Electrode catalyst and method for producing the same

Abstract

To provide an electrode catalyst that exhibits high power generation performance in the low current density region to the medium current density region, and a method for manufacturing the same. [Solution] The electrode catalyst comprises a solid carbon support and catalyst particles supported on the surface of the solid carbon support, wherein the pore loading rate ΔNp of the catalyst particles is 50% to 65%, the volume ratio ΔVs of small pores is 20% to 23%, and the volume ratio ΔVm of medium pores is 14% to 20%. Such an electrode catalyst is obtained by preparing an electrode catalyst precursor in which catalyst particles are supported on the surface of a solid carbon support, and heat-treating the electrode catalyst precursor in a non-oxidizing atmosphere at a temperature of 650°C to 750°C.

Description

[[Technical Field]]

[0001] The present invention relates to an electrode catalyst and a method for producing the same, and more particularly, to an electrode catalyst that exhibits high power generation performance in a range from a low current density region to a medium current density region, and a method for producing the same. [[Background Art]]

[0002] A polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which catalyst layers are bonded to both surfaces of an electrolyte membrane. A gas diffusion layer is normally disposed outside the catalyst layer. A laminate of the MEA and the gas diffusion layer is also called a membrane-electrode-gas diffusion layer assembly (MEGA). Further, a current collector (separator) having a gas flow path is disposed outside the gas diffusion layer. A polymer electrolyte fuel cell normally includes a structure (fuel cell stack) in which a plurality of such single cells each including the MEGA and the current collector are stacked.

[0003] The catalyst layer is generally formed of a mixture of an electrode catalyst, in which catalyst particles such as platinum or a platinum alloy are supported on a support surface, and a catalyst layer ionomer. The electrode reaction mainly occurs on the surfaces of the catalyst particles. Therefore, efforts have been made to make the catalyst particles as fine as possible and reduce the amount of platinum used per unit area of the electrode. However, under the operating environment of a fuel cell accompanied by potential fluctuations, there is a problem that the activity of the catalyst layer gradually decreases due to dissolution of catalyst particles, coarsening due to aggregation of catalyst particles, desorption of catalyst particles due to oxidation of the support, catalyst poisoning by the ionomer, and the like.

[0004] Various proposals have heretofore been made to solve this problem. For example, Patent Document 1 discloses that mesoporous carbon is heat-treated at a temperature of 1300°C or higher and lower than 1700°C, and metal particles are supported on the heat-treated mesoporous carbon. A catalyst obtained by this method is disclosed. The document states that heat treatment of mesoporous carbon at temperatures above 1300°C improves its oxidation resistance.

[0005] Patent Document 2 discloses an electrode catalyst obtained by heat-treating Pt-supported carbon at 800°C in a CO2 atmosphere. The document states: (A) When Pt-supported carbon is heat-treated in a CO2 atmosphere, CO gas is generated, and the Pt surface is reduced by the CO gas, thereby improving catalytic activity, and (B) The hydrophilicity of the carbon surface is improved, allowing it to perform well under low humidity conditions. It is stated.

[0006] In fuel cell electrodes, the surface of catalyst particles is coated with an ionomer. While the ionomer is a necessary component for proton conduction, its sulfonic acid groups act as a catalyst poisoning source. Therefore, coating the surface of catalyst particles with an ionomer tends to reduce their activity. In particular, catalyst poisoning by the ionomer is more likely to occur under low humidity conditions. On the other hand, Patent Document 1 discloses a method for heat-treating mesoporous carbon at a temperature of 1300°C or higher, and Patent Document 2 describes a method for heat-treating Pt-supported carbon in a CO2 atmosphere. However, it is difficult to suppress catalyst poisoning with these methods. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-108262 [Patent Document 2] Japanese Patent Publication No. 2010-161034 [Overview of the project] [Problems that the invention aims to solve]

[0008] The problem that this invention aims to solve is to provide an electrode catalyst that exhibits high power generation performance in the range from low current density to medium current density. Another problem that the present invention aims to solve is to provide a method for producing such an electrode catalyst. [Means for solving the problem]

[0009] To solve the above problems, the electrode catalyst according to the present invention is Solid carbon support, Catalyst particles supported on the surface of the solid carbon support and Equipped with, The pore-borne loading rate ΔNp of the catalyst particles, represented by the following formula (1), is 50% or more and 65% or less. The volume ratio ΔVs of the small pores, represented by the following equation (2), is 20% or more and 23% or less. The volume ratio ΔVm of the intermediate pores, expressed by the following equation (3), is between 14% and 20%.

[0010] however, ΔNp = Np × 100 / (Ns + Np) …(1) ΔVs = Vs × 100 / Vt …(2) ΔVm = Vm × 100 / Vt …(3) Ns is the number of catalyst particles whose catalyst-carbon distance is 0 nm or more and 2 nm or less. Np is the number of catalyst particles in which the catalyst-carbon distance is greater than 2 nm and less than or equal to 10 nm. Vt is the total volume of pores contained in the solid carbon support. Vs is the total volume of micropores contained in the solid carbon support. Vm is the total volume of the micropores contained in the solid carbon carrier. The aforementioned "small pores" refer to pores with a diameter of 6 nm or more and 10 nm or less. The term "medium-sized pore" refers to a pore with a diameter greater than 10 nm and less than or equal to 14 nm.

[0011] The method for producing an electrode catalyst according to the present invention is: A first step of preparing an electrode catalyst precursor in which catalyst particles are supported on the surface of a solid carbon carrier, a second step of heat-treating the electrode catalyst precursor at a temperature of 650°C or higher and 750°C or lower in a non-oxidizing atmosphere to obtain the electrode catalyst according to claim 1, comprising:

Effects of the Invention

[0012] When catalyst particles are supported on the surface of a solid carbon carrier and heat-treated at a temperature of 650°C or higher and 750°C or lower in a non-oxidizing atmosphere, an electrode catalyst that exhibits high power generation performance from a low current density region to a medium current density region can be obtained. In particular, the electrode catalyst exhibits high power generation performance under low-humidity conditions where poisoning by ionomers is likely to occur. This is considered to be due to the following reasons.

[0013] That is, when a solid carbon carrier is heat-treated under predetermined conditions, small pores on the carrier surface are appropriately oxidized by oxygen-containing functional groups on the carrier surface and oxygen adsorbed on the surface of the catalyst particles, and the proportion of mesopores increases. When new mesopores are formed, part of the catalyst particles supported on the outermost surface of the carrier penetrate into the mesopores. When a catalyst layer is formed using such an electrode catalyst, acid groups of the ionomer are less likely to adsorb onto the surface of the catalyst particles that have penetrated into the mesopores, and the contact area between the catalyst particles and the ionomer is reduced. As a result, it is considered that the catalytic activity is improved, and high power generation performance is exhibited from the low current density region to the medium current density region.

Brief Description of Drawings

[0014] [Figure 1] It is a diagram showing changes in Pt-carbon distance before and after heat treatment in Ar. [Figure 2] It is a diagram showing changes in carbon pore diameter before and after heat treatment in Ar. [Figure 3] It is a graph showing mass activity (@0.86V) of the electrode catalysts obtained in Example 1 and Comparative Example 1 at a cell temperature of 80°C under high-humidity conditions and low-humidity conditions. [Figure 4]The following shows the sulfonic acid adsorption rates of the electrode catalysts obtained in Example 1 and Comparative Example 1 under high humidity and low humidity conditions at a cell temperature of 80°C. [Modes for carrying out the invention]

[0015] [Configuration 1] Solid carbon support, Catalyst particles supported on the surface of the solid carbon support and Equipped with, The pore-borne loading rate ΔNp of the catalyst particles, represented by the following formula (1), is 50% or more and 65% or less. The volume ratio ΔVs of the small pores, represented by the following equation (2), is 20% or more and 23% or less. The volume ratio ΔVm of the micropores, expressed by the following equation (3), is 14% or more and 20% or less. Electrocatalyst.

[0016] however, ΔNp = Np × 100 / (Ns + Np) …(1) ΔVs = Vs × 100 / Vt …(2) ΔVm = Vm × 100 / Vt …(3) Ns is the number of catalyst particles whose catalyst-carbon distance is 0 nm or more and 2 nm or less. Np is the number of catalyst particles in which the catalyst-carbon distance is greater than 2 nm and less than or equal to 10 nm. Vt is the total volume of pores contained in the solid carbon support. Vs is the total volume of micropores contained in the solid carbon support. Vm is the total volume of the micropores contained in the solid carbon carrier. The aforementioned "small pores" refer to pores with a diameter of 6 nm or more and 10 nm or less. The term "medium-sized pore" refers to a pore with a diameter greater than 10 nm and less than or equal to 14 nm.

[0017] [Configuration 2] The solid carbon support is the electrode catalyst according to configuration 1, wherein the aggregate diameter determined by ultra-small-angle X-ray scattering (USAXS) is 100 nm or more and 200 nm or less.

[0018] [Configuration 3] The solid carbon support is an electrode catalyst according to configuration 1 or 2, wherein the mass fractal dimension of the aggregate diameter, as determined by ultra-small-angle X-ray scattering (USAXS), is 2.5 or more and 3.0 or less.

[0019] [Structure 4] The electrode catalyst according to any one of configurations 1 to 3, wherein the solid carbon support has a structure in which solid primary particles or aggregates thereof are fused in a dendritic manner.

[0020] [Composition 5] The solid carbon support has a surface functional group density of 5 μM / m². 2 An electrode catalyst described in any one of the following configurations 1 to 4.

[0021] [Composition 6] The first step involves preparing an electrode catalyst precursor in which catalyst particles are supported on the surface of a solid carbon support, The second step involves heat-treating the electrode catalyst precursor in a non-oxidizing atmosphere at a temperature of 650°C to 750°C to obtain the electrode catalyst described in any one of configurations 1 to 5. A method for producing an electrode catalyst equipped with the necessary components.

[0022] [Composition 7] A method for producing an electrode catalyst according to configuration 6, further comprising a third step of introducing surface functional groups to the surface of the solid carbon support after the second step.

[0023] [Structure 8] The method for producing an electrode catalyst according to configuration 7, wherein the third step is a step of treating the electrode catalyst with nitric acid.

[0024] One embodiment of the present invention will be described in detail below. [1. Electrocatalyst] The electrode catalyst according to the present invention is Solid carbon support, Catalyst particles supported on the surface of the solid carbon support and It is equipped with.

[0025] [1.1. Solid carbon carrier] [1.1.1. Definition] In the present invention, a solid carbon carrier is used as the carrier. Here, "solid carbon carrier" refers to a carrier whose primary particles are made of solid carbon. Therefore, carriers whose primary particles are made of hollow carbon, such as Ketjenbrak (registered trademark), and mesoporous carbon are not included in the "solid carbon carrier" as defined in this application.

[0026] [1.1.2. Aggregate diameter, Mass fractal dimension of aggregate diameter] Solid carbon carriers typically have a structure in which a small number of solid primary particles link together to form small aggregates, and then many of these aggregates link together to form larger aggregates (aglomerates).

[0027] The size of aggregates contained in a solid carbon support can be quantified by the aggregate diameter determined by ultra-small-angle X-ray scattering (USAXS). Generally, the smaller the aggregate diameter, the more dispersed the catalyst particles can be supported on the support surface. On the other hand, if the aggregate diameter becomes too small, it may become difficult to support the catalyst particles on the support surface. To support catalyst particles with high dispersion on the support surface, the aggregate diameter of the solid carbon support is preferably between 100 nm and 200 nm.

[0028] The amount of voids contained in the aggregate can be quantified by the mass fractal dimension of the aggregate diameter, which is determined by ultra-small-angle X-ray scattering (USAXS). A large mass fractal dimension of the aggregate diameter means that the amount of voids contained in the aggregate is small. When catalyst particles are supported on the surface of a support with a small amount of voids in the aggregate and heat-treated under predetermined conditions, ionomatous poisoning of the catalyst particles is suppressed, and catalytic activity is improved. To obtain such an effect, the mass fractal dimension of the aggregate diameter of the solid carbon support is preferably between 2.5 and 3.0.

[0029] Fuel cell carriers are required to be capable of supporting catalyst particles in high dispersion and to have excellent durability, as well as to be able to smoothly supply reaction gases to the catalyst particles supported on the carrier surface. For this reason, solid carbon carriers are preferably those that have a structure in which solid primary particles or their aggregates are fused together in a dendritic manner.

[0030] [1.1.3. Surface functional group density] The electrode catalyst according to the present invention is obtained by heat-treating an electrode catalyst precursor in a non-oxidizing atmosphere, as described later. Therefore, the functional group density on the surface of the solid carbon support becomes lower than before heat treatment. When the manufacturing conditions are optimized, the surface functional group density of the solid carbon support becomes 5 μM / m². 2 The following applies. Furthermore, if a treatment to increase the surface functional group density (activation treatment) is performed after heat treatment, the surface functional group density of the solid carbon support can be increased to 5 μM / m². 2 It can also be made super.

[0031] [1.2. Catalyst particles] [1.2.1. Composition] In the present invention, the material of the catalyst particles is not particularly limited, as long as it is a material having oxygen reduction activity and / or hydrogen oxidation activity. Examples of catalyst particle materials include: (a) Precious metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os), (b) Alloys containing two or more precious metal elements, (c) Alloys containing one or more noble metal elements and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.) These are some examples.

[0032] Among these, catalyst particles made of Pt or Pt alloy are preferred. This is because they have high activity for the electrode reaction in fuel cells. Examples of Pt alloys include Pt-Fe alloy, Pt-Co alloy, Pt-Ni alloy, Pt-Pd alloy, Pt-Cr alloy, Pt-V alloy, Pt-Ti alloy, Pt-Ru alloy, and Pt-Ir alloy.

[0033] [1.2.2. Particle size] The particle size of the catalyst particles is not particularly limited, and the optimal particle size can be selected according to the purpose. Generally, if the particle size of the catalyst particles is too small, the catalyst particles will dissolve easily. Therefore, a particle size of 1 nm or larger is preferable. On the other hand, if the particle size of the catalyst particles becomes too large, the mass activity decreases. Therefore, the particle size of the catalyst particles is preferably 20 nm or less. Preferably, the particle size of the catalyst particles is 10 nm or less, and more preferably 5 nm or less.

[0034] [1.2.3. Load] "Amount of catalyst particles supported" refers to the ratio of the mass of catalyst particles to the total mass of the electrode catalyst. In this invention, the amount of catalyst particles supported is not particularly limited, and the optimal amount can be selected according to the purpose. Generally, higher activity can be obtained as the amount of catalyst particles supported increases. Therefore, a supported amount of 5 mass% or more is preferable. More preferably, the supported amount is 10 mass% or more, 20 mass% or more, or 30 mass% or more. On the other hand, increasing the amount of catalyst particles beyond what is necessary does not make a difference in effect and is not beneficial. Therefore, the amount of particles to be supported is preferably 70 mass% or less. More preferably, the amount of particles to be supported is 60 mass% or less, or 40 mass% or less.

[0035] [1.3. Microstructure] [1.3.1. Intrapore Loading Rate] The "porosity loading rate of catalyst particles ΔNp" refers to the value expressed by the following equation (1). ΔNp = Np × 100 / (Ns + Np) …(1) however, Ns is the number of catalyst particles whose catalyst-carbon distance is 0 nm or more and 2 nm or less. Np is the number of catalyst particles whose catalyst-carbon distance is greater than 2 nm and less than or equal to 10 nm. "Catalyst-carbon distance" refers to the shortest distance between the surface of the catalyst particle and the outermost surface of the aggregate.

[0036] The catalyst-carbon distance can be measured by acquiring serial tilt images using a scanning transmission electron microscope (STEM) and reconstructing the 3D structure using the acquired serial tilt images. "The catalyst-carbon distance being between 0 nm and 2 nm" means that the catalyst particles are supported on the outermost surface of the aggregate. Immediately after the catalyst particles are supported on the surface of a solid carbon support, the probability of the catalyst particles being supported on the outermost surface of the aggregate increases.

[0037] The statement "the catalyst-carbon distance is greater than 2 nm and less than or equal to 10 nm" means that the catalyst particles are supported within pores formed on the surface of the aggregate. Typically, oxygen-containing functional groups are bonded to the surface of a solid carbon support. Additionally, oxygen may be adsorbed on the surface of the catalyst particles. Therefore, when catalyst particles are supported on the surface of a solid carbon support and then heat-treated in a non-oxidizing atmosphere, the carbon reacts with the oxygen-containing functional groups and / or the oxygen adsorbed on the surface of the catalyst particles, forming new pores on the surface of the aggregate. As a result, the probability of some of the catalyst particles that were supported on the outermost surface of the aggregate penetrating into the newly formed pores increases.

[0038] Immediately after supporting catalyst particles on the surface of a solid carbon support, the pore-containing rate of the catalyst particles is low. In contrast, when the solid carbon support is heat-treated in a non-oxidizing atmosphere after supporting catalyst particles on its surface, the pore-containing rate of the catalyst particles becomes higher than before the heat treatment. Since the catalyst particles supported in the pores are less likely to come into contact with the acidic groups of the ionomer, catalyst poisoning is suppressed. When manufacturing an electrocatalyst using the method described later, optimizing the manufacturing conditions results in a pore-containing rate of catalyst particles of 50% to 60%.

[0039] [1.3.2. Volume ratio of small pores and medium pores] The "volume ratio of small pores ΔVs" refers to the value expressed by the following equation (2). The "volume ratio of medium pores ΔVm" refers to the value expressed by the following equation (3). ΔVs = Vs × 100 / Vt …(2) ΔVm = Vm × 100 / Vt …(3) however, Vt is the total volume of pores contained in the solid carbon support. Vs is the total volume of micropores contained in the solid carbon support. Vm is the total volume of the micropores contained in the solid carbon carrier. The aforementioned "small pores" refer to pores with a diameter of 6 nm or more and 10 nm or less. The term "medium-sized pore" refers to a pore with a diameter greater than 10 nm and less than or equal to 14 nm.

[0040] As described above, solid carbon carriers have a structure in which a small number of solid primary particles link together to form aggregates, and a large number of aggregates further link together to form agglomerates. Therefore, when the pore size of a solid carbon carrier is measured, various pores with different diameters are detected.

[0041] Before heat treatment, solid carbon supports have a relatively high volume proportion of small pores and a relatively low volume proportion of medium pores. When such solid carbon supports are heat-treated under appropriate conditions, some of the small pores are oxidized and become medium pores. As a result, after heat treatment, the volume proportion of small pores decreases and the volume proportion of medium pores increases. Furthermore, the probability of some catalyst particles penetrating into the newly formed medium pores increases. When manufacturing an electrode catalyst using the method described later, optimizing the manufacturing conditions results in a volume proportion of 20% to 30% for small pores, and a volume proportion of 14% to 20% for medium pores.

[0042] [2. Method for manufacturing electrode catalysts] The method for producing an electrode catalyst according to the present invention is: The first step involves preparing an electrode catalyst precursor in which catalyst particles are supported on the surface of a solid carbon support, A second step is to heat-treat the electrode catalyst precursor in a non-oxidizing atmosphere at a temperature of 650°C to 750°C to obtain the electrode catalyst described in claim 1. It is equipped with. The method for manufacturing an electrode catalyst is: The process may further include a third step after the second step, in which surface functional groups are introduced onto the surface of the solid carbon support.

[0043] [2.1. 1st step] First, an electrode catalyst precursor is prepared in which catalyst particles are supported on the surface of a solid carbon support. The method of supporting the catalyst particles is not particularly limited, and known methods can be used. If the support of commercially available catalyst-supported carbon is a solid carbon support, the commercially available catalyst-supported carbon may be used as is as the electrode catalyst precursor.

[0044] [2.2. 2nd step] Next, the electrode catalyst precursor is heat-treated in a non-oxidizing atmosphere at a temperature of 650°C to 750°C. This yields the electrode catalyst according to the present invention.

[0045] [2.2.1. Heat treatment atmosphere] The atmosphere used for heat treatment must be a non-oxidizing atmosphere. Specifically, a "non-oxidizing atmosphere" refers to He atmospheres, Ar atmospheres, N2 atmospheres, etc., and does not include oxidizing atmospheres such as O2 atmospheres or CO2 atmospheres, or reducing atmospheres such as H2 atmospheres. When electrode catalyst precursors are heat-treated in a non-oxidizing atmosphere, their mass activity improves under low to medium humidity conditions compared to when no heat treatment is performed. This is thought to be due to the following reasons.

[0046] For example, as described in Patent Document 2, when an electrode catalyst precursor is heat-treated in a CO2 atmosphere, carbon reacts with CO2, generating CO gas. As a result, the surface of the aggregate is excessively oxidized by CO2, and coarse pores are formed on the surface of the aggregate. When such an electrode catalyst is mixed with an ionomer to form a catalyst layer, the acidic groups of the ionomer are adsorbed onto the surface of the catalyst particles, making the catalyst particles susceptible to poisoning by the ionomer.

[0047] In contrast, oxygen-containing functional groups are typically bonded to the surface of solid carbon supports. Furthermore, oxygen may be adsorbed on the surface of catalyst particles. Therefore, when an electrode catalyst precursor is heat-treated in a non-oxidizing atmosphere, the aggregate surface is moderately oxidized by the oxygen-containing functional groups and / or the oxygen adsorbed on the catalyst particle surface. As a result, new medium-sized pores are formed on the surface of the aggregate that are large enough to support catalyst particles but suppress the penetration of ions. In addition, all or part of the catalyst particles that were supported on the outermost surface of the aggregate penetrate into the newly formed medium-sized pores. As a result, ionomatous poisoning of the catalyst particles is suppressed, and it is thought that the mass activity is improved in the low to medium current density range.

[0048] [2.2.2. Heat treatment temperature] If the heat treatment temperature is too low, the effect of improving mass activity will be insufficient. Therefore, the heat treatment temperature must be 650°C or higher. Preferably, the heat treatment temperature is 660°C or higher, or 670°C or higher. On the other hand, if the heat treatment temperature is too high, the mass activity in the low to medium current density region may actually decrease. This is thought to be because, when the heat treatment temperature is too high, the medium pores on the surface of the aggregate become blocked by diffusion, increasing the probability that catalyst particles are supported on the outermost surface of the aggregate. Therefore, the heat treatment temperature needs to be 750°C or lower. Preferably, the heat treatment temperature is 740°C or lower, or 730°C or lower.

[0049] [2.3. Third step] Next, if necessary, surface functional groups are introduced to the surface of the solid carbon support. When an electrode catalyst precursor is heat-treated in a non-oxidizing atmosphere, oxygen-containing functional groups on the support surface are consumed in the formation of micropores, and the density of surface functional groups decreases. As a result, the hydrophobicity of the support surface increases. Therefore, when using an electrode catalyst in applications where hydrophilicity of the support surface is required, it is preferable to introduce surface functional groups to the support surface after heat treatment.

[0050] In the present invention, the method for introducing surface functional groups is not particularly limited, and the most suitable method can be selected depending on the purpose. For example, by treating the electrode catalyst with nitric acid after heat treatment, surface functional groups can be introduced to the support surface.

[0051] [3. Effect] In electrocatalysts using a support made of solid carbon particles with a small specific surface area and almost no primary pores, most of the supported catalyst particles reside on the support surface. Therefore, when a catalyst layer is fabricated using such an electrocatalyst, the catalyst particles come into direct contact with the ionomer, and the catalyst particles are poisoned by the acidic groups. This also reduces the catalytic activity.

[0052] Patent Document 1 discloses a method for heat-treating mesoporous carbon at a temperature of 1300°C to less than 1700°C. While this method is thought to stabilize the structure of mesoporous carbon, the high treatment temperature reduces the volume ratio of the intermediate pores, and therefore is considered to have little effect in suppressing catalyst poisoning. Patent Document 2 discloses a method for heat-treating Pt-supported carbon in a CO2 atmosphere. While this method is thought to reduce the oxide film on the Pt surface, it is considered that excessively large pores are formed due to the heat treatment being performed in a CO2 atmosphere, and therefore the effect of suppressing catalyst poisoning is considered to be small.

[0053] In contrast, when catalyst particles are supported on the surface of a solid carbon support and heat-treated at a temperature between 650°C and 750°C in a non-oxidizing atmosphere, an electrode catalyst exhibiting high power generation performance in the low to medium current density range can be obtained. In particular, it exhibits high power generation performance under low humidity conditions where poisoning by ions is likely to occur. This is thought to be due to the following reasons.

[0054] In other words, when a solid carbon support is heat-treated under predetermined conditions, the small pores on the support surface are moderately oxidized by oxygen-containing functional groups on the support surface and oxygen adsorbed on the surface of the catalyst particles, increasing the proportion of medium-sized pores. When new medium-sized pores are formed, some of the catalyst particles that were supported on the outermost surface of the support penetrate into these medium-sized pores. When a catalyst layer is formed using such an electrode catalyst, the acidic groups of the ionomer are less likely to adsorb onto the surface of the catalyst particles that have penetrated into the medium-sized pores, reducing the contact area between the catalyst particles and the ionomer. As a result, catalytic activity is improved, and it is believed that high power generation performance is observed from the low current density region to the medium current density region. [Examples]

[0055] (Example 1, Comparative Example 1) [1. Sample Preparation] [1.1. Fabrication of the cathode catalyst layer] For the cathode catalyst, 40% Pt / Vulcan (registered trademark) (TEC10V40E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was used. In Example 1, this cathode catalyst was heat-treated in Ar at 700°C for 2 hours. In Comparative Example 1, this cathode catalyst was used as is.

[0056] A catalyst ink was prepared by mixing and dispersing a cathode catalyst, water, ethanol, and an ionomer dispersion (D-2020) in predetermined ratios. After degassing the ink using a planetary agitator, it was coated onto a polytetrafluoroethylene sheet and dried to obtain a cathode catalyst layer. The Pt basis weight of the cathode catalyst layer was 0.168 mg / cm². 2 The ratio of the mass of ionomer to the mass of carbon (I / C) was set to 0.76.

[0057] [1.2. Fabrication of the Anode Catalyst Layer] 60% Pt / Vulcan® (TEC10E60TPM, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was used as the anode catalyst. The anode catalyst layer was prepared in the same manner as the cathode catalyst layer. The Pt basis weight was 0.2 mg / cm³. 2 The I / C was set to 1.0.

[0058] [1.3. Cell Creation] The cathode catalyst layer and the anode catalyst layer are each 1 cm 2 The corners were punched out. An electrolyte membrane was sandwiched between these, and the catalyst layer was transferred to the electrolyte membrane by hot pressing to obtain a MEA. A 10 μm thick fluorine-based polymer membrane was used as the electrolyte membrane. The hot pressing conditions were: temperature: 145°C, press pressure: 4.9 MPa, press time: 5 minutes. This MEA was sandwiched between carbon paper diffusion layers with a microporous layer to form a cell.

[0059] [2. Test Method] [2.1. Evaluation of carbon supports] A scanning transmission electron microscope (STEM), specifically a TalosF200X (Shottky-FEG) manufactured by FEI, was used. Powder containing the electrode catalyst was scraped from the cathode catalyst layer. This powder was dry-fixed to a copper TEM grid, and continuous tilt images were acquired using STEM. The acceleration voltage was 200kV, the resolution was 2048 pixels × 2048 pixels, and the instrument display magnification was 320,000x. The 3D structure was reconstructed using the acquired continuous tilt images, and the Pt-carbon distance and carbon pore diameter were analyzed.

[0060] [2.2. Evaluation of power generation performance] After the cell break-in period, the power generation performance was investigated under low humidity conditions (cell temperature: 80°C, humidity: 30%RH) and high humidity conditions (cell temperature: 80°C, humidity: 80%RH). In both low and high humidity conditions, the type and flow rate of gas used were hydrogen / 500 nccm on the anode side and air / 2000 nccm on the cathode side. The gas back pressure was set to 33 kPa at both electrodes.

[0061] [2.3. Evaluation of sulfonic acid adsorption rate] After evaluating the power generation performance of the cell, the sulfonic acid adsorption rate on the Pt surface was investigated. Under conditions of cell temperature: 40°C and humidity: 90%RH, hydrogen was flowed to the anode side and nitrogen to the cathode side. In this state, the potential was swept from the open-circuit voltage to the lower limit potential (75mV), and then further swept to the CO substitution potential (0.4V), where it was held for 5 minutes. After that, the gas on the cathode side was switched from nitrogen to CO, and left until the current stopped flowing. The sulfonic acid adsorption rate was calculated from the current change during this period.

[0062] [3. Results] [3.1. Evaluation of carbon supports] Figure 1 shows the change in the distance between Pt and carbon particles before and after heat treatment in Ar, as analyzed from 3D reconstructed images. As shown in Figure 1, in the region where the distance between Pt and carbon particles is 0 to 2 nm, representing the vicinity of the carbon support surface, the number of Pt particles decreased by approximately 10% due to heat treatment. On the other hand, in the region where the distance between Pt and carbon particles is 2 to 10 nm, representing the interior of the carbon, the number of Pt particles increased by approximately 10%.

[0063] Figure 2 shows the change in carbon pore size before and after heat treatment in Ar, as analyzed from 3D reconstructed images. As shown in Figure 2, the heat treatment reduced the volume of pores with a diameter of 6-10 nm (small pores) by approximately 2%. On the other hand, the heat treatment increased the volume of pores with a diameter of 10-14 nm (medium pores) by approximately 4%.

[0064] Table 1 shows the pore loading rate ΔNp, the volume percentage of small pores ΔVs, and the volume percentage of medium pores for the electrode catalysts obtained in Example 1 and Comparative Example 1. These results suggest that the pores of the carbon particles expanded due to heat treatment, and some of the Pt particles that were present on the surface of the carbon particles penetrated into the interior of the carbon particles.

[0065] [Table 1]

[0066] [3.2. Evaluation of power generation performance] Figure 3 shows the mass activity (@0.86V) of the electrode catalysts obtained in Example 1 and Comparative Example 1 at a cell temperature of 80°C under high humidity and low humidity conditions. The mass activity of Example 1 was higher than that of Comparative Example 1 under both high and low humidity conditions. In particular, the mass activity of Example 1 under low humidity conditions was significantly improved compared to that of Comparative Example 1.

[0067] [3.3. Evaluation of sulfonic acid adsorption rate] Figure 4 shows the sulfonic acid adsorption rates of the electrode catalysts obtained in Example 1 and Comparative Example 1 under high and low humidity conditions at a cell temperature of 80°C. The sulfonic acid adsorption rate of Example 1 was lower than that of Comparative Example 1 under both high and low humidity conditions. This result suggests that the poisoning of the Pt surface by sulfonic acid from the ionomer was suppressed.

[0068] From the above results, it can be said that the power generation performance of Example 1 improved because the heat treatment in a non-oxidizing atmosphere expanded the pores on the surface of the carbon particles, allowing Pt particles to penetrate into the expanded pores and reducing the surface area of ​​Pt that is in direct contact with the ionomer.

[0069] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0070] The electrode catalyst according to the present invention can be used as a catalyst contained in the catalyst layer of a polymer electrolyte fuel cell.

Claims

1. Solid carbon support, Catalyst particles supported on the surface of the solid carbon support and Equipped with, The pore-borne loading rate ΔNp of the catalyst particles, represented by the following formula (1), is 50% or more and 65% or less. The volume ratio ΔVs of the micropores, represented by the following equation (2), is 20% or more and 23% or less. The volume ratio ΔVm of the micropores, expressed by the following formula (3), is 14% or more and 20% or less. Electrocatalyst. however, ΔNp=Np×100 / (Ns+Np)…(1) ΔVs=Vs×100 / Vt…(2) ΔVm=Vm×100 / Vt…(3) Ns is the number of catalyst particles in which the catalyst-carbon distance is 0 nm or more and 2 nm or less. Np is the number of catalyst particles in which the catalyst-carbon distance is greater than 2 nm and less than or equal to 10 nm. Vt is the total volume of pores contained in the solid carbon support. Vs is the total volume of micropores contained in the solid carbon support. Vm is the total volume of the micropores contained in the solid carbon carrier. The aforementioned "small pores" refer to pores with a diameter of 6 nm or more and 10 nm or less. The term "medium-sized pore" refers to a pore with a diameter greater than 10 nm and less than or equal to 14 nm.

2. The electrode catalyst according to claim 1, wherein the solid carbon support has an aggregate diameter of 100 nm or more and 200 nm or less, as determined by ultra-small-angle X-ray scattering (USAXS).

3. The electrode catalyst according to claim 1, wherein the solid carbon support has a mass fractal dimension of aggregate diameter determined by ultra-small-angle X-ray scattering (USAXS) of 2.5 or more and 3.0 or less.

4. The electrode catalyst according to claim 1, wherein the solid carbon support has a structure in which solid primary particles or aggregates thereof are fused in a dendritic manner.

5. The solid carbon support has a surface functional group density of 5 μM / m². 2 The electrode catalyst according to claim 1, wherein the following applies:

6. The first step involves preparing an electrode catalyst precursor in which catalyst particles are supported on the surface of a solid carbon support, A second step is to heat-treat the electrode catalyst precursor in a non-oxidizing atmosphere at a temperature of 650°C to 750°C to obtain the electrode catalyst described in claim 1. A method for producing an electrode catalyst equipped with the following features.

7. The method for producing an electrode catalyst according to claim 6, further comprising a third step of introducing surface functional groups onto the surface of the solid carbon support after the second step.

8. The method for producing an electrode catalyst according to claim 7, further comprising the third step of treating the electrode catalyst with nitric acid.

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