WO2019212054A1 - Electrode sheet and manufacturing method thereof, and redox flow battery - Google Patents

Abstract

This electrode sheet includes first carbon nanotubes with an average fiber diameter of 100-1000 nm and second carbon nanotubes with an average fiber diameter of 1-30 nm, wherein the oxygen content of all carbon nanotubes, including the first carbon nanotubes and the second carbon nanotubes, is less than or equal to 0.3 mass%, and the oxygen content of the second carbon nanotubes is less than or equal to 1.2 mass%. An electrode sheet used in a redox flow battery is also provided.

Description

Electrode sheet, manufacturing method thereof, and redox flow battery

The present invention relates to an electrode sheet, a manufacturing method thereof, and a redox flow battery. The present invention particularly relates to an electrode sheet for a redox flow battery including carbon nanotubes, a method for producing the same, and a redox flow battery including the electrode sheet for redox flow battery.

A redox flow battery is known as a large capacity storage battery. A redox flow battery generally has an ion exchange membrane that separates an electrolytic solution and electrodes provided on both sides of the ion exchange membrane. Then, using an electrolytic solution containing a metal ion (active material) whose valence is changed by oxidation and reduction, charging and discharging are performed by simultaneously proceeding an oxidation reaction on one electrode and a reduction reaction on the other electrode. be able to.

The stationary storage battery is required to be small and have high output. Therefore, a redox flow battery is required to have a high current density. One factor that affects current density is cell resistivity. The cell resistivity is a numerical value determined as a result of combining resistance values generated in all elements through which current flows. Factors that contribute to the cell resistivity include, for example, the electrical resistance of the current collector, the electrical resistance of the electrode, the contact resistance between the electrode and the current collector, the reaction resistance at the electrode surface, the ion transfer resistance in the electrolyte, and the ions Proton transfer resistance in the exchange membrane is the main one. The reaction resistance on the electrode surface is particularly difficult to control. On the electrode surface of the redox flow battery, electrons are transferred (or received) to the electrode when the valence of the metal ion as the active material changes, and then the metal ion (including the electrolyte solution) whose valence has changed is the electrode. It needs to be quickly removed from the surface. Therefore, it is preferable that the redox flow battery is configured so that the electrolyte flows uniformly and at a constant speed in one direction.

Conventionally, it is considered that a carbon electrode of a redox flow battery is reacted through a functional group such as an OH group or a COOH group. For example, Patent Document 1 discloses that a porous plate is subjected to a heat treatment for improving hydrophilicity in order to improve wettability with an electrolytic solution and favorably perform an electrochemical reaction. In Patent Document 2, as a surface modification treatment other than heat treatment, the surface of a fiber cloth made of carbon fiber, graphite fiber, or carbon fiber / graphite fiber (composite fiber) is subjected to plasma treatment, photochemical treatment, or ion implantation treatment. Is disclosed. Further, Patent Document 3 discloses that a conventional heat treatment and / or chemical treatment method is applied to an electrode in order to clean an electrode formed from a carbon material and form a carbon surface that functions as an improved active catalyst site. Applying and activating is disclosed. Patent Document 4 discloses hydrophilizing the surface of carbon nanotubes contained in an electrode material.

JP2017-10809A JP3167295B WO2013 / 096276 WO2015 / 072452

As described above, in the redox flow battery, in order to improve the reactivity at the electrode, a functional group such as an OH group containing oxygen or a COOH group is attached to the surface of the electrode. However, if the amount of the functional group attached to the surface of the electrode increases too much, the hydrophilicity of the electrode becomes too large, and the movement of the metal ion, which is the active material that has finished charge transfer, from the electrode surface becomes slow, It was found that the cell resistivity was not lowered sufficiently.

The present invention has been made in view of the above problems, and an object thereof is to provide an electrode sheet capable of reducing cell resistivity, a method for producing the same, and a redox flow battery having low cell resistivity.

The configuration of the present invention for achieving the above object is as follows.
(1) An electrode sheet comprising first carbon nanotubes having an average fiber diameter of 100 to 1000 nm and second carbon nanotubes having an average fiber diameter of 1 to 30 nm, wherein the first carbon nanotubes and the second carbon nanotubes are The electrode sheet in which the oxygen content of the whole carbon nanotubes containing is 0.3% by mass or less and the oxygen content of the second carbon nanotubes is 1.2% by mass or less.
(2) The electrode sheet according to (1), wherein the second carbon nanotube is contained in an amount of 1 to 30% by mass with respect to the entire carbon nanotube.
(3) The electrode sheet according to (1) or (2), wherein the oxygen content of the entire carbon nanotube is 0.2% by mass or less.
(4) The electrode sheet according to any one of (1) to (3), wherein the oxygen content of the second carbon nanotube is 0.2% by mass or less.
(5) Any one of (1) to (4), wherein the second carbon nanotubes adhere to the surface of the first carbon nanotubes, and the second carbon nanotubes are entangled between the plurality of first carbon nanotubes. The electrode sheet according to 1.
(6) The electrode sheet according to any one of (1) to (5), an electrolyte solution inflow portion for allowing an electrolyte solution to flow into the electrode sheet, and a current collector for exchanging electrons between the electrode sheet A redox flow battery comprising: an electrolyte solution outflow member that causes the electrolyte solution that has passed through the electrode sheet to flow out; and an ion exchange membrane.
(7) A mixing step of mixing first carbon nanotubes having an average fiber diameter of 100 to 1000 nm and second carbon nanotubes having an average fiber diameter of 1 to 30 nm, and the carbon nanotubes mixed in the mixing step in a sheet form A battery electrode sheet manufacturing method comprising: forming a second carbon nanotube in an atmosphere of an inert gas or a reducing gas before the mixing step; The manufacturing method of an electrode sheet which performs the oxygen reduction process heated at below ° C.
(8) The electrode according to (7), wherein in the oxygen reduction step, the second carbon nanotube is heated in an atmosphere of an inert gas or a reducing gas at 2800 ° C. or higher and 3300 ° C. or lower for 0.5 hour or longer. Sheet manufacturing method.
(9) In the mixing step, the first carbon nanotubes and the second carbon nanotubes are dispersed in a medium containing the first carbon nanotubes and the second carbon nanotubes to form a dispersion, and the molding step includes The method for producing an electrode sheet according to (7) or (8), wherein the medium is removed from the dispersion, and the solid content from which the medium has been removed is formed into a sheet.

According to the present invention, it is possible to provide an electrode sheet capable of reducing cell resistivity, a method for manufacturing the same, and a redox flow battery having low cell resistivity.

It is a flowchart which shows an example of the manufacturing method of the electrode sheet for redox flow batteries concerning embodiment of this invention. It is the schematic which showed an example of the structure of a redox flow battery. It is the schematic which showed an example of the structure of the battery cell of a redox flow battery. It is the figure which looked at the electrolyte solution inflow part contained in the battery cell from the current collecting plate side.

Hereinafter, an electrode sheet for a redox flow battery to which the present invention is applied, a manufacturing method thereof, and a redox flow battery using an electrode sheet as an example of the present invention will be described. In the drawings, in order to make the features of the present invention easier to understand, the portions that become the features may be enlarged for the sake of convenience, and the dimensional ratios of the respective components may differ from the actual ones. In addition, the materials, dimensions, and the like exemplified in the following description are examples, and the present invention is not limited to these, and can be appropriately modified and implemented without changing the gist thereof.

As a result of intensive studies by the present inventors, the carbon electrode using carbon nanotubes with less oxygen content is more than the carbon electrode using carbon fibers introduced with a large amount of functional groups such as OH groups and COOH groups. Were also found to exhibit excellent properties. In addition, among carbon electrodes using carbon nanotubes, carbon electrodes using carbon nanotubes with few functional groups containing oxygen generated at the defects on the edges and basal surface achieve high current density by reducing cell resistivity. It became clear that we could do it. It is considered that high current density could be achieved by reducing the cell resistivity because the movement of metal ions from the electrode surface was facilitated by the decrease in the functional group exhibiting hydrophilicity.

Here, unless otherwise specified, “oxygen” refers to oxygen atoms, not oxygen as a molecule, and “amount of oxygen contained” refers to the mass-based content of oxygen atoms contained in the target. It means an amount (for example, mass%).

The “average fiber diameter” of the carbon nanotube is a value obtained by measuring the diameter of 100 randomly extracted fibers with a transmission electron microscope and calculating the arithmetic average value thereof. The “average fiber length” is a value obtained by measuring the length of 100 randomly extracted fibers with a transmission electron microscope and calculating the arithmetic average value thereof.

The carbon nanotubes used in the following embodiments may be single-walled (nanowalled nanotube, SWNT) or multi-walled (multi walled nanotube, MWNT) as long as the carbon six-membered ring network (graphene sheet) is formed in a tubular shape. Any of these may be used, and a mixture thereof may be used. In addition, the carbon nanotubes in the embodiment of the present invention have a carbon nanohorn structure closed by a five-membered ring at the tip, and some five-membered or seven-membered rings also exist in the conical / cylindrical portion. It may include a structure that is bent or uneven in a rule. However, SWNT and MWNT formed in a tubular shape with a six-membered carbon network (graphene sheet) are preferable.

<1. Electrode sheet>
<1-1. Compounding of carbon nanotubes>
The electrode sheet for redox flow battery according to the present embodiment (hereinafter also simply referred to as “electrode sheet”) includes a first carbon nanotube having an average fiber diameter of 100 to 1000 nm and a second carbon nanotube having an average fiber diameter of 1 to 30 nm. including. The carbon nanotubes contained in the electrode sheet are preferably composed only of first carbon nanotubes having an average fiber diameter of 100 to 1000 nm and second carbon nanotubes having an average fiber diameter of 1 to 30 nm.

By including the first carbon nanotube having a large average fiber diameter and the second carbon nanotube having a small average fiber diameter, a carbon nanotube having a wide fiber diameter distribution can be obtained. Carbon nanotubes with a wide distribution of fiber diameters are attached to each other, and a network in which a plurality of thick carbon nanotubes are physically connected via thin carbon nanotubes (hereinafter referred to as a physical network) is obtained. It will be easier. Even when incorporated in a battery and used for a long time, it is considered that the bonds between the respective carbon nanotubes can be maintained. Thereby, it is considered that the thick carbon nanotubes function as a main conductive material, and the thinner carbon nanotubes electrically connect the thick carbon nanotubes and efficiently support the conductive path. Further, it is considered that the gap between the thick carbon nanotubes, which is the main conductive material, can be filled with the thin carbon nanotubes, and the conductivity of the electrode can be further improved. When the conductivity of the electrode is increased, the cell resistivity of the redox flow battery is lowered, and the input / output characteristics at a large current can be improved.

“Adhesion” in the above description means a state where the second carbon nanotubes appear to be in contact with the surface of the first carbon nanotubes when the electrodes are observed with, for example, a transmission electron microscope (TEM). Further, when a structure in which at least a part of the second carbon nanotubes intersects with two or more first carbon nanotubes can be confirmed, it is determined that the electrode has a “tangled structure”.

The average fiber diameter of the first carbon nanotubes is preferably 100 to 300 nm, more preferably 100 to 200 nm, and still more preferably 100 to 150 nm. The average fiber length of the first carbon nanotubes is preferably 0.1 to 30 μm, more preferably 0.5 to 25 μm, and still more preferably 0.5 to 20 μm.

The average fiber diameter of the second carbon nanotube is 1 to 30 nm, preferably 5 to 25 nm, more preferably 5 to 20 nm. The average fiber length of the second carbon nanotube is preferably 0.1 to 10 μm, more preferably 0.2 to 8 μm, and still more preferably 0.2 to 5 μm.

When the average fiber diameters of the first carbon nanotubes and the second carbon nanotubes are in the above ranges, the electrode sheet has a structure capable of maintaining higher strength and higher conductivity. This is because the first carbon nanotube serves as a trunk and the second carbon nanotube is suspended in a branch shape between the plurality of first carbon nanotubes. When the average fiber diameter of the first carbon nanotubes is 100 nm or more, the trunk becomes stable and cracks are less likely to occur in the electrode structure, and it becomes easy to maintain sufficient strength. When the average fiber diameter of the second carbon nanotubes is 30 nm or less, the second carbon nanotubes can be sufficiently entangled with the first carbon nanotubes, and the conductivity is improved.

If the content of the second carbon nanotube is large, the content of the thin carbon nanotube is increased, so that it is considered that the thick carbon nanotube can be effectively combined. In this respect, the content of the second carbon nanotubes with respect to the total amount of the first carbon nanotubes and the second carbon nanotubes is preferably 1% by mass or more, more preferably 3% by mass or more, and 5% by mass. % Or more is more preferable.

By suppressing the content of the second carbon nanotubes, it is possible to prevent a decrease in the content of thick carbon nanotubes, and it is possible to suppress a decrease in strength and conductivity of the electrode layer. This is because thick carbon nanotubes greatly contribute to the strength and conductivity maintenance of the electrode layer. From this viewpoint, the content of the second carbon nanotubes with respect to the total amount of the first carbon nanotubes and the second carbon nanotubes is preferably 30% by mass or less, more preferably 20% by mass or less, and 15% by mass. More preferably, it is% or less. *

The content of the second carbon nanotube relative to the total amount of the first carbon nanotube and the second carbon nanotube can be calculated based on the charged amounts of the first carbon nanotube and the second carbon nanotube. You may calculate by observing with an electron microscope (TEM) and comparing with the observation image of the electrode by which the mixture ratio was decided previously.

<1-2. Carbon nanotube oxygen content>
The total oxygen content of the carbon nanotubes including the first carbon nanotubes and the second carbon nanotubes is 0.3% by mass or less, preferably 0.2% by mass or less, more preferably 0.1% by mass or less. It is possible to reduce the cell resistivity of the redox flow battery by suppressing the hydrophilicity of the carbon nanotubes and efficiently removing and discharging ions after reaction contained in the electrolyte described later from the electrode sheet. Because.

Carbon nanotubes have a very small oxygen content compared to carbon fibers and the like, but carbon nanotubes generally have different oxygen content when the average fiber diameter is different.

The reason for the manufacturing process is also considered to be large, but the first carbon nanotubes having an average fiber diameter of 100 to 1000 nm are generally easily available with a low oxygen content, and the oxygen content is usually 0.3% by mass. It is as follows. On the other hand, the amount of oxygen contained in the second carbon nanotubes having an average fiber diameter of 1 to 30 nm is usually about 1.8 to 2.5% by mass.

As described above, the second carbon nanotubes having an average fiber diameter of 1 to 30 nm usually have a larger oxygen content than the first carbon nanotubes. And when such carbon nanotubes are used for electrode materials, the amount of oxygen contained in carbon nanotubes has not been recognized so far, and there is a problem with cost. Has not been made. The present inventors have clarified that the cell resistivity of a redox flow battery using an electrode sheet can be significantly reduced by reducing the oxygen content of the second carbon nanotubes to 1.2 mass% or less.

The amount of oxygen contained in the first carbon nanotube is not particularly limited, but is preferably 0.2% by mass or less, more preferably 0.1% by mass or less. By reducing the amount of oxygen contained in the first carbon nanotubes, it is possible to suppress an increase in the cell resistivity of the redox flow battery using the electrode sheet.

The oxygen content of the second carbon nanotube is 1.2% by mass or less, preferably 1.0% by mass or less, more preferably 0.5% by mass or less, and further preferably 0.2% by mass or less. By reducing the oxygen content of the second carbon nanotube, the cell resistivity of the redox flow battery using the electrode sheet can be greatly reduced.

The oxygen content of the carbon nanotube can be measured using a temperature-programmed desorption gas analyzer, an inert gas melting-infrared absorption measuring device, etc. Will be described later).

<1-3. Components other than carbon nanotubes>
The electrode sheet may contain a carbon material other than carbon nanotubes (hereinafter referred to as “other carbon material”). The content of the other carbon material in the electrode sheet is preferably 400 parts by mass or less, more preferably 200 parts by mass or less, further preferably 100 parts by mass with respect to 100 parts by mass in total of the first carbon nanotubes and the second carbon nanotubes. It is as follows. This is because the cell resistivity of the redox flow battery using the electrode sheet can be reduced when the amount of the other carbon material is small. In addition, other carbon materials include graphite, carbon fiber, etc. in order to control the liquid permeability of the electrolyte by giving a larger gap to the electrode, or to give the electrode formed into a sheet shape strength. The conductive carbon additive may be included.

The carbon additive contained in the electrode preferably contains conductive carbon fibers in view of acid resistance, oxidation resistance, and ease of mixing with carbon nanotubes. The volume resistivity of the carbon fiber is preferably 10 ⁷ Ω · cm or less, and more preferably 10 ³ Ω · cm or less. The volume resistivity of the carbon fiber can be measured by the method described in Japanese Industrial Standards JIS R7609: 2007. In the electrode sheet, the space ratio (void ratio) excluding the region occupied by the carbon nanotube and the other conductive material is preferably 70% by volume or more and 90% by volume or less. By making the said ratio (void ratio) into the said range, the electroconductivity of an electrode sheet and the liquid permeability of electrolyte solution can be made compatible.

The average fiber diameter of the carbon fibers contained in the electrode sheet is preferably larger than 1 μm. When a carbon fiber having an average fiber diameter larger than that of the carbon nanotube is used, the pressure loss when the electrolytic solution is passed through the electrode sheet can be reduced. The average fiber diameter of the carbon fiber is preferably 2 to 100 μm, more preferably 5 to 30 μm. The average fiber length of the carbon fiber is preferably 0.01 to 20 mm, more preferably 0.05 to 8 mm, and still more preferably 0.1 to 1 mm.

The content of carbon fiber with respect to 100 parts by mass of carbon nanotubes contained in the electrode sheet is preferably 10 parts by mass or more, more preferably 40 parts by mass or more, and further preferably 70 parts by mass or more. This is to obtain the above-described effect by adding carbon fiber. Further, the content of carbon fiber with respect to 100 parts by mass of the carbon nanotubes contained in the electrode sheet is preferably 250 parts by mass or less, and more preferably 150 parts by mass or less. This is because the effect of adding the carbon nanotubes can be sufficiently obtained.

Furthermore, the electrode sheet may contain a water-soluble conductive polymer. The water-soluble conductive polymer aids dispersion by making the surface of the carbon nanotube hydrophilic by dispersing the carbon nanotube in the medium in the production method described later. However, the degree to which the surface of the carbon nanotube is hydrophilized is much smaller than that in which the OH group or COOH group is directly bonded to the surface of the carbon nanotube. As the water-soluble conductive polymer, a conductive polymer having a sulfone group is preferable, and specific examples include polyisothianaphthenesulfonic acid.

The content of the water-soluble conductive polymer in the electrode sheet is preferably 3.0 parts by mass or less, more preferably 2.0 parts by mass or less, with respect to 100 parts by mass of the carbon nanotubes in the electrode sheet. More preferably, it is 1.5 mass% or less. When an electrode is obtained by mixing carbon nanotubes and a water-soluble conductive polymer in an aqueous solution, a monomolecular layer of the water-soluble conductive polymer is formed on the surface of the carbon nanotube, so there are many water-soluble conductive polymers. It is because it does not need the amount of. Moreover, it is preferable that content of the water-soluble conductive polymer in an electrode sheet is 0.5 mass part or more with respect to 100 mass parts of carbon nanotube content in an electrode sheet. This is because the above-described effect due to the addition of the water-soluble conductive polymer can be sufficiently obtained.

<2. Manufacturing method of electrode sheet>
FIG. 1 is a flowchart showing an example of a method for producing an electrode sheet for a redox flow battery according to the present embodiment. The manufacturing method according to this example is a mixture in which first carbon nanotubes having an average fiber diameter of 100 to 1000 nm and second carbon nanotubes having an average fiber diameter of 1 to 30 nm are mixed and dispersed in a medium to obtain a dispersion. A dispersion step S20, a filtration step S30 for filtering the dispersion, a molding step S40 for shaping the solid content obtained by the filtration step S30 into a sheet, and a drying step S50 for drying the carbon nanotube mixture. . In the manufacturing method in this example, before the mixing / dispersing step S20, the oxygen content of the first carbon nanotube and / or the oxygen content of the second carbon nanotube is reduced as necessary. The second oxygen reduction step S12 may be included (in FIG. 1, the case where both are included) may be included. The manufacturing process shown in FIG. 1 is an example, and a process not shown in FIG. 1 may be added as necessary.

In the first oxygen reduction step S11, a process for reducing the oxygen content of the first carbon nanotubes is performed. The oxygen content of the first carbon nanotubes after the first oxygen reduction step S11 is as described for the oxygen content of the first carbon nanotubes included in the electrode sheet described above. As described above, the first carbon nanotubes having an average fiber diameter of 100 to 1000 nm used as the material for the electrode sheet usually have a sufficiently low oxygen content of 0.3% by mass or less. In such a case, the first oxygen reduction step S11 may not be performed. When the oxygen content of the first carbon nanotube exceeds 0.3% by mass, it is preferable to perform the first oxygen reduction step S11.

Specifically, as the first oxygen reduction step S11, it is preferable to heat-treat the first carbon nanotubes in an atmosphere of an inert gas such as argon gas, helium gas, nitrogen gas, or reducing gas. The heat treatment temperature is preferably 2500 ° C. or higher, more preferably 2600 ° C. or higher, further preferably 2700 ° C. or higher, and particularly preferably 2800 ° C. or higher. This is for sufficiently removing oxygen contained in the first carbon nanotubes. Moreover, it is preferable that the heat processing temperature is 3300 degrees C or less. This is to suppress the sublimation of the first carbon nanotubes and to secure a high yield and yield. Regarding the heat treatment time, from the viewpoint of reducing the oxygen content of the first carbon nanotubes, it is preferably held for 0.5 hours or more, more preferably 2.5 hours or more, within the above heat treatment temperature range. It is more preferable to hold for more than an hour. The upper limit of the heat treatment time is not particularly limited, but the holding time within the heat treatment temperature range is preferably 3 hours or less. This is to suppress the sublimation of the first carbon nanotubes, to secure a high yield and yield, and to reduce manufacturing costs such as electricity bills.

In the second oxygen reduction step S12, a process for reducing the amount of oxygen contained in the second carbon nanotube is performed. The oxygen content of the second carbon nanotubes after the second oxygen reduction step S12 is as described for the oxygen content of the second carbon nanotubes contained in the electrode sheet described above. In addition, when the oxygen content of the 2nd carbon nanotube used as a raw material is sufficiently small, it is not necessary to perform 2nd oxygen reduction process S12. Specifically, as the second oxygen reduction step S12, it is preferable to heat-treat the second carbon nanotubes in an atmosphere of an inert gas such as argon gas, helium gas, nitrogen gas, or reducing gas. About heat processing temperature and time, it is the same as that of 1st oxygen reduction process S11.

As described above, the oxygen content of the second carbon nanotubes having an average fiber diameter of 1 to 30 nm is generally about 1.8 to 2.5% by mass. In such a case, the second carbon nanotube needs to be subjected to the second oxygen reduction treatment step S12, and the oxygen content of the second carbon nanotube is reduced to 1.2 mass% or less. However, if the oxygen content of the second carbon nanotube used as a raw material is 1.2% by mass or less, the second oxygen reduction step S12 may not be performed.

In the mixing / dispersing step S20, the addition amount of the second carbon nanotubes with respect to the total addition amount of the first carbon nanotubes and the second carbon nanotubes is preferably 1% by mass or more, more preferably 3% by mass or more. Preferably, it is 5 mass% or more. The amount of the second carbon nanotubes added relative to the total amount of the first carbon nanotubes and the second carbon nanotubes is preferably 30% by mass or less, more preferably 20% by mass or less, and 15% by mass. More preferably, it is as follows. This is because the content of the second carbon nanotube with respect to the total amount of the first carbon nanotube and the second carbon nanotube is within the above range.

In the mixing / dispersing step S20, a dispersion in which the first carbon nanotubes and the second carbon nanotubes are dispersed in a medium is prepared. The medium is a liquid and is not particularly limited as long as it can be vaporized in the drying step S50 described later, but is preferably water and more preferably pure water. The dispersion is preferably produced by a shearing force or impact force device such as a wet jet mill, an ultrasonic irradiation device, or a combination thereof. When processing with a wet jet mill, a strong shearing force due to turbulent flow is generated by passing an aqueous solution through a nozzle at an ultrahigh speed, and carbon nanotubes having different average fiber diameters can be sufficiently dispersed by the shearing force. The pressure of the nozzle is preferably 100 MPa or more, more preferably 150 to 250 MPa. If it is this range, a 2nd carbon nanotube can fully be disperse | distributed, suppressing the damage of a 1st carbon nanotube. As the wet jet mill, for example, Starburst manufactured by Sugino Machine Co., Ltd. can be used.

In the mixing / dispersing step S20, the mixing order is not particularly limited. For example, after the first carbon nanotubes and the second carbon nanotubes are dry mixed, the medium may be mixed and the carbon nanotubes may be dispersed. After either one of the first and second carbon nanotubes is dispersed in the medium, the other may be dispersed in the medium, and these components may be mixed and dispersed simultaneously.

In the filtration step S30, the dispersion obtained in the mixing / dispersion step S20 is filtered, the medium is removed from the dispersion, and the solid content including the carbon nanotubes is recovered. The recovered solid content may be dehydrated (removed medium) as necessary.

In the molding step S40, the solid content obtained in the filtration step S30 is molded into a sheet shape to produce a molded body. Examples of the molding method include, but are not limited to, press molding and roll molding.

In the drying step S50, the molded body obtained in the molding step S40 is dried to obtain an electrode sheet. Examples of the drying method include heat drying, vacuum drying, and blast room temperature drying, which can be appropriately selected in consideration of the properties and state of the electrode sheet.

The obtained electrode has a structure in which loose carbon nanotubes are entangled with other carbon nanotubes, and thin carbon nanotubes are entangled with a plurality of thick carbon nanotubes. As a result, the manufactured electrode can easily maintain its shape during subsequent handling, and can withstand use in the intended application. In order to maintain the shape more stably, as described above, carbon fiber may be added to the electrode. In particular, when an electrode having a large shape is formed, the strength of the electrode is increased, which is effective.

<3. Redox flow battery configuration>
FIG. 2 is a schematic diagram illustrating an example of the configuration of the redox flow battery 1. The electrode sheet of this embodiment is used as an electrode layer of a positive electrode 120 and a negative electrode 130 described later, and details will be described later with reference to FIG. In the redox flow battery 1 shown in this example, a solution containing V ⁵⁺ / V ⁴⁺ (for example, a vanadium sulfate (V) / (IV) aqueous solution) is used as a positive electrode electrolyte, and V ³⁺ / V ²⁺ is used as a negative electrode electrolyte. Although the solution (for example, vanadium sulfate (III) / (II) aqueous solution) containing is used, it is not restricted to this. The redox flow battery 1 includes a battery cell 100, a positive electrode tank 200, a positive electrode supply pipe 210, a positive electrode discharge pipe 220, a positive electrode pump 230, a negative electrode tank 300, a negative electrode supply pipe 310, a negative electrode discharge pipe 320, A negative electrode pump 330 and a control unit (not shown) are provided. In addition, the structure of the redox flow battery shown here is only an example, and you may add structures, such as an inverter, a converter, various sensors, and a cooling device, as needed.

The battery cell 100 includes a case 110, a positive electrode 120, a negative electrode 130, and an ion exchange membrane 140. The positive electrode 120 is connected to the electric wiring 520, the negative electrode 130 is connected to the electric wiring 530, and the electric wirings 520 and 530 are connected to a load such as a generator or an external device. Hereinafter, reaction and charge transfer in the battery cell 100 during charging and discharging will be described. Details of the configuration of the members included in the battery cell 100 will be described later with reference to FIG. In FIG. 2, arrows indicating the valence change of ions, the movement direction of protons H ⁺ and electrons e ⁻ indicate the case where the solid line is charged and the dotted line is discharged.

During charging, ions contained in the positive electrode electrolyte are oxidized from V ⁴⁺ to V ⁵⁺ at the positive electrode 120, and ions contained in the negative electrode electrolyte are reduced from V ³⁺ to V ^{2+ at} the negative electrode 130. Therefore, at the time of charging, electrons e ⁻ are discharged from the positive electrode 120 through the electric wiring 520 to the outside, and electrons e ⁻ flow into the negative electrode 130 through the electric wiring 530 from the outside. Further, as a counter cation, proton H ⁺ moves through the ion exchange membrane 140 from the positive electrode 120 to the negative electrode 130 in the battery cell 100.

At the time of discharge, ions contained in the negative electrode electrolyte are oxidized from V ²⁺ to V ³⁺ in the negative electrode 130, and ions contained in the positive electrode electrolyte are reduced from V ⁵⁺ to V ^{4+ in} the positive electrode 120. Therefore, at the time of discharge, the electron e ⁻ is emitted from the negative electrode 130 through the electric wiring 530 to the outside, and the electron e ⁻ flows into the positive electrode 120 through the electric wiring 520 from the outside. Further, as a counter cation, proton H ⁺ moves through the ion exchange membrane 140 from the negative electrode 130 to the positive electrode 120 in the battery cell 100.

The positive electrode tank 200 stores a positive electrode electrolyte. By increasing the capacity of the positive electrode tank 200 together with the capacity of the negative electrode tank 300 described later, the charge / discharge capacity of the redox flow battery 1 can be increased. A plurality of positive electrode tanks 200 may be provided.

The positive electrode supply pipe 210 is connected to the positive electrode tank 200 and the positive electrode 120 of the battery cell 100. The positive electrode supply pipe 210 forms a path for transporting the positive electrode electrolyte from the positive electrode tank 200 to the positive electrode 120.

The positive electrode discharge pipe 220 is connected to the positive electrode tank 200 and the positive electrode 120 of the battery cell 100. The positive electrode discharge pipe 220 forms a path for transporting the positive electrode electrolyte from the positive electrode 120 to the positive electrode tank 200.

The positive electrode pump 230 circulates the positive electrode electrolyte through a path including the positive electrode tank 200 and the positive electrode 120. In FIG. 2, the positive electrode pump 230 is provided in the positive electrode supply pipe 210, but is not limited to the illustrated location as long as the positive electrode electrolyte can be circulated.

The negative electrode tank 300 stores a negative electrode electrolyte. A plurality of the negative electrode tanks 300 may be provided.

The negative electrode supply pipe 310 is connected to the negative electrode tank 300 and the negative electrode 130 of the battery cell 100. The negative electrode supply pipe 310 forms a path for transporting the negative electrode electrolyte from the negative electrode tank 300 to the negative electrode 130.

The negative electrode discharge pipe 320 is connected to the negative electrode tank 300 and the negative electrode 130 of the battery cell 100. The negative electrode discharge pipe 320 forms a path for transporting the negative electrode electrolyte from the negative electrode 130 to the negative electrode tank 300.

The negative electrode pump 330 circulates the negative electrode electrolyte through a path including the negative electrode tank 300 and the negative electrode 130. In FIG. 2, the negative electrode pump 330 is provided in the negative electrode supply pipe 310, but is not limited to the illustrated location as long as the positive electrode electrolyte can be circulated.

A control unit (not shown) processes various input signals and transmits output signals for operating each unit of the redox flow battery 1. Examples of the input signal include a signal from a sensor provided in each device, a signal based on an input from a user via an operation unit, and the like. The signal processing is performed by an arithmetic unit such as a CPU executing a program stored in a storage unit such as a memory. The output signal is transmitted to each device operable by an electrical signal. Examples of such devices include, but are not limited to, the positive electrode pump 230 and the negative electrode pump 330.

<4. Configuration of battery cell>
FIG. 3 is a schematic diagram illustrating an example of the configuration of the battery cell 100 included in the redox flow battery 1. The battery cell 100 includes a case 110, a positive electrode 120, a negative electrode 130, and an ion exchange membrane 140.

The case 110 is a container that accommodates the positive electrode 120, the negative electrode 130, the ion exchange membrane 140, the positive electrode electrolyte, and the negative electrode electrolyte. The shape, size, etc. of the case 110 can be appropriately designed according to the specifications, installation conditions, product design, etc. of the redox flow battery 1. The material of the case 110 can be appropriately selected according to the types of the positive electrode electrolyte and the negative electrode electrolyte, the usage environment, and the like, and a two-layer structure or the like can also be applied.

The positive electrode 120 includes a positive electrode current collector (current collector) 121, a positive electrode electrolyte inflow portion 122, a positive electrode layer 123, and a positive electrode electrolyte discharge portion 124. The negative electrode 130 includes a negative electrode current collector (current collector) 131, a negative electrode electrolyte inflow portion 132, a negative electrode layer 133, and a negative electrode electrolyte discharge portion 134.

In the redox flow battery 1 shown here, the positive electrode 120 and the negative electrode 130 have the same member configuration, and only the electrolyte supplied is different. Therefore, here, the configurations of the positive electrode 120 and the negative electrode 130 will be described simultaneously, and the respective configurations may be collectively referred to as follows. The positive electrode 120 and the negative electrode 130 are the electrode 120 (130), the positive electrode current collector plate 121 and the negative electrode current collector plate 131 are the current collector plate 121 (131), and the positive electrode electrolyte inflow portion 122 and the negative electrode electrolyte inflow portion 132 are the electrolyte inflow portion. 122 (132), the positive electrode layer 123 and the negative electrode layer 133 may be referred to as the electrode layer 123 (133), and the positive electrolyte discharge part 124 and the negative electrolyte discharge part 134 may be referred to as the electrolyte discharge part 124 (134). In addition to the configurations described here, the same configurations on the positive electrode side and the negative electrode side may be collectively referred to as shown here.

The current collecting plate 121 (131) emits electrons e ⁻ received from an electrode layer 122 (132) described later to the outside through the electric wiring 520 (530) and from the outside through the electric wiring 520 (530). This is a current collector that transfers the inflowing electrons e ⁻ to the electrode layer 122 (132). The current collector plate 121 (131) is not limited to a flat plate shape, and a plate or the like that is appropriately processed according to specifications or the like can be used. In the example shown in FIG. 3, the surface of the current collector 121 (131) facing the other electrode (the surface facing the negative current collector 131 in the positive current collector 121, the negative current collector). In 131, a concave portion for accommodating an electrolyte inflow portion 122 (132) to be described later is provided on the surface facing the positive electrode current collector plate 121). As a material of the current collector 121 (131), for example, a conductive material containing carbon can be used. Specifically, a conductive plastic made of graphite and a thermoplastic resin such as polyolefin, or a thermosetting resin such as graphite and an epoxy resin can be used. Among these, considering that it can be press-molded into a plate shape, it is preferable to use a molding material obtained by kneading and molding graphite and a thermoplastic resin, and carbon black having high conductivity such as acetylene black may be mixed.

The electrolyte inflow portion 122 (132) is provided in the concave portion provided in the current collector plate 121 (131), and allows the electrolyte inflow from the supply pipe 210 (310) to pass through the electrode layer 123 (133) described later. Liquid. The electrolyte inflow portion 122 (132) includes an outer frame 122a (132a), a support member 122b (132b), and a bottom portion 122c (132c).

The outer frame 122a (132a) and the support member 122b (132b) maintain the distance between these members in order to allow the electrolyte to flow between the current collector 121 (131) and the electrode layer 123 (133). The outer frame 122a (132a) and the support member 122b (132b) form a flow path for the electrolyte when viewed from the current collector 121 (131) side. The outer frame 122a (132a), the support member 122b (132b), and the flow path formed by these members will be described later with reference to FIG. The material of the outer frame 122a (132a) and the support member 122b (132b) preferably has conductivity in order to facilitate transfer of electrons between the electrode layer 123 (133) and the current collector 121 (131). . The outer frame 122a (132a) and the support member 122b (132b) may be made of the same material as the current collector plate 121 (131). Further, the outer frame 122a (132a) and the support member 122b (132b) may be integrated with the current collector 121 (131) in order to reduce the number of parts.

FIG. 4 is a view of the electrolyte inflow portion 122 (132) included in the battery cell 100 as viewed from the current collector 121 (131) side. Here, the up-down direction and the left-right direction are the same as the directions in the figure, and the front direction on the paper is the front direction and the back direction on the paper is the back direction. The outer frame 122a (132a) has a rectangular frame shape. At the lower end of the outer frame 122a (132a), there is provided an introduction hole C1 for connecting the supply pipe 210 (310) and introducing the electrolyte into the electrolyte inflow portion 122 (132). A plurality of support members 122b (132b) are provided, and extend inward from the left and right sides of the outer frame 122a (132a) at intervals. In the example shown in FIG. 4, the support member 122b (132b) is not formed on a straight line extending in the upward direction in the drawing from the introduction hole C1.

With this configuration, a trunk channel C2 extending upward from the introduction hole C1 and a plurality of branch channels C3 extending left and right at intervals in the trunk channel C2 are formed in the electrolyte inflow portion 122 (132). A bottom portion 122c (132c), which will be described later, is formed on the back side of the trunk channel C2 and the branch channel C3. Through these flow paths C1 to C3, the electrolytic solution spreads over a wide area in the electrolytic solution inflow portion 122 (132), and the electrolytic solution is sufficiently spread over the wide area of the electrode layer 123 (133) via the bottom portion 122c (132c) described later. Can be supplied.

In the example shown in FIG. 4, a plurality of sets of the outer frame 122a (132a) and the support member 122b (132b) are provided at intervals in the left-right direction. This interval forms the electrolyte discharge channel C4. The discharge channel C4 is provided through a bottom 122c (132c), an electrode layer 123 (133), and an electrolyte discharge unit 124 (134), which will be described later. The discharge channel C4 is connected to the discharge pipe 220 (320).

The shape of the flow path of the electrolytic solution in the electrolytic solution inflow portion 122 (132) described here is an example, and is not limited thereto. For example, the flow path may be formed radially from the introduction holes C1. In addition, the positive electrode electrolyte inflow portion 122 and the negative electrode electrolyte inflow portion 132 may have different shapes of flow paths.

Return to Fig. 3 again. The bottom 122c (132c) is provided between the support member 122b (132b) and an electrode layer 123 (133) described later. In this example, the bottom 122c (132c) is a flat plate provided in parallel to the current collector 121 (131), but is not limited thereto. The bottom 122c (132c) allows the electrolytic solution to pass through the electrode layer 123 (133). The material of the bottom portion 122c (132c) is the transmittance of the electrolyte solution in the horizontal direction in the drawing (right direction in the positive electrode 120 and left direction in the negative electrode 130) in the electrode layer 123 (133) of the electrolyte inflow portion 122 (132). (Hereinafter, the transmittance has the same meaning and may be simply referred to as “transmittance”) and is preferably selected in consideration of the ratio between the transmittance of the electrode layer 123 (133).

Here, the transmittance k (m ² ) is the sectional area S (m ² ) of the member through which the electrolytic solution having a viscosity μ (Pa · sec) is passed, the length L (m) of the member, and the flow rate Q (m ³⁾ From the differential pressure ΔP (Pa) between the liquid inflow side and the liquid outflow side of the member when the liquid is passed at / sec), the relationship of the liquid permeation flux (m / sec) represented by the following formula (1) ( Calculated from Darcy's law). Here, the cross-sectional area S is a cross-sectional area of the flow path in a plane perpendicular to the liquid flow direction. In this example, the electrolytic solution viewed from the current collector 121 (131) or the electrode layer 123 (133) side. This is the area of the inflow portion 122 (132).

The transmittance of the bottom portion 122c (132c) has a larger influence on the transmittance of the electrolyte inflow portion 122 (132) than the flow paths C1 to C4. Therefore, in order to adjust the transmittance of the electrolyte inflow portion 122 (132), it is preferable to adjust the transmittance of the bottom portion 122c (132c). The transmittance of the electrolyte inflow portion 122 (132) is preferably 100 times or more that of the electrode layer 123 (133), more preferably 300 times or more, and further preferably 1000 times or more.

When the transmittance of the electrolyte inflow portion 122 (132) is sufficiently higher than the transmittance of the electrode layer 123 (133), the electrolyte that has flowed into the electrolyte inflow portion 122 (132) has a low transmittance. Since it is blocked by the layer 123 (133), it spreads over the entire surface of the electrolyte inflow portion 122 (132), and the pressure of the electrolyte is equalized in the plane of the bottom portion 122c (132c). Therefore, the flow of the electrolyte passing through the electrode layer 123 (133) described later is a uniform flow in the sheet surface. Therefore, reactive species in the charge / discharge process can be replaced simultaneously and efficiently in the electrode layer 123 (133), the cell resistivity is lowered, and the charge / discharge capacity is improved. In addition, as described above, by adjusting the ratio of the transmittance between the electrolyte inflow portion 122 (132) and the electrode layer 123 (133), the flow of the electrolytic solution passing through the electrode layer 123 (133) is changed to the electrode layer 123 ( The direction can be more perpendicular to the surface 133). Therefore, the distance through which the electrolytic solution passes through the electrode layer 123 (133) where the electrolytic solution hardly flows can be minimized (the thickness of the electrode layer 123 (133)), and pressure loss can be reduced.

As the thickness of the member constituting the bottom portion 122c (132c) after incorporation into the battery cell 100 increases, the pressure required to spread the electrolyte over the entire surface of the bottom portion 122c (132c) can be reduced. Therefore, the thickness of the bottom portion 122c (132c) is preferably 0.08 mm or more, more preferably 0.1 mm or more, and further preferably 0.15 mm or more. As the thickness of the member constituting the bottom portion 122c (132c) after being assembled into the battery cell 100 is smaller, the electrode cell 100 and thus the redox flow battery 1 can be reduced in size. For this reason, the thickness of the bottom 122c (132c) is preferably 0.7 mm or less, and more preferably 0.5 mm or less.

The bottom 122c (132c) is preferably a porous sheet. The porous sheet may be a sponge-like member having voids or a member in which fibers are intertwined. Examples of the form in which fibers are intertwined include, for example, woven fabric, non-woven fabric, and paper in which a relatively short fiber is wound. Is mentioned. When the porous sheet is made of fibers, the average fiber diameter is preferably made of fibers larger than 1 μm. If the average fiber diameter of a porous sheet is larger than 1 micrometer, the liquid permeability of the electrolyte solution in a porous sheet can fully be ensured.

The material of the porous sheet is preferably one having corrosion resistance to the electrolyte. Specifically, an acidic solution is often used as the electrolyte solution of the redox flow battery 1. Therefore, the porous sheet preferably has acid resistance. Further, since the porous sheet may be oxidized by the reaction in the electrode layer 123 (133), it is preferable to have oxidation resistance. The porous sheet having acid resistance or oxidation resistance means that not only the shape of the porous sheet after use is maintained but also the chemical state of the surface does not change.

Further, the volume resistivity of the porous sheet is preferably 10 ⁷ Ω · cm or less, more preferably about 10 ³ Ω · cm or less. If the porous sheet has conductivity, the conductivity of the bottom 122c (132c) can be increased.

For these reasons, it is preferable that the fibers forming the porous sheet used as the bottom portion 122c (132c) have acid resistance, oxidation resistance, and conductivity. Examples of such fibers include carbon fibers, but metals may be used as long as such conditions are satisfied.

The electrode layer 123 (133) is a portion where oxidation and reduction of ions contained in the electrolytic solution are performed. The oxidation-reduction reaction of the electrolytic solution is as described above with reference to FIG. As the electrode layer 123 (133), the electrode sheet of this embodiment is used. In designing the electrode layer 123 (133), the ratio between the transmittance of the electrode layer 123 (133) and the transmittance of the electrolyte inflow portion 122 (132), and the transmittance of the electrode layer 123 (133) described later. It is preferable to consider the ratio between the rate and the transmittance of the electrolyte discharge part 124 (134).

Electrolyte discharge part 124 (134) discharges the electrolyte which passed electrode layer 123 (133) outside. The electrolytic solution that has passed through the electrolytic solution discharge unit 124 (134) is returned to the tank 200 (300) via the discharge channel C4 and the discharge pipe 220 (320).

The electrolyte discharge part 124 (134) preferably has a higher transmittance than the electrode layer 123 (133). If the transmittance of the electrolyte solution discharge portion 124 (134) is sufficiently high compared to the transmittance of the electrode layer 123 (133), the electrolyte solution that has passed through the electrode layer 123 (133) is transferred to the electrolyte solution discharge portion 124 (134). ) And quickly discharged to the discharge channel C4. In addition, by making the transmittance of the electrolyte solution discharge portion 124 (134) higher than the transmittance of the electrode layer 123 (133), the flow of the electrolyte solution passing through the electrode layer 123 (133) is changed to the electrode layer 123 (133). In the case of being directed in a direction perpendicular to the surface, the electrolyte can be discharged to the discharge channel C4 through the electrolyte discharge part 124 (134) without disturbing the flow of the electrolyte. Considering these matters, the transmittance of the electrolyte solution discharge portion 124 (134) is preferably 50 times or more, and more preferably 100 times or more that of the electrode layer 123 (133).

When the electrolyte discharge part 124 (134) becomes thick, the pressure required for the electrolyte to pass through the electrolyte discharge part 124 (134) can be reduced. Therefore, the thickness of the electrolytic solution discharge part 124 (134) is preferably 0.08 mm or more, more preferably 0.1 mm or more, and further preferably 0.15 mm or more. When the electrolyte discharge part 124 (134) is thinned, the distance between the electrode layer 123 (133) and the ion exchange membrane 140 is shortened, and the ion moving distance is shortened, so that the cell resistance of the redox flow battery 1 can be reduced. Therefore, the thickness of the electrolyte discharge part 124 (134) is preferably 0.7 mm or less, and more preferably 0.5 mm or less.

The electrolytic solution after passing through the electrode layer 123 (133) has a high ratio of the electrolytic solution after the oxidation reaction or reduction reaction occurs. Therefore, the electrolyte discharge unit 124 (134) quickly discharges the electrolyte, whereby ions after the valence changes from the vicinity of the electrode layer 123 (133) can be efficiently removed, and the electrode layer 123 (133) ), The efficiency of redox of ions contained in the electrolytic solution can be increased.

The electrolyte discharge part 124 (134) is preferably made of a porous member. Moreover, since the electrolyte discharge part 124 (134) is also required to have acid resistance, oxidation resistance, and conductivity, like the bottom part 122c (132c), the fibers forming the porous sheet have acid resistance, oxidation resistance, And what has electroconductivity is good. Examples of such fibers include carbon fibers, but metals may be used as long as such conditions are satisfied.

As the ion exchange membrane 140, a cation exchange membrane can be used. Specific examples include those disclosed in International Publication No. 2016/159348.

The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to specific embodiments, and various modifications and changes can be made within the scope where the effects of the present invention can be obtained. .

Hereinafter, examples of the present invention will be described. In addition, this invention is not limited only to a following example.

<1. Oxygen reduction process of carbon nanotube and measurement of oxygen content>
A first carbon nanotube (VGCF-H (registered trademark) manufactured by Showa Denko KK) (CNT1) having an oxygen content of 0.2 mass%, an average fiber diameter of 150 nm, and an average fiber length of 15 μm, and an oxygen content of 1.8 mass %, An average fiber diameter of 15 nm, and an average fiber length of 3 μm, a second carbon nanotube (manufactured by Showa Denko KK, VGCF-X (registered trademark)) (CNT2), respectively, using a graphitization furnace under an argon gas atmosphere, Heat treatment was performed under the heat treatment conditions shown in Table 1 to reduce the amount of oxygen contained in the first carbon nanotube and the second carbon nanotube. As shown in Table 1, the heat-treated first carbon nanotubes were designated as CNTs 11 to 13, the heat-treated second carbon nanotubes were designated as CNTs 21 to 23, and the first and second carbon nanotubes that had not been heat-treated were designated as CNT 10 and CNT20. The oxygen content of each sample was measured with an oxygen / nitrogen analyzer (LE-CO, TC-600).

Specific conditions for measuring the oxygen content of carbon nanotubes are as follows. Carbon monoxide generated by placing a nickel capsule into which a weighed 20 mg sample (carbon nanotube) was placed in a graphite crucible and heating the sample in a graphite crucible heated at an output of 5000 W using an oxygen / nitrogen analyzer. And carbon dioxide was quantified by infrared absorption method. Table 1 shows the measurement results of the oxygen content of the samples heat-treated under the respective heat treatment conditions shown in Table 1.

<2. Examples and Comparative Examples>
[Example 1]
CNT11 and CNT21 are added to pure water such that CNT11 is 90% by mass and CNT21 is 10% by mass with respect to the total amount of the first carbon nanotubes CNT11 and the second carbon nanotubes CNT21. 1.0 parts by mass of polyisothianaphthenesulfonic acid, which is a water-soluble conductive polymer, was added to a total of 100 parts by mass of CNT21 and CNT21 to prepare a mixed solution. The obtained liquid mixture was processed with the wet jet mill, and CNT11 and CNT21 were disperse | distributed to water. Carbon fiber (MLD-300, manufactured by Toray Industries, Inc.) having an average fiber diameter of 7 μm and an average fiber length of 0.13 mm was added to a dispersion liquid in which CNT11 and CNT21 were dispersed to prepare a mixed liquid. The amount of carbon fiber added was 100 parts by mass with respect to a total of 100 parts by mass of CNT11 and CNT21. This mixture was stirred with a magnetic stirrer to disperse the carbon fibers. The dispersion was filtered with a filter paper, and the resulting cake was dehydrated together with the filter paper, then compressed with a press and further dried, and the filter paper was peeled off to produce an electrode sheet containing carbon nanotubes. Table 2 shows the oxygen content [O] of the entire carbon nanotube (CNT) after the heat treatment (Table 1), based on the values in Table 1.

[Examples 2 to 4 and Comparative Examples 1 and 2]
Using the first and second carbon nanotubes of the type shown in Table 2, an electrode sheet was produced in the same process as in Example 1 with the addition amount shown in Table 2 for each component.

<3. Preparation of battery cell for evaluation>
For evaluation of the electrode sheet, a battery cell 100 having the configuration shown in FIG. 3 was produced. Hereinafter, a method for producing the evaluation battery cell will be described.

Grooves are formed as flow paths C1 to C4 as shown in FIG. 4 in the recesses of the current collector 121 (131) made of a carbon plastic molded body, and carbon fiber paper as a porous sheet is used as the bottom 122c (132c). (SGL Carbon Co., Ltd .: 39AA) was used. The thickness of this carbon fiber paper was 0.37 mm.

In FIG. 4, the total size of the electrolyte inflow portion 122 (132) is 50 mm × 50 mm, and a 24.5 mm × 50 mm outer frame 122a (132a) is opened in the width of 1 mm and 2 in the left-right direction. They were placed side by side. The thickness of the wall of the outer frame 122a (132a) was 1.5 mm, the width of the support member 122b (132b) was 1 mm, the width of the trunk channel C2 was 1 mm, and the width of the branch channel C2 was 3 mm. The height of the outer frame 122a (132a) is 1 mm, the height of the support member 122b (132b) is 0.63 mm, the thickness of the bottom 122c (132c) is 0.37 mm, and the outer frame 122a (132a) and the bottom 122c ( 132c) and the electrode layer 123 (133) side. As the introduction hole C1, a 0.8 mmφ hole was provided in the outer frame 122a (132a). A supply pipe 210 (310) was connected to the introduction hole C1. The electrolyte discharge channel C4 is provided between both sides of the two outer frames 122a (132a) and the two outer frames 122a (132a) so that the electrolyte is discharged in the direction of the arrow in FIG. . The width of the discharge channel provided between the two outer frames 122a is 1 mm.

As the electrode layer 123 (133), the electrode sheets produced in the examples and comparative examples were used. Two electrode sheets having a size of 24.5 mm × 50 mm are prepared, and the outer frame 122a (132a) and the bottom 122c (132c) are formed in accordance with the area surrounded by the two outer frames 122a (132a). Placed on the same surface.

As the electrolytic solution discharge part 124 (134), a carbon fiber paper (SGL, GDL10AA, average fiber diameter 12 μm), which is a porous sheet, was used. The thickness of this carbon fiber paper before being incorporated into the battery cell 100 was 0.25 mm.

Further, Nafion N212 (registered trademark, manufactured by DuPont) was used as the ion exchange membrane 140, and the battery cell 100 was assembled using the two electrode units having the above-described configuration as the positive electrode 120 and the negative electrode 130, respectively.

<4. Evaluation of electrode sheet>
Two 100 mL of an electrolyte solution having a vanadium ion concentration of 1.8 ^M containing equimolar amounts of V ³⁺ and V ⁵⁺ (V ^+3.5 ) was prepared as a positive electrode electrolyte and a negative electrode electrolyte. These electrolyte solutions were sent to the positive electrode and the negative electrode, respectively, with a tube pump.

The cell resistivity was measured at a current density of 100 mA / cm ² at room temperature (25 ° C.), and the cell resistivity was calculated using the charge / discharge curve at the third cycle. The cut-off voltage is 1.75V for charging and 1.0V for discharging. The cell resistivity is calculated by reading the voltage at the midpoint of the time when the cut-off voltage is reached, dividing the difference between the midpoint voltages of the charge and discharge curves by the current density, and then halving the value. is there. Table 2 shows the cell resistivity when the electrode sheets of Examples 1 to 4 and Comparative Examples 1 and 2 were used for the positive electrode layer and the negative electrode layer.

<5. Evaluation results>
In Comparative Example 1 in which the oxygen content [O] of the entire carbon nanotube was 0.4 mass%, which was larger than those in Examples 1 to 4 and Comparative Example 2, the cell resistivity showed a high value. In Comparative Example 2, the oxygen content [O] of the entire carbon nanotube is 0.3% by mass, which is the same as in Example 1, but the oxygen content Y2 of the second carbon nanotube is 1.8% by mass. Also in Comparative Example 2, the cell resistivity showed a high value. On the other hand, in Examples 1 to 4, the oxygen content [O] of the entire carbon nanotube is 0.3 mass% or less and the oxygen content Y2 of the second carbon nanotube is 1.2 mass% or less. A battery cell having a low cell resistivity was obtained.

Accordingly, the total amount of oxygen contained in the carbon nanotubes including the first carbon nanotubes having an average fiber diameter of 100 to 1000 nm and the second carbon nanotubes having an average fiber diameter of 1 to 30 nm and including the first carbon nanotubes and the second carbon nanotubes. The cell resistivity of the redox flow battery is reduced by the electrode sheet for redox flow battery in which [O] is 0.3 mass% or less and the oxygen content Y2 of the second carbon nanotube is 1.2 mass% or less. I found out that I could do it.

1: redox flow battery 100: battery cell 120: positive electrode 121: positive electrode current collector plate 122: positive electrode electrolyte inflow portion 123: positive electrode layer 124: positive electrode electrolyte discharge portion 130: negative electrode 131: negative electrode current collector plate 132: negative electrode electrolyte Inflow part 133: negative electrode layer 134: negative electrode electrolyte discharge part 140: ion exchange membrane 200: positive electrode tank 230: positive electrode pump 300: negative electrode tank 330: negative electrode pump C1: introduction hole C2: trunk channel C3: branch channel C4: discharge Flow path

Claims (9)

1. An electrode sheet comprising first carbon nanotubes having an average fiber diameter of 100 to 1000 nm and second carbon nanotubes having an average fiber diameter of 1 to 30 nm,
   The oxygen content of the entire carbon nanotube including the first carbon nanotube and the second carbon nanotube is 0.3% by mass or less,
   The electrode sheet whose oxygen content of the said 2nd carbon nanotube is 1.2 mass% or less.
2. 2. The electrode sheet according to claim 1, wherein the second carbon nanotube is contained in an amount of 1 to 30% by mass with respect to the entire carbon nanotube.
3. The electrode sheet according to claim 1 or 2, wherein the total oxygen content of the carbon nanotube is 0.2 mass% or less.
4. The electrode sheet according to any one of claims 1 to 3, wherein the oxygen content of the second carbon nanotube is 0.2 mass% or less.
5. The first carbon nanotube is attached to a surface of the first carbon nanotube, and the second carbon nanotube is entangled between the plurality of first carbon nanotubes. Electrode sheet.
6. The electrode sheet according to any one of claims 1 to 5, an electrolyte solution inflow portion for allowing an electrolyte solution to flow into the electrode sheet, a current collector for transferring electrons between the electrode sheet, A redox flow battery comprising: an electrolyte solution outflow portion for allowing the electrolyte solution that has passed through the electrode sheet to flow out; and an ion exchange membrane.
7. A mixing step of mixing first carbon nanotubes having an average fiber diameter of 100 to 1000 nm and second carbon nanotubes having an average fiber diameter of 1 to 30 nm, and forming the carbon nanotubes mixed in the mixing step into a sheet shape A method for producing an electrode sheet comprising a molding step,
   The method for producing an electrode sheet, wherein an oxygen reduction step of heating the second carbon nanotubes at 2500 ° C. to 3300 ° C. in an inert gas or reducing gas atmosphere before the mixing step.
8. The said oxygen reduction process manufactures the electrode sheet of Claim 7 which heats the said 2nd carbon nanotube by 2800 degreeC or more and 3300 degrees C or less for 0.5 hour or more in the atmosphere of inert gas or reducing gas. Method.
9. In the mixing step, the first carbon nanotubes and the second carbon nanotubes are dispersed in a medium containing the first carbon nanotubes and the second carbon nanotubes to form a dispersion liquid. The method for producing an electrode sheet according to claim 7 or 8, wherein the medium is removed from the liquid, and the solid content from which the medium has been removed is formed into a sheet shape.
   

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