JP7798284B2 - Anthraquinone active materials - Google Patents

Description

This disclosure relates to anthraquinone active materials for redox flow batteries.

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 sources. Redox flow batteries consist of cells that charge and discharge, and an electrolyte tank that stores power, and are characterized by the fact that charging and discharging is carried out by circulating the electrolyte with a pump.

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

Patent No. 6574382 Special table 2019-514170 publication

K. Lin, Q. Chen, M. R. Gerhardt, L. Tong, S. B. Kim, L. Eisenach, A. W. Valle, D. Hardee, R.G. Gordon, M.J. Aziz, M. P. Marshak, Science, 349 (2015) 1529-1532 D. G. Kwabi, K. Lin, Y. Ji. F. Kerr, M. Goulet, D. D. Porcellinis, D. P. Tabor, D. A. Pollack, A. Aspuru-Guzik, R. G. Gordon, M. J. Aziz, Joule 2, 19 (2018) 1894-1906

Non-patent documents 1 and 2 describe compounds in which various functional groups and elements are bonded to the 1st to 8th positions of anthraquinone. However, when active materials with specific substituents introduced at specific positions are used in redox flow batteries, the overvoltage (resistance) of the redox flow battery increases, resulting in reduced energy efficiency.

In light of the above circumstances, at least one embodiment of the present disclosure aims to provide an anthraquinone active material that can reduce the overvoltage of a redox flow battery.

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, comprising a compound represented by the following chemical formula:
One of R2 and R6 _is a hydroxyl group and the other is an alkoxy group, and R1 _, R4 _, R5 _and R8 _are functional groups other than a hydroxyl group and an alkoxy group, hydrogen or halogen _.

Research by the inventors of the present disclosure has shown that when a compound with a structure in which a hydroxyl group or an alkoxy group is introduced at the 1st, 4th, 5th, or 8th position of the anthraquinone skeleton is used as the negative electrode active material of a redox flow battery, the overvoltage tends to be high. In contrast, when the anthraquinone active material of the present disclosure is used, the hydroxyl group or alkoxy group is bonded to the 2nd, 3rd, 6th, or 7th position of the anthraquinone skeleton, which is thought to result in weaker coordination with potassium ions than coordination with active materials of the former structure, and therefore the overvoltage of the redox flow battery can be reduced compared to when an active material of the former structure is used.

1 is a graph showing the measurement results of the overvoltage of redox flow batteries using the compounds of each of the Examples and Comparative Examples as anthraquinone-based active materials in the negative electrodes.

The following describes an anthraquinone active material (hereinafter, simply referred to as "active material" unless there is a specific need to add "anthraquinone") according to an embodiment of the present disclosure. The embodiment described below represents one aspect of the present disclosure and does not limit the disclosure, and can be modified as desired within the scope of the technical concept of the present disclosure.

<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 the compound is converted into the reduced form.

In chemical formula (1), of R ₁ to R ₈ bonded to positions 1 to 8 of the anthraquinone skeleton, at least one of R ₂ , R ₃ , R ₆ , and R ₇ is a hydroxyl group or 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 sulfone group, amino group, nitro group, carboxyl group, phosphoryl group, thiol group, or alkyl ester, instead of hydrogen.

As will be specifically clarified in the examples described below, research by the inventors of the present disclosure has shown that when a compound having a structure in which a hydroxyl group or an alkoxy group is introduced at the 1st, 4th, 5th, or 8th position of the anthraquinone skeleton is used as the negative electrode active material in a redox flow battery, the overvoltage tends to be large.

<Variations of the active material of the present disclosure>
In the above basic structure of the active material of the present disclosure, it is sufficient that at least one of R ₂ , R ₃ , R ₆ , and R ₇ is a hydroxyl group or an alkoxy group, and there are no restrictions on how many of each functional group are bonded. However, a compound having a structure in which one of R ₂ or R ₆ is a hydroxyl group and the other is an alkoxy group may be used as the negative electrode active material of a redox flow battery. As will be made clear in the examples described below, research by the inventors of the present disclosure has shown that when a compound with such a structure is used as the active material of a redox flow battery, the overvoltage of the redox flow battery tends to be extremely small. Therefore, the use of a compound with such a structure as the active material can reduce the overvoltage of the redox flow battery.

A compound having a structure in which each of _R2 , _R3 , _R6 , and _R7 is either a hydroxyl group or an _alkoxy group may be used as the negative electrode active material of a redox flow battery. As will be apparent from the examples described below, research by the inventors of the present disclosure has shown that when a compound having _this structure is used as the active material of a redox flow battery, the effect of reducing overvoltage is somewhat inferior compared to when a compound having a structure in which one of R2 or R6 is a hydroxyl group and the other is an alkoxy group is used as the active material of a redox flow battery. However, the effect of reducing overvoltage is still observed compared to a compound that does not have a structure in which at least one of _R2 , _R3 , _R6 , and _R7 is a hydroxyl group or an alkoxy group. Therefore, when a compound having such a structure is used as the active material, the overvoltage of the redox flow battery can be reduced.

The alkoxy group bonded to at least one of R ₂ , R ₃ , R ₆ , and R ₇ may be O(CH ₂ ) _n COOH (n is a natural number of 1 to 6) having a carboxyl group. When the active material has a carboxyl group, the solubility in the electrolyte on the negative electrode side can be improved.

For any of the above compounds, if a hydroxyl group is bonded to the 1st, 4th, 5th, or 8th position of the anthraquinone skeleton, there is a possibility that the above-mentioned six-membered ring structure will be formed with the potassium ion in the electrolyte. Therefore, it is preferable that no hydroxyl group is bonded to the 1st, 4th, 5th, or 8th position of the anthraquinone skeleton, and it is preferable that a functional group other than a hydroxyl group or an alkoxy group, or hydrogen or halogen is bonded to these positions.

<Outline of the Example>
The overvoltage in a Redox flow battery was measured when potassium ferrocyanide trihydrate and potassium ferricyanide were used as the positive electrode active material and the compounds of Examples 1 to 6 and Comparative Examples 1 and 2 listed in Table 1 below were used as the negative electrode active material.

<Methods for obtaining and synthesizing each compound>
The compound of Example 1 (2,6-dihydroxyanthraquinone (2,6-DHAQ)) is available from Tokyo Chemical Industry Co., Ltd. under the product code A1894. The compound of Example 2 (2,6-bis(3'-carboxypropyloxy)-9,10-anthraquinone) is available from Tokyo Chemical Industry Co., Ltd. under the product code D5764.

The compound of Example 3 was synthesized using the procedure shown in the following chemical reaction formula (2). In summary, the synthesis begins with 2,6-DHAQ as the starting material, followed by the synthesis of an intermediate substance having an alkoxy group in which the hydrogen of one hydroxyl group is replaced with ethyl butanoate, and then the compound of Example 3 is synthesized from this intermediate substance.

40.0 g (167 mmol) of 2,6-DHAQ and 500 mL of N,N-dimethylformamide (DMF) were placed in a 1 L recovery flask. 23.1 g (167 mmol) of potassium carbonate was added while stirring, 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 and washed with distilled water. 6 M hydrochloric acid was added to the filtrate (pH > 9) while stirring. Hydrochloric acid was added until the pH of the filtrate was less than 3 and no carbon dioxide was generated upon addition of the hydrochloric acid, followed by stirring 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 suction filtered, washed with distilled water, and then vacuum dried at 80°C for 6 hours, yielding 11.4 g of a mixture of raw materials and intermediates. The resulting solid was pulverized into a powder and suspended in 200 mL of chloroform. Insoluble matter was removed by suction filtration, and the mixture was washed with 200 mL of chloroform until all soluble matter was dissolved. This procedure recovered 11.1 g of unreacted raw material. The filtrate was again suction filtered to completely remove insoluble matter, and the filtrate was concentrated under reduced pressure. The residue was suspended in distilled water, suction filtered, washed, and vacuum dried at 80°C for 4 hours, yielding 6.96 g of the intermediate product as a reddish-brown solid (yield: 12%).

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 reached less than 3. After stirring for 2 hours, the precipitate was separated by centrifugation. The supernatant and precipitate were each filtered under suction, and the residue was washed with distilled water. The residue was vacuum dried at 80°C for 4 hours to obtain 6.25 g of the compound of Example 3 (yield from the intermediate: 98%).

The compound of Example 4, 2,3,6,7-tetrahydroxyanthraquinone (2,3,6,7-THAQ), is synthesized from the first step of the following chemical reaction formula (3), the second step of the following chemical reaction formula (4), and the third step of the following chemical reaction formula (5).

A 500 mL beaker was charged with 42 g of ice and 100 mL of concentrated sulfuric acid. While carefully preventing the temperature of the reaction solution from exceeding 5°C, a mixed solution of 25.1 g (182 mmol) of 1,2-dimethoxybenzene (available from Tokyo Chemical Industry Co., Ltd.) and 17.3 mL (309 mmol) of acetaldehyde was added dropwise to the stirred reaction solution over 2.5 hours, followed by stirring at room temperature for 22 hours. The reaction solution was poured into a 1000 mL Erlenmeyer flask containing 350 mL of ethanol and washed with 60 mL of methanol. The precipitate was filtered under suction, washed with 160 mL of ethanol and 320 mL of distilled water, and then vacuum-dried at 60°C for 5 hours to obtain 22.3 g of a white solid (75% yield for chemical reaction equation (3)).

15.1 g (46.3 mmol) of the above white solid was placed in a 1 L recovery flask and suspended in 750 mL of acetic acid, followed by the addition of 85.4 g (287 mmol) of sodium dichromate dihydrate. The reaction solution was heated under reflux in an oil bath for 5 hours. After the reaction, the solution was allowed to cool and stand, and the resulting precipitate was filtered by suction. The residue was washed with distilled water and vacuum-dried at 70°C for 4 hours to yield 12.4 g of a yellow solid (yield of 82% for chemical reaction formula (4)).

18.8 g (57.3 mmol) of the above yellow solid was placed in a 1 L recovery flask, suspended in 250 mL of 47% hydrobromic acid, and heated to reflux in an oil bath at 150°C for 6 days. Over the course of 6 days, 90 mL of 47% hydrobromic acid was added. The reaction solution was allowed to cool, then transferred to a settling tube and centrifuged to remove the supernatant. 400 mL of distilled water was added to the residue, which was dispersed, and the mixture was centrifuged again to remove the supernatant. The insoluble matter was removed by suction filtration and washed with distilled water. The residue was vacuum-dried at 70-80°C for 13 hours to obtain 15.1 g of the compound of Example 4 (yield of 97% for chemical reaction formula (5)).

The compound of Example 5 is synthesized from the first step of the following chemical reaction formula (6) and the second step of the following chemical reaction formula (7).

8.13 g (29.9 mmol) of 2,3,6,7-THAQ and 400 mL of DMF were placed in a 1 L recovery flask. 18.26 g (217 mmol) of potassium ethoxide was added, and the mixture was heated to 65°C while stirring. 42.1 g (305 mmol) of potassium carbonate and 43.8 mL (306 mmol) of ethyl 4-bromobutanoate were then added, and the mixture was stirred at 95°C for 24 hours. After cooling, 240 mL of distilled water was added, and the precipitate was suction filtered. The residue was washed with distilled water and vacuum dried at 80°C for 1.5 hours, yielding 6.70 g of a yellow solid (yield of 31% for chemical reaction equation (6)).

6.70 g (9.19 mmol) of the above yellow solid was placed in a 500 mL recovery flask, followed by 100 mL of isopropyl alcohol and 200 mL of distilled water. 6.70 g (119 mmol) of potassium hydroxide was added, and the temperature was raised. The mixture was heated and stirred at 60°C for 18 hours. After cooling, 300 mL of distilled water was added, and insoluble matter was removed by suction filtration. 6 M hydrochloric acid was added with stirring until the pH was below 3. After stirring for 1 hour, the precipitate was separated by centrifugation. The precipitate was washed with distilled water and recovered by suction filtration. The residue was vacuum-dried at 70°C for 2.5 hours to obtain 5.08 g of the desired product (yield of chemical reaction formula (6) is 90%).

The mixture of Example 6 was synthesized using the procedure shown in the following chemical reaction formula (8). The synthesis is outlined as follows: From 2,3,6,7-THAQ as the starting material, an intermediate mixture is obtained in which the hydrogen atoms of two hydroxyl groups are replaced by ethyl butanoate, resulting in an alkoxy group. From this intermediate mixture, the mixture of Example 6 is obtained.

19.8 g (72.7 mmol) of 2,3,6,7-THAQ and 280 mL of DMF were placed in a 1 L recovery flask. 19.9 g (144 mmol) of potassium carbonate and 20.7 mL (144 mmol) of ethyl 4-bromobutanoate were then added. The mixture was then heated and stirred at 100°C for 23 hours. After cooling, 150 mL of distilled water was added and the precipitate was collected by suction filtration. 6 M hydrochloric acid was added to the filtrate with stirring until the pH reached approximately 3-4, and the precipitate was collected by centrifugation and suction filtration. This precipitate was subjected to Soxhlet extraction using chloroform. The extract was concentrated under reduced pressure to obtain 5.04 g of an intermediate mixture (yield: 14%).

5.15 g of the intermediate mixture was placed in a 500 mL recovery flask, to which 90 mL of isopropyl alcohol and 180 mL of distilled water were added. An additional 4.62 g (82.3 mmol) of potassium hydroxide was then added, and the mixture was heated and stirred at 60°C for 20 hours. After cooling, the mixture was poured into a 1 L beaker containing 300 mL of distilled water, and 2 M hydrochloric acid was added with stirring until the pH was below 3. After stirring for 1 hour, the precipitate was separated by centrifugation. This precipitate was washed with distilled water and recovered by suction filtration. The residue was vacuum dried at 70°C for 2.5 hours to obtain 3.71 g of the mixture of Example 6 (81% yield of the mixture from the intermediate mixture).

The compound of Comparative Example 1 (1,3,5,7-tetrahydroxyanthraquinone (1,3,5,7-THAQ)) was synthesized according to the procedure shown in the following chemical reaction equation (9). 3.00 g (19.5 mmol) of 3,5-dihydroxybenzoic acid (available from Tokyo Chemical Industry Co., Ltd.) and 39 mL of concentrated sulfuric acid were placed in a 100 mL recovery flask and stirred at 120°C for 2 hours. After cooling, the reaction solution was poured into a 300 mL beaker containing 100 g of ice, centrifuged, and the supernatant was removed by decantation. The residue was diluted with 100 mL of distilled water and subjected to suction filtration. The residue was washed with distilled water and vacuum dried at 75°C for 2 hours to obtain 2.15 g of the desired product (yield: 81%).

The compound of Comparative Example 2 is synthesized from the compound of Comparative Example 1 as the starting material through the first step of the following chemical reaction formula (10) and the second step of the following chemical reaction formula (11).

500 mg (1.84 mmol) of 1,3,5,7-THAQ and 25 mL of DMF were placed in a 100 mL recovery flask. 1.23 g (14.6 mmol) of potassium ethoxide was added and stirred for 20 minutes. 2.87 g (20.8 mmol) of potassium carbonate and 2.63 mL (18.3 mmol) of ethyl 4-bromobutanoate were then added and stirred at 95°C for 20 hours. After cooling, 15 mL of distilled water was added and the precipitate was suction filtered. The residue was washed with distilled water and vacuum dried at 60°C for 3 hours to obtain 889 mg of a yellow solid (yield of 90% for chemical reaction equation (10)).

870 mg (1.19 mmol) of the above yellow solid was placed in a 200 mL recovery flask, followed by 25 mL of isopropyl alcohol and 50 mL of distilled water. 838 mg (14.9 mmol) of potassium hydroxide was added, and the temperature was raised. The mixture was heated and stirred at 60°C for 17 hours. After cooling, the reaction solution was poured into a 300 mL beaker containing 150 mL of distilled water, and insoluble matter was removed by suction filtration. 2 mL of acetic acid was added to the filtrate to adjust the pH to less than 4, and the mixture was stirred at room temperature for 30 minutes. The precipitate was filtered by suction and washed with water, and then vacuum-dried at 60°C for 3 hours to obtain 714 mg of the desired product (yield of chemical reaction formula (11) is 97%).

<Preparation of Electrolyte>
The positive electrode electrolyte was prepared by dissolving 2.53 g (6.00 mmol) of potassium ferrocyanide trihydrate in a 1.0 mol/L aqueous potassium hydroxide solution and making up to 30 mL. The negative electrode electrolyte was prepared by dissolving the compounds or mixtures of Examples 1 to 6 and Comparative Examples 1 and 2 in the amounts shown in Table 1 in a 1.0 mol/L aqueous potassium hydroxide solution and making up to 25 mL.

<Configuration of the experimental equipment>
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 provided with a porous electrode (20 mm x 20 mm) made of carbon paper.

<Experimental Method>
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 Corporation). Using a pump (Smoothflow Pump QI-100-VF-P-S, Takunami Corporation), each electrolyte was passed through the flow path of each cell at 65 mL/min, circulating between each cell and each Schlenk flask.

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 via a cable, and the cells were charged to 50% of their theoretical capacity by constant-current, constant-voltage charging at a current value of 400 mA (current density 100 mA/cm ² ), an upper limit voltage of 1.4 V, and a cutoff current density of 2 mA/cm ² . Charging was performed at a current density of 33 mA/cm ² , and the voltage obtained 1 minute after current application was recorded and used as the charge voltage. The absolute value of the difference between the open-circuit voltage (OCV) at a state of charge (SOC) of 50% and the charge voltage was defined as the overvoltage during charging. For discharge, the voltage 1 minute after current application was defined as the discharge voltage, and the difference from the OCV was defined as the overvoltage during discharge. The overvoltage during charging and discharge was averaged to determine the overvoltage during charging and discharging.

<Experimental Results>
The results of the overvoltage measurements are shown in Figure 1. Compared to the compounds of Comparative Examples 1 and 2 in which hydroxyl groups are bonded to the 1- and 5-positions of the anthraquinone skeleton, the compounds of Examples 1 to 3 in which either hydroxyl groups or alkoxy groups are bonded to the 2- and 6-positions of the anthraquinone skeleton and the compounds of Examples 4 to 6 in which either hydroxyl groups or alkoxy groups are bonded to the 2-, 3-, 6-, and 7-positions of the anthraquinone skeleton showed lower overvoltages. From these results, it can be said that by using a compound in which a hydroxyl group or an alkoxy group is bonded to the 2-, 3-, 6-, or 7-position of the anthraquinone skeleton as the active material for the negative electrode of a redox flow battery, the overvoltage of the redox flow battery can be reduced. It can also be said that the effect of a compound in which either a hydroxyl group or an alkoxy group is bonded to the 2- and 6-positions of the anthraquinone skeleton is greater than that of a compound in which either a hydroxyl group or an alkoxy group is bonded to the 2-, 3-, 6-, or 7-position of the anthraquinone skeleton.

The contents described in each of the above embodiments can be understood, for example, as follows:

[1] An anthraquinone active material according to one embodiment is
An anthraquinone active material for a redox flow battery, comprising a compound represented by the following chemical formula:
At least one of R ₂ , R ₃ , R ₆ and R ₇ is a hydroxyl group or an alkoxy group.

Research by the inventors of the present disclosure has shown that when a compound with a structure in which a hydroxyl group or an alkoxy group is introduced at the 1st, 4th, 5th, or 8th position of the anthraquinone skeleton is used as the negative electrode active material of a redox flow battery, the overvoltage tends to be high. In contrast, when the active material of the present disclosure is used, the hydroxyl group or alkoxy group is bonded to the 2nd, 3rd, 6th, or 7th position of the anthraquinone skeleton, which is thought to result in weaker coordination with potassium ions than coordination with active materials of the former structure, and therefore the overvoltage of the redox flow battery can be reduced compared to when an active material of the former structure is used.

[2] An anthraquinone active material according to another embodiment is the anthraquinone active material according to [1],
One of _R2 and _R6 is a hydroxyl group, and the other is an alkoxy group.

Research by the inventors of the present disclosure has shown that when a compound in which a hydroxyl group is introduced into one of the 2- and 6-positions of the anthraquinone skeleton and an alkoxy group is introduced into the other is used as the active material of a redox flow battery, the overvoltage of the redox flow battery tends to be extremely small. Therefore, using a compound with such a structure as the active material can reduce the overvoltage of the redox flow battery.

[3] An anthraquinone active material according to yet another embodiment is the anthraquinone active material according to [1] or [2],
Each of R ₂ , R ₃ , R ₆ and R ₇ is either a hydroxyl group or an alkoxy group.

According to research by the inventors of the present disclosure, when a compound with this structure is used as the active material of a redox flow battery, the effect of reducing overvoltage is somewhat inferior compared to when a compound with the structure [2] above is used as the active material of a redox flow battery. However, compared to compounds that do not have the structure [1] above, the effect of reducing overvoltage is still observed. Therefore, when a compound with this structure is used as the active material, the overvoltage of a redox flow battery can be reduced.

[4] An anthraquinone active material according to yet another embodiment is an anthraquinone active material according to any one of [1] to [3],
At least one of the alkoxy groups is O(CH ₂ ) _n COOH (n is a natural number from 1 to 6).

With this configuration, the presence of a carboxyl group can improve the solubility of the active material in the electrolyte solution.

[5] An anthraquinone active material according to yet another embodiment is an anthraquinone active material according to any one of [1] to [4],
The R ₁ , R ₄ , R ₅ and R ₈ are functional groups other than hydroxyl and alkoxy groups, hydrogen, or halogen.

With this configuration, the active material does not strongly coordinate with the potassium ions, reducing the overvoltage of the redox flow battery.

Claims (3)

An anthraquinone active material for a redox flow battery, comprising a compound represented by the following chemical formula:
An anthraquinone active material _, wherein one of R2 and R6 _is a hydroxyl group and the other is an alkoxy group, and R1 _, R4 _, R5 , and R8 _{are functional} groups other than a hydroxyl group and an alkoxy group, hydrogen, or halogen . 2. The anthraquinone active material according to claim 1 _, wherein R3 _and R7 each represent a hydroxyl group or an alkoxy group. 3. The anthraquinone active material according to claim 1, wherein at least one of the alkoxy groups is O(CH ₂ ) _n COOH (n is a natural number from 1 to 6 ).