Catalyst particles, electrode catalyst layer, membrane electrode assembly, and polymer electrolyte fuel cell

Abstract

To provide a catalyst particle, an electrode catalyst layer, a membrane electrode assembly and a solid polymer electrolyte fuel cell, which are capable of increasing durability and power generation performance.SOLUTION: A catalyst particle 10 includes: a catalyst supporting carrier 15 comprising a conductive carrier 11 and a plurality of metal particles 12 supported by the conductive carrier 11; an inorganic film 13 covering a part of the catalyst supporting carrier 15; and an ionic liquid 14 located at a portion including the surface of the catalyst supporting carrier 15. A plurality of the metal particles 12 include a metal particle 12 having a surface, a part of which is exposed from the inorganic film 13 and comes into contact with the ionic liquid 14. The weight ratio of the inorganic film 13 to the total weight of the catalyst supporting carrier 15 and the inorganic film 13 is 0.01 or more and 0.2 or less, and the volume ratio of the ionic liquid 14 to the meso-pore volume of the catalyst supporting carrier 15 is 10% or more and 50% or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to catalyst particles, electrode catalyst layers, membrane electrode assemblies, and solid polymer fuel cells used in fuel cells. [Background technology]

[0002] A polymer electrolyte fuel cell comprises a membrane electrode assembly having a proton-conducting polymer electrolyte membrane and a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane in the thickness direction. One electrode catalyst layer constitutes the fuel electrode, which is the anode, and the other electrode catalyst layer constitutes the air electrode, which is the cathode. The electrode catalyst layer contains metal particles made of platinum-based noble metals, a carrier that supports the metal particles, and a polymer electrolyte.

[0003] The fuel electrode is supplied with a fuel gas containing hydrogen, and the air electrode is supplied with an oxidizing gas containing oxygen. In the electrode catalyst layer of the fuel electrode, protons and electrons are generated from the fuel gas. 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. In the electrode catalyst layer of 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 electrode reaction generates an electric current.

[0004] Since the main reactions in the electrode reaction described above occur in the electrode catalyst layer, the composition of the electrode catalyst layer is an important factor in improving the power generation performance of fuel cells. Therefore, much research has been conducted on the materials of the electrode catalyst layer and the ratios of each material. For example, if metal particles that act as catalysts in the electrode reaction dissolve into the polymer electrolyte, the output of the fuel cell will decrease. Therefore, Patent Document 1 proposes coating the metal particles with a porous inorganic material such as silica in order to suppress the dissolution of metal particles. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2008-4541 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, when metal particles are coated with a porous inorganic material, as in Patent Document 1, the leaching of metal particles is suppressed, but contact between the metal particles and the polymer electrolyte is hindered. As a result, proton conduction is inhibited and resistance increases, leading to the problem of a decrease in the output voltage of the fuel cell.

[0007] Platinum has traditionally been widely used as a metal particle, but because platinum is expensive, there is a demand to reduce the amount of platinum used in the electrode catalyst layer. In electrode catalyst layers with reduced platinum content, the decrease in output voltage mentioned above is particularly pronounced. [Means for solving the problem]

[0008] This paper describes various embodiments of catalyst particles, electrode catalyst layers, membrane electrode assemblies, and polymer electrolyte fuel cells for solving the above-mentioned problems. [Aspect 1] A catalyst particle comprising a conductive carrier and a catalyst support carrier composed of a plurality of metal particles supported on the conductive carrier, an inorganic coating covering a part of the catalyst support carrier, and an ionic liquid located in a portion including the surface of the catalyst support carrier, wherein the plurality of metal particles include metal particles whose surface is partially exposed from the inorganic coating and in contact with the ionic liquid, the weight ratio of the inorganic coating to the total weight of the catalyst support carrier and the inorganic coating is 0.01 or more and 0.2 or less, and the volume ratio of the ionic liquid to the mesopore volume of the catalyst support carrier is 10% or more and 50% or less.

[0009] According to the above configuration, the inorganic coating suppresses the dissolution of metal particles into the polymer electrolyte, and the presence of the ionic liquid suppresses the inhibition of proton conduction. Therefore, good durability and power generation performance can be obtained in a fuel cell using catalyst particles in the electrode catalyst layer.

[0010] [Aspect 2] The catalyst particles according to [Aspect 1], wherein the inorganic coating is made of silica formed from either tetraethoxysilane or triethoxymethylsilane. According to the above configuration, the inorganic coating is formed as a multilayer structure consisting of silica films, which makes it easier to retain ionic liquids inside the inorganic coating.

[0011] [Aspect 3] The catalyst particles according to [Aspect 1] or [Aspect 2], wherein the ionic liquid comprises 1-alkyl-3-methylimidazolium bis(trifluororomethanesulfonyl)imide. According to the above configuration, proton conduction via the ionic liquid is suitably possible.

[0012] [Aspect 4] The catalyst particles according to [Aspect 3], wherein the ionic liquid comprises 1-ethyl-3-methylimidazolium bis(trifluororomethanesulfonyl)imide. According to the above configuration, proton conduction via the ionic liquid is more preferably possible.

[0013] An electrode catalyst layer comprising catalyst particles according to any one of [Aspect 5], [Aspect 1] to [Aspect 4], and a polymer electrolyte. According to the above configuration, good durability and power generation performance can be obtained in a fuel cell using the electrode catalyst layer.

[0014] [Aspect 6] The electrode catalyst layer according to [Aspect 5] further comprising a fibrous material. According to the above configuration, the durability and drainage of the electrode catalyst layer are improved.

[0015] [Aspect 7] A membrane electrode assembly comprising a polymer electrolyte membrane and a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane, wherein at least one of the pair of electrode catalyst layers is the electrode catalyst layer described in [Aspect 5] or [Aspect 6].

[0016] According to the above configuration, good durability and power generation performance can be obtained in a fuel cell using the membrane electrode assembly.

[0017] [Embodiment 8] A solid polymer fuel cell comprising the membrane electrode assembly described in [Embodiment 7] and a pair of separators sandwiching the membrane electrode assembly. According to the above configuration, good durability and power generation performance can be obtained. [Advantages of the Invention]

[0018] According to the present invention, in a fuel cell, durability and power generation performance can be enhanced. [Brief Description of the Drawings]

[0019] [Figure 1] A diagram showing the structure of catalyst particles in one embodiment. [Figure 2] A diagram showing the structure of a membrane electrode assembly in one embodiment. [Figure 3] A diagram showing the structure of an electrode catalyst layer in one embodiment. [Figure 4] A diagram showing the structure of a solid polymer fuel cell in one embodiment. [Figure 5] (a), (b), and (c) are diagrams showing the element distribution in the catalyst particles of the examples. [Modes for Carrying Out the Invention]

[0020] Referring to FIGS. 1 to 4, one embodiment of catalyst particles, an electrode catalyst layer, a membrane electrode assembly, and a solid polymer fuel cell will be described. In each figure, for ease of understanding, the shapes and ratios of each part are exaggerated appropriately.

[0021] <00​​​​​​The inorganic coating 13 primarily covers the metal particles 12. A portion of the surface of the metal particles 12 is exposed from the inorganic coating 13, and at least a portion of this exposed portion is in contact with the ionic liquid 14. The inorganic coating 13 may also cover a portion of the surface of the conductive carrier 11. The thickness of the inorganic coating 13 may be uniform or non-uniform.

[0023] The plurality of metal particles 12 may include at least one of the following: metal particles 12 whose entire surface is covered by the inorganic coating 13, metal particles 12 whose entire surface is exposed from the inorganic coating 13, and metal particles 12 that are not in contact with the ionic liquid 14.

[0024] The ionic liquid 14 is in contact with a portion of the surface of the conductive carrier 11 and penetrates into at least a portion of the interior of the conductive carrier 11. Furthermore, if the inorganic coating 13 has gaps such as pores inside, the ionic liquid 14 may also penetrate into at least a portion of the interior of the inorganic coating 13. The ionic liquid 14 can penetrate into any gaps present in the catalyst support 15 and the inorganic coating 13.

[0025] According to the above configuration, since the inorganic coating 13 covers the metal particles 12, the elution of the metal particles 12 into the polymer electrolyte surrounding the catalyst particles 10 in the electrode catalyst layer is suppressed. On the other hand, if the inorganic coating 13 completely covers each metal particle 12, contact between the polymer electrolyte, which is a proton-conducting ionomer, and the metal particles 12, which act as a catalyst for the electrode reaction, is hindered. In contrast, in this embodiment, since the ionic liquid 14, which is capable of passing protons, is in contact with the metal particles 12, protons are conducted via the ionic liquid 14. Furthermore, if the ionic liquid 14 penetrates into the gaps within the inorganic coating 13, the ionic liquid 14 in the inorganic coating 13 also contributes to proton conduction. In addition, since a part of the surface of the conductive carrier 11 is exposed from the inorganic coating 13, a path for electron conduction is also secured.

[0026] Furthermore, in a configuration where the ionic liquid 14 is absent, as in the conventional design, if the inorganic coating 13 is formed from a hydrophilic material such as silica, the water generated by the electrode reaction, particularly at the air electrode, remains near the inorganic coating 13, resulting in a decrease in the drainage capacity of the electrode catalyst layer. In contrast, in this embodiment, by using an ionic liquid 14 with low hydrophilicity, the retention of water near the ionic liquid 14, i.e., near the catalyst particles 10, can be suppressed. This reduces the decrease in the drainage capacity of the electrode catalyst layer.

[0027] From the above, it can be seen that a fuel cell using the catalyst particles 10 of this embodiment can be obtained to produce good power generation performance. In particular, when a platinum-based metal is used as the metal particles 12, high output can be obtained even when the amount of metal particles 12 supported is reduced.

[0028] [Materials for catalyst particles] (Conductive carrier) The conductive carrier 11 is made of conductive fine particles and is not affected by the metal particles 12. The conductive carrier 11 has pores including mesopores. Preferably, the conductive carrier 11 is made of a carbon material. Examples of carbon materials used as the conductive carrier 11 include carbon black, graphite, graphite, activated carbon, and fullerene. The conductive carrier 11 contained in the electrode catalyst layer may consist of one type of material or two or more types of materials.

[0029] The average particle size of the conductive carrier 11 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 average particle size of the conductive carrier 11 is 10 nm or more, electron conduction paths are more easily formed within the electrode catalyst layer. Furthermore, if the average particle size of the conductive carrier 11 is 1000 nm or less, the increase in resistance due to an increase in the thickness of the electrode catalyst layer can be suppressed.

[0030] (metal particles) Examples of metal particles 12 include platinum group elements such as platinum, palladium, ruthenium, iridium, rhodium, and osmium, as well as metals such as gold, iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, and their alloys, oxides, complex oxides, and carbides. Among these, platinum, gold, palladium, rhodium, ruthenium, and their alloys are preferred due to their high catalytic activity. In particular, the metal particles 12 are preferably platinum or platinum alloys. The constituent materials of the metal particles 12 contained in the electrode catalyst layer may be one type or two or more types. The average particle size of the metal particles 12 is preferably 0.5 nm or more and 20 nm or less, and more preferably 1 nm or more and 5 nm or less. If the average particle size of the metal particles 12 is 0.5 nm or more, the stability as a catalyst is enhanced. If the average particle size of the metal particles 12 is 20 nm or less, the activity as a catalyst is enhanced. In this specification, the average particle size is the arithmetic mean particle size obtained from particle size measurement.

[0031] (Inorganic coating) The material of the inorganic coating 13 preferably contains one or more of silica (SiO2), zirconia (ZrO2), and titania (TiO2). In particular, the material of the inorganic coating 13 is preferably silica, and especially preferably silica obtained by hydrolysis and dehydration condensation of either tetraethoxysilane or triethoxymethylsilane. By using such silica, the inorganic coating 13 is formed as a multilayer structure consisting of silica films, making it easier to retain the ionic liquid 14 inside the inorganic coating 13. The composition of silica produced from tetraethoxysilane is (SiO2) n Furthermore, the composition of silica produced from triethoxymethylsilane is (SiO-Me-SiO) n Furthermore, the silica in question is porous silica (low-crystallinity silica).

[0032] The thickness of the inorganic coating 13 is preferably 1 nm or more and 100 nm or less, more preferably 10 nm or more and 50 nm or less, and even more preferably 20 nm or more and 40 nm or less. If the thickness of the inorganic coating 13 is 1 nm or more, the formation of the inorganic coating 13 is easy. Also, if the thickness of the inorganic coating 13 is 100 nm or less, sufficient gaps between layers in the multilayer structure are easily secured.

[0033] In the catalyst particles 10, the weight ratio of the inorganic coating 13 to the total weight of the inorganic coating 13 and the catalyst support 15 is 0.01 or more and 0.2 or less. The weight of the catalyst support 15 is the total weight of the conductive support 11 and the metal particles 12 supported on the conductive support 11.

[0034] If the weight ratio of the inorganic coating 13 is 0.01 or higher, the inorganic coating 13 is formed on the surface of the catalyst support 15 to the extent that the dissolution of the metal particles 12 is sufficiently suppressed. In addition, the ionic liquid 14 is easily retained within the inorganic coating 13. If the weight ratio of the inorganic coating 13 is 0.2 or lower, the thickness and formation area of ​​the inorganic coating 13 do not become too large, so that contact between the metal particles 12 and the ionic liquid 14 can be ensured to the extent that suitable proton conduction can be obtained. The weight ratio of the inorganic coating 13 can be determined, for example, using XPS (X-ray photoelectron spectroscopy).

[0035] (Ionic liquid) The ionic liquid 14 is preferably an imidazolium salt, and more specifically, the ionic liquid 14 preferably contains 1-alkyl-3-methylimidazolium bis(trifluoroomethanesulfonyl)imide. The alkyl group contained in the compound is, for example, a methyl group, an ethyl group, a butyl group, a pentyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, etc. Among these, the alkyl group is preferably an ethyl group. That is, the ionic liquid 14 preferably contains 1-ethyl-3-methylimidazolium bis(trifluoroomethanesulfonyl)imide.

[0036] In this case, it is preferable that the ionic liquid 14 consists only of 1-ethyl-3-methylimidazolium bis(trifluoroomethanesulfonyl)imide, but it may also contain other 1-alkyl-3-methylimidazolium bis(trifluoroomethanesulfonyl)imides. If the ionic liquid 14 contains multiple types of 1-alkyl-3-methylimidazolium bis(trifluoroomethanesulfonyl)imides, it is preferable that the content of 1-ethyl-3-methylimidazolium bis(trifluoroomethanesulfonyl)imide is 50% or more of the total mass of the ionic liquid 14.

[0037] In the catalyst particles 10, the volume of the ionic liquid 14 is preferably 10% to 50% of the mesopore volume of the catalyst support 15, and more preferably 10% to 30%. The above mesopore volume, i.e., the pore volume of the mesopore region, is the total volume of pores from 2 nm to 100 nm. The mesopore volume can be determined, for example, by a low-temperature nitrogen adsorption method.

[0038] If the volume ratio of the ionic liquid 14 is 10% or more, proton conduction between the polymer electrolyte and the metal particles 12 via the ionic liquid 14 becomes suitably possible. In particular, sufficient oxygen reduction activity can be obtained in the electrode catalyst layer of the air electrode. Also, if the volume ratio of the ionic liquid 14 is 50% or less, the amount of ionic liquid 14 will not become too large. Although the ionic liquid 14 has the function of passing protons, to what extent it is auxiliary compared to the polymer electrolyte, and if the amount of ionic liquid 14 becomes too large, the separation between the catalyst particles 10 and the polymer electrolyte will increase, and the resistance will increase. In contrast, if the volume ratio of the ionic liquid 14 is 50% or less, the increase in resistance caused by an excess of ionic liquid 14 can be suppressed.

[0039] [Method for producing catalyst particles] The catalyst particles 10 are produced by supporting metal particles 12 on a conductive carrier 11 to create a catalyst carrier 15, impregnating the catalyst carrier 15 with an ionic liquid 14, and then forming an inorganic film 13. A known film formation method such as the sol-gel method is used to form the inorganic film 13.

[0040] When forming the inorganic film 13 before impregnation with the ionic liquid 14, it is necessary to apply a surfactant to the catalyst support 15, form the inorganic film 13, and then remove the surfactant in order to adjust the charged state of the metal particle surface 12. However, since the surfactant removal process requires high-temperature heat treatment, the catalyst support 15 is exposed to high temperatures, resulting in degradation of the conductive support 11, i.e., deterioration of crystallization, if the conductive support 11 is made of carbon material. As a result, the durability of the electrode catalyst layer using catalyst particles 10 is reduced.

[0041] In contrast, by using a manufacturing method in which an inorganic film 13 is formed after impregnating the catalyst support 15 with an ionic liquid 14, it is possible to form the inorganic film 13 without the need for a surfactant. Therefore, it is possible to avoid heat treatment and suppress the deterioration of the conductive support 11, and the decrease in the durability of the electrode catalyst layer can be suppressed.

[0042] In the catalyst particles 10, the peak intensity ratio (G / D ratio) of the G-band to the D-band of the conductive carrier 11, which is a carbon material, as measured by Raman spectroscopy, is preferably 1.6 or more and 2.2 or less, and more preferably 1.8 or more and 2.0 or less. If the G / D ratio is within the above range, the crystallinity of the conductive carrier 11 is suitable, and thus the decrease in the durability of the electrode catalyst layer can be suppressed. Furthermore, by using a manufacturing method in which an inorganic film 13 is formed after impregnating the catalyst support 15 with an ionic liquid 14, it is easy to keep the G / D ratio of the conductive carrier 11 within the above range even in catalyst particles 10 having an inorganic film 13.

[0043] In this specification, the wavelength of the laser light used in Raman spectroscopy is 532 nm. The G band is defined as 1580 cm⁻¹. -1This refers to the nearby Raman Peak, and the D-band is 1360cm. -1 This refers to the nearby Raman Peak.

[0044] [Membrane electrode assembly and electrode catalyst layer] As shown in Figure 2, the membrane electrode assembly 20 comprises a polymer electrolyte membrane 21 and a pair of electrode catalyst layers. The pair of electrode catalyst layers are a fuel electrode catalyst layer 22A and an air electrode catalyst layer 22C.

[0045] The fuel electrode catalyst layer 22A constitutes the fuel electrode, which is the anode of the polymer electrolyte fuel cell. The air electrode catalyst layer 22C constitutes the air electrode, which is the cathode of the polymer electrolyte fuel cell. The fuel electrode catalyst layer 22A is a layer for separating the fuel gas into protons and electrons, while the air electrode catalyst layer 22C is a layer for receiving electrons from an external circuit and for oxidizing the protons transported through the polymer electrolyte membrane 21 with an oxidizing agent containing oxygen.

[0046] The polymer electrolyte membrane 21 is sandwiched between the fuel electrode catalyst layer 22A and the air electrode catalyst layer 22C in the thickness direction. The fuel electrode catalyst layer 22A is in contact with one of the two surfaces of the polymer electrolyte membrane 21, and the air electrode catalyst layer 22C is in contact with the other of the two surfaces of the polymer electrolyte membrane 21.

[0047] When viewed from a position opposite one of the surfaces of the polymer electrolyte membrane 21, the external shapes of the fuel electrode catalyst layer 22A and the air electrode catalyst layer 22C are approximately the same, and their external shapes are smaller than those of the polymer electrolyte membrane 21. The external shapes of the catalyst layers 22A, 22C and the polymer electrolyte membrane 21 are not particularly limited; for example, they may be rectangular.

[0048] The polymer electrolyte membrane 21 contains a polymer electrolyte. The polymer electrolyte used in the polymer electrolyte membrane 21 can be any polymer electrolyte having proton conductivity, such as a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. Examples of fluorine-based polymer electrolytes include Nafion (registered trademark: manufactured by DuPont), Flemion (registered trademark: manufactured by Asahi Glass Co., Ltd.), and Gore-Select (registered trademark: manufactured by Gore Japan LLC). Examples of hydrocarbon-based polymer electrolytes include engineering plastics and engineering plastics into which sulfonic acid groups have been introduced.

[0049] Figure 3 schematically shows the structure of the air electrode catalyst layer 22C. The air electrode catalyst layer 22C contains the catalyst particles 10 and polymer electrolyte 16 described above. Furthermore, the air electrode catalyst layer 22C may also contain a fibrous material 17.

[0050] The polymer electrolyte 16 has a mass-like structure formed by the aggregation of ionomer polymer electrolytes due to cohesive forces. These cohesive forces include Coulomb forces and van der Waals forces acting between the ionomers. The polymer electrolyte 16 can be any polymer electrolyte having proton conductivity, such as a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. An example of a fluorine-based polymer electrolyte is an electrolyte having a tetrafluoroethylene skeleton, such as Nafion (registered trademark: manufactured by DuPont). An example of a hydrocarbon-based polymer electrolyte is sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, etc. The polymer electrolyte contained in the air electrode catalyst layer 22C may be one type or two or more types.

[0051] If the polymer electrolyte used in the polymer electrolyte membrane 21 and the electrode catalyst layer are of the same type, the adhesion of the electrode catalyst layer to the polymer electrolyte membrane 21 is improved. The content of the polymer electrolyte 16 in the air electrode catalyst layer 22C is preferably 40 parts by mass or more and 140 parts by mass or less, when the content of the conductive carrier 11 in the air electrode catalyst layer 22C is 100 parts by mass. If the content of the polymer electrolyte 16 is 40 parts by mass or more, the decrease in proton conductivity due to the deficiency of the proton conduction pathway can be suppressed. As a result, it becomes easier to ensure a balance between proton conductivity and electronic conductivity in the air electrode catalyst layer 22C. Furthermore, if the content of the polymer electrolyte 16 is 140 parts by mass or less, a three-phase interface is more easily formed in the air electrode catalyst layer 22C, thereby increasing catalytic activity.

[0052] The fibrous material 17 is preferably composed of a material that is not affected by the catalyst particles 10 and the polymer electrolyte 16. Specifically, the fibrous material 17 is preferably a hydrophilic carbon fiber or a polymer fiber. Examples of hydrophilic carbon fibers include hydrophilized VGCF (Vapor Grown Carbon Fiber) and CNT (Carbon Nano Tube). An example of a polymer fiber is a nanofiber made of a polymer of amines having an imide structure or an azole structure.

[0053] The inclusion of the fibrous material 17 makes it less likely for cracks to occur in the air electrode catalyst layer 22C, thereby increasing the durability of the air electrode catalyst layer 22C. Furthermore, the inclusion of the fibrous material 17 ensures that adequate voids are maintained within the air electrode catalyst layer 22C, thereby improving the drainage performance of the air electrode catalyst layer 22C. Therefore, even during operation at high current densities where the amount of generated water increases, the occurrence of flooding, a phenomenon in which the diffusion pathway of gas is blocked due to the accumulation of generated water and power generation performance is reduced, can be suppressed.

[0054] The shape of the fibrous material 17 is not particularly limited; for example, the fibrous material 17 may have a hollow structure or a solid structure. Furthermore, the fibrous material 17 contained in the air electrode catalyst layer 22C may be of one type or of two or more types.

[0055] The fibrous material 17 may contain acidic or basic functional groups in its molecular structure. Examples of fibrous material 17 having acidic functional groups include hydrophilic carbon fibers, while examples of fibrous material 17 having basic functional groups include polymer fibers having imide or azole structures. Examples of acidic functional groups include carbonyl groups, and examples of basic functional groups include amine groups such as pyridine and imide. This makes it easier for the polymer electrolyte 16 to be present around the fibrous material 17.

[0056] If the fibrous material 17 contains acidic functional groups, hydrogen bonding occurs between the acidic functional groups and proton-conducting sites such as sulfonyl groups contained in the polymer electrolyte 16, making it easier for the polymer electrolyte 16 to exist around the fibrous material 17. On the other hand, if the fibrous material 17 contains basic functional groups, acidic proton-conducting sites such as sulfonyl groups contained in the polymer electrolyte 16 bond to the basic functional groups as an acid-base, making it easier for the polymer electrolyte 16 to exist around the fibrous material 17. Since acid-base bonding is stronger than hydrogen bonding, it is preferable that the fibrous material 17 contains basic functional groups. In other words, if the fibrous material 17 has basic functional groups containing nitrogen atoms in its molecular structure, the polymer electrolyte 16 is more likely to exist around the fibrous material 17.

[0057] The content of the fibrous material 17 in the air electrode catalyst layer 22C is preferably 5 parts by mass or more and 20 parts by mass or less, when the content of the conductive carrier 11 in the air electrode catalyst layer 22C is 100 parts by mass. If the content of the fibrous material 17 is 5 parts by mass or more, a network of fibrous material 17 is suitably formed. This suitably enables the construction of proton conduction pathways, electron conduction pathways, and improvement of the strength of the air electrode catalyst layer 22C. As a result, the power generation performance of the fuel cell can be improved. Furthermore, if the content of the fibrous material 17 is 20 parts by mass or less, the increase in resistance caused by the increase in the thickness of the air electrode catalyst layer 22C can be suppressed.

[0058] The fuel electrode catalyst layer 22A contains catalyst particles and a polymer electrolyte. The catalyst particles contained in the fuel electrode catalyst layer 22A may be the catalyst particles 10 described above, similar to the air electrode catalyst layer 22C, or they may have a different configuration from the catalyst particles 10, such as catalyst particles without an inorganic coating 13 and ionic liquid 14, i.e., catalyst particles consisting only of a catalyst support 15. Furthermore, the fuel electrode catalyst layer 22A may also contain the fibrous material 17 described above. Note that the materials and proportions of the catalyst particles, polymer electrolyte, and fibrous material contained in the air electrode catalyst layer 22C and the fuel electrode catalyst layer 22A may differ.

[0059] [Polymer electrolyte fuel cell] Referring to Figure 4, the configuration of a polymer electrolyte fuel cell equipped with the membrane electrode assembly 20 described above will be explained.

[0060] As shown in Figure 4, the polymer electrolyte fuel cell 30 comprises a membrane electrode assembly 20, a pair of gas diffusion layers 31A and 31C, and a pair of separators 32A and 32C. The membrane electrode assembly 20 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 22A and the gas diffusion layer 31C in contact with the air electrode catalyst layer 22C.

[0061] 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 22A, constitutes the fuel electrode, and the gas diffusion layer 31C, together with the air electrode catalyst layer 22C, constitutes the air electrode.

[0062] The laminate of the film electrode assembly 20 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 material of separators 32A and 32C is, for example, a carbon-based or metallic material. Preferably, the material of separators 32A and 32C is a material with good strength and moldability.

[0063] 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.

[0064] 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, the electrode reactions shown in (Equation 1) and (Equation 2) below proceed, 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. Fuel electrode: H2→ 2H + + 2e - ...(Formula 1) Air electrode: 1 / 2O2+ 2H + + 2e - → H2O ···(Equation 2)

[0065] The polymer electrolyte fuel cell 30 may be used in the single-cell state shown in Figure 4, 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.

[0066] Furthermore, the polymer electrolyte fuel cell 30 may be equipped with components such as gaskets to suppress gas leakage, in addition to the above-mentioned components. Also, 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 20.

[0067] [Examples] The catalyst particles, electrode catalyst layer, membrane electrode assembly, and polymer electrolyte fuel cell described above will be explained using specific examples and comparative examples.

[0068] (Example 1) <Generation of catalyst particles> Platinum-supported carbon (support density 30% by mass) was used as the catalyst support. Platinum-supported carbon was added to acetonitrile, and then an amount of ionic liquid equivalent to 50% of the mesopore volume of the platinum-supported carbon was added. This mixture was subjected to ultrasonic dispersion for 30 minutes, stirred overnight with a stirrer, and then the acetonitrile was removed with an evaporator to obtain a catalyst support impregnated with the ionic liquid. The ionic liquid used consisted solely of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. The mesopore volume of the platinum-supported carbon was determined using a low-temperature nitrogen adsorption method, targeting pores from 2 nm to 100 nm.

[0069] Next, the catalyst support impregnated with the above ionic liquid was added to water, and after ultrasonic stirring, a mixed solution containing TEOS (tetraethoxysilane) and ethanol was added. Then, sodium hydroxide was added and stirred for 2 hours, and the separated product was obtained using a centrifuge. The separated product was then dried at 80°C for 6 hours to obtain the catalyst particles of Example 1. These catalyst particles have an inorganic coating made of silica. As described above, the amount of inorganic coating on the catalyst particles was adjusted by the amount of TEOS, so that the weight ratio of the inorganic coating (weight of silica / total weight of silica and catalyst support) was 0.1.

[0070] <Fabrication of Membrane Electrode Assembly> A catalyst ink was prepared by subjecting a mixed solution containing the catalyst particles of Example 1, a polymer electrolyte, a fibrous material, and a dispersion medium to dispersion treatment. The following materials were used for each material other than the catalyst particles.

[0071] Polymer electrolyte: Fluorine-based polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.) Fibrous material: Carbon fiber (VGCF-H (registered trademark), manufactured by Showa Denko KK, average fiber length 6 μm, average fiber diameter 150 nm) Dispersion medium: A mixed solution of water and 1-propanol with a mass ratio of 1:1

[0072] The blending amounts of the polymer electrolyte and the fibrous material were such that, with the blending amount of the conductive carrier in the catalyst particles in the catalyst ink being 100 parts by mass, the polymer electrolyte was 70 parts by mass and the fibrous material was 20 parts by mass. The blending amount of the dispersion medium was set to an amount such that the solid content concentration in the catalyst ink was 10% by mass. The dispersion treatment was carried out for 六十二 minutes at a rotational speed of 600 rpm using a zirconia ball with a diameter of 3 mm and a planetary ball mill

[0073] Next, a coating film was formed by applying the catalyst ink to one side of a polymer electrolyte membrane (Nafion (registered trademark) 211, manufactured by DuPont) using a die coater. The shape of the coating film was square, and the length of one side was 50 mm. The application amount of the catalyst ink was set to an amount such that the platinum loading in the coating film was 0.1 mg / cm 2 [[ID=二十一]]And a drying treatment was carried out using an oven at 80 °C to volatilize the dispersion medium contained in the coating film, thereby forming an air electrode catalyst layer. [[ID=二十二]]

[0074] Next, a coating film was formed by applying the catalyst ink to the opposite side of the polymer electrolyte membrane from the side on which the air electrode catalyst layer was formed. The shape of the coating film was square, and the length of one side was 50 mm. The application amount of the catalyst ink was set to an amount such that the platinum loading in the coating film was 0.05 mg / cm <空氣極触媒層を形成した。<An air electrode catalyst layer was formed. 2The amount was determined to be such that the dispersion medium contained in the coating film was volatilized by a drying process using an 80°C oven, thereby forming the fuel electrode catalyst layer. This resulted in obtaining a film electrode assembly comprising the electrode catalyst layer of Example 1.

[0075] (Example 2) The film electrode assembly of Example 2 was obtained using the same materials and process as in Example 1, except that the amount of ionic liquid added in the catalyst particle generation process was changed to an amount equivalent to 30% of the mesopore volume of the platinum-supported carbon.

[0076] (Example 3) The film electrode assembly of Example 3 was obtained using the same materials and process as in Example 1, except that the amount of ionic liquid added in the catalyst particle generation process was changed to an amount equivalent to 10% of the mesopore volume of the platinum-supported carbon.

[0077] (Example 4) The film electrode assembly of Example 4 was obtained using the same materials and process as in Example 1, except that the amount of TEOS added was changed in the catalyst particle generation process so that the weight ratio of the inorganic coating was 0.2.

[0078] (Example 5) The film electrode assembly of Example 5 was obtained using the same materials and process as in Example 1, except that the amount of TEOS added was changed so that the weight ratio of the inorganic coating was 0.2 in the catalyst particle generation process, and the amount of ionic liquid added was changed to an amount equivalent to 30% of the mesopore volume of the platinum-supported carbon.

[0079] (Example 6) The film electrode assembly of Example 6 was obtained using the same materials and process as in Example 1, except that the amount of TEOS added was changed so that the weight ratio of the inorganic coating was 0.2 in the catalyst particle generation process, and the amount of ionic liquid added was changed to an amount equivalent to 10% of the mesopore volume of the platinum-supported carbon.

[0080] (Comparative Example 1) A film electrode assembly of Comparative Example 1 was obtained using the same materials and process as in Example 1, except that the amount of ionic liquid added in the catalyst particle generation process was changed to an amount equivalent to 60% of the mesopore volume of the platinum-supported carbon.

[0081] (Comparative Example 2) A film electrode assembly of Comparative Example 2 was obtained using the same materials and process as in Example 1, except that the amount of ionic liquid added in the catalyst particle generation process was changed to an amount equivalent to 5% of the mesopore volume of the platinum-supported carbon.

[0082] (Comparative Example 3) A film electrode assembly of Comparative Example 3 was obtained using the same materials and process as in Example 1, except that the amount of TEOS added was changed in the catalyst particle generation process so that the weight ratio of the inorganic coating was 0.2, and the amount of ionic liquid added was changed to an amount equivalent to 60% of the mesopore volume of the platinum-supported carbon.

[0083] (Comparative Example 4) A film electrode assembly of Comparative Example 4 was obtained using the same materials and process as in Example 1, except that the amount of TEOS added was changed so that the weight ratio of the inorganic coating was 0.2 in the catalyst particle generation process, and the amount of ionic liquid added was changed to an amount equivalent to 5% of the mesopore volume of the platinum-supported carbon.

[0084] (Comparative Example 5) A film electrode assembly of Comparative Example 5 was obtained using the same materials and process as in Example 1, except that the amount of TEOS added was changed so that the weight ratio of the inorganic coating was 0.3 in the catalyst particle generation process, and the amount of ionic liquid added was changed to an amount equivalent to 30% of the mesopore volume of the platinum-supported carbon.

[0085] (Comparative Example 6) A film electrode assembly of Comparative Example 6 was obtained using the same materials and process as in Example 1, except that the amount of TEOS added was changed so that the weight ratio of the inorganic coating was 0.005 in the catalyst particle generation process, and the amount of ionic liquid added was changed to an amount equivalent to 30% of the mesopore volume of the platinum-supported carbon.

[0086] (Comparative Example 7) The film electrode assembly of Comparative Example 7 was obtained using the same materials and process as in Example 1, except that impregnation of the platinum-supported carbon with an ionic liquid and formation of an inorganic coating were not performed. In other words, the catalyst particles of Comparative Example 7 consisted only of a catalyst-supported carrier and did not have an ionic liquid or an inorganic coating.

[0087] (evaluation) <Analysis of the coating state of inorganic films> The elemental distribution of the catalyst particles of Example 1 was analyzed by elemental mapping using TEM-EDS (energy-dispersive X-ray spectroscopy). Figure 5(a) shows the distribution of C, Figure 5(b) shows the distribution of Si, and Figure 5(c) shows the distribution of Pt. As shown in Figures 5(a) to (c), the distribution of Si overlaps with the distribution of Pt, confirming that an inorganic film made of silica is formed to cover the metal particles, which are mainly platinum.

[0088] <Evaluation of power generation performance> For each example and comparative example, samples were prepared by laminating carbon paper, which serves as a gas diffusion layer, to both sides of a membrane electrode assembly. Each sample was placed in a power generation evaluation cell, and current and voltage measurements were performed using a fuel cell measuring device. The cell temperature during measurement was set to 80°C. Humidification conditions were set to a relative humidity of 90% RH for the fuel electrode and 30% RH for the air electrode. Hydrogen was used as the fuel gas, and air was used as the oxidizing gas. In this case, hydrogen was flowed at a flow rate that resulted in an 80% hydrogen utilization rate, and air was flowed at a flow rate that resulted in an 40% oxygen utilization rate. The back pressure was set to 50 kPa.

[0089] In evaluating power generation performance, the current density was 1.5 A / cm². 2If the voltage is 0.65V or higher, it is marked as "○", and the current density is 1.5A / cm². 2 A voltage of 0.65V or less was marked with "×". A voltage of 0.65V or higher indicates that practically desirable power generation performance has been achieved.

[0090] <Durability evaluation> For each example and comparative example, the same sample used in the power generation performance evaluation described above was used, and the potential cycle test described in the "Cell Evaluation and Analysis Protocol" published by the New Energy and Industrial Technology Development Organization (NEDO) was performed. Current and voltage measurements were performed before and after the test under the same conditions as in the power generation performance evaluation described above, and the current density was 1.5 A / cm². 2 The voltage drop after the test compared to before the test was calculated.

[0091] In the durability evaluation, a voltage drop of 100mV or less was marked as "○", and a voltage drop exceeding 100mV was marked as "×". A voltage drop of 100mV or less indicates desirable durability for practical use, meaning the film electrode assembly can withstand long-term use.

[0092] Table 1 shows the weight ratio of the inorganic coating and the volume ratio of the ionic liquid, as well as the evaluation results of power generation performance and durability for each example and comparative example.

[0093] [Table 1]

[0094] As shown in Table 1, in Examples 1 to 6, where the weight ratio of the inorganic coating was 0.01 to 0.2 and the volume ratio of the ionic liquid was 10% to 50%, both power generation performance and durability were good.

[0095] On the other hand, in Comparative Examples 1-4, where the volume ratio of the ionic liquid was less than 10% or more than 50%, the power generation performance was low, suggesting that increased resistance occurred due to the inhibition of proton conduction. Similarly, in Comparative Example 5, where the weight ratio of the inorganic coating was greater than 0.2, the power generation performance was also low, suggesting that the excessive amount of inorganic coating inhibited proton conduction. Furthermore, in Comparative Examples 6 and 7, where the weight ratio of the inorganic coating was less than 0.01, both power generation performance and durability were low, suggesting that the elution of metal particles occurred.

[0096] As described above using the examples, the catalyst particles, electrode catalyst layer, membrane electrode assembly, and polymer electrolyte fuel cell of the above embodiment provide the following effects. (1) The catalyst particles contain an inorganic coating and an ionic liquid, which suppresses the elution of metal particles into the polymer electrolyte and inhibits proton conduction. Therefore, good durability and power generation performance can be obtained in fuel cells using catalyst particles in the electrode catalyst layer.

[0097] (2) Because the inorganic coating is made of silica formed from either tetraethoxysilane or triethoxymethylsilane, the inorganic coating is formed in a multilayer structure, which makes it easier to retain ionic liquid inside the inorganic coating.

[0098] (3) The ionic liquid contains 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, and in particular contains 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. With this configuration, proton conduction through the ionic liquid is suitably possible. [Explanation of symbols]

[0099] 10…Catalyst particles 11... Conductive carrier 12...metal particles 13...Inorganic coating 14…Ionic liquids 15…Catalyst support 16...polymer electrolyte 17…Fibrous material 20...Membrane electrode assembly 21...Polymer electrolyte membrane 22A…Fuel electrode catalyst layer 22C...Air electrode catalyst layer 30...Polymer fuel cell 31A, 31C… Gas diffusion layer 32A, 32C... Separators

Claims

1. A conductive carrier, and a catalyst support carrier composed of a plurality of metal particles supported on the conductive carrier, An inorganic coating covering a portion of the catalyst support, The catalyst support comprises an ionic liquid located in a portion including the surface of the catalyst support, The plurality of metal particles include metal particles whose surface is partially exposed from the inorganic coating and in contact with the ionic liquid. The weight ratio of the inorganic coating to the total weight of the catalyst support and the inorganic coating is 0.01 or more and 0.2 or less. The volume ratio of the ionic liquid to the mesopore volume of the catalyst support is 10% or more and 50% or less. Catalyst particles.

2. The inorganic coating consists of silica formed from either tetraethoxysilane or triethoxymethylsilane. Catalyst particles according to claim 1.

3. The aforementioned ionic liquid contains 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide. Catalyst particles according to claim 1.

4. The aforementioned ionic liquid contains 1-ethyl-3-methylimidazolium bis(trifluororomethanesulfonyl)imide. The catalyst particles according to claim 3.

5. Catalyst particles according to any one of claims 1 to 4, Polymer electrolytes, An electrode catalyst layer containing a catalyst.

6. Further containing fibrous material The electrode catalyst layer according to claim 5.

7. Polymer electrolyte membrane, The system comprises a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane, At least one of the pair of electrode catalyst layers is the electrode catalyst layer described in claim 5. Membrane electrode assembly.

8. The membrane electrode assembly according to claim 7, A pair of separators sandwiching the aforementioned film electrode assembly, A polymer electrolyte fuel cell equipped with the following features.

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