Method for manufacturing a catalyst layer for fuel cells

Abstract

This invention provides a method for manufacturing a catalyst layer for fuel cells that can selectively support a catalyst metal without increasing manufacturing costs. [Solution] One aspect of the present invention relates to a method for manufacturing a catalyst layer for a fuel cell, comprising: a carrier dispersion preparation step of preparing a carrier dispersion containing a carrier and an ionomer; a carrier dispersion coating step of coating the carrier dispersion onto a substrate; and a supporting step of subjecting the substrate coated with the carrier dispersion to solid-phase electrodeposition to support a catalyst metal on the carrier on the surface of the substrate.

Description

Technical Field

[0001] The present invention relates to a method for producing a catalyst layer for a fuel cell.

Background Art

[0002] A fuel cell electrochemically reacts hydrogen and oxygen to obtain electric power. In principle, the only product generated from the power generation of a fuel cell is water. Therefore, fuel cells have attracted attention as a clean power generation system that imposes almost no load on the global environment. A fuel cell is configured with a membrane electrode assembly (hereinafter also referred to as "MEA") as a basic unit, in which catalyst layers are disposed on both sides of an electrolyte membrane, and a gas diffusion layer is further disposed outside each catalyst layer. A polymer electrolyte having ion exchange groups (hereinafter also referred to as "ionomer") is generally used for the binder of the electrolyte membrane and the catalyst layer. During operation of the fuel cell, an electromotive force is obtained by supplying a fuel gas containing hydrogen to the catalyst layer on the anode (fuel electrode) side and an oxidizing gas containing oxygen to the catalyst layer on the cathode (air electrode) side, respectively. The oxidation reaction of hydrogen proceeds at the anode, and the reduction reaction of oxygen proceeds at the cathode, supplying an electromotive force to an external circuit. Various catalyst layers that can be used in fuel cells and methods for producing the same have been developed.

[0003] For example, Patent Document 1 describes a method for producing a fuel cell, which is characterized by comprising: a dispersing step of dispersing a carrier supporting a catalytic metal and an ion exchange resin in water using an ultrasonic homogenizer to prepare a dispersion liquid; a catalyst ink preparation step of adding an organic solvent to the dispersion liquid after the dispersing step and performing centrifugal stirring to prepare a catalyst ink; and a catalyst layer preparation step of applying the catalyst ink prepared in the catalyst ink preparation step onto an electrolyte membrane or a diffusion layer to prepare a catalyst layer.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

[0005] As described above, various catalyst layers and methods for manufacturing them that can be used in fuel cells have been developed. Conventional methods typically involve a step of applying a catalyst ink containing a carrier supporting a catalyst metal and an ionomer to a substrate and / or a solid electrolyte membrane. Since the catalyst ink contains a carrier supporting the catalyst metal, not only the surface of the carrier but also the surface of the catalyst metal supported on the carrier is coated with the ionomer. The ionomer coating the surface of the catalyst layer functions as a proton transport pathway, but it can also act as a barrier that hinders the supply of gas (especially oxidizing gases containing oxygen) to the catalyst metal, thereby reducing catalytic activity. When applying a catalyst ink containing a carrier supporting a catalyst metal and an ionomer, it is difficult to prevent the surface of the catalyst layer from being coated with the ionomer. Furthermore, increasing the amount of supported catalyst metal to improve catalytic activity increases the manufacturing cost of the catalyst layer. For this reason, there was room for improvement in achieving both proton transport capacity and catalytic activity in conventional methods.

[0006] Therefore, the present invention aims to provide a method for manufacturing a catalyst layer for fuel cells that can selectively support a catalyst metal without increasing manufacturing costs. [Means for solving the problem]

[0007] The inventors investigated various means to solve the above-mentioned problems. The inventors found that by applying a carrier dispersion containing a carrier and an ionomer but without a catalyst metal to a substrate, and then supporting the catalyst metal on the carrier on the surface of the substrate by solid-phase electrodeposition, the catalyst metal can be selectively supported on the carrier without increasing the amount of catalyst metal supported. Based on the above findings, the inventors completed the present invention.

[0008] In other words, the present invention encompasses the following aspects and embodiments. (Embodiment 1) A method for manufacturing a catalyst layer for a fuel cell, A carrier dispersion preparation step, in which a carrier dispersion containing a carrier and an ionomer is prepared. A carrier dispersion coating process, in which a carrier dispersion is applied to a substrate. A substrate coated with a carrier dispersion is subjected to solid-phase electrodeposition to support a catalyst metal on the carrier on the surface of the substrate; this is the supporting step. The method, including the method described above. (Embodiment 2) The method according to Embodiment 1, wherein the catalyst metal is platinum. (Embodiment 3) The method according to Embodiment 1 or 2, wherein the carrier is carbon. (Embodiment 4) The method according to any one of Embodiments 1 to 3, wherein in the loading step, the solid-phase electrodeposition includes placing a solid electrolyte membrane between the substrate and the anode, and applying a voltage between the substrate and the anode while pressing the substrate with the solid electrolyte membrane by the liquid pressure of the electrolyte containing a catalyst metal. (Embodiment 5) The method according to Embodiment 4, wherein the electrolyte is an aqueous solution of tetraammineplatinum(II) chloride. [Effects of the Invention]

[0009] The present invention makes it possible to provide a method for manufacturing a catalyst layer for fuel cells that can selectively support a catalyst metal without increasing manufacturing costs. [Brief explanation of the drawing]

[0010] [Figure 1] The images show a transmission electron microscope (TEM) image of the surface of the catalyst layer (Panel A), and a magnified TEM image of the area enclosed by a rectangle in Panel A (Panel B). [Figure 2] The image shows the elemental mapping of platinum on the surface of the catalyst layer using energy-dispersive X-ray spectroscopy (EDS). A is a TEM image of the surface of the catalyst layer, and B is the elemental mapping result of platinum in the region shown in panel A. [Figure 3] This is a TEM image of a cross-section of carbon paste on the surface of a substrate. [Figure 4]Figure 3 shows a magnified TEM image of the surface area enclosed by a rectangle (Panel A), and a magnified TEM image of the area shown in Panel A (Panel B). [Figure 5] Figure 3 shows a magnified TEM image of the area enclosed by the rectangle (1) (Panel A), and a magnified TEM image of the area containing the microparticles indicated by the arrow in Panel A (Panel B). [Figure 6] Figure 3 shows a magnified TEM image of the area enclosed by the rectangle (2) (Panel A), and a magnified TEM image of the area containing the microparticles indicated by the arrow in Panel A (Panel B). [Modes for carrying out the invention]

[0011] Preferred embodiments of the present invention will be described in detail below.

[0012] In each embodiment of the present invention, the fuel cell catalyst layer comprises at least a substrate, a carrier disposed on the surface of the substrate, a catalyst metal supported on the carrier, and an ionomer covering the surface of the carrier. The ionomer may cover not only the surface of the carrier but also at least a portion of the surface of the catalyst metal. Preferably, the ionomer does not substantially cover the surface of the catalyst metal.

[0013] In each embodiment of the present invention, the substrate constituting the catalyst layer for the fuel cell is usually in the form of a film. The substrate may be formed from a material commonly used in the art. Examples of such materials include thermosetting resins and thermoplastic resins, particularly thermoplastic resins such as polyethylene, polypropylene, cyclic polyolefin resins, polyphenylene ether resins, polyethylene naphthalate resins (PEN), polyetherimide, polyimide, polyphenylsulfone (PPS), polyethersulfone, polyphenylsulfone, or polysulfone.

[0014] In each embodiment of the present invention, examples of the carrier constituting the fuel cell catalyst layer include conductive carbon and oxides, as well as one or more mixtures thereof. The carbon is preferably carbon black (acetylene black, Ketjen black, and furnace black, etc.), activated carbon, graphite, glassy carbon, graphite, graphene, carbon fiber, carbon nanotubes, carbon nitride, carbon sulfide, carbon phosphide, channel black, roller black, disc black, oil furnace black, gas furnace black, lamp black, thermal black, or vulcan carbon, or one or more mixtures thereof. The oxide is preferably titanium oxide, niobium oxide, tin oxide, tungsten oxide, or molybdenum oxide, or one or more mixtures thereof. The carrier is preferably carbon, and more preferably carbon black.

[0015] In each embodiment of the present invention, examples of catalyst metals constituting the fuel cell catalyst layer include platinum, ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. The catalyst metal may consist of the metals exemplified above individually or as an alloy of two or more of them. The catalyst metal may also be an oxide, nitride, sulfide, or phosphide of the metals exemplified above. The catalyst metal is preferably platinum, a platinum alloy, or a composite containing platinum, and more preferably platinum. In the case of platinum alloys and platinum-containing composites, examples of metals other than platinum include ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, and yttrium. Platinum alloys and platinum-containing composites may contain two or more of the metals exemplified above.

[0016] In each aspect of the present invention, the ionomer constituting the catalyst layer for a fuel cell is usually a polymer electrolyte having ion exchange groups. Examples of the ion exchange groups contained in the polymer electrolyte include sulfonic acid groups, phosphoric acid groups, quaternary ammonium cation groups, and the like. Examples of the polymer constituting the polymer electrolyte include polymers containing perfluorocarbon, polyether ether ketone, polybenzimidazole, and the like as main components. The ionomer is preferably a perfluorocarbon sulfonic acid polymer.

[0017] One aspect of the present invention relates to a method for producing a catalyst layer for a fuel cell. The method of this aspect includes a carrier dispersion liquid preparation step, a carrier dispersion liquid application step, and a supporting step.

[0018] [1: Carrier dispersion liquid preparation step] This step includes preparing a carrier dispersion liquid containing a carrier and an ionomer.

[0019] The carrier and ionomer prepared in this step may be any material having the characteristics described above. Each material may be prepared by preparing one having predetermined characteristics by oneself, or may be prepared by purchasing a commercially available product or the like.

[0020] The carrier dispersion liquid may contain a solvent in addition to the carrier and the ionomer. The solvent is not particularly limited, and any liquid can be used. Examples of the solvent include water, alcohols, and mixtures of one or more thereof. Examples of the alcohol include methanol, ethanol, 1-propanol, 2-propanol, 2-methyl-2-propanol (tert-butyl alcohol), diacetone alcohol, ethylene glycol, propylene glycol, and the like.

[0021] In this process, the carrier dispersion is usually prepared by mixing the carrier and the ionomer, and optionally a solvent. The means of mixing the materials are not particularly limited. Examples of mixing means include ultrasonic homogenizers, jet mills, bead mills, ball mills, high shears, and film mixers. The specific conditions of the mixing means exemplified above (e.g., stirring speed, stirring time, and rotation speed) are not particularly limited and can be set appropriately within any range.

[0022] [2: Carrier dispersion coating process] This process includes applying a carrier dispersion to a substrate.

[0023] The base material used in this process may be any material having the characteristics described above. Each material may be prepared by the user themselves to have the specified characteristics, or it may be purchased as a commercially available product.

[0024] In this process, the means for applying the carrier dispersion to the substrate are not particularly limited. Examples of application methods include die coating, spin coating, screen printing, doctor blade, squeegee, spray coating, and applicator. The specific conditions for the application methods exemplified above are not particularly limited and can be set appropriately within any range.

[0025] This process may include removing the solvent from the carrier dispersion after coating. The removal of the solvent is not particularly limited and can be carried out by any means such as heating and drying. The specific conditions for solvent removal (e.g., temperature, pressure, and processing time) are not particularly limited and can be set appropriately within any range.

[0026] [3: Supporting process] This process includes subjecting a substrate coated with a carrier dispersion to solid-phase electrodeposition to support a catalyst metal on the carrier on the surface of the substrate.

[0027] The catalyst metal used in this process may be any material having the characteristics described above. Each material may be prepared by the user themselves, or it may be purchased commercially.

[0028] In the conventional method of coating a substrate and / or solid electrolyte membrane with a catalyst ink containing a carrier supporting a catalyst metal and an ionomer, since the catalyst ink contains a carrier supporting the catalyst metal, not only the surface of the carrier but also the surface of the catalyst metal supported on the carrier is coated with the ionomer. In contrast, it has been found that by subjecting a substrate coated with a carrier dispersion to solid-phase electrodeposition, the catalyst metal can be selectively supported on the carrier on the surface of the substrate. Solid-phase electrodeposition is known to be usable as a method for forming metal films of semiconductors and electronic components. However, it was not known that solid-phase electrodeposition could be applied to supporting catalyst metal on a carrier in a catalyst layer.

[0029] In this process, solid-phase electrodeposition can be carried out based on, for example, Japanese Patent Publication No. 7484865, Japanese Patent Publication No. 7505471, Japanese Patent Publication No. 7517250, and Japanese Patent Publication No. 7552548.

[0030] In one embodiment, solid-phase electrodeposition involves placing a solid electrolyte membrane between a substrate and an anode, and applying a voltage between the substrate and the anode while pressing the substrate with the solid electrolyte membrane using the liquid pressure of an electrolyte containing a catalyst metal. In this embodiment, the substrate is used as the cathode.

[0031] In this embodiment, the solid electrolyte membrane can be any material commonly used in solid-phase electrodeposition. Examples of such materials include fluorine-based ion-conducting polymers, such as proton-conducting ion-exchange polymers having perfluorosulfonic acid groups (i.e., perfluorosulfonic acid polymers).

[0032] In this embodiment, the anode is typically a plate-shaped metal plate. The anode can be any material commonly used in solid-phase electrodeposition. Preferably, the anode is made of a material insoluble in the electrolyte (e.g., titanium). The anode may also be coated with a catalytic metal contained in the electrolyte. For example, if the catalytic metal contained in the electrolyte is platinum, the anode is preferably a titanium-insoluble anode coated with platinum.

[0033] In this embodiment, the electrolyte is usually an aqueous solution containing a salt of the catalyst metal. For example, when the catalyst metal is platinum, the salt of the catalyst metal is preferably tetraammineplatinum(II) chloride, hexahydroxoplatinum nitrate, dinitrodiammineplatinum(II) nitrate, or hexahydroxoplatinum ammine complex, and more preferably tetraammineplatinum(II) chloride. When the catalyst metal is a platinum alloy, the electrolyte preferably contains, in addition to the platinum salt, a salt of a further metal that forms the platinum alloy as a salt of the catalyst metal. The concentration of the catalyst metal salt contained in the electrolyte is preferably in the range of 0.001 to 1 mol / L, and more preferably in the range of 0.01 to 0.1 mol / L.

[0034] In this embodiment, the electrolyte pressure is preferably in the range of 0.1 to 1 MPa, and more preferably in the range of 0.5 to 1 MPa.

[0035] In this embodiment, the applied voltage is preferably in the range of 0.1 to 10 V, and more preferably in the range of 1 to 10 V. The time for applying the voltage is preferably in the range of 1 to 120 seconds, and more preferably in the range of 1 to 60 seconds. The temperature for applying the voltage is preferably in the range of 20 to 100°C, and more preferably in the range of 50 to 100°C.

[0036] By carrying out this process under the conditions exemplified above, the catalyst metal can be selectively supported on the carrier on the surface of the substrate.

[0037] In the catalyst layer obtained by the method of this embodiment, the fact that the catalyst metal is selectively supported on the carrier on the surface of the substrate can be confirmed, for example, by observing the surface and / or cross-section of the catalyst layer with a transmission electron microscope (TEM). Alternatively, it can be confirmed by element mapping of the catalyst metal on the surface of the catalyst layer using an energy dispersive X-ray spectrometer (EDS).

[0038] In the catalyst layer obtained by the method of this embodiment, the composition and content of the carrier and the catalyst metal can be determined, for example, by dissolving and extracting the catalyst metal contained in the catalyst layer, and analyzing the metal elements contained in the extract by thermogravimetric analysis (TG) or inductively coupled plasma optical emission spectrometry (ICP).

[0039] The catalyst layer obtained by the method of this embodiment can be used, for example, as an electrode catalyst layer for fuel cells, water electrolysis systems or metal-air batteries applicable to uses such as power for moving bodies such as automobiles, railways, ships and aircraft, or power sources for domestic or commercial use. The method of this embodiment can selectively support the catalyst metal without increasing the manufacturing cost. Therefore, the method of this embodiment can provide fuel cells, water electrolysis systems or metal-air batteries at a lower manufacturing cost. [Examples]

[0040] Hereinafter, the present invention will be described more specifically with reference to Examples. However, the technical scope of the present invention is not limited to these Examples.

[0041] <I:Production of Catalyst Layer> A carrier dispersion liquid containing a carrier (carbon black) and an ionomer (perfluorocarbon sulfonic acid polymer) was prepared (carrier dispersion liquid preparation step). The prepared carrier dispersion liquid was applied onto a base material sheet to obtain a carbon paste-coated sheet (carrier dispersion liquid application step). An electrolytic solution (0.03 mol / L aqueous tetraammineplatinum(II) chloride solution) was prepared. A solid electrolyte membrane was disposed between the carbon paste-coated sheet and an anode (a titanium insoluble anode coated with platinum). While pressing the base material against the solid electrolyte membrane by the hydraulic pressure (0.6 MPa) of the platinum-containing electrolytic solution, solid-phase electrodeposition was performed by applying a voltage (5 V) between the base material and the anode at 70°C for 5 seconds, thereby allowing platinum to be supported on the carbon carrier on the surface of the carbon paste-coated sheet (supporting step).

[0042] <II:Evaluation of Catalyst Layer> The surface of the catalyst layer produced by the above procedure and the cross-section of the carbon paste contained in the catalyst layer were observed with a transmission electron microscope (TEM). Elemental mapping of platinum on the surface of the catalyst layer was also performed using an energy dispersive X-ray spectrometer (EDS).

[0043] A TEM image of the surface of the catalyst layer (Panel A) and an enlarged TEM image of the region surrounded by a rectangle in Panel A (Panel B) are shown in Figure 1. As shown in Figure 1, platinum fine particles were precipitated on the surface of the catalyst layer.

[0044] The results of elemental mapping of platinum on the surface of the catalyst layer obtained by EDS are shown in Figure 2. In the figure, A is a TEM image of the surface of the catalyst layer, and B is the elemental mapping result of platinum in the region shown in Panel A. From Figure 2, it was confirmed that the fine particles precipitated on the surface of the catalyst layer contain platinum.

[0045] Figure 3 shows a TEM image of the cross-section of the carbon paste on the surface of the substrate. Figure 4 shows a magnified TEM image of the surface region enclosed by a rectangle in Figure 3 (Panel A), and a magnified TEM image of the region indicated in Panel A (Panel B). Figure 5 shows a magnified TEM image of the interior (1) region enclosed by a rectangle in Figure 3 (Panel A), and a magnified TEM image of the region containing the fine particles indicated by the arrow in Panel A (Panel B). Figure 6 shows a magnified TEM image of the interior (2) region enclosed by a rectangle in Figure 3 (Panel A), and a magnified TEM image of the region containing the fine particles indicated by the arrow in Panel A (Panel B). As shown in Figures 3 to 6, in the catalyst layer, platinum fine particles were deposited not only on the surface of the carbon paste but also inside it.

[0046] From the above results, it has become clear that the method of the present invention can provide a catalyst layer in which the catalyst metal is supported not only on the surface of the support but also inside it.

[0047] It should be noted that the present invention is not limited to the embodiments described above, and various modifications are included. For example, the embodiments described above are described in detail to make the present invention easier to understand, and are not necessarily limited to those having all the configurations described. In addition, it is possible to add, delete, and / or replace some of the configurations in each embodiment with other configurations.

Claims

1. A method for manufacturing a catalyst layer for fuel cells, A carrier dispersion preparation step, in which a carrier dispersion containing a carrier and an ionomer is prepared. A carrier dispersion coating process, in which a carrier dispersion is applied to a substrate. A substrate coated with a carrier dispersion is subjected to solid-phase electrodeposition to support a catalyst metal on the carrier on the surface of the substrate; this is the supporting step. The method, including the method described above.

2. The method according to claim 1, wherein the catalyst metal is platinum.

3. The method according to claim 1, wherein the carrier is carbon.

4. The method according to claim 1, wherein the solid-phase electrodeposition step includes placing a solid electrolyte membrane between the substrate and the anode, and applying a voltage between the substrate and the anode while pressing the substrate with the solid electrolyte membrane by the liquid pressure of an electrolyte containing a catalyst metal.

5. The method according to claim 4, wherein the electrolyte is an aqueous solution of tetraammineplatinum(II) chloride.

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