Method for manufacturing carbon sheet and gas diffusion electrode

Abstract

To prevent a deposit of a resin composition from accumulating on a surface of a turn roll in an impregnation tank during a step of impregnating a long carbon fiber paper web with a resin composition liquid in a roll-to-roll manner to manufacture a carbon sheet.SOLUTION: A method for manufacturing a carbon sheet includes: an impregnation step of passing a long carbon fiber paper web through an impregnation tank containing a resin composition solution in a roll-to-roll manner; a forming step of heating and compressing the carbon fiber paper web impregnated with the resin composition; and a sintering step of carbonizing the resin composition. The method further includes scraping a surface of the turn roll inside the impregnation tank with a scraper.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a carbon sheet used in a fuel cell and a water electrolysis device, particularly a polymer electrolyte fuel cell or a water electrolysis device, and a method for manufacturing a gas diffusion electrode using the carbon sheet.

Background Art

[0002] A polymer electrolyte fuel cell that obtains electric power by an electrochemical reaction occurring at both electrodes by supplying a fuel gas containing hydrogen to the anode and an oxidizing gas containing oxygen to the cathode generally includes a separator, a gas diffusion electrode, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode, and a separator. A power generation unit called a stack in which a plurality of cells are stacked in series, with one cell formed by laminating these components in this order as one unit. Here, the three-layer part of the catalyst layer, the electrolyte membrane, and the catalyst layer is called a catalyst-coated membrane (CCM: Catalyst Coated Membrane), and the five-layer part including the gas diffusion electrodes arranged on both sides of the CCM is called a membrane electrode assembly (MEA: Membrane Electrode Assembly).

[0003] In the operation of a normal fuel cell, the following reaction proceeds on the anode side. H 2 →2H + +2e - (1) Also, on the cathode side, the following reaction proceeds. O 2 +4H + +4e - →2H 2 O (2).

[0004] That is, electric power as a fuel cell is obtained by the electrochemical reaction of hydrogen and oxygen. Also, water is generated at this time.

[0005] The gas diffusion electrode requires high gas diffusibility for diffusing the gas supplied from the separator to the catalyst layer, high water drainage for discharging the water generated during the electrochemical reaction to the separator, and high conductivity for extracting the generated current. Also, appropriate springiness is required so that the fastening pressure of the members is uniform in the plane within the fuel cell. Therefore, a carbon sheet made of conductive fibers such as carbon fiber is widely used (Patent Document 1). Further, for the purpose of moisturizing the CCM to prevent drying, conversely discharging excess water, or protecting the carbon fibers protruding from the carbon sheet so as not to damage the CCM, it is known to form a microporous layer (MPL: Micro Porous Layer) on the surface of the carbon sheet.

[0006] On the other hand, there is a water electrolysis device that utilizes a reaction in the direction opposite to that of the fuel cell reaction as follows. Anode side: 2H 2 O → O 2 + 4H + + 4e - (3) Cathode side: 2H + + 2e - → H 2 (4).

[0007] The water electrolysis device supplies water to the high-potential anode side to generate hydrogen and oxygen, and is industrially mainly used as a hydrogen production device. In the case of the water electrolysis device as well, an MEA with gas diffusion electrodes arranged on both sides of the CCM is used. Conventionally, a titanium fiber sheet or the like has been used as the gas diffusion electrode (Patent Document 2). When a reverse potential is applied to the carbon sheet in the presence of water, it decomposes into CO + while generating H 2 . C + 2H 2 O → CO 2 + 4H + + 4e - (5)

[0008] Therefore, when a carbon sheet is used on the high-potential anode side, the reaction of (5) above occurs instead of the reaction of (3) above, and the carbon sheet deteriorates, so its use has been avoided. However, in recent years, it has been proposed to use a carbon sheet, which is cheaper than an expensive titanium fiber sheet, for the gas diffusion electrode of the cathode where the potential is low and the concern about deterioration is small. Although drainage is not particularly required for the gas diffusion electrode for a water electrolysis device, gas diffusibility, conductivity, springiness, etc. are required in the same manner as in fuel cell applications.

[0009] When manufacturing the long carbon sheet used for the above gas diffusion electrode, it is known to provide a roll-to-roll impregnation step of attaching a resin composition to a long carbon fiber papermaking body (Patent Document 3).

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] However, when manufacturing a long carbon sheet by roll-to-roll, in the step of immersing the carbon fiber papermaking body in the resin composition liquid, deposits of the resin composition accumulate on the surface of the turn roll in the impregnation tank, and there has been a problem that the quality stability of the obtained carbon sheet deteriorates.

Means for Solving the Problems

[0012] In view of the above problems, the present invention provides a method for manufacturing a carbon sheet and a method for manufacturing a gas diffusion electrode described below. (1) An impregnation step of passing a long carbon fiber paper sheet through a roll-to-roll impregnation tank containing a resin composition solution, a molding step of heating and compressing the carbon fiber paper sheet impregnated with the resin composition, and a firing step of carbonizing the resin composition, and a method for manufacturing a carbon sheet for scraping the surface of a turn roll inside the impregnation tank with a scraper. (2) The method for manufacturing a carbon sheet according to (1), having a fluororesin coating layer on the surface of the scraper. (3) The method for manufacturing a carbon sheet according to (1) or (2), wherein the scraper is in the form of a thread. (4) The method for manufacturing a carbon sheet according to (3), wherein the scraper is in the form of a thread and has a diameter of 0.1 to 3 mm. (5) A method for manufacturing a gas diffusion electrode by forming a microporous layer on a carbon sheet manufactured by the method for manufacturing a carbon sheet according to any one of (1) to (4) to obtain a gas diffusion electrode.

Advantages of the Invention

[0013] The carbon sheet obtained by the method for manufacturing a carbon sheet of the present invention has good quality stability, and the quality of a fuel cell or a water electrolysis device using the same is also stable, and the performance is also improved.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0015] The long carbon fiber paper sheet used in the method for manufacturing a carbon sheet of the present invention is a paper sheet containing carbon fibers and a water-soluble resin.

[0016] Examples of carbon fibers include polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, and rayon-based carbon fibers. Among them, PAN-based carbon fibers and pitch-based carbon fibers are preferably used because of their excellent mechanical strength. Further, natural fibers and synthetic fibers such as rayon fibers, acrylic fibers, and cellulose fibers may be mixed.

[0017] The average diameter of the single fiber of the carbon fiber is preferably 3 to 20 μm. When the average diameter is 3 μm or more, the pore diameter of the carbon sheet becomes large, and the permeability of gases such as hydrogen, oxygen, and water vapor and liquid water is improved, so the performance of fuel cells and water electrolysis devices is improved. On the other hand, when the average diameter is 20 μm or less, the water vapor diffusivity becomes small, and it is possible to suppress the drying of the electrolyte membrane and the catalyst layer and the deterioration of performance. That is, the moisture retention is improved.

[0018] The average length of the single fiber of the carbon fiber is preferably 3 to 20 mm. When the average length is 3 mm or more, the mechanical strength, conductivity, and thermal conductivity of the carbon sheet become good. On the other hand, when the average length is 20 mm or less, the dispersibility of the carbon fiber during papermaking becomes good, and a homogeneous carbon sheet can be obtained.

[0019] The mass per unit area (basis weight) of the carbon fiber in the long carbon fiber paper sheet used in the present invention is preferably 10 to 50 g / m 2 When the basis weight of the carbon fiber is 10 g / m 2 or more, the mechanical strength of the carbon sheet becomes good. Further, when the basis weight of the carbon fiber is 50 g / m 2 or less, the gas permeability and drainage of the carbon sheet become good.

[0020] Here, the basis weight of the carbon fiber in the long carbon fiber paper sheet can be obtained by cutting a 10 cm square long carbon fiber paper sheet, holding it in an electric furnace at 600 °C for 15 minutes in a nitrogen atmosphere to remove organic substances, and then dividing the mass by the area of the carbon fiber paper sheet (0.01 m 2 ).

[0021] The water-soluble resin is used to bind the carbon fibers laminated by papermaking, and examples thereof include polyvinyl alcohol and polyvinyl acetate. In the long carbon fiber paper sheet, when the content of the water-soluble resin is based on 100 parts by mass of the carbon fibers, it is preferably 5 to 50 parts by mass.

[0022] The term "long" refers to a shape having a long strip shape in one direction with respect to a piece-shaped one (referred to as a single leaf) cut into a rectangle. The long carbon fiber paper sheet is wound in a roll shape before processing or during storage after processing, and is in a form applicable to a roll-to-roll process having a roll unwinding section and a winding section.

[0023] The method for manufacturing the carbon sheet of the present invention includes an impregnation step, a forming step, and a firing step. In the impregnation step, the long carbon fiber paper sheet is passed through an impregnation tank containing a resin composition solution by roll-to-roll. Thereby, the long carbon fiber paper sheet is impregnated with a resin composition containing a thermosetting resin such as a phenol resin, an epoxy resin, a melamine resin, or a furan resin. Next, in the forming step, the carbon fiber paper sheet impregnated with the resin composition is heated and compressed to shape it while curing the thermosetting resin, and then in the firing step, the resin composition is carbonized to obtain a carbon sheet in which the carbon fibers are bound by the carbide of the resin composition.

[0024] The carbon sheet is applied to the gas diffusion electrode of a fuel cell or a water electrolysis device as it is or after further processing. For example, in a fuel cell, a water repellent treatment may be applied to the carbon sheet to enhance its drainage property. It is also preferable to use, as the gas diffusion electrode, a carbon sheet having a microporous layer formed on its surface. At this time, forming a microporous layer on one surface of the carbon sheet is one of the preferred embodiments.

[0025] In addition, when applying the carbon sheet of the present invention to a gas diffusion electrode of a cell with a large-area cell having a side length exceeding 50 cm, the springiness of the carbon sheet manufactured using only one carbon fiber paper sheet may be insufficient, and the fastening pressure within the cell may become non-uniform in the in-plane direction. In such a case, it is preferable to stack a plurality of carbon fiber paper sheets impregnated with a resin composition in the forming process and heat-compress them to increase the thickness of the carbon sheet, thereby improving the springiness. Alternatively, a plurality of carbon sheets may be stacked and arranged as a gas diffusion electrode during MEA production. At this time, it is preferable to use a carbon sheet with a microporous layer and arrange it with the microporous layer side in contact with the CCM, and arrange the remaining carbon sheets without a microporous layer.

[0026] The basis weight of the carbon sheet manufactured by the manufacturing method of the present invention is preferably 20 to 60 g / m 2 . When the basis weight is 20 g / m 2 or more, the mechanical strength and conductivity of the carbon sheet are improved. On the other hand, when the basis weight is 60 g / m 2 or less, the gas diffusibility in the plane direction perpendicular to the surface of the carbon sheet (hereinafter, the plane direction perpendicular to the surface means the thickness direction) becomes good, and the power generation performance and water electrolysis performance are improved. When the basis weight of the carbon sheet is 30 to 40 g / m 2 , these effects are enhanced, which is more preferable.

[0027] The adjustment of the basis weight of the carbon sheet can be performed by controlling the amount of carbon fiber, resin carbide, etc., which are the constituent materials of the carbon sheet.

[0028] In order to enhance the drainage property, the carbon sheet may be one in which a water-repellent resin adheres internally by a water-repellent treatment. As the water-repellent resin, a fluororesin having a fluoroalkyl chain is preferable.

[0029] The amount of the water-repellent resin is not particularly limited, but when the total mass of the carbon sheet is 100% by mass, 0.1 to 20% by mass is preferable. Within this range, while the water-repellency is sufficiently exhibited, it is possible to suppress the water-repellent resin from blocking the pores that serve as gas diffusion paths or increasing the electrical resistance.

[0030] The thickness of the carbon sheet of the present invention is preferably 90 to 180 μm. Here, the thickness of the carbon sheet is the thickness when both surfaces are sandwiched with a pressure of 0.15 MPa. When the thickness is 90 μm or more, the mechanical strength is maintained and handling in the manufacturing process is easy. Also, the springiness increases and the in-plane uniformity of the fastening pressure in the cell of the fuel cell or the water electrolysis device is improved. On the other hand, when the thickness of the carbon sheet is 180 μm or less, the gas diffusibility in the plane perpendicular to the surface becomes high. Also, the conductive path in the plane perpendicular to the surface becomes short and the conductivity becomes good.

[0031] Next, the microporous layer will be described. The roles of the microporous layer include moisturizing the electrolyte membrane, reducing the interfacial electrical resistance between the catalyst layer and the gas diffusion electrode, and suppressing damage to the electrolyte membrane by carbon fibers protruding from the carbon sheet. It is preferably a layer containing carbon fine particles and a binder resin. As the carbon fine particles, carbon black is preferably used.

[0032] The specific surface area of the carbon fine particles is preferably 20 to 40 m 2 / g. When the specific surface area of the carbon fine particles is 20 m 2 / g or more, the dispersibility of the carbon fine particles in the microporous layer coating liquid increases, so a uniform microporous layer can be formed. Also, when the specific surface area of the carbon fine particles is 40 m 2 / g or less, the oxidation corrosion reaction of carbon is suppressed, so the durability of the fuel cell or the water electrolysis device is enhanced.

[0033] The binder resin is not particularly limited and may be selected from various thermoplastic resins and thermosetting resins. In particular, thermoplastic water-repellent resins such as fluororesins having a fluoroalkyl chain, which have high durability and are easy to adjust water management such as moisturizing and drainage, are preferably used.

[0034] Furthermore, the microporous layer may contain fine particles such as iridium oxide, ruthenium oxide, and titanium oxide that promote the electrolysis of water and suppress carbon deterioration.

[0035] The basis weight of the microporous layer of the present invention is 10 to 35 g / m 2It is preferable that the basis weight of the microporous layer is 10 g / m 2 When it is 2 or more, since it covers the carbon fibers protruding from the surface of the carbon sheet, it is possible to suppress the carbon fibers from damaging the electrolyte membrane. Also, drying of the electrolyte membrane can be prevented. Further, when the basis weight of the microporous layer is 35 g / m 2 When it is 4 or less, the gas diffusivity in the plane direction becomes good.

[0036] Next, an example of a method for manufacturing a long carbon fiber paper sheet, and further, an example of a method for manufacturing a carbon sheet by an impregnation step, a forming step, and a firing step will be described in detail. First, a method for manufacturing a long carbon fiber paper sheet will be described.

[0037] A carbon fiber bundle cut to a predetermined length is loosened in water to produce a uniformly dispersed carbon fiber dispersion, and the laminate of carbon fibers obtained by lifting it onto a papermaking wire continuously fed by a belt conveyor method is dried and wound up in a roll shape, thereby manufacturing a long carbon fiber paper sheet. Here, a surfactant, a thickener, or an antifoaming agent may be contained in the carbon fiber dispersion. Further, it is preferable to impregnate the paper sheet with an aqueous solution containing a water-soluble resin such as polyvinyl alcohol or polyvinyl acetate so that the shape of the carbon fiber paper sheet can be maintained and the water-soluble resin is adhered. After laminating the carbon fibers on the papermaking wire, the aqueous solution of the water-soluble resin may be impregnated by spray coating or curtain coating, or the water-soluble resin may be contained in the carbon fiber dispersion during papermaking. It is also preferable to have a step of drying after impregnating the carbon fiber paper sheet with the water-soluble resin.

[0038] Also, by increasing the speed of the papermaking wire, the carbon fibers can be oriented in the longitudinal direction. By orienting the carbon fibers, the elastic modulus in the carbon fiber orientation direction can be increased. When manufacturing a fuel cell, if it is arranged so that the carbon fiber orientation direction of the gas diffusion electrode is orthogonal to the separator flow path, the gas diffusion electrode can be prevented from being pushed into the groove of the flow path, and the drainage performance can be improved.

[0039] Next, each of the impregnation process, forming process, and firing process for producing a carbon sheet by binding intersections between carbon fibers with a resin carbide in the long carbon fiber paper sheet obtained above for improving mechanical strength and reducing electrical resistance will be described.

[0040] <Impregnation process> The impregnation process in the method for manufacturing a carbon sheet of the present invention is a roll-to-roll manufacturing process. The roll-to-roll manufacturing process has a roll unwinding section that holds a long carbon fiber paper sheet wound in a roll shape before processing and continuously feeds the carbon fiber paper sheet to a processing section, and a roll winding section that winds up the long base material after processing, and is a process of conveying the base material at a predetermined speed while applying a predetermined tension to the base material between the roll unwinding section and the roll winding section. In the present invention, after the impregnation process, the long carbon fiber paper sheet impregnated with the resin composition may be cut into single-sheet base materials and sent to the next process, or the long carbon fiber paper sheet may be passed through the forming process and the firing process in a roll-to-roll manner as it is.

[0041] In the impregnation process, the long carbon fiber paper sheet is passed through an impregnation tank containing a resin composition solution to obtain a long carbon fiber paper sheet to which the resin composition has adhered.

[0042] The resin composition solution contains a thermosetting resin as an essential component, and examples of the thermosetting resin include phenol resin, epoxy resin, melamine resin, furan resin, and the like. The resin composition solution may contain carbon powder, surfactant, and the like in addition to the resin component and the solvent. Examples of the carbon powder include carbon black, graphite, graphite, carbon nanotubes, carbon nanofibers, and the like.

[0043] Examples of the solvent for the resin composition solution include water, alcohols such as methanol, ethanol, isopropanol, n-butanol, ethylene glycol, and glycerin, ketones such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, esters such as ethyl acetate, butyl acetate, propylene glycol monomethyl ether acetate, and ethyl lactate, n-methyl-2-pyrrolidone, γ-butyrolactone, etc. A solvent obtained by mixing these can also be used.

[0044] The viscosity of the resin composition solution is preferably 1 to 100 mP·s. When the viscosity is 1 mP·s or more, sedimentation and separation of the components contained in the resin composition solution hardly occur, and a product with stable quality can be obtained. When the viscosity is 100 mP·s or less, the resin composition solution penetrates into the long carbon fiber paper web in a short time, improving productivity.

[0045] As shown in Fig. 1, the method of impregnating the resin composition solution of the present invention into a long carbon fiber paper web is to pass the long carbon fiber paper web 6 through an impregnation tank 1 containing the resin composition solution 5. At this time, in the turn roll 2 for reversing the traveling direction of the long carbon fiber paper web 6 in the impregnation tank 1, the surface thereof is scraped by a scraper 7.

[0046] Conventionally, since deposits of the resin composition gradually accumulate on the surface of the turn roll 2 in the impregnation tank 1 during the impregnation process, the concentration of the resin composition solution 5 impregnated into the long carbon fiber paper web 6 gradually changes, or the deposits are transferred from the turn roll 2 to the long carbon fiber paper web 6, so that the amount of the resin composition adhering to the long carbon fiber paper web 6 after impregnation is not stable. Therefore, there have been problems such as the basis weight and physical properties of the obtained carbon sheet deviating from the target values, and large variations in the in-plane direction. In addition, when the deposits on the surface of the turn roll 2 locally increase, abnormal tension is applied to the long carbon fiber paper web 6 starting from there, which may lead to meandering or breakage of the long carbon fiber paper web 6.

[0047] On the other hand, in the present invention, since the surface of the turn roll 2 is always scraped, the deposited resin composition can be removed, so the above problems can be suppressed.

[0048] As a method for scraping the surface of the turn roll 2 of the present invention, there is a method of installing a scraper 7 such as a plate shape, a brush shape, a cloth shape, or a thread shape along the longitudinal direction of the roll on the upper surface of the turn roll 2. The material of the scraper 7 can be appropriately selected such as metal, resin, ceramics, etc., but it is preferably a resin such as vinyl chloride, polyethylene, polypropylene, polyester, nylon, urethane, acrylate, fluororesin, etc. that does not damage the turn roll 2 and can be easily adhered to the surface of the turn roll 2 without gaps.

[0049] Since the upper surface of the turn roll 2 is always wet with the resin composition solution 5, the deposit of the resin composition scraped by the scraper 7 dissolves again in the resin composition solution 5. In particular, when using a thread-shaped scraper 7, it is preferable because the deposit hardly adheres to the scraper 7 and easily dissolves in the resin composition solution 5, so the change in the concentration of the resin composition solution 5 is reduced. Also, it is preferable to have a fluororesin coating layer on the surface of the scraper 7 because the scraped resin composition hardly adheres to the scraper 7.

[0050] When the scraper 7 is thread-shaped, as shown in FIG. 2, it is preferable to install the scraper 7 so that it crawls obliquely on the turn roll 2 because the scraper 7 can be strongly and uniformly pressed from one end to the other end of the turn roll 2. At this time, the positions A and A' for gripping the ends of the thread are preferably located inside the circle drawn by the roll surface when assuming a plane perpendicular to the axis of the turn roll 2 (FIG. 3). In this case, since the tensioned thread-shaped scraper 7 contacts the ends B and B' of the turn roll 2, the entire surface of the turn roll 2 can be scraped.

[0051] When the scraper 7 is filamentous, its diameter is preferably 0.1 to 3 mm. When the diameter is 0.1 mm or more, the resin composition firmly deposited on the surface of the turn roll 2 can be reliably scraped, and even if the impregnation process is operated for a long time, the scraper 7 can be prevented from breaking or deteriorating. On the other hand, when the diameter is 3 mm or less, the scraper 7 can be uniformly pressed against the surface of the turn roll 2, so that the deposits can be scraped over the entire surface of the turn roll 2.

[0052] Providing a coating layer of a water-repellent material such as a water-repellent resin or ceramics on the surface of the turn roll 2 is preferable because the resin composition deposited on the surface of the turn roll 2 can be easily scraped. Specific methods of coating include a method of coating a solution or dispersion of a water-repellent material by dip coating or spray coating and then melting it, and a method of directly forming a layer of a water-repellent material by thermal spraying, CVD, sputtering, etc. Examples of water-repellent resins include fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene·perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene·hexafluoropropylene copolymer (FEP), tetrafluoroethylene·ethylene copolymer (ETFE), silicone resins, polypropylene, etc. Examples of water-repellent ceramics include titanium oxide and zirconium oxide. Among these, PTFE, which is particularly excellent in solvent resistance and impact resistance, is preferable. That is, it is preferable to coat the surface of the turn roll 2 with PTFE by the above method to provide a PTFE coating layer.

[0053] The turn roll 2 may be completely immersed in the resin composition solution 5, or a part of the turn roll 2 may be exposed above the liquid level. It is preferable that the turn roll 2 is completely immersed because the resin composition deposited on the surface once is more likely to dissolve again in the resin composition solution 5. Further, a mechanism for stirring the resin composition solution 5 inside the impregnation tank 1, a mechanism for supplying the solution so that the liquid level becomes constant, and a squeezing mechanism for removing the excess solution contained in the long carbon fiber paper web 6 after impregnation can also be provided. Examples of the squeezing mechanism include a method of pressing a roll against the long carbon fiber paper web 6 from one side after impregnation, and a method of passing the long carbon fiber paper web 6 between two rolls to apply pressure from both sides.

[0054] Next, the carbon fiber paper web impregnated with the resin composition solution is dried at a temperature of 80 to 150°C in air to obtain a long carbon fiber paper web with the resin composition adhered thereto. Note that the processing may proceed in a long shape after the next forming step, or may proceed after cutting into sheets.

[0055] <Forming step> The method for manufacturing a carbon sheet of the present invention has a molding step of heating and compressing the carbon fiber paper web impregnated with the resin composition after the impregnation step. By heating at a temperature of 200 to 300°C in air, the thermosetting resin in the resin composition is cured, and components such as surfactants contained other than the resin are decomposed and removed. At this time, by pressing both sides of the carbon fiber paper web with a flat plate or the like, the flatness can be improved and the thickness can be adjusted to form a desired shape.

[0056] <Firing step> The method for manufacturing a carbon sheet of the present invention has a firing step of carbonizing the resin composition in order to increase the conductivity of the carbon sheet and enhance the long-term durability. In the firing step, for example, the resin composition adhered to the carbon fiber paper web after the molding step is carbonized at a temperature of 1,000 to 2,400°C in an inert atmosphere such as nitrogen to obtain a carbon sheet. Here, it is preferable to perform pre-carbonization treatment at a temperature of 300 to 1,000°C in an inert atmosphere before the carbonization treatment to decompose and remove impurities and approach the crystal structure of graphite, because the crystallinity during the carbonization treatment is increased and the above performance is enhanced.

[0057] The carbon sheet obtained by the method for producing a carbon sheet of the present invention can have sufficient mechanical strength and conductivity as a carbon sheet for a gas diffusion electrode of a fuel cell or a water electrolysis device.

[0058] Furthermore, a water repellent treatment may be performed to enhance the drainage property of the carbon sheet. As a method for subjecting the carbon sheet to a water repellent treatment, in addition to a method of immersing the carbon sheet in a dispersion of a water repellent resin, a method of applying a water repellent resin to the carbon sheet by dip coating, spray coating, or the like is also applicable. In the case of the method of immersing a long carbon sheet in a dispersion of a water repellent resin, it is preferable that the surface of the turn roll in the immersion tank is water repellent. After adhering a water repellent resin to the inside of the carbon sheet, a drying step and, if necessary, a heating step for spreading the water repellent resin in the carbon sheet may be added.

[0059] Next, a method for forming a microporous layer on the carbon sheet will be described. Either a long carbon sheet or a single-sheet carbon sheet may be used, but a long carbon sheet is preferable because the production rate and quality stability are increased.

[0060] The microporous layer can be formed by applying a coating liquid for forming a microporous layer in which carbon fine particles and a binder resin such as a water repellent resin are dispersed in a solvent such as water onto the carbon sheet and performing a heat treatment.

[0061] When preparing the coating liquid for forming the microporous layer, it is preferable to add a dispersant, a thickener, or the like to the coating liquid because the dispersion stability of the carbon fine particles and the binder resin is enhanced. As the dispersant, a nonionic surfactant is preferable because it contains less metal components, and examples thereof include "Triton (registered trademark)" X-100 (manufactured by Nacalai Tesque, Inc.) of the polyoxyethylene octylphenyl ether type. In addition, it is effective to add a thickener to keep the coating liquid at a high viscosity. As the thickener, for example, methyl cellulose-based, polyethylene glycol-based, polyvinyl alcohol-based, etc. are preferably used.

[0062] Furthermore, if necessary, fine particles for promoting the electrolysis of water, such as iridium oxide, ruthenium oxide, and titanium oxide, and fine particles for radical inactivation, such as cerium oxide and manganese oxide, are also added.

[0063] The mixture of the above components is kneaded using a homogenizer, a planetary mixer, an ultrasonic disperser, etc. to obtain a coating liquid for forming a microporous layer.

[0064] The coating of the coating liquid for forming a microporous layer on the carbon sheet can be carried out using various commercially available coating devices. As the coating method, screen printing, rotary screen printing, intaglio printing, gravure printing, spray coating, die coating, bar coating, blade coating, roll knife coating, etc. can be used.

[0065] After applying the microporous layer on the carbon sheet, the coating liquid is dried at a temperature of 60 to 150 °C, and then heated at a temperature of 250 to 380 °C to promote the decomposition and removal of additives such as dispersants and thickeners and the melting or curing of the binder resin. At this time, when the water-repellent treatment is performed on the binder resin in the microporous layer and the water-repellent resin attached to the inside of the carbon sheet, the water-repellent resin melts and spreads over the surfaces of other constituent materials, such as carbon fibers in the carbon sheet, carbides of the resin composition, and carbon fine particles in the microporous layer. As a result, the drainage performance of the carbon sheet and the microporous layer is enhanced.

[0066] In this way, a gas diffusion electrode with a microporous layer formed on the carbon sheet is obtained. Furthermore, the long or single-sheet gas diffusion electrode is cut to a predetermined size to fit the cell shape of the fuel cell or water electrolysis device to be applied.

[0067] Next, the microporous layer side surfaces of the gas diffusion electrodes are joined to both surfaces of the catalyst-coated electrolyte membrane (CCM) to manufacture a membrane electrode assembly (MEA).

[0068] As the electrolyte membrane, those with high proton conductivity, high oxidation resistance, and low gas crossover are preferred, and examples include those made of fluorine-based polymers and hydrocarbon-based polymers. Further, as the catalyst layer, a mixture of catalyst-supported carbon particles in which catalyst fine particles that are noble metals such as platinum, palladium, ruthenium, iridium, rhodium or their oxides are supported on carbon particles and a proton-conductive polymer is preferably applied.

[0069] Examples of the joining method of the gas diffusion electrode and the CCM include flat pressing and roll pressing, and they may be joined while heating. Further, as an aid for joining, an adhesive or an adhesive sheet may be provided at the periphery of the gas diffusion electrode and the CCM.

[0070] Furthermore, the obtained MEA is sandwiched from both sides by two separators for the anode and the cathode in which gas flow paths are formed to produce a fuel cell cell or an electrolyzer cell. At this time, gaskets for preventing the intrusion of gas and water on the anode side and the cathode side are arranged around the MEA.

[0071] Grooves are engraved on the surface of the separator in contact with the MEA to form gas or liquid water flow paths. Inlets / outlets are provided at the ends of the flow paths. In the case of a fuel cell cell, hydrogen is supplied to the anode side, air is supplied to the cathode side, and water is discharged. On the other hand, in the case of an electrolyzer cell, water is supplied to the anode side, oxygen is discharged, and hydrogen is discharged from the cathode side. Further, the separator is made of a conductive material such as stainless steel or carbon, and electrical connection can be made by connecting electrical wiring. In the case of a fuel cell cell, the cathode side becomes a high-potential power source for the external circuit. Conversely, in the case of an electrolyzer, by connecting a high-potential power source to the cathode side, the cathode side becomes a load for the external circuit and water electrolysis is performed. Furthermore, by providing a passage through which circulating water can be supplied inside the separator, the cell can be maintained at a predetermined temperature.

[0072] By connecting a plurality of cells thus produced, a unit of a fuel cell or a water electrolysis device can be manufactured. In the case of a fuel cell, about 200 to 500 cells are prepared and connected in series to generate a high voltage of 40 to 200 kV, and such a fuel cell stack can be used as a power source for a fuel cell vehicle or the like.

Example

[0073] Hereinafter, the present invention will be specifically described by way of examples. The performance evaluation method performed in the examples and the production method of the materials used in the examples are shown below.

[0074] <Method for Measuring Thickness of Carbon Sheet and Gas Diffusion Electrode> The thicknesses of the carbon sheet and the gas diffusion electrode were measured with a micrometer applying a load of 0.15 MPa.

[0075] <Method for Evaluating Power Generation Performance of Membrane Electrode Assembly (MEA)> A polymer electrolyte membrane (membrane thickness: 10 μm) was prepared using a 10 mass% dispersion of "Nafion (registered trademark)", a fluoropolymer (manufactured by Sigma-Aldrich). On the other hand, using platinum catalyst-supported carbon particles TEC10E50E (platinum loading rate 50 mass%, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) and a 10 mass% dispersion of "Nafion (registered trademark)", a fluoropolymer (manufactured by Sigma-Aldrich), a catalyst coating liquid was prepared so that the mass ratio of platinum catalyst-supported carbon particles to "Nafion (registered trademark)" was 2:1, and the catalyst coating liquid was applied to a commercially available PTFE film so that the platinum amount was 0.3 mg / cm 2 and a catalyst layer decal (catalyst layer thickness: 8 μm) was prepared. Next, a pair of catalyst layer decals cut into 7 cm squares was prepared, and they were overlapped facing each other so as to sandwich a polymer electrolyte membrane cut into 7 cm squares, and heat pressing was performed at 150 °C and 5 MPa for 3 minutes to obtain an electrolyte membrane with a catalyst layer (CCM).

[0076] A gas diffusion electrode cut into a 5 cm square on each side of the CCM was arranged so that the microporous layer was in contact with the catalyst layer, and hot-pressed at 130 °C and 0.1 MPa for 20 minutes to produce a membrane electrode assembly (MEA). This MEA was incorporated into a fuel cell, and at a cell temperature of 70 °C, hydrogen was supplied to the anode side and air was supplied to the cathode side, and the power generation state was maintained for 3 hours at a current density of 1 A / cm 2 The fuel utilization efficiency was 70%, the air utilization efficiency was 40%, and hydrogen on the anode side and air on the cathode side were humidified so that the dew points were 59 °C and 60 °C, respectively, and introduced into the cell. The voltage value at the time when 3 hours had elapsed was read as the power generation voltage and used as an index of power generation performance.

[0077] (Example 1) Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca (registered trademark)" T300 (average fiber diameter: 7 μm) was cut into a length of 12 mm, dispersed in water, and continuously paper-made. Further, a 10% by mass aqueous solution of polyvinyl alcohol was impregnated by spray coating, dried, and a long carbon fiber paper body with a basis weight of 30 g / m 2 and a width of 30 cm and a length of 100 m made of carbon fiber was obtained. The amount of polyvinyl alcohol adhered to 100 parts by mass of the paper body was 20 parts by mass.

[0078] Next, a resin composition solution was prepared by mixing flaky graphite (average particle diameter: 5 μm), a phenol resin (a mixture of a resol-type phenol resin and a novolac-type phenol resin in a mass ratio of 1:1), and methanol in a mass ratio of 5:5:90. The viscosity of the resin composition solution was 5 mPa·s.

[0079] As shown in Fig. 1, the long carbon fiber paper sheet 6 was continuously passed through the impregnation tank 1 filled with the resin composition solution 5 in a roll-to-roll manner, and then dried at 100 °C for 5 minutes. Here, as shown in Fig. 2, the turn roll 2 in the impregnation tank is made of SUS with a diameter of 20 cm and a length of 40 cm. As a scraper 7 on its upper surface, nylon thread EA628AW-10 with a thickness of 0.52 mm (fluororesin coating, manufactured by ESCO Co., Ltd.) was pressed against from one end to the other end of the turn roll 2 with tension applied. At this time, the scraper 7 was angled with respect to the axis of the turn roll 2 and arranged so as not to interfere with the conveyed long carbon fiber paper sheet 6.

[0080] An impregnation process was performed on 80 m of the 100 m long carbon fiber paper sheet to obtain a carbon fiber paper sheet containing the resin composition. The resin composition was 130 parts by mass with respect to 100 parts by mass of the carbon fibers in the obtained carbon fiber paper sheet. Also, no deposit of the resin composition occurred on the turn roll 2 in the impregnation tank during impregnation. After impregnation, the long carbon fiber paper sheet was cut out in 40 cm increments to obtain 200 sheets of 30 cm × 40 cm single-leaf carbon fiber paper sheets.

[0081] Next, the single-leaf carbon fiber paper sheets with the resin composition attached were sandwiched one by one between the upper and lower hot plates using a press molding machine and subjected to a heat compression treatment at 180 °C for 5 minutes. Here, a release paper was interposed between the paper sheet and the hot plate so that the hot plate and the paper sheet would not adhere, and spacers were arranged at the peripheral portions of the upper and lower hot plates to adjust the thickness of the paper sheet after heat compression. Then, heating was performed at 2,000 °C in a nitrogen atmosphere in a heating furnace for carbonization to obtain 200 sheets of single-leaf carbon sheets.

[0082] Furthermore, a water-repellent resin dispersion obtained by mixing 5 parts by mass of a dispersion of PTFE fine particles (manufactured by Daikin Industries, Ltd., "Polyflon (registered trademark)" D-210C) and 95 parts by mass of ion-exchanged water was spray-coated, and then dried at 100 °C for 5 minutes to obtain 200 sheets of water-repellent treated carbon sheets. The average value of the thickness at 0.15 MPa was 160 μm (standard deviation 3 μm), and the average value of the basis weight was 50 g / m 2 (standard deviation 1 g / m 2 ) was obtained.

[0083] As the carbon microparticles, "Denka Black (registered trademark)" (manufactured by Denka Co., Ltd.) was used. As the binder resin, PTFE, which is a fluororesin, was used. As the dispersant, "Triton (registered trademark)" X-100 (manufactured by Nacalai Tesque, Inc.) was used. Water was used as the dispersion medium. The mixing ratio was set as carbon microparticles / binder resin / dispersant = 75 parts by mass / 25 parts by mass / 150 parts by mass, and the coating liquid for forming the microporous layer was adjusted so that the non-decomposable components (carbon microparticles and binder resin) accounted for 20% by mass of the total amount. Here, the dispersion liquid "Polyflon (registered trademark)" D-210C (manufactured by Daikin Industries, Ltd.) in which PTFE particles are dispersed in water was used as the supply source of PTFE. Further, when adjusting the coating liquid for forming the microporous layer, a planetary mixer was used to disperse the raw materials so that the composition became uniform.

[0084] The coating liquid for forming the microporous layer was applied onto the water-repellent treated carbon sheet obtained above using a die coater, dried at 100 °C for 10 minutes, and then heated at 350 °C for 10 minutes to promote the adhesion between the binder resin and the carbon microparticles and decompose and remove the dispersant, thereby fabricating a gas diffusion electrode. Here, the coating amount was adjusted so that the basis weight of the microporous layer after heating was 20 g / cm 2 The average value of the thicknesses of the obtained 200 gas diffusion electrodes at 0.15 MPa was 180 μm.

[0085] Next, the power generation performance was evaluated as described in <Evaluation Method for Power Generation Performance of Membrane Electrode Assembly (MEA)>. The gas diffusion electrodes used were the 1st, 50th, 100th, 150th, and 200th of the 200 sheet-like gas diffusion electrodes arranged in the order in which the long carbon fiber paper-making body passed through the impregnation process. For each of these, two 5 cm square samples were cut out and used as the anode and cathode for each power generation performance evaluation. In the five evaluations conducted for each of the five gas diffusion electrodes, the maximum and minimum values of the power generation voltage were 0.75 V and 0.74 V, respectively.

[0086] (Example 2) As a scraper 7 on the upper surface of the turn roll 2 in the impregnation tank of the impregnation process, a carbon sheet and a gas diffusion electrode were produced in the same manner as in Example 1, except that the nylon thread EA628AW-1 with a thickness of 0.17 mm (fluororesin coating, manufactured by ESCO Co., Ltd.) was used instead.

[0087] During impregnation, no deposits of resin carbide occurred on the turn roll 2 in the impregnation tank. Also, the average value of the thickness of the 200 water-repellent treated carbon sheets obtained at 0.15 MPa was 160 μm (standard deviation 3 μm), and the average value of the basis weight was 50 g / m 2 (standard deviation 1 g / m 2 ) was obtained. Also, the average value of the thickness of the gas diffusion electrode after the formation of the microporous layer was 180 μm.

[0088] Furthermore, when the power generation performance was evaluated in the same manner as in Example 1 using the obtained gas diffusion electrode, in the five evaluations conducted for each of the five gas diffusion electrodes, the maximum and minimum values of the power generation voltage were 0.74 V and 0.73 V, respectively.

[0089] (Example 3) As the resin composition solution, a carbon sheet and a gas diffusion electrode were produced in the same manner as in Example 1, except that flaky graphite (average particle diameter: 5 μm), a phenolic resin (a mixture of resol type phenolic resin and novolac type phenolic resin in a mass ratio of 1:1), and isopropyl alcohol were mixed in a mass ratio of 5:5:90. The viscosity of the resin composition solution was 10 mPa·s.

[0090] During impregnation, no deposits of the resin composition occurred on the turn roll 2 in the impregnation tank. Also, the average value of the thickness of the 200 water-repellent treated carbon sheets obtained at 0.15 MPa was 160 μm (standard deviation 2 μm), and the average value of the basis weight was 50 g / m 2 (standard deviation 0.5 g / m 2 ) was obtained. Also, the average value of the thickness of the gas diffusion electrode after the formation of the microporous layer was 180 μm.

[0091] Furthermore, when the power generation performance was evaluated in the same manner as in Example 1 using the obtained gas diffusion electrode, the power generation voltage was 0.75 V in all five evaluations conducted for each of the five gas diffusion electrodes.

[0092] (Example 4) As the resin composition solution, a carbon sheet and a gas diffusion electrode were produced in the same manner as in Example 1, except that flaky graphite (average particle diameter: 5 μm), a phenol resin (a mixture of a resol type phenol resin and a novolak type phenol resin with a mass ratio of 1:1), and ethylene glycol were mixed at a mass ratio of 5:5:90. The viscosity of the resin composition solution was 50 mPa·s.

[0093] No deposits of the resin composition occurred on the turn roll 2 in the impregnation tank during impregnation. Also, the average value of the thickness of the 200 obtained water-repellent treated carbon sheets at 0.15 MPa was 160 μm (standard deviation 2 μm), and the average value of the basis weight was 50 g / m 2 (standard deviation 0.5 g / m 2 ) and the average value of the thickness of the gas diffusion electrode after forming the microporous layer was 180 μm.

[0094] Furthermore, when the power generation performance was evaluated in the same manner as in Example 1 using the obtained gas diffusion electrode, the power generation voltage was 0.75 V in all five evaluations conducted for each of the five gas diffusion electrodes. (Comparative Example 1) A carbon sheet and a gas diffusion electrode were produced in the same manner as in Example 1, except that the scraper 7 was not used on the turn roll 2 in the impregnation tank of the impregnation process.

[0095] During impregnation, deposits of the resin composition gradually precipitated on the surface of the turn roll 2 in the impregnation tank and had grown to a thickness of 1 cm when the carbon fiber paper web had passed 80 m. Also, the average value of the thickness of the 200 obtained water-repellent treated carbon sheets at 0.15 MPa was 161 μm (standard deviation 6 μm), and the average value of the basis weight was 47 g / m 2 (standard deviation 3 g / m 2 ) and the average value of the thickness of the gas diffusion electrode after forming the microporous layer was 185 μm.

[0096] Furthermore, when the power generation performance was evaluated in the same manner as in Example 1 using the obtained gas diffusion electrode, in the five evaluations conducted for each of the five gas diffusion electrodes, the maximum and minimum values of the power generation voltage were 0.70 V and 0.65 V, respectively.

Explanation of Signs

[0097] 1 Impregnation tank 2 Turn roll 3 Auxiliary roll 4 Auxiliary roll 5 Resin composition solution 6 Long carbon fiber paper 7 Scraper A, A’ Positions for gripping the ends of the yarn B, B’ Positions where the yarn contacts the ends of the turn roll

Claims

1. The method for producing a carbon sheet includes an impregnation step in which a long carbon fiber sheet is passed roll-to-roll through an impregnation tank containing a resin composition solution, a molding step in which the carbon fiber sheet impregnated with the resin composition is heated and compressed, and a firing step in which the resin composition is carbonized, and the surface of a turn roll inside the impregnation tank is scraped with a scraper.

2. The method for producing a carbon sheet according to claim 1, wherein the surface of the scraper has a coating layer of a fluororesin.

3. The method for producing a carbon sheet according to claim 1, wherein the scraper is filamentous.

4. The method for producing a carbon sheet according to claim 3, wherein the scraper is filamentous and has a diameter of 0.1 to 3 mm.

5. A method for producing a gas diffusion electrode, comprising forming a microporous layer on the carbon sheet produced by the method for producing a carbon sheet according to claim 1 to obtain a gas diffusion electrode.

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