1. Field of the Invention
The present invention relates to a fuel cell comprising a fuel cell unit composed of an electrolyte interposed between an anode electrode and a cathode electrode, and separators for supporting the fuel cell unit interposed therebetween.
2. Description of the Related Art
For example, the solid polymer type fuel cell comprises a fuel cell unit including an anode electrode and a cathode electrode disposed opposingly on both sides of an electrolyte composed of a polymer ion exchange membrane (cation exchange membrane) respectively, the fuel cell unit being interposed between separators. Usually, the solid polymer type fuel cell is used as a fuel cell stack obtained by stacking a predetermined number of the fuel cell units.
In such a fuel cell, a fuel gas such as a hydrogen-containing gas, which is supplied to the anode electrode, is converted into hydrogen ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte which is appropriately humidified. The electron, which is generated during this process, is extracted for an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas such as a gas containing oxygen or air is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen gas are reacted with each other on the cathode electrode, and thus water is produced.
In the fuel cell described above, an internal manifold is constructed in order to supply the fuel gas and the oxygen-containing gas (reaction gas) to the anode electrode and the cathode electrode of each of the stacked fuel cell units respectively. Specifically, the internal manifold includes a plurality of communication holes which are provided in an integrated manner to make communication with each of the fuel cell units and the separators which are stacked with each other. When the reaction gas is supplied to the supplying communication hole, the reaction gas is supplied in a dispersed manner to each of the fuel cell units, while the used reaction gas is integrally discharged to the discharging communication hole. The fuel cell is supplied with a cooling medium in order to cool the electrode power-generating surface. The internal manifold is provided with communication holes for the cooling medium in some cases, in the same manner as for the reaction gas.
As shown in FIG. 14, for example, Japanese Laid-Open Patent Publication No. 3-257760 discloses, as such a technique, a fuel cell in which a fuel cell unit 3 including an electric cell three-layered film 2 formed on a surface of a film formation substrate 1 is interposed between separators 4, and the separators 4 are formed with an internal manifold 5 for allowing the fuel gas and the oxygen-containing gas to flow.
However, in the conventional technique described above, in order to reliably effect the gas seal for the internal manifold 5, a seal plate 7 is installed via spacers 6a, 6b between the separators 4. Gaskets 8 are interposed between the separator 4 and the spacer 6a, between the spacer 6a and the seal plate 7, between the seal plate 7 and the spacer 6b, and between the spacer 6b and the separator 4 respectively. As a result, the following problem is pointed out. That is, the dimension of the fuel cell unit 3 in the stacking direction (direction of the arrow X) is considerably lengthy, the number of parts is increased, and the production cost becomes expensive.
Accordingly, as shown in FIG. 15, the following structure is adopted. That is, introducing sections 5c, which are used to make communication between a communication hole 5a of the internal manifold of the separator 4a and fluid flow passages 5b for allowing the reaction gas to flow into the surface of the separator 4a, are formed on the same plane as that of the fluid flow passages 5b. In order to ensure the sealing performance of the introducing sections 5c, a thin plate-shaped cover 9 is fitted to the introducing sections 5c to allow the gasket 8a to forcibly abut against the cover 9 (see FIG. 16).
However, a step is required to fit the considerably thin-walled cover 9 to the introducing section 5c as described above to assemble the fuel cell so that the surface of the cover 9 is flush with the surface of the separator 4a. An operation to stick (fit) the cover 9 is complicated. Further, the following problem is pointed out. That is, it is feared that the cover 9 may be lost during the assembling of the cell or during the stacking of the cell, resulting in leakage of the reaction gas. Further, any difference in height arises between the surface of the cover 9 and the surface of the separator 4a. It is impossible to apply the uniform tightening force to the separator 4a when the cell is tightened.
When a communication hole for the cooling medium is provided for the internal manifold of the separator, it is also necessary to use the thin plate-shaped cover. As a result, the same problem as that for the reaction gas described above arises.
In order to dissolve the inconvenience as described above, for example, a fuel cell stack disclosed in U.S. Pat. No. 6,066,409 is known. In the fuel cell stack, as shown in FIG. 17, a separator 4b is constructed by combining two separators 4b1, 4b2. An internal manifold is arranged at a central portion thereof. Specifically, communication holes, i.e., a supply port 5d1 and a discharge port 5d2 for the reaction gas on the first side, and a supply port 5e1 and a discharge port 5e2 for the reaction gas on the second side are formed to penetrate in the thickness direction of the separator 4b. 
As shown in FIG. 18, flow passage grooves 5f1, 5f2, which communicate with the supply port 5d1 and the discharge port 5d2 on the first side and which extend toward the outer circumferential side along a non-power-generating surface 4c1, are formed on the non-power-generating surface (non-reaction surface) 4c1 of the separator 4b. Further, flow passage grooves 5g1, 5g2, which communicate with the supply port 5e1 and the discharge port 5e2 for the reaction gas on the second side and which extend toward the outer circumferential side along the non-power-generating surface 4c1, are formed on the non-power-generating surface 4c1 of the separator 4b. A plurality of cooling air flow passage grooves 5j are formed in parallel to one another on the non-power-generating surface 4c1. Both ends of the cooling air flow passage grooves 5j are open toward the outside from the outer circumferential end of the non-power-generating surface 4c1.
Through-holes 5h1, 5h2 communicate with outer ends of the flow passage grooves 5f1, 5f2. The through-holes 5h1, 5h2 communicate with the reaction gas flow passage 5i on the side of the power-generating surface 4c2 of the separator 4b1 (see the separator 4b2 in FIG. 17). The reaction gas flow passage 5i is provided along the plane of the power-generating surface 4c2. A gasket 8b, which is used to prevent the different reaction gases from being mixed in the internal manifold, is interposed between the separators 4b1, 4b2.
In the arrangement as described above, when the first reaction gas is supplied to the supply port 5d1 which constitutes the internal manifold, the reaction gas is moved to the outer circumferential side of the separator 4b along the flow passage groove 5f1 communicating with the supply port 5d1. The reaction gas passes through the through-hole 5h1 communicating with the outer end of the flow passage groove 5f1, and it is supplied to the side of the power-generating surface 4c2. The reaction gas flow passage 5i is provided on the side of the power-generating surface 4c2. The reaction gas is supplied to an unillustrated fuel cell unit, while moving along the reaction gas flow passage 5i. The reaction gas, which is not used, is supplied from the through-hole 5h2 to the flow passage groove 5f2, and it is discharged to the outside from the discharge port 5d2 which constructs the internal manifold.
However, in the case of the conventional technique described above, the internal manifold is provided at the central portion of the separator 4b. For example, the following problem arises. That is, when the reaction gas supplied to the supply port 5d1 is fed from the through-hole 5h1 to the side of the power-generating surface 4c2 after being introduced into the flow passage groove 5f1, the pressure loss is increased, because the flow passage groove 5f1 is considerably lengthy. Further, the first reaction gas is introduced from the supply port 5d1 into the single flow passage groove 5f1, and it is further supplied to the reaction gas flow passage 5i via the single through-hole 5h1. Therefore, it is impossible to allow a large amount of the reaction gas to smoothly flow therethrough. As a result, the following problem is pointed out. That is, it is difficult to operate the fuel cell stack at a high current density.
The separator 4b is constructed by superimposing the pair of separators 4b1, 4b2. Therefore, it is necessary to mutually superimpose the cooling air flow passage grooves 5j formed on the non-power-generating surface 4c1 of the separators 4b1, 4b2. However, the cooling air flow passage groove 5j is a minute gap. It is considerably complicated to perform the operation for accurately superimpose the cooling air flow passage grooves 5j with each other. Further, the fuel cell stack is constructed such that the power-generating surface 4c2 is air-cooled by supplying the air to the cooling air flow passage groove 5j. For this reason, the following inconvenience arises. That is, the cooling ability is lowered as compared with the water-cooling, and it is difficult to perform the operation especially at a high current density.
Further, the cooling air flow passage groove 5j is provided at only the both side portions except for the central portion of the separator 4b which constructs the internal manifold. Therefore, the following problem arises. That is, it is impossible to effectively cool the entire surface of the power-generating surface 4c2, and the cooling efficiency is lowered.