JPH04366557A - Manufacture of porous carbon substrate - Google Patents

Abstract

PURPOSE:To easily form a seal part capable of certainly preventing the outflow of a reaction gas at the end part of a porous carbon substrate which is the constituting member of a fuel cell containing phosphoric acid as an electrolyte. CONSTITUTION:A porous carbon substrate 5 is formed by carbonizing treatment, and after it subjected to graphitizing treatment, the reaction gas passing directional end part of the porous carbon base plate 5 is subjected to the treatment of highly charging a fine carbon fiber 6 by means of vacuum sucking method to form a highly charged seal part 9. As the fine carbon fiber 6, for example, a porous carbon substrate graphitized by graphitizing treatment or a porous carbon substrate 5 carbonized by carbonizing treatment is crushed and used.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell using phosphoric acid as an electrolyte, and more particularly to a method for manufacturing a porous carbon substrate used as an electrode.

0002

2. Description of the Related Art Conventionally, fuel cells have been known as devices that directly convert chemical energy contained in fuel into electrical energy. Generally, this fuel cell has a pair of porous electrodes placed with an electrolyte-impregnated matrix in between, and a fuel gas such as hydrogen is brought into contact with the back surface of one electrode, and a gas such as oxygen is brought into contact with the back surface of the other electrode. This is a device that extracts electrical energy from between electrodes by bringing an oxidizing gas into contact and utilizing the electrochemical reaction that occurs. As long as fuel gas and oxidant gas are supplied to each electrode, electrical energy can be extracted with high conversion efficiency.

[0003] Furthermore, the porous electrode of the fuel cell as described above is composed of a porous carbon substrate, and the electromotive force of a unit cell composed of a pair of such porous electrodes and a matrix is at most It is about 1V. Therefore, in order to obtain a practical-scale power generation device using such unit cells, it is necessary to stack tens to hundreds of unit cells.

[0004] Normally, when stacking a large number of unit cells like this, a carbon separator with high density and high conductivity is used. Different separators are used depending on the shape of the electrodes constituting the unit cell. In other words, when both electrodes are made of smooth porous carbon substrates, a separator is used in which gas flow passages are formed in different directions on the upper and lower surfaces, and gas flow passages are provided on both electrodes. If a porous carbon substrate is used, a smooth separator is used.

As an example of a fuel cell, FIG. 4 shows a basic configuration example of a ribbed electrode type fuel cell unit cell using phosphoric acid as an electrolyte. In FIG. 4, a fuel electrode 2a and an oxidizer electrode 2b, which are a pair of ribbed electrodes 2, are arranged opposite to each other with a matrix layer 1 holding phosphoric acid as an electrolyte sandwiched therebetween, thereby forming a unit cell of a fuel cell. It is configured. A plurality of such unit cells are prepared and stacked with separators 3 in between to form a fuel cell.

In this case, the fuel electrode 2a and the oxidizer electrode 2b, which are the pair of ribbed electrodes 2, are usually made of carbon material, and each back surface has a plurality of grooves formed in parallel in directions perpendicular to each other. Catalyst layers 4a and 4b are formed on the opposing surfaces, respectively. The grooves formed in the fuel electrode 2a and the oxidizer electrode 2b are as follows:
It serves as a flow path for fuel gas and oxidant gas. Also,
The separator 3 is configured to be inductive and non-permeable.

[0007]

[Problems to be Solved by the Invention] By the way, in the fuel cell configured as described above using phosphoric acid as an electrolyte,
In order to maintain stable high performance over a long period of time, it is essential to proceed with a stable electromotive reaction over a long period of time. To achieve this, it is necessary to sufficiently seal a healthy catalyst layer and the active material (reactive gas) that facilitates these reactions, and to enhance the sealing function to separate the two from each other. . However, in the conventional fuel cell configured as described above using phosphoric acid as an electrolyte, the sealing function is insufficient, and the reaction gas mainly composed of hydrogen, which is the active material of the fuel electrode, and the activation of the oxidizer electrode. Reactive gas containing the substance oxygen sometimes leaked out from the electrode end. The reaction gas that has flowed out in this way may react in a way other than the electromotive reaction as external output from the battery, or a mixture of contradictory reaction gases (gas mainly composed of hydrogen and gas containing oxygen) may occur. Therefore, there was a problem that a stable electromotive reaction could not proceed for a long period of time.

[0008] In response to the need for such a sealing function, conventionally, for example, a PFE (polyfluoroethylene) film to prevent gas leakage has been attached to the edge of a porous carbon substrate constituting a porous electrode. A method of welding and a method of forming a highly filled seal portion at the end of the porous carbon substrate before graphitizing the porous carbon substrate have been considered. However, although the former method of PFE film welding has a sufficient sealing function, it requires a large number of processing steps and is difficult to realize. In addition, in the latter method of forming a highly filled seal part, it is difficult to improve the dimensional accuracy of the seal part, and the difference in density between the inside and the edge makes it difficult to form a porous carbon substrate after graphitization. There were problems such as distortion.

The present invention was proposed in order to solve the problems of the prior art as described above, and its purpose is to solve the problems of the prior art as described above. By providing an excellent method for manufacturing a porous carbon substrate, which can easily form a sealing part that can reliably prevent the outflow of reactive gases (gases mainly composed of hydrogen and gases containing oxygen). be.

0010

[Means for Solving the Problems] A method for producing a porous carbon substrate according to the present invention is used as an electrode of a fuel cell, and supports a catalyst layer on one side of the substrate for promoting electrode reaction, and supports a catalyst layer on the other side. In a method for manufacturing a porous carbon substrate in which a reactive gas containing hydrogen as a main component or an oxidizing reactive gas flows in a fixed direction on the surface, a porous carbon substrate is formed by carbonization treatment and graphitization treatment is performed. The method is characterized in that the end portion of the porous carbon substrate in the reaction gas flow direction is subsequently subjected to a treatment in which fine carbon fibers are highly filled. Moreover, as the high filling process in this case, it is possible to employ a vacuum suction method.

Further, the fine carbon fibers used in the present invention include fine carbon fibers obtained by crushing a porous carbon substrate graphitized by graphitization treatment at 1800° C. or higher, and fine carbon fibers obtained by crushing a porous carbon substrate graphitized by graphitization treatment at 1400° C. or higher. It is possible to use micro carbon fibers obtained by crushing a porous carbon substrate that has been carbonized.

0012

[Operation] The operation of the present invention having the above-mentioned structure is as follows. First, in the present invention, since the high-filling treatment of the carbon fibers is performed after the graphitization treatment, problems such as a decrease in dimensional accuracy and distortion that occur when the treatment is performed before the graphitization treatment do not occur.

[0013] The fine carbon fibers highly packed at the end of the porous carbon substrate then function effectively by impregnating the porous carbon substrate with phosphoric acid as an electrolyte.

[0014] That is, in the end portion highly filled with fine carbon fibers, the pore diameter formed by the substrate fibers is smaller than that in the other general portions of the substrate structure. Therefore, when a porous carbon substrate is impregnated with phosphoric acid, the phosphoric acid tends to be attracted to the edge of the substrate due to the capillary action at the edge of the substrate, and to be concentrated and retained at this edge. shows. As a result, the retained phosphoric acid produces a so-called wet sealing effect at the edge of the substrate. Therefore, when a porous carbon substrate subjected to such treatment is used as an electrode of a fuel cell, due to this wet sealing effect,
Leakage of reaction gas from the edge of the substrate can be reliably prevented.

Furthermore, as fine carbon fibers can be used by simply crushing porous carbon substrates graphitized by graphitization treatment or porous carbon substrates carbonized by carbonization treatment, fine carbon fibers are specially prepared. The advantage is that you don't have to.

[0016]

EXAMPLE An example of the method for manufacturing a porous carbon substrate according to the present invention will be described in detail below with reference to FIGS. 1 and 2.

That is, as an embodiment of the present invention, the following carbonization treatment (1), graphitization treatment (2), and high filling treatment (3) are sequentially performed to construct a fuel cell using phosphoric acid as an electrolyte. A porous carbon substrate, which is one component, was manufactured.

(1) Carbonization treatment First, carbon fiber 70 manufactured from petroleum pitch
30 parts by weight of a phenolic thermosetting resin was mixed with 30 parts by weight, the pulverized mixture was filled into a hopper, and uniformly dispersed from the hopper into a stainless steel container partitioned into 50 cm squares. Next, this dispersed powder was hot pressed in a flat press at 140°C and 8 atm for about 10 minutes to a thickness of 3 mm.
It was molded into. Next, this molded body is placed in an electric furnace,
950°C at a heating rate of 50°C/hour in an inert gas
The temperature was raised to 950° C. and maintained at this temperature for about 15 hours to carry out carbonization treatment of the phenolic thermosetting resin. After the carbonization treatment, it was slowly cooled and taken out into the air at about 250°C. Note that a substrate formed by such carbonization treatment is usually called a carbonized porous carbon substrate.

(2) Graphitization Treatment The porous carbon substrate formed through the carbonization treatment described above was further heat treated at a temperature of 2500° C. for 80 hours in an inert gas. Through this graphitization treatment, the porosity is 70%.
, a porous carbon substrate with a density of 0.50 to 0.60 g/cc was obtained. Note that a porous carbon substrate subjected to graphitization treatment in this manner is usually called a graphitized porous carbon substrate.

(3) High filling treatment The end portion of the porous carbon substrate graphitized by the graphitization treatment described above was highly filled with fine carbon fibers by the vacuum suction method shown in FIG. That is, as shown in FIG. 1, a container 7 containing fine carbon fibers 6 is placed on one side of the end of the porous carbon substrate 5, and a vacuum suction port 8 is placed on the opposite side of this end. Then, vacuum suction was applied to form a highly filled seal portion 9 at the end of the porous carbon substrate 5, as shown in FIG. In this case, as the fine carbon fibers 6 to be filled in the end portion, fine carbon fibers with a fiber length of 100 μm (micrometers) or less are used, which are obtained by crushing a porous carbon substrate graphitized by the graphitization process described above. did.

The porous carbon substrate 5 shown in FIG. 2 manufactured in this way was used as an electrode to construct a fuel cell unit cell as shown in FIG. In this case, the concentration of phosphoric acid impregnated into each battery component was as shown below. Moreover, each amount shown below was calculated from the volume of each member. Matrix layer: 100% of all voids are impregnated with 105% phosphoric acid. Catalyst layer: 20% of the total voids are impregnated with 105% phosphoric acid. Porous carbon substrate (electrode): 105 in 40% of total voids
Impregnated with % phosphoric acid. The fuel cell unit cell impregnated with phosphoric acid under these conditions was finally held at 100° C. for 24 hours. This operation moves phosphoric acid to the edge of the porous carbon substrate,
A wet seal portion was formed at the end.

The operation of the fuel cell having the porous carbon substrate according to the present invention obtained as described above will be explained with reference to FIG. Here, FIG. 3 is a characteristic diagram showing voltage characteristics of a fuel cell unit cell having a porous carbon substrate according to the present invention. That is, this FIG. 3 shows the rated output voltage (solid line) when the electromotive force test of the unit cell was conducted over a long period of time under the following conditions, and at the same time shows the transition of the open circuit voltage with the dotted line. Test conditions: Normal pressure, 220mA/cm2, 205℃
, UF = UA = 50%) As shown in FIG. 3, the open circuit voltage is high and remains stable over a long period of time. This proves that the porous carbon substrate according to the present invention has good sealing properties and there is no mutual contamination of active materials involved in electromotive reactions. Note that the present invention is not limited to the above-mentioned embodiments, and for example, the fine carbon fibers to be filled in the ends of the porous carbon substrate may be those obtained by crushing a graphitized porous carbon substrate. However, it is also possible to use fine carbon fibers obtained by crushing a carbonized porous carbon substrate.

[0023]

Effects of the Invention As explained above, in the method for manufacturing a porous carbon substrate according to the present invention, after graphitization treatment, fine carbon fibers are attached to the end of the porous carbon substrate in the reaction gas flow direction. By performing a high filling process, it is ensured that the reactive gas (gas mainly composed of hydrogen and gas containing oxygen) flows out to the edge of the porous carbon substrate, which is a component of a fuel cell that uses phosphoric acid as an electrolyte. It is possible to easily form a seal portion that can prevent damage.

[0024] Therefore, in a fuel cell using phosphoric acid as an electrolyte, it is possible to prevent problems such as a reaction other than an electromotive reaction of an outflowing reaction gas as an external output of the cell, or a mixture of contradictory reaction gases, which has been a problem in the past. Therefore, a stable electromotive reaction can proceed over a long period of time, and stable high performance can be maintained over a long period of time.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing a high-filling treatment state in which a porous carbon substrate is highly filled with fine carbon fibers using a vacuum suction method in an embodiment of the method for producing a porous carbon substrate according to the present invention. .

FIG. 2 is a schematic perspective view showing a porous carbon substrate manufactured by the process shown in FIG. 1.

FIG. 3 is a characteristic diagram showing the voltage characteristics of a fuel cell unit cell constructed using the porous carbon substrate of FIG. 2;

FIG. 4 is a configuration diagram showing a basic configuration example of a ribbed electrode type fuel cell unit cell using phosphoric acid as an electrolyte.

[Explanation of symbols]

1…Matrix layer 2… Ribbed electrode 3…Separator 4a, 4b...Catalyst layer 5...Porous carbon substrate ６　…　Micro carbon fiber 7…Container 8... Vacuum suction port 9… Highly filled seal section

Claims (1)

[Claims] Claim 1: Used as an electrode in a fuel cell, with a catalyst layer supported on one side for promoting the electrode reaction, and a reaction gas containing hydrogen as a main component or an oxidizing reaction supported on the other side. In a method for manufacturing a porous carbon substrate in which gas flows in a certain direction, a porous carbon substrate is formed by carbonization treatment, and after graphitization treatment is performed, at the end of the porous carbon substrate in the reaction gas flow direction, A method for manufacturing a porous carbon substrate, which comprises performing a process of highly filling micro carbon fibers.