Edge stress relief in diffusion media

#The present invention relates to PEM fuel cells, and more particularly to a diffusion media for a fuel cell.

#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. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on the active area portion of one face and the cathode catalyst on the active area portion of the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or bipolar plates which serve as current collectors for the anode and cathode, and contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.

#The electrically conductive bipolar 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's gaseous reactant's (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.

#The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:

#FIG. 1A is a partial cross sectional view of a fuel cell assembly according to the principles of the present invention;

#FIG. 1B is a graph of contact pressure along the width of the MEA/diffusion media interface of a fuel cell assembly according to the principles of the present invention;

#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.

#Fuel cell stacks typically comprise a plurality of fuel cells stacked one upon another and held in compression with respect to each other. The plurality of stacked fuel cells forms a fuel cell assembly which is compressed to hold the plurality of fuel cells in a compressive relation. FIG. 4 illustrates a partial cross-sectional view of a prior art fuel cell arrangement 110 including a first bipolar plate 112 and a second bipolar plate 114. Bipolar plates 112, 114 are also referred to herein as electrically conductive electrodes. A diffusion media 116, 118 is adjacent to each bipolar plate 112, 114, respectively. An MEA 120 is disposed between the diffusion media 116, 118. The MEA 120 includes an ionically conductive member 122 with an anode electrode 124 on one face and a cathode electrode 126 on a second opposite face. At the edges of the MEA 120, a first gasket layer 128 and a second gasket layer 130 are disposed, preventing any bare MEA 120 from being exposed to the bipolar plates 112, 114 and the harsh fuel cell environment. The fuel cell assembly requires a significant amount of compressive force to squeeze the fuel cells of the stack together. The need for compressive force arises from the internal gas pressure of the reactants within the fuel cells, in addition to the need to maintain good electrical contacts between the internal components of the fuel cells. Preferably, the active areas of the fuel cells are uniformly compressed to maximize the efficiency of the fuel cell stack assembly.

#The prior art assembly 110 with gaskets 128, 130 as shown in FIG. 4, typically induces significantly higher compression and shear stresses in the MEA 120 at the gasket edges 132, 134 during the stack build and compression process. This results, in part, because portions of the diffusion media 136, 138 above and under the gaskets 128, 130, respectively, have to be compressed more to accommodate the additional thickness of the gaskets 128, 130.