Electrode catalyst layer, membrane electrode assembly, and polymer electrolyte fuel cell

Abstract

To provide an electrode catalyst layer, a membrane-electrode assembly, and a solid polymer type fuel battery which enable the enhancement in substance's transportability and proton conductivity in an electrode catalyst layer, which allow a power generation performance to be exhibited over a long period of time, and which have good durability.SOLUTION: An electrode catalyst layer 10 hereof is arranged to be used for a solid polymer type fuel battery. The electrode catalyst layer comprises: a catalyst substance 12; a conductive carrier 13 serving to support the catalyst substance 12; a polyelectrolyte 14; and a fibrous substance 15 containing nitrogen atoms. According to energy dispersive X-ray spectroscopy on a particular region of a section of the electrode catalyst layer, the rate of a number of nitrogen atoms to a total number of atoms of elements of carbon, nitrogen, oxygen, fluorine, sulfur, and platinum is 2 at% or more and 20 at% or less. The particular region is a region including 50 area% or more of the fibrous substance containing nitrogen atoms, but not including the catalyst substance or the conductive carrier.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell. [Background Art]

[0002] A fuel cell is a power generation system that produces electricity from a chemical reaction between hydrogen and oxygen. Compared with conventional power generation methods, fuel cells have characteristics such as high efficiency, low environmental load, and low noise, and are attracting attention as a clean energy source in the future. In particular, polymer electrolyte fuel cells, which can be used near room temperature, are considered promising for applications such as vehicle-mounted power sources and household stationary power sources, and various research and development on polymer electrolyte fuel cells have been conducted in recent years. Challenges for their practical application include improving cell performance such as power generation characteristics and durability, infrastructure development, and reduction of manufacturing costs.

[0003] A polymer electrolyte fuel cell is generally constructed by stacking a large number of unit cells. A unit cell has a structure in which a membrane electrode assembly, in which a fuel electrode (anode) for supplying fuel gas and an oxygen electrode (cathode) for supplying an oxidant are bonded to both surfaces of a polymer electrolyte membrane, is sandwiched between separators having a gas flow path and a cooling water flow path. The fuel electrode (anode) and the oxygen electrode (cathode) are mainly composed of an electrode catalyst layer containing at least a catalyst substance such as a platinum-based noble metal, a conductive carrier and a polymer electrolyte, and a gas diffusion layer having both gas permeability and electrical conductivity.

[0004] In polymer electrolyte fuel cells, electricity can be generated through the following electrochemical reactions. First, in the fuel electrode catalyst layer, hydrogen contained in the fuel gas is oxidized by the catalyst to form protons and electrons. The generated protons pass through the polymer electrolyte in the electrode catalyst layer and the polymer electrolyte membrane in contact with the electrode catalyst layer to reach the oxygen electrode catalyst layer. Simultaneously, the generated electrons pass through the conductive carrier in the fuel electrode catalyst layer, the gas diffusion layer in contact with the fuel electrode catalyst layer, the separator, and the external circuit to reach the oxygen electrode catalyst layer. Then, in the oxygen electrode catalyst layer, the protons and electrons react with oxygen contained in an oxidizing gas such as air to produce water. In this series of reactions, the proton conduction resistance is greater than the electron conduction resistance, so efficiently conducting protons is important to improve reactivity and enhance the performance of the fuel cell.

[0005] The gas diffusion layer plays the role of diffusing the gas supplied from the separator and supplying it to the electrode catalyst layer. The pores in the electrode catalyst layer are located beyond the separator through the gas diffusion layer and serve as passages for transporting multiple substances. The pores in the fuel electrode are required to smoothly supply hydrogen contained in the fuel gas to the three-phase interface, which is the reaction site for oxidation-reduction. The pores in the oxygen electrode are required to smoothly supply oxygen contained in the oxidizer gas. Furthermore, the pores in the oxygen electrode are required to smoothly discharge the water generated by the reaction. Here, in order to smoothly supply gas and smoothly discharge the water generated, it is important that the electrode catalyst layer has sufficient gaps to allow the water generated to be discharged smoothly and does not have a dense structure.

[0006] To control the structure of the electrode catalyst layer so that it does not become dense and to improve power generation performance, for example, an electrode catalyst layer containing carbon or carbon fibers of different particle sizes has been proposed (Patent Documents 1 and 2).

[0007] Patent Document 1 describes how carbon particles with appropriately different particle sizes are combined to prevent dense packing of carbon particles in the electrode catalyst layer. Patent Document 2 describes how carbon fibers with different fiber lengths are included and their ratio is kept within a certain range to ensure that appropriate pores occupy a large proportion of the electrode catalyst layer. On the other hand, mixing large particles with small particles can cause the small particles to fill the gaps between the large particles, resulting in denser packing. Furthermore, if the carbon material consists only of particles, cracks in the catalyst layer are easily induced, which can lead to a decrease in durability.

[0008] Furthermore, in cases where carbon fibers are used, such as in Patent Document 2, although dense packing can be prevented, the proportion of electron conductors in the catalyst layer increases and the proportion of proton conductors decreases, resulting in increased proton transport resistance and a decrease in power generation performance. Since the power generation performance of a fuel cell depends greatly on mass transport, electron conductivity, and proton conductivity, ultimately, there are limits to improving power generation performance by methods that only increase electron conductivity, such as using combinations of carbon particles or carbon fibers. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Patent No. 3617237 [Patent Document 2] Patent No. 5537178 [Overview of the project] [Problems that the invention aims to solve]

[0010] The present invention has been made in view of the above-mentioned points, and aims to provide an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that can improve the mass transport and proton conductivity in the electrode catalyst layer, exhibit high power generation performance, and have good durability. [Means for solving the problem]

[0011] [1] An electrode catalyst layer used in a polymer electrolyte fuel cell, The electrode catalyst layer comprises a catalyst material, a conductive carrier supporting the catalyst material, a polymer electrolyte, and a fibrous material containing nitrogen atoms. An electrode catalyst layer characterized in that, as obtained by energy-dispersive X-ray spectroscopy of a specific region of the cross-section of the electrode catalyst layer, the ratio of nitrogen atoms to the total number of atoms of carbon, nitrogen, oxygen, fluorine, sulfur, and platinum elements is 2 at% or more and 20 at% or less, and the specific region contains 50 area % or more of a fibrous material containing the nitrogen atoms, and is a region that does not contain the catalyst material or the conductive carrier. [2] The electrode catalyst layer according to [1], characterized in that the average fiber diameter of the fibrous material containing nitrogen atoms is 50 nm or more and 400 nm or less. [3] The electrode catalyst layer according to [1] or [2], wherein the fibrous material containing nitrogen atoms is a polymer fiber. [4] The electrode catalyst layer according to [3], wherein the fibrous material containing the nitrogen atom has an azole structure. [5] The electrode catalyst layer according to any one of [1] to [4], wherein the peak of the fiber diameter distribution of the fibrous material containing nitrogen atoms is 150 nm or more and 250 nm or less. [6] The electrode catalyst layer according to any one of [1] to [5], characterized in that the content of the fibrous material containing nitrogen atoms in the electrode catalyst layer is 1% by weight or more and 10% by weight or less. [7] The electrode catalyst layer according to any one of [1] to [6], characterized in that the thickness of the electrode catalyst layer is 2 μm or more and 10 μm or less. [8] A membrane electrode assembly characterized in that an electrode catalyst layer according to any one of [1] to [7] is provided on at least one surface of a polymer electrolyte membrane. A polymer electrolyte fuel cell characterized by comprising the membrane electrode assembly described in [9] [8]. [Effects of the Invention]

[0012] According to one aspect of the present invention, it is possible to provide an electrode catalyst layer, a membrane electrode assembly and a polymer electrolyte fuel cell which improve mass transport properties and proton conductivity in the electrode catalyst layer, can exhibit high power generation performance, and have excellent durability. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] [Figure 1] FIG. 1 is a cross-sectional view schematically showing an example of the cross-sectional structure of the electrode catalyst layer of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an example of a cross-section of the electrode catalyst layer. [Figure 3] FIG. 3 is an explanatory diagram of the fiber diameter distribution of fibers. [Figure 4] FIG. 4 is an explanatory diagram of fiber diameters of fibers. [Figure 5] FIG. 5 shows a configuration example of the membrane electrode assembly according to the present embodiment, where FIG. 5(a) is a plan view of the membrane electrode assembly viewed from the oxygen electrode side of the electrode catalyst layer, and FIG. 5(b) is a cross-sectional view taken along line X-X' of FIG. 5(a). [Figure 6] FIG. 6 is an exploded perspective view showing a configuration example of a polymer electrolyte fuel cell. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. It should be noted that the present invention is not limited to the embodiments described below, and modifications such as design changes can be made based on the knowledge of those skilled in the art, and embodiments to which such modifications are added are also included in the scope of the present invention. In addition, each drawing is appropriately exaggerated for easy understanding.

[0015] The inventors of the present invention have conducted intensive studies on the initial power generation performance and durable power generation performance of polymer electrolyte fuel cells, and as a result, have found that gas diffusivity and proton conductivity in the electrode catalyst layer have a great influence on these performances. Then, by using a fibrous material containing nitrogen atoms in the electrode catalyst layer, wide voids are formed to improve gas diffusivity and reduce proton conduction resistance. Furthermore, the ratio of the number of nitrogen atoms to the total number of atoms of carbon, nitrogen, oxygen, fluorine, sulfur, and platinum elements obtained by energy dispersive X-ray spectroscopy for a specific region of a cross-section of the electrode catalyst layer is set to 2 at% or more and 20 at% or less, thereby successfully obtaining a polymer electrolyte fuel cell that exhibits high power generation performance and is excellent in durability. It has also been found that when the peak of the fiber diameter distribution of the fibrous material is 150 nm or more and 250 nm or less, proton conduction resistance is reduced without impairing conductivity, and gas diffusivity is improved. As a result, the inventors succeeded in obtaining a polymer electrolyte fuel cell that suppresses a decrease in output and deterioration of the electrode catalyst layer, and exhibits high power generation performance over a long period of time.

[0016] [Configuration of Electrode Catalyst Layer] Hereinafter, a specific configuration of the electrode catalyst layer according to the present embodiment will be described with reference to the drawings. As shown in the schematic diagram of FIG. 1, the electrode catalyst layer 10 according to the present embodiment is joined to the surface of a polymer electrolyte membrane 11, and is configured to include a catalyst material 12, a conductive carrier 13 carrying the catalyst material 12, a polymer electrolyte 14, and a fibrous material 15 containing nitrogen atoms. Portions where none of the above constituent elements are present form voids 4.

[0017] The fibrous material 15 contained in the electrode catalyst layer 10 according to the present embodiment is a fibrous material containing nitrogen atoms. An example of a nitrogen atom-containing fiber is a polymer fiber containing nitrogen atoms. It is preferable that the nitrogen atoms in the fibrous material constitute a Lewis basic group having an unshared electron pair. Fibrous materials containing nitrogen atoms preferably have an azole structure. An azole structure is a heterogeneous five-membered ring structure containing one or more nitrogen atoms, such as an imidazole structure, an oxazole structure, or a thiazole structure. Furthermore, fibrous materials containing nitrogen atoms preferably have a benzoazole structure, such as a benzimidazole structure or a benzoxazole structure. Specific examples of fibrous materials containing nitrogen atoms include polyazole polymers such as polybenzimidazole and polybenzoxazole. Furthermore, the fibrous material containing nitrogen atoms may be a polymer having a pyrrole ring structure or a pyridine ring structure. When a fibrous material contains nitrogen atoms, it can induce interaction between the lone pair of electrons on the nitrogen atoms and the protons of the polymer electrolyte. This improves the proton conductivity in the electrode catalyst layer 10, thereby improving the output characteristics. For example, it becomes possible to cover the fibrous material with a film of polymer electrolyte, thereby simultaneously improving both the mass transport and proton conductivity in the electrode catalyst layer 10. By converting the fibrous material into a polymer, its flexibility is increased, improving the strength of the electrode catalyst layer 10. Furthermore, by introducing nitrogen, the thermal stability of the electrode catalyst layer 10 is improved. Furthermore, it is preferable that the content of fibrous material 15 in the electrode catalyst layer 10 is between 1% by weight and 10% by weight. If the content of fibrous material 15 is less than the above range, the voids 4 become narrower, and sufficient drainage and gas diffusion may not be ensured. Also, cracks may occur in the electrode catalyst layer 10, which may lead to a decrease in durability. If the content of fibrous material 15 is greater than the above range, the proton conduction pathway by the polymer electrolyte 14 may be blocked, and resistance may increase.

[0018] In this embodiment, the ratio of nitrogen atoms to the total number of atoms of carbon, nitrogen, oxygen, fluorine, sulfur, and platinum elements, as obtained by energy-dispersive X-ray spectroscopy (EDX) of a specific region in the cross-section of the electrode catalyst layer, is 2 at% or more and 20 at% or less. The specific region is a cross-section of the electrode catalyst layer that contains 50 area or more of fibrous material containing nitrogen atoms and does not contain catalyst material or conductive support. The shape of the region (observation area) can be rectangular (including square), for example, a 150 nm × 150 nm square. The fibrous material containing nitrogen atoms may occupy 100% of the area in the specific region. There only needs to be one such specific region in the cross-section.

[0019] Figure 2 is a schematic diagram showing an example of a cross-section of an electrode catalyst layer. The cross-section of the electrode catalyst layer consists of a region R1 containing the catalyst material and conductive support, and a region R2 not containing the catalyst material and conductive support. Region R2 consists of a fibrous material 23 containing nitrogen atoms and a region n not containing the fibrous material 23 containing nitrogen atoms. In Figure 2, the above-mentioned specific region (observation area) is indicated by A, and includes only region R2 and does not include region R1. The shape of the region (observation area) can be rectangular (including square), for example, a 150 nm × 150 nm square. The fibrous material 23 containing nitrogen atoms may occupy 100% of the area in the specific region. There only needs to be one such specific region in the cross-section.

[0020] The elemental composition ratios in a specific region can be measured, for example, by performing elemental mapping using a transmission electron microscope (TEM-EDX) equipped with an energy-dispersive X-ray spectrometer. A suitable X-ray acceleration voltage in EDX is 200 kV. This acceleration voltage allows the electron beam to penetrate to a thickness of approximately nanometers in a specific region, providing information about the elements in the fiber and its surroundings.

[0021] In EDX analysis of a specific region, a nitrogen atom ratio of less than 2 at% indicates that the fiber density is too low. In EDX analysis of a specific region, a nitrogen atom ratio of more than 20 at% indicates that the fiber density is too high. In EDX analysis of a specific region, a nitrogen atom ratio of 2 at% to more than 20 at% indicates that the fiber density is appropriate.

[0022] If the fiber density is too low, the interaction between nitrogen-containing fibers and ions such as sulfonic acid groups in the polymer electrolyte weakens, leading to a lack of proton conduction pathways and an increase in resistance. Conversely, if the fiber density is too high, entanglement and aggregation of fibers can clog voids, preventing sufficient drainage and gas diffusion.

[0023] By appropriately setting the density of the fibers, the lone pairs of electrons on nitrogen atoms interact with the protons of the polymer electrolyte, improving the proton conductivity in the electrode catalyst layer and thus improving the output characteristics. The method for exposing the cross-section is the same as for observing the thickness of the electrode catalyst layer 10 described below.

[0024] The weight ratio of the fibrous material 15 contained in the electrode catalyst layer 10 can be determined by the ratio of the weight after removing other contained substances by chemical and electrochemical methods to the weight before removal. For example, the catalyst material can be dissolved by an acid such as aqua regia containing a strong oxidizing agent, and the conductive support can be burned off by high potential. In addition, the polymer electrolyte and polymer electrolyte membrane can be decomposed by hydrogen peroxide or the like.

[0025] The average fiber diameter of the fibrous material 15 contained in the electrode catalyst layer 10 according to this embodiment is preferably 50 nm or more and 400 nm or less. By setting the fiber diameter within this range, the voids 4 within the electrode catalyst layer 10 can be increased and the decrease in proton conductivity can be suppressed, enabling higher power output. If the average fiber diameter of the fibrous material 15 is smaller than the above range, the fibrous material may clog the voids, making it impossible to ensure sufficient drainage and gas diffusion. Since the fibrous material does not conduct electricity, the more it is added, the lower its electricity conductivity becomes. The thinner the fiber diameter, the more fibers can be added for the same amount of added material, and the surface area also increases, so the above effect can be brought to a wider area of ​​the catalyst layer. Therefore, thinner fibrous material is preferable. On the other hand, the size of the voids that can naturally occur when aggregates of catalyst support particles aggregate to form agglomerates is approximately 50 nm. Therefore, if the fiber diameter is less than 50 nm, the voids may be clogged, which may lead to a decrease in drainage. Furthermore, if the average fiber diameter of the fibrous material 15 is larger than the above range, it may inhibit the conduction of electrons and protons by the conductive carrier 13 and polymer electrolyte 14, leading to increased resistance.

[0026] Furthermore, the fiber length of the fibrous material 15 is preferably 1 μm to 80 μm, and more preferably 5 μm to 70 μm. By setting it within the above range, the strength of the electrode catalyst layer 10 can be increased, and consequently, the occurrence of cracks in the electrode catalyst layer 10 when forming the electrode catalyst layer 10 can be suppressed.

[0027] The fiber diameter of the fibrous material 15 can be obtained, for example, by observing the cross-section of the electrode catalyst layer 10 using a scanning electron microscope (SEM) and measuring the diameter of the exposed fibrous material 15 in the cross-section. If the fibrous material 15 is cut obliquely, the shape of the exposed cross-section may be elliptical. In that case, the fiber diameter of the fibrous material 15 can be obtained by measuring the diameter of a perfect circle fitted along the minor axis. The average fiber diameter can be obtained by measuring the fiber diameters of multiple locations, for example, 20 locations of the fibrous material 15, and taking the arithmetic mean. In this embodiment, it is preferable that the peak of the fiber diameter distribution of the nitrogen-containing fibrous material 15 contained in the electrode catalyst layer 10 is between 150 nm and 250 nm. If the fiber diameter of the fibrous material is smaller than the above range, the voids become narrower, and sufficient drainage and gas diffusion may not be ensured. In this case, water may accumulate in the electrode catalyst layer 10, which may lead to a decrease in output and accelerate the deterioration of the electrode catalyst layer. If the fiber diameter of the fibrous material is larger than the above range, the proton conduction pathway by the polymer electrolyte 14 and the electron conduction pathway by the conductive carrier 13 may be blocked, which may increase resistance.

[0028] Here, we will explain the peaks in the fiber diameter distribution of the fibrous material described above. Figure 3 is an explanatory diagram of the fiber diameter distribution of the fibrous material. The graph is a histogram showing the frequency of each fiber diameter of the fibrous material contained in the electrode catalyst layer 10, and represents the distribution of fiber diameters. Generally, histograms are used to see how quantitative data is distributed. A histogram is created by dividing the data into several classes, creating a frequency distribution table, and then plotting the data classes on the horizontal axis and the number of data points included in that class on the vertical axis.

[0029] In this embodiment, the histogram is created with a class width of 10 nm. For example, if the maximum fiber diameter obtained is 298 nm and the minimum is 102 nm, then with a class width of 10 nm, the smallest class will be "100 nm or more and less than 110 nm", and the largest class will be "290 nm or more and less than 300 nm". The number of classes (number of bars in the histogram) will be 20. In this case, each class in the histogram is represented as "100 + (n-1) × 10 nm or more and less than 100 + n × 10 nm" (n = 1 to 20). By determining the classes in this way and creating a frequency distribution table of the measured fiber diameters, a histogram representing the fiber diameter distribution is obtained. The peak of the fiber diameter distribution refers to the median of the class with the largest frequency in the frequency distribution table and the histogram. For example, in the histogram of fiber diameters of the fibrous material contained in the electrode catalyst layer 10, if the frequency of the class between 200 nm and 210 nm is the highest, the peak of the fiber diameter distribution of the fibrous material is 205 nm.

[0030] The fiber diameter of a fibrous material can be obtained, for example, by measuring the diameter of the exposed fibrous material when observing the cross-section of the electrode catalyst layer 10 using a scanning electron microscope (SEM), as shown in Figure 4. If the fibrous material is cut obliquely, the shape of the exposed cross-section may be elliptical. In that case, the fiber diameter can be obtained by measuring the diameter of a perfect circle fitted along the minor axis. In addition, the surface of the fibrous material may be exposed rather than the cross-section. In that case, the fiber diameter can be obtained by measuring the width of the fibers perpendicular to the major axis of the exposed fibrous material. By measuring the fiber diameters of multiple fibrous materials, a histogram representing the frequency of each fiber diameter can be obtained. The more measurement points there are for the fiber diameter, the clearer it is possible to identify the peak of the fiber diameter. In order to grasp the fiber diameter distribution of the fibrous material without bias within the electrode catalyst layer 10, it is preferable to measure similarly at at least 20 or more observation points. The observation magnification using a scanning electron microscope (SEM) is preferably 50,000x or higher, as this allows for clear confirmation of the contours of fibrous materials and accurate measurement of fiber diameter.

[0031] As a method for exposing the cross-section of the electrode catalyst layer 10, known methods such as ion milling and ultramicrotomes can be used. When performing the processing to expose the cross-section, it is particularly preferable to use cryo-ion milling, which processes the electrode catalyst layer 10 while cooling it, in order to reduce damage to the polymer electrolyte membrane 11 and the polymer electrolyte 14 that constitutes the electrode catalyst layer 10.

[0032] The thickness of the electrode catalyst layer 10 is preferably 30 μm or less. If the thickness is greater than 30 μm, cracks are more likely to occur, and when used in a fuel cell, the diffusivity and conductivity of the gas and water produced may decrease, potentially leading to a decrease in output. The thickness may be 10 μm or less.

[0033] The thickness of the electrode catalyst layer 10 is preferably 2 μm or more. If the thickness is greater than 10 μm, cracks are more likely to occur, and when used in a fuel cell, the diffusivity and conductivity of the gas and water produced will decrease, resulting in a decrease in output. The thickness of the electrode catalyst layer 10 may be 5 μm or more.

[0034] The thickness of the electrode catalyst layer 10 can be measured, for example, by observing the cross-section of the electrode catalyst layer 10 using a scanning electron microscope (SEM). Known methods such as ion milling and ultramicrotome can be used to expose the cross-section of the electrode catalyst layer 10. When performing the processing to expose the cross-section, it is particularly preferable to use cryo-ion milling, which cools the electrode catalyst layer 10 during processing, in order to reduce damage to the polymer electrolyte membrane 11 and the polymer electrolyte 14 constituting the electrode catalyst layer 10. For example, the thickness of the electrode catalyst layer can be measured by measuring its length within a field of view that encompasses the entire electrode catalyst layer at an observation magnification of approximately 1,000 to 10,000 times. To obtain a uniform thickness within the catalyst layer, it is preferable to perform similar measurements at at least 20 observation points.

[0035] The catalyst material 12 can be, for example, metals included in the platinum group, metals other than the platinum group, and alloys, oxides, complex oxides, and carbides of these metals. Metals included in the platinum group are platinum, palladium, ruthenium, iridium, rhodium, and osmium. Metals other than the platinum group can be iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum.

[0036] The conductive carrier 13 can be a carrier that is conductive and capable of supporting the catalyst material 12 without being eroded by the catalyst material 12. Carbon particles can be used for the conductive carrier 13. Examples of carbon particles include carbon black, graphite, activated carbon, carbon nanotubes, carbon nanofibers, and fullerenes. The particle size of the carbon particles is preferably between 10 nm and 1000 nm, and more preferably between 10 nm and 100 nm. A particle size of 10 nm or more prevents the carbon particles from becoming too densely packed in the electrode catalyst layer 10, thereby suppressing a decrease in the gas diffusivity of the electrode catalyst layer 10. A particle size of 1000 nm or less suppresses the formation of cracks in the electrode catalyst layer 10. The particle size of the carbon particles is the volume-average diameter determined by the laser diffraction / scattering method.

[0037] The polymer electrolyte 14 contained in the polymer electrolyte membrane 11 and the electrode catalyst layer 10 can be an electrolyte having proton conductivity. For example, fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes can be used. For fluorine-based polymer electrolytes, polymer electrolytes having a tetrafluoroethylene skeleton can be used. Nafion® manufactured by DuPont can be an example of a polymer electrolyte having a tetrafluoroethylene skeleton. For hydrocarbon-based polymer electrolytes, for example, sulfonated polyether ketones, sulfonated polyether sulfones, sulfonated polyether ether sulfones, sulfonated polysulfides, and sulfonated polyphenylenes can be used.

[0038] The polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 14 contained in the electrode catalyst layer 10 may be the same electrolyte or different electrolytes. However, considering the interfacial resistance at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 10, and the rate of dimensional change in the polymer electrolyte membrane 11 and the electrode catalyst layer 10 when humidity changes, it is preferable that the polymer electrolyte contained in the polymer electrolyte membrane 11 and the polymer electrolyte 14 contained in the electrode catalyst layer 10 are the same electrolyte or polymer electrolytes with similar coefficients of thermal expansion.

[0039] [Configuration of the membrane electrode assembly] Next, with reference to Figure 5, the specific configuration of the membrane electrode assembly 1 equipped with the electrode catalyst layer 10 according to this embodiment will be described. Figure 5 shows an example of the configuration of the membrane electrode assembly according to this embodiment, where (a) is a plan view of the membrane electrode assembly as seen from the oxygen electrode side of the electrode catalyst layer 10, and (b) is a cross-sectional view obtained by breaking along the XX' line in (a). The membrane electrode assembly 1 comprises a polymer electrolyte membrane 11 and electrode catalyst layers 10C and 10A bonded to the front and back surfaces of the polymer electrolyte membrane 11, respectively. In this embodiment, the electrode catalyst layer 10C formed on the upper surface (front surface) of the polymer electrolyte membrane 11 is the cathode-side electrode catalyst layer constituting the oxygen electrode, and the electrode catalyst layer 10A formed on the lower surface (back surface) of the polymer electrolyte membrane 11 is the anode-side electrode catalyst layer constituting the fuel electrode. Hereinafter, the pair of electrode catalyst layers 10C and 10A may be abbreviated as "electrode catalyst layer 10" when there is no need to distinguish between them. In the membrane electrode assembly 1 according to this embodiment, the electrode catalyst layer 10 only needs to be provided on at least one surface of the polymer electrolyte membrane 11. Furthermore, in order to prevent gas leakage from the outer peripheral portion of the polymer electrolyte membrane 11 where the electrode catalyst layer 10 is not bonded, the membrane electrode assembly 1 is provided with a gasket 16C on the oxygen electrode side and a gasket 16A on the fuel electrode side.

[0040] [Manufacturing method for membrane electrode assembly] The manufacturing method for the membrane electrode assembly 1 described above will be explained below. First, a catalyst ink is prepared. The catalyst material 12, conductive carrier 13, polymer electrolyte 14, and fibrous material 15 are mixed in a dispersion medium, and then the mixture is subjected to a dispersion treatment to produce the catalyst ink. The dispersion treatment can be carried out using, for example, a planetary ball mill, a bead mill, and an ultrasonic homogenizer.

[0041] The dispersion medium for the catalyst ink can be a solvent that does not erode the catalyst material 12, conductive carrier 13, polymer electrolyte 14, and fibrous material 15, and that can dissolve the polymer electrolyte 14 or disperse the polymer electrolyte 14 as a fine gel while maintaining high fluidity of the dispersion medium. The dispersion medium may contain water. The catalyst ink preferably contains a volatile liquid organic solvent. Since there is a risk of ignition if the solvent is a lower alcohol, it is preferable that water is mixed with such a solvent. Water can be mixed with the solvent to the extent that the catalyst ink does not become cloudy or gel due to the separation of the polymer electrolyte 14.

[0042] After applying the prepared catalyst ink to the substrate and drying it, the solvent is removed from the catalyst ink coating. This forms an electrode catalyst layer 10 on the substrate. A polymer electrolyte membrane 11 or a transfer substrate can be used as the substrate. When using a polymer electrolyte membrane 11 as the substrate, for example, a method can be used to form the electrode catalyst layer 10 by directly applying the catalyst ink to the surface of the polymer electrolyte membrane 11 and then removing the solvent from the catalyst ink coating. Subsequently, the catalyst ink is directly applied to the surface opposite the polymer electrolyte membrane 11 so as to face the electrode catalyst layer 10A across the polymer electrolyte membrane 11, and then the solvent is removed from the catalyst ink coating to form the electrode catalyst layer 10A, thereby obtaining the membrane electrode assembly 1. When using a transfer substrate, a catalyst layer substrate is prepared by applying a catalyst ink onto the transfer substrate and then drying it. Subsequently, for example, the electrode catalyst layer 10 and the polymer electrolyte membrane 11 are bonded together by heating and pressurizing while the surface of the electrode catalyst layer 10 on the catalyst layer substrate is in contact with the polymer electrolyte membrane 11. A membrane electrode assembly 1 can be manufactured by bonding the electrode catalyst layer 10 to both sides of the polymer electrolyte membrane 11.

[0043] To adjust the ratio of nitrogen atoms to the total number of atoms of carbon, nitrogen, oxygen, fluorine, sulfur, and platinum elements, obtained by energy-dispersive X-ray spectroscopy of a specific region of the electrode catalyst layer, to the above range, for example, this can be achieved by adjusting the fiber diameter of nitrogen-containing fibers in the electrode catalyst layer, adjusting the amount of polymer electrolyte in the electrode catalyst layer, or adjusting the amount of nitrogen-containing fibers in the electrode catalyst layer.

[0044] Furthermore, when using a gas diffusion layer 17 as a substrate, for example, a catalyst layer-attached gas diffusion layer 17 is prepared by applying a catalyst ink to the surface of the gas diffusion layer 17 and then drying it. Subsequently, the electrode catalyst layer 10 and the polymer electrolyte membrane 11 are bonded together by heating and pressurizing while the surface of the electrode catalyst layer 10 on the catalyst-attached gas diffusion layer 17 is in contact with the polymer electrolyte membrane 11. By bonding the electrode catalyst layer 10 to both sides of the polymer electrolyte membrane 11, a membrane electrode assembly 1 can be manufactured.

[0045] Various coating methods can be used to apply the catalyst ink to the substrate. Examples of coating methods include die coating, roll coating, curtain coating, spray coating, and squeegee coating. Die coating is preferred as the coating method. Die coating is preferred because it allows for stable film thickness during the coating period and enables intermittent coating. Methods for drying the catalyst ink coating include drying using a hot air oven, IR (far-infrared) drying, drying using a hot plate, and reduced-pressure drying. The drying temperature is 40°C to 200°C, preferably around 40°C to 120°C. The drying time is 0.5 minutes to 1 hour, preferably around 1 minute to 30 minutes.

[0046] When forming an electrode catalyst layer 10 on a transfer substrate, the pressure and temperature applied to the electrode catalyst layer 10 during transfer affect the power generation performance of the membrane electrode assembly 1. To obtain a membrane electrode assembly with high power generation performance, the pressure applied to the electrode catalyst layer 10 is preferably between 0.1 MPa and 20 MPa. A pressure of 20 MPa or less prevents excessive compression of the electrode catalyst layer 10. A pressure of 0.1 MPa or more prevents a decrease in power generation performance due to a decrease in bonding between the electrode catalyst layer 10 and the polymer electrolyte membrane 11. The bonding temperature is preferably near the glass transition temperature of the polymer electrolyte membrane 11 or the polymer electrolyte 14 contained in the electrode catalyst layer 10, considering the improvement of bonding at the interface between the polymer electrolyte membrane 11 and the electrode catalyst layer 10 and the suppression of interfacial resistance.

[0047] For the transfer substrate, for example, polymer films and sheets formed from fluororesins can be used. Fluorine resins have excellent transfer properties. Examples of fluororesins include ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), and polytetrafluoroethylene (PTFE). Examples of polymers that form polymer films include polyimide, polyethylene terephthalate, polyamide (nylon®), polysulfone, polyethersulfone, polyphenylene sulfide, polyether / etherketone, polyetherimide, polyarylate, and polyethylene naphthalate. A gas diffusion layer can also be used as the transfer substrate.

[0048] Here, by adjusting the blending ratio of the fibrous material 15, the blending ratio of the polymer electrolyte 14, the solvent composition of the catalyst ink, the dispersion strength during catalyst ink preparation, the heating temperature of the coated catalyst ink and its heating rate, the electrode catalyst layer 10 can be made to have sufficient gas diffusivity and proton conductivity. For example, the proportion of polymer electrolyte 14 in the electrode catalyst layer 10 is preferably about the same as to about half the weight of the conductive carrier 13. Also, the proportion of fibrous material 15 is preferably about the same as or less than the weight of the conductive carrier 13. The solid content ratio of the catalyst ink is preferably as high as possible within the range that allows for coating a thin film. Furthermore, the blending ratio of nitrogen-containing fibrous material 15 in the electrode catalyst layer 10 is preferably 1% by mass or more and 10% by mass or less. If the blending ratio of nitrogen-containing fibrous material 15 in the electrode catalyst layer 10 is less than 1% by mass, the effects of reducing proton conduction resistance and improving gas diffusion may not be sufficiently obtained, and cracks may occur when forming the electrode catalyst layer 10, reducing durability during long-term operation. On the other hand, if the blending ratio of fibrous material 15 in the electrode catalyst layer 10 is more than 10% by mass, the catalytic reaction may be inhibited, potentially reducing battery performance. The solid content ratio of the catalyst ink is preferably as high as possible within the range that allows for coating a thin film.

[0049] [Configuration of a polymer electrolyte fuel cell] Next, with reference to Figure 6, a specific example of the configuration of a polymer electrolyte fuel cell 3 equipped with the membrane electrode assembly 1 according to this embodiment will be described. Figure 6 is an exploded perspective view showing an example of the configuration of a polymer electrolyte fuel cell 3 with the membrane electrode assembly 1 attached. Note that Figure 6 is an example of a single-cell configuration, and the polymer electrolyte fuel cell 3 is not limited to this configuration; it may also be a configuration in which multiple single cells are stacked.

[0050] As shown in Figure 6, the polymer electrolyte fuel cell 3 comprises a membrane electrode assembly 1, a gas diffusion layer 17C on the oxygen electrode side, and a gas diffusion layer 17A on the fuel electrode side. The gas diffusion layer 17C is positioned opposite the electrode catalyst layer 10C, which is the cathode side electrode catalyst layer on the oxygen electrode side of the membrane electrode assembly 1. The gas diffusion layer 17A is positioned opposite the electrode catalyst layer 10A, which is the anode side electrode catalyst layer on the fuel electrode side of the membrane electrode assembly 1. The electrode catalyst layer 10C and the gas diffusion layer 17C constitute the oxygen electrode 2C, and the electrode catalyst layer 10A and the gas diffusion layer 17A constitute the fuel electrode 2A. In addition, gaskets 16C on the oxygen electrode side and gaskets 16A on the fuel electrode side are provided to prevent gas leakage from the outer peripheral portion of the polymer electrolyte membrane 11 where the electrode catalyst layer 10 is not bonded.

[0051] Furthermore, the polymer electrolyte fuel cell 3 includes a separator 18C positioned opposite the oxygen electrode 2C and a separator 18A positioned opposite the fuel electrode 2A. Separator 18C includes a gas channel 19C for reaction gas flow formed on the surface facing the gas diffusion layer 17C, and a cooling water channel 20C for cooling water flow formed on the surface opposite to the surface where the gas channel 19C is formed. Separator 18A has a similar configuration to separator 18C and includes a gas channel 19A formed on the surface facing the gas diffusion layer 17A, and a cooling water channel 20A formed on the surface opposite to the surface where the gas channel 19A is formed. Separators 18C and 18A are made of a conductive and gas-impermeable material.

[0052] Then, in the polymer electrolyte fuel cell 3, an oxidizing agent such as air or oxygen is supplied to the oxygen electrode 2C through the gas channel 19C of the separator 18C, and a fuel gas containing hydrogen or an organic fuel is supplied to the fuel electrode 2A through the gas channel 19A of the separator 18A, thereby generating electricity.

[0053] The polymer electrolyte fuel cell 3 according to this embodiment employs the membrane electrode assembly 1 according to this embodiment, thereby providing sufficient drainage and gas diffusion, enabling it to exhibit high power generation performance and high durability over the long term. In other words, according to this embodiment, it is possible to provide an electrode catalyst layer 10, a membrane electrode assembly 1, and a solid polymer fuel cell 3 that have sufficient gas diffusivity and proton conductivity in the operation of the solid polymer fuel cell 3, and that can exhibit high power generation performance and high durability over the long term. Therefore, the present invention can be suitably used in stationary cogeneration systems and fuel cell vehicles that utilize solid polymer fuel cells, and has great industrial value. [Examples]

[0054] The following describes a membrane electrode assembly according to an embodiment of the present invention. [Example A1] In Example A1, a platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte (20% Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and a fibrous material containing nitrogen atoms (polyazole fiber, average fiber diameter 100-400 nm, peak of average fiber diameter distribution 210 nm) were mixed. This mixture was dispersed using a planetary ball mill at 300 rpm for 60 minutes. At that time, zirconia balls with a diameter of 5 mm were added to fill approximately one-third of the zirconia container. The weight of the polymer electrolyte was set to 100% by weight relative to the weight of the carbon support in the platinum-supported carbon catalyst, the weight of the fibrous material containing nitrogen atoms was set to 10% by weight relative to the weight of the carbon support in the platinum-supported carbon catalyst, the proportion of water in the dispersion medium was set to 50% by weight, and the solid content concentration was adjusted to 10% by weight to prepare a catalyst ink.

[0055] A coating film was formed by applying catalyst ink to one side of a polymer electrolyte membrane (Nafion® 211, manufactured by Dupont) to a thickness of 200 μm using a slit die coater. Next, the polymer electrolyte membrane with the coating film was dried in an 80°C hot air oven until the coating film's tackiness disappeared, forming the cathode-side electrode catalyst layer. Then, a coating film was formed on the opposite side of the polymer electrolyte membrane to a thickness of 50 μm using a slit die coater. Next, the polymer electrolyte membrane with the coating film was dried in an 80°C hot air oven until the coating film's tackiness disappeared, forming the anode-side electrode catalyst layer. This obtained the membrane electrode assembly of Example A1. The nitrogen element composition ratio in the catalyst layer was 12%.

[0056] [Example A2] The film electrode assembly of Example A2 was obtained by the same method as in Example A1, except that the amount of fibrous material containing nitrogen atoms was doubled compared to Example A1 (20% by weight relative to the weight of the carbon support) when preparing the catalyst ink. The nitrogen element composition ratio in the catalyst layer was 20%.

[0057] [Example A3] The film electrode assembly of Example A3 was obtained by the same method as in Example A2, except that the amount of fibrous material containing nitrogen atoms was halved compared to Example A1 (5% by weight relative to the weight of the carbon support) when preparing the catalyst ink. The nitrogen element composition ratio in the catalyst layer was 2%.

[0058] [Comparative Example A1] Comparative Example A1 was obtained by the same method as in Example A1, except that carbon nanofibers (VGCF-H®, manufactured by Showa Denko Packaging) were added instead of fibrous material containing nitrogen atoms when preparing the catalyst ink. The nitrogen element composition ratio in the catalyst layer was 0%.

[0059] [Comparative example A2] A film electrode assembly of Comparative Example 2 was obtained by the same method as in Example A1, except that the amount of fibrous material containing nitrogen atoms was 1 / 10 of that in Example A1 (1% by weight relative to the weight of the carbon support) when preparing the catalyst ink. The nitrogen element composition ratio in the catalyst layer was 0.8%.

[0060] [Comparative example A3] A film electrode assembly of Comparative Example A3 was obtained by the same method as in Example A1, except that the amount of fibrous material containing nitrogen atoms was three times that of Example A1 (30% by weight relative to the weight of the carbon support) when preparing the catalyst ink. The nitrogen element composition ratio in the catalyst layer was 21%.

[0061] The following describes the results of comparing the fibrous material content, power generation performance, and durability of solid polymer fuel cells equipped with the membrane electrode assemblies of Examples A1 to A3 and Comparative Examples A1 to A3.

[0062] [Example B1] In Example 1, a platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte (Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and a fibrous material 15 (polyazole fiber, average fiber diameter 200 nm, peak fiber diameter distribution 205 nm, average fiber length approximately 20 μm) were mixed. This mixture was dispersed using a planetary ball mill (Fritsch P-7) at 500 rpm for 60 minutes. At that time, zirconia balls with a diameter of 5 mm were added to fill approximately one-third of the zirconia container. The catalyst ink was prepared by adjusting the mass of the polymer electrolyte to 100% by mass relative to the mass of the carbon particles, the mass of the fibrous material in the solid content to 1% by mass, the proportion of water in the dispersion medium to 50% by mass, and the solid content concentration to 10% by mass.

[0063] A coating film was formed by applying catalyst ink to one side of a polymer electrolyte membrane (Nafion® 211, manufactured by Dupont) to a thickness of 100 μm using a slit die coater. Next, the polymer electrolyte membrane with the coating film was dried in an 80°C hot air oven until the coating film lost its tackiness, forming the cathode-side electrode catalyst layer. Then, a coating film was formed on the opposite side of the polymer electrolyte membrane to a thickness of 50 μm using a slit die coater. Next, the polymer electrolyte membrane with the coating film was dried in an 80°C hot air oven until the coating film lost its tackiness, forming the anode-side electrode catalyst layer. This obtained the membrane electrode assembly 1 of Example B1.

[0064] [Example B2] The film electrode assembly of Example B2 was obtained by the same method as in Example B1, except that polyazole fibers with a peak in the fiber diameter distribution of 195 nm were used.

[0065] [Example B3] The film electrode assembly of Example B3 was obtained by the same method as in Example B1, except that polyazole fibers with a peak in the fiber diameter distribution of 225 nm were used.

[0066] [Example B4] The film electrode assembly of Example B4 was obtained by the same method as in Example B1, except that polyazole fibers with a peak in the fiber diameter distribution of 215 nm were used.

[0067] [Example B5] The film electrode assembly of Example B5 was obtained by the same method as in Example B1, except that polyazole fibers with a peak in the fiber diameter distribution of 205 nm were used.

[0068] [Example B6] The film electrode assembly of Example B6 was obtained by the same method as in Example B1, except that polyazole fibers with a peak in the fiber diameter distribution of 185 nm were used.

[0069] [Example B7] A catalyst ink was prepared in the same manner as in Example B6. A coating film was formed by applying the catalyst ink to the surface of a PTFE film to a thickness of 100 μm using a slit die coater. Next, the PTFE film with the coating film was dried in a hot air oven at 80°C until the coating film lost its tackiness, to obtain a transfer substrate with a cathode-side electrode catalyst layer. Next, a coating film was formed by applying the catalyst ink to the surface of another PTFE film to a thickness of 50 μm using a slit die coater. Next, the PTFE film with the coating film was dried in a hot air oven at 80°C until the coating film lost its tackiness, to obtain a transfer substrate with an anode-side electrode catalyst layer.

[0070] A transfer substrate with a cathode-side electrode catalyst layer and a transfer substrate with an anode-side electrode catalyst layer were placed on the front and back surfaces of a polymer electrolyte membrane (Nafion® 211, manufactured by Dupont) so that they faced each other, forming a laminate. Next, the laminate was hot-pressed at 120°C and 1 MPa to bond the electrode catalyst layers to the front and back surfaces of the polymer electrolyte membrane. Finally, the PTFE film was peeled off from each electrode catalyst layer to obtain the membrane electrode assembly of Example B7.

[0071] [Example B8] A film electrode assembly of Comparative Example B8 was obtained by the same method as in Example B1, except that a polyazole fiber with a peak in its fiber diameter distribution of 295 nm was used.

[0072] [Comparative Example B1] The film electrode assembly of Comparative Example B1 was obtained by the same method as in Example B1, except that polyazole fibers were not added when preparing the catalyst ink.

[0073] [Comparative example B2] A film electrode assembly of Comparative Example B2 was obtained by the same method as in Example B1, except that carbon nanofibers (VGCF®-H, manufactured by Showa Denko K.K.) were added instead of polyazole fibers when preparing the catalyst ink.

[0074] [Measurement of power generation performance] For measuring power generation performance, a JARI standard cell was used as the evaluation cell. This cell was constructed in accordance with the "Cell Evaluation and Analysis Protocol," a publication of the New Energy and Industrial Technology Development Organization (NEDO), by placing gas diffusion layers, gaskets, and separators on both sides of the membrane electrode assembly and tightening it to a predetermined surface pressure. IV measurements were then performed using the "standard" conditions described in the "Cell Evaluation and Analysis Protocol," as well as IV measurements with both the anode and cathode relative humidity set to RH100% (high humidity conditions). Regarding the evaluation of power generation performance, a "○" was given if the current at 0.6V under "standard" conditions was 25A or more, and the current at 0.6V under "high humidity" conditions was 30A or more. A "×" was given if either of these current values ​​was not met.

[0075] [Measurement of durability A and B] For durability measurement, the same single cell used for measuring power generation performance was used as the evaluation single cell. Durability was then measured using the humidity cycle test described in the "Cell Evaluation and Analysis Protocol" mentioned above. In the evaluation of durability A, a "○" was given if the hydrogen cross-leakage current after 8,000 cycles was less than 10 times the initial value, and a "×" was given if it was 10 times or more. In the evaluation of durability B, a "○" was given if the current after 10,000 cycles was 50% or more of the initial value, and a "×" was given if it was less than 50%.

[0076] [Measurement of fiber diameter distribution peaks] The fiber diameter distribution peak was measured by observing the cross-section of the film electrode assembly using a scanning electron microscope (SEM). Specifically, a small piece of film electrode assembly 1 was first bonded to a metal plate, and the cross-section of the electrode catalyst layer was exposed using a JEOL IB-19520CCP cross-sectional sample preparation device. Next, the exposed cross-section was observed at a magnification of 50,000x using a Hitachi High-Technologies FE-SEM S-4800, and the fiber diameter of the polymer fibers within the field of view was measured using the circle diameter measurement function. If the fibers were cut obliquely, the diameter of a perfect circle fitted along the short axis was measured. In cases where the surface of the fiber was exposed rather than the cross-section, the width of the fiber perpendicular to the long axis of the exposed fiber was measured. This was performed evenly at multiple observation points within the catalyst layer, and a data set of fiber diameters for 30 polymer fibers was obtained. Using this data set, a frequency distribution table was created with a class width of 10 nm, and a histogram representing the fiber diameter distribution was obtained. The median of the class with the highest frequency in the histogram was obtained as the peak of the fiber diameter distribution of polymer fibers.

[0077] [Measurement of nitrogen atom ratio in a specific region of the cross-section] The ratio of nitrogen atoms to the total number of atoms of carbon, nitrogen, oxygen, fluorine, sulfur, and platinum elements was measured using energy-dispersive X-ray spectroscopy in a specific region of the electrode catalyst layer. Specifically, a cross-section of the electrode catalyst layer was first obtained using cryo-ion milling, which processes the electrode catalyst layer while cooling it. Next, elemental mapping of a specific region of the cross-section was performed using a transmission electron microscope (TEM-EDX) equipped with energy-dispersive X-ray spectroscopy, and the elemental ratio of each element was obtained to calculate the nitrogen atomic fraction. The acceleration voltage was set to 200 kV. The specific region was defined as an area of ​​150 nm × 150 nm, where fibrous material containing nitrogen atoms occupied more than 50% of the field of view.

[0078] [Comparison results] Table 1 shows the conditions and results for each example and comparative example.

[0079] [Table 1]

[0080] As shown in Table 1, in both Examples A and B, the nitrogen atom content in the electrode catalyst layer was between 2% and 20% by weight. Furthermore, both Examples A and B received a "○" rating for power generation performance and durability. In other words, Examples A and B yielded membrane electrode assemblies capable of constructing fuel cells with excellent power generation performance and durability.

[0081] On the other hand, in comparative examples A1-A3 and B1-B2, the nitrogen atom ratio in the electrode catalyst layer was outside the range of 2 at% to 20 at%. Furthermore, at least one of the power generation performance and durability A and B was marked as "×". In other words, when the nitrogen element composition ratio in the electrode catalyst layer fell outside the above range, at least one of the power generation performance and durability decreased.

[0082] Therefore, it was found that when the nitrogen element composition ratio in the electrode catalyst layer is between 2 at% and 20 at%, a membrane electrode assembly capable of constructing a fuel cell with even better power generation performance can be obtained.

[0083] As shown in Table 1, the properties were particularly improved in the examples where the peak of the polymer fiber diameter distribution was between 150 nm and 250 nm. [Explanation of Symbols]

[0084] 1...Membrane electrode assembly, 2C...Oxygen electrode, 2A...Fuel electrode, 3...Solid polymer fuel cell, 4...Void, 10, 10C, 10A...Electrode catalyst layer, 11...Polymer electrolyte membrane, 12...Catalyst material, 13...Conductive carrier, 14...Polymer electrolyte, 15...Fibrous material containing nitrogen atoms, 16C, 16A...Gasket, 17C, 17A...Gas diffusion layer, 18C, 18A...Separator, 19C, 19A...Gas flow path, 20C, 20A...Cooling water flow path.

Claims

1. An electrode catalyst layer used in a polymer electrolyte fuel cell, The electrode catalyst layer comprises a catalyst material, a conductive carrier supporting the catalyst material, a polymer electrolyte, and a fibrous material containing nitrogen atoms. Energy-dispersive X-ray spectroscopy of a specific region in the cross-section of the electrode catalyst layer reveals that the ratio of nitrogen atoms to the total number of atoms of carbon, nitrogen, oxygen, fluorine, sulfur, and platinum elements is 2 at% or more and 20 at% or less, and the specific region contains 50 area % or more of a fibrous material containing the nitrogen atoms, and does not contain the catalyst material or the conductive carrier. The fibrous material containing nitrogen atoms is a polymer fiber having an azole structure, which is the electrode catalyst layer.

2. The electrode catalyst layer according to claim 1, characterized in that the average fiber diameter of the fibrous material containing nitrogen atoms is 50 nm or more and 400 nm or less.

3. The electrode catalyst layer according to claim 1 or 2, wherein the peak of the fiber diameter distribution of the fibrous material containing nitrogen atoms is 150 nm or more and 250 nm or less.

4. The electrode catalyst layer according to claim 1 or 2, characterized in that the content of the fibrous material containing nitrogen atoms in the electrode catalyst layer is 1% by weight or more and 10% by weight or less.

5. The electrode catalyst layer according to claim 1 or 2, characterized in that the thickness of the electrode catalyst layer is 2 μm or more and 10 μm or less.

6. A membrane electrode assembly characterized in that the electrode catalyst layer according to claim 1 or 2 is provided on at least one surface of a polymer electrolyte membrane.

7. A polymer electrolyte fuel cell characterized by comprising the membrane electrode assembly described in claim 6.

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