Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0267—Collectors; Separators, e.g. bipolar separators; Interconnectors having heating or cooling means, e.g. heaters or coolant flow channels
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0204—Non-porous and characterised by the material
  • H01M8/0223—Composites
  • H01M8/0228—Composites in the form of layered or coated products
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0204—Non-porous and characterised by the material
  • H01M8/0206—Metals or alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/023—Porous and characterised by the material
  • H01M8/0232—Metals or alloys
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0258—Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the configuration of channels, e.g. by the flow field of the reactant or coolant
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0271—Sealing or supporting means around electrodes, matrices or membranes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0271—Sealing or supporting means around electrodes, matrices or membranes
  • H01M8/0273—Sealing or supporting means around electrodes, matrices or membranes with sealing or supporting means in the form of a frame
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0271—Sealing or supporting means around electrodes, matrices or membranes
  • H01M8/028—Sealing means characterised by their material
  • H01M8/0282—Inorganic material
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0271—Sealing or supporting means around electrodes, matrices or membranes
  • H01M8/028—Sealing means characterised by their material
  • H01M8/0284—Organic resins; Organic polymers
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0297—Arrangements for joining electrodes, reservoir layers, heat exchange units or bipolar separators to each other
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
  • H01M8/2465—Details of groupings of fuel cells
  • H01M8/2483—Details of groupings of fuel cells characterised by internal manifolds
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0204—Non-porous and characterised by the material
  • H01M8/0206—Metals or alloys
  • H01M8/0208—Alloys
  • H01M8/021—Alloys based on iron
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
  • H01M8/2465—Details of groupings of fuel cells
  • H01M8/247—Arrangements for tightening a stack, for accommodation of a stack in a tank or for assembling different tanks
  • H01M8/248—Means for compression of the fuel cell stacks
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• the present invention relates to electrochemical cells and methods for assembling electrochemical cells.
• Electrochemical cells utilizing a proton exchange membrane can be configured in cells stacks having bipolar separator plates between adjacent cells.
• These bipolar separator plates are typically made from a variety of metals, such as titanium and stainless steel, and non-metallic conductors, such as graphitic carbon.
• Bipolar separator plates are typically fabricated by machining flow fields into a solid sheet of the material.
• the precursor material is formed by injection molding and converted to the conductive carbon form by high temperature firing under carefully controlled conditions.
• the flow fields are made up of a series of channels or grooves that allow passage of gases and liquids.
• Figure 1 is a face view of a prior art bipolar separator plate 10 made from a solid sheet of a conducting material.
• the central portion of the plate has a flow field 12 machined into its surface.
• the flow field may direct fluid flow across the surface of an electrode in many patterns, but is illustrated here as parallel serpentine channels.
• the plate Around the perimeter of the flow field 12, the plate provides a plurality of bolt holes 14 for assembling and securing a cell stack, various manifolds 16 for communicating fluids through the stack, and a flat surface 18 that allows the plate to be sealed against adjacent components of the cell stack.
• a bipolar separator plate for use in electrochemical cells must collect electrons liberated at one electrode (i.e., an anode), conduct the electrons through the plate, and deliver electrons to the face of another electrode (i.e., a cathode) on d e opposing side of the plate.
• the bipolar plate shown in Figure 1 collects and delivers electrons from electrodes of opposing cells through contact between the electrodes and the ridges 20 remaining between the channels 22 in the flowfield 12.
• Figure 2 is a schematic view of a proton exchange membrane (PEM) electrochemical cell configured as a hydrogen-air fuel cell stack 30.
• PEM proton exchange membrane
• This stack 30 comprises two identical fuel cells 32 each having a cathode 34, a PEM 36 and an anode 38.
• Flow fields 40 are provided on either side of the bipolar separator plate 42, as well as on the internal faces of the endplates 44. Electrons liberated at the anodes 38 induce electronic current flow to the cathode 34 of an adjacent cell on the other side of the plate 42 and, in the case of the last anode of the stack (here the anode on the right of the page), through an external circuit 46. Electrons are then combined with protons and oxygen at the cathodes 34 to form water. The electrical potential of the fuel cell 30 is increased by adding more cells 32 to the stack.
• the solid piece of graphite or metal used to fabricate the bipolar separator plate constrains the density of the final product to a density approximately the same as that of the original stock, thereby producing a very dense and heavy bipolar separator plate.
• machining each piece from a solid starting blank requires relatively expensive machining processes, as opposed to less expensive molding, casting or stamping processes.
• the molding step is inexpensive, however, the controlled sintering required to convert the precursor to the final product is slow and requires precise atmosphere and temperature control throughout the process.
• Another important aspect in fabricating an electrochemical fuel cell is the number of joints and junctions created in the cell. Reduction of the number of joints and junctions can greatly improve the performance of a electrochemical cell stack, for example if fabricated from a stack of flat components, because there are fewer potential leak points and fewer electronic contact resistances. A fabrication process that provides an electrochemical cell with a minimum of joints and junctions would be highly desirable.
• Assembling a PEM fuel cell stack using relatively flat components requires gas tight seals at each interface. Gaskets are typically used to create gas tight seals, however gaskets increase the number of parts that must be fabricated and aligned when the stack is assembled. A method and apparatus for forming gas tight bonds or seals at the interfaces between the components of an electrochemical cell would obviate the need for several gaskets and produce a more efficient cell.
• bipolar separator plate there remains a need for an improved bipolar separator plate. It would be desirable if the bipolar separator plate were thin, light weight, and could support high current densities. It would be further desirable if the bipolar separator plate reduced the number of joints or junctions in the individual cells or a cell stack and reduced the need for gaskets. Furthermore, it would be useful if the structure of the bipolar separator plate allowed the introduction of other specific properties, such as water permeability and reactant gas impermeability.
• the present invention provides a method for preparing a subassembly for an electrochemical cell.
• the method includes aligning a subassembly having two or more electrochemical cell components with one or more bonding elements disposed between the two or more electrochemical cell components.
• the bonding elements have a melting point temperature that is lower than the melting point temperature of any one of the two or more electrochemical cell components.
• the subassembly is compressed and heated to a temperature that is between about the melting point temperature of the bonding element and about the lowest melting point temperature of the any one of the two or more electrochemical cell components.
• the temperature is less than 800°C, more preferably below 250°C.
• the subassembly is then allowed to cool.
• the subassembly is preferably positioned into an electrochemical cell or an electrochemical cell stack.
• the two or more electrochemical cell components are preferably metal components selected from plates, shims, frames, flow fields or combinations thereof, such as stainless steel, titanium, nickel, nickel plated aluminum, nickel plated magnesium, or combinations thereof.
• the bonding element is preferably solder.
• the metal component is preferably dipped in a flux; then dipped in a bonding metal or solder, such as tin or a silver-tin alloy.
• the bonding metal, or solder can also be applied to the metal surface by electrodeposition or by various vacuum deposition techniques.
• Light or easily oxidized metal components such as those made from aluminum, magnesium, or alloys containing aluminum or magnesium are preferably coated with a layer of a corrosion resistant transition metal prior to the dipping the metal component in the flux.
• Suitable corrosion resistant transition metals include but are not limited to cobalt, copper, silver, nickel, gold or combinations thereof. Nickel is the most commonly used metal for the corrosion resistant layer.
• the two or more electrochemical cell components can be polymer components selected from frames, gaskets, membranes, shims, or combinations thereof where the bonding element is preferably an adhesive.
• the two or more electrochemical cell components may also comprise one or more metal components and one or more polymer components, where the bonding element is an adhesive.
• the two or more electrochemical cell components can include a plate and a flow field.
• the subassembly preferably includes a bipolar plate and a frame.
• the bipolar plate preferably has two plates, a flow field and a frame.
• the frame and flow field are disposed between the two plates with the frame disposed around the flow field.
• the frame has channels in fluid communication with the flow field.
• a fluid cooled bipolar plate assembly having two electronically conducting plates having opposing faces, an electronically conducting flow field bonded in electronic communication with a substantial portion of the opposing faces of the plates, between the two electronically conducting plates, and a frame disposed around a perimeter of the electronically conducting flow field and bonded between the two electronically conducting plates.
• the frame has channels for providing fluid communication between the flow field and a fluid source.
• an electronically conducting cathode flow field and an electronically conducting anode flow field are bonded to opposing sides of the assembly.
• the assembly is preferably bonded with an adhesive and/or solder.
• a bipolar plate for electrochemical cells having two or more porous, electrically conducting sheets selected from expanded metal mesh, woven metal mesh, metal foam, conducting polymer foam, porous conductive carbon material or combinations thereof.
• An electrically conducting gas barrier is disposed in electrical contact between the sheets.
• a cell frame is disposed around a periphery of any one of the two or more porous electrically conducting sheets.
• the cell frame has at least one surface that is bonded to the gas barrier.
• the cell frame includes channels in fluid communication with the porous electrically conducting sheet.
• the cell frame can be metallic, where it is bonded to the gas barrier with a metallic bond.
• the metallic bond is preferably formed by soldering the cell frame to the gas barrier.
• the cell frame can be polymeric, where it is bonded to the gas barrier with a polymeric bond.
• the polymeric bond is produced by an adhesive.
• Figure 1 is a face view of a typical metal separator plate with a serpentine flow field design.
• Figure 2 is a schematic cross-section of a PEM fuel cell. The relative thickness of the components has been greatly exaggerated for clarity.
• Figure 3 is a partial cross-section of a bipolar plate having a metal gas barrier with metal flow fields and polymer cell frames.
• Figure 4 is a schematic drawing of the flat components used in the three dimensional structure of a fluid cooled bipolar plate.
• Figure 5 is a graph showing the performance of a fuel cell stack of the present invention.
• the present invention relates to components for use in electrochemical cells and methods for fabricating those components, including bipolar separator plates. More particularly, the present invention relates to a method for bonding adjacent components of an individual cell and/or adjacent cells of a stack. The bonding provides gas tight seals which reduces or eliminates the need for certain gasket components and reduces or eliminates certain electronic contact resistances.
• One aspect of the invention provides a bipolar plate for an electrochemical cell having an electrically conducting flow field, at least one gas barrier and a cell frame.
• the components of the bipolar plate are bonded to each other to form gas tight bonds with either polymeric type bonds or metallic type bonds.
• Another aspect of the present invention provides a method for bonding conductive portions of a bipolar plate.
• the conductive pieces are tinned or plated to coat them with a conductive metal and subsequently bonded together under pressure and heat to form bonds between the parts.
• the component parts of a bipolar plate or other member of an electrochemical cell may be categorized as being either those that must be conductive for the stack to function or those that are not required to be conductive.
• the flow fields and the gas barriers of the bipolar plate must be fabricated from a conductive material, such as a metal or a conductive form of carbon, such as graphite.
• the cell frames and the sealing surfaces, also known as shims may be electronically insulating or conductive and can optionally be fabricated from various polymers, thereby resulting in a lower density component and significant overall weight savings for the cell or stack.
• FIG. 3 is a partial cross-section of a bipolar plate 48 having a metal gas barrier
• the metal gas barrier 50 extends beyond the flow fields 52, preferably to the edge of the stack, with separate cell frames 54 on both the anode and cathode sides of the gas barrier 50. Flow fields 52 are placed against the barrier 50, and inside the frames 54 as shown. Additional gasketing may be included between the barrier and the frame if desired.
• FIG 4 is a schematic drawing of the components used in the three dimensional structure of a fluid cooled bipolar plate.
• a fluid cooled bipolar plate 55 is shown assembled from a series of substantially planar components including two cooling fluid barriers 60 and a cooling fluid frame 62, with an electronically conducting flow field 63 therein, similar to the flow field 52 of Figure 3.
• the fluid cooled bipolar plate 55 may further include an anode cell frame 58, cathode cell frame 64 and sealing plates 56 (for contacting and securing a PEM or membrane and electrode (M&E) assembly).
• M&E membrane and electrode
• This bipolar plate includes an internal cooling flow field 63 that allows passage of a cooling fluid for cooling the stack.
• the cooling fluid flows from a cooling fluid inlet manifold 65, through the cooling flow field 63 within the cooling fluid frame 62, and into a cooling fluid outlet manifold (not shown) generally similar to, but opposed from, the inlet manifold 65.
• the flow field 67 in the anode cell frame 58, the flow field 63 in the cooling fluid frame 62 and the flow field 69 in the cathode cell frame 64 may be made from any porous conductive material as described above.
• Bonding the polymer elements of a lightweight PEM fuel cell stack, such as the anode and cathode cell frames, to metallic elements, such as the gas barriers may be done with an adhesive that bonds to both surfaces to form a gas-tight seal that remains stable under typical cell operating conditions.
• polymers were tested for fabricating components for PEM fuel cells (polycarbonate, polyethersulfone, polyetherimide, and polyimide). These polymers are representative of thermoplastics considered to be useful for this application.
• the polymer sheets were bonded to gold-plated titanium sheets using different adhesives to determine the combinations that produced adequate bonding for use in electrochemical cells.
• Gold plated titanium was chosen for these experiments because it is considered to be one of the most challenging materials, useful in conductive electrochemical cells components, to bond with a polymer.
• a variety of adhesives were used to bond 0.032" (0.813 mm) polymer sheets to gold plated 0.007" (0.178 mm) titanium sheets.
• the test specimens were approximately 2 centimeters wide and 2 centimeters long. The procedure involved preparing the surfaces, applying the adhesive, bonding, and curing the adhesive according to the individual manufacturers instructions. The bonded specimens were then exposed to water at 60 °C for 24 hours, after which they were examined to determine the quality of the bond.
• a representative group of adhesives showing reasonable bonding characteristics are included in Table I.
• bonds that are critical to the conductive path of electrons such as the bonding of flow fields to gas barriers
• bonds that are critical to seal against fluid leakage such as the bonding of gas barriers to metal frames.
• gas tight bonds it is beneficial, but typically not necessary, if the bond is also conductive.
• Conductive bonds can be created between a variety of metals by soldering the parts together. Soldering is a well known technique where a relatively low melting metal is used to bond two components together that are fabricated from metals having higher melting points than the low melting metal or solder. It is imperative that the low melting metal wets the higher melting metals to achieve a good bond.
• soldering for wire attachment for an electrical assembly involves heating the parts to above the melting point of the solder, then applying a flux to remove the oxide film on the metal and finally applying the solder, with the flux and the solder sometimes applied together.
• Soldering large areas requires a different process.
• the part is first coated with the bond- forming metal or alloy by one of several methods. It can first be coated with flux and then coated with solder in a process known as tinning.
• the flux can be applied in a variety of ways, however brushing and dipping are the most commonly used methods. Application of solder is typically accomplished by dipping the part in a container of molten solder.
• the bond forming metal or alloy can also be plated onto the metal surface by either electrodeposition or electroless deposition, both of which are well known processes. Also, the bond forming metal or alloy can be deposited onto die metal surface using vacuum deposition techniques, such as evaporation, chemical vapor desposition and sputtering. After depositing the bond forming metal by tinning or plating, the parts are bonded by clamping the flat surfaces to be bonded together and reheating the assembly to above the softening point of die solder. . Both the clamping and the heating can be accomplished simultaneously through the use of a hot press.
• the procedure for tinning stainless steel parts includes, dipping the part in an acid flux solution; slowly dipping the part in molten solder; dipping the part in the acid flux a second time; and dipping the part in molten solder a second time.
• the tinning procedure is carried out in an inert atmosphere.
• the clean stainless steel part is immersed in a water soluble flux, preferably an acid flux, and the excess solution is allowed to drain off.
• the part is then slowly lowered into the molten solder. It is important that the part be immersed slowly, because the flux is only active for removing the oxide film while the flux is hot, and a slow immersion process leads to a longer exposure time and better oxide removal.
• a silver solder such as eutectic tin- silver (96.5 Sn : 3.5 Ag) is preferred.
• a single immersion in the molten solder is sufficient, but in other cases a second dipping is preferred.
• the part is then removed from the flux and the excess solution allowed to drain off.
• the part should be sufficiently hot to boil the flux as it makes contact.
• the part is slowly lowered into the molten solder and removed.
• the first immersion in the solder covers most of the surface of the stainless steel part with a thin layer of solder, however occasionally there are areas where the flux did not remove all of the oxide film, that are poorly coated.
• the second dip in the flux with the part hot removes any remaining oxide film from the part and produces a shiny, mirror-like finish on the metal. It is important that all grease and superficial dirt be removed from the surface of the metal before soldering. Failure to clean and degrease the surface may leave a protective film on the oxide layer that prevents the flux from cleaning the surface, and prevents the solder from sticking.
• the parts are preferably thoroughly washed in deionized water to remove all remaining traces of the flux.
• the procedure for tinning titanium, aluminum, and magnesium containing parts includes plating the part with nickel, dipping the part in an acid flux solution, and dipping the part in molten solder. In order to nickel plate the part, the oxide film must be removed.
• the part After the part has been nickel plated, it is coated with flux and dipped in molten solder to produce a shiny tinned finish using the same cycle and silver solder as is used with stainless steel. Generally, only a single immersion is required for each the flux and the solder with nickel-plated aluminum.
• Both aluminum and titanium can be tinned directly if the proper combination of flux and solder are employed. With the use of a high viscosity liquid flux, aluminum and titanium can be directly tinned. In this process the part is first immersed in the flux, and then slowly immersed in molten solder, with a single cycle being sufficient.
• a solder with a more active component is preferred, such as eutectic tin-zinc (91% Sn, 9% Zn, melting point 199°C). It is important to note that for best results, all of the tinning operations described herein are preferably carried out in an oxygen free atmosphere, or an atmosphere with a substantially reduced oxygen content. It is not necessary, and indeed in most cases it is not possible, to exclude water, since water is a major component of many fluxes. As long as a tin based solder is being used, it doesn't matter what alloying elements are present in the solder when forming the final bonds, the tinned parts can readily be bonded together.
• eutectic tin-zinc 91% Sn, 9% Zn, melting point 199°C.
• solders with dissimilar compositions When solders with dissimilar compositions are used to tin metals to be bonded it is important to heat the clamped components to at least the melting point of the higher melting solder. While the metallurgical bond produced by soldering metal components together is preferred, it is not the only approach to joining metal components.
• Adhesives like those listed in Table II, can be used for some metal-to-metal bonds as well. In general, these adhesives are suitable for bonds where through-bond electrical conductivity is not required. Except for the Pd-Ag filled epoxy, none of the adhesives listed in Table II are conducting. The conductive adhesive bond may be substituted for a soldered joint in producing an electrochemical cell or stack in accordance with the present invention.
• Assembling a fully bonded electrochemical cell stack includes several steps. First, all of the metal parts are coated with die bonding metal, including the anode flow field, cathode flow field, and cooling flow field; the gas barriers; and the frame around the cooling flow field. These parts are then stacked into position, carefully positioned, and held in place using an alignment device or jig. The parts and the jig are then placed in a hot press to clamp the parts together, pressing the gas barriers firmly against the cooling frame and the flow fields firmly against the gas barriers. The press is heated to slightly above the melting point of the highest melting bonding metal present in order to remelt the bonding metal. The bonded assembly is then allowed to cool in the press.
• Sheets of hot melt adhesive are cut to match the size and shape of the anode and cathode cell frames. These are used to bond the cell frames to the cell shims through the use of the hot press. Those two piece subassemblies are bonded to the assembled bipolar plate by the same technique. All of the hot melt bonds can be made in a single step, if desired.
• An uncooled bipolar plate is somewhat easier to fabricate because it contains few parts.
• the flow fields are bonded to opposite sides of a gas barrier by soldering.
• the anode and cathode cell frames are attached to the gas barriers and the shims with hot melt adhesives in one or two pressing operations.
• the following examples show some of the preferred embodiments of the present invention, however they are not to be considered limiting in any sense.
• This example shows the fabrication of the core of a liquid cooled bipolar plate.
• Two titanium gas barriers were fabricated from 0.0045" (0.114 mm) metal sheet and plated with gold.
• a frame was fabricated from 0.032" (0.813 mm) polyethersulfone sheet.
• a flow field was fabricated from three layers of expanded titanium, one with a thickness of about 0.030" (0.762 mm) at full expansion and two which were expanded and subsequently flattened back to their original 0.003" (0.076 mm) thickness. All three sheets were spot welded together, with the 0.003" sheets welded to opposing faces of the 0.030" sheet, and the welded assembly was gold plated.
• This flow field was deliberately produced thicker than the frame to insure that the flow field, which compresses like a spring, would put pressure on the gas barriers to ensure electrical contact with the barriers.
• the faces of the polyethersulfone frame were coated with ECLECTIC E6000 adhesive.
• the frame was then pressed, by hand, against one gas barrier, with both parts kept in alignment by the use of a positioning jig fabricated for this purpose and equipped with alignment pins to insure that all of the parts stay precisely in alignment.
• the flow field was placed in the cavity produced by the frame on the barrier, and the second gas barrier was pressed on top of the other parts.
• the assembly, still in the assembly jig, was placed in a hot press. The components were then pressed together and cured under sufficient compression to keep the parts from moving at 60 °C for 24 hours.
• a face shim or sealing plate 56 was fabricated from 0.0045" (0.114 mm) titanium sheet, as illustrated in Figure 4.
• An anode cell frame was fabricated from 0.032" (0.813 mm) polyethersulfone sheet. The anode cell frame was used as a template to cut a sheet of hot melt adhesive (BEMIS number 3218). All three pieces were stacked in a positioning jig fabricated for this purpose and equipped with alignment pins to insure that all of the parts stay precisely in alignment with the adhesive between the metal shim and the polymer frame.
• the jig with the parts inside of it, was placed in a hot press, compressed to insure and maintain intimate contact between the sheet components, and heated to 150°C. (The temperature used for this step is a function of the adhesive used.) After several minutes, the press was cooled, and the subassembly removed.
• This subassembly is suitable for use as the frame around the anode flow field and the contact surface against the membrane portion of a membrane and electrode assembly (M&E) and used in assembling a fuel stack.
• M&E membrane and electrode assembly
• This example shows the tinning of stainless steel.
• a gas barrier was fabricated from 0.010" (0.254 mm) stainless steel sheet.
• the gas barrier and the other items required to carry out the tinning operation (flux, solder, heater, tongs, temperature indicator, etc.) are placed in a glove box and the atmosphere of the box thoroughly purged with argon to prevent oxidation of freshly cleaned surfaces.
• the barrier is immersed in an acid type liquid flux such as LA-CO N-3 (LA-CO Industries, Chicago, IL).
• the part is allowed to cool until the solder solidifies before being re-immersed in the flux. It is preferable to have the part remain sufficiently hot to boil the liquid slightly as it contacts the surface of the metal.
• the part is removed from the flux and slowly re-immersed in the solder, with the second cycle serving to remove the oxide film from the parts of the surface which were not stripped the first time and coat those areas with solder.
• After the part is removed from the molten solder it is allowed to cool in the air until the solder solidifies.
• the part may then be laid down on a heat resistant surface or other support to finish cooling. The result is a mirror-like finish on the tinned part.
• This example shows the tinning of aluminum.
• a cooling cell frame for a fluid cooled bipolar separator plate was fabricated from 0.020" (0.508 mm) aluminum sheet and plated with nickel.
• the gas barrier and the other items required to carry out the tinning operation were placed in a glove box and the atmosphere of the box was thoroughly purged with argon to prevent oxidation of freshly cleaned surfaces.
• the barrier was immersed in an acid type liquid flux such as LA-CO N-3 (LA-CO Industries, Chicago, IL).
• This example shows the bonding of aluminum and stainless steel components to form the core of a liquid cooled bipolar plate using solder.
• Two gas barriers and a flow field are tinned as described in Example 3.
• An aluminum cooling frame is tinned as described in Example 4.
• the components are stacked as shown in Figure 4 and placed in a hot press with a thermocouple in contact with the side of the frame and clamped firmly together. The press is heated until the thermocouple indicates that the load has reached and maintained a temperature of about 230 °C for about five minutes, at which time the power to the heaters is turned off and the entire assembly is allowed to cool while maintaining the assembly in a compressed state.
• Example 1 illustrates the performance of a fuel cell stack produced in accordance with the present invention.
• the method of Example 1 was used to prepare a set of four water cooled bipolar plate cores sized for use in a fuel cell stack with an active area for 125 cm 2 . Three uncooled bipolar plates were also used in the stack.
• the method of Example 2 was used to prepare eight anode cell frames with shims and eight cathode cell frames with shims for a stack of that size. Additional components (M&E's, flow fields, gaskets, etc.,) were also prepared by conventional means. These were used to fabricate an eight cell PEM fuel cell stack. The stack was operated using hydrogen fuel with air as the oxidant.
• Figure 5 illustrates the performance of that eight-cell stack, 125 cm 2 per cell active area while operating at an M&E temperature of 62 - 64°C, a fuel gas pressure of 15 psig, both fuel and oxidant gases humidified to a dew point of 27°C, and air supplied at four-fold stoichiometry.
• the stack provided an output of 967 W/kg and 846 W/L for the repeating units of this stack which was determined to be an efficiency of 53.4% (at a potential of 0.651 V/cell).

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• 2000
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