Water-repellent conductive porous substrate and method for manufacturing the same, and gas diffusion electrode and method for manufacturing the same

Abstract

To provide a gas diffusion electrode in which water easily flows in a constant direction with respect to the thickness direction and drainage performance is improved by providing a water-repellent conductive porous base material with a high fluorine deposition ratio between the front and back surfaces.SOLUTION: A method for manufacturing a water-repellent conductive porous base material includes a lamination step (A) of laminating a pair of conductive porous substrates, an adhesion step (B) of adhering a dispersion containing fluororesin to the pair of conductive porous substrates, a drying step (C) of drying the pair of conductive porous substrates after the lamination step and the adhesion step, and a peeling step (D) of peeling off the pair of conductive porous substrates to obtain a pair of water-repellent conductive porous substrates after the drying step.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for producing a water-repellent conductive porous substrate suitably used for a fuel cell, particularly a solid polymer fuel cell, and a gas diffusion electrode made of the water-repellent conductive porous substrate.

Background Art

[0002] A solid polymer fuel cell that obtains an electromotive force by supplying a fuel gas containing hydrogen to an anode and an oxidizing gas containing oxygen to a cathode and causing an electrochemical reaction to occur at both electrodes generally includes a separator, a gas diffusion electrode, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode, and a separator stacked in this order. 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. A gas diffusion electrode in which a microporous layer is formed on the surface of a conductive porous substrate made of carbon fiber or the like is widely used.

[0003] However, as a problem of such a gas diffusion electrode, for example, when the solid polymer fuel cell is operated at a relatively low temperature below 70°C and in a high current density region, the pores existing inside the gas diffusion electrode are blocked by a large amount of generated water, preventing the supply of gas, and thus the power generation performance deteriorates (flooding). Therefore, higher water drainage is required for the gas diffusion electrode.

[0004] On the other hand, a method is known in which a water-repellent conductive porous substrate is obtained by attaching a fluororesin having high water repellency to the conductive porous substrate, making it easier to discharge the generated water to the outside. Specifically, the conductive porous substrate is immersed in a liquid in which the fluororesin is dispersed to impregnate the fluororesin, and then dried to attach the fluororesin to the conductive porous substrate. However, in this method, since there is no directionality in the distribution of the fluororesin inside the gas diffusion electrode, it was insufficient to obtain sufficient water drainage for promoting the discharge of water from the catalyst layer side to the separator side.

[0005] Therefore, one method is to create a gradient in the amount of fluororesin deposited in the thickness direction within the conductive porous substrate, thereby facilitating the flow of water in a specific direction relative to the thickness. For example, a method has been proposed in which a conductive porous substrate impregnated with a fluororesin dispersion is placed on a rigid, gas-impermeable substrate surface, such as a glass plate, and dried to create a gradient in the distribution of fluororesin (see Patent Document 1). With this method, the solvent can be evaporated from only one side, causing a unidirectional flow of the solvent within the conductive porous substrate, and consequently, the fluororesin also moves to the same side, creating a gradient distribution. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2006 / 025908 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, in the method described in Patent Document 1, by covering one side of the conductive porous substrate with a non-porous member, the non-porous member is heated by the heat generated during drying, and drying also progresses from the side of the conductive porous substrate that is in contact with the non-porous member. As a result, fluororesin adheres to that side as well, and it was sometimes not possible to obtain a water-repellent conductive porous substrate with a large ratio of fluorine adhesion between the front and back sides.

[0008] The present invention aims to provide a method for manufacturing a water-repellent conductive porous substrate in which a fluororesin is attached to the conductive porous substrate in a water-repellent step, such that the ratio of fluorine attached to the front and back surfaces of the conductive porous substrate becomes larger. [Means for solving the problem]

[0009] As a result of diligent research to solve the above problems, the inventors have found that by laminating conductive porous substrates, the solvent moves toward the surface of the conductive porous substrates during the drying process, and a large amount of fluororesin adheres to the surfaces where the substrates are not in contact with each other.

[0010] The present invention, which solves the above problems, is a method for manufacturing a water-repellent conductive porous substrate, comprising the following steps (A) to (D). (A) Lamination process of stacking a pair of conductive porous substrates (B) Adhesion process: A dispersion containing fluororesin is attached to a pair of conductive porous substrates. (C) A drying step in which the pair of conductive porous substrates are dried after the lamination step and the adhesion step. (D) A peeling step after the drying step, in which a pair of conductive porous substrates are peeled off to obtain a pair of water-repellent conductive porous substrates. [Effects of the Invention]

[0011] By using the method for producing a water-repellent conductive porous substrate of the present invention, a water-repellent conductive porous substrate can be obtained in which the ratio of fluorine deposition on the front and back surfaces is large, and water flows easily in a certain direction relative to the thickness direction, i.e., the drainage properties are improved. [Modes for carrying out the invention]

[0012] The present invention provides a method for manufacturing a gas diffusion electrode, comprising: (A) a lamination step of laminating a pair of conductive porous substrates; (B) an adhesion step of attaching a dispersion containing a fluororesin to the pair of conductive porous substrates; (C) a drying step of drying the pair of conductive porous substrates after the lamination step and the adhesion step; and (D) a peeling step of peeling off the pair of conductive porous substrates after the drying step to obtain a pair of water-repellent conductive porous substrates.

[0013] The lamination process in this invention will now be described. Methods for lamination include laminating two conductive porous substrates on a flat plate, or unwinding conductive porous substrates from two roll-shaped conductive porous substrates and overlapping them using guide rolls or the like.

[0014] Next, the adhesion process will be described. In the adhesion process, it is preferable to allow a dispersion liquid containing fluororesin to penetrate into the interior of the conductive porous substrate and to adhere the fluororesin to the interior of the conductive porous substrate. For example, one method is to immerse the conductive porous substrate in a tank containing a dispersion liquid containing fluororesin. Alternatively, a coating method is used in which the dispersion liquid containing fluororesin is discharged from a slit die onto the laminated conductive porous substrate. Here, the adhesion process may be performed before or after the above-mentioned lamination process.

[0015] Examples of fluororesins used for adhesion include PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), ETFE (tetrafluoroethylene-ethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride).

[0016] Generally, fluororesins are insoluble in water and organic solvents, so when manufacturing dispersions for water-repellent treatment, it is preferable to use a dispersion of fluororesin processed into fine particles. Examples of dispersions of fine particles of fluororesin include "Polyflon®" D-210C, ND-110 (both manufactured by Daikin Industries, Ltd.), 120-JRB, and 31-JR (both manufactured by Mitsui Chemours Fluoroproducts Co., Ltd.).

[0017] The amount of fluororesin to be attached to the conductive porous substrate is not particularly limited, but it is preferably 0.1 to 20 parts by mass, based on 100 parts by mass of the entire conductive porous substrate. Within this range, sufficient water repellency is achieved while suppressing the fluororesin from blocking pores that serve as gas diffusion pathways or increasing electrical resistance.

[0018] Next, the drying process will be described. This process requires being carried out after the lamination process and the adhesion process. That is, in this process, the dispersion liquid containing the fluororesin is dried while adhering to and laminated on the conductive porous substrate. Examples of the drying method include putting it into a hot air dryer or a blowing type constant temperature dryer for drying, or drying by applying an IR heater or a dryer from above.

[0019] Also, the drying temperature is preferably 100 to 200 °C. When the drying temperature is 100 °C or higher, the movement of the solvent and the fluororesin is fast, and the ratio of the fluorine adhesion amount on the front and back surfaces of the conductive porous substrate becomes large. On the other hand, when the drying temperature is 200 °C or lower, the temperature unevenness in the plane of the conductive porous substrate becomes small, so the fluororesin distribution in the plane becomes uniform.

[0020] In the above drying process, the fluororesin adhering to the conductive porous substrate tends to move to the front and back surfaces of the conductive porous substrate along with the movement of the solvent volatilizing from the surface. At this time, when the fluororesin is adhered and dried only on, for example, one conductive porous substrate without using the manufacturing method of the present invention, the fluororesin moves to both the front and back surfaces along with the movement of the solvent volatilizing from the front and back surfaces of the conductive porous substrate, so the ratio of the fluorine adhesion amount on the front and back surfaces becomes small, and it becomes a water-repellent conductive porous substrate that cannot promote drainage in one direction. In contrast, in the present invention, the solvent volatilizes only from the surface where the conductive porous substrates are not in contact with each other. As a result, the fluororesin also moves to the same surface as above, and the fluorine adhesion amount increases only on the same surface as above. That is, in the conductive porous substrate after peeling, the ratio of the fluorine adhesion amount on the front and back surfaces becomes large. Therefore, the drainage property of the generated water is enhanced, so the fuel cell using this is suppressed from flooding and the performance is improved.

[0021] Next, the peeling process will be described. This process needs to be carried out after the above drying process. The method of peeling is not particularly limited. For example, there are methods such as gripping and peeling the conductive porous substrate with fingers, pasting and peeling a tape on the conductive porous substrate, and using a sharp object such as tweezers to insert the tip into the contact surface and peel. Also, it is preferably carried out while peeling along a roll-shaped member (guide roll) so that the substrate does not break during peeling.

[0022] As the conductive porous substrate in the present invention, porous substrates containing carbon fibers such as carbon fiber woven fabrics, carbon fiber papers, carbon fiber non-woven fabrics, carbon felts, carbon papers, and carbon cloths are preferably used. Among them, carbon paper is most preferable in terms of mechanical strength and the like. The above conductive porous substrate is composed of carbon fibers and a binder.

[0023] Examples of the above carbon fibers include polyacrylonitrile (PAN)-based, pitch-based, 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. Also, natural fibers and synthetic fibers such as rayon fibers, acrylic fibers, and cellulose fibers may be mixed.

[0024] <00001​​​​​​​​In polymer electrolyte fuel cells, the gas diffusion electrode requires high gas diffusivity to diffuse the gas supplied from the separator to the catalyst, and high drainage capacity to discharge the water generated by the electrochemical reaction back to the separator. For this reason, the conductive porous substrate preferably has a pore diameter peak in the range of 10 to 100 μm. The pore diameter and its distribution can be determined by measuring the pore diameter distribution using a mercury porosimeter.

[0027] Furthermore, it is preferable that the porosity of the conductive porous substrate be 80-95%. A porosity of 80% or more increases gas diffusivity and improves power generation performance. On the other hand, a porosity of 95% or less increases the mechanical strength of the conductive porous substrate and also improves conductivity. A porosity of 85-90% for the conductive porous substrate is more preferable as these effects are enhanced. The porosity of the conductive porous substrate can be measured using a hydrometer or the like.

[0028] The thickness of the conductive porous substrate of the present invention is preferably 90 to 180 μm. Here, the thickness of the conductive porous substrate is the thickness when both surfaces are sandwiched under a pressure of 0.15 MPa. If the thickness is 90 μm or more, mechanical strength is maintained. On the other hand, if the thickness is 180 μm or less, the resistance of the gas diffusion electrode manufactured using the conductive porous substrate is reduced and the gas diffusion in the direction perpendicular to the surface is improved.

[0029] The ratio of fluorine adhesion amounts on the front and back surfaces of the water-repellent conductive porous substrate of the present invention is 100 or more. When the ratio of fluorine adhesion amounts on the front and back surfaces is 100 or more, water flows more easily in one direction in the thickness direction within the gas diffusion electrode, improving the drainage performance of the gas diffusion electrode. Furthermore, by creating a gradient in the amount of fluororesin adhesion in the thickness direction within the conductive porous substrate, water flows even more easily in a constant direction relative to the thickness direction. It is particularly preferable that the gradient is such that the fluorine strength monotonically decreases from the surface with a large amount of fluorine adhesion to the surface with a small amount of fluorine adhesion.

[0030] In conventional manufacturing methods, as described above, the fluororesin tends to move between the front and back surfaces, so there is little difference in the amount of fluorine deposited on each surface, and therefore the fluorine deposition ratio is close to 1. Furthermore, in a method such as Patent Document 1, where one side of a conductive porous substrate is covered with a non-porous member, the fluororesin tends to tilt within the conductive porous substrate, and because it is heated by a heater during the drying process, the non-porous member is also heated, and drying progresses from the interface between the non-porous member and the conductive porous substrate. As a result, the fluororesin moves to the surface of the conductive porous substrate that is in contact with the non-porous member, increasing the amount of fluororesin deposited, and the fluorine deposition ratio between the front and back surfaces tends to decrease, making it difficult to achieve a fluorine deposition ratio of 100 or more.

[0031] In the present invention, the ratio of fluorine adhesion amounts on the front and back surfaces of a water-repellent conductive porous substrate is determined by measuring the fluorine adhesion amounts (F / C ratio) on two surfaces of the water-repellent conductive porous substrate and dividing the larger fluorine adhesion amount by the smaller fluorine adhesion amount.

[0032] The F / C ratio is determined as follows: First, a scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDX) system is used to scan a 600 μm × 400 μm area of ​​the surface of the water-repellent conductive porous substrate to measure the mass percentages of fluorine (F) and carbon (C). The measurement conditions are an acceleration voltage of 7 kV and a magnification of 200x. From the obtained measurements, the F / C ratio is calculated by dividing the mass percentage of F by the mass percentage of C.

[0033] Furthermore, the gradient of the fluorine deposition amount (F / C ratio) in the thickness direction of a water-repellent conductive porous substrate can be measured as follows. First, with the side with the highest fluorine deposition facing upwards, the substrate is cut with a sharp blade to randomly prepare 50 samples for cross-sectional observation perpendicular to the surface of the water-repellent conductive porous substrate. A scanning electron microscope (SEM)-energy-dispersive X-ray spectroscopy (EDX) system is used to perform a line scan perpendicular to the surface of these 50 cross-sections of the water-repellent conductive porous substrate, and the mass percentages of fluorine (F) and carbon (C) are measured. The measurement conditions are an acceleration voltage of 7kV, a magnification of 300x, and a line width of 20μm. From the obtained measurements, the degree of the gradient in the thickness direction can be measured by dividing the mass percentage of F by the mass percentage of C to determine the F / C ratio along the thickness direction.

[0034] Next, an example of a method for producing the water-repellent conductive porous substrate of the present invention will be described in detail, but the present invention is not limited to the following description.

[0035] The manufacturing method of the present invention is carried out, for example, by the following procedure. (1) Laminate the first conductive porous substrate and the second conductive porous substrate. (2) A dispersion containing fluororesin is applied to the laminated conductive porous substrate. (3) Dry the conductive porous substrate to which the dispersion containing fluororesin has been applied. (4) Separate the first conductive porous substrate from the second conductive porous substrate.

[0036] Here, since the migration of the fluororesin occurs during drying, it is not necessary to laminate the conductive porous substrate until the drying process is carried out. In other words, the order of steps (1) and (2) may be reversed.

[0037] First, we will describe a method for manufacturing a water-repellent conductive porous substrate using a sheet-shaped conductive porous substrate.

[0038] (1) Laminate a first conductive porous substrate that has not been treated with a water-repellent coating and a second conductive porous substrate. Here, the surfaces on which the first conductive porous substrate and the second conductive porous substrate come into contact are the surfaces on which the amount of fluororesin adhesion should be minimized.

[0039] During or after lamination, if the surfaces of the conductive porous substrates slide sideways against each other, there is a possibility of damaging the surfaces. Therefore, when laminating, it is preferable to first align the first and second conductive porous substrates at one end before laminating the entire structure. Lamination may be done without load, or with load applied via rollers. After lamination, it is also preferable to clamp the ends with clips or the like to prevent them from shifting positions.

[0040] (2) After laminating two conductive porous substrates, a dispersion containing fluororesin is applied. The dispersion containing fluororesin can be prepared by diluting the dispersion of fluororesin processed into fine particles with a solvent such as water. Here, the concentration of the dispersion containing fluororesin is adjusted as appropriate so that a predetermined amount of fluororesin adheres to the conductive porous substrate after drying. Methods of application include immersing the conductive porous substrate in the dispersion containing fluororesin, as well as applying the fluororesin to the conductive porous substrate by die coating, spray coating, etc.

[0041] (3) Next, the conductive porous substrate is dried. The drying temperature is preferably 100 to 200°C. If the drying temperature is 100°C or higher, the movement of the solvent and fluororesin is faster, and the ratio of fluorine adhesion on the front and back surfaces of the conductive porous substrate becomes larger. On the other hand, if the drying temperature is 200°C or lower, the temperature unevenness within the surface of the conductive porous substrate becomes smaller, resulting in a uniform distribution of fluororesin within the surface. Furthermore, during drying, it is preferable to grip the edges of the laminated conductive porous substrates with appropriate tension and suspend them in mid-air so that the solvent evaporates uniformly from the surfaces of the laminated conductive porous substrates that are not in contact with each other. One such method involves using a jig with clips attached to the four corners of a rectangular metal frame, and gripping the laminated conductive porous substrates, after the dispersion containing fluororesin has been applied, at four points with the clips for drying. Examples of drying methods include hot air dryers and infrared heaters.

[0042] (4) After drying, the first conductive porous substrate and the second conductive porous substrate are separated to obtain two water-repellent conductive porous substrates. For the separation method, each conductive porous substrate is grasped at its end and slowly separated so as not to break or tear. Separation may also be initiated by attaching adhesive tape to both surfaces of the end where separation is to begin and pulling.

[0043] Furthermore, applying the method described above to a long, roll-shaped conductive porous substrate is preferable because it allows for continuous processing, thereby increasing productivity. An example of a method for manufacturing a roll-shaped water-repellent conductive porous substrate is described below.

[0044] (1) Each roll of conductive porous substrate is unwound from the two unwinding shafts, and the two conductive porous substrates are transported by guide rolls so that they overlap and are stacked.

[0045] (2) Two stacked conductive porous substrates are introduced into an impregnation mechanism and impregnated with a dispersion containing fluororesin. Examples of impregnation mechanisms include an impregnation tank containing a dispersion containing fluororesin and a slit die coater for applying the dispersion containing fluororesin. The concentration of the dispersion containing fluororesin is adjusted as appropriate so that a predetermined amount of fluororesin adheres to the conductive porous substrate after drying.

[0046] (3) The conductive porous substrate to which the dispersion liquid containing fluororesin has been attached is introduced into a drying mechanism and dried. A floating dryer is preferred as the drying mechanism. By applying air from above and below, it is possible to prevent the two laminated conductive porous substrates from separating inside the dryer. The drying temperature is preferably 100 to 200°C, as described above.

[0047] (4) The two dried conductive porous substrates are separated by winding them onto two winding shafts positioned above and below the guide roll.

[0048] The water-repellent conductive porous substrate of the present invention may be used as is as a gas diffusion electrode, or a microporous layer may be formed on one side before being used as a gas diffusion electrode.

[0049] The microporous layer in this invention is a layer containing conductive particles and a water-repellent resin, which can improve the conductivity and drainage of the gas diffusion electrode. The microporous layer is preferably formed on the surface that is placed on the catalyst layer side when used as a gas diffusion electrode. Furthermore, it is preferable that the microporous layer is formed on the side of the water-repellent conductive porous substrate that has a large amount of fluorine attached, as this allows the generated water to flow quickly from the catalyst layer side to the separator side, thereby improving drainage.

[0050] Conductive particles such as carbon black, carbon nanofibers, carbon nanotubes, graphene, and carbon fibers can be used. Among these, inexpensive carbon black is preferred.

[0051] As the water-repellent resin contained in the above-mentioned microporous layer, a fluororesin having a fluoroalkyl chain is preferred in terms of chemical stability and water repellency. Similar to the fluororesins suitably used when treating the conductive porous substrate with water repellency, examples include PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer), ETFE (tetrafluoroethylene-ethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride).

[0052] The basis weight of the microporous layer is 10-35 g / m². 2 Preferably, the basis weight of the microporous layer is 10 g / m². 2 With the above configuration, the carbon fibers protruding from the surface of the conductive porous substrate are sufficiently covered by the microporous layer, thus preventing the carbon fibers from damaging the electrolyte membrane. Furthermore, this is preferable because it reduces the contact resistance between the gas diffusion electrode and the catalyst layer and prevents the electrolyte membrane from drying out. The basis weight of the microporous layer is 35 g / m². 2 The following conditions are preferable because they result in good drainage.

[0053] The thickness of the gas diffusion electrode of the present invention is preferably 90 to 200 μm. Here, the thickness of the gas diffusion electrode is the thickness when both surfaces are sandwiched under a pressure of 0.15 MPa. If the thickness of the gas diffusion electrode is 90 μm or more, mechanical strength is maintained, making handling in the manufacturing process easier. On the other hand, if the thickness of the gas diffusion electrode is 200 μm or less, gas diffusivity is increased and electrical resistance is reduced, thus improving the power generation performance of the fuel cell. The thickness of the gas diffusion electrode can be adjusted by appropriately adjusting the thickness of the conductive porous substrate and the microporous layer.

[0054] A microporous layer can be formed by applying a coating solution, in which conductive particles and a water-repellent resin are dispersed in a dispersion medium such as water or alcohol, onto a conductive porous substrate and then heat-treating it. Adding a dispersant and a thickener during the preparation of the coating solution is preferable because it increases the dispersion stability of the conductive particles and water-repellent resin in the solvent. Nonionic surfactants are preferred as dispersants because they contain few metal components, and examples include polyoxyethylene octylphenyl ether-based "Triton®" X-100 (manufactured by Nacalai Tesque Co., Ltd.). In addition, adding a thickener is effective in maintaining the viscosity of the coating solution. Suitable thickeners include methylcellulose-based, polyethylene glycol-based, and polyvinyl alcohol-based thickeners.

[0055] A mixture of the components listed above can be kneaded using a homogenizer, planetary mixer, ultrasonic disperser, etc., to obtain a coating liquid for forming a microporous layer.

[0056] The application of coating liquids for forming microporous layers to water-repellent conductive porous substrates can be performed using various commercially available coating devices. Coating methods include screen printing, rotary screen printing, intaglio printing, gravure printing, spray coating, die coating, bar coating, blade coating, and roll knife coating.

[0057] After applying a microporous layer to a water-repellent conductive porous substrate, the substrate is dried at a temperature of 60-150°C to remove the dispersion medium in the coating solution. Then, the conductive particles and water-repellent resin are sintered to promote the decomposition and removal of additives such as dispersants and thickeners, and the melting of the water-repellent resin. At this time, not only the water-repellent resin in the microporous layer but also the fluororesin attached to the water-repellent conductive porous substrate melts, and these melt and spread to wet the surfaces of other constituent materials such as carbon fibers, resin carbides, and conductive particles. As a result, the drainage of the water-repellent conductive porous substrate and the microporous layer is improved, and the performance of the fuel cell using it is improved. In this way, a gas diffusion electrode with a microporous layer formed on a water-repellent conductive porous substrate is obtained.

[0058] A membrane electrode assembly can be formed by bonding the above-mentioned gas diffusion electrode to at least one side of a solid polymer electrolyte membrane having catalyst layers on both sides. In this case, by arranging the microporous layer of the gas diffusion electrode substrate on the catalyst layer side, back diffusion of the generated water becomes easier, and the contact area between the catalyst layer and the gas diffusion electrode is increased, thereby reducing the contact electrical resistance.

[0059] The fuel cell of the present invention includes the gas diffusion electrode of the present invention, that is, it has separators on both sides of the membrane electrode assembly described above. In other words, the fuel cell is constructed by arranging separators on both sides of the membrane electrode assembly described above. Typically, a polymer electrolyte fuel cell is constructed by stacking multiple such membrane electrode assemblies, each sandwiched between separators via gaskets. The catalyst layer consists of a layer containing a polymer electrolyte and catalyst-supported carbon. Platinum is usually used as the catalyst. In fuel cells where a reformed gas containing carbon monoxide is supplied to the anode side, it is preferable to use platinum and ruthenium as the catalyst on the anode side. It is preferable to use a perfluorosulfonic acid-based polymer material with high proton conductivity, oxidation resistance, and heat resistance as the polymer electrolyte. Such fuel cell units and the configuration of fuel cells themselves are well known. [Examples]

[0060] The present invention will be specifically described below with reference to examples, but the present invention is not limited to the following examples.

[0061] <Method for evaluating the ratio of fluorine adhesion on the front and back surfaces of a water-repellent conductive porous substrate> The ratio of fluorine deposition on the front and back surfaces of a water-repellent conductive porous substrate was calculated by measuring the amount of fluorine deposition (F / C ratio) on two surfaces of the water-repellent conductive porous substrate and dividing the larger amount of fluorine deposition by the smaller amount of fluorine deposition.

[0062] The F / C ratio was determined as follows. First, a scanning electron microscope (SEM)-energy dispersive X-ray spectroscopy (EDX) system was used to scan a 600 μm × 400 μm area of ​​the surface of the water-repellent conductive porous substrate to be measured, and the mass percentages of fluorine (F) and carbon (C) were measured. The measurement conditions were an acceleration voltage of 7 kV and a magnification of 200x. From the obtained measurements, the F / C ratio was calculated by dividing the mass percentage of F by the mass percentage of C. This was done for both surfaces of the water-repellent conductive porous substrate, and the value obtained by dividing the larger F / C ratio by the smaller F / C ratio was defined as the fluorine deposition ratio.

[0063] <Fabrication of conductive porous substrates> Toray Industries, Inc.'s polyacrylonitrile-based carbon fiber "Torayca®" T300, with a carbon fiber diameter of 7 μm and a carbon fiber length of 12 mm, was dispersed in water and continuously produced using a wet papermaking method. Furthermore, a 10% by mass aqueous solution of polyvinyl alcohol was applied to the paper as a binder and dried, resulting in a carbon fiber basis weight of 30 g / m². 2 A carbon fiber paper was prepared. The amount of polyvinyl alcohol attached was 20 parts by mass per 100 parts by mass of the carbon fiber paper.

[0064] Next, a resin was prepared by mixing a resol-type phenolic resin and a novolac-type phenolic resin in a 1:1 mass ratio as the thermosetting resin, along with flake graphite (average particle size 5 μm) as the carbon powder and methanol as the solvent. These were mixed in a ratio of thermosetting resin / carbon powder / solvent = 15 parts by mass / 5 parts by mass / 80 parts by mass, and the mixture was stirred for 1 minute using an ultrasonic dispersion device to obtain a uniformly dispersed resin composition solution.

[0065] Next, a carbon fiber paper mass cut to 30 cm x 30 cm was horizontally immersed in a resin composition solution filled with an aluminum tray. Then, it was squeezed between two horizontally positioned rolls so that the total amount of thermosetting resin and flake graphite attached to 100 parts by mass of the paper mass was 130 parts by mass. At this time, the amount of resin component attached to the carbon fiber paper mass was adjusted by changing the clearance between the two horizontally positioned rolls. After impregnating the carbon fiber paper mass with the resin composition solution, it was heated at 100°C for 5 minutes to dry and produce a pre-impregnated body. Next, the material was heat-treated at 180°C for 5 minutes while being pressed with a flat plate press. Spacers were placed in the flat plate press during the pressing process to adjust the distance between the upper and lower press plates.

[0066] The heat-treated pre-impregnated material was introduced into a 2400°C heating furnace maintained in a nitrogen gas atmosphere, resulting in a thickness of 150 μm and a basis weight of 50 g / m². 2 A conductive porous substrate was obtained.

[0067] (Example 1) First, PTFE dispersion (Daikin Industries' "Polyflon®" D210-C) was diluted in a stainless steel tray to an appropriate concentration such that 5 parts by mass of fluororesin were added to 95 parts by mass of conductive porous substrate after drying. Two conductive porous substrates obtained in the above <Preparation of Conductive Porous Substrates> were then stacked in the solution and immersed to adhere the dispersion containing fluororesin to the two conductive porous substrates. Subsequently, using a jig with clips attached to the four corners of a rectangular metal frame, the stacked conductive porous substrates with the fluororesin-containing dispersion were gripped at four points with the clips and dried in a dryer at 100°C for 5 minutes. After that, the ends of each conductive porous substrate were grasped with fingers and slowly peeled off to produce water-repellent conductive porous substrates.

[0068] Table 1 shows the results of evaluating the ratio of fluorine adhesion on the front and back surfaces of the obtained water-repellent conductive porous substrates according to the <Method for Evaluating the Ratio of Fluorine Adhesion on the Front and Back Surfaces of Water-Repellent Conductive Porous Substrates> described above. In Table 1, the contact surface refers to the surface on which the first and second conductive porous substrates were in contact when they were laminated. The non-contact surface refers to a surface different from the surface on which the first and second conductive porous substrates were in contact. The fluorine adhesion ratio of the first water-repellent conductive porous substrate was 118. The fluorine adhesion ratio of the second water-repellent conductive porous substrate was 102.

[0069] (Example 2) A water-repellent conductive porous substrate was prepared in the same manner as in Example 1, except that a dispersion containing fluororesin was applied to two conductive porous substrates without lamination, and then they were laminated and dried. The results are shown in Table 1.

[0070] (Comparative Example 1) A water-repellent conductive porous substrate was prepared in the same manner as in Example 1, except that the conductive porous substrate was not laminated but instead a dispersion containing fluororesin was applied and dried on a single sheet. The results are shown in Table 1. The ratio of fluorine adhesion on the front and back surfaces was low at 1.1.

[0071] (Comparative Example 2) Comparative Example 2 was based on Example 1 of Patent Document 1. In a stainless steel tray, PTFE dispersion (Daikin Industries' "Polyflon®" D210-C) was diluted to an appropriate concentration so that 5 parts by mass of fluororesin were added to 95 parts by mass of conductive porous substrate after drying, to obtain a dispersion containing fluororesin. One conductive porous substrate obtained as described in <Preparation of Conductive Porous Substrate> above was immersed in this dispersion, thereby adhering the dispersion containing fluororesin to one conductive porous substrate. After adhering the dispersion containing fluororesin, the conductive porous substrate was placed on a glass plate. The conductive porous substrate was heated from above with a heater for 10 minutes to dry it, and a water-repellent conductive porous substrate was obtained. The results are shown in Table 1. Here, the contact surface in Table 1 refers to the surface that was in contact with the glass plate. Compared to Example 1, the amount of fluorine adhering to the surface in contact with the glass plate was higher, and the ratio of fluorine adhering to the front and back surfaces was 89 and 100 or less.

[0072] Table 1

Claims

1. A method for producing a water-repellent conductive porous substrate, comprising the following steps (A) to (D). (A) Lamination process of stacking a pair of conductive porous substrates (B) Adhesion process: A dispersion containing fluororesin is applied to a pair of conductive porous substrates. (C) A drying step in which the pair of conductive porous substrates are dried after the lamination step and the adhesion step. (D) A peeling step after the drying step, in which the pair of conductive porous substrates are peeled off to obtain a pair of water-repellent conductive porous substrates.

2. The method for producing a water-repellent conductive porous substrate according to claim 1, characterized in that the ratio of fluorine adhesion amounts on the front and back surfaces of the obtained water-repellent conductive porous substrate is 100 or more.

3. A method for manufacturing a gas diffusion electrode, characterized by forming a microporous layer on the side of a water-repellent conductive porous substrate, manufactured by the method for manufacturing a water-repellent conductive porous substrate according to claim 1 or 2, that has a larger amount of fluorine deposited on it.

4. A method for manufacturing a fuel cell, characterized by using a gas diffusion electrode, a catalyst layer, a polymer electrolyte membrane, and a separator, all manufactured by the method for manufacturing a gas diffusion electrode described in claim 3.

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