Electrode catalyst and its manufacturing method, as well as fuel cell electrode and fuel cell

Abstract

To provide a novel electrode catalyst in which the surfaces of catalyst particles are coated with a coating that satisfies a specific condition, a manufacturing method thereof, and an electrode for a fuel cell including the electrode catalyst, and a fuel cell.SOLUTION: A electrode catalyst includes catalyst particles and a nitrogen-containing carbon film covering the surface of the catalyst particles, and the nitrogen-containing carbon film contains micropores. An electrode for a fuel cell and the fuel cell is composed by using such an electrode catalyst. Such an electrode catalyst is obtained by forming an electrode catalyst precursor by coating the surface of the catalyst particles with a coating containing polymelamine and polydopamine, heat-treating the electrode catalyst precursor, and thermally decomposing the coating.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst and a method for producing the same, as well as an electrode for a fuel cell and a fuel cell, and more particularly to an electrode catalyst in which the surface of a catalyst particle is coated with a nitrogen-containing carbon film and a method for producing the same, as well as an electrode for a fuel cell and a fuel cell provided with such an electrode catalyst. [Background technology]

[0002] A polymer electrolyte fuel cell has a membrane electrode assembly (MEA) in which electrodes containing a catalyst are bonded to both sides of an electrolyte membrane. Current collectors (separators) equipped with gas flow channels are further arranged on both sides of the MEA. A polymer electrolyte fuel cell usually has a structure (fuel cell stack) in which multiple unit cells each consisting of an MEA and a current collector are stacked.

[0003] Fuel cell electrodes typically consist of a laminate consisting of a catalyst layer located on the electrolyte membrane side and a diffusion layer located on the gas flow path side. The catalyst layer generally consists of a mixture of an electrode catalyst, in which catalyst particles such as platinum or platinum alloy are supported on the surface of a carrier, and a catalyst layer ionomer. The electrode reaction occurs primarily on the surface of the catalyst particles. Therefore, efforts are being made to minimise the size of the catalyst particles and reduce the amount of platinum used per unit area of ​​the electrode. However, in the operating environment of a fuel cell, which involves potential fluctuations, the finer the catalyst particles, the more likely they are to dissolve, coarsen due to aggregation, and / or detach from the support, resulting in a gradual decrease in the activity of the catalyst layer.

[0004] Therefore, various proposals have been made in the past to solve this problem. For example, Non-Patent Document 1 states: (a) Irregular fcc-PtFe nanoparticles are supported on the surface of carbon particles, (b) By treating the nanoparticles with an aqueous solution of dopamine hydrochloride, the surface of the nanoparticles is coated with polydopamine. (c) The carbon-supported and dopamine-coated fcc-PtFe nanoparticles (fcc-PtFe / C) are heat-treated at 700°C to obtain ordered fct-PtFe nanoparticle catalysts coated with N-doped carbon (fct-Pt / Fe / C). A method for producing a PtFe nanoparticle catalyst is disclosed.

[0005] The same document states: (A) When PtFe / C is heat-treated without dopamine coating, the size of the nanoparticles increases to several tens of nanometers, whereas when PtFe / C coated with dopamine is heat-treated, the size of the nanoparticles remains almost the same (6.5 nm) as before the heat treatment. (B) The N-doped carbon shell-coated fct-PtFe / C catalyst exhibits 11.4 times higher mass activity and 10.5 times higher specific activity than the commercial Pt / C catalyst; and (C) Coating Pt / C with an N-doped carbon shell has negligible effect on activity. is stated.

[0006] Patent Document 1 describes a catalyst comprising catalyst particles and a carbon film covering the surface of the catalyst particles, wherein the content of chloride ions per unit surface area of ​​the catalyst particles is 12.5 μg / m 2 An electrocatalyst is disclosed that is less than The same document states: (a) When the surface of a catalyst particle is coated with a film derived from an organic substance and then heat-treated, an electrode catalyst in which the surface of the catalyst particle is coated with a carbon film is obtained; (b) When the obtained electrocatalyst is washed, the chloride ions adsorbed on the surface of the catalyst particles are desorbed, improving the initial activity compared to an electrocatalyst that has not been washed; and (c) The carbon film suppresses dissolution, aggregation, and / or detachment of catalyst particles and suppresses ionomer poisoning of catalyst particles, thereby improving initial activity and durability. is stated.

[0007] As synthesized, PtFe alloy nanoparticles have an irregular face-centered cubic (fcc) structure and low catalytic activity. When they are heat-treated at 700°C, they change to a regular face-centered tetragonal (fct) structure, improving the area-specific activity. However, the heat treatment process causes the nanoparticles to coarsen, resulting in a significant decrease in mass activity.

[0008] In contrast, Non-Patent Document 1 describes that coating the surface of PtFe alloy nanoparticles with dopamine and heat-treating them at 700°C suppresses the increase in particle size of the nanoparticles. This is explained as being because the thin carbon film (a thermal decomposition product of dopamine) covering the nanoparticle surface suppresses the aggregation of nanoparticles that occurs during the heat-treatment process. Furthermore, Non-Patent Document 1 describes that the fct-PtFe / C catalyst that has undergone such treatment has improved areal activity, mass activity, and durability compared to pure Pt / C catalysts and irregular fcc-PtFe / C catalysts. The improved durability is explained as being due to the carbon film covering the Pt surface suppressing the dissolution, aggregation, and desorption of Pt that occur during durability tests.

[0009] However, the effect of the carbon film itself formed by dopamine modification and heat treatment on the catalyst performance has not been fully elucidated. In fact, Non-Patent Document 1 reports that coating a pure Pt / C catalyst with dopamine and heat-treating it at 700°C did not improve the initial performance.

[0010] On the other hand, Patent Document 1 describes that coating the surface of a pure Pt / C catalyst with a dopamine-derived carbon film and then washing the electrode catalyst improves the initial activity and durability of the electrode catalyst. However, in order to further improve the performance of fuel cells, it is desirable to further improve the initial activity and durability of the electrode catalyst. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Publication No. 2020-136109 [Non-patent literature]

[0012] [Non-Patent Document 1] DY Chung, et al., J. Am. Chem. Soc., 2015, 137, 15478-15485 Summary of the Invention [Problem to be solved by the invention]

[0013] The problem to be solved by the present invention is to provide a novel electrode catalyst in which the surfaces of catalyst particles are coated with a film that satisfies specific conditions, and a method for producing the same. Another object of the present invention is to provide a novel electrode catalyst having excellent initial activity and durability, and a method for producing the same. Another object of the present invention is to provide a fuel cell electrode and a fuel cell that are provided with such an electrode catalyst. [Means for solving the problem]

[0014] In order to solve the above problems, the electrode catalyst according to the present invention comprises: Catalyst particles; a nitrogen-containing carbon film that covers the surface of the catalyst particles; Equipped with The nitrogen-containing carbonaceous membrane contains micropores.

[0015] The fuel cell electrode according to the present invention comprises a catalyst layer containing the electrode catalyst according to the present invention and a catalyst layer ionomer. The fuel cell according to the present invention comprises: an electrolyte membrane made of a solid polymer electrolyte; Electrodes bonded to both sides of the electrolyte membrane; Equipped with At least one of the electrodes is made of the fuel cell electrode according to the present invention.

[0016] The method for producing an electrode catalyst according to the present invention includes the steps of: a coating step of coating the surface of the catalyst particles with a coating containing polymelamine and polydopamine to obtain an electrode catalyst precursor; a thermal decomposition step of thermally decomposing the coating by heat-treating the electrode catalyst precursor to obtain an electrode catalyst in which the surfaces of the catalyst particles are coated with a nitrogen-containing carbon film; It is equipped with: [Effects of the Invention]

[0017] When the surface of a catalyst particle is coated with polymelamine and polydopamine and the polymelamine and polydopamine are thermally decomposed, an electrode catalyst is obtained in which the surface of the catalyst particle is coated with a nitrogen-containing carbon film containing micropores. The electrode catalyst thus obtained may have improved initial activity and durability compared to an electrode catalyst coated with a carbon film derived from polydopamine. This is because (a) The nitrogen-containing carbon film suppresses the grain growth of catalyst particles during heat treatment. (b) Because the nitrogen-containing carbon membrane has micropores, the transport of substances to the catalyst particle surface is not easily hindered even if the nitrogen-containing carbon membrane becomes thick, and (c) The nitrogen-containing carbon film suppresses dissolution, aggregation, and / or desorption of catalyst particles, and ionomer poisoning of catalyst particles. It is thought that... [Brief explanation of the drawings]

[0018] [Figure 1] 1 shows current-voltage curves under high humidity for the electrode catalysts obtained in Examples 1 and 3 and Comparative Example 1. [Figure 2] FIG. 2 is an enlarged view of the low load region of the current-voltage curve shown in FIG. [Figure 3] 1 shows the results of a durability test of the electrode catalysts obtained in Example 1 and Comparative Examples 1 and 2. [Figure 4] 1 shows the mass activity retention rates of the electrode catalysts obtained in Example 1 and Comparative Examples 1 and 2. [Figure 5] 1 is an HRTEM image of the electrode catalyst obtained in Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0019] An embodiment of the present invention will be described in detail below. [1. Electrocatalyst] The electrode catalyst according to the present invention comprises: Catalyst particles; a nitrogen-containing carbon film that covers the surface of the catalyst particles; It is equipped with: The electrode catalyst may further include a carrier that supports the catalyst particles.

[0020] 1.1. Catalyst particles [1.1.1. Composition] In the present invention, the material of the catalyst particles is not particularly limited. (a) Precious metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os), (b) an alloy containing two or more precious metal elements; (c) Alloys containing one or more precious metal elements and one or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.); etc.

[0021] Among these, Pt or a Pt alloy is preferred for the catalyst particles because it has high activity in the electrode reaction of the fuel cell. Examples of Pt alloys include Pt--Fe alloys, Pt--Co alloys, Pt--Ni alloys, Pt--Pd alloys, Pt--Cr alloys, Pt--V alloys, Pt--Ti alloys, Pt--Ru alloys, and Pt--Ir alloys.

[0022] [1.1.2. Particle size] The particle size of the catalyst particles is not particularly limited, and an optimal particle size can be selected depending on the purpose. Generally, if the particle size of the catalyst particles is too small, the catalyst particles tend to dissolve. Therefore, the particle size of the catalyst particles is preferably 1 nm or more. On the other hand, if the particle size of the catalyst particles is too large, the mass activity decreases. Therefore, the particle size of the catalyst particles is preferably 20 nm or less. The particle size of the catalyst particles is preferably 10 nm or less, and more preferably 5 nm or less.

[0023] 1.2. Carrier [1.2.1. Materials] The catalyst particles may be used in various applications as they are, or may be used in a state where they are supported on the surface of a carrier. Supporting the catalyst particles on the surface of a carrier allows fine catalyst particles to be stably dispersed, thereby reducing the amount of catalyst used. Examples of the carrier include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[0024] 1.2.2. Catalyst loading When catalyst particles are supported on the surface of a carrier, the amount of catalyst supported is not particularly limited, and an optimal amount can be selected depending on the purpose. Generally, if the amount of catalyst supported is too small, sufficient activity cannot be obtained. On the other hand, even if the amount of catalyst supported is greater than necessary, there is no difference in effect and it is not practically beneficial. For example, when catalyst particles made of Pt or a Pt alloy are supported on the surface of a carbon support, the amount of catalyst supported is preferably 5 wt % to 70 wt %.

[0025] [1.3. Nitrogen-containing carbon film] [1.3.1. Composition] The surface of the catalyst particle is coated with a nitrogen-containing carbon film. The "nitrogen-containing carbon film" refers to a film obtained by coating the surface of the catalyst particle with a film containing polymelamine and polydopamine and then thermally decomposing the film.

[0026] Pyrolysis of a coating containing only polydopamine results in a dense carbon film consisting essentially of carbon. On the other hand, pyrolysis of a coating containing polymelamine and polydopamine results in a nitrogen-containing carbon film containing micropores and having a higher nitrogen content than polydopamine-derived carbon films. The nitrogen contained in the nitrogen-containing carbon film is derived from polymelamine. The amount of nitrogen contained in the nitrogen-containing carbon film is usually 0.1 mass% to 35 mass%, depending on the production method.

[0027] The nitrogen-containing carbon film is (a) A catalyst obtained by coating the surface of a catalyst particle with a film made of a mixture of polymelamine and polydopamine and then thermally decomposing the film (simultaneous modification method), or (b) A catalyst obtained by coating the surface of a catalyst particle with polymelamine, then coating the surface of the catalyst particle with polydopamine, and then thermally decomposing the laminated film of polymelamine and polydopamine (sequential modification method); Either of the above may be used. In particular, the nitrogen-containing carbonaceous membrane obtained by the sequential modification method exhibits higher properties than the nitrogen-containing carbonaceous membrane obtained by the simultaneous modification method.

[0028] 1.3.2. Micropore volume "Micropore" refers to a pore with a diameter of 2 nm or less. The "micropore volume" refers to a value obtained by measuring the amount of nitrogen adsorption on the electrode catalyst and analyzing the result by the t-plot method using solid carbon as the reference isotherm.

[0029] As described above, pyrolysis of polydopamine in the presence of polymelamine produces a nitrogen-containing carbon membrane with micropores. If the volume of the micropores is too small, mass transfer to the catalyst particle surface may be hindered, resulting in a decrease in initial activity. Therefore, the volume of the micropores is preferably 0.001 cc / g or more. On the other hand, if the volume of the micropores is excessive, dissolution, aggregation, and / or detachment of the catalyst particles or ionomer poisoning of the catalyst particles may occur, so the volume of the micropores is preferably 0.3 cc / g or less.

[0030] Thickness The thickness of the nitrogen-containing carbon membrane affects the stability and activity of the catalyst particles. If the nitrogen-containing carbon membrane is too thin, dissolution, aggregation, and / or detachment of the catalyst particles is likely to occur. Furthermore, when such an electrode catalyst is applied to a fuel cell electrode, if the nitrogen-containing carbon membrane is too thin, the catalyst particles are likely to be poisoned by the catalyst layer ionomer. Therefore, the thickness of the nitrogen-containing carbon membrane is preferably 0.2 nm or more. The thickness of the nitrogen-containing carbon membrane is preferably 0.5 nm or more. On the other hand, if the nitrogen-containing carbonaceous membrane is too thick, the transport resistance of the reactants increases, which may result in a decrease in activity. Therefore, the thickness of the nitrogen-containing carbonaceous membrane is preferably 20 nm or less.

[0031] [1.4. Characteristics] [1.4.1. Initial activation] "Initial activity" refers to the current density at 0.86 V in the initial state (after break-in) of a polymer electrolyte fuel cell using the electrode catalyst according to the present invention as the cathode catalyst. When the manufacturing conditions of the electrode catalyst according to the present invention are optimized, the initial activity is 0.02 A / cm 2 @0.86V or higher.

[0032] [1.4.2. Activity after durability test] "Activity after durability test" refers to the current density at 0.86 V after a durability test of a polymer electrolyte fuel cell using the electrode catalyst according to the present invention as the cathode catalyst. "After durability test" refers to the time after the fuel cell was subjected to 10,000 cycles of potential fluctuations of 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds). When the manufacturing conditions of the electrode catalyst according to the present invention are optimized, the activity after the durability test is 0.02 A / cm 2 @0.86V or higher.

[0033] [1.4.3. ECSA retention rate] "ECSA retention rate (%)" refers to the ratio of ECSA (x) after the durability test to ECSA (x0) before the durability test (= x × 100 / x0). "After durability test" refers to the time after the fuel cell was subjected to 10,000 cycles of potential fluctuations of 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds). In the electrode catalyst according to the present invention, when the manufacturing conditions are optimized, the ECSA retention rate becomes 80% or more, and when the manufacturing conditions are further optimized, the ECSA retention rate becomes 90% or more, or even 95% or more.

[0034] [2. Electrodes for fuel cells] The fuel cell electrode according to the present invention includes a catalyst layer containing the electrode catalyst according to the present invention and a catalyst layer ionomer. The fuel cell electrode may further include a diffusion layer disposed on the current collector side. The content of the catalyst layer ionomer contained in the catalyst layer is not particularly limited, and an optimum content can be selected depending on the purpose. For example, when the catalyst particles are supported on a carbon support, the ratio of the weight (I) of the catalyst layer ionomer to the weight (C) of the carbon (=I / C) is preferably 0.3 or more and 2.0 or less.

[0035] [3. Fuel cell] The fuel cell according to the present invention comprises: an electrolyte membrane made of a solid polymer electrolyte; Electrodes bonded to both sides of the electrolyte membrane; It is equipped with:

[0036] [3.1. Electrolyte membrane] The electrolyte membrane is made of a solid polymer electrolyte. The composition of the solid polymer electrolyte is not particularly limited, and an optimum material can be selected depending on the purpose. Examples of solid polymer electrolytes include Nafion (registered trademark), Flemion (registered trademark), Aciplex (registered trademark), and Aquivion (registered trademark).

[0037] [3.2. Electrodes] The electrodes are bonded to both sides of the electrolyte membrane. In the fuel cell according to the present invention, at least one of the electrodes is the fuel cell electrode according to the present invention. The fuel cell electrode according to the present invention may be used on either the anode side or the cathode side. The fuel cell electrode according to the present invention is particularly preferably used as a cathode, since the presence or absence of a coating has a greater effect on the initial activity and durability of the fuel cell in the cathode than in the anode.

[0038] 4. Manufacturing method of electrode catalyst The method for producing an electrode catalyst according to the present invention includes the steps of: a coating step of coating the surface of the catalyst particles with a coating containing polymelamine and dopamine to obtain an electrode catalyst precursor; a thermal decomposition step of thermally decomposing the coating by heat-treating the electrode catalyst precursor to obtain an electrode catalyst in which the surfaces of the catalyst particles are coated with a nitrogen-containing carbon film; It is equipped with:

[0039] [4.1. Coating process] First, the surface of the catalyst particles is coated with a film containing polymelamine and polydopamine to obtain an electrode catalyst precursor (coating step). The coating step may involve coating the surface of the catalyst particles with a coating made of a mixture of polymelamine and polydopamine (simultaneous modification method). Alternatively, the coating step may include: a first coating step of coating the surface of the catalyst particles with polymelamine; a second coating step of further coating the surface of the catalyst particles with polydopamine; (sequential modification method).

[0040] 4.1.1. Catalyst particles The catalyst particles may be supported on the surface of a carrier, or may not be supported on the surface of a carrier. Other points regarding the catalyst particles are as described above, and therefore will not be described here.

[0041] [4.1.2. Coating] The coating contains polymelamine and polydopamine. The coating may be a mixture of polymelamine and polydopamine, or may be a laminated film of polymelamine and polydopamine. In order to obtain an electrode catalyst having excellent initial activity and durability, the coating is preferably a laminated film of polymelamine and polydopamine.

[0042] When forming a coating consisting of a mixture of polymelamine and polydopamine, for example, dopamine hydrochloride is added to a solution in which polymelamine has been dissolved, and then catalyst particles (or catalyst particles supported on a carrier) are added to this, stirred for a predetermined time, and dried. As a result, dopamine is polymerized to form polydopamine, and at the same time, the surface of the catalyst particles is coated with a coating consisting of a mixture of polymelamine and polydopamine (simultaneous modification method).

[0043] On the other hand, when forming a coating consisting of a laminated film, for example, first, catalyst particles (or catalyst particles supported on a carrier) are added to a solution in which polymelamine is dissolved, and the mixture is stirred for a predetermined time and dried. This results in catalyst particles coated with polymelamine. Next, the polymelamine-coated catalyst particles are added to a solution containing dopamine hydrochloride, and the mixture is stirred and dried. This causes dopamine to polymerize to form polydopamine, and at the same time, the surfaces of the polymelamine-coated catalyst particles are further coated with polydopamine (sequential modification method).

[0044] 4.1.3. Amount of polymelamine and polydopamine added The amounts of polymelamine and polydopamine to be added are not particularly limited, and optimal amounts can be selected depending on the purpose. By changing the amounts of these added, the thickness and nitrogen content of the nitrogen-containing carbonaceous film can be controlled.

[0045] Here, the "amount of polymelamine or polydopamine added" refers to the value expressed by the following formula (1). Content (mass%)=y×100 / (y+z)…(1) however, y is the mass of polymelamine or polydopamine contained in the electrode catalyst precursor, z is the mass of the catalyst particles contained in the electrode catalyst precursor (when the catalyst particles are supported on a support, it is the total mass of the catalyst particles and the support)

[0046] If the amount of polymelamine added is too small, micropores will not be formed and durability will also decrease. Therefore, the amount of polymelamine added is preferably 10 mass% or more. The amount of polymelamine added is more preferably 50 mass% or more. On the other hand, if the amount of polymelamine added is excessive, durability will be reduced. Therefore, the amount of polymelamine added is preferably 99 mass% or less, and more preferably 93 mass% or less.

[0047] If the amount of polydopamine added is too small, micropores will not be formed and durability will decrease. Therefore, the amount of polydopamine added is preferably 5 mass% or more. The amount of polydopamine added is more preferably 10 mass% or more. On the other hand, if the amount of polydopamine added is excessive, durability will decrease. Therefore, the amount of polydopamine added is preferably 50 mass% or less, and more preferably 24 mass% or less.

[0048] [4.2. Pyrolysis process] Next, the electrode catalyst precursor is heat-treated to pyrolyze the coating (thermal decomposition step), thereby obtaining an electrode catalyst in which the surfaces of catalyst particles are coated with a nitrogen-containing carbon film. The heat treatment conditions are not particularly limited as long as they are capable of carbonizing the coating. The heat treatment is preferably performed in an inert atmosphere at a temperature of 600°C or higher and 1000°C or lower for 0.5 to 10 hours.

[0049] [5. Effect] When the surface of a catalyst particle is modified with polydopamine alone and then pyrolyzed, a carbon film can be formed on the surface of the catalyst particle. However, because the polydopamine-derived carbon film is dense, if the carbon film becomes thick, mass transfer to the catalyst particle surface is hindered, resulting in a decrease in performance. On the other hand, when the surface of catalyst particles is modified with polymelamine alone and used without thermal decomposition, the performance improvement effect in the low load range is greater than when modified with a polydopamine-derived carbon film. However, when the polymelamine film is heat-treated, most of it is decomposed, and only a small amount remains as a carbon film. Furthermore, because the small amount of polymelamine-derived carbon film that remains is dense like the polydopamine-derived carbon film, performance decreases as the carbon film becomes thicker.

[0050] In contrast, when the surface of a catalyst particle is coated with polymelamine and polydopamine and the polymelamine and polydopamine are thermally decomposed, an electrode catalyst is obtained in which the surface of the catalyst particle is coated with a nitrogen-containing carbon film containing micropores. The electrode catalyst obtained in this manner may have improved initial activity and durability compared to an electrode catalyst coated with a carbon film derived from polydopamine. This is because (a) The nitrogen-containing carbon film suppresses the grain growth of catalyst particles during heat treatment. (b) Because the nitrogen-containing carbon membrane has micropores, the transport of substances to the catalyst particle surface is not easily hindered even if the nitrogen-containing carbon membrane becomes thick, and (c) The nitrogen-containing carbon membrane suppresses dissolution, aggregation, and / or detachment of catalyst particles, as well as ionomer poisoning of catalyst particles (a phenomenon in which sulfonic acid groups of ionomers are adsorbed onto the surface of catalyst particles, reducing active sites), It is thought that... [Example]

[0051] (Examples 1 to 3, Comparative Examples 1 and 2) 1. Sample Preparation 1.1. Preparation of electrode catalysts 1.1.1. Examples 1-2: Sequentially Modified Materials The catalyst used was 30 mass% Pt / Vulcan® (TEC10V30E) (hereinafter simply referred to as "Pt / C catalyst"). It was washed with high-temperature (100°C) Milli-Q® water (hereinafter simply referred to as "water") and dried. The washed Pt / C catalyst was dispersed in a mixed solvent of water and 2-propanol, to which a polymelamine (PME) solution was added and stirred at room temperature for 3 hours. After stirring, Powder A was recovered by suction filtration and vacuum-dried at 100°C. The PME addition amount (= the ratio of the mass of PME to the total mass of the Pt / C catalyst and PME) was 93 mass% (Example 1) or 19 mass% (Example 2). The molecular structure of polymelamine (poly(melamine-co-formaldehyde)) is shown in the following formula (2):

[0052] [ka]

[0053] Next, the dried Powder A, dopamine hydrochloride, and a Tris-HCl buffer solution adjusted to pH 8.5 were placed in a beaker and ultrasonically dispersed. The amount of dopamine hydrochloride added (= the mass ratio of dopamine hydrochloride to the total mass of the Pt / C catalyst and dopamine hydrochloride) was 30 mass%. The mass of dopamine excluding hydrochloric acid was 24 mass%. The dispersion was then stirred for 6 hours at room temperature in the air using a stirrer. This resulted in the polymerization of dopamine to form polydopamine (PDA). The molecular structure of polydopamine is shown in the following formula (3):

[0054] [ka]

[0055] After stirring, Powder B was collected by suction filtration and washed with water at room temperature. The washed Powder B was then vacuum dried at 100°C. The resulting Powder B was heat-treated in a tube furnace at 700°C for 2 hours while flowing Ar, carbonizing the PME and PDA. The heat-treated Powder C was washed with high-temperature (100°C) water to obtain an electrode catalyst.

[0056] 1.1.2. Example 3: Co-modification The Pt / C catalyst was dispersed in a mixed solvent of water and 2-propanol. To this was added a PME solution (PME content: 93 mass%) and dopamine hydrochloride (mass of dopamine excluding hydrochloric acid: 24 mass%), followed by stirring at room temperature for 3 hours. After stirring, Powder D was collected by suction filtration and vacuum dried at 100°C. The resulting Powder D was heat-treated in a tube furnace at 700°C for 2 hours while flowing Ar, carbonizing the PME and PDA. After the heat treatment, Powder D was washed with high-temperature (100°C) water to obtain an electrode catalyst.

[0057] 1.1.3. Comparative Example 1: Unmodified Material The Pt / C catalyst was directly subjected to the test. [1.1.4. Comparative example 2: PDA modified material] An electrode catalyst coated with a PDA-derived carbon film was obtained in the same manner as in Example 1, except that the PME coating on the Pt / C catalyst surface was omitted.

[0058] 1.2. Fuel Cell Construction The above electrode catalyst was dispersed in a water / ethanol / Nafion (registered trademark) ionomer solution (D-2020) to prepare a catalyst ink. The water / alcohol mass ratio in the catalyst ink was approximately 1. This catalyst ink was applied to a polytetrafluoroethylene sheet to prepare a cathode catalyst layer. The Pt basis weight of the cathode catalyst layer was 0.1 mg / cm. 2 The ratio of the mass of the ionomer to the mass of the carbon support (I / C) was set to about 1.

[0059] The anode catalyst layer contained 60 mass% Pt / Ketjen (registered trademark) as an electrode catalyst and Nafion (registered trademark) (D-2020) as an ionomer, with a Pt coverage of 0.2 mg / cm 2 and an I / C of 1.0 was used.

[0060] The cathode catalyst layer and the anode catalyst layer were thermally transferred (120°C, 50 kgf / cm) onto both sides of a Nafion (registered trademark) membrane (NR211). 2 , 5 min) to prepare a membrane electrode assembly (MEA). The electrode area was 1 cm 2 The MEA was sandwiched between paper diffusion layers (GDL) with a water-repellent layer to form a cell. 2. Test Method 2.1. Micropore volume The nitrogen adsorption amount of the obtained electrode catalyst was measured, and the obtained data was analyzed by the t-plot method using solid carbon as the reference isotherm to determine the inner and outer pore surface areas and the micropore volume.

[0061] [2.2. Initial power generation performance and power generation performance after endurance] The resulting cells were used to examine their power generation performance after break-in (initial power generation performance) and after potential cycling tests (post-durability power generation performance). The power generation performance was evaluated under high humidity conditions (cell temperature 60°C / humidity 80% RH). The potential cycling durability test involved 10,000 cycles of potential fluctuations between 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds).

[0062] [2.3. High-resolution transmission electron microscope observation] To investigate the state of the PME and PDA modified electrode catalysts after heat treatment, high-resolution transmission electron microscopy (HRTEM) was performed.

[0063] [3. Results] 3.1. Micropore volume Table 1 shows the pore internal surface area, pore external surface area, and micropore volume of each electrode catalyst analyzed by the t-plot method. The unmodified material (Comparative Example 1) and the PDA-modified material (Comparative Example 2) both had zero pore internal surface area and micropore volume. On the other hand, the sequentially modified materials (Examples 1 and 2) and the simultaneously modified material (Example 3) all had positive pore internal surface area and micropore volume. Furthermore, the greater the amount of PME modification, the higher these values ​​became. This is thought to be because the heat treatment at 700°C decomposed a portion of the PME, making the coating porous. Furthermore, weight analysis after heat treatment suggested that the remaining PME remained in the coating together with the PDA.

[0064] [Table 1]

[0065] [3.2. Initial power generation performance and power generation performance after endurance] FIG. 1 shows current-voltage curves under high humidity for the electrode catalysts obtained in Examples 1 and 3 and Comparative Example 1. (a) Example 1 shows less performance degradation due to durability tests than Comparative Example 1, and (b) Example 3 showed a decrease in initial power generation performance compared to Example 1. You can see that. Figure 2 shows an enlarged view of the low load region of the current-voltage curve shown in Figure 1. From Figure 2, it can be seen that in the low load region, Example 1 has higher initial power generation performance and higher post-endurance power generation performance than Comparative Example 1, and that the catalytic activity is improved.

[0066] Fig. 3 shows the results of the durability test of the electrode catalysts obtained in Example 1 and Comparative Examples 1 and 2. Fig. 3 shows that Example 1 exhibits a higher ECSA retention rate than Comparative Examples 1 and 2, indicating that it has higher durability. 4 shows the mass activity retention rates of the electrode catalysts obtained in Example 1 and Comparative Examples 1 and 2. It can be seen from FIG. 4 that Example 1 has a significantly improved mass activity retention rate compared to Comparative Examples 1 and 2.

[0067] [3.3. High-resolution transmission electron microscope observation] Figure 5 shows an HRTEM image of the electrode catalyst obtained in Example 1. The thickness of the nitrogen-containing carbon film covering the platinum surface was approximately 20 nm at its thickest point. Although not shown, it was apparent that the catalyst surface was covered with a carbon film even when modified with PDA alone (see Reference 1). The reason why the electrode catalyst of Example 1 was more durable than the electrode catalyst of Comparative Example 2 is thought to be that the chelating effect of nitrogen contained in the co-adsorbed PME inhibited the migration of dissolved platinum ions into the electrolyte, thereby suppressing catalyst degradation. [Reference 1] H. Yamada et al., J. Electrochem. Soc. 167 (2020) 084508

[0068] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0069] 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. An electrode catalyst comprising: (1) The electrode catalyst is Catalyst particles; a nitrogen-containing carbon film that covers the surface of the catalyst particles; Equipped with The nitrogen-containing carbonaceous membrane contains micropores. (2) The electrode catalyst has an ECSA retention rate of 80% or more after a durability test. however, The term "ECSA" refers to electrochemical active surface area. The "ECSA maintenance rate (%)" refers to the ECSA (x 0 ) to the ratio of ECSA (x) after the durability test (= x × 100 / x 0 ) The term "after durability test" refers to the time after a potential change of 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds) was applied 10,000 cycles to a polymer electrolyte fuel cell using the electrode catalyst as a cathode catalyst.

2. 2. The electrode catalyst according to claim 1, wherein the nitrogen-containing carbonaceous film has micropores with a volume of 0.001 cc / g or more and 0.3 cc / g or less.

3. 3. The electrode catalyst according to claim 1, wherein the nitrogen-containing carbonaceous film has a thickness of 0.2 nm or more and 20 nm or less.

4. The electrode catalyst according to claim 1 , further comprising a support for supporting the catalyst particles.

5. 5. The electrode catalyst according to claim 1, wherein the catalyst particles are Pt or a Pt alloy.

6. Initial activity is 0.02 A / cm 2 The electrode catalyst according to any one of claims 1 to 5, wherein the voltage at the electrode is 0.86 V or more. Here, the "initial activity" refers to the current density at 0.86 V in the initial state (after break-in) of a polymer electrolyte fuel cell using the electrode catalyst as a cathode catalyst.

7. The activity after the durability test was 0.02 A / cm 2 7. The electrode catalyst according to claim 1, wherein the voltage Vcc is 0.86 V or more. however, The "activity after durability test" refers to the current density at 0.86 V after a durability test of a polymer electrolyte fuel cell using the electrode catalyst as a cathode catalyst, The term "after durability test" refers to the time after the polymer electrolyte fuel cell was subjected to 10,000 cycles of potential fluctuation between 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds).

8. A fuel cell electrode comprising a catalyst layer containing the electrode catalyst according to any one of claims 1 to 7 and a catalyst layer ionomer.

9. an electrolyte membrane made of a solid polymer electrolyte; Electrodes bonded to both sides of the electrolyte membrane; Equipped with 9. A fuel cell, wherein at least one of the electrodes is the fuel cell electrode according to claim 8.

10. a coating step of coating the surface of the catalyst particles with a coating containing polymelamine and polydopamine to obtain an electrode catalyst precursor; a thermal decomposition step of thermally decomposing the coating by heat-treating the electrode catalyst precursor to obtain an electrode catalyst in which the surfaces of the catalyst particles are coated with a nitrogen-containing carbon film; A method for producing an electrode catalyst comprising:

11. The coating step includes: a first coating step of coating the surface of the catalyst particles with polymelamine; a second coating step of further coating the surface of the catalyst particles with polydopamine; The method for producing an electrode catalyst according to claim 10, comprising:

12. The method for producing an electrode catalyst according to claim 10 or 11, wherein the thermal decomposition step involves thermally decomposing the coating at a temperature of 600°C or higher and 1000°C or lower.

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