JPH0388272A - Electrode of phosphoric acid type fuel cell and manufacture thereof - Google Patents

Abstract

PURPOSE:To increase the output performance and life of a cell without the use of multilayer catalyst structure by using an electrode with one electrode catalyst layer in which the distribution of fluororesin is continuously, gradually increased from matrix side to electrode base material side. CONSTITUTION:The distribution of fluororesin 11 in an electrode catalyst layer 5a is continuously, gradually increased from a matrix 7 side to an electrode base material 3 side so that phosphoric acid in the electrode catalyst layer 5a is continuously, gradually decreased from the matrix 7 side to the electrode base material 3 side. The electrode catalyst layer 5a formed on the electrode base material 3 is not necessary to be formed with multilayer structure in which contents of fluororesin are different at every layer, and conflicting relation is satisfied with only one layer. Ion migration in a phosphoric acid solution film and gas supply are kept good. Output performance of a cell is increased and increased performance is retained for a long time.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrode for a phosphoric acid fuel cell and a method for manufacturing the same.

[Conventional technology]

FIG. 4 is a schematic perspective view showing the constituent members of a phosphorus type II fuel cell and their arrangement. In FIG. 4, the air electrode 1 and the fuel electrode 2 are, for example, rhinoceros
1. On top of the porous ribbed electrode base material 3.4 made of carbon fiber woven fabric with a thickness of 2 m to 2 m, air containing a catalyst in which noble metals such as platinum are supported on carbon black and a fluororesin as a binder is uniformly mixed. Electrode catalyst layer 5. The fuel electrode catalyst layer 6 is coated to a thickness of approximately Q, l Ig, and the electrode catalyst layer 5 of the air electrode 1 is coated with a small amount of fluororesin in SiC fine powder of approximately 0.1 cm. A thickly coated matrix 7 is provided, and this matrix 11 is impregnated with a phosphoric acid electrolyte.

As shown in FIG. 4, the phosphoric acid fuel cell is constructed by connecting a ribbed air electrode 1, a fuel electrode 2 to an air electrode catalyst layer 5 through a matrix 7. Air electrodes 1. are arranged so that the main surfaces of the fuel electrode catalyst layers 6 face each other. By installing a conductive and gas-impermeable separator plate 8 on the rib side of the fuel electrode 2 to prevent reaction gases from mixing with each other,
Air passage 9 to air electrode l. A fuel passage lO is formed in the fuel electrode 2.

As mentioned above, a phosphoric acid fuel cell is a battery impregnated with an electrolyte.
A large number of single cells each consisting of an air electrode 1 and a fuel electrode 2 each having an electrode catalyst layer 5.6 interposed therebetween are stacked with separator plates 8 in between, and the whole is tightened in the stacking direction to form a fuel cell cell stack. It is designed so that the desired output can be obtained by configuring it.

The output voltage that can be obtained when a phosphoric acid fuel cell having such a structure is operated at a certain current density is generally lower than that of the electrode 1 containing less fluororesin in the electrode catalyst layer 5.6.
．． When electrodes 1 and 2 are used, they exhibit good characteristics, but the characteristics deteriorate rapidly.On the contrary, when electrodes 1 and 2, which contain a large amount of fluororesin in the electrode catalyst layers 5 and 6, are used, the output voltage decreases. Although the properties are inferior, the deterioration of the properties is slow. As described above, regarding the amount of fluororesin in the bridge catalyst layer 5.6, the output voltage characteristics and the rate of decrease thereof have contradictory tendencies. This is because the amount of phosphoric acid in the electrode catalyst layers 5 and 6, the shape of phosphoric acid distribution W, and its change over time are controlled by the content of fluororesin, which is a water repellent. catalyst layer 5,
In electrode 1.2 equipped with 6, the output voltage characteristics of the battery are low due to the small amount of phosphoric acid in the [ffi catalyst layers 5 and 6, but this state can be maintained stably, so the deterioration in characteristics is comparatively low. When using the electrode 1.2 equipped with the electrode catalyst layer 5.6, which has a small surface area and a low content of resin, the amount of phosphoric acid in the electrode catalyst layer 5.6 is large and the movement of ions during phosphoric acid liquid exchange is reduced. Since it is easy to This results in a decline.

One of the solutions to this problem is to
The electrode catalyst layers 5 and 6 have a multilayer structure with respect to the fluororesin content. That is, after first forming an electrode catalyst layer with a high fluororesin content on the base materials 3 and 4 of the electrode 1.2, an electrode catalyst layer with a lower amount of fluororesin: '*m is formed thereon, and then A plurality of electrode catalyst layers are stacked to reduce the fluororesin content, and usually two or three layers are used. By making the t-electrode catalyst layer 5.6 have a multilayer structure in this way, the electrode base material 3
, 4 can maintain a low amount of phosphoric acid and ensure gas supply to the electrode catalyst layer 5.6.

[Problem to be solved by the invention]

However, as stated above, <conventional phosphorus II! ! It
For fuel cells, the output voltage characteristics and the speed of deterioration of those characteristics, i.e.
There is a contradictory relationship with the lifespan of a, and even if one can be improved, it is difficult to satisfy both.As a countermeasure, we have developed a multilayer electrode in which the amount of fluororesin is gradually reduced from the electrode base material side. There are also problems in forming the catalyst layer.

In order to form such a multi-layered electrode catalyst layer, the manufacturing process is 2 to 3 times longer than that of a normal electrode catalyst layer, and in addition, phosphoric acid and gas passages are required at the interface of each electrode catalyst layer. Since the connection between the battery and battery becomes discontinuous, it is not satisfactory even though the characteristics and life of the battery have been improved.

The present invention was developed in view of the above points, and its purpose is to reduce the amount of fluororesin contained in the upper part of one electrode catalyst layer to the surface, that is, to the cells, without making the electrode catalyst layer have a multilayer structure. An object of the present invention is to provide an electrode for a fuel cell in which the amount of phosphoric acid increases continuously from the matrix side toward the electrode base material side, and a method for manufacturing the same.

[Means to solve the problem]

In order to solve the above problems, the present invention forms an electrode catalyst layer on a carbon base material, then heats the surface of this electrode catalyst layer to a temperature higher than the evaporation temperature of the fluororesin, and The electrode is fabricated by heat treatment so that the temperature gradient is below the evaporation temperature of the fluororesin in the vicinity of The electrode is provided with an electrode catalyst layer that gradually increases toward the top.

[Effect]

As described above, in the uninvented electrode, the amount of fluororesin contained in one electrode catalyst layer increases continuously from the matrix side to the base material side, so that the soot layer of the electrode with different amounts of fluorine W can be used. Without forming multiple layers, the amount of phosphoric acid in the fuel cell decreases continuously from the matrix side to the base material side, and the movement of ions in the phosphoric acid and the supply of gas are sufficient. Since this phosphoric acid distribution state can be maintained for a long period of time, both the output characteristics and the life span of the battery can be improved at the same time.

〔Example〕

The present invention will be explained below based on examples.

Figure 1 shows the electrode catalyst layer 5 of the phosphoric acid fuel cell shown in Figure 4.
FIG. 6 is a partially enlarged sectional view schematically showing an electrode catalyst layer 5a according to the present invention corresponding to No. This figure shows only the distribution state of the fluororesin 11, which increases continuously toward .

The most common fluororesin 11 is polytetrafluoroethylene (hereinafter referred to as PTFE). P T F E contained in one electrode catalyst layer 5a
In order to give No. 11 a distribution state as shown in the 1s1 diagram, after preparing an electrode catalyst layer with a higher content of PTFEII than usual on the electrode base material 3, the side contacting the electrode base material 3 is prepared in an inert gas atmosphere. While keeping the temperature of the electrode catalyst layer below the evaporation temperature of PTFEll, the thickness of the electrode catalyst layer is adjusted so that the temperature of the surface side of the electrode catalyst layer on the opposite side is above the evaporation temperature of PTF E u. This can be obtained as the electrode catalyst layer 5a by performing heat treatment with a temperature gradient in the horizontal direction. Of course, in the present invention, other electrode catalyst layers corresponding to, for example, the electrode catalyst layer 6 shown in FIG. 4 can also be produced by the same method.

Next, I will talk about the method. First, a catalyst in which noble metals such as platinum are supported on carbon black and 6 wt% PTF
An electrode catalyst layer consisting of EII is formed by coating on a porous carbon electrode base material, but at this time care must be taken to ensure that the catalyst and PTFEll are uniformly mixed by using a surfactant or the like. The thickness of this electrode catalyst layer is made by thoroughly mixing carbon fibers that support a metal that will serve as a catalyst and carbon fibers that do not support a metal, so that the thickness of the electrode catalyst layer is less than L/200 pet. do. For example, in order to protect the thus obtained electrode corresponding to electrode 1 shown in FIG. This electrode is heated by irradiating infrared rays from the surface of the seed catalyst layer and keeping the temperature on the ME electrode base material side of the electrode catalyst layer at 500°C or less and the temperature on the surface side of the electrode catalyst layer at 8535 to 545°C for 1 minute. Electrode catalyst layer 5a
is obtained. The weight of the electrode after heat treatment was measured, and the average PTFE content of the electrode catalyst layer 5a was found to be 45 wt%.
It is.

The above heat treatment temperature was determined based on the diagram in FIG. 2 showing the results of thermogravimetric-differential thermal analysis (TG-DTA) of the electrode catalyst layer 5a measured in a sl atmosphere. Second
In the figure, song #(a) is a thermogravimetric analysis curve, and song *(
B) shows the differential thermal analysis curve.

Area A of song Ml (a) is the weight loss due to evaporation of P T F E 11, and area B of song f1 (b) is the weight loss due to evaporation of P T F E 11.
Endothermic peak due to melting of F E 11, region C is PTF
This represents an endothermic peak due to the evaporation of EII. From the curves (A) and (0) in Figure 2, P T F E
It can be seen that the 11 cD evaporation temperature is in the range of 530-575°C. In the present invention, in order to remove only a portion of PTFEll, it is suitable to set the temperature on the surface side of the soot layer of the electrode PfX medium layer in the heat treatment to 535 to 545°C. When this temperature was maintained for 1 minute, the temperature on the electrode base material side of the electrode catalyst layer 5a did not reach 500°C, and the electrode MllIj
It is possible to obtain the required temperature gradient in the thickness direction of the L15 monument.

The longitudinal section of the electrode having the electrode catalyst layer 5a according to the present invention was observed with a scanning electron microscope (SEM), and the results showed that 1jt
In the polar catalyst layer 5a, more P 1' F E u exists in the region closer to the electrode base material side in the thickness direction, and as it deviates toward the surface, it becomes more concentrated as shown in the schematic cross-sectional view in FIG. , PTFEu gradually and continuously decreases.
It can be kept secret.

Next, FIG. 3 shows the results of assembling and operating a cell using the electrode having the electrode FjlIs layer 5a obtained as described above as an air electrode. FIG. 3 is a diagram showing the relationship between operating time and output voltage. For comparison, the electrode catalyst layer described above is made of three layers of PTFE, and the average PTFE content in the electrode catalyst layer is 45w. The case of a conventional cell in which an electrode prepared so as to have a % of The curve (a) in FIG. 3 represents the non-inventive case. The curve (b) represents the conventional case. As shown in Figure 3, the conventional cell shown by the curve (b) has a low output from the beginning and also decreases greatly over time, whereas the output of the conventional cell shown by the curve (b) is low from the beginning and decreases significantly over time. The cell of the present invention (a) has a higher output than the conventional cell from the initial state, and its decrease over time is extremely small. In this way, the fuel cell using the electrode of the present invention solves the conflicting relationship between output and life caused by the PTFE content at once with one electrode catalyst layer formed on the electrode, and achieves good characteristics. It is very stable and can last and operate.

〔Effect of the invention〕

Phosphoric acid amount Conventionally, the fluororesin contained in the electrode catalyst layer provided on the electrode of a fuel cell was evenly distributed, so depending on the degree of fluororesin absorption, there is a conflicting relationship between the movement of ions in the phosphoric acid liquid film and the gas supply. However, as described in the examples, in the present invention, after forming the electrode catalyst layer, heat treatment is performed to impart a temperature gradient in the thickness direction. By applying this, the distribution of fluororesin in the electrode catalyst layer is gradually increased in the thickness direction from the matrix side to the electrode base material side, and the phosphoric acid in the electrode catalyst layer is removed from the electrode base material. Since the electrode catalyst layer formed on the electrode base material is made to gradually decrease in the thickness direction toward the side, the above-mentioned contradictory relationship can be achieved with just one layer, without having to make the electrode catalyst layer formed on the electrode base material a multilayer structure with different fluororesin contents. This solves the problems all at once, making it possible to maintain good conditions for both the movement of ions in the phosphoric acid liquid film and the gas supply.As a result, the output characteristics of the battery are improved, and these characteristics can be maintained for a long period of time. The great effect was that it was sustainable.

[Brief explanation of drawings]

FIG. 2 is a partially enlarged sectional view schematically showing the electrode catalyst layer of the electrode according to the present invention, FIG. 2 is a thermogravimetric analysis curve diagram and a differential thermal analysis curve diagram measured for the electrode catalyst layer of the electrode according to the present invention, Fig. 3 is a diagram showing the relationship between the operating time and output voltage of the fuel cell obtained by the invention, and Fig. 4 is a schematic perspective view showing the constituent members of the phosphoric acid fuel cell and their arrangement relationship. It is a diagram. l: air electrode, 2: fuel electrode, 3, 4: electrode base material, 5:
g! Gas electrode catalyst layer, 5a: Soot layer, 6: Fuel electrode catalyst layer, 7: Matrix, 8: Senof rate plate, : Air passage, 10: Fuel passage, No. llm 200 00 00 371/°C 00 00 000 20003000 1 000 soo. Figure 3

Claims (1)

[Scope of Claims] 1) An electrode catalyst layer in which a catalyst in which noble metals are supported on carbon black and a fluororesin are mixed on a surface opposite to the ribs on a porous carbon base material provided with ribs on a predetermined surface. and a matrix of SiC impregnated with an electrolyte interposed between each of the electrode catalyst layers of a pair of opposing electrodes, the electrode of a phosphoric acid fuel cell comprising: An electrode for a phosphoric acid fuel cell, wherein the fluororesin contained in the electrode catalyst layer gradually increases from the matrix side toward the base material in the thickness direction of the electrode catalyst layer. 2) In manufacturing the electrode according to claim 1, after forming an electrode catalyst layer containing a slightly larger amount of the fluororesin on the surface of the base material opposite to the ribs, the electrode catalyst layer is formed in an inert gas atmosphere. Heat treatment is performed such that the free surface of the electrode catalyst layer is heated to a temperature higher than the evaporation temperature of the fluororesin, and the temperature gradient near the side where the electrode catalyst layer contacts the base material is lower than the evaporation temperature of the fluororesin. Features: Phosphoric acid content fuel cell electrode manufacturing method.