Catalyst for electrode, composition for forming gas diffusion electrode, gas diffusion electrode, membrane / electrode assembly, and fuel cell stack

Abstract

The present invention provides a catalyst for electrodes, said catalyst having excellent catalytic activity and being capable of contributing to the cost reduction of a PEFC. This catalyst for electrodes comprises: a hollow carbon carrier which has nanopores having a pore diameter of from 1 nm to 20 nm; and a plurality of catalyst particles which are supported by the carrier. The catalyst particles contain Pt (zerovalent), and are supported by both inner portions and outer portions of the nanopores of the carrier. If an analysis of the particle size distribution of the catalyst particles is performed using three-dimensional reconstructed images obtained through a STEM-based electron tomography measurement, the proportion of the catalyst particles supported by the inner portions of the nanoparticles is 50% or more; at least one nanopore is formed in a cubic image having a side of from 20 nm to 50 nm, said cubic image being obtained from a three-dimensional reconstructed image of a catalyst aggregate; and this nanopore has the shape of a continuously extending intercommunicating pore.

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 suitably used for a gas diffusion electrode, and more preferably to an electrode catalyst used for a gas diffusion electrode of a fuel cell. The present invention also relates to a composition for forming a gas diffusion electrode, a membrane / electrode assembly, and a fuel cell stack, which contain the above-mentioned electrode catalyst particles.

Background Art

[0002] Research and development of a polymer electrolyte fuel cell (hereinafter, referred to as "PEFC" as necessary) are being conducted as a power source for fuel cell vehicles and household cogeneration systems. As a catalyst used for a gas diffusion electrode of PEFC, a noble metal catalyst composed of noble metal particles of a platinum group element such as platinum (Pt) is used.

[0003] For example, as a typical conventional catalyst, a powder of catalyst particles in which Pt fine particles are supported on conductive carbon powder, "Pt-supported carbon catalyst" (hereinafter, referred to as "Pt / C catalyst" as necessary) is known. The proportion of the cost occupied by noble metal catalysts such as Pt in the manufacturing cost of PEFC is large, which has become an issue for reducing the cost of PEFC and promoting its widespread use. In these research and developments, in order to reduce the amount of platinum used, conventionally, powder of catalyst particles having a core-shell structure formed of a core part made of a non-platinum element and a shell part made of Pt (hereinafter, referred to as "core-shell catalyst particles" as necessary) (hereinafter, referred to as "core-shell catalyst" as necessary) has been studied and many reports have been made.

[0004] For example, Patent Document 1 discloses a particle composite material (corresponding to core-shell catalyst particles) having a structure in which palladium (Pd) or a Pd alloy (corresponding to the core part) is coated with an atomic thin layer of Pt atoms (corresponding to the shell part). Further, this Patent Document 1 describes, as an example, core-shell catalyst particles having a structure in which the core part is Pd particles and the shell part is a layer made of Pt. On the other hand, as carriers for the catalyst for the electrode, there are hollow carbon having many pores inside the primary particles and solid carbon having fewer pores inside the primary particles compared to the hollow carbon, and studies have been made for improving the performance by taking advantage of the respective characteristics.

[0005] For example, Patent Document 2 discloses a study example in which hollow carbon is employed as a carrier. Further, Patent Document 3 discloses a study example in which solid carbon is employed as a carrier. For example, in Patent Document 2, as shown in FIG. 10, for a porous carrier (hollow carbon) 220 having an average particle diameter of 20 to 100 nm, the pore volume of pores P220 having a pore diameter of 4 to 20 nm and the mode diameter of the pore distribution are controlled within a predetermined range, and a configuration of an electrode catalyst 200 in which catalyst particles 230 are supported in the primary pores P220 of the carrier 220 is disclosed. In Patent Document 2, it is mentioned that this makes it possible to prevent the adsorption of the polymer electrolyte on the surface of the catalyst particles 230 present in the primary pores P220, and while preventing a decrease in the effective reaction surface area of the catalyst, it is possible to sufficiently ensure the gas transport property. Further, as a result, it is mentioned that a fuel cell catalyst layer excellent in power generation performance can be provided even when the amount of catalyst is reduced while the activity per unit weight of the catalyst is improved.

[0006] Also, for example, Patent Document 3 discloses an electrode catalyst (PtCo / C catalyst) for a fuel cell having a solid carbon carrier and catalyst particles including an alloy of platinum and cobalt supported on the carrier. In this electrode catalyst, the molar ratio of platinum and cobalt in the alloy is 4 to 11:1, and it is acid-treated at 70 to 90°C. In Patent Document 3, when a PtCo alloy is supported on a hollow carbon carrier, some of the PtCo alloy will be included inside the hollow carbon carrier. Even if an acid treatment is performed to suppress the elution of Co, it is difficult to sufficiently treat the PtCo alloy present inside the carrier. As a result, it has been regarded as a problem that Co is likely to elute from the PtCo alloy present inside the carrier. Therefore, Patent Document 3 mentions that by using a solid carbon carrier instead of a hollow carbon carrier, it is possible to avoid the inclusion of the PtCo alloy inside the carrier. Furthermore, it is disclosed that this enables sufficient acid treatment of the PtCo alloy and can suppress the elution of Co. As a result, it is mentioned that it is possible to achieve both the initial performance and the durability performance of the fuel cell.

[0007] Here, Patent Document 3 defines solid carbon as follows. That is, in Patent Document 3, solid carbon is carbon with fewer voids inside the carbon compared to hollow carbon. Specifically, it is mentioned that it is carbon with a ratio (t-Pot surface area / BET surface area) of the BET surface area determined by N2 adsorption and the external surface area calculated by t-Pot (calculated from the particle size to the surface area outside the particle) of 40% or more. Note that the "t-Pot surface area" described in Patent Document 3 is understood to indicate the "t-plot surface area" described in the technical report "Analysis of micropore surface area by t-plot method" published on the Internet by "MC Evatec Co., Ltd." on February 1, 2019. The analysis of the micropore surface area by the t-plot method is one of the methods for analyzing from the nitrogen adsorption isotherm (adsorption temperature: 77K). This method is a method of comparing and converting the data of the adsorption isotherm with a standard isotherm and graphing the relationship between the thickness t of the adsorption layer and the adsorption amount. In addition to being able to separate and quantify the specific surface area into the inside and outside of the pores, the tendency of the pores can be known from the shape of the graph. In addition, examples of solid carbon include, for example, the carbon described in Patent No. 4362116, and specifically, Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd. etc. can be mentioned, as is disclosed.

[0008] Furthermore, Patent Document 4 discloses an electrode catalyst (core-shell catalyst) in which catalyst particles are supported on both the inside and the outside of the mesopores of a hollow carbon carrier {more specifically, the nanopores formed in the primary particles of the hollow carbon carrier}. When analyzing the particle size distribution of the catalyst particles using a three-dimensional reconstruction image obtained by electron beam tomography measurement using STEM (scanning transmission electron microscope), this electrode catalyst has a configuration in which the proportion of the catalyst particles supported inside the mesopores {more specifically, the nanopores (Nanopore) formed in the primary particles of the hollow carbon carrier} is 50% or more.

[0009] Here, in this specification, the "nanopore" of the hollow carbon carrier refers to pores with a pore diameter of 1 to 20 nm, as will be described later.

[0010] Note that Non-Patent Documents 1 and 2 below disclose examples in which the ratio of the catalyst particles supported inside the pores {the above-mentioned nanopores (Nanopore)} and the ratio of the catalyst particles supported outside the pores {the above-mentioned nanopores (Nanopore)} of the catalyst particles supported on the hollow carbon carrier are analyzed by a method different from that of Patent Document 4 above. More specifically, in Non-Patent Document 1, the group of Strasser et al. at the Technical University of Berlin used a commercially available hollow carbon (trade name: "ketjenblack EC-300J", manufactured by Akzo Nobel, specific surface area: about 839 m 2 g -1) The results of simultaneously taking SEM (Scanning Electron Microscopy) images and TSEM (Transmission SEM) images of specific Pt / C catalyst particles with highly dispersed Pt catalyst particles in are reported for the same measurement area. For example, refer to Table 1, Figure 2, and the right column on P. 79 of Non-Patent Document 1. In their method, information on Pt catalyst particles existing only on the observed part (one-sided outer surface) of the outer surface of the hollow carbon support particles can be obtained from the SEM image. That is, information on the number of catalyst particles supported outside the nanopores of the hollow carbon support particles can be obtained. On the other hand, from the TSEM image (transmission image), information on all catalyst particles supported outside and inside the hollow carbon support particles (the above-mentioned primary particles) in the observed Pt catalyst particles can be obtained. And they have attempted to distinguish between Pt catalyst particles supported on the outer surface (outside the nanopores) and Pt catalyst particles supported inside the hollow carbon support particles based on the information from the TSEM image and the information from the SEM image.

[0011] Here, in Non-Patent Document 1, they did not measure the "opposite back surface" of the observed part ("one-sided outer surface") of the outer surface (outside the nanopores) of the hollow carbon support particles for the SEM image. They assume that the state of the "one-sided outer surface" and the state of the "opposite back surface" are the same state. That is, they assume that the number of catalyst particles supported on the "one-sided outer surface" is the same as the number of catalyst particles supported on the "opposite back surface". Next, in Non-Patent Document 2, the group of Dr. Uchida et al. at Yamanashi University used a STEM (Scanning Transmission Electron Microscope) device capable of taking SEM images and TEM (transmission electron microscopy) images of Pt catalyst particles, and a commercially available hollow carbon (product name: "Ketjenblack", manufactured by Ketjen Black International, specific surface area: about 875 m 2 g -1) reports the results of photographing a Pt / C catalyst with highly dispersed Pt catalyst particles. For example, refer to FIG. 1, Table 2, and the lower right column on P. 181 of Non-Patent Document 2. They first obtain information on the number of all Pt catalyst particles supported on hollow carbon carrier particles from the TEM image of the catalyst particles of a specific Pt / C catalyst of interest. Next, they obtain information on the number of Pt catalyst particles present only on the back surface of the hollow carbon carrier particles from the measurement of the SEM image of the same Pt / C catalyst particles as those photographed in the TEM image. Next, they use a special 3D sample holder to accurately rotate a specific Pt / C catalyst particle (measurement sample) of interest by 180° and measure the SEM image of only the back surface of the same Pt / C catalyst particle. Using this information, they attempt to distinguish between the Pt catalyst particles supported on the outer surface and the Pt catalyst particles supported inside among the Pt catalyst particles supported on the hollow carbon carrier particles.

[0012] Regarding the "internal loading rate" measured by this method, which is "100×(number of Pt catalyst particles supported inside) / (total number of Pt catalyst particles)", they reported that it was 62% for a commercially available 30 wt% Pt / C catalyst (product name: "TEC10E30E", manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., denoted as "c-Pt / CB" in this specification) and 50% or more for a commercially available 46 wt% Pt / C catalyst (product name: "TEC10E50E", manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., denoted as "Pt / CB" in this specification). As described above, the present inventors recognize that the analysis methods of Non-Patent Document 1 and Non-Patent Document 2 differ from the analysis method of Patent Document 4 in the following points. That is, the analysis method by electron beam tomography measurement in Patent Document 4 is a three-dimensional reconstruction method using an electron microscope. It reconstructs electron microscope images projected from various directions of the same field of view of a measurement sample of interest (the size of the measurement target sample is a lump in the range where its major axis or minor axis is about 100 to 300 nm, refer to FIGS. 11, 15, and 19 described later) into a three-dimensional image in a computer and creates a tomographic image (tomogram) using the computer.

[0013] On the one hand, the analysis method of Non-Patent Document 1 performs analysis using two-dimensional images such as SEM images and TSEM images obtained by photographing a measurement sample from a specific one direction. Further, the analysis method of Non-Patent Document 2 performs analysis using two-dimensional images such as SEM images obtained by photographing a measurement sample from two specific directions (directions of two axes orthogonal to each other obtained by rotating the sample holder by 180°) and a TSEM image obtained by photographing the measurement sample from a specific one direction. In the analysis methods of these Non-Patent Document 1 and Non-Patent Document 2, for example, when there are irregularities in the measurement sample (catalyst particles for electrodes), the inventors think that there is a high possibility that there are cases where it is impossible to sufficiently determine whether the supported position is inside or outside the hollow carbon support in the catalyst particles. The analysis method of Patent Document 4 uses a three-dimensional tomogram of a measurement sample, and since this can be observed from various reports, the inventors think that it is possible to more accurately grasp while visually confirming the supported position on the carrier of the catalyst particles contained in the catalyst for the electrode of the measurement sample to be focused on (the size of the measurement target sample is a lump in the range where the major axis or minor axis is about 100 to 300 nm, see FIGS. 11, 15, and 19 described later). In addition, the applicant of this patent application presents the following publications as publications in which the above-mentioned publicly known inventions are described.

Prior Art Documents

Patent Documents

[0014]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Documents

[0015]

Non-Patent Document 1

[0016] Towards the popularization of PEFCs, for the electrode catalyst, further improvement of catalytic activity is required to reduce the Pt usage amount and material cost. When the present inventors analyzed the particle size distribution of catalyst particles using a three - dimensional reconstructed image obtained by electron tomography measurement using STEM (scanning transmission electron microscope) for electrode catalysts such as Pt / C catalysts, there has been no report so far that an improved product having a structure in which catalyst particles are more supported inside than outside the nanopores of the primary particles of the hollow carbon carrier could be actually synthesized, and the present inventors have found that there is still room for improvement. The present invention has been made in view of such technical circumstances, and an object thereof is to provide an electrode catalyst having excellent catalytic activity that can contribute to cost reduction of PEFCs. Another object of the present invention is to provide a composition for forming a gas diffusion electrode, a gas diffusion electrode, a membrane - electrode assembly (MEA), and a fuel cell stack, which contain the above - mentioned electrode catalyst. [[Means for Solving the Problems]]

[0017] The present inventors have intensively studied a configuration for further improving the catalytic activity of an electrode catalyst in which catalyst particles of an electrode catalyst such as a Pt / C catalyst are supported in a large number in the nanopores of the primary particles of hollow carbon. As a result, it has been found that it is effective for improving the catalytic activity that the catalyst particles are supported on the carrier so as to satisfy the following conditions, and the present invention has been completed. More specifically, the present invention is composed of the following technical matters.

[0018] That is, the present invention includes 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, at least a part of the surface of the catalyst particles has a region composed of Pt(0 valent) formed thereon, the catalyst particles are supported both inside and outside the nanopores of the carrier, when analyzing the particle size distribution of the catalyst particles using a three-dimensional reconstruction image obtained by electron beam tomography measurement using STEM (scanning transmission electron microscope), the proportion of the catalyst particles supported inside the nanopores is 50% or more, in the three-dimensional reconstruction image of the STEM, focusing on a catalyst mass composed of the catalyst particles and the carrier with a size accommodated in a rectangular parallelepiped space with a side length of 60 to 300 nm, when looking at six square cross-sections of a cube image with a side length of 20 to 50 nm extracted from the internal region of the catalyst mass, at least one nanopore is formed in at least one cross-section, the nanopore formed in at least one of the six square cross-sections has at least one opening in contact with a first side of the four sides of the square cross-section and at least one opening in contact with a second side of the square cross-section parallel to the first side, and has a shape of a continuous through-hole extending continuously from the opening of the first side to the opening of the second side without being blocked, provides a catalyst for an electrode.

[0019] When observing the microstructure using the three-dimensional reconstruction image of STEM, the catalyst for an electrode of the present invention can exhibit excellent catalytic activity that can contribute to cost reduction of PEFC by satisfying the condition that (α) the proportion of the catalyst particles supported inside the nanopores is 50% or more and the condition that (β) the nanopores are formed in the shape of the above-mentioned continuous through-hole. The detailed reason why the catalyst for an electrode of the present invention has excellent catalytic activity has not been fully elucidated. However, the inventors of the present invention think as follows.

[0020] That is, when observing the fine structure using the three-dimensional reconstructed image of STEM, in the catalyst for an electrode of the present invention where the ratio of the catalyst particles supported inside the nanopores is 50% or more, compared with the conventional catalyst for an electrode, there are many highly active catalyst particles with a relatively small particle diameter inside the nanopores of the carrier. The catalyst particles supported inside the nanopores of such a carrier are supported on the carrier in a state where they are less likely to directly contact the polymer electrolyte present in the catalyst layer when used in the catalyst layer of the gas diffusion electrode of the PEFC. Therefore, in the catalyst for an electrode of the present invention, the decrease in catalytic activity due to the poisoning of the Pt component is reduced, and excellent catalytic activity can be exhibited when polarized compared with the conventional catalyst for an electrode. Also, in the catalyst for an electrode of the present invention, the dissolution of the Pt component from the catalyst particles is reduced. Furthermore, in the catalyst for an electrode of the present invention, from the viewpoint of more surely obtaining the effects of the present invention, when analyzing the particle size distribution of the catalyst particles using the three-dimensional reconstructed image obtained by electron beam tomography measurement using STEM (scanning transmission electron microscope), it is preferable that the ratio of the catalyst particles supported inside the nanopores is 70% or more. Also, when observing the fine structure using the three-dimensional reconstructed image of STEM, in the catalyst for an electrode of the present invention where the nanopores are formed in the shape of the above-described through-holes, when used in the catalyst layer of the gas diffusion electrode of the PEFC, it has excellent diffusibility of water and the protons and reaction gases (hydrogen, oxygen, or air) contained therein, and the catalyst particles supported inside the nanopores are easily utilized in the reaction. Therefore, also from this viewpoint, the catalyst for an electrode of the present invention can exhibit excellent catalytic activity.

[0021] Here, in the present invention, the "analysis method of the particle size distribution of the catalyst particles using a three-dimensional reconstructed image obtained by electron beam tomography measurement using STEM (scanning transmission electron microscope)" is an analysis method using STEM (scanning transmission electron microscope) at the UBE Science Analysis Center Co., Ltd., which performs electron beam tomography measurement and analyzes the obtained measurement data using image analysis software ("Avizo" manufactured by FEI) (analysis method name: "USAL-KM3D analysis method"). In the USAL-KM3D analysis method, a measurement sample to be measured is prepared according to the following procedures and conditions.

[0022] <Measurement sample preparation method and conditions> First, in order to optimally measure the structure of the measurement sample, it is prepared on a "Cu grid mesh with a carbon support film" for TEM observation by a dispersion method, which is a general sample preparation method for electron microscopes, so as to satisfy the following conditions. (A) On the above grid mesh, powder masses of the measurement target sample (catalyst for electrodes) (masses with a major axis or minor axis in the range of about 60 to 300 nm, see FIGS. 11, 15, and 19 described later) are present at an appropriate frequency such that they can be measured (observed) {number of particles (the number of observable catalyst particles is 100 or more, preferably 200 or more, more preferably 300 or more, and even more preferably 400 or more)}. (B) When the above grid mesh is rotated at a rotation angle of ±80° around its rotation axis, the captured image of the powder mass of the measurement sample does not overlap with the captured images of other fine powder masses. If the captured image of the powder mass of the measurement sample overlaps with the captured images of other fine powder masses, three-dimensional analysis cannot be performed. (C) A plurality of powder masses visible in the measurement area are arranged so as to be separated from each other to such an extent that 3D tomography observation is possible for the powder masses of the measurement sample. <Measurement conditions> 3D tomography observation is performed under conditions (such as adjustment of the acceleration voltage of the electron beam) that can three-dimensionally observe and distinguish nanopores of 1 nm or more among the pores contained in the powder masses of the above measurement target sample (catalyst for electrodes) without damaging the powder masses of the measurement target sample (catalyst for electrodes).

[0023] Furthermore, in the present invention, the "Nanopore" of the hollow carbon carrier refers to pores with a pore diameter of 1 to 20 nm, and the "Micropore" refers to pores with a pore diameter of less than 1 nm. And in the present invention, the "pore diameter of the Nanopore" refers to the "size of the entrance of the Nanopore". The "pore diameter of the Micropore" refers to the "size of the entrance of the Micropore". In the present invention, the "pore diameter (size of the pore entrance)" of the Nanopore refers to the size of the "entrance of the nanopore" that can be obtained by using a three-dimensional reconstructed image obtained by electron beam tomography measurement using a general STEM (scanning transmission electron microscope) to analyze the catalyst for the electrode. More preferably, in the present invention, the "pore diameter (size of the pore entrance)" of the Nanopore refers to the size of the "entrance of the nanopore" obtained by the above-mentioned "USAL-KM3D analysis method". More specifically, the "size of the entrance of the nanopore" refers to the diameter (equivalent circle diameter) of a circle having the same area as the area of the entrance of the nanopore obtained from the image of the entrance of the nanopore obtained by the "USAL-KM3D analysis method".

[0024] Also, in the present invention, a method using a three-dimensional reconstructed image of STEM for confirming the condition that the catalyst is formed such that the above-mentioned (β) nanopores have the above-mentioned communicating hole shape will be described. The confirmation of the condition (β) can be performed by using a three-dimensional reconstructed image obtained by analyzing the catalyst for the electrode by electron beam tomography measurement using a general STEM (scanning transmission electron microscope). However, from the viewpoint of more reliably confirming the condition (β), it is preferable to perform it using the three-dimensional reconstructed image of STEM obtained in the process of performing the above-mentioned USAL-KM3D analysis method. The procedure for confirming the condition (β) will be described below.

[0025] (D) First, obtain a three-dimensional reconstructed image of the catalyst to be measured by STEM. From the catalyst mass (a mass composed of catalyst particles and a hollow carbon carrier) shown in this three-dimensional reconstructed image, select a catalyst mass with a size that can be accommodated in a rectangular parallelepiped space (region of interest) with a side length of 60 to 300 nm. This procedure can be easily understood, for example, by referring to the cubic images extracted from the 3D-STEM images (three-dimensional reconstructed images) of the catalyst masses of the catalysts in Examples 1, 2, and Comparative Example 2 described later (refer to FIGS. 24(a), 24(f), and 24(k) described later).

[0026] (E) Next, extract a cubic image (with a side length of 20 to 50 nm) from the internal region of the catalyst mass selected in step (D). This procedure can be easily understood, for example, by referring to the three-dimensional images (three-dimensional reconstructed images of STEM) obtained for the catalyst masses of the catalysts in Examples 1, 2, and Comparative Example 2 described later (refer to FIGS. 24(b), 24(g), and 24(l) described later).

[0027] (F) Next, observe the three-dimensional image (three-dimensional reconstructed image of STEM) inside the catalyst mass obtained in step (E), and utilize the difference in luminance to segment the void portion (pore portions such as nanopores) and the portion of the hollow carbon carrier. More specifically, this three-dimensional image (three-dimensional reconstructed image of STEM) is composed of smaller cubic pixels (voxels). And each pixel (voxel) stores luminance (unitless). By setting an appropriate threshold for these luminances, the analyst can clearly segment (binarize) the void part (pores such as nanopores) and the hollow carbon carrier part of the three-dimensional image (three-dimensional reconstructed image of STEM). For a certain pixel (voxel), if its luminance is above the threshold, it is automatically judged as the carbon part. Also, for a certain pixel (voxel), if its luminance is below the threshold, it is automatically judged as the void part. This segmentation can be implemented by setting the same luminance threshold for all pixels (voxels) included in the same three-dimensional image (three-dimensional reconstructed image of STEM). For different three-dimensional images (three-dimensional reconstructed images of STEM), the analyst sets different luminance thresholds (thresholds suitable for segmentation). From the perspective of implementing segmentation more accurately, the size of the pixel (voxel) is preferably a cube with a side length of 1 nm or less. In the present invention, when measuring the porosity of the three-dimensional image (three-dimensional reconstructed image of STEM) of the catalyst mass described later, the catalyst particles are regarded as voids during this segmentation.

[0028] Regarding this step (F), for example, it can be easily understood by referring to the three-dimensional images extracted from the 3D-STEM images (three-dimensional reconstructed images) of the catalyst masses of the electrode catalysts in Example 1, Example 2, and Comparative Example 2 described later (refer to FIGS. 24(b), 24(g), and 24(l) described later). Furthermore, it can be easily understood by referring to three cross-sections (three cross-sections after segmentation) of the three-dimensional images (three-dimensional reconstructed images of STEM) obtained for the catalyst masses of the catalysts in Example 1, Example 2, and Comparative Example 2 (refer to FIGS. 24(c), 24(d), 24(e), 24(h), 24(i), 24(j), 24(m), 24(n), and 24(o) described later).

[0029] (G) Next, when looking at six square cross-sections of the three-dimensional image (3D reconstruction image of STEM) after performing segmentation in step (F), it is confirmed whether at least one nanopore (communication hole) of the following shape is formed in at least one cross-section. That is, it is confirmed whether the nanopore (communication hole) visible in the target square cross-section has at least one opening in contact with the first side and at least one opening in contact with the second side parallel to the first side among the four sides of the cross-section. Further, it is confirmed whether the nanopore has a shape of a communication hole that extends continuously from the opening on the first side to the opening on the second side without being blocked. For example, referring to the example of Example 1 described later, as shown in FIG. 24, the nanopore P1 visible in the target cross-section (x-y plane of the square) has two openings (opening A11 and opening A12) in contact with the first side L1. Further, this nanopore P1 has two openings (opening A21 and opening A22) in contact with the second side L2 parallel to the first side L1. Furthermore, this nanopore P1 has a shape of a communication hole that extends continuously from the openings on the first side L1 (opening A11 and opening A12) to the openings on the second side L2 (opening A21 and opening A22) without being blocked.

[0030] Also, from the viewpoint of more surely obtaining the effects of the present invention, in the catalyst for an electrode of the present invention, it is preferable that the nanopore (communication hole) visible in the square cross-section of the target cubic image has a branched shape (see FIG. 25). When the catalyst particles supported inside such a nanopore are used in the catalyst layer of the gas diffusion electrode of the PEFC, they are likely to be supported on the carrier in a state where it is difficult to come into contact with the polymer electrolyte present in the catalyst layer. Further, such a nanopore has excellent diffusibility of water and protons and reaction gases (hydrogen, oxygen, or air) contained therein. Therefore, when the catalyst for an electrode having such a nanopore is used in the catalyst layer of the gas diffusion electrode of the PEFC, the catalyst particles supported inside the nanopore are likely to be utilized in the electrode reaction of the PEFC.

[0031] Furthermore, from the viewpoint of more surely obtaining the effects of the present invention, for the same reason as described above, in the catalyst for an electrode of the present invention, it is preferable that the nanopores (communication holes) visible in the square cross-section of the focused cube image have two or more openings on the first side (see FIG. 25). Also, from the viewpoint of more surely obtaining the effects of the present invention, for the same reason as described above, in the catalyst for an electrode of the present invention, it is preferable that the nanopores (communication holes) visible in the square cross-section of the focused cube image have two or more openings on the second side (see FIG. 25). Furthermore, from the viewpoint of more surely obtaining the effects of the present invention, for the same reason as described above, in the catalyst for an electrode of the present invention, it is preferable that the nanopores (communication holes) visible in the square cross-section of the focused cube image have at least one opening on the third side perpendicular to the first side. For example, referring to the example of Example 1 described later, as shown in FIG. 25, the nanopore P1 visible in the cross-section (square x-y plane) to be focused also has one opening (opening A31) on the third side L3 perpendicular to the first side L1.

[0032] Also, from the viewpoint of more surely obtaining the effects of the present invention, for the same reason as described above, in the catalyst for an electrode of the present invention, it is preferable that the nanopores (communication holes) visible in the square cross-section of the focused cube image have at least one opening on the fourth side perpendicular to the first side. For example, referring to the example of Example 1 described later, as shown in FIG. 25, the nanopore P1 visible in the cross-section (square x-y plane) to be focused also has two openings (opening A41 and opening A42) on the fourth side L4 perpendicular to the first side L1.

[0033] Furthermore, from the viewpoint of more surely obtaining the effects of the present invention, in the catalyst for an electrode of the present invention, the porosity measured using a three-dimensional reconstructed image (the focused cube image) of STEM is preferably 35% or more, more preferably 40% or more, still more preferably 45% or more, further more preferably 50% or more, still further more preferably 55% or more, still further more preferably 60% or more, and still further more preferably 65% or more. On the other hand, from the viewpoint of durability, in the catalyst for an electrode of the present invention, the porosity measured using a three-dimensional reconstructed image (the focused cube image) of STEM is preferably 80% or less, and more preferably 75% or less.

[0034] Also, in the catalyst for an electrode of the present invention, the hollow carbon carrier preferably contains more nanopores having a pore diameter (the size of the pore entrance) of 1 to 10 nm among the nanopores. There is a report that the micelle diameter of the polymer electrolyte used in the catalyst layers of the anode and cathode of the MEA is about 10 nm (for example, Y.S. Kim, et al, DOE Hydrogen Program Merit Review and Peer Meeting FC16, (2009)). Therefore, by using a hollow carbon carrier containing more pores having a pore diameter (the size of the pore entrance) of 1 to 10 nm, it becomes difficult for the polymer electrolyte to penetrate into the nanopores, and the contact between the catalyst particles supported inside the nanopores and the polymer electrolyte is more surely prevented. Furthermore, in the catalyst for an electrode of the present invention, within the range where the effects of the present invention can be obtained, the hollow carbon carrier may further have micropores having a pore diameter of less than 1 nm.

[0035] Furthermore, from the viewpoint of more surely obtaining the effects of the present invention, the hollow carbon carrier is preferably one that can satisfy the above conditions (α) and (β) when used as a catalyst for an electrode among "CNovel (manufactured by Toyo Tanso Co., Ltd., product name, registered trademark)" (for example, the porous carbon described in Japanese Patent No. 5636171, Japanese Patent No. 5695147, Japanese Patent No. 5860600, Japanese Patent No. 5860601, and Japanese Patent No. 5860602). CNovel is a porous carbon comprising at least a nanopore (pore diameter of 1 to 20 nm) and a carbonaceous wall constituting the outer contour of the nanopore, wherein there is a portion having a layered structure in the carbonaceous wall, the carbonaceous wall forms a three-dimensional network structure, the nanopore is an open pore, and the nanopores are in a continuous shape (the shape of a through-hole. Refer to "through-hole P1 formed by connecting a plurality of nanopores P22" in FIG. 2 described later), and has a configuration that easily satisfies the above-mentioned condition (β) when used as a catalyst.

[0036] In addition, in the catalyst for an electrode of the present invention, the catalyst particles may be composed of Pt(0 valence). Furthermore, in the catalyst for an electrode of the present invention, the catalyst particles may be composed of a Pt alloy. The metal species that are elements of the alloy other than Pt are not particularly limited. From the viewpoint of obtaining excellent catalytic activity, the metal species that are elements of the alloy other than Pt are preferably at least one metal selected from Co and Ni. In addition, in the catalyst for an electrode of the present invention, the catalyst particles may be core-shell catalyst particles. In this case, from the viewpoint of obtaining excellent catalytic activity, the core-shell catalyst particles are preferably core particles and a Pt shell layer (a region composed of Pt(0 valence)) formed on at least a part of the surface of the core particles. The metal species constituting the core particles are not particularly limited, but from the viewpoint of obtaining excellent catalytic activity, it is preferably at least one of Pd, Ni, and Co. Also, the core particles may be an alloy of at least one of Pd, Ni, and Co and another metal. From the viewpoint of reducing the amount of noble metal used, the core particles may contain at least one of a base metal (base metal) other than the noble metal, and an oxide, nitride, and carbide of the base metal inside thereof.

[0037] Furthermore, in the catalyst for an electrode of the present invention, from the viewpoint of more surely obtaining the effects of the present invention, when analyzing the particle size distribution of the catalyst particles using a three-dimensional reconstruction image obtained by electron beam tomography measurement using STEM (scanning transmission electron microscope), it is preferable that the conditions of the following formula (1) are satisfied. (D10 / D20)≧0.80···(1) Here, in the above formula (1), D10 represents the additive average value of the equivalent spherical diameters of the catalyst particles supported inside the nanopores of the carrier, and D20 represents the additive average value of the equivalent spherical diameters of the catalyst particles supported outside the nanopores of the carrier. By supporting the catalyst particles on the hollow carbon carrier so as to simultaneously satisfy the conditions of the above formula (1), the catalyst for an electrode of the present invention can more surely exhibit excellent catalytic activity that can contribute to cost reduction of PEFC. Here, from the viewpoint of more surely obtaining the effects of the present invention, the value of (D10 / D20) in the above formula (1) is preferably 0.85 or more, and more preferably 0.90 or more.

[0038] Furthermore, in the catalyst for an electrode of the present invention, from the viewpoint of more surely obtaining the effects of the present invention, when performing analysis of the particle size distribution of the catalyst particles using a three-dimensional reconstruction image obtained by electron beam tomography measurement using STEM (scanning transmission electron microscope), it is more preferable that the conditions of the following formulas (2) and (3) are further simultaneously satisfied. D1≦D2···(2) (N1 / N2)>1.0···(3) Here, 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 nanopores of the carrier. 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 nanopores of the carrier. Also, in the above formula (2) and the above formula (3), N1 represents the frequency (number of particles) of the particle showing the maximum frequency (maximum number of particles) among the catalyst particles supported inside the nanopores of the carrier. In the above formula (1) and the above formula (2), N2 represents the frequency (number of particles) of the particle showing the maximum frequency (maximum number of particles) among the catalyst particles supported outside the nanopores of the carrier. By supporting the catalyst particles on the hollow carbon carrier so as to simultaneously satisfy the conditions of the above formulas (2) and (3), the catalyst for an electrode of the present invention can more surely exhibit excellent catalytic activity that can contribute to cost reduction of PEFC.

[0039] Furthermore, in the catalyst for an electrode of the present invention, at least a part of the region composed of Pt(zero valence) on the surface of the catalyst particles may be covered with a Pt oxide film within a range where the catalyst particles can exhibit excellent catalytic activity. Also, from the viewpoint of more surely obtaining the effects of the present invention, in the catalyst for an electrode of the present invention, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon carrier is 200 to 1500 m 2 / g, which is preferable. When the catalyst for an electrode is used for a cathode, from the viewpoint of more surely obtaining the effects of the present invention, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon carrier is 700 to 1500 m 2 / g, which is preferable, and 750 to 1400 m 2 / g, which is more preferable. Furthermore, when the catalyst for an electrode is used for a cathode, from the viewpoint that it is preferable to have a predetermined durability in consideration of the operating environment (temperature fluctuation range, potential fluctuation range) of the cathode, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon carrier is preferably 750 to 900 m 2 / g.

[0040] The present invention also provides a powder of a catalyst for an electrode containing 10 wt% or more of the catalyst for an electrode of the present invention described above. In the powder of the catalyst for an electrode, the "components other than the catalyst for an electrode of the present invention described above" are "catalysts for an electrode other than the catalyst for an electrode of the present invention described above". That is, the powder of the catalyst for an electrode of the present invention does not contain powders that do not function as a catalyst for an electrode. Since the powder of the catalyst for an electrode of the present invention contains the catalyst for an electrode of the present invention described above, it can exhibit excellent catalytic activity that can contribute to reducing the cost of PEFC. Here, from the viewpoint of more surely obtaining the effects of the present invention, the content ratio of the catalyst for an electrode of the present invention described above in the powder of the catalyst for an electrode of the present invention is preferably 30 wt% or more, more preferably 50 wt% or more, still more preferably 70 wt% or more, and most preferably 90 wt% or more.

[0041] In addition to the catalyst for an electrode of the present invention described above, the powder of the catalyst for an electrode of the present invention may contain a catalyst for an electrode having the following configuration (for convenience, referred to as "catalyst for electrode P"). That is, the catalyst for electrode P includes a 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 Pt(0 valence), the catalyst particles are supported on both the inside and the outside of the nanopores of the carrier, when the particle size distribution of the catalyst particles is analyzed using the above-mentioned "USAL-KM3D analysis method", the proportion of the catalyst particles supported inside the nanopores is less than "50%". The powder of the catalyst for an electrode of the present invention may be composed of the catalyst for an electrode of the present invention described above and the catalyst for electrode P. Also in this case, from the viewpoint of more surely obtaining the effects of the present invention, the content ratio of the catalyst for an electrode of the present invention in the powder of the catalyst for an electrode of the present invention is preferably 30 wt% or more, more preferably 50 wt% or more, still more preferably 70 wt% or more, and most preferably 90 wt% or more.

[0042] Furthermore, the powder of the catalyst for an electrode of the present invention may contain one or more types of conductive carbon carriers different from the hollow carbon carrier according to the electrode catalyst of the present invention within the range where the effects of the present invention can be obtained. For example, at least one of Ketjen black and acetylene black may be included. For example, 10 wt% to 100 wt% of different types of conductive carbon carriers may be included based on the weight of the hollow carbon carrier according to the electrode catalyst of the present invention.

[0043] Furthermore, the present invention provides a composition for forming a gas diffusion electrode containing the above-mentioned catalyst for an electrode of the present invention or the powder of the catalyst for an electrode of the present invention. Since the composition for forming a gas diffusion electrode of the present invention contains the catalyst for an electrode of the present invention or the powder of the catalyst for an electrode of the present invention, it is possible to easily manufacture a gas diffusion electrode having excellent catalytic activity (polarization characteristics) that can contribute to cost reduction of PEFCs.

[0044] The present invention also provides a gas diffusion electrode containing the catalyst for an electrode of the present invention or the powder of the catalyst for an electrode of the present invention described above. The gas diffusion electrode of the present invention is composed of including the catalyst for an electrode of the present invention. Therefore, it becomes easy to have a structure having excellent catalytic activity (polarization characteristics) that can contribute to cost reduction of PEFCs.

[0045] Furthermore, the present invention provides a membrane-electrode assembly (MEA) including the gas diffusion electrode of the present invention described above. Since the membrane-electrode assembly (MEA) of the present invention contains the gas diffusion electrode of the present invention, it becomes easy to have a structure having battery characteristics that can contribute to cost reduction of PEFCs.

[0046] The present invention also provides a fuel cell stack characterized by including the membrane-electrode assembly (MEA) of the present invention described above. According to the fuel cell stack of the present invention, since it includes the membrane-electrode assembly (MEA) of the present invention, it becomes easy to have a structure having battery characteristics that can contribute to cost reduction of PEFCs.

Advantages of the Invention

[0047] According to the present invention, a catalyst for an electrode having excellent catalytic activity that can contribute to cost reduction of PEFCs is provided. Further, according to the present invention, there are provided a composition for forming a gas diffusion electrode, a gas diffusion electrode, a membrane-electrode assembly (MEA), and a fuel cell stack including such a catalyst for an electrode.

Brief Description of the Drawings

[0048]

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Figure 19

Figure 20

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Figure 22

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Mode for Carrying Out the Invention

[0049] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

[0050] <Membrane Electrode Assembly (MEA)> FIG. 1 is a schematic cross-sectional view showing a preferred form of the MEA of the present invention. The MEA10 shown in FIG. 1 has a configuration including two flat plate-shaped gas diffusion electrodes (cathode 1 and anode 2) arranged to face each other, and a polymer electrolyte membrane (Polymer Electrolyte Membrane, hereinafter, referred to as "PEM" as necessary) 3 disposed between the cathode 1 and the anode 2. In the case of this MEA10, at least one of the cathode 1 and the anode 2 has a configuration containing an electrode catalyst 20 (Pt catalyst 20) described later. The MEA10 can be manufactured by laminating the cathode 1, the anode 2, and the PEM 3 as shown in FIG. 1 and then pressing them.

[0051] <Gas Diffusion Electrode (GDE)> The cathode 1, which is a gas diffusion electrode, has a configuration including a gas diffusion layer 1gd, a catalyst layer 1c formed on the surface of the gas diffusion layer 1gd on the PEM 3 side, and a water repellent layer (Micro Porous Layer, hereinafter, referred to as "MPL" as necessary) 1m disposed between the gas diffusion layer 1gd and the catalyst layer 1c. The anode 2, which is a gas diffusion electrode, also has a configuration including 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 disposed between the gas diffusion layer 2gd and the catalyst layer 2c, similar to the cathode 1.

[0052] (Catalyst Layer (CL)) At the cathode 1, the catalyst layer 1c is a layer where a reaction proceeds to generate water from the air (oxygen gas) sent from the gas diffusion layer 1gd and hydrogen ions moving through the PEM 3 from the anode 2. Also, at the anode 2, the catalyst layer 2c is a layer where a reaction proceeds to generate hydrogen ions and electrons from the hydrogen gas sent from the gas diffusion layer 2gd. At least one of the catalyst layer 1c of the cathode 1 and the catalyst layer 2c of the anode 2 contains the electrode catalyst 20 of the present invention.

[0053] (A preferred form of the electrode catalyst of the present invention) Hereinafter, a preferred form of the electrode catalyst of the present invention will be described with reference to FIGS. 2, 3, 24, and 25. FIG. 2 is a schematic cross-sectional view showing a preferred form of the electrode catalyst contained in at least one of the cathode catalyst layer 1c and the anode catalyst layer 2c of the MEA 10 shown in FIG. 1. FIG. 3 is an enlarged schematic cross-sectional view showing the schematic configuration of the electrode catalyst 20 shown in FIG. 2. As shown in FIGS. 2 and 3, the electrode catalyst 20 includes a carrier 22 which is a hollow carbon carrier and catalyst particles 23 supported on the carrier 22.

[0054] FIG. 24 is a table showing a plurality of 3D-STEM images (three-dimensional reconstructed images) obtained by electron beam tomography measurement using STEM for the electrode catalysts of Example 1, Example 2 (Examples of the present electrode catalyst 20), and Comparative Example 2, respectively. FIG. 25 is an enlarged view of a cross-section (x-y plane) of a cubic image extracted from a 3D-STEM image (three-dimensional reconstructed image) of the catalyst mass of the electrode catalyst of Example 1 (Example of the present electrode catalyst 20) shown in FIG. 24.

[0055] Also, the electrode catalyst 20 shown in FIGS. 2 to 3 preferably satisfies the following conditions from the viewpoint of more surely obtaining the effects of the present invention. That is, as described above, when observing the fine structure of the catalyst for electrode 20 using the three-dimensional reconstructed image of STEM, as described above, it has a configuration that satisfies the conditions: (α) the ratio of the catalyst particles supported inside the nanopores is 50% or more, and (β) the nanopores are formed to have the above-described shape.

[0056] Regarding the condition of (α) in more detail, when observing the fine structure of the catalyst for electrode 20 with the information of the three-dimensional reconstructed image of STEM obtained by the above-described procedures (A) to (C), the ratio of the catalyst particles 23 supported inside the nanopore P22 is 50% or more. Furthermore, from the viewpoint of more surely obtaining the effects of the present invention, when analyzing the particle size distribution of the catalyst particles 23 using the above-described three-dimensional reconstructed image, it is preferable that the ratio of the catalyst particles 23 supported inside the nanopore P22 is 55% or more, more preferably 60% or more, and still more preferably 70% or more for the catalyst for electrode 20.

[0057] Regarding the condition of (β) in more detail, when looking at the information of the three-dimensional reconstructed image of STEM obtained by the above-described procedures (D) to (G) for the catalyst for electrode 20, it has the following structure. Explaining in even more detail, when looking at six square cross-sections of a "cube image (with a side length of 20 to 50 nm)" further extracted from a "catalyst mass (a catalyst mass composed of catalyst particles 23 and a carrier 22 with a size accommodated in a rectangular parallelepiped space with a side length of 60 to 300 nm) constituting the catalyst for electrode 20" that can be seen in a 3D-STEM image (three-dimensional reconstructed image) of the catalyst for electrode 20, in at least one cross-section, it has a structure in which at least one "connected pore P1 where a plurality of nanopores P22 are connected" having the following shape is formed.

[0058] That is, in the electrode catalyst 20, when its fine structure is observed as a 3D-STEM image (three-dimensional reconstructed image), the nanopore P22 formed in at least one of the six square cross-sections of the three-dimensional image obtained by cutting out the inside of the above-mentioned catalyst mass has at least one opening in contact with the first side among the four sides of the square cross-section and at least one opening in contact with the second side of the square cross-section parallel to the first side. And this nanopore P22 has a shape of a continuous through-hole P1 that extends continuously from the opening on the first side to the opening on the second side without being blocked. Hereinafter, a more specific description will be given using the example of the electrode catalyst of Example 1 shown in FIGS. 24 and 25.

[0059] (D) First, obtain a three-dimensional reconstructed image of STEM for the electrode catalyst of Example 1 to be measured. From the catalyst mass reflected in this three-dimensional reconstructed image, select a catalyst mass having a size accommodated in a rectangular parallelepiped space (region of interest) with a side length of 60 to 300 nm (FIG. 24(a)). (E) Next, extract a cubic image (with a side length of 20 to 50 nm) from the internal region of the catalyst mass of the electrode catalyst of Example 1 selected in step (D) (FIG. 24(b)). (F) Next, observe the three-dimensional image (STEM three-dimensional reconstructed image) inside the catalyst mass of the electrode catalyst of Example 1 obtained in step (E), and utilize the difference in luminance to segment the void part (the part of pores such as nanopores) and the part of the hollow carbon carrier (FIG. 24(b)). (G) Next, when looking at the six square cross-sections of the three-dimensional image inside the catalyst mass of the electrode catalyst of Example 1 after performing segmentation in step (F), as shown in FIG. 25, the nanopore P1 visible in the cross-section of interest (the x-y plane of the square) has two openings (opening A11 and opening A12) in contact with the first side L1. Also, this nanopore P1 has two openings (opening A21 and opening A22) in contact with the second side L2 parallel to the first side L1. Furthermore, this nanopore P1 has a shape of a continuous through-hole P1 that extends continuously from the openings on the first side L1 (opening A11 and opening A12) to the openings on the second side L2 (opening A21 and opening A22) without being blocked.

[0060] From the viewpoint of more surely obtaining the effects of the present invention, as shown in the example of Example 1 in FIG. 25, the nanopores P1 of the catalyst for electrodes 20 preferably have a shape branched into a plurality. Furthermore, from the viewpoint of more surely obtaining the effects of the present invention, as shown in the example of Example 1 in FIG. 25, the nanopores P1 of the catalyst for electrodes 20 preferably have two or more openings on the first side. Also, from the viewpoint of more surely obtaining the effects of the present invention, as shown in the example of Example 1 in FIG. 25, the nanopores P1 of the catalyst for electrodes 20 preferably have two or more openings on the second side. Furthermore, from the viewpoint of more surely obtaining the effects of the present invention, as shown in the example of Example 1 in FIG. 25, the nanopores P1 of the catalyst for electrodes 20 preferably have at least one opening on the third side perpendicular to the first side. Explaining with reference to the example of Example 1, as shown in FIG. 25, the nanopore P1 visible in the cross-section of interest (the x-y plane of the square) also has one opening (opening A31) on the third side L3 perpendicular to the first side L1.

[0061] Also, from the viewpoint of more surely obtaining the effects of the present invention, as shown in the example of Example 1 in FIG. 25, the nanopores P1 of the catalyst for electrodes 20 preferably have at least one opening on the fourth side perpendicular to the first side. For example, explaining with reference to the example of Example 1, as shown in FIG. 25, the nanopore P1 visible in the cross-section of interest (the x-y plane of the square) also has two openings (opening A41 and opening A42) on the fourth side L4 perpendicular to the first side L1.

[0062] Furthermore, from the viewpoint of more surely obtaining the effects of the present invention, in the catalyst for an electrode 20, the porosity measured using the three-dimensional reconstructed image (the focused cube image) of STEM is preferably 35% or more, more preferably 40% or more, still more preferably 45% or more, further more preferably 50% or more, further more preferably 55% or more, further more preferably 60% or more, and further more preferably 65% or more. On the other hand, from the viewpoint of durability, in the catalyst for an electrode 20, the porosity measured using the three-dimensional reconstructed image (the focused cube image) of STEM is preferably 80% or less, and more preferably 75% or less.

[0063] Here, in the catalyst particle 23, a region made of Pt(0 valence) is formed on at least a part of its surface. However, a layer of Pt oxide may be formed on the region made of Pt(0 valence) on the surface of the catalyst particle 23 as long as the effects of the present invention can be obtained. As a more specific configuration of the catalyst particle 23, when the catalyst particle 23 is made of Pt(0 valence), it is not particularly limited. However, when the catalyst particle 23 is made of a Pt alloy, or when the catalyst particle 23 is a core-shell catalyst particle, it is preferably mentioned. When the catalyst particle 23 is made of a Pt alloy, the metal species that is an alloy element other than Pt is not particularly limited. From the viewpoint of obtaining excellent catalytic activity, the metal species that is an alloy element other than Pt is preferably at least one metal selected from Co and Ni.

[0064] When the catalyst particles 23 are core-shell catalyst particles, from the viewpoint of obtaining excellent catalytic activity, the core-shell catalyst particles are preferably core particles and a Pt shell layer (a region composed of Pt(0 valence)) formed on at least a part of the surface of the core particles. The metal species constituting the core particles is not particularly limited, but from the viewpoint of obtaining excellent catalytic activity, it is preferably at least one of Pd, Ni, and Co. Further, the core particles may be an alloy of at least one of Pd, Ni, and Co and another metal. From the viewpoint of reducing the amount of precious metal used, the core particles may contain a base metal (base metal) other than the precious metal, and at least one of an oxide of the base metal, a nitride of the base metal, and a carbide of the base metal inside.

[0065] Further, the catalyst 20 for an electrode preferably has an average crystallite size measured by powder X-ray diffraction (XRD) of 3 to 16.0 nm. Further, the Pt loading rate of the catalyst 20 for an electrode is preferably 5.6 to 66.5 wt%.

[0066] The carrier 22 is preferably one that can satisfy the above conditions (α) and (β) when used as a catalyst for an electrode. From this viewpoint, as the carrier 22, among CNovel (manufactured by Toyo Tanso Co., Ltd., product name, registered trademark), those that can satisfy the above conditions (α) and (β) when used as a catalyst for an electrode are preferable.

[0067] As shown in FIG. 2, in the present embodiment, the carrier 22 is a porous carbon provided with nanopores P22 (pore diameter of 1 to 20 nm, preferably pore diameter of 1 to 10 nm), micropores P24 (pore diameter of less than 1 nm), and a carbonaceous wall constituting the outer contour of the nanopores P22. There is a portion having a layered structure in this carbonaceous wall, and further, this carbonaceous wall forms a three-dimensional network structure. The portion having a layered structure in this carbonaceous wall has well-developed crystallinity.

[0068] Generally, this layered structure is formed by heat-treating a carbon material at a certain temperature or higher. However, generally, since the carbon material shrinks during the heat treatment, the pores are crushed and the specific surface area tends to decrease, and it has been difficult to obtain porous carbon with a high specific surface area by developing crystallinity. On the other hand, since the carrier 22 has nanopores P22 and a carbonaceous wall constituting the outer contour of the nanopores P22, it can withstand shrinkage during heat treatment, and a layered structure is sufficiently formed in this carbonaceous wall, and a sufficient specific surface area is also ensured.

[0069] In addition, since the carbonaceous wall of the carrier 22 forms a three-dimensional network structure, small catalyst particles on the order of several nanometers can be highly dispersed and supported, and it is suitable as a carrier for the catalyst layer of a fuel cell. Note that not all parts of the carbonaceous wall of the carrier 22 need to have a layered structure, and an amorphous part may exist in part. In addition, the carrier 22 has a specific surface area of 200 m 2 / g to 1500 m 2 / g is preferable. When the specific surface area is 200 m 2 / g or more, it becomes easier to more surely form a three-dimensional network structure. As a result, pores can be sufficiently formed, and it becomes easy to have sufficient gas adsorption ability. On the other hand, when the specific surface area is 1500 m 2 / g or less, it becomes easier to more surely shape the carbonaceous wall. As a result, it becomes easier to sufficiently form the nanopores P22.

[0070] Here, as shown in FIG. 2, in the carrier 22, the nanopores P22 are open pores, and have a configuration in which the nanopores P22 are continuously connected to form a connecting pore P1. By having this configuration, the flow of the reaction gas in the catalyst layer (catalyst layer 1c or catalyst layer 2c) can be smoothed. From the viewpoint that the carrier 22 has sufficient conductivity, the specific resistance is preferably 10.0×10 2 Ω·cm or less, more preferably 5.0×10 2 Ω·cm, and even more preferably 1.0×10 2 Ω·cm or less. In addition, within the range where the effects of the present invention can be obtained, the carrier 22 may contain pores with a pore diameter of less than 1 nm (relatively small pores among the pores classified as so-called micropores) and pores with a pore diameter exceeding 20 nm and not exceeding 50 nm (relatively large pores among the pores classified as so-called mesopores). Furthermore, the carrier 22 is preferably a hollow carbon carrier that has good dispersibility in the composition for forming a gas diffusion electrode containing the catalyst 20 for an electrode and has excellent electrical conductivity.

[0071] Here, as shown in FIG. 2, the catalyst particles 23 are supported both inside and outside the nanopores P22 of the carrier 22. And the catalyst 20 for an electrode simultaneously satisfies the following conditions (1) to (3) when measurements of electron beam tomography by 3D-STEM are carried out. (D10 / D20)≧0.80 ··· (1) D1≦D2 ··· (2) (N1 / N2)>1.0 ··· (3) Here, in formulas (1) to (3), D10 represents the additive average value of the sphere equivalent diameters of the catalyst particles 23 supported inside the nanopores P22 of the carrier 22.

[0072] Also, D20 represents the additive average value of the sphere equivalent diameters of the catalyst particles 23 supported outside the nanopores P22 of the carrier 22. Furthermore, D1 represents the sphere equivalent diameter (nm) of the particle showing the maximum frequency (maximum number of particles) among the catalyst particles 23 supported inside the nanopores P22 of the carrier 22. Also, D2 represents the sphere equivalent diameter of the particle showing the maximum frequency (maximum number of particles) among the catalyst particles 23 supported outside the nanopores P22 of the carrier 22. Furthermore, N1 represents the frequency (number of particles) of the particle showing the maximum frequency (maximum number of particles) among the catalyst particles 23 supported inside the nanopores P22 of the carrier 22. Also, N2 represents the frequency (number of particles) of the particle showing the maximum frequency (maximum number of particles) among the catalyst particles 23 supported outside the nanopores P22 of the carrier 22.

[0073] The electrode catalyst 20 that simultaneously satisfies the conditions of formulas (1) to (3) will have a large number of highly active catalyst particles 23 with a relatively small particle diameter existing inside the nanopores P22 of the carrier 22 as compared with the conventional electrode catalyst 200. The catalyst particles 23 supported inside the nanopores P22 of such a carrier 22 exhibit excellent catalytic activity when polarized as compared with the conventional electrode catalyst 200. Also, it will be supported on the carrier 22 in a state where it is difficult to directly contact a polymer electrolyte such as Nafion contained in the catalyst layer (catalyst layer 1c or catalyst layer 2c), and the dissolution of the Pt component is also reduced. Here, from the viewpoint of more surely obtaining the effects of the present invention, the value of (D10 / D20) in the above formula (1) is preferably 0.85 or more, and more preferably 0.90 or more.

[0074] The method for manufacturing the electrode catalyst 20 is not particularly limited as long as it includes a "carrier pretreatment step", a "Pt addition step", and a "reduction step" to satisfy the conditions of formulas (1) to (3), and it can be manufactured by a known method. In the carrier pretreatment step, the carrier 22 is put into ultrapure water, and a pH adjuster is further added to prepare a dispersion liquid with the pH adjusted to 2 to 5. Further, while stirring this dispersion liquid, the temperature is maintained at 80 to 99 °C, preferably 90 to 99 °C for a predetermined time (however, a state where it is not boiled is maintained). Then, the dispersion liquid is cooled to room temperature. Thereby, the gas inside the nanopores P22 of the carrier 22 is removed, and ultrapure water can sufficiently penetrate into the inside of the nanopores P22. And in the subsequent "Pt addition step", the Pt raw material will be sufficiently retained inside the nanopores P22 of the carrier 22. Thereby, a large number of precursors of Pt catalyst particles will be supported inside the nanopores P22 of the carrier 22.

[0075] In addition, the "ultrapure water" used for preparing the above-mentioned aqueous solution in this carrier pretreatment step is water with a specific resistance R (the reciprocal of the electrical conductivity measured by the JIS standard test method (JIS K0552)) represented by the following formula (4) of 3.0 MΩ·cm or more. Also, it is preferable that the "ultrapure water" has a water quality corresponding to "A3" or cleaner water quality defined in JIS K0557 "Water for Tests of Service Water and Wastewater".

[0076] This ultrapure water is not particularly limited as long as it has an electrical conductivity satisfying the relationship represented by the following formula (4). For example, as the above ultrapure water, ultrapure water produced using ultrapure water manufacturing devices "Milli-Q series" (manufactured by Merck KGaA) and "Elix UV series" (manufactured by Millipore Japan K.K.) can be mentioned. R = 1 / ρ ···(4) In the above formula (4), R represents the specific resistance, and ρ represents the electrical conductivity measured by the JIS standard test method (JIS K0552).

[0077] The next step after the "carrier pretreatment step" is the "Pt addition step". In this "Pt addition step", an aqueous solution in which a water-soluble Pt salt (N.E. CHEMCAT, trade name "A-salt" (Fe component concentration: 8 ppm or less)) is dissolved in ultrapure water is added at room temperature to the dispersion of the carrier 22 obtained through the "carrier pretreatment step". The next step after the "Pt addition step" is the "reduction step". In this "reduction step", the temperature of the liquid obtained through the "Pt addition step" is raised to 50°C or higher, and an aqueous solution in which a water-soluble reducing agent (preferably an acidic water-soluble reducing agent) is dissolved is added. After the addition of the reducing agent, the liquid temperature is maintained at 50°C or higher for a predetermined time to allow the reduction reaction to proceed, and then the temperature of the liquid is lowered to room temperature.

[0078] The next step after the "reduction step" is the "washing step". In this "washing step", the solid component and the liquid component in the liquid obtained through the "reduction step" are separated, and the solid component (a mixture of Pt / C catalyst and other impurities) is washed. For example, filtration means such as filter paper or filter cloth may be used to separate the solid component in the liquid obtained through the "reduction step" from the liquid component. The solid component may be washed using the above-mentioned ultrapure water, pure water (the specific resistance R represented by the above formula (4) is 0.1 MΩ·cm or more and less than 3.0 MΩ·cm), or pure warm water (pure water with a temperature of 40 to 80 °C). For example, when using pure warm water, the washing is repeated until the electrical conductivity of the filtrate after washing is less than 10 μS / cm. The next step after the "washing step" is the "drying step". In this "drying step", moisture is separated from the solid component (a mixture of Pt / C catalyst and water) obtained through the "washing step". First, the solid component is air-dried, and then dried in a dryer at a predetermined temperature and for a predetermined time. The next step after the "drying step" is the "grinding step". In this "grinding step", the solid component (Pt / C catalyst) obtained through the "drying step" is made into a catalyst powder using grinding means such as a mixer.

[0079] The polymer electrolyte contained in the catalyst layer 1c and the catalyst layer 2c is not particularly limited as long as it has proton conductivity, and known ones can be used. For example, the polymer electrolyte can be exemplified by known perfluorocarbon resins having sulfonic acid groups and carboxylic acid groups. Preferred examples of easily available polymer electrolytes having proton conductivity include Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Corporation), and Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.). And at least one of the catalyst layer 1c of the cathode 1 and the catalyst layer 2c of the anode 2 shown in FIG. 1 has a mass ratio N / C of the mass C of the carrier 22 and the mass N of the polymer electrolyte of 0.5 to 1.2, and more preferably a mass ratio N / C of 0.7 to 1.0.

[0080] (Gas Diffusion Layer (GDL)) The gas diffusion layer 1gd provided in the cathode 1 shown in FIG. 1 is a layer provided for supplying an oxidant gas (e.g., oxygen gas, air) to the catalyst layer 1c. Further, the gas diffusion layer 1gd has a role of supporting the catalyst layer 1c. Also, the gas diffusion layer 2gd provided in the anode 2 is a layer provided for supplying a reducing agent gas (e.g., hydrogen gas) to the catalyst layer 2c. Further, the gas diffusion layer 2gd has a role of supporting the catalyst layer 2c.

[0081] The gas diffusion layer (1gd) shown in FIG. 1 has a function and structure that allows hydrogen gas or air (oxygen gas) to pass through well and reach the catalyst layer. For this reason, the gas diffusion layer preferably has water repellency. For example, the gas diffusion layer has a water repellent component such as polyethylene terephthalate (PTFE). The members that can be used for the gas diffusion layer (1gd) are not particularly limited, and known members can be used. For example, carbon paper, carbon paper as the main raw material, and a secondary raw material composed of carbon powder, ion-exchanged water, and polyethylene terephthalate dispersion as an optional component are preferably applied to the carbon paper.

[0082] (Water repellent layer (MPL)) As shown in FIG. 1, a water repellent layer (MPL) 1m is disposed between the gas diffusion layer 1gd and the catalyst layer 1c in the cathode 1. The water repellent layer 1m has electronic conductivity, water repellency, and gas diffusibility, and is provided to promote the diffusion of the oxidant gas to the catalyst layer 1gd and the discharge of the reaction-generated water generated in the catalyst layer 1gd. The configuration of the water repellent layer 1m is not particularly limited, and a known configuration can be adopted.

[0083] (Polymer electrolyte membrane (PEM)) The polymer electrolyte membrane (PEM) 3 shown in FIG. 1 is not particularly limited as long as it has hydrogen ion conductivity, and a known one conventionally used in PEFC can be adopted. For example, a membrane containing, as a constituent component, those exemplified as the polymer electrolyte contained in the catalyst layer 1c and the catalyst layer 2c described above may be used.

[0084] <Modified forms of MEA> As described above, the preferred embodiments of the MEA of the present invention (and the catalyst layer and gas diffusion electrode of the present invention) have been described, but the MEA of the present invention is not limited to the configuration of MEA10 shown in FIG. 1. For example, the MEA of the present invention may have the configuration of MEA11 shown in FIG. 4. FIG. 4 is a schematic cross-sectional view showing another preferred form of the MEA of the present invention. The MEA11 shown in FIG. 4 has a configuration in which a gas diffusion electrode (GDE) 1A having the same configuration as the cathode 1 in the MEA10 shown in FIG. 1 is disposed only on one side of the polymer electrolyte membrane (PEM) 3. However, the catalyst layer 1c of the gas diffusion electrode (GDE) 1A has the configuration of the catalyst layer of the present invention. That is, in the catalyst layer 1c of GDE1A, the mass ratio N / C of the mass C of the carrier 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.

[0085] <Membrane-catalyst layer conjugate (CCM)> Next, the preferred embodiments of the membrane-catalyst layer conjugate (CCM: Catalyst Coated Membrane) of the present invention will be described. FIG. 5 is a schematic cross-sectional view showing a preferred form of the CCM of the present invention. The CCM12 shown in FIG. 5 has a configuration in which a polymer electrolyte membrane (PEM) 3 is disposed between the cathode catalyst layer 1c and the anode catalyst layer 2c. And at least one of the cathode catalyst layer 1c and the anode catalyst layer 2c has the configuration of the catalyst layer of the present invention. That is, 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 carrier 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.

[0086] <Modified forms of membrane-catalyst layer conjugate (CCM)> As described above, the preferred embodiments of the CCM of the present invention have been described, but the CCM of the present invention is not limited to the configuration of CCM12 shown in FIG. 5. For example, the CCM of the present invention may have the configuration of CCM13 shown in FIG. 6. FIG. 7 is a schematic cross-sectional view showing another preferred embodiment of the CCM of the present invention. The CCM13 shown in FIG. 6 has a configuration in which a catalyst layer 1c having the same configuration as the cathode 1 in the CCM12 shown in FIG. 5 is disposed only on one side of the polymer electrolyte membrane (PEM) 3. However, the catalyst layer 1c of the gas diffusion electrode (GDE) 1A has the configuration of the catalyst layer of the present invention. That is, in the catalyst layer 1c of the CCM13, the mass ratio N / C of the mass C of the carrier 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.

[0087] <Gas diffusion electrode (GDE)> Next, a preferred embodiment of the gas diffusion electrode (GDE) of the present invention will be described. FIG. 8 is a schematic cross-sectional view showing a preferred embodiment of the GDE of the present invention. The gas diffusion electrode (GDE) 1B shown in FIG. 7 has the same configuration as the cathode 1 mounted on the MEA10 shown in FIG. 1. However, the catalyst layer 1c of the gas diffusion electrode (GDE) 1B has the configuration of the catalyst layer of the present invention. That is, in the catalyst layer 1c of the gas diffusion electrode (GDE) 1B, the mass ratio N / C of the mass C of the carrier 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.

[0088] <Modified forms of the gas diffusion electrode (GDE)> As described above, the preferred embodiments of the GDE of the present invention have been described, but the GDE of the present invention is not limited to the configuration of the GDE1B shown in FIG. 7. For example, the GDE of the present invention may have the configuration of GDE1C shown in FIG. 8. FIG. 9 is a schematic cross-sectional view showing another preferred embodiment of the GDE of the present invention. The GDE1C shown in FIG. 8 has a configuration in which a water repellent layer (MPL) is not disposed between the catalyst layer 1c and the gas diffusion layer 1gd as compared with the GDE1B shown in FIG. 8.

[0089] <Composition for forming catalyst layer> Next, a preferred embodiment of the composition for forming a catalyst layer of the present invention will be described. The composition for forming a catalyst layer according to this embodiment contains a catalyst for an electrode 20, a polymer electrolyte, and a main component, and the mass ratio N / C of the mass C of the carrier 22 of the catalyst for an electrode 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0. Here, the composition of the liquid containing the polymer electrolyte is not particularly limited. For example, the liquid containing the polymer electrolyte may contain the polymer electrolyte having proton conductivity described above, water, and alcohol.

[0090] The composition ratios of the catalyst for an electrode 20, the polymer electrolyte, and other components (such as water and alcohol) contained in the composition for forming a catalyst layer are appropriately set so that the dispersion state of the catalyst for an electrode 20 in the resulting catalyst layer is good and the power generation performance of the MEA 10 including the catalyst layer can be improved. The composition for forming a catalyst layer can be prepared by mixing and stirring a liquid containing the catalyst for an electrode 20 and the polymer electrolyte. From the viewpoint of adjusting the coating property, polyhydric alcohol such as glycerin and / or water may be contained. When mixing the liquid containing the catalyst for an electrode 20 and the polymer electrolyte, a pulverizing mixer such as a ball mill or an ultrasonic disperser may be used. At least one of the catalyst layer 1c of the cathode 1 and the catalyst layer 2c of the anode 2 shown in FIG. 1 can be formed using a preferred embodiment of the composition for forming a catalyst layer of the present invention.

[0091] (Method for manufacturing a gas diffusion electrode) Next, an example of the method for manufacturing a gas diffusion electrode of the present invention will be described. The gas diffusion electrode only needs to be formed so as to include the catalyst layer of the present invention, and a known method can be adopted as the manufacturing method. It can be manufactured more reliably by using the composition for forming a catalyst layer of the present invention. For example, it may be manufactured by applying the composition for forming a catalyst layer on a gas diffusion layer (or the water repellent layer of a laminate having a water repellent layer formed on the gas diffusion layer) and drying it.

[0092] <Fuel cell stack> FIG. 9 is a schematic diagram showing a preferred embodiment of the fuel cell stack of the present invention. The fuel cell stack 30 shown in FIG. 9 has a configuration in which the MEA 10 shown in FIG. 1 is used as a single unit cell and a plurality of these unit cells are stacked. Further, the fuel cell stack 30 has a configuration in which the MEA 10 is disposed between the separator 4 and the separator 5. Gas flow paths are formed in the separator 4 and the separator 5, respectively.

Example

[0093] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to the following examples.

[0094] (I) Preparation of the electrode catalyst used in the cathode catalyst layer of the MEA (1) Production of the Pt / C catalyst used in the cathode of the MEA of Example 1 [Powder of carbon catalyst supporting Pt catalyst particles “Pt / C catalyst”] A powder of a Pt / C catalyst in which catalyst particles made of Pt are supported on the following carrier {Pt loading rate: 48.0 wt%, trade name “SA50BM-A207”, manufactured by N.E. CHEMCAT Corporation} was prepared. The powder of this Pt / C catalyst (hereinafter, referred to as “Pt / C catalyst A” as necessary) was adjusted by the following procedure.

[0095] (First step (carrier pretreatment step)) A dispersion in which a hollow carbon carrier, product name: “CNovel A” (BET specific surface area: 1200 m 2 / g) manufactured by Toyo Tanso Co., Ltd. was dispersed in an aqueous solution adjusted to pH = 2 to 5 (prepared by adding a pH adjuster to ultrapure water) was maintained at a temperature of 90 to 99 ° C. for about 0.5 hours while stirring (however, maintaining a non-boiling state). In addition, the "ultrapure water" used in this first step (carrier pretreatment step) was water with a specific resistance R (the reciprocal of the electrical conductivity measured by the JIS standard test method (JIS K0552)) represented by the following formula (4) of 3.0 MΩ·cm or more. Also, this "ultrapure water" has a water quality corresponding to "A3" or a cleaner water quality defined in JIS K0557 "Water for Use in Tests of Water Supply and Wastewater". This ultrapure water was produced using ultrapure water production apparatuses "Milli-Q series" (manufactured by Merck KGaA) and "Elix UV series" (manufactured by Millipore Japan Corporation). R = 1 / ρ (4) In the above general formula (4), R represents the specific resistance, and ρ represents the electrical conductivity measured by the JIS standard test method (JIS K0552).

[0096] (Second Step (Pt Addition Step)) An aqueous solution in which a water-soluble Pt salt (manufactured by N.E. CHEMCAT, product name "A-salt" (Fe component concentration: 8 ppm or less)) was dissolved in ultrapure water was added to the dispersion obtained through the first step to prepare a mixed solution, the pH was adjusted to 7 to 12, and the mixture was stirred while maintaining a predetermined temperature of 50°C or more for a predetermined time.

[0097] (Third Step (Reduction Step)) An aqueous solution in which an acidic water-soluble reducing agent was dissolved was added to the solution obtained through the second step to reduce the Pt ions in the mixed solution and obtain carbon supported with Pt catalyst particles "Pt / C" powder.

[0098] (Fourth Step (Washing Step)) The solid component and the liquid component in the solution obtained through the "third step" were separated using filter paper. Next, the solid component remaining on the filter paper (a mixture of the Pt / C catalyst and other impurities) was washed using the above-mentioned pure water and pure warm water. First, washing with pure water was performed. This washing was repeated until the electrical conductivity of the filtrate after washing was less than 20 μS / cm. Next, washing with pure warm water was performed. This washing was repeated until the electrical conductivity of the filtrate after washing was less than 10 μS / cm.

[0099] (Fifth Step (Drying Step)) The solid component (mixture of Pt / C catalyst and water) on the filter paper obtained after the "fourth step" was air-dried in this state in air. After this air-drying, the solid component on the filter paper was transferred to a magnetic dish and dried in an electric dryer at a predetermined temperature of 60 °C or higher for a predetermined time.

[0100] (Sixth step (grinding step)) The solid component (Pt / C catalyst) obtained after the "fifth step" was ground using a mixer to obtain a powder of Pt / C catalyst A.

[0101] (Measurement of loading rate (ICP analysis)) For this Pt / C catalyst A, the Pt loading rate (wt%) was measured by the following method. Pt / C catalyst A was immersed in aqua regia to dissolve the metal. Next, carbon, the insoluble component, was removed from the aqua regia. Next, the aqua regia from which carbon had been removed was subjected to ICP analysis. As a result of ICP analysis, for this Pt / C catalyst A, the Pt loading rate was 48.0 wt%.

[0102] (Surface observation and structure observation of the catalyst for the electrode) For the Pt / C catalyst A of this Example 1, in order to observe its three-dimensional structure, measurement of electron tomography using STEM (scanning transmission electron microscope) at the UBE Science Analysis Center Co., Ltd. was carried out using the "USAL-KM3D analysis method". Measurement of electron tomography using STEM (scanning transmission electron microscope) was carried out according to (A) to (C) and (D) to (G) of the above-described measurement sample preparation method, conditions, analysis procedure, and conditions. More detailed information is described below.

[0103] · STEM device: JEM-ARM200F atomic resolution analysis electron microscope manufactured by JEOL Ltd. · Data analysis software: 3D reconstruction software Composer, 3D data visualization software Visualizer-kai, and image analysis software Colorist manufactured by System Frontier · Measurement conditions Acceleration voltage: 60 kV Observation magnification: 800,000 to 1,000,000 times Inclination angle of the measurement sample: -80° to +80° Inclination step angle of the measurement sample: 2° Number of pixels: 512 × 512 pixels 512 × 512 pixels Pixel size: 0.350 nm / pixel to 0.500 nm / pixel Volume size: as shown in Fig. 11

[0104] For the Pt / C catalyst A, by image analysis of the three-dimensional reconstructed image (3D-STEM image) obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope), the Pt catalyst particles present inside the carbon carrier (hereinafter, internal particles) and the Pt catalyst particles present on the surface portion of the carbon carrier (hereinafter, external particles) were separated, and the particle size distribution of the Pt catalyst particles in each region was calculated. The three-dimensional reconstructed image (3D-STEM image) of the Pt / C catalyst A is shown in Fig. 12. The particle size analysis results (particle size distribution shown by the equivalent spherical diameter) of the catalyst particles supported inside the nano-pores of the carrier and the catalyst particles supported outside the nano-pores among the catalyst particles obtained by image analysis are shown in Figs. 13 and 14.

[0105] The 3D-STEM image was obtained by reconstructing a plurality of two-dimensional STEM images obtained by gradually inclining the sample stage under the above measurement conditions. In addition, the image analysis (particle size analysis) of the three-dimensional reconstructed image (3D-STEM image) was performed according to the following procedure. First, the observation region of the catalyst particles was selected from the three-dimensional reconstructed image, and each catalyst particle was labeled (not shown). Next, the volume of the labeled Pt catalyst particles was obtained, the diameter of a sphere (equivalent spherical diameter) having the same volume as this volume was calculated, and the particle size distribution (Figs. 13 and 14) was obtained.

[0106] Here, the equivalent spherical diameter was calculated by rounding off the numerical value below the decimal point (numerical value less than 1 nm) with the unit being nm. For this Pt / C catalyst A, the ratio of the catalyst particles supported inside the nanopores of the carrier and the ratio of the catalyst particles supported outside the nanopores of the carrier were determined. Also, the values of D10, D20, D1, D2, N1, and N2 were determined. The results are shown in Tables 2 and 3. Furthermore, the average value of the particle size of the catalyst particles of Pt / C catalyst A measured from the STEM image was 3.1 nm (average value of the particle size of the catalyst particles inside the nanopores: 3.1 nm, average value of the particle size of the catalyst particles outside the nanopores: 3.2 nm).

[0107] (2) Production of the Pt / C catalyst used for the cathode of the MEA in Example 2 [Powder of [Carbon catalyst supporting Pt catalyst particles "Pt / C catalyst"] Except for using the hollow carbon carrier, the product name of the prototype sample manufactured by Toyo Tanso Co., Ltd.: "CNovel B" (BET specific surface area: 800 m 2 / g), the powder of the Pt / C catalyst (hereinafter, referred to as "Pt / C catalyst B" as necessary) {Pt loading rate 48.0 wt%, product name "SA50BM-B237", manufactured by N.E.CHEMCAT} used for the cathode of the MEA in Example 2 was prepared under the same conditions and procedures as the Pt / C catalyst A used for the cathode of the MEA in Example 1.

[0108] <Surface observation and structure observation of the catalyst for the electrode> Regarding this Pt / C catalyst B in Example 2, in the same method and conditions as the Pt / C catalyst in Example 1, in order to observe its three-dimensional structure, electron tomography measurement using STEM (scanning transmission electron microscope) at the UBE Science Analysis Center Co., Ltd. was carried out using the "USAL-KM3D analysis method". Fig. 15 shows a STEM image showing the 3D-electron tomography measurement conditions (volume size) using STEM of the catalyst for the electrode (Pt / C catalyst B) in Example 2. Fig. 16 shows a 3D-STEM image (three-dimensional reconstructed image) of the catalyst for the electrode (Pt / C catalyst B) in Example 2. Fig. 17 shows a graph showing the particle size distribution (particle size distribution shown as the equivalent spherical diameter) of the catalyst particles supported inside the nanopores of the carrier among the catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst for the electrode (Pt / C catalyst B) of Example 2 shown in Fig. 16. Fig. 18 shows a graph showing the particle size distribution (particle size distribution shown as the equivalent spherical diameter) of the catalyst particles supported outside the nanopores of the carrier among the catalyst particles obtained by image analysis of the 3D-STEM image of the catalyst for the electrode (Pt / C catalyst B) of Example 2 shown in Fig. 16.

[0109] For this catalyst for the electrode (Pt / C catalyst B), the ratio of the catalyst particles supported inside the nanopores of the carrier and the ratio of the catalyst particles supported outside the nanopores of the carrier were determined. Also, the values of D10, D20, D1, D2, N1, and N2 were determined. The results are shown in Tables 2 and 3. Furthermore, the average value of the particle size of the catalyst particles of the catalyst for the electrode (Pt / C catalyst B) measured from the STEM image was 3.3 nm (average value of the particle size of the catalyst particles inside the nanopores: 3.2 nm, average value of the particle size of the catalyst particles outside the nanopores: 3.7 nm).

[0110] (3) Preparation of the powder of the Pt / C catalyst used for the cathode of the MEA of Comparative Example 1 As the Pt / C catalyst, a Pt / C catalyst with a Pt loading of 50 wt% manufactured by N.E. CHEMCAT Corporation (trade name: "SA50BK") was prepared. The carrier of this Pt / C catalyst is a commercially available hollow carbon carrier {manufactured by Lion Corporation, trade name "Carbon ECP" (registered trademark) (Ketjenblack EC300J), specific surface area 750 - 800 m 2 / g} was used.

[0111] <Surface Observation and Structural Observation of the Catalyst for the Electrode> Regarding this Pt / C catalyst of Comparative Example 1, in the same method and conditions as the Pt / C catalyst of Example 1, in order to observe its three-dimensional structure, electron tomography measurement using STEM (scanning transmission electron microscope) at the UBE Science Analysis Center Co., Ltd. was carried out using the "USAL-KM3D analysis method". Fig. 19 shows a STEM image indicating the 3D - electron beam tomography measurement conditions (volume size) of the Pt / C catalyst of Comparative Example 1 using STEM. Fig. 20 shows a 3D - STEM image (three - dimensional reconstructed image) of the Pt / C catalyst of Comparative Example 1. Fig. 21 shows a graph indicating the particle size distribution (particle size distribution shown by equivalent spherical diameter) of the catalyst particles supported inside the nanopores of the carrier among the catalyst particles obtained by image analysis of the 3D - STEM image of the Pt / C catalyst of Comparative Example 1 shown in Fig. 20. Fig. 22 shows a graph indicating the particle size distribution (particle size distribution shown by equivalent spherical diameter) of the catalyst particles supported outside the nanopores of the carrier among the catalyst particles obtained by image analysis of the 3D - STEM image of the Pt / C catalyst of Comparative Example 1 shown in Fig. 20.

[0112] For this Pt / C catalyst, the ratio of the catalyst particles supported inside the nanopores of the carrier and the ratio of the catalyst particles supported outside the nanopores of the carrier were determined. Also, the values of D10, D20, D1, D2, N1, and N2 were determined. The results are shown in Tables 2 and 3. Furthermore, the average value of the particle diameter of the catalyst particles of the Pt / C catalyst measured from the STEM image was 3.1 nm (average value of the particle diameter of the catalyst particles inside the nanopores: 3.1 nm, average value of the particle diameter of the catalyst particles outside the nanopores: 3.2 nm).

[0113] (4) Preparation of the Pt / C catalyst used for the cathode of MEA of Comparative Example 2 [Powder of [Carbon - supported Pt catalyst "Pt / C catalyst"] Except for using a hollow carbon carrier, "CNovel C" (BET specific surface area: 800 m 2 / g) manufactured by Toyo Tanso Co., Ltd. as a trial - produced sample product name, the powder of the Pt / C catalyst (hereinafter, referred to as "Pt / C catalyst C" as necessary) used for the cathode of the MEA of Example 2 was prepared under the same conditions and procedures as the Pt / C catalyst A used for the cathode of the MEA of Example 1 {Pt loading rate 48.0 wt%, product name "SA50BM - C207", manufactured by N.E.CHEMCAT Co., Ltd.}.

[0114] <Surface Observation and Structural Observation of the Electrode Catalyst> Regarding the Pt / C catalyst C of this Comparative Example 2 as well, in the same manner and under the same conditions as the Pt / C catalyst of Example 1, in order to observe its three-dimensional structure, electron tomography measurement using a STEM (scanning transmission electron microscope) at the UBE Science Analysis Center, Ltd. was carried out using the "USAL-KM3D analysis method". Regarding the catalyst for the electrode of this Comparative Example 2 (Pt / C catalyst C) as well, in the same manner as in Example 1, Example 2, and Comparative Example 1, information corresponding to the electron tomography measurement using a STEM (scanning transmission electron microscope) shown in FIGS. 11 to 14, FIGS. 15 to 17, and FIGS. 20 to 22 and the analysis results thereof was obtained, but the illustration is omitted. FIG. 23 shows a STEM image showing the 3D-electron tomography measurement conditions (volume size) using the STEM of the Pt / C catalyst of Comparative Example 2.

[0115] Regarding the catalyst for the electrode of this Comparative Example 2 (Pt / C catalyst C), the ratio of the catalyst particles supported inside the nanopores of the carrier and the ratio of the catalyst particles supported outside the nanopores of the carrier were determined. Also, the values of D10, D20, D1, D2, N1, and N2 were determined. The results are shown in Tables 2 and 3. Furthermore, the average value of the particle diameter of the catalyst particles of the catalyst for the electrode (Pt / C catalyst C) measured from the STEM image was 3.2 nm (average value of the particle diameter of the catalyst particles inside the nanopores: 2.9 nm, average value of the particle diameter of the catalyst particles outside the nanopores: 3.5 nm).

[0116] (5) Confirmation of the Fine Structure of the Electrode Catalysts of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 Using the Three-Dimensional Reconstruction Image of STEM Regarding the electrode catalysts of Example 1, Example 2, and Comparative Example 2, in order to confirm "whether the nanopores are formed in the shape of the communication holes according to the present invention" which is the condition of (β) described above, a study was carried out using the three-dimensional reconstruction image of STEM. Also, the porosity of each catalyst was determined using the three-dimensional reconstruction image of STEM. The results are shown in FIGS. 24 and 25.

[0117] As shown in FIGS. 24(c) and 25, among the six square cross-sections of the three-dimensional image inside the catalyst mass of the electrode catalyst of Example 1, the nanopore P1 visible in the cross-section of interest (the x-y plane of the square), which is the cross-section being focused on, has two openings (opening A11 and opening A12) in contact with the first side L1. Further, this nanopore P1 has two openings (opening A21 and opening A22) in contact with the second side L2 parallel to the first side L1. Furthermore, this nanopore P1 has the shape of a continuous through-hole P1 that extends continuously without being blocked from the openings (opening A11 and opening A12) on the first side L1 toward the openings (opening A21 and opening A22) on the second side L2.

[0118] Furthermore, as shown in FIGS. 24(c) and 25, the nanopore P1 inside the electrode catalyst of Example 1 has a shape that branches into a plurality of parts. Furthermore, as shown in FIGS. 24(c) and 25, the nanopore P1 inside the electrode catalyst of Example 1 has two or more openings (opening A11 and opening A12) on the first side L1. Also, as shown in FIGS. 24(c) and 25, the nanopore P1 inside the electrode catalyst of Example 1 has two or more openings (opening A21 and opening A22) on the second side L2. Furthermore, as shown in FIGS. 24(c) and 25, the nanopore P1 inside the electrode catalyst of Example 1 also has one opening (opening A31) on the third side L3 perpendicular to the first side L1. Also, as shown in FIGS. 24(c) and 25, the nanopore P1 inside the electrode catalyst of Example 1 also has two openings (opening A41 and opening A42) on the fourth side L4 perpendicular to the first side L1.

[0119] As shown in FIGS. 24(d) and 24(e), it was confirmed that for the electrode catalyst of Example 1, in the other two planes (y-z plane, z-x plane) of the three-dimensional image inside the catalyst mass, the nanopore P1 has the same shape as described above. Furthermore, as shown in FIGS. 24(h), 24(i) and 24(j), for the electrode catalyst of Example 2 as well, it was confirmed that, similar to the electrode catalyst of Example 1, a nanopore P22 having the shape of the through-hole P1 according to the present invention is formed. On the other hand, as shown in FIGS. 24(m), 24(n), and 24(o), it was confirmed that for the electrode catalyst of Comparative Example 1, nanopores P22 having the shape of the communication holes P1 according to the present invention were not formed. The nanopores P1 visible in the cross-sections (x-y plane, y-z plane, z-x plane) of the three-dimensional images inside the catalyst mass of the electrode catalyst of Comparative Example 1 have one opening in contact with the first side and an opening in contact with the second side parallel to the first side, but it was confirmed that they do not have the shape of the communication holes P1 that extend continuously without being blocked from the opening on the first side toward the opening on the second side.

[0120] (II) Preparation of P / C Catalysts Used for the Anodes of MEAs of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 The same Pt / C catalyst as the Pt / C catalyst used for the cathode of the MEA of Comparative Example 1 was used as the P / C catalyst for the anodes of the MEAs of Example 1, Example 2, Comparative Example 1, and Comparative Example 2.

[0121] <Example 1> An MEA having the same configuration as the MEA10 shown in FIG. 1 was fabricated by the following procedure.

[0122] (1) Fabrication of the Cathode GDL of the Cathode As the GDL, carbon paper (trade name "TGP-H-60" manufactured by Toray Industries, Inc.) was prepared. Ink for Forming the MPL of the Cathode 1.5 g of carbon powder (trade name "Denka Black" manufactured by Denki Kagaku Kogyo Co., Ltd.), 1.1 g of ion-exchanged water, and 6.0 g of a surfactant (trade name "Triton" (35 wt% aqueous solution) manufactured by The Dow Chemical Company) were placed in a Teflon (registered trademark) ball mill container containing a Teflon (registered trademark) ball and mixed. Next, 1.75 g of a polytetrafluoroethylene (PTFE) dispersion (trade name "31-JR" manufactured by Mitsui DuPont Fluorochemicals Co., Ltd.) was added to the ball mill container and mixed. Thereby, ink for forming the MPL of the cathode was prepared. MPL of the Cathode An ink for forming the MPL of the cathode was applied to one side of the GDL using a bar coater to form a coated film. Then, the coated film was sufficiently dried in a dryer, and further heat-pressing treatment was performed to create a laminate with the MPL formed on the GDL. Ink for forming the catalyst layer of the cathode Into a Teflon (registered trademark) ball mill container containing a Teflon (registered trademark) ball, the above-mentioned Pt / C catalyst A, ion-exchanged water, 10 wt% Nafion aqueous dispersion (product name "DE1021CS" manufactured by DuPont), and glycerin were put and mixed to prepare an ink for forming the catalyst layer of the cathode. For this ink, N / C = 0.7 was set. Also, carbon:ion-exchanged water:glycerin in the catalyst A for the electrode = 1:10:0.8 (mass ratio). Catalyst layer (CL) of the cathode The above-mentioned ink for forming the catalyst layer of the cathode was applied to the surface of the MPL of the laminate with the MPL formed on the MPL on the above-mentioned GDL by the bar coating method to form a coated film. After drying this coated film at room temperature for 30 minutes, it was dried at 60 °C for 1.0 hour to obtain a catalyst layer. In this way, a cathode, which is a gas diffusion electrode, was created. The Pt loading amount of the catalyst layer of the cathode was set to the value shown in Table 1.

[0123] (2) Preparation of the anode GDL of the anode As the GDL, the same carbon paper as that of the cathode was prepared. Ink for forming the MPL of the anode Into a Teflon (registered trademark) ball mill container containing a Teflon (registered trademark) ball, 1.5 g of carbon powder (product name "Denka Black" manufactured by Denki Kagaku Kogyo Co., Ltd.), 1.0 g of ion-exchanged water, and 6.0 g of a surfactant (product name "Triton" (35 wt% aqueous solution) manufactured by The Dow Chemical Company) were put and mixed. Next, 2.5 g of a polytetrafluoroethylene (PTFE) dispersion (product name "31-JR" manufactured by Mitsui DuPont Fluorochemicals Co., Ltd.) was put into the ball mill container and mixed. Thereby, an ink for forming the MPL for the anode was prepared. MPL of the anode An ink for forming the MPL of the anode was applied to one side of the GDL using a bar coater to form a coated film. Then, the coated film was sufficiently dried in a dryer, and further heat-pressing treatment was performed to create a laminate with the MPL formed on the GDL. Ink for forming the catalyst layer of the anode In a Teflon (registered trademark) ball mill container containing a Teflon (registered trademark) ball, SA50BK (Pt loading rate 50 wt%), ion-exchanged water, 5 wt% Nafion alcohol dispersion (product name "Nafion 5 wt.% dispersion", product number "274704" manufactured by SIGMA-ALDRICH), and glycerin were put and mixed to prepare an ink for forming the catalyst layer of the anode. For this ink, N / C = 1.2 was set. Also, carbon:ion-exchanged water:glycerin in SA50BK = 1:6:4 (mass ratio). Catalyst layer (CL) of the anode The above-mentioned ink for forming the catalyst layer of the anode was applied to the surface of the MPL of the laminate with the MPL formed on the GDL by the bar coating method to form a coated film. After drying this coated film at room temperature for 30 minutes, it was dried at 60 °C for 1.0 hour to obtain a catalyst layer. In this way, the anode, which is a gas diffusion electrode, was created. The Pt loading amount of the catalyst layer of the anode was 0.3 mg / cm 2 was set.

[0124] (3) Preparation of MEA A polymer electrolyte membrane (product name "Nafion NR212" manufactured by DuPont) was prepared. A laminate with this polymer electrolyte membrane placed between the cathode and the anode was created and heat-pressed using a hot press machine to prepare an MEA. The conditions for heat-pressing were pressing at 140 °C, 5 KN for 5 minutes, and further pressing at 140 °C, 25 KN for 3 minutes.

[0125] <Example 2> Each MEA was prepared under the same conditions and procedures as in Example 1, except that the following conditions for the catalyst layer of the cathode were changed. That is, in the preparation of the ink for forming the catalyst layer of the cathode, · Instead of the Pt / C catalyst A, the Pt / C catalyst B described above was used.

[0126] <Comparative Example 1> Except for changing the following conditions for the cathode catalyst layer, each MEA was fabricated under the same conditions and procedures as in Example 1. That is, in the preparation of the ink for forming the cathode catalyst layer, · Instead of the Pt / C catalyst A, the P / C catalyst (trade name: "SA-50BK") described above was used. · Instead of the 10 wt% Nafion aqueous dispersion, a 5 wt% Nafion alcohol dispersion (trade name "DE520CS" manufactured by DuPont; containing 48 wt% 1-propanol) was used. · 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 would be the values shown in Table 1. · The carbon:ion-exchanged water:glycerin in the P / C catalyst (trade name: "SA50BH") was 1:10:1 (mass ratio).

[0127] <Comparative Example 2> Except for changing the following conditions for the cathode catalyst layer, each MEA was fabricated under the same conditions and procedures as in Example 1. That is, in the preparation of the ink for forming the cathode catalyst layer, · Instead of the Pt / C catalyst A, the Pt / C catalyst C described above was used.

[0128] <Battery Performance Evaluation> The battery performances of the MEAs of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 were evaluated by the following battery performance evaluation method. The MEAs of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 were installed in a fuel cell single cell evaluation apparatus. Next, the power generation reaction in the MEA was allowed to proceed under the following conditions. The single cell (MEA) temperature was set at 80°C. At the anode, pure hydrogen at 1.0 atm humidified with saturated steam was supplied with the flow rate adjusted so that the utilization rate was 70%. Also, at the cathode, pure oxygen at 1.0 atm humidified with saturated steam at 80°C was supplied with the flow rate adjusted so that the utilization rate was 50%. The evaluation of the single cell (MEA) was carried out by controlling the current with an electronic load device attached to the fuel cell single cell evaluation apparatus, and the current-voltage curve obtained by scanning from 0 to 1.0 A / cm 2 up to was acquired as data. From the data of the above current-voltage curve, a graph was created by plotting the X-axis (current density) on a logarithmic scale (not shown), and the current density value at a voltage of 850 mV (current value per unit area of the electrode) was obtained.

[0129] By dividing the current density value thus obtained by the platinum weight per unit area of the cathode, it was calculated as the activity per unit weight (Mass.Act.) for the platinum contained in the cathode, and used as an index of the oxygen reduction ability of the catalyst contained in the cathode. The results are shown in Table 1. Note that Table 1 shows the results of comparing the Mass.Act. obtained in other examples as relative values (relative ratios) based on the Mass.Act. obtained in Comparative Example 1 (1.0).

[0130]

Table 1

Table 2

Table 3

[0131] From the results shown in Tables 1 to 3, it became clear that the MEAs of Example 1 and Example 2 had higher Pt mass activity compared to the MEAs of Comparative Example 1 and Comparative Example 2. In the above examples and comparative examples, the mode in which the catalyst particles are the simplest catalyst particles composed of Pt was examined. However, the characteristics of the catalyst for an electrode of the present invention are that when observing the fine structure using a three-dimensional reconstructed image of STEM, the ratio of the catalyst particles supported inside the above-described (α) nanopores is 50% or more (a condition regarding the supported position of the catalyst particles constituting the catalyst for an electrode), and (β) the nanopores are formed in a shape that becomes the above-described communication pores (a condition regarding the fine structure of the nanopores of the carrier constituting the catalyst for an electrode). Therefore, it is clear that the same results can be obtained even if the chemical components of the catalyst particles are changed. That is, if it has the characteristics of the three-dimensional structure of the present invention, it is clear that even if Pt alloy particles containing Pt or core-shell particles having a Pt shell layer are employed as the catalyst particles in the same manner as the particles composed of Pt, excellent Pt mass activity similar to that of the above-described examples is exhibited.

Industrial Applicability

[0132] The catalyst for an electrode of the present invention exhibits excellent catalytic activity. Further, the GDE, CCM, MEA, and fuel cell stack including the catalyst layer of the present invention exhibit excellent battery characteristics that can contribute to cost reduction of PEFC. Therefore, the present invention can be applied not only to the electrical equipment industry such as fuel cells, fuel cell vehicles, and portable mobile devices, but also to energy farms, cogeneration systems, etc., and contributes to the development of the energy industry and environmental technologies.

Explanation of Signs

[0133] 1 ··· Cathode, 1A, 1B, 1C ··· Gas diffusion electrode (GDE) 1c ··· Catalyst layer (CL), 1m ··· Water repellent layer (MPL), 1gd ··· Gas diffusion layer (GDL), 2 ··· Anode, 2c ··· Catalyst layer (CL), 2m ··· Water repellent layer (MPL), 2gd ··· Gas diffusion layer (GDL), 3 ··· Polymer electrolyte membrane (PEM), 4, 5 ··· Separator 10, 11 ··· Membrane electrode assembly (MEA), 12, 13 ··· Catalyst coated membrane (CCM) 20 ··· Catalyst for electrode, 22 ··· Hollow carbon carrier, 23 ··· Catalyst particles, 25 ··· Non-contact particles 30 ··· Fuel cell stack, P1 ··· Through-hole formed by connecting multiple nano-pores P22 P22 ··· Nano-pore of carrier, P24 ··· Micro-pore of carrier

Claims

1. A conductive hollow carbon support having nanopores with a pore diameter of 1 to 20 nm and micropores with a pore diameter of less than 1 nm, and a plurality of catalyst particles supported on the support, At least a part of the surface of the catalyst particles has a region composed of Pt(0 valence) formed thereon, The catalyst particles are supported both inside and outside the nanopores of the support, When analyzing the particle size distribution of the catalyst particles using a three-dimensional reconstructed image obtained by electron tomography measurement using STEM (scanning transmission electron microscope), the ratio of the catalyst particles supported inside the nanopores is 50% or more, In the three-dimensional reconstructed image of the STEM, paying attention to a catalyst mass composed of the catalyst particles and the support having a size accommodated in a rectangular parallelepiped space with a side length of 60 to 300 nm, when looking at six square cross-sections of a cubic image with a side length of 20 to 50 nm extracted from the internal region of the catalyst mass, at least one nanopore is formed in at least one cross-section, The nanopore formed in at least one of the six square cross-sections has at least one opening in contact with a first side among the four sides of the square cross-section and at least one opening in contact with a second side of the square cross-section parallel to the first side, and has a shape of a continuous through-hole that extends continuously from the opening of the first side to the opening of the second side without being blocked, A catalyst for an electrode.

2. The catalyst for an electrode according to claim 1, wherein the through-hole has a shape branched into a plurality.

3. The catalyst for an electrode according to claim 2, wherein the through-hole has two or more openings in the first side.

4. The catalyst for an electrode according to claim 2 or 3, wherein the through-hole has two or more openings in the second side.

5. The catalyst for an electrode according to at least one of claims 2 to 4, wherein the through-hole has at least one opening in a third side perpendicular to the first side.

6. The catalyst for an electrode according to at least one of claims 2 to 5, wherein the through-hole has at least one opening in a fourth side perpendicular to the first side.

7. The catalyst for an electrode according to at least one of claims 1 to 6, wherein the porosity measured using the three-dimensional reconstructed image of the STEM is 35% or more.

8. The catalyst for an electrode according to at least one of claims 1 to 7, wherein the pore diameter of the nanopore is 1 to 10 nm.

9. The catalyst for an electrode according to at least one of claims 1 to 8, wherein the hollow carbon carrier further has micropores having a pore diameter of less than 1 nm.

10. The catalyst for an electrode according to at least one of claims 1 to 9, wherein the catalyst particles are composed of Pt (zero valent).

11. The catalyst for an electrode according to at least one of claims 1 to 10, wherein the catalyst particles are composed of a Pt alloy.

12. The catalyst particles are core-shell catalyst particles, The core-shell catalyst particles have a core particle and a Pt shell layer corresponding to a region composed of the Pt (zero valent) formed on at least a part of the surface of the core particle. The catalyst for an electrode according to at least one of claims 1 to 11.

13. When analyzing the particle size distribution of the catalyst particles using the three-dimensional reconstructed image of the STEM, the catalyst for an electrode according to at least one of claims 1 to 12 satisfies the condition of the following formula (1). (D10 / D20) ≥ 0.80... (1) [In the formula (1), D10 represents the weighted average value of the equivalent spherical diameters of the catalyst particles supported inside the nanopores of the carrier, D20 represents the weighted average value of the equivalent spherical diameters of the catalyst particles supported outside the nanopores of the carrier. ]

14. When analyzing the particle size distribution of the catalyst particles using the three-dimensional reconstructed image of the STEM, in addition to the condition of the formula (1), the conditions of the following formula (2) and formula (3) are further satisfied simultaneously. The catalyst for an electrode according to at least one of claims 1 to 13. D1 ≤ D2... (2) (N1 / N2) > 1.0... (3) [In the formula (2) and the formula (3), D1 represents the equivalent spherical diameter of the particle showing the maximum frequency among the catalyst particles supported inside the nanopores of the carrier, D2 represents the equivalent spherical diameter of the particle showing the maximum frequency among the catalyst particles supported outside the nanopores of the carrier, N1 represents the frequency of the particle showing the maximum frequency among the catalyst particles supported inside the nanopores of the carrier, N2 represents the frequency of the particle showing the maximum frequency among the catalyst particles supported outside the nanopores of the carrier. ]

15. The electrode catalyst according to at least one of claims 1 to 14, wherein at least a part of the region composed of Pt(0 valence) on the surface of the catalyst particles is covered with a Pt oxide film.

16. The BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon carrier is 200 to 1500 m ２ / g, and the electrode catalyst according to at least one of claims 1 to 15.

17. A powder of an electrode catalyst containing 10 wt% or more of the electrode catalyst according to any one of claims 1 to 16.

18. A composition for forming a gas diffusion electrode, containing the electrode catalyst according to any one of claims 1 to 16 or the powder of the electrode catalyst according to claim 17.

19. A gas diffusion electrode containing the electrode catalyst according to any one of claims 1 to 16 or the powder of the electrode catalyst according to claim 17.

20. A membrane / electrode assembly (MEA) containing the gas diffusion electrode according to claim 19.

21. A fuel cell stack containing the membrane / electrode assembly (MEA) according to claim 20.

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