This invention relates to a production system for a fuel cell separator and a method of producing a fuel cell separator, and more particularly, to a system and method for producing a fuel cell separator wherein no ejector pins are used for lifting out a fuel cell separator from a mold, thereby being able to avoid forming ejector pin scars on the fuel cell separator, and to take out the fuel cell separator without giving any damage for the fuel cell separator.
A fuel cell which generates electric power by making use of fuel gas and oxidant gas, especially a solid polymer type fuel cell, is considered to be a new clean energy source in various applications including automobiles. A solid polymer fuel cell is configured in such a way that an ion conductive solid electrolyte membrane is sandwiched by an anode and a cathode each having a catalyst and functions as a gas diffusion electrode, and an outside of each electrode is further provided with a separator. The separator at the anode provides hydrogen as fuel gas, and the separator at the cathode provides oxygen as oxidizer gas, respectively, to the corresponding electrodes.
FIGS. 3(a) and 3(b) show an example of such a fuel cell separator. As shown in FIGS. 3(a) and 3(b), on a fuel cell separator 1, narrow grooves 1a are formed on a planar surface thereof. In order to increase an overall surface area for contacting between the gas diffusion electrode and the gas, the grooves 1a are meandering with a small pitch throughout the whole surface of the fuel cell separator. The grooves 1a may be formed on both surfaces of the fuel cell separator as shown in FIG. 3(b), or may be formed only on one surface of the fuel cell separator.
There are other types of structure of the fuel cell separator wherein both surfaces or one surface thereof is provided with a large number of projections where spaces between those projections are used as passages of the gas, or both surfaces or one surface thereof is provided with combinations of such projections and grooves.
In the fuel cell separator described above, the following characteristics are required.
(1) Gas non-permeability. This is a characteristic not to allow the hydrogen gas and/or oxygen gas supplied thereto to permeate through the fuel cell separator. Generally, a fuel cell is formed of many cell units stacked together where each cell unit includes a solid polymer electrolyte membrane at the center, gas diffusion electrodes at both outer sides of the electrolyte membrane, and the fuel cell separators further outside of both electrodes. The gas is flowing at least one side of the fuel cell separator. Therefore, if the fuel cell separator has gas permeability or gas leakage, an overall efficiency of power generation by the fuel cell will be decreased, or even the power generation itself cannot be performed.
(2) Electric conductivity. Because the fuel cell separator also acts as an electrode of the fuel cell, electric conductivity is an essential requirement.
(3) High surface accuracy or thickness accuracy. Because electric current flows through the contact area between the separator and the anode or cathode, insufficient surface accuracy, i.e, profile irregularity, deteriorates electric conductivity due to the insufficient contact area. Moreover, when the surface accuracy is insufficient, gaps may be formed between the anode and the cathode, which may cause breakage of the fuel cell separator when a force is applied to the separator in such a direction to press the gaps. When the surface accuracy is high, the contact resistance is small, resulting in the improvement in the fuel cell
In order to satisfy the requirements noted above, an example of conventional technology involves a mold process in which powder material is formed by mixing carbon powder and synthesis resin powder. The resultant powder material is supplied to a lower mold which is then covered by an upper mold of the press machine. The separator is formed by pressing the powder material by the press machine in the high temperature condition. Within the context of this specification, the term xe2x80x9cpowder materialxe2x80x9d is used as a generic term including material in the forms of powder, grain, and short fiber.
FIGS. 4(a)-4(c) further explain a production system for producing fuel cell separators using the mold technology. FIGS. 4(a)-4(c) show an upper mold 2, a mold frame 3 and a lower mold 4. As shown in FIG. 4(a), powder material a for a fuel cell separator is supplied to the lower mold 4 after separating the upper mold and the lower mold from one another. Then, as shown in FIG. 4(b), the upper mold 2 covers the lower mold 4 and applies pressure and heat to the powder material a to form the fuel cell separator 1.
After the molding is completed, as shown in FIG. 4(c), the upper mold 3 is lifted, and ejector pins 5 provided in the lower mold 4 move upward so as to separate the fuel cell separator 1 from the lower mold 4 until the position where the separator 1 can be retrieved.
A pattern 2a for an upper surface of the fuel cell separator 1 is provided on the upper mold 2. The pattern 2a is primarily gas passages to supply gas to a gas diffusion electrode of a fuel cell. An upper surface of the lower mold 4 has a pattern 4a for a lower surface of the fuel cell separator 1 to be used as gas passages.
While the pattern 2a of the upper mold is the same pattern formed on the fuel cell separator, the pattern 4a of the lower mold is a pattern different from that formed on the fuel cell separator 1. This is because ejector pins 5 are provided in the lower mold 4.
FIG. 5 shows an enlarged cross sectional view of the lower mold 4. On the upper surfaces of the ejector pins 5, patterns 5a are formed which are designed to be continuous to the pattern 4a. The pattern 4a and the pattern 5a are formed of concave and convex corresponding to the required pattern on the lower surface of the fuel cell separator 1. Thus, the lower mold 4 and the ejector pins 5 must be carefully prepared to avoid any level differences at boundary areas A where both members slidably contact with each other. If level differences exist, such differences are transferred to the fuel cell separator 1. The irregularity, i.e., level difference, of the fuel cell separator adversely affects a contact performance with a gas diffusion electrode, resulting in insufficient performance of the fuel cell.
However, in the configuration where ejector pins 5 are employed in the lower mold, it is practically difficult to precisely match the levels of the mold and ejector pins in the order of 0.01 mm. Consequently, on the fuel cell separator, a level difference of about 0.2 mm may be formed in the boundary areas A as ejector pin scars 1b as shown in FIGS. 3(a) and 3(b). Moreover, on the fuel cell separator 1, at the positions corresponding to the boundary areas of the ejector pins 5 and the lower mold 4, mold burrs or fins may be formed, which may obstruct the gas passages on the fuel cell separator 1. Furthermore, there is also a problem in that rigidity of the lower mold 4 is decreased because of the ejector pins.
In order to solve the aforementioned problems, the production system shown in FIGS. 6(a)-6(c) has been proposed. In this system, ejector pins are not provided in the lower mold. As shown in FIG. 6(a), by separating an upper mold 11 from a lower mold 12, the powder material a is supplied to the lower mold 12. Then, the upper mold 11 covers the lower mold 11 and applies heat and pressure to the powder material a for molding. After completing the mold process, the upper mold 11 is separated from the lower mold. The foregoing process is the same as that of the previous example shown in FIGS. 4(a) and 4(b). In this prior art, however, the fuel cell separator 1 is removed by further lifting the lower mold 12 so that the fuel cell separator 1 is retrieved when the separator is raised higher than the mold frame 13.
In this prior art example, since the ejector pins are not used, ejector pin scars are no longer formed on the fuel cell separator 1. However, since the lower mold 12 has to be lifted along the mold frame 13, a gap s is formed between the lower mold 12 and the mold frame 13. Thus, the melted material may drop in the gap s and obstruct the up/down movement of the lower mold 12. Hence, clean-up is frequently required to remove the debris in the gap, which requires additional works. Such clean-up works also result in frequent stops of the press machine, thereby decreasing the production efficiency. Further, when the up/down movement of the lower mold 12 is severely hampered, the mold may be inclined or may no longer be movable due to clinging.
The present invention has been made to solve the problems encountered in the conventional technologies described above. It is, therefore, an object of the present invention to provide a fuel cell separator production system and a production method thereof which are able to produce a fuel cell separator having no ejector pin scars.
It is another object of the present invention to provide a fuel cell production system and a fuel cell production method which require no additional works and is able to achieve a high production efficiency, and to take out the molded fuel cell separator without giving any damages to the fuel cell separator.
In order to achieve the aforementioned objectives, the production system of the fuel cell separator of the present invention is characterized as having an upper mold which has a pattern formed of concave and convex corresponding to one surface of a fuel cell separator, a lower mold which has a pattern formed of concave and convex corresponding to other surface of the fuel cell separator, a suction device which horizontally advances and retracts in a space between the upper mold and the lower mold, and one or more nozzles which spray a gas on the periphery of the pattern formed on the fuel cell separator on the lower mold. The mold in the production system does not include any ejector pins, thus, the patterns formed on the upper and lower molds are the same patterns to be formed on the corresponding surfaces of the fuel cell separator.
Preferably, the lower mold may include an opening for receiving powder material for forming the fuel cell separator. Further, it is preferable that the suction device is configured to move in the vertical direction between the upper mold and the lower mold as well as in the horizontal direction to and from the space between the upper mold and the lower mold.
In the fuel cell separator production method of the present invention where the fuel cell separator is formed by supplying powder material between the upper mold and the lower mold and pressing and heating the powder, material, the method further includes the steps of separating the upper mold and the lower mold from each other, spraying the gas on the periphery of the molded fuel cell separator on the lower mold, and releasing the fuel cell separator from the lower mold, and inserting the suction device between the upper mold and the lower mold, and taking out the fuel cell separator from the lower mold by the suction force of the suction device.
The fuel cell separator of the present invention produced through the aforementioned system and method has gas passages on one or both surfaces thereof on which no ejector pin scars are formed, thereby achieving a high contact performance with the gas diffusion electrode.