Electroconductive sheet, membrane electrode assembly, and fuel cell

Abstract

Provided are: an electroconductive sheet containing a fluororesin and a carbon-based electroconductive substance that contains carbon-based electroconductive fibers having an average fiber diameter of 0.1-0.9 µm, wherein the fluororesin content is 1-35 mass% with respect to the total solids of the electroconductive sheet, and the maximum peak in the pore size distribution is in the range of 0.1-5.0 µm; a membrane electrode assembly; and a fuel cell.

Description

Conductive sheets, membrane electrode assemblies, and fuel cells

[0001] This disclosure relates to conductive sheets, membrane electrode assemblies, and fuel cells.

[0002] Fuel cells are broadly classified into four types based on the type of electrolyte used: molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), phosphoric acid fuel cells (PAFCs), and polymer electrolyte fuel cells (PEFCs).

[0003] Among the above, solid polymer fuel cells include a membrane electrode assembly (MEA) in which a thin polymer electrolyte membrane is sandwiched between a gas diffusion layer (GDL) via a catalyst layer. The configuration comprising a catalyst layer and a gas diffusion layer is called a gas diffusion electrode (GDE).

[0004] The performance requirements for the gas diffusion layer have traditionally included gas diffusion performance to guide fuel gas or air into the catalyst layer and diffuse the fuel gas into the catalyst layer, water-repellent performance to discharge the generated water produced by the power generation reaction to the separator, high conductivity performance to extract the generated current to the outside without loss, and resistance to strongly acidic and strongly basic atmospheres caused by the generated ions.

[0005] Based on the required characteristics described above, conductive sheets such as carbon fiber sheets are often used for the gas diffusion layer.

[0006] Conventionally, conductive sheets used as gas diffusion layers comprise a carbon fiber layer, such as a carbon fiber sheet, and a microporous layer (so-called Micro Porous Layer: MPL, also called a microporous layer, fine porous layer, or water-repellent layer) on the surface of the carbon fiber layer, from the viewpoint of uniformly diffusing fuel gas into the catalyst layer and adjusting the wettability of the membrane electrode assembly.

[0007] Conventional gas diffusion layers are formed by applying a microporous layer-forming coating solution onto a carbon fiber layer, drying, and sintering to create a conductive sheet made of carbon fiber layers. Furthermore, for example, Patent Document 1 proposes a method for producing a gas diffusion layer by dispersing a fibril-like substance consisting of a conductive carbon material, a fluororesin, and an aromatic polyamide in a solvent, forming a paper mold, hot-pressing the resulting precursor, and then firing it.

[0008] Patent Document 1: International Publication No. 2012-026498

[0009] Conventional conductive sheets, when used as a gas diffusion layer in fuel cells and the like, can cause the polymer electrolyte membrane to become rough or rupture due to carbon-based conductive fibers. When the polymer electrolyte membrane becomes rough or ruptures, the battery performance tends to deteriorate. Therefore, the object of this disclosure is to provide a conductive sheet, a membrane electrode assembly, and a fuel cell that suppress the deterioration of the polymer electrolyte membrane (e.g., roughness or rupture of the membrane) and the deterioration of battery performance when used in a gas diffusion electrode.

[0010] The following embodiments are included as specific means for solving the problem: <1> A conductive sheet comprising a carbon-based conductive material containing carbon-based conductive fibers having an average fiber diameter of 0.1 μm or more and 0.9 μm or less, and a fluororesin, wherein the content of the fluororesin is more than 1% by mass and 35% by mass or less relative to the total solid content of the conductive sheet, and the maximum peak in the pore size distribution is 0.1 μm or more and 5.0 μm or less. <2> The conductive sheet according to <1>, wherein the carbon-based conductive fibers have an aspect ratio (average fiber length / average fiber diameter) of 20 or more and 100 or less. <3> The conductive sheet according to <1> or <2>, which is a single-layer conductive sheet for a gas diffusion layer in which the maximum peak of the pore size distribution in the thickness direction is uniform. <4> The conductive sheet according to any one of <1> to <3>, which is for a gas diffusion electrode. <5> A membrane electrode assembly comprising a polymer electrolyte membrane and a pair of gas diffusion electrodes that sandwich the polymer electrolyte membrane, wherein at least one of the pair of gas diffusion electrodes includes the conductive sheet described in any one of <1> to <4> above. <6> Thickness T of the polymer electrolyte membrane ＩＯＮ (μm) and the thickness T of the gas diffusion electrode ＧＤＥRatio (T) to (μm) ＩＯＮ / T ＧＤＥ ) is the membrane electrode assembly described in <5> above, wherein the ratio is 0.05 or more and 1.00 or less. <7> A fuel cell comprising the membrane electrode assembly described in <5> or <6> above, a separator, and

[0011] According to this disclosure, a conductive sheet, a membrane electrode assembly, and a fuel cell are provided that, when incorporated into a gas diffusion electrode, suppress degradation of the polymer electrolyte membrane (e.g., membrane roughness or tearing) and a decrease in battery performance.

[0012] Figure 1 is a schematic cross-sectional view showing an example of the layer configuration of a film electrode assembly according to the present disclosure. Figure 2 is a schematic cross-sectional view showing an example of the layer configuration in another embodiment of the film electrode assembly according to the present disclosure. Figure 3 is a pore distribution diagram of the conductive sheet of Example 1, with the vertical axis being [dV / d (log d) (cc / g)] and the horizontal axis being the pore diameter of the conductive sheet (μm). Figure 4 is a micrograph of the conductive sheet of Example 1 at a 100 μm scale. Figure 5 is a micrograph of the conductive sheet of Example 1 at a 50 μm scale. Figure 6 is a micrograph of the conductive sheet of Example 1 at a 10 μm scale.

[0013] The following describes an example embodiment of this disclosure. These descriptions and examples are illustrative and do not limit the scope of the invention. In numerical ranges described stepwise in this specification, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described stepwise. Also, in numerical ranges described in this specification, the upper or lower limit of that numerical range may be replaced with the value shown in the example.

[0014] Each component may contain multiple types of the corresponding substance. When referring to the amount of each component in a composition, if multiple types of the substance corresponding to each component are present in the composition, unless otherwise specified, it refers to the total amount of those multiple types of substances present in the composition.

[0015] When describing embodiments with reference to the drawings, components having substantially the same function will be given the same reference numeral throughout the drawings, and redundant explanations may be omitted.

[0016] <Measurement Methods for Various Physical Properties> The measurement methods used in this specification are as follows. The method for measuring the thickness of the conductive sheet is as follows: Using a thickness gauge (manufactured by Ono Sokki), the thickness of the conductive sheet is measured at 10 cm intervals at five arbitrary points, and the arithmetic mean of these measurements is taken as the thickness of the conductive sheet.

[0017] The method for measuring the water contact angle of a conductive sheet is as follows: Water is dropped onto the surface of the conductive sheet, and the contact angle is measured using a contact angle meter (KYOWA DMo-502) at 20°C to 25°C and 65% RH.

[0018] The method for measuring the pore size distribution of a conductive sheet is as follows: Using a fully automated pore size distribution analyzer (Anton Paar PoreMaster 60-GT), the pore size distribution from 0.01 μm to 100 μm is measured on the conductive sheet at 20°C. The observed maximum peak value is defined as the maximum peak in the pore size distribution when the vertical axis is [dV / d (log d) (cc / g)] and the horizontal axis is pore size (μm). Note that "dV / d (log d)" represents the cumulative pore volume distribution.

[0019] The method for measuring the breaking strength of a conductive sheet is as follows: A 13 cm sample is obtained from the conductive sheet to be measured. The breaking strength and elongation at break are measured using a Tensilon (Orientec RTC-1310A) with a chuck distance of 10 cm, a tensile speed of 50 mm / min, a temperature of 20°C to 25°C, and 65% RH conditions, and the tensile modulus is calculated.

[0020] The laminated structure of the conductive sheet can be confirmed by cutting the conductive sheet along the lamination direction and observing the cross-section with a scanning electron microscope (SU3500, manufactured by Hitachi, Ltd.).

[0021] The average fiber diameter, average fiber length, and average aspect ratio of carbon fibers are determined as follows: A cross-section in the lamination direction of a conductive sheet is photographed at a magnification of 1000x using a field emission scanning electron microscope. 500 locations are randomly selected from the observed surface, and the fiber diameter is measured at each location. The arithmetic mean of all these measurement results (n=500) is taken as the average fiber diameter. If the carbon fiber has a flattened cross-section, the arithmetic mean of the major and minor axes is taken as the average fiber diameter. The cross-section in the lamination direction of the conductive sheet, the cross-section, or the surface on the carbon fiber layer side of the conductive sheet is measured using an image analysis particle size distribution analyzer (Jusco International Co., Ltd., model IF-200nano), and the arithmetic mean based on the number of carbon fiber aggregates is taken as the average fiber length. The average aspect ratio is then calculated from the average fiber length and average fiber diameter.

[0022] The carbon fibers, carbon milled fibers, and carbon nanofibers described herein may consist of multiple fibers having different average fiber diameters. The carbon fibers, carbon milled fibers, and carbon nanofibers described herein may consist of multiple fibers having different average fiber lengths. The carbon fibers, carbon milled fibers, and carbon nanofibers described herein may consist of multiple fibers having different average fiber diameters and average fiber lengths.

[0023] <Conductive Sheet> The conductive sheet according to this disclosure comprises a carbon-based conductive material containing carbon-based conductive fibers having an average fiber diameter of 0.1 μm or more and 0.9 μm or less, and a fluororesin, wherein the fluororesin content is greater than 1% by mass and less than or equal to 35% by mass of the total solid content of the conductive sheet, and the maximum peak in the pore size distribution is between 0.1 μm and 5.0 μm.

[0024] The conductive sheet according to this disclosure contains a carbon-based conductive material having an average fiber diameter that is moderately thinner than conventional conductive materials. Therefore, even when this conductive sheet is provided in a gas diffusion electrode, it suppresses deterioration of the polymer electrolyte membrane (e.g., roughness or tearing of the membrane) due to protrusion of carbon-based conductive fibers. As a result, when the conductive sheet according to this disclosure is provided in a gas diffusion electrode, it provides superior battery life and performance. Furthermore, the conductive sheet according to this disclosure has a fluororesin content within the above range, and the maximum peak in the pore size distribution also satisfies the above range. As a result, the conductive sheet according to this disclosure has moderate water repellency and gas diffusivity. Therefore, when the conductive sheet according to this disclosure is provided in a gas diffusion electrode, it provides superior battery performance (for example, the cell voltage is more easily maintained at a high level, such as exceeding 0.40V). Due to these synergistic effects, when the conductive sheet according to this disclosure is provided in a gas diffusion electrode, it suppresses deterioration of the polymer electrolyte membrane (e.g., roughness or tearing of the membrane) and a decrease in battery performance.

[0025] [Characteristics of the conductive sheet] The conductive sheet has a maximum peak in its pore size distribution of 0.1 μm to 5.0 μm, preferably 0.2 μm to 3.0 μm, and more preferably 0.3 μm to 2.0 μm. When the maximum peak is 0.2 μm or larger, excessive reduction in pore size is suppressed, resulting in excellent permeability, i.e., gas diffusion. Furthermore, when incorporated into a membrane electrode assembly, excessive reduction in wettability is suppressed. As a result, the battery performance is excellent. When the maximum peak is 5.0 μm or smaller, excessive increase in pore size is suppressed, resulting in excellent water repellency. Furthermore, when incorporated into a membrane electrode assembly, excessive increase in wettability is suppressed. As a result, the battery performance is excellent.

[0026] There are no particular limitations on the specific methods for making the maximum peak in the pore size distribution fall within the above range, but examples include using a conductive sheet containing carbon-based conductive fibers with an average fiber diameter of 0.1 μm or more and 0.9 μm or less.

[0027] The water contact angle of the conductive sheet is preferably 130° or more and 160° or less, more preferably 140° or more and 150° or less. When the water contact angle is within the above range, the balance between water repellency and gas diffusibility is excellent. Further, when provided in the membrane electrode assembly, an excessive increase in wettability is suppressed. As a result, the battery performance is excellent.

[0028] Specific methods for setting the water contact angle within the above range are not particularly limited. For example, a method of adjusting the ratio of the fluororesin in the conductive sheet can be mentioned.

[0029] The breaking strength of the conductive sheet is not particularly limited. For example, it is preferably 0.5 MPa or more and less than 3 MPa, more preferably 0.8 MPa or more and 2.8 MPa or less, and even more preferably 1.0 MPa or more and 2.6 MPa or less. When the tensile elastic modulus of the conductive sheet according to the present disclosure is within the above range, the strength of the conductive sheet is more excellent, so that breakage and roughness of the conductive sheet itself are suppressed, and the battery performance is more excellent.

[0030] Specific methods for setting the breaking strength within the above range are not particularly limited. For example, a method of using a conductive sheet containing carbon-based conductive fibers having an average fiber diameter of 0.1 μm or more and 0.9 μm or less can be mentioned.

[0031] 〔Carbon-based conductive substance〕The carbon-based conductive substance may be dispersed, for example, between the fibers of aromatic polyamide pulp. By including the carbon-based conductive substance, conductivity is imparted to the conductive sheet.

[0032] The carbon-based conductive substance includes carbon-based conductive fibers having an average fiber diameter of 0.1 μm or more and 0.9 μm or less.

[0033] - Carbon-based conductive fibers The average fiber diameter of the carbon-based conductive fibers is 0.1 μm or more and 0.9 μm or less, preferably 0.1 μm or more and 0.7 μm or less, more preferably 0.1 μm or more and 0.6 μm or less, and even more preferably 0.1 μm or more and 0.5 μm or less. When the average fiber diameter of the carbon-based conductive fibers is within the above range, even when this conductive sheet is provided in the gas diffusion electrode, deterioration (such as roughness or breakage) of the polymer electrolyte membrane due to protrusion of the carbon-based conductive fibers is further reduced.

[0034] The aspect ratio (average fiber length / average fiber diameter) of the carbon-based conductive fibers is preferably 20 or more and 100 or less, more preferably 30 or more and 80 or less, and even more preferably 40 or more and 60 or less. When the aspect ratio is within the above range, even when this conductive sheet is provided in the gas diffusion electrode, roughness or breakage of the polymer electrolyte membrane due to protrusion of the carbon-based conductive fibers is further reduced.

[0035] Specific methods for making the average fiber diameter, aspect ratio, etc. of the carbon-based conductive fibers fall within the above ranges are not particularly limited. For example, a method of using carbon nanofibers, which will be described later, as the type of carbon-based conductive fibers can be mentioned.

[0036] The carbon-based conductive fibers are not particularly limited as long as their average fiber diameter is within the above range. For example, carbon fibers, carbon milled fibers, carbon nanofibers, etc. in the aspects described later can be mentioned. Among the above, as the carbon-based conductive fibers, from the viewpoint of adjusting the maximum peak in the pore size distribution of the conductive sheet to a suitable range, it is preferable to include carbon nanofibers, which will be described later. The carbon-based conductive fibers may be used alone or in combination of two or more.

[0037] - Other carbon-based conductive substances The carbon-based conductive substances may further contain other carbon-based conductive substances other than the carbon-based conductive fibers. The other carbon-based conductive substances are not particularly limited, and known carbon-based conductive substances used in the gas diffusion layer may be adopted. The other carbon-based conductive substances may be used alone or in combination of two or more.

[0038] Other carbon-based conductive materials include, for example, materials with a carbon content of 94% by mass or more and a resistivity of 100 Ω·cm or less, from the viewpoint of conductive performance. Specific examples of carbon-based conductive materials include carbon fibers, carbon black, graphite particles, carbon nanotubes, carbon milled fibers, carbon nanofibers, carbon nanohorns, and graphene.

[0039] Examples of graphite particles include flaky graphite, scale-like graphite, clay-like graphite, artificial graphite, expanded graphite, expanded graphite, leaf-like graphite, lump graphite, and spheroidal graphite. Spheroidal and flaky graphite are particularly preferred. The average particle size of the graphite particles is preferably 0.05 μm or more and 300.0 μm or less.

[0040] Examples of carbon black include acetylene black and Ketjenblack (registered trademark), which has a hollow shell structure. Ketjenblack is particularly preferred.

[0041] The average primary particle diameter of the carbon black is preferably, for example, 1.0 nm to 500.0 nm, more preferably 1.0 nm to 200.0 nm, and even more preferably 10.0 nm to 100.0 nm. The average secondary particle diameter of the carbon black is preferably, for example, 0.5 nm to 20.0 μm. If the average secondary particle diameter is 0.5 μm or more, further aggregation of the carbon black is suppressed when preparing the carbon black dispersion. If the average secondary particle diameter is 20.0 μm or less, the conductivity of the sheet is improved.

[0042] Examples of carbon fibers and carbon milled fibers include PAN-based carbon fibers, pitch-based carbon fibers, and phenol-based carbon fibers, and among these, it is preferable to include pitch-based carbon fibers.

[0043] In this specification, carbon milled fiber refers to fibrous carbon fiber that has been ground into a powder (milled) form using a pulverizer or the like.

[0044] When using at least one of carbon fibers and / or carbon milled fibers, the average fiber diameter is not particularly limited, but is preferably 3 μm or more and 20 μm or less, and more preferably 5 μm or more and 13 μm or less.

[0045] When the average fiber diameter is 3 μm or more, the strength of the individual fibers is high, making it easier to improve the strength of the conductive sheet. When the average fiber diameter is 20 μm or less, when used as a gas diffusion layer, the localized lifting of the conductive sheet, which is the carbon fiber layer, from the microporous layer is suppressed. As a result, the formation of surface irregularities caused by the lifting of the carbon fiber layer is suppressed, resulting in good surface smoothness and lower contact electrical resistance when used as a gas diffusion layer. In other words, it is superior in battery performance.

[0046] The average fiber length (so-called cut length) of carbon fibers or carbon milled fibers is not particularly limited, but it is preferably 20 mm or less. When the average fiber length is 20 mm or less, the uniform dispersion of the fibers improves, and the strength of the conductive sheet tends to improve.

[0047] The carbon content in carbon fibers and carbon milled fibers is preferably, for example, 94% by mass or more. A carbon content of 94% by mass or more improves the conductivity of the conductive sheet. Furthermore, even when a battery incorporating this conductive sheet is operated for a long period of time, the deterioration of the conductive sheet is suppressed.

[0048] Carbon nanofibers may be single fibers or aggregates. The average fiber diameter of the carbon nanofibers (single fibers or aggregates) is preferably, for example, 100 nm or more and 1000 nm or less.

[0049] If the average fiber diameter of the carbon nanofibers is 100 nm or more, the handling properties are good, and if it is 1000 nm or less, it is easy to increase the fiber density. The average fiber diameter of the carbon nanofibers is preferably 900 nm or less (i.e., 0.9 μm or less), more preferably 800 nm or less (i.e., 0.8 μm or less), even more preferably 600 nm or less (i.e., 0.6 μm or less), even more preferably 500 nm or less (i.e., 0.5 μm or less), even more preferably 400 nm or less (i.e., 0.4 μm or less), and even more preferably 300 nm or less. The average fiber diameter of the carbon nanofibers is preferably 110 nm or more (i.e., 0.11 μm or more), more preferably 120 nm or more (i.e., 0.12 μm or more), even more preferably 150 nm or more (i.e., 0.15 μm or more), even more preferably 200 nm or more (i.e., 0.20 μm or more), and particularly preferably greater than 200 nm (i.e., greater than 0.20 μm).

[0050] Furthermore, the average fiber length of the carbon nanofibers (single fibers or aggregates) is preferably 1 μm or more, and more preferably 10 μm or more. If the average fiber length is 1 μm or more, it is possible to suppress a decrease in conductivity, strength, and liquid retention. Also, if the average fiber length is 30 μm or less, the dispersibility of the carbon fibers is less likely to be impaired, and the carbon fibers are less likely to be oriented in the in-plane direction of the conductive sheet. As a result, it is easier to form conductive paths in the thickness direction of the conductive sheet. The average fiber length of the carbon nanofibers is preferably 10 μm or more and 30 μm or less, and more preferably 12 to 28 μm.

[0051] Carbon nanofibers can be produced, for example, by the method disclosed in International Publication No. 2020 / 045243. Specifically, a method for producing a carbon fiber aggregate includes: (1) a fibrous step of obtaining resin composite fibers by molding a resin composition consisting of a thermoplastic resin and 30 to 150 parts by mass of mesophase pitch per 100 parts by mass of the thermoplastic resin in a molten state to fibrose the mesophase pitch; (2) a stabilization step of stabilizing the resin composite fibers to obtain resin composite stabilized fibers; (3) a thermoplastic resin removal step of removing the thermoplastic resin from the resin composite stabilized fibers to obtain stabilized fibers; and (4) a carbonization and calcination step of heating the stabilized fibers in an inert atmosphere to carbonize or graphitize them to obtain a carbon fiber aggregate.

[0052] [Fluororesin] The conductive sheet according to this disclosure has a fluororesin content that is greater than 1% by mass and less than or equal to 35% by mass of the total mass of the conductive sheet.

[0053] From the viewpoint of appropriately adjusting the balance between gas diffusion and water repellency to achieve superior battery performance, the conductive sheet according to this disclosure preferably contains more than 1% by mass and 35% by mass or less, more preferably 5% by mass or more and 30% by mass or less, and more preferably 5% by mass or more and 25% by mass or less, relative to the total mass of the conductive sheet.

[0054] In this specification, "fluororesin" refers to a general term for synthetic resins containing fluorine.

[0055] When fluororesin is included in a conductive sheet, the fluororesin may be attached to the surface of, for example, aromatic polyamide pulp or carbon-based conductive material, or it may be fused to it.

[0056] The fluororesin is not particularly limited, and any known fluororesin used in gas diffusion layers, etc., may be used. The fluororesin may be used alone or in combination of two or more types.

[0057] Examples of fluororesins include tetrafluoroethylene resin (hereinafter sometimes referred to as "PTFE"), perfluoroalkoxy resin, tetrafluoroethylene-hexafluoropropylene copolymer resin, tetrafluoroethylene-ethylene copolymer resin, vinylidene fluoride resin, trifluoroethylene chloride, etc. Among the above, PTFE may be included as the fluororesin from the viewpoint of providing excellent heat resistance and sliding properties.

[0058] [Other Materials] The conductive sheet relating to this disclosure may further contain other materials other than carbon-based conductive materials and fluororesins. Examples of other materials include aromatic polyamide pulp and electrode catalysts.

[0059] Among the above, the conductive sheet preferably further contains aromatic polyamide pulp from the viewpoint of maintaining the strength of the conductive sheet.

[0060] Aromatic polyamide pulp (hereinafter also referred to as "aramid pulp") is a fiber of aromatic polyamide having amide bonds formed by the dehydration condensation of aromatic diamine and aromatic dicarboxylic acid components, for example, 85 mol% or more of the amide bonds. It is preferable that the fibers of the aramid pulp are highly fibrillated. Examples of aramids include poly(p-phenylene terephthalamide), copoly(p-phenylene-3,4'-oxydiphenylene-terephthalamide), poly(metaphenylene isophthalamide), poly(p-benzamide), poly(-4,4'-diaminobenzanilide), poly(p-phenylene-2,6-naphthalicamide), copoly(p-phenylene / 4,4'-(3,3'-dimethylbiphenylene)terephthalamide, poly(-orthophenylene terephthalamide), poly(p-phenylene phthalamide), and poly(metaphenylene isophthalamide).

[0061] Fibrillation refers to a method of randomly forming minute single fibers on the surface of a fiber. In this disclosure, fibrillation of aramid pulp is carried out by known methods. For example, it is carried out by adding a precipitating agent to an organic polymer solution as described in Japanese Patent Publication No. 35-11851 and Japanese Patent Publication No. 37-5752, and mixing them in a system that generates shear force. Alternatively, it can be carried out by applying mechanical shear force, such as beating, to a molded article having molecular orientation formed from an optically anisotropic polymer solution as described in Japanese Patent Publication No. 59-603, thereby randomly imparting minute single fibers.

[0062] The average fiber length of aramid pulp is not particularly limited, but from the viewpoint of high strength and high modulus of elasticity, it is preferably 0.1 mm to 100.0 mm, more preferably 0.5 mm to 10.0 mm, and even more preferably 0.5 mm to 5.0 mm.

[0063] The conductive sheet may further contain an electrode catalyst. When the conductive sheet is used as a gas diffusion electrode, it can be a single-layer gas diffusion electrode or a thin-film gas diffusion electrode. In this case, the conductive sheet is preferably a sheet containing aromatic polyamide pulp, a carbon-based conductive material, and an electrode catalyst.

[0064] Electrode catalysts: As electrode catalysts, catalysts used in known fuel cells such as polymer electrolyte fuel cells (PEFCs), phosphoric acid fuel cells (PAFCs), and biobatteries can be used. The type and content of the electrode catalyst may be appropriately selected depending on the application of the conductive sheet. Examples of electrode catalysts include carbon catalysts, platinum catalysts, iridium catalysts, and enzymes. Among the above, it is preferable to include a platinum catalyst as the electrode catalyst from the viewpoint of battery performance.

[0065] [Content of various materials] The content of carbon-based conductive fibers is preferably 75% by mass or more and 100% by mass or less, and more preferably 80% by mass or more and 100% by mass or less, relative to the total amount of carbon-based conductive material. When the content of carbon-based conductive fibers is 75% by mass or more and 100% by mass or less, it is easy to adjust the maximum peak in the pore size distribution of the conductive sheet to the above-mentioned preferred range.

[0066] The content of the carbon-based conductive material is preferably 50% to 85% by mass, and more preferably 55% to 80% by mass, relative to the total solid content of the conductive sheet. When the content of the carbon-based conductive material is 50% to 85% by mass, the decrease in water repellency is suppressed, and conductivity and battery performance are improved.

[0067] When aromatic polyamide pulp is included, the content of aromatic polyamide pulp is preferably 1% by mass or more and 20% by mass or less, and more preferably 3% by mass or more and 15% by mass or less, relative to the total solid content of the conductive sheet. When the content of aromatic polyamide pulp is 1% by mass or more, the durability and water repellency of the gas diffusion layer are better maintained. When the content of aromatic polyamide pulp is 20% by mass or less, the relative decrease in the proportion of carbon-based conductive material is suppressed, making it easier to diffuse fuel gas into the catalyst layer with high uniformity and to appropriately adjust the wettability of the membrane electrode assembly. As a result, the battery performance is better.

[0068] When aromatic polyamide pulp is included, the mass ratio of aromatic polyamide pulp to carbon-based conductive material (aromatic polyamide pulp / carbon-based conductive material) is not particularly limited, but is preferably in the range of 5 / 95 to 50 / 50, and more preferably in the range of 10 / 90 to 40 / 60. When the above mass ratio is 5 / 95 or higher, the reinforcing effect of aromatic polyamide pulp is more superior. When the above mass ratio is 50 / 50 or lower, the relative decrease in the proportion of carbon-based conductive material is suppressed, resulting in better battery performance. In particular, when the carbon-based conductive material includes at least one of carbon fibers and carbon nanofibers, when the above mass ratio is above the lower limit, the maximum peak in the pore size distribution of the conductive sheet is more easily adjusted to a suitable range, resulting in better battery performance.

[0069] When aromatic polyamide pulp and fluororesin are included, the mass ratio of aromatic polyamide pulp to fluororesin (aromatic polyamide pulp / fluororesin) is not particularly limited as long as the proportion of fluororesin to the total solid content of the conductive sheet is 1% by mass or less. The mass ratio of aromatic polyamide pulp to fluororesin (aromatic polyamide pulp / fluororesin) is preferably in the range of 10 / 90 to 70 / 30, and more preferably in the range of 20 / 80 to 60 / 40. When the above mass ratio is 10 / 90 or higher, the reinforcing effect is better due to the aromatic polyamide pulp. On the other hand, when the above mass ratio is 70 / 30 or lower, the water repellency is better due to the fluororesin.

[0070] [Uniformity in the Thickness Direction as a Gas Diffusion Layer] When a conductive sheet is used as a gas diffusion layer (GDL), uniformity of pore size in the thickness direction of the gas diffusion layer is an important factor in reducing the effect of water accumulation generated in fuel cells, such as flooding, which hinders air diffusion. A typical conventional gas diffusion layer (GDL) has a two-layer structure consisting of a microporous layer (MPL) and a conductive sheet made of a carbon fiber layer. The MPL plays a role in controlling gas permeability, and the conductive sheet plays a role in ensuring the strength of the substrate and conductivity in the thickness direction. Conventional conductive sheets tend to have a variable maximum peak in the pore size distribution in the thickness direction of the conductive sheet, resulting in a lack of uniformity. More specifically, it has been found that the pore size of the conductive sheet is very large, ranging from about 20 μm to 100 μm. When a gas diffusion layer with a conductive sheet of this size pore diameter is incorporated into a membrane electrode assembly, water generated in the fuel cell tends to condense to minimize surface energy, forming water puddles that inhibit air diffusion and cause a voltage drop known as flooding. Furthermore, conventional microporous layers (MPLs) have dense pores of less than 0.1 μm, made of carbon black or similar materials. As a result, water is primarily absorbed into these pores by capillary action, while gas permeability is extremely low, which can easily affect the power generation performance of the fuel cell. For this reason, conventional microporous layers (MPLs) are sometimes processed to increase gas permeability by applying a concentration gradient to reduce fluctuations in the maximum peak of the pore diameter distribution in the thickness direction. However, in this case, the effect of flooding caused by the large pore diameter in the conductive sheet remains unchanged.

[0071] In contrast, the conductive sheet according to this disclosure has a maximum peak in the pore size distribution of 0.1 μm to 5.0 μm, which allows for good gas permeability and suppression of water accumulation due to capillary action.

[0072] Furthermore, the conductive sheet according to this disclosure has a uniform maximum peak in the pore size distribution in the thickness direction of the conductive sheet. Therefore, it has superior gas permeability and diffusion, and when used in a battery, it has superior battery performance. From a similar viewpoint, the conductive sheet according to this disclosure is preferably a single-layer conductive sheet for a gas diffusion layer having a uniform maximum peak in the pore size distribution in the thickness direction.

[0073] In this specification, "thickness direction of the conductive sheet" refers to the direction perpendicular to the microporous layer when it is a gas diffusion layer. In this specification, "the maximum peak of the pore size distribution in the thickness direction of the conductive sheet is uniform" means that when the maximum peak of the pore size distribution of the conductive sheet is measured at regular intervals in the thickness direction from the edge of the sheet (for example, at the 5%, 10%, 50%, and 90% points in a straight line from the edge of the sheet in the thickness direction), the difference between the minimum and maximum values ​​at each obtained maximum peak is within 1.0 μm (preferably within 0.5 μm, more preferably within 0.3 μm, and even more preferably within 0.2 μm). The pore size distribution of the conductive sheet and the uniformity of its distribution can be confirmed by observation using a scanning electron microscope (SEM).

[0074] [Applications] The applications of the conductive sheet according to this disclosure are not particularly limited, but for example, it can be used as a gas diffusion layer, a gas diffusion electrode, etc. Among the above, the conductive sheet according to this disclosure is preferably used as a gas diffusion electrode from the viewpoint of suppressing degradation of the polymer electrolyte membrane and a decrease in battery performance when it is provided. If the conductive sheet according to this disclosure contains an electrode catalyst, the conductive sheet according to this disclosure may be used as a gas diffusion electrode with an integrated gas diffusion layer.

[0075] [Manufacturing Method] The method for manufacturing the conductive sheet according to this disclosure is not particularly limited, and known methods for manufacturing conductive sheets can be used. The method for manufacturing the conductive sheet according to this disclosure may be, for example, a method in which a conductive sheet forming composition containing a carbon-based conductive material is processed into a sheet by weaving or papermaking, the sheet is impregnated with a fluororesin-containing solution, and then sintered.

[0076] <Gas Diffusion Layer> The gas diffusion layer according to this disclosure has a conductive sheet according to this disclosure. According to the gas diffusion layer according to this disclosure, when a membrane electrode assembly including it is formed, degradation of the polymer electrolyte membrane and reduction in battery performance are suppressed.

[0077] The gas diffusion layer according to this disclosure is not particularly limited as long as it has a conductive sheet according to this disclosure, but it is preferable that it be a single layer made of the conductive sheet according to this disclosure when used in a membrane electrode assembly, from the viewpoint of further suppressing roughness and tearing of the polymer electrolyte membrane and deterioration of battery performance, respectively.

[0078] <Gas Diffusion Electrode> The gas diffusion electrode according to this disclosure comprises a conductive sheet according to this disclosure and a catalyst layer provided on the conductive sheet. According to the gas diffusion electrode according to this disclosure, degradation of the polymer electrolyte membrane and a decrease in battery performance are suppressed.

[0079] The gas diffusion electrode according to this disclosure may or may not have a microporous layer between the conductive sheet and the catalyst layer.

[0080] The microporous layer is not particularly limited, and known microporous layers used in gas diffusion electrodes can be employed. Examples of microporous layers include fluororesins and carbon-based conductive materials.

[0081] The catalyst layer is not particularly limited, and known catalyst layers used in gas diffusion electrodes can be employed. Examples of catalyst layers include electrode catalysts and carbon-based conductive materials.

[0082] <Membrane electrode assembly> The membrane electrode assembly according to this disclosure comprises a polymer electrolyte membrane and a pair of gas diffusion electrodes that sandwich the polymer electrolyte membrane, wherein at least one of the pair of gas diffusion electrodes includes a conductive sheet according to this disclosure.

[0083] The membrane electrode assembly described herein suppresses degradation of the polymer electrolyte membrane and a decrease in battery performance.

[0084] Figure 1 is a schematic cross-sectional view showing an example of the layer configuration of a membrane electrode assembly according to the present disclosure. The membrane electrode assembly 100 shown in Figure 1 comprises a polymer electrolyte membrane 10 and a pair of gas diffusion electrodes 20 that sandwich the polymer electrolyte membrane 10. At least one of the pair of gas diffusion electrodes 20 includes a conductive sheet according to the present disclosure. As shown in Figure 1, the gas diffusion electrode 20 is a laminate in which a catalyst layer 30 and a gas diffusion layer 40 are stacked in this order from the polymer electrolyte membrane 10 side. The catalyst layer 30 includes an electrode catalyst C. As shown in Figure 1, at least one of the pair of gas diffusion layers 40 is a conductive sheet according to the present disclosure, and it is preferable that both gas diffusion layers 40 are conductive sheets according to the present disclosure.

[0085] One of the pair of gas diffusion electrodes 20 is the cathode electrode, and the other is the anode electrode. Hydrogen is supplied to the anode electrode, and an oxidation reaction of hydrogen occurs in the catalyst layer. Oxygen-containing air is supplied to the cathode electrode, and an oxygen reduction reaction occurs. Protons generated at the anode electrode are then conducted to the cathode electrode via the polymer electrolyte membrane 10, and in this process electrons are extracted and electricity is generated. Water is generated at the cathode electrode, and this water wets the polymer electrolyte membrane 10, increasing its proton conductivity.

[0086] In conventional membrane electrode assemblies, a gas diffusion layer is provided, consisting of a microporous layer and a carbon fiber layer, arranged in that order from the polymer electrolyte membrane 10 side. In contrast, in the membrane electrode assemblies 100 shown in Figure 1, the gas diffusion layer 40 does not have a microporous layer, and the carbon fiber layer (i.e., the conductive sheet according to this disclosure) corresponds to the gas diffusion layer 40.

[0087] Although not shown in the figures, if at least one of the gas diffusion layers 40 (for example, the cathode side) includes the conductive sheet according to this disclosure, the membrane electrode assembly 100 may have a gas diffusion layer in which a microporous layer and a carbon fiber layer are provided in that order from the polymer electrolyte membrane 10 side.

[0088] The gas diffusion layer 40 may be a single layer, or the gas diffusion layer 40 may be formed as a laminate of two or more layers. When the carbon fiber layer forms the gas diffusion layer 40 as a laminate of two or more layers, at least one of the two or more laminates may be the conductive sheet according to the present disclosure.

[0089] FIG. 2 is a schematic cross-sectional view showing an example of the layer configuration in another embodiment of the membrane electrode assembly according to the present disclosure. The membrane electrode assembly 100 shown in FIG. 2 includes a polymer electrolyte membrane 10 and a pair of gas diffusion electrodes 20 sandwiching the polymer electrolyte membrane 10. At least one of the pair of gas diffusion electrodes 20 is the conductive sheet according to the present disclosure. In this case, the conductive sheet according to the present disclosure includes at least the electrode catalyst C in addition to the carbon-based conductive material.

[0090] Although not shown, if at least one of the gas diffusion electrodes 20 (for example, the cathode side) includes the conductive sheet according to the present disclosure, the membrane electrode assembly 100 may have the other gas diffusion electrode 20 (for example, the anode side) provided with a catalyst layer, a microporous layer, and a carbon fiber layer in this order from the polymer electrolyte membrane 10 side.

[0091] As shown in FIG. 2, the gas diffusion electrode 20 is a single-layer body in which the catalyst layer and the gas diffusion layer are integrated. The gas diffusion electrode 20 includes the electrode catalyst C. One of the pair of gas diffusion electrodes 20 is a cathode electrode, and the other is an anode electrode. Hydrogen is supplied to the anode electrode, and an oxidation reaction of hydrogen occurs. Air containing oxygen is supplied to the cathode electrode, and a reduction reaction of oxygen occurs.

[0092] The use of the membrane electrode assembly according to the present disclosure is not particularly limited, and for example, it can be used in a fuel cell or a water electrolysis device.

[0093] The thickness T of the polymer electrolyte membrane ＩＯＮ (μm) and the thickness T of the gas diffusion electrode ＧＤＥ (μm) ratio (T ＩＯＮ / T ＧＤＥ ) is preferably 1.00 or less, and more preferably 0.05 or more and 1.00 or less. In the conventional membrane electrode assembly, from the viewpoint of reducing roughness and breakage of the polymer electrolyte membrane, the thickness T of the polymer electrolyte membrane ＩＯＮ(μm) and the thickness T of the gas diffusion electrode ＧＤＥ Ratio (T) to (μm) ＩＯＮ / T ＧＤＥ ) is often greater than 1.00. In contrast, the film electrode assembly according to this disclosure has the above configuration, and the ratio (T ＩＯＮ / T ＧＤＥ Even if the value exceeds 1.00, it can reduce roughness and rupture of the polymer electrolyte membrane.

[0094] Thickness T of polymer electrolyte membrane ＩＯＮ The (μm) is not particularly limited, but may be, for example, 5.00 μm or more and 50.00 μm or less, or 15.00 μm or more and 30.00 μm or less.

[0095] Thickness T of polymer electrolyte membrane ＩＯＮ The (μm) is not particularly limited, but from the viewpoint of manufacturing cost and battery performance, for example, it is preferably 5.00 μm or more and 30.00 μm or less, and more preferably 15.00 μm or more and 20.00 μm or less.

[0096] Thickness T of the gas diffusion electrode ＧＤＥ The (μm) is not particularly limited, but from the viewpoint of manufacturing cost and battery performance, for example, it is preferably 40.00 μm or more and 150.00 μm or less, and more preferably 50.00 μm or more and 80.00 μm or less.

[0097] <Fuel Cell> The fuel cell according to this disclosure comprises a membrane electrode assembly and a separator. The fuel cell according to this disclosure suppresses the degradation of the polymer electrolyte membrane and the decrease in battery performance.

[0098] The separator (i.e., the polymer electrolyte membrane) is not particularly limited, and any known separator applicable to water electrolysis devices or fuel cells can be used.

[0099] The conductive sheet and its manufacturing method described below will be explained in detail with reference to examples. However, the scope of this disclosure is not limited to the following examples.

[0100] <Example 1> A sheet was prepared by wet papermaking using an aqueous dispersion (solid content concentration 1 wt%) consisting of 6 parts by mass of aromatic polyamide pulp (Teijin Limited, Twaron® 1094 pulp), 20 parts by mass of PTFE (a fluororesin manufactured by AGC Inc., AD911E), 9 parts by mass of Ketjenblack (a carbon-based conductive material manufactured by Lion, EC600JD), and 65 parts by mass of pitch-based ultrafine carbon fibers (Teijin Limited, average fiber diameter 0.3 μm, average fiber length 15 μm), which are carbon-based conductive fibers. The obtained sheet was processed using a laminate calender at a roll temperature of 200°C and a roll pressure of 40 kgf / cm². ２ After forming the material at a roll speed of 2 m / min, the conductive sheet was obtained by baking it in a hot air oven at 400°C for 1 hour.

[0101] In Example 1, the content and ratio of each material in the conductive sheet are as follows: • Total amount of carbon-based conductive material: 74% by mass • Content of carbon-based conductive fibers: 65% by mass • Content of aromatic polyamide pulp: 6% by mass • Content of fluororesin: Shown in Table 1. • Mass ratio (aromatic polyamide pulp / carbon-based conductive material): 6 / 74 (=8 / 93) • Mass ratio (aromatic polyamide pulp / fluororesin): 6 / 20 (=23 / 77) • Thickness T of the gas diffusion electrode ＧＤＥ : 74.00 μm

[0102] Figure 3 shows the pore distribution diagram for the conductive sheet of Example 1, with the vertical axis being [dV / d (log d) (cc / g)] and the horizontal axis being the pore diameter of the conductive sheet (μm). As shown in Figure 3, the gas diffusion layer of Example 1 has a maximum peak in the pore diameter distribution within the range of 0.2 μm to 5.0 μm.

[0103] Using the measurement method described above, the thickness-direction cross-section of the conductive sheet of Example 1 was observed using a scanning electron microscope (SEM). Figure 4 is a micrograph of the conductive sheet of Example 1 at a 100 μm scale. Figure 5 is a micrograph of the conductive sheet of Example 1 at a 50 μm scale. Figure 6 is a micrograph of the conductive sheet of Example 1 at a 10 μm scale. As shown in Figures 4 to 6, the conductive sheet of Example 1 shows that the variation range of the maximum peak in the pore size distribution of carbon fibers in the thickness direction is small, within 0.1 μm. In other words, the conductive sheet of Example 1 is a single-layer conductive sheet with a uniform maximum peak in the pore size distribution in the thickness direction.

[0104] <Example 2> A conductive sheet was obtained in the same manner as in Example 1, except that pitch-based ultrafine carbon fibers (manufactured by Teijin, average fiber diameter 0.3 μm, average fiber length 12 μm), which are carbon-based conductive fibers, were used. The content and ratio of each material in the conductive sheet in Example 2 were the same as in Example 1.

[0105] <Example 3> A conductive sheet was obtained using the same specifications as in Example 1, except that the content of PTFE, a fluororesin, was set to 10 parts by mass.

[0106] In Example 3, the content and ratio of each material in the conductive sheet are as follows: • Total amount of carbon-based conductive material: 82% by mass • Content of carbon-based conductive fibers: 72% by mass • Content of aromatic polyamide pulp: 7% by mass • Content of fluororesin: Shown in Table 1. • Mass ratio (aromatic polyamide pulp / carbon-based conductive material): 7 / 82 (=8 / 92) • Mass ratio (aromatic polyamide pulp / fluororesin): 7 / 10 (=41 / 59) • Thickness T of the gas diffusion electrode ＧＤＥ : 73.00 μm

[0107] <Comparative Example 1> A conductive sheet was obtained in the same manner as in Example 1, except that polyacrylonitrile carbon fiber (manufactured by Teijin, average fiber diameter 7.0 μm, fiber length 12 mm) was used.

[0108] <Comparative Example 2> When 74 parts by mass of Ketjenblack, a carbon-based conductive material, 6 parts by mass of aromatic polyamide pulp (Teijin Corporation, pulp of Twaron® 1094), and 20 parts by mass of PTFE, a fluororesin (AGC Corporation, AD911E), were used, it was not possible to produce a sheet by wet papermaking due to insufficient strength.

[0109] <Examples 4-5 and Comparative Example 3> Conductive sheets for each example were obtained in the same manner as in Example 1, except that the PTFE content was set to the amount shown in Table 1.

[0110] <Evaluation of Fiber Protrusion> For each example, a Catalyst Coated Membrane (CCM), in which a polymer electrolyte membrane and a catalyst layer are integrated, was fabricated by sandwiching the CCM between both sides in the thickness direction and pressing it to create a membrane electrode assembly (MEA). This membrane electrode assembly was incorporated into a single cell for a fuel cell, and power was generated with a battery temperature of 80°C, a fuel utilization efficiency of 70%, an air utilization efficiency of 40%, and humidification of the hydrogen on the anode side and the air on the cathode side to dew points of 80°C and 75°C, respectively. Then, the occurrence of deterioration and rupture of the polymer electrolyte membrane was evaluated based on the presence or absence of short circuits during power generation. The results are shown in Table 1. In Table 1, examples in which a short circuit occurred during power generation are indicated as [NG], and examples in which a short circuit did not occur are indicated as [OK].

[0111] As shown in Table 1, it was found that the conductive sheets of the examples, when applied to a membrane electrode assembly, suppressed deterioration such as roughening and tearing of the polymer electrolyte membrane and short circuits compared to the conductive sheets of Comparative Examples 1 and 2.

[0112] <Evaluation of Battery Performance> For each example, a Catalyst Coated Membrane (CCM), in which a polymer electrolyte membrane and a catalyst layer are integrated, was fabricated by sandwiching the CCM between two conductive sheets in the thickness direction and pressing them together to create a membrane electrode assembly (MEA). This membrane electrode assembly was incorporated into a single cell for a fuel cell, and power was generated with a battery temperature of 80°C, a fuel utilization efficiency of 70%, an air utilization efficiency of 40%, and humidification of the hydrogen on the anode side and the air on the cathode side to dew points of 80°C and 75°C, respectively. Current density: 0-6 A / cm² ２The cell voltage was measured, and current density-voltage curves were obtained for each cell. The current density obtained from the current density-voltage curves for each example cell was 3.5 A / cm². ２ The cell voltage at this time is shown in Table 1 under the item [Cell voltage at current density of 3.5 A]. In Table 1, [-] means that the measurement was not taken. Note that the thickness T of the polymer electrolyte membrane. ＩＯＮ The thickness was set to 20.00 μm.

[0113] As shown in Table 1, the conductive sheet of the example was found to have superior battery performance when incorporated into a film electrode assembly compared to the conductive sheet of the comparative example. In Table 1, "pore size" represents the "maximum peak in the pore size distribution." In Table 1, "ratio (T)" represents the "maximum peak in the pore size distribution." ＩＯＮ / T ＧＤＥ ) is "the thickness T of the polymer electrolyte membrane ＩＯＮ (μm) and the thickness T of the gas diffusion electrode ＧＤＥ Ratio (T) to (μm) ＩＯＮ / T ＧＤＥ It represents )

[0114]

[0115] The disclosure of Japanese Patent Application No. 2024-229279, filed on 25 December 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards described herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard were specifically and individually noted to be incorporated by reference.

[0116] (Explanation of symbols) 100 Membrane electrode assembly 10 Polymer electrolyte membrane 20 Gas diffusion electrode 30 Catalyst layer 40 Gas diffusion layer C Electrode catalyst 42 Microporous layer 44 Carbon fiber layer

Claims

1. A conductive sheet comprising a carbon-based conductive material containing carbon-based conductive fibers having an average fiber diameter of 0.1 μm or more and 0.9 μm or less, and a fluororesin, wherein the content of the fluororesin is more than 1% by mass and 35% by mass or less relative to the total solid content of the conductive sheet, and the maximum peak in the pore size distribution is 0.1 μm or more and 5.0 μm or less.

2. The conductive sheet according to claim 1, wherein the carbon-based conductive fiber has an aspect ratio (average fiber length / average fiber diameter) of 20 or more and 100 or less.

3. The conductive sheet according to claim 1, wherein the maximum peak of the pore size distribution in the thickness direction is uniform, which is a conductive sheet for a single-layer gas diffusion layer.

4. The conductive sheet according to claim 1, for use as a gas diffusion electrode.

5. A membrane electrode assembly comprising a polymer electrolyte membrane and a pair of gas diffusion electrodes that sandwich the polymer electrolyte membrane, wherein at least one of the pair of gas diffusion electrodes includes a conductive sheet as described in any one of claims 1 to 4.

6. Thickness T of the polymer electrolyte membrane ＩＯＮ (μm) and the thickness T of the gas diffusion electrode ＧＤＥ Ratio (T) to (μm) ＩＯＮ / T ＧＤＥ The membrane electrode assembly according to claim 5, wherein the coefficient of the coefficient is 0.05 or more and 1.00 or less.

7. A fuel cell comprising the membrane electrode assembly described in claim 5, and a separator.

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