Catalyst ink for forming electrode catalyst layer and method for producing membrane electrode junction

Abstract

To provide a catalyst ink that can be directly applied to a polymer electrolyte membrane without generating wrinkles or cracks and without lowering performance, and a membrane electrode junction using the catalyst ink.SOLUTION: A catalytic ink for an electrode catalyst layer includes catalyst-loaded carbon particles loaded with a catalyst, a polymer electrolyte, and at least one of carbon fiber and organic electrolyte fiber. A particle size distribution has peaks at least in a range of 0.1 μm or more and 1 μm or less and a range of 1 μm or more and 10 μm or less. Solid concentration is 8 wt.% or more and 20 wt.% or less. The catalyst ink is directly applied to a polymer electrolyte membrane 9 to manufacture a membrane electrode junction.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a catalyst ink for forming an electrode catalyst layer of a polymer electrolyte fuel cell and a method for manufacturing a membrane electrode assembly using the catalyst ink.

Background Art

[0002] A polymer electrolyte fuel cell has a structure in which a polymer electrolyte membrane is sandwiched between a cathode electrode catalyst layer and an anode electrode catalyst layer. A polymer electrolyte fuel cell having such a structure is expected to be used as a power source for automobiles, a stationary power source, etc. because it operates at room temperature and has a short startup time. As a conventional method for manufacturing a membrane electrode assembly, there are known a method of directly applying a catalyst ink to a polymer electrolyte membrane, and a method of applying a catalyst ink to a transfer substrate or a gas diffusion layer and then thermocompression bonding it to a polymer electrolyte membrane. The catalyst ink is composed of, for example, carbon particles carrying a catalyst, a polymer electrolyte, and a solvent.

[0003] Among them, the method of manufacturing a membrane electrode assembly by directly applying a catalyst ink to a polymer electrolyte membrane has characteristics such as good adhesion at the interface between the polymer electrolyte membrane and the electrode catalyst layer, and the catalyst layer is not crushed by thermocompression bonding. Therefore, this method for manufacturing a membrane electrode assembly can produce a membrane electrode assembly having excellent power generation performance and durability. However, in the conventional manufacturing method of directly applying a catalyst ink to an electrolyte membrane, when the catalyst ink is applied, the polymer electrolyte membrane swells or shrinks due to the solvent in the ink, resulting in problems such as wrinkles and cracks being likely to occur in the formed catalyst layer.

[0004] In response to the above problems, Patent Document 1 enables the formation of a catalyst layer directly on an electrolyte membrane by using a needle-like carbon material such as carbon nanotubes in the electrode catalyst layer. However, according to this method, since the utilization rate of the catalyst is low, the power generation performance may decrease. Furthermore, needle-like carbon materials such as carbon nanotubes are bulky, and due to the entanglement of the needle-like carbon materials, the catalyst ink may become highly viscous, making it difficult to apply. On the other hand, in Patent Document 2, the performance is improved by producing an electrode catalyst layer formed mainly of a fibrous proton conductive material. However, although performance improvement can be seen with this method, when directly applied to the electrolyte membrane, it may not be possible to obtain a film strength sufficient to suppress wrinkles and cracks in the catalyst layer.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] The present invention focuses on the above points, and an object thereof is to provide a catalyst ink that can be directly applied to a polymer electrolyte membrane without generating wrinkles or cracks and without degrading performance, and a membrane electrode assembly using the catalyst ink.

Means for Solving the Problems

[0007] To solve the above problems, a catalyst ink for an electrode catalyst layer according to one aspect of the present invention contains, in a solvent, catalyst-supported carbon particles which are carbon particles supporting a catalyst, and a polymer electrolyte, and further contains at least one of carbon fibers and organic electrolyte fibers, and has a particle size distribution having peaks in the ranges of at least 0.1 μm or more and 1 μm or less, and 1 μm or more and 10 μm or less, respectively. Another aspect of the present invention resides in a method for manufacturing a membrane electrode assembly by directly applying a catalyst ink to a polymer electrolyte membrane, wherein the catalyst ink for the electrode catalyst layer having the above-described configuration is used.

Advantages of the Invention

[0008] By using the catalyst ink according to the aspect of the present invention, the entanglement of carbon fibers and organic electrolyte fibers increases the membrane strength, and wrinkles and cracks can be suppressed even when the catalyst ink is directly applied to the electrolyte membrane. Further, by using the catalyst ink according to the aspect of the present invention, the carbon fibers impart electronic conductivity and the organic electrolyte fibers impart proton conductivity, thereby suppressing a decrease in performance. As a result, according to the aspect of the present invention, even when the catalyst ink is directly applied to the polymer electrolyte membrane during the manufacture of the membrane electrode assembly, it is possible to suppress wrinkles and cracks in the catalyst layer without degrading the power generation performance.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the present embodiment is not limited to the embodiments described below, and modifications such as design changes based on the knowledge of those skilled in the art can be added, and embodiments with such modifications are also included in the scope of the present embodiment. In the following detailed description, specific details are set forth in order to provide a thorough understanding of embodiments of the invention. However, it will be apparent that one or more embodiments may be practiced without these specific details. Also, well-known structures and devices may be shown in schematic form in order to simplify the drawings.

[0011] (Structure of a Polymer Electrolyte Fuel Cell) As shown in FIG. 1, on both sides of a polymer electrolyte membrane 2 constituting a polymer electrolyte fuel cell 1, a pair of electrode catalyst layers 3A and 3F facing each other with the polymer electrolyte membrane 2 interposed therebetween are disposed. On the surface of the electrode catalyst layer 3A opposite to the surface facing the polymer electrolyte membrane 2, a gas diffusion layer 4A is provided, and on the surface of the electrode catalyst layer 3F opposite to the surface facing the polymer electrolyte membrane 2, a gas diffusion layer 4F is provided, and they are disposed so as to face each other with the polymer electrolyte membrane 2 and the pair of electrode catalyst layers 3A and 3F interposed therebetween.

[0012] On the surface of the gas diffusion layer 4A opposite to the surface facing the electrode catalyst layer 3A, a separator 5A having a gas flow path 6A for reaction gas flow on the main surface facing this surface and a cooling water passage 7A for cooling water flow on the main surface opposite to the main surface having the gas flow path 6A is disposed. Further, on the surface of the gas diffusion layer 4F opposite to the surface facing the electrode catalyst layer 3F, a separator 5F having a gas flow path 6F for reaction gas flow on the main surface facing this surface and a cooling water passage 7F for cooling water flow on the main surface opposite to the main surface having the gas flow path 6F is disposed. Hereinafter, when there is no need to distinguish, the electrode catalyst layers 3A and 3F may be simply described as "electrode catalyst layer 3".

[0013] FIG. 2 is a schematic cross-sectional view showing a configuration example of an electrode catalyst layer according to the present embodiment. As shown in FIG. 2, an electrode catalyst layer 8 according to the present embodiment is joined to the surface of a polymer electrolyte membrane 9. The electrode catalyst layer 8 is composed of a catalyst 10, carbon particles 11 as a conductive carrier, a polymer electrolyte 12, carbon fibers 13, and organic electrolyte fibers 14. And portions where none of the components of the catalyst 10, carbon particles 11, polymer electrolyte 12, carbon fibers 13, and organic electrolyte fibers 14 exist are voids.

[0014] (Manufacture of Catalyst Ink) Next, a method for manufacturing a catalyst ink for forming an electrode catalyst layer according to the present embodiment will be described. The catalyst ink for forming an electrode catalyst layer is used to form the electrode catalyst layers 3 and 8 (electrode catalyst layers for a polymer electrolyte fuel cell) of the polymer electrolyte fuel cell 1. First, carbon particles 11 constituting catalyst-supported carbon particles carrying a catalyst 10 and a polymer electrolyte 12 are mixed and dispersed in a dispersion medium (solvent) to obtain a catalyst particle slurry. As the catalyst 10, for example, metals and alloys of these metals, oxides, double oxides, carbides, etc. can be used. Examples of the metal include platinum group elements (platinum, palladium, ruthenium, iridium, rhodium, osmium), iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, etc.

[0015] As the carbon particles 11, any particles can be used as long as they have conductivity and can carry the catalyst without being invaded by the catalyst. Generally, carbon particles are used. As the dispersion medium (solvent), any one of water and alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, pentanol, etc. can be selected and used. Also, as the dispersion medium, a dispersion medium in which two or more of the above-described dispersion media are mixed can be used. For mixing and dispersion, for example, a bead mill, a planetary mixer, a dissolver, etc. can be used.

[0016] As materials for the polymer electrolyte membranes 2 and 9 and the polymer electrolyte 12, any materials having proton conductivity may be used, and fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes can be used. As the fluorine-based polymer electrolyte, a polymer electrolyte having a tetrafluoroethylene skeleton can be used. For example, "Nafion (registered trademark)" manufactured by DuPont can be used. The polymer electrolyte 12 is in a state where the polymer electrolyte is aggregated. Next, at least one of the carbon fiber 13 and the organic electrolyte fiber 14 is added to the catalyst particle slurry produced by the above method, and mixed and dispersed to obtain a catalyst ink. For mixing and dispersing, for example, a bead mill, a planetary mixer, a dissolver, etc. can be used.

[0017] Examples of the carbon fiber 13 include carbon fiber, carbon nanotube, carbon nanohorn, conductive polymer nanofiber, etc. Only one of these fibers may be used alone, or two or more of them may be used in combination. The polymer electrolyte 12 and the polymer electrolyte constituting the organic electrolyte fiber 14 may be the same material or different from each other. Also, the polymer electrolytes constituting the polymer electrolyte 12 and the organic electrolyte fiber 14 respectively and the polymer electrolytes constituting the polymer electrolyte membranes 2 and 9 may be the same or different from each other.

[0018] (Manufacture of Membrane Electrode Assembly) The membrane electrode assembly is manufactured by joining the electrode catalyst layers 3 formed with the above catalyst ink to both surfaces of the polymer electrolyte membrane 2. As a method of joining the electrode catalyst layer 3 to the polymer electrolyte membrane 2, for example, a transfer substrate with an electrode catalyst layer coated with the catalyst ink is used, and the surface of the electrode catalyst layer of the transfer substrate with the electrode catalyst layer and the polymer electrolyte membrane are brought into contact and heated and pressed to join the polymer electrolyte membrane 2 and the electrode catalyst layer 3.

[0019] However, according to the above method, the adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2 is poor, and voids are likely to be formed at the interface between the electrode catalyst layer 3 and the polymer electrolyte membrane 2. As a result, problems such as a decrease in power generation performance due to interface resistance and a decrease in power generation performance due to flooding caused by water clogging in the voids tend to occur easily. On the other hand, the membrane electrode assembly can also be manufactured by a method in which catalyst ink is directly applied to the surface of the polymer electrolyte membrane 2 and then the solvent component (dispersion medium) is removed from the coating film of the catalyst ink. According to this method, the adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2 is good, and the above problems are less likely to occur.

[0020] However, in the method of directly applying the catalyst ink to the polymer electrolyte membrane 2, conventionally, due to the swelling of the polymer electrolyte membrane 2, wrinkles and cracks are likely to occur in the applied electrode catalyst layer 3, and as a result, a decrease in power generation performance and a decrease in durability are likely to occur. In contrast, if at least one of the carbon fiber 13 and the organic electrolyte fiber 14 is added to the catalyst ink as in this embodiment, the strength of the electrode catalyst layer 3 is increased. Specifically, a catalyst ink composed only of catalyst-supported carbon particles having a particle diameter in the range of 0.1 μm or more and 1 μm or less is likely to cause aggregation because the catalyst-supported carbon particles are small, and wrinkles and cracks are likely to occur in the applied electrode catalyst layer 3. Therefore, in this embodiment, by adding the above fiber 13 having a particle diameter in the range of 1 μm or more and 10 μm or less, aggregation of the catalyst-supported carbon particles with a small particle diameter can be suppressed, and thus wrinkles and cracks in the applied electrode catalyst layer 3 can be suppressed.

[0021] At this time, in the particle size distribution in the catalyst ink, if the peak in the range of 0.1 μm or more and 1 μm or less deviates from the range of 0.1 μm or more and 1 μm or less, aggregation cannot be completely suppressed, and wrinkles and cracks are likely to occur in the applied electrode catalyst layer 3. Further, in the particle size distribution in the catalyst ink, if the peak in the range of 1 μm or more and 10 μm or less becomes larger, when the coating film is applied thinly, the unevenness becomes prominent, which may cause damage to the polymer electrolyte membrane 2. On the other hand, in the case of a catalyst ink composed only of large particle diameters in the range of 1 μm or more and 10 μm or less, the specific surface area of the catalyst becomes insufficient and the catalytic activity decreases, which causes a performance degradation.

[0022] Also, when the viscosity of the catalyst ink is high, the coating and drying processes become difficult, and thus wrinkles and cracks are likely to occur in the electrode catalyst layer 3 during the coating and drying processes. Specifically, when the thixotropic index (TI value) of the viscosity at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s) exceeds 10, the coating becomes difficult and wrinkles and cracks are likely to occur. A catalyst ink composed only of small particle diameters is likely to cause aggregation of the particles, and as a result, the TI value becomes high. On the other hand, a catalyst ink composed of small particles showing a peak value of the particle size distribution in the range of 0.1 μm to 1 μm and large particles showing a peak value of the particle size distribution in the range of 1 μm to 10 μm is less likely to cause aggregation, so that the viscosity does not become high, and wrinkles and cracks are less likely to occur when coated on the electrode catalyst layer 3.

[0023] Here, in the catalyst ink of the present embodiment, as described above, the catalyst-supported carbon particles form a peak value of the particle size distribution in the range of 0.1 μm to 1 μm, and the added fibers form a peak value of the particle size distribution in the range of 1 μm to 10 μm. The formation of each of the above peak values can be achieved, for example, if 60% wt or more, preferably 80 wt% or more of the particle diameters of the target particles are within the respective ranges. When the solid content concentration of the catalyst ink becomes higher than 20 wt%, the viscosity becomes high and it becomes difficult to apply a thin film thickness, so that wrinkles and cracks are likely to occur when coated on the electrode catalyst layer 3.

[0024] On the other hand, when the viscosity of the catalyst ink is low, specifically, when the thixotropic index (TI value) of the viscosity at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s) is less than 1.5, wrinkles and cracks are likely to occur during the drying process of the catalyst ink when coated on the electrode catalyst layer 3. The average fiber diameter of the organic electrolyte fiber 14 is 2 μm or less. If the average fiber diameter is 2 μm or less, an appropriate fineness is ensured as the fiber material to be contained in the electrode catalyst layer. In order to improve the output of the polymer electrolyte fuel cell, it is desirable that the gas supplied to the electrode catalyst layer is appropriately diffused into the electrode catalyst layer through the pores of the electrode catalyst layer, and in particular, in the air electrode, the water generated by the electrode reaction is appropriately discharged through the pores. Further, due to the presence of the pores, an interface where the gas, the catalyst-supporting carbon, and the polymer electrolyte are in contact is easily formed, and the electrode reaction is promoted, so that the output of the polymer electrolyte fuel cell can be improved thereby.

[0025] From the above viewpoints, it is preferable that the electrode catalyst layer has pores with appropriate size and amount. If the average fiber diameter of the organic electrolyte fiber 14 is 2 μm or less, sufficient gaps are formed in the structure where the organic electrolyte fibers 14 are entangled in the electrode catalyst layer, and sufficient pores are ensured, so that the output of the fuel cell can be improved. Further, when the average fiber diameter of the organic electrolyte fiber 14 is 0.5 nm or more and 500 nm or less, the output of the fuel cell is particularly increased. The average fiber length of the organic electrolyte fiber 14 is preferably larger than the average fiber diameter and is 1 μm or more and 200 μm or less. If the average fiber length is within the above range, aggregation of the organic electrolyte fibers 14 in the electrode catalyst layer is suppressed, and pores are easily formed. Further, if the average fiber length is within the above range, a structure in which the organic electrolyte fibers 14 are entangled in the electrode catalyst layer is preferably formed, so that the strength of the electrode catalyst layer is increased and the effect of suppressing the occurrence of cracks is enhanced.

[0026] (Effect of this embodiment) According to this embodiment, even when the catalyst ink is directly applied to the polymer electrolyte membrane during the manufacture of the membrane electrode assembly, it is possible to suppress wrinkles and cracks in the catalyst layer without degrading the power generation performance. As a result, it is possible to manufacture a membrane electrode assembly having good power generation performance without reducing the yield.

[0027] Hereinafter, examples and comparative examples of the present invention will be described. (Example 1) Hereinafter, Example 1 of the present invention will be described. (Production of Catalyst Ink) Water was added to catalyst-supported carbon particles supporting 50 wt% of platinum (trade name: TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo K.K.) and a dispersion of a polymer electrolyte (trade name: Nafion dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and the mixture was mixed with a planetary mixer to prepare a catalyst particle slurry. Carbon fiber (trade name: VGCF-H, manufactured by Showa Denko) and 1-propanol were added to the above catalyst particle slurry, and the mixture was adjusted to a solid content concentration of 10 wt%, followed by dispersion with a bead mill disperser to obtain a catalyst ink. The measurement results of the particle size distribution of the above catalyst ink were as shown in Fig. 3. That is, the catalyst ink of Example 1 was a catalyst ink having a particle size distribution with peaks in the range of 0.1 μm to 1 μm and in the range of 1 μm to 10 μm, respectively. In addition, as a result of measuring the TI value of the viscosity at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s) of the catalyst ink of Example 1, it was 6.0.

[0028] <Measurement of Particle Size Distribution> Here, in this specification, the measurement of the particle size distribution in the catalyst ink was performed by the laser diffraction / scattering method. The measurement conditions are as follows. Measurement conditions: Sample permeability: Absorption Solvent refractive index: 1.333

[0029] <Measurement of Shear Rate> In this specification, the measurement of the shear rate (viscosity measurement method) was performed under the following conditions. Measurement conditions: Measuring device: Cone-plate viscometer Measurement range (shear rate): 1000 [1 / s] to 1 [1 / s] Measurement temperature: 23 °C

[0030] <TI Value> The TI value is the ratio of viscosities. The TI value in this specification was defined as follows. TI value = Viscosity at shear rate of 10 [1 / sec] / Viscosity at shear rate of 100 [1 / sec]

[0031] (Manufacture of Membrane Electrode Assembly) Next, the above catalyst ink was directly applied to both sides of the polymer electrolyte membrane by the die coating method to obtain a membrane electrode assembly. When the catalyst ink of Example 1 was directly applied to both sides of the polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, and good power generation performance was obtained.

[0032] (Example 2) Next, Example 2 of the present invention will be described. The catalyst ink of Example 2 was obtained by the same process as in Example 1, except that polymer electrolyte fibers were added instead of carbon fibers in the catalyst ink. The polymer electrolyte fibers were prepared by subjecting a dispersion of a polymer electrolyte (Nafion dispersion: manufactured by Wako Pure Chemical Industries, Ltd.) to electrospinning to form fibers and then cooling and pulverizing them. The average fiber diameter of the polymer electrolyte fibers was 150 nm, and the average fiber length was 10 μm. The measurement results of the particle size distribution of the catalyst ink of Example 2 showed peaks in two ranges, within the range of 0.1 μm to 1 μm and within the range of 1 μm to 10 μm. As a result of measuring the TI value of the viscosity at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s) of the catalyst ink of Example 2, it was 7.5. When the catalyst ink of Example 2 was directly applied to both sides of the polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, and good power generation performance was obtained.

[0033] (Example 3) Next, Example 3 of the present invention will be described. The catalyst ink of Example 3 was obtained by the same process as in Example 1, except that the same polymer electrolyte fibers as in Example 2 were added after adding the carbon fibers of Example 1. The measurement results of the particle size distribution of the catalyst ink of Example 3 showed peaks in the range of 0.1 μm to 1 μm and in the range of 1 μm to 10 μm, respectively. As a result of measuring the TI value of the viscosity of the catalyst ink of Example 3 at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s), it was 4.5. When the catalyst ink of Example 3 was directly coated on both sides of the polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, and good power generation performance was obtained.

[0034] (Comparative Example 1) A catalyst ink of Comparative Example 1 was obtained by the same process as in Example 1 above, except that carbon fibers were not added to the catalyst ink. The measurement results of the particle size distribution of the catalyst ink of Comparative Example 1 showed one peak in the range of 0.1 μm to 1 μm. As a result of measuring the TI value of the viscosity of the catalyst ink of Comparative Example 1 at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s), it was 11.0. When the catalyst ink of Comparative Example 1 was directly coated on both sides of the polymer electrolyte membrane, wrinkles and cracks occurred in the catalyst layer.

[0035] (Comparative Example 2) A catalyst ink of Comparative Example 2 was obtained by the same process as in Example 1 above, except that the catalyst ink was produced so that the solid content concentration was 5 wt%. The measurement results of the particle size distribution of the catalyst ink of Comparative Example 2 showed two peaks in the range of 0.1 μm to 1 μm and in the range of 1 μm to 10 μm. As a result of measuring the TI value of the viscosity of the catalyst ink of Comparative Example 2 at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s), it was 1.0. When the catalyst ink of Comparative Example 2 was directly coated on both sides of the polymer electrolyte membrane, wrinkles and cracks occurred in the catalyst layer.

[0036] (Comparative Example 3) As the organic electrolyte fiber, a catalyst ink of Comparative Example 2 was obtained by the same process as in Example 2, except that a polymer electrolyte fiber having an average fiber diameter of 3.0 μm and an average fiber length of 20 μm was used. Regarding the measurement result of the particle size distribution of the catalyst ink of Comparative Example 3, two peaks were observed, one in the range of 0.1 μm to 1 μm and the other in the range of 1 μm to 10 μm. As a result of measuring the TI value of the viscosity of the above catalyst ink at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s), it was 6.0. When the catalyst ink of Comparative Example 3 was directly applied to both sides of the polymer electrolyte membrane, although no wrinkles or cracks occurred in the catalyst layer, there was a slight decrease in power generation performance.

[0037] Moreover, the scope of the present invention is not limited to the illustrated and described exemplary embodiments, but also includes all embodiments that bring about an equivalent effect to the object of the present invention. Further, the scope of the present invention is not limited to the combination of the features of the invention defined by the claims, but may be defined by any desired combination of the specific features among all the disclosed respective features. Also, the entire content of Japanese Patent Application No. 2019-222431 (filed on December 9, 2019), for which this application claims priority, is incorporated herein by reference and forms a part of this disclosure.

Explanation of Signs

[0038] 1 Polymer electrolyte fuel cell 2, 9 Polymer electrolyte membrane 3, 8 Electrode catalyst layer 3A, 3F Electrode catalyst layer 10 Catalyst 11 Carbon particles 12 Polymer electrolyte 13 Carbon fiber 14 Organic electrolyte fiber

Claims

1. The catalyst-supporting carbon particles, which are carbon particles supporting a catalyst, and a polymer electrolyte are contained in a solvent, and at least one fiber selected from carbon fibers and organic electrolyte fibers is also contained, The particle size distribution has peaks in the range of at least 0.1 μm to 1 μm and in the range of at least 1 μm to 10 μm, A catalyst ink for forming an electrode catalyst layer, the catalyst ink having a solid content concentration of 8 wt % or more and 20 wt % or less.

2. 2. The catalyst ink for forming an electrode catalyst layer according to claim 1, wherein the carbon fibers contain one or more types selected from the group consisting of carbon nanofibers and carbon nanotubes.

3. 3. The catalyst ink for forming an electrode catalyst layer according to claim 1, wherein the organic electrolyte fibers have an average fiber diameter of 2 μm or less and an average fiber length of 1 μm or more and 200 μm or less.

4. A membrane electrode assembly using the catalyst ink for forming an electrode catalyst layer according to any one of claims 1 to 3.

5. A method for producing a membrane electrode assembly by directly applying a catalyst ink to a polymer electrolyte membrane, comprising the steps of: A method for producing a membrane electrode assembly, comprising the step of: using the catalyst ink for forming an electrode catalyst layer according to any one of claims 1 to 3 as the catalyst ink.

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