Abstract:
An interconnect element for electrically connecting an anode and a cathode in adjacent fuel cells in a fuel cell stack, wherein said interconnect element has at least one featured surface including dimples, bosses, and/or pins arranged in a two-dimensional pattern. Preferably, both surfaces are featured, as by mechanical dimpling, embossing, or chemical etching, so that protrusions of the interconnect surface extend into either or both of the adjacent gas flow spaces to make electrical contact with the surfaces of the anode and cathode. This permits conduction of heat from the anode. The protrusions create turbulence in gas flowing through the flow spaces, which increases hydrogen consumption at the anode and hence electric output of the cell.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to fuel cells; more particularly, to stacks comprising a plurality of individual cells being both physically separated and electrically connected by interconnect elements; and most particularly, to such a fuel cell stack wherein the interconnect elements are featured on their surfaces in a predetermined pattern to form direct electrical connections with the adjacent anode and/or cathode surfaces.  
         BACKGROUND OF THE INVENTION  
         [0002]    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 converted to CO 2  at the anode/electrolyte interface. Reformed gasoline is a commonly used fuel in automotive fuel cell applications.  
           [0003]    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, the interconnect elements typically being unfeatured flat plates formed of a superalloy or stainless steel. 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 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.  
           [0004]    For electrochemical reasons well known to those skilled in the art, an SOFC requires an elevated operating temperature, typically 750° C. or greater.  
           [0005]    For steric reasons, fuel cells are preferably 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. The flat interconnect forms the opposite wall of the passageway for gas flow past either electrode.  
           [0006]    One problem encountered in prior art cells fueled by reformate is that hydrogen utilization is relatively low. The flow space for reformate between each anode and its associated interconnect promotes laminar flow of gas across the anode surface. As hydrogen is depleted from the gas stream at the anode surface, it is not readily replaced. Increasing the flow rate through the flow space can decrease laminarity and increase turbulence but at an increase in throughput of unreacted hydrogen. What is needed is a means for increasing turbulence in the flowing reformate without increasing flow rate.  
           [0007]    Another problem encountered in prior art fuel cells is that localized high temperatures occur in regions of the anode supporting the highest rates of hydrogen/oxygen reaction. Such high temperatures, especially when unevenly distributed over the anode, can be damaging to the anode. Excess heat is abstracted from the fuel cell by the cooling effect of air passing between the interconnects and the cathodes, but this cooling is generalized over the entire interconnect surface and further depends upon radiative emission of heat from the anode into the interconnect through the hydrogen flow space. What is needed is a means for removing heat directly from the anode in local regions of high heat generation.  
           [0008]    Another problem encountered in prior art cells fueled by reformate is that, through non-uniform flow of reformate across the anode surface, local regions of low hydrogen concentration can occur. In these regions, oxygen ions electrically migrating through the electrolyte to the anode are not all consumed in reaction with hydrogen and carbon monoxide. Nickel in the anode can be oxidized by a surfeit of oxygen ions, leading to failure of the fuel cell. What is needed is a means for electrically restricting the flow of oxygen ions through the electrolyte, in any regions desired, to optimize the consumption of hydrogen and carbon monoxide without oxidizing nickel in the anode.  
           [0009]    It is an object of the present invention to reduce the formation of localized superheated regions in the anode.  
           [0010]    It is a further object of the invention to reduce the number of components in a fuel cell stack by eliminating the need for a filamentous pad between the electrodes and the interconnects.  
         SUMMARY OF THE INVENTION  
         [0011]    Briefly described, a fuel cell stack in accordance with the present invention has first and second flow spaces for the respective passage of reformate gas across the surface of the anodes and oxygen gas along the surface of the cathodes. Adjacent fuel cells are separated by a conductive interconnect element which connects the cells electrically and also forms a wall between a first flow space in one of the fuel cells and a second flow space in the adjacent fuel cell. In the prior art, the interconnects are planar and smooth-surfaced. In accordance with the present invention, the interconnects have one or both surfaces featured, as by mechanical dimpling or embossing, or by chemical etching, so that protrusions of the interconnect surface extend into either or both of the adjacent gas flow spaces. The protrusions increase the surface area of the interconnect, improving removal of heat from the anode side to the cathode side. Preferably, at least a portion of the protrusions are sufficiently high to make mechanical, and therefore electrical, contact with the surface of the adjacent anode and/or cathode. This permits the abstraction of heat from the anode by conduction in addition to radiation, and also obviates the need for filamentous pads in the flow spaces of the stack for electrical connection of the electrodes to the interconnects. Further, the pattern of protrusions electrically contacting the anode may be non-uniform and may be optimized to meter the flow of oxygen ions through the electrolyte to the anode in any regions desired, to optimize the consumption of hydrogen and carbon monoxide without oxidizing nickel in the anode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    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:  
         [0013]    [0013]FIG. 1 is a schematic cross-sectional view of a prior art two-cell stack of solid oxide fuel cells;  
         [0014]    [0014]FIG. 2 is an exploded isometric view of a single solid oxide fuel cell, showing the various elements;  
         [0015]    [0015]FIG. 3 is an isometric view of a fuel cell stack comprising five cells like the cell shown in FIG. 2;  
         [0016]    [0016]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;  
         [0017]    [0017]FIG. 5 is a schematic cross-sectional view showing a non-planar dimpled and bossed interconnect disposed between the anode and the cathode of adjacent fuel cells in a stack;  
         [0018]    [0018]FIG. 6 is a schematic cross-sectional view like that shown in FIG. 5, showing a non-planar etched interconnect disposed between the anode and the cathode of adjacent fuel cells in a stack;  
         [0019]    [0019]FIG. 7 is a plan view of a non-planar dimpled and bossed interconnect for a fuel cell stack, showing a regular two-dimensional arrangement of dimples and bosses;  
         [0020]    [0020]FIG. 8 is a cross-sectional view taken along line  8 - 8  in FIG. 7;  
         [0021]    [0021]FIG. 9 is a cross-sectional view taken along line  9 - 9  in FIG. 7; and  
         [0022]    [0022]FIG. 10 is an enlarged view of a portion of the view shown in FIG. 8, taken at circle  10 .  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]    Referring to FIG. 1, a prior art 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.  
         [0024]    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 anode  16  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.  
         [0025]    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. Four separate planar, non-featured interconnect elements are distributed through the stack, one each between adjacent 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 .  
         [0026]    Preferably, the interconnect elements 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. 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.  
         [0027]    Referring to FIGS. 1 and 2, 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.  
         [0028]    Referring to FIGS. 5 and 7- 10 , a portion  46  of fuel cell stack  10  includes cathode  18 , electrolyte  14 , anode  16 , cathode spacers  38 , and anode spacers  36  from each of fuel cells A and B. The fuel cells are joined by a first novel interconnect  48   a  in accordance with the invention. Interconnect  48   a  is non-planar and is featured to include dimples  50  and bosses  52  extending in opposite directions from a median plane of the interconnect and formed as by stamping or embossing a blank of planar sheet stock into a two-dimensional pattern  54  of dimples and bosses, as shown in FIGS.  7 - 10 . When installed into a stack, as shown in FIG. 5, featured interconnect  48   a  extends into both anode passageway  24  and cathode passageway  26 . The extension of the dimples into anode passageway  24  creates a tortuous pathway for reformate flowing through the passageway which increases turbulence and serves to provide fresh reformate having a high initial concentration of hydrogen continuously to the anode surface, thereby increasing overall consumption of hydrogen and electrical output of the stack.  
         [0029]    The dimples and bosses of interconnect  48   a  also serve to increase the interconnect surface area available for heat exchange between Cell B&#39;s anode  16  and cooling oxygen flowing through cathode passageway  26  of Cell A, thereby reducing the heat load on the anode.  
         [0030]    Preferably, dimples  50  depart from the median plane  51  of interconnect  48  sufficiently to make contact with the surface  20  of the anode, and bosses  52  depart from median plane  51  sufficiently to make contact with the surface  22  of the cathode, thereby completing the electrical circuit between Cells A and B, as shown in FIG. 5, obviating the need for a prior art filamentous foam  30  in passageways  24  and  26 , as shown in the prior art stack  10  (FIG. 1). Contact of dimples and bosses with the anode and the cathode also provides a conductive path for heat transfer from the anode to the cathode, greatly increasing the cooling effect of air flowing through the cathode passageway.  
         [0031]    It will be appreciated that whereas the features of interconnect embodiment  48   a  comprise dimples formed in inverse measure to bosses by deformation of a sheet, an interconnect embodiment within the scope of the invention may be featured with equivalent bosses formed on both sides as by molding or casting of the interconnect.  
         [0032]    It will be further appreciated that whereas interconnect embodiment  48   a  has a regular two-dimensional pattern of bosses and dimples over its entire surface, other feature patterns may be selected as desired, within the scope of the invention. For example, fewer dimples and/or bosses may be provided for contact in regions of the anode having inherently low hydrogen concentrations and correspondingly higher oxygen ion concentrations. By locally restricting the current flow through the anode and electrolyte in these regions, such an interconnect can prevent accumulation of a surfeit of oxygen ions and thereby prevent unwanted and deleterious oxidation of nickel in the anode.  
         [0033]    Referring to FIG. 6, another featured embodiment  48   b  of a non-planar interconnect may be formed from a planar blank as by machining or chemical etching to provide pins  56  extending toward and contacting both anode  16  and cathode  18 . Pins  56  are analogous to bosses  52  in embodiment  48   a . Embodiment  48   b  may also be formed in a two-dimensional pattern having regular spacing, like pattern  54 , or pins  56  may be variably spaced, as shown in FIG. 6., to match the concentration profile of hydrogen gas in reformate as it flows through the cell from left to right.  
         [0034]    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.