JP2006252798A - Manufacturing method of oxygen electrode catalyst for fuel cell, and of direct methanol fuel cell and catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a novel oxygen electrode catalyst with a particle size of 5 nm or less, active with oxygen reduction reaction, and at the same time, inactive with methanol oxidation reaction. <P>SOLUTION: The oxygen electrode catalyst for a fuel cell consists of binary system fine particles expressed in a general formula PtS (provided, a content of S is within the range of 3 at% to 50 at%) carried on a carbon carrier, with a particle size of 5 nm, a specific surface area of the carbon carrier of 300 m<SP>2</SP>/g, and the carrier being a kind of carrier selected from a group consisting of carbon black, acetylene black, and multi-wall carbon nanotube. The PtS catalyst carried by the carbon carrier can be used as an oxygen electrode catalyst for a direct methanol fuel cell. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel cell catalyst. More specifically, the present invention relates to a PtS catalyst suitable for an oxygen electrode catalyst for a direct methanol fuel cell and a method for producing the same.

  Conventionally, most of electric energy has been supplied by thermal power generation, hydroelectric power generation or nuclear power generation. However, thermal power generation not only causes large-scale environmental pollution because it burns fossil fuels such as oil and coal, but depletion of resources such as oil has become a problem. In addition, hydropower generation requires large-scale dam construction, and not only is there concern about the destruction of nature, but there are also limited areas for construction. Nuclear power generation is not only fatal in the radioactive contamination at the time of the accident, but also has a problem of decommissioning nuclear reactors that have reached the end of their life, and is moving in a direction that suppresses construction worldwide.

  Wind power generation and solar power generation have been used around the world as power generation methods that do not require large-scale facilities and do not cause environmental pollution. In Japan, wind power generation and solar power generation are actually used in some areas. It has been put into practical use. However, wind power generation has a drawback in that it cannot generate power without wind, and solar power generation cannot generate power without sunlight, and is affected by natural phenomena and cannot provide stable power supply. Further, in wind power generation, the frequency of the generated power fluctuates due to the strength of the wind, causing a failure of electrical equipment.

  Therefore, recently, research and development on a power generation device that can extract electric energy from hydrogen energy, such as a hydrogen fuel cell, has become active. Hydrogen is obtained by decomposing water and not only exists infinitely on the earth, but also contains a large amount of chemical energy per amount of substance. Moreover, when it is used as an energy source, harmful substances and global warming gases are used. It has the advantage that it does not occur.

  Research on fuel cells using methanol instead of hydrogen gas is also actively conducted. A methanol fuel cell that directly uses methanol, which is a liquid fuel, is expected to be a relatively small output power source for household and industrial use because it is an inexpensive fuel in addition to easy handling of the fuel. The theoretical output voltage of the methanol-oxygen fuel cell is 1.2 V (25 ° C.) which is almost the same as that of hydrogen fuel, and in principle the same characteristics can be expected.

  In a direct methanol fuel cell, methanol is oxidized at the anode and oxygen is reduced at the cathode to extract electric energy. Since these oxidation-reduction reactions are difficult to proceed at room temperature, a catalyst is used in direct methanol fuel cells. The present inventors have proposed that a PtRuP catalyst having a particle diameter of 2 to 3 nm is used as the methanol electrode catalyst to significantly improve the methanol oxidation activity (Japanese Patent Application Nos. 2003-433758 and 2004-2062322, and Japanese Patent Application No. 2004-206232). Application No. 2004-27134 and Japanese Patent Application No. 2004-373450).

  On the other hand, Pt is generally used as the oxygen electrode catalyst. The particle size of a general Pt catalyst is about 5 to 8 nm, and further refinement of the particle size is necessary to enhance the oxygen reduction activity. In a direct methanol fuel cell, a solid polymer electrolyte membrane (for example, Nafion membrane manufactured by DuPont) exists between a methanol electrode catalyst and an oxygen electrode catalyst. This solid polymer electrolyte membrane has a function of transporting protons originally generated at the methanol electrode from the methanol electrode to the oxygen electrode, but also has a property of permeating methanol to the oxygen electrode side at the same time. This phenomenon is called “methanol crossover”. Since the methanol that has reached the oxygen electrode by the methanol crossover is directly oxidized on the oxygen electrode Pt catalyst, a methanol oxidation reaction occurs on the oxygen electrode Pt catalyst, and the reaction sites for oxygen reduction are reduced. For this reason, the cathode overvoltage rises, which is a major cause of lowering the battery voltage. In order to reduce this methanol crossover, studies have been made to replace the solid polymer electrolyte membrane with a hydrocarbon-based sulfonic acid from the conventional Nafion (perfluorinated alkyl sulfonic acid). However, replacing the solid polymer electrolyte membrane with a hydrocarbon-based sulfonic acid suppresses methanol crossover, but there is a trade-off relationship that proton conductivity decreases. A solid polymer electrolyte membrane that maintains the same properties as Nafion membrane has not been developed yet.

  For this reason, Patent Document 1 proposes the use of Au or Ag which is active as an oxygen electrode catalyst for oxygen reduction reaction and inactive for methanol oxidation. If inert to methanol oxidation, the methanol oxidation reaction that reaches the oxygen electrode by methanol crossover does not occur, and methanol direct oxidation reaction does not occur on the catalyst, so oxygen reduction overvoltage can be suppressed. It can prevent the battery voltage from dropping. As the particle size of the catalyst decreases, the surface area increases and the catalytic activity improves. However, Patent Document 1 does not disclose the particle size of Au and Ag catalysts and the synthesis method thereof.

  Patent Document 2 proposes to use Au fine particles having a particle diameter of 1 to 200 nm as an oxygen reduction catalyst. However, this method is a method in which Au fine particles are generated on an Au conductive substrate by electroplating, and there is no disclosure regarding loading on a carbon carrier used directly as a carrier for a catalyst for a methanol fuel cell. In addition, the particle diameter of electrodeposited Au that effectively acts on the oxygen reduction reaction is 5 to 200 nm. In order to increase the oxygen reduction activity, the particle diameter needs to be 5 nm or less.

Japanese Patent Laid-Open No. 11-7964 JP 2003-249230 A

  An object of the present invention is to provide a novel oxygen electrode catalyst having a particle size of 5 nm or less and active for an oxygen reduction reaction, and at the same time inactive for a methanol oxidation reaction.

  Another object of the present invention is to provide a direct methanol fuel cell having a novel oxygen electrode catalyst having a particle size of 5 nm or less and active for oxygen reduction reaction and at the same time inactive for methanol oxidation reaction. Is to provide.

  Another object of the present invention is to provide a method for producing a novel oxygen electrode catalyst having a particle size of 5 nm or less and active for oxygen reduction reaction and at the same time inactive for methanol oxidation reaction. is there.

  As a result of intensive studies, the present inventors have found that when Pt particles are reduced and deposited on a carbon support, a PtS catalyst having a particle size of 5 nm or less can be synthesized by the presence of a supply source of S in the reaction solution. discovered. Furthermore, it has been discovered that PtS catalysts are highly active for oxygen reduction but inactive for methanol oxidation. When Pt is reduced and deposited on the carbon support in the absence of S, the particle size becomes 10 nm. However, when Pt is reduced and precipitated in an atmosphere in which S exists, the particle size is suppressed to 5 nm or less. For this reason, it is thought that the specific surface area of a catalyst increases and oxygen reduction activity improves. Although the mechanism of the catalyst particle size reduction by S is not clear at present, it is presumed that S has an effect of suppressing Pt particle growth. Further, when S having a high electronegativity is present on the Pt surface, a place having a partially low electron density is formed on the Pt surface. For this reason, the bond between the hydrogen of the methanol hydroxyl group is suppressed by the electrostatic repulsive force. For this reason, it is considered that methanol molecules are difficult to hydrogen bond on the PtS catalyst, and as a result, direct oxidation of methanol does not occur.

Therefore, the invention according to claim 1 as means for solving the above-described problems is represented by the following general formula.
PtS
The oxygen electrode catalyst for fuel cells is characterized by comprising binary fine particles represented by the formula (wherein the S content is in the range of 3 at% to 50 at%).

  The invention according to claim 2 as means for solving the problem is the oxygen electrode catalyst for a fuel cell according to claim 1, wherein the PtS binary fine particles are supported on a carbon support. .

  As a means for solving the above-mentioned problem, the invention according to claim 3 is characterized in that the particle size of the PtS binary fine particles supported on the carbon support is in the range of 1 nm to 5 nm. It is an oxygen electrode catalyst for fuel cells as described.

As a means for solving the above problem, the invention according to claim 4 is selected from the group consisting of carbon black, acetylene black and multi-wall carbon nanotubes, wherein the carbon support has a specific surface area of 300 m ² / g or less. The oxygen electrode catalyst for a fuel cell according to claim 2 or 3, wherein the catalyst is at least one kind of support.

As a means for solving the above problems, the invention according to claim 5 is directed to a direct methanol fuel cell having an oxygen electrode catalyst layer, a methanol electrode catalyst layer, and a solid electrolyte membrane interposed between the catalyst layers.
A direct methanol fuel cell, wherein the oxygen electrode catalyst layer comprises the oxygen electrode catalyst according to any one of claims 1 to 4.

  The invention according to claim 6 as means for solving the problem is the direct methanol fuel cell according to claim 5, wherein the methanol electrode catalyst layer comprises a PtRuP catalyst.

As a means for solving the above problems, the invention according to claim 7 includes an oxygen electrode catalyst layer, a methanol electrode catalyst layer, and a membrane electrode assembly having a solid electrolyte membrane interposed between these catalyst layers,
A membrane electrode assembly, wherein the oxygen electrode catalyst layer comprises the oxygen electrode catalyst according to any one of claims 1 to 4.

  As a means for solving the above-mentioned problems, the invention according to claim 8 is the membrane electrode assembly according to claim 7, wherein the methanol electrode catalyst layer comprises a PtRuP catalyst.

As a means for solving the above-mentioned problems, the invention according to claim 9 includes: (1) supplying a salt or complex of Pt, a supply source of S and a carbon support to at least one kind of alcohol;
(2) The mixed liquid obtained in the step (1) is refluxed in the vicinity of the boiling point of the alcohol in an inert gas atmosphere, and the PtS catalyst (S content is in the range of 3 at% to 50 at%) on the carbon support. And having a particle size of less than 5 nm),
(3) A method for producing a PtS catalyst, comprising the steps of filtering, washing, and drying a PtS catalyst deposited on a carbon support.

  The invention according to claim 10 as a means for solving the problem is the method for producing a PtS catalyst according to claim 9, wherein the supply source of S is a sulfur compound having a valence of less than +6. .

  As a means for solving the above-mentioned problems, the invention according to claim 11 is characterized in that the supply source of S is thiosulfate such as ammonium thiosulfate and sodium thiosulfate, sulfites such as ammonium sulfite and sodium sulfite, sodium sulfide, and trisulfide. The method for producing a PtS catalyst according to claim 9 or 10, which is at least one kind of sulfur compound selected from the group consisting of sulfides such as sodium and sodium tetrasulfide, sodium tetrathionate, and sodium hydrosulfite. It is.

  As a means for solving the above-mentioned problems, the invention according to claim 12 is characterized in that the alcohol is methyl alcohol, ethyl alcohol, ethylene glycol, tetraethylene glycol, glycerin, propylene glycol, isoamyl alcohol, n-amyl alcohol, sec-butyl. It is at least one alcohol selected from alcohol, n-butyl alcohol, isobutyl alcohol, allyl alcohol, n-propyl alcohol, 2-ethoxy alcohol, 1,2-hexadecanediol, sodium citrate and tannic acid. The method for producing a PtS catalyst according to claim 9.

As a means for solving the above-mentioned problems, the invention according to claim 13 includes: (1) a step of supplying a salt or complex of Pt, a reducing agent containing S and a carbon carrier into water;
(2) The mixed liquid obtained in the step (1) is heated to 50 ° C. or higher and electroless plating is performed on the carbon support to form a PtS catalyst (the content of S is in the range of 3 at% to 50 at%. Depositing a diameter of less than 5 nm),
(3) A method for producing a PtS catalyst, comprising the steps of filtering, washing, and drying a PtS catalyst deposited on a carbon support.

  The invention according to claim 14 as a means for solving the above-mentioned problem is that the reducing agent containing S is a sulfur compound having a valence of less than +6, The method for producing a PtS catalyst according to claim 13 It is.

  As a means for solving the above problems, the invention according to claim 15 is characterized in that the reducing agent containing S is a thiosulfate such as ammonium thiosulfate or sodium thiosulfate, a sulfite such as ammonium sulfite or sodium sulfite, sodium sulfide, The PtS catalyst according to claim 13 or 14, which is at least one sulfur compound selected from the group consisting of sulfides such as sodium trisulfide and sodium tetrasulfide, sodium tetrathionate, and sodium hydrosulfite. It is a manufacturing method.

As a means for solving the above-mentioned problems, the invention according to claim 16 includes (1) a step of supplying a salt or complex of Pt, a compound containing S and a carbon carrier into water,
(2) The mixed solution obtained in the step (1) is subjected to ultrasonic treatment to form a PtS catalyst on the carbon support (the S content is in the range of 3 at% to 50 at%, and the particle size is less than 5 nm. )
(3) A method for producing a PtS catalyst, comprising the steps of filtering, washing, and drying a PtS catalyst deposited on a carbon support.

  The invention according to claim 17 as means for solving the above-mentioned problems is the method for producing a PtS catalyst according to claim 13, wherein the compound containing S is a sulfur compound having a valence of less than +6. is there.

  As a means for solving the above-mentioned problems, the invention according to claim 18 is characterized in that the compound containing S is a thiosulfate such as ammonium thiosulfate or sodium thiosulfate, a sulfite such as ammonium sulfite or sodium sulfite, sodium sulfide, 18. The production of a PtS catalyst according to claim 16, wherein the PtS catalyst is at least one sulfur compound selected from the group consisting of sulfides such as sodium sulfide and sodium tetrasulfide, sodium tetrathionate, and sodium hydrosulfite. Is the method.

  According to the present invention, by including S in the Pt oxygen electrode catalyst of the direct methanol fuel cell, the particle size of the Pt catalyst is reduced to 5 nm or less, the catalyst surface area is increased, and as a result, the oxygen reduction activity is increased. I can do things. Further, since the PtS catalyst is inactive with respect to methanol oxidation, it is possible to suppress the oxygen reduction overvoltage due to methanol crossover and prevent voltage drop.

An oxygen electrode catalyst for a fuel cell according to the present invention has the following general formula supported on a carbon substrate,
PtS
It consists of the binary system fine particle shown by these. In the above formula, the S content is preferably in the range of 3 at% to 50 at%. When the S content is less than 3 at%, the particle size of the oxygen electrode catalyst cannot be reduced sufficiently. On the other hand, when the S content is more than 50 at%, the oxygen reduction activity of the oxygen electrode catalyst decreases, which is not preferable.

  The particle size of the PtS catalyst is preferably in the range of 1 nm to 5 nm. When the particle diameter is larger than 5 nm, the surface area per unit weight of the catalyst is decreased, and as a result, the oxygen reduction activity is decreased. On the other hand, when the particle size is less than 1 nm, the activity of the synthesized catalyst surface is extremely high, so a compound is formed in the surrounding material and the catalyst surface layer, and the activity of the catalyst itself is lowered.

As the PtS catalyst support, carbon black, acetylene black and multi-wall carbon nanotubes having a specific surface area of less than 300 m ² / g are suitable. Many micropores exist in the carbon support having a specific surface area of more than 300 m ² / g. For this reason, when a carbon support having a specific surface area larger than 300 m ² / g is used, the ratio of the catalyst particles buried in the micropores increases. The PtS catalyst buried in the micropores of the carbon support cannot contribute to the oxygen reduction reaction. For this reason, if a carbon support having a specific surface area greater than 300 m ² / g is used, the utilization efficiency of the catalyst is reduced and the oxygen reduction activity is reduced. Decreases. On the other hand, in the carbon support having a specific surface area of less than 300 m ² / g, since the above-mentioned micropores are few, the utilization efficiency of the PtS catalyst is improved and the oxygen reduction activity is increased. In particular, when the carbon support is acetylene black or multi-wall carbon nanotubes, since there are no micropores, all the PtS catalyst particles are deposited on the support surface, so that the catalyst utilization efficiency is increased and the oxygen reduction activity is improved. In general, the lower limit of the specific surface area of the carbon support is preferably about 20 m ² / g. When the specific surface area is less than 20 m ² / g, it becomes difficult to support the catalyst sufficiently.

  The PtS catalyst fine particles of the present invention can be produced by an electroless plating method, an alcohol reduction method, or an ultrasonic reduction method.

The production method of the PtS catalyst fine particles of the present invention by the alcohol reduction method is basically:
(1) supplying a Pt salt or complex, a source of S and a carbon support to at least one alcohol;
(2) the mixture obtained in step (1) is refluxed in the vicinity of the boiling point of the alcohol in an inert gas atmosphere to deposit a PtS catalyst on the carbon support;
(3) The step includes filtering, washing, and drying the PtS catalyst deposited on the carbon support.

  Examples of the salt or complex of Pt used in the present invention include platinum acetate, platinum nitrate, dinitrodiamine platinum complex, platinum ethylenediamine complex, platinum triphenylphosphine complex, platinum ammine complex, and bis (acetylacetonato) platinum (II). And hexachloroplatinic acid. These platinum compounds can be used alone or in combination of two or more.

  The source of S used in this alcohol reduction method includes thiosulfates such as ammonium thiosulfate and sodium thiosulfate, sulfites such as ammonium sulfite and sodium sulfite, and sulfides such as sodium sulfide, sodium trisulfide and sodium tetrasulfide. Products, sodium tetrathionate, sodium hydrosulfite, and the like can be used. Basically, sulfur compounds having a valence of less than +6 can be used. Sulfate cannot be used in the present invention as a source of S. Sulfate ion has a valence of +6 and reaches the highest valence. +6 valent S is in the same state as Ne, which is a rare gas, in the electron configuration. For this reason, + 6-valent S is extremely stabilized by the octet rule, and cannot form a compound with Pt.

Alcohols used in this alcohol reduction method are methyl alcohol, ethyl alcohol, ethylene glycol, tetraethylene glycol, glycerin, propylene glycol, isoamyl alcohol, n-amyl alcohol, sec-butyl alcohol, n-butyl alcohol, isobutyl alcohol, At least one alcohol selected from allyl alcohol, n-propyl alcohol, 2-ethoxy alcohol, 1,2-hexadecanediol, sodium citrate and tannic acid can be used. In addition, these alcohols can be used alone, or an alcohol to which water has been added can be used. In refluxing, in order to prevent oxidation of the fine particles, it is preferable to perform reflux while replacing the inside of the reaction system with an inert gas such as nitrogen or argon. The heating temperature and reflux time in the alcohol heating reflux treatment vary depending on the type of alcohol used. However, in general, the heating temperature is about 60 to 300 ° C., and the reflux time is in the range of 30 minutes to 6 hours.

The production method of the PtS catalyst fine particles of the present invention by the electroless plating method is basically:
(1) a step of supplying a salt or complex of Pt, a reducing agent containing S, and a carbon support in water;
(2) heating the mixed liquid obtained in the step (1) to 50 ° C. or higher to deposit a PtS catalyst on the carbon support by electroless plating;
(3) It comprises steps of filtering, washing, and drying the PtS catalyst deposited on the carbon support.

  The reducing agent containing S in the electroless plating method includes thiosulfates such as ammonium thiosulfate and sodium thiosulfate, sulfites such as ammonium sulfite and sodium sulfite, and sulfides such as sodium sulfide, sodium trisulfide and sodium tetrasulfide. Products, sodium tetrathionate, sodium hydrosulfite, and the like can be used. Basically, sulfur compounds having a valence of less than +6 can be used. Sulfate cannot be used in the present invention as a source of S. Sulfate ion has a valence of +6 and reaches the highest valence. +6 valent S is in the same state as Ne, which is a rare gas, in the electron configuration. For this reason, + 6-valent S is extremely stabilized by the octet rule, and cannot form a compound with Pt.

  In the case of electroless plating, the general bath temperature is 50 to 90 ° C., and the reduction time is 30 minutes to 4 hours.

The production method of the PtS catalyst fine particles of the present invention by the ultrasonic reduction method is basically:
(1) a step for supplying a salt or complex of Pt, a compound containing S, and a carbon support in water;
(2) sonicating the mixed solution obtained in step (1) to deposit a PtS catalyst on the carbon support; and (3) filtering, washing, and drying the PtS catalyst deposited on the carbon support. It consists of steps to do.

  The compounds containing S used in this ultrasonic reduction method include thiosulfates such as ammonium thiosulfate and sodium thiosulfate, sulfites such as ammonium sulfite and sodium sulfite, sodium sulfide, sodium trisulfide, sodium tetrasulfide and the like. Sulfides, sodium tetrathionate, sodium hydrosulfite, and the like can be used. Basically, sulfur compounds having a valence of less than +6 can be used. Sulfate cannot be used in the present invention as a source of S. Sulfate ion has a valence of +6 and reaches the highest valence. +6 valent S is in the same state as Ne, which is a rare gas, in the electron configuration. For this reason, + 6-valent S is extremely stabilized by the octet rule, and cannot form a compound with Pt.

  The loading of the catalyst used in the present invention is preferably in the range of 50 wt% to 70 wt%. When the loading ratio is less than 50 wt%, when a certain amount of catalyst is applied, the thickness of the catalyst coating increases, the diffusibility of the fuel decreases, and the battery characteristics deteriorate. On the other hand, when the loading ratio exceeds 70 wt%, it becomes impossible to deposit on the carbon base material in a state where the entire catalyst is loaded.

  As the methanol electrode catalyst used in a pair with the PtS oxygen electrode catalyst of the present invention, a methanol electrode catalyst conventionally used in direct methanol fuel cells (for example, PtRu) can be used. PtRuP catalyst particles described in the specifications of Japanese Patent Application No. 2003-433758, Japanese Patent Application No. 2004-206232, Japanese Patent Application No. 2004-271434, and Japanese Patent Application No. 2004-373450 relating to human prior applications are preferably used.

1.69 mmol of platinum hexachloroplatinate hexahydrate and 0.169 mmol of sodium thiosulfate were dissolved in 200 ml of ethanol / water solution (50 vol% / 50 vol%) to obtain a carbon support (Vulcan XC-72R, specific surface area of 254 m ² / g) 0.17 g was ultrasonically dispersed. Thereafter, this solution was refluxed for 2 hours at 95 ° C. in a nitrogen gas atmosphere, and PtS fine particles were deposited and supported on the carbon support. After completion of the reaction, the product was filtered, washed and dried to obtain a PtS catalyst.

Platinum hexachloroplatinate hexahydrate and 0.338 mmol of sodium sulfite are dissolved in 200 ml of ethanol / water solution (50 vol% / 50 vol%), and carbon support (Vulcan XC-72R, specific surface area 254 m ^{2 /} g) 0.17 g Was ultrasonically dispersed. Thereafter, this solution was refluxed for 2 hours at 95 ° C. in a nitrogen gas atmosphere, and PtS fine particles were deposited and supported on the carbon support. After completion of the reaction, the product was filtered, washed, and dried to obtain a PtS catalyst.

Comparative Example 1

1.69 mmol of hexachloroplatinic acid hexahydrate platinum is dissolved in 200 ml of ethanol / water solution (50 vol% / 50 vol%), and 0.17 g of carbon support (Vulcan XC-72R, specific surface area 254 m ^{2 /} g) is ultrasonicated. Dispersed. Thereafter, this solution was refluxed for 2 hours at 95 ° C. in a nitrogen gas atmosphere, and Pt fine particles were deposited and supported on the carbon support. After completion of the reaction, filtration, washing and drying were performed to obtain a Pt catalyst.

  The particle diameters of the catalysts obtained in Examples 1 and 2 and Comparative Example 1 were examined with an electron microscope. The composition was analyzed by fluorescent X-ray. The results are shown in Table 1. In Examples 1 and 2, the particle size of the catalyst is ˜4 nm, while in Comparative Example 1, it is as large as ˜8 nm. From this result, it was confirmed that the Pt catalyst was made fine by adding S.

After adding pure water and an alcohol solution of Nafion to the PtS catalyst obtained in Example 1 and stirring, the viscosity was adjusted to obtain an ink for an oxygen electrode catalyst. This was applied on a Teflon sheet so that the amount of PtS catalyst applied was 5 mg / cm ² . After drying the coating film, the Teflon sheet was peeled off. Further, after adding pure water and an alcohol solution of Nafion to the PtRuP catalyst supported on Vulcan XC-72R and stirring, the viscosity was adjusted to obtain a methanol electrode catalyst ink. This was coated on a Teflon sheet so that the coating amount of the PtRuP catalyst was 5 mg / cm ² . After drying the coating film, the Teflon sheet was peeled off. Thereafter, the PtS oxygen electrode catalyst and the PtRuP methanol electrode catalyst were bonded to both sides of the solid polymer electrolyte membrane (Nafion manufactured by DuPont) by hot pressing. A direct methanol fuel cell 10 shown in FIG. 1 was prepared using a 15 wt% aqueous methanol solution as a fuel.
In FIG. 1, 12 is an oxygen electrode side current collector, 14 is an oxygen electrode side diffusion layer, 16 is a solid polymer electrolyte membrane (Nafion manufactured by DuPont), 18 is a methanol electrode side diffusion layer, and 20 is a methanol electrode side current collector. 30 is a methanol introduction hole, 24 is an air introduction hole provided in the oxygen electrode side current collector 12, 26 is an oxygen electrode catalyst layer (PtS), and 28 is a methanol electrode catalyst layer (PtRuP). The oxygen electrode side current collector 12 has a function as a ventilation structure that takes in outside air (oxygen) through the air introduction hole 24 and also has a current collecting function. The solid polymer electrolyte membrane (Nafion manufactured by DuPont) 16 has a function of transporting protons generated at the methanol electrode to the oxygen electrode side, and a function as a separator for preventing a short circuit between the methanol electrode and the oxygen electrode. . In the direct methanol fuel cell configured as described above, the methanol supplied from the methanol electrode-side current collector 20 is led to the methanol electrode catalyst layer 28 via the methanol electrode-side diffusion layer 18 and is oxidized, and the electrons, protons And protons move to the oxygen electrode side through the solid polymer electrolyte membrane 16. At the oxygen electrode, oxygen taken in from the oxygen electrode side current collector 12 is reduced by electrons generated at the methanol electrode, and this reacts with the protons to generate water. In a direct methanol fuel cell, power generation occurs due to such methanol oxidation reaction and oxygen reduction reaction.

  A direct methanol fuel cell was produced in the same manner as in Example 3 except that the PtS catalyst synthesized in Example 2 was used as the oxygen electrode catalyst in Example 3.

Comparative Example 2

  A direct methanol fuel cell was produced in the same manner as in Example 3 except that the Pt catalyst of Comparative Example 1 was used as the oxygen electrode catalyst in Example 3.

The power density of the direct methanol fuel cells obtained in Examples 3 and 4 and Comparative Example 2 is shown in Table 2 below. In Examples 3 and 4, since a PtS catalyst having a particle diameter of ˜4 nm and high oxygen reduction activity and inert to methanol oxidation is used as an oxygen electrode catalyst, a high output density of 70 mW / cm ² or more is obtained. It has been. On the other hand, in Comparative Example 2, since a pure Pt catalyst having a particle diameter of ˜8 nm is used as the oxygen electrode catalyst, the output density is reduced due to low oxygen reduction activity and direct oxidation of methanol on the oxygen electrode Pt catalyst by methanol crossover. Is as low as 50 mW / cm ² .

1. Hexachloroplatinic acid hexahydrate platinum 1.69 mmol and sodium thiosulfate (0.0845 to 0.845 mmol, dissolved in 200 ml ethanol / water solution (50 vol% / 50 vol%), carbon support (Vulcan XC -72R, 0.17 g of specific surface area of 254 m ^{2 /} g) was ultrasonically dispersed, and then the solution was refluxed for 2 hours at 95 ° C. with stirring in a nitrogen gas atmosphere to deposit PtS fine particles on the carbon support. After completion of the reaction, the product was filtered, washed and dried to obtain a PtS catalyst.

A direct methanol fuel cell similar to that of Example 3 was prepared using the PtS catalyst having a different S concentration obtained in Example 5 as an oxygen electrode catalyst, and the output density was measured. Table 3 below shows the catalyst composition, particle size and power density. When the S composition of the PtS catalyst is 4 to 30 at%, a high output density of 69 to 78 mW / cm ² is obtained. However, it can be seen that when the S composition exceeds 50 at%, the oxygen reduction activity decreases due to the decrease in the Pt concentration and the output density decreases.

1.69 mmol of hexachloroplatinic acid hexahydrate platinum and 0.4225 mmol of sodium thiosulfate were dissolved in 200 ml of ethanol / water solution (50 vol% / 50 vol%) to obtain a carbon support (acetylene black, specific surface area 68 m ² / g). 0.17 g was ultrasonically dispersed. Thereafter, this solution was refluxed for 2 hours at 95 ° C. in a nitrogen gas atmosphere, and PtS fine particles were deposited and supported on the carbon support. After completion of the reaction, the product was filtered, washed and dried to obtain a PtS catalyst. As a result of examining the particle diameter of the catalyst with an electron microscope, it was 3 nm, and as a result of analyzing the composition with fluorescence X, it was Pt ₇₀ S ₃₀ .

Using the Pt ₇₀ S ₃₀ catalyst obtained in Example 6 as an oxygen electrode catalyst, a direct methanol fuel cell similar to that in Example 3 was produced, and the output density was measured. As a result, a high power density of 85 mW / cm ² was obtained. When the carbon support is acetylene black having a specific surface area of 68 m ² / g, there are no micropores in the support. Therefore, all the PtS catalyst is deposited on the surface of the acetylene black support without being buried in the fine pores. For this reason, it is considered that the number of catalyst particles contributing to the oxygen reduction reaction is increased as compared with Vulcan-XC72R, which is carbon black having a specific surface area of 254 m ² / g, and the total oxygen reduction activity is increased.

1.69 mmol of platinum hexachloroplatinate hexahydrate and 0.4225 mmol of sodium thiosulfate were dissolved in 200 ml of ethanol / water solution (50 vol% / 50 vol%) to obtain a carbon support (multi-wall carbon nanotube, specific surface area of 30 m ² / g) 0.17 g was ultrasonically dispersed. Thereafter, this solution was refluxed for 2 hours at 95 ° C. in a nitrogen gas atmosphere, and PtS fine particles were deposited and supported on the carbon support. After completion of the reaction, the product was filtered, washed and dried to obtain a PtS catalyst. As a result of examining the particle diameter of the catalyst with an electron microscope, it was 3 nm, and as a result of analyzing the composition with fluorescence X, it was Pt ₇₀ S ₃₀ .

Using the Pt ₇₀ S ₃₀ catalyst obtained in Example 8 as an oxygen electrode catalyst, a direct methanol fuel cell similar to that in Example 3 was produced, and the output density was measured. As a result, a high power density of 90 mW / cm ² was obtained. When the carbon support is a multi-wall carbon nanotube having a specific surface area of 30 m ² / g, there are no micropores in the support. Therefore, all the PtS catalyst is deposited on the surface of the acetylene black support without being buried in the fine pores. For this reason, it is considered that the number of catalyst particles contributing to the oxygen reduction reaction is increased as compared with Vulcan-XC72R, which is carbon black having a specific surface area of 254 m ² / g, and the total oxygen reduction activity is increased. Further, the multi-wall carbon nanotubes are bulky, and it was revealed by scanning electron microscope observation that many physical voids exist in the oxygen electrode catalyst layer supporting PtS catalyst particles. This physical void is considered to promote the diffusion of oxygen gas in the air and the diffusion and evaporation of water generated at the oxygen electrode, thereby improving the battery characteristics.

1.69 mmol of platinum hexachloroplatinate hexahydrate and 0.4225 mmol of sodium thiosulfate were dissolved in pure water, and 0.17 g of carbon support (Vulcan P, specific surface area 140 m ^{2 /} g) was added and stirred. . Thereafter, this solution was subjected to ultrasonic treatment for 6 hours, and PtS fine particles were deposited and supported on the carbon support. After completion of the reaction, the product was filtered, washed and dried to obtain a PtS catalyst. As a result of examining the particle diameter of the catalyst with an electron microscope, it was 3 nm, and as a result of analyzing the composition with fluorescence X, it was Pt ₇₀ S ₃₀ .

Using the Pt ₇₀ S ₃₀ catalyst obtained in Example 10 as an oxygen electrode catalyst, a direct methanol fuel cell similar to that in Example 3 was produced, and the output density was measured. As a result, a high power density of 80 mW / cm ² was obtained. When the carbon support is Vulcan P having a specific surface area of 140 m ² / g, the specific surface area is smaller than that of the Vulcan-XC72R support, and therefore there are few micropores present in the support. For this reason, it is considered that the PtS catalyst buried in the micropores is decreased, the number of catalyst particles contributing to the oxygen reduction reaction is increased, and the oxygen reduction activity is increased.

1.69 mmol of hexachloroplatinic acid hexahydrate and 0.169 mmol of sodium thiosulfate were dissolved in 400 ml of pure water. Carbon solution (Vulcan P, specific surface area 140 m ^{2 /} g) 0.17 g was ultrasonically dispersed in this aqueous solution. An aqueous sodium hydroxide solution was added dropwise, and the solution was adjusted to pH 12 using a pH meter. While stirring in the air, the liquid temperature was raised to 80 ° C. using a hot plate and electroless plating was performed at this temperature for 2 hours to deposit and carry PtS fine particles on the carbon support. After completion of the reaction, filtration, washing and drying were performed to obtain a PtS catalyst.

1.69 mmol of hexachloroplatinic acid hexahydrate and 0.338 mmol of sodium sulfite were dissolved in 400 ml of pure water. Carbon solution (Vulcan P, specific surface area 140 m ^{2 /} g) 0.17 g was ultrasonically dispersed in this aqueous solution. An aqueous sodium hydroxide solution was added dropwise, and the solution was adjusted to pH 12 using a pH meter. While stirring in the air, the liquid temperature was raised to 80 ° C. using a hot plate and electroless plating was performed at this temperature for 2 hours to deposit and carry PtS fine particles on the carbon support. After completion of the reaction, filtration, washing and drying were performed to obtain a PtS catalyst.

The particle size of the PtS catalyst obtained in Examples 12 and 13 was examined with an electron microscope. The composition was analyzed by fluorescent X-ray. Further, a direct methanol fuel cell was produced in the same manner as in Example 3, and the output density was measured. The results are shown in Table 4 below. Even in the electroless plating method and the ultrasonic reduction method, S can be contained in Pt, and the particle size of the PtS catalyst is reduced to 4 nm or less by containing S. Due to the increase in the active surface area due to the particle size reduction and the low methanol oxidation activity of the PtS catalyst, a high power density of 70 mW / cm ² or more was obtained.

  The PtS catalyst of the present invention is particularly useful as an oxygen electrode catalyst in a direct methanol fuel cell (DMFC), but can also be used as an oxygen electrode catalyst in a polymer electrolyte fuel cell (PEFC).

6 is a partial schematic configuration diagram of a direct methanol fuel cell manufactured in Example 3. FIG.

Explanation of symbols

10 Direct methanol fuel cell 12 Oxygen electrode side current collector 14 Oxygen electrode side diffusion layer 16 Solid polymer electrolyte membrane 18 Methanol electrode side diffusion layer 20 Methanol electrode side current collector 22 Methanol fuel tank 24 Air introduction hole 26 Oxygen electrode PtS Catalyst layer 28 Methanol electrode PtRuP catalyst layer 30 Methanol fuel introduction hole

Claims (18)

The following general formula
PtS
(Wherein the S content is in the range of 3 at 50 to 50 at%). The oxygen electrode catalyst for a fuel cell according to claim 1, wherein the PtS binary fine particles are supported on a carbon support. The oxygen electrode catalyst for a fuel cell according to claim 2, wherein the PtS binary fine particles supported on the carbon support have a particle diameter in the range of 1 nm to 5 nm. The specific surface area of the carbon support is 300 m ² / g or less, and is at least one kind of support selected from the group consisting of carbon black, acetylene black, and multi-wall carbon nanotubes. 3. The oxygen electrode catalyst for fuel cells according to 3. In a direct methanol fuel cell having an oxygen electrode catalyst layer, a methanol electrode catalyst layer, and a solid electrolyte membrane interposed between these catalyst layers,
The direct methanol fuel cell, wherein the oxygen electrode catalyst layer comprises the oxygen electrode catalyst according to any one of claims 1 to 4. 6. The direct methanol fuel cell according to claim 5, wherein the methanol electrode catalyst layer is made of a PtRuP catalyst. In a membrane electrode assembly having an oxygen electrode catalyst layer, a methanol electrode catalyst layer, and a solid electrolyte membrane interposed between these catalyst layers,
The membrane electrode assembly, wherein the oxygen electrode catalyst layer comprises the oxygen electrode catalyst according to any one of claims 1 to 4. The membrane electrode assembly according to claim 7, wherein the methanol electrode catalyst layer comprises a PtRuP catalyst. (1) supplying a Pt salt or complex, a source of S and a carbon support to at least one alcohol;
(2) The mixed liquid obtained in the step (1) is refluxed in the vicinity of the boiling point of the alcohol in an inert gas atmosphere, and the PtS catalyst (S content is within a range of 3 at% to 50 at%) on the carbon support. And having a particle size of less than 5 nm),
(3) A method for producing a PtS catalyst, comprising the steps of filtering, washing, and drying the PtS catalyst deposited on the carbon support. The method for producing a PtS catalyst according to claim 9, wherein the supply source of S is a sulfur compound having a valence of less than +6. Sources of S include thiosulfates such as ammonium thiosulfate and sodium thiosulfate, sulfites such as ammonium sulfite and sodium sulfite, sulfides such as sodium sulfide, sodium trisulfide and sodium tetrasulfide, sodium tetrathionate, hydrosulfite The method for producing a PtS catalyst according to claim 9 or 10, wherein the PtS catalyst is at least one sulfur compound selected from the group consisting of sodium. The alcohol is methyl alcohol, ethyl alcohol, ethylene glycol, tetraethylene glycol, glycerin, propylene glycol, isoamyl alcohol, n-amyl alcohol, sec-butyl alcohol, n-butyl alcohol, isobutyl alcohol, allyl alcohol, n-propyl alcohol. The method for producing a PtS catalyst according to claim 9, wherein the PtS catalyst is at least one alcohol selected from 1,2-ethoxy alcohol, 1,2-hexadecanediol, sodium citrate and tannic acid. (1) a step of supplying a salt or complex of Pt, a reducing agent containing S, and a carbon support in water;
(2) The mixed liquid obtained in the step (1) is heated to 50 ° C. or higher and electroless plating is performed on the carbon support to form a PtS catalyst (the content of S is in the range of 3 at% to 50 at%. Depositing a diameter of less than 5 nm),
(3) A method for producing a PtS catalyst, comprising the steps of filtering, washing, and drying the PtS catalyst deposited on the carbon support. The method for producing a PtS catalyst according to claim 13, wherein the reducing agent containing S is a sulfur compound having a valence of less than +6. The reducing agent containing S includes thiosulfates such as ammonium thiosulfate and sodium thiosulfate, sulfites such as ammonium sulfite and sodium sulfite, sulfides such as sodium sulfide, sodium trisulfide and sodium tetrasulfide, sodium tetrathionate, The method for producing a PtS catalyst according to claim 13 or 14, wherein the PtS catalyst is at least one sulfur compound selected from the group consisting of sodium sulfite. (1) a step of supplying a salt or complex of Pt in water, a compound containing S and a carbon carrier;
(2) The mixed liquid obtained in the step (1) is subjected to ultrasonic treatment to form a PtS catalyst on the carbon support (the content of S is in the range of 3 at% to 50 at%, and the particle size is less than 5 nm. )
(3) A method for producing a PtS catalyst, comprising the steps of filtering, washing, and drying the PtS catalyst deposited on the carbon support. The method for producing a PtS catalyst according to claim 13, wherein the compound containing S is a sulfur compound having a valence of less than +6. The compounds containing S include thiosulfates such as ammonium thiosulfate and sodium thiosulfate, sulfites such as ammonium sulfite and sodium sulfite, sulfides such as sodium sulfide, sodium trisulfide and sodium tetrasulfide, sodium tetrathionate, hydrosulfate The method for producing a PtS catalyst according to claim 16 or 17, wherein the PtS catalyst is at least one sulfur compound selected from the group consisting of phytosodium.

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