Membrane electrode assembly and solid polymer fuel cell

Abstract

A membrane electrode assembly according to the present invention includes pores. Pore diameters D are calculated from pore volumes measured by the mercury intrusion method. The distribution curve indicating the distribution of pore volumes in relation to pore diameters D has a first peak P1 in a first range in which the pore diameters D are 0.02-0.05 μm and a second peak P2 in a second range in which the pore diameters D are 0.3-3.0 μm.

Description

Membrane electrode assembly, and polymer electrolyte fuel cell.

[0001] This disclosure relates to a membrane electrode assembly and a polymer electrolyte fuel cell.

[0002] A polymer electrolyte fuel cell comprises a membrane electrode assembly and a pair of separators that sandwich the membrane electrode assembly. The membrane electrode assembly comprises a polymer electrolyte membrane, an anode catalyst layer, and a cathode catalyst layer. In the first example of the membrane electrode assembly, the electrode catalyst layer contains carbon particles having different particle sizes, thereby suppressing the dense distribution of pores in the electrode catalyst layer (see, for example, Patent Document 1). In the second example of the membrane electrode assembly, the electrode catalyst layer contains carbon fibers having different lengths, thereby suppressing the dense distribution of pores in the electrode catalyst layer (see, for example, Patent Document 2).

[0003] Japanese Patent Publication No. 10-241703 Japanese Patent Publication No. 2011-165362

[0004] On the other hand, even if the carbon particle combinations are in equal layers, the pore size and pore distribution will differ depending on the layer composition and layer formation conditions. Similarly, even if the carbon fiber combinations are in equal layers, the pore size and pore distribution will differ. Since the power generation performance of a fuel cell is greatly affected by the pore size and pore distribution, ultimately, there is still room for improvement in methods using carbon particle combinations or carbon fiber combinations in terms of enhancing power generation performance.

[0005] A membrane electrode assembly for solving the above problems is used in a polymer electrolyte fuel cell. The membrane electrode assembly comprises a polymer electrolyte membrane having a first surface and a second surface opposite to the first surface, a first electrode catalyst layer bonded to the first surface, and a second electrode catalyst layer bonded to the second surface. The first electrode catalyst layer includes a first catalyst material, a first conductive carrier supporting the first catalyst material, a first polymer electrolyte, and a fibrous material. The second electrode catalyst layer includes a second catalyst material, a second conductive carrier supporting the second catalyst material, and a second polymer electrolyte. The membrane electrode assembly includes pores. The diameter of the pores is calculated from the pore volume measured by the mercury intrusion method. The distribution curve showing the distribution of the pore volume with respect to the pore diameter has a first peak in a first range where the pore diameter is 0.02 μm or more and 0.05 μm or less, and a second peak in a second range where the pore diameter is 0.3 μm or more and 3.0 μm or less.

[0006] A solid polymer fuel cell for solving the above problems comprises the membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly.

[0007] According to the above-described membrane electrode assembly and polymer electrolyte fuel cell, the pair of electrode catalyst layers in the membrane electrode assembly include a relatively dense region containing small pores and a relatively sparse region having large pores. This makes it possible to increase the reaction efficiency with the dense region and improve the drainage of the generated water with the sparse region. As a result, the power generation performance of the polymer electrolyte fuel cell can be improved under both low-humidity and high-humidity conditions.

[0008] In the above-described membrane electrode assembly, the value obtained by dividing the height of the first peak by the height of the second peak may be 0.4 or greater.

[0009] In the above-described film electrode assembly, the value obtained by dividing the height of the second peak by the height of the first peak may be 0.10 or greater.

[0010] In each of the above-described membrane electrode assemblies, the height of the first peak relative to the second peak, and the height of the second peak relative to the first peak, are sufficiently high in the membrane electrode assemblies to achieve both the effects of dense regions and the effects of sparse regions.

[0011] In the above-described film electrode assembly, the full width at half maximum of the first peak may be 0.01 μm or more and 0.05 μm or less, and the full width at half maximum of the second peak may be 0.1 μm or more and 2 μm or less.

[0012] According to the above-described film electrode assembly, the peak areas in the first and second peaks are sufficiently large in the film electrode assembly to achieve both the effects of dense regions and the effects of sparse regions.

[0013] In the above-described membrane electrode assembly, the distribution curve showing the distribution of the pore volume with respect to the pore diameter may further have a third peak in a third range where the pore diameter is greater than 0.05 μm and less than 0.3 μm.

[0014] According to the above membrane electrode assembly, the membrane electrode assembly has a portion that is dense enough not to hinder gas diffusion, and has a portion that is highly reactive and conductive. This makes it possible to improve the battery performance of a polymer electrolyte fuel cell equipped with the membrane electrode assembly.

[0015] In the above-described film electrode assembly, the first electrode catalyst layer is a cathode catalyst layer, and the distribution curve of the cathode catalyst layer may have a fourth peak at the same pore diameter as the first peak, and a fifth peak at the same pore diameter as the second peak.

[0016] The above membrane electrode assembly improves drainage in the cathode catalyst layer, thereby facilitating the discharge of water generated at the oxygen electrode from the cathode catalyst layer. This improves the power generation performance of the polymer electrolyte fuel cell.

[0017] In the above-described membrane electrode assembly, the average fiber diameter of the fibrous material may be 300 nm or less.

[0018] In the above-described film electrode assembly, the thickness of the first electrode catalyst layer and the thickness of the second electrode catalyst layer may each be 5 μm or more and 20 μm or less.

[0019] According to the above-described membrane electrode assembly, the thickness of each electrode catalyst layer is 5 μm or more, which reduces variations in thickness within the electrode catalyst layer, thereby suppressing uneven distribution of catalyst material and polymer electrolyte contained in the electrode catalyst layer. The thickness of each electrode catalyst layer is 20 μm or less, which suppresses the occurrence of cracks in the electrode catalyst layer.

[0020] In the above-described membrane electrode assembly, the porosity, which is the value obtained by dividing the pore volume of the membrane electrode assembly by the sum of the volume of the first electrode catalyst layer and the volume of the second electrode catalyst layer, may be 80% or less.

[0021] According to the above-described membrane electrode assembly, the decrease in the mechanical strength of each electrode catalyst layer is suppressed, thereby increasing the durability of the membrane electrode assembly.

[0022] In the above-described membrane electrode assembly, the second electrode catalyst layer may contain a fibrous material. According to the above-described membrane electrode assembly, the durability of the second electrode catalyst layer can be increased by including a fibrous material in the second electrode catalyst layer.

[0023] In the above-described membrane electrode assembly, the fibrous material may include at least one of carbon nanofibers and polybenzimidazole nanofibers.

[0024] In the above-described membrane electrode assembly, the fibrous material of the first electrode catalyst layer may include polybenzimidazole nanofibers, the second electrode catalyst layer may include fibrous material, and the fibrous material of the second electrode catalyst layer may include carbon nanofibers.

[0025] The membrane electrode assembly and polymer electrolyte fuel cell of this disclosure can improve the power generation performance of the polymer electrolyte fuel cell under both low humidity and high humidity conditions.

[0026] Figure 1 is a cross-sectional view showing the structure of a membrane electrode assembly in one embodiment. Figure 2 is a schematic diagram illustrating the structure of the electrode catalyst layer of the membrane electrode assembly shown in Figure 1. Figure 3 is an example of a pore diameter distribution curve. Figure 4 is an exploded perspective view showing the structure of a polymer electrolyte fuel cell equipped with the membrane electrode assembly shown in Figure 1. Figure 5 shows the pore diameter distribution curves obtained for the electrode catalyst layers of each example and comparative example. Figure 6 is a table showing the evaluation results of the electrode catalyst layers of each example and comparative example.

[0027] An embodiment of a membrane electrode assembly and a polymer electrolyte fuel cell will be described with reference to Figures 1 to 6. [Electrode catalyst layer] The membrane electrode assembly will be described with reference to Figures 1 to 3. Figure 1 shows the cross-sectional structure along the thickness direction of the membrane electrode assembly.

[0028] As shown in Figure 1, the membrane electrode assembly 10 comprises a polymer electrolyte membrane 11, a cathode catalyst layer 12C, and an anode catalyst layer 12A. The polymer electrolyte membrane 11 is a solid polymer electrolyte membrane. The polymer electrolyte membrane 11 includes a first surface 11S1 and a second surface 11S2 which is the surface opposite to the first surface 11S1. The cathode catalyst layer 12C is bonded to the first surface 11S1. The cathode catalyst layer 12C is an example of a first electrode catalyst layer. The anode catalyst layer 12A is bonded to the second surface 11S2. The anode catalyst layer 12A is an example of a second electrode catalyst layer. The cathode catalyst layer 12C is an electrode catalyst layer provided in the oxygen electrode (cathode) 30C (see Figure 4). The anode catalyst layer 12A is an electrode catalyst layer provided in the fuel electrode (anode) 30A (see Figure 4). The outer periphery of the electrode catalyst layer 12 may be sealed with a gasket or the like (not shown).

[0029] Figure 2 schematically shows the structure of the electrode catalyst layer. The electrode catalyst layer 12 described below (see Figure 2) is an electrode catalyst layer applied to the cathode catalyst layer 12C, but the electrode catalyst layer 12 may be applied to both the cathode catalyst layer 12C and the anode catalyst layer 12A.

[0030] As shown in FIG. 2, the electrode catalyst layer 12 includes a catalyst material 21, a conductive carrier 22, a polymer electrolyte 23, and a fibrous material 24. The conductive carrier 22 supports the catalyst material 21. When the electrode catalyst layer 12 is applied to the anode catalyst layer 12A, the electrode catalyst layer 12 may not include the fibrous material 24. Here, the catalyst material 21 included in the cathode catalyst layer 12C is an example of a first catalyst material, and the catalyst material 21 included in the anode catalyst layer 12A is an example of a second catalyst material. The conductive carrier 22 included in the cathode catalyst layer 12C is an example of a first conductive carrier, and the conductive carrier 22 included in the anode catalyst layer 12A is an example of a second conductive carrier. The polymer electrolyte 23 included in the cathode catalyst layer 12C is an example of a first polymer electrolyte, and the polymer electrolyte 23 included in the anode catalyst layer 12A is an example of a second polymer electrolyte.

[0031] In the electrode catalyst layer 12, portions where the catalyst material 21, the conductive carrier 22, the polymer electrolyte 23, and the fibrous material 24 do not exist are voids. In the present embodiment, voids having a diameter of 0.01 μm or more and 5 μm or less are defined as pores among the voids. Each electrode catalyst layer 12 includes a large number of pores, and thus the membrane electrode assembly 10 includes a large number of pores.

[0032] In the membrane electrode assembly 10, the pore diameter of the pores calculated from the pore volume Vp measured by the mercury intrusion method is the pore diameter D. Here, the pore diameter D is defined as the diameter of the cylindrical model pores obtained by the mercury intrusion method.

[0033] Here, the distribution of the pore volume Vp described above will be explained. The distribution of the pore volume Vp is represented by a distribution function of the pore volume Vp (= dVp / dlogD) with respect to the pore diameter D (10 nm ≤ D ≤ 5.0 μm). The distribution of the pore volume Vp is obtained by the mercury intrusion method.

[0034] Because mercury has high surface tension, a predetermined pressure P must be applied to allow mercury to enter a pore. The distribution of the pore volume Vp and the specific surface area can be determined from the pressure P applied to allow mercury to enter the pore and the amount of mercury injected into the pore. The relationship between the applied pressure P and the pore diameter D into which mercury can enter at that pressure P can be expressed by equation (1), known as Washbum's equation. In equation (1) below, γ is the surface tension of mercury, and θ is the contact angle between mercury and the pore wall. In this embodiment, the pore diameter D is calculated using a surface tension γ of 0.48 N / m and a contact angle θ of 130°. D = -4γcosθ / P … Equation (1)

[0035] When actually performing measurements using the mercury intrusion method, the volume of mercury injected is recorded separately by applying different pressures P. Then, each pressure P is converted to a pore diameter D based on the above formula (1). Furthermore, assuming that the volume of injected mercury and the pore volume Vp are equal, the increase in pore volume dV, which is the increase in pore volume Vp when the pore diameter increases from D to D+dD, is plotted against the pore diameter D. In the distribution curve obtained by this plot, a point that is larger than the preceding and succeeding points, i.e., a peak, is the peak of the pore volume Vp distribution. However, even if a point is larger than the preceding and succeeding points in the distribution curve, if the value of dVp / dlogD at that point is 10% or less of the maximum value of dVp / dlogD, that point is considered noise rather than a peak.

[0036] From the above-mentioned perspective, in the membrane electrode assembly 10 of this disclosure, the distribution curve showing the distribution of pore volume Vp with respect to pore diameter D in the membrane electrode assembly 10 satisfies condition 1. (Condition 1) The curve has a first peak P1 (see Figure 3) in a first range where the pore diameter D is 0.02 μm or more and 0.05 μm or less, and a second peak P2 (see Figure 3) in a second range where the pore diameter D is 0.3 μm or more and 3.0 μm or less.

[0037] In the membrane electrode assembly 10, the functions required for improving the power generation performance are, for example, maintaining the three-phase interface in the electrode catalyst layer 12, gas diffusion in the electrode catalyst layer 12, and discharging the generated water in the electrode catalyst layer 12. 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 discharging the generated water are different from each other, and the pore diameter D suitable for improving the power generation performance needs to include these respective pore diameters D. Note that the three-phase interface is an interface formed by a polymer electrolyte, a catalyst, and a gas.

[0038] Since the membrane electrode assembly 10 has the first peak P1 (see FIG. 3), the electrode catalyst layer 12 can have a suitable structure from the viewpoints of maintaining the three-phase interface and gas diffusion. Thereby, in the membrane electrode assembly 10, the reaction efficiency, that is, the power generation performance can be enhanced. Further, since the membrane electrode assembly 10 satisfies the second peak P2 (see FIG. 3) simultaneously with the first peak P1, the electrode catalyst layer 12 can have a suitable structure also from the viewpoint of discharging the generated water. Thereby, it is possible to suppress a decrease in the power generation performance due to clogging of the generated water. Thus, by the electrode catalyst layer 12 simultaneously having a relatively dense region including small pores caused by the conductive carrier 22 and a relatively porous region including large pores caused by the fibrous material 24, the power generation performance of the solid polymer fuel cell 30 can be enhanced regardless of the humidification conditions.

[0039] Note that the distribution curve of the membrane electrode assembly 10 may have two or more peaks corresponding to the first peak P1 within the first range, and may have two or more peaks corresponding to the second peak P2 within the second range.

[0040] The distribution curve of the membrane electrode assembly 10 may satisfy at least one of Conditions 2 to 5. That is, the distribution curve of the electrode catalyst layer 12 may satisfy only any one of Conditions 2 to 5, or may satisfy two or more of Conditions 2 to 5.

[0041] (Condition 2) The value obtained by dividing the height of the first peak P1 by the height of the second peak P2 is 0.4 or more. (Condition 3) The value obtained by dividing the height of the second peak P2 by the height of the first peak P1 is 0.10 or more.

[0042] (Condition 4) The full width at half maximum of the first peak P1 is 0.01 μm or more and 0.05 μm or less, and the full width at half maximum of the second peak P2 is 0.1 μm or more and 2 μm or less. (Condition 5) A third peak P3 (see Figure 3) is further present in a third range where the pore diameter D is greater than 0.05 μm and less than 0.3 μm.

[0043] If the membrane electrode assembly 10 satisfies conditions 2 and 3, the height of the first peak P1 relative to the second peak P2, and the height of the second peak P2 relative to the first peak P1, are sufficiently high in the membrane electrode assembly 10 to achieve both the effects of dense regions and sparse regions. If the membrane electrode assembly 10 satisfies condition 4, the peak areas of the first peak P1 and the second peak P2 are sufficiently large in the membrane electrode assembly 10 to achieve both the effects of dense regions and sparse regions.

[0044] The definition of the full width at half maximum (FWHM) for each peak in condition 4 is as follows: When the pore volume at the peak position in the distribution function distribution f(D) = dVp / dlogD of the pore volume Vp is f(Dmax), the smallest pore diameter D that satisfies f(Dmax) / 2 is defined as minimum diameter D1, and the largest pore diameter D is defined as maximum diameter D2. In this case, the value obtained by subtracting the minimum diameter D1 from the maximum diameter D2 (D2-D1) is the FWHM.

[0045] When the membrane electrode assembly 10 satisfies condition 5, the membrane electrode assembly 10 has a portion that is dense enough not to hinder gas diffusion and has high reactivity and conductivity. This makes it possible to improve the battery performance of a polymer electrolyte fuel cell equipped with the membrane electrode assembly. For example, in the membrane electrode assembly 10, the first peak P1 and the second peak P2 may originate from the structure of the cathode catalyst layer 12C, and the third peak P3 may originate from the structure of the anode catalyst layer 12A. In this case, in addition to improving the drainage of the cathode catalyst layer 12C, the anode catalyst layer 12A has a portion in which the catalyst material 21, conductive carrier 22, and polymer electrolyte 23 are densely present to the extent that gas diffusion is not hindered. As a result, conductivity and reactivity are improved in the anode catalyst layer 12A, and in cooperation with the cathode catalyst layer 12C, the battery performance of the polymer electrolyte fuel cell 30 equipped with the membrane electrode assembly 10 is improved.

[0046] Figure 3 shows an example of a distribution curve that satisfies condition 1. As shown in Figure 3, the example distribution curve has a first peak P1, a second peak P2, and a third peak P3. Each peak may be the main peak among the peaks formed by a combination of multiple waveforms, or it may be a shoulder peak.

[0047] The first peak P1 is located within the first range, where the pore diameter D is between 0.02 μm and 0.05 μm. The second peak P2 is located within the second range, where the pore diameter D is between 0.3 μm and 3.0 μm. The third peak P3 is located within the third range, where the pore diameter D is greater than 0.05 μm and less than 0.3 μm.

[0048] The first division value (HP1 / HP2), obtained by dividing the height HP1 of the first peak P1 by the height HP2 of the second peak P2, is 0.4 or greater. The first division value may be less than 10. The second division value (HP2 / HP1), obtained by dividing the height HP2 of the second peak P2 by the height HP1 of the first peak P1, is 0.10 or greater. The second division value may be 1.3 or less. The full width at half maximum (FWHM) WP1 of the first peak P1 is 0.01 μm or more and 0.05 μm or less. The full width at half maximum (FWHM) WP2 of the second peak P2 is 0.1 μm or more and 2 μm or less.

[0049] Furthermore, the first division value may be 0.78 or greater, or 1.15 or greater. The first division value may be 7.63 or less, or 5.26 or less. The second division value may be 0.19 or greater. In addition, the full width at half maximum (FWHM) of the first peak P1 may be 0.03 μm or less. The FWHM of the second peak P2 may be 1 μm or less, or 0.5 μm or less.

[0050] The thickness of the electrode catalyst layer 12 may be, for example, 5 μm or more and 20 μm or less. By having an electrode catalyst layer 12 thickness of 20 μm or less, the occurrence of cracks in the electrode catalyst layer 12 is suppressed. Furthermore, when the membrane electrode assembly 10 including the electrode catalyst layer 12 is used in a polymer electrolyte fuel cell 30 (see Figure 4), a decrease in the diffusivity and conductivity of the gas and generated water is suppressed, and consequently, a decrease in the output of the polymer electrolyte fuel cell 30 is suppressed. In addition, by having an electrode catalyst layer 12 thickness of 5 μm or more, variations in thickness in the electrode catalyst layer 12 are less likely to occur, thereby suppressing uneven distribution of the catalyst material 21 and polymer electrolyte 23 contained in the electrode catalyst layer 12. As a result, when the membrane electrode assembly 10 including the electrode catalyst layer 12 is applied to a polymer electrolyte fuel cell 30 and the polymer electrolyte fuel cell 30 is operated for a long period of time, it is possible to improve the durability of the polymer electrolyte fuel cell 30.

[0051] The thickness of the electrode catalyst layer 12 can be measured, for example, by observing the 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 may include, for example, ion milling and ultramicrotome. When processing to expose the cross-section of the electrode catalyst layer 12, it is preferable to cool the electrode catalyst layer 12. This suppresses the degradation of the polymer electrolyte 23 contained in the electrode catalyst layer 12 due to heat.

[0052] The porosity is the value obtained by dividing the pore volume of the membrane electrode assembly 10 by the sum of the volumes of the cathode catalyst layer 12C and the anode catalyst layer 12A. The porosity may be 80% or less. The porosity can be adjusted by the content of fibrous material 24 and polymer electrolyte 23 in the electrode catalyst layers 12A and 12C. The porosity can be calculated from the sum of the pore volumes Vp for all pore diameters D (10 nm ≤ D ≤ 5.0 μm) measured by the mercury intrusion method, and the volume V0, which is the sum of the volumes of each electrode catalyst layer 12A and 12C used to measure the pore volume Vp. The volume of each electrode catalyst layer 12A and 12C is the value obtained by multiplying the area by the thickness. The porosity is the percentage of the pore volume V to the volume V0 of the electrode catalyst layer. By having a porosity of 80% or less, the decrease in the mechanical strength of the membrane electrode assembly 10 is suppressed, thereby increasing the durability of the membrane electrode assembly 10.

[0053] Conditions 1 to 4 may be satisfied by the cathode catalyst layer 12C, the anode catalyst layer 12A, or both electrode catalyst layers 12A and 12C of the membrane electrode assembly 10. If only one of the cathode catalyst layer 12C or the anode catalyst layer 12A satisfies each condition, it is preferable that the cathode catalyst layer 12C satisfies each condition. By satisfying each condition in the cathode catalyst layer 12C, both the drainage of water generated in the cathode catalyst layer 12C and the diffusivity of gas flowing into the cathode catalyst layer 12C can be improved. Consequently, the decrease in output in the polymer electrolyte fuel cell 30 can be suppressed.

[0054] In other words, the distribution curve of the cathode catalyst layer 12C may have a fourth peak at the same pore diameter D as the first peak P1 in the distribution curve of the film electrode assembly 10, and a fifth peak at the same pore diameter D as the second peak P2. The height and full width at half maximum of the fourth peak may be the same as those of the first peak P1. Alternatively, the height of the fourth peak may be lower than that of the first peak P1, and the full width at half maximum of the fourth peak may be smaller than that of the first peak P1. The height and full width at half maximum of the fifth peak may be the same as those of the second peak P2. Alternatively, the height of the fifth peak may be lower than that of the second peak P2, and the full width at half maximum of the fifth peak may be smaller than that of the second peak P2.

[0055] [Solid Polymer Fuel Cell] An example of a solid polymer fuel cell 30 will be described with reference to Figure 4. Figure 4 shows the structure of a single cell of the solid polymer fuel cell 30. The solid polymer fuel cell 30 may have multiple single cells. In this case, the solid polymer fuel cell 30 has a structure in which multiple single cells are stacked.

[0056] As shown in Figure 4, the polymer electrolyte fuel cell 30 comprises a membrane electrode assembly 10, a pair of gas diffusion layers, and a pair of separators. The pair of gas diffusion layers consists of a cathode gas diffusion layer 31C and an anode gas diffusion layer 31A. The pair of separators consists of a cathode separator 32C and an anode separator 32A.

[0057] The cathode gas diffusion layer 31C is in contact with the cathode catalyst layer 12C. The cathode catalyst layer 12C and the cathode gas diffusion layer 31C together constitute the oxygen electrode (cathode) 30C. The anode gas diffusion layer 31A is in contact with the anode catalyst layer 12A. The anode catalyst layer 12A and the anode gas diffusion layer 31A together constitute the fuel electrode (anode) 30A.

[0058] Within the first surface 11S1 of the polymer electrolyte membrane 11, the portion not covered by the cathode catalyst layer 12C is the outer periphery. The cathode gasket 13C is located in the outer periphery. Within the second surface 11S2, the portion not covered by the anode catalyst layer 12A is also the outer periphery. The anode gasket 13A is located in the outer periphery. The gaskets 13C and 13A prevent gas leakage from the outer periphery of each surface.

[0059] The cathode separator 32C and the anode separator 32A sandwich a multilayer structure consisting of a membrane electrode assembly 10 and two gas diffusion layers 31C and 31A in the thickness direction of the polymer electrolyte fuel cell 30. The cathode separator 32C faces the cathode gas diffusion layer 31C. The anode separator 32A faces the anode gas diffusion layer 31A.

[0060] In the cathode separator 32C, each of the two opposing surfaces has a plurality of grooves. Of the pair of surfaces, the grooves on the surface facing the cathode gas diffusion layer 31C are gas flow paths 32CG. Of the pair of surfaces, the grooves on the surface opposite to the opposing surface are cooling water flow paths 32CW. Each gas flow path 32CG extends along a first direction, and each cooling water flow path 32CW extends along a second direction perpendicular to the first direction.

[0061] In the anode separator 32A, each of the two opposing surfaces has a plurality of grooves. Of the pair of surfaces, the grooves on the surface facing the anode gas diffusion layer 31A are gas flow paths 32AG. Of the pair of surfaces, the grooves on the surface opposite to the opposing surface are cooling water flow paths 32AW. Each gas flow path 32CG extends along a first direction, and each cooling water flow path 32AW extends along a second direction.

[0062] Each separator 32C, 32A is formed from a material that is conductive and impermeable to gas.

[0063] In the polymer electrolyte fuel cell 30, an oxygen agent is supplied to the oxygen electrode 30C through the gas channel 32CG of the cathode separator 32C. In addition, fuel is supplied to the polymer electrolyte fuel cell 30 through the gas channel 32AG of the anode separator 32A. This allows the polymer electrolyte fuel cell 30 to generate electricity. The oxygen agent may be, for example, air and oxygen. The fuel may be, for example, a fuel gas containing hydrogen and an organic fuel.

[0064] In the polymer electrolyte fuel cell 30, the reaction shown in the following reaction equation (1) occurs at the fuel electrode 30A. In contrast, the reaction shown in the following reaction equation (2) occurs at the oxygen electrode 30C. ２ → 2H ＋ + 2e － ... Reaction equation (1) 1 / 2O ２ + 2H ＋ + 2e － → H ２ O... Reaction equation (2)

[0065] Thus, the polymer electrolyte fuel cell 30 in this embodiment is a fuel cell that generates water at the oxygen electrode 30C by supplying an oxygen-containing gas to the oxygen electrode 30C.

[0066] As shown in the above reaction equation (2), water is produced from oxygen, protons, and electrons in the oxygen electrode 30C. If the water produced in the oxygen electrode 30C is not discharged outside the oxygen electrode 30C, the supply of oxygen-containing gas to the oxygen electrode 30C will be obstructed by the water. This will reduce the power generation performance of the polymer electrolyte fuel cell 30. In this respect, the membrane electrode assembly 10 of this embodiment has high drainage capacity by satisfying the above conditions, and therefore, the cathode catalyst layer 12C of the oxygen electrode 30C can more significantly improve the power generation performance of the polymer electrolyte fuel cell 30 by satisfying the above conditions.

[0067] [Method for Manufacturing a Membrane Electrode Assembly] The method for manufacturing the membrane electrode assembly 10 described above is explained below. When manufacturing the membrane electrode assembly 10, first, the catalyst 21, the conductive carrier 22, the polymer electrolyte 23, and the fibrous material 24 are mixed in a dispersion medium. The mass of the polymer electrolyte 23 may be 50% by mass or more and 150% by mass or less relative to the mass of the conductive carrier 22. By having the mass of the polymer electrolyte 23 be 1 / 2 or more of the mass of the conductive carrier 22, a shortage of proton conduction paths can be suppressed, thereby suppressing the decrease in performance due to the increase in resistance in proton conduction. By having the mass of the polymer electrolyte 23 be 3 / 2 or less of the mass of the conductive carrier 22, flooding due to blockage of pores can be suppressed, thereby suppressing the decrease in performance.

[0068] The mass of the fibrous material 24 may be 5% by mass or more and 150% by mass or less relative to the mass of the conductive carrier 22. By having a mass of fibrous material 24 that is 1 / 20 or more of the mass of the conductive carrier 22, cracks in the electrode catalyst layers 12A and 12C are suppressed, and stress on the polymer electrolyte membrane 11 is suppressed, thereby preventing the polymer electrolyte membrane 11 from breaking. By having a mass of fibrous material 24 that is 3 / 2 or less of the mass of the conductive carrier 22, the proton conductivity and conductivity within the electrode catalyst layers 12A and 12C are suppressed, thereby preventing a decrease in performance.

[0069] The mass of the dispersion medium may be 70% to 95% by mass relative to the total mass of the catalyst ink. That is, the mass of the polymer electrolyte 23 may be 1 / 2 to 3 / 2 relative to the mass of the conductive carrier 22. The mass of the fibrous material 24 may be 1 / 20 to 3 / 2 relative to the mass of the conductive carrier 22. The mass of the dispersion medium may be 7 / 10 to 19 / 20 relative to the total mass of the catalyst ink. By having the mass of the dispersion medium be 7 / 10 to 19 / 20 relative to the total mass of the catalyst ink, the viscosity of the catalyst ink can be made suitable for coating.

[0070] Subsequently, the catalyst ink is prepared by subjecting the mixture to a dispersion treatment. The fibrous material 24 may be omitted from the materials constituting the catalyst ink for forming the anode catalyst layer 12A. The dispersion treatment can be carried out using, for example, a planetary ball mill, a bead mill, and an ultrasonic homogenizer.

[0071] The dispersion medium for the catalyst ink can be a solvent that does not erode the catalyst material 21, the conductive carrier 22, the polymer electrolyte 23, and the fibrous material 24, and that can dissolve the polymer electrolyte 23 or disperse the polymer electrolyte 23 as a fine gel while maintaining high fluidity of the dispersion medium. The dispersion medium preferably contains a volatile and liquid organic solvent.

[0072] The dispersion medium may contain water. Water is preferred as a dispersion medium because it has good affinity with the polymer electrolyte 23. If the solvent contains a lower alcohol, there is a risk of ignition, so it is preferable that the solvent contain water in addition to the lower alcohol. Water can be mixed with the solvent to the extent that the catalyst ink does not become cloudy or gel due to the separation of the polymer electrolyte 23.

[0073] After forming a coating film by applying the prepared catalyst ink to the substrate, the solvent is removed from the coating film by drying it. This forms an electrode catalyst layer 12 on the substrate. A polymer electrolyte membrane 11 or a transfer substrate can be used as the substrate. When using a polymer electrolyte membrane 11 as the substrate, for example, the electrode catalyst layer 12 can be formed by directly applying the catalyst ink to the surface of the polymer electrolyte membrane 11 to form a coating film, and then removing the solvent from the coating film.

[0074] When using a transfer substrate, a catalyst layer substrate is created by applying a catalyst ink onto the transfer substrate to form a coating film, and then drying the coating film. Subsequently, for example, the electrode catalyst layer 12 and the polymer electrolyte membrane 11 are joined by heating and pressurizing while the surface of the electrode catalyst layer 12 on the catalyst layer substrate is in contact with the polymer electrolyte membrane 11. A membrane electrode assembly 10 can be manufactured by joining the electrode catalyst layer 12 to both sides of the polymer electrolyte membrane 11.

[0075] Various coating methods can be used to apply the catalyst ink to the substrate. These methods may include, for example, die coating, roll coating, curtain coating, spray coating, and squeegee coating. Die coating is preferred. Die coating is preferred because it provides stable film thickness during the coating period and allows for intermittent coating.

[0076] Methods for drying the catalyst ink coating may include, for example, drying using a hot air oven, IR (far-infrared) drying, drying using a hot plate, and reduced-pressure drying. The drying temperature may be 40°C to 200°C, and preferably 40°C to 120°C. The drying time may be 0.5 minutes to 1 hour, and preferably 1 minute to 30 minutes.

[0077] When forming an electrode catalyst layer 12 on a transfer substrate, the pressure and temperature applied to the electrode catalyst layer 12 during transfer affect the power generation performance of the membrane electrode assembly 10. To obtain a membrane electrode assembly 10 with high power generation performance, the pressure applied to the multilayer including the electrode catalyst layer 12 is preferably between 0.1 MPa and 20 MPa. A pressure of 20 MPa or less prevents excessive compression of the electrode catalyst layer 12. A pressure of 0.1 MPa or more suppresses a decrease in bonding between the electrode catalyst layer 12 and the polymer electrolyte membrane 11, thereby preventing a decrease in power generation performance. The temperature at bonding is preferably near the glass transition temperature of the polymer electrolyte membrane 11 or the polymer electrolyte 23 contained in the electrode catalyst layer 12, from the viewpoint of improving the bonding at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 12 and suppressing interfacial resistance.

[0078] The transfer substrate may be, for example, a polymer film and a sheet body formed from a fluororesin. Fluororesins have excellent transfer properties. Examples of fluororesins include ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), and polytetrafluoroethylene (PTFE). The polymer forming the polymer film may be, for example, polyimide, polyethylene terephthalate, polyamide (nylon®), polysulfone, polyethersulfone, polyphenylene sulfide, polyether / etherketone, polyetherimide, polyarylate, and polyethylene naphthalate. The transfer substrate may also be gas diffusion layers 31A and 31C.

[0079] The size and distribution of pores in the electrode catalyst layer 12 can be adjusted by changing the temperature at which the catalyst ink coating is heated, the heating rate of the coating, the pressurizing conditions until the coating dries, the particle size of the conductive carrier 22, the mixing ratio of the catalyst substance 21 to the conductive carrier 22, the fiber diameter of the fibrous material 24, the mixing ratio of the fibrous material 24, the solvent composition of the catalyst ink, and the dispersion strength when preparing the catalyst ink.

[0080] For example, the higher the proportion of fibrous material 24, the larger the pore diameter D at the second peak P2 and the higher the porosity. The larger the average fiber diameter of the fibrous material 24, the larger the pore diameter D at the second peak P2. The higher the proportion of conductive carrier 22, the smaller the pore diameter D at the first peak P1 and the lower the porosity. The larger the particle size of the conductive carrier 22, the larger the pore diameter D at the first peak P1.

[0081] For example, the third peak P3 may be a single peak formed by the overlapping of the following two peaks: one peak originating from a conductive carrier 22 having a particle size larger than that of a conductive carrier 22 capable of forming the first peak P1, and the other originating from a fibrous material 24 having an average fiber diameter smaller than that of a fibrous material 24 capable of forming the second peak P2.

[0082] Furthermore, for example, the lower the drying rate of the coating film, the smaller the maximum pore diameter D in the film electrode assembly 10 and the smaller the average pore diameter D. The higher the drying rate of the coating film, the larger the maximum pore diameter D in the film electrode assembly 10 and the larger the average pore diameter D. The higher the pressure applied to the coating film, the smaller the maximum pore diameter D in the film electrode assembly 10 and the smaller the average pore diameter D.

[0083] The catalyst 21 may be, for example, a metal included in the platinum group, a metal other than the platinum group, and alloys, oxides, complex oxides, and carbides of these metals. Metals included in the platinum group include platinum, palladium, ruthenium, iridium, rhodium, and osmium. Metals other than the platinum group may be, for example, iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum.

[0084] The conductive carrier 22 can be a carrier that is conductive and capable of supporting the catalyst material 21 without being eroded by the catalyst material 21. The conductive carrier 22 may be carbon particles. The carbon particles may be, for example, carbon black, graphite, activated carbon, carbon nanotubes, carbon nanofibers, and fullerenes. The particle size of the carbon particles is preferably, for example, 10 nm or more and 1000 nm or less, and more preferably 10 nm or more and 100 nm or less. By having a particle size of 10 nm or more, the carbon particles do not become too densely packed in the electrode catalyst layer 12, thereby suppressing a decrease in the gas diffusivity of the electrode catalyst layer 12. By having a particle size of 1000 nm or less, the occurrence of cracks in the electrode catalyst layer 12 is suppressed.

[0085] The polymer electrolyte contained in the polymer electrolyte membrane 11 and the electrode catalyst layer 12 may be a proton-conducting electrolyte. The polymer electrolyte may be, for example, a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte. The fluorine-based polymer electrolyte may be a polymer electrolyte having a tetrafluoroethylene skeleton. The polymer electrolyte having a tetrafluoroethylene skeleton may be, for example, Nafion® manufactured by DuPont. The hydrocarbon-based polymer electrolyte may be, for example, a sulfonated polyether ketone, a sulfonated polyether sulfone, a sulfonated polyether ether sulfone, a sulfonated polysulfide, and a sulfonated polyphenylene.

[0086] 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, from the viewpoint of interfacial resistance at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 12, and the rate of dimensional change in the polymer electrolyte membrane 11 and the electrode catalyst layer 12 in response to changes in humidity, it is preferable that the polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 23 contained in the electrode catalyst layer 12 are the same electrolyte or similar electrolytes.

[0087] The average fiber diameter of the fibrous material 24 may be 300 nm or less. The fibrous material 24 may be an electronically conductive fiber, a proton-conducting fiber, or a polymer fiber that does not have electronic or proton conductivity. The electronically conductive fiber may be a carbon fiber, a carbon nanotube, a carbon nanohorn, or a conductive polymer nanofiber. From the viewpoint of conductivity and dispersibility, it is preferable to use carbon nanofiber as the fibrous material 24.

[0088] Electronically conductive fibers may have catalytic activity. Electronically conductive fibers having catalytic activity are preferred because they can reduce the amount of catalyst formed from noble metals used. When the electrode catalyst layer 12 is used as the cathode catalyst layer 12C, the electronically conductive fibers having catalytic activity may be a carbon alloy catalyst made from carbon nanofibers. Electronically conductive fibers having catalytic activity may also be fibers made from the electrode active material for the fuel electrode. The electrode active material may be a substance containing at least one transition metal element selected from the group consisting of Ta, Nb, Ti, and Zr. The substance containing the transition metal element may be, for example, a partial oxide of a carbonitride of a transition metal element, or a conductive oxide of a transition metal element, or a conductive oxynitride of a transition metal element.

[0089] Proton-conducting fibers can be any fibers produced from a polymer electrolyte having proton conductivity. Materials for producing proton-conducting fibers may include fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes. Examples of fluorine-based polymer electrolytes include Nafion® from DuPont, Flemion® from Asahi Glass Co., Ltd., Aciplex® from Asahi Kasei Corporation, and Gore Select® from Gore. Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketones, sulfonated polyethersulfones, sulfonated polyetherethersulfones, sulfonated polysulfides, sulfonated polyphenylenes, sulfonated polyimides, and acid-doped polybenzoazoles.

[0090] Polymer fibers that do not possess electronic or proton conductivity may be, for example, fibers made from polyimide and polybenzimidazole, or cellulose nanofibers. It is preferable that the polymer fibers are made from polybenzimidazole having basic functional groups, as this allows for simultaneous improvement of proton conductivity and gas diffusion in the electrode catalyst layer 12. Because polybenzimidazole nanofibers are strongly basic, the ionomer coating covering the surface of the polybenzimidazole nanofibers is stabilized by acid-base interactions. Furthermore, because polybenzimidazole nanofibers are resistant to high temperatures and acids, they are physically stable in the operating environment of the fuel cell, resulting in excellent long-term performance.

[0091] The polymer fibers may be produced from substances having basic functional groups other than polybenzimidazole. Substances other than polybenzimidazole may include, for example, azole compounds, compounds containing pyrrole rings, and compounds containing pyridine rings. Azole compounds may include, for example, imidazole, thiazole, and oxazole. If the electrode catalyst layer 12 contains sulfonyl groups as proton conducting sites, the polymer fibers are covered with a polymer electrolyte film by the bonding of the sulfonyl groups with basic functional groups. This simultaneously improves the proton conductivity and gas diffusivity in the electrode catalyst layer 12. The electrode catalyst layer 12 may contain one or more polymer fibers.

[0092] In polymer fibers, it is preferable that at least one of the average fiber diameter and the peak of the fiber diameter distribution is between 100 nm and 300 nm, more preferably between 150 nm and 250 nm, and even more preferably between 180 nm and 220 nm. The lower limit of the average fiber diameter may be any of 100 nm, 150 nm, and 200 nm. The upper limit of the average fiber diameter may be any of 250 nm, 220 nm, and 210 nm. The lower limit of the peak of the fiber diameter distribution may be any of 100 nm, 150 nm, and 200 nm. The upper limit of the peak of the fiber diameter distribution may be any of 250 nm, 220 nm, and 210 nm. By having the average fiber diameter and the peak of the fiber diameter distribution fall within the above ranges, it is possible to increase the voids within the electrode catalyst layer 12 and suppress the decrease in proton conductivity, thereby increasing the output of the polymer electrolyte fuel cell 30.

[0093] The fiber diameter of polymer fibers is measured, for example, using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). By measuring the fiber diameter of several randomly selected polymer fibers, a histogram representing the frequency of each fiber diameter can be obtained. From the viewpoint of clearly identifying the peak of fiber diameter, it is preferable to measure the fiber diameter of polymer fibers at at least 50 observation points, and more preferably at 100 observation points. The observation magnification of the scanning electron microscope (SEM) or transmission electron microscope (TEM) is preferably 10,000x or higher from the viewpoint of clearly confirming the outline of the polymer fiber and thereby accurately measuring the fiber diameter. The average fiber diameter is obtained from the arithmetic mean of the obtained fiber diameters.

[0094] In the case of the polymer fiber, the peak of the fiber length distribution is preferably 1 μm or more and 100 μm or less, more preferably 10 μm or more and 50 μm or less, and still more preferably 10 μm or more and 40 μm or less. When the peak of the fiber length distribution is within the above-described range, the occurrence of cracks in the electrode catalyst layer 12 can be suppressed during the formation of the electrode catalyst layer 12, and thus, the durability of the electrode catalyst layer 12 and the membrane electrode assembly 10 can be enhanced. In addition, the voids in the electrode catalyst layer 12 can be increased, whereby the output of the solid polymer fuel cell 30 can be increased.

[0095] The specific surface area of the polymer fiber may be 1 m ２ / g or more and 15 m ２ / g or less. The aspect ratio of the polymer fiber may be 20 or more and 400 or less. The aspect ratio of the polymer fiber is a value obtained by dividing the value of the peak in the fiber length distribution by the value of the peak in the fiber diameter distribution. When at least one of the specific surface area and the aspect ratio is within the above-described range, the occurrence of cracks in the electrode catalyst layer 12 can be suppressed when forming the electrode catalyst layer 12 containing the polymer fiber, and thus, the durability of the electrode catalyst layer 12 can be enhanced. In addition, the voids in the electrode catalyst layer 12 can be increased and a decrease in proton conductivity can be suppressed, whereby the output of the solid polymer fuel cell 30 can be increased. The specific surface area and the aspect ratio of the polymer fiber can be adjusted by the fiber diameter and the fiber length of the polymer fiber.

[0096] [Examples] Examples and comparative examples will be described with reference to FIGS. 5 and 6. [Example 1] A platinum-supported carbon catalyst, water, 1-propanol, a polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and polybenzimidazole nanofibers (average fiber diameter: 210 nm) were mixed. In the platinum-supported carbon catalyst, platinum particles were supported on carbon particles. The average particle diameter of the carbon particles was 30 nm, and the supported density of the platinum particles was 60% by mass.

[0097] Next, the mixture was dispersed using a planetary ball mill. During this process, approximately one-third of the zirconia container was filled with zirconia balls with a diameter of 5 mm. The processing time was set to 60 minutes, and the rotation speed to 300 rpm. This yielded a catalyst ink.

[0098] The catalyst ink was prepared such that the mass of the polymer electrolyte was 100% by mass relative to the mass of the carbon particles, the mass of the fibrous material was 5% by mass relative to the mass of the carbon particles, the proportion of water in the dispersion medium was 50% by mass, and the solid content in the catalyst ink was 10% by mass.

[0099] A catalyst film for the cathode layer 12C was formed by applying catalyst ink to the first surface 11S1 of the polymer electrolyte membrane 11 (Nafion® 211, manufactured by Dupont) using a slit die coater. Next, the polymer electrolyte membrane with the formed film was placed in a hot air oven at 80°C to dry the film until there was no change in weight. This obtained the catalyst layer 12C for the cathode. Subsequently, a catalyst film for the anode layer 12A was formed by applying the same catalyst ink to the second surface 11S2 of the polymer electrolyte membrane 11 using a slit die coater. Next, the polymer electrolyte membrane with the formed film was placed in a hot air oven at 80°C to dry the film until there was no change in weight. This obtained the membrane electrode assembly 10 of Example 1. The platinum content in the coating film on the first surface 11S1 of the polymer electrolyte membrane 11 was 0.4 mg / cm². ２ The catalyst ink was applied in such a manner. Furthermore, on the second surface 11S2, the platinum content in the coating film was 0.1 mg / cm³. ２ The catalyst ink was applied in such a manner.

[0100] [Example 2] The film electrode assembly of Example 2 was obtained by the same method as in Example 1, except that the mass of polybenzimidazole nanofibers (average fiber diameter 210 nm) was doubled when preparing the catalyst ink in Example 1.

[0101] [Example 3] The anode catalyst ink was prepared by the following method. Specifically, a platinum-supported carbon catalyst, water, 1-propanol, a polymer electrolyte (Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and carbon nanofibers (VGCF-H, average fiber diameter 150 nm, manufactured by Resonac Corporation) (VGCF is a registered trademark) were mixed. In the platinum-supported carbon catalyst, platinum particles were supported on carbon particles. The average particle diameter of the carbon particles was 45 nm, and the platinum particle support density was 20% by mass.

[0102] Next, the mixture was dispersed using a planetary ball mill. During this process, approximately one-third of the zirconia container was filled with zirconia balls with a diameter of 5 mm. The processing time was set to 60 minutes, and the rotation speed to 300 rpm. This yielded a catalyst ink.

[0103] The catalyst ink was prepared such that the mass of the polymer electrolyte was 100% by mass relative to the mass of the carbon particles, the mass of the fibrous material was 100% by mass relative to the mass of the carbon particles, the proportion of water in the dispersion medium was 50% by mass, and the solid content in the catalyst ink was 10% by mass.

[0104] Then, the film electrode assembly 10 of Example 3 was obtained by the same method as in Example 1, except that the anode catalyst ink was used when forming the coating film of the anode catalyst layer 12A.

[0105] [Example 4] The film electrode assembly 10 of Example 4 was obtained by the same method as in Example 3, except that the mass of polybenzimidazole nanofibers (average fiber diameter 210 nm) was doubled when preparing the catalyst ink for the cathode in Example 3.

[0106] [Example 5] An electrode assembly of Example 5 was obtained by the same method as in Example 1, except that an 80°C hot air oven was not used when drying the coating film of the cathode catalyst layer 12C and the anode catalyst layer 12A, and the coating film was left in a room temperature environment. The room temperature environment was 25°C and the relative humidity was 50%.

[0107] [Comparative Example 1] A film electrode assembly of Comparative Example 1 was obtained by the same method as in Example 1, except that the average particle size of the carbon particles in the platinum-supported carbon catalyst was set to 45 nm when preparing the catalyst ink.

[0108] [Comparative Example 2] In Comparative Example 1, when preparing the catalyst ink, carbon nanofibers (VGCF-H, average fiber diameter 150 nm, manufactured by Resonaq Corporation) were used instead of polybenzimidazole nanofibers (average fiber diameter 210 nm). In addition, the mass of the carbon nanofibers was set to 50% by mass relative to the mass of the carbon particles. Otherwise, the film electrode assembly of Comparative Example 2 was obtained by the same method as in Comparative Example 1.

[0109] [Comparative Example 3] A film electrode assembly of Comparative Example 3 was obtained by the same method as in Comparative Example 2, except that the anode catalyst ink used in Example 3 was used to form the coating film of the anode catalyst layer 12A.

[0110] [Comparative Example 4] A film electrode assembly of Comparative Example 4 was obtained by the same method as in Comparative Example 1, except that polybenzimidazole nanofibers (average fiber diameter 210 nm) were not incorporated into the catalyst ink when preparing the catalyst ink.

[0111] [Comparative Example 5] A film electrode assembly of Comparative Example 5 was obtained by the same method as in Comparative Example 2, except that the mass of carbon nanofibers (VGCF-H, average fiber diameter 150 nm, manufactured by Resonaq Corporation) was three times that of Comparative Example 2 when preparing the catalyst ink.

[0112] [Evaluation Method] [Area of ​​Electrode Catalyst Layer] When measuring the area of ​​the electrode catalyst layer 12, an image dimension measuring device (IM-6225, manufactured by Keyence Corporation) was used. To measure the area, first, test pieces were cut from the membrane electrode assembly 10 so that the areas of each electrode catalyst layer 12A, 12C and the polymer electrolyte membrane 11 were the same. Then, the test pieces were placed on the stage of the image dimension measuring device, and subsequently, the test pieces were imaged under conditions that allowed the edges of the test pieces to be distinguished. The area of ​​the imaged region was then automatically calculated using the image dimension measuring device.

[0113] [Pore Volume Distribution] The distribution of pore volume Vp was measured by the mercury intrusion method. Specifically, for the membrane electrode assemblies 10 of the examples and comparative examples, the distribution of pore volume Vp was measured in test specimens in which the area of ​​the electrode catalyst layer 12 was measured. Subsequently, the pore volume Vp of the test specimens was measured using an automated porosimeter (Micromeristics, Autopore IV9510). At this time, the volume of the measurement cell was set to approximately 5 cm³. ３ The pressure was set to 3 kPa and the mercury injection pressure was increased from 3 kPa to 400 MPa. This allowed us to obtain the amount of mercury injected at each pressure, i.e., the pore volume Vp. The mercury injection pressure was converted to pore diameter D using Washburn's equation, and then a distribution curve was created, which is a plot of the distribution function dVp / dlogD of pore volume Vp against pore diameter D. When converting to pore diameter D, the surface tension γ of mercury was set to 0.48 N / m, and the contact angle θ of mercury with respect to the pore wall was set to 130°. From the obtained distribution curve, the pore diameter D corresponding to the peak in the distribution curve was read.

[0114] [Thickness of the electrode catalyst layer] To measure the thickness of the electrode catalyst layer 12, first, the electrode catalyst layer 12 was processed using a cryocross-section polishing device (IB-19520CCP, manufactured by JEOL Ltd.), thereby exposing a cross-section along the thickness direction of the electrode catalyst layer 12. Subsequently, the cross-section was observed using a scanning electron microscope (SU8020, manufactured by Hitachi High-Tech Corporation), and the thickness of the electrode catalyst layer 12 was measured accordingly.

[0115] [Porrosion] For each electrode catalyst layer 12A and 12C, the volume of each electrode catalyst layer 12A and 12C was calculated by multiplying the thickness and area of ​​the electrode catalyst layer 12A and 12C. Next, the total volume V0, which is the sum of the volumes of each electrode catalyst layer 12A and 12C, was calculated. Then, the porosity (100 × V / V0) was calculated by dividing the sum of the pore volumes Vp for all pore diameters D (10 nm ≤ D ≤ 5.0 μm) in the membrane electrode assembly 10, measured by the mercury intrusion method, by the volume V0 of the electrode catalyst layer.

[0116] [Measurement of Power Generation Performance] For measuring power generation performance, a method conforming to the "Cell Evaluation and Analysis Protocol," a booklet published by the New Energy and Industrial Technology Development Organization (NEDO), was used. A JARI standard cell, in which a gas diffusion layer, gasket, and separator were placed on each surface of the membrane electrode assembly 10 and then tightened to a predetermined surface pressure, was used as the evaluation cell. Then, I-V measurement was performed in accordance with the method described in the "Cell Evaluation and Analysis Protocol." The conditions at this time were set to low humidity conditions. In addition, I-V measurement was performed with the relative humidity of the anode and cathode set to RH100%. The conditions at this time were set to high humidity conditions.

[0117] The power generation performance was evaluated at the following three levels: A: The current value is 45V or more when the cell voltage is 0.6V. B: The current value is 35V or more and less than 45V when the cell voltage is 0.6V. C: The current value is less than 35V when the cell voltage is 0.6V.

[0118] [Evaluation Results] The evaluation results for the film electrode assemblies of each example and comparative example are shown in Figures 5 and 6. The distribution curve of Example 1 had a first peak P1 and a second peak P2, with a pore diameter D of 0.031 μm for the first peak P1 and a pore diameter D of 0.89 μm for the second peak P2. Furthermore, the first division value obtained by dividing the height HP1 of the first peak P1 by the height HP2 of the second peak P2 was 1.71, and the second division value obtained by dividing the height HP2 of the second peak P2 by the height HP1 of the first peak P1 was 0.58. The full width at half maximum (FWHM) of the first peak P1 was WP1, which was 0.025 μm, and the full width at half maximum (FWHM) of the second peak P2 was WP2, which was 0.3 μm.

[0119] The distribution curve of Example 2 had a first peak P1 and two second peaks P2. It was found that the pore diameter D for the first peak P1 was 0.035 μm, the pore diameter D for the first second peak P2 was 0.55 μm, and the pore diameter D for the second second peak P2 was 1.12 μm. It was also found that the first division value obtained by dividing the height HP1 of the first peak P1 by the height HP2 of the first second peak P2 was 7.63, and the first division value obtained by dividing the height HP1 of the first peak P1 by the height HP2 of the second second peak P2 was 5.26. Furthermore, it was found that the second division value obtained by dividing the height HP2 of the first second peak P2 by the height HP1 of the first peak P1 was 0.13, and the second division value obtained by dividing the height HP2 of the second second peak P2 by the height HP1 of the first peak P1 was 0.19. It was observed that the full width at half maximum (FWHM) WP1 of the first peak P1 was 0.04 μm, the FWHM WP2 of the first second peak P2 was 0.5 μm, and the FWHM WP2 of the second second peak P2 was 0.5 μm.

[0120] The distribution curve of Example 3 had a first peak P1, a second peak P2, and a third peak P3. It was found that the pore diameter D for the first peak P1 was 0.028 μm, the pore diameter D for the second peak P2 was 1.60 μm, and the pore diameter D for the third peak P3 was 0.087 μm. It was also found that the first division value, obtained by dividing the height HP1 of the first peak P1 by the height HP2 of the second peak P2, was 1.15, and the second division value, obtained by dividing the height HP2 of the second peak P2 by the height HP1 of the first peak P1, was 0.87. The full width at half maximum (FWHM) WP1 of the first peak P1 was 0.05 μm, and the FWHM WP2 of the second peak P2 was 0.9 μm.

[0121] The distribution curve of Example 4 had a first peak P1, two second peaks P2, and a third peak P3. It was found that the pore diameter D for the first peak P1 was 0.039 μm, and the pore diameter D for the third peak P3 was 0.078 μm. It was also found that the pore diameter D for the first second peak P2 was 1.42 μm, and the pore diameter D for the second second peak P2 was 2.03 μm.

[0122] Furthermore, it was found that the first division value obtained by dividing the height HP1 of the first peak P1 by the height HP2 of the first second peak P2 was 1.54, and the second division value obtained by dividing the height HP2 of the first second peak P2 by the height HP1 of the first peak P1 was 0.65. It was also found that the first division value obtained by dividing the height HP1 of the first peak P1 by the height HP2 of the second second peak P2 was 1.34, and the second division value obtained by dividing the height HP2 of the second second peak P2 by the height HP1 of the first peak P1 was 0.74. It was found that the half-width WP1 of the first peak P1 was 0.05 μm, the half-width WP2 of the first second peak P2 was 1 μm, and the half-width WP2 of the second second peak P2 was 2 μm.

[0123] The distribution curve of Example 5 had a first peak P1 and a second peak P2, and it was found that the pore diameter D for the first peak P1 was 0.025 μm, and the pore diameter D for the second peak P2 was 1.60 μm. Furthermore, it was found that the first division value obtained by dividing the height HP1 of the first peak P1 by the height HP2 of the second peak P2 was 0.78, and the second division value obtained by dividing the height HP2 of the second peak P2 by the height HP1 of the first peak P1 was 1.28. It was found that the half-width WP1 of the first peak P1 was 0.025 μm, and the half-width WP2 of the second peak P2 was 0.5 μm.

[0124] The distribution curve of Comparative Example 1 was found to have two second peaks P2 and a third peak P3. Furthermore, in the distribution curve of Comparative Example 1, the pore diameter D for the first second peak P2 was found to be 2.17 μm, the pore diameter D for the second second peak P2 was found to be 2.76 μm, and the pore diameter D for the third peak P3 was found to be 0.066 μm. The distribution curve of Comparative Example 2 had only a third peak P3, and the pore diameter D for the third peak P3 was found to be 0.088 μm. The distribution curve of Comparative Example 3 had a second peak P2 and a third peak P3, and the pore diameter D for the second peak P2 was found to be 1.80 μm, and the pore diameter D for the third peak P3 was found to be 0.099 μm. The distribution curve of Comparative Example 4 had only a third peak P3, and the pore diameter D for the third peak P3 was found to be 0.047 μm. The distribution curve of Comparative Example 5 had only a third peak P3, and it was observed that the pore diameter D relative to the third peak P3 was 0.13 μm.

[0125] Thus, while the distribution curves of Examples 1 to 5 all have a first peak P1 and a second peak P2, none of the distribution curves of Comparative Examples 1 to 5 contained both the first peak P1 and the second peak P2.

[0126] The thickness of the cathode catalyst layer 12C was found to be 9 μm in Examples 1 and 3, 10 μm in Examples 2 and 4, and 7 μm in Example 5. The thickness of the cathode catalyst layer 12C was found to be 11 μm in Comparative Examples 1 and 3, 14 μm in Comparative Example 2, 5 μm in Comparative Example 4, and 18 μm in Comparative Example 5. The thickness of the anode catalyst layer 12A was found to be 4 μm in Examples 1 and 2 and Comparative Examples 1, 2 and 5, 10 μm in Examples 3 and 4 and Comparative Examples 3 and 4, and 2 μm in Example 5.

[0127] The area of ​​the film electrode assembly subjected to measurement with an automated porosimeter was 95.5 cm² in Example 1. ２ In Example 2, the measurement was 45.2 cm. ２ In Example 3, the measurement was 95.2 cm. ２ In Example 4, the measurement was 94.4 cm. ２ In Example 5, the measurement was 96.3 cm. ２ It was confirmed that the area of ​​the membrane electrode assembly subjected to measurement with an automated porosimeter was 50.2 cm² in Comparative Example 1. ２ In Comparative Example 2, the result was 39.8 cm. ２ In Comparative Example 3, the result was 39.5 cm. ２ In Comparative Example 4, the result was 37.0 cm. ２ In Comparative Example 5, the result was 37.8 cm. ２ It was confirmed that this was the case.

[0128] The porosity was found to be 48% in Example 1, 65% in Example 2, 66% in Example 3, 78% in Example 4, and 45% in Example 5. The porosity was found to be 68% in Comparative Example 1, 75% in Comparative Example 2, 81% in Comparative Example 3, 42% in Comparative Example 4, and 83% in Comparative Example 5.

[0129] The distribution curve of Example 1 can be said to have a first peak P1, which is due to the presence of carbon particles, and a second peak P2, which is due to the presence of polybenzimidazole nanofibers. Furthermore, in the distribution curve of Example 2, it can be said that the pore diameter D at the second peak P2 became larger than in Example 1 due to the increased amount of polybenzimidazole nanofibers added compared to Example 1.

[0130] In the film electrode assembly 10 of Example 3, the anode catalyst ink contains carbon particles with a larger particle size and carbon nanofibers with a smaller average fiber diameter compared to the catalyst ink of Example 1, so the anode catalyst layer 12A can be said to have a third peak P3. Furthermore, the pore diameter D at the second peak P2 in Example 3 is larger than the pore diameter D at the second peak P2 in Example 1. One reason why the pore diameter D at the second peak P2 differs between Example 1 and Example 3 is thought to be that the polybenzimidazole nanofibers have high flexibility, and therefore have a high degree of freedom in how the polybenzimidazole nanofibers are entangled, to the extent that the pore diameter D at the second peak P2 fluctuates.

[0131] In Example 5, the drying time was longer than in Example 1, which caused the carbon particles and polymer electrolytes to be more likely to be concentrated at the bottom, and the polybenzimidazole nanofibers to be more likely to be concentrated at the top. As a result, it is thought that the pores, which are due to the presence of polybenzimidazole nanofibers, tend to be larger. Consequently, the pore diameter D at the first peak P1 in the distribution curve of Example 5 is smaller than the pore diameter D at the first peak P1 in Example 1, and the pore diameter D at the second peak P2 in Example 5 is larger than the pore diameter D at the second peak P2 in Example 1.

[0132] In Comparative Example 1, the increased particle size of the carbon particles compared to Example 1 is thought to have altered the size distribution in the voids, which originates from the presence of polybenzimidazole nanofibers, resulting in the presence of multiple peaks. Furthermore, Comparative Examples 2, 4, and 5 do not contain polybenzimidazole nanofibers and therefore do not exhibit the second peak P2.

[0133] In Examples 1, 2, and 5, the power generation performance was B under both low and high humidity conditions, and in Examples 3 and 4, it was A under both low and high humidity conditions. In Comparative Examples 1, 2, 4, and 5, the power generation performance was C under both low and high humidity conditions, and in Comparative Example 3, it was C under low humidity conditions and B under high humidity conditions.

[0134] These results suggest that the power generation performance of the polymer electrolyte fuel cell 30 can be enhanced under both low and high humidity conditions if the distribution curve of the membrane electrode assembly 10 has one or more peaks in both the first range and the second range of pore diameter D. Furthermore, a comparison of Examples 1 to 5 suggests that the power generation performance of the polymer electrolyte fuel cell 30 can be further enhanced under both low and high humidity conditions if the distribution curve of the membrane electrode assembly 10 has a peak within the third range.

[0135] As described above, according to one embodiment of the membrane electrode assembly and the polymer electrolyte fuel cell, the following effects can be obtained: (1) The pair of electrode catalyst layers 12A and 12C in the membrane electrode assembly 10 include a relatively dense region containing small pores and a relatively sparse region having large pores. This makes it possible to increase the reaction efficiency with the dense region and to improve the drainage of the generated water with the sparse region. As a result, the power generation performance of the polymer electrolyte fuel cell 30 can be improved under both low humidity and high humidity conditions.

[0136] (2) The height of the first peak P1 relative to the second peak P2, and the height of the second peak P2 relative to the first peak P1, are sufficiently high in the film electrode assembly 10 to achieve both the effect of dense regions and the effect of sparse regions.

[0137] (3) The peak areas of the first peak P1 and the second peak P2 are sufficiently large in the film electrode assembly 10 to achieve both the effects of dense regions and the effects of sparse regions.

[0138] (4) The membrane electrode assembly 10 has a portion that is dense enough not to hinder gas diffusion, and has a portion that is highly reactive and conductive. This makes it possible to improve the battery performance of a polymer electrolyte fuel cell equipped with a membrane electrode assembly.

[0139] (5) The drainage capacity of the cathode catalyst layer 12C can be improved, thereby making it easier to discharge the water generated in the oxygen electrode 30C to the outside of the cathode catalyst layer 12C. This can improve the power generation performance of the polymer electrolyte fuel cell 30.

[0140] (6) By having a thickness of 5 μm or less for each electrode catalyst layer 12A and 12C, variations in thickness are less likely to occur in the electrode catalyst layers 12A and 12C, thereby suppressing uneven distribution of the catalyst material 21 and polymer electrolyte 23 contained in the electrode catalyst layers 12A and 12C. By having a thickness of 20 μm or less for each electrode catalyst layer 12A and 12C, the occurrence of cracks in the electrode catalyst layers 12A and 12C is suppressed.

[0141] (7) By having a porosity of 80% or less, the decrease in the mechanical strength of the electrode catalyst layer 12 is suppressed, thereby increasing the durability of the film electrode assembly 10.

[0142] 10...Membrane electrode assembly 11...Polymer electrolyte membrane 12...Electrode catalyst layer 12A...Anode catalyst layer 12C...Cathode catalyst layer 21...Catalyst material 22...Conductive carrier 23...Polymer electrolyte 24...Fibrous material 30...Solid polymer fuel cell 32A...Anode separator 32C...Cathode separator

Claims

1. A membrane electrode assembly used in a polymer electrolyte fuel cell, comprising: a polymer electrolyte membrane having a first surface and a second surface opposite to the first surface; a first electrode catalyst layer bonded to the first surface; and a second electrode catalyst layer bonded to the second surface, wherein the first electrode catalyst layer comprises a first catalyst material, a first conductive carrier supporting the first catalyst material, a first polymer electrolyte, and a fibrous material; the second electrode catalyst layer comprises a second catalyst material, a second conductive carrier supporting the second catalyst material, and a second polymer electrolyte; the membrane electrode assembly includes pores, and the diameter of the pores, calculated from the pore volume measured by the mercury intrusion method, is the pore diameter. A membrane electrode assembly having a distribution curve showing the distribution of pore volume with respect to pore diameter, having a first peak in a first range where the pore diameter is 0.02 μm or more and 0.05 μm or less, and a second peak in a second range where the pore diameter is 0.3 μm or more and 3.0 μm or less.

2. The membrane electrode assembly according to claim 1, wherein the value obtained by dividing the height of the first peak by the height of the second peak is 0.4 or more.

3. The membrane electrode assembly according to claim 1 or 2, wherein the value obtained by dividing the height of the second peak by the height of the first peak is 0.10 or more.

4. The film electrode assembly according to claim 1 or 2, wherein the full width at half maximum of the first peak is 0.01 μm or more and 0.05 μm or less, and the full width at half maximum of the second peak is 0.1 μm or more and 2 μm or less.

5. The membrane electrode assembly according to claim 1 or 2, wherein the distribution curve showing the distribution of the pore volume with respect to the pore diameter further has a third peak in a third range where the pore diameter is greater than 0.05 μm and less than 0.3 μm.

6. The film electrode assembly according to claim 1 or 2, wherein the first electrode catalyst layer is a cathode catalyst layer, and the distribution curve of the cathode catalyst layer has a fourth peak at the same pore diameter as the first peak, and a fifth peak at the same pore diameter as the second peak.

7. The membrane electrode assembly according to claim 1 or 2, wherein the average fiber diameter of the fibrous material is 300 nm or less.

8. The film electrode assembly according to claim 1 or 2, wherein the thickness of the first electrode catalyst layer and the thickness of the second electrode catalyst layer are each 2 μm or more and 20 μm or less.

9. The membrane electrode assembly according to claim 1 or 2, wherein the porosity, which is the value obtained by dividing the pore volume of the membrane electrode assembly by the sum of the volume of the first electrode catalyst layer and the volume of the second electrode catalyst layer, is 80% or less.

10. The film electrode assembly according to claim 1 or 2, wherein the second electrode catalyst layer contains a fibrous material.

11. The membrane electrode assembly according to claim 1 or 2, wherein the fibrous material comprises at least one of carbon nanofibers and polybenzimidazole nanofibers.

12. The membrane electrode assembly according to claim 5, wherein the fibrous material of the first electrode catalyst layer comprises polybenzimidazole nanofibers, the second electrode catalyst layer comprises fibrous material, and the fibrous material of the second electrode catalyst layer comprises carbon nanofibers.

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

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