Electrode catalyst layer

Abstract

The present invention aims to provide an electrode catalyst layer that is effective in preventing destruction of voids in the catalyst layer and catalyst-supporting particles due to the manufacturing process of a membrane electrode assembly and the internal stresses associated therewith when manufacturing a membrane electrode assembly for a polymer electrolyte fuel cell. The present invention provides an electrode catalyst layer comprising catalyst-supporting particles and a polymer electrolyte, wherein the electrode catalyst layer has a film thickness of 1 to 20 μm, and the L in the CIE LAB color system measured using a multi-angle spectrophotometer is * , a * , and b * But, L * ≧16.0, 0.0≦a * ≦5.0, 0.5≦b * ≦10.0

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst layer. [Background technology]

[0002] Fuel cells are devices that generate electricity and heat through a chemical reaction that converts hydrogen and oxygen into water. There are several types of fuel cells, including phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and polymer electrolyte fuel cells (PEFCs). Among these, polymer electrolyte fuel cells (PEFCs) have a structure in which a catalyst layer that constitutes the anode (fuel electrode) on one side of a solid polymer membrane and the cathode (air electrode) on the other side is provided, with a gas diffusion layer bonded to the outside of each catalyst layer. The catalyst layer is made of a catalyst-supported carrier in which particulate catalysts containing precious metals are highly dispersed and supported on the surface of nano-level support particles.

[0003] Known conventional polymer electrolyte fuel cells have a membrane electrode assembly that uses, as an electrode catalyst that satisfies both a large specific surface area and high conductivity, mesoporous carbon carrying catalytic metal particles or acetylene black carrying platinum or a platinum alloy. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 117369 [Patent Document 2] Patent Publication No. 2023-163196 Summary of the Invention [Problem to be solved by the invention]

[0005] The membrane electrode assembly (MEA) that constitutes a polymer electrolyte fuel cell is composed of multiple layers, each with different material properties, making it susceptible to internal stress during the fabrication process. The main factors contributing to the internal stress during the fabrication process of a membrane electrode assembly include thermal stress, mechanical stress, and humidity changes. For example, during the fabrication process, pressure is applied to the catalyst layer during the heat press to transfer the catalyst layer formed on the catalyst transfer film to the solid polymer electrolyte membrane, and during the tightening process when the membrane electrode assembly is assembled into the cell. Such pressure acts can destroy the pores in the catalyst layer and the catalyst support particles themselves, which then permanently break down, potentially resulting in reduced performance.

[0006] However, in all of these documents, the deterioration of performance due to pressurization was not sufficiently investigated, and there was still room for improvement. [Means for solving the problem]

[0007] As a result of extensive research into achieving the above object, the present inventors have found that the L * , a * , and b * It was found that an excellent elastic modulus can be obtained when the value of is in a specific range.

[0008] That is, the present invention provides the following configurations.

[0009] [1] An electrode catalyst layer comprising catalyst-supporting particles and a polymer electrolyte, wherein the electrode catalyst layer has a film thickness of 1 to 20 μm, and the L * , a * , and b * But, L * ≧16.0, 0.0≦a * ≦5.0, 0.5≦b * Electrode catalyst layer that satisfies ≦10.0.

[0010] [2] The electrode catalyst layer according to [1] above, wherein the catalyst support of the catalyst-supporting particles is made of silicon carbide.

[0011] [3] The electrode catalyst layer according to the above [1] or [2], wherein the polymer electrolyte is a fluorine-based polymer electrolyte. [Effects of the Invention]

[0012] According to the electrode catalyst layer of one aspect of the present invention, the elastic modulus of the catalyst layer can be adjusted to fall within a desired range, and as a result, when used in a fuel cell, degradation during the manufacturing process can be suppressed, and stable performance can be expected. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described. Note that the present invention is not limited to the embodiments described below, and modifications such as design changes based on the knowledge of those skilled in the art may be made, and such modified embodiments are also included in the scope of the present embodiments.

[0014] (electrode catalyst layer) The electrode catalyst layer of the present invention (also referred to as a cathode catalyst layer in this specification) is an electrode catalyst layer comprising catalyst-supporting particles and a polymer electrolyte, and is characterized in that the L * , a * , and b * But, L * ≧16.0, 0.0≦a * ≦5.0, 0.5≦b * ≦10.0.

[0015] Said L * L * ≧16.0, and L * Preferably, L is ≧18.0. * It is more preferable that the upper limit is ≧20.0. * Preferably, L is ≦80.0. * It is more preferable that L is ≦70.0. * It is particularly preferred that L is ≦60.0.* However, if the pressure is below the lower limit, the catalyst-supporting particles are easily destroyed by pressure, resulting in a decrease in performance. On the other hand, if the pressure is below the upper limit, the electrode catalyst layer exhibits a certain elastic modulus against pressure, which is suitable for producing a membrane electrode assembly. In addition, L * The lower and upper limits may be combined in any way within the range.

[0016] The above a * is 0.0≦a * ≦5.0 and 0.0≦a * ≦2.5, and 0.0≦a * It is more preferable that the ratio is ≦1.5. * However, if the particle size is within the above range, the catalyst-supporting particles are probably ubiquitous within the electrode catalyst layer, which is preferable.

[0017] The above b * is 0.5≦b * ≦10.0, and 1.0≦b * ≦5.0, and 2.0≦b * It is more preferable that the value is ≦3.0. * However, if the thickness is within the above range, the electrode catalyst layer exhibits a certain elastic modulus against pressure, which is preferable for producing a membrane electrode assembly.

[0018] The above-mentioned L * , a * , and b * can be combined in any way, for example, L * ≧20.0, 0.0≦a * ≦5.0, 0.5≦b * L may be ≦10.0 * ≧16.0, 0.0≦a * ≦1.5, 1.0≦b * L may be ≦5.0 * ≧16.0, 0.0≦a * ≦5.0, 2.0≦b * It may be ≦3.0.

[0019] The elastic modulus of a membrane layer made of a general polymer resin is controlled by the molecular structure and cross-linking structure of the resin. However, since the electrode catalyst layer of a fuel cell is a composite of a polymer electrolyte and catalyst-supporting particles, there are many unclear points about how to control the elastic modulus. In the present invention, the L * , a * , b * Focusing on this, it became clear that the elastic modulus of the catalyst layer falls within the desired range when it is within the above range. Although the details are unclear, it is assumed that the mechanism of action is as follows. In the electrode catalyst layer, it is assumed that the state of existence of the catalyst-supported particles and the polymer electrolyte affects the elastic modulus. Comparing the catalyst-supported particles and the polymer electrolyte, it is thought that aggregates of catalyst-supported particles have low elasticity, and that if the catalyst-supported particles are localized, they are easily induced to break when pressure is applied. On the other hand, it is assumed that the presence of the polymer electrolyte acts as a kind of cushioning material, suppressing the breakage of the catalyst-supported particles and improving the elastic modulus. Whether the catalyst-supported particles and the polymer electrolyte are in an appropriate state can be determined by the L * , a * , b * It is assumed that this was indirectly indicated by

[0020] Said L * , a * , b * If the modulus of elasticity is outside the range, the modulus of elasticity will be too low or too high, the pressure resistance will be insufficient, and the flow paths for gas and ions will not be secured. In other words, the conduction of gas, ions, and electrons will be hindered, and the active sites of the catalyst-supporting particles will decrease, which may hinder the performance of the fuel cell.

[0021] <Catalyst-supported particles> In the present invention, the catalyst-supported particles are composed of a catalyst, which is an element or compound having catalytic activity, and a catalyst carrier capable of supporting the catalyst. The catalyst-supported particles are not particularly limited, but examples thereof include catalyst-supported carbon particles and catalyst-supported ceramic particles.

[0022] The catalyst is not particularly limited, and examples thereof include metals such as platinum group elements (platinum, palladium, ruthenium, iridium, rhodium, and osmium), iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, as well as alloys, oxides, double oxides, and carbides of these metals.

[0023] The carbon particles are not particularly limited as long as they can support the catalyst and have electrical conductivity, and examples thereof include carbon black (acetylene black, furnace black, ketjen black, etc.), graphite, activated carbon, fullerene, etc.

[0024] The ceramic particles may be any material capable of supporting the catalyst, and preferably contain silicon, with silicon carbide, silicon oxycarbide, silicon nitroxycarbide, and silicon nitride being more preferred, and silicon carbide being particularly preferred. The ceramic particles may also be composited with a carbon material, which will be described later, and can be composited by adding a carbon material when producing the ceramic particles. By composited, the carbon material is incorporated into the ceramic particles that form a three-dimensional skeletal structure, which is preferable because it can impart electrical conductivity to the ceramic particles.

[0025] <Carbon materials> When ceramic particles are used as a catalyst support, it is desirable to blend a carbon material from the viewpoint of imparting electrical conductivity. The blending may involve adding the carbon material during production to form a composite, as described above, or adding the carbon material separately after production. However, from the viewpoint of the resulting power generation performance, the former is preferable. Examples of the carbon material include acetylene black, furnace black, and ketjen black.

[0026] When a carbon material is blended with catalyst-supported ceramic particles, the ratio of the carbon material to the catalyst-supported ceramic particles is preferably 5.5% by mass to 95.0% by mass, more preferably 10% by mass to 90% by mass, and particularly preferably 20% by mass to 80% by mass. When the ratio of the carbon material to the catalyst-supported ceramic particles is within the above range, it is preferable because the desired elastic modulus can be obtained.

[0027] The content of the catalyst-supported particles is preferably 1 to 80% by mass, more preferably 5 to 60% by mass, and particularly preferably about 10 to 40% by mass, relative to the total mass of the catalyst ink. When the ratio of catalyst-supported particles to the total mass of the catalyst ink is within the above range, it is possible to obtain the desired elastic modulus, which is preferable.

[0028] <Polymer electrolyte> The polymer electrolyte may be any one having proton conductivity, and may include fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes. Examples of fluorine-based polymer electrolytes include Nafion (registered trademark) manufactured by DuPont, Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., Aciplex (registered trademark) manufactured by Asahi Kasei Corporation, and Gore Select (registered trademark) manufactured by Gore. Examples of hydrocarbon-based polymer electrolytes include electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. Among these, Nafion (registered trademark)-based materials manufactured by DuPont are suitable for use as polymer electrolytes. Examples of hydrocarbon-based polymer electrolytes include electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. In particular, Nafion (registered trademark)-based materials manufactured by DuPont are suitable for use as polymer electrolytes.

[0029] The mass ratio of the catalyst-supporting particles to the polymer electrolyte (polymer electrolyte / catalyst-supporting particles) is preferably 0.2 to 0.7.

[0030] <Other> Furthermore, the catalyst layer may contain a fibrous material. By containing the fibrous material, a structure in which fibers are entangled is formed within the catalyst layer, which improves the strength and can suppress the occurrence of cracks. In addition, the formation of suitable voids is expected to improve power generation performance.

[0031] The average fiber diameter of the fibrous material is preferably 0.5 to 500 nm, more preferably 10 to 300 nm. The average fiber length is preferably 1 to 200 μm. From the viewpoint of void formation in the catalyst layer, it is preferable that the average fiber diameter and average fiber length are within the above ranges.

[0032] Examples of the fibrous material include conductive fibers and electrolyte fibers. Examples of the conductive fibers include carbon fibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. Examples of the electrolyte fibers include the above-mentioned polymer electrolytes processed into a fibrous form.

[0033] The solid content is preferably 1 to 80% by mass relative to the total mass of the catalyst ink. If it is less than 1% by mass, the concentration and elasticity of the electrode catalyst layer will be insufficient, and if it exceeds 80% by mass, the viscosity of the catalyst ink will increase and its stability over time may decrease.

[0034] (Method of manufacturing electrode catalyst layer) The electrode catalyst layer can be produced by preparing a catalyst ink, applying it to a substrate or a gas diffusion layer, drying it, and then thermocompressing the electrode catalyst layer onto a solid polymer electrolyte membrane. The catalyst ink is composed of catalyst-supported carbon particles, a polymer electrolyte, and a solvent, or catalyst-supported ceramic particles, a carbon material, a polymer electrolyte, and a solvent.

[0035] The solid polymer electrolyte membrane may be the same as the polymer electrolyte described above.

[0036] The solvent is not particularly limited as long as it can dissolve or disperse the polymer electrolyte and the catalyst. Examples of the solvent include water, alcohols (methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 3-butanol, pentanol, ethylene glycol, diacetone alcohol, 1-methoxy-2-propanol, etc.), ketones (acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, diisobutyl ketone, etc.), ethers (dioxane, tetrahydrofuran, etc.), sulfoxides (dimethyl sulfoxide, etc.), amides (dimethylformamide, dimethylacetamide, etc.), and the like, which can be used alone or in combination. The solvent used in the catalyst ink is preferably one that can be easily removed by heating, and in particular, one with a boiling point of 150° C. or less is preferably used.

[0037] The catalyst ink can be prepared by carrying out a dispersion treatment, and examples of the dispersion method include a ball mill, a bead mill, a roll mill, a shear mill, a wet mill, ultrasonic dispersion, and a homogenizer.

[0038] The catalyst ink can be applied to a substrate by a conventional coating method, such as a roll coater, air knife coater, blade coater, rod coater, reverse coater, bar coater, comma coater, die coater, gravure coater, screen coater, sprayer, or spinner. There are no particular limitations on the coating means as long as a similar membrane electrode assembly can be obtained in the end.

[0039] The desired electrode catalyst layer can be obtained by applying the catalyst ink to a substrate and volatilizing the solvent in the catalyst ink by heating. Drying methods include hot air drying and IR drying. The drying temperature is 40 to 200°C, preferably about 40 to 120°C. The drying time is 0.5 minutes to 1 hour, preferably about 1 to 30 minutes. The drying step may be performed using a single drying mechanism or a combination of multiple drying mechanisms. The average thickness of the electrode catalyst layer obtained by drying the catalyst ink is, for example, about 0.1 to 100 μm, preferably about 0.5 to 50 μm, and more preferably about 1 to 20 μm.

[0040] The substrate used in the transfer step is not particularly limited as long as it can be coated with a catalyst ink on at least one side, can form an electrode catalyst layer by heating, and can transfer the formed electrode catalyst layer to a solid polymer electrolyte membrane. For example, polymer films such as polyethylene terephthalate, polyamide, polyimide, polystyrene, polysulfone, polyethersulfone, polyphenylene sulfide, polyether ether ketone, polyetherimide, polybenzimidazole, polyamideimide, polyacrylate, polyethylene naphthalate, and polypalvanic acid aramid, or heat-resistant fluororesin films such as polytetrafluoroethylene, polychlorotrifluoroethylene, polyvinylidene fluoride, ethylene tetrafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and tetrafluoroperfluoroalkyl vinyl ether copolymer can be used.

[0041] Furthermore, the substrate may be subjected to a release treatment, or may have a multi-layer structure in which a release layer is integrated by co-extrusion or the like. The substrate may be a sheet, film, plate, membrane, or foil, or at least one of these may be adhered, bonded, adhered, or pasted.

[0042] (Membrane electrode assembly) The electrode catalyst layer of the present invention can be used in a membrane electrode assembly, which includes a proton-conductive solid polymer electrolyte membrane, an anode catalyst layer, a cathode catalyst layer, and gas diffusion layers provided on the outer sides of the anode and cathode catalyst layers.

[0043] The formation of the anode and cathode catalyst layers on the solid polymer electrolyte membrane and the formation of the gas diffusion layers thereon can be carried out by known, commonly used methods. [Example]

[0044] Examples of the present invention will be described below. The present invention is not limited to the examples shown below. Values ​​in the tables mean "parts by mass" unless otherwise specified.

[0045] (evaluation) The catalyst layers obtained in the examples and comparative examples were evaluated as follows.

[0046] [Elastic modulus measurement] The obtained electrode catalyst layer was cut into a 1 cm x 1 cm sample piece and measured using a surface force measurement device ESF-5000Plus (manufactured by ELIONIX) with an indentation load of 30 to 40 μN until the indentation depth reached 200 nm. The number of measurements, N=60, was used to evaluate the median value as the indentation modulus. The indentation modulus was evaluated as follows. Indentation modulus: 1000N / mm 2 If it is more than or equal to this, mark it as "◎" Indentation modulus: 500N / mm 2 More than 1000N / mm 2 If less than, mark "Yes" Indentation modulus: 500N / mm 2 If less than, "×"

[0047] [L * a * b * measurement] The obtained electrode catalyst layer was cut into a 6 cm diameter sample piece (film thickness is described in Examples and Comparative Examples), and measured using the L * , a * , and b * The values ​​were measured using a multi-angle spectrophotometer MA94 (X-rite) at a visual angle of 15° and a 10-degree field of view using a D65 light source.

[0048] Example 1 Catalyst-supported particles were prepared by mixing catalyst-supported particles A (Pt / SiC / KB, [Si] / [C] = 1:1.3, Pt loading 40% by mass) with a polymer electrolyte (Du Pont, Nafion (registered trademark) DE521) at a mass ratio of 0.2. This mixture, 1.0 g of n-propanol, 1.0 g of water, and zirconia balls (diameter 5 mm) were placed in a zirconia pot and mixed for 60 minutes in a planetary ball mill (Fritsch, P-7) to obtain a catalyst ink. The catalyst-supported particles were prepared by blending silicon carbide obtained by the method described in JP 2023-148962 A with a carbon material (Ketjenblack), and mixing with a dispersion containing a precious metal colloid.

[0049] The resulting catalyst ink was used to coat a 100 μm thick PTFE substrate with a platinum coating weight of 0.2 mg / cm using a K CONTROL COATER (manufactured by RK Print Coat Instrument) with a gap of 100 μm. 2 An electrode catalyst layer (film thickness: 5 μm) was produced so as to satisfy the following conditions.

[0050] Example 2 A catalyst ink and an electrode catalyst layer (film thickness 5 μm) were prepared according to the method described in Example 1, except that the mass ratio of catalyst-supporting particles to polymer electrolyte was set to 0.5.

[0051] Example 3 A catalyst ink and an electrode catalyst layer (film thickness 5 μm) were prepared according to the method described in Example 1, except that the weight ratio of catalyst-supporting particles to polymer electrolyte was set to 0.7.

[0052] (Comparative Example 1) 0.5 g of catalyst-supported particles B (Pt / CB, Tanaka Kikinzoku Co., Ltd., TEC10E50E, Pt loading 46 wt%) and a polymer electrolyte (DuPont, Nafion (registered trademark) DE521) were mixed at a mass ratio of 0.2. This mixture, 2.5 g of n-propanol, 2.5 g of water, and zirconia balls (diameter 5 mm) were placed in a zirconia pot and mixed for 60 minutes in a planetary ball mill (Fritsch, P-6) to obtain a catalyst ink.

[0053] A catalyst layer (film thickness 5 μm) was prepared in accordance with the method described in Example 1, except that the gap was set to 120 μm.

[0054] (Comparative Example 2) A catalyst ink and an electrode catalyst layer (film thickness 5 μm) were prepared according to the method described in Comparative Example 1, except that the mass ratio of catalyst-supporting particles to polymer electrolyte was set to 0.7.

[0055] [Table 1]

[0056] (Cathode catalyst layer) The electrode catalyst layers obtained in the examples and comparative examples were used as cathode catalyst layers.

[0057] (Anode catalyst layer) Carbon black (Pt / CB, Tanaka Kikinzoku Co., Ltd., TEC10E50E, Pt loading 46 wt%) carrying 0.5 g of platinum (Pt) was mixed with a polymer electrolyte (DuPont, Nafion® DE521) at a mass ratio of 0.7. This mixture, 2.5 g of n-propanol, 2.5 g of water, and zirconia balls (5 mm diameter) were placed in a zirconia pot and mixed for 60 minutes in a planetary ball mill (Fritsch, P-6). A catalyst ink was obtained by mixing in this ball mill.

[0058] Using the obtained catalyst ink, an anode catalyst layer was produced on a 100 μm thick PTFE substrate using a K CONTROL COATER (manufactured by RK Print Coat Instrument) with a gap of 120 μm so that the platinum coating amount was 0.2 mg / cm 2 .

[0059] (Fabrication of fuel cell electrode membrane (CCM)) The obtained anode catalyst layer, cathode catalyst layer, and solid polymer electrolyte membrane (Nafion NR212, manufactured by Du Pont) were hot pressed (140°C, pressure 2.86 kN) for 3 minutes in a hot press machine (TCMD-2.5, manufactured by Toho Kogyo Co., Ltd.) to prepare a CCM.

[0060] In the CCM, gas diffusion layers (GDL, manufactured by SGL, 22BB) were stacked on both sides of each catalyst layer, and a membrane electrode assembly (MEA) was obtained in which the cathode catalyst layer and the anode catalyst layer were stacked on the solid polymer electrolyte membrane so as to face each other.

[0061] A fuel cell using the obtained membrane electrode assembly exhibits stable power generation performance without deterioration due to pressure.

Claims

1. An electrode catalyst layer composed only of essential components: catalyst-supporting ceramic particles, a proton-conducting polymer electrolyte, and optional components: conductive fibers or electrolyte fibers, wherein the catalyst-supporting ceramic particles are composed of platinum, silicon carbide, and a carbon material, the mass ratio of the catalyst-supporting ceramic particles to the polymer electrolyte is 0.2 to 0.7, and the L*, a*, and b* in the CIE LAB color system, measured using a multi-angle spectrophotometer when the film thickness of the electrode catalyst layer is 1 to 20 μm, satisfy the following conditions: L*≧16.0, 0.0≦a*≦5.0, 0.5≦b*≦10.

0.

2. The electrode catalyst layer according to claim 1 , wherein the polymer electrolyte is a fluorine-based polymer electrolyte.

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