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

Abstract

Provided is a catalyst for electrode that has excellent catalytic activity and that is capable of contributing toward lower PEFC costs. This catalyst for electrode includes: a hollow carbon support having nanopores with a pore diameter of 1-20 nm; and a plurality of catalyst particles supported on the support. The catalyst particles are supported both inside and outside the nanopores of the support, are composed of (zerovalent) Pt, and when analysis of the particle size distribution of the catalyst particles is performed using three-dimensional, reconstructed images obtained through STEM-based electron tomography measurement, the percentage of catalyst particles supported inside the nanopores is 50% or more.

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 (t-Pot surface area / BET surface area) 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) 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 the "nanopores" formed in the "primary particles" of the hollow carbon support, as defined in the academic paper "M. Uchida, et al., Phys. Chem. Chem. Phys., 2013, 15 (27), 11236-1124 (see, for example, Fig. 1)."

[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, they provide information on the number of catalyst particles supported outside the nanopores of the hollow carbon support particles. 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) in the observed Pt catalyst particles. They then attempted to distinguish between the 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] Here, 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" and the number of catalyst particles supported on the "opposite back surface" are the same. 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 axis or minor axis in the range of approximately 100 to 300 nm, see Figures 11 and 15 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 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 and 15 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 analyzed the particle size distribution of catalyst particles in a Pt / C catalyst using three-dimensional reconstructed images obtained by electron tomography measurement using a STEM (scanning transmission electron microscope). They found that there had been no previous reports of the synthesis of an improved product in which catalyst particles were supported in greater numbers inside the nanopores of the 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 (Pt / C catalyst) having excellent catalytic activity that can contribute to cost reduction of PEFCs. Another object of the present invention is to provide a composition for forming a gas diffusion electrode, a gas diffusion electrode, a membrane electrode assembly (MEA), and a fuel cell stack, each of which contains the electrode catalyst. [Means for solving the problem]

[0017] The present inventors have conducted extensive research into a configuration that will further improve the catalytic activity of an electrode catalyst in which a large number of Pt / C catalyst particles 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 provides: A conductive hollow carbon support having nanopores with a pore size of 1 to 20 nm and a 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 and outside the nanopores of the support; When a 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 50% or more. An electrode catalyst is provided.

[0019] As described above, by supporting Pt / C catalyst particles on a hollow carbon support so as to satisfy the condition that the proportion of catalyst particles supported inside the nanopores is 50% or more, the electrode catalyst of the present invention can exhibit excellent catalytic activity that can contribute to reducing the cost 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 that, compared to conventional electrode catalysts, a Pt / C catalyst in which 50% or more of the catalyst particles are supported inside the nanopores has a large number of highly active catalyst particles with relatively small particle diameters 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.

[0020] In the present invention, the "nanopores" of the primary particles of the hollow carbon support refer to the "nanopores" formed in the "primary particles" of the hollow carbon support, as defined in the academic paper "M. Uchida, et al., Phys. Chem. Chem. Phys., 2013, 15 (27), 11236-1124 (see, for example, Fig. 1)." In the present invention, the "pore diameter of a nanopore" refers to the "size of the entrance of the nanopore." Furthermore, in the present invention, the "pore diameter (pore entrance size)" of a nanopore refers to the size of the "nanopore entrance" determined by the "USAL-KM3D analysis method" described below. More specifically, the "nanopore entrance size" refers to the diameter (circle equivalent diameter) of a circle having the same area as the area of ​​the nanopore entrance determined from an image of the nanopore entrance determined by the "USAL-KM3D analysis method." In addition, in the present invention, "hollow carbon" refers to carbon that has more pores (voids) inside it than the solid carbon described above, and refers to conductive carbon that contains the above-mentioned nanopores as part of the pores.

[0021] Furthermore, from the viewpoint of more reliably achieving the effects of the present invention, it is preferable that the hollow carbon support contains a larger number of nanopores with a pore diameter (pore entrance size) 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 the 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 larger number of pores with a pore diameter (pore entrance size) of 1 to 10 nm, it becomes more difficult for the polymer electrolyte to penetrate into the nanopores, and contact between the catalyst particles supported inside the nanopores and the polymer electrolyte is more reliably prevented.

[0022] 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.

[0023] <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: (P1) Powder agglomerates of the sample to be measured (electrode catalyst) (agglomerates with major or minor diameters in the range of approximately 100 to 300 nm, see Figures 11 and 15 described below) are arranged 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)}. (P2) When the grid mesh is rotated around its axis of rotation by an angle of ±80°, the photographed image of the powder mass of the measurement sample must not overlap with the photographed image of other fine powder masses. If the photographed image of the powder mass of the measurement sample overlaps with the photographed image of other fine powder masses, 3D analysis cannot be performed. (P3) The powder lumps visible in the measurement area are positioned far enough apart to enable 3D tomography observation of the powder lumps 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).

[0024] Furthermore, in the electrode catalyst of the present invention, from the viewpoint of more 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), it is preferable that the proportion of the catalyst particles supported inside the nanopores is 70% or more. 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 (1) 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). (D10 / D20)≦0.75 (1) Here, in the formula (1), D10 represents the arithmetic mean value of the sphere-equivalent diameters of the catalyst particles supported inside the nanopores of the support, and D20 represents the arithmetic mean value of the sphere-equivalent diameters of the catalyst particles supported outside the nanopores of the support. By supporting catalyst particles on a hollow carbon support so as to simultaneously satisfy the conditions of the above formula (1), the electrode catalyst of the present invention can more reliably exhibit excellent catalytic activity that can contribute to cost reduction of PEFCs.

[0025] Furthermore, in order to more reliably obtain the effects of the present invention, it is more preferable that the electrode catalyst of the present invention further simultaneously satisfies the conditions of the following formulas (2) and (3) 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). D1≦D2 (2) (N1 / N2)>2.0 (3) Here, in the formulas (2) and (3), D1 represents the spherical equivalent diameter of the particle that exhibits the greatest frequency (maximum particle number) among the catalyst particles supported inside the nanopores of the support, and D2 represents the spherical equivalent diameter of the particle that exhibits the greatest frequency (maximum particle number) among the catalyst particles supported outside the nanopores of the support. In addition, in the formulas (2) and (3), N1 represents the frequency (number of particles) of the particles exhibiting the maximum frequency (maximum number of particles) among the catalyst particles supported inside the nanopores of the support. In the formulas (1) and (2), N2 represents the frequency (number of particles) of the particles exhibiting the maximum frequency (maximum number of particles) among the catalyst particles supported outside the nanopores of the support. By supporting catalyst particles on a hollow carbon support so as to simultaneously satisfy the conditions of the above-mentioned formula (2) and formula (3), the electrode catalyst of the present invention can more reliably exhibit excellent catalytic activity that can contribute to cost reduction of PEFCs.

[0026] Furthermore, in the electrode catalyst of the present invention, at least a portion of the surface of the catalyst particle may be coated 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. Furthermore, in the electrode catalyst of the present invention, from the viewpoint of the availability of the support and the cost of raw materials, it is preferable that the hollow carbon support is Ketjen Black EC300J. In this case, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support (Ketjen Black EC300J) is 750 to 850 m 2 / g, and from the viewpoint of obtaining better initial performance of the electrode, it is preferable that the2 / g is more preferred.

[0027] 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.

[0028] 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 of the support and outside the nanopores, 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 of the above-mentioned electrode catalyst of the present invention in the powder of the electrode catalyst 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.

[0029] 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.

[0030] 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. 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.

[0031] 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]

[0032] 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]

[0033] [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 particle size distribution (particle size distribution shown in equivalent sphere diameter) of catalyst particles supported inside nanopores of a support among catalyst particles obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1 shown in FIG. [Figure 14] FIG. 13 is a graph showing the particle size distribution (particle size distribution shown in sphere-equivalent diameter) of catalyst particles supported outside the nanopores of the carrier among the catalyst particles obtained by image analysis of the 3D-STEM image of the electrode catalyst of Example 1 shown in FIG. 12. [Figure 15] 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 16] 1 is a 3D-STEM image (three-dimensional reconstruction image) of the electrode catalyst of Comparative Example 1. [Figure 17] 17 is a graph showing the particle size distribution (particle size distribution shown in sphere-equivalent diameter) of catalyst particles supported inside nanopores of a support among catalyst particles obtained by image analysis of the 3D-STEM image of the electrode catalyst of Comparative Example 1 shown in FIG. [Figure 18] FIG. 17 is a graph showing the particle size distribution (particle size distribution shown in sphere-equivalent diameter) of catalyst particles supported outside the nanopores of the carrier among the catalyst particles obtained by image analysis of the 3D-STEM image of the electrode catalyst of Comparative Example 1 shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0034] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[0035] <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.

[0036] <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.

[0037] (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.

[0038] (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 FIG. 2 is a schematic cross-sectional view showing a preferred embodiment of an electrode catalyst (Pt / C catalyst) contained in at least one of the cathode catalyst layer 1c and the anode catalyst layer 2c of the MEA 10 shown 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.

[0039] 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. Here, the catalyst particles 23 are made of Pt (zero valence). However, a Pt oxide layer may be formed on the surface of the catalyst particles 23 as long as the effects of the present invention can be obtained. The electrode catalyst 20 preferably has an average crystallite size of 3 to 16.0 nm as measured by powder X-ray diffraction (XRD). Here, the catalyst particles 23 are made of Pt (zero valence). However, a Pt oxide layer may be formed on the surface of the catalyst particles as long as the effects of the present invention can be obtained. The electrode catalyst 20 preferably has a Pt loading rate of 5.6 to 66.5 wt %.

[0040] The support 22 is not particularly limited as long as it is a hollow carbon support that is conductive, has nanopores with a pore diameter of 1 to 20 nm, can support the catalyst particles 23, and has a relatively large surface area. 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.

[0041] Examples of hollow carbon supports include Ketjenblack EC300J and Ketjenblack EC600JD. Commercially available products include those under the trade names "Carbon EPC" and "Carbon EPC600JD" (manufactured by Lion Chemical Corporation, etc.). Detailed characteristics of Ketjenblack EC300J and Ketjenblack EC600JD are described, for example, in a document published online by the Functional Carbon Filler Research Group entitled "Characteristics and Applications of the Conductive Carbon Black 'Ketjenblack EC'." Other examples of hollow carbon supports include those under the trade name "MCND (Mesoporous Carbon Nano-Dendrite)" (manufactured by Nippon Steel & Sumikin Chemical Co., Ltd.) and those under the trade name "Black pearls 2000" (manufactured by Cabot Corporation). Here, from the viewpoint of more reliably obtaining the effects of the present invention, the hollow carbon support is preferably at least one of Ketjen Black EC300J and Ketjen Black EC600JD. In the case of Ketjen Black EC300J, from the same viewpoint, the BET specific surface area (nitrogen adsorption specific surface area) of the hollow carbon support measured using nitrogen is 750 to 850 m 2 / g is preferred.

[0042] 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. When electron beam tomography measurement using 3D-STEM is carried out, the electrode catalyst 20 simultaneously satisfies the conditions of the following formulas (1) to (3). (D10 / D20)≦0.75 (1) D1≦D2 (2) (N1 / N2)>1.0 (3) Here, in the formulas (1) to (3), D10 represents the arithmetic mean value of the spherical equivalent diameter of the catalyst particles 23 supported inside the nanopores P22 of the support 22.

[0043] Furthermore, D20 represents the arithmetic mean value of the spherical equivalent diameter of the catalyst particles 23 supported outside the nanopores P22 of the support 22. Furthermore, D1 represents the sphere-equivalent diameter (nm) of the particles showing the maximum frequency (maximum particle number) among the catalyst particles 23 supported inside the nanopores P22 of the support 22. D2 represents the particle spherical equivalent diameter showing the maximum frequency (maximum particle number) of the catalyst particles 23 supported outside the nanopores P22 of the support 22. Furthermore, N1 represents the frequency (number of particles) of particles exhibiting the maximum frequency (maximum number of particles) among the catalyst particles 23 supported inside the nanopores P22 of the support 22. Furthermore, N2 represents the frequency (number of particles) of particles exhibiting the maximum frequency (maximum number of particles) among the catalyst particles 23 supported outside the nanopores P22 of the support 22.

[0044] Compared to a conventional electrode catalyst 200, an electrode catalyst 20 that simultaneously satisfies the conditions of formulas (1) to (3) has a large number of highly active catalyst particles 23 with relatively small particle diameters present inside the nanopores P22 of the support 22. The catalyst particles 23 supported inside the nanopores P22 of such a support 22 exhibit superior catalytic activity when formed into an electrode compared to a conventional electrode catalyst 200. In addition, the catalyst particles 23 are supported on the support 22 in a state where they are less likely to come into direct contact with the polymer electrolyte such as Nafion contained in the catalyst layer (catalyst layer 1c or catalyst layer 2c), which reduces dissolution of the Pt component.

[0045] Furthermore, the catalyst layer (catalyst layer 1c or catalyst layer 2c) may further contain another electrode catalyst (not shown) in addition to the electrode catalyst 20 according to the present invention. For example, the catalyst layer (catalyst layer 1c or catalyst layer 2c) may further contain the above-mentioned "electrode catalyst P" as another electrode catalyst. In this case, from the viewpoint of more reliably obtaining the effects of the present invention, the content ratio of the electrode catalyst 20 according to the present invention to the mass of all constituent materials in the catalyst layer (catalyst layer 1c or catalyst layer 2c) is preferably 10 wt% or more. Furthermore, from the viewpoint of more reliably obtaining the effects of the present invention, the content ratio of the electrode catalyst 20 according to the present invention to the mass of all constituent materials in the catalyst layer (catalyst layer 1c or catalyst layer 2c) 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. In this case, from the viewpoint of more reliably obtaining the effects of the present invention, the content ratio of the electrode catalyst 20 according to the present invention to the mass of all the electrode catalysts in the catalyst layer (catalyst layer 1c or catalyst layer 2c) is preferably 10 wt% or more. From the same viewpoint, the content ratio of the electrode catalyst 20 according to the present invention to the mass of all the electrode catalysts in the catalyst layer (catalyst layer 1c or catalyst layer 2c) 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.

[0046] 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 conditions of formulas (1) to (3). In the carrier pretreatment step, the carrier 22 is placed in deionized water (preferably deionized water with an electrical conductivity of 1 μS / cm or less, or more preferably "ultrapure water" as described below), and a pH adjuster is further added to prepare a dispersion with a pH adjusted to 9 to 13. 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 is not boiled). The dispersion is then 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.

[0047] 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."

[0048] 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).

[0049] The step following the "carrier pretreatment step" is the "Pt addition step." In this "Pt addition step," an aqueous solution in which a water-soluble Pt salt is 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 alkaline 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.

[0050] 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.

[0051] 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.

[0052] (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.

[0053] 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 smoothly and reach the catalyst layer. For this reason, the gas diffusion layer preferably has water repellency. For example, the gas diffusion layer contains a water repellent component such as polyethylene terephthalate (PTFE). The material that can be used for the gas diffusion layer (1gd) is not particularly limited, and known materials can be used. For example, preferred examples include carbon paper and carbon paper that is the main raw material and is coated with a secondary raw material that includes optional components such as carbon powder, ion-exchanged water, and polyethylene terephthalate dispersion as a binder.

[0054] (Water-repellent layer (MPL)) As shown in Fig. 1, the cathode 1 has a water-repellent layer (MPL) 1m disposed between the gas diffusion layer 1gd and the catalyst layer 1c. The water-repellent layer 1m has electronic conductivity, water repellency, and gas diffusivity, and is provided to promote the diffusion of the oxidant gas into the catalyst layer 1c and the discharge of the reaction product water generated in the catalyst layer 1c. The configuration of the water-repellent layer 1m is not particularly limited, and any known configuration can be adopted.

[0055] (Polymer electrolyte membrane (PEM)) 1 is not particularly limited as long as it has hydrogen ion conductivity, and any known PEM conventionally used in PEFCs can be used. For example, it may be a membrane containing, as a constituent component, any of the polymer electrolytes exemplified above for the catalyst layers 1c and 2c.

[0056] <Modified forms of MEA> As described above, the preferred embodiments of the MEA of the present invention (and the catalyst layer and the gas diffusion electrode of the present invention) have been described. However, the MEA of the present invention is not limited to the configuration of MEA10 shown in FIG. 1. For example, the MEA of the present invention may have the configuration of MEA11 shown in FIG. 4. FIG. 4 is a schematic cross-sectional view showing another preferred form of the MEA of the present invention. The MEA11 shown in FIG. 4 has a configuration in which a gas diffusion electrode (GDE) 1A having the same configuration as the cathode 1 in the MEA10 shown in FIG. 1 is disposed only on one side of the polymer electrolyte membrane (PEM) 3. However, the catalyst layer 1c of the gas diffusion electrode (GDE) 1A has the configuration of the catalyst layer of the present invention. That is, in the catalyst layer 1c of GDE1A, the mass ratio N / C of the mass C of the carrier 22 of the electrode catalyst 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0057] <Membrane-catalyst layer assembly (CCM)> Next, the preferred embodiments of the membrane-catalyst layer assembly (CCM) of the present invention will be described. FIG. 5 is a schematic cross-sectional view showing a preferred form of the CCM of the present invention. The CCM12 shown in FIG. 5 has a configuration in which a polymer electrolyte membrane (PEM) 3 is disposed between the cathode catalyst layer 1c and the anode catalyst layer 2c. And at least one of the cathode catalyst layer 1c and the anode catalyst layer 2c has the configuration of the catalyst layer of the present invention. That is, in at least one of the cathode catalyst layer 1c and the anode catalyst layer 2c, the mass ratio N / C of the mass C of the carrier of the electrode catalyst 20 to the mass N of the polymer electrolyte is 0.5 to 1.2, more preferably 0.7 to 1.0.

[0058] <Modified forms of membrane-catalyst layer assembly (CCM)> As described above, the preferred embodiments of the CCM of the present invention have been described. However, the CCM of the present invention is not limited to the configuration of CCM12 shown in FIG. 5. For example, the CCM of the present invention may have the configuration of CCM13 shown in FIG. 6. Fig. 7 is a schematic cross-sectional view showing another preferred embodiment of the CCM of the present invention. The 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 of 0.5 to 1.2, more preferably 0.7 to 1.0.

[0059] <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.

[0060] <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.

[0061] <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.

[0062] 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.

[0063] (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.

[0064] <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]

[0065] 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.

[0066] (I) Preparation of electrode catalyst for use in the cathode catalyst layer of the MEA

[0067] (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 49.0 wt%, trade name "SA50wBK", ​​manufactured by NECHEMCAT) in which catalyst particles made of Pt were supported on carbon black powder was prepared. This Pt / C catalyst powder (hereinafter referred to as "Pt / C catalyst A" as necessary) was prepared by the following procedure.

[0068] (First step (carrier pretreatment step)) A commercially available hollow carbon support (manufactured by Lion Corporation, trade name "Carbon ECP" (registered trademark) (Ketjenblack EC300J), specific surface area 750-800 m) 2 The dispersion was prepared by dispersing {1 / g} in an aqueous solution (prepared by adding a pH adjuster to ultrapure water) adjusted to pH 9 to 13, and the temperature was kept at 90 to 99°C for about 0.5 hours while stirring (however, the dispersion 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).

[0069] (2nd step (Pt addition step)) A mixed solution was prepared by adding an aqueous solution of water-soluble Pt salt 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.

[0070] (Third step (reduction step)) An aqueous solution containing an alkaline 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 powder of Pt catalyst particle-supported carbon "Pt / C."

[0071] (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.

[0072] (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.

[0073] (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.

[0074] <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 49.0 wt %.

[0075] <Surface and structural observation of electrode catalysts> To observe the three-dimensional structure of this Pt / C catalyst, electron tomography measurements were performed using a scanning transmission electron microscope (STEM) at UBE Scientific Analysis Center, Inc., using the "USAL-KM3D analysis method." Electron tomography measurements using a STEM (scanning transmission electron microscope) were carried out to meet the previously mentioned sample preparation methods and conditions (P1) to (P3) and measurement conditions. More detailed information is provided below.

[0076] ·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.

[0077] 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. The particle size analysis results (particle size distribution shown in spherical equivalent diameter) of catalyst particles loaded inside the nanopores of the support and catalyst particles loaded outside the nanopores obtained by image analysis are shown in Figures 13 and 14.

[0078] 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 and 14).

[0079] 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 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 of Pt / C catalyst A measured from the STEM image was 2.1 nm (average particle diameter of catalyst particles inside the nanopores: 1.7 nm, average particle diameter of catalyst particles outside the nanopores: 3.4 nm).

[0080] (2) 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.

[0081] <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. 15 shows STEM images showing the 3D electron tomography measurement conditions (volume size) using STEM for the Pt / C catalyst of Comparative Example 1. FIG. 16 shows a 3D-STEM image (three-dimensional reconstruction image) of the Pt / C catalyst of Comparative Example 1. Figure 17 shows a graph showing the particle size distribution (particle size distribution shown in equivalent sphere diameter) of catalyst particles supported inside the nanopores of the support among the catalyst particles obtained by image analysis of the 3D-STEM image of the Pt / C catalyst of Comparative Example 1 shown in Figure 16. FIG. 18 shows a graph showing the particle size distribution (particle size distribution shown in spherical equivalent diameter) of catalyst particles supported outside the nanopores of the carrier among the catalyst particles obtained by image analysis of the 3D-STEM image of the Pt / C catalyst of Comparative Example 1 shown in FIG. 16 .

[0082] For this Pt / C catalyst, the ratio of catalyst particles supported inside the nanopores of the support and the ratio of catalyst particles supported outside the nanopores of the support were determined. The values ​​of D10, D20, D1, D2, N1, and N2 were also determined. The results are shown in Tables 2 and 3. Furthermore, the average particle diameter of the Pt / C catalyst 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).

[0083] (II) Preparation of P / C catalyst used in the anode of MEAs in Example 1 and Comparative Example 1 The same Pt / C catalyst as Pt / C catalyst B used in the cathode of the MEA of Comparative Example 1 was used as the Pt / C catalyst for the anode of the MEA of Example 1 and Comparative Example 1.

[0084] Example 1 An MEA having a configuration similar to that of the MEA 10 shown in FIG. 1 was fabricated by the following procedure.

[0085] (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.

[0086] (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.

[0087] (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.

[0088] <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.

[0089] <Battery performance evaluation> The cell performance of the MEAs of Example 1 and Comparative Example 1 was evaluated by the following cell performance evaluation method. The MEAs of Example 1 and Comparative Example 1 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.

[0090] 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 and Comparative Examples as relative values ​​(relative ratios) with the Mass. Act. value obtained in Comparative Example 1 set as the standard (1.0).

[0091] [Table 1] [Table 2] [Table 3]

[0092] The results shown in Tables 1 to 3 reveal that the MEA of Example 1 has higher Pt mass activity than the MEA of Comparative Example 1. [Industrial Applicability]

[0093] 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]

[0094] 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...Pt / C catalyst, 22... Carrier, 23···Catalytic particles, 30. Fuel cell stack, P22···Nanopores 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, the catalyst particles are made of Pt (zero valence), the catalyst particles are supported both inside and outside the nanopores of the support; When a 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 50% or more, and the condition of the following formula (1) is satisfied: Catalyst for electrodes. (D10 / D20)≦0.75...(1) [In the formula (1), D10 represents the arithmetic mean of the spherical equivalent diameters of the catalyst particles supported inside the nanopores of the support, D20 represents the arithmetic mean of the spherical equivalent diameter of the catalyst particles supported outside the nanopores of the support.

2. When a 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 70% or more. The electrode catalyst according to claim 1 .

3. 2. The electrode catalyst according to claim 1, wherein, when an analysis of the particle size distribution of the catalyst particles is carried out using a three-dimensional reconstructed image obtained by electron beam tomography measurement using an STEM (scanning transmission electron microscope), in addition to the condition of formula (1), the conditions of the following formulas (2) and (3) are further simultaneously satisfied: D1≦D2 (2) (N1 / N2)>2.0...(3) [In the formula (2) and the formula (3), D1 represents the sphere-equivalent diameter of the particle exhibiting the highest frequency among the catalyst particles supported inside the nanopores of the support, D2 represents the particle equivalent sphere diameter exhibiting the maximum frequency among the catalyst particles supported outside the nanopores of the support, N1 represents the frequency of the particles having the highest frequency among the catalyst particles supported inside the nanopores of the support; N2 represents the frequency of the particles having the highest frequency among the catalyst particles supported outside the nanopores of the support.

4. At least a portion of the surface of the catalyst particle is coated with a Pt oxide coating. The electrode catalyst according to any one of claims 1 to 3.

5. The hollow carbon support has a BET specific surface area (nitrogen adsorption specific surface area) of 200 to 1500 m 2 / g; The electrode catalyst according to any one of claims 1 to 4.

6. The hollow carbon support is Ketjen black. The electrode catalyst according to any one of claims 1 to 5.

7. An electrode catalyst according to any one of claims 1 to 6, wherein the electrode catalyst is contained in an amount of 10 wt % or more. Electrode catalyst powder.

8. A composition for forming a gas diffusion electrode, comprising the electrode catalyst according to any one of claims 1 to 7.

9. A gas diffusion electrode comprising the electrode catalyst according to any one of claims 1 to 7.

10. A membrane electrode assembly (MEA) comprising the gas diffusion electrode of claim 7.

11. A fuel cell stack comprising the membrane electrode assembly (MEA) of claim 8.

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