Carbon sheet as well as gas diffusion electrode substrate, membrane electrode assembly, and fuel cell using same

Abstract

This carbon sheet includes at least a carbon fiber structure formed from carbon fibers, a binding agent, and graphite particles, wherein in a pore diameter distribution of the carbon sheet, the ratio of the volume of a second peak present in a region having a pore diameter of 0.5 µm to 3 µm to the volume of a first peak present in a region having a pore diameter of 20 µm to 100 µm is 0.06 to 0.50.　A carbon sheet which suppresses dry-up and achieves both conductivity and retention of moisture is provided in order to exhibit high power generation performance in a solid polymer fuel cell.

Description

Carbon sheet, and gas diffusion electrode substrate, membrane electrode assembly, and fuel cell using the same

[0001] The present invention relates to a carbon sheet suitable for use in an electrode of a fuel cell, particularly a polymer electrolyte fuel cell, and to a gas diffusion electrode substrate and a membrane electrode assembly using the same.

[0002] The electrodes used in polymer electrolyte fuel cells are sandwiched between two separators and arranged between them. They consist of a catalyst layer formed on the surface of the electrolyte membrane on both sides of the electrolyte membrane and a gas diffusion electrode substrate formed on the outside of this catalyst layer. The required performance of this gas diffusion electrode substrate includes, for example, gas diffusivity, conductivity for collecting electricity generated in the catalyst layer, drainage for efficient removal of moisture generated on the catalyst layer surface, and moisture retention to prevent the electrolyte membrane from drying out. Because fuel cells are systems that electrically extract the energy generated when hydrogen and oxygen react to produce water, a large amount of water (water vapor) is generated when the electrical load increases, i.e., when the current output to the outside of the cell increases. This water vapor condenses at low temperatures into droplets that clog the pores of the gas diffusion electrode substrate, reducing the amount of gas (oxygen or hydrogen) supplied to the catalyst layer. Eventually, when all the pores become clogged, power generation stops (a phenomenon known as flooding).

[0003] To prevent flooding, gas diffusion electrode substrates are required to have good drainage properties. To improve drainage properties, carbon sheets made of carbon fibers are typically treated with a water-repellent coating to enhance water repellency. However, if such a water-repellent carbon sheet is used as a gas diffusion electrode substrate without modification, the coarseness of the fibers can easily cause flooding by forming large droplets of water vapor upon condensation. For this reason, a coating solution containing dispersed conductive particles such as carbon black is sometimes applied to the water-repellent carbon sheet, followed by drying and sintering to form a layer known as a microporous layer.

[0004] Fuel cells used as a power supply source for automobiles are required to be able to generate electricity at high temperatures exceeding 90° C. At high temperatures, the electrolyte membrane tends to dry out, which reduces the ionic conductivity of the electrolyte membrane and reduces power generation performance (this phenomenon is called drying up).

[0005] In order to improve the drainage properties of a gas diffusion electrode substrate, Patent Document 1 proposes, in addition to a conventional water-repellent treatment, forming through-holes in the resin carbide that binds the carbon fibers together using organic particles or organic fibers. By forming through-holes in the bonded parts, paths for water and gas are formed, thereby simultaneously improving drainage and gas diffusion properties.

[0006] In Patent Document 2, an organic polymer compound binder is mixed with a thermosetting resin as a binding material, and the organic polymer binder is eliminated during heat treatment, forming pores around the area where the thermosetting resin remains as a carbonized resin. By using these pores as pathways, drainage and gas diffusion properties are improved.

[0007] In Patent Document 3, a water management sheet with controlled pore size is laminated on a substrate, thereby achieving both drainage and gas diffusion properties even in an environment with a large amount of generated water.

[0008] JP 2011-146373 A JP 2016-91996 A JP 2011-233274 A

[0009] The bonded portions of carbon fibers are the electron transfer paths of the gas diffusion electrode substrate and play an important role in maintaining conductivity. Patent Documents 1 and 2 form through-holes or pores at the bonded portions, which cuts the conductive paths and reduces conductivity. On the other hand, reducing the amount of organic particles or organic fibers added to ensure conductivity fails to form through-holes sufficient for drainage and gas diffusion. Furthermore, the through-holes fail to form pores with the water-retaining function necessary to suppress dry-up, making dry-up more likely to occur. Patent Document 3 also only controls the pore size of the water management sheet on the substrate and forms the pores using carbon black with a small particle size, so it fails to ensure a pore volume sufficient to exhibit water-retaining function to suppress dry-up.

[0010] The present inventors have discovered a carbon sheet that achieves both moisture retention and electrical conductivity by forming pores using a binder and graphite particles.

[0011] [1] A carbon sheet comprising at least a carbon fibrous structure made of carbon fibers, a binder, and graphite particles, wherein in the pore size distribution of the carbon sheet, the ratio of a second peak volume present in a pore size range of 0.5 μm to 3 μm to a first peak volume present in a pore size range of 20 μm to 100 μm is 0.06 to 0.50.

[0012] [2] The carbon sheet according to [1], wherein the carbon fiber structure is a carbon fiber paper sheet.

[0013] [3] The carbon sheet according to [1] or [2], wherein the aspect ratio of the graphite particles is 5 or more.

[0014] [4] The carbon sheet according to any one of [1] to [3], wherein the graphite particles have an average particle size of 1 μm or more and 20 μm or less.

[0015] [5] The carbon sheet according to any one of [1] to [4], wherein the graphite particles are scaly graphite or flake graphite.

[0016] [6] The carbon sheet according to any one of [1] to [5], wherein the mass of the graphite particles is 30 mass % or more and 50 mass % or less of the total mass of the carbon sheet.

[0017] [7] The carbon sheet according to any one of [1] to [6], wherein the mass of the graphite particles is 100 parts by mass or more and 300 parts by mass or less per 100 parts by mass of the binder.

[0018] [8] The electrical resistance in the perpendicular direction when pressed at 1.0 MPa is 0.5 mΩ cm ２ 4.5 mΩ・cm or more ２ The carbon sheet according to any one of claims [1] to [7], wherein:

[0019] [9] Thermal resistance in the direction perpendicular to the surface when pressurized at 1.0 MPa is 0.5 K cm ２ / W or more 3.3K・cm２ The carbon sheet according to any one of [1] to [8], wherein the thickness is 1 / W or less.

[0020]

[10] A water-repellent carbon sheet comprising the carbon sheet according to any one of [1] to [9], further comprising a water-repellent agent.

[0021]

[11] A gas diffusion electrode substrate having a microporous layer on one side of the carbon sheet according to any one of [1] to [9] or the water-repellent carbon sheet according to

[10] .

[0022]

[12] A membrane electrode assembly in which the carbon sheet according to any one of [1] to [9], the water-repellent carbon sheet according to

[10] , or the gas diffusion electrode substrate according to

[11] is used as a gas diffusion electrode, and the gas diffusion electrode is joined to at least one side of a solid polymer electrolyte membrane having catalyst layers on both sides.

[0023]

[13] A membrane electrode assembly in which the carbon sheet according to any one of [1] to [9], the water-repellent carbon sheet according to

[10] , or the gas diffusion electrode substrate according to

[11] is used as a gas diffusion electrode on a cathode side, and a gas diffusion electrode having a higher thermal resistance than the gas diffusion electrode on the cathode side is used on the anode side, and the gas diffusion electrode is joined to a solid polymer electrolyte membrane having catalyst layers on both sides.

[0024]

[14] A fuel cell having separators on both sides of the membrane electrode assembly according to

[12] or

[13] .

[0025]

[15] A transportation device using the fuel cell according to

[14] as a power supply source.

[0026]

[16] A method for producing the carbon sheet according to any one of [1] to [9], comprising impregnating carbon fibrous structures with a liquid composition containing a mixture of a thermosetting resin and a thermoplastic resin, and graphite particles.

[0027]

[17] A method for producing a carbon sheet according to

[16] , characterized in that a carbon fiber structure impregnated with a liquid composition containing a mixture of a thermosetting resin and a thermoplastic resin is heat-treated in an inert atmosphere at 1300°C or higher and 3000°C or lower.

[0028] By using the carbon sheet of the present invention, it is possible to achieve both electrical conductivity and moisture retention, making it difficult for the fuel cell to dry up and capable of exhibiting high power generation performance.

[0029] <Carbon Sheet> In the present invention, a carbon sheet is a porous structure in a sheet form formed by binding carbon fiber structures (e.g., a carbon fiber paper sheet that is only bonded with an organic polymer such as polyvinyl alcohol) with a binder, and generally has an average pore diameter of 10 to 100 μm. In the present invention, a carbon sheet that has been subjected to a water-repellent treatment is referred to as a "water-repellent carbon sheet" as described below. In the present invention, a carbon sheet or water-repellent carbon sheet is particularly suitable for use as a gas diffusion electrode substrate for a polymer electrolyte fuel cell, and therefore, hereinafter, the effects of using a carbon sheet as a gas diffusion electrode substrate may be described as the effects of a carbon sheet in a specific form.

[0030] Examples of carbon fibers that make up the carbon sheet include polyacrylonitrile (PAN)-based, pitch-based, and rayon-based carbon fibers. Among these, PAN-based carbon fibers are preferably used because of their excellent mechanical strength.

[0031] The carbon fiber structure may be in the form of a carbon fiber paper sheet, a carbon fiber woven fabric, a carbon fiber nonwoven fabric, etc. Among these, a carbon fiber paper sheet is preferred because it has an excellent property of absorbing dimensional changes in the direction perpendicular to the surface of the electrolyte membrane, i.e., excellent "springiness."

[0032] The carbon fibers constituting the carbon sheet preferably have an average single fiber diameter (hereinafter referred to as "carbon fiber diameter") in the range of 3 to 20 μm, more preferably in the range of 5 to 12 μm. When the carbon fiber diameter is 3 μm or more, more preferably 5 μm or more, the pore diameter becomes large, improving drainage and making it easier to suppress flooding. On the other hand, when the carbon fiber diameter is 20 μm or less, more preferably 12 μm or less, thickness unevenness becomes small, making it easier to control the thickness of the carbon sheet within the preferred range described below. Here, the carbon fiber diameter is determined by photographing the carbon fibers at 1,000x magnification using a microscope such as a scanning electron microscope, measuring the diameters of 30 randomly selected single fibers, and calculating the average value.

[0033] Furthermore, the average length of the single fibers constituting the carbon sheet (hereinafter referred to as "carbon fiber length") is preferably within the range of 3 to 20 mm, and more preferably within the range of 5 to 15 mm. When the carbon fiber length is 3 mm or more, more preferably 5 mm or more, the carbon sheet tends to have excellent mechanical strength, electrical conductivity, and thermal conductivity. On the other hand, when the carbon fiber length is 20 mm or less, more preferably 15 mm or less, the dispersion of the carbon fibers during papermaking is excellent, making it easier to obtain a homogeneous carbon sheet. Carbon fibers having such a carbon fiber length can be obtained by, for example, cutting continuous carbon fibers to the desired length. The carbon fiber length was determined by photographing the carbon fibers at 50x magnification using a microscope such as a scanning electron microscope, randomly selecting 30 different single fibers, and measuring their lengths to calculate the average value.

[0034] The thickness of the carbon sheet is preferably 50 to 230 μm, and more preferably 70 to 180 μm. By making the thickness of the carbon sheet 230 μm or less, more preferably 180 μm or less, gas diffusibility is likely to be increased and generated water is also likely to be discharged more easily. Furthermore, the size of the fuel cell as a whole can be made smaller. On the other hand, by making the thickness of the carbon sheet 50 μm or more, more preferably 70 μm or more, gas diffusion in the in-plane direction within the carbon sheet is efficient, and power generation performance is likely to be improved.

[0035] As the binder for binding the carbon fibers together, a resin or a carbide of a resin can be used, but one containing a carbide of a resin as the main component is preferred. As the resin, a thermosetting resin such as a phenolic resin, an epoxy resin, a melamine resin, or a furan resin is preferred, and it is more preferred to add a thermoplastic resin such as an acrylic resin, a polyvinylidene chloride resin, or a polytetrafluoroethylene resin to the thermosetting resin. This is because the addition of a thermoplastic resin improves the fluidity of the resin during molding, making it easier to form pores, as described below.

[0036] Furthermore, graphite particles such as artificial graphite, natural graphite, amorphous graphite, scaly graphite, flake graphite, expanded graphite, spherical graphite, and flake graphite are included in carbon sheets by mixing them with a binder to improve the mechanical properties, electrical conductivity, and thermal conductivity of the carbon sheet. Among these, flake graphite, scaly graphite, expanded graphite, and flake graphite are more preferred because their large aspect ratios make it easier for pores to be formed between the graphite particles and the binder, and their shapes make it easier to form conductive paths. Scaly graphite is massive graphite with relatively thick flakes, while flake graphite and flake graphite are thinner flakes than scaly graphite. Expanded graphite refers to graphite in which the spacing between the layers within the particles is expanded.

[0037] As a characteristic of the shape of the graphite particles, the aspect ratio, which is the long axis / short axis ratio of the particles, is preferably 5 or more, more preferably 5 or more and 7,000 or less, and even more preferably 15 or more and 5,000 or less. If the aspect ratio is less than 5, the pore volume formed by the graphite particles and the binder may not be sufficient to exhibit the water retention function. If the aspect ratio is 15 or more, a more preferable amount of pores is likely to be formed to exhibit the water retention function. If the aspect ratio is 7,000 or less, more preferably 5,000 or less, the graphite particles have sufficient strength, so that the shape of the graphite particles is less likely to be destroyed during processing and they can function as conductive paths within the carbon sheet.

[0038] The aspect ratio can be measured by magnifying and observing the particles using, for example, a scanning electron microscope (SEM). 100 graphite particles were randomly selected from an SEM photograph of the graphite particles, and the longest line segment connecting the outer ends of the graphite particles was measured to maximize the length. The longest line segment perpendicular to the particle diameter was defined as the particle diameter, and the longest line segment perpendicular to the particle diameter was defined as the minor axis. The ratio of the particle diameter to the minor axis of the graphite particles was measured, and the average value was calculated. The average particle diameter of the graphite particles is preferably 1 μm or more and 20 μm or less, and more preferably 3 μm or more and 10 μm or less. If the average particle diameter is less than 1 μm, the pore volume formed by the graphite particles and the binder may not be sufficient, resulting in reduced moisture retention. On the other hand, if the average particle diameter is greater than 20 μm, conductive paths may be difficult to form, resulting in increased conductive resistance. As with the aspect ratio, the average particle size can be determined by enlarging and observing the particles using, for example, a scanning electron microscope (SEM). In the SEM photograph of the graphite particles, 100 graphite particles were randomly selected, and the length connecting the outer ends of the 100 graphite particles was measured so that the longest length was the length, which was then taken as the particle size of the graphite particles, and the average value thereof was calculated.

[0039] The pore size distribution of the carbon sheet has a ratio of the second peak volume present in the 0.5 μm to 3 μm region to the first peak volume present in the first region of 20 μm to 100 μm of 0.06 to 0.50, preferably 0.20 to 0.40. The first region originates from pores formed by carbon fibers bound together by the binder, and serves as the main gas diffusion path and the discharge path of generated water. The second region originates from pores formed by the binder and graphite particles, and serves to moisturize the electrolyte membrane by containing generated water during power generation. When the ratio of the second peak volume to the first peak volume is 0.06 or more, preferably 0.20 or more, a pore volume containing generated water is ensured, improving the moisture retention of the carbon sheet. When the ratio of the second peak volume to the first peak volume is 0.50 or less, preferably 0.40 or less, the gas diffusion and drainage properties of the carbon sheet are improved.

[0040] In the present invention, when the carbon sheet is pressed at 1.0 MPa, the electrical resistance in the direction perpendicular to the surface is 0.5 mΩ cm ２4.5 mΩ・cm or more ２ The upper limit is preferably 4.5 mΩ cm or less. ２ Preferably, it is 4.0 mΩ cm or less. ２ It is more preferable that the conductive resistance is 4.5 mΩ cm or less. ２ or less, the resistance overvoltage of the fuel cell can be reduced, improving power generation performance. The conductive resistance can be reduced, for example, by using graphite particles with a large aspect ratio, or by increasing the ratio of graphite particles in the carbon sheet. There is no particular lower limit for the conductive resistance in the perpendicular direction when the carbon sheet is pressurized at 1.0 MPa, but in reality, it is 0.5 mΩ cm when pressurized at 1.0 MPa. ２ Since it is not easy to make it less than this, the lower limit is 0.5 mΩ cm when pressurized at 1.0 MPa. ２ is.

[0041] When the carbon sheet is pressurized at 1.0 MPa, the thermal resistance in the direction perpendicular to the surface is 0.5 K cm ２ / W or more 3.3K・cm ２ / W or less. The upper limit is 3.3 K cm ２ / W or less, and 3.0 K cm ２ / W or less. ２ / W or less, the heat generated by the power generation reaction in the fuel cell is easily released through the carbon sheet, the temperature rise of the electrolyte membrane can be suppressed, and the relative humidity is easily increased. Therefore, under high temperature conditions, drying up due to drying of the electrolyte membrane is suppressed, and power generation performance is improved. To reduce the thermal resistance, as in the case of conductive resistance, it can be adjusted by using graphite particles with a large aspect ratio, or by increasing the ratio of graphite particles in the carbon sheet. There is no particular limit to the lower limit of the thermal resistance in the direction perpendicular to the surface when the carbon sheet is pressurized at 1.0 MPa, but in reality, it is 0.5 K cm when pressurized at 1.0 MPa. ２ Since it is not easy to make it less than / W, the lower limit is 0.5 K cm when pressurized to 1.0 MPa. ２ / W.

[0042] To prevent flooding, the gas diffusion electrode substrate is required to have drainage properties. Water repellency can be enhanced by a water-repellent treatment, and the carbon sheet preferably has a water-repellent agent attached to the carbon fibers, binder, or graphite particles. A fluororesin is preferably used as the water-repellent agent. One or more resins selected from polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), perfluoroalkoxy fluororesin (PFA), ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF) can be used, and among these, PTFE or FEP, which exhibit strong water repellency, are more preferred as the water-repellent agent.

[0043] <Method for manufacturing carbon sheet> The method for manufacturing a carbon sheet of the present invention includes at least an impregnation step of adhering a binder and graphite particles to carbon fibrous structures. It preferably also includes a molding step of controlling the thickness of the carbon fibrous structures by hot pressing, a carbonization step of carbonizing the binder by heat treatment, and a water-repellent step of adhering a water-repellent agent to the carbonized carbon sheet.

[0044] [Impregnation Step] The method for producing a carbon sheet of the present invention includes a step of forming carbon fibers into a carbon fiber structure having a specific shape, such as a paper sheet, and then impregnating the carbon fiber structure with a liquid composition containing a binder and graphite particles.

[0045] In preparing the liquid composition, after adding the resin and graphite particles to the solvent, applying strong shear improves the dispersibility of the graphite particles and allows the graphite particles to adhere uniformly to the carbon sheet. One method for applying strong shear is to stir the mixture in a homogenizer at a rotation speed of 3,000 rpm or more for 10 minutes or longer.

[0046] The method of impregnating with the liquid composition may be a method of immersing in a liquid composition prepared by adding a solvent to a resin and graphite particles, a method of coating the liquid composition, etc. Among these, the method of immersing in a liquid composition is particularly preferred because of its excellent productivity.

[0047] When the liquid composition is impregnated, the carbon sheet is preferably impregnated so that the graphite particles account for 30 to 50 mass %, more preferably 40 to 50 mass % of the carbon sheet. When the graphite particles account for 30 mass % or more, more preferably 40 mass % or more of the carbon sheet, the conductivity of the carbon sheet can be significantly improved. When the graphite particles account for 50 mass % or less of the carbon sheet, the volume of the pores having a peak in the first region can be sufficiently secured, resulting in high drainage and gas diffusivity. When a carbonization step, described below, is performed after the impregnation step, it is preferable that the carbon sheet after the carbonization step satisfy the above ranges.

[0048] The graphite particles are preferably impregnated in an amount of 100 to 300 parts by mass, more preferably 250 to 300 parts by mass, per 100 parts by mass of the binder. When the amount of graphite particles is preferably 100 parts by mass or more, more preferably 250 parts by mass or more, per 100 parts by mass of the binder, the volume of the pores formed by the binder and the graphite particles increases, expanding the space available for water retention and improving moisture retention. When the amount of graphite particles is 300 parts by mass or less per 100 parts by mass of the binder, the graphite particles are bound by the binder, making them less likely to fall off even during power generation, thereby improving durability as a gas diffusion electrode substrate. When a carbonization step (described later) is performed after the impregnation step, the graphite particles and the binder (carbonized resin) in the carbon sheet after the carbonization step preferably satisfy the above-mentioned ranges.

[0049] [Forming Step] It is also preferable to heat and pressurize the carbon fibrous structures impregnated with the binder and graphite particles as described above before forming them into a shape prior to the heat treatment step described below. This forming step allows for more accurate control of the thickness and porosity of the resulting carbon sheet. In the forming step, the heating temperature is preferably 100 to 250°C, and the applied pressure is preferably 0.01 to 5 MPa.

[0050] [Carbonization Process] In order to carbonize the binder contained in the carbon sheet after the molding process, it is preferable to perform firing in an inert atmosphere. The inert atmosphere can be obtained by flowing an inert gas such as nitrogen gas or argon gas into the furnace. The maximum firing temperature is preferably in the range of 1,300 to 3,000°C, more preferably in the range of 1,500 to 3,000°C, and even more preferably in the range of 1,900 to 3,000°C. If the maximum temperature is 1,300°C or higher, carbonization of the binder progresses, resulting in a carbon sheet with excellent electrical and thermal conductivity. On the other hand, if the maximum temperature is 3,000°C or lower, the operating costs of the heating furnace are reduced.

[0051] [Water-repellent Step] Next, the carbon sheet obtained as described above is preferably subjected to a water-repellent treatment using a water-repellent treatment liquid in which a water-repellent agent containing a fluororesin is dispersed to form a water-repellent carbon sheet.

[0052] Methods for the water-repellent treatment include immersing the carbon sheet in a water-repellent treatment liquid and applying the water-repellent treatment liquid to the carbon sheet by die coating or the like. From the viewpoint of uniformly distributing the fluororesin in the carbon sheet in the direction perpendicular to the surface, the method of immersing the carbon sheet in a water-repellent treatment liquid is preferred. After the water-repellent treatment, it is preferable to carry out a heat-drying step or a sintering step.

[0053] <Gas diffusion electrode substrate> The above-mentioned carbon sheet or water-repellent carbon sheet can be used as a gas diffusion electrode substrate, but it is more preferable to form a microporous layer on one side of the carbon sheet or water-repellent carbon sheet and use it as a gas diffusion electrode substrate. The microporous layer is a porous layer containing carbon powder and a water repellent agent and typically has an average pore size of 0.01 μm to 1 μm. The average pore size of the microporous layer is measured by mercury intrusion porosimetry after scraping off the surface of the gas diffusion electrode substrate on which the microporous layer has been formed with a spatula or the like, and using the obtained specific surface area and total pore volume, the average pore size is calculated according to the following formula: (average pore size) = 4 × (total pore volume) / (specific surface area).

[0054] Examples of carbon powder contained in the microporous layer include carbon black such as furnace black, acetylene black, lamp black, and thermal black, graphite particles such as flake graphite, flaky graphite, amorphous graphite, artificial graphite, expanded graphite, and flake graphite, carbon nanotubes, and carbon nanofibers. Of these, carbon black is preferably used.

[0055] As the water repellent contained in the microporous layer, fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkylvinyl ether copolymer (PFA) are preferably used.

[0056] The basis weight of the microporous layer is not particularly limited, but is preferably 10 to 50 g / m ２ It is preferable that the range is 14 to 30 g / m ２ It is more preferable that the basis weight of the microporous layer is 10 g / m ２ More preferably, 14 g / m ２ When the microporous layer has a mass per unit area of ​​50 g / m or more, one surface of the carbon sheet can be covered with the microporous layer, which further promotes the back diffusion of the generated water and further suppresses the drying of the electrolyte membrane. ２ or less, more preferably 30 g / m ２ When the water content is less than 100%, the drainage property is further improved and flooding can be further suppressed.

[0057] The microporous layer is obtained by applying a coating liquid containing the above-mentioned carbon powder and water repellent agent to a carbon sheet. Coating 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. The coating is then preferably dried at a temperature of 80 to 180°C. After drying, the coating is preferably heated to a temperature of about 300 to 400°C, more preferably 340 to 390°C, to melt the water repellent agent.

[0058] [Membrane Electrode Assembly] In the present invention, a membrane electrode assembly can be formed by bonding the above-mentioned gas diffusion electrode substrate to at least one side of a solid polymer electrolyte membrane having catalyst layers on both sides. When a gas diffusion electrode having a microporous layer is used, it is preferable to place the microporous layer of the gas diffusion electrode substrate on the catalyst layer side, because this not only makes it easier for the generated water to back-diffuse, but also increases the contact area between the catalyst layer and the gas diffusion electrode substrate, thereby reducing the contact electrical resistance.

[0059] [Fuel Cell] The fuel cell of the present invention includes the gas diffusion electrode substrate of the present invention. That is, it has separators on both sides of the above-mentioned membrane electrode assembly. That is, a fuel cell is constructed by disposing separators on both sides of the above-mentioned membrane electrode assembly. Typically, a solid polymer 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 solid polymer electrolyte and catalyst-supported carbon. Platinum is usually used as the catalyst. The solid polymer electrolyte is preferably a perfluorosulfonic acid-based polymer material that has high proton conductivity, oxidation resistance, and heat resistance. The configuration of such a fuel cell unit and the fuel cell itself is well known.

[0060] The fuel cell of the present invention can be used as a power supply source for transportation equipment such as automobiles, ships, and trains.

[0061] In particular, when used as a power source for automobiles, high-temperature conditions are necessary. Current fuel cells installed in automobiles typically operate at 60–70°C, but large radiators are used to cool the heat generated by power generation. Raising the operating temperature to around 90°C would enable the radiator needed for heat rejection to be made smaller, and active development of fuel cells with higher operating temperatures is underway. Operating fuel cells at high temperatures requires preventing the electrolyte from drying out, so the carbon sheet with enhanced moisture retention of the present invention is preferably used. Furthermore, it is preferable to maintain a constant temperature distribution within the electrolyte membrane to prevent a decrease in proton conductivity due to drying of the electrolyte membrane. Because heat generated by chemical reactions is primarily generated in the catalyst layer on the cathode side, the cathode side of the electrolyte membrane becomes hotter, creating a temperature gradient within the electrolyte membrane. Therefore, by increasing the thermal resistance of the anode-side gas diffusion electrode substrate compared to the cathode-side gas diffusion electrode substrate, heat rejection from the anode side is suppressed. Maintaining a constant temperature distribution within the electrolyte membrane suppresses a decrease in the electrolyte membrane's proton conductivity, reducing cell resistance and improving power generation performance. The carbon sheet of the present invention has a low thermal resistance and is therefore preferably used as the cathode side gas diffusion electrode substrate, while the anode side gas diffusion electrode substrate is preferably a carbon sheet having a higher thermal resistance than the carbon sheet of the present invention.

[0062] Next, the carbon sheet of the present invention will be specifically described with reference to examples. The materials used in the examples, the methods for producing and evaluating the carbon sheet, and the methods for evaluating the cell performance of the fuel cell are described below.

[0063] <Average particle size and aspect ratio of graphite particles> Five measurement samples were prepared by cutting a carbon sheet into 6 mm squares, and the graphite particles contained in each sample were observed using a scanning electron microscope (SEM). 100 graphite particles were randomly selected from the SEM photographs of the graphite particles, and the length connecting the outer ends of the 100 graphite particles was measured so that the longest length was the longest, thereby defining the particle size of the graphite particles. The average value for the five samples was calculated. Similarly, the aspect ratio of the graphite particles was determined by measuring the ratio of the particle size to the minor axis of the 100 randomly selected graphite particles, where the length of the longest part of a line segment perpendicular to the particle size was defined as the minor axis, and the average value for the five samples was calculated.

[0064] <Pore size distribution> The carbon sheet is 6 cm ２ Five measurement samples cut into 1 / 4" strips were prepared and measured by mercury intrusion porosimetry at a measurement pressure range of 6 kPa to 123 MPa (pore diameters of 10 nm to 200 μm). The peak diameter was determined from the average value of each pore size distribution. For gas diffusion electrode substrates having a microporous layer, the fluororesin in the microporous layer was burned off by heating at 500°C in an air atmosphere for 1 hour, and carbon powder such as carbon black was scraped off with a spatula or the like before measurement. The measurement device used was an Autopore 9520 manufactured by Shimadzu Corporation. The pore size peak was determined by taking the maximum point in the target region of the curve of the logarithmic differential pore volume distribution graph obtained by pore measurement using a mercury porosimeter as the peak of that region, and the position of that peak was taken as the pore diameter.

[0065] The peak volume of the pores was determined by subtracting the integrated data of the smaller pore diameters from the integrated data of the larger pore diameters of the pores to be measured in the logarithmic differential pore volume distribution graph. For example, the peak volume in the second region (region of 0.5 μm to 3 μm) was determined by subtracting the integrated data of the smaller pore diameters from the integrated data of the smaller pore diameters.

[0066] By the above method, the first peak volume present in the pore diameter range of 20 μm to 100 μm, the second peak volume present in the pore diameter range of 0.5 μm to 3 μm, and the ratio between them were measured.

[0067] <Basis Weight of Carbon Sheet> Ten test pieces of 10 cm x 10 cm of carbon sheet were prepared, and the mass of each test piece was measured to determine the average basis weight of the ten test pieces.

[0068] <Thickness and density of carbon sheet> The thickness of the carbon sheet was measured using a digital thickness meter "Digimicro" manufactured by Nikon Corporation, where the carbon sheet was placed on a smooth surface plate and a load of 0.15 MPa was applied, and the difference in height between the case with and without the object was measured. Sampling was performed at 10 different locations on the carbon sheet, and the average of the measured height differences was used as the thickness. The density was also calculated from the thickness and basis weight of the carbon sheet.

[0069] <Conductive Resistance> Ten test pieces were prepared by cutting a carbon sheet into 30 mm x 30 mm pieces, and each test piece was sandwiched between two gold-plated plates, a uniform surface pressure of 1.0 MPa was applied, and a current of 1.0 A was passed through the test piece, and the resistance was measured one minute later. The resistance was multiplied by the area of ​​the carbon sheet to determine the conductive resistance in the direction perpendicular to the surface of the carbon sheet, and the average value of the 10 test pieces was calculated.

[0070] <Thermal Resistance> Ten test pieces were prepared by cutting the carbon sheet into 30 mm diameter circles, and each test piece was set in a thermal conductivity measuring device (IE-1230 (manufactured by Iwatsu Measurement Co., Ltd.)), and the thermal resistance was measured after 8 minutes under a pressure of 1.0 MPa. The thermal resistance in the direction perpendicular to the surface of the carbon sheet was determined by multiplying the resistance by the area of ​​the carbon sheet, and the average value of the 10 test pieces was calculated.

[0071] <Gas Diffusivity> Using a gas and water vapor diffusivity measuring device (MVDP-200C) manufactured by Seika Corporation, the gas for which diffusivity was to be measured was flowed on one side (primary side) of the carbon sheet, and nitrogen gas was flowed on the other side (secondary side). The differential pressure between the primary and secondary sides was controlled to approximately 0 Pa (0±3 Pa) (i.e., a state was created in which there was almost no gas flow due to the pressure difference and gas movement occurred only by molecular diffusion). The gas concentration when equilibrium was reached was measured using a gas concentration meter on the secondary side, and this value (%) was used as an index of gas diffusivity in the thickness direction. Ten test pieces were prepared by cutting the carbon sheet into 11 mm x 27 mm, and the gas diffusivity of each was measured to obtain the average value of the 10 pieces.

[0072] <Power Generation Performance of Polymer Electrolyte Fuel Cell> A catalyst solution was prepared by sequentially adding 1.00 g of platinum-loaded carbon (manufactured by Tanaka Kikinzoku Kogyo K.K., platinum loading: 50% by mass), 1.00 g of purified water, 8.00 g of Nafion (registered trademark) solution (5.0% by mass of Nafion (registered trademark) manufactured by Aldrich), and 18.00 g of isopropyl alcohol (manufactured by Nacalai Tesque, Inc.).

[0073] Next, the catalyst solution was sprayed onto a piece of "Naflon (registered trademark)" PTFE tape "TOMBO (registered trademark)" No. 9001 (manufactured by Nichias Corporation) cut to a size of 5 cm x 5 cm, and the tape was dried at room temperature to obtain a platinum content of 0.3 mg / cm. ２ A PTFE sheet with a catalyst layer was produced. Subsequently, a solid polymer electrolyte membrane "Nafion (registered trademark)" NRE-211CS (manufactured by DuPont) cut to 8 cm x 8 cm was sandwiched between two PTFE sheets with a catalyst layer, and pressed at 130°C for 5 minutes while applying a pressure of 5 MPa using a flat plate press, thereby transferring the catalyst layer to the solid polymer electrolyte membrane. After pressing, the PTFE sheet was peeled off, and a solid polymer electrolyte membrane with a catalyst layer was produced.

[0074] Next, the catalyst layer-equipped solid polymer electrolyte membrane was sandwiched between two gas diffusion electrode substrates prepared in each Example and Comparative Example, each cut to a size of 5 cm x 5 cm, and pressed at a temperature of 130°C for 5 minutes under a pressure of 3 MPa using a flat press to prepare a membrane electrode assembly.

[0075] The resulting membrane electrode assembly was assembled into a single cell for fuel cell evaluation. A serpentine separator with a single channel and a groove width, groove depth, and rib width of 1.0 mm was used. Unpressurized hydrogen was supplied to the anode side, and unpressurized air was supplied to the cathode side.

[0076] The measurements were carried out as follows. Both hydrogen and air were humidified using a humidifying pot set at 52°C to a humidity of 100%. The oxygen utilization rates in the hydrogen and air were 70 mol% and 40 mol%, respectively, and the cell temperature was 80°C. Under these conditions, a current density of 1.9 A / cm was obtained. ２ The output voltage at this point was measured and used as an index of resistance to dry-up.

[0077] [Example 1] <Preparation of Carbon Sheet> Toray Industries, Inc.'s polyacrylonitrile carbon fiber "TORAYCA" (registered trademark) T300 (average single fiber diameter: 7 μm) was cut into short fibers with an average length of 12 mm, dispersed in water, and continuously made into paper by a wet papermaking method. Furthermore, a 10% by mass aqueous solution of polyvinyl alcohol was applied as a binder to the paper and dried to prepare a carbon fiber paper body. The amount of polyvinyl alcohol applied was 22 parts by mass per 100 parts by mass of the carbon fiber paper body.

[0078] Next, a resin composition obtained by mixing a resol-type phenolic resin, which is a thermosetting resin, and a novolac-type phenolic resin, which is a thermoplastic resin, in a mass ratio of 1:1 was mixed with flake graphite (average particle size 5 μm) as graphite particles and methanol as a solvent in a blending ratio of resin composition / graphite particles / solvent = 13 mass % / 12 mass % / 75 mass %, and the mixture was stirred for 1 minute using an ultrasonic disperser to obtain a uniformly dispersed liquid composition.

[0079] Next, the carbon fiber paper sheet was immersed in an impregnation solution of the liquid composition filled in a tray, and squeezed between rolls to impregnate the carbon fiber paper sheet. It was then heated at 100°C for 5 minutes and dried. Next, a heating and pressurizing treatment was performed at 180°C for 5 minutes using a heating and pressurizing device consisting of upper and lower parallel hot plates. At this time, a spacer was placed between the upper and lower hot plates, and the distance between the upper and lower hot plates was adjusted so that the thickness after the heating and pressurizing treatment would be 160 μm.

[0080] The heat-treated substrate was introduced into a heating furnace maintained in a nitrogen gas atmosphere at a maximum temperature of 2400°C to obtain a carbon sheet. The weight per unit area of ​​the binder after heat treatment was calculated by measuring in advance the mass loss rate when only the binder was heat-treated under the same conditions, and multiplying the mass of the binder attached to the carbon fiber paper sheet before heat treatment by the mass loss rate. The graphite particles do not lose mass due to heat treatment. The weight per unit area of ​​the carbon sheet after heat treatment was 40.0 g / m ２ , binder basis weight is 6.1 g / m ２ , the graphite particle basis weight is 17.4 g / m ２ It was.

[0081] As a result of measurement as described above in <Pore size distribution>, the first peak volume, the second peak volume, and the ratio thereof were as shown in Table 1.

[0082] The carbon sheet prepared as described above was immersed in an aqueous dispersion of FEP resin ("Neoflon (registered trademark)" FEP dispersion ND-110 (manufactured by Daikin Industries, Ltd.)) to impregnate the carbon sheet with a water repellent agent. The sheet was then heated and dried in a dryer oven at 100°C for 5 minutes to prepare a water-repellent carbon sheet. The aqueous dispersion of the water repellent agent was diluted to an appropriate concentration so that the amount of water repellent agent was 5 parts by mass when the amount of the water-repellent carbon sheet after heating and drying was 95 parts by mass.

[0083] <Preparation of Gas Diffusion Electrode Substrate> Carbon powder: acetylene black: "Denka Black" (registered trademark) (manufactured by Denki Kagaku Kogyo Co., Ltd.), water repellent: PTFE resin ("Polyflon" (registered trademark) PTFE Dispersion D-201C (manufactured by Daikin Industries, Ltd.) which is an aqueous dispersion containing 60% by mass of PTFE resin), surfactant "TRITON" (registered trademark) X-100 (manufactured by Nacalai Tesque Inc.) 14.0 g, purified water was mixed using a disperser so that the carbon powder / PTFE resin / surfactant / purified water = 7.0% by mass / 2.5% by mass / 14.1% by mass / 76.4% by mass to prepare a coating liquid. This coating liquid was applied in a planar manner to one surface of the water-repellent carbon sheet prepared in the above <Preparation of Carbon Sheet> using a slit die coater, and then heated at a temperature of 120 ° C. for 10 minutes, and then at a temperature of 380 ° C. for 10 minutes. In this manner, a microporous layer was formed on the water-repellent carbon sheet to prepare a gas diffusion electrode substrate.

[0084] The results of measurements made according to the above <Gas diffusivity>, <Conductive resistance>, <Thermal resistance> and <Power generation performance of polymer electrolyte fuel cell> are shown in Table 1.

[0085] [Example 2] A carbon sheet was produced in the same manner as in Example 1, except that a carbon fiber paper body with a different basis weight of carbon fiber was produced. The basis weight of the carbon sheet after heat treatment was 38.8 g / m ２ , binder basis weight is 6.7 g / m ２ , the graphite particle basis weight is 18.4 g / m ２ It was.

[0086] [Example 3] A carbon sheet was produced in the same manner as in Example 1, except that a carbon fiber paper body with a different basis weight of carbon fiber was produced. The basis weight of the carbon sheet after heat treatment was 42.8 g / m ２ , binder basis weight is 6.1 g / m ２ , the graphite particle basis weight is 17.4 g / m ２ It was.

[0087] [Example 4] A carbon sheet was produced in the same manner as in Example 1, except that a carbon fiber paper body with a different basis weight of carbon fiber was produced. The basis weight of the carbon sheet after heat treatment was 47.3 g / m ２ , binder basis weight is 6.1 g / m ２ , the graphite particle basis weight is 17.4 g / m ２ It was.

[0088] [Example 5] Carbon fiber sheets with different basis weights of the carbon fibers were prepared, and carbon sheets were prepared in the same manner as in Example 1, except that a liquid composition with a different ratio of resin composition to graphite was used. The basis weight of the carbon sheet after heat treatment was 55.5 g / m ２ , binder basis weight is 12.2 g / m ２ , the graphite particle basis weight is 27.3 g / m ２ It was.

[0089] Comparative Example 1 A carbon sheet was produced in the same manner as in Example 1, except that a carbon fiber paper sheet was immersed in a liquid composition prepared by mixing a resin composition, graphite particles, and a solvent in a blending ratio of 35% by mass, 3% by mass, and 62% by mass. The weight of the carbon sheet after the heat treatment was 39.3 g / m ２ , binder basis weight is 12.5 g / m ２ , the graphite particle basis weight is 3.3 g / m ２ It was.

[0090] Comparative Example 2 A carbon sheet was produced in the same manner as in Example 1, except that spherical graphite particles (average particle size 12 μm, aspect ratio 4) were used as the graphite particles. The carbon sheet had a basis weight of 40.0 g / m after the heat treatment. ２ , binder basis weight is 6.3 g / m ２ , the graphite particle basis weight is 17.1 g / m ２ It was.

[0091] Comparative Example 3 A carbon sheet was produced in the same manner as in Example 1, except that acetylene black (AB) "Denka Black" was used instead of graphite particles. The carbon sheet had a basis weight of 39.0 g / m after heat treatment. ２ , binder basis weight is 6.3 g / m ２ , acetylene black basis weight is 16.2 g / m ２ It was.

[0092] [Example 6] A membrane electrode assembly was produced using the carbon sheet described in Example 1 as the gas diffusion electrode substrate on the cathode side and the carbon sheet described in Comparative Example 1 as the gas diffusion electrode substrate on the anode side, and the power generation performance was measured. The current density was 1.9 A / cm ２ The output voltage at this point was 0.52 V, which was a better value than that of Example 1 in which gas diffusion electrode substrates with low thermal resistance were used for both electrodes.

[0093] [Comparative Example 4] Carbon fiber sheets having different basis weights of carbon fibers were prepared and immersed in a liquid composition prepared by mixing resin composition / graphite particles / solvent at a blending ratio of 26% by mass / 24% by mass / 50% by mass. The carbon sheet basis weight after heat treatment was 60.2 g / m. ２ , binder basis weight is 13.4 g / m ２ , the graphite particle basis weight is 36.8 g / m ２ It was.

[0094]

Claims

1. A carbon sheet comprising a carbon fiber structure made of carbon fibers, a binder, and graphite particles, wherein the ratio of the second peak volume in the region of 0.5 μm to 3 μm to the first peak volume in the region of pore diameter 20 μm to 100 μm is 0.06 to 0.50 in the pore diameter distribution of the carbon sheet.

2. The carbon sheet according to claim 1, wherein the carbon fiber structure is a carbon fiber papermaking body.

3. The carbon sheet according to claim 1 or 2, wherein the aspect ratio of the graphite particles is 5 or more.

4. The carbon sheet according to claim 1 or 2, wherein the average particle size of the graphite particles is 1 μm or more and 20 μm or less.

5. The carbon sheet according to claim 1 or 2, wherein the graphite particles are scaly graphite or flake graphite.

6. The carbon sheet according to claim 1 or 2, wherein the mass of the graphite particles is 30% by mass or more and 50% by mass or less of the total mass of the carbon sheet.

7. The carbon sheet according to claim 1 or 2, wherein the mass of the graphite particles is 100 parts by mass or more and 300 parts by mass or less with respect to 100 parts by mass of the binder.

8. The carbon sheet according to claim 1 or 2, wherein the conductive resistance in the direction perpendicular to the surface when pressurized at 1.0 MPa is 0.5 mΩ・cm² ​​or more and 4.5 mΩ・cm² ​​or less.

9. The carbon sheet according to claim 1 or 2, wherein the thermal resistance in the direction perpendicular to the surface when pressurized at 1.0 MPa is 0.5 K·cm² / W or more and 3.3 K·cm² / W or less.

10. A water-repellent carbon sheet comprising a water-repellent agent in addition to the carbon sheet described in claim 1.

11. A gas diffusion electrode substrate having a microporous layer on one side of a carbon sheet as described in claim 1.

12. A membrane electrode assembly comprising a carbon sheet according to claim 1 or 2, a water-repellent carbon sheet according to claim 10, or a gas diffusion electrode substrate according to claim 11 as a gas diffusion electrode, wherein the gas diffusion electrode is bonded to at least one side of a solid polymer electrolyte membrane having catalyst layers on both sides.

13. A membrane electrode assembly comprising a carbon sheet according to claim 1 or 2, a water-repellent carbon sheet according to claim 10, or a gas diffusion electrode substrate according to claim 11 used as the cathode-side gas diffusion electrode, a gas diffusion electrode with greater thermal resistance than the cathode-side gas diffusion electrode used on the anode side, and a solid polymer electrolyte membrane having catalyst layers on both sides to which the gas diffusion electrode is bonded.

14. A fuel cell having separators on both sides of the membrane electrode assembly according to claim 12.

15. A fuel cell having separators on both sides of the membrane electrode assembly according to claim 13.

16. A transport device that uses the fuel cell described in claim 14 as a power source.

17. A transport device that uses the fuel cell described in claim 15 as a power source.

18. A method for producing a carbon sheet according to claim 1 or 2, comprising impregnating a carbon fiber structure into a liquid composition containing a mixture of a thermosetting resin and a thermoplastic resin and graphite particles.

19. A method for producing a carbon sheet according to claim 18, comprising heat-treating a carbon fiber structure impregnated in a liquid composition containing a mixture of a thermosetting resin and a thermoplastic resin in an inert atmosphere at a temperature of 1,300°C to 3,000°C.