Catalyst, electrode, membrane electrode assembly, fuel cell, and method for producing catalyst

Abstract

To produce a high-performance catalyst.SOLUTION: A catalyst includes a carbon support doped with a nitrogen atom and a first transition metal atom, the carbon support supporting thereon a plurality of fine particles containing a noble metal. The fine particles have an average particle size of 0.8 nm or more and 1.5 nm or less.SELECTED DRAWING: Figure 8

Description

[Technical Field]

[0001] The present disclosure relates to a catalyst, an electrode, a membrane electrode assembly, a fuel cell, and a method for producing a catalyst. [Background technology]

[0002] Nanometer-sized metal particles exhibit unique properties not observed in the bulk state, such as color development due to plasma vibration, increased catalytic activity, and a lower melting point. To date, extensive applied research has been conducted across a wide range of fields, taking advantage of their unique characteristics. There are two main methods for preparing them. One is the "top-down" method, in which bulk raw materials are pulverized by mechanical grinding or other methods, and the "milling method" is a representative example (see Non-Patent Document 1). The other is the "bottom-up" method, in which atoms or molecules are grown to the desired size. Metal nanoparticles have primarily been synthesized using the bottom-up method, which allows for easier control of the size as desired compared to the top-down method. Furthermore, bottom-up methods can be classified into gas-phase methods (dry methods) that use physical methods for synthesis reactions, such as the "sputtering method" and "spray pyrolysis method" (see Non-Patent Document 2), and liquid-phase methods (wet methods) that generate crystal nuclei in a liquid phase containing dissolved metal ions, promote nucleus growth, and synthesize nanoparticles, such as the "co-precipitation method," "sol-gel method," "microemulsion method," and "solvothermal method" (see Non-Patent Document 3).

[0003] The characteristics of each method are described below. The top-down method requires only one step: crushing bulk metal to nano-size, making it easy to operate. The greatest advantage of this method is that it produces highly crystalline fine particles. However, as mentioned above, this method has the drawback of being extremely difficult to control the size. Even if you actually try to create particles of the desired size, the size distribution may be large, and the desired specificity may not be achieved. The bottom-up methods are classified into gas phase and liquid phase methods and described below. Although the vapor phase method makes it easy to form highly crystalline particles consisting of only a few atoms, it has the drawback of low productivity and is not suitable for industrial use. In the liquid-phase method, if the temperature distribution in the reaction field is uniform, it is possible to produce nanoparticles with a relatively uniform size. Productivity is also significantly higher than with the gas-phase method. However, this method often uses a protective agent during the synthesis process to prevent nanoparticle aggregation and ensure dispersibility. Therefore, this method requires labor and cost to finally remove the protective agent from the metal particle surface and recover and concentrate only the metal nanoparticles. Furthermore, because this method synthesizes nanoparticles at low temperatures, they have low crystallinity. This reduces their catalytic activity. This method requires a heating step to increase crystallinity. As a result, even if this method produces nanoparticles with a uniform size, it is difficult to maintain that size distribution during the final heat treatment. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] C. Suryanarayana, Progress in Materials Science, 2001, 46, 1-184. [Non-patent document 2] MT Swihart, Current Opinion in Colloid and Interface Science, 2003, 8, 127-133. [Non-patent document 3] BL Cushing, VL Kolesnichenko, CJ O'Connor, Chem. Rev., 2004, 104, 3893-3946. Summary of the Invention [Problem to be solved by the invention]

[0005] For the reasons mentioned above, the size of metal nanoparticles that can be synthesized using conventional techniques is limited to around 1 nm. However, when considering size effects, in other words, to obtain a larger specific surface area, it is preferable to reduce the size to 1 nm or less. Ideally, it would be possible to prepare metal catalysts consisting of particles composed of only a few atoms, preferably single atoms. The key factor in this case is to reduce the size as much as possible while maintaining high crystallinity. Therefore, there is a strong demand for the development of a new method that combines the advantages of the top-down and bottom-up methods mentioned above. The present disclosure is intended to solve at least part of the above problems, and can be realized in the following forms. [Means for solving the problem]

[0006] The present inventors conducted extensive research to solve the above-mentioned problems. Specifically, they used a bottom-up method to prepare catalysts in which noble metal nanoparticles of approximately several nanometers in size were supported on specific carbon supports. These catalysts were then converted into electrodes and subjected to potential cycling over a specific potential range in an acidic environment. As a result, they found that the supported noble metal dissolved, reducing its size from its original size. Meanwhile, the dissolved noble metal ions were trapped at nitrogen atoms or metal atoms in the carbon skeleton, forming new microparticles. In other words, they found that potential cycling can induce the self-organization of subnanoscale (1 nm or less) metal particles, resulting in a high-performance catalyst. The catalysts prepared in this way are applicable to a wide range of fields, including electrode catalysts for polymer electrolyte fuel cells (PEFCs), metal-catalyzed batteries, sensors, and electrolysis, and have extremely high industrial applicability.

[0007] Using conventional metal nanoparticle synthesis methods, it is extremely difficult, both chemically and physically, to prepare uniformly distributed nanoparticle catalysts with sizes of 1 nm or less, and industrial productivity is also poor. However, when used as an electrode catalyst (e.g., in a fuel cell), it is predicted that the smaller the particle size, the larger the specific surface area and the better the catalytic ability (activity). One issue is that, when used in a fuel cell, highly dispersed supported nanoparticles generally deteriorate on the support during the power generation reaction (aggregation and coarsening due to Ostwald ripening). The smaller the particles, the greater the surface area and surface energy, making this tendency even more pronounced. In other words, there is a trade-off between nanoparticle activity and durability. One of the objectives of this disclosure is to solve this trade-off problem. The means of the present disclosure are as follows. [1] A catalyst in which a plurality of fine particles containing a noble metal are supported on a carbon support doped with nitrogen atoms and first transition metal atoms, The catalyst, wherein the fine particles have an average particle size of 0.8 nm or more and 1.5 nm or less. [Effects of the Invention]

[0008] The catalyst of the present disclosure forms sub-nanoparticles by applying a voltage having a potential cycle, resulting in a high-performance catalyst. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a conceptual diagram showing Ostwald ripening of Pt particles. [Figure 2] FIG. 1 is a conceptual diagram showing the mechanism of formation of sub-nanosized Pt particles. [Figure 3] FIG. 1 is a schematic diagram of an example of a polymer electrolyte fuel cell. [Figure 4] TEM images of various carbon supports. (A): TEM image of GCB. (B): TEM image of MPC. (C): TEM image of PMF. (D): TEM image of CB. [Figure 5]TEM images of various carbon supports after Pt loading. (A) TEM image of Pt / GCB. (B) TEM image of Pt / MPC. (C) TEM image of Pt / PMF. (D) TEM image of Pt / CB. [Figure 6] FIG. 2 is an explanatory diagram showing an example of a potential cycle waveform. [Figure 7] TEM images of Pt / GCB before and after potential cycling and graphs showing changes in particle size distribution. (A): TEM image before potential cycling. (B): TEM image after potential cycling. (C): Graph showing changes in particle size distribution before and after potential cycling. [Figure 8] TEM images of Pt / PMF before and after potential cycling and graphs showing changes in particle size distribution. (A): TEM image before potential cycling. (B): TEM image after potential cycling. (C): Dark-field image after potential cycling. (D): Graph showing changes in particle size distribution before and after potential cycling. (E): Graph showing the results of energy dispersive X-ray analysis. [Figure 9] Graphs showing the effect of potential cycling on electrochemical surface area (ECA) and mass activity (MAk) in the oxygen reduction reaction. (A) Graph showing electrochemical surface area (ECA). (B) Graph showing mass activity (MAk) in the oxygen reduction reaction. [Figure 10] Figures showing the influence of Pt loading method. (A): TEM image and particle size distribution graph of Pt / PMF prepared using the prior art. (B): TEM image and particle size distribution graph of Pt / PMF prepared using the colloidal method. (C): Graph showing mass activity retention. [Figure 11] FIG. 1 is an explanatory diagram showing an accelerated degradation protocol that simulates potential fluctuations in response to load in a fuel cell vehicle (FCV) recommended by the Fuel Cell Council of Japan (FCCJ). DETAILED DESCRIPTION OF THE INVENTION

[0010] Here, another example of the present disclosure is given. [2] The catalyst according to [1], wherein the microparticles have a standard deviation of 0% or more and 10% or less relative to the average particle diameter. The catalyst of the present disclosure has a small variation in particle size and high performance.

[0011] [3] The catalyst according to [1] or [2], wherein, when used in a fuel cell, at least one of the microparticles dissolves and becomes smaller as power is generated, and new microparticles containing a precious metal are generated on the carbon support from the metal ions produced by the dissolution.

[0012] [4] An electrode comprising the catalyst according to any one of [1] to [3]. The electrode of the present disclosure has high performance because sub-nanoparticles are formed by applying a voltage having a potential cycle.

[0013] [5] A membrane electrode assembly comprising the electrode according to [4] on the surface of an electrolyte membrane. The membrane electrode assembly of the present disclosure has high performance because sub-nanoparticles are formed by applying a voltage having a potential cycle.

[0014] [6] A fuel cell comprising the catalyst according to any one of [1] to [3]. The fuel cell of the present disclosure has high performance because sub-nanoparticles are formed by applying a voltage having a potential cycle.

[0015] [7] A catalyst in which a carbon support doped with nitrogen atoms and first transition metal atoms supports a plurality of fine particles containing a noble metal, The catalyst, wherein the fine particles include particles smaller than 0.8 nm. The catalysts of the present disclosure contain sub-nanoparticles and have high performance.

[0016] [8] The catalyst according to [7], wherein the microparticles have at least one peak below 0.8 nm in a particle size distribution diagram. The catalysts of the present disclosure contain sub-nanoparticles and have high performance.

[0017] [9] A catalyst in which a composite in which a carbon support doped with nitrogen atoms and first transition metal atoms supports a plurality of raw material fine particles containing a precious metal is subjected to application of a voltage having a potential cycle in an acidic environment, thereby dissolving and miniaturizing at least one of the raw material fine particles, and generating new fine particles on the carbon support from the metal ions produced by the dissolution. The catalyst of the present disclosure has high performance because sub-nanoparticles are formed by applying a voltage having a potential cycle.

[0018]

[10] The catalyst according to [9], wherein the potential cycle is a cycle repeated between a potential of 0 V or more and 1.0 V or less relative to a standard hydrogen electrode. The catalyst of the present disclosure has high performance because sub-nanoparticles are formed by applying a voltage having a specific potential cycle.

[0019]

[11] An electrode comprising the catalyst according to any one of [7] to

[10] . The electrodes of the present disclosure contain sub-nanoparticles and are highly efficient.

[0020]

[12] A membrane electrode assembly comprising the electrode according to

[11] on the surface of an electrolyte membrane. The membrane electrode assembly of the present disclosure contains sub-nanoparticles and has high performance.

[0021]

[13] A fuel cell comprising the catalyst according to any one of [7] to

[10] . The fuel cells of the present disclosure contain sub-nanoparticles and have high performance.

[0022]

[14] A method for producing a catalyst, comprising applying a voltage having a potential cycle in an acidic environment to a composite in which a carbon support doped with nitrogen atoms and first transition metal atoms supports a plurality of raw material fine particles containing a precious metal, thereby dissolving and miniaturizing at least one of the raw material fine particles, and generating new fine particles on the carbon support from metal ions produced by the dissolution. The manufacturing method of the present disclosure allows the formation of sub-nanoparticles, making it possible to manufacture a high-performance catalyst.

[0023]

[15] The method for producing a catalyst according to

[14] , wherein the potential cycle is a cycle that is repeated between a potential of 0 V or more and 1.0 V or less relative to a standard hydrogen electrode. The manufacturing method of the present disclosure allows for efficient formation of sub-nanoparticles, making it possible to manufacture a high-performance catalyst.

[0024] The present disclosure will be described in detail below. Note that when a numerical range is described using "-", it is intended to include both the lower limit and the upper limit unless otherwise specified. For example, the description "10-20" includes both the lower limit "10" and the upper limit "20". In other words, "10-20" has the same meaning as "10 or more and 20 or less".

[0025] 1. Catalyst A Catalyst A is a catalyst in which a carbon support doped with nitrogen atoms and first transition metal atoms supports a plurality of fine particles containing a noble metal. The average particle size of the fine particles is 0.8 nm to 1.5 nm.

[0026] (1) Carbon support The carbon support is doped with nitrogen atoms and first transition metal atoms. This "carbon support" is sometimes called a "noble metal-free carbon catalyst" or "carbon alloy." The presence of nitrogen atoms and first transition metal atoms in the carbon support serves as the starting point for the formation of ultrafine particles, which will be described later. On the other hand, if nitrogen atoms and first transition metal atoms are absent, ultrafine particles will not be formed. The first transition metal atom is at least one selected from the group consisting of scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). The doping amount of nitrogen atoms is not particularly limited, and from the viewpoint of promoting the miniaturization of fine particles by application of a voltage having a potential cycle, the doping amount of nitrogen atoms is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.5% by mass or more and 15% by mass or less, and even more preferably 1% by mass or more and 10% by mass or less, when the entire carbon support is taken as 100% by mass. The doping amount of the first transition metal atoms is not particularly limited, and from the viewpoint of promoting the miniaturization of the fine particles by application of a voltage having a potential cycle, the doping amount of the first transition metal atoms is preferably 0.1% by mass or more and 20% by mass or less, more preferably 0.5% by mass or more and 15% by mass or less, and even more preferably 1% by mass or more and 10% by mass or less, when the entire carbon support is taken as 100% by mass. The nitrogen adsorption specific surface area of ​​the carbon support is not particularly limited. From the viewpoint of improving the amount of supported fine particles, the nitrogen adsorption specific surface area of ​​the carbon support is set to 50 m 2 g -1 More than 2000m 2 g -1 Less than 150m is preferable 2 g -1 More than 800m 2 g -1 The following is more preferred:

[0027] (2) Multiple particles containing precious metals The noble metal is not particularly limited. It is preferable to use at least one noble metal selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru). Among these, from the viewpoint of catalytic performance, at least one selected from the group consisting of Pt, Rh, Pd, Ir, and Ru is more preferable, and at least one selected from the group consisting of Pt and Pd is even more preferable.

[0028] The content of the noble metal in the fine particles is not particularly limited, but is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. The content of the noble metal may be 100% by mass.

[0029] The number of fine particles supported on the carbon support is not particularly limited as long as it is two or more (plural).

[0030] The average particle size of the fine particles is not particularly limited. From the viewpoint of ensuring high activity, the average particle size of the fine particles is preferably 0.8 nm or more and 1.5 nm or less, more preferably 1.1 nm or more and 1.4 nm or less, and even more preferably 1.2 nm or more and 1.3 nm or less. The average particle size can be determined using the following method (method for determining average particle size). The synthesized catalyst is observed using a transmission electron microscope (TEM). The TEM photograph is printed out on paper, and the fine particles (black circular images) are considered to be spherical. The diameter from one end of the fine particle to the other is considered to be the diameter, and a total of 300 particles are measured randomly from the images in several fields of view (3-5 fields of view). The average diameter of the 300 particles is taken as the average particle size. Furthermore, the microparticles preferably have a standard deviation of 0% to 10% of the average particle size, which is calculated by creating a distribution chart of particle sizes of 300 particles.

[0031] (3) Manufacturing method of catalyst A There are no particular limitations on the method for producing catalyst A. A preferred example of the method for producing catalyst A will be described below. A preferred example of a method for producing catalyst A includes a step of mixing a noble metal salt, an alcohol having 1 to 5 carbon atoms, and a support to form a mixture, and a heating step of heating the mixture at 150°C or higher and 800°C or lower to produce catalyst A.

[0032] (3.1) Noble metal salts The noble metal contained in the noble metal is not particularly limited, but it is preferable to use at least one selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru). Among these, from the viewpoint of catalytic performance, at least one selected from the group consisting of Pt, Rh, Pd, Ir, and Ru is more preferable, and at least one selected from the group consisting of Pt and Pd is even more preferable.

[0033] As the noble metal salt, at least one selected from the group consisting of hexachloroplatinic acid (IV) hexahydrate (HPtCl·6H2O), tetraamminedichloroplatinum (Pt(NH3)4Cl2·xH2O), platinum (IV) bromide (PtBr4), and bis(acetylacetonato)platinum (II) ([Pt(C5H7O2)2]) can be suitably used.

[0034] (3.2) Alcohols with 1-5 carbon atoms As the alcohol having 1 to 5 carbon atoms, at least one selected from the group consisting of methanol, ethanol, propanol, isopropyl alcohol, 1-butanol, 2-butanol, t-butyl alcohol, 1-pentanol, and 3-pentanol can be suitably used. Among these, ethanol is preferred from the viewpoint of reducing the environmental load.

[0035] The ratio of the amount of alcohol to the amount of noble metal salt is not particularly limited. The concentration of the noble metal salt in the alcohol solution in which the noble metal salt is dissolved in alcohol is not particularly limited. The concentration of the noble metal salt is set to 0.1 mol L−1 from the viewpoint of obtaining highly active noble metal particles with an average particle size of 0.8 nm to 1.5 nm and uniform in size. -1 More than 50 molL -1 It is preferable that the concentration is less than 5 molL. -1 More than 40 molL -1 More preferably, it is 10 molL or less. -1 More than 30 molL -1 It is more preferable that:

[0036] (3.3) Carrier The support is the above-mentioned carbon support.

[0037] (3.4) Mixing ratio of carrier and alcohol The mixing ratio of the carrier and the alcohol is not particularly limited. From the viewpoint of sufficiently mixing the carrier and the alcohol to obtain highly active noble metal fine particles with a uniform size and an average particle size of 0.8 nm to 1.5 nm, the carrier is preferably mixed in a ratio of 2 mg to 200 mg, more preferably 10 mg to 100 mg, and even more preferably 30 mg to 80 mg, per mL of alcohol.

[0038] (3.5) Mixture The mixing method is not particularly limited. The components may be pulverized and mixed using a mortar and pestle, or may be pulverized and mixed using a dry pulverizer such as a ball mill, a vibration mill, a hammer mill, a roll mill, or a jet mill, or may be mixed using a mixer such as a ribbon blender, a Henschel mixer, or a V-type blender.

[0039] The mixing time is not particularly limited, and mixing is preferably carried out until the alcohol evaporates and the mixture becomes dry.

[0040] (3.6) Heating The heating temperature is 150°C or higher and 800°C or lower, preferably 150°C or higher and 400°C or lower, and more preferably 150°C or higher and 250°C or lower, from the viewpoint of obtaining highly active noble metal microparticles with a particle size of 0.8 nm or higher and 1.5 nm or lower and uniform in size. The heating is preferably carried out in an inert gas atmosphere. Suitable inert gases include rare gases such as argon gas and nitrogen gas. The heating may also be carried out in air.

[0041] (4) Amount of precious metals carried The amount of the noble metal supported is not particularly limited, and may be an amount required depending on the intended design, etc. From the viewpoints of catalyst performance and cost, the amount of the noble metal supported is preferably 5 parts by mass or more and 70 parts by mass or less, and more preferably 10 parts by mass or more and 50 parts by mass or less, per 100 parts by mass of the carbon support, in terms of metal.

[0042] (5) Effect of catalyst A When a voltage having a potential cycle is applied to catalyst A, the size of the fine particles decreases, and the catalyst becomes a high-performance catalyst.

[0043] 2.Catalyst B Catalyst B is a catalyst in which a carbon support doped with nitrogen and first transition metal atoms supports a plurality of fine particles containing a noble metal. The fine particles include particles smaller than 0.8 nm.

[0044] (1) Carbon support For catalyst B, the explanation for the "carbon support" in the "catalyst A" column applies as is, and the description thereof will be omitted. In other words, the "carbon support" explained in the "catalyst A" section applies as is.

[0045] (2) Multiple particles containing precious metals The noble metal is not particularly limited. It is preferable to use at least one noble metal selected from the group consisting of platinum (Pt), palladium (Pd), rhodium (Rh), gold (Au), silver (Ag), iridium (Ir), and ruthenium (Ru). Among these, from the viewpoint of catalytic performance, at least one selected from the group consisting of Pt, Rh, Pd, Ir, and Ru is more preferable, and at least one selected from the group consisting of Pt and Pd is even more preferable.

[0046] The content of the noble metal in the fine particles is not particularly limited, but is preferably 90% by mass or more, more preferably 95% by mass or more, and even more preferably 98% by mass or more. The content of the noble metal may be 100% by mass.

[0047] The number of fine particles supported on the carbon support is not particularly limited as long as it is two or more (plural).

[0048] Fine particles include particles smaller than 0.8 nm, the presence of which can be confirmed by observing the catalyst with a transmission electron microscope (TEM). Specifically, the catalyst is observed using a transmission electron microscope (TEM). TEM photographs are printed out on paper, and the noble metal particles (black circular images) are considered spherical, with the diameter measured from one end of the particle to the other. A total of 300 particles are randomly measured from images of several fields (3-5 fields). If particles smaller than 0.8 nm are present among the total of 300 particles, the particles are determined to contain particles smaller than 0.8 nm. Particles smaller than 0.8 nm are preferably between 0.2 nm and 0.8 nm, and more preferably between 0.3 nm and 0.7 nm.

[0049] The average particle size of the fine particles is not particularly limited, but is preferably 0.2 nm or more and 1.5 nm or less from the viewpoint of ensuring high activity. The average particle size can be determined using the following method (method for determining average particle size). The synthesized catalyst is observed using a transmission electron microscope (TEM). The TEM photograph is printed out on paper, and the fine particles (black circular images) are considered to be spherical. The diameter from one end of the fine particle to the other is considered to be the diameter, and a total of 300 particles are measured randomly from the images in several fields of view (3-5 fields of view). The average diameter of the 300 particles is taken as the average particle size. Furthermore, the fine particles preferably have at least one peak below 0.8 nm in a particle size distribution diagram, which is created from the particle sizes of 300 particles.

[0050] (3) Manufacturing method of catalyst B There are no particular limitations on the method for producing catalyst B. A preferred example of the method for producing catalyst B will be described below. Catalyst B can be suitably produced by applying a voltage having a potential cycle in an acidic environment to a composite in which a carbon support doped with nitrogen atoms and first transition metal atoms supports multiple raw material fine particles containing a precious metal, thereby dissolving and miniaturizing at least one of the raw material fine particles, and generating new fine particles on the carbon support from the metal ions generated by the dissolution. The above-mentioned catalyst A can be used as a composite in which a carbon support doped with nitrogen and first transition metal atoms supports multiple raw material fine particles containing a precious metal. Therefore, after producing the above-mentioned catalyst A, it can be suitably produced by applying a voltage having a potential cycle in an acidic environment to dissolve and miniaturize at least one of the raw material fine particles (fine particles in catalyst A with an average particle size of 0.8 nm to 1.5 nm) and generating new fine particles on the carbon support from the metal ions generated by the dissolution.

[0051] Here, the estimated mechanism by which fine particles in the sub-nano range (1 nm or less) are formed by the method for producing catalyst B will be described with reference to Figures 1 and 2. Pt particles will be exemplified as fine particles containing a noble metal. Figure 1 shows the case where a carbon support that is not doped with either nitrogen atoms or first transition metal atoms is used. The left side of Figure 1 shows a composite supported with Pt particles (raw material fine particles). When a voltage with a potential cycle is applied to this composite in an acidic environment, for example, Pt particles of 1.4 nm to 2 nm dissolve and form Pt. n+ As shown in the right figure, Pt n+ When a carbon support that is not doped with either nitrogen or first transition metal atoms is used, the Pt particles typically coarsen due to Ostwald ripening.

[0052] Figure 2 shows the case where a carbon support doped with nitrogen atoms and first transition metal atoms is used. The left image of Figure 2 shows a composite supported with Pt particles (raw material fine particles). When a voltage with a potential cycle is applied to this composite in an acidic environment, the Pt particles dissolve and Pt n+ Dissolved Pt n+Before reaching neighboring Pt particles, the Pt atoms are trapped by nitrogen atoms (N atoms) or Fe atoms (an example of first transition metal atoms) on the support, forming new Pt particles. At the same time, the remaining Pt particles also become smaller, and both become sub-nanosized (ultrafine particles), which is thought to result in a catalyst with a high specific surface area and high activity.

[0053] This section describes a composite in which a carbon support doped with nitrogen atoms and first transition metal atoms is supported with multiple raw material fine particles containing a precious metal. This composite corresponds to the above-mentioned "Catalyst A." Therefore, the composite can be produced by the above-mentioned "1.(3) Production method of Catalyst A."

[0054] The "acidic environment" is not particularly limited. Specific examples include immersing or contacting the composite in an acid solution. The acid solution is not particularly limited. For example, a perchloric acid solution is preferred as the acid solution, from the viewpoints of dissolving at least one of the microparticles to reduce their size and facilitating the generation of new microparticles on the carbon support from metal ions generated by the dissolution.

[0055] The potential cycle involves alternating low and high potentials. The low potential is preferably 0.0 V or more and 0.7 V or less, more preferably 0.5 V or more and 0.7 V or less, based on the standard hydrogen electrode. The high potential is preferably 0.8 V or more and 1.2 V or less, more preferably 0.9 V or more and 1.1 V or less, based on the standard hydrogen electrode. For example, potential cycling between 0.0 V and 1.0 V relative to the standard hydrogen electrode is preferred, and potential cycling between 0.6 V and 1.0 V is more preferred. When the low potential and the high potential are set within these preferred ranges, the fine particles are dissolved to form ultrafine particles of sub-nano size, and the newly generated fine particles also become ultrafine particles of sub-nano size.

[0056] The time for low potential per cycle is not particularly limited, but from the viewpoint of forming ultrafine particles, the time for low potential is preferably from 0.5 seconds to 300 seconds, more preferably from 1 second to 60 seconds, and even more preferably from 3 seconds to 10 seconds.

[0057] The high potential time per cycle is not particularly limited, and from the viewpoint of an appropriate amount of Pt dissolution, the high potential time is preferably from 0.5 seconds to 300 seconds, more preferably from 1 second to 60 seconds, and even more preferably from 3 seconds to 10 seconds.

[0058] The waveform of the potential cycle is not particularly limited, and examples of the waveform include a pulse wave, a periodic waveform, a square wave, a triangular wave, and a sine wave.

[0059] The number of potential cycles is not particularly limited, but from the viewpoint of forming sub-nano-sized ultrafine particles and improving catalytic performance, the number of potential cycles is preferably 1 to 100,000, more preferably 10 to 50,000, and even more preferably 100 to 10,000.

[0060] (4) Amount of precious metals carried The amount of the noble metal supported is not particularly limited, and may be an amount required depending on the intended design, etc. From the viewpoints of catalyst performance and cost, the amount of the noble metal supported is preferably 5 parts by mass or more and 70 parts by mass or less, and more preferably 10 parts by mass or more and 50 parts by mass or less, per 100 parts by mass of the carbon support, in terms of metal.

[0061] (5) Effect of catalyst B Catalyst B contains sub-nanoparticles and has high performance.

[0062] 3.Catalyst C Catalyst C is a catalyst obtained by applying a voltage having a potential cycle in an acidic environment to a composite in which a carbon support doped with nitrogen atoms and first transition metal atoms supports multiple raw material microparticles containing precious metals, thereby dissolving and miniaturizing at least one of the raw material microparticles and generating new microparticles on the carbon support from the metal ions produced by the dissolution.

[0063] (1) Carbon support In catalyst C, the explanation for the "carbon support" in the section "1. Catalyst A" is applied as is, and the description thereof is omitted. In other words, the "carbon support" explained in the section "1. Catalyst A" is applied as is.

[0064] (2) Multiple raw material particles containing precious metals Regarding catalyst C, with regard to the "multiple raw material fine particles containing precious metals," the explanation for "1. (2) multiple fine particles containing precious metals" in the "1. Catalyst A" column is applied as is, with the word "fine particles" read as "raw material fine particles," and the description is omitted.

[0065] (3) Application of a voltage with a potential cycle in an acidic environment Regarding the application of a voltage having a potential cycle in an acidic environment, the explanation in the section "2. Catalyst B" and "2. (3) Production method of catalyst B" applies as is, and the explanation is omitted here.

[0066] (4) Amount of precious metals carried The amount of the noble metal supported is not particularly limited, and may be an amount required depending on the intended design, etc. From the viewpoints of catalyst performance and cost, the amount of the noble metal supported is preferably 5 parts by mass or more and 70 parts by mass or less, and more preferably 10 parts by mass or more and 50 parts by mass or less, per 100 parts by mass of the carbon support, in terms of metal.

[0067] (5) Effect of catalyst C Catalyst C contains sub-nanoparticles and has high performance.

[0068] 4. Application of catalysts A, B, and C Catalysts A, B, and C can be applied to fuel cells. The potential cycle, which is one of the conditions for forming sub-nanoparticles described in "2.(3) Manufacturing method for catalyst B," corresponds to the operating potential range of a fuel cell. In particular, when considering its use in fuel cell vehicles (FCVs), it matches the range of potential fluctuations that occur when the load on an FCV changes. In other words, by installing catalysts A, B, and C in a fuel cell, highly active sub-nanoparticle catalysts are self-formed as the fuel cell is driven, and this activity continues indefinitely, demonstrating high durability. As a result, this is a groundbreaking catalyst that achieves both improved activity and maintained durability, which have been challenges in fuel cells.

[0069] In the case of catalyst A, the average particle size of the microparticles is between 0.8 nm and 1.5 nm in the initial state. However, when catalyst A is installed in a fuel cell, the particle size decreases as the fuel cell operates, and then a highly active sub-nanoparticle catalyst self-forms, maintaining its activity indefinitely. In the case of catalysts B and C, they are highly active sub-nanoparticle catalysts even in their initial state. By installing catalysts B and C in a fuel cell, highly active sub-nanoparticle catalysts are self-formed as the fuel cell is operated, and the activity continues indefinitely.

[0070] 5. Electrode The electrode containing the catalyst may be used as a cathode, an anode, or both a cathode and an anode. The electrodes of the present disclosure have high performance because they contain sub-nanoparticle catalysts.

[0071] 6.Membrane electrode assembly The membrane electrode assembly includes electrodes on the surface of an electrolyte membrane. The membrane electrode assembly of the present disclosure has high performance because the catalyst contains sub-nanoparticles.

[0072] 7.Fuel cell The fuel cell contains a catalyst. Examples of fuel cells include polymer electrolyte fuel cells (PEFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), alkaline electrolyte fuel cells (AFCs), and direct current fuel cells (DFCs). The fuel cell of the present disclosure has high performance because the catalyst contains sub-nanoparticles. An example of the configuration of a fuel cell will be described. This fuel cell 10 is a suitable example of a solid polymer fuel cell. As shown in FIG. 3, the fuel cell 10 includes a solid polymer electrolyte membrane 12 serving as an electrolyte membrane. The solid polymer electrolyte membrane 12 is made of, for example, perfluorosulfonic acid resin. An anode electrode 14 and a cathode electrode 16 are provided on both sides of the solid polymer electrolyte membrane 12 so as to sandwich it. The solid polymer electrolyte membrane 12 and the pair of anode electrode 14 and cathode electrode 16 sandwiching it form a membrane electrode assembly 18.

[0073] A gas diffusion layer 20 is provided on the outside of the anode electrode 14. The gas diffusion layer 20 is made of a porous material such as carbon paper, carbon cloth, or a porous metal, and functions to uniformly diffuse gas supplied from the separator 22 side to the anode electrode 14. Similarly, a gas diffusion layer 24 is provided on the outside of the cathode electrode 16. The gas diffusion layer 24 functions to uniformly diffuse gas supplied from the separator 26 side to the cathode electrode 16. While this diagram shows only one set of membrane electrode assembly 18, gas diffusion layers 20, 24, and separators 22, 26 configured as described above, an actual fuel cell 10 may have a stack structure in which multiple membrane electrode assemblies 18 and gas diffusion layers 20, 24 are stacked with separators 22, 26 interposed therebetween.

[0074] 8. Catalyst manufacturing method The method for producing a catalyst disclosed herein involves applying a voltage having a potential cycle in an acidic environment to a composite in which a carbon support doped with nitrogen atoms and first transition metal atoms supports a plurality of raw material microparticles containing a precious metal, thereby dissolving and miniaturizing at least one of the raw material microparticles and generating new microparticles on the carbon support from the metal ions produced by the dissolution.

[0075] (1) Carbon support In this production method, the explanation for the "carbon support" in the section "1. Catalyst A" applies as is, and the description thereof will be omitted. In other words, the "carbon support" explained in the section "1. Catalyst A" applies as is.

[0076] (2) Multiple raw material particles containing precious metals In this manufacturing method, with regard to the "plurality of raw material microparticles containing precious metals," the explanation for "1. (2) Multiple microparticles containing precious metals" in the "1. Catalyst A" column is applied as is, with the word "microparticles" read as "raw material microparticles," and the description is omitted.

[0077] (3) Applying a voltage with a potential cycle in an acidic environment Regarding the application of a voltage having a potential cycle in an acidic environment, the explanation in the section "2. Catalyst B" and "2. (3) Production method of catalyst B" applies as is, and the explanation is omitted here.

[0078] (4) Effect of manufacturing method This production method allows the formation of sub-nanoparticles, making it possible to produce high-performance catalysts. [Example]

[0079] The present disclosure will be explained more specifically with reference to examples. 1.Types of carbon supports Four types of carbon supports were prepared to investigate the influence of the type of carbon support (surface area, structure, defects, etc.) on Pt. Figure 4 shows transmission electron microscope (TEM) images of the four types of carbon supports that were compared. Figure 4(A) shows graphitized carbon black (GCB), which has a specific surface area of ​​150 m 2 g -1 The carbon surface has the advantage of being highly resistant due to the multi-layered graphene formation. Figure 4(B) shows a typical mesoporous carbon (MPC) with a specific surface area of ​​460 m 2 g -1 is. Figure 4(C) shows a carbon alloy (N-Fe-C, precious metal-free carbon (PMF)) in which nitrogen atoms (N) and iron atoms (Fe) are doped into the carbon skeleton of mesoporous carbon. The specific surface area is 560 m 2 g -1 is. Figure 4(D) shows carbon black (Ketjenblack) with a specific surface area of ​​800 m 2 g -1 All of these carbons are commercially available.

[0080] 2.Pt loading Figure 5 shows electron microscope (TEM) images of the carbon supports shown in Figure 4 after Pt was loaded onto them. (A), (B), (C), and (D) in Figure 5 correspond to (A), (B), (C), and (D) in Figure 4, respectively. "Pt / GCB" indicates graphitized carbon black loaded with Pt. "Pt / MPC" indicates mesoporous carbon loaded with Pt. "Pt / PMF" indicates a carbon alloy loaded with Pt and doped with nitrogen atoms (N) and iron atoms (Fe). "Pt / CB" indicates carbon black loaded with Pt. Only Pt / CB in Figure 5 (D) is a commercially available product; the others are loaded with Pt in accordance with prior art (Patent Application No. 2019-227955, paragraph

[0040] ). Compared to the commercially available Pt / CB in Figure 5(D), the Pt nanoparticles in Figure 5(A), (B), and (C) are smaller, but all of them were successfully dispersed and supported. Specifically, (A), (B), and (C) in Figure 5 were prepared as follows: 60 mg of hexachloroplatinic acid (IV) hexahydrate (HPtCl6·6H2O: Kanto Chemical, 98.5%) was placed in a beaker, and 1 mL of ethanol (C2H5OH) was added to dissolve it. 45 mg of carbon support was placed in a mortar, and the ethanol solution containing the Pt salt was added. The mixture was stirred and mixed until the ethanol evaporated and the mixture was dry. The resulting powder was transferred to a ceramic boat and heat-treated in a tubular furnace in an argon (Ar) atmosphere at 200°C for 2 hours. After cooling to room temperature, it was removed from the furnace and used as the catalyst.

[0081] 3. Electrode construction of Pt-loaded carbon supports Each carbon support carrying Pt was made into an electrode. The procedure was as follows: A specified amount (2 mg) of each carbon support carrying Pt was dispersed in ethanol and then dispersed and fixed on a disk-shaped carbon electrode substrate. After drying, a 0.2% Nafion solution was added dropwise to give a dry film thickness of 0.1 μm, and the electrode was then vacuum dried.

[0082] 4. Potential Cycling Treatment Next, the prepared electrode was immersed in an Ar-degassed 0.1 M HClO solution and connected to a potentiostat as the working electrode. A Pt wire and a saturated hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. Then, a potential cycle was repeated a specified number of times (0 to 100,000 times) using the waveform shown in Figure 6. Hereinafter, this process may be referred to as the "potential cycle process." 6, the low potential is 0.6 V and the high potential is 1.0 V based on the standard hydrogen electrode. The low potential time in one cycle is 3 seconds and the high potential time is 3 seconds, making one cycle 6 seconds.

[0083] 5. Changes in TEM images and particle size distribution before and after potential cycling (1) In the case of Pt / GCB, etc. Figures 7(A) and (B) show TEM images of Pt / GCB before and after potential cycling. Figure 7(C) shows the change in the particle size distribution of Pt particles in Pt / GCB before and after potential cycling. In the case of Pt / GCB, the average particle size (d) and its standard deviation (±σ) were 1.4±0.1 nm before potential cycling, but increased to 5.5±1.7 nm after potential cycling, and the distribution width also became considerably wider. The same phenomenon was also observed in the cases of Pt / MPC and Pt / CB. These phenomena are thought to be due to Ostwald ripening, in which Pt particles dissolved during potential cycling reprecipitate on nearby Pt particles, causing coarsening (see Figure 1).

[0084] (2) In the case of Pt / PMF In the case of Pt / PMF, as shown in Figure 8, the distribution of Pt particles before potential cycling (see Figure 8(A)) was significantly different from the initial Pt particles. After potential cycling, black dots smaller than the initial Pt particles were observed (see Figure 8(B)). These black dots were clearly visible as heavy elements in dark-field images (see Figure 8(C)). Energy-dispersive X-ray analysis (EDX) confirmed that these tiny dots were Pt particles (see Figure 8(E)). Thus, the ultrafine particles that appeared on the PMF after potential cycling were Pt particles. The change in particle size distribution revealed that the Pt size, which was 1.3±0.1 nm before potential cycling, changed to 0.5±0.1 nm after potential cycling (see Figure 8(D)). A significant feature that distinguishes this material from other carbon supports is the doping of N and Fe atoms into the carbon skeleton. Therefore, in the case of Pt / PMF, as shown in Figure 2, Pt ions dissolved during potential cycling are trapped by N or Fe atoms on the carbon before reaching neighboring Pt particles. It is presumed that nucleation occurs there, forming new ultrafine particles. Furthermore, the volume of the parent Pt particles (nanoparticles) decreases as they dissolve, and they too become smaller, down to 0.5 nm (sub-nanosize). Not all of the original particles become finer; as with Pt / GCB, some particles become coarser. However, overall, this is thought to have a positive effect on performance (see the evaluation of catalytic activity below).

[0085] 6. Evaluation of Catalytic Activity The catalytic activity of the electrodes after potential cycling was evaluated. Figure 9(A) plots the electrochemical surface area (ECA) of each catalyst obtained in an Ar-degassed 0.1 M HClO solution versus the number of potential cycling cycles. Figure 9(B) plots the mass activity (current per gram of platinum) for the oxygen reduction reaction in the same oxygen-saturated solution versus the number of potential cycling cycles. For Pt / GCB, Pt / MPC, and Pt / CB, the ECA value decreases with increasing potential cycling. This is presumably due to coarsening of Pt particles due to Ostwald ripening. As the ECA decreases, the mass activity also decreases, resulting in a decline in catalytic activity. On the other hand, for Pt / PMF, the ECA tends to be constant or slightly increases with the number of cycles, suggesting that the refined Pt particles are functioning as a catalyst. For Pt / PMF, the ECA value does not decrease, and therefore the mass activity does not decrease.

[0086] 7. Effect of Pt loading method We investigated the effect of the Pt support method. Figure 10 shows TEM images and particle size distributions of Pt / PMF synthesized using the prior art (patent application 2019-227955) and Pt / PMF synthesized using a conventional colloidal method. The prior art utilizes the characteristics of the synthesis method, resulting in uniform particle size. The standard deviation was only 7% of the average particle size (see Figure 10(A)). On the other hand, the colloidal method had a wider distribution, with a standard deviation of 16% of the average particle size (Figure 10(B)). We subjected these two types of Pt / PMF with different distribution widths to potential cycling as described above in "4. Potential Cycling Treatment" and compared their catalytic activity. We found that the Pt / PMF synthesized using the prior art (patent application 2019-227955) retained extremely high activity, even after many potential cycling cycles (Figure 10(C)). Note that in this experiment, electrodes after 60,000 potential cycling cycles were used. These experimental results showed that, as in the prior art, Pt / PMF prepared through a process of mixing a precious metal salt, an alcohol having 1 to 5 carbon atoms, and a carrier to form a mixture, and a heating process of heating the mixture at a temperature of 150°C to 800°C to form a catalyst, can be used to produce a catalyst with extremely high retention of catalytic activity even after repeated use.

[0087] 8.Other Figure 11 shows the accelerated degradation protocol recommended by the Fuel Cell Council of Japan (FCCJ), which simulates potential fluctuations in response to the load of a fuel cell vehicle (FCV). This accelerated degradation of platinum catalysts is used to evaluate their durability, a recommended domestic evaluation standard. This protocol uses square waves, maintaining potentials of 0.6 V and 1.0 V for three seconds. This is similar to the potential cycling process proposed in this disclosure (see Figure 6), and the potentials and retention times are within the ranges specified. In other words, applying this disclosure to a fuel cell for an FCV will result in spontaneous formation of sub-nano Pt particles within the cell, resulting in a catalyst that does not degrade in performance. In other words, the catalyst appears to be virtually unaffected by degradation.

[0088] 9. Effects of this embodiment This embodiment is extremely important in terms of reducing the cost of fuel cells, and is expected to greatly contribute to the widespread use of fuel cells themselves, the widespread use of fuel cell vehicles using such fuel cells, and the accelerating spread of stationary cogeneration systems.

[0089] The foregoing examples are for illustrative purposes only and are not to be construed as limiting the present disclosure. While the present disclosure has been described with reference to exemplary embodiments, the words used in describing and illustrating the present disclosure are understood to be descriptive and exemplary, rather than limiting. As detailed herein, changes may be made within the purview of the appended claims without departing from the scope or spirit of the present disclosure in its form. While specific structures, materials, and examples have been referenced herein in the detailed description of the present disclosure, it is not intended that the present disclosure be limited to the specifics disclosed herein; rather, the present disclosure extends to all functionally equivalent structures, methods, and uses within the scope of the appended claims.

[0090] The present disclosure is not limited to the above-described embodiments, and various modifications and variations are possible within the scope of the claims. [Explanation of symbols]

[0091] 10...Polymer fuel cell 12...Solid polymer electrolyte membrane 14...Anode electrode 16...Cathode electrode 18...Membrane electrode assembly 20...Gas diffusion layer 22...Separator 24...Gas diffusion layer 26...Separator

Claims

1. A catalyst in which a plurality of fine particles containing a precious metal are supported on a carbon support doped with nitrogen atoms and iron atoms, Before actual use in a fuel cell, the average particle size of the fine particles is 0.8 nm or more and 1.5 nm or less. A catalyst, wherein prior to said actual use, said fine particles comprise particles less than 0.8 nm.

2. The catalyst according to claim 1, wherein the fine particles have at least one peak at less than 0.8 nm in a particle size distribution diagram.

3. The catalyst according to claim 1, wherein the fine particles have a standard deviation of 0% to 10% of the average particle diameter.

4. 3. The catalyst according to claim 1 or 2, wherein, in actual use in a fuel cell, the catalyst is A catalyst in which at least one of the fine particles is dissolved and reduced in size, and new fine particles containing a precious metal are generated on the carbon support from metal ions produced by the dissolution.

5. An electrode comprising the catalyst according to any one of claims 1 to 4.

6. A membrane electrode assembly comprising the electrode according to claim 5 on a surface of an electrolyte membrane.

7. A fuel cell comprising the catalyst according to any one of claims 1 to 4.

8. A composite in which a carbon support doped with nitrogen atoms and iron atoms supports a plurality of raw material fine particles containing a precious metal, Before actual use in a fuel cell, a voltage having a potential cycle is applied in an acidic environment to dissolve and miniaturize at least one of the raw material microparticles, and new microparticles of less than 0.8 nm are generated on the carbon support from the metal ions produced by the dissolution.

9. The catalyst according to claim 8 , wherein the potential cycle is a repeated cycle between a potential of 0 V or more and 1.0 V or less relative to a standard hydrogen electrode.

10. An electrode comprising the catalyst according to claim 8 or claim 9.

11. A membrane electrode assembly comprising the electrode according to claim 10 on a surface of an electrolyte membrane.

12. A fuel cell comprising the catalyst according to claim 8 or 9.

13. A method for producing a catalyst, comprising applying a voltage having a potential cycle in an acidic environment to a composite in which a carbon support doped with nitrogen atoms and iron atoms supports a plurality of raw material microparticles containing a precious metal, thereby dissolving and miniaturizing at least one of the raw material microparticles, and generating new microparticles of less than 0.8 nm on the carbon support from metal ions produced by the dissolution.

14. The method for producing a catalyst according to claim 13, wherein the potential cycle is a cycle that is repeated between a potential of 0 V or more and 1.0 V or less relative to a standard hydrogen electrode.

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