JP2024075815A - Electrode, battery cell, cell stack, and redox flow battery - Google Patents

Abstract

The present invention provides an electrode having excellent battery reactivity and durability. The electrode has a porous substrate, and 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 and less than 30 m2/g. [Selected Figure]

Description

The present disclosure relates to electrodes, battery cells, cell stacks, and redox flow batteries.

Redox flow batteries are known as large-capacity storage batteries. For example, the redox flow battery shown in Patent Document 1 has a cell stack in which multiple battery cells are stacked. Charging and discharging are performed by circulating positive and negative electrode electrolytes through the battery cells.

The electrodes in the battery cells are made of conductive and liquid-permeable porous materials. For example, Patent Document 1 discloses an electrode made of an aggregate of carbon fibers.

JP 2002-246035 A

To improve the battery reaction at the electrode, it is preferable that the surface area of the electrode in contact with the electrolyte is large. However, in an electrolyte with high oxidizing power, the electrode gradually wears out due to the oxidizing power of the electrolyte. In particular, in a positive electrode electrolyte with high oxidizing power, an electrode with a large contact area with the electrolyte is easily worn out. When an electrode wears out, the battery performance of the battery cell that includes that electrode decreases.

Therefore, one of the objectives of this disclosure is to provide an electrode with excellent battery reactivity and durability. Another objective of this disclosure is to provide a battery cell, a cell stack, and a redox flow battery that can suppress the deterioration of battery performance during operation.

The electrode of the present disclosure comprises:
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 comprises:
The electrode of the present disclosure is provided.

The cell stack of the present disclosure includes:
The battery cell of the present disclosure is provided.

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

The electrodes disclosed herein have excellent battery reactivity and durability.

The battery cells, cell stacks, and redox flow batteries disclosed herein maintain excellent battery performance over the long term.

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

[Description of the embodiments of the present disclosure]
The present inventors have intensively studied the configuration of a battery having excellent battery reactivity and durability. As already explained, there is a trade-off between improved battery reactivity and durability in an electrode. In consideration of this point, the present inventors have arrived at a technical idea of defining the ratio of the specific mesopore surface area in an electrode and the upper limit of the specific mesopore 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. Hereinafter, embodiments of the present disclosure will be described in a list.

<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. Mesopores are pores with a diameter of 2 nm to 50 nm. Micropores are pores that are narrower than mesopores. Macropores are pores that are wider than mesopores. This definition conforms to the definition of the International Union of Pure and Applied Chemistry. Mesopores are more effective at improving battery reactions than macropores and micropores. This is because the electrolyte can easily enter the mesopores and the contact area between the inner surface of the mesopores and the electrolyte is large. The electrolyte cannot easily enter micropores that are narrower than mesopores. The electrolyte can easily enter macropores that are wider than mesopores, but the contact area of the macropores with 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 of the mesopore specific surface area are specified. Therefore, in the electrode according to the embodiment, although the ratio of mesopores that improve the battery reaction is high, the contact area with the electrolyte is not too large. Therefore, the electrode according to the embodiment has excellent battery reactivity and durability.

<2> As one embodiment of the electrode according to the present invention,
The BET specific surface area may be less than 40 m ² /g.

In the configuration of the above embodiment <2>, the BET specific surface area is not too large. In other words, the contact area between the electrode and the electrolyte is not too large. Therefore, even if the electrode is exposed to the electrolyte for a long period of time, extreme wear of the electrode is suppressed.

<3> As one embodiment of the electrode according to the present invention,
Examples include a form used in 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 have excellent durability. Therefore, the electrodes according to the embodiments are suitable as positive electrodes exposed to a positive electrode electrolyte with a high oxidizing power.

<4> As one embodiment of the electrode according to the present invention,
The substrate may include, as a conductive material, one or more elements selected from the group consisting of carbon, titanium, and tungsten.

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

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

By producing an electrode using at least one of carbon fibers and carbon particles as a raw material, it becomes easy to adjust the BET specific surface area and mesopore specific surface area of the electrode. For example, the BET specific surface area and mesopore specific surface area of the electrode can be adjusted by limiting the fiber length and fiber diameter of the carbon fibers. Also, the BET specific surface area and mesopore specific surface area of the electrode can be adjusted by limiting the particle size of the carbon particles. Here, the carbon binder residue is the carbonized binder. The binder bonds the carbon fibers together, bonds the carbon fibers and carbon particles, or bonds the carbon particles together during the manufacture of the electrode.

<6> As one embodiment of the electrode according to the present invention,
A catalyst supported on the substrate;
The catalyst may be formed of a non-carbon material.

Catalysts promote cell reactions. Therefore, having a catalyst supported on the substrate improves the cell reactivity of the electrode.

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

Metal oxide or metal carbide catalysts are difficult to oxidize. This makes it easier for the electrodes to maintain excellent battery reactivity for a long period of time.

<8> The battery cell according to the embodiment is
The electrode includes any one of the electrodes according to the above configuration <1> to <7>.

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

<9> The cell stack according to the embodiment is
The battery cell according to the above embodiment <8> is included.

The cell stack according to the embodiment is likely to maintain excellent battery performance over a long period of time. This is because the electrodes according to 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 according to the above embodiment <9> is provided.

The redox flow battery according to the embodiment is likely to maintain 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 wear out.

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

Example 1
<Overview of RF batteries>
An RF battery 1 according to an embodiment will be described with reference to Fig. 1 to Fig. 3. The RF battery 1 shown in Fig. 1 is charged and discharged with a positive electrode electrolyte solution containing a positive electrode active material and a negative electrode electrolyte solution containing a negative electrode active material. The positive electrode active material and the negative electrode active material are typically metal ions whose valence changes due to oxidation-reduction. The metal ions contained in the positive electrode electrolyte solution and the negative electrode electrolyte solution shown in Fig. 1 are only an example. Fig. 1 illustrates a Ti-Mn-based RF battery that contains Mn ions as the positive electrode active material and Ti ions as the negative electrode active material. In Fig. 1, the solid arrow indicates a charging reaction, and the dashed arrow indicates a discharging reaction.

The RF battery 1 is typically connected to a power grid 90 via an AC/DC converter 80 and a substation 81. The RF battery 1 charges with electricity generated by a power generation unit 91, or discharges the charged electricity to a load 92. The power generation unit 91 may be a power generation facility that uses 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, momentary sag compensation, emergency power supply, and output smoothing of natural energy power generation.

<RF battery configuration>
The RF battery 1 includes a battery cell 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 separator 101. The separator 101 is an ion exchange membrane that is impermeable to electrons but is permeable to, for example, hydrogen ions. The positive electrode cell 102 contains a positive electrode 2. The negative electrode cell 103 contains a negative electrode 3.

The positive electrode cell 102 and the negative electrode cell 103 constituting the battery cell 10 are respectively supplied with a positive electrode electrolyte and a negative electrode electrolyte. The RF battery 1 of this example has an outward pipe 108 and a return pipe 110 that connect the battery cell 10 and the positive electrode tank 12. The RF battery 1 of this example has an outward pipe 109 and a return pipe 111 that connect the battery cell 10 and the negative electrode tank 13. Pumps 112 and 113 are provided in each of the outward pipes 108 and 109. The positive electrode electrolyte is supplied from the positive electrode tank 12 to the positive electrode cell 102 through the outward pipe 108 by the pump 112. 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 through the outward pipe 109 by the pump 113. The negative electrode electrolyte that passes through the negative electrode cell 103 and is discharged from 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.

As shown in Figs. 2 and 3, the RF battery 1 is usually used in a form called a cell stack 100 in which multiple battery cells 10 are stacked. The cell stack 100 has a configuration in which a substack 200 shown in Fig. 3 is sandwiched between two end plates 220 from both sides. The two end plates 220 are fastened in a direction approaching each other by a fastening mechanism 230. Fig. 3 shows a cell stack 100 having multiple substacks 200. The substack 200 has a laminate in which a cell frame 30, a positive electrode 2, a diaphragm 101, and a negative electrode 3 are repeatedly stacked in this order. Supply and discharge plates 210 are arranged on both ends of the laminate. The supply and discharge plates 210 are connected to the forward pipes 108 and 109 and the return pipes 110 and 111 shown in Figs. 1 and 2 that constitute the above-mentioned circulation flow path. 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 body 32 provided around the bipolar plate 31. The positive electrode 2 is arranged to face one side of the bipolar plate 31. The negative electrode 3 is arranged to face the other side of the bipolar plate 31. The positive electrode 2 and the negative electrode 3 are stored inside the frame body 32, sandwiching the bipolar plate 31. A battery cell 10 is formed by placing the positive electrode 2 and the negative electrode 3 between the bipolar plates 31 of adjacent cell frames 30, with a diaphragm 101 sandwiched between them.

The frame body 32 of the cell frame 30 is provided with the liquid supply manifolds 33, 34, the liquid discharge manifolds 35, 36, the liquid supply slits 33s, 34s, and the 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 liquid discharge manifold 35 through the liquid discharge 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 discharged to the liquid discharge manifold 36 through the liquid discharge slit 36s. The liquid supply manifolds 33, 34 and the liquid discharge manifolds 35, 36 are provided penetrating the frame body 32, and the cell frames 30 are stacked to form flow paths for each electrolyte. Each of these flow paths is connected to the forward piping 108, 109 and the return piping 110, 111 shown in FIG. 1 via the supply and discharge plate 210. The cell stack 100 is capable of distributing the positive electrode electrolyte and the negative electrode electrolyte to the battery cell 10 through each of the above flow paths.

(Electrolyte)
The positive electrode electrolyte circulated to the battery cell 10 contains a positive electrode active material. Typical examples of the positive electrode active material include vanadium (V) ions, iron (Fe) ions, copper (Cu) ions, and manganese (Mn) ions. The negative electrode electrolyte contains a negative electrode active material. Typical examples of the negative electrode active material include V ions, chromium (Cr) ions, titanium (Ti) ions, cobalt (Co) ions, Cu ions, and zinc (Zn) ions.

The positive electrode active material and the negative electrode active material may have different types of metals that become ions, or may have the same types. Representative 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, a configuration in which the positive electrode active material contains Mn ions and the negative electrode active material contains Ti ions can provide a high electromotive force.

As already explained, the electrolyte of the RF battery 1 shown in Figs. 1 and 2 is a Mn-Ti-based electrolyte containing the active material shown in 3. above. 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 the positive electrode active material. In the negative electrode electrolyte, Ti ions function as the negative electrode active material. When Mn ions are used as the positive electrode active material, trivalent Mn ions (Mn ³⁺ ) are unstable, so that Mn ³⁺ in the positive electrode electrolyte may precipitate as MnO ₂ during charging. When the positive electrode electrolyte contains Ti ions, the precipitation of Mn ions can be suppressed by the Ti ions. The Ti ions contained in the positive electrode electrolyte and the 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 making the metal ion species in both electrolytes equal.

The solvent for the positive electrode electrolyte and the negative electrode electrolyte is preferably an acidic aqueous solution. Examples of the solvent include an aqueous solution of sulfuric acid (H ₂ SO ₄ ) and an aqueous solution of phosphoric acid (H ₃ PO ₄ ). In particular, an aqueous solution of sulfuric acid is preferable. The aqueous solution of sulfuric acid may contain phosphoric acid.

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

The electrode 2 in this example includes a porous substrate 20. The electrode 2 may further include a catalyst 22 supported on the substrate 20. The porous substrate 20 includes 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 pores that are thinner than the mesopores 21. The macropores 26 are pores that are thicker than the mesopores 21. In FIG. 4, these pores are shown in an exaggerated manner. In addition, in the schematic diagram of FIG. 4, the pores 21, 25, and 26 are shown individually, but in reality, the mesopores 21, the micropores 25, and the macropores 26 may be mixed in one pore.

In order to improve the reactivity between the electrode 2 and the electrolyte, it is preferable to increase the surface area of the electrode 2. On the other hand, if the surface area of the electrode 2 is large, the electrode 2 becomes more susceptible to wear by the electrolyte. The electrode 2 is worn out because the electrolyte containing the active material has an oxidizing power. The oxidizing power of the positive electrode electrolyte, particularly the positive electrode electrolyte containing Mn as an active material, is higher than the oxidizing power of the negative electrode electrolyte. Therefore, the electrode 2 used in the positive electrode cell 102 (FIGS. 1 and 2) is more susceptible to wear 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 substrate 20 and the inner peripheral surfaces of all holes 21, 25, 26 including the mesopore 21, divided by the mass (g) of the electrode 2. The surface area of the outer peripheral surface of the substrate 20 does not include the openings of the holes 21, 25, 26. Meanwhile, Y is the mesopore specific surface area (m ² /g). The mesopore specific surface area is the total area (m ² ) of the inner peripheral surfaces of the mesopores 21 divided 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 the mesopore specific surface area to the BET specific surface area.

If Y/X is 40% or more, it can be said that the ratio of mesopore specific surface area to the BET specific surface area is high. Compared to micropores 25 and macropores 26, mesopores 21 are more effective at improving the battery reactivity of electrode 2. Therefore, an electrode 2 with Y/X of 40% or more has excellent battery reactivity. Here, in the electrode 2 of this example, since the upper limit of the mesopore specific surface area is also specified, the upper limit of the BET specific surface area at Y/X is also limited to a certain 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 electrode 2 while suppressing wear of electrode 2 can be obtained.

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

The mesopore specific surface area of the electrode 2 in this example is 1.0 m ² /g or more and less than 30 m ² /g. If the mesopore specific surface area is smaller, the number of mesopores 21 that contribute to improving the battery reaction is reduced. If the mesopore specific surface area is larger, the surface area of the electrode 2 including the mesopores 21 is larger. In consideration of the balance between improving the battery reaction and durability, the mesopore specific surface area is, for example, 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 the 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 electrolyte, the more likely the electrode 2 is to be consumed. 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 electrolyte will not be too large. A more preferable BET specific surface area is 35 m ² /g or less. An even 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 electrolyte will be too small. The BET specific surface area is 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 substrate 20 contributes to the battery reaction. There are no particular limitations on the material of the substrate 20 as long as it is conductive. The material of the substrate 20 includes, for example, one or more elements selected from the group consisting of carbon (C), titanium (Ti), and tungsten (W). The substrate 20 may be composed of a single element or a compound of the above elements. The material of the substrate 20 is determined, for example, by X-ray diffraction.

An example of the carbon-containing substrate 20 is a substrate 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 portion of one carbon fiber 4 constituting the substrate 20. As shown in FIG. 5, the carbon-containing substrate 20 is preferably a substrate 20 in which the carbon fibers 4 and the carbon particles 5 are bound by the carbon binder residue 6. In the substrate 20 shown in FIG. 5, pores are formed by the gaps between the carbon fibers 4 to which the carbon particles 5 are attached.

The carbon fiber 4 plays a role in maintaining the three-dimensional structure of the electrode 2. The carbon fiber 4 is, for example, a fiber obtained by carbonizing a precursor composed of organic fibers. Examples of the organic fiber include acrylic fiber, phenol fiber, cellulose fiber, isotropic pitch fiber, and anisotropic pitch fiber. Examples of the acrylic fiber include acrylonitrile.

The carbon particles 5 increase the surface area of the electrode 2 and improve the battery reactivity of the electrode 2. The carbon particles 5 may be natural graphite or artificial graphite.

The carbon binder residue 6 serves to bind the carbon fibers 4 and the carbon particles 5 together. The carbon binder residue 6 is formed when the binder that binds the carbon fibers 4 and the carbon particles 5 together is carbonized by heat treatment during the manufacture of the electrode 2. Examples of binders include pitches such as coal tar pitch or coal-based pitch. Other examples of binders include resins such as polyacrylonitrile, and rubbers such as acrylonitrile-butadiene rubber.

The substrate 20 shown in FIG. 5 can be manufactured, for example, by attaching carbon particles 5 and a binder to carbon fibers 4, followed by a carbonization process, a graphitization process, and an oxidation treatment process. Any known method can be applied to each process.

The carbon particles 5 and binder can be attached to the carbon fibers 4, for example, by the following method. The binder is heated to melt, and the carbon particles 5 are dispersed in the obtained molten liquid. The carbon fibers 4 are immersed in the molten liquid in which the carbon particles 5 are dispersed. The binder is then solidified. Here, by controlling the weight ratio of the carbon fibers 4, the carbon particles 5, and the binder, an electrode 2 is obtained in which Y/X is 40% or more and Y is 1.0 ^m2 /g or more and less than 30 ^m2 /g.

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

The graphitization process is carried out to increase the crystallinity of the carbon binder residue 6 and other materials. It is a process in which heating is carried out at a higher temperature than in the carbonization process. The graphitization process is preferably carried out in an inert atmosphere such as a nitrogen atmosphere.

The oxidation treatment process is carried out to introduce oxygen functional groups onto the surface of the substrate 20. Examples of oxygen functional groups include hydroxyl groups, carbonyl groups, quinone groups, lactone groups, and free radical oxides. The oxygen functional groups contribute to improving the battery reaction of the substrate 20. They also improve the wettability of the substrate 20 to the electrolyte. Examples of oxidation treatment include wet chemical oxidation, electrolytic oxidation, and dry oxidation.

The substrate 20 may hold a catalyst 22 that promotes the cell reaction, as shown in FIG. 4. The catalyst 22 is exaggerated in FIG. 4. The catalyst 22 is, for example, a granular material. The catalyst 22 is a non-carbon-based material. For example, the catalyst 22 may be a metal oxide or a metal carbide. The metal oxide may be a composite oxide of the first and second elements listed below.

The first element and the second element are different elements. 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, the first element is difficult to dissolve in the electrolyte, and can exist for a long period of time. The second element contributes to improving the battery reaction of the electrode 2. The composition of the catalyst 22 is determined by X-ray diffraction.

<Action and effect>
The electrode 2 in which Y/X is 40% or more and Y is 1.0 ^m2 /g or more and less than 30 ^m2 /g has excellent battery reactivity and durability. Therefore, the battery cell 10 (FIG. 1), cell stack 100 (FIG. 3), and RF battery 1 including this electrode 2 maintain excellent battery performance for a long period of time.

<Test Example>
<Test Example 1>
In Test Example 1, the influence of the ratio of the specific surface area of mesopores in an electrode and the size of the specific surface area of mesopores on the battery reactivity and durability of the electrode was examined.

Positive and negative electrodes were prepared for Samples No. 1 to No. 9, which have different BET specific surface areas and mesopore specific surface areas. The positive and negative electrodes were the same. Both electrodes were carbon electrodes containing carbon fibers, carbon particles, and carbon binder residue. Hereinafter, when simply referred to as "electrode," it means both the positive and negative electrodes.

The BET specific surface area of each sample was measured as follows. Approximately 500 mg of the sample was weighed and dried in vacuum at 200° C. for several hours. The BET specific surface area (m ² /g) of the dried sample was measured using a specific surface area measuring device (Microtrack Bell; BELSORP MINI II). The specific surface area measuring device is a device that measures the amount of nitrogen adsorption by a gas adsorption method using nitrogen gas. The specific surface area measuring device analyzes the measurement results by a multipoint method based on the BET method, and automatically determines 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 represented as "X".

The mesopore specific surface area of each sample was measured using the 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 using the BJH method. In the analysis using the BJH method, a standard isotherm stored in the measuring device is used. In the analysis, a standard isotherm of a substance similar to the chemical properties of the surface of the sample is used. The standard isotherm used in this example is the standard isotherm of graphitized carbon black. The mesopore specific surface area of each sample is shown in Table 1. In the following explanation, the mesopore specific surface area may be represented as "Y". In Table 1, the ratio of the mesopore specific surface area to the BET specific surface area is shown as a percentage (%). In the following explanation, the ratio is represented as "Y/X".

Using the electrodes of Sample No. 1 to Sample No. 9, a single-cell RF battery was fabricated. The positive and negative electrodes of the RF battery were the same. The electrolyte used in the RF battery was a manganese-titanium electrolyte.

In order to evaluate the battery reactivity of the electrodes of each sample, the cell resistivity (Ω·cm ² ) of the RF battery was measured. A low cell resistivity means that the battery reactivity of the electrodes is good. The procedure for measuring the cell resistivity is as follows. Using the RF battery of each sample, charging and discharging were performed 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 charging was switched to discharging, and when the voltage reached the lower limit during discharging, the charging was switched to charging. After each cycle of charging and discharging, the cell resistivity (Ω·cm ² ) was obtained for each sample. The cell resistivity was obtained by determining the average charging voltage and the average discharging voltage in any one cycle out of the multiple cycles, and calculating {(difference between the average charging voltage and the average discharging voltage)/(average current/2)}×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 circulation resistance of the electrolyte increases, making it difficult to reduce 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 magnitude of Y/X, the cell resistivity was calculated by excluding the influence of distribution resistance from the actually measured cell resistivity. The cell resistivity is shown in Table 1.

To evaluate the durability of the electrodes of each sample, the electrode weight loss rate (%/year) was calculated according to the following procedure. First, the RF battery after measuring the cell resistivity was disassembled, and the positive and negative electrodes were taken out. The positive electrode was more worn out than the negative electrode. Therefore, the electrode weight loss rate of the positive electrode, which was more worn out, was used as an index of the electrode durability of each sample. To calculate the electrode weight loss rate, the weight loss rate of the positive electrode after charging and discharging was calculated. The loss rate is (W1-W2)/W1 expressed as a percentage (%). W1 is the weight of the positive electrode measured before being incorporated into the RF battery. W2 is the weight of the positive electrode measured after the removed positive electrode was washed and dried. The annualized value of the loss rate is the electrode weight loss rate. The electrode weight loss rate values are shown in Table 1.

As shown in Table 1, the cell resistivity of the RF batteries of Sample No. 1 to Sample No. 9 using electrodes 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 was 0.93 Ω·cm ² or less. This cell resistivity value was comparable to the cell resistivity of RF battery 1 of Sample No. 10 and Sample No. 11, which had a large BET specific surface area. Therefore, it was found that the electrodes of Sample No. 1 to Sample No. 9 had excellent battery reactivity.

The electrode weight loss rates of Samples No. 1 to No. 9 were significantly smaller than those of Samples No. 10 and 11. Therefore, it was found that the electrode consumption was significantly suppressed when the Y/X ratio of the electrode was 40% or more and Y was 1.0 m ² /g or more and less than 30 m ² /g.

Comparing the RF batteries of Sample No. 1 to Sample No. 9, the electrode weight loss rate in the RF batteries of Sample No. 1 to Sample No. 8, which have a BET specific surface area of 40 m ² /g or less, was significantly lower than the electrode weight loss rate in the RF battery of Sample No. 9. Therefore, it was found that in addition to limiting Y/X and Y, limiting X also has an important effect on the durability of the electrode.

Comparing the RF batteries of Sample No. 1 to Sample No. 9, it was found that the electrode weight loss rate is likely to be low if the mesopore specific surface area is 15 m ² /g or less, and further 10 m ² /g or less. However, if the mesopore specific surface area is small, the cell resistivity is likely to be large. When manufacturing an RF battery, the balance between battery reactivity and durability should be considered.

The cell resistivity of the RF battery of sample No. 10, in which Y/X is less than 40%, was low. It is presumed that the reason for the low cell resistivity of the RF battery of sample No. 10 is that the BET specific surface area and mesopore specific surface area of the electrodes of sample No. 10 are large. If the BET specific surface area and mesopore specific surface area are large, the contact area between the electrode and the electrolyte becomes large. However, because the contact area was too large, the weight loss rate of the electrodes of sample No. 10 was more than 100% per year.

Although Y/X is 40% or more, the cell resistivity of the RF battery of sample No. 11, in which Y is 30 ^m2 /g or more, was low. The reason for the low cell resistivity of the RF battery of sample No. 11 is presumed to be due to the large BET specific surface area and mesopore specific surface area of the electrode of sample No. 11. However, since the contact area between the electrode and the electrolyte is too large as described above, the weight loss rate of the electrode of sample No. 11 was 88% or more per year.

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 Liquid drainage manifold 33s, 34s Liquid supply slit, 35s, 36s Liquid drainage slit 80 AC/DC converter, 81 Transformer 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 and discharge plate, 220 End plate, 230 Fastening mechanism

Claims (10)

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 of claim 1, wherein the BET specific surface area is less than 40 m ² /g. The electrode according to claim 1 or 2, which is used as a positive electrode of a redox flow battery. The electrode according to any one of claims 1 to 3, wherein the substrate contains one or more elements selected from the group consisting of carbon, titanium, and tungsten as a conductive material. The electrode according to any one of claims 1 to 4, wherein the substrate comprises at least one selected from the group consisting of carbon fibers, carbon particles, and carbon binder residues. A catalyst supported on the substrate;
The electrode according to claim 1 , wherein the catalyst is made of a non-carbon material. The electrode according to claim 6, wherein the catalyst is a metal oxide or a metal carbide. Equipped with the electrode according to any one of claims 1 to 7.
Battery cell. A battery cell according to claim 8,
Cell stack. A cell stack according to claim 9,
Redox flow battery.

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