WO2024262555A1 - Gas diffusion layer and manufacturing method thereof, roll-shaped object of gas diffusion layer, and solid polymer fuel cell - Google Patents

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 its manufacturing method, roll of gas diffusion layer, and polymer electrolyte fuel cell

The present invention relates to a gas diffusion layer and a method for producing 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.

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). In addition, two sets of gas diffusion electrodes, each of which has a catalyst layer mainly 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 from the inside.
Such an 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 of which has gas flow paths for supplying fuel gas or oxidizing gas and discharging generated gas and excess gas.

The gas diffusion layer substrate is required to have the following three main functions:
The first function is to supply a fuel gas or an oxidizing gas uniformly to the precious metal catalyst in the catalyst layer from gas flow paths formed in a separator disposed on the outer side of the gas diffusion electrode substrate.
The second function is to discharge water produced 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.
As a gas diffusion layer substrate that satisfies these functions, a substrate having a porous structure made of a carbonaceous material is usually used. Specifically, a porous electrode substrate using carbon fibers such as carbon paper, carbon fiber cloth, and carbon fiber felt is generally 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 because they are porous materials, making them suitable materials for use as gas diffusion layer substrates.

In order to reduce the contact resistance between the gas diffusion layer substrate and the catalyst layer and to efficiently discharge water generated during power generation, a coating layer containing carbon fine particles and a water repellent agent may be provided on the surface of the gas diffusion layer substrate facing the catalyst layer.
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, pitch fluoride, and a fluorine-based solvent to the surface of the porous electrode substrate, followed by drying and sintering.
Furthermore, Patent Document 2 proposes a gas diffusion layer in which fine cracks are intentionally provided in advance in the coating layer, so that the structure of the coating layer is less likely to change before and after winding.
Furthermore, Patent Document 3 proposes a gas diffusion layer having a coating layer containing carbon powder and a water repellent agent.

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

A coating layer having cracks is likely to have poor adhesion to a porous electrode substrate, and therefore there is a demand for a gas diffusion layer having a coating layer in which the occurrence of cracks is suppressed.
An object of the present invention is to provide a gas diffusion layer in which the occurrence of cracks in a coating layer is suppressed, a method for producing the same, a roll of a gas diffusion layer, and a polymer electrolyte fuel cell.

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,
The coating layer is a gas diffusion layer comprising 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 according to [1], wherein a 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 according to [1] or [2], wherein the carbon powder A is at least one selected from the group consisting of carbon black, milled fiber, carbon nanotube, 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 comprising 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 any one of [1] to [4] above, wherein the coating layer contains a water repellent.
[6] The gas diffusion layer according to [3] or [4], wherein the aspect ratio of the pyrolytic graphite is 2 to 40.
[7] The gas diffusion layer according to [4] or [6], wherein a mass ratio represented by the carbon powder C/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.
[8] The gas diffusion layer according to any one of [1] to [7] above, having a thickness of 160 to 350 μm.
[9] The gas diffusion layer according to any one of the above [1] to [8], wherein the average pore size of the porous electrode substrate is 5 to 200 μm.
[10] The gas diffusion layer according to any one of [1] to [9], wherein the coating layer has a surface roughness of 3.0 μm or less.
[11] The gas diffusion layer according to any one of [1] to [10] above, wherein the porous electrode substrate contains carbon fibers.
[12] The gas diffusion layer according to [4], wherein the average particle size of the pyrolytic graphite is 3 to 50 μm.
[13] The gas diffusion layer according to [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] The gas diffusion layer according to [14], wherein the aspect ratio of the pyrolytic graphite is 2 to 40.
[16] The gas diffusion layer according to [14] or [15], wherein a mass ratio of the carbon black to 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 according to any one of the above [14] to [16], wherein the average particle size of the pyrolytic graphite is 3 to 50 μm.
[18] The gas diffusion layer according to any one of the above [14] to [17], 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 any one of [1] to [18] above, provided on the coating layer thereof with a protective layer, and wound into a roll.
[20] A polymer electrolyte fuel cell comprising the gas diffusion layer according to any one of [1] to [18].
[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 the steps of:
A method for producing a gas diffusion layer, comprising: applying a coating liquid, which is a mixture of carbon powder B having an average particle diameter of 1 to 50 μm and carbon powder A having an average particle diameter 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], 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], 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 a mass ratio represented by 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.

The present invention provides 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.

FIG. 1 is a cross-sectional view showing an example of a gas diffusion layer of the present invention. FIG. 2 is a perspective view showing an example of a roll of the gas diffusion layer of the present invention. 3 is a cross-sectional view taken along line A-A' of the roll of gas diffusion layer shown in FIG. 2. [0023] FIG. 1 is an exploded perspective view showing an example of the configuration of a polymer electrolyte fuel cell according to the present invention. 1A to 1F 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. 1 is a graph showing the measurement results of the peel strength of the gas diffusion layers obtained in Example 2 and Comparative Example 1.

The following describes in detail the forms for implementing the present invention, 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 by "to" means a numerical range including 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, in the drawings used in the following description, for convenience, characteristic parts may be shown enlarged in order to make the features easier to understand, and the dimensional ratios of each component may differ from the actual ones.

[Gas diffusion layer]
FIG. 1 shows an example of the gas diffusion layer of the present invention.
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 .

<Porous electrode substrate>
As the porous electrode substrate, any conductive porous material such as conductive paper, cloth, nonwoven fabric, etc., made from a conductive filler such as carbon powder, carbon fiber, metal fiber, resin, etc., can be used. In particular, it is preferable that the porous electrode substrate contains carbon fiber.
As the porous electrode substrate containing carbon fibers, a porous electrode substrate in which carbon fibers are bound by carbon is preferable. Hereinafter, the porous electrode substrate in which carbon fibers are bound by carbon is also particularly referred to as a "porous carbon electrode substrate".

(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. If the average fiber diameter of the carbon fibers is equal to or more than the lower limit, the distance between the fibers in the porous electrode substrate tends to be wider, and the gas permeability tends to be higher. 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. If 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 limit and the upper limit of the average fiber diameter of the carbon fibers can be arbitrarily combined, and for example, 2 to 30 μm are preferable, 3 to 20 μm are more preferable, and 4 to 8 μm are even more preferable.
The average fiber diameter of carbon fibers is determined by taking a photograph of the cross section of carbon fibers at 50 times or more magnification using a microscope such as a scanning electron microscope, randomly selecting 50 single fibers, measuring the diameters, and averaging the diameters. When the cross section of a carbon fiber has a long diameter and a short diameter, the long diameter is regarded as the fiber diameter of the fiber.

The average fiber length of the carbon fibers is preferably 2 μm or more, more preferably 3 μm or more. If the average fiber length of the carbon fibers is equal to or more 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. If the average fiber length of the carbon fibers is equal to or less than the upper limit, a porous electrode substrate with less dispersion spots can be obtained. The lower limit and the upper limit of the average fiber length of the carbon fibers can be arbitrarily combined, and for example, 2 to 30 mm are preferable, 2 to 12 mm are more preferable, and 3 to 9 mm are even more preferable.
The average fiber length of the carbon fibers is determined by, for example, photographing the carbon fibers at 50 times or more magnification using a microscope such as a scanning electron microscope, randomly selecting 50 single fibers, measuring their lengths, and averaging the lengths.

Examples of carbon fibers include polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, and phenol-based carbon fibers. 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.

(carbon)
The carbon functions as a binder to bind the carbon fibers together, and the uniformly dispersed carbon fibers are fixed together via the carbon.
Examples of carbon that bonds the 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 the porous electrode substrate described later.

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, and gas and liquid permeation and diffusion are facilitated.
The carbon content does not include the carbon fiber content.

(Other ingredients)
The porous electrode substrate may further contain carbon powder, which is expected to improve electrical conductivity.
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 %, based on the total mass of the porous electrode substrate. When the content of the carbon powder is equal to or more than the lower limit, a conductive path is formed by the carbon powder, and the 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.
The carbon powder will be described in detail in the section on the coating layer below.

(Physical properties of porous electrode substrate)
The gas permeability in the thickness direction of the porous electrode substrate is preferably 100 mL/(cm ² Pa hr) or more, more preferably 120 mL/(cm ² Pa hr) or more, even more preferably 150 mL/(cm ² Pa hr) or more, and particularly preferably 200 mL/(cm ² Pa hr) or more. If the gas permeability of the porous electrode substrate is the lower limit value or more, the fuel gas and the oxidizing gas are easily diffused and the reaction efficiency is improved. The gas permeability in the thickness direction of the porous electrode substrate is preferably 12000 mL/(cm ² Pa hr) or less, more preferably 5000 mL/(cm ² Pa hr) or less, even more preferably 2500 mL/(cm ² Pa hr) or less, and particularly preferably 1000 mL/(cm ² Pa hr) or less. If the gas permeability of the porous electrode substrate is equal to or less than the upper limit, the structure will not collapse and the shape can be well maintained even if liquid such as produced water passes through. The lower and upper limits of the gas permeability in the thickness direction of the porous electrode substrate can be arbitrarily combined, and for example, 100 to 12000 mL/(cm ² Pa hr) is preferable, 120 to 5000 mL/(cm ² Pa hr) is more preferable, 150 to 2500 mL/(cm ² Pa hr) is even more preferable, and 200 to 1000 mL/(cm ² Pa hr) is particularly preferable.
The gas permeability is measured by a method in accordance with JIS P 8117:2009.

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. If the thickness of the porous electrode substrate is equal to or more than the lower limit, the gas diffusion layer can be easily transported. 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. If 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 power generation performance can be maintained well. The lower limit and the upper limit of the thickness of the porous electrode substrate can be arbitrarily combined, for example, 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 determined by measuring the thickness at any ten points on the porous electrode substrate and averaging these values.

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. If the average pore diameter of the porous electrode substrate is equal to or more than the lower limit, the fuel gas and the oxidizing gas are easily diffused, and the reaction efficiency is improved. 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. If the average pore diameter of the porous electrode substrate is equal to or less than the upper limit, the fluid is uniformly transmitted through the gas diffusion layer, and reaction spots are unlikely to occur. The lower limit and the upper limit of the average pore diameter of the porous electrode substrate can be arbitrarily combined, and for example, 5 to 200 μm are preferable, 8 to 100 μm are more preferable, and 10 to 50 μm are even more preferable.
The porous electrode substrate is measured by mercury intrusion porosimetry, and the median diameter calculated from the obtained pore distribution is regarded as the average pore diameter.

The basis weight of the porous electrode substrate is preferably 50 g/m ² or more, more preferably 55 g/m ² or more, and even more preferably 60 g/m ² or more. If the basis weight of the porous electrode substrate is the lower limit or more, the handling property of the porous electrode substrate tends to be good. The basis weight of the porous electrode substrate is preferably 300 g/m ² or less, more preferably 270 g/m ² or less, and even more preferably 250 g/m ² or less. If the basis weight of the porous electrode substrate is the upper limit or less, the conductivity of the porous electrode substrate is easily ensured. The lower limit and the upper limit of the basis weight of the porous electrode substrate can be arbitrarily combined, for example, 50 to 300 g/m ² are preferable, 55 to 270 g/m ² are more preferable, and 60 to 250 g/m ² are even more preferable.

(Method of manufacturing porous electrode substrate)
An example of a method for producing a porous electrode substrate will be described below. Note that the method for producing a porous electrode substrate shown below is an example of a method for producing a porous carbon electrode substrate.
The porous electrode substrate can be obtained, for example, by subjecting a carbon fiber sheet described below to a carbonization treatment.

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 as the carbon fiber sheet.
The carbon fiber sheet can be obtained, for example, by forming a dispersion of carbon fibers or carbon fibers and organic fibers in a dispersion medium into a paper sheet, and adding a resin as necessary.
The carbon fiber sheet may be subjected to a heat and pressure treatment before the carbonization treatment.

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. If the thickness of the carbon fiber sheet is the lower limit or more, a porous electrode substrate having 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. If the thickness of the carbon fiber sheet is the upper limit or less, a porous electrode substrate having high conductivity is easily obtained. The lower limit and the upper limit of the thickness of the carbon fiber sheet can be arbitrarily combined, for example, 100 to 5000 μm are preferable, 150 to 4000 μm are more preferable, 170 to 3000 μm are more preferable, and 200 to 2000 μm are particularly preferable.
The thickness of the carbon fiber sheet is determined by measuring the thickness at any 10 points on the carbon fiber sheet and averaging the measured values. When the carbon fiber sheet is subjected to a heating and pressurizing treatment, the above-mentioned preferable thickness means the thickness of the carbon fiber sheet after the heating and pressurizing treatment.

The basis weight of the carbon fiber sheet is preferably 20 g/m ² or more, more preferably 40 g/m ² or more, and even more preferably 60 g/m ² or more. If the basis weight of the carbon fiber sheet is the lower limit or more, a porous electrode base material with good handleability is easily obtained. The basis weight of the carbon fiber sheet is preferably 500 g/m ² or less, more preferably 400 g/m ² or less, and even more preferably 300 g/m ² or less. If the basis weight of the carbon fiber sheet is the upper limit or less, a porous electrode base material with excellent conductivity is easily obtained. The lower limit and the upper limit of the basis weight of the carbon fiber sheet can be arbitrarily combined, for example, 20 to 500 g/m ² are preferable, 40 to 400 g/m ² are more preferable, and 60 to 300 g/m ² are even more preferable.

The carbon fibers used in the carbon fiber sheet are as explained 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 phenol 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. In addition, a water-dispersible phenolic resin or a water-soluble phenolic resin may be used as the phenolic resin.

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 form, are preferable.
In addition, when manufacturing a carbon fiber sheet, polyvinyl alcohol (PVA) or a heat-sealable polyester or polyolefin organic polymer binder may be used, but those that do not remain after the carbonization process or that are not mesh-like are not included in the "organic fiber".

As the polymer constituting the carbon fiber precursor fiber, a polymer with a residual mass of 20% by mass or more after carbonization treatment is preferred. 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 intertwining treatment described below. In other words, as the carbon fiber precursor fiber, acrylic fibers are preferred, and acrylic fibers containing 50% by mass or more of acrylonitrile units are more preferred.

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 fiber has excellent spinnability. 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 a highly circular fiber is preferred because it provides high mechanical strength after carbonization and can reduce production costs.

Fibrillar fibers refer to the entire fibers formed by partially branching small fibers (fibrils) that are components of fibers such as filaments and staples. Hereinafter, the fibers that are the source of branching of fibrillar fibers are also referred to as "trunks," and the branched small fibers are also referred to as "fibril portions."
The fibril fibers, when dispersed together with the carbon fibers, serve to prevent the carbon fibers from rebundling and to make the carbon fiber sheet self-supporting after heating and pressing.
Examples of fibrillated fibers include natural fibers such as wood pulp; and synthetic pulps such as fibrillated polyethylene fibers, acrylic fibers, and aramid fibers.
As the fibril fiber, fibril refined cellulose fiber obtained by beating Lyocell or Tencel, or fine cellulose may be used. These have a lower metal content than natural cellulose fibers, and are preferable from the viewpoint of preventing inhibition of proton conduction in fuel cells and deterioration of fluorine-based electrolyte membranes.

The average fiber length of the fibril fiber trunk is preferably 0.5 to 20 mm. When the average fiber length of the fibril fiber trunk 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 trunk is equal to or less than the upper limit, good dispersibility is easily obtained.
The average fiber diameter of the fibrillar fiber trunk is preferably 1 to 50 μm. When the average fiber diameter of the fibrillar fiber trunk is equal to or greater than the lower limit, good dispersibility is obtained. When the average fiber diameter of the fibrillar fiber trunk 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 part of the fibril fiber is preferably 0.01 to 30 μm. When the average fiber diameter of the fibril part of the fibril fiber is equal to or greater than the lower limit, the dehydration of the carbon fiber sheet during heating and pressurization and the gas permeability of the porous electrode substrate are easily ensured. When the average fiber diameter of the fibril part of the fibril fiber is equal to or less than the upper limit, the dispersibility is improved.

<Example of manufacturing method>
The method for producing a porous electrode substrate of the present embodiment includes the following steps (i) to (iv).
Step (i): A step of forming a precursor sheet from a dispersion 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 mechanical strength of the porous electrode substrate, the production of the porous electrode substrate is preferably carried out by a continuous method, but is not limited thereto, and may be carried out by a batch method.

Step (i):
For papermaking, a dispersion in which only carbon fibers are dispersed in a dispersion medium may be used, or a dispersion in which carbon fibers and organic fibers are dispersed in a dispersion medium may be used.
Examples of the dispersion medium 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 them, from the viewpoint of productivity, it is preferable to use water as the dispersion medium. The water may be deionized water.
An organic polymer binder (such as polyvinyl alcohol) that is burned off during carbonization may be added to the dispersion before papermaking. The organic polymer binder may be in the form of a solid such as fibers or particles, or in the form of a liquid.

When organic fibers are used, it is preferable to subject the precursor sheet obtained by papermaking to an entanglement treatment, thereby obtaining a precursor sheet having a three-dimensional entangled structure of the carbon fibers and the organic fibers and thus having a higher strength.
The entanglement method is not particularly limited, and examples thereof 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, the high-pressure liquid injection method is preferred because it is easy to suppress breakage of the carbon fibers due to the entanglement treatment and easy to obtain 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 is preferably 50% by mass or less, more preferably 40% by mass or less. The lower and upper limits of the content of the organic fibers can be arbitrarily combined, and are, for example, preferably 10 to 50% by mass, more preferably 15 to 40% by mass.

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

Step (ii):
Methods for adding a resin to a precursor sheet include, for example, a method in which the resin dispersion is sprayed or dropped onto the surface of the precursor sheet using a spray nozzle; a method in which the resin dispersion is caused to flow down the surface of the precursor sheet using a curtain coater; and a method in which the resin dispersion is uniformly coated onto the surface of the precursor sheet using a kiss coater.
When producing a porous electrode substrate containing carbon powder, it is preferable to add the 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.

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 the resin and the organic fiber relative to 100 parts by mass of the 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 more than the lower limit, the carbon fiber is 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, and it is easy to obtain a porous electrode base material with excellent gas permeability. The lower limit and the upper limit of the total amount can be arbitrarily combined, and for example, 50 to 180 parts by mass are preferable, 60 to 160 parts by mass are more preferable, and 80 to 140 parts by mass are more preferable.

Step (iii):
Examples of methods for heating and pressurizing a carbon fiber sheet include a method in which smooth hard plates are placed on both sides of the carbon fiber sheet to heat press it; and a method using a hot roll press or a continuous belt press.
When subjecting a carbon fiber sheet to a heat and pressure treatment, a release agent may be applied to the carbon fiber sheet or release paper may be sandwiched between the carbon fiber sheet and the hard plate, heated roll, or belt to prevent fibrous materials from adhering to the hard plate, roll, or belt.

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. If the temperature of the heating and pressurizing treatment is equal to or higher than the lower limit, carbonization is likely to proceed sufficiently. 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. If the temperature of the heating and pressurizing treatment is equal to or lower than the upper limit, burning of the resin and organic fiber is likely to be avoided. The lower limit and upper limit of the temperature of the heating and pressurizing treatment can be arbitrarily combined, and for example, 100 to 400°C are preferable, 120 to 200°C are more preferable, and 150 to 190°C are even more preferable.
The pressure of the heating and pressurizing treatment is preferably 0.05 MPa or more, and more preferably 1 MPa or more. If the pressure of the heating and pressurizing treatment is equal to or more than 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 less, and more preferably 15 MPa or less. If 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 destroyed 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 for example, 0.05 to 20 MPa is preferable, and 1 to 15 MPa is more preferable.
The time for the heat and pressure treatment is preferably from 30 seconds to 1 hour, and more preferably from 1 to 10 minutes.

Step (iv):
By carbonizing the carbon fiber sheet, 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.

The carbonization treatment of the carbon fiber sheet is preferably carried out in an inert atmosphere at 1000° C. or higher, since it is easy to obtain a porous electrode substrate having sufficient electrical conductivity. The temperature of the carbonization treatment is more preferably 1000 to 3000° C., further preferably 1000 to 2400° C., and particularly preferably 1000 to 2200° C.
The time for the carbonization treatment is preferably from 1 minute to 1 hour, and more preferably from 10 minutes to 1 hour.

Prior to the carbonization treatment, a pre-carbonization treatment may be performed in an inert atmosphere at a temperature of 300° C. or higher and lower than 1000° C. By performing 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 accumulation of various decomposition products on the inner wall 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 for the pre-carbonization treatment is preferably 1 minute to 1 hour, and more preferably 10 minutes to 1 hour.

The method for producing a porous electrode substrate may not include any one or more of the above steps (i) to (iii).
For example, the carbonization treatment may be performed without subjecting the carbon fiber sheet to a heating and pressurizing treatment.
A method in which organic fibers are used together with carbon fibers in step (i) and step (ii) is not carried out may also be used.
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 fall and accumulate.

(Other forms)
As the porous electrode substrate, a porous electrode substrate from which the carbonization process has been omitted, or commercially available carbon paper or the like may be used as the porous electrode substrate.
A porous electrode substrate that does not require the carbonization process can reduce energy costs compared to a substrate that requires carbonization. Examples of the porous electrode substrate that does not require the carbonization process include a carbon fiber web in which carbon fibers are bound with a binder filled with conductive material particles; a porous electrode substrate in which fine conductive materials such as carbon are bound with a binder such as a resin; and the like.

<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.

Each of the coating layer X and the coating layer Y preferably contains a water repellent agent.
The coating layer X and the coating layer Y may further contain optional components other than the carbon powder A, the carbon powder B and the water repellent agent, if necessary, within a range that does not impair the effects of the present invention.

When forming the coating layer, a portion of the coating liquid described below seeps 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 does not seep 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 included as needed, and optional components is defined as coating layer X. Also, 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 included as needed, and optional components is defined as coating layer Y.

[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 with the water repellent acting as a binder. In other words, the carbon powder A and the carbon powder B are incorporated into the network formed by the water repellent, forming a fine mesh structure.
It is preferable that the coating layer X contains a fibrous water repellent, which not only strengthens the mesh structure and improves the strength of the coating layer X, but also causes the fibrous water repellent to intertwine with the porous electrode substrate, thereby improving the adhesion between the coating layer X and the porous electrode substrate, thereby providing a gas diffusion layer for a polymer electrolyte fuel cell having a high peel strength of the coating layer X.

(Carbon powder A)
Examples of the 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.
The carbon powder A may be used alone or in combination of two or more kinds.

Carbon black has a significantly larger number of particles per unit mass than graphite powder, and at a certain critical concentration or higher, the agglomerates are linked together 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, for example, Denka Black (registered trademark) manufactured by Denka Co., Ltd. Commercially available ketjen black products include, for example, Ketjen Black EC manufactured by Lion Corporation Commercially available furnace black products include, for example, Vulcan XC72 manufactured by CABOT Corporation.

As the milled fiber, one produced by crushing virgin carbon fiber may be used, or one produced from recycled products such as carbon fiber reinforced thermosetting resin molded products, carbon fiber reinforced thermoplastic resin molded products, and prepregs may be used.
The carbon fiber used as the raw material for the milled fiber may be a PAN-based carbon fiber, a pitch-based carbon fiber, or a rayon-based carbon fiber.

Other graphites consist of highly crystalline graphite structures, with the average primary particle size generally ranging from a few micrometers to a few hundred micrometers.
Examples of other graphite include spherical graphite, flake graphite, lump graphite, amorphous graphite, artificial graphite, expanded graphite, etc. Among these, spherical graphite and flake graphite are preferred from the viewpoint of electrical conductivity.

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 particles of the carbon powder A, and sufficient gas permeability can be obtained. 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 limit and upper limit of the average particle diameter of the carbon powder A can be arbitrarily combined, and for example, is preferably 10 to 800 nm, more preferably 15 to 500 nm, and even more preferably 30 to 100 nm.
The average particle size of the carbon powder A was determined by photographing the surface of the coating layer with an electron microscope and analyzing the image by the method described below.

(Carbon powder B)
Examples of the carbon powder B include pyrolytic graphite, milled fiber, coke, activated carbon, and amorphous carbon. As the carbon powder B, pyrolytic graphite, milled fiber, coke, activated carbon, and amorphous carbon are preferred, and pyrolytic graphite is more preferred.
The carbon powder B may be used alone or in combination of two or more kinds.

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 electrical 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 is preferably 35 μm or less, and 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 preferable, and 5 to 11 μm is more preferable.
The average particle size of the carbon powder B was determined by photographing the surface of the coating layer with an electron microscope and analyzing the image by the method described later.

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. If the aspect ratio of carbon powder B is equal to or more than the lower limit, a sufficient effect of improving electrical conductivity can be obtained. The aspect ratio of carbon powder B is preferably 40 or less, more preferably 20 or less, and even more preferably 10 or less. If 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 limit and upper limit of the aspect ratio of carbon powder B can be arbitrarily combined, and for example, 2 to 40 are preferable, 3 to 20 are more preferable, and 4 to 10 are even more preferable.
The average thickness of the carbon powder B is obtained by photographing the carbon powder B 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 particles, measuring their thicknesses, and calculating the average value.
For those with catalog values, the catalog values may be used as the average particle size or average thickness of the carbon powder B as a simple measured value.

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, based on the total mass of coating layer X. If the content of carbon powder B is equal to or more than the lower limit, a sufficient effect of improving 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. If 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 limit and upper limit of the content of carbon powder B can be arbitrarily combined, and for example, 9 to 50% by mass is preferable, 15 to 50% by mass is more preferable, and 30 to 45% by mass is even more preferable.

The mass ratio represented by carbon powder A/carbon powder B, which represents the mass ratio of the content of carbon powder A to the content of 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. If the carbon powder A/carbon powder B ratio is equal to or greater than the lower limit, the contact area between carbon powder A and carbon powder B is large, and a sufficient effect of improving 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. If 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, so that a sufficient amount of water repellent, which is a binder, is present, and sufficient coating strength can be obtained. The lower and upper limits of the carbon powder A/carbon powder B ratio can be arbitrarily combined, and for example, 0.5 to 9.0 is preferable, 1.0 to 4.0 is preferable, 1.0 to 2.0 is preferable, 1.0 to 1.8 is more preferable, and 1.0 to 1.2 is even more preferable.

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

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 particularly preferred in order to turn the water repellent into fibers, and dispersion-type PTFE is even more preferred.

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, based on the total mass of the coating layer X. If the content of the water repellent is equal to or more than the lower limit, sufficient water repellency is obtained. If the content of the water repellent is equal to or less than the upper limit, a sufficient effect of improving electrical conductivity is obtained. The lower and upper limits of the oil content of the water repellent can be arbitrarily combined, for example, 10 to 40% by mass is preferable, 15 to 35% by mass is more preferable, and 20 to 30% by mass is even more preferable.

(Optional ingredients)
Examples of the optional components include surfactants, water-soluble polymers, thickeners, reinforcing agents, stabilizers, fillers, and crosslinking agents.
These optional components may be used alone or in combination of two or more.

(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. If the thickness of the coating layer X is equal to or more than the lower limit, the fuel gas and the oxidizing gas are easily diffused, and the reaction efficiency is improved. 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. If the thickness of the coating layer X is equal to or less than the upper limit, the increase in electronic resistance due to the coating layer X can be prevented, and the power generation performance can be maintained well. The lower limit and the upper limit of the thickness of the coating layer X can be arbitrarily combined, and for example, 3 to 100 μm are preferable, 5 to 50 μm are more preferable, and 10 to 40 μm are more preferable.
The thickness of the coating layer X is determined by measuring the thickness at any ten points on the coating layer and averaging these values.

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 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 more than the lower limit, the adhesion with the catalyst layer can be improved. The lower limit and the upper limit of the surface roughness of the coating layer X can be arbitrarily combined, and for example, 1.0 to 3.0 μm are preferable, 1.5 to 2.9 μm are more preferable, and 2.0 to 2.8 μm are more preferable.
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 to the porous electrode substrate.

[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 one in which the pyrolytic graphite and carbon powder C are bound together by 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, and the coating layer Y has a fine mesh structure.
The coating layer Y preferably contains a fibrous water repellent, which not only strengthens the mesh structure and improves the strength of the coating layer Y, but also causes the fibrous water repellent to intertwine with the porous electrode substrate, thereby improving the adhesion between the coating layer Y and the porous electrode substrate, thereby providing a gas diffusion layer for a polymer electrolyte fuel cell having a high peel strength of the coating layer Y.

(Pyrolytic graphite)
Pyrolytic graphite can be obtained by graphitizing powdered coke through heat treatment at 2500° C. or higher. It is believed that the use of pyrolytic graphite in the gas diffusion layer can suppress the occurrence of cracks on the surface of the coating layer by reducing the volumetric shrinkage rate when the ink dries, without reducing the 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 graphite itself has high thermal conductivity.

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. If the aspect ratio of pyrolytic graphite is equal to or more than the lower limit, a sufficient effect of improving electrical conductivity can be obtained. The aspect ratio of pyrolytic graphite is preferably 40 or less, more preferably 20 or less, and even more preferably 10 or less. If 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 limit and upper limit of the aspect ratio of pyrolytic graphite can be arbitrarily combined, and for example, 2 to 40 are preferable, 3 to 20 are more preferable, and 4 to 10 are even more preferable.
The average particle size of the pyrolytic graphite was measured using a laser diffraction particle size distribution analyzer, and was calculated as the 50% cumulative diameter in volume terms.
The average thickness of pyrolytic graphite was determined by photographing the graphite 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.
In addition, when it is difficult to measure the average particle size of pyrolytic graphite using a laser diffraction particle size distribution meter, photographs are taken at a magnification of 1000 times or more using a microscope such as a scanning electron microscope or a transmission electron microscope, 10 different pieces of pyrolytic graphite are randomly selected, their lengths are measured, and the average value is calculated, which can be used as a substitute for the average particle size.
In addition, when a catalog value is available, the catalog value may be adopted as the average particle size or average thickness of the pyrolytic graphite as a simple measured value.

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

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, based on the total mass of the coating layer Y. If the content of pyrolytic graphite is equal to or more than the lower limit, a sufficient effect of improving electrical conductivity can be obtained. The content of pyrolytic graphite is preferably 50% by mass or less, and more preferably 45% by mass or less. If 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 limit and upper limit of the content of pyrolytic graphite can be arbitrarily combined, and for example, 9 to 50% by mass is preferable, 15 to 50% by mass is more preferable, and 30 to 45% by mass is even more preferable.

(Carbon powder C)
Examples of the 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 further preferably carbon black.
The carbon powder C may be used alone or in combination of two or more kinds.
Carbon black, milled fiber and other graphites are as described in Carbon Powder A.

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. If the average particle diameter of the carbon powder C is equal to or more 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 C, and sufficient gas permeability can be obtained. 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. If 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 limit and the upper limit of the average particle diameter of the carbon powder C can be arbitrarily combined, and for example, 5 to 800 nm are preferable, 10 to 800 nm are more preferable, 15 to 500 nm are more preferable, and 30 to 100 nm are particularly preferable.
The average particle size of the carbon powder C was determined by photographing the surface of the coating layer with an electron microscope and analyzing the image by the method described later.

The mass ratio of the carbon powder C content to the pyrolytic graphite content, i.e., the carbon powder/pyrolytic graphite ratio, is preferably 0.5 or more, and more preferably 1.0 or more. If the carbon powder/pyrolytic graphite ratio is equal to or more than the lower limit, the contact area between the carbon powder and the pyrolytic graphite is large, and a sufficient effect of improving electrical conductivity can be 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. If 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, so that a sufficient amount of binder water repellent is present, and sufficient coating strength can be obtained. The lower and upper limits of the carbon powder/pyrolytic graphite ratio can be arbitrarily combined, and for example, 0.5 to 9.0 is preferable, 1.0 to 4.0 is more preferable, 1.0 to 2.0 is even more preferable, 1.0 to 1.8 is particularly preferable, and 1.0 to 1.2 is most preferable.

(Water repellent)
The water repellent is as described in the coating layer X, and the preferred embodiments are also the same. As the water repellent, a fluororesin is preferable from the viewpoint of being particularly excellent in water repellency.

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, based on the total mass of the coating layer Y. If the content of the water repellent is equal to or more than the lower limit, sufficient water repellency is obtained. If the content of the water repellent is equal to or less than 40% by mass, more preferably 35% by mass or less, and even more preferably 30% by mass or less, based on the total mass of the coating layer Y. If the content of the water repellent is equal to or less than the upper limit, sufficient conductivity improvement effect is obtained. The lower limit and upper limit of the content of the water repellent can be arbitrarily combined, for example, 10 to 40% by mass is preferable, 15 to 35% by mass is more preferable, and 20 to 30% by mass is even more preferable.

(Optional ingredients)
The optional components are as explained in the coating layer X.

(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. If the thickness of the coating layer Y is equal to or more than the lower limit, the fuel gas and the oxidizing gas are easily diffused, and the reaction efficiency is improved. 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. If the thickness of the coating layer Y is equal to or less than the upper limit, the increase in electronic resistance due to the coating layer can be suppressed, and the power generation performance can be maintained well. The lower limit and the upper limit of the thickness of the coating layer Y can be arbitrarily combined, and for example, 3 to 100 μm are preferable, 5 to 50 μm are more preferable, and 10 to 40 μm are more preferable.
The thickness of the coating layer Y is determined by measuring the thickness at any ten points on the coating layer and averaging these values.

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 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 more than the lower limit, the adhesion with the catalyst layer can be improved. The lower limit and upper limit of the surface roughness of the coating layer Y can be arbitrarily combined, for example, 1.0 to 3.0 μm are preferable, 1.5 to 2.9 μm are more preferable, and 2.0 to 2.8 μm are even more preferable.

(Other forms)
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 diffusion properties, and drainage properties, it is preferable that the coating layer is formed on only one surface of the porous electrode substrate.
In addition, when a 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 the solid polymer fuel cell described below.

<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 is 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 limit and upper limit of the thickness of the gas diffusion layer can be arbitrarily combined, and for example, 55 to 350 μm are preferable, and 100 to 250 μm are more preferable.
The thickness of the gas diffusion layer is determined by measuring the thickness at any ten points on the gas diffusion layer and averaging the measured values.

<Method of manufacturing gas diffusion layer>
An example of a method for producing a gas diffusion layer will be described below.
The method for producing a gas diffusion layer of the present embodiment includes the following steps (1) to (3) when the coating layer is coating layer X, for example.
Step (1): A step of applying a coating liquid containing carbon powder A, carbon powder B, a solvent, and, if necessary, a water repellent and optional components onto 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): A step of 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): A step of heating the porous electrode substrate on which the coating layer has been formed to a temperature greater than 300° C. and equal to or less than 400° C. to sinter the water repellent agent, thereby obtaining 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 the step (1).

(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, etc. 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 an optional component, and dispersion B containing a water repellent, a solvent, and optionally an optional component are each prepared, and dispersion A and dispersion B are mixed using a stirrer, etc. to prepare the coating liquid.
Examples of the solvent include water, organic solvents such as lower alcohols and acetone, mixed solvents of water and organic solvents, etc. From the viewpoints of cost and environmental load, water is preferred as the solvent.
From the viewpoint of increasing the wettability and improving the dispersibility of the carbon powder A, the carbon powder B, the pyrolytic graphite, and the carbon powder C, it is preferable that the dispersion liquid A contains at least one of an organic solvent and a surfactant.
When water is used as a solvent in preparing the dispersion B, the water repellent is preferably dispersed in water using a surfactant since the water repellent is difficult to disperse in water as it is.
Furthermore, as the dispersion liquid B, a dispersion in which a water repellent agent has been dispersed in advance may be used.

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 %, based on the total mass of dispersion A.
The content of the carbon powder B is preferably from 5 to 30% by mass, and more preferably from 5 to 10% by mass, based on the total mass of the dispersion A.
The content of the water repellent in the dispersion B used in the coating liquid for forming the coating layer X is preferably 1 to 20% by mass, more preferably 1 to 10% by mass, based on the total mass of the dispersion B.

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

The stirrer 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 turn the water repellent into fibers, it is preferable to keep the stirring temperature of the coating liquid at 30° C. or higher and mix and stir for 15 minutes or more under conditions of a stirring speed of 5000 rpm or higher when using a disper.

The viscosity of the coating liquid at 25°C is preferably 100 to 10,0000 mPa·s. If 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. If 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.

The coating liquid can be applied to the surface of the porous electrode substrate by a conventional method, such as a bar coating method, a blade method, a screen printing method, a spray method, a curtain coating method, a roll coating method, etc. By using these methods, a uniform coating film can be formed on the porous electrode substrate.
The thickness of the coating film is preferably 40 μm or more, more preferably 50 μm or more. If the thickness of the coating film is equal to or more than the lower limit, a coating film having a uniform thickness can be easily obtained. The thickness of the coating film is preferably 2000 μm or less, more preferably 1000 μm or less. If the thickness of the coating film is equal to or less than the upper limit, the occurrence of cracks in the coating layer can be further suppressed. The lower limit and the upper limit of the thickness of the coating film can be arbitrarily combined, and for example, 40 to 2000 μm are preferable, and 50 to 1000 μm are more preferable.
The coating speed of the coating liquid is preferably 1 to 20 m/min from the viewpoint of productivity.

Prior to step (1), the porous electrode substrate may be subjected to a water-repellent treatment as necessary 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 silicone resin or fluororesin are dispersed in a solvent, for example, the above-mentioned dispersion liquid B, can be used.

(Step (2))
For drying the coating film, for example, a plate heater, a heating roll, a hot air dryer, an IR heater, or the like can be used.
The atmospheric 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 speed of the coating film is increased. 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 limit and upper limit of the drying temperature can be arbitrarily combined, and for example, 50 to 300°C are preferable, 100 to 300°C are more preferable, and 150 to 300°C are even more preferable.
The drying time for the coating film is preferably 30 seconds to 20 minutes, and more preferably 30 seconds to 10 minutes, in consideration of productivity.

(Step (3))
In the manufacturing method of the gas diffusion layer of this embodiment, the dried "porous electrode substrate with a coating layer formed thereon" is sintered in an environment of more than 300° C. and not more than 400° C. to manufacture the gas diffusion layer.
In step (3), i.e., the sintering step, the water repellent 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 between pyrolytic graphite and carbon powder C. Therefore, the sintering temperature is preferably higher than 300° C. and not higher 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.
If the coating film contains a surfactant, the surfactant will be burned off during the sintering process.

<Action and effect>
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 generating cracks. The reason why the coating layer is suppressed from generating cracks is not clear, but is thought to be as follows.
One of the causes of cracks occurring in the coating layer is the aggregation of carbon powder when the coating film provided on the porous electrode substrate is dried to form the coating layer, and it is believed that cracks occur starting from the carbon powder aggregates. Carbon powder with a small particle size is particularly prone to aggregation. Since pyrolytic graphite has few impurities and a relatively large particle size compared to carbon powder, it is believed that the combination of carbon powder and pyrolytic graphite can suppress the aggregation of carbon powder and the occurrence of cracks.
In addition, since pyrolytic graphite contains few impurities, the combined use of carbon powder and pyrolytic graphite increases thermal conductivity and improves power generation performance.
Similarly, it is believed that the combined use of carbon powder A and carbon powder B suppresses the aggregation of carbon powder A, inhibits the occurrence of cracks, increases thermal conductivity, and improves power generation performance.

In consideration 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 roll-shaped gas diffusion layer will now be described.

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

<Core material>
The core material is preferably a hollow core material that is lightweight and easy to hold in an unwinding/winding device.
The core material may be paper or resin.
From the viewpoint of reducing dust generated when the core material is attached to the device, a resin core material is preferable. Examples of resins include polyethylene, ABS resin, polystyrene, polypropylene, polyvinyl chloride, and polyethylene terephthalate.
From the viewpoints of recycling the core material and being inexpensive, a paper core material is preferred. Even if the core material is made of paper, the use of a core material with a resin-coated surface can minimize the amount of dust generated during installation in the device.

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, the structure of the gas diffusion layer is less likely to change before and after winding. 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, and a decrease in productivity and an increase in temperature during transportation can be suppressed.
When the core material is hollow, the inner diameter of the core material is preferably 76.2 to 152.4 mm. 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, the core material has excellent durability even when used repeatedly. If the thickness of the core material is equal to or less than the upper limit, the weight of the roll of the gas diffusion layer can be prevented from increasing excessively.
The core material may be removed after the gas diffusion layer is wound into a roll.

<Protective Layer>
The protective layer is a sheet for protecting the coating layer of the gas diffusion layer, and 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 layer that does not adhere to the coating layer, and examples of the protective layer include paper and resin films.
As the paper, dust-free paper that generates little dust is preferable.
The resin film is preferably one that is less deformed when pressed against the carbon fiber in order to protect the coating layer. Examples of the material for the resin film include polyethylene, ABS resin, polystyrene, polypropylene, polyvinyl chloride, polyethylene terephthalate, and polytetrafluoroethylene.

The thickness of the protective layer is preferably 5 to 100 μm. When the thickness of the protective layer is equal to or more than the lower limit, damage to the coating layer caused by the carbon fibers piercing 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 roll of the gas diffusion layer does not become too large, and a decrease in productivity and an increase in transportation costs can be suppressed.
It is preferable that the width of the protective layer is equal to or greater than the width of the gas diffusion layer, with the difference being 200 mm or less. If 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 sufficiently obtained. In addition, it is possible to prevent the edge portion of the protective layer from damaging the coating layer. If the width of the protective layer is greater than that of the gas diffusion layer, with the difference being 200 mm or less, it is possible to prevent an increase in the cost of the protective layer. In addition, it is possible to maintain a good balance during winding, and the winding shape is likely to be stable.

[Polymer electrolyte fuel cell]
FIG. 4 shows an example of a polymer electrolyte fuel cell according to 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 a pair of separators 40A and 40B.
When the membrane-electrode assembly 30 and the pair of separators 40A, 40B are formed into one cell (single cell), the polymer electrolyte fuel cell 100 may be configured from one cell, or It may be an aggregate of a plurality of cells.

<Membrane-electrode assembly>
The membrane-electrode assembly 30 is composed of a polymer electrolyte membrane 31 and a pair of gas diffusion electrodes 32A, 32B.
The polymer electrolyte membrane 31 is sandwiched between a pair of gas diffusion electrodes 32A and 32B.

(Polymer electrolyte membrane)
The polymer electrolyte membrane 31 contains a polymer electrolyte.
Examples of the polymer electrolyte include a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte.
An example of the fluorine-based polymer electrolyte is 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.

(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 called the "oxygen electrode", and the catalyst layer 321A is also called the "oxygen electrode catalyst layer". The gas diffusion layer 10 provided on the gas diffusion electrode 32A is also called the "oxygen electrode gas diffusion layer".

The gas diffusion electrode 32B includes a catalyst layer 321B and a gas diffusion layer 10.
The gas diffusion electrode 32B is an electrode on the fuel electrode side, and hydrogen is supplied to the gas diffusion electrode 32B. The gas diffusion electrode 32B is also called the "fuel electrode", and the catalyst layer 321B is also called the "fuel electrode catalyst layer". The gas diffusion layer 10 provided on the gas diffusion electrode 32B is also called the "fuel electrode gas diffusion layer".

The catalyst layer 321A is a layer containing a catalyst and a binder, and is a reaction field where the reduction reaction of oxygen occurs.
The catalyst layer 321B is a layer containing a catalyst and a binder, and is a reaction field where the oxidation reaction of hydrogen occurs.
Examples of the catalyst include a catalyst support in which Pt (platinum), Ru (ruthenium) or the like is supported on a support such as carbon; a carbon alloy catalyst, and the like.
The binder is preferably a polymer compound having ion exchange ability, and specific examples thereof 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, power generation can be performed efficiently.

The gas diffusion layers 10 provided on the gas diffusion electrodes 32A and 32B are the gas diffusion layers of the present invention described above, and 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.

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

It is preferable that each of the separators 40A and 40B is made of a material that is conductive and gas impermeable. Examples of such materials include carbon.

<Method of Manufacturing Polymer Electrolyte Fuel Cell>
An example of a method for producing a polymer electrolyte fuel cell will be described below.
In the method for producing a polymer electrolyte fuel cell according to the present 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, etc. is applied onto 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 thereof include a bar coating method, a blade method, a screen printing method, a spray method, a curtain coating method, a roll coating method, etc. By using these methods, a uniform coating film of the catalyst layer can be formed on the coating layer of the gas diffusion layer.
The formed coating film of the catalyst layer is dried by a common method, and a gas diffusion electrode in which the catalyst layer is formed on the coating layer of the gas diffusion layer can be produced.

Next, the polymer electrolyte membrane is sandwiched between a pair of gas diffusion electrodes to obtain a membrane-electrode assembly.
The resulting 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 a plurality of single cells may be stacked and used as a polymer electrolyte fuel cell.

The present invention will be explained in more detail below using 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.

[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 off from an end of the test piece by 180-degree peeling at a tensile speed of 250 mm/min in an atmosphere of 23°C and 65% RH using a tensile testing machine (manufactured by Shimadzu Corporation, product name "Small Tabletop Tester EZ") to measure the peel strength.

<Measurement of surface roughness Ra>
The surface roughness Ra of the coating layer of the gas diffusion layer was measured using a surface roughness measuring device (manufactured by Mitutoyo Corporation, product name "Surfcom 1400D-LCD") with a cutoff value of 0.8 mm, a measurement section of 4 mm, a range of 320 μm, and five measurement sections.

<Calculation of average particle size of carbon powders A to C>
In order to evaluate the average particle diameters of the 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 surface of the coating layer.
The imaging conditions are as follows.
Acceleration voltage = 1 kV
Working distance: 3 mm
Detector: SE detector Deposition conditions: Osmium coat (Osmium deposition conditions)
Equipment: Meiwafosis Osmium Coater Tennant 20 type Deposition conditions: Auto mode Deposition thickness: 3 nm

The average particle diameters of the carbon powders A to C were calculated from the obtained SEM images by the following analytical method.
(Method of calculating average particle size of carbon powders A and C)
The SEM image, taken at a magnification of 50,000 times so that only carbon powder A or C was present within the observation field, was converted to a 32-bit image using the image analysis software "ImageJ". The length and value of the scale bar of the SEM image were read using the same software.
After reflecting the length per pixel in the observed image, the area in which the scale information and observation conditions were written was deleted. After subtracting the average brightness value from the obtained image data area, the outside of the image data area was interpolated with 0 so that the vertical and horizontal sizes of the image size were each a power of 2. The obtained image was converted into a two-dimensional autocorrelation function, and further normalized so that the maximum value was 1 by dividing the two-dimensional autocorrelation function by the maximum value of the two-dimensional autocorrelation function. After performing a circular average with the origin of the two-dimensional autocorrelation function as the center to obtain a one-dimensional autocorrelation function, the distance at which the autocorrelation function value closest to the origin attenuates to 0.5 was defined as r*, and the particle diameter of carbon powder A or C was calculated using the following formula. The particle diameter was calculated for the 13 images taken, and the average value was defined as the average particle diameter.
(Particle diameter of carbon powder A or C) = 3 x r *

(Method of Analyzing Particle Size of Carbon Powder B)
The SEM image of the 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 value of the scale bar of the SEM image were read using the same software.
After reflecting the length per pixel in the observed image, the longest distance between any two points on the contour of the carbon powder was measured to calculate the Feret's diameter of the carbon powder B, which was taken as the particle diameter of the carbon powder B. The particle diameters were calculated for the 13 captured images, and the average value was taken as the average particle diameter.

[Example 1]
<Production of Porous Electrode Substrate>
As the carbon fibers, PAN-based carbon fibers having an average fiber diameter of 7 μm and an average fiber length of 3 mm were used.
As the carbon fiber precursor fiber, an acrylic fiber (manufactured by Mitsubishi Chemical Corporation, product name "D122") having an average fiber diameter of 4 μm and an average fiber length of 3 mm was used.
As the fibrillated fiber, an easily splittable acrylic sea-island composite fiber (manufactured by Mitsubishi Chemical Corporation, product name "Bonnell MVP-C651", average fiber length: 3 mm) consisting of an acrylic polymer that can be fibrillated by beating and diacetate (cellulose acetate) was used.
A porous electrode substrate was produced as follows.

(1) Disintegration of Fibers Carbon fibers were dispersed in water to a fiber concentration of 1 mass % (10 g/L) and disintegrated through a mixer to prepare disintegrated slurry fibers (SA).
The carbon fiber precursor fibers were dispersed in water so that the fiber concentration was 1 mass % (10 g/L), and the dispersion was defibrated through a mixer to prepare defibrated slurry fibers (Sb).
Splittable acrylic sea-island composite fibers were dispersed in water to a fiber concentration of 1 mass % (10 g/L), and beaten and defibrated in a mixer to prepare defibrated slurry fibers (Sb').

(2) Production of precursor sheet Disintegrated slurry fiber (SA), disintegrated slurry fiber (Sb), disintegrated slurry fiber (Sb') and dilution water were weighed and dispersed so that the carbon fiber, carbon fiber precursor fiber and fibril fiber were in a mass ratio of 70:10:20 and the fiber concentration in the slurry was 1.44 g/L. For papermaking, a treatment device was used that consisted of a net drive unit, a sheet-like material conveying device consisting of a net made of a plastic net plain weave mesh of width 60 cm x length 585 cm connected in a belt shape and continuously rotated, a slurry supply unit of width 48 cm and a reduced pressure dehydration device arranged below the net. A pressurized water jet treatment device equipped with the following three water jet nozzles was arranged downstream of the treatment device.
Nozzle 1: hole diameter φ0.15 mm×50 holes, width direction hole pitch 1 mm (1001 holes/width 1 m), arranged in one row, nozzle effective width 500 mm.
Nozzle 2: hole diameter φ0.15 mm×50, one hole per width direction 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×100, 2 holes with a widthwise hole pitch of 1.5 mm arranged in 3 rows, row pitch 5 mm, nozzle effective width 500 mm.
The pressurized water jet pressure was set to 1 MPa (nozzle 1), 2 MPa (nozzle 2), and 1 MPa (nozzle 3), and the slurry in which the fibers were dispersed was fed from the slurry supply unit, and after reduced pressure dehydration, the slurry was passed through nozzles 1, 2, and 3 in this order to carry out an entanglement treatment, thereby obtaining a precursor sheet having a three-dimensional entangled structure. The precursor sheet was dried at 150° C. for 3 minutes using a pin tenter tester (manufactured by 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 well dispersed in the precursor sheet, and the handleability was also good.

(3) Resin Impregnation/Drying and Pressurized Heat Molding The obtained precursor sheet was impregnated with a phenolic resin dispersion and dried at an atmospheric temperature of 100° C. using a hot air dryer.
Next, both sides of this precursor sheet were sandwiched between papers coated with a silicone-based release agent, and press molding was performed in a double belt press at 190° C. and a belt speed of 0.2 m/min to obtain a carbon fiber sheet.

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

(5) Water-repellent treatment A water-repellent treatment liquid was prepared by mixing a PTFE dispersion (manufactured by Mitsui-Chemours Fluoroproducts Co., Ltd., product 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 solid content concentration in the water-repellent treatment liquid was 1 mass % for PTFE and 2 mass % for the surfactant, and distilled water was further added and the mixture was stirred at 1000 rpm for 10 minutes using a disper to prepare the water-repellent treatment liquid.
The porous electrode substrate was immersed in the water-repellent treatment liquid to be impregnated. 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 furnace to obtain a water-repellent treated porous electrode substrate.

<Preparation of Coating Solution>
Pyrolytic graphite (manufactured by Ito Graphite Industries Co., Ltd., product name "PC-H", average particle size 7.68 μm), acetylene black as carbon powder (manufactured by Denka Co., Ltd., product name "Denka Black (registered trademark)", average particle size 35 nm), and ion-exchanged water were mixed to prepare dispersion liquid 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 stirrer (manufactured by Primix Corporation, product name "Homomixer MARK-II") to prepare dispersion liquid A. Note that, when the particles of the pyrolytic graphite were confirmed at five points using the following scanning electron microscope (SEM) and the aspect ratio was calculated to be 5.
A coating liquid was prepared by adding a polytetrafluoroethylene (PTFE) dispersion as dispersion B to the obtained dispersion A. Specifically, dispersion A and dispersion B were mixed so that the amount of PTFE was 42 parts by mass per 100 parts by mass of carbon powder, and the mixture was stirred at 5000 rpm for 15 minutes with a disperser while maintaining the liquid temperature at 30° C., to obtain a coating liquid.
The solid content of the obtained coating liquid was 10.8% by mass. Here, the "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.

<Manufacture of gas diffusion layer>
The coating liquid was applied to one surface of the water-repellent treated porous electrode substrate by bar coating at a coating speed of 3.3 m/min so that the coating film had a thickness of 198 μm, forming a coating film on the surface of the porous electrode substrate.
Next, the porous electrode substrate was dried for 5 minutes using a hot air drying oven set at 150°C, and then sintered for 30 minutes at 360°C in a sintering oven to obtain a gas diffusion layer in which a coating layer having a thickness of 34 µm was formed on one side of the porous electrode substrate.
Three randomly selected points on the surface of the coating layer of the resulting gas diffusion layer were observed at a magnification of 200 times using a scanning electron microscope (manufactured by JEOL Ltd., product name "JSM-6390") The results are shown in Figure 5(a).

[Example 2]
Dispersion A was prepared in the same manner as in Example 1, except that the pyrolytic graphite and carbon powder were mixed so that the amount of pyrolytic graphite was 27 parts by mass per 100 parts by mass of the carbon powder.
A coating solution 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 solution, and three arbitrary points 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 solution is shown in Table 1.
The peel strength of the resulting gas diffusion layer was 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.

[Example 3]
Dispersion A was prepared in the same manner as in Example 1, except that the pyrolytic graphite and carbon powder were mixed so that the amount of pyrolytic graphite was 54 parts by mass per 100 parts by mass of the carbon powder.
A coating solution 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 solution, and three arbitrary points 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 solution is also shown in Table 1.

[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 amount of pyrolytic graphite was 81 parts by mass per 100 parts by mass of carbon powder.
A coating solution 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 solution, and three arbitrary points 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 solution is also shown in Table 1.

[Example 5]
Dispersion A was prepared in the same manner as in Example 1, except that the pyrolytic graphite and carbon powder were mixed so that the amount of pyrolytic graphite was 108 parts by mass per 100 parts by mass of the carbon powder.
A coating solution 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 solution, and three arbitrary points 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 solution is also shown in Table 1.

[Comparative Example 1]
Dispersion A was prepared in the same manner as in Example 1, except that no pyrolytic graphite was used.
A coating solution 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 solution, and three arbitrary points 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 solution is also shown in Table 1.
The peel strength of the resulting gas diffusion layer was 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.

As is clear from FIG. 5, the gas diffusion layers obtained in the Examples had less cracking in the coating layer than the gas diffusion layer obtained in Comparative Example 1.
As is apparent from FIG. 6, the gas diffusion layer obtained in Example 2 had a higher peel strength than the gas diffusion layer obtained in Comparative Example 1, and was excellent in adhesion of the coating layer to the porous electrode substrate.
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 show that by forming a coating layer using a combination of pyrolytic graphite and carbon powder, the occurrence of cracks can be suppressed and the peel strength and surface smoothness can be improved.

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 electrolyte fuel cell

Claims (24)

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 comprising 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. The gas diffusion layer according to claim 1, wherein the mass ratio of the carbon powder A to the carbon powder B is 0.5 to 9. The gas diffusion layer according to claim 1 or 2, wherein the carbon powder A is at least one selected from the group consisting of carbon black, milled fiber, carbon nanotube, 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. 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 comprising 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. The gas diffusion layer according to any one of claims 1 to 4, wherein the coating layer contains a water repellent. The gas diffusion layer according to claim 3 or 4, wherein the aspect ratio of the pyrolytic graphite is 2 to 40. The gas diffusion layer according to claim 4 or 6, wherein the mass ratio of the carbon powder C to the pyrolytic graphite is 0.5 to 9. The gas diffusion layer according to any one of claims 1 to 7, having a thickness of 160 to 350 μm. The gas diffusion layer according to any one of claims 1 to 8, wherein the average pore diameter of the porous electrode substrate is 5 to 200 μm. The gas diffusion layer according to any one of claims 1 to 9, wherein the surface roughness of the coating layer is 3.0 μm or less. The gas diffusion layer according to any one of claims 1 to 10, wherein the porous electrode substrate contains carbon fibers. The gas diffusion layer according to claim 4, wherein the average particle size of the pyrolytic graphite is 3 to 50 μm. The gas diffusion layer according to claim 4, wherein the carbon powder C has an average particle size of 30 to 100 nm. 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,
The gas diffusion layer, wherein the coating layer contains pyrolytic graphite, carbon black, and a fluororesin. The gas diffusion layer according to claim 14, wherein the aspect ratio of the pyrolytic graphite is 2 to 40. The gas diffusion layer according to claim 14 or 15, wherein the mass ratio of the carbon black to the pyrolytic graphite is 0.5 to 9. The gas diffusion layer according to any one of claims 14 to 16, wherein the average particle size of the pyrolytic graphite is 3 to 50 μm. The gas diffusion layer according to any one of claims 14 to 17, wherein the carbon black has an average particle size of 30 to 100 nm. A roll of a gas diffusion layer comprising a protective layer provided on the coating layer of the gas diffusion layer according to any one of claims 1 to 18 and wound into a roll. A polymer electrolyte fuel cell comprising a gas diffusion layer according to any one of claims 1 to 18. 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 the steps of:
A method for producing a gas diffusion layer, comprising: applying a coating liquid, which is 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 4,000 nm, to at least one surface of the porous electrode substrate. The method for manufacturing a gas diffusion layer according to claim 21, wherein the aspect ratio of the carbon powder B is 2 to 40. The method for manufacturing a gas diffusion layer according to claim 21 or 22, wherein the carbon powder B is pyrolytic graphite. The method for manufacturing a gas diffusion layer according to any one of claims 21 to 23, wherein the mass ratio represented by the carbon powder A/the carbon powder B is 0.5 to 9.

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