Abstract:
A non-bipolar fuel cell stack configuration where non-bipolar fuel cell arrays, manufactured on reel-to-reel sheets of porous plastic substrate material, are electronically connected in parallel with air and/or oxidizing gas flowing between the arrays. Separator plates of conventional type bipolar fuel cell stacks are eliminated in this approach and many of the electrical contact problems associated with conventional fuel cell stack are overcome. The present invention enables large power fuel cells, with relatively low total mass, to be readily manufactured at low cost.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to non-bipolar fuel cells and more specifically to high energy fuel cell stacks that deliver from tens of watts to megawatts of power. 
     The fundamental components of a prior art non-bipolar fuel cell array are shown in the schematic cross-sectional view of FIG.  1 . The basic components are the porous dielectric substrate  1 , the electrolyte  6 , the fuel electrode  2 , the oxidizer electrode  3 , the cell breaks  7  and  16 , the cell interconnects  12 , the external electrical circuit  20 , and the electrical load  9 . The fuel cell operates with the fuel  10  (such as hydrogen or methanol) dissolving in an electrolyte  6 . The dissolved fuel  10  catalytically breaks down into monatomic hydrogen  15  on the catalyzed fuel electrode  2 . The monatomic hydrogen  15  travels through the fuel electrode  2 , giving up an electron  19  to the electrode  2 , and forms a hydrogen ion  17  in the proton conductive electrolyte  5 . The electron  19  travels through the cell interconnects  12  to the adjacent cell oxidizer electrode  3 , which is formed over conduction electrode  4 . The hydrogen ion  17  travels though the conductive electrolyte  5  to the oxidizer electrode  3 . At the negative output terminal  22  of the array, electrons  19  flow though the electrical circuit  20  through an electrical load  9  and to the positive terminal of the array  23 . The array voltage is determined by the number of cells in the array connected in series. Each of the cells are electrically separated from the adjacent cells by dielectric occupied regions called cell breaks  7  and  16 . Adjacent cells are electrically connected by electron conductive vias or cell interconnections  12 . At the oxidizer electrode  3  and  4 , air  8  is catalytically reacted with the surface of the catalytic electrode  3  to form surface oxygen  13 , or oxygen ions  18  in the electrolyte. The oxygen electrode is made of layers of conductive metal films  4  and catalytic electrodes  3 . The oxygen ions  18  then receive the electrons from the electrodes  3  and form water  14  (a by-product) at the oxygen electrode  3 . On the fuel electrode  2  the fuel is gradually catalytically stripped of it&#39;s hydrogen  15  to leave carbon monoxide  24  on the surface of the electrode. The carbon monoxide  24  is oxidized to carbon dioxide  11  by taking the oxygen from water  10  in the fuel or by oxygen  25  which is diffused through the fuel enclosure wall  21 . The carbon dioxide  11  by-product diffuses out though the fuel enclosure wall  21  or through the cell break regions  7  and  16 . The water  14  by-product diffuses out from the oxygen electrode  3  to the surroundings. This particular example shows the fuel electrode  2  being pore free. This pore free electrode  2  can block fuel diffusion such as methanol  10  while passing monatomic hydrogen  15  to allow the fuel cell to efficiently utilize the methanol fuel. It may also add diatomic hydrogen diffusion impedance while preferentially having a low impedance to monatomic hydrogen, which has been catalyzed. Thus the pore free electrode  2  can also improve the performance of hydrogen fueled fuel cells. 
     By utilizing liquid methanol and water fuel, this type fuel cell packs more energy in a smaller space than conventional rechargeable batteries. The methanol fuel has effectively 5 to 13 Whr per cubic inch (20% to 50% efficiency) energy density. This is 3 to 9 times the energy density of today&#39;s best nickel cadmium batteries, and 40 to 120 times that of standard cellular phone battery packs. Also, these micro-fuel cells are lighter than conventional rechargeable batteries. The methanol fuel has effectively 1200 to 3000 Whr per kg energy per unit mass (20% to 50% efficiency). This is 2 to 5 times the 600 Whr per kg quoted for the latest rechargeable lithium ion batteries (Science News, Mar. 25, 1995). Various patents, such as U.S. Pat. No. 5,631,099, U.S. Pat. No. 5,759,712, U.S. Pat. No. 5,364,711, and U.S. Pat. No. 5,432,023 describe such non-bipolar fuel cells that run on hydrogen, hydrocarbon fuels, and oxygen. However, they do not describe how to assemble these fuel cells into larger parallel fuel cell stacks, which is the primary objective of this patent. 
     Our earlier U.S. Pat. Nos. 4,673,624 and 5,631,099 describe how to form non-bipolar stacks on insulator substrates. The method of stacking the fuel cells along a common fuel and electrical power connection is also mentioned in our U.S. Pat. No. 5,759,712. The present invention is intended to extend the micro-fuel cell principles set forth in these earlier patents and to show how these fuel cells are configurable into a stack to provide higher power capacity systems with air flow cooling. The present invention also shows how water is used along with air flow cooling to provide a heat and water exchanger. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to form modular power units that may be connected in parallel to deliver power in large quantity from tens of Watts to megawatts. These larger configurations of micro-fuel cells use active circulation and a variety of compatible reactants to achieve the high power outputs needed in many modern applications. An example of applications that realize a significant advantage from such power systems include, but are not limited to, household and building electrical power generators, portable electrical power generators, large power tools, utility power generators, telecommunications electrical power, and vehicle power. The principal advantages of using a fuel cells in these applications are that (1) the fuel cell realizes roughly twice the efficiency of the conventional heat engines, (2) they are quiet in operation, and (3) they are far less polluting. 
     The fuel cells of this invention may be formed on plastic sheets which make the manufacturing process suitable for large volume applications. Also, a critical component found in conventional bipolar fuel cell stacks, the electrically conductive separator plate, is completely eliminated in this approach. It is estimated that by eliminating this component from the fuel cell stack, a reduction in cost of from 10% to 20% is realized. As a result, these fuel cells have advantages over conventional fuel cell designs because of reduced mass, fewer components, and lower manufacturing and assembly costs. 
     This patent covers two embodiments for the packaging techniques of fuel cells for higher power applications. One preferred embodiment is a sealed assembly of multiple parallel arrays. A second preferred embodiment is a modular assembly of multiple parallel arrays. In the sealed assembly, the desired number of arrays are built up and sealed during the manufacturing phase. In the modular approach, the fuel cell arrays are assembled into fuel and air circulation frames to form modules that make connections for fuel, air, and electrical contact with a built-in bus structure. In the modular approach, the bus permits multiple modules to be connected in parallel by the enduser in as few as one module up to a large number of modules, depending on the application. This allows fuel cell energy systems to be sized appropriately to the application by simply adding or removing modules. The fuel cell modules may be installed or removed while the system is running, resulting in minimum down time for maintenance. They may also be adapted to offer a means for self-cleaning the cells, for purging the fuel lines, and for a “fail-safe” power shut down if unfavorable conditions exist. 
    
    
     These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a cross sectional view of a prior art nonbipolar fuel cell. 
     FIGS. 2A,  2 B and  2 C are typical layouts of the non-bipolar fuel cell array showing the air electrode side, a side view, and the fuel electrode side of the array. 
     FIG. 3 shows an exploded view of the sealed fuel cell stack embodiment. 
     FIG. 4 shows a fuel cell assembly with a fuel manifold pack sandwiched between two of the fuel cell arrays used in the sealed fuel cell assembly embodiment. 
     FIGS. 5A and 5B show a front and side view of the double-sided central air flow channel used in the sealed fuel cell assembly embodiment. 
     FIGS. 6A and 6B show a front and side view of a single-sided end-cap air flow channel used in both embodiments. 
     FIG. 7 shows a system level assembly for the sealed fuel cell stack embodiment. 
     FIG. 8 shows an exploded view of the fuel cell stack for the modular assembly embodiment. 
     FIG. 9 illustrates a typical fuel valve used in the modular assembly embodiment. 
     FIG. 10 is a perspective view of a system level assembly for the modular fuel cell stack embodiment. 
     FIG. 11 is a diagram of the reel-to-reel manufacturing approach for the non-bipolar fuel cells. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 2A,  2 B and  2 C illustrate the non-bipolar micro-fuel cell array  26  that is central to this invention and described in more detail in U.S. Pat. Nos. 4,673,624, 5,631,099, and 5,759,712. FIG. 2A shows the air side of the non-bipolar micro-fuel cell array. The air contact electrode  27  and air electrodes  28  are deposited on one side of the porous dielectric substrate  29 . The porous dielectric substrate  29  is made of dielectric materials such as polyimide or polyester plastics. The deposited electrodes  27  and  28  may be formed in a variety of methods as given in U.S. Pat. Nos. 4,673,624, 5,631,099, and 5,759,712. These electrodes, which represent small individual fuel cells, are connected in parallel with circuit traces along both edges of the array, or through the membrane  29  via interconnections  12 , as shown in FIG.  2 B. Bolt holes  30  and  33  for holding the assembly together are formed in the porous plastic  29 . Also, an electrical diode  31  is formed on the air side of the fuel cell array  26  to prevent reverse current flow through the array. The porous plastic substrate  29  is made to be electrically conductive through the fuel contact electrode  32  and the air electrical contact electrode  27 . Both fuel and negative electrical compression connections are provided for through holes  33 , with a thin sealant gasket  36  in the porous dielectric substrate  29 , while positive electrical compression connections are provided through holes  30 . 
     FIG. 2B is a side view of the non-bipolar fuel cell array. The porous dielectric substrate  29  is a thin membrane typically  5  to  200  microns thick. An electrolyte impregnates the porous dielectric substrate  29  in between the fuel cell&#39;s air electrodes  28  and fuel electrodes  35 , such as described in U.S. Pat. Nos. 4,673,624, 5,631,099, and 5,759,712. 
     FIG. 2C shows the fuel side of the non-bipolar micro-fuel cell array. The fuel electrodes  35  are deposited on the porous dielectric substrate  29  in the same manner as the air electrodes above. As with the air electrodes, the fuel electrodes  35  are connected in parallel with circuit traces along both edges of the array (not shown in this view). A gasket seal surface  36 , coated with a material such as a polyester epoxy, is added as a border around the fuel electrodes  35 . The fuel contact electrode  32  makes contact to the last fuel electrode  35  in the array inside the gasket seal. The air contact electrode  27  is shown separated from the fuel electrode  35  in the array on the outside of the gasket. These larger configurations of micro-fuel cells use active circulation and a variety of compatible reactants including, but not limited to, methanol, ethanol, hydrogen, reformate hydrogen, air, fluorine, chlorine, bromine, iodine, and oxygen to achieve the high power outputs needed in many modern applications. 
     FIG. 3 shows an exploded view for one embodiment of a non-bipolar fuel cell stack  100  configuration. The fuel cell stack  100 , as shown, is comprised of two fuel cell assemblies  38 , each made up of two fuel cell arrays  26  (as shown in FIGS. 2A,  2 B and  2 C), one central air flow manifold  39 , and two end plate air flow manifolds  40 . Larger stacks may be formed by inserting additional fuel cell assemblies  38  and central air flow manifolds  39  as desired. As shown, the air flow manifold  39  has air channels  41  to provide heat and water exchange to the air electrode side of the fuel cell assemblies  38 . The fuel bolt holes  33  and  42 , which are shown lined up, are used to provide a means for fueling, for negative electrical connections, and as a mechanical support by compressing the negative contact electrodes  32 ,  101  and  105  together on the fuel cell assemblies  38  and air flow manifolds  39 , respectively. In a similar manner, the air electrode bolt holes  30  and  43  provide both positive electrical connections and mechanical support by compressing the positive contact electrodes  27  and  37  together on the fuel cell assemblies  38  and air flow manifolds  39 , respectively. 
     FIG. 4 shows a fuel cell assembly  38  that is made up of two fuel cell arrays  26  and a porous fuel manifold  44 . In this assembly, a porous fuel manifold  44  is sandwiched between the fuel side of two fuel cell arrays  26 . This requires that the orientation of one of the arrays be flipped over horizontally so that the fuel side of both arrays face the porous fuel manifold  44 . This sandwiched fuel cell assembly  38  leaves air electrodes  28  exposed on both sides. The porous fuel manifold  44  is sealed along the surface of its rim  45 , using such techniques as ultrasonic thermal welding or an adhesive layer such as polyester epoxy. This assembly is like a bag that contains the fuel and exposes it over the fuel electrodes  35 . The fuel is delivered and removed from the porous fuel manifold  44  through the fuel and bolt holes  33 . A fuel gasket  36  seals the assembly between the fuel cell arrays  26  and porous fuel manifold  44 , leaving the fuel electrodes  35  exposed to the fuel. Compression holes  30  and  33  are also used to make electrical connections for the positive air electrode contact  27  and the negative fuel electrode contact  32 , respectively. 
     FIGS. 5A and 5B show a front and side view of the central air flow and support manifold  39  used in the fuel cell stack. As shown in FIG. 5B, this central air flow manifold  39  has two back-to-back air channels  41  separated by a center wall  46 . This structure, constructed from a dielectric material, consists of flow channel walls  47  supported by side wall support material  48  and has a positive contact electrode  37  and negative contact electrode  101  located at the two ends. These contact electrodes  37  and  101  may be made from solid metal, such as aluminum or a sheet of conductive copper formed around the end of the dielectric channel support material  48 . Air channels  41 , located on both sides of the manifold  39  and separated by a wall  46 , may be formed into a variety of patterns to provide a heat and moisture exchange mechanism over the air side surface of the fuel cell assemblies  26  (FIG.  4 ). For the serpentine pattern shown here, the air will continuously flow over the various portions of the fuel cell air electrodes  28 . The fuel cell air electrodes  28  have a water wicking mat surface, as described in U.S. Pat. Nos. 4,673,624, 5,631,099, and 5,759,712, that permits condensed water to humidify and move laterally into the air stream moving in the air flow channel  41 . The air flow channel  41  starts at one side of the central air flow manifold  39  and exits out the opposite side of the dielectric flow channel, as shown in FIG.  5 A. Side wall material  48  borders the flow channels walls  47 . This represents one possible configuration, although other arrangements that may produce better heat and water exchange by adjusting the channel wall thickness and spacing, as well as the flow pattern. Bolt holes  42  will accommodate the hollow fuel and stack compression bolts. Sealing around the fuel connections on the fuel cell array side may be accomplished with a rubber sealant gasket  36  when the fuel cell stack is assembled, although other methods may be used. In a similar manner, bolt holes  43  accommodate the air electrode compression bolts. 
     FIGS. 6A and 6B show a front and a side view of the end cap air flow and pressure support manifold  40  used in the fuel cell stack  100 . The end cap air channel manifolds  40  are the same as the central air flow channels  39  except that they have the air flow channel  41  on only the inner side of the assembly, as shown in FIG. 6B, and as a result provide end caps for the fuel cell stack assembly  100 . 
     An assembled system level fuel cell stack for this first embodiment of the patent is shown in FIG.  7 . The system, as illustrated, is comprised of eight (8) fuel cell assemblies  26 , seven (7) central air flow manifolds  39 , two (2) end cap air manifolds  40 , and other peripheral components discussed below. As shown, the fuel cell assembles  26  are compressed between the central air flow manifolds  39  with end cap manifold  40  included at each end of the stack. Hollow fuel bolts  49  and nuts  50  are used to pull the fuel contact electrodes  32  (on the fuel cells) and  101  (on the air flow manifolds) and  105  (on the end cap manifolds) together. A fuel line coupling  51  makes the connection to the hollow fuel bolts  49 , as shown. The negative electrical contact is made by means of cable  52  attached to fuel bolt  49 . In a like manner, solid compression bolts  54  are used to pull the air contact electrodes  27  (on the fuel cells) and  37  (on the air flow manifolds) together. The positive electrical contact is made by means of cable  53  attached to compression bolts  54  and nuts  50 . Fuel is delivered to the fuel cell stack by means of a fuel line  55 , through a fuel valve  56 , from a fuel tank  57 . The unused fuel exits though the hollow bolt  49  and can be, discarded, burned, or filtered and reused. Air flow channels  41  route the flow of air across the air electrodes of each fuel cell assembly. Air enters the fuel cell stack from one side and moves through the serpentine channels to exit on the opposite side of the stack. For safety purposes, electrical connections are made after the fuel connections are sealed off to avoid the possible presence of a destructive voltage in the vicinity of open fuel. 
     FIG. 8 shows an exploded view for a second alternate embodiment of a non-bipolar fuel cell modular configuration which can be used for applications with very large power requirements. The main difference here is that the fuel manifold  44  (FIG. 4) of the first embodiment, which is essentially a bag of fuel, has been replaced with a much larger serpentine fuel manifold  58  and in this approach the air and fuel manifolds  58  and  40 , respectively, along with individual fuel cell arrays  26 , forms a module  103  that slides into a common bus structure. Air flow channels  41  exist as before. This fuel distribution manifold  58  is made of the same material  47  but is thicker and has fuel flow channels  59  which are again arranged in a serpentine pattern. Fuel seal gasket  36  is used between the fuel cell array  26  and fuel manifold  58  to prevent fuel leakage. The fuel flows up from a fuel inlet connector  60 , through fuel ports  61  into the fuel channel  59 . The fuel inlet connector  60  is built into the positive contact electrode  37 , while the fuel outlet connector  62  is built into the negative electrical contact electrode  107 . The fuel electrodes  35  on the fuel cell array  26  are shown facing the fuel manifold  58  while the air electrodes  28  are shown facing the end cap air manifolds  40 . The module  103  is drawn together with solid bolts  54  and nuts  50  through holes  42  and  33 , and through holes  43  and  30 . In this case, the bolts are only used for alignment and compression of the assembly and electrical contact, since the fuel is brought into the fuel manifold  58  through inlet connector  60  and exits through fuel outlet connector  62 . This makes negative electrical contact through contact electrodes  107 ,  105  and  32  and positive electrical contact through contact electrodes  37  and  27 . The fuel manifold  58  also has a built-in dielectric handle  63  used for inserting and removing the module  103  into or from the system assembly. 
     FIG. 9 shows an exploded view of the fuel valve connector  64  along with a cross-sectional view of the mating fuel valve channel block  71  in the common bus structure. The fuel couplings are similar to the air inlet valve on car tires where a valve opens after the fuel cell module has made a gas tight seal. This connection is designed to have an o-ring seal  65  which makes a seal as it slides into the mating hole  80  of fuel supply channel block  71 . Two push buttons  66  and  73  make contact and open the valve inner seal cone  69  and the outer seal cone  79 . The cones  69  and  79  are held shut by air pressure and springs  67  and  74  around the sliding shafts  68  and  75 . Gas flows though the apertures  70  and  78 , and the aperture cage  72  when the valves are seated. Within the fuel supply channel block  71  there are also channels  76  included to distribute fuel to the valve flow channels  77 . 
     FIG. 10 shows a system level assembly of the fuel cell slide out modules  103  of this alternative embodiment. Each module  103 , which consists of two fuel cell arrays  26 , two air end cap manifolds  40 , and a fuel manifold  58 , may be installed in the channel block  82  by inserting valve connector  64  into mating hole  80  and secured in place with screws  83 . Air flow to all the modules enters the channel block  82  through port  84  and comes out of the individual modules  103  through their air ports  41 . The shaded areas of this figure indicate positive and negative electrical conductive regions along the bottom and top of the modules  103 , respectively, as discussed in FIG.  8 . Electrical connections within each module  103  are made by compressing these conductive areas together with bolts  54  and nuts  50 , as shown. Electrical connection between the modules  103  and the channel block  82  are made by spring loaded prongs  84 , which are located at both the top and bottom of the modules or by some comparable method. Electrical access to the system level assembly is made through wires  52  and  53  which connect to the assembly with hollow bolts  49  and  51  and nuts  50 . The fuel and oxidizer connections are made through double sealing connections by means of the fuel valve connector  64 , discussed earlier, which seals both the fuel line  55  and fuel cell modules  103  when they are separated. These connections only open when they are secured together. As before, fuel is delivered to the fuel cell stack by means of a fuel line  55  through a fuel valve  56 , from an external fuel tank  57 . The fuel enters through hollow bolt  51  with unused fuel exiting though the hollow bolt  49 . The unique quick disconnect fuel and electrical features, along with the electrical diodes (not shown) that are built into each fuel cell array  26 , allow the modules  103  to be removed and added to a running assembly of modules, by means of handle  63 , without disturbing the system. A critical component in this invention is a fuel cell that is formed on a plastic sheet, such as but not limited to the fuel cells described in U.S. Pat. Nos. 5,631,099, 4,673,624 and 5,759,712. The production technique enables the fuel cells to be produced in a reel-to-reel manufacturing method, similar to printing press processes, as shown in FIG.  11 . The production is envisioned as taking place in a vacuum system  85  in which the metal electrodes and catalysts are deposited onto a reeled porous plastic web  86 . In this manufacturing process, the porous plastic web  86  is fed from a source reel  87 , into the vacuum system  85 , over a number of position and tension reels  88 , and on to a take-up reel  89 . Sputter material sources  90  and heat sinks  91  are located at strategic positions along the porous plastic  86  path within the vacuum system  85 , where sputter masks  92  are used to deposit the appropriate patterns on the fuel cell arrays  26  (not shown). The electrolytes could also be deposited by means of a reeled vacuum deposition system or dip tank. The individual fuel cell devices can be cut off the rolls of fuel cells and assembled. The edge seals are expected to be heat sealed, with the cutting operation and heat seal operation envisioned as one and the same. A major advantage of this approach is that the production and disposal manufacturing processes do not present toxic waste by-products. 
     Both embodiments of this patent lend themselves to monitoring and diagnostic features which can help keep these energy sources safe during operation. The simplest of these devices is a diode included at the electrical output of each fuel cell array  26 . The diode prevents current from flowing backwards though a fuel cell in the event an internal short occurs. Fuses or current limiting electronics may also be used to prevent reverse current flow though an array. Monitoring devices may also be used to detect a significant fuel leak in a fuel cell module and shut down the module. To accomplish this, fuel valves  56  are electrically linked to the fuel cell outputs throughout the system. In the event of a fuel leak, the current output of the fuel cell module will tend to drop due to the mixing of fuel and oxidizer in the manifolds, significantly reducing the performance of the cell. If there is a large drop in current flowing in the solenoid of one of the valves, due to a fuel leak, the solenoid will drop out, closing the valve and shutting off the flow of fuel to that fuel cell. A simple light, visible on the outside of each module, may also provide a simple indicator of fuel cell operation. In a more sophisticated approach, electronic diagnostic circuits in the form of application specific integrated circuits may be added in the current conductors at the output of each fuel cell to monitor the cell current and voltage levels. These integrated circuits shut down a failing cell to prevent a catastrophic failure. In larger systems, the readout from these diagnostic circuits may be logged on a computer where the performance of each individual fuel cell, as well as the overall system performance, may be monitored and controlled at all times. Any number of more complex diagnostics concepts may be used, including, but not limited to, high frequency communications and/or non-contact radio communications through the bus output. Another feature that may be added is a “fail safe” device that is incorporated into the modules to have the current output of the fuel cells go through a solenoid valve that opens the reactant connections, such that the modules may only be turned on by “starting” them manually with an external electrical input, much like the pilot light on stoves and furnaces. On the other hand, they close automatically if the electrical output of a cell dropped below a threshold value. Along with diagnostics and safety features, additional features may include such items as the cleaning of the fuel cells by independently electrically pulsing the fuel cells. A more sophisticated valve arrangement may be added so that gas connections other than the reactant are selected to purge the cell and/or “bleed” the fuel lines. 
     While this invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which fall is defined in the following claims.