JP5068029B2 - Oxygen reduction composite catalyst, method for producing the same, and fuel cell using the same - Google Patents

Description

  The present invention relates to an oxygen reduction composite catalyst, a method for producing the same, and a fuel cell using the same.

  A fuel cell is a power generation system that supplies a hydrocarbon fuel such as hydrogen or alcohol and an oxidant such as oxygen and directly converts chemical energy obtained by the oxidation-reduction reaction into electric energy. Such fuel cells are attracting attention as a clean energy source compared to conventional power generation systems, and are particularly representative of polymer electrolyte fuel cells (PEFC) and direct methanol types (DMFC) using hydrogen as a fuel source. The direct oxidation fuel cell (DOFC) using a liquid hydrocarbon material as a direct fuel has been extensively studied for practical use because it can operate at a relatively low temperature and can be downsized.

  As an oxygen reduction catalyst, an electrode catalyst in which platinum or a platinum alloy is finely dispersed on a carbon support is widely used. Such platinum-based electrode catalysts generally have high oxygen reduction activity, but still have problems in terms of economy.

  On the other hand, a method for expressing oxygen reduction ability by modifying a carbon support surface using a metal complex of a macrocyclic organic compound such as phthalocyanine or porphyrin and a transition metal has been known for a long time. Development of an oxygen reduction catalyst using a macrocyclic organic compound or a so-called metal-N4 complex is also in progress (for example, Patent Documents 1 to 10).

  However, conventional oxygen reduction catalysts using macrocyclic organic compounds have lower oxygen reduction activity than the noble metal-based electrode catalyst. For example, when a high output voltage is required such as for automobiles, the output current is significantly reduced. Yes, it is difficult to use at a practical level. Further, since the two-electron reduction reaction is more likely to proceed than the four-electron reduction reaction, the concentration of hydrogen peroxide is high, and problems with durability such as corrosion of the catalyst itself and the PEFC electrolyte membrane have not been solved. Furthermore, a highly active cobalt or iron porphyrin-derived complex has a problem that it is very expensive. In addition, so-called macrocyclic complexes such as porphyrin or phthalocyanine have a handling difficulty that they are difficult to dissolve in a solvent.

  In addition to conventional noble metal particles, research is also being conducted to improve the functionality of fuel cell catalysts by using both organic compounds or organometallic complexes.

For example, Patent Document 11 discloses an additive that selectively causes an oxygen reduction reaction on a catalyst surface in an air electrode of a direct low-temperature fuel cell. In this patent document, in a low temperature fuel cell such as a direct methanol fuel cell, the fuel supplied to the fuel electrode penetrates the electrolyte, diffuses into the oxygen electrode, and reacts on the catalyst of the oxygen electrode, thereby reducing the electromotive force. As a means of solving the so-called crossover problem that causes a decrease in fuel efficiency and the like, a compound having an adsorption ability on the surface of catalyst particles such as platinum or a compound containing a metal complex thereof as an active ingredient is added onto the oxygen electrode catalyst. When added as an agent, the oxygen reduction selective reaction is allowed to proceed without being disturbed by the fuel compound.
However, in the above method, since the catalyst itself that drives the oxygen reduction reaction is a conventional noble metal, problems in terms of economy and the amount of resources have not been solved at all. Further, since the catalyst adsorbs the additive on the surface of the catalyst, there is a problem that the reaction area of the catalyst is reduced and the output current is lowered as compared with the conventional case.

Patent Document 12 discloses a polymer solid oxide fuel cell oxygen electrode, which is a catalyst comprising a carbon material carrying a transition metal macrocycle compound complex and a noble metal, wherein the carbon material has a BET specific surface area of 500 m ² / g or more. Catalysts for use are disclosed. In this patent document, the reason why the catalytic activity is higher in the coexisting state than the individual catalytic activity is that the two-electron reaction on the transition metal macrocycle compound complex together with the four-electron reduction reaction on the noble metal. Following this, it is assumed that the oxygen reduction reaction is carried out by two reduction reaction paths, such as further two-electron reduction reaction on the noble metal. However, in this patent document, the surface area of the carbon material that is the catalyst support is regulated, and there is a limit to the improvement of the catalyst activity by increasing the surface area of the support. In addition, the macrocyclic compound complex has a problem that it is still expensive as described above. There is still a need for further improvement in catalytic performance and cost reduction.

  In recent years, it has also become clear that a carbon support fired at high temperature can drive a four-electron reduction reaction of oxygen without using a conventional macrocyclic organic compound. For example, in Non-Patent Document 1, Non-Patent Document 2, and Patent Document 13, an oxygen reduction catalyst is prepared by carbonizing a nitrogen-containing polymer such as polyacrylonitrile. However, there is a problem that the oxygen reduction characteristic is inferior to the oxygen catalyst using the macrocyclic organic compound.

  Therefore, in order to obtain a high output energy density that is a characteristic of PEFC in practice, it is still inevitable to carry a platinum-based or platinum alloy.

JP-A-57-105969 JP-A-57-208073 JP-A-57-208074 JP 58-54565 A Japanese Patent Laid-Open No. 11-253811 JP 2000-157871 A JP 2003-109614 A JP 2004-41587 A JP 2004-206148 A JP 2005-203147 A JP 2005-228497 A JP 2003-109614 A JP 2004-330181 A S. Gupta, D. Tryk, I. Bae, W. Aldred, E. Yeager, J. Appl. Electrochem., 19 (1989) 19. Siyu Ye and Ashok K. Vijh, Electrochemistry Communications, vol. 5, No3, pp. 272-275.

  An object of the present invention is to provide an oxygen reduction catalyst for a fuel cell oxygen electrode, which is excellent in price and less resource-constrained and has a higher 4-electron reduction efficiency than before and has an excellent oxygen reduction activity. Further, another object of the present invention is to provide an oxygen reduction catalyst that can reduce the amount of use compared to the conventional case even when platinum is used, or is superior in characteristics and capable of obtaining a high output density. Is to provide. Furthermore, another object of the present invention is to provide a fuel cell membrane electrode assembly including the oxygen reduction catalyst and a fuel cell superior to the conventional one.

As a result of intensive studies to solve the above-mentioned problems, the present inventors have a surface portion capable of binding to a metal atom in at least one of a coordination bond and a covalent bond in an oxygen reduction catalyst for a fuel cell oxygen electrode. The oxygen reduction composite catalyst, in which a metal complex having at least a heteroatom-containing organic compound as a ligand is supported on a conductive support, is more effective than a conventional macrocyclic organic compound that modifies the surface of a conventional carbon support. It has been found that the four-electron reduction reaction efficiency can be improved and the oxygen reduction characteristics are excellent and effective.
Furthermore, the present inventors have provided both a metal complex having a heteroatom-containing organic compound as a ligand and a noble metal on a conductive carrier having a site capable of binding to a metal atom at least one of a coordination bond and a covalent bond. It has been found that an oxygen reduction catalyst having more excellent oxygen reduction characteristics can be obtained even when platinum is used in an amount equivalent to the conventional amount.

That is, in the present invention, a conductive carrier having a surface portion capable of binding to a metal atom at least one of a coordination bond and a covalent bond carries a metal complex having at least a heteroatom-containing organic compound as a ligand. And the conductive support is a carbon material obtained by mixing an organic compound containing a hetero atom in the molecule with sodium hydroxide and heating and carbonizing in an inert gas environment. Is an oxygen reduction composite catalyst characterized by
The conductive carrier contains at least one element selected from the group consisting of nitrogen, oxygen, sulfur, phosphorus, and carbon, and the metal atom in the metal complex is at least a coordinate bond or a covalent bond. On the other hand, they are combined.

  Further, the heteroatom-containing organic compound contains at least one atom selected from the group consisting of nitrogen, oxygen, sulfur or phosphorus, more specifically, a nitrogen-containing cyclic compound, and further, the nitrogen-containing cyclic compound Specific examples thereof include at least one nitrogen-containing aromatic compound selected from the group consisting of pyridines, bipyridines, terpyridines, phenanthrolines, pyrazines or imidazoles. Furthermore, tetracyanoquinodimethane or its salt is mentioned as an example of another hetero atom containing organic compound.

Examples of the central metal of the metal complex include a metal selected from at least one selected from the group consisting of fourth to sixth periodic transition metal elements. Specifically, one selected from cobalt, iron, and copper, or Two or more kinds of metals may be mentioned.
The conductive carrier further carries a noble metal or an alloy thereof, specifically, platinum or an alloy thereof.

Furthermore, in the method for producing an oxygen reduction composite catalyst of the present invention , an organic compound containing a heteroatom in the molecule is mixed with sodium hydroxide, and heated and carbonized in an inert gas environment, whereby coordinate bonding or sharing is achieved. A conductive carrier having a surface part capable of binding to a metal atom at least one of the bonds is obtained, and the conductive carrier carries at least a metal complex having a heteroatom-containing organic compound as a ligand, followed by heat treatment. Thus, an oxygen reduction composite catalyst is produced. Further, after the heat treatment, a noble metal or an alloy thereof is further supported.

  Furthermore, the present invention is a fuel cell using any one of the above oxygen reduction composite catalysts, and examples of the fuel cell include a solid polymer fuel cell and a direct oxidation fuel cell.

  According to the present invention, in an oxygen reduction catalyst for a fuel cell oxygen electrode, at least a heteroatom-containing organic compound is arranged on a conductive support having a surface portion capable of binding to a metal atom at least one of a coordination bond and a covalent bond. The oxygen reduction composite catalyst in which the metal complex having the ligand is supported can improve the efficiency of the four-electron reduction reaction of oxygen as compared with the conventional oxygen reduction catalyst in which the surface of the carbon support is modified with a conventional macrocyclic organic compound.

  Furthermore, the oxygen reduction composite catalyst of the present invention further supports a noble metal without modifying the metal complex by supporting a noble metal such as platinum, palladium, rhodium, iridium, ruthenium or osmium, and alloys containing them. The output current density can be improved as compared with the case where it is supported. Moreover, when obtaining the performance equivalent to the conventional one, the amount of noble metal used can be reduced.

In the first embodiment of the oxygen reduction composite catalyst of the present invention, a conductive support having a surface portion capable of binding to a metal atom at least one of a coordination bond and a covalent bond is a heteroatom-containing organic compound ligand. An oxygen reduction composite catalyst comprising at least a metal complex supported as a carrier, wherein the conductive support is mixed with sodium hydroxide and an organic compound containing a hetero atom in the molecule, and is heated and carbonized in an inert gas environment. It is characterized by being a carbon material .

  The conductive support contains at least one element selected from the group consisting of nitrogen, oxygen, sulfur, phosphorus, and carbon, and the metal atom in the metal complex and the element is at least one of a coordinate bond or a covalent bond. It is desirable to combine.

Since the above-mentioned conductive carrier is required to have high conductivity, it is desirable that a carbon material is a main component. Specifically, it is desirable to have a conductivity of 1 × 10 ⁻³ Scm ⁻¹ or more. The crystal structure is not particularly limited, but carbon black, amorphous carbon, carbon nanofilament and the like can be suitably used. Other examples include carbon nanotubes and nanohorns. Further, it is desirable that there are many surface sites that can be bonded to metal atoms so that the amount of the metal complex supported can be increased. Therefore, it is desirable that the content of at least one element selected from the group consisting of nitrogen, oxygen, sulfur, phosphorus or carbon is large, and at least one of nitrogen, oxygen, sulfur or phosphorus is added to the surface of the carbon material. It is desirable to contain 1 to 17 atomic% in the total amount. Further, it is desirable that the specific surface area of the conductive carrier is large so that the generated current per catalyst use amount is improved. Specifically, the BET specific surface area is desirably 450 m ² / g or more. Further, a method of manufacturing such a conductive support, obtained by a mixture of organic compounds containing hetero atoms in the molecule with sodium hydroxide, heating carbonization in an inert gas environment, such as argon gas be able to.

  The metal complex preferably has a heteroatom-containing organic compound as a ligand. As an example of the heteroatom-containing organic compound, an organic compound containing at least one atom selected from the group consisting of nitrogen, oxygen, sulfur or phosphorus, more specifically, a nitrogen-containing cyclic compound is desirable. . Specific examples of the nitrogen-containing cyclic compound include nitrogen-containing aromatic compounds such as pyridines, bipyridines, terpyridines, phenanthrolines, pyrazines and imidazoles. Furthermore, as a suitable example of another heteroatom-containing organic compound, tetracyanoquinodimethane (hereinafter abbreviated as TCNQ) or a salt thereof may be mentioned.

  Although it is not clear why it is desirable to have a heteroatom-containing organic compound as a ligand, it is considered that the metal complex functions as an oxygen reduction catalyst as is generally known. Further, it is considered that the reason why nitrogen-containing aromatic compounds are desirable is that they have a high coordination ability to the central metal, form stable complexes, and contribute to improvement of the durability of the catalyst. In addition, there is an advantage that it can be easily prepared at a lower cost than known macrocyclic substances such as porphyrin and phthalocyanine.

  Although the detailed reason why TCNQ or a salt thereof can be suitably used is unknown, it is possible to form a covalent bond or a coordinate bond with a central metal by a cyano group or π conjugation possessed by TCNQ or a salt thereof. The central metal portion can be regularly arranged to form a laminated structure, and as a result, the oxygen bridge structure (μ-oxo structure, μ-dioxo structure, di- It is considered that the oxygen reduction catalytic activity is improved by constructing a -μ-oxo structure, μ-hydroxo, di-μ-hydroxo structure, etc.).

  Examples of the central metal of the metal complex include a metal selected from at least one selected from the group consisting of fourth to sixth periodic transition metal elements. Specifically, one selected from cobalt, iron, and copper, or Two or more metals are desirable.

  Although it is known that these metal complexes generally have oxygen reduction catalytic ability, it is not practical as an oxygen reduction catalyst for fuel cells only by modification with a metal complex to a general-purpose carbon material such as ketjen black. There wasn't. In the present invention, a metal complex having a heteroatom-containing organic compound as a ligand is modified on a conductive carrier having a surface portion capable of binding to a metal atom at least one of a coordination bond and a covalent bond, As a result, the oxygen reduction ability was greatly improved.

  Although the details of this reason are not clear, it is considered that the metal or metal complex serving as the catalytic active point is directly bonded to the surface of the conductive support and can be dispersed uniformly on the surface of the conductive support. ing.

  Furthermore, regarding the coexistence effect of the conductive carrier and the metal complex, the nitrogen complex containing nitrogen compounds such as pyridines, bipyridines, terpyridines, phenanthrolines, pyrazines or imidazoles, or metal complexes using TCNQ or a salt thereof can be used. The method exhibited an effect superior to that of the porphyrin-derived complex, which is generally considered to have high oxygen reduction catalytic ability.

  The method for supporting the metal complex on the conductive carrier is not particularly limited and can be supported by a known method. For example, a method of immersing the conductive carrier in a solution in which a metal complex is dissolved, drying under reduced pressure, and solidifying can be used. In addition, a mechanical mixing method such as ball milling may be used for the metal complex powder and the conductive carrier.

  It is desirable to further heat-treat after supporting the metal complex. The heat treatment temperature is not limited because the optimum treatment temperature varies depending on the combination of the conductive carrier and the metal complex to be used, but it is desirable to carry out the heat treatment in the range of 250 ° C to 900 ° C. It is particularly desirable to carry out at 500 ° C to 900 ° C. Although details are not clear, the oxygen reduction ability is improved by heat treatment, so the binding site between the central metal and the conductive carrier or the binding site between the central metal and the heteroatom-containing organic compound ligand is modified. I think that I did. Furthermore, there is an effect of suppressing dissolution of the metal complex or metal particles in the electrolyte. At this time, the effect is not seen in the heat treatment at less than 250 ° C. In addition, when a heat treatment at 900 ° C. or higher is performed, there is a possibility that the binding site with the metal complex or the ligand in the conductive carrier is thermally decomposed and the interaction with the metal complex becomes impossible. Moreover, there is a possibility that the central metal is metallized to further agglomerate and enlarge, and the oxygen reducing ability is lowered.

  In addition, as a second embodiment of the present invention, it is desirable that a conductive support subjected to the first embodiment further carry a noble metal or a noble metal-containing alloy. As a result, the output current density can be improved as compared with the case where a noble metal is supported without modifying the metal complex. Moreover, when obtaining the performance equivalent to the conventional one, the amount of noble metal used can be reduced. Specific examples of the noble metal include at least one selected from platinum, palladium, rhodium, iridium, ruthenium or osmium, and alloys containing them.

  Although the details of this reason are not clear, the oxygen reduction catalytic activity of noble metals is improved and the output current density is improved by modifying the metal complex even in the high output voltage range where the output cannot be achieved only with the metal complex modification. Therefore, it is considered that the synergistic effect of the metal complex modified on the surface of the conductive support and the noble metal is expressed, and the overvoltage in oxygen reduction is reduced.

  The method for supporting the noble metal or the alloy containing the noble metal is not particularly limited, and a known method can be used. For example, platinum or platinum alloys are supported by dropping sodium formate into a suspension in which a conductive carrier surface-modified with a metal complex is dispersed in an aqueous solution of chloroplatinic acid or a mixed aqueous solution of chloroplatinic acid and other noble metal salts. The obtained oxygen reduction catalyst can be obtained.

  The step of supporting the noble metal or its alloy is preferably performed after the first step of supporting the metal complex on the conductive support. Although the detailed reason is not clear, it is considered that the metal complex needs to be bonded to the surface of the conductive support rather than the surface of the noble metal or its alloy particles in order to exhibit the above synergistic effect. is doing.

  Further, in the second embodiment of the present invention, since there was no change in the number of reaction electrons in the oxygen reduction reaction with or without the metal complex modification, the oxygen reduction reaction proceeds with a noble metal or an alloy thereof. I am considering. Therefore, when a metal complex is supported after supporting a noble metal or its alloy, the surface of the noble metal or its alloy particles may be coated, which may inhibit the oxygen reduction reaction.

  In addition, in the case of including the step of heat-treating the metal complex, the order of the second step for supporting the noble metal or its alloy is not particularly limited because it differs depending on the type of the metal complex and the noble metal used. When heat treatment is necessary for the second step, it can be performed together with the heat treatment of the metal complex.

  After producing the oxygen reduction composite catalyst as described above, the fuel cell is assembled as a catalyst for the oxygen electrode of the fuel cell by a conventionally known method, thereby obtaining a fuel cell with superior characteristics and higher power density than the conventional one. be able to. Further, even when a noble metal such as platinum or an alloy thereof is used, it is possible to obtain an excellent fuel cell capable of obtaining a high output density, or a fuel cell excellent in economic efficiency by reducing the amount used compared to the conventional one.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited at all by these.

[Example 1]
(Conductive carrier)
Separately, a carbon material was prepared and used for the conductive carrier supporting the catalyst. First, o-nitroaniline was used as an organic compound precursor, mixed and stirred in an aqueous potassium hydroxide solution, and then concentrated and dried. The obtained solid content was baked at 700 ° C. for 25 minutes in an argon atmosphere. The obtained carbon was sufficiently washed with water until neutral, and dried at 110 ° C. Further, using a planetary ball mill device (silicon nitride ball and pot) manufactured by Fritsch, the obtained carbon was finely pulverized in a dry manner, followed by drying at 700 ° C. for 30 minutes in a nitrogen stream, and a nitrogen-doped carbon carrier 1 Got.

(Catalyst preparation method)
A predetermined amount of the nitrogen-doped carbon support 1 was dispersed in acetone and further subjected to sonication to be homogenized. Bis (2,2′-bipyridine) cobalt tetracyanoquinodimethane (hereinafter abbreviated as Co (bpy) ₂ (TCNQ) ₂ ) was used as a supported metal complex. While gradually dropping an acetone solution prepared so as to have a metal concentration of 2 mass% with respect to the nitrogen-doped carbon carrier 1, ultrasonic treatment was further performed to homogenize. The acetone dispersion solution was dried and then dried at 120 ° C. in the atmosphere. The obtained metal complex-supported nitrogen-doped carbon support 1 was heated to 700 ° C. at a temperature increase rate of 50 ° C./min in an infrared image furnace under a nitrogen stream, and subjected to heat treatment for 30 minutes at that temperature, Catalyst 1 was obtained.

(Catalyst loading method)
4 mg of the prepared catalyst 1 was collected, dispersed in 200 μL of ethanol, and subjected to ultrasonic homogenization. Separately, 2 μL of 5% Nafion (registered trademark) solution was dissolved in 200 μL of ethanol, and 200 μL was collected from the ultrasonic homogenized Nafion-ethanol solution. In addition to the catalyst dispersion, 20 minutes of ultrasonic homogenization was performed. Processed. Further, 4 μL was taken from the homogenized catalyst / Nafion solution and dropped onto a substrate of a glassy carbon disk electrode (rotary ring disk electrode manufactured by Nisshin Keiki Co., Ltd., disk diameter 6 mm, platinum ring) and dispersed. Thereafter, the electrode surface was kept horizontal and air-dried, followed by drying at 60 ° C. in air for 10 minutes. Finally, homogeneity was confirmed with a microscope. Using the obtained catalyst-carrying electrode, the catalytic activity was evaluated by the following electrochemical measurement method.

(Evaluation method of catalyst activity)
The evaluation was carried out using a rotating ring / disk electrode evaluation device manufactured by Nisshin Keiki Co., Ltd., the working electrode fixed to the rotating device, the counter electrode as a platinum wire, and the reference electrode as a saturated calomel electrode. In addition, the electrolytic solution was a 0.5 mol / L sulfuric acid aqueous solution, the working electrode was rotated and maintained at 1000 rpm, and nitrogen was passed through the electrolytic aqueous solution to saturate the measurement system with nitrogen. It was confirmed that there was no electrochemical dissolution from the catalyst-carrying electrode with the nitrogen saturated electrolyte solution. This measurement was omitted for the confirmed electrode catalyst. In addition, measurement of nitrogen saturation system (50 mV / s and 5 to 10 mV / s) was performed with a stationary electrode as necessary. Switching to oxygen aeration, the measurement system was oxygen saturated at 1000 rpm. The ring potential was constant 1.35V (NHE [standard hydrogen electrode] reference) constant, the disk potential was swept from 0.9V (NHE reference) to the cathode at a scanning speed of 1mV / s to obtain a current-potential curve. When changing the number of revolutions, the electrolyte was replaced each time to eliminate the influence of the generated hydrogen peroxide.

Note that the plots for the disc potential of the disk current I _d and the ring current I _r of the rotating ring disk electrode was calculated the number of reaction electrons n based on the following equation.

n = 4 I _d / (I _d + I _r / N)
Here, N is the reaction supplement rate at the ring electrode, and is 0.446 in this example.

[Example 2]
Prepared in the same manner as in Example 1 except that an equimolar mixture of benzotriazole, urea, and oxalic acid was used instead of o-nitroaniline as the organic compound precursor of the conductive carrier, and the nitrogen-doped carbon carrier 2 and Catalyst 2 was obtained. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Example 3]
A nitrogen-doped carbon carrier 3 and a catalyst 3 were obtained in the same manner as in Example 1 except that equimolar mixture of benzotriazole in addition to o-nitroaniline was used as the organic compound precursor of the conductive carrier. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Example 4]
A nitrogen-doped carbon carrier 4 and a catalyst 4 were obtained in the same manner as in Example 1 except that equimolar oxyquinoline was mixed in addition to o-nitroaniline as the organic compound precursor of the conductive carrier. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Example 5]
Prepared in the same manner as in Example 1 except that (2,2′-bipyridine) cobalt tetracyanoquinodimethane (Co (bpy) (TCNQ) ₂ ) was used as the metal complex to be supported on the conductive support, and the catalyst 5 Got. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Example 6]
A catalyst was prepared in the same manner as in Example 2 except that (2,2′-bipyridine) cobalt tetracyanoquinodimethane (Co (bpy) (TCNQ) ₂ ) was used as the metal complex to be supported on the conductive support. Got. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Example 7]
Prepared in the same manner as in Example 3 except that (2,2′-bipyridine) cobalt tetracyanoquinodimethane (Co (bpy) (TCNQ) ₂ ) was used as the metal complex to be supported on the conductive support, and the catalyst 7 Got. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Example 8]
Prepared in the same manner as in Example 4 except that (2,2′-bipyridine) cobalt tetracyanoquinodimethane (Co (bpy) (TCNQ) ₂ ) was used as the metal complex to be supported on the conductive support, and catalyst 8 Got. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Example 9]
A catalyst 9 was obtained in the same manner as in Example 1 except that a tetrakis-methoxyphenyl-porphyrin cobalt complex (hereinafter abbreviated as CoPP) was used as the metal complex supported on the conductive carrier. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Example 10]
The acetone solution in which the catalyst 8 obtained in Example 8 was dispersed was subjected to ultrasonic treatment and homogenized. Next, hexachloroplatinic acid hexahydrate aqueous solution in an amount of 20 mass% by platinum atom calculation with respect to catalyst 8 is gradually added dropwise to the catalyst 8 dispersed acetone solution while ultrasonic homogenization treatment, and after completion of the addition, The homogenization process was continued. After the dispersion was dried on a hot water bath, the obtained powder was mechanically homogenized. Next, heat treatment was performed at 850 ° C. for 30 minutes under an argon stream to obtain Catalyst 10. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Comparative Example 1]
A catalyst 11 was obtained in the same manner as in Example 1 except that Ketjen Black [trade name Ketjen Black EC600JD (hereinafter abbreviated as EC600JD)] manufactured by Lion Corporation was used as the conductive carrier. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Comparative Example 2]
Prepared in the same manner as in Comparative Example 1 except that (2,2′-bipyridine) cobalt tetracyanoquinodimethane (Co (bpy) (TCNQ) ₂ ) was used as the metal complex to be supported on the conductive support, and the catalyst 12 Got. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Comparative Example 3]
A catalyst 13 was obtained in the same manner as in Example 9 except that Ketjen Black [trade name Ketjen Black EC600JD (hereinafter abbreviated as EC600JD)] manufactured by Lion Corporation was used as the conductive carrier. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 1.

[Comparative Example 4]
A catalyst 14 was obtained by supporting platinum in the same process as in Example 10, using a conductive carrier prepared in the same manner as in Example 8, except that the metal complex was not supported. Further, the catalyst loading method and the evaluation method were performed in the same manner as in Example 10.

  Table 1 is a list of materials of the conductive carriers of Examples 1 to 10 and Comparative Examples 1 to 4 and measured values of their physical properties.

  Table 2 is a list of combinations of conductive carriers and metal complexes used in Examples 1 to 10 and Comparative Examples 1 to 4.

  FIG. 1 is a comparison diagram of oxygen reduction characteristics of each catalyst in Examples 1 to 4 and Comparative Example 1. FIG. 2 is a comparison diagram of oxygen reduction characteristics of the catalysts for Examples 5 to 8 and Comparative Example 2 described above. From the comparison of the results, as a result of comparing the conductive support using the same metal complex, the oxygen reduction activity of the nitrogen-doped carbon support is larger than that of Ketjen Black, and the effect of the metal complex support of the nitrogen-doped carbon support in the present invention is It is clear.

  FIG. 3 is a comparison diagram of the number of oxygen reducing electrons of each catalyst in Examples 1 and 9 and Comparative Example 3 described above. From the comparison of the results, it is clear that the number of oxygen-reduced electrons is clearly improved and the production of hydrogen peroxide can be suppressed by the effect of the present invention.

  4, FIG. 5 and FIG. 6 are comparison charts of the cyclic voltammogram, the oxygen reduction characteristics and the number of oxygen reducing electrons in Example 10 and Comparative Example 4. Further, FIG. 7 shows the oxygen reduction current ratio and oxygen reduction electron number ratio based on Comparative Example 4 in Example 10 and Comparative Example 4. From the comparison of the results, the oxygen reduction current is improved by the synergistic effect with platinum even at a potential nobler than 0.7 V where the oxygen reduction activity is not expressed only by the supported metal complex, and the oxygen reduction activity can be improved. It is clear that the effects of the present invention are obtained.

Therefore, from the above results, the oxygen reduction composite catalyst of the present invention can improve the 4-electron reduction reaction efficiency of oxygen compared with the conventional macrocyclic organic compound modified with the conventional carbon support surface, and the oxygen reduction characteristics can be improved. It turns out that it is excellent and effective.
Furthermore, it can be seen that the oxygen reduction composite catalyst of the present invention is an oxygen reduction catalyst that is more excellent in oxygen reduction characteristics even in the same amount of platinum used as before.

2 is a comparison diagram of oxygen activity characteristics of each catalyst in Examples 1 to 4 and Comparative Example 1. FIG. 3 is a comparison diagram of oxygen activity characteristics of respective catalysts in Examples 5 to 8 and Comparative Example 2. FIG. 3 is a comparison diagram of the number of oxygen-reduced electrons of each catalyst in Examples 1 and 9 and Comparative Example 3. FIG. 4 is a cyclic voltammogram in Example 10 and Comparative Example 4. 6 is a comparison diagram of oxygen activity characteristics of respective catalysts in Example 10 and Comparative Example 4. FIG. 6 is a comparison diagram of the number of oxygen-reduced electrons of each catalyst in Example 10 and Comparative Example 4. FIG. FIG. 4 is a graph showing the oxygen reduction current ratio and oxygen reduction electron number ratio of Example 10 with respect to Comparative Example 4.

Claims (15)

The conductive support having a surface portion capable of binding to a metal atom at least one of a coordination bond and a covalent bond is an oxygen reduction composite catalyst comprising at least a metal complex having a heteroatom-containing organic compound as a ligand. And
The oxygen-reducing composite catalyst, wherein the conductive carrier is a carbon material obtained by mixing an organic compound containing a hetero atom in a molecule with sodium hydroxide and performing heat carbonization in an inert gas environment .   The conductive support contains at least one element selected from the group consisting of nitrogen, oxygen, sulfur, phosphorus or carbon, and the metal atom in the metal complex is at least a coordinate bond or a covalent bond. The oxygen reduction composite catalyst according to claim 1, which is bonded on the one hand.   The oxygen-reducing composite catalyst according to claim 1 or 2, wherein the conductive support is made of a carbon material containing at least one of nitrogen, oxygen, sulfur, and phosphorus.   The oxygen reduction composite catalyst according to claim 1, wherein the heteroatom-containing organic compound contains at least one atom selected from the group consisting of nitrogen, oxygen, sulfur, and phosphorus.   The oxygen reduction composite catalyst according to claim 4, wherein the heteroatom-containing organic compound is a nitrogen-containing cyclic compound.   6. The oxygen-reducing composite catalyst according to claim 5, wherein the nitrogen-containing cyclic compound is at least one nitrogen-containing aromatic compound selected from the group consisting of pyridines, bipyridines, terpyridines, phenanthrolines, pyrazines or imidazoles. .   The oxygen reduction composite catalyst according to claim 1, 4 or 5, wherein the heteroatom-containing organic compound is tetracyanoquinodimethane or a salt thereof.   The oxygen reduction composite catalyst according to claim 1 or 2, wherein a central metal of the metal complex is a metal selected from at least one member selected from the group consisting of fourth to sixth periodic transition metal elements.   The oxygen reduction composite catalyst according to claim 1, 2, or 8, wherein a central metal of the metal complex is one or more metals selected from cobalt, iron, and copper.   Furthermore, the oxygen reduction composite catalyst of any one of Claims 1-9 which carry | supports a noble metal or its alloy.   The oxygen-reducing composite catalyst according to claim 10, wherein the noble metal is platinum or an alloy thereof. An organic compound containing a heteroatom in the molecule is mixed with sodium hydroxide and heated and carbonized in an inert gas environment to form a surface site capable of binding to a metal atom at least one of a coordination bond and a covalent bond. An oxygen reduction composite catalyst is obtained by carrying at least a metal complex having a heteroatom-containing organic compound as a ligand on the conductive carrier and then heat-treating the conductive carrier. A method for producing a reduced composite catalyst.   The method for producing an oxygen reduction composite catalyst according to claim 12, wherein a noble metal or an alloy thereof is supported after the heat treatment.   A fuel cell comprising the oxygen reduction composite catalyst according to any one of claims 1 to 13.   The fuel cell according to claim 14, wherein the fuel cell is a solid polymer fuel cell or a direct oxidation fuel cell.

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