JP2006040633A - Electrode for fuel cell, its manufacturing method, and fuel cell using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrode for a fuel cell with a high utilization rate of catalyst, its manufacturing method, and a fuel cell using it. <P>SOLUTION: The electrode for the fuel cell is provided with a catalyst layer, and a gas diffusion layer fitted in that order on either face of a solid polymer electrolyte film, the catalyst layer contains carbon fiber with a content of the carbon fiber getting higher from the side near the solid polymer electrolyte film toward the gas diffusion layer side. The manufacturing method and the fuel cell using the same are also provided. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrode for a fuel cell, a method for producing the electrode, and a fuel cell using the same.

Fuel cells can be classified into solid polymer electrolyte type (PEFC), phosphoric acid type (PAFC), molten carbonate type (MCFC), solid oxide type (SOFC), alkaline type (AFC), etc., depending on the type of electrolyte. . Moreover, it can classify | categorize into a methanol fuel cell, a hydrazine fuel cell, etc. according to the kind of fuel.
A fuel cell is a device that obtains electric energy from hydrogen obtained by reforming a fuel such as natural gas, methanol, naphtha, and coal, and oxygen in the air, and can obtain clean and high power generation efficiency. In recent years, attention has been paid to small power supplies of about several hundred watts for mobile communication, construction, civil engineering, and the like, and there is an increasing tendency to make them portable.

A solid polymer electrolyte fuel cell has an ion exchange membrane such as a perfluorosulfonic acid membrane as an electrolyte, and anode and cathode electrodes are joined to both sides of the ion exchange membrane. The anode is hydrogen and the cathode is oxygen. Is a device that generates electricity through an electrochemical reaction. The electrochemical reaction occurring at each electrode is shown below.
Anode: H ₂ → 2H ⁺ + 2e ⁻
Cathode: 1 / 2O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O
Total reaction: H ₂ + 1 / 2O ₂ → H ₂ O
As is apparent from this reaction formula, the reaction at each electrode is carried out at a three-phase interface that can simultaneously exchange gas (hydrogen or oxygen), proton (H ⁺ ), and electron (e ⁻ ) as active materials. Only progress.

As an electrode having such a function, there is a solid polymer electrolyte-catalyst composite electrode comprising a solid polymer electrolyte, carbon particles, and a catalyst substance. For example, this electrode is a porous electrode in which carbon particles carrying a catalyst material and a solid polymer electrolyte are mixed and distributed three-dimensionally, and a plurality of pores are formed therein. The catalyst carrier carbon forms an electron conduction channel, the solid electrolyte forms a proton conduction channel, and the pores form a supply and discharge channel of oxygen or hydrogen and the product water. These three channels spread three-dimensionally in the electrode, and an infinite number of three-phase interfaces capable of simultaneously transferring gas, proton (H ⁺ ) and electron (e ⁻ ) are formed. Is provided.

  Conventionally, an electrode having such a structure is obtained by using a polymer film or a paste made of a catalyst-supported carbon particle in which noble metal particles such as platinum are supported on a carbon particle carrier in a highly dispersed state and a PTFE (polytetrafluoroethylene) particle dispersion. A method of forming a film by applying or spraying on a carbon electrode substrate of a conductive porous body (generally a dry film thickness of 3 to 30 μm) and drying by heating, and then applying and impregnating a solid polymer electrolyte solution thereon, catalyst After a paste composed of supported carbon particles, PTFE particles and a solid polymer electrolyte solution is applied or sprayed onto a carbon electrode substrate of a polymer film or a conductive porous body (generally a dry film thickness of 3 to 30 μm), A method of heating and drying, a solid polymer electrolyte membrane directly made of catalyst-supported carbon particles, PTFE particles and a solid polymer electrolyte solution paste It was produced by a method such as heating and drying after forming a film by coating or spraying (generally a dry film thickness of 3 to 30 μm). As the solid polymer electrolyte solution, a solution having the same composition as the above-described ion exchange membrane dissolved in alcohol and liquefied is used as the PTFE particle dispersion, and PTFE having a particle diameter of about 0.23 μm. A dispersion of particles is used.

However, in order to make the solid polymer electrolyte membrane and the catalyst layer more firmly adhere to each other when forming a film, heat compression is performed by a hot press or the like, so that the pores in the electrode are crushed, and the diffusion rate of the reaction gas is reduced. In addition, the proton conducting channel is electrically isolated and cannot be used for the electrochemical reaction, and as a result, the catalyst utilization rate is reduced.
Further, it is a fuel cell assembly in which a catalyst layer and a gas diffusion layer are provided on both surfaces of an electrolyte membrane, and it is proposed that the catalyst layer contains fibrous carbon to improve power generation characteristics (for example, Patent Document 1).

JP 2003-115302 A

  An object of the present invention is to provide a fuel cell electrode having a high utilization rate of a catalyst, a method for producing the same, and a fuel cell using the same by improving a micro three-dimensional structure.

The present invention provides the following electrode for a fuel cell, a method for producing the same, and a fuel cell using the same.
1. In a fuel cell electrode in which a catalyst layer and a gas diffusion layer are provided in this order on both sides of a solid polymer electrolyte membrane, the catalyst layer contains carbon fibers, and the content of carbon fibers in the catalyst layer is An electrode for a fuel cell, characterized in that the height increases from the side close to the molecular electrolyte membrane toward the gas diffusion layer side.
2. 2. The fuel cell electrode according to 1 above, wherein the catalyst layer contains a solid polymer electrolyte.
3. 3. The fuel cell electrode according to 1 or 2 above, wherein the catalyst substance in the catalyst layer is a platinum group metal or an alloy thereof.
4). 4. The fuel cell electrode according to any one of 1 to 3 above, wherein the gas diffusion layer forming material is a carbon fiber woven fabric or carbon paper subjected to a water repellent treatment.
5. In a method for producing a fuel cell electrode by providing a catalyst layer and a gas diffusion layer in this order on both sides of a solid polymer electrolyte membrane, the catalyst layer forming composition contains carbon fiber, and the carbon fiber in the catalyst layer The content of is increased from the side close to the solid polymer electrolyte membrane toward the gas diffusion layer side.
6). As the catalyst layer forming composition, at least two kinds of compositions having different carbon fiber contents are prepared, and a carbon fiber-free or low-content composition is applied to both surfaces of the solid polymer electrolyte membrane, and then carbon is added. 6. The method according to 5 above, wherein a composition having a high fiber content is applied.
7). 5. A fuel cell using the fuel cell electrode according to any one of 1 to 4 above.
8). 7. A fuel cell using the fuel cell electrode produced by the method according to 5 or 6 above.

In the fuel cell electrode of the present invention, since carbon fibers are unevenly distributed in the catalyst layer, pores in the electrode are not crushed, a microscopic three-dimensional structure is maintained, and a decrease in catalyst utilization rate is small. Therefore, a fuel cell using this has a higher output voltage than a cell using an electrode that does not contain it.
The fuel cell of the present invention can be adapted to make a small power source of about several hundred watts for mobile communication, construction, civil engineering, etc. portable.

  The fuel cell electrode in the present invention may be any of a solid polymer electrolyte type (PEFC), a phosphoric acid type (PAFC), a molten carbonate type (MCFC), a solid oxide type (SOFC), an alkaline type (AFC), and the like. Although it can be used for a fuel cell, it is particularly suitable for a solid polymer electrolyte fuel cell.

The solid polymer electrolyte membrane used in the present invention is preferably composed of an ion exchange resin membrane, and in particular, perfluorosulfonic acid, styrene-divinylbenzene, a thermosetting resin or a thermoplastic resin with a sulfonic acid or phosphoric acid group. A polymer electrolyte membrane into which is introduced is preferred.
The catalyst layer of the fuel cell electrode of the present invention is a solid polymer electrolyte-catalyst composite comprising a solid polymer electrolyte, carbon fiber, and a catalyst substance, and is a carbon carrying the solid polymer electrolyte and catalyst particles. A porous catalyst layer in which particles and carbon fibers are mixed and distributed three-dimensionally, and a plurality of pores are formed therein, and the carbon particles and carbon fibers that are the catalyst support are electrons A conduction channel, a solid polymer electrolyte forms a proton conduction channel, and pores form a supply and discharge channel of oxygen or hydrogen and a product water.

As the electrode catalyst material used in the present invention, platinum group metals such as platinum, rhodium, ruthenium, iridium, palladium, osmium, and alloys thereof are suitable. These catalytic materials and salts of the catalytic materials may be used alone or in combination. In particular, among these compounds, the form of the compound is an ammine represented by a metal salt or complex, particularly [Pt (NH ₃ ) ₄ ] X ₂ or [Pt (NH ₃ ) ₆ ] X ₄ (X is a monovalent anion). Complexes are preferred. Moreover, when using a metal compound, the mixture of several compounds may be used and double salt may be sufficient. For example, by using a mixture of a platinum compound and a ruthenium compound, formation of a platinum-ruthenium alloy can be expected by a reduction process.
The catalyst material has an appropriate size for increasing the activity. From this viewpoint, the average size of the catalyst material supported on the contact surface is preferably 2 to 4 nm.
In addition, in a study by K. Kinoshita et al. (J. Electrochem. Soc., 137, 845 (1990)), it is reported that the particle size of platinum having a high activity with respect to oxygen reduction is about 3 nm. It is also possible to mix carbon particles, and the carbon particles preferably have high catalytic activity. For example, when a platinum group metal compound is used, Denka Black (manufactured by Denki Kagaku Kogyo Co., Ltd.) Acetylene black such as Vulcan XC-72 (Cabot Corporation), Black Pearl 2000 (Cabot Corporation), etc. can be mixed.
Content of the catalyst substance with respect to the whole catalyst layer is 0.01-10 mass% normally, Preferably it is 0.3-5 mass%.

The carbon fiber used in the present invention is not particularly limited in terms of strength, elastic modulus, etc., but the fiber length is preferably less than or equal to the dry film thickness of the catalyst layer, more preferably less than or equal to half the dry film thickness of the catalyst layer, Specifically, it is preferably 1 to 30 μm. Moreover, the thickness of the carbon fiber used for this invention is although it does not specifically limit, Preferably it is 0.001-20 micrometers, More preferably, it is 0.01-10 micrometers.
The carbon fiber content with respect to the entire catalyst layer is preferably 0.01 to 30% by mass, more preferably 0.01 to 10% by mass.
In the electrode of the present invention, the carbon fibers are unevenly distributed in the catalyst layer so as to increase from the side close to the solid polymer electrolyte membrane toward the gas diffusion layer. For example, when the catalyst layer is divided into two layers in the thickness direction, the carbon fiber is preferably 0.1 to 30% by mass in the upper half layer, and more preferably 0.5 to 0.5%. It is desirable that it is 10% by mass.

  The solid polymer electrolyte used in the present invention may be composed of the same material as the solid polymer electrolyte membrane, and is preferably made of an ion exchange resin as in the case of the solid polymer electrolyte membrane. Perfluorosulfonic acid, styrene- A polymer electrolyte obtained by introducing a sulfonic acid or phosphoric acid type group into divinylbenzene, a thermosetting resin, or a thermoplastic resin is preferable.

  As the porous substrate for forming the gas diffusion layer used in the present invention, carbon fiber woven fabric, carbon paper, etc. are immersed in an aqueous dispersion such as PTFE or a fluororesin such as tetrafluoroethylene-hexafluoropropylene copolymer. And water-repellent treatment after drying and firing.

  As an electrode manufacturing method of the present invention, for example, paste A containing catalyst-carrying carbon particles, PTFE particles, a solid polymer electrolyte, and a solvent such as water is applied directly on the solid polymer electrolyte membrane as the first layer of the catalyst layer. Or after forming into a film by spraying and drying by heating, a paste B containing catalyst-carrying carbon particles, PTFE particles, solid polymer electrolyte, carbon fiber and water as a second layer is then formed on the catalyst layer of the first layer. The film is directly formed on the film by coating or spraying, and then dried by heating, and the carbon fiber is laminated so that the abundance thereof increases from the solid polymer electrolyte film to the outside. It is preferable to laminate | stack so that the dry film thickness of the whole catalyst layer at this time may be set to 3-30 micrometers. The change in the amount of carbon fiber may be continuous or stepwise.

  The electrode for a solid polymer electrolyte fuel cell according to the present invention is obtained by bonding a gas diffusion layer material subjected to water repellent treatment to a catalyst layer surface integrally formed with the solid polymer electrolyte by hot pressing or the like and joining them. It is.

  Examples of the fuel cell using the electrode of the present invention include a direct methanol supply type fuel cell (DMFC). The fuel cell of the present invention can be obtained by providing a cell having an anode electrode and a cathode electrode on both surfaces of the electrode of the present invention and a methanol fuel chamber and a cathode gas chamber on the back of each electrode.

Hereinafter, the present invention will be described with reference to examples.
[Example 1]
1 g of catalyst-supported carbon particles (Tanaka Kikinzoku Co., Ltd., TEC10V30E: Vulcan XC-72 (Cabot Corporation) carries 30% by mass of platinum), solid polymer electrolyte (Aldrich, 5% by mass of Nafion (registered trademark)) Solution) A catalyst paste A-1 consisting of 10 g and 2 g of water was applied to a solid polymer electrolyte membrane (manufactured by DuPont, Nafion (registered trademark) 112) (length 100 mm, width 100 mm, thickness 50 μm) placed on a hot plate. The film was directly laminated using a mask. Next, catalyst paste A-2 in which 0.5 g of carbon fiber having a fiber length of 15 μm was blended with catalyst paste A-1 (13 g) was laminated on the catalyst (A-1) layer in the same manner. The resulting catalyst layer A had a dry film thickness of 25 μm, and the amount of platinum supported in the catalyst layer A was 1.3 mg / cm ² .
Further, on the opposite surface of the solid polymer electrolyte membrane with the catalyst layer A formed on the catalyst layer A side, catalyst-supporting carbon particles (manufactured by Tanaka Kikinzoku Co., Ltd., TEC61V33: Vulcan XC-72 (manufactured by Cabot Corporation) with platinum / ruthenium A catalyst paste B-1 comprising 1 g of an alloy (supporting 30% by mass), 10 g of a solid polymer electrolyte (manufactured by Aldrich, Nafion (registered trademark) 5% by mass) and 2 g of water was laminated in the same manner, Catalyst paste B-2 in which 0.5 g of carbon fiber having a fiber length of 15 μm was blended with catalyst paste B-1 was laminated on the catalyst (B-1) layer in the same manner. The resulting catalyst layer B had a dry film thickness of 30 μm, and the amount of platinum supported in the catalyst layer B was 1.4 mg / cm ² .

[Example 2]
The solid polymer electrolyte membrane on which the catalyst layers A and B prepared in Example 1 were laminated was heat-pressed by hot pressing at 100 ° C. for 5 minutes, and carbon paper subjected to water repellent treatment with PTFE was sandwiched from both sides. Then, the electrode for a solid polymer electrolyte fuel cell was obtained by performing heat press again at 100 ° C. for 5 minutes.

[Example 3]
A cell was prepared by providing an anode electrode and a cathode electrode on both sides of the solid polymer electrolyte fuel cell electrode obtained in Example 2, and a fuel cell was prepared by providing a methanol fuel chamber and a cathode gas chamber on the back of each electrode. Then, the electrochemical evaluation was performed. The results are shown in FIG.

[Comparative Example 1]
In Example 1, a catalyst layer A (dry film thickness 25 μm) and a catalyst layer B (dry film thickness 30 μm) were prepared using only catalyst pastes A-1 and B-1 not containing carbon fiber, and Example 2 was further performed. Then, a solid polymer electrolyte fuel cell electrode was prepared in the same manner as described above, and then a fuel cell was produced in the same manner as in Example 3 and its electrochemical evaluation was performed. The results are shown in FIG.

[Comparative Example 2]
In Example 1, the catalyst layer A (dry film thickness 25 μm) was prepared using only the catalyst paste A-1 containing no carbon fiber, and the catalyst layer B (dry film) using only the paste B-2 containing carbon fiber. 30 μm) was prepared, and the electrode for a solid polymer electrolyte fuel cell was formed in the same manner as in Example 2. Then, a fuel cell was prepared in the same manner as in Example 3, and the electrochemical evaluation was performed. The results are shown in FIG.

  As shown in FIG. 1, the fuel cell of the present invention includes a battery using an electrode not containing carbon fibers in a catalyst layer (Comparative Example 1) and a battery using an electrode in which carbon fibers in the catalyst layer are not unevenly distributed (Comparative Example 2). It can be seen that the output voltage is higher than This is because the carbon fiber is unevenly distributed in the catalyst layer of the present invention, the pores in the catalyst layer are not crushed, and the microscopic three-dimensional structure is maintained, so that the catalyst utilization rate is reduced and high performance is achieved. This shows that a simple fuel cell can be obtained.

The current-voltage characteristics of Example 3 and Comparative Examples 1 and 2 are shown.

Claims (8)

  In a fuel cell electrode in which a catalyst layer and a gas diffusion layer are provided in this order on both sides of a solid polymer electrolyte membrane, the catalyst layer contains carbon fibers, and the content of carbon fibers in the catalyst layer is An electrode for a fuel cell, characterized in that the height increases from the side close to the molecular electrolyte membrane toward the gas diffusion layer side.   The fuel cell electrode according to claim 1, wherein the catalyst layer contains a solid polymer electrolyte.   3. The fuel cell electrode according to claim 1, wherein the catalyst substance in the catalyst layer is a platinum group metal or an alloy thereof.   The fuel cell electrode according to any one of claims 1 to 3, wherein the gas diffusion layer forming material is a carbon fiber woven fabric or carbon paper subjected to a water repellent treatment.   In a method for producing a fuel cell electrode by providing a catalyst layer and a gas diffusion layer in this order on both sides of a solid polymer electrolyte membrane, the catalyst layer forming composition contains carbon fiber, and the carbon fiber in the catalyst layer The content of is increased from the side close to the solid polymer electrolyte membrane toward the gas diffusion layer side.   As the catalyst layer forming composition, at least two kinds of compositions having different carbon fiber contents are prepared, and a carbon fiber-free or low-content composition is applied to both surfaces of the solid polymer electrolyte membrane, and then carbon is added. 6. The method according to claim 5, wherein a composition having a high fiber content is applied.   A fuel cell using the fuel cell electrode according to any one of claims 1 to 4.   A fuel cell using a fuel cell electrode produced by the method according to claim 5.

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