Abstract:
An apparatus for water management in a fuel cell. The apparatus includes a fuel cell having a first porous electrode layer, a second porous electrode layer, a proton-conducting membrane positioned between the first electrode and second electrode layers, and a first and second bi-polar distribution plate, wherein the first bi-polar distribution plate is positioned on a top of the first electrode layer and defining a first gas flow channel, and wherein the second bi-polar distribution plate is positioned on a bottom of the second electrode layer and defining a second gas flow channel. The apparatus further includes a mechanism for oscillating liquid water formed in the gas flow channel and configured to remove the liquid water.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Provisional Patent Application Ser. No. 60/839,024 filed on Aug. 21, 2006 entitled “WATER REMOVAL FROM GAS FLOW CHANNELS OF FUEL CELLS”, the entire contents of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The present invention relates to fuel cells, and more specifically to water management within a fuel cell. 
         [0003]    During fuel cell operation, water droplets frequently form on the surface of gas diffusion layers within the fuel cell. Generally, the water droplets migrate through the gas diffusion layers into gas flow channels. In the gas flow channels, movement of the water can be inhibited by pinning of the three-phase region, commonly referred to as the contact line region. In the contact line region, the gas, liquid and solid phases collide. 
         [0004]    A balance between keeping the membrane from becoming too dry or too wet must be maintained for efficient and reliable fuel cell operation. At high current densities, the production of liquid water may exceed the capacity of the gas streams to evaporate the water out of the fuel cell stack and drops of water will appear within the gas flow channels. If the water accumulation becomes too great, then the gas flow channel may become completely blocked by water and the fuel cell will “flood.” Therefore, the water drops must be removed from the gas flow channels for reliable operation. Efficient removal of the product water is an important step in fuel cell operation and enables increased commercial utilization of fuel cells. 
       SUMMARY 
       [0005]    In one embodiment, the invention provides an apparatus for water management in a fuel cell. The apparatus includes a fuel cell having a first porous electrode layer, a second porous electrode layer, a proton-conducting membrane positioned between the first electrode and second electrode layers, and a first and second bi-polar distribution plate, wherein the first bi-polar distribution plate is positioned on a top of the first electrode layer and defining a first gas flow channel, and wherein the second bi-polar distribution plate is positioned on a bottom of the second electrode layer and defining a second gas flow channel. The apparatus further includes a mechanism for oscillating liquid water formed in the gas flow channel and configured to remove the liquid water. 
         [0006]    In another embodiment, the invention provides a system for operating a fuel cell. The system includes a fuel cell having a first porous electrode layer, a second porous electrode layer, a proton-conducting membrane positioned between the first electrode and second electrode layers, and a first and second bi-polar distribution plate, wherein the first bi-polar distribution plate is positioned on a top of the first electrode layer and defining a first gas flow channel, and wherein the second bi-polar distribution plate is positioned on a bottom of the second electrode layer and defining a second gas flow channel. The system further includes a mechanism for oscillating liquid water formed in the gas flow channel. 
         [0007]    In another embodiment, the invention provides in a fuel cell system including a fuel cell, a method of water management for the fuel cell. The method includes passing a gas flow stream through a gas flow channel in the fuel cell, oscillating a liquid water drop in the gas flow channel to the natural frequency of the liquid water drop with a mechanism configured to oscillate a liquid water drop, and removing the liquid water drop from the gas flow channel. 
         [0008]    In another embodiment, the invention provides a fluidic oscillator for use with a fuel cell. The fuel cell has a first porous electrode layer, a second porous electrode layer, a proton-conducting membrane positioned between the first electrode and the second electrode layers, and a first and second bi-polar distribution plate, wherein the first bi-polar distribution plate is positioned on a top of the first electrode layer and defining a first gas flow channel, and wherein the second bi-polar distribution plate is positioned on a bottom of the second electrode layer and defining a second gas flow channel. The fluidic oscillator includes an inlet port for receiving a fluid flow, a first outlet port communicating between the inlet port and the first gas flow channel, a second outlet port communicating between the inlet port and the second gas flow channel, a first control port configured to transmit a signal from the first outlet port to the inlet port, and a second control port configured to transmit a signal from the second outlet port to the inlet port. The fluid flow has a flow characteristic wherein when the flow characteristic drops below a threshold parameter, the fluidic oscillator produces a cyclic force to oscillate the water droplet. 
         [0009]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a fuel cell according to one embodiment of the present invention. 
           [0011]      FIG. 2  is the fuel cell of  FIG. 1  and a mechanism embodying the present invention. 
           [0012]      FIG. 3  is a schematic of an oscillating liquid water drop. 
           [0013]      FIG. 4  is a schematic of a fluidic oscillator for use with the fuel cell of  FIG. 1  according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0015]    In addition, and as described in subsequent paragraphs, the specific mechanical configuration illustrated in the drawings are intended to exemplify embodiments of the invention and that other alternative mechanical configurations are possible. 
         [0016]      FIG. 1  illustrates a fuel cell  10  according to one embodiment of the present invention. In general, polymer electrolyte membrane (PEM) fuel cells  10  have a membrane electrode assembly  14  consisting of an ion-exchange, or electrolyte, membrane  18  disposed between two electrode layers, typically comprised of porous, electrically conductive sheet material. In some embodiments, the electrode layers are disposed between two gas diffusion layers, a cathode layer  22  and an anode layer  26 . In some embodiments, the gas diffusion layers may include a generally rough, nonwetting, chemically inhomogenous surface, such as provided by teflonated carbon paper with or without a microporous layer. In a fuel cell  10 , and as illustrated in  FIG. 1 , the membrane electrode assembly  14  is interposed between two separator plates  30 . The separator plates  30  are substantially impermeable to reactant fluid streams. The separator plates  30  generally define gas flow channels  34  and are bi-polar distribution plates. The gas flow channels  34  are formed within the separator plates  30  during the manufacturing process, such that the gas flow channels  34  can be stamped, machined, or the like into the separator plates  30 . 
         [0017]    Typically, a PEM fuel cell  10  operates via a controlled hydrogen-oxygen reaction, wherein the byproducts of the reaction are heat and water. Accordingly, water product  38  forms within the membrane electrode assembly  14  and further migrates through the layers  22 ,  26 . The water  38  further migrates to the gas flow channels  34 . Water droplets  38  move through the gas flow channels  34  and out of the fuel cell  10  via a gas flow. However, at times, the movement of the water droplets  38  in the gas flow channel  34  can be inhibited by pinning of the three-phase region. The three-phase region is the line of contact where the gas, liquid, and solid phases collide. At high gas flow rates, the water droplets  38  can be removed via gas flow. However, at low gas flow rates, the water droplets  38  can remain pinned to the gas diffusion layer  22 ,  26  and continue to grow. As the water droplets  38  grow, the gas flow channels  34  can become clogged with the water droplets  38 . 
         [0018]    When the gas flow channels  34  become clogged with water droplets  34 , it becomes necessary to dislodge the water droplets  38  for efficient fuel cell operation. The water droplets  38  can be removed with a mechanism  42  ( FIG. 2 ) configured to oscillate the water droplets  38  at or near the water droplets&#39; natural frequency. For oscillation near the natural frequency of the liquid-gas surface of the water droplet, minimal energy is required to induce large surface oscillations and relatively large inertia within the water droplet. As shown in  FIG. 3 , the oscillating force  46  acts on the liquid-gas surface of the water droplet  38 . The oscillating force  46  is a cyclic force acting on the water droplet  38 . The oscillation of the water droplet  38  utilizes the inertia of the water droplet to overcome the pinning energy of the drop contact line through oscillation at the drop surface, oscillation of the drop surface near the natural frequency, or oscillation of the drop surface at the natural frequency. The oscillating frequency can be kept constant (in which case the water droplets are permitted to grow until they reach a size at which the oscillating frequency matches the natural frequency of the water droplet), or can be varied to meet the natural frequency of various sizes of water droplets. 
         [0019]    The mechanism  42  for oscillating the water droplet  38  at or near its natural frequency to remove the water droplet  38  produces a cyclic force, that can include, but is not limited to a pulsed gas flow via a fluidic oscillator positioned substantially entirely in an inlet manifold  58  ( FIG. 2 ) of the fuel cell  10 , a cyclic acoustic wave, a pulsed electromagnetic wave, and mechanical vibrations. 
         [0020]      FIG. 4  shows a schematic illustration of one embodiment of a fluidic oscillator  50  according to the present invention. The fluidic oscillator  50  includes an inlet port  58 , first and second feedback conduits  62   a ,  62   b , and first and second outlet ports  66   a ,  66   b . The first and second feedback conduits  62   a ,  62   b  each have a control port  70   a ,  70   b , and a start port  74   a ,  74   b , respectively. The inlet port  58  is configured to receive a flow of fluid. The fluid flow is comprised of fuel cell components, such as hydrogen and water. The inlet port  58  includes a nozzle  78  that accelerates the fluid flow into a bridge  82 . The bridge  82  provides fluid communication between the inlet port  58  and the first and second outlet ports  66   a ,  66   b . The fluid flow from the nozzle  78  is a focused jet stream, resulting in a reduced static pressure. Each start port  74   a ,  74   b  communicates with the respective outlet ports  66   a ,  66   b . Each control port  70   a ,  70   b  communicates with the output end of the nozzle  78 , where the nozzle  78  meets the bridge  82 . The outlet ports  66   a ,  66   b  can further communicate with gas flow channels  34 . 
         [0021]    When operation of the fuel cell is initiated, the focused jet stream flowing out of nozzle  78  into bridge  82  flows into both outlet ports  66   a ,  66   b . The flow eventually favors one side or other (first outlet port  66   a  in this example), and fluid flow is directed from the inlet port  58  to the first outlet port  66   a . Fluid flow to the first outlet port  66   a  initiates a pressure pulse at the first start port  74   a  that travels through the first feedback conduit  62   a  to the first control port  70   a . The pressure pulse exits the first feedback conduit  62   a  at the first control port  70   a  and deflects the fluid flow to the second outlet port  66   b . Fluid flow to the second outlet port  66   b  initiates a pressure pulse at the second start port  74   b  that travels through the second feedback conduit  62   b  to the second control port  70   b . The pressure pulse exits the second feedback conduit  62   b  at the second control port  70   b  and deflects the fluid flow to the first outlet port  66   a . The fluidic oscillator  50  operates in this manner to create the fluid flow oscillations. Additionally, the focused jet of fluid flow from the nozzle  78  results in a low static pressure, which allows for a more easily deflected fluid flow when acted upon by the pressure pulses from the control ports  70   a ,  70   b  than if the fluid flow had a higher static pressure. The feedback conduits  62   a ,  62   b  provide closed loop feedback to deliver the pressure pulses to the control ports  70   a ,  70   b , so that the oscillator  50  is not vented. In a fuel cell application, it is desirable to avoid venting because the fluid flow contains hydrogen. 
         [0022]    The pressure pulse in the feedback conduits  62   a ,  62   b  is an acoustic wave, or operates at the speed of sound. The frequency of the oscillations can depend on any flow parameters that will affect the speed of sound, including, but not limited to, density, pressure, temperature, and relative humidity. The frequency of the oscillation can also be affected by factors, including, but not limited to the length of the feedback conduits, or another factor that will affect the time of travel of an acoustical wave from the start ends  74   a ,  74   b  to the control ports  70   a ,  70   b  for a given wave speed. 
         [0023]    The pulsed fluid flow causes water droplet motion. During the onset of motion within the water droplet, the inertia of the water droplet is significant; whereas, at steady flow, the inertia becomes insignificant. The timing and amplitude of the pulses, or the cyclic force, produces a more steady motion of the water droplets than steady shear flow by utilizing the transient liquid momentum to overcome the dissipation due to contact line motion. 
         [0024]    Various features and advantages of the invention are set forth in the following claims.