Membrane electrode assembly

Abstract

An object of the present disclosure is to provide a membrane electrode assembly excellent in durability and performance.SOLUTION: The membrane / electrode assembly according to this embodiment is a membrane / electrode assembly comprising a polymer electrolyte membrane, an anode catalyst layer disposed on one surface of the polymer electrolyte membrane, and a cathode catalyst layer disposed on the other surface of the polymer electrolyte membrane, the membrane electrode assembly comprises metal ions selected from cerium ions and manganese ions, and a host compound capable of forming an inclusion compound with the metal ions, the cathode catalyst layer comprises an electrode catalyst and an ionomer, the electrode catalyst is a metal-supported catalyst comprising a catalyst metal and a support that supports the catalyst metal, and the ionomer coverage of the catalyst metal in the cathode catalyst layer is 40% 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 problems with cerium ions is that their durability decreases due to a decrease in concentration caused by ion migration. Non-Patent Document 1 describes that the addition of 18-crown-6-ether can reduce the elution of cerium ions outside the MEA.

[0007] However, when an investigation was carried out using a membrane electrode assembly containing a radical quenching agent such as cerium ions and a crown ether compound such as 18-crown-6-ether, a decrease in initial voltage was observed, indicating that there is room for improvement in terms of performance.

[0008] 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]

[0009] The present inventors conducted extensive research to solve the above-mentioned problems and found that the reason for the voltage drop is that a crown ether compound contained in a membrane electrode assembly, such as an anode catalyst layer or an electrolyte membrane, migrates 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 discover that adjusting the ionomer coverage of the catalytic metal in the cathode catalyst layer to a predetermined value or less can suppress not only the voltage drop that occurs with use but also the initial voltage drop, leading to the present disclosure.

[0010] 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 includes a metal ion selected from cerium ions and manganese ions, and a host compound capable of forming an inclusion compound with the metal ion; the cathode catalyst layer comprises an electrode catalyst and an ionomer; the electrode catalyst is a metal-supported catalyst including a catalytic metal and a support that supports the catalytic metal; A membrane electrode assembly, wherein the ionomer coverage of the catalytic metal in the cathode catalyst layer is 40% or less. (2) The membrane electrode assembly according to (1), wherein the host compound is a crown ether compound or a salt thereof. (3) The membrane / electrode assembly according to (1) or (2), wherein the host compound has a molecular weight of 300 or more. (4) The membrane / electrode assembly according to any one of (1) to (3), wherein the host compound is a crown ether compound having an aromatic ring or an aliphatic ring or a salt thereof. (5) The membrane / electrode assembly according to any one of (1) to (4), wherein the ionomer contains a perfluorosulfonic acid polymer. (6) The ionomer has the following general formula: -(OCF2CFX)m -O p -(CF2) n -SO3H (In the formula, m represents an integer of 0 to 3, n represents an integer of 1 to 12, p represents 0 or 1, and X represents a fluorine atom or a trifluoromethyl group.) The membrane / electrode assembly according to any one of (1) to (5), comprising a fluororesin-based ionomer having a sulfonic acid group-containing side chain represented by the formula: (7) The membrane / electrode assembly according to any one of (1) to (6), wherein the catalytic metal of the cathode catalytic layer comprises at least one selected from the group consisting of platinum particles, platinum alloy particles, and platinum-containing composite particles. (8) The membrane / electrode assembly according to any one of (1) to (7), wherein the support of the cathode catalyst layer contains carbon having electronic conductivity or an oxide having electronic conductivity. (9) The membrane / electrode assembly according to any one of (1) to (8), wherein the pore volume distribution of the metal-supported catalyst has a peak pore size in the pore size range of 2.0 nm to 12.0 nm, and the pore volume of mesopores of 2 nm to 30 nm in the metal-supported catalyst is 1.9 cc / g or more. (10) A fuel cell comprising the membrane electrode assembly according to any one of (1) to (9). [Effects of the Invention]

[0011] The present disclosure makes it possible to provide a membrane electrode assembly that is excellent in durability and performance. [Brief explanation of the drawings]

[0012] [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

[0013] This embodiment is 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, wherein the membrane electrode assembly contains metal ions selected from cerium ions and manganese ions, and a host compound capable of forming an inclusion compound with the metal ions, the cathode catalyst layer contains an electrode catalyst and an ionomer, the electrode catalyst is a metal-supported catalyst containing a catalytic metal and a support that supports the catalytic metal, and the ionomer coverage of the catalytic metal in the cathode catalyst layer is 40% or less.

[0014] 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., the anode catalyst layer or the 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, reducing unevenness in concentration in the in-plane direction. Furthermore, in the membrane electrode assembly according to this embodiment, by setting the ionomer coverage of the metal-supported catalyst in the cathode catalyst layer within a predetermined range, poisoning of the cathode catalyst by a host compound such as a crown ether compound can be suppressed, thereby suppressing a decrease in initial voltage. For these reasons, this embodiment can provide a membrane electrode assembly with excellent durability.

[0015] The configuration of this embodiment will be described below.

[0016] [Membrane electrode assembly] 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.

[0017] 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.

[0018] The cathode catalyst layer functions as an air electrode (oxygen electrode). The cathode catalyst layer contains at least an electrode catalyst and an ionomer as an electrolyte. The electrode catalyst is a metal-supported catalyst in which catalytically active metal particles are supported on a support. In other words, the electrode catalyst is a metal-supported catalyst containing a catalytic metal and a support that supports the catalytic metal.

[0019] The method for supporting the catalytic metal on the carrier can be a conventional method. For example, a method can be used in which particulate catalytic metal is mixed with a carrier dispersion in which the carrier is dispersed, filtered, washed, redispersed in ethanol or the like, and then dried using a vacuum pump or the like. After drying, heat treatment may be performed as necessary.

[0020] The support is not particularly limited, and examples thereof include carbon and oxides. The carbon may be carbon having electronic conductivity. The oxide may be oxide having electronic conductivity. One type of support may be used alone, or two or more types may be used in combination.

[0021] 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.

[0022] 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.

[0023] The support is preferably a support having fine pores, and is preferably a carbon support having fine pores.

[0024] 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 -

[0025] The catalytic metal is not particularly limited, but examples thereof include platinum, palladium, rhodium, gold, silver, osmium, iridium, and alloys of two or more of these. The platinum alloy is also not particularly limited, but examples thereof include alloys of platinum with at least one of aluminum, chromium, manganese, iron, cobalt, nickel, gallium, zirconium, molybdenum, ruthenium, rhodium, palladium, vanadium, tungsten, rhenium, osmium, iridium, titanium, and lead. The catalytic metal is preferably at least one selected from the group consisting of platinum particles, platinum alloy particles, and platinum-containing composite particles. The catalytic metal may be used alone or in combination of two or more.

[0026] The content of the catalytic metal in the electrode catalyst of the cathode catalyst layer is not limited as long as it exhibits the above catalytic action and the ionomer coverage of the catalytic metal in the cathode catalyst layer is within the following range, but is, for example, 20 to 60 mass %, preferably 35 to 50 mass %, relative to the total mass of the electrode catalyst. Alternatively, the content of the catalytic metal in the electrode catalyst of the cathode catalyst layer is 0.01 to 2.0 mg / cm per unit area of ​​the carbon catalyst layer. 2 and preferably 0.1 to 0.5 mg / cm 2 is.

[0027] The metal-supported catalyst used in this embodiment preferably has a pore volume distribution with a peak pore size in the range of 2.0 nm to 12.0 nm. When the peak pore size in the pore volume distribution of the metal-supported catalyst is 2.0 nm or more, catalyst particles can be supported in the pores.

[0028] The peak pore diameter in the pore volume distribution is preferably 2.5 nm or more, preferably 3.0 nm or more. The peak pore diameter in the pore volume distribution is preferably 11.8 nm or less, preferably 11.5 nm or less, preferably 10 nm or less, preferably 8.0 nm or less, preferably 6.5 nm or less, preferably 6.0 nm or less, preferably 5.5 nm or less.

[0029] The metal-supported catalyst used in this embodiment preferably has a pore volume of 2-30 nm mesopores of 1.9 cc / g or more. When the pore volume of 2-30 nm mesopores in the metal-supported catalyst is 1.9 cc / g or more, a sufficient amount of catalyst particles can be supported in the pores. There is no particular upper limit to the pore volume of 2-30 nm mesopores in the metal-supported catalyst, but from the viewpoint of durability of the metal-supported catalyst, it is preferably 20.0 cc / g or less.

[0030] The pore volume of mesopores of 2 nm to 30 nm is preferably 4.0 cc / g or more, preferably 6.0 cc / g or more, preferably 7.5 cc / g or more, preferably 8.0 cc / g or more, preferably 8.5 cc / g or more, preferably 9.0 cc / g or more, preferably 9.5 cc / g or more, preferably 10.0 cc / g or more, preferably 10.5 cc / g or more. The pore volume of mesopores of 2 nm to 30 nm is preferably 18.0 cc / g or less, preferably 16.0 cc / g or less, preferably 14.0 cc / g or less.

[0031] The metal-supported catalyst used in this embodiment has a BET specific surface area of ​​150 m 2 / g or more. The BET specific surface area of ​​the metal-supported catalyst is preferably 150 m 2 When the BET specific surface area is 950 m / g or more, the particle size of the platinum catalyst can be kept small while maintaining a high loading rate, and the pores formed by the gaps between the particles can be increased. The upper limit of the BET specific surface area of ​​the metal-supported catalyst is not particularly limited, but from the viewpoint of the durability of the metal-supported catalyst, it is preferable that the upper limit be 950 m / g. 2 / g or less is preferable.

[0032] The BET specific surface area is preferably 200 m 2 / g or more, preferably 250m 2 / g or more, preferably 300m 2 / g or more, preferably 330m 2 / g or more, preferably 350m 2 / g or more, preferably 450m 2 / g or more. The BET specific surface area is preferably 900 m 2 / g or less, preferably 850m 2 / g or less, preferably 800m 2 / g or less, preferably 750m 2 / g or less.

[0033] A metal-supported catalyst is constructed by supporting catalytically active particulate catalytic metal on a support. As described above, the support preferably has pores (mesopores), and the particulate catalytic metal is preferably supported inside the pores. It is sufficient that at least a portion of the catalytic metal is supported inside the pores, and a portion may be supported on the surface of the support. However, from the viewpoint of reducing the ionomer coverage in the cathode catalyst layer, it is preferable that the catalytic metal be supported inside the pores.

[0034] When the supported metal catalyst has the pores and BET specific surface area as described above, the coverage of the ionomer on the catalytic metal in the cathode catalyst layer can be appropriately controlled.

[0035] The content of the electrode catalyst in the cathode catalyst layer is not particularly limited, but is, for example, 3 to 40 mass %, and preferably 5 to 38 mass %, relative to the total mass of the cathode catalyst layer.

[0036] The ionomer used as the electrolyte 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 ionomers such as perfluorosulfonic acid polymers; sulfonated resin-based ionomers such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfone, sulfonated polysulfide, and sulfonated polyphenylene; and sulfoalkylated resin-based ionomers such as sulfoalkylated polyether ether ketone, sulfoalkylated polyether sulfone, sulfoalkylated polyether ether sulfone, sulfoalkylated polysulfone, sulfoalkylated polysulfide, and sulfoalkylated polyphenylene. Of these, fluororesin-based ionomers are preferred. Further, the ionomer may be a perfluorosulfonic acid polymer and / or a polymer having the following general formula: -(OCF2CFX) m -O p -(CF2) n -SO3H (In the formula, m represents an integer of 0 to 3, n represents an integer of 0 to 12, p represents 0 or 1, and X represents a fluorine atom or a trifluoromethyl group.) Fluorine resin ionomers having sulfonic acid group-containing side chains represented by the formula:

[0037] [ka] [Available, for example, as Nafion® (EW (equivalent weight of ionomer) 1020, m=6.6)]

[0038] [ka] [Available, for example, as Aquivion® (EW 830, n=5.5)]

[0039] [ka] [Available, for example, as 3M™ (EW 850, p=4.7)] The ionomer may be used alone or in combination of two or more.

[0040] The ionomer may also be a fluorine-based ionomer with highly oxygen-permeable sulfonic acid group-containing side chains. The cyclic structure of the highly oxygen-permeable ionomer increases the molecular free volume, improving oxygen permeability and enabling higher output of fuel cells (Patent 6211249, Patent 6763300).

[0041] In this embodiment, the ionomer coverage of the catalytic metal in the cathode catalyst layer is 40% or less, preferably 38% or less, preferably 36% or less, preferably 34% or less, preferably 33% or less, preferably 32% or less, preferably 31% or less, preferably 30% or less, preferably 29% or less, and preferably 28% or less. When the ionomer coverage is 40% or less, voltage drop can be effectively suppressed, and this effect increases as the ionomer coverage decreases. One reason that voltage drop can be suppressed when the ionomer coverage is 40% or less is that crown ether that flows from the anode catalyst layer to the cathode catalyst layer can be prevented from being adsorbed onto the cathode catalyst surface via the ionomer.

[0042] In this embodiment, the lower limit of the ionomer coverage of the catalytic metal in the cathode catalyst layer is not particularly limited. The ionomer coverage of the catalytic metal in the cathode catalyst layer is, for example, 1% or more, in one embodiment, 2% or more, in one embodiment, 3% or more, in one embodiment, 4% or more, or in one embodiment, 5% or more.

[0043] The ionomer coverage in this embodiment is an index showing the degree to which the catalytic metal in the cathode catalyst layer is covered with the ionomer as a proton conductive material, and the ionomer coverage θ is defined as follows: θ=S C-S / (S C-S +S C-L ) ···(1) (wherein, (S C-S ) is the area where the catalytic metal in the cathode catalyst layer is in contact with the solid proton conducting material (i.e., the ionomer in this embodiment), and (S C-L ) is the area where the catalytic metal in the membrane electrode assembly is in contact with the liquid proton conducting material (S C-L ) where the liquid proton conducting material connects the catalytic metal and the solid proton conducting material in a state that allows proton conduction).

[0044] From equation (1), the ionomer coverage θ is the area where the catalyst metal is in contact with the solid proton conducting material (S C-S ) and the area where the catalyst metal is in contact with the liquid proton conducting material (S C-L ) and the area where the catalyst metal is in contact with the solid proton conducting material (S C-S ) is the ratio of

[0045] This coverage can be determined, for example, by determining the electric double layer capacitance formed at the catalyst-solid proton conductive material interface and the catalyst-liquid proton conductive material interface when the liquid conductive material holding portion of the carrier is filled with the liquid proton conductive material. The electric double layer capacitance is proportional to the electrochemically effective interface area. Therefore, the electric double layer capacitance formed at the catalyst-solid proton conductive material interface is (S C-S ), and the capacitance of the electric double layer formed at the catalyst-liquid proton conducting material interface is (S C-L )

[0046] Here, a method for measuring the capacitance of the electric double layer formed at the catalyst-solid proton conducting material interface and at the catalyst-liquid proton conducting material interface will be described.

[0047] In the catalyst layer, (1) Catalytic metal - solid proton conductor (CS) (2) Catalyst metal-liquid proton conductor (CL) (3) Porous support - solid proton conducting material (Cr-S) (4) Four types of interfaces between the porous support and the liquid proton-conducting material (Cr-L) can contribute to the electric double layer capacitance (Cdl).

[0048] As mentioned above, the electric double layer capacity is directly proportional to the electrochemically effective interface area. Therefore, the electric double layer capacity (Cdl C-S ), and the electric double layer capacitance (Cdl C-L ) can be obtained. C-S ) and (Cdl C-L ) is the value of (S C-S ) and (S C-L) will be used as

[0049] The contributions of the four interfaces to the electric double layer capacitance (Cdl) can be separated as follows.

[0050] First, the electric double layer capacitance is measured under high humidity conditions, such as 100% relative humidity, and low humidity conditions, such as 10% relative humidity. Examples of techniques for measuring the electric double layer capacitance include cyclic voltammetry and electrochemical impedance spectroscopy. From these comparisons, the contributions of the liquid proton-conducting material (water in this case), i.e., (2) and (4) above, can be separated from (1) and (3).

[0051] Furthermore, the catalytic metal can be deactivated. For example, when Pt is used as the catalytic metal, CO gas is supplied to the electrode under measurement to adsorb CO onto the Pt surface. This allows the Pt contribution to the electric double layer capacitance to be separated. By measuring the electric double layer capacitance under high and low humidification conditions in the deactivated state using the same method, as described above, the contribution of Pt, i.e., (1) and (2) above, can be separated from (3) and (4).

[0052] As a result, it is possible to separate all of the contributions of (1) to (4) above, and to determine the capacitance of the electric double layer formed at both the interfaces between the catalyst metal and the solid proton-conducting material and the liquid proton-conducting material.

[0053] That is, the measurement value in a highly humidified state (referred to as measurement value A) is the capacity of the electric double layer formed at all interfaces of (1) to (4) above. This is essentially the value of all the electric double layer capacities formed at both interfaces between the catalyst metal and the solid proton conductive material and the liquid proton conductive material for the entire catalyst layer. On the other hand, the measurement value in a low humidified state (referred to as measurement value B) is the capacity of the electric double layer formed at the interfaces of (1) and (3) above. Furthermore, the measurement value in a catalyst deactivated and highly humidified state (referred to as measurement value C) is the capacity of the electric double layer formed at the interfaces of (3) and (4) above, and the measurement value in a catalyst deactivated and low humidified state (referred to as measurement value D) is the capacity of the electric double layer formed at the interface of (3) above.

[0054] Therefore, the difference between measurement values ​​A and C is the electric double layer capacitance formed at the interface between (1) and (2), and the difference between measurement values ​​B and D is the electric double layer capacitance formed at the interface of (1). Then, by calculating the difference between these values, (AC) - (BD), the electric double layer capacitance formed at the interface of (2) can be found.

[0055] To summarize, the electric double layer capacity at the interface between the catalytic metal and the liquid proton conductive material can be calculated by subtracting the value of the electric double layer capacity measured under low humidification conditions from the value of the electric double layer capacity measured under high humidification conditions.

[0056] The electric double layer capacity at the interface between the catalytic metal and the solid proton conductive material and the electric double layer capacity at the interface between the catalytic metal and the liquid proton conductive material can be determined by measuring the electric double layer capacity after deactivation by adsorbing carbon monoxide onto the catalytic metal to deactivate it, and then subtracting the electric double layer capacity after deactivation from the overall electric double layer capacity before deactivation.

[0057] By combining these and subtracting the measured electric double layer capacity when the catalyst is deactivated and the humidity is low from the measured electric double layer capacity when the catalyst is deactivated and the humidity is high, the electric double layer capacity of the interface between the catalyst metal and the solid proton conductive material alone can be obtained.

[0058] In addition to the above, the contact area of ​​the catalyst metal with the solid proton conductive material and the exposed area to the conductive material support portion can also be determined by, for example, TEM (transmission electron microscope) tomography or the like.

[0059] The method of calculating the coverage of the catalyst metal by the solid proton conductive material using the electric double layer capacity can be simplified. As already explained, among the above (1) to (4), the measured value A in a highly humidified state is the electric double layer capacity formed at all interfaces of the above (1) to (4). On the other hand, the measured value B in a low humidified state is the electric double layer capacity formed at the interfaces of the above (1) and (3). In other words, the measured value A in a highly humidified state is the area (S) of both the interface between the catalyst metal and the solid proton conductive material (CS) and the interface between the catalyst metal and the liquid proton conductive material (CL). C-S +S C-L On the other hand, the measured value B in the low humidified state includes the area (S C-S ) but the area of ​​the catalyst metal-liquid proton conducting material (CL) interface (S C-L ) is not included. Therefore, the measured value B / measured value A is approximately the coverage factor θ = S C-S / (S C-S +S C-L ) can be seen.

[0060] By using such a simple method, it is not necessary to create a deactivated state using CO, and therefore the measurement can be carried out more quickly.

[0061] The cathode catalyst layer of this embodiment contains, between the catalytic metal and the solid proton-conducting material, a liquid proton-conducting material that can connect the catalytic metal and the solid proton-conducting material in a proton-conductive state, at least during measurement of the ionomer coverage. The introduction of the liquid proton-conducting material ensures a proton transport path between the catalytic metal and the solid proton-conducting material via the liquid proton-conducting material, enabling efficient transport of protons required for power generation to the surface of the catalytic metal. This liquid proton-conducting material only needs to be present between the catalytic metal and the solid proton-conducting material at least during measurement of the ionomer coverage, and can be disposed in the pores (secondary pores) between the porous carriers in the catalyst layer or the pores (micropores or mesopores: primary pores) within the porous carrier.

[0062] The liquid proton conducting material is not particularly limited as long as it has ion conductivity and can function to form a proton transport path between the catalytic metal and the solid proton conducting material. Specific examples include water, protic ionic liquids, aqueous perchloric acid solutions, aqueous nitric acid solutions, aqueous formic acid solutions, and aqueous acetic acid solutions.

[0063] When water is used as the liquid proton conductor, it can be introduced into the catalyst layer by wetting the catalyst layer with a small amount of liquid water or humidified gas before starting power generation. Alternatively, water produced by the electrochemical reaction during fuel cell operation can be used as the liquid proton conductor. Therefore, the liquid proton conductor does not necessarily need to be present when the fuel cell is in its initial state. For example, it is desirable to set the surface distance between the catalyst metal and the solid proton conductor to 0.5 nm or more. Maintaining this distance allows water (liquid proton conductor) to be interposed between the catalyst metal and the solid proton conductor (liquid conductor support) while maintaining a non-contact state between the catalyst metal and the solid proton conductor, thereby ensuring a proton transport pathway between the two.

[0064] When a liquid proton conductive material other than water, such as an ionic liquid, is used as the liquid proton conductive material, it is desirable to disperse the ionic liquid, the solid proton conductive material, and the catalyst in a solution when preparing the catalyst ink, but the ionic liquid may also be added when applying the catalyst to the catalyst layer substrate.

[0065] The content of the ionomer in the cathode catalyst layer is not limited as long as the ionomer coverage of the catalytic metal in the cathode catalyst layer is within the above range, but is, for example, 20 to 60 mass %, preferably 25 to 40 mass %, relative to the total mass of the cathode catalyst layer. When the ionomer content in the cathode catalyst layer is within the above range, the ionomer coverage of the catalytic metal in the cathode catalyst layer can be appropriately controlled.

[0066] The mass ratio (I / C) of the ionomer in the cathode catalyst layer to the support, particularly the carbon support, in the electrode catalyst is not limited as long as the ionomer coverage of the catalytic metal in the cathode catalyst layer is within the above range, but is 0.5 to 1.0, preferably 0.6 to 0.9, preferably 0.7 to 0.9, and preferably 0.7 to 0.8. When the mass ratio is within the above range, the ionomer coverage of the catalytic metal in the cathode catalyst layer can be appropriately controlled, and as the mass ratio decreases, the ionomer coverage also tends to decrease.

[0067] The thickness (dry film thickness) of the catalyst layer is, for example, 0.05 to 30 μm. The above thickness is applicable to both the cathode catalyst layer and the anode catalyst layer.

[0068] The ionomer coverage can be appropriately controlled within a desired range by adjusting, for example, the ionomer content, the composition of the catalyst layer, the mixing conditions, and the like.

[0069] The anode catalyst layer functions as a fuel electrode, that is, a hydrogen electrode.

[0070] The anode catalyst layer includes an electrode catalyst and an ionomer as an electrolyte. 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 host compound (e.g., crown ethers) capable of forming an inclusion compound with the metal ion.

[0071] The electrode catalyst is not particularly limited, but for example, the above-mentioned materials can be used.

[0072] The content of the catalytic metal in the electrode catalyst of the anode catalyst layer is not limited as long as it exhibits the above catalytic action, but is, for example, 10 to 70 mass %, preferably 15 to 50 mass %, relative to the total mass of the electrode catalyst. Alternatively, the content of the catalytic metal in the electrode catalyst of the anode catalyst layer is 0.01 to 2.0 mg / cm per unit area of ​​the anode catalyst layer. 2 and preferably 0.1 to 0.5 mg / cm 2 is.

[0073] The content of the electrode catalyst in the anode catalyst layer is not particularly limited, but is, for example, 3 to 40 mass %, and preferably 5 to 38 mass %, relative to the total mass of the anode catalyst layer.

[0074] 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).

[0075] The mass ratio (I / C) of the ionomer in the anode catalyst layer to the support in the electrode catalyst, particularly the carbon support, is not particularly limited, but is 0.5 to 1.2, preferably 0.7 to 1.0. By setting the mass ratio within the above range, good adhesion between the catalyst layer and the electrolyte membrane and good proton conductivity can be ensured.

[0076] The membrane electrode assembly according to this embodiment includes metal ions selected from cerium ions and manganese ions, and a host compound capable of forming an inclusion compound with the metal ions. The cerium ions and / or manganese ions function as radical quenchers, capturing and neutralizing hydrogen peroxide radicals, thereby suppressing deterioration of the membrane electrode assembly. Furthermore, by adding a host compound (e.g., crown ethers) 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.

[0077] 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)

[0078] The cerium ion may be either +3 or +4 valent, and the manganese ion may be either +3 or +4 valent.

[0079] 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.

[0080] 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.

[0081] In this embodiment, the host compound forms an inclusion compound with cerium ions or manganese ions as a guest compound. The inclusion compound refers to an adduct in which the metal ions as a guest compound are included in the host compound. Examples of host compounds that form inclusion compounds include crown ether compounds, cyclodextrin compounds, cyclophane compounds, and salts thereof. One type of host compound may be used alone, or two or more types may be used in combination.

[0082] The host compound is not particularly limited as long as it can form an inclusion compound with the metal ion. The host compound preferably 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. In one embodiment, the host compound is preferably a crown ether compound or a salt thereof. The crown ether compound is a compound having a ring with a repeating structure of (-CH2-CH2-Y-) units or (-CH2-CH2-CH2-Y-) units, where Y is at least one heteroatom selected from O, S, N, and P. The crown ether compound forms an inclusion compound by capturing a metal ion in the ring structure. The number of ring members in the crown ether compound is preferably 15 or more, and more preferably 18 or more. The crown ether compound forms an inclusion compound with a cerium ion or a manganese ion as a guest compound. The inclusion compound refers to an adduct in which the metal ion as a guest compound is included in the host compound. The crown ether compounds may be used alone or in combination of two or more.

[0083] Examples of crown ether compounds include crown ethers and crown ether derivatives. Examples of crown ethers include 15-crown-5-ether, 18-crown-6-ether, 21-crown-7-ether, and 24-crown-8-ether. In this embodiment, the host 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 migrate less to the cathode catalyst layer. 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.

[0084] In this embodiment, the host compound is preferably a crown ether compound or a salt thereof having a molecular weight of 300 or more. By using a crown ether compound having a molecular weight of 300 or more, 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.

[0085] 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.

[0086] The host compound and metal ions can be contained in the anode catalyst layer, the solid polymer electrolyte membrane, or both. In one embodiment, in the anode catalyst layer of the membrane electrode assembly, at least a portion of the host compound and the metal ions form an inclusion compound.

[0087] When the anode catalyst layer contains a host compound and metal ions, the anode catalyst layer contains at least an electrode catalyst, an ionomer as an electrolyte, metal ions selected from cerium ions and manganese ions, and a host compound. The metal ions can facilitate the conversion of hydroxide radicals generated from hydrogen peroxide to hydroxide ions, thereby suppressing deterioration of the anode catalyst layer.

[0088] The content of the metal ions and host 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 host compound. That is, when the metal ions and the host compound are added to the anode catalyst layer separately or simply mixed together, even if an inclusion compound is formed in the polymer electrolyte, the amount of the compound is not taken into consideration, and only the total amount of the compounded metal ions and the host 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 host compound that formed the inclusion compound. Furthermore, when there are metal ions and the host compound that do not form an inclusion compound in addition to the metal ions and the host compound that formed the inclusion compound, these are also included in the calculation.

[0089] The content of the metal ions in the anode catalyst layer is 0.1 to 20 μg / cm 2 The content of the host compound in the anode catalyst layer is preferably 0.1 to 20 μg / cm 3 . 2 It is preferable that:

[0090] In this embodiment, the molar ratio of the host compound to the metal ion ([moles of host compound] / [moles of metal ion]) is, for example, 0.1 to 10, preferably 0.2 to 7.5, and preferably 0.4 to 5.0. That is, the content of the host compound is, for example, 0.1 to 10 mol, preferably 0.2 to 7.5 mol, and preferably 0.4 to 5.0 mol per mol of the metal ion. Note that, even at this relative ratio, the inclusion compound is considered to be a mixture of both, as described above.

[0091] When the solid polymer electrolyte membrane contains a host compound and metal ions, the solid polymer electrolyte membrane containing the host compound and metal ions can be obtained, for example, by the following method. (1) A method in which 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 the membrane is immersed in a solution containing a host compound to incorporate the host 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 host 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 host 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 method in which a compound containing a metal ion (e.g., a cerium salt) is reacted with a host 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.

[0092] [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.

[0093] 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.

[0094] The anode catalyst layer may be formed by adding the metal ions and the host 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 host compound, and a solvent. The metal ions and the host compound may be added separately or in the form of a complex of the two.

[0095] In this embodiment, the cathode catalyst layer is formed so that the ionomer coverage of the catalytic metal in the cathode catalyst layer is 40% or less. The ionomer coverage is not particularly limited, but can be appropriately controlled within a desired range by adjusting, for example, the ionomer content, the composition of the catalyst layer, mixing conditions, etc.

[0096] [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.

[0097] 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.

[0098] 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]

[0099] The present embodiment will be described below using examples.

[0100] [Production example 1: Electrode catalyst] (Preparation of Platinum-Supported Catalyst A) A platinum-supported catalyst A was prepared, containing Pt particles as the catalytic metal and carbon black (VULCAN, manufactured by Cabot Corporation) as the support supporting the metal particles (metal loading ratio: 36 mass%). The platinum-supported catalyst A had a peak pore diameter of 11.5 nm, a mesopore volume of 2 to 30 nm of 1.9 cc / g, and a BET specific surface area of ​​159.0 m. 2 / g.

[0101] (Preparation of platinum-supported catalyst B) A platinum-supported catalyst B was prepared, containing Pt particles as the catalytic metal and Ketjen Black (product name: EC300J, manufactured by Lion Specialty Chemicals) as the support supporting the metal particles (metal loading ratio: 42 mass%). The peak pore diameter of platinum-supported catalyst B was 5.4 nm, the mesopore volume of 2 to 30 nm was 10.5 cc / g, and the BET specific surface area was 458.0 m. 2 / g.

[0102] (Preparation of platinum-supported catalyst C) A platinum-supported catalyst C was prepared, containing Pt particles as the catalytic metal and carbon black as the support supporting the metal particles (metal loading ratio: 48 mass%). The platinum-supported catalyst C had a peak pore diameter of 3.5 nm, a mesopore volume of 2 to 30 nm of 14.0 cc / g, and a BET specific surface area of ​​708.0 m. 2 / g.

[0103] [Production example 2: Inclusion compound (complex: Ce-ligand)] (Preparation of Complex A (Ce-B18CRE)) 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 (Complex A). 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.

[0104] (Preparation of Complex B (Ce-18CRE)) 18-Crown-6-ether (18CRE) (2.64 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 for 24 hours at room temperature. 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 (Complex B). FT-IR analysis confirmed that the peaks derived from the ether group were shifted to lower wavenumbers, confirming that CRE and Ce formed an inclusion compound.

[0105] [Example 1] (Formation of cathode catalyst layer) As an electrode catalyst, platinum-supported catalyst A 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.

[0106] The Pt content of the cathode catalyst layer is 0.2 mg / cm 2 The mass ratio (I / C) of the ionomer to the carrier was set to 0.9. The coverage of the catalyst metal with the ionomer in the cathode catalyst layer was measured by the method described below and was found to be 40%.

[0107] (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 A 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.

[0108] 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.

[0109] (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 and 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 water-repellent layers to prepare a test cell.

[0110] [Example 2] Except for changing the mass ratio (I / C) of ionomer to carrier in the cathode catalyst layer to 0.8, a membrane electrode assembly E2 was produced in the same manner as in Example 1. The ionomer coverage of the catalyst metal in the cathode catalyst layer was measured by the method described below and was found to be 36%.

[0111] [Example 3] Except for changing the mass ratio (I / C) of ionomer to carrier in the cathode catalyst layer to 0.7, a membrane / electrode assembly E3 was produced in the same manner as in Example 1. The ionomer coverage of the catalyst metal in the cathode catalyst layer was measured by the method described below and was found to be 33%.

[0112] [Example 4] A membrane / electrode assembly E4 was produced in the same manner as in Example 1, except that platinum-supported catalyst B was used instead of platinum-supported catalyst A and the mass ratio (I / C) of ionomer to carrier in the cathode catalyst layer was set to 1.0. The ionomer coverage of the catalyst metal in the cathode catalyst layer was measured by the method described below and was found to be 10%.

[0113] [Example 5] A membrane / electrode assembly E5 was produced in the same manner as in Example 1, except that platinum-supported catalyst C was used instead of platinum-supported catalyst A and the mass ratio (I / C) of ionomer to carrier in the cathode catalyst layer was set to 1.0. The ionomer coverage of the catalyst metal in the cathode catalyst layer was measured by the method described below and was found to be 28%.

[0114] [Example 6] A membrane / electrode assembly E6 was produced in the same manner as in Example 1, except that composite B was used instead of composite A. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 40%.

[0115] [Example 7] A membrane / electrode assembly E7 was produced in the same manner as in Example 2, except that composite B was used instead of composite A. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 36%.

[0116] [Example 8] A membrane / electrode assembly E8 was produced in the same manner as in Example 3, except that composite B was used instead of composite A. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 33%.

[0117] [Comparative Example 1] Except for not adding a composite to the anode catalyst layer and setting the mass ratio (I / C) of ionomer to carrier in the cathode catalyst layer to 1.0, a membrane / electrode assembly C1 was produced in the same manner as in Example 1. The ionomer coverage of the catalyst metal in the cathode catalyst layer was measured by the method described below and was found to be 47%.

[0118] Comparative Example 2 Except for changing the mass ratio (I / C) of ionomer to carrier in the cathode catalyst layer to 1.0, a membrane / electrode assembly C2 was produced in the same manner as in Example 1. The ionomer coverage of the catalyst metal in the cathode catalyst layer was measured by the method described below and was found to be 47%.

[0119] Comparative Example 3 Except for not adding a composite to the anode catalyst layer, a membrane electrode assembly C3 was produced in the same manner as in Example 1. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 40%.

[0120] Comparative Example 4 Except for not adding a composite to the anode catalyst layer, a membrane electrode assembly C4 was produced in the same manner as in Example 2. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 36%.

[0121] Comparative Example 5 Except for not adding a composite to the anode catalyst layer, a membrane electrode assembly C5 was produced in the same manner as in Example 3. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 33%.

[0122] Comparative Example 6 Except for not adding a composite to the anode catalyst layer, a membrane electrode assembly C6 was produced in the same manner as in Example 4. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 10%.

[0123] Comparative Example 7 Except for not adding a composite to the anode catalyst layer, a membrane electrode assembly C7 was produced in the same manner as in Example 5. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 28%.

[0124] [Comparative Example 8] A membrane electrode assembly C8 was produced in the same manner as in Comparative Example 2, except that composite B was used instead of composite A. The ionomer coverage of the catalytic metal in the cathode catalyst layer was measured by the method described below and was found to be 47%.

[0125] [Measurement of ionomer coverage of catalyst metal] The coverage of the catalyst metal with the solid proton conductive material (ionomer) was measured for the obtained membrane electrode assemblies E1 to E8 and C1 to C8. The coverage was measured by measuring the capacity of the electric double layer formed at the interface between the catalyst and the solid proton conductive material and the liquid proton conductive material (water), as in the embodiment already described.

[0126] A test cell (electrode area 1 cm) was prepared using the obtained membrane electrode assembly. 2 ) was fabricated and evaluated. The single cell used in this example was configured such that separators having gas flow channels were arranged on both sides of the membrane electrode assembly, and current collector plates were arranged on the outside (on both sides) of the separators, and end plates were arranged on the outsides of these. Gas piping required for the various evaluations (measurements) described below was then installed in this single cell. The gas piping was designed so that pressure, temperature, and humidity could be adjusted. Wiring from a potentiostat was also performed so that the current collector plates were on the anode and cathode sides.

[0127] For the membrane electrode assemblies E1 to E8 and C1 to C8, the electric double layer capacity was measured by electrochemical impedance spectroscopy in the activated state (before deactivation) under high humidity (100% RH) and low humidity (5% RH).Furthermore, the electric double layer capacity was measured in the deactivated state under high humidity (100% RH) and low humidity (5% RH).

[0128] The equipment used was an electrochemical measurement system HZ-3000 manufactured by Hokuto Denko Corporation and a frequency response analyzer FRA5020 manufactured by NF Corporation. The evaluation was carried out under the conditions of a cell temperature of 30°C, a frequency range of 20 kHz to 10 mHz, an amplitude of ±10 mV, a holding potential of 0.45 V, and supply gases of hydrogen / nitrogen (counter electrode / working electrode) at the humidity mentioned above.

[0129] [Performance evaluation] (Initial performance test) Membrane electrode assembly (electrode area: 12.96cm 2 The current-voltage characteristics were evaluated under low humidity conditions (cell temperature 95°C, humidity 30% RH), and a current of 1.5 A / cm 2 The performance (voltage) was measured. 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 anode gas was hydrogen, the anode gas flow rate was 1.0 L / min, the cathode gas was air, and the cathode gas flow rate was 2.0 L / min. The results are shown in Table 1. In addition, when each catalyst was combined with a crown ether, the addition of crown ether resulted in a 1.5A / cm 2 The voltage drop at the time of the test is shown in Table 1 as the difference in the initial voltage.

[0130] (Durability test: voltage drop rate) The above test cell (electrode area: 12.96 cm 2 ) was used in a low humidity environment (cell temperature 95°C, humidity 30% RH) and low current density (0.2 A / cm 2A durability test was conducted for 300 hours under the conditions of the initial performance test, with the cell pressure set to 150 kPa, the anode gas type being hydrogen, the anode gas flow rate being 1.0 L / min, the cathode gas type being air, and the cathode gas flow rate being 2.0 L / min. After the durability test, hydrogen / air was supplied and the current density was set to 1.5 A / cm under the conditions of the initial performance test described above. 2 The characteristics of the polymer electrolyte fuel cell were evaluated, and the cell voltage at the beginning of operation and the relationship between the cell voltage and the elapsed time after operation were measured. The results are shown in Table 1.

[0131] From Table 1, it can be seen that by setting the ionomer coverage of the catalytic metal in the cathode catalyst layer to 40% or less, preferably 36% or less, and preferably 33% or less, it is possible to suppress the voltage drop that occurs with use while also suppressing the drop in initial voltage.

[0132] [Table 1]

[0133] 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.

[0134] 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]

[0135] 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 includes a metal ion selected from cerium ions and manganese ions, and a host compound capable of forming an inclusion compound with the metal ion; the cathode catalyst layer comprises an electrode catalyst and an ionomer; the electrode catalyst is a metal-supported catalyst including a catalytic metal and a support that supports the catalytic metal; a coverage of the catalytic metal with an ionomer in the cathode catalytic layer being 40% or less;

2. 2. The membrane electrode assembly according to claim 1, wherein the host compound is a crown ether compound or a salt thereof.

3. 3. The membrane electrode assembly according to claim 2, wherein the molecular weight of the host compound is 300 or more.

4. 3. The membrane electrode assembly according to claim 2, wherein the host compound is a crown ether compound having an aromatic ring or an aliphatic ring, or a salt thereof.

5. 10. The membrane electrode assembly of claim 1, wherein the ionomer comprises a perfluorosulfonic acid polymer.

6. The ionomer has the following general formula: -(OCF ２ CFX) ｍ -O ｐ -(CF ２ ) ｎ -SO ３ H (In the formula, m represents an integer of 0 to 3, n represents an integer of 1 to 12, p represents 0 or 1, and X represents a fluorine atom or a trifluoromethyl group.) 2. The membrane electrode assembly according to claim 1, comprising a fluororesin-based ionomer having a sulfonic acid group-containing side chain represented by the formula:

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

8. 2. The membrane electrode assembly according to claim 1, wherein the support of the cathode catalyst layer comprises carbon having electronic conductivity or an oxide having electronic conductivity.

9. 2. The membrane electrode assembly according to claim 1, wherein the pore volume distribution of the metal-supported catalyst has a peak pore size in a pore size range of 2.0 nm to 12.0 nm, and the pore volume of mesopores of 2 nm to 30 nm in the metal-supported catalyst is 1.9 cc / g or more.

10. A fuel cell comprising the membrane electrode assembly according to any one of claims 1 to 9.

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