Abstract:
A fuel cell stack is disclosed that utilizes a porous material internally disposed in the fuel cell outlet manifolds, wherein the porous material facilitates the transport of liquid water from the plate outlets thereby minimizing the accumulation of liquid water in the fuel cell stack.

Description:
FIELD OF THE INVENTION 
     The invention relates to a fuel cell stack utilizing a porous material disposed in the fuel cell manifolds, wherein the porous material facilitates the transport of liquid water from fuel cell channels, thereby minimizing the accumulation of liquid water in the fuel cell stack. 
     BACKGROUND OF THE INVENTION 
     Fuel cell power systems convert a fuel and an oxidant to electricity. One type of fuel cell power system employs use of a proton exchange membrane (hereinafter “PEM”) to catalytically facilitate reaction of fuels (such as hydrogen) and oxidants (such as air or oxygen) to generate electricity. The PEM is a solid polymer electrolyte that facilitates transfer of protons from the anode to the cathode in each individual fuel cell of the stack of fuel cells normally deployed in a fuel cell power system. 
     In a typical fuel cell assembly (stack) within a fuel cell power system, individual fuel cells provide channels through which various reactants and cooling fluids flow. Fuel cell plates are typically designed with serpentine flow channels. Serpentine flow channels are desirable as they effectively distribute reactants over the active area of an operating fuel cell, thereby maximizing performance and stability. Movement of water from the channels to outlet manifolds of the fuel cell plates is caused by the flow of the reactants through the fuel cell. Drag forces pull the liquid water through the channels until the liquid water exits the fuel cell through the outlet manifold. However, when the fuel cell is operating at a lower power output, the velocity of the gas flow is too to low produce an effective drag force to transport the liquid water, and the liquid water accumulates in the flow channels. 
     A further limitation of utilizing gas flow drag forces to remove the liquid water is that the water encounters various surface irregularities with high or low surface energy or pinning points on the flow channel surfaces. Because the drag forces may not be strong enough to effectively transport the liquid water, the pinning points may cause the water to accumulate and pool, thereby stopping the water flow. Such pinning points are those commonly located where the channel outlets meet the fuel cell stack manifold. 
     Additionally, some current fuel cell assemblies utilize plates having hydrophilic surfaces. Water has been observed to form a film on the surface of the material that accumulates at the outlet of the flow channels and the perimeter of the plates. The water film can block the gas flow, which in turn reduces the driving force for removing liquid water and thus militates against the removal of the liquid water from the fuel cell stack. In the case of a fuel cell plate with a mildly hydrophobic surface, water has been observed to form large drops that protrude into the fuel cell stack outlet manifold blocking the exits of the channels of the fuel cell plates. The droplets are observed to remain at the plate edge until they can be intermittently removed by gas shear. The accumulation of water can cause gas flow blockages or flow imbalances that can have negative impacts on the performance of the stack. 
     It would be desirable to develop a fuel cell stack with an improved means for removing liquid water from fuel cell gas flow channels of the fuel cell stack, to minimize the accumulation of liquid water within the fuel cell stack. 
     SUMMARY OF THE INVENTION 
     Concordant and congruous with the present invention, a fuel cell stack with an improved means for removing liquid water from fuel cell flow channels of the fuel cell stack, to minimize the accumulation of liquid water within the fuel cell stack, has been discovered. 
     In one embodiment, the fuel cell assembly comprises a fuel cell stack including a plurality of fuel cell plates, wherein each plate includes at least one aperture formed therein, the apertures of said fuel cell plates substantially aligned to form a manifold; and a porous material disposed in the manifold, wherein the porous material is adapted to facilitate the flow of water from the fuel cell plates, through the porous material, and out of said fuel cell stack. 
     In another embodiment, the fuel cell assembly comprises a fuel cell stack including a plurality of fuel cell plates, wherein each plate includes at least two apertures formed therein, the apertures of said fuel cell plates substantially aligned to form an inlet manifold and an outlet manifold; and a porous material disposed in the manifold, wherein the porous material is adapted to facilitate the flow of water from the fuel cell plates, through the porous material, and out of said fuel cell stack through the manifold. 
     In another embodiment, the fuel cell assembly comprises a fuel cell stack including a plurality of fuel cell plates, wherein each plate includes at least two apertures formed therein, the apertures of said fuel cell plates substantially aligned to form an inlet manifold and an outlet manifold; a porous material disposed in the manifold, wherein the porous material is adapted to facilitate the flow of water from the fuel cell plates, through the porous material, and out of said fuel cell stack through the manifold; and a porous support adapted to maintain the position of said porous material against an inner surface of the manifold. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
         FIG. 1  is a cross-sectional view of a fuel cell stack incorporating a porous material in a fuel cell stack manifold according to an embodiment of the invention; 
         FIG. 2  is a top plan view of a fuel cell plate illustrative of the fuel cell plates of the fuel cell stack of the present invention; 
         FIG. 3  is a cross-sectional view of a fuel cell stack incorporating a porous material forming a point in a fuel cell stack manifold according to another embodiment of the invention; and 
         FIG. 4  is a fragmentary top plan view of a manifold of a fuel cell stack incorporating a porous support structure according to another embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description and appended drawings describe and illustrate various exemplary embodiments of the invention. The description and drawings serve to enable one skilled in the art to make and use the invention, and are not intended to limit the scope of the invention in any manner. 
       FIG. 1  shows a fuel cell assembly  10  including a plurality of stacked fuel cell plates  12 , an inlet manifold  14 , an outlet manifold  16 , and a porous material  18 .  FIG. 2  shows a top view of a typical fuel cell plate  12  including three inlet apertures  20 , three outlet apertures  22 , and a plurality of flow channels  24 . It is understood that the flow channels  24  include the channels disposed on a face of the fuel cell plate  12  as well as the gas passages disposed intermediate the faces of the fuel cell plate  12 . It is also understood that the material of construction, size, shape, quantity, and type of plates  12  in the fuel cell assembly  10 , and the configuration of the fuel cell plates  12  within the assembly  10 , may vary based on design parameters such as the amount of electricity to be generated, the size of the machine to be powered by the fuel cell assembly  10 , the volumetric flow rate of gases through the fuel cell assembly  10 , and other similar factors, for example. It is also understood that the plate  12  shown in  FIG. 2  may be used for an anode side (not shown) or for a cathode side (not shown) of the fuel cell assembly  10 . Further, it is understood that the plate  12  may have any number of inlet apertures  20  and outlet apertures  22 , as desired. As shown, the flow channels  24  are undulated, however, it is understood that the flow channels  24  may be substantially linear, serpentine, or other configuration, as desired. 
     The inlet manifold  14  includes an inlet  15 . The inlet manifold  14  is formed in the fuel cell assembly  10  by the inlet apertures  20  of the fuel cell plates  12 . The plates  12  are stacked with the inlet aperture  20  of each plate  12  substantially aligned with the inlet aperture  20  of an adjacent plate or plates  12 . It is understood that the diameter, quantity, and length of the inlet manifold  14  will depend on the size and quantity of inlet apertures  20  in the plates  12  and the number of plates  12  stacked in the fuel cell assembly  10 . 
     The outlet manifold  16  includes an outlet  17 . The outlet manifold  16  is formed in the fuel cell assembly  10  by the outlet apertures  22  of the fuel cell plates  12 . The plates  12  are stacked with the outlet aperture  22  of each plate  12  substantially aligned with the outlet aperture  22  of an adjacent plate or plates  12 . It is understood that the diameter, quantity, and length of the outlet manifold  16  will depend on the size and quantity of outlet apertures  22  in the plates  12  and the number of plates  12  stacked together in the fuel cell assembly  10 . 
     In the embodiment shown, the porous material  18  is a non-conductive foam having a first end  26  and a second end  28 . The porous material  18  may also include a hydrophilic coating (not shown). The first end  26  of the porous material  18  is positioned adjacent a dry end compression plate  30  of the fuel cell stack. The second end  28  of the porous material  18  has a substantially flat configuration and extends beyond a lower extremity of the fuel cell plates  12  of the assembly  10 . Alternatively, the second end  28  of the porous material  18  may form a point  34 , as illustrated in  FIG. 3 , have a rounded shape, or other shape, as desired. As shown in  FIG. 1 , the porous material  18  substantially fills the outlet manifold  16  and extends through the entire length of the outlet manifold  16  and into a portion of the wet end compression plate  32 . It is understood that the porous material  18  may extend through the wet end compression plate  30 , if desired. It is further understood that the porous material  18  may fill only a portion of the outlet manifold  16  and not extend into the wet end compression plate  32 . It is also understood that the porous material  12  may be disposed in an anode side outlet manifold (not shown), a cathode side outlet manifold (not shown), or both anode and cathode outlet manifolds. Further, it is understood that the porous material  18  may be any conductive or non-conductive open cell porous material, such as a fibrous material and a sponge or an assembly of a plurality of porous materials, for example. The porous material  18  has a hydrophilic coating such as silicon oxide (SiO x ) or other chemical coating having hydrophilic characteristics. Alternatively, the porous material  18  may be provided without a hydrophilic treatment. The porous material  18  may also be of constant pore size and porosity, or the porous material  18  may have a varying pore size. For example, the porous material  18  may have a higher density with small pore sizes where the fuel cell plates  12  abut the porous material  18  and a lower density with larger pore sizes throughout the remainder of the porous material  18  to reduce flow resistance. 
     Generally, during operation of a fuel cell power system, a stream of hydrogen is fed into the anode side of the fuel cell assembly  10 . Concurrently, a stream of oxygen is fed into the cathode side of the fuel cell assembly  10 . On the anode side, the hydrogen in the hydrogen stream is catalytically split into protons and electrons. The oxidation half-cell reaction is represented by: H 2             2H + +2e − . In a polymer electrolyte membrane fuel cell, the protons permeate through the membrane to the cathode side. The electrons travel along an external load circuit to the cathode side creating the current of electricity of the fuel cell assembly  10 . On the cathode side, the oxygen in the oxidant stream reacts with the protons permeating through the membrane and the electrons from the external circuit to form water molecules. This reduction half-cell reaction is represented by: 4H + +4e − +O 2           2H 2 O. Anode exhaust from the anode side flows through a backpressure control valve (not shown) to a combustor, or is alternatively recycled back to the anode inlet manifold. Cathode exhaust from the cathode side flows through a second backpressure control valve (not shown) to the combustor or to the ambient environment. A control module (not shown) regulates the conditions of the hydrogen stream, oxygen stream, and exhaust streams by operating various control valves (not shown), backpressure control valves (not shown), and compressors (not shown) in response to signals from pressure sensors (not shown) and electrical power sensors (not shown) connected to the fuel cell assembly  10 .
     When the invention according to a first embodiment is in operation, the above reactions take place within the fuel cell assembly  10 , and droplets of liquid water are formed in the channels  24  of the fuel cell plates  12  on the cathode sides of the fuel cell assembly  10 . Some water also may be transported into the anode flow channels, or may form in the anode channels via condensation resulting from consumption of the hydrogen. It is understood that the operation as described herein for the cathode side is similar to operation for the anode side of the fuel cell assembly  10 . The air stream flowing through the cathode side causes the water droplets to flow through the channels  24 , toward the outlet manifold  16 . Water vapor also flows towards the outlet manifold  16 . Once the water droplets contact the porous material  18 , the water is wicked away from the channels  24  by the porous material  18 . The hydrophilic coating on the porous material  18  will provide additional capillary force to attract the water droplets and the condensed water vapor. The exhaust gas streams also pass through the porous material  18 , and through the outlet manifold  16 . 
     It is expected that two different water transport mechanisms may be utilized to remove the water from the channels  24 , depending on the porous material  18  used. First, the porous material  18  may form a network of open continuous pores that are capable of utilizing capillary forces to move the water through the pores. A porous material  18  having varying pore sizes is provided. A portion of the porous material  18  having a higher density of pores is disposed immediately adjacent the fuel cell plates  12 . The high density pore portion provides small pore sizes in the porous material  18  to facilitate wicking of the water out of the fuel cell plates  12 . The remaining portion of the porous material  18  has a lower density of pores that provides larger pore sizes to provide a lower pressure drop region for the gas streams to flow through with a minimal pressure drop. 
     Second, the mechanism to remove water using the porous material  18  includes pores having a larger size. The porous material  18  is produced from a hydrophilic material and the liquid water and condensed water vapor form a film (not shown) on the fibers of the porous material  18 . The film forms a continuous path along the fibers from the channels  24  of the fuel cell plates  12  to the outlet manifolds  16  and to the outlet  17  of the fuel cell assembly  10 . 
     Both of the water transport mechanisms described above relies on gravity to remove the water from the porous material  18 . It is desirable, though not necessary, for a portion of the porous material  18  to be saturated to create a sufficient head to cause the water to drain from the porous material  18 . The head height varies inversely with the average pore size of the porous material. As shown in  FIG. 1 , the portion of the porous material  18  extending into the wet end compression plate  32  is the saturated portion. It is desirable that the saturated portion be below the fuel cell plates  12  rather than immediately adjacent any plates  12 , to minimize a pressure drop through the porous material  18  or blockage of the channels  24 . If a saturated portion is not created in the porous material  18 , a peristaltic pump  21  may used with the fuel cell assembly  10  to cause the water to flow through the assembly  10  and out of the porous material  18 . The peristaltic pump  21  may be a peristaltic pump such as the one disclosed by Anonymous, Pump to Remove Water from a Wick, Pub. No. 494084, O.G. June 2005. During operation of the fuel cell assembly  10 , it is anticipated that portions of an operational cycle will result in the outlet gas streams being less than saturated, here evaporation will aid water removal from the porous material  18 . 
       FIG. 4  shows a portion of a fuel cell assembly  10 ′ including a plurality of stacked fuel cell plates  12 ′, an inlet manifold (not shown), an outlet manifold  16 ′, a porous material  18 ′, and a porous or perforated support  19 ′. The fuel cell plate  12 ′, similar to the plate  12  shown in  FIG. 1 , includes two inlet apertures (not shown), two outlet apertures  22 ′ and a plurality of flow channels  24 ′. It is understood that the material of construction, size, shape, quantity, and type of plates  12 ′ in the fuel cell assembly  10 ′, and configuration of the fuel cell plates  12 ′ within the fuel cell assembly  10 ′ may vary based on design parameters such as the amount of electricity to be generated, the size of the machine to be powered by the fuel cell assembly  10 ′, the volumetric flow rate of gases through the fuel cell assembly  10 ′, and other similar factors, for example. It is also understood that the plate  12 ′ may be disposed on an anode side (not shown) or on a cathode side (not shown) of the fuel cell assembly  10 ′. Further, it is understood that the plate  12 ′ may have any number of inlet apertures and outlet apertures  22 ′, as desired. As shown, the flow channels  24 ′ are substantially linear, however, it is understood that the flow channels  24 ′ may be undulated, serpentine, or have another configuration, as desired. 
     The inlet manifold includes an inlet (not shown). The inlet manifold is formed in the fuel cell assembly  10 ′ by the inlet apertures  20 ′ of the fuel cell plates  12 ′. The plates  12 ′ are stacked one on top of another with the inlet aperture of each plate  12 ′ substantially aligned with the inlet aperture of an adjacent plate  12 ′. It is understood that the diameter, quantity, and length of the inlet manifold  14 ′ will depend on the size and quantity of inlet apertures in the plates  12 ′ and the number of plates  12 ′ stacked together in the fuel cell assembly  10 ′. 
     The outlet manifold  16 ′ includes an outlet (not shown). The outlet manifold  16 ′ is formed in the fuel cell assembly  10 ′ by the outlet apertures  22 ′ of the fuel cell plates  12 ′. The plates  12 ′ are stacked with the outlet aperture  22 ′ of each plate  12 ′ substantially aligned with the outlet aperture  22 ′ of an adjacent plate or plates  12 ′. It is understood that the diameter, quantity, and length of the outlet manifold  16 ′ will depend on the size and quantity of outlet apertures  22 ′ in the plates  12 ′ and the number of plates  12 ′ stacked together in the fuel cell assembly  10 ′. 
     In the embodiment shown, the porous material  18 ′ is a non-conductive foam having a hydrophilic surface  23 ′. As shown in  FIG. 4 , the porous material  18 ′ is disposed radially inward of an inner surface of the outlet manifold  22 ′. The porous material  18 ′ may extend through a portion of the outlet manifold  16 ′ or the entire manifold  16 ′ and into a portion of the wet end compression plate (not shown), as desired. The porous material  18 ′ may not extend around the entire perimeter of the manifold  22 ′ but may press against a portion of the manifold  22 ′ to support the required water flow. It is understood that the porous material  12 ′ may disposed in an anode side outlet manifold, a cathode side outlet manifold, or both anode and cathode outlet manifolds. It is also understood that the porous material  18 ′ may be any conductive or non-conductive open cell porous material, such as a fibrous material, a sponge, and an assembly of a plurality of porous materials, for example. The hydrophilic surface  23 ′ on the porous material  18 ′ may be a silicon oxide (SiO x ) or other chemical treatments or coatings that yield hydrophilic surface characteristics. Alternatively, the porous material  18 ′ may be provided without a coating. The porous material  18 ′ may also be of constant pore size, or the porous material  18 ′ may have varying pore sizes. The porous material  18 ′ may have a higher density where the fuel cell plates  12 ′ abut the porous material  18 ′ and a lower density throughout the remainder of the porous material  18 ′, for example. 
     In the embodiment shown in  FIG. 4 , the porous support  19 ′ is a perforated tube disposed radially inward of the porous material  18 ′ in the outlet manifold  16 ′. The porous support  19 ′ maintains a position of the porous material  18 ′. It is understood that the porous support  19 ′ could be a plastic screen or other similar structure capable of maintaining the position of the porous material  18 ′ against an inner surface of the manifold  16 ′. It is also understood that the porous support  19 ′ may be conductive or non-conductive as desired. Further, it is understood that the porous support  19 ′ may have hydrophilic surface properties such as that provided by a silicon oxide (SiO x ) coating, for example. 
     When the invention according to the embodiment shown in  FIG. 4  is in operation, the above reactions take place within the fuel cell assembly  10 ′, and droplets of liquid water are formed in the channels  24 ′ of the fuel cell plates  12 ′. It is understood that the operation as describe herein for the cathode side is similar to operation for the anode side of the fuel cell assembly  10 ′. The air stream flowing through the cathode side causes the water droplets to flow through the channels  24 ′, toward the outlet manifold  16 ′. Water vapor also flows towards the outlet manifold  16 ′. Once the water droplets or vapor contact the porous material  18 ′, the water is wicked away from the channels  24 ′ by the porous material  18 ′. The hydrophilic coating on the porous material  18 ′ acts to attract the water droplets and to transport the water to the outlet manifold  16 ′ of the fuel cell assembly  10 ′. It is understood that water from the water vapor may condense in the porous material  18 ′. Condensation of the vapor causes an additional pressure drop in the fuel cell assembly  10 ′. It is understood that any condensation that occurs is incidental to the invention, however alteration of the dimensions, configuration, and materials used for the porous material  18 ′, fuel cell plates  12 ′, and other components, as well as other design considerations, may be made to compensate for the additional pressure drop. The exhaust gas streams also pass through the porous material  18 ′, the porous support  19 ′ and through the outlet manifold  16 ′. The porous support  19 ′ promotes a low pressure drop to facilitate the flow of the exhaust gases through the porous material  18 ′ and manifold  16 ′. 
     It is expected that two different water transport mechanisms may be utilized to remove the water from the channels  24 ′ depending on the porous material  18 ′ used. First, the porous material  18 ′ may form a network of open continuous pores that are capable of utilizing capillary forces to move the water through the pores of the porous material  18 ′. A porous material  18 ′ having a varying density is provided. A portion of the porous material  18 ′ having a higher density is disposed immediately adjacent the fuel cell plates  12 ′. The high density portion provides small pore sizes in the porous material  18 ′ to facilitate wicking of the water. The remaining portion of the porous material  18 ′ has a lower density that provides larger pore sizes to provide a lower pressure drop region for the gas streams to flow through with a minimal pressure drop. 
     Second, the porous material  18 ′ includes pores having a larger size. The porous material  18 ′ is produced from a hydrophilic material or treated to have a hydrophilic surface property and the liquid water forms a film (not shown) on the fibers of the porous material  18 ′. The film forms a continuous path along the fibers from the exits of the fuel cell plates  12 ′ to the outlet manifolds  16 ′ and to the outlet  17 ′ of the fuel cell assembly  10 ′. Water is removed by dipping off the lowest point of the porous material  18 ′. 
     Both of the water transport mechanisms described above rely on gravity to remove the water from the porous material  18 ′. It is desirable, though not necessary, for a portion of the porous material  18 ′ to be saturated to create a sufficient head to cause the water to drain from the porous material  18 ′. A portion of the porous material  18 ′ may extend into the wet end compression plate  32 ′. The portion of the porous material  18 ′ may be the saturated portion. It is desirable that the saturated portion is below the fuel cell plates  12 ′ rather than immediately adjacent any plates  12 ′, to minimize a pressure drop through the porous material  18 ′ or blockage of the channels  24 ′. If a saturated portion is not created in the porous material  18 ′, a peristaltic pump may used with the fuel cell assembly  10 ′ to cause the water to flow through the assembly  10 ′ and out of the porous material  18 ′. The peristaltic pump  21  may be a peristaltic pump such as the one disclosed by Anonymous, Pump to Remove Water from a Wick, Pub. No. 494084, O.G. June 2005. Water is removed by dipping off the lowest point of the porous material  18 ′. 
     From the foregoing description, one ordinarily skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications to the invention to adapt it to various usages and conditions.