JP2008069494A - Composite material of metal and carbon fibers, method for producing the same, electrode for polymer electrolyte fuel cell, and polymer electrolyte fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a composite material of a metal and carbon fibers, which is capable of loading the metal on the carbon fibers uniformly, and excellent in productivity. <P>SOLUTION: This method for producing the composite material of the metal and carbon fibers includes: a process of installing a working electrode and a counter electrode in a solution containing a compound having an aromatic ring and a metal ion, impressing an electric voltage between the working electrode and counter electrode, and electrically plating the metal on a fibrillated polymer while forming the fibrillated polymer by performing an electrolytic oxidation polymerization of the compound having the aromatic ring on the working electrode; and a process of burning the fibrillated polymer plated with the metal to produce the carbon fibers loaded with the metal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a composite of metal and carbon fiber, a method for producing the same, an electrode for a solid polymer fuel cell containing the composite, and a solid polymer fuel cell including the electrode, and more particularly to a carbon fiber. The present invention relates to a method for producing a composite of metal and carbon fiber, which can uniformly carry a metal and has high productivity.

  In recent years, fuel cells have attracted attention as a battery with high power generation efficiency and a low environmental load, and extensive research and development has been conducted. Among fuel cells, polymer electrolyte fuel cells with high output density and low operating temperature are easier to reduce in size and cost than other types of fuel cells. It is expected to spread widely as a household cogeneration system.

  In general, in the above polymer electrolyte fuel cell, a pair of electrodes are arranged with a polymer electrolyte membrane sandwiched between them, and a fuel gas such as hydrogen is brought into contact with the surface of one electrode, and the surface of the other electrode is contacted. An oxygen-containing gas containing oxygen is brought into contact, and electric energy is taken out between the electrodes by using an electrochemical reaction that occurs at this time (see Non-Patent Documents 1 and 2). A catalyst layer is disposed on the electrode in contact with the polymer electrolyte membrane, and an electrochemical reaction occurs at the three-phase interface between the polymer electrolyte membrane, the catalyst layer, and the gas. Therefore, in order to improve the power generation efficiency of the polymer electrolyte fuel cell, it is necessary to increase the reaction field of the electrochemical reaction.

  In order to form a catalyst layer capable of increasing the reaction field of the electrochemical reaction, generally, a paste or slurry containing catalyst powder in which a noble metal catalyst such as platinum is supported on granular carbon such as carbon black, The method of apply | coating on electroconductive porous supports, such as carbon paper, is taken. However, the polymer electrolyte fuel cell including the catalyst layer formed by this method has low power generation efficiency.

  On the other hand, the present inventors polymerized a compound having an aromatic ring on a conductive porous support such as carbon paper to produce a fibril polymer, and then calcined the fibril polymer. It has been found that the power generation efficiency of a polymer electrolyte fuel cell can be improved by using the electrode prepared by producing a carbon fiber and supporting a noble metal on the carbon fiber by electroplating in the polymer electrolyte fuel cell. (See Patent Document 1).

The Chemical Society of Japan, “Chemical Review No. 49, Material Chemistry of New Batteries”, Academic Publishing Center, 2001, p. 180-182 “Polymer fuel cell <2001 edition>”, Technical Information Association, 2001, p. 14-15 International Publication No. 2004/063438 Pamphlet

  However, as a result of further investigation by the present inventors, the electrode disclosed in International Publication No. 2004/063438 has insufficient utilization efficiency of the noble metal because the noble metal is segregated on the surface of the carbon fiber. I understood that. Further, in the method described in International Publication No. 2004/063438, it was found that the electrolytic polymerization process of the monomer and the electroplating process of the metal are separately performed, so that the number of processes is large and there is room for improvement in productivity. .

  Accordingly, an object of the present invention is to solve the above-mentioned problems of the prior art, and to provide a method for producing a composite of metal and carbon fiber that is capable of uniformly supporting a metal on carbon fiber and has excellent productivity. There is to do. Another object of the present invention is to provide a composite of metal and carbon fiber produced by such a method, an electrode for a solid polymer fuel cell containing the composite, and a solid polymer fuel comprising the electrode. To provide a battery.

  As a result of intensive studies to achieve the above object, the present inventors prepared a solution containing a compound having an aromatic ring and a metal ion, a polymerization step of the compound having an aromatic ring in the solution, and a metal ion By simultaneously carrying out the electroplating step according to, the number of steps can be reduced, and the metal can be uniformly supported on the fibril polymer, and the metal obtained by firing the fibril polymer plated with the metal and The present inventors have found that a composite with carbon fiber functions as a catalyst layer of a polymer electrolyte fuel cell, and has completed the present invention.

That is, the method for producing a composite of a metal and carbon fiber according to the present invention includes:
A working electrode and a counter electrode are placed in a solution containing a compound having an aromatic ring and a metal ion, a voltage is applied between the working electrode and the counter electrode, and the compound having an aromatic ring is subjected to electrolytic oxidation polymerization on the working electrode. Electroplating a metal on the fibril polymer while producing the fibril polymer;
Firing the fibrillar polymer plated with the metal to produce carbon fibers carrying the metal.

  In the method for producing a composite of metal and carbon fiber of the present invention, the compound having an aromatic ring is preferably aniline, the metal is preferably Pt, and the working electrode is preferably carbon paper.

In a preferred embodiment of the method for producing a composite of metal and carbon fiber according to the present invention, the current density of the current applied between the working electrode and the counter electrode is -100 mA / cm ^{2 to} 50 mA / cm ² .

  In another preferred embodiment of the method for producing a composite of metal and carbon fiber of the present invention, the firing is performed in a non-oxidizing atmosphere.

  In addition, the composite of the metal and carbon fiber of the present invention is manufactured by the above-described method, and the electrode for the solid polymer fuel cell of the present invention includes a composite of the metal and carbon fiber. The polymer electrolyte fuel cell of the present invention includes the electrode for the polymer electrolyte fuel cell.

  According to the present invention, by preparing a solution containing a compound having an aromatic ring and a metal ion, and performing an electrolytic oxidation polymerization step of the compound having an aromatic ring in the solution and an electroplating step with the metal ion simultaneously, A composite of metal and carbon fiber in which the metal is uniformly supported on the carbon fiber can be produced with high productivity. In addition, an electrode for a polymer electrolyte fuel cell containing the composite and a polymer electrolyte fuel cell including the electrode can be provided.

<Metal and carbon fiber composite and method for producing the same>
Below, the composite_body | complex of the metal of this invention and carbon fiber and its manufacturing method are demonstrated in detail. In the method for producing a composite of metal and carbon fiber according to the present invention, a working electrode and a counter electrode are placed in a solution containing a compound having an aromatic ring and a metal ion, and a voltage is applied between the working electrode and the counter electrode. An electrooxidation polymerization of the compound having an aromatic ring on the working electrode to produce a fibril polymer, and electroplating a metal on the fibril polymer; and firing the fibril polymer plated with the metal. And a step of producing a metal-supported carbon fiber, and the composite of the metal and carbon fiber of the present invention is manufactured by such a method.

  In the production method of the present invention, since the electrolytic oxidation polymerization step and the metal plating electroplating step are simultaneously performed on the compound having an aromatic ring, the number of steps is less than the method described in International Publication No. 2004/063438. High productivity. Further, as in the method described in International Publication No. 2004/063438, when the plating step is performed after the polymerization step and the firing step, the metal tends to segregate near the surface of the carbon fiber layer formed on the support. However, in the present invention, since the polymerization process and the plating process are performed at the same time, the metal is uniformly supported in the fibril polymer, and the metal is uniformly supported to the inside by firing the metal supported fibril polymer. Carbon fiber is obtained. And the composite of the metal and carbon fiber manufactured by the method of the present invention has a large surface area per unit mass of the metal because the metal is uniformly supported to the inside of the carbon fiber, and the utilization efficiency of the metal is high. Therefore, even if the amount of metal used is reduced, sufficient catalytic action can be obtained.

  In the production method of the present invention, first, a solution containing a compound having an aromatic ring and a metal ion is prepared. Here, water is generally used as the solvent of the solution. In addition, examples of the compound having an aromatic ring include a compound having a benzene ring and a compound having an aromatic heterocycle. As the compound having a benzene ring, aniline and aniline derivatives are preferable, and an aromatic heterocycle is included. As the compound, pyrrole, thiophene and derivatives thereof are preferable. These compounds having an aromatic ring may be used singly or as a mixture of two or more. The concentration of the compound having an aromatic ring in the solution is preferably 0.05 to 3 mol / L, more preferably 0.25 to 1.5 mol / L.

  On the other hand, as the metal ions, noble metal ions such as Pt ions and Ru ions are preferable, and Pt ions are particularly preferable. The metal ion can be generated by dissolving a salt composed of a metal ion and an anion in a solvent such as water. Here, the concentration of the metal ion in the solution is preferably 0.1 mol / L or less, and more preferably 0.01 to 0.03 mol / L.

In the solution, it is preferable to mix an acid in addition to the compound having an aromatic ring and the metal ion. In this case, the negative ion of the acid is taken into the fibril polymer synthesized as a dopant to obtain a fibril polymer excellent in conductivity. By using this fibril polymer, the conductivity of the carbon fiber is finally improved. Further improvement can be achieved. Here, examples of the acid include HBF ₄ , H ₂ SO ₄ , HCl, HClO _{4, and the} like. The concentration of the acid is preferably in the range of 0.1 to 3 mol / L, and 0.5 to 2.5 mol / L. A range is more preferred.

  In addition to the compound having an aromatic ring, metal ions, and an acid, a soluble salt or the like may be appropriately added to the solution in order to adjust pH.

In the production method of the present invention, a working electrode and a counter electrode are installed in the solution, and a voltage higher than the oxidation potential of the compound having the aromatic ring between the working electrode and the counter electrode, preferably a current density of −100 mA / cm ^2. A voltage of ˜50 mA / cm ² is applied, and the compound having an aromatic ring is electrolytically oxidized and polymerized on the working electrode to form a fibril polymer, and then a metal is electroplated on the fibril polymer. The energization amount (unit: coulomb) is preferably set so that the polymerization amount of the compound having an aromatic ring and the metal plating amount are 2: 1. Here, as the working electrode and the counter electrode, a plate made of a highly conductive material such as stainless steel, platinum, and carbon, a porous support, and the like can be used, and as the working electrode, carbon paper is particularly preferable.

  As described above, by applying a voltage to a solution containing a compound having an aromatic ring and a metal ion, a fibrillated polymer plated with a metal is generated on the working electrode. In the fibril-like polymer plated with the metal, the fibril-like polymer usually has a three-dimensional continuous structure, a diameter of 30 to several hundred nm, preferably 40 to 500 nm, and a length of 0.5 to 100,000 μm. The thickness is preferably 1 to 10,000 μm. On the other hand, the supported metal corresponds to a metal ion contained in the solution, and as the metal, a noble metal such as Pt or Ru is preferable, and Pt is particularly preferable. Further, the particle size of the metal is preferably 50 nm or less, and more preferably 10 nm or less.

  In the production method of the present invention, the metal-supported fibrillar polymer obtained on the working electrode as described above is washed with a solvent such as water or an organic solvent, dried and then calcined by firing, preferably non- By firing and carbonizing in an oxidizing atmosphere, a metal-supported carbon fiber can be obtained. Here, the drying method is not particularly limited, and examples thereof include a method using a fluidized bed drying device, an air dryer, a spray dryer, etc., in addition to air drying and vacuum drying. In addition, the firing conditions are not particularly limited, and may be set as appropriate so as to obtain the optimum conductivity. Particularly, when high conductivity is required, the temperature is 500 to 3000 ° C., preferably 600 to Baking is preferably performed at 2800 ° C. for 0.5 to 6 hours. Note that examples of the non-oxidizing atmosphere include a nitrogen atmosphere, an argon atmosphere, and a helium atmosphere, and in some cases, a hydrogen atmosphere can also be used.

In the metal-supported carbon fiber, the carbon fiber usually has a three-dimensional continuous structure, has a diameter of 30 to several hundred nm, preferably 40 to 500 nm, and has a length of 0.5 to 100,000 μm, preferably 1 to 10,000 μm. And the surface resistance is 10 ⁶ to 10 ⁻² Ω, preferably 10 ⁴ to 10 ⁻² Ω. The carbon fiber has a residual carbon ratio of 95 to 30%, preferably 90 to 40%. Since the carbon fiber has a structure in which the entire carbon is three-dimensionally continuous, the carbon fiber has higher conductivity than the granular carbon. On the other hand, the particle size of the metal on the carbon fiber is preferably 50 nm or less, and more preferably 10 nm or less.

<Electrode for polymer electrolyte fuel cell>
Next, the polymer electrolyte fuel cell electrode of the present invention will be described in detail. An electrode for a polymer electrolyte fuel cell of the present invention comprises the above-mentioned composite of metal and carbon fiber and a porous support, and as an anode, an air electrode (oxygen electrode) Can also be used. Here, in the polymer electrolyte fuel cell electrode, the porous support functions as a gas diffusion layer, and the composite of metal and carbon fiber functions as a catalyst layer.

  The porous support for a polymer electrolyte fuel cell electrode of the present invention supplies a fuel such as hydrogen gas or an oxygen-containing gas such as oxygen or air to a composite (catalyst layer) of metal and carbon fiber. It functions as a gas diffusion layer and as a current collector that exchanges generated electrons. The material used for the porous support may be any material that is porous and has electronic conductivity, and specific examples include carbon paper and porous carbon cloth, and carbon paper is preferred. In the production of the above composite, by using a porous support such as carbon paper as a working electrode, a composite is formed on the porous support, so that the electrode for the polymer electrolyte fuel cell of the present invention is formed. Can be produced.

  An electrode for a polymer electrolyte fuel cell according to the present invention includes a porous support, a composite of the metal and carbon fiber disposed on the porous support, and a polymer electrolyte impregnated in the composite It is preferable to consist of. As the polyelectrolyte, an ion conductive polymer can be used. Examples of the ion conductive polymer include polymers having ion exchange groups such as sulfonic acid, carboxylic acid, phosphonic acid, and phosphonous acid. And the polymer may or may not contain fluorine. Examples of the ion conductive polymer include perfluorocarbon sulfonic acid polymers such as Nafion (registered trademark). The amount of impregnation of the polymer electrolyte is preferably in the range of 10 to 500 parts by mass with respect to 100 parts by mass of the composite. The thickness of the catalyst layer (composite) is not particularly limited, but is preferably in the range of 0.1 to 100 μm.

<Solid polymer fuel cell>
Next, the polymer electrolyte fuel cell of the present invention will be described in detail with reference to FIG. The illustrated polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) 1 and separators 2 located on both sides thereof. The membrane electrode assembly (MEA) 1 includes a solid polymer electrolyte membrane 3 and fuel electrodes 4A and air electrodes 4B located on both sides thereof. In the fuel electrode 4A, a reaction represented by 2H ₂ → 4H ⁺ + 4e ⁻ occurs, the generated H ⁺ passes through the solid polymer electrolyte membrane 3 to the air electrode 4B, and the generated e ⁻ is taken out to the outside. Current. On the other hand, in the air electrode 4B, a reaction represented by O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O occurs, and water is generated.

  The fuel electrode 4 </ b> A and the air electrode 4 </ b> B include a catalyst layer 5 and a porous support (gas diffusion layer) 6, and are arranged so that the catalyst layer 5 is in contact with the solid polymer electrolyte membrane 3. Here, the catalyst layer 5 is composed of a composite of the above-described metal and carbon fiber, and the metal is uniformly supported, and the surface area of the metal is very large. Therefore, the solid polymer electrolyte membrane 3 and the catalyst layer 5 The reaction field of electrochemical reaction at the three-phase interface with gas is very wide. Therefore, the polymer electrolyte fuel cell of the present invention has high power generation efficiency.

  In addition, as the solid polymer electrolyte membrane 3, an ion conductive polymer can be used. As the ion conductive polymer, those exemplified as the polymer electrolyte that can be impregnated in the catalyst layer are used. Can be used. Moreover, as the separator 2, the normal separator with which flow paths (not shown), such as fuel, air, and produced | generated water, were formed in the surface can be used.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples.

(Example 1)
To 500 mL of an aqueous solution (polymerization solution) containing 1 M H ₂ SO ₄ and 0.5 M aniline, 500 mL of an aqueous solution (plating solution) of 0.02 M H ₂ [PtCl ₆ ] was mixed. Next, a working electrode made of carbon paper [manufactured by Toray] is installed in the mixed solution, a platinum plate is used as a counter electrode, and a current of +50 mA / cm ² (used for polymerization) and -100 mA / cm at room temperature. By alternately applying a current of ² (used in Pt plating), platinum-supported fibrillar polyaniline was obtained. The obtained platinum-supported polyaniline was washed with ion-exchanged water, vacuum-dried for 24 hours, then heated together with carbon paper to 950 ° C at a heating rate of 10 ° C / min in an Ar atmosphere, and then held at 950 ° C for 1 hour. A platinum-supporting carbon fiber was obtained by firing treatment. An SEM photograph of the obtained platinum-supported carbon fiber is shown in FIG.

(Comparative Example 1)
A working electrode made of carbon paper [Toray] is installed in an aqueous solution containing 1M H ₂ SO ₄ and 0.5M aniline, a platinum plate is used as the counter electrode, and a current of +50 mA / cm ² is applied at room temperature. Then, electrolytic polymerization was performed, and fibrillar polyaniline was electrodeposited on the working electrode. The obtained polyaniline was washed with ion-exchanged water, vacuum-dried for 24 hours, then heated together with carbon paper to 950 ° C at a heating rate of 10 ° C / min in an Ar atmosphere, and then held at 950 ° C for 1 hour for firing treatment To obtain carbon fiber. Then, the obtained carbon fiber was placed in a 0.02M H ₂ [PtCl ₆ ] aqueous solution as a working electrode together with the carbon paper, a platinum plate was used as a counter electrode, and a current of −100 mA / cm ² was applied, Pt was electroplated to obtain platinum-supported carbon fibers. An SEM photograph of the obtained platinum-supported carbon fiber is shown in FIG.

  From FIG. 2, the platinum-supported carbon fiber of Example 1 prepared by simultaneously performing the aniline polymerization step and the platinum plating step shows no platinum particles even when observed with an SEM. It can be seen that it is uniformly supported inside. On the other hand, in the gold-supported carbon fiber of Comparative Example 1 prepared by separately performing the aniline polymerization step and the platinum plating step, platinum particles (white portions in the photograph) were observed and supported by observation with an SEM. It can be seen that the particle size of platinum is large.

<CV measurement>
Next, CV (cyclic voltammetry) measurement was performed on the platinum-supported carbon fiber obtained in Example 1. Specifically, carbon paper with platinum-supported carbon fibers prepared to have an electrode area of 1 cm ² is immersed in a 0.5 M H ₂ SO ₄ solution, a platinum plate is used as a counter electrode, and CV measurement of −0.2 V to 1.2 V is performed. went. In addition, Ag / AgCl was used as a standard electrode, and the voltage sweep rate was 100 mV / sec in an O ₂ bubbling atmosphere. The results are shown in FIG. In addition, as a comparison, CV measurement was also performed on carbon paper alone and carbon paper with carbon fiber not carrying platinum. In addition, the carbon paper with carbon fiber not carrying platinum was produced as follows.

(Production of carbon paper with carbon fiber not carrying platinum)
In an acidic aqueous solution containing aniline 0.5 mol / L and sulfuric acid 1.0 mol / L, carbon paper [manufactured by Toray] was installed as the working electrode, a platinum plate was installed as the counter electrode, and a constant current of 35 mA / cm ² at room temperature. Was applied, and electrolytic oxidation polymerization was performed until the total amount of electricity reached 3 C / cm ² , and polyaniline was electrodeposited on the working electrode. The obtained polyaniline was thoroughly washed with pure water and then vacuum-dried for 24 hours. Thereafter, the obtained polyaniline is heated to 950 ° C. at a temperature rising rate of 3 ° C./min in an Ar reduced pressure atmosphere together with the carbon paper, and kept at that temperature for 1 hour to perform a firing process, thereby producing carbon fibers on the carbon paper. did.

  From the results of FIG. 4, no peak was observed in the CV measurement of the carbon paper alone or carbon paper with carbon fiber, but the peak was observed in the CV measurement of the carbon paper with platinum-supported carbon fiber of Example 1. From this, it was confirmed that the composite of the metal and carbon fiber obtained according to the present invention has a catalytic action as a catalyst layer of a polymer electrolyte fuel cell.

It is sectional drawing of an example of the polymer electrolyte fuel cell using the electrode for polymer electrolyte fuel cells of this invention. 2 is a SEM photograph of the carbon fiber obtained in Example 1. 2 is a SEM photograph of the carbon fiber obtained in Comparative Example 1. It is a graph which shows the result of the CV (cyclic voltammetry) measurement with respect to the platinum carrying | support carbon fiber of an Example.

Explanation of symbols

1 Membrane electrode assembly (MEA)
2 Separator 3 Solid polymer electrolyte membrane 4A Fuel electrode 4B Air electrode 5 Catalyst layer (composite of metal and carbon fiber)
6 Porous support (gas diffusion layer)

Claims (9)

A working electrode and a counter electrode are placed in a solution containing a compound having an aromatic ring and a metal ion, a voltage is applied between the working electrode and the counter electrode, and the compound having an aromatic ring is subjected to electrolytic oxidation polymerization on the working electrode. Electroplating a metal on the fibril polymer while producing the fibril polymer;
Firing the fibrillated polymer plated with the metal to produce a carbon fiber carrying the metal, and a method for producing a composite of the metal and the carbon fiber.   4. The method for producing a composite of metal and carbon fiber according to claim 3, wherein the compound having an aromatic ring is aniline.   The method for producing a composite of metal and carbon fiber according to claim 1, wherein the metal contains at least Pt.   The said working electrode is carbon paper, The manufacturing method of the composite_body | complex of the metal and carbon fiber of Claim 1 characterized by the above-mentioned. ^{2. The} method for producing a composite of metal and carbon fiber according to claim 1, wherein a current density of a current applied between the working electrode and the counter electrode is −100 mA / cm ^{2 to} 50 mA / cm ² .   The method for producing a composite of metal and carbon fiber according to claim 1, wherein the firing is performed in a non-oxidizing atmosphere.   The composite_body | complex of the metal and carbon fiber manufactured by the method in any one of Claims 1-6.   An electrode for a polymer electrolyte fuel cell comprising the composite of metal and carbon fiber according to claim 7 and a porous support.   A polymer electrolyte fuel cell comprising the electrode for a polymer electrolyte fuel cell according to claim 8.