Catalyst electrode layer for fuel cells, and manufacturing method of electrode catalyst layer for fuel cells

Abstract

To provide a technique capable of achieving both proton conductivity and gas diffusion resistance in a catalyst electrode layer for fuel cells by controlling the coating structure of the ionomer on the catalyst support surface.SOLUTION: The catalyst electrode layer for fuel cells includes: a catalyst supporting material having a catalyst support, a catalyst metal supported on the catalyst support; and an ionomer partially covering the catalyst supporting material. The ionomer coverage, which is the ratio of the surface area covered with the ionomer to the surface area of the catalyst carrier obtained by three-dimensional transmission electron microscopy is 25% or more and 50% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a catalyst electrode layer for a fuel cell and a method for producing an electrode catalyst layer for a fuel cell. [Background technology]

[0002] For example, a polymer electrolyte fuel cell has an electrode catalyst layer containing a catalyst support material in which a catalytic metal such as Pt is supported on a conductive support. It is known that in an electrode catalyst layer, coating a larger amount of ionomer on the catalyst support forms a three-phase interface between the catalytic metal, the proton-conductive ionomer, and the reactant gas, thereby improving cell performance.

[0003] On the other hand, in an electrode catalyst layer, the surface of the catalytic metal supported on the carrier is coated with an ionomer, which can increase the gas diffusion resistance to the catalytic metal. To avoid the catalytic metal being coated with an ionomer, it has been proposed to retain the catalytic metal within the mesopores of the catalyst carrier (Patent Document 1). Patent Document 1 proposes that the ratio of the specific surface area of ​​the catalytic metal that can be reached by gas without passing through the electrolyte to the total specific surface area of ​​the catalytic metal, in other words, the catalytic metal exposure rate, be set to 50% or more, thereby reducing the gas diffusion resistance. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2016 / 067878 Summary of the Invention

[0005] However, in the method of Patent Document 1, the openings of the mesopores of the support particles may be coated with an ionomer. In such cases, the gas must diffuse through the ionomer to reach the catalyst metal. Therefore, the method of Patent Document 1 still has the problem of gas diffusion resistance caused by the ionomer. Furthermore, the method of Patent Document 1 does not take into consideration the use of a solid catalyst support. [Problem to be solved by the invention]

[0006] The present specification provides a technology for achieving both proton conductivity and gas diffusion resistance by controlling the coating structure of an ionomer on the surface of a catalyst carrier in a catalyst electrode layer for a fuel cell. [Means for solving the problem]

[0007] The present inventors have discovered that both gas diffusion and proton conductivity can be achieved by setting the ionomer coverage, which is the ratio of the surface area covered by ionomer to the surface area of ​​the catalyst support measured by three-dimensional transmission electron microscopy (3D-TEM), within a certain range. Conventionally, the ionomer coverage relative to the "catalyst" has been used as an indicator. However, the present inventors have used the "ionomer coverage relative to the catalyst support" as an indicator rather than the "ionomer coverage relative to the catalyst." Furthermore, they have discovered that by limiting the "ionomer coverage relative to the catalyst support," gas diffusion can be improved while maintaining proton conductivity, thereby improving catalytic activity. The "ionomer coverage relative to the catalyst support" has been found to be a more effective indicator of ionomer for improving catalytic activity.

[0008] The technology disclosed in this specification is embodied in an electrode catalyst layer for a fuel cell. The electrode catalyst layer includes a catalyst support material having a carbon-based catalyst support and a catalytic metal supported on the catalyst support, and an ionomer that partially covers the catalyst support material. Furthermore, the ionomer coverage, which is the ratio of the surface area covered by the ionomer to the surface area of ​​the catalyst support obtained by 3D-TEM, is 25% to 50%.

[0009] In this electrode catalyst layer, the surface of the catalyst support is partially coated with ionomer at a predetermined ratio. This maintains proton conductivity due to contact between the catalyst metal and the ionomer. Furthermore, regardless of the structure of the catalyst support (porous / solid), an area is secured on the catalyst support where gas can reach the catalyst metal without passing through the ionomer, reducing gas diffusion resistance to the catalyst metal. As a result, the electrode catalyst layer exhibits good catalytic activity, contributing to excellent cell performance.

[0010] The technology disclosed in this specification is also embodied in a method for manufacturing an electrode catalyst layer for a fuel cell. This manufacturing method includes the steps of preparing a catalyst ink containing a catalyst support material having a carbon-based catalyst support and a catalytic metal supported on the catalyst support, an ionomer, water, a bipolar solvent having a boiling point greater than 100°C and less than or equal to 170°C, and an aqueous medium containing ethanol, and supplying the catalyst ink onto a substrate and drying it to form the electrode catalyst layer.

[0011] This manufacturing method allows for the production of an electrode catalyst layer containing a catalyst-supporting material with an ionomer coverage of 25% to 50% by adjusting the ionomer coverage measured by 3D-TEM. This electrode catalyst layer exhibits good catalytic activity and contributes to excellent battery performance. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view showing the basic structure of a polymer electrolyte fuel cell according to one embodiment of the present invention. [Figure 2A] 1 is a cross-sectional view showing an example of a catalyst-supporting particle, which is a catalyst-supporting material according to one embodiment of the present invention. [Figure 2B] 1 is a cross-sectional view showing an example of a catalyst-supporting particle, which is a catalyst-supporting material according to one embodiment of the present invention. [Figure 3] FIG. 10 is a graph showing the relationship between the mass ratio of ethanol to diacetone alcohol in a catalyst ink and the ionomer coverage. [Figure 4] FIG. 1 is a graph showing the relationship between ionomer coverage and gas diffusion resistance. [Figure 5] FIG. 1 is a graph showing the relationship between the ionomer coverage and the proton resistance of the catalyst layer. [Figure 6] FIG. 10 is a diagram showing the relationship between power generation performance in an example and a comparative example. [Figure 7] FIG. 10 is a diagram showing the relationship between power generation performance (target output value) between an example and a comparative example. [Figure 8] FIG. 1 is a graph showing the relationship between the ionomer coverage and the power generation performance (target output value). DETAILED DESCRIPTION OF THE INVENTION

[0013] The fuel cell electrode catalyst layer (hereinafter also referred to as "catalyst layer" in this specification) disclosed herein comprises a catalyst support material having a carbon-based catalyst support and a catalytic metal supported on the catalyst support, and an ionomer that partially covers the catalyst support material. The ionomer coverage, which is the ratio of the surface area covered by the ionomer to the surface area of ​​the catalyst support measured by 3D-TEM, is 25% or more and 50% or less.

[0014] In another embodiment of the catalyst layer, the average thickness of the ionomer is 6 nm or more and 20 nm or less. When the average thickness is in this range, the catalyst support is coated with a sufficient amount of ionomer.

[0015] In another embodiment of the catalyst layer, the ionomer includes a sulfonic acid-based ionomer, because the sulfone-based ionomer may be advantageous in limiting the ionomer coverage and improving power generation performance.

[0016] In another embodiment of the catalyst layer, the catalyst support includes porous particles or solid particles, which may make it easier to control the ionomer coverage.

[0017] The membrane electrode assembly for a fuel cell disclosed in this specification includes the above-described electrode catalyst layer and an electrolyte layer. The fuel cell disclosed in this specification includes the above-described membrane electrode assembly for a fuel cell.

[0018] The membrane electrode assembly for a fuel cell and the fuel cell disclosed in this specification can include various embodiments of the catalyst layer described above.

[0019] In one embodiment of the method for manufacturing an electrode catalyst layer for a fuel cell disclosed herein, the bipolar solvent may contain diacetone alcohol. Diacetone alcohol may be preferable for adjusting the ionomer coverage. Furthermore, in one embodiment of this manufacturing method, the mass ratio of ethanol to the bipolar solvent is 0.10 or more and 0.50 or less. This mass ratio in this range makes it easy to adjust the ionomer coverage.

[0020] Hereinafter, an electrode catalyst layer for a fuel cell, a manufacturing method for an electrode catalyst layer, a membrane electrode assembly (MEA) for a fuel cell, a fuel cell, and the like disclosed in this specification will be described with reference to the drawings as appropriate. For convenience of explanation, an overview of a fuel cell will be described, and then the disclosure of this specification will be described. Note that the fuel cell in this specification is not particularly limited, but may be, for example, a polymer electrolyte fuel cell (PEFC). Furthermore, the fuel cell may be a battery mounted on a mobile body such as an FCEV, or may be a stationary fuel cell.

[0021] (fuel cell) Although not particularly limited, the fuel cell 2 is typically constructed by stacking or winding a plurality of cells 4. FIG. 1 shows an example of a fuel cell 2 in which the cells 4 are stacked. The cell 4 includes an electrolyte layer 6, an anode electrode catalyst layer (hereinafter also referred to as an anode catalyst layer) 8 and a cathode electrode catalyst layer (hereinafter also referred to as a cathode catalyst layer) 10 sandwiching the electrolyte layer 6, an anode gas diffusion layer 14, a cathode gas diffusion layer 16, and a pair of separators 20a, 20b. The electrolyte layer 6, the anode catalyst layer 8, and the cathode catalyst layer 10 form a membrane electrode assembly (MEA). Furthermore, the anode gas diffusion layer 14 and the cathode gas diffusion layer 16 may be joined to form a membrane electrode gas diffusion layer assembly (MEGA).

[0022] (electrolyte layer) For example, a fluorine-based ionomer or a hydrocarbon-based ionomer, which will be described later as an ionomer (polymer electrolyte), can be used. In this case, it is not necessary to use the same ionomer as that used in the catalyst layers 8 and 10.

[0023] The thickness of the electrolyte layer 6 is not particularly limited and may be appropriately determined in consideration of the characteristics of the resulting fuel cell 2. The thickness of the electrolyte layer is usually about 5 to 300 μm. The anode catalyst layer 8 and the cathode catalyst layer 10 will be described later.

[0024] (gas diffusion layer) There are no particular limitations on the anode gas diffusion layer 14 and the cathode gas diffusion layer 16, but known materials can be used as appropriate. The thickness of the substrate can be determined appropriately taking into consideration the properties of the resulting gas diffusion layers 14, 16, but it should be approximately 30 to 500 μm.

[0025] (separator) The pair of separators 20a, 20b are the anode separator 20a and the cathode separator 20b, which sandwich the anode gas diffusion layer 14 and the cathode gas diffusion layer 16 from the outside, respectively. Each separator 20a, 20b may be formed by corrugating a plate material such as carbon, such as carbon graphite or a carbon plate, or a metal material, such as stainless steel, to form a gas flow path between the separator and the gas diffusion layer. Note that gas seals and the like between the separators 20a, 20b and the electrolyte layer 6 are omitted in FIG. 1. Note that the side of each separator 20a, 20b that does not face the gas diffusion layers 14, 16 serves as a flow path for a refrigerant, such as water, during operation of the fuel cell 2.

[0026] (catalyst layer) The anode catalyst layer 8 and the cathode catalyst layer 10 each include a carbon-based catalyst support 32, a catalyst metal 36 supported on the catalyst support 32, and an ionomer 38 that partially coats the catalyst support 32. The catalyst layer in this specification may be either the anode catalyst layer 8 or the cathode catalyst layer 10. From the standpoint of gas diffusibility and the like, the cathode catalyst layer 10 may be preferable. As will be described later, the catalyst support 32 can take various shapes. In the following, the catalyst support 32 will be described using, as an example, catalyst-supported particles 30 as a catalyst support material in the form of particles. In the following, the anode catalyst layer 8 and the cathode catalyst layer 10 will be described collectively as catalyst layers 12.

[0027] Fig. 2 shows an overview of the catalyst-supported particle 30. As shown in Fig. 2, the catalyst-supported particle 30 has a carbon-based catalyst support 32, a catalyst metal 36, and an ionomer 38. Each material will be described later, and the coating structure of the ionomer 38 on the catalyst-supported particle 30 composed of these materials will be described below.

[0028] In the catalyst layer 12 shown in Fig. 2, the catalyst support 32 is in the form of particles. The catalyst support 32 may be porous particles or solid particles. An example of the catalyst support 32 in the form of porous particles is shown in Fig. 2A, and an example of the catalyst support 32 in the form of solid particles is shown in Fig. 2B.

[0029] As shown in Fig. 2A, the catalyst support 32 has the catalytic metal 36 on its surface, and when the catalyst support 32 is a porous particle, the catalytic metal 36 is provided inside the pores 34. As shown in Fig. 2B, when the catalyst support 32 is a solid particle, the catalytic metal 36 is provided only on its surface.

[0030] 2A and 2B, the ionomer 38 partially covers the surface of the catalyst support particle 30. As a result, the catalyst support particle 30 has a surface 30a that is not covered with the ionomer 38 and where the catalyst support 32 is exposed, and a surface 30b that is covered with the ionomer. By providing the surface 30a, the gas supplied from the outside comes into direct contact with the catalyst metal 36, either directly or via the pores 34.

[0031] The surface of the catalyst support 32 is coated with the ionomer 38 so that the ionomer coverage, which is the ratio of the surface area covered by the ionomer 38 to the surface area of ​​the catalyst support 32 measured by 3D-TEM, is 25% to 50%. Within this range, a catalyst layer 12 can be provided that achieves both gas diffusion resistance and proton conductivity and contributes to excellent power generation performance. This range of ionomer coverage is a range in which the amount of exposed surface of the catalyst support 32 is greater than that of conventional catalyst supports 32 of this type.

[0032] Here, 3D-TEM is a technique for analyzing the three-dimensional structure of a material by using computed tomography (CT) to capture TEM projection images taken while continuously tilting the target object using a TEM. This technique observes the three-dimensional structure of the material (dispersion state, defects, etc.) in three dimensions and enables quantitative evaluation of particle size, particle size distribution, volume, surface area, thickness, etc. through image analysis. Therefore, by observing the catalyst layer 12 containing catalyst-supported particles 30 using a 3D-TEM, it is possible to measure the surface area of ​​the catalyst support 32, the surface structure (identification of uncoated and coated regions with ionomer), the surface area coated with ionomer 38, and the thickness of the ionomer 38.

[0033] A known 3D-TEM can be used as the 3D-TEM, and a person skilled in the art can obtain the ionomer coverage of the catalyst layer 12 using a known 3D-TEM and a measurement program provided in the 3D-TEM. The sample for the 3D-TEM is not particularly limited, and may be a catalyst layer 12 in the form of an MEA or MEGA. The catalyst layer 12 may be, for example, a catalyst ink coated and dried, or may be thermocompressed. Preferably, the surface of the catalyst layer 12 formed is scraped off to obtain a portion of a predetermined size to be used as the sample. Details of the preparation of the test powder from the sample are disclosed in the Examples. When measuring the ionomer coverage, a target area (200 nm × 200 nm) is set for the test powder obtained from the catalyst layer 12 to be measured, and the measurement results for this target area are used as the ionomer coverage of the test powder.

[0034] The ionomer coverage rate can be obtained by 3D-TEM, for example, by the following method: That is, it is calculated as the ratio (%) of the surface area of ​​the surface 30b evaluated as being covered with the ionomer 38 to the surface area of ​​the catalyst support 32 (for example, the total surface area of ​​the surface 30a evaluated by 3D-TEM as being exposed on the catalyst support 32 and the surface 30b evaluated as being covered with the ionomer 38).

[0035] When the ionomer coverage is less than 25%, the proton resistance of the catalyst layer 12 measured by the AC impedance method is 1.5 Ω / cm 2 According to the present inventors, the proton resistance tends to exceed 1.5 Ω / cm. 2 If the ionomer coverage is not equal to or less than this, the intended power generation performance cannot be obtained. For example, from the viewpoint of proton resistance, the ionomer coverage is 30% or more, and in some cases, 35% or more is preferable.

[0036] Furthermore, if the ionomer coverage exceeds 50%, the gas diffusion resistance measured by the limiting current density method tends to exceed 23.5 s / m. According to the present inventors, the intended power generation performance cannot be obtained unless the gas diffusion resistance is 23.5 s / m or less. For example, from the viewpoint of gas diffusion resistance, the ionomer coverage may be preferably 48% or less, 45% or less, 40% or less, or 38% or less.

[0037] The range of the ionomer coverage can be set by appropriately combining the various upper and lower limits described above, in addition to 25% or more and 50% or less, and can be, for example, 30% or more and 40% or less.

[0038] A layer of ionomer 38 is formed on the surface 30b of the catalyst-supported particle 30 that is coated with ionomer 38. The thickness of the layer of ionomer 38 may be an average thickness of 6 nm or more and 20 nm or less, as measured by 3D-TEM. When the average thickness is within this range, the catalyst support 32 is coated with a sufficient amount of ionomer 38. Furthermore, when the average thickness is within this range, it is effective in ensuring an appropriate mass ratio between the catalyst metal 36 and the ionomer 38 and ensuring proton conductivity. In some cases, the average thickness may be preferably, for example, 7 nm or more, 8 nm or more, 9 nm or more, or 10 nm or more. In other cases, the average thickness may be, for example, 18 nm or less, 16 nm or less, 14 nm or less, 12 nm or less, or 10 nm or less.

[0039] The same sample as that used to measure the ionomer coverage can be used as the sample for measuring the average thickness of ionomer 38. When measuring the average thickness of ionomer 38, one target area (200 nm × 200 nm) is set for the test powder obtained from the catalyst layer 12 to be measured, and the median value (50% cumulative value) in the cumulative distribution on a volume basis of the thickness of ionomer 38 measured for this target area is taken as the average thickness of the ionomer of the test powder.

[0040] The material of the catalyst support 32 is not particularly limited, and known porous or non-porous (solid) carbon-based support materials can be appropriately selected and used. The shape of the catalyst support 32 may be particles of various shapes or a continuous body. Particles of various shapes, such as spherical, fibrous, and irregular shapes, can be used. Furthermore, examples of continuous bodies include entangled bodies such as knitted bodies, net-like bodies, cloth-like bodies, and nonwoven fabrics. In the case of porous materials, the shape, pore size, etc., are not particularly limited.

[0041] The material of the catalyst carrier 32 is not particularly limited, but examples thereof include carbon materials such as carbon black (e.g., ketjen black, oil furnace black, channel black, lamp black, thermal black, acetylene black), activated carbon, etc. Also included are carbon fibers such as multi-walled carbon nanotubes. These carbon fibers can be in the form of nonwoven fabric, carbon paper, carbon cloth, etc.

[0042] Furthermore, the catalyst support 32 may also contain porous metals such as Sn (tin) and Ti (titanium), or even conductive metal oxides, as part of the support. There are no particular limitations on the size or shape of the pores 34 of the catalyst support 32.

[0043] The catalytic metal 36 has a function of catalyzing an electrochemical reaction. Here, the catalytic metal used in the anode catalytic layer 8 is not particularly limited as long as it has a catalytic effect on the oxidation reaction of hydrogen, and known catalysts can be used in the same manner.

[0044] The catalytic metal 36 used in the cathode catalytic layer 10 is also not particularly limited as long as it has catalytic activity in the oxygen reduction reaction, and known catalysts can be used in the same manner. Specifically, it can be selected from metals such as platinum, ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper, silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, and alloys thereof. For example, from the standpoint of catalytic activity, platinum or a platinum alloy containing one or more elements selected from the group consisting of ruthenium, iron, nickel, manganese, cobalt, and copper may be preferred.

[0045] The shape and size of the catalytic metal 36 are not particularly limited, and may be the same as those of known catalytic components. Examples of shapes that can be used include granular, scale-like, and layer-like shapes, with granular shapes being preferred. The average particle size of the catalytic metal is not particularly limited.

[0046] The ionomer 38 has proton conductivity and also functions as a binder for the catalyst support 32 in the catalyst layer 12. The ionomer 38 is not particularly limited, and one or more of various known materials may be appropriately selected and used.

[0047] The ionomer 38 is roughly classified into fluorine-based ionomers and hydrocarbon-based ionomers, with the fluorine-based ionomers being preferred.

[0048] Examples of fluorine-based ionomers include perfluorocarbon sulfonic acid polymers, perfluorocarbon sulfonic acid polymers, trifluorostyrene sulfonic acid polymers, ethylene tetrafluoroethylene-g-styrene sulfonic acid polymers, ethylene-tetrafluoroethylene copolymers, and polyvinylidene fluoride-perfluorocarbon sulfonic acid polymers. Perfluorocarbon sulfonic acid polymers are sometimes preferred. Such fluorine-based polymer electrolytes are used.

[0049] Examples of hydrocarbon ionomers include sulfonated polyethersulfone (S-PES), sulfonated polyaryletherketone, sulfonated polybenzimidazole alkyl, phosphonated polybenzimidazole alkyl, sulfonated polystyrene, sulfonated polyetheretherketone (S-PEEK), and sulfonated polyphenylene (S-PPP).

[0050] In these ionomers, the proton-conducting protonic acid group may be a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a boronic acid group, or the like.

[0051] The proton conductivity of the ionomer 38 is not particularly limited and can be appropriately selected for use, for example, from the range of about 600 g / mol to 1500 g / mol.

[0052] In addition, the catalyst layer 12 may contain additives such as a water repellent such as polytetrafluoroethylene, polyhexafluoropropylene, or a tetrafluoroethylene-hexafluoropropylene copolymer, a dispersant such as a surfactant, a thickener such as glycerin, ethylene glycol (EG), polyvinyl alcohol (PVA), or propylene glycol (PG), or a pore-forming agent, as needed.

[0053] The thickness (dry film thickness) of the catalyst layer 12 is not particularly limited, but is about 1 μm or more and 50 μm or less. The above thickness applies to both the cathode catalyst layer 10 and the anode catalyst layer 8, and the thicknesses may be the same or different.

[0054] According to the catalyst layer 12 described above, by setting the ionomer coverage rate on the catalyst support 32 within a predetermined range, it is possible to easily achieve both proton conductivity and gas diffusivity, thereby improving catalytic activity and contributing to improving the power generation characteristics of the fuel cell.

[0055] (Method of manufacturing catalyst layer) The method for producing a catalyst layer disclosed in this specification (hereinafter also referred to as the present production method) comprises the steps of preparing a catalyst ink containing a catalyst material having a carbon-based catalyst support and a catalytic metal supported on the catalyst support, an ionomer, and an aqueous medium containing water, a bipolar solvent having a boiling point greater than 100°C and less than or equal to 170°C, and ethanol, and supplying the catalyst ink onto a substrate and drying it to form a catalyst layer. According to this production method, by using the aqueous medium, a catalyst layer can be obtained with a limited ionomer coverage on the surface of the catalyst support.

[0056] In this production method, the catalyst carrier, catalyst metal, ionomer, catalyst layer, etc., are the same as those described above for the catalyst carrier 32, catalyst metal 36, ionomer 38, and catalyst layer 12.

[0057] (Catalyst ink preparation process) The catalyst ink preparation step involves preparing or obtaining a catalyst ink containing a catalyst support material, an ionomer 38, and an aqueous medium containing water, a bipolar solvent, and ethanol. The catalyst ink can be obtained, for example, by adding water to the catalyst support material, then adding the bipolar solvent and ethanol to form a suspension, adding the ionomer, and then performing a dispersion process using an ultrasonic homogenizer or the like. If necessary, the catalyst ink can also be obtained by further performing a stirring process using high shear force.

[0058] The catalyst support material can be obtained by a known method. Methods for supporting the catalytic metal 36 on the catalyst support 32 are well known to those skilled in the art. Typically, the catalytic metal can be supported on the catalyst support 32 by known methods such as impregnation, liquid phase reduction support using an acid such as citric acid, evaporation to dryness, colloidal adsorption, spray pyrolysis, and reverse micelle (microemulsion) method.

[0059] Methods for preparing catalyst ink by mixing a catalyst support material, an ionomer 38, and an aqueous medium are also well known to those skilled in the art. The aqueous medium contains water, a bipolar solvent, and ethanol. The bipolar solvent is a solvent containing groups that contribute to the H-donating properties of the solvent, such as hydroxyl groups, amino groups, and amide groups, and groups that contribute to the H-accepting properties of the solvent, such as ether groups and ketone groups. The hydroxyl groups, amino groups, and amide groups are also groups that contribute to the H-accepting properties. Using a bipolar solvent makes it easier to control the coverage of the ionomer 38 on the surface of the catalyst support 32.

[0060] The bipolar solvent has a boiling point between 100°C and 170°C, which is higher than that of other solvents such as water (100°C) and ethanol (78°C). This suppresses adsorption between the catalyst support 32 and the ionomer 38 while also suppressing aggregation of the catalyst support 32. As a result, the coverage of the catalyst support 32 with the ionomer 38 can be controlled. Specifically, because the catalyst support 32 (e.g., carbon) and the ionomer 38 have hydrophobic sites, a high water content in the ink solvent causes adsorption and aggregation between the catalyst support 32 (e.g., carbon) and the ionomer 38 due to hydrophobic interactions. When the solvent contains water and ethanol, the ethanol, which has a lower boiling point, evaporates first during heating in the drying process, resulting in a high water content. This increases the likelihood of aggregation between the catalyst support 32 (e.g., carbon) and the ionomer 38. Adding a bipolar solvent, such as a high-boiling alcohol solvent, suppresses an increase in the water content in the solvent, even when the ethanol evaporates during heating. This makes it possible to suppress aggregation of the catalyst support 32 and the ionomer 38, and to control the coverage of the ionomer 38 on the catalyst support 32. The boiling point of the bipolar solvent is, for example, 110°C or higher, 120°C or higher, 130°C or higher, 140°C or higher, or 150°C or higher.

[0061] Examples of such high-boiling bipolar solvents include diacetone alcohol (166°C), acetylacetone (141°C), n-butyl alcohol (117°C), cyclohexanol (162°C), N,N-dimethylacetamide (165°C), N,N-dimethylformamide (153°C), 2-methoxyethanol (124°C), 2-ethoxyethanol (135°C), 1-hexanol (157°C), isoamyl alcohol (131°C), 1-pentanol (138°C), and 3-pentanol (116°C). These bipolar solvents can be used alone or in combination. Among these, diacetone alcohol may be preferred in terms of polarity and boiling point.

[0062] The mass ratio of ethanol to the volume of the bipolar solvent in the aqueous medium (ethanol / bipolar solvent) is related to the ionomer coverage on the surface of the catalyst support 32. This mass ratio can be adjusted appropriately to obtain a suitable ionomer coverage. This mass ratio is not particularly limited, but can be, for example, 0.10 or more and 0.50 or less. The higher the mass ratio of ethanol to the bipolar solvent, the lower the aggregation rate of the ionomer 38 on the surface of the catalyst support 32 and the higher the ionomer coverage tends to be. If this mass ratio is less than 0.10, the ionomer coverage is likely to be less than 25%. Furthermore, if this mass ratio exceeds 0.50, the ionomer coverage is likely to exceed 50%. The mass ratio is, for example, 0.12 or more, 0.13 or more, 0.14 or more, 0.15 or more, 0.20 or more, 0.25 or more, 0.30 or more, and, for example, 0.48 or less, 0.46 or less, 0.44 or less, 0.42 or less, or 0.40 or less.

[0063] The mass ratio of water to the mass of the bipolar solvent in the aqueous medium is not particularly limited, but may be, for example, 2.0 or more and 4.0 or less, or, for example, 2.1 or more, or 2.2 or more, or, for example, 3.4 or less, 3.2 or less, or 3.0 or less.

[0064] The mass ratio of water to the total mass of water, bipolar solvent, and ethanol in the aqueous medium is not particularly limited, but is, for example, 60% by mass to 75% by mass, or, for example, 65% by mass to 70% by mass. The content of the bipolar solvent to the total mass is also not particularly limited, but is, for example, 18% by mass to 32% by mass. It is also, for example, 20% by mass to 22% by mass, 24% by mass to 26% by mass, or, for example, 30% by mass to 28% by mass. The content of water to the total mass of the catalyst ink is also not particularly limited, but is, for example, 50% by mass to 70% by mass.

[0065] In addition to water, ethanol, and bipolar solvents, the aqueous medium may contain other solvents as long as the ionomer coverage is sufficient. Examples include methanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, and 2-methyl-2-propanol. Among these, it is preferable to use alcohols with a boiling point of 60°C or higher and 100°C or lower.

[0066] The concentration of the ionomer and catalyst support material in the catalyst ink is not particularly limited, but can be about 1 to 50% by mass, and more preferably about 5 to 30% by mass in the catalyst ink.

[0067] In addition to the components already described, when additives such as water repellents, dispersants, thickeners, and pore-forming agents are used in the catalyst ink, it is permissible to add these additives to the catalyst ink, provided that the addition does not interfere with the control of the coverage by the ionomer.

[0068] The present specification also provides such a catalyst ink.

[0069] (Catalyst layer formation process) The catalyst layer 12 formation step is a step of supplying the catalyst ink onto a substrate and drying it to form the catalyst layer 12. Here, the substrate can be the electrolyte layer 6 or the gas diffusion layers 14, 16. Alternatively, the catalyst layer 12 can be obtained by using a peelable substrate (such as a transfer sheet) such as a polytetrafluoroethylene (PTFE) sheet as the substrate, forming the catalyst layer 12 on the substrate, and then peeling the catalyst layer portion from the substrate.

[0070] The method for supplying the catalyst ink to the substrate is not particularly limited, and any known coating method may be appropriately selected, such as spraying, screen printing, doctor blade printing, gravure printing, or die coating.

[0071] Finally, the catalyst ink on the substrate is dried as a coating layer (film) of the catalyst ink in an air atmosphere or an inert gas atmosphere, for example, at room temperature to 180°C for 1 to 60 minutes, taking into consideration the type of solvent, etc. This forms the catalyst layer 12.

[0072] (Membrane electrode assembly) The membrane electrode assembly disclosed in this specification may include the above-mentioned catalyst layer 12. That is, the membrane electrode assembly includes a so-called MEA including an electrolyte layer 6 and an anode catalyst layer 8 and a cathode catalyst layer 10 arranged on one side thereof. The membrane electrode assembly also includes a so-called MEGA including an anode gas diffusion layer 14 arranged opposite the anode catalyst layer 8 and a cathode gas diffusion layer 16 arranged opposite the cathode catalyst layer 10.

[0073] At least one of the anode catalyst layer 8 and the cathode catalyst layer 10 is a catalyst layer 12 having an ionomer coverage within a predetermined range. Considering the improvement of proton conductivity and the diffusion characteristics of gases (especially O2), it is preferable that at least the cathode catalyst layer 10 is the catalyst layer 12 described above.

[0074] The fuel cell 2 disclosed in this specification includes a membrane electrode assembly such as the MEA or MEGA. The fuel cell 2 includes a cell 4 including an MEA and gas diffusion layers 14, 16 or an MEGA, and a pair of separators 20a, 20b. Such cells may be stacked to form a fuel cell having a stack structure including a current collector and a manifold.

[0075] (Method for manufacturing membrane electrode assembly, etc.) The method for producing the membrane electrode assembly and the fuel cell is not particularly limited, and a conventionally known method can be used. For example, an anode catalyst layer 8 and a cathode catalyst layer 10 may be supplied to an electrolyte layer 6, dried, and bonded to form an MEA. Furthermore, gas diffusion layers 14 and 16 may be bonded to form an MEGA, or a pair of separators 20a and 20b may be sandwiched and bonded to form a fuel cell 2, which is a single cell 4. The pressure and / or temperature conditions when bonding the catalyst layers 8 and 10, the gas diffusion layers 14 and 16, and the separators 20a and 20b, respectively, are set appropriately as needed.

[0076] Furthermore, the cells 4 manufactured in this manner may be stacked to form a stack structure of the fuel cell 2. Conventional knowledge known in the field of fuel cells may be referred to as appropriate. [Example]

[0077] Examples that embody the disclosure of this specification will be illustrated and described below, but the disclosure of this specification is not limited to the following examples.

[0078] In this example, a catalyst ink was prepared, an MEA was fabricated, and a fuel cell for evaluation was fabricated using this MEA. The ionomer coverage, ionomer thickness, gas diffusion resistance, catalyst layer proton resistance, and power generation characteristics of the MEA or fuel cell were evaluated. The fuel cell fabrication method and various evaluation methods are described below.

[0079] (1) Preparation of catalyst ink Catalyst-supported particles, which are catalyst-supported materials made by supporting platinum and cobalt on carbon particles, were prepared. Distilled water was added to the catalyst-supported particles, followed by the addition of diacetone alcohol and ethanol to form a suspension. The mass ratio of ethanol to diacetone alcohol and the mass ratio of diacetone alcohol to diacetone alcohol were determined as shown in Table 1. An ionomer having sulfonic acid groups was then added, and the mixture was thoroughly dispersed using an ultrasonic holder homogenizer. A high shear force was then applied using a rotary mixer (Filmix) manufactured by Primix Corporation to prepare four types of cathode catalyst inks. The amounts of distilled water, catalyst-supported particles, and ionomer used in all catalyst inks were the same. The amount of distilled water was 60% by mass of the total mass of the catalyst ink.

[0080] [Table 1]

[0081] (2) Fabrication of MEA In both the fuel cells of the Examples and Comparative Examples, MEAs were fabricated by forming a cathode catalyst layer and an anode catalyst layer on both sides of an electrolyte membrane. Specifically, the anode catalyst ink was applied directly to the electrolyte membrane using a die coater to a predetermined thickness, followed by drying to form the anode catalyst layer. The cathode catalyst layer was formed by applying the cathode catalyst ink to the surface of a PTFE film substrate using an applicator-type coater, drying the ink at 80°C for 5 minutes, and transferring the resulting film to the electrolyte membrane.

[0082] (3) Fuel cell construction (2) Carbon cloths functioning as gas diffusion layers were attached to both sides of the MEAs of the examples and comparative examples prepared in (2) to prepare fuel cells.

[0083] (4) Evaluation method (ionomer coverage) Powders scraped from the surface of the cathode catalyst layer of the MEAs of the examples and comparative examples were thoroughly mixed and homogenized to form test powders, which were then used for 3D-TEM observation. For the 3D-TEM, a 200 nm × 200 nm area was selected from the observation field of the test powder to be evaluated. The ionomer coverage, which is the ratio of the surface area covered by the ionomer to the surface area of ​​the catalyst support material, was calculated for the selected area. The results are shown in Table 1.

[0084] (Ionomer thickness) Test powders were prepared in the same manner as in the evaluation of the ionomer coverage, and a 200 nm x 200 nm evaluation area was selected for each test powder. A volume-based cumulative distribution of the thickness of the ionomer coating the catalyst support material was created for the selected area. The median value was obtained, and this median value (50% cumulative) was used as the average thickness of the ionomer in the test powder. The results are also shown in Table 1.

[0085] (gas diffusion resistance, catalyst layer proton resistance, power generation characteristics, etc.) The gas diffusion resistance (limiting current density method), catalyst layer proton resistance (AC impedance method), and power generation characteristics were measured, and the power generation characteristics (output) were evaluated using a fuel cell evaluation system manufactured by Toyo Corporation. The evaluation results are shown in Figures 3 to 8.

[0086] (result) (1: Relationship between the mass ratio of ethanol to diacetone alcohol in an aqueous medium, the ionomer coverage rate, and the average thickness of the ionomer) As shown in Table 1, the ionomer coverage in the cathode catalyst layer of the MEAs in Examples 1 and 2 was 32% and 36%, while the ionomer coverage in Comparative Examples 1 and 2 was 65% and 85%. Fig. 3 shows the relationship between the mass ratio (%) of ethanol to diacetone alcohol and the ionomer coverage. As shown in Fig. 3, it was found that as the mass ratio of ethanol increases, the ionomer coverage also tends to increase.

[0087] Furthermore, as shown in Table 1, the average thickness of the ionomer in Example 1 was approximately 8 nm, while the average thickness was approximately 5 nm in Comparative Example 2. From these results, it was found that the ionomer in Example 1 coated the catalyst support to a greater thickness, although the coverage was lower.

[0088] (2: Relationship between ionomer coverage and gas diffusion resistance) As shown in Fig. 4, it was found that an ionomer coverage of 50% or less is suitable for achieving one target value of gas diffusion resistance of 23.5 s / m or less. Note that in Fig. 4, from the results of Examples 1 and 2 and Comparative Examples 1 and 2, it was derived based on a statistical method that when the ionomer coverage is 50% or less, the gas diffusion resistance becomes 23.5 s / m or less.

[0089] (3: Relationship between ionomer coverage and catalyst layer proton resistance) As shown in Figure 5, the target value of the catalyst layer proton resistance was 1.5 Ω / cm 2 It was found that an ionomer coverage of 25% or more is suitable to satisfy the following: In Fig. 5, from the results of Examples 1 and 2 and Comparative Examples 1 and 2, it was found based on a statistical method that when the ionomer coverage is 25% or more, the proton resistance of the catalyst layer is 1.5 Ω / cm 2 It was derived that:

[0090] (4: Relationship between ionomer coverage and power generation characteristics) 6, it was found that Examples 1 and 2 had higher power generation performance than Comparative Examples 1 and 2. This is because Examples 1 and 2 had a gas resistance of 23.5 s / m or less and a catalyst layer proton resistance of 1.5 Ω / cm 2 This is thought to be to satisfy the following:

[0091] (5: Relationship between ionomer coverage and power generation characteristics) As shown in Fig. 7, it was found that Examples 1 and 2 had higher power generation performance than Comparative Examples 1 and 2. The vertical axis therefore indicates an output, which serves as an index, of 1. From these results, it can be seen that in Examples 1 and 2, the gas resistance was 23.5 s / m or less and the catalyst layer proton resistance was 1.5 Ω / cm 2 This is thought to be because the following is satisfied: Furthermore, as shown in Figure 8, it was found that when the ionomer coverage is 25% or more and 50% or less, the target output value can be sufficiently secured.

[0092] From the above results, it was found that by setting the ionomer coverage to 25% or more and 50% or less, both gas diffusion resistance and proton resistance can be achieved, resulting in high power generation performance. Furthermore, referring again to Figure 3, the results of Examples 1 and 2 and Comparative Examples 1 and 2 show that the mass ratio of ethanol corresponding to an ionomer coverage of 25% or more and 50% or less was 0.10 or more and 0.50 or less. Therefore, it was found that by using an aqueous medium and an ionomer in which the mass ratio of ethanol to diacetone alcohol is 0.10 or more and 0.50 or less, a catalyst layer with an ionomer coverage of 25% or more and 50% or less can be obtained. [Explanation of symbols]

[0093] 2 fuel cell, 4 cell, 6 electrolyte layer, 8 anode electrode catalyst layer, 10 cathode electrode catalyst layer, 14 anode electrode catalyst layer, 16 cathode electrode catalyst layer, 20a, 20b separator, 30 catalyst support material, 32 catalyst support, 34 hole, 36 catalyst metal, 38 ionomer

Claims

1. An electrode catalyst layer of a fuel cell, a catalyst support material having a catalyst support and a catalytic metal supported on the catalyst support; an ionomer partially coating the catalyst support material; Equipped with An electrode catalyst layer, wherein an ionomer coverage, which is the ratio of the surface area covered with the ionomer to the surface area of ​​the catalyst support obtained by three-dimensional transmission electron microscopy, is 25% or more and 50% or less.

2. 2. The electrode catalyst layer according to claim 1, wherein the average thickness of the ionomer is 6 nm or more and 20 nm or less.

3. The electrode catalyst layer according to claim 1 , wherein the ionomer comprises a sulfonic acid-based ionomer.

4. The electrode catalyst layer according to claim 1 , wherein the catalyst support is a porous particle or a solid particle.

5. An electrode catalyst layer according to any one of claims 1 to 4; A membrane electrode assembly for a fuel cell comprising:

6. A fuel cell comprising the membrane electrode assembly for a fuel cell according to claim 5 .

7. A method for manufacturing an electrode catalyst layer of a fuel cell, comprising: A step of preparing a catalyst ink including a catalyst support material having a carbon-based catalyst support and a catalyst metal supported on the catalyst support, an ionomer, an aqueous medium including water, a bipolar solvent having a boiling point higher than 100°C and lower than or equal to 170°C, and ethanol; supplying the catalyst ink onto a substrate and drying it to form the electrode catalyst layer; A manufacturing method comprising:

8. The method of claim 7 , wherein the bipolar solvent comprises diacetone alcohol.

9. The method according to claim 7 or 8, wherein the mass ratio of ethanol to the bipolar solvent is 0.10 or more and 0.50 or less.

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