Fuel cell assembly and method for making the same

[0001] (Related application) This application benefits from being based on US Provisional Application No. 60 / 202,827 filed May 8, 2000 and US Provisional Application No. 60 / 242,136 filed October 23, 2000. As such, those statements shall form part of this application. [0002] (Technical field) In the present invention, an electrolyte layer, a pair of gas diffusion electrode layers provided across the electrolyte layer, and a pair of distributions for defining passages of fuel gas and oxidant gas in contact with each gas diffusion electrode layer, respectively. It relates to a fuel cell having a plate and usually composed of a plurality of cells. [0003] (Background of invention) A fuel cell has a pair of electrodes attached to both sides of the electrolyte layer, and supplies fuel gas such as hydrogen and alcohol to one electrode and oxidizing agent gas such as oxygen and air to the other electrode to cause an electrochemical reaction by a catalyst. It is used to generate electricity, and there are phosphoric acid type, solid polymer type, molten carbonate type, etc. depending on the electrolyte used. [0004] Of these, polymer electrolyte electrolyte (SPE) fuel cells, which use an ion exchange membrane as the electrolyte, are attracting attention because they can be miniaturized, have a low operating temperature (100 ° C or less), and have high power generation efficiency. There is. [0005] As the solid polymer electrolyte (SPE), for example, a thin film of an ion exchange resin such as perfloolocarbon sulphonic acid (Nafion: trade name), phenol sulphonic acid, polystyrene sulphonic acid, or polytrifluorostyrene sulphonic acid is used, and these ions are used. Porous carbon paper containing a catalyst powder made of platinum or the like is provided as a gas diffusion electrode on both sides of the thin film of the exchange resin to form a membrane / electrode assembly (MEA), and a fuel gas passage is provided on one surface side of this MEA. An oxidant gas passage such as air is defined by a distribution plate (separator) on the other surface side to form a fuel cell. [0006] A large number of these fuel cell cells are connected in series for use, but usually, the fuel cell cells are stacked and the cells adjacent to each other in the stacking direction share a distribution plate between them to form a stack. When constructing the stack, it is necessary to block the passage defined on the surface of the MEA from the outside. Conventionally, each fuel cell has MEA and each distribution plate surrounded by packing, and each fuel cell is fastened in a laminated state by applying an external force in the stacking direction to seal the fuel cell. Further, by applying an external force, the MEA and the gas diffusion electrode are pressed into contact with each other to secure the contact area, and further, the electrode terminal from the outside and the gas diffusion electrode are pressed against each other to perform electrical contact with each other. [0007] However, since the volume of MEA changes depending on the water content, temperature, etc. of SPE, the external force changes in the fastening structure, and there is a concern that the sealing property may be impaired. Further, in order to stabilize the shape of the SPE, the outer circumference thereof may be surrounded by a frame, but since the frame and each distribution plate each thermally expand / contract, this also changes the above-mentioned external force in the conventional fastening structure. In addition, there is a concern that stress will be generated and each member will deteriorate over time, resulting in a decrease in durability. [0008] Further, the sealing structure by packing and the fastening structure for applying the above-mentioned external force increase the size of the entire device and hinder its application to a small fuel cell. Moreover, even if various measures for securing the seal are used, it is difficult to secure the required sealing performance for a long period of time because the thermal expansion / contraction of each part is non-uniform. [0009] (Outline of the invention) In view of such problems of the prior art, an object of the present invention is to provide a fuel cell capable of ensuring suitable sealing performance under all conditions. [0010] A second object of the present invention is to provide a fuel cell capable of ensuring electrical contact between an electrode and a gas diffusion electrode. [0011] A third object of the present invention is to provide a small and efficient fuel cell. [0012] A fourth object of the present invention is to provide a fuel cell that is easy to manufacture. [0013] According to the present invention, such an object is for defining a passage of a fuel gas and an oxidizing agent gas in contact with an electrolyte layer, a pair of gas diffusion electrode layers sandwiching the electrolyte layer, and each gas diffusion electrode layer. In a fuel cell composed of at least one cell having a pair of distribution plates, the electrolyte layer has a frame and an electrolyte held in the frame, and both distribution plates and the frame are combined. This is achieved by joining or adhering over the entire circumference of the outer circumference so that the inside becomes airtight. Here, both distribution plates and the frame can be joined by anode joining, diffusion joining, welding, various brazing, or the use of an adhesive. [0014] This eliminates the need for a structure that relies on an external force such as a sealing structure or a fastening structure by packing when joining the two distribution plates and the frame so as to secure the required airtightness. [0015] In particular, in the present invention, if both the distribution plates and the frame are made of materials having similar thermal expansion characteristics to each other, the frame of the electrolyte layer and each distribution plate are the same even if there is a temperature change. Thermal expansion / contraction occurs, and no internal stress is generated. This is effective for maintaining the sealing performance of the assembly or ensuring the durability of the assembly even under harsh conditions for a long period of time. [0016] It is preferable that both the distribution plates and the frame are made of a silicon substrate suitable for microfabrication. According to a preferred embodiment of the present invention, the two distribution plates and the frame are joined by anode bonding or the use of an adhesive over the entire outer circumference thereof. This eliminates the need for gaskets or fastening structures, simplifies the structure of the assembly and increases the reliability of the assembly under harsh conditions. [0017] (Form for carrying out the invention) Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. [0018] FIG. 1 shows the structure of a fuel cell stack to which the present invention is applied. Actually, a fuel cell is obtained by connecting a plurality of these stacks in series or in parallel, reforming alcohol or the like, or directly supplying hydrogen gas or the like as fuel, and further supplying air or the like as oxidant gas. Is configured. [0019] As shown in FIGS. 2 (a) and 2 (b), each fuel cell 1 is arranged via an electrolyte layer 2 in the center of the stacking direction and gas diffusion electrode layers 3 and 4 on the front and back surfaces thereof. It is composed of a pair of distribution plates 5. Here, each distribution plate 5 is shared with the next-stage or previous-stage cell adjacent to the cell 1 in the stacking direction by processing both sides thereof. [0020] The electrolyte layer 2 is composed of a grid-like frame 21 and SPE22 filled between the grids 21a of the frame 21, that is, the quadrangular through holes 21b as an electrolyte. As SPE22, for example, perfloolocarbon sulphonic acid (Nafion: trade name), phenol sulphonic acid, polystyrene sulphonic acid, polytrifluorostyrene sulphonic acid and the like are used. [0021] [0021] Here, the lattice-shaped frame 21 is formed by etching a silicon substrate and other processing, and has a quadrangular and annular peripheral portion and a quadrangular lattice portion provided inside the annular peripheral edge portion. As shown in FIGS. 2 (a) and 2 (b), each grid 21a of the grid portion of the grid frame 21 is provided with protrusions 21c so as to project toward the intermediate portion of the corresponding through holes 21b. There is. The protrusions 21c form ridges extending in the longitudinal direction of each grid 21a, defining a narrow intermediate portion within each through hole 21b. The protrusion 21c helps hold the SPE22 within each through hole 21b. [0022] Such protrusions can conveniently be formed at the same time as forming the grid-like frame 21. FIGS. 3 (a) to 3 (c) show the process of forming the electrolyte layer 2. First, as shown in FIG. 3A, resists 13 and 14 are formed in a predetermined pattern on both the front and back surfaces of a silicon substrate as a material for the grid-like frame 21. As shown in FIG. 3 (b), by anisotropic etching (wet etching) from both sides, a through hole 21b having a narrow middle portion in the thickness direction is formed by the protrusion 21c. .. Next, SPE22 is injected into each through hole 21b to make each surface of the electrolyte layer 2 a substantially flat surface. [0023] In this embodiment, quadrangular through holes 23a, 23b, 24a, and 24b are formed at diagonal positions of the grid-like frame 21. One diagonal through hole 23a, 23b of the lattice frame 21 constitutes a fuel gas supply path and a discharge path, and another diagonal through hole 24a, 24b constitutes an oxidant gas supply path and an discharge path. To do. [0024] On the other hand, the distribution plate 5 is also made by processing a silicon substrate and has almost the same shape as a quadrangle. Square recesses 51 and 52 having a flat bottom are formed on both the front and back surfaces of the central portion, and a large number of quadrangular pyramid-shaped protrusions 53 and 54 are provided on the flat bottom of the central recesses 51 and 52. ing. Further, on the surfaces of the recesses 51 and 52 and the protrusions 53 and 54, electrode terminal films 55 and 56 made of a gold (Au) plating layer for connecting the external circuit and the gas diffusion electrode layers 3 and 4 are known. The film is formed by the film method. [0025] FIGS. 4 (a) to 4 (c) show a method of forming each distribution plate 5. As shown in FIG. 4 (a), resists 15 and 16 are formed on each surface of the silicon substrate in a predetermined pattern, and as shown in FIG. 4 (b), the silicon substrate is anisotropically etched from both sides thereof. The recesses 51 and 52 and the protrusions 53 and 54 are formed at the same time. The distribution plates 5 at both ends of the fuel cell 1 in the stacking direction may be provided with a central recess and a quadrangular pyramid-shaped protrusion only on the inner surface. Next, as shown in FIG. 4C, electrode terminal films 55 and 56 are formed on the surfaces of the recesses 51 and 52 and the protrusions 53 and 54. [0026] The distribution plate 5 is isomorphic to the grid frame 21 and thus forms a quadrangle. Square through holes 57a, 57b, 58a, 58b are provided at each corner of the edge. The through holes 57a and 57b provided at one diagonal position constitute the fuel gas supply path and the discharge path, and the through holes 58a and 58b provided at the other diagonal positions form the oxidant gas supply path and the discharge path. Make up the road. Here, as shown in FIG. 1, the recess 51 and the through holes 58a and 58b are communicated with each other by the oxidant gas grooves 59a and 59b provided at the edge, and the recess 52 and the through holes 57a and 57b are communicated with each other. It is communicated by similar fuel gas grooves 60a and 60b. [0027] The gas diffusion electrode layers 3 and 4 are composed of platinum (Pt) catalyst layers 3b and 4b mixed with SPE22 provided on the surface side of the gas diffusion layers 3a and 4a made of carbon paper on the electrolyte layer 2 side. [0028] In this way, in each cell, the electrolyte layer 2 is sandwiched between the pair of distribution plates 5 via the gas diffusion electrode layers 3 and 4, and the anode is bonded over the entire circumference so as to surround the recesses 51 and 52. By doing so, the inside is hermetically sealed. Therefore, the oxidant gas passage 10 is defined on the central recess 51 side of each electrolyte layer 2, and the fuel gas passage 11 is defined on the central recess 52 side of each electrolyte layer 2. Each protrusion is substantially entirely covered with a gold-plated layer, constitutes an electrode terminal, and is lightly pressed against the grid-like frame 21 of the electrolyte layer 2. Therefore, each gas diffusion electrode layer 3 and 4 is electrically connected to the corresponding distribution plate 5 via a large number of protrusions in parallel with each other, and is surely connected between the electrolyte layer 2 and the external circuit. Continuity is ensured. [0029] The grid-like frame 21 and the distribution plate 5 can be joined by various methods. In particular, anodic bonding is preferable, and the procedure will be described below. An electrode film 9 and a glass film 8 made of heat-resistant hard glass such as Pyrex glass (registered trademark) are preliminarily formed on both sides of the lattice-shaped frame 21 of the electrolyte layer 2 by a sputtering method. .. Similarly, an electrode film 9 is formed on the entire outer circumference of both distribution plates 5. Next, the temperature is raised to about 400 ° C. at which sodium ions can be easily moved, and an electric field is applied to the temperature to move the ions. However, in the fuel cell, when the solid electrolyte is a polymer, heating the whole to about 400 ° C. may damage the solid electrolyte. Therefore, in this embodiment, a heater (not shown) is embedded in the lower layer of the electrode film 9 so that only the outer peripheral portion can be partially heated in the outer peripheral portion of both distribution plates 5. As this heater, for example, Si as an insulating layer₃N₄It is advisable to use polycrystalline Si sandwiched between. At this time, if the electrode terminal films 55 and 56 are also present in the lower layer of the heater, the heat utilization efficiency of the heater is lowered, so it is preferable not to arrange them in this portion. [0030] The grid-like frame 21 and the distribution plate 5 face each other, and 100 gf / cm in the stacking direction.²~ 2,000gf / cm²Apply some pressure. In addition, the polycrystalline Si heater is energized to locally raise the temperature of the joint to about 400 ° C. In this state, a voltage of 100V to 500V is applied between the grid frame 21 and the distribution plate 5 for 10 to 30 minutes. [0031] Alternatively, a structure in which the grid-like frame 21 and both distribution plates 5 are bonded to each other with an adhesive may be used. The effect that can be obtained is obtained as well. [0032] By flowing the fuel gas and the oxidant gas (air) through each of these fuel cell cells 1, an electrochemical reaction by the catalyst (Pt) occurs, and a potential difference is generated between the electrode terminal films 55 and 56, which is caused by this. As a stack connected in series, it can be supplied to an external circuit as a power source of a desired voltage. [0033] The fuel and oxidant of the fuel cell described above consisted of gas, but may contain liquid. [0034] Although the present invention has been described above with respect to specific examples, those skilled in the art may make various modifications to the present invention without departing from the technical fields of the present invention described in the appended claims. Needless to say. [Simple explanation of drawings] FIG. 1 is an exploded perspective view showing the structure of a fuel cell stack to which the present invention is applied. FIG. 2a is a partially enlarged cross-sectional view of the line IIa-IIa of FIG. FIG. 2b is a partially enlarged cross-sectional view of the line IIb-IIb of FIG. FIG. 3a is a cross-sectional view illustrating a procedure for manufacturing an electrolyte layer of a fuel cell to which the present invention has been applied. FIG. 3b is a cross-sectional view illustrating a procedure for manufacturing an electrolyte layer of a fuel cell to which the present invention is applied. FIG. 3c is a cross-sectional view illustrating a procedure for manufacturing an electrolyte layer of a fuel cell to which the present invention has been applied. FIG. 4a is a cross-sectional view illustrating a procedure for manufacturing a distribution plate of a fuel cell to which the present invention is applied. FIG. 4b is a cross-sectional view illustrating a procedure for manufacturing a distribution plate of a fuel cell to which the present invention is applied. FIG. 4c is a cross-sectional view illustrating a procedure for manufacturing a distribution plate of a fuel cell to which the present invention is applied.