Membrane electrode assembly

Abstract

An object of the present disclosure is to provide a membrane electrode assembly excellent in durability.SOLUTION: The present embodiment is a membrane electrode assembly, wherein the membrane electrode assembly comprises a metal ion selected from a cerium ion and a manganese ion, and a crown ether compound or a salt thereof capable of forming a clathrate compound with the metal ion, the cathode catalyst layer comprises an electrode catalyst and an electrolyte, the electrode catalyst is a metal-supporting carrier in which metal particles having catalytic activity are supported on a carrier having pores, the surface area inside / outside ratio is 1.20 or less, and the particle number inside / outside ratio is 0.70 or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a membrane electrode assembly. [Background technology]

[0002] A solid polymer electrolyte fuel cell generally comprises a membrane electrode assembly (MEA) that includes a solid polymer electrolyte membrane (electrolyte membrane), an anode catalyst layer disposed on one side of the solid polymer electrolyte membrane, and a cathode catalyst layer disposed on the other side of the solid polymer electrolyte membrane. The anode catalyst layer functions as the fuel electrode, and the cathode catalyst layer functions as the air electrode. Gas diffusion layers may also be disposed on both sides of the MEA; this configuration is called a membrane electrode gas diffusion layer assembly (MEGA). During power generation in solid polymer electrolyte fuel cells, hydrogen peroxide (H2O2) may be generated from water and oxygen in the catalyst layer, and hydroxyl radicals (·OH) may be generated from the hydrogen peroxide. These hydrogen peroxide and hydroxyl radicals can cause deterioration of the solid polymer electrolyte membrane and electrolyte resins, such as ionomers, contained in the catalyst layer.

[0003] Therefore, a technology has been proposed to neutralize the hydrogen peroxide radicals generated during fuel cell power generation by incorporating a radical quenching agent such as cerium ions into the MEA. The neutralization of hydrogen peroxide radicals refers to the reaction of hydrogen peroxide radicals with water, for example.

[0004] For example, Non-Patent Document 1 discloses a membrane electrode assembly (MEA) in which a coordination complex of 18-crown-6-ether / cerium ion (CRE / Ce) is embedded in a Nafion ionomer between the catalyst and the membrane. It is described (abstract, etc.) that Ce acts as a trap for HO- radicals, and CRE reduces the elution of cerium ions from the MEA during cell operation. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Vo Dinh Cong Tinh et al., "Enhancement of oxidative stability of PEM fuel cell by introduction of HO radical scavenger in Nafion ionomer", Journal of Membrane Science 613 (2020) 118517 Summary of the Invention [Problem to be solved by the invention]

[0006] As mentioned above, Non-Patent Document 1 discloses an anode catalyst layer containing cerium ions and 18-crown-6-ether as radical quenching agents. One of the issues with cerium ions is the reduced durability associated with a decrease in concentration due to ion migration. However, Non-Patent Document 1 describes that the addition of 18-crown-6-ether can reduce the elution of cerium ions outside the MEA. However, when a large amount of a crown ether compound such as 18-crown-6-ether is added to a membrane electrode assembly, the IV performance under high temperature and low humidity conditions decreases. Therefore, it has been found that there is room for improvement in the performance of membrane electrode assemblies containing a radical quenching agent such as cerium ions and a crown ether compound such as 18-crown-6-ether.

[0007] Therefore, an object of the present disclosure is to provide a membrane electrode assembly that is excellent in durability and performance. [Means for solving the problem]

[0008] The present inventors conducted extensive research to solve the above-mentioned problems and found that the reason for the performance degradation was that crown ether compounds contained in the anode catalyst layer and the electrolyte membrane migrated to the cathode catalyst layer, poisoning the cathode catalyst and reducing the proton conductivity of the cathode ionomer. Further research led the present inventors to find that catalyst poisoning by crown ether compounds can be suppressed by using a specific metal support as the electrode catalyst in the cathode catalyst layer, thereby providing a membrane electrode assembly with excellent durability, leading to the present disclosure.

[0009] Therefore, an example of the present embodiment is as follows. (1) A membrane electrode assembly having a solid polymer electrolyte membrane, an anode catalyst layer disposed on one surface of the solid polymer electrolyte membrane, and a cathode catalyst layer disposed on the other surface of the solid polymer electrolyte membrane, the membrane electrode assembly contains a metal ion selected from a cerium ion and a manganese ion, and a crown ether compound or a salt thereof capable of forming an inclusion compound with the metal ion, the cathode catalyst layer includes an electrode catalyst and an electrolyte; the electrode catalyst is a metal-supported carrier in which metal particles having catalytic activity are supported on a carrier having fine pores, When metal particles present on the outermost surface of the primary particle of the support are defined as outer particles, and metal particles present inside the outermost surface of the primary particle of the support are defined as inner particles, The ratio of the total surface area of ​​the outer particles to the total surface area of ​​the inner particles (total surface area of ​​outer particles / total surface area of ​​inner particles) is 1.20 or less; and A membrane electrode assembly in which the ratio of the total number of the external particles to the total number of the internal particles (total number of external particles / total number of internal particles) is 0.70 or less. (2) The membrane / electrode assembly according to (1), wherein the number of ring members in the cyclic structure of the crown ether compound is 15 or more. (3) The membrane / electrode assembly according to any one of (1) to (2), wherein the crown ether compound has a molecular weight of 300 or more. (4) The crown ether compound is dibenzo-15-crown-5-ether, benzo-18-crown-6-ether, dibenzo-18-crown-6-ether, benzo-21-crown-7-ether, dibenzo-21-crown-7-ether, benzo-24-crown-8-ether, dibenzo-24-crown-8-ether, cyclohexano-18-crown-6-ether, cyclohexano-21-crown-7-ether, cyclohexano-24-crown-8-ether, dicyclohexano-18-crown-6-ether, The membrane / electrode assembly according to any one of (1) to (3), wherein the at least one compound is selected from the group consisting of dicyclohexano-21-crown-7-ether, dicyclohexano-24-crown-8-ether, and compounds in which the aromatic ring or the aliphatic ring of these compounds is substituted with at least one substituent selected from a halogen atom, a hydroxy group, an amino group, a nitro group, a formyl group, an alkyl group having 1 to 6 carbon atoms, a hydroxyalkyl group having 1 to 6 carbon atoms, a carboxyalkyl group having 2 to 7 carbon atoms, and an aryl group having 6 to 14 carbon atoms. (5) The membrane / electrode assembly according to any one of (1) to (4), wherein the metal particles are at least one selected from the group consisting of platinum particles, platinum alloy particles, and platinum-containing composite particles. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to provide a membrane electrode assembly having excellent durability. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic cross-sectional view for explaining an example of the configuration of a membrane electrode assembly and a polymer electrolyte fuel cell according to the present embodiment, and is a cross-sectional view of a main part of a fuel cell 10 as an example. DETAILED DESCRIPTION OF THE INVENTION

[0012] This embodiment is a membrane electrode assembly having a solid polymer electrolyte membrane, an anode catalyst layer disposed on one side of the solid polymer electrolyte membrane, and a cathode catalyst layer disposed on the other side of the solid polymer electrolyte membrane, wherein the membrane electrode assembly contains metal ions selected from cerium ions and manganese ions, and a crown ether compound or a salt thereof capable of forming an inclusion compound with the metal ions; the cathode catalyst layer contains an electrode catalyst and an electrolyte; the electrode catalyst is a metal-supported support in which catalytically active metal particles are supported on a support having fine pores; and, when metal particles present on the outermost surfaces of primary particles of the support are defined as outer particles and metal particles present further inside than the outermost surfaces of the primary particles of the support are defined as internal particles, the ratio of the total surface area of ​​the external particles to the total surface area of ​​the internal particles (total surface area of ​​external particles / total surface area of ​​internal particles) is 1.20 or less, and the ratio of the total number of the external particles to the total number of the internal particles (total particle number of external particles / total particle number of internal particles) is 0.70 or less.

[0013] This embodiment can provide a membrane electrode assembly with excellent durability. In this embodiment, cerium ions and / or manganese ions, which function as radical quenchers, are added to the membrane electrode assembly (e.g., an anode catalyst layer or a solid polymer electrolyte membrane). The cerium ions and / or manganese ions can capture and neutralize hydrogen peroxide radicals, thereby suppressing deterioration of the membrane electrode assembly. Furthermore, by adding a host compound for the metal ions to the membrane electrode assembly, migration of the metal ions can be suppressed, thereby reducing unevenness in concentration in the in-plane direction. In this case, using a crown ether compound or a salt thereof as the host compound can further improve the effect of suppressing migration of metal ions. Furthermore, in the membrane electrode assembly according to this embodiment, the use of the specified metal-supported carrier as the electrode catalyst in the cathode catalyst layer can suppress poisoning of the cathode catalyst by the crown ether compound, thereby suppressing performance degradation under high loads. For these reasons, this embodiment can provide a membrane electrode assembly with excellent durability and performance.

[0014] The configuration of this embodiment will be described below.

[0015] The solid polymer electrolyte membrane prevents the flow of electrons and gases, and also prevents the flow of protons (H + ) from the anode-side catalyst layer to the cathode-side catalyst layer. As the solid polymer electrolyte membrane in this embodiment, an electrolyte membrane having proton conductivity known in the art can be used. As the solid polymer electrolyte membrane, for example, a membrane formed from a fluororesin having sulfonic acid groups (such as Nafion (manufactured by DuPont), Flemion (manufactured by AGC), and Aciplex (manufactured by Asahi Kasei Corporation)) can be used.

[0016] The thickness of the solid polymer electrolyte membrane is not particularly limited, but is, for example, 5 μm to 50 μm from the viewpoint of improving proton conductivity.

[0017] The cathode catalyst layer functions as an air electrode (oxygen electrode). The cathode catalyst layer includes at least an electrode catalyst and an electrolyte. In this embodiment, a metal-supported carrier in which catalytically active metal particles are supported on a carrier having fine pores is used as the electrode catalyst in the cathode catalyst layer. In a metal-supported catalyst, particulate catalytic metal is supported on a carrier. In the present disclosure, a state in which a catalyst is supported on a carrier having fine pores is a concept that includes at least one of a state in which a catalyst is supported on the surface of a carrier and a state in which a catalyst is supported on the inner wall surfaces of the pores of the carrier.

[0018] As a method for supporting metal particles on a carrier, a conventionally used method can be adopted. For example, a method can be mentioned in which catalytically active metal particles are mixed with a carrier dispersion liquid in which a carrier is dispersed, filtered, washed, redispersed in ethanol or the like, and then dried with a vacuum pump or the like. After drying, a heat treatment can be performed as necessary.

[0019] In this embodiment, a carrier having pores is used as the carrier.

[0020] In this embodiment, the electrode catalyst in the cathode catalyst layer is a metal-supported support in which catalytically active metal particles are supported on a support having fine pores, and when the metal particles present on the outermost surface of the primary particle of the support are defined as outer particles and the metal particles present inside the outermost surface of the primary particle of the support are defined as inner particles, the electrode catalyst satisfies the following requirements (1) and (2): (1) The ratio of the total surface area of ​​the outer particles to the total surface area of ​​the inner particles (total surface area of ​​the outer particles / total surface area of ​​the inner particles) (also called the surface area inner / outer ratio) is 1.20 or less. (2) The ratio of the total number of external particles to the total number of internal particles (total number of external particles / total number of internal particles) (also referred to as the internal / external particle ratio) is 0.70 or less.

[0021] One method for measuring the location of metal particles supported on a support is observation using an electron microscope. By gradually changing the observation angle of the electrode catalyst to be measured, taking multiple images, and then reconstructing them, it is possible to photograph the object in three dimensions. The coordinates of the center of gravity of the metal particles are determined three-dimensionally, and metal particles located on the outermost surface of the support's primary particles are determined to be external particles, while metal particles located inside the outermost surface are determined to be internal particles. When selecting particles to evaluate, the following procedure may be followed to eliminate variations.

[0022] The number of particles to be evaluated is, for example, 100 or more, preferably 200 or more, and preferably 1000 or more. The particles to be evaluated are preferably selected from at least two (preferably five or more) visual fields, and are selected so that the number of particles selected from each visual field is equal in each visual field.

[0023] In the electrode catalyst in the cathode catalyst layer of this embodiment, as described above, the ratio of the total surface area of ​​the outer particles to the total surface area of ​​the inner particles (total surface area of ​​outer particles / total surface area of ​​inner particles) is 1.20 or less. To address the problem of crown ethers migrating in trace amounts from the anode catalyst layer to the cathode catalyst layer poisoning the cathode catalyst and reducing power generation performance, it is effective to support platinum inside the carrier. A surface area ratio of 1.20 or less is preferred because catalyst poisoning by crown ethers is less likely to occur.

[0024] The inner / outer surface area ratio is preferably 0.30 or more, preferably 0.40 or more, preferably 0.50 or more, preferably 0.60 or more, preferably 0.70 or more, preferably 0.80 or more, preferably 0.90 or more, preferably 1.00 or more, preferably 1.10 or more. The inner / outer surface area ratio is preferably 1.18 or less, preferably 1.16 or less.

[0025] In the electrode catalyst in the cathode catalyst layer of this embodiment, as described above, the ratio of the total number of outer particles to the total number of inner particles (total number of outer particles / total number of inner particles) is 0.70 or less. To address the problem of crown ethers migrating in minute amounts from the anode catalyst layer to the cathode catalyst layer poisoning the cathode catalyst and reducing power generation performance, it is effective to support platinum inside the carrier. A particle number inner / outer ratio of 0.70 or less is preferred because catalyst poisoning by crown ethers is less likely to occur.

[0026] The inner / outer particle number ratio is preferably 0.35 or more, preferably 0.40 or more, preferably 0.45 or more, preferably 0.50 or more, preferably 0.55 or more, preferably 0.60 or more. The inner / outer particle number ratio is preferably 0.69 or less, preferably 0.68 or less.

[0027] The total number of internal particles and the total number of external particles can be calculated by the above-mentioned observation method using an electron microscope.

[0028] The total surface area of ​​particles is evaluated as follows: The particle diameter (r) of the measured particle is defined as the maximum diameter. The surface area of ​​each measured particle is calculated, and the total surface area is calculated by adding them all together. A spherical model is used to calculate the surface area (S), and S = 2π(r / 2) 2 The surface area calculated for each particle is summed up separately for the outer particles and the inner particles to obtain the total surface area of ​​the outer particles and the total surface area of ​​the inner particles, respectively.

[0029] The support is not particularly limited, and examples thereof include carbon and oxides. The carbon may be carbon having electronic conductivity. One type of support may be used alone, or two or more types may be used in combination.

[0030] Examples of the carbon support include carbon black (acetylene black, ketjen black, furnace black, etc.), activated carbon, graphite, glassy carbon, graphene, carbon fiber, carbon nanotubes, carbon nitride, carbon sulfide, carbon phosphide, etc. One type of carbon support may be used alone, or two or more types may be used in combination.

[0031] Examples of oxide supports include titanium oxide, niobium oxide, tin oxide, tungsten oxide, molybdenum oxide, etc. One type of oxide support may be used alone, or two or more types may be used in combination.

[0032] There are no particular limitations on the catalytic metal, so long as it exhibits catalytic activity in the reaction at the electrode. Air electrode (cathode): O2 + 4H + +4e - →2H2O Hydrogen electrode (anode): 2H2 → 4H + +4e -

[0033] The metal particles may be any metal having oxygen reduction catalytic activity, such as platinum, ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. Two or more of these metals may be used. The metal particles are preferably at least one selected from the group consisting of platinum particles, platinum alloy particles, and platinum-containing composite particles. Examples of metals other than platinum contained in platinum alloys and platinum-containing composite particles include ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. These metals may be used alone or in combination of two or more.

[0034] The average particle size of the metal particles is not particularly limited, but is preferably 1 to 10 nm. The particle size is calculated as a sphere-equivalent diameter from the volume of the particles in a transmission electron microscope photograph. The average particle size may be determined by measuring the particle sizes (sphere-equivalent diameters) of 100 to 1,000 particles using a transmission electron microscope, and averaging these values ​​to determine the average particle size of the metal particles.

[0035] The support may contain primary particles having pores, and may also contain secondary particles formed by aggregation of the primary particles having pores. In this embodiment, a pore is defined as a hole formed in a primary particle having a diameter of 1 to 300 nm and a depth of 1 nm or more from the outermost surface of the primary particle. A primary particle is the smallest particle unit of the support that cannot be decomposed.

[0036] The average particle diameter of the primary particles of the support may be, for example, 5 to 3,000 nm. The average particle diameter of the primary particles of the support is preferably 50 to 2,000 nm, and more preferably 100 to 1,500 nm. The particle diameter is calculated as a sphere-equivalent diameter from the volume of the particles in a transmission electron microscope photograph. The average particle diameter may be determined by measuring the particle diameters (sphere-equivalent diameters) of 100 to 1,000 support particles using a transmission electron microscope, and averaging these values ​​to determine the average particle diameter of the support particles.

[0037] The metal loading ratio of the metal particles loaded on the carrier is not particularly limited, but is, for example, 1 to 50 mass %, and preferably 29 to 48 mass %.

[0038] The content of the electrode catalyst in the cathode catalyst layer is not particularly limited, but is, for example, 3 to 40 mass % relative to the total mass of the catalyst layer.

[0039] The electrolyte used in the cathode catalyst layer is preferably an ionomer having sulfonic acid groups. Ionomers, also known as cation exchange resins, exist as clusters formed from ionomer molecules. The ionomer is not particularly limited, but any ionomer known in the art can be used. Examples of ionomers include fluororesin-based electrolytes such as perfluorosulfonic acid resins; sulfonated plastic-based electrolytes such as sulfonated polyether ketones, sulfonated polyether sulfones, sulfonated polyether ether sulfones, sulfonated polysulfones, sulfonated polysulfides, and sulfonated polyphenylenes; and sulfoalkylated plastic-based electrolytes such as sulfoalkylated polyether ether ketones, sulfoalkylated polyether sulfones, sulfoalkylated polyether ether sulfones, sulfoalkylated polysulfones, sulfoalkylated polysulfides, and sulfoalkylated polyphenylenes. The electrolytes may be used alone or in combination of two or more.

[0040] The anode catalyst layer functions as a fuel electrode, that is, a hydrogen electrode.

[0041] The anode catalyst layer includes an electrolyte such as an electrode catalyst and an ionomer. The ionomer is preferably an ionomer having a sulfonic acid group. Examples of ionomers having a sulfonic acid group include those described above. In one embodiment, the anode catalyst layer may include, in addition to the electrode catalyst and the ionomer, a metal ion selected from cerium ions and manganese ions, and a crown ether compound capable of forming an inclusion compound with the metal ion.

[0042] The electrode catalyst is not particularly limited, but for example, the above-mentioned materials can be used.

[0043] The ionomer having a sulfonic acid group is not particularly limited, but examples thereof include ion-conductive polymer electrolyte resins such as perfluorosulfonic acid ionomers. Specific examples of the ionomer having a sulfonic acid group include Nafion and Aquivion (Solvay).

[0044] The membrane electrode assembly according to this embodiment includes a metal ion selected from cerium ions and manganese ions, and a crown ether compound or its salt (host compound) capable of forming an inclusion compound with the metal ion. The cerium ion and / or manganese ion functions as a radical quencher, capturing and neutralizing hydrogen peroxide radicals, thereby suppressing deterioration of the membrane electrode assembly. Furthermore, by adding a crown ether compound, which is a host compound for the metal ions, to the membrane electrode assembly, migration of the metal ions can be suppressed, thereby reducing uneven concentration in the in-plane direction.

[0045] The metal ions are selected from cerium ions and manganese ions. The cerium ions and manganese ions function as radical quenching agents. The radical quenching agents can facilitate the conversion of hydroxide radicals generated from hydrogen peroxide to hydroxide ions, thereby suppressing deterioration of the anode catalyst layer. For example, the reaction of cerium ions to convert hydroxide radicals to hydroxide ions is as follows: Ce 3+ + OH (hydroxyl radical) → Ce 4+ +OH - (hydroxide ion)

[0046] The cerium ion may be either +3 or +4 valent, and the manganese ion may be either +3 or +4 valent.

[0047] The cerium salt for obtaining cerium ions is not particularly limited, and examples thereof include cerium nitrate, cerium carbonate, cerium acetate, cerium chloride, cerium sulfate, diammonium cerium nitrate, and tetraammonium cerium sulfate. One type of cerium salt may be used alone, or two or more types may be used in combination. The cerium salt may be an organic metal complex salt. Examples of the organic metal complex salt include cerium acetylacetonate.

[0048] The manganese salt for obtaining manganese ions is not particularly limited, and examples thereof include manganese nitrate, manganese carbonate, manganese acetate, manganese chloride, manganese sulfate, etc. One type of manganese salt may be used alone, or two or more types may be used in combination.

[0049] In this embodiment, the crown ether compound or a salt thereof, which is the host compound, forms an inclusion compound with a cerium ion or a manganese ion, which is the guest compound. The inclusion compound refers to an adduct in which the metal ion, which is the guest compound, is included in the host compound. The crown ether compound may be used alone or in combination of two or more.

[0050] A crown ether compound has a cyclic structure, and the number of ring members in the cyclic structure is preferably 15 or more, and more preferably 18 or more. A crown ether compound is a compound having a ring with a repeating structure of a (-CH2-CH2-Y-) unit or a (-CH2-CH2-CH2-Y-) unit, where Y is at least one heteroatom selected from O, S, N, and P. A crown ether compound captures a metal ion in this cyclic structure to form an inclusion compound. The number of ring members in a crown ether compound is preferably 15 or more, and more preferably 18 or more.

[0051] In this embodiment, the crown ether compound is preferably a compound having a molecular weight of not less than 300. By using a crown ether compound having a molecular weight of not less than 300, a migration suppression effect can be obtained, that is, migration of the crown ether compound from the anode catalyst layer or electrolyte membrane to the cathode catalyst layer can be suppressed.

[0052] Examples of the crown ether compound include crown ethers and crown ether derivatives. Examples of crown ethers having a molecular weight of 300 or more include 21-crown-7-ether and 24-crown-8-ether. In this embodiment, the crown ether compound is preferably a crown ether compound having an aromatic ring or an aliphatic ring. Crown ether compounds having an aromatic ring or an aliphatic ring have high hydrophobicity due to their structure, and therefore have an excellent migration suppression effect. Examples of crown ether compounds having an aromatic ring or an aliphatic ring include dibenzo-15-crown-5-ether, benzo-18-crown-6-ether, dibenzo-18-crown-6-ether, benzo-21-crown-7-ether, dibenzo-21-crown-7-ether, benzo-24-crown-8-ether, dibenzo-24-crown-8-ether, cyclohexano-18-crown-6-ether, cyclohexano-21-crown-7-ether, cyclohexano-24-crown-8-ether, dicyclohexano-18-crown-6-ether, ether, dicyclohexano-21-crown-7-ether, dicyclohexano-24-crown-8-ether, and compounds in which the aromatic ring or aliphatic ring of these compounds is substituted with at least one substituent selected from halogen atoms (e.g., fluorine atoms or bromine atoms), hydroxy groups, amino groups, nitro groups, formyl groups, alkyl groups having 1 to 6 carbon atoms (e.g., methyl groups, ethyl groups, propyl groups, and butyl groups), hydroxyalkyl groups having 1 to 6 carbon atoms, carboxyalkyl groups having 2 to 7 carbon atoms, and aryl groups having 6 to 14 carbon atoms (e.g., phenyl groups). The number of substituents is, for example, 1 to 6, 1 to 5, 1 to 4, 1 to 3, 1 to 2, or 1. These compounds may be used alone or in combination of two or more.

[0053] In the anode catalyst layer in the membrane electrode assembly of this embodiment, the crown ether compound and at least a portion of the metal ions form an inclusion compound.

[0054] Examples of the salt of the crown ether compound include nitrates, sulfates, carbonates, acetates, and propionates. One type of salt may be used alone, or two or more types may be used in combination.

[0055] The crown ether compound and metal ions can be contained in the anode catalyst layer, the solid polymer electrolyte membrane, or both.

[0056] When the anode catalyst layer contains a crown ether compound and a metal ion, the anode catalyst layer contains at least an electrode catalyst, an electrolyte, a metal ion selected from cerium ions and manganese ions, and a combination of the metal ion and the crown ether compound. The metal ion can facilitate conversion of hydroxide radicals generated from hydrogen peroxide to hydroxide ions, thereby suppressing deterioration of the anode catalyst layer.

[0057] The content of the metal ions and crown ether compound in the anode catalyst layer is preferably 0.1 to 20 mass% based on the total solid content of the anode catalyst layer. Regarding this content, the inclusion compound is considered to be a mixture of the metal ions and the crown ether compound. That is, when the metal ions and the crown ether compound are added to the anode catalyst layer separately or simply mixed together, the amount of the inclusion compound formed in the polymer electrolyte is not taken into consideration, and only the total amount of the incorporated metal ions and the crown ether compound is included in the calculation. Furthermore, when an inclusion compound is formed in advance and then added to the anode catalyst layer, the amount of the inclusion compound is considered to be the total amount of the metal ions and the crown ether compound that formed the inclusion compound. Furthermore, when there are metal ions and crown ether compounds that do not form an inclusion compound in addition to the metal ions and the crown ether compound that formed the inclusion compound, these are also included in the calculation.

[0058] In this embodiment, the molar ratio of the crown ether compound to the metal ion ([number of moles of crown ether compound] / [number of moles of metal ion]) is, for example, 0.1 to 10, preferably 0.2 to 7.5, and more preferably 0.4 to 5.0. That is, the content of the crown ether compound is, for example, 0.1 to 10 moles, preferably 0.2 to 7.5 moles, and more preferably 0.4 to 5.0 moles per mole of the metal ion. In this relative ratio, the inclusion compound is also considered to be a mixture of both, as described above.

[0059] When the solid polymer electrolyte membrane contains a crown ether compound and a metal ion, the solid polymer electrolyte membrane containing a crown ether compound and a metal ion can be obtained, for example, by the following method. (1) A solid polymer electrolyte membrane is immersed in a solution containing metal ions to exchange groups such as sulfonic acid groups with metal ions, and then immersed in a solution containing a crown ether compound to incorporate the crown ether compound into the membrane. (2) A method in which a compound containing a metal ion (e.g., a cerium salt) is added to a dispersion of a polymer electrolyte to ion-exchange groups such as sulfonic acid groups with metal ions, and then a solution or solid containing a crown ether compound is added to the dispersion, and the resulting liquid is used for coating to form a film. (3) A method in which a compound containing a metal ion (e.g., a cerium salt) is reacted with a crown ether compound in a solvent to form an inclusion compound, and then a solid polymer electrolyte membrane is immersed in a solution in which the inclusion compound is dissolved in a solvent to ion-exchange groups such as sulfonic acid groups with the inclusion compound, thereby incorporating the inclusion compound into the membrane. (4) A compound containing a metal ion (e.g., a cerium salt) is reacted with a crown ether compound in a solvent to form an inclusion compound, and then the inclusion compound or a solution thereof is added to a dispersion of a polymer electrolyte, and the resulting liquid is used to form a film by coating.

[0060] [Method for manufacturing membrane electrode assembly] The catalyst layer can be formed, for example, by the steps of preparing a catalyst ink (e.g., about 10% solids concentration) containing an electrode catalyst, an ionomer, and a solvent, applying the catalyst ink to the surface of a substrate and volatilizing the solvent in the coating to form a catalyst layer on the surface of the substrate, and transferring the catalyst layer on the surface of the substrate to an electrolyte membrane. Alternatively, the catalyst layer can be formed by directly applying the catalyst ink to a solid polymer electrolyte membrane instead of a substrate. A membrane-electrode assembly can be produced by forming a cathode catalyst layer and an anode catalyst layer on the solid polymer electrolyte membrane.

[0061] Examples of methods for applying the catalyst ink include spraying, blade coating using a doctor blade or applicator, die coating, reverse roll coating, and intermittent die coating.

[0062] The anode catalyst layer may be formed by adding the metal ions and the crown ether compound to a catalyst ink for forming the anode catalyst layer. Specifically, the catalyst ink for forming the anode catalyst layer may contain an electrode catalyst, an ionomer (e.g., an ionomer having a sulfonic acid group), the metal ions, a crown ether compound, and a solvent. The metal ions and the crown ether compound may be added separately or in the form of a complex of the two.

[0063] [Specific configurations of membrane electrode assembly and polymer electrolyte fuel cell] A solid polymer fuel cell has a membrane electrode assembly (MEA) as its basic unit, in which catalyst layers (electrodes) are bonded to both sides of a solid polymer electrolyte membrane. In solid polymer fuel cells, a gas diffusion layer is generally disposed on the outside of the catalyst layer. The gas diffusion layer supplies reactant gases and electrons to the catalyst layer, and is made of carbon paper, carbon cloth, or the like. The catalyst layer is the reaction site for electrode reactions.

[0064] The configurations of a membrane electrode assembly and a polymer electrolyte fuel cell will be described below with reference to FIG. 1. FIG. 1 is a schematic cross-sectional view illustrating an example of the configuration of a polymer electrolyte fuel cell according to this embodiment, showing a cross-sectional view of a main portion of a fuel cell 10 as an example. The polymer electrolyte fuel cell includes a stack of unit cells, each of which includes a power generation body and fuel cell separators disposed on both sides of the power generation body. A plurality of unit cells are stacked in the stacking direction, and the unit cells are electrically connected in series. As shown in FIG. 1, the fuel cell 10 includes a plurality of unit cells 1, which are basic units, stacked together. Each unit cell 1 is a polymer electrolyte fuel cell that generates an electromotive force through an electrochemical reaction between an oxidant gas (e.g., air) and a fuel gas (e.g., hydrogen). The unit cell 1 includes a membrane electrode & gas diffusion layer assembly (MEGA) 2 having gas diffusion layers (GDLs) 7 disposed on both sides thereof, and separators 3 in contact with the MEGA 2 to separate the MEGA 2. In this embodiment, the MEGA 2 is sandwiched between a pair of separators 3, 3.

[0065] The MEGA 2 includes a membrane electrode assembly (MEA) 4 and gas diffusion layers 7, 7 disposed on both sides thereof. The membrane electrode assembly 4 is composed of an electrolyte membrane 5 and a pair of electrodes 6, 6 bonded to sandwich the electrolyte membrane 5. The electrolyte membrane 5 is, for example, a proton-conductive ion-exchange membrane formed of a solid polymer material. The electrodes 6 include, for example, a porous carbon material supporting a catalyst such as platinum. The electrode 6 disposed on one side of the electrolyte membrane 5 functions as an anode, and the electrode 6 on the other side functions as a cathode. The gas diffusion layers 7 are formed of a gas-permeable conductive material. Examples of gas-permeable conductive materials include porous carbon materials such as carbon paper or carbon cloth, or porous metal materials such as metal mesh or metal foam. In this embodiment, the anode electrode is formed of an anode catalyst layer, and the cathode electrode is formed of a cathode catalyst layer. [Example]

[0066] The present embodiment will be described below using examples.

[0067] (Production example 1: Electrode catalyst A) A platinum-supported catalyst was prepared, containing Pt particles (average particle diameter: 3-4 nm) as the catalytic metal and carbon black (VULCAN, manufactured by Cabot Corporation, metal loading ratio: 48 wt%) as the support supporting the metal particles. The surface area / exterior ratio and particle number / exterior ratio of the prepared metal-supported support were calculated using the above-mentioned method, and were found to be 2.25 and 1.14, respectively. A transmission electron microtomography (3D-TEM) was used as the analytical instrument. Specifically, a JEOL TEM (JEM-z2500) was used. Projection images (TEM images) were taken while the sample was continuously tilted. A 3D image was then reconstructed by computed tomography (CT) to analyze the detailed structure of the material.

[0068] (Production example 2: Electrode catalyst B) A platinum-supported catalyst was prepared, containing Pt particles (average particle diameter: 2-3 nm) as the catalytic metal and carbon black (metal loading ratio: 29 wt%) as the support supporting the metal particles. The surface area inside-outside ratio and particle number inside-outside ratio of the prepared metal-supported support were calculated using the above-mentioned methods and were found to be 1.15 and 0.67, respectively.

[0069] (Production example 3: Electrode catalyst C) A platinum-supported catalyst was prepared containing Pt particles (average particle diameter: 2-3 nm) as the catalytic metal and carbon black (metal loading ratio: 48 wt%) as the support supporting the metal particles. The surface area inside-outside ratio and particle number inside-outside ratio of the prepared metal-supported support were calculated using the above-mentioned methods and were found to be 0.67 and 0.46, respectively.

[0070] (Production example 4: Electrode catalyst D) A platinum-supported catalyst was prepared, containing Pt particles (average particle diameter: 2-3 nm) as the catalytic metal and carbon black (Ketjenblack EC300J, Lion Specialty Chemicals, metal loading ratio 42 wt%) as the support for the metal particles. The surface area inside-outside ratio and particle number inside-outside ratio of the prepared metal-supported support were calculated using the above-mentioned methods and were found to be 0.36 and 0.39, respectively.

[0071] [Example 1] (Formation of cathode catalyst layer) Electrode catalyst B was dispersed in an ionomer solution (Nafion DE2020) containing water and ethanol using a bead mill to prepare a catalyst ink. The water / alcohol mass ratio in the catalyst ink was approximately 1. The catalyst ink was applied to a polytetrafluoroethylene sheet and dried to form a cathode catalyst layer.

[0072] The Pt content of the cathode catalyst layer is 0.2 mg / cm 2 The mass ratio (I / C) of the ionomer to the carbon support was set to 1.0.

[0073] (Formation of complex (Ce-ligand)) Benzo-18-crown-6-ether (also referred to as B18CRE) (3.12 g, 0.01 mol) and cerium(III) nitrate hexahydrate (4.34 g, 0.01 mol) were weighed into a 100 mL recovery flask, and ethanol (20 mL) and water (20 mL) were added. The mixture was stirred at room temperature for 24 hours. The solvent was then removed using an evaporator, and the mixture was vacuum dried at 60°C for 1 hour to obtain a white solid. FT-IR analysis confirmed that the peaks derived from the ether group were shifted to lower wavenumbers, confirming that the crown ether compound had encapsulated Ce.

[0074] (Formation of anode catalyst layer) A platinum-supported carbon catalyst (TEC10E30E, 30% platinum-supported carbon, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was used as the electrode catalyst. The electrode catalyst and the above composite were dispersed in an ionomer solution (DE2020) containing water, ethanol, and Nafion (registered trademark) to prepare a catalyst ink. This catalyst ink was applied to a polytetrafluoroethylene sheet and dried to form an anode catalyst layer.

[0075] The Pt content of the anode catalyst layer is 0.1 mg / cm 2 , cerium ion concentration is 6 μg / cm 2 The crown ether compound was added by the above method at a molar ratio of Ce:ligand = 1:1. The concentration of the crown ether compound was 13.5 μg / cm 2 The mass ratio of ionomer to carbon (I / C) was set to 1.0.

[0076] (Fabrication of membrane electrode assembly) The obtained cathode catalyst layer and anode catalyst layer were thermally transferred onto both sides of a Nafion (registered trademark) membrane (NR211), respectively, to prepare a membrane electrode assembly E1. The thermal transfer conditions were 140°C and 50 kgf / cm 2 (4.90 MPa) for 5 min. The electrode area of ​​the membrane electrode assembly for the initial performance test was 1 cm × 1 cm (1 cm 2 The electrode area of ​​the membrane electrode assembly for the durability test was 3.6 cm × 3.6 cm (12.96 cm 2 This membrane electrode assembly was sandwiched between paper diffusion layers (GDL) with a water-repellent layer to prepare a test cell.

[0077] [Example 2] A membrane / electrode assembly E2 was produced and evaluated in the same manner as in Example 1, except that electrode catalyst C was used as the electrode catalyst in the cathode catalyst layer.

[0078] [Example 3] A membrane electrode assembly E3 was produced and evaluated in the same manner as in Example 1, except that electrode catalyst D was used as the electrode catalyst in the cathode catalyst layer.

[0079] [Example 4] A membrane / electrode assembly E4 was produced and evaluated in the same manner as in Example 3, except that the amount of B18CRE used during complex formation was doubled (6.24 g, 0.02 mol).

[0080] [Comparative Example 1] A membrane / electrode assembly C1 was produced and evaluated in the same manner as in Example 1, except that electrode catalyst A was used as the electrode catalyst in the cathode catalyst layer and cerium (III) nitrate hexahydrate was added to the anode catalyst layer instead of the composite.

[0081] Comparative Example 2 A membrane / electrode assembly C2 was produced and evaluated in the same manner as in Example 1, except that electrode catalyst A was used as the electrode catalyst in the cathode catalyst layer.

[0082] Comparative Example 3 A membrane / electrode assembly C3 was produced and evaluated in the same manner as in Example 1, except that cerium (III) nitrate hexahydrate was added to the anode catalyst layer instead of the composite.

[0083] Comparative Example 4 A membrane / electrode assembly C4 was produced and evaluated in the same manner as in Example 2, except that cerium (III) nitrate hexahydrate was added to the anode catalyst layer instead of the composite.

[0084] Comparative Example 5 A membrane / electrode assembly C5 was produced and evaluated in the same manner as in Example 3, except that cerium (III) nitrate hexahydrate was added to the anode catalyst layer instead of the composite.

[0085] [evaluation] (Initial performance test) Membrane electrode assembly for initial performance testing (electrode area: 1 cm 2 The current-voltage characteristics were evaluated under low humidity conditions (95°C, 30% RH), and the current was 1.5 A / cm 2The performance (voltage) was measured at 1000 kJ / s. The sweep rate for evaluating the current-voltage characteristics was 20 mA / s, and the data was acquired by anode sweep. The cell pressure was 150 kPa, the cathode gas was air, and the cathode gas flow rate was 2.0 L / min. The results are shown in Table 1.

[0086] (Durability test: voltage drop rate) The above test cell (electrode area: 12.96 cm 2 The durability test was carried out in a low humidity environment (90°C, 40% RH) with the cell temperature at 90°C, hydrogen / air supplied, and a current density of 0.2 A / cm. 2 The initial characteristics of a polymer electrolyte fuel cell in this case and the characteristics after a durability test load were evaluated. Hydrogen and air were humidified and supplied to the cell so that the anode side had a dew point of 67°C and the cathode side had a dew point of 67°C, respectively, and the relationship between the cell voltage at the beginning of operation and the elapsed time after the start of operation was measured. The results are shown in Table 1. Furthermore, under the above cell evaluation conditions, the cell voltage was measured at the beginning of operation and after 300 hours had elapsed since the start of operation. The results are shown in Table 1.

[0087] [Table 1]

[0088] The upper and / or lower limit values ​​of the numerical ranges described herein can be arbitrarily combined to define a preferred range. For example, the upper and lower limit values ​​of the numerical ranges can be arbitrarily combined to define a preferred range, the upper limit values ​​of the numerical ranges can be arbitrarily combined to define a preferred range, and the lower limit values ​​of the numerical ranges can be arbitrarily combined to define a preferred range.

[0089] Although the present embodiment has been described in detail above, the specific configuration is not limited to this embodiment, and even if there are design changes within the scope that do not deviate from the gist of this disclosure, they are included in this disclosure. [Explanation of symbols]

[0090] 1: cell, 2: MEGA (membrane electrode gas diffusion layer assembly), 3: separator (fuel cell separator), 4: membrane electrode assembly (MEA), 6: electrode, 7: gas diffusion layer, 10: fuel cell, 21, 22: gas flow path

Claims

1. A membrane electrode assembly having a solid polymer electrolyte membrane, an anode catalyst layer disposed on one surface of the solid polymer electrolyte membrane, and a cathode catalyst layer disposed on the other surface of the solid polymer electrolyte membrane, the membrane electrode assembly contains a metal ion selected from a cerium ion and a manganese ion, and a crown ether compound or a salt thereof capable of forming an inclusion compound with the metal ion, the cathode catalyst layer includes an electrode catalyst and an electrolyte; the electrode catalyst is a metal-supported carrier in which metal particles having catalytic activity are supported on a carrier having fine pores, When metal particles present on the outermost surface of the primary particle of the support are defined as outer particles, and metal particles present inside the outermost surface of the primary particle of the support are defined as inner particles, the ratio of the total surface area of ​​the outer particles to the total surface area of ​​the inner particles (total surface area of ​​outer particles / total surface area of ​​inner particles) is 1.20 or less; and A membrane electrode assembly, wherein the ratio of the total number of the external particles to the total number of the internal particles (total number of external particles / total number of internal particles) is 0.70 or less.

2. 2. The membrane electrode assembly according to claim 1, wherein the number of ring members in the cyclic structure of the crown ether compound is 15 or more.

3. 2. The membrane electrode assembly according to claim 1, wherein the crown ether compound has a molecular weight of 300 or more.

4. The crown ether compound is selected from the group consisting of dibenzo-15-crown-5-ether, benzo-18-crown-6-ether, dibenzo-18-crown-6-ether, benzo-21-crown-7-ether, dibenzo-21-crown-7-ether, benzo-24-crown-8-ether, dibenzo-24-crown-8-ether, cyclohexano-18-crown-6-ether, cyclohexano-21-crown-7-ether, cyclohexano-24-crown-8-ether, dicyclohexano-18-crown-6-ether, 2. The membrane / electrode assembly according to claim 1, wherein the at least one compound is selected from the group consisting of dicyclohexano-21-crown-7-ether, dicyclohexano-24-crown-8-ether, ... and compounds in which the aromatic ring or the aliphatic ring of these compounds is substituted with at least one substituent selected from a halogen atom, a hydroxy group, an amino group, a nitro group, a formyl group, an alkyl group having 1 to 6 carbon atoms, a hydroxyalkyl group having 1 to 6 carbon atoms, a carboxyalkyl group having 2 to 7 carbon atoms, and an aryl group having 6 to 14 carbon atoms.

5. 2. The membrane electrode assembly according to claim 1, wherein the metal particles are at least one selected from the group consisting of platinum particles, platinum alloy particles, and platinum-containing composite particles.

Images