JP2021121983A - Operation method of redox flow battery - Google Patents

Abstract

To provide an operation method of a redox flow battery, capable of improving output performance (cell resistance) even without controlling the temperature of an electrolyte during operation of the redox flow battery to a low temperature.SOLUTION: An operation method of a redox flow battery of the present invention is an operation method of a redox flow battery 200 that includes a carbon nanotube-containing electrode layer 2 on an electrode. In the operation method, charging and discharging is performed in the state where the temperature of an electrolyte is set to 40°C or above and 90°C or below, the electrolyte being fed to the electrode layer 2 where an electrode reaction is carried out.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method of operating a redox flow battery, and more particularly to a method of operating a redox flow battery having an electrode layer containing carbon nanotubes in the electrode.

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, which is an active material whose valence changes due to redox, charging and discharging are performed by simultaneously proceeding with an oxidation reaction on one electrode and a reduction reaction on the other electrode. be able to.

By the way, it is general that whether or not a stationary storage battery can be introduced is determined in consideration of cost. The cost of a redox flow battery is basically determined by the current density. The current density that can be passed is determined by the cell resistance. The cell resistance is a numerical value determined as a result of totaling the resistance values generated in all the elements through which the current flows. For example, the electric resistance of the current collector plate (bipolar plate), the electric resistance of the electrode, and the electrode and the current collector plate. The main components are the contact resistance of the electrode, the reaction resistance on the electrode surface, the ion transfer resistance in the electrolytic solution, the proton transfer resistance in the ion exchange membrane, and the like. The reaction resistance on the electrode surface is particularly difficult to control. On the electrode surface, electrons are passed to (or received from) the electrode when the valence of the metal ion (active material) changes, but then the metal ion (containing electrolytic solution) whose valence has changed is transferred from the electrode surface. It needs to be removed promptly. Therefore, it is preferable that the redox flow battery is configured so that the electrolytic solution flows uniformly and at a constant speed in one direction. Further, since the transfer of electrons on the electrode surface is a kind of chemical reaction, it is considered that a higher temperature is preferable from the viewpoint of enhancing the reactivity.

However, in the conventional redox flow battery, it is common to suppress the temperature rise of the electrolytic solution during operation. For example, in Patent Document 1, a redox that improves heat dissipation performance and suppresses an increase in the temperature of the electrolytic solution by forming two or more unit containers in which the electrolytic solution tank for accommodating the electrolytic solution supplied to the battery cell is communicated. Flow batteries have been proposed. Further, in Patent Documents 2 and 3, in order to suppress the temperature rise of the electrolytic solution, a slit that becomes a part of the flow path of the electrolytic solution is provided in the frame of the battery cell, and the groove shape of the slit and the curved portion of the slit are provided. A redox flow battery that dissipates the heat of the electrolytic solution by defining the radius of curvature has been proposed.

Japanese Unexamined Patent Publication No. 2000-30729 Japanese Unexamined Patent Publication No. 2017-41452 Japanese Unexamined Patent Publication No. 2016-207669 International Publication No. 2015/074252 International Publication No. 2016/159348

In a redox flow battery using carbon fiber as an electrode constituting a battery cell, a functional group such as an OH group or a COOH group is generally added to the carbon fiber in order to improve the reactivity on the electrode surface. be. However, since the reaction is carried out via functional groups such as OH groups and COOH groups on the electrode surface using carbon fibers, decomposition of OH groups and COOH groups begins to occur when the temperature of the electrolytic solution exceeds 40 ° C. , The reactivity drops. Therefore, in the redox flow battery, as described in Patent Documents 1 to 3, it has been considered that the temperature of the electrolytic solution during operation should be lowered as much as possible.

According to Patent Document 4, the applicant uses first carbon nanotubes having an average fiber diameter of 100 nm or more and second carbon nanotubes having an average fiber diameter of 30 nm or less as electrode materials for a redox flow battery having a high electromotive force and a large charging capacity. The electrode material containing the above was proposed. Further, in Patent Document 4, as in Patent Documents 1 to 3, the electrolysis during operation of the redox flow battery shown in the examples is based on the knowledge that the temperature of the electrolytic solution during operation of the redox flow battery should be lowered as much as possible. The temperature of the liquid was controlled to a low temperature in the range of 15 to 25 ° C.
However, controlling the temperature of the electrolyte during operation of the redox flow battery to a low temperature requires the installation of a cooling device, a control device, etc., which complicates the device configuration and increases the device cost. There is.

An object of the present invention is to provide a method for operating a redox flow battery, which can improve output performance (cell resistance) without controlling the temperature of the electrolytic solution to a low temperature during operation of the redox flow battery. do.

The present inventors have investigated electrodes containing carbon nanotubes used in redox flow batteries. As a result, the electrode using carbon nanotubes has a very dense structure, but the surface of the electrode is chemically extremely stable, and there is no problem in reactivity even if the temperature rises in the electrolytic solution which is a strong acid. Rather, the reaction of the change in the valence of vanadium increases the reaction rate as the temperature rises, that is, by controlling the temperature of the electrolyte sent toward the electrode to a certain degree, the reactivity at the electrode is increased and the output is increased. It turned out that the performance can be improved.
Therefore, in the present invention, an extremely high reactivity can be ensured in the electrode layer by using an electrode layer containing carbon nanotubes for the electrode and setting the temperature of the electrolytic solution sent toward the electrode layer to 40 ° C. or higher and 90 ° C. or lower. The present invention has been completed. Specifically, the present invention provides the following.

(1) The present invention is a method of operating a redox flow battery in which an electrode layer containing carbon nanotubes is provided on an electrode, and the temperature of an electrolytic solution sent toward the electrode layer in which an electrode reaction is carried out is 40 ° C. or higher and 90 ° C. or higher. It is applied to the operation method of the redox flow battery, which is set as follows and charged / discharged.

(2) In the operation method of the redox flow battery according to (1) above, the content of the carbon nanotubes is 30 parts by mass or more with respect to 100 parts by mass of the total carbon contained in the electrode layer. May be good.

(3) In the operation method of the redox flow battery according to any one of (1) or (2) above, the electrode layer is the first carbon nanotube having an average fiber diameter of 100 to 1000 nm and an average fiber diameter of 30 nm or less. The configuration may include the second carbon nanotubes of the above.

(4) In the operation method of the redox flow battery according to the above (3), the second carbon nanotube is 0.05 with respect to a total of 100 parts by mass of the first carbon nanotube and the second carbon nanotube. The configuration may be ~ 30 parts by mass.

According to the present invention, it is possible to provide a method of operating a redox flow battery capable of improving output performance (cell resistance) without controlling the temperature of the electrolytic solution to a low temperature during operation of the redox flow battery. can.

It is sectional drawing which shows typically the battery cell which comprises the redox flow battery to which this invention is applied. It is the figure which showed typically so that the structure of the flow path (groove) formed in the liquid inflow part of the battery cell shown in FIG. 1 can be understood. It is a figure when the cell resistance with respect to the electrolytic solution temperature is plotted in Example and Comparative Examples 1 and 2.

Hereinafter, a method of operating the redox flow battery to which the present invention is applied will be described.

The operation method of the redox flow battery according to the present embodiment includes an electrode layer containing carbon nanotubes as an electrode of a battery cell constituting the redox flow battery, and is fed toward the electrode layer in which the electrode reaction is performed. It is characterized in that charging and discharging are performed by setting the temperature so that the temperature of the electrolytic solution (hereinafter, may be simply referred to as “the temperature of the electrolytic solution”) is 40 ° C. or higher and 90 ° C. or lower.

When carbon nanotubes were used for the electrode layer, it was clarified that the higher the temperature of the electrolytic solution, the lower the cell resistance, as shown in Examples described later. Therefore, by setting the temperature of the electrolytic solution to 40 ° C. or higher and 90 ° C. or lower and performing charging / discharging, the chemical reaction that transfers electrons on the electrode surface can be performed more quickly than when the temperature of the electrolytic solution is lower than 40 ° C. It can be advanced and the output performance can be improved. If the temperature of the electrolytic solution is less than 40 ° C., the reaction rate tends to decrease, and sufficient output cannot be obtained. On the other hand, if the temperature of the electrolytic solution exceeds 90 ° C., the electrolytic solution evaporates, and it is necessary to supply a large amount of electric power to the heating device for raising the temperature of the electrolytic solution, which is not preferable. From these points, the temperature of the electrolytic solution is preferably 60 ° C. or higher and 90 ° C. or lower, more preferably 70 ° C. or higher and 90 ° C. or lower, and further preferably 80 ° C. or higher and 90 ° C. or lower. If a material having sufficient heat resistance cannot be used as a component of the redox flow battery for economic reasons, the upper limit temperature of the electrolytic solution may be set as high as possible within the range of 90 ° C. or lower. preferable.

It was found that if a certain amount of carbon nanotubes was contained, the electrode reaction was selectively or preferentially performed on the surface of the carbon nanotubes. Therefore, there is no problem in including other carbon materials such as graphite and carbon fiber in order to improve the conductivity and impart larger voids. The content of the carbon nanotubes is preferably 30 parts by mass or more, more preferably 50 parts by mass or more, based on 100 parts by mass of the total carbon contained in the electrode layer. In the case of an electrode in which a plurality of layers are laminated, any one layer may contain a certain amount of carbon nanotubes.

The method for controlling the temperature of the electrolytic solution is not particularly limited, and heat generated by the discharge reaction of the battery cell, heat generated by self-discharge, heat generated by internal resistance, etc. can be used as a heat source or discharged from other devices. Heat or the like may be used as a heat source. It is also possible to use a heating means such as a conventionally known heating heater. It is preferable to utilize heat generation and exhaust heat in the battery cell because it is not necessary to supply electric power for heating or install new members. In order to suppress heat dissipation from the battery cell, the electrolytic solution tank, its piping, and the like, a part of these constituent members may be formed of a heat insulating layer or covered with a heat insulating material. Conventionally, the members constituting the battery cell have been provided with a heat dissipation function or a cooling function so that the temperature of the electrolytic solution does not rise too much. In the present invention, this is not necessary and the merit is great, but it goes without saying that a cooling device or the like may be installed.

Further, as a method of controlling the temperature of the electrolytic solution, a temperature detecting means for detecting the temperature of the electrolytic solution may be provided to monitor the temperature of the electrolytic solution, or the heat supply amount may be determined based on the detection result of the temperature measuring means. It may be controlled. The temperature detecting means of the electrolytic solution is not particularly limited, and examples thereof include a contact type temperature sensor such as a thermoelectric pair and a non-contact type temperature sensor such as a radiation thermometer. Examples of the location where the temperature measuring means is installed include an electrolytic solution tank and a pipe (for example, an inflow pipe and an outflow pipe) for supplying the electrolytic solution to the battery cell. The temperature is preferably measured at or near the surface of the electrode layer where the reaction of the electrolytic solution occurs, but the temperature of the electrolytic solution in the electrode layer is accurately measured during actual charging and discharging. For convenience, it is connected to the battery cell, more precisely the piping connected to the electrolytic cell to feed (supply) the electrolyte to the electrode layer, or to the battery cell to drain the electrolyte. It is preferable to install the temperature sensor in the installed pipe (for example, inflow pipe or outflow pipe) from the viewpoint of ease of installation of the temperature sensor. In particular, it is preferable to install a thermocouple at the inflow port of the inflow pipe or the outflow port of the outflow pipe near the electrode layer for measurement.

By the way, since the average fiber diameter of carbon nanotubes is generally 1 μm or less, which is smaller than the average fiber diameter of conventional carbon fibers, the size of pores in the sheet becomes smaller when formed in the electrode layer as a porous sheet. Since the permeability of the electrolytic solution is deteriorated, pressure loss is likely to occur. Therefore, the electrode is composed of an electrode layer, a liquid inflow portion that allows the passed electrolytic solution to flow into the electrode layer, and a liquid outflow member that causes the electrolytic solution that has passed through the electrode layer to flow out. Will pass through the electrode layer in the thickness direction, so that the pressure loss can be reduced as much as possible.

Hereinafter, the configuration of a battery cell, which is a main part of a redox flow battery, having an electrode layer containing carbon nanotubes on the electrode will be specifically described. FIG. 1 is a schematic view showing a cross section of a battery cell (single cell) constituting a redox flow battery. FIG. 2 is a plan view schematically showing an aspect of a liquid inflow portion of a battery cell constituting a redox flow battery. It should be noted that the drawings may be shown by enlarging the featured portions for convenience in order to make the features of the present invention easy to understand, and the dimensional ratios and the like of each component may be different from the actual ones. Further, the materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited thereto, and the present invention can be appropriately modified without changing the gist thereof.

The redox flow battery 200 of the present embodiment is a battery cell 20, an electrolytic solution tank (not shown) for accommodating the electrolytic solution, and an inflow pipe for flowing the electrolytic solution from the electrolytic solution tank into the battery cell 20. A pipe 11 and an outflow pipe 12 which is a pipe for flowing out the electrolytic solution from the battery cell 20 and returning it to the electrolytic solution tank, and a control unit (shown) that controls the supply of the electrolytic solution from the electrolytic solution tank to the battery cell 20. It is mainly composed of.
The battery cell 20 has an electrode layer 2, a current collector plate 40 having a liquid inflow portion 41 for flowing the passed electrolytic solution into the electrode layer 2, and an electrode layer 2 on the opposite side of the current collector plate 40. It is provided with a liquid outflow member 3 that flows out the electrolytic solution that has passed through the electrode layer 2. As will be described later, the electrolytic solution is supplied from an electrolytic solution tank (not shown) to the liquid inflow section 41 via the inflow pipe 11 (for example, pump transportation), passes through the electrode layer 2 and the liquid outflow member 3, and is passed through the outflow pipe 12 It is returned to the electrolyte tank and circulated through. Here, the liquid inflow portion 41 is preferably composed of a flow path 42 and a liquid inflow member 43 made of a porous sheet arranged between the electrode layer 2 and the current collector plate 40. In this case, the liquid inflow member 43 does not need to have reactivity to change the valence of the electrolytic solution (active material), but must have electron conductivity.

The battery cell 20 constituting the redox flow battery 200 shown in FIG. 1 includes two current collector plates 40 and 40, an ion exchange membrane 30, and a pair of electrodes provided between them (in FIG. 1, in FIG. 1). The case where it is composed of a single cell having a liquid inflow member 43, an electrode layer 2 and a liquid outflow member 3) is shown, but a plurality of these single cells are connected in series to form a plurality of pairs of electrodes. It can also be configured as a plurality of cells having. Further, in the battery cell (not shown) of the redox flow battery of another embodiment composed of a plurality of cells, the positive electrode and the negative electrode of each single cell are electrically connected in series, and the connection is the positive electrode side current collection. It also includes a cell configuration (also referred to as a laminated cell or a cell stack) having a current collector plate (also referred to as a “bipolar plate”) in which a plate and a current collector plate on the negative electrode side are integrated on the front and back sides.

Hereinafter, the electrode layer 2, the liquid outflow member 3, the ion exchange membrane 30, the current collector plate 40, and the liquid inflow portion 41 in the battery cell 20 constituting the redox flow battery 200 will be described.

[Electrode layer]
The electrode layer 2 contains carbon nanotubes. The average fiber diameter of the carbon nanotubes is preferably 1 μm or less, more preferably 1 to 300 nm, still more preferably 10 to 200 nm, and more preferably 15 to 150 nm.

The carbon nanotubes contained in the electrode layer 2 may be configured by mixing a plurality of types of carbon nanotubes having different average fiber diameters. In that case, for example, it is preferable to include 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 30 nm or less.

When the electrode layer 2 is composed of a mixture of a plurality of types of carbon nanotubes having different average fiber diameters, the electrode layer 2 is observed with a transmission electron microscope, and the one having a fiber diameter of more than 50 nm in the same field of view is the first. The carbon nanotube of 1 and the one having a fiber diameter of less than 50 nm are regarded as the second carbon nanotube, and the average fiber diameter is calculated as described above.

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

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

Further, when the average fiber diameters of the first carbon nanotubes and the second carbon nanotubes are in the above range, the electrode layer 2 has a structure capable of maintaining high strength and high conductivity. This is because the first carbon nanotube becomes a trunk and the second carbon nanotube is suspended in a branch shape between the plurality of first carbon nanotubes. For example, when the average fiber diameter of the first carbon nanotube is 100 nm or more, the trunk becomes stable, the structure of the electrode is less likely to crack, and it becomes easy to maintain sufficient strength. On the other hand, when the average fiber diameter of the second carbon nanotube is 30 nm or less, the second carbon nanotube can be sufficiently entangled with the first carbon nanotube, and the conductivity is improved. That is, by using an electrode having an electrode layer 2 containing two types of carbon nanotubes having different average fiber diameters, the cell resistance of the redox flow battery can be lowered and the electric capacity can be increased.

The content ratio of the second carbon nanotubes is preferably 0.05 to 30 parts by mass, more preferably 0.1 to 20 parts by mass, based on 100 parts by mass of the total of the first carbon nanotubes and the second carbon nanotubes. More preferably, it is 1 to 15 parts by mass. If the second carbon nanotube is included in this range, the electrode has a structure capable of maintaining high strength and high conductivity. This is because the second carbon nanotubes are included in this range, so that the first carbon nanotubes function as the main conductive material, and the second carbon nanotubes are placed between the first carbon nanotubes. It is thought that this is to connect electrically and efficiently support conductivity. When the ratio of the first carbon nanotubes and the second carbon nanotubes is within the above range, at least a part of the second carbon nanotubes may have a structure straddling two or more first carbon nanotubes or a second carbon nanotube. It becomes easy to form a structure in which at least a part of the carbon nanotubes of No. 1 intersects with two or more first carbon nanotubes. Therefore, as described above, effects such as a decrease in cell resistance and an increase in electric capacity can be expected.

Further, the electrode layer 2 may contain a conductive material other than carbon nanotubes. Specific examples thereof include conductive polymers, graphite, and conductive carbon fibers. It is preferable to contain conductive carbon fiber from the viewpoint of acid resistance, oxidation resistance, and ease of mixing with carbon nanotubes. The volume resistivity of the carbon fibers is preferably not more than 10 ⁷ Ω · cm, more preferably not more than 10 ³ Ω · cm. The volume resistivity of carbon fiber can be measured by the method described in Japanese Industrial Standard JIS R7609: 2007. When the ratio (porosity) of the space excluding the region occupied by the carbon nanotubes and other conductive materials of the electrode layer 2 is 70% or more and 90% or less, both the conductivity of the electrode and the air permeability of the electrolytic solution are compatible. can do.

The average fiber diameter of the carbon fibers contained in the electrode layer 2 is preferably larger than 1 μm. When carbon fibers having a larger average fiber diameter than carbon nanotubes are used, larger voids can be formed in the electrode layer 2, and the pressure loss when the electrolytic solution is passed through the electrodes can be reduced. In this case, it is preferable that the pressure loss when the electrolytic solution is passed through the electrode can be reduced and good conductivity can be provided. The average fiber diameter of the carbon fiber is preferably 2 to 100 μm, more preferably 5 to 30 μm. The average fiber length is preferably 0.01 to 20 mm, more preferably 0.05 to 8 mm, still more preferably 0.1 to 1 mm.

The content of the carbon fiber contained in the electrode layer 2 is preferably 70% by mass or less, more preferably 50% by mass or less of the electrode layer. In this case, it is preferable to obtain an electrode of a redox flow battery having a small pressure loss when the electrolytic solution is passed through the electrode. The amount of surface oxygen of the electrode layer 2 is an atomic ratio (O / C) of oxygen atoms to carbon atoms, and is measured by X-ray photoelectron spectroscopy (ESCA).

The thickness of the electrode layer 2 before being incorporated into the battery cell 20 is preferably 0.01 mm to 1 mm, more preferably 0.01 mm to 0.8 mm, and further preferably 0.02 to 0.5 mm. When the thickness of the electrode layer 2 before being incorporated into the battery cell 20 is 0.01 mm or more, the conductivity is good, which is preferable. When the thickness of the electrode layer 2 before being incorporated into the battery cell 20 is 1 mm or less, the liquid passage resistance does not become too large even when carbon nanotubes are used, and good liquid permeability can be obtained, which is preferable. Since the electrode layer 2 is compressed from both sides when it is incorporated into the battery, its thickness is smaller than the above.

Hereinafter, a method for manufacturing the electrode layer 2 will be described. The electrode layer 2 is formed into a sheet by preparing a dispersion liquid containing carbon nanotubes in advance, removing the dispersion medium by filtration, or performing coating, spin casting, spraying, etc., and then distilling off the dispersion medium. Can be molded. Since a large amount of dispersion liquid is used, it is preferable to use water in consideration of safety and environmental load resistance.

The method for preparing the dispersion liquid containing the carbon nanotubes is not particularly limited. For example, a ball mill, a paint shaker, an ultrasonic homogenizer, a jet mill and the like can be used. A wet jet mill is preferable because the carbon nanotubes can be uniformly dispersed while suppressing damage to the carbon nanotubes. Preliminary mixing may be carried out using a wet disperser or the like before the dispersion is carried out by a wet jet mill.

When the electrode layer 2 contains a plurality of types of carbon nanotubes and / or carbon fibers having different average fiber diameters, a plurality of types of carbon nanotubes and / or carbon fibers having different average fiber diameters are added to the dispersion medium as described above. The dispersion can be prepared and molded according to the above.

When preparing a dispersion liquid containing carbon nanotubes, adding a dispersant facilitates uniform mixing of the carbon nanotubes. Although known dispersants may be used, the water-soluble conductive polymer exhibits extremely excellent properties as a dispersant for carbon nanotubes. Further, since the carbon fiber is simple, it is preferable to disperse it in a dispersion liquid of carbon nanotubes by ultrasonic treatment.

It is considered that the water-soluble conductive polymer can make the surface of the carbon nanotubes hydrophilic and acts as a surfactant when obtaining a dispersion liquid for forming the electrode layer 2 into a sheet. The carbon nanotubes can be uniformly dispersed in the dispersion liquid, and the electrode layer 2 having a uniform porosity can be obtained. As the water-soluble conductive polymer, a conductive polymer having a sulfo group is preferable, and specific examples thereof include polyisothianaphthenic acid.

The amount of the water-soluble conductive polymer added is preferably 2 parts by mass or less, more preferably 1 part by mass or less, and further preferably 0.5 parts by mass or less with respect to 100 parts by mass of the carbon nanotube.

[Liquid outflow member]
The liquid outflow member 3 is a member provided for allowing the electrolytic solution that has passed through the electrode layer 2 to flow out to the outside of the electrode 10. The electrolytic solution that has passed through the liquid outflow member 3 flows out to the outflow pipe 12 and is returned to an electrolytic solution tank (not shown).

The liquid outflow member 3 may have a structure in which the electrolytic solution can easily flow as compared with the electrode layer 2. The liquid outflow member 3 can be configured in the same manner as the liquid outflow member described in, for example, Patent Document 5.

Further, the thickness of the liquid outflow member 3 is preferable because the thicker the thickness, the more the pressure required for the electrolytic solution to pass through the liquid outflow member 3 can be further reduced. The thickness of the liquid outflow member 3 after being incorporated into the battery cell 20 is preferably 0.08 mm or more, more preferably 0.1 mm to 0.7 mm, and further preferably 0.15 to 0.5 mm. It is preferable that the thickness of the liquid outflow member 3 after being incorporated into the battery cell 20 is 0.08 mm or more because the pressure required for passing the electrolytic solution can be reduced. If the thickness of the liquid outflow member 3 after being incorporated into the battery cell 20 is 0.7 mm or less, the distance between the electrode layer 2 and the ion exchange membrane 30 does not become too large (the ion movement resistance does not become too large). , It is preferable because the increase in cell resistance can be suppressed.

Further, the electrolytic solution after passing through the electrode layer 2 has a high proportion of the electrolytic solution after the oxidation reaction or the reduction reaction occurs. As described above, the liquid outflow member 3 rapidly outflows the electrolytic solution, so that the ions after the valence changes can be efficiently removed from the vicinity of the electrode layer 2, so that the reactivity can be enhanced. For example, when an electrolytic solution containing vanadium is used as the active material, V ⁴⁺ changes to V ⁵⁺ at the positive electrode and V ³⁺ changes to V ^{2+ at the negative electrode during the charging process.} Therefore, by efficiently removing the ions (V ⁵⁺ and V ²⁺ ) after the reaction, the ions (V ⁴⁺ and V ³⁺ ) before the reaction can be quickly supplied to the electrode layer 2 before and after the reaction. Ions can be efficiently replaced to increase reaction efficiency. In the discharge process, the change in the valence of the ions is reversed, but the ions before and after the reaction are efficiently replaced as in the charging process, and the reaction efficiency can be improved.

The liquid outflow member 3 is preferably made of a porous member, for example, a porous sheet. Since the liquid outflow member 3 is made of a porous sheet, the liquid outflow member 3 functions as a cushioning material for stress between the electrode layer 2 and the ion exchange membrane 30. Therefore, it is possible to suppress the occurrence of scratches or the like on the ion exchange membrane 30, and it is possible to stably support the electrode layer 2.

The porous sheet may be a sponge-like member having voids or a member formed by entwining fibers. For example, a woven fabric in which relatively long fibers are woven, felt in which fibers are entwined without being woven, paper in which relatively short fibers are squeezed into a sheet, or the like can be used. When the porous sheet is made of fibers, it is preferable that the average fiber diameter is made of fibers larger than 1 μm. When the average fiber diameter of the porous sheet is 1 μm or more, the liquid permeability of the electrolytic solution in the porous sheet can be sufficiently ensured. Since the liquid outflow member 3 is compressed from both sides when it is incorporated into the battery, its thickness is smaller than the above.

It is preferable that the porous sheet is not corroded by the electrolytic solution. Specifically, the redox flow battery often uses an acidic solution. Therefore, the porous sheet preferably has acid resistance. Further, since the porous sheet may be oxidized by the reaction, it is preferable that the porous sheet has oxidation resistance. Having acid resistance or oxidation resistance refers to a state in which the porous sheet after use maintains its shape.

Moreover, it is preferable that this porous sheet has conductivity. Here, conductivity and the volume resistivity is preferably not more than 10 ⁷ Ω · cm, more preferably a degree of conductivity 10 ³ Ω · cm or less. If the porous sheet has conductivity, the electrical conductivity in the liquid outflow member 3 can be enhanced. For example, when forming a porous sheet using fibers made of a conductive material, fibers made of a metal or alloy having acid resistance and oxidation resistance, or carbon fibers can be used.

[Ion exchange membrane]
As the ion exchange membrane 30, a known cation exchange membrane can be used. Specifically, a perfluorocarbon polymer having a sulfo group, a hydrocarbon polymer compound having a sulfo group, a polymer compound doped with an inorganic acid such as phosphoric acid, and a part of which is substituted with a proton conductive functional group. Examples thereof include organic / inorganic hybrid polymers obtained by impregnating a polymer matrix with a phosphoric acid solution or a sulfuric acid solution. Of these, a perfluorocarbon polymer having a sulfo group is preferable, and Nafion (registered trademark) is more preferable.

[Current collector plate]
The current collector plate 40 has a role of a current collector that transfers electrons to and from the electrode layer 2, and is a member that constitutes an electrode of a redox flow battery. The current collector plate 40 is usually plate-shaped, and a known material can be used. As the material, for example, a conductive material containing carbon can be used. Specific examples thereof include conductive plastics made of graphite and thermoplastic resins such as polyolefins, and thermosetting resins such as graphite and epoxy resins. Of 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. Carbon black having high conductivity such as acetylene black may be mixed.

[Liquid inflow part]
The liquid inflow portion 41 is a portion provided for allowing the electrolytic solution passed from the inflow port 42c through the inflow pipe 11 to flow into the electrode layer 2. Here, the liquid inflow portion 41 includes the outer frame 42a as shown in FIG. 2 in addition to the flow path 42 and the liquid inflow member 43.

A specific embodiment of the liquid inflow portion 41 is, for example, as shown in FIG. 1, flowing along a flow path 42 formed of a groove formed in a recess of a current collector plate and a flow path 42 (42d, 42e). It is preferably composed of a liquid inflow member 43 having electron conductivity for spreading the electrolytic solution over the entire surface of the electrode layer. As the liquid inflow member 43, the same porous sheet as the liquid outflow member 3 may be used, or a different material having the same characteristics may be used. Further, the outer frame 42a and the support member 42b forming the flow path as shown in FIG. 2 are preferably conductive in order to facilitate the transfer of electrons between the electrode layer 2 and the current collector plate 40. The outer frame 42a and the support member 42b can be made of the same material as the current collector plate. Further, it may be integrated with the current collector plate, and in order to form the outer frame 42a and the support member 42b, the flow paths 42d and 42e may be formed by forming a groove in the current collector plate 40.

It is preferable that the liquid inflow portion 41 has a structure in which the electrolytic solution easily flows as compared with the electrode layer 2. The liquid inflow section 41 can be configured in the same manner as the liquid inflow section as described in Patent Document 5, for example.

The thickness of the liquid inflow member 43 after being incorporated into the battery cell 20 is preferably 0.08 mm or more, more preferably 0.1 mm to 0.7 mm, still more preferably 0.15 to 0.5 mm. When the thickness of the liquid inflow member 43 after being incorporated into the battery cell 20 is 0.08 mm or more, the pressure required for passing the electrolytic solution to spread over the entire surface can be reduced, which is preferable. When the thickness of the liquid inflow member 43 after being incorporated into the battery cell 20 is 0.7 mm or less, the thickness of the liquid inflow portion 43 does not become too thick and the entire cell can be made smaller, which is preferable. Since the liquid inflow member 43 is compressed from both sides when it is incorporated into the battery cell 20, its thickness becomes smaller than the above.

Further, the shape of the groove forming the flow path 42 is not particularly limited as long as the in-plane variation in the amount of the electrolytic solution flowing into the electrode layer 2 can be suppressed. For example, as shown in FIG. 1, the groove may be formed in a rib shape or may be formed so as to spread radially.

By adopting the battery cell 20 as described above, the redox flow battery 200 has a small pressure loss and a high current density even when the electrode layer 2 having poor liquid permeability of the electrolytic solution is used. Can be done. In such a redox flow battery, the temperature of the electrolytic solution sent toward the electrode layer 2 of the battery cell 20 (the temperature of the electrolytic solution at the inflow port 42c) is set to 40 ° C. or higher and 90 ° C. or lower. High reactivity in the electrode layer 2 can be ensured, and high current density can be achieved.

Further, conventionally, the temperature control is performed so as to maintain a low temperature by giving a heat dissipation function or a cooling function to the members constituting the redox flow battery so that the temperature of the electrolytic solution does not rise too much. However, since it is not necessary in the present invention, there are also effects such as simplification of the battery configuration and reduction of the battery manufacturing cost. Needless to say, also in the present invention, a monitor device for monitoring the operating temperature, a cooling device, or the like may be installed as needed.

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to a specific embodiment, and various within the scope of the gist of the present invention described in the claims. Can be transformed / changed.

Hereinafter, examples of the present invention will be described. The present invention is not limited to the following examples.

[Example 1]
(Preparation of electrode layer)
A first carbon nanotube having an average fiber diameter of 150 nm and an average fiber length of 15 μm and a second carbon nanotube having an average fiber diameter of 15 nm and an average fiber length of 3 μm are combined with a total of 100 masses of the first carbon nanotube and the second carbon nanotube. 90 parts by mass and 10 parts by mass of each part are mixed in pure water, and polyisotianaftensulfonic acid, which is a water-soluble conductive polymer, is added to the total of the first carbon nanotubes and the second carbon nanotubes. A mixed solution was prepared by adding 1 part by mass to 100 parts by mass. The obtained mixed liquid was treated with a wet jet mill to obtain a dispersion liquid of carbon nanotubes. To this dispersion, 50 parts by mass of carbon fibers having an average fiber diameter of 7 μm and an average fiber length of 0.13 mm were added to a total of 100 parts by mass of the first and second carbon nanotubes and carbon fibers, and the mixture was stirred with a magnetic stirrer. Distributed. This dispersion was filtered on a filter paper, dehydrated together with the filter paper, and then the residue was compressed by a press and further dried to prepare an electrode layer 2 containing carbon nanotubes. The average thickness of the electrode layer 2 before assembly was 0.4 mm.

(Manufacturing of current collector plate / liquid inflow part)
As the current collector plate and the liquid inflow portion, the same current collector plate (bipolar plate) and the liquid inflow portion as in Example 1 of Patent Document 5 were used. That is, as shown in FIG. 1, a recess is formed on one side of the current collector plate 40 made of a carbon plastic molded body, and a groove is formed in the recess as a flow path 42 of the liquid inflow portion 41 to form a liquid inflow member 43. A carbon fiber paper (manufactured by SGL Carbon Co., Ltd .: 39AA), which is a porous sheet, was prepared. The average thickness of this carbon fiber paper before being incorporated into the cell is 0.37 mm.

As shown in FIG. 2, the flow paths 42A and 42B are composed of an outer frame 42a formed so as to be arranged side by side, and a support member 42b formed in a rib shape in the outer frame 42a. Specifically, the size of the entire liquid inflow portion 41 (recess) is 50 mm × 50 mm, and two outer frames 42a having a size of 24.5 mm × 50 mm are arranged side by side with a width of 1 mm. The liquid inflow member 43 was placed on the support member 42b so that the outer frame 42a and the upper surface of the liquid inflow member 43 were flush with each other. The liquid inflow port 42c was provided by forming a hole having a diameter of 0.8 mmφ. The inflow pipe 11 is connected to the liquid inflow port 42c, and the liquid discharge path is provided between both side surfaces of the outer frame 42a and the two outer frames 42a so as to be in the discharge direction shown by the arrow in FIG. The liquid discharge path between the two outer frames 42a utilizes the above-mentioned 1 mm wide space.

(Liquid outflow member)
As the liquid outflow member 3, carbon fiber paper (manufactured by SGL, GDL10AA), which is a porous sheet, was prepared. The average thickness of this carbon fiber paper before assembly is 0.25 mm.

(Battery assembly)
Using the electrode layer 2, the current collector plate 40 to which the liquid inflow portion 41 (flow path 42, liquid inflow member 43) is attached, and the liquid outflow member 3, the electrodes (liquid inflow member 43 and electrodes) are placed on the current collector plate 40. The layer 2 and the liquid outflow member 3) were assembled. In the electrode layer 2, two 24.5 mm × 50 mm electrode layers were arranged in parallel with a width of 1 mm so as to face the liquid inflow portion 41 of the current collector plate 40.

Further, using Nafion N212 (registered trademark, manufactured by DuPont) as the ion exchange membrane 30, the electrodes assembled on the two current collector plates 40 having the above-described configuration are used as positive and negative electrodes, respectively, and a frame, gasket, and terminal (not shown) are used. A redox flow battery having a battery cell having a single cell structure as shown in FIG. 1 was assembled via a push plate (a member that inputs and receives electric power between a current collector plate and an external power source).

In the redox flow battery assembled in this way, prepare two 100 mL of sulfuric acid aqueous ^{solutions having a vanadium ion (containing equal molar amounts of V + 3} and V ^{+ 4) concentration of 1.8 M as an electrolyte and send them into the battery cell.} The electrolytic solution was placed in a water bath to set the temperature of the water bath to 25 ° C., and the electrolytic solution was sent to the positive electrode and the negative electrode by a tube pump, respectively, and charging and discharging were repeated at a current density of ^{100 mA / cm 2.} The cutoff voltage is 1.75V for charging and 1.0V for discharging. The battery capacity of 100 ml of 1.8 M electrolyte is approximately 5 Ah in calculation. The second cycle was 4.7 Ah. When the cell resistance was calculated using the charge / discharge curve of 3 cycles, it was ^{0.72 Ωcm 2.}

Further, the electrolytic solution to be sent to the battery cell is put into a water bath, and the temperature of the water bath is changed to 30 ° C., 35 ° C., 40 ° C., 45 ° C., 50 ° C., 55 ° C., 60 ° C., 65 ° C., 70 ° C., 75 ° C., 80 ° C. , 85 ° C. and 90 ° C., and charging and discharging were carried out in the same manner. In the above charge / discharge, a thermometer was attached near the electrode layer 2 to measure the temperature of the electrolytic solution, and it was confirmed that the temperature was the same as the temperature of the water bath. Three cycles of charge / discharge were performed at each temperature, and the cell resistance was calculated using the charge / discharge curve of the third cycle. The cell resistance is a value obtained by reading the midpoint voltage at the time when the cutoff voltage is reached, dividing the difference between the midpoint voltage of the charge curve and the discharge curve by the current density, and further halving it.

[Comparative Example 1]
Instead of the electrode layer 2 used in Example 1, carbon fiber paper (manufactured by SGL Carbon Co., Ltd .: GDL10AA, average fiber diameter 12 μm) having a surface oxygen content (O / C) of 0.7% was used. A redox flow battery was produced in the same manner as in Example 1, and the cell resistance at each temperature was calculated.

[Comparative Example 2]
The carbon fiber paper used in Comparative Example 1 was calcined in air at 500 ° C. for 1 hour, and then incorporated into the electrode. A redox flow battery was produced in the same manner as in Example 1 except for this, and the cell resistance at each temperature was calculated. When the carbon fiber paper after firing was oxygen-analyzed, it was confirmed that the surface oxygen amount (O / C) was 1.3% and that functional groups such as OH groups and COOH groups were increased.

[Comparative Example 3]
The carbon fiber paper used in Comparative Example 1 was fired in nitrogen gas at 700 ° C. for 1 hour to volatilize functional groups such as OH groups and COOH groups on the surface, and then incorporated into the electrode. When the oxygen content of the carbon fiber paper after firing was analyzed, the surface oxygen content (O / C) was 0.2%. Except for this, a redox flow battery was produced in the same manner as in Example 1, and the cell resistance at each temperature was measured. However, the electromotive force exceeded 1.8 V from the beginning, and charging and discharging could not be performed at all.

[Result / Evaluation]
The measurement results of Example 1 and Comparative Examples 1 and 2 are shown in FIG. As shown in FIG. 3, in Example 1 in which carbon nanotubes were used for the electrode layer, low cell resistance could be stably maintained even when the electrolytic solution was operated at a temperature of 40 ° C. or higher, and moreover, 40. It can be seen that the cell resistance is even lower than when the battery is operated at a temperature lower than ° C., and that the battery characteristics are excellent. On the other hand, in Comparative Examples 1 and 2 in which carbon fiber paper having many functional groups such as OH groups and COOH groups on the surface was used for the electrode layer, the cell resistance increased as the temperature of the electrolytic solution increased. I understand. It is considered that this is because in Comparative Examples 1 and 2, when the functional groups such as OH group and COOH group contributing to the reaction on the surface of the carbon fiber paper start to be decomposed at 40 ° C. or higher, the reactivity is lowered. Further, in Comparative Example 3 in which the carbon fiber paper was fired in nitrogen gas at 600 ° C. or higher, it is considered that the carbon fiber paper does not function as an electrode because the functional groups added to the carbon paper have almost disappeared.

In the present embodiment, the operation method of the vanadium-based redox flow battery using vanadium (V) ion as the positive electrode active material and the negative electrode active material has been described, but it goes without saying that the present invention is not limited to this. The negative electrode electrolytic solution and the positive electrode electrolytic solution may be any aqueous solution containing at least one or more electrochemically active species. Examples of the electrochemically active species include metal ions such as manganese, titanium, chromium, bromine, iron, zinc, cerium and lead.

2 Electrode layer 3 Liquid outflow member 11 Inflow pipe 12 Outflow pipe 20 Battery cell 30 Ion exchange membrane 40 Current collector 41 Liquid inflow part 42 Flow path 43 Liquid inflow member 200 Redox flow battery

Claims (4)

It is a method of operating a redox flow battery having an electrode layer containing carbon nanotubes in the electrode.
A method for operating a redox flow battery, wherein charging and discharging are performed by setting the temperature of the electrolytic solution sent toward the electrode layer where the electrode reaction is performed to 40 ° C. or higher and 90 ° C. or lower. The method for operating a redox flow battery according to claim 1, wherein the content of the carbon nanotubes is 30 parts by mass or more with respect to a total of 100 parts by mass of carbon contained in the electrode layer. The redox flow according to any one of claims 1 or 2, wherein the electrode layer contains 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 30 nm or less. How to operate the battery. The operation of the redox flow battery according to claim 3, wherein the second carbon nanotube is 0.05 to 30 parts by mass with respect to a total of 100 parts by mass of the first carbon nanotube and the second carbon nanotube. Method.

Images