JPWO2018207367A1 - Aqueous solution secondary battery, charge / discharge method of aqueous solution secondary battery, electrolyte for aqueous solution secondary battery, flow battery system, and power generation system - Google Patents

Abstract

A positive electrode, a negative electrode, a positive electrode electrolyte, a negative electrode electrolyte, and a diaphragm, wherein at least one of the positive electrode electrolyte and the negative electrode electrolyte is an iodide ion and an oxidation reaction product of the iodide ion. Aqueous secondary battery comprising an organic compound capable of separating water.

Description

  The present invention relates to an aqueous secondary battery, an aqueous secondary battery charging / discharging method, an electrolyte for an aqueous secondary battery, a flow battery system, and a power generation system.

  In recent years, global environmental problems have become increasingly serious, and there is a strong demand for the realization of a sustainable society that does not depend on fossil fuels. In particular, global warming due to an increase in atmospheric carbon dioxide is a major issue on a global scale. For this reason, it is expected that the spread of renewable energies such as wind and solar that do not emit carbon dioxide during power generation will be promoted worldwide in the future. However, since the output of renewable energy greatly fluctuates depending on the weather, it is difficult to use it stably as it is. Therefore, in order to smooth out this fluctuation, the demand for a power storage device for power storage that is safe, inexpensive, and suitable for upsizing is increasing.

  Examples of large-capacity power storage devices for storing power include sodium-sulfur (NAS) batteries, lead-acid batteries, and redox flow batteries. Since the NAS battery has a large capacity and a long life, it has been proposed to use it for a peak shift use, a renewable energy power use for a system cooperation use, and the like. Lead-acid batteries have high reliability, backed by a history of more than 100 years, and have a low cost per unit of storage capacity, which is advantageous for upsizing. It has been proposed for a wide range of applications, such as leveling stations. A redox flow battery can be easily increased in capacity by increasing the capacity of a tank, and thus is suitable for power storage applications.

  On the other hand, various power storage devices have disadvantages. Since the operating temperature of the NAS battery is as high as 300 ° C., there is a danger of ignition, and in addition, there is a danger of generating a toxic gas such as a sulfurous acid gas when ignited. Since lead-acid batteries have a low volume energy density, they require a large installation area, and lead is regulated worldwide as typified by the RoHS Directive. It is also possible that The redox flow battery has a low risk of ignition and a high safety if it is an aqueous electrolyte, but has a problem that the volume energy density is low when metal ions such as vanadium are used as the positive and negative electrode active materials.

Therefore, development of a large-capacity power storage device that achieves both high safety and high weight and volume energy density is awaited. Above all, various studies have been made on a large-capacity power storage device using iodide ions (I ⁻ ) as an electrode active material as a large-capacity power storage device that can achieve safety and high energy density.

For example, Patent Document 1 discloses a redox flow battery using chromium ions as a negative electrode active material and halogen ions as a positive electrode active material. The use of iodide ions as halogen ions is also disclosed.
Patent Document 2 discloses a redox flow battery using an aqueous solution of a halogen oxo acid compound as a negative electrode electrolyte. Halogen oxo acid compounds also include iodate, and mention is made of using an oxidation-reduction reaction between iodate ions and iodide ions.
Patent Document 3 and Non-Patent Document 1 disclose a redox flow battery using a metal ion and an iodide ion polymer as components of an electrolytic solution. In particular, Non-Patent Document 1 discloses that ethanol or ethylene glycol is added to an aqueous electrolyte solution using zinc iodide as an active material, thereby suppressing iodine deposition.

JP-A-61-24172 JP-T-2006-520982 US Patent Application Publication No. 2015/0147673

B. Li et al. , Nature commun. , 6,6303 (2015)

Since iodide ions have high solubility in water and easily cause a redox reaction, they can be expected as an electrode active material capable of achieving high energy density and excellent battery characteristics of an aqueous secondary battery using water as an electrolyte. . However, iodide ions in the electrolytic solution are deposited on the electrode surface as iodine (I ₂ ) by an oxidation reaction to form a film, which may cause various problems. For example, when the resistance of the iodine film formed on the electrode surface increases, the oxidation current value corresponding to the charging current value of the battery may be reduced. In addition, the iodine molecules diffused from the iodine film into the aqueous solution sublimate out of the system, so that the amount of active material in the electrolyte decreases, and the battery capacity may decrease. Furthermore, in the case of a redox flow battery in which charge and discharge are performed by flowing an electrolytic solution, the flow path of the electrolytic solution may be blocked by the iodine film.
On the other hand, in an aqueous secondary battery, in general, when the charge rate is low, the concentration of the charge reaction product in the electrolytic solution is so low that a sufficient discharge output may not be obtained.

  The present invention is directed to an aqueous secondary battery and an aqueous secondary battery charging method, in which the formation of an iodine film on the electrode surface is suppressed, and the discharge characteristics are excellent at a low charge rate. An object of the present invention is to provide a battery electrolyte, and a flow battery system and a power generation system using the same.

<1> A positive electrode, a negative electrode, a positive electrode electrolyte, a negative electrode electrolyte, and a diaphragm are provided, and at least one of the positive electrode electrolyte and the negative electrode electrolyte is oxidized with iodide ions and the iodide ions. An aqueous secondary battery comprising an organic compound capable of separating a reaction product.
<2> The aqueous solution according to <1>, wherein the content of the organic compound is 1% by volume to 50% by volume of at least one of the positive electrode electrolyte and the negative electrode electrolyte containing the organic compound. Next battery.
<3> The aqueous secondary battery according to <1> or <2>, wherein the organic compound includes at least one selected from a ketone, a carboxylate, and a carbonate.
<4> The organic compound according to any one of <1> to <3>, wherein the organic compound includes at least one selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate. The aqueous secondary battery according to the above.
<5> A positive electrode active material reaction tank containing the positive electrode electrolyte, a negative electrode active material reaction tank containing the negative electrode electrolyte, a positive electrode electrolyte storage tank, a negative electrode electrolyte storage tank, and a positive electrode electrolyte feed A pump and a negative electrode electrolyte liquid supply pump, wherein the positive electrode liquid electrolyte transfer pump can circulate the positive electrode liquid electrolyte between the positive electrode liquid storage tank and the positive electrode active material reaction tank. And the negative electrode electrolyte solution pump is configured to circulate the negative electrode electrolyte between the negative electrode electrolyte storage tank and the negative electrode active material reaction tank. The aqueous secondary battery according to any one of 1> to <6>.
<6> The aqueous secondary solution according to any one of <1> to <5>, wherein the positive electrode electrolyte contains the iodide ion and the organic compound, and the negative electrode electrolyte contains a negative electrode active material. battery.
<7> The negative electrode active material contains at least one metal ion selected from the group consisting of zinc, chromium, titanium, iron, tin, vanadium, lead, manganese, cobalt, nickel, copper, and lithium. 2. The aqueous secondary battery according to item 1.
<8> The aqueous secondary battery according to <6> or <7>, wherein the negative electrode active material contains zinc ions.
<9> An aqueous solution comprising a positive electrode, a negative electrode, and an electrolytic solution, wherein the electrolytic solution contains iodide ions, a negative electrode active material, and an organic compound capable of separating an oxidation reaction product of the iodide ions. Secondary battery.
<10> The aqueous secondary battery according to <9>, wherein the content of the organic compound is 1% by volume to 50% by volume of the electrolytic solution.
<11> The aqueous secondary battery according to <9> or <10>, wherein the organic compound includes at least one selected from a ketone, a carboxylate, and a carbonate.
<12> The organic compound according to any one of <9> to <11>, wherein the organic compound includes at least one selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate. The aqueous secondary battery according to the above.
<13> The negative electrode active material contains at least one metal ion selected from the group consisting of zinc, chromium, titanium, iron, tin, vanadium, lead, manganese, cobalt, nickel, copper, and lithium. The aqueous secondary battery according to any one of <12> to <12>.
<14> The aqueous secondary battery according to any one of <9> to <13>, wherein the negative electrode active material contains zinc ions.
<15> The aqueous secondary battery according to any one of <9> to <14>, wherein the positive electrode is located below the negative electrode in a vertical direction.
<16> The aqueous solution according to any one of <9> to <15>, further including a diaphragm disposed between the positive electrode and the negative electrode to divide the electrolyte into a positive electrode electrolyte and a negative electrode electrolyte. Secondary battery.
<17> An active material reaction tank containing the electrolyte, an electrolyte storage tank, and an electrolyte feed pump are further provided. The electrolyte feed pump is provided with the electrolyte storage tank and the active material reaction tank. The aqueous secondary battery according to any one of <9> to <16>, wherein the secondary battery is configured to be able to circulate the electrolytic solution between the secondary battery and the battery.
<18> A method for charging and discharging an aqueous secondary battery using an electrolytic solution containing iodide ions as a positive electrode active material, including a step of separating an oxidation reaction product of iodide ions generated in the electrolytic solution. , An aqueous secondary battery charging and discharging method.
<19> Water, iodide ions, and an organic compound capable of separating an oxidation reaction product of iodide ions, used as an electrolytic solution in the charge / discharge method of the aqueous secondary battery according to <18>. Solution for an aqueous secondary battery.
<20> The electrolytic solution for an aqueous secondary battery according to <19>, wherein the content of the organic compound is 1% by volume to 50% by volume of the electrolytic solution for an aqueous secondary battery.
<21> Water, at least one selected from the group consisting of iodine (I ₂ ), triiodide ion (I ₃ ⁻ ), and pentaiodide ion (I ₅ ⁻ ), methyl ethyl ketone, methyl acetate, and ethyl acetate And at least one organic compound selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate and propylene carbonate.
<22> The electrolytic solution for an aqueous secondary battery according to <21>, wherein the content of the organic compound is 1% by volume to 50% by volume of the electrolytic solution for an aqueous secondary battery.
<23> A method for charging and discharging an aqueous secondary battery using an electrolyte containing iodide ions as a positive electrode active material, comprising a step of separating an oxidation reaction product of iodide ions generated in the electrolyte. The electrolyte solution for an aqueous secondary battery according to <21> or <21>, which is used as an electrolyte for a charge / discharge method for an aqueous secondary battery.
<24> The aqueous secondary battery according to any one of <1> to <17>, charging and discharging are controlled, and the charging potential of the positive electrode is set to the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation) And a control unit that sets the voltage to 1.5 V or less based on the flow battery system.
<25> A power generation system including a power generation device and the flow battery system according to <24>.

  According to the present invention, the formation of an iodine film on the electrode surface is suppressed, and the aqueous secondary battery and the method of charging the aqueous secondary battery, which are excellent in the discharge characteristics at a low charge rate, Provided are an electrolyte for a secondary battery, and a flow battery system and a power generation system using the same.

1 is a schematic cross-sectional view illustrating a configuration example of an aqueous secondary battery according to a first embodiment. FIG. 2 is a schematic cross-sectional view showing a configuration example in a case where the aqueous secondary battery of the first embodiment further includes a device for flowing an electrolytic solution. It is a schematic sectional view showing the example of composition of the aqueous solution type secondary battery of a 2nd embodiment. It is a schematic sectional view showing the example of composition in case the aqueous solution type secondary battery of a 2nd embodiment is further provided with diaphragm 5 between cathode 1 and anode 2. It is a schematic sectional view showing the example of composition when the aqueous solution type secondary battery of a 2nd embodiment is further provided with a device for flowing an electrolytic solution. It is a schematic sectional view showing the example of composition of a flow battery system. It is the schematic which shows the example of a structure of a power generation system. It is a figure which shows an example of the generated power short-time waveform of wind power generation. 4 is a graph showing measurement results of oxidation current values performed on an electrolyte solution to which methyl ethyl ketone was added and an electrolyte solution to which methyl ethyl ketone was not added. It is a photograph of the hydrophobic substance containing the oxidation reaction product of iodide ion. 4 is a graph showing an oxidation current value at an oxidation potential and a reduction current value at a reduction potential observed in a hydrophobic substance. 5 is a graph showing a potential waveform of normal pulse voltammetry. It is a graph which shows a normal pulse voltammogram (pulse width 50ms). It is a graph which shows a normal pulse voltammogram (pulse width 50ms). 5 is a graph showing a potential waveform of reverse pulse voltammetry. It is a graph which shows a reverse pulse voltammogram (initial potential 0.50V, 0.55V, and pulse width 50ms). It is a graph which shows a reverse pulse voltammogram (initial potential 0.50V, 0.55V, and pulse width 50ms). It is a reverse pulse voltammogram and is a graph which shows the effect of methyl ethyl ketone contained in an electrolyte solution (initial potential 0.60V and pulse width 50ms). It is a reverse pulse voltammogram and is a graph which shows the effect of the propylene carbonate contained in electrolyte solution (initial potential 0.60V and pulse width 50ms). It is a graph which shows a reverse pulse voltammogram (initial potential 0.90V-1.10V and pulse width 50ms). It is a graph which shows a reverse pulse voltammogram (initial potential 0.90V-1.10V and pulse width 50ms). It is a graph which shows a reverse pulse voltammogram (initial potential 1.40V, 1.50V, and pulse width 50ms). It is a graph which shows a reverse pulse voltammogram (initial potential 1.40V, 1.50V, and pulse width 50ms). It is a graph which shows a normal pulse voltammogram (pulse width 50, 500, and 5000 ms). It is a graph which shows a normal pulse voltammogram (pulse width 50, 500, and 5000 ms). It is a graph which shows a normal pulse voltammogram, and is a graph which showed the effect of methyl ethyl ketone contained in an electrolytic solution (pulse width 5000 ms). It is a graph which shows a normal pulse voltammogram, and is a graph which showed the effect of propylene carbonate contained in an electrolytic solution (pulse width 5000 ms). It is a schematic diagram which shows the electrode reaction of a flow battery system. It is a current potential curve of a positive electrode and a negative electrode of a flow battery.

  Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, the components (including the element steps and the like) are not essential unless otherwise specified. The same applies to numerical values and their ranges, and does not limit the present invention.

In the present disclosure, the numerical ranges indicated by using “to” include the numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In the numerical ranges described in stages in the present disclosure, the upper limit or lower limit described in one numerical range may be replaced with the upper limit or lower limit of the numerical range described in other stages. . Further, in the numerical range described in the present disclosure, the upper limit or the lower limit of the numerical range may be replaced with the value shown in the embodiment.
In the present disclosure, each component may include a plurality of corresponding substances. When there are a plurality of substances corresponding to each component in the composition, the content or content of each component is, unless otherwise specified, the total content or content of the plurality of substances present in the composition. Means quantity.
In the present disclosure, the term "film" or "film" refers to the case where the film or the film is formed over the entire region when the region where the film or the film is present is observed, and only a part of the region. The case where it is formed is also included.

  When an embodiment is described with reference to the drawings in the present disclosure, the configuration of the embodiment is not limited to the configuration illustrated in the drawings. Further, the size of the members in each drawing is conceptual, and the relative relationship between the sizes of the members is not limited to this.

<Aqueous solution type secondary battery (first embodiment)>
The aqueous secondary battery of the embodiment is
A positive electrode, a negative electrode, a positive electrode electrolyte, a negative electrode electrolyte, and a diaphragm, wherein at least one of the positive electrode electrolyte and the negative electrode electrolyte is an iodide ion and an oxidation reaction product of the iodide ion. And an organic compound that can be separated.

  As a result of investigations by the present inventors, it has been found that the aqueous secondary battery having the above-described structure is suppressed from forming an iodine film on the electrode surface and has excellent discharge characteristics at a low charge rate. More specifically, by containing an organic compound capable of separating an oxidation reaction product of iodide ions into the electrolytic solution, the dissolution of the iodine film formed on the electrode surface is promoted, and the electrolytic solution is separated from the electrolytic solution. It was found that by using the oxidation reaction product for the reduction reaction of the positive electrode, a sufficient discharge output was obtained even at a low charge rate.

  In any of the above-mentioned patent documents, the dissolution of the iodine film formed on the electrode surface is promoted by including an organic compound capable of separating the oxidation reaction product of iodide ions into the electrolytic solution, and the separated oxidation The technical idea of utilizing the reaction product for the reduction reaction of the positive electrode is not described.

  In the present disclosure, “aqueous solution-based secondary battery” means a secondary battery using an aqueous solution in which a necessary component is dissolved in water as an electrolyte. Specifically, a power storage device such as a redox flow battery (flow battery) is exemplified.

  In the present disclosure, “a positive electrode electrolyte” means an electrolyte that is in contact with a positive electrode, and “a negative electrode electrolyte” means an electrolyte that is in contact with a negative electrode. In the following description, both or one of the positive electrode electrolyte and the negative electrode electrolyte is also simply referred to as “electrolyte”.

In the present disclosure, “oxidation reaction product of iodide ion” means a substance generated by an oxidation reaction of iodide ion. Examples include iodine (I ₂ ), triiodide ion (I ₃ ⁻ ), pentaiodide ion (I ₅ ⁻ ), and combinations thereof.

  In the aqueous secondary battery of this embodiment, the electrolyte is divided into a positive electrode electrolyte and a negative electrode electrolyte. The aqueous secondary battery having such a configuration is also referred to as a “two-component aqueous secondary battery”.

  In the aqueous secondary battery of this embodiment, at least the positive electrode electrolyte of the positive electrode electrolyte and the negative electrode electrolyte is an organic compound capable of separating iodide ions and an oxidation reaction product of iodide ions (hereinafter simply referred to as “ It is preferable that the positive electrode electrolyte contains iodide ions and an organic compound, and the negative electrode electrolyte contains a negative electrode active material.

The aqueous secondary battery of the present embodiment is excellent in energy density because iodide ions are used as an active material contained in the electrolytic solution. Furthermore, the organic compound contained in the electrolytic solution separates the oxidation reaction product of iodide ions, thereby promoting the dissolution of the iodine film formed on the electrode surface and lowering the oxidation current value due to the formation of the iodine film. together but are suppressed, it can also be expected effect of inhibiting I ₂ molecules diffuse in the electrolyte solution from iodine coating is sublimation out of the system. Furthermore, since the oxidation reaction product separated by the organic compound can be used for the reduction reaction in the positive electrode, a high-power discharge can be obtained even in a state where the concentration of the charging reaction product is low and the charging rate is low.

In the aqueous secondary battery of this embodiment, an oxidation reaction product of iodide ions contained in the electrolytic solution is selectively separated using an organic compound. The mode of separating the oxidation reaction product of iodide ions using an organic compound is not particularly limited, but the electrolytic solution is separated into a hydrophobic substance containing the oxidation reaction product of iodide ions and a bulk aqueous solution. Is preferred.
The hydrophobic substance may be, for example, an organic compound obtained by forming a network structure with iodide ions and its oxidation reaction product, a cation, and a small amount of water. For example, it may be in a state of a hydrophobic liquid.

  The solubility of iodine molecules in hydrophobic substances is higher than that in water, and is more stable in hydrophobic substances. The iodine molecule has sublimability and a low solubility in water of about 1 mM. For this reason, iodine molecules existing in water are easily sublimated from the gas-liquid interface, but sublimation of iodine molecules existing in a hydrophobic substance is suppressed. Therefore, by separating the oxidation reaction product of iodide ions as a hydrophobic substance, sublimation of iodine molecules can be suppressed, and reduction of iodide ions in the electrolytic solution can be suppressed.

  When the oxidation reaction product of iodide ion is separated as a hydrophobic substance, the oxidation product of iodide ion is present in the separated hydrophobic substance at a higher concentration than in the bulk aqueous solution. Therefore, by contacting a hydrophobic substance with the positive electrode and using the oxidation reaction product contained therein for the reduction reaction of the positive electrode, a high-power discharge can be obtained even at a low charge rate. In addition, when the cathode is configured to be located at the lower part in the vertical direction as described later, the oxidation reaction product contained in the hydrophobic substance that precipitates because the specific gravity is larger than that of the bulk aqueous solution is reduced by the cathode at the reduction reaction. Can be used for

The organic compound is not particularly limited as long as it can separate the oxidation reaction product of iodide ion, but should be able to separate the oxidation reaction product of iodide ion as a hydrophobic substance for the reasons described above. preferably, it is more preferable affinity for I ₂ molecule is highly organic compounds than water. High organic compound also affinity than water for the I ₂ molecule, promotes the dissolution of the iodine coatings coordinated organic compound iodine film. For this reason, formation of an iodine film can be suppressed more efficiently. When the organic compound does not form a hydrophobic substance, it is preferably in a state of being dissolved in the electrolytic solution.

  Examples of the organic compound include at least one selected from the group consisting of ketones, carboxylic esters, and carbonates. Specific examples of the ketone include methyl ethyl ketone. Specific examples of the carboxylic acid ester include methyl acetate, ethyl acetate and the like. Specific examples of the carbonate ester include dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate.

  Among the above compounds, ketone is preferable from the viewpoint of lowering the viscosity of an aqueous solution (electrolyte solution) containing the ketone and lowering the battery resistance. Carboxylic acid esters are more chemically stable to iodine than ketones and are preferred from the viewpoint of increasing Coulomb efficiency. Carbonate is preferred because it has a high specific gravity and a high ability to separate an oxidation reaction product of iodide ions as a hydrophobic substance.

  The organic compound contained in the electrolyte may be only one kind or two or more kinds. The content of the organic compound in the electrolyte is preferably 1% by volume to 50% by volume at normal temperature (25 ° C.) and normal pressure, and more preferably 5% by volume to 40% by volume. When the content of the organic compound in the electrolytic solution is 1% by volume or more, the oxidation reaction product of iodide ions tends to be well separated even in a state of high charge, and when the content is 50% by volume or less, There is a tendency that a decrease in the conductivity of the electrolytic solution is suppressed.

  The content of the organic compound in the electrolytic solution can be identified, for example, by measuring the retention time corresponding to the concentration of the organic compound and the molecular weight of the monitor ion by gas chromatography.

  The electrolyte may contain a supporting electrolyte salt. The supporting electrolyte salt functions as an auxiliary agent for increasing the ionic conductivity of the electrolytic solution. When the electrolytic solution contains the supporting electrolyte salt, the ionic conductivity of the electrolytic solution increases, and the internal resistance of the aqueous secondary battery tends to decrease.

The supporting electrolyte salt is not particularly limited as long as it is a compound that dissociates in the electrolyte to form ions. Specifically, HCl, HNO ₃ , H ₂ SO ₄ , HClO ₄ , NaCl, Na ₂ SO ₄ , NaClO ₄ , KCl, K ₂ SO ₄ , KClO ₄ , NaOH, LiOH, KOH, alkyl ammonium salt, alkylimidazo Lium salts, alkylpiperidinium salts, alkylpyrrolidinium salts and the like can be mentioned. One type of supporting electrolyte salt may be used alone, or two or more types may be used in combination. Further, the salt containing iodine can serve as both the positive electrode active material and the supporting electrolyte salt.

  The electrolyte may contain a pH buffer. Examples of the pH buffer include an acetate buffer, a phosphate buffer, a citrate buffer, a borate buffer, a tartar buffer, a Tris buffer and the like.

  FIG. 1 is a schematic cross-sectional view illustrating an example of a configuration example of an aqueous secondary battery of the present embodiment. The solid arrows shown in FIG. 1 indicate the flow of electrons during charging, and the dotted arrows indicate the reactions of ions during charging.

  The aqueous secondary battery shown in FIG. 1 includes a positive electrode 1, a negative electrode 2, a positive electrode active material reaction tank 3, a negative electrode active material reaction tank 4, and a diaphragm 5. The positive electrode active material reaction tank 3 and the negative electrode active material reaction tank 4 contain a positive electrode electrolyte and a negative electrode electrolyte, respectively.

  In the aqueous secondary battery shown in FIG. 1, the positive electrode electrolyte contains iodide ions as a positive electrode active material and an organic compound capable of separating an oxidation reaction product of iodide ions, and the negative electrode electrolyte contains the negative electrode active material. Contains.

  The material of the positive electrode 1 is preferably selected from those having corrosion resistance to iodide ions contained in the positive electrode electrolyte. Specifically, a metal having high corrosion resistance, such as titanium, or a carbon material may be used. From the viewpoint of cost, a carbon material is preferable. The shape of the positive electrode 1 is not particularly limited. From the viewpoint of obtaining a larger output, a shape having a large specific surface area is preferable. Examples of the shape having a large specific surface area include a porous body, felt, and paper.

  The material of the negative electrode 2 is not particularly limited, and can be selected according to the type of the negative electrode active material to be used. For example, when zinc is used as the negative electrode active material, a material such as a carbon material, zinc, or a zinc-plated metal material on which zinc is deposited on the surface is used as the negative electrode. Examples of the shape of the negative electrode 2 include a mesh, a porous body, a punched metal, and a flat plate, but are not particularly limited.

  The positive electrode electrolyte contained in the positive electrode active material reaction tank 3 contains iodide ions as a positive electrode active material. The positive electrode electrolyte containing iodide ions can be prepared by dissolving an iodine compound in the electrolyte. Examples of the iodine compound include sodium iodide, potassium iodide, zinc iodide, hydrogen iodide, lithium iodide, ammonium iodide, barium iodide, calcium iodide, magnesium iodide, strontium iodide, and the like. The concentration of iodide ions in the positive electrode electrolyte is not particularly limited, but is preferably 0.01 M to 20 M, and more preferably 0.1 M to 10 M. When the iodide ion concentration is 0.01 M or more, a sufficient energy density tends to be obtained, and when it is 20 M or less, an organic compound described later tends to be sufficiently dissolved in the positive electrode electrolyte.

  The details and preferred embodiments of the components such as the organic compound contained in the positive electrode electrolyte are the same as the details and preferred embodiments of the components such as the organic compound contained in the electrolyte described above.

The negative electrode electrolyte contained in the negative electrode active material reaction tank 4 contains a negative electrode active material (X ⁺ ). A reduction reaction product (X) is generated by the reduction reaction of the negative electrode active material. The negative electrode active material is not particularly limited as long as the standard redox potential of the reaction system is lower than the standard redox potential of the positive electrode (0.536 V when the positive electrode active material is iodide ion). Examples include ions of zinc, chromium, titanium, iron, tin, vanadium, lead, manganese, cobalt, nickel, copper, lithium, quinone-based materials, viologen, and the like. Among these negative electrode active materials, metal ions are preferred, and zinc ions are more preferred. Zinc ions have higher solubility in water than other metal ions (for example, zinc chloride has a solubility of 30 M or more), have a low standard oxidation-reduction potential for dissolution-precipitation reaction (−0.76 V), and are inexpensive. The negative electrode electrolyte containing metal ions can be prepared by dissolving the metal compound in the electrolyte. For example, a negative electrode electrolyte containing zinc ions can be prepared by dissolving a zinc compound such as zinc chloride, zinc iodide, zinc bromide, zinc fluoride, zinc nitrate, zinc sulfate, and zinc acetate.

  The diaphragm 5 plays a role of separating the positive electrode electrolyte and the negative electrode electrolyte. The material of the diaphragm 5 is not particularly limited. For example, an ion exchange membrane having excellent acid resistance and high ionic conductivity is mentioned as a preferred example, but a porous membrane, a glass filter and the like can also be suitably used.

  FIG. 2 is a schematic cross-sectional view illustrating a configuration in a case where the aqueous secondary battery illustrated in FIG. 1 further includes a device for flowing an electrolytic solution. Description of the elements included in the aqueous secondary battery illustrated in FIG. 2 that are common to the elements included in the aqueous secondary battery illustrated in FIG. 1 will be omitted.

  The aqueous secondary battery shown in FIG. 2 includes a positive electrode 1, a negative electrode 2, a positive electrode active material reaction tank 3, a negative electrode active material reaction tank 4, and a diaphragm 5. Further, a positive electrode electrolyte storage tank 6 and a positive electrode electrolyte feed pump 8 for supplying the positive electrode electrolyte to the positive electrode active material reaction tank 3, and a negative electrode electrolyte for supplying the negative electrode electrolyte to the negative electrode active material reaction tank 4. The apparatus includes a liquid storage tank 7 and a negative electrode electrolyte solution supply pump 9, and an external power supply 10 for driving the positive electrode solution supply pump 8 and the negative electrode solution supply pump 9.

  In FIG. 2, arrows indicate the direction in which the electrolyte flows. In the case shown in FIG. 2, the cathode electrolyte is supplied from the cathode electrolyte storage tank 6 to the cathode active material reaction tank 3 by the cathode electrolyte feed pump 8, and the oxidation-reduction reaction of iodine proceeds. The negative electrode electrolyte is supplied from the negative electrode electrolyte storage tank 7 to the negative electrode active material reaction tank 4 by the negative electrode solution feed pump 9, and the oxidation-reduction reaction of the negative electrode active material proceeds. The positive electrode solution feed pump 8 and the negative electrode solution feed pump 9 are driven by an external power supply 10.

  As shown in FIG. 2, a battery that performs charging and discharging by flowing an electrolytic solution is called a redox flow battery. Since the capacity of the redox flow battery depends on the amount of the electrolyte, there is an advantage that the capacity can be easily increased by increasing the capacity of the tank for storing the electrolyte. In addition, by circulating the electrolytic solution, the dissolution of the iodine film by the organic compound contained in the electrolytic solution is promoted, so that the formation of the iodine film is effectively suppressed, and the clogging of the flow channel by the iodine film is effectively prevented. Is done.

  In the configuration shown in FIG. 2, the flow of the electrolyte in the aqueous secondary battery is performed by driving the positive electrolyte feed pump 8 and the negative electrolyte feed pump 9 by the external power supply 10. The positive electrode electrolyte solution pump 8 and the negative electrode electrolyte solution pump 9 may be driven by using the power generated by the secondary battery itself. In this case, a self-contained configuration without power supply from outside is provided.

<Aqueous solution type secondary battery (second embodiment)>
The aqueous secondary battery of this embodiment includes a positive electrode, a negative electrode, and an electrolytic solution, and the electrolytic solution can separate iodide ions, a negative electrode active material, and an oxidation reaction product of the iodide ions. And an organic compound.

  In the aqueous secondary battery of this embodiment, the electrolyte is not divided into a positive electrode electrolyte containing a positive electrode active material (iodide ion) and a negative electrode electrolyte containing a negative electrode active material. (Iodide ion) and a negative electrode active material. Hereinafter, the aqueous secondary battery having such a configuration is also referred to as “one-pack aqueous secondary battery”.

  In the aqueous secondary battery according to the present embodiment, the details and preferable aspects of the positive electrode, the negative electrode, the electrolytic solution, the active material (the positive active material and the negative active material), the organic compound, and the like are described in the aqueous secondary battery according to the first embodiment. The details and preferred embodiments of the positive electrode, the negative electrode, the electrolytic solution, the active material, the organic compound, and the like in are the same.

  FIG. 3 is a schematic cross-sectional view illustrating an example of a configuration example of the aqueous secondary battery of the present embodiment. Solid arrows indicate the flow of electrons during charging, and dotted arrows indicate the reactions of ions during charging.

  The aqueous secondary battery shown in FIG. 3 includes a positive electrode 1, a negative electrode 2, and an active material reaction tank 11. The active material reaction tank 11 contains an electrolytic solution. Further, the electrolytic solution contains an iodide ion as a positive electrode active material, an anode active material, and an organic compound capable of separating an oxidation reaction product of the iodide ion.

  In the aqueous secondary battery according to the present embodiment, the positive electrode is preferably located below the negative electrode in the vertical direction. When the positive electrode is located below the negative electrode, an oxidation reaction product of iodide ions separated by the organic compound precipitates in the electrolytic solution and collects on the positive electrode side. Thereby, the oxidation reaction product of iodide ions can be effectively used for the reduction reaction at the positive electrode.

  In the aqueous secondary battery of the present embodiment, as the negative electrode active material, it is preferable to use a material having a property that the reduction reaction product stays near the negative electrode. As a result, the charge reaction products generated at the positive electrode and the negative electrode can be present on the positive electrode side and the negative electrode side, respectively, and the two can be spatially separated. As a result, it is possible to suppress the occurrence of self-discharge due to contact between the charge reaction products generated at the positive electrode and the negative electrode, respectively.

  Advantages of using an aqueous secondary battery as a one-liquid type are as follows: (1) the diaphragm disposed between the positive electrode and the negative electrode can be omitted; (2) water molecules move between the positive electrode side and the negative electrode side during charging and discharging (3) Oxidation reaction products of iodide ions separated by the organic compound precipitate and collect on the positive electrode side, so that the electrolyte flows. (4) When an apparatus for flowing an electrolytic solution is provided, there is no need to distinguish between the positive electrode side and the negative electrode side, and the configuration can be simplified.

  If necessary, the aqueous secondary battery of the present embodiment may include a diaphragm 5 between the positive electrode 1 and the negative electrode 2. FIG. 4 is a schematic cross-sectional view illustrating a configuration in a case where the aqueous secondary battery illustrated in FIG. 3 further includes a diaphragm 5 between the positive electrode 1 and the negative electrode 2. The description of the components of the aqueous secondary battery shown in the drawing that are common to those of the aqueous secondary battery shown in FIG. 1 will be omitted.

  Details and preferable aspects of the diaphragm 5 included in the aqueous secondary battery shown in FIG. 4 are the same as the details and preferred aspects of the diaphragm 5 included in the aqueous secondary battery of the first embodiment. By providing a diaphragm in the one-component aqueous secondary battery, it is possible to further suppress the contact between the charge reaction products generated at the positive electrode and the negative electrode, thereby further improving the coulomb efficiency of the one-component aqueous secondary battery. Can be improved.

  If necessary, the aqueous secondary battery of the present embodiment may further include a device for flowing the electrolytic solution. FIG. 5 is a diagram showing an apparatus for flowing the electrolytic solution in the aqueous secondary battery shown in FIG. 4, an electrolytic solution storage tank 12 for supplying the electrolytic solution to the active material reaction tank 11, and an electrolytic solution sending pump 13. FIG. 5 is a schematic cross-sectional view showing a configuration in a case where the apparatus further includes an external power supply 10 for driving an electrolyte solution pump 13.

  Although the aqueous secondary battery shown in FIG. 5 includes both the diaphragm 5 and a device for flowing the electrolyte, the embodiment may not include the diaphragm 5.

  In FIG. 5, arrows indicate the direction in which the electrolyte flows. The electrolytic solution is supplied from the electrolytic solution storage tank 12 to the active material reaction tank 11 by the electrolytic solution sending pump 13. At the positive electrode, the oxidation-reduction reaction of iodide ions proceeds, and an oxidation reaction product is generated during the oxidation reaction. On the other hand, in the negative electrode, the oxidation-reduction reaction of the negative electrode active material proceeds, and a reduction reaction product is generated during the reduction reaction. The electrolyte supply pump 13 is driven by the external power supply 10.

  In the configuration shown in FIG. 5, the flow of the electrolyte in the aqueous secondary battery is performed by driving the electrolyte feed pump 13 by the external power supply 10, but the power generated by the aqueous secondary battery itself is used. The electrolyte feed pump 13 may be driven. In this case, a self-contained configuration without power supply from outside is provided.

<Charging / discharging method for aqueous secondary battery>
The charge / discharge method for an aqueous secondary battery of the present embodiment (hereinafter, also simply referred to as a charge / discharge method) is a method for charging / discharging an aqueous secondary battery using an electrolytic solution containing iodide ions as a positive electrode active material. And a step of separating an oxidation reaction product of iodide ions generated in the electrolytic solution.

When the above charging / discharging method is applied as a charging / discharging method for an aqueous secondary battery, an oxidation reaction product of iodide ions generated by charging is selectively separated from an electrolytic solution. Thus, reduction of the oxidation current value dissolution of the high resistance of the iodine film formed on the electrode surface is promoted is inhibited, that I ₂ molecules diffused from the iodine film in the electrolytic solution is sublimated out of the system Reduced concentration of iodide ions in the electrolyte is suppressed, and the separated oxidation reaction product is used for the reduction reaction at the positive electrode, improving the output characteristics of the aqueous secondary battery at a low charge rate. Can be expected.

  The method for separating the oxidation reaction product of iodide ions generated in the electrolytic solution is not particularly limited. For example, there is a method of separating an oxidation reaction product of iodide ions using an electrolytic solution used for the above-mentioned aqueous secondary battery.

<Electrolyte for Aqueous Solution Secondary Battery (First Embodiment)>
The electrolyte solution for an aqueous secondary battery of the present embodiment (hereinafter, also simply referred to as an electrolyte solution) includes water, iodide ions, and an organic compound capable of separating an oxidation reaction product of iodide ions, It is used as an electrolyte in the above-described method for charging and discharging an aqueous secondary battery.

<Electrolyte for Aqueous Solution Secondary Battery (Second Embodiment)>
The electrolytic solution for an aqueous secondary battery of the present embodiment includes water, iodide ions, and at least one selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate. And an organic compound.

When the electrolytic solution of each of the above embodiments is used for an aqueous secondary battery, an oxidation reaction product of iodide ions generated by charging is selectively separated from the electrolytic solution. Thus, reduction of the oxidation current value dissolution of the high resistance of the iodine film formed on the electrode surface is promoted is inhibited, that I ₂ molecules diffused from the iodine film in the electrolytic solution is sublimated out of the system Reduced concentration of iodide ions in the electrolyte is suppressed, and the separated oxidation reaction product is used for the reduction reaction at the positive electrode, improving the output characteristics of the aqueous secondary battery at a low charge rate. It is possible to expect effects such as being able to.

  The details and preferred embodiments of the iodide ions and the organic compound contained in the electrolyte are the same as the details and preferred embodiments of the iodide ions and the organic compound contained in the electrolyte used in the aqueous secondary battery described above.

<Flow battery system>
The flow battery system according to the present embodiment controls the charge and discharge of the aqueous secondary battery described above, and sets the charge potential of the positive electrode to 1.5 V or less based on the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). And a control unit that sets

  The details and preferred embodiments of the aqueous secondary battery used in the flow battery system are the same as the details and preferred embodiments of the aqueous secondary battery described above. The control unit in the flow battery system may be integrated with the aqueous secondary battery or may not be integrated.

In the above-mentioned flow battery system, the charging potential of the positive electrode is preferably set to 1.3 V or less, and is preferably set to 1.05 V or less based on the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). More preferred.

Hereinafter, the advantage of controlling the charging potential of the positive electrode so as not to exceed a reference value based on the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation) in the flow battery system will be described.

In the flow battery system, the positive electrode electrolyte contains iodide ions as a positive electrode active material. For this reason, iodide ions (I ⁻ ) are oxidized at the positive electrode by the charging reaction represented by the following formulas (1) and (2), and I ₃ ⁻ and I ₂ are generally generated, and the generated I ₃ ⁻ and I ₂ by discharge reaction shown in equation (1) and (2), is reduced at the cathode and I ^- to become.

3I ⁻ ⇔I ₃ ⁻ + 2e ⁻ (1)
2I ⁻ ⇔I ₂ + 2e ⁻ (2)

In the flow battery system, when the charging potential of the positive electrode exceeds 1.05 V with reference to the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation), the following equation is used together with the charging reaction represented by the above equations (1) and (2). IO shown in (3) ₃ ^- generation reaction occurs in (Reference 1: P.Beran and S.Bruchenstein, Voltammetry of Iodine (I) Cholride, Iodine and Iodate at Rotated Platinum Disk and Ring-Disk Electrodes, Analytical Chemistry, 40 , 1044 (1968)).

I ⁻ + 3H ₂ O → IO ₃ ⁻ + 6H ⁺ + 6e ⁻ (3)

  The reaction represented by the above formula (3) is reported to be an irreversible reaction, and the reaction rate of the reverse reaction is extremely low.

Further, by Dushman reaction represented by the following formula (4), the equation (3) IO ₃ generated by ^- _{I 2} is generated from.

IO ₃ ⁻ + 5I ⁻ + 6H ⁺ → 3I ₂ + 3H ₂ O (4)

  Further, the following formula (5) is obtained as the total reaction of the above formulas (3) and (4) (formula (3) + formula (4)).

I ₂ + 6H ₂ O → 2IO ₃ ⁻ + 12H ⁺ + 10e ⁻ (5)

According to the above document 1, the chemical reaction rate of the above equation (4) is faster than the electrochemical reaction of the above equation (3), and the chemical reaction rate of the equation (5) in which the equations (3) and (4) are constituent reactions. The rate-determining process is an electrochemical reaction of the formula (3). Therefore, the charge potential of the positive electrode, Ag / AgCl reference electrode ^- becomes particularly noticeable if it exceeds 1.05V relative to the potential of (Cl concentration saturation), 1.3V taking a safety margin, the 1.5V It may be set so as not to exceed, and it is assumed that the reactions of the above formulas (3) to (5) have occurred.

For this reason, when charging is repeated at a positive electrode charging potential exceeding 1.05 V with reference to the potential of the Ag / AgCl reference electrode, IO ₃ ⁻ is generated based on the equation (3) by the positive electrode charging reaction. IO ₃ ⁻ has a slow reaction rate of a discharge reaction, which is the reverse reaction of the formula (3), and is very unlikely to return to I ⁻ .

Furthermore, IO ₃ generated based on equation (3) ^- by reaction of formula (4), I in the electrolyte ^- reacted with, in the electrolytic solution without electron transfer at the positive electrode I ^- Is consumed.

In addition, I ₂ is reacted to form IO ₃ ⁻ by the reaction represented by the formula (5), which is the total reaction of the formulas (3) and (4), but similarly to the reaction represented by the formula (3), The reaction shown in the formula (5) is also an irreversible reaction. For this reason, the generated reaction rate of IO ₃ ⁻ is a reaction rate of a discharge reaction which is a reverse reaction of the formula (5), and it is very difficult to return to I ₂ .

Therefore, when charging is repeated at a positive electrode charging potential exceeding 1.05 V, the flow rate gradually increases due to an increase in the proportion of IO ₃ ⁻ that does not contribute to the discharge reaction and a decrease in the total concentration of I ⁻ and I _2. The battery system has a problem that the positive electrode discharge capacity and the positive electrode charge capacity decrease.

On the other hand, the flow battery system of the present embodiment includes a control unit that controls charging and discharging and sets the charging potential of the positive electrode to 1.05 V or less based on the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). ing. Thus, Ag / AgCl reference electrode ^- can set the potential of the (Cl concentration saturation) below 1.5V as a reference, IO ₃ during charging of the flow cell system ^- can inhibit the production reaction of. By suppressing the generation of IO ₃ ⁻ , the positive electrode discharge capacity and the positive electrode charge capacity can be maintained by maintaining the total concentration of iodine ions and iodine molecules during reversible charge and discharge. Therefore, in the present embodiment, it is possible to provide a flow battery system satisfying practical charging conditions.

  Note that the charging potential of the positive electrode in the flow battery system is not the charging voltage. The charging potential is a voltage indicated for an electrode having a certain reference potential. The charging voltage is the difference between the potential of the negative electrode and the potential of the positive electrode. Since the charging potential is based on a constant potential serving as a reference, a constant potential can be regarded as a constant value with respect to the reference electrode. However, in the case of the charging voltage between the negative electrode and the positive electrode, if the potential of the negative electrode and the positive electrode fluctuates in the same manner, the voltage becomes apparently constant. Accordingly, the potential of the positive electrode is not determined by the charging voltage, but needs to be measured with respect to the potential of the reference electrode for the positive electrode which is a reference.

(Control unit)
The flow battery system includes a control unit that controls charging and discharging, and sets the charging potential of the positive electrode to 1.5 V or less based on the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). Thus, Ag / AgCl reference electrode ^- can set the potential of the (Cl concentration saturation) below 1.5V as a reference, IO ₃ during charging of the flow cell system ^- can inhibit the production reaction of. By suppressing the generation of IO ₃ ⁻ , the total concentration of iodine ions and iodine molecules (I ⁻ , I ₃ ⁻ and I ₂ ) during reversible charge / discharge is maintained to maintain the positive electrode discharge capacity and the positive electrode charge capacity. The decrease is suppressed, and the cycle durability can be improved.

In addition, "the charging potential of the positive electrode is set to 1.5 V or less based on the potential of the Ag / AgCl reference electrode (Cl ^- concentration saturation)" basically means that the charging potential of the positive electrode is 1.5 V or less. This means charging the flow battery, and it is also acceptable that the positive electrode charging potential exceeds 1.5 V. For example, when it is inevitable that the charging potential of the positive electrode exceeds 1.5 V due to the influence of ripple noise or the like described later, the charging potential of the positive electrode may exceed 1.5 V.

  For example, the control unit performs constant-current charging until the voltage reaches the set voltage under the condition that the charging potential of the positive electrode does not exceed 1.5 V (vs. Ag / AgCl), and performs constant-voltage charging after the voltage reaches the set voltage. To control the flow battery.

Further, it is preferable that the control unit controls the positive electrode charging potential to 1.5 V or less based on the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). By controlling the charging potential of the positive electrode to 1.5 V (vs. Ag / AgCl) or less, deterioration of the positive electrode (particularly, carbon electrode) tends to be suppressed. Further, when the positive electrode electrolyte contains ethanol as a good solvent for iodine molecules, the decomposition of ethanol is further suppressed by controlling the charging potential of the positive electrode to 1.5 V (vs. Ag / AgCl) or less. There is a tendency.

In addition, “controlling the charging potential of the positive electrode to 1.5 V or less based on the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation)” means that the charging potential of the positive electrode is 1.5 V or less and the charging of the flow battery is performed. And the charging potential of the positive electrode is not allowed to exceed 1.5V.

For example, when the charging potential of the positive electrode exceeds 1.5 V (vs. Ag / AgCl) due to the influence of ripple noise or the like described later, the control unit controls the flow battery to cut off the excess using a high frequency filter or the like. Control. When the charging potential of the positive electrode falls within the range of 1.05 V to 1.5 V (vs. Ag / AgCl) even when ripple noise described later is superimposed, the control unit does not need to perform any special control. This is because the generation reaction of IO ₃ ⁻ represented by the above-described equation (3) is considered to be difficult to follow a high-frequency signal such as ripple noise.

In the flow battery system, the charging potential of the positive electrode is preferably set to less than 1.5 V with reference to the potential of the Ag / AgCl reference electrode (Cl ^- concentration saturation), more preferably to 1.3 V or less. It is more preferable to set the voltage to 1.05 V or less.
When operating the flow battery system under the condition that the charging potential of the positive electrode exceeds 1.5 V, during the operation period, the positive electrode electrolyte may be periodically sampled using a sampling unit described later. The concentration of the components contained in the positive electrode electrolyte may be adjusted by adding the electrolyte or the components contained therein to the positive electrode.

(Reference electrode for positive electrode)
The flow battery system may include a positive electrode reference electrode for measuring a positive electrode potential.
The positive electrode reference electrode may be any one that can be converted to a potential with respect to a standard hydrogen electrode potential and can exhibit a stable electrochemical potential. A reference electrode serving as an electrochemical potential reference is described in many textbooks as a basic matter of electrochemical chemistry (for example, Allen J. Bard and Larry R. Faulkner, “ELECTROCHEMICAL METHODDS” p. 3, (1980), John Wiley & Sons, Inc.). Examples of the reference electrode include an Ag / AgCl reference electrode and a saturated calomel electrode, and the Ag / AgCl reference electrode is preferable.

The reference electrode for the positive electrode is not limited to the Ag / AgCl reference electrode as long as it can convert the measured potential of the positive electrode to the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). May be used.

  Further, the flow battery system may further include a negative electrode reference electrode for measuring the potential of the negative electrode. The reference electrode may be provided at one place on the positive electrode, preferably at one place at each of the positive and negative electrodes, and more preferably at two or more places at each of the positive electrode and the negative electrode.

(Sampling unit)
The flow battery system may include a sampling unit that samples the positive electrode electrolyte. By sampling the positive electrode electrolyte in the sampling unit, it is possible to analyze the concentration of the component contained in the positive electrode electrolyte. You can analyze whether there is a shortage.

  The sampling unit may be disposed in, for example, a positive electrode electrolyte storage tank, or may be disposed in a circulation path. Further, the sampling unit may be configured to sample the positive electrode electrolyte every predetermined time.

(Density adjustment unit)
The flow battery system analyzes the cathode electrolyte sampled by the sampling unit and adjusts the concentration of the component contained in the cathode electrolyte circulating between the cathode and the cathode electrolyte storage tank based on the analysis result. A density adjusting unit may be provided. If the concentration of the components contained in the positive electrode electrolyte sampled by the sampling unit is insufficient compared with the specified amount, required amount, etc., the flow battery system is provided with the concentration adjusting unit. The concentration of the components added to the solution and contained in the positive electrode electrolyte can be adjusted.

  The concentration adjusting section may be configured to add each component to the positive electrode electrolyte stored in the positive electrode electrolyte storage tank, or may be configured to add each component to the positive electrode electrolyte flowing through the circulation path. You may. Further, the addition of the iodine compound, the additive, and the like to the positive electrode electrolyte may be performed during the operation of the flow battery or may be stopped.

(Potential measurement unit)
The flow battery system may include a potential measurement unit that measures a potential based on the concentrations of iodine ions and iodine molecules in the positive electrode electrolyte. The potential measurement unit includes, for example, a current collecting electrode for measuring a potential based on the concentration of iodine ions and iodine molecules, and a reference electrode serving as a reference for the electrochemical potential, and measures the reference electrode-based electrochemical potential. I do. By using the Nernst equation relating to the electrochemical potential, the concentrations of iodine ions and iodine molecules can be obtained from the measured electrochemical potential with reference to the reference electrode. Examples of the collecting electrode include a platinum electrode and a graphite electrode, and examples of the reference electrode include an Ag / AgCl electrode.

The control unit can estimate the state of charge (SOC) based on the potential measured by the potential measurement unit. For example, when only I ⁻ , I ₃ ⁻ , and I ₂ are considered as the oxidation-reduction substance, the SOC of 0% means that the cathode electrolyte basically does not include I ₃ ⁻ and I _2, but only I ⁻ Indicates a state where Furthermore, SOC is the 100%, I in essentially positive electrode electrolyte ^- not included, _{I 3} ^- shows a state that is only and _{I 2.}

  The potential measuring unit may be arranged, for example, in a positive electrode electrolyte storage tank, or may be arranged in a circulation path in which the positive electrode electrolyte circulates.

(Configuration example of flow battery system)
FIG. 6 shows an example of the flow battery system of the present embodiment. As shown in FIG. 6, the flow battery system 100 includes a positive electrode 111, a negative electrode 112, a diaphragm 115, a positive electrode reference electrode 113, a positive electrode electrolyte 116, a positive electrode electrolyte storage tank 118, and a negative electrode electrolyte 117. , A negative electrode electrolyte storage tank 119, circulation paths 120, 121 and pumps 122, 123 as liquid sending parts, and control of charging and discharging, and the charging potential of the positive electrode to the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). A control unit (not shown) for setting the potential to 1.05 V or less based on the potential is provided. The components of the flow battery system 100 are as described above.

  The flow battery system 100 illustrated in FIG. 6 includes a cell stack 130 including a plurality of single cells each including a positive electrode 111, a negative electrode 112, and a diaphragm 115. FIG. 6 shows the cell stack 130 having five single cells, but the number of single cells is not particularly limited. Further, in the flow battery system 100 shown in FIG. 6, the positive electrode reference electrode 113 and the negative electrode reference electrode 114 are arranged on the positive electrode 111 and the negative electrode 112 constituting the cell stack, and the potential measurement using the reference electrode is possible. It has become.

  In the flow battery system 100 shown in FIG. 6, the sampling unit 124 that samples the positive electrode electrolyte 116 and the potential measurement unit 125 that measures the potential based on the concentration of iodine ions and iodine molecules in the positive electrode electrolyte 116 include the positive electrode electrolyte. It is arranged in a storage tank 118.

  In the flow battery system 100 shown in FIG. 6, the positive electrode electrolyte 116 is circulated between the positive electrode chamber in which the positive electrode 111 is disposed and the positive electrode electrolyte storage tank 118, and the negative electrode chamber in which the negative electrode 112 is disposed and the negative electrode electrolyte Circulation paths 120 and 121 for circulating the negative electrode electrolyte 117 with the storage tank 119 and liquid sending pumps 122 and 123 are arranged as liquid sending units.

  The charging and discharging of the flow battery system 100 is controlled by a control unit (not shown). As described above, the control unit can set the charging potential of the positive electrode to 1.5 V (vs. Ag / AgCl) or less.

<Power generation system>
The power generation system according to the present embodiment includes a power generation device and the above-described flow battery system. The power generation system of the present embodiment can level and stabilize power fluctuations and stabilize power supply and demand by combining a flow battery system and a power generation device.

  The power generation system includes a power generation device. The type of power generation device is not particularly limited, and examples thereof include a power generation device that generates power using renewable energy, a hydraulic power generation device, a thermal power generation device, and a nuclear power generation device. Among them, a power generation device that generates power using renewable energy is preferable.

  A power generation device using renewable energy has a large amount of power generation that fluctuates due to weather conditions and the like. However, in combination with a flow battery system, power generation that fluctuates can be leveled to supply leveled power to a power system. .

  Examples of the renewable energy include wind power, solar light, wave power, tidal power, flowing water, tide, geothermal, and the like, and wind power or solar light is preferable.

  Power generated by using renewable energy such as wind power or solar power may be supplied to a high-voltage power system. Normally, wind power generation and solar power generation are affected by weather such as wind direction, wind power, and weather. It is not preferable to supply the generated power that is not constant to the high-voltage power system as it promotes the instability of the power system. The power generation system of the present embodiment can level the generated power waveform to a target power fluctuation level, for example, by superimposing the charge / discharge waveform of the flow battery system on the generated power waveform.

  When the above-described flow battery system is applied to such a renewable energy field, a charge potential exceeding 1.5 V (vs. Ag / AgCl) per single cell of the flow battery system is supplied to supply power to a high-voltage system. The required case may occur. When the charging potential of the positive electrode of the single cell is 1.5 V (vs. Ag / AgCl), the charging voltage of the entire single cell, that is, the potential difference between the positive electrode and the negative electrode exceeds 3 V. When each cell stack of the flow battery system has a configuration in which 20 cells are connected in series, the charging voltage of each cell stack is 60V. Further, when ten cell stacks are connected in series, the charging voltage becomes 600V. Charging of the flow battery system is performed by converting AC power generated by wind power generation or the like into DC power by an inverter. Therefore, the voltage range of the charge control voltage is determined by the relationship between the cell stack of the flow battery system and the output of the inverter.

  In the case where the charging voltage of the inverter is constant, if the number of single cells in the cell stack is small, the charging voltage applied to each single cell increases. Conversely, if the number of single cells in series in the cell stack is large, the charging voltage applied to each single cell decreases. Therefore, when a flow battery system is installed in a power generation system using renewable energy, the charging voltage applied per unit cell of the flow battery system is determined based on the inverter output and the number of single cells connected in series in the cell stack as basic parameters. .

  FIG. 2 shows an example of the power generation system of the present embodiment. FIG. 7 is a configuration diagram when the power generation system is applied to the wind power generation field. In FIG. 7, FB (Flow Battery) indicates a flow battery, and PCS (Power Conditioning System) indicates an inverter control system for AC / DC conversion. The FB and PCS in FIG. 7 correspond to the above-described flow battery system of the present embodiment.

  The generated power waveform shown in FIG. 7 is an example of a power waveform generated by the wind power generator. In the case of wind power generation, the generated power greatly varies depending on the strength and direction of the wind. When such fluctuating power is superimposed on a power system such as a transmission line, it affects the stabilization of the power system. Therefore, when supplying electric power by wind power generation to the electric power system, it is necessary to suppress fluctuations in the electric power of the electric power system.

  In order to suppress this variation, a charge / discharge waveform for reducing the variation of the generated power waveform is output from the flow battery system and superimposed on the generated power waveform. The flow battery system plays a role of leveling the generated power obtained by wind power generation and supplying it as stabilized power.

  Here, FIG. 8 shows a power waveform when the power waveform of the wind power generation is viewed on a shorter time scale. While power waveforms in a relatively long time region shown in regions (a) and (b) of FIG. 8 can be seen, the power waveforms on the shorter side than the region (a) and the regions (a) and (b) Meanwhile, pulse-like power generation waveforms on the order of microseconds to milliseconds are observed in three time regions longer than the region (b). At this time, the flow battery system uses the target output for a certain time width of the wind power as the center value, supplements the power by discharging if the generated power is lower than it, and charges using the generated power if it exceeds the target output. Alternatively, the charge / discharge may be controlled so as to approach the target output.

  The inverter is a converter for exchanging power between a charge / discharge signal of a flow battery, which is DC information, and generated power. The charging of the flow battery is performed by converting AC power from the wind power generation device into DC power. The inverter easily generates a pulsed high-frequency signal called ripple noise. In general, these high-frequency signals can be removed by installing capacitors capable of handling the respective frequency bands in the PCS. However, in a PCS in which these measures are not taken, a high-frequency ripple signal is applied to the flow battery.

  Ideally, the response speed of the flow battery can follow the output fluctuations of the PCS for wind power generation, but it is actually difficult. Due to the existence of the time constant (CR) defined by the electric double layer capacity at the electrode interface of the flow battery and the resistance of the flow battery, the signal of the power fluctuation in the microsecond order to tens of millisecond order region has a battery reaction I can't follow. In the case shown in FIG. 8, it is difficult to completely equalize power fluctuations in the time domain in which signals in the microsecond order to millisecond order are collected and high frequency signals in which ripple noise is also superimposed are collected by charging and discharging the flow battery. . In particular, in the case of a large-sized flow battery, the electric double layer capacity is increased by increasing the surface area of the electrode, and the time constant is increased, whereby this behavior becomes apparent.

  When the flow battery system is applied to wind power generation, photovoltaic power generation, and the like, frequent charging / discharging is repeated via an inverter during power generation in order to level the generated power. The power signal supplied to the flow battery, including the ripple noise generated from the inverter, includes a high-frequency power signal exceeding the following capability of the flow battery. When a high frequency power signal that cannot be followed by a flow battery is applied to the flow battery, that power is basically converted to heat. This heat tends to be concentrated on the electrode terminals of the flow battery, and easily affects the constituent materials of the flow battery.

  As described above, the charging voltage applied per unit cell of the flow battery system is determined based on the inverter output and the number of unit cells in series of the cell stack as basic parameters. There is a relationship. Therefore, it is preferable to design the flow battery system such that the positive electrode charging potential is 1.5 V (vs. Ag / AgCl) or less in consideration of the number of single cells in series, the number of cell stacks, and the charging voltage. Even if it is necessary to exceed 1.5 V in design, the charging potential of the positive electrode may be controlled to 1.5 V (vs. Ag / AgCl) or less in order to secure the life of the flow battery system. preferable.

  Due to the design of the flow battery system, when the flow battery system is operated under the condition that the charging potential of the positive electrode exceeds 1.5 V (vs. Ag / AgCl), during the operation period, the positive electrode electrolyte and the negative electrode electrolyte or are included in the same. It is preferred to carry out the addition of the components. However, when a volatile component is included, it is preferable to periodically analyze even in an operating environment where the positive electrode charging potential does not exceed 1.5 V, and to add the component if necessary.

  Even when the flow battery system is operated under the condition that the charging potential of the positive electrode does not exceed 1.5 V (vs. Ag / AgCl), regarding the ripple noise of the inverter, a signal such that the charging potential of the positive electrode exceeds 1.5 V is used. It seems reasonable to think that it is included. In order to promote the deterioration of the flow battery system due to the ripple noise of the inverter, it is preferable to install a capacitor having a bandwidth capable of absorbing the ripple noise in the PCS.

  Further, the power generation system may be a system that controls charging and discharging of the flow battery system according to the supply and demand of the generated power generated by the power generation device. For example, if the amount of power generated by the power generator exceeds the demand in the power system, the flow battery system performs charging, and the amount of power generated by the power generator reduces the demand in the power system. If it is less than the amount, the power generation system may be controlled so that the flow battery system discharges.

  The power generation system combines a power generation device that uses renewable energy with a flow battery system to enable the flow battery system to function as a low-cost, high-energy-density power storage system and to further reduce carbon dioxide emissions. It can help solve the global problem of controlling global warming.

  Hereinafter, the present invention will be described specifically with reference to Examples, but the scope of the present invention is not limited to these Examples.

[Example 1]
The following experiment was conducted to verify the effect of separating an oxidation reaction product of iodide ions using an organic compound contained in an electrolytic solution.

(1) The effect of improving the oxidation current value when the electrolytic solution contained an organic compound was examined by experiments. Specifically, an electrolyte solution in which 5% by volume of methyl ethyl ketone was added as an organic compound to an aqueous solution of 3 M sodium iodide (NaI) and an electrolyte solution in which methyl ethyl ketone was not added were prepared, and a glassy carbon electrode was used as an electrode. Was used to measure the magnitude of the oxidation current value at a specific potential. FIG. 9 shows the measurement results. The vertical axis in the figure represents the current value (mA), and the horizontal axis represents time (s).

The iodine film chemically reacts with iodide ions in the bulk aqueous solution to be dissolved as triiodide ions (I ₃ ⁻ ) (I ⁻ + I ₂ → I ₃ ⁻ ). However, when the formation rate of the iodine film exceeds the dissolution rate, the iodine film grows on the electrode surface and the electric resistance increases. Therefore, the oxidation current value is limited by the iodine film, and a steady state is established at a current value at which formation and dissolution of the iodine film are opposed. As a result of the experiment, the current value (mA) in the steady state where the current value is constant when the electrolytic solution to which methyl ethyl ketone is added is about 30% higher than that when the electrolytic solution to which methyl ethyl ketone is not added is used. Was. From the above results, it was found that the addition of the organic compound to the electrolytic solution promoted the dissolution of the iodine film and improved the oxidation current value.

(2) FIG. 10 is a photograph of a hydrophobic substance generated when the measurement of FIG. 9 is performed. This hydrophobic substance was in a liquid state, had a higher methyl ethyl ketone concentration than the bulk aqueous solution, and contained iodide ions and their oxidation reaction products, cations, and water. Also, the formed hydrophobic substance settled at the bottom of the container.

(3) FIG. 11 is a graph showing an oxidation current value at an oxidation potential and a reduction current value at a reduction potential observed in a hydrophobic substance containing methyl ethyl ketone and an oxidation reaction product of iodide ions. is there. In the graph, the vertical axis represents the current value (mA), and the horizontal axis represents time (s). A positive current value represents an oxidation current value, and a negative current value represents a reduction current value. As shown in FIG. 11, both the oxidation current and the reduction current were observed. From the above results, it was confirmed that both the oxidation reaction and the reduction reaction proceed in the hydrophobic substance.

[Example 2]
The oxidation reaction was examined by normal pulse voltammetry using an electrolyte containing methyl ethyl ketone or propylene carbonate as an organic compound, and the electrode reaction of the positive electrode was verified.
FIG. 12 is a graph showing a potential waveform of normal pulse voltammetry performed in Example 2. In FIG. 12, Ei denotes an initial potential, ΔEs denotes a pulse increment, tp denotes a pulse width, and τ denotes a pulse period. The potential waveform shown in FIG. 12 was input to an electrochemical cell using a potentiostat as an electrochemical measurement device, and current values corresponding to each pulse potential and pulse time were measured.

The potentiostat is a general device in electrochemical measurement, and controls a pulse potential shown in FIG. 12 with respect to a reference electrode potential as a potential reference, and is observed based on an electrochemical reaction progressing at a working electrode. This is a device that detects a current flowing through the device. Further, a counter electrode is provided so that a current flows through the counter electrode. Input resistance of the reference electrode is typically 10 ¹⁴ ohm level very large DC resistance, current of the electrochemical reactions proceeding at the working electrode is in a circuit configuration which flows all the counter electrode.

  The potentiostat includes three electrodes: a reference electrode serving as a potential reference, a working electrode subject to potential control, and a counter electrode. Recently, with the development of microcomputers, the waveform of the normal pulse voltammetry shown in FIG. 12 is generally integrated with a potentiostat function and can be programmed.

13A and 13B are graphs showing a normal pulse voltammogram obtained in Example 2 (pulse width: 50 ms). The voltammogram is a current-potential curve in which a current observed based on an electrochemical reaction is plotted against a potential.
FIG. 13A shows a case where a solution containing 95 vol% of a 20 mM sodium iodide aqueous solution containing 1 M sodium perchlorate (NaClO ₄ ) and 10 vol% of methyl ethyl ketone as a supporting electrolyte was used as an electrolytic solution, and glassy carbon (diameter: 1.6 mm), and the results when ΔEs = 0.05 V and τ = 20 s are shown.
FIG. 13B shows a solution containing 95 vol% of a 20 mM sodium iodide aqueous solution containing 1 M sodium perchlorate (NaClO ₄ ) and 10 vol% of propylene carbonate as a supporting electrolyte as an electrolyte, and glassy carbon ( The results are shown when ΔEs = 0.05 V and τ = 20 s using a diameter of 1.6 mm).
13A and 13B, the horizontal axis represents the potential (V vs. Ag / AgCl), and the vertical axis represents the current density (mA / cm ² ). The current density is a value obtained by dividing the current value 50 ms after the step of the oxidation potential by the electrode area (the same applies hereinafter). The measurement was performed in an environment at a liquid temperature of 25 ° C.

  Next, by reverse pulse voltammetry, a reduction reaction opposite to that of normal pulse voltammetry was investigated to verify the electrode reaction of the positive electrode.

  FIG. 14 is a graph showing a potential waveform of reverse pulse voltammetry performed in Example 2. Reverse pulse voltammetry can be performed using a programmed potentiostat as in normal pulse voltammetry. Ei is the initial potential, Ec is the potential at which the reaction of interest does not proceed (conditioning potential), ΔEs is the increment of the reverse pulse potential, tc is the time held at Ec, td is the time held at Ei, and tp is the reverse pulse width. Ec was an immersion potential, ΔEs = 0.05 V, tc = 10 s, and td = 2 s.

Reverse pulse voltammetry is obtained I in the normal pulse voltammetry ^- has a function of further examination of the behavior of the oxidation reaction. That is, it is possible to verify what electrochemical behavior the product generated at the initial potential shows. When the product generated at the initial potential is an oxidation reaction product, when the reverse pulse potential reaches a certain potential region, the behavior of the reduction reaction of the oxidation reaction product can be captured.

  The pulse potential of the reverse pulse repeats the increment of the reverse pulse potential, and is stepped in a lower direction with the initial potential Ei as a starting potential. When one potential step is completed, the potential is maintained at the conditioning potential Ec at which the oxidation-reduction reaction hardly proceeds. A time (tc) at which the boundary condition of the working electrode recovers to the same level as before the reaction is held in the conditioning potential Ec. After time tc, the potential is stepped to the initial potential Ei, and an oxidation reaction (generally an oxidation or reduction reaction) proceeds on the working electrode during td. After controlling the initial potential for the time td, a reverse pulse is applied. Reverse pulse voltammetry is performed by this repetition, and the reaction itself, the reaction mechanism, and the like can be closely examined based on the relationship between the current and the potential of the obtained reverse pulse.

  In both normal pulse voltammetry and reverse pulse voltammetry, the boundary conditions of the working electrode are common between the respective pulses.Therefore, in the relationship between the reaction product and the potential, the concentration of the reactant reacting at the initial potential can be considered to be constant. This is a great merit that the simplification can be made in examining the relationship between the current and the potential. Since the potential returns to the conditioning potential after the end of one pulse, the information of the arbitrary pulse potential does not accompany the history of the reactant generated at the pulse potential before the pulse. It can be said that the advantage of normal pulse voltammetry and reverse pulse voltammetry lies in that the electrochemical information to be observed can be simply extracted as described above.

FIGS. 15A and 15B are reverse pulse voltammograms performed in Example 2 (initial potentials 0.50 V, 0.55 V, and pulse width 50 ms). 15A and 15B are graphs showing the relationship between the step potential and the current value after holding at the initial potential (0.50 V, 0.55 V for 2 seconds).
FIG. 15A shows a solution containing 95 vol% of a 20 mM sodium iodide aqueous solution containing 1 M sodium perchlorate and 5 vol% of methyl ethyl ketone as a supporting electrolyte as an electrolytic solution, and glassy carbon (diameter 1.6 mm) for an electrode. And the result when the pulse width of the reverse pulse was set to 50 ms.
FIG. 15B shows a solution containing 95 vol% of a 20 mM aqueous solution of sodium iodide containing 1 M sodium perchlorate and 5 vol% of propylene carbonate as a supporting electrolyte, and glassy carbon (diameter 1.6 mm) for the electrode. ) Is used, and the result is shown when the pulse width of the reverse pulse is 50 ms.

As shown in FIGS. 15A and 15B, when the initial potential was set to 0.5 V and 0.55 V, a clear limit current relating to the reduction reaction was observed. Therefore, the products at the initial potentials of 0.5 V and 0.55 V are ions, and are I ₃ ⁻ under these conditions. That, I ^- I ₃ is an oxidation reaction product ^- was confirmed behavior is reduced.

FIGS. 16A and 16B are reverse pulse voltammograms performed in Example 2, and show the effect of methyl ethyl ketone or propylene carbonate contained in the electrolyte (initial potential 0.60 V and pulse width 50 ms).
FIG. 16A shows a 20 mM sodium iodide aqueous solution containing 95 vol% of 1 M sodium perchlorate as a supporting electrolyte and an aqueous solution containing 5 vol% of methyl ethyl ketone, and a 20 mM iodide containing 1 M sodium perchlorate as a supporting electrolyte. The results obtained when each of the sodium aqueous solutions was used as the electrolytic solution are shown.
FIG. 16B shows a 20 mM sodium iodide aqueous solution containing 95 vol% of 1 M sodium perchlorate as a supporting electrolyte, an aqueous solution containing 5 vol% of propylene carbonate, and a 20 mM iodine containing 1 M sodium perchlorate as a supporting electrolyte. The results obtained when each of the aqueous sodium chloride solutions was used as the electrolytic solution are shown.

  As shown in FIG. 16A and FIG. 16B, when methyl ethyl ketone or propylene carbonate was not contained in the electrolytic solution, it was observed that the reduction current value increased as the potential stepped lower. This behavior is a behavior observed in a solid phase electrochemical reaction (for example, a dissolution reaction from a metal plating surface). Since no diffusion overvoltage was required due to diffusion from the solution, it was confirmed that as the pulse potential of the reverse pulse increased, the current of the reduction reaction increased without showing a limiting current. When the electrolyte contained methyl ethyl ketone or propylene carbonate, the reduction current was the same as in the case of no addition in the range of 0.3 V to 0.6 V, but decreased in the range of 0.25 V or less. This behavior is thought to be due to the fact that the solution contains methyl ethyl ketone or propylene carbonate to promote reduction of the iodine film and dissolution of the iodine film.

FIGS. 17A and 17B are reverse pulse voltammograms (initial potentials of 0.9 V to 1.1 V and a pulse width of 50 ms) performed in Example 2. The initial potential is higher than the initial potentials in FIGS. 16A and 16B. The potential is set (similarly held for 2 seconds).
FIG. 17A shows the result under the same conditions as in FIG. 16A except for the initial potential, and FIG. 17B shows the result under the same conditions as in FIG. 16B except for the initial potential.

  The reduction current values observed in FIG. 17A and FIG. 17B are roughly divided into two groups. In one group, the initial potential was set to 0.9 V and 1.0 V, and it was observed that the reduction current value greatly increased as the reverse pulse potential was stepped lower. On the other hand, the other group is a case where the initial potential is 1.05 V and 1.1 V, and the reduction current value of the reverse pulse is lower than the case where the initial potential is 0.9 V and 1.0 V. Was. Since reverse pulse voltammetry looks at the difference in the reduction reaction rate of the generated species at the initial potential, the difference in the reverse pulse voltammogram between these groups is considered to be the product difference due to the difference in the initial potential. Is the simplest.

I ^- by the reaction shown in the following equation (1) and (2), _{I 3} ^- and to generate the _{I 2} are known.

2I ⁻ → I ₂ + 2e ⁻ (1)
3I ⁻ → I ₃ ⁻ + 2e ⁻ (2)

The standard oxidation-reduction potentials of the formulas (1) and (2) are approximately equal at 0.536 V (standard hydrogen electrode potential). Therefore, the oxidation reaction products of I ⁻ generated at the initial potential of the reverse pulse voltammetry are I ₂ and I ₃ ⁻ . The change of the standard electrode potential with respect to the temperature in the equation (7) is −0.148 mV per 1 ° C. (Reita Tamamushi, “Electrochemistry (Second Edition)” p.300, (1991), Tokyo Chemical Dojin). That is, in an environment of −25 ° C. where the temperature has dropped from 25 ° C. to 50 ° C., the standard electrode potential of the equation (2) changes only by 7.4 mV from 0.536 (standard hydrogen electrode potential). Although the electrochemical potential basically depends on the temperature, it is considered that the relationship between the battery reaction and the potential does not fluctuate as much as 100 mV at a practical living environment temperature as described above.

Here, when the initial potential exceeds 1.05 V, IO ₃ ⁻ is generated by the reaction represented by the following formula (3).

I ⁻ + 3H ₂ O → IO ₃ ⁻ + 6H ⁺ + 6e ⁻ (3)

As mentioned above, equation (3) is reported to be an irreversible reaction. Since Equation (3) is a irreversible reaction, even if the reverse pulse voltage reaches the reduction reaction area, resulting IO ₃ ^- very small rate of reaction, the resulting IO ₃ ^- is I by reduction ^- easily return to It is presumed that there is no.

Further, by Dushman reaction represented by the following formula (4), the formula (4) IO ₃ generated by ^- _{I 2} is generated from.

IO ₃ ⁻ + 5I ⁻ + 6H ⁺ → 3I ₂ + 3H ₂ O (4)

  Further, the following formula (5) is obtained as the total reaction of the above formulas (3) and (4) (formula (3) + formula (4)).

I ₂ + 6H ₂ O → 2IO ₃ ⁻ + 12H ⁺ + 10e ⁻ (5)

According to the above document 1, the chemical reaction rate of the above equation (4) is faster than the electrochemical reaction of the above equation (3), and the rate limiting of the equation (5) in which the equations (3) and (4) are constituent reactions. The process is an electrochemical reaction of the formula (3). Therefore, when the charge potential of the positive electrode exceeds 1.05 V with reference to the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation), it is estimated that the reactions of the above equations (3) to (5) have occurred. Is done.

In the reaction shown in the formula (5), which is the total reaction of the formulas (3) and (4), I ₂ is reacted to produce IO ₃ ⁻ , and as in the reaction shown in the formula (3), The reaction shown in (5) is also an irreversible reaction. For this reason, it is assumed that the generated IO ₃ ⁻ has a low discharge reaction rate and is unlikely to return to I ₂ .

Therefore, it is considered that IO ₃ ⁻ was generated when the initial potential exceeded 1.05 V, resulting in a reverse pulse voltammogram having a low reduction reaction rate.

Therefore, when methyl ethyl ketone or propylene carbonate is contained in the positive electrode electrolyte, it is considered that IO ₃ ⁻ is generated by an irreversible reaction under a charging potential condition in which the charging potential of the positive electrode exceeds 1.05 V. For this reason, it turns out that it is preferable to operate in a state where the charging potential of the positive electrode does not exceed 1.05 V.

FIGS. 18A and 18B are reverse pulse voltammograms performed in Example 2 (initial potentials of 1.4 V and 1.5 V and a pulse width of 50 ms), and are initially higher than the initial potentials in FIGS. 16A and 16B. The potential is set (similarly held for 2 seconds).
FIG. 18A shows the result under the same conditions as in FIG. 16A except for the initial potential, and FIG. 18B shows the result under the same conditions as in FIG. 16B except for the initial potential.

  As shown in FIGS. 18A and 18B, it is found that the reduction current value observed with the reverse pulse is lower when the initial potential is 1.5 V or more than when the initial potential is 1.4 V. Was. Furthermore, it was found that the reduction current value tends to decrease as the initial potential increases. From this, it was presumed that the electrode was inactivated by maintaining the initial potential at a noble potential of 1.5 V or more.

[Example 3]
19A and 19B are normal pulse voltammograms performed in Example 3 (pulse widths 50, 500, and 5000 ms). In the third embodiment, the current density is measured when the pulse width is set to 50 ms, 500 ms, and 5000 ms. Other conditions are the same as in the second embodiment.
FIG. 19A shows the result when a solution containing 5 vol% of methyl ethyl ketone in 95 vol% of a 3 M aqueous solution of sodium iodide was used as the electrolytic solution.
FIG. 19B shows the results when a solution containing 5 vol% of propylene carbonate in 95 vol% of a 3 M aqueous sodium iodide solution was used as the electrolyte.
The aqueous solution concentration on the order of 3M corresponds to the reaction active material concentration level in the actual flow battery. 19A and 19B, the horizontal axis represents the potential (V vs. Ag / AgCl), and the vertical axis represents the current density.

At tp = 50 ms, the current density monotonously increased with the increase in potential. I ₂ film generation amount at a time scale of the pulse width is 50ms is small, I ^- the electrode reaction was found to uninhibited largely by coating.

At tp = 500 ms, the current density decreased from around 1.5 V. In the time scale of the pulse width is 500 ms, _{I 2} film is thickened from the vicinity of 1.5V, I ^- were found to inhibit the electrode reaction.

  At tp = 5000 ms, the current values were almost equal from around 1.0 V to around 1.8 V. On a time scale with a pulse width of 5000 ms, it was confirmed that the thickness of the iodine film reached equilibrium in a potential region around 1.0 V to 1.8 V.

FIGS. 20A and 20B show the effect of methyl ethyl ketone or propylene carbonate contained in the electrolytic solution when tp = 5000 ms.
FIG. 20A shows the results when a 3 M aqueous sodium iodide solution (95 vol%) and a solution containing 5 vol% methyl ethyl ketone and a 3 M aqueous sodium iodide solution were used as electrolytes.
FIG. 20B shows the results when a solution containing 95 vol% of a 3 M aqueous sodium iodide solution and 5 vol% of methyl ethyl ketone, and a 3 M aqueous sodium iodide solution were used as electrolytes.

  As shown in FIGS. 20A and 20B, when the electrolytic solution contains methyl ethyl ketone or propylene carbonate, the current around 1.0 V to around 1.8 V is smaller than that when the electrolytic solution does not contain methyl ethyl ketone or propylene carbonate. It was observed large. This is probably because the iodine film was thinned by methyl ethyl ketone or propylene carbonate in the electrolytic solution, and the amount of iodide ions reacting on the electrode was increased.

Electrochemical measurement in which the supporting electrolyte concentration (sodium perchlorate in Example 2) is 50 times equivalent to the concentration of the reaction species of interest (I ^{− in} Example 2) is based on the effect of electrophoresis on mass transfer. Except for, the electrochemical reaction can be observed while keeping the electric double layer structure as the electrochemical reaction field constant. Therefore, there is an advantage that the existing electrochemical theory can be simply used in studying the electrochemical reaction mechanism based on the absolute reaction kinetics. Therefore, in Example 2, the study on the electrochemical reaction was performed in a system containing a reactive species on the order of mM and a supporting electrolyte. In the present embodiment, the electrochemical reaction was examined under the sodium iodide electrolyte conditions in the actual flow battery concentration range without the supporting electrolyte, but it basically corresponds to the examination result in the second embodiment. I understand. That is, the charging potential control in the potential region exceeding 1.05 V is performed by the above equation (3) (I ⁻ + 3H ₂ O → IO ₃ ⁻ + 6H ⁺ + 6e ⁻ ) and equation (5) (I ₂ + 6H ₂ O → 2IO ₃ ⁻⁾ + 12H ⁺ + 10e ⁻ ), which is not desirable because IO ₃ ⁻ is generated.

From the above results, it is preferable that the charge potential of the flow battery including methyl ethyl ketone or propylene carbonate in the positive electrode electrolyte does not exceed the positive electrode potential of 1.05 V (V vs. Ag / AgCl) in which IO ₃ ⁻ is not generated by charging. Understand.

  In a flow battery, it is undesirable that the positive electrode is covered with a thick iodine film, thereby narrowing the flow channel of the flow battery and obstructing the flow of the electrolyte itself. In this regard, it is desirable that the charging potential does not exceed 1.4V. The flow battery is a kind of secondary battery, and in order to stably manage the reaction active material involved in the charge / discharge reaction, the charge potential of 1.05 V (V vs. Ag / AgCl ) Charge control at the following potentials is preferred.

  From FIGS. 18A and 18B, it has been confirmed that the glassy carbon is oxidized by setting the potential to be nobler than 1.5 V. In addition to suppressing the decomposition of methyl ethyl ketone or propylene carbonate, the deterioration of the positive electrode is suppressed. From this point, it is understood that it is preferable to control the charging at a potential of 1.5 V or less in the actual flow battery.

[Example 4]
Next, with respect to the flow battery system shown in FIG. 6, the current potentials of the positive electrode and the negative electrode when a charge / discharge reaction was performed were examined. In this embodiment, a 1 M sodium iodide (NaI) aqueous solution to which 5 vol% of methyl ethyl ketone or propylene carbonate is added is used as a positive electrode electrolyte, and 1 M chloride containing 0.5 M zinc chloride (ZnCl ₂ ) is used as a negative electrode electrolyte. An aqueous solution of ammonium (NH ₄ Cl) was used, a carbon electrode was used as a positive electrode, and a zinc electrode (zinc-coated mesh electrode) was used as a negative electrode.

  In this example, the electrochemical reactions of the positive electrode and the negative electrode and the respective standard electrode potentials are as follows, and the open circuit voltage of the flow battery is about 1.3 V in a standard state. FIG. 21 shows a schematic diagram of the electrode reaction of the flow battery system in Example 4.

Negative electrode Zn⇔Zn ²⁺ + 2e ⁻ −0.76V

Positive electrode 3I ⁻ ⇔I ₃ ⁻ + 2e ⁻ 0.53 V
2I ⁻ ⇔I ₂ + 2e ⁻ 0.53V

FIG. 21 is a current-potential curve of a positive electrode and a negative electrode of a flow battery performed in Example 4. The current-potential curve was obtained by using the configuration of the flow battery system shown in FIG. 6 and setting the flow rate to 100 cm ³ / min and charging and discharging at a constant current.

The corresponding potential under various constant current controlled conditions is a value obtained by measuring the steady potential of each of the positive electrode and the negative electrode. The potential of each of the positive electrode and the negative electrode is a potential with respect to the Ag / AgCl reference electrode. Under the flow battery environment of this example, the current density at which the potential of the positive electrode becomes 1.05 V (Vvs. Ag / AgCl) is about 400 mA / cm ² .

Therefore, in the flow battery system of the present embodiment, it is preferable that the charge control condition of the flow battery is such that the charge current density does not exceed 400 mA / cm ² . This suppresses the potential of the positive electrode from reaching a potential region nobler than 1.05 V, and allows the operation of the flow battery while suppressing generation of IO ₃ ⁻ .

Note that since the negative electrode is basically a reaction between Zn and Zn ²⁺ , the potential may be controlled under the condition that the electrolytic solution is not decomposed.

Claims (25)

  A positive electrode, a negative electrode, a positive electrode electrolyte, a negative electrode electrolyte, and a diaphragm, wherein at least one of the positive electrode electrolyte and the negative electrode electrolyte is an iodide ion and an oxidation reaction product of the iodide ion. Aqueous secondary battery comprising an organic compound capable of separating water.   The aqueous secondary battery according to claim 1, wherein the content of the organic compound is 1% by volume to 50% by volume of at least one of the positive electrode electrolyte and the negative electrode electrolyte containing the organic compound.   3. The aqueous secondary battery according to claim 1, wherein the organic compound includes at least one selected from a ketone, a carboxylate, and a carbonate. 4.   The aqueous solution according to any one of claims 1 to 3, wherein the organic compound includes at least one selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate. Secondary battery.   A positive electrode active material reaction tank containing the positive electrode electrolyte, a negative electrode active material reaction tank containing the negative electrode electrolyte, a positive electrode electrolyte storage tank, a negative electrode electrolyte storage tank, and a positive electrode electrolyte feed pump, A negative electrode electrolyte solution pump, wherein the positive electrode solution solution pump is configured to circulate the positive electrode solution between the positive electrode solution storage tank and the positive electrode active material reaction tank. Wherein the negative electrode electrolyte solution pump is configured to be able to circulate the negative electrode electrolyte between the negative electrode electrolyte storage tank and the negative electrode active material reaction tank. The aqueous secondary battery according to claim 6.   The aqueous secondary battery according to any one of claims 1 to 5, wherein the positive electrode electrolyte includes the iodide ion and the organic compound, and the negative electrode electrolyte includes a negative electrode active material.   The negative electrode active material according to claim 6, wherein the negative electrode active material includes at least one metal ion selected from the group consisting of zinc, chromium, titanium, iron, tin, vanadium, lead, manganese, cobalt, nickel, copper, and lithium. Aqueous secondary battery.   The aqueous secondary battery according to claim 6, wherein the negative electrode active material includes zinc ions.   An aqueous secondary solution comprising a positive electrode, a negative electrode, and an electrolytic solution, wherein the electrolytic solution contains iodide ions, a negative electrode active material, and an organic compound capable of separating an oxidation reaction product of the iodide ions. battery.   The aqueous secondary battery according to claim 9, wherein the content of the organic compound is 1% to 50% by volume of the electrolytic solution.   The aqueous secondary battery according to claim 9, wherein the organic compound includes at least one selected from a ketone, a carboxylate ester, and a carbonate ester.   The aqueous solution according to any one of claims 9 to 11, wherein the organic compound includes at least one selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate, ethyl methyl carbonate, and propylene carbonate. Secondary battery.   The negative electrode active material contains at least one metal ion selected from the group consisting of zinc, chromium, titanium, iron, tin, vanadium, lead, manganese, cobalt, nickel, copper and lithium. 13. The aqueous secondary battery according to claim 12.   The aqueous secondary battery according to any one of claims 9 to 13, wherein the negative electrode active material includes zinc ions.   The aqueous secondary battery according to any one of claims 9 to 14, wherein the positive electrode is located below the negative electrode in the vertical direction.   The aqueous secondary solution according to any one of claims 9 to 15, further comprising a diaphragm disposed between the positive electrode and the negative electrode to divide the electrolyte into a positive electrode electrolyte and a negative electrode electrolyte. battery.   An active material reaction tank containing the electrolyte, an electrolyte storage tank, and an electrolyte feed pump are further provided, wherein the electrolyte feed pump is provided between the electrolyte storage tank and the active material reaction tank. The aqueous secondary battery according to any one of claims 9 to 16, wherein the electrolyte solution is configured to be able to circulate the electrolyte solution.   A method for charging and discharging an aqueous secondary battery using an electrolytic solution containing iodide ions as a positive electrode active material, comprising a step of separating an oxidation reaction product of iodide ions generated in the electrolytic solution, A method for charging and discharging a secondary battery.   Water, an iodide ion, and an organic compound capable of separating an oxidation reaction product of the iodide ion, which is used as an electrolytic solution in the charge / discharge method for an aqueous secondary battery according to claim 18. Electrolyte for aqueous secondary batteries.   The electrolyte solution for an aqueous secondary battery according to claim 19, wherein the content of the organic compound is 1% by volume to 50% by volume of the electrolyte solution for an aqueous secondary battery. Water, at least one selected from the group consisting of iodine (I ₂ ), triiodide ion (I ₃ ⁻ ) and pentaiodide ion (I ₅ ⁻ ), methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate And an at least one organic compound selected from the group consisting of ethyl methyl carbonate and propylene carbonate.   The electrolyte solution for an aqueous secondary battery according to claim 21, wherein the content of the organic compound is 1% by volume to 50% by volume of the electrolyte solution for an aqueous secondary battery.   A method for charging and discharging an aqueous secondary battery using an electrolytic solution containing iodide ions as a positive electrode active material, comprising a step of separating an oxidation reaction product of iodide ions generated in the electrolytic solution, The aqueous electrolyte solution for a secondary battery according to claim 21 or 21, which is used as an electrolyte solution for a method of charging and discharging a secondary battery. The aqueous secondary battery according to any one of claims 1 to 17, wherein charging and discharging are controlled, and the charge potential of the positive electrode is set based on the potential of an Ag / AgCl reference electrode (Cl ^- concentration saturation). A control unit for setting the voltage to 1.5 V or less.   A power generation system comprising: a power generation device; and the flow battery system according to claim 24.