WO2020184450A1 - Carbon positive electrode material for manganese/titanium-based redox flow battery, and battery provided with same - Google Patents

Abstract

This carbon positive electrode material for a manganese/titanium-based redox flow battery comprises carbon fibers (A), graphite particles (B), and a carbon material (C) for binding the carbon fibers (A) and the graphite particles (B), and satisfies the following conditions: (1) the particle diameter of the graphite particles (B) is at least 1 µm; (2) if the size of crystallites of the graphite particles (B) in the c-axis direction, determined by X-ray diffraction, is Lc(B), Lc(B) is at least 35 nm; (3) if the size of crystallites of the carbon material (C) in the c-axis direction, determined by X-ray diffraction, is Lc(C), Lc(C) is at least 10 nm; (4) if the size of crystallites of the carbon fibers (A) in the c-axis direction, determined by X-ray diffraction, is Lc(A), Lc(C)/Lc(A) is at least 1.0; (5) the number of bound oxygen atoms on the surface of the carbon positive electrode material is less than 1.0% of the total number of carbon atoms on the surface of the carbon positive electrode material; and (6) the BET specific surface area, determined on the basis of nitrogen adsorption quantity, is 0.5–1.5 m²/g.

Description

Carbon positive electrode material for manganese / titanium redox flow batteries and batteries equipped with it

The present invention relates to a carbon electrode material used for a positive electrode of a manganese / titanium-based redox flow battery. More specifically, the present invention has excellent oxidation resistance and low resistance, and is excellent in energy efficiency of the entire manganese / titanium-based redox flow battery. Regarding carbon positive electrode material.

The redox flow battery is a battery that utilizes redox in an aqueous solution of redox flow ions, and is a large-capacity storage battery with extremely high safety because it is a mild reaction only in the liquid phase.

As shown in FIG. 1, the main configuration of the redox flow battery is composed of external tanks 6 and 7 for storing electrolytic solutions (positive electrode electrolytic solution, negative electrode electrolytic solution) and an electrolytic cell EC. In the electrolytic cell EC, the ion exchange membrane 3 is arranged between the opposing current collector plates 1 and 1. In the redox flow battery, the electrolytic solution containing the active material is sent from the external tanks 6 and 7 to the electrolytic cell EC by the pumps 8 and 9, and the electrochemical energy conversion, that is, is performed on the electrode 5 incorporated in the electrolytic cell EC. Charging and discharging are performed. As the material of the electrode 5, a carbon material having chemical resistance, conductivity, and liquid permeability is used.

As the electrolytic solution used in the redox flow battery, an aqueous solution containing a metal ion whose valence changes due to redox is typically used. The electrolytic solution has changed from a type in which an aqueous solution of iron hydrochloric acid is used for the positive electrode and an aqueous solution of chromium in hydrochloric acid for the negative electrode to a type in which a sulfuric acid aqueous solution of vanadium having a high electromotive force is used for both electrodes, and the energy density has been increased.

In the case of a redox flow battery in which vanadium oxysulfate is used as the positive electrode electrolyte and vanadium sulfate is used as the negative electrode electrolytic solution, the electrolytic solution containing V ²⁺ is supplied to the liquid passage on the negative electrode side during discharge, and the positive electrode is positive. An electrolytic solution containing V ⁵⁺ (actually an ion containing oxygen) is supplied to the liquid passage on the side. In the liquid passage on the negative electrode side, V ²⁺ emits electrons in the three-dimensional electrode and is oxidized to V ³⁺ . The emitted electrons reduce V ⁵⁺ to V ⁴⁺ (actually oxygen-containing ions) in the three-dimensional electrode on the positive electrode side through an external circuit. Due to this redox reaction, SO ₄ ^2- in the negative electrode electrolyte is insufficient, and SO ₄ ^2- is excessive in the positive electrode electrolyte, so SO ₄ ^2- passes through the ion exchange membrane from the positive electrode side to the negative electrode side. The charge balance is maintained. Alternatively, the charge balance can be maintained by moving H ⁺ from the negative electrode side to the positive electrode side through the ion exchange membrane. When charging, the reaction opposite to discharging proceeds.

The electrode materials for redox flow batteries are particularly required to have the following performance.

1) Do not cause side reactions other than the desired reaction (high reaction selectivity), specifically, high current efficiency (η _I ).
2) The electrode reaction activity is high, specifically, the cell resistance (R) is small. That is, the voltage efficiency (η _V ) is high.
3) High battery energy efficiency (η _E ) related to 1) and 2) above.
η _E = η _I × η _V
4) The deterioration with repeated use is small (long life), specifically, the amount of decrease in battery energy efficiency (η _E ) is small.

For example, Patent Document 1 discloses a carbonaceous material having a specific pseudographite microcrystal structure with high crystallinity as an electrode material of an Fe—Cr battery capable of increasing the total energy efficiency of the battery. Specifically, it has pseudographite microcrystals having an average <002> plane spacing of 3.70Å or less and an average crystallite size of 9.0Å or more in the c-axis direction obtained by X-ray wide-angle analysis. A carbonaceous material having a total acidic functional group amount of at least 0.01 meq / g is disclosed.

Patent Document 2 describes a carbonaceous fiber made from polyacrylonitrile fiber as an electrode for an electric field layer of an iron-chromium-based redox flow battery or the like that enhances the energy efficiency of the battery and improves the charge / discharge cycle life. Carbon having a pseudo-graphite crystal structure with a <002> plane spacing of 3.50 to 3.60 Å obtained by line wide-angle analysis, and the number of bonded oxygen atoms on the carbon surface is 10 to 25% of the number of carbon atoms. Electrode materials are disclosed.

Patent Document 3 describes the <002> plane spacing obtained from X-ray wide-angle analysis as a carbon electrode material for vanadium-based redox flow batteries, which has excellent energy efficiency in the entire battery system and has little change in performance with long-term use. It has a pseudo-graphite crystal structure of 3.43 to 3.60 Å, a crystallite size of 15 to 33 Å in the c-axis direction, and a crystallite size of 30 to 75 Å in the a-axis direction, and XPS surface analysis. An electrode is disclosed in which the amount of surface acidic functional groups obtained from the above is 0.2 to 1.0% of the total number of surface carbon atoms, and the number of surface-bonded nitrogen atoms is 3% or less of the total number of surface carbon atoms.

Further, in Patent Document 4, the crystal structure on the carbonaceous fiber was obtained by X-ray wide-angle analysis as a carbon electrode material that enhances the overall efficiency of the vanadium-based redox flow battery and lowers the cell resistance at the time of initial charging. 002> It is composed of a carbon composite material to which carbon fine particles having an average primary particle diameter of 30 nm or more and 5 μm or less are attached, and the surface spacing is 3.43 to 3.70 Å. An electrode material having a obtained <002> plane spacing of 3.43 to 3.60 Å, a crystallite size of 15 to 35 Å in the c-axis direction, and a crystallite size of 30 to 75 Å in the a-axis direction. It is disclosed. In the above carbon composite material, carbonaceous fibers and carbon fine particles are preferably adhered to each other in close proximity or by an adhesive such as phenol resin, and by using the adhesive, carbon which is an electrochemical reaction field is used. It is stated that only the originally contacted portion of the carbonaceous fiber can be fixed without excessively reducing the surface of the quality fiber. In the column of Examples, the non-woven fabric is dipped in a solution containing 5% by weight (Example 1) of carbon fine particles (phenolic resin) or 5% by weight (Examples 2 to 4) of phenolic resin, and then carbonized. A carbonized fiber non-woven fabric obtained by a dry oxidation treatment is disclosed.

The development of the electrolytic solution used for the redox flow battery has been continued since then, and as an electrolytic solution having a higher electromotive force than the above-mentioned vanadium-based electrolytic solution and can be stably and inexpensively supplied, for example, Patent Document 5 (For example, a manganese / titanium-based electrolyte) has been proposed in which manganese is used for the positive electrode and chromium, vanadium, or titanium is used for the negative electrode.

Japanese Unexamined Patent Publication No. 60-232669 Japanese Unexamined Patent Publication No. 5-234612 Japanese Unexamined Patent Publication No. 2000-357520 Japanese Unexamined Patent Publication No. 2017-33758 Japanese Unexamined Patent Publication No. 2012-204135

However, the electrode material used for the vanadium-based electrolytic solution as in Patent Documents 2 to 4 is a redox flow battery using the manganese / titanium-based electrolytic solution described in Patent Document 5 (hereinafter, manganese / titanium-based redox flow battery). It has been found that when used as a carbonaceous electrode material (which may be abbreviated), the cell resistance increases remarkably during the initial charge and discharge, and the battery energy efficiency decreases.

As shown in the disproportionation reaction below, Mn ions are unstable in an aqueous solution and the reaction rate is slow, so that the cell resistance increases. It was also found that the electrode material deteriorates because the oxidizing power of Mn ions (positive electrode charging liquid) generated during charging is very strong. In particular, oxidation resistance to Mn ions is a characteristic strongly required for the positive electrode material of a manganese / titanium-based redox flow battery, and the above problem can be solved only by using the electrode material for the redox flow battery described in Patent Documents 2 to 4 described above. It was found that it was difficult to achieve both high oxidation resistance and low resistance.
_{^{2Mn 3+ + 2H 2 O⇔Mn 2+ +}} MnO 2 + 4H -

The present invention has been made in view of the above circumstances, and an object of the present invention is to improve the oxidation resistance to Mn ions (positive electrode charging liquid) even when a manganese / titanium-based electrolytic solution is used. It is an object of the present invention to provide a carbon positive electrode electrode material for a manganese / titanium-based redox flow battery, which can reduce cell resistance at the time of initial charge / discharge and improve battery energy efficiency.

The configuration of the carbon positive electrode material for a manganese / titanium-based redox flow battery according to the present invention that could solve the above problems is as follows.

1. 1. It is composed of carbonaceous fibers (A), graphite particles (B), and a carbonaceous material (C) that binds them together.
A carbon positive electrode material for a manganese / titanium-based redox flow battery, which is characterized by satisfying the following requirements.
(1) The particle size of the graphite particles (B) is 1 μm or more.
(2) When the size of the crystallites in the c-axis direction obtained by X-ray diffraction in the graphite particles (B) is Lc (B), Lc (B) is 35 nm or more.
(3) When the size of crystallites in the c-axis direction obtained by X-ray diffraction in the carbonaceous material (C) is Lc (C), Lc (C) is 10 nm or more.
(4) When the size of the crystallites in the c-axis direction obtained by X-ray diffraction in the carbonaceous fiber (A) is Lc (A), Lc (C) / Lc (A) is 1.0 or more.
(5) The number of bound oxygen atoms on the surface of the carbon positive electrode material is less than 1.0% of the total number of carbon atoms on the surface of the carbon positive electrode material.
(6) The BET specific surface area determined from the amount of nitrogen adsorbed is 0.5 to 1.5 m ² / g.
2. 2. The mass content of the graphite particles (B) and the carbonaceous material (C) with respect to the total amount of the carbonaceous fiber (A), the graphite particles (B), and the carbonaceous material (C) is 20% or more, respectively. The carbon positive electrode material according to 1 above, wherein the mass ratio of the carbonaceous material (C) to the graphite particles (B) is 0.2 to 3.0.
3. 3. The carbon positive electrode material according to 1 or 2 above, wherein the Lc (A) is 1 to 10 nm.
4. The carbon positive electrode material according to any one of 1 to 3 above, wherein the graphite particles (B) contain at least one selected from the group consisting of scaly graphite, flaky graphite, and expanded graphite.
5. A manganese / titanium-based redox flow battery provided with the carbon positive electrode material according to any one of 1 to 4 above.

The carbon positive electrode material of the present invention can realize both high oxidation resistance and low resistance, and is therefore particularly useful as a positive electrode material for manganese / titanium-based redox flow batteries. Further, the carbon positive electrode material of the present invention is suitably used for flow type and non-flow type redox batteries, or manganese / titanium-based redox flow batteries combined with a lithium, capacitor, and fuel cell system.

FIG. 1 is a schematic view of a redox flow battery. FIG. 2 is an exploded perspective view of a liquid flow type electrolytic cell having a three-dimensional electrode preferably used in the present invention. FIG. 3 shows No. 2A in Table 2A in Example 2 described later. It is an SEM photograph (magnification 100 times) of 1 (an example which satisfies the requirement of this invention). FIG. 4 shows No. 2A in Table 2A in Example 2 described later. 13 (comparative example not satisfying the requirements of the present invention) SEM photograph (magnification 100 times).

In particular, the present inventors have been diligently studying to provide a carbon electrode material preferably used for the positive electrode of a manganese / titanium-based redox flow battery using Mn ion as the positive electrode active material and Ti ion as the negative electrode active material. .. Unlike conventional V-based redox flow batteries and Fe-Cr-based redox flow batteries, it is important for manganese / titanium-based redox flow batteries to have oxidation resistance to Mn ions, but the electrodes proposed so far have been proposed. The material is not considered in this regard. Therefore, it has been found from the results of studies by the present inventors that it is difficult to achieve both oxidation resistance and low resistance when a conventional electrode material is used for a manganese / titanium-based redox flow battery.

In providing the above carbon positive electrode material, the present inventors first reviewed the requirements for particles exhibiting reactivity. Generally, carbon blacks such as acetylene black (acetylene soot), oil black (furness black, oil soot), and gas black (gas soot) are used as particles showing reaction activity in redox flow batteries; Soot, carbon fiber powder, carbon nanotubes (CNT, carbon nanotube), carbon nanofibers, carbon aerogel, mesoporous carbon, glassy carbon powder, activated carbon, graphene, graphene oxide, N-doped CNT, boron-doped CNT, Known carbon particles such as carbon particles such as fullerene, petroleum coke, acetylene coke, and smokeless carbon coke can be mentioned. Of these, carbon blacks having high reactivity and specific surface area and low crystallinity cannot be used because they are easily oxidized to the charging liquid of positive electrode manganese. On the other hand, it was not possible to exhibit sufficient reaction activity simply by using particles having high carbon crystallinity such as carbon particles such as CNT. Furthermore, these are rare and expensive, so they are not suitable as inexpensive electrode materials.

Therefore, the present inventors have focused on graphite particles as particles exhibiting reactivity, and have decided to adopt highly crystalline graphite particles satisfying the following requirements (1) and (2) as the graphite particles (B). did.
(1) The particle size is 1 μm or more. (2) When the size of the crystallites in the c-axis direction determined by X-ray diffraction in the graphite particles (B) is Lc (B), Lc (B) is 35 nm or more.
It has been found that if graphite particles satisfying these requirements are used, the carbon edge surface as a reaction field can be exposed without excess or deficiency, and both low resistance and high oxidation resistance can be achieved at the same time.

Further, as the carbonaceous material (C), it is a binding carbonaceous material that binds both carbonaceous fibers (A) and graphite particles (B), and satisfies the following requirements (3) and (4). We decided to use a highly crystalline carbonaceous material.
(3) When the size of the crystallite in the c-axis direction determined by X-ray diffraction is Lc (C), Lc (C) is 10 nm or more (4) Obtained by X-ray diffraction in the carbonaceous fiber (A). When the size of the crystallite in the c-axis direction is Lc (A), Lc (C) / Lc (A) is 1.0 or more. Here, "both carbonaceous fibers (A) and graphite particles (B)" (In other words, the carbonaceous material used in the present invention acts as a binder between the carbonaceous fiber and the graphite particles) means that the carbonaceous material acts as a binder on the surface of the carbonic fiber and the graphite particles and The inside (between carbonaceous fibers, including graphite particles) is strongly bonded, and when the electrode material as a whole is viewed, the carbonaceous fibers are covered with the carbonaceous material, and the surface of the graphite particles is exposed. It means that it is configured in.
However, it is preferable that the carbonaceous material after binding does not form a film. Here, "not in a film state" means that the carbonaceous material (C) does not form a webbed state like a whole foot (boxoku) or a foot in the carbonaceous fibers (A). This is because when the film state is formed, the liquid permeability of the electrolytic solution deteriorates, and the reaction surface area of the graphite particles cannot be effectively used.

For reference, FIG. 3 shows an SEM photograph showing a state in which both carbonaceous fibers (A) and graphite particles (B) are bound in the electrode material of the present invention. FIG. 3 shows No. 2A in Table 2A in Example 2 described later. 1 (an example of the present invention satisfying the requirements of the present invention) is an SEM photograph (magnification 100 times). From FIG. 3, the carbonaceous material (C) strongly binds the surface and the inside of the carbonaceous fiber (A) and the graphite particles (B), and the carbonaceous material (C) coats the carbonaceous fiber (A). It can be seen that the surface of the graphite particles (B) is exposed.
On the other hand, FIG. 4 is an SEM photograph showing a state in which both the carbonaceous fiber (A) and the graphite particles (B) are not bound in the electrode material of the present invention. FIG. 4 shows No. 2A in Table 2A in Example 2 described later. 13 (comparative example not satisfying the requirements of the present invention) SEM photograph (magnification 100 times).

In order to obtain such a bonded state, it is preferable to increase the content ratio of the carbonaceous material to the total amount of the carbonaceous fiber, the graphite particles and the carbonaceous material, and in the present invention, for example, it is 20% by mass or more. .. In this respect, the carbonaceous material in the present invention is different from the carbonaceous material described in Patent Document 4 described above. In Patent Document 4, the carbonaceous material used exhibits an action as a partial adhesive based on the idea that only the portion where the carbonaceous fiber and the carbon fine particles originally contacted can be fixed (adhered). This is because there is only recognition that it should be done. Therefore, in the examples of Patent Document 4, the content of the carbonaceous material is at most 14.4% by mass.

If such a binding carbonaceous material is used, the carbonaceous material strongly binds between carbonaceous fibers via graphite particles, so that an efficient conductive path can be formed, and the above-mentioned addition of graphite particles enables the formation of an efficient conductive path. It was found that the action was exerted more effectively and both low resistance and high oxidation resistance could be achieved.

Further, by using the highly crystalline carbonaceous materials of (3) and (4) above, not only the carbonaceous material itself is imparted with high oxidation resistance, but also the protective effect against oxidative deterioration of carbonaceous fibers is enhanced. It turned out to be. In the above-mentioned Patent Document 4, since the above (3) is not considered at all, it is considered that the desired oxidation resistance cannot be obtained.

Further, the carbon positive electrode material of the present invention satisfies the following requirement (5).
(5) When the number of bound oxygen atoms on the surface of the carbon positive electrode material is less than 1.0% of the total number of carbon atoms on the surface of the carbon positive electrode material, the ratio of the number of bound oxygen atoms to the total number of carbon atoms is calculated by O / C. It may be abbreviated.
According to the study results of the present inventors, in the manganese / titanium-based redox flow battery, unlike the conventional recognition, surprisingly, reducing the O / C to less than 1.0% results in lower resistance. It has become clear that both high oxidation resistance can be achieved.

This point will be described in detail.
Until now, in the electrode materials used for redox flow batteries, oxygen atoms introduced by oxidation treatment steps such as dry oxidation treatment after carbonization step and graphitization step have greatly contributed to the electrode reaction, resulting in low resistance. It was thought that it could be realized. Therefore, the O / C of the electrode material produced through the above oxidation treatment step was controlled as high as 1% or more.
However, according to the results of studies by the present inventors, in a manganese / titanium-based redox flow battery, durability such as oxidation resistance may be impaired due to defects in the carbon structure generated when an oxygen atom is introduced. It turned out for the first time. That is, the dry oxidation treatment carried out for introducing oxygen atoms sacrifices durability as the resistance is lowered, and there is a trade-off relationship.

Therefore, the present inventors used an electrode material in which the amount of oxygen atoms introduced was small and the O / C was controlled to less than 1.0% for a manganese / titanium-based redox flow battery, and surprisingly, the oxidation resistance was improved. We have found that low resistance can be achieved without loss. The reason for this is unknown in detail, but the site where the electrode reaction occurs is the edge surface of the graphite particles with a small amount of oxygen atoms introduced, and the presence or absence of oxygen atoms does not contribute to the electrode reaction as long as the graphite particles are exposed. It is considered that the durability of the carbonaceous material part was improved because the defects of the carbonaceous material part, which is responsible for the oxidation resistance, could be reduced by suppressing the amount of oxygen atoms introduced.

Further, the carbon positive electrode material of the present invention satisfies the following requirement (6).
(6) The BET specific surface area determined from the amount of nitrogen adsorbed is 0.5 to 1.5 m ² / g.
The BET specific surface area is also a requirement that contributes to both low resistance and high oxidation resistance.
Generally, when oxygen atoms are introduced, the BET specific surface area of the electrode material also increases with defects in the carbon structure. In the present invention, since the O / C is controlled to less than 1.0% from the viewpoint that the graphite particles need only be exposed, the BET specific surface area hardly increases and is derived from the carried graphite particles. When the BET surface area is within the above range, the effect of adding graphite particles is effectively exhibited and a desired effect can be obtained. Preferably, it is 0.5 to 1.0 m ² / g.

Since the electrode material of the present invention is configured as described above, it is possible to obtain an electrode having low resistance and long life by increasing reaction activity while maintaining high oxidation resistance. In particular, when used as an electrode material for an electrolytic cell of a positive electrode manganese-based redox flow battery, it is possible to reduce cell resistance during initial charge and discharge, improve battery energy efficiency, and improve oxidation resistance to positive electrode charging liquid. It becomes possible to provide an excellent carbon positive electrode material.

Hereinafter, the present invention will be described in detail for each constituent requirement with reference to FIG.

FIG. 2 is an exploded perspective view of a liquid flow type electrolytic cell preferably used in the present invention. In the electrolytic cell of FIG. 2, an ion exchange membrane 3 is arranged between two opposing current collector plates 1, 1, and spacers 2 are provided on both sides of the ion exchange membrane 3 along the inner surfaces of the current collector plates 1, 1. Passage passages 4a and 4b for the electrolytic solution are formed. The electrode material 5 is arranged in at least one of the liquid passages 4a and 4b. The current collector plate 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolytic solution. As shown in FIG. 2, if the electrode is composed of the electrode material 5 and the current collector plate 1 and the electrolytic solution passes through the electrode material 5 (three-dimensional electrode structure), the current collector plate 1 transports electrons. It is possible to improve the charge / discharge efficiency by using the entire surface of the pores of the electrode material 5 as an electrochemical reaction field while ensuring the above. As a result, the charging / discharging efficiency of the electrolytic cell is improved.

As described above, the electrode material 5 of the present invention is an electrode material using carbonaceous fiber (A) as a base material and carrying graphite particles (B) with a highly crystalline carbonaceous material (C). Satisfy the requirements of (6). The details of each requirement are as follows.

[Carbonaceous fiber (A)]
The carbonaceous fiber used in the present invention means a fiber obtained by heat-carbonizing a precursor of an organic fiber (details will be described later), and means a fiber in which 90% or more is composed of carbon in terms of mass ratio. (JIS L 0204-2). Precursors of organic fibers used as raw materials for carbonaceous fibers include acrylic fibers such as polyacrylonitrile; phenol fibers; PBO fibers such as polyparaphenylene benzobisoxazole (PBO); aromatic polyamide fibers; isotropic pitch and heterogeneity. Pitch fibers such as sex pitch fibers and mesophase pitch; cellulose fibers; and the like can be used. Among them, acrylic fiber, phenol fiber, cellulose fiber, isotropic pitch fiber, and anisotropic pitch fiber are preferable as the precursor of the organic fiber from the viewpoint of excellent oxidation resistance, strength and elasticity, and acrylic. Fiber is more preferred. The acrylic fiber is not particularly limited as long as it contains acrylonitrile as a main component, but the content of acrylonitrile in the raw material monomer forming the acrylic fiber is preferably 95% by mass or more, preferably 98% by mass or more. Is more preferable.

The mass average molecular weight of the organic fiber is not particularly limited, but is preferably 10,000 or more and 100,000 or less, more preferably 15,000 or more and 80,000 or less, and further preferably 20,000 or more and 50,000 or less. The mass average molecular weight can be measured by a method such as gel permeation chromatography (GPC) or solution viscosity.

The average fiber diameter of carbonaceous fibers is preferably 0.5 to 40 μm. If the average fiber diameter is smaller than 0.5 μm, the liquid permeability will deteriorate. On the other hand, if the average fiber diameter is larger than 40 μm, the reaction surface area of the fiber portion decreases and the cell resistance increases. Considering the balance between liquid permeability and reaction surface area, it is more preferably 3 to 20 μm.

In the present invention, it is preferable to use the carbonaceous fiber structure as a base material, which improves the strength and facilitates handling and processability. Specific examples of the structure include spun yarn, filament-focused yarn, non-woven fabric, knitted fabric, and woven fabric, which are sheet-like materials made of carbon fiber, and special knitted fabric or carbon described in JP-A-63-240177. Examples include paper made of fibers. Of these, non-woven fabrics made of carbon fibers, knitted fabrics, woven fabrics, special woven knitted fabrics, and paper made of carbon fibers are more preferable from the viewpoints of handling, processability, manufacturability, and the like.

When a non-woven fabric, knitted fabric, woven fabric or the like is used here, the average fiber length is preferably 30 to 100 mm. When paper made of carbon fibers is used, the average fiber length is preferably 5 to 30 mm. By setting the content within the above range, a uniform fiber structure can be obtained.

As described above, the carbonaceous fiber is obtained by heat-carbonizing a precursor of an organic fiber, but the above-mentioned "heat carbonization treatment" includes at least a flame resistance step and a carbonization (calcination) step. Is preferable. However, among these, the carbonization step does not necessarily have to be performed after the flame resistance step as described above, and after the graphite particles and the carbonaceous material are attached to the flame resistant fibers as described in Examples described later. A carbonization step may be performed, and in this case, the carbonization step after the flame resistance step can be omitted.

Of these, the flame-resistant step means a step of heating an organic fiber precursor preferably at a temperature of 180 ° C. or higher and 350 ° C. or lower in an air atmosphere to obtain a flame-resistant organic fiber. The heat treatment temperature is more preferably 190 ° C. or higher, and even more preferably 200 ° C. or higher. Further, it is preferably 330 ° C. or lower, and more preferably 300 ° C. or lower. By heating in the above temperature range, the content of nitrogen and hydrogen in the organic fiber can be reduced and the carbonization rate can be improved while maintaining the form of the carbonic fiber without thermally decomposing the organic fiber. During the flame resistance step, the organic fibers may be thermally shrunk and the molecular orientation may be disrupted to reduce the conductivity of the carbonaceous fibers. Therefore, it is preferable to perform the flame resistance treatment of the organic fibers under tension or stretching. It is more preferable to carry out flameproofing treatment under tension.

In the carbonization step, the flame-resistant organic fibers obtained as described above are preferably heated at a temperature of 1000 ° C. or higher and 2000 ° C. or lower in an inert atmosphere (preferably in a nitrogen atmosphere) to obtain carbonic fibers. Means a process. The heating temperature is more preferably 1100 ° C. or higher, and even more preferably 1200 ° C. or higher. Further, it is more preferably 1900 ° C. or lower. By performing the carbonization step in the above temperature range, carbonization of the organic fiber proceeds, and a carbonaceous fiber having a pseudographite crystal structure can be obtained.

Since each organic fiber has different crystallinity, the heating temperature in the carbonization step can be selected according to the type of the organic fiber used as a raw material. For example, when an acrylic resin (preferably polyacrylonitrile) is used as the organic fiber, the heating temperature is preferably 800 ° C. or higher and 2000 ° C. or lower, and more preferably 1000 ° C. or higher and 1800 ° C. or lower.

The flame resistance step and carbonization step described above are preferably carried out continuously, and the rate of temperature rise when the temperature is raised from the flame resistance temperature to the carbonization temperature is preferably 20 ° C./min or less, more preferably. Is 15 ° C./min or less. By setting the rate of temperature rise within the above range, it is possible to obtain carbonaceous fibers that retain the shape of the organic fibers and have excellent mechanical properties. The lower limit of the temperature rising rate is preferably 5 ° C./min or more in consideration of mechanical properties and the like.

As will be described in detail in the column of carbonaceous material (C) described later, the electrode material of the present invention is a carbonaceous fiber (A) and a carbonaceous material (C) as defined in (4) above. When the sizes of crystallites in the c-axis direction obtained by X-ray diffraction are Lc (A) and Lc (C), respectively, Lc (C) / Lc (A) satisfies 1.0 or more. Therefore, in the present invention, Lc (A) in the carbonaceous fiber (A) is not particularly limited as long as the above (4) is satisfied, but it is preferably 1 to 15 nm. As a result, the effects such as appropriate electron conductivity, oxidation resistance to sulfuric acid solvent and the like, and easy addition of oxygen functional groups are effectively exhibited. Lc (A) is more preferably 2 to 10 nm. The method for measuring Lc (A) will be described in detail in the column of Examples described later.

[Graphite particles (B)]
In the present invention, graphite particles are necessary to increase the change in valence (reactivity) due to redox and obtain high oxidation resistance. The carbon edge surface, which is the reaction field, is necessary for exhibiting high reactivity, but excessive exposure of the edge surface causes a decrease in oxidation resistance. According to the results of the study by the present inventors, when the size of the crystallites in the c-axis direction obtained by X-ray diffraction is Lc (B) for the graphite particles, the value of Lc (B) is the value of the carbon edge surface. We found that it correlates with the degree of exposure. Specifically, as defined in (2) above, Lc (B) is 35 nm or more, preferably 37 nm or more. As a result, the carbon edge surface as the reaction field can be exposed without excess or deficiency, and both low resistance and high oxidation resistance can be achieved at the same time. The upper limit of the above value is not particularly limited from the above viewpoint, but it is preferably about 50 nm or less in consideration of the balance between oxidation resistance and low resistance.

Graphite particles are generally roughly classified into natural graphite and artificial graphite. Examples of natural graphite include scaly graphite, scaly graphite, earthy graphite, spheroidal graphite, flaky graphite and the like, and examples of artificial graphite include expanded graphite and graphite oxide. In the present invention, either natural graphite or artificial graphite can be used, and among these, graphite oxide, scaly graphite, scaly graphite, earthy graphite, flaky graphite, and expanded graphite are carbon as a reaction field. It is preferable because it has an edge surface. Among them, scaly graphite, flaky graphite, and expanded graphite are more preferable because the carbon edge surface is very exposed and low resistance can be obtained, and the cost is low and the amount of resources is abundant. These scaly graphite, flaky graphite, and expanded graphite may be added alone, or two or more of them may be mixed and used. Here, scaly graphite means that the appearance is leaf-like. Scaly graphite is different from scaly graphite (which is lumpy in shape and is sometimes referred to as lump graphite).

As defined in (1) above, the graphite particles (B) used in the present invention have a particle size of 1 μm or more, preferably 3 μm or more. When the particle size is less than 1 μm, the ratio of being buried in the carbonaceous material increases, and graphite particles appear on the surface in a small amount, so that the specific surface area of the carbonaceous material increases too much. As a result, the effect of improving the oxidation resistance by adding the graphite particles (B) is not effectively exhibited, and the oxidation resistance tends to decrease.
Here, when the specific surface area of the carbonaceous material is increased, the reason why the effect of improving the oxidation resistance is not effectively exhibited is presumed as follows.
Normally, when graphite particles are buried, it is expected that the resistance will increase but the durability will also increase due to a trade-off, but in reality, the result will be high resistance and low durability. This is because the burial of the graphite particles does not effectively exert the effect of adding the graphite particles and increases the resistance, and the carbonaceous material (binder) coats the graphite particles, resulting in a high specific surface area of the carbonaceous material. , It is presumed that the durability will decrease.

Here, the "particle size" means the average particle size (D50) at a median 50% diameter in the particle size distribution obtained by a dynamic light scattering method or the like. Commercially available products may be used as the graphite particles, in which case the particle size described in the catalog can be adopted.

The graphite particles (B) used in the present invention are contained in an amount of 20% or more in terms of mass ratio to the total amount of the carbonaceous fibers (A), the graphite particles (B), and the carbonaceous material (C) described later. It is preferably 25% or more, and more preferably 25% or more. As a result, the above-mentioned effect due to the addition of graphite particles is effectively exhibited, and the oxidation resistance is particularly improved. The upper limit is not particularly limited from the viewpoint of oxidation resistance and the like, but it is preferably about 60% or less in consideration of the balance between oxidation resistance and low resistance. The content of the carbonaceous fiber (A) used for calculating the above content is the content of the structure when a structure such as a non-woven fabric is used as the base material.

In the present invention, the mass ratio of the carbonaceous material (C) described later to the graphite particles (B) is preferably 0.2 or more and 3.0 or less, and preferably 0.3 or more and 2.5 or less. More preferred. If the above ratio is less than 0.2, graphite particles are often shed off, and the effect of adding graphite is not particularly effective in improving the oxidation resistance. On the other hand, if the above ratio exceeds 3.0, the carbon edge surface of the graphite particles, which is the reaction field, is covered, and the desired low resistance cannot be obtained.

Graphite particles used in the present invention (B), BET specific surface area determined from nitrogen adsorption amount is preferably 3 ~ 20m ² / g, more preferably 5 ~ 15m ² / g. When the BET specific surface area is less than 3 m ² / g, the exposure of the edge surface of the graphite particles (B) is reduced, so that the desired low resistance cannot be obtained. On the other hand, when the BET specific surface area is 20 m ² / g or more, the specific surface area increases too much and the effect of improving the oxidation resistance by adding the graphite particles (B) is not effectively exhibited, and the oxidation resistance tends to decrease. Here, the above-mentioned "BET specific surface area obtained from the amount of adsorbed nitrogen" means the specific surface area calculated from the amount of gas molecules adsorbed by adsorbing gas molecules on solid particles.

[Carbonaceous material (C)]
The carbonaceous material used in the present invention is added as a binder for strongly binding carbonaceous fibers and graphite particles, which cannot be bound originally, and is a carbonaceous material having poor oxidation resistance. It has the effect of protecting the fibers. In the present invention, when the size of the crystallite in the c-axis direction obtained by X-ray diffraction in the carbonaceous material (C) is Lc (C) as defined in (3) above, Lc (C) is When the size of the crystallites in the c-axis direction in the carbonaceous fiber (A) determined by X-ray diffraction is Lc (A) as defined in (4) above and is 10 nm or more, Lc. (C) / Lc (A) needs to satisfy 1.0 or more.
By using a binding carbonaceous material that satisfies all of these requirements, not only high oxidation resistance is imparted to the carbonaceous material (C) itself, but also a carbonaceous material in which carbonaceous fibers are highly crystalline (carbonaceous material with high crystallinity). By being coated with C), the protective effect against oxidative deterioration of carbonaceous fibers is enhanced, and as a result, the oxidation resistance of the entire electrode material is also improved.

From the above viewpoint, Lc (C) is preferably 10 nm or more, and more preferably 12 nm or more. The upper limit of Lc (C) is not particularly limited from the above viewpoint, but it is preferably 40 nm or less in consideration of both oxidation resistance and low resistance.

If the ratio of Lc (C) / Lc (A) is less than 1.0, the above effect will not be effectively exhibited. The above ratio is preferably 2 or more, and more preferably 3 or more. On the other hand, if the above ratio exceeds 10, it becomes difficult to achieve both low resistance. The above ratio is preferably 8 or less.

The carbonaceous material (C) used in the present invention contains 20% or more of the above-mentioned carbonaceous fiber (A), graphite particles (B), and carbonaceous material (C) in terms of mass ratio to the total amount. Is preferable, and 30% or more is more preferable. By increasing the content of the carbonaceous material in this way, both the carbonaceous fiber and the graphite particles can be sufficiently bound, and the above-mentioned effect due to the addition of the carbonaceous material is effectively exhibited, and particularly the oxidation resistance is improved. improves. The upper limit is not particularly limited from the viewpoint of oxidation resistance and the like, but it is preferably about 60% or less in consideration of the liquid pressure loss and the like. More preferably, it is 50% or less.

The type of carbonaceous material (C) used in the present invention may be any as long as it can bind carbonaceous fibers (A) and graphite particles (B), and specifically, at the time of producing the electrode material of the present invention. It is not particularly limited as long as it exhibits binding property at the time of carbonization. Examples of such are pitches such as coal tar pitch and coal pitch; phenol resin, benzoxazine resin, epoxide resin, furan resin, vinyl ester resin, melanin-formaldehyde resin, urea-formaldehyde resin, resorcinol-formaldehyde. Examples thereof include resins such as resins, cyanate ester resins, bismaleimide resins, polyurethane resins and polyacrylonitrile; furfuryl alcohols; rubbers such as acrylonitrile-butadiene rubber. Commercially available products may be used for these.

Of these, pitches such as coal tar pitch and carboniferous pitch, which are particularly easily crystalline, are preferable because the desired carbonaceous material (C) can be obtained at a low firing temperature. Further, the polyacrylonitrile resin is also preferably used because the desired carbonaceous material (C) can be obtained by raising the firing temperature. Pitches are particularly preferable.
According to a preferred embodiment of the present invention, since the phenol resin is not used, there are merits such as no harmful effects (formaldehyde generation at room temperature and formaldehyde odor) associated with the phenol resin, and no odor is generated at room temperature. On the other hand, since the above-mentioned Patent Document 4 uses a phenol resin as an adhesive, in addition to the above-mentioned adverse effects, a separate facility for controlling the formaldehyde concentration in the work place to the control concentration or less is required, which is a cost aspect. , There are disadvantages in terms of work.

Here, the pitches that are particularly preferably used will be described in detail. For the coal tar pitch and coal-based pitch described above, the content of the mesophase phase (liquid crystal phase) can be controlled by the temperature and time of the infusibilization treatment. If the content of the mesophase phase is low, one that melts at a relatively low temperature or is in a liquid state at room temperature can be obtained. On the other hand, if the content of the mesophase phase is high, it melts at a high temperature and a high carbonization yield can be obtained. When the pitches are applied to the carbonaceous material (C), the content of the mesophase phase is preferably high (that is, the carbonization rate is high), for example, 30% or more is preferable, and 50% or more is more preferable. As a result, the fluidity at the time of melting can be suppressed, and the carbonaceous fibers can be bound to each other through the graphite particles without excessively covering the surface of the graphite particles. The upper limit is preferably 90% or less, for example, in consideration of the expression of binding property.

From the same viewpoint as above, the melting point of the pitches is preferably 100 ° C. or higher, more preferably 200 ° C. or higher. As a result, in addition to obtaining the above effects, it is possible to suppress the odor during the bonding process, which is also preferable in terms of processability. The upper limit is preferably 350 ° C. or lower, for example, in consideration of the development of binding property.

(Characteristics of Electrode Material of the Present Invention)
The electrode material of the present invention satisfies that the number of bound oxygen atoms on the surface of the carbon positive electrode material is less than 1.0% of the total number of carbon atoms on the surface of the carbon positive electrode material (O / C <1.0%). O / C can be measured by surface analysis such as X-ray photoelectron spectroscopy (XPS) and fluorescent X-ray analysis.

By using an electrode material having an O / C of less than 1.0%, the defect structure of the carbonaceous material (C) portion is reduced, and high durability can be obtained. Further, if the structure is such that the graphite particles can be sufficiently exposed, low resistance can be maintained. On the other hand, if an electrode material having an O / C of 1.0% or more and a high oxygen concentration is used, the defect structure of the carbonaceous material (C) portion increases, and the durability cannot be improved. As a result, the life is shortened.

Further, the electrode material of the present invention satisfies the BET specific surface area of 0.5 to 1.5 m ² / g determined from the amount of nitrogen adsorbed. If the BET specific surface area is less than 0.5 m ² / g, the exposure of the edge surface of the graphite particles (B) is reduced, so that the desired low resistance cannot be obtained. The preferred upper limit is 1.0 m ² / g. On the other hand, when the BET specific surface area exceeds 1.5 m ² / g, the specific surface area increases too much and the effect of improving the oxidation resistance by adding the graphite particles (B) is not effectively exhibited, and the oxidation resistance tends to decrease. ..

The basis weight of the electrode material of the present invention is 50 to 50 when the thickness of the spacer 2 sandwiched between the current collector plate 1 and the ion exchange membrane 3 (hereinafter referred to as "spacer thickness") is 0.3 to 3 mm. 500 g / m ² is preferable, and 100 to 400 g / m ² is more preferable. By controlling the basis weight within the above range, it is possible to prevent damage to the ion exchange membrane 3 while ensuring liquid permeability. In particular, in recent years, from the viewpoint of reducing resistance, the thickness of the ion exchange membrane 3 tends to be thin, and treatment and usage methods for reducing damage to the ion exchange membrane 3 are extremely important. From the above viewpoint, it is more preferable to use a non-woven fabric or paper having a flat surface processed on one side as a base material as the electrode material of the present invention. Any known method can be applied to the flattening method, and examples thereof include a method of applying a slurry to one side of a carbonaceous fiber and drying it; a method of impregnation and drying on a smooth film such as PET.

The thickness of the electrode material of the present invention is preferably at least larger than the spacer thickness. For example, when a carbonaceous fiber having a low density such as a non-woven fabric is used and graphite particles or a binding carbonaceous material used for the electrode material of the present invention are carried therein, the spacer thickness is 1.5 to 1.5 to. 6.0 times is preferable. However, if the thickness is too thick, the ion exchange membrane 3 may be pierced by the compressive stress of the sheet-like material. Therefore, it is recommended to use the electrode material of the present invention having a compressive stress of 9.8 N / cm ² or less. preferable. In order to adjust the compressive stress and the like according to the basis weight and thickness of the electrode material of the present invention, it is also possible to use the electrode material of the present invention in a laminated manner such as two layers or three layers. Alternatively, it can be combined with another form of electrode material.

(Method for manufacturing electrode material according to the present invention)
Next, a method for producing the electrode material of the present invention will be described. The electrode material of the present invention can be produced by adhering graphite particles and a precursor of a carbonaceous material (before carbonization) to a carbonaceous fiber (base material), and then undergoing a carbonization step and a graphitization step. .. In each step, a known method can be arbitrarily applied. In the present invention, in order to control the O / C to less than 1.0%, the oxidation treatment step (dry air oxidation) usually performed after the graphitization step is not performed.

Each process will be described below.

(Step of adhering graphite particles and precursors of carbonaceous material to carbonaceous fibers)
First, graphite particles and precursors of carbonaceous material are attached to carbonaceous fibers. A known method can be arbitrarily adopted for adhering the graphite particles and the precursor of the carbonaceous material to the carbonaceous fiber. For example, a method of heating and melting the above-mentioned carbonaceous material precursor, dispersing graphite particles in the obtained melt, immersing carbonaceous fibers in the melt dispersion, and then cooling to room temperature can be mentioned. Alternatively, as shown in Examples described later, the above carbonaceous material precursor and graphite particles are mixed with a solvent such as water or alcohol to which a binder (temporary adhesive) that disappears during carbonization such as polyvinyl alcohol is added. A method can be used in which the carbonaceous fibers are dispersed, the carbonaceous fibers are immersed in the dispersion, and then heated and dried. Here, the excess liquid among the melt dispersion liquid and the dispersion liquid in which the carbonaceous fiber is immersed can be passed through a nip roller provided with a predetermined clearance to squeeze the excess dispersion liquid when immersed in the dispersion liquid. Alternatively, the surface of the excess dispersion liquid when immersed in the dispersion liquid with a doctor blade or the like can be removed by scraping the surface.

After that, it is dried in an air atmosphere, for example, at 80 to 150 ° C.

(Carbonization process)
The carbonization step is performed to calcin the product after the attachment obtained in the above step. As a result, carbonaceous fibers are bound to each other via graphite particles. In the carbonization step, it is preferable to sufficiently remove the decomposition gas at the time of carbonization. For example, it is preferable to heat the gas at a temperature of 800 ° C. or higher and 2000 ° C. or lower under an inert atmosphere (preferably a nitrogen atmosphere). The heating temperature is more preferably 1000 ° C. or higher, further preferably 1200 ° C. or higher, even more preferably 1300 ° C. or higher, still more preferably 1500 ° C. or lower, still more preferably 1400 ° C. or lower.

As described above, the treatment corresponding to the carbonization step may be performed even after the fiber is made flame resistant, but the carbonization treatment performed after the fiber is made flame resistant may be omitted. That is, the method for producing the electrode material of the present invention is roughly classified into the following method 1 and method 2.
・ Method 1: Flame resistance of fiber → Carbonization of fiber → Adhesion of graphite particles and carbonaceous material → Carbonization → Graphite formation ・ Method 2: Flame resistance of fiber → Adhesion of graphite particles and carbonaceous material → Carbonization → Graphite According to the above method 1, the processing cost increases because carbonization is performed twice, but the sheet used as the electrode material is not easily affected by the difference in volume shrinkage ratio, so that the obtained sheet is deformed (warpage occurs). It has the advantage of being difficult to do. On the other hand, according to the above method 2, the processing cost can be reduced because the carbonization step may be performed once, but the sheet obtained by the difference in the volume shrinkage ratio at the time of carbonization of each material is easily deformed. Which of the above methods 1 and 2 should be adopted may be appropriately determined in consideration of these.

(Graphitization process)
The graphitization step is a step performed in order to sufficiently enhance the crystallinity of the carbonaceous material and to exhibit high oxidation resistance. After the carbonization step, the mixture is further heated in an inert atmosphere (preferably in a nitrogen atmosphere) at a temperature of 1800 ° C. or higher, which is higher than the heating temperature in the carbonization step, preferably 2000 ° C. The above is more preferable. The upper limit is preferably 3000 ° C. or lower in consideration of the load on the equipment.
On the other hand, Patent Document 4 described above is different from the production method of the present invention in that the graphitization step is not performed. Therefore, the electrode material of Patent Document 4 does not satisfy the requirement of the present invention [Lc of carbonaceous material (C) is 10 nm or more].

As described above, in the present invention, since the O / C is controlled to less than 1.0%, the oxidation treatment usually performed after the graphitization step is not performed. The oxidation treatment is carried out to introduce an oxygen functional group such as a hydroxyl group, a carbonyl group, a quinone group, a lactone group or a free radical oxide to the surface of the electrode material, and the O / C is carried out by the oxidation treatment. This is because is increased to 1.0% or more.

This application claims the benefit of priority based on Japanese Patent Application No. 2019-045663 filed on March 13, 2019. The entire contents of the specification of Japanese Patent Application No. 2019-045663 filed on March 13, 2019 are incorporated herein by reference.

The present invention will be described in more detail with reference to Examples and Comparative Examples. The present invention is not limited to the following examples. In the following,% means "mass%" unless otherwise specified.

In this example, the following items were measured. The details of the measurement method are as follows.

(1) Measurement of crystallite size (Lc) in the c-axis direction by X-ray diffraction For details, Lc (A) of carbonaceous fiber, Lc (B) of graphite particles, and Lc (C) of carbonaceous material. ) Was measured as follows.
Each of the carbonaceous fibers, graphite particles, and carbonaceous material (single substance) used in this example was sequentially subjected to the same heat treatment as in Example 2, and the measurement was performed using the final treated sample. Basically, carbon crystallinity is dominated by the influence of thermal energy given to the sample, and it is thought that the thermal history of the highest temperature given to the sample determines the crystallinity of Lc, but it depends on the degree of subsequent oxidation treatment. It is considered that the graphene laminated structure formed during the graphitization step is disturbed, and the crystallinity may be lowered due to the generation of defective structures. Therefore, the final processed sample was used.

Each single sample collected as described above was crushed in an agate mortar until the particle size was about 10 μm. Approximately 5% by mass of high-purity silicon powder for X-ray standard is mixed with the crushed sample as an internal standard substance, packed in a sample cell, and wide-angle X-ray is measured by the differential meter method using CuKα ray as a radiation source. did.

The carbonaceous fibers (A) and graphite particles (B) used for the electrode material of the present invention, and the carbonaceous material (C) for binding them are peak-separated from the chart obtained by the above wide-angle X-ray measurement. Therefore, each Lc value was calculated. Specifically, the peak where the apex is seen in the range of 26.4 ° to 26.6 °, which is twice the diffraction angle θ (2θ), is the graphite particle (B), and the range of 25.7 ° to 26.2 °. The peak in which the apex is seen was defined as the carbonaceous material (C). After determining the peak shape as a sine wave from each peak top, determine the peak shape as a sine wave from the base part seen around 24.0 ° to 25.0 °, and determine this as a carbonaceous fiber (A). ). When the peak tops of the graphite particles (B) and the carbonaceous material (C) cannot be separated, they are separated by determining the peak shape as a sine wave from the base portion seen near 24.0 ° to 26.0 °. did. From the three peaks separated by the above method, each Lc was calculated by the following method.

For the correction of the curve, the following simple method was used without correcting the so-called Lorentz factor, polarization factor, absorption factor, atomic scattering factor and the like. That is, the real intensity from the baseline of the peak corresponding to the <002> diffraction was re-plotted to obtain the <002> corrected intensity curve. From the length of the line segment (half-value width β) where the line parallel to the angle axis drawn to the height of 1/2 of the peak height intersects the correction intensity curve, the size of the crystallite in the c-axis direction is calculated by the following equation. I asked for Lc.
Lc = (k ・ λ) / (β ・ cosθ)

Here, the wavelength λ = 1.5418Å, the structural coefficient k = 0.9, β indicates the half width of the <002> diffraction peak, and θ indicates the <002> diffraction angle.

(2) Measurement of O / C by XPS surface analysis A ULVAC-PHI 5801MC device was used for the measurement of X-ray photoelectron spectroscopy, which is abbreviated as ESCA or XPS.
First, the sample was fixed on the sample holder with a Mo plate, sufficiently exhausted in the preliminary exhaust chamber, and then charged into the chamber of the measurement chamber. Monochrome AlKα wire was used as the radiation source, the output was 14 kV, 12 mA, and the degree of vacuum inside the device was 10 ^-8 torr.
All element scans were performed to examine the composition of surface elements, and narrow scans were performed on the detected and expected elements to evaluate the abundance ratio.
The ratio of the number of surface-bonded oxygen atoms to the total number of surface carbon atoms was calculated as a percentage (%), and O / C was calculated.

(3) BET specific surface area measurement Approximately 100 mg of a sample was collected, vacuum dried at 120 ° C. for 12 hours, weighed 90 g, and the BET specific surface area was measured using a specific surface area / pore distribution measuring device Gemini2375 (manufactured by Micromeritics). It was measured. Specifically, the amount of nitrogen gas adsorbed at the boiling point (-195.8 ° C.) of liquid nitrogen was measured in a relative pressure range of 0.02 to 0.95, and an adsorption isotherm of the sample was prepared. Based on the results in the range of relative pressure 0.02 to 0.15, the BET specific surface area per weight (unit: m ² / g) was determined by the BET method.

(4) Charge / discharge test Each electrode material obtained by the method described later is cut out into an electrode area of 8.91 cm ² with an electrode area of 2.7 cm in the vertical direction (liquid flow direction) and 3.3 cm in the width direction, and only on the positive electrode side. Introduced. At this time, the number of sheets was adjusted so that the basis weight in the cell was 230 to 350 g / m ² . Two electrode materials prepared as described below were laminated on the negative electrode side to assemble the cell shown in FIG. A Nafion 212 membrane was used as the ion exchange membrane, and the spacer thickness was 0.5 mm. The total cell resistance (Ω · cm ² ) was calculated from the voltage curve at the 10th cycle in the voltage range of 1.55 to 1.00 V at 144 mA / cm ² by the following formula.
A 5.0 moL / L sulfuric acid aqueous solution in which titanium oxysulfate and manganese oxysulfate were dissolved at 1.0 moL / L was used for both the positive electrode and the negative electrode electrolytic solutions, and for the positive electrode and the negative electrode electrolytic solutions. The amount of electrolyte was too large for the cell and piping. The liquid flow rate was 10 mL per minute, and the measurement was performed at 35 ° C.

here,
_VC50 is the charge voltage obtained from the electrode curve with respect to the amount of electricity when the charge rate is 50%.
V _D50 is the discharge voltage obtained from the electrode curve with respect to the amount of electricity when the charge rate is 50%.
I = current density (mA / cm ² )

<Method of manufacturing electrode material for negative electrode>
A plain weave cloth (thickness 1.0 mm, basis weight 600 g / m ² ) made of polyacrylonitrile fibers having an average fiber diameter of 16 μm was heated at 300 ° C. in an air atmosphere to make it flame resistant, and fired at 1000 ° C. for 1 hour in a nitrogen atmosphere. Then, it was heated at 600 ° C. for 8 minutes in an air atmosphere, and then calcined at 1800 ° C. for 1 hour in a nitrogen atmosphere. Further, the treatment was carried out at 700 ° C. for 15 minutes in an air atmosphere to obtain an electrode material for a negative electrode having a basis weight of 152 g / m ² and a thickness of 0.73 mm.

(5) Oxidation resistance test (5-1) Oxidation resistance of carbon particles (including graphite particles) 1.0 moL / L Titanium oxysulfate in 5.0 moL / L sulfuric acid aqueous solution and 1.0 moL / L manganese oxysulfate A battery composed of a 5.0 moL / L sulfuric acid aqueous solution and a platinum wire as a working electrode and an Ag / AgCl electrode as a reference electrode was charged until the open circuit voltage reached 1.266 V. The carbon particles used in the examples were immersed in the above electrolytic solution in an amount 40 times the amount of the carbon particles, and allowed to stand at 75 ° C. for 16 hours. After allowing to cool to room temperature, the open circuit voltage of the electrolytic solution (platinum wire at the working electrode and Ag / AgCl at the reference electrode) was measured, and the oxidation resistance was estimated based on the degree of voltage drop from 1.266V.
(5-2) Oxidation resistance of electrode material An electrolytic solution consisting of a 5.0 moL / L sulfuric acid aqueous solution of 1.0 moL / L titanium oxysulfate and a 5.0 moL / L sulfuric acid aqueous solution of 1.0 moL / L manganese oxysulfate. At a potential using a platinum wire as the working electrode and an Ag / AgCl electrode as the reference electrode, the battery was charged until the open circuit voltage reached 1.266 V. The prepared electrode material was immersed in a charging liquid 40 times as much as the weight of the electrode, and allowed to stand at 75 ° C. for 16 hours. After allowing to cool to room temperature, the open circuit voltage of the electrolytic solution (platinum wire at the working electrode and Ag / AgCl at the reference electrode) was measured, and the oxidation resistance was estimated based on the degree of voltage drop from 1.266V.

(6) Water flow test At a point 5 cm high from the electrode, drop one drop of ion-exchanged water onto the electrode from a 3 mmφ pipette, measure the time until the dropped water drop permeates, and use the following formula. The water flow rate was calculated by
Water flow rate (mm / sec)
= Thickness of electrode material (mm) ÷ Time until water droplets permeate (sec)

Example 1
In this example, using various carbon particles (A to F, A', a, b) shown in Table 1, the particle size and Lc (B) are measured, and an oxidation resistance test is performed to perform oxidation resistance. Gender was evaluated.

Of A to F, A to D are graphites specified in the present invention, A, B and C are scaly graphites, and D is flaky graphite. On the other hand, E is scaly graphite having a small Lc, and F is scaly graphite having a particle size of less than 1 μm and a small Lc. Further, A'is an example in which A is crushed by a bead mill for 6 hours with a Labostar mini machine manufactured by Ashizawa Finetech Co., Ltd., and Lc is small. a and b are carbon black.
Commercially available products are used for all the carbon particles, and the particle sizes shown in Table 1 are the values listed in the catalog. The particle size of A'was measured by laser diffraction.

These results are also shown in Table 1.

As shown in Table 1, all of A to D satisfying the requirements of graphite particles specified in the present invention (particle size: 1 μm or more, Lc of 35 nm or more) are excellent in oxidation resistance and high durability. Indicated. It is considered that these are because the excessive exposure of the edge surface, which is the starting point of oxidative deterioration, can be suppressed.

On the other hand, the oxidation resistance of both a and b using carbon rack instead of graphite was significantly reduced. It is considered that this is because the amorphous carbon portion is easily oxidatively deteriorated due to insufficient carbon crystallinity.

It was also found that even graphite particles having an Lc of less than 35 nm, such as E, F, and A', are also inferior in oxidation resistance. This is considered to be because the carbon edge surface is overexposed.

Example 2
In this example, using some of the carbon particles in Table 1, an electrode material was prepared as follows and various items were measured.

(No. 1)
No. In No. 1, polyacrylonitrile fibers are used as carbonaceous fibers, C in Table 1 (examples satisfying the requirements of the present invention) as graphite particles, and pitches of coal tar pitch MCP250 manufactured by JFE Chemical Co., Ltd. are used as carbonaceous materials as follows. To prepare an electrode material.

First, in ion-exchanged water, Kao's Leodor TW-L120 (nonionic surfactant) was 1.8%, polyvinyl alcohol (temporary adhesive) was 1.8%, and JFE Chemical's MCP250 (carbonaceous material). ) Was added to 14% and C in Table 1 was added to 9.8% as graphite powder, and the mixture was stirred with a mechanical stirrer for 1 hour to prepare a dispersion.

Carbon paper (CFP-030-PE manufactured by Nippon Polymer Sangyo Co., Ltd., grain size 30 g / m ² , thickness 0.51 mm) made of polyacrylonitrile fiber (average fiber diameter 10 μm) is immersed in the dispersion obtained in this manner. After that, the excess dispersion liquid was removed by passing it through a nip roller.
Next, it was dried at 150 ° C. for 20 minutes in an air atmosphere, carbonized (calcined) at 1000 ° C. for 1 hour in a nitrogen atmosphere, and then graphitized at 2000 ° C. for 1 hour. After graphitization, no treatment was carried out in an air atmosphere, and an electrode material (No. 1) having a thickness of 0.50 mm and a basis weight of 134.0 g / m ² was prepared.

(No. 2)
No. In No. 1, A of Table 1 (an example satisfying the requirements of the present invention) was used as the graphite powder; the content of graphite particles and carbonaceous material with respect to the total amount of carbonaceous fiber, graphite particles, and carbonaceous material is shown in Table 1. Except for the change as in 2, the above No. No. 1 in the same manner as in 1. An electrode material of 2 (thickness 0.48 mm, basis weight 144.0 g / m ² ) was produced.

(No. 3)
No. In No. 1, B of Table 1 (an example satisfying the requirements of the present invention) was used as the graphite powder; the content of graphite particles and carbonaceous material with respect to the total amount of carbonaceous fiber, graphite particles, and carbonaceous material is shown in Table 1. Except for the change as in 2, the above No. No. 1 in the same manner as in 1. An electrode material of 3 (thickness 0.45 mm, basis weight 126.0 g / m ² ) was prepared.

(No. 4)
No. In No. 1, D in Table 1 (an example satisfying the requirements of the present invention) was used as the graphite powder, and the above D was added to ion-exchanged water so as to be 4.9%; carbonaceous fibers and graphite particles. No. except that the contents of graphite particles and carbonaceous materials with respect to the total amount of carbonaceous materials were changed as shown in Table 2. No. 1 in the same manner as in 1. An electrode material of 4 (thickness 0.53 mm, basis weight 177.1 g / m ² ) was prepared.

(No. 5)
No. In No. 4, C in Table 1 (an example satisfying the requirements of the present invention) was used as the graphite powder; the content of graphite particles and carbonaceous material with respect to the total amount of carbonaceous fiber, graphite particles, and carbonaceous material is shown in Table 4. Except for the change as in 2, the above No. No. 4 in the same manner as in 4. An electrode material of 5 (thickness 0.51 mm, basis weight 164.0 g / m ² ) was prepared.

(No. 6)
No. In No. 1, carbon paper (CFP-010-PV manufactured by Nippon Polymer Sangyo Co., Ltd., grain size 10 g / m ² , thickness 0.19 mm) made of polyacrylonitrile fiber (average fiber diameter 10 μm) is used as the carbon fiber, and graphite is used. B of Table 1 (an example satisfying the requirements of the present invention) was used as the powder; the content of graphite particles and carbonaceous material with respect to the total amount of carbonic fiber, graphite particles and carbonaceous material is as shown in Table 2. No. except for the change. No. 1 in the same manner as in 1. An electrode material of 6 (thickness 0.16 mm, basis weight 51.4 g / m ² ) was prepared.

(No. 7)
No. In No. 1, carbon paper made of anisotropic pitch fibers (thickness 0.61 mm, grain size 30 g / m ² ) was used as the carbonaceous fibers; graphite particles and graphite particles with respect to the total amount of carbonaceous fibers, graphite particles, and carbonaceous materials. No. except that the content of carbonaceous material was changed as shown in Table 2. No. 1 in the same manner as in 1. An electrode material of 7 (thickness 0.47 mm, basis weight 127.0 g / m ² ) was produced.

(No. 8)
No. In No. 1, except that after graphitization, oxidation treatment was performed at 700 ° C. for 20 minutes in an air atmosphere. No. 1 in the same manner as in 1. An electrode material (comparative example) of 8 (thickness 0.50 mm, basis weight 134.0 g / m ² ) was prepared.

(No. 9)
No. In No. 2, except that after graphitization, oxidation treatment was performed at 700 ° C. for 20 minutes in an air atmosphere. No. 1 in the same manner as in 1. An electrode material (comparative example) of 9 (thickness 0.48 mm, basis weight 144.0 g / m ² ) was produced.

(No. 10)
No. In No. 1, D in Table 1 was used as the graphite powder, and the above D was added to ion-exchanged water so as to be 4.9%; graphite particles with respect to the total amount of carbonaceous fibers, graphite particles, and carbonaceous materials. And the content of carbonaceous materials was changed as shown in Table 2. No. 1 except that after graphitization, oxidation treatment was performed at 700 ° C. for 20 minutes in an air atmosphere. No. 1 in the same manner as in 1. An electrode material of 10 (thickness 0.55 mm, basis weight 114.0 g / m ² ) was prepared.

(No. 11)
A in Table 1 was used as the graphite powder, and TD-4304 manufactured by DIC Corporation was used as the carbonaceous material.
For details, see No. In 1, graphite powder was added to ion-exchanged water at 4.9% and phenol resin aqueous dispersion (TD4304) at 10%; graphite with respect to the total amount of carbonaceous fibers, graphite particles, and carbonaceous material. No. except that the contents of particles and carbonaceous materials were changed as shown in Table 2. No. 1 in the same manner as in 1. An electrode material (comparative example) of 11 (thickness 0.63 mm, basis weight 98.0 g / m ² ) was prepared.

(No. 12)
No. Reference numeral 12 denotes a comparative example simulating the above-mentioned Patent Document 3, in which carbonaceous fibers were treated as follows without using graphite particles and a carbonaceous material to obtain an electrode material.
Specifically, No. In No. 1, a marifleece woven fabric (thickness 0.81 mm, grain 100 g / m ² ) made of flame-resistant polyacrylonitrile fiber was carbonized (baked) at 1000 ° C. for 1 hour in a nitrogen atmosphere, and then graphite was carbonized at 1500 ° C. for 1 hour. Then, it was oxidized at 700 ° C. for 15 minutes to obtain No. An electrode material (comparative example) of 12 (thickness 0.78 mm, basis weight 50 g / m ² ) was prepared.
The rate of temperature rise when the temperature is raised from the flame resistance temperature to the carbonization temperature is No. Same as 1.

(No. 13)
No. In No. 1, graphite particles were not used; except that a spunlace non-woven fabric carbonized at 1000 ° C. (grain 50 g / m ² , thickness 0.85 mm) was used. No. 1 in the same manner as in 1. An electrode material (comparative example) of 13 (thickness 0.78 mm, basis weight 154.0 g / m ² ) was prepared.

Table 2 shows the above No. The measurement results of various items in 1 to 13 are shown.

No. Nos. 1 to 7 are electrode materials satisfying the requirements of the present invention, and all of them have obtained electrode materials having excellent oxidation resistance while maintaining low resistance.

On the other hand, No. Comparative examples 8 to 13 are examples in which the amount of oxygen introduced was increased by performing dry air oxidation treatment in an air atmosphere after graphitization to increase the O / C to about 2 to 3%, and This is an example in which the BET specific surface area is also high.
Of these, No. Reference numeral 8 denotes an example in which a carbonaceous material having a low crystallinity of Lc (C) of 1.5 nm was used, and the oxidation resistance was lowered.

Also No. No. 9 except that the dry air oxidation treatment was not performed. No. which is the same as 9. In comparison with 2 (O / C <1.0% of the present invention example), No. 9 is No. Compared with 2, the cell resistance was the same, but the oxidation resistance was lowered.

Also No. No. 10 except that the dry air oxidation treatment was not performed. No. which is the same as 10. In comparison with 3 (O / C <1.0% of the present invention example), No. 10 is No. Compared with No. 3, the cell resistance was the same, but the oxidation resistance was lowered.

No. In No. 11, a carbonaceous material having a low crystallinity of Lc (C) of 1.5 nm was used, and Lc (C) / Lc (A) was also as low as 0.5, and the cell resistance was about the same as that of the present invention. Although it was low, the oxidation resistance was significantly reduced.

No. Reference numeral 12 denotes an example in which neither graphite particles nor carbonaceous materials were used in simulating Patent Document 3, and the cell resistance was high and the oxidation resistance was further lowered.

No. Reference numeral 13 denotes an example in which a highly crystalline carbonaceous material is used without using graphite particles and the BET specific surface area is low, and the oxidation resistance is excellent, but the cell resistance is the highest.

According to the present invention, since it is possible to provide a carbon positive electrode material having low resistance and long life while maintaining high oxidation resistance, it is particularly useful as a positive electrode material for a redox flow battery using a manganese / titanium-based electrolytic solution. .. The carbon positive electrode material of the present invention is suitably used for flow type and non-flow type redox flow batteries, redox flow batteries combined with lithium, capacitor, and fuel cell systems.

1 Current collector plate 2 Spacer 3 Ion exchange membrane 4a, 4b Liquid passage 5 Electrode material 6 Positive electrolyte tank 7 Negative electrolyte tank 8, 9 Pump 10 Liquid inlet 11 Liquid outlet 12, 13 External flow path

Claims (5)

1. It is composed of carbonaceous fibers (A), graphite particles (B), and a carbonaceous material (C) that binds them together.
   A carbon positive electrode material for a manganese / titanium-based redox flow battery, which is characterized by satisfying the following requirements.
   (1) The particle size of the graphite particles (B) is 1 μm or more.
   (2) When the size of the crystallites in the c-axis direction obtained by X-ray diffraction in the graphite particles (B) is Lc (B), Lc (B) is 35 nm or more.
   (3) When the size of crystallites in the c-axis direction obtained by X-ray diffraction in the carbonaceous material (C) is Lc (C), Lc (C) is 10 nm or more.
   (4) When the size of the crystallites in the c-axis direction obtained by X-ray diffraction in the carbonaceous fiber (A) is Lc (A), Lc (C) / Lc (A) is 1.0 or more.
   (5) The number of bound oxygen atoms on the surface of the carbon positive electrode material is less than 1.0% of the total number of carbon atoms on the surface of the carbon positive electrode material.
   (6) The BET specific surface area determined from the amount of nitrogen adsorbed is 0.5 to 1.5 m ² / g.
2. The mass content of the graphite particles (B) and the carbonaceous material (C) with respect to the total amount of the carbonaceous fiber (A), the graphite particles (B), and the carbonaceous material (C) is 20% or more, respectively. The carbon positive electrode material according to claim 1, wherein the mass ratio of the carbonaceous material (C) to the graphite particles (B) is 0.2 to 3.0.
3. The carbon positive electrode material according to claim 1 or 2, wherein the Lc (A) is 1 to 10 nm.
4. The carbon positive electrode material according to any one of claims 1 to 3, wherein the graphite particles (B) include at least one selected from the group consisting of scaly graphite, flaky graphite, and expanded graphite.
5. A manganese / titanium-based redox flow battery provided with the carbon positive electrode material according to any one of claims 1 to 4.

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