Carbon fiber sheet, gas diffusion electrode substrate, fuel cell, and method for producing carbon fiber sheet

Abstract

The present invention addresses the problem of providing a carbon fiber sheet which is capable of suppressing drying of an electrolyte membrane by means of high thermal conductivity and which exhibits high gas diffusivity even in a high current density region. The present invention is a carbon fiber sheet obtained by bonding a carbon fiber structure with a binder containing at least a resin and fibrous carbon, the content of the fibrous carbon being 25% by mass or more per 100% by mass of the carbon fiber sheet, and the carbon fiber sheet having peaks in the pore diameter distribution of the carbon fiber sheet between 0.3-1.0 [mu] m and 20-100 [mu] m, respectively.

Description

Technical Field

[0001] The present invention relates to a carbon fiber sheet suitable for use in electrodes of fuel cells, particularly solid polymer fuel cells, a method for producing the same, and a gas diffusion electrode substrate and a fuel cell using the carbon fiber sheet. Background Art

[0002] Electrodes used in polymer electrolyte fuel cells are sandwiched between two separators in a polymer electrolyte fuel cell. They have a structure consisting of a catalyst layer formed on the surface of the electrolyte membrane and a gas diffusion electrode substrate formed outside the catalyst layer on both sides of the electrolyte membrane. Required properties of the gas diffusion electrode substrate include gas diffusivity, electrical conductivity for collecting electricity generated in the catalyst layer, water drainage for efficient removal of moisture generated on the catalyst layer surface, and thermal conductivity for dissipating reaction heat from the catalyst layer to the outside of the system.

[0003] The water-repellency of a gas diffusion electrode substrate is particularly important for preventing a sharp voltage drop (flooding) in high current density regions where large amounts of water are generated. To improve this water-repellency, a carbon fiber sheet composed of carbon fibers is typically treated with a hydrophobic treatment to enhance its water repellency. Furthermore, by forming pores of varying sizes within the gas diffusion electrode substrate, a gas diffusion electrode substrate has been designed with a controlled pore size distribution that maintains gas diffusion pathways even when water accumulates in some pores.

[0004] Furthermore, fuel cells used as automotive power sources will be required to deliver high power generation performance even under high-temperature conditions, with cell temperatures exceeding 90°C. At high temperatures, the electrolyte membrane dries out, reducing its ion conductivity and thus power generation performance (a phenomenon known as "dryout"). Therefore, to prevent the electrolyte membrane from reaching high temperatures, the gas diffusion electrode substrate must possess high thermal conductivity, enabling it to dissipate heat generated by the catalyst layer.

[0005] In Patent Document 1, fibrillated synthetic pulp is added to a carbon fiber sheet to form pores of 5 μm to 20 μm and pores of 20 μm to 50 μm, thereby achieving both gas diffusibility and drainage.

[0006] In Patent Document 2, carbon fibers with a fiber length of 1 mm or longer and short carbon fibers with a fiber length of less than 1 mm are used to form pores of different sizes in a carbon fiber sheet containing pulp, thereby improving gas diffusivity.

[0007] Patent Document 3 proposes a porous member containing vapor grown carbon fibers (VGCF) and supported only by conductive particles and a resin in order to ensure thermal conductivity.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Laid-Open No. 2011-181511

[0011] Patent Document 2: Japanese Patent Application Laid-Open No. 2013-218951

[0012] Patent Document 3: International Publication No. 2010 / 050219 Summary of the Invention

[0013] Problems to be solved by the invention

[0014] In Patent Document 1, high thermal conductivity is not achieved due to the use of pulp with a low degree of graphitization. In addition, pores larger than 5 μm and smaller than 20 μm tend to retain generated water, making it difficult for them to effectively function as gas diffusion paths. Therefore, there is a tendency for voltage to drop due to overflow in high current density areas. Patent Document 2 incorporates PAN-based or pitch-based carbon powder fibers (milled fibers), but this does not adequately suppress dry-out in terms of thermal conductivity. In addition, the pore diameter formed by the short carbon fibers is also on the order of several μm, which does not effectively function as a gas diffusion path under power generation conditions that generate large amounts of water. In Patent Document 3, since it does not use carbon fibers as a skeleton, it does not have pores larger than 20 μm and does not have drainage paths of sufficient size. Therefore, there is a problem of easily causing overflow in high current density areas.

[0015] An object of the present invention is to provide a carbon fiber sheet that can suppress drying out of an electrolyte membrane by virtue of its high thermal conductivity and exhibit high gas diffusivity even in a high current density region.

[0016] Means of solving problems

[0017] The present inventors have discovered a carbon fiber sheet that achieves both thermal conductivity and gas diffusion properties by bonding carbon fibers with a mixture containing fibrous carbon.

[0018] (1) A carbon fiber sheet, which is formed by bonding a carbon fiber structure with a binder containing at least a resin and fibrous carbon,

[0019] The content of the fibrous carbon in 100% by mass of the carbon fiber sheet is 25% by mass or more,

[0020] The pore size distribution of the carbon fiber sheet has peaks in the pore size range of 0.3 to 1.0 μm and in the pore size range of 20 to 100 μm.

[0021] (2) A carbon fiber sheet, which is formed by bonding a carbon fiber structure with a binder containing at least a resin and fibrous carbon,

[0022] The pore size distribution of the carbon fiber sheet has peaks in the pore size range of 0.3 to 1.0 μm and 20 to 100 μm.

[0023] The pore volume of pores with a pore diameter of 0.3 to 1.0 μm is 0.40 mL / g or more.

[0024] (3) The carbon fiber sheet according to (1), wherein the pore volume of the pores with a pore diameter of 0.3 to 1.0 μm is 0.40 mL / g or more.

[0025] (4) The carbon fiber sheet according to any one of (1) to (3), wherein the content of the resin in 100% by mass of the carbon fiber sheet is 5% by mass or more and 25% by mass or less.

[0026] (5) The carbon fiber sheet according to any one of (1) to (4), wherein the content of the fibrous carbon is 40% by mass or more in 100% by mass of the carbon fiber sheet.

[0027] (6) The carbon fiber sheet according to any one of (1) to (5), wherein the aspect ratio of the fibrous carbon is 10 or more and 400 or less.

[0028] (7) The carbon fiber sheet according to any one of (1) to (6), wherein the fiber diameter of the fibrous carbon is 100 nm or more and 1 μm or less.

[0029] (8) The carbon fiber sheet according to any one of (1) to (7), which has a peak with a half-value width of 0.3° or more and 1.0° or less in the range of 25° to 28° in an X-ray diffraction pattern.

[0030] (9) The carbon fiber sheet according to any one of (1) to (8), wherein the decrease in tensile stress when further stretched 0.2 mm after breaking is 1 N / cm or more and 6 N / cm or less.

[0031] (10) The carbon fiber sheet according to any one of (1) to (9), wherein the carbon fiber structure is a carbon fiber papermaking body.

[0032] (11) The carbon fiber sheet according to any one of (1) to (10), wherein the resin is a fluororesin.

[0033] (12) A gas diffusion electrode substrate having a microporous layer formed on one side of the carbon fiber sheet described in any one of (1) to (11).

[0034] (13) A fuel cell using the carbon fiber sheet described in any one of (1) to (11) as a gas diffusion electrode.

[0035] (14) A fuel cell using the gas diffusion electrode substrate described in (12) as a gas diffusion electrode.

[0036] (15) A transportation device using the fuel cell described in (13) or (14) as a power supply source.

[0037] (16) A method for producing a carbon fiber sheet according to any one of (1) to (11), the method comprising: an impregnation step of impregnating a carbon fiber structure with a liquid composition containing a resin and fibrous carbon as a binder; and a heat treatment step of melting the resin contained in the liquid composition impregnated in the impregnation step.

[0038] Effects of the Invention

[0039] By using the carbon fiber sheet of the present invention as a gas diffusion electrode substrate, thermal conductivity and gas diffusivity can be achieved simultaneously, and a solid polymer fuel cell can be achieved that exhibits high power generation performance in a high-temperature environment and a high current density range. DETAILED DESCRIPTION

[0040] <Carbon fiber sheet>

[0041] In the present invention, the carbon fiber sheet is particularly suitable for use as a gas diffusion electrode substrate for a solid polymer fuel cell. Therefore, the following may sometimes describe the effects of using the carbon fiber sheet as a gas diffusion electrode substrate as the effects when the carbon fiber sheet takes a specific form. A "carbon fiber sheet" is a structure having a porous structure, wherein the porous structure is composed of a network structure essentially composed of carbon fibers and voids between the carbon fibers. Specific examples of the carbon fiber sheet of the present invention include carbon fiber papermaking, woven fabrics, non-woven fabrics, and other carbon fiber structures bonded together by a binder. Among them, carbon fiber papermaking is particularly preferably used as the carbon fiber structure because of its excellent property of absorbing dimensional changes in the direction perpendicular to the surface of the electrolyte membrane, that is, "springiness". Here, "carbon fiber papermaking" refers to a structure in which carbon fibers are randomly dispersed in a two-dimensional plane.

[0042] Carbon fiber paper can be made by a wet papermaking method in which carbon fibers are dispersed in a liquid, or by a dry papermaking method in which carbon fibers are dispersed in air. The wet papermaking method is preferred due to its excellent productivity. Carbon fiber paper can also be made by mixing carbon powder or organic fibers to improve the conductivity and drainage of the gas diffusion electrode substrate. Furthermore, since form retention and operability are easily improved, carbon fibers can also be bonded to each other using organic polymers such as polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, and cellulose.

[0043] Examples of the carbon fibers constituting the carbon fiber sheet include polyacrylonitrile (PAN)-based, pitch-based, and rayon-based carbon fibers. PAN-based carbon fibers are preferably used because of their excellent mechanical strength.

[0044] The average single fiber diameter of the carbon fibers constituting the carbon fiber sheet (hereinafter referred to as the "carbon fiber diameter") is preferably within the range of 3 to 20 μm, more preferably 5 to 12 μm. A carbon fiber diameter of 3 μm or greater, more preferably 5 μm or greater, increases pore size, improves drainage, and facilitates flooding prevention. On the other hand, a carbon fiber diameter of 20 μm or less, more preferably 12 μm or less, minimizes thickness variation, making it easier to control the thickness within the preferred carbon fiber sheet thickness range described below. The carbon fiber diameter is determined by photographing the carbon fibers at 1000x magnification using a microscope such as a scanning electron microscope (SEM). Thirty randomly selected single fibers are then measured for their diameters, and the average value is calculated as the carbon fiber diameter.

[0045] When using a carbon fiber papermaking body as a carbon fiber structure, the average length of the carbon fibers constituting the carbon fiber papermaking body (hereinafter referred to as "carbon fiber length") is preferably in the range of 3 to 20 mm, more preferably in the range of 5 to 15 mm. A carbon fiber length of 3 mm or greater, more preferably 5 mm or greater, tends to produce a carbon fiber sheet with excellent mechanical strength, electrical conductivity, and thermal conductivity. On the other hand, a carbon fiber length of 20 mm or less, more preferably 15 mm or less, improves the dispersion of the carbon fibers during papermaking, making it easier to produce a homogeneous carbon fiber sheet. Carbon fibers having such a length can be obtained by, for example, cutting continuous carbon fibers into the desired length. The carbon fiber length is determined by photographing the carbon fibers at 50x magnification using a microscope such as an SEM. Thirty randomly selected individual fibers are then measured and their lengths are averaged to determine the average value.

[0046] The thickness of the carbon fiber sheet is preferably 50 to 230 μm, more preferably 70 to 180 μm. A carbon fiber sheet thickness of 230 μm or less, more preferably 180 μm or less, improves gas diffusion and facilitates the discharge of generated water. This also facilitates a reduction in the overall size of the fuel cell. On the other hand, a carbon fiber sheet thickness of 50 μm or greater, more preferably 70 μm or greater, improves in-plane gas diffusion efficiency within the sheet, and thus enhances power generation performance.

[0047] The pore size distribution of the carbon fiber sheet of the present invention has at least two peaks between 0.3 μm and 1.0 μm, and between 20 μm and 100 μm. The pores between 0.3 μm and 1.0 μm are formed by a binder containing at least a resin and fibrous carbon. Even under power generation conditions where a large amount of generated water is generated during power generation, the generated water does not remain and acts as a gas diffusion path, thereby effectively suppressing overflow. The volume of the pores between 0.3 μm and 1.0 μm is preferably 0.40 mL / g or more. More preferably, it is 0.50 mL / g or more and 1.5 mL / g or less. If the volume of the pores between 0.3 μm and 1.0 μm is less than 0.40 mL / g, overflow may occur due to the lack of gas diffusion paths. When the pore volume of pores between 0.3 μm and 1.0 μm exceeds 1.5 mL / g, the volume of pores between 20 μm and 100 μm decreases, leading to a decrease in gas diffusivity. Pores between 20 μm and 100 μm originate from the pores formed between carbon fibers bonded by a binder and serve as the primary gas diffusion pathway and discharge path for generated water. The pore size distribution can be measured using a mercury porosimeter, such as the Autopore 9520 manufactured by Shimadzu Corporation.

[0048] The carbon fiber sheet of the present invention preferably has a reduction in tensile stress when stretched further by 0.2 mm after breaking, preferably from 1 N / cm to 6 N / cm, more preferably from 1 N / cm to 4 N / cm. By setting the reduction to 6 N / cm or less per 0.2 mm, even if excessive tensile stress is applied to the carbon fiber sheet in a subsequent process using the carbon fiber sheet, such as in the process of manufacturing a membrane electrode assembly, and the carbon fiber sheet breaks, the reduction in tensile stress after the break is small, making it easier to recover from the process. The smaller the reduction in tensile stress immediately after the carbon fiber sheet breaks, the better. However, in the case of carbon fiber papermaking, it is not easy to reduce the reduction to less than 1 N / cm per 0.2 mm, so the lower limit is approximately 1 N / cm per 0.2 mm. The reduction in tensile stress immediately after the carbon fiber sheet breaks can be controlled by the type and amount of resin used as a binder. The reduction in tensile stress when a carbon fiber sheet is stretched 0.2 mm after breaking can be determined by measuring the difference between the tensile stress at the breaking point and the tensile stress after stretching an additional 0.2 mm immediately after breaking using a tensile tester such as the "Autograph" (registered trademark) manufactured by Shimadzu Corporation. The tensile stress is calculated by dividing the tensile force by the width of the test piece.

[0049] The binder used to bond the carbon fibers of the carbon fiber sheet of the present invention uses a mixture containing at least a resin and fibrous carbon. In a structure where the carbon fibers are bonded using a binder containing both resin and fibrous carbon, pores of 0.3 μm or more and 1.0 μm or less are easily formed within the binder. When carbon fibers are bonded using a binder containing only resin, where the fibrous carbon is contained within the carbon fiber sheet but not within the binder, the binder often lacks pores, and the webbed, extended binder hinders gas diffusion. Furthermore, fibrous carbon has superior thermal conductivity compared to resin. Therefore, the presence of fibrous carbon within the binder acts as a heat conduction path, improving the overall thermal conductivity of the carbon fiber sheet. Whether pores are formed within the binder by the fibrous carbon and resin can be confirmed by magnifying the webbed portion of the binder formed at the intersection of the carbon fibers using an SEM or other microscope and photographing it.

[0050] The resin contained in the binder can be either carbonized or uncarbonized resin, but uncarbonized resin is preferably used as the main component. Using uncarbonized resin eliminates the need for a high-temperature treatment step under an inert atmosphere, significantly reducing manufacturing costs. Preferred resins include phenolic resins, epoxy resins, and fluororesins, with fluororesins being more preferred. Examples of fluororesins include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA). PTFE or FEP are more preferred due to their high hydrophobicity. By mixing PTFE or FEP with fibrous carbon, pores with a hydrophobic surface are formed, making it difficult for water to accumulate within the pores, resulting in a carbon fiber sheet with high water-repellency. Using uncarbonized phenolic resin, epoxy resin, or fluororesin as the resin makes the carbon fiber sheet softer and easier to roll. Furthermore, the tensile stress of the carbon fiber sheet, when stretched 0.2 mm after rupture, can be reduced to 6 N / cm or less. Furthermore, since no thermosetting resin is contained as the binder, there is an advantage that the carbon fibers can be easily separated and recovered for reuse after the binder is burned off from the carbon fiber sheet.

[0051] Whether a fluororesin is used as the resin contained in the binder can be determined by SEM-EDX observation. If the shape of the binder bonding the carbon fibers together observed by SEM matches the shape obtained by F element mapping in the same field of view using EDX, it can be determined that a fluororesin is used as the binder.

[0052] The carbon fiber sheet of the present invention preferably contains 5% by mass or more of the aforementioned resin relative to 100% by mass of the carbon fiber sheet. Furthermore, as an upper limit, the resin content is preferably 25% by mass or less relative to 100% by mass of the carbon fiber sheet. A resin content of 5% by mass or more relative to the carbon fiber sheet increases the mechanical strength of the carbon fiber sheet, reducing the risk of breakage during the manufacturing process. When the resin content is 25% by mass or less relative to the carbon fiber sheet, the proportion of materials that interfere with electrical conductivity in the carbon fiber sheet is reduced, achieving a balance between mechanical strength and electrical conductivity of the carbon fiber sheet.

[0053] As the fibrous carbon contained in the binder, carbon nanofibers are preferably used for high thermal conductivity and easy formation of pores of 0.3 μm or more and 1.0 μm or less. Fibrous carbon has the function of a heat conduction path. The higher the degree of graphitization, the better the thermal conductivity. The degree of graphitization can be evaluated by an X-ray diffraction pattern. The narrower the half-peak width of the peak, the higher the degree of graphitization. Therefore, in the X-ray diffraction pattern of the carbon fiber sheet, it is preferred to have a peak with a half-peak width of 0.3° or more and 1.0° or less in the range of 25° to 28°, and more preferably a peak with a half-peak width of 0.3° or more and 0.6° or less. The half-peak width can be controlled by the manufacturing method of the fibrous carbon and the heat treatment temperature. When the half-peak width of the peak within this range is 1.0° or less, the degree of graphitization is high, and when it is 0.6° or less, the degree of graphitization is even higher, and the carbon fiber sheet contains a material with higher thermal conductivity, so the thermal conductivity of the entire sheet is excellent. The narrower the half-peak width of the peak within this range, the higher the degree of graphitization, which is preferred. However, since the degree of graphitization decreases as the fiber diameter of the fibrous carbon decreases, when fibrous carbon having a fiber diameter sufficient to form pores of 0.3 μm to 1.0 μm is attached to a carbon fiber sheet, the half-peak width is unlikely to be less than 0.3°, and the lower limit is approximately 0.3°. The half-peak width of the carbon fiber sheet can be calculated by measuring the X-ray diffraction pattern of the carbon fiber sheet using an X-ray diffractometer (D8 ADVANCE, manufactured by Bruker).

[0054] The aspect ratio of the fibrous carbon is preferably 10 or greater, more preferably 10 or greater and 400 or less, and even more preferably 20 or greater and 150 or less. When the aspect ratio is 10 or greater, and more preferably 20 or greater, pores of 0.3 μm or greater and 1.0 μm or less are easily formed within the binder, and even a small amount of fibrous carbon easily forms a heat conduction path within the binder, thereby improving the thermal conductivity of the carbon fiber sheet. If the aspect ratio is greater than 400, entanglement of the fibrous carbon fibers is likely to occur, and the viscosity of the liquid composition formed by dispersing the fibrous carbon in a solvent may sometimes increase. If the aspect ratio is 150 or less, the viscosity can be kept low even if the fibrous carbon concentration in the liquid composition is high.

[0055] When a carbon fiber sheet is produced by impregnating the sheet with a high-viscosity liquid composition, fibrous carbon adheres to the outer side of the sheet, while the amount of fibrous carbon adhered to the inner side of the sheet decreases. This makes it difficult to uniformly adhere the fibrous carbon throughout the thickness of the sheet. If the fibrous carbon is unevenly adhered throughout the thickness, areas with limited heat conduction pathways may form within the sheet, significantly reducing the overall thermal conductivity of the sheet. Therefore, it is preferable that the fibrous carbon adhere uniformly throughout the thickness of the sheet.

[0056] The fiber diameter of the fibrous carbon is preferably not less than 100 nm and not more than 1 μm. If it is 100 nm or more, it is easy to form pores of 0.3 μm or more in the binder. If it is less than 100 nm, the pores in the binder become smaller, and sometimes the gas diffusivity is reduced. If it is 1 μm or less, a heat conduction path can be formed with a smaller content, so that the thermal conductivity can be improved while maintaining the gas diffusivity. The fiber diameter can be measured by, for example, magnifying and observing the fibrous carbon using an SEM. In the SEM photograph of the fibrous carbon taken, 30 different fibrous carbons are randomly selected, and the diameter perpendicular to the fiber length is measured to find the average value. Regarding the aspect ratio, in the SEM photograph of the fibrous carbon taken in the same manner, 30 different fibrous carbons are randomly selected to measure the fiber length, and the average value is found. The ratio of the fiber length to the fiber diameter is taken as the aspect ratio.

[0057] The carbon fiber sheet of the present invention preferably contains 25% or more, and more preferably 40% or more, of fibrous carbon relative to 100% by mass of the carbon fiber sheet. Furthermore, as an upper limit, the fibrous carbon content is preferably 60% or less relative to 100% by mass of the carbon fiber sheet. When the fibrous carbon content is 25% or more, and preferably 40% or more, relative to the carbon fiber sheet, pores with a diameter of 0.3 μm or more and 1.0 μm or less are easily formed in the carbon fiber sheet. When the fibrous carbon content is 60% or less relative to the carbon fiber sheet, the volume of pores with a diameter of 20 μm or more and 100 μm or less can be sufficiently ensured, resulting in high water drainage and gas diffusion properties. When the carbon fiber sheet manufacturing method includes a heat treatment step, the carbon fiber sheet after the heat treatment step preferably meets the above range.

[0058] <Method for producing carbon fiber sheets>

[0059] Thus, the method for producing the carbon fiber sheet of the present invention is exemplified, but the carbon fiber sheet of the present invention is not limited to the following production method.

[0060] The method for producing a carbon fiber sheet of the present invention preferably comprises: an impregnation step of impregnating a carbon fiber structure with a liquid composition containing at least a resin and fibrous carbon; a molding step of controlling the thickness of the carbon fiber sheet by hot pressing; and a heat treatment step for melting the resin contained in the liquid composition impregnated in the impregnation step.

[0061] [Impregnation process]

[0062] The method for producing a carbon fiber sheet of the present invention preferably includes the steps of forming carbon fibers into a carbon fiber structure having a certain shape such as a paper body and then impregnating the structure with a liquid composition containing a resin and fibrous carbon.

[0063] As a method for impregnating the carbon fiber sheet with a liquid composition, a method of immersing the carbon fiber sheet in the liquid composition or a method of applying the liquid composition can be used. During application, the resin and fibrous carbon contained in the liquid composition tend to be higher on the coated surface and lower on the other surface. After application, a heat treatment such as hot pressing is used to melt the resin. Although the resin can be moved to a certain extent from the coated surface to the other surface, the fibrous carbon does not melt even when heated, making it difficult to move, and sometimes it is difficult to uniformly distribute the fibrous carbon throughout the carbon fiber sheet. Therefore, the method of immersing the carbon fiber sheet in a liquid composition is particularly preferred because it can uniformly distribute the fibrous carbon from both sides to the inside of the carbon fiber sheet and has excellent productivity. In addition, when using a fluororesin as the resin, the viscosity of the liquid composition can be reduced by using a dispersion of fluororesin particles as the fluororesin source of the liquid composition. Even if the fibrous carbon is mixed at a high concentration, the viscosity of the liquid composition is unlikely to increase, which is preferred. By making the liquid composition low in viscosity, fibrous carbon sufficient to ensure thermal conductivity can be attached to the interior of the carbon fiber sheet with just one impregnation.

[0064] The amount of fibrous carbon attached to the carbon fiber sheet can be controlled by adjusting the content of the fibrous carbon in the liquid composition.

[0065] The impregnation step is preferably performed before the molding step described below. Since the carbon fiber sheet before molding has a low density, the resin and fibrous carbon are easily attached to the inside of the carbon fiber sheet when it is impregnated with the liquid composition.

[0066] [Molding process]

[0067] The method for producing a carbon fiber sheet of the present invention preferably includes a molding step, after the impregnation step described above and before the heat treatment step described below, of heating and pressurizing the carbon fiber structure impregnated with the liquid composition at a temperature of 100°C or higher. This molding step allows for more accurate control of the thickness and porosity of the resulting carbon fiber sheet. Furthermore, by applying pressure while partially melting the resin on the surface of the carbon fiber sheet, the surface of the carbon fiber sheet becomes smooth. When the carbon fiber sheet is sandwiched between electrolyte membranes for fuel cell assembly, the binder containing resin and fibrous carbon may form protrusions that pierce the electrolyte membrane, causing short circuits. By smoothing the surface of the carbon fiber sheet, such short circuits can be significantly suppressed. In the molding step, the heating temperature is preferably 100-250°C. Furthermore, the applied pressure is preferably 0.01-5 MPa.

[0068] [Heat treatment process]

[0069] The carbon fiber sheet, impregnated with a liquid composition containing resin and fibrous carbon and optionally molded, is then subjected to a heat treatment step. The heat treatment temperature in this step is preferably between 300°C and 400°C. Heat treatment at temperatures above 300°C melts the resin contained in the impregnated liquid composition, allowing it to wet and spread. Furthermore, setting the temperature below 400°C eliminates the need for a large heating furnace, significantly reducing the production cost of the carbon fiber sheet.

[0070] Typically, the fibrous carbon content barely decreases during the heat treatment process, while the resin content significantly decreases during the heat treatment process. Therefore, by considering the reduction in resin during the heat treatment and determining the resin content in the liquid composition, the resin content of the carbon fiber sheet after the heat treatment process can be controlled. Since the extent of resin reduction during the heat treatment process varies depending on the heat treatment conditions, the resin content in the liquid composition should be appropriately adjusted based on these conditions.

[0071] [Hydrophobic process]

[0072] The carbon fiber sheet may be subjected to a hydrophobic treatment step using a hydrophobic treatment liquid in which a hydrophobic agent containing a fluororesin is dispersed.

[0073] Examples of hydrophobic treatment methods include immersing the carbon fiber sheet in a hydrophobic treatment solution and applying the hydrophobic treatment solution to the carbon fiber sheet by die coating or the like. From the perspective of uniformly distributing the fluororesin perpendicularly to the surface of the carbon fiber sheet, immersing the carbon fiber sheet in a hydrophobic treatment solution is preferred. Following the hydrophobic treatment, a heat drying step or a sintering step is preferably performed.

[0074] However, if a hydrophobic treatment is performed after forming a binder having pores, the hydrophobic agent adheres and covers the already formed pores, thereby blocking the pores and reducing gas diffusion. Therefore, it is preferable to adhere the hydrophobic agent during the impregnation step.

[0075] <Method for Manufacturing Carbon Fiber Sheet Having Microporous Layer>

[0076] [Coating process]

[0077] The carbon fiber sheet can be used as a gas diffusion electrode substrate, or a microporous layer can be formed on one side of the carbon fiber sheet to serve as a gas diffusion electrode substrate. The microporous layer typically contains carbon powder and a hydrophobic agent and is a porous layer with pores ranging from 0.01 μm to 1 μm.

[0078] Examples of the carbon powder contained in the microporous layer include carbon black, graphite particles, carbon nanotubes, carbon nanofibers, etc. Among them, carbon black is preferably used.

[0079] As the water-repellent agent contained in the microporous layer, fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) are preferably used.

[0080] The basis weight of the microporous layer is not particularly limited, but is preferably 10 to 50 g / m 2 In the range of 14 to 30 g / m 2 If the basis weight of the microporous layer is 10 g / m 2 More than 14 g / m 2 In the above, the surface of one side of the carbon fiber sheet can be covered by the microporous layer, which further promotes the back diffusion of generated water and further suppresses the drying of the electrolyte membrane. 2 Below, more preferably 30g / m 2 Below this value, the drainage performance is further improved, and overflow can be further suppressed.

[0081] The microporous layer is formed by applying a coating solution containing the aforementioned carbon powder or a hydrophobic agent to the carbon fiber sheet. Coating can be performed using a variety of coating methods, including screen printing, rotary screen printing, spraying, gravure printing, die coating, rod coating, and blade coating. The layer is then preferably dried at a temperature of 80-180°C. Furthermore, after drying, the layer is preferably heated at approximately 300-400°C, more preferably 340-390°C, to melt the hydrophobic agent.

[0082] [Membrane Electrode Assembly]

[0083] In the present invention, a membrane electrode assembly can be formed by joining the above-mentioned carbon fiber sheet or gas diffusion electrode substrate to at least one side of an electrolyte membrane having a catalyst layer on both sides. When using a gas diffusion electrode substrate having a microporous layer, by arranging the microporous layer of the gas diffusion electrode substrate on the catalyst layer side, in addition to being more likely to cause back diffusion of generated water, the contact area between the catalyst layer and the gas diffusion electrode substrate is increased, which can reduce the contact resistance, so it is preferred. As the catalyst used in the catalyst layer, platinum is generally used. The electrolyte membrane preferably uses a perfluorosulfonic acid-based polymer material with high proton conductivity, oxidation resistance and heat resistance.

[0084] Fuel Cell

[0085] The fuel cell of the present invention comprises the carbon fiber sheet or gas diffusion electrode substrate of the present invention. Specifically, separators are provided on both sides of the membrane electrode assembly. Specifically, a fuel cell is constructed by arranging separators on both sides of the membrane electrode assembly. Typically, a solid polymer fuel cell is constructed by stacking multiple layers of structures formed by sandwiching such membrane electrode assemblies with separators on both sides. The catalyst layer is composed of a layer containing a solid polymer electrolyte and catalyst-supported carbon. The construction of such a fuel cell unit and fuel cell itself is well known.

[0086] The fuel cell of the present invention can be used as a power supply source for transportation equipment such as automobiles, ships, and railways.

[0087] Especially when used as a power source for automobiles, they must operate under high temperature conditions. Current fuel cells installed in automobiles typically operate at temperatures between 60 and 70°C, but large radiators are used to cool the heat generated by power generation. Raising the operating temperature to around 90°C allows for a smaller radiator required for heat dissipation, leading to the development of fuel cells with higher operating temperatures. To operate fuel cells at high temperatures, it is necessary to prevent the electrolyte from drying out. Therefore, the use of the carbon fiber sheet with improved thermal conductivity of the present invention is preferred, as it facilitates heat dissipation from the electrolyte membrane.

[0088] Example

[0089] Next, the carbon fiber sheet of the present invention will be described in detail using examples. The following describes the materials used in the examples, the method for producing and evaluating the carbon fiber sheet, and the method for evaluating the cell performance of a fuel cell.

[0090] <Pore size distribution>

[0091] Prepare 5 pieces of carbon fiber and cut them into 2cm x 3cm squares. 2The measurement sample was measured by mercury intrusion porosimetry within the measurement pressure range of 6 kPa to 123 MPa (pore diameter of 10 nm to 200 μm), and the peak of the pore diameter was obtained from the average value of the obtained pore diameter distribution. Autopore 9520 manufactured by Shimadzu Corporation was used as the measurement device. Regarding the peak of the pore diameter, in the logarithmic differential pore volume distribution curve obtained by pore measurement with a mercury porosimeter, the maximum point of the target area was taken as the peak of this area.

[0092] In addition, the pore volume of pores with a pore diameter of 0.3 μm to 1.0 μm was obtained by subtracting the cumulative pore volume with a pore diameter of 1.0 μm or more from the cumulative pore volume with a pore diameter of 0.3 μm or more.

[0093] <Areal density of carbon fiber sheet>

[0094] Ten test pieces of 10 cm × 10 cm of carbon fiber sheets were made, the mass of each test piece was measured, and the average areal density of the ten test pieces was obtained.

[0095] <Thickness and density of carbon fiber sheet>

[0096] The thickness of the carbon fiber sheet was measured using a digital thickness gauge DIGIMICRO manufactured by Nikon Corporation while applying a load of 0.15 MPa to the carbon fiber sheet. In addition, the density was calculated based on the areal density of the carbon fiber sheet.

[0097] <Thermal resistance>

[0098] Ten test pieces were made by cutting the carbon fiber sheet into a circle with a diameter of 30 mm. Each test piece was placed in a thermal conductivity measurement device (IE-1230 manufactured by Iwatani Corporation), and the thermal resistance (K / W) when pressurized at 1.0 MPa was measured. The thermal resistance was multiplied by the area of the carbon fiber sheet, and the obtained value was taken as the thermal resistance in the direction perpendicular to the surface of the carbon fiber sheet (K·cm 2 / W), and the average value of the ten test pieces was obtained.

[0099] <Full width at half maximum in X-ray diffraction pattern>

[0100] The carbon fiber sheet was cut into a φ47 mm as a specimen. Using an X-ray diffractometer (D8 ADVANCE, manufactured by Bruker), while rotating the sample stage at a speed of 30 rpm, the range of 15 to 40° was measured at a step of 0.02°, and the peak with the maximum diffraction intensity was obtained. The full width at half maximum of this peak was used.

[0101] <Reduction amount of tensile stress when stretched 0.2 mm after rupture>

[0102] The tensile properties of a carbon fiber sheet were measured using the tensile test mode of the Autograph (registered trademark) AGS-X manufactured by Shimadzu Corporation. A test piece of carbon fiber sheet cut to a length of 15 cm and a width of 1.5 cm was mounted on the upper and lower sample mounting fixtures installed on the tensile testing machine. The carbon fiber sheet was stretched at a rate of 2 mm / min in the longitudinal direction to obtain a stress-displacement curve. The maximum stress in this curve was used as the rupture point of the carbon fiber sheet, and the difference between the tensile stress at the rupture point and the tensile stress after stretching 0.2 mm immediately after rupture was measured. The tensile stress is calculated by dividing the tensile force by the width of the test piece. This measurement was performed five times using different test pieces, and the average value was used as the reduction in tensile stress when stretching 0.2 mm after rupture.

[0103] Gas diffusivity

[0104] Using a gas and water vapor diffusion tester (MVDP-200C) manufactured by Seika Industry Co., Ltd., the gas to be measured was flowed along one side (the primary side) of a carbon fiber sheet, while nitrogen gas was flowed along the other side (the secondary side). The differential pressure between the primary and secondary sides was pre-controlled to approximately 0 Pa (0 ± 3 Pa) (i.e., a state where there is almost no gas flow due to the pressure difference, and gas movement occurs solely through molecular diffusion). The gas concentration at equilibrium was measured using a gas concentration meter on the secondary side, and this value (%) was used as an indicator of gas diffusivity in the thickness direction.

[0105] <Power Generation Performance of Polymer Electrolyte Fuel Cells>

[0106] A catalyst solution was prepared by sequentially adding 1.00 g of platinum-supported carbon (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., platinum supporting amount: 50 mass %), 1.00 g of purified water, 8.00 g of a "Nafion (registered trademark)" solution (5.0 mass % "Nafion (registered trademark)" manufactured by Aldrich Co., Ltd.), and 18.00 g of isopropyl alcohol (manufactured by Nacalai Tesque Co., Ltd.).

[0107] Next, the catalyst solution was sprayed onto a 50 mm × 50 mm cut PTFE tape "NAFLON (registered trademark)" "TOMBO (registered trademark)" No. 9001 (manufactured by Nichias Corporation) and dried at room temperature to produce a platinum-loaded film with a platinum content of 0.3 mg / cm 2Next, a solid polymer electrolyte membrane, "Nafion (registered trademark)" NRE-211CS (manufactured by DuPont), cut into 8 cm x 8 cm pieces, was sandwiched between two PTFE sheets with catalyst layers. The membrane was pressed at 130°C for 5 minutes while applying 5 MPa using a flatbed press to transfer the catalyst layer onto the electrolyte membrane. After pressing, the PTFE sheets were peeled off, producing an electrolyte membrane with a catalyst layer.

[0108] Next, the electrolyte membrane with the catalyst layer was sandwiched between two gas diffusion electrode substrates cut into 5 cm×5 cm produced in each example and comparative example, and pressed at 130°C for 5 minutes while applying 3 MPa of pressure using a flat plate press to produce a membrane electrode assembly.

[0109] The resulting membrane electrode assembly was assembled into a fuel cell for fuel cell evaluation. A single-channel serpentine separator with a groove width, groove depth, and rib width of 1.0 mm was used as the separator. Unpressurized hydrogen was supplied to the anode side, and unpressurized air was supplied to the cathode side, both at a relative humidity of 30%.

[0110] The measurement was performed as follows: The utilization rates of hydrogen and oxygen in air were set to 70 mol% and 40 mol%, respectively, and the cell temperature was set to 90°C. Under the above conditions, the current density was measured at 1.9 A / cm 2 The output voltage under the condition of MOSFET is used as an indicator of power generation performance.

[0111] (Example 1)

[0112] Toray Industries, Inc.'s polyacrylonitrile carbon fiber "Torayca (registered trademark)" T300 was cut into 7 mm lengths, dispersed in water for papermaking, and then impregnated with a 10% by mass aqueous dispersion of polyvinyl alcohol and dried to obtain a carbon fiber with a basis weight of approximately 20 g / m 2 The long carbon fiber paper body is wound into a roll.

[0113] A liquid composition was prepared by mixing "POLYFLON (registered trademark) PTFE DISPERSION D-210C" (DAIKIN INDUSTRIES Co., Ltd.), a fibrous carbon fiber (VGCF (registered trademark)) (fibrous carbon, aspect ratio: 53, fiber diameter 0.15 μm, manufactured by Resonac Co., Ltd.), a nonionic surfactant as a dispersant, and water in a mass ratio of 7:13:4:76 (PTFE resin aqueous dispersion: fibrous carbon: dispersant: water). A carbon fiber paper sheet cut into 10 cm x 10 cm pieces was immersed in the liquid composition and pulled up. The sheet was then heat-treated and squeezed with a mangle to remove excess liquid so that the fibrous carbon accounted for 48% by mass and the resin accounted for 17% by mass of the carbon fiber sheet. The sheet was then dried at 100°C for 10 minutes.

[0114] Next, the carbon fiber paper material immersed in the liquid composition was subjected to heat and pressure treatment at 180°C for 5 minutes while being pressed at 3 MPa using a flatbed press to obtain a carbon fiber sheet. This was further heat treated at 360°C for 10 minutes to obtain a carbon fiber sheet. The obtained carbon fiber sheet was measured using the various measurement methods described above, and the results are shown in Table 1.

[0115] (Example 2)

[0116] A carbon fiber sheet was obtained in the same manner as in Example 1, except that the carbon fiber paper was immersed in a liquid composition prepared by mixing an aqueous dispersion of PTFE resin: fibrous carbon: dispersant: water in a ratio of 4:7:2:87 (mass ratio). After heat treatment, excess liquid was removed using a roller compactor so that the fibrous carbon accounted for 36% by mass and the resin accounted for 13% by mass of the carbon fiber sheet (100% by mass).

[0117] (Comparative Example 1)

[0118] The basis weight of carbon fiber is about 30g / m 2 A long carbon fiber paper sheet was prepared, and the carbon fiber paper sheet was immersed in a liquid composition prepared by mixing an aqueous dispersion of PTFE resin: fibrous carbon: dispersant: water in a ratio of 7:4:1:88 (mass ratio). After heat treatment, excess liquid was removed using a roller compactor to obtain a carbon fiber sheet with a ratio of 22% by mass of fibrous carbon and 26% by mass of resin in 100% by mass of the carbon fiber sheet.

[0119] (Comparative Example 2)

[0120] A carbon fiber sheet was obtained in the same manner as in Example 1, except that a CNT paste (fibrous carbon, aspect ratio: 500, fiber diameter: 0.02 μm, manufactured by Cnano Co., Ltd.) was used instead of the vapor-grown carbon fiber. The carbon fiber paper sheet was immersed in a liquid composition prepared by mixing an aqueous dispersion of PTFE resin: fibrous carbon: dispersant: water in a ratio of 4:7:2:87 (mass ratio). After drying, the sheet was immersed again in the same liquid composition. After heat treatment, the excess liquid was removed using a roller compactor. The carbon fiber sheet was obtained by adjusting the mass ratio of fibrous carbon to 49% by mass and the mass ratio of resin to 17% by mass in the 100% by mass carbon fiber sheet.

[0121] (Comparative Example 3)

[0122] A carbon fiber sheet was obtained in the same manner as in Example 1 except that "DENKA BLACK (registered trademark)" (acetylene black, aspect ratio: 1, manufactured by DENKA Corporation) was used instead of vapor-grown carbon fiber and excess liquid was removed by a roller compactor to reduce the resin content to 18% by mass.

[0123] (Example 3)

[0124] A carbon fiber sheet was obtained in the same manner as in Example 1, except that the carbon fiber paper was immersed in a liquid composition prepared by mixing an aqueous dispersion of PTFE resin: fibrous carbon: dispersant: water in a ratio of 4:1:1:91 (mass ratio). After heat treatment, excess liquid was removed using a roller compactor to obtain a carbon fiber sheet having a composition of 26% by mass of fibrous carbon and 17% by mass of resin in 100% by mass of the carbon fiber sheet.

[0125] (Example 4)

[0126] A carbon fiber sheet was obtained in the same manner as in Example 1, except that the carbon fiber paper was immersed in a liquid composition prepared by mixing an aqueous dispersion of PTFE resin: fibrous carbon: dispersant: water in a ratio of 3:17:5:75 (mass ratio). After heat treatment, excess liquid was removed using a roller compactor to obtain a carbon fiber sheet having a composition of 61% by mass of fibrous carbon and 7% by mass of resin in 100% by mass of the carbon fiber sheet.

[0127] (Comparative Example 4)

[0128] A carbon fiber sheet was obtained in the same manner as in Example 1, except that carbon nanofibers (aspect ratio: 7, fiber diameter: 0.7 μm) were used as fibrous carbon instead of vapor-grown carbon fibers. The carbon fiber paper sheet was immersed in a liquid composition comprising an aqueous dispersion of PTFE resin: fibrous carbon: dispersant: water in a ratio of 7:13:5:75 (mass ratio). After drying, the sheet was immersed again in the same liquid composition. After heat treatment, the excess liquid was removed using a roller compactor. The carbon fiber sheet was prepared in the same manner as in Example 1, except that the fibrous carbon accounted for 50% by mass and the resin accounted for 16% by mass in the 100% by mass carbon fiber sheet.

[0129]

[0130] Industrial availability

[0131] The carbon fiber sheet and gas diffusion electrode substrate using the carbon fiber sheet of the present invention are preferably used as a gas diffusion electrode substrate for fuel cells, particularly polymer electrolyte fuel cells. Furthermore, the gas diffusion electrode substrate using the carbon fiber sheet of the present invention achieves a balance between thermal conductivity and gas diffusivity, suppressing dry-up of the electrolyte membrane while also exhibiting high gas diffusivity in high current density regions, thereby enabling the production of high-performance polymer electrolyte fuel cells.

Claims

1. A carbon fiber sheet formed by bonding a carbon fiber structure with a binder containing at least a resin and fibrous carbon, The content of the fibrous carbon in 100% by mass of the carbon fiber sheet is 25% by mass or more, The pore size distribution of the carbon fiber sheet has peaks in the pore size range of 0.3 to 1.0 μm and in the pore size range of 20 to 100 μm.

2. A carbon fiber sheet formed by bonding a carbon fiber structure with a binder containing at least a resin and fibrous carbon, The pore size distribution of the carbon fiber sheet has peaks in the pore size range of 0.3 to 1.0 μm and 20 to 100 μm. The pore volume of pores with a pore diameter of 0.3 to 1.0 μm is 0.40 mL / g or more. 3 . The carbon fiber sheet according to claim 1 , wherein the pore volume of pores with a pore diameter of 0.3 to 1.0 μm is 0.40 mL / g or more. The carbon fiber sheet according to claim 1 or 2, wherein a content of the resin in 100% by mass of the carbon fiber sheet is 5% by mass or more and 25% by mass or less. 5 . The carbon fiber sheet according to claim 1 , wherein the content of the fibrous carbon is 40% by mass or more in 100% by mass of the carbon fiber sheet. The carbon fiber sheet according to claim 1 or 2, wherein the aspect ratio of the fibrous carbon is 10 or more and 400 or less. 7 . The carbon fiber sheet according to claim 1 , wherein the fiber diameter of the fibrous carbon is 100 nm or more and 1 μm or less. 8 . The carbon fiber sheet according to claim 1 , having a peak with a half-value width of 0.3° to 1.0° in the range of 25° to 28° in an X-ray diffraction pattern. 9 . The carbon fiber sheet according to claim 1 , wherein the decrease in tensile stress when further stretched 0.2 mm after breaking is 1 N / cm to 6 N / cm.

10. The carbon fiber sheet according to claim 1 or 2, wherein the carbon fiber structure is a carbon fiber papermaking body.

11. The carbon fiber sheet according to claim 1 or 2, wherein the resin is a fluororesin.

12. A gas diffusion electrode substrate comprising the carbon fiber sheet according to claim 1 or 2 and a microporous layer formed on one surface thereof.

13. A fuel cell using the carbon fiber sheet according to claim 1 or 2 as a gas diffusion electrode.

14. A fuel cell using the gas diffusion electrode substrate according to claim 12 as a gas diffusion electrode.

15. Transportation equipment using the fuel cell according to claim 13 as a power supply source.

16. Transportation equipment using the fuel cell according to claim 14 as a power supply source.

17. A method for producing a carbon fiber sheet according to claim 1 or 2, comprising: an impregnation step of impregnating a carbon fiber structure with a liquid composition containing a resin and fibrous carbon as a binder; and a heat treatment step of melting the resin contained in the liquid composition impregnated in the impregnation step.

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