JP7131269B2 - Catalyst ink for electrode catalyst layer formation of polymer electrolyte fuel cells - Google Patents

Description

TECHNICAL FIELD The present invention relates to a catalyst ink for forming an electrode catalyst layer of a polymer electrolyte fuel cell.

A polymer electrolyte fuel cell, which has a structure in which a polymer electrolyte membrane is sandwiched between a cathode electrode catalyst layer and an anode electrode catalyst layer, operates at room temperature and has a short start-up time. It is
Conventional methods for manufacturing membrane electrode assemblies include a method in which a catalyst ink composed of carbon particles supporting a catalyst, a polymer electrolyte, and a solvent is directly applied to a polymer electrolyte membrane; A method is known in which a layer is coated and then thermocompression bonded to a polymer electrolyte membrane.
Among them, the method of manufacturing a membrane electrode assembly by directly applying a catalyst ink to a polymer electrolyte membrane has good adhesion at the interface between the polymer electrolyte membrane and the catalyst layer, and the catalyst layer is not crushed by thermocompression bonding. Due to the feature that there is no nucleation, a membrane electrode assembly having excellent power generation performance and durability can be produced.

However, in the conventional manufacturing method in which the catalyst ink is directly applied to the electrolyte membrane, when the catalyst ink is applied, the polymer electrolyte membrane swells or contracts due to the solvent in the ink, which causes wrinkles and cracks in the formed catalyst layer. The problem arises that it occurs.
In order to address the above problem, Patent Document 1 discloses that a needle-shaped carbon material such as a carbon nanotube is used for the electrode catalyst layer, so that the catalyst layer can be formed directly on the electrolyte membrane.
However, according to this method, there is a possibility that the power generation performance will be deteriorated due to the low utilization rate of the catalyst. Furthermore, acicular carbon materials such as carbon nanotubes are bulky, and the entanglement of the acicular carbon materials increases the viscosity of the catalyst ink, which may make application difficult.
On the other hand, in Patent Document 2, performance is improved by fabricating an electrode catalyst layer formed mainly of a fibrous proton-conducting material.
However, with this method, although the performance is improved, there is a possibility that the membrane strength that suppresses wrinkles and cracks in the catalyst layer when directly applied to the electrolyte membrane cannot be obtained.

WO2002/027844 JP 2007-220416 A

INDUSTRIAL APPLICABILITY The present invention is a catalyst for forming an electrode catalyst layer of a polymer electrolyte fuel cell, which can suppress the occurrence of wrinkles and cracks, can suppress deterioration of performance, and can be directly applied to a polymer electrolyte membrane. Intended to provide ink.

In order to solve the above problems, a catalyst ink for forming an electrode catalyst layer of a polymer electrolyte fuel cell according to one aspect of the present invention comprises catalyst-supporting carbon particles, carbon fibers, a polymer electrolyte, and organic electrolyte fibers in a solvent. The thixotropic index (TI value) of the viscosity at a shear rate of 10 (1/s) and the viscosity at a shear rate of 100 (1/s) is in the range of 1.5 to 10.
By using the catalyst ink, the entanglement of the carbon fibers and the organic electrolyte fibers increases the membrane strength, and even when the catalyst ink is directly applied to the electrolyte membrane, wrinkles and cracks can be suppressed.

According to the present invention, it is possible to suppress the occurrence of wrinkles and cracks, to suppress the deterioration of performance, and to form an electrode catalyst layer of a solid polymer fuel cell that can be directly applied to a polymer electrolyte membrane. of catalyst ink can be provided.

FIG. 1 is an exploded perspective view showing the internal structure of a polymer electrolyte fuel cell according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing a configuration example of an electrode catalyst layer according to this embodiment.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. In addition, this embodiment is not limited to the embodiment described below, and it is also possible to add modifications such as design changes based on the knowledge of those skilled in the art. Embodiments are also included in the scope of this embodiment.
Also, in the following detailed description, specific details are set forth in order to provide a thorough understanding of embodiments of the invention. It is evident, however, that one or more embodiments may be practiced without such specific details. In other instances, well-known structures and devices are shown in schematic form to simplify the drawings.

(Structure of polymer electrolyte fuel cell)
FIG. 1 is an exploded perspective view showing the internal structure of a polymer electrolyte fuel cell according to an embodiment of the present invention. As shown in FIG. 1, a pair of electrode catalyst layers 3A and 3F facing each other with the polymer electrolyte membrane 2 interposed are arranged on both sides of the polymer electrolyte membrane 2 constituting the polymer electrolyte fuel cell 1. It is A gas diffusion layer 4A is arranged on the surface of the electrode catalyst layer 3A opposite to the surface facing the polymer electrolyte membrane 2 . A gas diffusion layer 4F is arranged on the surface of the electrode catalyst layer 3F opposite to the surface facing the polymer electrolyte membrane 2 . The gas diffusion layers 4A, 4F are arranged to face each other with the polymer electrolyte membrane 2 and the pair of electrode catalyst layers 3A, 3F interposed therebetween.

A separator 5A is arranged on the surface of the gas diffusion layer 4A opposite to the surface facing the electrode catalyst layer 3A. The separator 5A has a gas channel 6A for circulating reaction gas on the main surface facing the gas diffusion layer 4A, and a cooling water passage 7A for circulating cooling water on the main surface opposite to the main surface having the gas channel 6A. . Furthermore, a separator 5F is arranged on the surface of the gas diffusion layer 4F opposite to the surface facing the electrode catalyst layer 3F. The separator 5F has a gas channel 6F for circulating reaction gas on the main surface facing the gas diffusion layer 4F, and a cooling water passage 7F for circulating cooling water on the main surface opposite to the main surface having the gas channel 6F. . Hereinafter, the electrode catalyst layers 3A and 3F may be simply referred to as "the electrode catalyst layer 3" when there is no need to distinguish between them.

FIG. 2 is a schematic cross-sectional view showing a configuration example of an electrode catalyst layer according to this embodiment. As shown in FIG. 2, the electrode catalyst layer 8 according to this embodiment is bonded to the surface of the polymer electrolyte membrane 9 . The electrode catalyst layer 8 comprises a catalyst 10 , carbon particles 11 as conductive carriers, a polymer electrolyte 12 and carbon fibers 13 , and an organic electrolyte fiber material 14 . The portions of the electrode catalyst layer 8 where none of the components of the catalyst 10, the carbon particles 11, the polymer electrolyte 12, the carbon fibers 13, and the organic electrolyte fibrous substance 14 are present are voids.

(Manufacture of catalyst ink)
Next, a method for producing the catalyst ink for forming the electrode catalyst layers 3 and 8 of the polymer electrolyte fuel cell 1 according to the present embodiment (catalyst ink for forming the electrode catalyst layer of the polymer electrolyte fuel cell). explain. First, catalyst-carrying carbon particles (hereinafter also simply referred to as carbon particles) 11 carrying a catalyst 10 and carbon fibers 13 are mixed and dispersed in a dispersion medium to obtain a catalyst particle slurry.
Examples of the catalyst 10 include metals such as platinum group elements (platinum, palladium, ruthenium, iridium, rhodium, osmium), iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, and these metal alloys, oxides, double oxides, carbides, etc. of these metals can be used.

As the carbon particles 11, any material may be used as long as it has conductivity and can support the catalyst without being attacked by the catalyst, but carbon particles are generally used.
Examples of carbon fibers 13 include carbon fibers, carbon nanotubes, carbon nanohorns, conductive polymer nanofibers, and the like. Only one type of these fibers may be used alone, or two or more types may be used in combination.
As the dispersion medium, one of water and alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, and pentanol is selected. can be used as Moreover, it is possible to use a solvent in which two or more of the above solvents are mixed. For mixing and dispersing, for example, a bead mill, planetary mixer, dissolver, or the like can be used.

Next, polymer electrolyte 12 and organic electrolyte fibers 14 are added to the catalyst particle slurry produced by the above method. As the polymer electrolyte membranes 2 and 9 and the polymer electrolyte 12, any material having proton conductivity may be used, and fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes can be used. As the fluorine-based polymer electrolyte, a polymer electrolyte having a tetrafluoroethylene skeleton, for example, "Nafion (registered trademark)" manufactured by DuPont can be used. The polymer electrolyte 12 is in a state in which the polymer electrolyte aggregates.

Aggregates of the polymer electrolyte 12 and polymer electrolytes forming the organic electrolyte fibers 14 may be the same or different from each other. In addition, the polymer electrolytes forming the aggregates of the polymer electrolyte 12 and the polymer electrolyte fibers 14 and the polymer electrolytes forming the polymer electrolyte membranes 2 and 9 may be the same or may be the same. can be different.
The average fiber diameter of the organic electrolyte fibers 14 is 2 μm or less. If the average fiber diameter is 2 μm or less, a fineness appropriate for the fiber material contained in the electrode catalyst layer is ensured.

In order to improve the output of polymer electrolyte fuel cells, the gas supplied to the electrode catalyst layer should be appropriately diffused into the electrode catalyst layer through the pores of the electrode catalyst layer. Therefore, it is desirable that the water generated by the electrode reaction is properly discharged through the pores. In addition, the presence of pores facilitates the formation of an interface between the gas, the catalyst-carrying carbon, and the polymer electrolyte, promoting the electrode reaction, which can also improve the output of the polymer electrolyte fuel cell. is.

From the above point of view, it is preferable that the electrode catalyst layer has pores of an appropriate size and amount. If the average fiber diameter of the organic electrolyte fibers 14 is 2 μm or less, sufficient gaps are formed in the structure in which the organic electrolyte fibers 14 are entangled in the electrode catalyst layer, and sufficient pores are secured. It is possible to improve the output. Furthermore, when the average fiber diameter of the organic electrolyte fibers 14 is 0.5 nm or more and 500 nm or less, the output of the fuel cell is particularly enhanced.

The average fiber length of the organic electrolyte fibers 22 is larger than the average fiber diameter, and is preferably 1 μm or more and 200 μm or less. When the average fiber length is within the above range, aggregation of the polymer electrolyte fibers 14 in the electrode catalyst layer is suppressed, and pores are easily formed. Further, when the average fiber length is within the above range, a structure in which the organic electrolyte fibers 14 are entangled in the electrode catalyst layer is preferably formed, so that the strength of the electrode catalyst layer is increased, and the effect of suppressing crack generation is enhanced. be done.
The ratio of the mass of the organic electrolyte fibers 14 to the combined mass of the carbon particles 11 and the carbon fibers 13 is preferably 0.1 or more and 3.0 or less. If the mass ratio of the carbon particles 11 and carbon fibers 13 to the organic electrolyte fibers 14 is within the above range, the conduction of protons in the electrode catalyst layer is promoted, so the output of the fuel cell can be improved.

If the ratio of the carbon particles 11 and the carbon fibers 13 is greater than the above ratio, the entanglement of the carbon fibers will increase the viscosity of the catalyst ink, making it difficult to apply, resulting in wrinkles and cracks in the catalyst layer. becomes. Specifically, when the thixotropic index (TI value) of the viscosity at a shear rate of 10 (1/s) and the viscosity at a shear rate of 100 (1/s) exceeds 10, coating becomes difficult and wrinkles and cracks occur. becomes more likely to occur. When the TI value is less than 1.5, the viscosity is too low, and wrinkles and cracks tend to occur during application. The thixotropic index (TI value) of the viscosity at a shear rate of 10 (1/s) and the viscosity at a shear rate of 100 (1/s) of the catalyst ink according to the present embodiment is in the range of 1.5 to 10. preferably within In addition, when the ratio of the carbon particles 11 and the carbon fibers 13 is increased with respect to the above ratio, the utilization rate of the catalyst becomes low, which causes deterioration of the performance.

On the other hand, if the ratio of the organic electrolyte fibers 14 is too high with respect to the above ratio, the membrane strength tends to be insufficient, which causes wrinkles and cracks when the catalyst ink is directly applied to the polymer electrolyte membrane. Moreover, when the ratio of the organic electrolyte fibers 14 is large, the contact angle of the catalyst ink with respect to the fluorine-based polymer electrolyte membrane is likely to fall below 50°. If the contact angle with respect to the fluorine-based polymer electrolyte membrane is less than 50°, the catalyst ink will permeate the membrane more, and wrinkles and cracks will tend to occur during application. Further, when the contact angle of the catalyst ink to the fluorine-based polymer electrolyte membrane exceeds 90°, the catalyst ink is repelled to the membrane, making application impossible. The contact angle of the catalyst ink according to the present embodiment with respect to the fluoropolymer electrolyte membrane is preferably in the range of 50° or more and 90° or less.

For the above reasons, the ratio of the mass of the organic electrolyte fibers 14 to the combined mass of the carbon particles 11 and the carbon fibers 13 is set to 0.1 or more and 3.0 or less, so that the catalyst ink can be manufactured while maintaining the film strength. In addition, since the organic electrolyte fibers exhibit good proton conductivity, deterioration of power generation performance can be suppressed.

(Manufacturing of membrane electrode assembly)
A membrane electrode assembly is manufactured by joining the electrode catalyst layers 3 to both surfaces of the polymer electrolyte membrane 2 . At this time, as a method of bonding the electrode catalyst layer 3 to the polymer electrolyte membrane 2, for example, a transfer base material with an electrode catalyst layer, which is a transfer base material coated with a catalyst ink, is used, and an electrode of the transfer base material with an electrode catalyst layer is used. There is a method of bonding the polymer electrolyte membrane 2 and the electrode catalyst layer 3 by bringing the surface of the catalyst layer and the polymer electrolyte membrane into contact with each other and applying heat and pressure.
However, according to the above method, the adhesion between the electrode catalyst layer 3 and the polymer electrolyte membrane 2 is poor, and voids are likely to be formed at the interface between the electrode catalyst layer 3 and the polymer electrolyte membrane 2 . As a result, problems such as deterioration of power generation performance due to interfacial resistance and deterioration of power generation performance due to flooding due to water clogging in gaps tend to occur.

On the other hand, the membrane electrode assembly can also be produced by a method of directly applying the catalyst ink to the surface of the polymer electrolyte membrane 2 and then removing the solvent component (dispersion medium) from the coating film of the catalyst ink. 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 less likely to occur. However, in the method of directly applying the catalyst ink to the polymer electrolyte membrane 2, swelling of the polymer electrolyte membrane 2 tends to cause wrinkles and cracks in the applied electrode catalyst layer 3, which leads to deterioration in power generation performance and durability. There was a problem that it is easy to cause a decrease.
In contrast, if appropriate amounts of the carbon fibers 13 and the organic electrolyte fibers 14 are added to the catalyst ink as in the present embodiment, the strength of the electrode catalyst layer 3 is increased. Even when directly applied, the electrode catalyst layer 3 is less prone to wrinkles and cracks, and the good proton conductivity of the organic electrolyte fibers makes it possible to suppress deterioration in performance.

(Effect of this embodiment)
According to this embodiment, even when the catalyst ink is directly applied to the polymer electrolyte membrane during the production of the membrane electrode assembly, it is possible to suppress the deterioration of the power generation performance, and suppress the wrinkles and cracks of the catalyst layer. It becomes possible to
Examples of the present invention and comparative examples will be described below.
(Example 1)
A first embodiment of the present invention will be described below.

(Manufacture of catalyst ink)
Water is added to catalyst-supporting carbon particles (trade name: TEC10E50E, manufactured by Tanaka Kikinzoku Co., Ltd.) supporting 50 wt% of platinum and carbon fibers (trade name: VGCF-H, manufactured by Showa Denko), and mixed with a planetary mixer. A catalyst particle slurry was prepared. At this time, the ratio of catalyst-carrying carbon particles to carbon fibers was set to 2:1.
Polymer electrolyte fibers as organic electrolyte fibers, a polymer electrolyte dispersion (trade name: Nafion dispersion, manufactured by Wako Pure Chemical Industries, Ltd.) and 1-propanol are added to the catalyst particle slurry, and dispersed using a bead mill disperser. , to obtain a catalyst ink. The polymer electrolyte fiber was produced by making a polymer electrolyte dispersion (Nafion dispersion: manufactured by Wako Pure Chemical Industries, Ltd.) fibrous using an electrospinning method, and then cooling and pulverizing the fiber. The polymer electrolyte fibers used as the organic electrolyte fibers had an average fiber diameter of 150 nm (0.15 μm) and an average fiber length of 10 μm.
At this time, the ratio of the mass of the organic electrolyte fibers to the combined mass of the catalyst-carrying carbon particles and the carbon fibers was adjusted to 0.5.
The viscosity of the above catalyst ink at a shear rate of 10 (1/s) and the viscosity at a shear rate of 100 (1/s) were measured and found to be 6.0.
Further, the contact angle of the catalyst ink with respect to the Nafion film was measured and found to be 60°.

(Manufacturing of membrane electrode assembly)
Then, the above catalyst ink was directly applied to both surfaces of the polymer electrolyte membrane by a die coating method to obtain a membrane electrode assembly.
When the catalyst ink of Example 1 was directly applied to both surfaces of the polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, and good power generation performance was obtained.

(Example 2)
Next, Example 2 of the present invention will be described. A catalyst ink of Example 2 was obtained in the same manner as in Example 1, except that polymer electrolyte fibers having an average fiber diameter of 1.0 μm and an average fiber length of 20 μm were used as the organic electrolyte fibers.
As a result of measuring the TI value of the viscosity of the catalyst ink at a shear rate of 10 (1/s) and at a shear rate of 100 (1/s), it was 8.0.
Further, the contact angle of the catalyst ink with respect to the Nafion film was measured and found to be 50°.
When the catalyst ink of Example 2 was directly applied to both surfaces of the polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, and good power generation performance was obtained.

(Example 3)
Next, Example 3 of the present invention will be described. The catalyst ink of Example 3 was prepared in the same manner as in Example 1, except that the ratio of the mass of the organic electrolyte fibers to the combined mass of the catalyst-supporting carbon particles and carbon fibers was adjusted to 2.0. Obtained.
As a result of measuring the TI value of the viscosity of the catalyst ink at a shear rate of 10 (1/s) and at a shear rate of 100 (1/s), it was 2.0.
Further, the contact angle of the catalyst ink with respect to the Nafion film was measured and found to be 50°.
When the catalyst ink of Example 3 was directly applied to both surfaces of the polymer electrolyte membrane, no wrinkles or cracks occurred in the catalyst layer, and good power generation performance was obtained.

(Comparative example 1)
A catalyst ink of Comparative Example 1 was obtained in the same manner as in Example 1 above, except that no carbon fiber was added to the catalyst ink.
As a result of measuring the TI value of the viscosity of the catalyst ink at a shear rate of 10 (1/s) and at a shear rate of 100 (1/s), it was 1.0.
Further, the contact angle of the catalyst ink with respect to the Nafion film was measured and found to be 40°.
When the catalyst ink of Comparative Example 1 was directly applied to both surfaces of the polymer electrolyte membrane, wrinkles and cracks occurred in the catalyst layer. The result was that the power generation performance was lower than that of Example 1. It is believed that if the catalyst ink did not contain carbon fibers, carbon particles and polymer fibers tended to agglomerate, and as a result, sufficient pores could not be formed in the catalyst layer, resulting in lower power generation performance.

(Comparative example 2)
A catalyst ink of Comparative Example 2 was obtained in the same manner as in Example 1, except that polymer electrolyte fibers having an average fiber diameter of 3.0 μm and an average fiber length of 20 μm were used as the organic electrolyte fibers.
The TI value of the viscosity of the above catalyst ink at a shear rate of 10 (1/s) and at a shear rate of 100 (1/s) was 12.0.
Further, the contact angle of the catalyst ink with respect to the Nafion film was measured and found to be 60°.
When the catalyst ink of Comparative Example 2 was directly applied to both surfaces of the polymer electrolyte membrane, the catalyst layer did not develop wrinkles or cracks, but the power generation performance was lowered.
Table 1 shows the evaluation results of Examples 1 to 3 and Comparative Examples 1 and 2.

DESCRIPTION OF SYMBOLS 1... Polymer electrolyte 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... Carbon fiber 14... Organic electrolyte fiber

Claims (5)

including catalyst-supporting carbon particles, carbon fibers, polymer electrolytes, and organic electrolyte fibers in a solvent;
The thixotropic index (TI value) of the viscosity at a shear rate of 10 (1/s) and the viscosity at a shear rate of 100 (1/s) is in the range of 1.5 or more and 10 or less. Catalyst ink for electrode catalyst layer formation in batteries. 2. The catalyst ink for forming an electrode catalyst layer of a polymer electrolyte fuel cell according to claim 1, wherein the contact angle of said catalyst ink with respect to a fluorine-based polymer electrolyte membrane is in the range of 50[deg.] or more and 90[deg.] or less. 3. The catalyst ink for forming an electrode catalyst layer of a polymer electrolyte fuel cell according to claim 1, wherein said carbon fibers contain one or more selected from carbon nanofibers and carbon nanotubes. 4. The catalyst for forming an electrode catalyst layer of a polymer electrolyte fuel cell according to claim 1, wherein the organic electrolyte fibers have an average fiber diameter of 2 μm or less and an average fiber length of 1 μm or more and 200 μm or less. ink. 5. The method according to any one of claims 1 to 4, wherein a ratio of the mass of the organic electrolyte fiber to the combined mass of the catalyst-carrying carbon particles and the carbon fiber is 0.1 or more and 3.0 or less. A catalyst ink for forming an electrode catalyst layer of polymer electrolyte fuel cells.

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