Use of ketazine-based redox species in electrolytes for redox flow batteries

Abstract

The present invention relates to an electrolyte solution for a redox flow battery with organic compounds from a specific group of ketazines of the following formula (I) as a redox system. The present invention further relates to a method of operating an electrically rechargeable redox flow battery and to the use of organic compounds of the formula (I) and the oligomers and polymers thereof as a redox system for storing and releasing electrical energy.

Description

[0001] Use of ketazine-based redox species in electrolytes for redox flow batteries

[0002] The invention relates to an electrolyte solution for redox flow batteries based on specific organic ketazine derivatives as redox-active species, a method for operating an electrically rechargeable redox flow battery based on this redox system, and the use of the specific group of ketazines as a redox system for storing and releasing electrical energy.

[0003] The harvesting of solar, wind, and hydropower represents a very good alternative to energy production based on fossil fuels, and increasing the share of these renewable energy generation methods is essential for slowing global warming. While the actual generation of sufficient quantities of renewable electrical energy as such represents "only" a scaling problem, the bottleneck today lies in energy storage, since in the current system, energy demand is neither synchronized nor synchronizable with the amount of energy generated. Therefore, sufficient and high-performance energy storage systems are needed to help decouple energy production and consumption in time. In this respect, redox flow batteries (RFBs) offer an easily scalable technological and ecological solution.The unique feature of this technology is that energy storage and release are decoupled from each other, so that only an increase in the electrolyte volume and the concentration of redox-active species is required to increase energy storage capacity. Storage systems based on liquid redox electrolytes have been known in principle since the last century. A more advanced type can be found, for example, in the vanadium redox flow battery based on aqueous solutions, which still represents the state of the art in this field. Aqueous vanadium-based cells (VRFBs) deliver energy densities of approximately 25 WhL. 1Various approaches toward broader commercialization exist, but obstacles to significant deployment include V2O5 precipitation at elevated temperatures, membrane compatibility, crossover of redox species across cell membranes, relatively low concentrations and cell voltages, the fluctuating price of vanadium, and—although vanadium has a better environmental footprint than other heavy metals—supply and sustainability issues. Other aqueous redox flow battery types (ARFBs) using alternative metals and also inorganic redox-active species (RAS) exhibit essentially similar disadvantages.

[0004] Instead of metal-based RAS, organic RAS can also be used in aqueous and non-aqueous RFBs. Organic RAS can potentially offer lower costs, improved sustainability, and lower toxicity. Furthermore, the flexibility and diversity of organic synthesis methods and organic structures allow for much better tailoring of their physicochemical properties. Challenges, however, include relatively low conductivities, electrolyte transitions between the anode and cathode compartments, and the procurement of organic RAS that are stable in multiple oxidation states. Organic RAS have been used in aqueous (AORFBs) and non-aqueous systems (NAORFBs). The greatest successes to date have been achieved with AORFBs, whose maximum voltage is limited by the narrow electrochemical window of water (1.23 V).The most intensively studied classes of organic RAS are quinones, phenothiazines, metallocenes, viologens, and TEMPO derivatives. The well-performing TMAP-TEMPO / BTMAP-viologen system delivers a cell voltage of 1.1 V and a capacity retention of 94% over 1000 cycles. A (ferrocenylmethyl)trimethylammonium / methylviologen system delivers a cell voltage of 1.05 V and a capacity retention of 91% over 700 cycles. Both systems represent the current state of the art, and their theoretical energy densities exceed those of VRFBs (41.8 WhL). 1 However, NARFBs still need to be optimized to achieve comparable cycle performance.

[0005] Different technical solutions for electrolyte solutions and redox flow batteries as such are also discussed in the patent literature.

[0006] For example, WO 2014 026 728 A1 describes a redox flow cell for storing electrical energy, comprising a reaction cell with two polarity-specific chambers for catholyte and anolyte, each of which is connected to a liquid reservoir and which is separated by a membrane for ion exchange, wherein the chambers are each filled with redox-active components that are present in bulk, dissolved or dispersed in an electrolyte solvent, as well as conductive salts dissolved therein and any further additives, characterized in that high-molecular compounds are provided as the redox-active components and a size-exclusion membrane is provided as the membrane for separating the high-molecular redox-active components that are present in bulk, in dissolved or dispersed form.

[0007] WO 2019 020 351 A1 discloses a method for operating an electrically rechargeable redox flow battery with the following steps: providing a redox flow battery comprising a first chamber and a second chamber, wherein the first chamber is separated from the second chamber by a membrane and wherein the first chamber comprises a cathode and the second chamber comprises an anode; introducing a first electrolyte as a catholyte into the first chamber and introducing a second electrolyte as anolyte into the second chamber, wherein the first electrolyte comprises a first reduction-oxidation pair and the second electrolyte comprises a second reduction-oxidation pair and the first and / or second electrolyte comprises a pH-stabilizing buffer for chemically stabilizing the reduction-oxidation pair; charging or discharging the redox flow battery.Against this background of the prior art, it is therefore an object of the invention to provide an improved electrolyte solution for redox flow batteries and an improved method for operating a redox flow battery using the electrolyte solution according to the invention. In particular, the electrolyte solution according to the invention is intended to enable improved cycle stability with high electrical power.

[0008] This object of the invention is achieved with an electrolyte solution having the features specified in claim 1 and with a method for operating an electrically rechargeable redox flow battery having the features specified in claim 8. Preferred developments of the invention are specified in the associated subclaims, the following description, and the drawing.

[0009] According to the invention, an electrolyte solution for a redox flow battery is provided, wherein the electrolyte solution comprises at least: a) a solvent; b) a conducting salt; c) organic compounds as a redox system, wherein the organic compounds are selected from the group consisting of the compounds according to the following formula I or mixtures thereof:

[0010] Formula I, where R 1are independently selected from the group consisting of substituted or unsubstituted aromatic C5-C6 hydrocarbons, five- or six-membered nitrogen or oxygen heterocycles, wherein the possible substitution of the aromatic hydrocarbons, the nitrogen or oxygen heterocycles at each bondable position is independently selected from the group consisting of nitrile, nitro, halogen, Cl-CIO alkyl esters, Cl-CIO alkoxy, Cl-CIO mono- or dialkylamino, straight-chain or branched CI-CIO alkyl; and the

[0011] R 2 are independently selected from the group consisting of H and straight-chain or branched CI -CIO alkyl or Cl -CIO alkoxy; where the organic compounds capable of redox reactions can be electrically neutral or charged in the solvent.

[0012] Surprisingly, it was discovered that the claimed group of ketazine compounds is highly suitable as a redox system in redox flow batteries. The ketazine-based redox species are based entirely on purely organic compounds, composed at most of carbon, hydrogen, nitrogen, or oxygen. No heavy metals are used in the compounds. The basic structure of the compounds is chosen such that the same compound can be used alone in the electrolyte solution according to the invention as an electrical storage device. Furthermore, the basic units can also be converted into a solid booster, which is then stored tightly as a solid in the anolyte and catholyte tanks and can be chemically oxidized by the soluble monomer form as a redox mediator. This can be used to significantly increase the total electrical capacity per unit volume of the system.The greatest advantage is that the organic compounds directly transfer two electrons, thus bypassing a radical intermediate. Skipping the radical intermediate leads to a significant extension of the electrolyte's lifetime. It has also been shown that a crossover of anolyte or catholyte across the separator into the other electrode area results in only a reversible energy loss. The organic compounds are not irreversibly deactivated but are subsequently fully regenerable. The organic compounds in the form of the ketazine components can be easily prepared from inexpensive starting materials in a two-step synthesis and, compared to metal-based redox systems, are significantly more environmentally friendly and, due to their low toxicity, easier to handle.The result is a highly electrically efficient and cost-effective electrolyte with improved sustainability compared to state-of-the-art electrolytes. This is particularly true compared to aqueous vanadium-based systems, which only provide a narrow voltage window between 1.23 and 1.4 V, exhibit poor availability of the metal component, are toxic to humans and the environment, can only be operated under strongly acidic conditions, and in which the mixing of anolyte and catholyte (crossover) leads to irreversible reactions. Many of the currently discussed non-aqueous redox flow systems based on heavy metals as redox-active species, for example in the form of cobaltocenes, exhibit similar disadvantages.

[0013] The electrolyte solution according to the invention is an electrolyte solution for a redox flow battery. A redox flow battery (RFB) or (redox) flow battery is an accumulator. The RFB stores electrical energy in the form of chemical compounds, whereby the compounds are dissolved in a solvent and thus mobile. In an RFB, two energy-storing electrolytes can circulate in separate circuits, with the circuits contacting each other via a separator that enables charge exchange. In the RFB, the dissolved substances are chemically reduced or oxidized, whereby electrical energy is stored during the charging process and released during the discharging process. The electrolyte solution according to the invention provides solutions for both a catholyte and / or an anolyte.

[0014] The electrolyte solution comprises a) a solvent. The redox system from the group of organic compounds according to the invention can be dissolved in various solvents. In principle, the use of water is possible for aqueous RFB (ARFB). However, the use of organic solvents such as acetonitrile, propylene carbonate, monoglyme, diglyme, dimethyl carbonate, ethyl methyl sulfone, dioxolane, and dimethyl sulfoxide is also possible. These solvents can be used for both the anolyte and the catholyte. The electrolyte solution comprises b) a conducting salt. The conducting salt in the electrolyte serves to improve the conductivity of the electrolyte and also ensures charge exchange across the separator. Conducting salts can be used, as are known in the field of batteries, or specifically redox flow batteries. The conducting salts can, for example, belong to the group of organic ammonium or imidazolium salts.

[0015] The electrolyte solution comprises c) organic compounds as a redox system, wherein the organic compounds are selected from the group consisting of the compounds according to the following formula I or mixtures thereof:

[0016] Formula I, where R 1 are independently selected from the group consisting of substituted or unsubstituted aromatic C5-C6 hydrocarbons, five- or six-membered nitrogen or oxygen heterocycles, wherein the possible substitution of the aromatic hydrocarbons, the nitrogen or oxygen heterocycles at each bondable position is independently selected from the group consisting of nitrile, nitro, halogen, Cl-CIO alkyl esters, Cl-CIO alkoxy, Cl-CIO mono- or dialkylamino, straight-chain or branched CI-CIO alkyl; and the

[0017] R 2are independently selected from the group consisting of H and straight-chain or branched CI -CIO alkyl or Cl -CIO alkoxy;

[0018] The most important components of the electrolyte are the organic compounds that form the electrolyte's redox system. The basic structure of this system is based on an aromatic ketazine framework, which bears substituents from two different groups to make it suitable as a redox species. The first group, R 1 encompasses cyclic structures based on hydrocarbons or hydrocarbon heterocycles, where the heterocycles carry at least additional non-metallic components such as oxygen or nitrogen. Further substitution R 2 is provided by hydrogen or non-cyclic hydrocarbons or non-cyclic alkoxy groups. The ketazine skeleton can be symmetrically substituted, i.e., the R 1 and the R 2are provided by the same substituents. However, it is also possible that the R 1 and the R 2 differ in their substituents. Thus, the R 1 and the R 2be selected independently of one another. The possible, independent substitution of the aromatic hydrocarbons, the nitrogen or oxygen heterocycles at any bondable site means that, for example, the hydrogens of the aromatic hydrocarbons can be replaced independently of one another by further groups. In the case of nitrogen heterocycles, for example, the hydrogens on the carbons of the heterocycle or also on the nitrogen can carry further substituents. One or more heteroatoms can be present in the heterocycles. The organic compounds form, at least in the uncharged state, an aromatic system extending over the entire basic molecular structure. The basic structure according to the invention and the substitution of the basic structure according to the invention lead to two electrons being transferred during oxidation or reduction of the compound.The radical intermediate is skipped or exhibits only a very short lifetime. Possible compounds and redox systems of these compounds with reversible loss and absorption of two electrons can be formed according to the invention as follows: In principle, one or more organic compounds can be present in the electrolyte of a half-cell.

[0019] The organic compounds capable of redox reactions can be electrically neutral or charged in the solvent. The organic compounds usable as a redox system according to the invention serve the purpose of absorbing or releasing electrical charges from the electrodes. It is therefore understandable that, as a function of the charging / discharging of a redox flow battery, the organic compounds absorb and release electrons, changing their charge state in the electrolyte. Preferably, the charge state of the organic compounds can change by two electrons simultaneously, generating an uncharged molecule from double cations and double cations from an uncharged molecule. These charged species, which arise during operation, are also subsumed under the term and structural formula of the organic compound.

[0020] In a preferred embodiment of the electrolyte solution, the R 1independently selected from the group consisting of substituted or unsubstituted aromatic C6 hydrocarbons, wherein the possible substitution of the aromatic hydrocarbons at each bondable position is independently selected from the group consisting of nitrile, nitro, halogen, Cl-CIO alkyl esters, Cl-CIO alkoxy, Cl-CIO mono- or dialkylamino, straight-chain or branched CI-CIO alkyl. The direct attachment of the selected group to R 1 with an aromatic hydrocarbon framework as a base to the ketazine framework can lead to particularly favorable redox compounds, which exhibit a 2-electron transfer in various solvents and thus do not exist as chemically very active radicals in the solvent. This can increase the chemical stability of the electrolyte and, in particular, the cycle stability of redox flow batteries. In this respect, this group leads to R 1to an improved electrochemical behavior of the redox-active species in the electrolyte. In a preferred embodiment of the electrolyte solution, the R 1 be selected from the group consisting of substituted or unsubstituted aromatic five- or six-membered nitrogen or oxygen heterocycles, wherein the possible substitution of the nitrogen heterocycles or the oxygen heterocycles at each bondable position is independently selected from the group consisting of straight-chain or branched Cl-C10 alkyl. The direct attachment of the selected group to R 1from oxygen or nitrogen heterocycles to the ketazine framework can lead to particularly favorable redox compounds that exhibit a 2-electron transfer in various solvents and thus do not exist as chemically very active radicals in the solvent. This can increase the chemical stability of the electrolyte and, in particular, the cycle stability of redox flow batteries. This group of R 1 leads to improved electrochemical behavior of the redox-active species in the electrolyte.

[0021] Within a preferred characteristic of the electrolyte solution, the R 1from the group consisting of substituted or unsubstituted pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, tetrazole, 1,2-oxazole, 1,3-oxazole, 1,3,4-oxadiazole, and 1,2,4-oxadiazole, or mixtures of at least two different compounds from this list. This group of nitrogen or oxygen substituents has proven particularly suitable for obtaining particularly long-term stable and electrochemically efficient redox systems in electrolytes.

[0022] Within a further preferred aspect of the electrolyte solution, the conducting salts can be selected from the group consisting of substituted or unsubstituted ammonium or imidazolium salts. In combination with nitrogen-based conducting salts, particularly efficient electrochemical electrolytes can be obtained, which have a long service life with frequent charge / discharge cycles and virtually unchanged capacity. Within a preferred embodiment of the electrolyte solution, the electrolyte solution can

[0023] Oligomers or polymers according to formula II or formula III include:

[0024] Formula II Formula III, where n = 1-12 and m = 2 to 1000. The electrochemical capacity of the electrolyte can be increased considerably by the use of covalently bridged monomers. By choosing the degree of polymerization, partially insoluble compounds, i.e. compounds that are solid in the solvent, are obtained which then remain stationary in the electrolyte. These polymers change their electrochemical properties through contact with the solvent-soluble monomers. The polymers can be stored in the tanks of a redox flow battery, for example as granulated solid booster material. This granulate is chemically charged by already charged redox species, i.e. reduced or oxidized, whereby the dissolved species itself is discharged. This is then recharged at the electrodes of the redox flow battery and therefore acts as a redox mediator.The oligomers or polymers can also be immobilized on surfaces in the electrolyte. For example, it is possible to functionalize the oligomers or polymers via a SiO2 group, whereby the oxygens from this functionalization can be used to physically or covalently bind or immobilize the booster to other surfaces. The compounds of formula II and formula III can be further functionalized at one or both end groups. Thus, further functionalization can improve interaction with other support materials. Such functionalization could be implemented, for example, as follows:

[0025] The boxes represent, for example, the surface of a support. Functionalization can also be carried out in a similar manner on the compounds of formula III. Both double and single functionalization are possible at one or both ends of the compounds.

[0026] Within a preferred embodiment of the electrolyte solution, the organic compound in the solvent can be present at a molar concentration of greater than or equal to 0.001 mol / L and less than or equal to 5 mol / L. This concentration of redox-active organic compound in the electrolyte has proven to be particularly efficient. Large amounts of charge can be reversibly stored and released in the electrolyte at low cost. The molar concentration can preferably be greater than or equal to 0.01 mol / L and less than or equal to 4 mol / L, further preferably greater than or equal to 0.1 mol / L and less than or equal to 3.5 mol / L, and furthermore preferably greater than or equal to 0.5 mol / L and less than or equal to 3 mol / L.

[0027] Furthermore, the invention relates to a method for operating an electrically rechargeable redox flow battery, the method comprising at least the steps:

[0028] - Providing a redox flow battery comprising two electrode chambers separated by a separator, each with an electrode, wherein one of the chambers is polarized as a cathode chamber with a cathode and the other chamber is polarized as an anode chamber with an anode;

[0029] - introducing the electrolyte solution according to the invention into at least the anode chamber and / or the cathode chamber;

[0030] - Charging or discharging the redox flow battery. The electrolyte according to the invention can be used particularly advantageously in a rechargeable redox flow battery. Here, the electrolyte solution can be used in isolation as anolyte, catholyte, or in the form of various organic compounds from the group according to the invention, simultaneously as anolyte and catholyte. Batteries with a high electrical storage capacity are obtained, whose cycle stability is significantly higher than described in the prior art. Furthermore, the electrolyte used is significantly more cost-effective and environmentally friendly than the systems discussed in the prior art. Furthermore, the advantages discussed in the context of the electrolyte according to the invention are explicitly referred to for the process according to the invention.

[0031] The method involves providing a redox flow battery comprising two electrode chambers separated by a separator, each with an electrode. One of the chambers is polarized as a cathode chamber with a cathode, and the other chamber is polarized as an anode chamber with an anode. The two chambers of the battery are separated by a separator, which separates the anode and cathode compartments and prevents the redox-active organic compound from passing from one compartment to the other. Depending on the design, the separator can be configured as a membrane, ion-selective membrane, or diaphragm.

[0032] In the process, the electrolyte solution according to the invention is introduced into at least the anode chamber and / or the cathode chamber. The electrolyte solution according to the invention can be operated with any additional or different electrolyte solution. However, it is also possible to use two different electrolyte solutions, each containing an organic compound, to operate the battery.

[0033] Ultimately, the process involves charging or discharging the redox flow battery. By introducing a current into the cell, the electrochemical properties of the organic compounds can be altered in such a way that the injected current is stored. During discharging, the current is provided again from the battery by reversibly changing the redox status of the redox-active compounds back to their original state.

[0034] Within a preferred aspect of the method, two different electrolyte solutions can be fed into the electrode chambers, wherein the catholyte electrolyte solution comprises organic compounds selected from the group consisting of alkoxy- or dialkylamino-substituted phenylketazines, alkylated pyrazole-imidazole-, 1,2,4-triazole-, 1,2,3-triazole-, and pyrrole-ketazines or their charged ions, and wherein the anolyte electrolyte solution comprises organic compounds selected from the group consisting of alkyl or alkoxy-pyridinium ketazines, 4-nitrophenylketazine, or their charged ions. In particular, the use of two different electrolyte solutions with different organic compounds, selected from the electrolyte solutions according to the invention, can contribute to particularly efficient operation of a redox flow battery. The redox flow battery can provide a particularly high storage capacity with simultaneously high cycle stability.This can be achieved in particular because the electrochemical reactions do not produce radicals, which tend to produce side reactions that are difficult to control. The radical stage is skipped with the electrolytes used in the invention, resulting in a highly controllable process. Furthermore, even the transfer of part of the anolyte or part of the catholyte into the respective other electrode compartment does not lead to irreversible reactions. This can improve the battery's handling and longevity.

[0035] Furthermore, the invention provides for the use of organic compounds according to the following formula I

[0036] Formula I, where R 1are independently selected from the group consisting of substituted or unsubstituted aromatic C5-C6 hydrocarbons, five- or six-membered nitrogen or oxygen heterocycles, wherein the possible substitution of the aromatic hydrocarbons, the nitrogen or oxygen heterocycles at each bondable site is independently selected from the group consisting of

[0037] Nitrile, nitro, halogen, Cl -CIO alkyl esters, Cl -CIO alkoxy, Cl -CIO mono- or dialkylamino, straight-chain or branched CI -CIO alkyl; and the

[0038] R 2 are independently selected from the group consisting of H and straight-chain or branched CI -CIO alkyl or Cl -CIO alkoxy; or their oligomers or polymers according to the following formulas II or III: , with n = 1-12 and m = 2 to 1000, or mixtures thereof, as a redox system for storing and releasing electrical energy. The ketazine basic structure with the substitution patterns listed according to the invention is suitable for the provision of mobile redox systems that are soluble or mobile in the electrolyte. Furthermore, this basic structure is also suitable in the form of oligomers or polymers as stationary energy storage devices.

[0039] It is also possible to use both mobile and stationary organic compounds simultaneously. The latter provides a safe and flexible overall system with high electrical storage capacity and long application times. Furthermore, it is possible for the oligomers or polymers to be permanently bound to a support medium via one or more functional groups. Examples of suitable support media include silica gel (SiCh), Alox (Al2O3), zirconium dioxide (ZrCh), cerium dioxide (CeCh), titanium dioxide (TiO2), and other oxide materials that can be used as stationary phases in chromatography and are insoluble in the aforementioned solvents. A possible modification of the compounds and attachment to other surfaces could be as follows:

[0040] Connection via one or two functional groups is possible. The functional connection can be made via identical or different bonds.

[0041] Examples

[0042] The properties of the organic compounds usable according to the invention are shown using the following ketazine compound 1, which forms the following redox system in redox flow batteries by releasing / absorbing two electrons:

[0043] Redox system - Ketazine 1

[0044] (4,4'-((1E, 1'E)-hydrazine-1,2-diylidenebis(ethan-1-yl-1-ylidene))bis(1-methylpyridin-1-ium))

[0045] I. Syntheses Preparation of the ketazine compound 1.

[0046] Hydrazine monohydrate is slowly added dropwise to a solution of l-(pyridin-4-yl)ethan-l-one (2 equiv.) in toluene. The solution is stirred for 24 hours. The solvent is evaporated, and the product (lE,2E)-l,2-bis(l-(pyridin-4-yl)ethylidene)hydrazine is sublimed from the residue at 2 mbar and 250°C.

[0047] Alternative preparation of the precursor of the ketazine compound 1 :

[0048] Two equivalents of 4-acetylpyridine are placed in a three-neck flask equipped with a magnetic stirrer, thermometer, and reflux condenser and mixed with chloroform. One equivalent of elydrazine monohydrate is then added to the reaction mixture through the second side neck. After refluxing for one hour, the mixture is cooled to room temperature in a water bath. Phosphorus pentoxide is quickly added to the suspensions. The mixture is then heated under reflux for three hours. After cooling to room temperature, 100 mL of water is added via a dropping funnel. The solution is adjusted to pEI 8. The aqueous phase is separated and washed three times with chloroform in a separatory funnel. The combined organic phases are washed with brine and dried over magnesium sulfate. The solvent is removed, and the product is purified by sublimation at 250 °C under vacuum. Production of a redox-active anolyte component

[0049] In a glove box under a protective gas atmosphere, dry dichloromethane and freshly sublimed 4,4'-bis(acetylpyridine ketazine) (1 equiv.) are placed in a baked Schlenk flask, and trimethyl oxonium tetrafluoroborate (2 equiv.) is added. The solution is stirred for 10 h. The solvent is then removed, and the solid is recrystallized from acetonitrile.

[0050] Production of a redox-active catholyte component in the form of a polymeric solid-

[0051] Boosters:

[0052] In a round-bottom flask containing toluene, freshly sublimed 4,4'-bis(acetylpyridine ketazine) (1 equiv.) is heated with dibromoethane (1 equiv.) under reflux for 14 h. The resulting polymer is filtered off with suction through a glass frit and washed with THF. The polymer is then dissolved in DMSO or THF, and AgBF4 (2 equiv.) is added to the solution. The AgBr residue is filtered off, and the clear solution is treated with water (25% of the total volume). The solvent is then evaporated off in a rotary evaporator, resulting in the polymer precipitating as a granular powder.

[0053] Preparation of a redox-active catholyte component: 2 equiv. of 4-methoxyacetophenone are added to a three-neck flask equipped with a magnetic stirrer, thermometer, and reflux condenser and mixed with chloroform. Then, 1 equiv. of hydrazine monohydrate is added to the reaction mixture through the second side neck with a septum. After refluxing for one hour, the mixture is cooled to room temperature in a water bath and acidified with dilute hydrochloric acid. The aqueous phase is washed three times with ethyl acetate. The combined organic phases are washed with brine, dried over magnesium sulfate, and finally the solvent is removed. The product (lE,2E)-l,2-bis(l-(4-methoxyphenyl)ethylidene)hydrazine is obtained by sublimation at 60 °C under vacuum (2 mbar).

[0054] II. Solubility and viscosity

[0055] Solubility in acetonitrile

[0056] The following table shows the solubility and viscosity of ketazine compound 1. This solubility is generally considered relatively high for a divalent salt in an organic solvent. Due to its low viscosity, this solution can be used as a redox mediator in the solid booster principle in redox flow batteries: The table shows the solubility of the mediator ketazine 1 in acetonitrile (ACN) and in acetonitrile with the conductive salt tetrabutylammonium tetrafluoroborate (TBABF4). The maximum concentrations are given in C max in mol l' 1 , the maximum theoretical volumetric charge density Qmax.th in Ah l' 1 the Emax.th in Whl' 1 (dimethylferrocene DMFc), as well as the viscosity r| of the electrolyte solutions in mPa s (T = 25 °C).

[0057] III. Voltage window

[0058] Figure 1 shows the potential window of ketazine compound 1. The potential window of ketazine species 1 was measured using a cyclic voltammogram (CV) against the catholytic redox flow standard MEEPT (10-[2-(2-methoxyethoxy)ethyl]-10H-phenothiazine; 0.01 M). Ketazine molecule 1 (0.01 M) formed the anolyte species in this measurement (left reversible part). The measurement was performed in acetonitrile as solvent with tetrabutylammonium tetrafluoridoborate (0.3 mol) as the conducting salt. The measurement was performed against the catholyte standard 10-[2-(2-methoxyethoxy)ethyl]-10H-phenothiazine (MEEPT), right reversible part. The voltage window is larger than the voltage window of known organic redox flow systems and larger than the stability window of water (1.23 V), which limits the applicability of most aqueous redox flow systems.

[0059] Figure 2 shows a comparable measurement to Figure 1. It shows the electrochemical behavior of ketazine species 1 compared to the catholyte standard dimethylferrocene (DMFc; 0.001 M). Both this figure and Figure 1 show a CV in which both species (anolyte and catholyte) are mixed in an electrolyte solution. It can be seen that the two species do not influence or interfere with each other, and the species crossover is completely reversible.

[0060] IV. Electrochemical Stability and Lifetime Due to the fact that the electrolyte according to the invention contains organic compounds as redox-active species, which are capable of skipping a radical step during charging / discharging through a two-electron transfer, the long-term stability of the electrolyte is significantly increased. This is shown in Figure 3. The cyclic voltammogram shows that ketazine 1 (left, reversible redox curves) still has almost the same end face shape and the same maximum current after the hundredth charge / discharge cycle. This is in comparison to the literature standard MEEPT (right, reversible redox curves), whose area and maximum currents have decreased significantly. The ketazine compounds usable according to the invention are therefore significantly more electrochemically stable over the long term.

[0061] Figure 4 shows a comparison of ketazine 1 against DMFc, and Figure 5 shows a comparison of ketazine 1 against MEEPT, each for 50 cycles. The Coulombic efficiencies (CE) comparing the current cycle to the first cycle are also shown. The results in Figure 5 were determined using a redox flow cell with the system ketazine 1 against DMFc in ACN as solvent with TBABF4 as the conducting salt. The results in Figure 6 were determined using a redox flow cell with the system ketazine 1 against MEEPT in ACN as solvent with TBABF4 as the conducting salt. In both Figures 5 and 6, it can be seen that after an initial formation phase of approximately five cycles up to the fiftieth cycle, a high capacity retention of the electrolyte according to the invention can be observed. The retention is also significantly higher compared to the known standard JV-butylphthalimide (NBuPh) / dimethylferrocene (DMFc). This is evident from Figure 6.In this figure, ketazine 1 is referred to as “AMAP.”

[0062] N. Thermal stability

[0063] To determine its thermal stability, the ketazine compound 1 was investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The measurements demonstrate that ketazines exhibit thermal stability above 250 °C. For the ketazine compound 1, the decomposition point determined by TGA (heating rate 10 K / min) with a mass loss of 5 weight percent is 262 °C (Figure 7). The DSC measurement shown in Figure 8 illustrates that the compound does not melt in a temperature range between -140 °C and 200 °C (heating rate 10 K / min). This behavior is already evident in the synthesis of the ketazine molecule 1, in which the non-methylated intermediate can be purified by sublimation, i.e., a solid-to-gas phase transition.

[0064] VI. Charges per molecular mass

[0065] Since two charges are transferred per molecule used according to the invention, compared to the prior art compounds, the ratio of molecular weight to transferable charge for the compounds explicitly shown in the application ranges from 100 g / mol charge to 175 g / mol charge. This is smaller than the redox flow standards TEMPO (156 g / mol charge) and ferrocene (186 g / mol charge), since these can only transfer one electron per molecule. The use of ferrocenylmethyl)trimethylammonium results in a 258.15 g / mol charge, methylviologen 93.13 g / mol charge, TMAP-TEMPO 272.43 g / mol charge, and BTMAP-viologen 179.29 g / mol charge.

[0066] If a total capacity of anolyte species is compared with catholyte species of the respective systems, the following results for the state of the art:

[0067] (Ferrocenylmethyl)trimethyl ammonium (258.15 g / mol charge) + methyl viologen (93.13 g / mol charge) = 351.28 g / mol charge

[0068] TMAP TEMPO (272.43 g / mol load) + TMAP Viologen (179.29 g / mol load) = 452.75 In comparison, the ketazine systems according to the invention are significantly more "material efficient." For a catholyte / analyte pair according to the invention, the following results:

[0069] V,7V'-Dimethyl-4,4'-Diacetylpyridiniumketazine 134.18 g / mol charge + 4,4'-Dimethoxy-acethophenylketazine = 148.19 g / mol charge = 282.37 g / mol charge

[0070] This sum is significantly below the mass sum of the systems known from the state of the art.

[0071] For the redox example investigated (N,N'-dimethyl-4,4'-diacetylpyridinium ketazine versus the redox standard 10-[2-(2-methoxyethoxy)ethyl]-10H-phenothiazine (MEEPT)) the following mass-to-charge ratios are obtained:

[0072] A / 7V'-Dimethyl-4,4'-Diacetylpyridiniumketazine 134.18 g / mol load + MEEPT 287.38 g / mol load = 421.66 g / mol load

[0073] This value is smaller compared to the state-of-the-art TMAP-TEMPO / TMAP viologen with 452.75 g / mol loading.

[0074] It should also be noted that the known non-aqueous systems still contain cations for both the anolytic redox species and the catholytic redox species. In the ketazine examples considered here, an anion pair shares a cation and is thus even more "material efficient" or "weight efficient."

[0075] No electrical weight efficiency was calculated with counterions here, as these can be varied arbitrarily for the systems. These anions can be selected from the group of BFF, PFe", TFSF, FSF, FTFSF, BETF, triflate, mesylate, acetate, nitrate, sulfate, chloride, BOB', DFOB".

[0076] The oligomeric and polymeric redox species from the functionalized ketazines according to the invention can have an average density of 1.5 kg / L. In the form of porous granules, these can reach an average density of approximately 0.7 kg / L. This results in a combined concentration of 2.48 mol / L for the oligomeric or polymeric anolyte and catholyte solid booster used according to the invention, and thus a volumetric charge density of 66.4 Ah / L.

[0077] VII. Cell structure

[0078] For operation in a redox flow battery, for example, two different ketazine species according to the invention can be used simultaneously. These are dissolved in a high concentration in an organic solvent and used as redox-active anolyte and catholyte components. The basic principle of the structure and reactions is shown in Figure 9. The structure of the redox flow battery consists of an anode tank 1, a cathode tank 10, the anode 2, the cathode 9, the actual conversion cell 6, and a porous separation membrane or anion-permeable membrane 5. In this example, the following compounds and the following reactions are considered as redox-active anolyte components:

[0079] In this example, the following compounds and the following reaction are considered as redox-active catholyte components:

[0080] The electrolyte solution according to the invention comprises, as a redox system in the anolyte tank 1, an organic compound 3, 4 according to the invention, wherein compound 4 is reduced during the battery charging process by absorbing two electrons at the anode 2. Compound 4 thus represents the oxidized form of compound 3 according to the invention. At the cathode, the redox-active compound 8 of the catholyte is oxidized to compound 7 by releasing two electrons. Compound 7 thus represents the oxidized form of compound 8 according to the invention. Accordingly, the reverse processes occur during battery discharging. During charging and discharging of the battery, the anions migrate through membrane 5 to equalize the charge.

[0081] Analogous to Figure 1, the same cell principle can also be operated with polymerized redox-active ketazine species. The polymerized redox-active ketazine species are stored, for example, in the tanks as granulated solid booster material. This granulate is chemically charged by already charged and mobile redox species, i.e., the monomers in solution. In the anolyte tank, the polymeric anolyte is reduced and the polymeric catholyte is oxidized, discharging the dissolved species themselves. The discharged species are then recharged at the electrodes and thus act as redox mediators.

Claims

Patent claims 1. An electrolyte solution for a redox flow battery, characterized in that the electrolyte solution comprises at least: a) a solvent; b) a conducting salt; c) organic compounds as a redox system, wherein the organic compounds are selected from the group consisting of the compounds according to the following formula I or mixtures thereof: Formula I, where the R 1are independently selected from the group consisting of substituted or unsubstituted aromatic C5-C6 hydrocarbons, five- or six-membered nitrogen or oxygen heterocycles, wherein the possible substitution of the aromatic hydrocarbons, the nitrogen or oxygen heterocycles at each bondable position is independently selected from the group consisting of nitrile, nitro, halogen, CI -CIO alkyl esters, CI -CIO alkoxy, Cl -CIO mono- or dialkylamino, straight-chain or branched CI -CIO alkyl; and the R 2 are independently selected from the group consisting of H and straight-chain or branched CI -CIO alkyl or Cl -CIO alkoxy; where the organic compounds capable of redox reactions can be electrically neutral or charged in the solvent.

2. Electrolyte solution according to claim 1, wherein the R 1are independently selected from the group consisting of substituted or unsubstituted aromatic C6- Hydrocarbons, wherein the possible substitution of the aromatic hydrocarbons at each bondable position is selected independently from the group consisting of nitrile, nitro, halogen, Cl -CIO alkyl esters, Cl -CIO alkoxy, Cl -CIO mono- or dialkylamino, straight-chain or branched CI -CIO alkyl.

3. Electrolyte solution according to claim 1, wherein the R 1 are selected from the group consisting of substituted or unsubstituted aromatic five- or six-membered nitrogen or oxygen heterocycles, wherein the possible substitution of the nitrogen heterocycles or the oxygen heterocycles at each bondable site is independently selected from the group consisting of straight-chain or branched Cl-C10 alkyl.

4. Electrolyte solution according to claim 1 or 3, wherein the R 1 are selected from the group consisting of substituted or unsubstituted pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, tetrazole, 1,2-oxazole, 1,3-oxazole, 1,3,4-oxadiazole and 1,2,4-oxadiazole or mixtures of at least two different compounds from this list.

5. Electrolyte solution according to one of the preceding claims, wherein the conducting salts are selected from the group consisting of substituted or unsubstituted ammonium or imidazolium salts.

6. Electrolyte solution according to one of the preceding claims, wherein the electrolyte solution comprises oligomers or polymers according to formula II or formula III: Formula II Formula III, where n = 1-12 and m = 2 to 1000.

7. Electrolyte solution according to one of the preceding claims, wherein the organic compound is present in the solvent in a molar concentration of greater than or equal to 0.001 mol / L and less than or equal to 5 mol / L.

8. A method for operating an electrically rechargeable redox flow battery, comprising at least the steps: - Providing a redox flow battery comprising two electrode chambers separated by a separator, each with an electrode, wherein one of the chambers is polarized as a cathode chamber with a cathode and the other chamber is polarized as an anode chamber with an anode; - feeding the electrolyte solution according to one of claims 1 to 7 into at least the anode chamber and / or the cathode chamber; - Charging or discharging the redox flow battery (1).

9. The method according to claim 8, wherein two different electrolyte solutions are fed into the electrode chambers, wherein the catholyte electrolyte solution comprises organic compounds selected from the group consisting of alkoxy- or dialkylamino-substituted phenylketazines, alkylated pyrazole-imidazole-, 1,2,4-triazole-, 1,2,3-triazole- and pyrrole-ketazines or their charged ions and wherein the anolyte electrolyte solution comprises organic compounds selected from the group consisting of alkyl- or alkoxy-pyridinium ketazines, 4-nitrophenylketazine or their charged ions.

10. Use of organic compounds according to the following formula I Formula I, where the R 1are independently selected from the group consisting of substituted or unsubstituted aromatic C5-C6 hydrocarbons, five- or six-membered nitrogen or oxygen heterocycles, wherein the possible substitution of the aromatic hydrocarbons, the nitrogen or oxygen heterocycles at each bondable position is independently selected from the group consisting of nitrile, nitro, halogen, CI -CIO alkyl esters, CI -CIO alkoxy, Cl -CIO mono- or dialkylamino, straight-chain or branched CI -CIO alkyl; and the R 2 are independently selected from the group consisting of H and straight-chain or branched CI -CIO alkyl or Cl -CIO alkoxy; or oligomers or polymers according to the following formulas II or III: Formula II Formula III, with n = 1-12 and m = 2 to 1000, or mixtures thereof, as a redox system for the storage and release of electrical energy.

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