Membrane electrode assembly and polymer electrolyte fuel cell

Abstract

To provide a membrane electrode assembly and a polymer electrolyte fuel cell that are capable of suppressing the occurrence of cracks in an electrode catalyst layer and stably obtaining high power generation performance.SOLUTION: A membrane electrode assembly 10 includes a polymer electrolyte membrane 11, a pair of electrode catalyst layers 12A and 12C in contact with the surface of the polymer electrolyte membrane 11 so as to sandwich the polymer electrolyte membrane 11, and gas diffusion layers 13A and 13C laminated on each of the pair of electrode catalyst layers 12A and 12C. The electrode catalyst layers 12A and 12C each include a catalytic substance, a carbon particle, a polymer electrolyte aggregate, and a fibrous substance, and the gas diffusion layers 13A and 13C have a Gurley value of 1.0 seconds or more and 3.0 seconds or less, which indicates the air permeability resistance in the thickness direction.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. A fuel gas containing hydrogen is supplied to the fuel electrode, and an oxidant gas containing oxygen is supplied to the air electrode. As a result, electrode reactions occur at the fuel electrode and the air electrode, as shown in the following (Equation 1) and (Equation 2), generating electricity. Fuel electrode: H2→ 2H + + 2e - ...(Formula 1) Air electrode: 1 / 2O2+ 2H + + 2e - → H2O (Eq. 2)

[0004] That is, as shown in Equation 1, protons and electrons are generated from the fuel gas supplied to the fuel electrode due to the action of the 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, as shown in Equation 2, the oxidant gas reacts with the protons and electrons that have moved from the fuel electrode to produce water. In this way, current is generated as the electrons pass through the external circuit (see, for example, Patent Document 1).

[0005] 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.

[0006] On the other hand, the gas diffusion layer is involved in supplying gas to the electrode catalyst layer, collecting electrons generated in the electrode reaction, and managing the amount of water in the cell by discharging 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 solid polymer fuel cells. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-115299 Summary of the Invention [Problem to be solved by the invention]

[0008] However, cracks may occur in the electrode catalyst layer due to shrinkage caused by drying during the formation of the electrode catalyst layer. When a crack exists in the electrode catalyst layer, the polymer electrolyte membrane is exposed at the cracked portion in the membrane electrode assembly. Such exposure of the polymer electrolyte membrane leads to a decrease in the durability of the membrane electrode assembly and a decrease in the power generation performance of the solid polymer fuel cell.

[0009] The electrical resistance of the gas diffusion layer is also one of the factors that affect power generation performance. The electrical resistance of the gas diffusion layer varies depending on the material, thickness, density, etc. of the gas diffusion layer. In particular, the thickness of the gas diffusion layer may vary within the gas diffusion layer or may fluctuate depending on the water retention state of the membrane electrode assembly during fuel cell operation. Therefore, if the electrical resistance of the gas diffusion layer has a significant impact on power generation performance, it becomes difficult to achieve stable power generation performance. [Means for solving the problem]

[0010] A membrane electrode assembly for solving the above problems comprises a polymer electrolyte membrane, a pair of electrode catalyst layers in contact with surfaces of the polymer electrolyte membrane with the polymer electrolyte membrane sandwiched therebetween, and a gas diffusion layer laminated on each of the pair of electrode catalyst layers, wherein the electrode catalyst layers contain a catalyst substance, carbon particles, aggregates of polymer electrolyte, and a fibrous material, and the Gurley value, which indicates the air permeation resistance in the thickness direction of the gas diffusion layer, is 1.0 seconds or more and 3.0 seconds or less.

[0011] According to the above configuration, since the electrode catalyst layer contains a fibrous material, the strength of the electrode catalyst layer is increased and the occurrence of cracks is suppressed. Furthermore, since the gas diffusion layer has sufficiently high air permeability and sufficient gas diffusion and drainage properties are obtained, the influence of the electrical resistance of the gas diffusion layer on the power generation performance of the fuel cell is minimized. Therefore, stable power generation performance can be achieved regardless of uneven electrical resistance.

[0012] In the above configuration, the fibrous material may include at least one of a polymer electrolyte fiber having proton conductivity and a carbon fiber. According to the above configuration, the fibrous material contains polymer electrolyte fibers, thereby improving the proton conductivity in the electrode catalyst layer, and the fibrous material contains carbon fibers, thereby improving the electron conductivity in the electrode catalyst layer.

[0013] In the above configuration, the carbon material constituting the carbon fiber may be any one of carbon fiber, carbon nanofiber, and carbon nanotube. According to the above configuration, the electronic conductivity of the electrode catalyst layer is suitably increased.

[0014] In the above configuration, the fibrous material may have an average fiber diameter of 0.1 μm or less, and an average fiber length of 1 μm or more and 200 μm or less. According to the above configuration, the fibrous material has a fineness suitable for inclusion in the electrode catalyst layer, and the fibrous material is suitably entangled in the electrode catalyst layer, thereby increasing the strength of the electrode catalyst layer and enhancing the effect of suppressing cracking.

[0015] In the above configuration, the fibrous material may include a polymer electrolyte fiber having proton conductivity, and the mass of the polymer electrolyte fiber contained in the electrode catalyst layer may be 0.01 to 0.3 times the mass of the carbon particles contained in the electrode catalyst layer.

[0016] According to the above configuration, the conduction of protons in the electrode catalyst layer is promoted, and therefore the output of the polymer electrolyte fuel cell can be improved.

[0017] In the above configuration, the catalyst substance may be in the form of particles, and the average particle size may be 0.5 nm or more and 20 nm or less. According to the above configuration, the stability and activity of the catalyst are improved.

[0018] In the above configuration, the carbon particles may have an average particle size of 10 nm or more and 1000 nm or less. This configuration facilitates the formation of an electron conduction path in the electrode catalyst layer. In addition, the electrode catalyst layer can be formed thin enough to prevent excessive resistance, thereby preventing a decrease in the output of the fuel cell.

[0019] A polymer electrolyte fuel cell for solving the above problems includes the above membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly. According to the above configuration, stable and high power generation performance can be obtained. [Effects of the Invention]

[0020] According to the present invention, it is possible to suppress the occurrence of cracks in the electrode catalyst layer, and to obtain stable, high power generation performance. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 2 is a diagram showing a cross-sectional structure of a membrane electrode assembly according to one embodiment of the membrane electrode assembly. [Figure 2] FIG. 2 is a diagram schematically illustrating a first configuration of an electrode catalyst layer according to an embodiment. [Figure 3] FIG. 4 is a diagram schematically illustrating a second configuration of an electrode catalyst layer according to an embodiment. [Figure 4] FIG. 1 is an exploded perspective view showing a polymer electrolyte fuel cell according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0022] An embodiment of a membrane electrode assembly and a polymer electrolyte fuel cell will be described with reference to FIGS. [Membrane electrode assembly] The structure of the membrane electrode assembly will be described with reference to FIGS.

[0023] 1, the membrane electrode assembly 10 includes a polymer electrolyte membrane 11, a pair of electrode catalyst layers, and a pair of gas diffusion layers. The pair of electrode catalyst layers is an anode catalyst layer 12A and an air cathode catalyst layer 12C. The pair of gas diffusion layers is an anode diffusion layer 13A and an air cathode diffusion layer 13C.

[0024] Polymer electrolyte membrane 11 is sandwiched between anode catalyst layer 12A and cathode catalyst layer 12C. Anode catalyst layer 12A is in contact with one of the two surfaces of polymer electrolyte membrane 11, and cathode catalyst layer 12C is in contact with the other of the two surfaces of polymer electrolyte membrane 11.

[0025] Anode diffusion layer 13A is laminated on anode catalyst layer 12A, and cathode diffusion layer 13C is laminated on cathode catalyst layer 12C. In other words, the laminate of polymer electrolyte membrane 11, anode catalyst layer 12A, and cathode catalyst layer 12C is sandwiched between anode diffusion layer 13A and cathode diffusion layer 13C.

[0026] Anode catalyst layer 12A and anode diffusion layer 13A constitute the anode of the polymer electrolyte fuel cell, while cathode catalyst layer 12C and anode diffusion layer 13C constitute the cathode of the polymer electrolyte fuel cell.

[0027] When viewed from a position facing one surface of polymer electrolyte membrane 11, anode catalyst layer 12A, cathode catalyst layer 12C, anode diffusion layer 13A, and cathode diffusion layer 13C have substantially the same external shape. Polymer electrolyte membrane 11 is larger than catalyst layers 12A and 12C and diffusion layers 13A and 13C. There are no particular limitations on the external shape of polymer electrolyte membrane 11, catalyst layers 12A and 12C, and diffusion layers 13A and 13C, and each may have, for example, a rectangular shape.

[0028] 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 Aquivion (registered trademark: manufactured by Solvay). Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, sulfonated polyimide, and acid-doped polybenzazoles.

[0029] Figure 2 schematically shows a first configuration of the electrode catalyst layer of this embodiment. As shown in Figure 2, each of anode catalyst layer 12A and cathode catalyst layer 12C includes catalyst material 21, carbon particles 22, polymer electrolyte aggregates 23, and polymer electrolyte fibers 24, which are fibrous polymer electrolytes. In catalyst layers 12A and 12C, aggregates 23 and polymer electrolyte fibers 24 are located around dispersed carbon particles 22, and voids H1 are formed between these components.

[0030] Examples of catalyst material 21 include platinum group elements such as platinum, palladium, ruthenium, iridium, rhodium, and osmium, metals such as iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, and alloys, oxides, and composite oxides thereof. It is particularly preferable that catalyst material 21 be platinum or a platinum alloy. The catalyst material 21 is in the form of particles, and its average particle size is preferably 0.5 nm or more and 20 nm or less, and more preferably 1 nm or more and 5 nm or less. If the average particle size of catalyst material 21 is equal to or greater than the lower limit, the catalyst's stability is enhanced. If the average particle size of catalyst material 21 is equal to or less than the upper limit, the catalyst's activity is enhanced.

[0031] The carbon particles 22 may be any carrier that is in the form of fine particles, is electrically conductive, and is not corroded by the catalyst. The carbon material used as the carbon particles 22 is, for example, a powdered carbon material made of carbon black, graphite, activated carbon, carbon nanotubes, fullerene, or the like. The average particle size of the carbon particles 22 is preferably 10 nm or more and 1000 nm or less, and more preferably 10 nm or more and 100 nm or less. If the average particle size of the carbon particles 22 is equal to or greater than the above-mentioned lower limit, an electron conduction path is easily formed in the electrode catalyst layer. If the average particle size of the carbon particles 22 is equal to or less than the above-mentioned upper limit, the electrode catalyst layer can be formed thin enough to prevent excessive resistance, thereby suppressing a decrease in the output of the fuel cell.

[0032] The catalyst substance 21 is preferably supported on carbon particles 22. When the carbon material supporting the catalyst substance 21 is in particulate form, the area of ​​the carbon material that can support the catalyst substance 21 can be increased, and the catalyst substance 21 can be supported on the carbon material at a high density. This makes it possible to improve catalytic activity.

[0033] The polymer electrolyte aggregates 23 are lumps of ionomer polymer electrolytes aggregated by cohesive forces. The cohesive forces include Coulomb forces and van der Waals forces acting between ionomers. The polymer electrolyte fibers 24 are polymer electrolytes that have an elongated shape due to crosslinking or other factors.

[0034] The polymer electrolyte constituting the aggregates 23 and the polymer electrolyte fibers 24 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 Aquivion (registered trademark: manufactured by Solvay). Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, sulfonated polyimide, and acid-doped polybenzazoles.

[0035] The polymer electrolyte constituting the aggregates 23 and the polymer electrolyte fibers 24 may be the same or different from each other. Furthermore, the polymer electrolyte constituting each of the aggregates 23 and the polymer electrolyte fibers 24 may be the same or different from the polymer electrolyte constituting the polymer electrolyte membrane 11. In order to improve the adhesion between the catalyst layers 12A, 12C and the polymer electrolyte membrane 11, the polymer electrolytes contained in the catalyst layers 12A, 12C and the polymer electrolyte membrane 11 are preferably made of the same material.

[0036] By including the polymer electrolyte fibers 24 in the catalyst layers 12A, 12C, the polymer electrolyte fibers 24 are entangled with each other and function as a support in the electrode catalyst layer, thereby suppressing cracking and the like in the catalyst layers 12A, 12C. Therefore, compared to conventional electrode catalyst layers composed of carbon particles 22 supporting a catalyst substance 21 and polymer electrolyte aggregates 23, the occurrence of cracks in the catalyst layers 12A, 12C can be suppressed.

[0037] The average fiber diameter of the polymer electrolyte fibers 24 is preferably 2 μm or less, and more preferably 0.1 μm or less. When the average fiber diameter is within the above range, the polymer electrolyte fibers 24 have a diameter small enough to be contained in the catalyst layers 12A and 12C.

[0038] To improve the output of a polymer electrolyte fuel cell, it is desirable that the gas supplied to the catalyst layers 12A, 12C be properly diffused throughout the catalyst layers 12A, 12C through the pores Hl of the catalyst layers 12A, 12C, and that water produced by the electrode reaction at the air electrode, in particular, be properly discharged through the pores Hl. Furthermore, the presence of the pores Hl facilitates the formation of an interface where the gas, catalyst, and polymer electrolyte come into contact, accelerating the electrode reaction, which also improves the output of the polymer electrolyte fuel cell.

[0039] From the above viewpoints, it is preferable that the catalyst layers 12A, 12C have pores Hl of an appropriate size and quantity. If the average fiber diameter of the polymer electrolyte fibers 24 is 1 μm or less, sufficient gaps are formed in the entangled structure of the polymer electrolyte fibers 24 in the electrode catalyst layer, ensuring sufficient pores Hl, thereby enabling an improvement in fuel cell output. Furthermore, if the average fiber diameter of the polymer electrolyte fibers 24 is 100 nm or more and 500 nm or less, the fuel cell output is particularly enhanced.

[0040] The average fiber length of the polymer electrolyte fibers 24 is greater than the average fiber diameter and is preferably 1 μm or more and 200 μm or less, and more preferably 1 μm or more and 150 μm or less. When the average fiber length is within the above range, aggregation of the polymer electrolyte fibers 24 in the catalyst layers 12A, 12C is suppressed, and voids H1 are easily formed. Furthermore, when the average fiber length is within the above range, an entangled structure of the polymer electrolyte fibers 24 is suitably formed in the catalyst layers 12A, 12C, thereby increasing the strength of the catalyst layers 12A, 12C and improving the effect of suppressing crack generation.

[0041] The total mass of the polymer electrolyte fibers 24 contained in the catalyst layers 12A, 12C is preferably 0.01 to 3.0 times the total mass of the carbon particles 22 contained in the catalyst layers 12A, 12C. If the mass ratio of the carbon particles 22 to the polymer electrolyte fibers 24 is within the above range, proton conduction in the catalyst layers 12A, 12C is promoted, thereby enabling an improvement in the output of the solid polymer fuel cell.

[0042] A second configuration of the electrode catalyst layer of this embodiment is shown in Fig. 3. As shown in Fig. 3, each of anode catalyst layer 12A and cathode catalyst layer 12C may contain carbon fibers 25 in addition to catalyst material 21, carbon particles 22, polymer electrolyte aggregates 23, and polymer electrolyte fibers 24.

[0043] The carbon fiber 25 is a fibrous structure containing carbon as a constituent element. The carbon material used as the carbon fiber 25 is, for example, a fibrous carbon material made of carbon fiber, carbon nanofiber, carbon nanotube, etc. In particular, it is preferable to use carbon nanofiber or carbon nanotube.

[0044] The average fiber diameter of the carbon fibers 25 is preferably 0.1 μm or less. If the average fiber diameter is 0.1 μm or less, the fiber diameter of the carbon fibers 25 becomes thin enough to be contained in the catalyst layers 12A and 12C.

[0045] The average fiber length of the carbon fibers 25 is preferably 1 μm or more and 200 μm or less. When the average fiber length of the carbon fibers 25 is within the above range, a structure in which the polymer electrolyte fibers 24 and the carbon fibers 25 are entangled is suitably formed in the catalyst layers 12A and 12C, thereby increasing the strength of the catalyst layers 12A and 12C and improving the effect of suppressing the occurrence of cracks.

[0046] In the second configuration, the fibrous materials, that is, the polymer electrolyte fibers 24 and the carbon fibers 25, are entangled and function as a support in the catalyst layers 12A, 12C, so that, similarly to the first configuration, the occurrence of cracks in the catalyst layers 12A, 12C can be suppressed. In the second configuration, the carbon fibers 25 may support the catalyst substance 21. Alternatively, the carbon particles 22 and the carbon fibers 25 may each support the catalyst substance 21. However, if the carbon particles 22 support the catalyst substance 21, this is preferable because the gaps formed by the carbon fibers 25 serve as drainage paths for water produced by power generation, improving the drainage properties of the catalyst layers 12A, 12C.

[0047] The catalyst layers 12A and 12C may contain only carbon fibers 25 as the fibrous material, without including polymer electrolyte fibers 24. Even in this configuration, the formation of an entangled structure of carbon fibers 25 makes it possible to suppress the occurrence of cracks. However, since carbon fibers 25 contribute only to electron conduction and not to proton conduction, if the fibrous material contained in the catalyst layers 12A and 12C is only carbon fibers 25, the proportion of polymer electrolyte contained in the catalyst layers 12A and 12C decreases, resulting in reduced proton conductivity in the catalyst layers 12A and 12C. While it is possible to compensate for proton conductivity by increasing the amount of polymer electrolyte aggregates 23, increasing the amount of aggregates 23 reduces pores Hl, thereby reducing gas diffusibility, drainage, and other properties.

[0048] In contrast, if the fibrous material contained in the catalyst layers 12A and 12C includes polymer electrolyte fibers 24, proton conduction in the catalyst layers 12A and 12C can be promoted while ensuring pores Hl, compared to a case in which the fibrous material is only carbon fibers 25.

[0049] Furthermore, in order to extract the electrons generated in the electrode reaction, the catalyst layers 12A and 12C also need to have electron conductivity. If the catalyst layers 12A and 12C contain polymer electrolyte fibers 24 and carbon fibers 25 as fibrous materials, the proton conductivity, electron conductivity, and the state of formation of pores Hl in the catalyst layers 12A and 12C will be favorable, thereby enabling an improvement in the output of the polymer electrolyte fuel cell.

[0050] The average fiber diameter and average fiber length of the polymer electrolyte fibers 24 and carbon fibers 25 can be measured, for example, by observing the cross section of the electrode catalyst layer using a scanning electron microscope. For example, the average fiber diameter is the average value of the maximum diameters of each fiber contained in three or more measurement regions of 30 μm × 30 μm size on the cross section. For example, the average fiber length is the average value of the maximum lengths of each fiber contained in three or more measurement regions of 30 μm × 30 μm size on the cross section.

[0051] The diffusion layers 13A and 13C may be made of, for example, carbon cloth, carbon paper, or nonwoven fabric. The diffusion layers 13A and 13C may be treated with a water-repellent coating. The Gurley value, which indicates the air resistance in the thickness direction of the diffusion layers 13A and 13C, is 1.0 seconds or more and 3.0 seconds or less. It is particularly preferable that the Gurley value of the diffusion layers 13A and 13C is 1.0 seconds. The Gurley value is the time required for a specified volume of air to permeate per unit area and unit pressure difference, and is a parameter expressed in hours per 100 mL. It is measured according to the measurement method specified in JIS P8117:2009.

[0052] When the Gurley value of the diffusion layers 13A, 13C is 1.0 seconds or more and 3.0 seconds or less, the gas diffusion and water discharge properties of the diffusion layers 13A, 13C are adequately achieved. As a result, the impact of the electrical resistance of the diffusion layers 13A, 13C on the power generation performance of the polymer electrolyte fuel cell is reduced. Therefore, even if the electrical resistance of the diffusion layers 13A, 13C varies due to, for example, variations in the thickness of the diffusion layers 13A, 13C or changes in the thickness of the diffusion layers 13A, 13C during fuel cell operation, the impact of such variations in electrical resistance on the power generation performance can be minimized. This improves the stability of the power generation performance of the polymer electrolyte fuel cell.

[0053] The diffusion layers 13A and 13C may be composed of a single layer or multiple layers. For example, the diffusion layers 13A and 13C may be a laminate of a carbon substrate such as carbon cloth or carbon paper and a microporous layer, which is a layer made of a porous material. The material of the microporous layer is, for example, a mixture of a fluororesin such as polytetrafluoroethylene (PTFE), perfluoroethylene-propene copolymer (FEP), or tetrafluoroethylene-ethylene copolymer (ETFE) with a carbon material such as carbon black particles, carbon fiber, or graphite. The Gurley values ​​of the diffusion layers 13A and 13C can be adjusted by the material and thickness of the diffusion layers 13A and 13C, the proportion of voids contained in the diffusion layers 13A and 13C, and the like.

[0054] [Polymer electrolyte fuel cell] The configuration of a polymer electrolyte fuel cell including the above-described membrane electrode assembly 10 will be described with reference to FIG.

[0055] 4, the polymer electrolyte fuel cell 30 includes a membrane electrode assembly 10 and a pair of separators 31A and 31C. The polymer electrolyte fuel cell 30 may further include a pair of gaskets 34A and 34C.

[0056] The membrane electrode assembly 10 is sandwiched between separators 31A and 31C. Separators 31A and 31C are made of a conductive and gas-impermeable material. Separator 31A faces anode diffusion layer 13A, and separator 31C faces cathode diffusion layer 13C. A gas flow path 32A is formed on the surface of separator 31A facing anode diffusion layer 13A, and a cooling water flow path 33A is formed on the surface opposite anode diffusion layer 13A. Similarly, a gas flow path 32C is formed on the surface of separator 31C facing cathode diffusion layer 13C, and a cooling water flow path 33C is formed on the surface opposite cathode diffusion layer 13C.

[0057] Gasket 34A surrounds the outer peripheries of anode catalyst layer 12A and anode diffusion layer 13A between polymer electrolyte membrane 11 and separator 31A. Gasket 34C surrounds the outer peripheries of cathode catalyst layer 12C and cathode diffusion layer 13C between polymer electrolyte membrane 11 and separator 31C. Gaskets 34A and 34C function to prevent gas supplied to catalyst layers 12A and 12C and diffusion layers 13A and 13C from leaking outside solid polymer fuel cell 30. Gaskets 34A and 34C may be one of the components of membrane electrode assembly 10.

[0058] 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.

[0059] The polymer electrolyte fuel cell 30 may be used in the form of a single cell as shown in FIG. 4, or multiple polymer electrolyte fuel cells 30 may be stacked and connected in series to be used as a single fuel cell.

[0060] [Method for manufacturing membrane electrode assembly] A method for manufacturing the membrane electrode assembly 10 will now be described. The catalyst layers 12A and 12C are formed by applying a catalyst layer slurry containing the materials for the catalyst layers 12A and 12C to a substrate to form a coating film, and then drying the coating film.

[0061] The catalyst layer slurry is produced by adding a powdered polymer electrolyte or an electrolyte solution in which the powdered polymer electrolyte is dissolved or dispersed, a catalyst material 21, carbon particles 22, and polymer electrolyte fibers 24 to a solvent and mixing them. The polymer electrolyte fibers 24 are formed, for example, by using an electrospinning method. When forming an electrode catalyst layer of the second configuration, carbon fibers 25 are further added to the catalyst layer slurry.

[0062] The solvent for the catalyst layer slurry is not particularly limited, and examples of the solvent include water, alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, and tert-butyl alcohol, ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, methyl butyl ketone, methyl isobutyl ketone, methyl amyl ketone, pentanone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, diethyl ketone, dipropyl ketone, and diisobutyl ketone, tetrahydrofuran, tetrahydropyran, dioxane, and diethylene glycol dimethyl ether. Examples of the solvent include ethers such as ether, anisole, methoxytoluene, diethyl ether, dipropyl ether, and dibutyl ether, amines such as isopropylamine, butylamine, isobutylamine, cyclohexylamine, diethylamine, and aniline, esters such as propyl formate, isobutyl formate, amyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, isopentyl acetate, methyl propionate, ethyl propionate, and butyl propionate, acetic acid, propionic acid, dimethylformamide, dimethylacetamide, and N-methylpyrrolidone. Examples of glycol and glycol ether solvents include ethylene glycol, diethylene glycol, propylene glycol, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diacetone alcohol, 1-methoxy-2-propanol, and 1-ethoxy-2-propanol.

[0063] The substrate for forming the catalyst layers 12A, 12C may be, for example, a transfer substrate that is peeled off after transferring the catalyst layers 12A, 12C to the polymer electrolyte membrane 11. The transfer substrate may be, for example, a resin film. The substrate for forming the catalyst layers 12A, 12C may be the polymer electrolyte membrane 11 or the diffusion layers 13A, 13C.

[0064] The method for applying the catalyst layer slurry to the substrate is not particularly limited, and examples of the application method include a doctor blade method, a die coating method, a dipping method, a screen printing method, a laminator roll coating method, and a spray method.

[0065] The coating film, which is the catalyst layer slurry applied to the substrate, can be dried by, for example, hot air drying, IR drying, etc. The drying temperature is preferably about 40° C. to 200° C., and more preferably about 40° C. to 120° C. The drying time is preferably about 0.5 minutes to 1 hour, and more preferably about 1 minute to 30 minutes.

[0066] When the substrate for forming the catalyst layers 12A and 12C is a transfer substrate, the catalyst layers 12A and 12C are bonded to the polymer electrolyte membrane 11 by thermocompression bonding, and then the transfer substrate is peeled off from the catalyst layers 12A and 12C. Then, the diffusion layers 13A and 13C are pressure-bonded to the catalyst layers 12A and 12C on the polymer electrolyte membrane 11. In this way, the membrane electrode assembly 10 is formed.

[0067] When the substrates for forming the catalyst layers 12A and 12C are the diffusion layers 13A and 13C, the catalyst layers 12A and 12C supported by the diffusion layers 13A and 13C are joined to the polymer electrolyte membrane 11 by thermocompression bonding, thereby forming the membrane electrode assembly 10.

[0068] When the substrate for forming the catalyst layers 12A and 12C is the polymer electrolyte membrane 11, the catalyst layers 12A and 12C are formed directly on the surfaces of the polymer electrolyte membrane 11, and then the diffusion layers 13A and 13C are pressure-bonded to the catalyst layers 12A and 12C, thereby forming the membrane electrode assembly 10.

[0069] If the manufacturing method uses the polymer electrolyte membrane 11 as the substrate for forming the catalyst layers 12A and 12C, high adhesion can be obtained between the polymer electrolyte membrane 11 and the catalyst layers 12A and 12C. Furthermore, since pressure is not required to bond the catalyst layers 12A and 12C, crushing of the catalyst layers 12A and 12C is also suppressed. Therefore, it is preferable to use the polymer electrolyte membrane 11 as the substrate for forming the catalyst layers 12A and 12C.

[0070] Here, because the polymer electrolyte membrane 11 has large swelling and shrinkage properties, when the polymer electrolyte membrane 11 is used as a substrate, the volume change of the substrate during the drying process of the coating film that becomes the catalyst layers 12A and 12C is larger than when a transfer substrate or diffusion layers 13A and 13C are used as the substrate. Therefore, when the electrode catalyst layer does not contain a fibrous material, as in the conventional case, cracks are likely to occur in the electrode catalyst layer. In contrast, the catalyst layers 12A and 12C of this embodiment contain a fibrous material, which suppresses the occurrence of cracks, and therefore are suitable for use in a manufacturing method that uses the polymer electrolyte membrane 11 as a substrate for forming the catalyst layers 12A and 12C. The polymer electrolyte fuel cell 30 is manufactured by assembling separators 31A and 31C to the membrane electrode assembly 10, and further providing a gas supply mechanism and the like.

[0071] [Example] The above-mentioned membrane electrode assembly and polymer electrolyte fuel cell will be described using specific examples and comparative examples.

[0072] Example 1 Platinum-supported carbon (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was used as the carbon particles supporting the catalyst material. 20 g of platinum-supported carbon was added to water and mixed, followed by addition of polymer electrolyte fibers (acid-doped polybenzoazoles), a polymer electrolyte dispersion (Nafion dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and 1-propanol, followed by stirring to prepare a slurry for the catalyst layer. The average fiber diameter of the polymer electrolyte fibers was 400 nm, and the average fiber length was 30 μm. The average fiber diameter was rounded to the nearest tenth, and the average fiber length was rounded to the nearest tenth.

[0073] In the catalyst layer slurry, the mass of the polymer electrolyte was adjusted to 100% by mass relative to the mass of the carbon particles, the mass of the polymer electrolyte fibers was adjusted to 10% by mass relative to the mass of the carbon particles, the proportion of water in the dispersion medium was adjusted to 50% by mass, and the solid content concentration was adjusted to 10% by mass.

[0074] The catalyst layer slurry was applied to a polymer electrolyte membrane (Nafion 212: manufactured by DuPont) using a die coating method, and then dried in an oven at 80°C to form a laminate consisting of a pair of electrode catalyst layers and a polymer electrolyte membrane. This laminate was sandwiched between two gas diffusion layers to obtain the membrane electrode assembly of Example 1. A laminate of a carbon substrate and a microporous layer was used as the gas diffusion layer. The gas diffusion layer had a Gurley value of 1.0 seconds and an electrical resistance of 8.0 mΩ cm. 2 is. Furthermore, a polymer electrolyte fuel cell of Example 1 was fabricated using the membrane electrode assembly and a JARI standard cell.

[0075] Example 2 Except for changing the gas diffusion layer, a membrane electrode assembly and a polymer electrolyte fuel cell of Example 2 were obtained using the same materials and processes as in Example 1. The gas diffusion layer of Example 2 had a Gurley value of 1.0 seconds and an electrical resistance of 11.5 mΩ cm 2 is. Example 3 Except for changing the gas diffusion layer, a membrane electrode assembly and a polymer electrolyte fuel cell of Example 3 were obtained using the same materials and processes as in Example 1. The gas diffusion layer of Example 3 had a Gurley value of 3.0 seconds and an electrical resistance of 9.8 mΩ cm 2 is.

[0076] (Comparative Example 1) Except for changing the gas diffusion layer, a membrane electrode assembly and a polymer electrolyte fuel cell of Comparative Example 1 were obtained using the same materials and processes as in Example 1. The gas diffusion layer of Comparative Example 1 had a Gurley value of 4.5 seconds and an electrical resistance of 11.0 mΩ cm 2 is.

[0077] (Comparative Example 2) Except for changing the gas diffusion layer, a membrane electrode assembly and a polymer electrolyte fuel cell of Comparative Example 2 were obtained using the same materials and processes as in Example 1. The gas diffusion layer of Comparative Example 2 had a Gurley value of 5.0 seconds and an electrical resistance of 8.0 mΩ cm 2 is.

[0078] (Comparative Example 3) Except for changing the gas diffusion layer, a membrane electrode assembly and a polymer electrolyte fuel cell of Comparative Example 3 were obtained using the same materials and steps as in Example 1. The gas diffusion layer of Comparative Example 3 had a Gurley value of 5.0 seconds and an electrical resistance of 15.0 mΩ cm 2 is.

[0079] Comparative Example 4 Except for changing the gas diffusion layer, a membrane electrode assembly and a polymer electrolyte fuel cell of Comparative Example 4 were obtained using the same materials and steps as in Example 1. The gas diffusion layer of Comparative Example 4 had a Gurley value of 17.0 seconds and an electrical resistance of 10.4 mΩ cm 2 is.

[0080] (Comparative Example 5) A membrane electrode assembly and a polymer electrolyte fuel cell of Comparative Example 5 were obtained using the same materials and steps as in Example 1, except that polymer electrolyte fibers were not added to the catalyst layer slurry.

[0081] (Evaluation method) <Crack> For each example and comparative example, the surface of the electrode catalyst layer was observed under a microscope (magnification: 200x) to check for cracking. Cases where cracks of 10 μm or more in length were observed were marked with ×, and cases where cracks of 10 μm or more in length were not observed were marked with ○.

[0082] <Power generation performance> In accordance with the "Cell Evaluation and Analysis Protocol" published by the New Energy and Industrial Technology Development Organization (NEDO), a JARI standard cell was used as the evaluation unit cell, in which gaskets and separators were placed on both sides of the membrane electrode assembly and clamped to apply a specified pressure. Then, IV measurements described in the "Cell Evaluation and Analysis Protocol" were carried out under standard conditions to determine the maximum power density.

[0083] (Evaluation results) The evaluation results of the Gurley value, electrical resistance, cracks, and power generation performance of the gas diffusion layer for each example and comparative example are shown in Table 1. Note that for comparative example 5, cracks were found in the electrode catalyst layer, so the power generation performance was not evaluated.

[0084] [Table 1]

[0085] As shown in Table 1, in Examples 1 to 3 and Comparative Examples 1 to 4, in which the electrode catalyst layer contained polymer electrolyte fibers, the occurrence of cracks was suppressed, whereas the occurrence of cracks was confirmed in Comparative Example 5, in which the electrode catalyst layer did not contain a fibrous material. Therefore, it was confirmed that the inclusion of a fibrous material in the electrode catalyst layer makes it possible to suppress the occurrence of cracks.

[0086] Furthermore, when Example 1 and Example 2, which have a Gurley value of 1.0 seconds, are compared, equivalent power generation performance is obtained despite the difference in electrical resistance. Furthermore, Example 3, which has a Gurley value of 3.0 seconds, also obtains power generation performance equivalent to Examples 1 and 2. On the other hand, when Comparative Example 2 and Comparative Example 3, which have a Gurley value of 5.0 seconds, are compared, Comparative Example 3, which has a higher electrical resistance, has a lower maximum power density, i.e., lower power generation performance. Furthermore, when Example 2 and Comparative Example 1 are compared, Comparative Example 1, which has a higher Gurley value, has lower power generation performance even though the electrical resistances are approximately the same. Furthermore, Comparative Example 4, which has a significantly higher Gurley value than the others, has a significantly lower power generation performance despite having a lower electrical resistance than Comparative Examples 1 and 3.

[0087] These results suggest that although both the air permeability and electrical resistance of the gas diffusion layer affect power generation performance, the influence of air permeability is greater. Furthermore, it suggests that if the air permeability is sufficiently high, the magnitude of the electrical resistance is less likely to affect power generation performance. In other words, it was shown that if the Gurley value of the gas diffusion layer is between 1.0 seconds and 3.0 seconds, stable high power generation performance can be obtained even if variations or fluctuations in electrical resistance occur.

[0088] As described above using the examples, the membrane electrode assembly and polymer electrolyte fuel cell of the above embodiment provide the following effects. (1) Because the catalyst layers 12A, 12C contain fibrous materials, the strength of the catalyst layers 12A, 12C is increased, reducing the occurrence of cracks. Furthermore, because the Gurley values ​​of the diffusion layers 13A, 13C are between 1.0 and 3.0 seconds, sufficient gas diffusion and drainage are achieved, minimizing the impact of the electrical resistance of the diffusion layers 13A, 13C on the power generation performance of the fuel cell. Therefore, the polymer electrolyte fuel cell 30 achieves stable power generation performance regardless of variations in the electrical resistance of the diffusion layers 13A, 13C.

[0089] (2) The catalyst layers 12A, 12C include proton-conductive polymer electrolyte fibers 24, thereby enhancing the proton conductivity of the catalyst layers 12A, 12C. On the other hand, the catalyst layers 12A, 12C include carbon fibers 25, thereby enhancing the electronic conductivity of the catalyst layers 12A, 12C. Furthermore, if the carbon material constituting the carbon fibers 25 is any of carbon fibers, carbon nanofibers, and carbon nanotubes, the electronic conductivity of the catalyst layers 12A, 12C is suitably enhanced.

[0090] (3) When the average fiber diameter of the fibrous material is 0.1 μm or less, the fibrous material is thin enough to be contained in the catalyst layers 12A and 12C. When the average fiber length of the fibrous material is 1 μm or more and 200 μm or less, the fibrous material is suitably entangled in the catalyst layers 12A and 12C, thereby increasing the strength of the catalyst layers 12A and 12C and improving the effect of suppressing cracking.

[0091] (4) If the mass of the polymer electrolyte fibers 24 contained in the catalyst layers 12A, 12C is 0.01 times or more and 0.3 times or less the mass of the carbon particles 22, the conduction of protons in the catalyst layers 12A, 12C is promoted, thereby improving the output of the solid polymer fuel cell 30.

[0092] (5) If the average particle size of the catalyst material 21 is 0.5 nm or more and 20 nm or less, the stability and activity of the catalyst are improved. Also, if the average particle size of the carbon particles 22 is 10 nm or more and 1000 nm or less, an electron conduction path is easily formed in the catalyst layers 12A, 12C, and the catalyst layers 12A, 12C can be formed thin enough not to cause excessive resistance, thereby suppressing a decrease in the output of the fuel cell. [Explanation of symbols]

[0093] 10...Membrane electrode assembly 11...Polymer electrolyte membrane 12A…Fuel electrode catalyst layer 12C: Air electrode catalyst layer 13A...Anode diffusion layer 13C...Air electrode diffusion layer 21...catalyst material 22...Carbon particles 23…aggregate 24...Polyelectrolyte fiber 25...Carbon fiber 30...Polymer fuel cell 31A, 31C...Separator 34A, 34C...Gasket

Claims

1. a polymer electrolyte membrane; a pair of electrode catalyst layers that sandwich the polymer electrolyte membrane and are in contact with the surfaces of the polymer electrolyte membrane; a gas diffusion layer laminated on each of the pair of electrode catalyst layers, the electrode catalyst layer includes a catalyst material, carbon particles, polymer electrolyte aggregates, and a fibrous material; a Gurley value indicating the air permeability resistance in the thickness direction of the gas diffusion layer is 1.0 seconds or more and 3.0 seconds or less; the fibrous material is a polymer electrolyte fiber having proton conductivity, The fibrous material has an average fiber diameter of 100 nm or more and 500 nm or less, and an average fiber length of 1 μm or more and 200 μm or less, the mass of the polymer electrolyte fibers is 0.01 to 0.3 times the mass of the carbon particles contained in the electrode catalyst layer, The gas diffusion layer has an electrical resistance of 8.0 mΩ·cm 2 or more and 11.5 mΩ·cm 2 or less. Membrane electrode assembly.

2. The catalyst material is in the form of particles, and the average particle size is 0.5 nm or more and 20 nm or less. The membrane electrode assembly according to claim 1 .

3. The carbon particles have an average particle size of 10 nm or more and 1000 nm or less. The membrane electrode assembly according to claim 1 or 2.

4. The membrane electrode assembly according to any one of claims 1 to 3, a pair of separators sandwiching the membrane electrode assembly; A polymer electrolyte fuel cell comprising:

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