Abstract:
A fuel cell includes a catalyst layer, a corrugated plate forming a plurality of channels that define a flow field in fluid communication with the catalyst layer, and a coating on at least one of the channels. The plate and coating are configured such that, if a gas flows through the channels, an obstruction blocking the at least one of the channels causes a pressure gradient between the channels that drives convection of the gas through the coating and around the obstruction.

Description:
BACKGROUND 
     Referring to  FIG. 1 , a prior art fuel cell  10  includes a membrane electrode assembly (MEA)  12  sandwiched between a pair of flow field plates  14 ,  16 . The MEA  12  includes a proton exchange membrane (PEM)  18  and catalyst layers  20 ,  22  bonded to opposite sides of the PEM  18 . The MEA  12  further includes gas diffusion layers  24 ,  26  (anode, cathode respectively) each in contact with one of the catalyst layers  20 ,  22 . As apparent to those of ordinary skill, the gas diffusion layer  24  and catalyst layer  20  may be collectively referred to as an electrode. Likewise, the gas diffusion layer  26  and catalyst layer  22  may also be collectively referred to as an electrode. 
     The flow field plate  14  includes at least one channel  28   n . As known in the art, the at least one channel  28   n  may form a spiral, “S,” or other shape on the face of the flow field plate  14  adjacent to the anode  24 . Hydrogen from a hydrogen source (not shown) flows through the at least one channel  28   n  to the anode  24 . The catalyst layer  20  promotes the separation of the hydrogen into protons and electrons. The protons migrate through the PEM  18 . The electrons travel through an external circuit  30 . 
     The flow field plate  16  also includes at least one channel  32   n . Similar to the at least one channel  28   n , the at least one channel  32   n  may form a spiral, “S,” or other shape on the face of the flow field plate  16  adjacent the cathode  26 . Oxygen from an oxygen or air source (not shown) flows through the at least one channel  32   n  and to the cathode  26 . The protons (generated as a result of hydrogen oxidation) that migrate through the PEM  18  combine with the oxygen and electrons returning from the external circuit  30  to form water and heat. 
     As known in the art, any suitable number of fuel cells  10  may be combined to form a fuel cell stack (not shown). Increasing the number of cells  10  in a stack increases the voltage output by the stack. Increasing the surface area of the cells  10  in contact with the MEA  12  increases the current output by the stack. 
     SUMMARY 
     A fuel cell includes a catalyst layer, a corrugated plate forming a plurality of channels that define a flow field in fluid communication with the catalyst layer, and a coating on at least one of the channels. The plate and coating are configured such that, if a gas flows through the channels, an obstruction blocking the at least one of the channels causes a pressure gradient between the channels that drives convection of the gas through the coating and around the obstruction. 
     A fuel cell includes a catalyst layer, a plate having a plurality of channels formed therein that define a flow field in fluid communication with the catalyst layer, and a porous matrix deposited on at least a portion of the plate. The plate and porous matrix are configured such that, if a gas flows through the channels, an obstruction blocking a particular channel causes a pressure gradient between the channels that drives convection of the gas through the coating and around the obstruction. 
     A fuel cell includes a plate having a surface defining a plurality of channels, a layer disposed on at least a portion of the surface and having a porosity between 0.20 and 0.99, and a catalyst in fluid communication with the channels. The plate and layer are configured such that, if a gas flows through the channels, an obstruction blocking a particular channel causes a pressure gradient between the channels that drives convection of the gas through the coating and around the obstruction. 
     While example embodiments in accordance with the invention are illustrated and disclosed, such disclosure should not be construed to limit the invention. It is anticipated that various modifications and alternative designs may be made without departing from the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view, in cross-section, of a prior art fuel cell. 
         FIG. 2  is a plan view, in cross-section, of a flow field plate of  FIG. 1 . 
         FIG. 3  is a perspective view of a flow field plate according to an embodiment of the invention. 
         FIG. 4  is a plot of experimental polarization curves for a serpentine nonporous cathode-side flow field, and serpentine and interdigitated porous cathode-side flow fields at 70° C. based on geometric land area. 
         FIG. 5  is a plot of experimental polarization curves for a serpentine nonporous cathode-side flow field, and serpentine and interdigitated porous cathode-side flow fields at 70° C. based on actual land area. 
         FIG. 6  is an end view, in cross-section, of a portion of a fuel cell according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Wider landing areas may increase cell conductivity and enhance electric current collection at an MEA. Inner areas of wider landing areas in nonporous flow fields, however, may suffer from reactant starvation due to relatively large reactant gas diffusion paths from the flow channels. Certain embodiments disclosed herein may enhance reactant distribution to catalysts, even with wider landing areas, resulting in improved fuel cell performance. 
     Stagnant zones may form in flow field channels downstream of obstructions. Such stagnant zones may impact MEA durability. Certain embodiments disclosed herein may prevent the formation of stagnant zones by permitting reactants to flow around any obstructions resulting in improved MEA durability. 
     Manifolds may have imperfections that affect the uniform distribution on reactants. As discussed below, certain embodiments may enhance reactant distribution to catalysts resulting in improved fuel cell performance. 
     Referring now to  FIG. 2 , the flow field plate  16  includes several parallel channels  32   n  ( 32   a ,  32   b ,  32   c ). The channels  32   n  are separated by wall portions  34 . In the illustration of  FIG. 2 , the flow of oxygen (air) is indicated by arrow. 
     An obstruction  36  has blocked the entire cross-section of the channel  32   b , thus obstructing the flow of oxygen downstream of the obstruction  36 . This may affect the durability of the fuel cell  10  illustrated in  FIG. 1 , may cause non-uniform distribution of reactants to the channels  32   n , may cause non-uniform current generation by the fuel cell  10 , and/or may affect the performance and durability of the fuel cell  10 . 
     Referring now to  FIG. 3 , an embodiment of an interdigitated flow field plate  38  includes inlet channels  40   n  ( 40   a ,  40   b ) and outlet channels  42   n  ( 42   a ,  42   b ) formed in a porous bulk media  43 . Wall portions  44  separate the channels  40   n ,  42   n.    
     Gases flowing into the inlet channels  40   n  (as indicated by light solid arrowed lines) may convect and/or diffuse through either or both of (1) the bulk media  43  (as indicated by dashed arrowed lines) and (2) an MEA (not shown) in contact with the plate  38 , and out of the outlet channels  42   n  (as indicated by heavy solid arrowed lines). As known in the art, pressure gradients drive convection whereas concentration gradients drive diffusion. 
     Convection may be the primary mechanism by which gasses move through the bulk media  43 . This convection may improve the distribution of gases to the MEA (not shown), as well as reduce the pressure needed to flow gases into the inlet channels  42   n  as compared with non-porous interdigitated flow fields. (High pressures are generally needed to flow gasses through the restricted flow path provided by a gas diffusion layer associated with a non-porous interdigitated flow field.) A reduction in pressure may reduce the amount of power needed to facilitate operation of the fuel cell in which the plate  38  is disposed. 
     Serpentine, “S” shaped, non-interdigitated, etc. channel configurations may be used in other embodiments. Pressure gradients within these embodiments (in the absence of channel obstructions) may be generally less than those within interdigitated embodiments. Diffusion, therefore, may be the primary mechanism by which gases move through the bulk media  43  in the absence of channel obstructions. In the presence of channel obstructions, however, convection may be the primary mechanism by which gases move through the bulk media  43 . 
     An obstruction  46  has filled the entire cross-section of the channel  40   a  as illustrated in  FIG. 3 . The porosity (which may range, for example, from 0.01 to 0.99) and tortuosity (which may be at least 1) of the plate  38 , however, is such that gases upstream of the obstruction  46  convect through the wall portions  44  defining the channel  40   a , as well as other portions of the bulk media  43  (as indicated by dashed arrowed lines), because of the pressure gradient within the channel  40   a  setup by the obstruction  46 . This convection may restore gas flow downstream of the obstruction  46  as illustrated. Gases may also diffuse through the wall portions  44  defining the channel  40   a , as well as other portions of the bulk media  43 , because of concentration gradients between the channels  40   n ,  42   n.    
     In other embodiments, the channels  40   n ,  42   n  (and/or plate  38 ) may be coated with various substances. For example, the channels  40   n  may be coated with Teflon and the channels  42   n  may be coated with a metal to alter the surface texture of pores within the channels  40   n ,  42   n . Of course, other coatings may also be used. 
     Several experiments were conducted to evaluate the performance of certain embodiments. Serpentine flow fields (5 cm 2 ) formed in both porous (61% total porosity and 95% open porosity) and nonporous (graphite) plates, as well as interdigitated flow fields (5 cm 2 ) formed in porous plates, were tested with woven gas diffusion electrodes having 5 grams of platinum nanoparticles per square meter and NAFION 117 membranes. 
     In a first experiment, the nonporous flow fields were used on both the anode and cathode sides of the cell. In a second experiment, the nonporous flow field was used on the anode side, while the serpentine porous flow field was used on the cathode side of the cell. In a third experiment, the nonporous flow field was used on the anode side, while the interdigitated porous flow field was used on the cathode side of the cell. 
     The cells were pre-conditioned by running them for 24 hours subject to room temperature at 0.5 volts with 1000 sccm air/300 sccm hydrogen at 100% relative humidity. This was followed by 4 hours of operation at an elevated temperature (70° C.) with all other parameters kept the same. 
     The effective current collector area for the tested porous flow fields was less than the current collector area for the nonporous flow field. As a result, the active area was normalized with the porosity of the plates to better assess the performance of the cells equipped with porous flow fields. 
     Referring now to  FIGS. 4 and 5 , the polarization curves reveal that while the serpentine nonporous cathode-side flow field appears to have a greater capacity to generate power relative to the serpentine porous cathode-side flow field based on geometric area, the serpentine and interdigitated porous cathode-side flow fields appear to have a greater capacity to generate power based on actual land area. 
     Multiphase computational fluid dynamic simulations were performed to study the dynamics of fluid flow within a single cathode-side channel, and within a cathode-side channel of a serpentine flow field. In the simulations, the channel dimensions (taken from a 5 cm 2  serpentine flow field) were 787.4×1016 microns. The flow rate (2e−5 kg/sec) was set according to the value used in the experiments detailed above. A hydrophilic media (contact angle=75°) with a surface tension of 0.07213 N/m was assumed for the single channel simulation, while a hydrophobic media (contact angle=133°) was assumed for the channel of the serpentine flow field simulation. The porosity was set to 0.61 with a permeability of 1e−9 m 2 . 
     An examination of the time evolution of reactant flow (as represented by contours of reactant velocity along and perpendicular to the landing area) under circumstances where a 1 mm thick obstruction has blocked the entire cross-section of both the single cathode-side channel and the cathode-side channel of the serpentine flow field revealed that reactants begin to flow through the porous matrix and around the obstruction after 5e−5 sec in both cases, thereby avoiding starvation downstream of the obstruction. 
     Referring now to  FIG. 6 , an embodiment of a fuel cell  48  includes a corrugated flow field plate  50  having opposing surfaces  52 ,  54 , a contact plate  56  in contact with, and sealed against, portions of the surface  52 , and an MEA  58 . The corrugated plate  50  and contact plate  56  define a plurality of channels  60  though which a coolant, such as water, may flow. The corrugated plate  50  and MEA  58  define a plurality of channels  62  through which a fuel, reactant, etc., may flow. 
     A porous matrix or coating, e.g., graphite, porous carbon, porous metal, etc.,  64  (having a porosity and tortuosity similar to that described above) has been deposited on the surface  54  of the corrugated plate  50 . (The MEA  58  is in contact with, and sealed against portions of the coating  64 .) This matrix  64  forms a porous layer through which gases flowing through the channels  62  may convect (and/or diffuse) in the presence of an obstruction as described herein. For example, an obstruction blocking one of the channels  62  may set up a pressure gradient between the channels  62  that drives convection of gases through the coating  64  in the vicinity of the obstruction, and around the obstruction thereby reestablishing flow of gases downstream of the obstruction. 
     The layer  64  may be thicker or thinner than the gas diffusion layer of the MEA  58 . For example, the layer  64  may have a thickness of 120 μm, or larger/smaller depending on, for example, the material used for the coating  64  and/or other design considerations. Any suitable thickness, however, may be used. 
     In other embodiments, different coatings  64  may be applied to different portions of the surface  54 . As an example, a coating having a relatively low porosity may be applied to those portions of the surface  54  that are in contact with the MEA  58  (i.e., the landing area), while a coating having a relatively high porosity may be applied to those portions of the surface  54  that define the channels  62 , etc. As another example, certain portions of the surface  54  may be masked prior to the application of the coating  64  so that the masked portions of the surface  54  are not coated. Other configurations are also possible. For example, a porous matrix or coating may be applied to flow field plates similar to that described with reference to  FIG. 1 ,  2  or  3 , or similar to that tested and discussed with reference to  FIGS. 4 and 5 , etc. 
     While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.