Membrane electrode assembly and solid polymer fuel cell

Abstract

A membrane electrode assembly and a solid polymer fuel cell capable of improving power generation performance are provided. One embodiment of the present invention relates to a membrane electrode assembly (10) for a solid polymer fuel cell comprising: a polymer electrolyte membrane (11), a fuel electrode side electrode catalyst layer (12A), and an oxygen electrode side electrode catalyst layer (12C), wherein the fuel electrode side electrode catalyst layer (12A) and the oxygen electrode side electrode catalyst layer (12C) each contain voids, each void comprising pores having a diameter within a range of 3 nm to 5.5 μm, and when a value obtained by integrating the pore volume of all pores in the fuel electrode side electrode catalyst layer (12A) and the oxygen electrode side electrode catalyst layer (12C) is defined as a first integrated volume, a value obtained by dividing the first integrated volume by the mass of the catalyst material contained in the two electrode catalyst layers is within a range of 2.8 to 4.5.

Description

Technical Field

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

[0002] Fuel cells generate electricity through a chemical reaction between hydrogen and oxygen. Compared to conventional power generation methods, fuel cells offer high efficiency, low environmental impact, and low noise levels, making them attractive as a clean energy source. In particular, solid polymer fuel cells, which can operate at near room temperature, are considered promising for applications such as in-vehicle power supplies and household stationary power sources.

[0003] A solid polymer fuel cell generally has a structure formed by stacking a plurality of single cells. A single cell has a structure formed by sandwiching a membrane electrode assembly between two separators. The membrane electrode assembly comprises a polymer electrolyte membrane, a fuel electrode (anode) for supplying fuel gas, and an oxygen electrode (cathode) for supplying an oxidant. The fuel electrode is bonded to the first surface of the polymer electrolyte membrane, and the oxygen electrode is bonded to the second surface opposite to the first surface. The separator has a gas flow path and a cooling water flow path. The fuel electrode and the oxygen electrode each have an electrode catalyst layer and a gas diffusion layer. In each electrode, the electrode catalyst layer is in contact with the polymer electrolyte membrane. The electrode catalyst layer contains a catalyst substance such as a platinum-based noble metal, a conductive carrier, and a polymer electrolyte. The gas diffusion layer has both gas permeability and conductivity.

[0004] Solid polymer fuel cells generate current through the following electrochemical reactions. First, in the fuel electrode's electrode catalyst layer, hydrogen contained in the fuel gas is oxidized by the catalyst, generating protons and electrons. The generated protons pass through the polymer electrolyte and polymer electrolyte membrane within the electrode catalyst layer and reach the oxygen electrode's electrode catalyst layer. Electrons generated simultaneously with the protons then pass through the conductive support, gas diffusion layer, separator, and external circuit within the electrode catalyst layer and reach the oxygen electrode's electrode catalyst layer. In the oxygen electrode's electrode catalyst layer, the protons and electrons react with oxygen contained in the oxidant gas to generate water.

[0005] The gas diffusion layer diffuses the gas supplied from the separator and supplies the gas to the electrode catalyst layer. The electrode catalyst layer has pores, and the pores of the electrode catalyst layer transport various substances such as gas and generated water. The pores of the fuel electrode need to have the function of smoothly supplying fuel gas to the three-phase interface as the reaction site of redox. The pores of the oxygen electrode need to have the function of smoothly supplying oxidant gas into the electrode catalyst layer. In order to smoothly supply fuel gas and oxygen, and then to improve the power generation performance of the fuel cell, the electrode catalyst layer needs to be provided with intervals between the pores to suppress the dense distribution of the pores. As a structure for suppressing the dense distribution of pores, for example, an electrode catalyst layer containing carbon particles or carbon fibers has been proposed (for example, refer to Patent Documents 1 and 2).

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 10-241703

[0009] Patent Document 2: Japanese Patent No. 5537178 Summary of the Invention

[0010] Problems to be solved by the invention

[0011] In Patent Document 1, by combining carbon particles having different particle sizes (average primary particle sizes) from each other, the distribution of pores in the electrode catalyst layer can be suppressed from becoming dense. In addition, in Patent Document 2, by combining carbon fibers having different lengths from each other, the distribution of pores in the electrode catalyst layer can be suppressed from becoming dense. On the other hand, even if the combination of carbon particles has the same particle size as the layers, the size of the pores and the distribution of the pores may be different from each other depending on the composition of the layers and the conditions for forming the layers. In addition, even if the combination of carbon fibers has the same length as the layers, the size of the pores and the distribution of the pores may be different from each other. Since the power generation performance of a fuel cell varies greatly depending on the size of the pores and the distribution of the pores, from the perspective of improving power generation performance, there is still room for improvement in the method of using a combination of carbon particles or a combination of carbon fibers.

[0012] The present invention has been made in view of the above-mentioned circumstances, and an object of the present invention is to provide a membrane electrode assembly and a polymer electrolyte fuel cell capable of improving power generation performance.

[0013] Means for solving problems

[0014] One form of a membrane electrode assembly for solving the above-mentioned problems is a membrane electrode assembly for a solid polymer fuel cell, comprising: a solid polymer electrolyte membrane comprising a first surface and a surface opposite to the first surface, namely a second surface; a fuel electrode side electrode catalyst layer comprising a first catalyst substance, a first conductive carrier carrying the first catalyst substance, and a first polymer electrolyte and bonded to the first surface; and an oxygen electrode side electrode catalyst layer comprising a second catalyst substance, a second conductive carrier carrying the second catalyst substance, a second polymer electrolyte, and a fibrous substance and bonded to the second surface, wherein the fuel electrode The fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer each contain voids, and the voids contain pores with a diameter in the range of 3 nm to 5.5 μm, the diameter of the pores, i.e., the pore diameter, is a value calculated from the pore volume measured by the mercury intrusion method, and when the value obtained by integrating the pore volume of all the pores in the fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer is set as the first integrated volume, the value obtained by dividing the first integrated volume by the mass of the catalyst substance contained in the two electrode catalyst layers, i.e., the mass of the catalyst substance, is in the range of 2.8 to 4.5.

[0015] One embodiment of a polymer electrolyte fuel cell for solving the above-mentioned problems is a polymer electrolyte fuel cell including the above-mentioned membrane electrode assembly.

[0016] Effects of the Invention

[0017] According to the present invention, the power generation performance of a polymer electrolyte fuel cell can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] [ Figure 1 ] is a cross-sectional view schematically showing the structure of the membrane electrode assembly according to the first embodiment of the present invention.

[0019] [ Figure 2 ] is a schematic diagram showing Figure 1 Schematic diagram of the structure of the electrode catalyst layer included in the membrane electrode assembly shown.

[0020] [ Figure 3 ] is a schematic diagram showing Figure 1 An exploded perspective view of the structure of a polymer electrolyte fuel cell including a membrane electrode assembly is shown.

[0021] [ Figure 4 ] is a graph showing the distribution curves of pore diameters in Examples and Comparative Examples.

[0022] [ Figure 5 ] is a graph showing the relationship between the pore diameter and the integrated pore volume ratio in Examples and Comparative Examples.

[0023] [ Figure 6 ] is a graph showing an example of a distribution curve of pore diameters in the electrode catalyst layer according to the second embodiment of the present invention. DETAILED DESCRIPTION

[0024] [First embodiment]

[0025] Reference Figures 1 to 5 The first embodiment of the electrode catalyst layer, membrane electrode assembly, and solid polymer fuel cell is described. The following describes the membrane electrode assembly, electrode catalyst layer, the structure of the single cell constituting the solid polymer fuel cell, the method for manufacturing the membrane electrode assembly, and examples.

[0026] [Membrane Electrode Assembly]

[0027] Reference Figure 1 , the structure of the membrane electrode assembly is described. Figure 1 The cross-sectional structure along the thickness direction of the membrane electrode assembly is shown.

[0028] like Figure 1 As shown, the membrane electrode assembly 10 includes a polymer electrolyte membrane 11, an oxygen electrode side electrode catalyst layer 12C, and a fuel electrode side electrode catalyst layer 12A. The polymer electrolyte membrane 11 is a solid polymer electrolyte membrane, that is, a solid polymer electrolyte membrane. Of a pair of opposing surfaces in the polymer electrolyte membrane 11, the oxygen electrode side electrode catalyst layer 12C is bonded to one surface, and the fuel electrode side electrode catalyst layer 12A is bonded to the other surface. In the polymer electrolyte membrane 11, the surface to which the fuel electrode side electrode catalyst layer 12A is bonded is the first surface, and the surface to which the oxygen electrode side electrode catalyst layer 12C is bonded is the second surface. The oxygen electrode side electrode catalyst layer 12C is an electrode catalyst layer constituting the oxygen electrode (cathode). The fuel electrode side electrode catalyst layer 12A is an electrode catalyst layer constituting the fuel electrode (anode). The outer periphery of the electrode catalyst layer 12 can be sealed with a gasket or the like not shown.

[0029] [Electrode catalyst layer]

[0030] Reference Figure 2 The configuration of the electrode catalyst layer included in the membrane electrode assembly 10 will be described in more detail. It should be noted that the electrode catalyst layer described below is applied to both the oxygen electrode side electrode catalyst layer 12C and the fuel electrode side electrode catalyst layer 12A. However, the following configuration may be applied to only one of the oxygen electrode side electrode catalyst layer 12C and the fuel electrode side electrode catalyst layer 12A.

[0031] like Figure 2As shown, the electrode catalyst layer 12 includes a catalyst material 21, a conductive support 22, a polymer electrolyte 23, and a fibrous material 24. The electrode catalyst layer 12 may not include the fibrous material 24. Specifically, the fuel electrode side electrode catalyst layer 12A may not include the fibrous material 24. In the electrode catalyst layer 12, the portion where the catalyst material 21, the conductive support 22, the polymer electrolyte 23, and the fibrous material 24 are absent is a void. In this embodiment, among the voids, those having a diameter of 3 nm or more and 5.5 μm or less are defined as "pores."

[0032] In the electrode catalyst layer 12 , the diameter of the pores calculated from the pore volume Vp measured by mercury intrusion porosimetry is the pore diameter D. The pore diameter D is defined as the diameter D of the pores modeled into a cylinder obtained by mercury intrusion porosimetry.

[0033] Specifically, the fuel electrode-side electrode catalyst layer 12A and the oxygen electrode-side electrode catalyst layer 12C each contain voids, each of which includes pores having a diameter within the range of 3 nm to 5.5 μm. The pore diameter D, or pore diameter, is a value calculated from the pore volume Vp measured by mercury intrusion porosimetry.

[0034] Here, the distribution of the pore volume Vp described above will be explained. The distribution of the pore volume Vp is represented by the distribution function (=dVp / dlogD) (Log differential pore volume distribution) of the pore volume Vp relative to the pore diameter D (3 nm ≤ D ≤ 5.5 μm). The distribution of the pore volume Vp is obtained by mercury porosimetry.

[0035] Since mercury has a high surface tension, a predetermined pressure P needs to be applied when mercury enters the pores. The distribution of the pore volume Vp and the specific surface area can be calculated based on the pressure P applied to allow mercury to enter the pores and the amount of mercury pressed into the pores. The relationship between the applied pressure P and the pore diameter D that mercury can enter under the pressure P can be expressed by the formula (1) called the Washburn equation. It should be noted that in the following formula (1), γ is the surface tension of mercury, and θ is the contact angle between mercury and the pore wall. In this embodiment, the surface tension γ is set to 0.48 N / m, and the contact angle θ is set to 130°, and the pore diameter D is calculated.

[0036] D=-4γcosθ / P…Formula (1)

[0037] It should be noted that when the mercury intrusion method is actually used for measurement, different pressures P are applied to record the volume of the mercury injected. Each pressure P is then converted to the pore diameter D based on the above formula (1). Furthermore, assuming that the volume of the mercury injected is equal to the pore volume Vp, the increase in pore volume Vp when the pore diameter increases from D to D+dD, i.e., the increase in pore volume dV, is plotted against the pore diameter D. The peak of this curve represents the peak of the pore volume Vp distribution.

[0038] In the electrode catalyst layer 12, the functions required to improve power generation performance include, for example, maintaining the three-phase interface within the electrode catalyst layer 12, gas diffusion within the electrode catalyst layer 12, and discharge of water generated within the electrode catalyst layer 12. Furthermore, to improve these functions, the electrode catalyst layer 12 needs to have voids. The appropriate amount of voids is determined by the amount of catalyst material undergoing the electrochemical reaction. Furthermore, the pore diameters D suitable for maintaining the three-phase interface, the pore diameters D suitable for gas diffusion, and the pore diameters D suitable for discharge of generated water do not necessarily have to be the same; the pore diameters D suitable for improving power generation performance need to include these pore diameters.

[0039] From the above viewpoint, at least one of the electrode catalyst layer 12 , that is, the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C, satisfies at least one of the following conditions 1 to 3.

[0040] [Condition 1]

[0041] In Condition 1, the peak of the distribution curve representing the distribution of pore volume Vp relative to pore diameter D is within the range of pore diameter D of 0.06 μm to 0.11 μm (0.06 μm ≤ D ≤ 0.11 μm). The peak of the distribution curve is preferably within the range of pore diameter D of 0.07 μm to 0.11 μm (0.07 μm ≤ D ≤ 0.11 μm). By ensuring that the peak of the distribution curve is within the range of pore diameter D of 0.06 μm to 0.11 μm, the electrode catalyst layer 12 can contain voids of a size sufficient to provide the electrode catalyst layer 12 with sufficient gas diffusivity and water drainage.

[0042] [Condition 2]

[0043] In the electrode catalyst layer 12, the value obtained by integrating the pore volume Vp of all pores is the first integrated volume (ΣVp1). The value obtained by integrating the pore volume Vp of pores with a pore diameter D of 50 nm or less is the second integrated volume (ΣVp2). Under condition 2, the percentage of the second integrated volume to the first integrated volume (ΣVp2 / ΣVp1×100) is within the range of 25% to 45%. It should be noted that each integrated volume can be calculated by integrating the pore volume Vp within the range of the pore diameter D corresponding to each integrated volume.

[0044] [Condition 3]

[0045] In the electrode catalyst layer 12, the value obtained by integrating the pore volume Vp of pores with a pore diameter D of 90 nm or greater is the third integrated volume (ΣVp3). Under condition 3, the percentage of the third integrated volume to the first integrated volume (ΣVp3 / ΣVp1×100) is within the range of 15% to 35%. It should be noted that the third integrated volume can be calculated by integrating the pore volume Vp within the range of pore diameter D of 90 nm or greater.

[0046] If condition 3 is satisfied, by including pores having relatively large diameters in the electrode catalyst layer 12 at the aforementioned ratio, the gas diffusivity and the discharge of generated water in the electrode catalyst layer 12 can be improved while maintaining a three-phase interface within the electrode catalyst layer 12. If condition 2 is satisfied, by including pores having relatively small diameters in the electrode catalyst layer 12 at the aforementioned ratio, the gas diffusivity and the discharge of generated water in the electrode catalyst layer 12 can be improved while maintaining a three-phase interface within the electrode catalyst layer 12.

[0047] It should be noted that the thickness of the electrode catalyst layer 12 is preferably in the range of 5 μm to 30 μm. By making the thickness of the electrode catalyst layer 12 less than 30 μm, cracks in the electrode catalyst layer 12 can be suppressed. In addition, when the electrode catalyst layer 12 is used in a solid polymer fuel cell, the reduction in the diffusivity and conductivity of the gas and the generated water can be suppressed, and thus the reduction in the output of the solid polymer fuel cell can be suppressed. In addition, by making the thickness of the electrode catalyst layer 12 more than 5 μm, it is difficult to produce thickness deviation in the electrode catalyst layer 12, and the distribution of the catalyst substance 21 and the polymer electrolyte 23 contained in the electrode catalyst layer 12 can be suppressed from becoming uneven. It should be noted that cracks or uneven thickness on the surface of the electrode catalyst layer 12 are not preferred. This is because when the electrode catalyst layer 12 is used as part of a solid polymer fuel cell and the solid polymer fuel cell is operated (used) for a long time, the durability of the solid polymer fuel cell is very likely to be adversely affected.

[0048] The thickness of the electrode catalyst layer 12 can be measured by, for example, observing a cross-section of the electrode catalyst layer 12 using a scanning electron microscope (SEM). Methods for exposing the cross-section of the electrode catalyst layer 12 include, for example, ion milling and ultramicrotomy. When exposing the cross-section of the electrode catalyst layer 12, the electrode catalyst layer 12 is preferably cooled. This can reduce damage to the polymer electrolyte 23 contained in the electrode catalyst layer 12.

[0049] [Structure of solid polymer fuel cells]

[0050] Reference Figure 3 The structure of a solid polymer fuel cell having a membrane electrode assembly 10 is described below. The structure described below is a structure in an example of a solid polymer fuel cell. Figure 3 The structure of a single cell included in a solid polymer fuel cell is shown. A solid polymer fuel cell can include a plurality of single cells and can be constructed by stacking a plurality of single cells.

[0051] like Figure 3 As shown, the polymer electrolyte fuel cell 30 includes a membrane electrode assembly 10, a pair of gas diffusion layers, and a pair of separators. The gas diffusion layers are an oxygen electrode-side gas diffusion layer 31C and a fuel electrode-side gas diffusion layer 31A. The separators are an oxygen electrode-side separator 32C and a fuel electrode-side separator 32A.

[0052] The oxygen electrode-side gas diffusion layer 31C is in contact with the oxygen electrode-side electrode catalyst layer 12C. The oxygen electrode-side electrode catalyst layer 12C and the oxygen electrode-side gas diffusion layer 31C form the oxygen electrode (cathode) 30C. The fuel electrode-side gas diffusion layer 31A is in contact with the fuel electrode-side electrode catalyst layer 12A. The fuel electrode-side electrode catalyst layer 12A and the fuel electrode-side gas diffusion layer 31A form the fuel electrode (anode) 30A.

[0053] In the polymer electrolyte membrane 11, the surface bonded to the oxygen electrode-side electrode catalyst layer 12C is the oxygen electrode surface, and the surface bonded to the fuel electrode-side electrode catalyst layer 12A is the fuel electrode surface. The portion of the oxygen electrode surface not covered by the oxygen electrode-side electrode catalyst layer 12C is the outer periphery. The oxygen electrode-side gasket 13C is located in the outer periphery. The portion of the fuel electrode surface not covered by the fuel electrode-side electrode catalyst layer 12A is the outer periphery. The fuel electrode-side gasket 13A is located in the outer periphery. Gaskets 13C and 13A suppress gas leakage from the outer periphery of each surface.

[0054] In the thickness direction of the solid polymer fuel cell 30, the oxygen electrode side separator 32C and the fuel electrode side separator 32A sandwich the multilayer structure formed by the membrane electrode assembly 10 and the two gas diffusion layers 31C and 31A. The oxygen electrode side separator 32C faces the oxygen electrode side gas diffusion layer 31C, while the fuel electrode side separator 32A faces the fuel electrode side gas diffusion layer 31A.

[0055] The oxygen electrode-side separator 32C has a plurality of grooves on each of its pair of opposing surfaces. The grooves on the surface of the pair that faces the oxygen electrode-side gas diffusion layer 31C serve as gas flow paths 32Cg. The grooves on the surface opposite the opposing surface serve as cooling water flow paths 32Cw.

[0056] The fuel electrode-side separator 32A has multiple grooves on each of its two opposing surfaces. The grooves on the surface facing the fuel electrode-side gas diffusion layer 31A serve as gas flow paths 32Ag. The grooves on the surface opposite the opposing surface serve as cooling water flow paths 32Aw.

[0057] Each of the separators 32C and 32A has electrical conductivity and is formed of a gas-impermeable material.

[0058] In the polymer electrolyte fuel cell 30, an oxidant is supplied to the oxygen electrode 30C via the gas flow path 32Cg of the oxygen electrode-side separator 32C. Furthermore, in the polymer electrolyte fuel cell 30, a fuel is supplied to the fuel electrode 30A via the gas flow path 32Ag of the fuel electrode-side separator 32A. This allows the polymer electrolyte fuel cell 30 to generate electricity. Examples of the oxidant include air and oxygen. Examples of the fuel include hydrogen-containing fuel gas and organic fuels.

[0059] In the polymer electrolyte fuel cell 30, the reaction represented by the following reaction formula (1) occurs in the fuel electrode 30A. Meanwhile, the reaction represented by the following reaction formula (2) occurs in the oxygen electrode 30C.

[0060] H2→2H + +2e - …Reaction (1)

[0061] 1 / 2O2+2H + +2e - →H2O…Reaction formula (2)

[0062] As described above, the polymer electrolyte fuel cell 30 of the present embodiment is a fuel cell that produces water in the oxygen electrode 30C by supplying a gas containing oxygen to the oxygen electrode 30C.

[0063] As described above, the electrode catalyst layer 12 of the present embodiment can be applied to the fuel electrode side electrode catalyst layer 12A, and can also be applied to the oxygen electrode side electrode catalyst layer 12C. Here, according to the above reaction formula (2), water is generated in the oxygen electrode 30C from oxygen, protons and electrons. In the case where the water generated in the oxygen electrode 30C is not discharged to the outside of the oxygen electrode 30C, the supply of oxygen-containing gas to the oxygen electrode 30C will be hindered by the water. As a result, the power generation performance of the solid polymer fuel cell 30 will be reduced. In this regard, the electrode catalyst layer 12 of the present embodiment has high drainage properties by satisfying the above conditions. Therefore, by applying such an electrode catalyst layer 12 to the oxygen electrode side electrode catalyst layer 12C possessed by the oxygen electrode 30C, the effect of improving the power generation performance of the solid polymer fuel cell 30 can be more significantly obtained.

[0064] [Method for Manufacturing Membrane Electrode Assembly]

[0065] Hereinafter, a method for producing the membrane electrode assembly will be described.

[0066] To manufacture the membrane electrode assembly 10, the catalyst material 21, the conductive support 22, the polymer electrolyte 23, and the fibrous material 24 are first mixed in a dispersion medium to form a mixture. This mixture is then subjected to a dispersion treatment to prepare a catalyst ink. It should be noted that the fibrous material 24 can be omitted from the materials contained in the catalyst ink. The dispersion treatment can be performed using, for example, a planetary ball mill, a bead mill, or an ultrasonic homogenizer.

[0067] The catalyst ink's dispersion medium can be a solvent that does not corrode the catalyst material 21, the conductive carrier 22, the polymer electrolyte 23, and the fibrous material 24. While maintaining high fluidity, it can dissolve the polymer electrolyte 23 or disperse it into a fine gel. The dispersion medium may contain water, which has good compatibility with the polymer electrolyte 23. The catalyst ink preferably contains a volatile liquid organic solvent. Because lower alcohols pose a fire hazard, mixing water with such a solvent is preferred. Water and a solvent may be mixed as long as the catalyst ink does not become cloudy or gelled due to separation of the polymer electrolyte 23.

[0068] After applying the prepared catalyst ink to a substrate and then drying it, the solvent is removed from the catalyst ink coating. Thus, the electrode catalyst layer 12 is formed on the substrate. The substrate can be a polymer electrolyte membrane 11 or a transfer substrate. When using a polymer electrolyte membrane 11 as the substrate, for example, the following method can be used: the catalyst ink is directly applied to the surface of the polymer electrolyte membrane 11, and then the solvent is removed from the catalyst ink coating to form the electrode catalyst layer 12.

[0069] When using a transfer substrate, a catalyst ink is applied to the transfer substrate and then dried to form a substrate with a catalyst layer. Then, for example, the electrode catalyst layer 12 of the catalyst-layered substrate is brought into contact with the polymer electrolyte membrane 11 by heating and pressurizing the substrate. This allows the electrode catalyst layer 12 and the polymer electrolyte membrane 11 to be bonded together. By bonding the electrode catalyst layer 12 to both surfaces of the polymer electrolyte membrane 11, the membrane electrode assembly 10 can be manufactured.

[0070] The catalyst ink can be applied to the substrate using various coating methods. Examples of coating methods include die coating, roller coating, curtain coating, spray coating, and doctor blade coating. Die coating is preferably used as the coating method. Die coating is preferred because the film thickness is stable during coating and intermittent coating can be performed. As a method for drying the coating of the catalyst ink, for example, drying using a hot air oven, IR (far infrared) drying, drying using a hot plate, and reduced pressure drying can be used. The drying temperature is in the range of 40°C to 200°C, preferably in the range of about 40°C to 120°C. The drying time is in the range of 0.5 minutes to 1 hour, preferably in the range of about 1 minute to 30 minutes.

[0071] In the case of forming the electrode catalyst layer 12 on the transfer substrate, the pressure and temperature applied to the electrode catalyst layer 12 when transferring the electrode catalyst layer 12 affect the power generation performance of the membrane electrode assembly 10. In order to obtain a membrane electrode assembly with high power generation performance, the pressure applied to the multilayer body is preferably in the range of 0.1 MPa to 20 MPa. By making the pressure below 20 MPa, the electrode catalyst layer 12 can be prevented from being excessively compressed. By making the pressure above 0.1 MPa, the reduction in power generation performance caused by the reduction in the bonding between the electrode catalyst layer 12 and the polymer electrolyte membrane 11 can be suppressed. When considering improving the bonding between the polymer electrolyte membrane 11 and the electrode catalyst layer 12 and suppressing the interface resistance, the temperature during bonding is preferably near the glass transition point of the polymer electrolyte membrane 11 or the polymer electrolyte 23 contained in the electrode catalyst layer 12.

[0072] Transfer substrate can, for example, use a polymer film and a sheet formed by a fluorine resin. The transferability of fluorine resin is excellent. As fluorine resin, for example, ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoro perfluoroalkyl vinyl ether copolymer (PFA) and polytetrafluoroethylene (PTFE) etc. can be used. The polymer forming the polymer film can, for example, include polyimide, polyethylene terephthalate, polyamide (nylon (registered trademark)), polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyetherimide, polyarylate and polyethylene naphthalate etc. Transfer substrate can also use a gas diffusion layer.

[0073] The size and distribution of the pores in the electrode catalyst layer 12 can be adjusted by adjusting the temperature at which the catalyst ink coating is heated, the rate at which the coating is heated, the pressurization conditions until the catalyst ink dries, the mixing ratio of the fibrous material 24, the solvent composition of the catalyst ink, and the dispersion strength during adjustment of the catalyst ink. For example, a higher mixing ratio of the fibrous material 24 increases the pore diameter D corresponding to the peak of the distribution curve, decreases the ratio of the second integrated volume to the first integrated volume, and increases the ratio of the third integrated volume to the first integrated volume.

[0074] The catalyst material 21 may be, for example, a metal included in the platinum group, a metal outside the platinum group, or an alloy, oxide, composite oxide, or carbide of these metals. The metals included in the platinum group include platinum, palladium, ruthenium, iridium, rhodium, and osmium. Metals outside the platinum group include iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum.

[0075] The conductive support 22 can use a support that has conductivity and can support the catalyst substance 21 without being corroded by the catalyst substance 21. The conductive support 22 can use carbon particles. The carbon particles can use (for example) carbon black, graphite, black lead, activated carbon, carbon nanotubes, carbon nanofibers and fullerenes. The particle size (average primary particle size) of the carbon particles is preferably in the range of about 10 nm to 1000 nm, more preferably in the range of about 10 nm to 100 nm. By making the particle size greater than 10 nm, the carbon particles will not be too densely packed in the electrode catalyst layer 12, thereby suppressing the reduction of gas diffusivity of the electrode catalyst layer 12. By making the particle size less than 1000 nm, cracks can be prevented from occurring in the electrode catalyst layer 12.

[0076] The polymer electrolyte contained in the polymer electrolyte membrane 11 and the electrode catalyst layer 12 can use an electrolyte with proton conductivity. The polymer electrolyte can use, for example, a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte. The fluorine-based polymer electrolyte can use a polymer electrolyte having a tetrafluoroethylene skeleton. It should be noted that, as a polymer electrolyte having a tetrafluoroethylene skeleton, Nafion (registered trademark) manufactured by DuPont can be cited. As a hydrocarbon-based polymer electrolyte, for example, sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene can be used.

[0077] The polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 23 contained in the electrode catalyst layer 12 may be the same electrolyte or different electrolytes. However, considering the interfacial resistance at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 12 and the dimensional change rate of the polymer electrolyte membrane 11 and the electrode catalyst layer 12 when the humidity changes, the polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 23 contained in the electrode catalyst layer 12 are preferably the same electrolyte or similar electrolytes.

[0078] Electron conductive fibers and proton conductive fibers can be used as the fibrous material 24. Examples of electron conductive fibers include carbon fibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. From the perspective of conductivity and dispersibility, carbon nanofibers are preferably used as the fibrous material 24.

[0079] From the perspective of reducing the amount of catalysts formed from precious metals, electron-conductive fibers with catalytic ability are more preferred. When the electrode catalyst layer 12 is used as an electrode catalyst layer constituting an oxygen electrode, that is, the oxygen electrode-side electrode catalyst layer 12C, examples of electron-conductive fibers with catalytic ability include carbon alloy catalysts made of carbon nanofibers. Electron-conductive fibers with catalytic ability may also be fibers formed from an electrode active material for a fuel electrode. Electrode active materials may include substances containing at least one transition metal element selected from the group consisting of Ta, Nb, Ti, and Zr. Examples of substances containing transition metal elements include partial oxides of carbonitrides of transition metal elements, conductive oxides of transition metal elements, and conductive oxynitrides of transition metal elements.

[0080] The proton conductive fiber can be any fiber formed of a polymer electrolyte having proton conductivity. Materials used to form the proton conductive fiber can include fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes. 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. Hydrocarbon-based polymer electrolytes include sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, sulfonated polyimide, and acid-doped polybenzoxazole.

[0081] The fibrous material 24 may be composed of only one of the aforementioned fibers or two or more thereof. Electron conductive fibers and proton conductive fibers may be used together as the fibrous material 24. Among the aforementioned fibrous materials 24, the fibrous material 24 preferably comprises at least one selected from the group consisting of carbon nanofibers, carbon nanotubes, and electrolyte fibers.

[0082] The fiber diameter of the fibrous material 24 is preferably in the range of 0.5 nm to 500 nm, more preferably in the range of 5 nm to 200 nm. By setting the fiber diameter in the range of 0.5 nm to 500 nm, the voids in the electrode catalyst layer 12 can be increased, thereby increasing the output of the solid polymer fuel cell 30. The fiber length of the fibrous material 24 is preferably in the range of 1 μm to 50 μm, more preferably in the range of 1 μm to 20 μm. By setting the fiber length in the range of 1 μm to 50 μm, the strength of the electrode catalyst layer 12 can be increased, thereby suppressing the generation of cracks in the electrode catalyst layer 12 when the electrode catalyst layer 12 is formed. In addition, the voids in the electrode catalyst layer 12 can be increased, thereby increasing the output of the solid polymer fuel cell 30.

[0083] The mass of the catalyst substance 21 in the electrode catalyst layer 12 can be determined from the amount of catalyst layer slurry applied or the dry mass. Alternatively, it can be determined by atomic absorption spectrometry (AAS), inductively coupled plasma analysis (ICP-AES), or the like.

[0084] [Effects of the First Embodiment]

[0085] As described above, in the membrane electrode assembly 10 of the present embodiment, among the voids contained in the electrode catalyst layer 12 composed of the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C, when the voids with a diameter in the range of 3 nm to 5.5 μm are set as pores, the diameter of the pores calculated from the pore volume measured by the mercury intrusion method is the pore diameter D, and when the value obtained by integrating the pore volume Vp of all pores is the first integrated volume (ΣVp1) and the value obtained by integrating the pore volume Vp of pores with a pore diameter D of 50 nm or less is the second integrated volume (ΣVp2), the percentage of the second integrated volume (ΣVp2) relative to the first integrated volume (ΣVp1) can be in the range of 25% to 55%, and further can be in the range of 25% to 45%.

[0086] In addition, in the membrane electrode assembly 10, among the voids contained in the electrode catalyst layer 12 composed of the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C, when the voids with a diameter in the range of 3 nm to 5.5 μm are set as pores, the diameter of the pores calculated from the pore volume measured by the mercury intrusion method is the pore diameter D, and when the value obtained by integrating the pore volume Vp of all pores is the first integrated volume (ΣVp1) and the value obtained by integrating the pore volume Vp of pores with a pore diameter D of 90 nm or more is the third integrated volume (ΣVp3), the percentage of the third integrated volume (ΣVp3) relative to the first integrated volume (ΣVp1) can be in the range of 10% to 35%, and further can be in the range of 15% to 35%.

[0087] According to the above-described configurations, the diffusibility of the gas and the dischargeability of the generated water can be improved while maintaining the three-phase interface, thereby improving the power generation performance.

[0088] Furthermore, in the membrane electrode assembly 10, as described later, the percentage of the integrated volume V (which is the integral of the pore volume Vp of all pores) relative to the volume Vo of the electrode catalyst layer 12 can be within a range of 65% to 90%. This configuration allows the electrode catalyst layer 12 to exhibit more adequate gas diffusivity and drainage properties.

[0089] In addition, in the membrane electrode assembly 10, the fibrous material 24 may include one or more fibrous materials selected from electron conductive fibers and proton conductive fibers, and the electron conductive fibers may include at least one selected from the group consisting of carbon nanofibers, carbon nanotubes, and transition metal-containing fibers.

[0090] In addition, in the membrane electrode assembly 10, when the fibrous material 24 contained in the fuel electrode side electrode catalyst layer 12A is set as the first fibrous material and the fibrous material 24 contained in the oxygen electrode side electrode catalyst layer 12C is set as the second fibrous material, the mass of the first fibrous material per unit volume of the fuel electrode side electrode catalyst layer 12A can be greater than the mass of the second fibrous material per unit volume of the oxygen electrode side electrode catalyst layer 12C.

[0091] According to the above configuration, by making the mass of the first fibrous material per unit volume of the fuel electrode-side electrode catalyst layer 12A greater than the mass of the second fibrous material per unit volume of the oxygen electrode-side electrode catalyst layer 12C, the fuel electrode-side electrode catalyst layer 12A is more likely to include pores having a larger pore diameter D than the oxygen electrode-side electrode catalyst layer 12C. As a result, the fuel gas flows more efficiently into the membrane electrode assembly 10.

[0092] In addition, in the membrane electrode assembly 10 , the oxygen electrode side electrode catalyst layer 12C may have a thickness within a range of 5 μm to 30 μm.

[0093] In addition, in the membrane electrode assembly 10 , the fuel electrode side electrode catalyst layer 12A may have a thickness within a range of 5 μm to 20 μm.

[0094] According to the above configurations, by setting the thickness of each electrode catalyst layer 12 to be below the upper limit, cracks can be suppressed in the electrode catalyst layer 12. Furthermore, when the electrode catalyst layer 12 is used in a solid polymer fuel cell 30, a decrease in the diffusibility of gas and generated water, as well as a decrease in conductivity, can be suppressed, thereby suppressing a decrease in the output of the solid polymer fuel cell 30. Furthermore, by setting the thickness of the electrode catalyst layer 12 to be above the lower limit, uneven thickness is less likely to occur in the electrode catalyst layer 12, and uneven distribution of the catalyst material 21 and polymer electrolyte 23 contained in the electrode catalyst layer 12 can be suppressed.

[0095] Furthermore, as described above, the polymer electrolyte fuel cell 30 of this embodiment includes the membrane electrode assembly 10 .

[0096] The above configurations ensure sufficient pores (spaces) around the catalyst material 21. This improves gas diffusivity and generated water discharge in the electrode catalyst layer 12 while maintaining a three-phase interface therein.

[0097] [Example]

[0098] Reference Figure 4 and Figure 5 , an embodiment of a membrane electrode assembly is described.

[0099] [Example 1]

[0100] A platinum-supported carbon catalyst (TEC10E30E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko Co., Ltd.) were mixed to obtain a mixture. It should be noted that in the platinum-supported carbon catalyst, the platinum catalyst is supported on carbon particles. The ratio of the mass of the carbon particles to the mass of the polymer electrolyte was set to 1:1. The mixture was then dispersed at 300 rpm for 60 minutes using a planetary ball mill. At this time, zirconia balls with a diameter of 5 mm were added to about one-third of the zirconia container. Thus, a catalyst ink for a fuel electrode was prepared. It should be noted that the catalyst ink for a fuel electrode was adjusted so that the mass of the polymer electrolyte relative to the mass of the carbon particles was 100 mass%, the mass of the fibrous substance relative to the mass of the carbon particles was 100 mass%, the proportion of water in the dispersion medium was 50 mass%, and the solid content in the catalyst ink was 10 mass%.

[0101] Separately, a platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko Co., Ltd.) were mixed to obtain a mixture. It should be noted that in the platinum-supported carbon catalyst, the platinum catalyst is supported on carbon particles. The ratio of the mass of the carbon particles to the mass of the polymer electrolyte was set to 1:1. The mixture was then dispersed at 300 rpm using a planetary ball mill for 60 minutes. At this time, zirconia balls with a diameter of 5 mm were added to about one-third of the zirconia container. Thus, a catalyst ink for an oxygen electrode was prepared. It should be noted that the catalyst ink for an oxygen electrode was adjusted so that the mass of the polymer electrolyte relative to the mass of the carbon particles was 100 mass%, the mass of the fibrous substance relative to the mass of the carbon particles was 100 mass%, the proportion of water in the dispersion medium was 50 mass%, and the solid content in the catalyst ink was 10 mass%.

[0102] Each catalyst ink was applied to both surfaces of a polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by Dupont) using a slot die coater to form a coating. It should be noted that each catalyst ink was applied to the polymer electrolyte membrane so that the coating thickness of the oxygen electrode catalyst ink on the cathode surface of the polymer electrolyte membrane was 150 μm, and the coating thickness of the fuel electrode catalyst ink on the anode surface was 100 μm. The polymer electrolyte membrane with the coating was then placed in a warm air oven at 80 degrees Celsius and dried until the coating was no longer sticky. Thus, the membrane electrode assembly of Example 1 was obtained.

[0103] [Example 2]

[0104] The membrane electrode assembly of Example 2 was obtained by the same method as in Example 1, except that multilayer carbon nanotubes (diameter 60 nm to 100 nm, manufactured by Tokyo Chemical Industry Co., Ltd.) were used instead of carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko K.K.).

[0105] [Example 3]

[0106] Each catalyst ink was prepared by the same method as in Example 1. Each catalyst ink was applied to the surface of the PTFE membrane using a slit die coating to form a coating. Then, it was placed in a warm air oven at 80 degrees to dry the coating until it was no longer sticky. Thus, a substrate with a catalyst layer was obtained. A substrate comprising an oxygen electrode side electrode catalyst layer and a substrate comprising a fuel electrode side electrode catalyst layer were prepared. It should be noted that each catalyst ink was applied to the polymer electrolyte membrane so that the coating thickness of the oxygen electrode catalyst ink on the cathode surface of the polymer electrolyte membrane was 150 μm, and the coating thickness of the fuel electrode catalyst ink on the anode surface was 60 μm. Then, each substrate with a catalyst layer was arranged on a pair of surfaces of a polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by Dupont) in a manner such that each surface was opposite to each other, thereby forming a stack. The two electrode catalyst layers were bonded to the polymer electrolyte membrane by hot pressing the stack under the conditions of 120°C and 5MPa. Next, the PTFE membrane was peeled off from each electrode catalyst layer, thereby obtaining the membrane electrode assembly of Example 3.

[0107] [Example 4]

[0108] When preparing the oxygen electrode catalyst ink, the amount of carbon nanofiber (VGCF (registered trademark)-H, manufactured by Showa Denko K.K.) added was set to one-fourth of that in Example 1. A membrane electrode assembly of Example 4 was obtained by the same method as in Example 1 except for the above.

[0109] [Example 5]

[0110] The membrane electrode assembly of Example 5 was obtained by the same method as Example 2 except that the amount of multiwalled carbon nanotubes (diameter 60 nm to 100 nm, manufactured by Tokyo Chemical Industry Co., Ltd.) added was half that of Example 2 when preparing the catalyst ink for the oxygen electrode.

[0111] [Example 6]

[0112] The membrane electrode assembly of Example 6 was obtained by the same method as Example 2 except that the amount of multiwalled carbon nanotubes (diameter 60 nm to 100 nm, manufactured by Tokyo Chemical Industry Co., Ltd.) added was set to one-fourth of that in Example 2 when preparing the catalyst ink for the oxygen electrode.

[0113] [Comparative Example 1]

[0114] The membrane electrode assembly of Comparative Example 1 was obtained by the same method as in Example 1, except that the coating amount of the oxygen electrode catalyst ink was 3 times that of Example 1 when forming the oxygen electrode side electrode catalyst layer.

[0115] [Comparative Example 2]

[0116] The membrane electrode assembly of Comparative Example 2 was obtained by the same method as in Example 1 except that the solid content ratio was changed to half of that in Example 1 when preparing the catalyst ink for the oxygen electrode.

[0117] [Comparative Example 3]

[0118] A membrane electrode assembly of Comparative Example 3 was obtained by the same method as in Example 1 except that no carbon nanofibers were added when preparing the catalyst ink for the oxygen electrode.

[0119] [Comparative Example 4]

[0120] A membrane electrode assembly of Comparative Example 4 was obtained by the same method as in Example 1, except that the amount of carbon nanofibers was doubled compared to that in Example 1 when preparing the catalyst ink for the oxygen electrode.

[0121] [Comparative Example 5]

[0122] A membrane electrode assembly of Comparative Example 5 was obtained by the same method as in Example 1, except that the amount of carbon nanofibers was set three times that of Example 1 when preparing the catalyst ink for the oxygen electrode.

[0123] [Numerical calculation based on pore volume Vp]

[0124] The distribution of pore volume Vp was measured by mercury intrusion porosimetry. Specifically, a membrane electrode assembly was formed on a polymer electrolyte membrane with only the oxygen electrode side electrode catalyst layer formed thereon, and the pore volume Vp was measured using an automatic porosimeter (Micromeritics, Autopore IV9510). The volume of the measurement cell was approximately 5 cm 3 , the pressure of mercury injection increased from 3kPa to 400MPa. Thus, the amount of mercury injected at each pressure, that is, the pore volume Vp, was obtained. The pressure of the injected mercury was converted into the pore diameter D using the Washburn equation, and a graph of the distribution function dVp / dlogD of the pore volume Vp relative to the pore diameter D was prepared. It should be noted that the surface tension γ was set to 0.48N / m, and the contact angle θ was set to 130°. Then, the pore diameter D corresponding to the peak of the graph was read as the pore diameter Dp.

[0125] Next, the volume of all pores with a pore diameter D of 3 nm or more and 5.5 μm or less is integrated to calculate the first integrated volume. In addition, the volume of pores with a pore diameter D of 90 nm or more is integrated to calculate the third integrated volume. Then, by dividing the third integrated volume by the first integrated volume and multiplying the quotient by 100, the percentage R(L) of the third integrated volume relative to the first integrated volume is calculated. In addition, the pore volume of pores with a pore diameter D of 50 nm or less is integrated to calculate the second integrated volume. Then, by dividing the second integrated volume by the first integrated volume and multiplying the quotient by 100, the percentage R(S) of the second integrated volume relative to the first integrated volume is calculated.

[0126] [Calculation of the integrated pore volume relative to the geometric volume of the electrode catalyst layer on the oxygen electrode side]

[0127] Next, the volume of all pores with a pore diameter D of 3 nm or more and 5.5 μm or less is integrated to calculate the integrated pore volume V. Furthermore, the geometric volume of the membrane electrode assembly used for measurement using an automatic porosimeter is multiplied by the thickness. Furthermore, the volume of the polymer electrolyte membrane is calculated by multiplying the area of ​​the membrane electrode assembly used for measurement using an automatic porosimeter by the thickness of the polymer electrolyte membrane. The geometric volume V0 of the oxygen electrode side electrode catalyst layer is calculated by subtracting the volume of the polymer electrolyte membrane from the volume of the membrane electrode assembly. Then, the percentage (V / V0) of the integrated pore volume V relative to the geometric volume V0 of the oxygen electrode side electrode catalyst layer is calculated.

[0128] [Measurement of thickness of electrode catalyst layer]

[0129] The thickness of the electrode catalyst layer was measured by observing the cross section of the electrode catalyst layer using a scanning electron microscope (SEM). Specifically, the cross section of the electrode catalyst layer was observed using a scanning electron microscope (manufactured by Hitachi High-Technologies, Ltd., FE-SEMS-4800) at a magnification of 1000 times. The thickness of the electrode catalyst layer was measured at 30 observation points in the cross section of the electrode catalyst layer. The average value of the thickness at the 30 observation points was defined as the thickness of the electrode catalyst layer.

[0130] [Measurement of power generation performance]

[0131] The power generation performance was measured using a method based on the "Battery Evaluation and Analysis Protocol," a pamphlet published by the New Energy and Industrial Technology Development Organization (NEDO). A JARI standard cell, obtained by placing a gas diffusion layer, a gasket, and a separator on each surface of a membrane electrode assembly and clamping them to a predetermined surface pressure, was used as the evaluation cell. Then, IV measurements were performed according to the method described in the "Battery Evaluation and Analysis Protocol." The conditions at this time were set to standard conditions. Furthermore, the IV measurements were performed with the relative humidity of the anode and the relative humidity of the cathode set to RH100%. The conditions at this time were set to high humidity conditions.

[0132] [Determination of durability]

[0133] The durability was measured using the same evaluation cells as those used in the power generation performance measurement, and the durability was measured according to the humidity cycle test described in the "Battery Evaluation and Analysis Protocol" above.

[0134] [Comparison results]

[0135] Table 1 shows the results of the following items for each of the electrode catalyst layers included in the membrane electrode assemblies of Examples 1 to 6 and the membrane electrode assemblies of Comparative Examples 1 to 5. Specifically, in each electrode catalyst layer, the pore diameter Dp corresponding to the peak in the distribution curve of the pore volume Vp, the percentage R(L) (%) of the third integrated volume relative to the first integrated volume, and the percentage R(S) (%) of the second integrated volume relative to the first integrated volume are shown in Table 1. Furthermore, in each electrode catalyst layer, the percentage V / V0 (%) of the first integrated volume V relative to the volume V0 of the electrode catalyst layer, and the thickness T (μm) of the electrode catalyst layer are shown in Table 1. Furthermore, Table 1 shows the results of power generation performance and durability measurements for each of the polymer electrolyte fuel cells including the membrane electrode assemblies of Examples 1 to 6 and the polymer electrolyte fuel cells including the membrane electrode assemblies of Comparative Examples 1 to 5.

[0136] In the power generation performance results, under standard conditions, a single cell with a current of 25 A or more at a voltage of 0.6 V was rated as "○," and a single cell with a current of less than 25 A was rated as "X." Furthermore, under high humidity conditions, a single cell with a current of 31 A or more at a voltage of 0.6 V was rated as "○," a single cell with a current of 30 A or more was rated as "△," and a single cell with a current of less than 30 A was rated as "X." Regarding durability, a single cell with a hydrogen crossover leakage current less than 8 times the initial value after 8,000 cycles was rated as "○," a single cell with a hydrogen crossover leakage current less than 10 times the initial value was rated as "△," and a single cell with a current of 10 times or more was rated as "X."

[0137] In each of the electrode catalyst layers of Examples 1 to 3 and the electrode catalyst layers of Comparative Examples 1 to 5, the distribution curve of the pore volume Vp is as follows: Figure 4 In addition, in each of the electrode catalyst layers of Examples 1 to 3 and Comparative Examples 1 to 5, a graph showing the relationship between the integrated pore volume fraction and the pore diameter D is shown as follows: Figure 5 shown.

[0138] [Table 1]

[0139]

[0140] As shown in Table 1, in all of Examples 1 to 3, it was confirmed that the pore diameter Dp corresponding to the peak in the pore volume Vp distribution curve was within the range of 0.06 μm to 0.11 μm. In all of Examples 1 to 3, it was confirmed that the value of the percentage R(L) of the third integrated volume relative to the first integrated volume was within the range of 15% to 35%, and the value of the percentage R(S) of the second integrated volume relative to the first integrated volume was within the range of 25% to 45%.

[0141] In all of Examples 1 to 3, it was confirmed that the thickness T of the electrode catalyst layer was within the range of 5 μm to 30 μm. Furthermore, in all of Examples 1 to 6, it was confirmed that the power generation performance was "○" or "△" and the durability was "○" or "△" regardless of the measurement conditions. In other words, it was confirmed that the membrane electrode assemblies of Examples 1 to 6 are capable of forming fuel cells with excellent power generation performance and durability.

[0142] On the other hand, in all of Comparative Examples 1 to 5, it was confirmed that the pore diameter Dp corresponding to the peak in the distribution curve of the pore volume Vp was not within the range of 0.06 μm to 0.11 μm. In all of Comparative Examples 1 to 5, it was confirmed that the value of the percentage R(L) of the third integrated volume relative to the first integrated volume was not within the range of 15% to 35%. In all of Comparative Examples 1 to 5, it was confirmed that the value of the percentage R(S) of the second integrated volume relative to the first integrated volume was not within the range of 25% to 45%.

[0143] In Comparative Example 1, it was confirmed that the thickness T of the electrode catalyst layer exceeded 30 μm. On the other hand, in Comparative Examples 2 to 5, it was confirmed that the thickness T of the electrode catalyst layer was within the range of 5 μm to 30 μm.

[0144] Comparative Examples 1 to 5 showed a "Poor" rating for power generation performance under at least one of standard conditions and high humidity conditions. Furthermore, durability was also shown as "Poor" for Comparative Examples 2, 3, and 5. This confirms that, compared to the aforementioned Examples, Comparative Examples 1 to 5 exhibited at least reduced power generation performance and durability.

[0145] As described above, according to the first embodiment of the electrode catalyst layer, the membrane electrode assembly, and the fuel cell, the following effects can be obtained.

[0146] (1) When the pore diameter Dp is within the range of 0.06 μm to 0.11 μm in the peak of the distribution curve representing the distribution of the pore volume Vp relative to the pore diameter D, the electrode catalyst layer 12 contains pores sufficient to have sufficient gas diffusivity and drainage properties, thereby improving the power generation performance.

[0147] (2) When the percentage of the second integral volume to the first integral volume is within the range of 25% to 45%, the three-phase interface can be maintained in the electrode catalyst layer 12 while improving the diffusivity of the gas and the discharge of the generated water, thereby improving the power generation performance.

[0148] (3) When the percentage of the third integral volume to the first integral volume is within a range of 15% to 35%, the three-phase interface can be maintained while improving the diffusivity of the gas and the discharge of the generated water, thereby improving the power generation performance.

[0149] (4) In the membrane electrode assembly, when the percentage of the integrated volume V obtained by integrating the pore volume of all pores relative to the volume V0 of the electrode catalyst layer is within the range of 65% to 90%, the electrode catalyst layer can have more sufficient gas diffusivity and drainage properties.

[0150] [Second embodiment]

[0151] Reference Figure 6 A second embodiment of a membrane electrode assembly and a solid polymer fuel cell will be described. In the second embodiment, the configuration of the membrane electrode assembly differs from that of the first embodiment. Therefore, these differences will be described in detail below. Configurations common to the first embodiment will be assigned the same reference numerals as in the first embodiment, and detailed descriptions of these configurations will be omitted.

[0152] [Membrane Electrode Assembly]

[0153] Reference Figure 6 , the structure of the membrane electrode assembly 10 is described. It should be noted that, in the present embodiment, the oxygen electrode side electrode catalyst layer 12C includes a fibrous substance 24. On the other hand, the fuel electrode side electrode catalyst layer 12A may include a fibrous substance 24 or may not include a fibrous substance 24. It should be noted that the fuel electrode side electrode catalyst layer 12A includes a first catalyst substance, a first conductive carrier, and a first polymer electrolyte. In addition, the oxygen electrode side electrode catalyst layer 12C includes a second catalyst substance, a second conductive carrier, a second polymer electrolyte, and a fibrous substance. The first catalyst substance may be the same as or different from the second catalyst substance. The first conductive carrier may be the same as or different from the second conductive carrier. The first polymer electrolyte may be the same as or different from the second polymer electrolyte.

[0154] Within each electrode catalyst layer 12, namely, within each of the fuel electrode-side electrode catalyst layer 12A and the oxygen electrode-side electrode catalyst layer 12C, the portion where the catalyst material 21, conductive support 22, polymer electrolyte 23, and fibrous material 24 are absent constitutes a void. In this embodiment, voids having a diameter of 3 nm to 5.5 μm are defined as "pores." That is, the membrane electrode assembly 10 contains voids, and the voids include pores having a diameter of 3 nm to 5.5 μm.

[0155] In the present embodiment, the method of calculating the pore diameter D, that is, the pore volume Vp of the pores, etc. is the same as the method of calculating the pore diameter D, that is, the pore volume Vp of the pores described in the first embodiment.

[0156] That is, the pore volume Vp of the pores, which is the pore diameter D measured by mercury intrusion, is calculated in the membrane electrode assembly 10. The pore diameter D is defined as the diameter D of the cylindrically modeled pore obtained by mercury intrusion.

[0157] Here, the distribution of the pore volume Vp described above is explained. The distribution of pore volume Vp is represented by the distribution function (= dVp / dlogD) (Log differential pore volume distribution) of pore volume Vp relative to pore diameter D (3 nm ≤ D ≤ 5.5 μm). The distribution of pore volume Vp is obtained using mercury porosimetry. The pore volume Vp is the total volume of pores with a certain pore diameter D.

[0158] Since mercury has a high surface tension, a predetermined pressure P needs to be applied when mercury enters the pores. The distribution of the pore volume Vp and the specific surface area can be calculated based on the pressure P applied to allow mercury to enter the pores and the amount of mercury pressed into the pores. The relationship between the applied pressure P and the pore diameter D that mercury can enter under the pressure P can be expressed by the formula (1) called the Washburn equation. It should be noted that in the following formula (1), γ is the surface tension of mercury, and θ is the contact angle between mercury and the pore wall. In this embodiment, the surface tension γ is set to 0.48 N / m, and the contact angle θ is set to 130°, and the pore diameter D is calculated.

[0159] D=-4γcosθ / P…Formula (1)

[0160] It should be noted that when the mercury intrusion method is actually used for measurement, different pressures P are applied to record the volume of the mercury injected. Each pressure P is then converted to the pore diameter D based on the above formula (1). Furthermore, assuming that the volume of the mercury injected is equal to the pore volume Vp, the increase in pore volume Vp when the pore diameter increases from D to D+dD, i.e., the increase in pore volume dV, is plotted against the pore diameter D. The peak of this curve represents the peak of the pore volume Vp distribution.

[0161] In the membrane electrode assembly 10, the functions required to improve power generation performance include, for example, maintaining the three-phase interface within the electrode catalyst layer 12 of the membrane electrode assembly 10, gas diffusion within the electrode catalyst layer 12, and discharge of water generated within the electrode catalyst layer 12. To enhance these functions, sufficient voids are required within the electrode catalyst layer 12. The required amount of voids is determined not by the volume of the electrode catalyst layer 12 but by the amount of catalyst material within the electrode catalyst layer 12, that is, the mass of the catalyst material contained in the electrode catalyst layer 12. Furthermore, the pore diameter D suitable for maintaining the three-phase interface, the pore diameter D suitable for gas diffusion, and the pore diameter D suitable for discharge of generated water are different from one another. Alternatively, the pore diameter D suitable for maintaining the three-phase interface, the pore diameter D suitable for gas diffusion, and the pore diameter D suitable for discharge of generated water do not necessarily have to be the same, but may fall within different ranges. Furthermore, the pore diameters D suitable for gas diffusion in the oxygen electrode side electrode catalyst layer 12C and the fuel electrode side electrode catalyst layer 12A are also different. The pore diameter D suitable for improving the power generation performance needs to include these various pore diameters D. It should be noted that the three-phase interface refers to an interface formed by a polymer electrolyte, a catalyst, and a gas.

[0162] From the above viewpoint, in the membrane electrode assembly 10, specifically, at least one of the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C satisfies at least one of the following conditions 4 to 7. More preferably, both the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C satisfy the following condition 7, and at least one of the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C satisfies at least one of the following conditions 4 to 6.

[0163] [Condition 4]

[0164] The peak of the distribution curve representing the distribution of pore volume Vp relative to pore diameter D is within the range of 0.06 μm to 0.11 μm (0.06 μm ≤ D ≤ 0.11 μm). By ensuring that the peak of the distribution curve is within the range of 0.06 μm to 0.11 μm, the electrode catalyst layer 12 can contain voids sufficient to provide sufficient gas diffusivity and water drainage.

[0165] Figure 6 An example of a distribution curve showing the distribution of the pore volume Vp with respect to the pore diameter D is shown.

[0166] like Figure 6As shown, the peaks of distribution curves B and C are within the range of pore diameter D of 0.06 μm or more and 0.11 μm or less. On the other hand, the peaks of distribution curves A and D are not within the range of pore diameter D of 0.06 μm or more and 0.11 μm or less. More specifically, the peak of distribution curve A is within the range of pore diameter D less than 0.06 μm. In contrast, the peak of distribution curve D is within the range of pore diameter D greater than 0.11 μm.

[0167] [Condition 5]

[0168] In the membrane electrode assembly 10, the value obtained by integrating the pore volume Vp of pores within the entire range of pore diameters D is the first integrated volume (ΣVp1). The value obtained by integrating the pore volume Vp of pores within the range of pore diameters D of 50 nm or less is the second integrated volume (ΣVp2). Under condition 5, the percentage of the second integrated volume to the first integrated volume (ΣVp2 / ΣVp1×100) is within the range of 25% to 45%.

[0169] [Condition 6]

[0170] In membrane electrode assembly 10, the value obtained by integrating the pore volume Vp of pores having a pore diameter D of 100 nm or greater is referred to as a third integrated volume (ΣVp3). Under condition 6, the percentage of the third integrated volume to the first integrated volume (ΣVp3 / ΣVp1×100) is within a range of 30% to 50%.

[0171] When condition 6 is satisfied, by including pores having relatively large diameters in the aforementioned ratio among the pores included in the membrane electrode assembly 10, a three-phase interface can be maintained within the electrode catalyst layer 12 of the membrane electrode assembly 10 while improving gas diffusivity and the discharge of generated water in the electrode catalyst layer 12. Furthermore, when condition 5 is satisfied, by including pores having relatively small diameters in the aforementioned range among the pores included in the electrode catalyst layer 12, a three-phase interface can be maintained within the electrode catalyst layer 12 while improving gas diffusivity and the discharge of generated water in the electrode catalyst layer 12.

[0172] [Condition 7]

[0173] In the membrane electrode assembly 10, the value obtained by integrating the pore volume Vp of the pores within the entire range of pore diameters D is referred to as the first integrated volume (ΣVp1). Under Condition 7, the first integrated volume is within a range of 2.8 to 4.5 relative to the mass of the catalyst material contained in the electrode catalyst layer 12, that is, the catalyst material mass M (ΣVp1 / M).

[0174] It should be noted that when both the oxygen electrode side electrode catalyst layer 12C and the fuel electrode side electrode catalyst layer 12A include fibrous material 24, the mass of the fibrous material (first fibrous material) 24 per unit volume in the fuel electrode side electrode catalyst layer 12A is preferably greater than the mass of the fibrous material (second fibrous material) 24 per unit volume in the oxygen electrode side electrode catalyst layer 12C. By making the mass of the fibrous material 24 per unit volume in the fuel electrode side electrode catalyst layer 12A greater than the mass of the fibrous material 24 per unit volume in the oxygen electrode side electrode catalyst layer 12C, the fuel electrode side electrode catalyst layer 12A is more likely to include pores having a larger pore diameter D than the oxygen electrode side electrode catalyst layer 12C. As a result, the fuel gas flows more efficiently into the membrane electrode assembly 10. It should be noted that the fibrous material 24 included in the oxygen electrode side electrode catalyst layer 12C and the fibrous material 24 included in the fuel electrode side electrode catalyst layer 12A may be the same fibrous material or different fibrous materials.

[0175] It should be noted that by observing the cross-section of the membrane electrode assembly 10 using a scanning electron microscope (SEM), the content of the fibrous material 24 in the oxygen electrode side electrode catalyst layer 12C and the content of the fibrous material 24 in the fuel electrode side electrode catalyst layer 12A can be compared. Specifically, the cross-section of the membrane electrode assembly 10 was observed at a magnification of 1000 times using a scanning electron microscope (manufactured by Hitachi High-Technologies, Ltd., FE-SEM S-4800). Thus, 30 observation points were randomly selected from the cross-sections of the oxygen electrode side electrode catalyst layer 12C and the fuel electrode side electrode catalyst layer 12A. Then, one observation point was randomly selected from the 30 observation points in each electrode catalyst layer 12, and the observation points of the oxygen electrode side electrode catalyst layer 12C and the observation points of the fuel electrode side electrode catalyst layer 12A were sequentially compared. By visually observing each observation point, it is determined whether the content of the fibrous substance 24 at the observation point on the oxygen electrode side electrode catalyst layer 12C or the content of the fibrous substance 24 at the observation point on the fuel electrode side electrode catalyst layer 12A is greater. Based on the results of the content determination at the 30 observation points, the content of the fibrous substance 24 in the oxygen electrode side electrode catalyst layer 12C and the content of the fibrous substance in the fuel electrode side electrode catalyst layer 12A can be compared by majority decision.

[0176] For example, at more than half of the observation points, the content of the fibrous substance 24 in the fuel electrode side electrode catalyst layer 12A is determined to be greater than the content of the fibrous substance 24 in the oxygen electrode side electrode catalyst layer 12C. In this case, it can be determined that the mass of the fibrous substance 24 per unit volume in the fuel electrode side electrode catalyst layer 12A is greater than the mass of the fibrous substance 24 per unit volume in the oxygen electrode side electrode catalyst layer 12C.

[0177] It should be noted that the oxygen electrode side electrode catalyst layer 12C preferably has a thickness in the range of 5 μm to 30 μm. By making the oxygen electrode side electrode catalyst layer 12C have a thickness of 30 μm or less, the generation of cracks in the oxygen electrode side electrode catalyst layer 12C can be suppressed. In addition, when the oxygen electrode side electrode catalyst layer 12C is used in a solid polymer fuel cell 30, the diffusivity of the gas and the generated water, as well as the reduction in conductivity, can be suppressed, thereby suppressing the reduction in the output of the solid polymer fuel cell 30. In addition, by making the oxygen electrode side electrode catalyst layer 12C have a thickness of 5 μm or more, it is difficult to generate uneven thickness in the oxygen electrode side electrode catalyst layer 12C, thereby suppressing the distribution of the catalyst substance 21 and the polymer electrolyte 23 contained in the oxygen electrode side electrode catalyst layer 12C from becoming uneven. It should be noted that cracks or uneven thickness on the surface of the oxygen electrode side electrode catalyst layer 12C are not preferred. This is because when the oxygen electrode side electrode catalyst layer 12C is used as part of the solid polymer fuel cell 30 and the solid polymer fuel cell 30 is operated (used) for a long time, the durability of the solid polymer fuel cell 30 is very likely to be adversely affected.

[0178] In addition, the fuel electrode side electrode catalyst layer 12A preferably has a thickness in the range of 5 μm to 20 μm. By making the fuel electrode side electrode catalyst layer 12A have a thickness of 20 μm or less, the generation of cracks in the fuel electrode side electrode catalyst layer 12A can be suppressed. In addition, when the fuel electrode side electrode catalyst layer 12A is used in a solid polymer fuel cell 30, the reduction in gas diffusivity and conductivity can be suppressed, and thus the reduction in the output of the solid polymer fuel cell 30 can be suppressed. In addition, by making the fuel electrode side electrode catalyst layer 12A have a thickness of 5 μm or more, it is difficult to generate uneven thickness in the fuel electrode side electrode catalyst layer 12A, and the distribution of the catalyst material 21 and the polymer electrolyte 23 contained in the fuel electrode side electrode catalyst layer 12A can be suppressed from becoming uneven. It should be noted that cracks or uneven thickness on the surface of the fuel electrode side electrode catalyst layer 12A are not preferred. This is because when the fuel electrode side electrode catalyst layer 12A is used as part of a solid polymer fuel cell 30 and the solid polymer fuel cell 30 is operated (used) for a long time, the durability of the solid polymer fuel cell 30 is very likely to be adversely affected.

[0179] It should be noted that the size and distribution of the pores in each electrode catalyst layer 12 can be adjusted by adjusting the temperature at which the catalyst ink coating is heated, the rate at which the coating is heated, the pressurization conditions until the catalyst ink dries, the mixing ratio of the fibrous material 24, the mixing ratio of the polymer electrolyte 23, the solvent composition of the catalyst ink, and the dispersion strength during adjustment of the catalyst ink. For example, a higher mixing ratio of the fibrous material 24 increases the pore diameter D corresponding to the peak of the distribution curve, while a lower mixing ratio of the polymer electrolyte 23 increases the pore volume Vp.

[0180] [Effects of the Second Embodiment]

[0181] As described above, for each of the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C of the present embodiment, when the value of the first integral volume relative to the mass of the catalyst substance is in the range of greater than 2.8 and less than 4.5, by including pores with relatively large diameters in this range, the three-phase interface can be maintained within the electrode catalyst layer 12 while improving the diffusivity of the gas in the electrode catalyst layer 12 and the discharge of the generated water.

[0182] In addition, in the membrane electrode assembly 10, for at least one of the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C, among the voids contained in the electrode catalyst layer 12, when the voids with a diameter in the range of 3 nm to 5.5 μm are set as pores, the diameter of the pores calculated from the pore volume measured by the mercury intrusion method is the pore diameter D, and when the value obtained by integrating the pore volume Vp of all pores is the first integrated volume (ΣVp1) and the value obtained by integrating the pore volume Vp of pores with a pore diameter D of 50 nm or less is the second integrated volume (ΣVp2), the percentage of the second integrated volume (ΣVp2) relative to the first integrated volume (ΣVp1) can be in the range of 25% to 55%, and further can be in the range of 25% to 45%.

[0183] In addition, in the membrane electrode assembly 10, for at least one of the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C, among the voids contained in the electrode catalyst layer 12, when the voids with a diameter in the range of 3 nm to 5.5 μm are set as pores, the diameter of the pores calculated from the pore volume measured by the mercury intrusion method is the pore diameter D, and when the value obtained by integrating the pore volume Vp of all pores is the first integrated volume (ΣVp1) and the value obtained by integrating the pore volume Vp of pores with a pore diameter D of 90 nm or more is the third integrated volume (ΣVp3), the percentage of the third integrated volume (ΣVp3) relative to the first integrated volume (ΣVp1) can be in the range of 10% to 35%, and further can be in the range of 15% to 35%.

[0184] According to the above-described configurations, the diffusibility of the gas and the dischargeability of the generated water can be improved while maintaining the three-phase interface, thereby improving the power generation performance.

[0185] In the membrane electrode assembly 10, the integrated volume V obtained by integrating the pore volume Vp of all pores relative to the volume V of the electrode catalyst layer 12 is o The percentage value may be in the range of 65% to 90%. According to the above configuration, the electrode catalyst layer 12 can have more sufficient gas diffusivity and water drainage.

[0186] In addition, in the membrane electrode assembly 10, the fibrous material 24 may include one or more fibrous materials selected from electron conductive fibers and proton conductive fibers, and the electron conductive fibers may include at least one selected from the group consisting of carbon nanofibers, carbon nanotubes, and transition metal-containing fibers.

[0187] In addition, in the membrane electrode assembly 10, when the fibrous material 24 contained in the fuel electrode side electrode catalyst layer 12A is set as the first fibrous material and the fibrous material 24 contained in the oxygen electrode side electrode catalyst layer 12C is set as the second fibrous material, the mass of the first fibrous material per unit volume of the fuel electrode side electrode catalyst layer 12A can be greater than the mass of the second fibrous material per unit volume of the oxygen electrode side electrode catalyst layer 12C.

[0188] According to the above configuration, by making the mass of the first fibrous material per unit volume of the fuel electrode-side electrode catalyst layer 12A greater than the mass of the second fibrous material per unit volume of the oxygen electrode-side electrode catalyst layer 12C, the fuel electrode-side electrode catalyst layer 12A is more likely to include pores having a larger pore diameter D than the oxygen electrode-side electrode catalyst layer 12C. As a result, the fuel gas flows more efficiently into the membrane electrode assembly 10.

[0189] In addition, in the membrane electrode assembly 10 , the oxygen electrode side electrode catalyst layer 12C may have a thickness within a range of 5 μm to 30 μm.

[0190] In addition, in the membrane electrode assembly 10 , the fuel electrode side electrode catalyst layer 12A may have a thickness within a range of 5 μm to 20 μm.

[0191] According to the above configurations, by setting the thickness of each electrode catalyst layer 12 to be below the upper limit, cracks can be suppressed in the electrode catalyst layer 12. Furthermore, when the electrode catalyst layer 12 is used in a solid polymer fuel cell 30, a decrease in the diffusibility and conductivity of gas and generated water can be suppressed, thereby suppressing a decrease in the output of the solid polymer fuel cell 30. Furthermore, by setting the thickness of the electrode catalyst layer 12 to be above the lower limit, uneven thickness is less likely to occur in the electrode catalyst layer 12, thereby suppressing uneven distribution of the catalyst material 21 and polymer electrolyte 23 contained in the electrode catalyst layer 12.

[0192] Furthermore, as described above, the polymer electrolyte fuel cell 30 of this embodiment includes the membrane electrode assembly 10 .

[0193] The above configurations ensure sufficient pores (spaces) around the catalyst material 21. This improves gas diffusivity and generated water discharge in the electrode catalyst layer 12 while maintaining a three-phase interface within the electrode catalyst layer 12.

[0194] [Example]

[0195] Reference Figure 6 , Table 2, describes an embodiment of a membrane electrode assembly.

[0196] [Example 7]

[0197] A platinum-loaded carbon catalyst (TEC10E50E, manufactured by Tanaka Precious Metals Industries, Ltd.), water, 1-propanol, a polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko Industries, Ltd.) were mixed to obtain a mixture. It should be noted that in the platinum-loaded carbon catalyst, the platinum catalyst is loaded on carbon particles. The mixture was dispersed at 300 rpm using a planetary ball mill for 30 minutes. At this time, zirconia balls having a diameter of 5 mm were added to about one-third of the zirconia container. It should be noted that the catalyst ink for the oxygen electrode was adjusted so that the mass of the polymer electrolyte relative to the mass of the carbon particles was 100 mass%, the mass of the carbon nanofibers relative to the mass of the carbon particles was 100 mass%, the proportion of water in the dispersion medium was 50 mass%, and the solid content concentration was 10 mass%.

[0198] In addition, a platinum-loaded carbon catalyst (TEC10E30E, manufactured by Tanaka Precious Metals Industries, Ltd.), water, 1-propanol, a polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and carbon nanofibers (VGCF (registered trademark) -H, manufactured by Showa Denko Industries, Ltd.) were mixed to obtain a mixture. It should be noted that in the platinum-loaded carbon catalyst, the platinum catalyst is loaded on carbon particles. The mixture was dispersed at 300 rpm for 30 minutes using a planetary ball mill. At this time, zirconia balls having a diameter of 5 mm were added to about one-third of the zirconia container. It should be noted that the catalyst ink for fuel electrodes was prepared in such a manner that the mass of the polymer electrolyte relative to the mass of the carbon particles was 100 mass%, the mass of the carbon nanofibers relative to the mass of the carbon particles was 100 mass%, the proportion of water in the dispersion medium was 50 mass%, and the solid component concentration was 10 mass%.

[0199] The oxygen electrode catalyst ink was applied to one surface of the polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by Dupont) using a slit die coating method to form a coating having a thickness of 150 μm. Subsequently, the polymer electrolyte membrane having the coating was placed in a warm air oven at 80 degrees, and the coating was dried until the coating was not sticky, thereby forming an oxygen electrode side electrode catalyst layer. Subsequently, the fuel electrode catalyst ink was applied to the other surface of the polymer electrolyte membrane using a slit die coating method to form a coating having a thickness of 100 μm. Subsequently, the polymer electrolyte membrane having the coating was placed in a warm air oven at 80 degrees, and the coating was dried until the coating was not sticky, thereby forming a fuel electrode side electrode catalyst layer. Thus, the membrane electrode assembly of Example 7 was obtained.

[0200] [Example 8]

[0201] In Example 7, when preparing the catalyst ink for the oxygen electrode, multilayer carbon nanotubes (diameter 60nm to 100nm, manufactured by Tokyo Chemical Industry Co., Ltd.) were used instead of carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko K.K.). Except for this, the membrane electrode assembly of Example 8 was obtained by the same method as in Example 7.

[0202] [Example 9]

[0203] The membrane electrode assembly of Example 9 was obtained by the same method as in Example 7 except that the amount of carbon nanofibers was changed to half of that in Example 7 when preparing the catalyst ink for the oxygen electrode.

[0204] [Example 10]

[0205] A membrane electrode assembly of Example 10 was obtained by the same method as in Example 7 except that the amount of carbon nanofibers was changed to one fifth of that in Example 7 when preparing the catalyst ink for the oxygen electrode.

[0206] [Example 11]

[0207] The membrane electrode assembly of Example 11 was obtained by the same method as in Example 7 except that the amount of the polymer electrolyte was changed to two-thirds of that in Example 7 when preparing the catalyst ink for the oxygen electrode.

[0208] [Example 12]

[0209] Each catalyst ink was prepared by the same method as in Example 7. The oxygen electrode catalyst ink was applied to the surface of the PTFE membrane using a slit die coating method to form a coating having a thickness of 150 μm. Subsequently, the PTFE membrane with the coating was placed in a warm air oven at 80 degrees, and the coating was dried until the coating was not sticky, thereby obtaining a transfer substrate with an oxygen electrode side electrode catalyst layer. Subsequently, the fuel electrode catalyst ink was applied to the surface of another PTFE membrane using a slit die coating method to form a coating having a thickness of 100 μm. Subsequently, the PTFE membrane with the coating was placed in a warm air oven at 80 degrees, and the coating was dried until the coating was not sticky, thereby obtaining a transfer substrate with a fuel electrode side electrode catalyst layer.

[0210] A polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by Dupont) was prepared. A transfer substrate with an oxygen electrode-side electrode catalyst layer was placed opposite one surface of the polymer electrolyte membrane, and a transfer substrate with a fuel electrode-side electrode catalyst layer was placed opposite the other surface of the polymer electrolyte membrane to form a laminate. The laminate was hot-pressed at a heating temperature of 120°C and a pressure of 1 MPa. Thus, the two electrode catalyst layers were bonded to the polymer electrolyte membrane. Next, the membrane electrode assembly of Example 12 was obtained by peeling the PTFE membrane from each electrode catalyst layer.

[0211] [Comparative Example 6]

[0212] A membrane electrode assembly of Comparative Example 6 was obtained by the same method as in Example 7, except that the amount of carbon nanofibers was doubled when preparing the oxygen electrode catalyst ink.

[0213] [Comparative Example 7]

[0214] A membrane electrode assembly of Comparative Example 7 was obtained by the same method as in Example 7, except that the amount of the polymer electrolyte was doubled when preparing the oxygen electrode catalyst ink.

[0215] [Comparative Example 8]

[0216] In Example 7, the membrane electrode assembly of Comparative Example 8 was obtained by the same method as in Example 7, except that carbon nanotubes (NC7000, manufactured by Nanocyl Corporation) were used instead of carbon nanofibers (VGCF (registered trademark)-H, manufactured by Showa Denko K.K.).

[0217] [Comparative Example 9]

[0218] A membrane electrode assembly of Comparative Example 9 was obtained by the same method as in Example 7, except that the amount of carbon nanofibers was doubled when preparing the fuel electrode catalyst ink.

[0219] [Comparative Example 10]

[0220] A membrane electrode assembly of Comparative Example 10 was obtained by the same method as in Example 7, except that carbon nanofibers were not added when preparing each catalyst ink.

[0221] [Numerical calculation based on pore volume Vp]

[0222] The distribution of pore volume Vp was measured by mercury intrusion porosimetry. Specifically, a membrane electrode assembly of approximately 25 square centimeters was prepared and the pore volume Vp was measured using an automatic porosimeter (Autopore IV 9510 manufactured by Micromeritics). The volume of the measurement cell was approximately 5 cm 3 , the pressure of mercury injection increased from 3kPa to 400MPa. Thus, the amount of mercury injected at each pressure, that is, the pore volume Vp, was obtained. The pressure of the injected mercury was converted into the pore diameter D using the Washburn equation, and a graph of the distribution function dVp / dlogD (Log differential pore volume distribution) of the pore volume Vp relative to the pore diameter D was prepared. It should be noted that the surface tension γ was set to 0.48N / m, and the contact angle θ was set to 130°. Then, the pore diameter D corresponding to the peak of the graph was read as the pore diameter Dp.

[0223] The volume of all pores with a pore diameter D of 3 nm or more and 5.5 μm or less is integrated to calculate the first integrated volume. The volume of pores with a pore diameter of 50 nm or less is integrated to calculate the second integrated volume, and the volume of pores with a pore diameter of 100 nm or more is integrated to calculate the third integrated volume. Then, by dividing the second integrated volume by the first integrated volume and multiplying the quotient by 100, the percentage R(S) of the second integrated volume relative to the first integrated volume is calculated. In addition, by dividing the third integrated volume by the first integrated volume and multiplying the quotient by 100, the percentage R(L) of the third integrated volume relative to the first integrated volume is calculated.

[0224] [Calculation of the first integrated volume relative to the mass of the catalyst]

[0225] The volume of all pores with a pore diameter D of 3 nm or more and 5.5 μm or less measured by mercury intrusion is integrated to calculate the first integral volume, which is then divided by the mass of the catalyst material. The mass of the catalyst material is the mass obtained by the amount of slurry applied to the catalyst layer or the dry mass. In the case of obtaining the mass of the catalyst material from the amount of application, the solid content (mass %) of the slurry for the catalyst layer is determined in advance and obtained based on the prescribed amount of application and solid content. In addition, in the case of obtaining the mass of the catalyst material from the dry mass, the electrode catalyst layer is processed into a predetermined size and its mass is weighed to obtain the mass.

[0226] [Calculation of the integrated pore volume relative to the geometric volume of the electrode catalyst layer]

[0227] Next, the volume of all pores with a pore diameter D of 3 nm or more and 5.5 μm or less is integrated to calculate the integrated pore volume V. Furthermore, the geometric volume of the membrane electrode assembly is calculated by multiplying the area of ​​the membrane electrode assembly used for measurement using an automatic porosimeter by its thickness. Furthermore, the volume of the polymer electrolyte membrane is calculated by multiplying the area of ​​the membrane electrode assembly used for measurement using an automatic porosimeter by the thickness of the polymer electrolyte membrane. The geometric volume V0 of the electrode catalyst layer is calculated by subtracting the volume of the polymer electrolyte membrane from the volume of the membrane electrode assembly. Then, the percentage of the integrated pore volume V relative to the geometric volume V0 of the electrode catalyst layer (V / V0) is calculated.

[0228] [Measurement of thickness of electrode catalyst layer]

[0229] The thickness of the membrane electrode assembly, cathode electrode catalyst layer, anode electrode catalyst layer, and polymer electrolyte membrane was measured by observing a cross-section of the membrane electrode assembly using a scanning electron microscope (SEM). Specifically, the cross-section of the membrane electrode assembly was observed using a scanning electron microscope (FE-SEM S-4800, manufactured by Hitachi High-Technologies, Ltd.) at a magnification of 1000x. The thickness of each layer was measured at 30 observation points in the cross-section of the membrane electrode assembly. The average thickness of the thicknesses at these 30 observation points was used as the thickness of each layer.

[0230] [Measurement of power generation performance]

[0231] The power generation performance was measured using a method based on the "Battery Evaluation and Analysis Protocol," a pamphlet published by the New Energy and Industrial Technology Development Organization (NEDO). A JARI standard cell, obtained by placing a gas diffusion layer, a gasket, and a separator on each surface of a membrane electrode assembly and clamping them to a predetermined surface pressure, was used as the evaluation cell. Then, IV measurements were performed according to the method described in the "Battery Evaluation and Analysis Protocol." The conditions at this time were set to standard conditions. Furthermore, the IV measurements were performed with the relative humidity of the anode and the relative humidity of the cathode set to RH100%. The conditions at this time were set to high humidity conditions.

[0232] [Determination of durability]

[0233] The durability was measured using the same evaluation cells as those used in the power generation performance measurement, and the durability was measured according to the humidity cycle test described in the "Battery Evaluation and Analysis Protocol" above.

[0234] [Comparison results]

[0235] Table 2 shows the results of each evaluation item for the membrane electrode assemblies of Examples 7 to 12 and Comparative Examples 6 to 10. Specifically, for each membrane electrode assembly, the first integrated volume relative to the mass M of the catalyst material constituting the membrane electrode assembly (ΣVp1 / M) and the thickness T (μm) of the electrode catalyst layer are shown in Table 2. Furthermore, for each membrane electrode assembly, the pore diameter Dp corresponding to the peak in the distribution curve of the pore volume Vp, the percentage R(L) (%) of the third integrated volume relative to the first integrated volume, and the percentage R(S) (%) of the second integrated volume relative to the first integrated volume are also shown in Table 2. Furthermore, for each membrane electrode assembly, the percentage V / V0 (%) of the first integrated volume V relative to the volume V0 of the electrode catalyst layer is shown in Table 2.

[0236] Table 2 shows the results of measuring power generation performance and durability for each of the polymer electrolyte fuel cells having the membrane electrode assemblies of Examples 7 to 12 and the polymer electrolyte fuel cells having the membrane electrode assemblies of Comparative Examples 6 to 10.

[0237] In the power generation performance results, under standard conditions, a single cell with a current of 25 A or greater at a voltage of 0.6 V was rated as "O," and a single cell with a current of less than 25 A was rated as "X." Furthermore, under high humidity conditions, a single cell with a current of 30 A or greater at a voltage of 0.6 V was rated as "O," and a single cell with a current of less than 30 A was rated as "X." Regarding durability, a single cell with a hydrogen crossover leakage current less than 10 times the initial value after 10,000 cycles was rated as "O," and a single cell with a current of 10 times or greater was rated as "X."

[0238] [Table 2]

[0239]

[0240] As shown in Table 2, in any of Examples 7 to 12, it was confirmed that the first integrated volume relative to the catalyst material mass M (ΣVp1 / M) was within the range of 2.8 to 4.5.

[0241] In all of Examples 7 to 12, it was confirmed that the power generation performance was "○" and the durability was "○" regardless of the measurement conditions. In other words, it was confirmed that the membrane electrode assemblies of Examples 7 to 12 are membrane electrode assemblies capable of forming solid polymer fuel cells with excellent power generation performance and durability.

[0242] On the other hand, in any of Comparative Examples 6 to 10, it was confirmed that the first integrated volume relative to the catalyst material mass M (ΣVp1 / M) was not within the range of 2.8 to 4.5.

[0243] Comparative Examples 6 to 10 showed a "Poor" rating for power generation performance under at least one of standard conditions and high humidity conditions. Furthermore, durability was also shown as "Poor" for Comparative Examples 7, 8, and 10. This confirms that, compared to the aforementioned Examples, Comparative Examples 6 to 10 exhibited at least reduced power generation performance and durability.

[0244] As described above, according to the second embodiment of the membrane electrode assembly and the fuel cell, the following effects can be obtained.

[0245] (5) For each of the fuel electrode side electrode catalyst layer 12A and the oxygen electrode side electrode catalyst layer 12C, when the value of the first integral volume relative to the mass of the catalyst material is in the range of greater than 2.8 and less than 4.5, by including pores with relatively large diameters in this range, the three-phase interface can be maintained within the electrode catalyst layer 12 while improving the diffusivity of the gas in the electrode catalyst layer 12 and the discharge of the generated water.

[0246] Explanation of symbols

[0247] 10…Membrane electrode assembly, 11…Polymer electrolyte membrane, 12…Electrode catalyst layer, 12A…Fuel electrode side electrode catalyst layer, 12C…Oxygen electrode side electrode catalyst layer, 13A…Fuel electrode side gasket, 13C…Oxygen electrode side gasket, 21…Catalyst material, 22…Conductive support, 23…Polymer electrolyte, 24…Fiber-like material, 30…Solid polymer fuel cell, 30A…Fuel electrode, 30C…Oxygen electrode, 31A…Fuel electrode side gas diffusion layer, 31C…Oxygen electrode side gas diffusion layer, 32A…Fuel electrode side separator, 32Ag, 32Cg…Gas flow path, 32Aw, 32Cw…Cooling water flow path, 32C…Oxygen electrode side separator.

Claims

1. A membrane electrode assembly for a solid polymer fuel cell, comprising: a solid polymer electrolyte membrane comprising a first surface and a second surface opposite to the first surface; a fuel electrode-side electrode catalyst layer comprising a first catalyst substance, a first conductive support supporting the first catalyst substance, and a first polymer electrolyte, and bonded to the first surface; an oxygen electrode-side electrode catalyst layer that includes a second catalyst substance, a second conductive carrier supporting the second catalyst substance, a second polymer electrolyte, and a fibrous substance and is bonded to the second surface; The fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer each contain voids including pores having a diameter within a range of 3 nm to 5.5 μm. The pore diameter is a value calculated from the pore volume measured by mercury intrusion porosimetry. When a value obtained by integrating the pore volumes of all the pores in the fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer is defined as a first integrated volume, The first integrated volume is divided by the mass of the catalyst material contained in the fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer, that is, the catalyst material mass (cm) 3 / g is in the range of 2.8 to 4.

5.

2. The membrane electrode assembly according to claim 1, wherein In at least one of the fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer, When a value obtained by integrating the pore volume of the pores having a pore diameter of 50 nm or less is defined as a second integrated volume, A percentage of the second integrated volume to the first integrated volume is within a range of 25% to 45%.

3. The membrane electrode assembly according to claim 1 or claim 2, wherein: In at least one of the fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer, When the value obtained by integrating the pore volume of the pores having a pore diameter of 90 nm or more is defined as the third integrated volume, A percentage of the third integrated volume to the first integrated volume is within a range of 15% to 35%.

4. The membrane electrode assembly according to claim 1 or claim 2, wherein: In at least one of the fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer, The peak of the distribution curve indicating the distribution of the pore volume with respect to the pore diameter is included in a range of the pore diameter of 0.06 μm to 0.11 μm.

5. The membrane electrode assembly according to claim 1 or claim 2, wherein: The fibrous material comprises one or more selected from electron conductive fibers and proton conductive fibers, The electron conductive fibers include at least one selected from the group consisting of carbon nanofibers, carbon nanotubes, and transition metal-containing fibers.

6. The membrane electrode assembly according to claim 1 or claim 2, wherein: The fuel electrode side electrode catalyst layer further comprises a fibrous substance, When the fibrous substance contained in the fuel electrode side electrode catalyst layer is a first fibrous substance, and the fibrous substance contained in the oxygen electrode side electrode catalyst layer is a second fibrous substance, The mass of the first fibrous substance per unit volume of the fuel electrode side electrode catalyst layer is greater than the mass of the second fibrous substance per unit volume of the oxygen electrode side electrode catalyst layer.

7. The membrane electrode assembly according to claim 1 or claim 2, wherein: The oxygen electrode side electrode catalyst layer has a thickness within a range of 5 μm to 30 μm.

8. The membrane electrode assembly according to claim 1 or claim 2, wherein: The fuel electrode side electrode catalyst layer has a thickness within a range of 5 μm to 20 μm.

9. The membrane electrode assembly according to claim 1, wherein In at least one of the fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer, When a value obtained by integrating the pore volume of the pores having a pore diameter of 50 nm or less is defined as a second integrated volume, A percentage of the second integrated volume to the first integrated volume is within a range of 25% to 55%.

10. The membrane electrode assembly according to claim 1 or claim 9, wherein: In at least one of the fuel electrode side electrode catalyst layer and the oxygen electrode side electrode catalyst layer, When the value obtained by integrating the pore volume of the pores having a pore diameter of 90 nm or more is defined as the third integrated volume, A percentage of the third integrated volume to the first integrated volume is within a range of 10% to 35%. 11 . A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 1 .

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