Patent Application
20050164070
1. Field of the Invention
This invention relates generally to bipolar plates for a fuel cell and, more particularly, to bipolar plates for a fuel cell that are extruded to provide the various flow channels within the plates.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electrochemical 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 disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. 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. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC 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. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for an automobile may have two hundred stacked fuel cells. The fuel cell stack receives a cathode input gas as a flow of air, typically forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several membranes in the stack. For the automotive fuel cell stack mentioned above, the stack would include about four hundred bipolar plates, and are typically made of stainless steel. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. The bipolar plates are made of a conductive material so that they conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid and the anode and cathode fluids for the electrochemical reaction flow, as is well understood in the art.
Currently, each bipolar plate is made by joining two separate plates after the flow channels in the plates have been formed. In one technique, each separate plate is formed by a stamping or etching process to form the channels in the plate. The two plates are then secured together by welding or brazing along the edges and predetermined weld lines to join the plates to form the bipolar plate and seal the channels. However, such a stamping, welding and/or brazing process is very labor intensive, and must provide a high seal integrity. If the seal integrity anywhere along the weld or braze line is compromised, and the flow channel leaks, the entire fuel cell stack cannot be used.
In accordance with the teachings of the present invention, an extruded bipolar plate for a fuel cell is disclosed. The bipolar plate is formed by an extrusion process where the extruder die forms linear channels in the extrusion to define the various flow channels in the plate. Because the bipolar plate is formed by the extrusion process, two separate plates do not need to be joined to form the bipolar plate. A variety of different shapes can be provided by the extrusion process to form the flow channels. In one design, the flow channels for the cooling fluid extend through the center of the bipolar plates, and flow channels for the anode and cathode fluids are provided on the outside of the bipolar plate. Further, the extrusion process can form recesses in the sides of the bipolar plates to receive end caps to secure the plates together. In addition, the end caps can include channels to control the flow of the fluids to the plates when the fuel cell stack is assembled.
Additional advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
The following description of the embodiments of the invention directed to an extruded bipolar plate for a fuel cell is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses.
The present invention is directed to extruded bipolar plates for a fuel cell stack.
The extrusion device 10 can be any extrusion device suitable for the purposes described herein, and many are known in the art that would be applicable. The metal can be any conductive metal suitable for a bipolar plate and applicable to be extruded. In one embodiment, the metal is aluminum, however, other metals, such as magnesium, stainless steel, titanium, etc., may be applicable.
The die 12 and the extrusion device 10 can form any desired design of flow channels within the bipolar plate for a particular fuel cell stack. The only requirement for the extrusion process is that the flow channels formed therein are linear because of the limitations of the extrusion process.
In addition to the cooling flow channels in the extruded bipolar plate, the extrusion process can also form recessed edges in the plate that provide a structure for securing the adjacent plates in the stack together.
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.
