Catalyst layer, electrolyte membrane-electrode assembly, fuel cell, and method for manufacturing catalyst layer

Abstract

A catalyst layer according to embodiment 1 comprises a conductive mesoporous material (12), and a catalyst (14) supported within at least the pores of the mesoporous material (12). In addition, the catalyst layer comprises: non-catalyst-supporting fibers (11) that do not support the catalyst (14); and a hydrogen ion conductive ionomer (13) covering at least a portion of the surfaces of the mesoporous material (12) and the non-catalyst-supporting fibers (11). The hydrophobicity of the non-catalyst-supporting fibers (11) is higher than that of the mesoporous material (12), and the ionomer (13) comprises a hydrophobic main chain and hydrophilic side chains. The ratio of the surface area of the portion coated with the ionomer (13) in the non-catalyst-supporting fibers (11) to the total surface area of the non-catalyst-supporting fibers (11) is greater than the ratio of the surface area of the portion coated with the ionomer (13) in the mesoporous material (12) to the total surface area of the mesoporous material (12).

Description

[Technical Field]

[0001] The present disclosure relates to a catalyst layer, an electrolyte membrane-electrode assembly using the catalyst layer, a fuel cell using the electrolyte membrane-electrode assembly, and a method for manufacturing the catalyst layer. [Background technology]

[0002] For example, Patent Document 1 discloses a catalyst layer in which a mesoporous material having a catalyst supported in the pores is coated with an ionomer. This catalyst layer includes a mesoporous material, a catalyst supported at least in the pores of the mesoporous material, non-catalyst-supported carbon, and an ionomer that coats them. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-181838 Summary of the Invention

[0004] The present disclosure provides a catalyst layer, an electrolyte membrane-electrode assembly, a fuel cell, and a method for manufacturing a catalyst layer that improves hydrogen ion conductivity while suppressing catalyst poisoning by ionomers, thereby enabling relatively highly efficient catalytic reactions.

[0005] The catalyst layer in the present disclosure is a catalyst layer for use in an electrolyte membrane-electrode assembly, and comprises a conductive mesoporous material and a catalyst supported at least in the pores of the mesoporous material. The catalyst layer also comprises catalyst-free fibers that do not support the catalyst, and a proton-conductive ionomer that coats at least a portion of the surfaces of the mesoporous material and the catalyst-free fibers. The catalyst-free fibers have a higher hydrophobicity than the mesoporous material, and the ionomer has a hydrophobic main chain and hydrophilic side chains. The ratio of the surface area of ​​the ionomer-coated portion of the catalyst-free fibers to the total surface area of ​​the catalyst-free fibers is greater than the ratio of the surface area of ​​the ionomer-coated portion of the mesoporous material to the total surface area of ​​the mesoporous material.

[0006] The electrolyte membrane-electrode assembly according to the present disclosure includes an electrolyte membrane, an anode provided on one main surface of the electrolyte membrane, and a cathode provided on the other main surface of the electrolyte membrane. The cathode includes the above-described catalyst layer.

[0007] Furthermore, a fuel cell according to the present disclosure includes the above-described electrolyte membrane-electrode assembly.

[0008] The present disclosure also provides a method for producing a catalyst layer for use in an electrolyte membrane-electrode assembly. The method includes a first step of mixing and dispersing a proton-conductive ionomer having a hydrophobic main chain and hydrophilic side chains, non-catalyst-supported fibers, and a hydrophilic solvent. The method also includes a second step of adding and mixing a mesoporous material having a catalyst supported in its pores to the mixture obtained in the first step to obtain an ink. The non-catalyst-supported fibers have a higher hydrophobicity than the mesoporous material.

[0009] The catalyst layer, electrolyte membrane-electrode assembly, fuel cell, and method for manufacturing a catalyst layer according to the present disclosure can improve hydrogen ion conductivity while suppressing catalyst poisoning by ionomers, thereby enabling a relatively highly efficient catalytic reaction. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell according to a first embodiment. [Figure 2] FIG. 2 is a schematic enlarged cross-sectional view of the cathode catalyst layer in the first embodiment. [Figure 3] FIG. 3 is a flowchart of a method for manufacturing a cathode catalyst layer according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] (Findings that formed the basis of this disclosure) At the time the inventors arrived at this disclosure, it was believed that the technology for forming a three-phase interface in the catalyst layer of a fuel cell would improve performance by bringing the catalyst into contact with an ionomer from the perspective of hydrogen ion supply. However, in recent years, it has become clear that contact between the ionomer and the catalyst poisons the catalyst, resulting in a decrease in performance.

[0012] Therefore, in order to address the issue of preventing catalyst poisoning by ionomers, the industry has generally designed products in such a way that the catalyst is encapsulated in a carbon support with large pores, such as a mesoporous material, to prevent contact between the ionomer and the catalyst.

[0013] However, the catalyst near the entrances to the pores of the mesoporous material comes into contact with the ionomer, and catalyst poisoning by the ionomer cannot be sufficiently suppressed, resulting in the following conflicting issues: Increasing the amount of ionomer increases ionomer catalyst poisoning, while decreasing the amount of ionomer causes the ionomer to become discontinuous within the catalyst layer, increasing the hydrogen ion resistance of the electrolyte membrane-electrode assembly.

[0014] Under these circumstances, the inventors came up with the idea of ​​connecting aggregates of ionomer and mesoporous material with non-catalyst-supported fibers coated with ionomer to solve these conflicting problems. The inventors came to the conclusion that this would improve the hydrogen ion conductivity in the catalyst layer and, at the same time, reduce the amount of ionomer adsorbed on the catalyst support by adsorbing the ionomer onto the non-catalyst-supported fibers.

[0015] The inventors then took a hint from the idea that changing the solvent polarity in the catalyst ink causes the hydrophilic groups of the ionomer to orient outward, and discovered that the following problem exists in realizing this idea. Specifically, the inventors discovered that the problem is to control the polarity of the solvent and the fiber in the catalyst ink for forming the catalyst layer in order to increase the adsorptive force between the catalyst-unsupported fiber and the ionomer compared to the adsorptive force between the ionomer and the catalyst support. The inventors then came up with the subject matter of the present disclosure in order to solve this problem.

[0016] Therefore, the present disclosure provides a catalyst layer, an electrolyte membrane-electrode assembly, a fuel cell, and a method for manufacturing a catalyst layer that can improve hydrogen ion conductivity and enable a relatively highly efficient catalytic reaction without causing a decrease in catalytic activity due to ionomer poisoning. According to the catalyst layer, the electrolyte membrane-electrode assembly, the fuel cell, and the method for manufacturing a catalyst layer of the present disclosure, the aggregates of the ionomer and the mesoporous material formed in the catalyst layer are connected by the ionomer adsorbed on the catalyst-unsupported fibers, thereby improving hydrogen ion conductivity in the catalyst layer. Therefore, the catalyst layer, the electrolyte membrane-electrode assembly, the fuel cell, and the method for manufacturing a catalyst layer of the present disclosure can improve hydrogen ion conductivity and enable a highly efficient catalytic reaction without causing a decrease in catalytic activity due to ionomer poisoning.

[0017] Hereinafter, embodiments will be described in detail with reference to the drawings. However, unnecessary detailed description may be omitted. For example, detailed description of well-known matters or redundant description of substantially the same configuration may be omitted. This is to avoid unnecessary redundancy in the following description and to facilitate understanding by those skilled in the art.

[0018] The accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the present disclosure, and are not intended to limit the subject matter described in the claims.

[0019] (Embodiment 1) Hereinafter, a first embodiment of the present disclosure will be described with reference to FIGS. 1 to 3. FIG.

[0020] [1-1.Configuration] (fuel cell) Fig. 1 is a schematic diagram showing an example of the configuration of a fuel cell 10, which is a basic unit of a fuel cell. The fuel cell 10 shown in Fig. 1 is a solid polymer electrolyte fuel cell that generates electricity by receiving supplies of hydrogen and oxygen.

[0021] 1, the fuel cell 10 has an anode separator 6a and a cathode separator 6c. The fuel cell 10 also has an electrolyte membrane-electrode assembly 5 provided between the anode separator 6a and the cathode separator 6c (sandwiched between the anode separator 6a and the cathode separator 6c).

[0022] The anode separator 6a has a flow path that allows hydrogen supplied to the fuel cell 10 to flow to the anode (fuel electrode) 2a of the electrolyte membrane-electrode assembly 5. The cathode separator 6c has a flow path that allows oxygen supplied to the fuel cell 10 to flow to the cathode (air electrode) 2c of the electrolyte membrane-electrode assembly 5.

[0023] When a plurality of fuel cells 10 are stacked in the thickness direction, they are connected as follows: That is, the surface of the anode separator 6a opposite to the surface on which the flow channels are formed is electrically connected to the surface of the cathode separator 6c of the fuel cell 10 adjacent to this anode separator 6a opposite to the surface on which the flow channels are formed. Also, the surface of the cathode separator 6c opposite to the surface on which the flow channels are formed is electrically connected to the surface of the anode separator 6a of the fuel cell 10 adjacent to this cathode separator 6c opposite to the surface on which the flow channels are formed.

[0024] (Electrolyte membrane-electrode assembly) 1, the electrolyte membrane-electrode assembly 5 includes an electrolyte membrane 1, an anode (fuel electrode) 2a including an anode catalyst layer 3a and an anode gas diffusion layer 4a, and a cathode (air electrode) 2c including a cathode catalyst layer 3c and a cathode gas diffusion layer 4c. The electrolyte membrane-electrode assembly 5 is configured such that the electrolyte membrane 1 is sandwiched between the anode 2a and the cathode 2c on both sides. The cathode catalyst layer 3c corresponds to an example of a catalyst layer of the present disclosure.

[0025] The anode catalyst layer 3a is disposed on one main surface of the electrolyte membrane 1, and the cathode catalyst layer 3c is disposed on the other main surface of the electrolyte membrane 1. The anode gas diffusion layer 4a is disposed on the side of the anode catalyst layer 3a opposite to the electrolyte membrane 1 side, and the cathode gas diffusion layer 4c is disposed on the side of the cathode catalyst layer 3c opposite to the electrolyte membrane 1 side.

[0026] An anode separator 6a is disposed on the anode gas diffusion layer 4a opposite to the anode catalyst layer 3a side, and a cathode separator 6c is disposed on the cathode gas diffusion layer 4c opposite to the cathode catalyst layer 3c side.

[0027] (electrolyte membrane) The electrolyte membrane 1 is responsible for ion (hydrogen ion) conduction between the anode 2a and the cathode 2c, and is required to have both hydrogen ion conductivity and gas barrier properties. In this embodiment, a perfluorosulfonic acid resin membrane is used as an example of the material for the electrolyte membrane 1. This membrane is preferred because it has high hydrogen ion conductivity and remains stable even in the power generation environment of the fuel cell.

[0028] (gas diffusion layer) The anode gas diffusion layer 4a and the cathode gas diffusion layer 4c are layers that have current collecting properties, gas permeability, and water repellency. Each of the anode gas diffusion layer 4a and the cathode gas diffusion layer 4c may include two layers: a substrate and a coating layer.

[0029] The substrate may be made of any material that has excellent electrical conductivity and gas and liquid permeability, and in this embodiment, carbon paper is used as the substrate material, for example.

[0030] The coating layer is interposed between the catalyst layer (anode catalyst layer 3a, cathode catalyst layer 3c) and the substrate to reduce the contact resistance between the catalyst layer and the substrate and improve liquid permeability (drainage).

[0031] The coating layer of this embodiment is formed mainly from a conductive material such as carbon black and a water-repellent resin such as polytetrafluoroethylene (PTFE).

[0032] (catalyst layer) The anode catalyst layer 3a is a layer that promotes the electrochemical reaction in the anode 2a, and the cathode catalyst layer 3c is a layer that promotes the electrochemical reaction in the cathode 2c.

[0033] The anode catalyst layer 3a contains a catalyst support material, a catalyst, and an ionomer (hydrogen ion conductive resin), and the catalyst is supported on at least the catalyst support material.

[0034] 2 is an enlarged cross-sectional view showing a schematic configuration of the cathode catalyst layer 3c. As shown in FIG. 2, the cathode catalyst layer 3c includes a mesoporous material 12, a catalyst 14, catalyst-unsupported fibers 11 that do not support the catalyst 14, and an ionomer 13.

[0035] The ionomer 13 is hydrogen ion conductive and coats at least a portion of the surfaces of the mesoporous material 12 and the catalyst-unsupported fibers 11. The ionomer 13 has a hydrophobic main chain and hydrophilic side chains. The catalyst 14 is supported at least in the pores of the mesoporous material 12. The hydrophobicity of the catalyst-unsupported fibers 11 is higher than that of the mesoporous material 12.

[0036] The ratio of the surface area of ​​the portion of the catalyst-unsupported fiber 11 that is coated with the ionomer 13 to the total surface area of ​​the catalyst-unsupported fiber 11 is greater than the ratio of the surface area of ​​the portion of the mesoporous material 12 that is coated with the ionomer 13 to the total surface area of ​​the mesoporous material 12.

[0037] (catalyst) In the present embodiment, the anode catalyst layer 3a also uses, as an example, a catalyst 14. There are no particular limitations on the material of the catalyst 14 as long as it has catalytic activity for hydrogen or oxygen, but from the viewpoint of improving catalytic activity, resistance to poisoning by carbon monoxide and the like, heat resistance, and the like, platinum, or a mixture or alloy containing platinum is preferred.

[0038] In this embodiment, the catalyst 14 is made of, for example, an alloy containing 30 to 90 atomic % of platinum and 10 to 70 atomic % of metals other than platinum.

[0039] The average particle size of catalyst 14 is not particularly limited, but from the viewpoint of improving the catalyst utilization rate and the supportability of the catalyst support material, the average particle size of catalyst 14 is preferably 1 to 30 nm. The average particle size of catalyst 14 is preferably a size that can fit into the pores of mesoporous material 12.

[0040] (Catalyst support material) In this embodiment, a mesoporous material 12, which is a type of porous carbon having interconnected pores, is used as an example of a catalyst support material used in the anode catalyst layer 3a and the cathode catalyst layer 3c.

[0041] The mesoporous material 12 is preferably a conductive material having pores with a diameter of 2 to 50 nm (mode radius of 1 to 25 nm) inside, and porous carbon is suitable for this purpose. Examples of porous carbon include carbon black such as Ketjen Black (registered trademark), acetylene black, and Vulcan (registered trademark), and carbon having a structure in which multiple graphene sheets are stacked to form mesoporous pores.

[0042] The mesoporous material 12 used in the cathode catalyst layer 3c of this embodiment has a pore mode radius of 10 nm and a pore volume of 2.0 cm before the catalyst 14 is supported in the pores. 3 / g. The mesoporous material 12 is configured to have a mode diameter of 500 nm.

[0043] (Non-catalyst-supported fiber) The material of the catalyst-unsupported fiber 11 may be carbon fiber, carbon nanotube, vapor grown carbon fiber (VGCF (registered trademark)), glass fiber, or various fibers that have been treated to be water-repellent, and which has a higher hydrophobicity than the mesoporous material 12.

[0044] In this embodiment, for example, vapor grown carbon fibers (VGCF-H manufactured by Showa Denko KK) having an average fiber diameter of 150 nm and an average fiber length of 6 μm are used as the catalyst-non-supporting fibers 11.

[0045] For example, when catalyst 14 is supported on carbon black, the hydrophobicity of the carbon black supporting catalyst 14 decreases because the hydrophilic catalyst 14 is supported on the water-repellent carbon black. Therefore, carbon black not supporting catalyst 14 becomes more hydrophobic than carbon black supporting catalyst 14. Therefore, carbon fiber that has not been subjected to hydrophilic treatment is more hydrophobic than carbon black supporting catalyst 14.

[0046] The fiber length of the catalyst-non-supporting fibers 11 is not particularly limited, but is preferably greater than 0.22 times the mode diameter of the mesoporous material 12 and equal to or less than the aggregate particle diameter of the mesoporous material 12 .

[0047] The aggregate particle size of the mesoporous material 12 can be confirmed by measuring the particle size distribution of a catalyst ink prepared by mixing and dispersing the catalyst layer material in a solvent. In this embodiment, the particle size distribution of the catalyst ink has a top peak particle size of 3 μm for aggregate particles and a particle size of 90% of the cumulative distribution of 6 μm.

[0048] Since the mode diameter of the mesoporous material 12 in this embodiment is 500 nm, the size of the aggregate particle diameter of the mesoporous material 12 is 12 times the mode diameter of the mesoporous material 12. Therefore, it is found that the fiber length of the catalyst-unsupported fiber 11 is preferably 12 times or less the mode diameter of the mesoporous material 12.

[0049] When spheres a of radius r are hexagonally close packed, connecting the centers of four adjacent spheres a results in a regular tetrahedron with a side length of 2r. Since the distance between the center of gravity of a regular tetrahedron and its vertices is calculated to be approximately 1.22r, the largest radius of a sphere that can be placed in the gap between four adjacent spheres a of radius r when spheres a are hexagonally close packed is approximately 0.22r.

[0050] Therefore, in order to connect the particles of the mesoporous material 12 with the catalyst-non-supporting fibers 11 , it is preferable that the fiber length of the catalyst-non-supporting fibers 11 is longer than 0.22 times the mode diameter of the mesoporous material 12 .

[0051] On the other hand, if the fiber length of the catalyst-free fibers 11 is longer than the aggregate particle size of the mesoporous material 12, the cathode catalyst layer 3c becomes sparse, the thickness of the cathode catalyst layer 3c increases, the diffusion path of hydrogen ions in the cathode catalyst layer 3c becomes longer, and the hydrogen ion conductivity decreases. Therefore, it is preferable that the fiber length of the catalyst-free fibers 11 be equal to or shorter than the aggregate particle size of the mesoporous material 12.

[0052] Therefore, the fiber length of the catalyst-unsupported fibers 11 is preferably greater than 0.22 times the mode diameter of the mesoporous material 12 and less than or equal to the aggregate particle diameter of the mesoporous material 12 (12 times the mode diameter of the mesoporous material 12).

[0053] Furthermore, if the catalyst-non-supporting fibers 11 are conductive, the contact points between the mesoporous material 12 and the catalyst-non-supporting fibers 11 become conductive contact points. This increases the number of conductive contact points in the cathode catalyst layer 3c, which not only improves the hydrogen ion conductivity as described above, but also improves the conductivity of the cathode catalyst layer 3c.

[0054] (ionomer) In this embodiment, the anode catalyst layer 3a also uses, as an example, an ionomer 13. The ionomer 13 that can be used in the anode catalyst layer 3a and the cathode catalyst layer 3c is preferably a polymer having a hydrophilic functional group (side chain) and a hydrophobic main chain. Examples of the ionomer 13 include ion-exchange polymers such as fluorine-based polymers having acidic functional groups and hydrocarbon-based polymers having acidic functional groups.

[0055] The ionomer 13 has a polymer main chain having a repeating structure and an acidic functional group. The acidic functional group is not particularly limited as long as it is a functional group that exhibits acidity, and examples thereof include a sulfonic acid group, a phosphoric acid group, and a carboxylic acid group. Among these, a hydrogen ion (H + ) From the viewpoint of conductivity, sulfonic acid groups are preferred.

[0056] A proposed mechanism for hydrogen ion conduction by ionomer 13 is that a hydrophilic core is formed by the acidic functional groups, a network of clusters in which water molecules are localized is formed within the core, and hydrogen ions move through this hydrophilic network.

[0057] Examples of fluorine-based polymers having acidic functional groups include perfluorosulfonic acid polymers, which have polytetrafluoroethylene (PTFE) units and perfluorosulfonic acid units, with the sulfonic acid being hydrophilic and the main chain made of polytetrafluoroethylene being hydrophobic.

[0058] In this embodiment, perfluorosulfonic acid polymer is used as the ionomer 13 in the anode catalyst layer 3a and the cathode catalyst layer 3c, for example.

[0059] Commercially available perfluorosulfonic acid polymers that can be used for the ionomer 13 include, for example, Nafion (registered trademark) manufactured by DuPont, Aquivion (registered trademark) manufactured by Solvay, Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., and Aciplex (registered trademark) manufactured by Asahi Kasei Corporation.

[0060] (Method of manufacturing catalyst layer) The cathode catalyst layer 3c in this embodiment is produced by the following steps.

[0061] FIG. 3 is a flow chart that schematically shows a method for manufacturing the cathode catalyst layer 3c.

[0062] (First step of the method for producing the catalyst layer) The first step is to adsorb ionomer 13 onto catalyst-unsupported fibers 11 in a hydrophilic solvent, i.e., to mix and disperse ionomer 13, catalyst-unsupported fibers 11, and a hydrophilic solvent. First, it is necessary to determine the solvent ratio that will result in a hydrophilic solvent.

[0063] In the first step, water 16, ethanol 17, and an ionomer solution 15 dispersed in water and ethanol are mixed to a composition of 85% water, 15% ethanol, and 3 wt% ionomer, and non-catalyst-supported fibers 11 are added and dispersed. Dispersion can be performed using a ball mill, a wet atomizer, ultrasonic waves, or the like.

[0064] The solvent ratio of water 16 and ethanol 17 as the hydrophilic solvent is selected by measuring the particle size distribution of ionomer 13 dispersed in a solvent with a certain solvent ratio using an ultrasonic particle size distribution analyzer. Specifically, it is preferable to select a solvent ratio that, when the proportion of water 16 is gradually increased from 50%, results in a mode diameter smaller than the mode diameter of ionomer 13 when the water proportion is 50%.

[0065] Ionomer 13 has hydrophilic sulfonic acid groups (side chains) and a hydrophobic main chain. When ionomer 13 is placed in a hydrophilic solvent, the main chain of ionomer 13 generates a repulsive force with the water and aggregates, and ionomer 13 takes a shape with the hydrophilic sulfonic acid groups (side chains) facing outward.

[0066] As a result, the peak particle size decreases in the particle size distribution of the ionomer 13. By selecting a solvent ratio that causes this change in shape to reduce the peak particle size in the particle size distribution, it is possible to create an environment in the ink where the repulsion between the hydrophobic main chain of the ionomer 13 and water is strong.

[0067] Water 16, ethanol 17, and ionomer solution 15 are added to achieve this hydrophilic solvent ratio, and then catalyst-free fibers 11 are added and dispersed (step S01). As a result, the highly hydrophobic catalyst-free fibers 11 generate a repulsive force with water, making them relatively more likely to adsorb the hydrophobic main chain of ionomer 13. The hydrophobic main chain of ionomer 13 is adsorbed around the catalyst-free fibers 11, and the hydrophilic sulfonic acid groups (side chains) of ionomer 13 are shaped to face outward, resulting in aggregates of catalyst-free fibers 11 and ionomer 13 being dispersed in the hydrophilic solvent.

[0068] (Second step of the method for producing the catalyst layer) The second step is a step of adding the mesoporous material 12 having the catalyst 14 supported in the pores to the mixed dispersion liquid prepared in the first step and mixing and dispersing them (step S02). That is, the second step is a step of adding and mixing the mesoporous material 12 having the catalyst 14 supported in the pores to the mixture prepared in the first step to obtain an ink. The dispersion method can be a ball mill, a wet atomization device, or ultrasonic waves.

[0069] The mesoporous material 12 having the catalyst 14 supported in the pores has lower hydrophobicity than the catalyst-unsupported fiber 11 due to the catalyst 14 supported in the pores.

[0070] Therefore, the repulsive force between the solvent and mesoporous material 12 in the hydrophilic solvent is weaker than the repulsive force between the solvent and non-catalyst-supported fibers 11, and the attractive force between the non-catalyst-supported fibers 11 and the hydrophobic main chain of the ionomer 13 is stronger than that between the mesoporous material 12. Therefore, the already formed adsorption between the non-catalyst-supported fibers 11 and the hydrophobic main chain of the ionomer 13 does not come off, and the mesoporous material 12 is dispersed.

[0071] (Third step of the method for producing the catalyst layer) The third step is to apply the ink prepared in the second step (step S03) and dry it to form the cathode catalyst layer 3c (step S04). The ink prepared in the second step may be applied directly to one main surface of the electrolyte membrane 1 and dried to form the cathode catalyst layer 3c on one main surface of the electrolyte membrane 1. Alternatively, the ink prepared in the second step may be applied to a substrate and dried, and the cathode catalyst layer 3c formed on the substrate may be transferred to the electrolyte membrane 1 to form the cathode catalyst layer 3c.

[0072] As a result, the following cathode catalyst layer 3c is obtained. That is, the obtained cathode catalyst layer 3c contains catalyst-free fibers 11 coated with ionomer 13, the ionomer 13, and mesoporous material 12 having catalyst 14 supported in the pores, in a dispersed or aggregated state. The obtained cathode catalyst layer 3c also has a structure in which the ratio of the surface area of ​​the portion of catalyst-free fibers 11 coated with ionomer 13 to the total surface area of ​​catalyst-free fibers 11 is greater than the ratio of the surface area of ​​the portion of mesoporous material 12 coated with ionomer 13 to the total surface area of ​​mesoporous material 12.

[0073] [1-2. Operation] The operation and function of the fuel cell 10 configured as above will now be described.

[0074] 1 and 2, the operation method and action of the fuel cell 10 will be described. The fuel cell 10 is operated as follows. That is, with the temperature of the fuel cell 10 maintained at 60°C, hydrogen with a dew point of 60°C is supplied to the flow path of the anode separator 6a at a flow rate of 100 cc / min, and air with a dew point of 60°C is supplied to the cathode separator 6c at a flow rate of 300 cc / min. An electronic load device is then electrically connected between the anode 2a and the cathode 2c.

[0075] Next, we will explain the transfer of substances and electrochemical reactions that occur within the operating fuel cell 10. Hydrogen supplied to the flow path of the anode separator 6a permeates the anode gas diffusion layer 4a and reaches the catalyst 14 of the anode catalyst layer 3a.

[0076] In the catalyst 14 of the anode catalyst layer 3a, hydrogen is converted into hydrogen ions (H + ) and electrons (e - ) and an oxidation reaction occurs in which they dissociate into

[0077] [ka]

[0078] The hydrogen ions (H + The hydrogen ions (H + ) passes through the ionomer 13 or water in the cathode catalyst layer 3c and reaches the catalyst 14 in the cathode catalyst layer 3c.

[0079] On the other hand, electrons (e - ) reaches the catalyst 14 of the cathode catalyst layer 3c as follows. - ) passes through the anode catalyst layer 3a, anode gas diffusion layer 4a, anode separator 6a, electronic load device, cathode separator 6c, cathode gas diffusion layer 4c, and cathode catalyst layer 3c in this order, and reaches the catalyst 14 in the cathode catalyst layer 3c.

[0080] Furthermore, oxygen in the air supplied to the flow path of the cathode separator 6c permeates the cathode gas diffusion layer 4c and reaches the catalyst 14 of the cathode catalyst layer 3c.

[0081] In the catalyst 14 of the cathode catalyst layer 3c, hydrogen ions (H + ) and oxygen (O) and electrons (e - ) combine to produce water in a reduction reaction.

[0082] [ka]

[0083] 2, the cathode catalyst layer 3c in this embodiment has the following structure: In other words, the cathode catalyst layer 3c contains catalyst-free fibers 11 coated with ionomer 13, the ionomer 13, and mesoporous material 12 having catalyst 14 supported in its pores, all present in a dispersed or aggregated state. The ratio of the surface area of ​​the portion of the catalyst-free fibers 11 coated with ionomer 13 to the total surface area of ​​the catalyst-free fibers 11 is greater than the ratio of the surface area of ​​the portion of the mesoporous material 12 coated with ionomer 13 to the total surface area of ​​the mesoporous material 12.

[0084] As a result, the aggregates of ionomer 13 and mesoporous material 12 formed in the cathode catalyst layer 3c are connected by the ionomer 13 coating the catalyst-free fiber 11, thereby improving the hydrogen ion conductivity in the cathode catalyst layer 3c.

[0085] Furthermore, the cathode catalyst layer 3c in this embodiment utilizes non-catalyst-supported fibers 11 coated with ionomer 13 to improve hydrogen ion conductivity within the cathode catalyst layer 3c. In the cathode catalyst layer 3c, the ratio of the surface area of ​​the portion of the mesoporous material 12 coated with ionomer 13 to the total surface area of ​​the mesoporous material 12 is not large, so there is no decrease in catalytic activity due to contact with the ionomer 13. As a result, hydrogen ions are conducted from the nearby ionomer 13 via water to the catalyst 14 inside and outside the mesoporous material 12 that is not in contact with the ionomer 13.

[0086] The water that serves as a path for hydrogen ions in the cathode catalyst layer 3c includes water that wets the surface of the mesoporous material 12 that includes pores.

[0087] This water, in addition to the water resulting from the reduction reaction of Chemical Formula 2, was left in the cathode catalyst layer 3c as follows: That is, prior to the aging step of preliminary operation (run-in power generation), which is called aging after assembly of the fuel cell 10, humidified gas with a dew point higher than the temperature of the fuel cell 10 is supplied to the flow paths of the anode separator 6a and the cathode separator 6c. Then, condensed water obtained by condensing the humidified gas in the electrolyte membrane-electrode assembly 5 and water generated during power generation in the aging step remain in the cathode catalyst layer 3c.

[0088] [1-3. Effects, etc.] As described above, the cathode catalyst layer 3c of the present embodiment is a catalyst layer used in the electrolyte membrane-electrode assembly 5, and comprises an electrically conductive mesoporous material 12 and a catalyst 14 supported at least in the pores of the mesoporous material 12. The cathode catalyst layer 3c also comprises catalyst-free fibers 11 that do not support the catalyst 14, and a proton-conductive ionomer 13 that covers at least a portion of the surfaces of the mesoporous material 12 and the catalyst-free fibers 11.

[0089] The hydrophobicity of the catalyst-unsupported fibers 11 is higher than that of the mesoporous material 12, and the ionomer 13 has a hydrophobic main chain and a hydrophilic side chain (functional group). The ratio of the surface area of ​​the portion of the catalyst-unsupported fibers 11 that is covered with the ionomer 13 to the total surface area of ​​the catalyst-unsupported fibers 11 is greater than the ratio of the surface area of ​​the portion of the mesoporous material 12 that is covered with the ionomer 13 to the total surface area of ​​the mesoporous material 12.

[0090] This allows the aggregates of mesoporous material 12 coated with ionomer 13 present in cathode catalyst layer 3c to be connected by ionomer 13 adsorbed on (coating) non-catalyst-supported fibers 11. As a result, the hydrogen ion conductivity in cathode catalyst layer 3c can be improved.

[0091] Furthermore, in the cathode catalyst layer 3c, the ratio of the surface area of ​​the portion of the mesoporous material 12 that is covered with the ionomer 13 to the total surface area of ​​the mesoporous material 12 is not large. Furthermore, to the catalyst 14 that is not in contact with the ionomer 13 inside and outside the mesoporous material 12, hydrogen ions are conducted from the nearby ionomer 13 using water as a medium, and the poisoning effect due to contact with the ionomer 13 does not increase.

[0092] Therefore, it is possible to obtain a cathode catalyst layer 3c that enables a relatively highly efficient catalytic reaction by improving hydrogen ion conductivity while suppressing a decrease in catalytic activity due to poisoning of the ionomer 13. That is, the cathode catalyst layer 3c of the present embodiment improves hydrogen ion conductivity while suppressing catalyst poisoning by the ionomer 13, thereby enabling a relatively highly efficient catalytic reaction.

[0093] As in this embodiment, in the cathode catalyst layer 3c, the fiber length of the catalyst-unsupported fibers 11 may be greater than 0.22 times the mode diameter of the mesoporous material 12 and less than the agglomerated particle diameter of the mesoporous material 12 (less than 12 times the mode diameter of the mesoporous material 12).

[0094] As a result, since the fiber length of the non-catalyst-supported fibers 11 is longer than the minimum gap of the mesoporous material 12, the ionomer 13 coating the non-catalyst-supported fibers 11 connects the ionomer 13 coating adjacent mesoporous materials 12, increasing the conduction paths for hydrogen ions.

[0095] Furthermore, by making the fiber length of the catalyst-free fibers 11 shorter than the aggregate particle diameter of the mesoporous material 12 (less than 12 times the mode diameter of the mesoporous material 12), a decrease in the density of the cathode catalyst layer 3c can be prevented, and the length of the hydrogen ion conduction path can be made the shortest possible. Therefore, the hydrogen ion conductivity of the cathode catalyst layer 3c can be further improved.

[0096] Furthermore, as in this embodiment, in the cathode catalyst layer 3c, the catalyst-non-supporting fibers 11 may be conductive.

[0097] As a result, the contact points between the mesoporous material 12 and the conductive catalyst-unsupported fibers 11 become conductive contacts, and the number of conductive contacts within the cathode catalyst layer 3c increases, thereby improving the conductivity of the cathode catalyst layer 3c and promoting the catalytic reaction.

[0098] The electrolyte membrane-electrode assembly 5 of the present embodiment includes an electrolyte membrane 1, an anode 2a provided on one main surface of the electrolyte membrane 1, and a cathode 2c provided on the other main surface of the electrolyte membrane 1. The cathode 2c includes a cathode catalyst layer 3c.

[0099] The fuel cell of this embodiment also includes an electrolyte membrane-electrode assembly 5 .

[0100] As a result, the electrolyte membrane-electrode assembly 5 and fuel cell of the present embodiment include the cathode catalyst layer 3c as described above, and therefore can improve hydrogen ion conductivity while suppressing catalyst poisoning by the ionomer 13, thereby enabling a relatively highly efficient catalytic reaction.

[0101] The method for producing the cathode catalyst layer 3c of this embodiment is a method for producing the cathode catalyst layer 3c used in the electrolyte membrane-electrode assembly 5. The method for producing the cathode catalyst layer 3c includes a first step of mixing and dispersing a hydrogen ion conductive ionomer 13 having a hydrophobic main chain and a hydrophilic side chain (functional group), catalyst-free fibers 11 that do not support a catalyst 14, and a hydrophilic solvent. The method for producing the cathode catalyst layer 3c also includes a second step of adding and mixing a mesoporous material 12 that supports the catalyst 14 in its pores to the mixture obtained in the first step to obtain an ink.

[0102] However, the hydrophobicity of the catalyst-unsupported fibers 11 is higher than the hydrophobicity of the mesoporous material 12 .

[0103] As a result, in the cathode catalyst layer 3c of this embodiment, in the first step of mixing and dispersing the ionomer 13, the catalyst-non-supporting fibers 11, and the hydrophilic solvent, the hydrophobic main chain of the ionomer 13 generates a repulsive force with the hydrophilic solvent. Therefore, in the hydrophilic solvent, the ionomer 13 takes a shape in which the hydrophilic functional groups (side chains) face outward and the hydrophobic main chains are aggregated inward.

[0104] When hydrophobic catalyst-free fibers 11 are added and dispersed in this mixture, the hydrophobic catalyst-free fibers 11 generate a repulsive force with the hydrophilic solvent. As a result, the hydrophobic main chains of the ionomer 13 and the catalyst-free fibers 11 attract each other and aggregate, forming a shape in which the hydrophilic functional groups (side chains) of the ionomer 13 face outward.

[0105] Next, in the second step, mesoporous material 12 having catalyst 14 supported in its pores is added to and mixed with the mixture obtained in the first step. The hydrophobicity of mesoporous material 12 having catalyst 14 supported in its pores is lower than that of non-catalyst-supported fibers 11. Therefore, the adsorption force between the hydrophobic main chain of ionomer 13 and mesoporous material 12 having catalyst 14 supported in the hydrophilic solvent is weaker than the adsorption force between the hydrophobic main chain of ionomer 13 and non-catalyst-supported fibers 11. Therefore, the mesoporous material 12 having catalyst 14 supported in its pores is dispersed in the ink without the adsorption between non-catalyst-supported fibers 11 and ionomer 13 being separated.

[0106] In the third step, the ink obtained in the second step is applied and dried to form a cathode catalyst layer 3c. The cathode catalyst layer 3c thus formed has a structure in which a large amount of ionomer 13 is adsorbed onto catalyst-unsupported fibers 11, and mesoporous material 12 with catalyst 14 supported in the pores is present around the ionomer 13.

[0107] The cathode catalyst layer 3c thus prepared in three separate steps has a structure in which aggregates of mesoporous material 12 coated with ionomer 13 present in the cathode catalyst layer 3c are connected to each other by ionomer 13 adsorbed on catalyst-free fibers 11. This enables the cathode catalyst layer 3c to improve the hydrogen ion conductivity within the cathode catalyst layer 3c.

[0108] Furthermore, in the cathode catalyst layer 3c thus prepared, the ratio of the surface area of ​​the portion of the mesoporous material 12 that is covered with the ionomer 13 to the total surface area of ​​the mesoporous material 12 does not become large. To the catalyst 14 that is not in contact with the ionomer 13 inside and outside the mesoporous material 12, hydrogen ions are conducted from the nearby ionomer 13 using water as a medium, and the poisoning effect due to contact with the ionomer 13 does not increase (no decrease in activity occurs).

[0109] As a result, it is possible to obtain, through the relatively simple process of ink preparation and coating, a cathode catalyst layer 3c having a structure in which a large amount of ionomer 13 is adsorbed around the catalyst-non-supporting fibers 11. The obtained cathode catalyst layer 3c can enable a relatively highly efficient catalytic reaction by achieving both improved hydrogen ion conductivity and suppressed decline in catalytic activity.

[0110] (Other embodiments) As described above, the first embodiment has been described as an example of the technology in the present disclosure. However, the technology in the present disclosure is not limited to this, and can also be applied to embodiments in which modifications, substitutions, additions, etc. are made. Furthermore, it is also possible to combine the components described in the first embodiment above to create new embodiments.

[0111] Therefore, other embodiments will be exemplified below.

[0112] In the first embodiment, the fuel cell 10 is described as having a catalyst layer of the present disclosure in the cathode catalyst layer 3c as an example of the electrolyte membrane-electrode assembly 5. The catalyst layer of the present disclosure may also be provided in the anode catalyst layer 3a.

[0113] When the catalyst layer of the present disclosure is used in the anode catalyst layer 3a, the amount of ionomer 13 that comes into direct contact with the catalyst 14 is reduced, and therefore, the decrease in catalytic activity can be suppressed even in the oxidation reaction (Chemical Formula 1) that occurs at the anode 2a, in which hydrogen dissociates into hydrogen ions and electrons.

[0114] Furthermore, by ensuring hydrogen ion conductivity by the ionomer 13 adsorbed on the catalyst-free fiber 11, it is possible to suppress a decrease in the hydrogen ion conductivity of the anode catalyst layer in this modification, which would otherwise be caused by a decrease in the amount of ionomer 13 coated on the mesoporous material 12.

[0115] Therefore, it is possible to suppress the decrease in catalytic activity due to poisoning of ionomer 13 without impairing hydrogen ion conductivity, and it is possible to provide a relatively highly efficient electrolyte membrane-electrode assembly according to this modification.

[0116] In the first embodiment, a perfluorosulfonic acid resin membrane has been described as an example of the electrolyte membrane 1. The electrolyte membrane 1 is one that conducts ions (hydrogen ions) between the anode 2a and the cathode 2c, and may be any membrane as long as it has both hydrogen ion conductivity and gas barrier properties.

[0117] Therefore, the electrolyte membrane 1 is not limited to a perfluorosulfonic acid resin membrane. Examples of the electrolyte membrane 1 include an ion-exchange fluorine-based resin membrane and an ion-exchange hydrocarbon-based resin membrane. Among these, a perfluorosulfonic acid resin membrane is preferred because it has high hydrogen ion conductivity and can exist stably even in the power generation environment of a fuel cell, for example.

[0118] The ion exchange capacity of the ion exchange resin is preferably 0.9 to 2.0 meq / g dry resin. An ion exchange capacity of 0.9 meq / g dry resin or more makes it easier to obtain high hydrogen ion conductivity, while an ion exchange capacity of 2.0 meq / g dry resin or less is preferred because swelling of the resin due to water absorption is suppressed, thereby suppressing dimensional changes in the electrolyte membrane 1.

[0119] The thickness of the electrolyte membrane 1 is preferably 5 μm or more and 50 μm or less. When the thickness is 5 μm or more, high gas barrier properties are obtained, and when it is 50 μm or less, high hydrogen ion conductivity is obtained.

[0120] In the first embodiment, the anode gas diffusion layer 4a and the cathode gas diffusion layer 4c are configured to include two layers, a carbon paper layer and a coating layer. The anode gas diffusion layer 4a and the cathode gas diffusion layer 4c may be any layer that has current collecting properties, gas permeability, and water repellency.

[0121] Therefore, the anode gas diffusion layer 4a and the cathode gas diffusion layer 4c are not limited to a two-layer structure consisting of a carbon paper substrate and a coating layer. The substrate may be any material that has excellent electrical conductivity and gas and liquid permeability, such as carbon paper, as well as porous materials such as carbon fiber cloth and carbon fiber felt.

[0122] The coating layer is interposed between the catalyst layer (anode catalyst layer 3a, cathode catalyst layer 3c) and the substrate to reduce the contact resistance between the catalyst layer and the substrate and improve liquid permeability (drainage).

[0123] The coating layer is formed mainly from, for example, a conductive material such as carbon black and a water-repellent resin such as polytetrafluoroethylene (PTFE).

[0124] In the first embodiment, an alloy containing platinum is used as an example of the catalyst 14. The catalyst 14 is not particularly limited as long as it has a catalytic action on hydrogen or oxygen.

[0125] Examples of materials for the catalyst 14 include metals such as platinum (Pt), ruthenium (Ru), iridium (Ir), rhodium (Rh), palladium (Pd), osmium (Os), tungsten (W), lead (Pb), iron (Fe), chromium (Cr), cobalt (Co), nickel (Ni), manganese (Mn), vanadium (V), molybdenum (Mo), gallium (Ga), and aluminum (Al), as well as mixtures and alloys of these metals.

[0126] Among these, the material of the catalyst 14 is preferably platinum or a mixture or alloy containing platinum, from the viewpoint of improving catalytic activity, resistance to poisoning by carbon monoxide and the like, heat resistance, and the like.

[0127] When the catalyst 14 is made of an alloy, the composition of the alloy may vary depending on the type of alloying metal, but the platinum content may be 30 to 90 atomic % and the other metal content may be 10 to 70 atomic %.

[0128] The average particle size of catalyst 14 is not particularly limited, but from the viewpoint of improving catalyst utilization rate and supportability on the carbon support, the average particle size of catalyst 14 is preferably 1 to 30 nm. The average particle size of catalyst 14 is preferably a particle size that can fit into the pores of mesoporous material 12.

[0129] In the first embodiment, carbon black is used as an example of a catalyst support material used in the anode catalyst layer 3 a and the cathode catalyst layer 3 c. The catalyst support material is not limited to carbon black. Alternatively, conductive porous carbon having internal pores (such as mesoporous carbon) can be used as the catalyst support material.

[0130] Before the catalyst 14 is supported on the mesoporous material 12, the mode radius of the pores is 1 to 25 nm, and the pore volume of the pores is 1.0 to 3.0 cm 3 / g. The pore volume of the pores may be 1.0 cm 3 / g or more, a large amount of catalyst 14 can be supported inside the mesoporous material 12, and 3If the pore size is 1 / g or less, the strength of the mesoporous material 12 as a structure is increased.

[0131] Furthermore, mesoporous material 12 may be configured to have a mode diameter of 200 to 1000 nm. If mesoporous material 12 has a mode diameter of 200 nm or more, the area where ionomer 13 penetrates into the pores becomes small relative to the pore volume of the pores, and therefore the proportion of catalyst 14 that is affected by poisoning by ionomer 13 becomes small.

[0132] Therefore, it is believed that catalytic activity can be improved by setting the mode diameter of the mesoporous material 12 to 200 nm or more. Furthermore, if the mode diameter of the mesoporous material 12 is 1000 nm or less, the reaction gas can be easily supplied to the catalyst 14 supported in the pores of the mesoporous material 12.

[0133] It should be noted that the above-described embodiments are intended to illustrate the technology of the present disclosure, and various modifications, substitutions, additions, omissions, etc. may be made within the scope of the claims or their equivalents. [Industrial Applicability]

[0134] The present disclosure is applicable to, for example, an electrolyte membrane-electrode assembly having a catalyst layer. Specifically, the present disclosure is useful for an electrode catalyst used in an electrolyte membrane-electrode assembly constituting, for example, a fuel cell, a hydrogen purifier cell, or a water electrolysis cell. [Explanation of symbols]

[0135] 1 Electrolyte membrane 2a Anode 2c cathode 3a Anode catalyst layer 3c Cathode catalyst layer 4a Anode gas diffusion layer 4c Cathode gas diffusion layer 5 Electrolyte membrane-electrode assembly 6a Anode separator 6c Cathode separator 10 Fuel cell 11. Catalyst-free fibers 12 Mesoporous materials 13 Ionoma 14 Catalyst 15 Ionomer solution 16 water 17 Ethanol

Claims

1. A catalyst layer used in an electrolyte membrane-electrode assembly, a conductive mesoporous material; a catalyst supported at least in the pores of the mesoporous material; a catalyst-unsupported fiber that does not support a catalyst; The mesoporous material and the catalyst-free fiber are coated with hydrogen at least partially on the surface thereof. an ionically conductive ionomer; the hydrophobicity of the catalyst-unsupported fibers is greater than the hydrophobicity of the mesoporous material; The ionomer has a hydrophobic main chain and hydrophilic side chains, The surface area of ​​the portion of the catalyst-unsupported fiber that is covered with the ionomer and the surface area of ​​the catalyst-unsupported fiber The ratio of the surface area of ​​the mesoporous material to the total surface area of ​​the fiber is the ratio of the surface area of ​​the porous membrane to the total surface area of ​​the mesoporous material; A catalyst layer characterized by:

2. The fiber length of the catalyst-unsupported fibers is greater than 0.22 times the mode diameter of the mesoporous material. and the mode diameter is 12 times or less than the mode diameter of the mesoporous material. The catalyst layer according to claim 1 .

3. The catalyst-free fiber is electrically conductive. The catalyst layer according to claim 1 .

4. an electrolyte membrane; and an anode provided on one main surface of the electrolyte membrane; a cathode provided on the other main surface of the electrolyte membrane, The cathode comprises a catalyst layer according to any one of claims 1 to 3. The electrolyte membrane-electrode assembly is characterized by:

5. The electrolyte membrane-electrode assembly according to claim 4 is provided. A fuel cell characterized by:

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