Membrane electrode assembly, polymer electrolyte fuel cell, and method for manufacturing membrane electrode assembly

Abstract

To provide an electrode catalyst layer for fuel cell in which the water retention under a low humidification condition is improved without blocking removal of water generated in electrode reaction, which shows the high power generation performance even under the low humidification condition and can be manufactured at a low cost.SOLUTION: A membrane electrode assembly for fuel cell according to the first embodiment of the present invention comprises: a polymer electrolyte membrane; and a pair of electrode catalyst layers which holds the polymer electrolyte membrane. At least one of the pair of electrode catalyst layers includes: a catalyst carrying particle; a polymer electrolyte; and a fibrous material whose average fiber diameter is between 10 nm and 300 nm. The mass of the fibrous material is between 0.02 time and 1.0 time of the mass of a carrier in the catalyst carrying particle. The mass of the polymer electrolyte is between 0.4 time and 1.0 time of the mass of the carrier in the catalyst carrying particle.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A polymer electrolyte fuel cell includes a membrane electrode assembly. The membrane electrode assembly includes a fuel electrode, an oxygen electrode, and a polymer electrolyte membrane sandwiched between the fuel electrode and the oxygen electrode. The fuel electrode and the oxygen electrode each include a gas diffusion layer and an electrode catalyst layer. The electrode catalyst layer includes a catalytic material, a carrier, and a polymer electrolyte.

[0003] Hydrogen contained in the fuel gas flows from the gas diffusion layer of the fuel electrode to the electrode catalyst layer of the fuel electrode. The hydrogen that flows into the electrode catalyst layer is oxidized to produce protons and electrons. The protons reach the electrode catalyst layer of the oxygen electrode through the polymer electrolyte of the fuel electrode and the polymer electrolyte membrane. The electrons reach the electrode catalyst layer of the oxygen electrode through the support of the fuel electrode and an external circuit. The oxygen contained in the oxidizer gas reacts with the protons and electrons in the electrode catalyst layer of the oxygen electrode, thereby producing water.

[0004] Substances involved in the electrode reaction flow through the pores of the electrode catalyst layer. For example, fuel gas flows through the pores of the fuel electrode and reaches the reaction site at the fuel electrode. Water, which conducts protons, flows through the pores of the fuel electrode and reaches the polymer electrolyte membrane. Oxidant gas reaches the reaction site at the oxygen electrode through the pores of the oxygen electrode. Water produced at the oxygen electrode is discharged from the oxygen electrode through the pores of the oxygen electrode.

[0005] The accumulation of excess water in the electrode catalyst layer hinders the movement of substances involved in the cell reaction. Smooth transport of substances through pores prevents the accumulation of excess water and the occurrence of flooding (see, for example, Patent Documents 1 to 4). On the other hand, excellent proton conductivity of a polymer electrolyte membrane requires the polymer electrolyte membrane to have an appropriate water content. The water content is the ratio of the required weight of water to the weight of the polymer electrolyte membrane when dry. A humidity adjustment film interposed between the electrode catalyst layer and the gas diffusion layer prevents a decrease in water content under low-humidity conditions and prevents the polymer electrolyte membrane from drying out (see, for example, Patent Document 5). A groove structure interposed between the electrode catalyst layer and the polymer electrolyte membrane also prevents a decrease in water content under low-humidity conditions and prevents the polymer electrolyte membrane from drying out (see, for example, Patent Document 6). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-120506 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-332041 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-87651 [Patent Document 4] Japanese Patent Application Laid-Open No. 2007-80726 [Patent Document 5] Japanese Patent Application Laid-Open No. 2006-252948 [Patent Document 6] Japanese Patent Application Laid-Open No. 2007-141588 Summary of the Invention [Problem to be solved by the invention]

[0007] There is a trade-off between (i) suppressing the accumulation of excess water and (ii) suppressing the decrease in moisture content, where increasing one effect reduces the other. Achieving both (i) suppressing the accumulation of excess water and (ii) suppressing the decrease in moisture content by using a structure other than a humidity adjustment film or groove structure could further expand the conditions for use of polymer electrolyte fuel cells, which have high power generation performance, and accelerate the spread of polymer electrolyte fuel cells. [Means for solving the problem]

[0008] A membrane electrode assembly for solving the above problems includes a polymer electrolyte membrane and a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane, at least one of the pair of electrode catalyst layers including a catalyst-supporting particle, a polymer electrolyte, and a fibrous material having an average fiber diameter of 10 nm or more and 300 nm or less, wherein the mass of the fibrous material is 0.02 to 1.0 times the mass of the support in the catalyst-supporting particle, and the mass of the polymer electrolyte is 0.4 to 1.0 times the mass of the support in the catalyst-supporting particle.

[0009] A polymer electrolyte fuel cell for achieving the above object includes an anode separator, an oxygen electrode separator, an anode diffusion layer located between the anode separator and the oxygen electrode separator, an oxygen electrode diffusion layer located between the anode diffusion layer and the oxygen electrode separator, and a membrane electrode assembly located between the anode diffusion layer and the oxygen electrode diffusion layer, wherein the membrane electrode assembly is the above-described membrane electrode assembly.

[0010] A method for manufacturing a membrane electrode assembly that solves the above problems includes forming an electrode catalyst layer by drying a coating of a catalyst ink and attaching the electrode catalyst layer to a polymer electrolyte membrane. The catalyst ink includes catalyst-supported particles containing a catalyst substance and a support, a polymer electrolyte, and a fibrous material having an average fiber diameter of 10 nm to 300 nm. The mass of the fibrous material is 0.02 to 1.0 times the mass of the support, and the mass of the polymer electrolyte is 0.4 to 1.0 times the mass of the support.

[0011] According to each of the above configurations, it is possible to improve the drainage of excess water under high load operating conditions and to suppress a decrease in the moisture content under low load operating conditions. In the membrane electrode assembly, the fibrous material may be carbon fiber or fluorine-based polymer electrolyte fiber, the catalyst-supporting particles may be platinum-supporting carbon particles, and the polymer electrolyte may be a fluorine-based polymer electrolyte.

[0012] In the membrane electrode assembly, the electrode catalyst layer may have a pore size distribution whose peak is in the range of 6 nm to 15 nm as measured by mercury intrusion porosimetry. According to each of the above configurations, it is possible to enhance the effectiveness of achieving both improved drainage of excess water under high load operating conditions and suppression of a decrease in moisture content under low load operating conditions. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a cross-sectional view showing a layer structure in one embodiment of a membrane electrode assembly. [Figure 2] FIG. 2 is a diagram schematically illustrating the internal configuration of an electrode catalyst layer according to an embodiment of the present invention. [Figure 3] FIG. 1 is an exploded perspective view showing a layer structure in one embodiment of a polymer electrolyte fuel cell. DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of the membrane electrode assembly and the polymer electrolyte fuel cell will be described below. [Configuration of membrane electrode assembly] 1, the membrane electrode assembly 10 includes a polymer electrolyte membrane 11, an anode catalyst layer 12A, and an oxygen electrode catalyst layer 12C. The polymer electrolyte membrane 11 is sandwiched between the anode catalyst layer 12A and the oxygen electrode catalyst layer 12C. The anode catalyst layer 12A is in contact with the polymer electrolyte membrane 11. The oxygen electrode catalyst layer 12C is in contact with the polymer electrolyte membrane 11. The anode catalyst layer 12A and the oxygen electrode catalyst layer 12C are each an electrode catalyst layer 12.

[0015] As shown in Figure 2, the electrode catalyst layer 12 comprises catalyst-supported particles. The catalyst-supported particles comprise catalyst particles 21 and a support 22. The electrode catalyst layer 12 further comprises a polymer electrolyte 23 and a fibrous material 24. The catalyst-supported particles, polymer electrolyte 23, and fibrous material 24 are uniformly mixed in the electrode catalyst layer 12. The catalyst-supported particles, polymer electrolyte 23, and fibrous material 24 define pores 25 in the electrode catalyst layer 12. The pores 25 are defined in a three-dimensional network.

[0016] Each electrode catalyst layer 12 satisfies the following [Condition 1] to [Condition 4]. The average fiber diameter is measured using a method in accordance with JIS R7607:2000. The compounding ratio R23 (=W23 / W22) is the ratio of the mass W23 to the mass W22. The compounding ratio R24 (=W24 / W22) is the ratio of the mass W24 to the mass W22. The mass W22 is the mass of the carrier 22 in one electrode catalyst layer 12. The mass W23 is the mass of the polymer electrolyte 23 in one electrode catalyst layer 12. The mass W24 is the mass of the fibrous material 24 in one electrode catalyst layer 12.

[0017] [Condition 1] The average fiber diameter of the fibrous material 24 is 10 nm or more and 300 nm or less. [Condition 2] The compounding ratio R24 is 0.02 or more and 1.0 or less. [Condition 3] The compounding ratio R23 is 0.4 or more and 1.0 or less.

[0018] The connections between the catalyst-supported particles form electron conduction paths. The connections between the polymer electrolytes 23 form proton conduction paths. The mixture of the catalyst-supported particles and the polymer electrolyte forms a three-dimensional mesh at the interface between the electrode and the electrolyte. The pores 25 separated by the three-dimensional mesh allow gas to spread across the interface between the electrode and the electrolyte.

[0019] The flexibility of the fibrous material 24 having an average fiber diameter of 10 nm or more is suitable for forming pores 25. The electrode catalyst layer 12 including the catalyst-supporting particles, the polymer electrolyte 23, and the fibrous material 24 is formed by applying ink. The fibrous material 24 having an average fiber diameter of 300 nm or less is suitable for dispersion in ink. When improvements in both the flexibility of the fibrous material 24 and its dispersibility in ink are required, the average fiber diameter of the fibrous material 24 is preferably 100 nm or more and 250 nm or less, and more preferably 150 nm or more and 200 nm or less.

[0020] The entanglement of the fibrous materials 24 increases the mechanical durability of the electrode catalyst layer 12. The electrode catalyst layer 12 with increased mechanical durability is, for example, less susceptible to cracking.

[0021] The pores 25 separated by the fibrous material 24 tend to widen due to the entanglement of the fibrous material 24. Widening the pores 25 by increasing the composition ratio of the fibrous material 24 contributes to the discharge of excess water when high-load operation is required. Suppressing the widening of the pores 25 by decreasing the composition ratio of the fibrous material 24 contributes to an improvement in the moisture content when low-load operation is required.

[0022] The grain boundaries between the polymer electrolyte 23 and the catalyst support particles, and the boundaries between the polymer electrolyte 23 and the fibrous material 24 form pores 25. However, the pores 25 separated by the polymer electrolyte 23 tend to be smaller and shorter than the pores 25 separated by the fibrous material 24. Narrowing the pores 25 by increasing the composition ratio of the polymer electrolyte 23 contributes to improving the water content when low-load operation is required. Enlarging the pores 25 by decreasing the composition ratio of the polymer electrolyte 23 contributes to discharging excess water when high-load operation is required.

[0023] The ion exchange groups of the polymer electrolyte 23 can hold water molecules. Increasing the ion exchange capacity of the polymer electrolyte 23 in the pores 25 defined by the polymer electrolyte 23 contributes to an increase in the water content. Increasing the ion exchange capacity of the polymer electrolyte 23 in the pores 25 defined by the polymer electrolyte 23 contributes to the discharge of excess water.

[0024] The pores 25 of the electrode catalyst layer 12, which have an average fiber diameter of 10 nm or more, a compounding ratio R24 of 1.0 or less, and a compounding ratio R23 of 1.0 or less, improve the drainage of excess water when high-load operation is required. When it is required to increase the effectiveness of improving the drainage of excess water, it is preferable to have a compounding ratio R24 of 0.05 or more and 1.0 or less, and a compounding ratio R23 of 0.8 or less. The pores 25 of the electrode catalyst layer 12 that satisfy the compounding ratio R23 of 0.4 or more and 1.0 or less suppress the decrease in water content when low-load operation is required. When it is required to increase the effectiveness in suppressing the decrease in water content, the compounding ratio R23 is preferably 0.5 or more and 0.8 or less.

[0025] When it is required to further suppress the decrease in water content, the peak observed in the pore size distribution, which is the size distribution of the pores 25 measured by mercury intrusion porosimetry, is preferably in the range of 6 nm to 15 nm.

[0026] [Structure of polymer electrolyte fuel cells] The structure of a polymer electrolyte fuel cell 30 including a membrane electrode assembly 10 will be described. The structure described below is one example of the structure of a polymer electrolyte fuel cell 30. Note that FIG. 3 shows a single cell structure included in the polymer electrolyte fuel cell 30. The polymer electrolyte fuel cell 30 may be configured to include a single cell structure, or may be configured to include a plurality of cell structures stacked together.

[0027] 2, a polymer electrolyte fuel cell 30 includes a membrane electrode assembly 10, two gas diffusion layers, and two separators. The two gas diffusion layers are an oxygen electrode diffusion layer 31C and a fuel electrode diffusion layer 31A. The two separators are an oxygen electrode separator 32C and a fuel electrode separator 32A.

[0028] The oxygen electrode diffusion layer 31C is in contact with the oxygen electrode catalyst layer 12C. The oxygen electrode catalyst layer 12C and the oxygen electrode diffusion layer 31C form the oxygen electrode 30C. The anode diffusion layer 31A is in contact with the anode catalyst layer 12A. The anode catalyst layer 12A and the anode diffusion layer 31A form the anode 30A.

[0029] The surface of the polymer electrolyte membrane 11 in contact with the oxygen electrode catalyst layer 12C is the oxygen electrode surface. The surface of the polymer electrolyte membrane 11 in contact with the anode catalyst layer 12A is the anode surface. The part of the oxygen electrode surface that is not covered by the oxygen electrode catalyst layer 12C is the oxygen electrode outer periphery. An oxygen electrode gasket 13C is located on the oxygen electrode outer periphery. The oxygen electrode gasket 13C prevents gas from leaking from the oxygen electrode outer periphery.

[0030] The portion of the anode surface that is not covered with anode catalyst layer 12A is the anode periphery. Anode gasket 13A is located on the anode periphery. Anode gasket 13A prevents gas from leaking from the anode periphery.

[0031] The membrane electrode assembly 10, the oxygen electrode diffusion layer 31C, and the fuel electrode diffusion layer 31A form a single multilayer body. The oxygen electrode separator 32C and the fuel electrode separator 32A sandwich the single multilayer body in the thickness direction of the cell structure. The oxygen electrode separator 32C and the fuel electrode separator 32A each have electrical conductivity and low gas permeability. The oxygen electrode diffusion layer 31C and the fuel electrode diffusion layer 31A each have gas diffusivity and electrical conductivity.

[0032] The oxygen electrode separator 32C has a first side surface facing the oxygen electrode diffusion layer 31C and a second side surface located on the opposite side of the first side surface. The first side surface of the oxygen electrode separator 32C defines gas flow channels 32Cg each having a plurality of grooves. The second side surface of the oxygen electrode separator 32C defines cooling water flow channels 32Cw each having a plurality of grooves.

[0033] The anode separator 32A has a first side surface facing the anode diffusion layer 31A and a second side surface opposite the first side surface. The first side surface of the anode separator 32A defines gas flow channels 32Ag, which are multiple grooves. The second side surface of the anode separator 32A defines cooling water flow channels 32Aw, which are multiple grooves.

[0034] Hydrogen contained in the fuel gas flows through gas flow channel 32Ag to anode catalyst layer 12A. The hydrogen that has flowed into anode catalyst layer 12A is oxidized to generate protons and electrons. The protons reach anode catalyst layer 12C through polymer electrolyte 23 in anode catalyst layer 12A and polymer electrolyte membrane 11. The electrons reach anode catalyst layer 12C through carrier 22 in anode catalyst layer 12A and an external circuit. Oxygen contained in the oxidizer gas reacts with the protons and electrons at anode catalyst layer 12C, thereby generating water.

[0035] [Detailed structure of membrane electrode assembly] An example of the polymer electrolyte membrane 11 is a fluorine-based polymer electrolyte membrane or a hydrocarbon-based polymer electrolyte membrane. An example of the fluorine-based polymer electrolyte membrane is at least one selected from the group consisting of 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). An example of a constituent material of the hydrocarbon-based polymer electrolyte membrane is at least one selected from the group consisting of sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene.

[0036] When it is required to improve the adhesion between the electrode catalyst layer 12 and the polymer electrolyte membrane 11, if the material of the polymer electrolyte 23 is a fluorine-based polymer electrolyte, it is preferable that the material of the polymer electrolyte membrane 11 is also a fluorine-based polymer electrolyte. Furthermore, if the material of the polymer electrolyte 23 is a hydrocarbon-based polymer electrolyte, it is preferable that the material of the polymer electrolyte membrane 11 is also a hydrocarbon-based polymer electrolyte. Furthermore, it is preferable that the material of the polymer electrolyte membrane 11 is the same as the material of the polymer electrolyte 23.

[0037] The catalyst particles 21 are particulate. When improvement of the activity of the catalyst particles 21 is required, the average particle size of the catalyst particles 21 is preferably 20 nm or less, and more preferably 5 nm or less. When stabilization of the activity of the catalyst particles 21 is required, the average particle size of the catalyst particles 21 is preferably 0.5 nm or more, and more preferably 1 nm or more.

[0038] The constituent materials of the catalyst particles 21 are platinum group metals, metals other than the platinum group metals, their alloys, oxides, and double oxides. The platinum group metal is at least one selected from the group consisting of platinum, palladium, ruthenium, iridium, rhodium, and osmium. The metal other than the platinum group metal is at least one selected from the group consisting of iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum. When improved activity of the catalyst particles 21 is required, the constituent material of the catalyst particles 21 is preferably platinum or a platinum alloy.

[0039] The support 22 is a particle that is conductive and is not eroded by the catalyst material. An example of the support 22 is carbon particles. When an expansion of the electron conduction path is required, the particle size of the support 22 is preferably 10 nm or more. When a reduction in the resistance value of the electrode catalyst layer 12 is required or an increase in the amount of catalyst particles 21 supported is required, the particle size of the support 22 is preferably 1000 nm or less, and more preferably 100 nm or less.

[0040] An example of the constituent material of the carrier 22 is at least one selected from the group consisting of carbon black, graphite, activated carbon, and fullerene. The carbon black is at least one selected from the group consisting of acetylene black, furnace black, and ketjen black.

[0041] The support 22 may be covered with a hydrophobic coating. The hydrophobic coating imparts hydrophobic properties to the support 22 and also provides gas permeability for fuel gas and oxidant gas. When improved gas permeability is required, the thickness of the hydrophobic coating is preferably 40 nm or less. When improved drainage of excess water is further required, the thickness of the hydrophobic coating is preferably 2 nm or more.

[0042] An example of a material for the hydrophobic coating is a fluorine-based compound having at least one polar group. An example of the polar group is at least one selected from the group consisting of a hydroxyl group, an alkoxy group, a carboxyl group, an ester group, an ether group, a carbonate group, and an amide group. A hydrophobic coating having a polar group is easily fixed to the outermost surface of the support 22. An example of the portion other than the polar group in the fluorine-based compound is a fluoroalkyl skeleton.

[0043] The polymer electrolyte 23 is a polymer electrolyte having proton conductivity. The polymer electrolyte 23 has, for example, a particulate shape or a shape in which a plurality of particles are united. An example of the polymer electrolyte 23 is a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. An example of the fluorine-based polymer electrolyte is at least one selected from the group consisting of 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). An example of the hydrocarbon-based polymer electrolyte is at least one selected from the group consisting of sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene.

[0044] The fibrous material 24 is at least one of an electron-conductive fiber and a proton-conductive fiber. The electron-conductive fiber is a fiber made of a carbon material having electron conductivity. The proton-conductive fiber is a fiber made of a polymer electrolyte having proton conductivity. The fibrous material 24 may be composed of only electron-conductive fibers, only proton-conductive fibers, or both electron-conductive fibers and proton-conductive fibers.

[0045] Examples of electron-conductive fibers include carbon fibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. When improved conductivity in the electrode catalyst layer 12 or improved dispersibility in the ink is required, the fibrous material 24 is preferably carbon nanofibers or carbon nanotubes.

[0046] The proton-conductive fiber is made of a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. An example of the fluorine-based polymer electrolyte is at least one selected from the group consisting of 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). An example of the hydrocarbon-based polymer electrolyte is at least one selected from the group consisting of sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene.

[0047] The constituent materials of the oxygen electrode diffusion layer 31C and the fuel electrode diffusion layer 31A are at least one selected from the group consisting of porous carbon materials such as carbon cloth, carbon paper, and nonwoven fabric. The oxygen electrode separator 32C and the fuel electrode separator 32A may each be a carbon type or a metal type. The oxygen electrode diffusion layer 31C and the oxygen electrode separator 32C may be integrated. The fuel electrode diffusion layer 31A and the fuel electrode separator 32A may also be integrated.

[0048] [Method for manufacturing membrane electrode assembly] The method for manufacturing a membrane electrode assembly includes a first step, a second step, and a third step. In the first step, a catalyst ink is manufactured. The catalyst ink includes catalyst particles 21, a support 22, a polymer electrolyte 23, a fibrous material 24, and a solvent. In the second step, the catalyst ink is applied to a substrate and then the solvent is evaporated to manufacture an electrode catalyst layer 12. In the third step, the electrode catalyst layer 12 is attached to a polymer electrolyte membrane 11 to manufacture a membrane electrode assembly 10.

[0049] The solvent constituting the catalyst ink does not corrode the catalyst-supported particles, the polymer electrolyte 23, and the fibrous material 24, but dissolves the polymer electrolyte 23 or disperses it as a fine gel. An example of the solvent constituting the catalyst ink is at least one selected from the group consisting of alcohols, ketone-based solvents, ether-based solvents, and polar solvents. An example of the alcohol is at least one selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, and pentanol. An example of the ketone-based solvent is at least one selected from the group consisting of acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methylcyclohexanone, acetonyl acetone, and diisobutyl ketone. An example of the ether-based solvent is at least one selected from the group consisting of tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, and dibutyl ether. An example of the polar solvent is at least one selected from the group consisting of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, and 1-methoxy-2-propanol. The solvent constituting the catalyst ink may contain water, which has a high affinity for the polymer electrolyte 23.

[0050] When improved dispersibility of catalyst-supported particles is required, the catalyst ink preferably contains a dispersant. Examples of dispersants include anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. When improved dispersibility of the catalyst ink is required, a dispersion treatment is preferably performed in the production of the catalyst ink. Examples of dispersion treatments include stirring with a ball mill and a roll mill, stirring with a shear mill, stirring with a wet mill, stirring by applying ultrasonic waves, and stirring with a homogenizer.

[0051] If it is required to suppress the occurrence of cracks on the surface of the electrode catalyst layer 12, the solid content of the catalyst ink is preferably 50 mass % or less.If it is required to improve the film formation rate of the electrode catalyst layer 12, the solid content of the catalyst ink is preferably 1 mass % or more.

[0052] Examples of methods for applying the catalyst ink to a substrate include doctor blade coating, dipping, screen printing, and roll coating. The substrate to which the catalyst ink is applied is a transfer sheet. Examples of materials that can be used to make the transfer sheet include fluororesins and organic polymer compounds other than fluororesins. Examples of fluororesins include ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), and polytetrafluoroethylene (PTFE). Examples of organic polymer compounds include polyimide, polyethylene terephthalate, polyamide, polysulfone, polyethersulfone, polyphenylene sulfide, polyether ether ketone, polyetherimide, polyarylate, and polyethylene naphthalate.

[0053] Example 1 [Production of catalyst ink] The catalyst-supported particles, polymer electrolyte 23, and fibrous material 24 shown below were mixed in a solvent and dispersed for 30 minutes using a planetary ball mill to prepare a catalyst ink. The solvent used for the catalyst ink was a mixed solvent of ultrapure water and 1-propanol. The volume ratio of ultrapure water to 1-propanol was 1:1. The catalyst ink was adjusted so that the solid content in the catalyst ink was 8% by mass.

[0054] Catalyst-supported particles: Platinum-supported carbon particles (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) Fibrous material 24: Carbon fiber Average fiber diameter of fibrous material 24: 150 nm ·Mixing ratio R24:0.05 Polymer electrolyte 23: Fluorine-based polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.) ·Mixing ratio R23:0.5

[0055] [Production of Electrode Catalyst Layer] A polytetrafluoroethylene (PTFE) sheet was used as the substrate to which the catalyst ink was applied. The catalyst ink was applied to the substrate using a doctor blade method, and the applied film was dried in an air atmosphere at 80°C, thereby producing an electrode catalyst layer 12. In this case, the amount of platinum supported was 0.1 mg / cm 2 The amount of catalyst ink applied was adjusted so that the platinum loading was 0.3 mg / cm2, thereby producing an anode catalyst layer 12A. The amount of catalyst ink applied was also adjusted so that the platinum loading was 0.3 mg / cm2, thereby producing an cathode catalyst layer 12C.

[0056] [Fabrication of Membrane Electrode Assembly] A portion of the oxygen electrode catalyst layer 12C formed on the substrate was punched out together with the substrate, and the punched oxygen electrode catalyst layer 12C was placed on the oxygen electrode surface of the polymer electrolyte membrane 11 described below. Also, a portion of the anode catalyst layer 12A formed on the substrate was punched out together with the substrate, and the punched anode catalyst layer 12A was placed on the anode surface of the polymer electrolyte membrane 11 described below. In this case, the size of the electrode catalyst layer 12 punched out together with the substrate was 5 cm x 5 cm. Polymer electrolyte membrane 11: Fluorine-based polymer electrolyte (Nafion (registered trademark) 211, manufactured by DuPont) Thickness of polymer electrolyte membrane 11: 25 μm

[0057] The anode catalyst layer 12A disposed on the anode surface and the cathode catalyst layer 12C disposed on the cathode surface were transferred at a transfer temperature of 130° C. and a transfer rate of 5.0×10 6 A transfer pressure of 100 Pa was applied to perform hot pressing, and a membrane electrode assembly 10 of Example 1 was obtained.

[0058] <Example 2> The compounding ratio R24 was changed to 0.5, and the compounding ratio R23 was changed to 0.6, and the other conditions were the same as those of Example 1, to obtain a membrane electrode assembly 10 of Example 2.

[0059] Example 3 The compounding ratio R24 was changed to 1.0, and the compounding ratio R23 was changed to 0.8, and the other conditions were the same as those of Example 1, to obtain a membrane electrode assembly 10 of Example 3.

[0060] Example 4 A membrane / electrode assembly 10 of Example 4 was obtained under the same conditions as in Example 1, except that the fibrous material 24 was changed to electrolyte fibers and the average fiber diameter was changed to 200 nm.

[0061] <Example 5> A membrane / electrode assembly 10 of Example 5 was obtained under the same conditions as in Example 2, except that the fibrous material 24 was changed to electrolyte fibers and the average fiber diameter was changed to 200 nm.

[0062] Example 6 A membrane / electrode assembly 10 of Example 6 was obtained under the same conditions as in Example 3, except that the fibrous material 24 was changed to electrolyte fibers and the average fiber diameter was changed to 200 nm.

[0063] <Comparative Example 1> A membrane / electrode assembly of Comparative Example 1 was obtained under the same conditions as in Example 1, except that the average fiber diameter was changed to 8 nm.

[0064] <Comparative Example 2> A membrane / electrode assembly of Comparative Example 2 was obtained under the same conditions as in Example 2, except that the average fiber diameter was changed to 8 nm.

[0065] <Comparative Example 3> The average fiber diameter was changed to 8 nm, and the other conditions were the same as those of Example 3, to obtain a membrane electrode assembly of Comparative Example 3.

[0066] <Comparative Example 4> A membrane / electrode assembly of Comparative Example 4 was obtained under the same conditions as in Example 4, except that the average fiber diameter was changed to 400 nm.

[0067] <Comparative Example 5> A membrane / electrode assembly of Comparative Example 5 was obtained under the same conditions as in Example 5, except that the average fiber diameter was changed to 400 nm.

[0068] <Comparative Example 6> A membrane / electrode assembly of Comparative Example 6 was obtained under the same conditions as in Example 6, except that the average fiber diameter was changed to 400 nm.

[0069] <Evaluation> [Power generation characteristics] The pore size distributions were measured by mercury porosimetry using the electrode catalyst layers 12 of Examples 1 to 6 and Comparative Examples 1 to 6. A peak was observed in the pore size distributions of the electrode catalyst layers 12 of Examples 1 to 6 between 6 nm and 15 nm. On the other hand, no peak was observed in the pore size distributions of the electrode catalyst layers 12 of Comparative Examples 1 to 6 between 6 nm and 15 nm.

[0070] Carbon paper was used for the oxygen electrode diffusion layer 31C and the fuel electrode diffusion layer 31A. The oxygen electrode diffusion layer 31C and the fuel electrode diffusion layer 31A were attached to each of the membrane electrode assemblies 10 obtained in Examples 1 to 6 and Comparative Examples 1 to 6, and the assembly was placed in a power generation evaluation cell, where current and voltage measurements were performed using a fuel cell measurement device. The conditions for current and voltage measurements are shown below.

[0071] [Measurement conditions] Fuel gas: Hydrogen Oxidizing gas: Air Cell temperature: 80℃ Control back pressure: 50kPa [Operating condition 1] - Relative humidity of fuel electrode: 100%RH Relative humidity of oxygen electrode: 100%RH [Operating condition 2] Fuel electrode relative humidity: 30%RH Relative humidity of oxygen electrode: 30%RH The gas flow rate was controlled so that the fuel utilization rate was constant under operating condition 1, which was a high-load operation, and operating condition 2, which was a low-load operation. The fuel utilization rate is the ratio of the flow rate of the reacting fuel gas to the flow rate of the supplied fuel gas. The fuel utilization rate is obtained by dividing the hydrogen consumption rate calculated from the current value by the hydrogen flow rate of the supplied fuel gas. Furthermore, since many cracks were observed in each of the membrane electrode assemblies 10 of Comparative Examples 1 and 6, current and voltage measurements were not performed on each of the membrane electrode assemblies 10 of Comparative Examples 1 and 6.

[0072] [Measurement results] The power generation performance of the fuel cells equipped with the membrane electrode assemblies 10 of Examples 1 to 6 and Comparative Examples 1 to 6 is shown in Table 1. In Table 1, the power generation performance is shown when the current density is 2.0 A / cm 2 The configurations for which the voltage is 0.65 V or higher when the current density is 2.0 A / cm are marked with a "○". 2 The configurations in which the voltage at this time was less than 0.65 V are marked with an "x" symbol. In addition, the configurations in which many cracks were observed in the electrode catalyst layer 12 are marked with an "*" symbol.

[0073] [Table 1] As shown in Table 1, the membrane electrode assemblies of Examples 1 to 6 exhibited good performance under two types of power generation conditions. On the other hand, the membrane electrode assemblies of Comparative Examples 2 to 5 did not exhibit good power generation performance under one or both of the power generation conditions. From the results of the power generation characteristics of the membrane electrode assemblies of the Examples and the membrane electrode assemblies of the Comparative Examples, it was confirmed that the membrane electrode assemblies of the Examples enhance the diffusibility of the reaction gas and do not inhibit the removal of water produced in the electrode reaction. [Explanation of symbols]

[0074] W22,W23,W24…mass 10...Membrane electrode assembly 11...Polymer electrolyte membrane 12...electrode catalyst layer 12A…Fuel electrode catalyst layer 12C: Oxygen electrode catalyst layer 13A...Fuel electrode gasket 13C...Oxygen electrode gasket 21...catalytic particles 22...Carrier 23...polymer electrolyte 24...Fibrous materials 25...pore 30...Polymer fuel cell 31A...Anode diffusion layer 31C...Oxygen electrode diffusion layer 32A...Fuel electrode separator 32C...Oxygen electrode separator 32Ag...gas flow path 32Aw...Cooling water flow path

Claims

1. a polymer electrolyte membrane; a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane, at least one of the pair of electrode catalyst layers includes catalyst-supporting particles, a polymer electrolyte, and a fibrous material having an average fiber diameter of 10 nm or more and 300 nm or less; the mass of the fibrous material is 0.02 times or more and 1.0 times or less the mass of the carrier in the catalyst-supporting particle, the mass of the polymer electrolyte is 0.5 to 0.8 times the mass of the carrier in the catalyst-supporting particle, the fibrous material is a carbon fiber or a fluorine-based polymer electrolyte fiber, A membrane electrode assembly, wherein the pore size distribution of the electrode catalyst layer measured by mercury porosimetry exhibits a peak in the range of 6 nm to 15 nm.

2. the fibrous material includes at least one of one or more electron-conducting fibers and one or more proton-conducting fibers; the catalyst-supporting particles are platinum-supporting carbon particles, The polymer electrolyte is a fluorine-based polymer electrolyte. The membrane electrode assembly according to claim 1 .

3. The membrane electrode assembly according to claim 1 or 2; a pair of gas diffusion layers sandwiching the membrane electrode assembly; a pair of separators facing each other with the membrane electrode assembly and the pair of gas diffusion layers interposed therebetween; A polymer electrolyte fuel cell comprising:

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