Method for manufacturing catalyst layers for polymer electrolyte fuel cells

Abstract

To improve the power generation performance.SOLUTION: Disclosed is a method of manufacturing a catalyst layer 1 for a solid polymer fuel cell which includes the steps of: applying a catalyst ink 3 containing a conductor on which a catalyst is supported, an ion conductor, a solvent, and a pore-forming agent soluble in the solvent; using the pore-forming agent as a precipitate 8 in one or more forms selected from the group consisting of fibrous, prismatic, and cylindrical by volatilizing the solvent from the catalyst ink 3 and forming the catalyst layer 1 containing the precipitates 8; and removing the precipitates 8 from the catalyst layer 1.SELECTED DRAWING: Figure 13

Description

[Technical Field]

[0001] This disclosure relates to a method for manufacturing a catalyst layer for a polymer electrolyte fuel cell, a membrane electrode assembly, and a polymer electrolyte fuel cell. [Background technology]

[0002] A technique is known for forming a catalyst layer having multiple micropores by incorporating a pore-forming agent, such as a water-soluble substance, into a catalyst ink (see, for example, Patent Document 1). Conventionally, water-soluble substances such as alcohols like polyvinyl alcohol and sugars like starch have been used. In order to effectively utilize electrode catalysts during power generation, it is necessary that the reaction gas is supplied to the entire catalyst layer and that the water generated by power generation is efficiently discharged. However, conventional pore-forming agents either precipitate as spherical deposits or form spherical pores by releasing gas. The pores formed by these pore-forming agents have narrow connections between them, which can restrict the supply of reaction gases and make it easier for water generated by power generation to accumulate. From the perspective of gas supply and generated water discharge, it is desirable to form elongated pores with a certain diameter in the catalyst layer, so that reaction gases can be supplied to the entire catalyst layer and generated water can be discharged through these pores. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2007-179792 [Overview of the project] [Problems that the invention aims to solve]

[0004] Even when a catalyst layer was formed using conventional pore-forming agents, the power generation performance (initial cell voltage) was not always sufficient, and improvements were needed. This disclosure aims to improve the power generation performance of polymer electrolyte fuel cells. [Means for solving the problem]

[0005] The means of this disclosure are shown below. [1] A step of applying a catalyst ink containing a conductor on which a catalyst is supported, an ion conductor, a solvent, and a pore-forming agent soluble in the solvent, The process involves volatilizing the solvent from the catalyst ink to form precipitates of one or more forms selected from the group consisting of fibrous, prismatic, and cylindrical shapes, and forming a catalyst layer containing the precipitates. A method for producing a catalyst layer for a polymer electrolyte fuel cell, comprising the step of removing the precipitate from the catalyst layer. [Effects of the Invention]

[0006] The manufacturing method disclosed herein can form a catalyst layer that is advantageous for improving power generation performance. [Brief explanation of the drawing]

[0007] [Figure 1] This is a schematic diagram illustrating the method for manufacturing the catalyst layer. [Figure 2] This is a schematic diagram illustrating the method for manufacturing the catalyst layer. [Figure 3] This is a schematic diagram illustrating the method for manufacturing the catalyst layer. [Figure 4] This is a schematic diagram illustrating the method for manufacturing the catalyst layer. [Figure 5] This is a schematic diagram illustrating the manufacturing method of a membrane electrode assembly. [Figure 6] This is a schematic diagram illustrating the manufacturing method of a membrane electrode assembly. [Figure 7] This is a schematic diagram of an example of a polymer electrolyte fuel cell. [Figure 8] This is an SEM image of the catalyst layer in Example 1. [Figure 9] This is an SEM image of the catalyst layer in Example 2. [Figure 10] This is an SEM image of the catalyst layer in Comparative Example 1. [Figure 11] It is a SEM image of the catalyst layer of Comparative Example 2. [Figure 12] It is a SEM image of α - cyclodextrin used as a raw material. [Figure 13] It is a graph showing the results of power generation evaluation.

Mode for Carrying Out the Invention

[0008] Here, another example of the present disclosure is shown. [2] The method for producing a catalyst layer for a solid polymer fuel cell according to [1], wherein the pore - forming agent is an oligosaccharide. [3] A solid polymer electrolyte membrane, The catalyst layer produced by the production method according to [1] or [2], and A membrane - electrode assembly in which the catalyst layer is joined to the solid polymer electrolyte membrane. [4] A solid polymer fuel cell comprising the membrane - electrode assembly according to [3].

[0009] Hereinafter, the present disclosure will be described in detail. In the description using "-" for a numerical range, unless otherwise specified, the lower limit value and the upper limit value are included. For example, in the description of "10 - 20", both the lower limit value "10" and the upper limit value "20" are included. That is, "10 - 20" has the same meaning as "10 or more and 20 or less".

[0010] 1. Method for Producing Catalyst Layer 1 for Solid Polymer Fuel Cell The method for producing catalyst layer 1 for a solid polymer fuel cell includes a step of applying catalyst ink 3, a step of forming a catalyst layer 1 containing precipitate 8 by volatilizing the solvent from the catalyst ink 3, and a step of removing precipitate 8 from the catalyst layer 1. In the step of applying catalyst ink 3, catalyst ink 3 containing a conductor on which a catalyst is supported, an ion conductor, a solvent, and a pore - forming agent soluble in the solvent is applied. In the step of forming a catalyst layer 1 containing precipitate 8 by volatilizing the solvent from the catalyst ink 3, the pore-forming agent is precipitate 8 in one or more forms selected from the group consisting of fibrous, prismatic, and cylindrical shapes. Figures 1-4 show an explanatory diagram of the manufacturing method of the catalyst layer 1. Figures 5 and 6 show an explanatory diagram of the manufacturing method of the film electrode assembly 7.

[0011] (1) Step of applying catalyst ink 3 (see Figures 1 and 2) In this step, a catalyst ink 3 containing a catalyst-supported conductor, an ion conductor, a solvent, and a solvent-soluble pore-forming agent is applied. In this step, the catalyst ink 3 is typically applied to a substrate 5.

[0012] (1.1) Catalyst ink 3 Catalyst ink 3 contains a conductor on which a catalyst is supported, an ion conductor, a solvent, and a pore-forming agent soluble in the solvent.

[0013] (1.1.1) Catalyst-supported conductor As catalysts (catalyst particles) in a conductor on which a catalyst is supported, platinum group elements such as platinum, palladium, ruthenium, iridium, rhodium, and osmium, as well as metals or alloys thereof such as iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum can be used. The particle size of the catalyst is not particularly limited. From the viewpoint of improving catalyst activity and catalyst stability, the particle size of the catalyst is preferably 0.5 nm to 20 nm, and more preferably 1 nm to 5 nm. If the catalyst is at least one noble metal selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru), the electrode reactivity is excellent, and the electrode reaction can be carried out efficiently and stably. The particle size of a catalyst can be determined, for example, by the following method: Observe the catalyst using a transmission electron microscope (TEM). Print out the TEM image on paper, treat the catalyst (black circular image) as spherical, and measure the diameter from edge to edge of the catalyst, randomly selecting a total of 300 particles from several fields of view (3-5 fields). The average of the diameters of these 300 particles is taken as the particle size. The conductor is an electronically conductive material (catalyst-supporting particle) that supports the catalyst. The conductor is not particularly limited. Carbon particles are preferably used as the conductor. The type of carbon particle is not limited. Suitable carbon particles include carbon black, graphite, activated carbon, carbon fiber, carbon nanotubes, fullerenes, etc. The particle size of the carbon particles is not particularly limited. From the viewpoint of forming good electron conduction paths and ensuring gas diffusion in the catalyst layer 1, the particle size of the carbon particles is preferably 10 nm to 1000 nm, and more preferably 10 nm to 100 nm. The particle size of a conductor can be determined, for example, by the following method: Observe the conductor using a transmission electron microscope (TEM). Print out the TEM image on paper, treat the conductor as spherical, and consider the distance from end to end of the conductor as the diameter. Randomly measure a total of 300 particles from several fields of view (3-5 fields). The average of the 300 counted diameters is taken as the particle size. The amount of conductor supporting the catalyst in the catalyst ink 3 is adjusted as appropriate so that the catalyst is present in the desired amount in the catalyst layer 1 of the air electrode (cathode) and the catalyst layer 1 of the fuel electrode (anode). In the case of the catalyst layer 1 of the air electrode (cathode), the amount of conductor added is adjusted so that, for example, when the total amount of catalyst ink is 100 parts by mass, it is between 0.04 parts by mass and 5.7 parts by mass. In the case of the catalyst layer 1 of the fuel electrode (anode), the amount of conductor added is adjusted so that, for example, when the total amount of catalyst ink is 100 parts by mass, it is between 0.04 parts by mass and 5.7 parts by mass.

[0014] (1.1.2) Ionic conductors As the ion conductor contained in catalyst ink 3, one having proton conductivity is used. As the ion conductor, for example, a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte can be used. As a fluorine-based polymer electrolyte, for example, a perfluorohydrocarbon electrolyte (e.g., Nafion® manufactured by DuPont) can be used. As a hydrocarbon-based polymer electrolyte, for example, electrolytes such as sulfonated polyimide (SPI), sulfonated polyetheretherketone (SPEEK), sulfonated polyphenylsulfone (SPPSU), and sulfonated polyethersulfone (SPES) can be used. A fluorine-based polymer electrolyte can be preferably used as the ion conductor. Furthermore, considering the adhesion between the catalyst layer 1 and the solid polymer electrolyte membrane 13, it is preferable to use the same material for the ion conductor as for the solid polymer electrolyte membrane 13. From the viewpoint of ensuring sufficient proton conductivity in the catalyst layer 1, the amount of ion conductor blended is preferably 0.02 parts by mass or more and 8.6 parts by mass or less, more preferably 0.2 parts by mass or more and 6.4 parts by mass or less, and even more preferably 0.4 parts by mass or more and 4.3 parts by mass or less, when the total amount of catalyst ink is 100 parts by mass.

[0015] (1.1.3) solvent The solvent is used as the dispersion medium for catalyst ink 3. The solvent is not particularly limited, but it is preferably a solvent with a boiling point of 10°C to 100°C, more preferably 15°C to 100°C, and even more preferably 25°C to 100°C. The solvent preferably contains at least a volatile organic solvent. As the organic solvent, at least one selected from the group consisting of alcohols, ketones, and ethers can be suitably used. From the viewpoint of solubility and volatility of ion conductors, alcohols having 1 to 5 carbon atoms are preferably used as the alcohol. At least one alcohol selected from the group consisting of ethanol, methanol, 1-propanol, 2-propanol, isopropyl alcohol, 1-butanol, 2-butanol, t-butyl alcohol, 1-pentanol, and 3-pentanol can be preferably used as the alcohol having 1 to 5 carbon atoms. Among these, ethanol is preferred from the viewpoint of reducing environmental impact. Suitable examples of ketones include acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, and diisobutyl ketone. Suitable examples of ethers include tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, and dibutyl ether. In addition to those listed above, polar solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, and 1-methoxy-2-propanol can also be used. Two or more solvents may be used in mixture form. From the viewpoint of forming a catalyst layer 1 that is advantageous for gas supply and discharge of generated water due to the volatilization of the solvent, the amount of solvent blended is preferably 80 parts by mass or more and 99.9 parts by mass or less, more preferably 85 parts by mass or more and 99 parts by mass or less, and even more preferably 80 parts by mass or more and 98 parts by mass or less, when the total amount of catalyst ink is 100 parts by mass. From the viewpoint of forming a catalyst layer 1 that is advantageous for gas supply and discharge of generated water through the volatilization of the solvent, a mixed solvent containing water is preferred as the organic solvent. When using a mixed solvent of an organic solvent and water, the ratio of the amount of organic solvent to the amount of water is not particularly limited. From the viewpoint of forming a catalyst layer 1 that is advantageous for gas supply and discharge of generated water due to the volatilization of the solvent, the mixing ratio (mass ratio) of organic solvent to water is preferably 20:80-80:20, more preferably 30:70-70:30, and even more preferably 40:60-60:40.

[0016] (1.1.4) Pore-forming agents soluble in solvents The pore-forming agent soluble in the solvent is not particularly limited as long as it can precipitate in a fibrous and / or prismatic form. From the viewpoint of being able to precipitate fibrous and / or prismatic precipitates 8 and being highly safe for the human body and the environment, oligosaccharides such as water-soluble cyclic oligosaccharides are preferred as pore-forming agents. Examples of oligosaccharides include α-cyclodextrin and β-cyclodextrin. In this disclosure, the background for using a pore-forming agent that can precipitate in a fibrous and / or prismatic shape and is soluble in a solvent will be explained. The membrane electrode assembly 7 that forms the core of the polymer electrolyte fuel cell 31 contains [1] hydrogen as protons (H +The system consists of an anode that separates gas and electrons, a cathode that uses protons, electrons, and oxygen to produce water, and a solid polymer electrolyte membrane 13 that separates the anode and cathode, allowing protons to pass through but not gas or electrons. The anode and cathode generally exist as a catalyst layer 1 composed of carbon (catalyst) supported with Pt and an electrolyte. In the anode and cathode reactions, the cathode reaction is the rate-limiting reaction, and it is necessary to create conditions that facilitate the cathode reaction in order to improve power generation performance. In order to facilitate the cathode reaction, it is thought that it is necessary to form paths that allow oxygen supply and discharge of produced water in the thickness direction of the catalyst layer 1 of the cathode, and pore-forming agents have been added. Metal salts and the like have been used as pore-forming agents, and voids have been formed in the catalyst layer 1 by removing the particulate precipitates during drying. The pores formed by this method are isotropic. On the other hand, the cathode reaction also requires the supply of electrons and protons, and electrons are transported by the carbon network of the catalyst, and protons are transported by the electrolyte network. These factors increase in resistance as the catalyst layer 1 is denser, the larger the surface area used for transport. To achieve a dense catalyst layer despite the presence of voids, it is necessary to form elongated voids. However, no conventional pore-forming agent has been presented that can create elongated shapes and be removed without damaging the solid polymer electrolyte membrane 13. In the technology disclosed here, we have succeeded in improving the power generation performance of a solid polymer fuel cell 31 by precipitating and removing a pore-forming agent (e.g., cyclic oligosaccharide) in an elongated shape, which is a substance that can be removed by washing with water without damaging the solid polymer electrolyte membrane 13. A preferred example is cyclic oligosaccharide, a substance known as dietary fiber, which also has the advantage of not requiring a special working environment for handling. Furthermore, this disclosure does not use pore-forming agents that are insoluble in the solvent. The reason is as follows: They become fibrous solids in the catalyst ink 3, are broken down during stirring, and cause unevenness and defects during coating.

[0017] (1.1.5) Other ingredients The catalyst ink 3 may contain a dispersant in order to disperse the conductor. Examples of dispersants include anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants.

[0018] (1.1.6) Solid content concentration of catalyst ink 3 The solid content concentration of the catalyst ink 3 is not particularly limited. From the viewpoint of suppressing variations in the amount of coating, the solid content concentration is preferably 0.1% by mass or more and 20% by mass or less, more preferably 1% by mass or more and 15% by mass or less, and even more preferably 2% by mass or more and 10% by mass or less.

[0019] (1.1.7) Amount of pore-forming agent From the viewpoint of precipitating the pore-forming agent in a fibrous and / or prismatic shape, the amount of the pore-forming agent is preferably 0.1 parts by mass or more and 5 parts by mass or less, more preferably 0.2 parts by mass or more and 2 parts by mass or less, and even more preferably 0.5 parts by mass or more and 1.5 parts by mass or less, when the total amount of catalyst ink is 100 parts by mass.

[0020] (1.2) Base material 5 The substrate 5 (transfer substrate) can be coated with catalyst ink 3 on at least one side, can be dried by heating (having the desired heat resistance), and can be transferred to the solid polymer electrolyte membrane 13 (having the desired release properties). For example, a polymer film is used as the substrate 5. The polymer contained in the polymer film is not particularly limited. Examples of polymers include fluororesins, polyolefin resins, polyethylene terephthalate, polyamide, polyimide, polystyrene, polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyetherimide, polybenzimidazole, polyamideimide, polyacrylate, polyethylene naphthalate, and polyparvanate aramid. Among these polymers, fluororesins are preferred from the viewpoint of having heat resistance and high transferability. Examples of fluororesins include polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, ethylenetetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and tetrafluoroperfluoroalkyl vinyl ether copolymer.

[0021] (1.3) Application The application method is not particularly limited. Examples of application methods include the doctor blade method, die coating method, dipping method, screen printing method, laminator roll coating method, and spray method. The coating thickness before drying is not particularly limited. From the viewpoint of suppressing variations in the amount of coating applied, the coating thickness is preferably 1 μm to 200 μm, more preferably 10 μm to 150 μm, and even more preferably 25 μm to 100 μm.

[0022] (2) Step of forming a catalyst layer 1 containing precipitate 8 In this step, the solvent is evaporated from the catalyst ink 3 to form a catalyst layer 1 containing precipitate 8 (see Figure 2-3). In other words, the catalyst ink 3 is dried. From the viewpoint of precipitating one or more precipitate forms selected from the group consisting of fibrous, prismatic, and cylindrical shapes, it is preferable to use natural drying to volatilize the solvent. For example, it is preferable to volatilize the solvent in an environment of 10°C to 50°C. However, heating at 0°C to 100°C is also acceptable. The thickness of the catalyst layer 1 is not particularly limited. From the viewpoint of gas supply and discharge of generated water, the catalyst layer 1 is preferably 0.1 μm to 20 μm. The aspect ratio when the precipitate 8 is fibrous is not particularly limited. From the viewpoint of smoothly supplying gas and discharging generated water through the void formed by removing the precipitate 8, the aspect ratio is preferably 3 to 20, more preferably 5 to 20, and even more preferably 10 to 15. The aspect ratio refers to the length in the axial direction of the precipitate 8 (fiber length) / fiber diameter. The aspect ratio can be determined by observing with an SEM, reading the fiber length and maximum diameter of five of the fibrous precipitates 8 in the image obtained by the SEM using the scale on the SEM screen, calculating the aspect ratio for each, and taking the average value. The diameter of the precipitate 8 when it is fibrous is not particularly limited. From the viewpoint of smoothly supplying gas and discharging generated water through the void formed by removing the precipitate 8, the diameter is preferably 0.5 μm to 4 μm, more preferably 0.5 μm to 2 μm, and even more preferably 1 μm to 2 μm. The diameter can be determined by observing with an SEM, reading the maximum diameter of five of the fibrous precipitates 8 in the image obtained by the SEM using the scale on the SEM screen, and taking the average value. The bottom area when the precipitate 8 is prismatic is not particularly limited. The bottom area (average value) is 0.05 μm, from the viewpoint of smoothly supplying gas and discharging generated water through the void formed by removing the precipitate 8. 2 5μm or more 2 The following is preferred: 0.5 μm 2 More than 4μm 2 The following is more preferable: 0.8 μm 2 3.1μm or more 2The following is more preferable. The bottom area can be obtained from the average value calculated by reading the size of five of the prismatic deposits 8 in the SEM image obtained by SEM observation according to the scale on the SEM screen and calculating the bottom area of each. The height is not particularly limited. From the viewpoint of smoothly performing gas supply and discharge of generated water through the pores formed by removing the deposits 8, the height is preferably 0.1 μm or more and 40 μm or less, more preferably 10 μm or more and 40 μm or less, and still more preferably 20 μm or more and 30 μm or less. The height can be obtained from the average value calculated by reading the height of five of the prismatic deposits 8 in the SEM image obtained by SEM observation according to the scale on the SEM screen. The shape of the bottom surface in the case where the deposit 8 is prismatic is not particularly limited, and examples include polygons such as triangles, quadrilaterals, pentagons, hexagons, heptagons, and octagons. When the deposit 8 is columnar, the bottom area is not particularly limited. The bottom area (average value) is preferably 0.05 μm 2 or more and 5 μm 2 or less, more preferably 0.5 μm 2 or more and 4 μm 2 or less, still more preferably 0.8 μm 2 or more and 3.1 μm 2 or less. The bottom area can be obtained from the average value calculated by reading the size of five of the columnar deposits 8 in the SEM image obtained by SEM observation according to the scale on the SEM screen and calculating the bottom area of each. The height is not particularly limited. From the viewpoint of smoothly performing gas supply and discharge of generated water through the pores formed by removing the deposits 8, the height is preferably 0.1 μm or more and 40 μm or less, more preferably 10 μm or more and 40 μm or less, and still more preferably 20 μm or more and 30 μm or less. The height can be obtained from the average value calculated by reading the height of five of the columnar deposits 8 in the SEM image obtained by SEM observation according to the scale on the SEM screen.

[0023] (3) Step of removing the deposit 8 from the catalyst layer 1 In this process, voids 12 are formed by removing precipitates 8 from the catalyst layer 1 (see Figure 4). The shape of the voids 12 is presumed to correspond to the shape of precipitates 8. It is preferable to remove the precipitate 8 from the catalyst layer 1 by dissolving the precipitate 8 using a solvent in which the pore-forming agent is soluble. Since the removal of precipitates 8 may be performed even after the membrane electrode assembly 7 is fabricated, the solvent used to dissolve the pore-forming agent is preferably one that does not dissolve the solid polymer electrolyte membrane 13, and water is preferably used. In this process, for example, the precipitate 8 is removed by washing the surface of the catalyst layer 1 with water (e.g., ultrapure water). This process may be performed at one or both of the following times: (i) After the catalyst layer 1 is formed on the substrate 5 (see Figure 3) (ii) After fabricating the membrane electrode assembly 7 (see Figure 6)

[0024] 2. Membrane electrode assembly 7 (MEA) As shown in Figure 6, the membrane electrode assembly 7 comprises a solid polymer electrolyte membrane 13 and a catalyst layer 1. The membrane electrode assembly 7 is manufactured, for example, as follows: A substrate 5 on which the catalyst layer 1 shown in Figure 4 is formed is bonded to both sides of a solid polymer electrolyte membrane 13. At this time, as shown in Figure 5, the catalyst layer 1 is bonded so that it faces the solid polymer electrolyte membrane 13. After that, the substrate 5 is peeled off to manufacture the membrane electrode assembly 7 shown in Figure 6. In the membrane electrode assembly 7, the catalyst layer 1 is bonded such that the side S1 not in contact with the substrate 5 is in contact with the solid polymer electrolyte membrane 13. The side S2 of the catalyst layer 1 that is in contact with the substrate 5 is not in contact with the solid polymer electrolyte membrane 13 (see Figures 5 and 6). One side of catalyst layer 1 is the anode catalyst layer 1A, which will be described later, and the other side is the cathode catalyst layer 1B.

[0025] 3. Polymer electrolyte fuel cell 31 As shown in Figure 7, the polymer electrolyte fuel cell 31 (PEFC) includes a membrane electrode assembly 7. On both sides of the polymer electrolyte membrane 13, an anode catalyst layer 1A and a cathode catalyst layer 1B are provided, sandwiching it. The membrane electrode assembly 7 is composed of the polymer electrolyte membrane 13 and the pair of anode catalyst layers 1A and cathode catalyst layers 1B that sandwich it.

[0026] A gas diffusion layer 20 is provided outside the catalyst layer 1A of the anode. The gas diffusion layer 20 is made of a porous material such as carbon paper, carbon cloth, or a porous metal, and has the function of uniformly diffusing the gas supplied from the separator 22 side into the catalyst layer 1A. Similarly, a gas diffusion layer 24 is provided outside the catalyst layer 1B of the cathode. The gas diffusion layer 24 has the function of uniformly diffusing the gas supplied from the separator 26 side into the catalyst layer 1B. In this figure, only one set of the membrane electrode assembly 7, gas diffusion layers 20, 24, and separators 22, 26 configured as described above is shown, but an actual polymer electrolyte fuel cell 31 may have a stack structure in which multiple membrane electrode assembly 7s and gas diffusion layers 20, 24 are stacked via separators 22, 26. [Examples]

[0027] The present disclosure will be further described by examples. 1. Example 1 (1) Preparation of catalyst layer 1 (1.1) Material • Catalyst (conductor on which the catalyst is supported): TEC10E50E manufactured by Tanaka Kikinzoku Kogyo Co., Ltd. (Carrier carbon: Ketjenblack, Platinum content: 46.8% by mass) • Ionomer dispersion (ion conductor dispersion): Manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. 20% by mass Nafion dispersion solution DE2020 CS type • Ethanol (solvent): Reagent-grade ethanol (99.5%) manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. • Water (solvent): Ultrapure water • Electrolyte membrane (solid polymer electrolyte membrane): Chemours Nafion NR-211 (thickness: 25 μm)

[0028] (1.2) Catalytic ink composition • Solid content (catalyst + ionomer): 7% by mass • Water:Ethanol ratio = 1:1 • Ionomer / carbon weight ratio (I / C): 0.75 (mass ratio) • α-cyclodextrin addition ratio: 1.0 part by mass per 100 parts by mass of total catalyst ink

[0029] (1.3) Procedure for preparing a catalyst layer transfer sheet (substrate on which a catalyst layer has been formed) In a beaker, the catalyst, ultrapure water, ionomer dispersion, and ethanol (containing α-cyclodextrin) were added in that order and stirred with an ultrasonic homogenizer. The stirred solution (catalyst ink) was dropped onto a PTFE sheet attached to a glass. It was spread with an applicator and allowed to air dry. The room temperature was adjusted to approximately 25°C using an air conditioner. In this way, a catalyst layer was formed. Afterwards, the surface of the catalyst layer was washed with ultrapure water.

[0030] (1.4) Equipment and supplies used • Ultrasonic homogenizer: UH-50, manufactured by SMT Co., Ltd. • Desktop coater: Mitsui Electric Machinery Co., Ltd. TC-3 model • Applicator: YBA-2 model, manufactured by Yoshimitsu Seiki Co., Ltd. • PTFE sheet: MSF-100, manufactured by Chuko Kasei Kogyo Co., Ltd.

[0031] (2) Observation using an electron microscope The surface of the catalyst layer before washing was observed using a scanning electron microscope (JEOL JSM6510LV).

[0032] (3) Evaluation of power generation performance A membrane electrode assembly was fabricated using the catalyst layer transfer sheet obtained in (1). After fabricating the membrane electrode assembly, the surface of the catalyst layer was washed with ultrapure water. Carbon paper was attached to both sides of the membrane electrode assembly as a gas diffusion layer, and it was installed in a power generation evaluation cell. Current and voltage measurements were performed at a cell temperature of 80°C using a fuel cell measurement device. Hydrogen was used as the fuel gas and air as the oxidizer gas, and flow rate control was performed with a constant utilization rate. The back pressure was set to 100 kPa.

[0033] 2. Example 2 The catalyst layer was formed in the same manner as in Example 1, except that the amount of α-cyclodextrin added was 2.0 parts by mass per 100 parts by mass of the total mass of the catalyst ink.

[0034] 3. Comparative Example 1 The catalyst layer was formed in the same manner as in Example 1, except that α-cyclodextrin was not added.

[0035] 4. Comparative Example 2 The catalyst layer was formed in the same manner as in Example 1, except that the amount of α-cyclodextrin added was 0.5 parts per 100 parts by mass of the total amount of catalyst ink.

[0036] 5. Observation results using an electron microscope The SEM images of the catalyst layers before washing in the examples and comparative examples are shown. In Example 1, fibrous precipitates 8 were observed as shown in Figure 8. In Example 2, prismatic precipitates 8 were observed as shown in Figure 9. On the other hand, no fibrous or prismatic precipitates were observed in either Comparative Examples 1 or 2. For reference, Figure 12 shows the α-cyclodextrin reagent used as a raw material, and it was confirmed that it was neither fibrous nor prismatic in form. Thus, in Examples 1 and 2, it can be seen that α-cyclodextrin precipitates upon drying of the catalyst ink, resulting in a fibrous or prismatic structure.

[0037] 6. Evaluation results of power generation performance The evaluation results are shown in Figure 13 and Table 1. It is known that power generation performance increases with a larger basis weight of Pt, which functions as a catalyst. Example 1, prepared using this technology, had a higher output despite having a Pt basis weight approximately 5% smaller than Comparative Example 1 (conventional technology). Example 2 also showed good power generation performance. To increase output, it is necessary to reduce overpotential, which is a factor that reduces output. The activation overpotential is a value that is determined to some extent by the type of catalyst. It is difficult to reduce both resistive overpotential and concentration overpotential simultaneously. In Examples 1 and 2, increased output was achieved by simultaneously reducing both resistive overpotential and concentration overpotential. Resistive overpotential is affected by electrical resistance and proton transport resistance. Concentration overpotential is affected by the supply of reactants. The factors that enabled the increase in output in Examples 1 and 2 are presumed to be as follows: It is presumed that the reaction gas was supplied to the entire catalyst layer and the generated water was discharged through the elongated voids formed by the removal of fibrous or prismatic precipitates. Furthermore, Examples 1 and 2 are completely different from the technology described in Patent Document 1 below. Specifically, conventional pore-forming agents used in Patent Document 1, etc., either precipitate in a spherical shape or form spherical pores by releasing gas. In the voids formed by these pore-forming agents, the supply of reaction gas is restricted because the parts where the voids connect become narrower, and it is conceivable that water generated by power generation tends to accumulate.

[0038] [Table 1]

[0039] 7. Effects of the Examples According to the examples, power generation efficiency can be improved by using a catalyst layer that is advantageous for gas supply and discharge of generated water.

[0040] The examples provided herein are for illustrative purposes only and should not be construed as limiting the present disclosure. While the present disclosure has been illustrated with examples of typical embodiments, the language used in the description and illustrations of the present disclosure should be understood as descriptive and illustrative, not limiting. As detailed herein, modifications are possible within the scope or essence of the present disclosure without departing in form. While specific structures, materials, and examples have been referenced herein, the present disclosure is not intended to be limited to the matters disclosed herein, but rather to encompass all functionally equivalent structures, methods, and uses within the scope of the present claims.

[0041] This disclosure is not limited to the embodiments detailed above, and various modifications or changes are possible within the scope of the claims. [Explanation of symbols]

[0042] 1...Catalyst layer 1A…Catalyst layer 1B…Catalyst layer 3… Catalyst ink 5...Base material 7...Membrane electrode assembly 8...Precipitate 12...Void 13...Solid polymer electrolyte membrane 20...Gas diffusion layer 22... Separator 24…Gas diffusion layer 26... Separator 31...Polymer fuel cell

Claims

[Claim 1] A step of applying a catalyst ink containing a catalyst-supported conductor, an ion conductor, a solvent, and a cyclic oligosaccharide soluble in the solvent, The process involves volatilizing the solvent from the catalyst ink to form precipitates of one or more forms selected from the group consisting of fibrous, prismatic, and cylindrical shapes from the cyclic oligosaccharide dissolved in the solvent, and forming a catalyst layer containing the precipitates. A method for producing a catalyst layer for a polymer electrolyte fuel cell, comprising the step of removing the precipitate from the catalyst layer.

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