Electrode catalyst layer

Abstract

The objective is to provide an electrode catalyst layer that is effective in preventing the destruction of voids in the catalyst layer and catalyst-supported particles due to internal stress during the manufacturing process of membrane electrode assemblies for polymer electrolyte fuel cells. [Solution] An electrode catalyst layer comprising catalyst-supported particles and a polymer electrolyte, wherein the thickness of the electrode catalyst layer is 1 to 20 μm, and the L in the CIE LAB color system measured using a multi-angle spectrophotometer. * a * , and b * However, L * ≥16.0, 0.0≦a * ≤5.0, 0.5 ≤b * The present invention provides an electrode catalyst layer that satisfies ≤10.0.

Description

Technical Field

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

Background Art

[0002] A fuel cell is a device that generates electric power and heat through a chemical reaction that obtains water from hydrogen and oxygen. There are multiple types of fuel cells, such as phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), and polymer electrolyte fuel cells (PEFC). Among these, a polymer electrolyte fuel cell (PEFC) has a structure in which a catalyst layer that forms an anode (fuel electrode) on one side of a polymer electrolyte membrane and a cathode (air electrode) 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-supported 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 catalyst metal particle-supported mesoporous carbon 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] The membrane electrode assembly that constitutes a solid polymer fuel cell is composed of multiple layers, and since each layer has different material properties, internal stress is likely to occur during the manufacturing process. The main factors of the manufacturing process of the membrane electrode assembly and the accompanying internal stress include thermal stress, mechanical stress, and humidity change. For example, in the manufacturing process, in the scene where the catalyst layer is pressurized, there are thermal pressing for transferring the catalyst layer formed on the catalyst transfer film to the solid polymer electrolyte membrane, and tightening when incorporating the membrane electrode assembly into the cell. Such pressurization acts may destroy the voids in the catalyst layer and the catalyst-supported particles themselves, and they will not recover afterwards, which may result in a decrease in performance.

[0006] However, in any of the documents, the study on the performance deterioration due to pressurization is not sufficient and there is still room for improvement.

Means for Solving the Problems

[0007] As a result of intensive studies to achieve the above object, the inventors have found that when the values of L * , a * , and b * in the CIE LAB color system of the electrode catalyst layer are within a specific range, an excellent elastic modulus is obtained.

[0008] That is, the following configurations of the present invention are provided.

[0009] 〔1〕 An electrode catalyst layer containing catalyst-supported particles and a polymer electrolyte, wherein when the film thickness of the electrode catalyst layer is 1 to 20 μm, L * , a * , and b * measured using a multi-angle spectrophotometer in the CIE LAB color system satisfy L * ≧16.0, 0.0≦a * ≦5.0, 0.5≦b * ≦10.0.

[0010] ​

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

[0012] According to one embodiment of the present invention, the elastic modulus of the catalyst layer can be adjusted to a desired range, and as a result, degradation during the manufacturing process can be suppressed when used in a fuel cell, and stable performance can be expected. [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] (electrode catalyst layer) The electrode catalyst layer of the present invention (also referred to as the cathode catalyst layer in this specification) is an electrode catalyst layer comprising catalyst-supported particles and a polymer electrolyte, wherein the L color of the CIE LAB color system, measured using a multi-angle spectrophotometer when the thickness of the electrode catalyst layer is 1 to 20 μm, is * a * , and b * However, L * ≥16.0, 0.0≦a * ≤5.0, 0.5 ≤b * It satisfies the condition ≤ 10.0.

[0015] Said L * L * ≥16.0, L * Preferably, L is ≥ 18.0. * It is more preferable if it is ≥20.0. Also, the upper limit is not particularly limited, but L * Preferably, L is ≤ 80.0. * It is more preferable that L be ≤ 70.0. * It is particularly preferable if ≤ 60.0.* However, if the value is below the lower limit, the catalyst-supported particles are easily destroyed by pressurization, leading to a decrease in performance. Also, if the value is below the upper limit, the electrode catalyst layer exhibits a constant elastic modulus under pressure, making it suitable for the fabrication of a film electrode assembly. Note L * The lower and upper limits may be combined in any way within the aforementioned range.

[0016] The a * is 0.0 ≤ a * ≤ 5.0 and 0.0 ≤ a * Preferably, ≤2.5, and 0.0 ≤ a * It is more preferable if it is ≤1.5. * However, if the range is as described above, it is preferable because the particle diameter of the catalyst-supported particles is likely within a suitable range, resulting in the catalyst-supported particles being ubiquitous within the electrode catalyst layer.

[0017] The aforementioned b * is 0.5 ≤ b * ≤ 10.0 and 1.0 ≤ b * Preferably, ≤ 5.0, and 2.0 ≤ b * It is more preferable if ≤ 3.0. * However, if the range is within the aforementioned range, the electrode catalyst layer exhibits a constant elastic modulus under pressure, which is preferable for the fabrication of a film electrode assembly.

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

[0019] The elastic modulus of a film layer made of a general polymer resin is controlled by the molecular structure and crosslinking structure of the resin. However, since the electrode catalyst layer of a fuel cell is a composite of a polymer electrolyte and catalyst-supported particles, there are many unknowns as to how to control its elastic modulus. In this invention, L in the CIE LAB color system * a * , b * Upon focusing on this, it became clear that the elastic modulus of the catalyst layer falls within the desired range when it is within the aforementioned range. Although the details are unclear, the following mechanism of action is assumed. In the electrode catalyst layer, it is presumed 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, aggregates of catalyst-supported particles have low elasticity, and it is thought that when catalyst-supported particles are localized, they are easily induced to break under pressure. On the other hand, it is assumed that the presence of the polymer electrolyte acts as a kind of cushioning material, suppressing the breakdown of catalyst-supported particles and improving the elastic modulus. Furthermore, whether the catalyst-supported particles and the polymer electrolyte are in a suitable state is determined by L in the CIE LAB color system. * a * , b * It is presumed that this was indirectly indicated by [the source].

[0020] Said L * a * , b * If the range is outside this range, the elastic modulus will be too low or too high, resulting in insufficient pressure resistance and inability to ensure gas and ion flow paths. In other words, the conduction of gas, ions, and electrons will be hindered, and the active sites of the catalyst-supported particles will decrease, potentially hindering the fuel cell's performance.

[0021] <Catalyst-supported particles> In the present invention, catalyst-supported particles consist 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 include catalyst-supported carbon particles and catalyst-supported ceramic particles.

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

[0023] Furthermore, the carbon particles are not particularly limited as long as they can support the catalyst and are conductive, but examples include carbon black (acetylene black, furnace black, Ketjen black, etc.), graphite, graphite, activated carbon, fullerene, etc.

[0024] The ceramic particles only need to be able to support the catalyst, and preferably have silicon, more preferably silicon carbide, silicon oxycarbide, silicon nitrooxycarbide, or silicon nitride, and particularly preferably silicon carbide. The ceramic particles may also be in a compounded state with a carbon material, which will be described later. Compounding is possible by adding the carbon material when manufacturing the ceramic particles. Compounding is preferable because it allows the carbon material to be incorporated into the ceramic particles that form a three-dimensional skeletal structure, thereby imparting conductivity to the ceramic particles.

[0025] <Carbon materials> When ceramic particles are used as catalyst supports, it is desirable to incorporate carbon materials from the viewpoint of imparting conductivity. This incorporation can involve adding and compounding the carbon material during manufacturing, as described above, or adding the carbon material separately after manufacturing; however, the former is preferable from the viewpoint of the resulting power generation performance. Examples of such carbon materials 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% to 95.0% by mass, more preferably 10% to 90% by mass, and particularly preferably 20% to 80% by mass. It is preferable that the ratio of the carbon material to the catalyst-supported ceramic particles is within the above range, as this allows for the acquisition of the desired elastic modulus.

[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. It is preferable that the ratio of catalyst-supported particles to the total mass of the catalyst ink is within the above range, as this allows for the acquisition of a desired modulus of elasticity.

[0028] <Polymer electrolyte> The polymer electrolyte can be any material that has proton conductivity, and fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes can be used. Examples of fluorine-based polymer electrolytes include Nafion® from DuPont, Flemion® from Asahi Glass Co., Ltd., Aciplex® from Asahi Kasei Corporation, and Gore Select® from Gore. Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketones, sulfonated polyethersulfones, sulfonated polyetherethersulfones, sulfonated polysulfides, and sulfonated polyphenylenes. Among these, DuPont's Nafion®-based materials can be suitably used as the polymer electrolyte. Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketones, sulfonated polyethersulfones, sulfonated polyetherethersulfones, sulfonated polysulfides, and sulfonated polyphenylenes. In particular, DuPont's Nafion®-based materials can be suitably used as the polymer electrolyte.

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

[0030] <Other> Furthermore, the catalyst may also contain fibrous material. By including fibrous material, an intertwined fibrous structure is formed within the catalyst layer, improving strength and suppressing crack formation. In addition, the formation of suitable voids can be expected to improve power generation performance.

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

[0032] Examples of fibrous materials include conductive fibers and electrolyte fibers. Conductive fibers include, for example, carbon fibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. Electrolyte fibers are, for example, the polymer electrolytes mentioned above that have been processed into fibers.

[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 may increase, and the stability over time may decrease.

[0034] (Method for manufacturing an electrode catalyst layer) The electrode catalyst layer can be manufactured by preparing a catalyst ink, coating and drying it on a substrate or gas diffusion layer, and then thermocompressing the electrode catalyst layer onto a solid polymer electrolyte membrane. Catalyst ink consists 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 can 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 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.

[0037] Catalytic inks can be produced by applying a dispersion treatment, and examples of dispersion methods include ball milling, bead milling, roll milling, shear milling, wet milling, ultrasonic dispersion, and homogenizing.

[0038] Conventional coating methods can be used to apply catalyst ink to a 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.

[0039] A desired electrode catalyst layer can be obtained by coating a substrate with catalyst ink 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 electrode catalyst layer obtained by drying the catalyst ink is, for example, 0.1 to 100 μm, preferably 0.5 to 50 μm, and more preferably about 1 to 20 μm.

[0040] The substrate used in the transfer process is not particularly limited as long as it can be coated with 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, 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.

[0041] 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.

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

[0043] The formation of anode and cathode catalyst layers on a solid polymer electrolyte membrane, and the formation of gas diffusion layers to them, can be carried out using known and conventional methods. [Examples]

[0044] 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 refer to "parts by mass".

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

[0046] [Measurement of elastic modulus] The obtained electrode catalyst layer was prepared as a 1cm × 1cm sample piece and measured using a surface force measuring device ESF-5000Plus (ELIONIX) with an indentation load of 30-40 μN until the indentation depth reached 200 nm. With a sample size of N=60, the median value was evaluated as the indentation modulus. The indentation modulus was evaluated based on the following criteria. The indentation modulus is 1000 N / mm 2 If it is greater than or equal to the above, use "◎". The indentation modulus is 500 N / mm². 2 More than 1000N / mm 2 If it is less than, use "〇". The indentation modulus is 500 N / mm². 2 If it is less than, mark it with "×".

[0047] [L * a * b * measurement] The obtained electrode catalyst layer was used as a 6 cm diameter sample piece (film thickness as described in the Examples and Comparative Examples), and the L in the CIE LAB color system was measured. * a * , and b * The values ​​were measured. The measurements were taken using a multi-angle spectrophotometer MA94 (X-rite) with a 15° field of view and a 10° field of view with a D65 light source.

[0048] (Example 1) As catalyst-supported particles, catalyst-supported particles A (Pt / SiC / KB, [Si] / [C]=1:1.3, Pt support amount 40% by mass) and a polymer electrolyte (Du Pont, Nafion® DE521) were mixed in a mass ratio of 0.2. This mixture, along with 1.0 g of n-propanol, 1.0 g of water, and a zirconia ball (5 mm in diameter), was 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 compounding a carbon material (Ketjenbrack) 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.

[0049] Using the obtained catalyst ink, a K CONTROL COATER (manufactured by RK Print Coat Instrument) was used, and a platinum basis weight of 0.2 mg / cm² was applied to a 100 μm thick PTFE substrate with a gap of 100 μm. 2 An electrode catalyst layer (thickness 5 μm) was fabricated to achieve the above result.

[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-supported 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-supported particles to polymer electrolyte was set to 0.7.

[0052] (Comparative Example 1) As catalyst-supported particles, 0.5 g of catalyst-supported particles B (Pt / CB, manufactured by Tanaka Kikinzoku, TEC10E50E, Pt load 46 wt%) and a polymer electrolyte (manufactured by Du Pont, Nafion® DE521) were mixed in a mass ratio of 0.2. This mixture, along with 2.5 g of n-propanol, 2.5 g of water, and a zirconia ball (5 mm in diameter), was placed in a zirconia pot and mixed for 60 minutes in a planetary ball mill (manufactured by Fritsch, P-6) to obtain a catalyst ink.

[0053] A catalyst layer (film thickness 5 μm) was prepared according to 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-supported 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, manufactured by Tanaka Kikinzoku, TEC10E50E, Pt load 46 wt%) supported with 0.5 g of platinum (Pt) was mixed with a polymer electrolyte (manufactured by DuPont, Nafion® DE521) in a mass ratio of 0.7. This mixture, along with 2.5 g of n-propanol, 2.5 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). This ball milling yielded a catalyst ink.

[0058] Using the obtained catalyst ink, an anode catalyst layer was fabricated on a 100 μm thick PTFE substrate with a gap of 120 μm using a K CONTROL COATER (manufactured by RK Print Coat Instrument Co., Ltd.) so that the platinum basis weight was 0.2 mg / cm2.

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

[0060] In the aforementioned CCM, gas diffusion layers (GDL, made of SGL, 22BB) were stacked on both sides of each catalyst layer to obtain a membrane electrode assembly (MEA) in which the cathode catalyst layer and the anode catalyst layer were stacked on a solid polymer electrolyte membrane so that they faced each other.

[0061] The resulting membrane electrode assembly fuel cell exhibits stable power generation performance without degradation due to pressurization.

Claims

1. An electrode catalyst layer comprising catalyst-supported carbon particles or catalyst-supported ceramic particles as essential components, a polymer electrolyte, and an optional component, a fibrous material, wherein the mass ratio of the catalyst-supported carbon particles or catalyst-supported ceramic particles to the polymer electrolyte is 0.2 to 0.7, and when the thickness of the electrode catalyst layer is 1 to 20 μm, the L*, a*, and b* in the CIE LAB color system measured using a multi-angle spectrophotometer satisfy L* ≥ 16.0, 0.0 ≤ a* ≤ 5.0, and 0.5 ≤ b* ≤ 10.

0.

2. The electrode catalyst layer according to claim 1, wherein the catalyst support for the catalyst-supported ceramic particles is made of silicon carbide.

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

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