Fuel cell gas diffusion layer substrate sheet and method for using same

Abstract

The purpose of the present invention is to provide a fuel cell gas diffusion layer substrate sheet which, at 30-120°C in an anode electrode gas diffusion layer of an anode electrode, is capable of improving the retention of water or water vapor generated in a cathode electrode when a fuel cell obtained by assembling the substrate sheet as the anode electrode gas diffusion layer is operated, and with which a fuel cell exhibiting high power generation performance is achieved. A fuel cell gas diffusion layer substrate sheet (1) according to the present invention includes carbon fibers (2), graphite particles (4), and a binder (6), and constitutes a porous sheet comprising a first surface and a second surface that is on the reverse side from the first surface. When the sheet is divided, in a thickness-direction cross section obtained by being cut perpendicularly to the surface extension direction, into three equal parts to form a first layer, a second layer, and a third layer from the first surface side toward the second surface side, the volume ratio at which the carbon fibers, graphite particles, and binder are present is less than 35% in the first layer, and is 35%-95% in at least one of the second layer and the third layer.

Description

Gas diffusion layer substrate sheet for fuel cells and method of using the same

[0001] The present invention relates to a fuel cell gas diffusion layer substrate sheet suitable for forming a gas diffusion layer for the anode electrode of a solid polymer fuel cell.

[0002] Solid polymer fuel cells (hereinafter simply referred to as "fuel cells"), which are widely used as power sources for automobiles and the like, are constructed by stacking multiple fuel cell cells separated by separators. In a single cell, a cathode electrode and an anode electrode are arranged on both sides of a polymer electrolyte membrane that selectively permeates specific ions. These electrodes consist of a catalyst layer made of a conductive material such as carbon supporting a catalyst such as platinum and an ion exchange resin, and a porous gas diffusion layer placed outside each catalyst layer, forming a membrane / electrode assembly. A separator is then placed outside the gas diffusion layer constituting this membrane / electrode assembly, forming a gas channel for supplying fuel gas or oxidizer gas and for discharging generated gas and excess gas, and the membrane / electrode assembly is sandwiched between the separators.

[0003] In the fuel cell with the above configuration, hydrogen gas or the like is supplied to the anode electrode as a fuel gas, and oxygen gas or air is supplied to the cathode electrode as an oxidizing gas. At each electrode, gas flows in the in-plane direction through a unique gas flow channel layer or the gas flow channel of the separator, and then the gas diffused in the gas diffusion layer is guided to the electrode catalyst where an electrochemical reaction proceeds. In this electrochemical reaction, hydrogen ions and water generated at the anode electrode permeate the electrolyte membrane in a hydrated state and reach the cathode electrode, where generated water is produced. Therefore, depending on the manner of water movement inside the membrane electrode assembly and the form of generated water produced by the electrochemical reaction, the anode electrode may dry out as power generation progresses, causing dry-up, while the cathode electrode may become excessively moist, causing flattening. If dry-up occurs at the anode electrode, the hydrogen gas dries out, reducing the proton conductivity of the electrolyte membrane, which is an ion exchange membrane, and thus decreasing the power generation performance of the fuel cell. On the other hand, if flattening occurs at the cathode electrode, water accumulates in the cathode electrode gas diffusion layer or gas flow channel layer (or the gas flow channel of the separator), obstructing the flow of oxidant gas. This prevents sufficient oxidant gas from being supplied to the membrane electrode assembly, and again, the power generation performance of the fuel cell decreases.

[0004] Conventionally, substrates containing carbon fibers, graphite, etc., have been used to form gas diffusion layers for the anode or cathode electrodes in fuel cells.

[0005] For example, Patent Document 1 discloses a porous gas diffusion layer substrate which consists of a carbon fiber aggregate in which carbon fibers are intertwined, and includes a carbide that binds the carbon fibers of the carbon fiber aggregate, and spheroidal graphite and / or artificial graphite with a median diameter in the range of 40 μm to 120 μm held between the carbon fibers of the carbon fiber aggregate.

[0006] Patent Document 2 describes a gas diffusion layer substrate for the anode electrode of a fuel cell, comprising (A) carbon fibers and (B) graphite particles, wherein the graphite particles (B) have a major axis of d Ｌ Let the minor axis be d. Ｓ In that case, the major axis d Ｌ and minor axis d Ｓ The ratio Dr(=d) Ｌ / d Ｓ A gas diffusion layer substrate for an anode electrode is disclosed, characterized in that it contains 5 to 100 graphite particles (B1).

[0007] Patent Document 3 discloses a porous carbon sheet containing carbon fibers and a binder, wherein, when the carbon sheet is divided into six equal parts in the thickness direction under compression in a section from one surface to the other surface, the layers obtained are numbered sequentially as layer 1, layer 2, layer 3, layer 4, layer 5, and layer 6 from the layer containing one surface to the layer containing the other surface, the layer with the highest packing density under compression is layer 2, and the relationship of the packing density under compression of layers 2, 3, 4, 5, and 6 is such that layer 2 has the highest packing density, and layer 3 has the second highest packing density.

[0008] Furthermore, Patent Document 4 discloses a porous carbon sheet containing carbon fibers and a binder, wherein, in the section from the surface with a 50% filling rate closest to one surface to the surface with a 50% filling rate closest to the other surface, the carbon sheet is divided into three equal parts perpendicular to the surface, and the layers obtained are characterized in that, if the layer with the highest filling rate closest to one surface is called layer X, the layer with a smaller filling rate than layer X closest to the other surface is called layer Y, and the layer located between layer X and layer Y is called layer Z, then the filling rates of the layers decrease in the order of layer X, layer Y, and layer Z.

[0009] Japanese Patent Publication No. 2020-87826, Japanese Patent Publication No. 2023-85031, WO 2016 / 060044 A1, WO 2017 / 069014 A1

[0010] The object of the present invention is to provide a fuel cell gas diffusion layer substrate sheet that provides a fuel cell that exhibits high power generation performance, by enabling good water retention in the anode gas diffusion layer of the anode electrode, which consists of the anode electrode gas diffusion layer and the cathode electrode gas diffusion layer, when the fuel cell, which is assembled as the anode electrode gas diffusion layer, is driven, for water or water vapor generated at the cathode electrode consisting of the cathode electrode catalyst layer and the cathode electrode gas diffusion layer, under conditions of 60°C to 120°C.

[0011] The present invention is as follows: [1] A porous fuel cell gas diffusion layer substrate sheet comprising carbon fibers, graphite particles, and a binder, and having a first surface and a second surface opposite to the first surface, wherein the fuel cell gas diffusion layer substrate sheet is divided into three equal parts in the thickness direction, and the layers are designated as a first layer, a second layer, and a third layer from the side of the first surface toward the side of the second surface, and the volume percentage (A) of the carbon fibers, graphite particles, and binder is less than 35% in the first layer and 35% to 95% in at least one of the second layer and the third layer, characterized in that the fuel cell gas diffusion layer substrate sheet is as follows: [2] The fuel cell gas diffusion layer substrate sheet according to item [1], wherein the volume percentage (A) in the second layer is 35% to 90%. [3] The fuel cell gas diffusion layer substrate sheet according to item [1], wherein the volume percentage (A) in the third layer is 35% to 90%. [4] A fuel cell gas diffusion layer substrate sheet according to item [1] above, wherein the binder contains resin carbide. [5] A fuel cell gas diffusion layer substrate sheet according to item [1] above, wherein the carbon fiber content in the fuel cell gas diffusion layer substrate sheet is 30 to 90% by mass. [6] A fuel cell gas diffusion layer substrate sheet according to item [1] above, wherein the graphite particle content in the fuel cell gas diffusion layer substrate sheet is 5 to 70% by mass. [7] A fuel cell gas diffusion layer substrate sheet according to item [1] above, wherein the thickness is 50 to 400 μm. [8] A fuel cell gas diffusion layer substrate sheet according to item [1] above, which is a gas diffusion layer substrate sheet for the anode electrode of a fuel cell. [9] A method for using the fuel cell gas diffusion layer substrate sheet described in item [1] above for manufacturing the anode gas diffusion layer in a fuel cell comprising an anode electrode separator, an anode electrode gas diffusion layer, an anode electrode catalyst layer, a solid polymer electrolyte membrane, a cathode electrode catalyst layer, a cathode electrode gas diffusion layer, and a cathode electrode separator, characterized in that the first surface of the fuel cell gas diffusion layer substrate sheet is arranged to face the anode electrode catalyst layer.

[0012] When the fuel cell gas diffusion layer substrate sheet of the present invention is used as a material for forming the anode gas diffusion layer in a fuel cell comprising an anode electrode separator, an anode electrode gas diffusion layer, an anode electrode catalyst layer, a solid polymer electrolyte membrane, a cathode electrode catalyst layer, a cathode electrode gas diffusion layer, and a cathode electrode separator, and the sheet is arranged so that its first surface faces the anode electrode catalyst layer, high power generation performance can be obtained at temperatures from 60°C to 120°C.

[0013] This is a schematic cross-sectional view showing the fuel cell gas diffusion layer substrate sheet of the present invention. This is a cross-sectional image of the fuel cell gas diffusion layer substrate sheet obtained in Example 1.

[0014] As shown in Figure 1, the fuel cell gas diffusion layer substrate sheet of the present invention comprises carbon fibers 2, graphite particles 4, and a binder 6 that binds the carbon fibers 2 together, the graphite particles 4 together, or the carbon fibers 2 and graphite particles 4 together. It is a porous, thin-walled plate-like article that is preferably used as a material for forming the anode gas diffusion layer in a fuel cell equipped with an anode electrode separator, an anode electrode gas diffusion layer, an anode electrode catalyst layer, a solid polymer electrolyte membrane, a cathode electrode catalyst layer, a cathode electrode gas diffusion layer, and a cathode electrode separator.

[0015] The carbon fibers may be any of the following: vapor-grown carbon fibers, carbon nanotubes (single-wall, double-wall, multi-wall, cup-laminated, etc.), polyacrylonitrile (PAN) carbon fibers, pitch carbon fibers, or rayon carbon fibers. The carbon fibers contained in the fuel cell gas diffusion layer substrate sheet of the present invention may be one type or two or more types.

[0016] The fiber diameter of the carbon fiber is not particularly limited, but the average fiber diameter is preferably 5 to 15 μm, more preferably 6 to 8 μm. The upper limit of the fiber length of the carbon fiber is usually 12 mm, and the lower limit is usually 2 mm. The average fiber length of the carbon fiber is preferably 2 to 9 mm, more preferably 3 to 6 mm.

[0017] The carbon fiber content in the fuel cell gas diffusion layer substrate sheet of the present invention is preferably 20 to 80% by mass, and more preferably 30 to 70% by mass, when the total content of carbon fibers, graphite particles, and binders is taken as 100% by mass.

[0018] The graphite particles may be derived from either natural graphite or artificial graphite. Alternatively, they may be expanded graphite particles obtained by special processing from flaky graphite, which is natural graphite. The graphite particles contained in the fuel cell gas diffusion layer substrate sheet of the present invention may be of only one type or of two or more types.

[0019] The shape of the graphite particles is not particularly limited and can be spherical, ellipsoidal, plate-shaped, linear, irregular, etc. In the present invention, it is preferable to include plate-shaped (flaky) graphite particles. The size of the graphite particles is also not particularly limited, and the average particle diameter d50 measured by laser diffraction is preferably 5 to 100 μm, more preferably 20 to 80 μm.

[0020] The proportion of graphite particles contained in the fuel cell gas diffusion layer substrate sheet of the present invention is preferably 5 to 60% by mass, and more preferably 10 to 50% by mass, when the total content of carbon fibers, graphite particles, and binders is taken as 100% by mass.

[0021] The binder is preferably a resin carbide. A preferred method for manufacturing the fuel cell gas diffusion layer substrate sheet of the present invention will be described later, but it is a method in which a composite fiber sheet, in which carbon fibers, graphite particles, and resin fibers are bonded to a resin adhesive, is heated to a high temperature to carbonize the resin fibers and resin adhesive, and the binder is derived from these carbides.

[0022] The binder content in the fuel cell gas diffusion layer substrate sheet of the present invention is preferably 5 to 60% by mass, and more preferably 10 to 50% by mass, when the total content of carbon fibers, graphite particles, and binder is taken as 100% by mass.

[0023] The thickness of the gas diffusion layer substrate sheet for fuel cells of the present invention is preferably 100 to 300 μm, more preferably 120 to 250 μm, from the viewpoint of rigidity and the power generation performance of the resulting fuel cell.

[0024] The basis weight of the fuel cell gas diffusion layer substrate sheet of the present invention is preferably 40 to 100 g / m², from the viewpoint of balancing gas diffusion and water retention. ２ More preferably 50 to 85 g / m ２ That is the case.

[0025] As described above, the fuel cell gas diffusion layer substrate sheet of the present invention is porous rather than solid. As shown in Figure 1, if this substrate sheet is divided into three equal parts in the thickness direction, that is, if the thickness direction cross section cut perpendicular to the surface stretching direction is divided into three equal parts, and these are designated as the first layer, second layer, and third layer from the first surface side to the second surface side, then the volume percentage (A) in which carbon fibers, graphite particles, and binders are present is less than 35% in the first layer, and 35% to 95% in at least one of the second and third layers. In other words, when the thickness direction cross section of the fuel cell gas diffusion layer substrate sheet of the present invention is viewed, each of the first, second, and third layers has a portion in which the material (carbon fibers, graphite particles, and binders) is present (hereinafter referred to as the "material-present portion") and a portion in which it is not present (hereinafter referred to as the "material-absent portion").

[0026] The volume percentage (A) of the first layer is less than 35%, preferably 5% to 30%, more preferably 10% to 25%. The volume percentage (A) is 35% to 95% in at least one of the second and third layers, and if the volume percentage (A) of one layer is 35% to 95%, the volume percentage (A) of the other layer may be less than 35% or more than 95%. The volume percentage (A) of the second layer is 35% to 95%, preferably 35% to 90%, more preferably 40% to 80%, and even more preferably 50% to 60%. The volume percentage (A) of the third layer is 35% to 95%, preferably 35% to 90%, more preferably 40% to 80%, and even more preferably 50% to 60%.

[0027] The above volume ratio (filling rate) can be obtained by a standard measurement method. Specifically, the volume ratio (filling rate) of the first to third layers can be obtained by three-dimensional X-ray CT measurement. Three-dimensional data of the fuel cell gas diffusion layer substrate sheet is obtained by scanning the entire area perpendicular to the surface of the carbon sheet at regular intervals of a certain length from one surface to the other using three-dimensional X-ray CT. By analyzing this three-dimensional data, the volume ratio (filling rate) at the measured surface can be obtained, and the volume ratio (filling rate) of a specific layer can be determined. The above-mentioned regular length (hereinafter referred to as slice pitch) can be set arbitrarily, and the slice pitch can be any value less than or equal to the average diameter of the single carbon fibers constituting the fuel cell gas diffusion layer substrate sheet.

[0028] The volume ratio (filling rate) of a surface at a predetermined position perpendicular to the surface of the fuel cell gas diffusion layer substrate sheet is determined by using an image processing program to take a slice image of that position in the 3D data, divide it into 256 steps based on brightness (maximum and minimum), and binarize it using the minimum 80 steps as a threshold. The ratio of the brighter, binarized area to the total area is the volume ratio (filling rate) of the surface at the predetermined position. Examples of such image processing programs include "ImageJ" and "J-trim".

[0029] The method for producing the fuel cell gas diffusion layer substrate sheet of the present invention is not particularly limited. A preferred production method comprises: an aggregate production step of producing a two-layer aggregate containing carbon fibers, graphite particles, and resin fibers to be carbonized in a later carbonization step, wherein the graphite particle content differs from that of the two-layer aggregate; a resin impregnation step of impregnating the two-layer aggregate with a carbon precursor resin to be carbonized in a later carbonization step as a resin adhesive to produce a resin-impregnated sheet; and a carbonization step of heating and firing the resin-impregnated sheet in a non-oxidizing atmosphere.

[0030] The resin fibers used in the above aggregate manufacturing process preferably include branched resin fibers (hereinafter referred to as "fibrillated resin fibers"), and may further include unbranched resin fibers.

[0031] The fibrillated resin fiber includes a trunk fiber portion extending in the length direction of the fiber, and a branched fiber portion branched from the trunk fiber portion and usually having a shorter diameter than the trunk fiber portion. Examples of constituent materials for the fibrillated resin fiber include acrylonitrile-based polymers, polyvinyl alcohol, polyolefins, polyurethanes, polyesters, polyamides, acrylic resins, aramids, polyacetals, polylactic acid, phenolic resins, and cellulose. Among these, acrylonitrile-based polymers are preferred.

[0032] The cross-sectional shape of the trunk fiber portion, which constitutes the fibrillated resin fiber and does not involve the branched fiber portion, is not particularly limited, and may be circular, elliptical, polygonal, or modified forms thereof. This cross-sectional shape is generally the same over the entire length direction of the fibrillated resin fiber. Further, the average fiber diameter of the trunk fiber portion is preferably 5 to 20 µm, more preferably 6 to 8 µm. In addition, the length of the trunk fiber portion, which is substantially the length of the fibrillated resin fiber, is preferably 1 to 15 mm, more preferably 1 to 10 mm.

[0033] The cross-sectional shape of the branched fiber portion, which constitutes the fibrillated resin fiber, is not particularly limited, and may be circular, elliptical, semicircular, fan-shaped, polygonal, or modified forms thereof. This cross-sectional shape may generally be the same or different over the entire length direction of the branched fiber portion.

[0034] The degree of fibrillation of the fibrillated resin fiber is generally quantified by freeness, specific surface area, and the like. The Canadian standard freeness measured in accordance with JIS P 8121-2 is preferably 50 to 700 mL, more preferably 150 to 600 mL.

[0035] As described above, a fibrillated resin fiber and an unbranched resin fiber can be used in combination, and a general linear resin fiber can be used as the unbranched resin fiber. The configuration of the linear resin fiber (structure such as cross-sectional shape, size such as fiber diameter and fiber length) is not particularly limited, and may be either a monofilament or a multifilament. Examples of constituent materials for the linear resin fiber include acrylonitrile-based polymer, polyvinyl alcohol, polyolefin, polyurethane, polyester, polyamide, acrylic resin, aramid, polyacetal, polylactic acid, phenolic resin, and cellulose. Among these, polyvinyl alcohol is preferable.

[0036] In the above-mentioned aggregate production step, preferably, a first slurry containing a plurality of carbon fibers (one or more types), a plurality of graphite particles (one or more types), a plurality of resin fibers (one or more types), water, and optionally a dispersant, an adhesive, an antifoaming agent, etc., is subjected to papermaking using a wet paper machine having a wire such as a fourdrinier wire, a cylinder wire or a rotoformer, after obtaining a first water-containing sheet, a method of performing papermaking while supplying a second slurry that contains carbon fibers and resin fibers and has a different graphite particle content to the surface of the first water-containing sheet can be applied. In the first slurry and the second slurry, when the total amount of carbon fibers, graphite particles, fibrillated resin fibers and linear resin fibers is taken as 100% by mass, the proportion of the used amount of each component is preferably 20 to 80% by mass, 5 to 60% by mass, 5 to 60% by mass and 1 to 30% by mass, respectively. Further, the solid content concentration in the first slurry and the second slurry is preferably 0.1% by mass or more.

[0037] The basis weight (total amount) of the two-layer aggregate to be subjected to the above resin impregnation step is preferably 30 to 100 g / m ２ .

[0038] The two-layer aggregate to be subjected to the above resin impregnation step may be one that has been previously subjected to an entanglement treatment such as a mechanical entanglement method (needle punching method, etc.), a high-pressure liquid jet method (water jet punching method, etc.), or a high-pressure gas jet method (steam jet punching method, etc.).

[0039] The above resin impregnation step is a step in which a resin-impregnated sheet is produced by impregnating the two-layer aggregate obtained above with a carbon precursor resin that will be carbonized in the subsequent carbonization step. The carbon precursor resin is not particularly limited, but thermosetting resins such as phenol resin, furan resin, epoxy resin, melamine resin, imide resin, urethane resin, aramid resin, urea resin, and unsaturated polyester resin are preferred because they have excellent wettability with carbon fibers, graphite particles, or resin fibers, and readily form conductive carbides in the subsequent carbonization step. Among these, phenol resin is particularly preferred because it has a high carbonization rate and becomes an excellent conductive material after carbonization.

[0040] The carbon precursor resin described above can be used as is, depending on its properties, but a solution (resin solution) obtained by dissolving the carbon precursor resin in a solvent, or a dispersion liquid (resin dispersion liquid) obtained by dispersing the carbon precursor resin in a dispersion medium may also be used. In the resin impregnation step described above, it is preferable to use a liquid containing the carbon precursor resin (hereinafter referred to as "carbon precursor resin-containing liquid").

[0041] When using the above-mentioned carbon precursor resin-containing liquid, methods such as immersing a two-layer aggregate in the carbon precursor resin-containing liquid, or coating the two-layer aggregate with the carbon precursor resin-containing liquid (kiss coat method, spray method, curtain coat method, roller contact method, etc.) can be applied. Of these, the method of immersing a two-layer aggregate in the carbon precursor resin-containing liquid is preferred. After using the above-mentioned carbon precursor resin-containing liquid, a resin-impregnated sheet without solvent or dispersion medium can be obtained by using a non-contact drying method such as blowing hot air, placing it in a high-temperature atmosphere, using an infrared heater, or using microwaves, or by using a contact drying method such as contacting it with a heated roll, plate, etc.

[0042] Other methods that do not use the carbon precursor resin-containing liquid include a method in which a resin film containing the carbon precursor resin is used to cover at least a portion of the surface of the two-layer aggregate, and the carbon precursor resin is dissolved by heating or a solvent to permeate the entire two-layer aggregate.

[0043] The resin-impregnated sheet obtained by the above resin impregnation process may contain the carbon precursor resin used as is, or, if the carbon precursor resin is a curable resin, it may be a cured resin formed by, for example, an operation to remove the solvent or dispersion medium (such as heating) as described above. In order to obtain a fuel cell gas diffusion layer substrate sheet with excellent structural stability in the subsequent carbonization process, in which the carbon fibers, graphite particles, and resin fibers contained in the above two-layer aggregate are sufficiently bound by the cured resin, a heating press (hydraulic press, hot press, belt press, roll press, etc.) can be performed on the two-layer aggregate while the carbon precursor resin is in contact with it.

[0044] In the resin impregnation process described above, the amount of carbon precursor resin impregnated into the two-layer aggregate is not particularly limited. From the viewpoint of conductivity and strength of the fuel cell gas diffusion layer substrate sheet obtained in the subsequent carbonization process, when the total amount of the two-layer aggregate and carbon precursor resin in the resin-impregnated sheet is set to 100% by mass, the content of the carbon precursor resin is preferably 10 to 70% by mass, more preferably 20 to 60% by mass.

[0045] The above carbonization process involves heating and firing the resin-impregnated sheet in a non-oxidizing atmosphere. The non-oxidizing atmosphere can be an atmosphere containing an inert gas such as argon or helium, or nitrogen gas. The heating temperature of the resin-impregnated sheet is preferably 1800°C to 2500°C, more preferably 1900°C to 2200°C, in order to avoid deterioration of the strength of the carbon fibers and to facilitate the carbonization of the resin fibers and carbon precursor resin. In addition, the above carbonization process can be carried out using a multi-stage heating method, which involves heating at a temperature lower than the preferred carbonization temperature. The heating time for the resin-impregnated sheet is set appropriately depending on its size, etc., but is usually 1 minute or more.

[0046] Through the carbonization process described above, the resin fibers and carbon precursor resin (resin adhesive) contained in the resin-impregnated sheet are carbonized, and the resulting carbides bind the carbon fibers together, the graphite particles together, or the carbon fibers and graphite particles together, thereby obtaining a fuel cell gas diffusion layer substrate sheet 1 having the structure shown in Figure 1.

[0047] The resulting fuel cell gas diffusion layer substrate sheet, although containing voids, has strong conductive paths due to its carbide-based phase. Furthermore, because the carbide is derived from resin, it possesses elasticity and exhibits high dimensional absorption when bonded to the electrolyte layer or catalyst layer during fuel cell manufacturing. Moreover, when a fuel cell is manufactured by forming an anode electrode gas diffusion layer using this fuel cell gas diffusion layer substrate sheet, the water or water vapor generated at the cathode electrode is dispersed in the anode electrode gas diffusion layer of the anode electrode, which consists of the anode electrode gas diffusion layer and the anode electrode catalyst layer. Under conditions of 60°C to 120°C, the diffusion of water vapor is slowed (water vapor does not escape easily), drying is suppressed, water retention is improved, and high power generation performance can be achieved.

[0048] When forming an anode gas diffusion layer using the fuel cell gas diffusion layer substrate sheet of the present invention, the surface (first surface) of the fuel cell gas diffusion layer substrate sheet on the first layer side may be treated with a water-repellent coating beforehand. Examples of water-repellent materials include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), and polyhexafluoropropylene (FEP). In the water-repellent treated fuel cell gas diffusion layer substrate sheet, the water-repellent material is either a film or scattered on the sheet.

[0049] When manufacturing a fuel cell, it is preferable to first fabricate a membrane / electrode assembly comprising an anode gas diffusion layer, a microporous layer, an anode-side catalyst layer, a solid polymer electrolyte membrane, a cathode-side catalyst layer, a microporous layer, and a cathode gas diffusion layer, and then to arrange an anode-side separator and a cathode-side separator on both sides, respectively.

[0050] In the resulting fuel cell, when oxidizing gas is supplied from an external source to the oxidizing gas channel of the cathode electrode separator, a portion of the oxidizing gas flowing along this channel enters the interior of the cathode electrode gas diffusion layer. The remaining unreacted oxidizing gas that does not enter flows along the oxidizing gas channel and is discharged to the outside of the fuel cell. Similarly, when fuel gas is supplied from an external source to the fuel gas channel of the anode electrode separator, a portion of the fuel gas flowing along this channel enters the interior of the anode electrode gas diffusion layer. The remaining unreacted fuel gas that does not enter flows along the fuel gas channel and is discharged to the outside of the fuel cell. Electricity is then extracted between the cathode electrode separator and the anode electrode separator through the reaction of the oxidizing gas and fuel gas.

[0051] 1. Manufacturing raw materials for fuel cell gas diffusion layer substrate sheets: The following manufacturing raw materials were used.

[0052] 1-1. Carbon fiber manufactured by Teijin Corporation (fiber length: 3 mm, fiber diameter: 7 μm) was used.

[0053] 1-2. Pulp fibers: Pulp fibers manufactured by Toyobo Co., Ltd. (fiber length: 2 mm, filtration capacity: 550 ml) were used.

[0054] 1-3. Resin fiber: Vinylon fiber manufactured by Kuraray Co., Ltd. (fiber length: 3 mm, fiber diameter: 11 μm) was used.

[0055] 1-4. Graphite particles: Graphite powder "CMX-40" (product name) manufactured by Nippon Graphite Co., Ltd. was used. It was flaky, and the average particle size d50 measured by laser diffraction was 40 μm.

[0056] 2. Method for Measuring the Volume Ratio of the First to Third Layers in a Fuel Cell Gas Diffusion Layer Substrate Sheet The volume ratio of the first to third layers in a fuel cell gas diffusion layer substrate sheet was measured using the following method. An X-ray CT image of the fuel cell gas diffusion layer substrate sheet was acquired using a high-resolution 3DX microscope "nano3DX" (product name) manufactured by Rigaku Corporation. The slice pitch for this measurement was 2.6 μm, and the measurement field was φ2.66 × H2.6 mm. ２The obtained 3D image was then divided into 256 steps based on brightness, from maximum to minimum, using the image processing software "ImageJ". The image was then binarized from the minimum to a threshold of 80, and the area of ​​the white parts was taken as the volume ratio (filling rate) of the image. The obtained values ​​for each layer were then averaged, and the area between the point with the 50% volume ratio closest to one side and the point with the 50% volume ratio closest to the other side was divided into three equal parts, which were designated as the first layer, second layer, and third layer, and the volume ratio (filling rate) of each layer was obtained.

[0057] 3. Manufacturing of gas diffusion layer substrate sheets for fuel cells A gas diffusion layer substrate sheet for fuel cells was manufactured using the above-mentioned raw materials.

[0058] Experimental Example 1 A first slurry containing 40 parts by mass of carbon fiber, 20 parts by mass of pulp fiber, 10 parts by mass of resin fiber, 30 parts by mass of graphite particles, and water was continuously formed using a short-wire paper machine to obtain a first paper-formed sheet in which the carbon fiber, pulp fiber, and resin fiber were intertwined, and the graphite particles were interposed in the gaps between the fibers. Next, a second slurry containing 35 parts by mass of carbon fiber, 15 parts by mass of pulp fiber, 10 parts by mass of resin fiber, 40 parts by mass of graphite particles, and water was supplied to the surface of this first paper-formed sheet and continuously formed using a cylinder paper machine to form a layer consisting of a second paper-formed sheet on one side of the layer consisting of the first paper-formed sheet. After that, water was squeezed out and the sheet was dried to obtain a long paper-formed sheet with a two-layer structure (hereinafter referred to as "paper-formed sheet W1"). The basis weight was 70 g / m². ２ The graphite particle content was 25% by mass in the first paper-formed sheet layer, 40% by mass in the second paper-formed sheet layer, and 32.5% by mass in the entire paper-formed sheet W1. Furthermore, the basis weight ratio of the first and second paper-formed sheet layers, calculated from the components excluding the graphite particles, was 7:3.

[0059] Next, the paper-formed sheet W1 was impregnated with a phenol resin solution, and the resulting resin-impregnated sheet was heated (120°C) to evaporate the solvent and dry it to obtain a fiber-bound sheet. The amount of phenol resin attached was 25 parts by mass per 100 parts by mass of the paper-formed sheet. After that, the paper-formed sheet was heat-pressed (250°C, 1 minute) using a double-belt press to obtain a composite fiber sheet. This composite fiber sheet was set on a first roll for unwinding and heat-treated (1 minute) in a graphitization furnace with an internal temperature of 2000°C and a nitrogen gas atmosphere to carbonize the pulp fibers, resin fibers, and phenol resin, thereby obtaining a fuel cell gas diffusion layer base sheet (hereinafter referred to as "base sheet H1") consisting of a graphite particle-supported carbon fiber sheet in which graphite particles are embedded and the resin carbides bind the carbon fibers together. The thickness was 180 μm. Figure 2 shows a cross-sectional image obtained by cutting the substrate sheet H1 perpendicular to its surface stretching direction and then photographing it with an electron microscope.

[0060] For the obtained base sheet H1, the volume percentage (filling rate) of solid components (carbon fibers, graphite particles, and carbides) in the first, second, and third layers was calculated according to the measurement method described in 2. above. The volume percentage A of the first layer was 31%, the volume percentage A of the second layer was 45%, and the volume percentage A of the third layer was 51% (see Table 1).

[0061] Experimental Example 2: The same procedure as in Experimental Example 1 was performed, except that the amount of the second slurry used was changed. A paper-formed sheet (hereinafter referred to as "paper-formed sheet W2") was obtained in which the graphite particle content was 17% by mass in the first paper-formed sheet layer, 28% by mass in the second paper-formed sheet layer, and 22.5% by mass in the entire paper-formed sheet, and the basis weight ratio of the first paper-formed sheet layer and the second paper-formed sheet layer, calculated from the components excluding graphite particles, was 7:3. The basis weight was 70 g / m². ２Subsequently, the same procedure as in Experimental Example 1 was performed on the paper-formed sheet W2, such as impregnating it with a phenol resin solution, to obtain a fuel cell gas diffusion layer substrate sheet (hereinafter referred to as "substrate sheet H2") with a thickness of 180 μm. Next, the volume percentage (filling rate) of solid components (carbon fibers, graphite particles, and carbides) in the first, second, and third layers of the obtained substrate sheet H2 was calculated according to the measurement method described in 2. above. The percentages were 25%, 35%, and 41%, respectively (see Table 1).

[0062] Experimental Example 3: The same procedure as in Experimental Example 1 was performed, except that the amounts of the first slurry and the second slurry used were changed. A paper-formed sheet (hereinafter referred to as "paper-formed sheet W3") was obtained in which the graphite particle content was 5% by mass in the first paper-formed sheet layer, 12% by mass in the second paper-formed sheet layer, and 8.5% by mass in the entire paper-formed sheet, and the basis weight ratio of the first paper-formed sheet layer and the second paper-formed sheet layer, calculated from the components excluding graphite particles, was 7:3. The basis weight was 70 g / m². ２ Subsequently, the same procedure as in Experimental Example 1 was performed on the paper-formed sheet W3, such as impregnating it with a phenol resin solution, to obtain a fuel cell gas diffusion layer substrate sheet (hereinafter referred to as "substrate sheet H3") with a thickness of 180 μm. Next, the volume percentage (filling rate) of solid components (carbon fibers, graphite particles, and carbides) in the first, second, and third layers of the obtained substrate sheet H3 was calculated according to the measurement method described in 2. above. The percentages were 20%, 26%, and 36%, respectively (see Table 1).

[0063] Experimental Example 4: The same procedure as in Experimental Example 1 was performed, except that the amounts of the first slurry and the second slurry used were changed. A paper-formed sheet (hereinafter referred to as "paper-formed sheet W4") was obtained in which the graphite particle content was 40% by mass in the first paper-formed sheet layer, 25% by mass in the second paper-formed sheet layer, and 32.5% by mass in the entire paper-formed sheet, and the basis weight ratio of the first paper-formed sheet layer and the second paper-formed sheet layer, calculated from the components excluding graphite particles, was 7:3. The basis weight was 70 g / m². ２Subsequently, the same procedure as in Experimental Example 1 was performed on the paper-formed sheet W4, such as impregnating it with a phenol resin solution, to obtain a fuel cell gas diffusion layer substrate sheet (hereinafter referred to as "substrate sheet H4") with a thickness of 180 μm. Next, the volume percentage (filling rate) of solid components (carbon fibers, graphite particles, and carbides) in the first, second, and third layers of the obtained substrate sheet H4 was calculated according to the measurement method described in 2. above. The percentages were 33%, 50%, and 40%, respectively (see Table 1).

[0064] Experimental Example 5: The same procedure as in Experimental Example 1 was performed, except that the amounts of the first slurry and the second slurry used were changed. A paper-formed sheet (hereinafter referred to as "paper-formed sheet W5") was obtained in which the graphite particle content was 40% by mass in the first paper-formed sheet layer, 25% by mass in the second paper-formed sheet layer, and 32.5% by mass in the entire paper-formed sheet, and the basis weight ratio of the first paper-formed sheet layer and the second paper-formed sheet layer, calculated from the components excluding graphite particles, was 7:3. The basis weight was 50 g / m². ２ Subsequently, the same procedure as in Experimental Example 1 was performed on the paper-formed sheet W5, such as impregnating it with a phenol resin solution, to obtain a fuel cell gas diffusion layer substrate sheet (hereinafter referred to as "substrate sheet H5") with a thickness of 140 μm. Next, the volume percentage (filling rate) of solid components (carbon fibers, graphite particles, and carbides) in the first, second, and third layers of the obtained substrate sheet H5 was calculated according to the measurement method described in 2. above. The percentages were 29%, 41%, and 34%, respectively (see Table 1).

[0065] Experimental Example 6: The same procedure as in Experimental Example 1 was performed, except that the amounts of the first slurry and the second slurry used were changed. A paper-formed sheet (hereinafter referred to as "paper-formed sheet W6") was obtained in which the graphite particle content was 5% by mass in the first paper-formed sheet layer, 12% by mass in the second paper-formed sheet layer, and 8.5% by mass in the entire paper-formed sheet, and the basis weight ratio of the first paper-formed sheet layer and the second paper-formed sheet layer, calculated from the components excluding graphite particles, was 7:3. The basis weight was 50 g / m². ２Subsequently, the same procedure as in Experimental Example 1 was performed on the paper-formed sheet W6, such as impregnating it with a phenol resin solution, to obtain a fuel cell gas diffusion layer substrate sheet (hereinafter referred to as "substrate sheet H6") with a thickness of 140 μm. Next, the volume percentage (filling rate) of solid components (carbon fibers, graphite particles, and carbides) in the first, second, and third layers of the obtained substrate sheet H6 was calculated according to the measurement method described in 2. above. The percentages were 21%, 29%, and 35%, respectively (see Table 1).

[0066] Experimental Example 7: The same procedure as in Experimental Example 1 was performed, except that the amounts of the first slurry and the second slurry used were changed. A paper-formed sheet (hereinafter referred to as "paper-formed sheet W7") was obtained in which the graphite particle content was 25% by mass in the first paper-formed sheet layer, 40% by mass in the second paper-formed sheet layer, and 32.5% by mass in the entire paper-formed sheet, and the basis weight ratio of the first paper-formed sheet layer and the second paper-formed sheet layer, calculated from the components excluding graphite particles, was 7:3. The basis weight was 30 g / m². ２ Subsequently, the same procedure as in Experimental Example 1 was performed on the paper-formed sheet W7, such as impregnating it with a phenol resin solution, to obtain a fuel cell gas diffusion layer substrate sheet (hereinafter referred to as "substrate sheet H7") with a thickness of 90 μm. Next, the volume percentage (filling rate) of solid components (carbon fibers, graphite particles, and carbides) in the first, second, and third layers of the obtained substrate sheet H7 was calculated according to the measurement method described in 2. above. The percentages were 27%, 28%, and 33%, respectively (see Table 1).

[0067] Experimental Example 8 A first slurry containing 60 parts by mass of carbon fiber, 25 parts by mass of pulp fiber, 15 parts by mass of resin fiber, and water was continuously formed using a short-wire paper machine to obtain a first paper-formed sheet in which the carbon fiber, pulp fiber, and resin fiber were intertwined. Next, a second slurry containing 60 parts by mass of carbon fiber, 25 parts by mass of pulp fiber, 15 parts by mass of resin fiber, and water was supplied to the surface of this first paper-formed sheet and continuously formed using a cylinder paper machine to form a layer consisting of a second paper-formed sheet on one side of the layer consisting of the first paper-formed sheet. After that, the sheets were dewatered and dried to obtain a long paper-formed sheet with a two-layer structure (hereinafter referred to as "paper-formed sheet W8"). The basis weight was 70 g / m².２ The basis weight ratio of the first paper-formed sheet layer and the second paper-formed sheet layer was 7:3.

[0068] Next, the same procedure as in Experimental Example 1 was performed on the paper-formed sheet W8, such as impregnating it with a phenol resin solution, to obtain a fuel cell gas diffusion layer substrate sheet (hereinafter referred to as "substrate sheet H8") with a thickness of 180 μm. For the obtained substrate sheet H8, the volume percentage (filling rate) of solid components (carbon fibers, graphite particles, and carbides) in the first, second, and third layers was calculated according to the measurement method described in 2. above. The percentages were 21%, 23%, and 23%, respectively (see Table 1).

[0069]

[0070] 4. Manufacturing and Evaluation of Fuel Cells Using the substrate sheets obtained in Experimental Examples 1-8, a membrane / electrode assembly was fabricated according to the following procedure. Then, the obtained membrane / electrode assembly was used in combination with a single-channel serpentine separator with a channel width of 1.0 mm, a rib width of 1.0 mm, and a groove depth of 1.0 mm, a current collector plate, etc., to fabricate a JARI standard cell. This cell was then incorporated into a single cell for fuel cell evaluation, with a current density of 2.4 A / cm². ２ The conditions were: cell temperature and dew point temperature of 40°C for each electrode, and current density of 3.2 A / cm². ２ The power generation performance (cell voltage measurement) was evaluated under conditions of cell temperature and dew point temperature of 70°C for each electrode.

[0071] <Method for Fabricating Membrane / Electrode Assembly> One side surface of a substrate sheet was treated with a water-repellent coating to create a water-repellent gas diffusion layer substrate for the anode electrode. Then, a paste for forming a microporous layer was applied to one side of the obtained water-repellent gas diffusion layer substrate for the anode electrode to fabricate an anode electrode gas diffusion layer laminate equipped with a microporous layer. On both sides of this anode electrode gas diffusion layer laminate, a separately prepared cathode electrode gas diffusion layer laminate, and an electrolyte layer consisting of the Nafion® series electrolyte membrane "NRE-212" (product name), a paste made of 50% Pt / C with a Pt load of 0.5 mg / cm³ was applied. ２ A membrane / electrode assembly was obtained using a fuel cell electrode film equipped with a catalyst layer.

[0072] Example 1 Using a water-repellent gas diffusion layer substrate for the anode electrode, which was prepared by water-repellent treatment of the first layer side surface of the substrate sheet H1 obtained in Experimental Example 1, a membrane / electrode assembly was prepared by the method described above, and then a single cell for fuel cell evaluation equipped with this membrane / electrode assembly was manufactured. The cell voltage was measured at 40°C and 70°C, and the power generation performance was determined according to the following criteria by comparing it with the measured values ​​in Comparative Example 3. The results are shown in Table 2. ◎: 10% higher than the measured values ​​of Comparative Example 3 at both 40°C and 70°C 〇: 5% higher than the measured values ​​of Comparative Example 3 at both 40°C and 70°C △: Equivalent to the measured values ​​of Comparative Example 3 at both 40°C and 70°C ×: Lower than the measured values ​​of Comparative Example 3 at both 40°C and 70°C

[0073] Examples 2-6: A single cell for fuel cell evaluation was manufactured in the same manner as in Example 1, except that a water-repellent gas diffusion layer substrate for the anode electrode was used, which was prepared by treating the first layer surface of the substrate sheets H2-H6 obtained in Experimental Examples 2-6 with a water-repellent treatment. The power generation performance was then evaluated (see Table 2).

[0074] Comparative Examples 1 and 2: A single cell for fuel cell evaluation was manufactured in the same manner as in Example 1, except that a water-repellent gas diffusion layer substrate for the anode electrode was used, which was prepared by treating the first layer side surface of the substrate sheets H7 and H8 obtained in Experimental Examples 7 and 8 with a water-repellent treatment, and the power generation performance was evaluated (see Table 2).

[0075] Comparative Example 3: A single cell for fuel cell evaluation was manufactured in the same manner as in Example 1, except that a water-repellent gas diffusion layer substrate for the anode electrode was used, which was prepared by treating the third layer side surface of the substrate sheet H1 obtained in Experimental Example 1 with a water-repellent treatment, and the power generation performance was evaluated (see Table 2).

[0076]

[0077] Furthermore, the present invention is not limited to the specific embodiments shown above, and various modified embodiments can be made within the scope of the present invention depending on the purpose and application.

[0078] The fuel cell gas diffusion layer substrate sheet of the present invention is suitable as a material for forming the anode electrode gas diffusion layer that constitutes the membrane / electrode assembly contained in a fuel cell. A fuel cell equipped with such an anode electrode gas diffusion layer has high power generation performance and can be used in transport fuel cells for vehicles, stationary fuel cells, and the like.

[0079] 1: Substrate sheet for gas diffusion layer in fuel cells 2: Carbon fiber 4: Graphite particles 6: Binding agent

Claims

1. A porous fuel cell gas diffusion layer substrate sheet comprising carbon fibers, graphite particles, and a binder, and having a first surface and a second surface opposite to the first surface, wherein the fuel cell gas diffusion layer substrate sheet is divided into three equal parts in the thickness direction, and the layers are designated as a first layer, a second layer, and a third layer from the side of the first surface toward the side of the second surface, and the volume percentage (A) of the carbon fibers, graphite particles, and binder is less than 35% in the first layer and 35% to 95% in at least one of the second layer and the third layer, characterized in that the fuel cell gas diffusion layer substrate sheet comprises carbon fibers, graphite particles, and a binder, and has a first surface and a second surface opposite to the first surface, wherein the volume percentage (A) of the carbon fibers, graphite particles, and binder is less than 35% in the first layer and 35% to 95% in at least one of the second layer and the third layer.

2. The gas diffusion layer substrate sheet for fuel cells according to claim 1, wherein the volume ratio (A) in the second layer is 35% to 90%.

3. The gas diffusion layer substrate sheet for fuel cells according to claim 1, wherein the volume ratio (A) in the third layer is 35% to 90%.

4. The fuel cell gas diffusion layer substrate sheet according to claim 1, wherein the binder contains resin carbides.

5. The fuel cell gas diffusion layer substrate sheet according to claim 1, wherein the carbon fiber content in the fuel cell gas diffusion layer substrate sheet is 30 to 90% by mass.

6. The fuel cell gas diffusion layer substrate sheet according to claim 1, wherein the content ratio of graphite particles contained in the fuel cell gas diffusion layer substrate sheet is 5 to 70% by mass.

7. A fuel cell gas diffusion layer substrate sheet according to claim 1, wherein the thickness is 50 to 400 μm.

8. The gas diffusion layer substrate sheet for a fuel cell according to claim 1, which is a gas diffusion layer substrate sheet for the anode electrode of a fuel cell.

9. A method for using a fuel cell gas diffusion layer substrate sheet according to claim 1 for use in manufacturing the anode gas diffusion layer in a fuel cell comprising an anode electrode separator, an anode electrode gas diffusion layer, an anode electrode catalyst layer, a solid polymer electrolyte membrane, a cathode electrode catalyst layer, a cathode electrode gas diffusion layer, and a cathode electrode separator, characterized in that the first surface of the fuel cell gas diffusion layer substrate sheet is arranged to face the anode electrode catalyst layer.

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