Electrode catalyst and method for producing the same

Abstract

To provide an electrode catalyst that exhibits high initial mass activity and can suppress the degradation of mass activity over time, and a method for producing the same. [Solution] The electrode catalyst comprises catalyst particles and a carbon film coating the surface of the catalyst particles, the carbon film containing micropores. The electrode catalyst has a CO adsorption ratio ΔV of 30% to 80%. Such an electrode catalyst is obtained by coating the surface of the catalyst particles with a first film containing a predetermined amount of polymelamine to form first coated particles, further coating the surface of the first film with a second film containing a predetermined amount of polydopamine to form second coated particles, and heat-treating the second coated particles under predetermined conditions.

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 having high initial mass activity and suppressed degradation of mass activity over time, and a method for producing the same. [Background technology]

[0002] A polymer electrolyte fuel cell (MLFC) comprises a membrane electrode assembly (MEA) in which electrodes containing catalysts are bonded to both sides of an electrolyte membrane. Separators with gas channels are further positioned on both sides of the MEA. A typical MFC has a structure (fuel cell stack) in which multiple single cells, each consisting of such an MEA and current collector, are stacked.

[0003] Fuel cell electrodes typically consist of a laminate 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 a carrier surface, and a catalyst layer ionomer. Electrode reactions mainly occur on the surface of the catalyst particles. Therefore, efforts are 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 fuel cells, which involves potential fluctuations, the finer the catalyst particles become, the more likely they are to dissolve, coarseen due to aggregation, and / or detach from the support. As a result, there is a problem in that the activity of the catalyst layer gradually decreases.

[0004] Therefore, various proposals have been made to solve this problem. For example, Patent Document 1 contains: (a) A dispersion of dopamine aqueous solution with Pt / C added is dried. (b) Heat the obtained solid powder in a nitrogen atmosphere. The HCM / Pt / C obtained by this process is disclosed. The document describes how this method can be used to coat the surface of an active species (Pt) with a porous carbon film (HCM) containing heteroatoms.

[0005] Patent Document 2 contains: (a) Pt is supported on the surface of a carbon support to form Pt / C, (b) The surface of Pt is coated with polydopamine to form Pt / C-PDA, (c) The polydopamine coating layer is impregnated with nickel nitrate to form Ni-impregnated Pt / C-PDA. (d) Heat-treat Ni-impregnated Pt / C-PDA at 900°C for 1 hour in a 90% Ar + 10% H2 atmosphere. A carbon alloy catalyst supported by (Pt2Ni1 / C) obtained by this method is disclosed. The document states that this method can yield an alloy catalyst having a core-shell structure in which the surface is made solely of Pt and the interior is made of a PtNi alloy.

[0006] Patent Document 3 contains: (a) Mix partially oxidized tantalum carbonitride with pyrene, (b) The mixture is carbonized at 600°C under a nitrogen atmosphere. A carbon-coated catalyst material obtained by this process is disclosed. The document states that using an organic compound having a structure in which three to five benzene rings are bonded (for example, pyrene), it is possible to graphitize the organic compound even at temperatures below 1000°C.

[0007] Patent Document 4 contains: (a) The surface of the catalyst particles is coated with a film containing polymelamine and polydopamine, (b) Heat treatment of the obtained electrode catalyst precursor An electrode catalyst obtained by this process is disclosed. The document states that the electrode catalyst obtained in this manner may exhibit improved initial activity and durability compared to electrode catalysts coated with a carbon film derived from polydopamine.

[0008] Furthermore, Patent Document 5 discloses an electrode catalyst including catalyst particles and a carbon film covering the surface of the catalyst particles, where the carbon film contains micropores and has an average thickness of 3.5 nm or more and 9.5 nm or less. It is described in the same document that when 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 the conventional case, and the mass activity retention rate is equal to or higher than the conventional case.

[0009] In an electrode for a fuel cell, the surface of the catalyst particles is covered with an ionomer. Although the ionomer is a component necessary for proton conduction, the sulfonic acid group of the ionomer serves as a catalyst poisoning source. Therefore, when the surface of the catalyst particles is covered with an ionomer, a decrease in activity is likely to occur. In addition, when the electrode for a fuel cell is used for a long time, the mass activity deteriorates with time. This is considered to be caused by a decrease in the surface area of the catalyst particles and catalyst poisoning.

[0010] On the other hand, as described in Patent Documents 1 to 5, when the surface of the catalyst particles is covered with a carbon film, the contact between the catalyst particles and the sulfonic acid group is suppressed, and a decrease in activity due to catalyst poisoning and a decrease in the surface area of the catalyst particles can be suppressed to some extent. However, in order to further improve the performance of the fuel cell, it is desired to further improve the initial mass activity of the electrode catalyst and further suppress the deterioration of the mass activity with time.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

[0012] The problem that this invention aims to solve is to provide an electrode catalyst with high initial mass activity and suppressed degradation of mass activity over time, as well as a method for producing the same. [Means for solving the problem]

[0013] To solve the above problems, the electrode catalyst according to the present invention is Catalyst particles and A carbon film covering the surface of the catalyst particles and Equipped with, The carbon film includes micropores, The CO adsorption ratio ΔV, expressed by the following equation (1), is between 30% and 80%.

[0014] ΔV(%) = (V1 - V2) × 100 / V1 …(1) however, V1 is the amount of CO adsorbed per unit mass of the catalyst particles, measured at room temperature for the catalyst particles that are not coated with the carbon film. V2 is the amount of CO adsorbed per unit mass of the catalyst particles coated with the carbon film, measured at -80°C.

[0015] The method for producing an electrode catalyst according to the present invention is: A first step is to coat the surface of catalyst particles or catalyst particles supported on a carrier with a first coating containing polymelamine to obtain first coated particles, A second step involves coating the surface of the first coated particle with a second coating containing polydopamine to obtain a second coated particle, A third step involves heat-treating the second coated particles to thermally decompose the first and second coatings, thereby obtaining an electrode catalyst in which the surface of the catalyst particles is coated with a carbon film. Equipped with, The first step includes coating the surface of the catalyst particles or the catalyst particles supported on the carrier with the first coating such that the amount of polymelamine added is 55 mass% or more and 85 mass% or less. The second step includes coating the surface of the first coated particles with the second coating so that the amount of polydopamine added is 20 mass% or more and 30 mass% or less. The third step includes heat-treating the second coated particles in an inert atmosphere at a temperature of 600°C to 1000°C.

[0016] however, The "amount added" of the polymelamine or polydopamine refers to the value represented by the following formula (4). Addition amount (mass%)=y×100 / (y+z) …(4) y is the mass of the polymelamine or polydopamine contained in the second coated particle. z is the mass of the catalyst particles contained in the second coated particle (or, if the catalyst particles are supported on the carrier, the total mass of the catalyst particles and the carrier). [Effects of the Invention]

[0017] When the surface of catalyst particles 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 particles is coated with a carbon film containing micropores. In this case, optimizing the manufacturing conditions so that the CO adsorption ratio ΔV is within a predetermined range improves the initial mass activity of the electrode catalyst and suppresses the degradation of mass activity over time. This is thought to be because, by setting ΔV within a predetermined range, the surface of the catalyst particles is coated with an appropriate amount of carbon film having an appropriate structure. [Brief explanation of the drawing]

[0018] [Figure 1] These are the pretreatment conditions for CO pulse measurement. [Figure 2] This is the CO adsorption ratio ΔV of the electrode catalysts obtained in Examples 1-4 and Comparative Examples 1-3. [Figure 3] These are the micropore capacities of the electrode catalysts obtained in Examples 1-4 and Comparative Examples 1-5. [Figure 4] This represents the micropore capacity of the carbon film of the electrode catalyst obtained in Examples 1-4 and Comparative Examples 1-3.

[0019] [Figure 5] This figure shows the relationship between the CO adsorption ratio ΔV and the initial activity improvement ratio. [Figure 6] This figure shows the relationship between the CO adsorption ratio ΔV and the activity improvement ratio after the durability test. [Figure 7] The ECSA maintenance rates ΔS (relative humidity: 30%RH, 80%RH) of the electrode catalysts obtained in Examples 1-4 and Comparative Examples 1-5 are shown. [Modes for carrying out the invention]

[0020] [Configuration 1] Catalyst particles and A carbon film covering the surface of the catalyst particles and Equipped with, The carbon film includes micropores, The CO adsorption ratio ΔV, expressed by the following equation (1), is between 30% and 80%. Electrocatalyst.

[0021] ΔV(%) = (V1 - V2) × 100 / V1 …(1) however, V1 is the amount of CO adsorbed per unit mass of the catalyst particles, measured at room temperature for the catalyst particles that are not coated with the carbon film. V2 is the amount of CO adsorbed per unit mass of the catalyst particles coated with the carbon film, measured at -80°C.

[0022] [Configuration 2] The electrode catalyst according to configuration 1, wherein the CO adsorption ratio ΔV is 45% or more and 58% or less.

[0023] [Configuration 3] The electrode catalyst according to Configuration 1 or 2, further comprising a carrier for supporting the catalyst particles.

[0024] [Configuration 4] The electrode catalyst according to Configuration 3, wherein the pore volume of the electrode catalyst is 0.025 mL / g or more and 0.045 mL / g or less.

[0025] [Configuration 5] The ECSA retention rate ΔS represented by the following formula (2) L , and the ECA retention rate ΔS represented by the following formula (3) H The electrode catalyst according to any one of Configurations 1 to 4, wherein each is 60% or more. ΔS L =S L2 ×100 / S L1 …(2) ΔS H =S H2 ×100 / S H1 …(3)

[0026] However, S L1 is the electrochemically active surface area measured under the condition of 3 x 100% RH for the electrode catalyst before the durability test, S L2 is the electrochemically active surface area measured under the condition of 3 x 100% RH for the electrode catalyst after the durability test, S H1 is the electrochemically active surface area measured under the condition of 80% RH for the electrode catalyst before the durability test, S H2 is the electrochemically active surface area measured under the condition of 80% RH for the electrode catalyst after the durability test, The "durability test" is a test in which a rectangular wave of 0.6 V (vs. RHE, 3 s) - 1.0 V (vs. RHE, 3 s) is applied to a membrane electrode assembly in which the electrode catalyst is added to a cathode catalyst layer for 10,000 cycles under the condition of 80% RH.

[0027] [Configuration 6] A first step is to coat the surface of catalyst particles or catalyst particles supported on a carrier with a first coating containing polymelamine to obtain first coated particles, A second step involves coating the surface of the first coated particle with a second coating containing polydopamine to obtain a second coated particle, A third step involves heat-treating the second coated particles to thermally decompose the first and second coatings, thereby obtaining an electrode catalyst in which the surface of the catalyst particles is coated with a carbon film. Equipped with, The first step includes coating the surface of the catalyst particles or the catalyst particles supported on the carrier with the first coating such that the amount of polymelamine added is 55 mass% or more and 85 mass% or less. The second step includes coating the surface of the first coated particles with the second coating so that the amount of polydopamine added is 20 mass% or more and 30 mass% or less. The third step includes heat-treating the second coated particles in an inert atmosphere at a temperature of 600°C to 1000°C. A method for manufacturing an electrode catalyst.

[0028] however, The "amount added" of the polymelamine or polydopamine refers to the value represented by the following formula (4). Addition amount (mass%)=y×100 / (y+z) …(4) y is the mass of the polymelamine or polydopamine contained in the second coated particle. z is the mass of the catalyst particles contained in the second coated particle (or, if the catalyst particles are supported on the carrier, the total mass of the catalyst particles and the carrier).

[0029] [Composition 7] The first step includes coating the surface of the catalyst particles or the catalyst particles supported on the carrier with the first coating so that the amount of polymelamine added is 75 mass% or more and 85 mass% or less. A method for producing an electrode catalyst as described in configuration 6.

[0030] One embodiment of the present invention will be described in detail below. [1. Electrocatalyst] The electrode catalyst according to the present invention is Catalyst particles and A carbon film covering the surface of the catalyst particles and It is equipped with. The electrode catalyst may further include a carrier that supports the catalyst particles.

[0031] [1.1. Catalyst particles] [1.1.1. Composition] In the present invention, the material of the catalyst particles is not particularly limited. 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] [1.1.2. 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. Carrier] [1.2.1. Materials] Catalyst particles can be used in their original state for various applications, or they can be used supported on a carrier surface. Supporting catalyst particles on a carrier surface allows for stable dispersion of fine catalyst particles, thus reducing the amount of catalyst used. Examples of carriers include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[0035] [1.2.2. Catalyst Loading Amount] When catalyst particles are supported on a carrier surface, the amount of catalyst supported is not particularly limited, and the optimal amount can be selected according to the purpose. Generally, if the amount of catalyst supported is too small, sufficient activity cannot be obtained. On the other hand, if the amount of catalyst supported is excessively large, 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 mass% to 70 mass%.

[0036] [1.3. Carbon film] [1.3.1. Composition] The surface of the catalyst particles is coated with a carbon film. In this invention, "carbon film" refers to a film obtained by coating the surface of the catalyst particles with a film containing polymelamine and polydopamine, and then thermally decomposing the film.

[0037] When a coating containing only polydopamine is thermally decomposed, a dense carbon film consisting substantially of only carbon is obtained. On the other hand, when a coating containing both polymelamine and polydopamine is thermally decomposed, a carbon film containing micropores and having a higher nitrogen content than the carbon film derived from polydopamine is obtained. The nitrogen contained in the carbon film originates from the polymelamine. The amount of nitrogen contained in the carbon film varies depending on the manufacturing method, but is usually between 0.1 mass% and 35 mass%.

[0038] Methods for forming a carbon film include: (a) A method in which the surface of catalyst particles is coated with a film consisting of a mixture of polymelamine and polydopamine, and the film is thermally decomposed (simultaneous modification method), (b) A method in which the surface of catalyst particles is coated with polymelamine, then the surface of the catalyst particles is coated with polydopamine, and then the laminated film of polymelamine and polydopamine is thermally decomposed (sequential modification method). These are some examples. To obtain an electrode catalyst exhibiting high activity, the carbon film is preferably obtained by a sequential modification method.

[0039] [1.3.2. Micropores] A "micropore" refers to a pore with a diameter of 2 nm or less. As described above, when polydopamine is thermally decomposed in the presence of polymelamine, a carbon film with micropores is obtained. If the volume of micropores in the carbon film becomes too small, mass transfer to the catalyst particle surface may be inhibited, and the initial activity may decrease. Therefore, the volume of micropores in the carbon film is preferably 0.010 mL / g or more. On the other hand, if the volume of micropores in the carbon film becomes excessive, the catalyst particles may dissolve, aggregate, and / or desorb, or the catalyst particles may be poisoned by ionoma. Therefore, the volume of micropores in the carbon film is preferably 0.020 mL / g or less.

[0040] [1.3.3. Thickness] The thickness of the carbon film affects the stability and activity of the catalyst particles. If the carbon film is too thin, the catalyst particles are more prone to dissolution, aggregation, and / or desorption. Furthermore, when such an electrode catalyst is applied to a fuel cell electrode, if the carbon film is too thin, the catalyst particles are more susceptible to poisoning by the catalyst layer ionomer. Therefore, a carbon film thickness of 0.2 nm or more is preferred. Preferably, the carbon film thickness is 0.5 nm or more. On the other hand, if the carbon film thickness becomes too thick, the transport resistance of the reactants increases, which may reduce the activity. Therefore, the carbon film thickness is preferably 20 nm or less.

[0041] [1.4. Characteristics] [1.4.1. CO adsorption ratio ΔV] "CO adsorption ratio ΔV" refers to the value expressed by the following equation (1). ΔV(%) = (V1 - V2) × 100 / V1 …(1) however, V1 is the amount of CO adsorbed per unit mass of the catalyst particles, measured at room temperature for the catalyst particles that are not coated with the carbon film. V2 is the amount of CO adsorbed per unit mass of the catalyst particles coated with the carbon film, measured at -80°C.

[0042] When catalyst particles are brought into contact with CO gas at room temperature before their surface is coated with a carbon film, CO is adsorbed onto almost the entire surface of the catalyst particles. On the other hand, when catalyst particles are brought into contact with CO gas at -80°C after their surface has been coated with a carbon film, CO is adsorbed only onto the surface of the catalyst particles that are not coated with the carbon film. Therefore, ΔV has a positive correlation with the coverage rate of the catalyst particle surface by the carbon film.

[0043] If ΔV becomes too small, the coverage by the carbon film becomes excessively small. As a result, the catalyst particles may be poisoned by the ionomer, and the initial activity may decrease. Also, when using an electrode catalyst, the catalyst particles are exposed to a strongly acidic atmosphere, causing dissolution and reprecipitation of the catalyst particles, and the surface area of ​​the catalyst particles gradually decreases. Therefore, ΔV needs to be 30% or more. Preferably, ΔV is 35% or more, 40% or more, or 45% or more.

[0044] On the other hand, if ΔV becomes too large, the coverage by the carbon film becomes excessively large. As a result, the transport resistance of the reactants increases, and the activity may actually decrease. Therefore, ΔV needs to be 80% or less. Preferably, ΔV is 75% or less, 60% or less, or 58% or less. In particular, when ΔV is between 45% and 58%, it exhibits high initial activity, high post-durability test activity, and a high ECSA maintenance rate, regardless of the relative humidity in the atmosphere.

[0045] [1.4.2. Micropore Capacity] The "micropore capacity" of an electrode catalyst refers to a value obtained by measuring the nitrogen adsorption amount of the electrode catalyst and analyzing it using the t-plot method with solid carbon as the reference isotherm. Furthermore, if the electrode catalyst includes a support and the support also has micropores, the "micropore capacity" of the electrode catalyst refers to the sum of the micropore capacity contained in the carbon film and the micropore capacity contained in the support.

[0046] As described above, when polydopamine is thermally decomposed in the presence of polymelamine, a carbon film with micropores is obtained. If the volume of micropores becomes too small, mass transfer to the catalyst particle surface may be inhibited, and the initial activity may decrease. Therefore, the volume of micropores is preferably 0.025 mL / g or more. More preferably, the volume of micropores is 0.030 mL / g or more, or 0.035 mL / g or more.

[0047] On the other hand, if the volume of micropores becomes excessive, dissolution, aggregation, and / or desorption of catalyst particles, or ionomatous poisoning of catalyst particles may occur. Therefore, the volume of micropores is preferably 0.045 mL / g or less. More preferably, the volume of micropores is 0.043 mL / g or less, or 0.040 mL / g or less.

[0048] [1.4.3. ECSA retention rate] ECSA maintenance rate ΔS under low humidity conditions L ", and "ECSA maintenance rate ΔS under high humidity conditions H " refers to the values ​​expressed by the following equations (2) and (3), respectively. ΔS L =S L2 ×100 / S L1 …(2) ΔS H =S H2 ×100 / S H1 …(3)

[0049] however, S L1This refers to the electrochemical surface area of ​​the electrode catalyst measured under 30% RH conditions before the durability test. S L2 The electrochemical surface area of ​​the electrode catalyst after the durability test was measured under conditions of 30% RH. S H1 This refers to the electrochemical surface area of ​​the electrode catalyst measured under 80% RH conditions before the durability test. S H2 The electrochemical surface area of ​​the electrode catalyst after the durability test, measured under 80% RH conditions, The aforementioned "durability test" is a test in which a rectangular wave of 0.6V (vs. RHE, 3s)-1.0V (vs. RHE, 3s) is applied for 10,000 cycles to a film electrode assembly in which the electrode catalyst is added to the cathode catalyst layer, under 80% RH conditions.

[0050] When ΔV is optimized in the electrode catalyst according to the present invention, the electrode catalyst exhibits high durability under both low humidity and high humidity conditions. When the manufacturing conditions are optimized, ΔS L and ΔS H These are each over 60%. Further optimization of the manufacturing conditions will result in ΔS L and ΔS H These figures are 70% or higher, 73% or higher, and 74% or higher, respectively.

[0051] [2. Electrodes for fuel cells] The electrode for a fuel cell according to the present invention comprises a catalyst layer including the electrode catalyst according to the present invention and a catalyst layer ionomer.

[0052] [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 will therefore be omitted.

[0053] [2.2. Catalyst layer ionomer] The catalyst layer includes a catalyst layer ionomer. In this invention, the type of catalyst layer ionomer is not particularly limited, and the most suitable material can be selected depending on the purpose.

[0054] The content of catalyst layer ionomer in the catalyst layer is not particularly limited, and the optimal content can be selected according to the purpose. For example, when catalyst particles are supported on a carbon support, the ratio of the weight of catalyst layer ionomer (I) to the weight of carbon (C) contained in the support and carbon film (=I / C) is preferably 0.3 or more and 2.0 or less.

[0055] [2.3. Diffusion layer] The fuel cell electrode according to the present invention may consist only of a catalyst layer, or it may be a laminate of a catalyst layer and a diffusion layer. The diffusion layer is arranged on the separator-side surface of the catalyst layer. In this invention, the type of diffusion layer is not particularly limited, and the most suitable one can be selected depending on the purpose. Generally, carbon paper, carbon cloth, etc., are used as the diffusion layer.

[0056] [3. Fuel cell] The fuel cell according to the present invention is An electrolyte membrane made of a solid polymer electrolyte, Electrodes bonded to both sides of the electrolyte membrane and It is equipped with.

[0057] [3.1. Electrolyte membrane] The electrolyte membrane consists of a solid polymer electrolyte. The composition of the solid polymer electrolyte is not particularly limited, and the optimal material can be selected according to the purpose. Examples of solid polymer electrolytes include Nafion®, Flemion®, Aciplex®, and Aquivion®.

[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 made of 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 preferred for use as a cathode. This is because the effect of the presence or absence of a coating on the initial activity and durability of the fuel cell is greater at the cathode than at the anode.

[0059] [4. Method for manufacturing electrode catalysts] The method for producing an electrode catalyst according to the present invention is: A first step is to coat the surface of catalyst particles or catalyst particles supported on a carrier with a first coating containing polymelamine to obtain first coated particles, A second step involves coating the surface of the first coated particle with a second coating containing polydopamine to obtain a second coated particle, A third step involves heat-treating the second coated particles to thermally decompose the first and second coatings, thereby obtaining an electrode catalyst in which the surface of the catalyst particles is coated with a carbon film. It is equipped with.

[0060] [4.1. 1st step] First, the surface of catalyst particles or catalyst particles supported on a carrier is coated with a first coating containing polymelamine to obtain first coated particles (first step).

[0061] Details regarding the catalyst particles and support are as described above, so we will omit further explanation. Specifically, the first coating is formed by adding catalyst particles or catalyst particles supported on a carrier to a solution in which polymelamine is dissolved, stirring for a predetermined time, and then drying. This yields catalyst particles whose surface is coated with a first coating containing polymelamine. In this case, the amount of polymelamine added affects the performance of the electrode catalyst. Details regarding the amount of polymelamine added will be described later.

[0062] [4.2. 2nd step] Next, the surface of the first coated particle is coated with a second coating containing polydopamine to obtain a second coated particle (second step).

[0063] Specifically, the formation of the second coating involves adding the first coated particles to a solution containing dopamine hydrochloride, stirring, and drying. This causes the dopamine to polymerize into polydopamine, and at the same time, the surface of the first coated particles is further coated with polydopamine. In this case, the amount of polydopamine added affects the performance of the electrode catalyst. Details regarding the amount of polydopamine to add will be described later.

[0064] [4.3. Third step] Next, the first and second coatings are thermally decomposed by heat treatment of the second coated particles (third step). This yields an electrode catalyst in which the surface of the catalyst particles is coated with a carbon film.

[0065] The heat treatment conditions are not particularly limited, as long as they are capable of carbonizing the coating. The third step preferably includes a step of heat-treating the second coating particles in an inert atmosphere at a temperature of 600°C to 1000°C. The heat treatment time should preferably be selected to be optimal depending on the heat treatment temperature. While the heat treatment time depends on the heat treatment temperature, a range of 0.5 to 10 hours is preferable.

[0066] [4.4. Amount of polymelamine and polydopamine to be added] The "amount added" of polymelamine or polydopamine refers to the value expressed by the following formula (4). Addition amount (mass%)=y×100 / (y+z) …(4) y is the mass of the polymelamine or polydopamine contained in the second coated particle. z is the mass of the catalyst particles contained in the second coated particle (or, if the catalyst particles are supported on the carrier, the total mass of the catalyst particles and the carrier).

[0067] [4.4.1. Amount of polymelamine added] The amount of polymelamine added affects the performance of the electrode catalyst. If the amount of polymelamine added is too low, the CO adsorption ratio ΔV may decrease. As a result, the activity improvement ratio compared to the unmodified product after initial and durability tests, and the durability of the electrode catalyst, decrease. Therefore, the amount of polymelamine added is preferably 55 mass% or more. More preferably, the amount added is 60 mass% or more, 70 mass% or more, or 75 mass% or more. On the other hand, if the amount of polymelamine added is excessive, the CO adsorption ratio ΔV may become excessively large. As a result, the activity improvement ratio compared to the unmodified product decreases after the initial and durability tests. Therefore, the amount of polymelamine added is preferably 85 mass% or less. More preferably, the amount added is 84 mass% or less.

[0068] [4.4.2. Dosage of polydopamine added] The amount of polydopamine added also affects the performance of the electrode catalyst. If the amount of polydopamine added is too low, micropores may not form in the carbon film, and the durability of the electrode catalyst may also decrease. Therefore, the amount of polydopamine added is preferably 20 mass% or more. More preferably, the amount added is 22 mass% or more. On the other hand, if the amount of polydopamine added is excessive, the initial activity may be lower than that of the unmodified product. Therefore, the amount of polydopamine added is preferably 30 mass% or less. More preferably, the amount added is 28 mass% or less, or 26 mass% or less.

[0069] [5. Effect] 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 catalyst poisoning sources. Therefore, coating the surface of catalyst particles with an ionomer tends to reduce their activity. Furthermore, prolonged use of fuel cell electrodes leads to a degradation of their mass activity over time. This is thought to be caused by a decrease in the surface area of ​​the catalyst particles and catalyst poisoning.

[0070] Causes of surface area reduction due to prolonged use include: (a) Elution of catalyst particles under low pH conditions due to sulfonic acid groups of ionomers, (b) Movement or aggregation of catalyst particles These are some examples. The increase in catalyst poisoning due to prolonged use is caused by: (a) Increased degree of contact between the sulfonic acid group of the ionomer and the catalyst particles. (b) Adsorption of free sulfonic acid groups generated by electrolyte decomposition onto catalyst particles These are some examples.

[0071] In contrast, when the surface of catalyst particles 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 particles is coated with a carbon film containing micropores. In this case, if the manufacturing conditions are optimized so that the CO adsorption ratio ΔV is within a predetermined range, the initial mass activity of the electrode catalyst is improved, and the degradation of mass activity over time is suppressed. This is thought to be because, by setting ΔV within a predetermined range, the surface of the catalyst particles is coated with an appropriate amount of carbon film having an appropriate structure.

[0072] In other words, by coating the surface of the catalyst particles with an appropriate amount of carbon film having a suitable structure, contact between the sulfonic acid groups of the ionomer and the catalyst particles is avoided, and the elution of the catalyst particles is suppressed even in a low pH environment. Furthermore, it is believed that coating the surface of the catalyst particles with an appropriate amount of carbon film having a suitable structure will reduce catalyst poisoning by ionomer sulfonic acid groups and free sulfonic acid groups. [Examples]

[0073] (Examples 1-4, Comparative Examples 1-5) [1. Sample Preparation] [1.1. Fabrication of Electrode Catalysts] [1.1.1. Examples 1-4, Comparative Examples 1-3] The following unmodified electrode catalysts were used. (a) Unmodified catalyst A: Commercially available 30 mass% Pt / Vulcan® (TEC10V30E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.). (b) Unmodified catalyst B: 30 mass% Pt / Vulcan® prepared using a known method (Pt-supported method by liquid-phase reduction).

[0074] [A. Adsorption of polymelamine (PME) onto unmodified catalysts] Dispersion A was obtained by dispersing either unmodified catalyst A or unmodified catalyst B in a 20 mass% aqueous solution of 2-propanol. Dispersion B was obtained by diluting a poly(melamine-co-formaldehyde) methylated solution (84 mass% 1-butanol solution, manufactured by Sigma-Aldrich) in a 20 mass% aqueous solution of 2-propanol.

[0075] A predetermined amount of dispersion B was added to dispersion A, and the mixture was stirred with a magnetic stirrer at room temperature for 3 hours. The solvent was then removed by suction filtration. For washing, ultrapure water was added to the powder on filter paper, stirred with a glass rod, and then filtered by suction, a process that was repeated twice. After filtering by suction, the powder was heated under vacuum at 100°C for 2 hours to allow for the thermosetting of the PME, thereby obtaining the PME-modified catalyst (first coated particles).

[0076] [B. Adsorption of polydopamine (PDA) onto PME-modified catalysts] A PME-modified catalyst was dispersed in 12 mM Tris-HCl buffer to obtain dispersion C. A predetermined amount of dopamine hydrochloride (Sigma-Aldrich) was added to dispersion C, and the mixture was stirred at room temperature for 6 hours. The solvent was then removed by suction filtration. For washing, ultrapure water was added to the powder on filter paper, stirred with a glass rod, and then filtered by suction, repeating this process twice. After filtration by suction, the powder was dried overnight under vacuum at 100°C to obtain PME / PDA modified catalyst (second coated particles).

[0077] [C. Formation of carbon film] The PME / PDA modified catalyst was heated at 800°C for 2 hours under Ar aeration (1 L / min). Afterward, it was washed by boiling, and the powder was dried overnight at 100°C under vacuum to obtain the electrode catalyst.

[0078] [1.1.2. Comparative Examples 4-5] Unmodified catalyst A (Comparative Example 4) or unmodified catalyst B (Comparative Example 5) was used as the electrode catalyst as is.

[0079] Table 1 shows the amount of electrode catalysts prepared (amount of PME per gram of unmodified catalyst and amount of PDA per gram of unmodified catalyst).

[0080] [Table 1]

[0081] [1.2. Fabrication of Membrane Electrode Assemblies (MEAs)] A catalyst ink was prepared by mixing an electrode catalyst, a mixed solvent of water and ethanol (1:1 by mass), and an ionomer (Nafion®, D2020, manufactured by DuPont) using an ultrasonic method. The ratio of the mass of the ionomer to the mass of carbon contained in the electrode catalyst (I / C) was set to 1.0. Note that "mass of carbon contained in the electrode catalyst" refers to the total mass of carbon contained in the support and the carbon film. The resulting catalyst ink was applied to a polytetrafluoroethylene (PTFE) sheet, and the coating was vacuum-dried at 80°C. This was used as the cathode-side catalyst layer.

[0082] Separately, a catalyst layer containing 60 mass% Pt / Ketjen® and ionomer (Nafion®, D2020, manufactured by DuPont) was prepared. The I / C ratio was 1.0, and the Pt basis weight was 0.2 mg / cm³. 2 This was used as the anode catalyst layer.

[0083] A cathode-side catalyst layer and an anode-side catalyst layer were transferred to both sides of the electrolyte membrane by hot pressing to obtain a MEA. A Nafion® membrane (NR-211, film thickness: 25 μm, manufactured by DuPont) was used as the electrolyte membrane. The hot pressing conditions were 120°C and 50 kgf / cm². 2 (4.9 MPa), 5 minutes.

[0084] [2. Test Method] [2.1. Evaluation of Electrocatalysts] [2.1.1. Measurement of CO adsorption amount] Figure 1 shows the pretreatment conditions for CO pulse measurement. Using a CO pulse measuring device manufactured by Okura Riken Co., Ltd., the electrode catalyst was pretreated under the conditions shown in Figure 1. Subsequently, CO pulse measurements were performed on the unmodified catalyst at room temperature, and on the carbon film coated catalyst in dry ice-Fluorinert refrigerant (-80°C), and the amount of CO adsorbed per unit mass of catalyst particles was determined.

[0085] [2.1.2. Micropore capacitance measurement] Using Autosorb-iQ (Quantachrome), nitrogen adsorption isotherms were measured at 77K after pretreatment (150°C, vacuum, 2 hours). The micropore volume was determined by analyzing the obtained adsorption data using the t-plot method.

[0086] [2.2. Cell Evaluation] [2.2.1. Cell Configuration and Evaluation Device] Cell evaluation was performed using the following cell configuration and evaluation equipment. Gas diffusion layer: Carbon paper (with microporous layer) Current collector: Integrated gold-plated copper plate (channel: 0.4mm pitch straight channel) Cell: 1cm 2 Rectangular cell

[0087] Evaluation bench: CO evaluation bench (manufactured by Chino Corporation) Electrochemical measuring device: • Potentiostat: POTENTIOSTAT / GALVANOSTAT 2100, PS530 (manufactured by Toho Giken Co., Ltd.) • Function generator: FG-02 (manufactured by Toho Giken Co., Ltd.) • Resistance measuring instrument: FG-100R (manufactured by Chino Corporation)

[0088] [2.2.2. Evaluation Items and Conditions] After performing cyclic voltammetry (CV), IV measurements and low potential maintenance in an overly humidified atmosphere (over-humidified acclimatization) were performed as part of the break-in period. Next, after performing IV measurements and electrochemical effective surface area (ECSA) measurements for performance evaluation, a potential fluctuation endurance test (ADT) was conducted. After the endurance test, the performance was evaluated again using the procedure described above. Table 2 shows the conditions for each evaluation item.

[0089] [Table 2]

[0090] [3. Results] [3.1. Evaluation of Electrocatalysts] Figure 2 shows the CO adsorption ratio ΔV of the electrode catalysts obtained in Examples 1-4 and Comparative Examples 1-3. Since ΔV is considered to be an index correlated with the coverage rate of the carbon film on the platinum surface, Comparative Examples 1 and 2 are estimated to have a high coverage rate (the proportion of the platinum surface covered by the carbon film). On the other hand, Comparative Example 3 is estimated to have a low coverage rate. In contrast, Examples 1-4 are considered to have a coverage rate that falls between these two.

[0091] Figure 3 shows the micropore capacity of the electrode catalysts obtained in Examples 1-4 and Comparative Examples 1-5. It can be seen that all electrode catalysts have micropores. Of these, the micropore capacity of Comparative Examples 4 and 5 is thought to be due to the micropore capacity derived from the support. The micropore capacity of the other samples is thought to be due to the micropore capacity derived from both the support and the carbon film.

[0092] Capacity V of micropores in a carbon film p3 It can be calculated based on the following formula. V p3 =V p1 -Vp2 ×(100-w) / 100 however, V p1 This refers to the capacity of the micropores of the carbon film modified catalyst. V p2 This refers to the capacity of the micropores of the unmodified carbon film catalyst. w is the ratio (mass%) of the mass of the carbon film to the total mass of the carbon film-modified catalyst.

[0093] Figure 4 shows the micropore capacity of the carbon films of the electrode catalysts obtained in Examples 1-4 and Comparative Examples 1-3. In the case of Comparative Example 3, the micropore capacity of the carbon film was almost zero. This is thought to be due to the excessively low amount of PME and PDA added. In contrast, in Examples 1-4 and Comparative Examples 1-2, the micropore capacity in the carbon films was 0.010 mL / g or higher in all cases.

[0094] [3.2. Cell Evaluation] [3.2.1. Relationship between CO adsorption ratio ΔV and initial activity improvement ratio] Figure 5 shows the relationship between the CO adsorption ratio ΔV and the initial activity improvement ratio. Here, the "initial activity improvement ratio" refers to the ratio of the initial mass activity of the carbon film-modified catalyst to the initial mass activity of the unmodified catalyst. The initial activity improvement ratio was determined under two conditions: high humidity conditions (temperature: 60°C, humidity: 80%RH) and low humidity conditions (temperature: 82°C, humidity: 30%RH). The mass activity was determined by dividing the current value at an IR correction voltage of 0.84V by the platinum mass.

[0095] Figure 5 shows that in the ΔV range of 30-80%, the initial activity improvement ratio is greater than 1 under both high and low humidity conditions, indicating that the carbon film improves the initial mass activity. This is thought to be because the carbon film suppresses the adsorption of sulfonic acid groups of the ionomer onto the platinum surface, thereby reducing platinum poisoning by the sulfonic acid groups.

[0096] If ΔV is lower than 30%, it is thought that the adsorption suppression of sulfonic acid groups is insufficient, and the presence of a small amount of carbon film exacerbates catalyst poisoning by the sulfonic acid groups of the ionomer, resulting in decreased activity compared to the unmodified catalyst. On the other hand, if ΔV is greater than 80%, it is thought that the carbon film becomes too thick, inhibiting the supply of oxygen and protons to the platinum surface. Of these, the ratio of activity improvement under high humidity conditions is lower than under low humidity conditions, suggesting that oxygen supply is likely inhibited under high humidity conditions.

[0097] [3.2.2. Relationship between CO adsorption ratio ΔV and activity improvement ratio after durability test] Figure 6 shows the relationship between the CO adsorption ratio ΔV and the activity improvement ratio after the endurance test. Here, the "activity improvement ratio after the endurance test" refers to the ratio of the mass activity of the carbon film-modified catalyst after the endurance test to the mass activity of the unmodified catalyst after the endurance test. The "endurance test" refers to the test (potential fluctuation endurance test) conducted under the conditions described in Table 2. The activity improvement ratio after durability testing was determined under two conditions: high humidity (temperature: 60°C, humidity: 80%RH) and low humidity (temperature: 82°C, humidity: 30%RH). Mass activity was calculated by dividing the current value at an IR correction voltage of 0.84V by the platinum mass.

[0098] Figure 6 shows that in the ΔV range of 30-80%, the activity improvement ratio after the durability test was greater than 2 under both high and low humidity conditions, indicating that the carbon film suppressed the decrease in mass activity after the durability test. This is thought to be because the carbon film prevented contact between the highly acidic ionomer and platinum, thereby suppressing the dissolution of platinum.

[0099] [3.2.3. ECSA retention rate] Figure 7 shows the ECSA maintenance rate ΔS (relative humidity: 30%RH, 80%RH) of the electrode catalysts obtained in Examples 1-4 and Comparative Examples 1-5. From Figure 7, it can be seen that the ECSA maintenance rate of the carbon film-modified catalyst is higher than that of the unmodified catalyst. This is thought to be because the carbon film suppressed the reduction in the surface area of ​​platinum.

[0100] In particular, Example 3 showed a high ECSA retention rate under both high and low humidity conditions. This is thought to be because ΔV was optimized (specifically, 45-58%), which suppressed contact between the ionomer and platinum under both high and low humidity conditions.

[0101] From these results, it was found that carbon film-modified catalysts with a ΔV of 30-80% exhibited higher initial mass activity compared to unmodified catalysts, and that the decrease in mass activity after durability testing was suppressed. Furthermore, it was found that further optimization of ΔV resulted in a high ECSA maintenance rate under both high and low humidity conditions.

[0102] 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]

[0103] 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 and A carbon film covering the surface of the catalyst particles and Equipped with, The carbon film includes micropores, The CO adsorption ratio ΔV, expressed by the following formula (1), is 30% or more and 80% or less. Electrocatalyst. ΔV(%)=(V 1 -V 2 )×100 / V 1 …(1) however, V 1 This refers to the amount of CO adsorbed per unit mass of the catalyst particles, measured at room temperature for the catalyst particles that are not coated with the carbon film. V 2 This is the amount of CO adsorbed per unit mass of the catalyst particles coated with the carbon film, measured at -80°C.

2. The electrode catalyst according to claim 1, wherein the CO adsorption ratio ΔV is 45% or more and 58% or less.

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

4. The electrode catalyst according to claim 3, wherein the volume of the micropores of the electrode catalyst is 0.025 mL / g or more and 0.045 mL / g or less.

5. The ECSA maintenance ratio ΔS is expressed by the following equation (2) L , and the ECA maintenance ratio ΔS, which is expressed by the following formula (3) H The electrode catalyst according to claim 1, wherein each of the above is 60% or more. ΔS L =S L2 ×100 / S L1 …(2) ΔS H =S H2 ×100 / S H1 …(3) however, S L1 The electrochemical surface area of ​​the electrode catalyst measured under 30% RH conditions before the durability test, S L2 The electrochemical surface area of ​​the electrode catalyst after the durability test, measured under conditions of 30% RH, S H1 The electrochemical surface area of ​​the electrode catalyst measured under 80% RH conditions before the durability test, S H2 The electrochemical surface area of ​​the electrode catalyst after the durability test, measured under 80% RH conditions, The aforementioned "durability test" is a test in which a rectangular wave of 0.6 V (vs. RHE, 3s) - 1.0 V (vs. RHE, 3s) is applied for 10,000 cycles to a film electrode assembly in which the electrode catalyst is added to the cathode catalyst layer, under conditions of 80% RH.

6. A first step is to coat the surface of catalyst particles or catalyst particles supported on a carrier with a first coating containing polymelamine to obtain first coated particles, A second step involves coating the surface of the first coated particle with a second coating containing polydopamine to obtain a second coated particle, A third step involves heat-treating the second coated particles to thermally decompose the first and second coatings, thereby obtaining an electrode catalyst in which the surface of the catalyst particles is coated with a carbon film. Equipped with, The first step includes coating the surface of the catalyst particles or the catalyst particles supported on the carrier with the first coating so that the amount of polymelamine added is 55 mass% or more and 85 mass% or less. The second step includes coating the surface of the first coated particles with the second coating so that the amount of polydopamine added is 20 mass% or more and 30 mass% or less. The third step includes heat-treating the second coated particles in an inert atmosphere at a temperature of 600°C to 1000°C. A method for manufacturing an electrode catalyst. however, The "amount added" of the polymelamine or polydopamine refers to the value represented by the following formula (4). Addition amount (mass%)=y×100 / (y+z)…(4) y is the mass of the polymelamine or polydopamine contained in the second coated particle. z is the mass of the catalyst particles contained in the second coated particle (or, if the catalyst particles are supported on the carrier, the total mass of the catalyst particles and the carrier).

7. The first step includes coating the surface of the catalyst particles or the catalyst particles supported on the carrier with the first coating so that the amount of polymelamine added is 75 mass% or more and 85 mass% or less. A method for producing an electrode catalyst according to claim 6.

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