Abstract:
A fuel cell stack includes a membrane electrode assembly and a bipolar plate. The bipolar plate has a corrugated portion defined by an adjacent pair of proximal and distal peak portions and a sidewall segment connecting the peak portions. The sidewall segment and membrane electrode assembly at least partially define a flow channel. The sidewall segment includes a shoulder portion defining a step spaced away from the peak portions.

Description:
TECHNICAL FIELD 
     This disclosure relates to proton exchange membrane (PEM) fuel cells and to the construction and arrangement of bipolar plates therein. 
     BACKGROUND 
     A proton exchange membrane fuel cell is an electrochemical energy conversion device that converts hydrogen and oxygen into water, and in the process produces electricity. Hydrogen fuel is channeled through flow fields to an anode on one side of the fuel cell. Oxygen (from the air) is channeled through flow fields to a cathode on the other side of the fuel cell. At the anode, a catalyst causes the hydrogen to split into hydrogen ions and electrons. A polymer electrolyte membrane disposed between the anode and cathode allows the positively charged ions to pass through it to the cathode. The electrons travel through an external circuit to the cathode, which creates an electrical current. At the cathode, the hydrogen ions combine with the oxygen to form water, which flows out of the cell. 
     SUMMARY 
     A vehicle includes a fuel cell stack arranged to provide power to move the vehicle. The fuel cell stack includes a membrane electrode assembly and a bipolar plate having a corrugated portion defined by (i) a series of alternating proximal and distal peak portions and (ii) respective sidewall segments connecting each adjacent pair of the proximal and distal peak portions. The proximal portions are in contact with the membrane electrode assembly such that each of the sidewall segments and membrane electrode assembly at least partially define a plurality of flow channels each having a width and a depth at least equal to the width. Each of the sidewall segments includes at least one shoulder portion such that the bipolar plate has a generally uniform thickness. The depth of one of the flow channels can be greater than the width of the one of the flow channels. The thickness of the bipolar plate can be approximately 250 microns. The bipolar plate can be formed from metal. The bipolar plate can be formed from stainless steel foil. 
     A fuel cell stack includes a membrane electrode assembly and a pair of bipolar plates electrically connected together. At least one of the bipolar plates has a corrugated segment defined by peak portions and a sidewall connecting the peak portions. The sidewall has end portions and a body portion disposed between the end portions. Each of the end portions is adjacent to one of the peak portions. The sidewall and membrane electrode assembly at least partially define a flow channel. The sidewall includes at least one shoulder portion formed in the body portion. The fuel cell stack can further include a plate disposed between and in contact with the pair of bipolar plates. The flow channel can have a width and a depth greater than the width. The at least one bipolar plate can have a generally uniform thickness. The thickness of the at least one bipolar plate can be approximately 100 microns. The at least one bipolar plate can be formed from metal. The at least one bipolar plate can be formed from stainless steel foil. 
     A vehicle includes a fuel cell stack arranged to provide power to move the vehicle. The fuel cell stack includes a membrane electrode assembly and a bipolar plate having an adjacent pair of proximal and distal peak portions and a sidewall segment connecting the peak portions. The sidewall segment and membrane electrode assembly at least partially define a flow channel. The sidewall segment includes a shoulder portion defining a step spaced away from the peak portions. The flow channel can have a width and a depth greater than the width. The bipolar plate can have a generally uniform thickness. The thickness of the bipolar plate can be approximately 100 microns. The bipolar plate can be formed from metal. The bipolar plate can be formed from stainless steel foil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic cross-sectional view of a bipolar plate flow channel. Channel width is labeled with a “W” and channel depth is labeled with a “D.” 
         FIG. 2  is a diagrammatic cross-sectional view of a conventional bipolar plate flow channel having a trapezoidal shape. 
         FIG. 3  is a diagrammatic cross-sectional view of a fuel cell stack disposed within a vehicle and including bipolar plates having flow channels defined at least partially by stepped sidewalls. 
         FIG. 4  is a diagrammatic cross-sectional view of a bipolar plate having flow channels at least partially defined by stepped sidewalls. 
         FIG. 5  is a diagrammatic cross-sectional view of a bipolar plate flow channel. The channel depth is at least equal to the channel width. Like numbered elements among the various figures can have similar descriptions. 
         FIG. 6  is a diagrammatic cross-sectional view of a bipolar plate flow channel. The sidewalls each include two shoulder projections. 
         FIG. 7  is a diagrammatic cross-sectional view of a junction between two adjacent fuel cells of a fuel cell stack. The bipolar plates of the fuel cells are in contact with each other. One of the bipolar plates has flow channels at least partially defined by stepped sidewalls. The other of the bipolar plates has flow channels which are trapezoidal in shape. 
         FIG. 8  is a diagrammatic cross-sectional view of a junction between two adjacent fuel cells of a fuel cell stack including a centerplate disposed between and in contact with bipolar plates of the fuel cells. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures can be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
     Candidate metallic bipolar plate (MBPP) materials can be formed into a series of channels having widths and depths designed to satisfy desired fuel cell performance criteria. To increase fuel cell performance, deep, narrow channels with vertical side wall geometries essentially mimicking a flat bottom “U” are preferred in certain circumstances. Such geometries, however, can be difficult or impossible to form from thin metallic materials in a cost effective manner. Formability limits of certain thin metallic materials, such as stainless steel foil, can thus restrict their usage as MBPP materials for fuel cell applications. For example, stamping deep, straight channels into thin metallic materials can produce excessive material thinning at channel geometry transition regions such as at channel edges. Such thinning can result in tearing of the plate during channel formation, assembly of the fuel cell, or operation of the fuel cell stack. Moreover, to the extent that the bipolar plate is a structural component of the fuel cell stack, such thinning can compromise the rigidity of the bipolar plate. 
     Conventional MBPP designs commonly feature channels with cross-sections resembling a flat-bottom “V” (or trapezoidal shape). These configurations tend to have moderate side wall angles and restricted channel depths in an effort to accommodate the forming limits of the precursor plate material and to minimize strain-induced thinning during the forming process. In some cases, base alloy processing steps can be altered to improve the ability of MBPP precursor materials to form past their normal limits. Alteration of the material base chemistry or manufacturing process, however, can detrimentally impact other characteristics desired of an alloy to be used in fuel cell applications such as corrosion resistance and electrical conductivity. Changes in material composition and processing can also be cost prohibitive. 
     In fuel cells, increasing flow channel cross-sectional area, particularly on the cathode side of the respective membrane electrode assembly (MEA), can substantially increase fuel cell performance. If the channel opening is too wide, however, the MEA can bow inward toward the channel. For this reason, it could be preferable for the channels to be formed with narrower openings and deeper channels. 
     The ability to form MBPPs with deeper channels, particularly when the channels are formed by a stamping process, can be improved by altering the forming limits of the precursor plate material at the expense of other characteristics as mentioned above. It has been discovered, however, that altering channel geometry to accommodate the inherent forming limits of the selected precursor material can also improve the ability to form MBPPs with deeper channels without significantly impacting such characteristics as corrosion resistance and electrical conductivity. Disclosed herein are examples of “stepped” sidewall MBPP channel geometries as shown, for example, in  FIG. 1 . Flow channels with stepped sidewalls can be distinguished from the more traditional trapezoidal channel configuration shown in  FIG. 2 . 
     The segments of the sidewall forming the shoulder (or step) need not form a 90 degree angle relative to each other. Any suitable angle (e.g., 80 degrees, 100 degrees, etc.) that permits deep channel formation without significant thinning can be used. Testing and/or simulation can determine optimum step dimensions. 
     Finite element analysis (FEA) of the stepped sidewall geometry (shown, for example, in  FIG. 1 ) has been compared to FEA of a traditional trapezoidal-shaped channel (shown, for example, in  FIG. 2 ) with equivalent depth. This comparison revealed that material thinning of the stepped geometry of  FIG. 1  is far less than that of the trapezoidal channel geometry of  FIG. 2 , and material strain across the stepped sidewall geometry of  FIG. 1  is more balanced. The FEA comparison also revealed that for the equivalent channel depth, D, the trapezoidal channel of  FIG. 2  is more likely to experience material failure in its highly strained upper radius zones, R. The FEA model results have been empirically verified in further studies. Usage of the stepped sidewall geometry similar to that illustrated in  FIG. 1  could allow for deeper channels with greater sidewall angles, A, to be formed from existing metallic materials while maintaining acceptable channel opening widths W. These two characteristics can result in improved fuel cell stack operational performance without diminishing the structural integrity of interfacing fuel cell stack components. 
     Referring to  FIG. 3 , a vehicle  98  such as a car can include a fuel cell stack  100  arranged, as known in the art, to provide power to move the vehicle  98 . The fuel cell stack  100  can include a plurality of fuel cells  102  electrically connected together. Each of the fuel cells  102  can include a membrane electrode assembly (MEA)  104  disposed between first and second bipolar plates  106 ,  108 . The membrane electrode assembly  104  includes a cathode portion on one side and an anode portion on the other side. Where the term “Gas” is used in the figures, it is intended to represent the fuel of the fuel cell  102  exposed to the anode side of the MEA  104 . In a hydrogen fuel cell, for example, the Gas would be hydrogen gas. Where the term “OX” is used in the figures, it is intended to represent oxygen (or air containing oxygen) exposed to the cathode side of the MEA  104 . 
     Referring to  FIG. 4 , each of the bipolar plates  106  can be stamp-formed from a precursor metal sheet such as a sheet of stainless steel foil or other appropriate conductive metallic material. Alternative forming methods such as hydro-forming and adiabatic forming can also be used. Each of the bipolar plates  106  defines adjacently aligned flow channels  110  (normal to the page) alternately disposed on opposing sides of the bipolar plate  106 . Further, each of the bipolar plates  106  includes at least partially stepped sidewalls  112  having shoulder portions  114 , and proximal and distal peak portions  116 ,  118  where the stepped sidewalls  112  connect with each other (giving the bipolar plate  106  a corrugated appearance). Hence, each of the stepped sidewalls  112 , in this example, have two end portions and a body portion disposed between the end portions. Each of the end portions is adjacent to one of the peak portion  116 ,  118 . The shoulder portions  114  are formed in the body portions. The proximal peak portions  116  of each bipolar plate  106  can be in direct contact with the MEA  104  ( FIG. 3 ). The distal peak portions  118  of adjacent bipolar plates can be aligned and in electrical contact with one another. 
     Particularly in instances in which the bipolar plates  106  are stamp-formed, the bipolar plates  106  can have a substantially uniform web thickness, T. Such thickness can be, for example, in the range of approximately 100 microns. Any suitable thickness, however, can be used (e.g., 80 to 250 microns, etc.) A similar description applies to the bipolar plates  108  of  FIG. 3 . 
     Referring to  FIG. 5 , a portion of a bipolar plate  206  includes at least partially stepped sidewalls  212  having shoulder portions  214  and proximal and distal peak portions  216 ,  218  respectively. The channel depth, D, in this example, is at least as equal to the channel width, W. In other examples, the channel depth, D, can be greater than the channel width, W. For example, D can be approximately 500 microns and W can be approximately 100 microns. 
     Referring to  FIG. 6 , a portion of a bipolar plate  306  includes at least partially stepped sidewalls  312  having shoulder portions  314  and proximal and distal peak portions  316 ,  318  respectively. In this example, each of the stepped sidewalls  312  can have two (or more) shoulder portions  114 . Other configurations are also contemplated. 
     Referring to  FIG. 7 , a portion of a fuel cell stack  400  includes MEAs  404  and bipolar plates  408 ,  420  in contact with each other and disposed between the MEAs  404 . In this example, the bipolar plate  408  includes stepped sidewalls  412  and the bipolar plate  420  does not. 
     Referring to  FIG. 8 , a portion of a fuel cell stack  500  includes MEAs  504 , bipolar plates  506 ,  508 , and a center plate  522 . The center plate  522  is disposed between and in contact with the bipolar plates  506 ,  508  to prevent nesting of adjacent bipolar plates and to increase the number of coolant flow channels associated with the bipolar plates  506 ,  508 . Other arrangements are also contemplated. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes can be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments can be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments may have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics can be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes can include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and can be desirable for particular applications.