Porous carbon sheet

Abstract

To provide a porous carbon sheet combining electrical conductivity and gas diffusivity.SOLUTION: A porous carbon sheet includes a porous carbon layer and a microporous layer. The microporous layer has an impregnated portion in which the microporous layer is impregnated and a non-impregnated portion in which the microporous layer is not impregnated. A rate of change in a thickness of the non-impregnated portion when 2 MPa pressure is applied relative to when no pressure is applied is greater than that of the impregnated portion when 2 MPa pressure is applied relative to when no pressure is applied.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a porous carbon sheet used in a gas diffusion layer for a fuel cell, a water electrolysis device, or a redox flow battery, and in particular to a porous carbon sheet suitable for a gas diffusion layer of a polymer electrolyte fuel cell used as a power source for a mobile body such as a fuel cell vehicle or a ship. [Background technology]

[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 been attracting attention as a clean energy source. Polymer electrolyte fuel cells, in particular, have a low standard operating temperature of around 100°C and a high energy density, making them expected to find a wide range of applications, including as a power generation device for relatively small-scale distributed power generation facilities and mobile vehicles such as fuel cell vehicles and ships.

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

[0004] Fuel cells are systems that electrically extract the energy that is generated when hydrogen and oxygen react to produce water. Therefore, when the electrical load increases, that is, when the current output from the cell increases, a large amount of water (water vapor) is generated. This water vapor condenses into droplets at low temperatures and blocks the pores in the gas diffusion layer, reducing the amount of gas (oxygen or hydrogen) supplied to the catalyst layer. When all the pores are eventually blocked, power generation stops (this phenomenon is called flooding).

[0005] Specifically, conductive porous substrates (porous carbon layers) made of carbon fibers, such as carbon felt, carbon paper, and carbon cloth, are used as gas diffusion layers. However, because the fibers are coarse, large water droplets are generated when water vapor condenses, easily causing flooding. For this reason, a microporous layer (also called a microporous layer) made of conductive fine particles such as carbonaceous powder is sometimes provided on the conductive porous substrate.

[0006] When a microporous layer is provided, the microporous layer may have a configuration in which a portion is impregnated with the conductive porous substrate and a portion is not impregnated. In order to improve the performance of the gas diffusion layer, various studies have been conducted on the thickness ratio of the impregnated portion to the non-impregnated portion of the microporous layer.

[0007] For example, Patent Document 1 discloses a gas diffusion layer in which the ratio of the thickness of the soaked portion to the thickness of the microporous layer taken as 100% is 30% or more and 70% or less, and describes that this method can be used to obtain a gas diffusion layer with high gas diffusivity.

[0008] Patent Document 2 also discloses a gas diffusion layer for a fuel cell, in which, with respect to a layer in which a microporous layer has permeated into a conductive porous substrate and a layer in which the microporous layer has not permeated, the thickness of the non-permeated layer is more than 0.0 μm and not more than 20.0 μm, and the thickness of the permeated layer is not more than 29% of the total thickness of the microporous layer, and describes that this method can be used to obtain a gas diffusion layer for a fuel cell that has low contact resistance with an electrode catalyst layer and high gas diffusivity. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2018 / 061833 [Patent Document 2] Patent Publication No. 2021-136056 Summary of the Invention [Problem to be solved by the invention]

[0010] However, neither Patent Document 1 nor Patent Document 2 mentions any consideration of how each layer of the gas diffusion layer changes due to pressure. Because high pressure is applied to the electrodes incorporated in fuel cells, in order to obtain a high-performance gas diffusion layer, it is necessary to consider the changes in each layer before and after pressure application.

[0011] Therefore, the present invention aims to provide a porous carbon sheet that combines electrical conductivity and gas diffusivity by controlling the structure of each layer of the porous carbon sheet under a pressure of 2 MPa, which is the same pressure as that generally applied to electrodes incorporated in solid polymer fuel cells. [Means for solving the problem]

[0012] In order to solve the above problems, the present invention employs the following means. [1] A porous carbon sheet having a porous carbon layer and a microporous layer, wherein the microporous layer has an impregnated portion that is impregnated into the porous carbon layer and an unimpregnated portion that is not impregnated into the porous carbon layer, and wherein the rate of thickness change of the unimpregnated portion when pressurized at 2 MPa relative to when not pressurized is greater than the rate of thickness change of the impregnated portion when pressurized at 2 MPa relative to when not pressurized. [2] The porous carbon sheet according to [1], wherein the surface roughness Sq (JIS B0601:2013) of the microporous layer when pressurized at 2 MPa is 3.0 μm or less. [3] The porous carbon sheet according to [1] or [2], wherein the thickness change rate of the non-impregnated portion when pressurized at 2 MPa is 50 to 100% compared to when not pressurized. [4] The porous carbon sheet according to any one of [1] to [3], wherein the rate of change in thickness of the impregnated portion when pressurized at 2 MPa relative to when not pressurized is 0.1 to 20%. [5] The porous carbon sheet according to any one of [1] to [4], wherein the porous carbon layer is composed of carbon fibers and a carbonized resin, and the basis weight of the carbonized resin is 0.7 to 1.0 times the basis weight of the carbon fibers. [6] The porous carbon sheet according to any one of [1] to [5], wherein the thickness of the impregnated portion at a pressure of 2 MPa is 0.60 times or more and less than 1.00 times the thickness of the porous carbon layer alone at a pressure of 2 MPa. [7] A fuel cell comprising the porous carbon sheet according to any one of [1] to [6]. [8] A water electrolysis device comprising the porous carbon sheet according to any one of [1] to [6]. [9] A redox flow battery comprising the porous carbon sheet according to any one of [1] to [6].

[10] A mobile object equipped with the fuel cell described in [7]. [Effects of the Invention]

[0013] According to the present invention, a porous carbon sheet having both electrical conductivity and gas diffusibility can be obtained. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a conceptual diagram showing an example of the volume fraction in the thickness direction of the porous carbon layer and the microporous layer of the porous carbon sheet of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] The porous carbon sheet of the present invention has a porous carbon layer and a microporous layer, that is, the microporous layer is provided on at least one side of the porous carbon layer.

[0016] In the present invention, the porous carbon layer is a porous body that has high gas diffusivity, allowing gas supplied from the separator to diffuse to the catalyst, high drainage properties, allowing water produced during the electrochemical reaction to be discharged to the separator, and high conductivity, allowing the generated current to be extracted. As the porous carbon layer, conductive porous substrates containing carbon fibers, such as carbon felt, carbon paper, and carbon cloth, are preferred because of their excellent corrosion resistance. Among these, a substrate composed of a conductive porous substrate containing carbon fibers and a carbonized resin is preferred in terms of mechanical strength. In particular, a substrate obtained by binding a carbon fiber sheet with a carbonized resin, i.e., carbon paper, is more preferred because of its excellent "springiness," i.e., ability to absorb dimensional changes in the thickness direction due to swelling and shrinkage of the electrolyte membrane. Here, carbonized resin refers to a carbonized product obtained by heat-treating a resin.

[0017] The porous carbon sheet of the present invention is preferably one that has been subjected to a water-repellent treatment by applying a water-repellent resin to the porous carbon layer. Examples of the water-repellent resin contained in the porous carbon layer include PTFE (polytetrafluoroethylene) (e.g., Teflon (registered trademark)), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (perfluoroalkoxy fluoride resin), ETFE (ethylene-tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride), with PTFE or FEP being preferred because of their strong water-repellent properties.

[0018] The amount of the water-repellent resin is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less when the mass of the entire porous carbon layer is taken as 100% by mass. If it is less than 0.1% by mass, the water repellency may not be sufficiently exhibited, and if it exceeds 20% by mass, the pores that serve as gas diffusion paths or drainage paths may be blocked, or electrical resistance may increase.

[0019] The water-repellent treatment of the porous carbon layer can be carried out by a commonly known treatment technique of immersing the porous carbon layer in a dispersion containing a water-repellent resin, or by a coating technique of applying a water-repellent resin to the porous carbon layer by die coating, spray coating, or the like. Dry process processing such as sputtering of the water-repellent resin can also be applied. After the water-repellent treatment, a drying step and / or a sintering step may be added as necessary.

[0020] The microporous layer is a layer having finer pores than the porous carbon layer. Since high conductivity is required for using the porous carbon sheet of the present invention as an electrode, the microporous layer is preferably a layer containing conductive fine particles such as carbon black, carbon nanotubes, carbon nanofibers, chopped carbon fibers, graphene, or graphite.

[0021] The conductive fine particles are preferably carbonaceous powders with a primary particle size of 20 to 70 nm. The primary particle size refers to the diameter of a single unagglomerated granular carbonaceous particle in the carbonaceous powder. For example, in the case of carbon black, it refers to the diameter of a single unagglomerated carbon black particle. A primary particle size of 20 nm or greater increases the pore mode diameter of the microporous layer, improving the gas diffusivity of the microporous layer. A primary particle size of 23 nm or greater is more preferable, and 26 nm or greater is even more preferable. A primary particle size of 70 nm or less makes the microporous layer more susceptible to collapse when pressure is applied. A primary particle size of 39 nm or less is more preferable because it makes the microporous layer more susceptible to collapse. Furthermore, conductive fine particles with a small primary particle size have a large specific surface area, which increases the specific surface area relative to the amount of additives, such as a water-repellent resin, that bind the conductive fine particles together, weakening the binding effect between the components contained in the microporous layer. This makes the microporous layer more susceptible to collapse when pressure is applied.

[0022] Furthermore, the microporous layer is required to have properties such as electrical conductivity, gas diffusion, water drainage, moisture retention, and thermal conductivity, as well as strong acid resistance on the anode side and oxidation resistance on the cathode side inside the fuel cell. Therefore, it is preferable for the microporous layer to contain a water-repellent resin such as a fluororesin in addition to the conductive fine particles. Examples of water-repellent resins contained in the microporous layer include PTFE, FEP, PFA, and ETFE, which are similar to the water-repellent resins preferably used to make porous carbon layers water-repellent. PTFE or FEP is preferred because of its particularly high water repellency. The content of the water-repellent resin in the microporous layer is preferably 1 to 70 parts by mass, more preferably 5 to 60 parts by mass, per 100 parts by mass of the conductive fine particles contained in the microporous layer. When the amount of water-repellent resin is 1 part by mass or more, the microporous layer has excellent drainage and mechanical strength, which is preferable. On the other hand, when the amount is 70 parts by mass or less, the microporous layer has excellent electrical conductivity, which is preferable. By adjusting the content of the water-repellent resin in the microporous layer, the binding effect and crushability of the microporous layer can be controlled.

[0023] The surface roughness of the porous carbon sheet is preferably small. A small surface roughness is preferable because it increases the contact area with adjacent catalyst layers when used in fuel cells, etc., and reduces contact resistance. Since the porous carbon sheet is used as an electrode under pressure, it is preferable that the surface roughness be small, especially when pressurized. Electrodes incorporated into polymer electrolyte fuel cells are generally subjected to a pressure of about 2 MPa. For this reason, when the porous carbon sheet is sandwiched between flat plates and pressurized at a pressure of 2 MPa, the surface roughness Sq (JIS B0601:2013) of the microporous layer is preferably 3.0 μm or less. If the surface roughness Sq exceeds 3.0 μm when pressurized at 2 MPa, non-contact areas may occur between adjacent catalyst layers.

[0024] The surface roughness Sq at a pressure of 2 MPa can be determined by X-ray CT measurement. The porous carbon sheet is sandwiched between flat indenters and pressurized at 2 MPa. From the tomographic image obtained by X-ray CT measurement, the highest pixel value is obtained for each pixel position in the XY plane relative to the thickness direction Z, and the standard deviation is calculated to calculate the surface roughness Sq at the time of pressure. For example, a high-resolution 3D X-ray microscope nano3DX manufactured by Rigaku Corporation can be used.

[0025] In the porous carbon sheet of the present invention, the microporous layer has an impregnated portion where the porous carbon layer is impregnated and an unimpregnated portion where the porous carbon layer is not impregnated.

[0026] In the porous carbon sheet of the present invention, the non-impregnated portion of the microporous layer has a greater thickness change rate at a pressure of 2 MPa relative to that at no pressure (hereinafter simply referred to as "thickness change rate") than the impregnated portion. The thickness change rate is calculated using the following formula, where a is the thickness at no pressure and b is the thickness at a pressure of 2 MPa. Thickness change rate (%) = (ab) / a × 100.

[0027] The difference between the thickness change rate of the non-impregnated portion and the thickness change rate of the impregnated portion is preferably 10% or more, and more preferably 30% or more. The non-impregnated portion is easily crushed when pressurized, which reduces the surface roughness of the non-impregnated portion when pressurized, thereby reducing the contact resistance with the catalyst layer. Furthermore, the impregnated portion is less likely to be crushed, which improves gas diffusion within the porous carbon sheet.

[0028] The thicknesses of the impregnated and non-impregnated portions can be determined by X-ray CT measurement. In a tomographic image obtained by X-ray CT, components that easily transmit X-rays (low density) are displayed in black, and components that easily absorb X-rays (high density) are displayed in white. In the present invention, porous carbon layer components such as carbon fibers are displayed in white, and microporous layer components such as conductive fine particles are displayed in a darker color. Note that voids where nothing is present are displayed in the darkest color. By determining a color threshold taking into account shape characteristics so that the microporous layer component and the porous carbon layer component can be distinguished, the areas of the microporous layer component and the porous carbon layer component on a certain plane can be determined.

[0029] To determine the thickness of the impregnated and non-impregnated portions, first calculate the volume fraction of the microporous layer and the porous carbon layer at each thickness direction position, using the entire measured area in the XY plane as the denominator and the area of ​​the microporous layer or porous carbon layer portion as the numerator. Then, using the thickness direction as the vertical axis and the volume fraction as the horizontal axis, determine the change in the volume fraction of the microporous layer and the porous carbon layer in the thickness direction. An example is shown in Figure 1. For a microporous layer volume fraction of 10 and a porous carbon layer volume fraction of 20, the portion where neither the microporous layer volume fraction 10 nor the porous carbon layer volume fraction 20 is zero is defined as the portion where the microporous layer and the porous carbon layer overlap, i.e., the impregnated portion, and its thickness is defined as the thickness 32 of the impregnated portion of the microporous layer. Similarly, the thickness of the portion where the volume fraction of the porous carbon layer is 0 and only the microporous layer is present is referred to as the thickness 31 of the non-impregnated portion of the microporous layer, and the thickness of the portion where the volume fraction of the microporous layer is 0 and only the porous carbon layer is present is referred to as the thickness 33 of the single porous carbon layer. As an apparatus for acquiring the image by X-ray CT, for example, a high-resolution 3D X-ray microscope nano3DX manufactured by Rigaku Corporation can be used. The thickness change rates of the impregnated and non-impregnated portions can be measured by performing measurements in an unpressurized state and in a state where the porous carbon sheet is sandwiched between flat indenters and pressurized at 2 MPa. The measurement in an unpressurized state should be performed before the measurement in a state pressurized at 2 MPa.

[0030] The thickness change rate of the non-impregnated portion of the porous carbon sheet of the present invention is preferably 50 to 100%. If the thickness change rate is less than 50%, the surface roughness of the non-impregnated portion when pressurized increases, which may result in non-contact areas between the adjacent catalyst layers. Furthermore, a thickness change rate of 100% refers to the case where the thickness of the non-impregnated portion is reduced to less than the minimum pixel size of an X-ray CT tomographic image when measured under a pressure of 2 MPa.

[0031] At a pressure of 2 MPa, the thickness of the impregnated portion is preferably 0.60 to 3.00 times the thickness of the single layer of porous carbon layer. A thickness of 0.60 or more is preferable because it reduces internal resistance. If it exceeds 3.00, the drainage ability of the porous carbon sheet decreases, which may result in a decrease in the power generation performance of the fuel cell. A thickness of less than 1.00 is more preferable because it allows gas to easily diffuse in the in-plane direction of the porous carbon sheet. If gas easily diffuses in the in-plane direction of the porous carbon sheet, gas can be sufficiently diffused in the in-plane direction inside the fuel cell, improving power generation performance.

[0032] The thickness of the impregnated portion can be controlled by adjusting the content of the water-repellent resin in the microporous layer. When the microporous layer is formed by applying a microporous layer coating liquid to the porous carbon layer by die coater coating, bar coating, or blade coating, the thickness can be controlled by adjusting the distance between the porous carbon layer and the nozzle or blade of the coating machine.

[0033] The ratio of the thickness of the impregnated portion to the total thickness of the microporous layer, including both the impregnated and non-impregnated portions, is called the penetration amount. The penetration amount is preferably 90% or less. A penetration amount of 90% or less improves the drainage of the porous carbon sheet, improving the power generation performance when incorporated into a fuel cell. The penetration amount tends to increase when the content of the water-repellent resin in the microporous layer coating liquid (described below) is low. Furthermore, when the microporous layer is formed by coating, the penetration amount tends to increase when the distance between the nozzle, blade, etc. of various coating machines and the porous carbon layer is short.

[0034] Since the porous carbon sheet is used as an electrode under pressure, it is preferable that the impregnated portion is resistant to crushing when pressure is applied, and the thickness change rate of the impregnated portion is preferably 0.1 to 20%. If the thickness change rate exceeds 20%, gas diffusivity may decrease. In other words, if the thickness change rate is 20% or less, electrical conductivity and gas diffusivity can be achieved at a higher level.

[0035] When the porous carbon layer is composed of carbon fiber and carbonized resin, the basis weight of the carbonized resin is preferably 0.7 to 1.0 times the basis weight of the carbon fiber in order to reduce the thickness change rate of the impregnated portion. The carbonized resin basis weight here refers to the basis weight of the carbonized resin only and does not include the basis weight of the carbon-based filler even if the porous carbon layer contains a carbon-based filler. Having a carbonized resin basis weight of 0.7 or more times the carbon fiber basis weight results in a porous carbon layer that is resistant to collapse under pressure. A sufficiently large amount of carbonized resin relative to the amount of carbon fiber forms many bonding points between the carbon fibers. It is believed that these bonding points are reinforced by the binder in the microporous layer impregnated into the porous carbon layer, making the impregnated portion less resistant to collapse under pressure. If the carbonized resin basis weight exceeds 1.0 times the carbon fiber basis weight, the gas diffusion properties of the porous carbon sheet may deteriorate.

[0036] Of the porous carbon layers, the porous carbon layer alone, which is not impregnated with a microporous layer, preferably has a greater thickness change rate than the impregnated portion. It is more preferable that the thickness change rate of the porous carbon layer alone is 5% or more greater than the thickness change rate of the impregnated portion. The porous carbon layer alone collapses under pressure, thereby maintaining its "springiness," a characteristic of absorbing dimensional changes in the thickness direction due to swelling and shrinkage of the electrolyte membrane. By increasing the ratio of the carbonized resin basis weight to the carbon fiber basis weight in the porous carbon layer, the thickness change rate of the porous carbon layer alone can be adjusted to be greater than the thickness change rate of the impregnated portion.

[0037] In addition, when a porous carbon layer is produced by impregnating a paper sheet containing carbon fibers with a resin composition and then firing the paper sheet, as in the production method described below, the carbonized resin weight per unit area and the carbon fiber weight per unit area of ​​the porous carbon layer can be controlled by controlling the amount of resin composition impregnated into the paper sheet containing carbon fibers. In this case, it should be noted that the carbon fibers and the resin components in the resin composition are carbonized during the carbonization step, resulting in a change in mass, and it is advisable to calculate the mass change rates of the carbon fibers and the resin components in the resin composition in advance.

[0038] Next, a preferred method for obtaining the porous carbon sheet of the present invention will be specifically described, but the present invention is not limited to the following description, and the description of a preferred embodiment in each individual description can also be construed as a description of the present invention as a general concept.

[0039] <Paper body and method for manufacturing paper body> In the present invention, a preferred embodiment of the porous carbon layer comprises a paper body containing carbon fibers and a carbonized resin. To obtain a paper body containing carbon fibers, a wet papermaking method in which carbon fibers are dispersed in a liquid or a dry papermaking method in which carbon fibers are dispersed in air is used. Among these, the wet papermaking method is preferably used because of its excellent productivity.

[0040] Examples of carbon fibers include polyacrylonitrile (PAN)-based, pitch-based, rayon-based, etc. Among these, PAN-based and pitch-based carbon fibers are preferably used in the present invention because of their excellent mechanical strength.

[0041] The carbon fiber in the present invention preferably has an average single fiber diameter in the range of 3 to 20 μm, more preferably in the range of 5 to 10 μm. When the average diameter is 3 μm or more, the pore size becomes large, improving drainage and preventing flooding. On the other hand, when the average diameter is 20 μm or less, water vapor diffusivity becomes small and drying-up can be prevented. In addition, using two or more types of carbon fibers having different average diameters is preferable because it can improve the surface smoothness of the porous carbon layer.

[0042] The carbon fibers contained in the paper body preferably have an average single fiber length in the range of 3 to 20 mm, more preferably in the range of 5 to 15 mm. An average length of 3 mm or more is preferred because the porous carbon sheet has excellent mechanical strength, electrical conductivity, and thermal conductivity. On the other hand, an average length of 20 mm or less is preferred because the carbon fibers are well dispersed during papermaking, resulting in a homogeneous porous carbon sheet. Carbon fibers having such an average length can be obtained by, for example, cutting continuous carbon fibers to the desired length.

[0043] In the present invention, paper can be made by mixing organic fibers with carbon fibers. The organic fibers are burned off in the firing step described below, improving the drainage and gas diffusion properties of the porous carbon sheet. Examples of organic fibers that can be used include polyethylene fibers, vinylon fibers, polyacetal fibers, polyester fibers, polyamide fibers, rayon fibers, and acetate fibers.

[0044] The paper body may contain an organic polymer as a binder for the purpose of improving the shape retention and handling properties of the paper body. Examples of the organic polymer that can be used include polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, and cellulose.

[0045] The paper body is preferably in the form of a sheet in which carbon fibers are randomly dispersed in a two-dimensional plane, for the purpose of maintaining isotropic in-plane electrical and thermal conductivities.

[0046] In the paper body, the weight of the carbon fiber is 10 to 60 g / m 2 It is preferable that the thickness is in the range of 20 to 50 g / m 2 It is more preferable that the weight of the carbon fiber is in the range of 10 g / m 2 When the weight is 60 g / m or more, the porous carbon sheet has excellent mechanical strength, which is preferable. 2When the porous carbon sheet has a thickness of 1 / 2 mm or less, the porous carbon sheet has excellent gas diffusivity and drainage property, which is preferable. When a plurality of sheets of paper are laminated together, the basis weight of the carbon fiber after lamination is preferably within the above range.

[0047] <Impregnation of Resin Composition> A method for obtaining a porous carbon layer consisting of a paper sheet containing carbon fibers and a carbonized resin includes impregnating the paper sheet with a resin composition containing a resin and then firing the paper sheet. It is preferable to use a resin that is carbonized during firing to form a conductive carbonized material. Hereinafter, a paper sheet containing carbon fibers impregnated with a resin composition will also be referred to as a "pre-impregnated body." Methods for impregnating the paper sheet with the resin composition include immersing the paper sheet in the resin composition, applying the resin composition to the paper sheet, and transferring a film made of a resin component onto the paper sheet. Of these, the method of immersing the paper sheet in the resin composition is preferred due to its excellent productivity.

[0048] In the present invention, the resin composition refers to a resin to which a solvent or the like is added as needed. Examples of the solvent that can be used include methanol, ethanol, and isopropyl alcohol. The resin composition may further contain additives such as a carbon-based filler and a surfactant.

[0049] The carbonization yield of the resin contained in the resin composition is preferably 40% by mass or more under the conditions of the subsequent firing step, since a carbonization yield of 40% by mass or more results in a porous carbon sheet with excellent mechanical properties, electrical conductivity, and thermal conductivity, which is preferable.

[0050] Examples of the resin include thermosetting resins such as phenolic resin, epoxy resin, melamine resin, and furan resin, etc. Among these, phenolic resin is preferably used because of its high carbonization yield.

[0051] Additives that can be added to the resin composition as needed can include a carbon-based filler for the purpose of improving the mechanical properties, electrical conductivity, and thermal conductivity of the porous carbon sheet. Examples of the carbon-based filler that can be used include carbon black, carbon nanotubes, carbon nanofibers, milled carbon fibers, and graphite.

[0052] The resin composition of the present invention is preferably in a liquid state at 25° C. and 0.1 MPa. When the resin composition is in a liquid state, it has excellent impregnation into the paper body, and the porous carbon sheet has excellent mechanical properties, electrical conductivity, and thermal conductivity, which is preferable.

[0053] <Laminating, heat treatment> After the pre-impregnated body is formed, it may be laminated or heat-treated before firing.

[0054] In order to obtain a porous carbon sheet with a predetermined thickness, multiple pre-impregnated bodies can be laminated together. In this case, multiple pre-impregnated bodies having the same properties can be laminated together, or multiple pre-impregnated bodies having different properties can be laminated together. Specifically, multiple pre-impregnated bodies having different average diameters and lengths of carbon fibers, carbon fiber basis weight of the paper body, and resin impregnation amounts can be laminated together.

[0055] In the present invention, the pre-impregnated body can be heat-treated for the purpose of thickening and partially crosslinking the resin composition. Methods for the heat treatment include blowing hot air onto the pre-impregnated body, heating the pre-impregnated body by sandwiching it between hot plates of a press or the like, or heating the pre-impregnated body by sandwiching it between continuous belts.

[0056] <Firing> To carbonize the resin in the pre-impregnated body, the body is calcined in an inert atmosphere. This calcination can be performed using a batch-type heating furnace or a continuous heating furnace. The inert atmosphere can be obtained by flowing an inert gas such as nitrogen gas or argon gas into the furnace.

[0057] The maximum firing temperature is preferably within the range of 1,300 to 3,000°C, more preferably within the range of 1,700 to 3,000°C, and even more preferably within the range of 1,900 to 3,000°C. A maximum temperature of 1,300°C or higher is preferred because it promotes carbonization of the resin component and results 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 preferred because it reduces the operating costs of the heating furnace.

[0058] In the present invention, the heating rate during firing is preferably within the range of 80 to 5,000°C / min. A heating rate of 80°C or higher is preferred because it provides excellent productivity. On the other hand, a heating rate of 5,000°C or lower is preferred because the carbonization of the resin proceeds slowly and a dense structure is formed, resulting in a porous carbon sheet with excellent electrical and thermal conductivity. When a continuous heating furnace is used, the heating rate is adjusted by adjusting the temperature gradient within the furnace and the conveying speed of the pre-impregnated body. Hereinafter, the fired pre-impregnated body will also be referred to as a "sintered carbon fiber body."

[0059] <Water-repellent finish> In the present invention, it is preferable to apply a water-repellent treatment to the sintered carbon fiber body for the purpose of improving drainage. That is, the sintered carbon fiber body that has been subjected to the water-repellent treatment can be preferably used as the porous carbon layer of the present invention. Methods for water-repellent treatment of the sintered carbon fiber body include immersing the sintered carbon fiber body in a dispersion containing a water-repellent agent, and applying the dispersion containing a water-repellent agent to the sintered carbon fiber body by die coating, spray coating, or the like. Note that after the water-repellent treatment, a drying process and / or a sintering process may be added as needed. The amount of the water-repellent resin applied is preferably 1 to 50 parts by mass, more preferably 3 to 40 parts by mass, per 100 parts by mass of the sintered carbon fiber body. When the amount of the water-repellent resin applied is 1 part by mass or more, the porous carbon sheet has excellent drainage properties, which is preferable. On the other hand, when it is 50 parts by mass or less, the porous carbon sheet has excellent conductivity, which is preferable.

[0060] <Formation of microporous layer> The microporous layer can be formed by applying a microporous layer coating liquid to one surface of the porous carbon layer.

[0061] The microporous layer coating liquid may contain a dispersion medium such as water or an organic solvent, or may contain a dispersion aid such as a surfactant. Water is preferred as the dispersion medium, and a nonionic surfactant is preferred as the dispersion aid. The inclusion of conductive fine particles is preferred because a microporous layer with excellent conductivity can be obtained, and the inclusion of a water-repellent resin is preferred because a microporous layer with excellent drainage properties and mechanical strength can be obtained.

[0062] The microporous layer coating liquid can be applied to the porous carbon layer using various commercially available coating devices. Usable coating methods include screen printing, rotary screen printing, spray atomization, intaglio printing, gravure printing, die coater coating, bar coating, and blade coating. Die coater coating is preferred because it allows for quantification of the coating amount regardless of the surface roughness of the porous carbon layer. The coating methods exemplified above are merely examples and are not necessarily limited to these.

[0063] In forming the microporous layer, the microporous layer coating liquid is preferably applied to one side and then dried at a temperature of 80 to 120°C. That is, the porous carbon layer coated with the microporous layer coating liquid is preferably placed in a dryer set at a temperature of 80 to 120°C and dried for 5 to 30 minutes. The drying air volume can be determined appropriately, but rapid drying is not preferred as it may induce microcracks on the surface. After drying, the layer is preferably placed in a muffle furnace, a calciner, or a high-temperature dryer and heated at 300 to 380°C for 5 to 20 minutes to melt the water-repellent resin and use it as a binder between the conductive fine particles to form the microporous layer.

[0064] <Membrane electrode assembly> In the present invention, a membrane electrode assembly can be formed by bonding the above-mentioned porous carbon sheet to at least one side of an electrolyte membrane having catalyst layers on both sides. Arranging the microporous layer side in contact with the catalyst layer side is preferable because it facilitates back-diffusion of 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. The electrolyte membrane is preferably made of a perfluorosulfonic acid-based polymer material, which has high proton conductivity, oxidation resistance, and heat resistance.

[0065] <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 configured to have separators on both ends of the above-mentioned membrane electrode assembly. The separator has a flow path for allowing 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 the fuel gas, oxidizing gas, etc. to flow in and out. A fuel cell stack can be constructed by stacking a plurality of the above-mentioned fuel cells.

[0066] <Water electrolysis device> A water electrolysis device is one aspect of the present invention. The water electrolysis device of the present invention includes the porous carbon sheet of the present invention. That is, the water electrolysis device includes a liquid electrolysis cell having separators on both sides of the above-mentioned membrane electrode assembly.

[0067] <Redox flow battery> A 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 a positive electrode and / or a negative electrode. The porous carbon sheet of the present invention can be used as an electrode in both flow-through type and flow-by type cells.

[0068] <Mobile object> A mobile body is one aspect of the present invention. The fuel cell of the present invention can be used as a power supply source for mobile bodies such as automobiles, ships, and trains. In other words, the mobile body of the present invention refers to a mobile body equipped with the fuel cell of the present invention. [Example]

[0069] The present invention will be described in more detail below with reference to examples. The methods for producing and evaluating the porous carbon sheets used in the examples are described below.

[0070] <Preparation of porous carbon layer> Toray Industries, Inc.'s Torayca (registered trademark) T300 polyacrylonitrile carbon fiber (average carbon fiber diameter: 7 μm) was cut and dispersed in water to form a continuous paper sheet using a wet papermaking method. A 10% by mass aqueous solution of polyvinyl alcohol was then applied as a binder, and the resulting sheet was dried to produce a paper sheet.

[0071] A resin composition was obtained by mixing a thermosetting resin (resol-type phenolic resin) and a thermoplastic resin (novolac-type phenolic resin) in a mass ratio of 1:1, using flake graphite (average particle diameter 5 μm) as a carbon-based filler, and methanol as a solvent, with the resin, flake graphite, and solvent being 10 mass%, 5 mass%, and 85 mass%, respectively, with the total mass of the resin composition being 100 mass%.

[0072] The paper sheet was immersed in the resin composition and then heated and dried to prepare a pre-impregnated paper sheet, which was then heat-treated at 200°C while being pressed at 3 MPa using a flat plate press.

[0073] The pre-impregnated body after the heat treatment was introduced into a heating furnace maintained in a nitrogen gas atmosphere at a maximum temperature of 2,400°C, to obtain a sintered carbon fiber body.

[0074] A diluted aqueous solution of PTFE dispersion (Polyflon (registered trademark) D-210C; manufactured by Daikin Industries, Ltd.; containing 60 parts by mass of PTFE in a dispersion medium (water)) was applied to the sintered carbon fiber body using a slit die coater so that the amount of PTFE was 5 parts by mass per 100 parts by mass of the conductive porous substrate, and the body was dried at 120°C to produce a porous carbon layer.

[0075] <Formation of microporous layer> A microporous layer coating liquid was prepared by mixing acetylene black "Denka Black (registered trademark)" (manufactured by Denki Kagaku Kogyo Kabushiki Kaisha), water-repellent resin "Polyflon (registered trademark)" PTFE dispersion D-210C (manufactured by Daikin Industries, Ltd.), surfactant "TRITON (registered trademark)" X-100 (manufactured by Nacalai Tesque, Inc.), and purified water in amounts of 7.0 parts by mass, 2.5 parts by mass, 14.1 parts by mass, and 76.4 parts by mass using a disperser. The microporous layer coating liquid was applied to one side of the porous carbon layer obtained in <Preparation of porous carbon layer> above using a die coater, and the coating was dried by heating at 120°C for 10 minutes. After drying by heating, the coating was sintered at 380°C for 3 minutes to prepare a porous carbon sheet having a microporous layer on the surface of the porous carbon layer. The coating amount of the microporous layer coating liquid was 15 g / m2, with a basis weight after sintering. 2 It was prepared so that

[0076] <Measurement of the thickness of the impregnated and non-impregnated parts of the porous carbon sheet> The unpressurized porous carbon sheet was subjected to X-ray CT analysis using a high-resolution 3D X-ray microscope, nano3DX (Rigaku Corporation), under conditions of a Cu X-ray source, a tube voltage of 40 kV, and a tube current of 30 mA. The thickness of the impregnated and non-impregnated portions was analyzed. In the tomographic image obtained with a resolution of 2.44 μm / pix, the microporous layer components, which transmit X-rays easily, appear black, while the porous carbon layer components, which absorb X-rays easily, appear white. To distinguish between the microporous layer and the porous carbon layer components, a color threshold was determined taking into account the morphological characteristics of each component. Specifically, the fibrous, relatively white portion of the tomographic image was determined to be the porous carbon layer, while the paste-like, relatively gray portion on the side where the microporous layer was applied was determined to be the microporous layer. A color threshold was determined to allow for the separation of each layer. At each thickness direction position every 2.44 μm, the volume fraction of the microporous layer and the porous carbon layer at each thickness direction position was calculated using the entire measured area in the XY plane as the denominator and the area of ​​the microporous layer or porous carbon layer within that as the numerator. Based on the volume fraction of the microporous layer and the volume fraction of the porous carbon layer, the thickness of each layer was calculated by defining the thickness of the portion where both the microporous layer and the porous carbon layer were present as the thickness of the impregnated portion of the microporous layer, the thickness of the portion where only the microporous layer was present as the thickness of the non-impregnated portion of the microporous layer, and the thickness of the portion where only the porous carbon layer was present as the thickness of the porous carbon layer alone. Next, the porous carbon sheet was sandwiched between flat indenters and similarly measured under a pressure of 2 MPa to measure the thicknesses of the impregnated portion, non-impregnated portion, and the porous carbon layer alone.

[0077] For each of the impregnated portion, the non-impregnated portion, and the single layer of the porous carbon layer, the thickness change rate was calculated using the following formula, where a is the thickness without pressure and b is the thickness with a pressure of 2 MPa. Thickness change rate (%) = (ab) / a × 100.

[0078] The thickness change rate was measured on three samples cut out from near the center of the porous carbon sheet in the width direction, and the average value of the results for the three samples was used as the thickness change rate for each layer of the porous carbon sheet (impregnated portion, non-impregnated portion, and single layer of porous carbon layer).

[0079] <Surface roughness measurement of porous carbon sheet> The porous carbon sheet was sandwiched between flat indenters and pressurized at 2 MPa. X-ray CT measurements were performed using a high-resolution 3D X-ray microscope, nano3DX (Rigaku Corporation), under conditions of a Cu X-ray source, a tube voltage of 40 kV, and a tube current of 30 mA. The surface roughness Sq (JIS B0601:2013) of the microporous layer was analyzed. From the tomographic image obtained with a resolution of 2.44 μm / pix, the surface roughness Sq was calculated by obtaining the highest pixel value for each pixel position in the XY plane relative to the thickness direction Z and calculating the standard deviation.

[0080] The surface roughness Sq of each of three samples cut out from near the center in the width direction of the porous carbon sheet was calculated, and the average value of the results for the three samples was taken as the surface roughness Sq of the porous carbon sheet.

[0081] <Proportion of non-contact area of ​​microporous layer> The porous carbon sheet was sandwiched between flat indenters and pressurized at 2 MPa and subjected to X-ray CT measurement using a high-resolution 3D X-ray microscope, nano3DX (Rigaku Corporation), with a Cu X-ray source, a tube voltage of 40 kV, and a tube current of 30 mA. The non-contact area between the flat indenter and the microporous layer was analyzed. From the tomographic image obtained with a resolution of 2.44 μm / pix, the proportion of non-contact areas was calculated using the total measured area in the XY plane relative to the thickness direction (Z) of the flat indenter as the denominator and the area within that where neither the microporous layer nor the porous carbon layer was present as the numerator.

[0082] The proportion of the non-contact portion of each of three samples cut out from near the center in the width direction of the porous carbon sheet was calculated, and the average value of the results for the three samples was taken as the proportion of the non-contact portion of the microporous layer.

[0083] If the proportion of the non-contact portion of the microporous layer is small, the contact area with the adjacent catalyst layer increases when the microporous layer is used in a fuel cell, etc., thereby reducing the contact resistance. That is, the proportion of the non-contact portion of the microporous layer can be used as an indicator of the contact resistance when the porous carbon sheet is used in a fuel cell.

[0084] <Measurement of in-plane air permeability of porous carbon sheet> A porous carbon sheet was punched into a doughnut shape with an outer diameter of φ40 mm and an inner diameter of φ10 mm. A pressure of 2 MPa was applied using a press device, and air was passed through it from the inside using a mass flow controller. The pressure was measured as the flow rate was increased in increments of 0.1 L / min, and the rate of change in air flow rate relative to pressure was calculated using the following equation 1. I=ΔQ / ΔP×μ×ln(r0 / r i ) / 2π (Equation 1) In Equation 1, I is the in-plane air permeability (μm 3 ), Q is the air flow rate (L / min), P is the pressure (kPa), μ is the air viscosity (18 kg / (m s)), r0 is the sample outer diameter (m), r i indicates the sample inner diameter (m). ΔQ / ΔP indicates the slope of the linear approximation of the air flow rate and pressure. The larger the in-plane air permeability I, the easier it is for gas to diffuse in the in-plane direction of the porous carbon sheet.

[0085] Example 1 A porous carbon layer was produced according to the above <Preparation of Porous Carbon Layer>. A pre-impregnated body was produced by adjusting the carbonized resin basis weight to 0.71 times the carbon fiber basis weight when the resin composition was impregnated into the paper sheet. A porous carbon sheet was then produced according to the above <Formation of Microporous Layer>. Acetylene black with an average primary particle size of 35 nm was used as the conductive fine particles when preparing the microporous layer coating liquid. The thicknesses of the impregnated and non-impregnated portions of the produced porous carbon sheet, as well as the single layer of porous carbon layer, were measured without pressure and under a pressure of 2 MPa according to the above <Measurement of Thickness of Impregnated and Non-Impregnated Portions of Porous Carbon Sheet>. The respective thickness change rates were calculated, and the thickness change rate of the non-impregnated portion was 100%, the thickness change rate of the impregnated portion was 10%, and the thickness change rate of the single layer of porous carbon layer was 19%. In addition, the surface roughness Sq of the microporous layer at a pressure of 2 MPa was 2.4 μm, the proportion of non-contact areas was less than 0.1%, and the in-plane air permeability of the porous carbon sheet at a pressure of 2 MPa was 569 μm 3 It was.

[0086] Example 2 A porous carbon sheet was produced and various measurements were carried out in the same manner as in Example 1, except that when the resin composition was impregnated into the paper body, the weight per unit area of ​​the carbonized resin was adjusted to be 0.54 times the weight per unit area of ​​the carbon fiber. As a result, the thickness change rate of the non-impregnated portion was 100%, the thickness change rate of the impregnated portion was 24%, and the thickness change rate of the single layer of the porous carbon layer was 22%. In addition, the surface roughness Sq of the microporous layer at a pressure of 2 MPa was less than 0.1 μm, the proportion of non-contact portions was less than 0.1%, and the in-plane air permeability of the porous carbon sheet at a pressure of 2 MPa was 44 μm 3 It was.

[0087] Example 3 A porous carbon sheet was produced and various measurements were carried out in the same manner as in Example 1, except that when the resin composition was impregnated into the paper body, the weight per unit area of ​​the carbonized resin was adjusted to be 0.46 times the weight per unit area of ​​the carbon fiber, and acetylene black with an average primary particle size of 48 nm was used as the conductive fine particles. As a result, the thickness change rate of the non-impregnated portion was 51%, the thickness change rate of the impregnated portion was 33%, and the thickness change rate of the single layer of the porous carbon layer was 32%. In addition, the surface roughness Sq of the microporous layer at a pressure of 2 MPa was 0.1 μm, the proportion of non-contact portions was less than 0.1%, and the in-plane air permeability of the porous carbon sheet at a pressure of 2 MPa was 224 μm 3 It was.

[0088] Example 4 A porous carbon sheet was produced and various measurements were carried out in the same manner as in Example 1, except that when the resin composition was impregnated into the paper body, the weight per unit area of ​​the carbonized resin was adjusted to be 0.70 times the weight per unit area of ​​the carbon fiber, and the microporous layer coating liquid was applied by positioning the die coater closer to the porous carbon layer than in Example 1 so that the impregnated portion would be thicker. As a result, the thickness change rate of the non-impregnated portion was 100%, the thickness change rate of the impregnated portion was 12%, and the thickness change rate of the single layer of the porous carbon layer was 20%. In addition, the surface roughness Sq of the microporous layer at a pressure of 2 MPa was 1.8 μm, the proportion of non-contact portions was less than 0.1%, and the in-plane air permeability of the porous carbon sheet at a pressure of 2 MPa was 340 μm 3 It was.

[0089] Example 5 A porous carbon sheet was produced and various measurements were carried out in the same manner as in Example 1, except that when the resin composition was impregnated into the paper body, the weight per unit area of ​​the carbonized resin was adjusted to be 0.74 times the weight per unit area of ​​the carbon fiber, and the microporous layer coating liquid was applied by positioning the die coater closer to the porous carbon layer than in Example 4 so that the impregnated portion would be even thicker. As a result, the thickness change rate of the non-impregnated portion was 100%, the thickness change rate of the impregnated portion was 12%, and the thickness change rate of the single layer of the porous carbon layer was 24%. Furthermore, the surface roughness Sq of the microporous layer at a pressure of 2 MPa was 1.8 μm, the proportion of non-contact portions was less than 0.1%, and the in-plane air permeability of the porous carbon sheet at a pressure of 2 MPa was 201 μm 3 It was.

[0090] (Comparative Example 1) A porous carbon sheet was produced and various measurements were carried out in the same manner as in Example 1, except that when the resin composition was impregnated into the paper body, the weight per unit area of ​​the carbonized resin was adjusted to be 0.61 times the weight per unit area of ​​the carbon fiber, and acetylene black with an average primary particle size of 72 nm was used as the conductive fine particles. As a result, the thickness change rate of the non-impregnated part was 34%, the thickness change rate of the impregnated part was 38%, and the thickness change rate of the single layer of the porous carbon layer was 34%. In addition, the surface roughness Sq of the microporous layer at a pressure of 2 MPa was 10.4 μm, the proportion of non-contact parts was 12%, and the in-plane air permeability of the porous carbon sheet at a pressure of 2 MPa was 21 μm 3 It was.

[0091] [Table 1] [Explanation of symbols]

[0092] 1 Horizontal axis (volume fraction) 2 Vertical axis (thickness direction) 10 Volume fraction of microporous layer 20 Volume fraction of porous carbon layer 30 Total thickness of porous carbon sheet 31 Thickness of unimpregnated part 32 Thickness of impregnated part 33 Thickness of a single porous carbon layer 34 Total thickness of the microporous layer [Industrial Applicability]

[0093] The porous carbon sheet of the present invention can be suitably used as an electrode for a fuel cell, a water electrolysis device, or a redox flow battery, and in particular as a gas diffusion electrode for a polymer electrolyte fuel cell used as a power source for mobile objects such as a fuel cell vehicle or a ship. Furthermore, a gas diffusion electrode using the porous carbon sheet of the present invention can achieve both electrical conductivity and gas diffusivity, and therefore can be used to produce a polymer electrolyte fuel cell that exhibits high power generation performance even in operating environments where a large amount of fuel gas is supplied and a large amount of water is generated, such as high humidity, low temperature, or high current density conditions.

Claims

1. A porous carbon sheet having a porous carbon layer and a microporous layer, wherein the microporous layer has an impregnated portion that is impregnated into the porous carbon layer and an unimpregnated portion that is not impregnated into the porous carbon layer, and wherein the rate of thickness change of the unimpregnated portion when pressurized by 2 MPa relative to when no pressure is applied is greater than the rate of thickness change of the impregnated portion when pressurized by 2 MPa relative to when no pressure is applied.

2. The porous carbon sheet according to claim 1, wherein the surface roughness Sq (JIS B0601:2013) of the microporous layer when pressurized at 2 MPa is 3.0 μm or less.

3. 2. The porous carbon sheet according to claim 1, wherein the thickness change rate of the non-impregnated portion when pressurized with 2 MPa is 50 to 100% compared to when no pressure is applied.

4. 2. The porous carbon sheet according to claim 1, wherein the rate of change in thickness of the impregnated portion when pressurized with 2 MPa relative to when no pressure is applied is 0.1 to 20%.

5. 2. The porous carbon sheet according to claim 1, wherein the porous carbon layer is composed of carbon fibers and a carbonized resin, and the basis weight of the carbonized resin is 0.7 to 1.0 times the basis weight of the carbon fibers.

6. 2. The porous carbon sheet according to claim 1, wherein the thickness of the impregnated portion at a pressure of 2 MPa is 0.60 to less than 1.00 times the thickness of the porous carbon layer alone at a pressure of 2 MPa.

7. A fuel cell comprising the porous carbon sheet according to any one of claims 1 to 6.

8. A water electrolysis device comprising the porous carbon sheet according to any one of claims 1 to 6.

9. A redox flow battery comprising the porous carbon sheet according to any one of claims 1 to 6.

10. A mobile object equipped with the fuel cell according to claim 7.

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