Patent Application
20050048349
1. Field of the Invention
This invention relates generally to fuel cells, and more particularly, to the manufacture of such fuel cells.
2. Background Information
Fuel cells are devices in which electrochemical reactions are used to generate electricity. A variety of materials may be suited for use as a fuel depending upon nature of the fuel cell. Organic materials, such as methanol or natural gas, are attractive fuel choices due to their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most currently available fuel cells are reformer-based fuel cell systems. However, because fuel processing is complex, and requires expensive components, which occupy comparatively significant volume, the use of reformer based systems is presently limited to comparatively large, high power applications.
Direct oxidation fuel cell systems may be better suited for a number of applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger scale applications. In fuel cells of interest here, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically conductive, but electronically non-conductive membrane (PCM). Typically, a catalyst, which enables direct oxidation of the fuel on the anode aspect of the PCM, is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). In the fuel oxidation process at the anode, the products are protons, electrons and carbon dioxide. Protons (from hydrogen in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons travel through an external circuit, which includes the load, and are united with the protons and oxygen molecules in the cathodic reaction, thus providing electrical power from the fuel cell.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, a mixture comprised predominantly of methanol and water is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. The fundamental reactions are the anodic oxidation of the methanol and water in the fuel mixture into CO2, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed at an acceptable rate (more specifically, slow oxidation of the fuel mixture will limit the cathodic generation of water, and vice versa).
Direct methanol fuel cells are being developed towards commercial production for use in portable electronic devices. Thus, the DMFC system, including the fuel cell and the other components should be fabricated using materials and processes that are not only compatible with appropriate form factors, but which are also cost effective. Furthermore, the manufacturing process associated with a given system should not be prohibitive in terms of associated labor or manufacturing cost or difficulty.
Typical DMFC systems include a fuel source, fluid and effluent management and air management systems, and a direct oxidation fuel cell (“fuel cell”). The fuel cell typically consists of a housing, hardware for current collection and fuel and air distribution, and a membrane electrode assembly (“MEA”) disposed within the housing.
A typical MEA includes a centrally disposed, protonically conductive, electronically non-conductive membrane (“PCM”). One example of a commercially available PCM is NAFION® a registered trademark of E.I. Dupont de Nemours and Company, a cation exchange membrane comprised of polyperflourosulfonic acid, in a variety of thicknesses and equivalent weights. The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, the electrode assembly typically includes a diffusion layer. The diffusion layer on the anode side is employed to evenly distribute the liquid fuel mixture across the anode face of the PCM, while allowing the gaseous product of the reaction, typically carbon dioxide, to move away from the anode face of the PCM. In the case of the cathode side, a diffusion layer is used to achieve a fast supply and even distribution of gaseous oxygen across the cathode face of the PCM, while minimizing or eliminating the collection of liquid, typically water, on the cathode aspect of the PCM. Each of the anode and cathode diffusion layers also assist in the collection and conduction of electric current from the catalyzed PCM.
As noted, the MEA is formed of a centrally disposed PCM that is sandwiched is between two catalyst layers. The catalyst layers of the MEA in some architectures can be arranged such that a gas diffusion layer (GDL) is adjacent the cathodic catalyst layer to allow oxygen to be transported to the cathode, and a liquid and gas diffusion layer (LDL/GDL) is adjacent the anodic catalyst layer that allow liquid fuel to be transported to the anode, and to allow carbon dioxide to travel away from the anode. Gaskets are often used to maintain the catalytic layers and the diffusion layers in place. Generally, the entire MEA is placed into a frame structure that both compresses the MEA and provides an electron path. Although this can provide some dimensional stability, the greater the compression that is required, the more physical components (i.e., screws, etc.) must be employed to assure adequate pressure. Those skilled in the art will recognize that sealing and application of significant pressure can be accomplished in various ways, but these aspects conventionally involve relatively large fastening components, such as screws, nuts and the like. Such components themselves can be expensive because they are specially machined. Furthermore, the assembly of devices that include these fasteners is a time consuming manual process that can also lead to inconsistency in results. Moreover, the additional parts can add weight, volume and cost to the fuel cell, which if used as a power source for hand-held electronic devices, should be of the smallest form factor possible.
As noted, it is also common to place gasketing around the exterior portions of the fuel cell to resist leaks of the fuel substance or water that is produced at the cathode out of the fuel cell. The gasketing can also be used to retain moisture in the fuel cell, as the NAFION® membrane operates ideally when sufficiently hydrated. The gasketing that is incorporated to prevent leakages is typically a deformable plastic material that is stretched and placed around the outer current conductor plates and usually handassembled around the lateral portions of the fuel cell. In accordance with a commonly owned, co-pending U.S. patent application Ser. No. 10/448,271, filed on May 30, 2003, by Hirsch, et al., for a FUEL EFFICIENT MEMBRANE ELECTRODE ASSEMBLY, which is incorporated by reference herein, a direct oxidation fuel cell is described in which the catalyst layers and diffusion layers can be extended beyond and overlap the gasketing to form an even greater seal than the gasketing would alone. In addition, the catalyzed portions of the membrane are extended into the area of the gasket to substantially resist the flow of fuel substance through any paths created at the edges of the diffusion layer between the diffusion layer and the gasket. Extending the catalytic layer into the area of the gasket allows methanol or other fuel substance to be oxidized on the catalyst prior to its leaking out of or around the diffusion layer. This aides in preventing undesired leakages but these gaskets must be manually placed in the proper location during manufacture. Additionally, the fuel cell still requires the use of screws, bolts and other fasteners to maintain the fuel cell components in place, and to maintain the proper compression required for electrical contact and leakage prevention.
More specifically, fuel cells contain a number of components. These components can include a fixture or base compression plate, a gasket, an anode current collector plate, a second gasket, a membrane electrode assembly, and yet another gasket, then the cathode current collector, a further gasket and a cathode compression plate. Depending on the size of the cell, a number of fasteners, typically four to eight screws and nuts may often be used to create compression required through all of these layers. Depending on the design of the current collectors, additional flow field plates may also be required at the anode, cathode or both.
Current fuel cell assembly techniques involve layering all of these components of the fuel cell by hand and then tightening the components together using several screws tightened to a specific torque. As this is accomplished by hand, this manufacturing technique results in variations in compressions from build to build, in addition to consuming significant assembly time per cell. In addition to DMFCs, other types of fuel cells, such as hydrogen-gas fueled fuel cells, conventionally require these manufacturing techniques, and have the same disadvantages.
Another aspect of typical fuel cell construction involves a hot pressing step. More specifically, during fuel cell construction, a membrane electrode assembly is first created, that includes a catalyzed membrane and a gas diffusion (GDL) layer material. (Sometimes, the gas diffusion layer material is simply referred to as a “diffusion layer.”) These components are laminated together in a hot press operation. This step, when performed separately, can also be time consuming.
There remains a need, therefore, for a process for manufacturing and assembling a fuel cell or a fuel cell array, which results in a product that does not require many screws, bolts or other fasteners to hold the components together or to maintain compression within the fuel cell. There remains a further need for a process for assembling a fuel cell, which either eliminates the need for gasketing between each layer of the fuel cell, or reduces the number of gasketing components needed. There remains yet a further need for a process that combines and automates several steps to save time and to produce more consistent results.
It is thus an object of the present invention to provide a cost-effective, highly efficient process for manufacturing a fuel cell or fuel cell array that allows the construction of a fuel cell with adequate compression without the use of screws, bolts and other fasteners. It is a further object of the invention to provide a fuel cell that has been produced by such process. It is yet a further object of the invention to provide a fuel cell that does not require multiple gaskets between and around the various layers and components of the fuel cell.
The deficiencies and disadvantages of prior techniques have been overcome by the solutions provided by the present invention, which is a process for manufacturing a fuel cell and an associated fuel cell array that includes a unique lead frame that integrates current collectors and other components of the fuel cell that are inserted into a mold and thereby sealed. More specifically, the fuel cell components are assembled on a lead frame structure, which is used to facilitate the molding process. The lead frame, containing the previously assembled components of the fuel cell, is inserted into a mold cavity formed by the mold plate and a moldable material, such as plastic, is introduced into the mold cavity to create the frame around the cell. For purposes of this description, the term “mold plate” includes any component that imparts a desired shape or form to the moldable material it receives and which allows the moldable material when solidified, to assume the desired shape as the fuel cell frame. Once the frame is set, the fuel cell frame seals the edges of the fuel cell against leaks. This eliminates the need for gaskets. The frame also holds the components of the cell in compression, without the need for screws and nuts, which are thus completely eliminated.
It should be understood that though, for purposes of illustration and description, an embodiment of the invention is described that includes an injection molding process, the scope of the invention is not limited thereto, and encompasses other types of molding techniques including introducing a liquid material into a mold cavity, and allowing or causing said material to become a solid (or effectively a solid) material. For example, it is contemplated that the invention includes such process steps as, for example, introducing a thermoset plastic that is heated and then allowed to set, using a thermal plastic that is introduced as a liquid and undergoes a physical or chemical change in the presence of a catalyst, (or as its temperature changes), and/or using other methods known to those skilled in the art. The method of the present invention can be readily adapted to incorporate any of these techniques, or combinations thereof.
In accordance with another aspect of the present invention, current collectors are designed such that they are integrated into the lead frame structural elements in such a manner that the current collectors can act as compression plates within the molded fuel cell. This eliminates the need for separate compression plate components. In accordance with the invention, the mold cavity is designed such that when it closes, the fuel cell is compressed to a predetermined thickness dictated by a desired internal pressure. This allows pressure to be placed evenly across the entire active area of the cell. After the fuel cell is thus constructed and removed from the mold, the now formed plastic frame as well as the structurally enhanced current collectors hold the fuel cell component under a continuous pressure across the surface of the fuel cell. Accordingly, the level of compression obtained by this manufacturing process is both consistent and predictable from cell to cell when compared to the variability in compression observed in cells that are manually assembled.
The current collector, which also serves as the compression plate, may also indude flow fields to aid in the flow of the products and reactants in desired directions throughout the fuel cell. The current collector may also be designed to perform gas or liquid diffusion functions, depending upon the overall system design. This is described in commonly-owned U.S. patent application Ser. No. 10/260,820, filed on Sep. 30, 2002 by Ren et al for a FLUID MANAGEMENT COMPONENT FOR USE IN A FUEL CELL, which is presently incorporated herein by reference.
The NAFION® membrane and the current collectors can both be initially fabricated with openings around the perimeters thereof such that plastic in the mold process can flow through the openings and can form internal fasteners that hold the edges of the current collectors and NAFION® membrane in place.
In accordance with yet a further aspect of the invention, instead of hot pressing segmented MEAs and then inserting them into a lead frame for molding, the diffusion layer materials are applied directly to the current collectors on the lead frame components and are thereafter hot pressed to the current collectors collectors, which are fastened or bonded to the MEA. This assembly is then insert molded to create a sealed fuel cell, or full fuel cell array. Fabricating the current collectors to provide gas diffusion properties may aid in this effort.
The invention description below refers to the accompanying drawings, of which:
In accordance with one aspect of the invention, the catalyzed membrane electrolyte, or CCM, 104 is die cut to include welding extension tabs, 110 and 112. The tabs include a plurality of openings that are collectively designated in
The lead frame assembly 100 of the present invention includes a lead frame component 140 on the anode side and a lead frame component 142 on the cathode side. In accordance with the invention, the lead frame components not only hold the assembly together, but also integrate current collector functionality. More specifically, an anode current collector 144 is incorporated into the anode lead frame component 140. Thus, the anode current collector includes the exterior frame portion 146, and a current collecting lead 148. Similarly, on the cathode side, a cathode current collector 150 is integrally formed as part of the cathode lead frame component 142 and includes an exterior frame portion 152 and a current collecting lead 154.
The assembled laminate cell 200 of the present invention, prior to the molding process, as viewed from the anode side is illustrated in
Similarly, on the cathode side illustrated in
After the MEA components are placed within the lead frame assembly, the components are spot welded together through holes in the NAFION®, 116 and 120, securing the components in place for the molding process. This process step is illustrated in the flow chart of
The lead frame assembly 200 (
Around the outside perimeter of the shut off surfaces such as the surface 514 of
Returning now to the flow chart of
As the plastic cures, as indicated in step 310, a dense, solid plastic frame is created around the fuel cell. This securely seals the edges of the fuel cell against leaks. The plastic forms a seal in the outer borders of the diffusion layers and tightly blocks any undesired pathways that could give rise to leaking fuel or water leakage. This eliminates the need for gasketing. The lead frame assembly itself is designed so that the more fragile internal components such as the catalyzed membrane and the active areas of the diffusion layers of the fuel cell are protected by the current collectors while the molding process takes place.
In addition to sealing the fuel cell, the frame manufactured in accordance with the present invention also holds the components of the cell in compression. Adequate compression is important for obtaining efficient current collection. Typically, compression is achieved by tight screws, bolts and other fasteners. However, in accordance with the present invention, compression in the fuel cell is introduced by the mold plates themselves. The mold cavity is designed in accordance with the invention such that when it closes, it compresses the cell to decrease the thickness dictated by the selected internal pressure. Pressure is placed substantially evenly across the entire active area of the cell during the molding process and after the molding is complete this pressure is maintained by the plastic frame, which is securely formed around the fuel cell components and remains stationary, applying substantially constant pressure. Alternatively, the mold may include compliant surfaces to apply pressure evenly across the entire active area of the cell regardless of the thickness of the cell. Alternatively, the compliant mold may be fabricated using methods known to those skilled in the art.
During the insert molding process, in order to minimize the heat experienced by the MEA and specifically the membrane electrolyte, while in the mold, a polymer with a relatively low melting temperature may be selected. This low temperature plastic may be injected at the lowest possible temperature in the workable range for the material. The temperature that will typically be used is much less than the maximum temperatures that is the MEA has been shown to endure. For instance where a NAFION® membrane electrolyte is used, it is desirable to maintain a temperature of less than 215 degrees Fahrenheit. The actual temperature threshold is a function of many factors including, but not limited to time and materials selected.
Additionally, a protective film or tape can optionally be placed across the surfaces of the current collectors prior to insertion into the mold if additional protection is desired from volatile vapors or other contaminants. This film or tape can also be used to help retain moisture in the cell.
After the introduction of the plastic, the fuel cell is held in place while the plastic cools to the mold temperature to allow full solidification of the plastic frame. The fuel cell is then removed from the mold as noted in step 312 of
The completed frame 600 around the fuel cell is illustrated in
The manufacturing time for fabricating a single insert-molded fuel cell in accordance with the present invention, once components are collected, involves 1-2 minutes for assembly of the lead frame, approximately one minute cycle time for the molding process (including the time needed for manually inserting the lead frame into the mold) and less than one minute to trim the excess lead frame material away from the finished cell. The steps in the process could potentially be automated, including assembly of the components onto the lead frame, insertion of the lead frame assembly into the mold, and trimming, thus further reducing cycle time. Furthermore, costly and potentially inconsistent manual work is also eliminated when automating the various steps. Manufacturing time is also greatly reduced by molding more than one cell at a time in a cavity. Additionally, the method described above can be applied to fuel cell arrays in which the cells may be previously connected in series before the lead frame is molded.
In accordance with another aspect of the invention, the manufacturing process of a fuel cell can be improved by combining the certain bonding techniques used to laminate fuel cell components with techniques for insert molding the fuel cell or fuel cell array.
More specifically, and as noted herein, typically a fuel cell is made by first creating the membrane electrode assembly by catalyst coating a protonically conductive membrane and then placing diffusion layer materials on each side. Typical diffusion layer materials include carbon cloth and/or carbon paper, however diffusion layers may further include plastics and metals. Typically, these items are placed in close proximity to each other and laminated together using hot press operation. Similar to an injection molding process, the hot press process involves bonding two or more components together by pressing two plates together and applying substantial force at the desired temperature.
In accordance with the present invention, instead of separately hot pressing segmented membrane electrode assemblies together in accordance with prior techniques and then inserting them to a lead frame for molding, these steps may be combined. More specifically, as illustrated in
As noted, the protonically conductive membrane 802 is striated with lines 810, 812, etc. Similarly, the anode diffusion layer 804 has lines such as the line 814. The cathode diffusion layer 806 has lines 816. These are perforations that divide individual membrane electrode assemblies for separate fuel cells for a fuel cell array.
On gas aspect of the diffusion layer opposite the membrane electrolyte, a current collector is provided, as described in detail above. In accordance with the present invention, as described in detail with reference to the prior figures, the anode current collector 820 is integrated into a lead frame structure 824 for the convenience of handling and support during the insert molding process. Similarly, the cathode current collector 830 is integrated into a cathode lead frame structure 834 in the manner described in detail with reference to
As illustrated in
More specifically,
Next, the components to be included in the fuel cell or fuel cell array are then assembled in accordance with step 906. The components include a protonically conductive, electronically non-conductive membrane that has been catalyzed. This membrane will be supplied with cutouts to allow for plastic flow through the membrane as described with reference to the previous embodiment, and if an array of cells is being created, cutouts will be provided between the individual MEAs. Optionally, the membrane could be supplied on a tape, which may be segmented to create individual cells in an array. If desired, additional materials such as Teflon® and other suitable materials can be added around the perimeter or other non-active portions of the membrane to strengthen the membrane and to improve leak-resistance. Adding such materials can also serve to lower the cost and simplify the combination hot press and molding process. Assembled with the catalyzed membrane are the lead frames with integrated current collectors, which have diffusion layers applied as described, on each aspect of the MEA.
Once all of the materials are assembled, the lead frames are spot welded together as illustrated in step 907, in order to maintain the various components in place in the mold device. For example, area 840 and 844 on the lead frame component 824 (
The next step 908 is to place the assembly into a suitable mold device, as described earlier in this application. The mold plates are then closed and heat is applied for a predetermined time period to hot press the components of the fuel cell as indicated in step 910. The time period may be on the order of approximately 4 minutes at a temperature of approximately 215 degrees Fahrenheit. However, those settings are for one example of the invention and the times may vary substantially while remaining within the scope of the present invention. The hot pressing step bonds the diffusion layers to the current collectors.
Now, without needing to move the assembly components to another device, a material suitable for molding is introduced into the mold cavities illustrated in step 912 of
The foregoing description has been directed to specific embodiments of the invention. It will be apparent, however, that other variations and other modifications may be made to the described embodiments, with the attainment of some or all of the advantages of such. Therefore, it is the object of the appended claims to cover all such variations and modifications as come with in the true spirit and scope of the invention.
