JP2002231262A - High polymer electrolyte fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high polymer electrolyte fuel cell causing no lowering of power generation efficiency even if it is operated for a long time, by preventing corrosion of a metal, because if metallic ions produced by the corrosion of a metallic separator are diffused on a high polymer electrolyte film, and are trapped by its ion exchange site, and ionic conductivity is lowered. SOLUTION: A cation exchange resin is disposed in at least a part of the gas passage of the separator. One having alkali salt or ammonium salt as an ion exchange group is used for this cation exchange resin.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a portable power supply,
The present invention relates to a polymer electrolyte fuel cell used for a power source for an electric vehicle, a home cogeneration system, and the like, and particularly to an improvement for preventing metal ions from being mixed.

[0002]

2. Description of the Related Art A fuel cell using a polymer electrolyte generates electric power and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen with an oxidizing gas containing oxygen such as air. . This fuel cell basically includes a polymer electrolyte membrane for selectively transporting hydrogen ions, and a pair of electrodes formed on both sides of the polymer electrolyte membrane, that is, an anode and a cathode. The electrode is usually composed mainly of carbon powder carrying a platinum group metal catalyst, and has a catalyst layer formed on the surface of the polymer electrolyte membrane, and gas permeability and electron conduction formed on the outer surface of the catalyst layer. It is composed of a diffusion layer having both properties. A gas sealing material or gasket is placed around the electrodes with a polymer electrolyte membrane between them so that the fuel gas and oxidizing gas supplied to the electrodes do not leak out and the two gases do not mix with each other. Is done. These sealing materials and gaskets are assembled in advance integrally with the electrodes and the polymer electrolyte membrane. This is called MEA (electrolyte membrane-electrode assembly). Outside of the MEA, it is mechanically fixed and the adjacent M
The EAs are electrically in series with each other, possibly in parallel,
A conductive separator plate for connection is arranged. A gas flow path for supplying a reaction gas to the electrode surface and carrying away generated gas and surplus gas is formed in a portion of the separator plate that contacts the MEA. Although the gas flow path can be provided separately from the separator plate, a method in which a groove is provided on the surface of the separator plate to form a gas flow path is generally used.

In order to supply the fuel gas and the oxidizing gas to these grooves, pipes for supplying the fuel gas and the oxidizing gas are branched into the number of separator plates to be used, and the branch destination is directly connected to the separator plate. A piping jig that connects to the groove is required. This jig is referred to as a manifold, and the type directly connected from the fuel gas and oxidizing gas supply pipes as described above is referred to as an external manifold. There is a type of this manifold called an internal manifold which has a simpler structure. In the internal manifold, a through hole is provided in a separator plate in which a gas flow path is formed, an inlet / outlet of the gas flow path is passed to this hole, and a fuel gas and an oxidizing gas are directly supplied from this hole. Since the fuel cell generates heat during operation, it is necessary to cool the fuel cell with cooling water or the like in order to maintain the cell in a favorable temperature state. Usually, a cooling unit for flowing cooling water is provided for every 1 to 3 cells. There are a type in which a cooling unit is inserted between separator plates, and a type in which a cooling water flow path is provided on the back surface of the separator plate to form a cooling unit, and the latter is often used. The MEA, the separator plate, and the cooling unit are alternately stacked, and 10 to 200 cells are stacked. The stacked body is generally sandwiched between end plates via a current collector plate and an insulating plate, and fixed from both ends with fastening bolts. This is the structure of a simple stacked battery. In such a polymer electrolyte fuel cell, the separator plate has high conductivity,
And high airtightness to fuel gas and oxidizing gas,
Furthermore, it is necessary to have high corrosion resistance to the reaction when redoxing hydrogen / oxygen.

[0004] For this reason, the conventional separator plate is usually made of a carbon material such as glassy carbon or expanded graphite, and the gas flow path is formed by cutting its surface or, in the case of expanded graphite, by molding with a mold. It had been. In the conventional method of cutting a carbon plate, it has been difficult to reduce not only the material cost of the carbon plate but also the cost for cutting the carbon plate. Also, the method using expanded graphite has a high material cost, and this is considered to be an obstacle for practical use. In recent years, attempts have been made to use a metal plate such as stainless steel in place of a conventionally used carbon material. However, in the above-described method using a metal plate, the metal plate is exposed to an oxidizing atmosphere having a pH of about 2 to 3 at a high temperature. When the metal plate is corroded, the electric resistance of the corroded portion increases, and the output of the battery decreases. When the metal plate is dissolved, the dissolved metal ions diffuse into the polymer electrolyte membrane and are trapped at ion exchange sites of the polymer electrolyte membrane. As a result, the ion conductivity of the polymer electrolyte itself is reduced. Due to these causes, there has been a problem that when the metal plate is used as it is for the separator plate and the battery is operated for a long period of time, the power generation efficiency gradually decreases. Further, even when the separator is made of a carbon material, it is also assumed that metal ions diffuse as impurities into the polymer electrolyte membrane from a gas pipe or the like using a metal.

[0005]

SUMMARY OF THE INVENTION The present invention improves the gas flow path of a separator used in a fuel cell to prevent diffusion of metal ions caused by a metal pipe and to use a metal separator. Even in such a case, it is an object of the present invention to provide a polymer electrolyte fuel cell having excellent durability by preventing diffusion of metal ions from a separator to a polymer electrolyte membrane.

[0006]

According to the present invention, there is provided a polymer electrolyte fuel cell comprising: a pair of electrodes having a catalyst layer and a gas diffusion layer; a polymer electrolyte membrane sandwiched between the pair of electrodes; And a separator having a gas flow path for supplying a fuel gas to one electrode, and a separator having a gas flow path for supplying an oxidizing gas to the other electrode, is stacked via the separator. And wherein a cation exchange resin is disposed in at least a part of a gas flow path of the separator. The cation exchange resin is preferably a fluorinated polymer having a sulfonic acid group. The sulfonic acid group is preferably an alkali metal salt or an ammonium salt.

According to the present invention, after the above-mentioned polymer electrolyte fuel cell has been operated for a predetermined time or when a predetermined decrease in output has been observed, an alkaline aqueous solution is filled in at least one of the gas flow paths, and Provided is a method for operating a polymer electrolyte fuel cell including a step of exchanging protons trapped in an ion exchange resin and discharging the protons from the fuel cell.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a polymer electrolyte fuel cell according to the present invention, a gas passage of a separator for supplying a fuel gas and an oxidizing gas to an electrode is coated with a cation exchange resin. The cation exchange resin captures metal ions mixed in the fuel gas or the oxidizing gas supplied to the gas flow path of the separator and suppresses diffusion of the metal ions into the electrode catalyst layer or the polymer electrolyte membrane. In a preferred aspect of the present invention, in the cation exchange resin, an ion exchange group such as a sulfonic acid group is an alkali metal salt or an ammonium salt. In this configuration, the ion exchange resin captures protons. When the cation exchange resin is represented by RH,
The reaction in which the sodium salt of an ion exchange group captures a proton is represented as follows. R-Na + H ⁺ = R-H + Na ⁺

As a result of the ion exchange resin capturing the protons as described above, the pH of the water in the fuel gas and the oxidizing gas supplied to the gas flow path is increased, and the dissolution and corrosion of the metal hardly occur. For example, iron does not dissolve or corrode in an atmosphere having a pH of 4 or more. Therefore, even if a metal separator is used, it is possible to prevent a decrease in the ion conductivity of the polymer electrolyte membrane due to corrosion and dissolution of the separator. As the cation exchange resin used in the present invention, inexpensive hydrocarbon cation exchange resins such as polystyrene, phenol and polyacrylic acid are preferable. Of course, a perfluorosulfonic acid-based ion exchange resin used as a polymer electrolyte membrane can also be used.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. The structural diagrams used here are for ease of understanding, and the relative sizes and positional relationships of each element are not always accurate. Embodiment 1 FIG. 1 is a longitudinal sectional view showing a unit cell of a polymer electrolyte fuel cell according to this embodiment. Reference numeral 10 denotes an electrolyte membrane-electrode assembly (MEA). This MEA comprises a polymer electrolyte membrane 11, an anode 12 and a cathode 13 sandwiching the polymer electrolyte membrane. An anode-side conductive separator plate 21 and a cathode-side conductive separator plate 31 are arranged outside the MEA, and current collecting plates 22 and 32 are arranged outside the anode-side conductive separator plate 21 and the cathode-side conductive separator plate 31. As a unit cell, a plurality of cells are connected in series and stacked,
A battery stack is configured. Conductive separator plate 21
Has a groove 41 that forms a fuel gas flow path on the surface in contact with the anode. Similarly, the conductive separator plate 31 has a groove 42 that forms a flow path for the oxidizing gas on the surface in contact with the cathode.
Having. Here, the anode-side conductive separator plate and the cathode-side conductive separator plate were independently manufactured, but the anode-side conductive separator plate and the cathode-side conductive separator plate were formed of a single separator plate, and one of them was used. The surface may be an anode-side conductive separator, and the other surface may be a cathode-side conductive separator.

Next, a method of manufacturing the MEA will be described. Platinum particles having an average particle size of about 30 °
The catalyst supported at a weight ratio of 0:50 was used as a catalyst for the electrode.
An ethyl alcohol dispersion of perfluorocarbon sulfonic acid powder (Flemion of Asahi Glass Co., Ltd.) was mixed with an isopropanol dispersion of the catalyst powder to form a paste. Using this paste as a raw material, a catalyst layer was formed on one surface of a carbon nonwoven fabric having a thickness of 250 μm by a screen printing method. The amount of platinum contained in this electrode was 0.1.
5 mg / cm ^2, the amount of perfluorocarbon sulfonic acid was adjusted to 1.2 mg / cm ^2. These electrodes have the same configuration for both the cathode and anode, and a catalyst layer printed on both sides of the center of a proton conductive polymer electrolyte membrane (Nafion membrane of DuPont, USA) having an area slightly larger than the electrodes has an electrolyte membrane. Joining was performed by hot pressing so as to be in contact with the side, thereby producing an MEA.

As the separator plate, a separator plate having a gas passage formed by cutting a carbon plate was used. Next, the same cation exchange resin powder as above in ethyl alcohol dispersion,
Ammonia was bubbled for 30 minutes. The cation exchange resin was applied to the inner wall surface of the gas flow path of the separator by screen printing using the dispersion of the cation exchange resin in which the ion exchange group was adjusted to an ammonium salt as a raw material. Heated. FIG.
An enlarged sectional view of a main part of (31) is shown. Separator 21
A coating layer 43 of a cation exchange resin is formed on the inner surface of the gas channel 41 (42) of (31). The separator was combined with the MEA described above, and 50 cells were laminated. The laminated cell was fastened to a stainless steel end plate via a current collector plate and an insulating plate with a fastening rod at a pressure of 20 kgf / cm ² . Note that a portion of the separator and the MEA that requires gas sealing secured the sealing properties by applying a thin layer of silicone grease without significantly lowering the conductivity. Further, a fuel cell using a gas diffusion layer not coated with a cation exchange resin was used as Comparative Example 1.

The fuel cell of this embodiment using a separator having a gas flow channel coated with a cation exchange resin and the fuel cell of Comparative Example 1 were maintained at 80 to 90 ° C., and the anode side was heated to 75 to 85 ° C. Hydrogen gas humidified and heated to a dew point was supplied, and air humidified and heated to a dew point of 70 to 80 ° C. was supplied to the cathode side. as a result,
When no current was output to the outside and there was no load, an open circuit voltage of 50 V was shown. These cells were subjected to a continuous power generation test under the conditions of a fuel utilization rate of 80%, an oxygen utilization rate of 40%, and a current density of 0.6 A / cm ² , and a change in output characteristics with time was examined. The result is shown in FIG. While the output of the battery of Comparative Example 1 decreased with time, the battery of Example 1 maintained a battery output of about 1250 W (27.5 V-45 A) over 8000 hours or more. In the battery of Example 1, since the gas flow channel is coated with a cation exchange resin, protons are trapped from moisture in the gas supplied to the gas flow channel, and p
H becomes high and suppresses metal corrosion. As a result, since the generation of metal ions diffusing into the catalyst layer and the polymer electrolyte membrane was prevented, the output fluctuation and the amount of decrease during the long driving time were reduced. Similar effects can be obtained by using an aqueous alkaline solution such as sodium hydroxide or potassium hydroxide instead of the aqueous ammonia used for the treatment of the ion exchange resin.

Example 2 A polymer electrolyte fuel cell was manufactured in the same manner as in Example 1. The difference from the first embodiment is
The point is that a metal separator is used as the conductive separators 21 and 31. In this embodiment, stainless steel SUS31 is used.
The six plates were processed into a wave shape by press working to form grooves or ribs for guiding gas. An elastic insulating material that forms a gas flow path that guides the gas from the supply side to the discharge side in cooperation with the groove or the rib, and acts as a gasket that prevents the gas from leaking outside from the gas flow path. Sheets were used in combination (WO 00/01025)
reference).

FIG. 4 shows the change over time of the output of the fuel cell stack of this embodiment. As a comparative example, a fuel cell stack in which the SUS316 separator was not coated with a cation exchange resin was referred to as a comparative example 2, and the results were also listed. Comparative Example 2
In the case of, a large output decrease was observed as the operation time progressed. This decrease in output was greater than in the case of Example 1. This is because Fe, C
It is considered that metal ions such as r and Ni diffused into the electrode catalyst and the polymer electrolyte membrane. FIG. 5 shows a change with time of the substitution rate of the ion exchange site in which the sulfonic acid group in the electrolyte membrane was replaced by the metal ion. Obviously, the fuel cell stack of this embodiment having the gas flow channel coated with the cation exchange resin has a smaller proportion of the sulfonic acid groups substituted by metal ions. Therefore, even when the metal plate is used as the separator, the fuel cell having the gas flow path of the present invention can perform a stable operation even in a long-term operation.

Example 3 A fuel cell using stainless steel SUS316 as a separator was manufactured in the same manner as in Example 2, and a continuous operation was performed under the same conditions as in Example 1. When the operation time reached 10,000 hours, a 0.1 M aqueous sodium hydroxide solution was circulated through the gas flow path of the fuel cell by a pump for 1 hour. Table 1 shows changes in the output of the fuel cell. By the process after the operation for 10,000 hours, the output reduction width with respect to the initial output was reduced to half.
This is presumably because the amount of impurity ions diffused into the MEA decreased due to the regeneration of the cation exchange resin by the treatment with the alkali. As a result of analyzing the replacement ratio of the ion exchange sites in the polymer electrolyte membrane by Fe, Cr, and Ni ions, as shown in Table 2, the replacement ratio was surely reduced after the alkali treatment.

[0017]

[Table 1]

[0018]

[Table 2]

According to the operating method of this embodiment, diffusion of metal ions into the electrode catalyst and the electrolyte membrane in the fuel cell can be prevented. Further, since the cation exchange resin can be regenerated without disassembling the fuel cell, it is effective in that the maintenance can be simplified.

[0020]

As described above, according to the present invention, by coating a gas flow path with a cation exchange resin, the corrosive atmosphere of metal can be eliminated, and the diffusion of metal ions to the electrode catalyst and the electrolyte membrane can be prevented. . Therefore, the fuel cell can be operated at a stable output for a long time. After a certain period of operation,
Alternatively, when a certain decrease in output is recognized, the output can be recovered by circulating alkali in the fuel cell, and the fuel cell can be operated with a stable output for a long time.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view illustrating a unit cell of a fuel cell according to an embodiment of the present invention.

FIG. 2 is an enlarged sectional view of a main part of a separator of the fuel cell.

FIG. 3 is a diagram showing output characteristics of the fuel cells of Example 1 and Comparative Example 1.

FIG. 4 is a diagram showing output characteristics of the fuel cells of Example 2 and Comparative Example 2.

FIG. 5 is a diagram showing the cation substitution rate of the electrolyte membranes of the fuel cells of Example 2 and Comparative Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 MEA 11 Polymer electrolyte membrane 12 Anode 13 Cathode 21 Anode-side conductive separator plate 22, 32 Current collector 31 Cathode-side conductive separator plate 41, 42 Gas flow path 43 Layer of cation exchange resin

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Masato Hosaka 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. F-term (reference) 5H026 AA06 CC03 EE18 5H027 AA06 KK51

Claims (4)

[Claims] 1. A pair of electrodes having a catalyst layer and a gas diffusion layer, a polymer electrolyte membrane sandwiched between the pair of electrodes, and a gas flow disposed outside the electrodes and supplying a fuel gas to one of the electrodes. A separator having a passage, and a unit cell including a separator having a gas flow path for supplying an oxidizing gas to the other electrode, a polymer electrolyte fuel cell in which the separator is stacked with the separator interposed therebetween, A polymer electrolyte fuel cell, wherein a cation exchange resin is disposed in at least a part of a flow path. 2. The polymer electrolyte fuel cell according to claim 1, wherein the cation exchange resin is a fluoropolymer having a sulfonic acid group. 3. The polymer electrolyte fuel cell according to claim 2, wherein said sulfonic acid group is an alkali metal salt or an ammonium salt. 4. After the polymer electrolyte fuel cell according to claim 1 has been operated for a predetermined time or when a predetermined decrease in output has been recognized, the at least one gas flow path is filled with an alkaline aqueous solution, A method for operating a polymer electrolyte fuel cell, comprising a step of exchanging protons trapped in a cation exchange resin and discharging the protons from the fuel cell.