Polycyclic organic compound or salt thereof, aqueous electrolyte containing that polycyclic organic compound or salt thereof, and redox flow battery employing that aqueous electrolyte

Abstract

To provide: a compound or a salt thereof having solubility in water in a wide pH range; an aqueous electrolyte containing the compound or salt thereof as an active material; and a redox flow battery employing the aqueous electrolyte.SOLUTION: The invention provides: a compound represented by the general formula (1) in the figure or a salt thereof; an aqueous electrolyte containing the compound or salt thereof as an active material; and a redox flow battery employing the aqueous electrolyte.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a polycyclic organic compound or a salt thereof, an aqueous electrolyte solution containing the polycyclic organic compound or the salt thereof, and a redox flow battery using the aqueous electrolyte solution. [Background technology]

[0002] In recent years, the use of renewable energy that does not emit greenhouse gases has become more common in order to realize a sustainable society. However, the amount of power generated by renewable energy is easily affected by the season, weather, etc., so there is a demand for storage batteries for power storage to ensure a stable supply of renewable energy. Redox flow batteries are known as such industrial storage batteries.

[0003] Currently, vanadium is used as an active material in the electrolyte in typical redox flow batteries, but because vanadium is a rare metal, there are concerns about supply shortages and high costs. In addition, because high-concentration sulfuric acid is used as the electrolyte, the search for alternative active materials to vanadium is ongoing.

[0004] For example, Patent Document 1 discloses a redox flow battery including a positive electrode electrolyte containing a first redox couple including water, a first organic compound having a specific structure, and a reduction product thereof, and a negative electrode electrolyte containing a second redox couple including water, a second organic compound having a specific structure, and a reduction product thereof. Patent Document 2 discloses an aqueous electrolyte solution containing water and a phenazine derivative having a specific structure or a salt thereof. Patent Document 3 discloses an electrolyte for a redox flow battery, which contains a phenazine compound having a specific structure and a solute. Patent Document 4 discloses a redox flow battery including a cathode, an anode, a charge carrier electrolyte, and a phenazine-based compound having a specific structure. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 9,614,245 [Patent Document 2] U.S. Pat. No. 10,454,124 [Patent Document 3] International Publication No. 2020 / 111725 [Patent Document 4] Special Publication No. 2020-527843 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the electrolytes of the redox flow batteries described in Patent Documents 1 to 3 have a redox potential of an organic compound contained in the negative electrode electrolyte close to the potential window of water, and thus have the possibility of a decrease in battery capacity due to hydrogen generation. In addition, the charge carrier medium of the redox flow battery described in Patent Document 4 is flammable, so there is a demand for a flame-retardant aqueous electrolyte.

[0007] The present invention has been made to solve the above problems, and an object of the present invention is to provide a polycyclic organic compound or a salt thereof having a redox potential that easily suppresses hydrogen generation, an aqueous electrolyte solution containing the polycyclic organic compound or the salt thereof as an active material, and a redox flow battery using the aqueous electrolyte solution. [Means for solving the problem]

[0008] Means for Solving the Problems The present inventors have conducted intensive research in light of the above-mentioned circumstances and have found that the above-mentioned problems can be solved by using an aqueous electrolyte solution containing a specific polycyclic organic compound or a salt thereof, and water, thereby completing the present invention.

[0009] That is, the gist of the present invention is as follows. [1] A compound represented by the following general formula (1) or a salt thereof: [ka] (In general formula (1), R 1 ~R 10 are each independently a group selected from a hydrogen atom, a halogen atom, an alkyl group, an aryl group, a hydroxy group, an alkoxy group, an acyloxy group, a carboxy group, an alkoxycarbonyl group, an acyl group, an amino group, an amide group, a nitrile group, a thiol group, a sulfonic acid group, a sulfate group, a phosphonic acid group, and a phosphate group; R 1 ~R 10 At least two of the groups are selected from a sulfonic acid group, a sulfonic acid group substituted with an alkyl group or an alkoxy group, a sulfate group, a sulfate group substituted with an alkyl group or an alkoxy group, and salts thereof. [2] An aqueous electrolyte solution comprising the compound or salt thereof according to [1] and water. [3] A redox flow battery comprising the aqueous electrolyte solution according to [2], a positive electrode, a negative electrode, and a separator. Effect of the Invention

[0010] Effects of the Invention According to the present invention, it is possible to provide a polycyclic organic compound or a salt thereof having a redox potential that easily suppresses hydrogen generation, an aqueous electrolyte solution containing the polycyclic organic compound or the salt thereof as an active material, and a redox flow battery using the aqueous electrolyte solution. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a diagram showing a cyclic voltammogram of an aqueous solution containing sodium salts of a compound represented by formula (1-1) and a compound represented by formula (1-2). [Diagram 2] FIG. 2 is a diagram showing a cyclic voltammogram of an aqueous solution containing sodium salts of the compound represented by formula (1-13) and the compound represented by formula (1-14). [Diagram 3] FIG. 2 is a diagram showing a cyclic voltammogram of an aqueous solution containing a sodium salt of the compound obtained in Comparative Synthesis Example 1. [Figure 4]FIG. 2 is a diagram showing a cyclic voltammogram of an aqueous solution containing a sodium salt of the compound obtained in Comparative Synthesis Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] One aspect of the present invention is a compound represented by the following general formula (1) or a salt thereof.

[0013] [ka]

[0014] In general formula (1), R 1 ~R 10 are each independently a group selected from a hydrogen atom, a halogen atom, an alkyl group, an aryl group, a hydroxy group, an alkoxy group, an acyloxy group, a carboxy group, an alkoxycarbonyl group, an acyl group, an amino group, an amide group, a nitrile group, a thiol group, a sulfonic acid group, a sulfate group, a phosphonic acid group, and a phosphate group; R 1 ~R 10 At least two of the above are groups selected from a sulfonic acid group, a sulfonic acid group substituted with an alkyl group or an alkoxy group, a sulfate group, a sulfate group substituted with an alkyl group or an alkoxy group, and salts thereof.

[0015] The compound represented by the general formula (1) has at least two groups selected from a sulfonic acid group, a sulfonic acid group substituted with an alkyl group or an alkoxy group, a sulfate group, and a sulfate group substituted with an alkyl group or an alkoxy group, and therefore has higher solubility in water than compounds conventionally used in electrolytes for redox flow batteries. Therefore, the compound represented by the general formula (1) or a salt thereof dissolves in water over a wide pH range, which increases the degree of freedom in designing an aqueous electrolyte and a redox flow battery using the same depending on the mode of use.

[0016] R 1 ~R 10 When is a halogen atom, R 1 ~R10 is preferably fluorine, chlorine, bromine or iodine, more preferably fluorine or chlorine, and even more preferably fluorine. R 1 ~R 10 When R is an alkyl group, 1 ~R 10 is preferably an alkyl group having 1 to 4 carbon atoms, more preferably a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an s-butyl group, a t-butyl group, or an isobutyl group, and even more preferably a methyl group or an ethyl group. R 1 ~R 10 When R is an aryl group, 1 ~R 10 is preferably an aryl group having 6 to 22 carbon atoms, more preferably a phenyl group or a naphthyl group, and even more preferably a phenyl group. R 1 ~R 10 When R is an alkoxy group, 1 ~R 10 is preferably an alkoxy group having 1 to 10 carbon atoms, more preferably a methoxy group, ethoxy group, n-propoxy group, isopropoxy group, n-butoxy group, s-butoxy group, t-butoxy group, or isobutoxy group, and even more preferably a methoxy group or an ethoxy group. R 1 ~R 10 When R is an acyloxy group, 1 ~R 10 is preferably an acyloxy group having 1 to 10 carbon atoms, more preferably an acetyloxy group, an n-propanoyloxy group, an isopropanoyloxy group, an n-butanoyloxy group, an s-butanoyloxy group, a t-butanoyloxy group, or an isobutanoyloxy group, and even more preferably an acetyloxy group. R 1 ~R 10 When R is an alkoxycarbonyl group, 1 ~R 10is an ester of a carboxy group and an alcohol having 1 to 4 carbon atoms, and is preferably a methoxycarbonyl group, an ethoxycarbonyl group, an n-propoxycarbonyl group, an isopropoxycarbonyl group, an n-butoxycarbonyl group, an s-butoxycarbonyl group, a t-butoxycarbonyl group, or an isobutoxycarbonyl group, and more preferably a methoxycarbonyl group or an ethoxycarbonyl group. R 1 ~R 10 When R is an acyl group, 1 ~R 10 is preferably an acetyl group, an n-propanoyl group, an isopropanoyl group, an n-butanoyl group, an s-butanoyl group, a t-butanoyl group, or an isobutanoyl group, and more preferably an acetyl group. R 1 ~R 10 is an amide group, R 1 ~R 10 is an amide of an amino group and a carboxylic acid having 1 to 4 carbon atoms, preferably a formamide group, an acetate amide group, a propionamide group, a butyrate amide group, an isobutyrate amide group, or a succinimide group, more preferably a formamide group or an acetate amide group.

[0017] R 1 ~R 10 When represents a sulfonic acid group substituted with an alkyl group or an alkoxy group, and a sulfate group substituted with an alkyl group or an alkoxy group, the alkyl group and the alkoxy group each preferably have 1 or more and 4 or less carbon atoms, more preferably 1. R 1 ~R 10 At least two of the groups are sulfonic acid groups, which is preferable since the solubility of the compound represented by general formula (1) in water is increased.

[0018] When the compound represented by formula (1) is a salt thereof, R 1 ~R 10The hydroxy group, carboxy group, thiol group, phosphonic acid group, phosphoric acid group, sulfonic acid group, sulfonic acid group substituted with an alkyl group or an alkoxy group, sulfate group, and sulfate group substituted with an alkyl group or an alkoxy group may form a salt, preferably an alkali metal salt, alkaline earth metal salt, or ammonium salt of these groups, and more preferably a lithium salt, sodium salt, potassium salt, or ammonium salt of these groups. When the compound represented by formula (1) is a salt thereof, R 1 ~R 10 The amino group may form a salt, preferably an inorganic acid salt of the amino group, more preferably a hydrochloride salt of the amino group.

[0019] The half-wave potential E of the compound represented by formula (1) or its salt based on a saturated KCl silver / silver chloride reference electrode 1 / 2 From the viewpoint of suppressing hydrogen generation by electrolysis of water, is preferably greater than −0.94, more preferably not less than −0.92, even more preferably not less than −0.90, and is preferably less than −0.50, more preferably not more than −0.60, even more preferably not more than −0.65.

[0020] In the general formula (1), R 4 ~R 8 is preferably a hydrogen atom, since synthesis is easy.

[0021] In addition, in the general formula (1), R 1 ~R 10 is not a hydroxy group, that is, when the compound represented by general formula (1) or a salt thereof does not have a hydroxy group, synthesis is easy and therefore it is preferable.

[0022] On the other hand, in the general formula (1), R 9 is a hydroxy group, the redox potential of the compound represented by formula (1) or a salt thereof shifts to the positive side, and the durability of the compound represented by formula (1) or a salt thereof is increased, which is preferable.

[0023] Examples of the compound represented by general formula (1) include compounds represented by formulas (1-1) to (1-16).

[0024] [ka] [ka]

[0025] (Preparation of compound represented by general formula (1)) Examples of methods for synthesizing the compound represented by general formula (1) include a method of condensing a catechol derivative with an o-phenylenediamine derivative and oxidizing the resulting dihydrophenazine, and a method of obtaining the corresponding compound represented by general formula (1) as a polycyclic organic compound by a Wall-Aue reaction, etc. Also, the compound represented by general formula (1) may be obtained by reacting the obtained polycyclic organic compound with fuming sulfuric acid to further introduce a sulfonic acid group into the polycyclic organic compound.

[0026] The compound represented by the general formula (1) thus obtained can be reacted with a cationic species or anionic species to prepare a salt of the compound represented by the general formula (1). 1 ~R 10The cation species or anion species may be any cation species or anion species that forms a salt with the hydroxy group, carboxy group, thiol group, phosphonic acid group, phosphoric acid group, sulfonic acid group, sulfonic acid group substituted with an alkyl group or alkoxy group, sulfate group, sulfate group substituted with an alkyl group or alkoxy group, and amino group (hereinafter, these are also collectively referred to as "salt-forming groups"). Examples of the cation species include alkali metal cations and ammonium. Examples of the alkali metal cation source include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide, and alkali metal alkoxides such as lithium methoxide, lithium ethoxide, sodium methoxide, sodium ethoxide, potassium methoxide, and potassium ethoxide. The alkali metal cation species may be used in the form of a solution, preferably an aqueous solution. Examples of the ammonium cation source include concentrated aqueous ammonia (about 28%). Examples of the anion species include R 1 ~R 10 The anion species is not particularly limited as long as it forms a salt with the amino group, and examples thereof include anions of inorganic acids. Examples of the source of the inorganic acid anion include inorganic acids and aqueous solutions thereof, and dilute hydrochloric acid is preferred.

[0027] The salt of the compound represented by general formula (1) can be obtained by suspending the compound represented by general formula (1) in a solvent that does not dissolve the compound represented by general formula (1), gradually adding the above-mentioned cation source or anion source, and distilling off the solvent after the salt of the compound represented by general formula (1) is completely dissolved in the solvent. The amount of cationic species or anionic species added may be 1 equivalent or more to the compound represented by the general formula (1), and it is preferable to contact the compound so that the total number of moles of sulfonic acid groups and sulfate groups in the compound represented by the general formula (1) and the amount of cations after addition are the same, or the total number of moles of amino groups in the compound represented by the general formula (1) and the amount of anions after addition are the same. That is, when a part of the sulfonic acid groups and sulfate groups contained in the compound represented by the general formula (1) are already in the form of salts of cationic species, it is preferable to contact the cationic species so that the total amount of the cationic species in the form of salts and the cationic species added is the same as the total number of moles of sulfonic acid groups and sulfate groups in the compound represented by the general formula (1), and when a part of the amino groups contained in the compound represented by the general formula (1) are already in the form of salts of anionic species, it is preferable to contact the cationic species so that the total amount of the anionic species in the form of salts and the anionic species added is the same as the total number of moles of amino groups in the compound represented by the general formula (1). When an excessive amount of a cation species or anion species is brought into contact with the aqueous electrolyte, the pH of the aqueous electrolyte changes significantly. Therefore, the amount of the cation species added is preferably up to 1.3 times the total molar number of the sulfonic acid group, carboxyl group, and phenolic hydroxyl group in the compound represented by general formula (1). As a solvent that does not dissolve the compound represented by formula (1), for example, methanol can be used. The solvent is used in an amount such that the molar concentration of the salt of the compound represented by formula (1) after dissolution is preferably 0.001M or more, more preferably 0.002M or more, and preferably 0.3M or less, more preferably 0.25M or less. The temperature at which the cation species or anion species is added dropwise to the suspension of the compound represented by general formula (1) is preferably 0°C or higher, more preferably 10°C or higher, and preferably 50°C or lower, more preferably 40°C or lower, and even more preferably room temperature (25°C).

[0028] The aqueous electrolyte of the present invention contains a compound represented by general formula (1) and / or a salt thereof, and water. The total concentration of the compound represented by formula (1) and its salt in the aqueous electrolyte is preferably 0.001M or more, more preferably 0.002M or more, and preferably 0.3M or less, more preferably 0.25M or less.

[0029] (water) The water contained in the aqueous electrolyte of the present invention is not particularly limited, but is preferably distilled water, ion-exchanged water, or ultrapure water, and more preferably ion-exchanged water from the viewpoint of cost.

[0030] (supporting electrolyte) The aqueous electrolyte of the present invention preferably contains, in addition to the compound represented by the above general formula (1) or a salt thereof, at least one selected from the group consisting of an alkali metal salt and an amine salt, which acts as a supporting electrolyte for increasing electrical conductivity. Examples of alkali metal salts include alkali metal halide salts; inorganic acid salts of alkali metals such as alkali metal sulfates, alkali metal nitrates, alkali metal phosphates, and alkali metal carbonates; and organic acid salts of alkali metals such as alkali metal oxalates and alkali metal citrates. Examples of the amine salt include ammonium chloride and ammonium sulfate. Among these, alkali metal halide salts are preferred, alkali metal chloride salts are more preferred, one selected from lithium chloride, sodium chloride and potassium chloride is more preferred, and potassium chloride is more preferred.

[0031] The concentration of the supporting electrolyte in the aqueous electrolyte is preferably 0.1 M or more, more preferably 0.5 M or more, and preferably 2 M or less, more preferably 1.5 M or less, in terms of the total concentration of the supporting electrolyte. The total concentration of the supporting electrolyte in the aqueous electrolyte is preferably 0.1M or more and 2M or less, and more preferably 0.5M or more and 1.5M or less.

[0032] The pH of the aqueous electrolyte is 6.5 or higher, preferably 7 or higher, more preferably 8 or higher, and 12.5 or lower, preferably 11 or lower, more preferably 10.7 or lower. The pH of the aqueous electrolyte is from 6.5 to 12.5, preferably from 7 to 11, and more preferably from 8 to 10.7.

[0033] [Method of manufacturing aqueous electrolyte] The method for producing the aqueous electrolyte of the present invention includes dissolving a compound represented by general formula (1) or a salt thereof in a solvent containing water. When the aqueous electrolyte contains a supporting electrolyte, the supporting electrolyte can be dissolved in the solvent containing water at any timing.

[0034] The structure of the compound contained in the aqueous electrolyte is measured by nuclear magnetic resonance (hereinafter sometimes abbreviated as NMR). Under an inert atmosphere, the aqueous electrolyte is dissolved in a deuterated solvent and placed in an NMR tube to perform NMR measurement. Alternatively, a double tube may be used as the NMR tube, with the aqueous electrolyte placed in one tube and the deuterated solvent placed in the other tube to perform NMR measurement. Examples of deuterated solvents include deuterated water, deuterated methanol, deuterated acetonitrile, and deuterated dimethyl sulfoxide. When determining the concentration of the constituents of the aqueous electrolyte, a specified amount of a standard substance is dissolved in the deuterated solvent, and the concentration of each constituent can be calculated from the ratio of the spectra. Alternatively, the concentration of one or more of the components constituting the aqueous electrolyte can be determined in advance by another analytical method such as gas chromatography, and the concentration can be calculated from the spectral ratio of the component with a known concentration to the other components. The nuclear magnetic resonance analyzer used is preferably a device with a proton resonance frequency of 400 MHz or more. The nuclides to be measured are 1 H, 13 C. 31 P, 19 F, 11 Examples include B. Since the compound represented by the general formula (1) is in a dissociated state in the aqueous electrolyte, it is sufficient to detect the anion species or cation species of the compound represented by the general formula (1). However, when inorganic cation species are taken into consideration, the concentration is calculated separately by ICP emission spectroscopy, and the composition is determined by combining the results with the NMR results.

[0035] [Redox flow battery] A redox flow battery according to one embodiment of the present invention includes a positive electrode side circulation mechanism and a negative electrode side circulation mechanism for circulating an aqueous electrolyte, and a battery cell. The battery cell circulates the aqueous electrolyte to promote an oxidation-reduction reaction and perform charging and discharging. The battery cell includes a positive electrode chamber for accommodating a positive electrode, a negative electrode chamber for accommodating a negative electrode, and a separator for separating the positive electrode chamber from the negative electrode chamber. The positive electrode chamber and the negative electrode chamber are capable of circulating the positive electrode side electrolyte and the negative electrode side electrolyte supplied by the positive electrode side circulation mechanism and the negative electrode side circulation mechanism, respectively. The redox flow battery according to this embodiment includes the aqueous electrolyte of the present invention as the negative electrode side electrolyte.

[0036] (Aqueous electrolyte) The aqueous electrolyte includes a positive electrode electrolyte and a negative electrode electrolyte.

[0037] (Positive electrode electrolyte) The positive electrode side electrolyte is an aqueous electrolyte similar to the negative electrode side electrolyte, and contains the same supporting electrolyte as that contained in the aqueous electrolyte of the present invention, which is the negative electrode side electrolyte. The active material contained in the positive electrode electrolyte (positive electrode active material) may be an active material commonly used in redox flow batteries, but is preferably an active material having a redox potential difference of 1.0 V or more with respect to the compound represented by general formula (1) or a salt thereof contained as an active material in the negative electrode electrolyte, more preferably 1.1 V or more, and even more preferably 1.2 V or more. By using such an active material, the electromotive force of the redox flow battery can be increased. Examples of the positive electrode active material having a redox potential difference of 1.0 V or more with respect to the compound represented by general formula (1) or a salt thereof include a compound having a nitroxyl radical structure, a ferrocyanide, a compound having a ferrocene structure, a quinone compound, a bromide ion, an iodide ion, and the like, and at least one compound selected from a compound having a nitroxyl radical structure, a ferrocyanide, and a compound having a ferrocene structure is preferred. Examples of compounds having a nitroxyl radical structure include 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (4-hydroxy-TEMPO), 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), 1-methyl-azaadamantane-N-oxyl (1-methyl-AZADO), azaadamantane-N-oxyl (AZADO), and the like. Examples of ferrocyanides include potassium ferrocyanide and sodium ferrocyanide. Examples of compounds having a ferrocene structure include (ferrocenylmethyl)trimethylammonium chloride, 1,1'-bis[3-(trimethylammonio)propyl]ferrocene dichloride, ferrocene-1,1'-dicarboxylic acid, ferrocene carboxylic acid, ferrocenesulfonic acid, ferrocene-1,1'-disulfonic acid, ferrocene acetic acid, and ferrocenylmethanol. Among these, in the aqueous electrolyte solution of the first embodiment, from the viewpoint of obtaining a redox flow battery with a large electromotive force, at least one compound selected from a compound having a nitroxyl radical structure and a compound having a ferrocene structure is more preferable, and a compound having a nitroxyl radical structure is even more preferable. In addition, in the aqueous electrolyte solution of the second embodiment, from the viewpoint of obtaining an aqueous electrolyte solution capable of driving a battery in a wide pH range, at least one compound selected from a compound having a nitroxyl radical structure, a ferrocyanide, and a compound having a ferrocene structure is preferable, sodium ferrocyanide, (ferrocenylmethyl)trimethylammonium chloride, and 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl are more preferable, and sodium ferrocyanide is even more preferable.

[0038] (Positive and negative electrodes) The positive and negative electrodes can be made of electronic conductors with a large specific surface area, such as porous metals, carbon felt, carbon paper, and carbon nanotube sheets. A positive terminal is connected to the positive electrode, and a negative terminal is connected to the negative electrode, and the positive and negative terminals are connected to a charge / discharge device. The charge / discharge device applies a voltage to the positive and negative electrodes when charging the battery cell, and extracts power from the battery cell via the positive and negative electrodes when discharging.

[0039] (Separator) As the separator, a porous membrane can be used, and examples thereof include an anion exchange membrane, a cation exchange membrane, a porous polypropylene membrane, a porous polyethylene membrane, an aromatic polyamide nonwoven fabric separator, a cellulose nonwoven fabric separator, etc. Among these, when anions such as chloride ions, bromide ions, iodide ions, and sulfate ions are used for charge compensation, an anion exchange membrane is preferred, and when cations such as sodium ions, potassium ions, lithium ions, and ammonium ions are used for charge compensation, a cation exchange membrane is preferred, and an anion exchange membrane is more preferred.

[0040] (Positive electrode side circulation mechanism and negative electrode side circulation mechanism) The positive electrode side circulation mechanism has a positive electrode tank, a positive electrode pump, and a positive electrode piping, and the positive electrode side electrolyte stored in the positive electrode tank flows into the positive electrode chamber of the battery cell through the positive electrode piping by the action of the positive electrode pump. The positive electrode side electrolyte charged and discharged in the battery cell flows out from the positive electrode chamber and returns to the positive electrode tank. A positive electrode filter that limits the loss of the positive electrode active material may be provided at the outlet of the positive electrode tank or between the positive electrode tank and the battery cell. The positive electrode filter can prevent the positive electrode active material from flowing into the battery cell, and can prevent the failure or deterioration of the battery cell due to the mixture of the positive electrode active material and the negative electrode active material. The positive electrode pump may be provided at any position in the positive electrode side circulation mechanism, but is preferably provided between the positive electrode tank and the battery cell. The negative electrode circulation mechanism includes a negative electrode tank, a negative electrode pump, a negative electrode piping, and an optional negative electrode filter, the arrangement and operation of which are similar to those of the corresponding parts of the positive electrode circulation mechanism. EXAMPLES

[0041] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0042] (DFT calculation) The free energies of the molecules in the oxidized and reduced states were calculated using density functional theory (DFT), and the redox potential was calculated by dividing the difference between them by the number of reactive electrons and the Faraday constant. The calculations were performed using Gaussian16, with the functional and basis set being ωB97X-D and 6-31+G(d,p), respectively. In addition, the continuum dielectric model (PCM) was applied to consider the effect of the solvent water. redox potential E calc is expressed by the following formula.

[0043]

number

[0044] In the formula, G red is the free energy of the molecule in the reduced state, Gox is the free energy of the molecule in the oxidation state, G H+ is the free energy of a proton, F is the Faraday constant, E abs is the absolute potential of the reference electrode.

[0045] Example 1 <Synthesis of compound represented by general formula (1)> Synthesis Example 1 (Synthesis of Compounds Represented by Formula (1-1) and Formula (1-2)) In a glass container equipped with a stirrer, 1,2-naphthoquinone-4-sodium sulfonate (5.20 g, 20 mmol) and methanol (100 mL) were mixed and cooled to 3°C. 3,4-Diaminobenzenesulfonic acid (3.76 g, 20 mmol) was added and stirred at 5°C or less for 5 hours. The temperature was raised to room temperature, 50 mL of isopropyl alcohol was added, and the mixture was stirred for 1 hour, after which the precipitated solid was filtered off. The solid was washed with isopropyl alcohol to obtain 5.92 g of a reddish brown solid. The reddish brown solid was dissolved in DO with one drop of 1M sodium hydroxide solution. 1 By measuring H-NMR, it was confirmed that the product was a mixture of the compounds represented by formula (1-1) and formula (1-2).

[0046] Synthesis Example 2 (Synthesis of Compounds Represented by Formula (1-13) and Formula (1-14)) In a glass vessel equipped with a stirrer, 4.44g (25mmol) of 2-hydroxy-1,4-naphthoquinone, 4.80g (25mmol) of 3,4-diaminobenzenesulfonic acid, and 250mL of water were added, and the mixture was heated and refluxed for 2 hours to react. After cooling the reaction liquid to room temperature, it was filtered under reduced pressure to obtain a reddish brown wet cake. This wet cake was dried to obtain 7.54g of a reddish brown powder. 4 mL of 20% fuming sulfuric acid and 0.7 g of the reddish brown powder obtained above were mixed in a glass container equipped with a stirrer and heated at 50°C for 4 hours. After cooling to room temperature, the reaction mixture was dropped into 5 mL of ice water, and 7 mL of saturated NaCl aqueous solution was added and stirred at room temperature. The reaction mixture was filtered under reduced pressure to obtain a reddish brown wet cake, to which 15 mL of 1 M sodium hydroxide aqueous solution was added to adjust the pH to 6-8. Then, 30 mL of isopropyl alcohol was added, and the mixture was stirred at room temperature for 2 hours and filtered under reduced pressure to obtain an orange wet cake. The wet cake was dried to obtain 0.80 g of orange powder. The orange powder was dissolved in DO with one drop of 1M sodium hydroxide solution. 1 By measuring H-NMR, it was confirmed that the product was a mixture of the compounds represented by formula (1-13) and formula (1-14).

[0047] Comparative synthesis example 1 In a glass vessel equipped with a stirrer, 4.44g (25mmol) of 2-hydroxy-1,4-naphthoquinone, 4.80g (25mmol) of 3,4-diaminobenzenesulfonic acid, and 250mL of water were added, and the mixture was heated and refluxed for 2 hours to react. After cooling the reaction liquid to room temperature, it was filtered under reduced pressure to obtain a reddish brown wet cake. This wet cake was dried to obtain 7.54g of a reddish brown powder. The reddish brown powder was dissolved in DO solvent with one drop of 1M sodium hydroxide solution added. 1 H-NMR measurement confirmed that the product was a mixture of the compounds shown below. [ka]

[0048] Comparative synthesis example 2 In a glass container equipped with a stirrer, 0.2881 g (1.5 mmol) of 2,7-dihydroxy-1,4-naphthoquinone, 0.30 g (1.5 mmol) of 3,4-diaminobenzenesulfonic acid, and 12 mL of water were added, and the mixture was heated and refluxed for 2 hours to react. After cooling the reaction liquid to room temperature, it was filtered under reduced pressure to obtain a reddish brown wet cake. This wet cake was dried to obtain a reddish brown powder. The reddish brown powder was dissolved in DO solvent with one drop of 1M sodium hydroxide solution added. 1 H-NMR measurement confirmed that the product was a mixture of the compounds shown below. [ka]

[0049] Example 2 <Electrochemical Measurement of Compound Represented by General Formula (1)> (Example 2-1) A mixture of the compound represented by formula (1-1) and the compound represented by formula (1-2) obtained in Synthesis Example 1 was weighed out to 20 mg and dissolved in 10 mL of 1 mol / L sodium hydroxide solution (pH>12) to prepare an electrolyte. A carbon electrode (glassy carbon disk electrode, electrode area 0.28 cm2) was placed in this solution as a working electrode. 2 Cyclic voltammetry measurements were performed at a scan rate of 50 mV / s using a silver / silver chloride electrode (Ag / AgCl (saturated KCl, BAS, model number 002012) as the reference electrode, a silver / silver chloride electrode (Ag / AgCl (saturated KCl, BAS, model number RE-1CP)) as the counter electrode, and a platinum wire (φ0.5) as the counter electrode. The obtained cyclic voltammogram is shown in Figure 1. The redox potential was -0.740 V vs Ag / AgCl. The redox potentials of the compounds represented by formula (1-1) and formula (1-2) predicted by DFT calculation are -0.809 V vs Ag / AgCl. The difference from the actual measured value is 69.1 mV, and the error is 9.3%.

[0050] (Example 2-2) Cyclic voltammetry measurement was carried out in the same manner as in Example 2-1, except that the mixture of the compound represented by formula (1-1) and the compound represented by formula (1-2) was replaced with the mixture of the compound represented by formula (1-13) and the compound represented by formula (1-14) obtained in Synthesis Example 2. The obtained cyclic voltammogram is shown in Figure 2. The redox potential was -0.900 V vs Ag / AgCl. The redox potentials of the compounds represented by formula (1-13) and formula (1-14) predicted by DFT calculations are -0.857 V vs Ag / AgCl. The difference from the actual measured value is 47.7 mV, with an error of 5.3%.

[0051] Example 3 For example, 1,2-naphthoquinone and 3,4-diaminobenzenesulfonic acid are condensed to first synthesize a 4-ring phenazine skeleton, and then the skeleton is sulfonated using fuming sulfuric acid or the like to obtain the compound represented by general formula (1) as a single compound or a mixture of positional isomers. [ka] Alternatively, for example, 2-hydroxy-1,4-naphthoquinone is sulfonated with fuming sulfuric acid or the like, and the resulting sulfonated product is isolated as necessary. Thereafter, the sulfonated product is condensed with 3,4-diaminobenzenesulfonic acid to obtain the compound represented by general formula (1) as a single compound or a mixture of positional isomers. [ka]

[0052] <Prediction of redox potential of the compound represented by general formula (1) by DFT calculation> Table 1 shows the redox potentials of the compounds represented by formulae (1-1) to (1-16) predicted by DFT calculation. [Table 1]

[0053] The estimation of redox potential by DFT calculations is described in J.Phys.Chem.A, Vol. 124, No. 35, pp. 7166-7176, 2020, and it is concluded that it is highly reliable and accurate for organic compound molecules. In addition, the calculated value of redox potential by DFT calculations shows good agreement with the measured value.

[0054] (Comparative Example 2-1) Cyclic voltammetry measurement was carried out in the same manner as in Example 2-1, except that the mixture of the compound represented by formula (1-1) and the compound represented by formula (1-2) was used as the mixture of the compounds obtained in Comparative Synthesis Example 1. The obtained cyclic voltammogram is shown in Figure 3. The redox potential was -0.945 V vs Ag / AgCl.

[0055] (Comparative Example 2-2) Cyclic voltammetry measurement was carried out in the same manner as in Example 2-1, except that the mixture of the compound represented by formula (1-1) and the compound represented by formula (1-2) was used as the mixture of the compounds obtained in Comparative Synthesis Example 2. The obtained cyclic voltammogram is shown in Figure 4. The redox potential was -0.960 V vs Ag / AgCl.

[0056] From the above results, it is considered that the compound represented by general formula (1) has a lower LUMO and a higher redox potential because it has an electron-withdrawing functional group. Therefore, it is considered that hydrogen generation can be suppressed in a redox flow battery equipped with an aqueous electrolyte containing the compound represented by general formula (1).

Claims

1. A compound represented by the following general formula (1) or a salt thereof: 【Chemical 1】 (In general formula (1), R １ ~R １０ are each independently a group selected from a hydrogen atom, a halogen atom, an alkyl group, an aryl group, a hydroxy group, an alkoxy group, an acyloxy group, a carboxy group, an alkoxycarbonyl group, an acyl group, an amino group, an amide group, a nitrile group, a thiol group, a sulfonic acid group, a sulfate group, a phosphonic acid group, and a phosphate group; R １ ~R １０ At least two of the groups are selected from a sulfonic acid group, a sulfonic acid group substituted with an alkyl group or an alkoxy group, a sulfate group, a sulfate group substituted with an alkyl group or an alkoxy group, and salts thereof.

2. In the general formula (1), R １ ~R １０ The compound or salt thereof according to claim 1 , wherein at least two of the following are sulfonic acid groups:

3. In the general formula (1), R １ ~R １０ is not a hydroxy group, or a salt thereof according to claim 1 .

4. The compound or salt thereof according to claim 1, wherein the compound represented by the general formula (1) is a compound represented by any one of the following formulas (1-1) to (1-12): 【Chemistry 2】

5. R ９ The compound or salt thereof according to claim 1 , wherein is a hydroxy group.

6. The compound or salt thereof according to claim 5, wherein the compound represented by the general formula (1) is a compound represented by any one of the following formulas (1-13) to (1-16): 【Chemistry 3】

7. In the general formula (1), R ４ ~R ８ The compound or salt thereof according to claim 1, wherein is a hydrogen atom.

8. Half-wave potential E with respect to a saturated KCl silver / silver chloride reference electrode (Ag / AgCl) １／２ The compound or salt thereof according to claim 1, wherein the ρ is greater than -0.94 and less than -0.

5.

9. 9. An aqueous electrolyte solution comprising the compound or salt thereof according to claim 1 and water.

10. A redox flow battery comprising the aqueous electrolyte solution according to claim 9, a positive electrode, a negative electrode, and a separator.

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