of the formula -(C n H 2n )-FG or formula-(C n H 2n )-Sp-(C n H 2n )-FG residues, Z is hydrogen or a covalent bond connecting the residue of formula (I) to the rest of the molecule, FG is a functional group selected from the following: -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、-N + R3R4C t H 2t -SO3 - or -NR2-SO2-R3, Brgp is a divalent bridging group connecting the residue of formula (I) to the rest of the molecule, Sp is -O-, -S-, -SO- or –SO2-, R1 is a C1-C4 alkyl group, 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 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, characterized in that it has a temperature of 30-90°C.

3. The aqueous electrolyte solution according to claim 1, characterized in that o is an integer of 3-20.

4. The aqueous electrolyte solution according to claim 1, characterized by having a temperature of 30-50°C.

5. The aqueous electrolyte solution according to claim 1 or 2, characterized by containing at least one conductive salt.

6. The aqueous electrolyte solution according to claim 1 or 2, characterized in that its state of charge is less than 90%. 7 . The aqueous electrolyte solution according to claim 1 , wherein Y is hydrogen and m is 1 or 2. 8 .

8. The aqueous electrolyte solution according to claim 1 or 2, characterized in that FG is a functional group selected from the following: -(O-CH2-CH2) o -OR2, -COR2, -COO - (Kat m+ ) 1 / m 、-SO3 - (Kat m+ ) 1 / m 、-SO4 - (Kat m+ ) 1 / m ,-PO4 2- (Kat m + ) 2 / m or-PO3 2- (Kat m+ ) 2 / m .

9. The aqueous electrolyte solution according to claim 1 or 2, characterized in that Kat is a hydrogen cation, an alkali metal cation, an alkaline earth metal cation or an ammonium cation. 10 . The aqueous electrolyte solution according to claim 9 , wherein the alkali metal cation is a sodium cation or a potassium cation, the alkaline earth metal cation is a magnesium cation or a calcium cation, and the ammonium cation is a quaternary ammonium cation.

11. The aqueous electrolyte solution according to claim 1 or 2, 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.

12. An aqueous electrolyte solution according to claim 1 or 2, characterized in that Z is a covalent group connecting the residue of formula (I) to a polymer backbone selected from the group consisting of polymethacrylate, polyacrylate, polystyrene, polyalkylene glycol, polyalkylene imine or polyvinyl ether, wherein the polymer backbone has 5 to 100 groups of formula (I).

13. An aqueous electrolyte solution according to claim 1 or 2, characterized in that it contains an oligomer or polymer having a repeating structural unit of formula (II) and optionally having additional structural units derived from a comonomer that promotes solubility in ME is a repeating structural unit derived from a polymerizable monomer, BG is a covalent bond or bridging group, FC represents a residue of the formula (X-C5H4)Fe(Y-C5H3-Z), X, Y and Z have the meanings defined in claim 1, and r is an integer ranging from 2 to 150.

14. An aqueous electrolyte solution according to claim 1 or 2, characterized in that the redox-active compound is a compound of formula (II) (X-C5H4)Fe(Y-C5H4) (II), wherein X and Y have the meanings defined in claim 1.

15. An aqueous electrolyte solution according to claim 1 or 2, characterized in that the redox-active compound is a compound of formula (III) <h2 style=";text-align:left;direction:ltr">[(X-C5H4)Fe(Y-C5H3-)]-R7-[(-C5H3-Y)Fe(C5H4-X)]<h2 style=";text-align:left;direction:ltr"> p <h2 style=";text-align:left;direction:ltr"> (III) wherein X and Y have the meanings defined in claim 1, R7 is a divalent to tetravalent organic group, and p represents an integer of 1-4.

16. The aqueous electrolyte solution according to claim 1 or 2, characterized in that n is 2.

17. A redox flow battery for storing electrical energy, comprising a reaction cell having two electrode chambers for a cathode electrolyte and an anode electrolyte, each of the two electrode chambers being connected to at least one liquid reservoir, wherein the electrode chambers are separated by a membrane, are provided with electrodes, and are each filled with an electrolyte solution containing a redox active component dissolved or dispersed in liquid form in an aqueous electrolyte solvent and optionally a conductive salt dissolved therein and possible further additives, characterised in that the anode electrolyte contains a water-soluble redox active component and the cathode electrolyte in the electrode chamber contains an aqueous electrolyte solution according to any one of claims 1 to 16.

18. The redox flow battery according to claim 17, characterized in that the anode electrolyte contains a compound containing one or more bipyridyl groups in the molecule as a redox active component.

19. A redox flow battery according to claim 17, characterised in that the electrode compartments for the cathode electrolyte and the anolyte are separated by a semi-permeable membrane which is impermeable to the redox couple in the cathode electrolyte, and in that the anode electrolyte contains a zinc salt as a redox active component.

20. A redox flow battery according to any one of claims 17 to 19, characterised in that it has a zinc solid anode and a redox couple zinc(II) / zinc(0).

21. A redox flow battery according to any one of claims 17 to 19, characterised in that the state of charge of the cathode electrolyte or the cathode electrolyte and the anode electrolyte is less than 90%.

22. Use of a redox flow battery according to any one of claims 17 to 19 for storing electrical energy for stationary and mobile applications, as stationary storage for emergency power supply, for peak load balancing, for intermediate storage of electrical energy from renewable energy sources.

23. Use according to claim 22, characterised in that it is used in the field of photovoltaic, hydroelectric and wind power generation or from gas, coal, biomass, tidal and marine power plants.

24. A redox flow battery for storing electrical energy, comprising a reaction cell having an electrode chamber for an electrolyte solution, the electrode chamber being connected to at least one liquid reservoir, wherein the electrode chamber is equipped with a cathode and an anode and is filled with an electrolyte solution containing redox-active components dissolved or dispersed in liquid form in an aqueous electrolyte solvent, and optionally conductive salts and possibly further additives dissolved therein, characterised in that the electrolyte solution contains an aqueous electrolyte solution according to any one of claims 1 to 16 and a zinc salt as a further redox-active component.

25. Redox flow battery according to claim 24, characterized in that it has a zinc solid anode and a redox couple zinc(II) / zinc(0).

26. The redox flow battery of claim 24, wherein the state of charge of the cathode electrolyte or the cathode electrolyte and the anode electrolyte is less than 90%.

27. Use of a redox flow battery according to claim 24 for storing electrical energy for stationary and mobile applications, as stationary storage for emergency power supply, for peak load balancing, for intermediate storage of electrical energy from renewable energy sources.

28. Use according to claim 27, characterised in that it is used in the field of photovoltaic, hydroelectric and wind power generation or from gas, coal, biomass, tidal and marine power plants.

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