Catalyst ink for forming electrode catalyst layer and method for manufacturing membrane electrode assembly

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 producing a membrane electrode assembly using the catalyst ink. [Background technology]

[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. Because such a structure operates at room temperature and has a short start-up time, it is expected to be used as a power source for automobiles and stationary power sources. Conventional methods for producing membrane electrode assemblies include a method in which a catalyst ink is directly applied to a polymer electrolyte membrane, and a method in which a catalyst ink is applied to a transfer substrate or a gas diffusion layer, and then the substrate or layer is thermocompressed to the polymer electrolyte membrane. The catalyst ink is composed of, for example, catalyst-supported carbon particles, a polymer electrolyte, and a solvent.

[0003] Among these, the method of producing a membrane electrode assembly by directly applying a catalyst ink to a polymer electrolyte membrane is characterized by good adhesion at the interface between the polymer electrolyte membrane and the electrode catalyst layer and by the fact that the catalyst layer is not crushed by thermocompression bonding. Therefore, this method of producing a membrane electrode assembly can produce a membrane electrode assembly with excellent power generation performance and durability. However, in conventional manufacturing methods in which catalyst ink is directly applied to an electrolyte membrane, the solvent in the ink causes the polymer electrolyte membrane to swell or shrink when the catalyst ink is applied, which creates the problem of making the formed catalyst layer prone to wrinkles and cracks.

[0004] To address the above-mentioned problem, Patent Document 1 uses a needle-shaped carbon material such as carbon nanotubes for the electrode catalyst layer, thereby enabling the catalyst layer to be formed directly on the electrolyte membrane. However, this method may result in a decrease in power generation performance due to low catalyst utilization, and furthermore, needle-shaped carbon materials such as carbon nanotubes are bulky, and the entanglement of the needle-shaped carbon materials makes the catalyst ink highly viscous, which may make it difficult to apply. On the other hand, in Patent Document 2, performance is improved by producing an electrode catalyst layer formed mainly of a fibrous proton-conductive material. However, although this method improves performance, it may not be possible to obtain a membrane strength sufficient to prevent wrinkling and cracking of the catalyst layer when the electrode catalyst layer is directly applied to the electrolyte membrane. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2002 / 027844 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-220416 Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in view of the above-mentioned points, and aims to provide a catalyst ink that can be applied directly to a polymer electrolyte membrane without causing wrinkles or cracks and without deteriorating performance, and a membrane electrode assembly using the catalyst ink. [Means for solving the problem]

[0007] In order to solve the above problems, a catalyst ink for an electrode catalyst layer according to one embodiment of the present invention comprises, in a solvent, catalyst-supported carbon particles, which are carbon particles carrying a catalyst, and a polymer electrolyte, and also comprises at least one of carbon fibers and organic electrolyte fibers, and has a particle size distribution with peaks in the ranges of at least 0.1 μm to 1 μm and at least 1 μm to 10 μm, respectively. Another aspect of the present invention is a method for producing a membrane electrode assembly by directly applying a catalyst ink to a polymer electrolyte membrane, wherein the catalyst ink is a catalyst ink for an electrode catalyst layer having the above-described configuration. [Effects of the Invention]

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

[0009] [Figure 1] 1 is an exploded perspective view showing the internal structure of a polymer electrolyte fuel cell according to one embodiment of the present invention. [Figure 2] 1 is a diagram illustrating the structure of a membrane electrode assembly for a polymer electrolyte fuel cell according to one embodiment of the present invention. [Figure 3] FIG. 10 is a diagram showing the measurement results of the particle size distribution of a catalyst ink according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. It should be noted that this 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 are possible, and embodiments with such modifications are also included in the scope of this embodiment. Furthermore, in the following detailed description, specific details are set forth in order to provide a thorough understanding of the embodiments of the present invention. However, it will be apparent that one or more embodiments may be practiced without such specific details. In addition, for the purpose of simplifying the drawings, well-known structures and devices may be shown in simplified form.

[0011] (Structure of polymer electrolyte fuel cells) 1, a pair of electrode catalyst layers 3A, 3F are arranged on both sides of a polymer electrolyte membrane 2 constituting a solid polymer fuel cell 1, facing each other with the polymer electrolyte membrane 2 sandwiched between them. A gas diffusion layer 4A is arranged on the surface of the electrode catalyst layer 3A opposite to the surface facing the polymer electrolyte membrane 2, and a gas diffusion layer 4F is arranged on the surface of the electrode catalyst layer 3F opposite to the surface facing the polymer electrolyte membrane 2, so as to face each other with the polymer electrolyte membrane 2 and the pair of electrode catalyst layers 3A, 3F sandwiched between them.

[0012] On the surface of the gas diffusion layer 4A opposite to the surface facing the electrode catalyst layer 3A, a separator 5A is disposed, the separator 5A having gas channels 6A for flowing reactant gases on its main surface facing the gas diffusion layer 4A and having cooling water channels 7A for flowing coolant on its main surface opposite to the main surface provided with the gas channels 6A. Furthermore, on the surface of the gas diffusion layer 4F opposite to the surface facing the electrode catalyst layer 3F, a separator 5F is disposed, the separator 5F having gas channels 6F for flowing reactant gases on its main surface facing the gas diffusion layer 4A and having cooling water channels 7F for flowing coolant on its main surface opposite to the main surface provided with the gas channels 6F. Hereinafter, when there is no need to distinguish between them, the electrode catalyst layers 3A and 3F may be simply referred to as "electrode catalyst layer 3."

[0013] Fig. 2 is a schematic cross-sectional view showing an example of the configuration of an electrode catalyst layer according to this embodiment. As shown in Fig. 2, an electrode catalyst layer 8 according to this embodiment is bonded 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 support, a polymer electrolyte 12 and carbon fibers 13, and organic electrolyte fibers 14. Portions where none of the components of the catalyst 10, carbon particles 11, polymer electrolyte 12 and carbon fibers 13, or organic electrolyte fibers 14 are present form pores.

[0014] (catalytic ink manufacturing) Next, a method for producing the catalyst ink for forming an electrode catalyst layer according to this 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, the carbon particles 11 constituting the catalyst-supported carbon particles supporting the catalyst 10 and the polymer electrolyte 12 are mixed and dispersed in a dispersion medium (solvent) to obtain a catalyst particle slurry. For example, metals and alloys, oxides, double oxides, carbides, etc. of these metals can be used as the catalyst 10. Examples of metals include platinum group elements (platinum, palladium, ruthenium, iridium, rhodium, osmium), iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, etc.

[0015] Any material may be used as the carbon particles 11 as long as it is conductive and can support the catalyst without being corroded by the catalyst, but carbon particles are generally used. The dispersion medium (solvent) can be any one selected from water or alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, and pentanol. It is also possible to use a mixture of two or more of the above-mentioned dispersion media. For mixing and dispersion, a bead mill, a planetary mixer, a dissolver, or the like can be used, for example.

[0016] The materials for the polymer electrolyte membranes 2, 9 and the polymer electrolyte 12 may be any material that has proton conductivity, and may include fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes. As the fluorine-based polymer electrolyte, a polymer electrolyte having a tetrafluoroethylene skeleton may be used. For example, "Nafion (registered trademark)" manufactured by DuPont may be used. The polymer electrolyte 12 is in a state where the polymer electrolyte is aggregated. Next, at least one of carbon fibers 13 and organic electrolyte fibers 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 dispersion, a bead mill, a planetary mixer, a dissolver, or the like 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 fibers 14 may be the same material or different materials. Furthermore, the polymer electrolytes constituting the polymer electrolyte 12 and the organic electrolyte fibers 14 may be the same material or different materials from the polymer electrolyte membranes 2 and 9.

[0018] (Manufacturing of membrane electrode assemblies) The electrode catalyst layers 3 formed from the catalyst ink are bonded to both sides of the polymer electrolyte membrane 2 to produce a membrane electrode assembly. As a method for bonding the electrode catalyst layer 3 to the polymer electrolyte membrane 2, for example, a method is available in which a transfer substrate with an electrode catalyst layer, which is formed by applying a catalyst ink to a transfer substrate, is used, and the surface of the electrode catalyst layer of the transfer substrate with the electrode catalyst layer is brought into contact with the polymer electrolyte membrane, and then heated and pressurized, thereby bonding the polymer electrolyte membrane 2 and the electrode catalyst layer 3.

[0019] However, the above method results in poor adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2, and voids are likely to form at the interface between the electrode catalyst layer 3 and the polymer electrolyte membrane 2. This tends to lead to problems such as a decrease in power generation performance due to interfacial resistance and a decrease in power generation performance due to flooding caused by water clogging in the voids. Alternatively, a membrane electrode assembly can also be produced by a method in which the 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 of the catalyst ink. This method provides good adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2, making the above-mentioned problems less likely to occur.

[0020] However, in the method of directly applying the catalyst ink to the polymer electrolyte membrane 2, there has traditionally been a problem in that swelling of the polymer electrolyte membrane 2 tends to cause wrinkles and cracks in the applied electrode catalyst layer 3, which can lead to a decrease in power generation performance and durability. In contrast, if at least one of carbon fibers 13 and organic electrolyte fibers 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 consisting only of catalyst-supporting carbon particles with a particle size in the range of 0.1 μm to 1 μm is prone to aggregation due to the small size of the catalyst-supporting carbon particles, which makes the applied electrode catalyst layer 3 prone to wrinkles and cracks. Therefore, in this embodiment, by adding the above-mentioned fibers 13 with a particle size in the range of 1 μm to 10 μm, aggregation of the catalyst-supporting carbon particles with a small particle size can be suppressed, thereby suppressing wrinkles and cracks in the applied electrode catalyst layer 3.

[0021] In this case, if the peak in the range of 0.1 μm or more and 1 μm or less in the particle size distribution in the catalyst ink falls outside the range of 0.1 μm or more and 1 μm or less, aggregation cannot be suppressed completely, and wrinkles and cracks are likely to occur in the applied electrode catalyst layer 3. Furthermore, if the particle size distribution in the catalyst ink has a peak in the range of 1 μm or more and 10 μm or less, when the coating is applied to form a thin film, the unevenness will become significant, which may cause damage to the polymer electrolyte membrane 2. On the other hand, catalyst inks consisting only of large particles with diameters in the range of 1 μm to 10 μm have an insufficient specific surface area of ​​the catalyst, which reduces catalytic activity and causes performance degradation.

[0022] Furthermore, if the viscosity of the catalyst ink is high, the coating and drying processes become difficult, making the electrode catalyst layer 3 prone to wrinkles and cracks during the coating and drying processes. Specifically, if the thixotropic index (TI value) between the viscosity at a shear rate of 10 (1 / s) and the viscosity at a shear rate of 100 (1 / s) exceeds 10, coating becomes difficult and wrinkles and cracks become more likely to occur. Catalyst inks consisting only of small particles tend to cause particle aggregation, resulting in a high TI value. On the other hand, catalyst inks consisting of small particles with a particle size distribution peak in the range of 0.1 μm to 1 μm and large particles with a particle size distribution peak in the range of 1 μm to 10 μm suppress aggregation, so they do not become highly viscous and are less likely to cause wrinkles or cracks when applied to the electrode catalyst layer 3.

[0023] In the catalyst ink of this embodiment, as described above, the catalyst-supported carbon particles form a particle size distribution peak in the range of 0.1 μm to 1 μm, and the added fibers form a particle size distribution peak in the range of 1 μm to 10 μm. The formation of each of the above peaks is possible, for example, if 60% wt or more, and preferably 80% wt or more, of the particle diameters of the target particles fall within the respective ranges. If the solids concentration of the catalyst ink is higher than 20 wt%, the viscosity will increase and it will be difficult to apply a thin film, making it more likely to wrinkle or crack when applied to 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, the catalyst ink is more likely to wrinkle or crack during the drying process when applied to the electrode catalyst layer 3. The average fiber diameter of the organic electrolyte fibers 14 is 2 μm or less. If the average fiber diameter is 2 μm or less, the fiber is ensured to be thin enough to be used as a fiber material contained in an electrode catalyst layer. To improve the output of a polymer electrolyte fuel cell, it is desirable that the gas supplied to the electrode catalyst layer be properly diffused throughout the electrode catalyst layer through the pores in the electrode catalyst layer, and that water produced by the electrode reaction, particularly at the air electrode, be properly discharged through the pores. Furthermore, the presence of pores facilitates the formation of an interface where the gas, catalyst-supported carbon, and polymer electrolyte come into contact, accelerating the electrode reaction, which also improves the output of the polymer electrolyte fuel cell.

[0025] From the above viewpoints, it is preferable that the electrode catalyst layer has pores of an appropriate size and quantity. If the average fiber diameter of the organic electrolyte fibers 14 is 2 μm or less, sufficient gaps are formed in the entangled structure of the organic electrolyte fibers 14 in the electrode catalyst layer, ensuring sufficient pores, thereby enabling an improvement in the output of the fuel cell. Furthermore, if the average fiber diameter of the organic electrolyte fibers 14 is 0.5 nm or more and 500 nm or less, the output of the fuel cell is particularly improved. The average fiber length of the organic electrolyte fibers 14 is preferably greater than the average fiber diameter and is between 1 μm and 200 μm. When the average fiber length is within this range, aggregation of the organic electrolyte fibers 14 in the electrode catalyst layer is suppressed, and pores are easily formed. Furthermore, when the average fiber length is within this range, an entangled structure of the organic electrolyte fibers 14 is suitably formed in the electrode catalyst layer, thereby increasing the strength of the electrode catalyst layer and enhancing the effect of suppressing cracking.

[0026] (Effects of this embodiment) According to this embodiment, even when a catalyst ink is applied directly to a polymer electrolyte membrane during the production of a membrane electrode assembly, it is possible to suppress wrinkles and cracks in the catalyst layer without reducing power generation performance, which makes it possible to produce a membrane electrode assembly with good power generation performance without reducing yield.

[0027] Examples and comparative examples of the present invention will be described below. (Example 1) Hereinafter, Example 1 of the present invention will be described. (Production of Catalyst Ink) Water was added to catalyst-supported carbon particles carrying 50 wt% of platinum (trade name: TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) and a dispersion of a polymer electrolyte (trade name: Nafion dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and 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, adjusted so that the solid content concentration became 10 wt%, and dispersed 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 respectively in the range of 0.1 μm to 1 μm and in the range of 1 μm to 10 μm. Further, 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 <​​​​​​​​​​​​​​​​​​​​​​The TI value in this specification is defined as follows: TI value = viscosity at shear rate 10 [1 / sec] / viscosity at shear rate 100 [1 / sec]

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

[0032] Example 2 Next, a second embodiment 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 to the catalyst ink instead of carbon fibers. The polymer electrolyte fibers were prepared by electrospinning a polymer electrolyte dispersion (Nafion dispersion, manufactured by Wako Pure Chemical Industries, Ltd.) into fibers, followed by cooling and pulverization. The average fiber diameter of the polymer electrolyte fibers was 150 nm, and the average fiber length was 10 μm. The particle size distribution of the catalyst ink of 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. The catalyst ink of Example 2 had a viscosity TI value of 7.5 when measured at a shear rate of 10 (1 / s) and at a shear rate of 100 (1 / s). When the catalyst ink of Example 2 was applied directly to both sides of a polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, and good power generation performance was obtained.

[0033] Example 3 Next, a third embodiment 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 the carbon fibers of Example 1 were added. 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. The catalyst ink of Example 3 had a TI value of 4.5 when the viscosity was measured at a shear rate of 10 (1 / s) and at a shear rate of 100 (1 / s). When the catalyst ink of Example 3 was applied directly to both sides of a polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, and good power generation performance was obtained.

[0034] (Comparative Example 1) The catalyst ink of Comparative Example 1 was obtained by the same process as in Example 1 above, except that no carbon fiber was added to the catalyst ink. 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. The TI values ​​of the viscosity of the catalyst ink of Comparative Example 1 at a shear rate of 10 (1 / s) and at a shear rate of 100 (1 / s) were measured and found to be 11.0. When the catalyst ink of Comparative Example 1 was applied directly to both sides of a polymer electrolyte membrane, wrinkles and cracks occurred in the catalyst layer.

[0035] (Comparative Example 2) The catalyst ink of Comparative Example 2 was obtained by the same process as in Example 1, except that the catalyst ink was produced so that the solid content concentration was 5 wt %. The particle size distribution of the catalyst ink of Comparative Example 2 showed two peaks, one in the range of 0.1 μm to 1 μm and the other in the range of 1 μm to 10 μm. The TI values ​​of the viscosity of the catalyst ink of Comparative Example 2 at a shear rate of 10 (1 / s) and at a shear rate of 100 (1 / s) were measured and found to be 1.0. When the catalyst ink of Comparative Example 2 was applied directly to both sides of the polymer electrolyte membrane, wrinkles and cracks occurred in the catalyst layer.

[0036] (Comparative Example 3) A catalyst ink of Comparative Example 2 was obtained by the same steps as in Example 2, except that polymer electrolyte fibers with an average fiber diameter of 3.0 μm and an average fiber length of 20 μm were used as the organic electrolyte fibers. The particle size distribution of the catalyst ink of Comparative Example 3 showed two peaks, one in the range of 0.1 μm to 1 μm and the other in the range of 1 μm to 10 μm. The TI values ​​of the above catalyst ink were measured for viscosity at a shear rate of 10 (1 / s) and at a shear rate of 100 (1 / s), and were found to be 6.0. When the catalyst ink of Comparative Example 3 was applied directly to both sides of the polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, but the power generation performance was slightly reduced.

[0037] Furthermore, the scope of the present invention is not limited to the exemplary embodiments shown and described, but includes all embodiments that achieve equivalent effects to the object of the present invention. Furthermore, the scope of the present invention is not limited to the combination of the features of the invention defined by the claims, but can be defined by any desired combination of specific features among all the respective disclosed features. The entire contents of Japanese Patent Application No. 2019-222431 (filed December 9, 2019), from which this application claims priority, are incorporated herein by reference. [Explanation of symbols]

[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 Polyelectrolyte 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 the catalyst-supporting carbon particles contain at least one fiber selected from carbon fibers and organic electrolyte fibers, 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, 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: A method for producing a membrane electrode assembly, wherein the catalyst ink for forming an electrode catalyst layer according to any one of claims 1 to 3 is used as the catalyst ink.

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