Membrane electrode assembly and polymer electrolyte fuel cell

Abstract

To provide a membrane electrode assembly and a polymer electrolyte fuel cell capable of improving power generation performance.SOLUTION: A membrane electrode assembly 10 includes, in this order, an anode gas diffusion layer 13A, an anode electrode catalyst layer 12A, a polymer electrolyte membrane 11, a cathode electrode catalyst layer 12C, and a cathode gas diffusion layer 13C. The anode gas diffusion layer 13A contains catalyst-supporting particles, a polymer electrolyte, and conductive fibers, and the mean flow pore diameter of the anode gas diffusion layer 13A is 0.3 to 10 μm.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a membrane electrode assembly and a polymer electrolyte fuel cell. [Background technology]

[0002] Fuel cells are attracting attention as a type of battery that can contribute to solving environmental and energy problems. Fuel cells generate electricity by utilizing a chemical reaction between a fuel such as hydrogen and an oxidant such as oxygen. Among fuel cells, polymer electrolyte fuel cells can be operated at low temperatures and can be made compact, so they are expected to be used as portable power sources, household power sources, and vehicle power sources.

[0003] A polymer electrolyte fuel cell includes a membrane electrode assembly having a fuel electrode (anode), an air electrode (cathode), and a polymer electrolyte membrane sandwiched between the fuel electrode and the air electrode. Each of the fuel electrode and the air electrode includes a laminate of an electrode catalyst layer and a gas diffusion layer.

[0004] A fuel gas containing hydrogen is supplied to the fuel electrode, and an oxidant gas containing oxygen is supplied to the air electrode. Protons and electrons are generated from the fuel gas supplied to the fuel electrode due to the action of a catalyst contained in the electrode catalyst layer. The protons are conducted by the polymer electrolyte contained in the electrode catalyst layer and the polymer electrolyte membrane, and move through the polymer electrolyte membrane to the air electrode. The electrons are extracted from the fuel electrode to an external circuit, and move through the external circuit to the air electrode. At the air electrode, the oxidant gas reacts with the protons and electrons that have moved from the fuel electrode to produce water. In this way, an electric current is generated as the electrons pass through the external circuit (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-115299 [Patent Document 2] Patent No. 5182908 Summary of the Invention [Problem to be solved by the invention]

[0006] Generally, an electrode catalyst layer contains a carbon material supporting a catalytic substance such as platinum and a polymer electrolyte. The carbon material contributes to electron conduction, and the polymer electrolyte contributes to proton conduction. Therefore, the type and ratio of the carbon material and polymer electrolyte are important factors for improving the power generation performance of a polymer electrolyte fuel cell. Furthermore, improving the gas diffusion within the electrode catalyst layer and the drainage of water generated during power generation also contributes to improved power generation performance.

[0007] Meanwhile, the gas diffusion layer is involved in supplying gas to the electrode catalyst layer, collecting electrons generated in the electrode reaction, and managing the water content within the cell by removing water generated during power generation. Therefore, the characteristics of the gas diffusion layer are also an important factor in improving the power generation performance of polymer electrolyte fuel cells. In particular, the mean flow pore size of the gas diffusion auxiliary layer provided in the gas diffusion layer has been extensively studied to improve performance. According to Patent Document 2, the mean flow pore size of the gas diffusion auxiliary layer in contact with the cathode side is preferably 20 μm or less, which allows for a high degree of freedom. However, the mean flow pore size of the gas diffusion auxiliary layer in contact with the anode side is preferably 0.1 μm or less. However, most commercially available gas diffusion auxiliary layers have mean flow pore sizes of approximately 3 to 20 μm, and those with a mean flow pore size of 0.1 μm or less are difficult to obtain. Furthermore, the gas diffusion auxiliary layer must be prepared separately from the cathode-side gas diffusion layer, which is costly.

[0008] Therefore, it is required to achieve high power generation performance even if the mean flow pore diameter of the anode-side gas diffusion layer is increased to some extent. The present invention has been made in view of the above-mentioned problems, and aims to provide a membrane electrode assembly and a solid polymer fuel cell that can achieve high power generation capacity even if the mean flow pore diameter of the anode-side gas diffusion layer is relatively large. [Means for solving the problem]

[0009] Various aspects of a membrane electrode assembly and a polymer electrolyte fuel cell for solving the above problems will be described below. [1] A membrane electrode assembly comprising an anode-side gas diffusion layer, an anode-side electrode catalyst layer, a polymer electrolyte membrane, a cathode-side electrode catalyst layer, and a cathode-side gas diffusion layer, in this order, wherein the anode-side gas diffusion layer contains catalyst-supporting particles, a polymer electrolyte, and conductive fibers, and the anode-side gas diffusion layer has a mean flow pore diameter of 0.3 to 10 μm.

[0010] According to the above configuration, pores are formed by the conductive fibers in the anode-side electrode catalyst layer, improving the diffusibility of the fuel gas and allowing the water vapor contained in the gas to sufficiently humidify the catalyst-electrolyte membrane. This reduces the need to increase water retention by reducing the mean flow pore diameter of the through-holes in the gas diffusion layer.

[0011] [2] The membrane electrode assembly according to [1], wherein the conductive fibers of the anode-side gas diffusion layer are carbon fibers. According to the above configuration, the conductivity of the catalyst layer is suitably increased.

[0012] [3] The membrane electrode assembly according to [1] or [2], wherein the cathode-side gas diffusion layer contains catalyst-supporting particles, a polymer electrolyte, and conductive fibers, and the mean flow pore diameter of the cathode-side gas diffusion layer is 0.3 to 10 μm.

[0013] According to the above configuration, pores are formed by the conductive fibers in the cathode-side catalyst layer, which increases the diffusibility of the oxidizing gas and allows the water vapor contained in the gas to sufficiently humidify the catalyst-electrolyte membrane. Furthermore, the appropriate mean flow pore size of the through-holes in the gas diffusion layer allows the generated water to be properly drained.

[0014] [4] The membrane electrode assembly according to [3], wherein the conductive fibers of the cathode-side gas diffusion layer are carbon fibers. According to the above configuration, the conductivity of the catalyst layer is suitably increased.

[0015] [5] The membrane / electrode assembly according to any one of [1] to [4], wherein the anode-side gas diffusion layer has a mean flow pore diameter of 2 to 10 μm. According to the above configuration, the power generation performance of the fuel cell is suitably improved.

[0016] [6] A polymer electrolyte fuel cell comprising the membrane electrode assembly according to any one of [1] to [5] and a pair of separators sandwiching the membrane electrode assembly. According to the above configuration, power generation performance is improved and a high output voltage can be obtained. [Effects of the Invention]

[0017] According to the present invention, there are provided a membrane electrode assembly and a polymer electrolyte fuel cell that can achieve high power generation capacity even if the mean flow pore diameter of the anode-side gas diffusion layer is relatively large. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 2 is a diagram showing a cross-sectional structure of a membrane electrode assembly according to an embodiment. [Figure 2] FIG. 3 is a diagram schematically illustrating the distribution of conductive fibers in an electrode catalyst layer according to an embodiment. [Figure 3] FIG. 1 is an exploded perspective view showing a polymer electrolyte fuel cell according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0019] An embodiment of a membrane electrode assembly and a polymer electrolyte fuel cell will be described with reference to FIGS.

[0020] [Configuration of membrane electrode assembly] The structure of the membrane electrode assembly will be described with reference to FIG. As shown in FIG. 1, the membrane electrode assembly 10 includes, in this order, an anode gas diffusion layer 13A, an anode electrode catalyst layer 12A, a polymer electrolyte membrane 11, a cathode electrode catalyst layer 12C, and a cathode gas diffusion layer 13C.

[0021] The polymer electrolyte membrane 11 is sandwiched in the thickness direction between the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C. The anode electrode catalyst layer 12A is in contact with one of the two surfaces of the polymer electrolyte membrane 11, and the cathode electrode catalyst layer 12C is in contact with the other of the two surfaces of the polymer electrolyte membrane 11.

[0022] The anode gas diffusion layer 13A is laminated on the anode electrode catalyst layer 12A, and the cathode gas diffusion layer 13C is laminated on the cathode electrode catalyst layer 12C. In other words, the laminate of the anode electrode catalyst layer 12A, the polymer electrolyte membrane 11, and the cathode electrode catalyst layer 12C is sandwiched between the anode gas diffusion layer 13A and the cathode gas diffusion layer 13C.

[0023] The anode-side electrode catalyst layer 12A and the anode-side gas diffusion layer 13A constitute the fuel electrode, which is the anode of the polymer electrolyte fuel cell, while the cathode-side electrode catalyst layer 12C and the cathode-side gas diffusion layer 13C constitute the air electrode, which is the cathode of the polymer electrolyte fuel cell.

[0024] When viewed from a position opposite one surface of the polymer electrolyte membrane 11, the anode electrode catalyst layer 12A, the cathode electrode catalyst layer 12C, and the anode gas diffusion layer 13A and the cathode gas diffusion layer 13C have substantially the same outer shape. The outer shape of the polymer electrolyte membrane 11 is preferably larger than the outer shapes of the anode electrode catalyst layer 12A, the cathode electrode catalyst layer 12C, the anode gas diffusion layer 13A, and the cathode gas diffusion layer 13C. There are no particular limitations on the outer shapes of the polymer electrolyte membrane 11 and the anode electrode catalyst layer 12A, the cathode electrode catalyst layer 12C, the anode gas diffusion layer 13A, and the cathode gas diffusion layer 13C, and they may be, for example, rectangular.

[0025] [Membrane electrode assembly materials] The materials and detailed configurations of the members of the membrane electrode assembly 10 will be described below. (polymer electrolyte membrane) The polymer electrolyte membrane 11 includes a polymer electrolyte. The polymer electrolyte used in the polymer electrolyte membrane 11 may be any polymer electrolyte having proton conductivity, such as a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. Examples of fluorine-based polymer electrolytes include Nafion (registered trademark: manufactured by DuPont), Flemion (registered trademark: manufactured by Asahi Glass Co., Ltd.), Aciplex (registered trademark: manufactured by Asahi Kasei Corporation), and Gore-Select (registered trademark: manufactured by Gore Japan LLC). Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. Among these, fluorine-based polymer electrolytes, specifically Nafion (registered trademark: manufactured by DuPont) and its related materials, are preferably used.

[0026] (electrode catalyst layer) The anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C each contain catalyst-supporting particles and a polymer electrolyte. (Catalyst-supported particles) The catalyst-supporting particles are carriers that support a catalytic substance. The anode-side electrode catalyst layer 12A further contains conductive fibers. The cathode-side electrode catalyst layer 12C does not necessarily have to contain conductive fibers, but it is preferable that it does.

[0027] (catalyst material) The catalytic substance has a particulate shape. The material of the catalytic substance is, for example, a metal, an alloy, a metal oxide, a metal double oxide, etc. The metal double oxide is an oxide containing two types of metals. Examples of metals contained in the catalytic material include platinum group elements such as platinum, palladium, ruthenium, iridium, rhodium, and osmium, gold, iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum.

[0028] When the catalytic substance is one or more metals selected from platinum, gold, palladium, rhodium, ruthenium, and iridium, the membrane electrode assembly 10 has excellent reactivity in the electrode reaction and can efficiently and stably proceed with the electrode reaction. In this case, the power generation performance of a polymer electrolyte fuel cell equipped with the membrane electrode assembly 10 is also improved.

[0029] From the viewpoint of enhancing the activity of the catalytic substance, the average particle diameter of the catalytic substance is preferably 20 nm or less, more preferably 5 nm or less. From the viewpoint of stabilizing the activity of the catalytic substance, the average particle diameter of the catalytic substance is preferably 0.5 nm or more, more preferably 1 nm or more. The average particle diameter of the catalytic substance is the average particle diameter determined by X-ray diffraction when measured in a state in which the catalytic substance is supported on a carrier, and is the arithmetic average particle diameter determined by particle size measurement when measured in a state in which the catalytic substance is not supported on a carrier.

[0030] (Carrier) The support is a conductive particle that is not eroded by the catalyst material. An example of the support is carbon particles. Specific examples of the support include carbon black, graphite, activated carbon, carbon fiber, carbon nanotubes, and fullerene. The catalyst-supported particles may have a catalyst substance loading density of 30 to 70 mass %.

[0031] The catalyst-supporting particles may be covered with a hydrophobic coating. The hydrophobic coating imparts hydrophobic properties to the catalyst-supporting particles and also has gas permeability that allows fuel gas and oxidant gas to pass through. From the viewpoint of improving water dischargeability in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C, the thickness of the hydrophobic coating is preferably 2 nm or more. From the viewpoint of improving gas permeability, the thickness of the hydrophobic coating is preferably 40 nm or less.

[0032] An example of a material for the hydrophobic coating is a fluorine-based compound having at least one polar group. Examples of polar groups include hydroxyl groups, alkoxy groups, carboxyl groups, ester groups, ether groups, carbonate groups, and amide groups. A hydrophobic coating having a polar group is easily fixed to the outermost surface of a support. The portion of the fluorine-based compound other than the polar group is preferably a structure consisting of fluorine and carbon because of its high hydrophobicity and chemical stability. However, the structure of the portion of the fluorine-based compound other than the polar group is not limited as long as it has a structure that provides sufficient hydrophobicity and chemical stability.

[0033] (polymer electrolyte) The polymer electrolyte may be any polymer electrolyte having proton conductivity, such as a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. An example of a fluorine-based polymer electrolyte is Nafion (registered trademark, manufactured by DuPont). Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. Among these, fluorine-based polymer electrolytes, specifically Nafion (registered trademark, manufactured by DuPont) and its related materials, are preferably used. The polymer electrolytes contained in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C are preferably made of the same material as that of the polymer electrolyte membrane 11.

[0034] The amount of polymer electrolyte in the electrode catalyst layer can be 0.5 to 1, where the mass of the carrier in the catalyst-supporting particle is taken as 1.

[0035] (Conductive fiber) The conductive fiber is a fiber having electronic conductivity, such as a carbon fiber. Specific examples of the conductive fiber include carbon fiber, carbon nanofiber, carbon nanotube, and conductive polymer nanofiber. Of these, carbon nanofiber and carbon nanotube are preferably used.

[0036] When the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C each contain conductive fibers, a structure is formed in which the conductive fibers are entangled, which favorably forms pores in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C. This improves gas diffusion and drainage in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C. Furthermore, the occurrence of cracks in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C is suppressed, improving the mechanical strength, and therefore improving the durability of the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C.

[0037] Furthermore, due to the entanglement of the conductive fibers, a structure in which the conductive fibers 20 are connected in a mesh-like pattern is formed in each of an in-plane region R1 extending along the thickness direction of the electrode catalyst layer and an in-plane region R2 extending in a direction along the surface of the electrode catalyst layer, as shown in the example of the cathode-side electrode catalyst layer 12C in Fig. 2. Therefore, the inclusion of conductive fibers increases the conductivity in both the direction along the surface and the thickness direction of the electrode catalyst layer.

[0038] The average fiber diameter of the conductive fibers is preferably 0.5 nm or more and 500 nm or less, and more preferably 10 nm or more and 300 nm or less. If the average fiber diameter is 0.5 nm or more, the flexibility of the conductive fibers is not excessively strong, and a structure in which the conductive fibers are entangled is suitably formed. If the average fiber diameter is 500 nm or less, it is possible to prevent the flexibility of the conductive fibers from being excessively weak, making it difficult to form a structure in which the conductive fibers are entangled. It is also possible to prevent pores from becoming coarse and increasing the resistance value. The average fiber diameter is determined by averaging the diameters of five arbitrarily selected conductive fibers in the cross sections of the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C photographed with a scanning electron microscope (SEM).

[0039] The average fiber length of the conductive fibers is preferably 1 μm or more and 200 μm or less, and more preferably 1 μm or more and 50 μm or less. If the average fiber length of the conductive fibers is within the above range, the mesh structure described above due to the entanglement of the conductive fibers is suitably formed. The average fiber length of the fibrous material 23 refers to the arithmetic mean value of the fiber lengths obtained by measuring the lengths of at least 10 conductive fibers. The average fiber length of the conductive fibers in the electrode catalyst layer can be determined by measuring the particle size distribution using a solution in which the electrode catalyst layer is dissolved in a solvent.

[0040] In this specification, conductive fibers refer to fibers with an aspect ratio of 10 or more. The aspect ratio of conductive fibers may be 20 or more. There is no particular upper limit to the aspect ratio of conductive fibers, but it may be 400 or less. In this specification, the aspect ratio of conductive fibers is a value calculated by dividing the average fiber length by the average fiber diameter.

[0041] The content of the conductive fibers in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C is preferably 0.1 to 2.0 times the weight of the carrier alone in the catalyst-supported particles. If the content of the conductive fibers is 0.1 times or more the weight of the carrier, a structure in which the conductive fibers are entangled is suitably formed. If the content of the conductive fibers is 2.0 times or less the weight of the carrier, an increase in the volume of the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C is suppressed, and therefore an increase in resistance value is suppressed.

[0042] The weight of the electrode catalyst layer is 0.05 to 0.2 mg / cm for the anode (fuel electrode) side electrode catalyst layer, and the amount of catalyst material such as platinum supported is 0.05 to 0.2 mg / cm 2 In order to achieve this, the amount of catalyst material such as platinum in the cathode (air electrode) side electrode catalyst layer is set to 0.05 to 0.4 mg / cm 2 can be adjusted so that

[0043] (gas diffusion layer) The anode side gas diffusion layer 13A and the cathode side gas diffusion layer 13C have gas diffusibility and electrical conductivity. (Porous conductive base material) The anode-side gas diffusion layer 13A and / or the cathode-side gas diffusion layer 13C preferably include a porous conductive substrate. The porous conductive substrate is a substrate that is gas permeable and conductive, such as a porous carbon material such as carbon cloth, carbon paper, carbon felt, or nonwoven fabric. From the viewpoint of exhibiting sufficient water repellency without a water-repellent treatment, it is preferable to use carbon paper, carbon cloth, or carbon felt that has been graphitized by baking at a high temperature of 2000°C or higher as the porous conductive substrate.

[0044] On the other hand, since firing at high temperatures increases production costs, instead of the porous conductive substrate, a sheet made of a polymer material that has been carbonized to an extent that electrical conductivity is ensured may be treated with a water repellent agent to be water repellent, and used as the anode-side gas diffusion layer 13A and / or the cathode-side gas diffusion layer 13C. Polyacrylonitrile is preferably used as the polymer material.

[0045] Furthermore, the porous conductive substrate does not have to be made of carbon as long as it has gas permeability, conductivity, and water repellency. For example, the porous conductive substrate may be made of a metal foam primarily containing iron, a metal foam primarily containing nickel, or a metal foam primarily containing iron and nickel.

[0046] Carbon felt and carbon cloth are formed by entangled carbon fibers, but because the carbon fibers themselves do not have binding properties, the carbon fibers may fall off when the entanglement is undone. Therefore, when the porous conductive substrate is made of carbon fibers, it is preferable to fix the shape of the porous conductive substrate by applying or impregnating it with a binder. Furthermore, using a water-repellent binder is advantageous in that it can reduce manufacturing costs. Examples of water-repellent binders that can be used include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA), which are stable materials.

[0047] (Microporous layer) The anode-side gas diffusion layer 13A and / or the cathode-side gas diffusion layer 13C may include a microporous layer formed on the surface of a porous conductive substrate. The microporous layer is provided on the surface of the anode-side gas diffusion layer 13A that contacts the anode-side electrode catalyst layer 12A and on the surface of the cathode-side gas diffusion layer 13C that contacts the cathode-side electrode catalyst layer 12C. The provision of the microporous layer makes it easier to maintain the moisture content in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C in an appropriate manner.

[0048] The microporous layer is composed of a mixture of a conductive material and a water-repellent resin. The conductive material is preferably carbon powder, and using carbon powder with a small particle size is preferable because it can smooth the surface of the microporous layer, i.e., the surfaces of the anode-side gas diffusion layer 13A and the cathode-side gas diffusion layer 13C. The water-repellent resin is, for example, a fluororesin such as polytetrafluoroethylene (PTFE), perfluoroethylene-propene copolymer (FEP), or tetrafluoroethylene-ethylene copolymer (ETFE). The pore size of the microporous layer is smaller than the pore size of the porous conductive substrate, and the mean flow pore size of the gas diffusion layer having the porous conductive substrate and the microporous layer is substantially determined by the microporous layer.

[0049] The thickness of the anode side gas diffusion layer 13A and the cathode side gas diffusion layer 13C is preferably 0.05 mm or more, more preferably 0.1 mm or more. The thickness of the anode side gas diffusion layer 13A and the cathode side gas diffusion layer 13C is preferably 2 mm or less, more preferably 1 mm or less, and even more preferably 0.35 mm or less. A thickness of 0.05 mm or more makes it easier to obtain sufficient tensile strength in the anode side gas diffusion layer 13A and the cathode side gas diffusion layer 13C, making them easier to handle. A thickness of 2 mm or less makes it easier to ensure sufficient gas permeability in the anode side gas diffusion layer 13A and the cathode side gas diffusion layer 13C.

[0050] The mean flow pore diameter of the anode-side gas diffusion layer 13A is 0.3 to 10 μm, and may be 2.0 μm or more, 3.0 μm or more, 8.0 μm or less, or 7.0 μm or less. The mean flow pore diameter of the cathode-side gas diffusion layer 13C is preferably 0.3 to 10 μm, and may be 2.0 μm or more, 3.0 μm or more, 8.0 μm or less, or 7.0 μm or less. The larger the mean flow pore diameter of the anode-side gas diffusion layer 13A or the cathode-side gas diffusion layer 13C, the more abundant the supply of fuel gas or oxidant gas and the water vapor contained in the gas, but the more drying of the catalyst layer and membrane due to gas desorption is promoted. Furthermore, the smaller the mean flow pore diameter, the more likely pore clogging occurs due to water produced by reactions during power generation. In particular, when the mean flow pore diameter of the anode-side gas diffusion layer is 0.3 to 10 μm, power generation performance is improved.

[0051] The mean flow pore size of the gas diffusion layer is measured in accordance with the through pore evaluation method using the bubble point method (JIS 3832-1990).

[0052] [Structure of polymer electrolyte fuel cells] The configuration of a polymer electrolyte fuel cell including the above-described membrane electrode assembly 10 will be described with reference to FIG.

[0053] 3, the polymer electrolyte fuel cell 30 includes a membrane electrode assembly 10 and a pair of separators 31A and 31C. The membrane electrode assembly 10 is sandwiched between the separator 31A and the separator 31C.

[0054] Separators 31A and 31C are made of a conductive and gas-impermeable material. Separator 31A faces anode-side gas diffusion layer 13A, and separator 31C faces cathode-side gas diffusion layer 13C. A gas flow path 32A is formed on the surface of separator 31A facing the anode-side gas diffusion layer 13A, and a cooling water flow path 33A is formed on the surface opposite the anode-side gas diffusion layer 13A. Similarly, a gas flow path 32C is formed on the surface of separator 31C facing cathode-side gas diffusion layer 13C, and a cooling water flow path 33C is formed on the surface opposite the cathode-side gas diffusion layer 13C.

[0055] During operation of the polymer electrolyte fuel cell 30, a fuel gas such as hydrogen flows through the gas flow channel 32A of the separator 31A, and an oxidant gas such as oxygen flows through the gas flow channel 32C of the separator 31C. Cooling water flows through the cooling water flow channels 33A and 33C of the separators 31A and 31C. The fuel gas is supplied from the gas flow channel 32A to the fuel electrode, and the oxidant gas is supplied from the gas flow channel 32C to the air electrode, causing an electrode reaction and generating an electromotive force between the fuel electrode and the air electrode. An organic fuel such as methanol may be supplied to the fuel electrode.

[0056] The polymer electrolyte fuel cell 30 may be used in the form of a single cell as shown in Fig. 3, or may be used as a single fuel cell by stacking and connecting a plurality of polymer electrolyte fuel cells 30 in series. In addition to the above components, the polymer electrolyte fuel cell 30 may also include a gasket or the like to prevent gas leakage.

[0057] (Mechanism of action) In this embodiment, the anode side electrode catalyst layer 12A contains catalyst-supporting particles, a polymer electrolyte, and conductive fibers, and the anode side gas diffusion layer 13A has a mean flow pore diameter of 0.3 to 10 μm, thereby improving the power generation performance of the fuel cell. Although the reason for this is not clear, it is thought that the inclusion of conductive fibers in the anode-side electrode catalyst layer 12A increases the number of pores in the anode-side electrode catalyst layer 12A, making it easier to supply moisture into the anode-side electrode catalyst layer 12A via the anode-side gas diffusion layer 13A, which has an appropriate pore size, and to the polymer electrolyte membrane 11 via the anode-side electrode catalyst layer 12A. In contrast, an anode-side electrode catalyst layer that does not contain conductive fibers has fewer pores, making it difficult to supply moisture from the anode-side gas diffusion layer into the anode-side electrode catalyst layer and to the polymer electrolyte membrane via the anode-side electrode catalyst layer. This is thought to instead promote the escape of moisture from the anode-side electrode catalyst layer to the outside through the anode-side gas diffusion layer, which has larger pore sizes.

[0058] [Method for manufacturing membrane electrode assembly] A method for manufacturing the membrane electrode assembly 10 will now be described. First, a catalyst ink is prepared as a coating liquid for forming the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C. The catalyst ink contains catalyst-supporting particles, a polymer electrolyte, and a solvent. When forming an electrode catalyst layer containing conductive fibers, the catalyst ink further contains conductive fibers.

[0059] The solvent functions as a dispersion medium for the catalyst ink. There are no particular limitations on the solvent, as long as it does not corrode the catalyst-supported particles, polymer electrolyte, or conductive fibers, and can dissolve the polymer electrolyte in a highly fluid state or disperse it as a fine gel. The solvent preferably contains a volatile organic solvent.

[0060] Examples of the solvent include alcohols, ketone solvents, ether solvents, and other polar solvents. Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, and pentanol. Examples of ketone solvents include acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, and diisobutyl ketone. Examples of ether solvents include tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, and dibutyl ether. Examples of other polar solvents include dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, and 1-methoxy-2-propanol.

[0061] The solvent may be a mixed solvent containing two or more of the above-mentioned materials. The solvent may also contain water, which has a high affinity for the polymer electrolyte. There are no particular restrictions on the amount of water added, as long as it does not cause cloudiness due to separation of the polymer electrolyte or gelation of the polymer electrolyte. When the solvent contains a lower alcohol, it is preferable to use a mixed solvent containing water to reduce the risk of fire.

[0062] The catalyst ink may contain a dispersant to improve the dispersibility of the catalyst-supported particles. Examples of dispersants include anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. If necessary, the catalyst ink may be subjected to a dispersion treatment. Examples of dispersion treatments include stirring with a ball mill or roll mill, stirring with a shear mill, stirring with a wet mill, stirring by applying ultrasonic waves, and stirring with a homogenizer. The viscosity of the catalyst ink and the size of the particles contained in the catalyst ink can be controlled by adjusting the conditions of the dispersion treatment.

[0063] The solid content of the catalyst ink is preferably 1% by mass or more and 50% by mass or less. If the solid content is 1% by mass or more, a decrease in the film formation rate is suppressed, and therefore a decrease in productivity is suppressed. If the solid content is 50% by mass or less, the viscosity of the catalyst ink is prevented from becoming excessively high, and therefore the occurrence of cracks in the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C can be suppressed.

[0064] The solid content of the catalyst ink can include polymer electrolytes, catalyst-supported particles, and conductive fibers. Even with the same solid content, the viscosity of the catalyst ink will be higher if the content of catalyst-supported particles is higher. The viscosity of the catalyst ink can also be adjusted by adding a dispersant. The proportion of catalyst-supported particles in the solid content is preferably 10% by mass or more and 80% by mass or less. The viscosity of the catalyst ink is preferably 0.1 mPa·s or more and 500 mPa·s or less, and more preferably 5 mPa·s or more and 100 mPa·s or less.

[0065] In general, the greater the content of polymer electrolyte in the catalyst ink, the smaller the volume of the pores formed in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C. The greater the content of catalyst-supported particles in the catalyst ink, the larger the volume of the pores formed in the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C. Furthermore, the inclusion of a dispersant in the catalyst ink reduces the volume of the pores. When the catalyst ink is subjected to a dispersion treatment, the longer the treatment time, the more the aggregates of the catalyst-supported particles are broken down, resulting in a smaller volume of the pores.

[0066] The catalyst ink may contain a pore-forming agent. After the anode-side electrode catalyst layer 12A and the cathode-side electrode catalyst layer 12C are formed, the pores can be formed by removing the pore-forming agent. Examples of pore-forming agents that can be used include substances that dissolve in acid, alkali, or water, sublimating substances such as camphor, and thermally decomposing substances. If the pore-forming agent is a substance that dissolves in hot water, the pore-forming agent may be removed by the water generated during power generation.

[0067] Examples of pore-forming agents that are soluble in acid, alkali, or water include acid-soluble inorganic salts, inorganic salts soluble in aqueous alkali solutions, metals soluble in acid or alkali, water-soluble inorganic salts, and water-soluble organic compounds. Examples of acid-soluble inorganic salts include calcium carbonate, barium carbonate, magnesium carbonate, magnesium sulfate, and magnesium oxide. Examples of inorganic salts soluble in aqueous alkali solutions include alumina, silica gel, and silica sol. Examples of metals soluble in acid or alkali include aluminum, zinc, tin, nickel, and iron. Examples of water-soluble inorganic salts include sodium chloride, potassium chloride, ammonium chloride, sodium carbonate, sodium sulfate, and monosodium phosphate. Examples of water-soluble organic compounds include polyvinyl alcohol and polyethylene glycol. One type of pore-forming agent may be used alone, but it is preferable to use two or more types in combination.

[0068] The catalyst ink is applied to a substrate to form a coating film, and the coating film is dried to form the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C. Examples of methods that can be used to apply the catalyst ink include doctor blade methods, dipping methods, screen printing methods, and roll coating methods.

[0069] The substrate may be, for example, a transfer substrate that is peeled off after transferring the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C to the polymer electrolyte membrane 11. The material of the transfer substrate may be, for example, a fluorine-based resin or an organic polymer compound other than a fluorine-based resin. Examples of fluorine-based resins include ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkylvinyl ether copolymer (PFA), and polytetrafluoroethylene (PTFE). Examples of organic polymer compounds include polyimide, polyethylene terephthalate, polyamide, polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyetherimide, polyarylate, and polyethylene naphthalate.

[0070] The substrate for forming the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C may be the polymer electrolyte membrane 11, or the anode gas diffusion layer 13A and the cathode gas diffusion layer 13C. When the substrate for forming the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C is a transfer substrate, the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C are bonded to the polymer electrolyte membrane 11 by thermocompression bonding, and then the transfer substrate is peeled off from the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C. Then, the anode gas diffusion layer 13A and the cathode gas diffusion layer 13C are laminated on the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C on the polymer electrolyte membrane 11. In this way, the membrane electrode assembly 10 is formed.

[0071] When the anode side gas diffusion layer 13A and the cathode side gas diffusion layer 13C are used as the base materials for forming the anode side electrode catalyst layer 12A and the cathode side electrode catalyst layer 12C, the anode side electrode catalyst layer 12A and the cathode side electrode catalyst layer 12C supported on the anode side gas diffusion layer 13A and the cathode side gas diffusion layer 13C are joined to the polymer electrolyte membrane 11 by thermocompression bonding, thereby forming the membrane electrode assembly 10.

[0072] When the substrate for forming the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C is a polymer electrolyte membrane 11, the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C are formed directly on the surface of the polymer electrolyte membrane 11, and then the anode gas diffusion layer 13A and the cathode gas diffusion layer 13C are laminated on the anode electrode catalyst layer 12A and the cathode electrode catalyst layer 12C, thereby forming the membrane electrode assembly 10.

[0073] When the anode-side gas diffusion layer 13A and the cathode-side gas diffusion layer 13C each include a microporous layer, the microporous layer is formed by applying a coating liquid prepared by dispersing a material for the microporous layer in a dispersion medium to the surface of the porous conductive substrate, and then removing the dispersion medium by drying the coating film.

[0074] The conductive material can be broken down into smaller particles by dispersing the coating liquid using a dispersing kneader such as a pot mill, ball mill, bead mill, disperser, planetary mixer, kneader, or ultrasonic disperser.

[0075] The coating liquid can be applied by known techniques such as an applicator method, a bar coater method, a doctor blade method, a screen printing method, a gravure printing method, a coater coating method, a spray coating method, a transfer method, an inkjet method, etc. Among these, the applicator method, the doctor blade method, and the coater coating method are preferably used because they require only a small load to set conditions for forming a smooth surface of the microporous layer.

[0076] The coating film is preferably dried slowly to prevent cracks from forming in the microporous layer. For example, it is preferable to use a dispersion medium with a high boiling point to slow down the drying rate, or to dry at a lower temperature for a longer period of time.

[0077] [Example] The above-mentioned membrane electrode assembly and polymer electrolyte fuel cell will be described using specific examples and comparative examples.

[0078] Example 1 <Preparation of catalyst ink> A platinum-supported carbon catalyst with a catalytic substance loading density of 50 mass % was used as the catalyst-supported particles, carbon fibers with an average fiber diameter of 150 nm and an average fiber length of 20 μm were used as the conductive fibers, and a fluorine-based polymer electrolyte was used as the polymer electrolyte. The platinum-supported carbon catalyst, the carbon fibers, and the polymer electrolyte solution (Nafion (registered trademark) dispersion liquid, manufactured by Wako Pure Chemical Industries, Ltd.) with a concentration of 25 mass % were mixed in a solvent to prepare a catalyst ink. The catalyst ink was subjected to a dispersion treatment for 30 minutes using a planetary ball mill.

[0079] The solvent used was a mixed solvent of ultrapure water and 1-propanol. The volume ratio of ultrapure water to 1-propanol in the mixed solvent was 1:1. The blending ratio of each material in the catalyst ink was such that the mass of the carrier in the catalyst-supported particles was 1, the mass of the conductive fiber was 0.5, and the mass of the polymer electrolyte was 0.6. The solid content in the catalyst ink was also adjusted to 15% by mass.

[0080] <Formation of electrode catalyst layer> A polytetrafluoroethylene sheet was used as a transfer substrate for forming the electrode catalyst layer, and the catalyst ink was applied to the substrate by the doctor blade method to form a coating film. The coating film was then dried at 80°C in the air to form the electrode catalyst layer. The amount of catalyst ink applied was adjusted to a platinum loading of 0.05 mg / cm when forming the electrode catalyst layer for the fuel electrode. 2 When forming the electrode catalyst layer of the air electrode, the platinum loading was 0.2 mg / cm 2 Each was adjusted so that In Example 1, both the electrode catalyst layer of the fuel electrode and the electrode catalyst layer of the air electrode contain conductive fibers.

[0081] <Fabrication of membrane electrode assemblies and polymer electrolyte fuel cells> The transfer substrate on which the electrode catalyst layer for the fuel electrode was formed and the transfer substrate on which the electrode catalyst layer for the air electrode was formed were each punched out into a square shape with a side length of 5 cm. The electrode catalyst layer for the fuel electrode was then transferred from the transfer substrate to one side of the polymer electrolyte membrane, and the electrode catalyst layer for the air electrode was transferred from the transfer substrate to the other side of the polymer electrolyte membrane. The electrode catalyst layers were transferred by hot pressing at a temperature of 120°C and a pressure of 5.0 x 106 Pa. A fluorine-based polymer electrolyte (Nafion (registered trademark) 211, manufactured by DuPont) was used as the polymer electrolyte membrane.

[0082] A laminate of a polymer electrolyte membrane and two electrode catalyst layers was sandwiched between a pair of gas diffusion layers to form a membrane electrode assembly of Example 1, and the membrane electrode assembly was sandwiched between separators to form a solid polymer fuel cell of Example 1.

[0083] A pair of gas diffusion layers was made of a carbon fiber sheet (manufactured by SGL) punched into a square shape with a side length of 5 cm. The carbon fiber sheet of Example 1 was carbon paper with a microporous layer formed thereon. The mean flow pore diameter of the carbon fiber sheet was measured by the method described below and was found to be 9.9 μm.

[0084] Example 2 Except for changing the pair of gas diffusion layers to carbon fiber sheets (manufactured by Freudenberg) which are nonwoven fabrics with microporous layers formed thereon, a membrane electrode assembly and a fuel cell of Example 2 were obtained using the same materials and processes as in Example 1. The mean flow pore diameter of the gas diffusion layers of Example 2 was 2.1 μm.

[0085] Example 3 Except for changing the pair of gas diffusion layers to carbon fiber sheets (manufactured by Freudenberg) which are nonwoven fabrics with microporous layers formed thereon, a membrane electrode assembly and a fuel cell of Example 3 were obtained using the same materials and processes as in Example 1. The mean flow pore diameter of the gas diffusion layers of Example 3 was 6.5 μm.

[0086] Example 4 Except for changing the pair of gas diffusion layers to carbon fiber sheets (manufactured by Freudenberg) which are nonwoven fabrics with microporous layers formed thereon, a membrane electrode assembly and a fuel cell of Example 4 were obtained using the same materials and processes as in Example 1. The mean flow pore diameter of the gas diffusion layers of Example 4 was 3.9 μm.

[0087] Example 5 Except for changing the pair of gas diffusion layers to carbon fiber sheets (manufactured by Freudenberg) which are nonwoven fabrics with microporous layers formed thereon, a membrane electrode assembly and a fuel cell of Example 5 were obtained using the same materials and processes as in Example 1. The mean flow pore diameter of the gas diffusion layers of Example 5 was 0.3 μm.

[0088] (Comparative Example 1) Except for changing the pair of gas diffusion layers to a carbon fiber sheet (manufactured by SGL) which is a nonwoven fabric with a microporous layer formed thereon, a membrane electrode assembly and a fuel cell of Comparative Example 1 were obtained using the same materials and processes as in Example 1. The mean flow pore diameter of the gas diffusion layer of Comparative Example 1 was 15 μm.

[0089] (Comparative Example 2) A membrane electrode assembly and a fuel cell of Comparative Example 2 were obtained using the same materials and processes as in Example 1, except that in the catalyst ink, spherical carbon particles with an average particle size of 2 nm were added as the conductive material instead of the conductive fibers, and the pair of gas diffusion layers was changed to carbon fiber sheets (manufactured by Freudenberg), which are carbon paper with a microporous layer formed thereon. The mean flow pore diameter of the gas diffusion layer of Comparative Example 3 was 0.3 μm.

[0090] (Comparative Example 3) A membrane electrode assembly and a fuel cell of Comparative Example 3 were obtained using the same materials and processes as in Example 1, except that in the catalyst ink, spherical carbon particles with an average particle size of 2 nm were added as the conductive material instead of the conductive fibers, and the pair of gas diffusion layers was changed to a carbon fiber sheet (manufactured by SGL), which is carbon paper with a microporous layer formed thereon. The mean flow pore diameter of the gas diffusion layer of Comparative Example 3 was 9.9 μm.

[0091] <Mean flow pore size measurement> The mean flow pore diameter of the gas diffusion layer in each example and comparative example was measured in accordance with JIS K 3832-1990. Specifically, a 25 mm diameter disk test piece was first prepared for the gas diffusion layer, and this test piece was thoroughly wetted with a test liquid having a surface tension of 15.7 dyn / cm. The test piece was placed in a measuring device, and air pressure was applied, gradually increasing from 0 to 1000 kPa. A flow rate curve was obtained by plotting the pressure on the x-axis and the flow rate on the x-axis. The pore diameter D can be calculated from the air pressure P using the bubble point equation 1 shown below. D=Cγ / P (Equation 1) where C is the pressure constant and γ is the surface tension of the liquid.

[0092] <Evaluation method> The fuel cells of each example and comparative example were placed in a power generation evaluation cell, and current and voltage measurements were performed using a fuel cell measurement device. The cell temperature during measurement was 80°C, and the relative humidity was 90% RH at the fuel electrode and 30% RH at the air electrode. Hydrogen was flowed as the fuel gas at a flow rate that resulted in a hydrogen utilization rate of 90%, and air was flowed as the oxidant gas at a flow rate that resulted in an oxygen utilization rate of 40%. The back pressure was 50 kPa, and the current density was 2.0 A / cm. 2 The voltage at this time was measured.

[0093] <Evaluation results> The conductive material contained in the anode electrode catalyst layer, the mean flow pore size of the anode-side gas diffusion layer, and the voltage measurement results for each example and comparative example are shown in Table 1. The voltage measurement results for Example 1 are set as reference value A, and the values for the other examples and comparative examples are expressed as the difference from reference value A.

[0094] [Table 1]

[0095] As shown in Table 1, in Examples 1 to 5 in which the electrode catalyst layer contains conductive fibers and the gas diffusion layer has a mean flow pore diameter of 0.3 to 10 μm, a higher output voltage was obtained compared to Comparative Examples 1 to 3 in which the gas diffusion layer has a mean flow pore diameter of 15 μm or more or the electrode catalyst layer contains conductive particles instead of conductive fibers.

[0096] As described above using the examples, the membrane electrode assembly and polymer electrolyte fuel cell of the above embodiment provide the following effects. (1) The anode-side electrode catalyst layer 12A contains conductive fibers, and the anode-side gas diffusion layer 13A has a mean flow pore diameter of 0.3 μm or more and 10 μm or less. This configuration improves the gas diffusibility of the anode-side electrode catalyst layer 12A, thereby improving the power generation performance of the fuel cell even when the gas diffusion layer has a mean flow pore diameter in a wide range, from 0.3 μm or more to 10 μm or less, and allowing the anode-side gas diffusion layer 13A to have a mean flow pore diameter in a wider range than when the electrode catalyst layer does not contain conductive fibers.

[0097] (2) When the conductive fibers are carbon fibers, the gas diffusibility of the anode-side electrode catalyst layer 12A is more suitably improved. (3) The anode gas diffusion layer 13A has a mean flow pore diameter of 0.3 μm or more and 10 μm or less. This configuration favorably improves the power generation performance of the fuel cell. [Explanation of symbols]

[0098] 10...membrane electrode assembly, 11...polymer electrolyte membrane, 12A...anode side electrode catalyst layer, 12C...cathode side electrode catalyst layer, 13A...anode side gas diffusion layer, 13C...cathode side gas diffusion layer, 20...conductive fiber, 30...solid polymer fuel cell, 31A, 31C...separator, 32A, 32C...gas flow path, 33A, 33C...cooling water flow path.

Claims

1. A membrane electrode assembly comprising, in this order, an anode-side gas diffusion layer, an anode-side electrode catalyst layer, a polymer electrolyte membrane, a cathode-side electrode catalyst layer, and a cathode-side gas diffusion layer, the anode-side gas diffusion layer includes catalyst-supporting particles, a polymer electrolyte, and conductive fibers; a mean flow pore diameter of the anode-side gas diffusion layer of 0.3 to 10 μm;

2. 2. The membrane electrode assembly according to claim 1, wherein the conductive fibers of the anode side gas diffusion layer are carbon fibers.

3. the cathode-side gas diffusion layer includes catalyst-supporting particles, a polymer electrolyte, and conductive fibers; 3. The membrane electrode assembly according to claim 1, wherein the cathode side gas diffusion layer has a mean flow pore diameter of 0.3 to 10 μm.

4. 4. The membrane electrode assembly according to claim 3, wherein the conductive fibers of the cathode gas diffusion layer are carbon fibers.

5. 3. The membrane electrode assembly according to claim 1, wherein the anode-side gas diffusion layer has a mean flow pore diameter of 2 to 10 μm.

6. A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 1 or 2 and a pair of separators sandwiching the membrane electrode assembly.

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