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

Abstract

The present invention addresses the problem of providing a porous carbon sheet which achieves both strength and electrical conductivity. The present invention provides a porous carbon sheet which contains carbon fibers and a carbonized resin. The porous carbon sheet contains carbon fibers of which the Raman peak intensity ratio (R value) at the surface is 5 to 30 times the R value at the surface of the carbonized resin. Here, the Raman peak intensity ratio (R value) refers to the peak intensity ratio of the D band to the G band measured by laser Raman spectrophotometry.

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 suitable for gas diffusion layers used in fuel cells, liquid electrolytic devices, and redox flow batteries, and more particularly to a porous carbon sheet suitable for gas diffusion layers in polymer electrolyte fuel cells used as power sources 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 them.

[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. As a material that possesses all of the above characteristics, porous carbon sheets are generally used as the gas diffusion layer. Specific examples of porous carbon sheets include conductive porous substrates such as carbon felt, carbon paper, and carbon cloth made from carbon fibers.

[0005] Furthermore, as a method for producing hydrogen, liquid electrolysis is attracting attention, particularly methods that use renewable energy as the power source for liquid electrolysis to produce clean hydrogen with a low environmental impact. Several methods of liquid electrolysis exist, but polymer electrolyte membrane (PEM) liquid electrolysis devices are being widely developed because they can utilize the technology of polymer electrolyte fuel cells (PEFCs) and can produce high-purity hydrogen.

[0006] In liquid electrolysis devices, the hydrogen and oxygen produced in the reaction may be subjected to high pressures of up to several tens of MPa, and large quantities of hydrogen need to be produced. Therefore, the gas diffusion layer used in liquid electrolysis devices requires high strength, high modulus of elasticity, and high conductivity.

[0007] One method for improving the conductivity of the gas diffusion layer is to carbonize the porous carbon sheet at high temperatures. However, if the carbonization temperature is too high, there is a concern that the strength of the porous carbon sheet will decrease. If the strength of the porous carbon sheet decreases, it will break due to pressure during the creation of the gas diffusion layer or fuel cell. As a result, it will no longer be able to absorb the dimensional change in the thickness direction due to the swelling and shrinkage of the electrolyte membrane. In addition, tensile forces acting on the porous carbon sheet during transport and winding during the creation of the gas diffusion layer or fuel cell may cause the porous carbon sheet to break, potentially reducing productivity. Various studies are being conducted to balance the strength and handling of the gas diffusion layer with conductivity and drainage properties.

[0008] For example, Patent Document 1 describes a technique for ensuring the strength of a porous carbon sheet while maintaining required properties such as gas diffusivity, by first making paper with carbon fibers and waste material together, then heating to remove the waste material, and finally carbonizing the impregnated resin composition.

[0009] Furthermore, Patent Document 2 describes a technique for improving the elasticity and flexibility of porous carbon sheets by joining the intersections of carbon fibers with amorphous carbon containing carbon particles made of graphite particles or carbon black.

[0010] Japanese Patent Publication No. 2018-018665 Japanese Patent Publication No. 2008-204823

[0011] However, in Patent Documents 1 and 2, it is necessary to increase the amount of resin composition in order to give strength to the porous carbon sheet, which is undesirable from the viewpoint of production cost, gas diffusion, and drainage. Furthermore, in Patent Document 1, the addition of waste material is necessary, which increases production costs. In addition, in Patent Document 2, there is a problem that the bonds at the intersections of carbon fibers are easily broken by pressure.

[0012] Therefore, the present invention aims to provide a porous carbon sheet with excellent strength without compromising properties such as conductivity and drainage, at a low cost.

[0013] To solve the above problems, the present invention employs the following means: [1] A porous carbon sheet comprising carbon fibers and a carbonized resin, wherein the carbon fibers have a Raman peak intensity ratio (R value) on the surface that is 5 to 30 times the R value on the surface of the carbonized resin. Here, the R value refers to the ratio of the peak intensity of the D band to the G band measured by laser Raman spectrophotometric method. [2] The porous carbon sheet according to [1], comprising carbon fibers having an R value of 0.35 to 1.10 on the surface. [3] The porous carbon sheet according to [1], comprising carbon fibers having an R value of 0.35 to 0.60 on the surface. [4] The porous carbon sheet according to [2] or [3], further comprising carbon fibers having an R value of 0.01 to 0.30 on the surface. [5] An electrical resistance of 25 mΩcm when an average surface pressure of 1.0 MPa is applied. ２[1] to [4] is a porous carbon sheet, the following: [6] A porous carbon sheet, the is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet is porous carbon, the porous carbon sheet, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous carbon, the porous carbon sheet, is porous

[12] A method for producing a porous carbon sheet, comprising a papermaking step of producing a plurality of carbon fiber paper bodies, an impregnation step of impregnating the plurality of carbon fiber paper bodies with a resin composition to obtain a plurality of pre-impregnated bodies, a lamination step of laminating the plurality of pre-impregnated bodies, and a firing step of firing the laminated pre-impregnated bodies to obtain a porous carbon sheet, wherein in the lamination step, at least one pre-impregnated body containing carbon fibers having a ratio of oxygen atoms to carbon atoms (O / C) of 0.10 or more to 0.30 or less on its surface is laminated.

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

[12] , wherein in the lamination step, a pre-impregnated body not containing carbon fibers having a ratio of oxygen atoms to carbon atoms (O / C) of 0.10 or more to 0.30 or less on its surface is laminated as at least one of the uppermost or lowermost layer.

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

[12] or

[13] , wherein the pre-impregnated body contains recycled carbon fibers.

[15] The method for manufacturing a porous carbon sheet according to

[14] , wherein in the lamination step, the pre-impregnated body that does not contain recycled carbon fibers is laminated on the uppermost or lowermost layer.

[0014] The present invention makes it possible to obtain a porous carbon sheet with excellent conductivity and strength.

[0015] This is a conceptual diagram showing the laser irradiation area in laser Raman spectrophotometric measurements. This is a conceptual diagram showing the laser irradiation area in laser Raman spectrophotometric measurements. This is a conceptual diagram showing the laser irradiation area in laser Raman spectrophotometric measurements. This is a conceptual diagram showing the laser irradiation area in laser Raman spectrophotometric measurements.

[0016] [Porous Carbon Sheet] A porous carbon sheet is one aspect of the present invention. The porous carbon sheet of the present invention includes carbon fibers and carbonized resin as essential components. As the porous carbon sheet, a porous material having gas diffusion, drainage, and conductivity, specifically a conductive porous substrate containing carbon fibers such as carbon felt, carbon paper, and carbon cloth, is selected because it has the above characteristics and excellent corrosion resistance. Furthermore, using a configuration in which carbonized resin is included in the conductive porous substrate containing carbon fibers is essential in terms of mechanical strength. In particular, it is preferable to use a substrate obtained by binding a carbon fiber paper body with 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.

[0017] The porous carbon sheet of the present invention contains carbon fibers in which the R value on the surface is 5 to 30 times the Raman peak intensity ratio (hereinafter referred to as "R value") on the surface of the carbonized resin. In the present invention, the R value is obtained by laser Raman spectrophotometric method and is 1580 cm². －１ 1350 cm relative to the nearby G band －１This refers to the peak intensity ratio of the nearby D band. The R value is used as an indicator of the degree of graphitization of carbon materials such as carbon fibers and carbonized resins. A smaller R value indicates less disorder in the graphite structure and higher crystallinity (degree of graphitization). In other words, the porous carbon sheet of the present invention is characterized in that the degree of graphitization on the surface of the carbon fibers is lower than the degree of graphitization on the surface of the carbonized resin. Since the surface of carbon fibers with a low degree of graphitization has many surface functional groups, it is possible to create a porous carbon sheet with high bonding properties with the carbonized resin and excellent strength and springiness. On the other hand, from the viewpoint of conductivity and thermal conductivity, a high degree of graphitization on the surface of the carbon fibers is preferable. In this regard, by combining a carbonized resin with a higher degree of graphitization on the surface than the degree of graphitization on the surface of the carbon fibers, it is possible to achieve both strength and conductivity and thermal conductivity. Specifically, the porous carbon sheet of the present invention includes carbon fibers in which the R value on the surface is 5 to 30 times the R value on the surface of the carbonized resin. By including carbon fibers whose R-value on the surface is five times or more the R-value on the surface of the carbonized resin, a porous carbon sheet can be obtained in which the adhesion between the carbon fibers and the carbonized resin is good, and which has excellent strength and spring properties. On the other hand, by including carbon fibers whose R-value on the surface is 30 times or less the R-value on the surface of the carbonized resin, a porous carbon sheet can be obtained in which the conductivity and thermal conductivity are excellent.

[0018] The R value can be measured by laser Raman spectrophotometric analysis, as described above. Measurement by laser Raman spectrophotometric analysis can be performed using a commercially available laser Raman microscope. Figures 1 to 3 are examples of magnified views of a porous carbon sheet. In the porous carbon sheet 1, the carbon resin 20 may simply adhere to the carbon fibers 10 as shown in Figures 2 and 3, or it may adhere to multiple carbon fibers 10 in a web-like manner as shown in Figure 1. For measurement, the R value at each location can be measured by irradiating the surface of the carbon fiber 10 and the surface of the carbon resin 20 with a laser emitted from a laser Raman microscope. If a single porous carbon sheet 1 contains multiple types of carbon fibers 10 with different surface R values, the average value of each R value is taken as the R value at the surface of the carbon fibers 10 constituting the porous carbon sheet 1.

[0019] As for the carbon fibers with a high R value, i.e., a low degree of graphitization, it is preferable to use carbon fibers with a ratio of oxygen atoms to carbon atoms (O / C) of 0.10 or more to 0.30 or less on the surface, or recycled carbon fibers as raw materials. Recycled carbon fibers refer to carbon fibers obtained by recycling composite materials of thermosetting resins or thermoplastic resins and the carbon fibers. Examples of recycling methods include pyrolysis, solvent dissolution, and supercritical water treatment, but regardless of the method used, the surface of the carbon fibers is oxidized by the recycling process, the number of functional groups on the surface increases, and carbon fibers with a ratio of oxygen atoms to carbon atoms (O / C) of 0.10 or more to 0.30 or less can be easily obtained. In other words, by using recycled carbon fibers in which the ratio of oxygen atoms to carbon atoms (O / C) on the surface can be easily adjusted to 0.10 or more to 0.30 or less, it is possible to achieve excellent manufacturing costs and CO ２ This method produces a porous carbon sheet with low emissions and excellent adhesion between carbon fibers and carbonized resin.

[0020] It is preferable that the porous carbon sheet contains carbon fibers having an R value of 0.35 or more and 1.10 or less on its surface. By setting the R value of the carbon fibers to 0.35 or more, a large number of functional groups are present on the surface of the carbon fibers, improving adhesion with the carbon resin and resulting in a porous carbon sheet with superior strength and spring properties. By setting the R value of the carbon fibers to 1.10 or less, more preferably 0.60 or less, a porous carbon sheet with superior electrical conductivity and thermal conductivity can be obtained.

[0021] Preferably, the porous carbon sheet contains carbon fibers with a surface R value of 0.01 to 0.30, in addition to carbon fibers with a surface R value of 0.35 to 1.10. By setting the surface R value of the carbon fibers to 0.01 or higher, a structurally stable porous carbon sheet can be obtained that takes advantage of the high strength of the carbon fibers themselves. By setting the surface R value of the carbon fibers to 0.30 or lower, a porous carbon sheet with superior conductivity and thermal conductivity can be obtained. In other words, by including carbon fibers with a surface R value of 0.01 to 0.30, which are difficult to obtain as recycled carbon fibers, in addition to carbon fibers with a surface R value of 0.35 to 1.10, which are easily obtained as recycled carbon fibers, a porous carbon sheet that achieves both strength and conductivity can be obtained.

[0022] The average diameter of the single fibers in the carbon fiber is preferably 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 10 μm or less. By setting the average diameter of the single fibers to preferably 3 μm or more, and more preferably 5 μm or more, the pore size becomes larger and drainage is improved, resulting in a porous carbon sheet with suppressed flooding. On the other hand, by setting the average diameter of the single fibers to 20 μm or less, and more preferably 10 μm or less, water vapor diffusion is reduced, resulting in a porous carbon sheet with suppressed drying up. Furthermore, by using two or more types of carbon fibers with different average diameters of single fibers, a porous carbon sheet with improved surface smoothness can be obtained.

[0023] The average length of the single fibers is preferably 3 mm or more and 20 mm or less, and more preferably 5 mm or more and 15 mm or less. By setting the average length of the single fibers to preferably 3 mm or more, and more preferably 5 mm or more, a porous carbon sheet with excellent mechanical strength, electrical conductivity, and thermal conductivity can be obtained. On the other hand, by setting the average length of the single fibers to preferably 20 mm or less, and more preferably 15 mm or less, the dispersion of carbon fibers during papermaking is excellent, and a homogeneous porous carbon sheet can be obtained. The carbon fibers having the above-mentioned average length can be obtained by methods such as cutting continuous carbon fibers to a desired length.

[0024] Examples of the carbonized resin include resins obtained by carbonizing thermosetting resins such as phenolic resins, epoxy resins, melamine resins, and furan resins. Among these, resins obtained by carbonizing phenolic resins are preferred because they have a high carbonization yield.

[0025] The porous carbon sheet has an electrical resistance of 25 mΩcm when subjected to an average surface pressure of 1.0 MPa. ２ The following is preferable. The electrical resistance can be calculated by measuring the voltage when a current is passed through a porous carbon sheet with an average surface pressure of 1.0 MPa applied. The electrical resistance is 25 mΩcm. ２ By following these steps, the conductivity necessary for the efficient operation of fuel cells, liquid electrolytic devices, and redox flow batteries can be obtained. Furthermore, the electrical resistance is 0.1 mΩcm. ２ It is preferable that the above conditions are met.

[0026] A water-repellent agent can also be applied to the surface of the porous carbon sheet. Examples of such water-repellent agents include PTFE (polytetrafluoroethylene) ("Teflon®," etc.), FEP (tetrafluoroethylene hexafluoropropylene copolymer), PFA (perfluoroalkoxy fluorine resin), ETFE (ethylene tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride), but PTFE or FEP are preferred from the viewpoint of exhibiting strong water repellency.

[0027] The amount of the water-repellent agent added is preferably 0.1 parts by mass or more and 20 parts by mass or less, when the total mass of the porous carbon sheet is 100 parts by mass. By adding 0.1 parts by mass or more of the water-repellent agent, sufficient water repellency can be achieved. On the other hand, by adding 20 parts by mass or less of the water-repellent agent, excellent gas diffusion, drainage, and conductivity can be obtained.

[0028] [Method for Manufacturing Porous Carbon Sheets] A method for manufacturing porous carbon sheets is one aspect of the present invention. The method for manufacturing porous carbon sheets of the present invention comprises a papermaking step of producing a carbon fiber paper body, an impregnation step of impregnating the carbon fiber paper body with a resin composition to obtain a pre-impregnated body, and a firing step of firing the pre-impregnated body to obtain a porous carbon sheet, wherein the pre-impregnated body contains carbon fibers in which the ratio of oxygen atoms to carbon atoms on the surface (O / C) is 0.10 or more and 0.30 or less.

[0029] The O / C ratio on the surface of the carbon fiber can be determined by dividing the ratio of oxygen atoms on the surface of the carbon fiber by the ratio of carbon atoms on the surface of the carbon fiber. The ratio of carbon atoms to oxygen atoms on the surface of the carbon fiber can be measured by X-ray photoelectron spectroscopy.

[0030] Next, the process for obtaining the porous carbon sheet of the present invention and preferred methods thereto will be described in detail, but the present invention is not limited to the following description.

[0031] (1) Papermaking process The method for producing a porous carbon sheet of the present invention includes a papermaking process for producing a carbon fiber papermaking body from carbon fibers. To produce the carbon fiber papermaking body, a wet papermaking method in which the carbon fibers are dispersed in a liquid and papermaking is performed, or a dry papermaking method in which the carbon fibers are dispersed in air and papermaking is performed. Among these, the wet papermaking method is preferred from the viewpoint of superior productivity.

[0032] Examples of carbon fibers used in carbon fiber papermaking include polyacrylonitrile (PAN)-based, pitch-based, and rayon-based carbon fibers. Among these, PAN-based and pitch-based carbon fibers are preferred from the viewpoint of superior mechanical strength.

[0033] In the present invention, it is important that the carbon fibers have a low degree of graphitization on their surface and a high number of functional groups on their surface. By using carbon fibers with a high number of functional groups, a porous carbon sheet with excellent adhesion between the carbon fibers and the carbonized resin can be produced. In order to produce such a porous carbon sheet, it is necessary to produce a carbon fiber papermaking body containing carbon fibers in which the O / C ratio on the surface of the carbon fibers is 0.10 or more and 0.30 or less. Specifically, when the carbon fiber papermaking body, which has become a pre-impregnated body through a subsequent impregnation process, is fired in a subsequent firing process, it is necessary that the pre-impregnated body contains carbon fibers in which the O / C ratio on the surface is 0.10 or more and 0.30 or less. Methods for obtaining carbon fibers with a high number of functional groups and an O / C ratio on the surface of 0.10 or more and 0.30 or less include gas-phase oxidation by heat treatment of the carbon fibers and liquid-phase oxidation by electrolytic treatment. On the other hand, it is preferable that the pre-impregnated body contains recycled carbon fibers in which the O / C ratio on the surface can be easily reduced to 0.10 or more and 0.30 or less.

[0034] Another preferred embodiment involves mixing organic fibers with the carbon fibers to produce the carbon fiber paper. Since the organic fibers are burned away in the firing process described later, a porous carbon sheet with improved drainage and gas diffusion properties can be obtained. Examples of the organic fibers that can be used include polyethylene fibers, vinylon fibers, polyacetal fibers, polyester fibers, polyamide fibers, rayon fibers, and acetate fibers.

[0035] Furthermore, to improve the shape retention and handling properties of the carbon fiber paper, it is also preferable to include an organic polymer as a binder in the carbon fiber paper. Examples of the organic polymer include polyvinyl alcohol, polyvinyl acetate, polyacrylonitrile, and cellulose.

[0036] In order to maintain isotropic conductivity and thermal conductivity within the plane of the carbon fiber paper, it is preferable to produce a sheet-like carbon fiber paper in which the carbon fibers are randomly dispersed in a two-dimensional plane.

[0037] (2) Impregnation Step In the method for manufacturing the porous carbon sheet of the present invention, an impregnation step is included in which the carbon fiber paper sheet is impregnated with a resin composition to obtain the preliminary impregnated body. As the resin composition, it is preferable to use one that carbonizes in the subsequent firing step to become a conductive carbide. As a method for impregnating the carbon fiber paper sheet with the resin composition, for example, a method of immersing the carbon fiber paper sheet in the resin composition, a method of applying the resin composition to the carbon fiber paper sheet, a method of laminating and transferring a film made of the resin composition onto the carbon fiber paper sheet, etc. can be mentioned. Among these, since the productivity is excellent, a method of immersing the carbon fiber paper sheet in the resin composition is preferable.

[0038] In the present invention, the resin composition refers to a resin to which a solvent or the like is added as necessary. As the solvent, for example, methanol, ethanol, isopropyl alcohol, etc. can be used. Further, the resin composition may further contain additives such as carbon-based fillers and surfactants.

[0039] The resin contained in the resin composition is preferably adjusted so that the carbonization yield becomes 40% by mass or more in the subsequent firing step. When the carbonization yield in the firing step is 40% by mass or more, the porous carbon sheet will have excellent mechanical properties, conductivity, and thermal conductivity. The carbonization yield is a value obtained by dividing the resin mass after passing through the firing step by the resin mass before passing through the firing step.

[0040] Examples of the resin include thermosetting resins such as phenol resin, epoxy resin, melamine resin, furan resin, etc. Among these, since the carbonization yield is high, phenol resin is preferable.

[0041] As necessary, the resin composition can contain additives such as carbon-based fillers for the purpose of improving the mechanical properties, conductivity, and thermal conductivity of the porous carbon sheet. As the carbon-based filler, for example, carbon black, carbon nanotubes, carbon nanofibers, mild carbon fibers, graphite, etc. can be used.

[0042] The resin composition is preferably in liquid form at 25°C and 0.1 MPa. Being in liquid form allows for excellent impregnation into the carbon fiber paper, resulting in a porous carbon sheet with superior mechanical properties, electrical conductivity, and thermal conductivity.

[0043] (3) Lamination process In the method for manufacturing a porous carbon sheet of the present invention, when a plurality of the pre-impregnated bodies are used to construct the porous carbon sheet of the present invention, by including a lamination process in which the plurality of pre-impregnated bodies are laminated, the porous carbon sheet can be made to a predetermined thickness in the firing process described later. In this case, multiple pre-impregnated bodies having the same configuration may be laminated, or multiple pre-impregnated bodies having different average diameters and lengths of carbon fibers, basis weights of carbon fibers in the carbon fiber papermaking body, and resin impregnation amounts may be laminated.

[0044] As an example, it is preferable to laminate a pre-impregnated material containing carbon fibers with an O / C ratio of 0.10 to 0.30 on its surface, as well as a pre-impregnated material without carbon fibers with an O / C ratio of 0.10 to 0.30 on its surface, so that it forms at least one of the uppermost or lowermost layer. In the process of obtaining carbon fibers with an O / C ratio of 0.10 to 0.30 on its surface, residue tends to remain on the surface of the carbon fibers. If carbon fibers with residue remain are used, aggregates of the carbon fibers may form in the papermaking process due to the residue, potentially causing the porous carbon sheet to contain these aggregates. When such a porous carbon sheet is incorporated into a fuel cell, the presence of these aggregates on the outermost surface of the porous carbon sheet may damage the electrolyte membrane. For these reasons, it is preferable to laminate a pre-impregnated material that does not contain carbon fibers with an O / C ratio of 0.10 to 0.30 on its surface, which is less prone to the formation of aggregates, so that it forms the surface (at least one of the uppermost or lowermost layer), thereby creating a porous carbon sheet without aggregates on its surface.

[0045] As another example, the pre-impregnated material containing recycled carbon fibers can be used. On the other hand, recycled carbon fibers with an O / C ratio of 0.10 to 0.30 on the surface tend to leave residue on the surface, which, as mentioned above, may damage the electrolyte membrane when incorporated into a fuel cell. Therefore, it is preferable to laminate the materials so that at least one of the uppermost or lowermost layers is a pre-impregnated material that does not contain recycled carbon fibers.

[0046] (4) Firing process The method for producing a porous carbon sheet of the present invention includes a firing process in which the pre-impregnated body is fired to obtain a porous carbon sheet. A porous carbon sheet can be obtained by firing the pre-impregnated body under an inert atmosphere in order to carbonize the resin contained in the pre-impregnated body. Examples of firing methods include using a batch-type heating furnace or using a continuous-type heating furnace. An inert atmosphere can be obtained by introducing an inert gas such as nitrogen gas or argon gas into the heating furnace.

[0047] As a pretreatment in the firing process, the pre-impregnated material may be heat-treated for the purpose of increasing the viscosity (partially crosslinking) of the resin composition. Examples of heat treatment methods include blowing hot air onto it, heating it by sandwiching it between hot plates such as those in a press, or heating it by sandwiching it between a continuous belt.

[0048] The maximum temperature during firing is preferably in the range of 1,300°C to 3,000°C, more preferably in the range of 1,700°C to 3,000°C, and even more preferably in the range of 1,900°C to 3,000°C. By setting the maximum temperature preferably to 1,300°C or higher, more preferably to 1,700°C or higher, and even more preferably to 1,900°C or higher, the carbonization of the resin progresses, resulting in a porous carbon sheet with excellent electrical and thermal conductivity. On the other hand, by setting the maximum temperature preferably to 3,000°C or lower, the operating costs of the heating furnace are reduced.

[0049] During firing, it is preferable that the heating rate of the pre-impregnated material is within the range of 10°C / min to 5,000°C / min. By setting the heating rate to preferably 10°C / min or more, and more preferably 80°C / min or more, the productivity of this process is improved. On the other hand, by setting the heating rate to preferably 5,000°C / min or less, the carbonization of the resin proceeds slowly, forming a dense structure in the porous carbon sheet, so that the porous carbon sheet has excellent conductivity and thermal conductivity. When using a continuous heating furnace, the heating rate can be set to the above range by adjusting the temperature gradient in the heating furnace and the transport speed of the pre-impregnated material.

[0050] (5) Water-repellent treatment In order to improve the drainage properties of the porous carbon sheet of the present invention obtained through the above-described steps, a water-repellent treatment may be applied. Examples of methods for water-repellent treatment of the porous carbon sheet include immersing the porous carbon sheet in a dispersion containing a water-repellent agent, applying a dispersion containing a water-repellent agent to the porous carbon sheet by die coating or spray coating, and applying a water-repellent agent by a dry process such as sputtering. After the water-repellent treatment, a drying step and a sintering step may be added as needed.

[0051] [Gas Diffusion Layer] The gas diffusion layer is formed by creating a microporous layer on the porous carbon sheet of the present invention. The microporous layer is a layer having pores even smaller than those of the porous carbon sheet, specifically preferably with an average pore diameter of 0.01 μm or more and 10 μm or less. When constructing the gas diffusion layer using the porous carbon sheet, high conductivity is required for the microporous layer. Therefore, it is preferable that the microporous layer is a layer containing conductive fine particles that exhibit high conductivity, such as carbon black, carbon nanotubes, carbon nanofibers, graphene, and graphite.

[0052] Furthermore, in addition to conductivity, gas diffusion, drainage, moisture retention, and thermal conductivity, the microporous layer is required to have strong acid resistance when used on the anode side of a fuel cell and oxidation resistance when used on the cathode side. Therefore, it is preferable that the microporous layer contains a water-repellent agent, such as a fluororesin, in addition to the conductive fine particles. Examples of fluororesins among the water-repellent agents include PTFE, FEP, PFA, and ETFE, similar to the fluororesins that are preferred when water-repellent treating the porous carbon sheet. Among these, PTFE or FEP is preferred from the viewpoint of particularly high water repellency. The content of the water-repellent agent 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. By preferably adding 1 part by mass or more, and more preferably 5 parts by mass or more, a microporous layer with excellent drainage and mechanical strength can be obtained. On the other hand, by preferably setting the amount of the water-repellent agent to 70 parts by mass or less, and more preferably 60 parts by mass or less, a microporous layer with excellent conductivity can be obtained.

[0053] The microporous layer can be formed by applying a microporous layer coating solution to one side of the porous carbon sheet.

[0054] Examples of dispersion media for the microporous layer coating liquid include water and organic solvents, with water being preferred. The dispersion media may also contain dispersion aids such as surfactants. Nonionic surfactants are preferred as the dispersion aids.

[0055] When applying the microporous layer coating solution to the porous carbon sheet, various commercially available coating devices can be used. For example, coating methods such as screen printing, rotary screen printing, spray atomization, intaglio printing, gravure printing, die coating, bar coating, and blade coating can be used. However, die coating is preferred from the viewpoint of being able to quantify the coating amount regardless of the surface roughness of the porous carbon sheet.

[0056] To form the microporous layer, it is preferable to dry the porous carbon sheet coated with the microporous layer coating liquid on one side at a temperature of 80°C to 120°C. Specifically, it is preferable to place the porous carbon sheet coated with the microporous layer coating liquid on one side into a dryer set to a temperature of 80°C to 120°C and dry it for 5 to 30 minutes. The amount of drying air is preferably adjusted appropriately so as not to induce the generation of microcracks on the surface of the microporous layer due to rapid drying. After drying, it is preferable to place the sheet into a muffle furnace, a firing furnace, or a high-temperature dryer and heat it at a temperature of 300°C to 380°C for 5 to 20 minutes so that the molten water-repellent resin acts as a binder between the conductive fine particles to form the microporous layer.

[0057] When measuring the R-value on the surface of the carbon fibers in the porous carbon sheet having the microporous layer formed on its surface, a cross-section can be prepared by cutting the porous carbon sheet in the thickness direction and measuring the R-value. The cross-section may be prepared using an ion milling device (such as the IM4000 model manufactured by Hitachi High-Technologies Corporation or its equivalent), or it may be prepared by cutting the porous carbon sheet with a sharp blade. In the prepared cross-section, the portion in which the microporous layer has not permeated the porous carbon sheet can be treated as the porous carbon sheet itself, and the R-value on the surface of the carbon fibers and carbonized resin can be measured.

[0058] [Membrane Electrode Assembly] The membrane electrode assembly is 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 (i.e., a gas diffusion layer) with the microporous layer formed on one side, it is preferable to arrange the microporous layer so that it is in contact with the catalyst layer, as this facilitates back diffusion of the generated water and increases the contact area between the catalyst layer and the gas diffusion layer, thereby reducing contact electrical resistance. Platinum is usually used as the catalyst for the catalyst layer. As the electrolyte membrane, it is preferable to use a perfluorosulfonic acid-based polymer material from the viewpoint of high proton conductivity, oxidation resistance, and heat resistance.

[0059] [Fuel Cell] A fuel cell is one aspect of the present invention. The fuel cell of the present invention refers to a fuel cell having separators at both ends of a membrane electrode assembly having a porous carbon sheet of the present invention. The separators at both ends each have a flow path to allow fuel gas to flow into and out of the anode-side gas diffusion layer, and to allow oxidizing gas to flow into and out of the cathode-side gas diffusion layer. The separators and the flow paths 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 fuel cells.

[0060] [Liquid Electrolytic Device] A liquid electrolytic device is one aspect of the present invention. The liquid electrolytic device of the present invention is a liquid electrolytic device that includes a membrane electrode assembly having the porous carbon sheet of the present invention as a component.

[0061] [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.

[0062] [Mobile Vehicles] Mobile vehicles are one aspect of the present invention. A mobile vehicle of the present invention refers to a mobile vehicle such as an automobile, ship, or railway powered vehicle that is equipped with the fuel cell of the present invention as a power supply source.

[0063] The present invention will be specifically described below with reference to examples. However, the embodiments of the present invention are not limited to these examples.

[0064] [Raw Materials] Carbon Fiber A1: Carbon fiber was obtained by recycling scraps of carbon fiber reinforced polymer (CFRP) containing Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca®" using a solvent dissolution method, and the carbon fiber extracted by dissolving the resin contained in the CFRP scraps was used. The O / C ratio on the surface of the carbon fiber was 0.19.

[0065] Carbon fiber A2: New polyacrylonitrile-based carbon fiber "Torayca®" manufactured by Toray Industries, Inc. was used. The O / C ratio on the carbon fiber surface was 0.05.

[0066] Carbon fiber A3: This carbon fiber was obtained by recycling CFRP scraps containing Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca®" using a thermal decomposition method. The carbon fiber was extracted by thermally decomposing the resin contained in the CFRP scraps. The O / C ratio on the carbon fiber surface was 0.26.

[0067] [Preparation of Porous Carbon Sheets] As part of the papermaking process, carbon fibers 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, which is used as a binder, was applied to the wet material and dried to produce a carbon fiber papermaking body.

[0068] Next, in the impregnation step, a resin was obtained by mixing a thermosetting resin (resol-type phenolic resin) and a thermoplastic resin (novolac-type phenolic resin) in a 1:1 mass ratio, along with flake graphite (average particle size 5 μm) and methanol, to obtain a uniformly dispersed resin composition. At this time, the resin, flake graphite, and methanol were mixed so that their masses were 10% by mass, 5% by mass, and 85% by mass, respectively, with the total mass of the resin composition being 100% by mass.

[0069] A pre-impregnated body was prepared by immersing a carbon fiber paper machine in a resin composition to impregnate it with the resin composition, and then heating and drying it.

[0070] Next, in some embodiments, two pre-impregnated bodies were laminated as part of the lamination process.

[0071] Next, in the firing process, the pre-impregnated material was heat-treated at 200°C while being pressurized at 3 MPa using a flat plate press, and then fired in a heating furnace with a maximum temperature of 2,400°C, maintained in a nitrogen gas atmosphere, to obtain a porous carbon sheet. In the example where two pre-impregnated materials were laminated in the lamination process, the pressurization and heat treatment in this firing process were adjusted so that the thickness was the same as that of a single pre-impregnated material.

[0072] In addition, when the porous carbon sheet was 100 parts by mass, a PTFE dispersion was applied with a slit die coater so that the PTFE was 5 parts by mass. Thereafter, it was dried at 120 ° C. to produce a water-repellent treated porous carbon sheet. The PTFE dispersion used at this time was a diluted aqueous solution of "Polyflon (registered trademark)" D-210C (manufactured by Daikin Industries, Ltd.; containing 60 parts by mass of PTFE in a dispersion medium (water)).

[0073] [Production of Gas Diffusion Layer] Acetylene black "Denka Black (registered trademark)" (manufactured by Denka Co., Ltd.), 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 were mixed using a disperser so as to be 7.0 parts by mass / 2.5 parts by mass / 14.1 parts by mass / 76.4 parts by mass, respectively, to prepare a microporous layer coating liquid. Next, the microporous layer coating liquid was applied to one side of the obtained porous carbon sheet using a die coater and heated and dried at 120 ° C. for 10 minutes. After heat drying, it was sintered at 380 ° C. for 3 minutes to produce a gas diffusion layer having a microporous layer on one surface of the porous carbon sheet. Here, the coating amount of the microporous layer coating liquid was adjusted so that the basis weight of the microporous layer after sintering was 15 g / m ２ Thus, the coating amount was adjusted.

[0074] [Measurement Method] (1) Measurement of graphitization degree of carbon fiber surface and carbonized resin surface of porous carbon sheet Using a laser Raman microscope "RAMANtouch" manufactured by Nanophoton Co., Ltd., laser wavelength 532 nm, laser output 10 kWcm －２ , diffraction grating 600 gr / mm, center wave number 460 cm －１ , exposure time 1 second, integration number 1 time, objective lens 50 times, scanning range 3 μm × 3 μm, the porous carbon sheet was irradiated with a laser. The obtained Raman peak near 1350 cm －１ D band and 1580 cm －１The R-values ​​on the carbon fiber surface and the carbide resin surface were calculated from the peak intensity ratio of the nearby G-band. Specifically, as shown in Figure 4, these measurements were performed individually on the upper surface A to A', the lower surface C to C', and the central part B to B' of the cross-section in the thickness direction of the porous carbon sheet. The central part of the cross-section was exposed by cutting the porous carbon sheet 1 with a sharp blade. At each part, the degree of graphitization of 10 randomly selected carbon fibers 10 and the carbide resin 20 attached to the carbon fibers 10 was measured, and the R-values ​​of the carbon fiber surface and the carbide resin surface were confirmed. In measuring the 10 carbon fibers 10, it was first confirmed whether multiple types of carbon fibers 10 with clearly different R-values ​​were included. If multiple types of carbon fibers 10 with different R-values ​​were confirmed, the average value was calculated separately for each type of carbon fiber 10. For the R-value of the carbide resin 20, the average value of the measurements from 10 locations (10 × n locations if n types of carbon fibers 10 were included) was adopted. For porous carbon sheet 1, which is formed by laminating two pre-impregnated bodies with different compositions but similar thicknesses, the R value will show that the upper surface, corresponding to the upper layer, strongly reflects the compositional conditions of one body, the lower surface, corresponding to the lower layer, strongly reflects the compositional conditions of the other body, and the central part of the cross-section strongly reflects the compositional conditions of both bodies.

[0075] (2) Number of carbon fiber aggregates First, 1 m of porous carbon sheet ２ During the process, the locations of fibrous carbon fiber aggregates present on the surface (or the upper surface if two pre-impregnated materials were stacked) were identified by visual inspection. Subsequently, the identified locations were observed with an optical microscope, and the number of aggregates with a fiber bundle width of 1 mm or more was counted.

[0076] (3) [Electrical Resistance of Porous Carbon Sheet] The electrical resistance of the porous carbon sheet was measured using the compression test mode of the "Autograph (registered trademark)" AGS-X manufactured by Shimadzu Corporation. The porous carbon sheet was cut to a size of 20 mm x 20 mm, sandwiched between rigid electrodes of smooth, gold-plated metal on the top and bottom, and an average surface pressure of 1.0 MPa was applied. When a current of 1 A was passed through the top and bottom electrodes in this state, the voltage between the top and bottom electrodes was measured to determine the electrical resistance per unit area (mΩcm). ２ ) was calculated.

[0077] (4) Tensile strength of the gas diffusion layer The tensile strength of the gas diffusion layer (a porous carbon sheet with a microporous layer formed on one side) was measured using the tensile test mode of the "Autograph®" AGS-X manufactured by Shimadzu Corporation. Test specimens of the gas diffusion layer, cut to a size of 6 cm in length and 1.5 cm in width, were attached to two upper and lower sample mounting fixtures attached to the tensile testing machine, and the material was pulled in the longitudinal direction at a speed of 2 mm / min to obtain a stress-displacement curve. The maximum stress in this curve was defined as the fracture point of the gas diffusion layer, and the tensile stress at the fracture point was measured. The tensile stress was calculated by dividing the tensile force by the width of the test specimen. This measurement was performed five times with different test specimens, and the average value was taken as the tensile strength.

[0078] (5) Spring properties of the gas diffusion layer The spring properties of the gas diffusion layer were measured using the compression test mode of the "Autograph (registered trademark)" AGS-X manufactured by Shimadzu Corporation. The gas diffusion layer was cut to a size of 20 mm x 20 mm, sandwiched between smooth metal rigid electrodes, and an average pressure of 3.0 MPa was applied. The thickness of the gas diffusion layer was measured after 25 seconds. Then, the average pressure was reduced to 1.0 MPa, and the thickness of the gas diffusion layer was measured again after 25 seconds. The difference between the thickness of the gas diffusion layer when an average pressure of 1.0 MPa was applied and the thickness of the gas diffusion layer when an average pressure of 3.0 MPa was applied was used as an index of spring properties.

[0079] (6) Electrical resistance of the gas diffusion layer The electrical resistance of the gas diffusion layer was measured using the compression test mode of the "Autograph®" AGS-X manufactured by Shimadzu Corporation. The gas diffusion layer was cut to a size of 20 mm x 20 mm, sandwiched between rigid electrodes of smooth, gold-plated metal on the top and bottom, and an average surface pressure of 1.0 MPa was applied. When a current of 1 A was passed through the upper and lower electrodes in this state, the voltage between the upper and lower electrodes was measured to determine the electrical resistance per unit area (mΩcm). ２ ) was calculated.

[0080] (Example 1) Using only carbon fiber A1, a pre-impregnated material containing only carbon fiber A1 (hereinafter referred to as "pre-impregnated material A1") was prepared, and a porous carbon sheet was prepared using one of these materials according to the method described above. Then, the R values ​​of the carbon fiber surface and the carbonized resin surface, the number of carbon fiber aggregates, and the electrical resistance were measured according to the method described above. Subsequently, a gas diffusion layer was prepared according to the method described above, and then the tensile strength, springiness, and electrical resistance were measured according to the method described above. The results are shown in Table 1.

[0081] (Example 2) A porous carbon sheet was prepared using carbon fiber A1 and carbon fiber A2 according to the method described above. Specifically, in the papermaking process, equal amounts of carbon fiber A1 and carbon fiber A2 were mixed to obtain a carbon fiber paper body, thereby preparing a pre-impregnated body containing carbon fiber A1 and carbon fiber A2 in a 1:1 ratio (hereinafter referred to as "pre-impregnated body A3"). A porous carbon sheet was then prepared using one of these pre-impregnated bodies according to the method described above. After that, the R values ​​of the carbon fiber surface and the carbonized resin surface, the number of carbon fiber aggregates, and the electrical resistance were measured according to the method described above. Subsequently, a gas diffusion layer was prepared according to the method described above, and then the tensile strength, springiness, and electrical resistance were measured according to the method described above. The results are shown in Table 1.

[0082] (Example 3) A porous carbon sheet and a gas diffusion layer were fabricated in the same manner as in Example 1, except that two pre-impregnated bodies A1 were prepared and the lamination process was carried out. Various measurements were then performed. The results are shown in Table 1.

[0083] (Example 4) One pre-impregnated material (hereinafter referred to as "pre-impregnated material A2") was prepared using only pre-impregnated material A1 and carbon fiber A2, and a porous carbon sheet was prepared by lamination according to the method described above. Then, with pre-impregnated material A2 as the upper surface, the R value of the carbon fiber surface and the carbonized resin surface, the number of carbon fiber aggregates, and the electrical resistance were measured according to the method described above. After that, a gas diffusion layer was prepared according to the method described above, and then the tensile strength, springiness, and electrical resistance were measured according to the method described above. The results are shown in Table 1.

[0084] (Example 5) A pre-impregnated material (hereinafter referred to as "pre-impregnated material A4") was prepared using only carbon fiber A3 instead of carbon fiber A1, and a porous carbon sheet and a gas diffusion layer were prepared in the same manner as in Example 1, except that one of these was used, and various measurements were performed. The results are shown in Table 1.

[0085] (Comparative Example 1) A porous carbon sheet and a gas diffusion layer were prepared in the same manner as in Example 1, except that one pre-impregnated body A2 was used, and various measurements were performed. The results are shown in Table 1.

[0086]

[0087] 1 Porous carbon sheet 10 Carbon fibers 20 Carbonized resin A-A' Upper surface B-B' Center of the cross section C-C' Lower surface

[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 carbonized resin, wherein the Raman peak intensity ratio (R value) on the surface is 5 to 30 times the R value on the surface of the carbonized resin. (Here, the R value refers to the ratio of the D-band peak intensity to the G-band peak intensity measured by laser Raman spectrophotometric method.) 2. The porous carbon sheet according to claim 1, comprising carbon fibers having an R value of 0.35 or more and 1.10 or less on its surface.

3. The porous carbon sheet according to claim 1, comprising carbon fibers having an R value of 0.35 or more and 0.60 or less on its surface.

4. The porous carbon sheet according to claim 2, further comprising carbon fibers having an R value of 0.01 or more and 0.30 or less on the surface.

5. The electrical resistance is 25 mΩcm when an average surface pressure of 1.0 MPa is applied. ２ The porous carbon sheet according to claim 1, which is as follows:

6. The porous carbon sheet according to claim 1, wherein the carbon fiber includes recycled carbon fiber.

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

8. A liquid electrolytic apparatus having a porous carbon sheet according to any one of claims 1 to 6.

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

10. A mobile body equipped with the fuel cell described in claim 7.

11. A method for producing a porous carbon sheet, comprising a papermaking step for producing a carbon fiber paper body, an impregnation step for impregnating the carbon fiber paper body with a resin composition to obtain a pre-impregnated body, and a firing step for firing the pre-impregnated body to obtain a porous carbon sheet, wherein the pre-impregnated body contains carbon fibers having a ratio of oxygen atoms to carbon atoms (O / C) of 0.10 or more and 0.30 or less on its surface.

12. A method for producing a porous carbon sheet, comprising a papermaking step of producing a plurality of carbon fiber paper bodies, an impregnation step of impregnating the plurality of carbon fiber paper bodies with a resin composition to obtain a plurality of pre-impregnated bodies, a lamination step of laminating the plurality of pre-impregnated bodies, and a firing step of firing the laminated pre-impregnated bodies to obtain a porous carbon sheet, wherein in the lamination step, at least one pre-impregnated body containing carbon fibers having a ratio of oxygen atoms to carbon atoms (O / C) of 0.10 or more and 0.30 or less on its surface is laminated.

13. The method for producing a porous carbon sheet according to claim 12, wherein in the lamination step, the pre-impregnated body that does not contain carbon fibers and has a ratio of oxygen atoms to carbon atoms (O / C) of 0.10 or more to 0.30 or less on its surface is laminated as at least one of the uppermost or lowermost layer.

14. The method for producing a porous carbon sheet according to claim 12, wherein the pre-impregnated body contains recycled carbon fibers.

15. The method for manufacturing a porous carbon sheet according to claim 14, wherein in the lamination step, the pre-impregnated body that does not contain recycled carbon fibers is laminated on the uppermost or lowermost layer, or at least one of the two.

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