JP2024063350A - Electrode catalyst layer and membrane electrode assembly - Google Patents

Abstract

[Problem] To provide an electrode catalyst layer, a membrane electrode assembly, and a solid polymer electrolyte fuel cell that can improve drainage and gas diffusion in an electrode catalyst and exhibit high power generation performance. [Solution] An electrode catalyst layer 10 includes a catalyst material 13, a conductive support 14, an ionomer 15, and a fibrous material 16. In a Voronoi diagram in which the center of gravity of each void V in the cross section of the electrode catalyst layer 10 is used as a generating point, when the standard deviation of the area of the Voronoi regions is ASD and the average area of the Voronoi regions is AAV, the degree of dispersion of the area of the Voronoi regions defined by ASD/AAV is 0.4 to 0.55. [Selected Figure] Figure 1

Description

The present invention relates to an electrode catalyst layer and a membrane electrode assembly.

In recent years, fuel cells have been attracting attention due to the growing interest in a decarbonized society. A fuel cell is a system that oxidizes fuel such as hydrogen using an oxidizing agent such as oxygen, and converts the resulting chemical energy into electrical energy. Fuel cells are classified into alkaline, phosphoric acid, polymer, molten carbonate, solid oxide, etc. depending on the type of electrolyte used. Polymer electrolyte fuel cells (PEFCs) that use solid polymer membranes operate at low temperatures and can be made small and lightweight while maintaining a high output density, so they are expected to be used as portable power sources, domestic power sources, and power sources for mobile vehicles.

A polymer electrolyte fuel cell (PEFC) comprises a polymer electrolyte membrane and a pair of electrodes. The pair of electrodes are called the fuel electrode (anode) and the air electrode (cathode), and are laminates that comprise a catalyst layer and a gas diffusion layer from the electrolyte membrane side. When a fuel gas containing hydrogen is supplied to the anode side and an oxidant gas containing oxygen is supplied to the cathode side, electricity is generated through the following chemical reactions:

Anode: _H2 → 2H+ + ^2e-
Cathode: 1/ _2O2 + 2H+ + ^2e- → _H2O
As shown in the chemical reaction formula above, fuel gas supplied to the anode electrode catalyst layer becomes protons and electrons through the action of the catalyst contained in the catalyst layer. The protons are conducted through the polymer electrolyte (ionomer) and polymer electrolyte membrane contained in the electrode catalyst layer, and move to the cathode electrode catalyst layer. The electrons pass through an external circuit and move to the cathode electrode catalyst layer, just like the protons. At the cathode electrode catalyst layer, the protons and electrons that have moved there react with oxidant gas supplied from the outside, producing water. Through this series of processes, the electrons pass through an external circuit, producing electrical energy.

To improve the performance of fuel cells, it is important to increase the electronic conductivity, proton conductivity, and gas diffusivity to the three-phase interface, which is the site of the redox reaction. The gas diffusion path within the electrode catalyst layer is the pores in the electrode catalyst layer, and these pores also serve as the discharge path for water produced by the reaction within the cathode electrode catalyst layer. Therefore, designing the three-dimensional structure within the electrode catalyst layer is important for improving the performance of fuel cells.

Ionomer and polymer electrolyte membranes have proton conductivity, but since moisture is required for proton conduction, fuel gas and oxidant gas are usually humidified before being supplied.

As for the configuration of the electrode catalyst layer, Patent Document 1 proposes, for example, an electrode catalyst layer containing carbon or carbon fibers of different particle sizes as a means of controlling the structure of the electrode catalyst layer so that it does not become dense and improving power generation performance (Patent Documents 1 and 2).

Japanese Patent Application Laid-Open No. 10-241703 Patent No. 5537178

Patent Documents 1 and 2 state that by including different carbon materials, pores are generated in the electrode catalyst layer, which is expected to improve drainage and gas diffusion. However, increasing the porosity increases the thickness, which lengthens the travel distance of electrons or protons and increases resistance. This reduces power generation performance. Furthermore, although there is a description of the size, shape, and content of the carbon material, there is no description of the structure of the catalyst layer, and its effects have not been specifically verified.

The present invention has been made with a focus on the above points, and aims to provide an electrode catalyst layer, a membrane electrode assembly, and a solid polymer fuel cell that can improve drainage and gas diffusion in the electrode catalyst and exhibit high power generation performance.

[1] An electrode catalyst layer comprising a catalyst material, a conductive support, an ionomer, and a fibrous material, in which, in a Voronoi diagram in which the center of gravity of each void in the cross section of the electrode catalyst layer is the generating point, the standard deviation of the area of the Voronoi regions is ASD and the average area of the Voronoi regions is AAV, the degree of dispersion of the area of the Voronoi regions defined by ASD/AAV is 0.4 to 0.55.

[2] The electrode catalyst layer according to claim 1, wherein the ratio of the area of all voids in the cross section to the total area of the cross section is 4 to 12%.

[3]: A membrane electrode assembly including the electrode catalyst layer described in [1] or [2].

The electrode catalyst layer of the present invention improves the material transport properties in the electrode catalyst layer, making it possible to provide an electrode catalyst layer, a membrane electrode assembly, and a solid polymer electrolyte fuel cell that are capable of exhibiting high power generation performance.

FIG. 2 is a diagram showing a schematic structure of a catalyst layer according to one embodiment of the present invention. FIG. 1 is a cross-sectional view illustrating a structure of a membrane electrode assembly according to one embodiment of the present invention. 1 is an exploded perspective view showing the internal structure of a unit cell of a polymer electrolyte fuel cell according to an embodiment of the present invention;

Embodiments of the present invention will be described below with reference to the drawings. Note that the present invention is not limited to the embodiments described below, and modifications such as design changes can be made based on the knowledge of those skilled in the art, and such modified embodiments are also included in the scope of the present invention. In addition, the drawings are appropriately exaggerated to make them easier to understand.

[Configuration of electrode catalyst layer]
As shown in FIG. 1, the electrode catalyst layer 10 according to this embodiment has a catalyst material 13, a conductive support 14, an ionomer 15, and a fibrous material 16.

(Catalyst material)
As the catalyst material 13 according to this embodiment, for example, metal group elements, metals, and their alloys, oxides, double oxides, carbides, etc. can be used. Examples of platinum group elements include platinum, palladium, ruthenium, iridium, rhodium, and osmium. Examples of metals include iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum. Among them, platinum and platinum alloys are preferable as the catalyst material 13. In addition, the particle size of these catalyst materials is preferably 0.5 to 20 nm because if it is too large, the catalytic activity decreases, and if it is too small, the stability of the catalyst material decreases. More preferably, it is 1 to 5 nm. The particle size of the catalyst material is the volume average diameter D50 measured by the laser diffraction/scattering method.

(Conductive Support)
Any conductive support 14 may be used as long as it is in the form of fine particles, has electrical conductivity, and is capable of supporting the catalytic substance without being affected by the catalytic substance and the polymer electrolyte, but carbon particles are generally used as the conductive support 14. Examples of the carbon particles that can be used include carbon black, graphite, activated carbon, carbon nanotubes, carbon nanofibers, and fullerenes.

The particle size of the carbon particles is preferably about 10 to 1000 nm, and more preferably about 10 to 100 nm. If the particle size is too small, it may become too dense in the electrode catalyst layer, reducing the gas diffusion properties of the electrode catalyst layer, and if the particle size is too large, it may cause cracks in the electrode catalyst layer or reduce the catalyst utilization rate, which is not preferable. The particle size of the carbon particles is the volume average diameter D50 measured by the laser diffraction/scattering method. In addition, by supporting the catalyst substance on a conductive support with a high specific surface area, the catalyst substance can be supported at a high density, improving the catalytic activity.

(Ionomer (Polyelectrolyte))
The ionomer contained in the polymer electrolyte membrane or the electrode catalyst layer may be any one having proton conductivity, and may be a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. The fluorine-based polymer electrolyte may be a polymer electrolyte having a tetrafluoroethylene skeleton, such as Nafion (manufactured by Chemours, registered trademark), and the hydrocarbon-based polymer electrolyte may be, for example, sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, or sulfonated polyphenylene. Considering the interface resistance between the polymer electrolyte membrane and the electrode catalyst layer and the dimensional change between the polymer electrolyte membrane and the electrode catalyst layer when the humidity changes, it is preferable that the ionomer contained in the polymer electrolyte membrane and the ionomer contained in the electrode catalyst layer are the same or have similar components.

(Fibrous material)
The fibrous material may be any material that can maintain its fibrous shape without being affected by the catalyst material and the ionomer, and examples of such materials include nanofibers made by processing polymer compounds, carbon, and conductive oxides into fibers. The fibrous material may be one of the fibers shown below, or two or more of them may be used in combination. In order to reduce the resistance of the electrode catalyst layer, the fibrous material preferably exhibits electronic conductivity or proton conductivity. In order to simultaneously improve the material transportability and proton conductivity in the electrode catalyst layer, it is preferable that the fibrous material contains at least a polymer fiber having proton conductivity or basicity.

Examples of electronically conductive fibrous materials include carbon nanofibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. In particular, carbon nanofibers are preferred in terms of conductivity and dispersibility. In addition, the use of catalytically conductive fibers is more preferable because it reduces the amount of catalyst made of precious metals used. When used as an air electrode for a polymer electrolyte fuel cell, for example, a carbon alloy catalyst made from carbon nanofibers can be exemplified. In addition, an electrode active material for an oxygen reduction electrode processed into a fiber form may be used, and for example, a material containing at least one transition metal element selected from Ta, Nb, Ti, and Zr may be used. Examples of the material include partial oxides of carbonitrides of these transition metal elements, or conductive oxides and conductive oxynitrides of these transition metal elements.

Examples of proton-conductive fibrous materials include nanofibers made by processing proton-conductive polymer electrolytes into fibers, and for example, fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes can be used. 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. In addition, acid-doped polybenzoazoles that exhibit proton conductivity by doping with acid can also be used suitably.

In addition, examples of fibrous materials include resin fibers having a basic functional group in the main chain skeleton or side chain functional group that can interact with acidic substances. These interact with the acidic substances contained in the polymer electrolyte, and the surface of the resin fiber is covered with the polymer electrolyte, making it possible to exhibit proton conductivity. In particular, by having a basic functional group containing a nitrogen (N) atom in its molecular structure, the polymer electrolyte can be uniformly covered on the polymer fiber, and the material transport property and proton conductivity in the electrode catalyst layer 10 can be improved at the same time. Examples of basic functional groups that can interact with acidic substances include =N- group, -NH ₂ group, >NH group, >N- group, ammonium group, amine derivatives, pyridine derivatives, imidazole derivatives, imidazolium group, etc. Specific examples of resin fibers include polybenzimidazole (PBI), polybenzoxazole, polybenzothioazole, polyvinyl imidazole, polyallylamine, etc., and in particular, polybenzimidazole (PBI) having an azole structure is preferable from the viewpoint of proton conductivity and processing.

The average fiber diameter of the fibrous material is preferably 100 to 400 nm, more preferably 100 to 250 nm. By setting the average fiber diameter within this range, the voids in the electrode catalyst layer can be increased and a decrease in conductivity can be suppressed, enabling a higher output. The average fiber length of the fibrous material is preferably 1 to 100 μm, more preferably 5 to 50 μm. By setting the average fiber length within this range, the strength of the electrode catalyst layer can be increased, and thus the occurrence of cracks when forming the electrode catalyst layer can be suppressed. In addition, the voids in the electrode catalyst layer can be increased, enabling a higher output.

(Structure of catalyst layer)
As shown in the structure of the catalyst layer in FIG. 1, the electrode catalyst layer 10 has voids V which are regions where none of the catalyst material 13, the conductive support 14, the ionomer 15, and the fibrous material 16 are present.

In a Voronoi diagram in which the center of gravity of each void V in the cross section of the electrode catalyst layer 10 is used as the generating point, when the standard deviation of the area of the Voronoi regions is ASD and the average area of the Voronoi regions is AAA, the degree of dispersion of the area of the Voronoi regions expressed as ASD/AAV satisfies 0.40 to 0.55. The degree of dispersion may be 0.54 or less, or may be 0.53 or less.

First, a method for detecting voids in the cross section of the electrode catalyst layer will be described. The voids V in the cross section of the electrode catalyst layer can be detected based on an image of the cross section of the electrode catalyst layer observed with a scanning electron microscope (SEM). For example, the void portion can be extracted by image processing from an SEM observation image taken in a field of view that includes the entire thickness of the electrode catalyst layer at an observation magnification of about 5000 to 20000 times.

The conditions for acquiring the electron microscope image are not particularly limited as long as the accelerating voltage does not damage the ionomer contained in the electrode catalyst layer, but for example, an accelerating voltage of 1 kV to 3 kV is preferable. In addition, when capturing an image, it is necessary to adjust the brightness and contrast of the image so that the brightness histogram falls within a range of 0 to 255. In addition, the size of the void that can be detected depends on the size and magnification of the image to be saved, and it is preferable that the resolution is at least 0.02 μm/px or more. In order to reduce errors, it is preferable to capture and measure images in the same manner in at least five or more, preferably 10 or more, fields of view. There are no limitations on the method for exposing the cross section of the electrode catalyst layer as long as the shape of the catalyst layer can be maintained during processing, and known methods such as ion milling and ultramicrotome can be used. The cross section is a cross section along the thickness direction.

Next, we will explain how to calculate the variance of the area of the Voronoi region in a Voronoi diagram with the center of gravity of each void V as the generating point.

A "Voronoi diagram" is a diagram in which perpendicular bisectors (Voronoi division lines) are drawn on the line connecting two adjacent kernel points on a plane, and the plane is divided into the regions (Voronoi regions) closest to each kernel point by connecting the perpendicular bisectors.

In this embodiment, a Voronoi diagram is obtained by using the centers of gravity of the voids V in the cross section of the catalyst layer as the generating points. The degree of dispersion of the areas of the Voronoi regions is defined as ASD/AAV, where ASD is the standard deviation of the areas of the Voronoi regions and AAV is the arithmetic mean area of the areas of the Voronoi regions. The smaller the value of this degree of dispersion, the greater the number of voids distributed in the cross section with high spatial dispersion.

In this specification, a Voronoi diagram is created based on each of the above SEM images of at least five or more fields of view, and the standard deviation and arithmetic mean area are calculated based on the areas of all the Voronoi regions included in each field of view. The Voronoi area may be 0.04 to 0.17 ^μm2 .

In addition, the percentage of the area of all voids V in the entire cross section can be calculated based on the void portion. The percentage of the area of all voids in the cross section with respect to the total area of the cross section may be 4 to 14%. This percentage may be 12% or less, 11% or less, or 5% or more.

(Thickness of electrode catalyst layer)
The thickness of the electrode catalyst layer is preferably 1 μm or more and 10 μm or less. If the thickness is thicker than 10 μm, cracks are likely to occur, and when used in a fuel cell, the diffusibility and conductivity of gas and generated water are reduced, resulting in a decrease in output. If the thickness is thinner than 1 μm, the layer thickness is likely to vary, and the catalyst material, conductive carrier, and ionomer are likely to become non-uniform. Cracks on the surface of the electrode catalyst layer and non-uniformity in thickness and material have a negative effect on the durability of the fuel cell when operated for a long period of time.

The thickness of the electrode catalyst layer can be measured, for example, by observing the cross section of the membrane electrode assembly using a scanning electron microscope (SEM). For example, it is possible to measure the thickness of the electrode catalyst layer by measuring the length within a field of view that contains the entire catalyst layer at an observation magnification of about 1000 to 100,000 times. In order to grasp the thickness without bias, it is preferable to measure similarly at at least 20 or more observation points. For example, known methods such as ion milling and ultramicrotome can be used to expose the cross section of the membrane electrode assembly.

[Configuration of membrane electrode assembly]
Next, referring to Fig. 2, a membrane electrode assembly including an electrode catalyst layer according to this embodiment will be specifically described. As shown in Fig. 2, the membrane electrode assembly 20 includes a polymer electrolyte membrane 1 and electrode catalyst layers 10 bonded to each surface of the polymer electrolyte membrane 1. In this embodiment, the electrode catalyst layer 10 formed on the upper surface of the polymer electrolyte membrane 1 is a cathode-side electrode catalyst layer 2 constituting an oxygen electrode, and the electrode catalyst layer 10 formed on the lower surface of the polymer electrolyte membrane is an anode-side electrode catalyst layer 3 constituting a fuel electrode. Hereinafter, the pair of electrode catalyst layers may be abbreviated to "electrode catalyst layers" when there is no need to distinguish between them. The outer periphery of the electrode catalyst layer may be sealed by a gasket or the like (not shown).

[Method of manufacturing electrode catalyst layer and membrane electrode assembly]
The manufacturing method of the above-mentioned electrode catalyst layer and membrane electrode assembly will be described below. First, a catalyst ink is prepared. At least the above-mentioned catalyst material, conductive support, ionomer, and fibrous material are mixed in a dispersion medium, and then the mixture is subjected to a dispersion treatment to prepare the catalyst ink. The dispersion treatment can be performed using, for example, a planetary ball mill, a bead mill, an ultrasonic homogenizer, or the like.

The solvent used as the dispersion medium of the catalyst ink may be any solvent that does not corrode the catalyst material, conductive carrier, polymer electrolyte, or fibrous material, and that can dissolve the polymer electrolyte or disperse it as a fine gel in a highly fluid state. Commonly used solvents include water, alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, and tert-butyl alcohol, ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, methyl butyl ketone, methyl isobutyl ketone, methyl amyl ketone, pentanone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, diethyl ketone, dipropyl ketone, and diisobutyl ketone, tetrahydrofuran, tetrahydropyran, dioxane, diethylene glycol dimethyl ether, and the like. Other examples of the solvent that may be used include ethers such as ether, anisole, methoxytoluene, diethyl ether, dipropyl ether, dibutyl ether, and the like, amines such as isopropylamine, butylamine, isobutylamine, cyclohexylamine, diethylamine, aniline, and the like, esters such as propyl formate, isobutyl formate, amyl formate, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, isopentyl acetate, methyl propionate, ethyl propionate, butyl propionate, and the like, as well as acetic acid, propionic acid, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, and the like. Examples of glycol and glycol ether solvents include ethylene glycol, diethylene glycol, propylene glycol, ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diacetone alcohol, 1-methoxy-2-propanol, and 1-ethoxy-2-propanol.
The solvent is preferably a mixture of water and alcohol, with a preferred water:alcohol weight ratio of 30:70 to 70:30.

It is desirable for the catalyst ink to contain at least a volatile liquid organic solvent, but since lower alcohols used as solvents have a high risk of ignition, it is preferable to use such solvents in a mixed solvent with water. There are no particular restrictions on the amount of water added, so long as it is enough to prevent the polymer electrolyte from separating and becoming cloudy or gelling.

The catalyst ink thus prepared is applied to a substrate and then dried, thereby removing the solvent components from the coating of the catalyst ink and forming an electrode catalyst layer on the substrate. The substrate may be a polymer electrolyte membrane, a transfer substrate, a gas diffusion layer, or the like.

When a polymer electrolyte membrane is used as the substrate, for example, a catalyst ink can be applied directly to the surface of the polymer electrolyte membrane, and then the solvent can be removed from the coating of the catalyst ink to form an electrode catalyst layer. Then, a catalyst ink can be applied directly to the surface opposite the polymer electrolyte membrane so as to face the electrode catalyst layer across the polymer electrolyte membrane, and then the solvent can be removed from the coating of the catalyst ink to form an electrode catalyst layer, thereby obtaining a membrane electrode assembly.

When using a transfer substrate or a gas diffusion layer, for example, an electrode catalyst layer is formed on the transfer substrate or the gas diffusion layer, and then, with the surface of the electrode catalyst layer in contact with the polymer electrolyte membrane, heating and pressure are applied to bond the electrode catalyst layer and the polymer electrolyte membrane, thereby producing a membrane electrode assembly.

The method of forming a catalyst layer directly on a polymer electrolyte membrane is preferred because it provides strong adhesion between the polymer electrolyte membrane and the catalyst layer and there is no risk of the catalyst layer being crushed.

Various coating methods can be used to apply the catalyst ink to the substrate. Examples of the coating methods include die coating, roll coating, curtain coating, spray coating, and squeegee. It is preferable to use die coating as the coating method. Die coating is preferable because the film thickness during the coating period is stable and intermittent coating can be performed. Methods for drying the coating film of the catalyst ink include drying using a hot air oven, IR (far infrared) drying, drying using a hot plate, and reduced pressure drying. The drying temperature is 40°C or higher and 200°C or lower, and preferably 40°C or higher and 120°C or lower. The drying time is 0.5 minutes or higher and 1 hour or lower, and preferably 1 minute or higher and 30 minutes or lower.

When forming an electrode catalyst layer on a transfer substrate or a gas diffusion layer, the pressure and temperature applied to the electrode catalyst layer during transfer affect the power generation performance of the membrane electrode assembly. In order to obtain a membrane electrode assembly with high power generation performance, the pressure applied to the electrode catalyst layer is preferably 0.1 MPa or more and 20 MPa or less. By setting the pressure to 20 MPa or less, excessive compression of the electrode catalyst layer is suppressed. By setting the pressure to 0.1 MPa or more, a decrease in power generation performance due to a decrease in the bonding strength between the electrode catalyst layer and the polymer electrolyte membrane is suppressed. In consideration of improving the bonding strength at the interface between the polymer electrolyte membrane and the electrode catalyst layer and suppressing the interface resistance, the temperature during bonding is preferably near the glass transition point of the polymer electrolyte membrane or the polymer electrolyte contained in the electrode catalyst layer.

For example, a polymer film and a sheet formed of a fluororesin can be used as the transfer substrate. Fluororesin has excellent transferability. Examples of fluororesins include ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkylvinylether copolymer (PFA), and polytetrafluoroethylene (PTFE). Examples of polymers that form the polymer film include polyimide, polyethylene terephthalate, polyamide (nylon (registered trademark)), polysulfone, polyethersulfone, polyphenylene sulfide, polyether ether ketone, polyetherimide, polyarylate, and polyethylene naphthalate.

Here, by adjusting the blending ratio of the ionomer (polymer electrolyte) in the electrode catalyst layer, the blending ratio of the fibrous material in the electrode catalyst, the solvent composition of the catalyst ink, the dispersion strength when preparing the catalyst ink, the heating temperature and heating rate of the applied catalyst ink, etc., it is possible to set the degree of dispersion of the area of the Voronoi region in the Voronoi diagram with the center of gravity of the void in the cross section of the electrode catalyst layer as the generating point to 0.55 or less.

For example, the amount of ionomer (polymer electrolyte) in the electrode catalyst layer is preferably about the same as or about half the mass of the conductive carrier, that is, 50 to 100 mass %.
The blending ratio of the fibrous material in the electrode catalyst layer is preferably about 1 to 100% by mass relative to the mass of the conductive carrier. If the blending ratio of the fibrous material 16 in the electrode catalyst layer is less than 1% by mass relative to the mass of the conductive carrier, the effect of reducing the proton conduction resistance and improving the gas diffusion is not sufficiently obtained, and cracks are generated when forming the electrode catalyst layer, resulting in a decrease in durability during long-term operation. On the other hand, if the blending ratio of the fibrous material in the electrode catalyst layer is more than 100% by mass relative to the mass of the conductive carrier, the catalytic reaction may be inhibited and the battery performance may decrease. In addition, when the fibrous material is a polymer fiber, it can be blended in the ink in the form of a dispersion liquid in advance to increase the dispersibility of the fibrous material in the electrode catalyst layer, and as a result, the Voronoi dispersion degree of the voids can be reduced.

The amount of the solvent in the ink for the catalyst layer is preferably 5 to 20% by mass, and may be 10% by mass or more, based on the total mass of the ink for the catalyst layer, of the catalyst substance, the conductive carrier, and the polymer electrolyte. The heating temperature after application of the ink is preferably 75 to 90° C.
The drying is preferably carried out by leaving the material stationary, but the material may be dried by moving it within a furnace, in which case the moving speed is preferably 4 m/min or less.
The solid content ratio of the catalyst layer ink is preferably as high as possible within the range that allows the ink to be applied in a thin film. The static viscosity of the catalyst ink may be 300 to 7000 mPa.

(Action and Effect)
That is, the electrode catalyst layer of this embodiment includes a catalyst material, a conductive carrier that supports the catalyst material, a polymer electrolyte, and a fibrous material, and in the cross section of the catalyst layer, the degree of dispersion of the Voronoi area of a Voronoi diagram with the center of gravity of the void as the generating point is 0.55 or less. With this configuration, the voids are spatially uniformly dispersed, so that drainage and gas diffusion are improved, and it is possible to provide an electrode catalyst layer for a polymer fuel cell that is capable of high output.
Furthermore, when the area fraction of voids is 4 to 12%, an appropriate amount of voids is ensured, and material transport is more efficient.

The electrode catalyst layer of this embodiment is highly suitable for use in, for example, polymer electrolyte fuel cells.

[Configuration of membrane electrode assembly]
Next, the polymer electrolyte fuel cell 11 will be described with reference to Fig. 3. Fig. 3 is an exploded perspective view showing an example of the configuration of a polymer electrolyte fuel cell 11 equipped with a membrane electrode assembly 20 including an electrode catalyst layer 10 according to this embodiment.
The membrane electrode assembly 20 includes a polymer electrolyte membrane 1 and electrode catalyst layers 2 (10), 3 (10) bonded to the front and back surfaces of the polymer electrolyte membrane 1, respectively. In this embodiment, the electrode catalyst layer 10 formed on the upper surface (front surface) of the polymer electrolyte membrane 1 is a cathode-side electrode catalyst layer 2 constituting an oxygen electrode, and the electrode catalyst layer 10 formed on the lower surface (back surface) of the polymer electrolyte membrane 1 is an anode-side electrode catalyst layer 3 constituting a fuel electrode. Hereinafter, the pair of electrode catalyst layers 2, 3 may be abbreviated as "electrode catalyst layer 10" when there is no need to distinguish them. In the membrane electrode assembly 20 according to this embodiment, the electrode catalyst layer 10 may be provided on at least one surface of the polymer electrolyte membrane 1. In addition, in order to prevent gas leakage from the outer periphery of the polymer electrolyte membrane 1 to which the electrode catalyst layer 10 is not bonded, a gasket 16C on the oxygen electrode side and a gasket 16A on the fuel electrode side are arranged in the membrane electrode assembly 20.

The solid polymer electrolyte fuel cell 11 further includes a gas diffusion layer 4 on the cathode (oxygen electrode) side and a gas diffusion layer 5 on the anode (fuel electrode) side. The gas diffusion layer 4 is disposed opposite the electrode catalyst layer 10, which is the cathode side electrode catalyst layer 2 on the oxygen electrode side of the membrane electrode assembly 20. The gas diffusion layer 5 is disposed opposite the electrode catalyst layer 10, which is the anode side electrode catalyst layer 3 on the fuel electrode side of the membrane electrode assembly 20. The electrode catalyst layer 2 and the gas diffusion layer 4 form the oxygen electrode 6, and the electrode catalyst layer 3 and the gas diffusion layer 5 form the fuel electrode 7.

The solid polymer electrolyte fuel cell 11 further includes a separator 30C arranged opposite the oxygen electrode 6, and a separator 30A arranged opposite the fuel electrode 7. The separator 30C includes a gas flow path 8 for flowing reactant gas formed on the surface facing the gas diffusion layer 4, and a cooling water flow path 9 for flowing cooling water formed on the surface opposite to the surface on which the gas flow path 8 is formed.

Separator 30A has the same configuration as separator 30C, and includes a gas flow path 8 formed on the surface facing gas diffusion layer 5, and a cooling water flow path 9 formed on the surface opposite to the surface on which gas flow path 8 is formed. Separators 30C and 30A are made of a material that is conductive and gas impermeable.

The polymer electrolyte fuel cell 11 generates electricity by supplying an oxidant such as air or oxygen to the oxygen electrode 6 through the gas flow path 8 of the separator 30C, and supplying a fuel gas containing hydrogen or an organic fuel to the fuel electrode 7 through the gas flow path 8 of the separator 30A.

[Example 1]
(Slurry for cathode catalyst layer)
20 g of platinum-supported carbon (TEC10E50E, manufactured by Tanaka Kikinzoku Co., Ltd.) (10 g of catalyst and 10 g of carbon (conductive carrier)) was placed in a container, water was added and mixed, and then 6 g of 1-propanol, polymer electrolyte (Nafion (registered trademark) dispersion, Wako Pure Chemical Industries: 60% by mass of polymer electrolyte relative to the mass of carbon (conductive carrier)), and 5 g of carbon nanofiber (manufactured by Showa Denko KK, product name "VGCF", fiber diameter about 150 nm, fiber length about 10 μm) as a fibrous substance (50% by mass relative to the mass of carbon (conductive carrier)) were added and stirred to obtain a slurry for a cathode catalyst layer. The mass ratio of water and 1-propanol, which are the solvents for the ink, was 60:40, and the amounts of water and 1-propanol were set so that the total mass of the platinum-supported carbon and polymer electrolyte accounted for 13% by mass of the slurry for the catalyst layer.
(Slurry for anode catalyst layer)
20 g of platinum-supported carbon (TEC10E50E, manufactured by Tanaka Kikinzoku Co., Ltd.) (10 g of catalyst and 10 g of carbon (conductive carrier)) was placed in a container, water was added and mixed, and then 1-propanol, 10 g of polymer electrolyte (Nafion (registered trademark) dispersion, Wako Pure Chemical Industries: 100% by mass of polymer electrolyte relative to the mass of carbon (conductive carrier)), and 5 g of carbon nanofiber (manufactured by Showa Denko KK, product name "VGCF", fiber diameter about 150 nm, fiber length about 10 μm) as a fibrous substance (50% by mass relative to the mass of carbon (conductive carrier)) were added and stirred to obtain a slurry for an anode catalyst layer. The mass ratio of water and 1-propanol, which are the solvents for the ink, was 70:30, and the amounts of water and 1-propanol were set so that the total mass of the platinum-supported carbon and polymer electrolyte accounted for 10% by mass of the slurry for the catalyst layer.
The obtained catalyst layer slurry was applied to both sides of a polymer electrolyte membrane (Nafion 212, manufactured by Chemours) by die coating and dried in an oven at 80° C. to obtain a membrane electrode assembly having electrode catalyst layers (cathode and anode) of Example 1. The degree of dispersion of the area of the Voronoi region defined by ASD/AAV in the cross section of the obtained catalyst layer was 0.51.

[Example 2]
In producing the slurry for the cathode catalyst layer, a membrane electrode assembly having an electrode catalyst layer (cathode) of Example 2 was obtained in the same procedure as in Example 1, except that resin fibers having an azole structure (fiber diameter: approximately 300 nm, fiber length: approximately 20 μm) were mixed as a fibrous material in advance in the form of a dispersion liquid, the amount of the fibrous material was 1 g, which was 10 mass % relative to the mass of the carbon (conductive support), and the amounts of water and 1-propanol were set so that the total mass of the platinum-supported carbon and the electrolyte accounted for 10 mass % of the slurry for the catalyst layer.

[Example 3]
In producing the slurry for the cathode catalyst layer, a membrane electrode assembly having an electrode catalyst layer (cathode) of Example 3 was obtained in the same procedure as in Example 1, except that a resin fiber having an azole structure (fiber diameter: approximately 200 nm, fiber length: approximately 20 μm) was used as the fibrous material, the amount of the fibrous material was 1 g, which was 10 mass % relative to the mass of the carbon (conductive carrier), and the amounts of water and 1-propanol were set so that the total mass of the platinum-supported carbon and the electrolyte accounted for 10 mass % of the slurry for the catalyst layer.

[Example 4]
In producing the slurry for the cathode catalyst layer, a membrane electrode assembly having an electrode catalyst layer (cathode) of Example 4 was obtained in the same procedure as in Example 1, except that resin fibers having an azole structure (fiber diameter: approximately 200 nm, fiber length: approximately 20 μm) were used as the fibrous material, the amount of the fibrous material was 0.5 g, which was 5 mass % relative to the mass of the carbon (conductive carrier), the amounts of water and 1-propanol were set so that the total mass of the platinum-supported carbon and the electrolyte accounted for 10 mass % of the slurry for the catalyst layer, and the mass ratio of water and 1-propanol, which are the solvents for the ink, was 70:30.

[Example 5]
In producing the slurry for the cathode catalyst layer, a membrane electrode assembly having an electrode catalyst layer (cathode) of Example 3 was obtained in the same procedure as in Example 1, except that resin fibers having an azole structure (fiber diameter: approximately 300 nm, fiber length: approximately 20 μm) were used as the fibrous material, the amount of the fibrous material was 1 g, which was 10 mass % relative to the mass of the carbon (conductive support), the amount of the polymer electrolyte added was 45 mass % relative to the mass of the carbon (conductive support), i.e., 4.5 g, which was 3/4 of Example 1, the amounts of water and 1-propanol were set so that the total mass of the platinum-supported carbon and the electrolyte accounted for 10 mass % of the slurry for the catalyst layer, and the mass ratio of water and 1-propanol, which was the ink solvent, was 70:30.

[Comparative Example 1]
In producing the slurry for the cathode catalyst layer, no fibrous material was added, the amounts of water and 1-propanol were set so that the total mass of the platinum-supported carbon and the electrolyte accounted for 7 mass% of the slurry for the catalyst layer, and the mass ratio of water and 1-propanol as the ink solvent was set to 70:30. The slurry was applied to a PET substrate and transferred to the electrolyte membrane by thermocompression bonding. Except for these, a membrane electrode assembly having an electrode catalyst layer (cathode) of Comparative Example 1 was obtained in the same manner as in Example 1.

[Comparative Example 2]
In producing the slurry for the cathode catalyst layer, a membrane electrode assembly having an electrode catalyst layer (cathode) of Comparative Example 2 was obtained by the same procedure as in Example 1, except that resin fibers having an azole structure (fiber diameter: approximately 200 nm, fiber length: approximately 20 μm) were used as the fibrous material, the amount of the fibrous material was 1 g, which was 10% by mass relative to the mass of the carbon (conductive support), the amounts of water and 1-propanol were set so that the total mass of the platinum-supported carbon and the electrolyte accounted for 9% by mass of the slurry for the catalyst layer, the amount of the polymer electrolyte added was 120% by mass relative to the mass of the carbon (conductive support), i.e., 12 g, which was twice that of Example 1, and the mass ratio of water and 1-propanol, which are the solvents of the ink, was 70:30.

The power generation evaluation and cross-sectional structure analysis of the membrane electrode assemblies of the examples and comparative examples were performed. In the power generation evaluation, a voltage of 0.630 V or more at 1.5 A/cm2 was evaluated as ◯, and an x was evaluated as below.

A cryo-cross-section polisher (JEOL Ltd.) was used to expose the cross-section, and a scanning electron microscope SU8010 (Hitachi High-Technologies Corporation) was used as the electron microscope.

The size of the image to be analyzed must be uniform, and the analysis was performed using an image of 10,000 times magnification, 1280 x 960 pixels. The extraction of voids was performed using the Trainable Weka Segmentation function in ImageJ Fiji, a free software widely used for processing and analyzing electron microscope images. A void in the cross section of the electrode catalyst layer is a region in which none of the catalyst material, conductive support, ionomer, or fibrous material exists, but in the extraction of voids, the outermost surface of the cross section is a void, and a portion whose depth is deeper than one primary particle diameter of the conductive support was extracted as a void. When the depth of the void is about one primary particle diameter of the conductive support or less, it was considered to be a depression that does not contribute to gas diffusion or drainage. Specifically, 15 or more regions of the conductive support and catalyst material, ionomer, fibrous material, and void regions were labeled and segmented.
Table 1 shows the degree of dispersion determined by the Voronoi division method using voids as the generating points, the ratio of voids to the cross section of the pores, and the results of the power generation performance evaluation.

It was found that MEAs with a degree of dispersion of 0.4 to 0.55, as determined by the Voronoi tessellation method, showed high performance. Furthermore, MEAs with voids accounting for 4 to 12% of the total cross section showed high performance. It is believed that the performance under high loads was improved as a result of optimizing the ink composition, drying temperature, etc., which increased the dispersibility of the voids and improved the material transport properties of the entire electrode catalyst layer.

2, 3, 10...electrode catalyst layer, 20...membrane electrode assembly, 13...catalyst material, 14...conductive support, 15...ionomer, 16...fibrous material, V...void.

Claims (3)

An electrode catalyst layer comprising a catalyst material, a conductive support, an ionomer, and a fibrous material,
An electrode catalyst layer, wherein in a Voronoi diagram using the center of gravity of each void in a cross section of the electrode catalyst layer as a generating point, when a standard deviation of the area of the Voronoi regions is ASD and an average area of the Voronoi regions is AAV, the degree of dispersion of the area of the Voronoi regions defined by ASD/AAV is 0.40 to 0.55. The electrode catalyst layer according to claim 1, wherein the ratio of the area of all voids in the cross section to the total area of the cross section is 4 to 14%. A membrane electrode assembly comprising the electrode catalyst layer according to claim 1 or 2.

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