Abstract:
An improved fuel cell is described. The invention addresses the problem of mechanical failure in thin electrolytes. One embodiment varies the thickness of the electrolyte and positions at least either the anode or cathode in the recessed region to provide a short travel distance for ions traveling from the anode to the cathode or from the cathode to the anode. A second embodiment uses a uniquely shaped manifold cover to allow close positioning of the anode to the cathode. Using the described structures results in a substantial improvement in fuel cell reliability and performance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 10/722,156, filed Nov. 24, 2003 now U.S. Pat. No. 7,459,225, by the same inventors, and claims priority therefrom. This divisional application is being filed in response to a restriction requirement in that prior application. The contents of the original U.S. patent application Ser. No. 10/722,156 are hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Fuel cells are electrochemical systems that directly produce electrical energy from two chemical compounds, typically a fuel and an oxidizer.  FIG. 1  shows a typical fuel cell system in which an electrolyte  104  separates a fuel and an oxidizer. Electrolyte  104  serves as a proton exchange membrane (PEM), which is a hydrogen ion conductor but an electronic insulator. Hydrogen atoms are catalyzed from Hydrogen gas or a hydrocarbon source at the anode and disassociate from their electrons. The electrons flow through an electrode assembly not shown, through an external load  116 , and back to the cathode  112 . The Hydrogen ions are conducted through the PEM  104  and combine at the cathode  112  with electrons and oxygen to form water or steam, a waste product. The electrical current passing from cathode  112  to anode  120  through the external load  116  provides useful electrical energyA more detailed description of fuel cell operation is provided in a  Fuel Cell Handbook , by Appleby, A. J. and Foulkes, F. R., and published by Van Nostrand Reinhold Co, New York, 1989. 
     In the illustrated system, electrolyte  104  is ideally made as thin as possible. Thin electrolytes are desirable because thinner structures are better ionic conductors and offer reduced electrical resistance. Typically, Ionic conductance is inversely proportional to thickness while electrical resistance is approximately proportional to thickness. High electrical resistance across the electrolyte increases power losses. 
     However, making electrolyte  104  thin increases fabrication difficulties and increases the probability of electrolyte failure. First, a thin electrolyte may not be effective at separating fuel and oxidizer. Fuel that diffuses through the electrolyte along with its electrons decreases cell efficiency because the electrons do not pass through the external circuit to provide useful energy. This situation is called fuel crossover. Fuel crossover oxidizes at the cathode  112  and generates heat. This is one limitation of using thin Nafion-based membranes with methanol fuel. Second, many fuel cell membrane technologies use soft or brittle materials. Thin electrolytes made from such materials are often mechanically unstable. If the membrane leaks or ruptures and allows bulk mixing of fuel and oxidizer, the cell fails and the device may explode or burn as catalytic materials in the anode and cathode permit runaway exothermic reactions. Fuel cell designers must balance safety and fuel crossover (which suggest thicker electrolytes) and ion conduction efficiency (which suggests thinner electrolytes). In order to solve the problem of structurally weak electrolytes, U.S. Pat. No. 4,863,813 by Dyer et al. eliminates a separating electrolyte and combines the oxidizer and the fuel in a common region  204  as shown in  FIG. 2 . In order to prevent a runaway fuel-oxidizer reaction, catalysts that enhance the reaction are shielded from the reacting species. To shield the catalysts, the Dyer patent teaches including the catalyst in electrode compositions and using selectively permeable electrodes. Thus, for example, the anode may be permeable to fuel but not to oxidizer. Designs of such selectively permeable electrodes are further described in Taylor et al (U.S. Pat. No. 5,102,750) and Ellgen et al (U.S. Pat. No. 5,162,166). However, the fabrication of such selectively permeable electrodes is difficult and the resulting constraints on electrode design results in non-optimal performance. 
     Thus an improved system of forming an electrolyte structure that maintains separation of the fuel and oxidizer yet avoids the tradeoff between mechanical robustness of the electrolyte is needed. For electrolytes in which fuel crossover is not significant, a method of mechanically stiffening a thinner electrolyte would allow better ion conduction and efficiency. 
     SUMMARY 
     A fuel cell that incorporates an improved electrolyte mechanical design is described. In one embodiment, the improved electrolyte is patterned and includes at least one recessed region to form a manifold. An electrode is positioned in a first manifold formed by the first recessed region. The positions of an anode electrode and a cathode electrode are arranged such that the distance between two electrodes, an anode and a cathode, is less than the average thickness of the electrolyte. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a conventional prior art structure for a fuel cell 
         FIG. 2  shows an improved prior art structure for a fuel cell that combines the fuel and the oxidizer. 
         FIG. 3  shows one embodiment of a co-planar fuel cell with separate manifolds. 
         FIG. 4  shows one embodiment of a coplanar fuel cell with a corrugated electrolyte. 
         FIG. 5  shows one embodiment of a coplanar fuel cell with grooved electrolytes. 
         FIG. 6  shows the use of a recessed electrolyte that includes stiffening ribs. 
         FIG. 7  shows one embodiment of the recessed fuel cells in which fuel cells are stacked. 
     
    
    
     DETAILED DESCRIPTION 
     A fuel cell system that allows use of a thicker electrolyte while still enabling close placement of an anode to a cathode is described. The system maintains separation of fuel and oxidizer chambers, reduces fuel cell mechanical failure, simplifies fabrication, and maintains fuel cell performance. 
       FIG. 3  shows a cross section of a fuel cell built on an electrolyte  304 . In the embodiment shown, electrolyte  304  is a thick film that may also serve as a substrate. Alternately, electrolyte  304  may be formed on top of a support substrate (not shown). A dual manifold cover  328  over electrolyte  304  creates separate manifolds including a fuel manifold  332  and oxidizer manifold  336 . Within fuel manifold  332  is an anode  308  and within oxidizer manifold  336  is a cathode  312 . 
     In one embodiment, Electrolyte  304  is fabricated from a solid acid material. Solid acid materials are electronic insulators yet still have high ionic conductivity. These characteristics make solid acids particularly suitable for fuel cells, as described in an article by Haile et al entitled “Solid Acid Fuel Cell”,  Nature  410, 19 Apr., 2001, pp 910-913. However, solid acid materials form brittle ceramic materials, which are unsuitable for fabrication of unsupported very thin electrolyte structures. 
     Anode  308  and cathode  312  are formed from a conductive material, typically an inert conductor such as graphite, and positioned adjacent to electrolyte  304 . Either anode  308  or cathode  312  or both anode and cathode also typically include a catalyst, such as platinum and/or ruthenium, to facilitate the reaction between fuel and oxidizer. Catalysts act as a substrate for molecular interactions and facilitate splitting of fuels and oxidizers such as hydrogen and oxygen, H2 and O2 into H and O, generally only the monatomic species are reactive. 
     In  FIG. 3 , anode  308  and cathode  312  are formed in a common plane on a first surface  316  of electrolyte  304 , although planarity is not a requirement. A small separation distance  318  separates anode edge  320  from closest cathode edge  324 . Separation distance  318  is preferably less than 10 micrometers. The current path  344  will be focused in the gap regions near the cathode edge  324  and anode edge  320 . 
     A separating wall  340  in manifold  328  separates fuel chamber  332  and oxidizer chamber  336 . Fuel manifold  332  typically contains a fuel such as Hydrogen or a hydrocarbon and an anode  308 . An oxidizer manifold  336  typically contains a cathode  312  and an oxidizer, such as oxygen. The thickness of separating wall  340  forms a minimum limit on the distance  318  separating anode from cathode. Because close positioning of anode  308  to cathode  312  is desirable, separating wall  340  is preferably thin, often less than 10 micrometers in width. A number of techniques may be used to form a manifold with such dimensions, including photolithographic techniques. 
     Arrows  344  illustrate the flow of fuel ions, such as hydrogen ions. Hydrogen ions generally flow from anode  308 , through electrolyte  304  to cathode  312  where they react with oxidizer. After reaction, the fuel cell outputs water. The ion flow causes a potential difference that drives electrons from the anode, through an electrical circuit, (not shown) to the cathode. The electron flow powers the external electrical circuit. Arrow  344  shows hydrogen ions flowing into one side of an electrolyte  304  and exiting the same side of the electrolyte  304  near a cathode. The novel ion path eliminates the thickness of the electrolyte as a determining factor in the path length of the hydrogen ions. Thus, cathode to anode spacing distances may be substantially less than electrolyte thickness. 
       FIGS. 4-6  show fuel cell structures in which the electrolyte, for example a solid acid, is molded, machined or otherwise shaped to create a non-planar surface that includes recessed regions. A manifold which may be planar or molded, machined or otherwise shaped, seals the recessed regions forming fuel and oxidation chambers in which electrodes and anodes are positioned. 
       FIG. 4  shows a fuel cell  400  embodiment that utilizes a corrugated electrolyte  404 . Corrugated electrolyte  404  includes at least two recessed regions  402 ,  406  formed from the corrugated structure. Anode  408  and cathode  412  are positioned in corresponding recessed regions  402 ,  406 . Each recessed region is typically triangular in cross section and typically ranges from 1 to 1000 micrometers in width at its widest point. The recessed regions  402 ,  406  are arranged to form fuel and oxidizer manifolds. 
     Shaping or corrugating the electrolyte may be accomplished by a number of techniques, including but not limited to molding or machining the electrolyte. In the case of a ceramic electrolyte, it may be patterned in the green state and subsequently fired, or it may be patterned after firing. When very small recessed regions are desired, photolithographic etching processes may be used to form the recessed regions. Typical photolithographic techniques are described in Wolf, S. and Tauber, R. N., Silicon Processing for the VLSI Era, Lattice Press, Sunset Beach, Calif., 1986 which is hereby incorporated by reference. Other methods of forming a corrugated electrolyte are available to those of ordinary skill in the art. 
     Raised portion  444  of corrugated electrolyte  404  forms a separating wall between fuel chamber and oxidizer chamber. The fuel and oxidizer chambers are formed from recessed regions  402 ,  406 . Manifold cover  432  of Fuel Cell  400  couples to raised portion  444  and seals fuel chamber  436  and oxidizer chamber  440 , the sealed chambers are fuel cell manifolds. When the height of raised portion  444  forms a plane with the tops of other raised portions  452 ,  456 , manifold cover  432  may be a planar structure that seals Fuel Cell manifolds. 
     Arrows  446  show the migration of fuel ions from anode  408 , through raised portion  444  to cathode  412 . The fuel ions enter the electrolyte and exit on the same side  448  of electrolyte  404 . An angled corner  445  at the top of raised portion  444  separates the entrance and exit points of fuel ions. Distance  414  between a closest point on anode  408  and a corresponding closest point on cathode  412  is substantially smaller than an average thickness  424  of electrolyte  404 . Distance  414  is also smaller than the median thickness of electrolyte  404 . As used herein, a median thickness is defined to be a thickness value at which half of the electrode is thicker than the median thickness, and half of the electrode is thinner than the median thickness. 
       FIG. 5  shows a fuel cell  500  where grooves have been formed in the electrolyte  504 . The grooves form recessed regions  506 ,  508  which when sealed form manifolds. Each recessed manifold forms an approximate box cross section with corners  512 ,  516 . Techniques for forming an electrolyte with the corresponding recessed regions include molding electrolyte  504 , micro-machining the electrolyte, jet-printing the electrolyte or other ceramic patterning techniques known to those of skill in the art. 
     A dividing segment  552  of electrolyte  504  separates adjacent recessed regions  506 ,  508 . The aspect ratio of dividing segment  556  is kept low, preferably the height  560  of dividing segment  552  is kept to less than 20 times the width  564 . The low aspect ratio maintains the strength of separating segment  552 . 
     Anode  532  and cathode  536  are formed inside corresponding recessed regions  506  and  508 . Conforming anode  532  and cathode  536  to recessed regions  506 ,  508  contours results in an overlap of anode segment  548  with electrolyte dividing segment  552 . Likewise, cathode segment  556  overlaps electrolyte dividing segment  552 . Thus anode segment  548  and cathode segment  556  abut the two sides of electrolyte dividing segment  552 . The thinness of dividing segment  552 , typically less than 50 microns, allows positioning of anode segment  548  in close proximity to cathode segment  556  thereby facilitating ionic exchange between the two segments. 
     Typical fuel cells repeat the anode-cathode structure increasing the voltage and current that may be generated across the fuel cell. For example, the structure may be repeated in series such that a third anode  540  in sequential recessed region  544  is followed by a corresponding cathode (not shown). Fuel surrounds each anode and oxidizer surrounds each cathode. Ionized fuel atoms, such as cationic positively charged hydrogen ions, flow from anode, through electrolyte  504 , to cathodes where they react with oxidizer. Alternatively, ionized oxidizer, such as anionic negatively charged oxygen ions, may flow from cathode, through electrolyte  504 , to anodes where they react with fuel. The majority of ions travel the shortest path from anode to cathode. This shortest path is through dividing segment  552 . The resulting potential difference can be used to drive electrical circuits. 
     In order to contain the fuel and the oxidizer, manifold cover  549  couples to electrolyte  504  creating a seal around at least two chambers suitable for containing fuel or oxidizer. The manifold cover may also provide structural support for raised portions of the electrolyte including separating segment  552 . The manifold cover may be made from variety of materials such as plastic or ceramic that are compatible with fuel, electrolyte, and catalyst. Some electrolytes operate at elevated temperatures, as high as several thousand degrees Centigrade. A manifold cover that covers both anode and cathode is typically sufficiently electrically insulating to prevent electrical shorting of the anode to the cathode. 
       FIG. 6  shows a cross sectional view of a fuel cell  600  in which the electrolyte  604  has been patterned to include at least two recessed regions  608 ,  612 . Electrolyte patterning may be achieved by micro-machining the electrolyte, molding the electrolyte or by other techniques known to those of skill in the art. Thicker areas  616  of electrolyte  604  act as “ribs” that provide strength to thinner areas  620  of electrolyte  604 . Thinner areas are typically less than 50 microns thick and thicker areas are typically at least 100 microns thick. Although the aspect ratio of height to width of the recessed regions may vary considerably, typically the aspect ratio is kept below 0.4. The low aspect ratio helps insure the presence of sufficient thick regions of the electrolyte to maintain the strength of recessed thinner regions of electrolyte  604 . 
     Fuel manifold cover  624  and oxidizer manifold cover  628  couple to opposite sides of electrolyte  604 . The recessed regions of electrolyte  604  coupled with the manifold cover to form corresponding fuel manifold  632  and oxidizer manifold  636 . The manifold covers may also serve to stiffen and strengthen electrolyte  604 . An anode adjacent to electrolyte  604  in fuel chamber  632  and a cathode adjacent to an opposite side of electrolyte  604  in oxidizer chamber  636  provides electrical contact points. Fuel ions enter a first side  638  of electrolyte  604  and exit the electrolyte  604  on a second side  640 . After exiting electrolyte  604 , the fuel ions interact with a catalyst and oxidizer in oxidizer chamber  636 . Alternatively, oxidizer ions enter a first side  640  of electrolyte  604  and exit a second side  638 . 
     Fuel cells are sometimes stacked to more efficiently utilize a volume of space. (See Appleby,  Fuel Cell Handbook , op cit.)  FIG. 7  is a cross sectional side view of a fuel cell  700  that shows one method of stacking a plurality of fuel cells  704 ,  708 , similar or equivalent to the fuel cell structures of  FIG. 5  and  FIG. 6 . Cells containing anodes  716 ,  718  are positioned adjacent to cells containing cathodes  720 . In the particular embodiment shown, four adjacent cathodes  720  surround anode  716 , however other arrangements may be used to fit different electrical or mechanical needs. The anodes and cathodes may be made from materials known in the art, typically including a conductor and a catalyst. 
     Stacking and electrically coupling together the cell outputs enables an increase in the voltage or current output by the stack. Parallel electrical connections increase the current while serial electrical connections increase the voltage output by the stack. 
     Electrical connections between adjacent cells may be made a number of ways. In one embodiment, no electrical connections are made in the manifold itself. Instead, the electrical connections are made at the endplates (not shown). The endplates form the terminations of the fuel cell structure in the front and back, parallel to the cross section of  FIG. 7 . Alternate methods of electrically interconnecting cells may also be used. 
     In the example fuel cell  700 , manifold covers between cells have been eliminated. Instead adjacent cell walls serve to seal each cell. Manifold covers may still be utilized on the top and bottom or on the sides of the cell stack. Although individual cells may be fragile, stacking increases the mechanical strength of the overall structure. 
     In the preceding description, a number of details have been provided. Such details include ideal dimensions, electrolyte shapes, examples of typical electrolyte material, and typical fuels for use in a fuel cell. Such details are provided to facilitate understanding of the invention and provide examples. However, such details should not be interpreted to limit the scope of the claim. The limits of the invention should only be defined by the claims which follow.