Patent Application
20170229714
Field of the Invention
This invention relates generally to bipolar plates for a fuel cell stack that include embossed features for sealing and, more particularly, to bipolar plates for a fuel cell stack that include embossed features, where bipolar plate halves are assembled so that the embossed features of opposing plate halves are offset relative each other to reduce the seal force and/or pressure variation around a seal perimeter.
Discussion of the Related Art
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell type for vehicles, and generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer, where the catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). The membranes block the transport of gases between the anode side and the cathode side of the fuel cell stack while allowing the transport of protons to complete the anodic and cathodic reactions on their respective electrodes.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. A fuel cell stack typically includes a series of flow field or bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
The fuel cell stack includes an active region in which the MEAs are located, which is the area in the stack where the electro-chemical reaction occurs. The reactant gases are fed to the channels in the active region from an inlet header or headers through a non-active feeder region that includes a sub-gasket between the cathode and anode flow channels instead of the MEA. The bipolar plates are typically assembled and welded together so that the reactant gases and the cooling fluid can be separated and coupled to appropriate manifold headers. The bipolar plates are generally configured so that the cathode reactant gas flow channels and the anode reactant gas flow channels are substantially the same size in both the active and non-active regions.
Various techniques are known in the art for fabricating the bipolar plates. In one design, stamped metal plate halves are assembled together so that anode flow channels are provided at one side of one of the plate halves, cathode flow channels are provided at an opposite side of the other plate half and cooling fluid flow channels are provided between the plate halves. The bipolar plates are stacked on top of each other and then compressed between end structures so as to seal the various channels therein.
The stamped shape of the plate halves define embossed features in the plate, such as grooves that ultimately define the reactant gas or cooling fluid flow channels and seals. The embossed features for channels can be in-phase and aligned or out of phase and misaligned. The anode to cathode channel alignment through an MEA can be substantially different, i.e., aligned or orthogonal. When an embossed feature is used for sealing, the embossed feature path essential follows the same path in the anode and the cathode plate halves. When opposing plate halves for a bipolar plate are stacked during the assembly process those areas where the plate halves contact each other can define bead seals, where the contact area is rigid, and those areas that are spaced apart from each other are deformable and resilient. The compressive force applied to the stack of bipolar plates during assembly causes the plate halves to be sealed together at the bead seals. Typically, a thin elastomer is placed on the plate halves at the bead seal locations to increase the sealing effect.
During the assembly process of the fuel cell stack the embossed features in the halve plates are aligned with each other as best as possible. However, because of inefficiencies in the assembly process and the number of plate halves required for a typical fuel cell stack, the embossed features in the plate halves are not perfectly aligned. This misalignment between the plate halves results in undesirable pressure variations on the plate halves that affects the plate sealing. Further, various locations in the feeder region and a transition region between the feeder region and the active region are curved to allow the reactant gases to be directed to the particular manifold. A straight embossed feature path will be less stiff that a curved embossed feature path. Header shapes and active area footprints require that the seal path embossed feature have turns to seal around the shapes, which will have a stiffer sealing-deflection response than a straight seal path. Plate halves will need to contact each other with a UEA subgasket separating the contact in the sealing areas. The corner or bend stiffness of a seal embossment feature as it encircles a header or active area shape will have sealing variations.
The present invention discloses and describes bipolar plates for a fuel cell stack that include opposing stamped metal plate halves having specialized embossed features so that when the stack is assembled, the embossed features of opposing plate halves are positioned relative to each other so that a centerline of opposing seal paths are offset to create offsets between more rigid sections and less rigid sections in the bipolar plate that allows for more uniform sealing along bead seals between the plate halves under the compressive assembly force.
Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the invention directed to bipolar plates for a fuel cell stack that include opposing stamped metal plate halves having specialized misaligned embossed features is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
The stack 10 is subject to a compressive force during stack assembly, as generally represented by compressive force arrows 38. The contact areas between the plate halves 18 and 20 and the plate halves 22 and 24 define bead seals 40 that seal the various channels 30, 34 and 36 in the stack 10. The configuration of the embossed features 26 provides more rigid areas 42 that are less likely to deform under the compressive force 38 and less rigid areas 44 that are more likely to deform under the compressive force 38.
The portion of the bipolar plates 12 and 14 shown in
As mentioned above, typically, even though perfect alignment of the plate halves 18, 20, 22 and 24 is desired, there is some misalignment between the plate halves 18, 20, 22 and 24 such that the seal integrity of the bead seals 40 is decreased especially in those regions where the channels, such as the cooling fluid flow channels 30, are curved.
As will be discussed in detail below, the present invention proposes configuring the embossed features in bipolar plate halves so that when the bipolar plate halves are aligned with each other during the stack assembly process, the embossed features are offset relative to each other so that the more rigid areas in one plate half are somewhat aligned with the less rigid more elastic areas in an adjacent plate half to provide a seal pressure along the seal path and across the seal path to have less variation. This offset between the embossed features of the plate halves provides less pressure variation along the curved areas of the embossed features, such as those discussed above with reference to
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
