Redox flow battery and its use

Abstract

A redox flow battery for storing electrical energy is described, comprising a reaction cell, having two electrode chambers for a catholyte and an anolyte, each connected to at least one reservoir, the electrode chambers being separated by a diaphragm, equipped with electrodes, and each filled with an electrolyte solution, comprising a redox-active component dissolved or dispersed in an aqueous electrolyte solvent, as well as a conductive salt dissolved therein and possibly further additives. A second embodiment relates to a redox flow battery for storing electrical energy, comprising a reaction cell, having an electrode chamber for an electrolyte solution, connected to at least one reservoir, the electrode chambers being equipped with a cathode and an anode, and filled with an electrolyte solution, comprising a redox-active component dissolved or dispersed in an aqueous electrolyte solvent, as well as a conductive salt dissolved therein and possibly further additives. The redox flow battery is characterized in that: · the at least one reservoir is an underground storage device, having a temperature of at least 30° C., · the concentration of the salt dissolved in the electrolyte solution is at least 10% by weight, and · the catholyte or the electrolyte solution comprises a selected redox-active and temperature-stable component. In a first embodiment, the anolyte comprises a water-soluble redox-active component, and in a second embodiment, the anolyte or electrolyte solution comprises a zinc salt.

Description

[0001] The present invention relates to a redox flow battery, also referred to as a redox flow battery (hereinafter also referred to as "RFB"), which uses an underground storage device as a storage tank. Such a system is used to store electrical energy based on electrochemical redox reactions.

[0002] RFBs usually consist of a galvanic cell, which is composed of two polarity-specific compartments (half-cells). They are usually separated by a diaphragm. In addition, two or more storage tanks are necessary, in which the electrolyte containing the redox-active material is present.

[0003] In addition to conventional RFBs, hybrid RFBs (hereinafter also referred to as “RFBs” or “hybrid RFBs”) are also known, in which at least one redox pair is composed of a combination of a solid and a salt, such as Zn as a redox pair. 2+ / Zn 0 . Such a system is described in WO 2017 / 084749 A1. During the charge and discharge process, the (non-redox active) ions in the electrolyte can exchange charge via the diaphragm. This traps the redox active substances in the corresponding half-cell. In contrast to RFBs, the diaphragm can also be omitted in hybrid RFBs. In addition, hybrid RFBs can also be of a construction type that only requires a single storage tank.

[0004] Due to their excellent scalability, RFBs are particularly suitable as stationary energy storage systems for different power and capacity requirements. For example, as buffer systems for renewable energy in private (e.g. single or multi-family homes) and industrial sectors (e.g. wind and solar power plants). They therefore have great potential for ensuring grid stability and for decentralized energy supply.

[0005] Existing RFBs are electrochemical energy storage devices. The substances required for establishing the potential at the electrode are redox-active substances that are dissolved or in particle form, which are converted into their respective other redox phases during the charging or discharging process in the electrochemical reactor. For this purpose, the electrolyte solution (catholyte, anolyte) is output from the tank and actively pumped to the electrode. The anode chamber and cathode chamber are separated in the reactor by a semipermeable membrane, which generally exhibits high selectivity for protons and / or is a size exclusion membrane. As long as the electrolyte solution is pumped, current will be output. The charging process is therefore 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 output power is a function of the size of the electrochemical reactor. The same applies to hybrid RFBs, in which the need for multiple tanks or for the presence of a diaphragm can be omitted.

[0006] RFBs have a complex system technology that roughly corresponds to a fuel cell. The typical structural size of a single reactor varies in the range of about 1-100 kW. The reactors can be combined very easily in a modular manner and the cell size can also be adjusted almost arbitrarily.

[0007] A variety of redox-active compounds have been proposed for use in batteries or batteries such as RFBs. An overview of the prior art can be found, for example, in the paper "Redox Species of Redox Flow Batteries: A Review" by Feng Pan and Quing Wang, Molecules 2015, 20, 20499-20517 or in the paper Angew. Chem. 2017, 56, 686-711 by Winsberg et al. Accordingly, RFBs containing 2,2,6,6-tetrasubstituted 1-piperidinyloxy compounds as cathode electrolytes are known. Such RFBs are also described, for example, in WO2014 / 026728 A1, WO2017 / 084749 A1 and WO2017 / 025177 A1.

[0008] Other RFBs containing 2,2,6,6-tetrasubstituted 1-piperidinyloxy compounds in the electrolyte are disclosed in WO 2018 / 032003 A1 and US 2018 / 0072669 A1.

[0009] Ferrocene, bis(η 5 -cyclopentadienyl)iron(II), [Fe(η 5 -C5H5)2] has been used in RFB. 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 functional ferrocenyland multiferrocenyl compounds", Bielefeld (2001). In addition, RFB containing functionalized ferrocene in the electrolyte is also known from WO2018 / 032003 A1 and US2018 / 0072669 A1.

[0010] To date, RFBs have been used in the literature mainly at room temperature. In the scientific literature, there is little evidence of studies at high temperatures, such as 30-50°C, and there are no systematic studies of the behavior of aqueous electrolytes containing 2,2,6,6-tetrasubstituted 1-piperidinyl compounds or functionalized ferrocenes at high temperatures. It is known that commercial vanadium RFBs (hereinafter also referred to as "V-RFBs") cause problems at temperatures above 40°C due to irreversible precipitation of the electrolyte.

[0011] V-RFBs are therefore limited by their application temperatures or by the required elaborate cooling. Special applications such as for cavernous storage devices, where inherently high temperatures usually occur, therefore generally exclude V-RFBs. Special HCl-containing electrolytes have been developed for V-RFBs, which are more temperature stable and thus allow temperatures up to about 45° C. and partly in special cases up to 60° C. (see US 2012 / 0077067 A1). However, these electrolytes are extremely corrosive. No other RFB systems have yet been described at these relatively high temperatures.

[0012] In particular, RFBs containing organic redox-active compounds have not been used at high temperatures so far. This is more difficult because many electrolytes containing organic redox-active compounds are unstable at high temperatures, especially in the charged state. In the present context, it should also be noted that the stability of many organic redox-active materials has been incorrectly assessed so far (M.-A. Goulet, MJ Aziz J. Electrochem. Soc. 2018, 165, A1466-A1477). The simple charge / discharge tests carried out so far do not provide any information about the aging behavior of the electrolyte. Therefore, the RFB containing TEMPTMA as a redox-active compound, which has been described as the most stable so far, is also affected by these decomposition methods. At high temperatures, side reactions are further accelerated and lead to rapid capacity loss of RFB.

[0013] Research has shown that many electrolytes based on redox-active organic compounds decompose faster than previously believed.

[0014] It has now been found that selected redox-active materials surprisingly exhibit high stability at high temperatures. They allow the preparation of aqueous electrolyte solutions that are stable at high temperatures. In addition, these redox-active compounds allow the preparation of RFBs that can be operated at high temperatures. Higher temperatures of the electrolyte occur when the RFB is operated in an environment with high temperatures, for example when storing electrolyte solutions at high temperatures and / or also in the electrode compartments at high power throughputs during charge / discharge operations.

[0015] The storage capacity of an RFB is known to be limited by the storage volume of the tanks used to store the electrolyte. As the capacity of an RFB increases, the number of tanks used to store the electrolyte in such a system also increases, and with it the complexity of the equipment technology.

[0016] Caverns as battery storage systems are known from WO 2018 / 007598 A1 and WO 2018 / 114012 A1. In addition, the brine4power project relates to such a system (www.ewe-gasspeicher.de / home / b4p). Caverns are suitable for storing large amounts of electrolyte solutions and allow the production of RFBs with large storage capacities.

[0017] For RFBs with high storage capacity, underground storage devices, such as the above-mentioned system with cavern storage, are suitable. Underground storage devices offer the possibility to store large to extremely large amounts of liquid in a simple manner. This allows easy storage of the electrolyte solutions required for the operation of the RFB.

[0018] High temperatures of at least 30°C and, depending on the depth and geological conditions, more than 80°C often prevail in underground storage devices, so that the liquids stored therein also acquire these temperatures during storage. When electrolyte solutions are stored in caverns at greater depths, temperatures of about 50-80°C, but at least 30-50°C, can be expected. In addition, the liquids stored in underground storage devices often have a high salt content. This can be due to the salts present in the surrounding rocks, or brine produced in salt domes when flushing caverns can be used as the fluid to be stored.

[0019] So far, mainly conventional RFBs have been described which use only solutions with low-concentration saline solutions. These systems (except for the known variants with cavernous storage) are not based on highly concentrated or saturated saline solutions ("brine").

[0020] It has been found that by using selected redox-active compounds in underground storage devices (where high temperatures prevail), it is possible to operate RFBs with high storage capacities, which have high stability and which allow a large number of charge and discharge cycles. The redox-active compounds used according to the invention make it possible to use RFBs with redox-active compounds for "high temperature" applications.

[0021] The present invention is based on the object of providing an RFB or a hybrid RFB having the above-mentioned property profile.

[0022] Another object of the present invention is to provide a RFB that can operate at high temperatures without significant decomposition of the electrolyte used.

[0023] A further sub-task is to provide a hybrid RFB which requires a small number of storage devices, preferably only one storage device, and in which the use of a separator in the electrolysis cell can also be omitted.

[0024] This object is solved by providing a device as claimed in claims 1 and 2 .

[0025] A further sub-task is to provide a hybrid RFB which is operated with a small number of storage devices, preferably only one storage device, and in which the use of separators in the electrolysis cell can also be omitted.

[0026] This sub-task is solved by providing a hybrid RFB as claimed in claim 2 .

[0027] In a first embodiment, the invention relates to a redox flow battery for storing electrical energy, comprising a reaction cell with two electrode chambers for a catholyte and an anolyte, each of which is connected to at least one reservoir, wherein the electrode chambers are separated by a diaphragm and are equipped with electrodes, and each is filled with an electrolyte solution comprising a redox-active component dissolved or dispersed in an aqueous electrolyte solvent, and a conductive salt dissolved therein, and possibly further additives. This first embodiment of the device is characterized in that:

[0028] The at least one reservoir is an underground storage device having a temperature of at least 30°C, characterized in that

[0029] The concentration of the salt dissolved in the electrolyte solution is at least 10% by weight, characterized in that

[0030] The anolyte comprises a water-soluble or water-dispersible redox-active component, and

[0031] The cathode electrolyte contains at least one compound or composition selected from one of the groups a), b), c), d) or e) as a redox-active component, wherein

[0032] a) is an organic compound comprising at least one redox-active residue of formula (I):

[0033] (X-C5H4)Fe(Y-C5H3-Z) (I),

[0034] b) is an organic compound comprising at least one redox-active residue of formula (Ia):

[0035]

[0036] in combination with hydrochloric acid and / or a salt selected from the group consisting of ammonium salts with inorganic or organic anions, preferably chloride or fluoride anions, salts with tetrafluoroborate anions or salts of trifluoromethanesulfonic acid,

[0037] c) is an organic compound comprising at least one redox-active residue of formula (Ib):

[0038]

[0039] d) is an organic compound comprising at least one redox-active residue of formula (Ic):

[0040]

[0041] e) is a water-soluble iron salt complexed with a nitrogen-containing ligand, wherein

[0042] X is of the formula -(C n H 2n )-FG or formula-(C w H 2w )-Sp-(C n H 2n )-FG or formula-(C n H 2n )-Brgp- group,

[0043] Y is hydrogen or of the formula -(C n H 2n )-FG or formula-(C w H 2w )-Sp-(C n H 2n )-FG group,

[0044] Z is hydrogen or a covalent bond linking the radical of formula (I) to the rest of the molecule,

[0045] 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 u+ ) 1 / u , -COOR2, -SO3 - (Kat u+ ) 1 / u , -SO3R2, -SO4 - (Katu+ ) 1 / u , -SO4R2, -PO4 2- (Kat u+ ) 2 / u , -PO4(R2)2, -PO3 2 (Kat u+ ) 2 / u ,-PO3(R2)2,-NR3R4R5 + (An m- ) 1 / m , -N + R3R4-C t H 2t -SO3 - Or –NR2-SO2-R3,

[0046] Brgp is a divalent bridging group which connects the group of formula (I) to the rest of the molecule,

[0047] Sp is -O- or -S-,

[0048] R1 is a C1-C4 alkyl group,

[0049] R2 is hydrogen or C1-C4 alkyl,

[0050] R3, R4 and R5 independently represent hydrogen or alkyl,

[0051] Kat is a u-valent cation,

[0052] An is an m-valent anion,

[0053] m and u are independently integers from 1 to 4,

[0054] n and w are independently integers from 2 to 4, and in particular 2, and

[0055] o is an integer of 1-50, preferably 3-20,

[0056] t is an integer from 2 to 5,

[0057] R7, R8, R9 and R 10 represents an alkyl group or R7, R8, R9 and R 10 Each together with the common carbon atom forms an alicyclic or heterocyclic residue,

[0058] Q is -O-, -S-, -NH-, -NR 13a -,-NR 13a R 13b + -(An m- ) 1 / m， -PR 13a -or–SiR13a R 13b -, or -Q- represents a covalent bond,

[0059] R 11 is -O or -S,

[0060] R 11 is -O or -S,

[0061] R 12 Is -CH2-, -O-, -S-, -SO-, -SO2-, –NR 13 -or–N + R 13 R 14 -(An m- ) 1 / m ,

[0062] R 13 , R 13a , R 13b and R 14 are independently of one another monovalent organic radicals, preferably alkyl and in particular C1-C4-alkyl, and

[0063] An and m have the meanings defined above.

[0064] In a second embodiment, the invention relates to a redox flow battery for storing electrical energy, comprising a reaction cell with an electrode compartment for an electrolyte solution, which is connected to at least one reservoir, wherein the electrode compartment is equipped with a cathode and an anode, optionally with a separator, which divides the electrode compartment into a cathode space and an anode space, the electrode compartment being filled with an electrolyte solution comprising a redox-active component dissolved or dispersed in an aqueous electrolyte solvent, and a conductive salt dissolved therein and optionally further additives. This second embodiment of the device is characterized in that:

[0065] The at least one reservoir is an underground storage device having a temperature of at least 30°C, characterized in that

[0066] The concentration of the salt dissolved in the electrolyte solution is at least 10% by weight, characterized in that

[0067] The anolyte comprises a water-soluble or water-dispersible redox-active component, and

[0068] The catholyte comprises as redox-active component at least one compound or composition selected from one of the groups a), b), c), d) or e), wherein these groups have the meanings defined above.

[0069] The present invention therefore relates to an RFB, also in the embodiment of a hybrid RFB, which uses underground storage devices (especially caverns) as storage tanks and thus uses brine liquid (ie concentrated salt solution, mainly sodium chloride or potassium chloride dissolved therein).

[0070] An underground storage device for the purposes of this specification is understood in a broad sense to mean an underground structure which can be filled with a large amount of electrolyte and drained. This can be a cave or a porous rock formation. In the underground storage device used according to the invention, a temperature of at least 30° C. is present.

[0071] A cave is understood to be a large underground cavity created naturally or artificially in an enclosed structure.

[0072] For example, caverns originally intended as natural gas storage facilities or oil storage facilities may be used as underground storage devices within the scope of the present invention. However, mined oil or gas deposits, or caverns specifically made for the purpose of running RFBs may also be used.

[0073] For operating the RFB according to the present invention, no above ground containers or tanks are required to store electrolyte. This can reduce the equipment and cost requirements for storing electrolyte for RFBs with high capacity. No extensive piping system is required to interconnect a large number of tanks or containers.

[0074] The underground storage device used according to the invention is, for example, an underground cavity or an underground rock formation, which can be located, for example, several hundred meters below the surface of the earth.

[0075] The electrolyte received in the underground storage device is typically the catholyte or the anolyte for the RFB, although they may preferably be present together in one electrolyte.

[0076] Salt dome caverns may be used as preferred underground storage means. They may be produced in a known manner by flushing or leaching salt out of underground salt formations. Thus, underground cavities may be produced using known methods to serve as underground storage means to store electrolytes for RFBs. Alternatively, existing caverns originally intended for natural gas or oil storage may be used to store electrolytes for RFBs.

[0077] It is to be understood that in addition to salt dome caverns, other cavern types, such as granite caverns, are also suitable for storing electrolytes for RFBs.

[0078] In a preferred embodiment, it can be provided that the cavern is delimited at least in sections, in particular completely by rock, for example by granite. Such a cavern was originally intended for storing natural gas or crude oil and can be converted into an underground storage device for RFB without any additional burden on the environment.

[0079] For example, gas or oil caverns that have been filled with salt water can be connected to the circuit of the RFB, and redox-active compounds can be added to the salt water during circulation. The salt water can be enriched with redox-active compounds by adding them to the salt water above ground. In this way, large storage capacities can be built with relatively little effort as battery storage for the RFB.

[0080] The underground storage device used according to the invention generally has a capacity of at least 20,000 m 3 , preferably 70000m 3 -1000000m 3 , or especially 500000m 3 -800000m 3 The volume.

[0081] As long as geological and technical boundary conditions allow, the volume of underground storage devices intended for battery storage of RFB is freely scalable and can also accommodate more than 1 million cubic meters of electrolyte.

[0082] Several underground storage units can also be interconnected, possibly in series or in parallel.

[0083] The redox active compound used in the RFB according to the present invention may be a low molecular weight organic molecule, an oligomer or a polymer. These molecules may be dissolved in an electrolyte solution or may be present as dispersed or suspended particles, for example as a dispersion, a microgel or a nanogel. The redox active compound comprises at least one group of formula (I), or a combination of at least one group of formula (Ia) with hydrochloric acid and / or a selected salt, or at least one group of formula (Ib) or (Ic).

[0084] The redox-active compounds used in the electrolyte solution according to the invention are preferably water-soluble. However, compounds which are dispersible in water can also be used.

[0085] Water-soluble iron salts can also be used in the cathode electrolyte of the RFB according to the invention. They are used in the form of complexes with nitrogen-containing ligands. Examples of suitable ligands are amino acids, especially glycine, amines, especially polyamines, or ethylenediamine derivatives, such as EDTA.

[0086] The water solubility of a compound is understood in the context of this specification to mean the solubility of at least 1 g of the compound in 1 L of water at 25°C.

[0087] In the context of the present description, a low molecular weight molecule is understood to mean a compound which has no repeating structural units derived from monomers and which contains at least one, preferably 1 to 6, particularly preferably 1 to 4, in particular 1 to 3 and very particularly preferably 1 or 2, groups of the formula (I), (Ia), (Ib) or (Ic).

[0088] In the context of this specification, an oligomer molecule is a compound having 2 to 10 repeating structural units derived from monomers, each of which carries a group of formula (I), (Ia), (Ib) or (Ic). The group of formula (I) may also be attached to the oligomer backbone via a spacer.

[0089] In the context of this specification, a polymer molecule is understood to be a compound having more than 10, preferably 11 to 50 repeating structural units derived from monomers, each of which carries a group of formula (I), (Ia), (Ib) or (Ic). The group of formula (I) may also be attached to the polymer backbone via a spacer.

[0090] In a preferred embodiment, redox-active components having 1 to 6, preferably 1 to 4, in particular 1 to 3 and most preferably 1 to 2 groups of the formula (I), (Ia), (Ib) or (Ic) in the molecule are used in the RFB or mixed RFB according to the invention.

[0091] Examples of preferably used low molecular weight, oligomeric and polymeric redox-active compounds are given below.

[0092] The molar amount of the redox-active compound containing at least one group of the formula (I), (Ia), (Ib) or (Ic) used in the RFB or hybrid RFB according to the invention can vary within wide ranges. Particular preference is given to using redox-active compounds containing groups of the formula (I), (Ia), (Ib) or (Ic) whose molecular weight varies in the range of 150 to 80,000 g / mol, preferably 250 to 50,000 g / mol and very preferably 500 to 20,000 g / mol.

[0093] The viscosity of the electrolyte used in the RFB or hybrid RFB according to the present invention is usually 1 mPas up to 10 3 mPas, particularly preferably 1-10 2 mPas and most preferably 1-20 mPas (measured with a rotational viscometer at 25° C., plate / plate).

[0094] The group of redox-active compounds used according to the invention is characterized by the presence of at least one group X on at least one cyclopentadienyl ring. The group X consists of a solubility-regulating group FG, which is preferably bonded to the cyclopentadienyl ligand of the ferrocene via an alkylene spacer having up to 4 carbon atoms or via an alkylene oxide-alkylene group having up to 4 carbon atoms as a spacer or via an alkylene sulfide-alkylene group having up to 4 carbon atoms as a spacer, or consists of an alkylene spacer with a divalent bridging group Brgp, which connects the group of formula (I) to the remainder of the molecule.

[0095] The alkylene spacer may be a group of the formula -CH2-CH(CH3)-, -(CH2)3-, -(CH2)4- or in particular -(CH2)2-.

[0096] The alkylene oxide spacer may be 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-.

[0097] The sulfide alkylene-alkylene spacer may be 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-.

[0098] The divalent bridging group Brgp can be any divalent group that connects the group of formula (I) to the rest of the molecule, for example, to the polymer backbone. Examples of bridging groups Brgp include alkylene, -O-, -S-, -SO-, -SO2-, -CO-, -CO-O-, -NR2-, -CO-NR2- and -SO2NR2-.

[0099] Particularly preferred are aqueous electrolyte solutions which contain a compound having at least one redox-active group of the formula (I) in which n is 2.

[0100] These compounds offer a particularly advantageous combination of water solubility and temperature stability. In addition, such compounds are particularly advantageously synthesized by treating the cyclopentadienyl ring of ferrocene with butyllithium and then reacting it with an alkylene oxide. Further residues can also be introduced via the OH groups formed. In addition, vinylferrocene can be functionalized by reacting with thiols containing FG groups.

[0101] In addition to the radical X, the radicals of the formula (I) in the redox-active compounds used according to the invention may contain further radicals on the cyclopentadienyl ring: n H 2n )-FG or formula-(C w H 2w )-O-(C n H 2n )-FG or formula-(C w H 2w )-S-(C n H 2n )-FG(group Y). However, Y is preferably hydrogen.

[0102] In the RFB according to the invention, an electrolyte solution with a high salt concentration is used, which contains water or water and an organic solvent, as well as the above-mentioned redox-active compounds, conductive salts and possibly other substances dissolved therein. The conductive salts are mainly used for charge balancing during the charging or discharging of the battery. Other substances dissolved in the electrolyte (also called auxiliary additives) have a positive influence on the stability or performance parameters of the battery.

[0103] Examples of electrolyte solvents are water or a mixture of an organic liquid miscible with water. Water is particularly preferred.

[0104] The conductive salt is usually an organic or inorganic salt. Examples are salts containing anions selected from the following: halides (fluoride, chloride, bromide, iodide), hydroxide, anions of inorganic acids (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: 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 n-butyl.

[0105] Particularly preferred electrolyte solutions for the RFB according to the present invention contain conductive salts containing anions selected from the group consisting of halide ions, hydroxide ions, phosphate ions, sulfate ions, perchlorate ions, hexafluorophosphate ions or tetrafluoroborate ions, in particular conductive additives containing these anions and cations selected from the group consisting of hydrogen ions, alkali metal or alkaline earth metal cations, and substituted or unsubstituted ammonium cations.

[0106] Examples of auxiliary additives are surfactants, viscosity modifiers, biocides, buffers, stabilizers, catalysts, conductive additives, antifreeze agents, temperature stabilizers and / or defoamers.

[0107] 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.

[0108] 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).

[0109] The electrolyte solution used for the RFB according to the invention has high temperatures. These occur during storage of the electrolyte solution in underground storage devices and therefore also in the electrode compartments of batteries or accumulators in which the electrolyte solution is used, for example during charging and / or discharging.

[0110] The aqueous electrolyte solution used preferably has a temperature of 30-90°C, in particular 40-70°C.

[0111] The aqueous electrolyte solution used contains at least one conductive salt, in particular such a salt produced during the cavern production process. The brine produced by this production method can preferably be used as the electrolyte solution in the RFB according to the invention.

[0112] The concentration of the salt dissolved in the electrolyte solution used according to the invention is very high. Preferably the concentration of the conductive salt is from 14 wt% to the saturation limit. Saturated salt solutions are often used, in which a portion of the salt is precipitated in the formation.

[0113] Particularly preferred RFBs according to the invention have a state of charge of the catholyte or of the catholyte and anolyte of less than 90%, in particular up to 80% and very preferably 70-80%. These RFBs are characterized by a particularly good temperature stability of the redox-active compounds in the catholyte and anolyte. Of course, the state of charge of the catholyte or of the catholyte and anolyte in the RFBs according to the invention can also be 100%.

[0114] 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 usable capacity of the electrolyte solution or battery relative to the nominal value. The state of charge is specified as a percentage of the fully charged state.

[0115] The state of charge can be determined, for example, via the off-circuit voltage (VOC). The no-load voltage is dependent on the state of charge; it increases as the state of charge increases. The cell voltage is measured in the “off-circuit” state, i.e. it is the cell voltage that would occur at a given state of charge without external load.

[0116] Also preferred in the operation of the RFB according to the invention is an aqueous electrolyte solution containing a compound having at least one redox-active group of the formula (I) in which Y is hydrogen and m is 1 or 2.

[0117] Also preferably used are aqueous electrolyte solutions containing compounds having at least one redox-active group of the formula (I), wherein FG is a functional group selected from the group consisting of: -(O-CH2-CH2) o -OR 2， -(O-CH2-CH2) o –NR3R4R5 + (An m- ) 1 / m , -COR2, –COO - (Kat u+ ) 1 / u, -SO3 - (Kat u+ ) 1 / u , -SO4 - (Kat u+ ) 1 / u , -PO4 2- (Kat u+ ) 2 / u ,-PO3 2- (Kat u+ ) 2 / u Or -NR3R4R5 + (An m- ) 1 / m .

[0118] Anion m- Examples are halide ions, such as fluoride ion or chloride ion, or hydroxide ion, anion of an inorganic acid (e.g. phosphate ion, sulfate ion, nitrate ion, hexafluorophosphate ion, tetrafluoroborate ion, perchlorate ion, chlorate ion, hexafluoroantimonate ion, hexafluoroarsenate ion, cyanide ion), or anion of an organic acid (e.g. acetate ion, formate ion, trifluoroacetate ion, trifluoromethanesulfonate ion, pentafluoroethanesulfonate ion, nonafluorobutane-sulfonate ion, butyrate ion, citrate ion, fumarate ion, glutarate ion, lactate ion, malate ion, malonate ion, oxalate ion, pyruvate ion, tartrate ion).

[0119] Preferred anions An m- Selected from halide ion, hydroxide ion, phosphate ion, sulfate ion, perchlorate ion, hexafluorophosphate ion or tetrafluoroborate ion.

[0120] Cationic Kat u+ Examples are hydrogen cations, metal cations, ammonium cations or phosphonium cations.

[0121] Preferred cation Kat u+ It is a hydrogen cation, an alkali metal cation, an alkaline earth metal cation or an ammonium cation, in particular a hydrogen cation, a sodium, potassium, magnesium or calcium cation, or a quaternary ammonium cation.

[0122] The aqueous electrolyte solution used is particularly preferably a compound having at least one redox-active group of formula (I), wherein Z is a covalent bond which connects the group 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).

[0123] Very preferably, the aqueous electrolyte solution used comprises oligomers or polymers which contain repeating structural units of the formula (II) and optionally further structural units derived from solubility-promoting comonomers:

[0124]

[0125] in

[0126] ME is a repeating structural unit derived from a polymerizable monomer,

[0127] BG is a covalent bond or bridging group,

[0128] FC is a group of formula (X-C5H4)Fe(Y-C5H3-Z),

[0129] X, Y and Z have the meanings defined above, and

[0130] r is an integer of 2-150, preferably 2-80 and most preferably 8-40.

[0131] Other particularly preferably used aqueous electrolyte solutions comprise oligomers or polymers which contain repeating structural units corresponding to the formula (V) and optionally further structural units derived from solubility-promoting comonomers:

[0132]

[0133] in

[0134] ME is a repeating structural unit derived from a polymerizable monomer,

[0135] BG is a covalent bond or bridging group

[0136] Pip is a pyridyl group of formula (Ia), (Ib) or (Ic), and

[0137] r is an integer from 2 to 150, provided that in the case where Pip is a group of formula (Ia), the electrolyte solution further comprises a salt selected from ammonium salts with inorganic or organic anions, salts with tetrafluoroborate anions or salts of trifluoromethanesulfonic acid.

[0138] The repeating units ME and BG form the backbone of the oligomer or polymer comprising several units of the redox active units of formula (I), (Ia), (Ib) or (Ic) defined above.

[0139] Examples of species that 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 their chemically modified derivatives, such as cellulose or cellulose ethers, and 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.

[0140] The following are examples of combinations of structural units ME and bridging groups BG for some of the above-mentioned substance classes. They are:

[0141]

[0142] Particularly preferred classes of substances forming the oligomeric or polymeric backbone are polymethacrylates, polyacrylates, polymethacrylamides, polyacrylamides, polystyrenes, polyethyleneimines and polyvinyl ethers.

[0143] The redox-active unit of formula (I), (Ia), (Ib) or (Ic) is covalently bonded to the polymer backbone.

[0144] The polymers containing the redox-active component may be linear polymers or they may be comb and star polymers, dendrimers, ladder polymers, ring polymers, polycathenes and polyrotaxanes.

[0145] Comb and star polymers, dendrimers, ladder polymers, ring polymers, polycormanes and polyrotaxanes are preferably 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.

[0146] 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 polyethylene glycol, polymethacrylic acid, polyacrylic acid or polystyrene sulfonates.

[0147] Preferred solubility-promoting comonomers are vinyl acetate, methyl vinyl ether, methacrylic acid, acrylic acid, alkyl methacrylates, ethyl acrylate, methacrylic acid amide, acrylic acid amide, vinyl sulfonates, vinylphosphonic acid or styrene sulfonates.

[0148] 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 methods. Examples of this are bulk polymerization, polymerization in solution or emulsion or suspension polymerization. These procedures are known to the person skilled in the art.

[0149] X is -O or -S-, preferably -O-.

[0150] Q is preferably -O- or -S-, especially -O-, or -Q- is preferably a covalent bond.

[0151] Low molecular weight compounds containing one or more groups of formula (I) generally have the following structure

[0152] (X-C5H4)Fe(Y-C5H3-)-R 22 ,

[0153] wherein X and Y have the meanings defined above, and R 22 is hydrogen or a monovalent or polyvalent, in particular monovalent to hexavalent organic radical, which may optionally contain further (X-C5H4)Fe(Y-C5H3-)- groups.

[0154] Very preferably, the aqueous electrolyte solution used comprises as redox-active compound a compound of the formula (III):

[0155] (X-C5H4)Fe(Y-C5H4) (III),

[0156] wherein X and Y have the meanings defined above.

[0157] Other very particularly preferably used aqueous electrolyte solutions comprise compounds of the formula (IV) as redox-active compounds:

[0158] [(X-C5H4)Fe(Y-C5H3-)]-R 23 -[(-C5H3-Y)Fe(C5H4-X)] p (IV),

[0159] Where X and Y are as defined above,

[0160] R 23 is a divalent to tetravalent organic radical, and

[0161] p is an integer from 1 to 4.

[0162] Other preferably used aqueous electrolyte solutions contain compounds having at least one redox-active group of formula (Ia), (Ib) or (Ic), which are covalently bonded 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 (Ia), (Ib) or (Ic).

[0163] The aqueous electrolyte solution preferably used contains a compound having one or more groups of formula (Ia), (Ib) or (Ic) and has the following structure:

[0164] Pip-R 15a -(Pip) x ,

[0165] wherein Pip is a pyridine group of formula (Ia), (Ib) or (Ic),

[0166] x is an integer from 0 to 5, preferably from 0 to 3, and

[0167] R 15a is a monovalent to hexavalent organic group or -O- or -S-, provided that in the case where Pip is a group of formula (Ia), the electrolyte solution further comprises hydrochloric acid and / or a salt selected from the following: an ammonium salt with an inorganic or organic anion, a salt with a tetrafluoroborate anion or a salt of trifluoromethanesulfonic acid.

[0168] Other particularly preferably used aqueous electrolyte solutions contain redox-active compounds of the formula (VIa) or (VIb):

[0169]

[0170]

[0171] Among them, R7, R8, R9, R 10 and R 11 has the meaning as defined above,

[0172] q is an integer from 1 to 3,

[0173] R 15 is a monovalent organic group, which is optionally attached to the piperidinyl group through an oxygen, sulfur or nitrogen atom, and

[0174] R 16 It is -O-, -S- or a divalent to tetravalent organic group.

[0175] Very preferably, the aqueous electrolyte solution used comprises a redox-active compound of the formula (VII), (VIII), (IX), (X) or (XI):

[0176]

[0177]

[0178] Among them, R7, R8, R9, R 10 , R 11 , R 12 , Q, ME, BG, r and q have the meanings as defined above,

[0179] R 16a is a di- to tetravalent organic radical, and

[0180] t is 0 or 1.

[0181] Other very particularly preferably used aqueous electrolyte solutions comprise compounds of the formula (VIa) as defined above, in which:

[0182] R 15 Yes-O-(C s H 2s )-R 23 ,-S-(C s H 2s )-R 23 or –NR2-(C s H 2s )-R 23 ,

[0183] s is an integer from 2 to 4,

[0184] R2 has one of the meanings defined above,

[0185] R 23 is a functional group selected from the following: –OH, -SH, -NO3, -NO2, -CN, -OR 17 , -SR 17 , -(O-CH2-CH2) v -OR 18 , -(O-CH2-CH2) v -SR 18 , -COR 18 , –COO - (Kat u+ ) 1 / u , -COOR 18 , -SO3 - (Kat u+ ) 1 / u , -SO3R 18 , -SO4- (Kat u+ ) 1 / u , -SO4R 18 , -PO4 2- (Kat u+ ) 2 / u ,-PO4(R 18 )2, -PO3 2- (Kat u+ ) 2 / u ,-PO3(R 18 )2 or –NR 19 R 20 R 21 + (An m- ) 1 / m ,

[0186] R 17 is a C1-C4 alkyl group,

[0187] R 18 is hydrogen or C1-C4 alkyl,

[0188] R 19 , R 20 , R 21 and R 22 are independently of one another hydrogen or alkyl, in particular each is methyl,

[0189] Kat is a u-valent cation,

[0190] An is an m-valent anion,

[0191] m and u are independently integers 1-4, and

[0192] v is an integer from 1 to 50.

[0193] In the preferred redox flow battery of the present invention, a redox active compound containing one or more groups of formula (Ia) and a stabilizing additive are used, the stabilizing additive being selected from ammonium salts of inorganic or organic anions, preferably ammonium salts of chloride or fluoride anions, salts of tetrafluoroborate anions or salts of trifluoromethanesulfonic acid. In particular, ammonium cations or metal cations are present as cations of these BF4 or trifluoromethanesulfonates.

[0194] Preferably hydrochloric acid or ammonium halide is used as additive. The latter is usually a quaternary ammonium halide, in particular an alkyl quaternary ammonium halide. The alkyl group in particular has 1 to 4 carbon atoms, which is preferably substituted with a hydroxyl group.

[0195] Particular preference is given to using quaternary ammonium chlorides or fluorides. The salts of the tetrafluoroborate anion or the trifluoromethanesulfonic acid are preferably salts with alkali metal cations, alkaline earth metal cations or with ammonium cations, in particular salts with sodium, potassium, magnesium or calcium cations, or salts with quaternary ammonium cations.

[0196] Very preferably, an aqueous electrolyte solution is used which contains a compound having at least one redox-active group of the formula (Ic) in which Y is -O-, -S-, -SO-, -SO2-, -NR 13 -or–N + R 13 R 14 -(An m- ) 1 / m They exhibit increased water solubility compared to compounds with Y=-CH2-. These compounds offer a particularly advantageous combination of water solubility and temperature stability.

[0197] Another preferred redox flow battery of the present invention comprises an iron solid cathode having a redox couple of iron(II) / iron(0) or iron(III) / iron(0); very preferably such a redox couple is used in combination with a zinc solid anode having a redox couple of zinc(II) / zinc(0).

[0198] Using R 22 , R 23 , R 15a , R 16 and R 16a A divalent to tetravalent organic group refers to an organic group which is attached to the rest of the molecule via 2, 3 or 4 covalent bonds.

[0199] Examples of the divalent organic group include an alkylene group, an alkyleneoxy group, a poly(alkyleneoxy group), an alkyleneamino group, a poly(alkyleneamino group), a cycloalkylene group, an arylene group, an aralkylene group, or a heterocyclic group.

[0200] The alkylene group may be branched or unbranched. The alkylene group generally contains 1 to 20 carbon atoms, preferably 2 to 4 carbon atoms. Examples of alkylene groups include methylene, ethylene, propylene and butylene. The alkylene group may be optionally substituted, for example, substituted with a carboxylic acid or sulfonic acid group, substituted with a carboxylate or sulfonic acid ester group, substituted with a carboxylic acid amide or sulfonic acid amide group, substituted with a hydroxyl or amino group, or substituted with a halogen atom.

[0201] Alkyleneoxy and poly(alkyleneoxy) groups may contain both branched and unbranched alkylene groups. The alkylene groups present in the alkyleneoxy or poly(alkyleneoxy) groups typically contain 2 to 4 carbon atoms, preferably 2 or 3 carbon atoms. The number of repeating units in the poly(alkyleneoxy) groups may vary within a wide range. The typical number of repeating units is 2 to 50. Examples of alkyleneoxy groups include: ethyleneoxy, propyleneoxy and butyleneoxy. Examples of poly(alkyleneoxy) groups are: poly(ethyleneoxy), poly(propyleneoxy) and poly(butyleneoxy).

[0202] Alkyleneamino and poly(alkyleneamino) groups may contain both branched and unbranched alkylene groups. The alkylene groups present in the alkyleneamino or poly(alkyleneamino) groups typically contain 2 to 4 carbon atoms, preferably 2 or 3 carbon atoms. The number of repeating units in the poly(alkyleneamino) group may vary over a wide range. A typical number of repeating units is 2 to 50. Examples of alkyleneamino groups include: ethyleneamino, isopropylamino and butyleneamino. Examples of poly(alkyleneamino) groups are: poly(ethyleneamino), poly(propyleneamino) and poly(butyleneamino).

[0203] Cycloalkylidene usually comprises 5,6 or 7 ring carbon atoms, each of which can be substituted independently of one another.The example of substituent comprises alkyl or two alkyl groups, which together with the ring carbon they are connected to can form another ring.The example of cycloalkylidene is cyclohexylene.Cycloalkylidene can be optionally substituted, for example, replace with carboxylic acid or sulfonic acid group, replace with carboxylate or sulfonic acid ester group, replace with carboxylic acid amide or sulfonic acid amide group, replace with hydroxyl or amino, or replace with halogen atom.

[0204] Arylene is usually a ring-forming aromatic group, containing 5-14 carbon atoms, each of which can be substituted independently of one another. The example of arylene is o-phenylene, m-phenylene, p-phenylene, o-biphenylene, m-biphenylene, p-biphenylene, 1-naphthyl, 2-naphthyl, 1-anthryl, 2-anthryl, 9-anthryl, 1-phenanthrenyl, 2-phenanthrenyl, 3-phenanthrenyl, 4-phenanthrenyl or 9-phenanthrenyl. Arylene can be optionally substituted, for example, substituted with carboxylic acid or sulfonic acid groups, substituted with carboxylate or sulfonate groups, substituted with carboxylic acid amide or sulfonic acid amide groups, substituted with hydroxyl or amino, or substituted with halogen atoms. Other examples of substituents include alkyl or two alkyl groups, which can form another ring together with the ring carbon atoms they are connected to.

[0205] Heterocyclic groups are generally ring-forming groups having 4-10 ring carbon atoms and at least one ring heteroatom, each of which can be substituted independently of one another. Examples of heteroatoms are oxygen, nitrogen, phosphorus, boron, selenium or sulfur. Examples of heterocyclic groups are furandiyl, thiophenediyl, pyrrolediyl or imidazolediyl. Heterocyclic groups are preferably aromatic. Heterocyclic groups can be optionally substituted, for example, with carboxylic acid or sulfonic acid groups, with carboxylate or sulfonate groups, with carboxylic acid amide or sulfonic acid amide groups, with hydroxyl or amino groups, or with halogen atoms. Another example of a substituent is an alkyl group, or two alkyl groups, which together with the ring carbon atoms to which they are attached can form another ring.

[0206] Aralkylene groups are usually aryl groups to which one or two alkyl groups are covalently bonded. Aralkyl groups may be covalently bonded to the remainder 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 with alkyl or halogen atoms. Examples of aralkylene groups are benzylidene or dimethylphenylene (xylylene).

[0207] R as a trivalent organic group 22 , R 23 , R 15a , R 16 and R 16a Examples are alkyltriyl, alkoxytriyl, trimer (alkyleneoxy), trimer (alkyleneamino), cycloalkyltriyl, aryltriyl, aralkyltriyl or heterocyclotriyl. These groups correspond to the divalent groups detailed above and differ in that they are linked to the rest of the molecule with three covalent bonds instead of two.

[0208] R as a tetravalent organic group 22 , R 23 , R 15a , R 16 and R 16a Examples are alkyltetrayl, alkoxytetrayl, tetra(alkyleneoxy), tetra(alkyleneamino), cycloalkyltetrayl, aryltetrayl, aralkyltetrayl or heterocyclotetrayl. These groups correspond to the divalent groups detailed above and differ in that they are linked to the rest of the molecule by four covalent bonds instead of two.

[0209] R as a monovalent organic group 22 , R 15 or R 15a represents an organic group attached to the rest of the molecule by a covalent bond.

[0210] Group R 15 or R 15aExamples of are alkyl, alkoxy, alkylamino, cycloalkyl, aryl, aralkyl or heterocyclic. These radicals correspond to the divalent radicals already described in detail above and differ in that they are linked to the rest of the molecule by one covalent bond instead of two. 15 or R 15a The bond to the remainder of the molecule can be via a carbon atom or via a heteroatom, for example via a nitrogen atom or an oxygen atom.

[0211] Preferred is the redox flow battery of the first embodiment, wherein the anolyte contains a compound containing one or more bipyridyl groups in the molecule as a redox active component.

[0212] These are generally compounds which contain at least one redox-active group of the formula (IX) or (X) in the molecule, in particular 1 to 6, in particular 1 to 4, very preferably 1 to 3 and most preferably 1 to 2 groups of the formula (IX) or (X) in the molecule:

[0213]

[0214] in

[0215] The lines extending from the nitrogen atoms in the structures of Formula IX and X represent covalent bonds connecting the structures of Formula IX and X to the rest of the molecule,

[0216] R 24 and R 25 are independently alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, heterocyclyl, halogen, hydroxy, amino, nitro or cyano,

[0217] An is an m-valent inorganic or organic anion,

[0218] b and c are independently integers from 0 to 4, preferably 0, 1 or 2, especially 0, and

[0219] m is an integer from 1 to 4.

[0220] The compound as the redox active component containing one or more bipyridyl groups in the molecule can be a low molecular weight organic molecule, an oligomer or a polymer. They preferably contain at least one group of formula (IX) or formula (X) which is connected to the rest of the molecule via a covalent bond.

[0221] The redox-active compounds containing groups of the formula (IX) or (X) are preferably water-soluble. However, they may also be compounds dispersible in water.

[0222] Anion m- The example is as mentioned above.

[0223] Preferred redox-active components for the anolyte are compounds of the formula IXa or Xa:

[0224]

[0225] in

[0226] R 24 , R 25 , An, b, c and m have the meanings as defined above,

[0227] R 26 and R 27 are independently of one another hydrogen, alkyl (optionally substituted with carboxylate, carboxylic acid amide, carboxylic acid, sulfonic acid or amino), cycloalkyl (optionally substituted with carboxylate, carboxylic acid amide, carboxylic acid, sulfonic acid or amino), aryl (optionally substituted with carboxylate, carboxylic acid amide, carboxylic acid, sulfonic acid or amino), or aralkyl (optionally substituted with carboxylate, carboxylic acid amide, carboxylic acid, sulfonic acid or amino), in particular C1-C6-alkyl, C1-C6-alkyl substituted with carboxylate groups, C1-C6-alkyl substituted with carboxylic acid amide groups, C1-C6-alkyl substituted with carboxylic acid groups, C1-C6-alkyl substituted with sulfonic acid groups or C1-C6-alkyl substituted with amino groups, and very preferably propionate, isobutyrate, ethyl or methyl, and

[0228] R 28 and R 29 are independently hydrogen, alkyl (optionally substituted with carboxylate, carboxylic acid amide, carboxylic acid, sulfonic acid or amino), cycloalkyl (optionally substituted with carboxylate, carboxylic acid amide, carboxylic acid, sulfonic acid or amino), aryl (optionally substituted with carboxylate, carboxylic acid amide, carboxylic acid, sulfonic acid or amino), or aralkyl (optionally substituted with carboxylate, carboxylic acid amide, carboxylic acid, sulfonic acid or amino), or two radicals R 28 and R 29 Together they form C1-C3-alkylene, in particular C1-C6-alkyl, C1-C6-alkyl substituted by a carboxylate group, C1-C6-alkyl substituted by a carboxylic acid amide group, C1-C6-alkyl substituted by a carboxylic acid group, C1-C6-alkyl substituted by a sulfonic acid group, or C1-C6-alkyl substituted by an amino group, or together they form ethylene, and very preferably propionate, isobutyrate, ethyl or methyl, or together they form ethylene.

[0229] Examples of compounds which are particularly preferably used are N,N'-diC1-C4-alkyldipyridyl cations with any anions for charge balance, such as N,N'-dimethyldipyridyl halides, for example N,N'-dimethyldipyridyl chloride.

[0230] Preferred redox flow batteries of the first embodiment comprise as anolyte an aqueous electrolyte solution having a temperature of at least 30° C., preferably 40-90° C. and in particular 40-70° C., comprising a redox-active compound containing at least one group of formula (IX) or (X).

[0231] In another preferred redox flow battery of this first embodiment, the electrode compartments for the catholyte and anolyte are separated by a semipermeable membrane which is impermeable to the redox pairs in the catholyte and anolyte. This can be, for example, a size exclusion membrane, such as a dialysis membrane.

[0232] In another preferred redox flow battery of this first embodiment, the electrode compartments for the catholyte and the anolyte are separated by a semipermeable membrane impermeable to the redox components in the catholyte, for example by a size exclusion membrane, and the anolyte comprises a zinc salt as redox active component.

[0233] The redox flow batteries of the second embodiment are characterized in that they need only have one underground storage device and do not require a membrane for separating the reaction cell into anode and cathode chambers.

[0234] This preferred redox flow battery of the second embodiment comprises no separator and only one underground storage device or several underground storage devices in series for the aqueous electrolyte solution.

[0235] Particularly preferred is a redox flow battery in which a zinc-solid anode having a redox couple zinc(II) / zinc(0) is used. Among them, the redox flow battery of the second embodiment is particularly preferred.

[0236] Examples of zinc salts used in the redox flow battery of the first and second embodiments include 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.

[0237] In a second embodiment and in a preferred embodiment of a redox flow battery of the first type according to the invention, zinc is used as redox-active anode material. It is particularly emphasized that zinc can be accommodated in different aggregation states within the battery.

[0238] In a second embodiment of the redox flow battery according to the invention and in a preferred variant of the first embodiment, zinc is used as redox-active anode material. It is particularly important to emphasize that zinc can be accommodated in different aggregation states within the battery.

[0239] During charging, the zinc(II) cations dissolved in the electrolyte are reduced to elemental zinc(0) on the anode surface. To this end, two electrons must be accepted. The active material zinc can therefore be present in the battery both in dissolved and solid form.

[0240] The zinc solid anode may always be present as a metal electrode, or it may be formed in situ on the conductive surface of the chamber during battery charging by reducing zinc cations. The zinc cations may serve primarily as the active material while serving secondarily as a conductive additive or as part of a conductive additive mixture.

[0241] During the charging process, 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 accepting two electrons, metallic zinc is deposited on the electrode surface. In the opposite case, metallic zinc is transferred from the electrode surface to zinc ions by releasing two electrons, which are enriched in the anolyte.

[0242] Particularly preferred redox flow batteries according to the invention have a zinc solid anode having the redox couple zinc(II) / zinc(0).

[0243] On the one hand, an advantage over the above-described fully organic redox flow batteries is reduced production costs. Zinc anodes can be produced significantly more cost-effectively than organic anodes. On the other hand, the redox couple Zn(II) / Zn(O) is characterized by very good stability against external environmental influences, such as sensitivity to oxygen. Conventional systems must be kept oxygen-free; this significantly complicates the battery construction and increases the operating costs. The use of zinc anodes can be completely omitted here.

[0244] Zinc also has a very high overvoltage in aqueous media, which allows for extremely high potential windows. 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, which is within the electrochemical voltage series without unwanted side reactions or decomposition of redox active materials, conductive additives, electrolytes or any other components in the entire battery. Conventional aqueous flow batteries are limited to a potential window of about 1.2V. When it is exceeded, hydrogen will be produced. By using a zinc anode, the potential window can be expanded to more than 2V. This leads to a considerable increase in electrical power / battery. Zinc has a very high overvoltage compared to hydrogen, so despite the high voltage of 2V, no hydrogen is formed at the anode, and the battery can operate safely.

[0245] The term "battery" is used in the context of this specification in its broadest sense. It may refer to a single rechargeable electrochemical cell or a combination of several such electrochemical cells.

[0246] In addition to the components described above, the redox flow battery according to the present invention may contain other elements or components conventionally used for such batteries.

[0247] In the redox flow battery according to the invention, the selected redox-active components are used, which are present in the chambers in dissolved, liquid or dispersed form.

[0248] The redox potential of the redox-active component can be determined, for example, by 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).

[0249] A first type of redox flow battery according to the invention comprises a semipermeable or microporous membrane. This fulfils the following functions:

[0250] Separates the anode and cathode compartments

[0251] · Trapping the redox active components in the catholyte and anolyte, i.e. trapping the cathode and anode active materials

[0252] • Permeability of the conductive salt of the electrolyte for balancing the charge, ie for the anions and / or cations of the conductive salt or for the charge carriers generally contained in the electrolyte.

[0253] The diaphragm can be a size exclusion membrane, such as a dialysis diaphragm, but can also be an ion selective membrane. The diaphragm prevents redox active compounds from being passed from the cathode compartment to the anode compartment and prevents redox active components of the anolyte from entering the cathode compartment. Dissolved zinc (II) cations are not necessary, but can also be blocked by the diaphragm.

[0254] Depending on the application, the material of the diaphragm 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. Diaphragms consisting of several materials (composite materials) are also possible.

[0255] The membrane and the resulting redox flow battery can be used in different configurations. Examples include flat membranes, bag filter configurations, and wound modules. These embodiments are known to those skilled in the art. Preferably, a flat membrane is used.

[0256] The thickness of the separator used according to the invention can vary within wide ranges. Typical thicknesses are 0.1 μm to 5 mm, particularly preferably 10 μm to 200 μm.

[0257] In addition to the above-mentioned redox-active components, electrolyte and optional separator, the redox flow battery according to the invention preferably comprises further components. These are:

[0258] Conveying devices such as pumps, as well as tanks and pipes, for transporting and storing redox-active components

[0259] An electrode, preferably consisting of or containing graphite, graphite nonwoven fabric, graphite paper, carbon nanotube blanket, activated carbon, carbon black or graphene,

[0260] Optional electrical conductors, for example selected from graphite or from metals.

[0261] The negative electrode preferably comprises zinc and may further comprise materials such as:

[0262] Stainless steel, Hastelloy or an alloy containing iron-chromium-nickel, graphite, graphite non-woven fabric, graphite paper, carbon nanotube blanket, activated carbon, carbon black or graphene.

[0263] The redox flow battery according to the invention contains electrical conductors as further optional but preferred components. Their task is to establish the best possible electrical contact between the electrode material and an external current source or current sink.

[0264] Aluminum, aluminum 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 as electrical conductors in the redox flow battery according to the invention.

[0265] All components capable of transferring electric current to an electrode are called current collectors. The redox reaction takes place at the electrodes (cathode and anode) in direct contact with the electrolyte.

[0266] The redox flow battery according to the invention may be used in a variety of fields. In the broadest sense, this may include storing electrical energy for stationary applications. The invention also relates to the use of a redox flow battery for these purposes.

[0267] Examples of applications include use as stationary storage devices for emergency power supply, peak load balancing, and intermediate storage of electrical energy from renewable energy sources, in particular photovoltaic, hydroelectric and wind power plants, from natural gas, coal, biomass, tidal and marine power plants, and in the electric power sector, for example as storage devices in land, air and water vehicles.

[0268] The redox flow batteries according to the invention can be interconnected in a manner known per se, either in series or in parallel.

[0269] The following examples illustrate the invention without limiting it.

[0270] Figure 1 and 2 The redox flow battery according to the invention and its function are explained exemplarily.

[0271] Figure 1 Schematic constructions A, B-1 and B-2 of electrochemical cells according to the invention are shown without underground storage.

[0272] Figure 2 Schematic configurations A, B-1 and B-2 of RFBs according to the invention, ie electrochemical cells with underground storage, are shown.

[0273] Figure 1 In construction form A, an RFB or hybrid RFB is shown which has two separate electrolyte circuits and which therefore has to be connected to at least two underground storage devices.

[0274] also, Figure 1 A hybrid RFB is shown in construction form B-1, which has an electrolyte circuit, which therefore has to be connected to at least one underground storage device. In construction form B-1, a diaphragm (12) is provided, which divides the electrochemical cell into a cathode chamber and an anode chamber. Construction form B-2 of the hybrid RFB according to the invention differs from construction form B-1 in that the diaphragm is missing.

[0275] Figure 1In the drawings, two polarity-specific chambers (one for the cathode electrolyte (1) and one for the anode electrolyte (2)) separated by a diaphragm (12) are shown in the constructions A and B-1 with electrodes (4, 5). The two electrodes (4, 5) are used to charge and discharge the battery via an electrical conductor (13). In the case of construction A, the electrolyte is supplied separately via inflow nozzles (6, 7) and discharged again via outflow nozzles (8, 9), respectively. In the case of construction B, there is only one inflow nozzle (10) and one outflow nozzle (11), respectively. In the case of construction B-2, only one chamber (3) with electrodes (4, 5) is provided.

[0276] If the RFB according to the invention is operated as a hybrid RFB, metallic zinc is deposited on the electrode (5), the anode, or the anode consists of zinc and the anolyte contains Zn. 2+ ions or the electrolyte solution contains Zn 2+ ion.

[0277] Figure 2 The structural form A of the RFB according to the invention is outlined in FIG. An electrochemical cell (14) is shown, which is connected to two caves (16, 17). They are filled with an anolyte (19) and a catholyte (20), respectively. Via a pipeline (23), the anolyte and the catholyte (19, 20) are conveyed from the corresponding caves (16, 17) to the electrochemical cell (14) by means of a pump (22) and reintroduced into the caves (16, 17) via a pipeline (24). In addition to the redox-active compounds, the anolyte (19) and the catholyte (20) have a high salt content (brine). The above-mentioned ferrocene, 2,2,6,6-tetrasubstituted pyridyl or iron compound is used as a redox-active component in the catholyte (20). For example, the above-mentioned bipyridyl compound or zinc salt is used as a redox-active component in the anolyte (19). Structural form A can be operated as a conventional RFB or as a hybrid RFB.

[0278] Figure 2Also outlined is structural form B of the RFB according to the invention. In this case, the construction is quite simplified. Here, the electrochemical cell (15) is connected to only one cavity (18). It is filled with an electrolyte solution (21). The electrolyte solution (21) is transported from the cavity (18) to the electrochemical cell (15) via a pipeline (23) with the aid of a pump (22) and reintroduced into the cavity (18) via a pipeline (24). In addition to the redox-active compounds, the electrolyte solution (21) has a high salt content (brine). In addition to the highly concentrated conductive salt (brine) the electrolyte solution (21) also contains two active substances, namely, a redox-active component for the anode and a redox-active component for the cathode. The above-mentioned ferrocene, 2,2,6,6-tetrasubstituted pyridyl or iron compound is used as the redox-active component for the cathode. Zinc salt is used as the redox-active component for the anode. Structural form B is operated as a mixed RFB. It can also be omitted Figure 2 The diaphragm shown in the structural form B is used.

[0279] The caverns (16, 17, 18) are each filled with a brine containing redox-active components dissolved in the brine or in solid form (ie dispersed). If necessary, further conductive additives and further auxiliary additives may additionally be dissolved.

Claims

1. A redox flow battery for storing electrical energy, comprising a reaction cell with two electrode chambers for a catholyte and an anolyte, the two electrode chambers being each connected to at least one reservoir, wherein the electrode chambers are separated by a diaphragm, are equipped with electrodes and are each filled with an electrolyte solution comprising a redox-active component dissolved or dispersed in an aqueous electrolyte solvent, as well as a conductive salt dissolved therein and possibly further additives, characterized in that the at least one reservoir is an underground storage device having a temperature of at least 30°C, The concentration of the salt dissolved in the electrolyte solution is at least 10% by weight. The anolyte comprises a water-soluble or water-dispersible redox-active component, and The cathode electrolyte contains at least one compound or composition selected from one of the groups a), b), c), d) or e) as a redox-active component, wherein a) is an organic compound comprising at least one redox-active group of formula (I): (X-C5H4)Fe(Y-C5H3-Z)(I), b) is an organic compound comprising at least one redox-active group of formula (Ia): in combination with hydrochloric acid and / or a salt selected from the group consisting of ammonium salts with inorganic or organic anions, salts with tetrafluoroborate anions or salts of trifluoromethanesulfonic acid, c) is an organic compound comprising at least one redox-active group of formula (Ib): d) is an organic compound comprising at least one redox-active group of formula (Ic): e) is a water-soluble iron salt complexed with a nitrogen-containing ligand, wherein X is of the formula -(C n H 2n )-FG or formula-(C w H 2w )-Sp-(C n H 2n )-FG or formula-(C n H 2n )-Brgp-, Y is hydrogen or of the formula -(C n H 2n )-FG or formula-(C w H 2w )-Sp-(C n H 2n )-FG group, Z is hydrogen or a covalent bond connecting the radical of formula (I) to the rest of the molecule of said organic compound, 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 u+ ) 1 / u , -COOR2, -SO3 - (Kat u+ ) 1 / u , -SO3R2, -SO4 - (Kat u+ ) 1 / u , -SO4R2, -PO4 2- (Kat u+ ) 2 / u , -PO4(R2)2, -PO3 2 (Kat u+ ) 2 / u ,-PO3(R2)2,-NR3R4R5 + (An m- ) 1 / m , -N + R3R4-C t H 2t -SO3 - Or –NR2-SO2-R3, Brgp is a divalent bridging group which connects the group of formula (I) to the rest of the molecule, Sp is -O- or -S-, 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 a u-valent cation, An is an m-valent anion, m and u are independently integers from 1 to 4, n and w are independently integers from 2 to 4, o is an integer from 1 to 50, t is an integer from 2 to 5, R7, R8, R9 and R 10 represents an alkyl group or R7, R8, R9 and R 10 Each together with the common carbon atom forms an alicyclic or heterocyclic group, Q is -O-, -S-, -NH-, -NR 13a -,-NR 13a R 13b + -(An m- ) 1 / m， -PR 13a -or–SiR 13a R 13b -,or -Q- represents a covalent bond, R 11 is -O or -S, R 12 is -CH2-, -O-, -S-, -SO-, -SO2-, –NR 13 -or–N + R 13 R 14 -(An m- ) 1 / m , R 13 , R 13a , R 13b and R 14 are independently a monovalent organic group, and An and m have the meanings defined above.

2. A redox flow battery according to claim 1, characterised in that the electrode compartments for the catholyte and the anolyte are separated by a semipermeable membrane which is impermeable to redox couples in the catholyte, and in that the anolyte comprises a zinc salt as a redox-active component.

3. The redox flow battery according to claim 1, characterized in that the anolyte contains a compound containing one or more bipyridyl groups in the molecule as a redox active component.

4. A redox flow battery for storing electrical energy, comprising a reaction cell with an electrode compartment for an electrolyte solution, connected to at least one reservoir, wherein the electrode compartment is equipped with a cathode and an anode and is filled with an electrolyte solution comprising a redox-active component dissolved or dispersed in an aqueous electrolyte solvent, and a conductive salt dissolved therein and optionally further additives, characterized in that the at least one reservoir is an underground storage device having a temperature of at least 30°C, The concentration of the salt dissolved in the electrolyte solution is at least 10% by weight. the electrolyte solution at the anode comprises a water-soluble or water-dispersible redox-active component, and The electrolyte solution at the cathode contains at least one compound or composition selected from one of the groups a), b), c), d) or e) as a redox active component, wherein a) is an organic compound comprising at least one redox-active group of formula (I): (X-C5H4)Fe(Y-C5H3-Z)(I), b) is an organic compound comprising at least one redox-active group of formula (Ia): in combination with hydrochloric acid and / or a salt selected from the group consisting of ammonium salts with inorganic or organic anions, salts with tetrafluoroborate anions or salts of trifluoromethanesulfonic acid, c) is an organic compound comprising at least one redox-active group of formula (Ib): d) is an organic compound comprising at least one redox-active group of formula (Ic): e) is a water-soluble iron salt complexed with a nitrogen-containing ligand, wherein X is of the formula -(C n H 2n )-FG or formula-(C w H 2w )-Sp-(C n H 2n )-FG or formula-(C n H 2n )-Brgp-, Y is hydrogen or of the formula -(C n H 2n )-FG or formula-(C w H 2w )-Sp-(C n H 2n )-FG group, Z is hydrogen or a covalent bond connecting the radical of formula (I) to the rest of the molecule of said organic compound, 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 u+ ) 1 / u , -COOR2, -SO3 - (Kat u+ ) 1 / u , -SO3R2, -SO4 - (Kat u+ ) 1 / u , -SO4R2, -PO4 2- (Kat u+ ) 2 / u , -PO4(R2)2, -PO3 2 (Kat u+ ) 2 / u ,-PO3(R2)2,-NR3R4R5 + (An m- ) 1 / m , -N + R3R4-C t H 2t -SO3 - Or –NR2-SO2-R3, Brgp is a divalent bridging group which connects the group of formula (I) to the rest of the molecule, Sp is -O- or -S-, 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 a u-valent cation, An is an m-valent anion, m and u are independently integers from 1 to 4, n and w are independently integers from 2 to 4, o is an integer from 1 to 50, t is an integer from 2 to 5, R7, R8, R9 and R 10 represents an alkyl group or R7, R8, R9 and R 10 Each together with the common carbon atom forms an alicyclic or heterocyclic residue, Q is -O-, -S-, -NH-, -NR 13a -,-NR 13a R 13b + -(An m- ) 1 / m， -PR 13a -or–SiR 13a R 13b -, or -Q- represents a covalent bond, R 11 is -O or -S, R 12 is -CH2-, -O-, -S-, -SO-, -SO2-, –NR 13 -or–N + R 13 R 14 -(An m- ) 1 / m , R 13 , R 13a , R 13b and R 14 are independently a monovalent organic group, and An and m have the meanings defined above.

5. Redox flow battery according to claim 4, characterized in that the electrode compartments for the catholyte and the anolyte do not contain a separator, and the anolyte contains a zinc salt as redox-active component.

6. Redox flow battery according to claim 2 or 5, characterized in that it comprises a zinc-solid anode with the redox couple zinc(II) / zinc(0).

7. Redox flow battery according to any one of claims 1 to 5, characterized in that the temperature in the underground storage device is 30-90°C.

8. A redox flow battery according to claim 7, characterised in that the temperature in the underground storage device is 40-70°C.

9. A redox flow battery according to any one of claims 1 to 5, characterised in that the concentration of the salt dissolved in the electrolyte solution is from 14 wt% to the saturation limit.

10. A redox flow battery according to any one of claims 1 to 5, characterised in that the state of charge of the catholyte or the catholyte and the anolyte is less than 90%.

11. A redox flow battery according to any one of claims 1 to 5, characterised in that n is an integer from 2 to 4.

12. The redox flow battery according to claim 11, characterized in that n is an integer of 2.

13. Redox flow battery according to any one of claims 1 to 5, characterised in that Y is hydrogen and m is 1 or 2.

14. A redox flow battery according to any one of claims 1 to 5, characterised in that FG is a functional group selected from the group consisting of: -(O-CH2-CH2) o -OR2, -(O-CH2-CH2) o -SR2, -COR2, –COO - (Kat u+ ) 1 / u , -SO3 - (Kat u+ ) 1 / u , -SO4 - (Kat u + ) 1 / u , -PO4 2- (Kat u+ ) 2 / u ,-PO3 2- (Kat u+ ) 2 / u Or -NR3R4R5 +( An m- ) 1 / m .

15. Redox flow battery according to any one of claims 1 to 5, characterised in that Kat is a hydrogen cation, an alkali metal cation, an alkaline earth metal cation or an ammonium cation.

16. A redox flow battery according to claim 15, characterised in that Kat is a hydrogen cation, a sodium, potassium, magnesium or calcium cation or a quaternary ammonium cation.

17. A redox flow battery according to any one of claims 1 to 5, characterized in that An m- Selected from halide ion, hydroxide ion, phosphate ion, sulfate ion, perchlorate ion, hexafluorophosphate ion or tetrafluoroborate ion.

18. Redox flow battery according to any one of claims 1 to 5, characterized in that Z is a covalent bond which connects the radical of formula (I) to a polymer backbone selected from the group consisting of polymethacrylates, polyacrylates, polystyrenes, polyalkylene glycols, polyalkyleneimines or polyvinyl ethers.

19. A redox flow battery according to claim 18, wherein the polymer backbone has 5 to 100 groups of formula (I).

20. Redox flow battery according to any one of claims 1 to 5, characterized in that it comprises an oligomer or polymer comprising repeating structural units of formula (II) and optionally further structural units derived from comonomers promoting solubility: in ME is a repeating structural unit derived from a polymerizable monomer, BG is a covalent bond or bridging group, FC is a group of formula (X-C5H4)Fe(Y-C5H3-Z), X, Y and Z have the meanings defined in claim 1, and r is an integer from 2 to 150.

21. A redox flow battery according to any one of claims 1 to 5, characterised in that the compound of group a) is a compound of formula (III): (X-C5H4)Fe(Y-C5H4)(III), wherein X and Y have the meanings defined in claim 1.

22. Redox flow battery according to any one of claims 1 to 5, characterized in that the compound of group a) is a compound of formula (IV): [(X-C5H4)Fe(Y-C5H3-)]-R 23 -[(-C5H3-Y)Fe(C5H4-X)] p (IV), wherein X and Y have the meanings defined in claim 1, R 23 is a di- to tetravalent organic radical, and p is an integer from 1 to 4.

23. A redox flow battery according to any one of claims 1 to 5, characterised in that the electrolyte solution comprises a compound having at least one redox-active group of formula (Ia), (Ib) or (Ic), which is covalently bonded to a polymer backbone selected from the group consisting of polymethacrylates, polyacrylates, polystyrenes, polyalkylene glycols, polyalkyleneimines or polyvinyl ethers.

24. A redox flow battery according to claim 23, wherein the polymer backbone has 5 to 100 groups of formula (Ia), (Ib) or (Ic).

25. Redox flow battery according to any one of claims 1 to 5, characterised in that the electrolyte solution comprises an oligomer or polymer comprising repeating structural units of formula (V) and optionally further structural units derived from comonomers promoting solubility: in ME is a repeating structural unit derived from a polymerizable monomer, BG is a covalent bond or bridging group Pip is a pyridyl group of formula (Ia), (Ib) or (Ic), and r is an integer from 2 to 150.

26. A redox flow battery according to any one of claims 1 to 5, characterised in that the electrolyte solution comprises a redox-active compound of formula (VIa) or (VIb): Among them, R7, R8, R9 and R 10 has the meaning defined in claim 1, q is an integer from 1 to 3, R 15 is a monovalent organic group, which is optionally linked to the piperidinyl group via an oxygen, sulfur or nitrogen atom, and R 16 It is -O-, -S- or a di- to tetravalent organic group.

27. A redox flow battery according to any one of claims 1 to 5, characterised in that the electrolyte solution comprises a redox-active compound of formula (VII), (VIII), (IX), (X) or (XI): Among them, R7, R8, R9, R 10 , R 11 , R 12 and Q have the meanings defined in claim 1, ME, BG and r have the meanings defined in claim 25, q has the meaning defined in claim 26, R 16a is a di- to tetravalent organic radical, and t is 0 or 1.

28. The redox flow battery of claim 26, characterized in that: R 15 Yes-O-(C s H 2s )-R 23 、-S-(C s H 2s )-R 23 or –NR2-(C s H 2s )-R 23 , s is an integer from 2 to 4, R2 has one of the meanings defined in claim 1, R 23 is a functional group selected from the following: –OH, -SH, -NO3, -NO2, -CN, -OR 17 , -SR 17 , -(O-CH2-CH2) v -OR 18 , -(O-CH2-CH2) v -SR 18 , -COR 18 , –COO - (Kat u+ ) 1 / u , -COOR 18 , -SO3 - (Kat u+ ) 1 / u , -SO3R 18 , -SO4 - (Kat u+ ) 1 / u , -SO4R 18 , -PO4 2- (Kat u+ ) 2 / u ,-PO4(R 18 )2, -PO3 2- (Kat u+ ) 2 / u ,-PO3(R 18 )2 or –NR 19 R 20 R 21 + (An m- ) 1 / m , R 17 is a C1-C4 alkyl group, R 18 is hydrogen or C1-C4 alkyl, R 19 , R 20 and R 21 are independently hydrogen or alkyl, Kat is a u-valent cation, An is an m-valent anion, m and u are independently integers 1-4, and v is an integer from 1 to 50.

29. A redox flow battery according to any one of claims 1 to 5, characterised in that it comprises a redox-active compound containing one or more groups of formula (Ia) and a stabilising additive, wherein the stabilising additive is selected from hydrochloric acid or quaternary ammonium alkyl halides, or tetrafluoroborates or trifluoromethanesulfonates of alkali metals, alkaline earth metals or ammonium.

30. The redox flow battery of claim 29, wherein the quaternary ammonium alkyl halide is a quaternary ammonium alkyl chloride or fluoride.

31. A redox flow battery according to claim 29, wherein the stabilizing additive is selected from sodium, potassium, magnesium, calcium or quaternary ammonium tetrafluoroborate or trifluoromethanesulfonate.

32. Redox flow battery according to any one of claims 1 to 5, characterized in that it comprises an iron solid cathode having the redox couple iron(II) / iron(0) or iron(III) / iron(0).

33. A redox flow battery according to any one of claims 1 to 5, characterised in that the underground storage means is a cavern.

34. A redox flow battery according to claim 33, wherein the cavern is a salt cavern.

35. Use of a redox flow battery according to any of claims 1 to 34 for electrical energy storage for stationary applications or for intermediate electrical energy storage from renewable energy sources.

36. Use of a redox flow battery according to claim 35 as stationary storage for emergency power supply, peak load balancing.

37. Use of a redox flow battery according to claim 35, wherein the renewable energy sources are in the field of photovoltaics, hydroelectricity and wind power.

38. Use of a redox flow battery according to any one of claims 1 to 34 for intermediate electrical energy storage from natural gas, coal, biomass, tidal and marine power plants.

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