JP7589762B2 - Membrane electrode assembly for polymer electrolyte fuel cell, polymer electrolyte fuel cell, and method for manufacturing membrane electrode assembly for polymer electrolyte fuel cell - Google Patents

Description

The present invention relates to a membrane electrode assembly for a polymer electrolyte fuel cell, a polymer electrolyte fuel cell, and a method for producing a membrane electrode assembly for a polymer electrolyte fuel cell.

Solid polymer fuel cells, which have a structure in which a polymer electrolyte membrane is sandwiched between a cathode electrode catalyst layer and an anode electrode catalyst layer, operate at room temperature and have a short start-up time, and are therefore expected to be used as a power source for automobiles and stationary power sources.
A conventional method for producing a membrane electrode assembly is known in which a catalyst ink consisting of catalyst-supported carbon particles, a polymer electrolyte, and a solvent is applied to a transfer substrate or a gas diffusion layer, and then the applied ink is thermocompressed to a polymer electrolyte membrane.
However, in the conventional method for producing a membrane electrode assembly by transfer, the adhesion between the electrode catalyst layer and the polymer electrolyte membrane is low, and gaps are easily generated between the electrode catalyst layer and the polymer electrolyte membrane. This causes problems such as a decrease in power generation performance due to interface resistance and a decrease in power generation performance due to flooding caused by water clogging the gaps.

To solve such problems, various techniques have been proposed. For example, Patent Document 1 discloses a technique in which ceramic particles are sprayed to form irregularities on the surface of a polymer electrolyte membrane, and an electrode catalyst layer is formed on the irregularities, so that the irregularities are embedded in the surface of the catalyst layer to improve adhesion. Patent Document 2 discloses a technique in which the interface between the electrode catalyst layer and the polymer electrolyte membrane is irradiated with laser light to heat it, thereby thermocompressing the interface to improve adhesion.
However, the techniques disclosed in Patent Documents 1 and 2 have the risk of reducing the durability of the membrane electrode assembly, and also of complicating the manufacturing process, resulting in reduced yields and increased costs.

JP 2007-26836 A JP 2009-176518 A

The present invention aims to provide a membrane electrode assembly for a polymer electrolyte fuel cell, a polymer electrolyte fuel cell, and a method for manufacturing a membrane electrode assembly for a polymer electrolyte fuel cell that has good adhesion at the interface between the electrode catalyst layer and the polymer electrolyte membrane.

A membrane electrode assembly for a polymer electrolyte fuel cell according to one embodiment of the present invention is a membrane electrode assembly for a polymer electrolyte fuel cell in which electrode catalyst layers are laminated on both sides of a polymer electrolyte membrane, the polymer electrolyte membrane containing a hydrocarbon-based polymer electrolyte, and no voids are present at the interface between the polymer electrolyte membrane and the electrode catalyst layers.
A membrane electrode assembly for a polymer electrolyte fuel cell according to another embodiment of the present invention is a membrane electrode assembly for a polymer electrolyte fuel cell in which electrode catalyst layers are laminated on both sides of a polymer electrolyte membrane, the electrode catalyst layers contain a catalyst, carbon particles, and a polymer electrolyte, the polymer electrolyte membrane contains a hydrocarbon-based polymer electrolyte, and at least one void is formed at an interface between the electrode catalyst layer and the polymer electrolyte membrane, and when a cross section of the membrane electrode assembly for a polymer electrolyte fuel cell cut along a plane perpendicular to the interface is observed with a scanning electron microscope, the height h of the void in a direction perpendicular to the interface is taken as h and the width w of the void in a direction parallel to the interface is taken as w, at each interface on both sides of the polymer electrolyte membrane, the height h of the void is 0.5 μm or less, and the sum of the widths w of the voids present within an area 30 μm long in a direction parallel to the interface is 10 μm or less.
A polymer electrolyte fuel cell according to yet another aspect of the present invention includes the polymer electrolyte fuel cell membrane electrode assembly according to the one or another aspect described above.

The present invention can provide a membrane electrode assembly for a polymer electrolyte fuel cell, a polymer electrolyte fuel cell, and a method for manufacturing a membrane electrode assembly for a polymer electrolyte fuel cell, which have good adhesion at the interface between the electrode catalyst layer and the polymer electrolyte membrane.

1 is an exploded perspective view showing the internal structure of a polymer electrolyte fuel cell according to one embodiment of the present invention; 1 is a diagram illustrating a structure of a membrane electrode assembly for a polymer electrolyte fuel cell according to one embodiment of the present invention. FIG. 4 is a diagram illustrating the structure of a membrane electrode assembly for a polymer electrolyte fuel cell according to another embodiment of the present invention. FIG. 2 is a schematic cross-sectional view illustrating an example of the structure of an interface between an electrode catalyst layer and a polymer electrolyte membrane. FIG. 4 is a schematic cross-sectional view illustrating another example of the structure of the interface between the electrode catalyst layer and the polymer electrolyte membrane.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the present embodiment is not limited to the embodiment described below, and modifications such as design changes based on the knowledge of those skilled in the art are possible, and such modified embodiments are also included in the scope of the present embodiment.
Moreover, in the following detailed description, specific details are set forth in order to provide a thorough understanding of the embodiments of the present invention. However, it will be apparent that one or more embodiments may be practiced without such specific details. In addition, for the sake of clarity in the drawings, well-known structures and devices may be shown in simplified form.

(Structure of a polymer electrolyte fuel cell)
1, a pair of electrode catalyst layers 3A, 3F are disposed on both sides of a polymer electrolyte membrane 2 constituting a solid polymer fuel cell 1, facing each other with the polymer electrolyte membrane 2 sandwiched therebetween. A gas diffusion layer 4A is disposed on the surface of the electrode catalyst layer 3A opposite to the surface facing the polymer electrolyte membrane 2, and a gas diffusion layer 4F is disposed on the surface of the electrode catalyst layer 3F opposite to the surface facing the polymer electrolyte membrane 2, so as to face each other with the polymer electrolyte membrane 2 and the pair of electrode catalyst layers 3A, 3F sandwiched therebetween.

On the surface of the gas diffusion layer 4A opposite to the surface facing the electrode catalyst layer 3A, a separator 5A is arranged, the main surface of which is provided with a gas flow path 6A for flowing reactive gas, and the main surface opposite to the main surface with the gas flow path 6A is provided with a cooling water passage 7A for flowing cooling water. Furthermore, on the surface of the gas diffusion layer 4F opposite to the surface facing the electrode catalyst layer 3F, a separator 5F is arranged, the main surface of which is provided with a gas flow path 6F for flowing reactive gas, and the main surface opposite to the main surface with the gas flow path 6F is provided with a cooling water passage 7F for flowing cooling water. Hereinafter, when there is no need to distinguish between them, the electrode catalyst layers 3A and 3F may be simply referred to as "electrode catalyst layer 3".

Fig. 2 is a schematic cross-sectional view showing an example of the configuration of the electrode catalyst layer according to the present embodiment. As shown in Fig. 2, the electrode catalyst layer 8 according to the present embodiment is bonded to the surface of a polymer electrolyte membrane 9, and is composed of a catalyst 10, carbon particles 11 as a conductive carrier, and a polymer electrolyte 12. In the electrode catalyst layer 8, portions where none of the components of the catalyst 10, the carbon particles 11, and the polymer electrolyte 12 are present are voids.
The polymer electrolyte membrane 9 according to the present embodiment may be a hydrocarbon-based polymer electrolyte membrane including a hydrocarbon-based polymer electrolyte, or may be a hydrocarbon-based polymer electrolyte membrane composed only of a hydrocarbon-based polymer electrolyte. In the present embodiment, the term "hydrocarbon-based polymer electrolyte membrane" refers to a membrane containing, for example, more than 50% by mass of a hydrocarbon-based polymer electrolyte, which will be described later, relative to the total mass of the polymer electrolyte membrane 9.

(Production of catalyst ink)
Next, a method for producing a catalyst ink for forming the electrode catalyst layers 3, 8 (electrode catalyst layers for a polymer electrolyte fuel cell) of the polymer electrolyte fuel cell 1 according to this embodiment will be described. First, carbon particles 11 carrying a catalyst 10 are mixed and dispersed in a dispersion medium to obtain a catalyst particle slurry.

As the catalyst 10, for example, metals such as platinum group elements (platinum, palladium, ruthenium, iridium, rhodium, osmium), iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, etc., and alloys, oxides, double oxides, carbides, etc. of these metals can be used.
Any material may be used as the carbon particles 11 as long as it is conductive and can support the catalyst 10 without being corroded by the catalyst 10, but carbon particles are generally used. Examples of carbon particles that can be used include carbon black, graphite, activated carbon, carbon nanotubes, carbon nanofibers, and fullerenes. If the particle size of the carbon particles is too small, it is difficult to form an electron conduction path, and if it is too large, the gas diffusion property of the electrode catalyst layer 8 decreases and the utilization rate of the catalyst decreases, so the particle size is preferably in the range of 10 nm to 1000 nm. More preferably, the particle size is in the range of 10 nm to 100 nm.

As the dispersion medium, for example, any one of water or alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, pentanol, etc. can be selected and used. It is also possible to use a mixture of two or more of the above-mentioned solvents. For mixing and dispersion, for example, a device such as a bead mill, a planetary mixer, or a dissolver can be used.

Next, the polymer electrolyte 12 is added to the catalyst particle slurry produced by the above method. As the polymer electrolyte 12, for example, a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte can be used. As the fluorine-based polymer electrolyte, for example, Nafion (registered trademark) manufactured by DuPont, Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., Aciplex (registered trademark) manufactured by Asahi Kasei Corporation, Gore Select (registered trademark) manufactured by Gore, etc. can be used. As the hydrocarbon-based polymer electrolyte, for example, electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene can be used. Among them, Nafion (registered trademark)-based materials manufactured by DuPont can be preferably used as the polymer electrolyte.

(Production of Membrane Electrode Assembly)
The membrane electrode assembly is manufactured by bonding the electrode catalyst layer 3 to both sides of the polymer electrolyte membrane 2. At this time, as a method for bonding the electrode catalyst layer 3 to the polymer electrolyte membrane 2, for example, a transfer substrate with an electrode catalyst layer formed by applying a catalyst ink to the transfer substrate is used, and the surface of the electrode catalyst layer of the transfer substrate with the electrode catalyst layer is brought into contact with the polymer electrolyte membrane and heated and pressurized to bond the polymer electrolyte membrane 2 and the electrode catalyst layer 3. When the transfer substrate with the electrode catalyst layer is used to bring the polymer electrolyte membrane 2 and the electrode catalyst layer 3 into contact with each other and heat and pressurize the electrodes, the pressure and temperature applied to the electrode catalyst layer 3 may affect the power generation performance of the membrane electrode assembly. In order to obtain a membrane electrode assembly with high power generation performance, it is desirable that the pressure applied to the laminate is in the range of 0.1 MPa to 20 MPa. If the pressure applied to the laminate is more than 20 MPa, the electrode catalyst layer 3 is overcompressed, and if it is less than 0.1 MPa, the adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2 may decrease, resulting in a decrease in power generation performance. In addition, in consideration of improving the bonding strength at the interface between the polymer electrolyte membrane 2 and the electrode catalyst layer 3 and suppressing the interface resistance, the temperature during bonding is preferably near the glass transition point of the polymer electrolyte membrane 2 or the polymer electrolyte 12 of the electrode catalyst layer 3.

However, the above method results in poor adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2, and voids are likely to form at the interface between the electrode catalyst layer 3 and the polymer electrolyte membrane 2. This tends to lead to problems such as a decrease in power generation performance due to interface resistance and a decrease in power generation performance due to flooding caused by water clogging the voids.

On the other hand, a membrane electrode assembly can also be manufactured by a method in which a catalyst ink is directly applied to the surface of the polymer electrolyte membrane 2, and then the solvent component (dispersion medium) is removed from the coating of the catalyst ink. As a method for directly applying the catalyst ink to the polymer electrolyte membrane 2, various coating methods can be used, such as die coating, roll coating, curtain coating, spray coating, and squeegee. In particular, die coating is preferred. Die coating has a stable film thickness in the middle of the coating and can also be used for intermittent coating. Furthermore, as a method for drying the applied catalyst ink, for example, a hot air oven, IR (far infrared) drying, a hot plate, and reduced pressure drying can be used. The drying temperature is in the range of 40°C to 200°C, preferably in the range of 40°C to 120°C. The drying time is in the range of 0.5 minutes to 1 hour, preferably in the range of 1 minute to 30 minutes.

According to this method, the adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2 is good, and the above problems are unlikely to occur. However, when applying the catalyst ink directly to the polymer electrolyte membrane 2, the swelling of the polymer electrolyte membrane 2 tends to cause wrinkles and cracks in the applied electrode catalyst layer 3, which can lead to reduced power generation performance and reduced durability. In particular, fluorine-based polymer electrolyte membranes have a low glass transition point and are prone to swelling, so wrinkles and cracks are likely to occur in the electrode catalyst layer 3 during the process of applying the catalyst ink directly to the polymer electrolyte membrane 2 and drying it.

In contrast, the hydrocarbon-based polymer electrolyte has a high glass transition point and is less likely to swell in the process of applying and drying the catalyst ink directly to the polymer electrolyte membrane 2. Therefore, by using a hydrocarbon-based polymer electrolyte membrane, which is a membrane containing a hydrocarbon-based polymer electrolyte, as in the present embodiment, even when the catalyst ink is applied directly to the polymer electrolyte membrane 2, wrinkles and cracks are less likely to occur in the electrode catalyst layer 3, and a membrane electrode assembly with good adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2 can be obtained. In addition, as the hydrocarbon-based polymer electrolyte contained in the hydrocarbon-based polymer electrolyte membrane, for example, electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene can be used.

The above-mentioned effects achieved when a hydrocarbon-based polymer electrolyte membrane is used as the polymer electrolyte membrane 2 will be described in detail below.
Ink containing a catalyst and alcohol is sometimes used as a catalyst ink for manufacturing an electrode catalyst layer, but the catalyst ink itself has a risk of ignition (combustion). Therefore, when using the catalyst ink, water is sometimes added to the catalyst ink to reduce the ignition (combustibility) of the ink itself.
Although the addition of water to the catalyst ink reduces the ignition (combustibility) of the ink itself, it has the disadvantage of slowing down the drying speed of the catalyst ink. Therefore, when manufacturing an electrode catalyst layer using the catalyst ink with added water, there is a need to increase the drying temperature of the catalyst ink from the usual temperature of about 80° C. to, for example, about 90° C.

Here, many of the fluoropolymer electrolyte membranes used as the polymer electrolyte membrane have a low glass transition point. Therefore, when a fluoropolymer electrolyte membrane is used as the polymer electrolyte membrane, the drying temperature of the catalyst ink may exceed the glass transition point of the fluoropolymer electrolyte membrane. In this case, the fluoropolymer electrolyte membrane swells, and the adhesion between the electrode catalyst layer and the fluoropolymer electrolyte membrane tends to decrease.
In contrast, the hydrocarbon-based polymer electrolyte membrane used in this embodiment often has a higher glass transition point than the fluorine-based polymer electrolyte membrane. For example, the glass transition point of the hydrocarbon-based polymer electrolyte membrane is 100° C. or higher. Therefore, when a hydrocarbon-based polymer electrolyte membrane is used as the polymer electrolyte membrane, even if the drying temperature of the catalyst ink is increased to, for example, about 90° C., the drying temperature rarely exceeds the glass transition point of the hydrocarbon-based polymer electrolyte membrane. As a result, swelling of the hydrocarbon-based polymer electrolyte membrane is extremely reduced, and the adhesion between the electrode catalyst layer and the hydrocarbon-based polymer electrolyte membrane tends to be improved compared to the adhesion between the electrode catalyst layer and the fluorine-based polymer electrolyte membrane.

On the other hand, a method of applying catalyst ink directly to the polymer electrolyte membrane 2 without causing wrinkles or cracks in the fluorine-based polymer electrolyte membrane is to add fibrous material 13 to the catalyst ink. If fibrous material 13 is added to the catalyst ink, the strength of the electrode catalyst layer 3 is increased, so that even when the catalyst ink is applied directly to the polymer electrolyte membrane 2, wrinkles and cracks are less likely to occur in the electrode catalyst layer 3, making it possible to obtain a membrane electrode assembly with good adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2.

FIG. 3 shows an example of the configuration of a membrane electrode assembly for a polymer electrolyte fuel cell, which includes an electrode catalyst layer 3 formed by adding a fibrous material 13 to a catalyst ink.
An electron conductive fiber and a proton conductive fiber can be used as the fibrous material 13. As the fibrous material 13, only one of the fibers shown below may be used alone, or two or more of them may be used in combination, or an electron conductive fiber and a proton conductive fiber may be used in combination.

Examples of the electronically conductive fiber according to this embodiment include carbon fiber, carbon nanotube, carbon nanohorn, conductive polymer nanofiber, etc. In particular, carbon nanofiber is 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 of a polymer electrolyte fuel cell, for example, a carbon alloy catalyst made from carbon nanofiber can be exemplified. In addition, an electrode active material for an oxygen reduction electrode may be processed into a fiber form, and for example, a material containing at least one transition metal element selected from Ta, Nb, Ti, and Zr may be used. Examples include partial oxides of carbonitrides of these transition metal elements, or conductive oxides and conductive oxynitrides of these transition metal elements.

The proton conductive fiber according to this embodiment may be a polymer electrolyte having proton conductivity processed into a fiber shape, and may be, for example, a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. As the fluorine-based polymer electrolyte, for example, Nafion (registered trademark) manufactured by DuPont, Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., Aciplex (registered trademark) manufactured by Asahi Kasei Corporation, Gore Select (registered trademark) manufactured by Gore, etc. may be used. As the hydrocarbon-based polymer electrolyte, for example, electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene may be used. Among them, Nafion (registered trademark)-based materials manufactured by DuPont may be preferably used as the polymer electrolyte.

The fiber diameter of the fibrous material 13 is preferably in the range of 0.5 nm to 500 nm, more preferably in the range of 5 nm to 200 nm. By setting the fiber diameter in this range, the number of pores in the electrode catalyst layer 3 can be increased, enabling a higher output.
The fiber length of the fibrous material 13 is preferably in the range of 1 μm to 40 μm, more preferably in the range of 1 μm to 20 μm. By setting the fiber length in this range, the strength of the electrode catalyst layer 3 can be increased, and the occurrence of wrinkles and cracks during formation can be suppressed. In addition, the number of pores in the electrode catalyst layer 3 can be increased, enabling higher output.

Here, the voids 14 in this embodiment will be described in detail with reference to Fig. 4. It is more preferable that the voids 14 do not exist at the interface between the electrode catalyst layer 8 and the polymer electrolyte membrane 9, but the voids 14 may occur. Here, the above-mentioned "there are no voids 14" means that even when the magnification of a scanning electron microscope (SEM) is set to 4000 times and the interface between the electrode catalyst layer 8 and the polymer electrolyte membrane 9 is observed, the presence of the voids 14 cannot be confirmed at the interface.
The cause of the voids 14 is the generation of minute irregularities on the surface of the electrode catalyst layer 8 when the electrode catalyst layer 8 is formed on a transfer substrate (not shown). As a result, when the electrode catalyst layer 8 is transferred to the polymer electrolyte membrane 9, voids 14 due to the irregularities are generated at the interface between the polymer electrolyte membrane 9 and the electrode catalyst layer 8.

Furthermore, even in a method in which the catalyst ink is applied directly to the polymer electrolyte membrane 9 without using a transfer substrate, if wrinkles or cracks occur in the electrode catalyst layer 8 formed by application, a corresponding void 14 will occur at the interface between the polymer electrolyte membrane 9 and the electrode catalyst layer 8.
In particular, when there is a gap 14 at the interface between the electrode catalyst layer 8 and the polymer electrolyte membrane 9 whose height h, which is the length in the direction perpendicular to the interface, exceeds 0.5 μm, or when there are a large number of gaps 14 with a height h of 0.5 μm or less in a certain region, problems such as a decrease in power generation performance and a decrease in durability are likely to occur.

However, in a fuel cell, water is generated by power generation, and when the fuel cell is in use, the generated water seeps into the polymer electrolyte membrane 9, causing the polymer electrolyte membrane 9 to swell. Therefore, it was found that even if there is a gap 14 between the electrode catalyst layer 8 and the polymer electrolyte membrane 9, if the height h of the gap 14 is 0.5 μm or less and the total width w of the gap 14 existing within an area having a length l of 30 μm in the direction parallel to the interface is 10 μm or less, the gap 14 will be filled by the swelling of the polymer electrolyte membrane 9.

In the example shown in FIG. 4, two gaps 14, 14 exist within a region having a length l of 30 μm in the direction parallel to the interface, and the sum of the widths w1, w2 of both gaps 14, 14 is 10 μm or less.
In this embodiment, when a cross section of a membrane electrode assembly for a polymer electrolyte fuel cell is cut along a plane perpendicular to the interface and observed by SEM, the length of the gap 14 in a direction perpendicular to the interface is defined as height h, and the length of the gap 14 in a direction parallel to the interface is defined as width w.

Therefore, by satisfying the above two numerical conditions for the voids 14 occurring at the interface between the polymer electrolyte membrane 9 and the electrode catalyst layer 8, it becomes possible to prevent a decrease in power generation performance due to the interfacial resistance between the electrode catalyst layer 8 and the polymer electrolyte membrane 9, and a decrease in power generation performance due to flooding caused by clogging of the voids 14 with water. The height h of the voids 14 needs to be 0.5 μm or less, and is more preferably 0.3 μm or less. This is because if the height h of the voids 14 is 0.3 μm or less, the voids 14 are easily filled even if the swelling rate of the polymer electrolyte membrane 9 is low.
Furthermore, when the sum of the widths w of the voids 14 existing within a region having a length l of 30 μm in the direction parallel to the interface exceeds 10 μm, the width of the voids 14 becomes wider, and the voids 14 are difficult to fill even if the polymer electrolyte membrane 9 swells.

The voids 14 can be confirmed by observing the cross section of the membrane electrode assembly for polymer electrolyte fuel cells cut along a plane perpendicular to the interface using a SEM. The type of SEM is not particularly limited, but for example, the S-4800 manufactured by Hitachi High-Technologies Corporation can be used. The magnification during SEM observation is not particularly limited, but can be, for example, 4000x.

The above-mentioned effects can be achieved if the height h and width w of the voids 14 present at the interface between one side of the polymer electrolyte membrane 9 and the electrode catalyst layer 8 are within the above ranges, but it is more preferable that the height h and width w of the voids 14 present at the interface with the electrode catalyst layer 8 on both sides of the polymer electrolyte membrane 9 are within the above ranges.

Furthermore, as shown in FIG. 5, it is even more preferable that the voids 14 that exist at the same position in the direction parallel to the interface on both sides of the polymer electrolyte membrane 9, or that exist so as to overlap each other, simultaneously satisfy the above range. In other words, by having the voids 14 that exist within a region of 30 μm in length in the direction parallel to the interface on both sides of the interface of the polymer electrolyte membrane 9 both satisfy the above two numerical conditions, the reaction efficiency on the anode side and the cathode side can be further increased.

The thickness of the electrode catalyst layer 8 is preferably 5 μm or more and 30 μm or less, and particularly preferably 20 μm or less. If the thickness of the electrode catalyst layer 8 is greater than 30 μm, more precisely, if it is greater than 20 μm, the electrode catalyst layer 8 is likely to crack, and further, when the electrode catalyst layer 8 is used in a fuel cell, the diffusibility and conductivity of gas and generated water may decrease, resulting in a decrease in output. If the thickness of the electrode catalyst layer 8 is thinner than 5 μm, the layer thickness is likely to vary, and the catalyst and polymer electrolyte inside may become non-uniform.
Also, for example, the blending ratio of the polymer electrolyte 12 in the electrode catalyst layer 8 is preferably about the same as or about half the weight of the carbon particles 11. Also, the blending ratio of the fibrous material 13 is preferably about the same as or about half the weight of the carbon particles 11. The solids ratio of the catalyst ink is preferably as high as possible within the range that allows it to be applied to a thin film.

(Effects of this embodiment)
According to this embodiment, it is possible to manufacture a membrane electrode assembly having good adhesion between the electrode catalyst layer 8 and the polymer electrolyte membrane 9 and having excellent power generation performance and durability, without using any complicated steps.

Examples and comparative examples of the present invention will be described below.
Example 1
A platinum-supported carbon catalyst (TEC10E50E, Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, and a polymer electrolyte (Nafion (registered trademark) dispersion, Wako Pure Chemical Industries, Ltd.) were mixed, and each component was dispersed using a bead mill disperser to a degree that did not result in overdispersion, to produce a catalyst ink. The solids content of the catalyst ink thus produced was 10 mass %. The mass ratio of water to 1-propanol was 1:1. The conditions for dispersing each component using the bead mill disperser were set as follows. The following conditions were also used in common for the following examples and comparative examples.
Number of passes: 5 times Ball (bead) diameter: 0.3 mm
Agitator peripheral speed: 10 m/sec.

The hydrocarbon-based polymer electrolyte membrane was produced by sulfonating super engineering plastics using a known method.

The produced catalyst ink was directly applied to both surfaces of a hydrocarbon-based polymer electrolyte membrane using a slit die coater and dried to form an electrode catalyst layer, thereby obtaining a membrane electrode assembly.
The membrane electrode assembly thus obtained was first sliced using a microtome (EM UC7 ultramicrotome manufactured by Leica). Next, the interface between the electrode catalyst layer and the polymer electrolyte membrane of the sliced membrane electrode assembly was observed using a SEM (S-4800 manufactured by Hitachi High-Technologies Corporation) set at a magnification of 4000 times.
In the membrane electrode assembly of Example 1, there was no void at the interface between the electrode catalyst layer and the polymer electrolyte membrane, and therefore the adhesion between the electrode catalyst layer and the polymer electrolyte membrane was good, and the assembly showed good power generation performance and durability.

Example 2
A membrane/electrode assembly of Example 2 was obtained in the same manner as in Example 1, except that the amount of the electrode catalyst layer (catalyst ink) applied on the cathode side was doubled.
In the membrane electrode assembly of Example 2, there was no void at the interface between the electrode catalyst layer and the polymer electrolyte membrane, and therefore the adhesion between the electrode catalyst layer and the polymer electrolyte membrane was good, and the assembly showed good power generation performance and durability.

Example 3
A membrane electrode assembly of Example 3 was obtained in the same manner as in Example 1, except that a planetary ball mill disperser was used to disperse the catalyst ink. The conditions for dispersing each component using the ball mill disperser were set as follows. The following conditions were also used in the following Examples and Comparative Examples.
Dispersion time: 3 hours Ball diameter: 3 mm
The catalyst ink of Example 3 had a lower degree of dispersion than the catalyst ink of Example 1, which was dispersed using a bead mill disperser. Therefore, at the interface between the electrode catalyst layer and the polymer electrolyte membrane of the membrane electrode assembly of Example 3, there were multiple voids with a height h of 0.3 μm to 0.4 μm, and the total width w of the multiple voids present within an area 30 μm long in the direction parallel to the interface was 6 μm. The power generation performance and durability of the membrane electrode assembly of Example 3 were good.

Example 4
A membrane/electrode assembly of Example 4 was obtained in the same manner as in Example 1, except that a platinum-cobalt alloy carbon catalyst was used instead of the platinum-supported carbon catalyst.
Compared with the ink of Example 1, the catalyst ink of Example 4 caused cracks in part of the electrode catalyst layer when applied to the polymer electrolyte membrane. As a result, at the interface between the electrode catalyst layer and the polymer electrolyte membrane of the membrane electrode assembly of Example 4, there were multiple voids with a height h of 0.1 μm to 0.2 μm, and the total width w of the multiple voids present within an area 30 μm long in the direction parallel to the interface was 10 μm. The power generation performance and durability of the membrane electrode assembly of Example 4 were good.

Example 5
A membrane/electrode assembly of Example 5 was obtained in the same manner as in Example 1, except that the catalyst ink of Example 1 was mixed with carbon nanofibers (VGCF-H (registered trademark), manufactured by Showa Denko KK).
In the membrane electrode assembly of Example 5, there was no void at the interface between the electrode catalyst layer and the polymer electrolyte membrane, and therefore the adhesion between the electrode catalyst layer and the polymer electrolyte membrane was good, and the assembly also showed good power generation performance and durability.

Example 6
A membrane electrode assembly of Example 6 was obtained in the same manner as in Example 1, except that the catalyst ink of Example 1 was mixed with carbon nanofibers (VGCF-H (registered trademark), manufactured by Showa Denko K.K.), and a fluorine-based polymer electrolyte membrane was used as the polymer electrolyte membrane.
In the membrane electrode assembly of Example 6, there was no void at the interface between the electrode catalyst layer and the polymer electrolyte membrane, and therefore the adhesion between the electrode catalyst layer and the polymer electrolyte membrane was good, and the assembly also showed good power generation performance and durability.

(Example 7)
A membrane/electrode assembly of Example 7 was obtained in the same manner as in Example 3, except that the catalyst ink of Example 3 was mixed with carbon nanofibers (VGCF-H (registered trademark), manufactured by Showa Denko KK).
The catalyst ink of Example 7 had a lower degree of dispersion than the catalyst ink of Example 3. Therefore, at the interface between the electrode catalyst layer and the polymer electrolyte membrane of the membrane electrode assembly of Example 7, there were multiple voids with a height h of 0.4 μm to 0.5 μm, and the total width w of the multiple voids present within an area 30 μm long in the direction parallel to the interface was 9 μm. The power generation performance and durability of the membrane electrode assembly of Example 7 were good.

(Example 8)
A platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.), water, 1-propanol, a polymer electrolyte (Nafion (registered trademark) dispersion, manufactured by Wako Pure Chemical Industries, Ltd.), and carbon nanofibers (VGCF-H (registered trademark), manufactured by Showa Denko K.K.) were mixed and used with a bead mill disperser to produce a catalyst ink.
The produced catalyst ink was directly applied to both surfaces of a polymer electrolyte membrane (Nafion 211 (registered trademark), manufactured by DuPont) using a slit die coater and dried to form an electrode catalyst layer, thereby obtaining a membrane electrode assembly.
In the membrane electrode assembly of Example 8, there was no void at the interface between the electrode catalyst layer and the polymer electrolyte membrane, and therefore the adhesion between the electrode catalyst layer and the polymer electrolyte membrane was good, and the assembly showed good power generation performance and durability.

(Example 9)
A membrane/electrode assembly of Example 9 was obtained in the same manner as in Example 8, except that the amount of the electrode catalyst layer (catalyst ink) applied on the cathode side was doubled.
In the membrane electrode assembly of Example 9, there was no void at the interface between the electrode catalyst layer and the polymer electrolyte membrane, and therefore the adhesion between the electrode catalyst layer and the polymer electrolyte membrane was good, and the assembly showed good power generation performance and durability.

(Example 10)
A membrane/electrode assembly of Example 10 was obtained in the same manner as in Example 8, except that a ball mill disperser was used to disperse the catalyst ink.
The catalyst ink of Example 10 had a lower degree of dispersion than the catalyst ink of Example 8, which was dispersed using a bead mill disperser. Therefore, at the interface between the electrode catalyst layer and the polymer electrolyte membrane of the membrane electrode assembly of Example 10, there were multiple voids with a height h of 0.3 μm to 0.4 μm, and the total width w of the multiple voids present within an area 30 μm long in the direction parallel to the interface was 6 μm. The power generation performance and durability of the membrane electrode assembly of Example 10 were good.

Example 11
A membrane/electrode assembly of Example 11 was obtained in the same manner as in Example 8, except that a platinum-cobalt alloy carbon catalyst was used instead of the platinum-supported carbon catalyst.
Compared with the ink of Example 8, the catalyst ink of Example 11 caused cracks in part of the electrode catalyst layer when applied to the polymer electrolyte membrane. As a result, at the interface between the electrode catalyst layer and the polymer electrolyte membrane of the membrane electrode assembly of Example 11, there were multiple voids with a height h of 0.1 μm to 0.2 μm, and the total width w of the multiple voids present within an area 30 μm long in the direction parallel to the interface was 10 μm. The power generation performance and durability of the membrane electrode assembly of Example 11 were good.

(Comparative Example 2)
A membrane electrode assembly of Comparative Example 2 was obtained in the same manner as in Example 8, except that the membrane electrode assembly was produced by a method in which the catalyst ink was applied to a transfer substrate and then transferred to a polymer electrolyte membrane.
In the membrane electrode assembly of Comparative Example 2, voids with a height h exceeding 0.5 μm were generated at the interface between the electrode catalyst layer and the polymer electrolyte membrane, resulting in deterioration of power generation performance and durability.

(Comparative Example 3)
A membrane/electrode assembly of Comparative Example 3 was obtained in the same manner as in Example 8, except that the amount of the electrode catalyst layer (catalyst ink) applied on the cathode side was four times as much.
In the membrane electrode assembly of Comparative Example 3, wrinkles and cracks occurred in the electrode catalyst layer, resulting in reduced power generation performance and durability. At this time, multiple voids with heights h of 0.1 μm to 0.3 μm existed at the interface between the electrode catalyst layer and the polymer electrolyte membrane, and the total width w of the multiple voids existing within an area 30 μm long in the direction parallel to the interface was 13 μm.

Reference Signs List 1 Solid polymer fuel cell 2 Polymer electrolyte membrane 3A, 3F Electrode catalyst layer 4A, 4F Gas diffusion layer 5A, 5F Separator 6A, 6F Gas flow path 7A, 7F Cooling water passage 8 Electrode catalyst layer 9 Polymer electrolyte membrane 10 Catalyst 11 Carbon particles 12 Polymer electrolyte 13 Fibrous material 14 Void portion

Claims (7)

A membrane electrode assembly for a polymer electrolyte fuel cell, comprising electrode catalyst layers laminated on both sides of a polymer electrolyte membrane,
The electrode catalyst layer contains a catalyst, carbon particles, and a polymer electrolyte,
The polymer electrolyte membrane contains a fluorine-based polymer electrolyte,
At least one void is formed at the interface between the electrode catalyst layer and the polymer electrolyte membrane,
When a cross section of the membrane electrode assembly for a polymer electrolyte fuel cell is cut along a plane perpendicular to the interface and observed with a scanning electron microscope, the height h of the void portion is 0.5 μm or less at each of the interfaces on both sides of the polymer electrolyte membrane, and the sum of the widths w of the void portions present within a region 30 μm long in the direction parallel to the interface is 10 μm or less. The membrane electrode assembly for a polymer electrolyte fuel cell according to claim 1, wherein the height h is 0.3 μm or less. The electrode catalyst layer further contains a fibrous material,
the fibrous material is a proton-conducting fiber,
The fiber diameter of the fibrous material is in the range of 0.5 nm to 500 nm,
3. The membrane/electrode assembly for a polymer electrolyte fuel cell according to claim 1, wherein the fiber length of the fibrous material is within a range of 1 μm or more and 40 μm or less. The fiber diameter of the fibrous material is within a range of 5 nm to 200 nm,
4. The membrane electrode assembly for a polymer electrolyte fuel cell according to claim 3, wherein the fiber length of the fibrous material is within a range of 1 μm or more and 20 μm or less. The membrane electrode assembly for a polymer electrolyte fuel cell according to any one of claims 1 to 4, wherein the thickness of the electrode catalyst layer is 20 μm or less. A solid polymer fuel cell comprising a membrane electrode assembly for a solid polymer fuel cell according to any one of claims 1 to 5. A method for producing a membrane electrode assembly for a polymer electrolyte fuel cell in which electrode catalyst layers are laminated on both sides of a polymer electrolyte membrane, comprising the steps of:
The method includes at least a step of bonding the electrode catalyst layer to the polymer electrolyte membrane,
The electrode catalyst layer contains a catalyst, carbon particles, and a polymer electrolyte,
The polymer electrolyte membrane contains a fluorine-based polymer electrolyte,
At least one void is formed at the interface between the electrode catalyst layer and the polymer electrolyte membrane,
a method for producing a membrane electrode assembly for a polymer electrolyte fuel cell, in which, when a cross section of the membrane electrode assembly for a polymer electrolyte fuel cell is cut along a plane perpendicular to the interface and observed with a scanning electron microscope, the height h of the void portion is 0.5 μm or less at each of the interfaces on both sides of the polymer electrolyte membrane, and the sum of the widths w of the void portions present within a region 30 μm long in the direction parallel to the interface is 10 μm or less.

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