JPH07246336A - Anode electrode catalyst for fuel cell and method for producing the same - Google Patents

Abstract

(57) [Abstract] [Purpose] The most important issue in the development of fuel cells is poisoning of the catalytic metal by carbon monoxide in the supplied fuel. Solving this issue is the key to the practical application of fuel cells. It is a point. It is an object of the present invention to provide a fuel cell anode electrode catalyst which is hardly affected by carbon monoxide poisoning and a method for producing the same. A method for producing an anode electrocatalyst for a fuel cell, which comprises an alloy of 1 to 60 atomic% of tin, and at least one metal of platinum, palladium and ruthenium, and an electrode catalyst comprising a tin-platinum alloy. Using this catalyst as the anode of a fuel cell, even if it is operated while supplying a fuel containing carbon monoxide of about 100 ppm, there is almost no adverse effect due to poisoning, and it is easy to perform normal methanol reforming and subsequent shift reaction. Since the carbon monoxide content in the produced fuel can be easily adjusted to about 100 ppm, the produced fuel can be used as it is without further purification. Especially catalysts prepared by the above method comprises Pt ₃ Sn alloy phase, the Pt ₃ Sn alloy phase has a strong poisoning resistance, show better electrode active.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anode electrocatalyst for a fuel cell, which is used as an anode of a fuel cell and is composed of an alloy of tin and a noble metal, and a method for producing the same.
In particular, the present invention relates to a fuel cell anode electrode catalyst composed of an alloy of tin and platinum having a Pt ₃ Sn alloy phase and a method for producing the same.

[0002]

2. Description of the Related Art Electrochemical cells, such as polymer electrolyte fuel cells, are attracting attention as a power source for electric vehicles and spacecraft because they are more compact and have higher current densities than phosphoric acid fuel cells. . Also, in the development of this field, various electrode structures, catalyst production methods, system configurations, etc. have been proposed. The conventional fuel cell electrode structure is, for example, a collector for cathode / cathode /
It has a five-layer sandwich structure of polymer solid electrolyte (ion exchange membrane) / anode / collector for anode.

The fuel supplied to this fuel cell is produced, for example, by reforming methanol, and this fuel contains carbon dioxide and carbon monoxide in addition to hydrogen. Conventionally, carbon black supporting a catalyst and polytetrafluoroethylene (hereinafter referred to as PT) are used as the anode of the fuel cell.
A powder mixture of FE) supported on a substrate such as a gas permeable carbon woven fabric is generally used. However, this anode catalyst is often poisoned by carbon monoxide contained in the fuel in a polymer electrolyte fuel cell having a low operating temperature, and the catalytic activity is often greatly reduced.

In order to avoid this drawback, it is desirable to use pure hydrogen as a fuel, but not only is pure hydrogen expensive, but its storage is also costly. For example, liquid hydrogen is stored in a storage tank or in the atmosphere. It is easily scattered and is technically difficult to store. Therefore, if carbon monoxide is removed from the methanol reformed fuel, the problem of poisoning does not occur as in the case of pure hydrogen supplied from the tank, but it is necessary to completely remove carbon monoxide by multi-step treatment. Is difficult and costly, as is the case with pure hydrogen. In the past, avoiding poisoning of fuel cell electrodes by carbon monoxide has been of utmost concern in the field, and the poisoning has been a serious obstacle to the practical application of fuel cells, and various electrode materials Despite the proposal, a fuel cell electrode having sufficient resistance to poisoning has not been developed yet.

[0005]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an alloy catalyst of tin and a noble metal, particularly platinum, which has not been used for the anode of a fuel cell and has excellent poisoning resistance, and a method for producing the same. For that purpose, the reformed gas can be used as a fuel even in a polymer solid oxide fuel cell which is liable to be poisoned by carbon monoxide because of its low operating temperature.

[0006]

The anode electrocatalyst for a fuel cell according to the present invention is for a fuel cell comprising an alloy of 1 to 60 atomic% of tin and at least one noble metal of platinum, palladium and ruthenium. An anode electrocatalyst and an anode electrocatalyst for a fuel cell, comprising an alloy of 8 to 48 atomic% tin and 52 to 98 atomic% platinum and containing a Pt ₃ Sn alloy phase, the latter catalyst being preferably platinum-supported. It is obtained by treating the catalyst with a tin chloride solution and subjecting it to a reducing heat treatment.

The details of the present invention will be described below. Although a catalyst containing tin and a noble metal has been proposed in the past (for example, Japanese Patent Laid-Open No. 3-42040), the catalyst is for treating harmful gases and has a structure in which a noble metal is coated with tin. On the other hand, the catalyst according to the present invention is an anode electrode catalyst for a fuel cell and contains alloyed tin and noble metal. The thus constructed anode catalyst of the present invention has a high resistance to poisoning by carbon monoxide in the noble metal in the catalyst and has a long duration even if the carbon monoxide concentration in the fuel supplied to the fuel cell is relatively high. It enables you to drive with stable activity.

The first fuel cell anode electrode catalyst according to the present invention is an alloy comprising 1 to 60 atomic% of tin and at least one noble metal of platinum, palladium and ruthenium. In addition to conventional alloys, includes amorphous or amorphous alloys, solid solutions and intermetallic compounds. Therefore, the first electrocatalyst according to the present invention is not only a solid solution or a supersaturated solid solution obtained by melting and mixing tin and a noble metal and further rapidly cooling, but also a metal represented by SnMe _x (Me is platinum, palladium or ruthenium). It is also possible to prepare an intercalation compound and use it as it is. It may also be produced by a method using a tin chloride solution described below.

This electrocatalyst uses 1 to 60 atomic% of tin. If the atomic% of tin is less than 1%, there is little effect of preventing poisoning of precious metal catalysts, and if it exceeds 60%, tin is present in the electrolyte when the cation exchange membrane, which is the polymer solid electrolyte of the fuel cell, is an acid type. Elutes easily, and the absolute amount of the noble metal, which is the main catalyst substance, is insufficient, and the catalytic activity is reduced. The noble metal used is selected from platinum, palladium and ruthenium, and each metal is used alone or in combination thereof, and a particularly preferred noble metal is platinum. Components other than tin are usually composed of these noble metals, but other components may be contained in a slight amount.

In addition to the above, the present invention provides an anode electrocatalyst for a fuel cell, which comprises an alloy of 8 to 48 atomic% tin and 52 to 98 atomic% platinum and contains a Pt ₃ Sn alloy phase. The alloy of this composition contains a Pt ₃ Sn alloy phase in the alloy phase, that is, an alloy phase having an atomic ratio of platinum: tin of 3: 1. The Pt ₃ Sn alloy phase has excellent electrode characteristics such as poisoning resistance. is doing. When the tin content exceeds the upper limit of 48 atom%, the Pt ₃ Sn alloy phase disappears and becomes a PtSn phase, and the tin content is the lower limit of 8 atom%.
If it is less than Ptα, a single layer is formed. The particle size is preferably 10 to 60Å, and if it exceeds 60Å, the catalyst surface area will decrease.
If it is less than 10Å, the catalytic activity tends to decrease.

The specific surface area of the carrier carrying the catalyst is 60-
2000 m ² / g is preferable, and even if it exceeds 2000 m ² / g, there is no particular problem, but the particle size becomes finer and the catalytic activity slightly decreases,
If it is less than 60 m ² / g, the particle size becomes too large. Of the two types of tin-noble metal alloys and tin-platinum alloy catalysts of the present invention described above, the former method for producing the catalyst is not particularly limited, and each metal is directly attached to the surface of the carrier by sputtering or the like, or a corresponding metal. A method in which a compound solution is applied to a carrier and an alloy is formed on the surface of the carrier by utilizing a thermal decomposition method can be adopted.

On the other hand, in order to produce the latter catalyst, it is necessary to use not only a method of forming an alloy having a constant composition but also a method of forming a Pt ₃ Sn alloy phase. First, a platinum catalyst in which platinum is supported on a carrier such as carbon black is prepared according to a conventional method. The catalyst is then suspended in a tin chloride solution and the pH is adjusted so that a thin layer of tin hydroxide is coated on the platinum catalyst when the pH is neutral or alkaline, eg above 7, and then the solution is heated. The coated tin hydroxide is converted to tin oxide. A catalyst layer containing a Pt ₃ Sn alloy phase is obtained by separating the catalyst having the tin oxide coating by filtration or the like and performing heat reduction treatment in a reducing atmosphere containing hydrogen. In order to satisfactorily form the Pt ₃ Sn alloy phase, the pH is adjusted to 7 or higher, the solution heating is performed at 80 to 100 ° C, and the heating during hydrogen reduction is performed at 300 to 1000 ° C. If the heating is carried out at a temperature higher than 1000 ° C, the particle size may become large and a liquid phase may occur, and if it is lower than 300 ° C, tin oxide may not be sufficiently reduced.

The reason why carbon monoxide poisoning is suppressed by the combination of a noble metal and tin, that is, the addition of tin to a noble metal catalyst has not been clarified, but the adsorption site of carbon monoxide is occupied by tin and carbon monoxide is occupied. It is a synergistic effect of reducing the amount of adsorption or preventing adsorption itself and of oxidizing carbon monoxide once adsorbed by the function of tin as an oxidation catalyst to remove carbon monoxide. Presumed to be. Although it depends on the amount of tin, when the fuel cell is operated using the electrode catalyst of the present invention, even if the supplied fuel contains about 100 ppm of carbon monoxide, the catalyst activity is reduced to such an extent that the catalytic activity is lowered. Is almost never poisoned,
Stable operation can be continued.

In order to incorporate the above catalyst into a fuel cell as an anode, a carrier such as carbon black is loaded with a first metal by a thermal decomposition method or the like, and then a second metal is further loaded to form an alloy. Alternatively, after supporting each metal on the carrier by sputtering, or after forming a noble metal, particularly an alloy of platinum and tin on the carrier using a tin chloride solution as described above, it is formed into a thin film with an ion exchange resin, PTFE, or the like. Formed on a substrate or an electrolyte membrane as a porous body,
This is fixed to a predetermined position of the fuel cell, and this substrate or the electrolyte membrane with alloy catalyst is installed at a predetermined position of the fuel cell together with a separator having a gas supply groove. The cathode, which is the counter electrode of the fuel cell, is not particularly limited, and a conventional electrode such as one prepared by mixing carbon black supporting a catalyst and PTFE powder, supporting the mixture on a substrate, and firing the mixture may be used. Accordingly, the electrolyte solution may be impregnated and used.

In the fuel cell thus manufactured, since the anode which is easily poisoned by carbon monoxide is composed of the electrocatalyst of the present invention, the fuel is 100 ppm as described above.
Even if it contains a small amount of carbon monoxide, it has almost no effect on operation, and the maximum amount of carbon monoxide contained in the fuel produced by methanol reforming is about 100 ppm. It is possible to use the produced fuel as it is.

[0016]

[Examples] Next, examples of the fuel cell anode electrode catalyst according to the present invention will be described, but the examples do not limit the present invention.

Example 1 Platinum and tin targets were simultaneously subjected to argon sputtering on a glass plate with a lead terminal having a diameter of 10 mm in a chamber under reduced pressure to form a platinum-tin alloy thin film having a thickness of 0.5 μm. This was fixed to one end surface of a stainless steel rod having a diameter of 8 mm and attached to a rotary electrode device.
Regarding the alloy composition, one of the samples prepared at the same time was used for IC
It was determined by quantitative analysis by the P method.

Three rotating electrodes of platinum: tin = 49: 51 (atomic%) were separately immersed in a 0.1 M aqueous solution of perchloric acid to obtain pure hydrogen, hydrogen containing 40 ppm of carbon monoxide, and carbon monoxide. Hydrogen containing 100 ppm was separately bubbled for 1 hour for poisoning, and then the voltage-current characteristic of each rotary electrode was measured while continuing bubbling and rotating at 1500 rpm with the platinum electrode as the counter electrode. The result is shown in FIG. As shown by (1) in FIG. 1, in the alloy composition of this example, the voltage-current characteristic curves match regardless of the carbon monoxide concentration, and it is understood that the effect of carbon monoxide does not occur.

[0018]

Example 2 A rotary electrode was manufactured in the same manner as in Example 1 except that the atomic ratio of platinum: tin was set to 79:21. Two of these rotating electrodes were separately immersed in a 0.1 M aqueous solution of perchloric acid, and pure hydrogen and hydrogen containing 100 ppm of carbon monoxide were bubbled separately for 1 hour to poison and then continue bubbling. The voltage-current characteristics of each rotating electrode were measured. As shown in FIG. 1, it can be seen that the voltage-current characteristic curves of pure hydrogen (2a) and carbon monoxide-containing hydrogen (2b) are almost the same, and there is almost no effect of carbon monoxide poisoning.

[0019]

Comparative Example 1 A rotary electrode was manufactured in the same manner as in Example 1 except that plain platinum was used instead of the platinum-tin alloy. Two of these rotating electrodes were separately immersed in a 0.1 M aqueous solution of perchloric acid, and pure hydrogen and hydrogen containing 100 ppm of carbon monoxide were bubbled separately for 1 hour, respectively, and then exposed, while continuing to bubble. The voltage-current characteristics of each rotating electrode were measured. As shown in FIG. 1, when pure hydrogen (3a) is used, carbon monoxide poisoning does not occur, so that the reaction current is lower than that of the platinum-tin alloy electrode of this comparative example as long as the activity of the tin addition does not decrease. It is getting bigger. However, when carbon monoxide-containing hydrogen (3b) was used as a fuel, it was found that carbon monoxide poisoning was large and current could not be extracted.

[0020]

Example 3 A palladium-tin alloy was prepared in the same manner as in Example 1 except that platinum was replaced with palladium, and a rotating electrode was prepared using the alloy, and carbon monoxide was added to 100%.
Voltage of the rotating electrode using hydrogen containing ppm as fuel
The current characteristics were measured. Although the characteristics were slightly inferior to those of the rotating electrode of Example 1, a stable current could be taken out for a long period of time.

[0021]

EXAMPLE 4 A ruthenium-tin alloy was produced in the same manner as in Example 1 except that palladium was replaced with ruthenium, and a rotating electrode was produced using the alloy, and hydrogen containing 100 ppm of carbon monoxide was used. Was used as a fuel, and the voltage-current characteristics of the rotating electrode were measured. The properties were substantially the same as the rotating electrode of Example 3.

[0022]

Example 5 Platinum and tin were applied to the surface of an anode substrate having a diameter of 20 mm in a chamber under reduced pressure under the same conditions as in Example 1.
Sputtering was performed at a rate of 0.18 mg / cm ^{2 to} prepare a platinum-tin alloy fuel cell anode. The above anode substrate is a water-repellent treated layer that functions as a current collector.
A mixture (2: 1) of commercially available carbon black (specific surface area 200 m ² / g) and commercially available PTFE powder was hot-press molded at 360 ° C. and 5 kg / cm ² on the surface of m carbon paper.

On the other hand, 10 g of carbon powder was impregnated with a chloroplatinic acid aqueous solution (platinum concentration: 150 g / liter) and then subjected to a thermal decomposition treatment to prepare a platinum carbon catalyst having a platinum loading of 30% by weight. The carbon catalyst was immersed in a commercially available ion exchange resin dispersion solution (Nafion solution) and then dried to form an ion exchange resin layer on the surface. The catalyst powder was fractionated so that the amount of supported platinum was 0.3 mg / cm ^{2 on} average, and redispersed in alcohol. Next, this dispersion was filtered under a weak suction to deposit the catalyst powder on a filter paper having a diameter of 50 mm so that alcohol remained slightly, and then the filter paper functioning as a current collector was treated to have a water-repellent diameter of 20 mm. ,thickness
130 ° C, 5 kg / c with 360 μm carbon paper
A cathode with a current collector was prepared by hot pressing at a pressure of m ² .

These electrodes are adhered to both sides of Nafion (made by DuPont) which is a perfluorocarbon type ion exchange membrane having a diameter of 30 mm and a thickness of 150 μm and having bolt holes in the vicinity of four corners, and is further vertically attached to the outer surfaces of both electrodes. 5cm, side 5c
m, thickness 10 mm, a large number of concave surfaces on the inner surface, and a pair of brass fastening plates with bolt holes near the four corners are located, and bolts are inserted into the bolt holes at the four corners of both fastening plates, The fuel cell was assembled by tightening the nut at the tip. Using these three fuel cells, the operation was continued while separately supplying three types of fuel in the same manner as in Example 1. However, all the fuel cells had almost the same initial activity and the performance deteriorated even after 3 days. It can be seen that there is no poisoning due to carbon monoxide.

[0025]

[Example 6] Platinum is 30% by weight relative to the carbon carrier.
A total of four supported catalysts were prepared. The ratio of tin to the platinum amount of each catalyst is 10 atom%, 20 atom%, 30 atom% and
The total four catalysts were separately dispersed in an aqueous solution of stannous chloride adjusted to a concentration of 50% using an ultrasonic homogenizer, and then the pH of the aqueous solution was adjusted to 8.2 with ammonia. I adjusted it to be.

Next, the aqueous solution was heated to 96 ° C. with stirring to deposit tin oxide on the catalyst. The catalyst was filtered off, separated and dried, then in a stream of nitrogen containing 20% hydrogen.
Heat treatment was performed at 900 ° C. for 30 minutes. When these four catalysts were identified by XRD (X-ray diffraction), the alloy phases shown in Table 1 were confirmed. Further, the particle size was measured by TEM, the specific surface area was measured, and further the metal (platinum) surface area was measured from the carbon monoxide adsorption amount. The respective values are summarized in Table 1.

[0027]

[Table 1]

The above four kinds of catalysts were separately immersed in a commercially available solid polymer (composition: perfluorocarbon sulfonic acid) dispersion solution, and then dried to deposit about 30% by weight of the solid polymer on the surface. Then, the catalyst was transferred onto a water repellent carbon paper by a filtration transfer method,
Hot pressing was performed at 0 ° C. to obtain fuel cell anode electrodes each having a composition as described above and having a thickness of 20 μm.

[0029]

[Comparative Example 2] Carbon powder (10 g) was impregnated with a chloroplatinic acid aqueous solution (platinum concentration: 5 g / liter), and then reduction treatment was carried out using hydrazine as a reducing agent to obtain platinum loadings of 20 each.
Two types of platinum carbon catalysts, wt% and 40 wt%, were prepared. Both carbon catalysts are commercially available solid polymers (composition:
Perfluorocarbon sulfonic acid) dispersion solution, then dried to deposit about 30% by weight of the above solid polymer on the surface, and then transfer the catalyst onto the water repellent carbon paper by the filter transfer method. Then, hot pressing was performed at 130 ° C. to obtain fuel cell anode electrodes each having a composition as described above and having a thickness of 20 μm.

Four types of electrodes of Example 6 and 2 of Comparative Example 2
The activity per unit gram of platinum of the electrodes of various types was measured,
The results are shown in Fig. 2. In FIG. 2, the vertical axis represents the activity per unit surface area of the catalyst (calculated from the specific surface area obtained by TEM), and the horizontal axis represents the atomic% of tin. In the figure, □ is the activity at a cell voltage of 0.7 V, and + is the activity at a cell voltage of 0.5 V. As can be seen from FIG. 2, the Pt ₃ Sn alloy phase formed in the alloy had the highest tin content of 20 atomic%. Further, although the values of the specific surface areas obtained by TEM in Table 1 are almost the same, the adsorbed amount of carbon monoxide decreased as the added amount of tin increased, and the poisoning by carbon monoxide was reduced. I find it difficult to receive. Similarly, using the electrode having a tin content of 20 atomic% in Example 6 and the electrode not containing tin in Comparative Example 2, the relationship between the current density and the cell voltage was measured. The results are shown in Fig. 3. As can be seen from FIG. 3, there is almost no difference in the cell voltage between the two electrodes while the current density is low, but the electrode of Example 6 had a considerably higher cell voltage at high current efficiency.

[0031]

The present invention is a fuel cell anode electrocatalyst comprising an alloy of 1 to 60 atomic% of tin and at least one metal of platinum, palladium and ruthenium (claim 1). The anode electrocatalyst of the present invention, which is an alloy of tin and a noble metal, has a greatly reduced carbon monoxide poisoning amount as compared with the conventional fuel cell catalyst, and continues to operate at a relatively high activity for a long period of time. It becomes possible to do. Further, even if the content of carbon monoxide in the fuel supplied to the fuel cell is relatively large, there is almost no effect on the activity, so refining of the supplied fuel is unnecessary, and the labor and cost required for refining can be reduced. You can When the noble metal used is platinum (claim 2), the catalytic activity is high and the activity is maintained for a long time, which is particularly preferable.

Particularly in the case of a platinum-tin alloy, when the composition of the alloy is 8 to 48 atom% of tin and 52 to 98 atom% of platinum and an electrode is manufactured by a predetermined method, a Pt ₃ Sn alloy phase is formed in the alloy. (Claim 3). This Pt ₃ Sn alloy phase has a particularly high poisoning resistance to carbon monoxide. The grain size of this alloy is preferably 10 to 60Å (claim 4), and when it exceeds 60Å, the catalyst surface area decreases, and when it is less than 10Å, the catalytic activity tends to decrease.

[0033] When carrying the alloy on a carrier, it is desirable specific surface area of the carrier is 60~2000m ² / g (claim 5), 2000m ² / g exceeds the catalytic activity particle size becomes finer Is slightly decreased, and if it is less than 60 m ² / g, the particle size tends to be too large.

Further, in order to manufacture an electrode containing the above-mentioned Pt ₃ Sn alloy phase, a platinum-supported catalyst is suspended in a tin chloride solution and then added with p.
Pt ₃ Sn alloy was prepared by adjusting H to coat tin hydroxide on the catalyst, heating the solution to convert the tin hydroxide to tin oxide, and heat treating the catalyst in a reducing atmosphere containing hydrogen. Forming a phase (claim 6). According to this method, the Pt ₃ Sn alloy phase is surely generated in the manufactured electrode, and the electrode having the above-mentioned high electrode characteristics can be obtained. In order to obtain better electrode performance, the pH is adjusted to 7 or higher in the above-mentioned manufacturing method, the solution heating is performed at 80-100 ° C, and the heat treatment is performed at 300-1000.
It is desirable to carry out at a temperature of ℃ (Claim 7).

[Brief description of drawings]

FIG. 1 is a graph showing voltage-current characteristics of rotating electrodes in Examples 1 and 2 and Comparative Example 1.

FIG. 2 is a graph showing the relationship between the activity per unit surface area of the catalyst of the electrodes of Example 6 and Comparative Example 2 and the weight% of tin.

FIG. 3 is a graph showing the relationship between the cell current and the current density of the electrodes of Example 6 and Comparative Example 2.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (71) Applicant 391016716 STONHART Associates Incorporated STONEHART ASSOCIATES INCORPORATED United States 06443 Connecticut, Madison, Cottage Road 17, P-O Box 1220 (72) Inventor Masanori Watanabe 872, 2412 Wada-cho, Kofu-shi, Japan (72) Inventor Yumi Yamamoto 2-73 Shinmachi, Hiratsuka-shi, Kanagawa Tanaka Kikinzoku Kogyo Co., Ltd. Technology Development Center

Claims (7)

[Claims] 1. A fuel cell anode electrocatalyst comprising an alloy of 1 to 60 atomic% tin and at least one metal of platinum, palladium and ruthenium. 2. A fuel cell anode electrocatalyst comprising an alloy of 1 to 60 atomic% tin and 99 to 40 atomic% platinum. 3. A fuel cell anode electrocatalyst comprising an alloy of 8 to 48 atomic% tin and 52 to 98 atomic% platinum and containing a Pt ₃ Sn alloy phase. 4. The electrode catalyst according to claim 3, which has a particle size of 10 to 60Å. 5. The electrode catalyst according to claim 3, wherein a catalyst containing a Pt ₃ Sn alloy phase is supported on a carrier, and the carrier has a specific surface area of 60 to 2000 m ² / g. 6. A platinum-supported catalyst is suspended in a tin chloride solution, the pH is adjusted to coat the catalyst with tin hydroxide, the solution is heated to convert the tin hydroxide to tin oxide, and A method for producing an anode electrocatalyst for a fuel cell, characterized in that the catalyst is heat-treated in a reducing atmosphere containing hydrogen to form a Pt ₃ Sn alloy phase. 7. The pH is adjusted to 7 or more, and the solution heating is 80 to
The method according to claim 6, wherein the heat treatment is performed at 100 ° C and the heat treatment is performed at 300 to 1000 ° C.