JP2001143714A - Solid polymer fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a solid polymer fuel cell having a light weight and a small size, which includes an anode having a catalyst layer excellent in carbon monoxide poisoning resistance, and can effectively generate electricity even when using reformed gas from methanol, etc. SOLUTION: Discolsed herein is a solid polymer fuel cell, which includes an anode having a catalyst layer of a thickness of 1 to 15 μm comprising platinum, or a platinum-ruthenium alloy containing 60% by weight of platinum, or a platinum-molybdenum alloy containing 60% by weight of platinum. Also, the fuel cell includes an electrolytic membrane having a thickness of 5 to 80 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a polymer electrolyte fuel cell, and more particularly to a polymer electrolyte fuel cell having an anode catalyst layer having excellent resistance to poisoning with carbon monoxide.

[0002]

2. Description of the Related Art A fuel cell is a cell that directly converts the reaction energy of a gas serving as a fuel into electric energy. Among them, a hydrogen / oxygen fuel cell is basically composed of only water as a reaction product. There is a feature that there is. Among the hydrogen / oxygen fuel cells, polymer electrolyte fuel cells that use an ion exchange membrane as the electrolyte membrane can operate from room temperature and have a high power density. With increasing social demands, they are expected to be used as power supplies for electric vehicles, stationary power supplies, and the like.

In a polymer electrolyte fuel cell, a proton conductive ion exchange membrane is usually used as an electrolyte membrane.
In particular, an ion exchange membrane made of a perfluorocarbon polymer having a sulfonic acid group has excellent basic characteristics. In the polymer electrolyte fuel cell, gas diffusion electrode layers are formed on both surfaces of the ion exchange membrane, and hydrogen as a fuel and oxygen or air as an oxidant are respectively supplied to an anode (hydrogen electrode) and a cathode (oxygen electrode or Power is generated by supplying it to the cathode.

The electrode catalyst contained in the gas-diffusible electrode layer has a high output. Therefore, a noble metal catalyst mainly composed of platinum or a platinum alloy catalyst is usually supported on a conductive carbon black carrier having a large specific surface area. Used with good dispersibility. Since the reaction in the electrode layer proceeds only at the three-phase interface where the electrolyte, the electrode catalyst, and the fuel gas are simultaneously present, in a polymer electrolyte fuel cell, an ion exchange resin is contained in the electrode layer, and the electrode catalyst is formed by the ion exchange resin. Performance is improved by a method of enlarging the three-phase interface by coating.

[0005]

The use of hydrogen gas (reformed gas) obtained by steam reforming methane, methanol, etc., as the fuel gas sent to the anode has been studied. For example, when using methanol, 25
Using a catalyst such as a Cu-Zn system at a temperature of 0 to 300 ° C, methanol is reacted stepwise as follows. _{CH 3 OH = 2H 2 + CO} -90kJ / mol, CO + H 2 O = H 2 + CO 2 + 40kJ / mol.

That is, methanol is reacted with steam in a reformer to convert hydrogen and carbon monoxide (CO), and then CO is further shifted with steam in a shift converter, so that hydrogen gas becomes a main component. Obtain reformed gas. Then, the reformed gas is supplied to the anode as fuel.

However, even when the above-described shift reaction is performed, about 1 vol% of CO is contained in the obtained hydrogen gas, and CO becomes a catalyst poison of the platinum-based catalyst in the anode.
In particular, in a polymer electrolyte fuel cell operated at a low temperature of 100 ° C. or less, it is known that CO poisons the platinum catalyst remarkably, thereby greatly reducing the cell characteristics.

[0008] As a method for avoiding the influence of CO in the reformed gas, the following method can be mentioned. A method in which a catalyst having excellent resistance to CO poisoning is used for the anode. A method for incorporating a function for reducing CO into a reformer. A method of reducing the gas discharged from the reformer to a concentration of several hundred ppm or less through a device that oxidizes and removes CO.

A method has also been proposed in which air or oxygen is directly introduced into fuel gas to oxidize and remove CO adsorbed on the surface of the platinum catalyst of the anode. However, a mechanism for controlling the flow rate of air or oxygen is required, and the weight and size of the entire fuel cell system cannot be reduced.

Conventionally, a platinum-ruthenium alloy catalyst has been known as an anode catalyst having excellent resistance to CO poisoning. However, although alloying a platinum catalyst with ruthenium increases CO poisoning resistance, it reduces hydrogen oxidizing activity and causes the output characteristics of the fuel cell to deteriorate. Therefore, the anode catalyst layer has a two-layer structure, a layer made of a platinum-ruthenium catalyst having a low ruthenium content is formed on the electrolyte membrane side, and a layer made of a platinum-ruthenium catalyst having a high ruthenium content is formed outside the layer. A battery has been proposed (JP-A-10-270057). However, this method uses a large amount of ruthenium and tends to increase the cost.

In view of the above problems, the development of an anode having a catalyst layer that uses a small amount of ruthenium and has excellent resistance to CO poisoning enables the use of a reformed gas such as methanol as a fuel gas. There is a need for the development of a lightweight and small polymer electrolyte fuel cell that can generate power efficiently.

[0012]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and has a solid polymer electrolyte membrane and an anode and a cathode disposed on both sides of the electrolyte membrane, respectively. In a polymer electrolyte fuel cell in which a gas containing mainly hydrogen and carbon monoxide is supplied, and an oxidant gas is supplied to a cathode, an anode has a thickness of 1 to 15 μm, and platinum and a mass ratio of platinum. Platinum-ruthenium alloy containing 60% or more of platinum, and 60% by mass
A solid polymer comprising a catalyst layer containing at least one metal catalyst selected from the group consisting of platinum-molybdenum alloys containing platinum, wherein the electrolyte membrane has a thickness of 5 to 80 μm; -Type fuel cell is provided.

In the present invention, since the electrolyte membrane is thin, oxygen supplied to the cathode passes through the electrolyte membrane, reaches the anode catalyst layer, and adsorbs on the catalyst surface of the anode catalyst layer.
Can be removed by oxidation. Furthermore, since the thickness of the anode catalyst layer is small, CO in the anode catalyst layer can be sufficiently oxidized and removed.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In a solid polymer fuel cell according to the present invention, an anode and a cathode are respectively arranged on both surfaces of a solid polymer electrolyte membrane, and a gas containing mainly hydrogen and containing CO is supplied to the anode. An oxidant gas is supplied to the cathode. Here, the gas mainly containing hydrogen and containing CO is a fuel gas, specifically, a gas obtained by reforming a fuel such as methanol or the like.
It is a gas containing 1 ppm. CO is 500 volpp
If it exceeds m, the CO in the anode catalyst layer may not be sufficiently oxidized and removed. Also, 10 volpp
When it is less than m, the effect of the present invention is small because the catalyst poisoning of the anode is originally small. The oxidizing gas supplied to the cathode is a gas containing oxygen, such as air and oxygen.

In the anode and the cathode, a catalyst layer containing a catalyst is disposed adjacent to the electrolyte membrane, and a gas diffusion layer is disposed outside the catalyst layer, and the anode and the cathode are constituted by the catalyst layer and the gas diffusion layer. Is preferred. A separator having ribs, for example, is disposed outside the gas diffusion layer, and a fuel gas or an oxidizing gas can be supplied to the groove of the separator to be supplied to the catalyst layer through the gas diffusion layer.

In the present invention, the thickness of the anode catalyst layer is 1 to 15 μm. If the thickness exceeds 15 μm, oxygen supplied through the electrolyte membrane does not reach the entire anode, and CO adsorbed on the catalyst surface of the anode cannot be sufficiently removed. On the other hand, if the thickness of the catalyst layer is less than 1 μm, sufficient hydrogen oxidation activity to obtain a high output cannot be obtained. The thickness of the anode catalyst layer is particularly preferably 5 to 10 μm.
The thickness of the electrolyte membrane in the present invention is 5 to 80 μm. If it is less than 5 μm, the film strength is weak, and if it exceeds 80 μm, the amount of oxygen passing from the cathode to the anode is so small that CO cannot be oxidized and removed sufficiently. The thickness of the electrolyte membrane is 10
It is preferably from 60 to 60 μm.

During operation of the solid polymer electrolyte fuel cell of the present invention, the amount of oxygen permeated by the electrolyte membrane is 1 × 10 ^-4 to
It is preferably 5 × 10 ⁻³ cm ³ / (cm ² · s), and particularly preferably 3 × 10 ⁻⁴ to 2 × 10 ⁻³ cm ³ / (cm ² · s).
It is preferred that Within this range, the anode C
O can be sufficiently removed by oxidation. Here, the amount of oxygen permeation (cm ³ / (cm ² · s)) is the amount (cm ³ ) of oxygen permeating per 1 cm ^{2 of} membrane area per second under operating conditions
³ ) is shown. The amount of oxygen permeated by the electrolyte membrane varies depending on the thickness and structure of the electrolyte membrane, but is also affected by the oxygen concentration in the gas supplied to the air electrode, the gas utilization rate, the gas supply speed, and the like. It is preferable that the fuel cell be operated so that the oxygen permeation amount is in the above range by adjusting these.

In the present invention, the metal catalyst of the anode catalyst layer is at least one metal catalyst selected from the group consisting of platinum, platinum-ruthenium alloy, and platinum-molybdenum alloy. However, here, platinum-ruthenium alloy and platinum-
For molybdenum alloys, platinum is 60% of the total mass of the alloy
Contains more. These catalysts have high hydrogen oxidation reaction activity. Further, the metal catalyst may include one or more additional metals selected from the group consisting of tungsten, tin, rhenium and palladium while maintaining the platinum content at 60% or more. When the above-mentioned additional metal is contained, the resistance to CO poisoning increases. The additional metal may be present as an intermetallic compound of platinum, a platinum-ruthenium alloy or a platinum-molybdenum alloy, or may form an alloy.

When the additive metal is contained in the metal catalyst, the additive metal is contained in an amount of 2 to 20 per unit mass of the metal catalyst.
%, Particularly preferably 3 to 15%, more preferably 5 to 10%. If it is less than 2%, the effect of improving the resistance to CO poisoning is small, and if it exceeds 20%, the oxidation reaction activity of hydrogen may be low. Further, the metal catalyst preferably has a large specific surface area in order to increase the oxidation reaction activity of hydrogen,
Therefore, the average particle diameter is 1 to 30 nm, particularly 2 to 20 n
m, more preferably 3 to 5 nm.

In the catalyst layer of the anode, the metal catalyst may be included as fine metal particles, or may be supported on a carrier such as carbon and included as a supported catalyst. For supported catalyst, the carrier specific surface area preferably carbon 50~1500m ² / g, especially specific surface area of 70～800M ²
/ G of carbon is preferred. If the carbon has a specific surface area in this range, the metal catalyst can be supported with good dispersibility, and the metal catalyst hardly aggregates. For this reason, an electrode catalyst having excellent electrode reaction activity can be obtained. As the above carbon,
Known carbon materials can be used, and among them, carbon black such as channel black, furnace black, thermal black and acetylene black, activated carbon and the like can be preferably used.

Further, in the polymer electrolyte fuel cell, when operating at a high current density, it is necessary that the electrode have excellent gas diffusivity, so that the thickness of the electrode layer is reduced and the per unit area is reduced. It is necessary to secure the amount of catalyst. Therefore, when the electrode catalyst is a supported catalyst, the metal catalyst preferably accounts for 20 to 70% (mass ratio) of the total amount of the metal catalyst and the carrier.

In the present invention, the catalyst layer of the anode preferably contains an ion exchange resin in order to obtain a high output. As the ion exchange resin, a fluorinated polymer having a sulfonic acid group, particularly a perfluorocarbon polymer having a sulfonic acid group is preferable. Above all, CF ₂ =
Polymerized units based on CF ₂ and CF ₂ = CF- (OCF ₂ CF
_{X) m -O p - (CF} 2) n -SO 3 H in based polymer units (wherein, X is a fluorine atom or a trifluoromethyl group, m is an integer of 0 to 3, n is 1 to An integer of 12 and p is 0 or 1.).

It is preferable that the catalyst layer of the cathode contains an ion exchange resin as in the case of the anode. As the ion exchange resin for the cathode and the resin constituting the solid polymer electrolyte membrane, those similar to the ion exchange resin contained in the catalyst layer for the anode can be preferably used.

The method for producing the anode in the present invention includes, for example, the following method. A slurry in which carbon powder and polytetrafluoroethylene (PTFE) are dispersed is applied to a surface of carbon paper, carbon cloth, or the like, and is fired in the air to form a gas diffusion layer. Next, a liquid in which a platinum-supported carbon catalyst and an ion exchange resin composed of a perfluorocarbon polymer having a sulfonic acid group are dissolved or dispersed (hereinafter referred to as catalyst ink) on the slurry-coated surface of the gas diffusion layer. ), And dried to form a catalyst layer having a thickness of 1 to 30 µm, thereby obtaining an anode comprising a catalyst layer and a gas diffusion layer.

The cathode can be manufactured in the same manner as the anode. However, in the case of a cathode, the thickness is preferably about 5 to 50 μm in order to efficiently discharge generated water. The anode and the cathode obtained as described above are opposed to each other with the catalyst layer facing inward, and hot-pressed with an ion-exchange membrane interposed therebetween to perform membrane-
An electrode assembly is formed. This assembly functions as a power generation unit of the polymer electrolyte fuel cell. Here, a bonding method (JP-A-7-2207) is used as a bonding method between the ion exchange membrane and the electrode.
41).

In addition, as a method of producing an anode, a cathode and a membrane-electrode assembly, a method of directly forming an anode and a cathode on an ion-exchange membrane using the above catalyst ink, and a method of forming an electrode on a flat plate Any method such as transferring the electrode to an ion exchange membrane to form an anode or a cathode can be used.

[0027]

EXAMPLES Hereinafter, specific embodiments of the present invention will be described with reference to Examples (Examples 1 and 2) and Comparative Examples (Example 3), but the present invention is not limited thereto.

[Example 1] Carbon black powder (trade name:
A catalyst in which platinum is supported on Vulcan XC-72R (manufactured by Cabot) (carbon black and platinum in a mass ratio of 60:
40) was used as an electrode catalyst. Next, polymerized units based on CF ₂ = CF ₂ and CF ₂ = CFOCF ₂ CF (CF ₃ ) O (C
The above electrode catalyst is dispersed in a solution obtained by dissolving a copolymer comprising F ₂ ) ₂ SO ₃ H and polymerized units in ethanol,
A catalyst dispersion was prepared. The mass ratio between the electrode catalyst and the ion exchange resin in the catalyst dispersion was 70:30.

As the solid polymer electrolyte membrane, a perfluorocarbon sulfonic acid type ion exchange membrane (trade name: Flemion, manufactured by Asahi Glass Co., Ltd.) having an ion exchange capacity of 1.0 meq / g dry resin and a film thickness of 50 μm was used. The catalyst dispersion was sprayed on one surface of the electrolyte membrane to form an anode catalyst layer having a thickness of 10 μm. Similarly, the above catalyst dispersion was sprayed on the other surface of the electrolyte membrane to form a cathode catalyst layer having a thickness of 30 μm, thereby obtaining a membrane-catalyst layer assembly.

A carbon paper was used as a gas diffusion layer, and the above-mentioned joined body was sandwiched between two pieces of carbon paper, and was incorporated into a measuring cell. As fuel gas, H ₂ 75vol
%, CO ₂ 25 vol%, and CO ₂₀₀ vol ppm A power generation test was performed at normal pressure and a cell temperature of 80 ° C. using air as an oxidizing gas using a reforming simulation gas. The cell voltage was measured at current densities of 0.5 A / cm ² and 1.0 A / cm ² . Continuously, current density 0.5A
/ Cm ^2, a continuous operation test by constant current driving was performed, and cell voltages after 200 hours and 1000 hours were measured. Table 1 shows the results. At this time, the oxygen permeation amount of the electrolyte membrane was 3 × 10 ⁻⁴ cm ³ / (cm ² · s).

[Example 2] An anode and a cathode were formed in the same manner as in Example 1 except that a platinum-ruthenium alloy (platinum content: 75% by mass) was used instead of platinum as the electrode catalyst, and a membrane-catalyst layer bonding was performed. I got a body. Using this joined body, it was assembled in a measuring cell in the same manner as in Example 1, and evaluated in the same manner as in Example 1. Table 1 shows the results. At this time, the oxygen permeation amount of the electrolyte membrane was 3.9 × 10 ⁻⁴ cm ³ / (cm ² · s).

Example 3 A membrane-catalyst layer assembly was obtained in the same manner as in Example 1, except that the thickness of the anode catalyst layer was changed to 60 μm. Using this joined body, it was assembled in a measuring cell in the same manner as in Example 1, and evaluated in the same manner as in Example 1. Table 1 shows the results
Shown in At this time, the oxygen permeation amount of the electrolyte membrane is:
It was 3.2 × 10 ⁻⁴ cm ³ / (cm ² · s).

[0033]

[Table 1]

[0034]

According to the present invention, when fuel gas containing CO is used, CO poisoning on the platinum catalyst or platinum alloy catalyst of the anode can be suppressed. Therefore, the polymer electrolyte fuel cell of the present invention can obtain stable output characteristics over a long period of time.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Masako Kawamoto 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi, Kanagawa F-term in Asahi Glass Co., Ltd. HH05 5H026 AA06 CX03 CX05 EE02 EE05 EE19 HH00 HH03 HH05

Claims (4)

[Claims] 1. A solid polymer electrolyte membrane and an anode and a cathode arranged on both sides of the electrolyte membrane, respectively. A gas containing mainly hydrogen and containing carbon monoxide is supplied to the anode, and an oxidant gas is supplied to the cathode. Is supplied, the anode has a thickness of 1 to 15 μm, and contains platinum, a platinum-ruthenium alloy containing 60% or more platinum by mass, and 60% or more by mass. A solid polymer fuel comprising: a catalyst layer containing at least one metal catalyst selected from the group consisting of platinum-molybdenum alloys containing platinum; and the electrolyte membrane has a thickness of 5 to 80 μm. battery. 2. The polymer electrolyte fuel cell according to claim 1, wherein the gas supplied to the anode contains 10 to 500 vol ppm of carbon monoxide. 3. The polymer electrolyte fuel cell according to claim 1, wherein the catalyst layer contains a fluorine-containing ion exchange resin. 4. The method for operating a solid polymer electrolyte fuel cell according to claim 1, 2 or 3, wherein the electrolyte membrane comprises:
Oxygen transmission rate 1 × 10 ^{-4 to} 5 × 10 ^-3 cm ³ / (cm ² ·
An operation method of a polymer electrolyte fuel cell, which is operated in s).