WO2022209182A1

WO2022209182A1 - Electrode, battery cell, cell stack, and redox flow battery

Info

Publication number: WO2022209182A1
Application number: PCT/JP2022/001845
Authority: WO
Inventors: 康一橋本; 雄大池上; 正幸大矢; 雍容董; 吉恭川越; 賢一伊藤; 貴五十嵐; 良平岩原
Original assignee: 住友電気工業株式会社; 東洋紡株式会社
Priority date: 2021-04-02
Filing date: 2022-01-19
Publication date: 2022-10-06

Abstract

An electrode comprising a porous substrate, wherein the ratio of the mesopore specific surface area to the BET specific surface area is 40% or more, and the mesopore specific surface area is 1.0 m²/g or more but less than 30 m²/g.

Description

Electrodes, battery cells, cell stacks, and redox flow batteries

The present disclosure relates to electrodes, battery cells, cell stacks, and redox flow batteries.
This application claims priority based on Japanese Patent Application No. 2021-063777 filed in Japan on April 2, 2021, and incorporates all the contents described in the Japanese application.

A redox flow battery is known as a large-capacity storage battery. For example, the redox flow battery disclosed in Patent Document 1 includes a cell stack in which a plurality of battery cells are stacked. Charging and discharging are performed by circulating the positive electrode electrolyte and the negative electrode electrolyte in the battery cell.

A conductive and liquid-permeable porous body is used for the electrodes provided in the battery cells. For example, Patent Literature 1 discloses an electrode composed of an aggregate of carbon fibers.

JP-A-2002-246035

The electrodes of the present disclosure are
An electrode comprising a porous substrate,
The ratio of the mesopore specific surface area to the BET specific surface area is 40% or more,
The mesopore specific surface area is 1.0 m ² /g or more and less than 30 m ² /g.

The battery cell of the present disclosure is
An electrode of the present disclosure is provided.

The cell stack of the present disclosure is
A battery cell of the present disclosure is provided.

The redox flow battery of the present disclosure comprises
A cell stack of the present disclosure is provided.

FIG. 1 is an operating principle diagram of a redox flow battery according to an embodiment. FIG. 2 is a schematic configuration diagram of a redox flow battery according to an embodiment. FIG. 3 is a schematic configuration diagram showing an example of a cell stack. FIG. 4 is a schematic diagram of an electrode according to the embodiment. FIG. 5 is a partially enlarged view of the electrode according to the embodiment.

[Problems to be Solved by the Present Disclosure]
In order to improve the battery reaction at the electrodes, it is preferable that the electrodes have a large surface area in contact with the electrolyte. However, in an electrolytic solution with a high oxidizing power, the electrode is gradually consumed by the oxidizing power of the electrolytic solution. In particular, in a positive electrode electrolyte having a high oxidizing power, an electrode having a large contact area with the electrolyte is easily consumed. When the electrode is consumed, the battery performance of the battery cell provided with the electrode deteriorates.

Therefore, one object of the present disclosure is to provide an electrode that is excellent in battery reactivity and durability. Another object of the present disclosure is to provide a battery cell, a cell stack, and a redox flow battery capable of suppressing deterioration of battery performance due to operation.

[Effect of the present disclosure]
The electrodes of the present disclosure are excellent in battery reactivity and durability.

The battery cell, cell stack, and redox flow battery of the present disclosure maintain excellent battery performance over a long period of time.

[Description of Embodiments of the Present Disclosure]
The present inventors have extensively studied the configuration of a battery that is excellent in battery reactivity and durability. As already explained, there is a trade-off relationship between the improvement in battery reactivity and the durability of the electrode. In consideration of this point, the present inventors arrived at a technical idea of defining the ratio of the mesopore specific surface area to the electrode and the upper limit of the mesopore specific surface area. Based on this technical idea, the present inventors have completed the electrode, battery cell, cell stack, and redox flow battery of the present disclosure described below. Embodiments of the present disclosure are listed and described below.

<1> The electrode according to the embodiment is
An electrode comprising a porous substrate,
The ratio of the mesopore specific surface area to the BET specific surface area is 40% or more,
The mesopore specific surface area is 1.0 m ² /g or more and less than 30 m ² /g.

The pores formed in the porous body are classified into micropores, mesopores, and macropores. A mesopore is a pore having a diameter of 2 nm or more and 50 nm or less. Micropores are pores that are smaller than mesopores. Macropores are pores that are thicker than mesopores. This definition complies with the definition of the International Union of Pure and Applied Chemistry. Mesopores are more effective in improving battery reaction than macropores and micropores. This is because the electrolyte can easily enter the mesopores, and the contact area between the inner peripheral surface of the mesopores and the electrolyte is large. It is difficult for the electrolytic solution to enter the micropores, which are narrower than the mesopores. The macropores, which are thicker than the mesopores, easily allow the electrolyte to enter, but the contact area of the macropores with respect to the electrolyte per unit volume is small.

In the electrode according to the embodiment, the ratio of the mesopore specific surface area to the BET specific surface area and the upper limit and lower limit of the mesopore specific surface area are specified. Therefore, in the electrodes of the embodiments, the contact area with the electrolytic solution does not become too large even though the proportion of mesopores that improve the battery reaction is high. Therefore, the electrodes according to the embodiments are excellent in battery reactivity and durability.

<2> In the electrode according to the embodiment,
The BET specific surface area may be less than 40 m ² /g.

In the configuration of form <2> above, the BET specific surface area does not become too large. In other words, the contact area between the electrodes and the electrolytic solution does not become too large. Therefore, even if the electrodes are exposed to the electrolytic solution for a long period of time, extreme wear of the electrodes is suppressed.

<3> The electrode according to the embodiment is
It may be used for the positive electrode of a redox flow battery.

In general, the oxidizing power of the positive electrode electrolyte is higher than that of the negative electrode electrolyte. As already mentioned, the electrodes according to the embodiments are excellent in durability. Therefore, the electrode according to the embodiment is suitable as a positive electrode exposed to a positive electrode electrolyte having a high oxidizing power.

<4> In the electrode according to the embodiment,
The base material may contain, as a conductive material, one or more elements selected from the group consisting of carbon, titanium, and tungsten.

Carbon, titanium, and tungsten are excellent in conductivity and corrosion resistance. Therefore, these elements are suitable as base materials for electrodes.

<5> In the electrode according to the embodiment,
The substrate may contain at least one selected from the group consisting of carbon fibers, carbon particles, and carbon binder residues.

By using at least one of carbon fiber and carbon particles as a raw material to produce the electrode, it becomes easy to adjust the BET specific surface area and the mesopore specific surface area of the electrode. For example, the BET specific surface area and mesopore specific surface area of the electrode are adjusted by limiting the fiber length and fiber diameter of the carbon fiber. In addition, the BET specific surface area and the mesopore specific surface area of the electrode are adjusted by limiting the particle size of the carbon particles. Here, the carbon binder residue is carbonized binder. The binder binds carbon fibers together, binds carbon fibers and carbon particles, or binds carbon particles together when the electrode is manufactured.

<6> The electrode according to the embodiment is
comprising a catalyst held on the substrate;
The catalyst may be made of a non-carbon material.

The catalyst promotes the cell reaction. Therefore, the battery reactivity of the electrode is improved by holding the catalyst on the substrate.

<7> In the electrode of form <6> above,
The catalyst may be a metal oxide or metal carbide.

　Metal oxide or metal carbide catalysts are difficult to oxidize. Therefore, the electrode tends to maintain excellent battery reactivity over a long period of time.

<8> The battery cell according to the embodiment is
The electrode according to any one of the above modes <1> to <7> is provided.

The battery cell according to the embodiment easily maintains excellent battery performance over a long period of time. This is because the electrodes of the embodiment provided in the battery cells are less likely to wear out.

<9> The cell stack according to the embodiment is
The battery cell of form <8> is provided.

The cell stack according to the embodiment easily maintains excellent battery performance over a long period of time. This is because the electrodes of the embodiment provided in the battery cells that make up the cell stack are less likely to wear out.

<10> The redox flow battery according to the embodiment is
The cell stack of form <9> is provided.

The redox flow battery according to the embodiment easily maintains excellent battery performance over a long period of time. This is because the electrodes of the embodiment provided in the redox flow battery are less likely to be consumed.

[Details of the embodiment of the present disclosure]
Specific examples of electrodes, battery cells, cell stacks, and redox flow batteries of the present disclosure will be described with reference to the drawings. Hereinafter, the redox flow battery may be referred to as "RF battery". The same reference numerals in the drawings indicate the same or corresponding parts. The present invention is not limited to these exemplifications, but is indicated by the scope of the claims, and is intended to include all modifications within the scope and meaning equivalent to the scope of the claims.

<Example 1>
≪Overview of RF battery≫
An RF battery 1 according to an embodiment will be described with reference to FIGS. 1 to 3. FIG. The RF battery 1 shown in FIG. 1 performs charging and discharging with a positive electrode electrolyte containing a positive electrode active material and a negative electrode electrolyte containing a negative electrode active material. The positive electrode active material and the negative electrode active material are typically metal ions whose valences change due to oxidation-reduction. The metal ions contained in the positive electrode electrolyte and the negative electrode electrolyte shown in FIG. 1 are examples. FIG. 1 illustrates a Ti—Mn-based RF battery containing Mn ions as a positive electrode active material and Ti ions as a negative electrode active material. In FIG. 1, the solid line arrows indicate the charge reaction, and the dashed line arrows indicate the discharge reaction.

The RF battery 1 is typically connected to the power system 90 via the AC/DC converter 80 and the substation equipment 81 . The RF battery 1 charges the power generated by the power generation unit 91 or discharges the charged power to the load 92 . The power generation unit 91 is a power generation facility using natural energy such as solar power generation or wind power generation, or a general power plant. The RF battery 1 is used, for example, for load leveling applications, momentary sag compensation, emergency power supply applications, and output smoothing applications for natural energy power generation.

<<Configuration of RF battery>>
The RF battery 1 includes battery cells 10 , a positive electrode tank 12 and a negative electrode tank 13 . The battery cell 10 is responsible for charging and discharging. The positive electrode tank 12 stores a positive electrode electrolyte. The negative electrode tank 13 stores a negative electrode electrolyte.

(battery cell)
The battery cell 10 is separated into a positive electrode cell 102 and a negative electrode cell 103 by a diaphragm 101 . The diaphragm 101 is an ion exchange membrane that is impermeable to electrons but permeable to hydrogen ions, for example. The cathode cell 102 incorporates the cathode electrode 2 . The negative electrode cell 103 incorporates the negative electrode 3 .

A positive electrode electrolyte and a negative electrode electrolyte are supplied to the positive electrode cell 102 and the negative electrode cell 103 that constitute the battery cell 10, respectively. The RF battery 1 of this example includes an outbound pipe 108 and a return pipe 110 that connect between the battery cell 10 and the positive electrode tank 12 . The RF battery 1 of this example includes an outward pipe 109 and a return pipe 111 that connect between the battery cell 10 and the negative electrode tank 13 .

Pumps

112 and 113 are provided in the respective

outbound pipes

108 and 109, respectively. The positive electrode electrolyte is supplied from the positive electrode tank 12 by the pump 112 to the positive electrode cell 102 through the outgoing pipe 108 . The positive electrode electrolyte discharged from the positive electrode cell 102 through the positive electrode cell 102 is returned to the positive electrode tank 12 through the return pipe 110 . The negative electrode electrolyte is supplied from the negative electrode tank 13 to the negative electrode cell 103 by the pump 113 through the outgoing pipe 109 . The negative electrode electrolyte discharged from the negative electrode cell 103 through the negative electrode cell 103 is returned to the negative electrode tank 13 through the return pipe 111 . In other words, the

outward pipes

108 and 109 and the

return pipes

110 and 111 form a circulation flow path.

The RF battery 1 is usually used in a form called a cell stack 100 in which a plurality of battery cells 10 are stacked, as shown in FIGS. The cell stack 100 has a configuration in which the sub-stack 200 shown in FIG. 3 is sandwiched between two end plates 220 from both sides. The two end plates 220 are tightened toward each other by a tightening mechanism 230 . FIG. 3 shows a cell stack 100 comprising multiple substacks 200 . The substack 200 includes a laminate in which the cell frame 30, the positive electrode 2, the diaphragm 101, and the negative electrode 3 are repeatedly laminated in this order. Supply/discharge plates 210 are arranged at both ends of the laminate. The supply/discharge plate 210 is connected to the

outgoing pipes

108 and 109 and the

return pipes

110 and 111 shown in FIGS. The number of stacked battery cells 10 in the cell stack 100 can be selected as appropriate.

The cell frame 30 has a bipolar plate 31 arranged between the positive electrode 2 and the negative electrode 3 and a frame 32 provided around the bipolar plate 31 . The positive electrode 2 is arranged on one side of the bipolar plate 31 so as to face the bipolar plate 31 . On the other side of the bipolar plate 31, the negative electrodes 3 are arranged so as to face each other. The positive electrode 2 and the negative electrode 3 are housed inside the frame 32 with the bipolar plate 31 interposed therebetween. One battery cell 10 is formed by arranging the positive electrode 2 and the negative electrode 3 between the bipolar plates 31 of the adjacent cell frames 30 with the diaphragm 101 interposed therebetween.

The frame 32 of the cell frame 30 is formed with

liquid supply manifolds

33, 34,

liquid discharge manifolds

35, 36,

liquid supply slits

33s, 34s, and

liquid discharge slits

35s, 36s. In this example, the positive electrode electrolyte is supplied from the liquid supply manifold 33 to the positive electrode 2 through the liquid supply slit 33s. The positive electrode electrolyte supplied to the positive electrode 2 is discharged to the drainage manifold 35 through the drainage slit 35s. Similarly, the negative electrode electrolyte is supplied from the liquid supply manifold 34 to the negative electrode 3 through the liquid supply slit 34s. The negative electrode electrolyte supplied to the negative electrode 3 is drained to the drain manifold 36 through the drain slit 36s. The

liquid supply manifolds

33 and 34 and the

liquid discharge manifolds

35 and 36 are provided through the frame 32 , and the cell frames 30 are stacked to form flow paths for the respective electrolytes. Each of these flow paths communicates with the

outgoing pipes

108 and 109 and the

return pipes

110 and 111 shown in FIG. The cell stack 100 can circulate the positive electrode electrolyte and the negative electrode electrolyte through the battery cells 10 through the respective channels.

(Electrolyte)
The positive electrode electrolyte solution circulated through the battery cells 10 contains a positive electrode active material. The positive electrode active material is typically vanadium (V) ions, iron (Fe) ions, copper (Cu) ions, or manganese (Mn) ions. The negative electrode electrolyte contains a negative electrode active material. The negative electrode active material is typically V ions, chromium (Cr) ions, titanium (Ti) ions, cobalt (Co) ions, Cu ions, or zinc (Zn) ions.

The positive electrode active material and the negative electrode active material may have different types of metals to be ions, or may be the same. Typical combinations of positive electrode active materials and negative electrode active materials are shown below.
1. Positive electrode active material: V ions (V ⁴⁺ /V ⁵⁺ ), negative electrode active material: V ions (V ²⁺ /V ³⁺ )
2. Positive electrode active material: Fe ions (Fe ²⁺ /Fe ³⁺ ), negative electrode active material: Cr ions (Cr ²⁺ /Cr ³⁺ )
3. Positive electrode active material: Mn ions (Mn ²⁺ /Mn ³⁺ ), negative electrode active material: Ti ions (Ti ³⁺ /Ti ⁴⁺ )
4. Positive electrode active material: Fe ions (Fe ²⁺ /Fe ³⁺ ), negative electrode active material: Ti ions (Ti ³⁺ /Ti ⁴⁺ )
5. Positive electrode active material: Mn ion (Mn ²⁺ /Mn ³⁺ ), negative electrode active material: Zn ion (Zn ²⁺ /Zn)
6. Positive electrode active material: Fe ions (Fe ²⁺ /Fe ³⁺ ), negative electrode active material: Co ions (Co ⁺ /Co ²⁺ )
7. Positive electrode active material: Cu ion (Cu ⁺ /Cu ²⁺ ), negative electrode active material: Cu ion (Cu ⁺ /Cu)
In particular, when the positive electrode active material contains Mn ions and the negative electrode active material contains Ti ions, a high electromotive force can be obtained.

The electrolyte solution of the RF battery 1 shown in FIGS. is a Mn—Ti-based electrolytic solution containing an active material shown in. In this case, both the positive electrode electrolyte and the negative electrode electrolyte may contain Mn ions and Ti ions. In the positive electrode electrolyte, Mn ions function as a positive electrode active material. Ti ions function as a negative electrode active material in the negative electrode electrolyte. When Mn ions are used as the positive electrode active material, since trivalent Mn ions (Mn ³⁺ ) are unstable, Mn ³⁺ in the positive electrode electrolyte may precipitate as MnO ₂ during charging. When the positive electrode electrolyte contains Ti ions, the Ti ions can suppress deposition of Mn ions. Ti ions contained in the positive electrode electrolyte and Mn ions contained in the negative electrode electrolyte do not function as active materials. The Mn ions contained in the negative electrode electrolyte are mainly for equalizing the metal ion species in both electrolytes.

The solvent for the positive electrode electrolyte and the negative electrode electrolyte is preferably an acidic aqueous solution. The solvent is, for example, a sulfuric acid (H ₂ SO ₄ ) aqueous solution and a phosphoric acid (H ₃ PO ₄ ) aqueous solution. In particular, an aqueous sulfuric acid solution is preferred. Phosphoric acid may be contained in the sulfuric acid aqueous solution.

(electrode)
In this example, the structure of the positive electrode 2 will be described with reference to FIG. 4 as a representative example. FIG. 4 is a diagram schematically showing a cross section of the positive electrode 2. As shown in FIG. The negative electrode 3 of this example has the same configuration as the positive electrode 2 . Unlike this example, the negative electrode 3 may have a configuration different from that of the positive electrode 2 . In the following description, the positive electrode 2 will simply be called the electrode 2 .

The electrode 2 of this example has a porous base material 20 . Electrode 2 may further comprise a catalyst 22 retained on substrate 20 . The porous substrate 20 comprises mesopores 21 , micropores 25 and macropores 26 . The mesopores 21 are pores with a diameter of 2 nm or more and 50 nm or less. The micropores 25 are finer pores than the mesopores 21 . The macropores 26 are pores thicker than the mesopores 21 . These pores are exaggerated in FIG. In addition, although the schematic diagram of FIG. 4 shows each of the

pores

21, 25, and 26 individually, in reality, the mesopores 21, the micropores 25, and the macropores 26 are mixed in one pore. There is

It is preferable to increase the surface area of the electrode 2 in order to improve the reactivity between the electrode 2 and the electrolytic solution. On the other hand, when the surface area of the electrode 2 becomes large, the electrode 2 is easily consumed by the electrolyte. The reason why the electrode 2 is consumed is that the electrolytic solution containing the active material has an oxidizing power. The oxidizing power of the positive electrode electrolyte, particularly the oxidizing power of the positive electrode electrolyte containing Mn as an active material, is higher than that of the negative electrode electrolyte. Therefore, the electrode 2 used in the positive electrode cell 102 (FIGS. 1 and 2) wears out more easily than the negative electrode 3 used in the negative electrode cell 103 .

In the electrode 2 of this example, Y/X is 40% or more and Y is 1.0 m ² /g or more and less than 30 m ² /g. X is the BET specific surface area (m ² /g). The BET specific surface area is the total area (m ² ) of the surface area of the outer peripheral surface of the base material 20 and the area of the inner peripheral surfaces of all the

pores

21, 25, and 26 including the mesopores 21, and the mass (g) of the electrode 2. It is divided by The surface area of the outer peripheral surface of the substrate 20 does not include the openings of the

holes

21 , 25 and 26 . On the other hand, Y is the mesopore specific surface area (m ² /g). The mesopore specific surface area is obtained by dividing the total area (m ² ) of the inner peripheral surfaces of the mesopores 21 by the mass (g) of the electrode 2 . These specific surface areas are determined by a commercially available specific surface area measuring device. Y/X is the ratio of mesopore specific surface area to BET specific surface area.

When Y/X is 40% or more, it can be said that the ratio of the mesopore specific surface area to the BET specific surface area is high. The mesopores 21 are more effective in improving the battery reactivity of the electrode 2 than the micropores 25 and the macropores 26 . Therefore, the electrode 2 with Y/X of 40% or more is excellent in battery reactivity. Here, in the electrode 2 of this example, since the upper limit of the mesopore specific surface area is also defined, the upper limit of the BET specific surface area in Y/X is also limited to some extent. By limiting the upper limit of the BET specific surface area, the BET specific surface area does not become too large. Therefore, the effect of improving the battery reactivity of the electrode 2 while suppressing consumption of the electrode 2 is obtained.

As Y/X increases, the mesopore specific surface area in the BET specific surface area increases. The mesopores 21 contribute to improving the battery reaction. Therefore, it is preferable that Y/X is large. Y/X is preferably 60% or more. Y/X is more preferably 70% or more.

The mesopore specific surface area of the electrode 2 of this example is 1.0 m ² /g or more and less than 30 m ² /g. As the mesopore specific surface area decreases, the number of mesopores 21 that contribute to the improvement of the battery reaction decreases. A large mesopore specific surface area increases the surface area of the electrode 2 including the mesopores 21 . Considering the balance between the improvement of battery reaction and durability, the mesopore specific surface area is 2.0 m ² /g or more and 28 m ² /g or less, 3.0 m ² /g or more and 27 m ² /g or less, or 5.0 m ² /g or more and 25 m ² /g or less.

In order to ensure durability of the electrode 2, the BET specific surface area of the electrode 2 is preferably less than 40 m ² /g. The larger the surface area of the electrode 2 in contact with the electrolytic solution, the more easily the electrode 2 is worn. If the BET specific surface area of the electrode 2 is less than 40 m ² /g, the contact area of the electrode 2 with the electrolytic solution does not become too large. A more preferable BET specific surface area is 35 m ² /g or less. A more preferable BET specific surface area is 30 m ² /g or less. If the BET specific surface area is too small, the contact area of the electrode 2 with the electrolytic solution will be too small. The BET specific surface area is preferably 2.0 m ² /g or more, or 5.0 m ² /g or more. The range of the BET specific surface area is, for example, 5.0 m ² /g or more and 30 m ² /g or less.

The base material 20 contributes to the battery reaction. The material of the base material 20 is not particularly limited as long as it has conductivity. The material of the base material 20 includes, for example, one or more elements selected from the group consisting of carbon (C), titanium (Ti), and tungsten (W). The base material 20 may be composed of a single element, or may be composed of a compound of the above elements. The material of the base material 20 is obtained by, for example, X-ray diffraction.

The base material 20 containing carbon is, for example, a base material 20 containing at least one selected from the group consisting of carbon fibers, carbon particles, and carbon binder residues. FIG. 5 is a cross-sectional view showing an enlarged part of one carbon fiber 4 forming the base material 20. As shown in FIG. As shown in FIG. 5, the substrate 20 containing carbon is preferably a substrate 20 in which carbon fibers 4 and carbon particles 5 are bound together by a carbon binder residue 6 . In the substrate 20 shown in FIG. 5, pores are formed by gaps between the carbon fibers 4 to which the carbon particles 5 are attached.

The carbon fiber 4 has a role of maintaining the three-dimensional structure of the electrode 2. The carbon fibers 4 are, for example, fibers obtained by carbonizing a precursor composed of organic fibers. Organic fibers are, for example, acrylic fibres, phenolic fibres, cellulose fibres, isotropic pitch fibres, or anisotropic pitch fibres. Acrylic fibers are, for example, acrylonitrile.

The carbon particles 5 have the role of increasing the surface area of the electrode 2 and improving the battery reactivity of the electrode 2 . The carbon particles 5 may be natural graphite or artificial graphite.

The carbon binder residue 6 has the role of binding the carbon fibers 4 and the carbon particles 5 together. The carbon binder residue 6 is obtained by carbonizing the binder that binds the carbon fibers 4 and the carbon particles 5 by heat treatment during the manufacture of the electrode 2 . Binders are, for example, pitches such as coal tar pitch or coal-based pitch. Other binders are, for example, resins such as polyacrylonitrile, or rubbers such as acrylonitrile-butadiene rubber.

The base material 20 shown in FIG. 5 can be produced, for example, by adhering the carbon particles 5 and the binder to the carbon fibers 4, followed by a carbonization process, a graphitization process, and an oxidation treatment process. In each step, any known method can be applied.

In order to adhere the carbon particles 5 and the binder to the carbon fibers 4, for example, the following method can be used. The binder is heated and melted, and the carbon particles 5 are dispersed in the resulting melt. The carbon fibers 4 are immersed in the melt in which the carbon particles 5 are dispersed. Then, the binder is solidified. Here, by controlling the weight ratio of the carbon fibers 4, the carbon particles 5, and the binder, the electrode 2 that satisfies Y/X of 40% or more and Y of 1.0 m ² /g or more and less than 30 m ² /g. is obtained.

The carbonization process is performed to calcine the product obtained in the above process. The carbonization step is preferably carried out in an inert atmosphere such as a nitrogen atmosphere. The temperature of the carbonization step is, for example, 800° C. or higher and 2000° C. or lower. The carbonization step carbonizes the binder to form a carbon binder residue 6 .

The graphitization process is performed to increase the crystallinity of the carbon binder residue 6 and the like. This is a step of heating at a higher temperature than the carbonization step. The graphitization step is preferably performed in an inert atmosphere such as a nitrogen atmosphere.

The oxidation treatment step is performed to introduce oxygen functional groups to the surface of the base material 20 . Oxygen functional groups are, for example, hydroxyl groups, carbonyl groups, quinone groups, lactone groups, free-radical oxides. The oxygen functional groups contribute to improving the battery reaction of the substrate 20 . Moreover, the wettability of the base material 20 with respect to the electrolytic solution is improved. The oxidation treatment includes, for example, wet chemical oxidation, electrolytic oxidation, and dry oxidation.

As shown in FIG. 4, the substrate 20 may hold a catalyst 22 that promotes the cell reaction. Catalyst 22 is exaggerated in FIG. The catalyst 22 is, for example, granular. Catalyst 22 is a non-carbon based material. For example, the catalyst 22 is metal oxide, metal carbide. The metal oxide is, for example, a composite oxide of the following first element and second element.

The first element and the second element are elements different from each other. The first element is one element selected from the group consisting of Ti, tin (Sn), cerium (Ce), W, tantalum (Ta), molybdenum (Mo), and niobium (Nb). The second element is one element selected from the group consisting of Nb, Mn, Fe, Cu, Ti, Sn, and Ce. The first element contributes to improving oxidation resistance. That is, since the first element is difficult to dissolve in the electrolytic solution, it can exist for a long period of time. The second element contributes to improving the battery reaction of the electrode 2 . The composition of catalyst 22 is determined by X-ray diffraction.

Electrode 2 in which Y/X is 40% or more and Y is 1.0 m ² /g or more and less than 30 m ² /g is excellent in battery reactivity and durability. Therefore, the battery cell 10 (FIG. 1), the cell stack 100 (FIG. 3), and the RF battery 1 having this electrode 2 maintain excellent battery performance over a long period of time.

<Test example>
<<Test Example 1>>
In Test Example 1, the effects of the ratio of the mesopore specific surface area and the size of the mesopore specific surface area in the electrode on the battery reactivity and durability of the electrode were investigated.

　Sample No. 1 to sample no. 9, a plurality of positive electrodes and a plurality of negative electrodes having different BET specific surface areas and mesopore specific surface areas were prepared. The positive and negative electrodes are the same. Both electrodes were carbon electrodes containing carbon fibers, carbon particles and carbon binder residues. Hereinafter, when simply referred to as an "electrode", it means both a positive electrode and a negative electrode.

The BET specific surface area of each sample was measured as follows. About 500 mg of sample was weighed out and vacuum dried at 200° C. for several hours. The BET specific surface area (m ² /g) of the obtained dried sample was measured using a specific surface area measuring device (BELSORP MINI II manufactured by Microtrack Bell). The specific surface area measuring device is a device for measuring the nitrogen adsorption amount by a gas adsorption method using nitrogen gas. The specific surface area measuring device analyzes the measurement results by a multi-point method based on the BET method, and automatically obtains the BET specific surface area (m ² /g). The BET specific surface area of each sample is shown in Table 1. In the following description, the BET specific surface area may be expressed as "X".

The mesopore specific surface area of each sample was measured with the above specific surface area measuring device. The mesopore specific surface area is obtained in the process of obtaining the BET specific surface area. The specific surface area measuring device obtains the mesopore specific surface area by analyzing the nitrogen adsorption isotherm obtained in the nitrogen adsorption process by the BJH method. Analysis by the BJH method uses a standard isotherm stored in the measuring device. For the analysis, standard isotherms of substances similar to the surface chemistry of the sample are used. The standard isotherm used in this example is that of graphitized carbon black. The mesopore specific surface area of each sample is shown in Table 1. In the following description, the mesopore specific surface area may be expressed as "Y". Table 1 shows the ratio of the mesopore specific surface area to the BET specific surface area in percentage (%). In the following description, the ratio is expressed as "Y/X".

　Made multiple RF batteries. Each RF cell was sample no. 1 to sample no. 9 electrodes. The positive and negative electrodes in each RF cell were the same. The electrolyte used in the RF battery was a manganese-titanium based electrolyte.

To evaluate the battery reactivity of each sample electrode, the cell resistivity (Ω·cm ² ) of each RF battery was measured. A low cell resistivity in an RF battery means that the electrode has good battery reactivity. The cell resistivity measurement procedure is as follows. The RF battery of each sample was charged and discharged at a constant current with a current density of 256 mA/cm ² . In this test, multiple cycles of charging and discharging were performed. In the test, the upper and lower limits of the switching voltage were set, and when the voltage reached the upper limit during charging, the battery was switched to discharging, and when the voltage reached the lower limit during discharging, the battery was switched to charging. After each cycle of charging and discharging, the cell resistivity (Ω·cm ² ) was determined for each sample. The cell resistivity is obtained by obtaining the average voltage during charging and the average voltage during discharging in any one cycle among multiple cycles, {(difference between average voltage during charging and average voltage during discharging) / (average current / 2)} × It was obtained from the cell effective area. The temperature of the electrolyte was 35°C. Here, when the mesopore specific surface area increases, the resistance component derived from the flow resistance of the electrolytic solution increases, making it difficult to decrease the cell resistivity. In this test, in order to appropriately evaluate the degree of reduction in cell resistivity based on the BET specific surface area, mesopore specific surface area, and the size of Y/X, the cell resistance was obtained by eliminating the effect of flow resistance from the actually measured cell resistivity. asked for a rate. Table 1 shows the cell resistivity.

　In order to evaluate the durability of the electrode of each sample, the electrode weight reduction rate (%/year) was obtained according to the following procedure. First, the RF battery after measuring the cell resistivity was disassembled, and the positive electrode and the negative electrode were taken out. The positive electrode was consumed more than the negative electrode. Therefore, the electrode weight reduction rate of the positive electrode, which is rapidly consumed, is used as an index of the durability of the electrode in each sample. In obtaining the electrode weight reduction rate, the weight reduction rate of the positive electrode after charging and discharging was obtained. The reduction rate is (W1-W2)/W1 expressed as a percentage (%). W1 is the weight of the positive electrode measured prior to incorporation into the RF battery. W2 is the weight of the positive electrode measured after washing and drying the positive electrode taken out. A value obtained by annualizing the reduction rate is the electrode weight reduction rate. Table 1 shows the values of the electrode weight reduction rate.

As shown in Table 1, sample No. having an electrode in which Y/X is 40% or more and Y is 1.0 m ² /g or more and less than 30 m ² /g. 1 to sample no. The cell resistivity of RF battery No. 9 was 0.93 Ω·cm ² or less. This cell resistivity value is obtained from sample No. 1 having a large BET specific surface area. 10 and sample no. It was comparable to the cell resistivity of RF battery 1 of 11. Therefore, sample no. 1 to sample no. 9 electrode was found to be excellent in battery reactivity.

Sample no. 1 to sample no. The electrode weight reduction rate of sample No. 9 is the same as that of sample No. 9. 10 and sample no. It was much smaller than the electrode weight reduction rate in No. 11. Therefore, it was found that electrode wear is greatly suppressed when the Y/X ratio of the electrode is 40% or more and the Y ratio is 1.0 m ² /g or more and less than 30 m ² /g.

Sample no. 1 to sample no. When comparing RF batteries No. 9, sample No. 9 with a BET specific surface area of 40 m ² /g or less. 1 to sample no. The electrode weight reduction rate in the RF battery of No. 8 is that of the sample No. 8. It was significantly lower than the electrode weight loss rate in No. 9 RF battery. Therefore, in addition to limiting Y/X and Y, limiting X has also been found to have a significant effect on electrode durability.

Sample no. 1 to sample no. 9, it was found that if the mesopore specific surface area is 15 m ² /g or less, and further 10 m ² /g or less, the electrode weight reduction rate tends to be low. However, when the mesopore specific surface area decreases, the cell resistivity tends to increase. When fabricating RF batteries, a balance between battery reactivity and durability should be considered.

Sample No. where Y/X is less than 40%. The cell resistivity of 10 RF batteries was low. Sample no. The reason for the low cell resistivity of the RF battery of sample no. This is presumably because the BET specific surface area and the mesopore specific surface area of No. 10 electrodes are large. The larger the BET specific surface area and the mesopore specific surface area, the larger the contact area between the electrode and the electrolytic solution. However, because the contact area was too large, sample No. The weight loss rate of 10 electrodes was over 100%/year.

Sample No. Y/X is 40% or more, but Y is 30 m ² /g or more. The cell resistivity of 11 RF batteries was low. Sample no. The reason for the low cell resistivity of the RF battery of sample no. It is presumed that this is because the BET specific surface area and the mesopore specific surface area of the electrode No. 11 are large. However, as described above, the contact area between the electrode and the electrolytic solution was too large, so sample No. The weight reduction rate of 11 electrodes was 88%/year or more.

1 Redox flow battery (RF battery)
2 positive electrode 3 negative electrode 4 carbon fiber 5 carbon particles 6 carbon binder residue 10 battery cell 12 positive electrode tank 13 negative electrode tank 20 substrate 21 mesopores 22 catalyst 25 micropores 26 macropores 30 cell frame 31 bipolar plate 32

frame

33, 34

liquid supply manifold

35, 36

drainage manifold

33s, 34s

liquid supply slit

35s, 36s drainage slit 80 AC/DC converter 81 substation 90 power system 91 power generation unit 92 load 100 Cell stack 101 Diaphragm 102 Positive electrode cell 103

Negative electrode cell

108, 109 Forward piping 110, 111 Return piping 112, 113 Pump 200 Substack 210 Supply/discharge plate 220 End plate 230 Tightening mechanism

Claims

An electrode comprising a porous substrate,
The ratio of the mesopore specific surface area to the BET specific surface area is 40% or more,
The mesopore specific surface area is 1.0 m 2 /g or more and less than 30 m 2 /g,
electrode.
2. The electrode according to claim 1, wherein said BET specific surface area is less than 40 m< 2 >/g.
The electrode according to claim 1 or claim 2, which is used for the positive electrode of a redox flow battery.
The electrode according to any one of claims 1 to 3, wherein the base material contains, as a conductive material, one or more elements selected from the group consisting of carbon, titanium, and tungsten.
The electrode according to any one of claims 1 to 4, wherein the substrate contains at least one selected from the group consisting of carbon fibers, carbon particles, and carbon binder residues.
comprising a catalyst held on the substrate;
The electrode according to any one of claims 1 to 5, wherein the catalyst is made of a non-carbon material.
The electrode according to claim 6, wherein the catalyst is metal oxide or metal carbide.
An electrode according to any one of claims 1 to 7,
battery cell.
A battery cell comprising the battery cell according to claim 8,
cell stack.
comprising a cell stack according to claim 9,
redox flow battery.