JPH061700B2 - Composite electrode for fuel cell - Google Patents

Description

TECHNICAL FIELD The present invention relates to the structure of a composite electrode for a fuel cell, and more particularly to the structure of a water repellent agent for a composite electrode electrode catalyst layer.

[Conventional technology]

A fuel cell directly converts the chemical energy of the fuel into electric energy, and its structure is such that an anode 9A and a cathode 9B are arranged with a matrix 8 containing phosphoric acid interposed between them as shown in FIG. Are stacked via a separate plate 10, a fuel gas and an oxidant gas are supplied to each electrode from an external gas supply system, and the fuel gas and the oxidant gas are electrochemically supplied on the electrode catalyst layers 5A and 5B of each electrode. Reaction,
As a result, electric energy is taken out of the system.

Each of the electrodes of the anode 9A and the cathode 9B is constituted by depositing an electrode catalyst layer 5 on a ribbed electrode substrate 4 as shown in FIG. In the electrode catalyst layer 5, the catalyst fine particles 7 in which the precious metal fine particles 1 are carried on the surface of the catalyst carrier 2 are the fine particles 3 of the fluororesin.
It is configured by being bound by. Inside the electrode catalyst layer 5, the gas from the ribbed electrode base material 4 side and the electrolytic solution from the matrix 8 side come into contact with each other to form a three-phase interface and an electrochemical reaction proceeds.

In order to efficiently and stably carry out this electrochemical reaction, it is necessary to increase the number of three-phase interfaces between the catalyst fine particles 7 in the electrode catalyst layer and the gas of the electrolytic solution and to improve the diffusion of the supply gas and the generated gas. is necessary. For example, the electrode reaction of the anode on the fuel gas supply side is H ₂ → 2H ⁺ + 2e (1), and the electrode reaction of the cathode on the oxidant gas supply side is 1 / 2O ₂ + 2H. ^＋＋ 2e → H ₂ O ……… (2). As shown by the above reaction, water vapor is generated in the cathode electrode by the electrode reaction, but since the diffusivity of this water vapor is smaller than the hydrogen diffusivity required for the anode reaction, the electrolytic mass in the electrode is It is necessary to make the cathode relatively smaller than the anode to increase the diffusion amount of water vapor.

Therefore, conventionally, the calcination temperature of each electrode containing a fluororesin as a water repellent was changed to change the water repellency of the electrode catalyst layer, thereby adjusting the electrolytic mass of the anode and the cathode.

[Problems to be solved by the invention]

However, recently, instead of forming the anode 9A and the cathode 9B of the fuel cell separately, a composite electrode in which the anode and the cathode are integrally formed has been proposed. As shown in FIG. 5, this composite electrode has a structure in which an anode 9A and a cathode 9B are joined via a separate plate 10.
Such a composite electrode includes a ribbed electrode substrate and a separate plate 10.
The electrode catalyst layers 5A and 5B on the respective electrodes are simultaneously adhered to each other to form the composite electrode substrate after being bonded via. The composite electrode has the effect of reducing the electrical resistance and thermal resistance between the separate plate 10 and the anode 9A or the cathode 9B and facilitating the handling of the electrode. However, in such a composite electrode, since the electrode catalyst layer 5 is fired at the same temperature, there is a problem that the water repellency of each electrode cannot be adjusted by changing the firing temperature.

The present invention has been made in view of the above points, and an object thereof is to provide a composite electrode capable of making the anode and the cathode have different water repellency even if they are fired at the same temperature.

[Means for solving problems]

According to the invention described above, the electrode catalyst layer containing the water repellent agent is thermocompression bonded to both surfaces of the composite electrode substrate in which the ribbed electrode base material is bonded via the separate plate to integrally integrate the anode and cathode electrodes. In the formed composite electrode for a fuel cell, the content of the water repellent 3 contained in the anode side electrode catalyst 5A is higher than the content of the water repellent 3 contained in the cathode side electrode catalyst layer 5B. This is achieved by providing less electrode catalyst layer. A fluororesin dispersion or the like is used as the water repellent. The amount of the water repellent agent in the anode side electrode catalyst layer is 30 to 80 in the cathode side electrode catalyst layer.
% To be adjusted.

[Function] The fluororesin has water repellency. Since the amount of fluororesin in the electrode catalyst layer on the anode side is smaller than the amount of fluororesin in the electrode catalyst layer on the cathode side, the water repellency of the anode electrode catalyst layer is better than that of the cathode even if it is fired at the same temperature. Also becomes smaller.

〔Example〕

Water containing a surfactant is added to the carbon catalyst carrier 2 supporting the platinum fine particles 1 to disperse the catalyst carrier well by ultrasonic waves, and 40 to 60% by weight of fluororesin dispersion is added as a fluororesin. Well mixed to prepare a dispersion liquid of the catalyst carrier and the fluororesin. An aqueous solution organic solvent is added to and mixed with this dispersion liquid to coagulate the fluororesin and the catalyst carrier together for solid-liquid separation. The obtained solid content is thoroughly kneaded to prepare an electrode paste. This paste is rolled by a calender roll, formed into a sheet, and then dried to prepare an electrode film used for the cathode side electrode catalyst layer. The electrode film used for the electrode catalyst layer on the anode side is the same as that described above, but the amount of fluororesin is adjusted to 30 to 80% of that of the cathode electrode film. Both electrode films are simultaneously bound to both surfaces of a monolithic composite electrode substrate having a porous ribbed electrode base material on both sides of a separate plate at a fluororesin melting temperature of 320 to 380 ° C. A fuel cell was constructed using the obtained composite electrode, and the cell was tested for current-voltage characteristics and durability.

[Comparative example]

The electrode film used for the electrode catalyst layer on the cathode side was adhered to both surfaces of the composite electrode substrate to form a composite electrode, and the characteristics of the fuel cell using this composite electrode were measured.

Fig. 1 shows the current-voltage characteristics of the fuel cell, and Fig. 2 shows the current density.
The durability of the relationship between time and voltage at 0 mA / cm ² is shown. In FIG. 1 and FIG. 2, 21A and 21B are 22 when the anode and the cathode of the conventional method which does not adopt the composite electrode method are separately formed.
A and 22B are cases according to the comparative example, and 23A and 23B are cases according to the example of the present invention. The characteristics in the case of this example are excellent.

〔The invention's effect〕

According to the present invention, the anode and cathode electrodes are integrally formed by thermocompression bonding the electrode catalyst layer containing the water repellent agent on both surfaces of the composite electrode substrate in which the ribbed electrode base material is joined via the separate plate. In the composite electrode for a fuel cell, the anode side is provided with the electrode catalyst layer in which the content of the water repellent agent contained in the electrode catalyst layer is smaller than the content of the water repellent agent contained in the cathode side electrode catalyst layer. Side electrode catalyst layer is less water repellent than the cathode side electrode catalyst layer, the anode contains more electrolyte relative to the cathode, and the cathode contains less electrolyte, which is why In addition, the diffusion of water molecules will be good, and the characteristics and durability of the fuel cell will be good.

[Brief description of drawings]

1 is a current-voltage characteristic diagram of a fuel cell using a composite electrode according to an embodiment of the present invention, and FIG. 2 is a time-voltage characteristic diagram of a fuel cell using a composite electrode according to an embodiment of the present invention. FIG. 4 is a perspective view showing the structure of a conventional fuel cell, FIG. 4 is a sectional view showing the structure of an electrode, and FIG. 5 is a perspective view showing the structure of a composite electrode. 3: Water repellent agent, 5A: Anode-side electrode back layer, 5B: Cathode-side electrode catalyst layer.

Claims (1)

[Claims] 1. An anode and a cathode are integrally formed by thermocompression bonding an electrode catalyst layer containing a water repellent on both surfaces of a composite electrode substrate in which ribbed electrode base materials are joined via a separate plate. The composite electrode for a fuel cell is provided with an electrode catalyst layer in which the content of the water repellent agent contained in the electrode catalyst layer on the anode side is smaller than the content of the water repellent agent contained in the electrode catalyst layer on the cathode side. Characteristic composite electrode for fuel cell.