Electrode material and method for manufacturing the same, as well as electrode, membrane electrode assembly and solid polymer fuel cell.

Abstract

This invention provides an electrode material that offers a fuel cell electrode capable of achieving both high activity and voltage cycle durability. [Solution] An electrode material comprising a porous carbon carrier and a catalyst composite supported on the porous carbon carrier, wherein the catalyst composite comprises a Pt-rich phase and a Ta-rich phase composed of Ta oxide. The Ta-rich phase has a structure interposed in the gaps of the Pt-rich phase, thereby suppressing aggregation of the Pt-rich phase, and thus an electrode using this electrode material has high activity and potential cycle durability.

Description

[Technical Field]

[0001] The present invention relates to an electrode material suitable for electrodes in polymer electrolyte fuel cells, and to an electrode, a membrane electrode assembly, and a polymer electrolyte fuel cell using the same. [Background technology]

[0002] Polymer electrolyte fuel cells (PEFCs) are already commercially available in fuel cell vehicles (FCVs) that use them as a power source, and their applications are expected to expand and become widespread in trucks, buses, ships, and construction machinery. Generally, a PEFC has a structure in which a membrane electrode assembly (MEA), which has a pair of electrodes placed on both sides of a solid polymer electrolyte membrane, is sandwiched between separators in which gas channels are formed. Fuel cell electrodes (especially PEFC electrodes) generally consist of an electrode catalyst layer made of an electrode material with electrode catalytic activity and a polymer electrolyte, and a gas diffusion layer that combines gas permeability and electronic conductivity.

[0003] Currently, electrode materials widely used for PEFCs consist of catalyst nanoparticles dispersed and supported on a porous carbon support. While Pt catalysts are widely used, alloy catalysts composed of Pt with alloys of Co or Ni are known to have superior catalytic activity. Recently, research has also been conducted on ternary or multi-component alloy catalysts, and there have been reports of achieving high initial ORR activity and durability (for example, Non-Patent Document 1).

[0004] On the other hand, electrode materials using porous carbon supports have the problem that catalyst fine particles are detached due to oxidative corrosion of the carbon support as a result of repeated start-stop cycles under the operating conditions of a PEFC. To address this problem, the present inventors have previously reported electrode materials that suppress oxidative corrosion by supporting a metal oxide on the surface of a porous carbon support and then supporting a Pt catalyst on top of that, thereby reducing direct contact between the Pt catalyst and the porous carbon support, in order to improve the durability of catalyst start-stop cycle tests (for example, Patent Documents 1 and 2). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 7228942 [Patent Document 2] Patent No. 7570140 [Non-patent literature]

[0006] [Non-Patent Document 1] H. Chen, C. Guan, and H. Feng, ACS Applied Nano Mater., 5 (7), 9810-9817 (2022) [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In the operation of PEFCs, in addition to the oxidative corrosion of the porous carbon support due to the repeated start-stop cycles mentioned above, there is also the problem that the particle size of the Pt catalyst increases due to Pt dissolution and precipitation associated with load fluctuations, leading to a decrease in the electrochemical effective surface area (ECSA) and oxygen reduction reaction (ORR) activity of Pt. The electrode materials described in the aforementioned Patent Documents 1 and 2 had room for improvement in terms of load fluctuation cycle durability in order to obtain practical electrode performance.

[0008] Under these circumstances, the object of the present invention is to provide an electrode material that can achieve both high activity and potential cycle durability and provide an electrode for a fuel cell, a method for manufacturing the same, and an electrode, a membrane electrode assembly, and a polymer electrolyte fuel cell using the same. [Means for solving the problem]

[0009] As a result of diligent research to solve the above problems, the inventors of the present invention have found that a catalyst composite of a ternary alloy composed of Pt, Ta, and Co can achieve both high activity and voltage cycle durability, and is particularly excellent in load fluctuation cycle characteristics, leading to the present invention.

[0010] That is, the present invention relates to the following invention. <1> An electrode material comprising a porous carbon carrier and a catalyst composite supported on the porous carbon carrier, where the catalyst composite includes a Pt-rich phase and a Ta-rich phase composed of a tantalum oxide. <2> The electrode material according to <1>, wherein the catalyst composite has a structure in which the Ta-rich phase is interposed in the gaps of the Pt-rich phase. <3> The electrode material according to <1> or <2>, wherein the Pt-rich phase and the Ta-rich phase are formed by phase separation of a PtTaCo composite. <4> The electrode material according to any one of <1> to <3>, wherein the Pt-rich phase is PtCo alloy particles. <5> The electrode material according to any one of <1> to <4>, wherein the Ta-rich phase contains a crystalline tantalum oxide. <6> The electrode material according to any one of <1> to <4>, wherein the Ta-rich phase is an oxygen-deficient tantalum oxide. <7> The electrode material according to any one of <1> to <6>, wherein, with respect to the total of Pt, Co, and Ta (100 atomic %), it is 30 to 70 atomic % of Pt, 25 to 60 atomic % of Ta, and 5 to 30 atomic % of Co. <8> The electrode material according to any one of <1> to <6>, wherein, with respect to the total of Pt, Co, and Ta (100 atomic %), it is 40 to 60 atomic % of Pt, 25 to 40 atomic % of Ta, and 10 to 30 atomic % of Co. <9> The electrode material according to any one of <1> to <6>, wherein, with respect to the total of Pt, Co, and Ta (100 atomic %), it is 30 to 50 atomic % of Pt, 40 to 60 atomic % of Ta, and 5 to 20 atomic % of Co. <10> The electrode material according to any one of <1> to <9>, wherein the porous carbon carrier is particulate solid carbon and / or mesoporous carbon.

[0011] <11> An electrode comprising the electrode material according to any one of <1> to <10> and a proton-conductive electrolyte material. <12> A membrane electrode assembly having a solid polymer electrolyte membrane, a cathode joined to one side of the solid polymer electrolyte membrane, and an anode joined to the other side of the solid polymer electrolyte membrane, wherein either one or both of the anode or cathode is the electrode according to <11>. <13> A solid polymer fuel cell comprising the membrane electrode assembly according to <12>.

[0012] <1A> A method for producing an electrode material according to any one of <1> to <10>, the production method including the following steps (1) to (2). Step (1): By distilling off the solvent from the solution obtained by mixing a dispersion liquid in which a porous carbon carrier is dispersed in a solvent and a solution containing a Pt raw material compound, a Ta raw material compound, and a Co raw material compound, a dried product in which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon carrier is obtained. Step (2): The dried product obtained in step (1) is heat-treated in a non-oxidizing atmosphere in the first heat treatment to pyrolyze the catalyst composite precursor into a PtTaCo composite, and then the PtTaCo composite is heat-treated at a higher temperature in the second heat treatment to obtain a catalyst composite phase-separated into a Pt-rich phase and a Ta-rich phase composed of Ta oxide. <2A> The production method according to <1A>, wherein in step (2), the temperature of the first heat treatment is 150°C or higher and 500°C or lower. <3A> The production method according to <1A> or <2A>, wherein in step (2), the temperature of the second heat treatment is 600°C or higher and 1100°C or lower.

[0013] <1B> A method for producing an electrode material according to any one of <1> to <10>, the production method including the following steps (1) to (2). Step (1): A step of obtaining a dried product in which a catalyst composite precursor containing Pt, Ta, and Co is supported on a porous carbon carrier by the following steps (a) to (c). Step (a): A solution containing a Ta raw material compound is added to and mixed with a dispersion liquid in which a porous carbon carrier is dispersed in a solvent, and the solvent is distilled off to obtain a first dried product in which a precursor containing Ta is supported on the porous carbon carrier. Step (b): A step in which a second dried product is obtained by dispersing the first dried product obtained in step (a) in a solvent, adding a solution containing one of the raw material compounds selected from the group consisting of Pt raw material compounds and Co raw material compounds to the dispersion, mixing, and distilling off the solvent, thereby obtaining a second dried product in which a precursor containing Ta and an element derived from one of the raw material compounds is supported on the porous carbon carrier. Step (c): A third dry product is obtained by dispersing the second dry product obtained in step (b) in a solvent, adding a solution which is one raw material compound to the other raw material compound, mixing the solution, and distilling off the solvent, thereby obtaining a third dry product in which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon support. Step (2): The dried material obtained in Step (1) is subjected to a first heat treatment in a non-oxidizing atmosphere to thermally decompose the catalyst composite precursor to form a PtTaCo composite, and then the PtTaCo composite is subjected to a second heat treatment at a higher temperature to obtain a catalyst composite in which the phases of a Pt-rich phase and a Ta-rich phase consisting of Ta oxide are separated. <2B> The manufacturing method described in <1B>, wherein a Pt raw material compound is used in step (b) and a Co raw material compound is used in step (c). <3B> The manufacturing method according to <1B> or <2B>, wherein in step (2), the temperature of the first heat treatment is 150°C or higher and 500°C or lower. <4B> The manufacturing method according to any one of <1B> to <3B>, wherein in step (2), the temperature of the second heat treatment is 600°C or higher and 1100°C or lower. [Effects of the Invention]

[0014] According to the present invention, an electrode material and a method for manufacturing the same are provided, which can achieve both high activity and potential cycle durability, and in particular excellent load fluctuation cycle characteristics for fuel cell electrodes. Additionally, an electrode, a membrane electrode assembly, and a polymer electrolyte fuel cell using the electrode material are provided. [Brief explanation of the drawing]

[0015] [Figure 1] This is a schematic diagram of the electrode material of the present invention. [Figure 2A] This is a diagram illustrating the phase separation of the PtTaCo composite in the electrode material of the present invention. [Figure 2B] This is a conceptual diagram illustrating the load fluctuation cycle durability and start-stop cycle durability of the electrode material of the present invention. [Figure 3] This is a schematic cross-sectional view of the membrane electrode assembly of the present invention. [Figure 4] This is a conceptual diagram showing a typical configuration of the solid polymer fuel cell of the present invention. [Figure 5] This is a flowchart of the procedure (simultaneous loading method) for preparing the electrode material (PtTaCo / KB) in Example A. [Figure 6] This is the STEM-EDS analysis result for Example 1A (Pt3Ta4Co1 / KB). [Figure 7] These are the X-ray diffraction (XRD) patterns of the electrode materials for Example 1A and Comparative Example 1A (Pt3Co1 / KB). [Figure 8] This figure shows the electrochemical effective surface area (ECSA) of Example 1A (Pt3Ta4Co1 / KB) and Comparative Example 1A (Pt3Co1 / KB). [Figure 9] This figure shows the mass activity (MA) of Example 1A and Comparative Example 1A. [Figure 10] This figure shows the conditions for the load fluctuation cycle test. [Figure 11] This figure shows the relationship between the number of load fluctuation cycles and ECSA in Example 1A and Comparative Example 1A. [Figure 12] This figure shows the relationship between the number of load fluctuation cycles and MA in Example 1A and Comparative Example 1A. [Figure 13] This figure shows the relationship between the number of load fluctuation cycles and the electrochemical effective surface area (ECSA) in Example 2A (Pt7Ta2Co1 / KB), Reference Example 1 (Pt / C standard catalyst), and Reference Example 2 (Pt3Co1 / C standard catalyst). [Figure 14] This figure shows the relationship between the number of load fluctuation cycles and mass activity (MA) in Example 2A, Reference Example 1, and Reference Example 2. [Figure 15]This diagram shows the conditions for the start-up / shutdown cycle test. [Figure 16] This figure shows the relationship between the number of start-stop cycle tests and ECSA in PEFC single cells (MEA) for Example 1A, Comparative Example 1A, and Reference Example 1. [Figure 17] This is a flowchart of the procedure (simultaneous loading method) for preparing the electrode material (PtTaCo / MC) in Example B. [Figure 18] This is a STEM image of the electrode material (Pt3Ta2Co1 / MC) from Example 1B. [Figure 19] This is a STEM image of the electrode material (Pt3Ta4Co1 / MC) from Example 2B. [Figure 20] This is a STEM image of the electrode material (Pt3Co1 / MC) of Comparative Example 1B. [Figure 21] This figure shows the ECSA of the electrode material in Example 2B and other electrode materials. [Figure 22] This figure shows the mass activity (MA) of the electrode material in Example 2B and other electrode materials. [Figure 23] This is a flowchart of the procedure (sequential loading method) for preparing the electrode material (PtTaCo / KB) in Example C. [Figure 24] These are STEM images of the electrode materials from Example 1C (Pt3Ta2Co1 / KB, sequential loading method) and Example 3A (Pt3Ta2Co1 / KB, simultaneous loading method). [Figure 25] These are the XRD patterns of the electrode materials for Example 1C (sequential loading method) and Example 3A (simultaneous loading method). [Figure 26] This is an electrochemical evaluation of the electrode materials in Example 1C (sequential loading method) and Example 3A (simultaneous loading method) ((a)ECSA, (b)SA, (c)MA). [Modes for carrying out the invention]

[0016] The present invention will be described in detail below with reference to examples. However, the present invention is not limited to the following embodiments and can be modified and implemented as such without departing from the spirit of the invention. The dimensions, materials, and other specific numerical values ​​shown in the embodiments are merely examples to facilitate understanding of the invention and do not limit the present invention unless otherwise specified. Furthermore, in all drawings, similar components are denoted by the same reference numerals, and explanations are omitted where appropriate.

[0017] In this specification, when the expression "~" is used, it is used to mean an expression that includes the numbers before and after it. Also, in this specification, the expression "A and / or B" includes "A only," "B only," and "both A and B."

[0018] <1. Electrode materials and methods for manufacturing electrode materials> <1-1. Electrode material> The electrode material of the present invention comprises a porous carbon support and a catalyst composite supported on the porous carbon support, wherein the catalyst composite comprises a Pt-rich phase and a Ta-rich phase consisting of Ta oxide.

[0019] Figure 1 shows a conceptual schematic diagram of the electrode material of the present invention. As shown in Figure 1, the electrode material 1 according to the present invention is composed of a porous carbon carrier 2 and a catalyst composite 3 supported (fixed) on the surface (inner surface of the pores or outer surface of the pores) of the porous carbon carrier 2.

[0020] The porous carbon support 2 plays a role in improving electronic conductivity when electrodes are formed, and also serves as the framework for the electrodes. Details of the porous carbon support 2 will be described later.

[0021] The catalyst complex 3 contains a Pt-rich phase 3A and a Ta-rich phase 3B, and functions as an electrode catalyst component. The catalyst complex 3 is dispersed and supported on the surface of the porous carbon support 2 (inner and outer pore surfaces), and since a portion of the surface of the porous carbon support 2 is exposed, when an electrode is constructed using this electrode material, the porous carbon support 2 comes into contact with each other, forming low-resistance conductive paths and resulting in an electrode with excellent electronic conductivity.

[0022] Catalyst complex 3 is typically formed by phase-separating a PtTaCo complex, which consists of a first component platinum (Pt), a second component tantalum (Ta), and a third component cobalt (Co), into a Pt-rich phase 3A and a Ta-rich phase 3B, as shown in Figure 2A. In this specification, "PtTaCo complex" refers to the ternary solid intermediate before phase separation. The PtTaCo complex is typically obtained by heating a catalyst complex precursor containing Pt, Ta, and Co to thermally decompose the precursor. Further details will be described later in the section on the method for producing electrode materials. Furthermore, in this specification, "(solid) phase separation" refers to the phenomenon in which a solid in question separates into multiple solid phases.

[0023] The Pt-rich phase is a solid phase mainly composed of particulate Pt, and the Ta-rich phase is a solid phase composed of Ta oxide. In Figure 1, the Ta-rich phase is schematically shown as particulate, but its form is not limited to particles and may be a continuous phase, film-like, island-like, etc. Details of the Pt-rich and Ta-rich phases will be described later.

[0024] The catalyst complex 3 formed by phase separation typically has a nanocomposite structure in which at least a portion of the Ta-rich phase 3B is interposed in the gaps of the Pt-rich phase. In this specification, "nanocomposite structure" means a structure in which solid phases of metals (including alloys) or metal compounds (typically metal oxides) on the order of nm size (less than 10 nm) are integrated. The particles constituting the nanocomposite structure may be crystalline or amorphous, and the shape, size, and distribution of the particles are not particularly limited.

[0025] The electrode material of the present invention having catalyst composite 3 has the following advantages (i) to (iii).

[0026] (i) The presence of the Ta-rich phase in the gaps of the Pt-rich phase allows the Pt-rich phase to be dispersed and maintained on an nm scale, resulting in the excellent catalytic activity attributed to the Pt-rich phase.

[0027] (ii) Because it is a nanocomposite structure in which a Ta-rich phase (Ta oxide phase) is interposed between Pt-rich phases, aggregation of the Pt-rich phase is suppressed, and load fluctuation cycle durability is ensured (see Figure 2B left).

[0028] (iii) The porous carbon support surrounding the Pt-rich phase undergoes oxidative corrosion, but the porous carbon support surrounding the Ta-rich phase is less susceptible to oxidative corrosion. Therefore, although the porous carbon support in contact with the Pt-rich phase undergoes oxidative corrosion in the initial state, after corrosion progresses, the arrangement in which the support is in contact with the Ta oxide phase is maintained, and the fixation of the catalyst complex onto the support is maintained via the Ta-rich phase. This ensures start-stop cycle durability (see Figure 2B, right). As will be described later, the sequential loading method allows for a process design in which a Ta precursor, followed by a Pt precursor (or Co precursor), and then another Co precursor (or Pt precursor) are loaded onto a porous carbon support, and then heat-treated in a non-oxidizing atmosphere to separate the Pt-rich phase from the Ta-rich phase. This allows the Ta-rich phase (Ta oxide phase) derived from the initially loaded Ta precursor to be preferentially positioned at the support interface, making it easier to reduce the proportion of the Pt-rich phase in direct contact with the support. Therefore, the sequential loading method is advantageous from the viewpoint of durability design.

[0029] The electrode material of the present invention can achieve both high catalytic activity and potential cycle durability (load fluctuation cycle durability and start-stop cycle durability), and can provide fuel cell electrodes with particularly excellent load fluctuation cycle characteristics. Therefore, a polymer electrolyte fuel cell equipped with electrodes formed from this electrode material exhibits excellent electrode performance, high durability, and can generate electricity for a long period of time.

[0030] The electrode material of the present invention is suitable as an electrode material for solid polymer fuel cells, but it can also be used for other applications (for example, electrodes for solid polymer water electrolysis).

[0031] The components of the electrode material of the present invention will be described in detail below. In the following description, the electrode material of the present invention will be described assuming its use as an electrode for a polymer electrolyte fuel cell (PEFC), but the electrode material of the present invention is not limited to this application.

[0032] In the following, the cathode conditions of the PEFC refer to the conditions at the cathode during normal operation of the PEFC, meaning a temperature of room temperature to approximately 150°C and the supply of oxygen-containing gas such as air (oxidizing atmosphere). The anode conditions refer to the conditions at the anode during normal operation of the PEFC, meaning a temperature of room temperature to approximately 150°C and the supply of hydrogen-containing fuel gas (reducing atmosphere).

[0033] Furthermore, in this specification, "potential cycle durability" refers to the durability against potential fluctuations such as fuel cell startup and shutdown and load fluctuations, and can be evaluated using the startup / shutdown cycle test and load fluctuation cycle test of the "NEDO PEFC Cell Evaluation and Analysis Protocol" described in the examples.

[0034] [Porous carbon carrier] The porous carbon support is included in the electrode material of the present invention and plays a role in improving conductivity (electron conductivity) when an electrode is formed, and also serves as the framework of the electrode.

[0035] In this specification, "porous carbon" means "a carbon material having multiple pores" that forms the framework (foundation) of an electrode material. In this specification, "pore" refers to pores with a diameter of 150 nm or less (especially pores with a diameter of 100 nm or less). "Mesopore region pores" refers to pores with a diameter of 2 nm to 50 nm. Furthermore, in this specification, "micropore region pores" refers to pores with a diameter of less than 2 nm, and "macropore region pores" refers to pores with a diameter greater than 50 nm and less than or equal to 150 nm.

[0036] Porous carbon supports are preferable if they have a large surface area and the catalyst composite is dispersed and supported on the surface (inner and outer pore surfaces) at the nanoscale. The BET specific surface area of ​​the support is not limited, but for example, 100 m 2 / g~1500m 2 It is / g.

[0037] The size and shape of the electrode material of the present invention depend on the size and shape of the porous carbon support, which is the skeletal material. Therefore, the size and shape of the porous carbon support are determined within a range that allows the electrode material to be in continuous contact when forming an electrode for a fuel cell, and that creates a space that allows for smooth diffusion of gases such as hydrogen and oxygen, and discharge of water (steam) within the fuel cell electrode.

[0038] The porous carbon carrier is porous carbon having numerous pores. Any porous carbon can be used as the porous carbon carrier, as long as it does not impair the objectives of the present invention.

[0039] As the porous carbon, porous carbon having multiple pores that form a three-dimensional network structure is preferred. Here, "having multiple pores that form a three-dimensional network structure" means that the porous carbon has multiple pores on its surface and inside, and adjacent pores connect to each other so that the multiple pores are connected in three dimensions, and connecting pores with openings are formed on the surface.

[0040] The pore structure (pore size, shape, etc.) of porous carbon can be confirmed by observing it with an electron microscope. Examples of electron microscopes include field emission scanning electron microscopes (FESEM), transmission electron microscopes (TEM), and scanning transmission electron microscopes (STEM).

[0041] The porous carbon used in this invention may be of one type, or two or more carbon materials with different sizes (particle size, fiber diameter, and fiber length) and crystallinity may be used in any proportion.

[0042] The porous carbon support may be made of highly crystalline carbon. "Highly crystalline carbon" is carbon that has been graphitized (crystallized) by high-temperature heat treatment, etc., and has superior oxidation resistance compared to amorphous or low-crystalline carbon. Highly crystalline carbon can be either homemade or commercially available.

[0043] Suitable examples of porous carbon supports include particulate solid carbon. Suitable solid carbon materials include carbon black (CB) and graphitized carbon black (GCB), which is obtained by graphitizing (crystallizing) carbon black. The particulate solid carbon preferably has a secondary particle size of 0.03 to 500 μm (primary particle size of approximately 10 nm to 100 nm).

[0044] Solid carbon can be used for both homemade and commercially available products. Examples of commercially available products include Lion Specialty Chemicals' "Ketjenblack" series (product number: EC600JD, etc.), Cabot's "Vulcan" series (product number: XC-72, etc.), Cabot's "GCB" series (product number: GCB200, etc.), and Tokai Carbon's "Tokablack" series (product number: Tokablack #3800, etc.). Commercially available solid carbon can be crushed and adjusted to the desired particle size before use.

[0045] Another preferred example of a porous carbon support is mesoporous carbon. Mesoporous carbon (hereinafter sometimes referred to as "MC") is a porous carbon having numerous pores in the mesoporous region.

[0046] As mesoporous carbon, porous carbon having pores in the mesopore region (2-50 nm) can be used, but preferably the pore diameter is between 3 nm and 40 nm. Within this range, even when electron-conducting oxides or electrode catalysts are fixed (supported) on the inner wall of the pores, the diffusion of substances into the pores is not significantly hindered and proceeds smoothly.

[0047] Furthermore, as will be described later, when manufacturing electrodes for fuel cells, the electrode material of the present invention is mixed with a proton-conducting electrolyte material (ionomer). However, since the proton-conducting electrolyte material (ionomer) has a molecular size of 10 nm or more, it cannot penetrate into the mesopores, which have a small pore diameter. Therefore, it is possible to suppress ionomer-derived poisoning of the electrode catalyst metal supported in the pores of mesoporous carbon via the aforementioned electron-conducting oxide.

[0048] The mesoporous carbon according to the present invention may contain regions other than pores in the mesopore region (2 nm to 50 nm), such as micropore regions and macropores, but it is preferable that the proportion of pores in the mesopore region is high.

[0049] The pore structure (pore size, shape, etc.) of mesoporous carbon can be confirmed by observation with an electron microscope. Examples of electron microscopes include field emission scanning electron microscopes (FESEM) and scanning transmission electron microscopes (STEM).

[0050] In mesoporous carbon, the pores in the mesoporous regions preferably have a three-dimensional network structure, with some or all of the pores in a mesoporous region communicating with pores in adjacent mesoporous regions, in addition to individual pores independent of other pores. The presence of these communicating pores promotes the diffusion of substances within the pores of the mesoporous carbon.

[0051] Mesoporous carbon may be graphitized. Graphitization improves crystallinity, leading to increased crystallinity and thus improved electronic conductivity.

[0052] The mesoporous carbon used in the electrode material for fuel cells of the present invention may be synthesized as appropriate or a commercially available product may be used. Examples of commercially available products include the CNovel series (designed mesopore size: 5~150nm) manufactured by Toyo Tanso Co., Ltd., which is a mesoporous carbon with MgO as a template.

[0053] Commercially available mesoporous carbon may be crushed to adjust the particle size to the desired size before use. When using crushed graphitized mesoporous carbon, the exposed surface may have many defects and insufficient graphitization; therefore, a second graphitization treatment (heat treatment under an inert atmosphere) may be performed.

[0054] Furthermore, the porous carbon carrier may be porous carbon having an electronically conductive oxide layer on its surface. Here, "having an electronically conductive oxide layer on its surface" means that part or all of the surface of the porous carbon carrier is covered with an electronically conductive oxide layer. Unlike carbon materials, the electron-conducting oxide layer does not undergo oxidative decomposition. Therefore, by supporting the catalyst composite on the electron-conducting oxide layer and preventing direct contact with the porous carbon support, durability (especially during the startup and shutdown of fuel cells under high potential loads) is improved.

[0055] The electron-conductive oxide layer on the surface of the porous carbon support can be any electron-conductive oxide that is stable under the cathode conditions of a PEFC, and examples include electron-conductive oxides mainly composed of an oxide of one metal element selected from tin (Sn), molybdenum (Mo), niobium (Nb), tantalum (Ta), titanium (Ti), and tungsten (W). In this specification, "mainly electron-conductive oxide" means (A) an oxide consisting only of the matrix oxide, and (B) an oxide doped with another element, in which the matrix oxide is present in an amount of 80 mol% or more. The oxides constituting the electron-conductive oxide phase may have oxygen vacancies.

[0056] Examples of electronically conductive oxide layers include Ta2O5 layers and Nb2O5 layers. Ta2O5 and Nb2O5 layers can be doped with other elements as needed.

[0057] The thickness of the electronically conductive oxide layer depends on the type and amount of electronically conductive oxide, but is preferably 1 to 10 nm. Furthermore, it is preferable that the electronically conductive oxide layer covers the entire surface of the porous carbon support, but it may also cover only a portion of the surface.

[0058] [Catalytic complex] The catalyst composite in the electrode material of the present invention is supported on a porous carbon support, contains a Pt-rich phase and a Ta-rich phase, and functions as an electrode catalyst component. The catalyst composite is dispersed and supported (fixed) on the surface (inner surface and outer surface of pores) of the porous carbon support. Although the catalyst composite contains a Pt-rich phase and a Ta-rich phase as essential components, it may also contain other components (other phases) as long as it does not impair the objective of the present invention.

[0059] The catalyst composite in the electrode material of the present invention preferably has a structure in which a Ta-rich phase, consisting of Ta oxide, is interposed in the gaps between Pt-rich phases. In other words, a preferred embodiment of the catalyst composite has a nanocomposite structure in which the Ta-rich phase is present filling the gaps between Pt-rich phases (see Figure 1). Catalyst complexes having such a structure can typically be formed by phase-separating a PtTaCo complex, which consists of a first component platinum (Pt), a second component tantalum (Ta), and a third component cobalt (Co), into a Pt-rich phase and a Ta-rich phase.

[0060] Here, "PtTaCo complex" refers to the ternary solid intermediate before phase separation, while "catalytic complex" refers to the final configuration including the Pt-rich and Ta-rich phases obtained after phase separation. Furthermore, as will be explained in the manufacturing method described later, the "catalyst complex precursor" refers to the mixed state of element-derived components immobilized on a support. This is converted to a PtTaCo complex (before phase separation) by a first heat treatment, and then further converted to a catalyst complex (after phase separation) by a second heat treatment.

[0061] The PtTaCo composite may be crystalline, amorphous, or a mixture of crystalline and amorphous materials.

[0062] The proportions of the first to third components in the PtTaCo complex are determined within the range where phase separation of the PtTaCo complex occurs.

[0063] A suitable composition example for the PtTaCo composite is 30-70 atomic percent of Pt, 25-60 atomic percent of Ta, and 5-30 atomic percent of Co, relative to the total amount of Pt, Co, and Ta (100 atomic percent).

[0064] A suitable composition example for the PtTaCo composite is one in which, relative to the total amount of Pt, Co, and Ta (100 atomic%), Pt accounts for 40-60 atomic%, Ta for 25-40 atomic%, and Co for 10-30 atomic%. The Pt3Ta2Co1 disclosed in the examples below falls within this composition range.

[0065] A suitable composition example for the PtTaCo composite is one in which, relative to the total amount of Pt, Co, and Ta (100 atomic%), Pt accounts for 30-50 atomic%, Ta for 40-60 atomic%, and Co for 5-20 atomic%. The Pt3Ta4Co1 disclosed in the examples below falls within this composition range.

[0066] Other suitable composition examples for the PtTaCo composite include Pt 60-80 atomic%, Ta 10-30 atomic%, and Co 1-20 atomic%, relative to the total of Pt, Ta, and Co (100 atomic%), and Pt 65-75 atomic%, Ta 15-25 atomic%, and Co 5-15 atomic%. Furthermore, the Pt7Ta2Co1 disclosed in the examples described later falls within this compositional range.

[0067] The PtTaCo composite before phase separation consists of the first to third components (Pt, Ta, Co) as basic components, but may also contain other elements (e.g., Ni) to the extent that it does not impair the objectives of the present invention.

[0068] The amounts of each component in the PtTaCo composite can be determined by inductively coupled plasma emission spectrometry (ICP).

[0069] The form of the catalyst complex after phase separation can be arbitrary as long as it does not impair the objective of the present invention, and examples include particulate, island-like, film-like, wire-like, and so on. From the viewpoint of conductivity when forming electrodes, it is preferable that the catalyst composite is in particulate form, and that the particulate catalyst composite does not completely cover the surface of the porous carbon support, leaving a portion of the surface of the porous carbon support exposed, and that it is dispersed and supported to such an extent that direct contact between the porous carbon support and other porous carbon support is not hindered.

[0070] When the catalyst composite is supported on the surface of a porous carbon support, its size is typically 1 to 10 nm in diameter, preferably 2 to 5 nm. The "size of the catalyst composite" can be obtained by the average particle size of any 20 catalyst composites examined from electron microscope images. When calculating the average particle size from electron microscope images, if the shape of the fine particles is not spherical, the length in the direction showing the maximum length of the particle is used as its particle size.

[0071] Furthermore, the amount of catalyst composite supported is appropriately determined within a range that allows for sufficient electrode catalytic activity when the electrode is constructed. Typically, the catalyst composite is 20-75% by mass relative to the total weight of the electrode material, or 15-60% by mass based on the amount of Pt. Within this range, excellent catalytic activity per unit mass is achieved, and the desired electrode reaction activity corresponding to the supported amount can be obtained.

[0072] As described above, the electrode material of the present invention can be obtained by phase-separating the PtTaCo composite into a Pt-rich phase and a Ta-rich phase, thereby obtaining the electrode material of the present invention (after phase separation) having a catalyst composite (nanocomposite structure composite) that is a nanocomposite structure containing a Pt-rich phase and a Ta-rich phase, supported on a porous carbon support. The phase separation treatment method can be any method that can significantly separate the phases of the PtTaCo composite, and depending on the type and composition of the PtTaCo composite, examples include heat treatment, chemical treatment, and electrochemical treatment (including combinations thereof) under a non-oxidizing gas atmosphere. Among these, heat treatment under a non-oxidizing gas atmosphere is preferred, as will be described later in "1-2. Method for manufacturing electrode material of the present invention".

[0073] The following provides a detailed explanation of the Pt-rich and Ta-rich phases.

[0074] (Pt-rich phase) The "Pt-rich phase" constituting the catalyst complex is a solid phase containing 50 atomic percent or more of Pt, preferably 70 atomic percent or more (including cases where it is 100 atomic percent). The Pt-rich phase is typically formed by phase separation of a PtTaCo complex.

[0075] The atoms other than Pt that constitute the Pt-rich phase are mainly Co, which is present in the PtTaCo composite before phase separation, but other elements such as Ta may also be included, as long as they do not impair the effects of the present invention.

[0076] The Pt-rich phase is particulate. Furthermore, the Pt-rich phase is not limited to crystals, but may be amorphous, or a mixture of crystalline and amorphous materials.

[0077] The particle size of the Pt-rich phase depends on the particle size and morphology of the PtTaCo composite before phase separation, but typically it is 1 to 5 nm (preferably 1 to 3 nm). The "particle size of the Pt-rich phase" can be obtained by the average value of the particle sizes of any 20 Pt-rich phases examined from the electron microscope image. When calculating the average particle size from the electron microscope image, if the shape of the fine particles is other than spherical, the length in the direction indicating the maximum length of the particle is taken as its particle size.

[0078] (Ta-rich phase) The "Ta-rich phase" contained in the catalyst composite is a solid phase composed of tantalum oxide. The Ta-rich phase is typically formed by phase separation of the PtTaCo composite. In this specification, "tantalum oxide" means an oxide of tantalum (Ta), and the form of the tantalum oxide phase is not limited to crystals, and includes any of crystals, amorphous substances, and mixtures of crystals and amorphous substances. Also, when distinguishing and expressing tantalum oxide as crystalline and amorphous (non-crystalline), they are respectively expressed as "crystalline tantalum oxide" and "amorphous tantalum oxide". In addition, in this specification, "crystalline tantalum oxide" is a concept that includes not only crystals of Ta2O5 (TaOx, x = 2.5), which is the most stable oxide, but also crystals of oxygen-deficient tantalum oxide (TaOx, 0.1 < x < 2.5) in an oxygen-deficient state.

[0079] The tantalum oxide constituting the Ta-rich phase may contain atoms other than Ta and oxygen. Depending on the method and conditions of the phase separation treatment, in the Ta-rich phase, the Co component may be dissolved in the tantalum oxide when phase-separating the PtTaCo composite.

[0080] The tantalum oxide constituting the Ta-rich phase may be only crystalline tantalum oxide, may be only amorphous tantalum oxide, or may be a mixture of crystalline tantalum oxide and amorphous tantalum oxide.

[0081] The form of the Ta-rich phase may be particles or not, as long as it does not impair the object of the present invention. As described above, typically, a part of the Ta-rich phase exists so as to be interposed in the Pt-rich phase. When the Ta-rich phase is in the form of particles, its particle size depends on the particle size and morphology of the PtTaCo composite before phase separation, but is typically 1 to 5 nm (preferably 1 to 3 nm). The "particle size of the Ta-rich phase" can be obtained by the average value of the particle diameters of any 20 Ta-rich phases examined from electron microscope images. When calculating the average particle size from electron microscope images, if the shape of the fine particles is not spherical, the length in the direction showing the maximum length of the particle is used as its particle size.

[0082] <1-2. Method for manufacturing electrode materials> The method for manufacturing the electrode material of the present invention is not particularly limited, and a suitable method can be selected as appropriate depending on the type of porous carbon support and catalyst composite that constitute the electrode material. However, the manufacturing method described below (hereinafter referred to as "the method for manufacturing the electrode material of the present invention") is preferred.

[0083] The methods for manufacturing electrode materials of the present invention are broadly classified into "simultaneous loading method" and "sequential loading method". Hereinafter, "simultaneous loading method" refers to a preparation method in which precursors of multiple elements constituting a catalyst are mixed and loaded together in a dispersion solvent of a carrier, and then dried and heat treated. "Sequential loading method" refers to a preparation method in which the precursors of the multiple elements are sequentially loaded in a predetermined order in a dispersion solvent of a carrier, and then dried and heat treated.

[0084] The "simultaneous loading method" and the "sequential loading method" in the method for manufacturing electrode materials of the present invention will be described in detail below.

[0085] "Simultaneous support method" The method for manufacturing electrode materials of the present invention (simultaneous loading method) includes the following steps (1) to (2). Step (1): A step to obtain a dried product on which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon carrier by distilling off the solvent from a solution obtained by mixing a dispersion of porous carbon carriers in a solvent with a solution containing Pt raw material compound, Ta raw material compound, and Co raw material compound. Step (2): The dried material obtained in Step (1) is subjected to a first heat treatment in a non-oxidizing atmosphere to thermally decompose the catalyst composite precursor to form a PtTaCo composite, and then the PtTaCo composite is subjected to a second heat treatment at a higher temperature to obtain a catalyst composite in which the phases of a Pt-rich phase and a Ta-rich phase are separated.

[0086] The simultaneous support method has the advantage of reliably forming a nanocomposite structure in which the Ta-rich phase self-organizes and interposes in the gaps between the Pt-rich phases during the phase separation process, by immobilizing the raw material compounds on a support in a single step and applying stepwise heat treatment. Furthermore, because the proximity of each element is high in the precursor stage, the formation of the PtCo alloy phase proceeds stably in the subsequent heat treatment. In addition, the process configuration is relatively simple, and it tends to be easier to optimize the conditions during scale transition.

[0087] The following provides a more detailed explanation of steps (1) and (2) of the simultaneous loading method.

[0088] [Process (1)] Step (1) is a step in which a porous carbon support is dispersed in a solvent, and a solution containing a Pt raw material compound, a Ta raw material compound, and a Co raw material compound is mixed with the solution obtained by distilling off the solvent, thereby obtaining a dried product on which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon support.

[0089] In step (1), first, a dispersion of porous carbon support in a solvent is mixed with a solution containing Pt raw material compound, Ta raw material compound, and Co raw material compound to form a mixed solution.

[0090] The porous carbon support is as described in <1-1. Electrode Materials> above, so its explanation will be omitted.

[0091] The solvent used to disperse the porous carbon support is appropriately selected depending on the type of porous carbon support, but organic solvents with excellent dispersibility of carbon materials (e.g., ethanol, dichloroethane, etc.) are preferably used.

[0092] The Pt raw material compound, Ta raw material compound, and Co raw material compound are compounds that serve as raw materials for the catalyst complex (and its precursor, the PtTaCo complex) in the electrode material of the present invention, and there are no restrictions on their type as long as the desired catalyst complex (and its precursor, the PtTaCo complex) is formed. For example, the Pt raw material compound, Ta raw material compound, and Co raw material compound can be those exemplified in the sequential loading method described later. A preferred example is a combination of Pt acetylacetonate, tantalum ethoxide, and cobalt nitrate, which will be described later in the examples.

[0093] The solvent used to dissolve the raw material compound should be one that can dissolve the raw material compound and has high compatibility with the solvent used in the dispersion of the porous carbon support. In the case of the above combination of raw material compounds, for example, a hydrophobic solvent (e.g., dichloromethane) can be selected for Pt acetylacetonate, and ethanol can be selected for tantalum ethoxide and cobalt nitrate.

[0094] The proportions of the Pt raw material compound, Ta raw material compound, and Co raw material compound are determined within a range that allows for phase separation of the PtTaCo complex, depending on the desired composition of the catalyst complex.

[0095] For example, the proportions of Pt raw material compounds, Ta raw material compounds, and Co raw material compounds are as follows: Pt 30-70 atomic%, Ta 25-60 atomic%, and Co 5-30 atomic%, relative to the total of Pt, Ta, and Co (100 atomic%).

[0096] Other preferred ratios include 40-60 atomic% Pt, 25-40 atomic% Ta, and 10-30 atomic% Co relative to the total of Pt, Co, and Ta (100 atomic%). The Pt3Ta2Co1 disclosed in the examples below falls within this compositional range.

[0097] Other preferred ratios include 30-50 atomic% Pt, 40-60 atomic% Ta, and 5-20 atomic% Co, relative to the total of Pt, Co, and Ta (100 atomic%). The Pt3Ta4Co1 disclosed in the examples below falls within this compositional range.

[0098] Other preferred ratios include Pt 60-80 atomic%, Ta 10-30 atomic%, and Co 1-20 atomic%, relative to the total of Pt, Ta, and Co (100 atomic%), or Pt 65-75 atomic%, Ta 15-25 atomic%, and Co 5-15 atomic%. Furthermore, the Pt7Ta2Co1 disclosed in the examples described later falls within this compositional range.

[0099] There are no particular restrictions on the method of mixing the dispersion containing the porous carbon support with the solution containing the raw material compound, and they can be mixed by conventionally known means. The mixing order is also arbitrary; for example, the raw material compounds may be individually dissolved in solvents and each solution may be mixed with a dispersion containing a porous carbon support, or a solution containing any two or more of the raw material compounds may be mixed with a dispersion containing a porous carbon support, and then the solution containing the remaining raw material compounds may be mixed.

[0100] Furthermore, the dispersion or solution prepared in step (1) may contain other components (e.g., dispersants, pH adjusters, etc.) to the extent that they do not impair the objectives of the present invention. The other components are preferably those that vaporize in subsequent drying and heat treatment steps.

[0101] Next, the solvent is removed from the mixed solution by distillation to obtain a dried product in which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon support. The dried product (catalyst complex precursor) is a precursor in which the Pt raw material compound, Ta raw material compound, and Co raw material compound are dispersed and supported together on the surface (inner and outer pore surfaces) of a porous carbon support, and is a precursor that becomes a PtTaCo complex after processing in a subsequent step (step (2)).

[0102] There are no particular restrictions on the method of removing the solvent by distillation, but one method is to evaporate the solvent by reducing the pressure and drying the mixture. A specific example is to use a vacuum device such as a rotary evaporator to reduce the pressure and dry the mixed solution.

[0103] [Process (2)] Step (2) is a step in which the dried material obtained in step (1) is subjected to a first heat treatment in a non-oxidizing atmosphere to thermally decompose the catalyst composite precursor to form a PtTaCo composite, and then the PtTaCo composite is subjected to a second heat treatment at a higher temperature to obtain a catalyst composite in which the phases of a Pt-rich phase and a Ta-rich phase consisting of Ta oxide are separated.

[0104] (First heat treatment) The dried material obtained in step (1) (catalyst composite precursor supported on a porous carbon support) contains substances derived from each of the raw materials. This is thermally decomposed by a first heat treatment in a non-oxidizing atmosphere and transformed into a PtTaCo composite.

[0105] A "non-oxidizing atmosphere" refers to an inert atmosphere such as nitrogen or argon, or a reducing atmosphere containing hydrogen.

[0106] The first heat treatment conditions should be determined within a range that thermally decomposes the catalyst complex precursor containing Pt, Ta, and Co to form a PtTaCo complex. The heat treatment temperature is determined considering the type of raw material compound of the catalyst complex precursor, but for example, it may be 150°C or higher, 180°C or higher, or 200°C or higher, with upper limits such as 500°C or lower, 400°C or lower, or 300°C or lower. As shown in the examples described later, for example, when the raw material compounds are Pt acetylacetonate, tantalum ethoxide, and cobalt nitrate, a PtTaCo composite can be formed by heat treatment at 200°C to 300°C under an inert atmosphere.

[0107] (Second heat treatment) The PtTaCo composite obtained in the first heat treatment is subjected to a second heat treatment at a higher temperature than the first heat treatment to produce a catalyst composite that undergoes phase separation into a Pt-rich phase and a Ta-rich phase. Furthermore, because the system of this invention contains Ta (Ta: 3007°C, Ta2O5: 1784°C), aggregation tends to be less likely to occur.

[0108] The temperature of the second heat treatment is higher than that of the first heat treatment, and should be such that the PtTaCo composite obtained in the first heat treatment undergoes phase separation. Specific heat treatment temperatures are 600°C or higher, 650°C or higher, 700°C or higher, and 750°C or higher, with upper limits being, for example, 1100°C or lower, 1000°C or lower, 900°C or lower, and 850°C or lower.

[0109] By performing heat treatment under these conditions, the PtTaCo composite can be separated into a Pt-rich phase and a Ta-rich phase without significantly decomposing the porous carbon support.

[0110] The oxygen necessary for the formation of the Ta-rich phase (Ta oxide) may originate from organic groups derived from the raw material compounds, functional groups on the support surface, residual moisture, or trace amounts of oxygen allowed in the inert atmosphere. If necessary, the composition and crystallinity of the Ta oxide may be adjusted by adding an annealing or mild oxidation process under low oxygen partial pressure after phase separation.

[0111] "Sequential loading method" The method for manufacturing electrode materials of the present invention (sequential loading method) includes the following steps (1) to (2). Step (1): A step to obtain a dried product in which a catalyst composite precursor containing Pt, Ta, and Co is supported on a porous carbon support by the following steps (a) to (c). Step (a): A step in which a porous carbon carrier is dispersed in a solvent, a solution containing a Ta raw material compound is added to the dispersion and mixed, and the solvent is removed by distillation to obtain a first dried product in which a precursor containing Ta is supported on the porous carbon carrier. Step (b): A step in which a second dried product is obtained by dispersing the first dried product obtained in step (a) in a solvent, adding a solution containing one of the raw material compounds selected from the group consisting of Pt raw material compounds and Co raw material compounds to the dispersion, mixing, and distilling off the solvent, thereby obtaining a second dried product in which a precursor containing Ta and an element derived from one of the raw material compounds is supported on the porous carbon carrier. Step (c): A third dry product is obtained by dispersing the second dry product obtained in step (b) in a solvent, adding a solution which is one raw material compound to the other raw material compound, mixing the solution, and distilling off the solvent, thereby obtaining a third dry product in which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon support. Step (2): The dried material obtained in Step (1) is subjected to a first heat treatment in a non-oxidizing atmosphere to thermally decompose the catalyst composite precursor to form a PtTaCo composite, and then the PtTaCo composite is subjected to a second heat treatment at a higher temperature to obtain a catalyst composite in which the phases of a Pt-rich phase and a Ta-rich phase are separated.

[0112] The sequential loading method, with its process design that involves first placing a Ta precursor on the carrier surface, followed by sequentially loading Pt and Co precursors, has the advantage of preferentially positioning the Ta-rich phase (Ta oxide) at the carrier interface after phase separation. This is expected to reduce the proportion of direct contact between the Pt-rich phase and the carrier, thereby improving stabilization during start-up / stop-down and load fluctuation cycles.

[0113] The following provides a more detailed explanation of steps (1) (steps (a) to (c)) and step (2) of the sequential loading method. Note that explanations of aspects common to the simultaneous loading method (porous carbon carrier, atmosphere, etc.) will be omitted as appropriate.

[0114] [Process (1)] Step (1) is a step in which a Ta precursor, a Pt precursor (or Co precursor), and a Co precursor (or Pt precursor) are sequentially supported on a porous carbon support by steps (a) to (c) to obtain a dried product on which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon support.

[0115] "Process (a)" Step (a) is a step in which a Ta raw material compound (Ta precursor) is supported on porous carbon. More specifically, step (a) is a step of obtaining a first dried product on which a precursor containing Ta is supported on the porous carbon carrier by adding a solution containing a Ta raw material compound to a dispersion in which a porous carbon carrier is dispersed in a solvent, mixing the solutions, and then removing the solvent by distillation.

[0116] As the Ta raw material compound, for example, tantalum alkoxide can be used. As the tantalum alkoxide, tantalum methoxide, tantalum propoxide, tantalum butoxide, tantalum methoxyethoxide, and tantalum ethoxyethoxide can be used. Among these, tantalum ethoxide, tantalum propoxide, and tantalum butoxide are preferred.

[0117] The solvent is preferably one that can ensure solubility while suppressing the hydrolysis and condensation of the alkoxide, and examples include acetone, acetylacetone, toluene, xylene, kerosene, or ethanol. It is preferable to control the water content as needed to form a desirable precursor layer.

[0118] The concentrations of porous carbon and tantalum alkoxide can be appropriately determined within the range that allows for the manufacture of the electrode material of the present invention.

[0119] The method of removing the solvent is arbitrary as long as it does not impair the objective of the present invention, but removal of the solvent by reduced pressure is preferred.

[0120] "Step (b)" and "Step (c)" Step (b) is a step of supporting a Pt precursor or a Co precursor on porous carbon on which the first dried product containing the Ta precursor obtained in step (a) is supported. More specifically, step (b) is a step in which a second dried product is obtained by dispersing the first dried product obtained in step (a) in a solvent, adding a solution containing one of the raw material compounds selected from the group consisting of Pt raw material compounds and Co raw material compounds to the dispersion, mixing, and distilling off the solvent, thereby supporting a precursor containing Ta and an element derived from one of the raw material compounds on the porous carbon carrier.

[0121] Step (c) is a step of supporting a Co precursor (or Pt precursor) on porous carbon on which a second dried product containing the Ta precursor and Pt precursor (or Co precursor) obtained in step (b) is supported. More specifically, step (c) is a step in which a third dried product is obtained by dispersing the second dried product obtained in step (b) in a solvent, adding a solution which is one of the raw material compounds to the other, mixing it, and distilling off the solvent, thereby supporting a catalyst composite precursor containing Pt, Ta, and Co on the porous carbon support.

[0122] In other words, steps (b) and (c) are steps in which a Pt raw material compound and a Co raw material compound are sequentially supported on a porous carbon carrier on which a first dried product containing the Ta precursor obtained in step (a) is supported. The order in which the Pt raw material compound and the Co raw material compound are supported is arbitrary as long as it does not impair the objective of the present invention, but typically the Pt raw material compound is used in step (b) and the Co raw material compound is used in step (c).

[0123] The Pt starting compound (Pt precursor) is not particularly limited as long as it contains Pt and can form a PtTaCo complex by the heat treatment described later. For example, Pt(II) acetylacetonate (Pt(acac)2), hexachloroplatinum(IV) acid (H2PtCl6·nH2O), tetraammineplatinum(II) salt ([Pt(NH3)4]X2, X=Cl - Examples include platinum(II) salts (PtCl2, etc.), platinum organic complexes, etc. Note that against anion species (Cl - NO3 -These substances (etc.) are removed or inactivated by the final heat treatment, and are therefore permissible to the extent that they do not interfere with the process. For Pt(acac)2, organic solvents such as dichloromethane or toluene are used. For H2PtCl6 and ammine complexes, water or an ethanol / water mixed solvent is preferred, making it applicable to aqueous processes as well.

[0124] The Co raw material compound (Co precursor) is not particularly limited as long as it contains Co and can form a PtTaCo complex (before phase separation) by the heat treatment described later. Examples include cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O), cobalt(II) chloride hexahydrate (CoCl2·6H2O), and cobalt(II) acetate (Co(OAc)2·nH2O). Since water or an ethanol / water mixed solvent is preferred as the solvent, the process can also be applied to aqueous systems.

[0125] In steps (b) and (c), it is desirable that the solvent system of the selected precursor does not dissolve or disintegrate the Ta precursor supported in step (a), so it is preferable to appropriately adjust the polarity, pH, and water content of the solvent. Furthermore, drying conditions should be such that reduced pressure (e.g., 5-50 kPa) and a temperature of approximately 40-60°C are used, taking care to prevent localized decomposition and re-aggregation of the precursor due to excessive heating. The elemental ratio in the third dried product is adjusted by adjusting the amounts added in steps (a) to (c) so that the final Pt:Ta:Co (atomic ratio) is within the desired range (e.g., the range described in the claim).

[0126] [Process (2)] Step (2) is a step in which the dried material obtained in step (1) is subjected to a first heat treatment in a non-oxidizing atmosphere to thermally decompose the catalyst composite precursor to form a PtTaCo composite, and then the PtTaCo composite is subjected to a second heat treatment at a higher temperature to obtain a catalyst composite in which the phases of a Pt-rich phase and a Ta-rich phase consisting of Ta oxide are separated.

[0127] Step (2) in the sequential loading method is substantially the same as that in the simultaneous loading method described above.

[0128] (First heat treatment) The dried material obtained in step (1) contains catalyst composite precursors supported on a porous carbon support, each containing substances derived from the respective raw materials. This is thermally decomposed by a first heat treatment in a non-oxidizing atmosphere and transformed into a PtTaCo composite.

[0129] A "non-oxidizing atmosphere" refers to an inert atmosphere such as nitrogen or argon, or a reducing atmosphere containing hydrogen.

[0130] The first heat treatment conditions should be determined within a range that thermally decomposes the catalyst complex precursor containing Pt, Ta, and Co to form a PtTaCo complex. The heat treatment temperature is determined considering the type of raw material compound of the catalyst complex precursor, but for example, it may be 150°C or higher, 180°C or higher, or 200°C or higher, with upper limits such as 500°C or lower, 400°C or lower, or 300°C or lower. As shown in the examples described later, for example, when the raw material compounds are Pt acetylacetonate, tantalum ethoxide, and cobalt nitrate, a PtTaCo composite can be formed by heat treatment at 200°C to 300°C under an inert atmosphere.

[0131] (Second heat treatment) The PtTaCo composite obtained in the first heat treatment is subjected to a second heat treatment at a higher temperature than the first heat treatment to generate a catalyst composite that undergoes phase separation into a Pt-rich phase and a Ta-rich phase consisting of Ta oxide. The catalyst composite according to the present invention contains Ta (Ta: 3007°C, Ta2O5: 1784°C), which has a high melting point, and therefore tends to be less prone to aggregation as a catalyst composite.

[0132] The temperature of the second heat treatment is higher than that of the first heat treatment, and should be such that the PtTaCo composite obtained in the first heat treatment undergoes phase separation. Specific heat treatment temperatures are 600°C or higher, 650°C or higher, 700°C or higher, and 750°C or higher, with upper limits being, for example, 1100°C or lower, 1000°C or lower, 900°C or lower, and 850°C or lower.

[0133] By performing heat treatment under these conditions, the PtTaCo composite can be separated into a Pt-rich phase and a Ta-rich phase without significantly decomposing the porous carbon support.

[0134] <2. Electrode> The electrode of the present invention comprises the electrode material of the present invention described above and a proton-conducting electrolyte material. In the electrode of the present invention, the electrode materials of the present invention are in contact with each other to form a conductive path. The electrode of the present invention can be suitably used as an electrode for fuel cells. Furthermore, the electrode material of the present invention can also be used as an electrode for purposes other than fuel cells (for example, an electrode for a polymer electrolyte water electrolysis device).

[0135] The following describes an electrode for a fuel cell formed using the electrode material of the present invention. Specifically, we will describe a case in which the above-mentioned electrode material is used as an electrode in a PEFC.

[0136] The electrode of the present invention may consist only of the electrode material described above, but it may also include a proton-conducting electrolyte material (hereinafter sometimes referred to as "proton-conducting electrolyte material" or simply "electrolyte material") that is typically used as an electrolyte in a fuel cell. The electrolyte material included in the electrode of the fuel cell together with the electrode material may be the same as or different from the electrolyte material used for the electrolyte membrane of the fuel cell. From the viewpoint of improving the adhesion between the fuel cell electrode and the electrolyte membrane, it is preferable to use the same material.

[0137] Proton-conducting electrolyte materials are used for the electrodes and electrolyte membranes of PEFCs. These proton-conducting electrolyte materials are broadly classified into fluorine-based electrolyte materials, which contain fluorine atoms in all or part of their polymer backbone, and hydrocarbon-based electrolyte materials, which do not contain fluorine atoms in their polymer backbone. Both types can be used as electrolyte materials.

[0138] Suitable examples of fluorine-based electrolyte materials 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.).

[0139] Suitable examples of hydrocarbon-based electrolyte materials include polymers such as polysulfonic acid, polystyrene sulfonic acid, polyarylether ketone sulfonic acid, polyphenyl sulfonic acid, polybenzimidazole sulfonic acid, polybenzimidazole phosphonic acid, and polyimide sulfonic acid, as well as polymers having side chains such as alkyl groups.

[0140] The mass ratio of the electrode material to the electrolyte material mixed with the electrode material should be appropriately determined to provide good proton conductivity within the electrode formed using these materials, and to allow smooth gas diffusion and water vapor discharge within the electrode. However, if the amount of electrolyte material mixed with the electrode material is too large, proton conductivity will improve, but gas diffusivity will decrease. Conversely, if the amount of electrolyte material mixed is too small, gas diffusivity will improve, but proton conductivity will decrease. Therefore, the mass ratio of the electrolyte material to the electrode material is preferably in the range of 10 to 50 mass%. If this mass ratio is less than 10 mass%, the continuity of the proton-conducting material will deteriorate, and sufficient proton conductivity for a fuel cell electrode cannot be ensured. Conversely, if it is greater than 50 mass%, the continuity of the electrode material will deteriorate, and it may not be possible to have sufficient electronic conductivity for a fuel cell electrode. Furthermore, the diffusivity of gases (oxygen, hydrogen, water vapor) inside the electrode may decrease.

[0141] The electrode for fuel cell of the present invention may contain components other than the electrode material and proton conductive material described above, as long as the objective of the present invention is not impaired. For example, the electrode material may contain conductive materials other than the porous carbon support included in the electrode material described above (hereinafter referred to as "other conductive materials"). Including other conductive materials may increase the number of conductive paths connecting the electrode materials, thereby improving the overall conductivity of the electrode.

[0142] Other conductive materials that can be used include known conductive materials used in fuel cell electrodes. These are typically carbon-based conductive materials, such as particulate carbon (including chain-linked carbon particles) like carbon black and activated carbon, and fibrous carbon such as carbon fibers and carbon nanotubes (CNTs). Mesoporous carbon can also be used as another conductive material.

[0143] Although the electrode material of the present invention has been described as an electrode for a PEFC, it can also be used as an electrode in various other fuel cells, such as alkaline fuel cells and phosphoric acid fuel cells. Furthermore, it can be suitably used as an electrode for a water electrolysis device that uses a polymer electrolyte membrane similar to that of a PEFC. Furthermore, the electrode for fuel cells containing the electrode material of the present invention has excellent electrochemical catalytic activity for the reduction of oxygen and the oxidation of hydrogen, and can therefore be used as a cathode and anode. In particular, it has excellent electrochemical catalytic activity for oxygen reduction, and since electrochemical oxidative decomposition of the conductive material that serves as the support does not occur under the operating conditions of the fuel cell, it can be particularly suitable for use as a cathode.

[0144] Furthermore, the fuel cell electrode of the present invention can be used as an electrode in various fuel cells other than PEFCs, such as alkaline fuel cells and phosphoric acid fuel cells. It can also be suitably used as an electrode for water electrolysis devices that use a solid polymer electrolyte membrane similar to that of PEFCs.

[0145] <3. Membrane electrode assembly (MEA)> The membrane electrode assembly of the present invention is a membrane electrode assembly having a solid polymer electrolyte membrane, a cathode bonded to one side of the solid polymer electrolyte membrane, and an anode bonded to the other side of the solid polymer electrolyte membrane, wherein either one or both of the cathode and the anode are electrodes of the present invention.

[0146] As a preferred embodiment of the present invention, a membrane electrode assembly using a fuel cell electrode containing the electrode material of the present invention as the cathode will be described. Figure 3 schematically shows the cross-sectional structure of a membrane electrode assembly according to an embodiment of the present invention. As shown in Figure 3, the membrane electrode assembly 10 has a structure in which the cathode 4 and anode 5 are arranged facing the solid polymer electrolyte membrane 6.

[0147] The cathode 4 is composed of an electrode catalyst layer 4a and a gas diffusion layer 4b. The electrode of the present invention is used in the electrode catalyst layer 4a.

[0148] Conventional known gas diffusion layers can be used as the gas diffusion layer 4b. For example, conductive carbon-based sheet-like members having a pore size distribution of about 100 nm to 90 μm, which have been conventionally used as gas diffusion layers in PEFCs, can be used. Preferably, water-repellent treated carbon paper, carbon cloth, carbon nonwoven fabric, etc., can be used. Alternatively, sheet-like members other than carbon-based materials such as stainless steel may also be used. There are no particular restrictions on the thickness of such gas diffusion layer 4b, but it is usually about 50 μm to 1 mm. Furthermore, the gas diffusion layer 4b may have a microporous layer on one side consisting of an aggregate of carbon fine particles with an average particle size of about 10 to 100 nm and a water-repellent material.

[0149] Anode 5 is composed of an electrode catalyst layer 5a and a gas diffusion layer 5b. As the electrode catalyst layer 5a, in addition to the electrode of the present invention, other known electrode catalyst layers for anodes can also be used. For example, an electrode can be formed on the gas diffusion layer 5b by coating and drying an electrode material, which is manufactured by supporting noble metal particles as a catalyst on the surface of a support made of carbon-based materials such as graphite, carbon black, activated carbon, carbon nanotubes, and glassy carbon, with a dispersion of an electrolyte material for a fuel cell. The gas diffusion layer 5b of anode 5 can be the same as the gas diffusion layer 4b described for cathode 4.

[0150] As the solid polymer electrolyte membrane 6, any known electrolyte membrane for PEFCs that has proton conductivity and possesses chemical and thermal stability may be used. Although the thickness is exaggerated in Figure 3, the thickness of the solid polymer electrolyte membrane 6 is usually around 2 to 50 μm in order to reduce electrical resistance.

[0151] Examples of electrolyte materials constituting the solid polymer electrolyte membrane 6 include fluorine-based electrolyte materials and hydrocarbon-based electrolyte materials. Electrolyte membranes formed from fluorine-based electrolyte materials are particularly preferred due to their excellent heat resistance and chemical stability. Specific examples of suitable materials 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.).

[0152] The embodiments of the film electrode assembly of the present invention have been described above with reference to the drawings, but these are merely examples of the present invention, and various other configurations can also be adopted.

[0153] <4. Polymer electrolyte fuel cell> The polymer electrolyte fuel cell (single cell) of the present invention comprises the membrane electrode assembly of the present invention, and typically has a structure in which the membrane electrode assembly is sandwiched between separators in which gas channels are formed.

[0154] Figure 4 is a conceptual diagram showing a typical configuration of the solid polymer fuel cell of the present invention. As shown in Figure 4, in the solid polymer fuel cell 20, hydrogen is supplied to the anode 5, and (Reaction 1) 2H2 → 4H + +4e - By this, the generated protons (H + ) are supplied to the cathode 4 through the solid polymer electrolyte membrane 6, and the generated electrons are supplied to the cathode through the external circuit 21. (Reaction 2) O2 + 4H + +4e - → 2H2O reacts with oxygen to generate water. This electrochemical reaction between the anode and the cathode generates a potential difference between the two electrodes. In the solid polymer fuel cell of the present invention, since the components other than the membrane electrode assembly of the present invention are the same as those of a known solid polymer fuel cell, a detailed description thereof will be omitted. Actually, a fuel cell stack is formed by stacking the solid polymer fuel cells (single cells) of the present invention by a number corresponding to the power generation performance, and it is used by assembling other accompanying devices such as a gas supply device and a cooling device.

Examples

[0155] Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited thereto. In the following, Ketjen black may be described as "KB" and mesoporous carbon may be described as "MC". Also, the "porous carbon carrier" may sometimes be simply described as the "carbon carrier".

[0156] A. Electrode material (A) (PtTaCo / KB, co-impregnation method) A1. Preparation of electrode material As the electrode material (A) of the example, as shown in the flowchart of Figure 5, the electrode material of Example 1A below was manufactured by the co-impregnation method.

[0157] The porous carbon carrier and metal catalyst precursor compound (raw material compound) used are as follows. <Porous carbon carrier (carbon carrier)> As the carbon support, Ketjen black (KB) (EC600JD, Lion Specialty Chemicals Co., Ltd.) was used. <Pt raw material compound> As the Pt raw material compound, Platinum(II) acetylacetonate (Sigma Aldrich) (hereinafter sometimes referred to as "Pt(acac)2") was used. <Ta raw material compound> As the Ta raw material compound, tantalum ethoxide (Ta(OC2H5)5, High-Purity Chemical Research Institute Co., Ltd.) was used. <Co raw material compound> As the Co raw material compound, cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, Kishida Chemical Co., Ltd.) was used.

[0158] <Example 1A: Pt3Ta4Co1 / KB> Step (1) First, as raw material compounds of Pt, Co and Ta, Pt(acac)2 was dissolved in dichloromethane (10 mL), (Ta(OC2H5)5 and (Co(NO3)2·6H2O were respectively dissolved in ethanol. The charge amount was such that Pt:Ta:Co (atomic ratio) = 3:4:1. Next, each reagent solution was added to a dispersion of 125 mg of Ketjen black (KB) previously ultrasonicated in an ethanol solution, and finally about 20% by volume ratio of the total volume of the solvent was added with acetone to make a dispersion (total amount of ethanol: 110 mL, acetone: 30 mL). The charge amounts of the Pt raw material compound (Pt(acac)2), the Ta raw material compound (Ta(OC2H5)5) and the Co raw material compound (Co(NO3)2·6H2O) were set to be 46.7 wt% as the loading amount with respect to the entire electrode material of the PtTaCo composite (loading amount as Pt: 20 wt%). Next, the eggplant flask containing the sample was set in a rotary evaporator equipped with a vacuum function and a rotation function, and the pressure was reduced while applying ultrasonic waves until all the solvent evaporated, and it was rotated until the solvent completely evaporated and evaporated to dryness to obtain a dry powder in which a precursor of the PtTaCo composite was adsorbed on the KB surface (inner and outer surfaces of the pores).

[0159] Process (2) The powder obtained in step (1) was heated to 210°C in an Ar atmosphere (heating rate 1°C / min) and held for 3 hours, and then heated to 240°C in an Ar atmosphere (heating rate 1°C / min) and held for 3 hours. After that, it was heated to 800°C in an Ar atmosphere over 30 minutes and held for 2 hours to prepare the electrode material (Pt3Ta4Co1 / KB) of Example 1A.

[0160] <Example 2A: Pt7Ta2Co1 / KB> The electrode material for Example 2A (Pt7Ta2Co1 / KB) was prepared in the same manner as in Example 1A, except that the amount of raw material compound charged in step (1) was changed to Pt:Ta:Co (atomic ratio) = 7:2:1, and the holding time at 800°C in step (2) was set to 30 minutes.

[0161] <Comparative example 1A: Pt3Co1 / KB> The electrode material for Comparative Example 1A (Pt3Co1 / KB) was prepared in the same manner as in Example 1A, except that the amount of raw material compound charged in step (1) above was changed to Pt:Ta:Co (atomic ratio) = 3:0:1 (without using the Ta compound).

[0162] <Reference example 1> For Reference Example 1, a Pt / C catalyst (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E, Pt 46wt%) was used as the electrode material.

[0163] <Reference example 2> As a reference example 2, a Pt3Co1 / C catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC36E52, Pt 46 wt%, Co 5 wt%, Pt:Co (atomic ratio) = 3:1) was used.

[0164] A2. Physical property evaluation A2-1. Microstructure evaluation Figure 6 shows the STEM-EDS analysis results of the electrode material of Example 1A prepared by the method described above. Figure 6(a) is the STEM image, and (b) is the EDS mapping image (Pt+Ta+Co).

[0165] The STEM image in Figure 6(a) shows that particles with a diameter of approximately 2 nm are uniformly and highly dispersed on the carbon support (KB). The EDS mapping image in Figure 6(b) confirmed that particles composed of PtCo and particles composed of Ta overlapped. Therefore, it was determined that in the electrode material of Example 1A, the elements Pt, Co, and Ta were not alloyed as a whole, but rather separated into a Pt-rich phase and a Ta-rich phase, with the Ta-rich phase filling the space between the Pt-rich phases.

[0166] A2-2. Evaluation using XRD Figure 7 shows the X-ray diffraction (XRD) patterns of the electrode materials for Example 1A (Pt3Ta4Co / KB) and Comparative Example 1A (Pt3Co / KB). As shown in Figure 7, since a diffraction peak originating from Pt3Co was observed in Example 1A, similar to Comparative Example 1A, it was determined that the Pt-rich particles in Example 1A are based on a Pt-Co alloy. Furthermore, since a peak originating from Ta oxide (TaOx) is present around 57 degrees in Example 1A, it was determined that the Ta-rich phase in Example 1A is Ta oxide (TaOx). Thus, the XRD evaluation also suggested that phase separation of the catalyst was occurring.

[0167] A3. Electrochemical evaluation (half-cell) A3-1. Preparation of evaluation electrodes The electrodes for evaluation were fabricated using the following procedure. First, a mixture of 19 mL of ultrapure water and 6 mL of 2-propanol was added to a sample bottle containing the electrode material powder. Subsequently, 5% Nafion dispersion was added to achieve an I / C (ionomer / carbon) ratio of 0.5. The sample bottle was then immersed in ice water and ultrasonically stirred for 30 minutes to obtain the electrode material dispersion. The amount of electrode material powder was determined so that when 10 μL of the electrode material dispersion was dropped onto the electrode, the Pt mass per unit area on the electrode was 10 μg-Pt·cm. -2The following procedure was followed: 10 μL of the prepared electrode material dispersion was dropped onto a glassy carbon (GC) disk electrode using a micropipette, fixed to a rotating device, and dried at room temperature for approximately 30 minutes at a rotation speed of 300 rpm to form a Nafion film and fix the electrode material onto the GC electrode, thereby obtaining an electrode (working electrode) for evaluation.

[0168] A3-2. Evaluation of Cyclic Voltammetry (CV) The electrode materials of Example 1A (Pt3Ta4Co / KB) and Comparative Example 1A (Pt3Co / KB) were evaluated by cyclic voltammetry (CV). The electrochemical effective surface area (ECSA) was calculated from the amount of hydrogen adsorption obtained from the CV. Note that ECSA corresponds to the effective surface area of ​​Pt contained in the electrode material.

[0169] The measurement conditions for CV are as follows. Assuming that one hydrogen atom is adsorbed per platinum atom, the value is 210 μC / cm². 2 This will result in the amount of electricity. Measurement: Three-electrode cell (working electrode: fuel cell electrode for evaluation, counter electrode: graphite or Pt, reference electrode: Ag / AgCl) Electrolyte: 0.1M HClO4 (pH: approx. 1) Measurement potential range: 0.05~1.2V (reversible hydrogen electrode reference) Scanning speed: 50 mV / s Hydrogen adsorption amount: Calculated from the peak area showing hydrogen adsorption of 0.05~0.4V. Electrochemical Effective Surface Area (ECSA): Calculated using the following formula. ECSA=(hydrogen adsorption amount)[μC] / 210[μC / cm 2 ]

[0170] A3-3. Evaluation of ORR activity ORR activity was evaluated for the electrode materials of Example 1A (Pt3Ta4Co / KB) and Comparative Example 1A (Pt3Co / KB). ORR activity is determined by linear sweep voltammetry (LSV) using the rotating disk electrode method (RDE method), and the resulting activation-controlled current (i k The Mass activity (activity per unit Pt mass, mass activity) calculated based on the above was used as the indicator. Mass holding = i k Pt mass on the electrode Activation dominant current (i k ) refers to the current-potential curve obtained by rotational electrode measurement, where i is set at any potential. -1 and ω -1 / 2 The intercept was obtained by plotting the data and creating a Koutecky-Levich plot, then extrapolating the resulting straight line. The specific procedure is as follows: First, bubble O2 at 50 mL / min for 30 minutes, then 0.2V RHE From there, towards the noble direction, 1.2V at 10mV / s. RHE The potential was scanned up to and measured. During the measurement, O2 was purged at 50 mL / min. RHE This is the potential relative to the reversible hydrogen electrode (RHE).

[0171] Figure 8 shows the ECSA evaluation results and Figure 9 shows the mass activity (MA) evaluation results for Example 1A (Pt3Ta4Co / KB), Comparative Example 1A (Pt3Co / KB), and Reference Example 1 (Pt / C, TEC10E50E). As shown in Figure 8, the ECSA value for Example 1A was higher than that of Comparative Example 1A and Reference Example 1, confirming improved catalytic performance. The improvement in ECSA in Example 1A was determined to be due to the suppression of growth caused by aggregation of Pt-rich particles, which are the actual catalytic component, by the presence of the Ta-rich phase. Furthermore, as shown in Figure 9, the MA value for Example 1A was higher than that of Comparative Example 1A and Reference Example 1. Therefore, it was determined that Example 1A had superior performance compared to Comparative Example 1A from the perspective of MA. The improvement in MA in Example 1A was suggested to be influenced by factors such as spillover effects.

[0172] A3-4. Load fluctuation cycle test The load fluctuation cycle test was performed by applying a potential cycle that simulated load fluctuations, according to the method recommended by the Fuel Cell Commercialization Promotion Council (FCCJ) (Proposal for Targets, Research and Development Issues and Evaluation Methods for Solid Polymer Fuel Cells, published May 2023). The load fluctuation cycle shown in Figure 10 is a cycle that accelerates degradation involving the dissolution and reprecipitation of the catalyst itself, and was performed at 0.6~0.95V. RHE The experiment was conducted by applying a square wave for 6 seconds, with each cycle lasting 3 seconds.

[0173] For Example 1A (Pt3Ta4Co / KB) and Reference Example 1 (Pt / C, TEC10E50E), the ECSA from 0 (initial) cycles to 30,000 cycles and 0.9V RHE MA was measured in [location]. Figure 11 shows the results for cycle count and ECSA (relative value), and Figure 12 shows the results for cycle count and MA (relative value).

[0174] As shown in Figure 11, in Reference Example 1, ECSA decreased as the number of cycles increased, falling to about 50% after 30,000 cycles, whereas in Example 1A, the decrease in ECSA was smaller, and the ECSA retention rate remained at about 90% even after 30,000 cycles. Furthermore, as shown in Figure 12, MA decreased to a similar extent in both Example 1A and Reference Example 1 as the number of cycles increased. Observation of Example 1A after the test revealed that fine particles remained, suggesting that the presence of Ta suppressed the aggregation of PtCo particles.

[0175] Furthermore, as a long-term test of load fluctuation cycles, load fluctuation cycles (400,000 cycles) were performed in Example 2A (Pt7Ta2Co1 / KB), Reference Example 1 (Pt / C), and Reference Example 2 (Pt3Co1 / C). Figure 13 shows the relationship between the number of load fluctuation cycles and the electrochemical effective surface area (ECSA) for each electrode, and Figure 14 shows the relationship between the number of load fluctuation cycles and the mass activity (MA).

[0176] As shown in Figure 13, in the initial stages of the load fluctuation cycle (cycle count 0), the ECSA of Example 2A was equal to or greater than that of Reference Example 1 and greater than that of Reference Example 2. Subsequently, as the number of load fluctuation cycles increased, the ECSA of Reference Example 1 decreased and that of Reference Example 2 remained stable, whereas in Example 2A, although the ECSA decreased as the number of load fluctuation cycles increased, it maintained a value greater than that of both Reference Example 1 and Reference Example 2. Therefore, Example 2A was determined to have superior durability from the perspective of ECSA compared to Reference Examples 1 and 2.

[0177] As shown in Figure 14, in the initial stage of the load fluctuation cycle (cycle count 0), Example 2A showed a MA equivalent to that of Reference Example 1 and a smaller value than that of Reference Example 2. Subsequently, as the number of load fluctuation cycles increased, the MA decreased significantly in Reference Examples 1 and 2, but the decrease in MA in Example 2A was more gradual compared to Reference Examples 1 and 2. Therefore, Example 2A was judged to have superior durability compared to Reference Examples 1 and 2, even from the perspective of MA.

[0178] Thus, in Example 2A, high dispersion on the Ketjenblack (KB) surface was maintained even after 400,000 load fluctuation cycles. Compared to conventional catalysts, it was determined that this method prevents detachment and enlargement of catalyst particles due to dissolution during load fluctuation cycle testing.

[0179] A4. Electrochemical evaluation (start-up / shutdown cycle test, MEA (PEFC single cell)) The electrode materials of Example 1A (Pt3Ta4Co / KB), Comparative Example 1A (Pt3Co / KB), and Reference Example 1 (Pt / C) underwent start-stop cycle tests using the method recommended by the FCCJ (Proposal for Goals, Research and Development Issues, and Evaluation Methods for Polymer Electrode Fuel Cells, published January 2011). The start-stop cycle test is a cycle test that accelerates carbon corrosion, specifically the 1.0~1.5V shown in Figure 15. RHEThe rectangular wave is applied for 2 seconds per cycle, and the degradation behavior of the electrode catalyst after the cycle test is evaluated as the ECSA retention rate.

[0180] The MEA (PEFC single cell) and conditions used for evaluation are as follows: (solid electrolyte membrane) Nafion membrane (manufactured by DuPont, Nafion 212) (anode) • Electrode catalyst layer: Pt / C catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) • Gas diffusion layer: Carbon fiber gas diffusion layer (EC-TP1-060T, manufactured by Toray Industries, Inc.) (Cathode) • Electrode catalyst layer: Example 1A, Comparative Example 1A, Reference Example 1 • Gas diffusion layer: Carbon fiber gas diffusion layer with microporous layer (22BB, manufactured by SGL Carbon, Germany)

[0181] A self-made jig for evaluating single-cell power generation, incorporating the PEFC single cell to be measured, was placed in a constant temperature chamber set to 80°C, and the test was conducted under the following conditions. (Anode condition) Electrode area: 1cm 2 Supply gas type: 100% H2 Gas supply rate: 348 mL / min Supply gas humidification temperature: 80℃ (relative humidity: 100%) (Cathode condition) Electrode area: 1cm 2 Supply gas type: Air Gas supply rate: 830 mL / min Supply gas humidification temperature: 80℃ (relative humidity: 100%)

[0182] Figure 16 shows the relationship between the number of start-stop cycle tests and ECSA for the electrode materials of Example 1A, Comparative Example 1A, and Reference Example 1. The ECSA of Example 1A (Pt3Ta4Co / KB) decreased up to 5000 cycles, but became stable after 5000 cycles, and after 30,000 cycles, it showed an ECSA retention rate far exceeding that of Comparative Example 1A (Pt3Co / KB) without Ta and Reference Example 1 (Pt / C) which is a standard catalyst. Thus, in the PEFC single cell (MEA) test, a clear improvement in start-stop durability was confirmed for Example 1A. Although the ECSA of Example 1A also decreased initially (up to 5000 cycles), it was due to the oxidative corrosion of the carbon support (KB) that was in contact with the Pt-rich phase among the Pt-rich phase and Ta-rich phase constituting the catalyst composite. After that, the carbon support and the Ta-rich phase came into contact, and it was judged that the progress of oxidative corrosion was suppressed.

[0183] B. Electrode Material (B) (PtTaCo / MC, co-impregnation method) B1. Preparation of Electrode Material (B) As the electrode material (B) of the examples, the electrode materials of the following Examples 1B and 2B were produced by the co-impregnation method as shown in the flowchart of FIG. 17.

[0184] The porous carbon support and metal catalyst precursor compound (raw material compound) used were as follows. <Porous Carbon Support (Carbon Support)> As the carbon support, the following mesoporous carbon (MC) (manufactured by Toyo Tanso Co., Ltd., "Porous Carbon CNovel MH-18 (grade name)") was used. Average particle size: 2 μm Designed pore diameter: 5 nm Specific surface area: 1200 m 2 / g Total pore volume: 1.4 mL / g Micropore volume: 0.4 mL / g Graphitized treatment available <Pt Raw Material Compound> As the Pt raw material compound, Platinum(II) acetylacetonate (Sigma Aldrich) (hereinafter sometimes referred to as "Pt(acac)2") was used. <Ta Raw Material Compound> As the Ta raw material compound, tantalum ethoxide (Ta(OC2H5)5, High-Purity Chemical Research Institute Co., Ltd.) was used. <Co raw material compound> As the Co raw material compound, cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, Kishida Chemical Co., Ltd.) was used.

[0185] <Example 1B: Pt3Ta2Co / MC> (Preparation of carbon support (grinding and heat treatment of MC)) The carbon support was prepared as follows. First, the above MC (MH-18, initial average particle size: 2 μm) was subjected to ball milling treatment (2 hours) and ground to about 0.5 to 1 μm. Next, the ground product was heat-treated at 1500 °C in an argon atmosphere for 2 hours to graphitize the surface exposed by grinding, thereby obtaining an MC support (hereinafter referred to as "graphitized MC" or simply "MC") composed of graphitized MC.

[0186] Step (1) As raw material compounds of Pt, Co, and Ta, Pt(acac)2 was dissolved in dichloromethane, (Ta(OC2H5)5, and (Co(NO3)2·6H2O were respectively dissolved in ethanol. The charging amounts were such that Pt:Ta:Co (atomic ratio) = 3:2:1. Next, each reagent solution was added to a dispersion of graphitized MC that had been ultrasonically dispersed in an ethanol solution in advance, and finally, about 20% of acetone was added by volume ratio of the entire solvent to obtain a dispersion. The charging amounts of the Pt raw material compound (Pt(acac)2), Ta raw material compound (Ta(OC2H5)5), and Co raw material compound (Co(NO3)2·6H2O) were set so that the loading amount with respect to the entire electrode material of the PtTaCo composite was 34.4 wt% (loading amount as Pt: 20 wt%). Next, the round-bottom flask containing the sample was placed in a rotary evaporator equipped with a vacuum and rotation function. The pressure was reduced while applying ultrasound until all the solvent evaporated, and the evaporator was rotated until the solvent was completely evaporated to dryness, thereby obtaining a dry powder in which the precursor of the PtTaCo composite was adsorbed onto the MC surface (inner and outer pore surfaces).

[0187] Process (2) The powder obtained in step (1) was heated to 210°C in an Ar atmosphere (heating rate 1°C / min) and held for 3 hours, and then heated to 240°C in an Ar atmosphere (heating rate 1°C / min) and held for 3 hours. Subsequently, it was heated to 800°C in an Ar atmosphere over 30 minutes and held for 2 hours to prepare the electrode material (Pt3Ta2Co1 / MC) of Example 1B, which consists of graphitized MC supporting Pt3Ta2Co1.

[0188] <Example 2B: Pt3Ta4Co / MC> The electrode material of Example 2B (Pt3Ta4Co1 / MC) was prepared, except that the amounts of Pt, Ta, and Co raw material compounds charged in step (1) were set to Pt:Ta:Co (atomic ratio) = 3:4:1.

[0189] <Comparative example 1B: Pt3Co1 / MC> In step (1), the electrode material (Pt3Co1 / MC) of Comparative Example 1B was prepared, except that the Ta raw material compound was not used and the amounts of the Pt raw material compound and Co raw material compound charged were set to Pt:Co (atomic ratio) = 3:1.

[0190] B2. Physical property evaluation Figures 18 to 20 show the microstructure observations of the electrode materials of Example 1B, Example 2B, and Comparative Example 1B. In all electrode materials, it was confirmed that catalyst particles (catalyst composites) with a diameter of 3 nm or less were uniformly and highly dispersed on the carbon support (MC). The crystallite sizes of PtCo calculated from the XRD peaks were 1.5 nm for Example 1B (Pt3Ta2Co1), 1.7 nm for Example 2B (Pt3Ta4Co1), and 2.4 nm for Comparative Example 1B (Pt3Co1). A tendency for the crystallite size to decrease with the addition of Ta was observed.

[0191] B2. Electrochemical Evaluation (Half Cell) For the electrode materials of Example 2B and Comparative Example 1B, evaluation by electrochemical measurement was performed. The method for preparing the evaluation electrode and the evaluation method are the same as those of "A2. Electrochemical Evaluation" of the above-described electrode catalyst A.

[0192] CV measurement and ORR activity evaluation were performed on the electrodes using the electrode materials of Example 2B (Pt3Ta4Co / MC) and Comparative Example 1B (Pt3Co / MC), and ECSA and MA were determined. The results of ECSA are shown in Fig. 21, and the results of MA are shown in Fig. 22. For comparison, the results of Example 1A (Pt3Ta4Co / KB), Comparative Example 1A (Pt3Co / KB), and Reference Example 1 (Pt / KB) are also shown in Figs. 21 and 22.

[0193] Example 2B (Pt3Ta4Co / MC) using mesoporous carbon (MC) as the carrier showed larger values of both ECSA and MA as compared with Example 1A (Pt3Ta4Co / KB) using Ketjen black (KB) as the carrier. Since the ECSA of Example 2B is larger than that of Comparative Example 1B, it can be said that the catalytic particles (catalytic complex) in the electrode material containing Ta result in a large effective surface area in terms of Pt. In addition, Example 2B showed a performance exceeding 500 A / g in MA indicating the activity per Pt mass, and it was confirmed that it has a catalytic activity about three times higher than that of Reference Example 1 which is a standard catalyst.

[0194] C. Electrode Material (C) (PtTaCo / KB, Sequential Loading Method) C1. Preparation of Electrode Material As the electrode material (C) of the example, the electrode material of Example 1C below was produced by the sequential loading method as shown in the flowchart of Fig. 23.

[0195] The porous carbon carrier and metal catalyst precursor compound (raw material compound) used are the same as those in the above-described "A. Electrode Material (A) (PtTaCo / KB, Simultaneous Loading Method)", and thus the description is omitted.

[0196] <Example 1C: Pt3Ta2Co1 / KB, sequential loading method> Step (1) Step (a): An ethanol solution in which a predetermined amount of Ta precursor (Ta(OC2H5)5) was dissolved was added to a eggplant flask containing a dispersion of a predetermined amount of Ketjen Black (KB) ultrasonically dispersed in an ethanol solution, and the mixture was stirred well. Subsequently, the eggplant flask containing the sample was set in a rotary evaporator equipped with a vacuum function and a rotation function, and the pressure was reduced while applying ultrasonic waves until all the solvent evaporated, and it was rotated until the solvent completely evaporated and evaporated to dryness, thereby obtaining a dry powder (the first dried product) in which the Ta precursor was fixed on the KB surface (inner and outer pore surfaces). Step (b): Dichloromethane in which a predetermined amount of Pt raw material compound (Pt(acac)2) was dissolved was added to the KB on which the dry powder (the first dried product) was fixed, and the mixture was stirred well. Subsequently, the eggplant flask containing the sample was set in a rotary evaporator, and it was rotated while applying ultrasonic waves until the solvent completely evaporated and evaporated to dryness, thereby obtaining a dry powder (the second dried product) in which the Ta precursor and the Pt precursor were fixed on the KB surface (inner and outer pore surfaces). Step (c): Ethanol in which a predetermined amount of Co raw material compound (Co(NO3)2·6H2O) was dissolved was added to the KB on which the dry powder (the second dried product) was fixed, and the mixture was stirred well. Subsequently, the eggplant flask containing the sample was set in a rotary evaporator, and it was rotated while applying ultrasonic waves until the solvent completely evaporated and evaporated to dryness, thereby obtaining a dry powder (the third dried product) in which the Ta precursor, the Pt precursor, and the Co precursor were fixed on the KB surface (inner and outer pore surfaces).

[0197] In the above step (1), the charged amounts of the Pt raw material compound (Pt(acac)2), the Ta raw material compound (Ta(OC2H5)5), and the Co raw material compound (Co(NO3)2·6H2O) were such that Pt:Ta:Co = 3:2:1 (atomic ratio), and the loading amount with respect to the entire electrode material of the PtTaCo composite was 46.7 wt% (loading amount as Pt: 20 wt%).

[0198] Process (2) The powder obtained in step (1) (third dried product) was heated to 210°C in an Ar atmosphere (heating rate 1°C / min) and held for 3 hours, and then heated to 240°C in an Ar atmosphere (heating rate 1°C / min) and held for 3 hours. After that, it was heated to 800°C in an Ar atmosphere over 30 minutes and held for 2 hours to prepare the electrode material for Example 1C (Pt3Ta2Co1 / KB, sequential loading method).

[0199] <Example 3A: Pt3Ta2Co1 / KB, simultaneous loading method> In the method for manufacturing the electrode material described above as "A. Electrode Material (A) (PtTaCo / KB, simultaneous loading method)", the electrode material of Example 3A (Pt3Ta2Co1 / KB, simultaneous loading method) was prepared in the same manner as in Example 1A, except that the amount of raw material compound charged in step (1) was changed to Pt:Ta:Co (atomic ratio) = 3:2:1.

[0200] C2. Physical Property Evaluation C2-1. Microstructure Evaluation Figure 24 shows the results of evaluating the microstructure of the electrode materials of Example 3A and Example 1C, prepared using the method described above, by scanning transmission electron microscopy (STEM). From the STEM image in Figure 24(a), it can be confirmed that in Example 3A, particles with a particle size of approximately 2-3 nm are uniformly and highly dispersedly supported on the carbon support (KB). In contrast, in Example 1C, which was prepared by the sequential loading method, the dispersibility of the particles is as good as that of the simultaneous loading method, and it can be seen that the particles are uniformly dispersed and supported on the carbon support. The representative particle size of Example 1C is approximately 5 nm, but all are controlled to the nanometer scale, indicating that a microstructure effective for electrode reactions has been formed.

[0201] C2-2. Evaluation using XRD The electrode materials prepared as described above were evaluated by X-ray diffraction (XRD). Figure 25 shows the X-ray diffraction (XRD) patterns of the electrode materials for Example 3A (simultaneous loading method) and Example 1C (sequential loading method). As shown in Figure 25, in all loading methods, characteristic diffraction peaks attributable to the Ta2O5 phase were observed in the diffraction patterns, confirming the formation of the tantalum oxide phase, and no peak disappearance or new phase formation dependent on the synthesis conditions was observed. In addition, a relative shift near Pt(111), suggesting a reduction in interplanar spacing corresponding to the Pt-Co alloy phase, was observed for the noble metal component, indicating that alloying proceeded stably in all methods.

[0202] Crystallite size evaluation revealed that the crystallite size based on Pt(111) was 2.12 nm for Example 3A (simultaneous loading method) and 2.09 nm for Example 1C (sequential loading method). No significant difference in crystallite growth behavior was observed between the loading methods. In other words, it was determined that the formation of fine crystals was maintained in the sequential loading method as in the simultaneous loading method.

[0203] Based on the above, it was found that the formation of the Ta2O5 phase and the Pt-Co alloy phase was reproducibly achieved in all loading methods, and that the alloying and phase separation behaviors were similarly reproducible. In particular, the sequential loading method was found to show results equivalent to the simultaneous loading method in terms of indicators such as crystallite size, peak position, and peak width.

[0204] C3. Electrochemical evaluation (half-cell) The electrode materials of Example 3A (simultaneous loading method) and Example 1C (sequential loading method) were evaluated by electrochemical measurements. The method for preparing the evaluation electrodes and the evaluation method were the same as those described in "A2. Electrochemical Evaluation" for electrode catalyst A above. Unless otherwise specified, the specific activity (SA) and mass activity (MA) were 0.90V. RHE The values ​​in were used.

[0205] Figure 26 shows the results of CV measurements and ORR activity evaluations of electrodes using the electrode materials of Example 3A (simultaneous loading method) and Example 1C (sequential loading method), and how the electrochemical effective surface area (ECSA), specific activity (SA), and mass activity (MA) were determined.

[0206] As shown in Figure 26, the SA values ​​for Example 3A and Example 1C were similar, indicating that the reactivity per active site was equivalent under the same measurement conditions. Therefore, it was determined that the quality of the active sites was maintained regardless of the support method, and that the Pt-Co alloy phase could be formed. On the other hand, in Example 1C using the sequential loading method, ECSA decreased compared to Example 3A using the simultaneous loading method, and as a result, MA tended to decrease relatively. This result is thought to be partly due to the increase in grain size of the Pt-Co alloy phase, as explained in the microstructure observation above.

[0207] As described above, in the comparison between the simultaneous loading method and the sequential loading method, SA was similar, and the quality of the active sites was maintained regardless of the loading method. However, in the sequential loading method, ECSA decreased due to the increase in particle size, and as a result, MA tended to decrease relatively. However, the sequential loading method is characterized by the expectation of improved durability due to the formation of a Ta-rich phase on the carbon support surface, which contributes to the practically important performance of maintaining activity during long-term operation. Therefore, even if the simultaneous loading method may be advantageous in initial ECSA and MA indicators, the sequential loading method has design potential that contributes to the practically important performance of maintaining activity during long-term operation. [Industrial applicability]

[0208] The electrode material of the present invention provides a fuel cell electrode with excellent electrode catalytic activity, electronic conductivity, gas diffusion properties, and excellent durability, making it promising as an electrode component for polymer electrolyte fuel cells used in the automotive, power, gas, and home appliance industries. In particular, it is expected to be used in fuel cell vehicles (passenger cars and commercial vehicles) where load fluctuations are severe. [Explanation of Symbols]

[0209] 1 Electrode material 2. Porous carbon carrier 3. Catalyst complex 3A Pt-rich phase 3B Ta-Rich Phase 4. Fuel cell electrodes (cathodes) 4a Electrode catalyst layer (cathode) 4b Gas diffusion layer 5 Fuel cell electrode (anode) 5a Electrode catalyst layer (anode) 5b Gas diffusion layer 6 Solid polymer electrolyte membrane 10 Membrane electrode assembly (MEA) 20 Solid polymer fuel cell 21 External circuit

Claims

1. The porous carbon support comprises a catalyst composite supported on the porous carbon support. The electrode material is characterized in that the catalyst composite comprises a Pt-rich phase and a Ta-rich phase consisting of a Ta oxide.

2. The electrode material according to claim 1, wherein the catalyst composite has a structure in which the Ta-rich phase is interposed in the gaps between the Pt-rich phases.

3. The electrode material according to claim 1 or 2, wherein the Pt-rich phase and the Ta-rich phase are formed by phase-separating a PtTaCo composite.

4. The electrode material according to claim 1, wherein the Pt-rich phase is PtCo alloy particles.

5. The electrode material according to claim 1, wherein the Ta-rich phase includes a crystalline Ta oxide.

6. The electrode material according to claim 1, wherein the Ta-rich phase is an oxygen-deficient Ta oxide.

7. The electrode material according to claim 1, wherein the total amount of Pt, Co, and Ta (100 atomic%) is 30 to 70 atomic percent of Pt, 25 to 60 atomic percent of Ta, and 5 to 30 atomic percent of Co.

8. The electrode material according to claim 1, wherein the total amount of Pt, Co, and Ta (100 atomic%) is 40-60 atomic%, Ta is 25-40 atomic%, and Co is 10-30 atomic%.

9. The electrode material according to claim 1, wherein the total amount of Pt, Co, and Ta (100 atomic%) is 30 to 50 atomic percent of Pt, 40 to 60 atomic percent of Ta, and 5 to 20 atomic percent of Co.

10. The electrode material according to claim 1, wherein the porous carbon support is particulate solid carbon and / or mesoporous carbon.

11. An electrode comprising the electrode material described in claim 1 and a proton-conducting electrolyte material.

12. A membrane electrode assembly comprising a solid polymer electrolyte membrane, a cathode bonded to one side of the solid polymer electrolyte membrane, and an anode bonded to the other side of the solid polymer electrolyte membrane, wherein either the anode or the cathode, or both, are the electrodes described in claim 11.

13. A polymer electrolyte fuel cell comprising the membrane electrode assembly described in claim 12.

14. A method for manufacturing an electrode material according to claim 1, comprising the following steps (1) to (2). Step (1): A step to obtain a dried product on which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon carrier by distilling off the solvent from a solution obtained by mixing a dispersion of porous carbon carrier in a solvent with a solution containing a Pt raw material compound, a Ta raw material compound, and a Co raw material compound. Step (2): The dried material obtained in step (1) is subjected to a first heat treatment in a non-oxidizing atmosphere to thermally decompose the catalyst composite precursor to form a PtTaCo composite, and then the PtTaCo composite is subjected to a second heat treatment at a higher temperature to obtain a catalyst composite in which the phases of a Pt-rich phase and a Ta-rich phase consisting of Ta oxide are separated.

15. A method for manufacturing an electrode material according to claim 1, comprising the following steps (1) to (2). Step (1): A step to obtain a dried product in which a catalyst composite precursor containing Pt, Ta, and Co is supported on a porous carbon support by the following steps (a) to (c). Step (a): A step to obtain a first dried product in which a precursor containing Ta is supported on the porous carbon carrier by adding a solution containing a Ta raw material compound to a dispersion in which a porous carbon carrier is dispersed in a solvent, mixing the solution, and then removing the solvent by distillation. Step (b): A step in which a second dried product is obtained by dispersing the first dried product obtained in step (a) in a solvent, adding a solution containing one of the raw material compounds selected from the group consisting of Pt raw material compounds and Co raw material compounds to the dispersion, mixing, and distilling off the solvent, thereby obtaining a second dried product in which a precursor containing Ta and an element derived from one of the raw material compounds is supported on the porous carbon carrier. Step (c): A third dry product is obtained by dispersing the second dry product obtained in step (b) in a solvent, adding a solution which is one raw material compound to the other raw material compound, mixing the solution, and distilling off the solvent, thereby obtaining a third dry product in which a catalyst composite precursor containing Pt, Ta, and Co is supported on the porous carbon carrier. Step (2): The dried material obtained in step (1) is subjected to a first heat treatment in a non-oxidizing atmosphere to thermally decompose the catalyst composite precursor to form a PtTaCo composite, and then the PtTaCo composite is subjected to a second heat treatment at a higher temperature to obtain a catalyst composite in which the phases of a Pt-rich phase and a Ta-rich phase consisting of Ta oxide are separated.

16. The manufacturing method according to claim 15, wherein a Pt raw material compound is used in step (b) and a Co raw material compound is used in step (c).

17. The manufacturing method according to claim 14 or 15, wherein in step (2), the temperature of the first heat treatment is 150°C or more and 500°C or less.

18. The manufacturing method according to claim 14 or 15, wherein in step (2), the temperature of the second heat treatment is 600°C or more and 1100°C or less.

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