Electrode catalyst, gas diffusion electrode forming composition, gas diffusion electrode, membrane electrode assembly, and fuel cell stack

Abstract

The present invention provides an electrode catalyst with excellent catalytic activity that can contribute to cost reduction in PEFC. The electrode catalyst comprises: a hollow carbon support having nanopores with a pore diameter of 1 to 20 nm; and a plurality of catalyst particles supported on the support. The catalyst particles are supported both inside and outside the nanopores of the support. The catalyst particles are composed of Pt (0 valence). When particle size distribution analysis of the catalyst particles is performed using a three-dimensional reconstructed image obtained by electron beam tomography measurement using STEM, the ratio of the catalyst particles supported inside the nanopores is 50% or greater.

Description

Technical Field

[0001] The present invention relates to an electrode catalyst using hollow carbon as a carrier. More specifically, it relates to an electrode catalyst suitable for a gas diffusion electrode, and more particularly to an electrode catalyst suitable for a gas diffusion electrode of a fuel cell.

[0002] The present invention also relates to a composition for forming a gas diffusion electrode, a membrane electrode assembly, and a fuel cell stack containing the electrode catalyst particles. Background Art

[0003] Polymer Electrolyte Fuel Cell (hereinafter referred to as "PEFC" as needed) is being researched and developed as a power source for fuel cell vehicles and household cogeneration systems.

[0004] The catalyst used in the gas diffusion electrode of PEFC uses a noble metal catalyst composed of noble metal particles of a platinum group element such as platinum (Pt).

[0005] For example, as a typical conventional catalyst, a “Pt-supported carbon catalyst” is known, which is a catalyst particle powder in which Pt fine particles are supported on conductive carbon powder (hereinafter referred to as a “Pt / C catalyst” as needed).

[0006] The cost of precious metal catalysts such as Pt accounts for a large proportion of the production cost of PEFC, and there are technical problems in reducing the cost of PEFC and in promoting its widespread use.

[0007] In these research and development efforts, in order to reduce the amount of platinum used, the current focus is on powders (hereinafter referred to as "core-shell catalysts") of catalyst particles (hereinafter referred to as "core-shell catalyst particles" as needed) having a core structure formed by a core portion composed of non-platinum elements and a shell portion composed of Pt, and many reports have been completed.

[0008] For example, Patent Document 1 discloses a particle composite material (corresponding to a core-shell catalyst particle) having a structure in which palladium (Pd) or a Pd alloy (corresponding to a core) is covered with an atomically thin layer of Pt atoms (corresponding to a shell). Patent Document 1 also describes, as an example, a core-shell catalyst particle having a structure in which the core is a Pd particle and the shell is a layer composed of Pt.

[0009] On the other hand, as supports for electrode catalysts, there are hollow carbons having many pores inside primary particles, and solid carbons having fewer pores inside primary particles than hollow carbons. Research is being conducted on these to improve performance by flexibly utilizing their respective characteristics.

[0010] For example, Patent Document 2 discloses a research example in which hollow carbon is used as a support, and Patent Document 3 discloses a research example in which solid carbon is used as a support.

[0011] For example, Patent Document 2 discloses the structure of an electrode catalyst 200 as follows: Figure 10 As shown, for a porous carrier (hollow carbon) 220 with an average particle size of 20 to 100 nm, the pore volume of pores P220 with a pore diameter of 4 to 20 nm and the mode diameter of the pore distribution are controlled within a specified range, and catalyst particles 230 are loaded in the primary pores P220 of the carrier 220.

[0012] Patent Document 2 describes that this prevents the adsorption of the polymer electrolyte onto the surfaces of the catalyst particles 230 within the primary pores P220, thereby preventing a decrease in the effective reaction surface area of ​​the catalyst and ensuring sufficient gas transportability. It further describes that this results in increased catalyst activity per unit weight, enabling the provision of a fuel cell catalyst layer exhibiting excellent power generation performance even with a reduced catalyst dosage.

[0013] For example, Patent Document 3 discloses an electrode catalyst (PtCo / C catalyst) for a fuel cell comprising a solid carbon support and catalyst particles containing an alloy of platinum and cobalt supported on the support. The molar ratio of platinum to cobalt in the alloy of the electrode catalyst is 4 to 11:1, and the electrode catalyst has been subjected to an acid treatment at 70 to 90°C.

[0014] In Patent Document 3, when a PtCo alloy is supported on a hollow carbon support, a portion of the PtCo alloy is contained inside the hollow carbon support. Even if an acid treatment is performed to suppress the dissolution of Co, it is difficult to fully treat the PtCo alloy present inside the support. As a result, Co is easily dissolved from the PtCo alloy present inside the support, which has become a technical problem.

[0015] Therefore, Patent Document 3 describes that by using a solid carbon support instead of a hollow carbon support, the PtCo alloy can be prevented from being contained within the support. It further discloses that this allows the PtCo alloy to be adequately acid-treated and suppresses the dissolution of Co. It states that, as a result, both initial performance and durability of the fuel cell can be achieved.

[0016] However, in Patent Document 3, medium solid carbon is defined as follows.

[0017] That is, Patent Document 3 states that "solid carbon" is carbon with fewer voids inside the carbon than hollow carbon, and specifically refers to carbon in which the ratio of the BET surface area obtained by N2 adsorption to the external surface area obtained by t-Pot (the surface area outside the particle is calculated based on the particle size) (t-Pot surface area / BET surface area) is 40% or more.

[0018] The "t-Pot surface area" described in Patent Document 3 refers to a parameter representing the "t-plot surface area" described in, for example, the technical report "Micropore Surface Area Analysis Using the t-plot Method," published online by MC Evolve Technologies on February 1, 2019. Micropore surface area analysis using the t-plot method is one method of analysis based on the nitrogen adsorption isotherm (adsorption temperature: 77K). This method compares and converts the adsorption isotherm data with a standard isotherm to plot the relationship between the thickness t of the adsorption layer and the amount of adsorption. This method not only allows the specific surface area to be quantified by differentiating it into the inside and outside of the pores, but also allows the pore orientation to be understood from the shape of the graph.

[0019] Examples of solid carbon include carbon described in Japanese Patent No. 4362116. Specifically, Japanese Patent No. 4362116 discloses DENKA BLACK (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd.

[0020] Patent Document 4 discloses an electrode catalyst (core-shell catalyst) in which catalyst particles are supported both inside and outside the mesopores of a hollow carbon support (more specifically, nanopores formed in the primary particles of the hollow carbon support). This electrode catalyst has a structure in which, when analyzing the particle size distribution of the catalyst particles using a three-dimensional reconstructed image obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope), the ratio of catalyst particles supported inside the mesopores (more specifically, nanopores formed in the primary particles of the hollow carbon support) is 50% or greater.

[0021] In this specification, the "nanopore" of the hollow carbon carrier refers to the academic paper: "M. Uchida, et al., "Phys. Chem. Chem. Phys.", 2013, 15(27), 11236-1124 (for example, see Figure 1 The term "nanopore" refers to a "nanopore" formed on a "primary particle" of a hollow carbon carrier as defined in the "Nanopore" in the present invention.

[0022] In addition, the following non-patent documents 1 and 2 disclose an example of analyzing the ratio of catalyst particles supported inside the pores {the above-mentioned nanopores} and the ratio of catalyst particles supported outside the pores {the above-mentioned nanopores} using a method different from that of the above-mentioned patent document 4, regarding catalyst particles supported on a hollow carbon carrier.

[0023] More specifically, in Non-Patent Document 1, Strasser et al. reported that a group at the Technical University of Berlin made a high dispersion of Pt catalyst particles in a commercially available hollow carbon (trade name: "ketjenblack EC-300J", manufactured by Akzo Nobel, with a specific surface area of ​​about 839 m 2 g -1 ) is obtained by simultaneously taking SEM (Scanning Electron Microscopy) images and TSEM (Transmission Scanning Electron Microscopy) images of the specific Pt / C catalyst particles of interest in the same measurement area. For example, refer to Table 1 of Non-Patent Document 1. Figure 2 and page 79, right column.

[0024] In their method, based on the SEM image, information on the Pt catalyst particles that only exist in the observed portion (the outer surface of one side) of the outer surface of the hollow carbon support particles is obtained. That is, information on the number of catalyst particles outside the nanopores of the hollow carbon support particles is obtained. On the other hand, based on the TSEM image (transmission image), information on all catalyst particles outside and inside the observed hollow carbon support particles (the above-mentioned primary particles) is obtained. And, based on the information from the TSEM image and the information from the SEM image, they implemented an attempt to distinguish between the Pt catalyst particles supported on the outer surface (outside the nanopores) and the Pt catalyst particles supported on the inside of the Pt catalyst particles supported on the hollow carbon support particles.

[0025] However, in Non-Patent Document 1, they did not measure the "opposite inner surface" of the outer surface (outside the nanopores) of the hollow carbon support particles, which is opposite to the observed portion ("one outer surface"). They assumed that the state of the "one outer surface" and the state of the "opposite inner surface" were the same. In other words, they assumed that the number of catalyst particles supported on the "one outer surface" was the same as the number of catalyst particles supported on the "opposite inner surface."

[0026] Then, in non-patent document 2, the Yamanashi University team of Uchida et al. reported the results obtained as follows: using a STEM (Scanning Transmission Electron Microscope) device that can take SEM images and TEM (transmission electron microscopy) images of Pt catalyst particles, the Pt catalyst particles were highly dispersed in commercially available hollow carbon (trade name: "Ketjenblack", manufactured by Ketjen Black International, with a specific surface area of ​​about 875m 2 g -1 ) and the Pt / C catalyst obtained by the method of the present invention. Figure 1 , Table 2 and lower right column of page 181.

[0027] First, based on the TEM image of the catalyst particles of the specific Pt / C catalyst they were interested in, they obtained information on the number of all Pt catalyst particles supported on the hollow carbon support particles. Next, based on the measurement of the SEM image of the same Pt / C catalyst particles as the particles taken using the TEM image, they obtained information on the number of Pt catalyst particles that only existed on the inner surface of the hollow carbon support particles. Next, they used a special 3D sample holder to accurately rotate the specific Pt / C catalyst particles (measurement sample) of interest by 180°, thereby measuring the SEM image of the same Pt / C catalyst particles limited to the inner surface. Using this information, an attempt was made to distinguish between the Pt catalyst particles supported on the outer surface and the Pt catalyst particles supported on the inner surface of the Pt catalyst particles supported on the hollow carbon support particles.

[0028] Regarding the "internal loading rate" measured using this method = "100×(number of Pt catalyst particles loaded inside) / (total number of Pt catalyst particles)", they reported that it was 62% in the commercially available 30wt% Pt / C catalyst (trade name: "TEC10E30E", manufactured by Tanaka Metal Industries, Ltd., marked as "c-Pt / CB" in the manual), and more than 50% in the commercially available 46wt% Pt / C catalyst (trade name: "TEC10E50E", manufactured by Tanaka Metal Industries, Ltd., marked as "Pt / CB" in the manual).

[0029] Based on the above description, the inventors of the present invention have recognized that the analysis methods of Non-Patent Documents 1 and 2 differ from the analysis method of Patent Document 4 in the following respects.

[0030] That is, the analysis method using electron beam tomography measurement in Patent Document 4 is a three-dimensional reconstruction method using an electron microscope. The method is as follows: a measurement sample of interest (the size of the measurement target sample is a block with a major axis or a minor axis in the range of about 100 to 300 nm, see the following) is formed. Figure 11 and Figure 15 ) in the same field of view, projected from various directions, and after the obtained electron microscope image is reconstructed into a three-dimensional image in a computer, a tomographic image (tomogram) is produced using a computer.

[0031] On the other hand, the analysis method of non-patent document 1 is to photograph the measurement sample from a specific direction to obtain an SEM image and a TSEM image, and then use such two-dimensional images for analysis. In addition, the analysis method of non-patent document 2 is to photograph the measurement sample from two specific directions (two mutually orthogonal axial directions obtained by rotating the sample holder 180°) to obtain an SEM image, and photograph the measurement sample from a specific direction to obtain a TSEM image, and then use such two-dimensional images for analysis. The inventors of the present invention believe that in these analysis methods of non-patent document 1 and non-patent document 2, for example, when the measurement sample (catalyst particles for electrode) has unevenness, there is a high possibility that it is not possible to fully determine whether the supported position is inside or outside the hollow carbon support in the catalyst particles.

[0032] In the analysis method of Patent Document 4, a three-dimensional tomographic image (tomogram) of the measurement sample is used, and it can be observed based on various reports. Therefore, the inventors of the present invention believe that it is possible to visually confirm and more accurately grasp the measurement sample of interest (the size of the measurement target sample is a block with a major or minor axis in the range of about 100 to 300 nm, see the following) Figure 11 and Figure 15 ) is the supporting position of the catalyst particles contained in the electrode catalyst on the carrier.

[0033] In addition, the applicant of this patent provides the following publications as publications describing the publicly known inventions in the above-mentioned documents.

[0034] [Prior art literature]

[0035] Patent Literature

[0036] Patent Document 1: U.S. Patent Application Publication No. 2007 / 31722

[0037] Patent Document 2: Japanese Patent Application Laid-Open No. 2013-109856

[0038] Patent Document 3: WO2016 / 063968

[0039] Patent Document 4: WO2019 / 221168

[0040] Non-patent literature

[0041] Non-Patent Literature 1: Nature Materials, Vol. 19 (January 2020), 77-85

[0042] Non-patent document 2: Journal of Power Sources, 315 (2016), 179-191 Summary of the Invention

[0043] [Technical problem to be solved by the invention]

[0044] In order to popularize PEFC, electrode catalysts are required to have a further improvement in catalytic activity in order to reduce the amount of Pt used and reduce material costs.

[0045] The inventors of the present invention have discovered that, for Pt / C catalysts, there have been no reports to date on the actual synthesis of an improved product as described below, and there is still room for improvement. The improved product has the following structure: that is, when a three-dimensional reconstructed image is obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope) and the particle size distribution analysis of the catalyst particles is performed using this three-dimensional reconstructed image, a large number of catalyst particles are supported at a position closer to the inside than the outside of the nanopores of the primary particles of the hollow carbon support.

[0046] The present invention has been made in view of the above-mentioned technical problems, and an object of the present invention is to provide an electrode catalyst (Pt / C catalyst) having excellent catalytic activity that can contribute to cost reduction of PEFC.

[0047] Another object of the present invention is to provide a composition for forming a gas diffusion electrode containing the electrode catalyst, a gas diffusion electrode, a membrane electrode assembly (MEA), and a fuel cell stack.

[0048] [Technical solutions for solving technical problems]

[0049] The inventors of the present invention have conducted intensive research on a structure capable of further improving the catalytic activity of electrode catalysts comprising catalyst particles having a large amount of Pt / C catalyst supported within the nanopores of hollow carbon primary particles.

[0050] As a result, the inventors have found that when catalyst particles are supported on a carrier so as to satisfy the following conditions, it is effective in improving the catalytic activity, thereby completing the present invention.

[0051] More specifically, the present invention is composed of the following technical contents.

[0052] That is, the present invention provides an electrode catalyst comprising: a conductive hollow carbon support having nanopores with a pore diameter of 1 to 20 nm; and a plurality of catalyst particles supported on the support.

[0053] The catalyst particles are composed of Pt (0 valence),

[0054] The catalyst particles are supported inside and outside the nanopores of the carrier.

[0055] When a particle size distribution analysis of the catalyst particles was performed using a three-dimensional reconstructed image obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope), the ratio of the catalyst particles supported inside the nanopores was greater than 50%.

[0056] By supporting Pt / C catalyst particles on the hollow carbon support so that the ratio of catalyst particles supported inside the nanopores is 50% or more as described above, the electrode catalyst of the present invention can exhibit excellent catalytic activity that can contribute to cost reduction of PEFC.

[0057] The detailed reasons why the electrode catalyst of the present invention has excellent catalytic activity have not yet been fully elucidated.

[0058] However, the inventors of the present invention believe that the reason is as follows: that is, in Pt / C catalysts where the ratio of catalyst particles supported inside nanopores is 50% or more compared to conventional electrode catalysts, a large number of highly active catalyst particles with relatively small particle sizes exist inside the nanopores of the carrier.

[0059] The catalyst particles supported on the nanopores of the carrier are supported on the carrier in a state where they are not easily in direct contact with the polymer electrolyte present in the catalyst layer. Therefore, the electrode catalyst of the present invention can reduce the decrease in catalytic activity caused by poisoning of the Pt component and, compared with existing electrode catalysts, can exhibit more excellent catalytic activity during electric polarization. In addition, the electrode catalyst of the present invention can also reduce the dissolution of the Pt component from the catalyst particles.

[0060] In the present invention, the "nanopore" of the primary particle of the hollow carbon carrier refers to the academic paper: "M. Uchida, et al., "Phys. Chem. Chem. Phys.", 2013, 15(27), 11236-1124 (for example, see Figure 1 The term "nanopore" refers to a "nanopore" formed on a "primary particle" of a hollow carbon carrier as defined in the "Nanopore" in the present invention.

[0061] Furthermore, in the present invention, the "pore diameter of a nanopore" means the "size of the entrance of the nanopore".

[0062] In the present invention, the "pore diameter" (size of the pore entrance) of a nanopore refers to the size of the "nanopore entrance" as determined using the "USAL-KM3D analysis method" described below. More specifically, the "nanopore entrance size" refers to the diameter (equivalent circle diameter) of a circle having the same area as the entrance, obtained from an image of the nanopore entrance obtained using the "USAL-KM3D analysis method."

[0063] In the present invention, "hollow carbon" refers to carbon having more pores (voids) inside the carbon than the above-mentioned solid carbon, and means conductive carbon including the above-mentioned nanopores in a part of the pores.

[0064] Furthermore, from the perspective of more reliably achieving the effects of the present invention, the hollow carbon carrier preferably contains more nanopores with a pore diameter (the size of the pore entrance) of 1 to 10 nm. It has been reported that the micelle diameter of the polymer electrolyte used in the anode and cathode catalyst layers of MEAs is approximately 10 nm (for example, YSKim, et al., DOE Hydrogen Program Merit Review and Peer Meeting FC16, (2009)). Therefore, by using a hollow carbon carrier containing more pores with a pore diameter (the size of the pore entrance) of 1 to 10 nm, the polymer electrolyte is less likely to penetrate the nanopores, and contact between the catalyst particles supported inside the nanopores and the polymer electrolyte can be more reliably prevented.

[0065] In addition, in the present invention, "a method for analyzing the particle size distribution of the above-mentioned catalyst particles using a three-dimensional reconstructed image obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope)" is an analysis method using the STEM (scanning transmission electron microscope) of UBE Scientific Analysis Center Co., Ltd., which means an analysis method in which electron beam tomography measurement is performed and image analysis of the obtained measurement data is performed using image analysis software ("Avizo" manufactured by FEI Corporation) (analysis method name: "USAL-KM3D analysis method").

[0066] In the USAL-KM3D analysis method, a measurement sample to be measured is prepared according to the following steps and conditions.

[0067] <Measurement sample preparation method and conditions>

[0068] First, in order to optimally measure the structure of the measurement sample, a dispersion method, a common electron microscope sample preparation method, was used on a "Cu grid with a carbon support film" for TEM observation to satisfy the following conditions.

[0069] (P1) Powder of the sample to be measured (electrode catalyst) (a block with a major or minor diameter of about 100 to 300 nm, see the following) Figure 11 and Figure 15 ) exists on the above-mentioned grid at a moderate frequency that can be measured (observed) {the number of particles (the number of catalyst particles that can be observed is 100 or more, preferably 200 or more, more preferably 300 or more, and further preferably 400 or more)}.

[0070] (P2) The grid is rotated at an angle of ±80° about its rotation axis so that the image of the sample powder block does not overlap with the images of other microparticle blocks. If the image of the sample powder block overlaps with the images of other microparticle blocks, three-dimensional analysis cannot be performed.

[0071] (P3) The powder lumps of the measurement sample are arranged so as to be separated from each other to enable 3D cross-sectional imaging observation, so that the lumps visible within the measurement area are spaced apart from each other.

[0072] <Measurement conditions>

[0073] For the nanopores larger than 1 nm in the pores contained in the powder block of the above-mentioned measurement object sample (catalyst for electrode), 3D tomographic observation is carried out under conditions that enable three-dimensional observation and distinction without destroying the powder block of the measurement object sample (catalyst for electrode) (for example, adjusting the acceleration voltage of the electron beam, etc.).

[0074] In addition, in the electrode catalyst of the present invention, from the perspective of more reliably obtaining the effects of the present invention, when a three-dimensional reconstructed image is obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope), and the particle size distribution analysis of the above-mentioned catalyst particles is performed using the three-dimensional reconstructed image, the ratio of the above-mentioned catalyst particles supported inside the above-mentioned nanopores is preferably greater than 70%.

[0075] Furthermore, in the electrode catalyst of the present invention, from the perspective of more reliably obtaining the effects of the present invention, when a three-dimensional reconstructed image is obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope) and the particle size distribution analysis of the above-mentioned catalyst particles is performed using the three-dimensional reconstructed image, it is preferred to satisfy the conditions of the following formula (1).

[0076] (D10 / D20)≤0.75 (1)

[0077] In the above formula (1), D10 represents the arithmetic mean of the equivalent spherical diameters of the catalyst particles supported inside the nanopores of the above carrier, and D20 represents the arithmetic mean of the equivalent spherical diameters of the catalyst particles supported outside the nanopores of the above carrier.

[0078] By supporting the catalyst particles on the hollow carbon support so as to simultaneously satisfy the conditions of the above-mentioned formula (1), the electrode catalyst of the present invention can more reliably exhibit excellent catalytic activity that can contribute to cost reduction of PEFC.

[0079] In addition, in the electrode catalyst of the present invention, from the perspective of more reliably obtaining the effects of the present invention, when a three-dimensional reconstructed image is obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope) and the particle size distribution analysis of the above-mentioned catalyst particles is performed using the three-dimensional reconstructed image, it is more preferable to also simultaneously satisfy the conditions of the following formulas (2) and (3).

[0080] D1≤D2 (2)

[0081] (N1 / N2)>2.0 (3)

[0082] In the above formula (2) and the above formula (3), D1 represents the equivalent spherical diameter of the particle showing the maximum frequency (maximum number of particles) among the catalyst particles supported inside the nanopore of the support. In the above formula (2) and the above formula (3), D2 represents the equivalent spherical diameter of the particle showing the maximum frequency (maximum number of particles) among the catalyst particles supported outside the nanopore of the support.

[0083] In the above formula (2) and the above formula (3), N1 represents the frequency (number of particles) of the particles showing the maximum frequency (maximum number of particles) among the catalyst particles supported inside the nanopore of the support. In the above formula (1) and the above formula (2), N2 represents the frequency (number of particles) of the particles showing the maximum frequency (maximum number of particles) among the catalyst particles supported outside the nanopore of the support.

[0084] By supporting the catalyst particles on the hollow carbon support so as to simultaneously satisfy the conditions of the above-mentioned formulae (2) and (3), the electrode catalyst of the present invention can more reliably exhibit excellent catalytic activity that can contribute to cost reduction of PEFC.

[0085] In addition, in the electrode catalyst of the present invention, at least a portion of the surface of the catalyst particle may be covered with a Pt oxide film within a range in which the catalyst particle can exhibit excellent catalytic activity.

[0086] In addition, from the viewpoint of more reliably obtaining the effects of the present invention, in the electrode catalyst of the present invention, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support is preferably 200 to 1500 m 2 / g.

[0087] In addition, when the electrode catalyst is used in the cathode, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support is preferably 700 to 1500 m 2 / g, more preferably 750 to 1400 m 2 In addition, when the electrode catalyst is used in the cathode, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon carrier is preferably 750 to 900 m2 from the viewpoint of preferably maintaining a predetermined durability according to the operating environment (temperature fluctuation range, potential fluctuation range) of the cathode. 2 / g.

[0088] Furthermore, in the electrode catalyst of the present invention, the hollow carbon support is preferably Ketjen Black EC300J (Ketjen EC300J) from the viewpoint of the ease of obtaining the support and the cost of raw materials. In this case, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support (Ketjen Black EC300J) is preferably 750 to 850 m 2 / g, and more preferably 800 to 850 m / g from the viewpoint of obtaining better initial electrode performance. 2 / g.

[0089] The present invention also provides an electrode catalyst powder, which contains 10 wt % or more of the electrode catalyst of the present invention.

[0090] In the electrode catalyst powder, "components other than the electrode catalyst of the present invention" are "electrode catalysts other than the electrode catalyst of the present invention." That is, the electrode catalyst powder of the present invention does not include any powder that does not function as an electrode catalyst.

[0091] The electrode catalyst powder of the present invention contains the electrode catalyst of the present invention described above, and therefore can exhibit excellent catalytic activity that can contribute to cost reduction of PEFC.

[0092] Among them, from the perspective of more reliably obtaining the effects of the present invention, the content ratio of the above-mentioned electrode catalyst of the present invention in the electrode catalyst powder of the present invention is preferably 30 wt% or more, more preferably 50 wt% or more, further preferably 70 wt% or more, and most preferably 90 wt% or more.

[0093] The electrode catalyst powder of the present invention may contain, in addition to the electrode catalyst of the present invention described above, an electrode catalyst having the following composition (referred to as "electrode catalyst P" for convenience of explanation).

[0094] That is, the electrode catalyst P comprises: a hollow carbon support having nanopores with a pore diameter of 1 to 20 nm; and a plurality of catalyst particles supported on the support.

[0095] The catalyst particles are composed of Pt (0 valence),

[0096] The catalyst particles are supported inside and outside the nanopores of the carrier.

[0097] When the particle size distribution analysis of the catalyst particles was performed using the aforementioned “USAL-KM3D analysis method,” the ratio of the catalyst particles supported inside the nanopores was less than 50%.

[0098] The electrode catalyst powder of the present invention may be composed of the electrode catalyst of the present invention and the electrode catalyst P described above.

[0099] In this case, from the perspective of more reliably obtaining the effects of the present invention, the content ratio of the above-mentioned electrode catalyst of the present invention in the electrode catalyst powder of the present invention is also preferably 30 wt% or more, more preferably 50 wt% or more, further preferably 70 wt% or more, and most preferably 90 wt% or more.

[0100] The present invention also provides a composition for forming a gas diffusion electrode, comprising the electrode catalyst of the present invention or the electrode catalyst powder of the present invention.

[0101] The gas diffusion electrode-forming composition of the present invention contains the electrode catalyst or the electrode catalyst powder of the present invention, and therefore can easily produce a gas diffusion electrode having excellent catalytic activity (polarization characteristics) that can contribute to cost reduction of PEFC.

[0102] The present invention also provides a gas diffusion electrode comprising the electrode catalyst of the present invention or the electrode catalyst powder of the present invention.

[0103] The gas diffusion electrode of the present invention is composed of the electrode catalyst of the present invention, and therefore can easily form a structure having excellent catalytic activity (polarization characteristics) that can contribute to cost reduction of PEFC.

[0104] The present invention also provides a membrane electrode assembly (MEA) comprising the above-mentioned gas diffusion electrode of the present invention.

[0105] Since the membrane electrode assembly (MEA) of the present invention includes the gas diffusion electrode of the present invention, it is easy to form a structure having cell characteristics that can contribute to cost reduction of PEFC.

[0106] The present invention also provides a fuel cell stack comprising the membrane electrode assembly (MEA) of the present invention.

[0107] According to the fuel cell stack of the present invention, since it includes the membrane electrode assembly (MEA) of the present invention, it is easy to form a structure having cell characteristics that can contribute to cost reduction of PEFC.

[0108] [Effects of the Invention]

[0109] According to the present invention, it is possible to provide an electrode catalyst having excellent catalytic activity that can contribute to cost reduction of PEFC.

[0110] The present invention can also provide a gas diffusion electrode-forming composition, a gas diffusion electrode, a membrane electrode assembly (MEA), and a fuel cell stack containing such an electrode catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0111] Figure 1 It is a schematic cross-sectional view showing a preferred embodiment of the MEA of the present invention.

[0112] Figure 2 Yes Figure 1 Schematic cross-sectional view of a preferred embodiment of the electrode catalyst of the present invention contained in at least one of the cathode catalyst layer and the anode catalyst layer of the MEA shown.

[0113] Figure 3 Yes Figure 2 The diagram shows an enlarged cross-sectional view schematically illustrating the schematic structure of an electrode catalyst.

[0114] Figure 4 It is a schematic cross-sectional view showing another preferred embodiment of the MEA of the present invention.

[0115] Figure 5 It is a schematic cross-sectional view showing a preferred embodiment of the CCM of the present invention.

[0116] Figure 6 It is a schematic cross-sectional view showing another preferred embodiment of the CCM of the present invention.

[0117] Figure 7 It is a schematic cross-sectional view showing a preferred embodiment of the GDE of the present invention.

[0118] Figure 8 It is a schematic cross-sectional view showing another preferred embodiment of the GDE of the present invention.

[0119] Figure 9 This is a schematic diagram showing a preferred embodiment of the fuel cell stack of the present invention.

[0120] Figure 10 This is a schematic cross-sectional view showing a conventional electrode catalyst.

[0121] Figure 11 This is a STEM image showing the measurement conditions (volume size) of 3D electron beam tomography of the electrode catalyst of Example 1 using STEM.

[0122] Figure 12 This is a 3D-STEM image (three-dimensional reconstruction image) of the electrode catalyst of Example 1.

[0123] Figure 13 Yes means through Figure 12 The graph shown is a graph showing the particle size distribution (particle size distribution expressed as equivalent spherical diameter) of catalyst particles supported inside the nanopores of the support among the catalyst particles obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1.

[0124] Figure 14 Yes means through Figure 12 The graph shown is a graph showing the particle size distribution (particle size distribution expressed as equivalent spherical diameter) of catalyst particles supported outside the nanopores of the support among the catalyst particles obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1.

[0125] Figure 15 This is a STEM image showing the measurement conditions (volume size) of 3D electron beam tomography using STEM of the electrode catalyst of Comparative Example 1.

[0126] Figure 16 This is a 3D-STEM image (three-dimensional reconstructed image) of the electrode catalyst of Comparative Example 1.

[0127] Figure 17 Yes means through Figure 16 The graph shown is a graph of the particle size distribution (particle size distribution expressed as equivalent spherical diameter) of catalyst particles supported inside the nanopores of the support among the catalyst particles obtained by image analysis of a 3D-STEM image of the electrode catalyst of Comparative Example 1.

[0128] Figure 18 Yes means through Figure 16 The graph shown is a graph showing the particle size distribution (particle size distribution expressed as equivalent spherical diameter) of catalyst particles supported outside the nanopores of the support among the catalyst particles obtained by image analysis of a 3D-STEM image of the electrode catalyst of Comparative Example 1. DETAILED DESCRIPTION

[0129] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to appropriate drawings.

[0130] <Membrane Electrode Assembly (MEA)>

[0131] Figure 1 It is a schematic cross-sectional view showing a preferred embodiment of the MEA of the present invention.

[0132] Figure 1 The MEA 10 shown has a structure including two flat-plate-shaped gas diffusion electrodes (a cathode 1 and an anode 2 ) arranged to face each other, and a polymer electrolyte membrane (hereinafter referred to as “PEM” as needed) 3 arranged between the cathode 1 and the anode 2 .

[0133] In the case of this MEA 10 , at least one of the cathode 1 and the anode 2 has a structure in which an electrode catalyst 20 (Pt catalyst 20 ) described later is included.

[0134] The cathode 1, anode 2 and PEM 3 are placed as follows Figure 1 The MEA 10 can be manufactured by laminating the components in the manner shown and then performing pressure bonding.

[0135] Gas Diffusion Electrode (GDE)

[0136] The cathode 1, a gas diffusion electrode, has the following structure: a gas diffusion layer 1gd and a catalyst layer 1c formed on the surface of the gas diffusion layer 1gd on the PEM 3 side. The cathode 1 also has a water-repellent layer (microporous layer, hereinafter referred to as "MPL") 1m disposed between the gas diffusion layer 1gd and the catalyst layer 1c.

[0137] Like the cathode 1 , the anode 2 as a gas diffusion electrode also has the following structure: a gas diffusion layer 2gd; a catalyst layer 2c formed on the surface of the gas diffusion layer 2gd on the PEM 3 side; and an MPL 2m arranged between the gas diffusion layer 2gd and the catalyst layer 2c.

[0138] (Catalyst layer (CL))

[0139] In the cathode 1 , the catalyst layer 1 c is a layer that performs a reaction to generate water from air (oxygen) fed from the gas diffusion layer 1 gd and hydrogen ions moving from the anode 2 in the PEM 3 .

[0140] In the anode 2 , the catalyst layer 2 c is a layer that performs a reaction of generating hydrogen ions and electrons from hydrogen gas transported from the gas diffusion layer 2 gd.

[0141] At least one of the catalyst layer 1 c of the cathode 1 and the catalyst layer 2 c of the anode 2 contains the electrode catalyst 20 of the present invention.

[0142] (A preferred embodiment of the electrode catalyst of the present invention)

[0143] Below, using Figure 2 A preferred embodiment of the electrode catalyst of the present invention will be described.

[0144] Figure 2 Yes Figure 1 1 is a schematic cross-sectional view of a preferred embodiment of an electrode catalyst (Pt / C catalyst) contained in at least one of the cathode catalyst layer 1c and the anode catalyst layer 2c of the MEA 10 shown. Figure 3 Yes Figure 2 FIG. 2 is an enlarged cross-sectional view schematically showing the schematic structure of the electrode catalyst 20. FIG.

[0145] like Figure 2 and Figure 3 As shown, the electrode catalyst 20 includes a carrier 22 which is a hollow carbon carrier and catalyst particles 23 supported on the carrier 22 .

[0146] In addition, from the viewpoint of more reliably obtaining the effects of the present invention, Figures 2 and 3 The electrode catalyst 20 shown preferably satisfies the following conditions.

[0147] The catalyst particles 23 are composed of Pt (valence 0). However, a Pt oxide layer may be formed on the surface of the catalyst particles 23 within a range that can achieve the effects of the present invention.

[0148] The electrode catalyst 20 preferably has an average crystallite size of 3 to 16.0 nm as measured by powder X-ray diffraction (XRD).

[0149] The catalyst particles 23 are composed of Pt (valence 0). However, a Pt oxide layer may be formed on the surface of the catalyst particles within a range that can achieve the effects of the present invention.

[0150] Furthermore, the Pt supporting rate of the electrode catalyst 20 is preferably 5.6 to 66.5 wt %.

[0151] The support 22 is not particularly limited as long as it is a hollow carbon support that is conductive, has nanopores with a pore diameter of 1 to 20 nm, can support the catalyst particles 23 , and has a relatively large surface area.

[0152] In addition, within the scope of being able to obtain the effects of the present invention, the carrier 22 may also include: pores with a pore diameter less than 1 nm (classified as so-called "micropores", which are relatively small pores among pores), pores with a pore diameter greater than 20 nm and less than 50 nm (classified as so-called "mesopores", which are relatively large pores among pores).

[0153] Furthermore, the support 22 is preferably a hollow carbon support having good dispersibility in the gas diffusion electrode forming composition containing the electrode catalyst 20 and excellent electrical conductivity.

[0154] Examples of hollow carbon supports include Ketjen Black EC300J and Ketjen Black EC600JD. Examples of commercially available products include those sold under the trade names "Carbon EPC" and "Carbon EPC600JD" (products manufactured by Lion Chemical Co., Ltd.). Detailed characteristics of Ketjen Black EC300J and Ketjen Black EC600JD are described in, for example, the document "Characteristics and Applications of Conductive Carbon Black 'Ketjen Black EC'" published online by the Functional Carbon Filler Research Association.

[0155] Examples of other hollow carbon supports include "MCND (Mesoporous Carbon Nano-Dendrite)" (manufactured by Nippon Steel & Sumikin Chemicals Co., Ltd.) and "Black Pearls 2000" (manufactured by Cabot Corporation).

[0156] Among them, from the viewpoint of more reliably obtaining the effects of the present invention, the hollow carbon support is preferably at least one of Ketjen Black EC300J and Ketjen Black EC600JD. Furthermore, in the case of Ketjen Black EC300J, from the same viewpoint, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support measured by nitrogen is preferably 750 to 850 m 2 / g.

[0157] Among them, such as Figure 2As shown, the catalyst particles 23 are supported inside and outside the nanopores P22 of the carrier 22 .

[0158] Furthermore, when the electrode catalyst 20 is measured by electron beam tomography using 3D-STEM, the conditions of the following formulas (1) to (3) are simultaneously satisfied.

[0159] (D10 / D20)≤0.75 (1)

[0160] D1≤D2 (2)

[0161] (N1 / N2)>1.0 (3)

[0162] In Formulas (1) to (3), D10 represents the arithmetic mean of the equivalent spherical diameters of the catalyst particles 23 supported inside the nanopores P22 of the support 22 .

[0163] In addition, D20 represents the arithmetic mean of the equivalent spherical diameters of the catalyst particles 23 supported outside the nanopores P22 of the support 22 .

[0164] Furthermore, D1 represents the equivalent spherical diameter (nm) of the particles showing the maximum frequency (maximum number of particles) among the catalyst particles 23 supported inside the nanopores P22 of the support 22 .

[0165] Furthermore, D2 represents the equivalent spherical diameter of the particles showing the maximum frequency (maximum number of particles) among the catalyst particles 23 supported outside the nanopores P22 of the support 22 .

[0166] Furthermore, N1 represents the frequency (number of particles) of particles showing the maximum frequency (maximum number of particles) among the catalyst particles 23 supported inside the nanopores P22 of the support 22 .

[0167] Furthermore, N2 represents the frequency (number of particles) of particles showing the maximum frequency (maximum number of particles) among the catalyst particles 23 supported outside the nanopores P22 of the support 22 .

[0168] Compared to the conventional electrode catalyst 200, the electrode catalyst 20 that satisfies the conditions of formulas (1) to (3) simultaneously has a large number of highly active catalyst particles 23 with a relatively small particle size within the nanopores P22 of the carrier 22. Compared to the conventional electrode catalyst 200, the catalyst particles 23 supported within the nanopores P22 of the carrier 22 exhibit superior catalytic activity during electrode polarization. In addition, the catalyst particles 23 are supported on the carrier 22 in a state that is not easily in direct contact with a polymer electrolyte such as Nafion contained in the catalyst layer (catalyst layer 1c or catalyst layer 2c), which can also reduce the dissolution of the Pt component.

[0169] Furthermore, the catalyst layer (catalyst layer 1c or catalyst layer 2c) may contain other electrode catalysts (not shown) in addition to the electrode catalyst 20 of the present invention. For example, the catalyst layer (catalyst layer 1c or catalyst layer 2c) may contain the aforementioned "electrode catalyst P" as another electrode catalyst.

[0170] In this case, from the perspective of more reliably achieving the effects of the present invention, the content ratio of the electrode catalyst 20 involved in the present invention is preferably 10 wt% or more relative to the mass of all constituent materials in the catalyst layer (catalyst layer 1c or catalyst layer 2c). In addition, from the perspective of more reliably achieving the effects of the present invention, the content ratio of the electrode catalyst 20 involved in the present invention is preferably 30 wt% or more, more preferably 50 wt% or more, further preferably 70 wt% or more, and most preferably 90 wt% or more relative to the mass of all constituent materials in the catalyst layer (catalyst layer 1c or catalyst layer 2c).

[0171] In this case, from the perspective of more reliably achieving the effects of the present invention, the content of the electrode catalyst 20 according to the present invention is preferably 10 wt% or more relative to the mass of the total electrode catalyst in the catalyst layer (catalyst layer 1c or catalyst layer 2c). Furthermore, from the same perspective, the content of the electrode catalyst 20 according to the present invention is preferably 30 wt% or more, more preferably 50 wt% or more, even more preferably 70 wt% or more, and most preferably 90 wt% or more relative to the mass of the total electrode catalyst in the catalyst layer (catalyst layer 1c or catalyst layer 2c).

[0172] The method for producing the electrode catalyst 20 includes a "support pretreatment step," a "Pt addition step," and a "reduction step" for satisfying the conditions of Formulas (1) to (3), but is not particularly limited and can be produced by a known method.

[0173] In the carrier pretreatment step, the carrier 22 is placed in deionized water (preferably deionized water with a conductivity of 1 μS / cm or less, or more preferably "ultrapure water" as described below), and a pH adjuster is added to adjust the pH to 9 to 13 to prepare a dispersion. The dispersion is then maintained at a temperature of 80 to 99°C, preferably 90 to 99°C, for a predetermined period of time (but not boiling) while stirring. The dispersion is then cooled to room temperature.

[0174] This removes the gas inside the nanopores P22 of the support 22, allowing ultrapure water to fully penetrate the nanopores P22. Furthermore, in the subsequent "Pt addition step," the Pt raw material is fully retained within the nanopores P22 of the support 22. This allows a large amount of Pt catalyst particle precursors to be supported within the nanopores P22 of the support 22.

[0175] In this support pretreatment step, the "ultrapure water" used to prepare the aqueous solution is water having a specific resistance R (the inverse of the electrical conductivity measured by the JIS standard test method (JIS K0552)) of 3.0 MΩ·cm or greater, as shown in the following formula (4). Furthermore, the "ultrapure water" preferably has a water quality equivalent to "A3" or higher as defined in JIS K0557, "Water for Use in Water and Drainage Tests."

[0176] The ultrapure water is not particularly limited as long as it is water having a conductivity that satisfies the relationship shown in the following formula (4). For example, the ultrapure water may be ultrapure water produced using an ultrapure water production device "Milli-Q Series" (manufactured by Merck Co., Ltd.) or "Elix UV Series" (manufactured by Millipore Co., Ltd., Japan).

[0177] R=1 / ρ (4)

[0178] In the above formula (4), R represents specific resistance, and ρ represents electrical conductivity measured by the JIS standard test method (JIS K0552).

[0179] The step following the "support pretreatment step" is the "Pt addition step." In this "Pt addition step," an aqueous solution of a water-soluble Pt salt dissolved in ultrapure water is added to the dispersion of the support 22 obtained in the "support pretreatment step" at room temperature.

[0180] The step following the Pt addition step is the reduction step. In this reduction step, the solution obtained in the Pt addition step is heated to 50°C or higher, and then an aqueous solution containing a water-soluble reducing agent (preferably an alkaline water-soluble reducing agent) is added. After the addition of the reducing agent, the solution temperature is maintained at 50°C or higher for a predetermined period to allow the reduction reaction to proceed. The solution is then cooled to room temperature.

[0181] The process after the "reduction process" is the "cleaning process". In this "cleaning process", the solid component and the liquid component in the solution obtained through the "reduction process" are separated, and then the solid component (a mixture of the Pt / C catalyst and impurities other than it) is cleaned. For example, the solid component in the solution obtained through the "reduction process" can be separated from the liquid component by filtering methods such as filter paper and filter cloth. In the cleaning of the solid component, the above-mentioned ultrapure water, pure water (the resistivity R shown in the above-mentioned formula (4) is 0.1MΩ·cm or more and less than 3.0MΩ·cm), and pure warm water (pure water with a temperature adjusted to 40 to 80°C) can be used. For example, when using pure warm water, the cleaning is repeated until the conductivity of the filtrate after cleaning is less than 10μS / cm.

[0182] The next step after the "washing step" is the "drying step." In this drying step, water is separated from the solid component (a mixture of the Pt / C catalyst and water) obtained in the "washing step." First, the solid component is air-dried, then dried in a dryer at a predetermined temperature and for a predetermined time.

[0183] The step following the “drying step” is the “crushing step.” In this “crushing step,” the solid component (Pt / C catalyst) obtained in the “drying step” is converted into catalyst powder using a crushing device such as a mixer.

[0184] For the polymer electrolyte contained in catalyst layer 1c and catalyst layer 2c, as long as it has hydrogen ion conductivity, it is not particularly limited, and known compounds can be used. For example, as the polymer electrolyte, known perfluorocarbon resins with sulfonic acid groups and carboxylic acid groups can be exemplified. As the polymer electrolyte with hydrogen ion conductivity that can be easily obtained, Nafion (registered trademark, manufactured by DuPont Co., Ltd.), Aciplex (registered trademark, manufactured by Asahi Kasei Corporation), and Furemion (registered trademark, manufactured by Asahi Glass Co., Ltd.) can be preferably exemplified.

[0185] And, in Figure 1 In at least one of the catalyst layer 1c of the cathode 1 and the catalyst layer 2c of the anode 2, the mass ratio N / C of the mass C of the support 22 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0186] (Gas Diffusion Layer (GDL))

[0187] Figure 1 The gas diffusion layer 1gd included in the cathode 1 is provided to supply an oxidizing gas (eg, oxygen or air) to the catalyst layer 1c. The gas diffusion layer 1gd also serves to support the catalyst layer 1c.

[0188] The gas diffusion layer 2gd included in the anode 2 is provided to supply a reducing gas (for example, hydrogen) to the catalyst layer 2c. The gas diffusion layer 2gd also serves to support the catalyst layer 2c.

[0189] Figure 1 The gas diffusion layer (IGD) shown here has the function and structure to allow hydrogen or air (oxygen) to pass smoothly through the catalyst layer. Therefore, the gas diffusion layer is preferably water-repellent. For example, the gas diffusion layer contains a water-repellent component such as polyethylene terephthalate (PTFE).

[0190] There are no particular limitations on the materials that can be used for the gas diffusion layer (IGD), and known materials can be used. Preferred examples include carbon paper and materials made from carbon paper as a main material and coated with auxiliary materials, wherein the auxiliary materials include optional carbon powder, ion-exchanged water, and a polyethylene terephthalate dispersion as a binder.

[0191] (Water repellent layer (MPL))

[0192] like Figure 1 As shown, in the cathode 1, a water-repellent layer (MPL) 1m is disposed between the gas diffusion layer 1gd and the catalyst layer 1c. The water-repellent layer 1m has electronic conductivity, water repellency, and gas diffusivity. It is provided to facilitate the diffusion of oxidant gas into the catalyst layer 1c and the removal of reaction water generated in the catalyst layer 1c. The structure of the water-repellent layer 1m is not particularly limited, and a known structure can be employed.

[0193] (Polymer Electrolyte Membrane (PEM))

[0194] for Figure 1 The polymer electrolyte membrane (PEM) 3 shown is not particularly limited as long as it has hydrogen ion conductivity, and any known polymer electrolyte membrane currently used in PEFC can be employed. For example, a membrane containing the substances exemplified as polymer electrolytes contained in the catalyst layers 1c and 2c described above can be used as constituent components.

[0195] MEA deformation method

[0196] While preferred embodiments of the MEA of the present invention (and the catalyst layer of the present invention, and the gas diffusion electrode of the present invention) have been described above, the MEA of the present invention is not limited thereto. Figure 1 The structure of MEA10 is shown.

[0197] For example, the MEA of the present invention may also have Figure 4 The structure of MEA11 is shown.

[0198] Figure 4 It is a schematic cross-sectional view showing another preferred embodiment of the MEA of the present invention. Figure 4 The MEA 11 shown has a structure in which a gas diffusion electrode (GDE) 1A is arranged on only one surface of a polymer electrolyte membrane (PEM) 3. The gas diffusion electrode (GDE) 1A has Figure 1 The structure of the cathode 1 in the MEA 10 shown in FIG. The catalyst layer 1c of the gas diffusion electrode (GDE) 1A has the structure of the catalyst layer of the present invention. Specifically, in the catalyst layer 1c of the GDE 1A, the mass ratio N / C of the mass C of the support 22 of the electrode catalyst 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0199] Catalyst membrane layer assembly (CCM)

[0200] Next, preferred embodiments of the catalyst-layer membrane assembly (CCM) of the present invention will be described.

[0201] Figure 5 It is a schematic cross-sectional view showing a preferred embodiment of the CCM of the present invention. Figure 5 The illustrated CCM 12 has a structure in which a polymer electrolyte membrane (PEM) 3 is disposed between a cathode catalyst layer 1c and an anode catalyst layer 2c. Furthermore, at least one of the cathode catalyst layer 1c and the anode catalyst layer 2c has the catalyst layer structure of the present invention. Specifically, in at least one of the cathode catalyst layer 1c and the anode catalyst layer 2c, the mass ratio (N / C) of the mass C of the support of the electrode catalyst 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0202] <Deformation of Catalyst Membrane Layer Assembly (CCM)>

[0203] The preferred embodiments of the CCM of the present invention have been described above, but the CCM of the present invention is not limited to Figure 5 The structure of CCM12 is shown.

[0204] For example, the CCM of the present invention may also have Figure 6 The structure of CCM13 is shown.

[0205] Figure 7 It is a schematic cross-sectional view showing another preferred embodiment of the CCM of the present invention. Figure 6 The CCM 13 shown has a structure in which a catalyst layer 1c is arranged on only one surface of a polymer electrolyte membrane (PEM) 3. The catalyst layer 1c has Figure 5The cathode 1 in the CCM 12 shown in FIG. The catalyst layer 1c of the gas diffusion electrode (GDE) 1A has the structure of the catalyst layer of the present invention. Specifically, the mass ratio N / C of the support mass C of the electrode catalyst 20 to the mass N of the polymer electrolyte in the catalyst layer 1c of the CCM 13 is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0206] Gas Diffusion Electrode (GDE)

[0207] Next, preferred embodiments of the gas diffusion electrode (GDE) of the present invention will be described.

[0208] Figure 8 It is a schematic cross-sectional view showing a preferred embodiment of the GDE of the present invention. Figure 7 The gas diffusion electrode (GDE) 1B shown has Figure 1 The structure is the same as that of the cathode 1 mounted on the MEA 10 shown in FIG. The catalyst layer 1c of the gas diffusion electrode (GDE) 1B has the structure of the catalyst layer of the present invention. Specifically, the mass ratio N / C of the mass C of the support 22 of the electrode catalyst 20 to the mass N of the polymer electrolyte in the catalyst layer 1c of the gas diffusion electrode (GDE) 1B is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0209] Gas Diffusion Electrode (GDE) Deformation

[0210] The preferred embodiments of the GDE of the present invention have been described above, but the GDE of the present invention is not limited to Figure 7 The structure of Gde1B is shown.

[0211] For example, the GDE of the present invention may also have Figure 8 The structure of Gde1C is shown.

[0212] Figure 9 : is a schematic cross-sectional view showing another preferred embodiment of the GDE of the present invention. Figure 8 Compared to GDE1B, Figure 8 The GDE 1C shown has a structure in which no water repellent layer (MPL) is arranged between the catalyst layer 1 c and the gas diffusion layer 1 gd.

[0213] <Catalyst layer-forming composition>

[0214] Next, preferred embodiments of the catalyst layer-forming composition of the present invention will be described.

[0215] The catalyst layer-forming composition of this embodiment contains an electrode catalyst 20, a polymer electrolyte, and a main component. The mass ratio N / C of the mass C of the support 22 of the electrode catalyst 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0216] The composition of the solution containing the polymer electrolyte is not particularly limited. For example, the solution containing the polymer electrolyte may contain the above-mentioned polymer electrolyte having hydrogen ion conductivity, water, and alcohol.

[0217] By appropriately setting the composition ratio of the electrode catalyst 20, polymer electrolyte, and other components (water, alcohol, etc.) contained in the catalyst layer forming composition, the dispersion state of the electrode catalyst 20 in the obtained catalyst layer becomes good, thereby improving the power generation performance of the MEA10 containing the catalyst layer.

[0218] The catalyst layer-forming composition can be prepared by mixing and stirring the electrode catalyst 20 and a solution containing a polymer electrolyte. From the perspective of adjusting the coating properties, a polyol such as glycerol and / or water may also be contained. When mixing the electrode catalyst 20 and the solution containing the polymer electrolyte, a pulverizing mixer such as a ball mill or ultrasonic disperser can be used.

[0219] for Figure 1 At least one of the catalyst layer 1 c of the cathode 1 and the catalyst layer 2 c of the anode 2 shown can be formed using a preferred embodiment of the catalyst layer-forming composition of the present invention.

[0220] (Method for Manufacturing Gas Diffusion Electrode)

[0221] Next, an example of a method for producing a gas diffusion electrode of the present invention will be described. A gas diffusion electrode can be formed to include the catalyst layer of the present invention, and its production method can employ known methods. Using the catalyst layer-forming composition of the present invention allows for more reliable production.

[0222] For example, the catalyst layer-forming composition can be produced by applying the composition onto the gas diffusion layer (or the water-repellent layer of a laminate having a water-repellent layer formed on the gas diffusion layer) and drying the coating.

[0223] Fuel cell stack

[0224] Figure 9 This is a schematic diagram showing a preferred embodiment of the fuel cell stack of the present invention.

[0225] Figure 9 The fuel cell stack 30 shown has Figure 1The MEA 10 shown is a single cell structure formed by stacking a plurality of cells. The fuel cell stack 30 has a structure in which the MEA 10 is disposed between a separator 4 and a separator 5. Gas flow paths are formed in each of the separators 4 and 5.

[0226] [Example]

[0227] Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to the following Examples.

[0228] (I) Preparation of electrode catalyst used in the cathode catalyst layer of MEA

[0229] (1) Production of Pt / C Catalyst Used in the Cathode of MEA of Example 1

[0230] [Pt / C catalyst powder containing Pt catalyst particles]

[0231] A Pt / C catalyst powder (Pt loading rate: 49.0 wt %, trade name: "SA50wBK", ​​manufactured by Enyi Kaijia Co., Ltd.) in which catalyst particles composed of Pt are supported on carbon black powder was prepared.

[0232] The powder of the Pt / C catalyst (hereinafter referred to as "Pt / C catalyst A" as needed) was prepared according to the following procedure.

[0233] (First Step (Carrier Pretreatment Step))

[0234] A commercially available hollow carbon support {manufactured by Lion Corporation, trade name “Carbon ECP” (registered trademark) (Ketjen Black EC300J), with a specific surface area of ​​750 to 800 m 2 / g} was dispersed in an aqueous solution adjusted to pH 9 to 13 (prepared by adding a pH adjuster to ultrapure water), and the obtained dispersion was maintained at 90 to 99°C for about 0.5 hours (but not boiling) while stirring.

[0235] The "ultrapure water" used in the first step (support pretreatment step) is water having a specific resistance R (the inverse of the electrical conductivity measured by the JIS standard test method (JIS K0552)) of 3.0 MΩ·cm or greater, as shown in the following formula (4). Furthermore, the "ultrapure water" has a water quality equivalent to "A3" or higher as defined in JIS K0557, "Water for Use in Water and Drainage Tests."

[0236] This ultrapure water was produced using ultrapure water production equipment "Milli-Q Series" (manufactured by Merck Co., Ltd.) and "Elix UV Series" (manufactured by Nippon Millipore Co., Ltd.).

[0237] R=1 / ρ (4)

[0238] In the above general formula (4), R represents specific resistance, and ρ represents electrical conductivity measured by the JIS standard test method (JIS K0552).

[0239] (Second Step (Pt Adding Step))

[0240] An aqueous solution of a water-soluble Pt salt dissolved in ultrapure water is added to the dispersion obtained in the first step to prepare a mixed solution, which is then adjusted to pH 7-12 and maintained at a predetermined temperature of 50° C. or higher for a predetermined time while being stirred.

[0241] (Third Step (Reduction Step))

[0242] An aqueous solution containing an alkaline water-soluble reducing agent is added to the solution obtained in the second step to reduce the Pt ions in the mixed solution, thereby obtaining carbon "Pt / C" powder supporting Pt catalyst particles.

[0243] (Fourth step (cleaning step))

[0244] Use filter paper to separate the solid and liquid components in the solution obtained through the "third step". Then, use the above-mentioned pure water and pure warm water to clean the solid components (a mixture of Pt / C catalyst and impurities other than it) remaining on the filter paper. First, use pure water to clean. Repeat this cleaning until the conductivity of the cleaned filtrate is less than 20μS / cm. Then, use pure warm water to clean. Repeat this cleaning until the conductivity of the cleaned filtrate is less than 10μS / cm.

[0245] (Fifth Step (Drying Step))

[0246] The solid content (a mixture of Pt / C catalyst and water) on the filter paper obtained in the "fourth step" is air-dried in this state. After air-drying, the solid content on the filter paper is transferred to a magnetic dish and dried in an electric dryer at a predetermined temperature of 60°C or above for a predetermined time.

[0247] (Sixth Step (Crushing Step))

[0248] The solid component (Pt / C catalyst) obtained in the "fifth step" was pulverized using a mixer to obtain a powder of Pt / C catalyst A.

[0249] <Determination of loading rate (ICP analysis)>

[0250] The Pt loading rate (wt %) of the Pt / C catalyst A was measured by the following method.

[0251] Pt / C catalyst A was immersed in aqua regia to dissolve the metal. The insoluble carbon was then removed from the aqua regia. The aqua regia after carbon removal was then subjected to ICP analysis.

[0252] As a result of ICP analysis, the Pt loading rate of the Pt / C catalyst A was 49.0 wt %.

[0253] <Surface and structural observation of electrode catalysts>

[0254] In order to observe the three-dimensional structure of the Pt / C catalyst, electron beam tomography was measured using a STEM (scanning transmission electron microscope) of UBE Scientific Analysis Center Co., Ltd. using the "USAL-KM3D analysis method."

[0255] Electron beam tomography measurements using a STEM (scanning transmission electron microscope) were performed by satisfying the aforementioned measurement sample preparation methods and conditions (P1) to (P3) and measurement conditions. More details are described below.

[0256] STEM apparatus: JEM-ARM200F atomic resolution analytical electron microscope manufactured by JEOL Ltd.

[0257] Data analysis software: SYSTEM IN FRONTIER's 3D reconstruction software Composer, 3D data visualization software Visualizer-kai, and image analysis software Colorist.

[0258] Measurement conditions

[0259] Accelerating voltage: 60 kV;

[0260] Observation magnification: 800,000-1,000,000 times;

[0261] The inclination angle of the test sample is -80° to +80°;

[0262] Tilt step angle of the measuring specimen: 2°;

[0263] Pixel count: 512×512 pixels512×512 pixels;

[0264] Pixel size: 0.350nm / pixel ~ 0.500nm / pixel;

[0265] Volume dimensions: shown in Figure 11 .

[0266] For Pt / C catalyst A, electron beam tomography measurements using STEM (scanning transmission electron microscopy) were performed to obtain a three-dimensional reconstructed image (3D-STEM image). By analyzing this image, the Pt catalyst particles present within the carbon support (hereinafter referred to as internal particles) and the Pt catalyst particles present on the surface of the carbon support (hereinafter referred to as external particles) were separated, and the particle size distribution of the Pt catalyst particles in each region was calculated.

[0267] The three-dimensional reconstructed image (3D-STEM image) of Pt / C catalyst A is shown in Figure 12 middle.

[0268] The particle size analysis results (particle size distribution expressed as equivalent spherical diameter) of the catalyst particles supported inside the nanopores of the support and the catalyst particles supported outside the nanopores among the catalyst particles obtained by image analysis are shown in FIG. Figure 13 、 Figure 14 middle.

[0269] The sample stage is tilted stepwise under the above-mentioned measurement conditions to obtain a plurality of two-dimensional STEM images, and the obtained plurality of two-dimensional STEM images are reconstructed to obtain a 3D-STEM image.

[0270] In addition, the image analysis (particle size analysis) of the three-dimensional reconstructed image (3D-STEM image) is performed according to the following steps. First, the observation area of ​​the catalyst particles is selected based on the three-dimensional reconstructed image, and each catalyst particle is labeled (not shown). Next, the volume of the labeled Pt catalyst particle is calculated, and the diameter of a sphere with the same volume as the volume (equivalent spherical diameter) is calculated to obtain the particle size distribution ( Figure 13 、 Figure 14 ).

[0271] Here, when calculating the equivalent spherical diameter, the unit is set to nm, and the values ​​after the decimal point (values ​​less than 1 nm) are rounded off.

[0272] For this Pt / C catalyst A, the ratio of catalyst particles supported inside the nanopores of the support and the ratio of catalyst particles supported outside the nanopores of the support were determined. The values ​​of D10, D20, D1, D2, N1, and N2 were also determined. The results are shown in Tables 2 and 3.

[0273] The average particle size of the catalyst particles of Pt / C catalyst A measured from STEM images was 2.1 nm (the average particle size of the catalyst particles inside the nanopores was 1.7 nm, and the average particle size of the catalyst particles outside the nanopores was 3.4 nm).

[0274] (2) Preparation of Pt / C Catalyst Powder Used in the Cathode of MEA of Comparative Example 1

[0275] As a Pt / C catalyst, a Pt / C catalyst (trade name: "SA50BK") with a Pt loading rate of 50 wt% manufactured by Enyi Kaijia Co., Ltd. was prepared. As a support for the Pt / C catalyst, a commercially available hollow carbon support {Lion Corporation, trade name "Carbon ECP" (registered trademark) (Ketjen Black EC300J), with a specific surface area of ​​750 to 800 m 2 / g}.

[0276] <Surface and structural observation of electrode catalysts>

[0277] For the Pt / C catalyst of Comparative Example 1, in order to observe its three-dimensional structure using the same method and conditions as the Pt / C catalyst of Example 1, the "USAL-KM3D analysis method" was also adopted, and electron beam tomography measurements were performed using the STEM (scanning transmission electron microscope) of UBE Scientific Analysis Center Co., Ltd.

[0278] Figure 15 This is an STEM image showing the measurement conditions (volume size) of 3D electron beam tomography using STEM of the Pt / C catalyst of Comparative Example 1.

[0279] Figure 16 A 3D-STEM image (three-dimensional reconstructed image) of the Pt / C catalyst of Comparative Example 1 is shown.

[0280] Figure 17 Indicates passing Figure 16 The graph shown is a graph showing the particle size distribution (particle size distribution expressed as equivalent spherical diameter) of catalyst particles supported inside the nanopores of the support among the catalyst particles obtained by image analysis of a 3D-STEM image of the Pt / C catalyst of Comparative Example 1.

[0281] Figure 18 Indicates passing Figure 16 The graph shown is a graph showing the particle size distribution (particle size distribution expressed as equivalent spherical diameter) of catalyst particles supported outside the nanopores of the support among the catalyst particles obtained by image analysis of a 3D-STEM image of the Pt / C catalyst of Comparative Example 1.

[0282] For this Pt / C catalyst, the ratio of catalyst particles supported inside the nanopores of the support and the ratio of catalyst particles supported outside the nanopores of the support were determined. The values ​​of D10, D20, D1, D2, N1, and N2 were also determined. The results are shown in Tables 2 and 3.

[0283] The average particle size of the Pt / C catalyst particles measured from STEM images was 3.1 nm (the average particle size of the catalyst particles inside the nanopores was 3.1 nm, and the average particle size of the catalyst particles outside the nanopores was 3.2 nm).

[0284] (II) Preparation of P / C Catalyst Used in the Anode of MEA of Example 1 and Comparative Example 1

[0285] As the Pt / C catalyst used in the anode of the MEA of Example 1 and Comparative Example 1, the same Pt / C catalyst as the Pt / C catalyst B used in the cathode of the MEA of Comparative Example 1 was used.

[0286] <Example 1>

[0287] Follow the steps below to make a Figure 1 The MEA 10 shown is an MEA with the same structure.

[0288] (1) Preparation of cathode

[0289] Cathode GDL

[0290] As the GDL, carbon paper (manufactured by Toray Industries, Ltd., trade name "TGP-H-60") was prepared.

[0291] Ink for forming cathode MPL

[0292] Into a Teflon (registered trademark) ball mill container filled with Teflon (registered trademark) balls, 1.5 g of carbon powder (manufactured by Denki Kagaku Kogyo Co., Ltd., trade name "DENKA BLACK"), 1.1 g of ion-exchanged water, and 6.0 g of a surfactant (manufactured by Dow Chemical, trade name "Triton" (35 wt% aqueous solution)) were added and mixed.

[0293] Next, 1.75 g of a polytetrafluoroethylene (PTFE) dispersion (trade name "31-JR" manufactured by Mitsui DuPont Fluorochemicals, Inc.) was added to the ball mill container and mixed. Thus, an ink for forming the MPL of the cathode was prepared.

[0294] MPL of cathode

[0295] Using a bar coater, the cathode MPL-forming ink was applied to one surface of the GDL to form a coating film. The coating film was then fully dried in a dryer and then heat-pressed to produce a laminate with the MPL formed on the GDL.

[0296] Ink for forming cathode catalyst layer

[0297] The Pt / C catalyst A, ion-exchanged water, a 10 wt% aqueous Nafion dispersion (manufactured by DuPont, trade name "DE1021CS"), and glycerol were added to a Teflon (registered trademark) ball mill container filled with Teflon (registered trademark) balls and mixed to prepare an ink for forming the cathode catalyst layer. The ink had an N / C ratio of 0.7. The mass ratio of carbon in electrode catalyst A to ion-exchanged water to glycerol was 1:10:0.8.

[0298] Cathode catalyst layer (CL)

[0299] The cathode catalyst layer ink described above was applied to the surface of the MPL, a laminate formed on the GDL and MPL on MPL, using a bar coater to form a coating film. This coating film was dried at room temperature for 30 minutes and then at 60°C for 1 hour to form a catalyst layer. In this manner, a cathode serving as a gas diffusion electrode was fabricated. The Pt loading in the cathode catalyst layer was the value shown in Table 1.

[0300] (2) Anode production

[0301] Anode GDL

[0302] As the GDL, the same carbon paper as that for the cathode was prepared.

[0303] Ink for forming MPL of anode

[0304] Into a Teflon (registered trademark) ball mill container filled with Teflon (registered trademark) balls, 1.5 g of carbon powder (manufactured by Denki Kagaku Kogyo Co., Ltd., trade name "DENKA BLACK"), 1.0 g of ion-exchanged water, and 6.0 g of a surfactant (manufactured by Dow Chemical, trade name "Triton" (35 wt% aqueous solution)) were added and mixed.

[0305] Next, 2.5 g of a polytetrafluoroethylene (PTFE) dispersion (manufactured by Mitsui DuPont Fluorochemicals, Inc., trade name "31-JR") was added to the ball mill container and mixed. Thus, an MPL-forming ink for the anode was prepared.

[0306] Anode MPL

[0307] Using a bar coater, the anode MPL-forming ink was applied to one surface of the GDL to form a coating film. The coating film was then fully dried in a dryer and then subjected to heat and pressure bonding to produce a laminate with the MPL formed on the GDL.

[0308] Ink for forming a catalyst layer of anode

[0309] SA50BK (Pt loading 50 wt%), ion-exchanged water, a 5 wt% Nafion alcohol dispersion (manufactured by SIGMA-ALDRICH, trade name "Nafion 5 wt.% Dispersion," product number "274704"), and glycerol were added to a Teflon (registered trademark) ball mill container filled with Teflon (registered trademark) balls and mixed to prepare an ink for forming the anode catalyst layer. The ink had an N / C ratio of 1.2. The mass ratio of carbon in SA50BK to ion-exchanged water to glycerol was 1:6:4.

[0310] Anode catalyst layer (CL)

[0311] The ink for forming the anode catalyst layer was applied to the surface of the MPL layer, which was a laminate formed on the GDL, using a bar coater to form a coating film. The coating film was dried at room temperature for 30 minutes and then at 60°C for 1.0 hour to form a catalyst layer. In this manner, an anode serving as a gas diffusion electrode was produced. The Pt loading of the anode catalyst layer was 0.3 mg / cm 2 .

[0312] (3) MEA production

[0313] A polymer electrolyte membrane (manufactured by DuPont, trade name "Nafion NR212") was prepared. A laminate with the polymer electrolyte membrane placed between the cathode and anode was prepared and heat-pressed using a hot press to produce an MEA. The heat-pressing conditions were: 140°C, 5 kN for 5 minutes, followed by 140°C, 25 kN for 3 minutes.

[0314] <Comparative Example 1>

[0315] Each MEA was produced according to the same conditions and procedures as in Example 1 except that the following conditions were changed regarding the cathode catalyst layer.

[0316] That is, in the preparation of the ink for forming the cathode catalyst layer,

[0317] Instead of the Pt / C catalyst A, the above-mentioned P / C catalyst (trade name: "SA-50BK") was used.

[0318] · A 5 wt % Nafion alcohol dispersion (manufactured by DuPont, trade name “DE520CS”; containing 48 wt % of 1-propanol) was used instead of the 10 wt % Nafion aqueous dispersion.

[0319] The composition of the ink for forming the cathode catalyst layer and the coating conditions of the ink were adjusted so that the Pt loading and N / C became the values ​​shown in Table 1.

[0320] The carbon in the P / C catalyst (trade name: "SA50BH"): ion-exchanged water: glycerol = 1:10:1 (mass ratio).

[0321] Battery Performance Evaluation

[0322] The cell performance of the MEAs of Example 1 and Comparative Example 1 was evaluated according to the following cell performance evaluation method.

[0323] The MEAs of Example 1 and Comparative Example 1 were set in a fuel cell evaluation device.

[0324] Next, a power generation reaction was performed in the MEA under the following conditions.

[0325] The cell (MEA) temperature was 80°C. Pure hydrogen at 1.0 atmosphere pressure, humidified with saturated water vapor, was supplied to the anode, and its flow rate was adjusted to achieve a utilization rate of 70%. Separately, pure oxygen at 1.0 atmosphere pressure, humidified with 80°C saturated water vapor, was supplied to the cathode, and its flow rate was adjusted to achieve a utilization rate of 50%.

[0326] In the evaluation of the single cell (MEA), the current was controlled by the electronic load device attached to the fuel cell evaluation device, and the data was obtained at a range of 0 to 1.0 A / cm 2 The current-voltage curve is obtained by scanning the current value.

[0327] Based on the data of the above current-voltage curve, a graph (not shown) with the X-axis (current density) plotted on a logarithmic scale was prepared to obtain the current density value (the current value per unit area of ​​the electrode) at a voltage of 850 mV.

[0328] The current density thus obtained was divided by the platinum weight per unit area of ​​the cathode to calculate the activity per unit weight of platinum contained in the cathode (Mass. Act.), which was used as an indicator of the oxygen reduction ability of the catalyst contained in the cathode.

[0329] In Table 1, the Mass. Act. obtained in Comparative Example 1 is used as a reference (1.0), and relative values ​​(relative ratios) are shown, and the results of the comparison are shown by comparing with the Mass. Act. obtained in other Examples and Comparative Examples.

[0330]

[0331] The results shown in Tables 1 to 3 show that the MEA of Example 1 has a higher Pt mass activity than the MEA of Comparative Example 1.

[0332] [Industrial Applicability]

[0333] The electrode catalyst of the present invention exhibits excellent catalytic activity. In addition, the GDE, CCM, MEA, and fuel cell stack including the catalyst layer of the present invention exhibit excellent cell characteristics that can contribute to cost reduction of PEFC.

[0334] Therefore, the present invention can be applied not only to electrical equipment industries such as fuel cells, fuel cell vehicles, and portable mobile devices, but also to ENE-FARM (home fuel cell combined heat and power system), cogeneration system, etc., which will contribute to the development of the energy industry and environmental protection technology-related industries.

[0335] [Explanation of symbols]

[0336] 1: cathode;

[0337] 1A, 1B, 1C: gas diffusion electrode (GDE);

[0338] 1c: catalyst layer (CL);

[0339] 1m: water repellent layer (MPL);

[0340] 1gd: gas diffusion layer (GDL);

[0341] 2: anode;

[0342] 2c: catalyst layer (CL);

[0343] 2m: water repellent layer (MPL);

[0344] 2gd: gas diffusion layer (GDL);

[0345] 3: Polymer electrolyte membrane (PEM);

[0346] 4, 5: diaphragm;

[0347] 10, 11: membrane electrode assembly (MEA);

[0348] 12, 13: catalyst membrane layer assembly (CCM);

[0349] 20: Pt / C catalyst;

[0350] 22: carrier;

[0351] 23: catalyst particles;

[0352] 30: fuel cell stack;

[0353] P22: Nanopores of the carrier.

Claims

1. A catalyst for an electrode, wherein: The invention comprises: a conductive hollow carbon carrier having nanopores with a pore diameter of 1 to 20 nm; and a plurality of catalyst particles supported on the carrier. The catalyst particles are composed of 0-valent Pt, The catalyst particles are supported inside and outside the nanopores of the carrier, When a particle size distribution analysis of the catalyst particles is performed using a three-dimensional reconstructed image obtained by electron beam tomography measurement using a scanning transmission electron microscope (STEM), the ratio of the catalyst particles supported inside the nanopores is 50% or more, and When a particle size distribution analysis of the catalyst particles is performed using a three-dimensional reconstructed image obtained by electron beam tomography measurement using a scanning transmission electron microscope (STEM), the following condition (1) is satisfied: (D10 / D20)≤0.75 (1) In the formula (1), D10 represents the arithmetic mean of the equivalent spherical diameters of the catalyst particles supported inside the nanopores of the support, D20 represents the arithmetic mean of the equivalent spherical diameters of the catalyst particles supported outside the nanopores of the support.

2. The electrode catalyst according to claim 1, wherein When the particle size distribution of the catalyst particles was analyzed using a three-dimensional reconstructed image obtained by electron beam tomography measurement using a scanning transmission electron microscope (STEM), the ratio of the catalyst particles supported inside the nanopores was greater than 70%.

3. The electrode catalyst according to claim 1, wherein When a three-dimensional reconstructed image is obtained by electron beam tomography measurement using a scanning transmission electron microscope (STEM), and the particle size distribution analysis of the catalyst particles is performed using the three-dimensional reconstructed image, not only the condition of the above formula (1) but also the conditions of the following formulas (2) and (3) are satisfied simultaneously, D1≤D2 (2) (N1 / N2)>2.0 (3) In the above formula (2) and the above formula (3), D1 represents the equivalent spherical diameter of the particle showing the highest frequency among the catalyst particles supported inside the nanopores of the support, D2 represents the equivalent spherical diameter of the particle showing the highest frequency among the catalyst particles supported on the outside of the nanopores of the support, N1 represents the frequency of the particle showing the highest frequency among the catalyst particles supported inside the nanopores of the support, N2 represents the frequency of particles showing the highest frequency among the catalyst particles supported on the outside of the nanopores of the support.

4. The electrode catalyst according to any one of claims 1 to 3, wherein At least a portion of the surface of the catalyst particles is covered with a Pt oxide film.

5. The electrode catalyst according to any one of claims 1 to 3, wherein The BET specific surface area of ​​the hollow carbon carrier is 200 to 1500 m 2 / g, where BET specific surface area refers to nitrogen adsorption specific surface area.

6. The electrode catalyst according to claim 5, wherein The hollow carbon carrier is Ketjen Black EC300J.

7. A catalyst powder for an electrode, wherein: Contains 10 wt % or more of the electrode catalyst according to any one of claims 1 to 6.

8. A composition for forming a gas diffusion electrode, wherein: Contains the electrode catalyst according to any one of claims 1 to 6.

9. A gas diffusion electrode, wherein: Contains the electrode catalyst according to any one of claims 1 to 5.

10. A membrane electrode assembly MEA, wherein: A gas diffusion electrode according to claim 9.

11. A fuel cell stack, wherein: A membrane electrode assembly MEA according to claim 10 is included.

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