Electrode catalyst, fuel cell electrode, and fuel cell

Abstract

To provide a novel electrode catalyst having high initial mass activity and a high mass activity retention rate.SOLUTION: An electrode catalyst includes a catalyst particle and a carbon film covering the surface of the catalyst particle. The carbon film includes micropores, and the average thickness of the carbon film is 3.5 nm or more and 9.5 nm or less. An electrode for a fuel cell includes a catalyst layer including such an electrode catalyst and a catalyst layer ionomer. A fuel cell includes an electrolyte membrane made of a solid polymer electrolyte and electrodes bonded to both sides of the electrolyte membrane, and at least one of the electrodes is made of such an electrode for a fuel cell.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst, a fuel cell electrode, and a fuel cell, and more particularly to an electrode catalyst in which the surfaces of catalyst particles are coated with a carbon film, and a fuel cell electrode and a fuel cell equipped 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] Patent Document 2 discloses an electrode catalyst that includes catalyst particles and a nitrogen-containing carbon film that covers the surfaces of the catalyst particles, the nitrogen-containing carbon film having micropores. The same document states: (A) 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; and (B) The electrode catalyst obtained in this manner may have improved initial activity and durability compared to an electrode catalyst coated with a polydopamine-derived carbon film. is stated.

[0008] Patent Document 3 discloses a catalyst for polymer electrolyte fuel cells obtained by mixing a catalytic metal-supported carbon material, in which a catalytic metal component is supported on a support made of a porous carbon material, with a catalytic metal-unsupported carbon material made of a dendritic graphitic carbon material. The document describes that when a catalyst layer is formed using such a catalyst, a water-repellent catalyst metal-unsupported carbon material is placed near a hydrophilic catalyst metal-supported carbon material, and therefore water vapor generated in the catalyst metal-supported carbon material is quickly discharged outside the catalyst layer via the catalyst metal-unsupported carbon material, thereby suppressing flooding.

[0009] 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.

[0010] 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.

[0011] However, the effect of the carbon film itself, which is produced by dopamine modification and heat treatment, on the catalyst performance has not been fully elucidated. In fact, Non-Patent Document 1 reports that even when a pure Pt / C catalyst was coated with dopamine and heat-treated at 700°C, the initial performance did not improve. This is thought to be because impurities introduced into the catalyst during the modification process adversely affected its performance.

[0012] 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.

[0013] Furthermore, Patent Document 2 describes that coating the surface of catalyst particles with a nitrogen-containing carbonaceous film containing micropores improves low-load activity and durability. However, in order to further improve fuel cell performance, it is desirable to further improve activity at medium and high loads.

[0014] Furthermore, Patent Document 3 describes that in order to support the catalytic metal component not only on the surface of the support but also inside the pores, the support must have many mesopores with diameters of 4 nm or more and less than 10 nm. However, in order to support catalyst particles inside the mesopores, the size of the catalyst particles and the manufacturing process may be limited. In addition, protons cannot reach catalyst particles supported deep inside the mesopores, so they cannot be used for electrode reactions. Therefore, the size of the primary particle diameter of the support may be limited. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Japanese Patent Application Publication No. 2020-136109 [Patent Document 2] Japanese Patent Publication No. 2022-142887 [Patent Document 3] Patent No. 6496531 [Non-patent literature]

[0016] [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]

[0017] The problem to be solved by the present invention is to provide a novel electrode catalyst having high initial mass activity and mass activity retention rate. Another problem to be solved by the present invention is to provide a novel electrode catalyst that has high initial activity in the middle load range and high activity retention rate in the middle load range. 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]

[0018] In order to solve the above problems, the electrode catalyst according to the present invention comprises: Catalyst particles; a carbon film covering the surface of the catalyst particle; Equipped with the carbon membrane contains micropores; The carbon film has an average thickness of 3.5 nm or more and 9.5 nm or less.

[0019] 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.

[0020] Furthermore, the fuel cell according to the present invention has 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. [Effects of the Invention]

[0021] By coating the surface of catalyst particles with polymelamine and polydopamine and then thermally decomposing the polymelamine and polydopamine, an electrode catalyst is obtained in which the surface of the catalyst particles is coated with a carbon film containing micropores. In this case, if the thickness of the carbon film is 3.5 nm or more and 9.5 nm or less, the initial mass activity is improved compared to conventional methods, and the mass activity retention rate is also equal to or better than conventional methods. Furthermore, by further optimizing the thickness of the carbon film, the initial activity in the mid-load range is equal to or better than conventional methods, and the activity retention rate in the mid-load range is also equal to or better than conventional methods. Moreover, these effects are obtained even when the catalyst particles are not supported in mesopores or when the particle size of the catalyst particles is relatively large.

[0022] this is, (a) The carbon film suppresses the grain growth of catalyst particles during heat treatment. (b) By making the thickness of the carbon film 3.5 nm or more, the contact area between the catalyst particle surface and the ionomer is reduced, thereby suppressing ionomer poisoning of the catalyst particles, as well as dissolution, aggregation, and / or detachment of the catalyst particles; and (c) By making the carbon film thickness 9.5 nm or less, the increase in transport resistance of reactants to the catalyst particle surface is suppressed. It is thought that... [Brief explanation of the drawings]

[0023] [Figure 1] FIG. 1 is a graph showing the relationship between the thickness of the carbon film and the initial mass activity. [Figure 2] FIG. 10 is a graph showing the relationship between the thickness of the carbon film and the mass activity retention rate. [Figure 3] FIG. 10 is a graph showing the relationship between the thickness of the carbon film and the initial activity in the middle load range. [Figure 4] FIG. 10 is a graph showing the relationship between the thickness of the carbon film and the activity retention rate in the medium load range.

[0024] An embodiment of the present invention will be described in detail below. [1. Electrocatalyst] The electrode catalyst according to the present invention has the following configuration.

[0025] [Configuration 1] Catalyst particles; a carbon film covering the surface of the catalyst particle; Equipped with the carbon membrane contains micropores; The average thickness of the carbon film is 3.5 nm or more and 9.5 nm or less. Electrocatalyst.

[0026] [Configuration 2] 2. The electrocatalyst of claim 1, wherein the initial mass activity is 600 A / g or greater.

[0027] [Configuration 3] 3. The electrode catalyst according to aspect 1 or 2, wherein the mass activity retention rate is 50% or more.

[0028] [Configuration 4] Initial mid-load activity is 1.0A / cm 2 4. The electrocatalyst of any one of configurations 1 to 3, wherein the voltage is 0.6V or greater.

[0029] [Configuration 5] 5. The electrode catalyst according to any one of configurations 1 to 4, wherein the activity retention rate in a medium load range is 80% or more.

[0030] [Configuration 6] 6. The electrode catalyst according to any one of configurations 1 to 5, wherein the carbon film has micropores with a volume of 0.001 cc / g or more and 0.3 cc / g or less.

[0031] [Configuration 7] 7. The electrode catalyst according to any one of configurations 1 to 6, further comprising a carbon support that supports the catalyst particles.

[0032] [Configuration 8] 8. The electrocatalyst of any one of aspects 1 to 7, wherein the catalyst particles are Pt or a Pt alloy.

[0033] 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.

[0034] 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.

[0035] [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.

[0036] 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. The material of the support is not particularly limited, but a carbon support is preferred, such as carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[0037] 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 %.

[0038] [1.3. Carbon Film] [1.3.1. Composition] The surface of the catalyst particle is coated with a carbon film. The "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.

[0039] 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 carbon film containing micropores and a higher nitrogen content than polydopamine-derived carbon films. The nitrogen contained in the carbon film is derived from polymelamine. The nitrogen content in the carbon film is typically 0.1 mass% to 35 mass%, depending on the production method.

[0040] The 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 carbon membrane obtained by the sequential modification method exhibits higher properties than the carbon membrane obtained by the simultaneous modification method.

[0041] 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.

[0042] As mentioned above, thermal decomposition of polydopamine in the presence of polymelamine results in a 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.

[0043] Thickness The thickness of the carbon film affects the stability and activity of the catalyst particles. If the carbon film 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 carbon film is too thin, the catalyst particles are likely to be poisoned by the catalyst layer ionomer. Therefore, the carbon film thickness must be 3.5 nm or more. The carbon film thickness is preferably 4.0 nm or more.

[0044] On the other hand, if the carbon membrane is too thick, the transport resistance of the reactants increases, which may result in a decrease in activity. Therefore, the thickness of the carbon membrane must be 9.5 nm or less. The thickness of the carbon membrane is preferably 9.0 nm or less, 8.5 nm or less, 8.0 nm or less, 7.5 nm or less, or 7.0 nm or less.

[0045] [1.4. Characteristics] 1.4.1. Initial Mass Activity "Initial mass activity (A / g)" refers to the current value per platinum mass at 0.86 V in the initial state (after break-in) of a polymer electrolyte fuel cell using the electrode catalyst of the present invention as the cathode catalyst. In the electrocatalyst according to the present invention, when the manufacturing conditions are optimized, the initial mass activity becomes 600 A / g or more, and when the manufacturing conditions are further optimized, the initial mass activity becomes 650 A / g or more, 700 A / g or more, 750 A / g or more, or 800 A / g or more.

[0046] [1.4.2. Mass activity maintenance rate] "Mass activity retention rate (%)" refers to the ratio of mass activity (MA) after durability testing to the initial mass activity (MA0) (=MA×100 / MA0). "Mass activity after durability test" refers to the current value per platinum mass 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. The "durability test" refers to a test in which a potential change of 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds) is applied to the fuel cell for 10,000 cycles. When the manufacturing conditions of the electrode catalyst according to the present invention are optimized, the mass activity retention rate becomes 50% or more.

[0047] [1.4.3. Early mid-load activity] Initial mid-load activity (A / cm 2 "@0.6V)" refers to the current density at 0.6V 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 in the medium load range is 1.0 A / cm 2 By optimizing the manufacturing conditions, the initial mid-load range activity is 1.2A / cm 2 @0.6V or above, or 1.5A / cm 2 @0.6V or higher.

[0048] [1.4.4. Medium load range activity maintenance rate] "Mid-load range activity retention rate (%)" refers to the ratio of mid-load range activity (I) after the durability test to the initial mid-load range activity (I0) (=I×100 / I0). "Medium load range activity after durability test" refers to the current density at 0.6 V after a durability test of a polymer electrolyte fuel cell using the electrode catalyst according to the present invention as the cathode catalyst. The "durability test" refers to a test in which a potential change of 0.6 V (held for 3 seconds) and 1.0 V (held for 3 seconds) is applied to the fuel cell for 10,000 cycles. In the electrode catalyst according to the present invention, when the manufacturing conditions are optimized, the activity retention rate in the medium load range is 80% or more, and when the manufacturing conditions are further optimized, the activity retention rate in the medium load range is 85% or more.

[0049] [2. Electrodes for fuel cells] The fuel cell electrode according to the present invention has the following configuration.

[0050] [Configuration 9] A fuel cell electrode comprising a catalyst layer containing the electrode catalyst according to any one of configurations 1 to 8 and a catalyst layer ionomer.

[0051] [2.1. Electrocatalyst] The catalyst layer contains the electrode catalyst according to the present invention. Details of the electrode catalyst are as described above, and therefore will not be described here.

[0052] [2.2. Catalyst Layer Ionomer] The catalyst layer contains a catalyst layer ionomer. In the present invention, the type of catalyst layer ionomer is not particularly limited, and an optimum material can be selected depending on the purpose.

[0053] 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.

[0054] 2.3. Diffusion Layer The fuel cell electrode according to the present invention may consist of only a catalyst layer, or may be a laminate of a catalyst layer and a diffusion layer. The diffusion layer is disposed on the surface of the catalyst layer facing the separator. In the present invention, the type of the diffusion layer is not particularly limited, and an optimum one can be selected depending on the purpose. Generally, carbon paper, carbon cloth, etc. are used as the diffusion layer.

[0055] [3. Fuel cell] The fuel cell according to the present invention has the following configuration.

[0056] [Configuration 10] an electrolyte membrane made of a solid polymer electrolyte; Electrodes bonded to both sides of the electrolyte membrane; Equipped with 10. A fuel cell, wherein at least one of the electrodes is the fuel cell electrode according to aspect 9.

[0057] [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).

[0058] [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.

[0059] 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 carbon film; It is equipped with:

[0060] [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).

[0061] 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.

[0062] [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.

[0063] 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).

[0064] 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).

[0065] 4.1.3. Amount of polymelamine and polydopamine added The amounts of polymelamine and polydopamine to be added are not particularly limited, and can be selected optimally depending on the purpose. By changing the amounts of these added, the thickness and nitrogen content of the carbon film can be controlled.

[0066] 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)

[0067] If the amount of polymelamine added is too small, durability will decrease. Furthermore, the amount of micropores generated will decrease, resulting in a decrease in performance. Therefore, the amount of polymelamine added is preferably 20 mass% or more. The amount of polymelamine added is more preferably 30 mass% or more, 40 mass% or more, or 50 mass% or more. On the other hand, if the amount of polymelamine added is excessive, the carbon membrane may become too thick and its performance may deteriorate. Therefore, the amount of polymelamine added is preferably 90 mass% or less. The amount of polymelamine added is more preferably 85 mass% or less.

[0068] 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.

[0069] [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 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.

[0070] [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.

[0071] In contrast, if the surface of catalyst particles (active species) is coated with polymelamine and polydopamine and then thermally decomposed, an electrode catalyst is obtained in which the surface of the catalyst particles is coated with a carbon film containing micropores. When the surface of the catalyst particles is covered with a carbon film, the contact area between the catalyst particles and the ionomer is reduced. As a result, ionomer poisoning (a phenomenon in which sulfonic acid groups in the ionomer adsorb to the platinum surface, reducing active sites) is suppressed, improving catalytic activity. Furthermore, dissolution of the catalyst particles is suppressed, improving durability. In other words, coating the surface of the catalyst particles with a carbon film derived from polymelamine and polydopamine suppresses dissolution of the catalyst particles and improves durability without reducing power generation performance.

[0072] In this case, if the carbon film thickness is set to 3.5 nm or more and 9.5 nm or less, the initial mass activity is improved compared to conventional methods, and the mass activity retention rate is equal to or better than conventional methods. Furthermore, if the carbon film thickness is further optimized, the initial activity in the mid-load range is equal to or better than conventional methods, and the activity retention rate in the mid-load range is also equal to or better than conventional methods.

[0073] this is, (a) The carbon film suppresses the grain growth of catalyst particles during heat treatment. (b) By making the thickness of the carbon film 3.5 nm or more, the contact area between the catalyst particle surface and the ionomer is reduced, thereby suppressing ionomer poisoning of the catalyst particles, as well as dissolution, aggregation, and / or detachment of the catalyst particles; and (c) By making the carbon film thickness 9.5 nm or less, the increase in transport resistance of reactants to the catalyst particle surface is suppressed. It is thought that...

[0074] The modification of catalyst particles with polydopamine and polymelamine can be applied to the support on which the catalyst particles are supported, or to the catalyst particles themselves. Furthermore, because micropores are introduced into the carbon film during the heat treatment process after modification, there is no limit to the size of the catalyst particles to which this method can be applied. Furthermore, the thickness of the carbon film can be changed by adjusting the amount of polydopamine and polymelamine added. As a result, carbon films thick enough to allow protons to reach the surface of the catalyst particles can be easily fabricated. [Example]

[0075] (Examples 1 to 2, Comparative Examples 1 to 4) 1. Sample Preparation 1.1. Preparation of electrode catalysts 1.1.1. Examples 1-2 and Comparative Examples 2-4: Sequentially Modified Materials The catalyst used was 30 mass% Pt / Vulcan® (TEC10V30E) (hereinafter simply referred to as "Pt / C catalyst"). This 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 for 2 hours.

[0076] Next, the dried Powder A, dopamine hydrochloride, and Tris-HCl buffer adjusted to pH 8.5 were placed in a beaker and ultrasonically dispersed. The dispersion was then stirred for 6 hours at room temperature in the air using a stirrer. This allowed the dopamine to polymerize and become polydopamine (PDA).

[0077] 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.

[0078] The modification amount of the electrode catalyst (the ratio of the mass of the carbon film to the total mass of the electrode catalyst) was adjusted by the amounts of PME and PDA added. The amount of PME added (= the ratio of the mass of PME to the total mass of the Pt / C catalyst and PME) was 41.2 mass% (Comparative Example 2), 73.2 mass% (Comparative Example 3), 57.2 mass% (Example 1), 73.2 mass% (Example 2), or 93.1 mass% (Comparative Example 4), respectively. The amount of dopamine added (= the mass ratio of dopamine to the total mass of Pt / C catalyst and dopamine, the mass ratio of dopamine excluding hydrochloric acid) was 1.2 mass% (Comparative Example 2), 4.8 mass% (Comparative Example 3), 24.5 mass% (Example 1), 24.2 mass% (Example 2), or 22.9 mass% (Comparative Example 4), respectively.

[0079] 1.1.2. Comparative Example 1: Unmodified Material The Pt / C catalyst was directly subjected to the test.

[0080] 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.

[0081] 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.

[0082] 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.

[0083] 2. Test Method [2.1. Content and average thickness of carbon film] The contents (modification amounts) of Pt, the carrier, and the carbon film were evaluated by thermogravimetric analysis (TG). The electrode catalyst with the highest modification amount (Comparative Example 4) was observed with a transmission electron microscope (TEM) to evaluate the average thickness of the carbon film. Furthermore, the average thickness of the carbon film of each sample was calculated based on the modification amount and average thickness.

[0084] [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).

[0085] [3. Results] [3.1. Content and average thickness of carbon film] Table 1 shows the contents of Pt, the carrier, and the carbon film, as well as the average thickness of the carbon film.

[0086] [Table 1]

[0087] [3.2. Initial power generation performance and power generation performance after endurance] 3.2.1. Initial Mass Activity Figure 1 shows the relationship between carbon film thickness and initial mass activity. It was found that all electrode catalysts modified with carbon films had higher initial mass activity than unmodified catalysts. It was also found that the initial mass activity was maximized when the carbon film thickness was approximately 6 nm. Furthermore, it was found that the initial mass activity was approximately 600 A / g or more when the carbon film thickness was 3.5 to 9.5 nm.

[0088] [3.2.2. Mass activity maintenance rate] Figure 2 shows the relationship between carbon membrane thickness and mass activity retention. There was a tendency for the mass activity retention to increase as the carbon membrane became thicker. This is thought to be because the contact area between the catalyst particles and the ionomer decreased as the carbon membrane became thicker, suppressing catalyst particle degradation.

[0089] [3.2.3. Medium load range activity] Figure 3 shows the relationship between carbon film thickness and initial activity in the medium load range. It was found that the initial activity in the medium load range was maximized when the carbon film thickness was 3 nm. It was also found that the initial activity in the medium load range dropped sharply when the carbon film thickness exceeded 6 nm. This is thought to be because the transport resistance of reactants increases when the carbon film thickness exceeds 6 nm.

[0090] [3.2.4. Medium load range activity maintenance rate] Figure 4 shows the relationship between the thickness of the carbon membrane and the activity retention rate in the medium load range. There was a tendency for the activity retention rate in the medium load range to increase as the thickness of the carbon membrane increased. This is because (a) As the thickness of the carbon film increases, the contact area between the catalyst particles and the ionomer decreases, suppressing the deterioration of the catalyst particles, or (b) The distance between the catalyst particles and the ionomer is large, causing a local increase in pH inside the carbon film. It is thought that...

[0091] 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]

[0092] 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. Catalyst particles; a carbon film covering the surface of the catalyst particle; Equipped with the carbon membrane contains micropores; The average thickness of the carbon film is 3.5 nm or more and 9.5 nm or less. Electrocatalyst.

2. 2. The electrocatalyst according to claim 1, having an initial mass activity of 600 A / g or more.

3. 2. The electrode catalyst according to claim 1, wherein the mass activity retention rate is 50% or more.

4. Initial mid-load activity is 1.0 A / cm 2 The electrode catalyst according to claim 1, wherein the voltage is 0.6 V or more.

5. 2. The electrode catalyst according to claim 1, wherein the activity retention rate in a medium load range is 80% or more.

6. 2. The electrode catalyst according to claim 1, wherein the volume of the micropores in the carbon film is 0.001 cc / g or more and 0.3 cc / g or less.

7. 2. The electrode catalyst according to claim 1, further comprising a carbon support that supports the catalyst particles.

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

9. 9. A fuel cell electrode comprising a catalyst layer comprising the electrode catalyst according to claim 1 and a catalyst layer ionomer.

10. an electrolyte membrane made of a solid polymer electrolyte; Electrodes bonded to both sides of the electrolyte membrane; Equipped with 10. A fuel cell, wherein at least one of the electrodes is the fuel cell electrode according to claim 9.

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