Abstract:
A fuel cell has a hydrogen flow path adapted to pass hydrogen into communication with an anode catalyst of an MEA. A coolant flow path is adapted to pass coolant through the fuel cell to cool the fuel cell. An enclosure encompasses at least a portion of the hydrogen flow path, the coolant flow path, or both. A hydrogen vent is adapted to vent hydrogen from the enclosure without reliance upon any electrical device. The hydrogen vent can prevent a frame front from passing into the enclosure and can be made of a porous material such as cellulose, plastic (for example, a foamed plastic) or metal (for example a sintered metal). A method of manufacturing a fuel cell includes passively venting hydrogen to maintain a hydrogen concentration level within the enclosure below about 4 percent. Additional enclosures with hydrogen vents may also be provided.

Description:
FIELD OF THE INVENTION 
     The present invention relates to fuel cells; and more particularly, to various enclosures in a fuel cell system. 
     BACKGROUND OF THE INVENTION 
     Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. The oxygen can be either a pure form (O 2 ) or air (a mixture of O 2  and N2). PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, gas impermeable, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell&#39;s gaseous reactants over the surfaces of the respective anode and cathode catalysts. 
     The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. By way of example, some typical arrangements for multiple cells in a stack are shown and described in U.S. Pat. No. 5,763,113. 
     The electrically conductive plates sandwiching the MEAs may contain an array of grooves in the faces thereof that define a reactant flow field for distributing the fuel cell&#39;s gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels. 
     In a fuel cell stack, a plurality of cells are stacked together in electrical series while being separated by a gas impermeable, electrically conductive bipolar plate. In some instances, the bipolar plate is an assembly formed by securing a pair of thin metal sheets having reactant flow fields formed on their external face surfaces. Typically, an internal coolant flow field is provided between the metal plates of the bipolar plate assembly. Various examples of a bipolar plate assembly of the type used in PEM fuel cells are shown and described in commonly-owned U.S. Pat. No. 5,776,624. 
     Hydrogen has been known to accumulate in the coolant system of the fuel cell. For example, hydrogen has been found to migrate into the coolant flow field and accumulate in the cooling system. Hydrogen may also disassociate from the coolant itself. Previous approaches to venting accumulated hydrogen from a fuel cell have included use of a hydrogen detector and/or fan to actively ventilate the reservoir. These electrical devices consume electricity, reducing the efficiency of the fuel cell. In addition, they can result in ventilating coolant vapor, thereby evaporating the coolant. 
     SUMMARY OF THE INVENTION 
     The present invention is capable of eliminating one or more of the disadvantages discussed above. In addition, it has been discovered that hydrogen may also accumulate in a fuel cell housing enclosure and/or in fuel cell system enclosures other than the coolant flow path. For example, hydrogen may leak from various pipes and fittings of the hydrogen flow path, including the hydrogen supply. Certain aspects of the present invention can also enable the venting of accumulated hydrogen from these other enclosures of a fuel cell. 
     In accordance with one aspect of the present invention a fuel cell is provided. The fuel cell has a hydrogen flow path adapted to pass hydrogen into communication with an anode catalyst of an MEA. A coolant flow path is adapted to pass coolant through the fuel cell to cool the fuel cell. An enclosure encompasses at least a portion of the hydrogen flow path, the coolant flow path, or both. A hydrogen vent is adapted to vent hydrogen from the enclosure without reliance upon any electrical device. 
     In accordance with another aspect of the present invention a method of manufacturing a fuel cell is provided. The method includes creating a hydrogen fuel flow path to conduct hydrogen through the fuel cell. An enclosure is created which captures hydrogen that leaks, directly or indirectly, from the hydrogen flow path. The concentration of hydrogen which leaks into the enclosure is passively maintained below a level of about 4 percent. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is an exploded isometric view of a PEM fuel stack; and 
         FIG. 2  is a schematic view of a fuel cell including various enclosures, each having a hydrogen vent according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
       FIG. 1  schematically depicts a partial PEM fuel cell stack having a pair of membrane-electrode-assemblies (MEAs)  8  and  10  separated from each other by a non-porous, electrically-conductive bipolar plate  12 . Each of the MEAs  8 ,  10  have a cathode face  8   c ,  10   c  and an anode face  8   a ,  10   a . The MEAs  8  and  10 , and bipolar plate  12 , are stacked together between non-porous, electrically-conductive, liquid-cooled bipolar plates  14  and  16 . The bipolar plates  12 ,  14  and  16  each include flow fields  18 ,  20  and  22  having a plurality of flow channels formed in the faces of the plates for distributing fuel and oxidant gases (i.e., H 2  &amp; O 2 ) to the reactive faces of the MEAs  8  and  10 . Nonconductive gaskets or seals  26 ,  28 ,  30 , and  32  provide a seal and electrical insulation between the several plates of the fuel cell stack. Porous, gas permeable, electrically conductive sheets  34 ,  36 ,  38  and  40  press up against the electrode faces of the MEAs  8  and  10  and serve as primary current collectors for the electrodes. Primary current collectors  34 ,  36 ,  38  and  40  also provide mechanical supports for the MEAs  8  and  10 , especially at locations where the MEAs are otherwise unsupported in the flow field. Suitable primary current collectors include carbon/graphite paper/cloth, fine mesh noble metal screens, open cell noble metal foams, and the like which conduct current from the electrodes while allowing gas to pass therethrough. 
     Bipolar plates  14  and  16  press up against the primary current collector  34  on the cathode face  8   c  of MEA  8  and primary current collector  40  on the anode face  10   a  of MEA  10 , while the bipolar plate  12  presses up against the primary current collector  36  on the anode face  8   a  of MEA  8  and against the primary current collector  38  on the cathode face  10   c  of MEA  10 . An oxidant gas such as oxygen or air is supplied to the cathode side of the fuel cell stack from a storage tank  46  via appropriate supply plumbing  42 . Similarly, a fuel such as hydrogen is supplied to the anode side of the fuel cell from a storage tank  48  via appropriate supply plumbing  44 . In a preferred embodiment, the oxygen tank  46  may be eliminated, and air supplied to the cathode side from the ambient. Likewise, the hydrogen tank  48  may be eliminated and hydrogen supplied to the anode side from a reformer which catalytically generates hydrogen from methanol or a liquid hydrocarbon (e.g., gasoline). Exhaust plumbing (not shown) for both the H 2  and O 2 /air sides of the MEAs is also provided for removing H 2 -depleted anode gas from the anode flow field and O 2 -depleted cathode gas from the cathode flow field. Coolant plumbing  50  and  52  is provided for supplying and exhausting liquid coolant to the bipolar plates  14  and  16 , as needed. Each of the bipolar plates  14  and  16  include a plurality of flow channels forming a coolant flow field. 
     Referring to  FIG. 2 , a cooling system for a fuel cell stack  60  is illustrated. The cooling system includes inlet line or pipe  50  in fluid communication, via an appropriate manifold to the coolant flow fields of bipolar plates  14  and  16 . After traveling through the coolant flow fields, the coolant passes out of the fuel cell stack  60  via an appropriate header to coolant outlet line  52 . An electronic control valve  62  controls passage of the coolant through bypass line  68  and/or through line  64  to a radiator and/or fan  66 . Thus, as the temperature of the fuel cell  60  increases, more of the coolant flows through the radiator  66  under the influence of the control valve  62 . Upon exiting the control valve  62 , the coolant from the fuel cell stack  60  mixes with coolant flowing from the coolant reservoir  72  through line  70 , as necessary. 
     A pump  74  pumps the coolant back into the fuel cell stack  60  via line or pipe  76 . A drain line  84  is also provided to permit coolant to be drained from the fuel cell stack  60  into the coolant reservoir  72  by manipulation of a drain valve  86 . As the coolant travels through the fuel cell stack  60  it comes into close proximity to hydrogen also traveling through the fuel cell stack  60 . In some cases, for example, the hydrogen and the coolant may be traveling in adjacent channels of their respective flow fields separated only by a sealant. Hydrogen has the potential to migrate into the coolant flow channels or the enclosure that defines the coolant flow path. 
     Any hydrogen that might have migrated into the coolant enclosure tends to accumulate in the highest point within the coolant system. Typically, this highest point is in the coolant reservoir  72 . Consequently, it is desirable to locate a hydrogen vent  88  in a wall of the coolant enclosure defining the flow path; preferably in a wall  90  of the coolant reservoir  72 . The coolant flow path is defined by the enclosure created by, e.g., the flow channels in the fuel cell stack  60 , the coolant reservoir  72  and the lines  50 ,  52 ,  64 ,  68 ,  70  and  76 . Thus, the hydrogen vent  88  may be placed within a wall of any of these enclosure components of the coolant flow path. 
     The hydrogen vent  88  of this embodiment is adapted to allow hydrogen to pass therethrough while simultaneously preventing any coolant (including evaporating coolant vapor) from passing therethrough. In addition, the hydrogen vent  88  is preferably adapted to prevent a frame front from passing into the enclosure through the hydrogen vent  88 . The hydrogen vent  88  provides pores (represented by the cross-hatching) which are sufficiently large to allow hydrogen molecules to pass therethrough. The pores are also preferably sufficiently small that coolant, including coolant vapors, cannot pass therethrough. Thus, the hydrogen vent  88  is passive. As used herein “passive” means that the hydrogen vent does not require any electrical or other active components to function. For example, the hydrogen vent  88  requires no electrical components such as a sensor, controller, or fan are required. 
     In addition, the hydrogen vent  88  is preferably adapted to passively vent hydrogen such that the hydrogen remains below about 4 percent within the enclosure  72 ; and more preferably, below about 1 percent. The hydrogen vent  88  is preferably made of a porous material selected from the group consisting of cellulose, plastic (for example, a foamed plastic) or metal (for example, a sintered metal). 
     The fuel cell system has various enclosures, including a fuel cell stack enclosure  92 . This fuel cell stack enclosure  92  encompasses an area surrounding a part of the coolant flow path. For example, it includes inlet coolant  51 , the coolant flow fields, and coolant outlet  53 . Similarly, the fuel cell enclosure  92  includes an area surrounding a part of the hydrogen flow path and the oxygen flow path. With respect to the hydrogen flow path, for example, the fuel cell enclosure surrounds the inlet  94 , hydrogen flow field, and the outlet  96 . 
     The fittings and headers which connect the various components of the hydrogen flow path within the fuel cell enclosure  92  are potential sources for hydrogen leaks. Similarly, hydrogen which may have migrated into the cooling system may potentially leak from corresponding fittings and manifolds from the coolant flow path within the fuel cell enclosure  92 . Thus, a hydrogen vent  98  is located within a wall  100  of the fuel cell enclosure  92 . This hydrogen vent  98  has the same properties discussed above with respect to the hydrogen vent  88  of the coolant flow reservoir  72 . 
     The fuel cell system enclosure defining the coolant flow path, including lines  50 ,  52 ,  64 ,  68 ,  76  and reservoir  72 , the hydrogen supply tank  48  and the fuel cell enclosure  92  are all located within an overall fuel cell system enclosure  110 . This system enclosure  110  encompasses an area surrounding the coolant reservoir  72  and the fuel cell stack enclosure  92 . Thus, hydrogen vented through the hydrogen vent  88  of the coolant reservoir  72  or through the hydrogen vent  98  of the fuel cell enclosure  92  is still contained within the system enclosure  110 . In addition, the hydrogen supply tank  48  is located within the system enclosure  110 . As indicated previously, the hydrogen tank  48  may be replaced with a reformer. Thus, the entire hydrogen flow path is enclosed within the system enclosure  110 , although H 2 -depleted gas leaving the fuel cell stack  60  via outlet  96  exits the system enclosure  110 . 
     As indicated above, this system enclosure  110  includes various potential hydrogen sources. For example, hydrogen may be vented into the system enclosure  110  by the coolant reservoir  72  hydrogen vent  88  or by the fuel cell enclosure  92  hydrogen vent  98 . In addition, hydrogen may potentially leak from the hydrogen supply tank  48  or reformer and associated hydrogen flow lines  111  and fittings. Thus, a hydrogen vent  108  is located within a wall  112  of the system enclosure  110  to vent hydrogen to the atmosphere. This hydrogen vent  108  has the same properties discussed above with respect to the previously identified hydrogen vents  88 ,  98 . 
     Of course, many alternatives to the previously described preferred embodiment can be envisioned by those skilled in the art based upon the above description. For example, the hydrogen supply tank may be located outside of the system enclosure, but have its own hydrogen supply enclosure encompassing the area around the hydrogen supply tank. In addition, an oxygen supply tank may additionally be located within the system enclosure of  FIG. 2 . 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.