Abstract:
A fuel cell having an optimized flow space for the passage of hydrogen gas across the surface of an anode. The invention prevents destructive oxidation of the anode by preventing the buildup of locally high levels of oxygen. The anode surface itself may be shaped in lateral plan to follow the natural contours of gas flow to eliminate hydrogen stagnation areas on the anode surface. Alternatively, the anode surface or the cathode surface may be coated in regions of anode stagnation to prevent the fuel cell reactions from occurring in those regions. Alternatively, the gas seals may be formed to cover the anode surface in stagnation regions. Alternatively, the cathode and/or electrolyte may be shaped or thickened to reduce or prevent diffusion of oxygen ions therethrough.

Description:
TECHNICAL FIELD 
     The present invention relates to fuel cells; more particularly, to such fuel cells having a solid oxide electrolyte; and most particularly, to such a fuel cell wherein the permeation of oxygen ion to regions of the anode having localized low hydrogen concentration is controlled to prevent localized areas of high oxygen ion concentration which can cause corrosion and failure of the anode. 
     BACKGROUND OF THE INVENTION 
     Fuel cells which generate electric current by controllably combining elemental hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by a permeable electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Either pure hydrogen or reformate is flowed along the outer surface of the anode and diffuses into the anode. Oxygen, typically from air, is flowed along the outer surface of the cathode and diffuses into the cathode. Each O 2  molecule is split and reduced to two O −2  ions at the cathode/electrolyte interface. The oxygen ions diffuse through the electrolyte and combine at the anode/electrolyte interface with four hydrogen ions to form two molecules of water. The anode and the cathode are connected externally through the load to complete the circuit whereby four electrons are transferred from the anode to the cathode. When hydrogen is derived from “reformed” hydrocarbons, the “reformate” gas includes CO which is also converted to CO 2  at the anode/electrolyte interface. 
     A single cell is capable of generating a relatively small voltage and wattage, typically about 0.7 volts and less than about 2 watts per cm 2  of active area. Therefore, in practice it is usual to stack together in electrical series a plurality of cells. Because each anode and cathode must have a free space for passage of gas over its surface, the cells are separated by perimeter spacers which are vented to permit flow of gas to the anodes and cathodes as desired but which form seals on their axial surfaces to prevent gas leakage from the sides of the stack. Adjacent cells are connected electrically by “interconnect” elements in the stack, and the outer surfaces of the anodes and cathodes are electrically connected to their respective interconnects by electrical contacts disposed within the gas-flow space, typically by a metallic foam or a metallic mesh which is readily gas-permeable or by conductive filaments. The outermost, or end, interconnects of the stack define electrical terminals, or “current collectors,” connected across a load. 
     For electrochemical reasons well known to those skilled in the art, an SOFC requires an elevated operating temperature, typically 750° C. or greater. 
     For steric reasons, fuel cells may be rectangular in plan view. Typically, gas flows into and out of the cells through a vertical manifold formed by aligned perforations near the edges of the components, the hydrogen flowing from its inlet manifold to its outlet manifold across the anodes in a first direction, and the oxygen flowing from its inlet manifold to its outlet manifold across the cathodes in a second direction. Thus, fuel cells are typically square in horizontal plan, and the anodes and cathodes have square corners. 
     A serious problem can arise in operation of a fuel cell formed as just described. The anode typically includes a relatively active metal such as nickel (Ni). In a cell having a square hydrogen flow path, the corners and sides of the square may be stagnant areas in which hydrogen is not readily replenished, allowing the partial pressure of O −2  to build up in the anode and at the anode/electrolyte interface. O −2  which is not scavenged immediately by hydrogen or CO can attack and oxidize nickel in the anode. The mismatch in thermal expansion coefficient between Ni and NiO causes volume changes which can lead to stress and eventual cracking and failure of the cell. 
     What is needed is a means for preventing the formation of local areas of high oxygen ion concentration at the anode to protect the anode from corrosive attack. 
     It is a principal object of the present invention to prevent formation of a locally corrosive concentration of O −2  at the anode of a solid oxide fuel cell. 
     It is a further object of the invention to increase the uniformity of gas distribution over the surface of an anode in a solid oxide fuel cell. 
     SUMMARY OF THE INVENTION 
     Briefly described, a fuel cell in accordance with the invention has a flow space for the passage of hydrogen gas across the surface of an anode. In the prior art, hydrogen gas may eddy and stagnate in corners or along edges of the flow space, resulting in locally low levels of hydrogen and correspondingly permitting locally high levels of oxygen ion in the anode, which can cause undesirable destructive oxidation of the anode. The invention prevents such destructive oxidation by preventing the buildup of such locally high levels of oxygen ion. 
     In a first embodiment of the invention, the anode surface itself is shaped in lateral plan to follow the natural contours of gas flow through the space and to eliminate corners or other areas on the anode surface on which gas may eddy and stagnate. Thus, no combustion reaction is possible in these regions of the anode, and oxygen ion therefore is not drawn to these regions. 
     In a second embodiment, the sidewall of the flow space is shaped by configuring the aperture in the spacer which defines the sidewall of the flow space in such a way that the spacer occludes the otherwise stagnant areas of the rectangular anode, preventing hydrogen from reaching the anode surface in these regions. 
     In a third embodiment, the anode surface on which gas may eddy and stagnate is dielectric coated in the regions of eddying and stagnation to prevent the migration of hydrogen into the anode. 
     In a fourth embodiment, the cathode surface corresponding to the anode regions of eddying and stagnation is eliminated to prevent the migration of oxygen ion to the anode in those regions. 
     In a fifth embodiment, the cathode surface corresponding to the anode regions of eddying and stagnation is much thicker than in cathode regions corresponding to the laminar flow regions of the anode to reduce the migration of oxygen ion to the anode in the stagnation regions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention will be more fully understood and appreciated from the following description of certain exemplary embodiments of the invention taken together with the accompanying drawings, in which: 
     FIG. 1 is a schematic cross-sectional view of a two-cell stack of solid oxide fuel cells in accordance with the invention; 
     FIG. 2 is an exploded isometric view of a single solid oxide fuel cell, showing the various elements; 
     FIG. 3 is an isometric view of a fuel-cell stack comprising five cells like the cell shown in FIG. 2; 
     FIG. 4 is an isometric view like that shown in FIG. 3, partially exploded, showing the addition of current collectors, end plates, and bolts to form a complete fuel cell stack ready for use; 
     FIG. 5 is a schematic plan view of a prior art anode surface in a solid oxide fuel cell, showing flow eddying and stagnation along the flow boundaries and at the corners of the anode surface; and 
     FIG. 6 is a view like that shown in FIG. 5, showing an anode surface in accordance with the invention in which high oxygen ion concentrations in the anode are prevented. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a fuel cell stack  10  includes elements normal in the art to solid oxide fuel cell stacks comprising more than one fuel cell. The example shown includes two fuel cells A and B, connected in series, and is of a class of such fuel cells said to be “anode-supported” in that the anode is a structural element having the electrolyte and cathode deposited upon it. Element thicknesses as shown are not to scale. 
     Each fuel cell includes an electrolyte element  14  separating an anodic element  16  and a cathodic element  18 . Each anode and cathode is in direct chemical contact with its respective surface of the electrolyte, and each anode and cathode has a respective free surface  20 , 22  forming one wall of a respective passageway  24 , 26  for flow of gas across the surface. Anode  16  of fuel cell B faces and is electrically connected to an interconnect  28  by filaments  30  extending across but not blocking passageway  24 . Similarly, cathode  18  of fuel cell A faces and is electrically connected to interconnect  28  by filaments  30  extending across but not blocking passageway  26 . Similarly, cathode  18  of fuel cell B faces and is electrically connected to a cathodic current collector  32  by filaments  30  extending across but not blocking passageway  26 , and anode  16  of fuel cell A faces and is electrically connected to an anodic current collector  34  by filaments  30  extending across but not blocking passageway  24 . Current collectors  32 , 34  may be connected across a load  35  in order that the fuel cell stack  10  performs electrical work. Passageways  24  are formed by anode spacers  36  between the perimeter of anode  16  and either interconnect  28  or anodic current collector  34 . Passageways  26  are formed by cathode spacers  38  between the perimeter of electrolyte  14  and either interconnect  28  or cathodic current collector  32 . Spacers  36 , 38  also serve to seal the perimeter of the stack against gas leakage and may be augmented by seals  37  (FIG. 2) specifically formulated for sealing against the surface of electrolyte  14 ; for example, compressed phlogopite mica can form an excellent gas seal. 
     Referring to FIGS. 2 through 4, a plurality of individual fuel cells  11  may be stacked together to form a stack  12  (FIGS. 3 and 4) similar to schematic stack  10  shown in FIG.  1 . Stack  12  comprises five such cells. To form a complete working fuel cell assembly  13  (FIG.  4 ), stack  12  is sandwiched between an anodic current collector  34  and a cathodic current collector  32  which in turn are sandwiched between a top plate  15  and a gas-manifold base  17 , the entire assembly being sealingly bound together by bolts  19  extending through bores in top plate  15  and threadedly received in bores in base  17 . 
     Preferably, the interconnect and the current collectors are formed of an alloy which is chemically and dimensionally stable at the elevated temperatures necessary for fuel cell operation, generally about 750° C. or higher, for example, Hastalloy, Haines 230, or a stainless steel. The electrolyte is formed of a ceramic oxide and preferably includes zirconia stabilized with yttrium oxide (yttria), known in the art as YSZ. The cathode is formed of, for example, porous lanthanum strontium manganate or lanthanum strontium iron, and the anode is formed, for example, of a mixture of nickel and YSZ. 
     Referring to FIGS. 1,  2 , and  5 , in operation, hydrogen or reformate gas  21  is provided via supply conduits  23  to passageways  24  at a first edge  25  of the anode free surface  20 , flows parallel to the surface of the anode across the anode in a first direction, and is removed via exhaust conduits  27  at a second and opposite edge  29  of anode surface  20 . Hydrogen (and CO if the fuel gas is reformate) also diffuses into the anode to the interface with the electrolyte. Oxygen  31 , typically in air, is provided via supply conduits  33  to passageways  26  at a first edge  39  of the cathode free surface  22 , flows parallel to the surface of the cathode in a second direction orthogonal to the first direction of the hydrogen, and is removed via exhaust conduits  41  at a second and opposite edge  43  of cathode surface  22 . Molecular oxygen gas (O 2 ) diffuses into the cathode and is catalytically reduced to two O −2  ions by accepting four electrons from the cathode and the cathodic current collector  32  (cell B) or the interconnect  28  (cell A) via filaments  30 . The electrolyte is permeable to the O −2  ions which pass through the electrolyte and combine with four hydrogen atoms to form two water molecules, giving up four electrons to the anode and the anodic current collector  34  (cell A) or the interconnect  28  (cell B) via filaments  30 . Thus cells A and B are connected in series electrically between the two current collectors, and the total voltage and wattage between the current collectors is the sum of the voltage and wattage of the individual cells in a fuel cell stack. 
     FIG. 5 illustrates schematically a practical problem that is well known in the construction and operation of rectangular fuel cell stacks such as stack  12 . The cathode side of a fuel cell typically is flooded with an excess of oxygen in the form of air. However, on the anode side reformate fuel gas is metered across the electrode surface at a relatively low rate of flow, ideally but not practically at a flow rate sufficiently low that all the fuel is consumed by the cell and none is passed through. At such low flow rates, as shown in FIG. 5, eddying and stagnation  40  of the fuel gas flow  42  can occur along the sides and at the corners of the rectangular anode surface  20 . This is especially undesirable because in these areas there is relatively low partial pressure of hydrogen, as hydrogen is consumed in the anode faster than it can be replaced. The result is that an undesirably high partial pressure of O −2  can arise in the anode in these areas, resulting in oxidation of anode nickel which can lead to structural failure of the cell. Such oxidation does not occur in anode regions having a hydrogen supply sufficient to scavenge O −2  ions as they emerge from the electrolyte. 
     Known approaches to remedying this problem involve either using pressure gradients to cause the flow to be more uniform and/or providing aerodynamically improved entry and exit manifolding to expand and contract the flow smoothly. The former approach is undesirable because it results in reduced system efficiency due to pressure increase in the fuel flow, and the latter approach is undesirable because it requires very substantial increase in the size and shape of the stack to accommodate the smoothing manifolds. 
     The problem may be remedied in accordance with the present invention, as shown in FIG. 6, by any of a number of physical and/or chemical configurations as described below, all of which act to prevent the buildup of unacceptably high O −2  ion concentrations in the areas  44  of the anode surface wherein hydrogen eddying and stagnation may or does occur, as shown in FIG.  5 . 
     In a first embodiment, anode  16  may be shaped physically in plan view by known techniques during manufacture of the cell to match the substantially laminar portion of the fuel gas flow. Areas  44  represent regions where no anode material exists; thus, no nickel corrosion can occur. 
     In a second embodiment, a square anode  16  may be covered by a dielectric sealing material, for example, YSZ, deposited by known techniques in areas  44 , thus making the anode non-conductive in those areas as well as inhibiting the permeation of H 2  and O −2 . 
     In a third embodiment, the central aperture in seal  37  may be formed in the shape of the anode surface in FIG. 6 such that the electrolyte is sealed on either the anode side or the cathode side against permeation by O −2  over a region identical to area  44  in the anode; in other words, a seal “mask.” Seal  37  on either or both sides of electrolyte  14  may be thus modified. 
     In a fourth embodiment, either or both of electrolyte  14  and cathode  18  may be formed to match the shape of the laminar flow anode surface in FIG. 6, the actual anode surface being rectangular; thus permeation of O −2  ions into areas  44  of the anode is not possible. 
     In a fifth embodiment, the cathode may be formed with significantly increased thickness in areas corresponding to areas  44  in the anode, thus increasing the length of the diffusion path and decreasing the rate of permeability of the cathode to oxygen ions in those areas. 
     Techniques for forming the anode, electrolyte, cathode, spacers, and seals in the configurations just recited are well within the skill of one skilled in the art of fuel cell manufacture; therefore, such techniques need not be recited here. 
     While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.