Electrode material, electrode, membrane electrode assembly, and solid polymer-type fuel cell

Abstract

Provided is an electrode material which can achieve both high activity and potential cycle durability and from which a fuel cell electrode having, in particular, excellent load fluctuation cycle characteristics is formed. This electrode material contains a conductive carrier and a catalyst composite supported on the conductive carrier, wherein the catalyst composite includes a PtTaCo composite composed of platinum (Pt) as a first component, tantalum (Ta) as a second component, and cobalt (Co) as a third component. A catalyst composite obtained after phase-separating the PtTaCo composite in the electrode material contains Pt-rich particles and Ta-rich particles.　

Description

Electrode materials, electrodes, membrane electrode assemblies, and polymer electrolyte fuel cells

[0001] (Citation of Related Applications) This application claims the benefit of and priority to Japanese Patent Application No. 2023-125985, filed on August 2, 2023. Priority to the aforementioned patent application is expressly claimed, and the entire disclosure of that application is incorporated herein by reference for all purposes.

[0002] The present invention relates to an electrode material suitable for electrodes of polymer electrolyte fuel cells, and to an electrode, a membrane electrode assembly, and a polymer electrolyte fuel cell using the same.

[0003] Fuel cell vehicles (FCVs) powered by polymer electrolyte fuel cells (PEFCs) are already commercially available, and their widespread use and widespread adoption are expected in trucks, buses, ships, construction machinery, and other applications. PEFCs generally have a structure in which a membrane electrode assembly (MEA), in which a pair of electrodes are arranged on both sides of a solid polymer electrolyte membrane, is sandwiched between separators with gas flow channels. Fuel cell electrodes (particularly PEFC electrodes) generally consist of an electrode catalyst layer made of an electrode material with electrocatalytic activity and a polymer electrolyte, and a gas diffusion layer that is both gas permeable and electronically conductive.

[0004] Currently, electrode materials for PEFCs are widely used, which consist of electrode catalyst particles (typically Pt or Pt alloy particles) dispersed on a carbon support. However, electrode materials using such carbon supports have the following problems: Pt peels off due to carbon corrosion during start-up and shutdown under PEFC operating conditions; Pt dissolution and precipitation due to load fluctuations increases the particle size of the Pt catalyst, resulting in a decrease in the electrochemically effective surface area (ECSA) and oxygen reduction reaction (ORR) activity of Pt.

[0005] It has been shown that the use of an electronically conductive oxide support such as niobium-doped tin oxide (Nb-SnO2) as an electrode catalyst support improves the durability of the catalyst in start-stop cycle tests (see Patent Document 1). It is also widely known that alloy catalysts composed of Pt with Co or Ni can exhibit activity greater than that of pure Pt catalysts (see, for example, Patent Document 2). Recently, research has been conducted on ternary or multi-element alloy catalysts, and it has been reported that they have achieved high initial ORR activity and durability (see, for example, Non-Patent Document 1).

[0006] Japanese Patent No. 5322110 Japanese Patent Application Laid-Open No. 2021-093270

[0007] H. Chen, C. Guan, and H. Feng, ACS Applied Nano Mater., 5 (7), 9810-9817 (2022)

[0008] However, the above-mentioned conventional electrode materials have room for improvement in terms of potential cycle durability (particularly load change cycle durability) in order to obtain practical electrode performance.

[0009] Under these circumstances, an object of the present invention is to provide an electrode material that can achieve both high activity and potential cycle durability and that can provide an electrode for a fuel cell, as well as an electrode, a membrane electrode assembly, and a polymer electrolyte fuel cell that use the same.

[0010] The present inventors have conducted extensive research to solve the above problems and have found that a catalyst composite composed of Pt, Ta, and Co can achieve both high activity and potential cycle durability, and is particularly excellent in load variation cycle characteristics, thereby completing the present invention.

[0011] That is, the present invention relates to the following inventions. <1> An electrode material comprising a conductive support and a catalytic composite supported on the conductive support, wherein the catalytic composite contains a PtTaCo composite composed of platinum (Pt) as a first component, tantalum (Ta) as a second component, and cobalt (Co) as a third component. <2> The electrode material according to <1>, in which the PtTaCo composite constituting the catalytic composite is phase-separated. <3> The electrode material according to <2>, in which the catalytic composite after phase separation contains Pt-rich particles and Ta-rich particles. <4> The electrode material according to <3>, in which the Pt-rich particles are a PtCo alloy. <5> The electrode material according to <3> or <4>, in which the Ta-rich particles are Ta oxide. <6> The electrode material according to any one of <1> to <5>, in which Pt accounts for 60 to 80 atomic %, Ta for 10 to 30 atomic %, and Co for 1 to 20 atomic %, relative to the total (100 atomic %) of Pt, Ta, and Co. <7> The electrode material according to any one of <1> to <6>, wherein the conductive support is a carbon support. <8> The electrode material according to <7>, wherein the carbon support is highly crystalline carbon.

[0012] <9> An electrode comprising the electrode material according to any one of <1> to <8> and a proton-conductive electrolyte material. <10> 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 or both of the anode and the cathode are the electrodes according to <9>. <11> A polymer electrolyte fuel cell comprising the membrane electrode assembly according to <10>.

[0013] The present invention provides an electrode material that can achieve both high activity and potential cycle durability and that provides a fuel cell electrode that is particularly excellent in load variation cycle characteristics, as well as an electrode, a membrane electrode assembly, and a polymer electrolyte fuel cell that use the same.

[0014] 1 is a conceptual schematic diagram of an electrode material of the present invention, where (a) is a first embodiment (PtTaCo composite, before phase separation) and (b) is a second embodiment (nanocomposite structure composite, after phase separation).

[0023] FIG. 1 is an explanatory diagram of phase separation of a PtTaCo composite in a catalyst composite of an electrode material of the present invention, where (a) is before phase separation and (b) is after phase separation.

[0024] FIG. 1 is a cross-sectional schematic diagram of a membrane electrode assembly of the present invention.

[0025] FIG. 1 is a conceptual diagram showing a typical configuration of a polymer electrolyte fuel cell of the present invention.

[0026] FIG. 2 is a flowchart of the preparation procedure for an electrode material (PtTaCo / KB) of an example.

[0027] FIG. 3 is a diagram showing the conditions of a load cycle test.

[0028] FIG. 4 is a diagram showing the relationship between the number of load cycles and electrochemically effective surface area (ECSA) in Example 1.

[0029] FIG. 5 is a diagram showing the relationship between the number of load cycles and mass activity (MA) in Example 1.

[0029] FIG. 6 is a diagram showing the results of STEM-EDS analysis before (initial: 0 cycles) and after (400,000 cycles) a load cycle test in Example 1.

[0015] The present invention will be described in detail below with reference to illustrative examples. The present invention is not limited to the following embodiments, and can be modified as desired without departing from the spirit of the present invention. The dimensions, materials, and other specific numerical values ​​shown in the embodiments are merely examples for facilitating understanding of the invention, and do not limit the present invention unless otherwise specified. In addition, in all drawings, similar components are designated by similar reference numerals, and descriptions thereof will be omitted where appropriate.

[0016] In this specification, when the expression "~" is used, it is used as an expression that includes the numerical values ​​before and after it. Furthermore, in this specification, the expression "A and / or B" includes "A only," "B only," and "both A and B."

[0017] <1. Electrode Material> The electrode material of the present invention includes a conductive support and a catalyst composite supported on the conductive support, wherein the catalyst composite contains a PtTaCo composite that is composed of platinum (Pt) as a first component, tantalum (Ta) as a second component, and cobalt (Co) as a third component.

[0018] The electrode material of the present invention can be made into a catalyst composite having a nanocomposite structure by subjecting the PtTaCo composite that constitutes the catalyst composite to phase separation treatment. Here, in this specification, "phase separation (of a solid)" refers to the phenomenon in which a target solid separates into multiple solid phases. Also, in this specification, "nanocomposite structure" refers to a structure in which nanometer-sized (less than 10 nm) particles of metal (including alloys) or metal compound (typically metal oxide) are integrated. The particles that constitute the nanocomposite structure may be crystalline or amorphous, and the shape and size of the particles are arbitrary and may be the same or different.

[0019] Furthermore, in this specification, the electrode material before the catalyst composite (PtTaCo composite) is phase-separated may be referred to as the "electrode material of the present invention (before phase separation)," and the electrode material after the phase separation may be referred to as the "electrode material of the present invention (after phase separation)."

[0020] Fig. 1(a) shows a conceptual schematic diagram of the electrode material of the present invention (before phase separation). As shown in Fig. 1(a), the electrode material 1 of the present invention is composed of a conductive support 2 and a catalyst complex 3 supported (fixed) on the surface of the conductive support 2 (the inner surface of the pores and the outer surface of the pores).

[0021] The conductive support 2 serves to improve the electronic conductivity when the electrode is formed, and also serves as a skeleton of the electrode. A carbon support is typically used as the conductive support, but an electronically conductive oxide support or a metal support may also be used.

[0022] The catalyst composite 3 in the electrode material (before phase separation) of the present invention is a PtTaCo composite composed of platinum (Pt) as a first component, tantalum (Ta) as a second component, and cobalt (Co) as a third component. The catalyst composite 3 is dispersed and supported on the surfaces (inner surfaces and outer surfaces of the pores) of the conductive support 2, and a portion of the surface of the conductive support 2 is exposed. Therefore, when an electrode is constructed using this electrode material, the conductive supports 2 come into contact with each other, forming low-resistance conductive paths, resulting in an electrode with excellent electronic conductivity.

[0023] The electrode material (after phase separation) of the present invention is an electrode material obtained by phase separation of the PtTaCo composite that constitutes the catalyst composite of the electrode material (before phase separation) of the present invention. Figure 1(b) shows a conceptual schematic diagram of the electrode material (after phase separation) of the present invention. Figure 2 shows an explanatory diagram of the phase separation of the PtTaCo composite.

[0024] The electrode material (after phase separation) of the present invention can be produced, for example, by subjecting the electrode material (before phase separation) of the present invention to a phase separation treatment to phase separate the PtTaCo composite. With this production method, the PtTaCo composite contained in the electrode material of the present invention is phase-separated into Pt-rich particles 3A and Ta-rich particles 3B (details of the Pt-rich particles and Ta-rich particles will be described later). As a result, the electrode material (after phase separation) of the present invention has a structure in which a nanocomposite catalyst composite 3 (nanocomposite structure composite) containing nm-sized Pt-rich particles 3A and Ta-rich particles 3B is supported on a conductive support 2.

[0025] The electrode material of the present invention can provide a fuel cell electrode that is both highly active and has excellent potential cycle durability, and is particularly excellent in load variation cycle characteristics. Therefore, a fuel cell equipped with an electrode formed from the electrode material not only exhibits excellent electrode performance but also has high durability and can generate power for a long period of time.

[0026] The components of the electrode material of the present invention will be described in detail below. Note that, although the following description will be made assuming that the electrode material of the present invention is used in an electrode for a polymer electrolyte fuel cell (PEFC), the electrode material of the present invention is not limited to this application.

[0027] In the following, the cathode conditions of a PEFC refer to the conditions at the cathode during normal operation of the PEFC, where the temperature is from room temperature to about 150°C and a gas containing oxygen such as air is supplied (oxidizing atmosphere), and the anode conditions refer to the conditions at the anode during normal operation of the PEFC, where the temperature is from room temperature to about 150°C and a fuel gas containing hydrogen is supplied (reducing atmosphere).

[0028] [Conductive Support] The conductive support is included in the electrode material of the present invention and serves to improve the electrical conductivity (electronic conductivity) when the electrode is formed, and also serves as the skeleton of the electrode. The conductive support may be any one that is electrically conductive and can support the catalyst complex. Typically, a carbon support is used as the conductive support, but an electronically conductive oxide support or a metal support may also be used.

[0029] The conductive support preferably has a large surface area and the catalyst complex is distributed on the surface in nano-size. The BET specific surface area of ​​the support is not limited, but is, for example, 100 m ２ / g~1500m ２ / g.

[0030] The size and shape of the electrode material of the present invention depend on the size and shape of the conductive support, which is its framework material, and are therefore determined within a range that allows continuous contact of the electrode material when the fuel cell electrode is formed, and that allows for the formation of a space large enough to allow smooth diffusion of gases such as hydrogen and oxygen and the discharge of water (vapor) within the fuel cell electrode.

[0031] [Carbon Support] The carbon support, which is a suitable example of the conductive support, can be any carbon support used in electrode materials, as long as it does not impair the objectives of the present invention. The shape and size of the carbon support can be selected appropriately taking into account the intended use of the electrode, etc. However, in gas diffusion electrode applications such as fuel cell electrodes, electrical conductivity and gas diffusibility within the electrode are required when the electrode is formed. Therefore, in order to achieve both electrical conductivity and gas diffusibility, if the carbon support is particulate, it is preferable that the particle size is 0.03 to 500 μm, and if it is fibrous, it is preferable that the diameter is 2 nm to 20 μm and the total length is approximately 0.03 to 500 μm.

[0032] As the carbon support, carbon materials of any shape and size such as granular, fibrous, etc. Examples of carbon materials include carbon black, acetylene black, ketjen black, activated carbon, mesoporous carbon, carbon nanotubes, carbon nanohorns, carbon nanorings, carbon nanofibers, carbon nanowires, and fullerenes.

[0033] The carbon support used in the present invention may be one type, or two or more types of carbon materials differing in size (particle size, fiber diameter, and fiber length), crystallinity, etc. may be used in any ratio.

[0034] The carbon support is preferably highly crystalline carbon. "Highly crystalline carbon" is carbon that has been graphitized (crystallized) by high-temperature heat treatment or the like, and has superior oxidation resistance compared to amorphous or low-crystalline carbon. Highly crystalline carbon can be either homemade or commercially available.

[0035] A suitable example of the carbon support is particulate solid carbon. Examples of suitable solid carbon include carbon black (CB) and highly crystalline carbon black (GCB) 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).

[0036] The solid carbon can be either homemade or commercially available. Examples of commercially available products include the "Ketjenblack" series (product number: EC600JD, etc.) from Lion Specialty Chemicals Co., Ltd., the "Vulcan" series (product number: XC-72, etc.) from Cabot Corporation, the "GCB" series (product number: GCB200, etc.) from Cabot Corporation, and the "Tokablack" series (product number: Tokablack #3800, etc.) from Tokai Carbon Co., Ltd. Commercially available solid carbon can be pulverized to adjust the particle size to the desired size before use.

[0037] Another suitable example of a carbon support is mesoporous carbon. Mesoporous carbon (hereinafter sometimes referred to as "MC") is a porous carbon having many pores in the mesopore region.

[0038] As the mesoporous carbon, porous carbon having pores in the mesopore region (2 to 50 nm) can be used, and the pore diameter is preferably 3 to 40 nm. Within this range, even when an electronically conductive oxide or an electrode catalyst is fixed (supported) on the inner wall of the pore, the diffusion of substances into the pore interior is not significantly hindered and proceeds smoothly.

[0039] Furthermore, as will be described later, when producing a fuel cell electrode, the electrode material of the present invention is mixed with a proton-conductive electrolyte material (ionomer). However, since the proton-conductive electrolyte material (ionomer) has a size of several tens of nanometers, it cannot penetrate into mesopores with small pore diameters. This makes it possible to suppress poisoning of the electrode catalyst metal supported in the pores of mesoporous carbon via the electronically conductive oxide, which is caused by the ionomer.

[0040] The mesoporous carbon according to the present invention may contain regions (micropore region, macropore) other than the mesopore region (2 nm to 50 nm), but it is preferable that the proportion of pores in the mesopore region is high.

[0041] The pore structure (pore diameter, shape, etc.) of mesoporous carbon can be confirmed by observation with an electron microscope, such as a field emission scanning electron microscope (FESEM) or a scanning transmission electron microscope (STEM).

[0042] The pores in the mesopore region of the mesoporous carbon preferably have a three-dimensional network structure, including not only individual pores independent of other pores but also interconnected pores in which some or all of the pores in the mesopore region are interconnected with pores in adjacent mesopore regions. The presence of interconnected pores promotes the diffusion of substances within the pores of the mesoporous carbon.

[0043] The mesoporous carbon used in the electrode material of the present invention may be suitably synthesized or may be a commercially available product, such as the CNovel series (designed mesopore diameter: 5 to 150 nm) manufactured by Toyo Tanso Co., Ltd., which is a mesoporous carbon templated with MgO.

[0044] The carbon support may also have an electronically conductive oxide layer on its surface. Here, "having an electronically conductive oxide layer on its surface" means that a part or all of the surface of the carbon support is covered with an electronically conductive oxide layer. Unlike carbon materials, an electronically conductive oxide layer is not oxidatively decomposed. Therefore, by supporting the catalyst composite on the electronically conductive oxide layer and preventing it from coming into direct contact with the carbon support, durability (for example, durability during start-up and shutdown of a fuel cell subjected to a high load) is improved.

[0045] The electronically conductive oxide layer on the surface of the carbon support may be any electronically conductive oxide that is stable under the cathode conditions of a PEFC, and examples thereof include electronically conductive oxides primarily 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, the term "electronically conductive oxide primarily composed of" refers to (A) an oxide consisting solely of a matrix oxide, and (B) an oxide doped with another element, containing 80 mol % or more of the matrix oxide.

[0046] Suitable examples of the electron-conductive oxide layer include a Ta2O5 layer and a Nb2O5 layer. The Ta2O5 layer and the Nb2O5 layer can be doped with other elements as required.

[0047] The thickness of the electron-conductive oxide layer is preferably 1 to 10 nm, although it depends on the type and amount of the electron-conductive oxide. The electron-conductive oxide layer preferably covers the entire surface of the carbon support, but may cover only a part of the surface.

[0048] [Catalyst Composite] The catalyst composite in the electrode material of the present invention (before phase separation) contains a PtTaCo composite composed of platinum (Pt) as a first component, tantalum (Ta) as a second component, and cobalt (Co) as a third component.

[0049] The proportions of the first to third components in the PtTaCo composite are determined within a range in which phase separation of the PtTaCo composite occurs.

[0050] A preferred composition example of the PtTaCo composite is 60 to 80 atomic % of Pt, 10 to 30 atomic % of Ta, and 1 to 20 atomic % of Co relative to the total (100 atomic %) of Pt, Ta, and Co, or 65 to 75 atomic % of Pt, 15 to 25 atomic % of Ta, and 5 to 15 atomic % of Co. Note that PtTaCo disclosed in the examples below falls within this composition range.

[0051] The PtTaCo composite constituting the catalyst composite has the first to third components (Pt, Ta, Co) as its basic components, but may contain other elements (e.g., Ni, etc.) within the scope of the present invention.

[0052] The amount of each component in the catalyst composite can be determined by inductively coupled plasma emission spectrometry (ICP).

[0053] The form of the catalyst complex supported on the conductive support may be any form as long as it does not impair the object of the present invention, and examples thereof include particulate, island, film, wire, etc. From the viewpoint of conductivity when an electrode is formed, it is preferable that the catalyst complex is particulate, the particulate catalyst complex does not completely cover the surface of the conductive support, leaving part of the surface of the conductive support exposed, and the conductive support is supported in a dispersed state to such an extent that direct contact between the conductive support and other conductive supports is not inhibited.

[0054] The size of the catalyst composite is typically 1 to 10 nm in diameter, preferably 2 to 5 nm. The "size of the catalyst composite" can be obtained by averaging the particle diameters 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 other than spherical, the length in the direction showing the maximum length of the particle is taken as the particle size.

[0055] The amount of the catalyst composite supported is determined appropriately within a range that provides sufficient electrode catalytic activity when the electrode is constructed. Typically, the catalyst composite is 5 to 60 mass% of the total weight of the electrode material, and is 0.5 to 30 mass% based on the amount of Pt. Within this range, excellent catalytic activity per unit mass is achieved, and the desired electrode reaction activity can be obtained according to the supported amount.

[0056] The PtTaCo composite constituting the catalyst composite of the electrode material (before phase separation) of the present invention may be crystalline, amorphous, or a mixture of crystalline and amorphous.

[0057] As described above, the electrode material of the present invention (before phase separation) can be made into the electrode material of the present invention (after phase separation) having a structure in which the catalyst composite (PtTaCo composite) is supported on a conductive support as a nanocomposite structure catalyst composite containing Pt-rich particles and Ta-rich particles by phase-separating the catalyst composite (PtTaCo composite) into Pt-rich particles and Ta-rich particles. The phase separation treatment method may be any treatment method that can significantly phase-separate the PtTaCo composite, and examples thereof include high-temperature heat treatment, chemical treatment, and electrochemical treatment (including combinations thereof).

[0058] "Pt-rich particles" are particles formed by phase separation of a PtTaCo composite and containing Pt as the main component (Pt 50 atomic % or more). Since the atoms other than Pt that make up the Pt-rich particles are Co contained in the PtTaCo composite before phase separation, Pt-rich particles are typically PtCo alloy particles.

[0059] The Pt-rich particles may have any shape as long as it does not impair the object of the present invention. Furthermore, the Pt-rich particles are not limited to being crystalline, and may be amorphous or a mixture of crystalline and amorphous.

[0060] The particle size of the Pt-rich particles is typically 1 to 5 nm (preferably 1 to 3 nm), depending on the particle size and shape of the PtTaCo composite before phase separation. The "particle size of the Pt-rich particles" can be obtained by averaging the particle sizes of any catalyst composite (20 particles) examined from an electron microscope image. When calculating the average particle size from an electron microscope image, if the shape of the fine particles is other than spherical, the length in the direction showing the maximum length of the particle is taken as the particle size.

[0061] The "Ta-rich particles" are particles formed by phase separation of a PtTaCo composite, and are composed of Ta oxide. In this specification, "Ta oxide" refers to an oxide of tantalum (Ta), and Ta, which is the most stable oxide, is used. ２ O ５This concept includes not only crystals of (TaOx, x=2.5) but also crystals of oxides in an oxygen-deficient state (TaOx, 0.1<x<2.5).

[0062] The Ta oxide constituting the Ta-rich particles may contain atoms other than Ta and oxygen. Depending on the method and conditions of the phase separation treatment, the Ta-rich particles may contain a Co component in a solid solution in the Ta oxide when the PtTaCo composite is phase-separated.

[0063] The Ta-rich particles may have any shape as long as it does not impair the object of the present invention. Furthermore, the Ta-rich particles are not limited to being crystalline, and may be amorphous or a mixture of crystalline and amorphous.

[0064] The particle size of the Ta-rich particles is typically 1 to 5 nm (preferably 1 to 3 nm), depending on the particle size and shape of the PtTaCo composite before phase separation. The "particle size of the Ta-rich particles" can be obtained by averaging the particle sizes of any catalyst composite (20 particles) examined from an electron microscope image. When calculating the average particle size from an electron microscope image, if the shape of the fine particles is other than spherical, the length in the direction showing the maximum length of the particle is taken as the particle size.

[0065] Furthermore, as shown in the examples, the Ta-rich particles may contain Ta-core Pt-shell particles. The Ta-core Pt-shell particles consist of a Ta particle core (approximately 1 to 3 nm) and a fine Pt shell (approximately 0.1 to 3 nm) on the surface. In this configuration, the fine Pt particles are stable, contributing to high activity.

[0066] <2. Electrode> The electrode of the present invention comprises the electrode material of the present invention and a proton-conductive 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 a fuel cell. Note that the electrode material of the present invention can also be used as an electrode other than an electrode for a fuel cell (for example, an electrode for a solid polymer water electrolysis device).

[0067] A fuel cell electrode formed using the electrode material of the present invention will be described below, specifically, the case where the above-mentioned electrode material is used as an electrode in a PEFC will be described.

[0068] The electrode of the present invention may be composed of only the above-mentioned electrode material, but also contains a proton-conductive electrolyte material (hereinafter sometimes referred to as a "proton-conductive electrolyte material" or simply an "electrolyte material") that is typically used in the electrolyte of a fuel cell. The electrolyte material contained in the fuel cell electrode together with the electrode material may be the same as or different from the electrolyte material used in the fuel cell electrolyte membrane. From the viewpoint of improving adhesion between the fuel cell electrode and the electrolyte membrane, it is preferable to use the same material.

[0069] Proton-conductive electrolyte materials are used as electrolyte materials for the electrodes and electrolyte membranes of PEFCs. These proton-conductive electrolyte materials are roughly classified into fluorine-based electrolyte materials that contain fluorine atoms in all or part of the polymer skeleton, and hydrocarbon-based electrolyte materials that do not contain fluorine atoms in the polymer skeleton. Both of these can be used as electrolyte materials.

[0070] Specific examples of suitable 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.).

[0071] Specific examples of suitable hydrocarbon-based electrolyte materials include polymers such as polysulfonic acid, polystyrene sulfonic acid, polyaryl ether ketone sulfonic acid, polyphenyl sulfonic acid, polybenzimidazole sulfonic acid, polybenzimidazole phosphonic acid, and polyimide sulfonic acid, as well as polymers of these having a side chain such as an alkyl group.

[0072] The mass ratio of the electrode material to the electrolyte material mixed with the electrode material may be appropriately determined so as to provide good proton conductivity within the electrode formed using these materials and to facilitate smooth gas diffusion and water vapor discharge within the electrode. However, if too much electrolyte material is mixed with the electrode material, proton conductivity improves but gas diffusibility decreases. Conversely, if too little electrolyte material is mixed, gas diffusibility improves but proton conductivity decreases. 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 deteriorates, making it impossible to ensure sufficient proton conductivity for a fuel cell electrode. Conversely, if it is greater than 50 mass%, the continuity of the electrode material deteriorates, potentially making it impossible to provide sufficient electronic conductivity for a fuel cell electrode. Furthermore, the diffusibility of gases (oxygen, hydrogen, water vapor) within the electrode may decrease.

[0073] The fuel cell electrode of the present invention may contain components other than the above-described electrode material and proton-conducting material, as long as the object of the present invention is not impaired. For example, it may contain a conductive material (hereinafter referred to as "other conductive material") other than the carbon support contained in the above-described electrode material. The inclusion of other conductive materials may increase the number of conductive paths connecting the electrode materials, thereby improving the conductivity of the electrode as a whole.

[0074] As the other conductive material, known conductive materials used in fuel cell electrodes can be used. Typical examples include carbon-based conductive materials, such as particulate carbon (including chain-linked carbon particles) such as carbon black and activated carbon, and fibrous carbon such as carbon fiber and carbon nanotubes (CNT). Mesoporous carbon can also be used as the other conductive material.

[0075] Although the fuel cell electrode containing the electrode material of the present invention has been described as a PEFC electrode, it can also 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 a water electrolysis device using a polymer electrolyte membrane similar to that used in PEFCs. Fuel cell electrodes containing the electrode material of the present invention have excellent electrochemical catalytic activity for oxygen reduction and hydrogen oxidation, and can therefore be used as cathodes and anodes. In particular, they have excellent electrochemical catalytic activity for oxygen reduction, and electrochemical oxidative decomposition of the conductive material, which serves as the carrier, does not occur under the operating conditions of the fuel cell, making them particularly suitable for use as cathodes.

[0076] 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, and can also be suitably used as an electrode for water electrolysis devices that use a solid polymer electrolyte membrane similar to that used in PEFCs.

[0077] <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 or both of the cathode and the anode are the above-described electrodes of the present invention.

[0078] 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 a cathode will be described. Figure 3 is a schematic diagram showing 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 a cathode 4 and an anode 5 are arranged facing a solid polymer electrolyte membrane 6.

[0079] 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 for the electrode catalyst layer 4a.

[0080] The gas diffusion layer 4b can be a conventionally known gas diffusion layer. For example, a conductive carbon-based sheet-like material having a pore size distribution of approximately 100 nm to 90 μm, which has been conventionally used as a gas diffusion layer in PEFCs, can be used. Suitable examples include carbon paper, carbon cloth, and carbon nonwoven fabric that have been subjected to a water-repellent treatment. Sheet-like materials other than carbon-based materials, such as stainless steel, can also be used. The thickness of such a gas diffusion layer 4b is not particularly limited, but is typically approximately 50 μm to 1 mm. Furthermore, the gas diffusion layer 4b may have a microporous layer on one side thereof, which is composed of an aggregate of carbon fine particles with an average particle size of approximately 10 to 100 nm and a water-repellent material.

[0081] The 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 anode electrode catalyst layers can also be used. For example, an electrode in which the electrode catalyst layer 5a is manufactured by applying and drying a dispersion of an electrode material carrying noble metal particles as a catalyst and an electrolyte material for a fuel cell on the surface of a conductive support made of a carbon-based material such as graphite, carbon black, activated carbon, carbon nanotubes, or glassy carbon, and then forming the electrode catalyst layer 5a on the gas diffusion layer 5b can be used. The gas diffusion layer 5b of the anode 5 can be the same as the gas diffusion layer 4b described for the cathode 4.

[0082] Any known electrolyte membrane for PEFCs may be used as the solid polymer electrolyte membrane 6 as long as it has proton conductivity, chemical stability, and thermal stability. Although the thickness is exaggerated in Figure 3, the thickness of the solid polymer electrolyte membrane 6 is usually about 0.007 to 0.05 mm in order to reduce electrical resistance.

[0083] 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.).

[0084] Although the embodiments of the membrane electrode assembly of the present invention have been described above with reference to the drawings, these are merely examples of the present invention, and various other configurations can also be adopted.

[0085] 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 generally has a structure in which the membrane electrode assembly is sandwiched between separators having gas flow paths formed therein.

[0086] 4 is a conceptual diagram showing a typical configuration of a polymer electrolyte fuel cell according to the present invention. As shown in FIG. 4, in a polymer electrolyte fuel cell 20, hydrogen is supplied to the anode 5, and (reaction 1) 2H → 4H + +4e - The generated proton (H + ) is 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, resulting in (Reaction 2) O2 + 4H + +4e - → 2H2O reacts with oxygen to produce water. This electrochemical reaction between the anode and cathode generates a potential difference between the two electrodes. In the polymer electrolyte fuel cell of the present invention, the components other than the membrane electrode assembly of the present invention are the same as those of known polymer electrolyte fuel cells, so detailed explanations will be omitted. In practice, a fuel cell stack is formed by stacking the polymer electrolyte fuel cells (single cells) of the present invention in a number appropriate for the power generation performance, and the stack is used by assembling other associated devices such as a gas supply device and a cooling device.

[0087] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. In the following, Ketjen Black may be referred to as "KB."

[0088] A. Electrode Material (PtTaCo / KB) A1. Preparation of Electrode Material As the electrode material of the example, the electrode material of Example 1 below was produced according to the flowchart shown in FIG.

[0089] The conductive support (carbon support) and metal catalyst precursor compound (raw material compound) used are as follows. <Conductive support (carbon support)> Ketjen black (KB) (EC600JD, Lion Specialty Chemicals Co., Ltd.) was used as the carbon support. <Pt raw material compound> Platinum (II) acetylacetonate (Sigma Aldrich) (hereinafter sometimes referred to as "Pt(acac)2") was used as the Pt raw material compound. <Ta raw material compound> Tantalum ethoxide (Ta(OC ２ H ５ ) ５ Cobalt (II) nitrate hexahydrate (Co(NO)) was used as the Co raw material compound. ３ ) ２ ・6H ２ O, Kishida Chemical Co., Ltd.) was used.

[0090] Example 1: Pt ７ Ta ２ Co １ / KB> Step (1) First, as raw material compounds of Pt, Co, and Ta, Pt(acac)2 was dissolved in dichloromethane (10 mL) and (Ta(OC)2 was dissolved in ethanol. ２ H ５ ) ５ , (Co(NO ３ ) ２ ・6H ２ Each of the reagent solutions was dissolved in the solution. The atomic ratio of Pt:Ta:Co was 7:2:1 in terms of the amount of the reagents used. Next, each of the reagent solutions was added to a dispersion of 125 mg of Ketjen Black (KB) that had been ultrasonically dispersed in an ethanol solution in advance, and finally, acetone was added in an amount of about 20% by volume of the total solvent to prepare a dispersion (total amount of ethanol: 110 mL, acetone: 30 mL). The Pt raw material compound (Pt(acac)2) and Ta raw material compound (Ta(OC ２ H ５ ) ５ ) and Co source compound (Co(NO ３ ) ２ ・6H ２The amount of the PtTaCo composite was 40 wt % of the total electrode material. The flask containing the sample was then placed in a rotary evaporator equipped with a vacuum and rotation function, and the pressure was reduced while applying ultrasonic waves until all the solvent was evaporated. The mixture was then rotated until the solvent was completely evaporated, resulting in a dry powder in which the precursor of the PtTaCo composite was adsorbed on the KB surface (the inner and outer surfaces of the pores).

[0091] Step (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, then further heated to 240°C in an Ar atmosphere (heating rate: 1°C / min) and held for 3 hours. Thereafter, the powder was further heated to 800°C in 30 minutes in an Ar atmosphere and held for 30 minutes to obtain Pt ７ Ta ２ Co １ The electrode material of Example 1 (Pt ７ Ta ２ Co １ / KB) was prepared.

[0092] Comparative Example 1 As an electrode material in Comparative Example 1, a Pt / C catalyst (Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E, Pt 46 wt %) was used.

[0093] Comparative Example 2: As Comparative 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)

[0094] A2. Electrochemical evaluation (half cell)

[0095] The electrodes for evaluation were prepared by the following procedure. First, a mixed solution of 19 mL of ultrapure water and 6 mL of 2-propanol was added to a sample bottle containing electrode material powder, followed by the addition of a 5% Nafion dispersion so that I / C (ionomer / carbon) = 0.3. After that, the sample bottle was immersed in ice water and subjected to ultrasonic stirring for 30 minutes to prepare an 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 17.3 μg-Pt cm. -210 μL of the prepared electrode material dispersion was dropped onto a glassy carbon (GC) disk electrode using a micropipette, and the electrode was fixed to a rotating device and dried at room temperature for about 30 minutes at a rotation speed of 300 rpm to form a Nafion membrane and fix the electrode material onto the GC electrode, thereby obtaining an electrode (working electrode) for evaluation.

[0096] A2-1. Load Fluctuation Cycle Test The load fluctuation cycle test was conducted by applying a potential cycle simulating load fluctuations according to the method recommended by the Fuel Cell Commercialization Council of Japan (FCCJ) (Proposal of Goals, Research and Development Issues and Evaluation Methods for Polymer Electrolyte Fuel Cells, published in May 2023). The load fluctuation cycle shown in Figure 6 is a cycle that promotes deterioration accompanied by dissolution and redeposition of the catalyst itself, and is 0.6 to 0.95 V. RHE The experiment was carried out by applying a square wave of 1000 kJ / s for 6 seconds, with 3 seconds per cycle.

[0097] For the electrode material of Example 1, the ECSA and 0.9V were measured every 100,000 cycles from 0 (initial) cycle to 400,000 cycles. RHE For Comparative Example 1 (Pt / C catalyst (TEC10E50E)) and Comparative Example 2 (PtCo / C catalyst (TEC36E52)), a load fluctuation test was carried out up to 100,000 cycles under the same conditions, and the ECSA and MA at 0.9V were measured. RHE The MA was measured.

[0098] A2-2. Cyclic voltammetry (CV) evaluation Example 1 (Pt ７ Ta ２ Co １ / KB), Comparative Example 1 (Pt / C) and Comparative Example 2 (Pt ３ Co １ The electrode material (Pt / C) was evaluated by cyclic voltammetry (CV). The electrochemical effective surface area (ECSA) was calculated from the amount of hydrogen adsorption determined by CV. The ECSA corresponds to the effective surface area of ​​Pt contained in the electrode material.

[0099] The CV measurement conditions are as follows: Assuming that one atom of H is adsorbed per one atom of Pt, the CV is 210 μC / cm 2Measurement: Three-electrode cell (working electrode: fuel cell electrode for evaluation, counter electrode: graphite or Pt, reference electrode: Ag / AgCl) Electrolyte: 0.1 M HClO4 (pH: approx. 1) Measurement potential range: 0.05 to 1.2 V (based on the reversible hydrogen electrode) Scan rate: 50 mV / s Hydrogen adsorption amount: Calculated from the peak area showing hydrogen adsorption between 0.05 and 0.4 V Electrochemically effective surface area (ECSA): Calculated using the following formula: ECSA = (hydrogen adsorption amount) [μC] / 210 [μC / cm 2 ]

[0100] CV evaluation was performed before and after the load change cycle test on an electrode using the electrode material of Example 1. In the initial state of 0 cycles, a hydrogen adsorption / desorption peak was observed on the low potential side of the CV, and a CV curve was obtained in which the oxygen adsorption reaction on the Pt surface began near 0.8 V RHE. As the number of cycles increased, the hydrogen adsorption / desorption peak in the CV curve became smaller. Figure 7 shows the results of Example 1 (Pt ７ Ta ２ Co １ / KB), Comparative Example 1 (Pt / C) and Comparative Example 2 (Pt ３ Co １ 7 shows the relationship between the number of load cycles and the electrochemically effective surface area (ECSA) for each electrode material (1 / C). As shown in FIG. 7, at the beginning of the load cycle (number of cycles: 0), Example 1 exhibited an ECSA equivalent to that of Comparative Example 1 and greater than that of Comparative Example 2. Thereafter, as the number of load cycles increased, the ECSA of Comparative Example 1 decreased, while that of Comparative Example 2 remained flat. However, although the ECSA of Example 1 decreased as the number of load cycles increased, it maintained a value greater than that of Comparative Examples 1 and 2. Therefore, it was determined that Example 1 had superior durability to Comparative Examples 1 and 2 from the viewpoint of ECSA.

[0101] A2-3. Evaluation of ORR activity Example 1 (Pt ７ Ta ２ Co １ / KB), Comparative Example 1 (Pt / C) and Comparative Example 2 (Pt ３ Co １The ORR activity of the electrode material (C) was evaluated. The ORR activity was measured by linear sweep voltammetry (LSV) using the rotating disk electrode (RDE) method, and the resulting activation-dominant current (i k The mass activity (activity per unit Pt mass, mass activity) calculated based on the mass activity of Pt was used as an index. ｋ / Pt mass on the electrode Activation-dominated current (i k ) is the current-potential curve obtained by rotating electrode measurement at any potential i -1 and ω -1 / 2 A Koutecky-Levich plot was created by plotting the voltage at 0.2 V and the resulting line was extrapolated to obtain the intercept. RHE 1.2 V at 10 mV / s in the noble direction RHE The potential was scanned up to 1000 kJ / s, and measurements were performed. During the measurements, O2 was constantly purged at a rate of 50 mL / min. RHE is the potential relative to the reversible hydrogen electrode (RHE).

[0102] When the linear sweep voltammogram (LSV) of the electrode using the electrode material of Example 1 was evaluated before and after the load change cycle at 1600 rpm, the decrease in the ORR onset potential was slightly large from 0 to 100,000 cycles, and the decrease in the onset potential was gradual at 200,000, 300,000, and 400,000 cycles. This is because Pt ７ Ta ２ Co １It can be inferred that the surface of the composite changed to a stable catalyst structure in which the PtCo alloy and Ta oxide (TaOx) were phase-separated through Co elution, Pt dissolution and precipitation, and potential cycling. Figure 8 shows the relationship between the number of load cycles and mass activity (MA) for each electrode material of Example 1, Comparative Example 1, and Comparative Example 2. As shown in Figure 8, at the beginning of the load cycle (0 cycles), Example 1 exhibited an MA equivalent to that of Comparative Example 1 and a lower MA than that of Comparative Example 2. Subsequently, as the number of load cycles increased, the MA of Comparative Examples 1 and 2 decreased significantly, but the decrease in MA of Example 1 was more moderate than that of Comparative Examples 1 and 2. Therefore, Example 1 was determined to have superior durability compared to Comparative Examples 1 and 2, even from the perspective of MA.

[0103] A3. Physical Property Evaluation (Microstructure Evaluation) Figure 9 shows the results of STEM-EDS analysis of the electrode material of Example 1. Figures 9(a) to 9(c) show the results before the load variation cycle test (initial: 0 cycle), where (a) is a STEM image, (b) is an EDS mapping image (Pt + Ta + Co), and (c) is an EDS line profile (Pt, Ta). Figures 9(d) to 9(f) show the results after the load variation cycle test (400,000 cycles), where (d) is a STEM image, (e) is an EDS mapping image (Pt + Ta + Co), and (f) is an EDS line profile (Pt, Ta).

[0104] The STEM image of the electrode material of Example 1 before the load cycle test in Figure 9(a) shows that the catalyst particles are uniformly and highly dispersed on the carbon support (KB) with a diameter of 2 to 5 nm overall. The mapping image in Figure 9(b) shows that the elemental distributions of Pt, Ta, and Co almost overlap in most of the catalyst particles, indicating that the initial catalyst particles (before the cycle test) exist in a composite state of Pt, Ta, and Co. The line analysis image in Figure 9(c) confirms that there is a slight increase in Ta on the particle surface.

[0105] On the other hand, in the electrode material of Example 1 after the load variation cycle test (400,000 cycles), the STEM image of FIG. 9(d) and the mapping image of FIG. 9(e) show that the Pt particle size has grown slightly compared to the initial state, and large particles with a diameter exceeding 5 nm have been confirmed. Also, as shown by the white arrows in FIG. 9(d), many particles that are almost unchanged from the initial state were confirmed. Furthermore, from FIG. 9(e), it can be confirmed that the catalyst composite after the load variation cycle test (400,000 cycles) has partially phase-separated, and there are Pt-rich particles and Ta-rich particles. And, from FIG. 9(f), it was confirmed that Pt is almost uniformly present within the Pt-rich particles, while the Ta distribution is concentrated at the center of the particles. From these results, it was determined that at least a portion of the catalyst composite after the load variation cycle test (400,000 cycles) has a structure in which Pt-rich particles surround Ta-rich particles, a so-called "Ta core Pt shell" structure.

[0106] Thus, from the comparative analysis of catalyst particles before and after the load change cycle test in the electrode material of Example 1, it is presumed that the nanocomposite structure composite containing Pt-rich particles and Ta-rich particles formed by phase separation of the PtTaCo composite (catalyst composite) contributes to the improvement of the performance of the electrode material of Example 1. ７ Ta ２ Co １ ) maintained a high degree of dispersion on the Ketjenblack (KB) surface even after 400,000 load cycles, and it was determined that, compared to conventional catalysts, this catalyst can prevent catalyst particle enlargement due to detachment or dissolution / precipitation during load cycle testing.

[0107] The electrode material of the present invention can provide a fuel cell electrode having excellent electrocatalytic activity, electronic conductivity, gas diffusivity, and durability, and is therefore promising as an electrode component for polymer electrolyte fuel cells used in the automotive, electric power, gas, and home appliance industries. In particular, it is expected to be used for fuel cell vehicles (passenger cars and commercial vehicles) that are subject to large load fluctuations.

[0108] REFERENCE SIGNS LIST 1 Electrode material 2 Conductive support 3 Catalyst composite 3A Pt-rich particles 3B Ta-rich particles 4 Fuel cell electrode (cathode) 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. An electrode material comprising a conductive support and a catalytic complex supported on the conductive support, the catalytic complex containing a PtTaCo complex composed of platinum (Pt) as a first component, tantalum (Ta) as a second component, and cobalt (Co) as a third component.

2. The electrode material according to claim 1, in which the PtTaCo composite constituting the catalyst composite is phase-separated.

3. The electrode material according to claim 2, wherein the catalyst composite after phase separation contains Pt-rich particles and Ta-rich particles.

4. The electrode material according to claim 3, wherein the Pt-rich particles are a PtCo alloy.

5. The electrode material according to claim 3 or 4, wherein the Ta-rich particles are Ta oxide.

6. An electrode material according to any one of claims 1 to 5, wherein the content of Pt is 60 to 80 atomic %, the content of Ta is 10 to 30 atomic %, and the content of Co is 1 to 20 atomic %, relative to the total content (100 atomic %) of Pt, Ta, and Co.

7. The electrode material according to any one of claims 1 to 6, wherein the conductive support is a carbon support.

8. The electrode material according to claim 7, wherein the carbon support is highly crystalline carbon.

9. An electrode comprising the electrode material according to any one of claims 1 to 8 and a proton-conducting electrolyte material.

10. 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 the anode or the cathode, or both, are the electrodes described in claim 9.

11. A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 10.

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