JP6935816B2 - Aqueous secondary battery, charging / discharging method of aqueous secondary battery, electrolytic solution for aqueous secondary battery, flow battery system and power generation system - Google Patents

Description

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

In recent years, global environmental problems have become more 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 carbon dioxide in the atmosphere has become a major issue on a global scale. Therefore, it is expected that the spread of renewable energies such as wind power and solar power, which do not emit carbon dioxide during power generation, will continue to be promoted worldwide. However, since the output of renewable energy fluctuates greatly depending on the weather, it is difficult to use it in a stable manner as it is. Therefore, in order to level this fluctuation, there is an increasing demand for a power storage storage device that is safe, inexpensive, and suitable for upsizing.

Large-capacity storage devices for power storage include sodium-sulfur (NAS) batteries, lead-acid batteries, redox flow batteries, and the like. Since NAS batteries have a large capacity and a long life, they have been proposed to be used for peak shift applications, grid linkage applications of renewable energy power, and the like. Lead-acid batteries are highly reliable, backed by a history of more than 100 years, have a low cost per unit storage capacity, and are advantageous for upsizing. Therefore, they use nighttime power for households and businesses, and generate renewable energy. It has been proposed for a wide range of applications such as leveling of places. Redox flow batteries are suitable for power storage applications because they can be easily increased in capacity by increasing the capacity of the tank.

On the other hand, various power storage devices also have disadvantages. Since the operating temperature of NAS batteries is as high as 300 ° C., there is a risk of ignition, and there is a risk of generating toxic gas such as sulfurous acid gas when ignited. Lead-acid batteries have a low volumetric energy density, so they require a large installation area, and lead is subject to global regulation as represented by the RoHS Directive, so it will be subject to regulation in the future. It is also possible that A redox flow battery has a low risk of ignition and high safety if it is an aqueous electrolyte, but it has a problem that its volumetric energy density is low when a conventional metal ion such as vanadium is used as a positive or negative electrode active material.

Therefore, the development of a large-capacity power storage device that has both high safety and high weight and volumetric energy density is awaited. Among them, a large-capacity power storage device using iodide ion (I ⁻ ) as an electrode active material has been studied in various ways as a large-capacity power storage device capable of achieving safety and high energy density.

For example, Patent Document 1 discloses a redox flow battery in which a chromium ion is used as a negative electrode active material and a halogen ion is used as a positive electrode active material. It is also disclosed that iodide ions are used as halogen ions.
Patent Document 2 discloses a redox flow battery using an aqueous solution of a halogen oxoacid compound as a negative electrode electrolytic solution. The halogen oxo acid compound also contains iodate, and it is also mentioned that the oxidation-reduction reaction between iodate ion and iodide ion is used.
Patent Document 3 and Non-Patent Document 1 disclose a redox flow battery containing 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 electrolytic solution containing zinc iodide as an active material, thereby suppressing the precipitation of iodine.

Japanese Unexamined Patent Publication No. 61-24172 Special Table 2016-520982 U.S. Patent Application Publication No. 2015/01476773

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

Since iodide ions have high solubility in water and are prone to redox reaction, they can be expected as an electrode active material that can achieve high energy density and excellent battery characteristics of an aqueous secondary battery using water as an electrolytic solution. .. However, iodide ions in the electrolytic solution may _{precipitate as iodine (I 2} ) on the electrode surface by an oxidation reaction to form a film, which may cause various problems. For example, the iodine film formed on the electrode surface has a high resistance, which may reduce the oxidation current value corresponding to the charging current value of the battery. In addition, the iodine molecules diffused from the iodine film into the aqueous solution sublimate out of the system, which may reduce the amount of active material in the electrolytic solution and reduce the battery capacity. Further, in the case of a redox flow battery in which the electrolytic solution is flowed to charge and discharge, the flow path of the electrolytic solution may be blocked by the iodine film.
On the other hand, in an aqueous solution-based secondary battery, in a low charge rate state, the concentration of the charge reaction product in the electrolytic solution is generally low, so that a sufficient discharge output may not be obtained.

The present invention provides a method for charging an aqueous solution-based secondary battery and an aqueous solution-based secondary battery in which the formation of an iodine film on the electrode surface is suppressed and excellent in discharge characteristics at a low charge rate, and an aqueous solution-based secondary battery used for these. An object of the present invention is to provide an electrolytic solution for a battery, and a flow battery system and a power generation system using the same.

<1> A positive electrode, a negative electrode, a positive electrode electrolytic solution, a negative electrode electrolytic solution, and a diaphragm are provided, and at least one of the positive electrode electrolytic solution and the negative electrode electrolytic solution is an iodide ion and oxidation of the iodide ion. An aqueous secondary battery containing an organic compound capable of separating the reaction product.
<2> The aqueous solution system 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 electrolytic solution and the negative electrode electrolytic solution containing the organic compound. Next battery.
<3> The aqueous secondary battery according to <1> or <2>, wherein the organic compound contains at least one selected from a ketone, a carboxylic acid ester, and a carbonic acid ester.
<4> In any one of <1> to <3>, the organic compound contains 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 described.
<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 solution. A pump and a negative electrode electrolyte feed pump are further provided, and the positive electrode electrolyte feed pump can circulate the positive electrode electrolyte between the positive electrode electrolyte storage tank and the positive electrode active material reaction tank. The negative electrode electrolytic solution feeding pump is configured so that the negative electrode electrolytic solution can be circulated between the negative electrode electrolytic solution 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 solution-based secondary according to any one of <1> to <5>, wherein the positive electrode electrolytic solution contains the iodide ion and the organic compound, and the negative electrode electrolytic solution 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, <6>. The aqueous secondary battery described in 1.
<8> The aqueous solution-based 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 an iodide ion, a negative electrode active material, and an organic compound capable of separating the oxidation reaction product of the iodide ion. System secondary battery.
<10> The aqueous solution-based 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 contains at least one selected from a ketone, a carboxylic acid ester, and a carbonic acid ester.
<12> In any one of <9> to <11>, the organic compound contains 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 described.
<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, <9>. The aqueous secondary battery according to any one of <12>.
<14> The aqueous solution-based secondary battery according to any one of <9> to <13>, wherein the negative electrode active material contains zinc ions.
<15> The aqueous solution-based secondary battery according to any one of <9> to <14>, wherein the positive electrode is located below the negative electrode in the vertical direction.
<16> The aqueous solution according to any one of <9> to <15>, further comprising a diaphragm arranged between the positive electrode and the negative electrode to separate the electrolytic solution into a positive electrode electrolytic solution and a negative electrode electrolytic solution. System secondary battery.
<17> An active material reaction tank for accommodating the electrolytic solution, an electrolytic solution storage tank, and an electrolytic solution feeding pump are further provided, and the electrolytic solution feeding pump includes the electrolytic solution storage tank and the active material reaction tank. The aqueous secondary battery according to any one of <9> to <16>, which is configured to be able to circulate the electrolytic solution between the two batteries.
<18> A method for charging and discharging an aqueous secondary battery using an electrolytic solution containing iodide ions as a positive electrode active material, which comprises a step of separating an oxidation reaction product of iodide ions generated in the electrolytic solution. , A method for charging and discharging an aqueous secondary battery.
<19> Containing water, iodide ion, and an organic compound capable of separating the iodide ion oxidation reaction product, it is used as an electrolytic solution for the charging / discharging method of the aqueous solution-based secondary battery according to <18>. Electrolyte for aqueous secondary batteries.
<20> The electrolytic solution for an aqueous solution-based 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 the aqueous solution-based secondary battery.
<21> the water and iodine _(I 2), triiodide _{(I 3} ^-), and penta-iodide ion _{(I 5} ^-) and at least one member selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate , At least one organic compound selected from the group consisting of dimethyl carbonate, ethyl methyl carbonate and propylene carbonate, and an electrolytic solution for an aqueous secondary battery.
<22> The electrolytic solution for an aqueous solution-based 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 the aqueous solution-based secondary battery.
<23> A method for charging and discharging an aqueous secondary battery using an electrolytic solution containing iodide ions as a positive electrode active material, which comprises a step of separating an oxidation reaction product of iodide ions generated in the electrolytic solution. The electrolytic solution for an aqueous secondary battery according to <21> or <21>, which is used as an electrolytic solution for a method of charging and discharging an aqueous secondary battery.
<24> The aqueous secondary battery according to any one of <1> to <17>, charge / discharge is controlled, and the charging potential of the positive electrode is set to the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). A flow battery system including a control unit that sets the voltage to 1.5 V or less based on the above.
A power generation system including the <25> power generation device and the flow battery system according to <24>.

According to the present invention, a method for charging an aqueous solution-based secondary battery and an aqueous solution-based secondary battery in which formation of an iodine film on the electrode surface is suppressed and excellent in discharge characteristics at a low charge rate, and an aqueous solution system used for these. An electrolytic solution for a secondary battery, and a flow battery system and a power generation system using these are provided.

It is schematic cross-sectional view which shows the structural example of the aqueous solution type secondary battery of 1st Embodiment. It is a schematic cross-sectional view which shows the structural example in the case where the aqueous solution type secondary battery of 1st Embodiment further includes the apparatus for flowing an electrolytic solution. It is schematic cross-sectional view which shows the structural example of the aqueous solution system secondary battery of 2nd Embodiment. It is a schematic cross-sectional view which shows the structural example in the case where the aqueous solution type secondary battery of 2nd Embodiment further includes a diaphragm 5 between a positive electrode 1 and a negative electrode 2. It is schematic cross-sectional view which shows the structural example in the case where the aqueous solution type secondary battery of 2nd Embodiment further includes the apparatus for flowing an electrolytic solution. It is the schematic sectional drawing which shows the structural example of the flow battery system. It is the schematic which shows the structural example of the power generation system. It is a figure which shows an example of the generated power short-time waveform of the wind power generation. It is a graph which shows the measurement result of the oxidation current value performed in the electrolytic solution which added methyl ethyl ketone and the electrolytic solution which did not add methyl ethyl ketone. It is a photograph of a hydrophobic substance containing an oxidation reaction product of iodide ion. It is a graph which shows the oxidation current value at an oxidation potential and the reduction current value at a reduction potential observed in a hydrophobic substance. It is a graph which shows the potential waveform of normal pulse voltammetry. It is a graph which shows the normal pulse voltammogram (pulse width 50ms). It is a graph which shows the normal pulse voltammogram (pulse width 50ms). It is a graph which shows the potential waveform of reverse pulse voltammetry. It is a graph which shows the reverse pulse voltammogram (initial potential 0.50V, 0.55V and pulse width 50ms). It is a graph which shows the reverse pulse voltammogram (initial potential 0.50V, 0.55V and pulse width 50ms). It is a reverse pulse voltammogram and is a graph showing the effect of methyl ethyl ketone contained in the electrolytic solution (initial potential 0.60 V and pulse width 50 ms). It is a reverse pulse voltammogram and is a graph showing the effect of propylene carbonate contained in the electrolytic solution (initial potential 0.60 V and pulse width 50 ms). It is a graph which shows the reverse pulse voltammogram (initial potential 0.90V-1.10V and pulse width 50ms). It is a graph which shows the reverse pulse voltammogram (initial potential 0.90V-1.10V and pulse width 50ms). It is a graph which shows the reverse pulse voltammogram (initial potential 1.40V, 1.50V and pulse width 50ms). It is a graph which shows the reverse pulse voltammogram (initial potential 1.40V, 1.50V and pulse width 50ms). It is a graph which shows the normal pulse voltammogram (pulse width 50, 500 and 5000 ms). It is a graph which shows the normal pulse voltammogram (pulse width 50, 500 and 5000 ms). It is a graph which shows the normal pulse voltammogram, and is the graph which showed the effect of methyl ethyl ketone contained in an electrolytic solution (pulse width 5000 ms). It is a graph which shows the normal pulse voltammogram, and is the 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 element steps and the like) are not essential unless otherwise specified. The same applies to the numerical values and their ranges, and does not limit the present invention.

The numerical range indicated by using "~" in the present disclosure includes the numerical values before and after "~" as the minimum value and the maximum value, respectively.
In the numerical range described stepwise in the present disclosure, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical range described stepwise. .. Further, in the numerical range described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the value shown in the examples.
In the present disclosure, each component may contain a plurality of applicable substances. When a plurality of substances corresponding to each component are present in the composition, the content or content of each component is the total content or content of the plurality of substances present in the composition unless otherwise specified. Means quantity.
In the present disclosure, the term "membrane" or "film" is used only in a part of the region in addition to the case where the membrane or the region in which the coating exists is formed in the entire region. The case where it is formed is also included.

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

<Aqueous solution-based secondary battery (first embodiment)>
The aqueous solution-based secondary battery of the present embodiment is
A positive electrode, a negative electrode, a positive electrode electrolytic solution, a negative electrode electrolytic solution, and a diaphragm are provided, and at least one of the positive electrode electrolytic solution and the negative electrode electrolytic solution is an iodide ion and an oxidation reaction product of the iodide ion. Includes separable organic compounds.

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

In all of the above-mentioned patent documents, the electrolytic solution contains an organic compound capable of separating the oxidation reaction product of iodide ions, thereby promoting the dissolution of the iodine film formed on the electrode surface and the separated oxidation. The technical idea of using the reaction product for the reduction reaction of the positive electrode is not described.

In the present disclosure, the "aqueous solution-based secondary battery" means a secondary battery in which an aqueous solution in which necessary components are dissolved in water is used as an electrolytic solution. Specific examples thereof include power storage devices such as redox flow batteries (flow batteries).

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

In the present disclosure, the "iodide ion oxidation reaction product" means a substance produced by the iodide ion oxidation reaction. For example, iodine (I ₂ ), triiodide ion (I ₃ ⁻ ), pentaiodide ion (I ₅ ⁻ ) and combinations thereof can be mentioned.

In the aqueous solution-based secondary battery of the present embodiment, the electrolytic solution is divided into a positive electrode electrolytic solution and a negative electrode electrolytic solution. An aqueous solution-based secondary battery having such a configuration is also referred to as a "two-component aqueous solution secondary battery".

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

Since the aqueous secondary battery of the present embodiment uses iodide ions as the active material contained in the electrolytic solution, it is excellent in energy density. Furthermore, the organic compound contained in the electrolytic solution separates the oxidation reaction product of iodide ions, which promotes the dissolution of the iodine film formed on the electrode surface and reduces the oxidation current value due to the formation of the iodine film. Is suppressed, and the effect of suppressing the sublimation of _{I 2} molecules diffused from the iodine film into the electrolytic solution to the outside of the system can be expected. Further, since the oxidation reaction product separated by the organic compound can be used for the reduction reaction in the positive electrode, a high output discharge can be obtained even in a low charge rate state in which the concentration of the charge reaction product is dilute.

In the aqueous solution-based secondary battery of the present embodiment, the oxidation reaction product of the iodide ion contained in the electrolytic solution is selectively separated by using an organic compound. The mode for separating the oxidation reaction product of iodide ion 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 ion and a bulk aqueous solution. Is preferable.
The hydrophobic substance may be, for example, an organic compound obtained by forming a network structure with iodide ion and its oxidation reaction product, a cation, and a small amount of water. For example, it may be in a hydrophobic liquid state.

The solubility of iodine molecules in hydrophobic substances is higher than that in water, and they are more stable in hydrophobic substances. In addition to having sublimation properties, iodine molecules have a low solubility in water of about 1 mM. Therefore, the iodine molecules present in water are easily sublimated from the gas-liquid interface, but the iodine molecules present in the hydrophobic substance are suppressed from sublimation. Therefore, by separating the iodide ion oxidation reaction product as a hydrophobic substance, the sublimation of the iodine molecule can be suppressed and the decrease of the iodide ion in the electrolytic solution can be suppressed.

When the iodide ion oxidation reaction product is separated as a hydrophobic substance, the iodide ion oxidation reaction product is present in the separated hydrophobic substance at a higher concentration than in the bulk aqueous solution. Therefore, by bringing a hydrophobic substance into contact with the positive electrode and using the oxidation reaction product contained therein for the reduction reaction of the positive electrode, a high output discharge can be obtained even in a low charge rate state. Further, as described later, when the positive electrode is configured to be located at the lower part when viewed in the vertical direction, 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 positive electrode. 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 for the reason described above, the organic compound can be separated from the oxidation reaction product of iodide ion as a hydrophobic substance. preferably, it is more preferable affinity for I ₂ molecule is highly organic compounds than water. In _{an organic compound having a higher affinity for I 2} molecules than water, the organic compound coordinates with the iodine film and promotes the dissolution of the iodine film. Therefore, the formation of the 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 an electrolytic solution.

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

Among the above compounds, ketone is preferable from the viewpoint of reducing the viscosity of the aqueous solution (electrolyte solution) containing the ketone and reducing the battery resistance. Carboxylic acid esters are more chemically stable to iodine than ketones and are preferred from the standpoint of increasing coulombic efficiency. Carbonic acid esters are preferable because they have a high specific density and have a high ability to separate iodide ion oxidation reaction products as a hydrophobic substance.

The amount of the organic compound contained in the electrolytic solution may be only one type or two or more types. The content of the organic compound in the electrolytic solution 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 ion tends to be separated well even in a high charge rate state, and when it is 50% by volume or less, The decrease in conductivity of the electrolytic solution tends to be suppressed.

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

The electrolytic solution 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 a supporting electrolyte salt, the ionic conductivity of the electrolytic solution tends to increase, 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 electrolytic solution 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, alkylammonium salt, alkyl imidazole. Examples thereof include lithium salt, alkyl piperidinium salt, and alkyl pyrrolidinium salt. 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 electrolytic solution may contain a pH buffer. Examples of the pH buffer include acetate buffer, phosphate buffer, citric acid buffer, borate buffer, tartrate buffer, Tris buffer and the like.

FIG. 1 is a schematic cross-sectional view showing an example of a configuration example of the aqueous solution-based secondary battery of the present embodiment. The solid arrow shown in FIG. 1 indicates the flow of electrons during charging, and the dotted arrow indicates the reaction of ions during charging.

The aqueous solution-based 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 electrolytic solution and a negative electrode electrolytic solution, respectively.

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

The material of the positive electrode 1 is preferably selected from those having corrosion resistance to iodide ions contained in the positive electrode electrolytic solution. Specific examples thereof include metals having high corrosion resistance such as titanium and carbon materials. From a cost standpoint, carbon materials are preferred. 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 and the like. For example, when zinc is used as the negative electrode active material, a carbon material, zinc, a zinc-plated metal material, or the like in 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 punching metal, a flat plate, and the like, but the shape is not particularly limited.

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

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

The negative electrode electrolytic solution contained in the negative electrode active material reaction tank 4 contains a negative electrode active material (X ⁺ ). The reduction reaction of the negative electrode active material produces a reduction reaction product (X). 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 thereof include ions such as zinc, chromium, titanium, iron, tin, vanadium, lead, manganese, cobalt, nickel, copper, lithium, quinone-based materials, and viologens. Among these negative electrode active materials, metal ions are preferable, and zinc ions are more preferable. Zinc ion has a higher solubility in water than other metal ions (for example, the solubility of zinc chloride is 30 M or more), the standard redox potential of the dissolution-precipitation reaction is low (-0.76 V), and it is inexpensive. The negative electrode electrolytic solution containing metal ions can be prepared by dissolving the compound of the metal in the electrolytic solution. For example, a negative electrode electrolyte solution 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 serves to separate 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 preferable example, but a porous membrane, a glass filter and the like can also be preferably used.

FIG. 2 is a schematic cross-sectional view showing a configuration when the aqueous solution-based secondary battery shown in FIG. 1 further includes a device for flowing an electrolytic solution. Of the elements included in the aqueous solution-based secondary battery shown in FIG. 2, the elements common to the elements included in the aqueous solution-based secondary battery shown in FIG. 1 will not be described.

The aqueous solution-based 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, the positive electrode electrolyte storage tank 6 and the positive electrode electrolyte feed pump 8 for supplying the positive electrode electrolyte solution to the positive electrode active material reaction tank 3, and the negative electrode electrolysis for supplying the negative electrode electrolyte solution to the negative electrode active material reaction tank 4. It includes a liquid storage tank 7, a negative electrode electrolyte feed pump 9, and an external power source 10 for driving the positive electrode electrolyte feed pump 8 and the negative electrode electrolyte feed pump 9.

In FIG. 2, the arrow indicates the direction in which the electrolytic solution flows. In the case shown in FIG. 2, the positive electrode electrolytic solution is supplied from the positive electrode electrolyte storage tank 6 to the positive electrode active material reaction tank 3 by the positive electrode electrolyte liquid feed pump 8, and the redox 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 electrolyte liquid feed pump 9, and the redox reaction of the negative electrode active material proceeds. The positive electrode electrolyte liquid feed pump 8 and the negative electrode electrolyte liquid feed pump 9 are driven by an external power source 10.

As shown in FIG. 2, a battery that charges and discharges by flowing an electrolytic solution is called a redox flow battery. Since the battery capacity of the redox flow battery depends on the amount of the electrolytic solution, there is an advantage that the capacity can be easily increased by increasing the capacity of the tank for storing the electrolytic solution. Further, 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 path due to the iodine film is effectively prevented. Will be done.

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

<Aqueous solution-based secondary battery (second embodiment)>
The aqueous secondary battery of the present embodiment includes a positive electrode, a negative electrode, and an electrolytic solution, and the electrolytic solution can separate an iodide ion, a negative electrode active material, and an oxidation reaction product of the iodide ion. Includes organic compounds.

In the aqueous secondary battery of the present embodiment, the electrolytic solution is not divided into a positive electrode electrolytic solution containing a positive electrode active material (iodide ion) and a negative electrode electrolytic solution containing a negative electrode active material, and the same electrolytic solution is the positive electrode active material. Contains (iodide ion) and negative electrode active material. Hereinafter, the aqueous solution-based secondary battery having such a configuration is also referred to as a "one-component aqueous solution-based secondary battery".

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

FIG. 3 is a schematic cross-sectional view showing an example of a configuration example of the aqueous solution-based secondary battery of the present embodiment. The solid arrow indicates the flow of electrons during charging, and the dotted arrow indicates the reaction of ions during charging.

The aqueous solution-based 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, a negative electrode active material, and an organic compound capable of separating an oxidation reaction product of the iodide ion.

In the aqueous solution-based secondary battery of the present embodiment, it is preferable that the positive electrode is located below the negative electrode in the vertical direction. By locating the positive electrode below the negative electrode, the oxidation reaction product of iodide ions separated by the organic compound precipitates in the electrolytic solution and collects on the positive electrode side. As a result, the oxidation reaction product of iodide ion can be effectively used for the reduction reaction at the positive electrode.

In the aqueous solution-based secondary battery of the present embodiment, it is preferable to use a substance having a property that the reduction reaction product stays in the vicinity of the negative electrode as the negative electrode active material. As a result, the charging reaction products generated in the positive electrode and the negative electrode can be present on the positive electrode side and the negative electrode side, respectively, and both can be spatially separated. As a result, it is possible to prevent the charge reaction products generated at the positive electrode and the negative electrode from coming into contact with each other to cause self-discharge.

The advantages of making the aqueous secondary battery a one-component type are (1) the diaphragm placed between the positive electrode and the negative electrode can be omitted, and (2) water molecules move between the positive electrode side and the negative electrode side with charge and discharge. Therefore, it is possible to prevent the amount of the electrolytic solution from becoming non-uniform. (3) Since the oxidation reaction product of the iodide ion separated by the organic compound precipitates and collects on the positive electrode side, the electrolytic solution is allowed to flow. (4) When a device for flowing the electrolytic solution is provided, it is not necessary to distinguish between the positive electrode side and the negative electrode side, and the configuration can be simplified.

If necessary, the aqueous solution-based 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 showing a configuration in which the aqueous solution-based secondary battery shown in FIG. 3 further includes a diaphragm 5 between the positive electrode 1 and the negative electrode 2. Of the elements included in the aqueous solution-based secondary battery shown in the figure, the elements common to the elements included in the aqueous solution-based secondary battery shown in FIG. 1 will not be described.

The details and preferred embodiments of the diaphragm 5 included in the aqueous solution-based secondary battery shown in FIG. 4 are the same as the details and preferred embodiments of the diaphragm 5 included in the aqueous solution-based secondary battery of the first embodiment. By providing a diaphragm on the one-component aqueous solution-based secondary battery, it is possible to further suppress contact between the charging reaction products generated at the positive electrode and the negative electrode, respectively, and further improve the Coulomb efficiency of the one-component aqueous solution-based secondary battery. Can be improved.

If necessary, the aqueous solution-based secondary battery of the present embodiment may further include a device for flowing the electrolytic solution. FIG. 5 shows an electrolytic solution storage tank 12 for supplying the electrolytic solution to the active material reaction tank 11 and an electrolytic solution feeding pump 13 as devices for the aqueous secondary battery shown in FIG. 4 to flow the electrolytic solution. It is a schematic cross-sectional view which shows the structure in the case where the external power source 10 for driving the electrolytic solution liquid feed pump 13 is further provided.

The aqueous solution-based secondary battery shown in FIG. 5 is provided with both the diaphragm 5 and the device for flowing the electrolytic solution, but may be in a mode in which the diaphragm 5 is not provided.

In FIG. 5, the arrow indicates the direction in which the electrolytic solution flows. The electrolytic solution is supplied from the electrolytic solution storage tank 12 to the active material reaction tank 11 by the electrolytic solution feeding pump 13. At the positive electrode, the redox reaction of iodide ions proceeds, and an oxidation reaction product is produced during the oxidation reaction. On the other hand, in the negative electrode, the redox reaction of the negative electrode active material proceeds, and a reduction reaction product is produced during the reduction reaction. The electrolytic solution feed pump 13 is driven by an external power source 10.

In the configuration shown in FIG. 5, the flow of the electrolytic solution in the aqueous secondary battery is performed by driving the electrolytic solution pump 13 with the external power source 10, but the electric power generated by the aqueous secondary battery itself is used. The electrolytic solution feed pump 13 may be driven. In this case, the configuration is a self-sustaining system without external power supply.

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

When the above charging / discharging method is applied as a charging / discharging method for an aqueous secondary battery, the 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 Since the separated oxidation reaction product, which is suppressed and the decrease in the concentration of iodine ion in the electrolytic solution is suppressed, is used for the reduction reaction in the positive electrode, the output characteristics of the aqueous secondary battery at a low charge rate are improved. You can expect effects such as being able to do it.

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

<Electrolyte solution for aqueous secondary battery (first embodiment)>
The electrolytic solution for an aqueous solution-based secondary battery of the present embodiment (hereinafter, also simply referred to as an electrolytic solution) contains water, iodide ions, and an organic compound capable of separating the oxidation reaction product of iodide ions. It is used as an electrolytic solution in the above-mentioned method for charging and discharging an aqueous secondary battery.

<Electrolyte solution for aqueous secondary battery (second embodiment)>
The electrolytic solution for an aqueous secondary battery of the present embodiment is at least one selected from the group consisting of water, iodide ions, methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate, ethyl methyl carbonate and propylene carbonate. Includes organic compounds.

When the electrolytic solution of each of the above embodiments is used in an aqueous solution-based secondary battery, the 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 Since the separated oxidation reaction product, which is suppressed and the decrease in the concentration of iodine ion in the electrolytic solution is suppressed, is used for the reduction reaction in the positive electrode, the output characteristics of the aqueous secondary battery at a low charge rate are improved. You can expect effects such as being able to do it.

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

<Flow battery system>
The flow battery system of the present embodiment controls charging and discharging with the above-mentioned aqueous secondary battery, 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).} It is provided with a control unit to be set to.

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

In the above flow battery system, the charging potential of the positive electrode is preferably set to 1.3 V or less, preferably 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 in the flow battery system so as not to exceed the reference value with reference to the ^{potential of the Ag / AgCl reference electrode (Cl − concentration saturation) will be described.}

In a flow battery system, the positive electrode electrolyte contains iodide ions as the positive electrode active material. Therefore, the charging reaction represented by the following formulas (1) and (2) usually oxidizes the ^{iodide ion (I −} _{) at the positive electrode to generate I 3} ⁻ and I ₂ , and the produced I ₃ ^−. And I ₂ are reduced at the positive electrode to become ^{I −} by the discharge reaction represented by the formulas (1) and (2).

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 respect to the potential of the Ag / AgCl reference electrode (Cl −} concentration saturation), the following formula is used together with the charging reactions represented by the above formulas (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) has been reported to be an irreversible reaction, and the reaction rate of the reverse reaction is extremely small.

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

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 formula (4) is faster than the electrochemical reaction of the above formula (3), and the formula (3) and the formula (4) are constituent reactions of the formula (5). The rate-determining process is the electrochemical reaction of equation (3). Therefore, it becomes particularly remarkable 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), but 1.3 V and 1.5 V are set with a safety margin. It may be set so as not to exceed, and it is presumed that the reactions of the above formulas (3) to (5) have occurred.

Therefore, 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 the discharge reaction, which is the reverse reaction of the formula (3), and it is very difficult 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.

Further, the reaction shown in the formula (5), which is the total reaction of the formulas (3) and (4), reacts with I ₂ to generate IO ₃ ^−, which is the same as the reaction shown in the formula (3). , The reaction represented by the formula (5) is also an irreversible reaction. Therefore, the generated IO ₃ ⁻ has a slow reaction rate of the discharge reaction, which is the reverse reaction of the equation (5), and is very difficult to return to _{I 2.}

Therefore, when charging is repeated at a positive electrode charging potential exceeding 1.05 V, the proportion of _{IO 3} ^{− that does not contribute to the discharge reaction increases, and the total concentration of I −} and I ₂ decreases, so that the flow gradually flows. The battery system has a problem that the positive electrode discharge capacity and the positive electrode charge capacity are lowered.

On the other hand, the flow battery system of the present embodiment includes a control unit that controls charging / 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. IO ₃ ^- a by suppressing generation, it is possible to maintain the positive electrode discharge capacity and the positive electrode charge capacity to maintain the total concentration of iodide ion and iodine molecule when reversible charge and discharge. Therefore, in the present embodiment, it is possible to provide a flow battery system that satisfies a practical charging condition.

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 potentials of the negative electrode and the positive electrode. Since the charging potential is based on a constant potential that serves as a reference, the 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, when the potential fluctuates in the same way between the negative electrode and the positive electrode, the voltage becomes apparently constant. Therefore, the potential of the positive electrode is not determined by the charging voltage, and it is necessary to measure the potential of the reference electrode for the positive electrode as 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. IO ₃ ^- By suppressing the formation of reversible charge and discharge an iodine ion and iodine molecule when (I ^-, I ₃ ^- and I ₂₎ to maintain the total concentration of the positive electrode discharge capacity and the positive electrode charge capacity The decrease is suppressed and the cycle durability can be improved.

In principle, "setting the charging potential of the positive electrode to 1.5V 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 1.5V or less.} It means charging the flow battery, and it is permissible that the charging potential of the positive electrode exceeds 1.5 V. For example, when it is unavoidable that the charging potential of the positive electrode exceeds 1.5V due to the influence of ripple noise or the like described later, the charging potential of the positive electrode may exceed 1.5V.

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

Further, the control unit preferably controls 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).} By controlling the charging potential of the positive electrode to 1.5 V (vs. Ag / AgCl) or less, deterioration of the positive electrode (particularly, the carbon electrode) tends to be suppressed. When the positive electrode electrolytic solution 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.

“Controlling the charging potential of the positive electrode to 1.5V or less based on the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation)” means charging the flow battery with the charging potential of the positive electrode set to 1.5V or less. It means that the charging potential of the positive electrode does not 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 uses a high-frequency filter or the like to cut the excess amount of the flow battery. Control. If the charging potential of the positive electrode is within the range of 1.05V to 1.5V (vs. Ag / AgCl) even if the ripple noise described later is superimposed, the control unit does not need to perform any special control. _{This is because the formation reaction of IO 3} ⁻ represented by the above-mentioned equation (3) is considered to be difficult to follow a high frequency signal such as ripple noise.

Further, 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), and more preferably set to 1.3 V or less. It is preferably set to 1.05 V or less, and more preferably set 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, the positive electrode electrolyte may be periodically sampled using the sampling unit described later during the operation period, and the concentration adjusting unit described later may be used. It may be used to add an electrolytic solution or a component contained therein to the positive electrode to adjust the concentration of the component contained in the positive electrode electrolytic solution.

(Reference electrode for positive electrode)
The flow battery system may include a positive electrode reference electrode for measuring the positive electrode potential.
The reference electrode for the positive electrode may be any one that can be converted into a potential with respect to the standard hydrogen electrode potential and exhibits a stable electrochemical potential. The reference electrode that serves as an electrochemical potential reference is shown in many textbooks as a basic matter of electrochemical potential (for example, Allen J. Bard and Larry R. Faulkner, "ELECTROCHEMICAL METHODS" p.3, (1980), John. Willey & Sons, Inc.). Examples of the reference electrode include an Ag / AgCl reference electrode, a saturated calomel ejector, and the like, and an 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 the measured positive electrode potential can be converted into the potential of the Ag / AgCl reference electrode (Cl ^{− concentration saturation), and other reference electrodes can be used.} You may use it.

In addition, the flow battery system may further include a negative electrode reference electrode for measuring the negative electrode potential. The reference electrode may be installed at one location on the positive electrode, preferably at one location on each of the positive electrode and the negative electrode, and more preferably at a plurality of locations on the positive electrode and the negative electrode.

(Sampling section)
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 components contained in the positive electrode electrolyte. For example, the concentration of the components contained in the positive electrode electrolyte can be adjusted to a specified amount, a required amount, or the like. It is possible to analyze whether there is a shortage in comparison.

The sampling unit may be arranged in the positive electrode electrolyte storage tank, for example, or may be arranged in the circulation path. Further, the sampling unit may have a configuration in which the positive electrode electrolytic solution is sampled at predetermined time intervals.

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

The concentration adjusting unit may have, for example, a configuration in which each component is added to the positive electrode electrolyte stored in the positive electrode electrolyte storage tank, or a configuration in which each component is added to the positive electrode electrolyte flowing through the circulation path. You may. Further, the addition of iodine compounds, additives, etc. to the positive electrode electrolytic solution may be performed while the flow battery is in operation or stopped.

(Potential measurement unit)
The flow battery system may include a potential measuring unit that measures a potential based on the concentrations of iodine ions and iodine molecules in the positive electrode electrolyte. The potential measuring unit has, for example, a current collecting electrode for measuring the potential based on the concentration of iodine ions and iodine molecules, and a reference electrode as a reference for the electrochemical potential, and measures the electrochemical potential with reference to the reference electrode. do. By using the Nernst equation for the electrochemical potential, the concentrations of iodine ions and iodine molecules can be obtained from the measured electrochemical potential of the reference electrode reference. Examples of the current 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 charge state (SOC: State Of Charge) based on the potential measured by the potential measurement unit. For example, ^{when only I −} , I ₃ ⁻ , and I ₂ are considered as redox substances, a SOC of 0% means that the positive electrode electrolyte basically does not contain _{I 3} ⁻ and I ₂ and only ^{I −.} Indicates the state of. Further, 100% SOC basically means a state in which I ⁻ is not contained in the positive electrode electrolyte and only _{I 3} ⁻ and I _{2 are contained.}

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

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

The flow battery system 100 shown in FIG. 6 includes a cell stack 130 including a plurality of single cells including a positive electrode 111, a negative electrode 112, and a diaphragm 115. FIG. 6 shows a 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, a positive electrode reference electrode 113 and a negative electrode reference electrode 114 are arranged on the positive electrode 111 and the negative electrode 112 constituting the cell stack, and 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 for sampling the positive electrode electrolyte 116 and the potential measuring unit 125 for measuring the potential based on the concentrations of iodine ions and iodine molecules in the positive electrode electrolyte 116 are the positive electrode electrolyte. It is located in the 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 arranged and the positive electrode electrolyte storage tank 118, and the negative electrode chamber and the negative electrode electrolyte in which the negative electrode 112 is arranged are circulated. Circulation paths 120, 121 and liquid feeding pumps 122, 123 for circulating the negative electrode electrolytic solution 117 between the storage tank 119 and the liquid feeding pumps 122, 123 are arranged as liquid feeding portions.

The charge / discharge 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 of the present embodiment includes a power generation device and the above-mentioned flow battery system. By combining the flow battery system and the power generation device, the power generation system of the present embodiment can level and stabilize electric power fluctuations and stabilize the supply and demand of electric power.

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 hydroelectric power generation device, a thermal power generation device, and a nuclear power generation device. Of these, a power generation device that uses renewable energy to generate electricity is preferable.

The amount of power generated by a power generation device using renewable energy fluctuates greatly depending on weather conditions, etc., but by combining it with a flow battery system, it is possible to level the fluctuating generated power and supply the leveled power to the power system. ..

Examples of renewable energy include wind power, solar power, wave power, tidal power, running water, tidal power, geothermal power, etc., but wind power or solar power is preferable.

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

When the above-mentioned flow battery system is applied to such a renewable energy field, a charging potential exceeding 1.5 V (vs. Ag / AgCl) is applied to a single cell of the flow battery system in order to supply electric power to a high-voltage system. It may be required. When the charging potential of the positive electrode of the single cell is 1.5V (vs. Ag / AgCl), the total charging voltage of the single cell, that is, the potential difference between the positive electrode and the negative electrode becomes a value exceeding 3V. 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 10 cell stacks are connected in series, the charging voltage becomes 600V. The flow battery system is charged by converting AC power generated by wind power generation or the like into DC power with an inverter. Therefore, the voltage range of the charge control voltage is determined in relation to the cell stack of the flow battery system and the output of the inverter.

When the charging voltage of the inverter is constant and the number of single cells in the cell stack is small, the charging voltage applied to each single cell becomes large. On the contrary, when the number of single cells in the cell stack is large, the charging voltage applied to each single cell becomes small. Therefore, when the flow battery system is installed in a power generation system using renewable energy, the charging voltage applied per single cell of the flow battery system is determined by the inverter output and the number of single cells in the cell stack as basic parameters. ..

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

The generated power waveform shown in FIG. 7 is an example of the power waveform generated by the wind power generation device. In the case of wind power generation, the generated power fluctuates greatly depending on the strength of the wind, the direction of the wind, and the like. When the fluctuating electric power is superimposed on the electric power system such as a transmission line, it affects the stabilization of the electric power system. Therefore, when supplying electric power generated 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 fluctuation, a charge / discharge waveform that alleviates the fluctuation 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 wind power generation is viewed on a shorter time scale. While the power waveform in the relatively long time region shown in the regions (a) and (b) of FIG. 8 can be seen, the region (a) and the region (b) are on the shorter time side than the region (a). In the three time domains on the longer side than the region (b), pulse-shaped power generation waveforms on the order of microseconds to milliseconds can be seen. At this time, the flow battery system uses the target output of a certain time width of the wind power generation as the center value, supplements the power by discharging if the generated power is lower than that, and charges using the generated power if it exceeds the target output. However, the charge / discharge may be controlled so as to approach the target output.

The inverter is a converter for exchanging electric power between the charge / discharge signal of the flow battery, which is DC information, and the generated electric power. The flow battery is charged by converting AC power from the wind power generator into DC power. Inverters tend to generate pulsed high-frequency signals called ripple noise. Generally, these high frequency signals can be removed by installing a capacitor corresponding to each frequency band 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.

It would be ideal if the response speed of the flow battery could follow all the output fluctuations of the PCS of 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 battery reaction occurs in the signal of the power fluctuation in the microsecond order to several tens of milliseconds order. I can't keep up. In the case shown in FIG. 8, it is difficult to completely level the power fluctuation in the time domain in which signals in the microsecond order to millisecond order region are collected and high frequency signals in which ripple noise is superimposed are collected by charging and discharging the flow battery. .. In particular, in the case of a large flow battery, this behavior becomes apparent as the surface area of the electrode increases, the electric double layer capacity increases, and the time constant increases.

When the flow battery system is applied to wind power generation, photovoltaic power generation, etc., frequent charging / discharging is repeated via an inverter during power generation in order to level the generated power. The power signals supplied to the flow battery, including the ripple noise generated from the inverter, include high-frequency power signals that exceed the follow-up capacity of the flow battery. When a high-frequency power signal that the flow battery cannot follow is applied to the flow battery, the power is basically converted into heat. This heat tends to be concentrated on the electrode terminals of the flow battery, and tends to adversely affect the constituent materials of the flow battery.

As described above, the charging voltage applied per single cell of the flow battery system is determined by the inverter output and the number of single cell series of the cell stack as basic parameters, but the required storage capacity and the number of cell stack series are the same. There is also a relationship. Therefore, it is preferable to design the flow battery system so that the charging potential of the positive electrode 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 accept that it exceeds 1.5V by design, it is possible to control the charging potential of the positive electrode to 1.5V (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), it is included in the positive electrode electrolyte and the negative electrode electrolyte during the operation period. It is preferable to carry out the addition of components. However, when a volatile component is contained, it is preferable to perform periodic analysis even in an operating environment where the charging potential of the positive electrode does not exceed 1.5 V, and 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 1.5V (vs. Ag / AgCl), the signal that the charging potential of the positive electrode exceeds 1.5V with respect to the ripple noise of the inverter. It seems reasonable to think that is included. Since the ripple noise of the inverter accelerates the deterioration of the flow battery system, 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 the charge / discharge of the flow battery system according to the supply and demand of the generated power generated by the power generation device. For example, when the supply amount of the generated power generated by the power generation device exceeds the demand amount in the power system, the flow battery system charges and the supply amount of the generated power generated by the power generation device is the demand in the power system. Below the volume, 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, so that the flow battery system functions as a low-cost, high-energy density power storage system, and further reduces carbon dioxide emissions. , Helps solve the global issue of curbing global warming.

Hereinafter, the present invention will be specifically described 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 the iodide ion oxidation reaction product using the organic compound contained in the electrolytic solution.

(1) The effect of improving the oxidation current value by containing an organic compound in the electrolytic solution was investigated experimentally. Specifically, an electrolytic solution in which 5% by volume of methyl ethyl ketone was added as an organic compound and an electrolytic solution in which methyl ethyl ketone was not added were prepared in a 3M aqueous solution of sodium iodide (NaI), respectively, 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. In the figure, the vertical axis represents the current value (mA) and the horizontal axis represents the time (s).

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

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

(3) FIG. 11 is a graph showing the oxidation current value at the oxidation potential and the reduction current value at the reduction potential observed in a hydrophobic substance containing an oxidation reaction product of methyl ethyl ketone and iodide ion. be. In the graph, the vertical axis represents the current value (mA), the horizontal axis represents the time (s), the positive current value represents the oxidation current value, and the negative current value represents the 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]
Using an electrolytic solution containing methyl ethyl ketone or propylene carbonate as an organic compound, the oxidation reaction was investigated by normal pulse voltammetry, and the electrode reaction of the positive electrode was verified.
FIG. 12 is a graph showing the potential waveform of normal pulse voltammetry carried out in Example 2. In FIG. 12, Ei represents the initial potential, ΔEs represents the pulse increment, tp represents the pulse width, and τ represents the pulse period. The potential waveform shown in FIG. 12 was input to the electrochemical cell using a potentiostat, which is an electrochemical measuring device, and the current values corresponding to each pulse potential and pulse time were measured.

The potentiostat is a general device in electrochemical measurement, and it is observed based on the electrochemical reaction that proceeds at the working electrode by controlling the pulse potential shown in FIG. 12 with respect to the reference electrode potential that is the reference of the potential. It is a device that detects the current. In addition, a counter electrode is provided so that current flows through the counter electrode. The input resistance of the reference electrode is very large, DC resistance, which is usually at the ^{level of 10 14} ohms, and the circuit configuration is such that all the current of the electrochemical reaction proceeding at the working electrode flows to the opposite electrode.

The potentiostat includes a reference electrode as a reference for these potentials, a working electrode as a target for potential control, and three counter electrodes. Recently, with the development of microcomputers, the waveform of normal pulse voltammetry shown in FIG. 12 can be generally programmed by being integrated with the potentiostat function.

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

Next, the reduction reaction, which is the opposite reaction to the normal pulse voltammetry, was investigated by reverse pulse voltammetry, and the electrode reaction of the positive electrode was verified.

FIG. 14 is a graph showing the potential waveform of reverse pulse voltammetry carried out in Example 2. Reverse pulse voltammetry can be performed using programmed potentiostats similar to 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 reverse pulse potential increment, tc is the time held in Ec, td is the time held in Ei, and tp is the reverse pulse width. Ec was the immersion potential, ΔEs = 0.05V, tc = 10s, and td = 2s.

Reverse pulse voltammetry has a function that can more closely examine the behavior of the oxidation reaction of ^{I − obtained by normal pulse voltammetry.} That is, it is possible to verify what kind of electrochemical behavior the product produced at the initial potential exhibits. When what is produced 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 is stepped in a low direction with the initial potential Ei as the starting potential by repeating the reverse pulse potential increment. When one potential step is completed, the redox reaction is held at the conditioning potential Ec, which is the most difficult to proceed. The conditioning potential Ec holds the time (tk) for the boundary conditions of the working electrode to recover to the same level as before the reaction. After tk hours, the potential is stepped to the initial potential Ei, during which an oxidation reaction (generally an oxidation or reduction reaction) proceeds on the working electrode. After controlling the td time to the initial potential, a reverse pulse is applied. By repeating this process, reverse pulse voltammetry is performed, and the reaction itself, the reaction mechanism, and the like can be closely examined based on the relationship between the obtained reverse pulse current and potential.

In both normal pulse voltammetry and reverse pulse voltammetry, the boundary conditions of the working electrode are the same between each pulse, so the concentration of the reactant that reacts at the initial potential can be regarded as constant in the relationship between the reaction product and the potential. It is possible, and it is a great merit that it can be simplified when examining the relationship between current and potential. Since it returns to the conditioning potential after the end of one pulse, the information of the arbitrary pulse potential is not accompanied by the history of the reactants generated at the pulse potential before the pulse. As mentioned above, the strength of normal pulse voltammetry and reverse pulse voltammetry can be said to be that the electrochemical information to be observed can be simply extracted.

15A and 15B are reverse pulse voltammograms performed in Example 2 (initial potentials 0.50V, 0.55V and pulse width 50ms). In FIGS. 15A and 15B, 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) is shown graphically.
In FIG. 15A, 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 was used as an electrolytic solution, and glassy carbon (1.6 mm in diameter) was used for the electrode. Is used, and the result is shown when the pulse width of the reverse pulse is 50 ms.
In FIG. 15B, a solution containing 95 vol% of a 20 mM sodium iodide aqueous solution containing 1 M sodium perchlorate and 5 vol% of propylene carbonate as the supporting electrolyte was used as the electrolytic solution, and glassy carbon (1.6 mm in diameter) was used 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 potentials were 0.5V and 0.55V, a clear critical current for the reduction reaction was observed. Therefore, the product at the initial potentials of 0.5V and 0.55V is an ion, which is I ₃ ⁻ under this condition. That, I ^- I ₃ is an oxidation reaction product ^- was confirmed behavior is reduced.

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

As shown in FIGS. 16A and 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 was stepped down. This behavior is observed in a solid-phase electrochemical reaction (for example, a dissolution reaction from a metal-plated surface). Since the diffusion overvoltage associated with the diffusion from the solution is not required, it was confirmed that the reduction reaction current increases as the pulse potential of the reverse pulse increases without showing the limit current. When the electrolytic solution contained methyl ethyl ketone or propylene carbonate, the reduction current was the same in the region of 0.3V to 0.6V as in the case of no addition, but the reduction current decreased in the region of 0.25V or less. This behavior is considered to be because the reduction of the iodine film and the dissolution of the iodine film are promoted by the inclusion of methyl ethyl ketone or propylene carbonate in the solution.

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

The reduction current values observed in FIGS. 17A and 17B can be roughly divided into two groups. One group was when the initial potentials were 0.9V and 1.0V, and it was observed that the reduction current value increased significantly as the reverse pulse potential was stepped down. On the other hand, the other group is the case where the initial potential is 1.05V and 1.1V, and the reduction current value of the reverse pulse is lower than that when the initial potential is 0.9V and 1.0V. rice field. Since reverse pulse voltammetry looks at the difference in the reduction reaction rate of the chemical species generated at the initial potential, it is considered that the difference in the reverse pulse voltammetry between these groups is the difference in the product 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 redox potentials of formulas (1) and (2) are approximately equal at 0.536 V (standard hydrogen electrode potential), respectively. Therefore, the oxidation reaction products of ^{I −} produced at the initial potential of reverse pulse voltammetry _{are I 2} and I ₃ ⁻ . The change of the standard electrode potential of the formula (7) with respect to temperature is −0.148 mV per 1 ° C. (Reita Tamamushi, “Electrochemistry (2nd Edition)” p.300, (1991), Tokyo Chemical Co., Ltd.). That is, in an environment of -25 ° C, which is a decrease of 50 ° C from 25 ° C, the standard electrode potential (standard electrode potential) of the formula (2) changes by only 7.4 mV from 0.536 (standard hydrogen electrode potential). The electrochemical potential basically depends on the temperature, but as shown above, it is considered that the relationship between the battery reaction and the potential does not fluctuate significantly at the 100 mV level in the practical living environment temperature.

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) has been 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 such thing.

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

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 formula (4) is faster than the electrochemical reaction of the above formula (3), and the rate-determining rate of the formula (5) having the formulas (3) and (4) as constituent reactions. The process is an electrochemical reaction of formula (3). Therefore, 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), it is presumed that the reactions of the above formulas (3) to (5) have occurred. Will be done.

The reaction shown in Equation (5) is the total reaction of the formula (3) and (4), by the reaction I ₂ IO ₃ ^- but is generated in the same manner as in the reaction shown in equation (3), wherein The reaction shown in (5) is also an irreversible reaction. Therefore, it is presumed that the generated IO ₃ ⁻ has a slow discharge reaction rate and is _{difficult 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 small 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. Therefore, it can be seen that it is preferable to operate in a state where the charging potential of the positive electrode does not exceed 1.05 V.

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

As shown in FIGS. 18A and 18B, it was found that the reduction current value observed by the reverse pulse is lower when the initial potential is 1.5 V or higher than when the initial potential is 1.4 V. rice field. Furthermore, it was found that the reduction current value tends to decrease as the initial potential is increased. From this, it was estimated that the electrode was inactivated by holding the initial potential at a noble potential of 1.5 V or higher.

[Example 3]
19A and 19B are normal pulse voltammograms performed in Example 3 (pulse widths 50, 500 and 5000 ms). In Example 3, the current densities are measured when the pulse widths are 50 ms, 500 ms, and 5000 ms. Other conditions are the same as in Example 2.
FIG. 19A shows the results when a solution containing 5 vol% of methyl ethyl ketone in 95 vol% of a 3M sodium iodide aqueous solution 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 3M sodium iodide aqueous solution was used as the electrolytic solution.
The aqueous solution concentration on the order of 3M corresponds to the reaction active material concentration level in the actual flow battery. In FIGS. 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 increased monotonically with increasing potential. On a time scale with a pulse width of 50 ms, the _{amount of I 2} film formed was small, and ^{it was found that the I −} electrode reaction was not significantly inhibited by the film.

At tp = 500 ms, the current density decreased from around 1.5 V. On a time scale with a pulse width of 500 ms, it was found that the _{I 2} film thickened ^{from around 1.5 V and inhibited the I − electrode reaction.}

At tp = 5000 ms, the current values were almost the same 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 an equilibrium in the potential region of about 1.0 V to about 1.8 V.

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 solution containing 95 vol% of a 3M sodium iodide aqueous solution and 5 vol% of methyl ethyl ketone and a 3M sodium iodide aqueous solution were used as electrolytic solutions, respectively.
FIG. 20B shows the results when a solution containing 95 vol% of a 3M sodium iodide aqueous solution and 5 vol% of methyl ethyl ketone and a 3M sodium iodide aqueous solution were used as electrolytic solutions, respectively.

As shown in FIGS. 20A and 20B, when the electrolytic solution contains methyl ethyl ketone or propylene carbonate, the current of about 1.0 V to about 1.8 V is compared with the case where the electrolytic solution does not contain methyl ethyl ketone or propylene carbonate. It was greatly observed. It is considered that this is 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 increased.

The electrochemical measurement in which the supporting electrolyte concentration (sodium perchlorate in Example 2) is set at a level 50 times equal to the concentration of the reaction species of ^{interest (I − in Example 2) is the effect of electrophoresis on substance transfer.} Except for, the electrochemical reaction can be observed while keeping the electrolytic double layer structure, which is an electrochemical reaction field, constant. Therefore, there is an advantage that the existing electrochemical theory can be simply used in the study of the electrochemical reaction mechanism based on the absolute reaction kinetics. Therefore, in Example 2, the study on the electrochemical reaction was carried out in a system containing a reaction species on the order of mM and a supporting electrolyte. In this example, the electrochemical reaction was examined under the condition of the sodium iodide electrolyte solution in the actual flow battery concentration range, which does not contain the supporting electrolyte, but it basically corresponds to the examination result in Example 2. I understand. That is, the charging potential control in the potential region exceeding ^{1.05 V is performed by the above equations (3) (I −} + 3H ₂ O → IO ₃ ⁻ + 6H ⁺ + 6e ⁻ ) and equation (5) (I ₂ + 6H ₂ O → 2IO ₃ ^−). As shown in + 12H ⁺ + 10e ⁻ ), it is not desirable because _{IO 3} ^{− is generated.}

From the above results, the charging potential of the flow cell containing a methyl ethyl ketone or propylene carbonate in the cathode electrolyte IO ₃ ^- that it is preferable not to exceed the positive electrode potential 1.05V (V vs.Ag/AgCl) not generating the charging Recognize.

In a flow battery, it is not desirable that the positive electrode is covered with a thick iodine film to narrow the flow path of the flow battery and hinder the flow of the electrolytic solution itself. In this respect, 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 reactive active material involved in the charge / discharge reaction, the charging potential of 1.05 V (V vs. Ag / AgCl) shown in Examples 2 and 3 is used. ) Charging control at the following potential is preferable.

From FIGS. 18A and 18B, it has been confirmed that glassy carbon is oxidized by setting the potential to a potential higher than 1.5V. Therefore, 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 can be seen that it is preferable to control charging at a potential of 1.5 V or less in an 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 the charge / discharge reaction was carried out were examined. In this example, a solution obtained by adding 5 vol% of methyl ethyl ketone or propylene carbonate to a 1 M sodium iodide (NaI) aqueous solution is used as the positive electrode electrolytic solution, and 1 M chloride containing _{0.5 M zinc chloride (ZnCl 2) is used as the negative electrode electrolytic solution.} An ammonium (NH ₄ Cl) aqueous solution 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 embodiment, the electrochemical reaction of the positive electrode and the negative electrode and the standard electrode potential of each are as follows, and the open circuit voltage of the flow battery is about 1.3 V in the standard state. Further, 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 ⁻ 0.53V
2I ⁻ ⇔ I ₂ + 2e ⁻ 0.53V

FIG. 21 is a current potential curve of the positive electrode and the negative electrode of the flow battery carried out in Example 4. ^{The current potential curve is obtained under the condition that the flow rate is 100 cm 3} / min and the flow rate is charged and discharged with a constant current in the configuration of the flow battery system shown in FIG.

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 potentials of the positive electrode and the negative electrode are the potentials for 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 this embodiment, it is preferable that the charge control condition of the flow battery is such that the charging current density ^{does not exceed 400 mA / cm 2.} Thereby it is prevented that the potential of the positive electrode reaches a noble potential region than 1.05V, IO ₃ ^- may operate the flow battery by suppressing the generation of.

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 does not decompose.

Claims (25)

A positive electrode, a negative electrode, a positive electrode electrolytic solution, a negative electrode electrolytic solution, and a diaphragm are provided, and at least one of the positive electrode electrolytic solution and the negative electrode electrolytic solution is an iodide ion and an oxidation reaction product of the iodide ion. look including a separable organic compound, the organic compound is a ketone, comprising at least one selected from carboxylic acid ester and carbonate, aqueous secondary battery. 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 electrolytic solution and the negative electrode electrolytic solution containing the organic compound. The claim that the oxidation reaction product of the iodide ion comprises at least one selected from the group consisting _{of iodine (I 2} ), triiodide ion (I ₃ ⁻ ) and pentaiodide ion (I ₅ ^−). 1 or the aqueous secondary battery according to claim 2. The aqueous solution according to any one of claims 1 to 3, wherein the organic compound contains at least one selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate, ethyl methyl carbonate and propylene carbonate. System 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, a positive electrode electrolyte feed pump, and the like. A negative electrode electrolytic solution feeding pump is further provided, and the positive electrode electrolytic solution feeding pump is configured to be able to circulate the positive electrode electrolytic solution between the positive electrode electrolytic solution storage tank and the positive electrode active material reaction tank. The negative electrode electrolytic solution feeding pump is configured so that the negative electrode electrolytic solution can be circulated between the negative electrode electrolytic solution storage tank and the negative electrode active material reaction tank. The aqueous secondary battery according to any one of claim 4. The aqueous secondary battery according to any one of claims 1 to 5, wherein the positive electrode electrolytic solution contains the iodide ion and the organic compound, and the negative electrode electrolytic solution contains a negative electrode active material. The sixth aspect of claim 6, wherein 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. Aqueous secondary battery. The aqueous secondary battery according to claim 6 or 7, wherein the negative electrode active material contains zinc ions. Includes a positive electrode, a negative electrode, and an electrolyte, the electrolyte solution and an iodide ion, and the negative electrode active material, viewed contains a organic compound capable of separating the oxidation reaction product of said iodide ions, said organic compound Is an aqueous secondary battery containing at least one selected from ketones, carboxylic acid esters and carbonate esters. The aqueous secondary battery according to claim 9, wherein the content of the organic compound is 1% by volume to 50% by volume of the electrolytic solution. The claim that the oxidation reaction product of the iodide ion comprises at least one selected from the group consisting _{of iodine (I 2} ), triiodide ion (I ₃ ⁻ ) and pentaiodide ion (I ₅ ^−). 9 or the aqueous secondary battery according to claim 10. The aqueous solution according to any one of claims 9 to 11, wherein the organic compound contains at least one selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate, ethyl methyl carbonate and propylene carbonate. System secondary battery. 9 to claims, wherein 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. The aqueous solution-based secondary battery according to any one of claims 9 to 13, wherein the negative electrode active material contains zinc ions. The aqueous solution-based 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 solution system secondary according to any one of claims 9 to 15, further comprising a diaphragm arranged between the positive electrode and the negative electrode to separate the electrolytic solution into a positive electrode electrolytic solution and a negative electrode electrolytic solution. battery. An active material reaction tank for accommodating the electrolytic solution, an electrolytic solution storage tank, and an electrolytic solution feed pump are further provided, and the electrolytic solution feed pump is located between the electrolytic solution storage tank and the active material reaction tank. The aqueous secondary battery according to any one of claims 9 to 16, which is configured to be able to circulate the electrolytic solution. A charging and discharging method of the aqueous secondary battery using the electrolytic solution containing iodide ions as a cathode active material, comprising the step of separating the oxidation reaction product of iodide ions generated in the electrolyte, the separation Is a method for charging and discharging an aqueous secondary battery, which is carried out using an organic compound containing at least one selected from a ketone, a carboxylic acid ester and a carbonic acid ester. It comprises water, iodide ions, and an organic compound capable of separating the oxidation reaction product of iodide ions, wherein the organic compound contains at least one selected from ketones, carboxylic acid esters and carbonate esters, and is claimed. Item 18. An aqueous electrolytic solution for an aqueous secondary battery, which is used as an electrolytic solution for the method for charging and discharging an aqueous secondary battery. The electrolytic solution for an aqueous solution-based secondary battery according to claim 19, wherein the content of the organic compound is 1% by volume to 50% by volume of the electrolytic solution for an aqueous solution-based secondary battery. And water, iodine (I _2), triiodide (I 3 ^_-), and penta-iodide ion (I 5 ^_-) and at least one member selected from the group consisting of methyl ethyl ketone, methyl acetate, ethyl acetate, dimethyl carbonate , At least one organic compound selected from the group consisting of ethyl acetate and propylene carbonate, and an aqueous electrolytic solution for a secondary battery. The electrolytic solution for an aqueous solution-based secondary battery according to claim 21, wherein the content of the organic compound is 1% by volume to 50% by volume of the electrolytic solution for an aqueous solution-based secondary battery. An aqueous solution-based secondary battery charging / discharging method using an electrolytic solution containing iodide ions as a positive electrode active material, which comprises a step of separating an oxidation reaction product of iodide ions generated in the electrolytic solution. The electrolytic solution for an aqueous secondary battery according to claim 21 or claim 21, which is used as an electrolytic solution for a method of charging and discharging a secondary battery. The aqueous secondary battery according to any one of claims 1 to 17, and the charge / discharge are controlled, and the charging potential of the positive electrode is set with reference to the potential of the Ag / AgCl reference electrode (Cl ⁻ concentration saturation). A flow battery system including a control unit set to 1.5 V or less. A power generation system including the power generation device and the flow battery system according to claim 24.

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