Membrane electrode assembly and fuel cell

Abstract

The objective is to provide a membrane electrode assembly that, when used as a membrane electrode assembly for a polymer electrolyte fuel cell, not only maintains the durability of the catalyst layer but is also expected to maintain power generation performance even when operated for long periods in a high-potential environment. [Solution] A membrane electrode assembly is provided, comprising a proton-conducting solid electrolyte membrane, an anode catalyst layer, a cathode catalyst layer, and a gas diffusion layer provided outside the anode and cathode catalyst layers, wherein the cathode catalyst layer includes catalyst-supported ceramic particles, a carbon material, and an ionomer, and the absolute value (x) calculated from the formula |(ΔV1-ΔV2) / ΔV1| is in the range of 0 to 0.5.

Description

Technical Field

[0001] The present invention relates to a membrane electrode assembly and a solid polymer fuel cell.

Background Art

[0002] A fuel cell is a device that generates electric power and heat through a chemical reaction that produces water from hydrogen and oxygen. There are multiple types of fuel cells, such as phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and polymer electrolyte fuel cells (PEFCs). Among these, a polymer electrolyte fuel cell (PEFC) has a structure in which a catalyst layer that forms an anode (fuel electrode) is provided on one side of a solid polymer membrane, and a catalyst layer that forms a cathode (air electrode) is provided on the other side, and a gas diffusion layer is adhered to the outside of each catalyst layer. The catalyst layer is composed of a catalyst support carrier in which particulate catalysts containing a noble metal are highly dispersed and supported on the surface of nanoscale carrier particles.

[0003] Conventional polymer electrolyte fuel cells are known to have a membrane electrode assembly that uses mesoporous carbon supporting catalyst metal particles or platinum or platinum alloy supported acetylene black as an electrode catalyst that satisfies high specific surface area and high conductivity.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, it was found that in all membrane electrode assemblies, prolonged operation under high-potential conditions can cause a rapid deterioration in the power generation performance of the polymer electrolyte fuel cell. Furthermore, the timing of this deterioration varies depending on the type of carrier used, making it difficult to predict in advance. The objective of the present invention is to provide a membrane electrode assembly that, when used as a membrane electrode assembly for a polymer electrolyte fuel cell, is expected to not only maintain the durability of the catalyst layer but also maintain power generation performance even when operated for a long period of time in a high-potential environment. [Means for solving the problem]

[0006] As a result of diligent research to achieve the above objective, the inventors have found that by keeping the absolute value of the difference in the rate of change of gas diffusion resistance within a specific range, it is possible to maintain the durability of the catalyst layer and the power generation performance under high potential environments. In other words, the present invention provides the following configuration.

[0007] [1] A solid electrolyte membrane having proton conductivity, an anode catalyst layer, a cathode catalyst layer, and a gas diffusion layer provided outside the anode and cathode catalyst layers, wherein the cathode catalyst layer comprises catalyst-supported ceramic particles, a carbon material, and an ionomer. A membrane electrode assembly in which the absolute value (x) calculated by the measurement method described below is within the range of 0 to 0.5. (Measurement method) Let ΔV1 be the rate of change in gas diffusion resistance during the start-stop endurance test of the film electrode assembly from 0 to 2500 cycles. Let ΔV2 be the rate of change in gas diffusion resistance during the start-stop endurance test of the film electrode assembly from 2500 to 5000 cycles. The absolute value (x) is calculated from the formula |(ΔV1-ΔV2) / ΔV1|.

[0008] [2] The film electrode assembly according to [1] above, wherein the ratio of carbon material to catalyst-supported ceramic particles is 5.5% by mass to 95.0% by mass.

[0009] [3] The film electrode assembly according to [1] or [2] above, wherein the mass ratio of the total mass of the catalyst-supported ceramic particles and carbon material to the ionomer is 1:0.01 to 1:30.

[0010] [4] The film electrode assembly according to any one of [1] to [3] above, wherein the catalyst-supported ceramic particles have the element silicon.

[0011] [5] A solid polymer fuel cell comprising a membrane electrode assembly as described in any one of [1] to [4] above. [Effects of the Invention]

[0012] According to a polymer electrolyte fuel cell equipped with a membrane electrode assembly of one aspect of the present invention, it is possible to maintain a constant porosity of the catalyst layer even during long-term operation in a high-potential environment while maintaining the durability of the catalyst layer, and thus maintain high power generation performance. Furthermore, according to the present invention, the degradation period of a polymer electrolyte fuel cell can be easily predicted, and it is expected that this can be used as an indicator for maintenance and for inspecting defective products in the manufacturing process. [Modes for carrying out the invention]

[0013] The embodiments of the present invention will be described below. However, 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 within the scope of this invention.

[0014] (Membrane electrode assembly) The membrane electrode assembly of the present invention comprises a proton-conducting solid electrolyte membrane, an anode catalyst layer, a cathode catalyst layer, and a gas diffusion layer provided outside the anode and cathode catalyst layers, wherein the absolute value (x) is in the range of 0 to 0.5. In polymer electrolyte fuel cells, fuel gas is continuously supplied and discharged, allowing the fuel gas to reach every corner of the catalyst layer, thereby increasing the number of reaction sites. Furthermore, by efficiently discharging reaction-generated water from the catalyst layer, flooding can be suppressed, and stable power generation performance can be expected even when exposed to high-potential environments for extended periods. In conventional polymer electrolyte fuel cells, battery degradation during actual fuel cell operation was predicted based on durability tests spanning tens to hundreds of thousands of cycles. However, this invention has found that by setting the degradation rate value within a specific cycle range to a desired value, a catalyst layer exhibiting excellent power generation performance over long periods can be obtained without conducting the aforementioned durability tests. Moreover, it is expected that applying this invention will make it possible to easily provide catalyst layers of stable quality.

[0015] The absolute value (x) of the difference in the rate of change of gas diffusion resistance is thought to be related to the timing of the buckling point at which corrosion degradation of the catalyst layer progresses rapidly. In order to delay the timing of the buckling point, it is desirable for the value to be in the range of 0 to 0.5, preferably in the range of 0 to 0.3, and more preferably in the range of 0 to 0.2.

[0016] The absolute value (x) is calculated from the formula |(ΔV1-ΔV2) / ΔV1|, where ΔV1 is the rate of change in gas diffusion resistance during 0 to 2500 cycles of the start-stop endurance test of the film electrode assembly, and ΔV2 is the rate of change in gas diffusion resistance during 2500 to 5000 cycles of the start-stop endurance test. In this specification, "||" indicates an absolute value.

[0017] <Solid electrolyte membrane> The solid electrolyte membrane only needs to have proton conductivity, and fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes can be used. As the fluorine-based polymer electrolyte, for example, Nafion (registered trademark) manufactured by DuPont, Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., Aciplex (registered trademark) manufactured by Asahi Kasei Corporation, Gore Select (registered trademark) manufactured by Gore & Associates, etc. can be used. Further, as the hydrocarbon-based polymer electrolyte, electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, etc. can be used. Among them, Nafion (registered trademark) - based materials manufactured by DuPont can be preferably used as the polymer electrolyte. As the hydrocarbon-based polymer electrolyte, electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, etc. can be used. In particular, Nafion (registered trademark) - based materials manufactured by DuPont can be preferably used as the polymer electrolyte.

[0018] <Anode catalyst layer> The anode catalyst layer is composed of catalyst - supported carbon particles and an ionomer. The catalyst - supported carbon particles are not particularly limited. Examples of the catalyst include platinum group elements (platinum, palladium, ruthenium, iridium, rhodium, osmium), metals such as iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, etc., and alloys, oxides, double oxides, carbides, etc. of these metals. Further, the carbon particles are not particularly limited as long as they can support the catalyst and have conductivity. Examples include carbon black (acetylene black, furnace black, ketjen black, etc.), graphite, carbon, activated carbon, fullerene, etc.

[0019] As the ionomer, any material having proton conductivity may be used, and fluorine-based polymer resins and hydrocarbon-based polymer resins are preferably used. For example, Nafion (registered trademark) manufactured by DuPont, sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, etc. may be mentioned. The ionomer may be the same as that used for the above-mentioned solid electrolyte membrane, and the same is preferable because the adhesion between the catalyst layer and the solid electrolyte membrane is excellent.

[0020] Furthermore, it may contain a fibrous substance. By containing the fibrous substance, a structure in which the fibers are intertwined is formed in the catalyst layer, thereby improving the strength and suppressing crack generation. In addition, improvement in power generation performance can be expected by forming suitable voids. The average fiber diameter of the fibrous substance is preferably 0.5 to 500 nm, more preferably 10 nm to 300 nm. Also, 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 the average fiber length are within the above ranges.

[0021] Examples of the fibrous substance include conductive fibers and electrolyte fibers. Examples of the conductive fibers include carbon fibers, carbon nanotubes, carbon nanohorns, conductive polymer nanofibers, etc. Examples of the electrolyte fibers include those obtained by processing the above-mentioned polymer electrolyte into a fibrous form.

[0022] <Cathode catalyst layer> The cathode catalyst layer is composed of catalyst-supported ceramic particles, a carbon material, and an ionomer. The catalyst-supported ceramic particles are not particularly limited. Examples of the catalyst include platinum group elements (platinum, palladium, ruthenium, iridium, rhodium, osmium), metals such as iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, etc., and alloys, oxides, double oxides, carbides, etc. of these metals. More preferably, they are platinum, platinum cobalt, and platinum nickel.

[0023] The ceramic particles only need to be able to support the catalyst, and preferably have silicon, for example, silicon carbide, silicon oxycarbide, silicon nitrooxycarbide, and silicon nitride. The ceramic particles may also be in a composite state with a carbon material, which will be described later. This composite state can be achieved by adding the carbon material when manufacturing the ceramic particles. By compounding, the carbon material is incorporated into the ceramic particles that form a three-dimensional skeletal structure, which is preferable as it allows for the imparting of conductivity to the ceramic particles.

[0024] The particle diameter of the ceramic particles is preferably 10 to 1000 nm, and more preferably 10 to 100 nm. A particle diameter of 1000 nm or less is preferable because it allows for a thinner catalyst layer, reducing resistance and contributing to improved power generation performance. A particle diameter of 10 nm or more is also preferable because it allows for good voids in the catalyst layer.

[0025] Examples of carbon materials include acetylene black, furnace black, and Ketjen black.

[0026] When the carbon material is composed of acetylene black, the average diameter of the primary particles of the carbon material is preferably 10 nm to 200 nm, more preferably 20 nm to 100 nm, and even more preferably 30 nm to 50 nm. When the average diameter of the primary particles of the carbon material is 10 nm to 200 nm, good conductivity can be achieved.

[0027] When the carbon material is composed of carbon nanofibers or carbon nanotubes, the average diameter of the carbon material is preferably between 10 nm and 200 nm, and the length of the carbon material is preferably between 1 μm and 20 μm.

[0028] When a carbon material is compounded with catalyst-supported ceramic particles, the ratio of the carbon material to the catalyst-supported ceramic particles is preferably 5.5% to 95.0% by mass, more preferably 10% to 90% by mass, and particularly preferably 20% to 80% by mass. Having the ratio within the above range is preferable because it allows for a smaller ΔV2.

[0029] The morphology and size of ceramic particles and carbon materials can be measured, for example, by observation using a transmission electron microscope or scanning electron microscope. Furthermore, the average diameter of primary particles can be determined, for example, from microscope images using image analysis-based particle size distribution measurement software.

[0030] The ionomer used is the same as the ionomer used in the anode catalyst layer mentioned above.

[0031] The mass ratio of the total mass of the catalyst-supported ceramic particles and carbon material to the ionomer is preferably 1:0.01 to 1:30, and more preferably 1:0.1 to 1:10. Having the mass ratio within this range is preferable from the viewpoint of forming a catalyst layer with voids that are excellent for gas diffusion.

[0032] (Method for manufacturing the catalyst layer) The catalyst layer can be manufactured by preparing a slurry for the catalyst layer, coating and drying it on a substrate or gas diffusion layer, and then heat-pressing the catalyst layer onto a solid polymer film. The slurry for the catalyst layer consists of catalyst-supported carbon particles, an ionomer, and a solvent, or catalyst-supported ceramic particles, a carbon material, an ionomer, and a solvent.

[0033] The solvent is not particularly limited as long as it can dissolve or disperse the ionomer and catalyst. Examples 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.), etc., which can be used individually or in combination. Furthermore, 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 lower is preferred.

[0034] The concentration of the solute (catalyst-supported carbon particles, ionomer or catalyst-supported ceramic particles, carbon material, ionomer) in the catalyst layer slurry is, for example, 1 to 80% by mass, preferably 5 to 60% by mass, and more preferably 10 to 40% by mass.

[0035] Catalyst layers can be prepared by applying a dispersion treatment. Dispersion methods include ball milling, bead milling, roll milling, shear milling, wet milling, ultrasonic dispersion, and homogenizing.

[0036] Conventional coating methods can be used to apply the catalyst layer slurry onto the substrate. Specific coating methods include, for example, roll coaters, air knife coaters, blade coaters, rod coaters, reverse coaters, bar coaters, comma coaters, die coaters, gravure coaters, screen coaters, sprayers, and spinners. Furthermore, there are no particular restrictions on the coating method, as long as a similar film electrode assembly can ultimately be obtained.

[0037] A catalyst layer can be obtained by applying a slurry for the catalyst layer onto 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 40 to 120°C. The drying time is 0.5 minutes to 1 hour, preferably 1 minute to 30 minutes. Furthermore, the drying process may consist of a single drying mechanism or a combination of multiple drying mechanisms. The average thickness of the catalyst layer obtained by drying the catalyst layer slurry is, for example, 0.1 to 100 μm, preferably 0.5 to 50 μm, and more preferably about 1 to 20 μm.

[0038] The substrate used in the transfer process is not particularly limited as long as it can be coated with a catalyst layer slurry on at least one side, can form a catalyst layer by heating, and can transfer the formed catalyst layer to a solid polymer film. For example, polymer films such as polyethylene terephthalate, polyamide, polyimide, polystyrene, polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyetherimide, polybenzimidazole, polyamideimide, polyacrylate, polyethylene naphthalate, and polyparvanate aramid can be used, 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.

[0039] Alternatively, these substrates may be treated with a release agent, or they may be multilayered in which a release layer is integrated by co-extrusion or the like. The base material may be a sheet, film, plate, membrane, or foil, or at least one of these may be bonded, adhered, fused, or laminated.

[0040] (Method for manufacturing a membrane electrode assembly) Methods for manufacturing a membrane electrode assembly include forming a catalyst layer on a transfer substrate or gas diffusion layer and then forming the catalyst layer on a solid polymer film by thermocompression bonding, or directly forming the catalyst layer on a solid polymer film. One possible method is to form a catalyst layer directly onto a solid polymer film. This method is preferable because it provides high adhesion between the solid polymer film and the catalyst layer, and there is no risk of the catalyst layer being crushed. [Examples]

[0041] The following describes embodiments of the present invention. The present invention is not limited to the embodiments shown below. Unless otherwise specified, the values ​​in the table mean "parts by weight".

[0042] (evaluation) Using the MEAs obtained in the examples and comparative examples, single cells were assembled and installed in a power generation evaluation device (FCE-1, manufactured by Panasonic Production Engineering Co., Ltd.), and the following evaluations were performed.

[0043] [Startup / Shutdown Durability Test] Durability tests were conducted under conditions of a cell temperature of 80°C and relative humidity of 100%RH, with hydrogen gas supplied to the anode side and nitrogen gas to the cathode side, within a potential range of 1.0V-1.5V.

[0044] [Measurement of oxygen gas diffusion resistance in the catalyst layer] Under conditions of a cell temperature of 80°C and relative humidity of 80%RH, hydrogen gas was supplied to the anode side, and nitrogen gas diluted to an oxygen concentration of 2% was supplied to the cathode side. The back pressure was varied to 0, 20, 50, 80, and 100 kPa, and the critical current density (Ilim) was measured for each, and the gas diffusion resistance (Rtotal) was calculated. From the pressure dependence data of Rtotal, the gas diffusion resistance (Rother) of the catalyst layer was calculated.

[0045] Let ΔV1 be the rate of change in gas diffusion resistance during the start-stop endurance test of the film electrode assembly from 0 to 2500 cycles. Let ΔV2 be the rate of change in gas diffusion resistance during the start-stop endurance test of the film electrode assembly from 2500 to 5000 cycles. The absolute value (x) is calculated from the formula |(ΔV1-ΔV2) / ΔV1|. The absolute value (x) can be evaluated as follows: If the absolute value (x) is in the range of 0 to 0.2, then it is marked with "◎". If the absolute value (x) is in the range of 0 to 0.5 (excluding the above range), then "○" If the absolute value (x) is outside the range of 0 to 0.5, mark it as "×".

[0046] (Example 1) (Preparation of anode catalyst ink) Carbon black (Pt / CB, manufactured by Tanaka Kikinzoku, TEC10E50E, Pt load 46 wt%) supported with 0.45 g of platinum (Pt) and ionomer (manufactured by DuPont, Nafion® DE521) were mixed in a volume ratio of 1.0. This mixture, along with 2.5 g of ethanol, 2 g of water, and a zirconia ball (5 mm in diameter), was placed in a zirconia pot and mixed for 60 minutes using a planetary ball mill (manufactured by Fritsch, P-6). The mixture obtained from this ball milling will hereafter be referred to as anode catalyst ink.

[0047] (Preparation of cathode catalyst ink) As an electrode catalyst, platinum-supported silicon carbide ([Si] / [C]=1:1.3, Pt support amount 40% by mass) and ionomer (Du Pont, Nafion® DE521) were mixed in a mass ratio of 0.5. This mixture, along with 2.5 g of ethanol, 2 g of water, and a zirconia ball (5 mm in diameter), was placed in a zirconia pot and mixed for 60 minutes using a planetary ball mill (Fritsch, P-7). The electrode catalyst was prepared by compounding a carbon material with silicon carbide obtained by the method described in Japanese Patent Application Publication No. 2023-148962 and mixing it with a dispersion containing a noble metal colloid.

[0048] (Fabrication of membrane electrode assemblies (MEAs)) Using the obtained anode and cathode catalyst inks, an anode catalyst layer and a cathode catalyst layer were fabricated on a solid electrolyte membrane (DuPont, Nafion NR212) using a spray coating apparatus (A-Sing Technologies) such that the platinum weight of the anode was 0.5 mg / cm² and the platinum weight of the cathode was 0.3 mg / cm². A fuel cell electrode membrane (CCM), consisting of an anode catalyst layer or a cathode catalyst layer and a polymer electrolyte membrane, was hot-pressed (140°C, pressure 2.86kN) for 3 minutes using a hot press machine (Shinto Kogyo Co., Ltd., CYPM-10).

[0049] In the above CCM, gas diffusion layers (GDL, 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 anode catalyst layer were stacked on a polymer electrolyte membrane so that they faced each other. The gas diffusion resistance was measured every 2,500 cycles of the start-stop endurance test, and the ΔV1 of the cell consisting of the MEA was 1.52 × 10⁻⁶. -3 ΔV² is 1.36 × 10⁻⁶ -3 It was confirmed that the absolute value (x) is 0.105.

[0050] (Example 2) The MEA was obtained and its absolute value (x) was determined by the same method as in Example 1, except that the electrode catalyst of the cathode catalyst layer was changed to platinum-supported silicon nitride. The electrode catalyst was prepared by compounding a carbon material with silicon nitride obtained by the method described in Japanese Patent Application Publication No. 2024-021564 and mixing it with a dispersion containing a noble metal colloid.

[0051] (Example 3) The MEA was obtained and its absolute value (x) was determined by the same method as in Example 1, except that the electrode catalyst in the cathode catalyst layer was changed to platinum-supported silicon oxycarbide. The electrode catalyst was prepared by the method described in Patent No. 7533800.

[0052] (Comparative Example 1) An MEA was obtained in the same manner as in the example, except that the electrode catalyst of the cathode catalyst layer was changed to platinum-supported carbon used in the anode catalyst layer. The gas diffusion resistance was measured every 2,500 cycles of the start-stop endurance test, and the ΔV1 of the cell consisting of the MEA was 2.0 × 10⁻⁶. -3 ΔV² is 1.0 × 10⁻⁶ -4 It was confirmed that the absolute value (x) is 6.0.

[0053] [Table 1]

[0054] As shown in Table 1, in Comparative Example 1, the rate of change in gas diffusion resistance in the second half of the start-stop endurance test (2500-5000 cycles) was significantly larger than the rate of change in gas diffusion resistance in the first half of the start-stop endurance test (0-2500 cycles). It was confirmed that the power generation performance deteriorated rapidly with the rapid increase in gas diffusion resistance. On the other hand, in Examples 1-3, no rapid increase in gas diffusion resistance was observed, and the condition of the catalyst layer was considered to be maintained, resulting in stable power generation performance over a long period of time.

Claims

1. The system comprises a proton-conducting solid electrolyte membrane, an anode catalyst layer, a cathode catalyst layer, and a gas diffusion layer provided outside the anode and cathode catalyst layers. The cathode catalyst layer comprises catalyst-supported ceramic particles, a carbon material, and an ionomer. A membrane electrode assembly in which the absolute value (x) calculated by the measurement method described below is within the range of 0 to 0.

5. (Measurement method) ΔV1 is defined as the rate of change in gas diffusion resistance during the start-stop durability test of the film electrode assembly from 0 to 2500 cycles. ΔV2 is defined as the rate of change in gas diffusion resistance during 2500 to 5000 cycles of the start-stop durability test of the film electrode assembly. The absolute value (x) is calculated from the formula |(ΔV1 - ΔV2) / ΔV1|.

2. The film electrode assembly according to claim 1, wherein the ratio of carbon material to catalyst-supported ceramic particles is 5.5% by mass to 95.0% by mass.

3. The film electrode assembly according to claim 1, wherein the mass ratio of the total mass of the catalyst-supported ceramic particles and carbon material to the ionomer is 1:0.01 to 1:

30.

4. The membrane electrode assembly according to claim 1, wherein the catalyst-supported ceramic particles have the element silicon.

5. A solid polymer fuel cell comprising a membrane electrode assembly according to any one of claims 1 to 4.

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