Electrode catalyst layer, membrane electrode assembly, and solid polymer fuel cell

Abstract

The present invention provides an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that can suppress the decrease in battery performance that occurs in conjunction with the amount of catalyst. [Solution] The membrane electrode assembly 10 comprises a polymer electrolyte membrane 11 and a pair of electrode catalyst layers 12A and 12C sandwiching the polymer electrolyte membrane 11. The electrode catalyst layer 12C used in the air electrode of a polymer electrolyte fuel cell includes a conductive carrier supporting a catalyst material, a polymer electrolyte, and a fibrous material. In the electrode catalyst layer 12C, the amount of catalyst material supported per unit area (mg / cm³) 2 The thickness (μm) of the electrode catalyst layer 12C relative to ) is 15 to 30.

Description

[Technical Field]

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

[0002] Fuel cells are attracting attention as a battery that can contribute to solving environmental and energy problems. Fuel cells generate electricity by utilizing the chemical reaction between a fuel such as hydrogen and an oxidizer such as oxygen. Among fuel cells, polymer electrolyte membrane fuel cells can operate at or near room temperature, and are therefore expected to be used as power sources for homes and vehicles. For this reason, research and development is underway to solve problems such as improving battery performance, including power generation performance and durability, and reducing manufacturing costs, in order to commercialize polymer electrolyte membrane fuel cells.

[0003] A polymer electrolyte fuel cell comprises a membrane electrode assembly having an electrode catalyst layer constituting the fuel electrode, which is the anode; an electrode catalyst layer constituting the air electrode, which is the cathode; and a polymer electrolyte membrane sandwiched between these two electrode catalyst layers (see, for example, Patent Document 1).

[0004] The fuel electrode is supplied with a fuel gas containing hydrogen, and the air electrode is supplied with an oxidizing gas containing oxygen. Protons and electrons are generated from the fuel gas supplied to the fuel electrode by the action of the catalyst contained in the electrode catalyst layer. The protons are conducted by the polymer electrolyte contained in the electrode catalyst layer and polymer electrolyte membrane, and move through the polymer electrolyte membrane to the air electrode. Electrons are extracted from the fuel electrode into an external circuit and move through the external circuit to the air electrode. At the air electrode, water is produced by a reaction between the oxidizing gas and the protons and electrons that have moved from the fuel electrode. This series of electrochemical reactions generates an electromotive force. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2021-118181 [Overview of the project] [Problems that the invention aims to solve]

[0006] Because precious metals such as platinum used as catalysts are expensive, reducing the amount of catalyst used is desired to lower manufacturing costs. However, since the catalyst is contained in the electrode catalyst layer together with the support that holds it, reducing the amount of catalyst also reduces the amount of support. As a result, the internal structure such as voids and the thickness of the electrode catalyst layer may change, potentially degrading properties such as mass transport, drainage, and durability. Therefore, reducing the amount of catalyst may lead to a decrease in battery performance that goes beyond a simple decrease in reactivity due to the reduction in catalyst. For this reason, knowledge about the amount of catalyst that takes into account the correlation with other components in the electrode catalyst layer is needed. [Means for solving the problem]

[0007] This paper describes embodiments of an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that address the above-mentioned problems.

[0008] [Aspect 1] An electrode catalyst layer for use in the air electrode of a polymer electrolyte fuel cell, comprising a conductive carrier supporting a catalyst material, a polymer electrolyte, and a fibrous material, wherein the amount of catalyst material supported per unit area (mg / cm²) 2 An electrode catalyst layer having a thickness (μm) of 15 or more and 30 or less relative to ).

[0009] According to the above configuration, good power generation performance and durability can be obtained even when the amount of catalyst material used is reduced. Therefore, the decrease in battery performance that occurs in conjunction with the amount of catalyst can be suppressed.

[0010] [Aspect 2] The electrode catalyst layer according to [Aspect 1], wherein the content of the fibrous material in the electrode catalyst layer is 2 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the conductive carrier. According to the above configuration, the effects of improving power generation performance and durability through the use of fibrous material can be accurately obtained.

[0011] [Aspect 3] A membrane electrode assembly comprising a polymer electrolyte membrane and a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane, wherein one of the pair of electrode catalyst layers is the electrode catalyst layer described in [Aspect 1] or [Aspect 2]. According to the above configuration, a membrane electrode assembly capable of obtaining good power generation performance and durability can be obtained.

[0012] [Aspect 4] A solid polymer fuel cell comprising the membrane electrode assembly described in [Aspect 3] and a pair of separators sandwiching the membrane electrode assembly. According to the above configuration, good power generation performance and durability can be obtained.

Advantages of the Invention

[0013] According to the present disclosure, it is possible to suppress a decrease in battery performance caused by an accompanying catalyst amount.

Brief Description of the Drawings

[0014] [Figure 1] FIG. 1 is a diagram showing a cross-sectional structure of a membrane electrode assembly according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an internal structure of an electrode catalyst layer according to an embodiment. [Figure 3] FIG. 3 is a diagram showing a perspective structure of a solid polymer fuel cell according to an embodiment in an exploded manner.

Modes for Carrying Out the Invention

[0015] Referring to the drawings, an embodiment of an electrode catalyst layer, a membrane electrode assembly, and a solid polymer fuel cell will be described. [Configuration of Membrane Electrode Assembly and Electrode Catalyst Layer] As shown in FIG. 1, the membrane electrode assembly 10 includes a polymer electrolyte membrane 11 and a pair of electrode catalyst layers, namely a fuel electrode catalyst layer 12A and an air electrode catalyst layer 12C.

[0016] The polymer electrolyte membrane 11 is sandwiched between the fuel electrode catalyst layer 12A and the air electrode catalyst layer 12C in the thickness direction. The fuel electrode catalyst layer 12A contacts one of the two surfaces of the polymer electrolyte membrane 11, and the air electrode catalyst layer 12C contacts the other of the two surfaces of the polymer electrolyte membrane 11. The fuel electrode catalyst layer 12A constitutes the fuel electrode which is the anode of the solid polymer fuel cell. The air electrode catalyst layer 12C constitutes the air electrode which is the cathode of the solid polymer fuel cell.

[0017] When viewed from a position facing one surface of the polymer electrolyte membrane 11, the outer shapes of the fuel electrode catalyst layer 12A and the air electrode catalyst layer 12C are substantially the same shape, and these outer shapes are smaller than the outer shape of the polymer electrolyte membrane 11. The shapes of the outer shapes of the catalyst layers 12A, 12C and the polymer electrolyte membrane 11 are not particularly limited, and for example, they may be rectangular.

[0018] The polymer electrolyte membrane 11 contains a polymer electrolyte. The polymer electrolyte used for the polymer electrolyte membrane 11 may be any polymer electrolyte having proton conductivity, and for example, it may be a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. As the fluorine-based polymer electrolyte, a polymer electrolyte having a tetrafluoroethylene skeleton can be used. Examples of the fluorine-based polymer electrolyte are Nafion (registered trademark: manufactured by DuPont), Flemion (registered trademark: manufactured by AGC), Gore-Select (registered trademark: manufactured by Gore), etc. Examples of the hydrocarbon-based polymer electrolyte are sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, etc. [[ID=憨11]]

[0019] The catalyst layers 12A and 12C contain a catalyst, a conductive carrier, a polymer electrolyte, and a fibrous material. Figure 2 schematically shows the internal structure of the catalyst layers 12A and 12C. As shown in Figure 2, the catalyst layers 12A and 12C contain a catalyst support carrier 20, which is a conductive carrier 21 supporting the catalyst 22, a polymer electrolyte 23, and a fibrous material 24. In the catalyst layers 12A and 12C, the polymer electrolyte 23 and fibrous material 24 are located around the dispersed catalyst support carrier 20, and voids 26 are formed between these components.

[0020] The conductive carrier 21 only needs to be conductive and capable of supporting the catalyst 22 without being affected by the catalyst 22. As the conductive carrier 21, carbon materials such as carbon black, graphite, activated carbon, carbon nanotubes, carbon nanofibers, and fullerenes can be used.

[0021] The shape of the conductive carrier 21 is not particularly limited. The conductive carrier 21 may be particulate or fibrous. From the viewpoint of enabling smooth transfer of electrons affected by the catalytic substance 22, it is preferable that the conductive carrier 21 has a shape that allows the catalytic substance 22 to be supported on its outer surface.

[0022] The particle size of the particulate conductive carrier 21 is preferably 10 nm or more and 1000 nm or less, and more preferably 10 nm or more and 100 nm or less. If the particle size of the conductive carrier 21 is above the lower limit, electron conduction paths are more easily formed inside the catalyst layers 12A and 12C. If the particle size of the conductive carrier 21 is below the upper limit, the increase in resistance due to the increase in the thickness of the catalyst layers 12A and 12C can be suppressed.

[0023] The catalyst material 22 is particulate. The catalyst material 22 consists of, for example, metals, alloys, metal oxides, metal complex oxides, metal carbides, etc. Examples of metals contained in the catalyst material 22 include platinum group elements such as platinum, palladium, ruthenium, iridium, rhodium, and osmium, as well as gold, iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum. In particular, from the viewpoint of obtaining high catalytic activity, the catalyst material 22 is preferably composed of platinum, gold, palladium, rhodium, ruthenium, or alloys thereof. The particle size of the catalyst material 22 is preferably 0.5 nm or larger from the viewpoint of increasing the stability of the catalyst material. Furthermore, from the viewpoint of increasing catalytic activity, the particle size of the catalyst material 22 is preferably 20 nm or less, more preferably 10 nm or less, and even more preferably 5 nm or less. The particle sizes of the conductive carrier 21 and catalyst material 22 are defined as the median diameter (D50) in the particle size distribution determined by laser diffraction / scattering.

[0024] The polymer electrolyte 23 can be any polymer electrolyte having proton conductivity, such as a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. As the polymer electrolyte 23, the polymer electrolytes exemplified as materials for the polymer electrolyte membrane 11 can be used.

[0025] The electrolyte constituting the polymer electrolyte membrane 11 and the polymer electrolyte 23 may be the same electrolyte or different electrolytes. If these polymer electrolytes are of the same type or have similar coefficients of thermal expansion, it is possible to reduce the interfacial resistance between the polymer electrolyte membrane 11 and the catalyst layers 12A and 12C, and to reduce the difference in the rate of dimensional change between the polymer electrolyte membrane 11 and the catalyst layers 12A and 12C when humidity changes.

[0026] The content ratio of the polymer electrolyte 23 in the catalyst layers 12A and 12C may be, for example, 50 parts by mass or more and 100 parts by mass or less, based on 100 parts by mass of the conductive carrier 21 contained in the catalyst layers 12A and 12C.

[0027] The fibrous material 24 only needs to be able to maintain its fibrous shape without being affected by the catalyst material 22 and the polymer electrolyte 23. For example, the fibrous material 24 may be polymer fibers, carbon fibers, nanofibers made by processing conductive oxides into fibers, etc. One type of fiber may be used alone as the fibrous material 24, or two or more types of fibers may be used in combination.

[0028] From the viewpoint of suppressing the increase in resistance of catalyst layers 12A and 12C by assisting mass transport, it is preferable that the fibrous material 24 has electronic conductivity or proton conductivity. From the viewpoint of improving other mass transport properties simultaneously with proton conductivity, it is preferable that the fibrous material 24 contained in catalyst layers 12A and 12C contains polymer fibers that exhibit proton conductivity or basicity.

[0029] Examples of electronically conductive fibrous material 24 include carbon fibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. Carbon nanofibers are particularly preferred due to their good conductivity and dispersibility. Furthermore, using an electronically conductive fiber with catalytic activity can reduce the amount of catalyst material 22 used. For example, a carbon alloy catalyst made from carbon nanofibers can be used as the fibrous material 24 in the air electrode catalyst layer 12C. Alternatively, the fibrous material 24 may be an electrode active material for an oxygen reduction electrode that has been processed into fibers. As the electrode active material, for example, a material containing at least one transition metal element selected from tantalum, niobium, titanium, and zirconium can be used. Examples of such materials are conductive oxides or conductive oxynitrides of these transition metal elements, or partial oxides of carbonitrides of these transition metal elements.

[0030] An example of a proton-conducting fibrous material 24 is a resin fiber having a proton-interacting structure or functional group in its backbone. The proton-interacting structure or functional group preferably contains an atom with a lone pair of electrons, such as a nitrogen atom, oxygen atom, phosphorus atom, or sulfur atom, and is particularly preferably a nitrogen atom. Examples of structures or functional groups containing a nitrogen atom include amino groups, imino groups, azole groups which are unsaturated heterocyclic five-membered ring structures containing one or more nitrogen atoms, such as pyrrolidyl groups, pyrimidyl groups, and isothiazoles; functional groups which have six or more unsaturated heterocyclic ring structures, such as pyrrolidyl groups, piperidyl groups, and piperadyl groups; amide structures, imide bonds, and derivatives thereof. Specific examples of resin fibers include fibers made of polybenzimidazole, polybenzoxazole, polybenzothioazole, polyvinylimidazole, polyallylamine, etc. In particular, from the viewpoint of good proton conductivity and processability, polybenzimidazole, polybenzoxazole, and polybenzothioazole having an azole structure are preferred, and among these, polybenzimidazole is preferred due to its high nitrogen atom content.

[0031] The average fiber length of the fibrous material 24 is preferably 1 μm or more and 80 μm or less, and more preferably 5 μm or more and 70 μm or less. If the average fiber length of the fibrous material 24 is within the above range, it is possible to increase the strength of the catalyst layers 12A and 12C, thereby suppressing the occurrence of cracks in the catalyst layers 12A and 12C.

[0032] The average fiber diameter of the fibrous material 24 is preferably 50 nm or more and 400 nm or less. If the average fiber diameter of the fibrous material 24 is 50 nm or more, the blockage of the voids 26 in the catalyst layers 12A and 12C by the fibrous material 24 is suppressed, resulting in good gas diffusion and drainage. Furthermore, if the average fiber diameter of the fibrous material 24 is 400 nm or less, the increase in resistance caused by the fibrous material 24 inhibiting the conduction of electrons and protons is suppressed.

[0033] The average fiber diameter of the fibrous material 24 is determined by cross-sectional observation of the catalyst layers 12A and 12C using a scanning electron microscope (SEM). The average fiber diameter is the average value of the diameters of 10 or more arbitrarily selected fibrous materials 24 within the observation area of ​​the scanning electron microscope. For measuring the diameter of the fibrous material 24, for example, a vertical distance measurement function can be used. For example, cryo-ion milling can be used to expose the cross-sections of the catalyst layers 12A and 12C.

[0034] The content ratio of the fibrous material 24 in the catalyst layers 12A and 12C is preferably 2 parts by mass or more and 100 parts by mass or less, based on 100 parts by mass of the conductive carrier 21 contained in the catalyst layers 12A and 12C. If the content ratio of the fibrous material 24 is 2 parts by mass or more relative to the conductive carrier 21, the strength of the catalyst layers 12A and 12C is increased, and the occurrence of cracks in the catalyst layers 12A and 12C is suppressed. If the content ratio of the fibrous material 24 is 100 parts by mass or less relative to the conductive carrier 21, aggregation of the fibrous material 24 and a decrease in the drainage performance of the catalyst layers 12A and 12C are suppressed. Furthermore, the thickness of the catalyst layers 12A and 12C is suppressed, which also suppresses a decrease in drainage performance and makes it easier to secure conduction paths for electrons and protons.

[0035] [Composition of the air electrode catalyst layer] The characteristics of the air electrode catalyst layer 12C will be explained further. In the air electrode catalyst layer 12C, the amount of catalyst material 22 supported per unit area X (mg / cm²) 2 The ratio Rc (Rc=Y / X) of the thickness Y (μm) of the air electrode catalyst layer 12C to ) is between 15 and 30.

[0036] When the amount of catalyst material 22 supported changes, the content of conductive carrier 21 in the air electrode catalyst layer 12C also changes, which can alter the balance of internal structures such as voids in the air electrode catalyst layer 12C and affect the conductivity and drainage of electrons and protons. The thickness of the air electrode catalyst layer 12C may also change. The thickness of the air electrode catalyst layer 12C affects the strength of the air electrode catalyst layer 12C and its durability in maintaining performance over long-term use.

[0037] In this embodiment, by defining the ratio of the amount of catalyst material 22 supported to the thickness of the air electrode catalyst layer 12C, the thickness of the air electrode catalyst layer 12C is made suitable for the amount of catalyst material 22 supported, thereby achieving a good balance in the internal structure.

[0038] In detail, if the above ratio Rc is 15 or higher, the air electrode catalyst layer 12C will not become too thin relative to the amount of catalyst material 22 supported, so the catalyst support carrier 20 within the air electrode catalyst layer 12C will not become too dense, and sufficient voids 26 will be secured. Therefore, good drainage performance can be obtained. Also, if the air electrode catalyst layer 12C is thin, variations in the thickness of the air electrode catalyst layer 12C are likely to occur, which can lead to uneven degradation of the air electrode catalyst layer 12C and a decrease in durability. If the above ratio Rc is 15 or higher, the air electrode catalyst layer 12C will not become excessively thin, so this decrease in durability can be suppressed. In addition, the decrease in strength of the air electrode catalyst layer 12C can also be suppressed, so the occurrence of defects such as cracks can be suppressed.

[0039] On the other hand, if the above ratio Rc is 30 or less, the air electrode catalyst layer 12C does not become too thick relative to the amount of catalyst material 22 supported, making it easier to secure conduction paths and suppressing a decrease in drainage performance.

[0040] The amount X of catalyst material 22 can be determined from the composition of the materials used when manufacturing the air electrode catalyst layer 12C. Furthermore, in the air electrode catalyst layer 12C after manufacturing, the amount X of catalyst material 22 can be determined by performing X-ray fluorescence analysis on the air electrode catalyst layer 12C. Specifically, a calibration curve is created using a sample of a membrane electrode assembly equipped with an electrode catalyst layer containing the same catalyst material as the target of measurement, with a known amount of catalyst material, and the amount X is determined by interpolation. If the catalyst material contained in the other electrode catalyst layer separated by the polymer electrolyte membrane affects the measurement results depending on the measurement depth, the other electrode catalyst layer can be wiped off with a solvent or the like. For example, a Rigaku ZSX Primus IV can be used as the X-ray fluorescence analyzer.

[0041] The thickness Y of the air electrode catalyst layer 12C can be determined by observing the cross-section of the air electrode catalyst layer 12C using a scanning electron microscope, for example, by using the function to measure the perpendicular distance. Specifically, a tangent line is drawn to the part of the cross-sectional contour of the air electrode catalyst layer 12C corresponding to one end in the thickness direction of the air electrode catalyst layer 12C, and a perpendicular line is drawn from the tangent line toward the other end in the thickness direction, perpendicular to the tangent line. The length from one end to the other end of the perpendicular line is the thickness of the air electrode catalyst layer 12C at this measurement point. The thickness of the air electrode catalyst layer 12C can be obtained by measuring the thickness of the air electrode catalyst layer 12C at 10 or more locations that are at least 200 μm apart from each other, and taking the arithmetic mean of the measured values.

[0042] As a scanning electron microscope, for example, the SU-8020 manufactured by Hitachi High-Tech Corporation can be used. Regarding the main measurement conditions for the scanning electron microscope, the acceleration voltage may be 5kV, and the measurement magnification should be appropriately selected according to the thickness of the air electrode catalyst layer 12C so as not to cause measurement errors.

[0043] As a method for exposing the cross-section of the air electrode catalyst layer 12C, it is particularly preferable to use a cryo-ion milling method, in which the air electrode catalyst layer 12C is processed while being cooled, in order to reduce damage to the polymer electrolyte membrane 11 and the polymer electrolyte 23 that constitutes the air electrode catalyst layer 12C.

[0044] From the perspective of reducing the amount of catalyst material 22 used, the amount X of catalyst material 22 loaded is 0.5 mg / cm³. 2 Preferably, the amount X of the catalyst 22 supported is 0.1 mg / cm³, from the viewpoint of suppressing a decrease in the reactivity of the electrochemical reaction. 2 It is preferable that the above conditions are met.

[0045] Furthermore, the loading rate, which represents the loading density of the catalyst material 22, is preferably 30% by mass or more and 70% by mass or less, expressed as the mass of the loaded catalyst material 22 relative to the mass of the catalyst support carrier 20.

[0046] In order to reduce the amount of catalyst material 22 used while suppressing a decrease in reactivity, it is desirable to increase the specific surface area of ​​the catalyst material 22 by using catalyst material 22 with a small particle size, thereby increasing the reaction field. In order to obtain a sufficient reaction field using catalyst material 22 with a small particle size, it is preferable to use a conductive carrier 21 with a large specific surface area, or to reduce the loading density of catalyst material 22 on the conductive carrier 21, so as to suppress the concentration of catalyst material 22.

[0047] On the other hand, conductive carriers 21 with a large specific surface area often have a low degree of crystallinity and are easily oxidized, which tends to reduce the durability of the electrode catalyst layer. Also, if the loading density of the catalyst material 22 is reduced, the amount of conductive carrier 21 increases relative to the amount of catalyst material 22, so the inside of the air electrode catalyst layer 12C becomes denser and drainage performance tends to decrease.

[0048] The inclusion of fibrous material 24 in the air electrode catalyst layer 12C makes it possible to improve these problems. Specifically, since the fibrous material 24 assists in the transport of electrons and protons, conduction paths are more easily maintained, and the decrease in durability is suppressed. This assistance in the formation of conduction paths is also effective when the amount of conductive support 21 is reduced due to a reduction in the amount of supported catalyst material 22, or when the conductive support 21 is lost due to long-term use. In addition, the entanglement of the fibrous material 24 makes it easier to secure voids 26 within the air electrode catalyst layer 12C, thus suppressing a decrease in drainage performance.

[0049] Thus, the inclusion of the fibrous material 24 contributes to improved power generation performance and durability. Therefore, it also has the effect of suppressing the decrease in power generation performance and durability that occurs when the amount of catalyst material 22 is reduced. These effects can be accurately obtained if the content ratio of the fibrous material 24 is 2 parts by mass or more relative to the conductive carrier 21.

[0050] Furthermore, since the output of the fuel cell depends more on the oxygen reduction activity of the air electrode than on the fuel electrode, good power generation performance and durability can be obtained if at least the air electrode catalyst layer 12C has the above-described configuration. The fuel electrode catalyst layer 12A may have the same configuration as the air electrode catalyst layer 12C, or it may have a different configuration from the air electrode catalyst layer 12C. For example, the composition and thickness of the catalyst material 22 in the fuel electrode catalyst layer 12A may be the same as or different from that of the air electrode catalyst layer 12C. Also, the ratio of the thickness of the fuel electrode catalyst layer 12A to the amount of catalyst material 22 in the fuel electrode catalyst layer 12A may be between 15 and 30, or outside this range.

[0051] [Polymer electrolyte fuel cell] Referring to Figure 3, the configuration of a polymer electrolyte fuel cell equipped with the membrane electrode assembly 10 described above will be explained.

[0052] As shown in Figure 3, the polymer electrolyte fuel cell 30 comprises a membrane electrode assembly 10, a pair of gas diffusion layers 31A and 31C, and a pair of separators 32A and 32C. The membrane electrode assembly 10 is sandwiched between the gas diffusion layer 31A and the gas diffusion layer 31C, with the gas diffusion layer 31A in contact with the fuel electrode catalyst layer 12A and the gas diffusion layer 31C in contact with the air electrode catalyst layer 12C.

[0053] The gas diffusion layers 31A and 31C are layers for uniformly diffusing the supplied gas and possess gas diffusivity and conductivity. The gas diffusion layers 31A and 31C include, for example, porous materials such as carbon cloth, carbon paper, and nonwoven fabric. The gas diffusion layer 31A, together with the fuel electrode catalyst layer 12A, constitutes the fuel electrode, and the gas diffusion layer 31C, together with the air electrode catalyst layer 12C, constitutes the air electrode.

[0054] The laminate of the film electrode assembly 10 and the gas diffusion layers 31A and 31C is sandwiched between separators 32A and 32C. Separators 32A and 32C are gas-impermeable and conductive. The materials of separators 32A and 32C are, for example, carbon-based or metallic materials.

[0055] Separator 32A faces the gas diffusion layer 31A, and separator 32C faces the gas diffusion layer 31C. On separator 32A, a gas channel 33A is formed on the surface facing the gas diffusion layer 31A, and a cooling water channel 34A is formed on the surface opposite to the gas diffusion layer 31A. Similarly, on separator 32C, a gas channel 33C is formed on the surface facing the gas diffusion layer 31C, and a cooling water channel 34C is formed on the surface opposite to the gas diffusion layer 31C.

[0056] When the polymer electrolyte fuel cell 30 is in use, a fuel gas such as hydrogen flows through the gas channel 33A of separator 32A, and an oxidizing gas such as oxygen flows through the gas channel 33C of separator 32C. Cooling water also flows through the cooling water channels 34A and 34C of each separator 32A and 32C. When the fuel gas is supplied to the fuel electrode from the gas channel 33A and the oxidizing gas is supplied to the air electrode from the gas channel 33C, an electrochemical reaction proceeds, and an electromotive force is generated between the fuel electrode and the air electrode. Organic fuel such as methanol may be supplied to the fuel electrode.

[0057] The polymer electrolyte fuel cell 30 may be used in the form of a single cell as shown in Figure 3, or multiple polymer electrolyte fuel cells 30 may be stacked and connected in series to form a single fuel cell. The polymer electrolyte fuel cell 30 can be used by assembling it with a gas supply device, a cooling device, and other ancillary devices.

[0058] In addition to the above-mentioned components, the polymer electrolyte fuel cell 30 may also be equipped with components such as gaskets to suppress gas leakage. The gaskets are arranged to surround the outer periphery of the catalyst layers 12A and 12C. Furthermore, the gas diffusion layer 31A and the separator 32A may be an integrated structure, and the gas diffusion layer 31C and the separator 32C may be an integrated structure. Alternatively, the gas diffusion layers 31A and 31C may be components that constitute the membrane electrode assembly 10.

[0059] [Method for manufacturing a membrane electrode assembly] A method for manufacturing the membrane electrode assembly 10, including the method for manufacturing the catalyst layers 12A and 12C, will be described. First, a catalyst ink, which is a coating liquid for forming catalyst layers 12A and 12C, is prepared. The catalyst ink comprises a catalyst support 20, a polymer electrolyte 23, a fibrous material 24, and a dispersion medium.

[0060] The dispersion medium is not particularly limited as long as it does not erode the catalyst support 20, the polymer electrolyte 23, and the fibrous material 24, and can dissolve the polymer electrolyte 23 in a highly fluid state or disperse it as a fine gel. The dispersion medium preferably contains a volatile organic solvent.

[0061] The dispersion medium is, for example, alcohols, ketones, ethers, and other polar solvents. Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, etc. Examples of ketones include acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, diisobutyl ketone, etc. Examples of ethers include tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, dibutyl ether, etc. Examples of other polar solvents include amines, esters, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, 1-methoxy-2-propanol, etc.

[0062] The dispersion medium may be a mixture containing two or more of the materials described above. The dispersion medium may also contain water. The amount of water is not particularly limited, as long as it does not cause clouding or gelation of the catalyst ink. If the dispersion medium contains a lower alcohol, it is preferable to use a mixture of the alcohol and water to reduce the risk of ignition. After mixing the materials for the catalyst ink, a dispersion treatment may be performed. Dispersion treatment can be carried out using, for example, a planetary ball mill, a bead mill, or an ultrasonic homogenizer.

[0063] Catalyst layers 12A and 12C are formed by applying a catalyst ink to a substrate to form a coating film and drying the coating film. As the substrate, a transfer substrate that is peeled off after transferring the catalyst layers 12A and 12C to the polymer electrolyte membrane 11, the polymer electrolyte membrane 11, or gas diffusion layers 31A and 31C can be used.

[0064] For example, the transfer substrate may be a film made of a fluorine-based resin with excellent transferability, such as ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), or polytetrafluoroethylene (PTFE), or a film made of polymers such as polyimide, polyethylene terephthalate, polyamide, polysulfone, polyethersulfone, polyphenylene sulfide, polyether / etherketone, polyetherimide, polyarylate, or polyethylene naphthalate.

[0065] As a method for applying the catalyst ink, known methods such as die coating, roll coating, curtain coating, spray coating, kiss coating, comma coating, bar coating, spin coating, and squeegee can be used. Among these, die coating is preferred as the method for applying the catalyst ink. Die coating is preferred because it allows for stable film thickness in the middle of the coating period and enables intermittent coating.

[0066] For drying the coating film, methods such as drying using a hot air oven, IR (far-infrared) drying, drying using a hot plate, and reduced-pressure drying can be used. The drying temperature may be, for example, 40°C to 200°C, and preferably 40°C to 120°C. The drying time may be, for example, 0.5 minutes to 1 hour, and preferably 1 minute to 30 minutes.

[0067] When the substrate for forming the catalyst layers 12A and 12C is a transfer substrate, the catalyst layers 12A and 12C are bonded to the polymer electrolyte membrane 11 by thermocompression bonding, and then the transfer substrate is peeled off from the catalyst layers 12A and 12C. This forms the membrane electrode assembly 10.

[0068] When the substrate for forming the catalyst layers 12A and 12C is a polymer electrolyte membrane 11, the catalyst layers 12A and 12C are formed directly on the surface of the polymer electrolyte membrane 11. This forms the membrane electrode assembly 10.

[0069] When the substrate for forming the catalyst layers 12A and 12C is the gas diffusion layer 31A and 31C, the catalyst layers 12A and 12C, supported by the gas diffusion layer 31A and 31C, are joined to the polymer electrolyte membrane 11 by thermocompression bonding, thereby forming the membrane electrode assembly 10.

[0070] When catalyst layers 12C and 12A formed on a substrate are bonded to a polymer electrolyte membrane 11 by pressurization and heating, the pressure and heat applied to the catalyst layers 12C and 12A affect the power generation performance of the fuel cell. Therefore, the pressurization pressure is preferably between 0.1 MPa and 20 MPa. If the pressure is 20 MPa or less, excessive compression of the catalyst layers 12C and 12A is suppressed, and if the pressure is 0.1 MPa or more, sufficient bonding between the catalyst layers 12C and 12A and the polymer electrolyte membrane 11 is obtained, thereby suppressing a decrease in the power generation performance of the fuel cell.

[0071] Furthermore, it is preferable to set the heating temperature to near the glass transition temperature of the polymer electrolyte contained in the polymer electrolyte membrane 11 or the catalyst layers 12C, 12A. This improves the bonding between the catalyst layers 12C, 12A and the polymer electrolyte membrane 11, while suppressing an increase in interfacial resistance.

[0072] [Examples] The membrane electrode assembly and polymer electrolyte fuel cell equipped with the electrode catalyst layer described above will be explained using specific examples and comparative examples.

[0073] (Materials for the air electrode catalyst layer) The following materials were used for the air electrode catalyst layer. The catalyst support was selected from P1, P2, and P3, and the fibrous material was selected from F1 and F2. The specific surface area of ​​each catalyst support (P1, P2, and P3) was 100 m². 2 / g or more 500m 2 It is within the range of / g or less.

[0074] Catalyst support P1: Particles on a conductive support made of Ketjenblack, on which platinum, the catalyst material, is supported (the catalyst load ratio is expressed as a mass ratio of Ketjenblack:platinum = 50:50, and the particle size of the catalyst material is 2.5 nm). Catalyst-supported carrier P2: Particles on a conductive carrier made of Ketjenblack, on which platinum, the catalyst material, is supported (the catalyst support ratio is expressed as Ketjenblack:platinum = 60:40 by mass ratio, and the particle size of the catalyst material is 3.2 nm). Catalyst-supported carrier P3: Particles on which platinum, the catalyst material, is supported on a conductive carrier made of acetylene black (the catalyst support ratio is expressed as acetylene black:platinum = 50:50 by mass ratio, and the particle size of the catalyst material is 2.0 nm). Polymer electrolyte: 20% Nafion® dispersion (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Fibrous material F1: Carbon fiber (VGCF-H, manufactured by Resonaq Corporation) Fibrous material F2: Polybenzimidazole fibers (average fiber diameter 150 nm, fiber length 15 μm)

[0075] (Materials for the fuel electrode catalyst layer) The following materials were used for the fuel electrode catalyst layer. Catalyst support: Particles consisting of a conductive support made of Ketjenblack, on which platinum, the catalyst material, is supported (the catalyst load ratio is expressed as Ketjenblack:platinum = 60:40 by mass ratio, and the particle size of the catalyst material is 3.2 nm). Polymer electrolyte: 20% Nafion® dispersion (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) Fibrous material: Carbon fiber (VGCF-H, manufactured by Resonaq Corporation) Furthermore, the catalyst support described above is the same catalyst support P2 used in the air electrode catalyst layer.

[0076] (Preparation of catalyst ink) Catalyst inks for forming the air electrode catalyst layer and the fuel electrode catalyst layer were prepared, respectively. The following manufacturing process for the catalyst inks is common to both the air electrode catalyst layer and the fuel electrode catalyst layer.

[0077] A mixture was obtained by mixing a catalyst support, a polymer electrolyte, a fibrous material, and a dispersion medium. The amount of polymer electrolyte was 100 parts by mass, with 100 parts by mass being the conductive support contained in the catalyst support. Ethanol was used as the dispersion medium. The amount of dispersion medium was adjusted so that the solid content concentration in the mixture was 12% by mass. A catalyst ink was obtained by dispersing the mixture using a planetary ball mill at 300 rpm for 120 minutes. The dispersion treatment was carried out by adding zirconia balls with a diameter of 5 mm to a zirconia container containing the mixture, filling it to about one-third of the total volume.

[0078] (Formation of electrode catalyst layer and film electrode assembly) A catalyst ink for the air electrode catalyst layer was applied to one side of a polymer electrolyte membrane (Nafion® 211, manufactured by DuPont) using a slit die coater to form a coating film. Then, the polymer electrolyte membrane with the coating film was dried in an 80°C hot air oven until the surface of the coating film was no longer sticky, thereby forming the air electrode catalyst layer. This resulted in obtaining a laminate of the air electrode catalyst layer and the polymer electrolyte membrane.

[0079] Next, a catalyst ink for the fuel electrode catalyst layer was applied to the other side of the polymer electrolyte membrane using a slit die coater to form a coating film. Then, the laminate with the coated film was dried in an 80°C hot air oven until the surface of the coating film was no longer sticky, thereby forming the fuel electrode catalyst layer. This obtained a membrane electrode assembly.

[0080] (Fabrication of polymer electrolyte fuel cells) As a polymer electrolyte fuel cell, a JARI standard cell was fabricated in accordance with the "Cell Evaluation and Analysis Protocol," a publication of the New Energy and Industrial Technology Development Organization (NEDO). This cell was constructed by placing gas diffusion layers, gaskets, and separators on both sides of the membrane electrode assembly and tightening them to achieve a predetermined surface pressure.

[0081] (Examples / Comparative Examples) The membrane electrode assemblies and polymer electrolyte fuel cells of Examples 1-8 and Comparative Examples 1-4 were fabricated by changing the type of catalyst support, the type of fibrous material, the content ratio of the fibrous material, and the thickness of the catalyst layer in the air electrode catalyst layer. As described above, the catalyst support was selected from P1-P3, and the fibrous material was selected from F1 and F2. In Comparative Examples 1 and 3, no fibrous material was added to the catalyst ink. The composition and thickness of the fuel electrode catalyst layer are common to each example and each comparative example.

[0082] (Evaluation of power generation performance) For each example and comparative example, the fabricated JARI standard cell was used as the evaluation cell, and the IV measurement described in the "Cell Evaluation and Analysis Protocol" was performed to evaluate the power generation performance.

[0083] In evaluating power generation performance, the current density was 1.5 A / cm². 2 When the cell voltage was 0.67V or higher, it was classified as particularly good ("A"), when it was between 0.65V and 0.67V, it was classified as good ("B"), and when it was below 0.65V, it was classified as poor ("C").

[0084] (Durability evaluation) The above power generation performance was defined as the initial power generation performance. For each example and comparative example, the single evaluation cell after the IV measurement in the initial power generation performance evaluation was subjected to the potential fluctuation cycle test described in the "Cell Evaluation Analysis Protocol". In the potential fluctuation cycle test, 10,000 cycles of start-up and stop tests and 10,000 cycles of load response tests were performed. After the potential fluctuation cycle test, IV measurements were performed under the same conditions as the initial power generation performance evaluation.

[0085] And then, the retention rate Rr of the cell voltage (Rr (%) = (cell voltage after the test / cell voltage before the test) × 100) when the current density is 1.5 A / cm 2 was determined. In the evaluation of durability, when the retention rate Rr is 80% or more, it is particularly good "A", when it is 60% or more and less than 80%, it is good "B", and when it is less than 60%, it is poor "C".

[0086] (Evaluation results) Table 1 shows, for the air electrode catalyst layer of each example and each comparative example, the type of catalyst-supporting carrier, the type of fibrous material, the content ratio of the fibrous material, the loading amount X (mg / cm 2 ) of the catalyst substance, the thickness Y (μm) of the catalyst layer, and the ratio Rc of the thickness Y to the loading amount X. The content ratio of the fibrous material is shown in parts by mass when the conductive carrier is 100 parts by mass.

[0087] Note that, for the fuel electrode catalyst layer of each example and each comparative example, the content ratio of the fibrous material is 25 parts by mass, the loading amount of the catalyst substance is 0.4 mg / cm 2 and the thickness of the catalyst layer is 8 μm. Also, the ratio of the thickness to the loading amount of the catalyst substance in the fuel electrode catalyst layer is 20.

[0088] Furthermore, Table 1 shows the evaluation results of the power generation performance and durability for each example and each comparative example.

[0089]

Table 1

[0090] As shown in Table 1, Examples 1-8, where the ratio Rc was between 15 and 30, showed high initial power generation performance and durability. On the other hand, Comparative Examples 1 and 4, where the ratio Rc was less than 15, showed high initial power generation performance but low durability. Similarly, Comparative Example 3, which did not contain fibrous material, also showed high initial power generation performance but low durability, even though its ratio Rc was 15. Therefore, it is suggested that the inclusion of fibrous material contributes to improved durability. Furthermore, Comparative Example 2, where the ratio Rc was greater than 30, showed high durability but low power generation performance.

[0091] As described above using the examples, the electrode catalyst layer, membrane electrode assembly, and polymer electrolyte fuel cell of the above embodiment provide the following effects. (1) In the air electrode catalyst layer 12C, the amount of catalyst material 22 supported X (mg / cm³) 2 The thickness Y (μm) of the catalyst layer relative to the ) is 15 to 30. This allows for good power generation performance and durability even when the amount of catalyst material 22 used is reduced. Therefore, the decrease in battery performance that occurs in conjunction with the amount of catalyst can be suppressed.

[0092] (2) The content of the fibrous material 24 in the air electrode catalyst layer 12C is 2 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the conductive carrier 21. This ensures that the effects of improving power generation performance and durability due to the fibrous material 24 are accurately obtained. [Explanation of symbols]

[0093] 10...Membrane electrode assembly 11...Polymer electrolyte membrane 12A…Fuel electrode catalyst layer 12C...Air electrode catalyst layer 20…Catalyst support 21... Conductive carrier 22...catalyst material 23...polymer electrolyte 24…Fibrous material 30...Polymer fuel cell 31A, 31C... Gas diffusion layer 32A, 32C... Separators

Claims

1. An electrode catalyst layer for use in the air electrode of a polymer electrolyte fuel cell, comprising a conductive carrier supporting a catalytic material, a polymer electrolyte, and a fibrous material, Amount of the catalyst material supported per unit area (mg / cm²) ２ The thickness (μm) of the electrode catalyst layer relative to the given element is 15 or more and 30 or less. Electrocatalyst layer.

2. The content of the fibrous material in the electrode catalyst layer is 2 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the conductive carrier. The electrode catalyst layer according to claim 1.

3. Polymer electrolyte membrane, The system comprises a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane, One of the pair of electrode catalyst layers is the electrode catalyst layer according to claim 1 or 2. Membrane electrode assembly.

4. The membrane electrode assembly according to claim 3, The system comprises a pair of separators that sandwich the aforementioned membrane electrode assembly. Polymer electrolyte fuel cell.

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