Electrolyte for redox flow battery and redox flow battery system

Abstract

To provide an electrolyte for a redox flow battery that can reduce deposition of a metal forming an organometallic complex.SOLUTION: There is provided the electrolyte for a redox flow battery, including: an organometallic complex; and an auxiliary complexing agent including at least one type selected from the group consisting of a polycarboxylic acid, a polyol, and an aminocarboxylic acid.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an electrolyte for a redox flow battery and a redox flow battery system. [Background technology]

[0002] Patent Document 1 discloses an electrolyte used in a redox flow battery, which is a negative electrode electrolyte containing a metal complex such as chromium. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 57-9072 Summary of the Invention [Problem to be solved by the invention]

[0004] In electrolytes for redox flow batteries that use metal complexes as active materials, precipitation of the metals that make up the metal complexes affects the efficiency and life of the redox flow battery. Therefore, it is desirable to reduce metal precipitation.

[0005] A metal complex is a compound in which a ligand is bonded to a metal element. When the hydrogen ion exponent (pH) of an aqueous solution of a metal complex becomes high, i.e., when the solution becomes highly alkaline, the metal that makes up the metal complex is likely to precipitate. When an aqueous solution of a metal complex is used as the electrolyte of a redox flow battery, as the state of charge (SOC) of the electrolyte becomes high, the water in the electrolyte is electrolyzed to produce hydrogen gas and hydroxide ions (OH - ) is easily generated. In addition, when the electrolyte is exposed to air, the metal complex is oxidized, and the OH - Increases. OH - When the pH of the electrolyte increases due to an increase in -This tends to combine with the metal complex to form a metal hydroxide. Metal hydroxides are difficult to dissolve, so once they precipitate, they remain precipitated. The precipitated metal hydroxide does not function as an active material. Therefore, when metal hydroxide precipitates, the amount of the metal complex, which is the active material in the electrolyte, decreases, and the capacity of the redox flow battery decreases.

[0006] An object of the present disclosure is to provide an electrolyte solution for a redox flow battery that can reduce precipitation of metals that constitute an organometallic complex. [Means for solving the problem]

[0007] The electrolyte for a redox flow battery according to the present disclosure includes an organometallic complex and an auxiliary complexing agent, the auxiliary complexing agent including one or more compounds selected from the group consisting of polycarboxylic acids, polyols, and aminocarboxylic acids. [Effects of the Invention]

[0008] The electrolyte solution for a redox flow battery according to the present disclosure can reduce precipitation of metals constituting an organometallic complex. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of a redox flow battery system according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Description of the embodiments of the present disclosure] First, embodiments of the present disclosure will be listed and described.

[0011] (1) The electrolyte for a redox flow battery according to the present disclosure includes an organometallic complex and an auxiliary complexing agent, the auxiliary complexing agent including one or more compounds selected from the group consisting of polycarboxylic acids, polyols, and aminocarboxylic acids.

[0012] The organometallic complex functions as an active material. The organometallic complex is composed of a metal and a ligand that bonds with the metal to form a complex. The polycarboxylic acid, polyol, and aminocarboxylic acid have the effect of reducing precipitation of the metal that constitutes the metal complex. The redox flow battery electrolyte of the present disclosure contains an auxiliary complexing agent composed of the specific organic compound described above, thereby reducing precipitation of the metal that constitutes the organometallic complex. Therefore, the redox flow battery electrolyte of the present disclosure can reduce the decrease in battery capacity due to metal precipitation.

[0013] (2) In the redox flow battery electrolyte solution (1) above, the polycarboxylic acid may be at least one selected from the group consisting of tartaric acid, citric acid, adipic acid, and maleic acid.

[0014] The four types of polycarboxylic acids mentioned above are safe, inexpensive, readily available, and easy to handle.

[0015] (3) In the redox flow battery electrolyte solution of (1) or (2) above, the aminocarboxylic acid may be at least one selected from the group consisting of ethylenediaminetetraacetic acid, 1,2-propanediaminetetraacetic acid, 1,3-propanediaminetetraacetic acid, and diethylenetriaminepentaacetic acid.

[0016] The four aminocarboxylic acids mentioned above are safe, inexpensive, readily available, and easy to handle.

[0017] (4) In the redox flow battery electrolyte solution according to any one of (1) to (3), the polyol may be at least one selected from the group consisting of ethylene glycol, propylene glycol, butanediol, erythritol, xylitol, and triethanolamine.

[0018] The six types of polyol acids described above are safe, inexpensive, readily available, and easy to handle.

[0019] (5) In the redox flow battery electrolyte solution according to any one of (1) to (4) above, the molar concentration of the auxiliary complexing agent may be 0.001 mol / L or more and 0.5 mol / L or less.

[0020] When the molar concentration of the auxiliary complexing agent is 0.001 mol / L or more, metal deposition is easily reduced, and when the molar concentration of the auxiliary complexing agent is 0.5 mol / L or less, the auxiliary complexing agent is easily dissolved in the electrolyte.

[0021] (6) In the redox flow battery electrolyte solution according to any one of (1) to (5) above, the auxiliary complexing agent may contain two or more of the polycarboxylic acid, the polyol, and the aminocarboxylic acid.

[0022] When the auxiliary complexing agent contains two or more of the above organic compounds, the effect of reducing metal deposition may be enhanced compared to when only one organic compound is contained.

[0023] (7) In the redox flow battery electrolyte solution (6) above, the auxiliary complexing agent may contain either one of the polycarboxylic acid and the polyol, and the aminocarboxylic acid.

[0024] When the auxiliary complexing agent contains at least an aminocarboxylic acid, it is easy to effectively reduce metal deposition.

[0025] (8) In the redox flow battery electrolyte solution (7) above, the auxiliary complexing agent may contain tartaric acid.

[0026] Tartaric acid, a type of polycarboxylic acid, is highly effective in reducing metal precipitation. By including tartaric acid in the auxiliary complexing agent, metal precipitation can be reduced more effectively.

[0027] (9) A redox flow battery system according to the present disclosure includes the redox flow battery electrolyte solution according to any one of (1) to (8) above.

[0028] The redox flow battery system of the present disclosure includes the electrolytic solution for a redox flow battery of the present disclosure, and thus can reduce the precipitation of the metal constituting the organometallic complex contained in the electrolytic solution. Therefore, the redox flow battery system of the present disclosure can reduce the decrease in battery capacity.

[0029] [Details of Embodiments of the Present Disclosure] Hereinafter, specific examples of the electrolytic solution for a redox flow battery and a redox flow battery system according to embodiments of the present disclosure will be described. In the drawings, the same reference numerals in the figures indicate the same or corresponding parts. The sizes of the members shown in each drawing are expressed for the purpose of clarifying the description, and do not necessarily represent actual dimensional relationships. Note that the present invention is not limited to these examples, and is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0030] Prior to the description of the electrolytic solution for a redox flow battery of the embodiment, an overview of the redox flow battery system will be described. The redox flow battery system of the embodiment includes the electrolytic solution for a redox flow battery of the embodiment. Hereinafter, the redox flow battery may be referred to as an "RF battery".

[0031] <RF Battery System> Referring to FIG. 1, an example of the RF battery system 1 of the embodiment will be described. The RF battery system 1 is an electrolytic solution circulation type secondary battery. The RF battery system 1 performs charging and discharging by the redox reaction between the positive electrode active material contained in the positive electrode electrolytic solution and the negative electrode active material contained in the negative electrode electrolytic solution.

[0032] The RF battery system 1 is connected to a substation 71 that is connected to a power grid. An AC / DC converter 7 is provided between the RF battery system 1 and the substation 71. A power generation unit 8 and a load 9 are connected to the power grid. The RF battery system 1 can charge the power generated by the power generation unit 8 and discharge the charged power to the load 9. The power generation unit 8 is, for example, a power generation facility that uses natural energy, or other general power generation facility. Examples of power generation facility that uses natural energy include solar power generation and wind power generation. The RF battery system 1 is used, for example, for load leveling, momentary sag compensation, emergency power supply, and output smoothing of natural energy power generation.

[0033] As shown in FIG. 1, an RF battery system 1 includes a battery cell 100. An electrolyte is supplied to the battery cell 100. The RF battery system 1 shown in FIG. 1 includes the battery cell 100, an electrolyte, tanks 2p and 2n in which the electrolyte is stored, pipes 3p and 3n connecting the tanks 2p and 2n to the battery cell 100, and pumps 40 provided on the pipes 3p and 3n. The tank 2p stores a positive electrode electrolyte. The tank 2n stores a negative electrode electrolyte. The positive electrode electrolyte circulates between the tank 2p and the battery cell 100 through the pipe 3p. The negative electrode electrolyte circulates between the tank 2n and the battery cell 100 through the pipe 3n.

[0034] (battery cell) The battery cell 100 includes a positive electrode 104, a negative electrode 105, and a diaphragm 101. The diaphragm 101 is disposed between the positive electrode 104 and the negative electrode 105. The battery cell 100 is separated into a positive electrode cell 102 and a negative electrode cell 103 by the diaphragm 101. The positive electrode 104 is disposed in the positive electrode cell 102. The negative electrode 105 is disposed in the negative electrode cell 103. A positive electrode electrolyte is supplied to the positive electrode cell 102. A negative electrode electrolyte is supplied to the negative electrode cell 103. The battery cell 100 can be configured in any known manner as appropriate.

[0035] The pipes 3p and 3n have the same configuration. Each of the pipes 3p and 3n includes a first pipe 31 and a second pipe 32. A pump 40 is provided in the first pipe 31. The pump 40 circulates the electrolyte in each of the tanks 2p and 2n to the battery cell 100. The first pipe 31 in the pipe 3p is a pipe that sends positive electrode electrolyte from the tank 2p to the battery cell 100. The second pipe 32 in the pipe 3p is a pipe that returns positive electrode electrolyte from the battery cell 100 to the tank 2p. That is, the positive electrode electrolyte is supplied to the positive electrode cell 102 from the tank 2p through the first pipe 31. The positive electrode electrolyte discharged from the positive electrode cell 102 is returned to the tank 2p through the second pipe 32. The first pipe 31 in the pipe 3n is a pipe that sends negative electrode electrolyte from the tank 2n to the battery cell 100. The second pipe 32 in the pipe 3n is a pipe that returns the negative electrode electrolyte from the battery cell 100 to the tank 2n. That is, the negative electrode electrolyte is supplied from the tank 2n to the negative electrode cell 103 through the first pipe 31. The negative electrode electrolyte discharged from the negative electrode cell 103 is returned to the tank 2n through the second pipe 32. When charging or discharging is performed, the electrolyte is circulated by the pump 40. When charging or discharging is not performed, the pump 40 is stopped and the electrolyte is not circulated.

[0036] The RF battery system 1 may include a single battery cell 100 or multiple battery cells 100. The RF battery system 1 shown in FIG. 1 includes a cell stack 200 in which multiple battery cells 100 are stacked. The cell stack 200 is configured by repeatedly stacking a cell frame 120, a positive electrode 104, a diaphragm 101, and a negative electrode 105 in this order. End plates 210 are arranged on both ends of the cell stack 200. The cell stack 200 is integrated by clamping the end plates 210 together with clamping members 230. The configuration of the cell stack 200 can be any known configuration as appropriate.

[0037] The cell frame 120 has a bipolar plate 121 and a frame body 122. The bipolar plate 121 is disposed between the positive electrode 104 and the negative electrode 105. The frame body 122 is provided around the bipolar plate 121. Inside the frame body 122, a recess is formed by the bipolar plate 121 and the frame body 122. The recesses are provided on both sides of the bipolar plate 121. The positive electrode 104 and the negative electrode 105 are housed in each recess, with the bipolar plate 121 sandwiched between them.

[0038] 1, a single battery cell 100 is formed by placing a positive electrode 104 and a negative electrode 105 between the bipolar plates 121 of adjacent cell frames 120 with a diaphragm 101 sandwiched between them. An annular sealing member 127, for example, is placed between the frames 122 of each cell frame 120. The number of stacked battery cells 100 in the cell stack 200 can be selected as appropriate.

[0039] Although detailed illustration is omitted, the frame 122 has a liquid supply manifold that supplies each electrolyte solution and a liquid discharge manifold that discharges each electrolyte solution. Each manifold is provided to penetrate the frame 122, and a flow path for each electrolyte solution is formed by stacking the cell frames 120. Each of these flow paths is connected to the first pipe 31 and the second pipe 32, respectively.

[0040] <Electrolyte> The RF battery electrolyte of the embodiment is an aqueous solution containing an organometallic complex and an auxiliary complexing agent. One of the features of the RF battery electrolyte of the embodiment is that the auxiliary complexing agent contains a specific organic compound. Hereinafter, the RF battery electrolyte may be simply referred to as "electrolyte." Furthermore, the organometallic complex may be simply referred to as "metal complex."

[0041] (organometallic complexes) Organometallic complexes function as the active material in RF batteries. Organometallic complexes are compounds consisting of a metal and a ligand. Organometallic complexes are formed by bonding a metal to a ligand made of an organic compound.

[0042] <metal> Metals are metals used as active materials in RF batteries. Specifically, metals are transition metals whose valence changes through oxidation-reduction reactions. Transition metals are elements that belong to groups 3 to 12 of the periodic table. The periodic table is a long-form periodic table established by IUPAC (International Union of Pure and Applied Chemistry). Metals usually exist in the form of metal ions.

[0043] The metal is, for example, at least one selected from the group consisting of chromium (Cr), titanium (Ti), iron (Fe), vanadium (V), and manganese (Mn). A metal complex of Fe, V, or Mn functions as a positive electrode active material. An electrolyte containing such a metal complex can be used as a positive electrode electrolyte. A metal complex of Cr, Fe, Ti, or V functions as a negative electrode active material. An electrolyte containing such an organometallic complex can be used as a negative electrode electrolyte.

[0044] <Ligand> A ligand is an organic compound that bonds with a metal to form a complex. A ligand has one or more coordinating atoms that bond to the metal. The number of coordinating atoms can be, for example, between 1 and 10. Multidentate ligands with multiple coordinating atoms tend to bond more strongly with the metal and form more stable complexes than monodentate ligands with only one coordinating atom. The more coordinating atoms there are, the more likely it is that a stable complex will be formed.

[0045] The ligand may be a chelating agent made of an organic compound capable of coordinating to a metal. Examples of the ligand include aminocarboxylic acid chelating agents, carboxylic acid chelating agents, and amino acid chelating agents. Examples of aminocarboxylic acid chelating agents include ethylenediaminetetraacetic acid (EDTA), propanediaminetetraacetic acid (PDTA), and diethylenetriaminepentaacetic acid (DTPA). PDTA includes 1,2-PDTA and 1,3-PDTA. Examples of carboxylic acid chelating agents include citric acid, tartaric acid, gluconic acid, malic acid, and maleic acid. Examples of amino acid chelating agents include glutamic acid, aspartic acid, and histidine. Among these, aminocarboxylic acid chelating agents are more likely to form stable complexes with metals.

[0046] The molar concentration of the organometallic complex is, for example, 0.3 M or more and 1.5 M or less. The unit M stands for "moles / liter (mol / L)." The higher the concentration of the organometallic complex, the greater the amount of active material contained in the electrolyte, and the higher the capacity of the RF battery. When the molar concentration of the organometallic complex is 0.3 M or more, it is easy to increase the capacity of the RF battery. When the molar concentration of the organometallic complex is 1.5 M or less, it is easy for the organometallic complex to dissolve sufficiently in the electrolyte. The molar concentration of the organometallic complex may further be 0.5 M or more and 1.3 M or less, or 0.8 M or more and 1.1 M or less.

[0047] The metal and ligand are bonded in a 1:1 molar ratio. In other words, the metal and ligand are present in equal amounts. The molar concentration of the metal and the molar concentration of the ligand in the electrolyte are equal. The molar concentration of an organometallic complex refers to the respective molar concentrations of the metal and the ligand. For example, if the molar concentration of the organometallic complex is 0.8M, then the respective molar concentrations of the metal and the ligand are 0.8M.

[0048] The molar concentration of an organometallic complex can be determined from the respective molar concentrations of the metal and ligand that constitute the organometallic complex. The molar concentration of the metal can be determined, for example, using an inductively coupled plasma (ICP) optical emission spectrometer. The molar concentration of the ligand can be determined, for example, using an organic elemental analyzer. An organic elemental analyzer can analyze the composition of an organic compound by measuring the types and content ratios of elements such as carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) that constitute the organic compound.

[0049] (auxiliary complexing agent) The auxiliary complexing agent contains one or more organic compounds selected from the group consisting of polycarboxylic acids, polyols, and aminocarboxylic acids. These organic compounds have the effect of reducing the deposition of the metals that constitute the metal complex. The polycarboxylic acids include polycarboxylic acids and their salts. The aminocarboxylic acids include aminocarboxylic acids and their salts.

[0050] The polycarboxylic acid is, for example, at least one selected from the group consisting of tartaric acid, citric acid, adipic acid, and maleic acid. The aminocarboxylic acid is, for example, at least one selected from the group consisting of EDTA, PDTA, and DTPA. PDTA includes 1,2-PDTA and 1,3-PDTA. The polyol is, for example, at least one selected from the group consisting of ethylene glycol, propylene glycol, butanediol, erythritol, xylitol, and triethanolamine (TEA). Among them, tartaric acid, which is a type of polycarboxylic acid, and the above-mentioned aminocarboxylic acids are highly effective in reducing metal deposition.

[0051] The mechanism by which the auxiliary complexing agent reduces metal precipitation is not clear, but it is thought to be as follows: Even if the metal is released from the ligand in the electrolyte, the metal ion released from the ligand bonds with the auxiliary complexing agent to form a complex. Therefore, the metal ion is not released from the OH group in the electrolyte. - This can reduce the precipitation of metal hydroxides by bonding with the metal.

[0052] The auxiliary complexing agent may contain only one or more of polycarboxylic acids, polyols, and aminocarboxylic acids. When the auxiliary complexing agent contains two or more of these organic compounds, the combination of organic compounds may be any of a combination of polycarboxylic acid and aminocarboxylic acid, a combination of polyol and aminocarboxylic acid, a combination of polycarboxylic acid and polyol, or a combination including all of polycarboxylic acid, polyol, and aminocarboxylic acid. Combining two or more organic compounds may enhance the effect of reducing metal deposition. When the auxiliary complexing agent contains two or more organic compounds, the molar concentrations of the respective organic compounds may be equal or different. For example, in an auxiliary complexing agent containing a first auxiliary complexing agent made of a first organic compound and a second auxiliary complexing agent made of a second organic compound, the molar concentrations of the first auxiliary complexing agent and the second auxiliary complexing agent may be equal or different. The second organic compound is an organic compound different from the first organic compound.

[0053] The auxiliary complexing agent may contain either one of a polycarboxylic acid and a polyol, and an aminocarboxylic acid. When the auxiliary complexing agent contains at least an aminocarboxylic acid, metal deposition is more likely to be effectively reduced. In particular, when the auxiliary complexing agent contains a polycarboxylic acid and an aminocarboxylic acid, and the polycarboxylic acid is tartaric acid, metal deposition can be more effectively reduced.

[0054] The molar concentration of the auxiliary complexing agent is, for example, 0.001M or more and 0.5M or less. The higher the concentration of the auxiliary complexing agent, the more the precipitation of the metal constituting the metal complex can be reduced. When the molar concentration of the auxiliary complexing agent is 0.001M or more, metal precipitation is easily reduced. When the molar concentration of the auxiliary complexing agent is 0.5M or less, the auxiliary complexing agent is easily dissolved sufficiently in the electrolyte. The molar concentration of the auxiliary complexing agent may further be 0.05M or more and 0.5M or less, or 0.1M or more and 0.4M or less.

[0055] The molar concentration of the auxiliary complexing agent can be determined, for example, by a CHN elemental analyzer.

[0056] A method for producing an electrolytic solution according to an embodiment will be described. The method for producing an electrolytic solution includes, for example, preparing a stock solution containing an organometallic complex and adding an auxiliary complexing agent to the stock solution. The stock solution can be prepared, for example, by adding a chelating agent to an aqueous solution containing metal ions and stirring the mixture. The chelating agent is an organic compound that functions as a ligand. By mixing the metal ions and the chelating agent, the metal ions and the organic compound bond to form an organometallic complex. The amount of chelating agent added is equal to the amount of metal ions. That is, the chelating agent is added so that the molar ratio of the metal ions to the chelating agent is 1:1. The molar concentrations of the metal ions and the chelating agent are adjusted so that the organometallic complex has a predetermined molar concentration. Alternatively, the stock solution may be prepared by dissolving an organometallic complex synthesized by reacting metal ions with a chelating agent in water. The molar concentration of the organometallic complex is, for example, 0.3 M or more and 1.5 M or less. The molar concentrations of the metal ions and the chelating agent are, for example, 0.3M or more and 1.5M or less.

[0057] A neutralizing agent may be added to the stock solution containing the organometallic complex. The organometallic complex becomes more soluble in water when neutralized with an alkali. The neutralizing agent is, for example, at least one selected from the group consisting of potassium hydroxide (KOH), sodium hydroxide (NaOH), and ammonium hydroxide (NH4OH). When preparing a stock solution by adding a chelating agent to an aqueous solution containing metal ions, stirring the aqueous solution containing the metal ions and the chelating agent while neutralizing the solution facilitates the reaction between the metal ions and the chelating agent, making it easier to form an organometallic complex.

[0058] The stock solution containing the organometallic complex may be stirred while heated. This makes the organometallic complex more soluble in water. When preparing the stock solution by adding a chelating agent to an aqueous solution containing metal ions, heating the aqueous solution while stirring promotes the reaction between the metal ions and the chelating agent, making it easier to form the organometallic complex. This shortens the stirring time. The aqueous solution is heated, for example, to a temperature of 60°C or higher and 120°C or lower.

[0059] An electrolyte solution is produced by adding an auxiliary complexing agent to the above-mentioned stock solution. The amount of the auxiliary complexing agent added is adjusted so that the auxiliary complexing agent has a predetermined molar concentration. The molar concentration of the auxiliary complexing agent is, for example, 0.001 M or more and 0.5 M or less. The stock solution to which the auxiliary complexing agent has been added may be heated. Heating makes the auxiliary complexing agent more soluble. The stock solution is heated, for example, to 60°C or more and 120°C or less.

[0060] When the electrolyte solution of the embodiment is used as the negative electrode electrolyte, the pH of the electrolyte solution may be adjusted to be weakly alkaline. Weakly alkaline refers to a pH greater than 7 and equal to or less than 12. If the pH of the electrolyte solution is too low, hydrogen is likely to be generated due to a side reaction when the electrolyte solution is charged. If the electrolyte solution is weakly alkaline, the generation of hydrogen due to a side reaction during charging is likely to be reduced. Furthermore, during use of the electrolyte solution, the pH of the electrolyte solution may increase due to charging the electrolyte solution or exposure to air. If the electrolyte solution is weakly alkaline before use, it is easy to prevent the pH of the electrolyte solution from becoming excessively high during use. The pH of the electrolyte solution before use may be 11 or less. If the pH of the electrolyte solution before use is 11 or less, the electrolyte solution is likely to maintain a weakly alkaline state during use.

[0061] [Test Example 1] A stock solution containing an organometallic complex of Cr and PDTA was prepared. This stock solution was produced by adding KOH to water containing the organometallic complex while stirring and neutralizing it. The organometallic complex is a salt synthesized by reacting Cr with PDTA. The molar concentration of the organometallic complex was adjusted to 0.8M. An auxiliary complexing agent was added to the prepared stock solution to produce a sample electrolyte containing the organometallic complex and the auxiliary complexing agent. The metal constituting the organometallic complex is Cr. The ligand constituting the organometallic complex is PDTA. Here, the organometallic complex composed of Cr and PDTA is referred to as "Cr-PDTA." The molar concentrations of Cr and PDTA are both 0.8M. The pH value of the electrolyte is between 8 and 11.

[0062] The organometallic complexes and auxiliary complexing agents contained in the electrolyte samples are shown in Table 1. The number in parentheses after the name of the organometallic complex indicates the molar concentration of the organometallic complex. The number in parentheses after the name of the auxiliary complexing agent indicates the molar concentration of the auxiliary complexing agent. The auxiliary complexing agents in Samples No. 1 to No. 6 contain one or two organic compounds selected from the group consisting of polycarboxylic acids, polyols, and aminocarboxylic acids.

[0063] (Sample No. 1-Sample No. 3) Samples No. 1 to No. 3 used an auxiliary complexing agent containing only one organic compound. Samples No. 1 to No. 3 added PDTA as an auxiliary complexing agent. Samples No. 1 to No. 3 differ in the amount of PDTA added. Sample No. 1 added 0.1M PDTA. Sample No. 2 added 0.2M PDTA. Sample No. 3 added 0.3M PDTA.

[0064] (Sample No. 4-Sample No. 6) Samples No. 4 to No. 6 used auxiliary complexing agents containing two organic compounds. Samples No. 4 to No. 6 contained tartaric acid, TEA, or citric acid as an auxiliary complexing agent in addition to PDTA. All samples No. 4 to No. 6 contained 0.1M PDTA. Sample No. 4 contained 0.1M tartaric acid. Sample No. 5 contained 0.1M TEA. Sample No. 6 contained 0.1M citric acid. For samples No. 4 to No. 6, the molar concentration of the auxiliary complexing agent, which is a combination of two organic compounds, is 0.2M.

[0065] (Sample No. 100) For comparison, Sample No. 100 was prepared, which is an electrolyte solution without the addition of the auxiliary complexing agent. Sample No. 100 contained no auxiliary complexing agent, but instead contained 0.1M potassium tetraborate (K2B4O7). K2B4O7 is a pH adjuster. Samples No. 1 to No. 6 did not contain K2B4O7.

[0066] The stability of the organometallic complexes was evaluated for Samples No. 1 to No. 6 and Sample No. 100. The stability of the organometallic complexes was evaluated by the time it took for the metal to precipitate while the electrolyte was maintained at a constant temperature. The longer the time it took for the metal to precipitate, the greater the effectiveness of the auxiliary complexing agent in reducing metal precipitation.

[0067] The stability was evaluated as follows. The electrolyte was charged until the SOC of the electrolyte reached 75%. The pH value of the electrolyte after charging was 9 or more and 11 or less. During charging, the trivalent Cr ions (Cr 3+ ) is a divalent Cr ion (Cr 2+ When the electrolyte is charged until the SOC reaches 75%, the divalent Cr ions (Cr 2+ The ratio of ) was 75 mol %. The charged electrolyte was placed in a thermostatic chamber and maintained at a constant temperature. The electrolyte maintained at a constant temperature was visually observed, and the number of days until metal precipitates were observed in the electrolyte was measured.

[0068] In this test, the electrolyte for Samples 1 through 6 and Sample 100 was maintained at temperatures of 35°C, 45°C, and 55°C. The electrolyte for Samples 4 through 6 and Sample 100 was maintained at 75°C. Tables 1 and 2 show the number of days required for metal deposits to be observed after maintaining the electrolyte at each temperature. The values listed for each temperature in Tables 1 and 2 indicate the number of days required for metal deposits to be observed at that temperature. A value preceded by a ">" indicates a period exceeding the specified number of days. For example, ">194" means that no metal deposits were observed even after 194 days. A value preceded by a ">" indicates a period less than the specified number of days. For example, "<1" means that metal deposits were observed in less than one day.

[0069] [Table 1]

[0070] [Table 2]

[0071] As shown in Table 1, at temperatures of 35°C, 45°C, and 55°C, samples No. 1 to No. 6 took longer to deposit metals than sample No. 100. This is thought to be due to the reduction in metal deposition caused by the auxiliary complexing agent, which is a specific organic compound. Furthermore, the results for samples No. 1 to No. 3 show that the higher the molar concentration of the auxiliary complexing agent, the more easily metal deposition is reduced.

[0072] The higher the temperature of the electrolyte, the easier it is for metal to deposit. As shown in Table 2, at 75°C, the deposition time for Samples No. 4 to No. 6 is longer than that for Sample No. 100. Samples No. 4 to No. 6 exhibit reduced metal deposition compared to Sample No. 100, even under high-temperature electrolyte conditions.

[0073] Additionally, the test results reveal the following: The deposition times of samples No. 4 to No. 6 are comparable to or longer than those of sample No. 2. Therefore, when the auxiliary complexing agent contains two organic compounds, it may be more effective in reducing metal deposition than when the auxiliary complexing agent contains only one organic compound. Furthermore, of all the samples, sample No. 4 showed reduced metal deposition over a long period of time. Therefore, when the auxiliary complexing agent contains tartaric acid, metal deposition can be more effectively reduced. [Explanation of symbols]

[0074] 1 RF battery system 7 AC / DC converters, 71 substation equipment 8 power generation unit, 9 load 2p, 2n tank 3p, 3n piping 31 First pipe, 32 Second pipe 40 Pump 100 battery cells 101 diaphragm, 102 positive electrode cell, 103 negative electrode cell 104 positive electrode, 105 negative electrode 120 Cell Frame 121 bipolar plate, 122 frame 127 Sealing material 200 cell stack 210 end plate, 230 fastening member

Claims

1. an organometallic complex and an auxiliary complexing agent; The auxiliary complexing agent includes one or more selected from the group consisting of polycarboxylic acids, polyols, and aminocarboxylic acids. Electrolyte for redox flow batteries.

2. 2. The electrolyte solution for a redox flow battery according to claim 1, wherein the polycarboxylic acid is at least one selected from the group consisting of tartaric acid, citric acid, adipic acid, and maleic acid.

3. 3. The electrolyte solution for a redox flow battery according to claim 1, wherein the aminocarboxylic acid is at least one selected from the group consisting of ethylenediaminetetraacetic acid, 1,2-propanediaminetetraacetic acid, 1,3-propanediaminetetraacetic acid, and diethylenetriaminepentaacetic acid.

4. 3. The electrolyte solution for a redox flow battery according to claim 1, wherein the polyol is at least one selected from the group consisting of ethylene glycol, propylene glycol, butanediol, erythritol, xylitol, and triethanolamine.

5. 3. The electrolyte solution for a redox flow battery according to claim 1, wherein the molar concentration of the auxiliary complexing agent is 0.001 mol / L or more and 0.5 mol / L or less.

6. 3. The electrolyte solution for a redox flow battery according to claim 1, wherein the auxiliary complexing agent comprises two or more of the polycarboxylic acid, the polyol, and the aminocarboxylic acid.

7. 7. The electrolyte solution for a redox flow battery according to claim 6, wherein the auxiliary complexing agent contains the aminocarboxylic acid and one of the polycarboxylic acid and the polyol.

8. The electrolyte for a redox flow battery according to claim 7 , wherein the auxiliary complexing agent comprises tartaric acid.

9. The redox flow battery electrolyte according to claim 1 is provided. Redox flow battery system.

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