JPS6231788B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fuel cell electrode containing a conductive water repellent made of partially fluorinated graphite in a fuel cell using phosphoric acid as an electrolyte.

Conductive water repellents are important for preventing wetting of solids by liquids in solid-liquid three-phase reaction systems involving electrochemical reactions. For example, in a fuel cell that uses phosphoric acid as an electrolyte, when the cell is operated, the electrolytic layer in the electrode gets wetted by the electrolyte, so the ratio of solid-liquid three-phase interfaces that cause cell reactions decreases. Since the performance of the battery tends to deteriorate, in order to prevent this, it is necessary to mix a water repellent agent in the electrode to prevent wetting of the dissolving medium layer.

Conventionally, polytetrafluoroethylene has generally been used as this water repellent. However, in this case, the water repellency improves as the amount of polytetrafluoroethylene mixed increases, but since polytetrafluoroethylene is an insulator, increasing the amount mixed increases the internal resistance of the electrode and improves battery performance. There is a problem in that it causes a decrease in That is,
Attempts to extend battery life by improving water repellency have the opposite effect of decreasing battery performance. Therefore, in order to improve this drawback, attempts have been made to mix a small amount of graphite fluoride ((CF)n), which has greater water repellency than polytetrafluoroethylene, and use a small amount of polytetrafluoroethylene as a binder. ing. In this case, the graphite fluoride is a combination of fluorine atoms and carbon atoms in a ratio of 1:1. The water repellency of graphite fluoride is greater than that of polytetrafluoroethylene, and its contact angle with water is approximately 140 degrees, which is 30 degrees larger than that of polytetrafluoroethylene, which is approximately 110 degrees. Therefore, this method is considered to be effective in preventing wetting of the catalyst, but since graphite fluoride is an insulator like polytetrafluoroethylene, the internal resistance of the electrode increases and the battery There is a problem that performance is degraded.

An object of the present invention is to provide a gas diffusion electrode for a fuel cell that has high water repellency and contains a conductive substance and has low internal resistance.

The present invention was completed based on the experimental result that in a compound of fluorine and carbon, when the ratio of fluorine atoms to one carbon atom is lower than 1, both electrical conductivity and water repellency are exhibited. be.

Partially fluorinated graphite represented by the general formula CFx (0<x<1) is already known (Kagaku, 1975
February issue, p.23). However, its conductivity and water repellency are not yet known, and its use as a conductive water repellent as in the present invention is new.

To produce such partially fluorinated graphite, for example, 1 mole of graphite and F ₂ gas are mixed.
0.5x mol in a reaction vessel and heated to 380℃~450℃
The desired atomic weight ratio can be easily obtained by heating to
You can get CFx.

In the present invention, a conductive water repellent made of the above-mentioned partially fluorinated graphite is added to a fuel cell electrode in order to enhance the water repellency of the electrode and reduce the internal resistance of the electrode. . It is sufficient to use a small amount of insulating polytetrafluoroethylene as a binder.

To manufacture such an electrode, a mixture of a catalyst, a binder, and partially fluorinated graphite may be applied to a substrate of carbon fiber, a wire mesh, or a porous metal plate, and then fired. As the catalyst, platinum, rhodium, indium, ruthenium, osmium, silver, copper, cobalt, etc. supported on a carrier such as acetylene black can be used.

Another method for manufacturing such electrodes is to mix a carrier such as acetylene black, a binder, and partially fluorinated graphite in advance, and then mix this with a solution of the salt of the catalyst metal described above, and then add hydrogen to the solution. It is also possible to carry out a method in which the material is reduced and supported by sintering.

The present invention will be explained in more detail below using Examples.

Examples 1 to 5 1 mol of graphite (average particle size 30 μm) and F ₂ gas were 0.005 mol (Example-1), 0.1 mol (Example-2), 0.2 mol (Example-3), and 0.3 mol, respectively. (Example-4) and 0.4 mol (Example-5) were sealed in a reaction container and kept at 400°C for 1 hour.
_{CF 0.01} , CF _0.2 , CF _0.4 , _{CF 0.6} , _{and CF 0.8 were produced .}

In order to examine the water repellency of these partially fluorinated graphites, the contact angle with respect to water was measured. The results are shown in FIG. The larger the contact angle, the greater the water repellency. FIG. 1 also shows the contact angle of polytetrafluoroethylene for reference. From Figure 1 x
When the value of is 0.01 or more, the contact angle is larger than that of polytetrafluoroethylene, indicating that the material has excellent water repellency.

Next, the resistance value was measured to examine the conductivity of this CFx. The results are shown in FIG. When the value of x is 1, it becomes an insulator, but when x is smaller than 1, it has electronic conductivity. In particular, when x becomes 0.2 or less, the resistance value becomes 1000Ω·cm ^-1 or less.

Therefore, as is clear from FIGS. 1 and 2, CFx satisfying 0.01≦x≦0.2 is preferable from the viewpoint of conductivity and water repellency.

Example 6 Carbon fiber cloth (thickness 0.3 mm) was used as the electrode substrate.
90% porosity) was used. As an electrode catalyst, 10 wt% (wt%) of platinum was supported on acetylene black, and 15 wt% of CFx (x = 0.01) was used.
After mixing, 10wt% polytetrafluoroethylene was further mixed as a binder to form a paste.
It was applied to the electrode substrate. The amount of catalyst was selected so that the amount of platinum per 1 cm ² of the electrode substrate was 0.5 mg. The thickness of the applied catalyst layer is approximately 0.1 mm. This electrode was baked at 300 to 350°C for 15 minutes to obtain an electrode. Next, this electrode was used in a fuel cell that uses phosphoric acid as an electrolyte and hydrogen and air as fuel and oxidizing agents, and its characteristics were measured. Regarding the results, the cell voltage-current density characteristics are shown in A of Figure 3, and the current density is
A in FIG. 4 shows the discharge characteristics of the cell voltage over time when the voltage was 15 mA/cm ² .

Examples 7 to 10 The value of x of CFx is 0.2 (Example-7)
Electrodes were manufactured in the same manner as in Example 6, except that the values were 0.4 (Example 8), 0.6 (Example 9), and 0.8 (Example 10), and used in a fuel cell in the same manner. Characteristics were measured. The results are shown in FIGS. 3 and 4. In the figure, B is x=0.2, C is x=0.4,
D represents x=0.6 and E represents x=0.8, respectively.

Example 11 CFx (x=0.01, 0.2,
0.4, 0.6, 0.8) were mixed at 15 wt%, a platinum salt solution was mixed therein, and the mixture was reduced to support platinum. The amount of supported platinum is 10wt%, which is the same as in Examples 6 to 10.
And so. The catalyst containing CFx obtained by this preparation method was applied onto a carbon fiber cloth in the same manner as in Examples 6 to 10 to prepare an electrode. The performance of the fuel cell using this electrode was very similar to that shown in FIGS. 3 and 4. That is, it has been found that the same effect can be obtained even if carbon powder and CFx (0<x<1) are mixed in advance and platinum is supported on the mixture.

Comparative Example 1 A gas diffusion electrode for a fuel cell was manufactured according to a conventional method. That is, in the method of Example 6, the amount of CFx added was 0%, and instead, 30 wt% of polytetrafluoroethylene was added. The electrode thus obtained was used in Example-6
It was also used in a fuel cell and its characteristics were measured. The results are shown in G in FIG. 3 and G in FIG. 4.

3 and 4, the electrode using the conductive water repellent according to the present invention has cell voltage-current density characteristics,
It is recognized that this material is significantly superior to conventional products in terms of discharge characteristics and discharge aging characteristics.

In addition, the amount of polytetrafluoroethylene added
I was able to reduce it to 1/3 or less.

Furthermore, when the internal resistance of the electrodes obtained in Examples 6 and 7 and Comparative Example 1 was measured, it was 4 to 5 Ωcm ^-1 in Comparative Example 1, while it was 0.2 to 0.5 in Examples 6 and 7. We were able to reduce it by 1/10 to 1/20 to Ωcm ^-1 . Therefore, if the electrode of the present invention is used, the voltage drop will be small even when a large current is drawn, and
In addition, since it has high water repellency, it prevents the catalyst from getting wet, making it possible to produce a long-life fuel cell.

As described above, according to the present invention, it is possible to obtain a gas diffusion electrode for a fuel cell that has high water repellency and low internal resistance.

[Brief explanation of the drawing]

FIG. 1 is a characteristic diagram showing the relationship between the x value of CFx and the contact angle with water, and FIG. 2 is a characteristic diagram showing the relationship between the x value of CFx and resistance. FIG. 3 is a cell voltage-current density characteristic diagram of the fuel cell electrode according to the present invention, and FIG. 4 is a characteristic diagram showing the temporal change characteristics of the cell voltage during constant current discharge.

Claims (1)

[Claims] 1. In a fuel cell electrode using phosphoric acid as an electrolyte, a catalyst, a water-repellent binder made of polytetrafluoroethylene, and a compound with the general formula _CF
An electrode for a fuel cell, characterized in that it is made of a mixture of conductive water-repellent partially fluorinated graphite expressed by the formula (0.01≦X≦0.2) and has a fired layer.