JPH0722020B2 - Method for manufacturing gas diffusion electrode - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a novel gas diffusion electrode, especially a high-efficiency electrode for maintaining a good reaction interface between an electrode catalyst or a layer thereof and an electrolytic solution during cell formation. The present invention relates to a high-performance fuel cell gas diffusion electrode.

[Technical background of the invention and its problems] Concentrated acidity such as phosphoric acid or sulfuric acid generated by using hydrogen as a negative electrode active material, which is hydrogen mixed with impurities such as carbon dioxide obtained by reforming hydrocarbons In a fuel cell using a solution as an electrolyte, generally, a catalyst mixture prepared by binding an electrode catalyst prepared by supporting a noble metal catalyst on carbon powder with a binder such as a fluororesin is applied to a porous carbon substrate and baked. A unit cell is formed by interposing a matrix layer holding an electrolyte as described above between a pair of gas diffusion electrodes formed by forming an electrode catalyst layer on the substrate.

BACKGROUND ART Conventionally, development of carbon powder used as a carrier for supporting catalyst powder in such a gas diffusion electrode for a fuel cell has been advanced, and in order to be used for electromotive reaction over a long period of time, graphitization has progressed as the carbon powder. Materials such as graphite powder, acetylene black or graphitized furnace black are known to be suitable.

However, these highly graphitized carbon materials are highly conductive and have excellent resistance to oxidative dissolution, that is, they are expected to have high performance and long life, but are themselves water repellent and Since it is used by kneading with a fluorocarbon resin that is originally water-repellent, the electrode catalyst or electrode catalyst layer is very difficult to get wet with the electrolyte solution during battery formation, so the interface involved in the electromotive reaction often becomes narrow, which is high. It was difficult to manufacture a high-performance electrode.

[Object of the Invention]

Thus, the present invention solves the above-mentioned difficulties and improves the wettability of the electrode catalyst or the electrode catalyst layer and the electrolytic solution during the formation of the cell, and can form and maintain a good reaction interface between them. It is an object of the present invention to provide a method for manufacturing a gas diffusion electrode for use in a gas.

[Outline of Invention]

The present invention is to apply a catalyst mixture obtained by binding an electrode catalyst in which a noble metal-based catalyst is supported on graphitized carbon powder with a fluororesin to a porous carbon substrate and then firing it to form an electrode on the substrate. In a gas diffusion electrode formed with a catalyst layer, hydrophilic carbon powder is further added to the catalyst mixture to improve the wetting of the electrode catalyst or the electrode catalyst layer and the electrolytic solution at the time of forming a battery, thereby improving the wettability between them. The present invention provides a high-performance gas diffusion electrode for a fuel cell, characterized in that a good reaction interface is formed and maintained.

[Specific Description of the Invention]

The present invention will be described in detail. In the method for producing a gas diffusion electrode according to the present invention, graphitization which has been conventionally used as a carbon powder carrying a noble metal powder such as platinum or platinum group element as a catalyst is advanced. It is characterized in that carbon powder is used and hydrophilic carbon powder is further added.

In general, the hydrophilicity of carbon powder varies depending on the amount of various hydrophilic functional groups on the surface of carbon powder, such as hydroxyl group, carboxyl group, ketone group, lactone, quinone, etc., and the number of activated carbon> carbon black> graphite. Many in order. When these carbon powders are heated, these functional groups are reduced and graphitization proceeds, that is, the bond distance between carbon and carbon atoms becomes shorter.
Therefore, the water repellency gradually becomes stronger.

In the present invention, the hydrophilicity of various carbon powders is obtained by dispersing carbon powders in water and kneading them together with a fluororesin dispersion to obtain a carbon paper which is coated with carbon and then fired at 0.05cc of 105cc. The judgment is made by measuring the contact angle of the liquid drop when% phosphoric acid is dropped. When the contact angle (θ) of the liquid droplet at that time is small and forms an acute angle, it is hydrophilic, and when it is large and obtuse, it is judged to be water repellent. The above-mentioned activated carbon,
Non-graphitized non-graphitized amorphous carbon powder such as carbon black is hydrophilic with an acute contact angle, and graphite powder or graphitized crystalline carbon powder such as graphitized carbon black is obtuse angled. Has a contact angle and is highly water repellent.

When the contact angle of the liquid droplets is measured as described above for various carbon powders that are commercially available now and are easily available, the contact angle of activated carbon manufactured by Kanto Kagaku Co., Ltd. is 82-86 ° The carbon powder of trade name CSX-99 is 72 to 88 ° and is hydrophilic, and therefore has good wettability. On the other hand, the contact angle of trade name Denka Black manufactured by Tokyo Denki Kagaku Kogyo Co., Ltd. 11
6 to 123 °, graphite powder manufactured by Nippon Graphite Co., Ltd., trade name AUP is 115 to 13
The 3 ° USSP has a large contact angle such as 111 to 122 °, and the water repellency becomes strong.

The water repellency depends on the amount of the fluororesin dispersion liquid used as the binder, and the greater the amount, the greater the water repellency.

The amount of the hydrophilic carbon powder added to the highly graphitized carbon powder having high water repellency is preferably in the range of 10 to 70% by volume with respect to the former. The amount of hydrophilic carbon powder added varies depending on the type of carbon powder to be combined, but if too much hydrophilic carbon powder is mixed, the wettability will improve, but the resistance to oxidative dissolution will weaken, and the catalytic function will decline and the electrode will deteriorate. Is not preferable because it is accelerated, and it is preferable to set it within the above range. When combining both carbon powders, it is desirable to select those having the same specific surface area as much as possible.

In producing a gas diffusion electrode using these carbon powders, for example, platinum or a platinum group metal powder is added to graphitized carbon powder in an amount of 5 to 15% by weight based on the carbon powder. The mixture is mixed and supported, and hydrophilic carbon powder is added in the amount described above. Then, appropriate water is added and the mixture is agitated and mixed so as to sufficiently wet the inside of the pores of the carbon powder. Fluorine resin as a binder, such as polytetrafluoroethylene (PT
Dispersion (FE, trade name Teflon) is added with stirring and kneaded to obtain a catalyst mixture.

In the production method according to the present invention, hydrophilic carbon powder is used together with water-repellent carbon powder having advanced graphitization, but the catalyst (for example, platinum or platinum group metal powder) is carbon powder having advanced graphitization. When mixed, they are supported on the carbon powder, and hardly supported on the hydrophilic carbon powder added thereafter. Such a hydrophilic carbon powder on which a catalyst is not supported is obtained by allowing an electrolyte such as phosphoric acid to partially and efficiently exist in the vicinity of a water-repellent carbon powder (a carbon powder with advanced graphitization) supporting a catalyst. , Has a function as a wetting control agent for balancing water repellency / hydrophilicity.

The catalyst mixture thus obtained is applied to a porous carbon substrate and fired to form an electrode catalyst layer. As the carbon substrate, for example, graphitized fibers are thinned by a papermaking method. Flat plate-like carbon paper having a bulk specific gravity of 0.28 to 0.60 or ribbed carbon paper can be used. For the coating, for example, the substrate can be satisfactorily coated by vacuum suction from the back surface and spraying from the front surface. Then, after pressing, firing is carried out in an atmosphere of an inert gas such as helium or nitrogen to obtain a desired gas diffusion electrode.

Of course, this manufacturing method shows an example, and is not limited to this.

When the gas diffusion electrode thus obtained is used, the wettability between the electrolytic solution and the electrode catalyst or the electrode catalyst layer is good, a good reaction interface is maintained between them, and a high-performance and long-life fuel cell is obtained. It is possible to do this, as will be apparent from the examples below. It should be understood that the present invention should not be limited to these embodiments, but may have various modifications and applications.

Example of Invention

Example 1 Platinum was added to graphite powder (USSP manufactured by Nippon Graphite Co., Ltd., average particle size 1 μ, specific surface area 438 m ² / g) in an amount of 20% by weight of platinum to obtain a total amount of 20 g,
8 g of carbon black powder (CSX-99 manufactured by US Botbot Inc.) with good conductivity and hydrophilicity (about 60
(% By volume), add 500 ml of pure water to this, and mix well with a mixer so that the inside of the pores of the carbon powder are sufficiently wet. To this, 12 ml of 60% by weight polytetrafluoroethylene (PTFE) dispersion was gradually added with stirring to form a catalyst mixture.

This catalyst was applied to carbon paper (Kureha Chemical Industry Co., Ltd. E
-715, porosity 77%, thickness 0.4mm, area resistance 0.4Ωcm) While vacuum suctioning the back side, spray gun with nozzle pressure 3Kg / cm
Spray evenly with ² . Then, it was pressed with a roller at 2 kg / cm ² , dried in helium gas at 330 ° C. for 15 minutes, and baked to prepare an electrode.

Using the electrodes obtained in this way, make a single cell,
A mixed gas of 80% hydrogen + 20% carbon dioxide, which is a simulated gas of natural gas reforming, is used as the fuel, air is used as the oxidizer, and phosphoric acid is used as the electrolyte. The gas flow rate is three times the theoretical amount of the maximum polarization current value. The polarization characteristics were measured at 200 ° C. and normal pressure.

For comparison, a unit cell was prepared using the conventional electrode prepared in the same manner as in Example 1 except that the hydrophilic carbon black powder was not used, and the polarization characteristics were measured in the same manner.

The results of these measurements are also shown in the graph of FIG. In this graph, curve 1 shows the case of the battery using the electrode of the present invention, and curve 2 shows the case of the battery using the conventional electrode. As is clear from this, when using the electrode according to the present invention, by adding and mixing the hydrophilic carbon powder, it was possible to maintain a good reaction interface between the electrolytic solution and the electrode catalyst or its layer. As compared with the conventional electrode using no hydrophilic carbon powder, a higher battery voltage could be obtained when polarized with a constant current.

Next, FIG. 2 shows the results of comparison of changes in battery voltage with time, that is, life characteristics when the same two single cells were polarized at a rated current of 220 mA / cm ² . In the figure, the curve 1 shows the case of the electrode of the present invention, and the curve 2 shows the case of the conventional electrode, and it is clear that the case of the present invention exhibits longer life characteristics.

Example 2 Example 1 was replaced with the following graphitized carbon powder (contact angle was in the range of about 100 to 200 °) instead of the graphite powder of Example 1 (USSP of Nippon Graphite Co., Ltd.). Electrodes and batteries were prepared in the same manner as in, and measurements were performed, and almost the same results were obtained. AUP made by Nippon Graphite Co., Ltd. (specific surface area 150 m ² /
g), its CSSP (specific surface area 200 m ² / g), Lonza, Switzerland
Graphite HSAG-13,14,15,17 (specific surface area 113,12
0,123,205m ² / g), Tokyo Denki Kagaku Kogyo Denka Black (average particle size 420Å, specific surface area 70m ² / g).

Example 3 An electrode was obtained in the same manner as in Example 1 except that the following hydrophilic carbon powder (contact angle was in the range of about 50 to 90 °) was used in place of the carbon black powder in Example 1, and When a battery was made and measured in the same manner, almost the same results as in Example 1 were obtained.

Vulcan XC-72, Vulcan XC-72R (specific surface area 254 m ² / g, average particle size 30 mμ), Regal660R (specific surface area 112 m ² / g, average particle size 24 m
μ), Monarch 1100 (specific surface area 240 m ² / g, average particle size 14 m
μ), Monarch 1300 (specific surface area 560 m ² / g, average particle size 13 m
μ).

Example 4 Instead of the carbon paper in Example 1, carbon paper with ribs manufactured by Nippon Carbon Co., Ltd. (density 0.45 g / cm ³ ,
When a thickness of 2 mm and a porosity of 75 to 80%) were used and electrodes and batteries were prepared in the same manner using the catalyst mixture of Examples 1 to 3, the same results as in Example 1 were obtained. Was obtained.

[Brief description of drawings]

Drawing FIG. 1 is a graph showing the polarization characteristics of a unit cell using the electrode of the present invention and a unit cell using the conventional electrode in comparison,
The figure is a graph showing the life characteristics of the same cells in comparison.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Ho Jo Jo 1 Komukai Toshiba-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Tokyo Shibaura Electric Co., Ltd. Research Institute (56) Reference JP-A-57-141871 (JP, A) )

Claims (1)

[Claims] 1. A catalyst mixture prepared by supporting a noble metal-based catalyst on graphitized carbon powder and binding with a fluororesin is applied to a porous carbon substrate and baked to form an electrode catalyst layer on the substrate. In a method for producing a gas diffusion electrode comprising a step of forming, a noble metal catalyst is supported on a graphitized carbon powder, and then a hydrophilic carbon powder is added to the gas diffusion electrode. Production method.