Abstract:
In a fuel cell assembly, nickel-based anodes are readily oxidized when exposed to oxygen as may happen through atmospheric invasion of the assembly during cool-down following shutdown of the assembly. Repeated anode oxidation and reduction can be destructive of the anodes, leading to cracking and failure. To prevent such oxygen migration, oxygen getter devices containing oxygen-gettering material such as metallic nickel are provided in the fuel passageways leading to and from the anodes. Oxidation of the oxygen-gettering material is readily reversed through reduction by fuel when the assembly is restarted.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 10/178,826, now abandoned, which was filed on Jun. 24, 2002. 
    
    
     This invention was made with Government support under DE-FC26-02NT41246 awarded by DOE. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates to hydrogen/oxygen fuel cells having a solid-oxide electrolytic layer separating an anode layer from a cathode layer; more particularly, to fuel cell stack assemblies and systems comprising a nickel-based anode; and most particularly, to such fuel cell assemblies and systems wherein the anode is protected from oxidation, especially during cool-down after the assembly has been shut down. 
     BACKGROUND OF THE INVENTION 
     Fuel cells which generate electric current by the electrochemical combination of hydrogen and oxygen are well known. In one form of such a fuel cell, an anodic layer and a cathodic layer are separated by an electrolyte formed of a ceramic solid oxide. Such a fuel cell is known in the art as a “solid oxide fuel cell” (SOFC). Hydrogen, either pure or reformed from hydrocarbons, 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  anions catalytically by the cathode. The oxygen anions transport 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 a 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 via an oxidation process similar to that performed on the hydrogen. Reformed gasoline is a commonly used fuel in automotive fuel cell applications. 
     A single cell is capable of generating a relatively small voltage and wattage, typically between about 0.5 volt and about 1.0 volt, depending upon load, and less than about 2 watts per cm 2  of cell surface. 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. The perimeter spacers include dielectric layers to insulate the interconnects from each other. Adjacent cells are connected electrically by “interconnect” elements in the stack, the outer surfaces of the anodes and cathodes being 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 electric terminals, or “current collectors,” which may be connected across a load. 
     A complete SOFC system typically includes auxiliary subsystems for, among other requirements, generating fuel by reforming hydrocarbons; tempering the reformate fuel and air entering the stack; providing air to the hydrocarbon reformer; providing air to the cathodes for reaction with hydrogen in the fuel cell stack; providing air for cooling the fuel cell stack; providing combustion air to an afterburner for unspent fuel exiting the stack; and providing cooling air to the afterburner and the stack. A complete SOFC assembly also includes appropriate piping and valving, as well as a programmable electronic control unit (ECU) for managing the activities of the subsystems simultaneously. 
     The anodes of cells in a fuel cell assembly typically include metallic nickel and/or a nickel cermet (Ni-YSZ) which are readily oxidized. During operation of an assembly, the anodes are in a reduced state. A problem exists in that the anodes are vulnerable to oxidation by atmospheric oxygen which can enter the stacks via the reformate passageways during cool-down of the assembly. The grain growth and contraction of the metallic nickel in the anode during the oxidation/reduction cycle is not readily managed by the ceramic component. Repeated oxidation and reduction of nickel in the anodes can lead to unwanted structural changes and can result in catastrophic cracking of the anodes. 
     It is a principal object of the present invention to protect the nickel anodes of a fuel cell from structural degradation by periodic oxidation and reduction of the nickel. 
     It is a further object of the present invention, through such prevention, to improve the reliability and extend the lifetime of solid oxide fuel cells. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Briefly described, in a fuel cell assembly, for example, a solid-oxide fuel cell assembly, metallic nickel in a Ni-YSZ based anode is readily oxidized when exposed to oxygen as may happen through atmospheric invasion of the assembly during cool-down following shutdown of the assembly. Anodes are in an oxidized equilibrium state when the assembly is fabricated and are then reduced by reformate when the assembly is first turned on. Repeated anode oxidation and reduction can be destructive of the three-dimensional structure of the anodes and can lead to cracking and failure of the anodes and thus the entire assembly. To prevent such oxygen migration and re-oxidation, oxygen getter devices, themselves containing oxygen-scavenging material such as metallic nickel, are provided in the reformate passageways leading to and from the anodes. When oxidized, the oxygen-gettering material is readily reversed through reduction by reformate when the assembly is restarted. 
    
    
     
       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; 
         FIG. 2  is a schematic elevational view of two fuel cell stacks electrically connected in series; and 
         FIG. 3  is a schematic mechanization diagram of an SOFC assembly, showing the incorporation of oxygen getters in fuel passageways leading into and out of the anodes. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a fuel cell stack  10  includes elements known in the art of solid oxide fuel cell stacks comprising more than one fuel cell. The example shown includes two identical fuel cells  11 , 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  11  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 a first fuel cell  11  faces and is electrically connected to an interconnect  28  by filaments  30  extending across but not blocking passageway  24 . Similarly, cathode  18  of a second fuel cell  11  faces and is electrically connected to interconnect  28  by filaments  30  extending across but not blocking passageway  26 . Similarly, cathode  18  faces and is electrically connected to a cathodic current collector  32  by filaments  30  extending across but not blocking passageway  26 , and anode  16  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 . Anode spacer  36  and cathode spacer  38  are formed from sheet stock in such a way to yield the desired height of the anode passageways  24  and cathode passageways  26 . 
     Preferably, the interconnect and the current collectors are formed of an alloy, typically a “superalloy,” which is chemically and dimensionally stable at the elevated temperatures necessary for fuel cell operation, generally about 750° C. or higher, for example, Hastelloy, Haynes 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 of, for example, a mixture of nickel and YSZ. 
     In operation ( FIG. 1 ), reformate gas  21  is provided 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 at a second and opposite edge  29  of anode surface  20 . Hydrogen and CO diffuse into the anode to the interface with the electrolyte. 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 the cathode in a second direction which can be orthogonal to the first direction of the reformate (second direction shown in the same direction as the first for clarity in  FIG. 1 ), and is removed 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  or the interconnect  28  via filaments  30 . The electrolyte ionically conducts or transports O −2  anions to the anode electrolyte innerface where they combine with four hydrogen atoms to form two water molecules, giving up four electrons to the anode and the anodic current collector  34  or the interconnect  28  via filaments  30 . Thus cells  11  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. 
     Referring to  FIG. 2 , the cells  11  are arranged side-by-side rather than in overlapping arrangement as shown in  FIG. 1 . Further, the side-by-side arrangement may comprise a plurality of cells  11 , respectively, such that each of first stack  44  and second stack  46  shown in  FIG. 2  is a stack of identical fuel cells  11 . The cells  11  in stack  44  and stack  46  are connected electrically in series by interconnect  47 , and the stacks are connected in series. 
     Referring to  FIG. 3 , the diagram of a solid-oxide fuel cell assembly  12  includes auxiliary equipment and controls for stacks  44 , 46  electrically connected as in  FIG. 2 . 
     A conventional high speed inlet air pump  48  draws inlet air  50  through an air filter  52 , past a first MAF sensor  54 , through a sonic silencer  56 , and a cooling shroud  58  surrounding pump  48 . 
     Air output  60  from pump  48 , at a pressure sensed by pressure sensor  61 , is first split into branched conduits between a feed  62  and a feed  72 . Feed  62  goes as burner cooling air  64  to a stack afterburner or tail gas combustor  66  via a second MAF sensor  68  and a burner cool air control valve  70 . 
     Feed  72  is further split into branched conduits between an anode air feed  74  and a cathode air feed  75 . Anode feed  74  goes to a hydrocarbon fuel vaporizer  76  via a third MAF sensor  78  and reformer air control valve  80 . A portion of anode air feed  74  may be controllably diverted by control valve  82  through the cool side  83  of reformate pre-heat heat exchanger  84 , then recombined with the non-tempered portion such that feed  74  is tempered to a desired temperature on its way to vaporizer  76 . 
     Cathode air feed  75  is controlled by cathode air control valve  86  and may be controllably diverted through cathode bypass feed  87  by cathode air preheat bypass valve  88  through the cool side  90  of cathode air pre-heat heat exchanger  92  on its way to stacks  44 , 46 . After passing through the cathode sides of the cells in stacks  44 , 46 , the partially spent, heated air  93  is fed to afterburner  66 . 
     A hydrocarbon fuel feed pump  94  draws fuel from a storage tank  96  and delivers the fuel via a pressure regulator  98  and filter  100  to a fuel injector  102  which injects the fuel into vaporizer  76 . The injected fuel is combined with air feed  74 , vaporized, and fed to a reformer catalyst  104  in main fuel reformer  106  which reforms the fuel to, principally, hydrogen and carbon monoxide. Reformate  108  from catalyst  104  is fed to the anodes in stacks  44 , 46 . Unconsumed fuel  110  from the anodes is fed to afterburner  66  where it is combined with air supplies  64  and  93  and is burned. The hot burner gases  112  are passed through a cleanup catalyst  114  in main reformer  106 . The effluent  115  from catalyst  114  is passed through the hot sides  116 , 118  of heat exchangers  84 ,  92 , respectively, to heat the incoming cathode and anode air. The partially-cooled effluent  115  is fed to a manifold  120  surrounding stacks  44 , 46  from whence it is eventually exhausted  122 . 
     Still referring to  FIG. 3 , a first oxygen getter device  124  is provided in the conduit feeding fuel such as, for example, pure hydrogen or reformate  108  to the anodes (not visible) in stacks  44 , 46 . A second and substantially identical oxygen getter device  126  is similarly provided in the conduit feeding spent fuel  110  from the anodes to afterburner  66 . As described above, during cool-down of the fuel cell stacks after shut-down of the assembly, it is important to prevent migration of oxygen into anode passageways  24  wherein anode surface  20 , comprising metallic nickel in a ceramic matrix (nickel/YSZ cermet), would be subject to damaging oxidation. Each getter includes a passageway  128  having an inlet  130  and an outlet  132  through which fuel is passed during operation of the fuel cell assembly. Within the passageway is a readily-oxidized material  134  (oxygen-reducing means), for example, nickel metal foam, nickel wire or nickel mesh, which is capable of gettering oxygen by reaction therewith but which does not present a significant obstruction to flow of fuel through the passageway. Nickel in the getters reacts with oxygen to produce nickel oxide, NiO, when the assembly is shut down, thus protecting the nickel-containing anodes from oxidation. When the assembly is turned back on, reformate is again produced which, in passing through the getters, reduces the NiO back to metallic nickel, allowing the getters to be used repeatedly. 
     An SOFC assembly  1000  in accordance with the invention is especially useful as an auxiliary power unit (APU)  1000 - 1  for vehicles  136  on which the APU may be mounted as shown in  FIG. 3 , such as, for example, cars  136 - 1  and trucks  136 - 2 , boats and ships  136 - 3 , and airplanes  136 - 4 , wherein motive power is supplied by a conventional engine and the auxiliary electrical power needs are met by an SOFC assembly. 
     An SOFC assembly in accordance with the invention is also useful as a stationary power plant such as, for example, in a household or for commercial usage. 
     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.