JPH09266005A - Solid high polymer fuel cell system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To miniaturize a system and to operate the system economically and stably over a long time by lowering the carbon monoxide concentration of reformed fuel gas supplied to a cell fuel electrode to a practical level. SOLUTION: This system comprises a fuel reforming system which reforms hydrogen carbide or alcohol fuel and a fuel cell main body 1 in which a platinum alloy is used as the electrode catalyst of a cell fuel electrode 2 into which fuel gas reformed by the fuel reforming system is introduced, platinum as the catalyst of a cell oxidizer electrode 3 into which an oxidizer is introduced, and a solid high polymer film as the electrolyte. In this case, the fuel reforming system is provided with a catalyst layer 11 for selectively oxidizing carbon monoxide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polymer electrolyte fuel cell system, and more particularly, to improve a reformed gas supplied to a fuel electrode using a platinum alloy as an electrode catalyst so as to be suitable for a hydrocarbon or alcohol fuel. The present invention relates to a solid polymer fuel cell system.

[0002]

2. Description of the Related Art Conventionally, a fuel cell is known as a system for directly converting chemical energy contained in fuel into electric energy. For this fuel cell, various systems are usually tried depending on the type of electrolyte.

By the way, in a solid polymer fuel cell, a solid polymer such as Nafion is used as an electrolyte, and a pair of porous electrodes composed of a fuel electrode and an oxidizer electrode are opposed to each other with the electrolyte layer sandwiched therebetween to form a fuel cell. However, by bringing an oxidizer such as air into contact with the back surface of the fuel electrode, the electrochemical energy generated at this time is used to convert chemical energy into electric energy.

Since the electrolyte of this solid polymer fuel cell is composed of a solid polymer, the structure is simple and a high current density can be achieved. Therefore, because the entire system can be made compact, it can be expected as an in-vehicle power supply,
Research for practical use is being actively conducted.

Noble metals such as platinum are used as electrode catalysts for both the fuel electrode and the oxidizer electrode, and they are devised to increase the rate of the above-mentioned electrochemical reaction. In this case, the operating temperature is usually about 100 ° C. or less, and a high current density is realized under a relatively mild condition.

[0006] In such a polymer electrolyte fuel cell, the electrode catalyst of the fuel electrode is poisoned by a very small amount (ppm order) of carbon monoxide in the reformed fuel gas, and the performance immediately deteriorates. Can not stand.

[0007] Therefore, it is known that the electrode catalyst is improved, and if, for example, a platinum ruthenium alloy is used, the CO poisoning resistance is somewhat improved, but it is still several hundred ppm.
When carbon monoxide is present to some extent, the performance is deteriorated and it is difficult to put it into practical use.

Therefore, under the present circumstances, it has been practically used only for a special purpose using pure hydrogen as a fuel.
On the other hand, as a versatile fuel cell, it is necessary that a hydrocarbon fuel such as natural gas or city gas, or an alcohol fuel such as methanol can be applied to the fuel, and this kind of raw fuel is hydrogenated by steam reforming. Is converted into reformed fuel gas containing as a main component and supplied to the fuel cell.

[0009]

The steam reforming technology that has been widely used in the past will be described below by taking the case of using methanol as a raw fuel as an example. In the presence of a catalyst such as Cu-ZnO or the like, methanol and water vapor are reacted at 300 to 350 ° C to convert it into a reformed gas containing hydrogen as a main component. Further, a Cu-based catalyst or the like is used at 200 to 250 ° C. It reacts to reduce carbon monoxide. If this is expressed by a reaction formula, it becomes like (1) and (2).

[0010]

[Equation 1]

The reaction of the above equation (2) is an exothermic reaction, and the equilibrium shifts to the right side when the temperature is low, and the carbon monoxide concentration can be lowered, but the reaction becomes slow when the temperature is low, and at present, 200 The limit is about ℃. As a result, a carbon monoxide concentration of about 0.1% to 0.5% can be achieved, but even the above-mentioned several hundreds of ppm cannot be achieved.

Therefore, a special refining step is required to reduce carbon monoxide to an extremely low concentration. In the chemical industry that requires a large amount of high-purity hydrogen, carbon monoxide is removed to the ppm order by PSA (pressure swing adsorption method) or the like, but it is economically useful only in a large scale. Further, when economic efficiency is not important, for example, in a laboratory, it can be removed by membrane separation or the like.

On the other hand, in the case of a solid polymer fuel cell, the scale is about several hundred kW, and if 200 kW is taken as an example, the required hydrogen is about 200 Nm ³ / H, and there is no purification method with good cost performance. It is difficult to realize a polymer electrolyte fuel cell system using hydrocarbons, alcohols, etc. as fuel.

The present invention has been made to solve the above-mentioned problems, and by reducing the carbon monoxide concentration of the reformed fuel gas supplied to the cell fuel electrode to a practical level, it is compact, economical and for a long time. An object is to provide a polymer electrolyte fuel cell system that can be stably operated.

[0015]

In order to achieve the above object, the present invention constitutes a solid polymer fuel cell system by the following means. The invention corresponding to claim 1 is a fuel reforming system for reforming a hydrocarbon or alcohol fuel, and platinum as an electrode catalyst of a cell fuel electrode into which reformed fuel gas reformed by this fuel reforming system is introduced. In a solid polymer fuel cell system comprising an alloy, platinum as a catalyst of a battery oxidizer electrode into which an oxidant is introduced, and a fuel cell main body using a solid polymer membrane as an electrolyte, monoxide is added to the fuel reforming system. A catalyst layer for selectively oxidizing carbon is provided.

The invention according to claim 2 is such that the catalyst layer for selectively oxidizing carbon monoxide according to the invention according to claim 1 comprises a platinum-supported catalyst. The invention corresponding to claim 3 is
The catalyst layer for selectively oxidizing carbon monoxide of the invention according to claim 1 is composed of a ruthenium-supported catalyst.

The invention according to claim 4 comprises a catalyst layer for selectively oxidizing carbon monoxide according to the invention according to claim 1, which is composed of a mixture of a platinum-supported catalyst and a ruthenium-supported catalyst. In the invention corresponding to claim 5, the catalyst layer for selectively oxidizing carbon monoxide of the invention according to claim 1 is formed of platinum-supported catalyst, ruthenium-supported catalyst, or a mixture of platinum-supported catalyst and ruthenium-supported catalyst. And a copper-copper oxide catalyst.

According to the invention of claim 6, the temperature of the catalyst layer for selectively oxidizing carbon monoxide of the invention of claim 1 is 100 ° C. to 250 ° C. Invention corresponding to claim 7 supplies air or oxygen for the oxidation of carbon monoxide invention corresponding to claim 1 in the range of O ₂ /CO=0.5~3.0.

The invention according to claim 8 uses the exhaust gas of the battery oxidizer electrode as the air or oxygen source supplied to the catalyst layer of the invention according to claim 1. Therefore, in the polymer electrolyte fuel cell system of the invention according to any one of claims 1 to 8, since the CO concentration at the cell inlet can be 50 ppm or less, the performance and durability of the fuel cell body are significantly improved. In addition, the CO selective oxidation method using a catalyst can make the system compact and make effective use of energy as compared with other methods.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a system diagram showing a configuration example of a polymer electrolyte fuel cell system using methanol as a fuel.

In FIG. 1, reference numeral 1 is a battery fuel electrode 2 using an electrolyte as a solid polymer and a platinum alloy as a fuel electrode catalyst, and a battery oxidizer electrode 3 using platinum as an oxidizer electrode catalyst.
In the fuel cell body having the above, a water cooling unit 4 for cooling the heat generating portion is formed in the fuel cell body 1, and cooling water is supplied to the water cooling unit 4 through a water treatment device 5. Reference numeral 6 is a steam generator to which water for cooling the heat generating portion of the fuel cell body 1 by the water cooling portion 4 is supplied as makeup water.

Further, 7 is supplied with methanol fuel M,
A heating evaporator that heats and vaporizes this methanol fuel M,
Reference numeral 8 is a reformer that is supplied by mixing the vaporized methanol gas in the heating evaporator 7 with the steam S generated in the steam generator 6 at an appropriate ratio. The reformer 8 mixes methanol gas and steam. The gas is reacted in the reforming catalyst layer to be converted into the reformed gas G containing hydrogen as a main component.

Further, 9 is supplied with reformed gas G from the reformer 8 and carbon monoxide (CO) contained in the reformed gas G is supplied.
A carbon monoxide (CO) shifter for reducing the gas, 10 is a heat exchanger for cooling the reformed gas G in which the CO gas has been lowered by the CO shifter, and 11 is a reformer cooled by the heat exchanger 10. In the carbon monoxide (CO) selective oxidation catalyst layer, which is supplied by being mixed with a part of the oxidant electrode exhaust gas containing oxygen discharged from the cell oxidant electrode 3 of the fuel cell body 1 in the gas G, this CO selection is performed. The oxidation catalyst layer 11 selects CO to reduce its concentration and introduces it into the cell fuel electrode 2 of the fuel cell body 1.

On the other hand, 12 is an air compressor for compressing the air A and supplying it to the cell oxidant electrode 3 of the fuel cell body 1. OG is an oxidant electrode exhaust gas discharged from the cell oxidant electrode 3 of the fuel cell body 1, and FG is a fuel cell body 1
Is the fuel electrode exhaust gas discharged from the cell fuel electrode 2.

Next, the operation of the polymer electrolyte fuel cell system using the above-mentioned methanol as a fuel will be described. When the methanol fuel M flows into the heating evaporator 7, the methanol fuel M is heated and vaporized, mixed with the steam S from the steam generator 6 at an appropriate ratio, and then flows into the reformer 8 at about 300 ° C. . In the reformer 8, this mixed gas is reacted in the reforming catalyst layer to be converted into reformed gas G containing hydrogen as a main component.

Next, when the reformed gas G flows into the CO shift converter 9 at a temperature of about 200 to 250 ° C., the reformed gas G reduces carbon monoxide to about 0.5% and then the heat exchanger 10 Is cooled to a temperature of about 70 to 170 ° C. and mixed with a part of the oxidant electrode exhaust gas OG containing oxygen discharged from the cell oxidant electrode 3 of the fuel cell body 1.

In this case, the reformed gas G and the oxidant electrode exhaust gas O
The mixing ratio with G is in the range of O ₂ / CO (molar ratio) = 0.5 to 3, and an appropriate value may be selected, and it depends on the catalyst performance and the apparatus performance.

When the reformed gas G mixed with the oxidant electrode exhaust gas OG flows into the CO selective oxidation catalyst layer 11, CO is selectively oxidized and the CO concentration is reduced to several tens ppm or less, and then the fuel cell body 1 Is led to the cell fuel electrode 2.

On the other hand, the air A, which is an oxidant, is supplied to the air compressor 1.
The cell oxidant electrode 13 of the fuel cell body 1 compressed by 2
After being used for power generation with the cell fuel electrode 2 described above, the exhaust gases FG and OG of the respective electrodes are used as heat sources of the heating evaporator 7.

Further, since heat is generated in the fuel cell main body 1 during power generation, cooling water is supplied to the water cooling section 4 through the water treatment device 5, and after cooling the fuel cell main body 5, the steam generator 6 is cooled. Supplied as make-up water.

In such a solid polymer fuel cell system, the lower the CO concentration of the supplied fuel is, the more economically and durable it is, but the CO poisoning resistance is required in order to achieve ppm order. An excellent platinum alloy, for example, a Pt-Pu alloy is used as an electrode catalyst.

In addition, the battery oxidizer electrode 3 normally has CO in the air.
Since it contains only ppm or less, Pt is used as an electrode catalyst as usual. In this case, although depending on the operating temperature, a certain amount of CO concentration is allowed, and according to the knowledge of the present inventors, if the operating temperature is about 80 to 100 ° C, it is about several tens of ppm. CO will not be a practical problem. Further, this operating temperature is a temperature that can be reasonably realized by an ordinary polymer electrolyte fuel cell, and there is no problem in terms of materials.

Various methods are conceivable for reducing CO, but here, as described above, the CO concentration is several tens.
Considering that it can be sufficiently satisfied if it is in the order of ppm, a method of selectively oxidizing CO with a catalyst that can be the cheapest and compact apparatus is used. In this case, as the oxygen source to be oxidized, the oxygen utilization rate of the battery cathode exhaust gas is 40 to
The efficiency is better as it is closer to 0.5 at about 60%, which is preferable but depends on the performance of the CO selective oxidation catalyst layer 11 and the performance of the catalytic reactor. Of course, the larger this value is, the more C
Although the O concentration can be made as low as possible, the efficiency of the entire system deteriorates, which is not preferable.

Here, the biggest problem is the performance of the CO selective oxidation catalyst layer 11, and it is no exaggeration to say that the effect of the system of the present embodiment is halved depending on this performance. In the present invention, a platinum catalyst supported on a high surface area carrier such as γ-alumina is suitable for the CO selective oxidation catalyst layer 11. The same applies to a supported ruthenium-supported catalyst, and platinum and a ruthenium-supported catalyst may be mixed and used. These catalysts have a power to selectively oxidize only CO by suppressing the oxidation of hydrogen that is present in a large amount in the reformed gas.

Further, a copper-copper oxide catalyst is added to these catalysts.
Addition of about 20% improves the response to fluctuations in the CO concentration at the catalyst layer inlet, and stabilizes the CO concentration at the outlet of the catalyst layer. In the actual system, the CO transformer 9 in the previous stage is used to
The CO concentration is reduced to about 0.5% and enters the CO selective oxidation catalyst layer 11, but it is unavoidable that the inlet CO concentration fluctuates due to fluctuations in the flow rate, temperature and the like.

Next, the important factor is the temperature of the CO selective catalyst tank 11. The temperature of the CO selective catalyst tank 11 is 100
It is good to keep at ~ 250 ° C. The lower limit temperature is C of the catalyst
Although it depends on the O-oxidizing ability, the effect of coexisting water vapor is significant, and at 100 ° C. or less, it condenses on the catalyst and significantly reduces the catalyst performance. When the upper limit temperature becomes higher, the selectivity of CO oxidation is lost, and a side reaction due to a large amount of hydrogen, carbon dioxide gas, etc. present in the reformed gas also occurs, and conversely C
The outlet concentration of O becomes high.

In order to keep the temperature of the catalyst layer within the above temperature range, it is important to control the inlet temperature of the catalyst layer. Considering the heat generation of the CO oxidation reaction and the like, it is preferable to control the temperature within the range of 70 to 170 ° C. .

Here, a specific example of the CO selective catalyst used in the system of the present embodiment will be described. Example 1 (Catalyst A) Commercially available γ-alumina granules (3φ spheres, porosity of about 40)
%) With platinum impregnated on the surface as much as 0.5% (W
t) After being supported, it was adjusted by firing in a reducing atmosphere at 400 ° C.

Example 2 (Catalyst B) 1% (Wt) of ruthenium was supported on the surface of a similar γ-alumina granule by the impregnation method as much as possible, and then calcined in a reducing atmosphere at 350 ° C.

Example 3 (Catalyst C) A similar γ-alumina granule was impregnated with 30% copper (W).
t) After being supported, it was adjusted by firing in a reducing atmosphere at 300 ° C.

Various combinations of catalyst layers were formed with the catalysts A, B, and C, and the reformed gas was simulated (CO 0.3%, CO
_A CO selective oxidation test was carried out by a usual flow method using 20% 20%, H ₂ O 10%, H ₂ balance) gas as a test gas. In this case, the space velocity was set to 10000 H ⁻¹ based on the actual gas flow rate (STP) and the precious metal catalyst volume. The results are shown in Table 1.

[0042]

[Table 1] In the above embodiment, the system for reforming the alcohol fuel is described, but the reforming of the hydrocarbon fuel can be performed in the same manner as described above.

[0043]

As described above, according to the polymer electrolyte fuel cell system of the present invention, the concentration of carbon monoxide at the cell inlet can be reduced to 50 ppm or less, so that the performance and durability of the fuel cell body are significantly improved. Further, the catalytic carbon monoxide selective oxidation method makes the system compact, and is more advantageous in energy efficiency than other methods, and can greatly contribute to the practical application of the polymer electrolyte fuel cell system.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a polymer electrolyte fuel cell system according to the present invention.

[Explanation of symbols]

 1 ... Fuel cell main body 2 ... Cell fuel electrode 3 ... Cell oxidizer electrode 4 ... Water cooling unit 5 ... Water treatment device 6 ... Steam generator 7 ... Heating evaporator 8 ... Reformer 9 ...... CO converter 10 ...... Heat exchanger 11 ...... CO selective oxidation catalyst layer

Claims (8)

[Claims] 1. A fuel reforming system for reforming hydrocarbon or alcohol fuel, and a platinum alloy and an oxidizer as an electrode catalyst of a cell fuel electrode into which reformed fuel gas reformed by this fuel reforming system is introduced. In a solid polymer fuel cell system composed of platinum as a catalyst of a cell oxidizer electrode into which is introduced and a fuel cell main body using a solid polymer membrane as an electrolyte, carbon monoxide is selectively used in the fuel reforming system. A polymer electrolyte fuel cell system, characterized in that a catalyst layer for oxidizing the polymer is provided. 2. The catalyst layer for selectively oxidizing carbon monoxide is composed of a platinum-supported catalyst.
The polymer electrolyte fuel cell system described. 3. The polymer electrolyte fuel cell system according to claim 1, wherein the catalyst layer that selectively oxidizes carbon monoxide comprises a ruthenium-supported catalyst. 4. The solid polymer fuel cell system according to claim 1, wherein the catalyst layer for selectively oxidizing carbon monoxide is composed of a mixture of a platinum-supported catalyst and a ruthenium-supported catalyst. 5. A catalyst layer for selectively oxidizing carbon monoxide comprises a platinum-supported catalyst, a ruthenium-supported catalyst, some of a mixture of a platinum-supported catalyst and a ruthenium-supported catalyst, and a copper-copper oxide catalyst. The polymer electrolyte fuel cell system according to claim 1, wherein: 6. The polymer electrolyte fuel cell system according to claim 1, wherein the temperature of the catalyst layer for selectively oxidizing carbon monoxide is 100 ° C. to 250 ° C. 7. The solid polymer fuel cell system according to claim 1, wherein air or oxygen for oxidizing carbon monoxide is supplied in the range of O ₂ /CO=0.5 to 3.0. . 8. The exhaust gas from the oxygen electrode of the battery is used as the air or oxygen source supplied to the catalyst layer.
The polymer electrolyte fuel cell system described.