Fuel cell assembly

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 (3a, 4a) 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.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows 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 toFIGS. 2aand2b, each fuel cell1comprises a central electrolyte layer2, a pair of gas diffusion electrode layers3and4(seeFIGS. 2aand2b) placed on either side of the central electrolyte layer2, and a pair of flow distribution plates5placed on either outer side of the gas diffusion electrode layers3and4. The outer side of each flow distribution plate5is similarly formed as the inner side thereof so as to serve as the flow distribution plate for the adjacent fuel cell.

The electrolyte layer2comprises a grid frame21, and solid polymer electrolyte (SPE)22which is filled into rectangular through holes21bdefine between adjacent bars21a of the grid frame21. The SPE22may be made from such materials as perfluorocarbonsulfonic acid (Nafion: tradename), phenolsulfonic acid, polyethylenesulfonic acid, polytrifluorosulfonic acid, and so on.

The grid frame21is 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 bar21ain the grid area of the grid frame21is provided with a projection21cat an intermediate part thereof so as to project into an intermediate part of the corresponding through hole21bas best shown inFIGS. 2aand2b. The projection21cis in the shape of a ridge extending along the length of the bar21a, and produces a narrower middle part in each through hole21b. The projection21chelps to retain the SPE22in each through hole21b.

Such a projection can be conveniently formed at the same time as forming the grid frame21.FIGS. 3ato3cillustrate the process of forming the electrolyte layer2. First of all, a suitably patterned photoresist layer13and14is placed on each side of a silicon wafer serving as the material for the grid frame21as shown inFIG. 3a. An anisotropic etching is performed from both sides of the wafer as illustrated inFIG. 3b,and this produces a plurality of through holes21beach of which is narrowed in a middle part by a projection21c. Then, SPE22is filled into each of the through holes21bso as to define a substantially flush planar surface on each side of the electrolyte layer2.

In this embodiment, a rectangular through hole23a,23b,24aand24bis formed in each corner portion of the fringe portion of the grid frame21. One of the diagonally opposing pairs of these through holes23aand23bserve as inlet and outlet for the full gas. The remaining opposing pair of these through holes24aand24bserve as inlet and outlet for the oxidizing gas.

Each flow distribution plate5is also formed by working a silicon wafer, and has a substantially conformal rectangular shape. A rectangular recess51or52having a flat bottom is formed centrally on each side of the flow distribution plate5, and a plurality of projections53or54each 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 layer55or56by suitable means for electrically connecting the gas diffusion electrode layers3and4to an external circuit.

FIGS. 4ato4cshow the process of forming each flow distribution plate5. A suitably patterned photoresist layer15and16is formed on each side of a silicon wafer as shown inFIG. 4a, and the silicon wafer is etched from both sides to form the recesses51and52and projections53and54at the same time as shown inFIG. 4b. The distribution plate5on 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 layer55and56is formed over the surface of the recesses51and52and projections53and54as shown inFIG. 4c.

The distribution plate5is conformal to the grid frame21, and therefore has a rectangular shape. A rectangular through hole57a,57b,58aor58bis formed in each corner portion of the fringe portion thereof. One of the diagonally opposing pairs of these through holes57aand57bserve as inlet and outlet for the fuel gas. The remaining opposing pair of these through holes58aand58bserve as inlet and outlet for the oxidizing gas. As shown inFIG. 1, grooves59aand59bformed in the fringe portion communicate the recess51with the through holes58aand58bfor the oxidizing gas, and similar grooves60aand60bcommunicate the recess52with the through holes57aand57bfor the fuel gas.

Each gas diffusion electrode layer3and4is formed in a plane passing through the free ends of the corresponding projections53and54, and comprises a gas diffusion layer3aand4aformed 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 layer3band4bformed as a porous layer placed on the surface thereof facing the electrolyte layer2.

FIGS. 5ato5eshow the process of forming each gas diffusion layer3and4. First of all, a flow distribution plate S is formed according to the process illustrated inFIGS. 4aand4c. The recesses51and52are each filled with a sacrificial material17so as to define a flush outer surface with this sacrificial material17and the electrode terminal layer55and56on the top regions of the projections53and54. Alternatively, the sacrificial material17may be deposited to such an extent as to entirely bury the projections53and54therein, and etched back until a flush outer surface is defined with this sacrificial material17and the electrode terminal layer SS and56on the top regions of the projections53and54(FIG. 5a). An iron or nickel layer18is formed on each outer surface of this assembly (FIG. 5b). 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 layers3aand4a) is grown thereon at 300 to 600° C. under an atmospheric condition (FIG. 5c). 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 material17is removed by using hydrogen fluoride (HF) to define the air passages10and the fuel gas passages11(FIG. 5d). At the same time, the iron or nickel layer18on the recesses51and52is also substantially entirely removed so that it would not hamper the diffusion of the gases.

A platinum catalyst layer3band4bconsisting of a porous film is deposited on the surface of each carbon nana-tube film3aand4ato a thickness in the range of 10 nm to 100 nm by sputtering or evaporation (FIG. 5e). Finally, a SPE layer made of similar material as the SPE22is 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 plates5are placed on either side of an electrolyte layer2via a gas diffusion electrode layer3or4, and these components are joined by anodic bonding along the parts surrounding the recesses. Therefore, a plurality of narrow passages11are defined in one of the central recesses52of each flow distribution plate5for the fuel gas, and a plurality of similar narrow passages10are defined in the other of the central recesses51of the flow distribution plate5for the oxidizing gas.

The SPE layer which is placed on the platinum catalyst layer3band4bon the surface of each gas diffusion layer3aand4afacing away from the flow distribution plate5serves as a bonding agent, and this contributes to a favorable bonding between the platinum catalyst layer3band4band the SPE22.

The adhesion between the grid frame21and the distribution plates5can be accomplished in a number of different ways. Preferably, anodic bonding is used as described in the following. An electrode layer9and a layer8of heat resistance and hard glass, for instance, made of Pyrex glass (tradename) are formed along the peripheral surface of the grid frame21of the electrolyte layer2on each side thereof by sputtering, and a similar electrode layer9is formed along the peripheral part of the opposing surface of the distribution plates5. 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 layer9to selectively heat only the peripheral part of the flow distribution plates. The heater may consist of polycrystalline silicon sandwiched between insulating layers such as Si3N4layers. If the electrode terminal layer55and56extend under the heater, the thermal efficiency of the heater will be impaired. Therefore, it is preferable to omit the electrode terminal layer55and56from under the heater.

The grid frame21and the distribution plates5are placed one over another, and compressed at a pressure of 100 gf/cm2to 2,000 gf/cm2. 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 layer9of the grid frame21and the electrode layer9of the distribution plate5for 10 to 30 minutes.

Alternatively, a bonding agent may be used for attaching the grid frame21and the distribution plates5together. 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 cell1, an electrochemical reaction takes places by virtue of the platinum catalyst, and an electric voltage develops between the electrode terminal layers55and56. 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.