Sheet material, electrode, and redox flow battery

Abstract

A sheet material according to the present invention is a liquid-permeable sheet material that is composed of a plurality of fibers and binding sections that are present at intersections of the fibers and bind the fibers to each other. The fibers are composed of carbon fibers, and the binding sections are composed of a carbide. The carbon fibers and the carbide each have peaks in the D band near 1360 cm–1 and the G band near 1580 cm–1 in spectra calculated by laser Raman spectroscopy with an excitation wavelength of 532 nm. The peak area ratio (D / G) of the D band to the G band of the carbide is 0.8 times to 2.0 times the peak area ratio (D / G) of the D band to the G band of the carbon fibers. The sheet material has a density of less than 0.20 g / cm3 and a hardness (Asker C hardness) of 30 or more as measured using a type C durometer hardness tester specified in JISK7312.

Description

Sheet material, electrode, and redox flow battery

[0001] The present invention relates to a sheet material used in a flow-through electrolytic cell in which a liquid is circulated, such as in water electrolysis, organic hydride electrolysis, or a redox flow battery.

[0002] Redox flow batteries are batteries that utilize the oxidation-reduction of redox ions in an aqueous solution while circulating (flowing) an electrolyte within the cell. Because the reaction is mild and occurs only in the liquid phase, they are extremely safe, large-capacity storage batteries, and are expected to become widespread as storage batteries for stabilizing power grids by storing and discharging electricity generated by wind power, solar power, and other sources.

[0003] Redox flow batteries have shifted from a type that uses an iron hydrochloric acid solution for the positive electrode and a chromium hydrochloric acid solution for the negative electrode to a type that uses a vanadium sulfuric acid solution, which has a high electromotive force, for both electrodes, resulting in higher energy density.In recent years, development has progressed to further increase the active material concentration, further increasing energy density.

[0004] As shown in Figure 1, a redox flow battery is composed of an external tank that stores an electrolyte and an electrolytic cell. A pump sends the electrolyte containing the active material from the external tank to the electrolytic cell, while electrochemical energy conversion, i.e., charging and discharging, occurs on electrodes built into the electrolytic cell.

[0005] To popularize redox flow batteries, there is a strong demand for lower battery costs. Generally, the selection of inexpensive, general-purpose materials can reduce battery costs. On the other hand, the development of high-performance cell components (sheet materials, ion exchange membranes, current collector plates, etc.) can reduce the amount of materials used and running costs. In particular, sheet materials are extremely important cell components, as they are not only responsible for the reaction of the active materials but also serve as a flow path for the electrolyte. By imparting low cell resistance and high permeability to sheet materials, battery efficiency and system efficiency can be improved, greatly contributing to the widespread use of redox flow batteries.

[0006] Therefore, research has been conducted into reducing the resistance between carbon fibers in sheet materials by binding them with a carbide resin binder. As an example of such a sheet material, Patent Document 1 discloses a carbon fiber nonwoven fabric with low compressive strain due to the ratio and density adjustment of the carbon fiber to resin carbide. Patent Document 2 also discloses a carbon electrode material in which carbon fibers and high-specific-surface-area carbon particles are bound together with a carbonaceous material. In this case, the mesopore specific surface area and the number of surface oxygen atoms are controlled to improve the reactivity between the carbon electrode material and the active material in the electrolyte, and high-specific-surface-area carbon particles are used. Patent Document 3 also discloses a carbon electrode material in which high-specific-surface-area carbon fine particles are attached to carbonaceous fibers to reduce reaction resistance. The high-specific-surface-area carbon fine particles increase the electrochemical reaction area, improving the reactivity between the carbon electrode material and the active material in the electrolyte.

[0007] Japanese Patent Application Publication No. 2019-175833 International Publication No. WO2020 / 184665 Japanese Patent No. 6617464

[0008] To further improve the performance of redox flow batteries, i.e., to increase their energy conversion efficiency, it is necessary to reduce both cell resistance and the pump output required for electrolyte delivery. To reduce the cell resistance associated with the sheet material, it is necessary to improve the conductivity of the constituent materials of the sheet material, the contact between the sheet material and the current collector plate and the carbon fibers that make up the sheet material, and the reactivity of the sheet material with the active material in the electrolyte. For example, while increasing the density of the sheet material is an effective way to improve the contact between the sheet material and the current collector plate and the carbon fibers that make up the sheet material, decreasing the density of the sheet material is effective for reducing the pump output required for electrolyte delivery. Therefore, it is extremely difficult to achieve both reduced cell resistance and improved electrolyte delivery.

[0009] In Patent Document 1, the density of the carbon fiber nonwoven fabric is 0.20 g / cm ３Although this high temperature improves the contact between the carbon fiber nonwoven fabric and the current collector plate, and between the carbon fibers that make up the carbon fiber nonwoven fabric, it makes it difficult for the electrolyte to flow, increasing the pump output required to deliver the electrolyte.In addition, the binder solution used is made by dissolving phenolic resin in an organic solvent, which requires solvent recovery to reduce environmental impact, resulting in higher manufacturing costs.

[0010] In Patent Document 2, the carbon electrode material contains carbon particles with a high specific surface area, which results in insufficient bonding between the carbonaceous fibers. Furthermore, oxidation treatment is performed in an air atmosphere before and after graphitization, and multiple oxidation treatments embrittle the carbon electrode material, reducing the hardness of the sheet and thereby reducing the contact between the carbon electrode material and the current collector plate.

[0011] In Patent Document 3, in order to suppress a decrease in the carbonaceous fiber surface, which is the electrochemical reaction field of the carbon electrode material, the carbon fine particles with a high specific surface area and the carbonaceous fibers are merely brought into close proximity to each other or are merely bound together by a small amount of phenolic resin, and therefore the binding between the carbonaceous fibers is insufficient to improve the contact between the carbonaceous fibers.

[0012] As described above, it is difficult to simultaneously reduce cell resistance and improve electrolyte transportability. The present invention was made in response to these problems in the prior art, and its object is to provide a sheet material that can reduce cell resistance to improve energy efficiency, improve electrolyte permeability, and reduce energy loss during electrolyte transport. Another object is to provide a redox flow battery that achieves excellent charge / discharge performance by using this sheet material as an electrode in the redox flow battery.

[0013] As a result of extensive research, the present inventors have found that the above problems can be solved by the following means, and have arrived at the present invention. That is, the present invention has the following configuration.

[0014] [1] A liquid-permeable sheet material comprising carbon fibers and charcoal bound to the carbon fibers, wherein the carbon fibers and the charcoal are each found to have a wavelength of 1360 cm in a spectrum determined by laser Raman spectroscopy at an excitation wavelength of 532 nm. －１ D band and 1580 cm －１a peak area ratio of the D band to the G band (D / G) of the carbide is 0.8 to 2.0 times the peak area ratio of the D band to the G band (D / G) of the carbon fiber; and a density of 0.20 g / cm ３ [2] A liquid-permeable sheet material having a hardness (Asker C hardness) of 30 or more measured using a type C durometer hardness tester specified in JIS K7312. [2] The sheet material according to the above [1], wherein the peak area ratio (D / G) of the charcoal is 2.0 or more and 3.4 or less, and the peak area ratio (D / G) of the carbon fiber is 1.0 or more and 3.0 or less. [3] The sheet material according to the above [1] or [2], wherein the carbon fiber constitutes a nonwoven fabric, and at least a part of the charcoal is in the form of particles and bound to the carbon fiber. [4] A liquid-permeable sheet material having a BET specific surface area of ​​1.5 m ２ The sheet material according to any one of [1] to [3] above, having a tensile strength of 1 / g or more and a thickness of 1 mm or more and 5 mm or less. [5] The sheet material according to any one of [1] to [4] above, having an average fiber diameter of the carbon fibers of 10 to 30 μm. [6] The sheet material according to any one of [1] to [5] above, wherein the carbide is a carbide of a phenolic resin. [7] The sheet material according to any one of [1] to [6] above, wherein the melting point of the phenolic resin is 75 to 400°C. [8] The sheet material according to any one of [1] to [7] above, further comprising at least one of graphitizable carbon and carbon black. [9] An electrode comprising the sheet material according to any one of [1] to [8] above and a current collector.

[10] A redox flow battery comprising the electrode according to [9] above.

[11] A method for producing a sheet material according to any one of the above [1] to [8], wherein a carbon fiber precursor sheet is immersed in an aqueous dispersion containing a resin binder, pulled out, and then carbonized and graphitized, and then oxidized.

[0015] The sheet material of the present invention maintains high hardness even when it has a low density, thereby achieving both high liquid permeability (preferably high water permeability) due to low density and high contactability due to maintained hardness. While achieving both high liquid permeability and high contactability, the relationship between the graphitization degree of the carbide and the carbon fiber is controlled within a certain range, thereby providing a sheet material that not only reduces cell resistance and improves energy efficiency, but also improves electrolyte permeability and reduces energy loss during electrolyte delivery. Furthermore, the sheet material of the present invention can be produced at low cost by simultaneously carbonizing the carbon fiber precursor sheet and the carbide precursor. Redox flow batteries using the sheet material of the present invention can achieve excellent charge / discharge performance.

[0016] It is a schematic diagram of a redox flow battery. It is a schematic diagram showing a unit cell used in the redox flow battery. It is an SEM photograph of the sheet material obtained in Example 1. It is an SEM photograph of the sheet material obtained in Comparative Example 6.

[0017] In this specification, "to" means a range including the upper and lower limits. Furthermore, in this specification, unless otherwise specified, "carbide" means a carbide other than carbon fiber, and for example, refers to a carbide that is bonded to carbon fiber, and may be expressed as "carbide A." Graphitizable carbon, carbon black, and the like, which will be described later, are not included in carbide A because they do not bond carbon fibers.

[0018] The present inventors have conducted extensive research to provide a sheet material for electrodes that is particularly suitable for use in vanadium-based redox flow batteries, and have found that by bonding carbide A to carbon fibers and increasing the hardness of the sheet material, the contact resistance between the sheet material and the current collector plate can be reduced even if the density of the sheet material is low, and the low density of the sheet material also improves the liquid permeability, thereby completing the present invention.

[0019] Hereinafter, the present invention will be described in detail for each of its constituent elements with reference to FIGS.

[0020] FIG. 1 is a schematic diagram of a redox flow battery suitable for use in the present invention. FIG. 2 is a diagram showing a single cell, in which an ion exchange membrane 3 is disposed between two opposing current collector plates 1, 1, and electrolyte passages 4a, 4b are formed on both sides of the ion exchange membrane 3 by spacers 2 along the inner surfaces of the current collector plates 1, 1. A sheet material 5 is disposed in at least one of the passages 4a, 4b. The current collector plate 1 is provided with a liquid inlet 10 and a liquid outlet 11 for the electrolyte. As shown in FIG. 2, an electrode is composed of the electrode material 5 and the current collector plate 1, and the structure allows the electrolyte to pass through the electrode material 5 (a three-dimensional electrode structure). The sheet material of the present invention can be used for this electrode material 5.

[0021] The sheet material of the present invention is a sheet material in which carbide A is bound to carbon fibers, and the density of the sheet material is 0.20 g / cm ３ and the hardness (Asker C hardness) measured using a type C durometer hardness tester specified in JIS K7312 is 30 or more. As described above, the sheet material of the present invention can achieve both low resistance and high liquid permeability.

[0022] In the sheet material of the present invention, carbide A obtained by carbonizing a carbide precursor and carbon fibers obtained by carbonizing a carbon fiber precursor are bound together. When the carbon fibers are sufficiently bound together by carbide A, the contact resistance between the carbon fibers is reduced and the strength of the sheet material is increased, thereby reducing the contact resistance between the sheet material and the current collector plate when a cell is assembled.

[0023] The average fiber diameter of the carbon fibers is preferably 10 to 30 μm. An average fiber diameter greater than 10 μm improves liquid permeability. An average fiber diameter smaller than 30 μm increases the surface area, allowing for lower cell resistance. Considering both liquid permeability and cell resistance, the average fiber diameter is more preferably 10 to 20 μm. The average fiber length of the carbon fibers is preferably 10 mm or more, and more preferably 30 mm or more. This is because the longer the fiber length, the fewer fiber ends there are, making it less likely for the fibers to puncture the diaphragm. Furthermore, the average fiber length is preferably 120 mm or less, and more preferably 100 mm or less. Within this range, processability is good and productivity is improved.

[0024] Examples of fibers that are carbon fiber precursors include rayon fibers, acrylic fibers, and lignin fibers. However, acrylic fibers (polyacrylonitrile-based fibers) are preferred from the viewpoints of mechanical strength and cost. Furthermore, flame-resistant yarns obtained by heat treating acrylic fibers in air at 200 to 300°C (flame-resistant treatment) may also be used as the carbon fiber precursor fibers. When flame-resistant yarns are not used, it is preferable to form the carbon fiber precursor fibers into a sheet and then perform the flame-resistant treatment. Hereinafter, the carbon fiber precursor fibers formed into a sheet will be referred to as a carbon fiber precursor sheet.

[0025] The carbon fiber precursor sheet may be formed as a woven fabric, a knitted fabric, a nonwoven fabric, or paper (for example, a sheet of a carbon fiber precursor formed by a paper machine). When carbonizing a carbide precursor and a carbon fiber precursor simultaneously to reduce costs, a woven fabric, a knitted fabric, or a nonwoven fabric that can be formed into a sheet using only the carbon fiber precursor fiber is preferred. Furthermore, since liquid permeability deteriorates when the sheet material is made dense, it is preferred that the sheet material be formed as a nonwoven fabric whose density can be adjusted by orienting the fibers more randomly.

[0026] For example, a carbon fiber precursor sheet can be produced by cutting carbon fiber precursor fibers into lengths of 15 mm to 152 mm and then forming them into a nonwoven fabric by any known method. For example, staple fibers are defibrated and then a web is produced using a carding machine. Furthermore, the webs are laminated in cross layers or parallel layers, and the fibers are entangled by needle punching or water jet processing to obtain a carbon fiber precursor sheet. This carbon fiber sheet is then carbonized. In other words, a carbon fiber sheet is a carbonized carbon fiber precursor sheet.

[0027] The carbide precursor may be any material that can adhere to the surface of the carbon fiber precursor in the carbon fiber precursor sheet and that can be carbonized together with the sheet during carbonization of the carbon fiber precursor sheet to bond the carbon fibers together, and a resin binder can be used. Examples of such resin binders include thermosetting resins such as phenolic resins, epoxy resins, melamine resins, and furan resins. Carbonized carbide A obtained by carbonizing these resins is preferably used because it is a non-graphitizable carbon that not only bonds the fibers together but also can impart a high surface area through the activation treatment described below. Among these, phenolic resins are particularly preferred because of their high carbonization yield. Using a phenolic resin as the carbide precursor allows for a high specific surface area while maintaining the hardness of the sheet material, and improves the reactivity of the sheet material with the active material in the electrolyte, resulting in a sheet material that combines reduced cell resistance and high liquid permeability.

[0028] From the viewpoint of ease of handling, the resin binder is preferably a particulate resin binder that is a particulate solid at room temperature of 5 to 35°C. The shape and size of the particles are not limited. If the resin binder is in liquid form, it is difficult to uniformly disperse the resin binder in water, and it must be dissolved in an organic solvent before use, which makes it difficult to handle. Furthermore, liquid resin binders contain a large amount of volatile components, which may worsen the working environment.

[0029] The melting point of the resin binder is preferably 75°C to 400°C. When a particulate resin binder is used, if the melting point of the particulate resin binder is 75°C or higher, it is less likely to form a film when heated, and at least a portion of it can be bound to the fibers in a particulate state. When the melting point of the particulate resin binder is 400°C or lower, it is easily melted, and the binding strength to the carbon fibers can be strengthened. When a particulate resin binder is used, if at least a portion of it is bound to the carbon fibers in a particulate state, deformation and cracking of the sheet material due to heat treatment can be suppressed. Note that the above "particulate" means that it is not formed into a film, and the shape and size of the particles are not limited. The melting point of the resin binder is preferably 75 to 200°C, more preferably 75 to 150°C, and even more preferably 78 to 110°C.

[0030] The volume average particle diameter of the particulate resin binder is preferably 1 to 100 μm, more preferably 5 to 40 μm. When the volume average particle diameter of the particulate resin binder is 100 μm or less, it can be uniformly dispersed throughout the fiber sheet. When the average particle diameter of the particulate resin binder is 1 μm or more, it is possible to prevent particles from passing through the fiber sheet, thereby preventing a decrease in the amount of particle adhesion. The volume average particle diameter is preferably 5 to 70 μm, more preferably 10 to 40 μm.

[0031] The sheet material of the present invention has a hardness (Asker C hardness) of 30 or more as measured using a Type C durometer hardness tester specified in JIS K7312. The sheet density of the sheet material of the present invention is reduced to impart high liquid permeability, but by increasing the Asker C hardness, the contact resistance with the current collector plate and the contact resistance between the fibers when a cell is assembled can be reduced. The Asker C hardness is more preferably 30 or more, and even more preferably 35 or more. The Asker C hardness may also be approximately 50 to 95, or approximately 60 to 85. The Asker C hardness can be controlled by controlling the sheet material density, the amount of carbide A, and the binding state of carbide A.

[0032] The sheet material of the present invention preferably contains 5 to 95 mass% of carbide A, more preferably 10 to 80 mass% of the total weight. When the content is 5 mass% or more, the binding strength between the carbon fibers and the hardness of the sheet are sufficient. When the content is 95 mass% or less, the voids in the sheet material increase, improving liquid permeability.

[0033] In the sheet material of the present invention, the total content of carbon fiber and carbide A is preferably 80 to 100 mass%, more preferably 85 to 100 mass%, and even more preferably 90 to 100 mass%, of the total weight.

[0034] The sheet material of the present invention has a density of 0.20 g / cm ３ When the density is in this range, the liquid permeability is good. The density is preferably less than 0.18 g / cm ３ More preferably, 0.15 g / cm or less ３ The density is more preferably 0.05 to 0.18 g / cm. ３is preferably 0.06 to 0.15 g / cm ３ More preferably, 0.07 to 0.11 g / cm ３ is even more preferred.

[0035] The sheet material of the present invention preferably has a thickness of 1 to 5 mm, more preferably 1.5 to 4 mm, and even more preferably 2.0 to 3.0 mm. A thickness of 1 mm or more can widen the liquid passage space of an electrolytic cell containing the sheet material, thereby improving liquid permeability. A thickness of 5 mm or less makes it easier to achieve the desired resistance reduction effect. Furthermore, the thickness of the sheet material of the present invention is preferably at least greater than the spacer thickness. For example, the thickness is preferably 1.10 to 3.00 times the spacer thickness, more preferably 1.15 to 2.50 times, and even more preferably 1.20 to 2.00 times. The sheet material of the present invention can also be used in a laminated state. Alternatively, it can be combined with other materials.

[0036] When the sheet material of the present invention was analyzed by Raman spectroscopy, －１ D band and 1580 cm －１ A peak appears in the G band near the D band. When the crystallinity is high, a single peak appears in the G band. As the crystallinity decreases (the defect structure increases), a peak appears near the D band. In other words, it is thought that the larger the peak area ratio of the D band to the G band (D / G ratio), the more defect structures in the carbon crystal and the more active sites for reaction with the electrolyte. On the other hand, it is thought that the smaller the peak area ratio of the D band to the G band (D / G ratio), the fewer defect structures in the carbon crystal and the better the electronic conductivity.

[0037] The carbon fibers and carbide A contained in the sheet material of the present invention each have a wavelength of 1360 cm in the spectrum determined by laser Raman spectroscopy at an excitation wavelength of 532 nm. －１ D band and 1580 cm －１The peak area ratio (D / G) of carbide A in the sheet material of the present invention, measured by laser Raman spectroscopy, is preferably 0.8 to 2.0 times the peak area ratio (D / G) of carbon fiber measured by laser Raman spectroscopy. It is more preferably 0.9 to 1.9 times, and even more preferably 1.0 to 1.8 times. Within the above range, the crystallinity of the carbon fiber can be improved, high conductivity can be imparted, and high reactivity can be imparted to carbide A. A peak area ratio (D / G) of 0.8 to 2.0 times means that the graphitization of carbide A is suppressed compared to the graphitization of carbon fiber. This is significant because, even when the same graphitization treatment is performed, the fiber is sufficiently graphitized to increase conductivity, while carbide A is not excessively graphitized, aiming to improve the specific surface area through activation. When the peak area ratio (D / G) of carbide A is 0.8 times or more the peak area ratio (D / G) of the carbon fiber, the specific surface area of ​​carbide A can be improved by activation. When the peak area ratio (D / G) of carbide A is 2.0 times or less the peak area ratio (D / G) of the carbon fiber, the conductivity of carbide A increases. In addition, the durability of carbide A against oxidation degradation due to charge and discharge is improved.

[0038] In the sheet material of the present invention, the peak area ratio (D / G) of the carbon fiber is preferably 1.0 to 3.0, more preferably 1.3 to 2.7, and even more preferably 1.5 to 2.5. When the peak area ratio (D / G ratio) of the carbon fiber is 3.0 or less, the crystal structure defects are low and the conductivity is high. When the peak area ratio (D / G ratio) of the carbon fiber is 1.0 or more, high-temperature treatment exceeding 2000°C is not required, thereby reducing production costs.

[0039] In the sheet material of the present invention, the peak area ratio (D / G) of carbide A is preferably 2.0 to 3.4. More preferably, it is 2.2 to 3.2, and even more preferably, it is 2.4 to 3.0. When the peak area ratio (D / G) of carbide A is 3.4 or less, the crystal structure defects are low and the conductivity is high. When the peak area ratio (D / G) of carbide A is 2.0 or more, the specific surface area can be easily improved by activation.

[0040] Here, the sheet material of the present invention may further contain carbide A having a different peak area ratio than the above and / or carbon fiber having a different peak area ratio than the above, but it is preferable that they do not contain any of them.

[0041] The sheet material of the present invention has a specific surface area (BET specific surface area) determined by the BET method from the amount of nitrogen adsorption of 1.5 m ２ It is preferable that the specific surface area is 2 m / g or more. ２ / g or more, more preferably 15m ２ / g or more. BET specific surface area is 1.5 m ２ / g or more, the contact area with the electrolyte increases, and the reactivity between the sheet material and the active material in the electrolyte increases. ２ If the specific surface area is less than 1.5 to 1000 m / g, the sheet will not become brittle and will be able to maintain its hardness. ２ / g, and 1.5 to 200m ２ / g, and 2 to 100m ２ / g, and 15 to 80m ２ / g.

[0042] The sheet material of the present invention may contain graphitizable carbon to improve conductivity, provided that the surface area of ​​the sheet material is not significantly reduced. Examples of graphitizable carbon include carbides (different from the carbide A) made from pitches, such as coal tar pitch and coal-based pitch, as precursors. The content of graphitizable carbon (preferably, carbides made from pitches as precursors) is preferably 0 to 95% by mass, more preferably 80% by mass or less, of the total weight of the sheet material of the present invention. At 95% by mass or less, the density of the sheet material is not significantly increased, resulting in good liquid permeability. Furthermore, the ratio of graphitizable carbon (e.g., carbides made from pitches as precursors) to carbide A in the sheet material of the present invention is preferably 0 to 95% by mass, more preferably 80% by mass or less. At 95% by mass or less, the carbide surface of the resin binder exposed on the surface of the sheet material is not blocked, ensuring sufficient reactive surface area effective for charge and discharge, thereby suppressing an increase in resistance.

[0043] To obtain a high specific surface area, the sheet material of the present invention may contain high-specific surface area carbon particles to the extent that the hardness of the sheet material is not significantly reduced. Examples of high-specific surface area carbon particles include carbon blacks such as acetylene black (acetylene soot), oil black (furnace black, oil soot), ketjen black, and gas black (gas soot). Other examples include carbon nanotubes (CNTs), carbon nanofibers, carbon aerogels, mesoporous carbon, graphites, graphene, graphene oxide, N-doped CNTs, boron-doped CNTs, and fullerenes. The high-specific surface area carbon particles preferably account for 0 to 30% by mass, more preferably 20% by mass or less, of the total weight of the sheet material of the present invention. A content of 30% by mass or less does not interfere with the binding of fibers by the resin binder. The ratio of the high-specific surface area carbon to the carbide A in the resin binder is preferably 0 to 30% by mass, and more preferably 20% by mass or less. When the ratio is 30% by mass or less, the falling off of carbon particles can be suppressed.

[0044] The specific surface area of ​​the carbon particles is, for example, 50 to 1000 m before the oxidation treatment described below. ２ / g, preferably 100 to 700 m ２ / g, more preferably 200 to 500m ２ The specific surface area can be specified, for example, as the BET specific surface area determined by nitrogen adsorption.

[0045] Next, a method for producing the sheet material of the present invention will be described.

[0046] The sheet material is obtained by forming a sheet precursor by attaching a carbide precursor to a carbon fiber precursor sheet or a carbon fiber sheet, and carbonizing this sheet material precursor. Specifically, the sheet precursor is obtained by impregnating a carbon fiber precursor sheet or a carbon fiber sheet with a dispersion of the carbide precursor, or by spray-coating the dispersion of the carbide precursor or dry-coating the carbide precursor onto a carbon fiber precursor sheet or a carbon fiber sheet. In order to uniformly apply the carbide precursor to the inside of the carbon fiber precursor sheet or the carbon fiber sheet, it is preferable to impregnate the carbon fiber precursor sheet or the carbon fiber sheet with a dispersion of the carbide precursor.

[0047] The sheet material can be obtained by a carbonization treatment in which the sheet precursor is heated in an inert atmosphere to carbonize it. From the viewpoint of processing costs, it is preferable to attach a carbide precursor to a carbon fiber precursor sheet, and then carbonize the carbide precursor and the carbon fiber precursor sheet simultaneously.

[0048] In the examples of the present invention described below, a particulate resin binder is attached to a carbon fiber precursor sheet to form a sheet material precursor, and this sheet precursor is carbonized to produce a sheet material. To apply the resin binder, a method of immersing the carbon fiber precursor sheet in a dispersion of the resin binder in water and drying it, a method of spraying the dispersion onto the carbon fiber precursor sheet and then heating and drying it, or a method of dry-spraying the resin binder can be used. A binder that disappears during carbonization, such as polyvinyl alcohol, may be added to this dispersion as a temporary binder. Furthermore, known dispersants, penetrants, antifoaming agents, etc. may be added to improve the handleability of the dispersion.

[0049] In order to impart uniform hardness to the sheet material of the present invention, it is necessary to apply the resin binder uniformly not only to the surface but also to the inside of the carbon fiber precursor sheet, and therefore a method of immersing the carbon fiber precursor sheet in the dispersion is more preferable. In the immersion process, excess liquid can be removed by passing the carbon fiber precursor sheet through nip rollers having a predetermined clearance, or by scraping off the surface of the excess dispersion liquid when immersed in the dispersion liquid with a doctor blade or the like, or by squeezing with a nip roller.

[0050] A particularly preferred method of impregnation is to immerse a carbon fiber precursor sheet in an aqueous dispersion containing a resin binder, remove it, and dry it. Dispersing the resin binder in the aqueous dispersion maintains the particulate nature of the resin binder, allowing it to be used to bind the carbon fibers together. The concentration of the resin binder in the aqueous dispersion is, for example, 1 to 50 mass%, preferably 2 to 40 mass%, and more preferably 3 to 25 mass%. The binder concentration in the aqueous dispersion is, for example, 0.1 to 10 mass%, preferably 0.3 to 5 mass%, and more preferably 0.5 to 3 mass%. The drying temperature may be below the melting point of the resin binder or above it, but a temperature above the melting point is preferred in terms of enhancing binding properties. The drying temperature is, for example, 70 to 200°C, preferably 90 to 170°C, and more preferably 100 to 140°C.

[0051] Before carbonizing the sheet material precursor, it may be subjected to heat pressing using a heat press roller, a heat press plate, or the like. The heat pressing is performed for the purpose of controlling the thickness of the sheet and aligning fibers exposed perpendicular to the sheet surface parallel to the sheet surface to prevent piercing of the diaphragm. The thickness of the sheet can also be adjusted by heat pressing. The heat pressing temperature is preferably 80°C or higher and 300°C or lower, and more preferably 100°C or higher and 280°C or lower. When the heat pressing temperature is 80°C or higher, the resin binder is easily softened, making it easy to adjust the thickness. When the heat pressing temperature is 300°C or lower, the generation of decomposition gases from the resin binder and the carbon fiber precursor sheet is suppressed, making it easy to prevent deterioration of the working environment.

[0052] The sheet material of the present invention, in which carbon fibers and carbide A are bound together, is formed by carbonizing a sheet material precursor in which a resin binder is attached to a carbon fiber precursor sheet. In the carbonization treatment, it is necessary to remove decomposition gases generated during carbonization; for example, the carbonization treatment is performed by heating in a carbonization furnace at a temperature of 500°C to 1500°C under an inert atmosphere (preferably a nitrogen atmosphere). In order to sufficiently remove the decomposition gases, the heating temperature is preferably 900°C or higher. In consideration of production costs, a temperature of 1500°C or lower is preferable, and 1200°C or lower is more preferable. The carbonization treatment may be performed in multiple stages to control the amount of decomposition gas generated in the carbonization furnace.

[0053] In the present invention, graphitization is preferably performed after the carbonization treatment to increase the crystallinity of the sheet material and improve its electronic conductivity. In the graphitization treatment, heating is performed in an inert atmosphere (preferably a nitrogen atmosphere) in a graphitization furnace at a temperature of 1300°C to 2400°C. Heating at a temperature higher than the heating temperature in the carbonization treatment is preferred, with 1500°C or higher being preferred. In terms of production costs and imparting a large surface area in the oxidation treatment described below, treatment at 2000°C or lower is preferred.

[0054] In the present invention, dry oxidation treatment is preferably performed before or after the graphitization treatment to increase the surface area of ​​the sheet material. In particular, to prevent a decrease in sheet hardness due to the oxidation treatment, it is particularly preferable to perform a single oxidation treatment after the graphitization treatment. The oxidation treatment is a dry heat treatment performed in an acidic gas atmosphere (preferably an air atmosphere). The heating temperature is preferably 500°C to 900°C. This introduces oxygen functional groups, such as hydroxyl groups, carbonyl groups, quinone groups, and lactone groups, to the surface of the sheet material, activating the surface and achieving a high surface area. In other words, the oxidation treatment is an activation treatment. Activation can increase the number of active sites and reduce cell resistance. Activation also reduces density and improves liquid permeability. The heating temperature is preferably 550°C or higher, more preferably 700°C or lower. Heating temperatures below 500°C increase the treatment time and reduce productivity. At temperatures above 900° C., the surface of the sheet material is rapidly oxidized, making the sheet material embrittled and reducing its hardness.

[0055] The crystalline structure of the sheet material of the present invention depends on the temperature during the graphitization treatment and the precursor raw material of the sheet material. To make the peak area ratio (D / G) of carbide A 0.8 to 2.0 times the peak area ratio (D / G) of carbon fiber, one method involves graphitizing a carbon fiber precursor sheet at high temperature, applying a resin binder, and then graphitizing at low temperature. However, this method increases the number of graphitization treatments, raising production costs. To make the peak area ratio (D / G) of carbide A 0.8 to 2.0 times the peak area ratio (D / G) of carbon fiber in a single graphitization treatment, it is preferable to use a resin binder that is more difficult to graphitize than the carbon fiber precursor.

[0056] In the sheet material of the present invention, it is preferable that at least a portion of the carbide A is bound to the carbon fiber in a particulate form without being coated. When a resin binder, which is a carbide precursor, and a carbon fiber precursor are simultaneously carbonized to reduce the energy and cost required for the carbonization process, if the carbonization process is performed in a state in which the resin binder coats the carbon fiber precursor, the sheet material may be distorted or cracked due to fiber shrinkage, resulting in an unflat sheet material. Therefore, by carbonizing at least a portion of the carbide A of the resin binder in a particulate form without being coated, a smooth sheet material can be produced.

[0057] This application claims the benefit of priority based on Japanese Patent Application No. 2023-120037, filed on July 24, 2023. The entire contents of the specification of Japanese Patent Application No. 2023-120037, filed on July 24, 2023, are incorporated herein by reference.

[0058] The present invention will be described in detail below with reference to examples, but these examples are not intended to limit the scope of the present invention. Measurement methods for the physical properties of the sheet material will be described below, but the physical properties described in this specification, including the examples, are based on values ​​determined by the following methods.

[0059] First, the methods for measuring the physical properties of the sheet materials of the examples and comparative examples will be described. (Thickness and density of sheet material) A constant pressure thickness measuring instrument (PG-16 manufactured by Teclock Corporation) was used to measure the thickness of the sheet material at a pressure of 5 cm. ２The thickness was determined by measuring five points on the sample with a measuring probe at a pressure of 7 gf / cm 2 , and the density was calculated from the weight and area of ​​the sheet material.

[0060] (Hardness of Sheet Material) A type C durometer hardness tester (Asker C hardness tester) specified in JIS K7312 was used. Specifically, a sheet material was cut out to a width of 20 mm and a length of 50 mm, and the hardness was measured 30 seconds after the pressure surface of the Asker C hardness tester was pressed against the sheet material as an evaluation sample. Note that since the thickness of the sheet material was less than 10 mm, multiple sheets were stacked to a thickness exceeding 10 mm for measurement.

[0061] (Measurement of peak area ratio (D / G ratio) by laser Raman spectroscopy) A Raman-11 manufactured by Nanophoton Inc. was used, with a 100x objective lens (NA = 0.9), a 300 gr / mm grating, and an excitation laser wavelength of 532 nm. The field of view (spot) was adjusted to either carbide A (carbide A portion) or carbon fiber (carbon fiber portion), and the laser intensity was adjusted using an ND filter so as to obtain a spectrum with a good S / N ratio. Measurement was performed under conditions where the carbon structure would not change due to laser irradiation. The obtained spectrum was analyzed using a 700 to 2000 cm －１ The wavenumber range was linearly baseline corrected, and the －１ Nearby is D band, 1580 cm －１ Depending on the peak shape, the area around 1620 cm was designated as the G band. Depending on the peak shape, the area around 1620 cm was designated as the D' band, and 0 to 2 other peaks (1160 cm －１ Near 1530 cm －１ The peak area ratio (D / G ratio) was calculated from the peak area at 1360 cm obtained by peak separation. －１ The peak area of ​​the D band in the vicinity of 1580 cm －１ This is the value obtained by dividing the peak area of ​​the G band in the vicinity.

[0062] (BET specific surface area (BET: m ２Approximately 50 mg of the sheet material was weighed out and vacuum dried for 24 hours at 130° C. The amount of nitrogen adsorption of the dried sample was measured using an automatic specific surface area measuring device (Shimadzu Corporation, GEMINI VII) by a gas adsorption method using nitrogen gas, and the BET specific surface area (m / g) was determined by a multipoint method based on the BET method.

[0063] (Measurement of total cell resistance) The sheet material was placed in a 16 cm2 area with a length of 10 cm in the vertical direction (fluid flow direction) and a width of 1.6 cm2. ２ The cell shown in Figure 2 was assembled. The ion exchange membrane used was an APS4 membrane manufactured by Asahi Glass Co., Ltd., and the spacer thickness was 2.0 mm. ２ The total cell resistance was calculated from the voltage curve after two cycles in the voltage range of 1.70 to 1.00 V using the following formula. The cathode electrolyte used was a 2.5 mol / L aqueous sulfuric acid solution containing 1.7 mol / L vanadium oxysulfate, and the anode electrolyte used was a 2.5 mol / L aqueous sulfuric acid solution containing 1.7 mol / L vanadium sulfate. The electrolyte volume was 10 mL, the flow rate was 20 mL per minute, and measurements were taken at 30°C. The charge voltage V corresponding to the amount of electricity when the charge rate is 50% is Ｃ５０ , discharge voltage V Ｄ５０ are calculated from the voltage curve, and the current density is calculated as I (mA / cm ２ ) and the total cell resistance (Ω cm2) was calculated using the following formula: Total cell resistance = (V Ｃ５０ -V Ｄ５０ ) / (2・I)[Ω・cm ２ ] where V Ｃ５０ is the charging voltage for the amount of electricity when the charging rate is 50% calculated from the voltage curve, V Ｄ５０ is the discharge voltage relative to the amount of electricity when the charge rate is 50%, calculated from the voltage curve, and I = current density [mA / cm ２ ].

[0064] (Measurement of liquid permeability) The sheet material was placed in a 16 cm2 area, 10 cm in the vertical direction (liquid passage direction) and 1.6 cm in the width direction. ２A sample was cut out and assembled into the cell shown in Figure 2 in the same manner as in the measurement of the total cell resistance. Pure water at 25°C was placed downstream of the cell, and pure water was sucked into the cell from the upstream side using an aspirator to fill the cell. The pressure-reducing valve of the aspirator was adjusted to 0.02 MPa, and the time required for 10 g (≒ 10 mL) of water to be discharged downstream was measured, and the flow rate [mL / min] at that time was also measured.

[0065] (Observation of the Morphology of Carbide A) The sheet material was cut into 5 mm pieces, and Au was vapor-deposited using a sputtering device. The surface of the sheet material was observed under a scanning electron microscope at 1000 magnifications to see how the carbide A had adhered.

[0066] (Example 1) In Example 1, the following steps were carried out. <Production of carbon fiber precursor sheet> As a carbon fiber precursor sheet, flame-resistant polyacrylonitrile cotton having an average fiber diameter of 15 μm was defibrated with a carding machine to form a web. The obtained web was punched to a punching density of 250 holes / cm. ２ Needle punched at a weight of 250 g / m ２A flame-resistant polyacrylonitrile nonwoven fabric having a thickness of 3.3 mm was obtained. <Preparation of Dispersion> A dispersion was prepared by adding 1.0 wt% of polyvinyl alcohol as a temporary binder, 10.0 wt% of uncured phenolic resin particles (melting point approximately 90°C, average particle size 20 μm) as a resin binder, and 1.0 wt% of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant to ion-exchanged water. <Preparation of Sheet Material Precursor> The flame-resistant polyacrylonitrile nonwoven fabric obtained was immersed in the obtained Dispersion 1, passed through nip rollers to remove excess dispersion, and then dried in an air atmosphere at 120°C for 20 minutes to obtain a sheet material precursor. <Heat Pressing> The obtained sheet material precursor was heat-pressed with a heat press roller at 200°C, with a gauge pressure of 3 MPa and a heat press roll clearance of 2.0 mm. <Carbonization, graphitization, and oxidation treatment> The hot-pressed sheet material precursor was heated to 1000°C at a heating rate of 5°C / min in nitrogen gas and held at this temperature for 1 hour to perform carbonization (firing). The temperature was further raised to 1500°C at a heating rate of 5°C / min in nitrogen gas and held at this temperature for 1 hour to perform graphitization. Finally, the sheet material precursor was subjected to oxidation treatment at 650°C in an air atmosphere until the weight yield reached 94%. By performing the above steps, in Example 1, a sheet material having a weight of 229 g / m ２ A sheet material having a thickness of 2.4 mm was obtained. Fig. 3 is an SEM photograph of the sheet material obtained in Example 1. From Fig. 3, it can be seen that in the sheet material of Example 1, carbide A is present that is bound to the carbon fibers without forming a film.

[0067] (Example 2) In Example 2, in the step of <Preparation of Dispersion Liquid> in Example 1, 1.0 wt % of polyvinyl alcohol as a temporary binder, 10.0 wt % of uncured phenolic resin particles (melting point approximately 100°C, average particle size 20 μm) as a resin binder, and 1.0 wt % of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant were added to ion-exchanged water to prepare a dispersion liquid. A sheet material was prepared in the same manner as in Example 1 except for this step. In Example 2, a sheet material having a weight of 257 g / m was prepared. ２ A sheet material having a thickness of 2.5 mm was obtained.

[0068] (Example 3) In Example 3, in the <carbonization, graphitization, and oxidation treatment> step of Example 1, the graphitization treatment involved heating the material in nitrogen gas at a heating rate of 5°C / min to 1500°C, holding the material at this temperature for 1 hour, and then heating the material in nitrogen gas at a heating rate of 5°C / min to 2000°C, holding the material at this temperature for 1 hour to perform graphitization. The carbonization treatment and oxidation treatment were carried out in the same manner as in Example 1. A sheet material was produced in the same manner as in Example 1, except for the <carbonization, graphitization, and oxidation treatment> step. In Example 3, a sheet material having a weight of 223 g / m was produced. ２ A sheet material having a thickness of 2.5 mm was obtained.

[0069] (Example 4) In Example 4, in the step of <Preparation of Dispersion Liquid> in Example 1, a dispersion liquid was prepared by adding 1.0 wt % of polyvinyl alcohol as a temporary binder, 5.0 wt % of uncured phenolic resin particles (melting point approximately 80°C, average particle size 20 μm) and 5.0 wt % of high-softening-point coal tar pitch (melting point approximately 320°C or higher, average particle size 20 μm) as binders, and 1.0 wt % of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant to ion-exchanged water. A sheet material was prepared in the same manner as in Example 1 except for this step. In Example 4, a dispersion liquid was prepared using a weight of 227 g / m. ２ A sheet material having a thickness of 2.5 mm was obtained.

[0070] (Example 5) In Example 5, in the <Preparation of Dispersion Liquid> of Example 1, 1.0 wt % of polyvinyl alcohol as a temporary binder, 8.5 wt % of uncured phenol resin particles (melting point approximately 90°C, average particle diameter 20 μm) as a resin binder, and 1.0 wt % of phenol resin particles (melting point approximately 90°C, average particle diameter 20 μm) as a specific surface area 300 m ２ A dispersion was prepared by adding 1.5% by weight of carbon black particles having a particle size of 229 g / m² and 1.0% by weight of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant. A sheet material was prepared in the same manner as in Example 1 except for this step. In Example 5, a sheet material having a particle size of 229 g / m² was prepared. ２ A sheet material having a thickness of 2.4 mm was obtained.

[0071] In Example 6, in the step of <Preparation of carbon fiber precursor sheet> in Example 1, flame-resistant polyacrylonitrile cotton having an average fiber diameter of 15 μm was defibrated with a carding machine to form a web as a carbon fiber precursor sheet. The obtained web was punched to a punching density of 250 holes / cm.２ Needle punching was performed at a weight of 300 g / m ２ In the step of <Preparation of Dispersion> of Example 1, 1.0 wt % of polyvinyl alcohol was used as a temporary binder, 4.5 wt % of uncured phenol resin particles (melting point: approximately 90°C, average particle size: 20 μm) and 4.5 wt % of high-softening-point coal tar pitch (melting point: approximately 320°C or higher, 20 μm) were used as binders, and a flame-resistant polyacrylonitrile nonwoven fabric having a specific surface area of ​​300 m was obtained. ２ A dispersion was prepared by adding 1.0 mass % of carbon black particles having a molecular weight of 1 / g and 1.0 wt % of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant. A sheet material was prepared in the same manner as in Example 1 except for the steps of <Preparation of carbon fiber precursor sheet> and <Preparation of dispersion>. In Example 6, a 1 m ２ Weight per unit (hereinafter simply referred to as "weight"): 227 g / m ２ A sheet material having a thickness of 2.4 mm was obtained.

[0072] In Example 7, in the step of <Preparation of carbon fiber precursor sheet> in Example 1, flame-resistant polyacrylonitrile cotton having an average fiber diameter of 15 μm was defibrated with a carding machine to form a web as a carbon fiber precursor sheet. The obtained web was punched to a punching density of 250 holes / cm. ２ Needle punching was performed at a weight of 300 g / m ２ A flame-retardant polyacrylonitrile nonwoven fabric having a thickness of 3.8 mm was obtained. In addition, in the step of <Preparation of dispersion> in Example 1, 1.0 wt % of polyvinyl alcohol was added as a temporary binder, 20 wt % of uncured phenolic resin particles (melting point approximately 90°C, average particle size 20 μm) as a resin binder, and 1.0 wt % of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant to prepare a dispersion. A sheet material was prepared in the same manner as in Example 1, except for the steps of <Preparation of carbon fiber precursor sheet> and <Preparation of dispersion>. In Example 7, a carbon fiber precursor sheet having a weight of 346 g / m was prepared. ２ A sheet material having a thickness of 2.5 mm was obtained.

[0073] In Comparative Example 1, in the step of <Preparation of carbon fiber precursor sheet> in Example 1, flame-resistant polyacrylonitrile cotton having an average fiber diameter of 15 μm was defibrated with a carding machine to form a web as a carbon fiber precursor sheet. The obtained web was punched to a punching density of 250 holes / cm. ２ Needle punching was performed at a weight of 400 g / m ２ A flame-resistant polyacrylonitrile nonwoven fabric having a thickness of 4.3 mm was obtained. A sheet material was produced in the same manner as in Example 1 except for this step. In Comparative Example 1, a flame-resistant polyacrylonitrile nonwoven fabric having a weight of 227 g / m ２ A sheet material having a thickness of 3.0 mm was obtained.

[0074] (Comparative Example 2) In Comparative Example 2, in the step of <Preparation of Dispersion> in Example 1, 20.0 wt % of uncured phenolic resin particles (melting point approximately 90°C, average particle size 20 μm) were added as the resin binder, and 1.0 wt % of Rheodol TWL-120 (manufactured by Kao Corporation) was added as the dispersant to prepare a dispersion. Furthermore, in the <Processing of Dispersion into Carbon Fiber Precursor Sheet> in Example 1, a flame-resistant polyacrylonitrile nonwoven fabric was immersed in the resin binder dispersion, and then passed through a nip roller to remove excess dispersion. This process was then repeated three times to dry the fabric in an air atmosphere at 120°C for 20 minutes, yielding a sheet material precursor. A sheet material was prepared in the same manner as in Example 1, except for the steps of <Preparation of Dispersion> and <Processing of Dispersion into Carbon Fiber Precursor Sheet>. In Comparative Example 2, a sheet material with a weight of 576 g / m was prepared. ２ A sheet material having a thickness of 2.4 mm was obtained.

[0075] (Comparative Example 3) In Comparative Example 3, in the step of <Preparation of carbon fiber precursor sheet> of Example 1, a carbon fiber precursor sheet was prepared by defibrating flame-resistant polyacrylonitrile cotton having an average fiber diameter of 15 μm with a carding machine to form a web, and punching the obtained web to a punching density of 250 threads / cm. ２ Needle punching was performed at a weight of 400 g / m ２A flame-retardant polyacrylonitrile nonwoven fabric having a thickness of 4.3 mm was obtained. Furthermore, in the step of <Preparation of dispersion> in Example 1, 1.0 wt % of polyvinyl alcohol as a temporary binder, 1.0 wt % of uncured phenolic resin particles (melting point approximately 90°C, average particle size 20 μm) as a resin binder, and 1.0 wt % of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant were added to ion-exchanged water to prepare a dispersion. A sheet material was prepared in the same manner as in claim 1, except for the steps of <Preparation of carbon fiber precursor sheet> and <Preparation of dispersion>. In Comparative Example 3, a carbon fiber precursor sheet having a weight of 232 g / m was prepared. ２ A sheet material having a thickness of 2.8 mm was obtained.

[0076] (Comparative Example 4) In Comparative Example 4, in the step of <Preparation of Dispersion> in Example 1, a dispersion was prepared by adding 1.0 wt % of polyvinyl alcohol as a temporary binder, 10.0 wt % of high-softening-point coal tar pitch (melting point of approximately 320°C or higher, average particle size of 20 μm), and 1.0 wt % of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant to ion-exchanged water. A sheet material was prepared in the same manner as in Example 1 except for this step. In Comparative Example 4, a sheet material with a weight of 221 g / m was prepared. ２ A sheet material having a thickness of 2.5 mm was obtained.

[0077] (Comparative Example 5) In Comparative Example 5, the <Preparation of dispersion> step of Comparative Example 4 was carried out instead of the <Preparation of dispersion> step of Example 1, and the <Carbonization, graphitization, and oxidation treatment> step of Example 1 was carried out instead of the <Carbonization, graphitization, and oxidation treatment> step of Example 3. A sheet material was produced in the same manner as in Comparative Example 1 except for the <Preparation of dispersion> step and the <Carbonization, graphitization, and oxidation treatment> step. In Comparative Example 5, a sheet material having a weight of 218 g / m ２ A sheet material having a thickness of 2.6 mm was obtained.

[0078] (Comparative Example 6) In Comparative Example 6, in the step of <Preparation of Dispersion> in Example 1, 1.0 wt % of polyvinyl alcohol as a temporary binder, 10.0 wt % of uncured phenolic resin particles (melting point approximately 70°C, average particle size 20 μm) as a resin binder, and 1.0 wt % of Rheodol TWL-120 (manufactured by Kao Corporation) as a dispersant were added to ion-exchanged water to prepare a dispersion. Aside from this step, a sheet material was prepared in the same manner as in Example 1. However, frequent cracks and curvatures occurred, making it impossible to obtain a flat surface. Figure 4 is an SEM photograph of the sheet material obtained in Comparative Example 6. As can be seen from Figure 4, the sheet material of Comparative Example 6 has carbide A coated and bonded to the carbon fibers.

[0079] Table 1 shows the results of measuring the physical properties of the sheet materials of the examples and comparative examples.

[0080]

[0081] Examples 1 to 7 are sheet materials of the present invention, and sheet materials having low resistance and high liquid permeability were obtained. As described above, this is thought to be due to the following: (1) carbide A is bound to the carbon fiber, and this carbide A has a peak area ratio of the D band to the G band (D / G) of 0.8 to 2.0 times the peak area ratio (D / G) of the carbon fiber, thereby achieving both the conductivity and surface area of ​​the constituent material (carbon fiber) of the sheet material; (2) by improving the hardness of the sheet material, the contact resistance between the sheet material and the current collector plate can be reduced even if the sheet material is low in density; and (3) because the sheet material has low density, the liquid permeability is also improved, thereby improving the reactivity between the sheet material and the active material in the electrolyte.

[0082] The sheet material of Comparative Example 1 is an example in which carbide A is not bound to the carbon fibers. Although the liquid permeability is good, it is thought that the sheet hardness is low and the specific surface area is also low, so that the resistance-lowering effect was not obtained.

[0083] Although the sheet material of Comparative Example 2 has carbide A bound to the carbon fibers, the high density of the sheet material not only deteriorates the liquid permeability, but also prevents the supply of sufficient active material for the reaction due to the deteriorated liquid permeability, which is thought to be why the resistance reduction effect was not achieved.

[0084] In the sheet material of Comparative Example 3, although carbide A was bound to the carbon fibers, the hardness of the sheet material was low and the contact resistance could not be reduced, which is thought to be why the resistance-lowering effect was not obtained.

[0085] In the sheet materials of Comparative Examples 4 and 5, carbide A is bound to the carbon fiber, but the (D / G) of carbide A is less than 0.8 times the (D / G) of the carbon fiber, which is considered to be why a high specific surface area could not be achieved and the effect of reducing resistance was not obtained.

[0086] The sheet material of Comparative Example 6 was subject to frequent bending and cracking, and a flat sheet could not be obtained. This is because the carbon fibers were coated with carbide A, and deformation due to shrinkage of the carbon fiber precursor sheet during heat treatment could not be suppressed.

[0087] According to the present invention, it is possible to provide a sheet material with excellent system efficiency, which not only reduces cell resistance during initial charge and discharge but also reduces pump load during liquid delivery and energy loss due to self-discharge. Therefore, the present invention is useful as an electrode sheet material for a flow-through electrolytic cell in which a liquid is circulated, and is particularly useful as an electrode sheet material for constituting electrodes of a redox battery using a vanadium-based electrolyte. The sheet material of the present invention can be suitably used for water electrolysis, organic hydride electrolysis synthesis, etc.

Claims

1. A carbon fiber and a carbide bonded to the carbon fiber, wherein the carbon fiber and the carbide each have a wavelength of 1,360 cm in a spectrum obtained by laser Raman spectroscopy with an excitation wavelength of 532 nm. －１ D band and 1580 cm －１ a peak area ratio (D / G) of the carbide to the D band to the G band is 0.8 to 2.0 times the peak area ratio (D / G) of the carbon fiber to the D band to the G band; and a density of 0.20 g / cm ３ and has a hardness (Asker C hardness) of 30 or more as measured using a type C durometer hardness tester specified in JIS K7312.

2. The sheet material according to claim 1, wherein the peak area ratio (D / G) of the carbide is 2.0 or more and 3.4 or less, and the peak area ratio (D / G) of the carbon fiber is 1.0 or more and 3.0 or less.

3. The sheet material according to claim 1, wherein the carbon fibers form a nonwoven fabric, and at least a portion of the carbide is in particulate form and bound to the carbon fibers.

4. BET specific surface area is 1.5m ２ The sheet material according to claim 1, wherein the elastic modulus is 1 mm or more and the thickness is 5 mm or less.

5. The sheet material according to claim 1, wherein the carbon fibers have an average fiber diameter of 10 to 30 μm.

6. The sheet material according to claim 1, wherein said carbonized material is a carbonized material of a phenolic resin.

7. The sheet material according to claim 1, wherein the melting point of said phenolic resin is 75 to 400°C.

8. The sheet material according to claim 1, further comprising at least one of graphitizable carbon and carbon black.

9. An electrode comprising the sheet material according to any one of claims 1 to 8 and a current collector.

10. A redox flow battery comprising the electrode according to claim 9.

11. A method for producing a sheet material according to any one of claims 1 to 8, comprising immersing a carbon fiber precursor sheet in an aqueous dispersion containing a resin binder, removing the sheet from the aqueous dispersion, carbonizing the sheet, graphitizing the sheet, and then oxidizing the sheet.

Images