Electrode catalyst, fuel cell, and electrode catalyst production method

Abstract

The present invention provides an electrode catalyst in which a metal catalyst is supported on a carrier. The metal catalyst contains platinum or a platinum alloy. The carrier is a porous body. When the content of the metal catalyst in the electrode catalyst as measured by inductively coupled plasma mass spectrometry is denoted as A (mass%) and the content of the metal catalyst in the electrode catalyst as measured by X-ray photoelectron spectroscopy is denoted as B (mass%), the value B / A of the content B with respect to the content A is 0.50-1.00. It is also preferable that, when the outer edge length of a particle of the carrier in a transmission electron microscope image of the electrode catalyst is denoted as L and the number of particles of the metal catalyst that are in contact with the outer edge of the particle of the carrier and located on the outer side of the outer edge in the transmission electron microscope image is denoted by N, the value N / L of N with respect to L is not more than 19 μm-1.

Description

Electrode catalyst, fuel cell, and method for manufacturing an electrode catalyst

[0001] This invention relates to an electrode catalyst and a fuel cell containing the same. Furthermore, this invention relates to a method for producing an electrode catalyst.

[0002] From the perspective of preventing global warming, CO ２ Fuel cells that can generate electricity without emitting emissions are attracting attention. Conventional fuel cells have had the problem of requiring large amounts of platinum, an expensive material, to increase the reaction rate in the cathode. Therefore, attempts have been made to improve the activity of the platinum catalyst used in the cathode and reduce the amount of platinum catalyst used.

[0003] For example, Patent Document 1 proposes a supported metal catalyst in which platinum particles are supported on a carrier, wherein 50% or more of the platinum particles are supported in the surface region of the carrier. The document states that in such a supported metal catalyst, the utilization efficiency of the catalyst component is enhanced because the platinum particles, which are the catalyst component, are supported in shallow positions within the pores of the carrier.

[0004] US2023 / 085417A1

[0005] However, in the supported metal catalyst described in Patent Document 1, the position of the platinum particles was not sufficiently optimized, and as a result, there was room for improvement in catalytic activity.

[0006] Therefore, the object of the present invention is to provide an electrode catalyst having high catalytic activity.

[0007] The present invention provides an electrode catalyst in which a metal catalyst is supported on a carrier, wherein the metal catalyst contains platinum or a platinum alloy, the carrier is a porous material, and when the content of the metal catalyst in the electrode catalyst measured by inductively coupled plasma mass spectrometry is A (mass%), and the content of the metal catalyst in the electrode catalyst measured by X-ray photoelectron spectroscopy is B (mass%), the value of content B relative to content A, B / A, is 0.50 or more and 1.00 or less.

[0008] The present invention also relates to an electrode catalyst in which a metal catalyst is supported on a carrier, wherein the metal catalyst contains platinum or a platinum alloy, the carrier is a porous material, and when L is the outer edge length of the carrier particles in a transmission electron microscope image of the electrode catalyst, and N is the number of metal catalyst particles that are in contact with the outer edge of the carrier particles and located outside the outer edge in the transmission electron microscope image, the value of N relative to L, N / L, is 19 μm. －１ The following is an electrode catalyst provided.

[0009] The present invention also relates to a method for producing an electrode catalyst in which a metal catalyst is supported on a carrier containing a porous body, comprising: stirring a mixture containing the carrier, a water-soluble metal salt, water, and a hydrophobic solvent; separating solid components from the mixture; reducing the metal salt contained in the solid components to produce the metal catalyst; and calculating the BET specific surface area SSA of the carrier from nitrogen gas adsorption. Ｎ２ The BET specific surface area SSA of the carrier calculated from water vapor adsorption is Ｈ２Ｏ Ratio SSA Ｈ２Ｏ / SSA Ｎ２ The present invention provides a method for manufacturing an electrode catalyst in which the coefficient is 0.0010 or more and 0.30 or less.

[0010] The present invention also provides a method for producing an electrode catalyst in which a metal catalyst is supported on a carrier containing a porous body, wherein a mixture containing the carrier, a colloidal solution, and a hydrophobic solvent is stirred, and solid components are separated from the mixture, wherein the colloidal solution uses water as the dispersion medium and the metal catalyst or its precursor is dispersed in the mixture.

[0011] Figure 1 is a transmission electron microscope image of the electrode catalyst of Example 1. Figure 2 is a transmission electron microscope image of the electrode catalyst of Comparative Example 1.

[0012] The present invention will be described below based on its preferred embodiments. The present invention relates to an electrode catalyst. The electrode catalyst of the present invention is characterized by a metal catalyst supported on a carrier.

[0013] The support for the electrode catalyst is a porous body having pores. Preferably, at least a portion of the pores of the support communicate with the surface of the support. Whether or not a support has pores communicating with its surface can be confirmed by observing the support with a scanning electron microscope or by analyzing the adsorption isotherm measured by nitrogen adsorption and calculating the pore size distribution. From the viewpoint of ensuring a sufficient amount of metal catalyst supported by the support while effectively suppressing the decrease in catalytic activity of the electrode catalyst caused by the metal catalyst being supported inside the pores of the support (at a position far from the surface of the support), it is preferable that the support has a pore size of an appropriate size. Specifically, it is preferable that the support has mesopores. Mesopores are pores with a diameter of 2.0 nm to 50.0 nm. Whether or not a support has mesopores can be confirmed, for example, by nitrogen adsorption. In detail, if at least one peak is observed in the range of 2.0 nm to 50.0 nm when the pore size distribution of the support is measured by nitrogen adsorption, the support can be considered to have mesopores.

[0014] When operating a fuel cell incorporating the electrode catalyst of the present invention, from the viewpoint of facilitating gas transport to the metal catalyst supported within the pores of the carrier and discharging water produced by the reaction between oxygen and hydrogen (hereinafter also referred to as "produced water"), the mode pore diameter of the carrier in the range of 2.0 nm or more and 50.0 nm or less is preferably 2.0 nm or more, more preferably 2.5 nm or more, still more preferably 3.0 nm or more. Further, from the viewpoint of facilitating the support of the metal catalyst near the surface of the carrier within the pores of the carrier, and making it difficult for the electrolyte to enter the pores when the electrode catalyst of the present invention is incorporated into a fuel cell, thereby suppressing a decrease in catalytic activity caused by the metal catalyst being covered with the electrolyte, the mode pore diameter of mesopores of the carrier is preferably 20.0 nm or less, more preferably 15.0 nm or less, still more preferably 10.0 nm or less. The mode pore diameter of the carrier refers to the peak position in the pore size distribution of the carrier measured by a nitrogen adsorption method. The advantages of the metal catalyst being supported near the surface of the carrier within the pores of the carrier will be described later. In the present specification, "mode pore diameter of mesopores" means the mode pore diameter in the range of 2.0 nm or more and 50.0 nm or less.

[0015] From the viewpoint of supporting the metal catalyst on the carrier in a highly dispersed state, the BET specific surface area SSA of the carrier calculated from nitrogen gas adsorption Ｎ２ is preferably 150 m ２ / g or more, more preferably 250 m ２ / g or more, still more preferably 450 m ２ / g or more. Further, when operating a fuel cell incorporating the electrode catalyst of the present invention, from the viewpoint of facilitating gas transport to the metal catalyst supported within the pores of the carrier and discharging produced water, the BET specific surface area SSA Ｎ２ is preferably 3000 m ２ / g or less, more preferably 2500 m ２ / g or less, still more preferably 2000 m ２ / g or less. The "highly dispersed state" means a state in which the particle size of the metal catalyst is appropriately controlled, there is little contact between particles of the metal catalyst, and the particles are uniformly present. The particle size of the metal catalyst will be described later.

[0016] From the viewpoint of supporting a larger amount of catalyst metal inside the pores of the carrier, the total pore volume of the carrier is preferably 0.2 mL / g or more, more preferably 0.5 mL / g or more, and even more preferably 1.0 mL / g or more. Furthermore, from the viewpoint of reducing the bulk of the electrode catalyst of the present invention and making the catalyst layer sufficiently thin when the electrode catalyst is incorporated into the catalyst layer of a fuel cell, and from the viewpoint of facilitating the diffusion of gas and generated water, the total pore volume of the carrier is preferably 5.0 mL / g or less, more preferably 4.5 mL / g or less, and even more preferably 4.0 mL / g or less. The pore diameter and total pore volume of the carrier described above can be measured by nitrogen adsorption.

[0017] A carrier having the pore diameter and total pore volume described above can be produced by, for example, thermally decomposing an organometallic compound such as magnesium citrate, and then removing the resulting metal or metal oxide component by acid treatment (for example, the method described in International Publication No. 2023 / 090060). This publication is incorporated herein by reference as part of this specification. Another method of production is to use a porous body of metal or oxide as a template, inject a carbon source such as furfuryl alcohol into the pores of the template to form a composite, then carbonize the carbon source by heat treatment, and further remove the template by acid treatment or the like (for example, the method described in Japanese Patent Application Publication No. 2019-169317). This publication is incorporated herein by reference as part of this specification. Commercially available carriers can also be used. For example, "Knobel" (registered trademark, manufactured by Toyo Tanso Co., Ltd.), a porous carbon manufactured by a casting method, can be used.

[0018] It is also preferable that the support material has appropriate hydrophobicity. Details regarding the hydrophobicity of the support material will be described later.

[0019] From the viewpoint of improving gas permeability and draining water generated by power generation, the most frequent pore size of the mesopores in the support (electrode catalyst) on which the metal catalyst is supported is preferably 2.0 nm or larger, more preferably 2.5 nm or larger, and even more preferably 3.0 nm or larger. Furthermore, from the viewpoint of making it difficult for the electrolyte to enter the pores when the electrode catalyst of the present invention is incorporated into a fuel cell, and suppressing the decrease in catalytic activity caused by the metal catalyst being covered with the electrolyte, the most frequent pore size of the mesopores in the support (electrode catalyst) on which the metal catalyst is supported is preferably 20 nm or smaller, more preferably 15 nm or smaller, and even more preferably 10 nm or smaller.

[0020] From the viewpoint of facilitating the transport of gas to the metal catalyst supported within the pores of the carrier and the discharge of generated water, the total pore volume of the carrier (electrode catalyst) with the metal catalyst supported is preferably 0.1 mL / g or more, more preferably 0.25 mL / g or more, even more preferably 0.5 mL / g or more, and particularly preferably 0.70 mL / g or more. Furthermore, from the viewpoint of reducing the bulk of the carrier and thereby reducing the thickness of the electrode layer containing the electrode catalyst, and thereby facilitating the transport of gas to the metal catalyst supported within the pores of the carrier and the discharge of generated water, the total pore volume of the carrier (electrode catalyst) with the metal catalyst supported is preferably 5.0 mL / g or less, more preferably 4.0 mL / g or less, and even more preferably 3.0 mL / g or less. The average pore diameter and total pore volume of the carrier with the metal catalyst supported can be measured by nitrogen adsorption.

[0021] Particularly preferred materials as supports for electrode catalysts are porous carbon, carbon black, acetylene black, carbon nanotubes, graphene, and conductive oxides. Therefore, it is preferable that the support contains one or more of the above-mentioned materials, and more preferably that the support contains porous carbon, as this prevents ionomer poisoning by supporting the metal catalyst within the pores and results in high activity. Examples of the conductive oxides include tin oxide doped with tantalum, niobium, or antimony.

[0022] The metal catalyst preferably contains platinum or a platinum alloy. This is because platinum and platinum alloys exhibit high electrochemical catalytic activity for the reduction of oxygen (and oxidation of hydrogen) in the temperature range around 80°C, which is the operating temperature of polymer electrolyte fuel cells. Platinum alloys are alloys containing platinum and other metals, and are cost-effective because they reduce the amount of expensive platinum used. The other metal is at least one selected from the elements of Group 3 to Group 12 of the periodic table (excluding platinum). As the other metal, those that show high catalytic activity when alloyed with platinum are preferably used. Specifically, for example, Sc is a Group 3 element. Ti, Zr, and Hf are Group 4 elements. V, Nb, and Ta are Group 5 elements. Cr, Mo, and W are Group 6 elements. Mn and Re are Group 7 elements. Fe and Ru are Group 8 elements. Co, Rh, and Ir are Group 9 elements. Ni and Pd are Group 10 elements. Examples of Group 11 elements include Cu, Ag, and Au. Examples of Group 12 elements include Zn. These metallic elements can be used individually or in combination of two or more. In particular, from the viewpoint of catalytic activity, the other metal is preferably at least one selected from the group consisting of Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ta, Ru, and W, and more preferably at least one selected from the group consisting of Co and Ru.

[0023] From the viewpoint of sufficiently increasing the catalytic activity of the electrode catalyst, the total content of platinum and platinum alloy contained in the metal catalyst is preferably 20% by mass or more, more preferably 30% by mass or more, even more preferably 40% by mass or more, even more preferably 80% by mass or more, even more preferably 90% by mass or more, and ideally 100% by mass. The total content of platinum and platinum alloy contained in the metal catalyst can be quantified, for example, by elemental analysis using an ICP emission spectrometer.

[0024] From the viewpoint of reducing costs and improving catalytic activity, the molar ratio of platinum (Pt) to the other metals (M) in the platinum alloy is preferably Pt:M = 1:1 to 20:1, more preferably Pt:M = 2:1 to 19:1, and even more preferably Pt:M = 3:1 to 18:1. Pt:M can be measured by elemental analysis using an ICP emission spectrometer.

[0025] Generally, in fuel cell electrode catalysts, potential fluctuations during power generation cause dissolution and reprecipitation of the metal catalyst, leading to degradation where the particle size of the metal catalyst increases and the active specific surface area decreases. This degradation progresses through Ostwald growth, where smaller metal catalyst particles preferentially dissolve, while larger metal catalyst particles reprecipitation and grow. Therefore, the wider the particle size distribution of the metal catalyst, the more easily degradation progresses. From the viewpoint of suppressing the above-mentioned degradation and improving the durability of the electrode catalyst of the present invention, it is preferable that the variation in the particle size of the metal catalyst is small. Specifically, the coefficient of variation of the particle size of the metal catalyst (the value obtained by dividing the standard deviation of the particle size by the average particle size and multiplying by 100) is preferably 55% or less, more preferably 50% or less, and even more preferably 45% or less. While a smaller coefficient of variation of the particle size of the metal catalyst is preferable, it is realistic to set it to 5% or more.

[0026] From the viewpoint of increasing the surface area of ​​the metal catalyst and enhancing the catalytic activity of the electrode catalyst, the average particle size of the metal catalyst is preferably 10 nm or less, more preferably 8 nm or less, and even more preferably 5 nm or less. Furthermore, from the viewpoint of ensuring durability, the average particle size of the metal catalyst is preferably 1 nm or more, more preferably 1.5 nm or more, and even more preferably 2 nm or more.

[0027] The standard deviation and average value (average particle diameter) of the particle size of the metal catalyst described above are calculated based on TEM images obtained by observing the electrode catalyst with a transmission electron microscope (hereinafter also referred to as "TEM"). Specifically, 100 particles of the metal catalyst observed in the TEM image in which the particle size outline is clearly visible are arbitrarily selected, and the average particle diameter and standard deviation are calculated based on the particle diameter of the selected particles. If the particle of an individual metal catalyst is not circular, its major axis is used as a substitute for its particle diameter.

[0028] In the electrode catalyst of the present invention, when the content of the metal catalyst in the electrode catalyst measured by inductively coupled plasma mass spectrometry (hereinafter also referred to as "ICP-MS") is A (mass%), and the content of the metal catalyst in the electrode catalyst measured by X-ray photoelectron spectroscopy (hereinafter also referred to as "XPS") is B (mass%), it is preferable that B / A is 0.50 or more. XPS measurement detects not only the metal catalyst supported on the surface of the support, but also the metal catalyst supported in the interior (near the surface) up to a depth of about 3 to 10 nm from the surface. In contrast, ICP-MS measurement detects all the metal catalyst contained on the surface and inside the support. Therefore, a B / A of 0.50 or more means that the proportion of the metal catalyst supported near the surface of the support is sufficiently large. When the proportion of the metal catalyst supported near the surface of the support is large, the metal catalyst comes into contact more easily with the reaction substrate (e.g., oxygen gas), which improves the power generation efficiency of the electrode catalyst. From the viewpoint of more reliably obtaining such effects, a B / A ratio of 0.65 or higher is more preferable, and a ratio of 0.7 or higher is even more preferable.

[0029] Furthermore, B / A is preferably 1.00 or less, more preferably 0.99 or less, and even more preferably 0.98 or less. The advantages of setting B / A to 1.00 or less will be described later.

[0030] From the viewpoint of sufficiently increasing the catalytic activity of the electrode catalyst, the content A of the metal catalyst in the electrode catalyst, as measured by ICP-MS, is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. Furthermore, from the viewpoint of suppressing a decrease in durability due to aggregation of metal catalyst particles, the content A is preferably 70% by mass or less.

[0031] From the viewpoint of sufficiently increasing the catalytic activity of the electrode catalyst, the content B of the metal catalyst in the electrode catalyst, as measured by XPS, is preferably 20% by mass or more, more preferably 30% by mass or more, and preferably 40% by mass or more. Furthermore, the content B may be, for example, 70% by mass or less.

[0032] Details of the measurement methods for content A and B will be explained in the examples described later.

[0033] As described above, in the electrode catalyst of the present invention, the proportion of the metal catalyst supported near the surface of the support is large. Here, "near the surface of the support" can be classified into the following (1) and (2): (1) Outside the pores of the support (i.e., the surface of the support) (2) Inside the pores of the support, in a position close to the surface of the support

[0034] In the electrode catalyst of the present invention, it is preferable that the proportion of metal catalyst supported at position (1) is small (and the proportion of metal catalyst supported at position (2) is large). The reason for this is as follows: When the metal catalyst is supported at position (2), compared to when it is supported at position (1), the surface of the metal catalyst is less likely to be covered by the electrolyte when the electrode catalyst of the present invention is incorporated into a fuel cell. Therefore, the decrease in catalytic activity caused by the metal catalyst surface being covered by the electrolyte is suppressed.

[0035] The proportion of metal catalyst supported at position (1) can be evaluated using TEM. Specifically, in the TEM image of the electrode catalyst, L is the length of the outer edge of the support particles, and N is the number of metal catalyst particles (particles supported at position (1)) that are in contact with and located outside the outer edge of the support particles. A small value of N relative to L, N / L, indicates a small proportion of metal catalyst supported at position (1). Specifically, N / L is 19 μm. －１ Preferably, it is 15 μm －１ More preferably, the following: 13 μm －１ More preferably, the following is preferred: 11 μm －１ The following is particularly preferred: 5 μm －１ The following is most preferable: The lower limit of N / L is 0.0 μm. －１ It can be made to the above, 0.5 μm －１ Preferably, the size is 3.0 μm or larger. －１ It is more preferable that the above is true, and 4.2 μm －１ The above is even more preferable. A more detailed method for measuring N / L will be described in the examples below.

[0036] Furthermore, the fact that the B / A ratio is 1.00 or less suggests that the proportion of metal catalyst supported at position (1) is small. In other words, when a large amount of metal catalyst is supported at position (1), the detection of peaks originating from the support is inhibited by the metal catalyst supported at position (1) in XPS measurements, and B tends to be a large value. As a result, the B / A ratio may exceed 1.00.

[0037] From the viewpoint of suppressing poisoning of the metal catalyst by chloride ions and enhancing the catalytic activity of the electrode catalyst, the chlorine element content in the electrode catalyst of the present invention is preferably 1% by mass or less, more preferably 0.1% by mass or less, even more preferably 0.01% by mass or less, and ideally 0% by mass. The chlorine ion content in the electrode catalyst can be measured, for example, by X-ray fluorescence analysis or inductively coupled plasma emission spectroscopy. In order to keep the chlorine element content in the electrode catalyst within the above range, the electrode catalyst can be manufactured in the absence of chlorine elements, for example, by using raw materials that do not contain chlorine.

[0038] Next, the method for producing the electrode catalyst of the present invention (hereinafter also referred to as the "first production method") will be described. The first production method is broadly divided into the following steps (1) to (3): (1) A step of stirring a mixed solution containing a carrier, a water-soluble metal salt, water, and a hydrophobic solvent (stirring step). (2) A step of separating solid components from the mixed solution (separation step). (3) A step of reducing the metal salt contained in the solid components to produce a metal catalyst (reduction step). These steps will be described in order below.

[0039] 1. Stirring Step As described above, in this step, a mixture containing a support, a water-soluble metal salt, water, and a hydrophobic solvent is stirred. Details of the support used in this step are as described above, and the above explanation is applied as appropriate. From the viewpoint of facilitating the deposition of the metal catalyst near the surface of the support within its pores, it is preferable that the support has sufficient hydrophobicity. If the support has sufficient hydrophobicity, the water containing the water-soluble metal salt will not easily penetrate deep into the pores of the support, and will tend to remain in shallow positions (near the surface of the support) within the pores of the support. As a result, it is possible to facilitate the deposition of the metal catalyst in shallow positions within the pores of the support.

[0040] The hydrophobicity of the carrier is calculated from the BET specific surface area (SSA) of the carrier, derived from nitrogen gas adsorption. Ｎ２ The BET specific surface area (SSA) of the carrier calculated from water vapor adsorption. Ｈ２Ｏ Ratio SSA Ｈ２Ｏ / SSA Ｎ２ It can be evaluated by [this method]. For details, see SSA Ｈ２Ｏ / SSAＮ２ It is preferable that it is 0.30 or less, more preferably 0.25 or less, and even more preferably 0.20 or less. SSA Ｈ２Ｏ / SSA Ｎ２ A value of 0.30 or less indicates that the carrier is sufficiently hydrophobic. SSA Ｈ２Ｏ / SSA Ｎ２ There is no particular restriction on the lower limit, but it can be, for example, 0.0010 or higher, and especially 0.0030 or higher. BET Specific Surface Area SSA Ｈ２Ｏ and SSA Ｎ２ The measurement method will be explained in the examples described later.

[0041] The hydrophobicity of a support can be adjusted by graphitization heat treatment or the addition of a water-repellent agent. For example, if the support is a carbon support, its hydrophobicity can be increased by graphitizing it through high-temperature heat treatment. Before high-temperature heat treatment, carbon supports have crystalline structure defects and functional groups, which contribute to their high hydrophilicity. By subjecting such a carbon support to heat treatment at 900°C or higher, the defects can be crystallized or the functional groups removed, thereby increasing the hydrophobicity of the support. It is also possible to increase the water repellency (hydrophobicity) of a support by impregnating and supporting it with a water-repellent material.

[0042] The carrier may be a commercially available or synthetic product used as is, or it may be used after activation treatment. The activation treatment can be carried out, for example, by heating the carrier under vacuum. From the viewpoint of effectively activating the carrier, the heating temperature for the activation treatment is preferably 60°C to 200°C, more preferably 100°C to 180°C, and even more preferably 120°C to 160°C. From the same viewpoint, the heating time for the activation treatment is preferably 1 hour to 24 hours, more preferably 6 hours to 20 hours, and even more preferably 8 hours to 16 hours.

[0043] The water-soluble metal salt used in the stirring step is a component that is reduced in the subsequent reduction step to become a metal catalyst. The water-soluble metal salt preferably contains a water-soluble platinum salt, and if the metal catalyst is an alloy of platinum and another metal, the water-soluble metal salt preferably contains a water-soluble platinum salt in addition to the water-soluble platinum salt of the other metal. The water-soluble platinum salt is Pt(NO) ３ ) ２ , Pt(NO ３ ) ４ , Pt(NH ３ ) ４ Cl ２ , Pt(NH ３ ) ４ (OH) ２ , Pt(NH ３ ) ４ (NO ３ ) ２ , Pt(NH ３ ) ６ Cl ４ , Pt(NH ３ ) ６ (OH) ４ H ２ Pt(OH) ６ , Pt(NH ３ ) ６ (NO ３ ) ４ , Pt(NH ３ ) ４ (CH ３ COO) ２ , (H ３ NCH ２ CH ２ OH) ２ [Pt(OH) ６ ], K ２ (Pt(NO ２ ) ４ Na ２ Pt(OH) ６ _K ２ Pt(OH) ６ H ２ PtCl ６ _K ２ PtCl ６ _K ２ PtCl ２ , PtCl ２ , PtCl ４The following can be used. In particular, Pt(NO) is a suitable water-soluble metal salt because it does not contain chlorine, which acts as a catalyst poison, and is easy to concentrate to high levels. ３ ) ２や Pt(NO ３ ) ４ It is preferable to use the above. In addition, as the water-soluble salt of the other metal, nitrates such as cobalt nitrate, nickel nitrate, and ruthenium nitrate; and chlorides such as cobalt chloride and nickel chloride can be used. As mentioned above, if chloride ions remain in the electrode catalyst, the catalytic activity of the electrode catalyst may decrease. Therefore, it is preferable that the water-soluble metal salt used in this process does not contain chlorine.

[0044] As the hydrophobic solvent used in this process, a solvent that does not mix with water in the mixture (separates into layers) can be used. From the viewpoint of reliably preventing miscibility between the hydrophobic solvent and water in the mixture, the hydrophobic solvent is preferably soluble in 100 mL of water at 20°C with a solubility of 10.0 g / 100 mL or less, more preferably 1 g / 100 mL or less, and even more preferably 0.1 g / 100 mL or less.

[0045] The hydrophobic solvent preferably contains one or more selected from hydrocarbons, ethers, esters, and halogenated solvents, and more preferably contains hydrocarbons. This is because hydrocarbons have high chemical stability.

[0046] Examples of the hydrocarbons mentioned above include linear aliphatic hydrocarbons such as pentane, hexane, heptane, and octane; cyclic aliphatic hydrocarbons such as cyclopentane, cyclohexane, methylcyclohexane, cycloheptane, and cyclooctane; and aromatic hydrocarbons such as benzene, toluene, o-xylene, m-xylene, and p-xylene.

[0047] Examples of the ether include diethyl ether, ethyl propyl ether, dipropyl ether, dibutyl ether, and tert-butyl methyl ether. Examples of the ester include ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, tert-butyl acetate, amyl acetate, and ethyl propionate. Examples of the halogenated solvent include dichloromethane, chloroform, carbon tetrachloride, 1,2-dichloroethane, and chlorobenzene.

[0048] From the viewpoint of reliably supporting the metal salt near the surface of the support, the volume ratio of water to the hydrophobic solvent in the mixture (water / hydrophobic solvent) is preferably 0.1% to 30% by volume, more preferably 0.5% to 20% by volume, and even more preferably 1% to 10% by volume. Thus, the main component of the liquid component of the mixture used in this manufacturing method is the hydrophobic solvent.

[0049] From a similar viewpoint, the mass of the carrier relative to the volume of the hydrophobic solvent contained in the mixture (carrier / hydrophobic solvent) is preferably 0.01 g / mL or more and 0.5 g / mL or less, more preferably 0.015 g / mL or more and 0.25 g / mL or less, and even more preferably 0.02 g / mL or more and 0.1 g / mL or less.

[0050] From the viewpoint of increasing the productivity of this manufacturing method, the ratio of the metal salt to the aqueous solution in the mixture (metal salt / aqueous solution) is preferably 10% by mass or more, more preferably 20% by mass or more, and even more preferably 30% by mass or more. Furthermore, from the viewpoint of completely dissolving the metal salt in the solvent in the mixture, the ratio of the metal salt to the aqueous solution contained in the mixture (metal salt / aqueous solution) is preferably less than or equal to the ratio of the metal salt to the aqueous solution in a saturated aqueous solution of the metal salt. For example, if the metal salt is platinum nitrate, the ratio of the metal salt to the aqueous solution (metal salt / aqueous solution) is preferably 60% by mass or less, more preferably 50% by mass or less, and particularly preferably 45% by mass or less. The mass of the aqueous solution used in calculating the mass ratio of the metal salt / aqueous solution is the total mass of water and water-soluble substances contained in the mixture. That is, the above "metal salt / aqueous solution" means "metal salt / (water + water-soluble substance)". The water-soluble substance is a substance that is mainly dissolved in water in the mixture, and for example, metal salts and inorganic acids described later fall into this category.

[0051] From the viewpoint of improving the productivity of this manufacturing method, the mass ratio of metal ions to the carrier in the mixture (metal ions / carrier) is preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. Furthermore, if an excessive amount of metal catalyst is supported on the carrier, the distance between metal particles becomes close, which may easily lead to degradation due to dissolution and reprecipitation. Therefore, from the viewpoint of suppressing such degradation, the mass ratio of metal ions to the carrier in the mixture (metal ions / carrier) is preferably 80% by mass or less, more preferably 75% by mass or less, and even more preferably 70% by mass or less. The mass of metal ions used in calculating the above-mentioned mass ratio of metal ions / carrier refers to the mass derived from metal ions among the mass of metal salts contained in the mixture.

[0052] The mixture may contain other components besides the carrier, metal salt, water, and hydrophobic solvent described above. Examples of such components include inorganic acids such as nitric acid. Depending on the type of metal salt, the solubility of the metal salt in water can be increased by including an inorganic acid in the mixture. The mass ratio of inorganic acid to water (inorganic acid / water) is, for example, 10% by mass or more and 65% by mass or less.

[0053] There are no particular restrictions on the mixing order of the carrier, metal salt, water, hydrophobic solvent, and other components contained in the mixture. For example, the mixture can be prepared by separately preparing a carrier dispersion containing the carrier and hydrophobic solvent, and an aqueous solution of the metal salt containing the metal salt, water, and other components as needed, and then mixing the carrier dispersion and the aqueous solution of the metal salt. Alternatively, the mixture may be prepared by sequentially adding the aqueous solution of the metal salt and the carrier to the hydrophobic solvent. In the carrier dispersion prepared here, the mass of the carrier relative to the volume of the hydrophobic solvent (carrier / hydrophobic solvent) is preferably within the numerical range described above as the carrier / hydrophobic solvent ratio in the mixture. Furthermore, in the aqueous solution of the metal salt, the ratio of the metal salt to the aqueous solution (metal salt / aqueous solution) is preferably within the numerical range described above as the metal salt / aqueous solution ratio in the mixture.

[0054] The carrier dispersion prepared by mixing the carrier and a hydrophobic solvent may be sonicated and / or vigorously stirred before being mixed with the metal salt aqueous solution. Vigorous stirring can be performed, for example, using a paint shaker (manufactured by Seiwa Giken Co., Ltd.).

[0055] By performing the stirring step described above, the metal salt aqueous solution can be attached to the vicinity of the surface of the support within the pores of the support. The reason for this is not clear, but the inventors assume the following: In the mixture, the hydrophobic solvent in which the support is dispersed and the metal salt aqueous solution are in phase separation. In this stirring step, stirring the mixture causes the metal salt aqueous solution to come into uniform contact with the surface of the support. At this time, the interface between the hydrophobic solvent and the metal salt aqueous solution is more energetically unstable than the interface between the support surface and the metal salt aqueous solution, so the metal salt aqueous solution tries to penetrate into the pores of the support surface to reduce the energetically unstable interface. On the other hand, the metal salt aqueous solution does not easily penetrate deep into the pores of the support and tends to remain near the surface of the support within the pores. This is especially pronounced when a hydrophobic support, such as porous carbon, is used as the support.

[0056] In order to ensure sufficient contact between the metal salt aqueous solution and the surface of the carrier, it is preferable to vigorously stir the mixture during the stirring step. Vigorous stirring can be performed, for example, by the paint shaker described above. The stirring time during the stirring step is preferably 0.5 hours to 12 hours, more preferably 1 hour to 8 hours, and even more preferably 1.5 hours to 4 hours.

[0057] 2. Separation Process The separation process is a process of separating the solid components from the mixture, that is, a process of removing the liquid components from the mixture. Through this process, water is removed from the water containing the metal salt that has entered the pores of the carrier, and the metal salt is supported in the pores of the carrier. The separation process can be carried out by filtration and / or drying, but preferably, filtration is performed to remove most of the hydrophobic solvent and water, which are liquid components, and then drying is performed to remove the remainder of the liquid components.

[0058] Drying of the solids can be carried out under heating or under reduced pressure. When drying is carried out under heating, from the viewpoint of improving productivity and suppressing thermal decomposition of metal salts, the drying temperature is preferably 60°C to 200°C, more preferably 100°C to 180°C, and even more preferably 120°C to 150°C. When drying is carried out under reduced pressure, it is preferable to reduce the pressure to 10,000 Pa or less, more preferably 1,000 Pa or less, and even more preferably 100 Pa or less. In either case, the drying time is preferably 0.5 hours to 12 hours, more preferably 2 hours to 8 hours, and even more preferably 2 hours to 6 hours. There are no particular restrictions on the drying atmosphere; for example, an inert atmosphere such as nitrogen or argon or an air atmosphere can be used.

[0059] Before drying the solids under heat or reduced pressure, they may be left to stand at room temperature and allowed to air dry.

[0060] If necessary, the stirring and separation steps may be repeated multiple times. In this case, the solid obtained in the separation step is used as a support for the next stirring step. By performing the stirring and separation steps multiple times in this manner, an electrode catalyst with a higher amount of metal catalyst can be obtained.

[0061] 3. Reduction Process The reduction process is a process of reducing the metal salt contained in the solid to produce a metal catalyst. This makes it possible to obtain an electrode catalyst in which the metal catalyst is supported on a carrier. The reduction of the metal salt can be carried out by well known methods, and one example is a method of heat treatment in a hydrogen-containing atmosphere. The hydrogen concentration in the hydrogen-containing atmosphere is preferably 1% to 100% by volume, more preferably 2% to 50% by volume, and even more preferably 3% to 4% by volume, with the remainder being an inert gas such as nitrogen or argon. The heating temperature in the reduction process is preferably 100°C to 1200°C, more preferably 120°C to 1000°C, and even more preferably 150°C to 900°C. The heating time (reduction treatment time) is preferably 10 minutes to 30 hours, more preferably 0.5 hours to 20 hours, and even more preferably 1 hour to 16 hours.

[0062] A slow oxidation step may be performed on the electrode catalyst obtained by the reduction treatment, if necessary. Performing a slow oxidation step prevents rapid oxidation of the metal catalyst when the metal catalyst is exposed to the atmosphere after reduction, which could cause the electrode catalyst to burn due to the heat of oxidation. The slow oxidation step can be performed by heating the electrode catalyst in an atmosphere containing oxygen and / or water (water vapor). The oxygen concentration in the atmosphere is preferably 0.1% to 10% by volume, more preferably 0.25% to 5% by volume, and even more preferably 0.5% to 3% by volume. The water vapor concentration in the atmosphere is preferably 0.5% to 20% by volume, more preferably 1% to 15% by volume, and even more preferably 2% to 10% by volume. The remainder of the atmosphere is an inert gas such as nitrogen or argon. The temperature of the slow oxidation step can be room temperature or higher and 40°C or lower.

[0063] Furthermore, an acid treatment step may be performed on the electrode catalyst obtained by the reduction treatment, if necessary. For example, if the metal catalyst is an alloy of platinum and a transition metal, the durability and activity of the electrode catalyst can be improved by performing an acid treatment step. Inorganic acids such as nitric acid can be used as the acid in the acid treatment step.

[0064] According to the above production method, in the stirring step, the aqueous metal salt solution is adsorbed near the surface within the pores of the carrier, so after water removal and reduction of the metal salt, an electrode catalyst in which a metal catalyst is supported near the surface of the carrier within the pores of the carrier can be obtained.

[0065] Next, another method for producing the electrode catalyst of the present invention (hereinafter also referred to as "the second production method") will be described. The second production method is broadly divided into the following steps (1) and (2). (1) A step of stirring a mixed solution containing a carrier, a colloidal solution, and a hydrophobic solvent (stirring step). (2) A step of separating a solid content from the mixed solution (separation step). Hereinafter, the second production method will be described focusing on differences from the first production method. For points not particularly described below, the above description regarding the first production method is appropriately applied to the second production method.

[0066] The second production method differs from the first production method in that a colloidal solution is used in place of the water-soluble metal salt and water in the stirring step. The colloidal solution used in the second production method uses a metal catalyst or a precursor thereof as a dispersoid and water as a dispersion medium. Details of the metal catalyst are as described above. The precursor of the metal catalyst is a metal salt that becomes the metal catalyst through reduction treatment, and is poorly soluble or insoluble in water. Examples of such metal salts include Pt(NO ２ )(NH ３ ) ２ Platinum salts such as the above can be mentioned.

[0067] The mixed solution may contain components other than the carrier, the colloidal solution, and the hydrophobic solvent. For example, the mixed solution may contain a water-soluble salt of a metal other than platinum. As such a water-soluble salt, those exemplified as the water-soluble salt of another metal in the first production method can be used. Further, in order to improve the stability of the colloidal solution, the colloidal solution may contain a dispersant such as an organic protective agent. Examples of the organic protective agent include polyvinylpyrrolidone, polyacrylamine, and polyethyleneimine. Furthermore, in order to lower the viscosity of the colloidal solution, additional water may be added to the mixed solution.

[0068] From the viewpoint of increasing the productivity of the second manufacturing method, the concentration of the dispersed phase in the colloidal solution is preferably 0.05 g / mL or higher, more preferably 0.1 g / mL or higher, and even more preferably 0.2 g / mL or higher. Furthermore, from the viewpoint of increasing the stability of the colloidal solution, the concentration of the dispersed phase in the colloidal solution is preferably 5 g / mL or lower, more preferably 4 g / mL or lower, and even more preferably 3 g / mL or lower.

[0069] The following are examples of methods for preparing colloidal solutions. One method for preparing a colloidal solution in which the dispersed phase is a metal catalyst is to add an ethylene glycol solution of sodium hydroxide to an ethylene glycol solution of platinum chloride and heat it with microwaves. Another method for preparing a colloidal solution in which the dispersed phase is a precursor of a metal catalyst is to use Pt(NO). ２ ) ２ (NH ３ ) ２ One method involves adding nitric acid and then heating the mixture.

[0070] From the viewpoint of facilitating contact between the carrier and the colloidal solution, the volume ratio of the colloidal solution to the hydrophobic solvent in the mixture (colloidal solution / hydrophobic solvent) is preferably 0.1% to 30% by volume, more preferably 0.5% to 20% by volume, and even more preferably 1% to 10% by volume.

[0071] From the viewpoint of reliably supporting the metal catalyst near the surface of the support, the volume ratio of water to the hydrophobic solvent contained in the mixture (water / hydrophobic solvent) is preferably 0.1% to 30% by volume, more preferably 0.5% to 20% by volume, and even more preferably 1% to 10% by volume. The "volume of water" referred to here is the total volume of water in the colloidal solution and any additional water added as appropriate for purposes such as viscosity adjustment.

[0072] There are no particular restrictions on the mixing order of the carrier, colloidal solution, and hydrophobic solvent contained in the mixture, but it is preferable to prepare the mixture by adding the hydrophobic solvent to the colloidal solution and then adding the carrier.

[0073] In the case where a colloidal solution containing a metal catalyst is used in the second production method, unlike the first production method, the metal catalyst itself is directly supported on a carrier. Therefore, the reduction step in the first production method does not need to be performed in the second production method. However, the reduction step may also be performed in the second production method for the purpose of improving catalytic activity or the like. Further, in the second production method, a precursor of a metal catalyst (for example, Pt(NO ２ ) ２ (NH ３ ) ２ ) when a colloidal solution containing is used, a reduction step is performed after the separation step to reduce the precursor of the metal catalyst to the metal catalyst. Since the details of the reduction step are the same as those in the first production method, the description thereof is omitted here.

[0074] When the colloidal solution contains a dispersant, a step of removing the dispersant may be performed in the separation step. Examples of the method for removing the dispersant include a method of washing the solid content (electrode catalyst) obtained in the separation step with washing water, and a method of heating the solid content (electrode catalyst) obtained in the separation step to volatilize the dispersant.

[0075] The electrode catalyst of the present invention obtained by the above-described method is suitably used, for example, for electrodes of various fuel cells. Specifically, the electrode catalyst of the present invention is used for an anode and / or a cathode of a polymer electrolyte fuel cell or a phosphoric acid fuel cell. Among others, the electrode catalyst of the present invention is preferably used for a cathode. The above-described fuel cell may comprise, for example, a cathode including the electrode catalyst of the present invention, an anode, and an electrolyte disposed between the cathode and the anode. Further, the electrode catalyst of the present invention can also be used for an anode catalyst layer and / or a cathode catalyst layer of a water electrolysis apparatus.

[0076] The above embodiments of the present invention encompass the following technical concepts: [1] An electrode catalyst in which a metal catalyst is supported on a carrier, wherein the metal catalyst contains platinum or a platinum alloy, the carrier is a porous material, and when the content of the metal catalyst in the electrode catalyst measured by inductively coupled plasma mass spectrometry is A (mass%), and the content of the metal catalyst in the electrode catalyst measured by X-ray photoelectron spectroscopy is B (mass%), the value of content B relative to content A, B / A, is 0.50 or more and 1.00 or less. [2] An electrode catalyst in which a metal catalyst is supported on a carrier, wherein the metal catalyst contains platinum or a platinum alloy, the carrier is a porous material, when the outer edge length of the carrier particles in the transmission electron microscope image of the electrode catalyst is L, and the number of metal catalyst particles in contact with the outer edge of the carrier particles and located outside the outer edge in the transmission electron microscope image is N, the value of N relative to L, N / L, is 19 μm －１ The following are electrode catalysts: [3] The electrode catalyst according to [1] or [2], wherein the carrier contains porous carbon. [4] The electrode catalyst according to any one of [1] to [3], wherein the carrier has mesopores. [5] The electrode catalyst according to any one of [1] to [4], wherein the coefficient of variation of the particle size of the metal catalyst is 55% or less. [6] The electrode catalyst according to any one of [1] to [5], wherein the content of chlorine element in the electrode catalyst is 1% by mass or less. [7] When L is the outer edge length of the carrier particles in the transmission electron microscope image of the electrode catalyst, and N is the number of metal catalyst particles that are in contact with the outer edge of the carrier particles and located outside the outer edge in the transmission electron microscope image, the value of N relative to L, N / L, is 19 μm －１The electrode catalyst according to any one of [1] to [6] below. [8] The electrode catalyst according to any one of [1] to [7], wherein the metal catalyst is a platinum alloy containing platinum and other metals, the molar ratio of platinum (Pt) to the other metal (M) in the platinum alloy is Pt:M = 1:1 to 20:1, and the other metal is at least one selected from the group consisting of Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ta, Ru, and W. [9] The electrode catalyst according to [8], wherein the other metal is at least one selected from the group consisting of Co and Ru.

[10] The electrode catalyst according to any one of [1] to [9], wherein the most frequent pore size of the electrode catalyst in the range of 2.0 nm to 50.0 nm is 2.0 nm to 20 nm.

[11] A fuel cell comprising: a cathode containing the electrode catalyst described in any one of [1] to

[10] ; an anode; and an electrolyte disposed between the cathode and the anode.

[12] A method for producing an electrode catalyst in which a metal catalyst is supported on a carrier containing a porous body, comprising: stirring a mixture containing the carrier, a water-soluble metal salt, water, and a hydrophobic solvent; separating solids from the mixture; reducing the water-soluble metal salt contained in the solids to produce the metal catalyst; and calculating the BET specific surface area SSA of the carrier from nitrogen gas adsorption. Ｎ２ The BET specific surface area SSA of the carrier calculated from water vapor adsorption is Ｈ２Ｏ Ratio SSA Ｈ２Ｏ / SSA Ｎ２ A method for producing an electrode catalyst, wherein the ratio is 0.0010 or more and 0.30 or less.

[13] The method for producing an electrode catalyst according to

[12] , wherein the hydrophobic solvent contains a hydrocarbon.

[14] The water-soluble metal salt is Pt(NO ３ ) ２A method for producing an electrode catalyst according to

[12] or

[13] , including the following:

[15] A method for producing an electrode catalyst according to any one of

[12] to

[14] , wherein the ratio of the metal salt to the total amount of water and water-soluble substance contained in the mixture (water-soluble metal salt / (water + water-soluble substance)) is 10% by mass or more and 60% by mass or less.

[16] A method for producing an electrode catalyst in which a metal catalyst is supported on a carrier containing a porous body, wherein a mixture containing the carrier, a colloidal solution and a hydrophobic solvent is stirred, and solid components are separated from the mixture, wherein the colloidal solution uses water as the dispersion medium and the metal catalyst or its precursor is the dispersed phase.

[17] A method for producing an electrode catalyst in which the dispersed phase of the colloidal solution is Pt(NO ２ ) ２ (NH ３ ) ２ The method for producing the electrode catalyst described in

[16] .

[18] The BET specific surface area SSA of the carrier calculated from nitrogen gas adsorption. Ｎ２ The BET specific surface area SSA of the carrier calculated from water vapor adsorption is Ｈ２Ｏ Ratio SSA Ｈ２Ｏ / SSA Ｎ２ A method for producing an electrode catalyst according to

[16] or

[17] , wherein the ratio is 0.0010 or more and 0.30 or less.

[0077] The present invention will be described in more detail below with reference to examples. However, the scope of the present invention is not limited to these examples. Unless otherwise specified, "%" means "mass%".

[0078] In the following examples and comparative examples, carrier 1 or carrier 2, which is porous carbon having mesopores, was used as the carrier. The BET specific surface area SSA of carrier 1 and carrier 2 was measured by the following method. Ｎ２ and SSA Ｈ２Ｏ The total pore volume and most frequent pore diameter are shown in Table 1. Carriers 1 and 2 were heated under vacuum at 150°C for 12 hours to activate them before use.

[0079] [BET Specific Surface Area] BET specific surface area (SSA) by nitrogen adsorption method Ｎ２The measurement was performed using the following procedure. As a pretreatment, carrier 1 and carrier 2 were heated at 400°C for 3 hours under reduced pressure of 20 Pa. For nitrogen adsorption and desorption measurements, a gas adsorption analyzer (Micromeritics 3Flex) was used, and the specific surface area SSA was calculated using the BET (Brunauer-Emmett-Teller) method from the adsorption isotherm at liquid nitrogen temperature (77 K). Ｎ２ The BET specific surface area SSA was determined by the water vapor adsorption method. Ｈ２Ｏ The measurement was performed using the following procedure. As a pretreatment, carrier 1 and carrier 2 were heated at 100°C for 4 hours under reduced pressure of 20 Pa. Subsequently, a gas adsorption analyzer (Micromeritics 3Flex) was used for water vapor adsorption and desorption measurements, and the specific surface area SSA was calculated from the adsorption isotherm using the BET method. Ｈ２Ｏ They sought it.

[0080] [Total Pore Volume and Mode Pore Diameter] The mode pore diameter and total pore volume were measured by analyzing nitrogen adsorption / desorption isotherms. A gas adsorption analyzer (Micromeritics 3Flex) was used for nitrogen adsorption / desorption measurements. As a pretreatment, carrier 1 and carrier 2 were heated at 400°C for 3 hours under reduced pressure of 20 Pa. The total pore volume of carrier 1 and carrier 2 was determined from the obtained adsorption isotherms using the single-point method. The mode pore diameter between 2.0 nm and 50.0 nm was calculated by analyzing the obtained adsorption isotherms using the BJH method. The analysis software used was 3Flex version 5.02, which was included with the gas adsorption analyzer.

[0081] [Example 1] (Stirring step) Pt(NOx) in 40 mL of hexane ３ ) ２ After adding 0.8 mL of nitric acid solution (containing 25% by mass of Pt), carrier 1 was added. The mixture was then stirred for over 2 hours using a paint shaker (manufactured by Seiwa Giken Co., Ltd.).

[0082] (Separation process) The mixture obtained in the stirring process was filtered to remove most of the liquid components and recover the solids. These solids were air-dried at room temperature for about 1 hour, and then dried further at 100°C for 2 hours under a nitrogen stream. The dried solids obtained in this way were used as a carrier, and the stirring and separation processes were repeated two more times. The stirring and separation processes were performed a total of three times. Hereinafter, the number of times the stirring and separation processes are performed will be referred to as the "number of loadings". In Example 1, the number of loadings was 3.

[0083] (Reduction Process) Next, the dried solid was subjected to a reduction treatment at 680°C for 3 hours under a nitrogen atmosphere containing 4% by volume of hydrogen. Furthermore, after confirming that the furnace temperature had dropped to 40°C or below, it was subjected to a slow oxidation treatment for 20 minutes under a nitrogen atmosphere containing 1% by volume of oxygen, and then removed into the air to obtain an electrode catalyst. XRF measurements showed that no chlorine element was detected in the electrode catalyst. This was also the case in Examples 2 to 9 described later. Table 1 shows the ratio of the mass of the support to the volume of the hydrophobic solvent (support / hydrophobic solvent) and the ratio of the volume of the metal salt aqueous solution to the pore volume of the support (metal salt aqueous solution / pore volume of the support) in the above mixture.

[0084] [Example 2] In the stirring process, Pt(NO ３ ) ２ An electrode catalyst was obtained in the same manner as in Example 1, except that 2.4 mL of nitric acid solution (containing 25% by mass of Pt) was used, carrier 2 was used instead of carrier 1, the solid components were naturally dried at room temperature for 12 hours or more during the separation process, and the number of loading steps was limited to one.

[0085] [Example 3] In the stirring process, Pt(NO ３ ) ２ An electrode catalyst was obtained in the same manner as in Example 1, except that 1.2 mL of nitric acid solution (containing 25% by mass of Pt) was used, carrier 2 was used instead of carrier 1, and the number of loading steps was reduced to two.

[0086] [Example 4] (Stirring step) Pt(NOx) in 40 mL of hexane ３ ) ２2.35 mL of nitric acid solution (containing 25% by mass of Pt) and 0.145 g of cobalt nitrate hexahydrate were added, and then carrier 1 was added. The mixture was then stirred for a little over 2 hours using a paint shaker (manufactured by Seiwa Giken Co., Ltd.). (Separation step) The liquid component was mostly removed by filtering the mixture obtained in the stirring step, and the solid component was recovered. This solid component was air-dried at room temperature for about 0.5 hours, and then dried at 120°C for 2 hours under a nitrogen stream. (Reduction step) The procedure was carried out in the same manner as in Example 1, except that the temperature was 700°C and the time was 12 hours. (Acid treatment step) The powder obtained in the reduction step was dispersed in 2M nitric acid. The amount of nitric acid used was adjusted so that the ratio of the volume of nitric acid to the mass of the powder (nitric acid / powder) was 19 mL / g. After stirring at 60°C for 18 hours, the solution was filtered and the solid component was recovered. The recovered solid component was dispersed in pure water and filtered again. Filtration and dispersion in pure water were repeated until the conductivity of the filtrate was 0.1 mS / cm or less. The resulting filtered solid was dried in a vacuum at 60°C for 4 hours to obtain an electrode catalyst.

[0087] [Example 5] In the stirring process, Pt(NO ３ ) ２ An electrode catalyst was obtained in the same manner as in Example 4, except that 2.2 mL of nitric acid solution (containing 25% by mass of Pt) and 0.452 g of cobalt nitrate hexahydrate were used.

[0088] [Example 6] In the stirring step, Pt(NO) was added to 24 mL of hexane. ２ ) ２ (NH ３ ) ２ A colloidal solution (containing 34% by mass of Pt) was prepared by dispersing with 60% nitric acid as the dispersion medium. 1.63 g of this solution and 0.271 g of cobalt nitrate hexahydrate were added, and then 0.6 g of carrier 1 was added to obtain a mixture. The mixture was then stirred for over 2 hours using a paint shaker (manufactured by Seiwa Giken Co., Ltd.). Except for this point, the procedure was the same as in Example 4 to obtain the electrode catalyst.

[0089] [Example 7] In the stirring step, Pt(NO) was added to 24 ml of hexane. ２ ) ２ (NH ３ ) ２A colloidal solution of 1.7 g using 60% nitric acid as the dispersion medium and 0.087 g of cobalt nitrate hexahydrate were added, followed by the addition of 0.6 g of carrier 1. The mixture was then stirred for over 2 hours using a paint shaker (manufactured by Seiwa Giken Co., Ltd.). Except for this point, the procedure was the same as in Example 4 to obtain the electrode catalyst.

[0090] [Example 8] In the stirring step, Pt(NO) is added to 24 mL of hexane. ２ ) ２ (NH ３ ) ２ After adding 1.65 g of a colloidal solution using as the dispersed phase and 60% nitric acid as the dispersion medium, 0.6 g of support 1 was added. The mixture was then stirred for more than 2 hours using a paint shaker (manufactured by Seiwa Giken Co., Ltd.). Except for this point, the procedure was the same as in Example 4 to obtain the electrode catalyst.

[0091] [Example 9] In the stirring step, Pt(NO) was added to 24 ml of hexane. ２ ) ２ (NH ３ ) ２ A colloidal solution of 1.6 g using 60% nitric acid as the dispersion medium and 0.3 g of ruthenium nitrate solution (containing 10% ruthenium) were added, and then 0.6 g of carrier 1 was added. The mixture was then stirred for more than 2 hours using a paint shaker (manufactured by Seiwa Giken Co., Ltd.). Except for this point, the procedure was the same as in Example 4 to obtain the electrode catalyst.

[0092] [Comparative Example 1] In Comparative Example 1, platinum was supported on carrier 2 by a liquid-phase reduction method. Carrier 2 and Pt(NO ３ ) ２ A mixture was obtained by adding a nitric acid solution (containing 25% by mass of Pt), water, and ethanol to a container in that order. The volume ratio of water to ethanol in the mixture was 1:1. Next, the mixture was stirred at 85°C for 16 hours to reduce platinum ions and support the resulting platinum particles onto carrier 2. The solid components were separated from the liquid components by filtration, and the recovered solid components were vacuum dried. Furthermore, the mixture was reduced at 200°C for 3 hours under a nitrogen atmosphere containing 4% by volume of hydrogen to reduce the platinum atoms supported on carrier 2 in the form of platinum hydroxide to zero-valent platinum. Finally, the mixture was slowly oxidized at 40°C or below for 20 minutes under a nitrogen atmosphere containing 1% by volume of oxygen to obtain an electrode catalyst.

[0093] [Comparative Example 2] In Comparative Example 2, platinum was supported on carrier 2 by a liquid-phase reduction method. 15% Pt (NO ２ ) ２ (NH ３ ) ２ A mixture was obtained by adding 118.5 g of pure water and 1 g of carrier 1 to 6.5 g of nitric acid solution. After adding 12.5 mL of methanol to this mixture, the mixture was stirred at 95°C for 7 hours to support platinum on carrier 1. The solid was separated from the liquid component by filtration, and the recovered solid was vacuum dried. The vacuum-dried solid was dispersed in 1 M nitric acid and stirred at 95°C for 2 hours. The pH of the solution was then adjusted to 2 using NaOH. Then, 0.16 g of cobalt sulfate (in CO equivalent) was added to the solution. An aqueous solution of sodium borohydride (prepared by dissolving 1 g of sodium borohydride in 100 ml of pure water) was added dropwise to the solution after the addition. After stirring for 1 hour, the solid was separated from the liquid component by filtration, and the recovered solid was vacuum dried. The vacuum-dried solid was heat-treated at 800°C for 1 hour under a nitrogen atmosphere containing 4 volume% hydrogen to produce an electrode catalyst.

[0094] [Evaluation] The total pore volume and most frequent pore size of the manufactured electrode catalyst were measured using the nitrogen adsorption method described above. The results are shown in Table 2. In addition, the B / A ratio, the average value (average particle size), standard deviation, coefficient of variation, and N / L of the metal catalyst were measured using the following method. Furthermore, the performance of the electrode catalyst was evaluated using the following method.

[0095] [Measurement of Pt content A by ICP-MS] 30 mg of the electrode catalyst was immersed in acid to dissolve the metal contained in the electrode catalyst. The Pt concentration of the obtained metal solution was measured using an ICP emission spectrometer (ICP-MS, Hitachi High-Tech Corporation PS3520UVDDII). The Pt content A (mass%) was determined by ICP-MS. In addition, in Examples 4 to 7 and 9, the ratio of platinum to cobalt or ruthenium (Pt:M) was determined. The results are shown in Table 2.

[0096] [Measurement of Pt content B by XPS] Using an X-ray photoelectron spectrometer (PHI Quantes, ULVAC-PHI), photoelectron spectra from the inner-shell energy levels of platinum and carbon atoms on the surface of the electrode catalyst were measured. AlKα rays were used as the X-ray source. In the obtained photoelectron spectra, the binding energy was corrected using the C1s peak originating from the 1s orbital of the carbon atom. Then, the Pt content B (mass%) of the electrode catalyst was determined by XPS from the quantitative values ​​of the detected elements. These results are shown in Table 2. The value of B / A was calculated by dividing the content B obtained by the above method by the content A. These results are shown in Table 2.

[0097] [Average value, standard deviation, and coefficient of variation of particle size of metal catalyst] The electrode catalyst was observed using a TEM (JEM-2100F, manufactured by JEOL Ltd.) and a TEM image was obtained. Next, 100 particles of the metal catalyst observed in the TEM image that had clearly visible particle size contours were arbitrarily selected, and the particle size (major diameter if the particle was not circular) of these particles was measured. Based on the measured particle size, the average value (average particle size) and standard deviation of the particle size of the metal catalyst were calculated. The coefficient of variation was calculated by dividing the calculated standard deviation by the average particle size and multiplying by 100. These results are shown in Table 2. Also, the TEM images of Example 1 and Comparative Example 1 are shown in Figures 1 and 2, respectively.

[0098] [Measurement of N / L] Three different fields of view of the catalyst particles were randomly selected from the TEM images. Next, the perimeter L of the support and the number N of metal catalyst particles that are in contact with the outer edge of the support and located outside the outer edge were measured, and the ratio N / L of the two was calculated. The N / L of the electrode catalysts of each example and comparative example was obtained by calculating the arithmetic mean of the N / L values ​​calculated for each of the three fields of view. These results are shown in Table 2. In the electrode catalyst of Comparative Example 1 shown in Figure 2, a portion of the metal catalyst particles that are in contact with the outer edge of the support particles and located outside the outer edge are indicated by white arrows.

[0099] [Evaluation of Electrode Catalyst Performance 1] A. Preparation of the Cathode Catalyst Layer The cathode catalyst layer was obtained by the following method. The electrode catalyst prepared in the Examples and Comparative Examples and a 3 mm diameter yttrium-stabilized zirconia ball were placed in a container and ultrapure water was added. Then, the mixture was stirred at 200 rpm for 10 minutes using a planetary ball mill (PM-200, manufactured by Lechner). A mixture of 2-propanol and ionomer (20% Aquivion dispersion, manufactured by Sigma-Aldrich) was added to the stirred mixture and stirred at 370 rpm for 1 hour using a planetary ball mill. The ratio I / C, which is the ratio of the mass I of the ionomer to the amount C of ionomer contained in the electrode catalyst, was set to 0.7, the mass ratio of ultrapure water to 2-propanol was 1:1, and the solid content concentration was 12%. The dispersion obtained in this manner was coated onto a polytetrafluoroethylene sheet using a bar coater to form a coating film, and then the coating film was dried at 60°C to obtain an electrode catalyst layer. The amount of catalyst per unit area of ​​the electrode catalyst layer was 0.3 mg / cm². ２ That's what I decided.

[0100] B. Preparation of the Anode Catalyst Layer The anode catalyst layer was obtained by the following method. Platinum-supported carbon black (TEC10E50E) manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., the electrode catalyst, and yttrium-stabilized zirconia balls with a diameter of 3 mm were placed in a container, and ultrapure water was added. Then, the mixture was stirred at 200 rpm for 10 minutes using a planetary ball mill (PM-200, manufactured by Lechner). After stirring, a mixture of 2-propanol and ionomer (20% Aquivion dispersion, manufactured by Sigma-Aldrich) was added, and the mixture was stirred at 370 rpm for 1 hour using a planetary ball mill. The ratio of the mass I of the ionomer to the amount C of ionomer contained in the electrode catalyst, I / C, was set to 0.7, the mass ratio of ultrapure water to 2-propanol was 1:1, and the solid content concentration was 8%. The dispersion obtained in this manner was coated onto a polytetrafluoroethylene sheet using a bar coater to form a coating film, and then the coating film was dried at 60°C to obtain an electrode catalyst layer. The amount of catalyst per unit area of ​​the electrode catalyst layer was 0.05 mg / cm². ２ That's what I decided.

[0101] C. Manufacturing of Membrane Electrode Assembly and Solid Polymer Fuel Cell: The anode catalyst layer and cathode catalyst layer, formed on a polytetrafluoroethylene sheet, were cut into 50 mm squares. These were then superimposed on an electrolyte membrane of Nafion® (NR-211, manufactured by Chemours), and heated at 150°C and 3 kgf / cm². ２ Under these conditions, the material was hot-pressed in air for 10 minutes to transfer the material. In this way, a cathode catalyst layer and an anode catalyst layer were formed on each surface of a solid polymer electrolyte membrane made of Nafion®, and a membrane electrode assembly (CCM) was obtained. The CCM was sandwiched between a pair of gas diffusion layers (manufactured by SGL Carbon Co., Ltd., model number: 22BB). Furthermore, this was sandwiched between a separator consisting of a pair of carbon plates in which gas channels were formed, and a solid polymer fuel cell was fabricated. The fuel cell obtained in this way corresponds to a JARI standard cell.

[0102] D. Current density 2A / m ２ The cell voltage was measured, and hydrogen gas was supplied to the anode side of the fuel cell, while air was supplied to the cathode side. The flow rates were set so that the utilization rate of both hydrogen gas and air was 50%. The gases were humidified using an external humidifier before being supplied to the fuel cell. The temperature of the fuel cell was adjusted to 65°C. The humidity of the supplied gases was adjusted so that the relative humidity was 50% RH on the anode side and 50% RH on the cathode side. The current density at this time was 2 A / m³. ２ The cell voltage was measured. The results are shown in Table 2.

[0103] [Measurement of ORR Mass Activity] Hydrogen gas was supplied to the anode side and oxygen to the cathode side of the fuel cell obtained by the method described above. The flow rates were set so that the utilization rate of hydrogen gas was 70% and the utilization rate of oxygen was a maximum of 8.4%. The pressure of the supplied gases was set to atmospheric pressure. The gases were humidified using an external humidifier before being supplied to the fuel cell. The temperature of the fuel cell was adjusted to 65°C. The humidity of the supplied gases was adjusted so that the relative humidity was 50% RH on the anode side and 50% RH on the cathode side. The cell voltage, current density, and resistance were measured at this time. Next, the current density was plotted on the horizontal axis and the IR-free voltage (a voltage value corrected for the voltage drop due to resistance, obtained by adding the measured voltage and the voltage calculated by multiplying the resistance value and current density) on the vertical axis. The equation of the regression line was obtained from 3 to 4 points on the low current density side of this plot. Extrapolating this regression line, we find the current density value when the value on the vertical axis is 0.9V, and then measure the cathode at 1 cm. ２ The current value per gram of platinum obtained in this way was divided by the amount of platinum per gram (A / g). －Ｐｔ ) was determined. The results are shown in Table 2.

[0104]

[0105]

[0106] As is clear from Table 2, in each example, the ORR mass activity and current density of 2 A / m² were compared with Comparative Examples 1 and 2. ２ The voltage at this point is high, indicating that the electrode catalyst has excellent catalytic activity and mass transport capabilities.

[0107] In the electrode catalysts of Comparative Examples 1 and 2, the B / A ratio was greater than 1.00. This is thought to be because, in these electrode catalysts, platinum is supported on the surface (outside the pores) of the support, forming a platinum-covered layer on the surface of the support. In other words, in the XPS measurement, the detection of peaks originating from carbon atoms constituting the support was inhibited by the platinum supported on the support surface, resulting in a large value for the Pt content B, which is thought to be the reason why the B / A ratio exceeded 1.

[0108] As described in detail above, the present invention provides an electrode catalyst having high catalytic activity.

Claims

1. An electrode catalyst in which a metal catalyst is supported on a carrier, wherein the metal catalyst contains platinum or a platinum alloy, the carrier is a porous material, and when the content of the metal catalyst in the electrode catalyst measured by inductively coupled plasma mass spectrometry is A (mass%), and the content of the metal catalyst in the electrode catalyst measured by X-ray photoelectron spectroscopy is B (mass%), the value of content B relative to content A, B / A, is 0.50 or more and 1.00 or less.

2. An electrode catalyst in which a metal catalyst is supported on a carrier, wherein the metal catalyst contains platinum or a platinum alloy, the carrier is a porous material, and when L is the outer edge length of the carrier particles in a transmission electron microscope image of the electrode catalyst, and N is the number of metal catalyst particles that are in contact with the outer edge of the carrier particles and located outside the outer edge in the transmission electron microscope image, the value of N relative to L, N / L, is 19 μm. －１ The following is an electrode catalyst.

3. The electrode catalyst according to claim 1 or 2, wherein the carrier comprises porous carbon.

4. The electrode catalyst according to claim 1 or 2, wherein the carrier has mesopores.

5. The electrode catalyst according to claim 1 or 2, wherein the coefficient of variation of the particle size of the metal catalyst is 55% or less.

6. The electrode catalyst according to claim 1 or 2, wherein the content of chlorine element in the electrode catalyst is 1% by mass or less.

7. In the transmission electron microscope image of the electrode catalyst, let L be the outer edge length of the support particles, and let N be the number of metal catalyst particles that are in contact with the outer edge of the support particles and located outside the outer edge in the transmission electron microscope image, then the value of N relative to L, N / L, is 19 μm. －１ The electrode catalyst according to claim 1, which is as follows:

8. The electrode catalyst according to claim 1 or 2, wherein the metal catalyst is a platinum alloy containing platinum and other metals, the molar ratio of platinum (Pt) to the other metal (M) in the platinum alloy is Pt:M = 1:1 to 20:1, and the other metal is at least one selected from the group consisting of Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Nb, Mo, Ta, Ru, and W.

9. The electrode catalyst according to claim 8, wherein the other metal is at least one selected from the group consisting of Co and Ru.

10. The electrode catalyst according to claim 1 or 2, wherein the most frequent pore size in the range of 2.0 nm to 50.0 nm is 2.0 nm to 20.0 nm.

11. A fuel cell comprising: a cathode containing the electrode catalyst according to claim 1 or 2; an anode; and an electrolyte disposed between the cathode and the anode.

12. A method for producing an electrode catalyst in which a metal catalyst is supported on a porous material carrier, comprising: stirring a mixture containing the carrier, a water-soluble metal salt, water, and a hydrophobic solvent; separating solid matter from the mixture; reducing the water-soluble metal salt contained in the solid matter to produce the metal catalyst; and calculating the BET specific surface area SSA of the carrier from nitrogen gas adsorption. Ｎ２ The BET specific surface area SSA of the carrier calculated from water vapor adsorption is Ｈ２Ｏ Ratio SSA Ｈ２Ｏ / SSA Ｎ２ A method for manufacturing an electrode catalyst, wherein the ratio is 0.0010 or more and 0.30 or less.

13. The method for producing an electrode catalyst according to claim 12, wherein the hydrophobic solvent contains a hydrocarbon.

14. The water-soluble metal salt is Pt(NO ３ ) ２ A method for producing an electrode catalyst according to claim 12, including the method described in claim 12.

15. A method for producing an electrode catalyst according to any one of claims 12 to 14, wherein the ratio of the water-soluble metal salt to the total amount of water and water-soluble substance contained in the mixed liquid (water-soluble metal salt / (water + water-soluble substance)) is 10% by mass or more and 60% by mass or less.

16. A method for producing an electrode catalyst in which a metal catalyst is supported on a carrier containing a porous material, comprising: stirring a mixture containing the carrier, a colloidal solution, and a hydrophobic solvent; and separating solid components from the mixture, wherein the colloidal solution uses water as the dispersion medium and the metal catalyst or its precursor as the dispersed phase.

17. The dispersoid of the colloidal solution is Pt(NO ２ ) ２ (NH ３ ) ２ The method for producing an electrode catalyst according to claim 16, wherein 18. BET specific surface area SSA of the carrier calculated from nitrogen gas adsorption. Ｎ２ The BET specific surface area SSA of the carrier calculated from water vapor adsorption is Ｈ２Ｏ Ratio SSA Ｈ２Ｏ / SSA Ｎ２ A method for producing an electrode catalyst according to claim 16 or 17, wherein the ratio is 0.0010 or more and 0.30 or less.

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