Gas diffusion layer and manufacturing method thereof, roll-shaped object of gas diffusion layer, and solid polymer fuel cell

Abstract

The present invention provides a gas diffusion layer in which the occurrence of a crack in a coating layer is suppressed and a manufacturing method thereof, a roll-shaped object of the gas diffusion layer, and a solid polymer fuel cell. The gas diffusion layer has a porous electrode substrate and a coating layer formed on at least one surface of the porous electrode substrate, wherein the coating layer contains carbon powder A having an average particle diameter of 5 to 800 nm and carbon powder B having an average particle diameter of 1 to 50 μm.

Description

Gas diffusion layer and method of manufacturing the same, roll of gas diffusion layer, and polymer electrolyte fuel cell

[0001] The present invention relates to a gas diffusion layer, a method for manufacturing the same, a roll of a gas diffusion layer, and a polymer electrolyte fuel cell. This application claims priority based on Japanese Patent Application No. 2023-100085, filed on June 19, 2023, the contents of which are incorporated herein by reference.

[0002] A polymer electrolyte fuel cell is a device that generates electromotive force by electrochemically reacting a fuel gas such as hydrogen with an oxidizing gas such as oxygen. A polymer electrolyte fuel cell has a polymer electrolyte membrane that selectively conducts hydrogen ions (protons). Two gas diffusion electrodes, each consisting of a catalyst layer primarily composed of carbon powder carrying a precious metal catalyst and a gas diffusion layer substrate, are bonded to both sides of the polymer electrolyte membrane. This assembly consisting of a polymer electrolyte membrane and two gas diffusion electrodes is called a membrane electrode assembly (MEA). Separators are installed on both sides of the MEA, each with gas flow paths for supplying fuel gas or oxidizing gas and discharging product gas and excess gas.

[0003] A gas diffusion layer substrate is required to have three main functions. The first function is to uniformly supply fuel gas or oxidizing gas to the precious metal catalyst in the catalyst layer from gas flow channels formed in a separator arranged outside the gas diffusion electrode substrate. The second function is to discharge water generated by the reaction in the catalyst layer. The third function is to conduct electrons required for the reaction in the catalyst layer or electrons generated by the reaction in the catalyst layer to the separator. Gas diffusion layer substrates that fulfill these functions typically use substrates with a porous structure made of a carbonaceous material. Specifically, porous electrode substrates using carbon fibers such as carbon paper, carbon fiber cloth, and carbon fiber felt are commonly used. These porous electrode substrates not only exhibit high electrical conductivity due to the carbon fibers, but also have high permeability to liquids such as fuel gas and generated water due to their porous nature, making them suitable materials for gas diffusion layer substrates.

[0004] In order to reduce the contact resistance between the gas diffusion layer substrate and the catalyst layer and efficiently discharge water generated during power generation, a coating layer containing carbon fine particles and a water repellent agent is sometimes provided on the catalyst layer side of the gas diffusion layer substrate. For example, Patent Document 1 proposes a gas diffusion layer in which a coating layer is formed on the surface of a porous electrode substrate by applying a paste composition containing conductive carbon particles, fluorinated pitch, and a fluorine-based solvent to the surface of the porous electrode substrate, followed by drying and sintering. Patent Document 2 also proposes a gas diffusion layer in which fine cracks are intentionally formed in the coating layer in advance, thereby preventing changes in the structure of the coating layer before and after winding. Patent Document 3 also proposes a gas diffusion layer having a coating layer containing carbon powder and a water repellent agent.

[0005] JP 2010-129451 A JP 2016-12558 A International Publication No. 2018 / 016626

[0006] A coating layer having cracks tends to have poor adhesion to a porous electrode substrate. Therefore, a gas diffusion layer having a coating layer in which cracking is suppressed is desired. One object of the present invention is to provide a gas diffusion layer in which cracking is suppressed in the coating layer, a method for producing the same, a roll of the gas diffusion layer, and a polymer electrolyte fuel cell.

[0007] The present invention has the following aspects. [1] A gas diffusion layer having a porous electrode substrate and a coating layer formed on at least one surface of the porous electrode substrate, wherein the coating layer contains carbon powder A having an average particle size of 5 to 800 nm and carbon powder B having an average particle size of 1 to 50 μm. [2] The gas diffusion layer of [1] above, wherein the mass ratio of the carbon powder A to the carbon powder B is 0.5 or more or 1.0 or more, and 9.0 or less, 4.0 or less, 2.0 or less, 1.8 or less, or 1.2 or less. [3] The gas diffusion layer of [1] or [2] above, wherein the carbon powder A is at least one selected from the group consisting of carbon black, milled fiber, carbon nanotubes, carbon nanofibers, coke, activated carbon, and amorphous carbon, and the carbon powder B is at least one selected from the group consisting of pyrolytic graphite, milled fiber, coke, activated carbon, and amorphous carbon. [4] A gas diffusion layer having a porous electrode substrate and a coating layer formed on at least one surface of the porous electrode substrate, wherein the coating layer contains at least one carbon powder C selected from the group consisting of carbon black, milled fiber, carbon nanotubes, carbon nanofibers, coke, activated carbon, and amorphous carbon, and pyrolytic graphite. [5] The gas diffusion layer of any of [1] to [4] above, wherein the coating layer contains a water repellent. [6] The gas diffusion layer of [3] or [4] above, wherein the aspect ratio of the pyrolytic graphite is 2 to 40. [7] The gas diffusion layer of [4] or [6] above, wherein the mass ratio expressed as carbon powder C / pyrolytic graphite is 0.5 or more or 1.0 or more, and 9.0 or less, 4.0 or less, 2.0 or less, 1.8 or less, or 1.2 or less. [8] The gas diffusion layer of any of [1] to [7] above, wherein the thickness is 160 to 350 μm. [9] The gas diffusion layer of any one of [1] to [8] above, wherein the porous electrode substrate has an average pore size of 5 to 200 μm.

[10] The gas diffusion layer of any one of [1] to [9] above, wherein the coating layer has a surface roughness of 3.0 μm or less.

[11] The gas diffusion layer of any one of [1] to

[10] above, wherein the porous electrode substrate contains carbon fiber.

[12] The gas diffusion layer of [4] above, wherein the average particle size of the pyrolytic graphite is 3 to 50 μm.

[13] The gas diffusion layer of [4] above, wherein the average particle size of the carbon powder C is 30 to 100 nm.

[14] A gas diffusion layer having a substrate in which carbon fibers are bonded with carbon, and a coating layer formed on at least one surface of the substrate, wherein the coating layer contains pyrolytic graphite, carbon black, and a fluororesin.

[15] The gas diffusion layer of

[14] above, wherein the aspect ratio of the pyrolytic graphite is 2 to 40.

[16] The gas diffusion layer of

[14] or

[15] above, wherein the mass ratio expressed by the carbon black / the pyrolytic graphite is 0.5 or more or 1.0 or more, and 9.0 or less, 4.0 or less, 2.0 or less, 1.8 or less, or 1.2 or less.

[17] The gas diffusion layer of any of

[14] to

[16] above, wherein the average particle size of the pyrolytic graphite is 3 to 50 μm.

[18] The gas diffusion layer of any of

[14] to

[17] above, wherein the average particle size of the carbon black is 30 to 100 nm.

[19] A roll of a gas diffusion layer, obtained by providing a protective layer on the coating layer of the gas diffusion layer of any of [1] to

[18] above, and winding the resulting gas diffusion layer into a roll.

[20] A polymer electrolyte fuel cell, comprising the gas diffusion layer of any of [1] to

[18] above.

[21] A method for producing a gas diffusion layer having a porous electrode substrate and a coating layer formed on at least one surface of the porous electrode substrate, comprising applying a coating liquid comprising a mixture of carbon powder B having an average particle size of 1 to 50 μm and carbon powder A having an average particle size of 5 to 800 nm to at least one surface of the porous electrode substrate.

[22] The method for producing a gas diffusion layer according to

[14] above, wherein the aspect ratio of the carbon powder B is 2 to 40.

[23] The method for producing a gas diffusion layer according to

[21] or

[22] above, wherein the carbon powder B is pyrolytic graphite.

[24] The method for producing a gas diffusion layer according to any of

[21] to

[23] above, wherein the mass ratio of the carbon powder A / the carbon powder B is 0.5 or more or 1.0 or more, and 9.0 or less, 4.0 or less, 2.0 or less, 1.8 or less, or 1.2 or less.

[0008] According to the present invention, it is possible to provide a gas diffusion layer in which the occurrence of cracks in the coating layer is suppressed, a method for producing the same, a roll of the gas diffusion layer, and a polymer electrolyte fuel cell.

[0009] 1 is a cross-sectional view showing an example of a gas diffusion layer of the present invention; 2 is a perspective view showing an example of a roll of a gas diffusion layer of the present invention; 3 is a cross-sectional view taken along line AA' of the roll of a gas diffusion layer shown in FIG. 2; 4 is an exploded perspective view showing an example of the configuration of a polymer electrolyte fuel cell of the present invention; 5 are scanning electron microscope photographs of the surfaces of the coating layers of the gas diffusion layers obtained in the examples and comparative examples, where (a) is the observation result of Example 1, (b) is the observation result of Example 2, (c) is the observation result of Example 3, (d) is the observation result of Example 4, (e) is the observation result of Example 5, and (f) is the observation result of Comparative Example 1; and 6 is a graph showing the measurement results of the peel strength of the gas diffusion layers obtained in Example 2 and Comparative Example 1.

[0010] Below, embodiments for carrying out the present invention will be described in detail, but the present invention is not limited to the embodiments described below, and various modifications are possible without departing from the gist of the present invention. In this specification and claims, a numerical range expressed as "to" means a numerical range that includes the numerical values ​​before and after "to" as the lower and upper limits. For example, A to B is equivalent to A or more and B or less. In addition, each drawing used in the following description may conveniently show characteristic portions in an enlarged manner to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones.

[0011] [Gas Diffusion Layer] An example of the gas diffusion layer of the present invention is shown in Figure 1. The gas diffusion layer 10 of this embodiment has a porous electrode substrate 11 and a coating layer 12 formed on one surface of the porous electrode substrate 11.

[0012] <Porous electrode substrate> Any conductive porous material such as conductive paper, cloth, or nonwoven fabric made from a conductive filler such as carbon powder, carbon fiber, metal fiber, or resin can be used as the porous electrode substrate. In particular, the porous electrode substrate preferably contains carbon fiber. As the porous electrode substrate containing carbon fiber, a porous electrode substrate in which carbon fibers are bound by carbon is preferred. Hereinafter, a porous electrode substrate in which carbon fibers are bound by carbon will also be referred to as a "porous carbon electrode substrate."

[0013] (Carbon Fiber) The average fiber diameter of the carbon fibers is preferably 2 μm or more, more preferably 3 μm or more, and even more preferably 4 μm or more. When the average fiber diameter of the carbon fibers is equal to or greater than the lower limit, the distance between fibers in the porous electrode substrate tends to be wider, resulting in higher gas permeability. The average fiber diameter of the carbon fibers is preferably 30 μm or less, more preferably 20 μm or less, and even more preferably 8 μm or less. When the average fiber diameter of the carbon fibers is equal to or less than the upper limit, the porous electrode substrate can be provided with appropriate flexibility. The lower and upper limits of the average fiber diameter of the carbon fibers can be arbitrarily combined, and for example, 2 to 30 μm is preferred, 3 to 20 μm is more preferred, and 4 to 8 μm is even more preferred. The average fiber diameter of the carbon fibers is determined, for example, by photographing the cross section of the carbon fiber at 50x magnification or more using a microscope such as a scanning electron microscope, measuring the diameter of 50 randomly selected single fibers, and averaging the diameters. Note that when the cross section of the carbon fiber has a major axis and a minor axis, the major axis is taken as the fiber diameter of the fiber.

[0014] The average fiber length of the carbon fibers is preferably 2 μm or more, more preferably 3 μm or more. When the average fiber length of the carbon fibers is equal to or greater than the lower limit, the porous electrode substrate can have sufficient strength. The average fiber length of the carbon fibers is preferably 30 mm or less, more preferably 12 mm or less, and even more preferably 9 mm or less. When the average fiber length of the carbon fibers is equal to or less than the upper limit, a porous electrode substrate with less dispersion unevenness can be obtained. The lower and upper limits of the average fiber length of the carbon fibers can be arbitrarily combined, and for example, 2 to 30 mm is preferred, 2 to 12 mm is more preferred, and 3 to 9 mm is even more preferred. The average fiber length of the carbon fibers is determined, for example, by photographing the carbon fibers at a magnification of 50 times or more using a microscope such as a scanning electron microscope, randomly selecting 50 single fibers, measuring their lengths, and averaging the measured lengths.

[0015] Examples of carbon fibers include polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, phenol-based carbon fibers, etc. Among these, it is preferable that the carbon fibers include at least one of PAN-based carbon fibers and pitch-based carbon fibers, since fibers with a large fiber diameter are easily available.

[0016] (Carbon) Carbon functions as a binder for binding carbon fibers together. A plurality of uniformly dispersed carbon fibers are fixed together via the carbon. Examples of carbon that binds carbon fibers include carbonized resins and organic fibers. The resins and organic fibers used as raw materials for carbon will be described in detail in the section on the manufacturing method of a porous electrode substrate below.

[0017] The carbon content relative to the total mass of the porous electrode substrate is preferably 10 to 40 mass%, more preferably 15 to 40 mass%. If the carbon content is equal to or greater than the lower limit, the strength of the porous electrode substrate is easily ensured and the carbon fibers are less likely to fall off. If the carbon content is equal to or less than the upper limit, sufficient voids are easily ensured, facilitating the permeation and diffusion of gases and liquids. Note that the carbon content does not include the content of carbon fibers.

[0018] (Other Components) The porous electrode substrate may further contain carbon powder. When the porous electrode substrate further contains carbon powder, improved conductivity can be expected. When the porous electrode substrate contains carbon powder, the content of the carbon powder is preferably 1 to 20 mass%, more preferably 1 to 15 mass%, relative to the total mass of the porous electrode substrate. When the content of the carbon powder is equal to or greater than the lower limit, a conductive path is formed by the carbon powder, and conductivity is likely to be improved. When the content of the carbon powder is equal to or less than the upper limit, the porous electrode substrate is likely to be prevented from becoming brittle or difficult to bend. Carbon powder will be described in detail in the section on coating layer below.

[0019] (Physical properties of porous electrode substrate) The gas permeability in the thickness direction of the porous electrode substrate is 100 mL / (cm ２ .Pa.hr) or more, and 120 mL / (cm ２ .Pa.hr) or more is more preferable, and 150 mL / (cm ２ .Pa.hr) or more is more preferable, and 200 mL / (cm ２ When the gas permeability of the porous electrode substrate is equal to or greater than the lower limit, the fuel gas and the oxidizing gas are easily diffused, improving the reaction efficiency. The gas permeability in the thickness direction of the porous electrode substrate is preferably 12,000 mL / (cm ２ ·Pa·hr) or less, and 5000 mL / (cm ２ ・Pa ・hr) or less is more preferable, and 2500 mL / (cm ２ .Pa.hr) or less is more preferable, and 1000 mL / (cm ２ When the gas permeability of the porous electrode substrate is equal to or less than the upper limit, the structure does not collapse and the shape can be well maintained even when a liquid such as produced water passes through. The upper and lower limits of the gas permeability in the thickness direction of the porous electrode substrate can be arbitrarily combined, and may be, for example, 100 to 12,000 mL / (cm ２ .Pa.hr), and 120 to 5000 mL / (cm ２ 150 to 2500 mL / (cm ２ .Pa.hr), and 200 to 1000 mL / (cm ２The gas permeability is measured by a method in accordance with JIS P 8117:2009.

[0020] The thickness of the porous electrode substrate is preferably 30 μm or more, more preferably 55 μm or more, and even more preferably 100 μm or more. When the thickness of the porous electrode substrate is equal to or greater than the lower limit, transport of the gas diffusion layer is facilitated. In addition, a coating layer can be easily formed on at least one surface of the porous electrode substrate. The thickness of the porous electrode substrate is preferably 800 μm or less, more preferably 350 μm or less, and even more preferably 250 μm or less. When the thickness of the porous electrode substrate is equal to or less than the upper limit, an increase in electrical resistance can be suppressed and good power generation performance can be maintained. The lower and upper limits of the thickness of the porous electrode substrate can be arbitrarily combined, and for example, are preferably 30 to 800 μm, more preferably 55 to 350 μm, and even more preferably 100 to 250 μm. The thickness of the porous electrode substrate is the average value obtained by measuring the thickness at any 10 positions on the porous electrode substrate.

[0021] The average pore diameter of the porous electrode substrate is preferably 5 μm or more, more preferably 8 μm or more, and even more preferably 10 μm or more. When the average pore diameter of the porous electrode substrate is equal to or greater than the lower limit, fuel gas and oxidizing gas are easily diffused, improving reaction efficiency. The average pore diameter of the porous electrode substrate is preferably 200 μm or less, more preferably 100 μm or less, and even more preferably 50 μm or less. When the average pore diameter of the porous electrode substrate is equal to or less than the upper limit, fluids uniformly permeate the gas diffusion layer, making it difficult for reaction spots to occur. The lower and upper limits of the average pore diameter of the porous electrode substrate can be arbitrarily combined, and are, for example, preferably 5 to 200 μm, more preferably 8 to 100 μm, and even more preferably 10 to 50 μm. The average pore diameter is determined by measuring the porous electrode substrate by mercury intrusion porosimetry and calculating the median diameter from the obtained pore distribution.

[0022] The basis weight of the porous electrode substrate is 50 g / m ２ More than 55 g / m ２ More preferably, 60 g / m or more ２When the basis weight of the porous electrode substrate is equal to or greater than the lower limit, the handling properties of the porous electrode substrate tend to be improved. ２ Preferably, 270 g / m or less ２ More preferably, 250 g / m or less ２ If the basis weight of the porous electrode substrate is equal to or less than the upper limit, the conductivity of the porous electrode substrate is easily ensured. The lower and upper limits of the basis weight of the porous electrode substrate can be arbitrarily combined, and are, for example, 50 to 300 g / m ２ is preferred, and 55 to 270 g / m ２ More preferably, 60 to 250 g / m ２ is more preferable.

[0023] (Method for manufacturing porous electrode substrate) An example of a method for manufacturing a porous electrode substrate will be described below. The method for manufacturing a porous electrode substrate described below is an example of a method for manufacturing a porous carbon electrode substrate. The porous electrode substrate can be obtained, for example, by carbonizing a carbon fiber sheet described below.

[0024] <<Carbon Fiber Sheet>> From the viewpoint of improving the strength of the carbon fiber sheet, a sheet in which carbon fibers are bonded together with at least one of a resin and an organic fiber is preferred. The carbon fiber sheet can be obtained, for example, by papermaking a dispersion in which carbon fibers or carbon fibers and organic fibers are dispersed in a dispersion medium, and adding a resin as necessary. The carbon fiber sheet may be subjected to a heat and pressure treatment before the carbonization treatment.

[0025] The thickness of the carbon fiber sheet is preferably 100 μm or more, more preferably 150 μm or more, even more preferably 170 μm or more, and particularly preferably 200 μm or more. When the thickness of the carbon fiber sheet is equal to or greater than the above-mentioned lower limit, a porous electrode substrate with excellent gas permeability is easily obtained. The thickness of the carbon fiber sheet is preferably 5000 μm or less, more preferably 4000 μm or less, even more preferably 3000 μm or less, and particularly preferably 2000 μm or less. When the thickness of the carbon fiber sheet is equal to or less than the above-mentioned upper limit, a porous electrode substrate with high conductivity is easily obtained. The lower and upper limits of the thickness of the carbon fiber sheet can be arbitrarily combined; for example, 100 to 5000 μm is preferable, 150 to 4000 μm is more preferable, 170 to 3000 μm is even more preferable, and 200 to 2000 μm is particularly preferable. The thickness of the carbon fiber sheet is determined by measuring the thickness at any 10 locations on the carbon fiber sheet and averaging the measured values. When the carbon fiber sheet is subjected to a heat and pressure treatment, the above-mentioned preferable thickness means the thickness of the carbon fiber sheet after the heat and pressure treatment.

[0026] The weight of the carbon fiber sheet is 20 g / m ２ More than 40 g / m ２ More preferably, 60 g / m or more ２ When the basis weight of the carbon fiber sheet is equal to or greater than the above lower limit, a porous electrode substrate with good handleability is easily obtained. ２ Preferably, 400 g / m or less ２ More preferably, 300 g / m or less ２ When the basis weight of the carbon fiber sheet is equal to or less than the above upper limit, a porous electrode substrate with excellent conductivity is easily obtained. The lower and upper limits of the basis weight of the carbon fiber sheet can be arbitrarily combined, and are, for example, 20 to 500 g / m ２ is preferred, and 40 to 400 g / m ２ More preferably, 60 to 300 g / m ２ is more preferable.

[0027] The carbon fibers used in the carbon fiber sheet are as described above in the section on the porous electrode substrate. The resin used in the carbon fiber sheet is preferably a thermosetting resin such as a phenolic resin or a furan resin, but is not limited thereto. Examples of the phenolic resin include a resol-type phenolic resin and a novolac-type phenolic resin. Alternatively, a water-dispersible phenolic resin or a water-soluble phenolic resin may be used as the phenolic resin.

[0028] As the organic fibers used for the carbon fiber sheet, for example, carbon fiber precursor fibers, which are fibers that have a relatively large residual mass after carbonization, and fibrillar fibers, which are fibers that can bind carbon fibers in a mesh-like structure, are preferred. Note that, when producing the carbon fiber sheet, polyvinyl alcohol (PVA) or heat-sealable polyester or polyolefin organic polymer binders may be used, but those that do not remain after the carbonization treatment or that are not mesh-like are not included in the "organic fibers."

[0029] The polymer constituting the carbon fiber precursor fiber is preferably a polymer having a residual mass of 20% by mass or more after carbonization treatment. Examples of such polymers include acrylic polymers, cellulose polymers, and phenolic polymers. Among these, it is preferable to use an acrylic polymer containing 50% by mass or more of acrylonitrile units, considering that it has excellent spinnability, can bond carbon fibers together from low to high temperatures, has a large residual mass at the time of carbonization, and further has fiber elasticity and fiber strength during the entanglement treatment described below. That is, as the carbon fiber precursor fiber, an acrylic fiber is preferred, and an acrylic fiber containing 50% by mass or more of acrylonitrile units is more preferred.

[0030] The average fiber length of the carbon fiber precursor fiber is preferably 2 to 30 mm, since good dispersibility can be obtained. The average fiber diameter of the carbon fiber precursor fiber is preferably 1 to 5 μm. When the average fiber diameter of the carbon fiber precursor fiber is equal to or greater than the lower limit, the spinnability is excellent. When the average fiber diameter of the carbon fiber precursor fiber is equal to or less than the upper limit, breakage due to shrinkage during heating and pressurizing treatment or carbonization treatment can be easily suppressed. The cross-sectional shape of the carbon fiber precursor fiber is not particularly limited, but one with high circularity is preferred, since it has high mechanical strength after carbonization and can reduce production costs.

[0031] Fibrillar fibers refer to the entire fiber formed by partially branching small fibers (fibrils) that are components of fibers such as filaments or staples. Hereinafter, the fibers from which fibril fibers branch are also referred to as the "trunk," and the branched small fibers are also referred to as the "fibril portion." When dispersed together with carbon fibers, fibrillar fibers prevent the carbon fibers from re-bundling and also serve to make the carbon fiber sheet self-supporting after heating and pressurization. Examples of fibrillar fibers include natural fibers such as wood pulp; and synthetic pulps such as fibrillated polyethylene fibers, acrylic fibers, and aramid fibers. Furthermore, fibrillar refined cellulose fibers obtained by beating lyocell or Tencel, or fine cellulose, may also be used as fibrillar fibers. These fibers contain less metal than natural cellulose fibers and are therefore preferred from the viewpoint of preventing proton conduction inhibition and deterioration of fluorine-based electrolyte membranes in fuel cells.

[0032] The average fiber length of the fibril fiber trunks is preferably 0.5 to 20 mm. When the average fiber length of the fibril fiber trunks is equal to or greater than the lower limit, the mechanical strength of the carbon fiber sheet is easily ensured. When the average fiber length of the fibril fiber trunks is equal to or less than the upper limit, good dispersibility is easily obtained. The average fiber diameter of the fibril fiber trunks is preferably 1 to 50 μm. When the average fiber diameter of the fibril fiber trunks is equal to or greater than the lower limit, good dispersibility is obtained. When the average fiber diameter of the fibril fiber trunks is equal to or less than the upper limit, breakage due to shrinkage during heat treatment is easily suppressed. The average fiber diameter of the fibril portion of the fibril fiber is preferably 0.01 to 30 μm. When the average fiber diameter of the fibril portion of the fibril fiber is equal to or greater than the lower limit, dehydration during heating and pressurization of the carbon fiber sheet and gas permeability of the porous electrode substrate are easily ensured. When the average fiber diameter of the fibril portion of the fibril fiber is equal to or less than the upper limit, dispersibility is improved.

[0033] <<Example of Manufacturing Method>> The manufacturing method of the porous electrode substrate of this embodiment includes the following steps (i) to (iv). Step (i): A step of obtaining a precursor sheet by papermaking a dispersion liquid in which carbon fibers, or carbon fibers and organic fibers are dispersed in a dispersion medium. Step (ii): A step of adding a resin to the precursor sheet to obtain a carbon fiber sheet. Step (iii): A step of heating and pressurizing the carbon fiber sheet. Step (iv): A step of carbonizing the carbon fiber sheet after the heating and pressurizing treatment to obtain a porous electrode substrate. From the viewpoints of productivity and the mechanical strength of the porous electrode substrate, the porous electrode substrate is preferably manufactured by a continuous method, but is not limited thereto, and may be manufactured by a batch method.

[0034] Step (i): For papermaking, a dispersion liquid in which only carbon fibers are dispersed in a dispersion medium may be used, or a dispersion liquid in which carbon fibers and organic fibers are dispersed in a dispersion medium may be used. Examples of dispersion media include water; and organic solvents such as methanol, ethanol, ethylene glycol, and propylene glycol. These dispersion media may be used alone or in combination of two or more. Among these, water is preferably used as the dispersion medium from the viewpoint of productivity. The water may be deionized water. An organic polymer binder (such as polyvinyl alcohol) that is burned off during the carbonization treatment may be added to the dispersion liquid before papermaking. The organic polymer binder may be in a solid form such as fibers or particles, or in a liquid form.

[0035] When organic fibers are used, it is preferable to subject the precursor sheet obtained by papermaking to an entanglement treatment. This results in a precursor sheet with a higher strength and an entangled structure in which the carbon fibers and organic fibers are three-dimensionally entangled. The entanglement treatment method is not particularly limited, but examples include mechanical entanglement methods such as needle punching; high-pressure liquid injection methods such as water jet punching; and high-pressure gas injection methods such as steam jet punching, and these may be combined. Among these, high-pressure liquid injection methods are preferred because they easily suppress breakage of the carbon fibers during the entanglement treatment and easily achieve appropriate entanglement. When organic fibers are used, the content of the organic fibers relative to the total mass of the precursor sheet is preferably 10% by mass or more, more preferably 15% by mass or more, and preferably 50% by mass or less, and more preferably 40% by mass or less. The lower and upper limits of the organic fiber content can be arbitrarily combined, for example, 10 to 50% by mass is preferred, and 15 to 40% by mass is more preferred.

[0036] The precursor sheet is preferably dried at 90 to 120° C. before step (ii). Examples of drying methods include heating using a high-temperature atmospheric furnace, a far-infrared heating furnace, a hot plate, a hot roll, or the like.

[0037] Step (ii): Examples of methods for adding a resin to a precursor sheet include spraying or dripping a resin dispersion onto the surface of the precursor sheet using a spray nozzle; flowing the resin dispersion down onto the surface of the precursor sheet using a curtain coater; and uniformly coating the surface of the precursor sheet with the resin dispersion using a kiss coater. When producing a porous electrode substrate containing carbon powder, it is preferable to add carbon powder to the resin dispersion. From the viewpoint of production costs, the dispersion medium used in the resin dispersion is preferably water, alcohol, dimethylformamide, dimethylacetamide, or a mixture thereof. When water is used as the dispersion medium, a dispersant such as a surfactant may be used.

[0038] The carbon fiber sheet after the addition of the resin dispersion may be dried, for example, at 60 to 110°C, more preferably 70 to 100°C. The resin in the carbon fiber sheet may be uniform in the thickness direction or may have a concentration gradient. The total amount of resin and organic fiber per 100 parts by mass of carbon fiber in the carbon fiber sheet is preferably 50 parts by mass or more, more preferably 60 parts by mass or more, and even more preferably 80 parts by mass or more. If the total amount is equal to or greater than the lower limit, the carbon fibers are less likely to fall off. The total amount is preferably 180 parts by mass or less, more preferably 160 parts by mass or less, and even more preferably 140 parts by mass or less. If the total amount is equal to or less than the upper limit, sufficient voids are easily secured, making it easier to obtain a porous electrode substrate with excellent gas permeability. The lower and upper limits of the total amount can be arbitrarily combined; for example, 50 to 180 parts by mass is preferred, 60 to 160 parts by mass is more preferred, and 80 to 140 parts by mass is more preferred.

[0039] Step (iii): Examples of a method for heat-pressing a carbon fiber sheet include a method of applying smooth rigid plates to both sides of the carbon fiber sheet and hot pressing it, and a method using a hot roll press or a continuous belt press. When heat-pressing a carbon fiber sheet, a release agent may be applied to the carbon fiber sheet, or release paper may be sandwiched between the carbon fiber sheet and the rigid plate, heated roll, or belt to prevent fibrous materials from adhering to the rigid plate, roll, or belt.

[0040] The temperature of the heating and pressurizing treatment varies depending on the type and content of the resin and organic fiber contained in the carbon fiber sheet, but is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 150°C or higher. When the temperature of the heating and pressurizing treatment is above the lower limit, carbonization easily proceeds. The temperature of the heating and pressurizing treatment is preferably 400°C or lower, more preferably 200°C or lower, and even more preferably 190°C or lower. When the temperature of the heating and pressurizing treatment is below the upper limit, burning of the resin and organic fiber is easily avoided. The lower and upper limits of the temperature of the heating and pressurizing treatment can be arbitrarily combined; for example, 100 to 400°C is preferred, 120 to 200°C is more preferred, and 150 to 190°C is more preferred. The pressure of the heating and pressurizing treatment is preferably 0.05 MPa or higher, more preferably 1 MPa or higher. When the pressure of the heating and pressurizing treatment is above the lower limit, the surface of the carbon fiber sheet is easily smoothed. The pressure of the heating and pressurizing treatment is preferably 20 MPa or lower, and more preferably 15 MPa or lower. When the pressure of the heating and pressurizing treatment is equal to or less than the upper limit, the carbon fibers are less likely to be broken during the heating and pressurizing treatment. The lower and upper limits of the pressure of the heating and pressurizing treatment can be arbitrarily combined, and are preferably 0.05 to 20 MPa, and more preferably 1 to 15 MPa. The time of the heating and pressurizing treatment is preferably 30 seconds to 1 hour, and more preferably 1 to 10 minutes.

[0041] Step (iv): By subjecting the carbon fiber sheet to a carbonization treatment, the resin and organic fibers contained in the carbon fiber sheet are carbonized, and in the resulting porous electrode substrate, the carbonized resin and fibrous carbonized resin become carbon that binds the carbon fibers.

[0042] The carbonization treatment of the carbon fiber sheet is preferably carried out in an inert atmosphere at 1000° C. or higher, since this makes it easier to obtain a porous electrode substrate with sufficient conductivity. The temperature of the carbonization treatment is more preferably 1000 to 3000° C., even more preferably 1000 to 2400° C., and particularly preferably 1000 to 2200° C. The time of the carbonization treatment is preferably 1 minute to 1 hour, and more preferably 10 minutes to 1 hour.

[0043] Prior to the carbonization treatment, a pre-carbonization treatment may be carried out in an inert atmosphere at a temperature of 300°C or higher but lower than 1000°C. By carrying out the pre-carbonization treatment, it becomes easier to discharge decomposition gas containing a large amount of sodium generated in the early stage of carbonization, and it becomes easier to suppress adhesion or deposition of various decomposition products on the inner walls of the carbonization furnace, or the occurrence of corrosion or black stains due to the decomposition products. The temperature of the pre-carbonization treatment is more preferably 300 to 800°C. The time of the pre-carbonization treatment is preferably 1 minute to 1 hour, more preferably 10 minutes to 1 hour.

[0044] The method for producing a porous electrode substrate may not include one or more of the steps (i) to (iii). For example, the method may involve carbonization of a carbon fiber sheet without heating and pressurizing it. Alternatively, the method may involve using organic fibers together with carbon fibers in step (i) without performing step (ii). Alternatively, instead of step (i), a precursor sheet may be produced by a dry method in which carbon fibers or carbon fibers and organic fibers are dispersed in the air and allowed to settle.

[0045] (Other embodiments) As the porous electrode substrate, a porous electrode substrate from which the carbonization process has been omitted, or commercially available carbon paper, etc. may be used as the porous electrode substrate. A porous electrode substrate from which the carbonization process has been omitted can reduce energy costs compared to when carbonization is performed. Examples of porous electrode substrates from which the carbonization process has been omitted include a carbon fiber web in which carbon fibers are bound with a binder filled with conductive material particles; and a porous electrode substrate in which fine conductive materials such as carbon are bound with a binder such as a resin.

[0046] <Coating Layer> Examples of the coating layer include the following coating layer X and coating layer Y. Coating layer X: A coating layer containing carbon powder A having an average particle size of 5 to 800 nm and carbon powder B having an average particle size of 1 to 50 μm. Coating layer Y: A coating layer containing at least one carbon powder C selected from the group consisting of carbon black, milled fiber, carbon nanotube, carbon nanofiber, coke, activated carbon, and amorphous carbon, and pyrolytic graphite.

[0047] It is preferable that each of the coating layers X and Y contains a water repellent. The coating layers X and Y may further contain optional components other than the carbon powder A, the carbon powder B, and the water repellent, as necessary, within a range that does not impair the effects of the present invention.

[0048] When forming a coating layer, a portion of the coating liquid described below permeates into the porous electrode substrate, making it difficult to clearly define the boundary between the coating layer and the porous electrode substrate. However, in the present invention, the portion where the coating liquid has not permeated into the porous electrode substrate is defined as the coating layer. That is, in the case of coating layer X, the portion of the layer consisting only of carbon powder A and carbon powder B, a water repellent agent that is included as needed, and optional components is defined as coating layer X. In addition, in the case of coating layer Y, the portion of the layer consisting only of carbon powder C and pyrolytic graphite, a water repellent agent that is included as needed, and optional components is defined as coating layer Y.

[0049] [Coating Layer X] When the coating layer X contains carbon powder A, carbon powder B, and a water repellent, the coating layer X is formed by binding the carbon powder A and the carbon powder B together with the water repellent, which acts as a binder. In other words, the carbon powder A and the carbon powder B are incorporated into a network formed by the water repellent, resulting in a fine mesh structure. The coating layer X preferably contains a fibrous water repellent. This not only strengthens the mesh structure and improves the strength of the coating layer X, but also causes the fibrous water repellent to become entangled with the porous electrode substrate, thereby further improving the adhesion between the coating layer X and the porous electrode substrate, resulting in a gas diffusion layer for a polymer electrolyte fuel cell having high peel strength from the coating layer X.

[0050] (Carbon Powder A) Examples of carbon powder A include carbon black, milled fiber, carbon nanotubes, carbon nanofibers, coke, activated carbon, amorphous carbon, and graphite other than pyrolytic graphite (hereinafter also referred to as "other graphite"). Carbon powder A is preferably carbon black, milled fiber, carbon nanotubes, carbon nanofibers, coke, activated carbon, or amorphous carbon, more preferably carbon black, milled fiber, or other graphite, and even more preferably carbon black. These carbon powders A may be used alone or in combination of two or more.

[0051] Carbon black has a significantly larger number of particles per unit mass than graphite powder, and above a certain critical concentration, the agglomerates are connected in a three-dimensional network to form macroscopic conductive paths. Examples of carbon black include acetylene black, ketjen black, furnace black, channel black, lamp black, and thermal black. Commercially available acetylene black products include "Denka Black (registered trademark)" manufactured by Denka Co., Ltd. Commercially available ketjen black products include "Ketjen Black EC" manufactured by Lion Corporation Commercially available furnace black products include "Vulcan XC72" manufactured by CABOT Corporation.

[0052] The milled fiber may be produced by pulverizing virgin carbon fiber, or may be produced from recycled products such as carbon fiber reinforced thermosetting resin molded products, carbon fiber reinforced thermoplastic resin molded products, prepregs, etc. The carbon fiber used as the raw material for the milled fiber may be PAN-based carbon fiber, pitch-based carbon fiber, or rayon-based carbon fiber.

[0053] Other graphite has a highly crystalline graphite structure, and the average particle size of its primary particles is generally several micrometers to several hundred micrometers. Examples of other graphite include spherical graphite, flake graphite, lump graphite, amorphous graphite, artificial graphite, and expanded graphite. Among these, spherical graphite and flake graphite are preferred from the viewpoint of electrical conductivity.

[0054] The average particle diameter of the carbon powder A is 5 to 800 nm. When the average particle diameter of the carbon powder is equal to or greater than the lower limit, the pores of the porous electrode substrate can be prevented from being filled with small-diameter particles of the carbon powder A, thereby achieving sufficient gas permeability. When the average particle diameter of the carbon powder A is equal to or less than the upper limit, a uniform coating liquid can be easily obtained. The average particle diameter of the carbon powder A is preferably 10 nm or more, more preferably 15 nm or more, and even more preferably 30 nm or more, and is preferably 800 nm or less, more preferably 500 nm or less, and particularly preferably 100 nm or less. The lower and upper limits of the average particle diameter of the carbon powder A can be arbitrarily combined; for example, 10 to 800 nm is preferred, 15 to 500 nm is more preferred, and 30 to 100 nm is even more preferred. The average particle diameter of the carbon powder A was determined by photographing the surface of the coating layer with an electron microscope and analyzing the image using the method described below.

[0055] (Carbon Powder B) Examples of carbon powder B include pyrolytic graphite, milled fiber, coke, activated carbon, and amorphous carbon. As carbon powder B, pyrolytic graphite, milled fiber, coke, activated carbon, and amorphous carbon are preferred, and pyrolytic graphite is more preferred. These carbon powders B may be used alone or in combination of two or more.

[0056] The average particle diameter of the carbon powder B is 1 to 50 μm. When the average particle diameter of the carbon powder B is equal to or greater than the lower limit, a sufficient effect of improving conductivity can be obtained. When the average particle diameter of the carbon powder B is equal to or less than the upper limit, a uniform coating liquid can be easily obtained. The average particle diameter of the carbon powder B is preferably 3 μm, more preferably 5 μm or more, and preferably 35 μm or less, more preferably 11 μm or less. The lower and upper limits of the average particle diameter of the carbon powder B can be arbitrarily combined, and for example, 3 to 35 μm is preferred, and 5 to 11 μm is more preferred. The average particle diameter of the carbon powder B was determined by photographing the surface of the coating layer with an electron microscope and analyzing the image using the method described below.

[0057] The aspect ratio of carbon powder B, which is the ratio of the average particle diameter (μm) of carbon powder B to the average thickness (μm) of carbon powder B, is preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more. When the aspect ratio of carbon powder B is equal to or greater than the lower limit, a sufficient improvement in conductivity can be achieved. The aspect ratio of carbon powder B is preferably 40 or less, more preferably 20 or less, and even more preferably 10 or less. When the aspect ratio of carbon powder B is equal to or less than the upper limit, the viscosity increase during mixing is small, and a uniform coating liquid can be easily obtained. The lower and upper limits of the aspect ratio of carbon powder B can be arbitrarily combined, and for example, 2 to 40 is preferred, 3 to 20 is more preferred, and 4 to 10 is even more preferred. The average thickness of carbon powder B was determined by taking photographs at a magnification of 1000 times or more using a microscope such as a scanning electron microscope or a transmission electron microscope, randomly selecting 10 different carbon powder B samples, measuring their thicknesses, and calculating the average value. When a catalogue value is available, the catalogue value may be used as the average particle size or average thickness of the carbon powder B as a simple measurement value.

[0058] The content of carbon powder B is preferably 9% by mass or more, more preferably 15% by mass or more, and even more preferably 30% by mass or more, relative to the total mass of coating layer X. When the content of carbon powder B is equal to or greater than the lower limit, a sufficient improvement in conductivity can be obtained. The content of carbon powder B is preferably 50% by mass or less, more preferably 50% by mass or less, and even more preferably 45% by mass or less. When the content of carbon powder B is equal to or less than the upper limit, an excessive increase in viscosity can be suppressed, and sufficient coatability can be obtained. The lower and upper limits of the content of carbon powder B can be arbitrarily combined; for example, 9 to 50% by mass is preferred, 15 to 50% by mass is more preferred, and 30 to 45% by mass is even more preferred.

[0059] The mass ratio of the carbon powder A content to the carbon powder B content, expressed as carbon powder A / carbon powder B, i.e., the carbon powder A / carbon powder B ratio, is preferably 0.5 or more, and more preferably 1.0 or more. When the carbon powder A / carbon powder B ratio is equal to or greater than the lower limit, the contact area between the carbon powder A and the carbon powder B is large, and a sufficient improvement in conductivity can be obtained. The carbon powder A / carbon powder B ratio is preferably 9.0 or less, more preferably 4.0 or less, even more preferably 2.0 or less, particularly preferably 1.8 or less, and most preferably 1.2 or less. When the carbon powder A / carbon powder B ratio is equal to or less than the upper limit, the amount of carbon powder around each pyrolytic graphite particle is not excessive, and therefore a sufficient amount of water repellent agent as a binder is present, resulting in sufficient coating strength. The lower and upper limits of the carbon powder A / carbon powder B ratio can be arbitrarily combined, and are, for example, preferably 0.5 to 9.0, more preferably 1.0 to 4.0, more preferably 1.0 to 2.0, more preferably 1.0 to 1.8, and even more preferably 1.0 to 1.2.

[0060] (Water repellent agent) Examples of the water repellent agent include fluororesins and silicone resins. Among these, fluororesins are preferred from the viewpoint of being particularly excellent in water repellency. These water repellents may be used alone or in combination of two or more. The water repellent agent may be used by dispersing it in a solvent such as water.

[0061] Examples of fluororesins include tetrafluoroethylene-hexafluoropropylene copolymer, polytetrafluoroethylene (PTFE), tetrafluoroethylene-ethylene copolymer, etc. Among these, PTFE is preferred. Among these, PTFE produced by emulsion polymerization is more preferred, and dispersion-type PTFE is even more preferred, in order to turn the water repellent into fiber.

[0062] When the coating layer X contains a water repellent, the content of the water repellent is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, relative to the total mass of the coating layer X. When the content of the water repellent is equal to or greater than the aforementioned lower limit, sufficient water repellency can be obtained. When the content of the water repellent is equal to or greater than the aforementioned upper limit, sufficient conductivity improvement effect can be obtained. The lower and upper limits of the oil content of the water repellent can be arbitrarily combined, and for example, 10 to 40% by mass is preferred, 15 to 35% by mass is more preferred, and 20 to 30% by mass is even more preferred.

[0063] (Optional Components) Examples of optional components include surfactants, water-soluble polymers, thickeners, reinforcing agents, stabilizers, fillers, crosslinking agents, etc. These optional components may be used alone or in combination of two or more.

[0064] (Physical Properties of Coating Layer X) The thickness of the coating layer X is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more. When the thickness of the coating layer X is equal to or greater than the lower limit, the fuel gas and oxidizing gas are easily diffused, improving reaction efficiency. In addition, when the porous electrode substrate contains carbon fibers, the carbon fibers can be prevented from breaking through the coating layer and reaching the catalyst layer or the polymer electrolyte membrane. The thickness of the coating layer X is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 40 μm or less. When the thickness of the coating layer X is equal to or less than the upper limit, an increase in electronic resistance due to the coating layer X can be suppressed, and good power generation performance can be maintained. The lower and upper limits of the thickness of the coating layer X can be arbitrarily combined, and are, for example, preferably 3 to 100 μm, more preferably 5 to 50 μm, and even more preferably 10 to 40 μm. The thickness of the coating layer X is the average value obtained by measuring the thickness at any 10 locations on the coating layer.

[0065] The surface roughness of the coating layer X is preferably 3.0 μm or less, more preferably 2.9 μm or less, and even more preferably 2.8 μm or less. If the surface roughness of the coating layer X is equal to or less than the above upper limit, the contact resistance with the catalyst layer can be reduced. The surface roughness of the coating layer X is preferably 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more. If the surface roughness of the coating layer X is equal to or greater than the above lower limit, the adhesion with the catalyst layer can be improved. The lower and upper limits of the surface roughness of the coating layer X can be arbitrarily combined, and for example, are preferably 1.0 to 3.0 μm, more preferably 1.5 to 2.9 μm, and even more preferably 2.0 to 2.8 μm. The surface roughness of the coating layer is the surface roughness Ra measured by the method described below on the surface of the coating layer opposite the porous electrode substrate.

[0066] [Coating Layer Y] When the coating layer Y contains pyrolytic graphite, carbon powder C, and a water repellent, the coating layer Y containing pyrolytic graphite, carbon powder C, and a water repellent is formed by binding the pyrolytic graphite and carbon powder C together with the water repellent, which acts as a binder. In other words, the pyrolytic graphite and carbon powder are incorporated into a network formed by the water repellent, resulting in a fine mesh structure. The coating layer Y preferably contains a fibrous water repellent. This not only strengthens the mesh structure and improves the strength of the coating layer Y, but also causes the fibrous water repellent to become entangled with the porous electrode substrate, thereby further improving the adhesion between the coating layer Y and the porous electrode substrate, resulting in a gas diffusion layer for a polymer electrolyte fuel cell having high peel strength from the coating layer Y.

[0067] (Pyrolytic graphite) Pyrolytic graphite can be obtained by heat treating powdered coke at 2500°C or higher to graphitize it. It is believed that using pyrolytic graphite in the gas diffusion layer reduces the volumetric shrinkage rate when the ink dries, thereby suppressing the occurrence of cracks on the surface of the coating layer, without reducing electrical conductivity. The heat treatment temperature is preferably 2500 to 3500°C. The heat treatment is preferably carried out in an inert gas. Pyrolytic graphite obtained by such heat treatment contains few impurities, and the thermal conductivity of the graphite itself is high.

[0068] The aspect ratio of pyrolytic graphite, which is the ratio of the average particle size (μm) of pyrolytic graphite to the average thickness (μm) of pyrolytic graphite, is preferably 2 or more, more preferably 3 or more, and even more preferably 4 or more. When the aspect ratio of pyrolytic graphite is equal to or greater than the lower limit, a sufficient improvement in conductivity can be achieved. The aspect ratio of pyrolytic graphite is preferably 40 or less, more preferably 20 or less, and even more preferably 10 or less. When the aspect ratio of pyrolytic graphite is equal to or less than the upper limit, the viscosity increase during mixing is small, and a uniform coating liquid can be easily obtained. The lower and upper limits of the aspect ratio of pyrolytic graphite can be arbitrarily combined; for example, 2 to 40 is preferred, 3 to 20 is more preferred, and 4 to 10 is even more preferred. The average particle size of pyrolytic graphite is measured using a laser diffraction particle size distribution analyzer to determine the 50% cumulative diameter in volume equivalent. The average thickness of pyrolytic graphite is determined by taking photographs at a magnification of 1000 times or more using a microscope such as a scanning electron microscope or a transmission electron microscope, randomly selecting 10 different pieces of pyrolytic graphite, measuring their thicknesses, and calculating the average value. If it is difficult to measure the average particle size of pyrolytic graphite using a laser diffraction particle size distribution analyzer, photographs can be taken at a magnification of 1000 times or more using a microscope such as a scanning electron microscope or a transmission electron microscope, randomly selecting 10 different pieces of pyrolytic graphite, measuring their lengths, and calculating the average value, which can be used as a substitute for the average particle size. Furthermore, when catalog values ​​are available, the catalog values ​​may be used as a simplified measurement value for the average particle size or average thickness of pyrolytic graphite.

[0069] The average particle size of the pyrolytic graphite is preferably 3 μm or more, more preferably 4 μm or more, and even more preferably 5 μm or more. When the average particle size of the pyrolytic graphite is equal to or greater than the lower limit, a sufficient effect of improving conductivity can be obtained. The average particle size of the pyrolytic graphite is preferably 50 μm or less, more preferably 35 μm or less, and even more preferably 11 μm or less. When the average particle size of the pyrolytic graphite is equal to or less than the upper limit, a uniform coating liquid can be easily obtained. The lower and upper limits of the average particle size of the pyrolytic graphite can be arbitrarily combined, and for example, 3 to 50 μm is preferred, 4 to 35 μm is more preferred, and 5 to 11 μm is even more preferred.

[0070] The content of pyrolytic graphite is preferably 9% by mass or more, more preferably 15% by mass or more, and even more preferably 30% by mass or more, relative to the total mass of the coating layer Y. When the content of pyrolytic graphite is equal to or greater than the lower limit, a sufficient effect of improving conductivity can be obtained. The content of pyrolytic graphite is preferably 50% by mass or less, more preferably 45% by mass or less. When the content of pyrolytic graphite is equal to or less than the upper limit, an excessive increase in viscosity can be suppressed, and sufficient coatability can be obtained. The lower and upper limits of the content of pyrolytic graphite can be arbitrarily combined; for example, 9 to 50% by mass is preferred, 15 to 50% by mass is more preferred, and 30 to 45% by mass is even more preferred.

[0071] (Carbon Powder C) Examples of carbon powder C include carbon black, milled fiber, carbon nanotubes, carbon nanofibers, coke, activated carbon, amorphous carbon, and graphite other than pyrolytic graphite. Carbon powder C is preferably carbon black, milled fiber, carbon nanotubes, carbon nanofibers, coke, activated carbon, or amorphous carbon, more preferably carbon black, milled fiber, or other graphite, and even more preferably carbon black. These carbon powders C may be used alone or in combination of two or more. Carbon black, milled fiber, and other graphite are as described for carbon powder A.

[0072] The average particle diameter of the carbon powder C is preferably 5 nm or more, more preferably 10 nm or more, even more preferably 15 nm or more, and particularly preferably 30 nm or more. When the average particle diameter of the carbon powder C is equal to or greater than the lower limit, filling of the pores of the porous electrode substrate with small-diameter particles of the carbon powder C can be suppressed, thereby achieving sufficient gas permeability. The average particle diameter of the carbon powder C is preferably 800 nm or less, more preferably 500 nm or less, and even more preferably 100 nm or less. When the average particle diameter of the carbon powder C is equal to or less than the upper limit, a uniform coating liquid can be easily obtained. The lower and upper limits of the average particle diameter of the carbon powder C can be arbitrarily combined; for example, 5 to 800 nm is preferred, 10 to 800 nm is more preferred, 15 to 500 nm is even more preferred, and 30 to 100 nm is particularly preferred. The average particle diameter of the carbon powder C was determined by photographing the surface of the coating layer with an electron microscope and analyzing the image using the method described below.

[0073] The mass ratio of the carbon powder C to the pyrolytic graphite, i.e., the carbon powder / pyrolytic graphite ratio, which represents the mass ratio of the carbon powder C content to the pyrolytic graphite content, is preferably 0.5 or more, and more preferably 1.0 or more. When the carbon powder / pyrolytic graphite ratio is equal to or greater than the lower limit, the contact area between the carbon powder and the pyrolytic graphite is large, and a sufficient improvement in electrical conductivity is obtained. The carbon powder / pyrolytic graphite ratio is preferably 9.0 or less, more preferably 4.0 or less, even more preferably 2.0 or less, particularly preferably 1.8 or less, and most preferably 1.2 or less. When the carbon powder / pyrolytic graphite ratio is equal to or less than the upper limit, the amount of carbon powder around each pyrolytic graphite particle is not excessive, and a sufficient amount of water repellent as a binder is present, resulting in sufficient coating strength. The lower and upper limits of the carbon powder / pyrolytic graphite ratio can be arbitrarily combined, and for example, is preferably 0.5 to 9.0, more preferably 1.0 to 4.0, even more preferably 1.0 to 2.0, particularly preferably 1.0 to 1.8, and most preferably 1.0 to 1.2.

[0074] (Water Repellent Agent) The preferred embodiments of the water repellent agent are the same as those described for the coating layer X. As the water repellent agent, a fluororesin is preferred from the viewpoint of particularly excellent water repellency.

[0075] When the coating layer Y contains a water repellent, the content of the water repellent is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more, relative to the total mass of the coating layer Y. When the content of the water repellent is equal to or greater than the lower limit, sufficient water repellency can be obtained. When the content of the water repellent is equal to or greater than the total mass of the coating layer Y, the content of the water repellent is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably 30% by mass or less. When the content of the water repellent is equal to or less than the upper limit, sufficient conductivity improvement effect can be obtained. The lower and upper limits of the content of the water repellent can be arbitrarily combined, and for example, 10 to 40% by mass is preferred, 15 to 35% by mass is more preferred, and 20 to 30% by mass is more preferred.

[0076] (Optional Components) The optional components are as described in the coating layer X.

[0077] (Physical Properties of Coating Layer Y) The thickness of the coating layer Y is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more. When the thickness of the coating layer Y is equal to or greater than the lower limit, the fuel gas and oxidizing gas are easily diffused, improving reaction efficiency. In addition, when the porous electrode substrate contains carbon fibers, the carbon fibers can be prevented from breaking through the coating layer and reaching the catalyst layer or the polymer electrolyte membrane. The thickness of the coating layer Y is preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 40 μm or less. When the thickness of the coating layer Y is equal to or less than the upper limit, an increase in electronic resistance due to the coating layer can be prevented, and good power generation performance can be maintained. The lower and upper limits of the thickness of the coating layer Y can be arbitrarily combined, and are, for example, preferably 3 to 100 μm, more preferably 5 to 50 μm, and even more preferably 10 to 40 μm. The thickness of the coating layer Y is the average value obtained by measuring the thickness at any 10 locations on the coating layer.

[0078] The surface roughness of the coating layer Y is preferably 3.0 μm or less, more preferably 2.9 μm or less, and even more preferably 2.8 μm or less. If the surface roughness of the coating layer Y is equal to or less than the above upper limit, the contact resistance with the catalyst layer can be reduced. The surface roughness of the coating layer Y is preferably 1.0 μm or more, more preferably 1.5 μm or more, and even more preferably 2.0 μm or more. If the surface roughness of the coating layer Y is equal to or greater than the above lower limit, the adhesion with the catalyst layer can be improved. The lower and upper limits of the surface roughness of the coating layer Y can be arbitrarily combined, and for example, are preferably 1.0 to 3.0 μm, more preferably 1.5 to 2.9 μm, and even more preferably 2.0 to 2.8 μm.

[0079] (Other embodiments) Although the coating layer 12 shown in Fig. 1 is formed on one surface of the porous electrode substrate 11, the coating layer may be formed on both surfaces of the porous electrode substrate. However, in consideration of productivity, gas diffusibility, and drainage, it is preferable that the coating layer is formed on only one surface of the porous electrode substrate. Note that when the coating layer is formed on only one surface of the porous electrode substrate, it is preferable to provide the coating layer on the surface of the porous electrode substrate that comes into contact with the catalyst layer in the solid polymer fuel cell, from the viewpoint of reducing the contact resistance between the catalyst layer and the porous electrode substrate when used in a solid polymer fuel cell described below.

[0080] <Physical Properties of Gas Diffusion Layer> In order to achieve good electrical conductivity and drainage, the thickness of the gas diffusion layer is preferably 55 μm or more, more preferably 100 μm or more, and preferably 350 μm or less, more preferably 250 μm or less. If the thickness of the gas diffusion layer is equal to or greater than the lower limit, handling is possible. If the thickness of the gas diffusion layer is equal to or less than the upper limit, good electrical conductivity is obtained. The lower and upper limits of the thickness of the gas diffusion layer can be arbitrarily combined, and for example, 55 to 350 μm is preferable, and 100 to 250 μm is more preferable. The thickness of the gas diffusion layer is the average value obtained by measuring the thickness at any 10 points on the gas diffusion layer.

[0081] <Method for Manufacturing Gas Diffusion Layer> An example of a method for manufacturing a gas diffusion layer will be described below. The method for manufacturing a gas diffusion layer according to this embodiment, for example, when the coating layer is coating layer X, includes the following steps (1) to (3). Step (1): Applying a coating liquid containing carbon powder A, carbon powder B, a solvent, and, if necessary, a water repellent and optional components to at least one surface of a porous electrode substrate to form a coating film on at least one surface of the porous electrode substrate. Step (2): Drying the porous electrode substrate on which the coating film has been formed in an environment of 50°C to 300°C to remove the solvent in the coating film and form a coating layer on at least one surface of the porous electrode substrate. Step (3): Heating the porous electrode substrate on which the coating layer has been formed to a temperature higher than 300°C but not higher than 400°C to sinter the water repellent and obtain a gas diffusion layer. When the coating layer is coating layer Y, a coating liquid containing pyrolytic graphite, carbon powder C, a solvent, and, if necessary, a water repellent and optional components is used in step (1).

[0082] (Step (1)) The coating liquid is obtained by mixing carbon powder A, carbon powder B, and a solvent, or pyrolytic graphite, carbon powder C, and a solvent, and optionally a water repellent and optional components, using a stirrer or the like. When a water repellent is used, for example, dispersion A containing carbon powder A, carbon powder B, and a solvent, or pyrolytic graphite, carbon powder C, and a solvent, and optionally optional components, and dispersion B containing a water repellent, a solvent, and optionally optional components are separately prepared, and dispersion A and dispersion B are mixed using a stirrer or the like to prepare the coating liquid. Examples of solvents include water; organic solvents such as lower alcohols and acetone; and mixed solvents of water and organic solvents. From the viewpoints of cost and environmental impact, water is preferred as the solvent. From the viewpoints of increasing the wettability and improving dispersibility of carbon powder A, carbon powder B, pyrolytic graphite, and carbon powder C, it is preferred that dispersion A contains at least one of an organic solvent and a surfactant. When water is used as a solvent in preparing the dispersion liquid B, it is preferable to disperse the water repellent in water using a surfactant, since the water repellent is difficult to disperse in water as it is. Alternatively, a dispersion in which the water repellent is dispersed in advance may be used as the dispersion liquid B.

[0083] The content of carbon powder A in dispersion A used in the coating liquid for forming coating layer X is preferably 5 to 30 mass %, more preferably 5 to 10 mass %, relative to the total mass of dispersion A. The content of carbon powder B is preferably 5 to 30 mass %, more preferably 5 to 10 mass %, relative to the total mass of dispersion A. The content of water repellent agent in dispersion B used in the coating liquid for forming coating layer X is preferably 1 to 20 mass %, more preferably 1 to 10 mass %, relative to the total mass of dispersion B.

[0084] The content of pyrolytic graphite in dispersion A used in the coating liquid for forming coating layer Y is preferably 5 to 30 mass %, and more preferably 5 to 10 mass %, relative to the total mass of dispersion A. The content of carbon powder C is preferably 5 to 30 mass %, and more preferably 5 to 10 mass %, relative to the total mass of dispersion A. The content of water repellent agent in dispersion B used in the coating liquid for forming coating layer Y is preferably 1 to 20 mass %, and more preferably 1 to 10 mass %, relative to the total mass of dispersion B.

[0085] The agitator used in preparing the coating liquid is not particularly limited, and examples thereof include a disper, a homogenizer, a sand mill, a jet mill, a ball mill, a bead mill, etc. Among these, a disper and a homogenizer are preferred from the viewpoints of easy operation and shortening the processing time. In order to fiberize the water repellent agent, it is preferred to keep the agitation temperature of the coating liquid at 30°C or higher and mix and agitate the coating liquid for 15 minutes or longer at a stirring speed of 5000 rpm or higher using a disper.

[0086] The viscosity of the coating liquid at 25°C is preferably 100 to 10,000 mPa·s. When the viscosity of the coating liquid is equal to or greater than the lower limit, the coating liquid is less likely to penetrate excessively into the porous electrode substrate, and the thickness of the coating layer can be easily maintained. When the viscosity of the coating liquid is equal to or less than the upper limit, the preparation time of the coating liquid can be shortened, and good productivity can be maintained.

[0087] The coating liquid can be applied to the surface of the porous electrode substrate by any conventionally known method, including bar coating, blade coating, screen printing, spray coating, curtain coating, and roll coating. These methods allow for the formation of a uniform coating film on the porous electrode substrate. The thickness of the coating film is preferably 40 μm or more, more preferably 50 μm or more. A coating film having a uniform thickness can be easily obtained if the thickness of the coating film is equal to or greater than the aforementioned lower limit. The thickness of the coating film is preferably 2000 μm or less, more preferably 1000 μm or less. A coating film having a thickness equal to or less than the aforementioned upper limit can further suppress the occurrence of cracks in the coating layer. The lower and upper limits of the coating film thickness can be arbitrarily combined, and are preferably, for example, 40 to 2000 μm, more preferably 50 to 1000 μm. From the viewpoint of productivity, the coating liquid application speed is preferably 1 to 20 m / min.

[0088] Prior to step (1), the porous electrode substrate may be subjected to a water-repellent treatment as needed to impart water repellency to the porous electrode substrate. For the water-repellent treatment, a dispersion liquid in which particles of a water-repellent agent such as a silicone resin or a fluororesin are dispersed in a solvent, such as the above-mentioned dispersion liquid B, can be used.

[0089] (Step (2)) For example, a plate heater, a heated roll, a hot air dryer, an IR heater, or the like can be used to dry the coating film. The ambient temperature (drying temperature) when drying the coating film is preferably 50°C or higher, more preferably 100°C or higher, and even more preferably 150°C or higher. If the drying temperature is equal to or higher than the lower limit, the drying rate of the coating film increases. In addition, a uniform coating layer can be easily formed. The drying temperature is preferably 300°C or lower, more preferably 300°C or lower, and even more preferably 300°C or lower. If the drying temperature is equal to or lower than the upper limit, the evaporation rate of the solvent does not become too fast, and the occurrence of cracks can be further suppressed. The lower and upper limits of the drying temperature can be arbitrarily combined; for example, 50 to 300°C is preferred, 100 to 300°C is more preferred, and 150 to 300°C is even more preferred. In consideration of productivity, the drying time of the coating film is preferably 30 seconds to 20 minutes, and more preferably 30 seconds to 10 minutes.

[0090] (Step (3)) In the manufacturing method of the gas diffusion layer of this embodiment, the gas diffusion layer is manufactured by sintering the dried "porous electrode substrate with a coating layer formed thereon" in an environment of more than 300°C and not more than 400°C. In step (3), i.e., the sintering step, the water repellent agent contained in the coating film is heated to near its melting point to melt the water repellent particles and control their shape, thereby controlling the pore structure of the coating layer and strengthening the binding between carbon powder A and carbon powder B or the binding between pyrolytic graphite and carbon powder C. Therefore, the sintering temperature is preferably more than 300°C and not more than 450°C, more preferably 320 to 420°C, and even more preferably 340 to 400°C. The sintering time is preferably 1 to 90 minutes, more preferably 1 to 60 minutes, and even more preferably 10 to 30 minutes. Note that if a surfactant is contained in the coating film, the surfactant will be burned off during the sintering step.

[0091] <Effects> The gas diffusion layer of the present embodiment described above includes a coating layer containing carbon powder A and carbon powder B, or pyrolytic graphite and carbon powder C, and the coating layer is suppressed from developing cracks. While the reason for the suppression of cracking in the coating layer is unclear, the following is believed to be the cause. One cause of cracking in the coating layer is agglomeration of carbon powder when the coating film provided on the porous electrode substrate is dried to form the coating layer, and cracks are thought to originate from the carbon powder agglomerates. Carbon powder with a small particle size is particularly prone to agglomeration. Because pyrolytic graphite has few impurities and a relatively large particle size compared to carbon powder, the combined use of carbon powder and pyrolytic graphite is thought to suppress the agglomeration of carbon powder and thus suppress the development of cracks. Additionally, because pyrolytic graphite has few impurities, the combined use of carbon powder and pyrolytic graphite increases thermal conductivity and improves power generation performance. Similarly, when carbon powder A and carbon powder B are used in combination, it is believed that aggregation of carbon powder A is suppressed, cracking can be suppressed, thermal conductivity is increased, and power generation performance is improved.

[0092] In view of the productivity and processability of the gas diffusion layer, the gas diffusion layer is preferably in the form of a roll. An example of the gas diffusion layer in the form of a roll will be described below.

[0093] 2 and 3 show an example of a roll of a gas diffusion layer (hereinafter simply referred to as a "roll") of the present invention. A roll 20 of a gas diffusion layer of this embodiment is a roll in which a laminate 22 (hereinafter also referred to as a "gas diffusion layer with a protective layer"), in which a protective layer 23 is provided on the coating layer 12 of a gas diffusion layer 10, is wound around a cylindrical core material 21 in a roll shape. In the roll 20 of a gas diffusion layer of this embodiment, the laminate 22 is wound around the core material 21 so that the protective layer 23 faces inside, but the laminate 22 may also be wound around the core material 21 so that the protective layer 23 faces outside.

[0094] <Core Material> The core material is preferably a lightweight, hollow core material that is easy to hold in an unwinding / rewinding device. Examples of the core material include paper and resin. From the viewpoint of reducing dust generated when the core material is attached to the device, a resin core material is preferred. Examples of resin include polyethylene, ABS resin, polystyrene, polypropylene, polyvinyl chloride, and polyethylene terephthalate. From the viewpoint of recycling the core material and being inexpensive, a paper core material is preferred. Furthermore, even if the core material is made of paper, by using a core material with a resin-coated surface, dust generated when the core material is attached to the device can be minimized.

[0095] The outer diameter of the core material is preferably 82.4 to 172.4 mm. If the outer diameter of the core material is equal to or greater than the lower limit, structural changes are less likely to occur before and after winding of the gas diffusion layer. If the outer diameter of the core material is equal to or less than the upper limit, the winding diameter does not become too large, thereby suppressing a decrease in productivity and an increase in weight during transportation. When the core material is hollow, the inner diameter of the core material is preferably 76.2 to 152.4 mm. Furthermore, the thickness of the core material is preferably 4 to 15 mm. If the thickness of the core material is equal to or greater than the lower limit, excellent durability is achieved even with repeated use. If the thickness of the core material is equal to or less than the upper limit, excessive weight increase of the rolled gas diffusion layer can be suppressed. The core material may be removed after the gas diffusion layer is wound into a roll.

[0096] <Protective Layer> The protective layer is a sheet for protecting the coating layer of the gas diffusion layer. Furthermore, by providing the protective layer, it is possible to prevent foreign matter such as carbon fibers and carbides that have fallen off from the porous electrode substrate from adhering to the coating layer. The protective layer may be any material that does not adhere to the coating layer, and examples thereof include paper and resin film. As paper, dust-free paper that generates little dust is preferred. As resin film, a resin film that does not deform much when pressed against the carbon fibers is preferred in order to protect the coating layer. Examples of materials for the resin film include polyethylene, ABS resin, polystyrene, polypropylene, polyvinyl chloride, polyethylene terephthalate, and polytetrafluoroethylene.

[0097] The thickness of the protective layer is preferably 5 to 100 μm. When the thickness of the protective layer is equal to or greater than the lower limit, damage to the coating layer due to piercing of the carbon fibers into the protective layer can be further suppressed. When the thickness of the protective layer is equal to or less than the upper limit, the winding diameter of the rolled gas diffusion layer does not become too large, thereby suppressing a decrease in productivity and an increase in transportation costs. The width of the protective layer is preferably equal to or greater than the width of the gas diffusion layer, with the difference being 200 mm or less. When the width of the protective layer is equal to or greater than the width of the gas diffusion layer, the effect of the protective layer can be fully obtained. In addition, damage to the coating layer by the edge portions of the protective layer can be suppressed. When the width of the protective layer is greater than the width of the gas diffusion layer, with the difference being 200 mm or less, an increase in the cost of the protective layer can be suppressed. In addition, good balance can be maintained during winding, and the winding shape is likely to be stable.

[0098] [Polymer electrolyte fuel cell] Figure 4 shows an example of a polymer electrolyte fuel cell of the present invention. The polymer electrolyte fuel cell 100 of this embodiment includes a membrane-electrode assembly (MEA) 30 and a pair of separators 40A, 40B. The membrane-electrode assembly 30 is sandwiched between the pair of separators 40A, 40B. When the membrane-electrode assembly 30 and the pair of separators 40A, 40B form one cell (single cell), the polymer electrolyte fuel cell 100 may be composed of one cell or may be an assembly of multiple cells.

[0099] <Membrane-Electrode Assembly> The membrane-electrode assembly 30 is composed of a polymer electrolyte membrane 31 and a pair of gas diffusion electrodes 32 A and 32 B. The polymer electrolyte membrane 31 is sandwiched between the pair of gas diffusion electrodes 32 A and 32 B.

[0100] (Polymer Electrolyte Membrane) The polymer electrolyte membrane 31 includes a polymer electrolyte. Examples of the polymer electrolyte include a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte. Examples of the fluorine-based polymer electrolyte include a polymer electrolyte having a tetrafluoroethylene skeleton. Examples of the hydrocarbon-based polymer electrolyte include sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether sulfone, sulfonated polysulfide, and sulfonated polyphenylene.

[0101] (Gas diffusion electrode) The gas diffusion electrode 32A includes a catalyst layer 321A and a gas diffusion layer 10. The gas diffusion electrode 32A is an electrode on the oxygen electrode side, and oxygen is supplied to the gas diffusion electrode 32A. The gas diffusion electrode 32A is also referred to as the "oxygen electrode," and the catalyst layer 321A is also referred to as the "oxygen electrode catalyst layer." The gas diffusion layer 10 provided on the gas diffusion electrode 32A is also referred to as the "oxygen electrode gas diffusion layer."

[0102] The gas diffusion electrode 32B includes a catalyst layer 321B and a gas diffusion layer 10. The gas diffusion electrode 32B is the electrode on the fuel electrode side, and hydrogen is supplied to the gas diffusion electrode 32B. The gas diffusion electrode 32B is also referred to as the "fuel electrode," and the catalyst layer 321B is also referred to as the "fuel electrode catalyst layer." The gas diffusion layer 10 provided on the gas diffusion electrode 32B is also referred to as the "fuel electrode gas diffusion layer."

[0103] The catalyst layer 321A is a layer containing a catalyst and a binder, and is a reaction field where an oxygen reduction reaction occurs. The catalyst layer 321B is a layer containing a catalyst and a binder, and is a reaction field where a hydrogen oxidation reaction occurs. Examples of the catalyst include a catalyst support in which platinum (Pt), ruthenium (Ru), or the like is supported on a support such as carbon; a carbon alloy catalyst; etc. The binder is preferably a polymer compound with ion exchange ability, and specific examples include fluorine-based ion exchange resins and hydrocarbon-based ion exchange resins. The thickness of each of the catalyst layers 321A and 321B is preferably 2 to 15 μm. If the thickness of the catalyst layer is within the above range, efficient power generation is possible.

[0104] The gas diffusion layer 10 provided on the gas diffusion electrodes 32A, 32B is the gas diffusion layer of the present invention described above, and therefore a description thereof will be omitted. The gas diffusion layer 10 is disposed so that the surface on the coating layer side faces the catalyst layer 321A or 321B.

[0105] <Separator> Of the pair of opposing surfaces of the separator 40A, a plurality of groove-shaped gas flow channels 41A are formed on the opposing surface facing the gas diffusion electrode 32A. A plurality of groove-shaped cooling water flow channels (not shown) may be formed on the surface of the separator 40A opposite to the opposing surface. Of the pair of opposing surfaces of the separator 40B, a plurality of groove-shaped gas flow channels 41B are formed on the opposing surface facing the gas diffusion electrode 32B. A plurality of groove-shaped cooling water flow channels (not shown) may be formed on the surface of the separator 40B opposite to the opposing surface.

[0106] Each of the separators 40A and 40B is preferably made of a material that is electrically conductive and gas impermeable, such as carbon.

[0107] <Method for Manufacturing a Polymer Electrolyte Fuel Cell> An example of a method for manufacturing a polymer electrolyte fuel cell will be described below. In the method for manufacturing a polymer electrolyte fuel cell of this embodiment, a catalyst layer is first formed on the coating layer of the gas diffusion layer described above to obtain a gas diffusion electrode. Specifically, a catalyst ink containing a catalyst, a binder, a solvent, and the like is applied to the coating layer of the gas diffusion layer to obtain a coating film of the catalyst layer. The method for applying the catalyst ink is not particularly limited, but examples include bar coating, blade coating, screen printing, spraying, curtain coating, and roll coating. These methods can form a uniform coating film of the catalyst layer on the coating layer of the gas diffusion layer. The formed coating film of the catalyst layer is dried by a common method to produce a gas diffusion electrode in which the catalyst layer is formed on the coating layer of the gas diffusion layer.

[0108] Next, the polymer electrolyte membrane is sandwiched between a pair of gas diffusion electrodes to obtain a membrane-electrode assembly. The obtained membrane-electrode assembly is sandwiched between a pair of separators to obtain a single cell. The obtained single cell may be used as a polymer electrolyte fuel cell, or multiple single cells may be stacked and used as a polymer electrolyte fuel cell.

[0109] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to the examples described below, and various modifications are possible without departing from the gist of the present invention.

[0110] [Measurement Method] <Measurement of Peel Strength> A test piece having a width of 15 mm was cut out from the gas diffusion layer, and the interface between the porous electrode substrate and the coating layer was peeled from the edge of the test piece by 180-degree peeling at a tension speed of 250 mm / min in an atmosphere of 23°C and 65% RH using a tensile tester (manufactured by Shimadzu Corporation, product name "Small Tabletop Tester EZ"), to measure the peel strength.

[0111] <Measurement of Surface Roughness Ra> Using a surface roughness measuring device (manufactured by Mitutoyo Corporation, product name "Surfcom 1400D-LCD"), the surface roughness Ra of the coating layer of the gas diffusion layer was measured with a cutoff value of 0.8 mm, a measurement interval of 4 mm, a range of 320 μm, and five measurement intervals.

[0112] <Calculation of Average Particle Diameter of Carbon Powders A to C> To evaluate the average particle diameter of carbon powders A, B, and C, the surface of the osmium-coated coating layer was imaged using a field emission scanning electron microscope (FE-SEM, model number: "JSM-7610F", manufactured by JEOL Ltd.) to obtain a secondary electron image (SEM image) of the coating layer surface. The imaging conditions were as follows: Acceleration voltage = 1 kV WD: 3 mm Detector: SE detector Vapor deposition conditions: osmium coat (Osmium vapor deposition conditions) Apparatus: Osmium Coater Tennant 20 model manufactured by Meiwafosis Co., Ltd. Vapor deposition conditions: Auto mode Vapor deposition thickness: 3 nm

[0113] The average particle diameters of carbon powders A to C were calculated from the obtained SEM images using the following analytical method. (Method for calculating the average particle diameter of carbon powders A and C) SEM images taken at a magnification of 50,000x, with only carbon powder A or C present within the observation field, were converted into 32-bit images using the image analysis software "ImageJ." The software was used to read the length and numerical value of the scale bar in the SEM image. After reflecting the length per pixel in the observed image, the areas containing scale information and observation conditions were deleted. The average brightness value was subtracted from the obtained image data area, and the area outside the image data area was interpolated with zeros so that the vertical and horizontal dimensions of the image were each a power of two. The obtained image was converted into a two-dimensional autocorrelation function, and then normalized to a maximum value of 1 by dividing the two-dimensional autocorrelation function by its maximum value. After performing circular averaging around the origin of the two-dimensional autocorrelation function to obtain a one-dimensional autocorrelation function, the distance closest to the origin at which the autocorrelation function value decays to 0.5 was defined as r*, and the particle diameter of carbon powder A or C was calculated using the following formula. Particle diameters were calculated for the 13 captured images, and the average value was taken as the average particle diameter: (Particle diameter of carbon powder A or C) = 3 x r*

[0114] (Method for analyzing particle size of carbon powder B) An SEM image of carbon powder B on the surface of the coating layer taken at a magnification of 10,000 times was converted into an 8-bit image using the image analysis software "ImageJ." The length and numerical value of the scale bar on the SEM image were read using the same software. After reflecting the length per pixel in the observed image, the Feret's diameter of carbon powder B was calculated by measuring the longest distance between any two points on the outline of the carbon powder, and this was taken as the particle size of carbon powder B. Particle sizes were calculated for the 13 images taken, and the average value was taken as the average particle size.

[0115] Example 1 Production of Porous Electrode Substrate PAN-based carbon fibers with an average fiber diameter of 7 μm and an average fiber length of 3 mm were used as carbon fibers. Acrylic fibers (manufactured by Mitsubishi Chemical Corporation, product name "D122") with an average fiber diameter of 4 μm and an average fiber length of 3 mm were used as carbon fiber precursor fibers. Splittable acrylic sea-island composite fibers (manufactured by Mitsubishi Chemical Corporation, product name "Bonnel M.V.P.-C651", average fiber length: 3 mm) composed of an acrylic polymer that fibrillates by beating and diacetate (cellulose acetate) were used as fibrillated fibers. A porous electrode substrate was produced as follows.

[0116] (1) Fiber defibration Carbon fibers were dispersed in water to a fiber concentration of 1% by mass (10 g / L) and defibrated in a mixer to prepare defibrated slurry fibers (SA). Carbon fiber precursor fibers were dispersed in water to a fiber concentration of 1% by mass (10 g / L) and defibrated in a mixer to prepare defibrated slurry fibers (Sb). Splittable acrylic sea-island composite fibers were dispersed in water to a fiber concentration of 1% by mass (10 g / L) and defibrated in a mixer to prepare defibrated slurry fibers (Sb').

[0117] (2) Precursor Sheet Production Disaggregated slurry fibers (SA), disaggregated slurry fibers (Sb), disaggregated slurry fibers (Sb'), and dilution water were weighed and dispersed so that the carbon fiber, carbon fiber precursor fiber, and fibrillar fiber were mixed in a mass ratio of 70:10:20, and the fiber concentration in the slurry was 1.44 g / L. For papermaking, a processing device was used, consisting of a net drive unit, a sheet-like material conveying device consisting of a 60 cm wide x 585 cm long plastic plain weave mesh connected in a belt-like shape and continuously rotating, a slurry supply unit with a width of 48 cm, and a vacuum dehydration device located below the net. A pressurized water jet processing device equipped with the following three water jet nozzles was located downstream of the processing device. Nozzle 1: A row of 50 nozzles with a hole diameter of 0.15 mm, each with a widthwise hole pitch of 1 mm (1001 nozzles / 1 m width), and an effective nozzle width of 500 mm. Nozzle 2: Hole diameter φ0.15 mm x 50, 1 hole widthwise hole pitch 1 mm (1001 holes / width 1 m), arranged in one row, nozzle effective width 500 mm. Nozzle 3: Hole diameter φ0.15 mm x 100, 2 holes widthwise hole pitch 1.5 mm, arranged in three rows, row pitch 5 mm, nozzle effective width 500 mm. Pressurized water jet pressure was 1 MPa for nozzle 1, 2 MPa for nozzle 2, and 1 MPa for nozzle 3. The slurry containing dispersed fibers was introduced from the slurry supply unit, and after dehydration under reduced pressure, it was passed through nozzle 1, nozzle 2, and nozzle 3 in that order to apply an entanglement treatment and obtain a precursor sheet with a three-dimensional entangled structure. The precursor sheet was dried at 150 ° C. for 3 minutes using a pin tenter tester (Tsujii Senki Kogyo Co., Ltd., product name "PT-2A-400") to obtain a precursor sheet. The carbon fibers, carbon fiber precursor fibers, and fibrillar fibers were dispersed well in the precursor sheet, and the handling properties were also good.

[0118] (3) Resin impregnation / drying and pressurized heat molding The obtained precursor sheet was impregnated with a phenolic resin dispersion and dried using a hot air dryer at an ambient temperature of 100° C. Next, the precursor sheet was sandwiched between papers coated with a silicone-based release agent on both sides, and press molding was performed using a double belt press at 190° C. and a belt speed of 0.2 m / min to obtain a carbon fiber sheet.

[0119] (4) Carbonization Treatment The obtained carbon fiber sheet was carbonized in a carbonization furnace under conditions of 2000°C in a nitrogen gas atmosphere to obtain a porous electrode substrate. The obtained porous electrode substrate was smooth and free from warping or undulation. The thickness of the obtained porous electrode substrate was 155 μm, and the gas permeability was 950 mL / (cm ２ The average pore size was 35 μm and the basis weight was 57 g / m ２ It was.

[0120] (5) Water-repellent treatment A water-repellent treatment liquid was prepared by mixing PTFE dispersion (manufactured by Mitsui Chemours Fluoroproducts Co., Ltd., trade name "31-JR"), polyoxyethylene (10) octylphenyl ether as a surfactant, and distilled water. Specifically, the PTFE dispersion and the surfactant were mixed so that the solids concentration in the water-repellent treatment liquid was 1% by mass of PTFE and 2% by mass of surfactant, and distilled water was added. The water-repellent treatment liquid was prepared by stirring at 1000 rpm for 10 minutes using a disperser. The porous electrode substrate was impregnated by immersion in the water-repellent treatment liquid. The impregnated porous electrode substrate was passed through two pairs of nip rolls to remove excess water-repellent treatment liquid, and then dried in a drying oven to obtain a water-repellent treated porous electrode substrate.

[0121] <Preparation of Coating Liquid> Pyrolytic graphite (manufactured by Ito Graphite Industries Co., Ltd., trade name "PC-H", average particle size 7.68 μm), acetylene black as carbon powder (manufactured by Denka Company, trade name "Denka Black (registered trademark)", average particle size 35 nm), and ion-exchanged water were mixed to prepare dispersion A. Specifically, pyrolytic graphite and carbon powder were mixed so that the pyrolytic graphite was 14 parts by mass per 100 parts by mass of carbon powder, ion-exchanged water was further added, and the mixture was stirred at 10,000 rpm for 1 minute while cooling using a mixer (manufactured by Primix Corporation, product name "Homomixer MARK-II") to prepare dispersion A. Note that the particles of the pyrolytic graphite were observed at five points using a scanning electron microscope (SEM) described below, and the aspect ratio was calculated to be 5. A polytetrafluoroethylene (PTFE) dispersion was added as dispersion B to the obtained dispersion A to prepare a coating liquid. Specifically, dispersion A and dispersion B were mixed so that 42 parts by mass of PTFE was used per 100 parts by mass of carbon powder, and the mixture was stirred at 5000 rpm for 15 minutes using a disperser while maintaining the liquid temperature at 30°C to obtain a coating liquid. The solid content of the obtained coating liquid relative to the total mass was 10.8 mass%. Here, "solid content" refers to the total content of all components contained in the coating liquid, excluding the solvent, calculated as pure content. The composition of the coating liquid is shown in Table 1.

[0122] <Manufacture of Gas Diffusion Layer> A coating solution was applied to one surface of a water-repellent treated porous electrode substrate using a bar coater at a coating speed of 3.3 m / min to form a coating film with a thickness of 198 μm on the surface of the porous electrode substrate. The coating was then dried for 5 minutes in a hot air oven set at 150°C, and then sintered in a sintering furnace at 360°C for 30 minutes to obtain a gas diffusion layer in which a 34 μm-thick coating layer was formed on one surface of the porous electrode substrate. Three randomly selected locations on the surface of the coating layer of the resulting gas diffusion layer were observed at 200x magnification using a scanning electron microscope (manufactured by JEOL Ltd., product name "JSM-6390"). The results are shown in Figure 5(a).

[0123] Example 2 Dispersion A was prepared in the same manner as in Example 1, except that pyrolytic graphite and carbon powder were mixed so that 27 parts by mass of pyrolytic graphite was used per 100 parts by mass of carbon powder. A coating liquid was prepared in the same manner as in Example 1, except that the obtained dispersion A was used. A gas diffusion layer was manufactured using this coating liquid, and three random locations on the surface of the coating layer were observed using a scanning electron microscope. The results are shown in Figure 5(b). The composition of the coating liquid is also shown in Table 1. The peel strength of the obtained gas diffusion layer was also measured. The results are shown in Figure 6. The surface roughness Ra of the coating was also measured. The results are shown in Table 2.

[0124] [Example 3] Dispersion A was prepared in the same manner as in Example 1, except that pyrolytic graphite and carbon powder were mixed so that the pyrolytic graphite was 54 parts by mass per 100 parts by mass of carbon powder. A coating liquid was prepared in the same manner as in Example 1, except that the obtained dispersion A was used. A gas diffusion layer was manufactured using this coating liquid, and three random locations on the surface of the coating layer were observed using a scanning electron microscope. The results are shown in Figure 5(c). The composition of the coating liquid is also shown in Table 1.

[0125] [Example 4] Dispersion A was prepared in the same manner as in Example 1, except that pyrolytic graphite and carbon powder were mixed so that the pyrolytic graphite was 81 parts by mass per 100 parts by mass of carbon powder. A coating liquid was prepared in the same manner as in Example 1, except that the obtained dispersion A was used. A gas diffusion layer was manufactured using this coating liquid, and three random locations on the surface of the coating layer were observed using a scanning electron microscope. The results are shown in Figure 5(d). The composition of the coating liquid is also shown in Table 1.

[0126] [Example 5] Dispersion A was prepared in the same manner as in Example 1, except that pyrolytic graphite and carbon powder were mixed so that 108 parts by mass of pyrolytic graphite was used per 100 parts by mass of carbon powder. A coating liquid was prepared in the same manner as in Example 1, except that the obtained dispersion A was used. A gas diffusion layer was manufactured using this coating liquid, and three random locations on the surface of the coating layer were observed using a scanning electron microscope. The results are shown in Figure 5(e). The composition of the coating liquid is also shown in Table 1.

[0127] Comparative Example 1 Dispersion A was prepared in the same manner as in Example 1, except that pyrolytic graphite was not used. A coating liquid was prepared in the same manner as in Example 1, except that the obtained dispersion A was used. A gas diffusion layer was produced using this coating liquid, and three random locations on the surface of the coating layer were observed using a scanning electron microscope. The results are shown in Figure 5(f). The composition of the coating liquid is also shown in Table 1. The peel strength of the obtained gas diffusion layer was also measured. The results are shown in Figure 6. Furthermore, the surface roughness Ra of the coating was measured. The results are shown in Table 2.

[0128]

[0129]

[0130] As is clear from Fig. 5, the gas diffusion layers obtained in each example had less cracking in the coating layer than the gas diffusion layer obtained in Comparative Example 1. Furthermore, as is clear from Fig. 6, the gas diffusion layer obtained in Example 2 had higher peel strength and better adhesion of the coating layer to the porous electrode substrate than the gas diffusion layer obtained in Comparative Example 1. Furthermore, as is clear from Table 2, the gas diffusion layer obtained in Example 2 had a smoother surface of the coating layer than the gas diffusion layer obtained in Comparative Example 1. These results demonstrate that the use of a combination of pyrolytic graphite and carbon powder to form a coating layer can suppress cracking and improve peel strength and surface smoothness.

[0131] REFERENCE SIGNS LIST 10 Gas diffusion layer 11 Porous electrode substrate 12 Coating layer 20 Roll of gas diffusion layer 21 Core material 22 Laminate 23 Protective layer 30 Membrane-electrode assembly 31 Polymer electrolyte membrane 32A Gas diffusion electrode 32B Gas diffusion electrode 321A Catalyst layer 321B Catalyst layer 40A Separator 40B Separator 41A Gas flow path 41B Gas flow path 100 Solid polymer fuel cell

Claims

1. A gas diffusion layer having a porous electrode substrate and a coating layer formed on at least one surface of the porous electrode substrate, The coating layer is a gas diffusion layer containing carbon powder A having an average particle size of 5 to 800 nm and carbon powder B having an average particle size of 1 to 50 μm.

2. 2. The gas diffusion layer according to claim 1, wherein a mass ratio of the carbon powder A to the carbon powder B is 0.5 to 9.

3. 2. The gas diffusion layer according to claim 1, wherein the carbon powder A is at least one selected from the group consisting of carbon black, milled fiber, carbon nanotubes, carbon nanofiber, coke, activated carbon, and amorphous carbon, and the carbon powder B is at least one selected from the group consisting of pyrolytic graphite, milled fiber, coke, activated carbon, and amorphous carbon.

4. A gas diffusion layer having a porous electrode substrate and a coating layer formed on at least one surface of the porous electrode substrate, The coating layer is a gas diffusion layer containing at least one carbon powder C selected from the group consisting of carbon black, milled fiber, carbon nanotube, carbon nanofiber, coke, activated carbon, and amorphous carbon, and pyrolytic graphite.

5. The gas diffusion layer according to claim 1 or 4, wherein the coating layer contains a water repellent agent.

6. 5. The gas diffusion layer according to claim 3, wherein the aspect ratio of the pyrolytic graphite is 2 to 40.

7. 5. The gas diffusion layer according to claim 4, wherein a mass ratio of the carbon powder C to the pyrolytic graphite is 0.5 to 9.

8. The gas diffusion layer according to claim 1 or 4, having a thickness of 160 to 350 μm.

9. 5. The gas diffusion layer according to claim 1, wherein the porous electrode substrate has an average pore size of 5 to 200 μm.

10. 5. The gas diffusion layer according to claim 1, wherein the surface roughness of the coating layer is 3.0 μm or less.

11. The gas diffusion layer according to claim 1 or 4, wherein the porous electrode substrate comprises carbon fibers.

12. 5. The gas diffusion layer according to claim 4, wherein the average particle size of the pyrolytic graphite is 3 to 50 μm.

13. 5. The gas diffusion layer according to claim 4, wherein the carbon powder C has an average particle size of 30 to 100 nm.

14. A gas diffusion layer having a substrate in which carbon fibers are bonded by carbon and a coating layer formed on at least one surface of the substrate, The gas diffusion layer, wherein the coating layer contains pyrolytic graphite, carbon black, and a fluororesin.

15. 15. The gas diffusion layer according to claim 14, wherein the aspect ratio of the pyrolytic graphite is 2 to 40.

16. The gas diffusion layer according to claim 14 or 15, wherein a mass ratio of the carbon black to the pyrolytic graphite is 0.5 to 9.

17. The gas diffusion layer according to claim 14 or 15, wherein the average particle size of the pyrolytic graphite is 3 to 50 μm.

18. 16. The gas diffusion layer according to claim 14, wherein the carbon black has an average particle size of 30 to 100 nm.

19. A roll of a gas diffusion layer, comprising the gas diffusion layer according to claim 1, 4 or 14, and a protective layer provided on the coating layer, and wound into a roll.

20. A polymer electrolyte fuel cell comprising the gas diffusion layer according to claim 1, 4 or 14.

21. A method for manufacturing a gas diffusion layer having a porous electrode substrate and a coating layer formed on at least one surface of the porous electrode substrate, comprising: a coating liquid containing a mixture of carbon powder B having an average particle diameter of 3 to 50 μm and carbon powder A having an average particle diameter of 5 to 4000 nm, and the coating liquid is applied to at least one surface of the porous electrode substrate.

22. The method for manufacturing a gas diffusion layer according to claim 21, wherein the carbon powder B has an aspect ratio of 2 to 40.

23. The method for producing a gas diffusion layer according to claim 21 or 22, wherein the carbon powder B is pyrolytic graphite.

24. The method for producing a gas diffusion layer according to claim 21 or 22, wherein a mass ratio of the carbon powder A to the carbon powder B is 0.5 to 9.