Manufacturing method for gas diffusion layer

Abstract

To provide a manufacturing method capable of highly productive mass production of a gas diffusion layer that exhibits excellent power generation performance when used as a gas diffusion electrode in a fuel cell.SOLUTION: The manufacturing method for a gas diffusion layer in which a microporous layer is formed on at least one surface of a conductive porous substrate, includes a coating step (1) for applying a microporous layer forming coating liquid, which contains at least conductive microparticles and a dispersion medium, onto the conductive porous substrate, a pressing step (2) in which at least two ends of a portion coated with the microporous layer forming coating liquid are pressed using a pressing roll, and a drying step (3) in which the microporous layer forming coating liquid coated on the conductive porous substrate is dried to form a microporous layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a gas diffusion layer used in a fuel cell, and more particularly to a method for manufacturing a gas diffusion layer having excellent smoothness. [Background technology]

[0002] Fuel cells are a mechanism that electrically extracts the energy generated when hydrogen and oxygen react to produce water. They are expected to be a clean energy source because they are highly energy efficient and their only waste product is water.

[0003] The electrodes used in polymer electrolyte fuel cells are sandwiched between two separators in the polymer electrolyte fuel cell and have a structure consisting of a catalyst layer formed on the surface of the polymer electrolyte membrane and a gas diffusion layer formed on the outside of this catalyst layer on both sides of the polymer electrolyte membrane. Gas diffusion layers are generally required to have gas diffusibility, electrical conductivity, and drainage properties. To obtain a gas diffusion layer with these properties, a conductive porous substrate that combines gas diffusibility and electrical conductivity is generally used.

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

[0005] To minimize flooding, gas diffusion layers are required to have good drainage properties. To improve drainage properties, conductive porous substrates are usually treated to have a water-repellent property.

[0006] However, if a water-repellent treated conductive porous substrate is used as a gas diffusion layer as is, the coarse fibers make it easy for the condensed water vapor to form large droplets, which can easily cause flooding. For this reason, a microporous layer (also called a microporous layer) can be formed by applying a coating liquid containing dispersed conductive particles to a water-repellent treated conductive porous substrate, drying and sintering the coating liquid.

[0007] The coating liquid used is a microporous layer-forming coating liquid in which conductive fine particles are dispersed. In addition to the conductive fine particles, a water repellent and a dispersant are also used as ingredients of the microporous layer-forming coating liquid.

[0008] Conventionally, a typical method for manufacturing a gas diffusion layer involves continuously transporting a long conductive porous substrate and passing it through processes such as water-repellent treatment, drying, application of a coating liquid for forming a microporous layer, drying, and sintering. As a method for applying the coating liquid for forming a microporous layer, a die coater or the like is mainly used, as typified by Patent Document 1, which allows for the quantification of the amount of coating regardless of the surface roughness of the conductive porous substrate. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2022-42074 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-145536 [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-346833 Summary of the Invention [Problem to be solved by the invention]

[0010] When a coating device such as a die coater is used that ejects a coating liquid for forming a microporous layer using a nozzle that is wide in the width direction, the coating liquid ejected from both widthwise ends of the tip of the coating device is pressed weakly against the interior of the conductive porous substrate, making it difficult for the coating liquid to penetrate into the conductive porous substrate. As a result, both ends of the conductive porous substrate are raised by the coating liquid. The raised coating liquid for forming a microporous layer may be transferred and deposited on the pass line after coating, or may become thick only at the ends when the substrate is wound up, which may cause rupture of the gas diffusion layer.

[0011] In relation to the above-mentioned problem, Patent Document 2 describes a method for producing a prepreg in which a glass substrate is impregnated with a resin composition, in which raised portions are smoothed using a pressure roll, and a method for scraping off raised resin using a resin scraping bar. However, even if these methods are applied to gas diffusion layers, the scraping bar cannot completely smooth the surface. Furthermore, the description of the method using a pressure roll is not sufficient to allow it to be applied to gas diffusion layers.

[0012] Patent Document 3 describes a method of smoothing a conductive porous substrate after it has been made water-repellent by pressing it with a roll. However, even if this method is applied to a conductive porous substrate coated with a microporous layer-forming coating liquid, there remains the problem that the surface cannot be completely smoothed. [Means for solving the problem]

[0013] The present invention for solving the above problems is as follows. [1] A method for producing a gas diffusion layer in which a microporous layer is formed on at least one surface of a conductive porous substrate, the method comprising the steps of: a coating step (1) of coating a microporous layer-forming coating liquid containing at least conductive fine particles and a dispersion medium onto the conductive porous substrate; a pressing step (2) of pressing at least both coating ends of the coated portion where the microporous layer-forming coating liquid has been applied using a pressing roll; and a drying step (3) of drying the microporous layer-forming coating liquid applied onto the conductive porous substrate to form a microporous layer, in this order. [2] The method for producing a gas diffusion layer according to [1], wherein the pressure roll is a release roll. [3] The method for producing a gas diffusion layer according to [1] or [2], wherein in the pressing step (2), only both coated ends are pressed using a pressing roll. [4] The method for producing a gas diffusion layer according to [1] or [2], wherein in the pressing step (2), the central portion and both ends of the coating are pressed using one pressing roll or a plurality of pressing rolls. [5] The method for producing a gas diffusion layer according to [3], wherein in the pressing step (2), both coated ends are pressed using a pressing roll having a face length of 20 to 60 mm. [6] The method for producing a gas diffusion layer according to [4], wherein in the pressing step (2), pressing is performed using a roll having different diameters at both ends and the center, and the face length at each end being 20 to 60 mm. [7] The method for producing a gas diffusion layer according to [4], wherein in the pressing step (2), the center of the coating is pressed using at least one roll, and each of the coating ends is pressed using a roll having a face length of 20 to 60 mm. [8] The method for producing a gas diffusion layer according to any one of [1] to [7], further comprising a slitting step (4) of trimming at least a part of both coated ends after the pressing step (2) or the drying step (3). [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a method for mass-producing, with high productivity, gas diffusion layers that exhibit good power generation performance when used as gas diffusion electrodes for fuel cells. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 2 is a schematic diagram showing an example of a manufacturing process for a gas diffusion layer. [Figure 2] FIG. 2 is a cross-sectional view of the tip of the discharge part of the die coater as viewed from the conveyance direction. [Figure 3] 1 is a cross-sectional view showing an example of a pressing step of the present invention, as viewed from the conveyance direction. [Figure 4] 1 is a cross-sectional view showing an example of a pressing step of the present invention, as viewed from the conveyance direction. [Figure 5] 1 is a cross-sectional view showing an example of a pressing step of the present invention, as viewed from the conveyance direction. DETAILED DESCRIPTION OF THE INVENTION

[0016] The gas diffusion layer produced in the present invention comprises a microporous layer formed on at least one surface of a conductive porous substrate. By using a conductive porous substrate, a gas diffusion layer having gas diffusibility, drainage property, and electrical conductivity can be obtained. The conductive porous substrate preferably has an average pore diameter of 10 μm or more and 100 μm or less, since the above effects are enhanced.

[0017] Examples of conductive porous substrates that can be used include porous substrates containing carbon fibers such as carbon fiber paper, carbon felt, carbon paper, and carbon cloth, and metal porous substrates such as foamed sintered metal, metal mesh, and expanded metal. Among these, it is preferable to use a porous substrate containing carbon fibers because of its excellent corrosion resistance, and it is particularly preferable to use a substrate made by binding a carbon fiber paper with carbon or a resin carbide, i.e., carbon paper, because of its excellent "springiness" (i.e., the ability to absorb dimensional changes in the thickness direction of the electrolyte membrane).

[0018] In the present invention, the microporous layer is formed through a coating step (1) of coating a microporous layer-forming coating liquid onto a conductive porous substrate, a pressing step (2) of pressing at least both ends of the coated portion of the area where the microporous layer-forming coating liquid is applied using a pressure roll, and a drying step (3) of drying the microporous layer-forming coating liquid applied to the conductive porous substrate. Since the microporous layer is required to be conductive, the microporous layer-forming coating liquid is prepared by dispersing conductive fine particles in a dispersion medium. The dispersion medium is selected appropriately from water, alcohol, etc.

[0019] Examples of conductive fine particles used in the present invention include carbon black, carbon nanotubes, carbon nanofibers, chopped carbon fibers, graphene, graphite, etc. When carbon black is used, acetylene black is preferably used because it contains few impurities and is less likely to reduce the activity of the catalyst.

[0020] In addition to conductive fine particles, the microporous layer preferably contains a water repellent such as a fluororesin. The inclusion of a water repellent can improve drainage of water generated during electrochemical reactions and suppress flooding. Examples of water repellents used in the microporous layer include fluororesins such as PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (perfluoroalkoxyalkane), ETFE (ethylene tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride). PTFE or FEP is preferred because of its particularly high water repellency. To obtain a microporous layer containing a water repellent, the water repellent can be added to the coating liquid for forming the microporous layer.

[0021] The coating liquid for forming the microporous layer preferably contains a surfactant as a dispersant to aid in the dispersion of the conductive fine particles. To prevent short circuits and deterioration of battery performance due to metal contamination, nonionic surfactants with low metal ion content are preferred. From this perspective, methylcellulose-based and polyethylene glycol-based surfactants are preferred, and among them, ethers of alkylphenols and polyethylene glycols, ethers of higher aliphatic alcohols and polyethylene glycols, and polyvinyl alcohol-based surfactants are particularly preferred.

[0022] Here, an example of the manufacturing method of the present invention will be described in detail using Figures 1, 2, 3, 4 and 5, but the present invention is not limited to these embodiments, and the descriptions of preferred configurations and numerical ranges in the descriptions of individual embodiments can also be interpreted as descriptions of the manufacturing method of the gas diffusion layer of the present invention as a general concept.

[0023] 1 shows an example of a manufacturing process using the manufacturing method of the present invention. First, in a coating step 2, a coating liquid for forming a microporous layer is applied onto a conductive porous substrate 1 using a coating device 3. Next, in a pressing step 5, a pressure roll 6 presses the area where the coating liquid for forming a microporous layer is applied. The resulting product then passes through a drying step 4 and a heat treatment step 9 to form a gas diffusion layer in which a microporous layer is formed on the conductive porous substrate 1. Finally, the gas diffusion layer is trimmed in a slitting step 10 and wound into a roll in a winding step 11.

[0024] In the present invention, the 50 mm widthwise regions on both ends of the portion coated with the microporous layer-forming coating liquid (hereinafter also referred to as the coated portion) are referred to as both coated end portions, and the portion of the coated portion other than the both coated end portions is referred to as the coated center portion.

[0025] The conductive porous substrate 1 to be subjected to the coating step 2 is preferably subjected to a water-repellent treatment. Examples of water-repellent agents include fluororesins similar to those used in the microporous layer coating liquid, but PTFE or FEP, which exhibit strong water repellency, are preferred. For the water-repellent treatment, it is preferable to use a water-repellent dispersion in which the water-repellent agent is dispersed in water or another dispersing medium. The dispersant is preferably one that can be evaporated or decomposed and removed at a temperature lower than the melting point of the water-repellent agent.

[0026] The amount of water repellent applied in the water repellent treatment is not particularly limited, but is preferably 0.1 to 20 parts by mass per 100 parts by mass of the conductive porous substrate. If the amount is less than 0.1 part by mass, the water repellency may not be sufficient. If the amount is more than 20 parts by mass, the pores that serve as gas diffusion paths or drainage paths may be blocked, or electrical resistance may increase.

[0027] As a method of water-repellent treatment, in addition to a method of immersing the conductive porous substrate 1 in a water-repellent dispersion, a method of applying the water-repellent dispersion by die coating, spray coating, etc. After the water-repellent treatment, a drying step and further a heat treatment step may be added as necessary before the substrate is subjected to the application step 2. However, from the viewpoint of productivity, it is also preferable to perform these drying and heat treatment steps together in the drying step 4 and heat treatment step 9 after the pressing step 5.

[0028] As the coating device 3 in the coating step 2, various commercially available coating machines can be used. For example, coating machines capable of coating by methods such as screen printing, rotary screen printing, spray atomization, intaglio printing, gravure printing, die coater coating, bar coating, and blade coating can be used. Among these, a coating machine capable of die coater coating, i.e., a die coater, is preferred because it allows for quantification of the coating amount regardless of the surface roughness of the conductive porous substrate 1.

[0029] 2 is a schematic diagram showing the application of a microporous layer-forming coating liquid 20 using a die coater. When the microporous layer-forming coating liquid 20 is discharged from both widthwise ends of the die tip 12, the microporous layer-forming coating liquid 20 discharged flows into the spaces between both ends of the die tip 12 and the conductive porous substrate 1, weakening the force pressing it toward the conductive porous substrate 1. As a result, the amount of the microporous layer-forming coating liquid 20 that seeps into the conductive porous substrate 1 decreases near both ends of the die tip, resulting in a mounded state. This mounded microporous layer-forming coating liquid 21 is formed continuously in the transport direction. If no particular treatment such as pressing step 5 is performed, the raised microporous layer-forming coating liquid 21 may be transferred to pass roll 8 in FIG. 1 and deposited on pass roll 8, or only the end portions of the gas diffusion layer wound up in winding step 11 may become thicker, which may cause breakage of the conductive porous substrate 1 or the gas diffusion layer.

[0030] To prevent the above-described breakage, the production method of the present invention includes a pressing step 5. The pressing step 5 may be configured to include a pressing roll 6 on the surface on which the microporous layer-forming coating liquid is applied and a back roll 7 on the opposite surface, as shown in Figure 1.

[0031] FIG. 3 is a schematic diagram showing an example of pressing step 5. As shown in FIG. 3, pressing only both coated ends is one preferred embodiment. By pressing both coated ends of the portion of the conductive porous substrate 1 coated with the microporous layer-forming coating liquid 20 with end pressing rolls 61, the raised microporous layer-forming coating liquid 21 can be permeated into the conductive porous substrate 1. This makes the entire surface coated with the microporous layer-forming coating liquid 20 flat. Flattening the coated surface can prevent the conductive porous substrate 1 from breaking as described above. When pressing both coated ends, either all or part of the coated ends may be pressed. Specifically, it is preferable to press the portion where the raised microporous layer-forming coating liquid 21 is present.

[0032] When pressing step 5 is performed in the embodiment shown in Fig. 3, the face lengths of the two end pressing rolls 61 are preferably 20 to 60 mm. Pressing may be performed using multiple rolls as end pressing rolls 61, and in that case, the total face length of the rolls is preferably 20 to 60 mm for each of the two coating ends. By setting the length to 20 mm or more, the raised microporous layer-forming coating liquid 21 can be stably pressed even when the conductive porous substrate 1 meanders. By setting the length to 60 mm or less, it is possible to avoid pressing areas other than the raised microporous layer-forming coating liquid 21, and a stable, constant pressure can be applied to the raised microporous layer-forming coating liquid 21.

[0033] FIG. 4 is a schematic diagram showing another example of pressing step 5. As shown in FIG. 4, pressing both coated end portions and the coated center portion with a plurality of rolls is also a preferred embodiment. By pressing both coated end portions of the conductive porous substrate 1 with end pressing rolls 61 and pressing the coated center portion with a center pressing roll 62, the entire coated portion can be made flat, and breakage of the conductive porous substrate 1 as described above can be suppressed. When pressing the coated center portion, the entire coated center portion may be pressed, or only a portion of it may be pressed.

[0034] Furthermore, the end pressing roll 61 and the center pressing roll 62 do not have to be one roll each, and may be configured with a plurality of rolls.

[0035] The face length of the roll pressing both coating ends is preferably 20 to 60 mm. When multiple rolls are used as end pressing rolls 61 as described above, the total face length of the rolls is preferably 20 to 60 mm for each of both coating ends.

[0036] FIG. 5 is a schematic diagram showing another example of pressing step 5. As shown in FIG. 5, one preferred embodiment is to press the entire coated area with a single pressing roll composed of a roll center 63 and roll end portions 64. In this case, the roll center 63 and roll end portions 64 may have the same diameter, but the roll diameter of the roll end portions 64 is preferably 0.01 to 3 mm larger than the roll diameter of the roll center 63. By increasing the roll diameter by 0.01 mm or more, the linear pressure at the roll end portions 64 can be greater than that at the roll center 63, allowing the raised microporous layer-forming coating liquid 21 to properly penetrate. Furthermore, by not increasing the roll diameter by more than 3 mm, excessive depression of the coated end portions due to excessive pressure can be prevented, the entire coated area can be made flat, and the above-mentioned breakage of the conductive porous substrate 1 can be prevented.

[0037] The pressure roll 6 used in the pressing step 5 is preferably a releasable roll. Here, the pressure roll 6 being a releasable roll means that the microporous layer-forming coating liquid is less likely to adhere to the pressure roll 6 when pressed against it. Specifically, the contact angle of the pressure roll surface with water is preferably 60° or more, more preferably 70° or more. If the contact angle is less than 60°, the microporous layer-forming coating liquid may adhere to the pressure roll 6.

[0038] Here, the contact angle of the pressure roll surface with water can be determined by measuring using a contact angle meter (for example, a fully automatic contact angle meter manufactured by Kyowa Interface Science Co., Ltd.) in a room at a room temperature of 23°C and a humidity of 60%.

[0039] The means for making the pressing roll 6 a release roll include, for example, coating the surface of the pressing roll 6 with a fluororesin, a silicone resin, or the like, surface treatment with hard chrome plating having excellent abrasion resistance, or providing the surface of the pressing roll 6 with irregularities, or a combination of these means.

[0040] The linear pressure applied in pressing step 5 is preferably 0.00080 to 0.25 kgf / mm. By setting the linear pressure to 0.00080 kgf / mm or more, the raised microporous layer-forming coating liquid 21 can be appropriately impregnated into the conductive porous substrate 1. By setting the linear pressure to 0.25 kgf / mm or less, breakage of the conductive porous substrate 1 due to pressing pressure can be prevented, allowing stable transport. Linear pressure can be applied by the weight of the roll itself, or by applying an arbitrary load to both ends of the roll shaft (not shown) using a roll pressing device such as a cylinder or motor.

[0041] The back roll 7 in the pressing step 5 is a roll that supports the load from the pressing roll 6 and is driven to send out the substrate in the conveying direction. For the above purpose, the face length of the back roll 7 needs to be longer than the width of the conductive porous substrate 1. The back roll 7 is preferably a hard metal roll so as to withstand the pressure from the pressing roll 6. It is also preferable to mirror-finish the surface of the back roll 7 by hard chrome plating or the like.

[0042] The present invention includes a drying step 4 of removing the dispersion medium from the coating liquid for forming a microporous layer to form a microporous layer after the pressing step 5. The drying temperature is preferably a temperature at which the dispersion medium is removed but the surfactant is not, and specifically, is preferably 20°C or higher and 150°C or lower, more preferably 60°C or higher and 120°C or lower.

[0043] The present invention may include a heat treatment step 9 after the drying step 4, in which the surfactant in the microporous layer is removed and the water repellent agent is melted once to bind and sinter the conductive fine particles. In the heat treatment step 9, the heat treatment is performed at a temperature at which the surfactant can be removed, i.e., a temperature at which the surfactant can evaporate or thermally decompose to reduce its weight, and at a temperature above the melting point of the water repellent agent. To completely remove the surfactant by heat treatment in a short time, the heat treatment is preferably performed at a temperature of 290°C or higher, more preferably 300°C or higher. The upper limit of the heat treatment temperature is preferably below the decomposition temperature of the water repellent agent. When a fluororesin is used as the water repellent agent, the heat treatment is preferably performed at 400°C or lower, more preferably 350°C or lower.

[0044] Hot air is preferred as the heat source in the heat treatment step 9. After being heated by a heater or the like, the hot air is introduced into the heat treatment furnace through the air inlet, and heat is supplied to the substrate to be heated inside the furnace. The surfactant vapor and decomposition products generated by the heat treatment are discharged outside the heat treatment furnace through the exhaust port along with the hot air. To improve safety, it is preferable to use a so-called single-pass method, in which the hot air discharged out of the heat treatment furnace through the exhaust port is not returned to the heating section by a heater or the like.

[0045] Finally, the gas diffusion layer is wound up in winding step 11. When winding up in winding step 11, a general resin film or paper sheet may be wound together as a protective sheet to protect the microporous layer. Examples of resins used for the resin film include flexible materials such as polyethylene, polypropylene, polyethylene terephthalate (PET), polyimide, and polyvinyl chloride. Paper sheets can be of any type, including low-dust-generating general kraft paper, neutral kraft paper, dust-free paper, miscellaneous industrial paper, wrapping paper, and specialty paper.

[0046] Before the winding step 11, a slitting step 10 may be performed in which the edge portions including at least a part of both coated ends are trimmed and the film is slit to the product width as needed. The type of blade used for slitting in the slitting step 10 may be a shear cutting blade or a score cutting blade.

[0047] In the present invention, as one embodiment for efficiently producing a gas diffusion layer, a conductive porous substrate 1 as shown in FIG. 1 is preferably wound into a long roll and then continuously processed from unwinding to rewinding.

[0048] Furthermore, guide rolls may be appropriately arranged between the unwinding of the conductive porous substrate 1 and the heat treatment step 9 to facilitate smooth transport of the conductive porous substrate 1. The substrate is preferably transported by driving a winder. If necessary, the conductive porous substrate may be nipped to remove tension while being transported. Furthermore, a device such as a fan for cooling the substrate may be installed after the heat treatment step 9.

[0049] The gas diffusion layer produced by the production method of the present invention is pressure-bonded to both sides of an electrolyte membrane having catalyst layers provided on both sides so that the catalyst layer and the gas diffusion layer are in contact with each other, and further, components such as a separator are incorporated to assemble a unit cell for use as a fuel cell. When a gas diffusion layer having a microporous layer provided on only one side is used, it is preferable to assemble it so that the microporous layer and the catalyst layer are in contact with each other.

[0050] [Example] The present invention will be specifically described below with reference to examples.

[0051] <Whether or not the coating liquid is attached to the pass roll> When a long conductive porous substrate of 800 m was continuously processed into a gas diffusion layer, after the coating liquid for forming a microporous layer was applied, the presence or absence of the coating liquid on the pass roll that the coated surface came into contact with was first judged visually. If the substrate broke during the process, the length of conveyance up to the break was judged.

[0052] <Presence or absence of fracture> When a long conductive porous substrate of 800 m was continuously processed into a gas diffusion layer, the occurrence of breakage was judged by the presence or absence of breakage on the pass roll and the winder.

[0053] Example 1 [Preparation of conductive porous substrate] Toray Industries, Inc.'s PAN-based carbon fiber "TORAYCA (registered trademark)" T300 (average diameter: 7 μm) was cut to an average length of 12 mm, dispersed in water, and continuously made into a paper body by a wet papermaking method. Furthermore, a 10% by mass aqueous solution of polyvinyl alcohol was applied as a binder to the paper body and dried, resulting in a carbon fiber basis weight of 26 g / m. 2 The amount of polyvinyl alcohol attached was 18 parts by mass per 100 parts by mass of carbon fiber.

[0054] Next, a phenolic resin was prepared by mixing a mixture of resol-type phenolic resin and novolac-type phenolic resin so that the non-volatile content was in a 1:1 mass ratio, and then the mixture was mixed with flake graphite powder (average particle size 5 μm) as carbon powder and methanol as a solvent in a compounding ratio of phenolic resin (non-volatile content) / carbon powder / solvent = 10 parts by mass / 1 part by mass / 85 parts by mass to obtain a uniformly dispersed resin composition.

[0055] Next, the short carbon fiber sheet was continuously immersed in the resin composition and squeezed between rolls in a resin impregnation process, after which it was wound into a roll to obtain a precursor fiber sheet. The rolls were smooth metal rolls designed to allow excess resin composition to be removed with a doctor blade. Two rolls were placed horizontally with a certain clearance between them, and the short carbon fiber sheet was pulled up vertically to adjust the overall amount of resin composition deposited. The amount of phenolic resin (non-volatile content) deposited on the precursor fiber sheet was 130 parts by mass per 100 parts by mass of carbon fiber.

[0056] The hot plates were set parallel to each other in the press molding machine, and a spacer was placed on the lower hot plate to maintain a constant distance between the upper and lower hot plates. The distance between the hot plates was adjusted so that the precursor sheet after pressure treatment had a desired carbon short fiber density. The precursor fiber sheet was then sandwiched between release papers on the top and bottom, and heated and pressurized in the press molding machine while being transported intermittently, thereby pressurizing the precursor sheet.

[0057] The pressurized precursor fiber sheet was introduced into a heating furnace maintained in a nitrogen gas atmosphere at a maximum temperature of 2,400°C, and passed through a carbonization process in which the sheet was fired while continuously traveling through the heating furnace. The sheet was then wound into a roll to obtain a conductive porous substrate. The width of the conductive porous substrate was 550 mm.

[0058] [Preparation of coating liquid for forming microporous layer] A coating liquid for forming a microporous layer was prepared by kneading 10 parts by mass of acetylene black, 6 parts by mass of a PTFE particle dispersion "Polyflon (registered trademark)" D-210C (manufactured by Daikin Industries, Ltd.), 10 parts by mass of a surfactant "TRITON (registered trademark)" X-100 (manufactured by Nacalai Tesque, Inc.) with a planetary mixer.

[0059] [Preparation of water repellent] A water repellent agent was prepared by diluting a dispersion of PTFE particles, "Polyflon (registered trademark)" D-210C (manufactured by Daikin Industries, Ltd.), with water so that the PTFE content was 3% by mass.

[0060] [Preparation of gas diffusion layer] The conductive porous substrate wound into a roll with a width of 550 mm was set on an unwinder and unwound. The unwound conductive porous substrate was first passed through an impregnation tank filled with the water repellent prepared according to the above [Preparation of water repellent] to impregnate the conductive porous substrate with PTFE for water repellency treatment, and then dried in a dryer set at 100°C. Next, in the coating step, the microporous layer-forming coating liquid prepared according to the above [Preparation of microporous layer-forming coating liquid] was applied to the center of the conductive porous substrate using a die coater with a coating width of 510 mm. At this time, the basis weight of the microporous layer after sintering was 25 g / m 2 The coating amount was adjusted so that the coating amount was 0.0020 kgf / mm. Next, in the pressing step, pressing was performed in the manner shown in FIG. 3. Specifically, two release rolls with a face length of 60 mm and a diameter of 70 mm and coated with a fluororesin were used as end pressing rolls 61, with 50 mm corresponding to both coating ends and 10 mm corresponding to the area not coated with the microporous layer-forming coating liquid. After the pressing step, the film was transported to the drying step, during which it passed through a pass roll so that the surfaces coated with the microporous layer-forming coating liquid were in contact. In the drying step, water was removed with hot air at 140°C, and sintering was performed in a sintering machine set at a temperature of 350°C, and a gas diffusion layer was obtained that was continuously wound into a roll by a winder.

[0061] A total of 800 m of gas diffusion layers were produced using the above method, and it was visually confirmed that the microporous layer-forming coating liquid did not adhere to the pass roll during transport, and that the conductive porous substrate did not break during transport.In addition, there was no breakage of the gas diffusion layer on the winder.

[0062] Example 2 A gas diffusion layer was obtained in the same manner as in Example 1, except that in the pressing step, the entire surface of the coated area was pressed with a single release roll having a face length of 550 mm and a roll surface coated with a fluororesin. The results are shown in Table 1.

[0063] Example 3 In the pressing step, pressing was performed in the manner shown in FIG. 4. Specifically, two release rolls with a face length of 50 mm and a diameter of 62 mm, whose roll surfaces were coated with a fluororesin, were used as end pressing rolls 61. They were pressed at a linear pressure of 0.0030 kgf / mm, with 30 mm of each roll corresponding to both coating ends and 20 mm of each roll corresponding to the portion not coated with the microporous layer-forming coating liquid. Furthermore, as center pressing roll 62, one release roll with a face length of 410 mm and a diameter of 60 mm, coated in the same manner as the release roll, pressed the center of the coating at a linear pressure of 0.0010 kgf / mm. The three rolls were spaced 20 mm apart. A gas diffusion layer was obtained in the same manner as in Example 1, except as described above. The results are shown in Table 1.

[0064] Example 4 In the pressing step, pressing was performed in the manner shown in Figure 5. Specifically, pressing was performed using one pressing roll having a roll center 63 with a face length of 410 mm and a diameter of 60 mm, and roll end portions 64 with a face length of 60 mm and a diameter of 62 mm. At this time, the pressing roll and the center of the coating portion were aligned in the width direction. The linear pressure was set so that the linear pressure at both roll end portions 64 was 0.0020 kgf / mm. A gas diffusion layer was obtained in the same manner as in Example 1 except for the above. The results are shown in Table 1.

[0065] Example 5 A gas diffusion layer was obtained in the same manner as in Example 1, except that in the pressing step, a non-releasable rubber roll was used as the pressing roll. After the start of processing, adhesion of the microporous layer-forming coating liquid to the pass roll that first came into contact with the coating surface was gradually observed, but an 800 m gas diffusion layer was produced without breakage.

[0066] (Comparative Example 1) A gas diffusion layer was produced in the same manner as in Example 1, except that in the pressing step, pressing was not performed with a pressing roll.

[0067] Immediately after the start of processing, adhesion of the coating liquid for forming a microporous layer to the pass roll after the coating step was observed, and when a total of 300 m of gas diffusion layer had been produced, excessive tension was applied to the conductive porous substrate at a location where the coating liquid for forming a microporous layer that had adhered to the pass roll during transportation had accumulated to a large extent, causing the conductive porous substrate to break.

[0068] (Comparative Example 2) A gas diffusion layer was produced in the same manner as in Example 1, except that in the pressing step, the pressing roll was changed to a 50 mm-wide scraper made of reinforced polyester, the scraper was brought into contact with both coated ends 30 mm apart and a 20 mm apart from the portion where the microporous layer-forming coating liquid was not applied at a linear pressure of 0.0020 kgf / mm, and the raised microporous layer-forming coating liquid was scraped off.

[0069] Adhesion of the microporous layer-forming coating liquid to the pass roll was observed immediately after the start of processing, as in Comparative Example 1. When a total of 600 m of gas diffusion layer had been produced, the thickness of both ends of the gas diffusion layer wound into a roll became excessive, and the gas diffusion layer broke on the winder.

[0070] (Comparative Example 3) A gas diffusion layer was produced in the same manner as in Example 1, except that in the pressing step, instead of a pressing roll, a resin sponge 50 mm wide, 30 mm thick, and 70 mm long was used, and pressed at a linear pressure of 0.0010 kgf / mm so as to contact the sponge over 30 mm of both coated ends and 20 mm of the portion not coated with the microporous layer-forming coating liquid.

[0071] As in Comparative Examples 1 and 2, adhesion of the coating liquid for forming a microporous layer to the pass roll was observed immediately after the start of processing, and the gas diffusion layer broke on the winder when a total of 400 m of gas diffusion layer had been produced, as in Comparative Example 2.

[0072] [Table 1] [Explanation of symbols]

[0073] 1 Conductive porous substrate 2 Application process 3 Coating equipment 4 Drying process 5 Pressing process 6 Pressing roll 7 Back Roll 8-pass roll 9. Heat treatment process 10 Slitting process 11 Winding process 12 Die tip 20 Coating liquid for forming microporous layer 21. Coating liquid for forming raised microporous layer 61 End pressure roll 62 Center pressure roll 63 Center of roll 64 Both ends of the roll [Industrial Applicability]

[0074] According to the present invention, it becomes possible to mass-produce with high productivity gas diffusion layers that have good power generation performance when used as gas diffusion electrodes for fuel cells.

Claims

1. A method for producing a gas diffusion layer in which a microporous layer is formed on at least one surface of a conductive porous substrate, the method comprising the steps of: a coating step (1) of coating a microporous layer-forming coating liquid containing at least conductive fine particles and a dispersion medium onto the conductive porous substrate; a pressing step (2) of pressing at least both coating ends of the portion coated with the microporous layer-forming coating liquid using a pressing roll; and a drying step (3) of drying the microporous layer-forming coating liquid coated on the conductive porous substrate to form a microporous layer.

2. The method for producing a gas diffusion layer according to claim 1, wherein the pressure roll is a release roll.

3. The method for producing a gas diffusion layer according to claim 1, wherein in the pressing step (2), only both ends of the coating are pressed with a pressing roll.

4. 2. The method for producing a gas diffusion layer according to claim 1, wherein in the pressing step (2), the central portion and both ends of the coating are pressed using one pressing roll or a plurality of pressing rolls.

5. 4. The method for producing a gas diffusion layer according to claim 3, wherein in the pressing step (2), both ends of the coating are pressed using a pressing roll having a face length of 20 to 60 mm.

6. 5. The method for producing a gas diffusion layer according to claim 4, wherein the pressing step (2) uses a roll having different diameters at both ends and the center, and the face lengths at both ends are 20 to 60 mm.

7. 5. The method for producing a gas diffusion layer according to claim 4, wherein in the pressing step (2), the center of the coating is pressed using at least one roll, and both end portions of the coating are pressed using rolls each having a face length of 20 to 60 mm.

8. The method for producing a gas diffusion layer according to claim 1, further comprising a slitting step (4) of trimming at least a part of both coated ends after the pressing step (2) or the drying step (3).

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