Electrode catalyst, 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 carrier which has nanopores having a pore diameter of from 1 nm to 20 nm and micropores having a pore diameter of less than 1 nm; and a plurality of catalyst particles which are supported by the carrier. The catalyst particles are supported by both inner portions and outer portions of mesopores of the carrier, and contain Pt (zerovalent). 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 condition of formula (S1), namely (100 × (N10 / N20) ≤ 8.0) is satisfied. In the formula, N10 represents the number of noble metal particles that are not in contact with pores having a pore diameter of 1 nm or more; and N20 represents the number of catalyst particles that are supported by the inner portions of the nanopores of the carrier.

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst using a hollow carbon support, more particularly to an electrode catalyst suitable for use in a gas diffusion electrode, and more particularly to an electrode catalyst suitable for use in 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, each of which contains the electrode catalyst particles. [Background technology]

[0002] 2. Description of the Related Art Polymer electrolyte fuel cells (hereinafter referred to as "PEFC" as necessary) are being researched and developed as power sources for fuel cell vehicles and home cogeneration systems. The catalyst used in the gas diffusion electrode of a PEFC is a precious metal catalyst made of precious metal particles of a platinum group element such as platinum (Pt).

[0003] For example, a typical conventional catalyst is a "Pt-supported carbon catalyst" (hereinafter referred to as "Pt / C catalyst" as necessary), which is a powder of catalyst particles in which Pt fine particles are supported on conductive carbon powder. The cost of precious metal catalysts such as Pt accounts for a large proportion of the manufacturing cost of PEFCs, and this is a challenge to reducing the cost of PEFCs and promoting their widespread use. In the course of these research and development efforts, in order to reduce the amount of platinum used, powders (hereinafter referred to as "core-shell catalysts" as needed) of catalyst particles having a core-shell structure (hereinafter referred to as "core-shell catalyst particles" as needed) formed from a core portion made of a non-platinum element and a shell portion made of Pt have been studied, and many reports have been published.

[0004] For example, Patent Document 1 discloses a particle composite (corresponding to a core-shell catalyst particle) having a configuration in which palladium (Pd) or a Pd alloy (corresponding to a core portion) is coated with an atomically thin layer of Pt atoms (corresponding to a shell portion). Furthermore, Patent Document 1 describes, as an example, a core-shell catalyst particle having a configuration in which the core portion is a Pd particle and the shell portion is a layer made of Pt. On the other hand, as supports for electrode catalysts, there are hollow carbon, which has many pores inside the primary particles, and solid carbon, which has fewer pores inside the primary particles than hollow carbon, and studies are being conducted to improve performance by taking advantage of the characteristics of each.

[0005] For example, Patent Document 2 discloses a study example in which hollow carbon is used as a carrier, and Patent Document 3 discloses a study example in which solid carbon is used as a carrier. For example, Patent Document 2 discloses the configuration of an electrode catalyst 200 in which, as shown in FIG. 10, a porous support (hollow carbon) 220 having an average particle diameter of 20 to 100 nm has the pore volume and mode diameter of the pore distribution of pores P220 having a pore diameter of 4 to 20 nm controlled within a predetermined range, and catalyst particles 230 are supported in the primary pores P220 of the support 220. Patent Document 2 mentions that this prevents the polymer electrolyte from being adsorbed onto the surface of the catalyst particles 230 present in the primary pores P220, making it possible to prevent a decrease in the effective reaction surface area of ​​the catalyst while ensuring sufficient gas transportability. Furthermore, it mentions that as a result, the activity per catalyst weight is improved, and a catalyst layer for a fuel cell that exhibits excellent power generation performance can be provided even when the catalyst amount is reduced.

[0006] Furthermore, for example, Patent Document 3 discloses an electrode catalyst (PtCo / C catalyst) for fuel cells that has a solid carbon support and catalyst particles containing an alloy of platinum and cobalt supported on the support. This electrode catalyst has a platinum to cobalt molar ratio of 4 to 11:1 in the alloy, and is acid-treated at 70 to 90°C. In Patent Document 3, when a PtCo alloy is supported on a hollow carbon support, a portion of the PtCo alloy is contained inside the hollow carbon support, and 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 support, and as a result, it is considered a problem that Co is easily eluted from the PtCo alloy present inside the support. Therefore, Patent Document 3 mentions that by using a solid carbon support instead of a hollow carbon support, it is possible to prevent the PtCo alloy from being contained within the support. Furthermore, it discloses that this allows the PtCo alloy to be sufficiently acid-treated, thereby suppressing the elution of Co. It also mentions that as a result, it is possible to achieve both good initial performance and durability of the fuel cell.

[0007] Here, Patent Document 3 defines solid carbon as follows: That is, Patent Document 3 states that solid carbon is carbon with fewer voids inside it compared to hollow carbon, and specifically, it is carbon for which the ratio of the BET surface area determined by N2 adsorption to the external surface area determined by t-Pot (the surface area of ​​the outside of a particle calculated from the particle size) (t-Pot surface area / BET surface area) is 40% or more. The "t-Pot surface area" described in Patent Document 3 is understood to refer to the "t-plot surface area" described, for example, in the technical report "Analysis of Micropore Surface Area Using the t-plot Method" published online by MC Evatec Co., Ltd. on February 1, 2019. Analysis of micropore surface area using the t-plot method is one method of analysis based on nitrogen adsorption isotherms (adsorption temperature: 77 K). This method compares and converts adsorption isotherm data with standard isotherms to plot a graph of the relationship between the adsorption layer thickness t and the amount of adsorption. In addition to being able to separate and quantify the specific surface area into the inside and outside of the pores, the shape of the graph also allows for understanding the pore trends. Furthermore, examples of solid carbon include the carbon described in Japanese Patent No. 4362116, and it is disclosed that specific examples include Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Kabushiki Kaisha.

[0008] Furthermore, Patent Document 4 discloses an electrode catalyst (core-shell catalyst) in which catalyst particles are supported both inside and outside the mesopores of a hollow carbon support (more specifically, nanopores formed in the primary particles of the hollow carbon support). This electrode catalyst has a configuration in which, when the particle size distribution of the catalyst particles is analyzed using three-dimensional reconstructed images obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), the proportion of catalyst particles supported inside the mesopores (more specifically, nanopores formed in the primary particles of the hollow carbon support) is 50% or more.

[0009] In this specification, the "nanopores" of the hollow carbon support refer to pores with a pore diameter of 1 to 20 nm, as will be described later.

[0010] The following Non-Patent Documents 1 and 2 disclose examples in which the proportion of catalyst particles supported inside pores (the above-mentioned nanopores) and the proportion of catalyst particles supported outside pores (the above-mentioned nanopores) of catalyst particles supported on hollow carbon supports are analyzed using a method different from that used in the above-mentioned Patent Document 4. More specifically, in Non-Patent Document 1, a group of Strasser et al. at the Technical University of Berlin reported that a commercially available hollow carbon (product name: "ketjenblack EC-300J", manufactured by Akzo Nobel, specific surface area: about 839 m) 2 g -1They reported the results of simultaneously capturing SEM (Scanning Electron Microscopy) and TSEM (Transmission SEM) images of specific Pt / C catalyst particles of interest in the same measurement area for a Pt / C catalyst in which Pt catalyst particles are highly dispersed on a substrate. See, for example, Table 1, Figure 2, and the right column on page 79 of Non-Patent Document 1. In their method, SEM images provide information on the Pt catalyst particles present only on the observed portion (one of the outer surfaces) of the hollow carbon support particles. In other words, information on the number of catalyst particles supported outside the nanopores of the hollow carbon support particles is obtained. On the other hand, TSEM images (transmission images) provide information on all catalyst particles supported on the exterior and interior of the hollow carbon support particles (the primary particles mentioned above) for the observed Pt catalyst particles. They then attempted to distinguish between Pt catalyst particles supported on the exterior surface (outside the nanopores) and those supported inside the hollow carbon support particles using information from TSEM and SEM images.

[0011] In Non-Patent Document 1, they did not measure the "opposite back surface" of the observed portion ("one side outer surface") of the outer surface of the hollow carbon support particle (outside the nanopores) in the SEM image. They assumed that the state of the "one side outer surface" and the state of the "opposite back surface" are the same. In other words, they assumed that the number of catalyst particles supported on the "one side outer surface" is the same as the number of catalyst particles supported on the "opposite back surface." Next, in Non-Patent Document 2, Uchida and his group at Yamanashi University used a scanning transmission electron microscope (STEM) device capable of taking SEM images and transmission electron microscopy (TEM) images of Pt catalyst particles to measure the surface area of ​​a commercially available hollow carbon (product name: "Ketjenblack", manufactured by Ketjen Black International, specific surface area: approximately 875 m 2 g -1(2004) reported the results of photographing a Pt / C catalyst in which Pt catalyst particles were highly dispersed. For example, see Figure 1, Table 2, and the bottom right column on page 181 of Non-Patent Document 2. First, they obtained information on the number of Pt catalyst particles supported on hollow carbon support particles from a TEM image of a specific Pt / C catalyst particle of interest. Next, they measured SEM images of the same Pt / C catalyst particle as the TEM image to obtain information on the number of Pt catalyst particles present only on the back surface of the hollow carbon support particle. Next, they used a special 3D sample holder to rotate the specific Pt / C catalyst particle of interest (measurement sample) precisely 180 degrees to measure an SEM image of only the back surface of the same Pt / C catalyst particle. Using this information, they attempted to distinguish between Pt catalyst particles supported on the outer surface and those supported internally on hollow carbon support particles.

[0012] The "internal loading rate" measured by this method = "100 × (number of Pt catalyst particles loaded internally) / (total number of Pt catalyst particles)" and they reported that it was 62% for a commercially available 30 wt% Pt / C catalyst (trade name: "TEC10E30E", manufactured by Tanaka Metal Industries Co., Ltd., referred to as "c-Pt / CB" in the specification) and over 50% for a commercially available 46 wt% Pt / C catalyst (trade name: "TEC10E50E", manufactured by Tanaka Metal Industries Co., Ltd., referred to as "Pt / CB" in the specification). As explained above, the present inventors recognize that the analysis methods of Non-Patent Documents 1 and 2 differ from the analysis method of Patent Document 4 in the following points. That is, the analysis method using electron beam tomography measurement in Patent Document 4 is a three-dimensional reconstruction method using an electron microscope, in which electron microscope images of the same field of view of a target measurement sample (the size of the measurement target sample is a mass with a major or minor axis in the range of approximately 100 to 300 nm, see Figures 11, 16, and 21 described below) projected from various directions are reconstructed into a three-dimensional image in a computer, and a cross-sectional image (tomogram) is created using the computer.

[0013] On the other hand, the analysis method of Non-Patent Document 1 uses two-dimensional images, such as SEM images and TSEM images, taken from a specific direction of the measurement sample. Furthermore, the analysis method of Non-Patent Document 2 uses two-dimensional images, such as SEM images taken from two specific directions of the measurement sample (directions of two mutually perpendicular axes obtained by rotating the sample holder by 180°), and a TSEM image taken from a specific direction of the measurement sample. The present inventors believe that with the analysis methods of Non-Patent Document 1 and Non-Patent Document 2, for example, if the measurement sample (electrode catalyst particles) has irregularities, there is a high possibility that some catalyst particles may exist for which it is not possible to fully determine whether they are supported inside or outside the hollow carbon support. The analytical technique of Patent Document 4 uses a three-dimensional tomogram of the measurement sample, which can be observed from various reports, and the inventors believe that this makes it possible to visually confirm and more accurately grasp the support positions on the carrier of the catalyst particles contained in the electrode catalyst of the measurement sample of interest (the size of the measurement target sample is a mass with its major axis or minor axis in the range of approximately 100 to 300 nm; see Figures 11, 16, and 21 described below). The applicant of the present 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] U.S. Patent Application Publication No. 2007 / 31722 [Patent Document 2] Japanese Patent Application Laid-Open No. 2013-109856 [Patent Document 3] WO2016 / 063968 publication [Patent Document 4] WO2019 / 221168 publication [Non-patent literature]

[0015] [Non-Patent Document 1] Nature Materials Vol 19 (January 2020)77-85 [Non-patent document 2] Journal of Power Sources 315(2016)179-191 Summary of the Invention [Problem to be solved by the invention]

[0016] Toward the widespread use of PEFCs, further improvement in catalytic activity of electrode catalysts is required to reduce the amount of Pt used and material costs. The present inventors have analyzed the particle size distribution of catalyst particles in electrode catalysts such as Pt / C catalysts using three-dimensional reconstructed images obtained by electron tomography measurement using a STEM (scanning transmission electron microscope). They have found that there have been no reports to date of actually synthesizing an improved product in which catalyst particles are supported in greater numbers inside the nanopores of primary particles of a hollow carbon support than outside them, and that there is still room for improvement. The present invention has been made in view of the above technical circumstances, and has an object to provide an electrode catalyst having excellent catalytic activity that can contribute to reducing the cost 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, each of which contains the electrode catalyst. [Means for solving the problem]

[0017] The present inventors have conducted extensive research into a configuration that achieves further improvement in catalytic activity for an electrode catalyst in which a large number of catalyst particles of an electrode catalyst, such as a Pt / C catalyst, are supported within the nanopores of primary particles of hollow carbon. As a result, the inventors have found that supporting catalyst particles on a carrier so as to satisfy the following conditions is effective in improving catalytic activity, and have completed the present invention. More specifically, the present invention comprises the following technical features.

[0018] That is, the present invention is The present invention comprises a conductive hollow carbon support having nanopores with a pore diameter of 1 to 20 nm, and a plurality of catalyst particles supported on the support, a region made of Pt (zero valence) is formed on at least a portion of the surface of the catalyst particle, the catalyst particles are supported both inside and outside the nanopores of the support; When an analysis of the particle size distribution of the catalyst particles is carried out using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), the condition of the following formula (S1) is satisfied: An electrode catalyst is provided. 100×(N10 / N20)≦8.0···(S1) Here, in the formula (S1), N10 represents the number of non-contact particles (n101+n102), which is the sum of (I) the number of precious metal particles (n101) that are not in contact with pores having a pore diameter of 1 nm or more that can be confirmed by the electron beam tomography measurement and (II) the number of precious metal particles (n102) that are not in contact with the hollow carbon support itself and exist outside it.

[0019] N20 indicates the number of catalyst particles supported inside the nanopores of the support. Furthermore, regarding the number of non-contact particles and non-contact particles, N10 (= n101 + n102), if the presence of (II) precious metal particles is not confirmed by the electron beam tomography measurement, i.e., if the number of (II) precious metal particles, n102 = 0 (or n102 ≒ 0), the inventors believe that the non-contact particles can be considered as follows. That is, in this case, "non-contact particles" are "precious metal particles (catalyst particles) that are not in contact with pores with a pore diameter of 1 nm or more that can be confirmed by the electron beam tomography measurement." In other words, in this case, "non-contact particles" are "catalyst particles that are in contact with pores with a pore diameter of less than 1 nm that cannot be confirmed by the electron beam tomography measurement." In fact, the presence of (I) precious metal particles was not confirmed in the electrode catalyst of the present invention prepared by the inventors.

[0020] In this specification, to distinguish from nanopores, "pores with a pore diameter of less than 1 nm" are referred to as "micropores," and pores with a pore diameter of more than 20 nm are referred to as macropores. The inventors believe that it is highly likely that the non-contact particles in this case (catalyst particles in contact with pores (micropores) with a pore diameter of less than 1 nm that cannot be confirmed by the electron beam tomography measurement) are embedded in the micropores and are unable to effectively contribute to the progress of the electrode reaction. As shown in Figures 2 and 10, when micropores are present in the hollow carbon support used, non-contact particles (catalyst particles) embedded in the micropores are thought to be present in both the conventional electrode catalyst 200 and the electrode catalyst 20 of the present invention (see non-contact particle 25 in Figure 2 and non-contact particle 250 in Figure 10).

[0021] In the present invention, by supporting catalyst particles of an electrode catalyst such as a Pt / C catalyst on a hollow carbon support so as to satisfy the condition of the above formula (S1), the electrode catalyst of the present invention can exhibit excellent catalytic activity that can contribute to cost reduction of PEFCs. The detailed reasons why the electrode catalyst of the present invention has excellent catalytic activity have not been fully elucidated. However, the present inventors believe as follows: In electrode catalysts such as Pt / C catalysts in which catalyst particles supported inside nanopores satisfy the condition of formula (S1), there are fewer non-contact particles (catalyst particles) embedded in the micropores of the support than in conventional electrode catalysts, and a relatively large number of highly active catalyst particles are present inside the nanopores of the support. The catalyst particles supported inside the nanopores of such a support are supported on the support in a state where they are unlikely to come into direct contact with the polymer electrolyte present in the catalyst layer. Therefore, the electrode catalyst of the present invention is less susceptible to a decrease in catalytic activity due to poisoning of the Pt component, and can exhibit superior catalytic activity when formed into an electrode compared to conventional electrode catalysts. Furthermore, the electrode catalyst of the present invention also reduces dissolution of the Pt component from the catalyst particles.

[0022] Furthermore, from the viewpoint of more reliably achieving the effects of the present invention, the value of [100×(N10 / N20)] in formula (S1) is preferably 6.5 or less, and more preferably 1.0 or less.

[0023] Here, in the present invention, the "nanopore" of the hollow carbon support refers to a pore having a pore diameter of 1 to 20 nm, and the "micropore" refers to a pore having a pore diameter of less than 1 nm. 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 a nanopore refers to the size of the "nanopore entrance" that can be determined using a three-dimensional reconstructed image obtained when an electrode catalyst is analyzed by electron tomography measurement using a general STEM (scanning transmission electron microscope). More preferably, in the present invention, the "pore diameter (pore entrance size)" of a nanopore refers to the size of the "nanopore entrance" determined by the above-mentioned "USAL-KM3D analysis method." More specifically, the "size of the nanopore entrance" refers to the diameter (equivalent circle diameter) of a circle having the same area as the area of ​​the nanopore entrance, as determined from an image of the entrance of the nanopore obtained by the "USAL-KM3D analysis method."

[0024] In addition, in the present invention, the term "analysis method for the particle size distribution of the catalyst particles using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope)" refers to an analysis method using a STEM (scanning transmission electron microscope) at UBE Scientific Analysis Center, Inc., in which electron tomography measurement is performed and the obtained measurement data is subjected to image analysis using image analysis software ("Avizo" manufactured by FEI) (name of analysis method: "USAL-KM3D analysis method"). In the USAL-KM3D analysis method, the measurement sample to be measured is prepared according to the following procedure and conditions.

[0025] <Measurement sample preparation method and conditions> First, to enable optimal measurement of the structure of the measurement sample, a sample is prepared on a "Cu grid mesh with carbon support film" for TEM observation using the dispersion method, a common electron microscope sample preparation method, so as to satisfy the following conditions: (A) Powder agglomerates of the sample to be measured (electrode catalyst) (agglomerates with major or minor diameters in the range of approximately 60 to 300 nm, see Figures 11, 16, and 21 described below) are placed on the grid mesh at a suitable frequency that is measurable (observable) {particle number (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 grid mesh is rotated around its axis of rotation by an angle of ±80°, the image of the powder mass of the measurement sample must not overlap with the image of other fine powder masses. If the image of the powder mass of the measurement sample overlaps with the image of other fine powder masses, 3D analysis cannot be performed. (C) The powder masses visible in the measurement area are positioned far enough apart to allow 3D tomography observation of the powder masses of the measurement sample. <Measurement conditions> 3D tomography observation is performed under conditions (e.g., adjustment of the electron beam acceleration voltage) that allow three-dimensional observation and differentiation of nanopores of 1 nm or larger contained in the powder mass of the above-mentioned measurement sample (electrode catalyst) without damaging the powder mass of the measurement sample (electrode catalyst). Then, for the powder mass of the sample to be measured (electrode catalyst), the measurement data obtained when measuring the "distance (shortest path) from the entrance of the nanopore to the catalyst particle loading position" described below is analyzed by image analysis using image analysis software ("Avizo" manufactured by FEI).

[0026] Furthermore, in the electrode catalyst of the present invention, from the viewpoint of more reliably obtaining the effects of the present invention, In the three-dimensional reconstructed STEM image, when focusing on a catalyst mass consisting of the catalyst particles and the support, which has a size that can be accommodated in a rectangular parallelepiped space with a side of 60 to 300 nm, and when six square cross sections of a cubic image with a side of 20 to 50 nm extracted from the internal region of the catalyst mass are viewed, 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 preferably has at least one opening adjacent to a first of the four sides of the square cross section and at least one opening adjacent to a second side of the square cross section that is parallel to the first side, and has the shape of a communicating hole that extends continuously from the opening on the first side to the opening on the second side without being blocked.

[0027] When the microstructure of the electrode catalyst of the present invention is observed using a three-dimensional reconstructed STEM image, it is found that the catalyst satisfies the conditions (α) of the aforementioned formula (S1) and (β) that the nanopores are formed to have the above-mentioned interconnected pore shape, thereby more reliably exhibiting excellent catalytic activity that can contribute to reducing the cost of PEFCs. The detailed reason why the electrode catalyst of the present invention has excellent catalytic activity more reliably when it satisfies the above condition (β) has not been fully elucidated. However, the present inventors believe as follows: When observing the microstructure using a three-dimensional reconstructed STEM image, the present inventors believe that an electrode catalyst in which (β) nanopores are formed to have the above-mentioned shape of interconnected pores, when used in the catalyst layer of a gas diffusion electrode of a PEFC, has excellent diffusivity for water and the protons contained therein, as well as for reactant gases (hydrogen, oxygen, or air), and that the catalyst particles supported inside the nanopores are more easily utilized in the reaction.

[0028] Next, a method for confirming the conditions under which the catalyst is formed so that the nanopores (β) have the shape of the above-mentioned interconnected pores, using a three-dimensional reconstructed STEM image, will be described. The condition (β) can be confirmed by using a three-dimensional reconstructed image obtained by analyzing the electrode catalyst by electron 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 use a three-dimensional reconstructed STEM image obtained in the process of implementing the above-mentioned USAL-KM3D analysis method. The procedure for checking the condition (β) is explained below.

[0029] (D) First, a three-dimensional reconstructed STEM image of the catalyst to be measured is obtained. From the catalyst masses (masses consisting of catalyst particles and hollow carbon support) shown in this three-dimensional reconstructed image, a catalyst mass of a size that can be accommodated in a rectangular parallelepiped space (region of interest) with sides of 60 to 300 nm is selected. 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 Example 1, Example 2, and Comparative Example 2 described below (see Figures 27(a), 27(f), and 27(k) described below).

[0030] (E) Next, a cubic image (20 to 50 nm on a side) is extracted from the internal region of the catalyst mass selected in step (D). This procedure can be easily understood by referring to, for example, the stereoscopic images (three-dimensional reconstructed STEM images) obtained for the catalyst masses of the catalysts of Example 1, Example 2, and Comparative Example 2 described below (see Figures 27(b), 27(g), and 27(l) described below).

[0031] (F) Next, the three-dimensional image of the interior of the catalyst mass obtained in step (E) (a three-dimensional reconstructed STEM image) is observed, and the difference in brightness is used to segment the voids (pores such as nanopores) and the hollow carbon support. More specifically, this 3D image (a 3D reconstructed STEM image) is divided into smaller cubic pixels (voxels). Each pixel (voxel) stores a unitless brightness value. The analyst then sets an appropriate threshold for these brightness values ​​to clearly segment (binarize) the 3D image (a 3D reconstructed STEM image) into voids (pores such as nanopores) and hollow carbon support portions. If the brightness of a pixel (voxel) is above the threshold, it is automatically determined to be a carbon portion. Conversely, if the brightness of a pixel (voxel) is below the threshold, it is automatically determined to be a void portion. This segmentation can be performed by setting the same brightness threshold for all pixels (voxels) contained in the same 3D image (a 3D reconstructed STEM image). For different 3D images (a 3D reconstructed STEM image), the analyst sets different brightness thresholds (thresholds appropriate for segmentation). From the viewpoint of performing segmentation more accurately, it is preferable that the size of a pixel (voxel) is a cube with one side of 1 nm or less. In the present invention, when measuring the porosity of a three-dimensional image (three-dimensional reconstructed STEM image) of a catalyst mass, which will be described later, the catalyst particles are considered to be voids during this segmentation.

[0032] This step (F) can also be easily understood by referring to, for example, a stereoscopic image extracted from a 3D-STEM image (three-dimensional reconstructed image) of the catalyst mass of the electrode catalyst of Example 1, Example 2, and Comparative Example 2 (see Figures 27(b), 27(g), and 27(l) described below). Furthermore, it can also be easily understood by referring to three cross sections (three cross sections after segmentation) of a stereoscopic image (three-dimensional reconstructed STEM image) obtained for the catalyst mass of the catalyst of Example 1, Example 2, and Comparative Example 2 (see Figures 27(c), 27(d), 27(e), 27(h), 27(i), 27(j), 27(m), 27(n), and 27(o) described below).

[0033] (G) Next, when looking at the six square cross sections of the 3D image (3D reconstructed STEM image) after performing segmentation in step (F), check whether at least one nanopore (communicating pore) of the following shape has been formed in at least one cross section. That is, it is confirmed whether the nanopore (communicating hole) visible in the square cross section of interest has at least one opening that contacts a first side of the four sides of the cross section and at least one opening that contacts a second side that is parallel to the first side. Furthermore, it is confirmed whether the nanopore has the shape of a communicating 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 Example 1 described later, as shown in Figure 28, nanopore P1 visible in the cross section of interest (square xy plane) has two openings (openings A11 and A12) that are in contact with a first side L1. This nanopore P1 also has two openings (openings A21 and A22) that are in contact with a second side L2 that is parallel to the first side L1. Furthermore, this nanopore P1 has the shape of a communicating hole that extends continuously without being blocked from the openings on the first side L1 (openings A11 and A12) to the openings on the second side L2 (openings A21 and A22).

[0034] Furthermore, from the viewpoint of more reliably achieving the effects of the present invention, it is preferable that the nanopores (communicating pores) visible in the square cross section of the focused cubic image in the electrode catalyst of the present invention have a multi-branched shape (see Figure 28). When catalyst particles supported inside such nanopores are used in the catalyst layer of a PEFC gas diffusion electrode, they are easily supported on the support in a state where they are less likely to come into contact with the polymer electrolyte present in the catalyst layer. Furthermore, such nanopores have excellent diffusivity for water and the protons contained therein, as well as for reactant gases (hydrogen, oxygen, or air). Therefore, when an electrode catalyst having such nanopores is used in the catalyst layer of a PEFC gas diffusion electrode, the catalyst particles supported inside the nanopores are more easily utilized in the electrode reaction of the PEFC.

[0035] Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, for the same reasons as those described above, it is preferable that in the electrode catalyst of the present invention, the nanopores (communicating pores) visible in the square cross section of the focused cubic image have two or more openings on the first side (see Figure 28). Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, for the same reasons as those described above, it is preferable that in the electrode catalyst of the present invention, the nanopores (communicating pores) visible in the square cross section of the focused cubic image have two or more openings on the second side (see Figure 28). Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, for the same reasons as those described above, it is preferable that in the electrode catalyst of the present invention, the nanopores (communicating pores) visible in the square cross section of the focused cubic 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 below, as shown in Figure 28, the nanopore P1 visible in the cross section of interest (square xy plane) also has one opening (opening A31) on the third side L3 perpendicular to the first side L1.

[0036] Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, for the same reasons as those described above, it is preferable that in the electrode catalyst of the present invention, the nanopores (communicating pores) visible in the square cross section of the focused cubic 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 below, as shown in Figure 28, the nanopore P1 visible in the cross section of interest (square xy plane) also has two openings (opening A41 and opening A42) on the fourth side L4 perpendicular to the first side L1.

[0037] Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, the porosity of the electrode catalyst of the present invention measured using a three-dimensional reconstructed STEM image (a cube image of interest) is preferably 35% or more, more preferably 40% or more, more preferably 45% or more, even more preferably 50% or more, even more preferably 55% or more, even more preferably 60% or more, and even more preferably 65% ​​or more. On the other hand, from the viewpoint of durability, the porosity of the electrode catalyst of the present invention measured using a three-dimensional reconstructed STEM image (a cube image of interest) is preferably 80% or less, more preferably 75% or less.

[0038] Furthermore, in the electrode catalyst of the present invention, the hollow carbon support preferably contains a greater number of nanopores with pore diameters (pore entrance sizes) of 1 to 10 nm. It has been reported that the micelle diameter of the polymer electrolyte used in the catalyst layers of the anode and cathode of an MEA is approximately 10 nm (e.g., YSKim, et al., DOE Hydrogen Program Merit Review and Peer Meeting FC16, (2009)). Therefore, by using a hollow carbon support containing a greater number of pores with pore diameters (pore entrance sizes) of 1 to 10 nm, the polymer electrolyte is less likely to penetrate into the nanopores, and contact between the catalyst particles supported inside the nanopores and the polymer electrolyte is more reliably prevented. Furthermore, in the electrode catalyst of the present invention, the hollow carbon support may further have micropores with a pore diameter of less than 1 nm, as long as the effects of the present invention can be obtained.

[0039] Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, the hollow carbon support is preferably "CNovel (product name, registered trademark, manufactured by Toyo Tanso Co., Ltd.)" (e.g., porous carbons described in Japanese Patent Nos. 5636171, 5695147, 5860600, 5860601, and 5860602), which can satisfy the above-mentioned conditions (α) and (β) when used as an electrode catalyst. CNovel is a porous carbon that has at least nanopores (pore diameters of 1 to 20 nm) and carbonaceous walls that form the outer shell of these nanopores. The carbonaceous walls have portions that form a layered structure, the carbonaceous walls form a three-dimensional network structure, and the nanopores are open pores with a continuous shape (the shape of interconnected pores; see "Nanopore P22 multiple interconnected interconnected pores P1" in Figure 2 described below). When used as a catalyst, it has a configuration that easily satisfies the above-mentioned condition (β).

[0040] In the electrode catalyst of the present invention, the catalyst particles may be made of Pt (zero valence). Furthermore, in the electrode catalyst of the present invention, the catalyst particles may be made of a Pt alloy. The metal species that constitutes the alloy other than Pt is not particularly limited. From the viewpoint of obtaining excellent catalytic activity, the metal species that constitutes the alloy other than Pt is preferably at least one metal selected from Co and Ni. Furthermore, in the electrode catalyst 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 preferably comprise a core particle and a Pt shell layer (a region consisting of Pt (zero valence)) formed on at least a portion of the surface of the core particle. The metal species constituting the core particle is not particularly limited, but from the viewpoint of obtaining excellent catalytic activity, it is preferably at least one of Pd, Ni, and Co. Alternatively, the core particle may be an alloy of at least one of Pd, Ni, and Co with another metal. From the viewpoint of reducing the amount of precious metal used, the core particle may contain, inside thereof, a base metal other than the precious metal, and at least one of an oxide of a base metal, a nitride of a base metal, and a carbide of a base metal.

[0041] Furthermore, in order to more reliably obtain the effects of the present invention, it is preferable that the electrode catalyst of the present invention satisfies the condition of the following formula (S2) when an analysis of the particle size distribution of the catalyst particles is carried out using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope). 100×{N10 / (N20+N30)}≦5.0···(S2) Here, N10 in formula (S2) has the same meaning as N10 in formula (S1). In addition, in the formula (S2), N20 has the same meaning as N10 in the formula (S1). Furthermore, in formula (S2), N30 represents the number of catalyst particles supported outside the nanopores of the support.

[0042] By supporting catalyst particles on a hollow carbon support so as to simultaneously satisfy the conditions of the above formula (S2), the electrode catalyst of the present invention has fewer non-contact particles (catalyst particles) embedded in the micropores of the support compared to conventional electrode catalysts, and a relatively large number of highly active catalyst particles are present inside the nanopores of the support. As a result, the electrode catalyst of the present invention can more reliably exhibit excellent catalytic activity that can contribute to cost reduction of PEFCs. Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, the value of [100×{N10 / (N20+N30)}] in formula (S2) is preferably 3.0 or less, and more preferably 1.0 or less.

[0043] Furthermore, when an analysis of the particle size distribution of the catalyst particles is carried out using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), For the catalyst particles supported inside the nanopores of the support, it is preferable that the average distance from the entrance of the nanopore to the supporting position of the catalyst particle is 5.0 nm or more. The inventors believe that catalyst particles that satisfy this condition are supported inside the nanopores of the support and can be sufficiently prevented from contacting the polymer electrolyte membrane. Furthermore, the inventors believe that catalyst particles that satisfy this condition are supported inside the nanopores of the support but are present at a moderate depth from the entrance of the nanopores, making it relatively easy to obtain a supply of protons, oxygen gas, and hydrogen gas.

[0044] Here, the "distance from the nanopore entrance to the catalyst particle loading position" refers to the length of the shortest line drawn from the nanopore entrance of interest along the nanopore's uneven inner wall (which may have irregularities or curves) to the catalyst particle of interest using a 3D image of the support's internal three-dimensional structure, including the nanopores, obtained by electron beam tomography. Furthermore, if there are multiple nanopore entrances for the catalyst particle of interest inside the nanopore, the distance (shortest path) from each nanopore entrance to the catalyst particle loading position is determined, and the shortest distance among these is selected. Furthermore, the "average distance from the nanopore entrance to the catalyst particle loading position" refers to the arithmetic mean of all "distances from the nanopore entrance to the catalyst particle loading position" within the target sample (electrode catalyst) powder mass.

[0045] To ensure the effects of the present invention, the average distance from the entrance of the nanopore to the catalyst particle loading position of the catalyst particles loaded inside the nanopores of the carrier is preferably 5.0 to 8.5 nm, more preferably 5.0 to 5.5 nm. By setting the loading position of the catalyst particles to a range of preferably 8.5 nm or less, more preferably 5.5 nm or less, the supply of protons to the catalyst particles inside the nanopores tends to be more easily maintained even when the MEA is required to generate electricity under higher-than-normal temperature conditions and lower-than-normal humidification conditions.

[0046] Furthermore, in the electrode catalyst of the present invention, from the viewpoint of further reliably obtaining the effects of the present invention, when an analysis of the particle size distribution of the catalyst particles is carried out using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), it is preferable that the catalyst particles supported inside the nanopores of the support are located such that the distance from the entrance of the nanopore to the supporting position of the catalyst particle is in the range of 0 to 27 nm. The inventors believe that catalyst particles that satisfy this condition are supported inside the nanopores of the support but are present at a moderate depth from the entrance of the nanopores, and this more reliably achieves the effect of obtaining a sufficient supply of reactant gas and protons while avoiding contact with the polymer electrolyte mentioned above. From the same viewpoint as above, it is more preferable that the catalyst particles supported inside the nanopores are located such that the distance from the entrance of the nanopore to the position where the catalyst particle is supported is in the range of 0 to 18 nm.

[0047] Furthermore, in the electrode catalyst of the present invention, from the viewpoint of further reliably obtaining the effects of the present invention, when an analysis of the particle size distribution of the catalyst particles is carried out using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), it is preferable that the particle size of the catalyst particles supported inside the nanopores of the support is greater than 0 nm and not more than 7 nm. The present inventors believe that catalyst particles that satisfy this condition have an appropriate particle size and therefore have a sufficient reaction surface area, and can sufficiently promote electrode reactions even when supported inside the nanopores of the support.

[0048] Furthermore, in the electrode catalyst of the present invention, from the viewpoint of further reliably obtaining the effects of the present invention, when the particle size distribution of the catalyst particles is analyzed using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), the proportion of the catalyst particles supported inside the nanopores is preferably 50% or more, and more preferably 70% or more.

[0049] By supporting Pt / C catalyst particles on a hollow carbon support in a manner that satisfies the above conditions, a large number of highly active catalyst particles with relatively small particle diameters are present inside the nanopores of the support, compared to conventional electrode catalysts. The catalyst particles supported inside the nanopores of such a support are supported on the support in a state where they are unlikely to come into direct contact with the polymer electrolyte present in the catalyst layer. Therefore, the electrode catalyst of the present invention is less susceptible to a decrease in catalytic activity due to poisoning of the Pt component, and can exhibit superior catalytic activity when formed into an electrode compared to conventional electrode catalysts. Furthermore, the electrode catalyst of the present invention also reduces dissolution of the Pt component from the catalyst particles.

[0050] Furthermore, in the electrode catalyst of the present invention, at least a portion of the region consisting of Pt (zero valence) on the surface of the catalyst particle may be covered with a Pt oxide film, to the extent that the catalyst particle can exhibit excellent catalytic activity. In order to more reliably obtain the effects of the present invention, the electrode catalyst of the present invention is characterized in that the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support is 200 to 1500 m 2 / g is preferred. Furthermore, when the electrode catalyst is used in a cathode, in order to more reliably obtain the effects of the present invention, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support is set to 700 to 1500 m 2 / g, and 750 to 1400m 2 / g. Furthermore, when the electrode catalyst is used in a cathode, it is preferable that the catalyst has a predetermined durability in consideration of the operating environment of the cathode (temperature fluctuation range, potential fluctuation range). From this viewpoint, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support is 750 to 900 m. 2 / g is preferred.

[0051] The present invention also provides an electrode catalyst powder containing 10 wt % or more of the above-mentioned electrode catalyst of the present invention. In addition, in the electrode catalyst powder, "components other than the above-mentioned electrode catalyst of the present invention" means "electrode catalysts other than the above-mentioned electrode catalyst of the present invention." In other words, the electrode catalyst powder of the present invention does not include powders that do not function as an electrode catalyst. The electrode catalyst powder of the present invention contains the above-mentioned electrode catalyst of the present invention, and therefore can exhibit excellent catalytic activity that can contribute to reducing the cost of PEFCs. Here, from the viewpoint of more reliably obtaining the effects of the present invention, the content ratio of the above-mentioned electrode catalyst of the present invention in the electrode catalyst powder of the present invention is preferably 30 wt % or more, more preferably 50 wt % or more, even more preferably 70 wt % or more, and most preferably 90 wt % or more.

[0052] The electrode catalyst powder of the present invention may contain, in addition to the above-mentioned electrode catalyst of the present invention, an electrode catalyst (for convenience, referred to as "electrode catalyst P") having the following configuration. That is, the electrode catalyst P includes a hollow carbon support having nanopores with a pore diameter of 1 to 20 nm, and a plurality of catalyst particles supported on the support, the catalyst particles are made of Pt (zero valence), The catalyst particles are supported both inside the nanopores and outside the pores of the support, 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 electrode catalyst powder of the present invention may be composed of the above-mentioned electrode catalyst of the present invention and the electrode catalyst P. In this case, too, from the viewpoint of more reliably obtaining the effects of the present invention, the content ratio of the above-mentioned electrode catalyst of the present invention in the electrode catalyst powder of the present invention is preferably 30 wt % or more, more preferably 50 wt % or more, even more preferably 70 wt % or more, and most preferably 90 wt % or more.

[0053] Furthermore, the electrode catalyst powder of the present invention may contain one or more types of conductive carbon supports different from the hollow carbon support of the electrode catalyst of the present invention, as long as 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, the different types of conductive carbon supports may be included in an amount of 10 wt % to 100 wt % based on the weight of the hollow carbon support of the electrode catalyst of the present invention.

[0054] Furthermore, the present invention provides a composition for forming a gas diffusion electrode, which contains the above-mentioned electrode catalyst of the present invention or a powder of the electrode catalyst of the present invention. The composition for forming a gas diffusion electrode of the present invention contains the electrode catalyst of the present invention or a powder of the electrode catalyst of the present invention, and therefore can easily produce a gas diffusion electrode having excellent catalytic activity (polarization characteristics) that can contribute to cost reduction of PEFCs.

[0055] The present invention also provides a gas diffusion electrode containing the above-mentioned electrode catalyst of the present invention or powder of the electrode catalyst of the present invention. The gas diffusion electrode of the present invention is configured to contain the electrode catalyst of the present invention, and therefore can be easily configured to have excellent catalytic activity (polarization characteristics) that can contribute to cost reduction of PEFCs.

[0056] Furthermore, the present invention provides a membrane electrode assembly (MEA) comprising the gas diffusion electrode of the present invention described above. The membrane-electrode assembly (MEA) of the present invention includes the gas diffusion electrode of the present invention, and therefore can be easily configured to have cell characteristics that can contribute to cost reduction of PEFCs.

[0057] The present invention also provides a fuel cell stack comprising the above-mentioned membrane electrode assembly (MEA) of the present invention. According to the fuel cell stack of the present invention, since it contains the membrane electrode assembly (MEA) of the present invention, it is easy to provide a configuration having cell characteristics that can contribute to reducing the cost of PEFC. [Effects of the Invention]

[0058] According to the present invention, an electrode catalyst having excellent catalytic activity that can contribute to cost reduction of PEFCs is provided. Furthermore, 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, each of which contains the electrode catalyst. [Brief explanation of the drawings]

[0059] [Figure 1] FIG. 1 is a schematic cross-sectional view showing a preferred embodiment of an MEA of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view showing a preferred embodiment of the electrode catalyst of the present invention contained in at least one of the cathode catalyst layer and the anode catalyst layer of the MEA shown in FIG. [Figure 3] FIG. 3 is an enlarged schematic cross-sectional view showing the schematic configuration of the electrode catalyst shown in FIG. 2. [Figure 4] FIG. 2 is a schematic cross-sectional view showing another preferred embodiment of the MEA of the present invention. [Figure 5] FIG. 1 is a schematic cross-sectional view showing a preferred embodiment of a CCM of the present invention. [Figure 6] FIG. 2 is a schematic cross-sectional view showing another preferred embodiment of the CCM of the present invention. [Figure 7] 1 is a schematic cross-sectional view showing a preferred embodiment of a GDE of the present invention. [Figure 8] FIG. 2 is a schematic cross-sectional view showing another preferred embodiment of the GDE of the present invention. [Figure 9] 1 is a schematic diagram showing a preferred embodiment of a fuel cell stack of the present invention. [Figure 10] FIG. 1 is a schematic cross-sectional view showing a conventional electrode catalyst. [Figure 11] 1 is a STEM image showing the 3D electron tomography measurement conditions (volume size) using a STEM for the electrode catalyst of Example 1. [Figure 12] 1 is a 3D-STEM image (three-dimensional reconstruction image) of the electrode catalyst of Example 1. [Figure 13] 13 is a graph showing the distribution state of Pt catalyst particles located outside and inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1 shown in FIG. [Figure 14] 13 is a graph showing the distribution state of Pt catalyst particles located inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1 shown in FIG. [Figure 15] FIG. 13 is another graph showing the distribution state of Pt catalyst particles located inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1 shown in FIG. [Figure 16] 10 is a STEM image showing the 3D electron tomography measurement conditions (volume size) using a STEM for the electrode catalyst of Example 2. [Figure 17] 1 is a 3D-STEM image (three-dimensional reconstruction image) of the electrode catalyst of Example 2. [Figure 18] 18 is a graph showing the distribution state of Pt catalyst particles located outside and inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 2 shown in FIG. [Figure 19] 18 is a graph showing the distribution state of Pt catalyst particles located inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 2 shown in FIG. 17. [Figure 20] FIG. 18 is another graph showing the distribution state of Pt catalyst particles located inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 2 shown in FIG. [Figure 21] 1 is a STEM image showing the 3D electron tomography measurement conditions (volume size) using a STEM for the electrode catalyst of Comparative Example 1. [Figure 22] 1 is a 3D-STEM image (three-dimensional reconstruction image) of the electrode catalyst of Comparative Example 1. [Figure 23] 23 is a graph showing the distribution state of Pt catalyst particles located outside and inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Comparative Example 1 shown in FIG. [Figure 24] 23 is a graph showing the distribution state of Pt catalyst particles located inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of a 3D-STEM image of the electrode catalyst of Comparative Example 1 shown in FIG. [Figure 25] FIG. 23 is another graph showing the distribution state of Pt catalyst particles located inside nanopores of a carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Comparative Example 1 shown in FIG. [Figure 26] 1 is a STEM image showing the 3D electron tomography measurement conditions (volume size) using a STEM for the electrode catalyst of Comparative Example 1. [Figure 27] 1 is a table showing a plurality of 3D-STEM images (three-dimensional reconstructed images) obtained by electron tomography measurement using a STEM for each of the electrode catalysts of Example 1 and Example 2 and the electrode catalyst of Comparative Example 2. [Figure 28] 28 is an enlarged view of a cross section (xy plane) of a cube image extracted from a 3D-STEM image (three-dimensional reconstructed image) of the catalyst mass of the electrode catalyst of Example 1 shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0060] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0061] <Membrane-electrode assembly (MEA)> FIG. 1 is a schematic cross-sectional view showing a preferred embodiment of the MEA of the present invention. The MEA 10 shown in FIG. 1 has a configuration including two flat gas diffusion electrodes (cathode 1 and anode 2) arranged facing each other, and a polymer electrolyte membrane (hereinafter referred to as "PEM" as necessary) 3 arranged between the cathode 1 and the anode 2. In the case of this MEA 10, at least one of the cathode 1 and the anode 2 contains an electrode catalyst 20 (Pt catalyst 20) described later. The MEA 10 can be manufactured by stacking the cathode 1, anode 2, and PEM 3 as shown in FIG. 1, and then pressing them together.

[0062] <Gas diffusion electrode (GDE)> The cathode 1, which is a gas diffusion electrode, includes a gas diffusion layer 1gd and a catalyst layer 1c formed on the surface of the gas diffusion layer 1gd facing the PEM 3. The cathode 1 further includes 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. Similar to the cathode 1, the anode 2, which is a gas diffusion electrode, has a configuration including a gas diffusion layer 2gd, a catalyst layer 2c formed on the PEM 3 side of the gas diffusion layer 2gd, and an MPL 2m disposed between the gas diffusion layer 2gd and the catalyst layer 2c.

[0063] (Catalyst layer (CL)) In the cathode 1, the catalyst layer 1c is a layer where a reaction proceeds between air (oxygen gas) sent from the gas diffusion layer 1gd and hydrogen ions moving through the PEM 3 from the anode 2 to produce water. In 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.

[0064] (A preferred embodiment of the electrode catalyst of the present invention) A preferred embodiment of the electrode catalyst of the present invention will be described below with reference to FIGS. 2, 3, 27 and 28. FIG. FIG. 2 is a schematic cross-sectional view showing a preferred embodiment of an 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. FIG. 3 is an enlarged schematic cross-sectional view showing the general configuration of the electrode catalyst 20 shown in FIG. As shown in FIGS. 2 and 3, the electrode catalyst 20 includes a support 22 which is a hollow carbon support, and catalyst particles 23 supported on the support 22.

[0065] FIG. 27 is a table listing multiple 3D-STEM images (three-dimensional reconstructed images) obtained by electron tomography measurement using a STEM for each of the electrode catalysts of Example 1 and Example 2 (examples of the electrode catalyst 20) and the electrode catalyst of Comparative Example 2. Figure 28 is an enlarged view of the cross section (xy 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 (an example of the electrode catalyst 20) shown in Figure 27.

[0066] Moreover, the electrode catalyst 20 shown in FIGS. 2 and 3 preferably satisfies the following conditions in order to more reliably obtain the effects of the present invention. That is, as described above, the electrode catalyst 20 has a configuration that satisfies the condition (α) of formula (S1) and the condition (β) that the nanopores are formed to have the shape of the above-mentioned communicating pores when the microstructure is observed using a three-dimensional reconstructed image of STEM.

[0067] To explain the condition (α) in more detail, when the microstructure of the electrode catalyst 20 is observed using the information of the three-dimensional reconstructed STEM image obtained by the above-mentioned steps (A) to (C), the value of [100 × (N10 / N20)] in formula (S1) is 8.0 or less. Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, in the electrode catalyst 20, the value of [100×(N10 / N20)] in the formula (S1) is preferably 6.5 or less, and more preferably 1.0 or less.

[0068] To explain the condition (β) in more detail, the electrode catalyst 20 has the following structure when viewed from the information of the three-dimensional reconstructed STEM image obtained by the steps (D) to (G) described above. To explain in more detail, when six square cross sections of a "cubic image (each side 20 to 50 nm)" extracted from the "catalyst mass constituting the electrode catalyst 20 (a catalyst mass consisting of catalyst particles 23 and a support 22 of a size that can be accommodated in a rectangular space with a side of 60 to 300 nm)" that can be seen in a 3D-STEM image (three-dimensional reconstruction image) are viewed, the electrode catalyst 20 has a structure in which at least one "communicating hole P1 formed by connecting multiple nanopores P22" having the following shape is formed in at least one cross section.

[0069] That is, when the microstructure of the electrode catalyst 20 is viewed in a 3D-STEM image (three-dimensional reconstruction 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 interior of the catalyst mass has at least one opening that contacts a first of the four sides of the square cross section and at least one opening that contacts a second side of the square cross section that is parallel to the first side. The nanopore P22 has a shape that forms a communicating hole P1 that extends continuously without blocking from the opening on the first side to the opening on the second side. Hereinafter, a more specific explanation will be given using the example of the electrode catalyst of Example 1 shown in FIGS.

[0070] (D) First, a three-dimensional reconstructed STEM image is obtained for the electrode catalyst of Example 1 that is the measurement target. From the catalyst masses that appear in this three-dimensional reconstructed image, a catalyst mass of a size that can be accommodated in a rectangular parallelepiped space (region of interest) with sides of 60 to 300 nm is selected (FIG. 27(a)). (E) Next, a cubic image (each side is 20 to 50 nm) is extracted from the inner region of the catalyst mass of the electrode catalyst of Example 1 selected in step (D) (FIG. 27(b)). (F) Next, a three-dimensional image (a three-dimensional reconstructed STEM image) of the interior of the catalyst mass of the electrode catalyst of Example 1 obtained in step (E) is observed, and the void portions (pore portions such as nanopores) and hollow carbon support portions are segmented using the difference in brightness (Figure 27(b)). (G) Next, when viewing six square cross sections of the three-dimensional image of the interior of the catalyst mass of the electrode catalyst of Example 1 after performing segmentation in step (F), as shown in Figure 28, the nanopore P1 visible in the cross section of interest (the square xy plane) has two openings (openings A11 and A12) that are adjacent to the first side L1. This nanopore P1 also has two openings (openings A21 and A22) that are adjacent to the second side L2 that is parallel to the first side L1. Furthermore, this nanopore P1 has the shape of a communicating hole P1 that extends continuously without being blocked from the openings on the first side L1 (openings A11 and A12) to the openings on the second side L2 (openings A21 and A22).

[0071] In order to more reliably obtain the effects of the present invention, it is preferable that the nanopores P1 of the electrode catalyst 20 have a shape that is branched into multiple parts, as shown in the example of Example 1 in FIG. Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, it is preferable that the nanopores P1 of the electrode catalyst 20 have two or more openings on the first side, as shown in the example of Example 1 in FIG. Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, it is preferable that the nanopores P1 of the electrode catalyst 20 have two or more openings on the second side, as shown in the example of Example 1 in FIG. Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, it is preferable that the nanopore P1 of the electrode catalyst 20 has at least one opening on a third side perpendicular to the first side, as shown in the example of Example 1 in Figure 28. Explaining this with reference to the example of Example 1, as shown in Figure 28, the nanopore P1 visible in the cross section of interest (square xy plane) also has one opening (opening A31) on a third side L3 perpendicular to the first side L1.

[0072] Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, it is preferable that the nanopore P1 of the electrode catalyst 20 has at least one opening on the fourth side perpendicular to the first side, as shown in the example of Example 1 in Fig. 28. For example, referring to the example of Example 1, as shown in Fig. 28, the nanopore P1 visible in the cross section of interest (the square xy plane) also has two openings (opening A41 and opening A42) on the fourth side L4 perpendicular to the first side L1.

[0073] Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, the porosity of the electrode catalyst 20 measured using a three-dimensional reconstructed STEM image (a cube image of interest) is preferably 35% or more, more preferably 40% or more, more preferably 45% or more, even more preferably 50% or more, even more preferably 55% or more, even more preferably 60% or more, and even more preferably 65% ​​or more. On the other hand, from the viewpoint of durability, the porosity of the electrode catalyst 20 measured using a three-dimensional reconstructed STEM image (a cube image of interest) is preferably 80% or less, more preferably 75% or less.

[0074] Here, a region made of Pt (zero valence) is formed on at least a part of the surface of the catalyst particle 23. However, a layer of Pt oxide may be formed on the region made of Pt (zero valence) on the surface of the catalyst particle 23 as long as the effects of the present invention can be obtained. A more specific configuration of the catalyst particles 23 is not particularly limited when the catalyst particles 23 are made of Pt (zero valence), but it is preferable that the catalyst particles 23 are made of a Pt alloy and are core-shell catalyst particles. When the catalyst particles 23 are made of a Pt alloy, the metal species other than Pt that constitutes the alloy is not particularly limited. From the viewpoint of obtaining excellent catalytic activity, the metal species other than Pt that constitutes the alloy is preferably at least one metal selected from the group consisting of Co and Ni.

[0075] When the catalyst particles 23 are core-shell catalyst particles, from the viewpoint of obtaining excellent catalytic activity, the core-shell catalyst particles preferably comprise a core particle and a Pt shell layer (a region consisting of Pt (zero valence)) formed on at least a portion of the surface of the core particle. The metal species constituting the core particle is not particularly limited, but from the viewpoint of obtaining excellent catalytic activity, it is preferably at least one of Pd, Ni, and Co. Furthermore, the core particle may be an alloy of at least one of Pd, Ni, and Co with another metal. From the viewpoint of reducing the amount of precious metal used, the core particle may contain, inside thereof, a base metal other than the precious metal, and at least one of an oxide of a base metal, a nitride of a base metal, and a carbide of a base metal.

[0076] The electrode catalyst 20 preferably has an average crystallite size of 3 to 16.0 nm as measured by powder X-ray diffraction (XRD). The electrode catalyst 20 preferably has a Pt loading rate of 5.6 to 66.5 wt %.

[0077] The carrier 22 is preferably one that can satisfy the above conditions (α) and (β) when used as an electrode catalyst. From this viewpoint, the carrier 22 is preferably one of CNovel (a product name, registered trademark, manufactured by Toyo Tanso Co., Ltd.) that can satisfy the above conditions (α) and (β) when used as an electrode catalyst.

[0078] As shown in Fig. 2, in this embodiment, the support 22 is porous carbon having nanopores P22 (pore diameter of 1 to 20 nm, preferably 1 to 10 nm), micropores P24 (pore diameter of less than 1 nm), and carbonaceous walls that form the outer peripheries of the nanopores P22. The carbonaceous walls have portions that form a layered structure, and furthermore, the carbonaceous walls form a three-dimensional network structure. The portions that form the layered structure in the carbonaceous walls have a well-developed crystallinity.

[0079] This layered structure is usually produced by heating a carbon material above a certain temperature. However, carbon materials generally shrink during the heat treatment, which tends to collapse the pores and reduce the specific surface area. Therefore, it has been difficult to obtain porous carbon with a high specific surface area by developing crystalline structures. In contrast, the support 22 has nanopores P22 and carbonaceous walls that form the outer shell of these nanopores P22, so it can withstand shrinkage during heat treatment, a layered structure is sufficiently formed in the carbonaceous walls, and a sufficient specific surface area is secured.

[0080] Furthermore, the support 22 has a three-dimensional network structure in its carbonaceous walls, which allows it to support highly dispersed catalyst particles as small as a few nanometers, making it suitable as a support for the catalyst layer of a fuel cell. Note that the entire carbonaceous wall of the support 22 does not need to have a layered structure, and some amorphous portions may be present. The carrier 22 has a specific surface area of ​​200 m 2 / g~1500m 2 / g. The specific surface area is preferably 200m 2 / g or more, a three-dimensional network structure can be more reliably formed. This allows sufficient pores to be formed, and the material is more likely to have sufficient gas adsorption capacity. On the other hand, 2 When the carbonaceous wall thickness is 1 / g or less, the carbonaceous wall can be formed more reliably, which makes it easier to form the nanopores P22 sufficiently.

[0081] 2, the support 22 has a structure in which the nanopores P22 are open pores and the nanopores P22 are continuously connected to form connected pores P1. This structure can facilitate the flow of reaction gas in the catalyst layer (catalyst layer 1c or catalyst layer 2c). From the viewpoint of having sufficient conductivity, the carrier 22 has a specific resistance of 10.0×10 2 It is preferable that the resistance is 5.0×10 Ω·cm or less. 2 Ω·cm is more preferable, and 1.0×10 2 It is more preferable that the resistivity is Ω·cm or less. Furthermore, the support 22 may contain pores with a pore diameter of less than 1 nm (relatively small pores classified as so-called micropores) and pores with a pore diameter of more than 20 nm and not more than 50 nm (relatively large pores classified as so-called mesopores), within the range in which the effects of the present invention can be obtained. Furthermore, the support 22 is preferably a hollow carbon support that has good dispersibility in the composition for forming a gas diffusion electrode containing the electrode catalyst 20 and has excellent electrical conductivity.

[0082] As shown in FIG. 2, the catalyst particles 23 are supported both inside the nanopores P22 of the support 22 and outside the nanopores P22. The electrode catalyst 20 satisfies the condition of the following formula (S1) when measured by electron tomography using 3D-STEM. 100×(N10 / N20)≦8.0···(S1) Here, in formula (S1), N10 represents the number of non-contact particles 25 (n101+n102), which is the sum of (I) the number of precious metal particles (n101) that are not in contact with pores with a pore diameter of 1 nm or more that can be confirmed by electron beam tomography measurement and (II) the number of precious metal particles (n102) that are not in contact with the hollow carbon support 22 itself and exist outside it. N20 indicates the number of catalyst particles 23 supported inside the nanopores P22 of the support 22.

[0083] Compared to the conventional electrode catalyst 200 (see Figure 10), the electrode catalyst 20 that satisfies the conditions of formula (S1) has fewer non-contact particles 25 (catalyst particles that do not contribute much to the electrode reaction) embedded in the micropores P24 of the support 22, and a relatively large number of highly active catalyst particles 23 are present inside the nanopores P22 of the support 22. The catalyst particles 23 supported inside the nanopores P22 of the support 22 are supported on the support 22 in a state where they are unlikely to come into direct contact with the polymer electrolyte present in the catalyst layer (catalyst layer 2c or catalyst layer 1c in FIG. 1). Therefore, the electrode catalyst 20 of this embodiment is less susceptible to a decrease in catalytic activity due to poisoning of the Pt component, and can exhibit superior catalytic activity when made into an electrode compared to the conventional electrode catalyst 200. Furthermore, the electrode catalyst 20 of this embodiment also reduces dissolution of the Pt component from the catalyst particles 23. Here, in order to more reliably obtain the effects of the present invention, the value of [100×(N10 / N20)] in formula (S1) is preferably 6.5 or less, and more preferably 1.0 or less.

[0084] It is preferable that the electrode catalyst 20 further satisfies the condition of the following formula (S2) when measured by electron tomography using 3D-STEM. 100×{N10 / (N20+N30}≦5.0···(S2) Here, N10 in formula (S2) has the same meaning as N10 in formula (S1). In addition, in the formula (S2), N20 has the same meaning as N10 in the formula (S1). Furthermore, in formula (S2), N30 represents the number of catalyst particles 23 supported outside the nanopores P22 of the support 22.

[0085] By supporting catalyst particles on the support 22 so as to simultaneously satisfy the conditions of the above formula (S2), the electrode catalyst 20 has fewer non-contact particles 25 (catalyst particles) embedded in the micropores P24 of the support 22, and a relatively large number of highly active catalyst particles 23 are present inside the nanopores P22 of the support 22, compared to the conventional electrode catalyst 200. As a result, the electrode catalyst 20 can more reliably exhibit excellent catalytic activity that can contribute to cost reduction of PEFCs. Here, in order to more reliably obtain the effects of the present invention, the value of [100×{N10 / (N20+N30}] in formula (S2) is preferably 3.0 or less, and more preferably 1.0 or less.

[0086] Furthermore, when the particle size distribution of the catalyst particles 23 of the electrode catalyst 20 is analyzed using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), it is preferable that the average distance from the entrance of the nanopore P22 to the loading position of the catalyst particles 23 loaded inside the nanopore P22 of the carrier 22 is 5.0 nm or more. The present inventors believe that catalyst particles 23 that satisfy this condition are supported inside the nanopores P22 of the support 22 and can be sufficiently prevented from contacting the polymer electrolyte membrane. Furthermore, catalyst particles 23 that satisfy this condition are supported inside the nanopores P22 of the support 22, but are present at a moderate depth from the entrance of the nanopores, and are therefore relatively likely to be supplied with protons, oxygen gas, and hydrogen gas.

[0087] To more reliably obtain the effects of the present invention, the average distance from the entrance of the nanopores P22 to the position where the catalyst particles 23 are supported inside the nanopores P22 of the carrier 22 is preferably 5.0 to 8.5 nm, and more preferably 5.0 to 5.5 nm, for the catalyst particles 23 supported inside the nanopores P22. By setting the support position of the catalyst particles 23 to a range of preferably 8.5 nm or less, more preferably 5.5 nm or less, the supply of protons to the catalyst particles 23 inside the nanopores P22 tends to be more easily maintained even when the MEA 10 is required to generate electricity under higher-than-normal temperature conditions and lower-than-normal humidification conditions.

[0088] Furthermore, when the particle size distribution of the catalyst particles 23 of the electrode catalyst 20 is analyzed using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), it is more preferable that the catalyst particles 23 supported inside the nanopores P22 of the support 22 are located such that the distance from the entrance of the nanopores P22 to the supporting position of the catalyst particles 23 is in the range of 0 to 27 nm. The catalyst particles 23 that satisfy this condition are supported inside the nanopores P22 of the support 22, but are present at a moderate depth from the entrance of the nanopores P22, and it is believed that this more reliably achieves the effect of obtaining a sufficient supply of reactant gas and protons while avoiding contact with the polymer electrolyte mentioned above. From the same viewpoint as above, it is more preferable that the catalyst particles 23 loaded inside the nanopores P22 of the support 22 are located such that the distance from the entrance of the nanopores P22 to the loading position of the catalyst particles 23 is in the range of 0 to 18 nm.

[0089] Furthermore, when the particle size distribution of the catalyst particles 23 of the electrode catalyst 20 is analyzed using a three-dimensional reconstructed image obtained by electron beam tomography measurement using a STEM (scanning transmission electron microscope), it is preferable that the particle size of the catalyst particles 23 supported inside the nanopores P22 of the support 22 is greater than 0 nm and not more than 7 nm. The catalyst particles 23 that satisfy this condition have an appropriate particle size and therefore have a sufficient reaction surface area, and are thought to be able to sufficiently promote the electrode reaction even when supported inside the nanopores P22 of the support 22.

[0090] Furthermore, when the particle size distribution of the catalyst particles 23 of the electrode catalyst 20 is analyzed using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), it is preferable that the proportion of the catalyst particles 23 supported inside the nanopores P22 is 50% or more, and more preferably 70% or more.

[0091] By supporting catalyst particles 23 on the support 22 so as to satisfy the above conditions, a large number of highly active catalyst particles 23 with relatively small particle diameters are present inside the nanopores P22 of the support 22, compared to conventional electrode catalysts. The catalyst particles 23 supported inside the nanopores P22 of the support 22 are supported on the support in a state where they are unlikely to come into direct contact with the polymer electrolyte present in the catalyst layer (catalyst layer 1c or catalyst layer 2c). Therefore, the electrode catalyst 20 is less susceptible to a decrease in catalytic activity due to poisoning of the Pt component, and can exhibit superior catalytic activity when made into an electrode compared to the conventional electrode catalyst 200. Furthermore, the electrode catalyst 20 also reduces dissolution of the Pt component from the catalyst particles 23.

[0092] The method for producing the electrode catalyst 20 is not particularly limited, and the electrode catalyst 20 can be produced by a known method, except that it includes a "carrier pretreatment step," a "Pt addition step," and a "reduction step" to satisfy the formula (1), the formula (2), and the other conditions described above. In the carrier pretreatment step, the carrier 22 is placed in ultrapure water, and a pH adjuster is added to prepare a dispersion having a pH adjusted to 2 to 5. The dispersion is then stirred and maintained at a temperature of 80 to 99°C, preferably 90 to 99°C, for a predetermined period of time (but without boiling), after which the dispersion is cooled to room temperature. This removes gas from inside the nanopores P22 of the support 22, allowing the ultrapure water to sufficiently penetrate into the nanopores P22. Then, in the subsequent "Pt addition step," the Pt raw material is sufficiently retained inside the nanopores P22 of the support 22. As a result, a large number of precursors of Pt catalyst particles are supported inside the nanopores P22 of the support 22.

[0093] The "ultrapure water" used to prepare the above-mentioned aqueous solution in this carrier pretreatment step is water with a resistivity R (the reciprocal of electrical conductivity measured by the JIS standard test method (JIS K0552)) of 3.0 MΩ cm or higher, as expressed by the following formula (4). Furthermore, it is preferable that the "ultrapure water" has a water quality equivalent to or higher than "A3" as specified in JIS K0557 "Water used in testing water and wastewater."

[0094] There are no particular limitations on the ultrapure water, as long as it has an electrical conductivity that satisfies the relationship expressed by the following formula (4): For example, the ultrapure water can be produced using an ultrapure water production system such as the "Milli-Q Series" (manufactured by Merck Ltd.) or the "Elix UV Series" (manufactured by Nihon Millipore K.K.). R=1 / ρ (4) In the above formula (4), R represents the resistivity, and ρ represents the electrical conductivity measured according to the JIS standard test method (JIS K0552).

[0095] The next step after the "carrier pretreatment step" is the "Pt addition step." In this "Pt addition step," an aqueous solution of water-soluble Pt salt (NECHEMCAT Corporation, product name "A-salt" (Fe component concentration: 8 ppm or less)) dissolved in ultrapure water is added at room temperature to the dispersion of the carrier 22 obtained through the "carrier pretreatment step." The step following 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 containing a water-soluble reducing agent (preferably an acidic water-soluble reducing agent) 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 liquid temperature is lowered to room temperature.

[0096] The process following the "reduction process" is the "washing process." In this "washing process," the solid components in the liquid obtained through the "reduction process" are separated from the liquid components, and the solid components (a mixture of Pt / C catalyst and other impurities) are washed. For example, the solid components in the liquid obtained through the "reduction process" may be separated from the liquid components using a filtration method such as filter paper or filter cloth. The solid components may be washed using the above-mentioned ultrapure water, pure water (resistivity R expressed by the above-mentioned equation (4) is 0.1 MΩ·cm or more and less than 3.0 MΩ·cm), or pure hot water (pure water at a temperature of 40 to 80°C). For example, when pure hot water is used, washing is repeated until the electrical conductivity of the filtrate after washing is less than 10 μS / cm. The process following the "washing process" is the "drying process." In this "drying process," water is separated from the solid component (a mixture of Pt / C catalyst and water) obtained after the "washing process." First, the solid component is air-dried, and then it is dried in a dryer at a specified temperature for a specified time. The process following the "drying process" is the "pulverization process." In this "pulverization process," the solid component (Pt / C catalyst) obtained in the "drying process" is pulverized into catalyst powder using a pulverizing means such as a mixer.

[0097] The polymer electrolyte contained in catalyst layer 1c and catalyst layer 2c is not particularly limited as long as it has hydrogen ion conductivity, and known polymer electrolytes can be used. For example, known perfluorocarbon resins having sulfonic acid groups and carboxylic acid groups can be exemplified as polymer electrolytes. Preferred examples of readily available polymer electrolytes having hydrogen ion 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.). In 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, the mass ratio N / C of the mass C of the carrier 22 to the mass N of the polymer electrolyte is set to 0.5 to 1.2, and more preferably, the mass ratio N / C is set to 0.7 to 1.0.

[0098] (Gas Diffusion Layer (GDL)) 1, the gas diffusion layer 1gd is provided to supply an oxidant gas (e.g., oxygen gas or air) to the catalyst layer 1c, and also serves to support the catalyst layer 1c. The gas diffusion layer 2gd provided in the anode 2 is a layer provided to supply a reducing agent gas (for example, hydrogen gas) to the catalyst layer 2c and also serves to support the catalyst layer 2c.

[0099] 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 applied to the carbon paper are preferably mentioned.

[0100] (Water repellent layer (MPL)) As shown in FIG. 1, in the cathode 1, a water repellent layer (MPL) 1m is disposed between the gas diffusion layer 1gd and the catalyst layer 1c. The water repellent layer 1m has electron 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.

[0101] (Polymer electrolyte membrane (PEM)) The polymer electrolyte membrane (PEM) 3 shown in FIG. 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.

[0102] <Modified forms of MEA> As described above, the preferred embodiments of the MEA of the present invention (and the catalyst layer of the present invention, the gas diffusion electrode of the present invention) have been described, but the MEA of the present invention is not limited to the configuration of the MEA10 shown in FIG. 1. For example, the MEA of the present invention may have the configuration of the MEA11 shown in FIG. 4. Fig. 4 is a schematic cross-sectional view showing another preferred embodiment of the MEA of the present invention. The MEA 11 shown in Fig. 4 has a configuration in which a gas diffusion electrode (GDE) 1A having a configuration similar to that of the cathode 1 in the MEA 10 shown in Fig. 1 is arranged on only one side of a 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, the catalyst layer 1c of the GDE 1A has a mass ratio N / C of the mass C of the support 22 of the electrode catalyst 20 to the mass N of the polymer electrolyte, which is 0.5 to 1.2, preferably 0.7 to 1.0.

[0103] <Membrane / catalyst layer assembly (CCM)> Next, a preferred embodiment of the catalyst coated membrane (CCM) of the present invention will be described. Fig. 5 is a schematic cross-sectional view showing a preferred embodiment of a CCM of the present invention. The CCM 12 shown in Fig. 5 has a configuration in which a polymer electrolyte membrane (PEM) 3 is disposed between a cathode catalyst layer 1c and an anode catalyst layer 2c. 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 support of the electrode catalyst 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0104] <Deformation of catalyst-coated membrane assemblies (CCM)> Although a preferred embodiment of the CCM of the present invention has been described above, the CCM of the present invention is not limited to the configuration of the CCM 12 shown in FIG. For example, the CCM of the present invention may have the configuration of the CCM 13 shown in FIG. Fig. 7 is a schematic cross-sectional view showing another preferred embodiment of the CCM of the present invention. The CCM 13 shown in Fig. 6 has a configuration in which a catalyst layer 1c having a configuration similar to that of the cathode 1 in the CCM 12 shown in Fig. 5 is disposed on only one side of a 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, the catalyst layer 1c of the CCM 13 has a mass ratio N / C of the mass C of the support of the electrode catalyst 20 to the mass N of the polymer electrolyte, which is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0105] <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 a GDE of the present invention. The gas diffusion electrode (GDE) 1B shown in Fig. 7 has a configuration similar to that of the cathode 1 mounted on the MEA 10 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 support 22 of the electrode catalyst 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0106] <Modification of Gas Diffusion Electrode (GDE)> Although the preferred embodiment of the GDE of the present invention has been described above, the GDE of the present invention is not limited to the configuration of the GDE 1B shown in FIG. For example, the GDE of the present invention may have the configuration of a GDE1C shown in FIG. Fig. 9 is a schematic cross-sectional view showing another preferred embodiment of the GDE of the present invention. Compared to GDE 1B shown in Fig. 8, GDE 1C shown in Fig. 8 does not have a water-repellent layer (MPL) between the catalyst layer 1c and the gas diffusion layer 1gd.

[0107] <Composition for forming catalyst layer> Next, a preferred embodiment of the catalyst layer-forming composition of the present invention will be described. The composition for forming a catalyst layer of this embodiment contains an electrode catalyst 20, a polymer electrolyte, and a main component, and the mass ratio N / C of the mass C of the support 22 of the electrode catalyst 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0. The composition of the polymer electrolyte-containing liquid is not particularly limited. For example, the polymer electrolyte-containing liquid may contain the above-mentioned polymer electrolyte having hydrogen ion conductivity, water, and alcohol.

[0108] The composition ratio of the electrode catalyst 20, polymer electrolyte, and other components (water, alcohol, etc.) contained in the catalyst layer-forming composition is appropriately set so that the electrode catalyst 20 is well dispersed in the resulting catalyst layer and the power generation performance of the MEA 10 including the catalyst layer can be improved. The catalyst layer-forming composition can be prepared by mixing and stirring a liquid containing the electrode catalyst 20 and a polymer electrolyte. To adjust the coating properties, the composition may contain a polyhydric alcohol such as glycerin and / or water. When mixing the liquid containing the electrode catalyst 20 and the polymer electrolyte, a grinding mixer such as a ball mill or 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.

[0109] (Method of manufacturing gas diffusion electrode) Next, an example of a method for manufacturing the gas diffusion electrode of the present invention will be described. The gas diffusion electrode may be formed so as to include the catalyst layer of the present invention, and a known method can be used for the manufacturing method. The use of the composition for forming a catalyst layer of the present invention allows for more reliable manufacturing. For example, the catalyst layer may be produced by applying the catalyst layer-forming composition onto the gas diffusion layer (or onto the water-repellent layer of a laminate in which a water-repellent layer is formed on a gas diffusion layer), followed by drying.

[0110] <Fuel cell stack> FIG. 9 is a schematic diagram showing a preferred embodiment of a fuel cell stack of the present invention. The fuel cell stack 30 shown in Fig. 9 has a configuration in which a plurality of unit cells, each of which is the MEA 10 shown in Fig. 1, are stacked one on top of the other. The fuel cell stack 30 has a configuration in which the MEA 10 is disposed between a separator 4 and a separator 5. The separator 4 and the separator 5 each have a gas flow path formed therein. [Example]

[0111] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0112] (I) Preparation of electrode catalyst for use in the cathode catalyst layer of the MEA (1) Preparation of Pt / C catalyst used in the cathode of the MEA of Example 1 [Pt catalyst particle-supported carbon catalyst "Pt / C catalyst" powder] A Pt / C catalyst powder (Pt loading rate 48.0 wt%, product name SA50BM-A207, manufactured by NECHEMCAT) in which catalyst particles made of Pt were supported on the following support was prepared. This Pt / C catalyst powder (hereinafter referred to as "Pt / C catalyst A" as necessary) was prepared by the following procedure.

[0113] (First step (carrier pretreatment step)) Hollow carbon support: Toyo Tanso Co., Ltd., prototype product name: "CNovel A" BET specific surface area: 1200 m 2 The dispersion liquid was dispersed in an aqueous solution (prepared by adding a pH adjuster to ultrapure water) adjusted to pH = 2 to 5, and the temperature was kept at 90 to 99°C for about 0.5 hours while stirring (however, the liquid was kept in a state where it was not boiled). The "ultrapure water" used in this first step (carrier pretreatment step) had a resistivity R (the reciprocal of electrical conductivity measured by the JIS standard test method (JIS K0552)) of 3.0 MΩ cm or higher, as expressed by the following formula (4). Furthermore, this "ultrapure water" had a water quality equivalent to or higher than "A3" as specified in JIS K0557 "Water used in testing water and wastewater." This ultrapure water was produced using ultrapure water production equipment "Milli-Q Series" (manufactured by Merck Ltd.) and "Elix UV Series" (manufactured by Nihon Millipore Co., Ltd.). R=1 / ρ (4) In the above general formula (4), R represents the resistivity, and ρ represents the electrical conductivity measured according to the JIS standard test method (JIS K0552).

[0114] (2nd step (Pt addition step)) A mixed solution was prepared by adding an aqueous solution of water-soluble Pt salt (NECHEMCAT, product name "A-salt" (Fe component concentration: 8 ppm or less)) dissolved in ultrapure water to the dispersion obtained through the first step, and the pH was adjusted to 7 to 12. The mixture was stirred for a predetermined time while maintaining a predetermined temperature of 50°C or higher.

[0115] (Third step (reduction step)) An aqueous solution containing an acidic water-soluble reducing agent was added to the liquid obtained through the second step, and the Pt ions in the mixed liquid were reduced to obtain a Pt catalyst particle-supported carbon "Pt / C" powder.

[0116] (Fourth step (cleaning step)) Filter paper was used to separate the solid and liquid components from the liquid obtained through the "third step." Next, the solid content remaining on the filter paper (a mixture of Pt / C catalyst and other impurities) was washed using the pure water and pure warm water described above. First, the pure water wash was performed. This wash was repeated until the electrical conductivity of the filtrate after washing was less than 20 μS / cm. Next, the pure warm water wash was performed. This wash was repeated until the electrical conductivity of the filtrate after washing was less than 10 μS / cm.

[0117] (5th process (drying process)) The solid component on the filter paper (a mixture of Pt / C catalyst and water) obtained through the "fourth step" was air-dried in this state. After this air-drying, the solid component on the filter paper was transferred to a porcelain dish and dried in an electric dryer at a predetermined temperature of 60°C or higher for a predetermined time.

[0118] (Sixth step (crushing step)) The solid component (Pt / C catalyst) obtained in the "fifth step" was pulverized using a mixer to obtain a powder of Pt / C catalyst A.

[0119] <Measurement of loading rate (ICP analysis)> The Pt loading rate (wt%) of this Pt / C catalyst A was measured by the following method. Pt / C catalyst A was immersed in aqua regia to dissolve the metal. Next, the insoluble carbon was removed from the aqua regia. The carbon-free aqua regia was then subjected to ICP analysis. As a result of ICP analysis, the Pt loading rate of this Pt / C catalyst A was 48.0 wt %.

[0120] <Surface and structural observation of electrode catalysts> In order to observe the three-dimensional structure of Pt / C catalyst A of Example 1, electron tomography measurements were carried out using a STEM (scanning transmission electron microscope) at UBE Scientific Analysis Center, Inc., using the "USAL-KM3D analysis method."

[0121] Electron tomography measurements using a STEM (scanning transmission electron microscope) were carried out according to the previously described sample preparation method and conditions, and the analytical procedures and conditions (A) to (C) and (D) to (G). Further details are provided below. ·STEM equipment: JEOL JEM-ARM200F atomic resolution analytical electron microscope Data analysis software: System In Frontier's 3D reconstruction software Composer, 3D data visualization software Visualizer-kai, and image analysis software Colorist Measurement conditions Accelerating voltage: 60 kV Magnification: 800,000 to 1,000,000 times Measurement sample tilt angle: -80° to +80° Tilt step angle of measurement sample: 2° Number of pixels: 512 × 512 pixels 512 × 512 pixels Pixel size: 0.350 nm / pixel ~ 0.500 nm / pixel Volume size: shown in Figure 11.

[0122] For Pt / C catalyst A, three-dimensional reconstructed images (3D-STEM images) obtained by electron tomography measurement using a scanning transmission electron microscope (STEM) were analyzed to separate the Pt catalyst particles present inside the carbon support (hereinafter referred to as internal particles) and the Pt catalyst particles present on the surface of the carbon support (hereinafter referred to as external particles), and the particle size distribution of the Pt catalyst particles in each region was calculated. A three-dimensional reconstruction image (3D-STEM image) of Pt / C catalyst A is shown in Figure 12. FIG. 13 is a graph showing the distribution of Pt catalyst particles located outside and inside the nanopores of the carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1 shown in FIG. 12. FIG. 14 shows a graph illustrating the distribution of Pt catalyst particles located inside the nanopores of the carbon support in the depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1 shown in FIG. 12. Furthermore, FIG. 15 shows another graph showing the distribution of Pt catalyst particles located inside the nanopores of the carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1 shown in FIG. 12.

[0123] The 3D-STEM image was obtained by reconstructing multiple 2D STEM images obtained by tilting the sample stage stepwise under the above measurement conditions. Image analysis (particle size analysis) of the three-dimensional reconstructed image (3D-STEM image) was performed using the following procedure. First, an observation area 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 particle was determined, and the diameter of a sphere with the same volume (sphere-equivalent diameter) was calculated to determine the particle size distribution (Figures 13, 14, and 15).

[0124] Here, the sphere-equivalent diameter was calculated in units of nm, with decimal values ​​(values ​​less than 1 nm) rounded off. For this Pt / C catalyst A, the proportion of catalyst particles supported inside the nanopores of the support and the proportion of catalyst particles supported outside the nanopores of the support were determined. The N10, N20, and N30 values ​​were also determined. The results are shown in Tables 1 and 2. The presence of the above-mentioned (II) noble metal particles was not confirmed in the electron beam tomography measurement of the electrode catalyst of Example 1. In other words, there were no noble metal particles that were not in contact with the hollow carbon support itself and were present outside it (n102 = 0). Furthermore, the average particle diameter of the catalyst particles of Pt / C catalyst A measured from the STEM image was 3.1 nm (average particle diameter of catalyst particles inside the nanopores: 3.1 nm, average particle diameter of catalyst particles outside the nanopores: 3.2 nm).

[0125] (2) Preparation of Pt / C catalyst used in the cathode of the MEA of Example 2 [Pt catalyst particle-supported carbon catalyst "Pt / C catalyst" powder] The hollow carbon support was manufactured by Toyo Tanso Co., Ltd., prototype product name: "CNovel B" (BET specific surface area: 800 m 2 A powder of a Pt / C catalyst (hereinafter referred to as "Pt / C catalyst B" as needed) used in the cathode of the MEA of Example 2 (Pt loading rate 48.0 wt%, trade name "SA50BM-B237", manufactured by NECHEMCAT) was prepared under the same conditions and procedures as those of Pt / C catalyst A used in the cathode of the MEA of Example 1, except that Pt / C catalyst A (48.0 wt % Pt / g) was used.

[0126] <Surface and structural observation of electrode catalysts> For Pt / C catalyst B of Example 2, in order to observe its three-dimensional structure using the same method and conditions as for the Pt / C catalyst of Example 1, electron tomography measurements were carried out using a STEM (scanning transmission electron microscope) at UBE Scientific Analysis Center, Inc., using the "USAL-KM3D analysis method." FIG. 16 shows STEM images showing the 3D electron tomography measurement conditions (volume size) using a STEM for the electrode catalyst of Example 2. FIG. 17 shows a 3D-STEM image (three-dimensional reconstruction image) of the electrode catalyst of Example 2. FIG. 18 shows nanopores in the carbon support obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 2 shown in FIG. External and internal 10 is a graph showing the distribution of Pt catalyst particles located in the pore depth direction. FIG. 19 shows nanopores in the carbon support obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 2 shown in FIG. internal 10 is a graph showing the distribution of Pt catalyst particles located in the pore depth direction. FIG. 20 shows nanopores in the carbon support obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 2 shown in FIG. internal 10 shows another graph illustrating the distribution of Pt catalyst particles located in the pore depth direction.

[0127] For this electrode catalyst (Pt / C catalyst B), the ratio of catalyst particles supported inside the nanopores of the support and the ratio of catalyst particles supported outside the nanopores of the support were determined. The N10, N20, and N30 values ​​were also determined. The results are shown in Tables 1 and 2. The presence of the above-mentioned (II) noble metal particles was not confirmed in the electron beam tomography measurement of the electrode catalyst of Example 2. That is, there were no noble metal particles that were not in contact with the hollow carbon support itself and were present outside it (n102 = 0). Furthermore, the average particle diameter of the catalyst particles of the electrode catalyst (Pt / C catalyst B) measured from the STEM image was 3.3 nm (average particle diameter of catalyst particles inside the nanopores: 3.2 nm, average particle diameter of catalyst particles outside the nanopores: 3.7 nm).

[0128] (3) Preparation of Pt / C catalyst powder for use in the cathode of the MEA of Comparative Example 1 A Pt / C catalyst with a Pt loading rate of 50 wt% manufactured by NECHEMCAT (product name: "SA50BK") was prepared as the Pt / C catalyst. The support for this Pt / C catalyst was a commercially available hollow carbon support (manufactured by Lion Corporation, product name "Carbon ECP" (registered trademark) (Ketjenblack EC300J), specific surface area 750 to 800 m). 2 / g} was used.

[0129] <Surface and structural observation of electrode catalysts> For the Pt / C catalyst of Comparative Example 1, in order to observe its three-dimensional structure, electron tomography measurements using a STEM (scanning transmission electron microscope) were carried out at UBE Scientific Analysis Center, Inc. using the "USAL-KM3D analysis method" under the same method and conditions as for the Pt / C catalyst of Example 1. FIG. 21 shows STEM images showing the 3D electron tomography measurement conditions (volume size) using STEM for the Pt / C catalyst of Comparative Example 1. FIG. 22 shows a 3D-STEM image (three-dimensional reconstruction image) of the Pt / C catalyst of Comparative Example 1. FIG. 23 shows a graph illustrating the distribution of Pt catalyst particles located outside and inside the nanopores of the carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Comparative Example 1 shown in FIG. 22. FIG. 24 shows a graph illustrating the distribution of Pt catalyst particles located inside the nanopores of the carbon support in the depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Comparative Example 1 shown in FIG. 22. Furthermore, Figure 25 shows another graph showing the distribution state of Pt catalyst particles located inside the nanopores of the carbon support in the pore depth direction, obtained by image analysis of the 3D-STEM image of the electrode catalyst of Comparative Example 1 shown in Figure 22.

[0130] For this Pt / C catalyst, the ratio of catalyst particles supported inside the nanopores of the support and the ratio of catalyst particles supported outside the nanopores of the support were determined. The N10, N20, and N30 values ​​were also determined. The results are shown in Tables 1 and 2. The presence of the above-mentioned (II) noble metal particles was not confirmed in the electron beam tomography measurement of the electrode catalyst of Comparative Example 1. In other words, there were no noble metal particles that were not in contact with the hollow carbon support itself and were present outside it (n102 = 0). Furthermore, the average particle size of the Pt / C catalyst measured from the STEM image was 3.1 nm (average particle size of catalyst particles inside the nanopores: 3.1 nm, average particle size of catalyst particles outside the nanopores: 3.2 nm).

[0131] (4) Preparation of Pt / C catalyst used in the cathode of the MEA of Comparative Example 2 [Pt catalyst particle-supported carbon catalyst "Pt / C catalyst" powder] The hollow carbon support was manufactured by Toyo Tanso Co., Ltd., prototype product name: "CNovel C" (BET specific surface area: 800 m 2 A powder of a Pt / C catalyst (hereinafter referred to as "Pt / C catalyst C" as needed) used in the cathode of an MEA in Example 2 (Pt loading rate 48.0 wt%, trade name "SA50BM-C207", manufactured by NECHEMCAT) was prepared under the same conditions and procedures as those for Pt / C catalyst A used in the cathode of an MEA in Example 1, except that 48.0 wt % Pt / C catalyst A was used in the cathode of an MEA in Example 2.

[0132] <Surface and structural observation of electrode catalysts> For the Pt / C catalyst C of Comparative Example 2, in order to observe its three-dimensional structure using the same method and conditions as the Pt / C catalyst of Example 1, electron tomography measurements were carried out using a STEM (scanning transmission electron microscope) at UBE Scientific Analysis Center, Inc., using the "USAL-KM3D analysis method." As with Examples 1, 2, and Comparative Example 1, information corresponding to the electron tomography measurements and analysis results using a STEM (scanning transmission electron microscope) was also obtained for this Comparative Example 2 electrode catalyst (Pt / C catalyst C), but is not shown in the figures. FIG. 26 shows a STEM image showing the 3D electron tomography measurement conditions (volume size) using STEM for the Pt / C catalyst of Comparative Example 2.

[0133] For this electrode catalyst (Pt / C catalyst C) of Comparative Example 2, the proportion of catalyst particles supported inside the nanopores of the support and the proportion of catalyst particles supported outside the nanopores of the support were determined. The values ​​of D10, D20, D1, D2, N1, and N2 were also determined. The results are shown in Tables 2 and 3. Furthermore, the average particle diameter of the catalyst particles in the electrode catalyst (Pt / C catalyst C) measured from the STEM image was 3.2 nm (average particle diameter of catalyst particles inside the nanopores: 2.9 nm, average particle diameter of catalyst particles outside the nanopores: 3.5 nm).

[0134] (5) Confirmation of the microstructure of the electrode catalysts of Example 1, Example 2, Comparative Example 1, and Comparative Example 2 using three-dimensional reconstructed STEM images The electrode catalysts of Examples 1, 2, and Comparative Example 2 were examined using three-dimensional reconstructed STEM images to confirm the aforementioned condition (β), "whether the nanopores are formed to have the shape of the continuous pores according to the present invention." The porosity of each catalyst was also determined using the three-dimensional reconstructed STEM images. The results are shown in Figures 27 and 28.

[0135] 27(c) and 28, the nanopore P1 visible in the cross section of interest (the square xy plane) of the six square cross sections of the three-dimensional image of the interior of the catalyst mass of Example 1 for the electrode catalyst has two openings (opening A11 and opening A12) that contact the first side L1. This nanopore P1 also has two openings (opening A21 and opening A22) that contact the second side L2 that is parallel to the first side L1. Furthermore, this nanopore P1 has the shape of a communicating hole P1 that extends continuously without being blocked 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).

[0136] Furthermore, as shown in FIG. 27(c) and FIG. 28, the nanopores P1 inside the electrode catalyst of Example 1 have a shape that is branched into multiple parts. Furthermore, as shown in FIG. 27(c) and FIG. 28, 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. Furthermore, as shown in FIGS. 27(c) and 28, 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 FIG. 27(c) and FIG. 28, the nanopore P1 inside the electrode catalyst of Example 1 also has one opening (opening A31) on a third side L3 perpendicular to the first side L1. Furthermore, as shown in Figures 27(c) and 28, the nanopore P1 inside the electrode catalyst of Example 1 also has two openings (opening A41 and opening A42) on a fourth side L4 perpendicular to the first side L1.

[0137] As shown in Figures 27(d) and 27(e), it was confirmed that the nanopores P1 of the electrode catalyst of Example 1 also have the same shape as described above on the other two planes (yz plane, zx plane) of the three-dimensional image inside the catalyst mass. Furthermore, as shown in Figures 27(h), 27(i) and 27(j), it was confirmed that the electrode catalyst of Example 2 also had nanopores P22 formed having the shape of the communicating pores P1 according to the present invention, similar to the electrode catalyst of Example 1. 27(m), 27(n), and 27(o), it was confirmed that nanopores P22 having the shape of the communicating pores P1 according to the present invention were not formed in the electrode catalyst of Comparative Example 1. The nanopores P1 visible in the cross sections (xy plane, yz plane, zx plane) of the three-dimensional image of the interior of the catalyst block of the electrode catalyst of Comparative Example 1 have one opening in contact with the first side and one opening in contact with the second side parallel to the first side, but do not have the shape of communicating pores P1 that extend continuously without being blocked from the opening on the first side to the opening on the second side.

[0138] (II) Preparation of P / C catalysts used in the anodes of MEAs in Example 1, Example 2, Comparative Example 1, and Comparative Example 2 The same Pt / C catalyst as that used in the cathode of the MEA of Comparative Example 1 was used as the Pt / C catalyst in the anode of the MEAs of Example 1, Example 2, Comparative Example 1, and Comparative Example 2.

[0139] Example 1 An MEA having a configuration similar to that of the MEA 10 shown in FIG. 1 was fabricated by the following procedure.

[0140] (1) Creating the cathode Cathode GDL Carbon paper (manufactured by Toray Industries, Inc., product name "TGP-H-60") was prepared as the GDL. Ink for forming cathode MPL 1.5 g of carbon powder (trade name "Denka Black" manufactured by Denki Kagaku Kogyo Kabushiki Kaisha), 1.1 g of ion-exchanged water, and 6.0 g of surfactant (trade name "Triton" (35 wt % aqueous solution) manufactured by Dow Chemical Company) were placed in a Teflon (registered trademark) ball mill container containing Teflon (registered trademark) balls and mixed. Next, 1.75 g of polytetrafluoroethylene (PTFE) dispersion (manufactured by DuPont-Mitsui Fluorochemicals, product name "31-JR") was added to the ball mill container and mixed, thereby producing an ink for forming the MPL of the cathode. Cathode MPL The cathode MPL ink was applied to one side of the GDL using a bar coder to form a coating film, which was then thoroughly dried in a dryer and further subjected to a heat-pressing process to form a laminate with the MPL formed on the GDL. Ink for forming the cathode catalyst layer The above-mentioned Pt / C catalyst A, ion-exchanged water, 10 wt% Nafion aqueous dispersion (manufactured by DuPont under the trade name "DE1021CS"), and glycerin were mixed in a Teflon ball mill container containing Teflon balls to prepare an ink for forming a cathode catalyst layer. The N / C ratio for this ink was 0.7. The carbon:ion-exchanged water:glycerin ratio in the electrode catalyst A was 1:10:0.8 (mass ratio). Cathode catalyst layer (CL) The ink for forming the cathode catalyst layer was applied to the surface of the MPL of the laminate formed by the MPL on the GDL by bar coating to form a coating film. This coating film was dried at room temperature for 30 minutes, and then dried at 60 ° C for 1.0 hour to form a catalyst layer. In this way, a cathode, which is a gas diffusion electrode, was prepared. The Pt loading of the cathode catalyst layer was adjusted to the values ​​shown in Table 1.

[0141] (2) Creating the anode Anode GDL The GDL was the same carbon paper as the cathode. Ink for forming anode MPL 1.5 g of carbon powder (trade name "Denka Black" manufactured by Denki Kagaku Kogyo Kabushiki Kaisha), 1.0 g of ion-exchanged water, and 6.0 g of surfactant (trade name "Triton" (35 wt % aqueous solution) manufactured by Dow Chemical Company) were placed in a Teflon (registered trademark) ball mill container containing Teflon (registered trademark) balls and mixed. Next, 2.5 g of polytetrafluoroethylene (PTFE) dispersion (manufactured by DuPont-Mitsui Fluorochemicals, product name "31-JR") was added to the ball mill container and mixed, thereby preparing an ink for forming the MPL for the anode. Anode MPL The ink for forming the anode MPL was applied to one side of the GDL using a bar coder to form a coating film. The coating film was then thoroughly dried in a dryer and further subjected to a heat-pressing process to create a laminate with the MPL formed on the GDL. Ink for forming the anode catalyst layer SA50BK (Pt loading: 50 wt%), ion-exchanged water, a 5 wt% Nafion alcohol dispersion (manufactured by SIGMA-ALDRICH under the trade name "Nafion 5 wt.% dispersion", product number "274704"), and glycerin were mixed in a Teflon ball mill container containing Teflon balls to prepare an ink for forming the anode catalyst layer. The N / C ratio for this ink was 1.2. The mass ratio of carbon in the SA50BK to ion-exchanged water to glycerin was 1:6:4. Anode catalyst layer (CL) The ink for forming the anode catalyst layer was applied to the surface of the MPL of the laminate formed by the above-mentioned GDL and MPL by bar coating to form a coating film. This coating film was dried at room temperature for 30 minutes, and then dried at 60°C for 1.0 hour to form a catalyst layer. In this way, an anode, which is a gas diffusion electrode, was produced. The Pt loading in the anode catalyst layer was 0.3 mg / cm. 2 It was decided.

[0142] (3) Creation of MEA A polymer electrolyte membrane (trade name "Nafion NR212" manufactured by DuPont) was prepared. This polymer electrolyte membrane was placed between a cathode and an anode to form a laminate, which was then thermocompressed using a hot press to form an MEA. The thermocompression conditions were 140°C, 5 KN for 5 minutes, and then 140°C, 25 KN for 3 minutes.

[0143] <Example 2> Each MEA was prepared under the same conditions and procedures as in Example 1, except that the conditions for the cathode catalyst layer were changed as follows. That is, in the preparation of the ink for forming the cathode catalyst layer, Instead of Pt / C catalyst A, the previously mentioned Pt / C catalyst B was used.

[0144] <Comparative Example 1> Each MEA was prepared under the same conditions and procedures as in Example 1, except that the conditions for the cathode catalyst layer were changed as follows. That is, in the preparation of the ink for forming the cathode catalyst layer, Instead of Pt / C catalyst A, the previously mentioned P / C catalyst (product name: "SA-50BK") was used. Instead of the 10 wt% Nafion aqueous dispersion, a 5 wt% Nafion alcohol dispersion (manufactured by DuPont under the trade name "DE520CS"; containing 48 wt% 1-propanol) was used. The composition of the ink for forming the cathode catalyst layer and the coating conditions for the ink were adjusted so that the Pt loading and N / C would be the values ​​shown in Table 1. The mass ratio of carbon:ion-exchanged water:glycerin in the P / C catalyst (product name: "SA50BH") was 1:10:1.

[0145] <Comparative Example 2> Each MEA was prepared under the same conditions and procedures as in Example 1, except that the conditions for the cathode catalyst layer were changed as follows. That is, in the preparation of the ink for forming the cathode catalyst layer, Instead of Pt / C catalyst A, the previously mentioned Pt / C catalyst C was used.

[0146] <Battery performance evaluation> The cell performance of the MEAs of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 was evaluated by the following cell performance evaluation method. The MEAs of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 were placed in a single fuel cell evaluation device. Next, the power generation reaction was allowed to proceed within the MEA under the following conditions. The single cell (MEA) temperature was set to 80°C. Pure hydrogen at 1.0 atmospheres, humidified with saturated steam, was supplied to the anode with the flow rate adjusted to achieve a utilization rate of 70%. Pure oxygen at 1.0 atmospheres, humidified with saturated steam at 80°C, was supplied to the cathode with the flow rate adjusted to achieve a utilization rate of 50%. The evaluation of the single cell (MEA) was carried out by controlling the current using an electronic load device attached to the fuel cell single cell evaluation device, with the current value being 0 to 1.0 A / cm 2 The current-voltage curve obtained by scanning up to 1000 kJ / s was obtained as data. A graph (not shown) was created by plotting the data from the current-voltage curve with the X axis (current density) on a logarithmic scale, and the current density value (current value per unit area of ​​the electrode) at a voltage of 850 mV was obtained.

[0147] The current density value thus obtained was divided by the platinum weight per unit area of ​​the cathode to calculate the activity per unit weight (Mass Act.) of the platinum contained in the cathode, which was used as an index of the oxygen reduction ability of the catalyst contained in the cathode. The results are shown in Table 1. Table 1 shows the results of comparing the Mass. Act. values ​​obtained in other Examples as relative values ​​(relative ratios) with the Mass. Act. value obtained in Comparative Example 1 as the reference (1.0).

[0148] [Table 1] [Table 2]

[0149] The results shown in Tables 1 and 2 reveal that the MEAs of Examples and 2 have higher Pt mass activity than the MEAs of Comparative Examples 1 and 2. In the above examples and comparative examples, we have examined the simplest form of catalyst particles, which are catalyst particles made of Pt. However, a feature of the electrode catalyst of the present invention is that, when the microstructure is observed using a three-dimensional reconstructed STEM image, it has a three-dimensional structure that satisfies the previously mentioned conditions of formula (S1) (conditions regarding the support positions of the catalyst particles that constitute the electrode catalyst and the number of catalyst particles at those support positions) and (β) the condition that the nanopores are formed to have the above-mentioned interconnected pores (conditions regarding the microstructure of the nanopores in the support that constitute the electrode catalyst). Therefore, it is clear that similar results can be obtained even if the chemical components of the catalyst particles are changed. In other words, as long as the three-dimensional structure of the present invention is maintained, it is clear that the same excellent Pt mass activity as in the above examples can be obtained even if Pt alloy particles containing Pt or core-shell particles with a Pt shell layer are used as catalyst particles, as well as Pt particles. [Industrial Applicability]

[0150] The electrode catalyst of the present invention exhibits excellent catalytic activity, and the GDE, CCM, MEA, and fuel cell stack that include the catalyst layer of the present invention exhibit excellent cell characteristics that can contribute to cost reduction of PEFCs. 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 ENE-FARM, cogeneration systems, etc., and contributes to the development of the energy industry and environmental technology. [Explanation of symbols]

[0151] 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... Membrane / catalyst layer assembly (CCM) 20... Electrode catalyst (Pt / C catalyst), 22···Hollow carbon support (CNovel), 23···Catalytic particles, 25...Non-contact particles 30. Fuel cell stack, P1...P22 is a connected nanopore P22...Nanopores of the support, P24...Micropores of the support

Claims

1. The present invention includes a conductive hollow carbon support having nanopores with a pore size of 1 to 20 nm, and a plurality of catalyst particles supported on the support, a region made of Pt (zero valence) is formed on at least a portion of the surface of the catalyst particle, the catalyst particles are supported both inside and outside the nanopores of the support; When an analysis of the particle size distribution of the catalyst particles is performed using a three-dimensional reconstructed image obtained by electron tomography measurement using a STEM (scanning transmission electron microscope), the condition of the following formula (S1) is satisfied: Catalyst for electrodes. 100×(N10 / N20)≦8.0...(S1) [In the formula (S1), N10 represents the number of non-contact particles (n101+n102), which is the sum of the number of noble metal particles (n101) that are not in contact with pores having a pore diameter of 1 to 20 nm that can be confirmed by the electron beam tomography measurement and the number of noble metal particles (n102) that are not in contact with the hollow carbon support itself and exist outside it, N20 indicates the number of catalyst particles supported inside the nanopores of the support.

2. In the three-dimensional reconstructed image of the STEM, when focusing on a catalyst mass made up of the catalyst particles and the support, which has a size that can be accommodated in a rectangular parallelepiped space with a side of 60 to 300 nm, and when six square cross sections of a cubic image with a side of 20 to 50 nm extracted from an internal region of the catalyst mass are viewed, 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 contacting a first side of four sides of the square cross section and at least one opening contacting a second side of the square cross section that is parallel to the first side, and has the shape of a communicating hole that extends continuously without being blocked from the opening on the first side to the opening on the second side. The electrode catalyst according to claim 1 .

3. The electrode catalyst according to claim 2 , wherein the communicating pores have a shape branched into multiple pores.

4. The electrode catalyst according to claim 3 , wherein the communicating hole has two or more openings on the first side.

5. The electrode catalyst according to claim 3 or 4, wherein the communicating hole has two or more openings on the second side.

6. 6. The electrode catalyst according to claim 3, wherein the communicating hole has at least one opening on a third side perpendicular to the first side.

7. 7. The electrode catalyst according to claim 3, wherein the communicating hole has at least one opening on a fourth side perpendicular to the first side.

8. 8. The electrode catalyst according to claim 1, wherein the porosity measured using the three-dimensional reconstructed image of the STEM is 35% or more.

9. 9. The electrode catalyst according to claim 1, wherein the nanopores have a pore diameter of 1 to 10 nm.

10. 10. The electrode catalyst according to claim 1, wherein the hollow carbon support further has micropores with a pore diameter of less than 1 nm.

11. 11. The electrode catalyst according to claim 1, wherein the catalyst particles are made of Pt (zero valence).

12. 11. The electrode catalyst according to claim 1, wherein the catalyst particles are made of a Pt alloy.

13. the catalyst particles are core-shell catalyst particles, and a core particle and a Pt shell layer as a region made of Pt (zero valence) are formed on at least a part of the surface of the core particle; The electrode catalyst according to any one of claims 1 to 12.

14. When an analysis of the particle size distribution of the catalyst particles is performed using the three-dimensional reconstructed image of the STEM, the condition of the following formula (S1) is satisfied: The electrode catalyst according to any one of claims 1 to 13. 100×{N10 / (N20+N30)}≦5.0...(S2) [In the above formula (S2), N10 has the same meaning as N10 in formula (S1). N20 has the same meaning as N10 in formula (S1), N30 indicates the number of catalyst particles supported outside the nanopores of the support.

15. When an analysis of the particle size distribution of the catalyst particles is performed using the three-dimensional reconstructed image of the STEM, For the catalyst particles supported inside the nanopores of the support, The average distance from the entrance of the nanopore to the supporting position of the catalyst particle is 5.0 nm or more. The electrode catalyst according to any one of claims 1 to 14.

16. When an analysis of the particle size distribution of the catalyst particles is performed using the three-dimensional reconstructed image of the STEM, The catalyst particles supported inside the nanopores of the support are present at a distance from the entrance of the nanopore to the supporting position of the catalyst particles in a range of 0 to 27 nm. The electrode catalyst according to any one of claims 1 to 15.

17. When an analysis of the particle size distribution of the catalyst particles is performed using the three-dimensional reconstructed image of the STEM, The particle size of the catalyst particles supported inside the nanopores of the support is greater than 0 nm and not greater than 7 nm. The electrode catalyst according to any one of claims 1 to 16.

18. When a particle size distribution of the catalyst particles is analyzed using a three-dimensional reconstructed image of the STEM, the proportion of the catalyst particles supported inside the nanopores is 50% or more. The electrode catalyst according to any one of claims 1 to 17.

19. When a particle size distribution of the catalyst particles is analyzed using a three-dimensional reconstructed image of the STEM, the proportion of the catalyst particles supported inside the nanopores is 70% or more. The electrode catalyst according to claim 18.

20. At least a portion of the region consisting of Pt (zero valence) on the surface of the catalyst particle is covered with a Pt oxide coating. The electrode catalyst according to any one of claims 1 to 19.

21. The hollow carbon support has a BET specific surface area (nitrogen adsorption specific surface area) of 200 to 1500 m ２ The electrode catalyst according to any one of claims 1 to 20, wherein the average molecular weight of the electrode catalyst is 1 / g.

22. 22. An electrode catalyst powder containing 10 wt % or more of the electrode catalyst according to claim 1.

23. A composition for forming a gas diffusion electrode, comprising the electrode catalyst according to any one of claims 1 to 21 or the electrode catalyst powder according to claim 22.

24. A gas diffusion electrode containing the electrode catalyst according to any one of claims 1 to 21 or the electrode catalyst powder according to claim 22.

25. 25. A membrane electrode assembly (MEA) comprising the gas diffusion electrode of claim 24.

26. 26. A fuel cell stack comprising the membrane electrode assembly (MEA) of claim 25.

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