Redox flow battery system

Abstract

A redox flow battery system includes a battery cell, a positive electrolyte to be supplied to the battery cell, and a negative electrolyte to be supplied to the battery cell. The positive electrolyte contains a positive active material formed of an organic compound. The negative electrolyte contains a negative active material formed of a metal chelate complex. The organic compound contains a phosphate group or a sulfo group.

Description

REDOX FLOW BATTERY SYSTEM

[0001] The present disclosure relates to redox flow battery systems. The present application claims priority to Japanese Patent Application No. 2023-108724 filed on June 30, 2023, the entire contents of which are incorporated herein by reference.

[0002] A redox flow battery in PTL 1 includes a positive electrolyte containing bromine as a positive active material and a negative electrolyte containing a metal chelate complex consisting of chromium ethylenediaminetetraacetate (CrEDTA) as a negative active material.

[0003] Japanese Unexamined Patent Application Publication No. 57-9072

[0004] A redox flow battery system according to an aspect of the present disclosure includes a battery cell, a positive electrolyte to be supplied to the battery cell, and a negative electrolyte to be supplied to the battery cell. The positive electrolyte contains a positive active material formed of an organic compound. The negative electrolyte contains a negative active material formed of a metal chelate complex. The organic compound contains a phosphate group or a sulfo group.

[0005] FIG. 1 is a schematic diagram illustrating the configuration of a redox flow battery system according to an embodiment.FIG. 2 schematically illustrates the structure of a first example of a water-soluble derivative of TEMPO.FIG. 3 schematically illustrates the structure of a second example of a water-soluble derivative of TEMPO.FIG. 4 schematically illustrates the structure of a third example of a water-soluble derivative of TEMPO.FIG. 5 schematically illustrates the structure of a fourth example of a water-soluble derivative of TEMPO.FIG. 6 schematically illustrates the structure of a fifth example of a water-soluble derivative of TEMPO.FIG. 7 schematically illustrates the structure of a first example of a water-soluble derivative of ferrocene.FIG. 8 schematically illustrates the structure of a second example of a water-soluble derivative of ferrocene.FIG. 9 schematically illustrates the structure of a third example of a water-soluble derivative of ferrocene.FIG. 10 schematically illustrates the structure of a fourth example of a water-soluble derivative of ferrocene.

[0006] In a redox flow battery system, the electromotive force is generated by the difference in oxidation-reduction potential between a positive active material and a negative active material. It is desirable to increase the electromotive force in a redox flow battery system.

[0007] For example, aqueous electrolytes, which contain water as a solvent, are used as electrolytes for a redox flow battery system. To increase the energy density of a redox flow battery system, it is desirable that the electrolytes have high active material concentration. It is desirable that the active materials have high solubility in water.

[0008] The redox flow battery in PTL 1 produces a corrosive gas during a battery reaction because the positive electrolyte contains bromine as a positive active material. The corrosive gas corrodes the constituent members of the redox flow battery system.

[0009] An object of the present disclosure is to provide a redox flow battery system which has high electromotive force, which includes active materials with high solubility in water, and whose constituent members are less likely to corrode.

[0010] A redox flow battery system according to an aspect of the present disclosure has high electromotive force and includes active materials with high solubility in water. The constituent members of the redox flow battery system according to the aspect of the present disclosure are less likely to corrode. (Description of Embodiments of Present Disclosure)

[0011] First, embodiments of the present disclosure will be listed and described.

[0012] (1) A redox flow battery system according to an aspect of the present disclosure includes a battery cell, a positive electrolyte to be supplied to the battery cell, and a negative electrolyte to be supplied to the battery cell. The positive electrolyte contains a positive active material formed of an organic compound. The negative electrolyte contains a negative active material formed of a metal chelate complex. The organic compound contains a phosphate group or a sulfo group.

[0013] (2) In the redox flow battery system according to (1) above, the organic compound may contain a phosphate group.

[0014] (3) A redox flow battery system according to an aspect of the present disclosure includes a battery cell, a positive electrolyte to be supplied to the battery cell, and a negative electrolyte to be supplied to the battery cell. The positive electrolyte contains a positive active material formed of an organic compound. The negative electrolyte contains a negative active material formed of a metal chelate complex. The organic compound contains a carboxy group, sulfur trioxide, or sulfur tetroxide.

[0015] The positive active material has a relatively high oxidation-reduction potential, whereas the negative active material has a relatively low oxidation-reduction potential. The redox flow battery system according to (1) or (3) tends to have a large difference in oxidation-reduction potential between the positive active material and the negative active material. The redox flow battery system according to (1) or (3) can have high electromotive force.

[0016] The positive active material and the negative active material both dissolve easily in water. The positive active material and the negative active material in the redox flow battery system according to (1) or (3) have high solubility in water. The redox flow battery system according to (1) or (3) can have high energy density.

[0017] The positive active material produces no corrosive gas during a battery reaction, unlike the positive active material in PTL 1, which contains bromine. The constituent members of the redox flow battery system according to (1) or (3) are less likely to corrode.

[0018] Since the positive active material in the redox flow battery system according to (2) contains a phosphate group, the electron transfer rate constant, described later, is high, which results in superior battery reactivity. When the positive active material contains a phosphate group, the pH of the positive electrolyte can be easily maintained at a constant level by its buffer action. In addition, when the positive active material contains a phosphate group, the positive active material can become a divalent ion.

[0019] The organic compound may contain two or more of a phosphate group, a sulfo group, a carboxy group, sulfur trioxide, and sulfur tetroxide. When two or more of these are contained in the organic compound, the positive active material has a higher solubility, and the redox flow battery system can have a higher energy density.

[0020] (4) In the redox flow battery system according to any one of (1) to (3) above, the positive active material may be a water-soluble derivative of TEMPO. TEMPO is a trivial name. The IUPAC (International Union of Pure and Applied Chemistry) name for TEMPO is 2,2,6,6-tetramethylpiperidine-1-oxyl.

[0021] (5) In the redox flow battery system according to any one of (1) to (3) above, the positive active material may be a water-soluble derivative of ferrocene. Ferrocene is a trivial name. The IUPAC name for ferrocene is bis(η5-cyclopentadienyl)iron(II).

[0022] The redox flow battery system according to (4) or (5) has high electromotive force and includes active materials with high solubility in water. The constituent members of the redox flow battery system according to (4) or (5) are less likely to corrode.

[0023] (6) In the redox flow battery system according to any one of (1) to (5) above, the battery cell may include a positive electrode, a negative electrode, and a membrane. The positive active material in the positive electrolyte and the negative active material in the negative electrolyte may be anions. The membrane may be a cation-exchange membrane.

[0024] Since the positive active material in the positive electrolyte and the negative active material in the negative electrolyte in the redox flow battery system according to (6) are in the form of an anion, a cation-exchange membrane can be used as the membrane.

[0025] (7) In the redox flow battery system according to any one of (1) to (6) above, a central metal of the metal chelate complex may be one selected from the group consisting of chromium, titanium, iron, and vanadium.

[0026] The redox flow battery system according to (7) can have high electromotive force. The positive active material and the negative active material in the redox flow battery system according to (7) can have high solubility in water. (Details of Embodiments of Present Disclosure)

[0027] A specific example of a redox flow battery system according to an embodiment of the present disclosure will be described below with reference to the drawings. In the drawings, the same reference signs denote components with the same names. (Embodiment) (Redox Flow Battery System)

[0028] A redox flow battery system 1 according to a first embodiment will be described with reference to FIG. 1. The redox flow battery system 1 includes battery cells 100 that charge and discharge, a positive electrolyte 31a to be supplied to the battery cells 100, and a negative electrolyte 41a to be supplied to the battery cells 100. A feature of the redox flow battery system 1 is that the positive electrolyte 31a and the negative electrolyte 41a are a particular combination of positive and negative electrolytes. (Overview)

[0029] The redox flow battery system 1 charges to store electric power generated by a power generation unit 8 and discharges to supply the stored electric power to a load 9. The redox flow battery system 1 is typically connected to an alternating current / direct current converter 7. The alternating current / direct current converter 7 is connected to a transformer facility 71. The transformer facility 71 is connected to the power generation unit 8 and to the load 9. An example of the power generation unit 8 is a solar photovoltaic power generator, wind power generator, or another power generator provided in a common power plant. An example of the load 9 is a consumer of electric power. The redox flow battery system 1 uses the positive electrolyte 31a and the negative electrolyte 41a, described later. The redox flow battery system 1 charges and discharges by utilizing the difference between the oxidation-reduction potential of a positive active material contained in the positive electrolyte 31a and the oxidation-reduction potential of a negative active material contained in the negative electrolyte 41a. The redox flow battery system 1 can be used to perform, for example, load leveling, voltage sag compensation, or power smoothing for natural energy. The natural energy is, for example, energy obtained by solar photovoltaic power generation or wind power generation. The redox flow battery system 1 can also be used as an emergency power supply. (Positive Electrolyte)

[0030] The positive electrolyte 31a is supplied to a positive cell 102 of each battery cell 100 by a positive electrolyte circulation mechanism 3 described later. The positive electrolyte 31a contains a positive active material formed of an organic compound. The organic compound contains a sulfo group (SO3H), a phosphate group (H2PO4), sulfur trioxide (SO3), a carboxy group (COOH), or sulfur tetroxide (SO4). The organic compound is an organic compound that is not an organometallic complex or is an organic compound that is an organometallic complex. The organic compound that is not an organometallic complex contains a sulfo group, a phosphate group, sulfur trioxide, or sulfur tetroxide. The organic compound that is an organometallic complex contains a sulfo group, a phosphate group, or a carboxy group. A sulfo group, a phosphate group, or a carboxy group may be contained in an organic acid or may be contained in a salt formed by neutralization. Simply the term "organic compound" hereinafter refers to an organic compound that is not an organometallic complex. An organic compound that is an organometallic complex is simply referred to as "organometallic complex". In the positive electrolyte 31a, the organometallic complex may be present as a complex ion or may be present as an electrically neutral complex.

[0031] The positive active material has a relatively high oxidation-reduction potential. As described later, the negative active material has a relatively low oxidation-reduction potential. Thus, there tends to be a large difference between the oxidation-reduction potential of the positive active material and the oxidation-reduction potential of the negative active material. The redox flow battery system 1 can have high electromotive force. The electromotive force tends to become higher as the oxidation-reduction potential of the positive active material becomes higher. The positive active material has a standard oxidation-reduction potential of, for example, 0.75 V or more, 0.80 V or more, or 0.85 V or more. The upper limit of the standard oxidation-reduction potential of the positive active material is not particularly limited and is, for example, 1.0 V from a practical viewpoint. The positive active material has a standard oxidation-reduction potential of, for example, 0.75 V to 1.0 V, 0.80 V to 1.0 V, or 0.85 V to 1.0 V. The method for determining the standard oxidation-reduction potential of the positive active material will be described in the Test Examples.

[0032] The positive active material dissolves relatively easily in water. The positive active material has relatively high solubility in water. Here, the solubility of the positive active material in water is the maximum amount of positive active material that can dissolve in water at 25°C and is a measure of how soluble the positive active material is in water. The unit for solubility is moles per liter (mol / L). A positive active material having a higher solubility is more soluble in water. The positive active material has a solubility of, for example, 0.8 mol / L or more, 1.0 mol / L or more, or 1.2 mol / L or more. The upper limit of the solubility of the positive active material is not particularly limited and is, for example, 2.0 mol / L from a practical viewpoint. The positive active material has a solubility of, for example, 0.8 mol / L to 2.0 mol / L, 1.0 mol / L to 2.0 mol / L, or 1.2 mol / L to 2.0 mol / L. The method for determining the solubility of the positive active material will be described in the Test Examples.

[0033] The positive active material produces no corrosive gas during a battery reaction, unlike the positive active material in PTL 1, which contains bromine. Thus, the constituent members of the redox flow battery system 1 are less likely to corrode.

[0034] An example of the organic compound is a water-soluble derivative of TEMPO. A positive active material formed of a water-soluble derivative of TEMPO has a relatively high oxidation-reduction potential. A positive active material formed of a water-soluble derivative of TEMPO has relatively high solubility in water. The water-soluble derivative of TEMPO contains, for example, one selected from the group consisting of sulfonic acid, sultone, phosphoric acid, sulfurous acid, and sulfuric acid as a portion of the molecular structure of TEMPO. The water-soluble derivative of TEMPO is based on TEMPO and has one selected from the group consisting of a sulfo group, a phosphate group, sulfur trioxide, and sulfur tetroxide as a substituent. Examples of water-soluble derivatives of TEMPO include 2,2,6,6-tetramethylpiperidine-1-oxyl-4-sulfonate, as illustrated in FIG. 2, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-oxypropanesulfonate, as illustrated in FIG. 3, 4-phosphonooxy-2,2,6,6-tetramethylpiperidine-1-oxyl, as illustrated in FIG. 4, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-oxypropanesulfite, as illustrated in FIG. 5, and 2,2,6,6-tetramethylpiperidine-1-oxyl-4-oxypropanesulfate, as illustrated in FIG. 6.

[0035] An example of the organometallic complex is a water-soluble derivative of ferrocene. A positive active material formed of a water-soluble derivative of ferrocene has a relatively high oxidation-reduction potential. A positive active material formed of a water-soluble derivative of ferrocene has relatively high solubility in water. The water-soluble derivative of ferrocene contains, for example, one selected from the group consisting of sulfonic acid, phosphoric acid, succinic acid, and malonic acid as a portion of the molecular structure of ferrocene. The water-soluble derivative of ferrocene is based on ferrocene and has one selected from the group consisting of a sulfo group, a phosphate group, and a carboxy group as a substituent. Specific examples of water-soluble derivatives of ferrocene include 1-sulfonylferrocene, as illustrated in FIG. 7, 1-phosphorylferrocene, as illustrated in FIG. 8, 3-ferrocenoylpropionic acid, as illustrated in FIG. 9, and succinic acid ferrocenol ester, as illustrated in FIG. 10. Although these are 1-substituted derivatives, they may also be 1,1'-derivatives.

[0036] The positive active material may be in the form of an anion. An anion of an organic compound or an anion of an organometallic complex may be present in the positive electrolyte 31a. For example, an anion of a water-soluble derivative of TEMPO or an anion of a water-soluble derivative of ferrocene may be present in the positive electrolyte 31a. When the positive active material is in the form of an anion and the negative active material described later is in the form of an anion, a cation-exchange membrane can be used as a membrane 101 described later. The positive active material may be in the form of a cation. A cation of an organic compound in which a plurality of cations are attached to an anion of the organic compound or a cation of an organometallic complex in which a plurality of cations are attached to an anion of the organometallic complex may be present in the positive electrolyte 31a. For example, a cation of a water-soluble derivative of TEMPO or a cation of a water-soluble derivative of ferrocene may be present in the positive electrolyte 31a.

[0037] The solvent for the positive electrolyte 31a is, for example, water. Water can be used as the solvent for the positive electrolyte 31a since the positive active material dissolves easily in water. (Negative Electrolyte)

[0038] The negative electrolyte 41a is supplied to a negative cell 103 of each battery cell 100 by a negative electrolyte circulation mechanism 4 described later. The negative electrolyte 41a contains a negative active material formed of a metal chelate complex. In the negative electrolyte 41a, the metal chelate complex may be present as a complex ion or may be present as an electrically neutral complex. The negative active material has a relatively low oxidation-reduction potential. The electromotive force tends to become higher as the oxidation-reduction potential of the negative active material becomes lower and accordingly there is a larger difference between the oxidation-reduction potential of the negative active material and the oxidation-reduction potential of the positive active material. The negative active material has a standard oxidation-reduction potential of, for example, -0.7 V or less, -0.9 V or less, -1.0 V or less, or -1.1 V or less. The lower limit of the standard oxidation-reduction potential of the negative active material is not particularly limited and is, for example, -1.5 V from a practical viewpoint. The negative active material has a standard oxidation-reduction potential of, for example, -1.5 V to -0.7 V, -1.5 V to -0.9 V, -1.5 V to -1.0 V, or -1.5 V to -1.1 V.

[0039] The negative active material dissolves easily in water. The negative active material has high solubility in water. The negative active material has a solubility of, for example, 0.8 mol / L or more, 1.0 mol / L or more, or 1.2 mol / L or more. The upper limit of the solubility of the negative active material is not particularly limited and is, for example, 2.0 mol / L from a practical viewpoint. The negative active material has a solubility of, for example, 0.8 mol / L to 2.0 mol / L, 1.0 mol / L to 2.0 mol / L, or 1.2 mol / L to 2.0 mol / L.

[0040] The central metal of the metal chelate complex is, for example, one selected from the group consisting of chromium (Cr), titanium (Ti), iron (Fe), and vanadium (V). When one of these is the central metal of the metal chelate complex, a negative active material formed of the metal chelate complex tends to increase electromotive force and dissolves easily in water. The chelate ligand of the metal chelate complex is, for example, one selected from the group consisting of propylenediaminetetraacetic acid (PDTA), ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), 1,2-diaminocyclohexane-N,N'-tetraacetic acid (CyDTA), and triethanolamine (TEA). When one of these is the chelate ligand of the metal chelate complex, a negative active material formed of the metal chelate complex tends to increase electromotive force and high solubility in water. The metal chelate complex is, for example, one selected from the group consisting of chromium propylenediaminetetraacetate (CrPDTA), CrEDTA, chromium diethylenetriaminepentaacetate (CrDTPA), chromium 1,2-diaminocyclohexane-N,N'-tetraacetate (CrCyDTA), vanadium diethylenetriaminepentaacetate (VDTPA), and iron triethanolamine (FeTEA). Negative active materials formed of these metal chelate complexes tend to increase electromotive force and dissolve easily in water. The standard oxidation-reduction potentials of these metal chelate complexes are as follows: CrPDTA: -1.1 V CrEDTA: -1.0 V CrDTPA: -1.4 V CrCyDTA: -1.2 V VDTPA: -1.2 V FeTEA: -1.0 V

[0041] The negative active material may be in the form of an anion. An anion of a metal chelate complex may be present in the negative electrolyte 41a. The anion of the metal chelate complex may be an anion type complex in which any of, for example, PDTA, EDTA, DTPA, CyDTA, DTPA, or TEA is coordinated. The negative active material may be in the form of a cation. A cation of a metal chelate complex in which a plurality of cations are attached to an anion of the metal chelate complex may be present in the negative electrolyte 41a. The cation of the metal chelate complex may be a cation type complex in which any of, for example, PDTA, EDTA, DTPA, CyDTA, DTPA, or TEA is coordinated.

[0042] The solvent for the negative electrolyte 41a is, for example, water. Water can be used as the solvent for the negative electrolyte 41a since the negative active material dissolves easily in water. (Battery Cell)

[0043] The battery cells 100 charge and discharge. The battery cells 100 are connected to the alternating current / direct current converter 7. Each battery cell 100 is separated into the positive cell 102 and the negative cell 103 by the membrane 101. The membrane 101 is, for example, an ion-exchange membrane. The ion-exchange membrane is, for example, an ion-exchange membrane through which electrons do not permeate and hydrogen ions permeate. The membrane 101 in this embodiment is a cation-exchange membrane. The positive cell 102 has a positive electrode 104 built thereinto. The negative cell 103 has a negative electrode 105 built thereinto. The positive cell 102 and the negative cell 103 can have known configurations.

[0044] The redox flow battery system 1 may include a single battery cell 100 or may include a plurality of battery cells 100. In this embodiment, as illustrated in FIG. 1, the redox flow battery system 1 includes a cell stack 200 including a plurality of battery cells 100 stacked together. In the cell stack 200, cell frames 120, the positive electrodes 104, the membranes 101, and the negative electrodes 105 are repeatedly stacked in sequence. End plates 210 are disposed at both ends of the cell stack 200. The end plates 210 are fastened with fastening members 230 to form the unitary cell stack 200. A known configuration can be used as appropriate as the configuration of the cell stack 200.

[0045] Each cell frame 120 includes 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 surrounds the outer periphery of the bipolar plate 121. The bipolar plate 121 and the frame body 122 form recesses inside the frame body 122. The recesses are provided on both sides of the bipolar plate 121. One of the recesses has the positive electrode 104 housed therein, whereas the other recess has the negative electrode 105 housed therein. The bipolar plate 121 is sandwiched between the positive electrode 104 and the negative electrode 105. The membrane 101 is sandwiched between the adjacent cell frames 120. The positive electrode 104 and the negative electrode 105 are disposed between the adjacent bipolar plates 121. In this way, one battery cell 100 is formed. For example, ring-shaped sealing members 127 are disposed between the frame bodies 122. The number of battery cells 100 in the cell stack 200 can be selected as appropriate.

[0046] The frame bodies 122 include liquid supply manifolds that supply the electrolytes and liquid discharge manifolds that discharge the electrolytes. The liquid supply manifolds and the liquid discharge manifolds are not illustrated in FIG. 1. Each manifold extends through the frame bodies 122. The cell frames 120 are stacked together to form flow channels for the electrolytes. The flow channels are connected to a first pipe 33 or 43 and a second pipe 34 or 44 described later. (Positive Electrolyte Circulation Mechanism and Negative Electrolyte Circulation Mechanism)

[0047] The positive electrolyte circulation mechanism 3 includes a positive electrolyte tank 31, the first pipe 33, the second pipe 34, and a pump 36. The positive electrolyte tank 31 contains the positive electrolyte 31a. The first pipe 33 is a flow channel for the positive electrolyte 31a from the positive electrolyte tank 31 toward the positive cells 102. The first pipe 33 connects the positive electrolyte tank 31 to the positive cells 102. The second pipe 34 is a flow channel for the positive electrolyte 31a from the positive cells 102 toward the positive electrolyte tank 31. The second pipe 34 connects the positive cells 102 to the positive electrolyte tank 31. The pump 36 pumps the positive electrolyte 31a from the positive electrolyte tank 31. The pump 36 is disposed in the first pipe 33.

[0048] The negative electrolyte circulation mechanism 4 includes a negative electrolyte tank 41, the first pipe 43, the second pipe 44, and a pump 46. The negative electrolyte tank 41 contains the negative electrolyte 41a. The first pipe 43 is a flow channel for the negative electrolyte 41a from the negative electrolyte tank 41 toward the negative cells 103. The first pipe 43 connects the negative electrolyte tank 41 to the negative cells 103. The second pipe 44 is a flow channel for the negative electrolyte 41a from the negative cells 103 toward the negative electrolyte tank 41. The second pipe 44 connects the negative cells 103 to the negative electrolyte tank 41. The pump 46 pumps the negative electrolyte 41a from the negative electrolyte tank 41. The pump 46 is disposed in the first pipe 43. (Test Example 1)

[0049] Test Example 1 is a test example in which the organic compound that forms the positive active material is an organic compound that is not an organometallic complex. The standard oxidation-reduction potential, diffusion coefficient, value correlated with electron transfer rate constant, and solubility of the organic compound of each sample were determined by calculation. The calculated values are shown in Table 1. (Sample No. 1-1)

[0050] The organic compound of Sample No. 1-1 has the structure illustrated in FIG. 2. The organic compound of Sample No. 1-1 contains a sulfo group. The organic compound of Sample No. 1-1 is a water-soluble derivative of TEMPO. The water-soluble derivative of TEMPO of Sample No. 1-1 is 2,2,6,6-tetramethylpiperidine-1-oxyl-4-sulfonate. (Sample No. 1-2)

[0051] The organic compound of Sample No. 1-2 has the structure illustrated in FIG. 3. The organic compound of Sample No. 1-2 contains a sulfo group. The organic compound of Sample No. 1-2 is a water-soluble derivative of TEMPO. The water-soluble derivative of TEMPO of Sample No. 1-2 is 2,2,6,6-tetramethylpiperidine-1-oxyl-4-oxypropanesulfonate. (Sample No. 1-3)

[0052] The organic compound of Sample No. 1-3 has the structure illustrated in FIG. 4. The organic compound of Sample No. 1-3 contains a phosphate group. The organic compound of Sample No. 1-3 is a water-soluble derivative of TEMPO. The water-soluble derivative of TEMPO of Sample No. 1-3 is 4-phosphonooxy-2,2,6,6-tetramethylpiperidine-1-oxyl. (Sample No. 1-4)

[0053] The organic compound of Sample No. 1-4 has the structure illustrated in FIG. 5. The organic compound of Sample No. 1-4 contains sulfur trioxide. The organic compound of Sample No. 1-4 is a water-soluble derivative of TEMPO. The sulfite of Sample No. 1-4 is 2,2,6,6-tetramethylpiperidine-1-oxyl-4-oxypropanesulfite. (Sample No. 1-5)

[0054] The organic compound of Sample No. 1-5 has the structure illustrated in FIG. 6. The organic compound of Sample No. 1-5 contains sulfur tetroxide. The organic compound of Sample No. 1-5 is a water-soluble derivative of TEMPO. The sulfate of Sample No. 1-5 is 2,2,6,6-tetramethylpiperidine-1-oxyl-4-oxypropanesulfate. (Standard Oxidation-Reduction Potential)

[0055] The standard oxidation-reduction potential is determined with respect to the potential of a standard hydrogen electrode as follows: 1. Structure files of reduced and oxidized forms of the organic compound of each sample are created. 2. For each of the structures of the reduced and oxidized forms, the total energy in a vacuum and the vibrational energy are calculated. Each energy is determined using UB3LYP / 6-311+g as a basis function in density functional theory (DFT) by structural optimization calculation in a vacuum state. 3. For each of the structures of the reduced and oxidized forms, the solvent effect is calculated. The solvent effect is determined using the polarizable continuum model (PCM) by designating the permittivity of the aqueous solvent and performing structural optimization. 4. For each of the structures of the reduced and oxidized forms, the potential in water is calculated. The potential in water is "(total energy in vacuum) + (solvent effect) - (vibrational energy)" 5. The oxidation-reduction potential on a vacuum basis is calculated from the difference between the potentials of the reduced and oxidized forms in water. The calculated value of the potential of 4-hydroxy-TEMPO (TEMPOL) and the correction value of the measured value of the potential of TEMPOL are derived on a water-soluble TEMPO derivative basis, and the calculated value and the correction value are added as a correction term for the results of calculation of the oxidation-reduction potential. Specifically, the correction value is the difference between the calculated and measured values of the potential of the substance used as a reference. The value obtained by adding the correction value to the calculated value of the potential of the substance to be estimated is the estimated value of the potential of the substance to be estimated. (Diffusion Coefficient)

[0056] The diffusion coefficient is the diffusion coefficient of an aqueous solution of a target for calculation at a concentration of 0.3 mol / L. The diffusion coefficient of the aqueous solution is determined using classical molecular dynamics (MD). The diffusion coefficient of the aqueous solution is the value at room temperature (25°C). (Value Correlated with Electron Transfer Rate Constant)

[0057] The value correlated with electron transfer rate constant is determined as follows. For a water-soluble derivative of TEMPO, there is a high correlation between the electron density of the nitrogen atom in the ring-shaped structure and the electronic transfer rate of the water-soluble derivative of TEMPO. The electron density of the nitrogen atom corresponds to the electron density of the nitroxyl radical. The structure of the target for calculation is optimized by DFT, and the natural bond orbital (NBO) charge of the optimized structure is calculated. It is assumed that, as the NBO charge of the nitroxyl radical becomes higher, the electron density becomes lower, and the electron transfer rate constant becomes higher. (Solubility)

[0058] The calculated value of solubility is determined as follows. The solubility of a water-soluble derivative of TEMPO in water is calculated by the conductor-like screening model for realistic solvation (COSMO-RS). The solubility is corrected using a correlation equation. The correlation equation is determined from calculated and measured values for molecules of known solubility. Specifically, the structure is optimized with the quantum chemistry calculation software "TURBOMOLE 2022" using TZVP as a basis function. For the optimized structure, calculation is performed by COSMO-RS using the software "COSMOtherm 2022".

[0059]

[0060] As shown in Table 1, the standard oxidation-reduction potential of the organic compound of each sample was 0.80 V or more. The standard oxidation-reduction potentials of Sample Nos. 1-1, 1-3, 1-4, and 1-5 were 0.83 V or more and were 0.85 V or more. The solubility of the organic compound of each sample was 1.70 mol / L or more. The solubilities of Sample Nos. 1-1, 1-2, 1-4, and 1-5 were 2.20 mol / L or more. The solubilities of Sample Nos. 1-1, 1-2, and 1-5 were 2.40 mol / L or more. A positive electrolyte containing a positive active material formed of the organic compound of each sample and a negative electrolyte containing a negative active material formed of, for example, CrPDTA, CrEDTA, CrDTPA, CrCyDTA, VDTPA, or FeTEA can be used to construct a redox flow battery system having high electromotive force and including active materials with high solubility in water. Specifically, a redox flow battery system having an electromotive force of 1.6 V or more, 1.8 V or more, or 2.0 V or more can be constructed.

[0061] As shown in Table 1, the diffusion coefficient of the organic compound of each sample was 2.30 × 10-6cm2 / s or more. "cm2 / s" is "square centimeters per second". The diffusion coefficients of Sample Nos. 1-1, 1-3, 1-4, and 1-5 were 3.00 × 10-6cm2 / s or more. The diffusion coefficients of Sample Nos. 1-1, 1-3, and 1-5 were 4.50 × 10-6cm2 / s or more. The value correlated with electron transfer rate constant, that is, the NBO charge of the nitroxyl radical, of the organic compound of each sample was 0.250 C or more. "C" is "coulombs". The NBO charges of the nitroxyl radicals of Sample Nos. 1-1, 1-3, 1-4, and 1-5 were 0.300 C or more and were 0.320 C or more. A positive electrolyte containing a positive active material formed of the organic compound of each sample can be used to construct a redox flow battery system with superior battery reactivity. (Test Example 2)

[0062] Test Example 2 is a test example in which the organic compound that forms the positive active material is an organic compound that is an organometallic complex. The calculated values of the standard oxidation-reduction potential, diffusion coefficient, and solubility of the organometallic complex of each sample were determined. The calculated values were determined by the same methods as in Test Example 1. The calculated values are shown in Table 2. (Sample No. 2-1)

[0063] The organometallic complex of Sample No. 2-1 has the structure illustrated in FIG. 7. The organometallic complex of Sample No. 2-1 contains a sulfo group. The organometallic complex of Sample No. 2-1 is a water-soluble derivative of ferrocene. The water-soluble derivative of ferrocene of Sample No. 2-1 is 1-sulfonylferrocene. (Sample No. 2-2)

[0064] The organometallic complex of Sample No. 2-2 has the structure illustrated in FIG. 8. The organometallic complex of Sample No. 2-2 contains a phosphate group. The organometallic complex of Sample No. 2-2 is a water-soluble derivative of ferrocene. The water-soluble derivative of ferrocene of Sample No. 2-2 is 1-phosphorylferrocene. (Sample No. 2-3)

[0065] The organometallic complex of Sample No. 2-3 has the structure illustrated in FIG. 9. The organometallic complex of Sample No. 2-3 contains a carboxy group. The organometallic complex of Sample No. 2-3 is a water-soluble derivative of ferrocene. The water-soluble derivative of ferrocene of Sample No. 2-3 is 3-ferrocenoylpropionic acid. (Sample No. 2-4)

[0066] The organometallic complex of Sample No. 2-4 has the structure illustrated in FIG. 10. The organometallic complex of Sample No. 2-4 contains a carboxy group. The organometallic complex of Sample No. 2-4 is a water-soluble derivative of ferrocene. The water-soluble derivative of ferrocene of Sample No. 2-4 is succinic acid ferrocenol ester.

[0067]

[0068] As shown in Table 2, the standard oxidation-reduction potential of the organometallic complex of each sample was 0.75 V or more. The standard oxidation-reduction potentials of Sample Nos. 2-1 and 2-2 were 0.80 V or more. The standard oxidation-reduction potential of Sample No. 2-1 was 0.82 V or more. The solubility of the organometallic complex of each sample was 0.75 mol / L or more. The solubilities of Sample Nos. 2-1, 2-2 and 2-3 were 0.80 mol / L or more. The solubilities of Sample Nos. 2-1 and 2-2 were 0.85 mol / L or more. A positive electrolyte containing a positive active material formed of the organometallic complex of each sample and a negative electrolyte containing a negative active material formed of, for example, CrPDTA, CrEDTA, CrDTPA, CrCyDTA, VDTPA, or FeTEA can be used to construct a redox flow battery system having high electromotive force and including active materials with high solubility in water.

[0069] As shown in Table 2, the diffusion coefficient of the organometallic complex of each sample was 4.50 × 10-6cm2 / s or more. The diffusion coefficients of Sample Nos. 2-1, 2-3, and 2-4 were 6.50 × 10-6cm2 / s or more. The diffusion coefficient of Sample No. 2-1 was 7.00 × 10-6cm2 / s or more. A positive electrolyte containing a positive active material formed of the organometallic complex of each sample can be used to construct a redox flow battery system with superior battery reactivity. Specifically, a redox flow battery system having an electromotive force of 1.6 V or more, 1.8 V or more, or 2.0 V or more can be constructed.

[0070] The present invention is not limited to these examples and is indicated by the claims, and all changes that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

[0071] 1 redox flow battery system 3 positive electrolyte circulation mechanism 31 positive electrolyte tank 31a positive electrolyte 33 first pipe 34 second pipe 36 pump 4 negative electrolyte circulation mechanism 41 negative electrolyte tank 41a negative electrolyte 43 first pipe 44 second pipe 46 pump 7 alternating current / direct current converter 71 transformer facility 8 power generation unit 9 load 100 battery cell 101 membrane 102 positive cell 103 negative cell 104 positive electrode 105 negative electrode 120 cell frame 121 bipolar plate 122 frame body 127 sealing member 200 cell stack 210 end plate 230 fastening member

Claims

1. A redox flow battery system comprising: a battery cell; a positive electrolyte to be supplied to the battery cell; and a negative electrolyte to be supplied to the battery cell, wherein the positive electrolyte contains a positive active material formed of an organic compound, the negative electrolyte contains a negative active material formed of a metal chelate complex, and the organic compound contains a phosphate group or a sulfo group.

2. The redox flow battery system according to claim 1, wherein the organic compound contains a phosphate group.

3. A redox flow battery system comprising: a battery cell; a positive electrolyte to be supplied to the battery cell; and a negative electrolyte to be supplied to the battery cell, wherein the positive electrolyte contains a positive active material formed of an organic compound, the negative electrolyte contains a negative active material formed of a metal chelate complex, and the organic compound contains a carboxy group, sulfur trioxide, or sulfur tetroxide.

4. The redox flow battery system according to any one of claims 1 to 3, wherein the positive active material is a water-soluble derivative of TEMPO.

5. The redox flow battery system according to any one of claims 1 to 3, wherein the positive active material is a water-soluble derivative of ferrocene.

6. The redox flow battery system according to any one of claims 1 to 5, wherein the battery cell includes a positive electrode, a negative electrode, and a membrane, the positive active material in the positive electrolyte and the negative active material in the negative electrolyte are anions, and the membrane is a cation-exchange membrane.

7. The redox flow battery system according to any one of claims 1 to 6, wherein a central metal of the metal chelate complex is one selected from the group consisting of chromium, titanium, iron, and vanadium.

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