Fuel cell stacks typically comprise a plurality of fuel cells stacked one upon the other and held in compression with respect to each other. The plurality of stacked fuel cells form a fuel cell assembly which is compressed to hold the plurality of fuel cells in a compressive relation. Typically, each fuel cell comprises an anode layer, a cathode layer, and an electrolyte interposed between the anode layer and the cathode layer. The fuel cell assembly requires a significant amount of compressive force to squeeze the fuel cells of the stack together. The need for the compressive force comes about from the internal gas pressure of the reactants within the fuel cells plus the need to maintain good electrical contact between the internal components of the cells. Generally, the per area unit force is about 195–205 psi total which is distributed evenly over the entire active area of the cell (typically 77–155 square inches for automotive size stacks). Thus, for a fuel cell with an area of about 80 square inches, the typical total compressive force of these size stacks is about 15,500 to 16,500 pounds.
Typical prior art fuel cell stack structures have focused on the use of rigid end plates and tie rods to apply and maintain a compressive force on the fuel cell assembly. The fuel cell assembly to be compressed is interposed between a pair of rigid end plates. The end plates are then compressed together by tie rods that extend through the end plates and impart a compressive force on the end plates. This structure and other similar structures typically used, resulted in fuel cell stack structures of varying lengths in order to achieve the desired compression of the fuel cell assembly. Additionally, the electrolyte membrane that has been used in fuel cells was approximately 0.007 inches thick and would slip or stress relax over time thus requiring the fuel cell stack structure to be further compressed in order to maintain a desired compressive force on the fuel cell assembly.
Today's current fuel cell assemblies utilize a membrane that is 0.018 microns thick and is reinforced. As a result the slip or stress relaxation is much lower and is not as big a concern as in prior art fuel cell assemblies. Therefore, the fuel cell assemblies using the new, thinner and reinforced membrane are not required to be further compressed after a period of time to account for relaxation or stress relief of the membrane.
Fuel cell stack structures can be utilized in a variety of applications. Because the space in which the fuel cell stack structures are typically employed have a fixed volume, such as in an automobile or bus, it would be desirable if the fuel cell stack structures were of a uniform size so that different fuel cell stack structures could be interchanged.
Therefore, it would be desirable to provide fuel cell stack structures that are of a uniform length. Furthermore, it would be desirable to build and provide a fuel cell stack structure of uniform length regardless of the number of fuel cells that comprise the fuel cell assembly while still imparting a desired compressive loading on the active area of the fuel cell assembly.
Because the current fuel cell stack structures utilize thin and reinforced membranes that exhibit substantially less dimensional slip and stress relaxation, other compressive loading approaches are now feasible for the fuel cell stack structures. These compressive loading techniques enable the fuel cell stack structure to be of a uniform size. Furthermore, because the characteristics of the fuel cell assembly and the reinforced membranes are better understood, the distance that a fuel cell assembly needs to be compressed to obtain a given compressive loading on the active area of the fuel cell assembly can be determined by the number of fuel cells that comprise the fuel cell assembly. This enables the use of a fixed distance of compression when assembling a fuel cell stack structure as opposed to prior art use of compressing a fuel cell stack structure until a predetermined compressive load is applied to the active area.