JP2005056746A - Method for manufacturing electrocatalyst for direct methanol fuel cells - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a direct methanol fuel cells having cell characteristics superior in suppressing a CO poisoning on a platinum surface at a carbon electrode for supporting a platinum catalyst, particularly in case it is used in an anode electrode, thereby maintaining an efficient and durable electrode reaction. <P>SOLUTION: This method for manufacturing a direct methanol fuel cells comprises coating and treating an electrocatalyst by an anatase crystalline titanium oxide sol that has been stabilized through dispersion with one or more acids selected from a hydroxycarboxylic acid. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for producing a direct methanol fuel cell electrode catalyst, and more particularly to a highly efficient and inexpensive method for producing a direct methanol fuel cell electrode catalyst.

The conventional internal combustion engine power generation method using petroleum or coal as raw material converts chemical energy into mechanical energy and then converts it into electrical energy, so it not only has low energy conversion efficiency but also harmful gas generated during combustion of the raw material. Due to air pollution caused by CO2 and global warming due to CO ₂ gas, construction of next-generation energy systems is now essential.
A fuel cell is an electrochemical conversion device that supplies hydrogen and oxygen as fuel to two electrodes, reacts using a catalyst, and converts the chemical energy of the fuel directly into electrical energy. In recent years, it has attracted attention as a power generation system that also has pollution.

There are several types of such fuel cells depending on the electrolyte used, and the phosphoric acid type (PAFC), molten carbonate type (MCFC), solid oxide type (SOFC), and solid polymer type (PEFC). ), Direct methanol fuel cells (DMFC), etc., and application development is being promoted taking into consideration features such as operating temperature, power generation efficiency, productivity, and size.
Among these, PEFC is being developed for practical use in automobiles, household cogeneration systems, electronic terminal equipment power supplies, etc. due to its small size, light weight, and ease of handling.

Under such circumstances, a direct methanol fuel cell that uses methanol, which is relatively inexpensive and easy to handle, as a fuel does not require a fuel reformer, so that the entire cell can be reduced in size and weight. It is attracting attention as a portable power source.
The electrode reaction of the direct methanol fuel cell is as follows.

CH ₃ OH + H ₂ O → CO ₂ + 6H ⁺ + 6e ⁻ (fuel electrode: anode electrode)
3 / 2O ₂ + 6H ⁺ + 6e ⁻ → 3H ₂ O (Air electrode: cathode electrode)
As the electrode material used for such a reaction, platinum or carbon powder carrying platinum and ruthenium is generally used in order to efficiently promote the methanol oxidation reaction particularly at the anode electrode. However, since direct methanol fuel cells directly oxidize methanol on the electrode surface, there is a problem of catalyst poisoning in which the catalyst surface is covered with CO generated in the reaction process and the electrode performance deteriorates. There is an urgent need to develop a catalyst that can efficiently oxidize the catalyst.

As an electrode catalyst for highly activated fuel cells, a compound comprising an oxide or hydroxide containing Si and an element selected from Nb, Ni, Sn, Ta, Ti, and Zr on a conductive carbon powder carrier There is known a fuel cell electrode catalyst made of a Pt material to which is deposited. (For example, refer to Patent Document 1.)
However, the effect of adding such a metal oxide or hydroxide is an effect of suppressing elution and sintering of the catalytically active metal at the cathode electrode, and the problem of catalyst poisoning has not been solved.

As another technique, a technique related to an electrode for a fuel cell including a catalyst made of a noble metal metal and a photocatalyst activated by irradiation with light such as titanium dioxide is also disclosed. (For example, see Patent Document 2.)
However, this technology is a technology for electrodes that can be used for general purposes in a polymer electrolyte fuel cell, and there is no description of a direct methanol fuel cell, and no mention is made of catalyst poisoning of the electrode. .

In such a situation, one of the inventors of the present invention directly mixed a colloidal solution of titanium oxide with an electrode catalyst such as platinum as a catalyst for a direct methanol fuel cell, and supported the titanium oxide on platinum. Attempts were made to solve the poisoning problem and the technology was disclosed. (For example, Non-Patent Document 1)
However, although the titanium oxide described in Non-Patent Document 1 has a tendency to eliminate CO poisoning to some extent, the effect is not yet sufficient, and further improvement is necessary.

  Thus, as an electrode catalyst for direct methanol fuel cells, no excellent solution technology has yet been found as a method for solving the problem of CO poisoning of platinum catalysts.

Japanese Patent Laid-Open No. 9-167620

JP 2000-243406 A

Koichi Kobayashi and three others, “Preparation and Evaluation of Direct Methanol Fuel Cell Catalysts” (Proceedings of the 9th Fuel Cell Symposium), published on May 15, 2002, p. 338-343

JP 2001-206720 A

As described above, although various proposals have been made as methods for improving the activity of the electrode catalyst for fuel cells, in reality, the performance and life requirements required for the electrode catalyst for direct methanol fuel cells are achieved. Has not reached.
Therefore, the present inventors have been able to produce a catalyst such as platinum, which has been a problem in the past, for an electrode catalyst used in a direct methanol fuel cell that can be manufactured at a relatively low cost and is expected to promote its spread. We intensively studied how to solve the problem of CO poisoning.
As a result, a catalyst electrode obtained by coating an electrode catalyst such as platinum with a titanium oxide sol dispersed and stabilized with hydroxycarboxylic acid is converted into a catalyst electrode carrying titanium oxide as disclosed in the prior art. In comparison, the present inventors have found that the present invention is significantly effective in eliminating CO poisoning, and completed the present invention based on such a technique.

  That is, the present invention provides a direct methanol fuel cell electrode catalyst characterized in that the electrode catalyst is coated with anatase crystalline titanium oxide sol dispersed and stabilized with one or more acids selected from hydroxycarboxylic acids. Regarding the method.

The electrode obtained by the method for producing an electrocatalyst for a direct methanol fuel cell (hereinafter abbreviated as DMFC) according to the present invention, particularly when used as an anode electrode, is capable of CO poisoning of the platinum surface on the platinum catalyst-supporting carbon electrode. Suppress and thus maintain an efficient and sustained electrode reaction. Therefore, according to the present invention, a DMFC having excellent battery characteristics can be provided.
In addition, according to the present invention, by supporting the anatase-type titanium oxide sol used in the present invention on a platinum-ruthenium electrode catalyst, it is possible to obtain a DMFC electrode catalyst having further performance exceeding the above performance.

The present invention is described in further detail below.
First, the anatase type crystalline titanium oxide sol dispersed and stabilized with one or more acids selected from hydroxycarboxylic acids used in the present invention consists of crystalline primary particles. Further, since the sol particles have an average particle diameter of about several nanometers to several tens of nanometers by a dynamic scattering method, the sol particles can be obtained as a sol having a higher concentration than the amorphous titanium oxide sol.
In addition, the anatase type crystalline of titanium oxide is one of the crystalline forms of titanium oxide such as rutile type and brookite type, which are produced at the lowest temperature. In the present invention, the titanium oxide sol is dried, and this is powder X-ray. An anatase-type titanium oxide sol in which an anatase-type diffraction peak is observed when subjected to diffraction measurement is used.

  Such a sol can be manufactured, for example, by a method described in Patent Document 3. To illustrate the production method, first, a water-soluble titanium oxide compound such as titanium tetrachloride is neutralized and decomposed with an alkali such as ammonium carbonate to obtain a hydrous titanium oxide gel, which is filtered and washed to remove residual impurities. Remove. Next, the gel is hydrothermally treated at a temperature of 100 ° C. or higher to grow crystallites and particle diameters, thereby obtaining an anatase type crystalline titanium oxide sol. Next, an anatase type crystalline titanium oxide sol which is dispersed and stabilized with one or more acids selected from the hydroxycarboxylic acids used in the present invention is obtained by adding a hydroxycarboxylic acid thereto and subjecting it to a second hydrothermal treatment. be able to.

Such a sol has fine particles, thereby increasing the specific surface area, and the characteristics of the sol can be used more effectively than a general powder. The average particle diameter of the titanium oxide sol used in the present invention exhibits the best electrode characteristics when it is in the range of 5 nm to 15 nm. Instead of such sols, for example, titanium oxide powder or colloidal solution of titanium oxide having an average particle size of several hundred nm that can be obtained by a pulverization method is used for the fuel cell electrode catalyst as in the present invention. In this case, since the coating state of titanium on the noble metal such as platinum and carbon becomes rough, not only the activity of the electrode catalyst is reduced or unstable electrode characteristics are generated, but also the coating strength to the electrode is also reduced. There is also a problem in practical use.
As described above, even if an electrode catalyst comprising platinum or a combination of platinum and ruthenium is coated with ordinary titanium oxide, the catalytic activity and practical use are insufficient, but the dispersion is stabilized by the hydroxycarboxylic acid used in the present invention. The anatase-type titanium oxide sol in which the titanium oxide nanoparticles are dispersed is immersed in the electrode material or spray-coated on the electrode material, or added, mixed, dried and heated. By performing the coating treatment, the effect as an electrode catalyst for DMFC, that is, the effect of improving catalytic activity and suppressing CO poisoning is exhibited.

  The sol dispersion stabilizer used in the present invention must be one or more acids selected from hydroxycarboxylic acids such as lactic acid, citric acid, glycolic acid, malic acid, tartaric acid, and mandelic acid, for the reason. However, the use of mineral acids such as nitric acid and hydrochloric acid, or organic acids such as carboxylic acid and polycarboxylic acid, which are other inorganic acids, makes it possible to obtain excellent DMFC electrode catalyst characteristics as required in the present invention. I can't get it. Further, among the above hydroxycarboxylic acids, the use of citric acid, malic acid or tartaric acid exhibits the best effect of reducing CO poisoning to a catalyst such as platinum.

Further, regarding the amount of hydroxycarboxylic acid in the anatase crystalline titanium oxide sol, the amount of hydroxycarboxylic acid is 0.01 to 0.1 in terms of hydroxycarboxylic acid / titanium oxide (TiO ₂ ) molar ratio with respect to the amount of titanium oxide. The above-mentioned effect is exhibited most in the range.
Furthermore, the electrode catalyst used for coating with the sol used in the present invention is platinum or platinum and ruthenium. The platinum and ruthenium are alloys, and the molar ratio thereof is approximately 1: 0.5 to 1.2, but the effect of reducing CO poisoning is most effective when the ratio is in the vicinity of 1: 1.

The method for producing an electrode catalyst for DMFC of the present invention comprises the above-mentioned electrode catalyst such as a platinum catalyst, and after adding the sol in advance and performing a coating treatment, this is generally used as an electrode material such as carbon powder. And can be manufactured by baking.
Alternatively, it can be produced by adding the titanium oxide sol used in the present invention to a commercially available platinum-conductive carbon fine powder and mixing them well, followed by firing.

Regarding the amount of titanium oxide with respect to an electrode catalyst such as a platinum catalyst, it is approximately 10 to 40% by mass as TiO ₂ / Pt or TiO ₂ / (Pt—Ru).
As a mixing method, for example, anatase-type titanium oxide sol and the above-mentioned noble metal-conductive carbon powder may be directly mixed, or they may be dispersed in ultrapure water or the like and then mixed into a slurry.
Next, the dispersion slurry is dried and pulverized, and then heated at a temperature of about 100 to 400 ° C. in a nitrogen atmosphere, whereby the DMFC electrode catalyst of the present invention can be produced.

In performing the coating treatment as described above, a thickener such as a water-soluble polymer can be added to the dispersion slurry in order to adjust the viscosity of the anatase-type titanium oxide sol.
Examples of such thickeners include polysaccharides, polyvinyl alcohol, and polyethylene oxide. Furthermore, in order to make it easy to carry anatase type titanium oxide sol on a noble metal catalyst such as platinum, alcohols such as methanol and ethanol, aliphatic surfactants, fluorine surfactants, silicone surfactants, etc. are used. The coating process can also be performed.

  Hereinafter, the details of the invention described so far will be further described with specific examples, but the present invention is not limited only to these examples.

[Titanium oxide sol-coated Pt / C catalyst]
Pt / C (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., specific surface area 329.2 m ² / g, Pt / C = 46.2 / 53.8 (mass ratio)) 0.70 g, titanium oxide sol (manufactured by Taki Chemical Co., Ltd., Tynock SA) -1, TiO ₂ 10% by mass, anatase type, average particle size 7 nm, dispersant: malic acid used (malic acid / TiO ₂ molar ratio 0.03)) 1.32 g and ultrapure water 50 g were mixed to obtain a suspension It was.
Ultrasonic dispersion treatment was performed for 1 hour so that the titanium oxide sol and Pt / C in the suspension were uniformly mixed.
The suspension subjected to the ultrasonic dispersion treatment was dried at 60 ° C., and then the dried product was pulverized in a mortar and then baked at 250 ° C. for 2 hours in an N ₂ gas atmosphere to be coated with TiO ₂ . A Pt / C catalyst was obtained.

[Titanium oxide sol-coated Pt-Ru / C catalyst]
To 0.70 g of Pt-Ru / C (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., specific surface area 329.2 m ² / g, Pt / Ru / C = 29.6 / 23.0 / 47.4 (mass ratio)), titanium oxide sol (Taki Chemical ( Co., Ltd., Tainok SA-2, TiO ₂ 10 mass%, anatase type, average particle size 8 nm, dispersant: citric acid used (citric acid / TiO ₂ molar ratio 0.03)) 0.86 g, ultrapure water 50 g mixed A suspension was obtained.
Ultrasonic dispersion treatment was performed for 1 hour so that the titanium oxide sol and Pt—Ru / C in the suspension were uniformly mixed.
The suspension subjected to the ultrasonic dispersion treatment was dried at 60 ° C., and then the dried product was pulverized in a mortar and then baked at 250 ° C. for 2 hours in an N ₂ gas atmosphere to be coated with TiO ₂ . A Pt-Ru / C catalyst was obtained.

[Titanium oxide sol-coated Pt / C catalyst]
Pt / C (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., specific surface area 329.2 m ² / g, Pt / C = 46.2 / 53.8 (mass ratio)) 0.70 g, titanium oxide sol (manufactured by Taki Chemical Co., Ltd., Tynock SA) -3, TiO ₂ 10% by mass, anatase type, average particle size 7 nm, dispersant: tartaric acid used (tartaric acid / TiO ₂ molar ratio 0.03)) 1.32 g and ultrapure water 50 g were mixed to obtain a suspension.
Ultrasonic dispersion treatment was performed for 1 hour so that the titanium oxide sol and Pt / C in the suspension were uniformly mixed.
The suspension subjected to the ultrasonic dispersion treatment was dried at 60 ° C., and then the dried product was pulverized in a mortar and then baked at 250 ° C. for 2 hours in an N ₂ gas atmosphere to be coated with TiO ₂ . A Pt / C catalyst was obtained.
[Comparative Example 1]

[Titanium oxide non-treated Pt / C catalyst]
Pt / C (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., specific surface area 329.2 m ² / g, Pt / C = 46.2 / 53.8 (mass ratio)) 0.70 g was mixed with 50 g of ultrapure water in the same manner as in Example 1. The ultrasonic dispersion treatment for 1 hour was performed.
The suspension subjected to ultrasonic dispersion treatment was dried at 60 ° C., and then the dried product was pulverized in a mortar and calcined at 250 ° C. for 2 hours in an N ₂ gas atmosphere to obtain a Pt / C catalyst.
[Comparative Example 2]

[Titanium oxide non-treated Pt-Ru / C catalyst]
Pt-Ru / C (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., specific surface area 329.2 m ² / g, Pt / Ru / C = 29.6 / 23.0 / 47.4 (mass ratio)) 0.70 g was mixed with 50 g of ultrapure water. In the same manner as in Example 1, ultrasonic dispersion treatment for 1 hour was performed.
The suspension subjected to the ultrasonic dispersion treatment was dried at 60 ° C., and then the dried product was pulverized in a mortar and calcined at 250 ° C. for 2 hours in an N ₂ gas atmosphere to obtain a Pt—Ru / C catalyst.
[Comparative Example 3]

[Titanium oxide powder-coated Pt / C catalyst]
Pt / C (Tanaka Kikinzoku Kogyo Co., Ltd., specific surface area 329.2m ² / g, Pt / C = 46.2 / 53.8 (mass ratio)) 0.70g, titanium oxide powder (Taki Chemical Co., Ltd., Tynock) A-100, TiO ₂ 84 mass%, anatase type, average particle diameter 2 μm, specific surface area 300 m ² / g) 0.16 g and ultrapure water 50 g were mixed to obtain a suspension.
An ultrasonic dispersion treatment for 1 hour was performed so that the titanium oxide powder and Pt / C in the suspension were uniformly mixed.
The suspension subjected to the ultrasonic dispersion treatment was dried at 60 ° C., and then the dried product was pulverized in a mortar and then baked at 250 ° C. for 2 hours in an N ₂ gas atmosphere to be coated with TiO ₂ . A Pt / C catalyst was obtained.
[Comparative Example 4]

[Nitric acid-stable titanium oxide sol-coated Pt / C catalyst]
Pt / C (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., specific surface area 329.2m ² / g, Pt / C = 46.2 / 53.8 (mass ratio)) 0.70g, nitrate stable titanium oxide sol (manufactured by Taki Chemical Co., Ltd.) , Tynock I-3, TiO ₂ 10% by mass, anatase type, average particle size 12nm, dispersant: nitric acid (nitric acid / TiO ₂ molar ratio 0.01)) 1.32g, ultrapure water 50g was mixed and the suspension was mixed Obtained.
Ultrasonic dispersion treatment was performed for 1 hour so that the titanium oxide sol and Pt / C in the suspension were uniformly mixed.
The suspension subjected to the ultrasonic dispersion treatment was dried at 60 ° C., and then the dried product was pulverized in a mortar and then baked at 250 ° C. for 2 hours in an N ₂ gas atmosphere to be coated with TiO ₂ . A Pt / C catalyst was obtained.
Table 1 shows the components of the electrode catalysts obtained in Examples 1 to 3 and Comparative Examples 1 to 4.

[Production of anode electrode (fuel electrode)]
The catalyst was sufficiently obtained by mixing 0.20 g of the catalyst obtained in Examples 1 to 3 and Comparative Examples 1 to 4, 1.53 g of 5 mass% Nafion solution and 0.34 g of ultrapure water, and stirring for 1 hour. A paste dispersed in was obtained.
The obtained paste was applied onto carbon paper that had been subjected to a water-repellent treatment with polytetrafluoroethylene so as to be 0.80 mg / cm ^{2 in} terms of Pt. After drying this, the paper was cut to produce a 4 cm ² anode electrode.

[Production of membrane electrode assembly (MEA)]
Each of the anode electrodes described above and the Pt / C electrode catalyst prepared by the above method using the catalyst of Comparative Example 1 was used as a cathode electrode, preliminarily 1% with 3% hydrogen peroxide, 1 hour with pure water, and 1M A membrane electrode assembly (MEA) was produced by thermocompression bonding at 130 ° C. and 1 MPa for 2 minutes together with a Nafion 112 membrane that had been boiled with sulfuric acid for 1 hour and further with pure water for 1 hour.

[Evaluation of electrode performance and DMFC]
A 5 mass% aqueous methanol solution was used as the fuel, the amount of fuel supplied to the anode was 0.8 ml / min, and the fuel supply temperature was 95 ° C. Further, the fuel cell was operated at a cell temperature of 80 ° C. with an oxygen supply amount to the cathode of 100 ml / min.
In addition, as a method for confirming MEA performance, a fuel cell evaluation device (Globe Tech. Inc.) is used for load fluctuation (IV) characteristics, and an impedance analyzer (Zehner GmbH) is used for impedance measurement. The impedance was measured under the conditions of a voltage of 0.30 V, a frequency of 50 mHz to 15 kHz, and a superimposed voltage of 5 mV.
When the electrode catalysts manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 were subjected to the above performance evaluation test, the following results of IV characteristics and impedance characteristics were obtained.

[IV characteristics]
The IV characteristic of DMFC using the produced electrode catalyst on the anode side was measured. The results of measurement using each electrode catalyst are shown in FIG. The IV characteristic is a characteristic indicating battery performance. The higher the IV curve, the better the battery performance. In FIG. 1, since only the catalyst type is changed, the higher the IV curve, the better the catalyst performance.
From the measurement results, comparing the electrode using the catalyst of Example 1 and the electrode using the catalyst of Comparative Example 1, it can be seen that the performance of the catalyst supporting TiO ₂ is improved. The same applies to the comparison between Example 2 and Comparative Example 2.
Furthermore, when the electrode using the catalyst of Example 1 and the electrode using the catalyst of Comparative Example 3 are compared, almost no improvement in the battery performance of Comparative Example 3 is observed. That is, this experiment shows that the platinum catalyst is not simply coated with TiO ₂ , and the superiority of the technology of the present invention can be known.
Further, when the electrode using the catalyst of Example 1 and the electrode using the catalyst of Comparative Example 4 are compared, the titanium oxide sol (malic acid stable type) of the present invention is the oxidation of Comparative Example 4 even if the same titanium oxide sol is used. It is clear that the catalyst performance is remarkably improved as compared with the electrode using titanium sol (nitric acid stable type). From this fact, the superiority of the technology of the present invention can be heard.
In addition, as a result of measuring about the catalyst of Example 3, IV characteristic was the same as the measurement result by the catalyst of Example 1 substantially.

[Impedance characteristics]
The impedance characteristic of DMFC using the produced electrode catalyst on the anode side was measured. The Nyquist plot is used as the display method for the impedance characteristics. In the Nyquist plot, the size of the semicircle of the arc (arc) indicates the superiority or inferiority of the catalyst activity. Therefore, the smaller the size of the semicircle, the better the catalytic activity. The results of measurement using the catalysts of Example 1 and Comparative Examples 1, 3 and 4 are shown in FIG.
From the measurement results, when the electrode using the catalyst of Example 1 and the electrode using the catalyst of Comparative Example 1 are compared, it can be seen that the catalyst supporting TiO ₂ of the present invention is remarkably superior in catalytic activity.
For example, comparing the electrode using the titanium oxide sol of the present invention with the nitric acid stable titanium oxide sol of Comparative Example 4, it can be clearly seen that the catalytic activity of the titanium oxide sol of the present invention is improved.
In addition, as a result of measuring about the catalyst of Example 3, the impedance characteristic was the same as the measurement result by the catalyst of Example 1.

It is a graph which shows the load fluctuation (IV) characteristic of DMFC which used the catalyst of the Example and the comparative example for the anode pole. It is a graph which shows the impedance characteristic of DMFC which used the catalyst of the Example and the comparative example for the anode pole.

Claims (6)

A method for producing an electrode catalyst for a direct methanol fuel cell, wherein the electrode catalyst is coated with anatase crystalline titanium oxide sol dispersed and stabilized with one or more acids selected from hydroxycarboxylic acids. The method for producing an electrode catalyst for a direct methanol fuel cell according to claim 1, wherein the anatase type crystalline titanium oxide sol has an average particle size in the range of 5 to 15 nm. The amount of hydroxycarboxylic acid of the anatase type crystalline titanium oxide sol is in the range of 0.01 to 0.1 in terms of hydroxycarboxylic acid / titanium oxide (TiO ₂ ) molar ratio with respect to the amount of titanium oxide. A method for producing an electrode catalyst for a direct methanol fuel cell. The method for producing an electrode catalyst for a direct methanol fuel cell according to claim 1, 2 or 3, wherein the hydroxycarboxylic acid is citric acid, malic acid or tartaric acid. The method for producing an electrode catalyst for a direct methanol fuel cell according to any one of claims 1 to 4, wherein the electrode catalyst is an anode electrode catalyst. The method for producing an electrode catalyst for a direct methanol fuel cell according to any one of claims 1 to 5, wherein the electrode catalyst is platinum or platinum and ruthenium.

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