Abstract:
A method of manufacturing an electrically conductive interconnect for a solid oxide fuel cell stack, including the steps of (a) making a metal substrate having a first surface configured for electrical contact with an anode of the solid oxide fuel cell stack and a second surface configured for electrical contact with a cathode of the solid oxide fuel cell stack; (b) depositing a layer comprising metallic cobalt over at least a portion of at least one of the first and second surfaces; and (c) subjecting the metallic cobalt to reducing conditions, thereby causing at least a portion of the metallic cobalt to diffuse into the metal substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of co-pending U.S. patent application Ser. No. 11/499,583 filed on Aug. 4, 2006, which is hereby incorporated by reference in its entirety. 
     
    
     GOVERNMENT-SPONSORED STATEMENT 
       [0002]    This invention was made with United States Government support under Government Contract/Purchase Order No. DE-FC26-02NT41246. The Government has certain rights in this invention. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention relates to fuel cells, more particularly to solid-oxide fuel cells, and most particularly to a solid oxide fuel cell stack that includes a cobalt-containing interconnect surface. 
       BACKGROUND OF THE INVENTION 
       [0004]    A fuel cell is an energy conversion device that generates electricity and heat by electrochemically combining a gaseous fuel, for example, hydrogen, carbon monoxide, or a hydrocarbon, with an oxidant such as air or oxygen, across an ion-conducting electrolyte. The fuel cell converts chemical energy into electrical energy, which may then be used by a high-efficiency electric motor, or stored. A solid oxide fuel cell (SOFC) is frequently constructed of solid-state materials, typically utilizing an ion conductive oxide ceramic as the electrolyte. A conventional electrochemical cell in a SOFC is comprised of an anode and a cathode with an electrolyte disposed therebetween. The oxidant passes over the oxygen electrode or cathode while the fuel passes over the fuel electrode or anode, generating electricity, water, and heat. 
         [0005]    In a typical SOFC, a fuel flows to the anode where it is oxidized by oxygen ions from the electrolyte, producing electrons that are released to the external circuit, and mostly water and carbon dioxide are removed in the fuel flow stream. At the cathode, the oxidant accepts electrons from the external circuit to form oxygen ions. The oxygen ions migrate across the electrolyte to the anode. The flow of electrons through the external circuit provides for consumable or storable electricity. However, each individual electrochemical cell generates a relatively small voltage. Higher voltages may be attained by electrically connecting a plurality of electrochemical cells in series to form a stack. 
         [0006]    U.S. Pat. No. 6,737,182, the disclosure of which is incorporated herein by reference, discloses a solid oxide fuel cell stack comprising an electrochemical cell that has an electrolyte disposed between and in ionic communication with a first and second electrode, and an interconnect that is in fluid and thermal communication with at least a portion of the electrochemical cell, the interconnect being configured to receive electrical energy and thereby act as a heating element. 
         [0007]    U.S. Patent Application Publication No. 2005/0153190, the disclosure of which is incorporated herein by reference, discloses a solid oxide fuel cell stack that comprises flexible thin foil interconnect elements and thin spacer elements that can conform to nonplanarities in the stack&#39;s electrolyte elements, thereby avoiding the inducing of torsional stresses in the electrolyte elements. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a method of manufacturing an electrically conductive interconnect for a solid oxide fuel cell stack. The method of manufacturing includes the steps of (a) making a metal substrate having a first surface configured for electrical contact with an anode of the solid oxide fuel cell stack and a second surface configured for electrical contact with a cathode of the solid oxide fuel cell stack; (b) depositing a layer comprising metallic cobalt over at least a portion of at least one of the first and second surfaces; and (c) subjecting the metallic cobalt to reducing conditions, thereby causing at least a portion of the metallic cobalt to diffuse into the metal substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
           [0010]      FIG. 1  is a schematic cross-sectional view of a two-cell stack of solid oxide fuel cells in accordance with the present invention. 
           [0011]      FIG. 2  is a graph containing a series of power vs. time curves that demonstrate the advantage of coating a chromium alloy interconnect with a cobalt-containing layer in accordance with the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    Solid oxide fuel cell stacks typically include interconnects fabricated from metallic materials, which are commonly chromium-containing metal alloys. Fuel cell cathodes are typically formed from mixed oxides such as perovskites ABO 3 , where A represents a metal such as lanthanum, cerium, calcium, sodium, strontium, lead, praseodymium, rare earth metals and mixtures thereof, and B represents titanium, niobium, iron, cobalt, manganese, nickel and mixtures thereof. 
         [0013]    Under typical high temperature operating conditions, e.g., about 750° C., the chromium included in the alloy volatilizes and reacts with oxygen and moisture from the air to generate chromium oxide and other related species, as shown below: 
         [0000]      2Cr+1.5O 2 →Cr 2 O 3  
 
         [0000]      Cr 2 O 3  +O 2 (g)+H 2 O(g)→2CrO 2 (OH) 2 (g)
 
         [0000]    Cr 2 O 3  and CrO 2 (OH) 2  in the gas phase undergo reaction with the cathode and degrade its performance and durability. This adverse effect is prevented or mitigated by the present invention. 
         [0014]    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. 
         [0015]    Each fuel cell includes a solid electrolyte  14  separating an anode  16  and a cathode  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 , and 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 . 
         [0016]    Current collectors  32 ,  34  may be connected across a load  35  to enable the fuel cell stack  10  to perform 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 . 
         [0017]    Interconnect  28  disposed between anode  16  and cathode  18  comprises a first surface  28   a  in electrical contact with anode  16  and a second surface  28   b  in electrical contact with cathode  18 . Interconnect  28  is formed from a metal or metal alloy that typically includes chromium, for example, an iron-chromium alloy. 
         [0018]    In the operation of fuel cell stack  10 , reformate gas  21  is provided to passageways  24  at a first edge  25  of the anode free surface  20 , flows parallel to the surface  20  of anode  16  across the anode in a first direction, and is removed at a second and opposite edge  29  of anode surface  20 . Hydrogen and CO diffuse into anode  16  to the interface with electrolyte  14 . Oxygen  31 , typically in air, is provided to passageways  26  at a first edge  39  of the cathode free surface  22 , flows parallel to the surface of cathode  18  in a second direction (omitted for clarity in  FIG. 1 ) that is orthogonal to the first direction of the reformate flow, and is removed at a second and opposite edge  43  of cathode surface  22 . Molecular oxygen gas diffuses into cathode  18  and is catalytically reduced to two oxygen ions by accepting four electrons from cathode  18  and cathodic current collector  32  of cell B or interconnect  28  of cell A via filaments  30 . Electrolyte  14  is permeable to the oxygen ions that pass by electric field through the electrolyte and combine with four hydrogen atoms to form two water molecules, giving up four electrons to anode  16  and anodic current collector  34  of cell A or interconnect  28  of cell B via filaments  30 . Thus, cells A and B are connected in series electrically between the two current collectors  32  and  34 , and the total voltage and wattage between the current collectors is the sum of the voltage and wattage of the individual cells in fuel cell stack  10 . 
         [0019]    In accordance with the present invention, at least a portion of at least one of surfaces  28   a  and  28   b  of interconnect  28  comprises a layer of metallic cobalt, cobalt oxide, or a mixture thereof. A layer of metallic cobalt, which may be formed by, for example, electroplating, has a thickness preferably of about 0.5 micron to about 10 microns, more preferably, about 2.5 microns to about 5 microns. The metallic cobalt layer may be subjected to oxidizing conditions by, for example, heating in an oxygen-containing atmosphere to a temperature of about 800° C. for a period of about 15 minutes to about 8 hours, causing at least a portion of the metallic cobalt to be oxidized to cobalt oxide. The metallic cobalt can also be diffused into the surface of the chromium alloy substrate by heating to about 800° C. in a vacuum or in a non-oxidative atmosphere for a period of about 15 minutes to about 8 hours. This latter treatment produces a cobalt rich surface that, upon subsequent exposure to a controlled oxygen-containing atmosphere during the cooling phase of the cycle, can form a cobalt oxide layer. 
         [0020]      FIG. 2  is a graph containing a series of plots of specific power in mW/cm 2  vs. time in hours that demonstrate the beneficial effect of coating a chromium alloy sample, representative of a fuel cell interconnect, with a cobalt-containing layer in accordance with the present invention. 
         [0021]    Tests were carried out using a button cell having a 2.83 cm 2  active area and 5% A-site deficient LSCF6428 lanthanum-strontium-iron-cobaltite (La 0.6 Sr 0.4 ) 0.95 Co 0.2 Fe 0.8 O 3 ) cathode. A series of uncoated and coated Crofer 22 APU alloy discs, representing the interconnect alloy, were placed on top of a Ag current collecting mesh that is in contact with a fully covered Ag—Pd metallization layer of the cathode. Crofer discs were coated with Co-containing layers of 0.1 mil (2.5 microns) and 0.2 mil (5 microns). Before being placed on the cathode for testing, the electroplated Crofer discs were vacuum-treated and pre-oxidized at 800° C. for 4 hours to form a continuous Co oxide layer on the Crofer disc surface. 
         [0022]    The results of coated Crofer samples are compared with the cells containing no Cr source (curve  1  of  FIG.2 ) and uncoated Crofer discs (curves  2  and  3  of  FIG. 2 ). As shown by the test results, Cr poisoning of the cathode was significantly reduced for the Co-coated Crofer discs (curves  4  and  5  of  FIG.2 ) compared with the uncoated Crofer disc, with a fade rate of  0 . 01 ˜ 0 . 03  %/h vs.  0 . 16 ˜ 0 . 27  %/h at 100-200 hrs. Even though initial power densities of the Co-coated samples were slightly lower than that of the no-Cr sample, possibly due to initial Cr poisoning before testing, their fade rate were comparable to the baseline cathode performance of the no-Cr baseline source. 
         [0023]    As demonstrated by the foregoing results, the layer of metallic cobalt, cobalt oxide, or mixture thereof is highly is highly effective in preventing formation of chromium oxide and other related species, and its subsequent detrimental reaction with the cathode. In addition, the resulting surface has high electrical conductivity that is stable over extended time in the high temperature operating environment. Similar results have also been obtained by deposition of the Co layer using other processes such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). 
         [0024]    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 should be recognized that the invention is not limited to the described embodiments but has full scope defined by the language of the following claims.