Electrode catalyst for fuel cell, method for selecting same, and fuel cell equipped with same

Abstract

The present invention provides a fuel cell electrode catalyst comprising catalytic metal particles and conductive support particles supporting the catalytic metal particles. In this fuel cell electrode catalyst, the proportion of the surface area of ​​the catalytic metal particles, as calculated from a transmission electron microscope image, occupied by catalytic metal particles having a particle size of 4.5 nm or less is 5% or less.

Description

Technical Field

[0001] The present invention relates to an electrode catalyst for a fuel cell, a method for selecting the electrode catalyst, and a fuel cell equipped with the electrode catalyst. Background Art

[0002] Polymer electrolyte fuel cells (PEFs), fuel cells that generate electricity through an electrochemical reaction between a fuel gas and an oxidant gas, are attracting significant attention as energy sources. Because they can operate at room temperature and offer high power density, they are being actively researched as suitable for automotive applications.

[0003] In polymer electrolyte fuel cells, a membrane electrode assembly (MEA) (hereinafter referred to as "fuel electrode-polymer electrolyte membrane-air electrode") is generally used, in which electrodes composed of catalyst layers (a fuel electrode (anode catalyst layer) and an air electrode (cathode catalyst layer)) are bonded to both sides of a polymer electrolyte membrane serving as an electrolyte membrane. Alternatively, a gas diffusion layer may be further bonded to both sides of the MEA, resulting in a membrane electrode gas diffusion layer assembly (MEA-gas diffusion layer) (hereinafter referred to as "MEGA").

[0004] Each electrode is formed by a catalyst layer, which is a layer used to carry out an electrode reaction using the electrode catalyst contained in the catalyst layer. In order for the electrode reaction to proceed, a three-phase interface is required where the electrolyte, electrode catalyst, and reactant gas coexist. Therefore, the catalyst layer is generally composed of a layer containing the electrode catalyst and electrolyte. In addition, the gas diffusion layer is a layer used to supply the reactant gas to the catalyst layer and transfer electrons. It is made of a porous and electronically conductive material.

[0005] As an electrode catalyst used in such a solid polymer fuel cell, for example, Japanese Patent Application Laid-Open No. 2018-190545 discloses an electrode catalyst for a fuel cell, which comprises catalyst metal particles and carrier particles supporting the catalyst metal particles, wherein the catalyst metal particles contain platinum or a platinum alloy, and the carrier particles have a BET specific surface area of ​​700 m 2 / g or more of the carbonaceous material, the average particle size of the catalytic metal particles is 2.5 to 4.5 nm, and the standard deviation of the particle size of the catalytic metal particles is 1.30 nm or less.

[0006] International Publication No. 2016 / 063968 discloses a fuel cell electrode catalyst comprising a solid carbon support and an alloy of platinum and cobalt supported on the support. Summary of the Invention

[0007] Fuel cells deteriorate with years of use. One of the main factors is the presence of fine particles of precious metals, such as platinum and / or platinum alloys, contained in the electrode catalyst for fuel cells, which are difficult to detect by X-ray diffraction (XRD). In fuel cells, the surface of the active material (precious metal) on the electrode catalyst reacts, and the surface area of ​​the precious metal is an important physical property in the performance of the fuel cell. Through first-principle calculations, etc., it can also be known that the precious metal of the fine particles is easier to dissolve than the precious metal of large particles, but the dissolved precious metal is re-precipitated around the precious metal particles. Therefore, if the dissolution and re-precipitation of the precious metal of the fine particles occur repeatedly, the precious metal is enlarged and the surface area is reduced. That is, since the electrode catalyst for fuel cells contains a large amount of such fine particles of precious metals with a high dissolution rate and a reduced surface area, degradation occurs with the use of the fuel cell over the years.

[0008] Therefore, the present invention provides a fuel cell electrode catalyst capable of suppressing a decrease in the surface area of ​​a noble metal after a durability test, a method for selecting the same, and a fuel cell including the same.

[0009] The present inventors have discovered that, in a fuel cell electrode catalyst comprising catalyst metal particles and conductive carrier particles carrying the catalyst metal particles, by specifying the proportion of the surface area of ​​the catalyst metal particles occupied by fine particles that are difficult to detect even by XRD and can cause degradation of the fuel cell, a fuel cell electrode catalyst can be selected in which the precious metal content of the fine particles is reduced. As a result, a fuel cell electrode catalyst can be obtained that can suppress the decrease in the surface area of ​​the precious metal after a durability test.

[0010] That is, the gist of the present invention is as follows.

[0011] (1) A fuel cell electrode catalyst comprising catalyst metal particles and conductive carrier particles supporting the catalyst metal particles.

[0012] The ratio of the surface area of ​​the catalytic metal particles having a particle diameter of 4.5 nm or less to the surface area of ​​the catalytic metal particles calculated from a transmission electron microscope image is 5% or less.

[0013] (2) The fuel cell electrode catalyst described in (1) may be an electrode catalyst for a cathode catalyst layer.

[0014] (3) A solid polymer fuel cell comprising a membrane electrode assembly having an anode catalyst layer, a cathode catalyst layer, and a solid polymer electrolyte membrane disposed between the anode catalyst layer and the cathode catalyst layer.

[0015] The electrode catalyst contained in the cathode catalyst layer is the electrode catalyst for fuel cells described in (1).

[0016] (4) A method for selecting an electrode catalyst for a fuel cell comprising catalyst metal particles and conductive support particles supporting the catalyst metal particles, comprising:

[0017] A fuel cell electrode catalyst is selected in which the ratio of the surface area of ​​the catalytic metal particles having a particle diameter of 4.5 nm or less to the surface area of ​​the catalytic metal particles calculated from a transmission electron microscope image is 5% or less.

[0018] According to the present invention, it is possible to provide a fuel cell electrode catalyst capable of suppressing a decrease in the surface area of ​​a noble metal after a durability test, a method for selecting the same, and a fuel cell including the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, in which like symbols represent like elements, and wherein:

[0020] Figure 1 This is a graph showing the relationship between the particle size of the platinum-cobalt alloy particles and the number ratio of the platinum-cobalt alloy particles before and after the endurance test for the platinum-cobalt alloy-supported carbon electrode catalyst of Comparative Example.

[0021] Figure 2 This is a graph showing the relationship between the particle size of platinum-cobalt alloy particles and the weight change before and after the endurance test for a platinum-cobalt alloy-supported carbon electrode catalyst of a comparative example.

[0022] Figure 3 This is a graph showing the relationship between the ratio of the surface area of ​​platinum-cobalt alloy particles with a diameter of 4.5 nm or less to the surface area of ​​platinum-cobalt alloy particles and the surface area reduction rate of the platinum-cobalt alloy particles after the durability test for the platinum-cobalt alloy-supported carbon electrode catalysts of the comparative example group and the example group. DETAILED DESCRIPTION

[0023] Hereinafter, embodiments of the present invention will be described in detail.

[0024] The fuel cell electrode catalyst, the method for selecting the same, and the fuel cell including the same of the present invention are not limited to the following embodiments, and can be implemented in various forms by implementing changes and improvements that can be made by those skilled in the art without departing from the scope of the present invention.

[0025] The present invention relates to an electrode catalyst for a fuel cell, comprising catalyst metal particles and conductive carrier particles carrying the catalyst metal particles, wherein the ratio of the surface area of ​​the catalyst metal particles calculated from a transmission electron microscope (TEM) image to the surface area of ​​the catalyst metal particles occupied by catalyst metal fine particles, i.e., catalyst metal particles having a particle size of 4.5 nm or less, which are the cause of fuel cell degradation, is below a certain value.

[0026] The catalytic metal particles are not limited as long as they exhibit a catalytic effect in the following reaction at the electrodes of the membrane electrode assembly, and any known material in the relevant technical field can be used.

[0027] Air electrode (cathode catalyst layer): O2+4H + +4e - →2H2O

[0028] Hydrogen electrode (anode catalyst layer): 2H2→4H + +4e -

[0029] Examples of the catalytic metal particles include noble metals and their alloys, such as platinum, platinum alloys, palladium, rhodium, gold, silver, osmium, iridium, and mixtures of two or more thereof. Platinum alloys are not limited, but examples include alloys of platinum with at least one of aluminum, chromium, manganese, iron, cobalt, nickel, gallium, zirconium, molybdenum, ruthenium, rhodium, palladium, vanadium, tungsten, rhenium, osmium, iridium, titanium, and lead.

[0030] The catalyst metal particles are preferably platinum and / or a platinum alloy, such as an alloy of platinum and cobalt.

[0031] Conductive carrier particles can be any carrier known in the art without limitation. Examples of conductive carrier particles include carbon materials such as carbon black, carbon nanotubes, and carbon nanofibers, carbon compounds such as silicon carbide, tin oxide, or mixtures of two or more thereof.

[0032] The BET specific surface area of ​​the conductive carrier particles is not limited, but is usually 600 m 2 / g~900m 2 / g.

[0033] The content of the catalytic metal particles is not limited, but is usually 5% to 70% by weight, preferably 10% to 60% by weight, relative to the total weight of the fuel cell electrode catalyst.

[0034] In the fuel cell electrode catalyst of the present invention, the proportion of the surface area of ​​the catalytic metal particles having a particle diameter of 4.5 nm or less in the surface area of ​​the catalytic metal particles calculated from a transmission electron microscope (TEM) image is 5% or less.

[0035] The ratio of the surface area of ​​the catalytic metal particles having a particle diameter of 4.5 nm or less to the surface area of ​​the catalytic metal particles calculated from the TEM image was calculated as follows.

[0036] (1) The fuel cell electrode catalyst to be used is imaged using a TEM. 100 catalyst metal particles are randomly selected from the TEM images of at least four viewing fields, and a particle size distribution is generated. The catalyst metal particles selected from the TEM images can be confirmed using EDX. The particle size of the catalyst metal particles is the average value of the major and minor diameters of the particles.

[0037] (2) The ratio of the surface area in each particle size range is calculated from the particle size distribution information obtained in (1) and the weight information of the catalyst metal contained in the fuel cell electrode catalyst.

[0038] Total weight of catalyst metal particles in each particle size range = average volume of one particle in each particle size range × density of catalyst metal × frequency (number) calculated from particle size distribution in each particle size range

[0039]

[0040]

[0041] Total surface area of ​​catalytic metal particles in each particle size range = average surface area of ​​one catalytic metal particle in each particle size range × number of catalytic metal particles in each particle size range

[0042]

[0043] (3) The ratio of the surface area of ​​the catalytic metal particles having a particle diameter of 4.5 nm or less to the total surface area of ​​the catalytic metal particles is calculated.

[0044] In the electrode catalyst for fuel cells of the present invention, by setting the proportion of the surface area of ​​catalyst metal particles with a particle size of 4.5 nm or less in the surface area of ​​catalyst metal particles calculated from the TEM image within the above-mentioned range, the electrode catalyst for fuel cells contains precious metal particles that are the cause of fuel cell degradation in a smaller amount than before. As a result, it is possible to suppress the decrease in the surface area of ​​the precious metal after the durability test of the electrode catalyst for fuel cells.

[0045] The fuel cell electrode catalyst of the present invention can be used as an electrode catalyst for the cathode catalyst layer and / or the anode catalyst layer in a fuel cell. The fuel cell electrode catalyst of the present invention is preferably used as an electrode catalyst for the cathode catalyst layer in a fuel cell. By using the fuel cell electrode catalyst of the present invention as an electrode catalyst for the cathode catalyst layer, the dissolution of precious metals in the cathode catalyst layer, which may dissolve precious metals, can be suppressed, thereby improving the durability of the fuel cell.

[0046] The fuel cell electrode catalyst of the present invention has a BET specific surface area of ​​600 m 2 / g~900m 2 / g, and particles having a surface area calculated from a TEM image of catalytic metal particles occupied by catalytic metal particles having a particle size of 4.5 nm or less within the above range can be selected from the produced catalytic metal particle-supported particles, or particles can be produced by a known method.

[0047] The fuel cell electrode catalyst of the present invention can be prepared, for example, as follows: (1) Support particles such as carbon particles having a specific BET specific surface area and a noble metal precursor such as a platinum precursor, for example, a dinitrosodiammineplatinum nitrate solution, are suspended in a solvent such as pure water to obtain a suspension.

[0048] (2) The noble metal precursor in the suspension obtained in (1) is reduced to a noble metal using a reducing agent such as ethanol or sodium borohydride at room temperature (about 20°C) to 100°C to obtain a dispersion. When the noble metal in the fuel cell electrode catalyst is an alloy, such as a platinum alloy, a solution containing the metal contained in the alloy, such as cobalt, in the form of ions is added to the dispersion containing platinum, and the metal is precipitated in the form of a hydroxide or carbonate by adjusting the pH.

[0049] (3) The dispersion obtained in (2) is filtered, and the resulting filter cake is dried at 80°C to 120°C for 1 to 12 hours to obtain a powder.

[0050] (4) The powder obtained in (3) is calcined in an inert atmosphere, such as a nitrogen or argon atmosphere, at 100° C. to 1200° C. for 1 to 8 hours to obtain catalyst metal particle-supported particles.

[0051] The calcination in (4) is performed to improve the durability of the fuel cell electrode catalyst when used at high temperatures. This calcination is performed within a range where the pore diameter and pore volume of the support particles do not change, and is preferably performed under the conditions of (4).

[0052] When the catalytic metal particle-supported particles obtained in (4) contain impurities such as the metal added in (2), the catalytic metal particle-supported particles obtained in (4) can be added to a solution containing an acid, an alkali, etc. to dissolve and remove the impurities.

[0053] (5) From the catalytic metal particle-supported particles obtained in (4), those in which the ratio of the surface area of ​​the catalytic metal particles having a particle size of 4.5 nm or less to the surface area of ​​the catalytic metal particles calculated from a TEM image falls within the above range are selected.

[0054] The present invention also relates to a solid polymer fuel cell, which is a fuel cell having an electrode catalyst for a fuel cell of the present invention, that is, a solid polymer fuel cell having a membrane electrode assembly, wherein the membrane electrode assembly has an anode catalyst layer, a cathode catalyst layer, and a solid polymer electrolyte membrane arranged between the anode catalyst layer and the cathode catalyst layer, and the electrode catalyst of the anode catalyst layer and / or the cathode catalyst layer is the electrode catalyst for a fuel cell of the present invention.

[0055] Here, the solid polymer electrolyte membrane is preferably an electrolyte membrane having proton conductivity. As the electrolyte membrane having proton conductivity, any electrolyte membrane known in the art can be used without limitation. For example, a membrane formed of a fluororesin having a sulfonic acid group as an electrolyte (such as Nafion (manufactured by DuPont), FLEMION (manufactured by AGC), and Aciplex (manufactured by Asahi Kasei Corporation) can be used.

[0056] The thickness of the solid polymer electrolyte membrane is not limited, but is usually 5 μm to 50 μm in order to enhance the proton conductivity.

[0057] The anode catalyst layer becomes the fuel electrode, that is, the hydrogen electrode, and the cathode catalyst layer becomes the air electrode (oxygen electrode). Each catalyst layer contains an electrode catalyst and an electrolyte.

[0058] The anode catalyst layer and / or the cathode catalyst layer contains the fuel cell electrode catalyst of the present invention as an electrode catalyst. The fuel cell electrode catalyst of the present invention is as described above.

[0059] When the anode catalyst layer or the cathode catalyst layer does not contain the fuel cell electrode catalyst of the present invention as an electrode catalyst, an electrode catalyst known in the relevant technical field may be used as the electrode catalyst.

[0060] In the solid polymer fuel cell of the present invention, the cathode catalyst layer preferably includes the fuel cell electrode catalyst of the present invention. By including the fuel cell electrode catalyst of the present invention, the cathode catalyst layer can suppress the dissolution of precious metals in the cathode catalyst layer, which may have the potential for dissolution of precious metals, and can improve the durability of the fuel cell.

[0061] The content of the electrode catalyst in each catalyst layer is not limited, but is usually 5% by weight to 40% by weight relative to the total weight of the catalyst layer.

[0062] As electrolyte, it is not limited, preferably ionomer.Ionomer is also referred to as cation exchange resin, exists in the form of the cluster formed by ionomer molecule.As ionomer, ionomer known in the technical field can be used, it is not limited, for example, fluorine resin electrolytes such as perfluorosulfonic acid resin materials can be used, sulfonated plastic electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfone, sulfonated polysulfide, sulfonated polyphenylene, sulfonated plastic electrolytes such as sulfoalkylated polyether ether ketone, sulfoalkylated polyether sulfone, sulfoalkylated polyether ether sulfone, sulfoalkylated polysulfone, sulfoalkylated polysulfide, sulfoalkylated polyphenylene, or mixtures thereof of two or more thereof etc.

[0063] The thickness of each catalyst layer is not limited, but is generally 1 μm to 20 μm in order to ensure the amount of catalyst required for power generation and to keep the proton resistance low.

[0064] The polymer electrolyte fuel cell of the present invention can be produced by a method known in the art.

[0065] The polymer electrolyte fuel cell of the present invention can be produced, for example, as follows.

[0066] (1) The fuel cell electrode catalyst of the present invention and an electrolyte, for example, an electrolyte having the same composition as the solid polymer electrolyte membrane, are suspended in a solvent, such as pure water, to prepare a catalyst ink. In this case, ultrasonic dispersion or the like may be used to obtain a uniform catalyst ink.

[0067] (2) The catalyst ink obtained in (1) is spread and attached to a removable substrate, such as a Teflon sheet, to form a catalyst layer precursor. Spreading and attachment can be performed by, for example, utilizing gravity, spray force, or electrostatic force.

[0068] (3) The catalyst layer precursor on the substrate is dried to prepare the catalyst layer on the substrate, and the catalyst layer is peeled off from the substrate to obtain the catalyst layer.

[0069] Among them, in (2) to (3), the catalyst ink is dispersed and attached to the substrate, and then the catalyst layer is obtained by drying and peeling. However, the catalyst ink can also be directly dispersed and attached to the surface of the solid polymer electrolyte membrane and then dried to prepare the catalyst layer.

[0070] (4) The catalyst layer obtained in (3) is used as an air electrode and as a fuel electrode, for example, the catalyst layer obtained in (3) or a catalyst layer prepared by using a commercially available Pt / C catalyst instead of the fuel cell electrode catalyst of the present invention in (1). A layer assembly is obtained by arranging an air electrode on one surface and a fuel electrode on the other surface of a solid polymer electrolyte membrane. Depending on the situation, a diffusion layer, such as a conductive porous sheet formed of an air-permeable or liquid-permeable material such as carbon cloth or carbon paper, may be arranged on the outside of each of the air electrode and the fuel electrode.

[0071] (5) The layer assembly obtained from (4) configured in the manner of (diffusion layer-) air electrode-solid polymer electrolyte membrane-fuel electrode (-diffusion layer) is pressed using a hot press at usually 100°C to 200°C, for example, 140°C, for usually 5 seconds to 600 seconds, for example, 300 seconds, to obtain a membrane electrode assembly or MEGA.

[0072] (6) Spacers for gas circulation are placed on both sides of the membrane electrode assembly or MEGA obtained in (5) to obtain a single cell. A plurality of these single cells are stacked to obtain a solid polymer fuel cell.

[0073] The solid polymer fuel cell of the present invention has improved durability and power generation performance.

[0074] Hereinafter, several embodiments of the present invention will be described, but the present invention is not limited to the contents shown in these embodiments.

[0075] I. Preliminary Research

[0076] i. Sample preparation

[0077] <Preparation of a Platinum-Cobalt Alloy-Supported Carbon Electrode Catalyst of Comparative Example>

[0078] The carbon support (BET specific surface area: 1000 m 2 / g~1300m 2 / g) is suspended in dilute nitric acid, and then a dinitrosodiammineplatinum nitric acid solution is added to the suspension and stirred. Ethanol is further added to the suspension and heated (room temperature (about 20°C) to 100°C) to reduce the platinum ions by the reducing effect of ethanol to obtain carbon loaded with platinum.

[0079] The obtained platinum-loaded carbon is suspended in a cobalt nitrate solution, and then an alkaline solution such as ammonia is added to the suspension to adjust the pH of the suspension to above 7, so that cobalt is precipitated on the platinum-loaded carbon in the form of cobalt hydroxide. Thereafter, the platinum-loaded carbon on which the cobalt hydroxide is adsorbed is obtained by filtration.

[0080] The platinum-loaded carbon adsorbed with cobalt hydroxide is dried at 80°C to 120°C for 1 to 12 hours, and the resulting powder is calcined at 100°C to 1200°C for 1 to 8 hours under an inert gas (nitrogen or argon) atmosphere to obtain platinum-cobalt alloy-loaded carbon formed by alloying platinum and cobalt.

[0081] The obtained platinum-cobalt-loaded carbon is suspended in dilute nitric acid, and the suspension is adjusted to a temperature range of room temperature (about 20°C) to 100°C and stirred for more than 30 minutes to remove cobalt on the surface of the alloy particles and obtain a carbon electrode catalyst loaded with platinum-cobalt alloy.

[0082] ii. Analysis

[0083] (TEM analysis)

[0084] The platinum-cobalt alloy-supported carbon electrode catalyst of the comparative example was imaged using TEM. From the resulting TEM images of at least four viewing fields, 100 platinum-cobalt alloy particles (Pt alloy particles) were randomly selected to generate a particle size distribution. The particle size of the platinum-cobalt alloy particles is the average of the major and minor diameters of the particles.

[0085] Next, for the carbon electrode catalyst supported on the platinum-cobalt alloy, the load response durability was evaluated based on the method described in the "I-4-1 MEA durability (electrolyte membrane·catalyst)" item of "Goals, research and development topics and evaluation methods for solid polymer fuel cells", [online], January 2014, Fuel Cell Practical Promotion Council, [retrieved February 24, 2014], network <URL: http: / / fccj.jp / pdf / 23_01_kt.pdf>.

[0086] Specifically, first, MEGA was produced using a carbon electrode catalyst supporting a platinum-cobalt alloy. Next, the obtained MEGA was subjected to a potential cycle test (durability test) under the following test conditions.

[0087] (Test conditions)

[0088] Temperature: 80℃

[0089] Gas pressure: normal pressure

[0090] Relative humidity: 100%

[0091] Anode: H2

[0092] Cathode: N2

[0093] Load response: 0.6V~1.0V

[0094] Number of cycles: 100,000 cycles

[0095] After the potential cycling test, the catalyst layer of the MEGA was measured again by TEM. 100 platinum-cobalt alloy particles were randomly selected from the obtained TEM image, and the particle size distribution was prepared in the same manner as above.

[0096] The obtained particle size distribution information and the following formula were used to analyze the weight change of platinum-cobalt alloy particles of various particle sizes before and after the endurance test of the platinum-cobalt alloy-supported carbon electrode catalyst.

[0097] Total weight of Pt alloy particles in each particle size range = average volume of one Pt alloy particle in each particle size range × density of Pt alloy × frequency (number) calculated from particle size distribution in each particle size range

[0098] iii. Results

[0099] Figure 1 The figure shows the relationship between the particle size of the platinum-cobalt alloy particles and the number ratio of the platinum-cobalt alloy particles before and after the endurance test for the carbon electrode catalyst supported by the platinum-cobalt alloy of the comparative example.

[0100] Figure 2 Graph 1 shows the relationship between the particle size of the platinum-cobalt alloy particles and the weight change before and after the endurance test for the platinum-cobalt alloy carbon electrode catalyst.

[0101] Depend on Figure 1 It can be seen from the durability test that the number of platinum-cobalt alloy particles with a particle size of 4.5 nm or less is reduced. Figure 2 It is found that the weight change of platinum-cobalt alloy particles with a particle size of 4.5 nm or less before and after the endurance test is negative, and therefore, the platinum-cobalt alloy particles with a particle size of 4.5 nm or less are reduced by elution or the like during the endurance test.

[0102] Therefore, by Figure 1 and 2 It is found that platinum-cobalt alloy particles having a particle size of 4.5 nm or less are easily eluted.

[0103] II. Research

[0104] i. Sample preparation

[0105] <Preparation of Platinum-Cobalt Alloy-Supported Carbon Electrode Catalyst of Comparative Example Group>

[0106] Several groups of platinum-cobalt alloy-supported carbon electrode catalysts of comparative examples were obtained in the same manner as in the preparation of the platinum-cobalt alloy-supported carbon electrode catalysts of comparative examples.

[0107] TEM images of each platinum-cobalt alloy-supported carbon electrode catalyst in the comparative example group were taken. From the resulting TEM images of at least four fields of view, 100 platinum-cobalt alloy particles were randomly selected to generate a particle size distribution. The particle size of the platinum-cobalt alloy particles is the average of the major and minor axes of the particles.

[0108] The obtained particle size distribution information and the following formula were used to calculate the ratio of the surface area of ​​the platinum-cobalt alloy particles having a particle size of 4.5 nm or less to the surface area of ​​the platinum-cobalt alloy particles for each platinum-cobalt alloy-supported carbon electrode catalyst of the comparative example group.

[0109] Total weight of Pt alloy particles in each particle size range = average volume of one Pt alloy particle in each particle size range × density of Pt alloy × frequency (number) calculated from particle size distribution in each particle size range

[0110]

[0111]

[0112] Total surface area of ​​Pt alloy particles in each particle size range = average surface area of ​​one Pt alloy particle in each particle size range × number of Pt alloy particles in each particle size range

[0113]

[0114] <Preparation of Platinum-Cobalt Alloy-Supported Carbon Electrode Catalyst of Example Group>

[0115] In the preparation of the carbon electrode catalyst supported on the platinum-cobalt alloy of the comparative example group, a carbon support (BET specific surface area: 1000 m 2 / g~1300m 2 / g) was changed to a carbon support (BET specific surface area: 600 m 2 / g~900m 2 / g) to prepare several carbon electrode catalysts loaded with platinum-cobalt alloy, calculate the proportion of the surface area of ​​the platinum-cobalt alloy particles with a particle size of 4.5 nm or less in the surface area of ​​the platinum-cobalt alloy particles in each carbon electrode catalyst loaded with platinum-cobalt alloy by the same method as the calculation method in each carbon electrode catalyst loaded with platinum-cobalt alloy in the comparative example group, and select from these several carbons loaded with platinum-cobalt alloy the material in which the proportion of the surface area of ​​the platinum-cobalt alloy particles with a particle size of 4.5 nm or less in the surface area of ​​the platinum-cobalt alloy particles is 5% or less. Otherwise, the preparation of the carbon loaded with platinum-cobalt alloy is carried out in the same manner as the comparative example group to obtain a carbon electrode catalyst loaded with platinum-cobalt alloy.

[0116] ii. Analysis

[0117] (Measurement of surface area reduction rate after durability test)

[0118] For the carbon electrode catalysts loaded with platinum-cobalt alloy in the comparative example group and the embodiment group, load response durability was evaluated based on the method described in the "I-4-1 MEA durability (electrolyte membrane·catalyst)" item of "Goals, research and development topics and evaluation methods for solid polymer fuel cells", [online], January 2014, Fuel Cell Practical Promotion Council, [retrieved February 24, 2014], network <URL: http: / / fccj.jp / pdf / 23_01_kt.pdf>, and the changes in the surface area of ​​platinum before and after the evaluation were analyzed.

[0119] Specifically, first, MEGA was prepared using each carbon electrode catalyst supported by a platinum-cobalt alloy of the comparative example group and the embodiment group. Next, cyclic voltammetry (CV) was performed on the obtained MEGA to calculate the electrochemical active area (ECSA) of platinum. Next, a potential cycle test (endurance test) was performed on the MEGA according to the following test conditions. After the potential cycle test, CV was performed again to calculate the ECSA of platinum. Based on the ECSA of platinum calculated by CV before and after the potential cycle test, the surface area decrease rate of the carbon electrode catalyst supported by the platinum-cobalt alloy after the endurance test was calculated.

[0120] (Test conditions)

[0121] Temperature: 80℃

[0122] Gas pressure: normal pressure

[0123] Relative humidity: 100%

[0124] Anode: H2

[0125] Cathode: N2

[0126] Load response: 0.6V~1.0V

[0127] Number of cycles: 100,000 cycles

[0128] iii. Results

[0129] Figure 3 The figure shows the relationship between the ratio of the surface area of ​​the platinum-cobalt alloy particles with a diameter of 4.5 nm or less to the surface area of ​​the platinum-cobalt alloy particles and the surface area reduction rate of the platinum-cobalt alloy particles after the durability test for the platinum-cobalt alloy-supported carbon electrode catalysts of the comparative example group and the example group.

[0130] Depend on Figure 3 It can be seen that the carbon electrode catalyst loaded with platinum-cobalt alloy in the embodiment group, that is, the carbon electrode catalyst loaded with platinum-cobalt alloy in which the proportion of the surface area occupied by platinum-cobalt alloy particles below 4.5 nm is less than 5%, has a lower surface area decrease rate of platinum-cobalt alloy particles after the durability test than the carbon electrode catalyst loaded with platinum-cobalt alloy in the comparison example group.

Claims

1. A fuel cell electrode catalyst, characterized in that comprising catalyst metal particles and conductive carrier particles supporting the catalyst metal particles, The ratio of the surface area of ​​the catalytic metal particles having a particle diameter of 4.5 nm or less to the surface area of ​​the catalytic metal particles calculated from the transmission electron microscope image is greater than 0% and is 5% or less.

2. The fuel cell electrode catalyst according to claim 1, wherein The electrode catalyst is an electrode catalyst for a cathode catalyst layer.

3. A solid polymer fuel cell, characterized in that: A membrane electrode assembly is provided, The membrane electrode assembly comprises an anode catalyst layer, a cathode catalyst layer, and a solid polymer electrolyte membrane disposed between the anode catalyst layer and the cathode catalyst layer. The electrode catalyst contained in the cathode catalyst layer is the fuel cell electrode catalyst according to claim 1 .

4. A method for selecting an electrode catalyst for a fuel cell, characterized in that: The fuel cell electrode catalyst comprises catalytic metal particles and conductive support particles supporting the catalytic metal particles. The selection method includes selecting a fuel cell electrode catalyst in which the ratio of the surface area of ​​the catalytic metal particles having a particle size of 4.5 nm or less to the surface area of ​​the catalytic metal particles calculated from a transmission electron microscope image is greater than 0% and less than 5%.

Images