JP7207130B2 - flow battery - Google Patents

Description

The present disclosure relates to flow batteries.

A flow battery that charges and discharges by circulating an electrolytic solution is known. Flow batteries have many advantages such as low cost, safety, and long life. For example, Patent Literatures 1 and 2 disclose flow batteries using polyoxometalate as a mediator.

JP 2016-085955 A JP 2017-147045 A

For example, when polyoxometalate is used as a mediator on the positive electrode side, charging and discharging may not be possible if the redox potential of the positive electrode solid active material is high. The present disclosure has been made in view of the above circumstances, and a main object thereof is to provide a flow battery that can be charged and discharged even when the oxidation-reduction potential of the positive electrode solid active material is high.

In order to solve the above problems, the present disclosure provides a flow battery having a positive electrode, a negative electrode, and a separator, wherein the positive electrode contains a positive electrode solid active material containing Na element and a positive electrode mediator. and an electrolyte, wherein the positive electrode solid active material has an oxidation-reduction potential of 0.35 V (vs. Ag/AgCl) or more, and the electrolyte acts as the positive electrode mediator to oxidize the positive electrode solid active material. A flow battery containing a first mediator having a redox potential higher than the redox potential and a second mediator having a redox potential lower than the redox potential of the positive electrode solid active material is provided.

According to the present disclosure, by using a combination of a first mediator and a second mediator as positive mediators, a flow battery that can be charged and discharged even when the positive electrode solid active material has a high oxidation-reduction potential is provided. be able to.

In the present disclosure, even when the oxidation-reduction potential of the positive electrode solid active material is high, there is an effect that a chargeable/dischargeable flow battery can be provided.

1 is a schematic diagram illustrating a flow battery in accordance with the present disclosure; FIG. 4 shows the results of oxidation-reduction potential measurement for the positive electrode solid active material and the positive electrode mediator in Example 1. FIG. 4 shows the results of a charge/discharge test on the flow battery obtained in Example 1. FIG. 3 is a discharge curve of flow batteries obtained in Examples 1-3. 4 shows the results of a charge/discharge test for the flow battery obtained in Comparative Example 1. FIG.

The flow battery in the present disclosure will be described in detail below.

FIG. 1 is a schematic diagram illustrating a flow battery in the present disclosure. The flow battery 100 shown in FIG. 1 has a case 40 and a separator 30 that separates the interior of the case 40 into a cathode compartment 11 and an anode compartment 21 . A positive current collector 12 and a positive diffusion layer 13 are arranged in the positive electrode chamber 11 . A negative electrode current collector 22 and a negative electrode diffusion layer 23 are arranged in the negative electrode chamber 21 .

A circulation path 18 is provided between the positive electrode chamber 11 and the positive electrode reservoir container 14 , and a circulation pump 19 is attached in the middle of the circulation path 18 . On the other hand, a circulation path 28 is provided between the anode chamber 21 and the anode reservoir container 24 , and a circulation pump 29 is attached in the middle of the circulation path 28 .

In addition, a positive electrode solid active material 15 containing Na element and an electrolytic solution 16 containing a positive electrode mediator are stored in the positive electrode reservoir container 14, and a filter 17 prevents the positive electrode solid active material 15 from flowing out. ing. On the other hand, a negative electrode solid active material 25 and an electrolytic solution 26 containing a negative electrode mediator are stored in the negative electrode reservoir container 24 , and a filter 27 prevents the negative electrode solid active material 25 from flowing out.

The present disclosure is characterized in that the positive electrode solid active material 15 has a high oxidation-reduction potential. In addition, the electrolytic solution 16 contains a first mediator having an oxidation-reduction potential higher than that of the positive electrode solid active material 15 and an oxidation-reduction potential lower than that of the positive electrode solid active material 15 as positive electrode mediators. and a second mediator having

According to the present disclosure, by using a combination of a first mediator and a second mediator as positive mediators, a flow battery that can be charged and discharged even when the positive electrode solid active material has a high oxidation-reduction potential is provided. be able to.

Conventionally, a relatively low-potential material such as lithium iron phosphate (for example, a material of 0.30 V vs. Ag/AgCl or less) has been used as a positive electrode solid active material for a flow battery (redox flow battery). On the other hand, from the viewpoint of improving the energy density, materials with relatively high potential (for example, 0.35 V vs. Ag/AgCl or higher) such as sodium vanadate phosphate and Prussian blue analogues are preferable. Here, Patent Document 1 discloses a polyoxometalate as a positive electrode mediator. However, when polyoxometallate is used in combination with a relatively high-potential material, the potential of polyoxometallate cannot be made higher than the potential of the positive electrode solid active material during charging, making charging difficult. Become.

More specifically, when the potential of the positive electrode mediator is higher than the potential of the positive electrode solid active material during charging, electrons start to flow from the positive electrode mediator to the positive electrode solid active material. Conversely, when the potential of the positive electrode mediator is lower than the potential of the positive electrode solid active material during discharge, electrons start to flow from the positive electrode solid active material to the positive electrode mediator. The polyoxometalates (for example, silvanadomolybdic acid, phosphovanadomolybdic acid, silicotungstic acid, etc.) described in Patent Document 1 do not have an oxidation-reduction potential of 0.35 V or more, so the oxidation-reduction potential is 0.35 V or higher. When used in combination with a positive electrode solid active material of 35 V or higher, charging becomes difficult. In contrast, in the present disclosure, by using the first mediator having a higher oxidation-reduction potential than the positive electrode solid active material, even if the positive electrode solid active material has a higher oxidation-reduction potential, , charging becomes possible. Furthermore, discharge becomes possible by using the second mediator having a redox potential lower than that of the positive electrode solid active material.

1. Positive Electrode The positive electrode in the present disclosure has a positive electrode solid active material containing Na element and an electrolytic solution containing a positive electrode mediator. Also, the positive electrode usually has a positive electrode current collector. The structure of the positive electrode is not particularly limited as long as the electrolyte can be circulated and ions can be exchanged with the negative electrode through the separator.

(1) Positive electrode solid active material The positive electrode solid active material in the present disclosure contains Na element. Na ions migrate between the positive and negative electrodes through the separator.

The redox potential of the positive electrode solid active material is usually 0.35 V (vs. Ag/AgCl) or higher, may be 0.37 V (vs. Ag/AgCl) or higher, and may be 0.39 V (vs. Ag /AgCl) or more, or 0.41 V (vs. Ag/AgCl) or more. The oxidation-reduction potential of the positive electrode solid active material can be obtained, for example, by measurement by a cyclic voltammetry (CV) method.

An example of the positive electrode solid active material is sodium vanadate phosphate (Na ₃ V ₂ (PO ₄ ) ₃ ). Another example of the positive electrode solid active material is a Prussian blue analog represented by the general formula Na ₂ [M(CN) ₆ ]. M is preferably at least one element selected from Fe, Ni, Cu, Co, and Mn. Among them, M preferably contains at least Fe. Moreover, M may contain the same element (for example, Fe) having different valences.

Examples of the shape of the positive electrode solid active material include particulate, fibrous, sheet, and porous. When the shape of the positive electrode solid active material is particulate, the average particle diameter (D ₅₀ ) is, for example, 100 μm or more and 10 mm or less. Moreover, the surface of the positive electrode solid active material may be coated with carbon. The carbon coating can prevent the components of the positive electrode solid active material from dissolving in the electrolytic solution. As a method for forming the carbon coating, for example, there is a method of coating the surface of the positive electrode solid active material with a substance that serves as a carbon source and then firing it in an inert atmosphere.

The positive electrode may have a conductive material in contact with the positive electrode solid active material. Conductive materials include, for example, natural graphite (flaky graphite, flake graphite), graphite such as artificial graphite, acetylene black, carbon black, ketjen black, carbon whiskers, needle coke, carbon fiber, metals (e.g., copper, nickel, aluminum, silver, and gold).

The positive electrode solid active material and the conductive material may or may not always be in contact. In the former case, the positive electrode solid active material and the conductive material are preferably bound by a binder. Examples of binders include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorine-containing resins such as fluororubber, thermoplastic resins such as polypropylene and polyethylene, ethylene-propylene-diene rubber (EPDM), and sulfonated EPDM. , natural butyl rubber (NBR), styrene-butadiene rubber (SBR), and polyacrylonitrile (PAN). On the other hand, in the latter case, the positive electrode solid active material and the conductive material may come into contact when the electrolyte is circulated. Moreover, the positive electrode does not have to have a conductive material.

(2) Electrolyte The electrolyte in the positive electrode contains a positive mediator. Moreover, the electrolytic solution usually contains a solvent and may further contain a supporting electrolyte.

The electrolytic solution has, as a positive electrode mediator, a first mediator having an oxidation-reduction potential higher than that of the positive electrode solid active material. The difference between the oxidation-reduction potential of the first mediator and the oxidation-reduction potential of the positive electrode solid active material is, for example, 0.05 V (vs. Ag/AgCl) or more, and 0.1 V (vs. Ag/AgCl) or more. It may be 0.3 V (vs. Ag/AgCl) or more.

First mediators include, for example, hydroquinone sulfonate. Examples of hydroquinonesulfonates include sodium hydroquinonesulfonate, potassium hydroquinonesulfonate, and lithium hydroquinonesulfonate, with sodium hydroquinonesulfonate being preferred.

The concentration of the first mediator in the electrolytic solution is, for example, 0.1% by weight or more, may be 1% by weight or more, or may be 3% by weight or more. On the other hand, the concentration of the first mediator in the electrolytic solution is, for example, 15% by weight or less, may be 10% by weight or less, or may be 7% by weight or less.

The electrolytic solution has, as a positive electrode mediator, a second mediator having a redox potential lower than that of the positive electrode solid active material. The difference between the oxidation-reduction potential of the positive electrode solid active material and the oxidation-reduction potential of the second mediator is, for example, 0.05 V (vs. Ag/AgCl) or more, and 0.1 V (vs. Ag/AgCl) or more. It may be 0.3 V (vs. Ag/AgCl) or more.

The second mediator preferably has a high molecular weight. The molecular weight of the second mediator is, for example, 1000 or more. Also, the second mediator is preferably a polyoxometalate (polyacid). Polyoxometalates may be isopolyacids or heteropolyacids, the latter being preferred. Heteropolyacids include silvanadomolybdic acid (H _4+x [SiV _x Mo _12-x O ₄₀ ] (0≦x≦4)) and phosphovanadomolybdic acid (H _3+x [PV _x Mo _12-x O ₄₀ ]). (0≦x≦4)) and silicotungstic acid (H ₄ [SiW ₁₂ O ₄₀ ]). The above x in silvanadomolybdic acid and phosphovanadomolybdic acid is preferably 1.5 or more and 3.5 or less.

The concentration of the second mediator in the electrolytic solution is, for example, 0.1% by weight or more, may be 1% by weight or more, or may be 3% by weight or more. On the other hand, the concentration of the second mediator in the electrolytic solution is, for example, 15% by weight or less, may be 10% by weight or less, or may be 7% by weight or less.

The solvent contained in the electrolytic solution may be water or a non-aqueous solvent. Also, the supporting salt is usually a Na salt. The Na salt may be an organic salt or an inorganic salt. Specific examples of Na salts include sodium sulfate.

(3) Positive Electrode Current Collector Examples of the positive electrode current collector include carbon paper, aluminum, copper, titanium, stainless steel, nickel, iron, platinum, baked carbon, conductive polymer, and conductive glass. Examples of the shape of the positive electrode current collector include foil-like, film-like, sheet-like, net-like, punched or expanded shapes, laths, porous bodies, foams, and fibers formed. The shape of the positive electrode current collector is, for example, 500 μm or more and 1 cm or less. In the field of flow batteries, current collectors are sometimes referred to as electrodes.

A positive electrode diffusion layer may be arranged between the positive electrode current collector and the separator. By providing the positive electrode diffusion layer, the battery reaction proceeds smoothly. Examples of the positive electrode diffusion layer include carbon felt.

(4) Positive Electrode In the positive electrode, an electrolytic solution containing a positive mediator is circulated. For example, as shown in FIG. 1, the reaction field and the energy storage field can be separated by circulating the electrolytic solution without circulating the positive electrode solid active material. As a result, it is possible to achieve both miniaturization and high capacity of the power generation element. On the other hand, although not shown, the positive electrode solid active material may be circulated together with the electrolyte.

2. Negative Electrode The negative electrode in the present disclosure is not particularly limited, but preferably has a negative electrode solid active material and an electrolytic solution containing a negative electrode mediator. Moreover, the negative electrode usually has a negative electrode current collector. The structure of the negative electrode is not particularly limited as long as the electrolyte can be circulated and ions can be exchanged with the positive electrode through the separator.

The negative electrode solid active material is a material having a lower oxidation-reduction potential than the positive electrode solid active material. The negative electrode solid active material may or may not contain Na element. Examples of negative electrode solid active materials include sodium titanium phosphate (NaTi ₂ (PO ₄ ) ₃ ) and titanium pyrophosphate (TiP ₂ O ₇ ). Also, the negative electrode solid active material and the conductive material may or may not always be in contact. In the former case, the negative electrode solid active material and the conductive material are preferably bound together by a binder.

The electrolyte in the negative electrode contains a negative mediator. Moreover, the electrolytic solution usually contains a solvent and may further contain a supporting salt. A known mediator can be used as the negative mediator. The second mediator described above may be used as the negative mediator. Further, the solvent and the supporting salt are the same as the solvent and the supporting salt for the negative electrode described above. Also, the negative electrode current collector and the negative electrode diffusion layer are the same as the positive electrode current collector and the positive electrode diffusion layer described above, so descriptions thereof will be omitted here.

At the negative electrode, the electrolytic solution is circulated. For example, as shown in FIG. 1, the reaction field and the energy storage field can be separated by circulating the electrolytic solution without circulating the negative electrode solid active material. As a result, it is possible to achieve both miniaturization and high capacity of the power generation element. On the other hand, although not shown, the negative electrode solid active material may be circulated together with the electrolyte.

3. Separator The separator in the present disclosure has ion permeability and has the function of preventing mixing of the positive electrode component and the negative electrode component. Examples of separators include ion conductive polymer membranes (ion exchange membranes) capable of conducting ions, ion conductive solid electrolyte membranes, gel membranes, and microporous membranes. Examples of ion conductive polymer membranes include perfluorocarbon materials (tetrafluoroethylene-perfluorovinyl copolymers) composed of perfluoro side chains having a hydrophobic tetrafluoroethylene skeleton and sulfonic acid groups. . Examples of the ion conductive solid electrolyte membrane include cation conductive glass (oxide glass).

4. Flow Battery A flow battery in the present disclosure has a positive electrode, a negative electrode and a separator as described above. The flow battery may further have functional units such as a current measuring unit, a voltage measuring unit, a resistance measuring unit, a temperature measuring unit, a discharge control unit, and a charge control unit. In addition, although the use of the flow battery is not particularly limited, for example, it can be used for charging a storage battery. By connecting the storage battery to the flow battery, the storage battery can be easily charged. Moreover, as a storage battery, a vehicle-mounted storage battery is mentioned, for example.

The present disclosure is not limited to the above embodiments. The above embodiment is an example, and any device that has substantially the same configuration as the technical idea described in the claims of the present disclosure and produces the same effect is the present invention. It is included in the technical scope of the disclosure.

[Example 1]
(Preparation of positive electrode material)
Sodium sulfate (Na ₂ SO ₄ , manufactured by WAKO) was dissolved in water to obtain an aqueous solution with a concentration of 0.5M. Next, sodium hydroquinone sulfonate (manufactured by WAKO) and silicovanadomolybdic acid (H ₆ SiV ₂ Mo ₁₀ O ₄₀ , manufactured by Nippon Inorganic Chemical Co., Ltd.) were prepared as positive electrode mediators at 4% by weight and 6% by weight, respectively. It was dissolved in the above aqueous solution at a weight percent ratio. As a result, an electrolytic solution containing a positive mediator was obtained.

In addition, a Prussian blue analog (Na ₂ [(Ni, Fe) (CN) ₆ ]) as a positive electrode solid active material, Ketjen Black (ECP600JD, manufactured by Lion Specialty Chemical) as a conductive material, PTFE (Polyflon PTFE-F) as a binder , F-104, manufactured by Daikin Industries) was prepared. A positive electrode mixture was obtained by weighing at a weight ratio of positive electrode solid active material:conductive material:binder=85:10:5, mixing in a mortar, and molding.

(Preparation of negative electrode material)
Sodium sulfate (Na ₂ SO ₄ , manufactured by WAKO) was dissolved in water to obtain an aqueous solution with a concentration of 0.5M. Next, as negative mediators, 1,3-diamino-2-hydroxypropane-N,N,N',N'tetraacetic acid (manufactured by Aldrich) and disodium catechol-3,5-disulfonate (Tyrone, Tokyo Kasei Co., Ltd.) were prepared and dissolved in the above aqueous solution at a ratio of 1M and 1M, respectively. As a result, an electrolytic solution containing a negative electrode mediator was obtained.

Also prepared were NaTi ₂ (PO ₄ ) ₃ as a negative electrode solid active material, Ketjenblack (ECP600JD, manufactured by Lion Specialty Chemical) as a conductive material, and PTFE (Polyflon PTFE-F, F-104, manufactured by Daikin Industries) as a binder. . A negative electrode mixture was obtained by weighing at a weight ratio of negative electrode solid active material: conductive material: binder = 85:10:5, mixing in a mortar, and molding.

(Preparation of flow battery)
A flow battery shown in FIG. 1 was fabricated. Nafion 324 (manufactured by Aldrich) was used as a separator. Carbon felt (manufactured by Nippon Carbon Co., Ltd.) was used as the positive electrode diffusion layer and the negative electrode diffusion layer. Ti metal (manufactured by Niigata) was used as the positive electrode current collector and the negative electrode current collector. Luer tip syringes (manufactured by Osaka Chemical Co., Ltd.) were used as reservoir containers for storing the positive electrode solid active material and the negative electrode solid active material. Quartz Wool (manufactured by Tosoh Corporation) was used as a filter for supplementing the solid active material. A silicon tube was used for the circulation path, and Masterflex (manufactured by Yamato Scientific Co., Ltd.) was used for the liquid feed pump. Using these, a flow battery was produced.

[Example 2]
A flow battery was fabricated in the same manner as in Example 1, except that potassium hydroquinonesulfonate was used as the positive mediator instead of sodium hydroquinonesulfonate.

[Example 3]
A flow battery was fabricated in the same manner as in Example 1, except that lithium hydroquinone sulfonate was used as the positive mediator instead of sodium hydroquinone sulfonate.

[Comparative Example 1]
In the same manner as in Example 1, except that sodium hydroquinone sulfonate was not used as the positive mediator and the proportion of silicovanadomolybdic acid (H ₆ SiV ₂ Mo ₁₀ O ₄₀ , manufactured by Nippon Inorganic Chemical Co., Ltd.) was changed to 10% by weight. A flow battery was fabricated.

[evaluation]
(Oxidation-reduction potential measurement)
The positive electrode solid active material and the positive electrode mediator in Example 1 were subjected to oxidation-reduction potential measurement. Measurements were made by cyclic voltammetry (CV). In the measurement of the positive electrode solid active material, 0.1 g of the positive electrode solid active material was pressed and hardened to the tip of the Ti electrode as a working electrode, and 0.1 g of the negative electrode solid active material was pressed and hardened to the tip of the Ti electrode to serve as the counter electrode. , and a saturated _KCl Ag/ _AgCl electrode as a reference electrode. On the other hand, in the measurement of the positive mediator, the positive mediator was dissolved in an aqueous Na ₂ SO ₄ solution, a glassy carbon electrode was used as the working electrode, a platinum wire was used as the counter electrode, and a saturated KCl Ag/AgCl electrode was used as the reference electrode. , and a sweep rate of 20 mV/sec. The oxidation-reduction potential was defined as the point at which the slope of the peak curve on the oxidation side changed to the peak curve on the reduction side and became zero.

The results are shown in FIG. As shown in FIG. 2, the redox potential of the positive electrode solid active material (Na ₂ [(Ni, Fe)(CN) ₆ ]) was 0.48 V (vs. Ag/AgCl). Furthermore, the redox potential of sodium hydroquinone sulfonate was higher than that of the positive electrode solid active material, and the redox potential of silicovanadomolybdic acid was lower than that of the positive electrode solid active material.

(Charging and discharging test)
A charge/discharge test was performed on the flow batteries obtained in Examples 1 to 3 and Comparative Example 1. A multi-electrochemical measurement system (HZ-Pro, manufactured by Hokuto Denko) was used for the charge/discharge test. The battery was charged to +1.6 V in a constant current mode of +20 mA/cm ² during charging, and discharged to +1.3 V in a constant current mode of −20 mA/cm ² during discharging. Also, the flow rate of the flow battery was set to 10 mL/s.

As shown in FIG. 3, it was confirmed that the flow battery obtained in Example 1 was chargeable and dischargeable. Moreover, as shown in FIG. 4, it was confirmed that Examples 1 to 3 had a predetermined discharge capacity. In particular, Example 1 (sodium hydroquinone sulfonate) provided a higher capacity than Example 2 (potassium hydroquinone sulfonate) and Example 3 (lithium hydroquinone sulfonate). This result suggested that too many types of cations passing through the separator reduced the discharge capacity. On the other hand, as shown in FIG. 5, in Comparative Example 1, charging and discharging could not be performed. It is presumed that the reason for this is that the oxidation-reduction potential of the positive electrode solid active material was too high, and normal charging was not possible.

DESCRIPTION OF SYMBOLS 11... Positive electrode chamber 12... Positive electrode collector 13... Positive electrode diffusion layer 14... Positive electrode reservoir container 15... Positive electrode solid active material 16... Electrolytic solution 21... Negative electrode chamber 22... Negative electrode collector 23... Negative electrode diffusion layer 24... Negative electrode reservoir container DESCRIPTION OF SYMBOLS 25... Negative electrode solid active material 26... Electrolyte solution 30... Separator 100... Flow battery

Claims (1)

A flow battery having a positive electrode, a negative electrode, and a separator,
The positive electrode has a positive electrode solid active material containing Na element and an electrolytic solution containing a positive electrode mediator,
Prussian, wherein the positive electrode solid active material has an oxidation-reduction potential of 0.35 V (vs. Ag/AgCl) or more and is represented by the general formula Na ₂ [M(CN) ₆ ], where M contains at least Fe is a blue analogue,
The electrolytic solution contains, as the positive electrode mediator, a first mediator that is hydroquinone sulfonate having an oxidation-reduction potential higher than the oxidation-reduction potential of the positive electrode solid active material, and a second mediator that is a heteropolyacid with a low redox potential ;
The flow battery , wherein the concentrations of the first mediator and the second mediator in the electrolytic solution are 1% by weight or more and 10% by weight or less, respectively .

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