Catalyst layer

Abstract

To provide a catalyst layer excellent in proton conductivity and flooding tolerance.SOLUTION: A catalyst layer includes an ionomer / catalyst composite and an ionomer / base material composite. The ionomer / catalyst composite includes: an electrode catalyst having catalyst particles carried on the surface of a conductive particle; and a first ionomer coating at least the surface of the conductive particle. The ionomer / base material composite includes a sheet-like base material particle and a second ionomer coating the surface of the base material particle. The ion exchange capacity of the first ionomer is less than that of the second ionomer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a catalyst layer, and more particularly to a catalyst layer having excellent proton conductivity and flooding resistance. [Background technology]

[0002] A polymer electrolyte fuel cell is equipped with a membrane electrode assembly (MEA) in which catalyst layers are bonded to both sides of an electrolyte membrane. The catalyst layer is the reaction field for electrode reactions and is generally made of a composite of carbon carrying an electrode catalyst such as platinum and a solid polymer electrolyte (catalyst layer ionomer). A gas diffusion layer is usually further 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. Furthermore, a separator with a gas flow path is disposed on the outside of the gas diffusion layer. A polymer electrolyte fuel cell generally has a structure (stack structure) in which a plurality of unit cells each consisting of such an MEA, gas diffusion layer, and separator are stacked.

[0003] Polymer electrolyte fuel cells are required to exhibit high power generation performance in a wide range of temperature and humidity environments, from low temperature and high humidity to high temperature and low humidity. To achieve both of these performances, it is necessary to improve the catalyst layer where the reaction occurs. To improve power generation performance under high temperature and low humidity conditions, it is necessary to lower proton resistance and reduce IR loss. In addition, to improve power generation performance under low temperature and high humidity conditions, it is possible to reduce gas diffusion resistance by increasing the porosity of the catalyst layer and improving water discharge performance. However, these properties are in a trade-off relationship: for example, increasing the volume fraction of ionomer reduces proton resistance, but also reduces porosity.

[0004] Therefore, various proposals have been made in the past to solve this problem. For example, Patent Document 1 states: (a) preparing a first ink containing platinum-supported carbon and a first ionomer made of perfluorosulfonic acid having a side chain with 4 or more carbon atoms; (b) preparing a second ink containing carbon black and a second ionomer having a side chain with 3 or less carbon atoms; (c) mixing the first ink and the second ink to form a cathode ink; (d) Apply the cathode ink to the substrate and allow it to dry. The cathode catalyst layer obtained by the above process is disclosed. The document describes that this method can produce a catalyst layer in which a catalyst member in which the surface of platinum-supported carbon is coated with a first ionomer and a proton-conducting member in which the surface of carbon black is coated with a second ionomer are mixed.

[0005] Patent Document 2 states: (a) preparing composite particles containing a catalyst and a cation exchange resin; (b) preparing coated particles in which the surfaces of composite particles are coated with a water-repellent material; (c) The coated particles are attached to the surface of the solid polymer electrolyte membrane, and the coated particles are pressed against the solid polymer electrolyte membrane. The resulting membrane electrode assembly is disclosed. The document describes that this method can improve the water repellency of the catalyst layer without reducing the gas diffusibility and electron conductivity of the catalyst layer.

[0006] Patent Document 1 describes that shortening the side chains of the fluorine-based ionomer used in the proton-conducting member increases crystallinity, suppressing its swelling and thereby suppressing flooding. However, while the method described in Patent Document 1 can suppress the swelling of the ionomer used in the proton-conducting member, it cannot control the retention of water in the voids around it. Furthermore, it cannot control the retention of water around the catalyst member.

[0007] Patent Document 2 discloses a catalyst layer made using coated particles in which the surfaces of catalyst-containing composite particles are coated with a water-repellent material. However, the catalyst layer is made only of the coated particles, and does not contain any other proton conductors. Therefore, the catalyst layer described in this document has low proton conductivity and porosity. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2019-040705 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-149503 Summary of the Invention [Problem to be solved by the invention]

[0009] The problem to be solved by the present invention is to provide a catalyst layer having excellent proton conductivity and flooding resistance. [Means for solving the problem]

[0010] In order to solve the above problems, the catalyst layer according to the present invention has the following configuration. (1) The catalyst layer is an ionomer / catalyst composite; Ionomer / substrate composite and It is equipped with: (2) The ionomer / catalyst composite is an electrode catalyst in which catalyst particles are supported on the surfaces of conductive particles; a first ionomer that coats at least the surfaces of the conductive particles; It is equipped with: (3) The ionomer / substrate composite is Sheet-like base particles; a second ionomer that coats the surface of the base particle; It is equipped with: (4) The ion exchange capacity of the first ionomer is smaller than the ion exchange capacity of the second ionomer. [Effects of the Invention]

[0011] In a catalyst layer containing an ionomer / catalyst composite and an ionomer / substrate composite, if the ion exchange capacity of the first ionomer is smaller than that of the second ionomer, the water repellency around the ionomer / catalyst composite becomes higher than that around the ionomer / substrate composite. Therefore, water is more likely to be present around the ionomer / substrate composite than around the ionomer / catalyst composite. As a result, the reactant gas can reach the surface of the catalyst particles without being obstructed by water. Furthermore, by adding a sheet-like ionomer / substrate composite to the catalyst layer, the proton resistance of the catalyst layer is reduced and the porosity within the catalyst layer is increased. As a result, the gas diffusion resistance of the catalyst layer is reduced, and high performance is demonstrated even under low-temperature and high-humidity conditions. [Brief explanation of the drawings]

[0012] [Figure 1] 1 shows the results of a power generation test (current density at 60° C. and 80% RH) of the fuel cells obtained in Example 1 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the present invention will be described in detail below. [1. Catalyst layer] The catalyst layer according to the present invention comprises: an ionomer / catalyst composite; Ionomer / substrate composite and It is equipped with:

[0014] 1.1. Ionomer / catalyst composite "Ionomer / catalyst complex" an electrode catalyst in which catalyst particles are supported on the surfaces of conductive particles; a first ionomer that coats at least the surfaces of the conductive particles; A composite comprising:

[0015] [1.1.1. Conductive particles] [A. Materials] The conductive particles are carriers for supporting catalyst particles. In the present invention, the material of the conductive particles is not particularly limited as long as it is conductive and can be used in a fuel cell environment.

[0016] Examples of conductive particles include: (a) carbon black, (b) Porous carbon such as mesoporous carbon; (c) SnO2, non-stoichiometric titanium oxide (TiO x Conductive oxide particles made of conductive metal oxides or composite metal oxides such as etc. The conductive particles may be any one of these, or may be a combination of two or more of these.

[0017] [B. Average particle size] The "average particle size of conductive particles" is the median diameter (D 50 ) The average particle size of the conductive particles is not particularly limited, and an optimal particle size can be selected depending on the purpose. Generally, if the average particle size of the conductive particles is too small, the connection between the particles may become poor, resulting in increased electronic resistance. Therefore, the average particle size is preferably 0.1 μm or more. The average particle size is more preferably 0.3 μm or more, and even more preferably 0.5 μm or more. On the other hand, if the average particle size of the conductive particles is too large, the uniformity of the catalyst layer is lost and, in some cases, the electrolyte membrane may be damaged during pressing. Therefore, the average particle size is preferably 20 μm or less. The average particle size is more preferably 10 μm or less, and even more preferably 5 μm or less.

[0018] 1.1.2. Catalyst particles [A. Materials] In the present invention, the material of the catalyst particles is not particularly limited as long as it has activity in the hydrogen oxidation reaction or the oxygen reduction reaction. Examples of materials for catalyst particles include: (a) Precious metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os), (b) an alloy containing two or more precious metal elements; (c) Alloys containing one or more precious metal elements and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.); etc.

[0019] Among these, Pt or a Pt alloy is preferred for the catalyst particles because it has high activity in the electrode reaction of the fuel cell. Examples of Pt alloys include Pt--Fe alloys, Pt--Co alloys, Pt--Ni alloys, Pt--Pd alloys, Pt--Cr alloys, Pt--V alloys, Pt--Ti alloys, Pt--Ru alloys, and Pt--Ir alloys.

[0020] [B. Average particle size] "Average particle size of catalyst particles" refers to the average value of the maximum dimension of catalyst particles measured under a microscope for 20 or more catalyst particles selected at random. The average particle size of the catalyst particles is not particularly limited, and an optimum average particle size can be selected depending on the purpose. Generally, if the average particle size of the catalyst particles is too small, the catalyst particles tend to dissolve. Therefore, the average particle size is preferably 1 nm or more. On the other hand, if the average particle size of the catalyst particles is too large, the mass activity decreases. Therefore, the average particle size of the catalyst particles is preferably 20 nm or less. The average particle size is preferably 10 nm or less, and more preferably 5 nm or less.

[0021] [C. Loading Amount] The term "catalyst particle loading amount" refers to the ratio of the mass of the catalyst particles to the total mass of the electrode catalyst. The amount of catalyst particles supported is not particularly limited, and an optimal amount can be selected depending on the purpose. Generally, if the amount of catalyst particles supported is too small, the thickness of the catalyst layer required to obtain a predetermined basis weight increases, and the electronic resistance, proton resistance, and / or gas diffusion resistance of the catalyst layer increase. Therefore, the amount of catalyst particles supported is preferably 5 mass% or more. The amount is more preferably 10 mass% or more, and even more preferably 15 mass% or more. On the other hand, if the amount of catalyst particles loaded is excessive, the catalyst particles will aggregate on the surface of the support (conductive particles), which will actually reduce the activity of the electrode catalyst. Therefore, the amount of catalyst particles loaded is preferably 60 mass% or less. The amount is more preferably 50 mass% or less, and even more preferably 40 mass% or less.

[0022] [1.1.3. First ionomer] [A. Materials] The surface of the electrode catalyst is coated with a first ionomer. The first ionomer may coat only the surfaces of the conductive particles, or may coat the surfaces of the catalyst particles in addition to the surfaces of the conductive particles. In order to reduce poisoning of the catalyst particles by the first ionomer, it is preferable that the first ionomer coat only the surfaces of the conductive particles.

[0023] In the present invention, the material of the first ionomer is not particularly limited. Examples of the first ionomer material include: (a) perfluorocarbon sulfonic acid polymers such as Nafion®, Flemion®, Aciplex®, and Aquivion®; (b) Highly oxygen-permeable ionomer etc.

[0024] The first ionomer may consist of any one of these, or may consist of two or more of them. When the first ionomer contains two or more types of ionomers, the first ionomer may be a mixture of two or more types of ionomers, or may be a laminate film of two or more types of ionomers. For example, the first ionomer may be a laminated membrane having a first layer made of a highly oxygen-permeable ionomer formed on the surface of the electrode catalyst and a second layer made of a perfluorocarbon sulfonic acid polymer formed on the surface of the first layer.

[0025] Here, "highly oxygen-permeable ionomer" refers to a polymer compound that contains an acid group and a cyclic structure within its molecular structure. Highly oxygen-permeable ionomers have a high oxygen permeability coefficient due to the cyclic structure within their molecular structure. Therefore, when used as a first ionomer, the oxygen transfer resistance at the interface with the catalyst particles becomes relatively small. In other words, the "highly oxygen-permeable ionomer" refers to an ionomer having an oxygen permeability coefficient higher than that of perfluorocarbon sulfonic acid polymers such as Nafion (registered trademark).

[0026] Generally, the power generation performance of a fuel cell in the high current density region is rate-determined by the diffusion of oxygen to the catalyst particle surface. In contrast, coating the surface of the catalyst particle with a highly oxygen-permeable ionomer improves the oxygen permeability of the catalyst layer, thereby improving the performance of the fuel cell. The molecular structure of the highly oxygen-permeable ionomer is not particularly limited as long as it exhibits relatively low oxygen transfer resistance. In particular, ionomers containing a cyclic structure (aliphatic cyclic structure) in their molecular structure have lower oxygen transfer resistance than ionomers that do not contain a cyclic structure, and are therefore suitable as polymers for coating electrode catalysts.

[0027] Examples of highly oxygen-permeable ionomers include: (a) an electrolyte polymer containing a perfluorocarbon unit having an aliphatic ring structure and an acid group unit having a perfluorosulfonic acid group in a side chain; (b) an electrolyte polymer containing a perfluorocarbon unit having an aliphatic ring structure and an acid group unit having a perfluoroimide in the side chain; (c) an electrolyte polymer containing a unit in which perfluorosulfonic acid is directly bonded to a perfluorocarbon having an aliphatic ring structure; (See References 1-4). [Reference 1] Japanese Patent Application Laid-Open No. 2003-036856 [Reference 2] International Publication No. 2012 / 088166 [Reference 3] JP 2013-216811 A [Reference 4] JP 2006-152249 A

[0028] [B. I1 / C1] "I1 / C1" refers to the ratio of the mass of the first ionomer (I1) to the mass of the conductive particles (C1). The first ionomer is primarily used to exchange protons with the catalyst particles. In order to obtain high power generation performance in a wide range of temperature and humidity conditions, from low temperature and high humidity conditions to high temperature and low humidity conditions, it is preferable that I1 / C1 satisfy certain conditions.

[0029] The first ionomer is primarily responsible for localized proton conduction around the catalyst particles. Therefore, the smaller I1 / C1 is, the better, as long as a minimum level of proton conductivity is ensured. However, if I1 / C1 is too small, it may be difficult to transport protons to the catalyst particle surface. Therefore, I1 / C1 is preferably 0.1 or greater. I1 / C1 is more preferably 0.2 or greater, and even more preferably 0.4 or greater. On the other hand, if I1 / C1 is too large, the porosity around the electrode catalyst becomes too small, which may increase gas diffusion resistance. Therefore, I1 / C1 is preferably 1.2 or less. I1 / C1 is more preferably 1.0 or less, and even more preferably 0.8 or less.

[0030] [C. Ion exchange capacity] In the present invention, the ion exchange capacity of the first ionomer is smaller than the ion exchange capacity of the second ionomer described below. When the ion exchange capacity of the first ionomer that coats the periphery of the electrode catalyst is relatively small, the periphery of the electrode catalyst becomes relatively hydrophobic. As a result, water generated by the electrode reaction is more easily discharged to the outside of the ionomer / catalyst composite, and flooding (blockage of voids by liquid water) is suppressed.

[0031] Here, the difference (IEC2-IEC1) between the ion exchange capacity of the second ionomer (IEC2) and the ion exchange capacity of the first ionomer (IEC1) is defined as "ΔIEC." If ΔIEC is too small, the flooding suppression effect will be insufficient. Therefore, ΔIEC is preferably 0.02 meq / g or more. ΔIEC is more preferably 0.05 meq / g or more, and even more preferably 0.1 meq / g or more. On the other hand, if IEC1 is too small relative to IEC2, the proton resistance of the ionomer / catalyst composite may increase. Furthermore, if IEC2 is too large relative to IEC1, too much water may accumulate around the ionomer / substrate composite, making it difficult to supply the reaction gas around the ionomer / catalyst composite. Therefore, ΔIEC is preferably 0.9 meq / g or less. ΔIEC is more preferably 0.5 meq / g or less, and even more preferably 0.4 meq / g or less.

[0032] [1.2. Ionomer / Substrate Composite] "Ionomer / substrate composite" is Sheet-like base particles; a second ionomer that coats the surface of the base particle; A composite comprising:

[0033] [1.2.1. Base material particles] [A. Materials] In the present invention, the ionomer / substrate composite functions as a proton conductor in the catalyst layer. Therefore, the material of the substrate particles is not particularly limited as long as it does not inhibit proton conduction. The material of the substrate particles may be a conductive material or an insulating material.

[0034] In the present invention, the base particle is a sheet-like particle. The term "sheet-like particle" refers to a flat or thin plate-like particle whose dimensions in the x-axis direction and y-axis direction (in-plane directions of the plate) are greater than its dimension in the z-axis direction (thickness direction of the plate). When an ionomer / substrate composite is used as a proton-conducting material, the amount of voids introduced into the catalyst layer can be controlled over a relatively wide range depending on the shape of the substrate particles. In particular, when sheet-shaped particles are used as the substrate particles, it is possible to improve the proton conductivity in a specific direction (for example, the thickness direction of the catalyst layer).

[0035] Examples of sheet-like base particles include: (a) Sheet-like particles made of conductive materials such as graphene sheets, graphite sheets, and conductive metal oxide nanosheets; (b) Sheet-like particles made of insulating materials such as mica, boron nitride, and silicates; etc. The base particles may consist of any one of these, or may consist of two or more of these.

[0036] Among these, the substrate particle is preferably a graphene sheet and / or a graphite sheet. When a graphene sheet and / or a graphite sheet is used as the substrate particle, high performance is exhibited under both low-temperature, high-humidity conditions and high-temperature, low-humidity conditions. This is thought to be because the use of a graphene sheet and / or a graphite sheet as the substrate introduces a relatively large amount of voids into the catalyst layer.

[0037] [B. Average particle size] The "average particle size of the base material particles" refers to the median diameter (D50 ) The average particle size of the base particles is not particularly limited, and an optimal particle size can be selected depending on the purpose. Generally, if the average particle size of the base particles is too small, the particles may not be bonded well together, resulting in increased proton resistance. Therefore, the average particle size is preferably 0.1 μm or more. The average particle size is more preferably 0.5 μm or more, and even more preferably 1.0 μm or more. On the other hand, if the average particle size of the substrate particles is too large, the uniformity of the catalyst layer is lost and, in some cases, the electrolyte membrane may be damaged during pressing. Therefore, the average particle size is preferably 50 μm or less. The average particle size is more preferably 10 μm or less, and even more preferably 5 μm or less.

[0038] [1.2.2. Second Ionomer] [A. Materials] The surface of the base particle is coated with a second ionomer. In the present invention, the material of the second ionomer is not particularly limited. Examples of the second ionomer material include: (a) perfluorocarbon sulfonic acid polymers such as Nafion®, Flemion®, Aciplex®, and Aquivion®; (b) Highly oxygen-permeable ionomer etc. The second ionomer may be the same material as the first ionomer, or may be a different material. Other aspects of the second ionomer are the same as those of the first ionomer, so a description thereof will be omitted.

[0039] [B. I2 / C2] "I2 / C2" refers to the ratio of the mass of the second ionomer (I2) to the mass of the base particle (C2). The second ionomer is primarily used to transport protons in the thickness direction of the catalyst layer. In order to obtain high power generation performance in a wide range of temperature and humidity conditions, from low temperature and high humidity to high temperature and low humidity, it is preferable that I2 / C2 satisfy certain conditions.

[0040] The second ionomer is primarily responsible for proton conduction in the thickness direction of the catalyst layer. To reduce the proton resistance in the thickness direction of the catalyst layer, I2 / C2 is preferably 0.5 or more. I2 / C2 is more preferably 1.0 or more, and even more preferably 2.0 or more. On the other hand, even if I2 / C2 is made larger than necessary, there is no difference in the effect and it is of no practical benefit. Therefore, I2 / C2 is preferably 10.0 or less. I2 / C2 is more preferably 7.5 or less, and even more preferably 5.0 or less.

[0041] [C. Ion exchange capacity] The ion exchange capacity of the second ionomer is greater than that of the first ionomer. When the ion exchange capacity of the second ionomer is relatively large, water tends to be present around the ionomer / substrate composite. As a result, flooding is more likely to be suppressed. Other aspects of the ion exchange capacity are as described above, and therefore will not be described here.

[0042] 1.3. Composition and structure of catalyst layer 1.3.1. Ionomer / catalyst complex content The "content (X) of the ionomer / catalyst composite" refers to the value represented by the following formula (1). X (mass%) = W1 × 100 / W0…(1) however, W0 is the total mass of the ionomer / catalyst composite and the ionomer substrate composite contained in the catalyst layer, W1 is the mass of the ionomer / catalyst composite contained in the catalyst layer.

[0043] In the present invention, the value of X is not particularly limited, and an optimum value can be selected depending on the purpose. Generally, if X is too small, the thickness of the catalyst layer required to obtain a certain catalyst particle basis weight becomes large, and gas diffusion resistance may increase. Therefore, X is preferably 10 mass% or more. X is more preferably 20 mass% or more, and even more preferably 30 mass% or more.

[0044] On the other hand, if X is too large, the porosity in the catalyst layer decreases, which may increase gas diffusion resistance. Therefore, X is preferably 95 mass% or less. X is more preferably 80 mass% or less, and even more preferably 60 mass% or less.

[0045] [1.3.2. Porosity] "Porosity" refers to a value calculated by the following formula (2). Porosity=(V-V0) / V…(2) however, V is the apparent volume of the catalyst layer. V0 is the true volume of the catalyst layer (the volume when the voids in the catalyst layer are assumed to be zero).

[0046] The porosity affects the gas diffusion resistance of the catalyst layer. To reduce the gas diffusion resistance of the catalyst layer, the porosity is preferably 0.57 or more. The porosity is more preferably 0.64 or more, and even more preferably 0.67 or more. On the other hand, excessive porosity may result in poor bonding between conductive particles and / or between ionomers, resulting in increased electronic resistance and / or proton resistance. Therefore, the porosity is preferably 0.8 or less. The porosity is more preferably 0.75 or less, and even more preferably 0.7 or less.

[0047] The catalyst layer according to the present invention includes an ionomer / catalyst composite and an ionomer / substrate composite, and therefore a relatively large amount of voids is likely to be formed around the electrode catalyst, resulting in a lower gas diffusion resistance in the catalyst layer according to the present invention than in conventional catalyst layers.

[0048] [2. Manufacturing method of catalyst layer] The catalyst layer according to the present invention comprises: (a) preparing an ionomer / catalyst composite; (b) preparing an ionomer / substrate composite; (c) A mixed powder containing an ionomer / catalyst composite and an ionomer / substrate composite is applied to the surface of the substrate using a dry coating method. This is obtained by:

[0049] [2.1. 1st step] First, an ionomer / catalyst composite is prepared (first step). The ionomer / catalyst composite specifically comprises: (a) dissolving or dispersing an electrode catalyst and a first ionomer in a solvent; (b) Drying the dispersion This is obtained by:

[0050] The conditions of the dispersion liquid, such as the type of solvent and the concentration of the dispersion liquid, are not particularly limited, and the optimum conditions can be selected depending on the purpose. The method for drying the dispersion is not particularly limited, and an optimum method can be selected depending on the purpose. Examples of the method for drying the dispersion include spray drying and freeze drying.

[0051] [2.2. 2nd process] Next, an ionomer / substrate composite is produced (second step). Specifically, the ionomer / substrate composite comprises: (a) dissolving or dispersing the base particles and the second ionomer in a solvent; (b) Drying the dispersion This is obtained by:

[0052] The conditions of the dispersion liquid, such as the type of solvent and the concentration of the dispersion liquid, are not particularly limited, and the optimum conditions can be selected depending on the purpose. The method for drying the dispersion is not particularly limited, and an optimum method can be selected depending on the purpose. Examples of the method for drying the dispersion include spray drying and freeze drying. When the dispersion is dried using the freeze-drying method, if the base particles are sheet-like particles, it is preferable to freeze-dry the dispersion after foaming. When the dispersion is foamed, the sheet-like particles are oriented on the surface of the foam film, allowing the second ionomer to be uniformly coated on the surface of the sheet-like particles.

[0053] [2.3. Third step] Next, a mixed powder containing the ionomer / catalyst composite and the ionomer / substrate composite is applied to the surface of a substrate (e.g., an electrolyte membrane) using a dry coating method (third step), thereby obtaining a catalyst layer according to the present invention.

[0054] Here, the term "dry coating method" refers to a method of forming a coating film using a dry paint that does not contain a solvent. In the present invention, the type of dry coating method is not particularly limited, and various methods can be used depending on the purpose. Examples of dry coating methods include electrostatic screen printing, electrostatic spraying, and fluidized bed dipping.

[0055] [3. Effect] In a catalyst layer containing an ionomer / catalyst composite and an ionomer / substrate composite, if the ion exchange capacity of the first ionomer is smaller than that of the second ionomer, the water repellency around the ionomer / catalyst composite becomes higher than that around the ionomer / substrate composite. Therefore, water is more likely to be present around the ionomer / substrate composite than around the ionomer / catalyst composite. As a result, the reactant gas can reach the surface of the catalyst particles without being obstructed by water. Furthermore, by adding a sheet-like ionomer / substrate composite to the catalyst layer, the proton resistance of the catalyst layer is reduced and the porosity within the catalyst layer is increased. As a result, the gas diffusion resistance of the catalyst layer is reduced, and high performance is demonstrated even under low-temperature and high-humidity conditions. [Example]

[0056] (Example 1, Comparative Examples 1 and 2) 1. Sample Preparation 1.1. Preparation of ionomer (IEC1.3) / catalyst composite 10 g of 29 mass% platinum-supported carbon (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10V30E), 51.3 g of water, 13.2 g of 25.7 mass% ionomer solution (Solvay, D79, IEC=1.3 meq / g), and 2.6 g of 1-propanol were weighed and mixed using a planetary mixer. This mixture was dispersed using a high-pressure homogenizer (Sugino Machine Corporation, Starbast Mini, nozzle diameter: 100 μm, pressure: 75 MPa) to obtain a dispersion. This dispersion was whipped into a cream using a former and transferred to a stainless steel pad coated with polytetrafluoroethylene (PTFE). The foamy dispersion was then placed in a pre-cooled vacuum freeze dryer to freeze. After confirming that the dispersion had cooled to -40°C or below, it was vacuum dried for at least 24 hours. After vacuum drying, the dried material was pulverized in a mill to obtain an ionomer (IEC1.3) / catalyst composite. The I1 / C1 ratio was 0.5.

[0057] 1.2. Preparation of ionomer (IEC1.0) / catalyst composite 10 g of 29 mass% platinum-supported carbon (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10V30E), 50.8 g of water, and 16.3 g of 20.9 mass% ionomer solution (Chemours, D2020, IEC=1.0 meq / g) were weighed and mixed using a planetary mixer. This mixture was dispersed using the high-pressure homogenizer (nozzle diameter: 100 μm, pressure: 75 MPa) to obtain a dispersion. This dispersion was whipped into a cream using a former and transferred to a PTFE-coated stainless steel pad. The foamy dispersion was then placed in a pre-cooled vacuum freeze dryer to freeze. After confirming that the dispersion had cooled to -40°C or below, it was vacuum dried for at least 24 hours. After vacuum drying, the dried product was pulverized in a mill to obtain an ionomer (IEC 1.0) / catalyst composite. The I1 / C1 ratio was 0.5.

[0058] 1.3. Preparation of ionomer (IEC1.3) / graphite sheet composite 1.0 g of graphite (SG-BH8, manufactured by Ito Graphite Industries Co., Ltd.), 44.5 g of water, and 11.7 g of a 25.7 mass% ionomer solution (D79, manufactured by Solvay, IEC=1.3 meq / g) were weighed and mixed using a planetary mixer. This mixture was dispersed using the above-mentioned high-pressure homogenizer (nozzle diameter: 100 μm, pressure: 245 MPa) to cleave the graphite. This resulted in a dispersion containing dispersed graphite sheets. This dispersion was whipped into a cream using a former and transferred to a PTFE-coated stainless steel pad. The foamy dispersion was then placed in a pre-cooled vacuum freeze dryer to freeze. After confirming that the dispersion temperature had dropped below -40°C, it was vacuum dried for at least 24 hours. After vacuum drying, the dried material was pulverized in a mill to obtain an ionomer (IEC 1.3) / graphite sheet composite. The I2 / C2 ratio was 3.0.

[0059] 1.4. Preparation of Ionomer (IEC1.3) / Carbon Black Composite 2.0 g of carbon black (Vulcan® XC-72, manufactured by Cabot Corporation), 134.7 g of water, and 23.3 g of a 25.7 mass% ionomer solution (D79, manufactured by Solvay, IEC=1.3 meq / g) were weighed and mixed using a planetary mixer. This mixture was dispersed using the above-mentioned high-pressure homogenizer (nozzle diameter: 100 μm, pressure: 150 MPa) to obtain a dispersion. This dispersion was treated with a spray dryer (Buchi Mini Spray Dryer B290, air supply, inlet temperature: 220°C, liquid flow rate: 5 mL / min) to obtain an ionomer (IEC 1.3) / carbon black composite. The I2 / C2 ratio was 3.0.

[0060] 1.5. Fabrication of catalyst layer and membrane electrode assembly 1.5.1. Example 1 An ionomer (IEC 1.0) / catalyst composite and an ionomer (IEC 1.3) / graphite sheet composite were mixed in a mass ratio of 2:1. This mixed powder was applied to one side of an electrolyte membrane (NR211, manufactured by Chemours) using an electrostatic screen method to form a 10 mm x 10 mm cathode catalyst layer. The platinum weight was 0.15 mg. Pt / cm 2 It was decided. Furthermore, a typical catalyst sheet made of platinum-supported carbon and Nafion (registered trademark) was thermally transferred onto the other surface of the electrolyte membrane to form an anode catalyst layer, thereby obtaining a membrane electrode assembly.

[0061] 1.5.2. Comparative Example 1 The ionomer (IEC1.3) / catalyst composite and the ionomer (IEC1.3) / graphite sheet composite were mixed at a mass ratio of 2: 1. Thereafter, a cathode catalyst layer and a membrane electrode assembly were produced in the same manner as in Example 1.

[0062] 1.5.3. Comparative Example 2 An ionomer (IEC 1.0) / catalyst composite and an ionomer (IEC 1.3) / carbon black composite were mixed at a mass ratio of 2: 1. Thereafter, a cathode catalyst layer and a membrane electrode assembly were produced in the same manner as in Example 1.

[0063] 2. Test Method The fabricated membrane electrode assembly was assembled into a small cell and subjected to a power generation test under the following test conditions: cell temperature: 60°C, bubbler temperature: 55°C, back pressure: 50 kPa, hydrogen flow rate: 0.5 L / min, and air flow rate: 2 L / min.

[0064] [3. Results] 1 shows the results of a power generation test (current density at 60° C. and 80% RH) of the fuel cells obtained in Example 1 and Comparative Examples 1 and 2. The following can be seen from FIG.

[0065] (1) In the catalyst layer of Comparative Example 1, the IEC of the first ionomer and the IEC of the second ionomer were the same. In Comparative Example 1, a slight tendency toward flooding was observed. (2) The catalyst layer of Example 1 was constructed by increasing the water repellency around the electrode catalyst by lowering the IEC of the first ionomer compared to the IEC of the second ionomer. Example 1 had a higher current density than Comparative Example 1. This is thought to be because the water repellency of the ionomer (IEC 1.0) / catalyst composite was greater than that of the ionomer (IEC 1.3) / graphite sheet composite, which made it easier for water to move from the vicinity of the electrode catalyst to the vicinity of the ionomer (IEC 1.3) / graphite sheet composite, thereby suppressing flooding.

[0066] (3) The catalyst layer of Comparative Example 2 used the same type of ionomer as in Example 1, but the base particles were carbon black instead of sheet-like particles. Comparative Example 2 had a lower current density than Comparative Example 1. This result indicates that it is effective to use sheet-like particles as the base particles of the ionomer / substrate composite. The reason why the current density improved when sheet-like particles were used as the base particles is thought to be because the use of sheet-like particles increased the amount of voids in the catalyst layer.

[0067] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0068] The catalyst layer according to the present invention can be used as a catalyst layer on the cathode or anode side of a polymer electrolyte fuel cell or a water electrolysis device.

Claims

1. A catalyst layer comprising: (1) The catalyst layer is an ionomer / catalyst composite; Ionomer / substrate composite and It is equipped with: (2) The ionomer / catalyst composite is an electrode catalyst in which catalyst particles are supported on the surfaces of conductive particles; a first ionomer that coats at least the surfaces of the conductive particles; It is equipped with: (3) The ionomer / substrate composite is Sheet-like base particles; a second ionomer that coats the surface of the base particle; It is equipped with: (4) The ion exchange capacity of the first ionomer is smaller than the ion exchange capacity of the second ionomer.

2. The catalyst layer according to claim 1 , wherein the conductive particles include carbon black, porous carbon, and / or conductive oxide particles.

3. The catalyst layer according to claim 1 or 2, wherein the substrate particles include graphene sheets and / or graphite sheets.

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