JP5417038B2 - Method for producing catalyst electrode used for membrane electrode assembly, catalyst electrode used for membrane electrode assembly, method for producing membrane electrode assembly, membrane electrode assembly, and fuel cell - Google Patents

Description

  The present invention relates to a method for producing a catalyst electrode used for a membrane electrode assembly for a fuel cell, a catalyst electrode used for a membrane electrode assembly, a method for producing a membrane electrode assembly, a membrane electrode assembly, and a fuel cell. It is.

  A fuel cell that generates electricity by an electrochemical reaction between a fuel gas and an oxidant gas has attracted attention as an energy source. This type of fuel cell includes a membrane electrode assembly formed by bonding a catalyst electrode to both surfaces of an electrolyte membrane. In such a membrane electrode assembly, conventionally, carbon black carrying a catalyst metal such as platinum has been used for the catalyst electrode.

  In recent years, in order to improve the electron conductivity and gas diffusibility in the catalyst electrode, a technique using carbon nanotubes instead of carbon black has been proposed for the catalyst electrode used in the membrane electrode assembly (see below). (See Patent Documents 1 and 3). In this technique, a plurality of carbon nanotubes oriented perpendicular to the surface of the substrate are grown on the substrate, and a catalytic metal such as platinum is supported thereon, and then the surface is covered with an ionomer (electrolyte). To produce a catalyst electrode. When the surface of a carbon nanotube is coated with an ionomer, generally, a solution in which an ionomer is dispersed (hereinafter referred to as an ionomer dispersion solution) is dropped onto the carbon nanotube and dried.

JP 2005-35396 A JP 2006-085929 A JP 2007-257886 A

Yoshiaki Nakayama, et al., “High-volume synthesis of carbon nanocoils, nanochaplets and related materials by green engineering and development research on highly functional composite materials”, Internet <URL: http://www.osaka.jst.go.jp/ kadai / pdf / h1305.pdf>

  However, when the above-mentioned carbon nanotubes oriented perpendicular to the surface of the substrate are used for the catalyst electrode used in the membrane electrode assembly, the surface of the plurality of carbon nanotubes carrying the catalyst metal depending on the shape of the carbon nanotube. It has been found that there is a case where a plurality of carbon nanotubes may be agglomerated due to the tension of the ionomer dispersion solution when the is coated with ionomer. In this specification, “a plurality of carbon nanotubes agglomerate” means that a plurality of carbon nanotubes grown in a substantially uniform distribution on the substrate gather locally and the distribution of the plurality of carbon nanotubes is not good. It means to be uniform.

  The present invention has been made to solve the above-described problems. When carbon nanotubes are used as catalyst electrodes used in membrane electrode assemblies for fuel cells, a plurality of carbon nanotubes at the time of production of the catalyst electrodes are used. The purpose is to prevent aggregation.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  [Application Example 1] A method for producing a catalyst electrode used in a membrane electrode assembly for a fuel cell, wherein a plurality of carbon nanotubes are oriented on a substrate perpendicularly to the surface of the substrate and have a corrugated shape A carbon nanotube growth step for growing the catalyst, a catalyst metal supporting step for supporting the catalyst metal on the plurality of carbon nanotubes by dropping a catalyst metal salt solution onto the plurality of carbon nanotubes, drying and firing reduction, and An ionomer dispersion solution in which an ionomer is dispersed in the plurality of carbon nanotubes supporting the catalyst metal is dropped and dried, whereby the surface of the plurality of carbon nanotubes supporting the catalyst metal is coated with the ionomer. A coating step, wherein the carbon nanotube growth step includes the compounding step. For carbon nanotubes, the wavelength of the wave shape, 0.3 ([mu] m) or more, to be less than 5 ([mu] m), comprising the step of growing the plurality of carbon nanotubes, producing a catalyst electrode.

  The inventor of the present application has repeated experiments, and the wavelength of the wave shape is 0.3 (μm) or more and less than 5 (μm) by the method for producing a catalyst electrode of Application Example 1, that is, in the carbon nanotube growth step. Preferably, the plurality of carbon nanotubes by the ionomer dispersion solution described above are grown in the ionomer coating step by growing the plurality of carbon nanotubes to be 0.5 (μm) or more and less than 1.8 (μm). It was found that agglomeration can be prevented. That is, the method for producing a catalyst electrode of Application Example 1 can prevent aggregation of a plurality of carbon nanotubes during production of the catalyst electrode.

  [Application Example 2] The method of manufacturing a catalyst electrode according to Application Example 1, wherein the carbon nanotube growth step is performed by adding the plurality of carbon nanotubes at a density at which adjacent carbon nanotubes contact each other on the substrate. A method for producing a catalyst electrode, comprising a step of growing.

  By the method for producing a catalyst electrode of Application Example 2, aggregation of a plurality of carbon nanotubes during production of the catalyst electrode can be further prevented.

[Application Example 3]
A catalyst electrode used in a membrane electrode assembly for a fuel cell, which is produced by the production method described in Application Example 1 or described above.

  [Application Example 4] A method for producing a membrane electrode assembly for a fuel cell, comprising: a catalyst electrode production process for producing a catalyst electrode according to Application Example 3; and a catalyst electrode produced on the substrate at least on an electrolyte membrane. A catalyst electrode transfer step of transferring to one surface of the fuel cell membrane electrode assembly.

  According to the manufacturing method of the fuel cell membrane electrode assembly of Application Example 4, the catalyst electrode in which the carbon nanotubes are not aggregated, that is, the in-plane distribution of the carbon nanotubes can be joined to the surface of the electrolyte membrane.

  Application Example 5 A fuel cell membrane electrode assembly manufactured by the manufacturing method according to Application Example 4, which is a fuel cell membrane electrode assembly.

  Application Example 6 A fuel cell comprising a fuel cell membrane electrode assembly according to Application Example 5 and a separator sandwiching the fuel cell membrane electrode assembly.

It is explanatory drawing which shows schematic structure of the fuel cell as one Example of this invention. 5 is an explanatory view showing a manufacturing process of the membrane electrode assembly 10. FIG. It is explanatory drawing which shows the relationship between the wavelength in the waveform shape of a carbon nanotube, and aggregation of a carbon nanotube.

Hereinafter, embodiments of the present invention will be described based on examples.
A. Fuel cell configuration:
FIG. 1 is an explanatory diagram showing a schematic configuration of a fuel cell as one embodiment of the present invention. In FIG. 1, the cross-sectional structure of the fuel cell 100 is schematically shown. As shown in the figure, the fuel cell 100 is configured by sandwiching both surfaces of a membrane electrode assembly 10 with a cathode side separator 20 and an anode side separator 30.

  The membrane electrode assembly 10 is configured by joining an anode and a cathode to both surfaces of an electrolyte membrane 12 having proton conductivity, respectively. The anode and the cathode each include a catalyst layer 14 and a gas diffusion layer 16. In this embodiment, Nafion (registered trademark) is used as the electrolyte membrane 12. Another electrolyte membrane may be used as the electrolyte membrane 12. In this embodiment, the catalyst layer 14 is a layer composed of carbon nanotubes carrying platinum and an ionomer (electrolyte). The membrane electrode assembly 10 and the catalyst layer 14 will be described in detail later. In this embodiment, carbon cloth is used as the gas diffusion layer 16. As the gas diffusion layer 16, other materials having gas diffusibility and conductivity such as carbon paper may be used.

  Ribs and grooves are formed on the surface of the cathode-side separator 20 that is in contact with the gas diffusion layer 16 on the cathode side, and these grooves form air as an oxidant gas and cathode off-gas discharged from the cathode. A cathode gas flow path 20p is formed. Ribs and grooves are also formed on the surface of the anode-side separator 30 on the side in contact with the gas diffusion layer 16 on the anode side, and these grooves are discharged from hydrogen as fuel gas and from the anode. The anode gas flow path 30p for flowing the anode off gas is configured. Although not shown, the cathode side separator 20 and the anode side separator 30 are also formed with cooling water passages for flowing cooling water. As materials for the cathode side separator 20 and the anode side separator 30, various materials having conductivity such as carbon and metal can be applied.

B. Manufacturing process of membrane electrode assembly:
FIG. 2 is an explanatory view showing a manufacturing process of the membrane electrode assembly 10. First, an iron catalyst, which is a growth nucleus of carbon nanotubes, is deposited almost uniformly on a silicon substrate by sputtering or the like. By a CVD (Chemical Vapor Deposition) method, it is perpendicular to the surface of the silicon substrate. A plurality of carbon nanotubes (CNTs) that are oriented and have a wave shape are grown (step S100). Here, “vertically oriented” does not need to be strictly vertically oriented. For example, the angle formed by the straight line connecting the surface of the silicon substrate and the root and tip of the carbon nanotube is 90 ± 10 °. It may be within the range. By forming growth nuclei (for example, iron catalyst) for growing carbon nanotubes on a silicon substrate in advance at a high density, the silicon substrate is oriented almost perpendicularly to the surface of the silicon substrate, Brush-like carbon nanotubes grow at a density at which adjacent carbon nanotubes are in contact with each other. In this embodiment, the growth conditions of the carbon nanotubes are controlled so that the wavelength in the wave shape of the carbon nanotubes is 0.5 to 1.8 (μm). In this embodiment, the reason why the shape of the carbon nanotube is a wave shape and the wavelength in the wave shape of the carbon nanotube is 0.5 to 1.8 (μm) will be described later.

  Next, a platinum salt solution is dropped on the entire surface of the carbon nanotubes grown in a brush shape, dried and baked and reduced, thereby supporting platinum (Pt) as a catalyst metal on the carbon nanotubes (step S110). In this example, the height from the surface of the silicon substrate to the tip of the carbon nanotube corresponding to the thickness of the catalyst layer 14 was set to 30 to 120 (μm). When the thickness of the catalyst layer 14 is less than 30 (μm), the surface area of the carbon nanotubes is relatively small, so that platinum is supported on the surface of the carbon nanotubes in a highly dispersed state, that is, uniformly dispersed. Since the thickness of the catalyst layer 14 is more difficult than when the thickness is 30 (μm), and the thickness of the catalyst layer 14 is larger than 120 (μm), the proton moving distance becomes relatively long. This is because the power generation performance in a power generation region in which a relatively high current flows is lower than when the thickness of the catalyst layer 14 is 120 (μm) or less.

  Next, an ionomer dispersion solution (for example, Nafion dispersion solution (“Nafion” is a registered trademark)) is dropped on the carbon nanotubes supporting platinum, and the surface of the carbon nanotubes supporting platinum is ionized by drying. Covering with (electrolyte) (step S120). Through the above steps, a catalyst electrode to be the catalyst layer 14 in the membrane electrode assembly 10 is produced.

  Next, the catalyst electrode (catalyst layer 14) produced by the above-described process is thermally transferred to both surfaces of the electrolyte membrane 12 (step S130). Furthermore, the membrane electrode assembly 10 is completed by hot press bonding the gas diffusion layer 16 to the surface of the catalyst layer 14.

C. Experiment:
In the manufacturing process of the catalyst electrode and the manufacturing process of the membrane electrode assembly 10 described above, the inventor of the present application coats the surface of the carbon nanotube carrying platinum with an ionomer (step S120 in FIG. 2). Depending on the shape, it was experimentally confirmed that the ionomer dispersion solution may cause a problem of aggregation of the brush-like carbon nanotubes. Hereinafter, the relationship between the wavelength and the amplitude of the wave shape of the carbon nanotube and the aggregation of the carbon nanotube will be described.

  FIG. 3 is an explanatory view showing the relationship between the wavelength in the corrugated shape of the carbon nanotube and the aggregation of the carbon nanotube.

  In this experiment, first, seven types of carbon nanotubes having different shapes were grown on a silicon substrate by the steps S100 and 110 shown in FIG. 2, and samples each carrying platinum were prepared. As shown in the figure, the wavelengths in the corrugated shape of the carbon nanotubes of Examples 1 to 4 are about 0.5 (μm), 0, 75 to 0, 85 (μm), and 0.85 to 1.57, respectively. μm), 1.34 to 1.79 (μm). Moreover, the wavelength in the waveform shape of the carbon nanotubes of Comparative Examples 1 to 3 was about 5 (μm), 8.1 (μm), 10 (μm) or more (wavelength measurement impossible, almost straight). In addition, each of these wavelengths was computed from the observation result by the scanning electron microscope of each sample. For some samples, the amplitude of the carbon nanotube in the wave shape was also calculated.

  In the carbon nanotube of Example 2, the amplitude in the portion where the wavelength was 0.75 (μm) was 0.7 (μm). That is, in this part, amplitude / wavelength≈0.93. Further, in the carbon nanotube of Example 2, the amplitude in the portion where the wavelength was 0.85 (μm) was 0.61 (μm). That is, in this part, amplitude / wavelength≈0.72.

  Further, in the carbon nanotube of Example 3, the amplitude in the portion where the wavelength was 1.21 (μm) was 0.47 (μm). That is, in this portion, amplitude / wavelength≈0.39. In addition, in the carbon nanotube of Example 3, the amplitude in the portion where the wavelength was 1.57 (μm) was 0.81 (μm). That is, in this portion, amplitude / wavelength≈0.52.

  In addition, in the carbon nanotube of Example 4, the amplitude in the portion where the wavelength was 1.34 (μm) was 1.11 (μm). That is, in this part, amplitude / wavelength≈0.83. Further, in the carbon nanotube of Example 4, the amplitude at the wavelength portion of 1.79 (μm) was 1.51 (μm). That is, in this part, amplitude / wavelength≈0.84.

  Further, in the carbon nanotube of Comparative Example 2, the amplitude in the portion where the wavelength was 8.1 (μm) was 2.7 (μm). That is, in this part, amplitude / wavelength≈0.33.

  Next, in the step S120 shown in FIG. 2, for each sample, the surface of the carbon nanotube carrying platinum was covered with an ionomer to produce a catalyst electrode. And about each sample, the surface of the brush-like carbon nanotube was observed with the scanning electron microscope. In Comparative Examples 1 to 3, condensation occurred in the brush-like carbon nanotubes. On the other hand, in Examples 1-4, condensation did not arise in the brush-like carbon nanotube. In addition, such a result was obtained because the wavelength in the wave shape of the carbon nanotube is about 0.5 to 1.79 (μm), and the amplitude / wavelength is 0.39 to 0.00. In the case of 93, it is considered that the adjacent carbon nanotubes are in contact with each other in advance, and the movement of each carbon nanotube in the direction along the surface of the silicon substrate is restricted.

  From the above experimental results, in the manufacturing process of the catalyst electrode and the manufacturing process of the membrane electrode assembly 10 of the present example, the carbon nanotubes grown on the silicon substrate have a wave shape in step S100 shown in FIG. Furthermore, the growth conditions of the carbon nanotubes are adjusted so that the wavelength in the corrugated shape is 0.5 to 1.8 (μm) and the amplitude / wavelength is 0.3 to 1.0. Were determined. In addition, since the shape of the carbon nanotubes grown on the silicon substrate varies, it may be difficult to grow the carbon nanotubes having the wave shape that does not cause the aggregation described above on the entire surface of the silicon substrate. Therefore, among the plurality of carbon nanotubes grown on the silicon substrate, the abundance ratio of the carbon nanotubes having a wave shape that does not cause the aggregation described above is 50% or more, preferably 80% or more. Determine the growth conditions for carbon nanotubes.

  According to the manufacturing process of the catalyst electrode (catalyst layer 14) used in the membrane electrode assembly 10 of the present embodiment described above and the manufacturing process of the membrane electrode assembly 10, the catalyst electrode used in the membrane electrode assembly 10 is When carbon nanotubes are used, aggregation of a plurality of carbon nanotubes due to the tension of the ionomer dispersion solution can be prevented.

D. Variations:
Although the embodiments of the present invention have been described above, the present invention is not limited to such embodiments, and can be implemented in various modes without departing from the scope of the present invention. For example, the following modifications are possible.

D1. Modification 1:
In the above embodiment, platinum is used as the catalyst metal supported on the carbon nanotubes, but other catalyst metals may be used instead of platinum as the catalyst metal.

D2. Modification 2:
In the above embodiment, the catalyst layer 14 including carbon nanotubes is formed on both surfaces of the electrolyte membrane 12. However, the catalyst layer 14 may be bonded to at least one surface of the electrolyte membrane 12.

D3. Modification 3:
In the above embodiment, the catalyst electrode 14 is formed on both surfaces of the electrolyte membrane 12 by thermal transfer, and then the gas diffusion layer 16 is bonded to the surface of the membrane electrode assembly 10. Bonding may be omitted.

DESCRIPTION OF SYMBOLS 100 ... Fuel cell 10 ... Membrane electrode assembly 12 ... Electrolyte membrane 14 ... Catalyst layer 16 ... Gas diffusion layer 20 ... Cathode side separator 20p ... Cathode gas channel 30 ... Anode side separator 30p ... Anode gas channel

Claims (6)

A method for producing a catalyst electrode used in a membrane electrode assembly for a fuel cell, comprising:
A carbon nanotube growth step for growing a plurality of carbon nanotubes having a corrugated shape while being oriented perpendicularly to the surface of the substrate on the substrate;
A catalyst metal supporting step of supporting a catalyst metal on the plurality of carbon nanotubes by dropping a catalyst metal salt solution on the plurality of carbon nanotubes, drying and firing reduction, and
An ionomer dispersion solution in which an ionomer is dispersed is dropped onto a plurality of carbon nanotubes supporting the catalyst metal and dried to dry the ionomer so that the surfaces of the plurality of carbon nanotubes supporting the catalyst metal are coated with the ionomer. A coating process,
The carbon nanotube growth step includes:
About 50% or more of the plurality of carbon nanotubes, the wavelength in the corrugated shape is 0.3 (μm) or more and less than 5 (μm), and the adjacent carbon nanotubes on the substrate Including a step of growing the plurality of carbon nanotubes such that the densities are in contact with each other ,
A method for producing a catalyst electrode. A method for producing a catalyst electrode according to claim 1,
The carbon nanotube growth step includes:
About 80% or more of the plurality of carbon nanotubes, the wavelength in the corrugated shape is 0.3 (μm) or more and less than 5 (μm), and the adjacent carbon nanotubes on the substrate Including a step of growing the plurality of carbon nanotubes such that the densities are in contact with each other,
A method for producing a catalyst electrode. A catalyst electrode used in a fuel cell membrane electrode assembly,
The catalyst electrode manufactured by the manufacturing method of Claim 1 or 2. A method for producing a fuel cell membrane electrode assembly comprising:
A catalyst electrode manufacturing process for manufacturing the catalyst electrode according to claim 3,
A catalyst electrode transfer step of transferring the catalyst electrode produced on the substrate to at least one surface of the electrolyte membrane;
A method for producing a membrane electrode assembly for a fuel cell. A fuel cell membrane electrode assembly comprising:
The membrane electrode assembly for fuel cells manufactured by the manufacturing method of Claim 4. A fuel cell,
A fuel cell membrane electrode assembly according to claim 5,
A separator for sandwiching the fuel cell membrane electrode assembly;
A fuel cell comprising:

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