JPH11111305A - Fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the resistance to toxicity against carbon monoxide and oxidization activity of methanol by including Pt and Re in an anode catalyst contained in an anode of a gas diffusing electrode, and setting the Re content at a specified ratio. SOLUTION: Content ratio of Re with respect to the total metal in an anode catalyst is set at 1-80 atom %. Pt is preferably included as the alloy with at least one kind of metal to be selected except for Pt from among platinum group elements, Au, Ag, Cr, Fe, Co, Ni, Mo, W, Zn, and Sn. When particle size of the anode catalyst is preferably set at 1-30 nm so as to obtain high activity. As the anodeic catalyst, a catalyst with the catalyst grains carried by a carrier is preferable. As a carrier, a carbon material having conductivity for functioning as a collector and corrosion resistance under the catalyst using condition, for example, conductive carbon black is used. Specific surface area thereof is preferably set at 50-2,000 m<2> /g.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a fuel cell characterized by an anode catalyst contained in an anode comprising a gas diffusion electrode.

[0002]

2. Description of the Related Art A polymer electrolyte fuel cell using hydrogen as a fuel has been conventionally known as a typical one using a fluororesin-based ion exchange membrane as an electrolyte. It has the feature that a density is obtained and only water is generated in principle. For this reason, with the increasing social demands for energy and global environmental issues in recent years,
There are great expectations for power sources for electric vehicles and stationary power sources.

In a polymer electrolyte fuel cell, use of hydrogen gas obtained by steam reforming a hydrocarbon-based material such as methane or methanol as a fuel has been considered. For example, the methanol reforming reaction is a reaction performed at 250 to 300 ° C. using a Cu—Zn-based catalyst or the like, and proceeds stepwise as in the following equation. _{CH 3 OH = 2H 2 + CO} -90kJ / mol CO + H 2 O = H 2 + CO 2 + 40kJ / mol

[0004] That is, methanol is reacted with steam in a reformer to convert the gas into reformed gas (hydrogen and carbon monoxide), and then the carbon monoxide is further subjected to a shift reaction with steam in a shift converter, whereby hydrogen gas is mainly converted. The reformed gas is supplied to the anode of the fuel cell body as a component.

[0005] Usually, even if such a shift reaction is performed,
About 1% of carbon monoxide is contained in the obtained hydrogen gas, which becomes a catalyst poison of the platinum-based catalyst used for the anode. In particular, in a polymer electrolyte fuel cell characterized by a low-temperature operation of 100 ° C. or less, the catalytic poisoning effect of carbon monoxide on the platinum catalyst is remarkable, and the cell characteristics are greatly reduced. In order to avoid the influence of carbon monoxide in the reformed gas, a function for reducing the concentration of carbon monoxide is incorporated in the reformer, or the catalyst used for the anode is exposed to carbon monoxide. It is considered to improve poison resistance. However, conventionally used platinum catalysts and platinum alloy catalysts did not have sufficient poisoning resistance to carbon monoxide.

On the other hand, a methanol fuel cell using methanol directly as a fuel is expected to be a relatively small-output power source for home use and industrial use because the fuel is easy to handle and inexpensive. The theoretical output voltage of a methanol-oxygen fuel cell is 1.2 V (25
° C), and similar characteristics can be expected in principle.

[0007] For this reason, many studies have been made on the oxidation reaction of methanol at the anode, but a methanol oxidation catalyst having sufficient activity has not been found yet.
For example, when the oxidation catalyst is a platinum catalyst, the overvoltage of the anodic oxidation reaction at the methanol electrode becomes considerably large. Therefore, the terminal voltage of the methanol fuel cell is already low even under a light load condition in combination with the overvoltage of the oxygen reduction reaction at the air electrode or the oxygen electrode, and further decreases as the output current increases, and the value can be expected from thermodynamic data. It is much smaller than the value.

Conventionally, in addition to platinum alone as a conductive carbon carrier, a platinum-ruthenium alloy (Japanese Unexamined Patent Publication No.
440) or a platinum-tin alloy (JP-A-2-11445)
Attempts have been made to improve the methanol oxidation activity by carrying 2). However, even when such a platinum-based catalyst is used in a large amount, the oxidation reaction of methanol is slow, a large current cannot be taken out, and the development of a catalyst having more excellent methanol oxidation activity is desired.

In the conventional methanol fuel cell, the supplied methanol does not react at the methanol electrode, but reaches the air electrode or the oxygen electrode as it passes through the electrolyte. The reaction caused a decrease in battery performance. In order to reduce the amount of cross leak of methanol,
There is a need to develop a catalyst having excellent methanol oxidation activity.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a hydrogen fuel solid polymer fuel cell and a methanol fuel cell having excellent methanol oxidation activity with improved poisoning resistance to carbon monoxide. It is in.

[0011]

According to the present invention, an anode comprising a gas diffusion electrode is a catalyst containing platinum and rhenium, and the ratio of rhenium to all metals in the catalyst is low. Provided is a fuel cell, which is 1 to 80 atomic%.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The anode catalyst according to the present invention comprises:
It is a catalyst containing platinum and rhenium. Rhenium is
It is preferably contained as an alloy with platinum, but may be contained as a compound such as rhenium oxide.
The structure of the alloy of rhenium and platinum is not particularly limited, and may be any structure such as a solid solution, an amorphous alloy, and an intermetallic compound.

The content ratio of rhenium to the total metal in the anode catalyst is 1 to 80 atomic%, and 10 to 30 atomic%.
Preferably it is atomic%. In this specification,
The ratio of rhenium in the catalyst (unit: atomic%) indicates the ratio of rhenium in the total of the metal elements constituting the catalyst (excluding the carrier). If the rhenium content is out of the above range, the resistance to poisoning of metals constituting the catalyst, particularly platinum, to carbon monoxide is reduced in the hydrogen fuel cell, and the methanol oxidation activity is significantly reduced in the methanol fuel cell.

In the present invention, platinum is an alloy with at least one metal selected from the group consisting of platinum group elements other than platinum, gold, silver, chromium, iron, cobalt, nickel, molybdenum, tungsten, zinc and tin. It is preferably contained as From the viewpoints of stability and high activity, it is particularly preferable to contain platinum as an alloy of one or more metals selected from chromium, molybdenum, tungsten, and tin.

The composition of the above-mentioned platinum alloy is 30 to 90 atomic% of platinum, 10% of alloying element (ie, element other than platinum element).
Is preferably 70 to 70 atomic%, particularly preferably 50 to 80 atomic% of platinum and 20 to 50 atomic% of alloying element. As a method of alloying, it is preferable to perform heat treatment at a temperature of 600 to 900 ° C. in an atmosphere of an inert gas such as argon, helium, or nitrogen to achieve alloying.

Next, a method for producing an anode catalyst according to the present invention will be described. For example, first, a supported catalyst supporting platinum black, platinum alloy black, or platinum or a platinum alloy was dispersed in water or alcohol containing a rhenium compound such as ammonium perrhenate, potassium perrhenate or sodium perrhenate. Thereafter, a rhenium compound is deposited on the catalyst by evaporation to dryness or the like, and then calcined to deposit rhenium oxide (Re ₂ O ₇ ). Then, this is subjected to a hydrogen reduction treatment in a hydrogen atmosphere to produce a rhenium-containing catalyst.

The average particle diameter of the anode catalyst is preferably from 1 to 30 nm, and more preferably from 2 to 5 nm in order to obtain high activity.
Is particularly preferred. The fuel cell of the present invention is not particularly limited, but is preferably a polymer electrolyte fuel cell using hydrogen as a fuel, or a methanol fuel cell.

In a polymer electrolyte fuel cell, since operation at a high current density and high gas diffusivity are required, the thickness of the electrode layer is reduced, and the catalyst particles are dispersed in the electrode layer with good dispersibility. It is important to secure the quantity. A fine powder catalyst containing platinum black or the like is suitable for thinning the electrode layer and using the catalyst at a high density.

Further, as the anode catalyst in the present invention, a catalyst in which catalyst particles are supported on a conductive carrier is suitable for obtaining catalyst particles having a preferable particle size with good dispersibility.
In order to secure a sufficient amount of the supported catalyst, it is preferable that the catalyst is supported at 10 to 60% by weight, particularly 20 to 40% by weight based on the total weight of the supported catalyst.

As the carrier used for the above-mentioned supported catalyst, a carbon material having conductivity which functions as a current collector as a carrier for an electrode catalyst and corrosion resistance under the conditions where the catalyst is used are preferable. Among them, conductive carbon black, acetylene black, graphite and the like are preferable, and the specific surface area of the carrier is 50 to 2000 m ² / g, particularly 200 to 800 m ^2.
2 / g is preferred.

In the present invention, the gas diffusion electrode constituting the anode using the above catalyst can be produced according to a conventional method, and is obtained by holding the above catalyst with a hydrophobic resin binder such as polytetrafluoroethylene. This gas diffusion electrode is preferably made of a porous sheet.
On the other hand, the cathode is obtained by holding a catalyst such as platinum supported on carbon with a hydrophobic resin binder such as polytetrafluoroethylene, and is preferably a porous gas diffusion electrode like the anode.

In a polymer electrolyte fuel cell using a cation exchange membrane as an electrolyte, each of the above gas diffusion electrodes and a perfluorosulfonic acid type ion exchange membrane are brought into close contact with each other by a hot press method or the like. If necessary, the gas diffusion electrode may include a solid polymer electrolyte. As the current collector, a conductive carbon plate having a groove serving as a passage for the fuel gas or the oxidizing gas can be used.

[0023]

The present invention will now be described with reference to Examples (Examples 1, 3, 5, 7,
9, 11) and Comparative Examples (Examples 2, 4, 6, 8, 10, 1)
This will be described in more detail with reference to 2). The average particle size of each catalyst was measured by powder X-ray diffraction.
Analyzed by P emission analysis.

"Example 1" An aqueous solution of chloroplatinic acid containing 10 g of platinum in terms of metal and a 35% by weight aqueous solution of formalin were added to ion-exchanged water, cooled to -10 ° C and stirred. Here 40
A weight% aqueous sodium hydroxide solution was added dropwise and the mixture was stirred at 60 ° C. for 1 hour. After filtration and washing, the mixture was heated at 140 ° C under reduced pressure for 6 hours.
After drying for an hour, platinum particles were obtained. The average particle size of the platinum particles was about 2.5 nm.

1 g of the platinum particles was dispersed in 200 ml of ion-exchanged water, and 0.2 g of ammonium perrhenate was added thereto.
50 g of an aqueous solution containing g were added and stirred. Next, a 30 ml aqueous solution containing 0.4 g of sodium borohydride was added dropwise, followed by stirring for 1 hour. This was filtered and washed, dried under reduced pressure at 140 ° C. for 6 hours, and then subjected to a reduction treatment at 700 ° C. for 3 hours in an electric furnace in an argon atmosphere containing 3% by volume of hydrogen.

The composition of platinum and rhenium in the obtained catalyst was 90 atomic% of platinum and 10 atomic% of rhenium. The average particle size of this catalyst was 4.0 nm.

Example 2 The platinum particles produced in Example 1 were used as a catalyst.

Example 3 Pt / C with a loading of 30% by weight manufactured by E-TEK (Pt / C in the present specification indicates a platinum catalyst supported on a carbon support. The specific surface area of the carbon support is 5 g (approximately 250 m ² / g) was dispersed in ion-exchanged water, and ammonium perrhenate containing 1.5 g as a rhenium amount was added thereto, and the mixture was evaporated to dryness with stirring. This was calcined at 650 ° C. for 2 hours in an electric furnace, and then maintained in an argon atmosphere containing 3% by volume of hydrogen.
Heat treatment was performed at 0 ° C. for 3 hours. The composition of platinum and rhenium in the obtained catalyst was 75 atomic% of platinum and 25 atomic% of rhenium. The average particle size of this catalyst was 4.5 nm.

"Example 4" 30% by weight of loading used in Example 3
Of Pt / C was used for the catalyst as it was.

Example 5 10 g of a carbon black carrier (Cabot Corporation product name: Vulcan XC-72R) was dispersed in ion-exchanged water, 800 ml of an aqueous chloroplatinic acid solution containing 3 g of platinum and 1.5 g of ruthenium. Was added, and 10 ml of a 35% by weight aqueous solution of formalin was added, and the mixture was cooled to −10 ° C. and stirred. This is 4
A 0% by weight aqueous solution of sodium hydroxide was added dropwise and refluxed for 1 hour. This was filtered and washed, dried under reduced pressure at 140 ° C. for 5 hours, and then placed at 650 ° C. in an electric furnace under an argon atmosphere.
For 3 hours to obtain a platinum-ruthenium alloy-supported catalyst. The average particle size of the supported catalyst was about 3.0 nm.

1 g of the supported catalyst was dispersed in 200 ml of ion-exchanged water, and 0.3 g of ammonium perrhenate was added thereto.
50 g of an aqueous solution containing g were added and stirred. Next, a 30 ml aqueous solution containing 0.4 g of sodium borohydride was added dropwise, followed by stirring for 1 hour. This was filtered and washed, dried under reduced pressure at 140 ° C. for 6 hours, and then subjected to a reduction treatment at 680 ° C. for 3 hours in an electric furnace in an argon atmosphere containing 3% by volume of hydrogen.

The composition of platinum, ruthenium and rhenium in the metal of the obtained supported catalyst was 50 atomic% of platinum, 40 atomic% of ruthenium and 10 atomic% of rhenium. The average particle size of the supported catalyst was 4.5 nm.

Example 6 The platinum-ruthenium alloy supported catalyst prepared in Example 5 was used.

Example 7 30 wt% Pt manufactured by E-TEK
/ C in 2 g of ion-exchanged water and dilute NH
_The pH was adjusted to 8 with ₄ OH water. Chromium nitrate containing 0.15 g of chromium was added thereto, stirred for about 2 hours, filtered, and dried at 140 ° C. under reduced pressure for 6 hours. Next, the inside of the electric furnace was kept in an argon atmosphere containing 3% by volume of hydrogen and heat-treated at 700 ° C. for 2 hours to obtain a platinum-chromium alloy-supported catalyst. The average particle size of this supported catalyst was about 3.5 nm.

Next, in the same manner as in Example 1 except that this platinum-chromium alloy-supported catalyst was used instead of the platinum particles,
A supported catalyst in which rhenium was contained in a platinum-chromium alloy was obtained. The composition of platinum, chromium and rhenium in the metal of the obtained supported catalyst was 50 atomic% of platinum, 40 atomic% of chromium, and 10 atomic% of rhenium. The average particle size of this supported catalyst was 4.8 nm.

Example 8 The catalyst supported on the platinum-chromium alloy prepared in Example 7 was used.

Example 9 30 wt% Pt manufactured by E-TEK
/ C was dispersed in ion-exchanged water, and an aqueous solution of stannous chloride (SnCl ₂ ) containing 0.28 g of tin was added with stirring. A 30 ml aqueous solution of 0.5 g of sodium borohydride was added dropwise thereto, and the mixture was stirred for 1 hour. After filtering and washing this, it was dried at 140 ° C. under reduced pressure for 6 hours. Next, the inside of the electric furnace was kept in an argon atmosphere containing 3% by volume of hydrogen and heat-treated at 700 ° C. for 2 hours to obtain a platinum-tin alloy supported catalyst. The average particle size of this supported catalyst was about 3.2 nm.

Next, rhenium was contained in the platinum-tin alloy supported catalyst in the same manner as in Example 1 except that this platinum-tin alloy supported catalyst was used instead of the platinum particles. The composition of platinum, tin and rhenium in the metal of the obtained supported catalyst was 50 atomic% of platinum, 40 atomic% of tin and 10 atomic% of rhenium. The average particle size of the supported catalyst was 5.1.
nm.

"Example 10" The platinum-tin alloy supported catalyst produced in Example 9 was used.

Example 11 30% by weight of P manufactured by E-TEK
2 g of t / C was dispersed in ion-exchanged water, and an aqueous solution of potassium molybdate containing 0.3 g of molybdenum was added with stirring. A 30 ml aqueous solution of 0.5 g of sodium borohydride was added dropwise thereto, and the mixture was stirred for 1 hour. After filtering and washing this, it was dried at 140 ° C. under reduced pressure for 6 hours. Next, the inside of the electric furnace was kept under an argon atmosphere containing 3% by volume of hydrogen and heat-treated at 700 ° C. for 2 hours to obtain a platinum-molybdenum alloy-supported catalyst. The average particle size of the supported catalyst was about 3.7 nm.

Next, a platinum-molybdenum alloy-supported catalyst was prepared in the same manner as in Example 1, except that the platinum-molybdenum alloy-supported catalyst was used instead of the platinum particles, and that 0.6 g of ammonium perrhenate was used. Contained rhenium. The composition of platinum, molybdenum and rhenium in the metal of the obtained supported catalyst was 40 atomic% of platinum, 30 atomic% of molybdenum and 30 atomic% of rhenium. The average particle size of this supported catalyst was 4.8 nm.

Example 12 The platinum-molybdenum alloy-supported catalyst produced in Example 11 was used.

[Evaluation Result 1] The thickness of the ion exchange membrane was 8
0μm of perfluorocarbon sulfonic acid type ion-exchange membrane (Asahi Glass Co. Product name: Flemion membranes) using a surface area per 0.8 mg / cm ² platinum amount apparently effective electrode area is 10 cm ² gas diffusion electrodes (E A gas diffusion electrode having an electrode effective area of 10 cm ² was prepared by using the alloy catalysts obtained in Examples 1 to 12 so that the amount of platinum became 0.5 mg / cm ² per apparent surface area. As an anode, a membrane-electrode assembly was produced by a hot press method.

This assembly was assembled in a measuring cell, and air was used as an oxidizing gas, a) pure hydrogen was used as a fuel gas, b) hydrogen containing 10 ppm of carbon monoxide, c) was used.
A power generation test was performed at 1 atmosphere and a cell temperature of 70 ° C. using three types of hydrogen, hydrogen containing 100 ppm of carbon monoxide. The cell voltage at a current density of 400 mA / cm ² is shown in the middle column of Table 1.

[Evaluation Result 2] Film prepared in Evaluation Result 1
The electrode assembly was assembled into a half-cell for a methanol fuel cell using the electrode assembly, and the electrode potential of the methanol oxidation reaction was measured at 1 atm and 80 ° C. In the right column of Table 1, 0.4 of the methanol electrode is shown.
Specific activity (unit: mg / Pt) at V and current density 50
Shows the methanol electrode potential at mA / cm ² (iR free potential minus ohmic loss, unit: mV) (based on hydrogen electrode).

From these results, it can be seen that the use of a rhenium-containing platinum alloy catalyst for the anode improves the carbon monoxide poisoning resistance, and that the use of the rhenium-containing platinum alloy catalyst of the present invention improves the methanol oxidation activity. It turns out that it improves.

[0047]

[Table 1]

[0048]

According to the present invention, there is provided a catalyst containing platinum and rhenium, wherein the content ratio of rhenium to all metals in the catalyst is 1 to 5.
By using a catalyst that is 80 atomic% for the anode,
In a polymer electrolyte fuel cell, the carbon monoxide resistance at the anode is improved, and in a methanol fuel cell, the methanol oxidation activity at the anode is improved.

Claims (4)

[Claims] An anode catalyst contained in an anode comprising a gas diffusion electrode is a catalyst containing platinum and rhenium, and a content ratio of rhenium to all metals in the catalyst is 1 to 80 atomic%. A fuel cell, characterized by: 2. Platinum is a platinum group element other than platinum, gold, silver,
The fuel cell according to claim 1, which is contained as an alloy with one or more metals selected from the group consisting of chromium, iron, cobalt, nickel, molybdenum, tungsten, zinc, and tin. 3. The anode catalyst has an average particle diameter of 1 to 3.
3. The fuel cell according to claim 1, wherein the thickness is 0 nm. 4. The method according to claim 1, wherein the anode catalyst has a specific surface area of 50 to 20.
4. The fuel cell according to claim 1, wherein the fuel cell is supported on a conductive support of 00 m ² / g.