JPS58126674A - Oxygen electrode for fuel cell and its manufacture - Google Patents

Abstract

PURPOSE:To provide a high performance oxygen electrode having large reactivity and generating large electric power per unit area by directly allowing to carry a positive active mass on a cation exchange membrane by means of vapor deposition or heat bonding. CONSTITUTION:A fine particle electrode active mass 11 which is formed on a cation exchange membrane by means of vapor deposition or heat bonding is half embeded into the cation exchange membrane, and this fine particle has a vacant space to pass easily air (oxygen) 13 to the surface of a cation. Since particles are contacted each other, an electrode active mass layer has conductivity so as to function as an electrode. The reaction takes place at the area (three phase reaction zone) where a positive active mass 11, a cation exchange membrane 10, and air 13 are in contact each other. Most of water produced by reaction passes the vacant space of the electrode active mass in a form of steam and is released to the air. Therefore, the reaction takes place continuously.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a structure of an oxygen electrode for a fuel cell and a method for manufacturing the same.

As a typical example of conventional fuel cells, the structure of a methanol/oxygen (air) fuel cell is shown in FIG. Methanol is decomposed into electrons, carbon dioxide gas, and hydrogen ions at the loaded electrode 2. At this time, electrons enter the negative electrode 2 and are taken out as a current. Carbon dioxide gas is discharged to the outside from the gas outlet 3. The hydrogen ions then move through the cation exchanger [4] into the cathode solution 5, and at the oxygen electrode 6, the hydrogen ions, oxygen (present in the air), and electrons moving from the negative electrode 2 are transferred to the cathode. Electrons move from the negative electrode 2 to the oxygen electrode (positive electrode) 6 by zero or more reactions that react and generate water, and this is the station where the electromotive force of the fuel cell is generated.

At this time, since the cathode solution 5 is diluted with the generated water, the hydrogen ion concentration decreases. Therefore, the speed of the cathode reaction is reduced and the current density is reduced, which is a problem.

As a solution to this problem, a method has been proposed in which a cathode reaction is carried out without using a cathode solution by press-bonding an oxygen electrode and a cation exchange membrane. However, if the oxygen electrode used in this case can directly support only the positive electrode active material on the cation exchange membrane without using the positive electrode active material or a water-repellent material, the reactivity will be higher; It is thought that a large current density can be obtained.

The present invention is characterized in that the positive electrode active material is directly supported on the cation exchange membrane by vapor deposition or thermocompression bonding.

Examples of the present invention will be shown below.

FIG. 2 shows the electrode structure in the present invention. 10 is a cation exchange membrane, and in the example, cation exchange membrane (Chel2
The company's registered trademark) was used. The reason for this is that Nafion has excellent heat resistance and oxidation resistance, and is suitable for processing at relatively high temperatures as in the production method of the present invention. 11 is a layer of positive electrode active material, and claim 1fj
The product manufactured by the manufacturing method described in claim 12 is a platinum O vapor deposited film, and the product manufactured by the manufacturing method described in claim 3 is platinum black fine particles. Figure #I3 is a schematic enlarged view of Figure 92. The structure produced by either vapor deposition or thermocompression bonding is considered to be basically the same. That is, the particulate electrode active material 11' is partially embedded in the cation exchange membrane, and these particulates have voids that allow air (WI element) to easily pass through to the cation surface. Furthermore, since the fine particles are in contact with each other, the layer of the electrode active material has conductivity e and can sufficiently fulfill the role of an electrode. The reaction is carried out using the positive electrode active material 11', the cation exchange membrane,
This occurs near the region where the three components of air and air are in contact with each other (the so-called three-phase reaction zone). Furthermore, most of the water produced by this reaction is released into the air through the gaps in the electrode active material in the form of water vapor, so that the reaction is carried out extremely smoothly.

Next, a method for manufacturing an oxygen electrode for a fuel cell according to claim 2 will be described.

j14wJ shows a cross section of a vacuum evaporation apparatus used in an example of this manufacturing method. Place the sulfur ion membrane 15 cut to the desired size on the sample stage 16, attach the platinum (1! polar active material) wire twisted with the tungsten wire 17 to the heater holder 18, and place it on the sample stand 16. bell 20
Place it on top of the stand 21. Then, the inside of the glass is evacuated using a vacuum pump and an oil diffusion pump connected to the exhaust port 22. In this state, when a current of 5 Å or more is passed through the tungsten wire 17 by the heater power supply 24, heat is generated, the platinum wire is also heated and evaporated, and platinum particles are deposited on the Nafion film. When the vapor deposition is completed, the leak valve 25 is opened to return the inside of the glass bell to normal pressure, and the Nafion film is taken out. The above are the steps of this manufacturing method.

The oxygen electrode produced by this manufacturing method is characterized by the fact that a homogeneous platinum vapor deposited film is obtained over the entire Nafion membrane, and the platinum particles are extremely fine, with a particle size of 1 μm or less.

Therefore, the electrode properties are uniform over the entire surface of the membrane, and the three-phase reaction zone is densely present, so that an extremely large current density can be obtained.

Next, a method for manufacturing an oxygen electrode for a fuel cell according to claim 5 will be described.

As the Nafion membrane is heated and the temperature is raised, it gradually softens and gradually becomes plastic. In the present invention, this property is utilized to make the Nafion membrane carry an electrode active material. In this example, platinum black powder (10 μs in particle size) is placed as an electrode active material on a Nafion membrane 25 cut into a desired size, and rollers 27 and 22 rotate at a constant speed.
It is sandwiched in between and rolled while applying a pressure of about 40 positive/-. At this time, the roller 27 uses a heat generating mechanism and performs this work while heating the ion membrane to around 200°C. Then, due to the thermoplastic nature of the Nafion membrane, platinum black is half-buried therein and is sent out to the outside of the roller as an oxygen electrode 28.

The pressure, heating temperature, and rotation speed of the roller can be changed arbitrarily, but it is sufficient that the platinum black powder is buried in the Nafion membrane.

Since platinum black has a very porous surface, its surface area is extremely large compared to its particle size, and therefore there are many active sites. Therefore, even if platinum black powder with a larger particle size than platinum particles obtained by vapor deposition is used, its electrode performance is extremely high and a large current density can be obtained. Moreover, since the powder particles penetrate relatively deeply into the sulfur ion membrane, it has the advantage of being extremely mechanically strong. Furthermore, since this manufacturing method allows oxygen electrodes to be manufactured continuously and stably, it is an extremely advantageous manufacturing method when considering industrialization.

FIG. 6 shows a cross section of a methanol/air fuel cell actually assembled using the oxygen electrode produced according to the present invention. Compared to conventional fuel cells, the fuel cell using the present invention does not require a cathode solution, so the size of the battery cell is reduced. This characteristic is useful for improving the portability of small fuel cells and reducing the space required for large fuel cell power plants, and is one of the features of the present invention.

The operating conditions are as follows. Methanol is used as fuel, and water and phosphoric acid are added to it in a weight ratio of 2:1:1.
Anode solution 32 was prepared as follows. Negative electrode 5
In No. 4, a carbon plate on which platinum was supported was used. Then, the positive electrode of the present invention was fixed with the ion membrane surface 55 facing the γ node solution and the surface 35' carrying the electrode active material facing the air.

The lead wire 36 was fixed to the electrode active material supporting surface 55' using gold paste.

When this battery is operated near room temperature, the terminal voltage r:
Very high values of L6v and current density of 40 m/- were obtained, and this power hardly decreased even after 72 hours or more as long as fuel was supplied. Further, electrodes produced by either of the methods described in claim 2 and claim 3 exhibited substantially the same performance. The above results prove that the present invention is not only unique, but also that its structure is very suitable for conducting battery reactions. That is, since the electrode active material is directly supported on the cation exchange membrane, the hydrogen ions that have passed through the cation exchange membrane are immediately utilized for the cathode reaction in the three-phase reaction zone, resulting in extremely high efficiency. Furthermore, since a water-repellent substance is not required, the three-phase reaction zone that can participate in the reaction per unit area is extremely wide and is greater than 0, which makes it possible for the oxygen electrode according to the present invention to generate extremely large electric power per unit area. . In addition, since no substances are used for water repellency, there is a three-phase reaction zone and the outside air (the atmosphere which is the #1 source).
The distance between the electrode active material and the electrode active material is approximately the same as the particle size of the electrode active material. Therefore, not only oxygen from the atmosphere is easily supplied to the three-phase reaction zone, but also water generated in the three-phase reaction zone is quickly released outside the battery as water vapor, which has been a problem in the past. , dilution of hydrogen ions by water, and isolation of the three-phase reaction zone from the outside air by water droplets were also solved.

This is thought to have made it possible to maintain the characteristics of the battery stably for a long period of time even when the battery was operated to obtain large amounts of power.

Although each of the present inventions relating to the production of oxygen electrodes has its own unique characteristics, it became clear from this example that it is suitable for obtaining an oxygen electrode with unprecedented high performance.

[Brief explanation of drawings]

FIG. 1 shows a cross section of a conventionally used methanol/air fuel cell. 1... Anode solution (methanol aqueous solution) 2.
... Negative electrode (anode) 3 ... Carbon dioxide gas outlet 4 ... Cation exchange membrane 5 ... Cathode solution 6 ... Oxygen electrode 7 ...Fuel supply port 9...Battery tank FIG. 2 shows a cross section of the oxygen electrode of the present invention. (Actually, it is a single membrane, but to show the solution side and air side sides, the cation exchange membrane and the electrode active material supported on it are shown as layers.) 10...Cation exchange membrane 11. ...Surface (layer) 1 on which electrode active material is supported
2... Anode solution 13... Air Figure 3 shows a partially enlarged view of Figure 2. 10.12.13... Same as Figure 2 14...
Particles of electrode active material FIG. 4 shows a cross section of a vacuum evaporation apparatus used in the method for producing an oxygen electrode for a fuel cell as set forth in claim 2. 15...Cation exchange II (sulfur ion) 16
...Sample stand 17...Twisted platinum wire (electrode active material) and tungsten wire 18...Heater holder 19...Heater electrode support 20...Glass value 21...Glass mirror stand 22...Exhaust port 23...Leak intake port 24...Heater power source Fig. 5 1 shows a hot rolling mill used in the method for manufacturing an oxygen electrode for a fuel cell according to claim 2. 25... Cation exchange membrane 26... Electrode active material powder 27... Heating roller 28... Electrode active material embedded in the cation exchange membrane Material (reaction layer) 29...Roller 30...Sample stage FIG. 6 shows a cross section of a methanol/air fuel cell used in an example of the present invention. 31... Fuel supply port 52... Anode solution (mixture of methanol, water, and phosphoric acid) 33... Carbon dioxide gas outlet 54... Negative electrode 35 ...Cation exchange membrane 35'... Schematic representation of the electrode active material embedded in the cation exchange membrane as a reaction layer 36...Lead wire 57... ...Battery tank and above Figure 1 Figure 2 Figure 3 Figure 1 μ Figure 4 Figure 5 Figure 6 -347-

Claims (1)

[Scope of Claims] t. An oxygen electrode for a fuel cell, characterized in that a positive electrode active material and a cation exchange membrane are integrated, and a cathode solution is not required. 2 Accumulate the positive electrode active material on the cation exchange membrane by vapor deposition,
A method for producing an oxygen electrode for a fuel cell, characterized in that the oxygen electrode is an oxygen electrode for a fuel cell. 5. Bonding fine particles of the positive electrode active material to the cation exchange membrane by thermocompression,
A method for producing an oxygen electrode for a fuel cell according to claim 2, characterized in that the oxygen electrode for a fuel cell is made by supporting the oxygen electrode.