Patent Publication Number: US-2022223880-A1

Title: Solid oxide fuel cells with cathode functional layers

Description:
RELATED APPLICATION 
     This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/310,358, filed Mar. 18, 2016, the entire disclosure of which is hereby incorporated herein by reference. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with United States Government support under Contract No. DEAR0000494 awarded by the Department of Energy Advanced Projects Research Agency-Energy (ARPA-E). The United States Government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     In various embodiments, the present invention relates to solid oxide fuel cells, and in particular to solid oxide fuel cells with cathode-side functional interlayers. 
     BACKGROUND 
     Solid oxide fuel cells (SOFCs) are highly efficient, environmentally friendly electrochemical devices that are capable of directly converting chemical energy stored in hydrogen or hydrocarbon fuels into electrical energy. During SOFC operation, oxygen ions migrate from a cathode to an anode through a dense electrolyte. At the anode, the oxygen ions oxidize the fuel, resulting in the generation of electrons that may be directed through an external circuit. SOFCs may be used for, for example, off-grid and portable power generation. 
     Although SOFCs have many advantages over other types of fuel cells, many conventional SOFCs operate at high temperatures (e.g., 800° C. or above), which can lead to degradation of the cell components, longer startup times, and high maintenance costs. While lower-temperature operation would address many of these issues, operation at lower temperatures also presents operational challenges. For example, internal losses within the electrolyte and cathode resistance are typically higher during operation at lower temperature. Thus, there is a need for SOFC structures that have improved performance during low-temperature operation. 
     SUMMARY 
     In accordance with various embodiments of the present invention, SOFCs incorporate thin functional layers between the electrolyte and the cathode in order to reduce the interfacial resistance between the electrolyte and the cathode. While not wishing to be bound by any particular theory of operation, the present inventors believe that functional layers in accordance with embodiments of the present invention enhance the bonding and/or adhesion at the interface between the cathode and the solid electrolyte, thereby expediting oxygen ion transport within the SOFC. Exemplary functional layers in accordance with embodiments of the invention include cobalt-doped gadolinium-doped ceria (Co-GDC, for example, Co-Ce 0.9 Gd 0.1 O 1.95 ) and cobalt-doped samarium doped ceria (Co-SDC, for example, Co-Ce 0.8 Sm 0.2 O 1.9 ). The functional layer may be disposed and in direct contact with the solid electrolyte and the cathode material, and the functional layer may be fabricating utilizing, for example, wet chemical or solid-state techniques. 
     Functional layers in accordance with embodiments of the present invention may also provide one or more additional advantages for SOFC operation. For example, various functional layers (e.g., Co-GDC) have high ionic transference numbers (i.e., fractions of their total current carrying capability carried via ionic transport), and thus may be utilized to retard, block, or substantially minimize electronic conduction in the SOFC when utilized with mixed-conducting electrolytes (i.e., electrolytes that are both ionic and electronic conductors) such as GDC. This reduction in electronic conduction may advantageously improve, for example, the open-circuit voltage of the SOFC. 
     Furthermore, functional layers in accordance with embodiments of the invention may have high oxygen exchange coefficients, thereby enabling faster surface oxygen adsorption and desorption, and thus faster oxygen lattice incorporation. Thus, the functional layer may enhance mass transport at the interface between the cathode and the electrolyte, reducing interfacial resistance to oxygen introduction into the electrolyte, increasing SOFC performance, and decreasing the area-specific resistance of the SOFC. Moreover, the increased oxygen exchange coefficients of functional layers in accordance with embodiments of the invention advantageously enable lower-temperature SOFC operation. For example, oxygen surface exchange within various functional layers may occur at temperatures as low as approximately 400° C., enabling SOFC operation at temperatures ranging from, for example approximately 400° C. to approximately 750° C., or even approximately 400° C. to approximately 550° C. In accordance with various embodiments of the invention, high oxygen surface coefficients are enabled by the agglomeration or redistribution of one or more elements of the functional layer (e.g., a dopant such as Co) toward or at grain boundaries in the functional layer, thereby forming dopant-rich phases within the functional layer. 
     In addition, constituent elements and/or species within various functional layers in accordance with embodiments of the invention act as sintering agents that enable increased densification of the functional layer at lower temperatures. For example, Co-GDC may be densified at temperatures as low as 900° C., while GDC requires temperatures of approximately 1400° C. for densification. The ability to densify functional layers at lower temperatures maintains a greater variety of materials compatibility options with regard to other materials within the SOFC (e.g., with cathode and electrode materials for chemical compatibility to avoid materials reactions between layers) and may also help to minimize thermal-mismatch issues resulting from different coefficients of thermal expansion between layers (i.e., stresses than may arise during temperature cycling during, e.g., fabrication and/or operation, due to differential contraction and/or expansion). The use of high-density functional layers may also minimize or reduce ohmic losses that might arise due to the use of porous or poorly sintered layers. Finally, the ability to sinter functional layers in accordance with embodiments of the invention at lower temperatures may also prevent or reduce diffusion of various elements from the functional layer (e.g., Co or other metals) into the electrolyte of the SOFC, thereby maintaining higher performance of the SOFC. In fact, various embodiments of the invention feature SOFCs with multilayer electrolytes, in which the concentration of a mobile species (e.g., a metal such as Co) within a first layer of the electrolyte (where the first electrolyte is closer to the cathode) is higher than that within a second layer of the electrolyte (where the second electrolyte is closer to the anode), where the first layer of the electrolyte is adjacent to the functional layer. In such SOFCs, the concentration of the mobile species is higher still (i.e., higher than in any layer of the electrolyte) within the functional layer. Such structures may advantageously resist interdiffusion of the mobile species and thereby maintain high SOFC performance. 
     In an aspect, embodiments of the invention feature a solid oxide fuel cell that includes, consists essentially of, or consists of a cathode, a solid electrolyte, an anode, and a functional layer. The solid electrolyte conducts oxygen ions from the cathode to the anode. At the anode, the oxygen ions react with a hydrogen-containing fuel to generate electricity. The functional layer is disposed between the cathode and the solid electrolyte. The functional layer reduces interfacial resistance (to, e.g., ionic conduction) between the cathode and the solid electrolyte. 
     Embodiments of the invention may include one or more of the following in any of a variety of combinations. The functional layer may be disposed in direct contact with the cathode and/or with the solid electrolyte. The functional layer may include, consist essentially of, or consist of cobalt-doped gadolinium-doped ceria and/or cobalt-doped samarium-doped ceria. The cathode may include, consist essentially of, or consist of lanthanum strontium cobalt ferrite, lanthanum strontium manganite, lanthanum strontium cobaltite, barium strontium cobalt ferrite, samarium strontium cobaltite, samarium-doped ceria, and/or gadolinium-doped ceria. The solid electrolyte may include, consist essentially of, or consist of yttria-stabilized zirconia, scandia-stabilized zirconia, lanthanum strontium magnesium gallate, samarium-doped ceria, and/or gadolinium-doped ceria. The anode may include, consist essentially of, or consist of a composite that includes, consists essentially of, or consists of one or more metals (e.g., nickel) and/or one or more metal oxides (e.g., nickel oxide) and a material of the solid electrolyte (e.g., gadolinium-doped ceria). The anode may include, consist essentially of, or consist of a composite that includes, consists essentially of, or consists of one or more metals (e.g., nickel) and/or one or more metal oxides (e.g., nickel oxide) and yttria-stabilized zirconia. The thickness of the functional layer may range from approximately 0.1 μm to approximately 20 μm, from approximately 0.5 μm to approximately 15 μm, from approximately 1 μm to approximately 12 μm, from approximately 1 μm to approximately 10 μm, or from approximately 1 μm to approximately 6 μm. The hydrogen-containing fuel may include, consist essentially of, or consist of hydrogen and/or a hydrocarbon. The functional layer may contain one or more metals or other dopants (e.g., cobalt, samarium, etc.) at a composition ranging from approximately 0.1 mol % to approximately 10 mol %, from approximately 0.2 mol % to approximately 8 mol %, or from approximately 0.5 mol % to approximately 5 mol %. The functional layer and at least a portion of the solid electrolyte (e.g., one or more layers of an electrolyte composed of multiple layers, or a portion of a single-layer electrolyte) may both contain one or more metals or other dopants (e.g., cobalt, samarium, etc.). The concentration of at least one said metal or other dopant (e.g., cobalt, samarium, etc.) within the at least a portion of the solid electrolyte may be less than the concentration within the functional layer. One or more layers of the solid electrolyte may have a concentration of at least one said metal or other dopant (e.g., cobalt, samarium, etc.) greater than that within one or more other portions or layers of the solid electrolyte and/or less than that within the functional layer. 
     In another aspect, embodiments of the invention feature a method of electrochemically converting a hydrogen-containing fuel to electricity using a solid oxide fuel cell that includes, consists essentially of, or consists of (i) a cathode, (ii) an anode, (iii) a solid electrolyte disposed between the anode and the cathode, and (iv) a functional layer disposed between the cathode and the solid electrolyte. Oxygen or an oxygen-containing gas is ionized at the cathode, thereby producing oxygen ions. The oxygen ions are conducted from the cathode to the anode, through the solid electrolyte, whereby the functional layer reduces interfacial resistance between the cathode and the solid electrolyte. The oxygen ions are reacted with the fuel at the anode, thereby generating electricity. 
     Embodiments of the invention may include one or more of the following in any of a variety of combinations. The functional layer may be disposed in direct contact with the cathode and/or with the solid electrolyte. The functional layer may include, consist essentially of, or consist of cobalt-doped gadolinium-doped ceria and/or cobalt-doped samarium-doped ceria. The cathode may include, consist essentially of, or consist of lanthanum strontium cobalt ferrite, lanthanum strontium manganite, lanthanum strontium cobaltite, barium strontium cobalt ferrite, samarium strontium cobaltite, samarium-doped ceria, and/or gadolinium-doped ceria. The solid electrolyte may include, consist essentially of, or consist of yttria-stabilized zirconia, scandia-stabilized zirconia, lanthanum strontium magnesium gallate, samarium-doped ceria, and/or gadolinium-doped ceria. The anode may include, consist essentially of, or consist of a composite that includes, consists essentially of, or consists of one or more metals (e.g., nickel) and/or one or more metal oxides (e.g., nickel oxide) and a material of the solid electrolyte (e.g., gadolinium-doped ceria). The anode may include, consist essentially of, or consist of a composite that includes, consists essentially of, or consists of one or more metals (e.g., nickel) and/or one or more metal oxides (e.g., nickel oxide) and yttria-stabilized zirconia. The thickness of the functional layer may range from approximately 0.1 μm to approximately 20 μm, from approximately 0.5 μm to approximately 15 μm, from approximately 1 μm to approximately 12 μm, from approximately 1 μm to approximately 10 μm, or from approximately 1 μm to approximately 6 μm. The hydrogen-containing fuel may include, consist essentially of, or consist of hydrogen and/or a hydrocarbon. The functional layer may contain one or more metals or other dopants (e.g., cobalt, samarium, etc.) at a composition ranging from approximately 0.1 mol % to approximately 10 mol %, from approximately 0.2 mol % to approximately 8 mol %, or from approximately 0.5 mol % to approximately 5 mol %. The functional layer and at least a portion of the solid electrolyte (e.g., one or more layers of an electrolyte composed of multiple layers, or a portion of a single-layer electrolyte) may both contain one or more metals or other dopants (e.g., cobalt, samarium, etc.). The concentration of at least one said metal or other dopant (e.g., cobalt, samarium, etc.) within the at least a portion of the solid electrolyte may be less than the concentration within the functional layer. One or more layers of the solid electrolyte may have a concentration of at least one said metal or other dopant (e.g., cobalt, samarium, etc.) greater than that within one or more other portions or layers of the solid electrolyte and/or less than that within the functional layer. The oxygen ions may be reacted with the fuel at a temperature ranging from approximately 300° C. to approximately 800° C., from approximately 350° C. to approximately 750° C., from approximately 400° C. to approximately 750° C., from approximately 400° C. to approximately 700° C., from approximately 450° C. to approximately 650° C., from approximately 450° C. to approximately 600° C., or from approximately 500° C. to approximately 650° C. 
     In yet another aspect, embodiments of the invention feature a method of fabricating a solid oxide fuel cell. An anode layer is provided. A solid electrolyte layer is disposed over the anode layer. A functional layer is disposed (e.g., deposited) over the solid electrolyte layer. A cathode layer is disposed over the functional layer, thereby forming the solid oxide fuel cell. 
     Embodiments of the invention may include one or more of the following in any of a variety of combinations. The functional layer may be disposed in direct contact with the cathode layer and/or with the solid electrolyte layer. The functional layer may include, consist essentially of, or consist of cobalt-doped gadolinium-doped ceria and/or cobalt-doped samarium-doped ceria. The cathode layer may include, consist essentially of, or consist of lanthanum strontium cobalt ferrite, lanthanum strontium manganite, lanthanum strontium cobaltite, barium strontium cobalt ferrite, samarium strontium cobaltite, samarium-doped ceria, and/or gadolinium-doped ceria. The solid electrolyte layer may include, consist essentially of, or consist of yttria-stabilized zirconia, scandia-stabilized zirconia, lanthanum strontium magnesium gallate, samarium-doped ceria, and/or gadolinium-doped ceria. The anode layer may include, consist essentially of, or consist of a composite that includes, consists essentially of, or consists of one or more metals (e.g., nickel) and/or one or more metal oxides (e.g., nickel oxide) and a material of the solid electrolyte (e.g., gadolinium-doped ceria). The anode layer may include, consist essentially of, or consist of a composite that includes, consists essentially of, or consists of one or more metals (e.g., nickel) and/or one or more metal oxides (e.g., nickel oxide) and yttria-stabilized zirconia. The thickness of the functional layer may range from approximately 0.1 μm to approximately 20 μm, from approximately 0.5 μm to approximately 15 μm, from approximately 1 μm to approximately 12 μm, from approximately 1 μm to approximately 10 μm, or from approximately 1 μm to approximately 6 μm. The hydrogen-containing fuel may include, consist essentially of, or consist of hydrogen and/or a hydrocarbon. The functional layer may contain one or more metals or other dopants (e.g., cobalt, samarium, etc.) at a composition ranging from approximately 0.1 mol % to approximately 10 mol %, from approximately 0.2 mol % to approximately 8 mol %, or from approximately 0.5 mol % to approximately 5 mol %. The functional layer and at least a portion of the solid electrolyte layer (e.g., one or more layers of an electrolyte layer composed of multiple layers, or a portion of a single-layer electrolyte layer) may both contain one or more metals or other dopants (e.g., cobalt, samarium, etc.). The concentration of at least one said metal or other dopant (e.g., cobalt, samarium, etc.) within the at least a portion of the solid electrolyte layer may be less than the concentration within the functional layer. One or more layers of the solid electrolyte layer may have a concentration of at least one said metal or other dopant (e.g., cobalt, samarium, etc.) greater than that within one or more other portions or layers of the solid electrolyte layer and/or less than that within the functional layer. 
     The solid electrolyte layer may be disposed over the anode layer before the functional layer is deposited. The anode layer and/or the solid electrolyte layer may be provided by tape casting. Disposing the solid electrolyte layer over the anode layer may include, consist essentially of, or consist of laminating the solid electrolyte layer to the anode layer (by, e.g., hot pressing and/or cold pressing). The solid electrolyte may be provided or formed before the solid electrolyte layer is disposed over the anode layer. The solid oxide fuel cell may be annealed after the cathode layer is disposed over the functional layer. At least a portion of the solid oxide fuel cell (e.g., the anode layer and the solid electrolyte layer; the anode layer, the solid electrolyte layer, and the functional layer; or the anode layer, the solid electrolyte layer, the functional layer, and the cathode layer) may be annealed after the solid electrolyte layer is disposed over the anode layer. The functional layer and the cathode layer may be annealed together (e.g., co-fired) after the cathode layer is disposed over the functional layer. The solid oxide fuel cell may be annealed at a temperature ranging from approximately 600° C. to approximately 1300° C., from approximately 650° C. to approximately 1250° C., from approximately 700° C. to approximately 1200° C., from approximately 750° C. to approximately 1150° C., from approximately 800° C. to approximately 1100° C., from approximately 850° C. to approximately 1050° C., from approximately 900° C. to approximately 1000° C., or from approximately 900° C. to approximately 950° C. The solid oxide fuel cell may be annealed at a temperature no greater than approximately 1100° C., no greater than approximately 1050° C., no greater than approximately 1000° C., or no greater than approximately 950° C. The solid oxide fuel cell may be annealed at a temperature no less than approximately 800° C., no less than approximately 850° C., or no less than approximately 900° C. The solid oxide fuel cell may be annealed for a time period ranging from approximately 0.3 hour to approximately 10 hours, from approximately 0.4 hour to approximately 8 hours, from approximately 0.5 hour to approximately 5 hours, or from approximately 1 hour to approximately 4 hours. The solid oxide fuel cell may be annealed for a time period no greater than 20 hours. The solid oxide fuel cell may be annealed for a time period no greater than 10 hours. The solid oxide fuel cell may be annealed for a time period no less than 0.5 hours. The solid oxide fuel cell may be annealed for a time period no less than 1 hour. The thickness of the functional layer may be selected based at least in part on an intended temperature of operation (e.g., the temperature of the fuel and/or of the solid oxide fuel cell itself) of the solid oxide fuel cell. 
     These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and may exist in various combinations and permutations. As used herein, the terms “approximately” and “substantially” mean±10%, and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Nonetheless, such other materials may be present, collectively or individually, in trace amounts. For example, a structure consisting essentially of multiple metals will generally include only those metals and only unintentional impurities (which may be metallic or non-metallic) that may be detectable via chemical analysis but do not contribute to function. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIG. 1  is a schematic diagram of solid oxide fuel cell incorporating a functional layer in accordance with various embodiments of the invention; 
         FIGS. 2A and 2B  are schematic diagrams of test structures for the evaluation of cathode performance in accordance with various embodiments of the invention; 
         FIG. 3  is a graph of area specific resistance for various cathode test structures in accordance with various embodiments of the invention; 
         FIG. 4  is a graph of cell performance at various operating temperatures for a cell incorporating a 6 μm thick functional layer in accordance with various embodiments of the invention; 
         FIG. 5  is a graph of cell performance at various operating temperatures for a cell incorporating a 1 μm thick functional layer in accordance with various embodiments of the invention; 
         FIG. 6  is a graph of cell performance at an operating temperature of 650° C. for fuel cells with functional layers of various thicknesses in accordance with various embodiments of the invention; 
         FIG. 7  is a graph of cell performance at an operating temperature of 500° C. for fuel cells with functional layers of various thicknesses in accordance with various embodiments of the invention; and 
         FIG. 8  is a graph of oxygen surface exchange coefficients of two exemplary materials in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically depicts a SOFC  100  in accordance with embodiments of the present invention. As shown, the SOFC  100  features a cathode  110 , a solid electrolyte  120 , an anode  130 , and a functional layer  140 . During operation of SOFC  100 , oxygen from an oxygen source  150  (e.g., air) is ionized by the cathode  110 . The resulting oxygen ions are conducted from the cathode  110 , through the solid electrolyte  120 , to the anode  130 . At the anode  130 , the oxygen ions are reacted with a hydrogen-containing fuel  160  to produce electricity. As shown, the generated electricity may be flowed through an external load  170  and back to the cathode  110  to support further ionization at the cathode  110 . The electrochemical reaction may also generate by-products such as, e.g., water and carbon dioxide at the anode  130 . The hydrogen-containing fuel  160  may include, consist essentially of, or consist of, for example, hydrogen gas and/or a hydrocarbon fuel such as natural gas, propane, gasoline, diesel, or biofuel. In various embodiments, the hydrogen-containing fuel  160  may include, consist essentially of, or consist of a mixture of H 2 , CO, H 2 O, CO 2 , and CH 4 . For increased power generation, multiple SOFCs  100  may be linked together in a stacked structure. While  FIG. 1  depicts SOFC  100  in a planar configuration, in various embodiments of the invention the SOFC may be arranged in a concentric tubular configuration. For example, the oxygen source  150  may be flowed through a tubular cathode  110 , around which are disposed tubular electrolyte  120  and anode  130  (with a tubular functional layer  140  disposed between the cathode  110  and the solid electrolyte  120 ), while the hydrogen-containing fuel  160  may be flowed around the outside of the anode  130 . 
     As detailed herein, the presence and use of the functional layer  140  within SOFC  100  may provide various beneficial effects enhancing the overall performance of SOFC  100 . For example, the functional layer  140  may reduce interfacial resistance between the cathode  110  and the electrolyte  120 . In various embodiments, the functional layer  140  has an oxygen exchange coefficient higher than that of the electrolyte  120  and/or the cathode  110 , and therefore enhances oxygen transport within SOFC  100 . As a result of faster oxygen transport and/or lower interfacial resistance, the functional layer  140  may also decrease the area-specific resistance of the SOFC  100 . Functional layers  140  may also retard or block electronic conduction, as they may have ionic transference numbers higher than those for the electrolyte  120 . In accordance with various embodiments of the invention, the functional layer  140  may have a thickness ranging from approximately 0.1 μm to approximately 20 μm, from approximately 1 μm to approximately 15 μm, or from approximately 1 μm to approximately 10 μm. In various embodiments, the thickness of the functional layer  140  is no greater than 20 μm, as thicker functional layers may exhibit deleteriously high ohmic resistance and thus limit the performance of SOFC  100 . In various embodiments, the thickness of the functional layer  140  is no less than 0.1 μm, as thinner functional layers may not be fully continuous and may therefore not provide beneficial effects. 
     In accordance with embodiments of the invention, the presence of the functional layer  140  permits efficient operation of the SOFC  100  at lower-than-conventional temperatures. For example, SOFC  100  may be operated at temperatures below approximately 800° C., e.g., at temperatures ranging from approximately 400° C. to approximately 750° C. Moreover, the thickness of the functional layer  140  may be selected based on, at least in part, the desired operating temperature for the SOFC  100 . For example, at relatively higher temperatures (e.g., ranging from approximately 550° C. to approximately 750° C.), the performance of the SOFC  100  may be limited by the open-circuit voltage, and thus relatively thicker functional layers  140  (for example, 5 μm or thicker, e.g., a thickness ranging from approximately 5 μm to approximately 20 μm, or ranging from approximately 5 μm to approximately 10 μm) may be utilized to enhance the open-circuit voltage and thus the overall performance of the SOFC  100 . Alternatively, at relatively lower temperatures (e.g., ranging from approximately 400° C. to approximately 550° C.), the performance of the SOFC  100  may be limited by ohmic resistance (that is, although the presence of the functional layer  140  still reduces the interfacial resistance between the cathode  110  and the electrolyte  120 , the ohmic resistance of the functional layer  140  itself may impact performance at lower temperatures), and thus relatively thinner functional layers  140  (for example, 5 μm or thinner, e.g., a thickness ranging from approximately 0.1 μm to approximately 5 μm, or ranging from approximately 1 μm to approximately 5 μm) may be utilized to minimize or reduce ohmic resistance and thus improve the overall performance of the SOFC  100 . 
     In various embodiments of the invention, the cathode  110  includes, consists essentially of, or consists of, for example, one or more of lanthanum strontium cobalt ferrite (LSCF) (e.g., La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ ), lanthanum strontium manganite (LSM) (e.g., La 0.8 Sr 0.2 MnO 3_δ ), lanthanum strontium cobaltite (LSC) (e.g., La 0.6 Sr 0.4 Co 3-δ ), barium strontium cobalt ferrite (BCSF) (e.g., Ba 0.5 Sr 0.5 CO 0.8 Fe 0.2 O 3-δ ), samarium strontium cobaltite (SSC) (e.g., Sm 0.5 Sr 0.5 CoO 3-δ ), or doped ceria with dopant concentrations ranging from 5 to 30 mol % (or ranging from 10 to 20 mol %), for example samarium-doped ceria (SDC) (e.g., Co-Ce 0.8 Sm 0.2 O 1.9 ), gadolinium-doped ceria (GDC) (e.g., Co-Ce 0.9 Gd 0.1 O 1.95 ), or ceria doped with any one or more of Y, La, Pr, Sm, or Nd. 
     For example, the cathode  110  may include, consist essentially of, or consist of a mixture of SSC and GDC, e.g., in a ratio of 3:7 to 7:3 by mass. The cathode  110  may be both an ionic conductor and an electronic conductor, and the cathode  110  may be porous to promote oxygen access for ionization and to provide electrochemically active triple phase boundaries (TPBs) where the electrolyte  120  (or functional layer  140 ), air, and cathode  110  meet. For example, the cathode  110  may have a porosity ranging from approximately 30% to approximately 60%. 
     In general, the solid electrolyte  120  is a dense (e.g., ceramic) material that conducts oxygen ions while minimizing electronic conduction therewithin in order to prevent current leakage and corresponding electrical losses. The thickness of the solid electrolyte  120  may range from, for example, approximately 5 μm to approximately 40 μm, or even from approximately 10 μm to approximately 30 μm. In various embodiments of the invention, the solid electrolyte  120  includes, consists essentially of, or consists of, for example, one or more of yttria-stabilized zirconia (YSZ, e.g., Y 0.16 Zr 0.84 O 2-x ), scandia-stabilized zirconia (ScSZ, e.g., Sc 0.2 Ce 0.01 Zr 0.79 O 2-x ), lanthanum strontium magnesium gallate (LSGM, e.g., La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3-x , La 0.9 Sr 0.1 Ga 0.8 Mg 0.2 O 3-x , or La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 3-x ), SDC, or GDC. Such solid electrolytes  120  may have dopant concentrations ranging from, for example, approximately 5 to approximately 30 mol %, or from approximately 10 to approximately 20 mol %. In various embodiments, the solid electrolyte  120  may include, consist essentially of, or consist of one or more doped cerias such as yttria-doped ceria (YDC, e.g., Y 0.1 Ce 0.9 O 1.95 ), neodymium-doped ceria (NdDC, e.g., Nd 0.1 Ce 0.9 O 1.95 ), praseodymium-doped ceria (PrDC, e.g., Pr 0.1 Ce 0.9 O 1.95 ), and/or lanthanum-doped ceria (LaDC, e.g., La 0.1 Ce 0.9 O 1.95 ). Such solid electrolytes  120  may have dopant concentrations ranging from, for example, approximately 5 to approximately 30 mol %, or from approximately 10 to approximately 20 mol %. 
     In various embodiments, the electrolyte  120  is composed of two or more layers having different compositions. In other embodiments, the electrolyte  120  may have a composition in which the concentration of one or more elements, dopants, or species varies along at least a portion of the thickness of electrolyte  120 . For example, in an SOFC  100  in which the functional layer  140  contains a dopant such as Co, all or a portion of the electrolyte  120  may also contain the dopant, but at a concentration different from (e.g., less than) that at which it is present within the functional layer  140 . For example, the electrolyte  120  may have a first layer, adjacent to the functional layer  140 , which contains a first concentration of the dopant, where the first concentration is less than the concentration of the dopant within the functional layer  140 . The electrolyte  120  may also have a second layer, adjacent to the first layer and/or to the anode  130 , which contains the dopant at a second concentration less than the first concentration (and the second concentration may even be approximately zero). Similarly, all or a portion of the thickness of the electrolyte  120  (e.g., a portion of the electrolyte  120  adjacent to the functional layer  140 , or even the entire electrolyte  120 ) may have a varying (e.g., graded) composition of the dopant that decreases as a function of distance away from the functional layer  140 . Such compositional profiles may resist interdiffusion within the various layers of the SOFC  100 , thereby maintaining high SOFC performance. 
     Like the cathode  110 , the anode  130  is preferably a porous conductor of both ions and electrons in order to promote the electrochemical reaction. In various embodiments of the invention, the anode  130  includes, consists essentially of, or consists of, for example, a composite containing a nickel oxide and one or more oxide materials (e.g., one or more materials present within electrolyte  120 , although such materials need not be preset in electrolyte  120 ). Other oxide materials present within anodes  130  in accordance with embodiments of the invention may include yttria-stabilized zirconia (YSZ) ranging in yttria content from 3 to 8 mol %, a doped ceria-based material, and/or a specific doped ceria material found in the solid electrolyte  120  (e.g., GDC). For example, the anode  130  may include, consist essentially of, or consist of a cermet of nickel oxide and yttria-stabilized zirconia. In such a cermet the nickel oxide to yttria-stabilized zirconia ratio may range from, for example, approximately 1:2 to 3:1 by weight. In another example, the anode  130  may include, consist essentially of, or consist of a cermet of nickel oxide and GDC. In other embodiments, the anode  130  may include, consist essentially of, or consist of a ceramic oxide material containing strontium, iron, cobalt, and molybdenum (i.e., SCFM), for example SrFe 0.1 Co 0.45 Mo 0.45 O 3 , SrFe 0.2 Co 0.4 Mo 0.4 O 3 , SrFe 0.34 Co 0.33 Mo 0.33 O 3 , or SrFe 0.5 Co 0.25 Mo 0.25 O 3 . 
     In various embodiments, the SOFC  100  may include an anode functional layer (not depicted in  FIG. 1 ) disposed between the anode  130  and the electrolyte  120 . The anode functional layer may be, for example, a mixture of the anode material and a ceria-based electrolyte. For example, the anode functional layer may include, consist essentially of, or consist of a cermet of nickel oxide and GDC. In such a cermet the nickel oxide to GDC ratio may range from, for example, approximately 1:2 to 3:1 by weight. In another example, the anode functional layer may include, consist essentially of, or consist of a ceramic oxide material containing strontium, iron, cobalt, and molybdenum (i.e., SCFM), for example SrFe 0.1 Co 0.45 Mo 0.45 O 3 , SrFe 0.2 Co 0.4 Mo 0.4 O 3 , SrFe 0.34 Co 0.33 Mo 0.33 O 3 , or SrFe 0.5 Co 0.25 Mo 0.25 O 3 , and GDC. In such a cermet the SFCM to GDC ratio may range from, for example, approximately 1:2 to 3:1 by weight. In various embodiments, the thickness of the anode functional layer may range from approximately 5 μm to approximately 50 μm, or from approximately 10 μm to approximately 25 μm. 
     In accordance with various embodiments of the present invention, the functional layer  140  is disposed at the interface between the cathode  110  and the solid electrolyte  120  and reduces the interfacial resistance (e.g., to ionic conduction) therebetween. In various embodiments, the functional layer  140  includes, consists essentially of, or consists of a ceramic material doped with one or more metals (e.g., cobalt). The functional layer  140  may contain cobalt and/or one or more other metals in the range of, for example, 0.1 mol % to 10 mol % of the ceramic material. In various embodiments, the cobalt and/or other metal concentration ranges from 0.5 mol % to 5 mol % of the ceramic material. In various embodiments, the ceramic material of the functional layer  140  is one or more materials of the cathode  110  and/or of the solid electrolyte  120 . For example, the functional layer  140  may include, consist essentially of, or consist of cobalt-doped GDC and/or cobalt-doped SDC. In addition, the doped ceria composition in the functional layer  140  may have a different dopant than the electrolyte. For example, the functional layer  140  may include, consist essentially of, or consist of cobalt-doped SDC while the electrolyte may include, consist essentially of, or consist of GDC. 
     Various techniques may be utilized for the fabrication of the SOFC  100  and its constituent layers in accordance with embodiments of the present invention. For example, the anode  130  and/or the solid electrolyte  120  may be fabricated by tape casting of the individual layers. In various embodiments, the anode  130  and solid electrolyte  120  are laminated together by, for example, hot isostatic pressing, before application of the functional layer  140 . The functional layer  140  and/or the cathode  110  may be applied as a suspension or paste (e.g., mixed with one or more liquid agents) via, e.g., drop coating, spin coating, spray coating, blade coating, or screen printing. In addition, the functional layer  140  may be deposited by physical deposition (PVD) techniques including sputtering, evaporation, and pulsed laser deposition (PLD). Such techniques may be advantageous for the formation of thin layers having thicknesses less than 5 μm, or even less than 1 μm. In various embodiments, the functional layer  140  and the cathode  110  layer are co-fired, i.e., subjected together to a single annealing step. In a co-fired process the functional layer  140  is first deposited on the SOFC  100 , followed by deposition of the cathode  110  on the functional layer  140 . Then, the SOFC  100  with both layers is co-fired, annealing both layers with the same thermal exposure. For example, the SOFC  100  may be annealed at a temperature ranging from approximately 800° C. to approximately 1100° C. and/or for a time period ranging from approximately 0.5 hour to approximately 5 hours. 
     Alternatively, the functional layer  140  may be applied and then annealed for sintering before application of the cathode  110 . In such as process the functional layer  140  and the cathode  110  may be fired in separate annealing steps to allow optimization of the materials parameters of the individual layers, or for other production considerations such as minimization of contamination. After application of the functional layer  140 , the partial SOFC  100  (i.e., without the cathode  110 ) may be annealed. For example, the partial SOFC  100  may be annealed at a temperature ranging from approximately 800° C. to approximately 1100° C. and/or for a time period ranging from approximately 0.5 hour to approximately 5 hours. Then, after application of the cathode  110 , the entire SOFC  100  may be annealed. For example, the SOFC  100  may be annealed at a temperature ranging from approximately 800° C. to approximately 1100° C. and/or for a time period ranging from approximately 0.5 hour to approximately 5 hours. 
     Various functional layers  140  in accordance with embodiments of the invention contain species such as Co that may act as sintering agents, thereby enabling the use of sintering anneals at lower temperatures and/or for shorter times. For example, Co-containing functional layers  140  may be sintered at temperatures of approximately 950° C. or below, or even 900° C. or below, while still achieving sufficient layer density to enable high performance of SOFC  100 . 
     Examples 
     In order to evaluate the effect of the functional layer  140  on cathode performance, a functional layer  140  composed of cobalt-doped GDC was prepared by solid-state synthesis. Specifically, GDC (Ce 0.9 Gd 0.1 O 1.95 ) and cobalt oxide (Co 3 O 4 ) (here 2 mol %; embodiments of the invention may utilize, e.g., approximately 0.1 mol % to approximately 10 mol %) were mixed and ball milled for a period of 24 hours. The resulting mixture was dried, heat treated at 700° C. for 2 hours, and then ball milled again for 24 hours to produce the cobalt-doped GDC. 
     In order to study the effect of this functional layer  140  on cathode performance, symmetrical test structures were fabricated, as shown in  FIGS. 2A and 2B . As shown, test structure  200  featured a cathode  110  on either side of a solid electrolyte  120 . The test structure  210  was identical to test structure  200 , but with the addition of the cobalt-doped GDC functional layer  140  between each of the cathodes  110  and the solid electrolyte  120 . In this series of tests, GDC was utilized as the solid electrode  120 , while a mixture of SSC (Sm 0.5 Sr 0.5 CoO 3-δ ) and GDC (Ce 0.9 Gd 0.1 O 1.95 ) (SSC-GDC) in a ratio of 7:3 by mass or LSCF (La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ ) and GDC (Ce 0.9 Gd 0.1 O 1.95 ) in a ratio of 1:1 by mass (LSCF-GDC) was utilized for the cathodes  110 . Once the functional layers  140  and the cathodes  110  were applied to the electrolytes  120  in the test structures  200 ,  210 , the structures were co-fired at temperatures ranging from 800° C. to 1100° C. for a period of 2 hours. 
       FIG. 3  is a graph of the area specific resistance (ASR) of the cathode  110  for various test structures  200 ,  210  as a function of operating temperature. Curve  300  represents test structure  200  (i.e., without the functional layer  140 ) utilizing LSCF-GDC as the cathode  110  after co-firing at a temperature of 1100° C. Curve  310  represents test structure  200  (i.e., without the functional layer  140 ) utilizing SSC-GDC as the cathode  110  after co-firing at a temperature of 950° C. Curve  320  represents test structure  210  (i.e., with the functional layer  140 ) utilizing SSC-GDC as the cathode  110  after co-firing at a temperature of 900° C. Curve  330  represents test structure  210  (i.e., with the functional layer  140 ) utilizing SSC-GDC as the cathode  110  after co-firing at a temperature of 950° C. As shown, the ASR of the test structures  210  is much improved when compared to the performance of the test structures  200  over the entire range of operating temperatures studied. In addition, the presence of the functional layer  140  within the test structures  210  decreased cathode polarization by approximately one order of magnitude (10×) and decreased the activation energy significantly. 
     Performance of full SOFCs incorporating the cobalt-doped GDC functional layer  140  was also investigated. In the SOFCs, the anode was a mixture of nickel oxide and GDC (Ni-GDC) in a ratio of 2:3 by mass, the electrolyte was GDC 20 μm thick, and the cathode was SSC-GDC in a ratio of 7:3 by mass. These compositions and thicknesses are only exemplary; in various embodiments, the anode may include, consist essentially of, or consist of a cermet of nickel oxide and yttria-stabilized zirconia. In such a cermet the nickel oxide to yttria-stabilized zirconia ratio may range from, for example, approximately 1:2 to 3:1 by weight. The anode and electrolyte layers were fabricated via tape casting and then laminated together using a hot press. The resulting anode/electrolyte assemblies were annealed at 1450° C. for a time period of 4 hours. The functional layer was deposited on the sintered half-cell by drop coating to a thickness ranging from 1 μm to 10 μm. The SSC-GDC cathode material was applied directly to the functional layer, and the resulting assemblies were co-fired at 950° C. for a time period of 2 hours. In addition, control SOFCs lacking the cobalt-doped GDC functional layer were similarly fabricated for comparative purposes. The various SOFCs were evaluated at a range of temperatures utilizing humidified hydrogen gas as fuel. 
       FIG. 4  is a graph of SOFC performance at temperatures ranging from 500° C. to 650° C. for a SOFC incorporating a cobalt-doped GDC functional layer having a thickness of 6 μm. Curves  400 ,  410 ,  420 ,  430  indicate the cell voltage (in volts) as a function of current density (in amps/cm 2 ) for operating temperatures of 500° C., 550° C., 600° C., and 650° C. respectively. Curves  440 ,  450 ,  460 ,  470  indicate the power density (in watts/cm 2 ) as a function of current density (in amps/cm 2 ) for operating temperatures of 500° C., 550° C., 600° C., and 650° C. respectively. 
       FIG. 5  is a graph of SOFC performance at temperatures ranging from 500° C. to 650° C. for a SOFC incorporating a cobalt-doped GDC functional layer having a thickness of 1 μm. Curves  500 ,  510 ,  520 ,  530  indicate the cell voltage (in volts) as a function of current density (in amps/cm 2 ) for operating temperatures of 500° C., 550° C., 600° C., and 650° C. respectively. Curves  540 ,  550 ,  560 ,  570  indicate the power density (in watts/cm 2 ) as a function of current density (in amps/cm 2 ) for operating temperatures of 500° C., 550° C., 600° C., and 650° C. respectively. 
       FIG. 6  is a graph of SOFC performance as a function of cobalt-doped GDC functional layer thickness at an operating temperature of 650° C. Curves  600 ,  610 ,  620  indicate the cell voltage (in volts) as a function of current density (in amps/cm 2 ) for SOFCs having no functional layer (i.e., a functional layer thickness of zero) and having functional layers of thicknesses 1 μm and 6 μm, respectively. Curves  630 ,  640 ,  650  indicate the power density (in watts/cm 2 ) as a function of current density (in amps/cm 2 ) for SOFCs having no functional layer and having functional layers of thicknesses 1 μm and 6 μm, respectively. 
       FIG. 7  is a graph of SOFC performance as a function of cobalt-doped GDC functional layer thickness at an operating temperature of 500° C. Curves  700 ,  710 ,  720  indicate the cell voltage (in volts) as a function of current density (in amps/cm 2 ) for SOFCs having no functional layer and having functional layers of thicknesses 1 μm and 6 μm, respectively. Curves  730 ,  740 ,  750  indicate the power density (in watts/cm 2 ) as a function of current density (in amps/cm 2 ) for SOFCs having no functional layer and having functional layers of thicknesses 1 μm and 6 μm, respectively. 
     As shown in  FIGS. 4-7 , the use of a functional layer  140  in accordance with embodiments of the present invention can improve performance of an SOFC dramatically. For example, as shown in  FIGS. 4 and 6 , the SOFC having the 6 μm thick functional layer exhibits an output power density of 1.8 W/cm 2  at an operating temperature of 650° C., dramatically higher than the performance of the SOFC without the functional layer. The thicker functional layer, which improves open circuit voltage, performs better within this regime in which performance is limited by open circuit voltage. Moreover, as shown in  FIGS. 5 and 7 , at lower operating temperatures (500° C.-600° C.), the SOFC having the 1 μm thick functional layer exhibits superior performance primarily due to the reduced ohmic resistance within the layer, even though the open circuit voltage is not greatly improved. Thus, functional layers in accordance with embodiments of the invention may have their thicknesses selected based on, at least in part, the intended operating temperature of the SOFC. Since the functional layer may improve performance via improvements in open circuit voltage and/or ohmic resistance, functional layers of various thicknesses may be utilized to improve SOFC performance over a range of different operating conditions. 
       FIG. 8  is a graph of oxygen surface exchange coefficients of an exemplary functional layer material (Co-GDC) and an exemplary electrolyte material (GDC). Here, the Co-GDC layer contains 2 mol % Co, and, as shown, the Co-GDC layer exhibits a higher oxygen surface exchange coefficient over the entire temperature range of approximately 400° C. to approximately 750° C. In various embodiments, this higher oxygen surface exchange coefficient is due at least in part to agglomeration or enrichment of the Co at grain boundaries within the GDC. The higher oxygen surface exchange coefficient of the functional layer enhances mass transport at the interface between cathode and electrolyte, reducing interfacial resistance to oxygen introduction in the electrolyte, increasing cell performance, and decreasing ASR. Moreover, the use of such functional layers in accordance with embodiments of the invention enables enhanced SOFC performance at lower temperatures, for example between approximately 400° C. and approximately 750° C. 
     The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.