Anthraquinone active materials

Abstract

This anthraquinone active substance for redox flow batteries includes a first compound represented by a chemical formula. With respect to R1 to R8, at least one is a hydroxyl group and at least one is an alkoxy group.

Description

[Technical Field]

[0001] The present disclosure relates to anthraquinone active materials for redox flow batteries. This application claims priority based on Patent Application No. 2021-208037, filed with the Japan Patent Office on December 22, 2021, the contents of which are incorporated herein by reference. [Background technology]

[0002] Redox flow batteries are suitable for storing large amounts of power because the amount of power stored can be freely designed according to the capacity of the electrolyte tank, and are expected to be used to level out power supply and demand, including natural energy. Redox flow batteries consist of cells that charge and discharge, and an electrolyte tank that stores power, and are characterized by charging and discharging by circulating the electrolyte with a pump.

[0003] Currently, redox flow batteries that use vanadium as the active material in the electrolyte are mainstream. However, due to the recent rise in vanadium prices, development of redox flow batteries that use organic substances or metal complexes as the active material is underway. For example, Patent Document 1 describes a redox flow battery that uses anthraquinone or naphthoquinone as the negative electrode active material, and exemplifies numerous anthraquinones having sulfo groups. Patent Document 2 describes a redox flow battery that uses, as the active material, a composition containing a coordination compound in which a redox non-innocent ligand is coordinated to a metal center, rather than the active material itself, and exemplifies numerous anthraquinones in which various functional groups are bonded to positions 1 to 8 of the anthraquinone as the redox non-innocent ligand. Non-Patent Documents 1 and 2 also describe compounds in which various functional groups or elements are bonded to positions 1 to 8 of anthraquinone. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6574382 [Patent Document 2] Special Publication No. 2019-514170 [Non-patent literature]

[0005] [Non-Patent Document 1] K.Lin,Q.Chen,MRGerhardt,L.Tong,SBKim,L.Eisenach,AWValle,D.Hardee,R,G.Gordon,M,J.Aziz,MPMarshak,Science,349(2015) 1529-1532 [Non-patent document 2] DGKwabi,K.Lin,Y.Ji.F.Kerr,M.Goulet,DDPorcellinis,DPTabor,DAPollack,A.Aspuru-Guzik,RGGordon,MJAziz,Joule 2,19(2018) 1894-1906 Summary of the Invention [Problem to be solved by the invention]

[0006] Any functional group or element can be bonded to positions 1 to 8 of anthraquinone, but certain combinations of these can cause practical problems. For example, Non-Patent Document 2 describes the problem that 2,6-dihydroxyanthraquinone (2,6-DHAQ) experiences a large decrease in capacity during charge and discharge, and describes the problem that 2,6-bis(3'-carboxypropyloxy)-9,10-anthraquinone (2,6-DBEAQ) can suppress the decrease in capacity during discharge but has a low cell voltage.

[0007] In view of the above circumstances, at least one embodiment of the present disclosure has an object to provide an anthraquinone active material that has a good balance between the cell voltage of a redox flow battery and suppression of capacity loss during discharge. [Means for solving the problem]

[0008] In order to achieve the above object, the anthraquinone active material according to the present disclosure is an anthraquinone active material for a redox flow battery, which contains a first compound represented by the following chemical formula: [ka] Among the R1 to R8, R 2 but is a hydroxyl group, R 6 but It is an alkoxy group. [Effects of the Invention]

[0009] According to the anthraquinone active material of the present disclosure, the presence of a hydroxyl group makes the cell voltage of the redox flow battery appropriate, and the presence of an alkoxy group makes it possible to suppress a decrease in capacity during discharge of the redox flow battery, thereby achieving a good balance between the cell voltage and suppression of a decrease in capacity during discharge. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a graph showing the experimental results of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0011] An anthraquinone active material (hereinafter, simply referred to as "active material" unless there is a particular need to add "anthraquinone") according to an embodiment of the present disclosure will be described below. The embodiment described below shows one aspect of the present disclosure, does not limit the disclosure, and can be arbitrarily modified within the scope of the technical concept of the present disclosure.

[0012] <Basic structure of the active material of the present disclosure> The active material of the present disclosure is an active material that dissolves in the electrolyte on the negative electrode side of a redox flow battery in a discharged state, and contains a compound represented by the following chemical formula (1): When this compound is used as the active material on the negative electrode side of a redox flow battery, it is converted by an oxidation-reduction reaction into either this compound or a reduced form in which the oxygen atoms double-bonded to the 9th and 10th positions of the anthraquinone skeleton are converted to hydroxyl groups. Specifically, when the redox flow battery performs a discharge operation, an oxidation reaction occurs in which the reduced form is converted into this compound, and when the redox flow battery performs a charge operation, a reduction reaction occurs in which this compound is converted into the reduced form.

[0013] [ka]

[0014] In chemical formula (1), of R1 to R8 bonded to the 1st to 8th positions of the anthraquinone skeleton, at least one is a hydroxyl group and at least one is an alkoxy group (-OR). In the alkoxy group, R bonded to the oxygen atom has 1 to 6 carbon atoms, and when it has 4 to 6 carbon atoms, it has a linear or branched structure. The bond between the carbon atoms constituting R is not limited to a single bond and may include a double bond or a triple bond. Furthermore, R may include an ether bond. Furthermore, at least one of the carbons constituting R may be bonded to a halogen or any functional group, such as a hydroxyl group, sulfo group, amino group, nitro group, carboxyl group, phosphoryl group, thiol group, or alkyl ester, instead of hydrogen.

[0015] When a compound having such a structure is used as the negative electrode active material of a redox flow battery, the cell voltage of the redox flow battery becomes appropriate because the active material has a hydroxyl group, and the presence of an alkoxy group makes it possible to suppress a decrease in capacity during discharge of the redox flow battery, thereby achieving a good balance between the cell voltage and suppression of a decrease in capacity during discharge.

[0016] <Variations of the active material of the present disclosure> In the above-described basic structure of the active material of the present disclosure, it is sufficient that R1 to R8 each contain at least one hydroxyl group and at least one alkoxy group; the number of these groups is not limited. However, compounds with a structure limited to one hydroxyl group may be used as the negative electrode active material of a redox flow battery. Active materials with such structures have a simpler structure than active materials with multiple hydroxyl groups, making their synthesis and raw material procurement easier. Furthermore, in alkaline electrolytes, the hydroxyl groups of the active material function as acids, neutralizing the alkali in the electrolyte. Therefore, an excess amount of alkali must be added to the electrolyte to neutralize it. However, an active material with fewer hydroxyl groups is advantageous over an active material with two or more hydroxyl groups in that the amount of alkali added to the electrolyte can be reduced.

[0017] Furthermore, a compound having a structure in which the number of hydroxyl groups and the number of alkoxy groups are limited to one may be used as the negative electrode active material of a redox flow battery. This active material has a simpler structure than an active material having multiple hydroxyl groups and multiple alkoxy groups, making it easier to synthesize and procure raw materials.

[0018] Among compounds with a structure limited to one hydroxyl group and one alkoxy group, a structure in which a hydroxyl group is bonded to the 2-position and an alkoxy group is bonded to the 6-position is preferred. The raw material for synthesizing this compound is 2,6-dihydroxyanthraquinone (2,6-DHAQ), which is mass-produced industrially, making it easy to procure. This compound can be synthesized from 2,6-DHAQ by reacting it with an organic alkylating agent (RX) in the presence of a base or acid, as shown in the following chemical reaction formula (2). In chemical reaction formula (2), R is any alkyl group, and X is any leaving group, such as halogen, tosylate, mesylate, sulfonate, phosphate, or 1-imino-2-(trichloro)ethyloxy. Furthermore, bases such as NaH, NaOH, KOH, and K2CO3 can be used. Furthermore, alkoxides, triethylamine, diisopropylethylamine, diazabicycloundecene, and the like can also be used.

[0019] [ka]

[0020] In addition, the alkoxy group attached to the 6th position is replaced by O(CH2) n It may also be COOH (n is a natural number of 1 to 6). When the active material has a carboxyl group, the solubility in the electrolyte on the negative electrode side can be improved.

[0021] A compound with a structure in which the number of hydroxyl groups is limited to two but the number of alkoxy groups is not limited (however, there must be one or more alkoxy groups) may be used as the negative electrode active material for a redox flow battery. In an active material with this structure, the electron donating properties of the two hydroxyl groups are combined to further improve the electron density of the anthraquinone skeleton, shifting the redox potential to a lower potential, thereby increasing the cell voltage of the redox flow battery.

[0022] Furthermore, a compound having a structure in which the number of hydroxyl groups and the number of alkoxy groups are limited to two may be used as the negative electrode active material of a redox flow battery. An active material having such a structure has a simpler structure than an active material having three or more hydroxyl groups or three or more alkoxy groups, making it easier to synthesize the active material and to procure the raw materials.

[0023] In compounds with a structure limited to two hydroxyl groups and two alkoxy groups, two of R2, R3, R6, and R7 are hydroxyl groups and the remaining two are alkoxy groups. The raw material for synthesizing this compound is 2,3,6,7-tetrahydroxyanthraquinone (2,3,6,7-THAQ), which can be synthesized in high yield and is therefore easily procured. This compound can be synthesized from 2,3,6,7-THAQ by reacting it with an organic alkylating agent (RX) in the presence of a base, as shown in the following chemical reaction formula (3).

[0024] [ka]

[0025] In a compound in which two of R2, R3, R6, and R7 are hydroxyl groups and the remaining two are alkoxy groups, each of the two alkoxy groups is an O(CH2) n It may be COOH (n is a natural number from 1 to 6). Compounds with this structure can have the following three structures: compounds (4) to (6). The active material has a carboxyl group, which can improve the solubility in the electrolyte on the negative electrode side. For example, when n=3, the active material exhibits a suitable solubility of 0.6M / 1M-KOH in an alkaline electrolyte.

[0026] [ka]

[0027] [ka]

[0028] [ka]

[0029] The active material is not limited to the compound (first compound) having the above structure, but may be a mixture of the first compound and a second compound represented by the following chemical formula (7).

[0030] [ka]

[0031] The second compound is a compound represented by chemical formula (7) in which at least one of R1' to R8' bonded to positions 1 to 8 of the anthraquinone skeleton is a hydroxyl group and the rest are hydrogen atoms, or a compound represented by chemical formula (7) in which at least one of R1' to R8' is an alkoxy group and the rest are hydrogen atoms, or a mixture thereof. Three examples of mixtures of the first compound and the second compound are shown in the following chemical formulas (8) to (10).

[0032] [ka]

[0033] [ka]

[0034] [ka]

[0035] When an active material is used as a mixture of the first and second compounds having the above structure, the cell voltage and the viscosity of the electrolyte can be adjusted by adjusting the mixing ratio of anthraquinones having different properties.

[0036] In order to obtain the above-mentioned effects, the ratio of the mass of the first compound to the mass of the active material is preferably 0.3 or more, more preferably 0.4 or more, even more preferably 0.5 or more, even more preferably 0.6 or more, even more preferably 0.7 or more, even more preferably 0.8 or more, even more preferably 0.9 or more, even more preferably 0.95 or more, and most preferably 0.99 or more.

[0037] As exemplified by the following chemical formula (11), the first compound preferably has a structure in which, when the molecule of the first compound is inverted with respect to the center C of the central six-membered ring of the anthraquinone skeleton, the combination of R1 to R8 differs from that before inversion. That is, the first compound is preferably an anthraquinone that does not have i-symmetry. Because the molecule of the first compound having such a structure has a dipole moment, polarization occurs within the molecule, improving solubility in a polar electrolyte solution and enabling the capacity density of the redox flow battery to be improved.

[0038] [ka]

[0039] If this compound containing sulfo groups is dissolved in a neutral to alkaline electrolyte and used as an active material, the redox potential of the active material may increase, the cell voltage may decrease, and the voltage efficiency may decrease, so it is preferable that the compound represented by chemical formula (1) does not contain sulfo groups. However, if the compound represented by chemical formula (1) contains hydroxyl groups, the redox potential tends to decrease due to this effect, so as long as the number of sulfo groups is less than the number of hydroxyl groups, the compound represented by chemical formula (1) can contain such a compound. [Example]

[0040] In Example 1, which used potassium ferrocyanide trihydrate and potassium ferricyanide as the positive electrode active material and (2-(3'-carboxypropyloxy)-6-hydroxy-9,10-anthraquinone (2,6-MHMBEAQ) as the negative electrode active material, the cell voltage and the rate of capacity decrease during discharge in a Redox flow battery were measured.

[0041] The positive electrode electrolyte was prepared by dissolving 5.76 g (13.6 mmol) of potassium ferrocyanide trihydrate and 1.80 g (5.45 mmol) of potassium ferricyanide in 1.0 mol / L aqueous potassium hydroxide solution and making up to 68.2 mL. The negative electrode electrolyte was prepared by dissolving 0.816 g (2.5 mmol) of 2,6-MHMBEAQ in 1 mol / L aqueous potassium hydroxide solution and making up to 25 mL.

[0042] 2,6-MHMBEAQ was synthesized according to the procedure shown in the following chemical reaction formula (12). In summary, the synthesis begins with 2,6-DHAQ as the starting material, followed by the synthesis of an intermediate having an alkoxy group in which the hydrogen of one hydroxyl group is replaced by ethyl butanoate, and then from this intermediate the target substance, 2,6-MHMBEAQ, is synthesized.

[0043] [ka]

[0044] A 1-L recovery flask was charged with 40.0 g (167 mmol) of 2,6-DHAQ (Tokyo Chemical Industry Co., Ltd.) and 500 mL of N,N-dimethylformamide (DMF). While stirring, 23.1 g (167 mmol) of potassium carbonate was added, followed by 23.9 mL (167 mmol) of ethyl 4-bromobutanoate. The mixture was then heated and stirred at 100°C for 17 hours. After cooling, 600 mL of distilled water was added, and the precipitate was suction filtered. The filter cake was washed with distilled water. While stirring, 6 M hydrochloric acid was added to the filtrate (pH > 9). Hydrochloric acid was added until the pH of the filtrate became less than 3 and no carbon dioxide was generated upon addition of the hydrochloric acid. The mixture was then stirred at room temperature for 1 hour. The precipitate was transferred to a 200 mL centrifuge tube and centrifuged to separate the precipitate. The precipitate was filtered under suction, washed with distilled water, and then dried under vacuum at 80°C for 6 hours, yielding 11.4 g of a mixture of raw materials and intermediates. The resulting solid was pulverized into powder and suspended in 200 mL of chloroform. Insoluble material was removed by suction filtration, and the mixture was washed with 200 mL of chloroform until all soluble material was dissolved. This procedure yielded 11.1 g of unreacted raw materials. The filtrate was again filtered under suction to completely remove insoluble material, and the filtrate was concentrated under reduced pressure. The residue was suspended in distilled water, filtered under suction, washed, and dried under vacuum at 80°C for 4 hours, yielding 6.96 g of the intermediate as a reddish-brown solid (yield: 12%).

[0045] Next, 6.96 g (19.6 mmol) of the intermediate was placed in a 1 L recovery flask, along with 190 mL of isopropyl alcohol and 380 mL of distilled water. 4.48 g (79.9 mmol) of potassium hydroxide was added, and the mixture was heated and stirred at 60 °C for 20 hours. After cooling, 550 mL of distilled water was added, and the mixture was transferred to a 2 L Erlenmeyer flask. 2 M hydrochloric acid was added with stirring until the pH was below 100. After stirring for 2 hours, the precipitate was separated by centrifugation. The supernatant and precipitate were each filtered with suction, and the residue was washed with distilled water. The residue was dried under vacuum at 80 °C for 4 hours to obtain 6.25 g of the target product (98% yield from the intermediate).

[0046] The redox flow battery used in the measurements was one manufactured by the inventors of the present disclosure. This redox flow battery has a configuration in which a positive electrode cell and a negative electrode cell are separated by an ion exchange membrane (Nafion (registered trademark), NR-212). Each cell has a 21 mm x 21 mm serpentine flow path formed as a flow path for the electrolyte. Each cell is equipped with a porous electrode (20 mm x 20 mm) made of carbon paper.

[0047] Each electrolyte was placed in a Schlenk flask, and dissolved oxygen was removed by bubbling inert gas (nitrogen) for at least 5 minutes. Each Schlenk flask was kept at 30°C using an aluminum block thermostatic bath (ALB-121, Synix Co., Ltd.). Using a pump (Smoothflow Pump QI-100-VF-PS, Takunami Co., Ltd.), each electrolyte was circulated through the flow path of each cell at 65 mL / min between each cell and each Schlenk flask.

[0048] A charge / discharge device (ACD-01, Asuka Electronics Co., Ltd.) was electrically connected to the current collector (a carbon separator made by the inventors of the present disclosure using conductive carbon resin) provided on each cell with a cable, and a current value of 400 mA (current density 100 mA / cm) was applied. 2 ) and a charge-discharge cycle was carried out in which constant current charging with an upper cutoff voltage set to 1.4 V and constant current discharging with a lower cutoff voltage set to 0.6 V were repeated.

[0049] During this cycle, the voltage (cell voltage) between the positive electrode and the negative electrode during discharge was measured, and the discharge capacity was measured for each cycle. The results of the discharge capacity measurements for each cycle are shown in Figure 1. The relative capacity on the vertical axis of Figure 1 is the ratio of the discharge capacity for each cycle to the discharge capacity for the second cycle. The rate of capacity decline was calculated from the rate of change between the capacity value for the second cycle and the capacity value for the 3834th cycle using the following formula, where n = 3834. Capacity reduction rate=(1-(Dn / D2) 1 / t ) x 100 Dn: Relative capacity at the nth cycle D2: Relative capacity at the second cycle (D2=1) n: number of cycles t: number of days elapsed

[0050] In the aforementioned Patent Document 2, the cell voltages of redox flow batteries in which potassium ferrocyanide was used as the positive electrode active material and 2,6-DHAQ and 2,6-DBEAQ were used as the negative electrode active materials (referred to as Comparative Examples 1 and 2, respectively) are listed in Table S2, and the rate of capacity decline during discharge is listed in Figure S7. The cell voltages and rate of capacity decline during discharge for Example 1 and Comparative Examples 1 and 2 are shown in Table 1 below.

[0051] [Table 1]

[0052] Comparing Comparative Example 1 and Comparative Example 2, the former has a higher cell voltage but a higher rate of capacity loss during discharge than the latter. That is, having only hydroxyl groups results in a higher cell voltage but a higher rate of capacity loss during discharge, while having only alkoxy groups results in a lower rate of capacity loss during discharge but a lower cell voltage. In contrast, by having one hydroxyl group and one alkoxy group as in Example 1, the cell voltage and rate of capacity loss during discharge are between those of Comparative Example 1 and Comparative Example 2. As a result, Example 1 can be said to have a better balance between the cell voltage and rate of capacity loss during discharge than Comparative Examples 1 and 2, and can be said to be able to provide an appropriate cell voltage for the redox flow battery and suppress the rate of capacity loss during discharge.

[0053] The contents described in each of the above embodiments can be understood, for example, as follows.

[0054] [1] An anthraquinone active material according to one embodiment is An anthraquinone active material for a redox flow battery, comprising a first compound represented by the following chemical formula: [ka] At least one of R1 to R8 is a hydroxyl group, and at least one is an alkoxy group.

[0055] According to the anthraquinone active material of the present disclosure, the presence of a hydroxyl group makes the cell voltage of the redox flow battery appropriate, and the presence of an alkoxy group makes it possible to suppress a decrease in capacity during discharge of the redox flow battery, thereby achieving a good balance between the cell voltage and suppression of a decrease in capacity during discharge.

[0056] [2] An anthraquinone active material according to another embodiment is the anthraquinone active material according to [1], Only one of R1 to R8 is a hydroxyl group.

[0057] According to this seventh aspect, the structure of the active material is simpler than that of an active material having a plurality of hydroxyl groups, making it easier to synthesize the active material and to procure the raw materials.

[0058] [3] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to [2], Only one of R1 to R8 is an alkoxy group.

[0059] According to this configuration, the structure of the active material is simpler than that of an active material having a plurality of hydroxyl groups and a plurality of alkoxy groups, and the synthesis method and procurement of raw materials are easier.

[0060] [4] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to [1], Only two of the R1 to R8 are hydroxyl groups.

[0061] With this configuration, the electron-donating properties of the two hydroxyl groups are combined to further improve the electron density of the anthraquinone skeleton, shifting the redox potential to a lower potential, thereby increasing the cell voltage of the redox flow battery.

[0062] [5] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to [4], Only two of the R1 to R8 are alkoxy groups.

[0063] According to this configuration, the structure of the active material is simpler than that of an active material having three or more hydroxyl groups or alkoxy groups, making it easier to synthesize the active material and to procure the raw materials.

[0064] [6] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to [5], Two of R2, R3, R6, and R7 are hydroxyl groups, and the remaining two are alkoxy groups.

[0065] According to this configuration, the raw material for synthesizing this active material is 2,3,6,7-tetrahydroxyanthraquinone, which can be synthesized with a high yield, making it easy to procure the raw material.

[0066] [7] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to any one of [1] to [6], The R2 is a hydroxyl group, and the R6 is an alkoxy group.

[0067] According to this configuration, the raw material for synthesizing this active material is 2,6-dihydroxyanthraquinone, which is mass-produced industrially, making it easy to procure the raw material.

[0068] [8] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to any one of [1] to [7], The alkoxy group is O(CH2) n COOH (n is a natural number from 1 to 6).

[0069] According to this configuration, the presence of a carboxyl group can improve the solubility of the active material in the electrolyte solution.

[0070] [9] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to any one of [1] to [8], When the molecule of the first compound is inverted about the center of the central six-membered ring of the anthraquinone skeleton, the first compound has a structure in which the combination of R1 to R8 is different from that before inversion.

[0071] According to this configuration, the molecules of the first compound have a dipole moment, which causes polarization within the molecules, improving the solubility in a polar electrolyte solution and enabling the capacity density of the redox flow battery to be improved.

[0072]

[10] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to any one of [1] to [9], The first compound does not contain a sulfo group.

[0073] When the first compound contains a sulfo group, dissolving the first compound in a neutral to alkaline electrolyte and using it as an active material may increase the redox potential of the active material, resulting in a decrease in cell voltage and voltage efficiency. On the other hand, if the first compound does not contain a sulfo group, these problems can be avoided.

[0074]

[11] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to any one of [1] to [9], the first compound comprises a sulfo group; The number of the sulfo groups is less than the number of the hydroxyl groups.

[0075] If the first compound has hydroxyl groups, the oxidation-reduction potential tends to decrease due to this effect. Therefore, if the number of sulfo groups is less than the number of hydroxyl groups, the inconvenience caused by the active material having sulfo groups can be suppressed.

[0076]

[12] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to any one of [1] to

[11] , Further comprising a second compound represented by the following chemical formula: [ka] The second compound is a compound in which at least one of R1' to R8' is a hydroxyl group and the rest are hydrogen atoms, or a compound in which at least one of R1' to R8' in the chemical formula is an alkoxy group and the rest are hydrogen atoms, or a mixture thereof.

[0077] According to this configuration, the cell voltage and the viscosity of the electrolyte can be adjusted by adjusting the mixing ratio of anthraquinones having different properties.

Claims

1. An anthraquinone active material for a redox flow battery, comprising a first compound represented by the following chemical formula: 【Chemistry 1】 The R １ ~R ８ An anthraquinone active material in which R 2 is a hydroxyl group and R 6 is an alkoxy group.

2. The R １ ~R ８ 2. The anthraquinone active material according to claim 1, wherein only one of the groups is a hydroxyl group.

3. The R １ ~R ８ The anthraquinone active material according to claim 2 , wherein only one of the groups is an alkoxy group.

4. The R １ ~R ８ 2. The anthraquinone active material according to claim 1, wherein only two of the groups are hydroxyl groups.

5. The R １ ~R ８ The anthraquinone active material according to claim 4 , wherein only two of the groups are alkoxy groups.

6. The R ２ , R ３ , R ６ , R ７ 6. The anthraquinone active material according to claim 5, wherein two of the groups are hydroxyl groups and the remaining two are alkoxy groups.

7. The alkoxy group is O(CH ２ ) ｎ The anthraquinone active material according to any one of claims 1 to 6, which is COOH (n is a natural number from 1 to 6).

8. When the molecule of the first compound is inverted with respect to the center of the central six-membered ring of the anthraquinone skeleton, R １ ~R ８ The anthraquinone active material according to any one of claims 1 to 6, wherein the combination of (a) and (b) has a structure different from that before the inversion.

9. 7. The anthraquinone active material according to claim 1, wherein the first compound does not contain a sulfo group.

10. the first compound comprises a sulfo group; 7. The anthraquinone active material according to claim 1, wherein the number of said sulfo groups is smaller than the number of said hydroxyl groups.

11. Further comprising a second compound represented by the following chemical formula: 【Chemistry 2】 The second compound is １ '~R ８ a compound in which at least one of R ' is a hydroxyl group and the rest are hydrogen atoms, or the R １ '~R ８ 7. The anthraquinone active material according to claim 1, wherein at least one of the groups represented by the formula (I) and the group represented by the formula (II) is an alkoxy group and the remaining groups are hydrogen atoms, or a mixture thereof.

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