Porous carbon sheet, fuel cell, liquid electrolyte device, redox flow battery, mobile body, and method for producing porous carbon sheet

Abstract

The purpose of the present invention is to provide a porous carbon sheet that has excellent gas diffusivity and electrical conductivity, and to form a porous carbon sheet that includes carbon fibers and a binder and has a higher filling rate near both surfaces thereof in the thickness direction of the porous carbon sheet than the filling rate in the interior thereof.

Description

Porous carbon sheet, fuel cell, liquid electrolyzer, redox flow battery, mobile device, and method for manufacturing porous carbon sheet

[0001] The present invention relates to a porous carbon sheet used in the gas diffusion layer of fuel cells, liquid electrolytic devices, and redox flow batteries, and more particularly to a porous carbon sheet suitable for the gas diffusion layer of polymer electrolyte fuel cells used as a power source for mobile devices such as fuel cell vehicles and ships.

[0002] Fuel cells are a type of power generation device that extracts electrical energy by electrochemically oxidizing fuels such as hydrogen and methanol, and have recently attracted attention as a clean energy source. Among them, polymer electrolyte fuel cells have a low standard operating temperature of around 100°C and high energy density, making them promising for a wide range of applications as power generation devices for relatively small-scale distributed power generation facilities, as well as for mobile devices such as fuel cell vehicles and ships.

[0003] The basic structure of a polymer electrolyte fuel cell consists of a polymer electrolyte membrane, catalyst layers formed on both sides of the polymer electrolyte membrane, a gas diffusion layer formed outside the catalyst layer, and two separators that sandwich these components.

[0004] The gas diffusion layer requires high gas diffusivity to diffuse the gas supplied from the separator to the catalyst, high drainage to discharge water generated by the electrochemical reaction back to the separator, and high conductivity to extract the generated electric current. Carbon paper, a base material obtained by binding carbon fiber paper with carbonized resin, is commonly used as a material that possesses all of the above characteristics.

[0005] Various studies have been conducted on the structure of carbon paper in order to achieve both gas diffusivity and conductivity, and techniques for adjusting the distribution of carbonized resin in carbon paper are known. Patent document 1 describes a technique in which a solution containing a resin composition is applied to a carbon fiber papermaking body and the resin composition is removed from one side with a doctor blade, and a technique in which a large amount of resin composition is distributed on the surface by volatilizing the solvent from the surface during the drying process.

[0006] Furthermore, Patent Document 2 describes a technique for controlling the density distribution within a porous carbon sheet by laminating two high-density sheets together, sandwiching a low-density sheet between them, and then integrating them.

[0007] International Publication No. 2016 / 060044, Japanese Patent Publication No. 2007-176750

[0008] However, in Patent Document 1, it is difficult to control the carbonized resin, and the uneven distribution of the carbonized resin on the carbon paper surface is insufficient. Because the carbonized resin remains in the interior of the carbon paper more than necessary, it is undesirable from the viewpoint of gas diffusion in the in-plane direction. Furthermore, in Patent Document 2, it is not possible to unevenly distribute the resin only near the surface, and since multiple sheets are laminated, there is a problem that the resulting porous carbon sheet becomes thick.

[0009] Therefore, the present invention aims to provide a porous carbon sheet with excellent conductivity without impairing gas diffusion in the perpendicular and in-plane directions.

[0010] To solve the above problems, the present invention adopts the following configuration: [1] A porous carbon sheet comprising carbon fibers and a binder, wherein the packing density near both surfaces in the thickness direction of the porous carbon sheet is higher than the packing density inside. [2] The porous carbon sheet according to [1], wherein when the thickness of the porous carbon sheet is divided into seven equal parts to obtain seven layers, the seven layers obtained are sequentially numbered as layer 1, layer 2, layer 3, layer 4, layer 5, layer 6, and layer 7 from the layer containing one surface to the layer containing the other surface, the structural uneven distribution of layer 1 and layer 7 is 1.5 times or more the structural uneven distribution of the innermost sparse layer. Here, the structural uneven distribution of each layer refers to the ratio of the volume of the non-void portion of each layer when the volume of the non-void portion of the entire porous carbon sheet is taken as 100%. [3] The porous carbon sheet according to [2], wherein the structural uneven distribution of layer 1 and layer 7 is 15% or more each, and the structural uneven distribution of the innermost sparse layer is 13% or less. [4] The porous carbon sheet according to [2] or [3], wherein the innermost sparse layer is layer 4. [5] A porous carbon sheet according to any one of [2] to [4], wherein the structural unevenness of layer 1 is the greatest among layers 1, 2, and 3, and the structural unevenness of layer 7 is the greatest among layers 5, 6, and 7. [6] A porous carbon sheet according to any one of [2] to [5], wherein the structural unevenness of the innermost sparse layer is 5.0% or more and 10.0% or less. [7] A porous carbon sheet according to any one of [1] to [6], wherein the coverage rate of both surfaces is 50% or more and 80% or less. [8] A porous carbon sheet having a structure that includes multiple porous carbon sheets according to any one of [1] to [7]. [9] A fuel cell having a porous carbon sheet according to any one of [1] to [8].

[10] A liquid electrolytic device having a porous carbon sheet according to any one of [1] to [8].

[11] A redox flow battery having a porous carbon sheet according to any one of [1] to [8].

[12] A mobile body equipped with the fuel cell according to [9].

[13] A method for producing a porous carbon sheet, comprising, in this order, a coating step of applying conductive particles and / or resin to both surfaces of a carbon fiber papermaking body, and a firing step of firing the carbon fiber papermaking body at a high temperature.

[14] The method for producing a porous carbon sheet according to

[13] , wherein in the coating step, two or more liquids containing conductive particles and / or resin of different compositions are used for the coating.

[15] A method for producing a porous carbon sheet according to

[13] or

[14] , comprising a step of stacking a plurality of porous carbon sheets or intermediates thereafter, after the impregnation step.

[0011] The present invention makes it possible to obtain a porous carbon sheet with excellent gas diffusivity and conductivity.

[0012] This figure schematically shows a cross-section of the porous carbon sheet of the present invention.

[0013] The following describes preferred embodiments of the porous carbon sheet according to the present invention. However, the present invention is not limited to the following embodiments and can be modified in various ways depending on the purpose and application.

[0014] A porous carbon sheet according to an embodiment of the present invention (hereinafter sometimes referred to as "the porous carbon sheet of the present invention") includes carbon fibers and a binder as essential components. As the porous carbon sheet of the present invention, a porous body having gas diffusion, drainage, and conductivity is preferred, and conductive porous substrates containing carbon fibers such as carbon felt, carbon paper, and carbon cloth are preferred because they have the above characteristics and also have excellent corrosion resistance. In particular, it is preferable to use a substrate composed of a conductive porous substrate containing carbon fibers and a carbonized resin in terms of mechanical strength, and it is even more preferable to use a substrate obtained by binding a carbon fiber paper body with a carbonized resin, i.e., carbon paper, because it has excellent properties for absorbing dimensional changes in the thickness direction due to swelling and shrinkage of the electrolyte membrane, i.e., "springiness". Here, carbonized resin refers to a carbide obtained by heat-treating a resin.

[0015] In this invention, the term "binding agent" refers to a component other than the carbon fibers in a porous carbon sheet, which plays a role in bonding the carbon fibers together. In carbon paper, carbonized resin is included as the binding agent.

[0016] The porous carbon sheet of the present invention has a structure in which non-void areas are unevenly distributed on both surfaces. Specifically, in the thickness direction of the porous carbon sheet, the packing density near both surfaces is required to be higher than the packing density inside. Here, packing density refers to the ratio of the volume of non-void areas to the sum of the volumes of void areas and non-void areas, which is taken as 100%. Furthermore, the area near both surfaces is the range of 20 μm inward from both surfaces of the porous carbon sheet, and the inside refers to the area of ​​the porous carbon sheet other than the area near both surfaces. By adopting a structure with the packing density distribution described above, contact resistance with other components such as separators and catalyst layers is reduced, and conductivity is improved. In addition, it is known that if the porous carbon sheet bends in the flow path of a separator which is responsible for supplying gas and discharging generated water, it can obstruct the supply of gas and the discharge of generated water from the gas flow path. However, by adopting the above structure, the surface side of the porous carbon sheet becomes rigid, and bending can be suppressed. Typically, a porous carbon sheet having the above structure can be obtained by having conductive particles or binders unevenly distributed on both surfaces.

[0017] The packing density of a porous carbon sheet can be measured by preparing a sample that allows observation of the cross-section of the porous carbon sheet using an ion milling device, magnifying the cross-section with a scanning electron microscope and taking photographs, binarizing the void and non-void areas, and then calculating the proportion occupied by the non-void area. Although fuzzy material may protrude from the surface of the porous carbon sheet, in this invention, this is not included in the structure of the porous carbon sheet. That is, if there is fuzzy material protruding from the surface in the cross-sectional image, this fuzzy material is ignored, and the surface and the areas near both surfaces are defined.

[0018] One way to represent the uneven distribution structure of a porous carbon sheet is to compare layers obtained by dividing the thickness of the porous carbon sheet into multiple equal parts. In the porous carbon sheet of the present invention, the seven layers obtained by dividing the thickness of the porous carbon sheet into seven equal parts are designated as layer 1, layer 2, layer 3, layer 4, layer 5, layer 6, and layer 7 in order from the layer containing one surface to the layer containing the other surface. In this case, it is preferable that the structural uneven distribution rate of layer 1 and layer 7 is 1.5 times or more, and more preferably 2 times or more, than the structural uneven distribution rate of the innermost sparse layer. By adopting the above structure, a porous carbon sheet with excellent conductivity and excellent gas diffusion in the in-plane direction can be obtained. Here, the structural uneven distribution rate of each layer indicates the ratio of the volume of non-voids in each layer when the volume of non-voids in the entire porous carbon sheet is taken as 100%. The innermost sparse layer is the layer with the lowest structural uneven distribution rate among layers 2 to 6. The innermost sparse layer is preferably one of layers 3 to 5, and more preferably layer 4. By having the most sparse layer near the center of the thickness direction of the porous carbon sheet, a porous carbon sheet with excellent in-plane gas diffusion properties can be obtained.

[0019] Preferably, the structural segregation rates of layer 1 and layer 7 are 15% or more, and the structural segregation rate of the innermost sparse layer is 13% or less. By having the above structural segregation rate distribution, a porous carbon sheet with excellent conductivity and excellent in-plane gas diffusion can be obtained.

[0020] Furthermore, it is preferable that the structural uneven distribution rate is highest in layer 1 among layers 1, 2, and 3, and that the structural uneven distribution rate is highest in layer 7 among layers 5, 6, and 7. By adopting the above structural uneven distribution rate, the above effects can be more effectively achieved.

[0021] On the other hand, the structural segregation rate within the porous carbon sheet is sufficient as long as it does not impair conductivity or mechanical strength. If the structural segregation rate within the porous carbon sheet is too high, gas diffusion in the in-plane direction may deteriorate. For the above reasons, it is preferable that the structural segregation rate of the innermost layer is 5.0% or more and 10.0% or less. If the structural segregation rate of the innermost layer is 5.0% or more, conductivity is improved. If the structural segregation rate of the innermost layer is 10.0% or less, a porous carbon sheet with excellent gas diffusion in the in-plane direction can be obtained. One method for obtaining a porous carbon sheet with the above-mentioned preferred structure is to control the structure so that there are few conductive particles and binders inside the porous carbon sheet, and many conductive particles and binders on both surfaces.

[0022] The structural segregation rate of a porous carbon sheet can be calculated by analyzing cross-sectional images. First, a sample is prepared using an ion milling device or the like to allow observation of the cross-section of the porous carbon sheet, and the cross-section is magnified and photographed using a scanning electron microscope. After photography, the obtained cross-sectional image is binarized to distinguish between voids and non-voids, allowing the area of ​​the non-voids to be measured. The binarized image is divided into seven equal parts, with the thickness of the porous carbon sheet divided into seven equal parts, and these layers are designated as Layer 1, Layer 2, Layer 3, Layer 4, Layer 5, Layer 6, and Layer 7, starting from the layer containing one surface and moving towards the layer containing the other surface. The area of ​​the non-voids in each layer is measured and can be calculated using the following formula. Note that layer n represents one of Layers 1 to 7. Structural segregation rate of layer n (%) = Area of ​​non-voids in layer n ÷ Sum of non-void areas in all layers × 100.

[0023] The porosity of the porous carbon sheet is preferably between 50% and 80%. The porosity of the porous carbon sheet can be measured by preparing a sample that allows the cross-section of the porous carbon sheet to be observed using an ion milling apparatus, magnifying the cross-section with a scanning electron microscope and taking photographs, then binarizing the void and non-void areas. A porosity of 50% or more is preferable because it results in excellent gas diffusion. If the porosity exceeds 80%, the mechanical strength of the porous carbon sheet may decrease.

[0024] The in-plane air permeability of the porous carbon sheet was 1,000 μm under conditions of compression from both surfaces at a pressure of 1 MPa.３ 3,000 μm or more ３ Preferably, the following: 1,000 μm ３ If the surface area is greater than 3,000 μm, the gas will diffuse sufficiently within the plane, improving the power generation performance of the fuel cell. ３ The following conditions allow for the creation of a porous carbon sheet with a good balance of mechanical strength.

[0025] The coverage rate of both surfaces of the porous carbon sheet is preferably 50% or more and 80% or less. The coverage rate of the porous carbon sheet refers to the proportion of the non-porous area when the porous carbon sheet is viewed from the surface side. The coverage rate of the porous carbon sheet can be measured by magnifying the surface of the porous carbon sheet with a scanning electron microscope, taking a photograph, and binarizing the porous and non-porous areas. The coverage rate is calculated as the ratio of the non-porous area to the area of ​​the surface image. It is preferable that the coverage rate of both surfaces of the porous carbon sheet is 50% or more, as this results in excellent conductivity and mechanical strength. It is preferable that the coverage rate of both surfaces of the porous carbon sheet is 80% or less, as this results in excellent gas diffusion in the direction perpendicular to the surface. It is more preferable that the coverage rate of both surfaces of the porous carbon sheet is 70% or less, and even more preferable that it is 65% or less.

[0026] The gas permeability resistance in the direction perpendicular to the surface of the porous carbon sheet is preferably 0.5 Pa or more and 30 Pa or less. A gas permeability resistance of 0.5 Pa or more allows for a porous carbon sheet with a good balance with mechanical strength. A resistance of 30 Pa or less provides excellent gas diffusion. From the viewpoint of improving the power generation performance of the fuel cell, the gas permeability resistance in the direction perpendicular to the surface of the porous carbon sheet is more preferably 20 Pa or less, and even more preferably 15 Pa or less.

[0027] Porous carbon sheets treated with a water-repellent resin are suitably used as gas diffusion layers. Examples of water-repellent resins include PTFE (polytetrafluoroethylene) (e.g., "Teflon®"), FEP (tetrafluoroethylene hexafluoropropylene copolymer), PFA (perfluoroalkoxy fluoride resin), ETFE (ethylene tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride), but PTFE or FEP, which exhibit strong water repellency, are preferred.

[0028] The amount of water-repellent resin is not particularly limited, but it is preferably 0.1% by mass or more and 20% by mass or less, when the total mass of the porous carbon sheet is considered to be 100% by mass. If the amount is less than 0.1% by mass, the water repellency may not be sufficiently exhibited. On the other hand, if it is 20% by mass or less, it is preferable as it will have excellent gas diffusion, drainage, and conductivity.

[0029] Methods for treating porous carbon sheets with water-repellent properties include the commonly known technique of immersing the porous carbon sheet in a dispersion containing a water-repellent resin, as well as coating techniques such as die coating and spray coating, which involve applying the water-repellent resin to the porous carbon sheet. Dry processes such as sputtering of the water-repellent resin can also be applied. After the water-repellent treatment, drying and even sintering processes may be added as needed.

[0030] A layer with a microporous layer formed on the porous carbon sheet of the present invention is suitably used as a gas diffusion layer. The microporous layer is a layer having even finer pores than the porous carbon sheet, preferably with an average pore diameter of 0.01 to 10 μm. Since high conductivity is required for the porous carbon sheet of the present invention to be used as a gas diffusion layer, the microporous layer is preferably a layer containing conductive fine particles such as carbon black, carbon nanotubes, carbon nanofibers, chopped carbon fibers, graphene, and graphite.

[0031] Furthermore, the microporous layer is required to have properties such as conductivity, gas diffusion, water drainage, moisture retention, and thermal conductivity. In addition, if used on the anode side of a fuel cell, it is required to have strong acid resistance, and if used on the cathode side, it is required to have oxidation resistance. Therefore, in addition to conductive fine particles, it is preferable to include a water-repellent resin, such as a fluororesin. Examples of fluororesins included in the microporous layer are the same as those preferred when water-repellent treating porous carbon sheets, such as PTFE, FEP, PFA, and ETFE. PTFE or FEP are preferred because they have particularly high water repellency. The content of the water-repellent resin in the microporous layer is preferably 1 to 70 parts by mass, and more preferably 5 to 60 parts by mass, per 100 parts by mass of conductive fine particles contained in the microporous layer. If the amount of water-repellent resin is 1 part by mass or more, the microporous layer will have excellent drainage and mechanical strength, which is preferable. On the other hand, if it is 70 parts by mass or less, the microporous layer will have excellent conductivity, which is preferable.

[0032] A method for manufacturing a porous carbon sheet is one aspect of the present invention. The method for manufacturing a porous carbon sheet of the present invention comprises, in this order, a coating step of applying conductive particles and / or resin to both surfaces of a carbon fiber papermaking body, and a firing step of firing the carbon fiber papermaking body at a high temperature.

[0033] Next, a preferred method for obtaining the porous carbon sheet of the present invention will be specifically described. However, the present invention is not limited to the following description, and any description of preferred embodiments within each description can also be interpreted as a description of the present invention as a broader concept.

[0034] <Papermaking Process> In the method for producing porous carbon sheets of the present invention, a carbon fiber papermaking body is used, but the carbon fiber papermaking body can also be obtained by a papermaking process in which carbon fiber raw materials are produced. To produce the carbon fiber papermaking body, a wet papermaking method in which carbon fibers are dispersed in a liquid and papermaking is performed, or a dry papermaking method in which carbon fibers are dispersed in air and papermaking is performed. Among these, the wet papermaking method is preferred because it offers superior productivity.

[0035] Carbon fibers used in carbon fiber papermaking include polyacrylonitrile (PAN), pitch, and rayon-based carbon fibers. Among these, PAN and pitch-based carbon fibers are preferred due to their superior mechanical strength.

[0036] The average diameter of the single carbon fibers constituting the carbon fiber paper is preferably in the range of 3 to 20 μm, and more preferably in the range of 5 to 10 μm. When the average diameter is 3 μm or more, the pore size becomes larger, improving drainage and suppressing flooding. On the other hand, when the average diameter is 20 μm or less, the water vapor diffusion is reduced, suppressing drying up. Furthermore, using two or more types of carbon fibers with different average diameters is preferable because it can improve the surface smoothness of the porous carbon sheet.

[0037] The average length of the individual carbon fibers contained in the carbon fiber papermaking body is preferably in the range of 3 to 20 mm, and more preferably in the range of 5 to 15 mm. A single carbon fiber length of 3 mm or more is preferable because it results in a porous carbon sheet with excellent mechanical strength, electrical conductivity, and thermal conductivity. On the other hand, a single carbon fiber length of 20 mm or less is preferable because it allows for excellent dispersion of the carbon fibers during papermaking, resulting in a homogeneous porous carbon sheet. Carbon fibers having such an average length can be obtained by methods such as cutting continuous carbon fibers to the desired length.

[0038] One preferred embodiment is to mix organic fibers with carbon fibers during the papermaking process. Since the organic fibers are burned away in the firing process described later, the drainage and gas diffusion properties of the porous carbon sheet are improved. Examples of organic fibers that can be used include polyethylene fibers, vinylon fibers, polyacetal fibers, polyester fibers, polyamide fibers, rayon fibers, and acetate fibers.

[0039] Furthermore, it is preferable to include an organic polymer as a binder in the carbon fiber papermaking body in order to improve its shape retention and handling properties. Examples of suitable organic polymers include polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, and cellulose.

[0040] For the purpose of maintaining in-plane conductivity and thermal conductivity isotropically, the carbon fiber paper body is preferably in the form of a sheet in which carbon fibers are randomly dispersed within a two-dimensional plane.

[0041] <Applying Step> As a method for obtaining a porous carbon sheet having a high structural uneven distribution rate on both surface sides, a method of applying conductive particles and / or resin to both surfaces of a carbon fiber paper body and then firing the same may be mentioned. In the applying step, the step of applying conductive particles and / or resin to both surfaces of the carbon fiber paper body is performed. By applying conductive particles and / or resin to both surfaces of the carbon fiber paper body, the conductive particles and / or binders are unevenly distributed on both surfaces, and as a result, a porous carbon sheet having a high structural uneven distribution rate on both surface sides can be obtained.

[0042] As methods for applying conductive particles and / or resin to both surfaces of a carbon fiber paper body, a method of coating a liquid containing conductive particles and / or resin on both surfaces of the carbon fiber paper body, and a method of sandwiching the carbon fiber paper body between films containing conductive particles and / or resin and performing transfer, are usable. When a method of coating a liquid containing conductive particles and / or resin is used as the method for applying conductive particles and / or resin, the application to the carbon fiber paper body is simple, and the resulting porous carbon sheet is excellent in mechanical properties, conductivity and thermal conductivity, which is preferable. As for the coating method, screen printing, rotary screen printing, spray coating, intaglio printing, gravure printing, die coater coating, bar coating, blade coating and the like can be used.

[0043] Examples of the resin to be applied to both surfaces of the carbon fiber paper body include thermosetting resins such as phenolic resins, epoxy resins, melamine resins, and furan resins. It is preferable to use a resin that carbonizes into a conductive carbide during firing, and phenolic resins are preferably used due to their high carbonization yield. Hereinafter, a product obtained by applying conductive particles and / or resin to a paper body containing carbon fibers is also referred to as a "pre-impregnated body".

[0044] As the conductive particles to be applied to the carbon fiber paper body, carbon black, carbon nanotubes, carbon nanofibers, milled carbon fibers, graphite and the like can be used.

[0045] The aforementioned liquid containing conductive particles and / or resin refers to a solution obtained by adding conductive particles and / or resin to a solvent or dispersion medium. Examples of solvents or dispersion mediums include water, ethanol, and isopropyl alcohol. The liquid may also contain additives such as carbon-based fillers and surfactants.

[0046] In the application process, it is preferable to apply the conductive particles and / or resin using two or more liquids with different compositions. In particular, it is preferable to prepare a liquid containing conductive particles and a liquid containing resin separately and apply each liquid to the carbon fiber papermaking structure. A liquid containing both conductive particles and resin may have high viscosity. When a highly viscous liquid is applied, the surface of the carbon fiber papermaking structure may be coated with conductive particles and resin, which may impair the gas diffusion properties of the porous carbon sheet. In addition, the conductive particles and resin may not penetrate into the interior of the carbon fiber papermaking structure, which may impair conductivity and mechanical strength. By applying a liquid containing conductive particles and a liquid containing resin separately to the carbon fiber papermaking structure, it is possible to apply conductive particles and resin to the interior of the carbon fiber papermaking structure appropriately without impairing the gas diffusion properties of the resulting porous carbon sheet. In addition to applying a liquid containing only conductive particles and a liquid containing only resin separately, it is also preferable to apply a liquid with a high concentration of conductive particles and a low concentration of resin, or a liquid with a low concentration of conductive particles and a high concentration of resin separately.

[0047] When applying two or more liquids containing conductive particles and / or resin with different compositions, the order is not particularly limited. A liquid containing a large amount of conductive particles may be applied first, followed by a liquid containing a large amount of resin, or vice versa. Furthermore, the application of the liquids containing conductive particles or resin may be repeated in any order, and the amount of each liquid applied does not have to be equal. In addition, one or more different liquids containing conductive particles and / or resin may be used on both surfaces of the carbon fiber papermaking body.

[0048] <Heat Treatment Process> Between the impregnation process and the firing process, a heat treatment process may be inserted to thicken the resin or partially crosslink it. Methods of heat treatment include blowing hot air, heating by sandwiching the material between hot plates such as those in a press, or heating by sandwiching it between a continuous belt. In addition, multiple layers of pre-impregnated material may be stacked before the heat treatment. By stacking multiple layers of pre-impregnated material and heat treating them, a thick porous carbon sheet can be produced.

[0049] <Firing Process> By firing the pre-impregnated material in an inert atmosphere, the resin applied to the pre-impregnated material is carbonized, and a porous carbon sheet can be obtained. This firing can be carried out using either a batch-type or a continuous-type heating furnace. The inert atmosphere can be obtained by flowing an inert gas such as nitrogen gas or argon gas through the furnace.

[0050] The maximum firing temperature is preferably in the range of 1,300 to 3,000°C, more preferably in the range of 1,700 to 3,000°C, and even more preferably in the range of 1,900 to 3,000°C. A maximum temperature of 1,300°C or higher is preferable because it promotes carbonization of the resin, resulting in a porous carbon sheet with excellent electrical and thermal conductivity. On the other hand, a maximum temperature of 3,000°C or lower is preferable because it reduces the operating cost of the heating furnace.

[0051] During firing, it is preferable that the heating rate of the pre-impregnated material is within the range of 80 to 5,000°C / min. A heating rate of 80°C / min or higher is preferable because it results in better productivity. On the other hand, a heating rate of 5,000°C / min or lower is preferable because the carbonization of the resin proceeds slowly, forming a dense structure, resulting in a porous carbon sheet with excellent conductivity and thermal conductivity. When using a continuous heating furnace, the heating rate is adjusted by adjusting the temperature gradient inside the furnace and the transport speed of the pre-impregnated material. Multiple porous carbon sheets obtained by firing may be laminated. For example, a thick porous carbon sheet can be made by joining porous carbon sheets with an adhesive.

[0052] <Water-repellent treatment> As described above, a porous carbon sheet that has been treated with a water-repellent coating to improve drainage is preferably used as a gas diffusion layer. Methods for water-repellent treatment of a porous carbon sheet include immersing the porous carbon sheet in a dispersion containing a water-repellent resin, or applying a dispersion containing a water-repellent resin to the porous carbon sheet by die coating, spray coating, etc. After the water-repellent treatment, a drying process and a sintering process may be added as needed. The amount of water-repellent resin to be added is preferably 1 to 50 parts by mass, and more preferably 3 to 40 parts by mass, per 100 parts by mass of the porous carbon sheet before water-repellent treatment. If the amount of water-repellent resin added is 1 part by mass or more, the porous carbon sheet will have excellent drainage properties, which is preferable. On the other hand, if the amount is 50 parts by mass or less, the porous carbon sheet will have excellent conductivity, which is preferable.

[0053] <Formation of Microporous Layer> As described above, a porous carbon sheet with a microporous layer formed on one side is suitably used as a gas diffusion layer. One method for forming the microporous layer is to apply a microporous layer coating solution.

[0054] It is preferable to include conductive fine particles in the microporous layer coating solution because it provides a microporous layer with excellent conductivity, and it is also preferable to include a water-repellent resin because it provides a microporous layer with excellent drainage and mechanical strength. The microporous layer coating solution may also contain a dispersion medium such as water or an organic solvent, or a dispersion aid such as a surfactant. Water is preferred as the dispersion medium, and a nonionic surfactant is preferred as the dispersion aid.

[0055] The microporous coating solution can be applied to porous carbon sheets using various commercially available coating devices. While screen printing, rotary screen printing, spray aeration, intaglio printing, gravure printing, die coating, bar coating, and blade coating are all possible coating methods, die coating is preferred because it allows for precise quantification of the coating amount regardless of the surface roughness of the porous carbon sheet. The coating methods described above are merely examples and are not necessarily limited to these.

[0056] In forming the microporous layer, it is preferable to apply the microporous layer coating solution to one side and then dry it at a temperature of 80 to 120°C. Specifically, it is preferable to place the porous carbon sheet coated with the microporous layer coating solution into a dryer set to a temperature of 80 to 120°C and dry it for 5 to 30 minutes. The amount of drying air can be determined as appropriate, but rapid drying is undesirable as it may induce microcracks on the surface. After drying, it is preferable to place the sheet into a muffle furnace, firing furnace, or high-temperature dryer and heat it at 300 to 380°C for 5 to 20 minutes to melt the water-repellent resin and form a microporous layer as a binder that connects conductive fine particles.

[0057] <Membrane Electrode Assembly> A membrane electrode assembly can be formed by bonding the porous carbon sheet of the present invention to at least one side of an electrolyte membrane having catalyst layers on both sides. When using a porous carbon sheet with a microporous layer formed on it to serve as a gas diffusion layer, it is preferable to arrange it so that the microporous layer side is in contact with the catalyst layer side. This facilitates back diffusion of the generated water, increases the contact area between the catalyst layer and the porous carbon sheet, and reduces contact electrical resistance. Platinum is usually used as the catalyst for the catalyst layer. It is preferable to use a perfluorosulfonic acid-based polymer material with high proton conductivity, oxidation resistance, and heat resistance for the electrolyte membrane.

[0058] <Fuel Cell> A fuel cell is one aspect of the present invention. The fuel cell of the present invention is a fuel cell having the porous carbon sheet of the present invention. That is, it refers to a fuel cell having separators at both ends of the membrane electrode assembly described above. The separator has a flow path to allow fuel gas to flow into the anode-side gas diffusion layer and oxidizing gas to flow into the cathode-side gas diffusion layer. The separator and the flow path can be of any shape that allows fuel gas and oxidizing gas to flow in and out. A fuel cell stack can be constructed by stacking multiple of the above fuel cells.

[0059] <Liquid Electrolysis Apparatus> A liquid electrolysis apparatus is one aspect of the present invention. The liquid electrolysis apparatus of the present invention has the porous carbon sheet of the present invention. That is, it has a liquid electrolysis cell having separators on both sides of the membrane electrode assembly described above.

[0060] <Redox Flow Battery> The redox flow battery is one aspect of the present invention. The redox flow battery of the present invention uses the porous carbon sheet of the present invention as the positive electrode and / or negative electrode. The porous carbon sheet of the present invention can be used as an electrode in either a flow-through type or a flow-by type cell.

[0061] <Mobile Vehicles> Mobile vehicles are one aspect of the present invention. The fuel cell in the present invention can be used as a power source for mobile vehicles such as automobiles, ships, and railways. In other words, the mobile vehicle of the present invention refers to a mobile vehicle equipped with the fuel cell of the present invention.

[0062] The present invention will be specifically described below with reference to examples. However, the present invention is not limited in any way by the following examples.

[0063] <Preparation of Porous Carbon Sheets> Carbon fibers (Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca®") were cut to an average length of 7 mm, dispersed in water, and continuously paper-made using a wet papermaking method to obtain a wet material. Furthermore, a 10% by mass aqueous solution of polyvinyl alcohol was applied to the wet material as a binder, and dried to produce a carbon fiber paper-made body.

[0064] Next, liquids A, B, and C to be applied in the application process were prepared as follows.

[0065] First, a resin was prepared by mixing a thermosetting resin (resol-type phenolic resin) and a thermoplastic resin (novolac-type phenolic resin) in a 1:1 mass ratio. This mixture was then mixed with flake graphite (average particle size 5 μm) as the graphite and methanol as the solvent, and uniformly dispersed to obtain solution A. At this time, the total mass of solution A was 100% by mass, and the resin, graphite, and solvent were mixed so that their masses were 25% by mass, 5% by mass, and 70% by mass, respectively.

[0066] Next, a resin was prepared by mixing a thermosetting resin (resol-type phenolic resin) and a thermoplastic resin (novolac-type phenolic resin) in a 1:1 mass ratio, and methanol was used as the solvent. The mixture was then uniformly dispersed to obtain solution B. At this time, the total mass of solution B was 100% by mass, and the resin and solvent were mixed so that their masses were 30% by mass and 70% by mass, respectively.

[0067] Next, flake graphite (average particle size 5 μm) was mixed with methanol as the solvent to obtain a uniformly dispersed solution C. At this time, the total mass of solution C was 100% by mass, and the graphite and solvent were mixed so that their masses were 10% by mass and 90% by mass, respectively.

[0068] A pre-impregnated body was prepared by applying one or more of liquids A, B, and C to both sides of a carbon fiber paperboard, and then heating and drying it. Next, it was heat-treated at 200°C while being pressed at 3 MPa using a flat plate press.

[0069] The pre-impregnated material, after heat treatment, was fired in a furnace maintained in a nitrogen gas atmosphere with a maximum temperature of 2,400°C to obtain a porous carbon sheet.

[0070] <Evaluation> [Basis weight of porous carbon sheet] A 10 cm square sample was cut from the porous carbon sheet, and the mass [g] of the sample was measured using a precision balance. The obtained mass was measured against the sample area (0.01 m²). ２ The value obtained by dividing by ) is the basis weight [g / m²] of the porous carbon sheet. ２ ]

[0071] [Thickness of the porous carbon sheet] The compression test mode of the "Autograph®" AGS-X manufactured by Shimadzu Corporation was used. A porous carbon sheet was cut to a size of 20 mm x 20 mm, sandwiched between smooth metal rigid electrodes, and the thickness of the porous carbon sheet when an average pressure of 1.0 MPa was applied was defined as the thickness of the porous carbon sheet.

[0072] [Filling density of porous carbon sheets] First, a sample was prepared using an ion milling system IM4000 manufactured by Hitachi High-Technologies Corporation, which allowed observation of a cross-section perpendicular to the sheet plane of a porous carbon sheet. Next, using a scanning electron microscope S-4800 manufactured by Hitachi, Ltd., the cross-section of the sample was magnified to approximately 500 times, and photographs were taken at five locations to obtain five images. Next, the void and non-void regions were binarized in each image. In the binarization process, the point where the degree of separation of the two peaks obtained when plotting the grayscale histogram of the image was highest was used as the brightness threshold for binarization (Otsu's binarization method). That is, the image was divided into the dark side of brightness (void region) and the bright side of brightness (non-void region, carbon fiber sheet constituent components such as carbon fibers and binders). Next, the area extending 20 μm inward from the surface of the porous carbon sheet was defined as the vicinity of the porous carbon sheet (the area extending 20 μm inward from the upper surface was defined as the vicinity of the upper surface, and the area extending 20 μm inward from the lower surface was defined as the vicinity of the lower surface), and the area other than the vicinity of both surfaces of the porous carbon sheet was defined as the interior of the porous carbon sheet. The percentage of non-voids in each of these areas was calculated. For five images, the packing density near both surfaces and in the interior was calculated, and five packing density values ​​were obtained for each image. The average value of the five values ​​obtained for each image was calculated, and this average value was defined as the packing density near both surfaces and in the interior of the porous carbon sheet.

[0073] [Structural uneven distribution rate of porous carbon sheet] Binarization of cross-sectional images was performed in the same manner as described in [Filling density of porous carbon sheet] above. Next, in the binarized images, the porous carbon sheet was divided into seven layers obtained by dividing the thickness of the porous carbon sheet into seven equal parts, and the layers were numbered sequentially as Layer 1, Layer 2, Layer 3, Layer 4, Layer 5, Layer 6, and Layer 7, starting from the layer containing one surface and moving towards the layer containing the other surface, and the area of ​​the non-void portion of each layer was measured. The structural uneven distribution rate of each layer was calculated by dividing the area of ​​the non-void portion of each layer by the total area of ​​the non-void portions of all layers. The structural uneven distribution rate of each layer was calculated for five images, and five structural uneven distribution rate values ​​were obtained for each layer. The average value of the five values ​​obtained for each layer was calculated, and this average value was taken as the structural uneven distribution rate of each layer of the porous carbon sheet.

[0074] [Porosity of Porous Carbon Sheet] Binarization of cross-sectional images was performed in the same manner as described in [Filling Ratio of Porous Carbon Sheet] above. Next, the ratio of voids to the total area of ​​the porous carbon sheet, including both voids and non-voids, was calculated as the porosity. The average value of the porosities obtained from five images was calculated and used as the porosity of the porous carbon sheet.

[0075] [Coverage of the Porous Carbon Sheet Surface] Using a Hitachi S-4800 scanning electron microscope, five images were taken from the surface of the porous carbon sheet at approximately 200x magnification at five locations, acquiring five images in total. For each image, the void and non-void areas were binarized, and the ratio of the non-void area to the total area of ​​the porous carbon sheet (including both void and non-void areas) was calculated as the coverage rate. In the binarization process, the point where the separation of the two peaks obtained when plotting the grayscale histogram of the image was highest was used as the brightness threshold (Otsu's binarization method). That is, the image was divided into the dark side (void area) and the bright side (carbon fiber, binder, and other carbon fiber sheet components). The average value of the coverage rates from the five locations was calculated and used as the coverage rate for the measured surface of the porous carbon sheet. Coverage rates were measured on both the top and bottom surfaces of the porous carbon sheet.

[0076] [In-plane air permeability of porous carbon sheets] A porous carbon sheet sample, punched out in the shape of a donut with an outer diameter of φ40 mm and an inner diameter of φ10 mm, was compressed in the thickness direction at a pressure of 1 MPa using a press device. Air was then flowed from the inside to the outside of the porous carbon sheet using a mass flow controller. At this time, the flow rate of the air and the pressure applied to the air to achieve that flow rate were measured. The pressure was measured when the flow rate was increased by 0.1 L / min increments, and the ratio of the change in air flow rate to pressure, i.e., the in-plane air permeability, was calculated from the following equation 1. I = ΔQ / ΔP × μ × ln(r0 / ri) / 2π ... (Equation 1) In Equation 1, I is the in-plane air permeability (μm ３ Q is the airflow rate (L / min), P is the pressure (kPa), and μ is the air viscosity (1.8 × 10⁻¹⁰). ―５kg / (m·s)), r0 represents the sample outer diameter (m), and ri represents the sample inner diameter (m), respectively. Furthermore, ΔQ / ΔP represents the slope obtained when linear approximation is performed on the air flow rate and pressure. It can be said that the larger the value of in-plane air permeability I, the more easily gas diffuses in the in-plane direction of the porous carbon sheet.

[0077] [Gas Permeation Resistance of Porous Carbon Sheet] A porous carbon sheet cut into a circular shape is sandwiched between disks with an inner diameter of 40 mm, and 14 cc / cm is placed in the hollow part of the disk on one side ２ / sec of air is supplied, and the value obtained by measuring the differential pressure between the upstream side and the downstream side when air permeates through the porous carbon sheet within a range of 40 mm inner diameter is defined as the gas permeation resistance of the porous carbon sheet. It can be said that the smaller the value of gas permeation resistance, the more easily gas diffuses in the through-plane direction of the porous carbon sheet.

[0078] [Electrical Resistance of Porous Carbon Sheet] The compression test mode of "Autograph (Registered Trademark)" AGS-X manufactured by Shimadzu Corporation was used. A porous carbon sheet was cut into a size of 20 mm × 20 mm to obtain a sample. The sample was sandwiched between smooth rigid metal electrodes with gold plating on the upper and lower sides, and the sample was compressed at an average pressure of 1.0 MPa through the rigid electrodes. In this state, by measuring the voltage between the upper and lower electrodes when a current of 1 A was passed through the upper and lower electrodes, the electrical resistance per unit area was calculated. It can be said that the smaller the electrical resistance, the higher the conductivity of the porous carbon sheet.

[0079] (Example 1) A porous carbon sheet was produced in accordance with the above <Production of Porous Carbon Sheet>. Here, in the applying step, Liquid A was applied to both surfaces of a carbon fiber paper body to produce a porous carbon sheet. The application amount of the liquid was adjusted so that the basis weight of the porous carbon sheet was about 40 g / m ２ . For the produced porous carbon sheet, the basis weight, thickness, filling rate, structural uneven distribution rate, porosity, coverage rate, gas permeation resistance, in-plane air permeability and electrical resistance were measured in accordance with the above <Evaluation>. The results are as shown in Table 1.

[0080] (Example 2) A porous carbon sheet was produced and various measurements were carried out in the same manner as in Example 1, except that in the applying step, Liquid C was first applied to both surfaces, and then Liquid B was applied to both surfaces. The results are as shown in Table 1.

[0081] (Example 3) In the application process, liquid B was first applied to both surfaces, and then liquid C was applied to both surfaces. Otherwise, the porous carbon sheet was prepared and various measurements were performed in the same manner as in Example 1. The results are shown in Table 1.

[0082] (Example 4) In the application process, liquid C was applied to the upper surface of the porous carbon sheet and liquid B to the lower surface. Otherwise, the porous carbon sheet was prepared and various measurements were performed in the same manner as in Example 1. The results are shown in Table 1.

[0083] (Comparative Example 1) In the application process, liquid A was applied only to the upper surface of the carbon fiber paper, but otherwise, a porous carbon sheet was prepared and various measurements were performed in the same manner as in Example 1. The results are shown in Table 1.

[0084] (Comparative Example 2) In the application process, liquid C was first applied only to the top surface, and then liquid B was applied only to the top surface. Otherwise, the porous carbon sheet was prepared and various measurements were performed in the same manner as in Example 1. The results are shown in Table 1.

[0085] (Comparative Example 3) In the impregnation process, a pre-impregnated carbon fiber paper was prepared by impregnating a carbon fiber paper machine with liquid A. A porous carbon sheet was then prepared and various measurements were taken in the same manner as in Example 1. The results are shown in Table 1.

[0086]

[0087] 1. Porous carbon sheet 11. Layer 1 12. Layer 2 13. Layer 3 14. Layer 4 15. Layer 5 16. Layer 6 17. Layer 7

[0088] The porous carbon sheet of the present invention can be suitably used as an electrode in fuel cells, liquid electrolytic devices, and redox flow batteries, and in particular as a gas diffusion electrode in polymer electrolyte fuel cells used as a power source for mobile devices such as fuel cell vehicles and ships.

Claims

1. A porous carbon sheet comprising carbon fibers and a binder, wherein the packing density near both surfaces in the thickness direction of the porous carbon sheet is higher than the packing density inside.

2. The porous carbon sheet according to claim 1, wherein when the thickness of the porous carbon sheet is divided into seven equal parts, the seven layers obtained are designated as layer 1, layer 2, layer 3, layer 4, layer 5, layer 6, and layer 7 in order from the layer containing one surface to the layer containing the other surface, and the structural uneven distribution of layer 1 and layer 7 is 1.5 times or more the structural uneven distribution of the innermost, least porous layer. Here, the structural uneven distribution of each layer refers to the ratio of the volume of non-porous parts in each layer to the volume of non-porous parts in the entire porous carbon sheet, which is set to 100%.

3. The porous carbon sheet according to claim 2, wherein the structural distribution rate of layer 1 and layer 7 is 15% or more, and the structural distribution rate of the innermost sparse layer is 13% or less.

4. The porous carbon sheet according to claim 2, wherein the innermost sparse layer is layer 4.

5. The porous carbon sheet according to claim 2, wherein the structural uneven distribution of layer 1 is the greatest among layers 1, 2, and 3, and the structural uneven distribution of layer 7 is the greatest among layers 5, 6, and 7.

6. The porous carbon sheet according to claim 2, wherein the structural uneven distribution rate of the innermost sparse layer is 5.0% or more and 10.0% or less.

7. The porous carbon sheet according to claim 1 or claim 2, wherein the coverage rate of both surfaces is 50% or more and 80% or less.

8. A porous carbon sheet having a structure comprising a plurality of porous carbon sheets as described in claim 1.

9. A fuel cell having a porous carbon sheet according to any one of claims 1 to 8.

10. A liquid electrolytic apparatus having a porous carbon sheet according to any one of claims 1 to 8.

11. A redox flow battery having a porous carbon sheet according to any one of claims 1 to 8.

12. A mobile body equipped with the fuel cell described in claim 9.

13. A method for manufacturing a porous carbon sheet, comprising, in this order, a coating step of applying conductive particles and / or resin to both surfaces of a carbon fiber papermaking body, and a firing step of firing the carbon fiber papermaking body at a high temperature.

14. A method for producing a porous carbon sheet according to claim 13, wherein in the application step, two or more liquids containing conductive particles and / or resins of different compositions are used for application.

15. A method for producing a porous carbon sheet according to claim 13, comprising a step of laminating a plurality of porous carbon sheets or intermediates thereof after the impregnation step.

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