Redox flow cell for storing and utilizing electrical energy

Abstract

This cell is a low-cost and durable redox flow cell that uses less corrosive and more active components. The flow cell contains a reaction cell with two electrolyte chambers, one for the catholyte and one for the anolyte, each communicating with at least one liquid reservoir separated by an ion-conducting membrane and equipped with electrodes, each filled with electrolyte solutions. The redox-active components are dissolved or dispersed in an electrolytic solvent, and possibly conductive salts and other additives are also dissolved in these solutions.The redox flow cell is characterized in that the anolyte contains a redox active component having one to six radicals of formula i or one to six radicals of formula ii in the molecule and the catholyte is a redox active component having one to six radicals of formula iii in the molecule or iron salts, or that the anolyte and catholyte contain an oxido-active fragment having one to six residues of formula i or formula ii in combination with one to six residues of formula iii in the molecule, where r1 is a covalent cc bond or a divalent bridging group, r2 and r3 independently of each other, alkyl, alkoxy, .

Description

[0001] The invention relates to a redox flow cell, also commonly referred to as a redox flow battery, for storing electrical energy. The redox flow cell comprises two polarity-specific chambers, each containing a redox-active chemical compound in either dissolved form or dispersed in an electrolyte solvent, and connected to a liquid reservoir. This creates two independent circuits for the redox-active compounds, which are, for example, dissolved in water or an organic solvent, or dispersed in an electrolyte solvent. These circuits are separated by a membrane between the polarity-specific chambers. Ion exchange between the two chambers occurs via this membrane.

[0002] The cells are particularly suitable for stationary storage applications, for example as a buffer battery for wind power or solar plants or as power and control reserves for load balancing in power grids, but also as mobile energy storage, for example for the operation of electric cars and electronic devices.

[0003] Redox flow batteries (RFBs) are electrochemical energy storage devices. The compounds required to set the potential at the electrodes are dissolved, redox-active species that are converted to their respective opposite redox states during the charging and discharging process in an electrochemical reactor. For this purpose, the electrolyte solutions (catholyte, anolyte) are drawn from a tank and actively pumped to the electrodes. The anode and cathode compartments in the reactor are separated by an ion-selective membrane, which usually exhibits high selectivity for cations, especially protons (e.g., Nafion™). Membranes also exist that selectively allow negatively charged ions to pass through while blocking positively charged ions. Furthermore, size-selective membranes (e.g., dialysis or ultrafiltration membranes) are used, which allow both anions and cations to pass through.

[0004] The anode and cathode compartments within the meaning of this invention are defined as follows: The cathode compartment contains the catholyte as the electrolyte and is bounded by the cathode and the membrane surface facing the cathode. The anode compartment contains the anolyte as the electrolyte and is bounded by the anode and the membrane surface facing the anode.

[0005] At the cathode, reduction of the redox-active component occurs during discharge, and oxidation occurs during charging. At the anode, oxidation of the redox-active component occurs during discharge, and reduction occurs during charging.

[0006] Exemplary descriptions of the reactions in a redox flow cell during the charging process: Anode A + e -< → A -< or A -< + e -< → A 2-< or A n+< + xe -< → A( nx< ) +< or A n-< + xe -< → A (n+x)-<

[0007] Here, A is the redox-active component, and n and x can take on a natural number >= one. Electrons are symbolized by e -<. Cathode K → K +< + e -< or K -< → K + e -< or K n+< → K (n+y)+< + ye -< or K n-< → K (ny)-< + ye -<

[0008] Here, K is the redox-active component, and n and y can take on a natural number greater than or equal to one. Electrons are symbolized by e - <.

[0009] When the cell is discharged, the above reactions are reversed.

[0010] As long as electrolyte solution is being pumped, current can be drawn from (discharge) or fed into the system (charge). Therefore, the amount of energy that can be stored in a regenerative braking system (RFB) is directly proportional to the size of the storage tanks. The power that can be drawn from the system, however, is a function of the size of the electrochemical reactor.

[0011] RFBs have a complex system technology (BoP - Balance of Plant) that is roughly equivalent to that of a fuel cell. Typical sizes of individual reactors range from approximately 2 to 50 kW. The reactors can be easily combined modularly, and the tank size can also be adjusted almost arbitrarily. RFBs that operate with vanadium compounds as the redox couple on both sides (VRFB) are of particular importance. This system was first described in 1986 (AU 575247 B) and currently represents the technical standard.

[0012] Other inorganic, low-molecular-weight redox couples (redox-active compounds) were investigated, including those based on... Cer (B. Fang, S. Iwasa, Y. Wei, T. Arai, M. Kumagai: "A study of the Ce(III) / Ce(IV) redox couple for redox flow battery application", Electrochimica Acta 47, 2002, 3971-3976), Ruthenium (M. H. Chakrabarti, E. Pelham, L. Roberts, C. Bae, M. Saleem: "Ruthenium based redox flow battery for solar energy storage", Energy Conv. Manag. 52, 2011,2501-2508] Chrom (C-H. Bae, E. P. L. Roberts, R. A. W. Dryfe: "Chromium redox couples for application to redox flow batteries", Electrochimica Acta 48, 2002, 279-87) Uran (T. Yamamura, Y. Shiokawa, H. Yamana, H. Moriyama: "Electrochemical investigation of uranium ß-diketonates for all-uranium redox flow battery", Electrochimica Acta 48, 2002, 43-50) Mangan (F. Xue, Y. Wang, W. Hong Wang, X. Wang: "Investigation on the electrode process of the Mn(II) / Mn(III) couple in redox flow battery", Electrochimica Acta 53, 2008, 6636-6642) Eisen (L. W. Hruska, R. F. Savinell: "Investigation of Factors Affecting Performance of the Iron-Redox Battery", J.Electrochem. Soc.,128:1, 1981, 18-25). .

[0013] Organic and semi-organic systems in aqueous solutions are also attracting attention. For example, in January 2014, the anthraquinone-disulfonic acid / bromine system was published, which allows for very high current densities. However, the use of elemental bromine places high demands on the materials of all battery components and on the system's safety. (B. Huskinson, MP Marshak, C. Suh, S. Er, MR Gerhardt, CJ Galvin, X. Chen, A. Aspuru-Guzik, RG Gordon, MJ Aziz: "A metal free organic-inorganic aqueous flow battery", Nature 505, 2014, 195-198). Quinones are also being tested as fully organic systems in aqueous solution (B. Yang, L. Hoober-Burkhard, F. Wang, GK Surya Prakash, SR Narayanan: "An inexpensive aqueous flow battery for large-scale electrical energy storage based on eater-double organic redox couples": J. Electrochem. Soc., 161 (9), 2014, A1361 - A1380).However, the current densities that can be meaningfully applied in redox systems are limited to less than 5 mA / cm², and the maximum achievable capacity is below 10 Ah / I. The stable radical molecule 2,2,6,6-tetramethyl-1-piperidonyloxyl (TEMPO) has also been used in redox flow batteries together with... N-Methylphthalimide is used. (Z. Li, S. Li, S. Liu, K. Huang, D. Fang, F. Wang, S. Peng: "Electrochemical properties of an allorganic redox flow battery using 2,2,6,6-Tetramethyl-1-Piperidonyloxyl and N-Methylphthalimide": Electrochemical and Solid-State Letters, 14 (12), 2011, A171-A173) Due to the resulting potentials and solubilities of the starting materials, this material system cannot be readily used in aqueous media, but requires hazardous substances as solvents, such as acetonitrile. Furthermore, the achievable current densities of this system are at least 100 times lower (< 0.35 mA / cm²) than those of the material systems proposed in the invention. Other electrolyte systems, such as LiPF 6 and TEMPO (X. Wie, W. Xu, M. Vijayakumar, L. Cosimbescu, T. Liu, V. Sprenkle, W. Wang: "TEMPO-based catholyte for high-energy density redox flow batteries" Adv. Mater. 2014 Vol.26, 45, p7649-7653) also require organic solvents and conducting salts which can release toxic gases, such as hydrogen fluoride, in the event of failure, and thus place high demands on system safety.

[0014] Redox flow cells with semipermeable membranes, in which high-molecular-weight compounds are used as the redox pair, are known from WO 2014 / 026728 A1. In the example, a poly(2,2,6,6-tetramethylpiperidinyloxymethacrylate-co-poly(ethyleneglycolmethylether-methacrylate)) is used as the catholyte and a poly(4,4'-bipyridine-co-poly(ethyleneglycol)) as the anolyte.

[0015] The object of this invention is to provide a redox flow cell with selected redox-active material systems that can be operated safely, cost-effectively, and efficiently, that contains an electrolyte solution with improved pumpability, that can continue to function even with cross-contamination via membrane defects, and that can achieve a higher potential level compared to known solutions. The redox-active components used according to the invention are characterized by a significantly reduced viscosity compared to the polymeric redox-active compounds known from WO 2014 / 026728 A1. Compared to known polymeric redox systems, the viscosity of concentrated solutions is significantly lower at a comparable capacity (1 mol / L redox-active units), resulting in lower pressure losses when pumping the solutions and thus improved energy efficiency. For example, concentrated solutions of N-Dimethylviologenchloride has a viscosity of 5 mPas at room temperature, while concentrated solutions of the N -Methyl viologen polymers exhibit a viscosity of 20 mPas at room temperature with the same capacity. Furthermore, the material systems used according to the invention are also characterized by lower corrosivity compared to acid-based electrolytes.

[0016] The problem is solved by providing redox flow cells with selected redox-active material systems that can be operated without catalysts, are highly soluble in water, inexpensive, and compatible with each other. The redox-active material systems can also be used as dispersions.

[0017] The present invention relates to a redox flow cell for storing electrical energy, comprising a reaction cell with two electrode chambers for catholyte and anolyte, each connected to at least one liquid reservoir separated by an ion-conducting membrane, and equipped with electrodes, wherein the electrode chambers are each filled with electrolyte solutions containing redox-active components dissolved or dispersed in an electrolyte solvent, as well as optionally dissolved conducting salts and possibly further additives. The redox flow cell according to the invention is characterized in that the anolyte contains a redox-active component comprising one to six, preferably one to four, in particular one to three, and most preferably one to two residues of formula I in the molecule, or comprising one to six, preferably one to four,in particular one to three and most preferably one to two residues of formula II in the molecule and that the catholyte contains a redox-active component which contains one to six, preferably one to four, in particular one to three and most preferably one to two residues of formula III in the molecule or which contains iron salts or that the anolyte and catholyte contain a redox-active component which contains one to six, preferably one to four, in particular one to three and most preferably one to two residues of formula I or formula II in combination with one to six, preferably one to four, in particular one to three and most preferably one to two residues of formula III in the molecule , wherein The lines extending from the nitrogen atoms in the structures of formulas I and II and the line extending from the 4-position in the structure of formula III represent covalent bonds connecting the structures of formulas I, II, and III to the rest of the molecule; R1 is a covalent C-C bond or a divalent bridging group, in particular a covalent C-C bond, an arylene group, or a heteroarylene group, and most preferably a covalent C-C bond, a phenylene group, a biphenylene group, or a thiophenediyl group; R2 and R3 independently represent alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, heterocyclyl, halogen, hydroxy, amino, nitro, or cyano; X represents a q-valent inorganic or organic anion or a mixture of such anions; b and c independently represent integers from 0 to 4, preferably 0, 1, or 2; q is an integer from 1 to 3 is, a is a number with the value 2 / q, and R 4 , R 5 ,R6 and R7 can independently represent alkyl, cycloalkyl, aryl, or aralkyl, in particular C1-C6 alkyl, and most preferably ethyl or methyl.

[0018] Preferably, redox-active components used in the anolyte contain one to four residues of formula 1a and / or formula IIa in the molecule. wherein the lines extending from the nitrogen atoms in the structures of formulas Ia and IIa represent covalent bonds which connect the structures of formulas Ia and IIa with the rest of the molecule, and R 2 , R 3 , X, a, b, c and q have the meaning defined above.

[0019] Redox-active components preferably used in the anolyte are compounds of the formulas Ib, IIb, IV, V, VII, VIIa, VIIb VIII, VIIIa, VIIIb, IX, IXa, IXb, X, Xa, Xb, XI, XIa, XIb, XII, XIIa and XIIb wherein R1, R2, R3, R4, R5, R6, R7 and X have the meaning defined above; R8 and R10 independently represent hydrogen, optionally an alkyl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group; optionally a cycloalkyl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group; optionally an aryl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group; or optionally an aralkyl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group, in particular a C1-C6 alkyl; a C1-C6 alkyl substituted with a carboxylic ester group; a C1-C6 alkyl substituted with a carboxylic amide group; or a C1-C6 alkyl substituted with a carboxylic acid group. 1-C6 alkyl, C1-C6 alkyl substituted with a sulfonic acid group,or C1-C6 alkyl substituted with an amino group, and most preferably propionate, isobutionate, ethyl or methyl, R9 is a divalent to hexavalent, in particular a divalent to tetravalent organic bridging group, R12 is a covalent bond or a divalent to hexavalent, in particular a divalent to tetravalent organic bridging group, R14 is a covalent bond or a divalent organic bridging group, R15 is a divalent to hexavalent, in particular a divalent to tetravalent organic bridging group, R18 is an o-positively charged divalent to hexavalent, in particular a divalent to tetravalent organic residue which is covalently linked to the nitrogen atom of the bipyridyl residue via a carbon atom, in particular a divalent to tetravalent quaternary ammonium residue, a divalent to tetravalent quaternary phosphonium residue, a divalent to trivalent ternary sulfonium residue or an o-positively charged divalent to six-valuedin particular a divalent to tetravalent heterocyclic residue, R 19 is an o-fold, preferably singly positively charged divalent organic residue which is covalently linked to the nitrogen atom of the bipyridyl residue via a carbon atom, in particular a quaternary ammonium residue, a quaternary phosphonium residue, a ternary sulfonium residue or an o-fold, preferably singly positively charged divalent heterocyclic residue, R 20 and R 21 are hydrogen independently of each other, optionally an alkyl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group, optionally a cycloalkyl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group, optionally an aryl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group, or optionally a carboxylic ester, carboxylic amide, carboxylic acid,R20 and R21 together form a C1-C3 alkylene group, in particular a C1-C6 alkyl, a C1-C6 alkyl substituted with a carboxylic ester group, a C1-C6 alkyl substituted with a carboxylic amide group, a C1-C6 alkyl substituted with a carboxylic acid group, a C1-C6 alkyl substituted with a sulfonic acid group, or a C1-C6 alkyl substituted with an amino group, or together they represent ethylene, and most preferably propionate, isobutionate, ethyl, or methyl, or together they represent ethylene, R22 being a divalent organic bridging group, R23 being a u-fold negatively charged divalent to hexavalent, in particular divalent to tetravalent, organic residue covalently bonded to the nitrogen atom of the bipyridyl residue via a carbon atom is, in particular, an alkylene residue substituted with one or two carboxyl or sulfonic acid groups,a phenylene residue substituted with one or two carboxyl or sulfonic acid groups or a divalent heterocyclic residue substituted with one or two carboxyl or sulfonic acid groups, R 24 is a u-fold, preferably singly negatively charged divalent organic residue covalently linked to the nitrogen atom of the bipyridyl residue via a carbon atom, in particular an alkylene residue substituted with a carboxyl or sulfonic acid group, a phenylene residue substituted with a carboxyl or sulfonic acid group, or a divalent heterocyclic residue substituted with a carboxyl or sulfonic acid group, a, b, c, and q have the meaning defined above, d is an integer from 1 to 5, preferably from 1 to 3, e is a number with the value (2 + 2d + 2t) / q, g is an integer from 1 to 5, preferably from 1 to 3, h is an integer from 1 to 5, preferably from 1 to 3, wherein the sum of g and h is an integer from 2 to 6,preferably from 2 to 4, i is a number with the value 2h / q, j is an integer from 1 to 5, preferably from 1 to 3, k is a number with the value (2 + 2j) / q, o is an integer from 1 to 4, p is a number with the value (o + 2h) / q, r is a number with the value (3 + 3j) / q, t is 0 or, if R 9 is a divalent organic bridging group, means 0 or 1, u is an integer from 1 to 4, z is a number with the value 2 / q, z1 is a number with the value (o+2) / q, Y, in the case that 2h - u or 2 (2 - u) - u is greater than 0, means a v- or x-valent inorganic or organic anion or a mixture of such anions, or in the case that 2h - u or 2 (2 - u) - u is less than 0, denotes a v- or x-valent inorganic or organic cation or a mixture of such cations, v is an integer from -1 to -3 or from +1 to +3, x is an integer from -1 to -3 or from +1 to +3,w is 0 or a positive number with the value (-u + 2h) / v, y is 0 or a positive number with the value (2 - u) (j + 1) / x, Y1 in the case that 2 - 2u is less than 0, denotes an x1-valent inorganic or organic cation or a mixture of such cations, x1 is an integer from -1 to -3 or from +1 to +3, and y1 is 0 or a positive number with the value (2 - 2u) / x1.

[0020] Particularly preferred redox-active components used in the anolyte are compounds of formulas IVa, Va, VIIc, VIIIc, IXc and Xc wherein R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R 10 , R 12 , R 14 , R 15 and X have the meaning defined above, and b, c, d, e, g, h, i, j, k and q have the meaning defined above.

[0021] Preferred redox-active components used in the catholyte are compounds of formulas purple, IIIb, IIIc, VI, VIa and / or VIb, as well as those of formulas VII, VIIa, VIIb, VIII, VIIIb, VIIIa, IX, IXa, IXb, X, Xa and / or Xb defined above. wherein R4, R5, R6, R7, X, o, u and q have the meaning defined above, R11 is a divalent to tetravalent organic bridging group, R13 is hydrogen, alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, heterocyclyl, halogen, hydroxy, amino, nitro or cyano, and R16 is an o-fold, preferably a singly positively charged monovalent organic residue, in particular a quaternary ammonium residue, a quaternary phosphonium residue, a ternary sulfonium residue or an o-fold, preferably a singly positively charged monovalent heterocyclic residue, and R17 is an m-fold positively charged divalent to tetravalent organic residue, in particular a divalent to tetravalent quaternary ammonium residue, a divalent to tetravalent quaternary phosphonium residue, a divalent to tetravalent ternary R 25 is a sulfonium residue or an m-fold positively charged divalent to tetravalent heterocyclic residue, and is a u-fold, preferably a singly negatively charged monovalent residue.in particular a carboxyl or sulfonic acid residue or a u-fold, preferably a singly negatively charged monovalent heterocyclic residue, R 26 an m-fold negatively charged divalent to tetravalent organic residue, in particular an alkylene residue substituted with one or two carboxyl or sulfonic acid groups, or a phenylene residue substituted with one or two carboxyl or sulfonic acid groups or a divalent heterocyclic residue substituted with one or two carboxyl or sulfonic acid groups, Z a q-valent inorganic or organic cation or a mixture of such cations, f an integer from 1 to 3, l a number with the value o / q or u / q, m an integer from 1 to 4, and n a number with the value m / q.

[0022] Particularly preferred redox-active components used in the catholyte are compounds of the formulas VI, VIa, VIIc, VIIIc, IXc and / or Xc defined above.

[0023] Particularly preferred redox-active compounds used according to the invention are those of formulas Ib, IIb, VIId, VIIe, VIIId and / or VIIIe wherein R2, R3, R4, R5, R6, R7, R8, R10, R14, R19 and X have the meaning defined above, R20 and R21 independently represent hydrogen, optionally an alkyl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group, optionally a cycloalkyl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group, optionally an aryl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group, or optionally an aralkyl substituted with a carboxylic ester, carboxylic amide, carboxylic acid, sulfonic acid or amino group, or two residues R20 and R21 together form a C1-C3 alkylene group, in particular a C1-C6 alkyl, with a C1-C6 alkyl substituted with a carboxylic acid ester group, C1-C6 alkyl substituted with a carboxylic acid amide group, C1-C6 alkyl substituted with a carboxylic acid group,a C1-C6 alkyl substituted with a sulfonic acid group, or a C1-C6 alkyl substituted with an amino group, or together represent ethylene, and most preferably propionate, isobutionate, ethyl, or methyl, or together represent ethylene, where a, b, c, and q have the meaning defined above, and s is a number with the value 3 / q.

[0024] Of these compounds that are particularly preferred, those of formulas Ib, IIb, VIId, VIIe, VIIId and / or VIIIe are used in the anolyte and those of formulas VIId, VIIe, VIIId and / or VIIIe in the catholyte.

[0025] The catholyte most preferably contains compounds of the formulas IIIa, IIIb or IIIc defined above, and the anolyte contains compounds of the formulas Ib or IIb defined above.

[0026] In particular, the catholyte contains compounds of formula IIIb defined above and the anolyte contains compounds of formula Ib defined above.

[0027] Examples of preferred compounds of formula IIIb are salts of 2,2,6,6-tetramethylpiperidine-4-(N,N,N-trialkylammonium), in particular salts of 2,2,6,6,-tetramethylpiperidine-4-(N,N,N-trimethylammonium) and most especially 2,2,6,6-tetramethylpiperidine-4-(N,N,N-trimethylammonium) chloride.

[0028] Examples of preferred compounds of formula Ib are salts of N,N'-dialkyl viologen, in particular salts of N,N'-dimethyl viologen and especially N,N'-dialkyl viologen chloride.

[0029] Particularly preferred redox-active compounds are those of the formulas VII, VIIa, VIIb, VIIc, VIId, VIIe, VIII, VIIIa, VIIIb, VIIIc, VIIId, VIIIe, IX, IXa, IXb, IXc, X, Xa, Xa and Xc defined above. These contain both electroactive bipyridyl groups and electroactive nitroxide groups and can be used in both the catholyte and the anolyte, preferably the same compounds in both chambers.

[0030] The redox-active compounds of the formulas VII, VIIa, VIIb, VIIc, VIId, VIIe, VIII, VIIIa, VIIIb, VIIIc, VIIId, VIIIe, IX, IXa, IXb, IXc, X, Xa, Xb, and Xc defined above are combination molecules consisting of a positive and a negative redox-active unit (TEMPO and viologen). Up to now, these redox-active units have only been used in the form of two different substances. The combination molecules particularly preferred according to the invention can be both oxidized and reduced. One of the resulting advantages is that the solutions are no longer irreversibly damaged by mixing, for example, due to membrane defects. Furthermore, the potentials can be adjusted by selecting the two redox-active units and thus optimized for various application scenarios.

[0031] If one of the residues R2, R3, R4, R5, R6, R7, R8, R10, R13, R20 and / or R21 denotes alkyl, the alkyl group can be either branched or unbranched. An alkyl group typically contains one to twenty carbon atoms, preferably one to ten. Examples of alkyl groups are: methyl, ethyl, n-propyl, isopropyl, n -Butyl, sec -Butyl, tert .-Butyl, pentyl, n-hexyl, n-heptyl, 2-ethylhexyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, n-hexadecyl, n-heptadecyl, n-octadecyl, n-nonadecyl, or eicosyl. Alkyl groups with one to six carbon atoms are particularly preferred. Alkyl groups may optionally be substituted, for example, with carboxyl or sulfonic acid groups, with carboxyl ester or sulfonic acid ester groups, with carboxylamide or sulfonamide groups, with hydroxyl or amino groups, or with halogen atoms.

[0032] If one of the R2, R3, and / or R13 substituents represents an alkoxy, the alkoxy group can consist of a single alkyl unit, which may be either branched or unbranched. An alkoxy group typically contains one to twenty carbon atoms, preferably one to ten. Examples of alkoxy groups are: methoxy, ethoxy, isopropoxy, n-butoxy, sec.-butoxy, tert.-butoxy, pentyloxy, n-hexyloxy, n-heptyloxy, 2-ethylhexyloxy, n-octyloxy, n-nonyloxy, n-decyloxy, n-tridecyloxy, n-tetradecyloxy, n-pentadecyloxy, n-hexadecyloxy, n-octadecyloxy, or eicosyloxy. Alkoxy groups with one to six carbon atoms are particularly preferred.

[0033] If one of the residues R2, R3, and / or R13 denotes a haloalkyl group, the haloalkyl group can be either branched or unbranched. A haloalkyl group typically contains one to twenty carbon atoms, each of which is independently substituted with one or more halogen atoms, preferably one to ten. Examples of halogen atoms are fluorine, chlorine, bromine, or iodine. Fluorine and chlorine are preferred. Examples of haloalkyl groups are: trifluoromethyl, difluoromethyl, fluoromethyl, bromodifluoromethyl, 2-chloroethyl, 2-bromoethyl, 1,1-difluoroethyl, 2,2,2-trifluoroethyl, 1,1,2,2-tetrafluoroethyl, 2-chloro-1,1,2-trifluoroethyl, pentafluoroethyl, 3-bromopropyl, 2,2,3,3-Tetrafluoropropyl, 1,1,2,3,3,3-Hexafluoropropyl, 1,1,1,3,3,3-Hexafluoropropyl, 3-Bromo-2-methylpropyl, 4-Bromobutyl, Perfluoropentyl.

[0034] If any of the residues R2, R3, R4, R5, R6, R7, R8, R10, R13, R20, and / or R21 denotes a cycloalkyl, the cycloalkyl group is typically a cyclic group containing three to eight, preferably five, six, or seven ring carbon atoms, each of which may be independently substituted. Examples of substituents are alkyl groups or two alkyl groups which, together with the ring carbons to which they are bonded, may form another ring. Examples of cycloalkyl groups are cyclopropyl, cyclopentyl, or cyclohexyl. Cycloalkyl groups may optionally be substituted, for example, with carboxyl or sulfonic acid groups, with carboxyl ester or sulfonic acid ester groups, with carboxylamide or sulfonamide groups, with hydroxyl or amino groups, or with halogen atoms.

[0035] If one of the residues R2, R3, R4, R5, R6, R7, R8, R10, R13, R20, and / or R21 denotes aryl, the aryl group is typically a cyclic aromatic group containing five to fourteen carbon atoms, each of which may be independently substituted. Examples of substituents are alkyl groups or two alkyl groups, which together with the ring carbon atoms to which they are bonded may form another ring. Examples of aryl groups are phenyl, biphenyl, anthryl, or phenantolyl. Aryl groups may optionally be substituted, for example, with carboxyl or sulfonic acid groups, with carboxyl ester or sulfonic acid ester groups, with carboxylamide or sulfonamide groups, with hydroxyl or amino groups, or with halogen atoms.

[0036] If one of the residues R2, R3, and / or R13 denotes heterocyclyl, the heterocyclyl group typically comprises a cyclic group with four to ten ring carbon atoms and at least one ring heteroatom, each of which may be independently substituted. Examples of substituents are alkyl groups, or two alkyl groups which, together with the ring carbons to which they are bonded, may form another ring. Examples of heteroatoms are oxygen, nitrogen, phosphorus, boron, selenium, or sulfur. Examples of heterocyclyl groups are furyl, thienyl, pyrrolyl, or imidazolyl. Heterocyclyl groups are preferably aromatic. Heterocyclyl groups may optionally be substituted, for example, with carboxyl or sulfonic acid groups, with carboxyl ester or sulfonic acid ester groups, with carboxylamide or sulfonamide groups, with hydroxyl or amino groups, or with halogen atoms.

[0037] If one of the residues R2, R3, R4, R5, R6, R7, R8, R10, R13, R20, and / or R21 denotes an aralkyl group, then the aralkyl group is typically an aryl group (aryl having already been defined) to which an alkyl group is covalently bonded. The aralkyl group on the aromatic ring may be substituted, for example, with alkyl groups or with halogen atoms. Benzyl is an example of an aralkyl group. Aralkyl groups may optionally be substituted, for example, with carboxyl or sulfonic acid groups, with carboxyl ester or sulfonic acid ester groups, with carboxylamide or sulfonamide groups, with hydroxyl or amino groups, or with halogen atoms.

[0038] If one of the R2, R3, and / or R13 substituents denotes an amino group, the amino group can be unsubstituted or bear one, two, or three substituents, preferably alkyl and / or aryl groups. Alkyl substituents can be either branched or unbranched. A mono- or dialkylamino group typically contains one or two alkyl groups with one to twenty carbon atoms, preferably with one to six carbon atoms. Examples of monoalkylamino groups are methylamino, ethylamino, propylamino, or butylamino. Examples of dialkylamino groups are diethylamino, dipropylamino, or dibutylamino. Examples of trialkylamino groups are triethylamino, tripropylamino, or tributylamino.

[0039] If one of the substituents R2, R3 and / or R13 denotes halogen, it is understood to be a covalently bonded fluorine, chlorine, bromine or iodine atom. Fluorine or chlorine are preferred.

[0040] If R1 denotes a divalent bridging group, then this refers to a divalent inorganic or organic residue. Examples of divalent inorganic residues are -O-, -S-, -SO-, -SO₂-, -OP(O)O-, or -NH-. Examples of divalent organic residues are alkylenes, cycloalkylenes, arylenes, aralkylenes, or heterocyclylenes.

[0041] If R14 and R22 denote a divalent organic bridging group, then this refers to an organic residue that is linked to the rest of the molecule via two covalent bonds. Examples of divalent organic residues R14 or R22 are alkylene, alkylenoxy, poly(alkylenoxy), alkyleneamino, poly(alkyleneamino), cycloalkylene, arylene, aralkylene, or heterocyclylene.

[0042] Alkylene groups can be either branched or unbranched. An alkylene group typically contains one to twenty carbon atoms, preferably two to four. Examples of alkylene groups are methylene, ethylene, propylene, and butylene. Alkylene groups may optionally be substituted, for example, with carboxyl or sulfonic acid groups, with carboxyl ester or sulfonic acid ester groups, with carboxylamide or sulfonamide groups, with hydroxyl or amino groups, or with halogen atoms.

[0043] Alkylenoxy and poly(alkylenoxy) groups can contain both branched and unbranched alkylene groups. An alkylene group occurring in an alkylenoxy or poly(alkylenoxy) group typically contains two to four carbon atoms, preferably two or three. The number of repeating units in poly(alkylenoxy) groups can vary widely. Typical numbers of repeating units range from 2 to 50. Examples of alkylenoxy groups are ethyleneoxy, propylenoxy, and butylenoxy. Examples of poly(alkylenoxy) groups are poly(ethyleneoxy), poly(propylenoxy), and poly(butylenoxy).

[0044] Alkyleneamino and poly(alkyleneamino) groups can contain both branched and unbranched alkylene groups. An alkylene group occurring in an alkyleneamino or poly(alkyleneamino) group typically contains two to four carbon atoms, preferably two or three. The number of repeating units in poly(alkyleneamino) groups can vary widely. Typical numbers of repeating units range from 2 to 50. Examples of alkyleneamino groups are ethyleneamino, propyleneamino, and butyleneamino. Examples of poly(alkyleneamino) groups are poly(ethyleneamino), poly(propyleneamino), and poly(butyleneamino).

[0045] Cycloalkylene groups typically contain five, six, or seven ring carbon atoms, each of which can be independently substituted. Examples of substituents include alkyl groups or two alkyl groups that, together with the ring carbons to which they are bonded, can form another ring. An example of a cycloalkylene group is cyclohexylene. Cycloalkylene groups may optionally be substituted, for example, with carboxyl or sulfonic acid groups, with carboxyl ester or sulfonic acid ester groups, with carboxylamide or sulfonamide groups, with hydroxyl or amino groups, or with halogen atoms.

[0046] Arylene groups are typically cyclic aromatic groups containing five to fourteen carbon atoms, each of which can be independently substituted. Examples of arylene groups include o-phenylene, m-phenylene, p-phenyl, o-biphenylyl, m-biphenylyl, p-biphenylyl, 1-anthryl, 2-anthryl, 9-anthryl, 1-phenantolyl, 2-phenantolyl, 3-phenantolyl, 4-phenantolyl, and 9-phenantolyl. Arylene groups may be substituted, for example, with carboxyl or sulfonic acid groups, carboxyl ester or sulfonic acid ester groups, carboxylamide or sulfonamide groups, hydroxyl or amino groups, or halogen atoms. Other examples of substituents are alkyl groups or two alkyl groups which, together with the ring carbon atoms to which they are bonded, can form another ring.

[0047] 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 are oxygen, nitrogen, phosphorus, boron, selenium, or sulfur. Examples of heterocyclyl groups are furandiyl, thiopheniyl, pyrroliyl, or imidazoliyl. Heterocyclyl groups are preferably aromatic. Heterocyclyl groups can optionally be substituted, for example, with carboxyl or sulfonic acid groups, with carboxyl ester or sulfonic acid ester groups, with carboxylamide or sulfonamide groups, with hydroxyl or amino groups, or with halogen atoms. Further examples of substituents are alkyl groups, or two alkyl groups that, together with the ring carbons to which they are bonded, can form another ring.

[0048] Aralkylene groups are typically aryl groups to which one or two alkyl groups are covalently bonded. Aralkyl groups can be covalently linked to the rest of the molecule via their aryl and alkyl groups or via two alkyl groups. The aralkylene group on the aromatic ring can be substituted, for example, with alkyl groups or with halogen atoms. Examples of aralkylene groups are benzylene and dimethylphenylene (xylylene).

[0049] If one of the residues R 9 , R 11 , R 12 or R 15 represents a divalent to hexavalent organic bridging group, then this refers to an organic residue that is connected to the rest of the molecule via two, three, four, five or six covalent bonds.

[0050] Examples of divalent organic residues are alkylene, alkylenoxy, poly(alkylenoxy), alkyleneamino, poly(alkyleneamino), cycloalkylene, arylene, aralkylene, and heterocyclylene. These residues have already been described in detail above.

[0051] Examples of trivalent organic residues are alkyltriyl, alkoxytriyl, tris-poly(alkylenoxy), tris-poly(alkylenamino), cycloalkyltriyl, aryltriyl, aralkyltriyl, and heterocyclyltriyl. 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 by three covalent bonds instead of two.

[0052] Examples of tetravalent organic residues are alkylquaternyl, alkoxyquaternyl, quater-poly(alkylenoxy), quater-poly(alkylenamino), cycloalkylquaternyl, arylquaternyl, aralkylquaternyl, and heterocyclylquaternyl. 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 by four covalent bonds instead of two.

[0053] Examples of pentavalent organic residues are alkylquinquinyl, alkoxyquinquinyl, quinqui-poly(alkylenoxy), quinqui-poly(alkylenamino), cycloalkylquinquinyl, arylquinquinyl, aralkylquinquinyl, and heterocyclylquinquinyl. These residues correspond to the divalent residues described in detail above, with the difference that they are linked to the rest of the molecule by five covalent bonds instead of two.

[0054] Examples of hexavalent organic residues are alkylhexyl, alkoxyhexyl, hexyl poly(alkylenoxy), hexyl poly(alkylenamino), cycloalkylhexyl, arylhexyl, aralkylhexyl, and heterocyclylhexyl. 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 by six covalent bonds instead of two.

[0055] R 16 is an o-positively charged, preferably a singly positively charged monovalent organic residue. This is generally an alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, or heterocyclyl containing one to four positively charged residues, in particular quaternary ammonium residues, quaternary phosphonium residues, ternary sulfonium residues, or a singly to quadruply charged monovalent heterocyclic residue. Charge balance is achieved via the anion(s) X q-<. The o-positively charged residue is preferably linked to the piperidine-1-oxyl residue via the heteroatom of the o-positively charged residue.Particularly preferred examples of residues R 16 are the residues -N +< R 26 R 27 R 28 , -P +< R 26 R 27 R 28 , -S +< R 26 R 27 or -Het +< , wherein R 26 , R 27 and R 28 are independently hydrogen, alkyl, cycloalkyl, aryl, aralkyl or heterocyclyl, in particular C 1 -C 6 alkyl, cyclohexyl, phenyl or benzyl, and Het is a monovalent and singly positively charged heterocyclic residue having one to three ring nitrogen atoms or one ring nitrogen atom and one to two ring oxygen atoms or ring sulfur atoms, particularly preferably a monovalent imidazolium, pyridinium, guanidinium, uronium, thiouronium, piperidinium or morpholinium residue.

[0056] R 19 is an o-positively charged, preferably a singly positively charged, divalent organic residue. This is generally an alkylene, haloalkylene, cycloalkylene, arylene, aralkylene, or heterocyclylene containing one to four positively charged residues, in particular one to four quaternary ammonium residues, one to four quaternary phosphonium residues, one to four ternary sulfonium residues, or a singly to quadruply positively charged divalent heterocyclic residue. Charge balance is achieved via the anion(s) X q-<. The o-positively charged residue is preferably linked to the piperidine-1-oxyl residue via the heteroatom of the singly positively charged residue. The linkage of R 19 to the nitrogen atom of the bipyridyl residue occurs via a carbon atom of the R 19 residue.Particularly preferred examples of residues R 19 are the residues -N +< R 26 R 27 R 29 -, -P +< R 26 R 27 R 29 -, -S +< R 26 R 29 - or -Het +< -, wherein R 26 and R 27 are independently hydrogen, alkyl, cycloalkyl, aryl, aralkyl or heterocyclyl, in particular C 1 -C 6 -alkyl, cyclohexyl, phenyl or benzyl, R 29 is a divalent organic residue, and Het is a divalent and singly positively charged heterocyclic residue having one to three ring nitrogen atoms or one ring nitrogen atom and one to two ring oxygen atoms or ring sulfur atoms, particularly preferably a monovalent imidazolium, pyridinium, guanidinium, uronium, thiouronium, piperidinium or morpholinium residue.

[0057] R 17 is an m-fold positively charged divalent to tetravalent organic residue. This is an organic residue that has m positively charged groups and is linked to the rest of the molecule via two, three, or four covalent bonds. Examples of positively charged residues are quaternary ammonium, quaternary phosphonium, ternary sulfonium, or an m-fold positively charged divalent to tetravalent heterocyclic residue. The m-fold positively charged residue is preferably linked to the piperidine-1-oxyl residues via the heteroatoms of the m-fold positively charged residue.Particularly preferred examples of residues R 17 are the residues -N +< R 30 R 31 -[R 32 -N +< R 30 R 31 ] f -, -P +< R 30 R 31 -[R 32 -PR 30 R 31 ] f -, -S +< R 30 -[R 24 -S +< R 30 ] f - or [Het m+< ,]f- wherein R 30 and R 31 are independently alkyl, cycloalkyl, aryl, aralkyl or heterocyclyl, in particular C 1 -C 6 -alkyl, cyclohexyl, phenyl or benzyl, f has the meaning defined above, R 32 represents an f+1 valent organic residue and Het represents a di- to tetravalent and m-positively charged heterocyclic residue comprising one to three ring nitrogen atoms or one ring nitrogen atom and one to two ring oxygen atoms or comprising ring sulfur atoms, particularly preferably a divalent to tetravalent imidazolium, pyridinium, guanidinium, uronium, thiouronium, piperidinium or morpholinium residue.

[0058] R 18 is an o-positively charged divalent to tetravalent organic residue. This is an organic residue that possesses o positively charged groups and is linked to the rest of the molecule via two, three, or four covalent bonds. R 18 is covalently linked to the nitrogen atom of the bipyridyl residue(s) via a carbon atom. Examples of positively charged residues include quaternary ammonium, quaternary phosphonium, ternary sulfonium, or an o-positively charged divalent to tetravalent heterocyclic residue. The o-positively charged residue is preferably linked to the piperidine-1-oxyl residues via the heteroatoms of the o-positively charged residue.Particularly preferred examples of residues R 18 are the residues -[N +< R 30 R 31 ] g -[R 33 -N +< R 30 R 31 ] h -, -[P +< R 30 R 31 ] g -[R 33 -PR 30 R 31 ] h -, -[S +< R 30 ] g -[R 33 -S +< R 30 ] h - or [Het m+< ,] g+h -, wherein R 30 and R 31 are independently alkyl, cycloalkyl, aryl, aralkyl or heterocyclyl, in particular C 1 -C 6 alkyl, cyclohexyl, phenyl or benzyl, g and h have the meanings defined above, R 33 represents a g+h valent organic residue and Het represents a divalent to tetravalent and o-positively charged heterocyclic residue containing one to three ring nitrogen atoms or has a ring nitrogen atom and one to two ring oxygen atoms or ring sulfur atoms, particularly preferably a divalent to tetravalent imidazolium, pyridinium, guanidinium, uronium, thiouronium, piperidinium or morpholinium residue.

[0059] Examples of divalent organic residues R 29, R 32 and R 33 are alkylenes, cycloalkylenes, arylenes, aralkylenes or heterocyclylenes. These residues have already been described in detail above.

[0060] Examples of trivalent organic residues R32 and R33 are alkyltriyl, cycloalkyltriyl, aryltriyl, aralkyltriyl, and heterocyclyltriyl. 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 by three covalent bonds instead of two.

[0061] Examples of tetravalent organic residues R32 and R33 are alkylquaternyl, cycloalkylquaternyl, arylquaternyl, aralkylquaternyl, and heterocyclylquaternyl. 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 by four covalent bonds instead of two.

[0062] R 23 is a u-positively charged divalent to tetravalent organic residue. This is an organic residue that has u negatively charged groups and is linked to the rest of the molecule via two, three, or four covalent bonds. R 23 is covalently linked to the nitrogen atom of the bipyridyl residue(s) via a carbon atom. Examples of negatively charged residues include alkylene or arylene residues substituted with carboxylic or sulfonic acid residues, wherein hydrocarbon units of the alkylene residues or several arylene residues may be interrupted by one or more -O, -CO-O, -CO-NH, or -NH groups, or a divalent to tetravalent heterocyclic residue substituted with up to two carboxylic or sulfonic acid residues. The connection of the u-positively charged residue to the piperidine-1-oxyl residues preferably occurs via carbon atoms of the u-positively charged residue.Particularly preferred examples of residues R 23 are alkylene residues or arylene residues substituted with one or two carboxylic acid or sulfonic acid residues.

[0063] R 24 is a u-positively charged, preferably a singly negatively charged, divalent organic residue. This is generally an alkylene, haloalkylene, cycloalkylene, arylene, aralkylene, or heterocyclylene containing one to four singly negatively charged residues, in particular an alkylene or arylene residue with one to four carboxylic acid or sulfonic acid substituents, wherein hydrocarbon units of the alkylene residues or several arylene residues may be interrupted by one or more -O-, -CO-O-, -CO-NH-, or -NH- groups, or a one- to four-positively charged divalent heterocyclic residue. Charge balance is achieved via the anion(s) X q-< or via the cation(s) Y x+<. The u-positively charged residue is preferably linked to the piperidine-1-oxyl residue via a carbon atom of the u-positively charged residue.The bonding of R 24 to the nitrogen atom of the bipyridyl residue occurs via a carbon atom of the R 24 residue.

[0064] R 25 is a u-positively charged, preferably a singly negatively charged, monovalent organic residue. This is generally an alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, or heterocyclyl containing one to four singly negatively charged residues, in particular one to four carboxylic acid residues or one to four sulfonic acid residues, or a monovalent heterocyclic residue substituted with one to four carboxylic acid residues or one to four sulfonic acid residues. Charge balance is achieved via the cation(s) Z q+<. The u-positively charged residue is preferably linked to the piperidine-1-oxyl residue via a carbon atom of the singly negatively charged residue.

[0065] The redox-active components used according to the invention, with one to six residues of formula I or with one to six residues of formula II in the molecule, have counterions X q-<. These counterions balance the ionic charges that arise during charging or discharging. The counterions X q-< can be inorganic or organic q-valent anions.

[0066] Examples of inorganic anions x q-< are halide ions, such as fluoride, chloride, bromide or iodide, or hydroxide ions or anions of inorganic acids, such as phosphate, sulfate, nitrate, hexafluorophosphate, tetrafluoroborate, perchlorate, chlorate, hexafluoroantimonate, hexafluoroarsenate, cyanide.

[0067] Examples of organic anions X q-< are anions of mono- or polyvalent carboxylic acids or mono- or polyvalent sulfonic acids, where these acids can be saturated or unsaturated. Examples of organic acid anions are acetate, formate, trifluoroacetate, trifluoromethanesulfonate, pentafluoroethanesulfonate, nonofluorobutanesulfonate, butyrate, citrate, fumarate, glutarate, lactate, malate, malonate, oxalate, pyruvate, or tartrate.

[0068] Furthermore, the redox-active components used according to the invention can contain inorganic cations, such as mono- or polyvalent metal ions, or organic cations, such as ammonium, imidazolium, pyridinium, guanidinium, uronium, thiouronium, piperidinium, morpholinium, or phosphonium. Charge balance is achieved by the anions X q-< .

[0069] In some cases, the redox-active components used according to the invention carry one or more negative charges. Here, charge balance is achieved through counterions Y< x< or Z< q+<. However, there may also be cases in which the redox-active components have a zwitterionic structure and do not require any further anions or cations for charge balance.

[0070] The anions Yx can be inorganic or organic x-valent anions. Examples of anions Yx correspond to those mentioned above for anions Xq.

[0071] The cations Y x+< can be inorganic or organic x-valent cations. Examples include x-valent metal ions or x-valent organic cations such as ammonium, imidazolium, pyridinium, guanidinium, uronium, thiouronium, piperidinium, morpholinium, or phosphonium. Monovalent or divalent metal ions, especially alkali or alkaline earth cations, are preferred.

[0072] The cations Z q+< can be inorganic or organic q-valent cations. Examples include q-valent metal ions or q-valent organic cations such as ammonium, imidazolium, pyridinium, guanidinium, uronium, thiouronium, piperidinium, morpholinium, or phosphonium. Monovalent or divalent metal ions, especially alkali or alkaline earth cations, are preferred.

[0073] Preferably, compounds containing halide ions, hydroxide ions, phosphate ions, sulfate ions, perchlorate ions, hexafluorophosphate ions or tetrafluoroborate ions are used in the redox flow cells according to the invention, as well as preferably cations selected from the group of hydrogen ions (H+), alkali or alkaline earth metal cations (e.g. lithium, sodium, potassium, magnesium, calcium), and substituted or unsubstituted ammonium cations (e.g. tetrabutylammonium, tetramethylammonium, tetraethylammonium), wherein the substituents can generally be alkyl groups.

[0074] Preferably, redox-active compounds containing one to four, in particular one to two, structural units of formula I or II are used, in which R 1 is a covalent CC bond or -O-, -NH-, arylene or heteroarylene, and most preferably a covalent CC bond, phenylene, biphenylene or thiophendiyl.

[0075] Other preferably used redox-active compounds are those containing one to four structural units, in particular one to two structural units of formula I and / or II, especially those of formulas Ia or IIa and most preferably those of formulas Ib, IIb, IV, V, VII, VIII, IX, IXa, X or Xa, and most preferably those of formulas IVa, Va, VIIa, VIIb, VIIc, VIId, VIIIa, VIIIb, VIIIc, VIIId, IXa, IXb, Xa or Xb, in which b and c are equal to zero or in which b and c represent 1 or 2 and R 2 and R 3 respectively represent methyl, ethyl, methoxy, ethoxy, trifluoromethyl, difluoromethyl, fluorine, chlorine, hydroxy, amino or nitro.

[0076] Further favored redox compounds are those of the formula III, VII, VIIa, VIIb VIII, VIIIa, VIIIb, IX, IXa, IXb, X, Xa or Xb, in particular those of the formulas IIIa, IIIb, V, VIII, VII, VII, VIII, VIII, VIII, VIII, V VIIIe, IXc or Xc, in which R 4 , R 5 , R 6 and R 7 mean C 1 -C 6 -Alkyl and especially preferred Ethyl or Methyl.

[0077] Other preferably used redox-active compounds are those of formula IV, V, VII, VIIa, VIIb, VIII, VIIIa or VIIIb, in particular those of formulas IVa, Va, VIIc or VIIIc and most preferably those of formulas Ib, VIId, VIIe, VIIId or VIIIe, in which R 8 or R 8 and R 10 represent hydrogen, C 1-C 6 alkyl, C 1-C 6 alkyl substituted with a carboxylic acid alkyl ester group, C 1-C 6 alkyl substituted with a carboxylic acid amide group, C 1-C 6 alkyl substituted with a carboxylic acid group, C 1-C 6 alkyl substituted with a sulfonic acid group or C 1-C 6 alkyl substituted with an amino group, and most preferably hydrogen, ethyl or methyl.

[0078] Other preferably used redox-active compounds are those of formula IIb, in which R 20 and R 21 represent hydrogen, C 1-C 6 alkyl substituted with a carboxylic acid alkyl ester group, C 1-C 6 alkyl substituted with a carboxylic acid amide group, C 1-C 6 alkyl substituted with a carboxylic acid group, C 1-C 6 alkyl substituted with a sulfonic acid group or C 1-C 6 alkyl substituted with an amino group and, most preferably, hydrogen, ethyl or methyl, or in which the residues R 20 and R 21 together form a C 1-C 3 alkylene group, in particular ethylene.

[0079] Other preferred redox-active compounds are those of formula IIIa, in which R 13 is hydrogen, C 1-C 6 alkyl, C 1-C 6 alkoxy, C 1-C 6 partial or perfluoroalkyl, C 1-C 6 partial or perchloroalkyl, C 1-C 6 fluorochloroalkyl, phenyl, benzyl, fluorine, chlorine, hydroxy, amino or nitro.

[0080] Other preferably used redox-active compounds are those of formula IV or V, in particular those of formula IVa or Va, in which R 9 is alkylene, poly(alkyleneamino), arylene, aryltriyl, arylquaternyl, heterocyclylene, heterocyclyltriyl or heterocyclylquaternyl, most preferably C 2 -C 6 alkylene, di-(C 2 -C 6 alkyleneamino), tri-(C 2 -C 6 alkyleneamino), quater-(C 2 -C 6 alkyleneamino), phenylene, phenyltriyl or phenylquaternyl.

[0081] Other preferably used redox-active compounds are those of formula VII or VIII, in particular those of formulas VIIc or VIIIc, in which R 12 is alkylene, alkyltriyl, alkylquaternyl, alkyloxydiyl, alkyloxytriyl, alkyloxyquaternyl, arylene, aryltriyl, arylquaternyl, heterocyclylene, heterocyclyltriyl or heterocyclylquaternyl, most preferably C 2 -C 6 alkylenes, such as ethylene or propylene, or C 2 -C 6 alkoxydiyl, such as 1,2-dioxyethylene or 1,3-dioxypropylene, or C 3 -C 6 alkoxytriyl, such as a 1,2,3-propanetriol residue or a trimethylolpropane residue, or C 4 -C 6 alkoxyquaternyl, such as a pentaerythritol residue, or phenylene, phenyltriyl or phenylquaternyl.

[0082] Other preferably used redox-active compounds are those of formula IX or X, in particular those of formulas VIId or VIIId, in which R 14 is alkylene, alkyleneamino, poly(alkyleneamino), arylene or heterocyclylene, most preferably C 2 -C 6 -alkylene, C 2 -C 6 -alkyleneamino or phenylene.

[0083] Other preferably used redox-active compounds are those of formula IX or X, in particular those of formula IXc or Xc, in which R 15 is alkylene, alkyltriyl, alkylquaternyl, arylene, aryltriyl, arylquaternyl, heterocyclylene, heterocyclyltriyl or heterocyclylquaternyl, most preferably C 2 -C 6 -alkylenes, such as ethylene or propylene, or phenylene, phenyltriyl or phenylquaternyl.

[0084] Other preferably used redox-active compounds are those of formula VI, in which R 11 is alkylene, alkyltriyl, alkylquaternyl, alkyloxydiyl, alkyloxytriyl, alkyloxyquaternyl, arylene, aryltriyl, arylquaternyl, heterocyclylene, heterocyclyltriyl or heterocyclylquaternyl, most preferably C 2 -C 6 alkylenes, such as ethylene or propylene, or C 2 -C 6 alkoxydiyl, such as 1,2-dioxyethylene or 1,3-dioxypropylene, or C 3 -C 6 alkoxytriyl, such as a 1,2,3-propanetriol residue or a trimethylolpropane residue, or C 4 -C 6 alkoxyquaternyl, such as a pentaerythritol residue, or phenylene, phenyltriyl or phenylquaternyl.

[0085] The indices b and c are preferably each 0 or independently 1 or 2.

[0086] Index a is preferably 1 or 2 and especially 2.

[0087] Index q is preferably 1 or 2 and in particular 1.

[0088] Index d is preferably 1 or 2 and in particular 1.

[0089] Index g is preferably 1 or 2 and in particular 1.

[0090] Index h is preferably 1 or 2 and particularly 1.

[0091] Compounds of formulas VII or VIII are particularly preferred, especially those of formulas VIIa or VIIIa and most preferably those of formulas VIIc or VIIIc in which index g is 1 and index h means 1 or 2 or in which index g is 1 or 2 and index h means 1.

[0092] Index i is preferably 1 or 2 and in particular 1.

[0093] Index j is preferably 1 or 2 and in particular 1.

[0094] Index k is preferably 1, 2 or 4 and in particular 2 or 4.

[0095] Index f is preferably 1 or 2 and in particular 1.

[0096] Index m is preferably 1 or 2 and particularly 1.

[0097] Index n is preferably 1 / 2, 1 or 2 and particularly 1 / 2 or 1.

[0098] Index l is preferably 1 / 2 or 1 and especially 1.

[0099] Index o is preferably 1 or 2 and especially 1.

[0100] Index p is preferably 6, 5, 4, 3, 5 / 2, 2 or 3 / 2 and particularly 3 or 5.

[0101] Index r is preferably 9, 6, 9 / 2 or 3 and particularly 9 or 6.

[0102] Index s is preferably 3 or 3 / 2 and especially 3.

[0103] The index t is preferably 0.

[0104] Index u is preferably 1 or 2, in particular 1.

[0105] Index v is preferably -1 or -2 and especially -1.

[0106] Index x is preferably -1 or -2 and especially -1.

[0107] Index x1 is preferably -1 or -2 and especially -1.

[0108] Index y is preferably 1 or 2 and especially 2.

[0109] Index z is preferably 1 or 2 and especially 2.

[0110] Index z1 is preferably 3 or 1.5 and particularly 1.5.

[0111] The iron salts used as catholytes according to the invention are water-soluble iron salts in oxidation states II and / or III. Iron salts can contain any anions, as long as this does not jeopardize the water solubility of these salts.

[0112] In this description, water solubility of a compound is understood to mean a solubility of at least 1 g of the compound in 1 l of water at 25°C.

[0113] Examples of iron salts are combinations of Fe(II) chloride with Fe(III) chloride or of Fe(II) sulfate with Fe(III) sulfate. Besides iron salts with inorganic anions, iron salts with organic anions can also be used, for example, Fe(II) acetate with Fe(III) acetate.

[0114] The redox system used according to the invention offers many advantages compared to other material systems for redox flow batteries. For example, no noble metal catalysts are required, as is the case with iron / chromium cell types, because the reaction kinetics of viologen are significantly faster than those of chromium. Furthermore, the redox-active materials (TEMPO derivatives, iron(II / III) chloride, and dimethyl viologen chloride) are highly water-soluble even at neutral pH (more than 2 mol / L at room temperature). This results in high storage capacities (over 53 Ah / L with 2 mol / L of active materials). Moreover, the solutions do not require aggressive acids as electrolytes, which are frequently used in prior art systems (e.g., vanadium systems).

[0115] The redox-active materials used according to the invention are also compatible with each other, i.e., a mixed solution of TEMPO derivatives and viologen derivatives or of iron(II / III) chloride and viologen derivatives can be prepared, and this solution can be used as both an anolyte and a catholyte. This significantly reduces the problem of cross-contamination via membrane defects, which is a major issue for long-term stability in other organic / partially organic redox flow systems.

[0116] In particular, combination molecules that combine both TEMPO and viologen functionality in a single molecule significantly reduce the problem of cross-contamination via membrane defects. Should cross-contamination occur, the same substance is still present in both the anolyte and catholyte. These combination molecules thus simulate substances—usually metals / metal salts—that can assume at least three different redox states, as is the case, for example, with the metal vanadium.

[0117] Another advantage is that the formation of hydrogen, which is detrimental to system operation and safety, can be suppressed, since viologen is able to oxidize hydrogen (CL Bird, AT Kuhn, Chem. Soc. Rev.: "Electrochemistry of the viologens" 40, 1981, p49-82). Hydrogen produced during charging is no longer available to the battery system and therefore results in a loss of battery efficiency. Viologen derivatives can thus also be used as redox-active additives for other redox flow battery systems. Furthermore, an advantage of this system is the possibility of rebalancing the capacity using light, in which the viologen molecule can be photoinduced to be converted into its reduced form (TW Ebbesen, G. Levey, LK Patterson "Photoreduction of methyl viologen in aqueous neutral solution without additives" Nature, 1982 vol 298, p545-548). This eliminates the need for costly external rebalancing cycles.Rebalancing is a necessary step in vanadium systems to ensure that the same amount of charge carriers is present on both the anode and cathode sides.

[0118] The redox-active components are used in dissolved form; this also includes their use as a dispersion.

[0119] The molar masses of the redox-active components used according to the invention, containing residues of formula I, II, or III, or of formulas I and III, or II and III, can vary widely. Redox-active components containing residues of formula I, II, or III, or of formulas I and III, or II and III, with molar masses of less than 500 g / mol are particularly preferred.

[0120] The viscosity of the electrolyte used according to the invention is typically in the range of 1 mPas to 10 3< mPas, particularly preferably 10 -2< to 10 2< mPas and most preferably 1 to 20 mPas (measured at 25 °C with a rotational viscometer, plate / plate).

[0121] The redox-active components used according to the invention can be produced using standard methods of organic synthesis. These procedures are known to those skilled in the art.

[0122] The redox flow cell according to the invention can contain, in addition to the redox-active components described above, other elements or components that are common for such cells.

[0123] In the redox flow cell according to the invention, selected redox-active components are used in both chambers, which are separated from each other by an ion-conducting membrane and which are present in the chambers in dissolved or dispersed form.

[0124] The electrolyte contains the redox-active components. An organic solvent and / or water are also used. Furthermore, the electrolyte may contain at least one conducting salt. Additional additives may also be used. Examples include surfactants, viscosity modifiers, pesticides, buffers, stabilizers, catalysts, conducting additives, antifreeze, temperature stabilizers, and / or antifoaming agents.

[0125] Examples of electrolyte solvents include water, alcohols (e.g., ethanol), carbonic acid esters (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., gamma-butyrolactone), and lactams (e.g., NMethyl-2-pyrrolidone), nitro compounds (e.g., nitromethane), ethers (e.g., tetrahydrofuran), chlorinated hydrocarbons (e.g., dichloromethane), carboxylic acids (e.g., formic acid, acetic acid), mineral acids (e.g., sulfuric acid, hydrogen halides or hydrohalic acids), and mixtures thereof are used. Water, carbonic acid esters (e.g., propylene carbonate), nitriles (e.g., acetonitrile), and mixtures thereof are preferred. Water is particularly preferred.

[0126] Examples of conducting salts are salts containing anions selected from the group consisting of halide ions (fluoride ion, chloride ion, bromide ion, iodide ion), hydroxide ions, anions of inorganic acids (e.g., phosphate ions, sulfate ions, nitrate ions, hexafluorophosphate ions, tetrafluoroborate ions, perchlorate ions, chlorate ions, hexafluoroantimonate ions, hexafluoroarsenate ions, cyanide ions), or anions of organic acids (e.g., acetate ions, formation ions, trifluoroacetic acid ions, trifluoromethanesulfonate ions, pentafluoroethanesulfonate ions, nonofluorobutanesulfonate ions, butyrate ions, citrate ions, fumarate ions, glutarate ions, lactate ions, malate ions, malonate ions, oxalate ions, pyruvate ions, tartrate ions). Chloride and fluoride ions, hydroxide ions, phosphate ions, sulfate ions, perchlorate ions, hexafluorophosphate ions, and tetrafluoroborate ions are particularly preferred. as well as cations selected from the group of hydrogen ions (H+), alkali 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 can generally be alkyl groups. Hydrogen ions, lithium ions, sodium ions, potassium ions, tetrabutylammonium ions, and mixtures thereof are particularly preferred. In particular the conducting 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 and their mixtures where n-Bu represents the n-butyl group.

[0127] Examples of electrolyte additives are surfactants, which can be nonionic, anionic, cationic, or amphoteric. Nonionic surfactants are particularly preferred (e.g., polyalkylene glycol ethers, fatty alcohol propoxylates, alkyl glucosides, alkyl polyglucosides, octylphenol ethoxylates, nonylphenol ethoxylates, saponins, phospholipids).

[0128] Other examples of electrolyte additives are buffers (e.g., carbonic acid-bicarbonate buffer, carbonic acid-silicate buffer, acetic acid-acetate buffer, phosphate buffer, ammonia buffer, citric acid or citrate buffer, tris(hydroxymethyl)aminomethane, 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, 4-(2-hydroxyethyl)-piperazine-1-propanesulfonic acid, 2-(N-morpholino)ethanesulfonic acid, barbital acetate buffer).

[0129] The selection of redox-active components is such that the redox-active component in the catholyte has a different, preferably higher, more positive redox potential than that of the redox-active component in the anolyte. The term "redox-active component" here refers to all redox states associated with that component, i.e., all its reduction / oxidation states. The redox-active component can assume ≥ 2 oxidation and / or reduction states. If the redox-active component assumes more than 2 oxidation and / or reduction states, the redox potential of at least two oxidation and / or reduction states must differ sufficiently to allow a potential difference to develop between the catholyte and anolyte.

[0130] According to the invention, the potential difference between the redox reactions of the redox-active components taking place in the anolyte and catholyte respectively lies between greater than 0 V and 4.0 V; preferably between 0.5 and 2.5 V; particularly preferably between 0.9 and 1.6 V.

[0131] 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).

[0132] The redox flow cell according to the invention contains an ion-conducting membrane. This membrane fulfills the following functions: Separation of anode and cathode compartments; retention of both redox-active components; permeability for the conducting salts of the electrolyte, which serve to balance the charge, i.e., for anions and / or cations of the conducting salt or for the charge carriers contained in the electrolyte in general.

[0133] The proposed membrane, for example a membrane permeable to ions of the conducting salt or a dialysis membrane, separates the redox-active components with comparatively low molar masses in the two chambers.

[0134] Depending on the application, the membrane materials can consist of plastics, ceramics, glasses, metals, or textiles. Examples of materials include organic polymers such as cellulose or modified cellulose, for example, cellulose ethers or cellulose esters, polyethersulfone, polysulfone, polyvinylidene fluoride, polyester, polyurethanes, polyamides, polypropylene, polyvinyl chloride, polyacrylonitrile, polystyrene, polyvinyl alcohol, polyphenylene oxide, polyimides, polytetrafluoroethylene, and their derivatives, as well as ceramics, glasses, or felts. Membranes made of multiple materials (composites) are also possible.

[0135] The membranes and the resulting redox flow cells can be used in various forms. Examples include flat membranes, pocket filter designs, and wound modules. These embodiments are known to those skilled in the art. Flat membranes are preferred.

[0136] The membrane used according to the invention can be supported for better stability, e.g. by a sieve-shaped or perforated plastic material or fabric.

[0137] The thickness of the membrane used according to the invention can vary widely. Typical thicknesses are in the range between 0.1 µm and 5 mm, particularly preferably between 10 µm and 200 µm.

[0138] In addition to the electroactive components, electrolytes, and membranes described above, the redox flow cell according to the invention contains further components. These are: Conveying equipment, such as pumps, as well as tanks and pipes for the transport and storage of redox-active components; electrodes, preferably consisting of or containing graphite, graphite fleece, graphite paper, carbon nano-tube carpets, activated carbon, carbon black or graphene; optionally current collectors, such as those made of graphite or metals

[0139] The positive electrode may contain or consist of the following additional materials: Precious metal-coated or diamond-coated titanium, niobium, tungsten, graphite, silicon carbide or tantalum, in particular platinum and / or iridium and / or ruthenium oxide-coated titanium, diamond or with electrically conductive components, e.g. B. Boron, doped diamond, glassy carbon (Lothar Dunsch: Electrochemical reactions on glassy carbon, Zeitschrift für Chemie, 14, 12, p463-468, December 1974), indium tin oxide, lead, lead-silver alloy, e.g., lead-silver alloy with 1% silver, tin, tin oxide, carbon black, spinels (as described, e.g., in EP 0042984), perovskites (CaTiO3), delafossites (containing copper and / or iron oxide), antimony, bismuth, cobalt, platinum and / or platinum moraine, palladium and / or palladium moraine, manganese, polypyrrole (as described, e.g., in EP 0042984).(described in EP 0191726 A2, EP 0206133 A1), stainless steel, Hastelloy or iron-chromium-nickel alloys containing nickel. Nickel-containing positive electrodes are preferably used when the electrolyte has an alkaline pH value of >= 7-8.

[0140] The following known coating processes can be used for coated electrode materials: chemical vapor deposition (CVD), physical vapor deposition (PVD), electroplating, electroless deposition from a liquid solution containing the metal in dissolved form and a reducing agent, wherein the reducing agent causes the deposition of the desired metal on a surface.

[0141] The negative electrode may contain or consist of the following materials: zinc, stainless steel, Hastelloy or iron-chromium-nickel alloys, graphite, graphite fleece, graphite paper, carbon nano-tube carpets, activated carbon, soot or graphene.

[0142] Nickel-containing negative electrodes are preferably used when the electrolyte has an alkaline pH value of >= 7-8.

[0143] The redox flow cells according to the invention include current collectors as a further optional but preferred component. These have the function of establishing the best possible electrical contact between the electrode material and the external current source or sink.

[0144] Aluminum, aluminum alloys, 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, zirconium can be used as current collectors in the redox flow cells according to the invention.

[0145] The following well-known coating processes can be used, among others, to manufacture coated current collectors: chemical vapor deposition (CVD), physical vapor deposition (PVD), electroplating, electroless deposition from a liquid solution containing the metal in dissolved form and a reducing agent, wherein the reducing agent causes the deposition of the desired metal on a surface.

[0146] The redox flow cell according to the invention can be used in a wide variety of fields. In the broadest sense, this can include the storage of electrical energy for mobile and stationary applications. The invention also relates to the use of the redox flow cell for these purposes.

[0147] Examples of applications include use as stationary storage for emergency power supply, peak load balancing, and intermediate storage of electrical energy from renewable energy sources, particularly in the photovoltaic and wind power sectors, from gas, coal, biomass, tidal and marine power plants, and use in the field of electromobility, such as storage in land, air and water vehicles.

[0148] The redox flow cell according to the invention is preferably used as a stationary storage device for electrical energy.

[0149] The redox flow cells according to the invention can be interconnected in series or in parallel in a manner known per se.

[0150] The following examples illustrate the invention without limiting it thereto. Example 1: Iron / Viologen redox flow battery

[0151] Theoretical potential of the cell (E 0< is defined as the redox potential in water at 20 °C against a silver / silver chloride reference electrode): E 0< Fe 2+< / Fe 3+< = 0,77 V E 0< MV 2+< / MV +< = -0,43 V → Cell voltage = 1,2 V

[0152] An electrolyte solution was prepared consisting of 1 mol / L each of FeCl₂ and dimethyl viologen chloride, dissolved in a 2 mol / L NaCl electrolyte solution. These substances are commercially available. The electrolyte solution was tested in a redox flow cell with an active area of ​​5 cm². Charging and discharging processes were performed both statically (without pumping) and with pumped liquid. Energy densities of up to 120 mW / cm² were achieved. The storage capacity was 25 Ah / L. Taking overvoltages into account, a cell voltage of approximately 1.0 V was observed.

[0153] In Figure 1The OCV curve of this cell is plotted as a function of its state of charge. The OCV curve shows the relationship between the cell voltage and the state of charge. The cell voltage is measured in an "open circuit," meaning it is the open-circuit voltage (OCV) that results at a given state of charge without an external load. The higher these voltage values, the higher the energy content and the more efficiently the system can be operated.

[0154] Figure 2 The charging curve of this cell is shown. Example 2: TEMPO ammonium chloride / Viologen redox flow battery

[0155] Theoretical potential of the cell: E 0< TEMPO-N +< / TEMPO-N 2+< = 0,78 V E 0< MV 2+< / MV +< = -0,43 V → Cell voltage = 1,21 V

[0156] Two electrolyte solutions were prepared: The solution for the working electrode (positive terminal of the battery) was prepared from 1.0 g of TEMPO ammonium chloride (structure shown below) and 0.55 g of NaCl in 10 ml of water. The solution for the counter electrode (negative terminal of the battery) was prepared from 1.5 g of dimethyl viologen chloride and 0.55 g of NaCl in 10 ml of water. The solutions were tested in a redox flow cell with an active area of ​​5 cm² (analogous to Example 1). The cell was cyclically charged and discharged.

[0157] Structure of TEMPO ammonium chloride:

[0158] In Figure 3 The OCV curve of this cell is plotted as a function of its state of charge.

[0159] In Figure 4 It is shown that the achievable potential level of the single cell is higher when a redox system consisting of small molecules is used instead of a polymeric redox system. Figure 4The OCV curve of a cell containing the TEMPO-ammonium chloride described above, as well as dimethyl viologen with chloride as the counterion, as redox-active components is shown (upper curve). Also shown is the OCV curve of a cell containing a TEMPO- and a viologen-based polymer as redox-active components. It can be seen that the cell voltage of the system with the small molecules is increased by approximately 0.2 V; that is, the energy density of the system with small molecules is increased by more than 15%, even at the same concentration. Example 3: Methylviologen-TEMPO redox flow battery

[0160] Theoretical potential of the cell: E 0< MV-TEMPO 2+< / MV-TEMPO 3+< = 0,68 V E 0< MV-TEMPO 2+< / MV-TEMPO +< = -0,46 V → Cell voltage = 1,14 V

[0161] An electrolyte solution was prepared from 213 mg of methylviologen-TEMPO (with the following structure) and 235 mg of NaCl in 4 ml of water. This solution was used for both the working electrode (positive terminal of the battery) and the counter electrode (negative terminal of the battery) and tested in a redox flow cell with an active area of ​​less than 5 cm² (analogous to Example 1, but without pumping). The cell was cyclically charged and discharged. Furthermore, an OCV curve was recorded.

[0162] Structure of Methylviologen-TEMPO:

[0163] In Figure 5 The OCV curve of this cell is plotted as a function of its state of charge (SOC).

[0164] Figure 6 The charging curve of this cell is shown. Example 4: Propanoate Viologist-TEMPO Redox Flow Battery

[0165] Theoretical potential of the cell: E 0< MV-TEMPO 2+< / MV-TEMPO 3+< = 0,67 V E 0< MV-TEMPO 2+< / MV-TEMPO +< = -0,49 V → Cell voltage = 1,16V

[0166] An electrolyte solution was prepared from 110 mg of propanoate viologen-TEMPO (with the following structure) and 117 mg of NaCl in 2 ml of water. This solution was used for both the working electrode (positive terminal of the battery) and the counter electrode (negative terminal of the battery) and tested in a redox flow cell with an active area of ​​less than 5 cm² (analogous to Example 1, but without pumping). The cell was cyclically charged and discharged. Furthermore, an OCV curve was recorded.

[0167] Structure of Propanoateviologen-TEMPO:

[0168] In Figure 7 The OCV curve of this cell is plotted as a function of its state of charge (SOC).

[0169] Figure 8 The charging curve of this cell is shown. Synthesis examples Example 5: Synthesis of TEMPO ammonium chloride

[0170] 4-Oxo-2,2,6,6-tetramethylpiperidin-1-oxyl (2)

[0171] 20 g 4-oxo-2,2,6,6-tetramethylpiperidine(1) 2 g of Na₂WO₄ × 2 H₂O and 2 g of Na₂H₂EDTA were dissolved in 133 ml of water at room temperature. 26.6 ml of 30% hydrogen peroxide was added while stirring. The reaction progress was monitored by gas chromatography (GC) until complete conversion of the (1) A further 5 ml portion of hydrogen peroxide was added. The red reaction solution was separated from the green precipitate and washed with 150 ml of water. The aqueous phase was extracted seven times with 50 ml of dichloromethane and dried over magnesium sulfate. The solvent was removed and the product (yield 60%) was dried under vacuum. 4-(Dimethylamino)-2,2,6,6-tetramethylpiperidin-1-oxyl (3)

[0172] 2 g 4-oxo-2,2,6,6-tetramethylpiperidine N -oxyl (2)The compounds were dissolved in 20 ml of dry methanol, and 7.3 g of dimethylamine hydrochloride were added under an argon atmosphere. The reaction mixture was treated with 444 mg of NaBH₃CN while cooling and stirring. After 48 hours, the mixture was alkalized with sodium hydroxide solution and extracted three times with 50 mL of dichloromethane. The organic phase was dried over magnesium sulfate, the solvent was removed, and the crude product was dried under vacuum. The resulting crude product was used in the next step without further purification. 1-Oxyl- N,N,N -2,2,6,6-heptamethylpiperidin-4-ammoniumchlorid (4, kurz: TEMPO-Ammoniumchlorid)

[0173] The crude product 4-(dimethylamino)-2,2,6,6-tetramethylpiperidin-1-oxyl (3)The solution was completely dissolved in 20 ml of diethyl ether, solids were removed by filtration, and a solution of 1.42 g of methyl iodide in 5 ml of diethyl ether was added. After stirring the solution at room temperature for 20 hours, the resulting precipitate was separated and washed with 20 ml of diethyl ether. The precipitate was dissolved in 50 ml of water, and an ion exchange of the counterion from iodide to chloride was carried out using an ion exchange resin (Dowex Marathon A2, chloride form). The resulting solution was freeze-dried, and the product was obtained as an orange powder (yield 89%). Example 6: Synthesis of the methyl viologen-TEMPO

[0174] 1-Oxyl-2,2,6,6-tetramethylpiperidin-4-yl-4-(chloromethyl)benzoate (7)

[0175] To a solution of 5 g 4-Hydroxy-2,2,6,6-tetramethylpiperidine- N -oxyl (6)In 80 ml of dry chloroform and 8 ml of dry triethylamine, 4.3 mL of 4-(chloromethyl)benzoyl chloride (5) were added dropwise with stirring at room temperature. After six hours, the reaction mixture was added to a mixture of 300 ml of ice water and 50 ml of 5% bicarbonate solution, stirred, and extracted three times with 200 ml of chloroform. The organic phase was washed with 200 ml of water, dried over magnesium sulfate, and the solvent was removed under vacuum. After drying under vacuum, the crude product was obtained as an orange powder (95% yield). The crude product was used in the next step without further purification. 1-(4-(((1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-[4,4'-bipyridine]-1-ium chloride (8)

[0176] To 4.5 g of the crude product of 1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl-4-(chloromethyl)benzoate (7)2.2 g of 4,4'-bipyridine were added to 80 ml of acetonitrile, and the solution was stirred at 80 °C for 72 hours. The reaction mixture was precipitated in 450 ml of cold ethyl acetate, the resulting precipitate was separated, and dried under vacuum. The product was obtained as an orange solid (78% yield). 1-(4-(((1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-1'-methyl-[4,4'-bipyridine]-1,1'-diium chloride (9, short: methylviologen-TEMPO)

[0177] Variant A: To a solution of 2 g 1-(4-(((1-Oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-[4,4'-bipyridine]-1-ium chloride (8) Chloromethane (pressure 2 bar) was added to 8 ml of water in a pressure reactor. The reaction solution was stirred for 35 hours at 95 °C, and the product was obtained as a solid (yield 95%) by freeze-drying.

[0178] Variant B: To a solution of 0.5 g 1-(4-(((1-Oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-[4,4'-bipyridine]-1-ium chloride (8)0.14 ml of methyl iodide was added to 12 ml of DMSO. The reaction mixture was stirred at 60 °C for 6 hours and then precipitated in 150 ml of ethyl acetate. The precipitate was dissolved in water, and the counterion was exchanged from iodide to chloride using an ion exchange resin (Dowex Marathon A2, chloride form). The resulting solution was freeze-dried, and the product was obtained as an orange powder (82% yield). Example 7: Synthesis of the propanoate viologen-TEMPO

[0179] 3-(1'-(4-(((1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-[4,4'-bipyridine]-1,1'-diium-1-yl)propanoate chloride (10, short: propanoateviologen-TEMPO)

[0180] Add 2 g of 1-(4-(((1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-[4,4'-bipyridine]-1-ium chloride (8) 3 ml of acrylic acid were added to 10 ml of acetonitrile while stirring. The solution was stirred at 60 °C for 30 minutes, cooled, and precipitated in cold ethyl acetate. The precipitate was separated, dried under vacuum, and the desired product was obtained as a powder (yield 56%). Example 8: Synthesis of multifunctionalized viologens

[0181] 1-Methyl-[4,4'-bipyridine]-1-ium chloride (11)

[0182] In a pressure reactor, 100 g of 4,4'-bipyridine were dissolved in 200 ml of acetonitrile and 200 ml of toluene. After the addition of 32.4 g of chloromethane, the solution was stirred at 70 °C for 26 hours. The solvent was removed under vacuum, and the product (yield 98%) was obtained as a grey powder. 1',1'"-(oxybis(ethane-2,1-diyl))bis(1-methy)-[4,4'-bipyridine]-1,1'-diium)chloride (13)

[0183] Add 0.2 g of 1-methyl-[4,4'-bipyridine]-1-ium chloride (11) 61 µl of 1-bromo-2-(2-bromoethoxy)ethane were added (12) 18 mg tetrabutylammonium iodide and 2 ml DMSO were added. The reaction mixture was stirred at 110 °C for 3 days, cooled, and precipitated in cold ethyl acetate. The precipitate was dissolved in water, and an ion exchange of the counterion from iodide to chloride was carried out using an ion exchange resin (Dowex Marathon A2, chloride form). The resulting solution was freeze-dried, and the product was obtained as an orange powder (73% yield). Analytical data on the target products from examples 5 to 8

[0184] 1H NMR spectra were recorded on a Bruker Fourier 300 (300 MHz). TEMPO radicals were reduced with phenylhydrazine or hydrazine hydrate, ensuring the absence of paramagnetic species that could interfere with the measurements.

[0185] Cyclic voltammograms were measured in a 3-electrode setup, with a glassy carbon disc electrode serving as the working electrode, a platinum wire as the counter electrode, and a silver / silver chloride electrode as the reference. Aqueous sodium chloride solutions (0.1 mol / l) were used as the electrolyte. 1-Oxyl- N , N , N -2,2,6,6-heptamethylpiperidin-4-aminiumchlorid (4, kurz: TEMPO-Ammoniumchlorid)

[0186] 1<H NMR (DMSO, 300 MHz) δ: 3.73 (1H, m); 3.02 (9H, s); 1.99 (2H, m); 1.55 (2H, m); 1.09 (12H, d).

[0187] Figure 9 shows a cyclic voltammogram of the substance in aqueous sodium chloride solution (0.1 mol / l), measured against a silver / silver chloride reference electrode. 1-(4-(((1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-1'-methyl-[4,4'-bipyridine]-1,1'-diium chloride (9, short: methylviologen-TEMPO)

[0188] 1<H NMR (D 2 O, 300 MHz) δ: 9.09 (2H, d); 8.94 (2H, d); 8.44 (4H, m); 8.01 (2H, d); 7.52 (2H, d); 5.93 (2H, s); 5.28 (1H, m); 4.40 (3H, s); 2.13 (2H, m); 1.82 (2H, m); 1.24 (12H, s).

[0189] Figure 10 shows a cyclic voltammogram of the substance in aqueous sodium chloride solution (0.1 mol / l), measured against a silver / silver chloride reference electrode. 3-(1'-(4-(((1-oxyl-2,2,6,6-tetramethylpiperidin-4-yl)oxy)carbonyl)benzyl)-[4,4'-bipyridine]-1,1'-diium-1-yl)propanoate chloride (10, short: propanoateviologen-TEMPO)

[0190] 1<H NMR (D 2 O, 300 MHz) δ: 9.05 (4H, m); 8.43 (4H, m); 8.02 (2H, d); 7.51 (2H, d); 5.92 (2H, s); 5.31 (1H, m); 4.81 (2H, t); 2.87 (2H, t); 2.16 (2H, m); 1.84 (2H, m); 1.26 (12H, d).

[0191] Figure 11 Figure 1 shows a cyclic voltammogram of the substance in aqueous sodium chloride solution (0.1 mol / l), measured against a silver / silver chloride reference electrode. The solid lines represent individual measurements of the anodic and cathodic regions, respectively, while the dashed line represents a measurement over the entire region. 1',1 "'-(Oxybis(ethane-2,1-diyl))bis(1-methyl-[4,4'-bipyridine]-1,1'-diium)chloride (13)

[0192] 1< H NMR (DMSO, 300 MHz) δ: 9,09 (4H, m); 8,99 (4H, m); 8,50 (8H, m); 4,88 (4H, m); 4,45 (6H, s); 4,06 (4H, m).

Claims

1. Redox flow cell for storage of electrical energy comprising a reaction cell having two electrode chambers for catholyte and anolyte, which are each connected to at least one store for liquid and are separated by an ion-conducting membrane, and which are equipped with electrodes, wherein the electrode chambers are each filled with electrolyte solutions comprising redox-active components disolved or dispersed in an electrolyte solvent, as well as optionally conducting salts dissolved therein and optionally further additives, wherein the anolyte contains a redox-active component comprising one to six residues of formula I in the molecule or comprising one to six residues of formula II in the molecule, wherein the catholyte contains a redox-active component comprising one to six residues of formula III in the molecule or comprising iron salts or wherein anolyte and catholyte contain a redox-active component comprising one to six residues of formula I or of formula II in combination with one to six residues of formula III in the molecule wherein the lines going off the nitrogen atoms in the structures of formulae I and II and the line going off the 4-position in the structure of formula III represent covalent bonds connecting the structures of formulae I, II and III with the remainder of the molecule, R1 is a covalent C-C-bond or a divalent bridge group, R2 and R3 independently of one another represent alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, heterocyclyl, halogen, hydroxy, amino, nitro or cyano, X is a q-valent inorganic or organic anion or a mixture of such anions, b and c independently of one another are integers from 0 to 4, q is an integer from 1 to 3, a is a number of value 2 / q, and R4, R5, R6 and R7 independently of one another represent alkyl, cycloalkyl, aryl or aralkyl.

2. Redox flow cell according to claim 1, wherein in the anolyte a compound of formulae Ib, IIb, IV, V, VII, VIIa, VIIb VIII, VIIIa, VIIIb, IX, IXa, IXb, X, Xa, Xb, XI, XIa, XIb, XII, XIIa and / or XIIb is used as redox-active component wherein R1, R2, R3, R4, R5, R6, R7 and X have the meaning defined in claim 1, R8 and R10 independently of one another represent hydrogen, alkyl that is optionally substituted with a carboxylic ester group, carboxylic amide group, carboxylic acid group, sulfonic acid group or amino group, cycloalkyl that is optionally substituted with a carboxylic ester group, carboxylic amide group, carboxylic acid group, sulfonic acid group or amino group, aryl that is optionally substituted with a carboxylic ester group, carboxylic amide group, carboxylic acid group, sulfonic acid group or amino group, aralkyl that is optionally substituted with a carboxylic ester group, carboxylic amide group, carboxylic acid group, sulfonic acid group or amino group, preferably C1-C6-alkyl or C1-C6-alkyl that is substituted with a carboxylic ester group, or C1-C6-alkyl that is substituted with a carboxylic amide group, or C1-C6-alkyl that is substituted with a carboxylic acid group, or C1-C6-alkyl that is substituted with a sulfonic acid group, or C1-C6-alkyl that is substituted with an amino group, and especially preferred hydrogen, propionate, isobutionate, ethyl or methyl, R9 is a divalent to hexavalent, preferably a divalent to tetravalent organic bridge group, R12 is a covalent bond or a divalent to hexavalent, preferably a divalent to tetravalent organic bridge group, R14 is a covalent bond or a divalent organic bridge group, R15 is a divalent to hexavalent, preferably a divalent to tetravalent organic bridge group, R18 is an o-times positively charged divalent to hexavalent, preferably divalent to tetravalent organic residue, which is covalently connected via a carbon atom with the nitrogen atom of the bipyridyl residue, preferably a divalent to tetravalent quaternary ammonium residue, a divalent to tetravalent quaternary phosphonium residue, a divalent to trivalent ternary sulfonium residue or an o-times positively charged divalent to hexavalent, preferably a divalent to tetravalent heterocyclic residue, R19 is an o-times, preferably single positively charged divalent organic residue, which is via a carbon atom covalently connected with the nitrogen atom of the bipyridyl residue, preferably a quaternary ammonium residue, a quaternary phosphonium residue, a ternary sulfonium residue or an o-times, preferably single positively charged divalent heterocyclic residue, R20 and R21 independently of one another represent hydrogen, alkyl which is optionally substituted with a carboxylic ester group, a carboxylic amide group, a carboxylic acid group, a sulfonic acid group or an amino group, cycloalkyl which is optionally substituted with a carboxylic ester group, a carboxylic amide group, a carboxylic acid group, a sulfonic acid group or an amino group, aryl which is optionally substituted with a carboxylic ester group, a carboxylic amide group, a carboxylic acid group, a sulfonic acid group or an amino group, or aralkyl which is optionally substituted with a carboxylic ester group, a carboxylic amide group, a carboxylic acid group, a sulfonic acid group or an amino group, or two of the residues R20 and R21 together form a C1-C3-alkylene group, preferably C1-C6-alkyl, C1-C6-alkyl substituted with a carboxylic ester group, C1-C6-alkyl substituted with a carboxylic amide group, C1-C6-alkyl substituted with a carboxylic acid group, C1-C6-alkyl substituted with a sulfonic acid group, or C1-C6-alkyl substituted with an amino group or together represent ethylene, and especially preferred hydrogen, propionate, isobutionate, ethyl or methyl or together are ethylene, R22 is a divalent organic bridge group, R23 represents an u-times negatively charged divalent to hexavalent, preferably divalent to tetravalent organic residue, which is via a carbon atom covalently connected with the nitrogen atom of the bipyridyl residue, preferably an alkylene residue substituted with one or two carboxyl- or sulfonic acid groups, a phenylene residue substituted with one or two carboxyl- or sulfonic acid groups or a divalent hetercyclic residue substituted with one or two carboxyl- or sulfonic acid groups, R24 is an u-times, preferably a single negatively charged divalent organic residue, which is via a carbon atom covalently connected with the nitrogen atom of the bipyridyl residue, preferably an alkylene residue substituted with one carboxyl- or sulfonic acid group, a phenylene residue substituted with one carboxyl- or sulfonic acid group, or a divalent heterocyclic residue substituted with one carboxyl- or sulfonic acid group, a, b, c and q have the meaning defined in claim 1, d is an integer from 1 to 5, preferably from 1 to 3, e is a number having the value (2 + 2d + 2t) / q, g is an integer from 1 to 5, preferably from 1 to 3 h is an integer from 1 to 5, preferably from 1 to 3, wherein the sum of g and h is an integer from 2 to 6, preferably from 2 to 4, i is a number with the value 2h / q, j is an integer from 1 to 5, preferably from 1 to 3, k is a number with the value (2 + 2j) / q, o is an integer from 1 to 4, p is a number with the value (o + 2h) / q, r is a number with the value (3 + 3j) / q, t is 0 or, if R9 is a divalent organic bridge group, represents 0 or 1, u is an integer from 1 to 4, z is a number with the value 2 / q, z1 is a number with the value (o+2) / q Y in case that 2h - u or 2 (2 - u) - u are greater than 0, is a v- or x-valent inorganic or organic anion or represents a mixture of such anions, or in case that 2h - u or 2 (2 - u) - u are smaller than 0, is a v- or x-valent inorganic or organic cation or represents a mixture of such cations, v is an integer from -1 to -3 or from +1 to +3, x is an integer from -1 to -3 or from +1 to +3 ist, w is 0 or a positive number with value (-u + 2h) / v, y is 0 or a positive number with value (2 - u) (j + 1) / x, Y1 in case 2 - 2u is smaller than 0, is a x1-valent inorganic or organic cation or a mixture of such cations, x1 is an integer from -1 to -3 or from +1 to +3 ist, and y1 is 0 or a positive number with value (2 - 2u) / x1.

3. Redox flow cell according to claim 2, wherein in the anolyte a compound of formulae IVa, Va, VIIc, VIIIc, IXc and / or Xc is used as redox-active component wherein R2, R3, R4, R5, R6, R7 and X have the meaning defined in claim 1, R8, R9, R10, R12, R14 and R15 have the meaning defined in claim 2, b, c and q have the meaning defined in caim 1, and d, e, g, h, i, j and k have the meaning defined in claim 2.

4. Redox flow cell according to claim 1, wherein in the catholyte a compound of formulae IIIa, IIIb, IIIc, VI, VIa and / or VIb or of formulae VII, VIIa, VIIb, VIII, VIIIa, VIIIb, IX, IXa, IXb, X, Xa and / or Xb defined in claim 2 are used as redox-active component wherein R4, R5, R6, R7, X and q have the meaning defined in claim 1, o and u have the meaning defined in claim 2, R11 is a divalent to tetravalent organic bridge group, R13 is hydrogen, alkyl, alkoxy, haloalkyl, cycloalkyl, aryl, aralkyl, heterocyclyl, halogen, hydroxy, amino, nitro or cyano, and R16 is an o-times, preferably a single positively charged monovalent organic residue, preferably a quaternary ammonium residue, a quaternary phosphonium residue, a ternary sulfonium residue or an o-times, preferably a single positively charged monovalent heterocyclic residue, R17 is a m-times positively charged divalent to tetravalent organic residue, preferably a divalent to tetreavalent quaternary ammonium residue, a divalent to tetravalent quaternary phosphonium residue, a divalent to tetravalent ternary sulfonium residue or a m-times positively charged divalent to tetravalent heterocyclic residue, R25 is an u-times, preferably a single negatively charged monovalent residue, preferably a carboxyl residue or a sulfonic acid residue or an u-times, preferably a single negatively charged monovalent heterocyclic residue, R26 is a m-times negatively charged divalent to tetravalent organic residue, preferably an alkylene residue substituted with one or two carboxyl groups or sulfonic acid groups, or a phenylene residue substituted with one or two carboxyl groups or sulfonic acid groups, or a divalent heterocyclic residue substituted with one or two carboxyl groups or sulfonic acid groups, Z is a q-valent inorganic or organic cation or a mixture of such cations, f is an integer from 1 to 3, l is a number with the value o / q or u / q, m is an integer from 1 to 4, and n represents a number with the value m / q.

5. Redox flow cell according to one of the claims 3 or 4, wherein in the catholyte a compound of formulae VI, VIa, VIIc, VIIIc, IXc and / or Xc defined in claim 3 or 4 is used as redox-active compound.

6. Redox flow cell according to claim 1, wherein a compund of formulae Ib, IIb, VIId, VIIe, VIIId and / or VIIIe is used as redox-active compound wherein R2, R3, R4, R5, R6, R7 and X have the meaning defined in claim 1, R8, R10, R14 and R19 have the meaning defined in claim 2, R20 and R21 independently of one another are hydrogen, alkyl which is optionally substituted with a carboxylic ester group, a carboxylic amide group, a carboxylic acid group, a sulfonic acid group or an amino group, cycloalkyl which is optionally substituted with a carboxylic ester group, a carboxylic amide group, a carboxylic acid group, a sulfonic acid group or an amino group, aryl which is optionally substituted with a carboxylic ester group, a carboxylic amide group, a carboxylic acid group, a sulfonic acid group or an amino group, or aralkyl which is optionally substituted with a carboxylic ester group, a carboxylic amide group, a carboxylic acid group, a sulfonic acid group or an amino group, or two of the residues R20 and R21 together form a C1-C3-alkylene group, preferably C1-C6-alkyl, C1-C6-alkyl which is substituted with a carboxylic ester group, C1-C6-alkyl which is substituted with a carboxylic amide group, C1-C6-alkyl which is substituted with a carboxylic acid group, C1-C6-alkyl which is substituted with a sufonic acid group, or C1-C6-alkyl which is substituted with an amino group, or together form ethylene, and especially preferred hydrogen, propionate, isobutionate, ethyl or methyl or together represent ethylene, a, b, c and q have the meaning defined in claim 1, s is a number with value 3 / q, and wherein the compounds of formulae Ib, IIb, VIId, VIIe, VIIId and / or VIIIe are used in the anolyte and the compounds of formulae VIIc, VIId, VIIIc and / or VIIId are used in the catholyte.

7. Redox flow cell according to claim 1, wherein the catholyte contains compounds of formulae IIIa, IIIb or IIIc according to claim 4 and the anolyte contains compounds of formulae Ib or IIb according to claim 2.

8. Redox flow cell according to claim 7, wherein the catholyte contains a compound of formula IIIb and the anolyte contains a compound of formula lb.

9. Redox flow cell according to claim 8, wherein the compound of formula IIIb is a salt of 2,2,6,6-tetramethylpiperidine-4-(N,N,N-trialkylammonium) and wherein the compound of formula Ib is a salt of N, N'-dialkylviologen.

10. Redox flow cell according to claim 1, wherein in the catholyte and / or in the anolyte compounds of formulae VII, VIIa, VIIb, VIIc, VIId, VIIe, VIII, VIIIa, VIIIb, VIIIc, VIIId, VIIIe, IX, IXa, IXb, IXc, X, Xa, Xb or Xc defined in claims 2, 3 or 6 are used.

11. Redox flow cell according to one of claims 1 to 10, wherein compounds are used comprising halogenide ions, hydroxide ions, phosphate ions, sulfate ions, perchlorate ions, hexafluorophosphate ions or tetrafluoroborate ions as well as preferably cations selected from the group of hydrogen ions, alkali or earth alkaline metal cations, as well as of the substituted or unsubstituted ammonium cations.

12. Redox flow cell according to one of claims 1 to 11, wherein redox-active compounds comprising one to four structural units of formula III, VII, VIIa, VIIb VIII, VIIIa, VIIIb, IX, IXa, IXb, X, Xa or Xb, preferably those of formulae IIIa, IIIb, V, VIa, VIIc, VIId, VIIe, VIIIc, VIIId, VIIIe, IXc or Xc are used, in which R4, R5, R6 and R7 each are C1-C6-alkyl and especially preferred represent ethyl or methyl.

13. Redox flow cell according to one of claims 2, 3 or 6, wherein redox-active compounds of formulae IV, V, VII, VIIa, VIIb, VIII, VIIIa or VIIIb, preferably those of formulae IVa, Va, VIIc or VIIIc and most preferably those of formulae Ib, VIId, VIIe, VIIId or VIIIe are used, in which R8 or R8 and R10 are hydrogen, C1-C6-alkyl, C1-C6-alkyl which is substituted with a carboxylic alkylester group, C1-C6-alkyl which is substituted with a carboxylic amide group, C1-C6-alkyl which is substituted with a carboxylic acid group, C1-C6-alkyl which is substituted with a sulfonic acid group or C1-C6-alkyl which is substituted with an amino group, and very preferably preferred represent hydrogen, propionate, isobutionate, ethyl or methyl.

14. Redox flow cell according to claim 4, wherein redox-active compounds of formula IIIa are used, in which R13 is hydrogen, C1-C6-alkyl, C1-C6-alkoxy, C1-C6-partial- or perfluoroalkyl, C1-C6-partial- or perchloroalkyl, C1-C6-fluorochloroalkyl, phenyl, benzyl, fluorine, chlorine, hydroxy, amino or nitro.

15. Use of the redox flow cell according to one of the claims 1 to 14 for storage of electrical energy for stationary and mobile applications, preferably as stationary repository for emergency power supply, for peak load adjustment, and for the intermediate storage of electrical energy from replenishable energy sources, especially in the sector of photovoltaics and wind power or from gas-, coal-, biomass-, tidal- or marine-power plants, and for applications in the field of electromobility, as repository in land, air and water vehicles.