Abstract:
In a fuel cell assembly typically comprising a plurality of cells each comprising an electrolyte layer ( 2 ), a pair of gas diffusion electrode layers ( 3, 4 ), and a pair of flow distribution plates ( 5 ), each flow distribution plate is provided with a central recess ( 51, 52 ) having a number of projections ( 53, 54 ) formed therein; and an electrode terminal layer ( 55, 56 ) is formed on each projection to establish a connection with an external circuit; each gas diffusion electrode layer defining the passages for fuel and oxidizer gases by covering the central recess, and provided with a porous layer ( 3   a,    4   a ) typically in the form of a nano-tube carbon film, formed over each flow distribution plate. Because the porous layer is directly formed on each flow distribution plate, the thickness of each gas diffusion electrode layer can be freely controlled, and the overall thickness of the assembly can be minimized so as to allow a compact design.

Description:
This application claims the benefit of U.S. Provisional Application Nos. 60/202,827, filed May. 8, 2000, and 60/242,136, filed Oct. 23, 2000, both of which are herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a fuel cell assembly typically comprising a plurality of fuel cells each including an electrolyte layer, a pair of gas diffusion electrode layers placed on either side of the electrolyte layer, and a pair of flow distribution plates placed on either outer side of the gas diffusion electrode layers to define passages for distributing fuel gas and oxidizing gas in cooperation with the opposing surfaces of the gas diffusion electrode layers. 
     BACKGROUND OF THE INVENTION 
     A fuel cell comprises an electrolyte layer and a pair of electrodes placed on either side of the electrolyte layer, and generates electricity through an electrochemical reaction between fuel gas such as hydrogen and alcohol and oxidizing gas such as oxygen and air, which are supplied to the corresponding electrodes, with the aid of a catalyst. Depending on the electrolytic material used for the electrolyte layer, the fuel cell may be called as the phosphoric acid type, solid polymer type or molten carbonate type. 
     In particular, the solid polymer electrolyte (SPE) type fuel cell using an ion-exchange resin membrane for the electrolyte layer is considered to be highly promising because of the possibility of compact design, low operating temperature (100° C. or lower), and high efficiency. 
     The SPE typically consists of an ion-exchange resin membrane made of perfluorocarbonsulfonic acid (Nafion: tradename), phenolsulfonic acid, polyethylenesulfonic acid, polytrifluorosulfonic acid, and so on. A porous carbon sheet impregnated with a catalyst such as platinum powder is placed on each side of the ion-exchange resin membrane to serve as a gas diffusion electrode layer. This assembly is called as a membrane-electrode assembly (MEA). A fuel cell can be formed by defining a fuel gas passage on one side of the MEA and an oxidizing gas passage on the other side of the MEA by using flow distribution plates (separators). 
     Typically, such fuel cells are stacked, and the flow distribution plates are shared by the adjacent fuel cells in the same stack. When forming such a stack, it is necessary to seal off the passages defined on the surfaces of the MEAs from outside. Conventionally, gaskets were placed in the periphery of the interface between each adjoining pair of a MEA and a distribution plate. The contact area between the MEA and the gas diffusion electrode was ensured by pressing them together by applying an external force, typically with the aid of a suitable fastener. The required electric connection between the gas diffusion electrode and an electrode terminal connected to an external circuit was also ensured by pressing them together by applying an external force. 
     However, because the material used for the gas diffusion electrode, such as a carbon sheet, has surface irregularities, and the electrode terminal for connection with an external circuit is allowed to contact the gas diffusion electrode while providing flow paths for the fuel and oxidizer, the contact area between them is very much limited. Also, the SPE can function as an ion-exchange membrane only when impregnated with water, and the SPE when impregnated with water significantly changes its volume depending on the temperature. The flow distribution plates also expand and contract according to the temperature. The resulting stress affects the pressure that is applied to the fuel cell, and this prevents an accurate control of the pressure acting between the different layers of the fuel cell. In particular, it tends to prevent a reliable electric contact to be established between the electrode terminal and the gas diffusion electrode. 
     The carbon sheet is preferred as the material for the gas diffusion electrode, but cannot be made as thin as desired (in the order of a few μm) in view of the handling. This tends to undesirably increase the thickness of each fuel cell. Also, the carbon sheet is required to be porous, but the catalyst in the form of fine powder tends to fill the pores of the carbon sheet. Such a loss of porosity of the carbon sheet reduces the diffusion rate of the fuel gas and oxidizer gas which in turn reduces the efficiency of the device. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide a fuel cell assembly which can ensure a favorable electric connection between the gas diffusion electrode and the electrode terminal for external connection at all times. 
     A second object of the present invention is to provide a fuel cell assembly which can ensure a low electric contact resistance between the gas diffusion electrode and the electrode terminal while allowing unhampered gas diffusion so that a high efficiency of the device may be ensured. 
     A third object of the present invention is to provide a fuel cell assembly which is suitable for compact design. 
     A fourth object of the present invention is to provide a fuel cell assembly which is easy to manufacture. 
     According to the present invention, such objects can be accomplished by providing a fuel cell assembly at least one cell comprising an electrolyte layer, a pair of gas diffusion electrode layers interposing the electrolyte layer between them, and a pair of flow distribution plates for defining passages for fuel and oxidizer gases that contact the gas diffusion electrode layers, characterized by that: each flow distribution plate is provided with a central recess having a number of projections formed therein; and an electrode terminal layer is formed on each projection to establish an electric connection with an external circuit; each gas diffusion electrode layer defining the passages for fuel and oxidizer gases by covering the central recess, and provided with a porous layer formed over each flow distribution plate so as to cover the electrode terminal layer. 
     Because the porous gas diffusion electrode layers cover the electrode terminal layers on each distribution plate, a low electric contact resistance can be ensured between the electrode terminal layers and the gas diffusion layers without applying any external pressure. Because the porous gas diffusion electrode layers are formed directly over the surfaces of the flow distribution plates, the thickness of each diffusion electrode layer can be controlled at will, and this contributes to a compact design. Also, because each gas diffusion electrode layer essentially consists of a carbon film such as carbon nano-tube which has numerous fine through holes across its thickness, it can offer a substantially larger surface area than a comparable solid carbon sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Now the present invention is described in the following with reference to the appended drawings, in which: 
         FIG. 1  is an exploded perspective view of a part of a fuel cell assembly embodying the present invention; 
         FIG. 2   a  is a sectional view taken along line IIa—IIa of  FIG. 1 ; 
         FIG. 2   b  is a sectional view taken along line IIb—IIb of  FIG. 1 ; 
         FIGS. 3   a  to  3   c  are sectional views of the electrolyte layer in different steps of the fabrication process; 
         FIGS. 4   a  to  4   c  are sectional views of the flow distribution plate in different steps of the fabrication process; and 
         FIGS. 5   a  to  5   e  are sectional views of the gas diffusion electrodes in different steps of the fabrication process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows the structure of a part of a fuel cell assembly embodying the present invention. In practice a plurality of cells are formed into a stack, and a number of such stacks are connected in series and/or parallel, and fuel consisting of reformed alcohol, hydrogen gas or the like is supplied to each fuel cell stack along with oxidizing gas such as air. 
     Referring also to  FIGS. 2   a  and  2   b , each fuel cell  1  comprises a central electrolyte layer  2 , a pair of gas diffusion electrode layers  3  and  4  (see  FIGS. 2   a  and  2   b ) placed on either side of the central electrolyte layer  2 , and a pair of flow distribution plates  5  placed on either outer side of the gas diffusion electrode layers  3  and  4 . The outer side of each flow distribution plate  5  is similarly formed as the inner side thereof so as to serve as the flow distribution plate for the adjacent fuel cell. 
     The electrolyte layer  2  comprises a grid frame  21 , and solid polymer electrolyte (SPE)  22  which is filled into rectangular through holes  21   b  define between adjacent bars  21  a of the grid frame  21 . The SPE  22  may be made from such materials as perfluorocarbonsulfonic acid (Nafion: tradename), phenolsulfonic acid, polyethylenesulfonic acid, polytrifluorosulfonic acid, and so on. 
     The grid frame  21  is formed by etching or otherwise working a silicon wafer, and is provided with a rectangular and annular fringe portion and a rectangular grid area defined inside the annular fringe portion. Each bar  21   a  in the grid area of the grid frame  21  is provided with a projection  21   c  at an intermediate part thereof so as to project into an intermediate part of the corresponding through hole  21   b  as best shown in  FIGS. 2   a  and  2   b . The projection  21   c  is in the shape of a ridge extending along the length of the bar  21   a , and produces a narrower middle part in each through hole  21   b . The projection  21   c  helps to retain the SPE  22  in each through hole  21   b.    
     Such a projection can be conveniently formed at the same time as forming the grid frame  21 .  FIGS. 3   a  to  3   c  illustrate the process of forming the electrolyte layer  2 . First of all, a suitably patterned photoresist layer  13  and  14  is placed on each side of a silicon wafer serving as the material for the grid frame  21  as shown in  FIG. 3   a . An anisotropic etching is performed from both sides of the wafer as illustrated in  FIG. 3   b,  and this produces a plurality of through holes  21   b  each of which is narrowed in a middle part by a projection  21   c . Then, SPE  22  is filled into each of the through holes  21   b  so as to define a substantially flush planar surface on each side of the electrolyte layer  2 . 
     In this embodiment, a rectangular through hole  23   a ,  23   b ,  24   a  and  24   b  is formed in each corner portion of the fringe portion of the grid frame  21 . One of the diagonally opposing pairs of these through holes  23   a  and  23   b  serve as inlet and outlet for the full gas. The remaining opposing pair of these through holes  24   a  and  24   b  serve as inlet and outlet for the oxidizing gas. 
     Each flow distribution plate  5  is also formed by working a silicon wafer, and has a substantially conformal rectangular shape. A rectangular recess  51  or  52  having a flat bottom is formed centrally on each side of the flow distribution plate  5 , and a plurality of projections  53  or  54  each having the shape of a truncated pyramid are formed on the flat bottom. The surface of the recesses and the projections are coated with a gold plate layer serving as an electrode terminal layer  55  or  56  by suitable means for electrically connecting the gas diffusion electrode layers  3  and  4  to an external circuit. 
       FIGS. 4   a  to  4   c  show the process of forming each flow distribution plate  5 . A suitably patterned photoresist layer  15  and  16  is formed on each side of a silicon wafer as shown in  FIG. 4   a , and the silicon wafer is etched from both sides to form the recesses  51  and  52  and projections  53  and  54  at the same time as shown in  FIG. 4   b . The distribution plate  5  on the upper end or lower end of each fuel cell stack may be provided with a recess and projections only on inner side thereof. Thereafter, electrode terminal layer  55  and  56  is formed over the surface of the recesses  51  and  52  and projections  53  and  54  as shown in  FIG. 4   c.    
     The distribution plate  5  is conformal to the grid frame  21 , and therefore has a rectangular shape. A rectangular through hole  57   a ,  57   b ,  58   a  or  58   b  is formed in each corner portion of the fringe portion thereof. One of the diagonally opposing pairs of these through holes  57   a  and  57   b  serve as inlet and outlet for the fuel gas. The remaining opposing pair of these through holes  58   a  and  58   b  serve as inlet and outlet for the oxidizing gas. As shown in  FIG. 1 , grooves  59   a  and  59   b  formed in the fringe portion communicate the recess  51  with the through holes  58   a  and  58   b  for the oxidizing gas, and similar grooves  60   a  and  60   b  communicate the recess  52  with the through holes  57   a  and  57   b  for the fuel gas. 
     Each gas diffusion electrode layer  3  and  4  is formed in a plane passing through the free ends of the corresponding projections  53  and  54 , and comprises a gas diffusion layer  3   a  and  4   a  formed by a porous carbon film having minute holes extending across its thickness (carbon nano-tube: see Langmuir, Vol. 15, No. 3, 1999, pp 750–758, American Chemical Society), and a platinum catalyst layer  3   b  and  4   b  formed as a porous layer placed on the surface thereof facing the electrolyte layer  2 . 
       FIGS. 5   a  to  5   e  show the process of forming each gas diffusion layer  3  and  4 . First of all, a flow distribution plate S is formed according to the process illustrated in  FIGS. 4   a  and  4   c . The recesses  51  and  52  are each filled with a sacrificial material  17  so as to define a flush outer surface with this sacrificial material  17  and the electrode terminal layer  55  and  56  on the top regions of the projections  53  and  54 . Alternatively, the sacrificial material  17  may be deposited to such an extent as to entirely bury the projections  53  and  54  therein, and etched back until a flush outer surface is defined with this sacrificial material  17  and the electrode terminal layer SS and  56  on the top regions of the projections  53  and  54  ( FIG. 5   a ). An iron or nickel layer  18  is formed on each outer surface of this assembly ( FIG. 5   b ). Then, a carbon layer is deposited on the entire surface of the assembly by CVD at 600° C., for instance, and a carbon nano-tube film (gas diffusion layers  3   a  and  4   a ) is grown thereon at 300 to 600° C. under an atmospheric condition ( FIG. 5   c ). In this step, a part of the carbon fails to grow into fibers. The part of the carbon which has failed to turn into fibers including that which has only partially grown into fibers is removed by adding oxygen. The sacrificial material  17  is removed by using hydrogen fluoride (HF) to define the air passages  10  and the fuel gas passages  11  ( FIG. 5   d ). At the same time, the iron or nickel layer  18  on the recesses  51  and  52  is also substantially entirely removed so that it would not hamper the diffusion of the gases. 
     A platinum catalyst layer  3   b  and  4   b  consisting of a porous film is deposited on the surface of each carbon nana-tube film  3   a  and  4   a  to a thickness in the range of 10 nm to 100 nm by sputtering or evaporation ( FIG. 5   e ). Finally, a SPE layer made of similar material as the SPE  22  is formed over the entire surface of the assembly to a thickness in the range of 1 to 10 μm by spin-coating although it is not shown in the drawing. 
     In this manner, in each fuel cell, a pair of flow distribution plates  5  are placed on either side of an electrolyte layer  2  via a gas diffusion electrode layer  3  or  4 , and these components are joined by anodic bonding along the parts surrounding the recesses. Therefore, a plurality of narrow passages  11  are defined in one of the central recesses  52  of each flow distribution plate  5  for the fuel gas, and a plurality of similar narrow passages  10  are defined in the other of the central recesses  51  of the flow distribution plate  5  for the oxidizing gas. 
     The SPE layer which is placed on the platinum catalyst layer  3   b  and  4   b  on the surface of each gas diffusion layer  3   a  and  4   a  facing away from the flow distribution plate  5  serves as a bonding agent, and this contributes to a favorable bonding between the platinum catalyst layer  3   b  and  4   b  and the SPE  22 . 
     The adhesion between the grid frame  21  and the distribution plates  5  can be accomplished in a number of different ways. Preferably, anodic bonding is used as described in the following. An electrode layer  9  and a layer  8  of heat resistance and hard glass, for instance, made of Pyrex glass (tradename) are formed along the peripheral surface of the grid frame  21  of the electrolyte layer  2  on each side thereof by sputtering, and a similar electrode layer  9  is formed along the peripheral part of the opposing surface of the distribution plates  5 . Then, with this assembly heated to about 400° C. at which sodium ions become highly mobile, an electric field is produced in the assembly so as to move ions. In the fuel cell assembly of the present invention, if the electrolyte consists of solid polymer, heating the entire assembly to the temperature of 400° C. may damage the solid electrolyte. Therefore, according to this embodiment, a heater (not shown in the drawing) is placed under the electrode layer  9  to selectively heat only the peripheral part of the flow distribution plates. The heater may consist of polycrystalline silicon sandwiched between insulating layers such as Si 3 N 4  layers. If the electrode terminal layer  55  and  56  extend under the heater, the thermal efficiency of the heater will be impaired. Therefore, it is preferable to omit the electrode terminal layer  55  and  56  from under the heater. 
     The grid frame  21  and the distribution plates  5  are placed one over another, and compressed at a pressure of 100 gf/cm 2  to 2,000 gf/cm 2 . Electric current is conducted through the polycrystalline silicon heater to locally heat the bonded area to a temperature in the order of 400° C. At the same time, a voltage in the order of 100 to 500 V is applied between the electrode layer  9  of the grid frame  21  and the electrode layer  9  of the distribution plate  5  for 10 to 30 minutes. 
     Alternatively, a bonding agent may be used for attaching the grid frame  21  and the distribution plates  5  together. In either case, it is possible to eliminate the need for any sealing arrangements or clamping arrangements to achieve a desired sealing capability, and this allows a compact design of the fuel cell assembly. 
     As the fuel gas and oxidizing gas (air) are conducted through this fuel cell  1 , an electrochemical reaction takes places by virtue of the platinum catalyst, and an electric voltage develops between the electrode terminal layers  55  and  56 . A number of such fuel cells are stacked so that a desired voltage can be obtained. 
     Although the fuel and oxidant for the fuel cells described herein consist of gases, they may also include liquids. 
     Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.