Conductive sheet, membrane electrode assembly, and fuel cell

Abstract

The present invention provides: a conductive sheet that includes a carbon-based conductive substance, does not substantially contain a fluororesin, and has a local maximum peak of 0.1-5.0 μm in a pore size distribution; 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, a gas diffusion layer comprises 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 suppressing damage to the ion exchange membrane by the fibers contained in the carbon fiber layer, from the viewpoint of uniformly diffusing fuel gas into the catalyst layer, and from the viewpoint of adjusting the wettability of the membrane electrode assembly.

[0007] Conventional gas diffusion layers are formed by applying a microporous layer-forming coating solution (for example, a mixture containing carbon black and a fluorine-based water repellent) onto a carbon fiber layer, drying, and sintering to create a microporous layer on top of the carbon fiber layer. For example, Patent Document 1 describes a method for producing a gas diffusion layer with a microporous layer by coating the surface of a carbon fiber sheet with a slurry containing a carbon-based conductive material and a fluorine-based resin at appropriate concentrations using spray or knife coating. Furthermore, Patent Document 2 proposes a method for producing a gas diffusion layer by dispersing a fibril-like material consisting of a conductive carbon material, a fluorine-based resin, and an aromatic polyamide in a solvent, forming a paper mold, hot-pressing the resulting precursor, and then firing it. Patent Document 1: Japanese Patent Application Publication No. 7-220734 Patent Document 2: International Publication No. 2012-026498

[0008] However, in conductive sheets used for gas diffusion electrodes in fuel cells, fluororesin is generally used from the standpoint of protecting battery performance. However, in recent years, there has been a growing trend towards environmentally friendly manufacturing that does not rely on fluorine-containing materials.

[0009] This disclosure is made in view of the above-mentioned conventional circumstances, and aims to provide a conductive sheet, a membrane electrode assembly, and a fuel cell that exhibit excellent battery performance when mounted in a fuel cell.

[0010] The following embodiments are included as specific means for solving the problem: <1> A conductive sheet containing a carbon-based conductive material, substantially free of fluororesin, and having a maximum peak in the pore size distribution of 0.1 μm or more and 5.0 μm or less. <2> The conductive sheet according to <1>, having a water contact angle of 120° or less. <3> The conductive sheet according to <1> or <2>, which is a gas diffusion electrode. <4> A membrane electrode assembly comprising a polymer electrolyte membrane and a pair of gas diffusion electrodes sandwiching the polymer electrolyte membrane, wherein at least one of the pair of gas diffusion electrodes contains the conductive sheet according to any one of <1> to <3>. <5> A fuel cell comprising the membrane electrode assembly according to <4> and a separator.

[0011] According to this disclosure, a conductive sheet, a membrane electrode assembly, and a fuel cell are provided that exhibit excellent battery performance when mounted on a fuel cell.

[0012] 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. Figure 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.

[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 tensile modulus of a conductive sheet is as follows: A 13 cm sample is obtained from the conductive sheet to be measured. Using a Tensilon (Orientec RTC-1310A), the breaking strength and elongation at breaking are measured 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 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.

[0022] <Conductive Sheet> The conductive sheet according to this disclosure contains a carbon-based conductive material, substantially does not contain fluororesin, and has a maximum peak in the pore size distribution of 0.1 μm or more and 5.0 μm or less.

[0023] According to this disclosure, having the above configuration makes it easier to diffuse the fuel gas uniformly into the catalyst layer and to appropriately adjust the wettability of the membrane electrode assembly. Furthermore, water repellency is easily ensured. As a result, excellent battery performance is achieved even without substantially containing fluororesin.

[0024] [Characteristics of the conductive sheet] The conductive sheet preferably has a maximum peak in the pore size distribution of 0.1 μm to 5.0 μm, more preferably 0.2 μm to 3.0 μm, and even more preferably 0.3 μm to 2.0 μm. When the maximum peak is within the above range, excessive small or large pore sizes are suppressed, and excellent permeability, i.e., gas diffusion, is achieved even without substantially containing fluororesin. Furthermore, when incorporated into a membrane electrode assembly, excessive decrease or increase in wettability is suppressed. As a result, excellent battery performance is achieved.

[0025] The conductive sheet has a maximum peak in its pore size distribution of 0.1 μm or larger, preferably 0.2 μm or larger, and more preferably 0.3 μm or larger. 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, even without substantially containing fluororesin. Furthermore, when incorporated into a membrane electrode assembly, excessive reduction in wettability is suppressed. As a result, excellent battery performance is achieved.

[0026] The conductive sheet has a maximum peak in its pore size distribution of 5.0 μm or less, preferably 3.0 μm or less, and more preferably 2.0 μm or less. When the maximum peak is 5.0 μm or less, excessive pore size enlargement is suppressed, resulting in excellent water repellency even without substantially containing fluororesin. Furthermore, when incorporated into a membrane electrode assembly, excessive increase in wettability is suppressed. As a result, excellent battery performance is achieved.

[0027] There are no particular limitations on the specific methods for setting the maximum peak in the pore size distribution within the above range, but one example is a method in which the carbon-based conductive material includes at least one selected from the group consisting of carbon black, carbon fibers, carbon milled fibers, and carbon nanofibers.

[0028] The water contact angle of the conductive sheet is not particularly limited and may be 140° or less, 90° or less, 20° to 80°, or 25° to 40°. The water contact angle of the conductive sheet may exceed 140°, 140° to 160°, or 140° to 150°. Conventional conductive sheets used in gas diffusion layers tend to have a water contact angle exceeding 140° from the viewpoint of providing water repellency. In contrast, the conductive sheet according to this disclosure has the above-described configuration, and therefore, even if the water contact angle of the conductive sheet is 90° or less, it is excellent in gas diffusion and the ability to adjust the wetting of the film electrode assembly, and is excellent in battery performance.

[0029] Furthermore, in conventional fuel cells, if a hydrophilic gas diffusion layer (for example, a gas diffusion layer with a water contact angle of 140° or less) is used, the output range of the battery is 1 W / cm². ２ A low operating environment, such as less than 1 W / cm², is required. In contrast, the conductive sheet according to this disclosure has the aforementioned configuration, and therefore has an output range of 1 W / cm². ２ Even with these limitations, it exhibits excellent gas diffusion properties and moisture control capabilities for the membrane electrode assembly, resulting in superior battery performance.

[0030] There are no particular limitations on the specific methods used to set the water contact angle within the above range, but examples include adjusting the proportion of fluororesin in the conductive sheet.

[0031] The conductive sheet according to this disclosure preferably has a tensile modulus of 200 MPa or more and 900 MPa or less, more preferably 300 MPa or more and 700 MPa or less, and even more preferably 300 MPa or more and 500 MPa or less. When the tensile modulus of the conductive sheet according to this disclosure is within the above range, the strength of the conductive sheet is superior, and therefore the battery performance is superior.

[0032] There are no particular limitations on the specific methods for achieving the tensile modulus within the above range, but one example is a method in which the carbon-based conductive material includes at least one of carbon fibers and carbon milled fibers (preferably carbon milled fibers).

[0033] [Carbon-based Conductive Material] The carbon-based conductive material may be dispersed, for example, between the fibers of aromatic polyamide pulp. By including the carbon-based conductive material, conductivity is imparted to the conductive sheet.

[0034] The carbon-based conductive material is not particularly limited, and known carbon-based conductive materials used in gas diffusion layers may be employed. The carbon-based conductive material may be used alone or in combination of two or more.

[0035] As the carbon-based conductive material, for example, from the viewpoint of conductive performance, it is preferable to use a material having a carbon content of 94% by mass or more and a specific resistance of 5 Ω·cm or less. Specific examples of the carbon-based conductive material include carbon fiber, carbon black, graphite particles, carbon nanotubes, carbon milled fiber, carbon nanofiber, carbon nanohorn, graphene, and the like.

[0036] Among the above, as the carbon-based conductive material, it is preferable to include at least one selected from the group consisting of graphite particles, carbon black, carbon fiber, carbon milled fiber, and carbon nanofiber, more preferably to include at least one selected from the group consisting of carbon black, carbon fiber, carbon milled fiber, and carbon nanofiber, and even more preferably to include at least one of carbon milled fiber and carbon nanofiber. When including at least one selected from the above group, it is easy to adjust the range of the maximum peak in the pore size distribution of the conductive sheet to the above-mentioned preferred range.

[0037] - Graphite Particles Examples of the graphite particles include flaky graphite, scaly graphite, earthy graphite, artificial graphite, expanded graphite, exfoliated graphite, leaf-shaped graphite, massive graphite, and spherical graphite. In particular, spherical and flaky graphite are preferred. The average particle size of the graphite particles is preferably 0.05 μm or more and 300.0 μm or less.

[0038] - Carbon Black Examples of the carbon black include acetylene black and ketjen black (registered trademark) having a hollow shell structure. In particular, ketjen black is preferred.

[0039] The average primary particle diameter of the carbon black is preferably, for example, 1.0 nm or more and 500.0 nm or less, more preferably 1.0 nm or more and 200.0 nm or less, and even more preferably 10.0 nm or more and 100.0 nm or less. The average secondary particle diameter of the carbon black is preferably, for example, 0.5 nm or more and 20.0 μm or less. If the average secondary particle diameter is 0.5 μm or more, aggregation of the carbon black is further suppressed when preparing a dispersion of the carbon black. If the average secondary particle diameter is 20.0 μm or less, the conductivity of the sheet is improved.

[0040] - Carbon fiber and carbon milled fiber Examples of the carbon fiber and carbon milled fiber include PAN-based carbon fiber, pitch-based carbon fiber, phenol-based carbon fiber, etc. Among these, it is preferable to contain pitch-based carbon fiber.

[0041] In this specification, the carbon milled fiber refers to a fibrous carbon fiber crushed with a crusher or the like into a powder (milled) state.

[0042] When using at least one of carbon fiber and / or carbon milled fiber, its average fiber diameter is not particularly limited, but for example, it is preferably 3 μm or more and 20 μm or less, and more preferably 4 μm or more and 13 μm or less.

[0043] The average fiber diameter is a value measured by microscopic observation. When the carbon fiber has a flat cross section, the arithmetic mean value of the major axis and the minor axis is defined as the average fiber diameter.

[0044] When the average fiber diameter is 3 μm or more, the strength of the single fiber is high, and it is easy 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 carbon fiber layer is suppressed from locally floating from the microporous layer. Therefore, the formation of surface irregularities due to the floating of the carbon fiber layer is suppressed, the surface smoothness is good, and the contact electrical resistance is small when used as a gas diffusion layer. That is, the battery performance is more excellent.

[0045] 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 gas diffusion layer tends to improve.

[0046] The carbon content of 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 sheet. Furthermore, even when a battery incorporating this conductive sheet is operated for a long period of time, the deterioration of the sheet is suppressed.

[0047] Carbon nanofibers may be single fibers or aggregates. The average fiber diameter of carbon nanofibers is preferably, for example, 100 nm or more and 1000 nm or less. The average fiber diameter of carbon nanofibers is preferably 900 nm or less, more preferably 800 nm or less, even more preferably 600 nm or less, even more preferably 500 nm or less, even more preferably 450 nm or less, and even more preferably 400 nm or less. The average fiber diameter of carbon nanofibers is preferably 110 nm or more, more preferably 120 nm or more, even more preferably 150 nm or more, and even more preferably 200 nm or more. If the average fiber diameter of carbon nanofibers is 100 nm or more, handling properties are good, and if it is 1000 nm or less, it is easy to increase the fiber density.

[0048] The average fiber length of the carbon nanofibers 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 100 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, conductive paths in the thickness direction of the conductive sheet are easily formed. From the viewpoint of sheet strength, for example, the average fiber length of the carbon nanofibers is preferably 10 μm or more and 100 μm or less, and more preferably 12 μm or more and 80 μm or less.

[0049] 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 to obtain 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 to obtain resin composite stabilized fibers by stabilizing the resin composite fibers; (3) a thermoplastic resin removal step to obtain stabilized fibers by removing the thermoplastic resin from the resin composite stabilized fibers; and (4) a carbonization and calcination step to obtain a carbon fiber aggregate by heating the stabilized fibers in an inert atmosphere to carbonize or graphitize them.

[0050] [Fluororesin] The conductive sheet relating to this disclosure is substantially free of fluororesin. In this specification, "substantially free of fluororesin" means that the fluororesin content relative to the total solid content of the conductive sheet is 0% by mass or 1% by mass or less.

[0051] From the viewpoint of providing a conductive sheet with superior battery performance, the proportion of the fluororesin in the conductive sheet according to this disclosure is preferably 0% by mass or 1% by mass or less, preferably 0.5% by mass or less, more preferably 0.2% by mass or less, even more preferably close to 0% by mass, and particularly preferably 0% by mass.

[0052] In this specification, "fluororesin" refers to a general term for synthetic resins containing fluorine.

[0053] 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.

[0054] 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.

[0055] 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, and the like. Among the above, PTFE is preferred as the fluororesin from the viewpoint of having excellent heat resistance and sliding properties.

[0056] [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, electrode catalysts, ionomer resins, and the like.

[0057] Among the above, the conductive sheet preferably further contains aromatic polyamide pulp, from the viewpoint of suitably adjusting gas diffusion properties and water repellency to achieve superior battery performance.

[0058] The conductive sheet may further contain an electrode catalyst. When the conductive sheet is used as a gas diffusion electrode, it can be configured as a single-layer gas diffusion electrode or a thin-film gas diffusion electrode. In the case of containing an electrode catalyst, the conductive sheet is preferably a sheet comprising aromatic polyamide pulp, a carbon-based conductive material, and the electrode catalyst.

[0059] 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).

[0060] 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.

[0061] 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 10.0 mm, more preferably 0.2 mm to 5.0 mm, even more preferably 0.3 mm to 3.0 mm, and particularly preferably 0.2 mm to 1.0 mm.

[0062] 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.

[0063] [Content of various materials] The content of carbon-based conductive material is preferably 50% by mass or more and 85% by mass or less, and more preferably 55% by mass or more and 80% by mass or less, relative to the total solid content of the conductive sheet. When the content of carbon-based conductive material is 50% by mass or more and 85% by mass or less, the decrease in water repellency is suppressed and conductivity and battery performance are better.

[0064] 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 2% 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 of the conductive sheet is improved. 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 with high uniformity into the catalyst layer and to appropriately adjust the wettability of the membrane electrode assembly. In addition, water repellency is easily ensured. As a result, the battery performance is improved even without substantially containing fluororesin.

[0065] 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 1 / 99 to 50 / 50, and more preferably in the range of 3 / 97 to 40 / 60. When the above mass ratio is 1 / 99 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.

[0066] 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 100 / 0 or 6 / 1 or less, and more preferably in the range of 100 / 0 or 5 / 1 or less. When the above mass ratio is within the above range, the reinforcing effect is more superior due to the aromatic polyamide pulp.

[0067] [Applications of the conductive sheet] 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, a nonionic surfactant, an enhancer (e.g., a papermaking strength enhancer). Among the above, the conductive sheet according to this disclosure is preferably used as a gas diffusion electrode from the viewpoint of providing excellent battery performance when it is included. If the conductive sheet according to this disclosure includes an electrode catalyst, the conductive sheet according to this disclosure may be used as a gas diffusion electrode with an integrated gas diffusion layer.

[0068] [Method for Manufacturing Conductive Sheets] The method for manufacturing conductive sheets according to this disclosure is not particularly limited, and known methods for manufacturing conductive sheets can be used. The method for manufacturing conductive sheets according to this disclosure may, for example, involve manufacturing a conductive sheet by weaving or papermaking a composition for forming conductive sheets containing a carbon-based conductive substance, and then forming a sheet. The method for manufacturing conductive sheets according to this disclosure may, for example, involve manufacturing a conductive sheet by weaving or papermaking a composition for forming conductive sheets containing a carbon-based conductive substance, impregnating the sheet with a fluororesin-containing solution, and then sintering it.

[0069] <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. The gas diffusion electrode according to this disclosure offers superior battery performance.

[0070] The gas diffusion electrode according to this disclosure may or may not have a microporous layer between the conductive sheet and the catalyst layer, but from the viewpoint of achieving superior battery performance and reducing costs, it is preferable that there is no microporous layer.

[0071] 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.

[0072] <Membrane electrode assembly> The membrane electrode assembly according to the present 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 the present disclosure.

[0073] The film electrode assembly described herein offers superior battery performance when mounted on a battery.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] 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.

[0078] The gas diffusion layer 40 may be a single layer, or it may be formed as a laminate of two or more layers. If 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 layers of the laminate may be the conductive sheet according to this disclosure.

[0079] Figure 2 is a schematic cross-sectional view showing an example of a layer configuration in another embodiment of the membrane electrode assembly according to the present disclosure. The membrane electrode assembly 100 shown in Figure 2 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 is a conductive sheet according to the present disclosure. In this case, the conductive sheet according to the present disclosure includes at least an electrode catalyst C in addition to a carbon-based conductive material.

[0080] Although not shown in the figures, if at least one of the gas diffusion electrodes 20 (for example, the cathode side) includes the conductive sheet according to this disclosure, the membrane electrode assembly 100 may be such that the other gas diffusion electrode 20 (for example, the anode side) is a gas diffusion electrode provided with a catalyst layer, a microporous layer, and a carbon fiber layer in that order from the polymer electrolyte membrane 10 side.

[0081] As shown in Figure 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 contains an electrode catalyst C. Of the pair of gas diffusion electrodes 20, one 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. Air containing oxygen is supplied to the cathode electrode, and an oxygen reduction reaction occurs.

[0082] The applications of the membrane electrode assembly relating to this disclosure are not particularly limited, but for example, it can be used in fuel cells and water electrolysis devices.

[0083] <Fuel Cell> The fuel cell according to this disclosure comprises a membrane electrode assembly and a separator according to this disclosure. The fuel cell according to this disclosure has excellent battery performance.

[0084] The separator is not particularly limited, and any known separator applicable to water electrolysis devices or fuel cells can be used.

[0085] 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.

[0086] <Examples 1 and 2> The following raw materials were prepared: • Carbon-based conductive material: Carbon nanofiber (PotenCia®, manufactured by Teijin Limited) • Carbon-based conductive material: Conductive carbon black (Ketjen Black, manufactured by Lion Corporation) • Aromatic polyamide pulp: Para-aramid fibril (JSF-8086, manufactured by Teijin Aramid Co., Ltd.) • Fluororesin: 61% by mass PTFE dispersion (AD911, particle size 0.25 μm, manufactured by AGC Inc.)

[0087] Carbon nanofibers and a nonionic surfactant were added to deionized water and dispersed using a Primix Disperser to prepare a 0.5% by mass carbon nanofiber dispersion. The mass ratio of carbon nanofibers to nonionic surfactant (carbon nanofibers:nonionic surfactant) was 1:1.

[0088] Conductive carbon black and a nonionic surfactant were added to deionized water and dispersed using Primix Filmix to prepare a 0.5% by mass conductive carbon black dispersion. The mass ratio of conductive carbon black to nonionic surfactant (conductive carbon black:nonionic surfactant) was 1:2.

[0089] Para-aramid fibrils were added to deionized water and dispersed using a high-pressure homogenizer to prepare a 0.5% by mass para-aramid fibril dispersion.

[0090] A 61% by mass PTFE dispersion was diluted with deionized water to prepare a 0.5% by mass PTFE dispersion.

[0091] Each dispersion prepared as described above has a solid content of 25 g / m² of each raw material other than fluororesin. ２Each component was weighed and mixed, and the mixture was diluted with deionized water so that the total solid content of the raw materials was 0.05% by mass. Then, a papermaking strength enhancer (acrylamide-based) equivalent to 0.5% of the solid content was added to obtain a mixed dispersion. The amount of fluororesin solids in the mixed dispersion was adjusted by dilution so that the ratio of fluororesin to the total solid content of the conductive sheet was as shown in Table 1. The obtained mixed dispersion was made into sheets using a wet papermaking method with a rectangular sheet machine manufactured by Kumagai Riki Kogyo Co., Ltd. The obtained sheets were pressed, dewatered and dried, then smoothed using a calender with metal rollers at 200°C and a linear pressure of 10 kg / cm, and then heat-treated at 400°C for 30 minutes in an air atmosphere to obtain the conductive sheets for each example.

[0092] In Example 1, the content and ratio of each material in relation to the conductive sheet are as follows: • Total amount of carbon-based conductive material: 93% 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): 1 / 93 • Mass ratio (aromatic polyamide pulp / fluororesin): 1 / 0

[0093] In Example 2, the content and ratio of each material in the conductive sheet are as follows: • Total amount of carbon-based conductive material: 92% 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 / 92 • Mass ratio (aromatic polyamide pulp / fluororesin): 7 / 1

[0094] <Comparative Example 2> A commercially available carbon fiber paper (Toray Industries, Inc., TGP-H-030, 100 μm thick) was used as a conductive sheet as is.

[0095] <Comparative Example 3> A commercially available carbon fiber paper (Toray Industries, Inc., TGP-H-030, 100 μm thick) was impregnated with a PTFE dispersion solution that was appropriately diluted, dried, and then heat-treated at 400°C for 30 minutes to produce a single-layer water-repellent conductive sheet.

[0096] <Comparative Example 4> Using a commercially available carbon fiber paper (manufactured by Toray Industries, Inc., TGP-H-030, thickness 100 μm), a PTFE dispersion was appropriately diluted, impregnated, and dried, and a mixed paste of 77% by mass of acetylene black and 23% by mass of PTFE was applied to the surface so that the solid content was 30 g / m ２ After drying at 60°C for 24 hours and then heat-treating at 400°C for 30 minutes, a water-repellent conductive sheet having a microporous layer containing a carbon-based conductive material and a fluororesin and a carbon fiber layer containing a fluororesin was produced.

[0097] Table 1 shows the maximum peaks in the pore size distribution measured by the above method for the conductive sheets of each example. In Table 1, in Comparative Example 4, for each of the microporous layer and the carbon fiber layer, the values of the maximum peaks in the pore size distribution are listed as [microporous layer / carbon fiber layer].

[0098] <Evaluation of Battery Performance> Using the conductive sheet of each example as a gas diffusion layer on the cathode (anode: air electrode) side, a cell was fabricated. The cell has the configuration shown in FIG. 1. Using each cell, the cell voltage was measured at a cell temperature of 80°C and a current density of 0 to 6 A / cm ２ and current density-voltage curves were obtained for each. For each cell of each example, the cell voltages when the current density obtained from the current density-voltage curve was 1 A / cm ２ , 2 A / cm ２ , 3 A / cm ２ and 4 A / cm ２ are shown in Table 1. In Table 1, [Stop] means that the cell voltage is below 0.2 V. In Table 1, [-] means that it was not measured. In Table 1, *1 indicates the maximum peak value in the pore size distribution of the microporous layer (MPL) alone.

[0099] The details of the device environment are as follows. - Separator: JARI-2, 0.3 mm / 0.3 mm straight 1 cm ２• Gas diffusion layer: 1 cm x 1 cm • Humidifier: Cathode 80°C / Anode 75°C • Gas supply: Anode (hydrogen electrode) 0.4 L / min, Cathode (air electrode) 0.95 L / min • Back pressure: Anode 100 kPa / Cathode 100 kPa • Ion exchange membrane (CCM) coated with electrode catalyst (platinum-supported carbon) • Catalyst amount (i.e., platinum amount) on the anode (cathode: hydrogen electrode) side: 0.1 mg / cm ２ - Catalytic amount (i.e., platinum amount) on the cathode (anode: air electrode) side: 0.4 mg / cm² ２ A carbon fiber layer with conductivity and water repellency was used as an anode (an example of a gas diffusion electrode) by coating carbon fiber paper manufactured by SGL Carbon Japan Co., Ltd. (specifically SGL25BC, a water-repellent carbon fiber paper with a PTFE content of 5% by mass) with a mixed paste (specifically, a mixed paste with a solid content of 23% by mass of PTFE and a solid content of 77% by mass of carbon black). The carbon fiber layer has a maximum peak in its pore size distribution in the range of 0.1 μm to less than 0.2 μm.

[0100]

[0101] As shown in Table 1, the conductive sheets of Examples 1 and 2 were found to have cell voltages that were equal to or higher than those of the conductive sheets of Comparative Examples 1, 3, and 4, in which the proportion of fluororesin in the conductive sheet exceeded 1% by mass, and the conductive sheet of Comparative Example 2, which substantially did not contain fluororesin but had a maximum peak in the pore size distribution exceeding 5.0 μm, demonstrating superior battery performance.

[0102] The disclosure of Japanese Patent Application No. 2024-229277, 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.

[0103] (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

Claims

1. A conductive sheet containing a carbon-based conductive material, substantially free of fluororesin, and having a maximum peak in the pore size distribution of 0.1 μm or more and 5.0 μm or less.

2. The conductive sheet according to claim 1, wherein the water contact angle is 120° or less.

3. A conductive sheet according to claim 1 or claim 2, which is a gas diffusion electrode.

4. A membrane electrode assembly comprising a polymer electrolyte membrane and a pair of gas diffusion electrodes sandwiching the polymer electrolyte membrane, wherein at least one of the pair of gas diffusion electrodes includes the conductive sheet described in claim 1 or claim 2.

5. A fuel cell comprising the membrane electrode assembly and separator described in claim 4.

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