Patent Publication Number: US-2011059383-A1

Title: Combined cell structure for solid oxide fuel cell

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Provisional Patent Application No. 61/240,095 filed in the U.S. Patent and Trademark Office on Sep. 4, 2009, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate to combined cell structures for solid oxide fuel cells. 
     2. Description of Related Art 
     Solid oxide fuel cells (SOFCs) have the advantages of no pollution, high-efficiency power generation, and the like. SOFCs are used in stationary power generation systems, small power supplies and vehicle power sources. An SOFC cell may be manufactured as a tube-type cell, a flat-tube-type cell or a flat-plate-type cell. The tube-type or flat-tube-type cells may be cathode supported cells, segmented in series cells, anode supported cells, or the like. 
     Currently, anode supported SOFC cells are frequently used for small SOFC systems in the range of 1 to 10 KW. On the other hand, cathode supported SOFC cells or segmented in series cells are frequently used for large SOFC systems in the range of 100 KW or more. 
     SUMMARY OF THE INVENTION 
     In embodiments of the present invention, a combined cell structure for a solid oxide fuel cell (“SOFC combined cell structure”) is used to easily manufacture a large SOFC system using a plurality of anode supported SOFC cells. 
     In other embodiments, an SOFC combined cell structure is durable against thermal and mechanical stresses (which are typically generated with a plurality of anode supported SOFC cells that are combined in series), enables simplification of manifold design, and prevents increases in current collecting resistance. 
     According to embodiments of the present invention, a combined cell structure for a solid oxide fuel cell includes first and second cells each cell having a first electrode, a second electrode and an electrolyte layer between the first and second electrodes. The combined cell structure further includes a support member for connecting the first and second cells. The support member may include a first sub-support member passing through a hollow portion of the first cell, and a second sub-support member passing through a hollow portion of the second cell. One end of the first sub-support member is fixedly coupled to one end of the second sub-support member. 
     In one embodiment, the support member includes a solid rod passing though the unit cells. When the support member includes sub-support members, each of the first and second sub-support members may include a solid rod. 
     In one embodiment, the support member includes a hollow tube passing through the unit cells. When the support member includes sub-support members, each of the first and second sub-support members may include a hollow tube. The hollow tube may have a plurality of openings or holes between both ends. 
     The first and second support members may be formed of stainless steel, nickel or a nickel alloy. 
     One end of the first sub-support member may include a first coupling, and one end of the second support member may include a second coupling. The first and second couplings may be directly coupled to each other. Alternatively, the combined cell structure may include an adapter for connecting an end of the first sub-support member to an end of the second sub-support member. 
     The combined cell structure may include a porous member between the first electrodes of the unit cells and the support member. The porous member may include metal felt, metal mesh or a combination thereof. 
     The combined cell structure may include a connector for connecting the first and second unit cells. The connector may contact the first or second sub-support member. The connector may be configured to resiliently deform in response to stress from the first and/or second cells. The connector may be connected to at least one of the first and second unit cells. 
     The combined cell structure may further include a sealing member between at least one unit cell and the connector. For example, the sealing member may be between the first unit cell and the connector and/or between the second cell and the connector. 
     The combined cell structure may include a current collector in contact with the second electrode. 
     The first electrode may take any shape, for example, a circular shape, an elliptical shape or a polygonal tube shape. The first electrode may be an anode, and the second electrode may be a cathode. 
     The ends of the unit cells that are not connected to each other (unconnected ends) may be opened. In some embodiments, one of the unconnected ends of a cell stack may be opened, and the other of the unconnected ends may be closed. 
     The combined cell structure may further include at least one third cell between the first and second cells and connected to the first and second cells in series in the longitudinal direction. 
     According to other embodiments of the present invention, a combined cell structure for a solid oxide fuel cell includes a first sub-cell and a second sub-cell. The first sub-cell includes a first cell having a first electrode for forming a tubular support, a second electrode on the first electrode, and an electrolyte layer between the first and second electrodes. A rod-shaped first sub-support member passes through an interior of the first electrode in a longitudinal direction. The second sub-cell includes a second cell having a first electrode for forming a tubular support, a second electrode on the first electrode and an electrolyte layer between the first and second electrodes. A rod-shaped second sub-support member passes through an interior of the first electrode in a longitudinal direction. One end of the first sub-support member is fixedly connected to an end of the second sub-support member such that the first and second cells are connected in series in a longitudinal direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic front view of an SOFC combined cell structure according to an embodiment of the present invention. 
         FIG. 1B  is a cross-sectional view of a tube-type SOFC unit cell according to an embodiment of the present invention. 
         FIG. 1C  is a cross-sectional view of a flat-tube-type SOFC unit cell according to another embodiment of the present invention. 
         FIG. 2  is an enlarged, partial cross-sectional view of a combined cell structure according to an embodiment of the present invention. 
         FIG. 3A  is a cross-sectional view of the sub-support member depicted in  FIG. 2 . 
         FIG. 3B  is a cross-sectional view of the connector depicted in  FIG. 2 . 
         FIG. 3C  is a front view of the connector depicted in  FIGS. 2 and 3B . 
         FIG. 4  is a cross-sectional view of an SOFC stack including the combined cell structure of  FIG. 2 . 
         FIG. 5  is a cross-sectional view of an SOFC stack including a combined cell structure including a sub-support member according to another embodiment of the present invention. 
         FIG. 6  is a cross-sectional view of the sub-support member depicted in the combined cell structure of  FIG. 5 . 
         FIG. 7  is a partial cross-sectional view of a combined cell structure including a sub-support member according to another embodiment of the present invention. 
         FIG. 8  is a cross-sectional view of the sub-support member depicted in the combined cell structure of  FIG. 7 . 
         FIGS. 9A to 9C  are cross-sectional diagrams illustrating a process of manufacturing an SOFC stack having the combined cell structure of  FIG. 7 . 
         FIG. 10  is a cross-sectional view of an SOFC stack including a combined cell structure including a sub-support member according to still another embodiment of the present invention. 
         FIG. 11  is a cross-sectional view of the sub-support member depicted in the combined cell structure of  FIG. 10 . 
         FIG. 12  is a partial cross-sectional view of a combined cell structure including a sub-support member according to yet another embodiment. 
         FIG. 13  is a cross-sectional view of the sub-support member depicted in the combined cell structure of  FIG. 12 . 
         FIGS. 14A to 14C  are cross-sectional diagrams illustrating a process of manufacturing an SOFC stack having the combined cell structure of  FIG. 12 . 
         FIG. 15  is a cross-sectional view of an SOFC stack having a combined cell structure including a sub-support member according to still yet another embodiment of the present invention. 
         FIG. 16  is a cross-sectional view of the sub-support member depicted in the combined cell structure of  FIG. 15 . 
         FIG. 17A  is a cross-sectional diagram illustrating the connection of two sub-support members to an adapter according to an embodiment of the present invention. 
         FIG. 17B  is a cross-sectional diagram illustrating the connection of two sub-support members to an adapter according to another embodiment of the present invention. 
         FIG. 18  is a cross-sectional view of the connection of two sub-support members according to yet another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, exemplary embodiments of the present invention will be described with reference to the accompanying drawings. In the following description, detailed discussion of known functions and structures may be omitted. In the drawings, like elements are represented by like reference numerals, and the dimensions of components may be exaggerated for clarity. 
     The term “manifold,” as used herein, refers to a structure having a flow path for the smooth supply, distribution or discharge of a fluid. With respect to the drawings and their related descriptions in this specification, a housing or boundary wall forming a manifold is designated by a reference numeral and referred to as the manifold, for convenience of illustration. 
       FIG. 1A  is a schematic front view of an SOFC combined cell structure according to an embodiment of the present invention.  FIGS. 1B and 1C  are cross-sectional views of SOFC unit cells, which may be used in the combined cell structure of  FIG. 1A . Referring to  FIG. 1A , the combined cell structure  100  includes a plurality of tube-type or flat-tube-type SOFC cells  10  connected to each other along a longitudinal direction of the combined cell structure  100 . A solid support member  30  passes through the combined SOFC cells  10  from one end of the combined structure  100  to the other end of the combined structure  100 . The plurality of SOFC cells  10  are connected in series and are mechanically and/or physically stabilized by the support member  30 . As used herein, “connected in series” refers to a structure in which the tube-type or flat-tube-type SOFC cells (each having a length) are connected to each other along the longitudinal direction. A connector  20  may be disposed between adjacent SOFC cells. 
     In one embodiment, the support member  30  may pass through the hollow portions of the respective SOFC cells  10 . Alternatively, the support member  30  may be divided into a plurality of sub-support members, where each SOFC cell includes a sub-support member, and the sub-support members are connected to each other along the longitudinal direction to thereby connect the adjacent SOFC cells. Each of the SOFC cells provided with a sub-support member may be referred to as an SOFC sub-cell. The SOFC sub-cells become unit cell structures making up the combined cell structure. 
     Each of the SOFC cells  10  includes a first electrode  11 , a second electrode  15  and an electrolyte  13  between the first and second electrodes  11  and  15 . The first electrode  11  is an anode or cathode. When the first electrode  11  is an anode, the second electrode  15  is a cathode. When the first electrode  11  is a cathode, the second electrode  15  is an anode. The electrolyte  13  is an ion conductive oxide material for transporting oxygen ions or protons. The SOFC cell  10  becomes a unit in which electricity and water are produced by the electrochemical reaction of hydrogen and oxygen respectively supplied to the anode and cathode. 
     In one embodiment, a porous Ni/YSZ cermet may be used as the material of the first electrode  11 . A porous mixed conducting oxide may be used as the material of the second electrode  15 . Yttria stabilized zirconia (YSZ) may be used as the material of the electrolyte  13 . 
     In one embodiment, the SOFC cells through which the support member  30  passes may be tube-type SOFC cells  10   a  having a generally circular cross-section, as illustrated in  FIG. 1B , or flat-tube-type SOFC cells  10   b  having generally elliptical cross-sections, as illustrated in  FIG. 1C . When the cells are tube-type SOFC cells  10   a , the support member  30  may pass through a hollow portion  2   a  of each of the SOFC cells  10   a , as depicted in  FIG. 1B . When the cells are flat-tube-type SOFC cells  10   b , the cells may include three hollow portions, and the support member  30  may pass through the central hollow portion  2   b , as depicted in  FIG. 10 . 
     According to some embodiments, the combined cell structure includes a plurality of anode supported SOFC cells connected in series. However, the combined cell structure can also include a plurality of tube-type or flat-tube-type cathode supported SOFC cells, segmented in series cells or the like. 
       FIG. 2  is a partial cross-sectional view of a combined cell structure according to another embodiment.  FIG. 3A  is a cross-sectional view of the support member (including sub-support members) in the combined cell structure of  FIG. 2 .  FIG. 3B  is a cross-sectional view of a connector in the combined cell structure of  FIG. 2 .  FIG. 3C  is a front view of the connector depicted in  FIGS. 2 and 3B . Referring to  FIG. 2 , the combined cell structure  200  includes a plurality of sub-cells  210   a ,  210   b  and  210   c  connected to each other along the fuel flow direction. Connectors  220  are disposed between adjacent sub-cells. A sealing member  250  may be provided between each of the sub-cells and the connector  220 . 
     Each of the sub-cells  210   a ,  210   b  and  210   c  includes a first electrode  11  for forming a tube-type or flat-tube-type anode support body, a solid electrolyte  13  formed on the outer circumferential surface of the first electrode  11 , a second electrode  15  formed on the solid electrolyte  13 , and at least two connected sub-support members  230  passing through a hollow portion of the first electrode  11 . A porous member  240  may be provided between the support member  230  and the first electrode  11 . 
     As illustrated in  FIG. 3A , the sub-support member  230  has a rod-shaped body  231  with a length. The body  231  may be a solid rod having no interior void space and may have a generally circular cross-section or a generally polygonal (i.e., a polygon inscribed or circumscribed in a circle) cross-section. One end of the body  231  may have a female threaded coupling  233 , and the other end of the body  231  may have a male threaded coupling  235 . The sub-support members  230  are connected by connecting a female threaded coupling  233  of one sub-support member with a male threaded coupling  235  of another sub-support member. The support member  230  may be made of stainless steel, nickel, a nickel alloy, or the like. 
     The connector  220  may be disposed to avoid direct contact between adjacent SOFC cells in the combined cell structure  200 . The connector  220  may be formed of a conductive metal material. In one embodiment, as illustrated in  FIGS. 3B and 3C , the connector  220  includes a disk-shaped first portion  221   a  and a second portion  221   b  extending from an edge of the first portion  221   a  along the thickness direction. 
     The area of the first portion  221   a  is similar to the cross-sectional area of the sub-cell. A first hole  223  is provided at the center of the first portion  221   a . The sub-support member  230  can be inserted longitudinally through the first hole  223 . A plurality of second holes  224  are provided around the first hole  223  and pass through the first portion  221   a  in the thickness direction. The plurality of second holes  224  allow a fluid to flow through the first portion  221   a.    
     A projection  221   e  protrudes from the first portion in a direction opposite the direction in which the second portion  221   b  extends. The projection  221   e  is inserted in the hollow portion of the first of two adjacent sub-cell. The projection  221   e  may be a circular ring surrounding the plurality of second holes  224  at a side of the first portion  221   a . The circular ring may be a solid line or dotted line. 
     The thickness of at least a portion of the second portion  221   b  is less than the thickness of the sub-support member  230 . The second portion has an end  221   d  bent for insertion into the hollow portion of the second of the two adjacent sub-cells. The bent portion may include a stepped portion  221   c . The stepped portion  221   c  may face one side (along the longitudinal direction) of the second sub-cell. If the thickness of at least a portion of the second portion  221   b  is thinner than the sub-support member  230 , the connector  220  resiliently reacts when compressive stress is applied between the cells and the support members during operation of the combined cell structure  200 , thereby reducing undesired thermal stress generated in the combined cell structure  200 . 
     The porous member  240  may have a porous structure in which a fluid may flow along the outer circumferential surface of the support member  230 . The porous member  240  is formed of a material with good conductivity so that the sub-support members  230  are electrically connected to the first electrode  11  in each of the sub-cells. The porous member  240  may be formed of a nickel felt, a metal felt (made of a metal other than nickel), a metal mesh, or the like. 
     The sealing member  250  seals the sub-cell and the connector. The sealing member  250  may include PYREX®, ceramic/glass composites, Thermiculite® #866, and the like. In another embodiment, the boundary portion between the sub-cell and the connector may be directly connected using a brazing technique. 
     Hereinafter, the process of manufacturing a sub-cell in the combined cell structure according to embodiments of the present invention will be described. First, an yttria-stabilized zirconia (YSZ) powder mixed with 40 vol°/0 nickel (Ni) is kneaded by adding activated carbon, an organic binder and water to the YSZ powder, and the kneaded slurry is extrusion-molded. After drying the extrusion-molded slurry, an anode support tube is prepared by sintering the dried slurry at about 1300° C. 
     Subsequently, the YSZ powder is prepared as an electrolyte slurry, and the electrolyte slurry is dip-coated on the anode support tube using a slurry coating technique. The electrolyte slurry coated on the anode support tube is dried at room temperature and then sintered at about 1400° C. 
     Then, a (La,Sr)MnO 3  (LSM) powder is prepared as a cathode slurry, and the cathode slurry is dip-coated on the electrolyte layer of the anode support tube. The cathode slurry coated on the electrolyte layer of the anode support tube is dried and then sintered at about 1200° C. The manufactured SOFC cell has an outer diameter of about 20 mm, an inner diameter of about 16 mm and a length of about 300 mm. 
     An SOFC sub-cell is then manufactured by preparing a sub-support member  230  formed of stainless steel, surrounding the sub-support member  230  with a nickel felt and then inserting the sub-support member  230  into a hollow portion of the SOFC cell. 
       FIG. 4  is a cross-sectional view of an SOFC stack including the combined cell structure of  FIG. 2 . Referring to  FIG. 4 , the SOFC stack according to embodiments of the present invention is manufactured by preparing individual combined cell structures and stacking a plurality of the prepared combined cell structures. Here, an individual combined cell structure is formed by connecting the sub-support members  230  of a plurality of tube-type or flat-tube-type sub-cells  210   a ,  210   b ,  210   c  and  210   d  to each another. In each of the combined cell structures, a connector  220  may be inserted between the sub-cells, and a boundary between the SOFC cell and the connector  220  may be sealed by a sealing member. 
     To form the cathode current corrector, a silver (Ag) wire may be wound on the second electrode of each of the sub-cells. Alternatively, a porous cathode current collecting layer on which a La 0.9 Sr 0.1 CoO 3  powder is coated (using a plasma spray technique) may be formed on the second electrode of each of the sub-cells. A wire or mesh formed of stainless steel and a Ni-based heat-resistant alloy may be used as the material of the cathode current collector. In one embodiment, for example, a Ag wire  250  is used as the cathode current collector, and the sub-support member  230  is used as the anode current collector. 
     One end of the combined cell structure (for example, a structure having four connected sub-cells  210   a ,  210   b ,  210   c  and  210   d ) may be connected to a first manifold  280   a  by a first end connector  270   a . The other end of the combined cell structure may be connected to a second manifold  280   b  by a second end connector  270   b . In such an embodiment, the two end connectors  270   a  and  270   b  connect the combined cell structure to the two manifolds  280   a  and  280   b  so that a fluid can flow therethrough while allowing the connected sub-support members  230  passing through the sub-cells to be fixed to the two manifolds  280   a  and  280   b.    
     In one embodiment, each of the end connectors  270   a  and  270   b  includes a rod-shaped body  271  connected to the sub-support member  230 , and a shielding portion  272  in the form of a band surrounding the body that provides support between the body and the manifold. At least one opening  273  is provided in the shielding portion to allow fuel to flow through the shielding portion. The body may be inserted into an opening in the manifold. A projection  274  may be provided to at least one surface of the shielding portion. A corner of the connector  220  or the manifold  280   a  or  280   b  may contact the shielding portion between the body and the projection. In one embodiment, the end connector  270   a  or  270   b  and the manifold  280   a  or  280   b  may be electrically isolated from each other by a separate insulating member  275  or insulative coating layer. 
     The cell stack including a plurality of combined cell structures may be configured such that the cathode current collector wire  260  of at least one combined cell structure is connected to the sub-support members  230  of at least one other combined cell structure. In such an embodiment, the wire  260  of the first combined cell structure is electrically connected to (e.g., by physically contacting) at least one of the two manifolds  280   a  or  280   b  (e.g., via the end connector), thereby electrically connecting the wire  260  with the sub-support members  230  of the second combined cell structure. 
     Hereinafter, operation of the SOFC stack according to embodiments of the present invention will be described with reference to the drawings. As shown in  FIGS. 2 and 4 , fuel flows from the first manifold  280   a  through openings in the end connectors to enter the combined cell structures. Then, the fuel flows through the sub-cells to the second manifold  280   b  by passing through a porous member  240  extending between the sub-support members  230  and first electrodes  11  of each of the sub-cells. An oxidant circulates about the exterior of the combined cell structures. Oxygen in the air may be used as the oxidant. The fuel may include methane, propane, butane or the like. 
     In each of the combined cell structures, electricity is generated by the electrochemical reaction of hydrogen (fuel) and oxygen (oxidant). Here, the hydrogen is supplied to the first electrode via a passage between the first electrode and the sub-support member of each of the sub-cells. The oxygen is supplied to the second electrode on the outer surface of each of the sub-cells. That is, the fuel that flows into the combined cell structure is reformed at an atmospheric temperature of about 600 to 1000° C. and converted into a reformate containing oxygen. Through the aid of an anode catalyst, the hydrogen supplied to the first electrode is bonded to oxygen ions at a high temperature, thereby producing water and electrons. Meanwhile, through the aid of a cathode catalyst and at a high temperature, the oxygen supplied to the second electrode is bonded to electrons that have been moved from the first electrode through an external circuit or load (not shown) connected to the SOFC stack, and thus converted into oxygen ions. The oxygen ions are moved to the second electrode by passing through an electrolyte. The water produced by the reaction of the hydrogen and the oxygen ions is discharged along with unreacted fuel to the second manifold  280   b  along the fuel flow direction between the sub-support members  230  and the first electrode. The electrons produced by the reaction of the hydrogen and the oxygen ions at the first electrode supply electric power to the load while moving toward the second electrode. The electrochemical reactions respectively generated at the first and second electrodes (anode and cathode) of each of the sub-cells are represented by the following Reaction Formula 1. 
     
       
         
           
             
               
                 
                   
                     
                       
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       FIG. 5  is a cross-sectional view of an SOFC stack including a combined cell structure according to still another embodiment of the present invention.  FIG. 6  is a cross-sectional view of a sub-support member in the SOFC stack of  FIG. 5 . Referring to  FIG. 5 , the SOFC stack is manufactured by stacking or arranging a plurality of combined cell structures. Here, each of the combined cell structures is formed by connecting a plurality of sub-cells  211   a ,  211   b ,  211   c  and  211   d  by connecting their respective sub-support members  230   a . The SOFC stack illustrated in  FIG. 5  is substantially identical to the SOFC stack illustrated in  FIG. 4 , except that the stack of  FIG. 5  has a different serial connection structure than the stack of  FIG. 4 . In particular, in  FIG. 4 , the cathode current collector wire of a first combined cell structure is connected in series to the end connector of a second combined cell structure. In contrast, in  FIG. 5 , the cathode current collector wire of a first combined cell structure is connected in series to the sub-support members  230   a  of a second combined cell structure. 
     As illustrated in  FIG. 6 , the sub-support member  230   a  has a rod-shaped body  231   a  with a length. The rod-shaped body  231   a  may be a solid rod with no interior space. One end of the rod-shaped body  231   a  has a female threaded coupling  233 , and the other end of the rod-shaped body  231   a  has a male threaded coupling  235 . To connect two sub-support members  230   a , the male threaded coupling  235  of a first sub-support member is coupled to the female threaded coupling  233  of a second sub-support member. The sub-support member  230   a  has a ring  237  with a thickness extending radially from an end of the body  231   a . In some embodiments, the ring  237  extends from the end of the body  231   a  having the female threaded coupling  233 . The ring  237  includes a plurality of openings  238  for allowing fuel to flow through the ring  237 . 
     Referring back to  FIG. 5 , in some embodiments of the SOFC stack, the plurality of combined cell structures are connected by the cathode current collector wire  260 . In particular, the cathode collector wire  260  of a first combined cell structure is connected to the sub-support members  230   a  of at least one second combined cell structure. That is, the cathode current collector wire  260  of the first combined cell structure is electrically connected to (e.g., by physically contacting) the rings  237  of the sub-support members  230   a  of the second combined cell structure  230   a.    
       FIG. 7  is a partial cross-sectional view of combined cell structure  300  according to still another embodiment of the present invention.  FIG. 8  is a cross-sectional view of a support member  330  that may be used in the combined cell structure of  FIG. 7 . Referring to  FIG. 7 , the combined cell structure  300  includes a plurality of sub-cells  310   a ,  310   b  and  310   c  connected to each other along a fuel flow direction. Connectors  220  may be disposed between adjacent sub-cells. The plurality of sub-cells are connected to each other along a longitudinal direction by connection of their respective sub-support members  330 . Each of the sub-cells may have a porous member  240  disposed between the sub-support member  330  and the first electrode  11 . A sealing member  250  may be provided between each of the sub-cells and the connector  220 . 
     As illustrated in  FIG. 8 , according to some embodiments of the present invention, the support member  330  includes a generally tubular body  331  having a length. The body  331  has a hollow portion  332  and may have a generally circular cross-section or a generally polygonal (i.e., a polygon inscribed or circumscribed in a circle) cross-section. One end of the body  331  may have a female threaded coupling  333 , and the other end of the body  331  may have a male threaded coupling  335 . Adjacent sub-support members may be connected by coupling the female threaded coupling  331  of a first sub-support member with the male threaded coupling  335  of a second sub-support member. The female threaded coupling  333  is provided on an inner surface of the tubular body  331 , and the male threaded coupling  335  is provided on an outer surface of the body  331 . The sub-support member  330  may be formed of a solid material (such as stainless steel), having a strength. 
       FIGS. 9A to 9C  depict various steps in a process of manufacturing an SOFC stack using the combined cell structure of  FIG. 7 . First, as illustrated in  FIG. 9A , the first, second and third sub-cells  310   a ,  310   b  and  310   c  are prepared, and the connector  220  is connected to one end of each of the sub-cells. A sealing member  250  is provided between each of the sub-cells and the connector  220 . Each of the sub-cells and the connector  220  may be connected to each other using a brazing technique or the like. 
     Subsequently, as illustrated in  FIG. 9B , the sub-support members  330  of adjacent sub-cells  310   a ,  310   b  and  310   c  are connected to each other by screwing the male threaded coupling  335  of one sub-support member  330  into the female threaded coupling  333  of an adjacent sub-support member  330 . Then, a fourth sub-cell  310   d  is prepared, and the male threaded coupling  335  of the support member  330   a  of the fourth sub-cell  310   d  is coupled to the female threaded coupling  333  of the support member  330  of the third sub-cell  310   c . The female threaded coupling may be omitted from the support member  330   a  of the fourth sub-cell  310   d , and that end of the sub-cell  310   d  may be closed by a cap  390  having a thickness establishing a distance from the support member  330   a.    
     The male threaded coupling  335  of the support member  330  in the first sub-cell  310   a  may be connected to the female threaded coupling  333   a  of an end connector  370  fixedly connected to a first manifold  380   a . The end connector  370  may have a generally tubular body  371  for supplying fuel to the hollow portion  332  of the sub-support member  330 . 
     Subsequently, as illustrated in  FIG. 9C , an SOFC stack is manufactured by appropriately stacking or arranging a plurality of combined cell structures, each having first to fourth sub-cells  310   a ,  310   b ,  310   c  and  310   d  that are connected to each other along the fuel flow direction. The plurality of combined cell structures may be fixedly connected to the manifold by the end connector  370 . 
     In some embodiments, the generally tubular body  371  of the end connector  370  may be positioned between the first manifold  380   a  and a second manifold  380   b  so that fuel can be supplied to each of the combined cell structures by flowing from the second manifold  380   b  to the first manifold  380   a . The first and second manifolds  380   a  and  380   b  may form a two-level structure at one side of the combined cell structures. 
     Then, a cathode current collector  360  is formed by winding an Ag wire on the second electrode of each of the sub-cells  310   a ,  310   b ,  310   c  and  310   d  of each of the combined cell structures. The plurality of combined cell structures are electrically connected in series through the end connector  370  in the first manifold  380   a.    
     Hereinafter, operation of the SOFC stack according to embodiments of the present invention will be described with reference to the drawings. Referring to  FIG. 9C , fuel is supplied from the second manifold  380   b  to the hollow portion  332  of the sub-support members  330  of each of the combined cell structures via the tubular body  371  of the end connector  370 . When the fuel reaches the end of the hollow portion of the sub-support member  330   a  of the fourth sub-cell  310   d  (positioned at one end of each combined cell structure), the fuel then flows in the opposite direction and passes through the porous members  240  between the sub-support members  330   a  and  330  and the first electrodes of the sub-cells. 
     Most of the fuel supplied to each of the combined cell structures is converted at a high temperature to a reformate containing hydrogen. The hydrogen is distributed and supplied (via the porous members  240 ) to the first electrodes of the respective sub-cells  310   a ,  310   b ,  310   c  and  310   d.    
     The hydrogen supplied to the first electrodes electrically reacts with the oxygen supplied to the second electrodes (from the air), thereby producing electricity and water. The electricity is supplied to an external load (not shown) connected to the anode and cathode of the SOFC stack. The water is discharged along with any unreacted fuel to the first manifold  380   a  along the fuel flow direction. 
       FIG. 10  is a cross-sectional view illustrating an SOFC stack including a combined cell structure according to still another embodiment of the present invention.  FIG. 11  is a cross-sectional view of a sub-support member used in the combined cell structure depicted in  FIG. 10 . Referring to  FIG. 10 , according to embodiments of the present invention, the SOFC stack is manufactured by appropriately arranging a plurality of combined cell structures. Here, each of the combined cell structures is formed by connecting the respective sub-support members  330   b  of a plurality of sub-cells  311   a ,  311   b ,  311   c  and  311   d . The SOFC stack depicted in  FIG. 10  is substantially identical to the SOFC stack illustrated in  FIG. 9 , except that the stack of  FIG. 9  has a different serial connection structure than the stack of  FIG. 10 . In particular, in  FIG. 9 , the cathode current collector wire of a first combined cell structure is connected in series to the end connector of a second combined cell structure. In contrast, in  FIG. 10 , the cathode current collector wire of a first combined cell structure is connected in series to the sub-support members  330   b  of a second combined cell structure. 
     As illustrated in  FIG. 11 , the sub-support member  330   b  includes a generally tubular body  331   a  having a hollow portion  332 . One end of the body  331   a  has a female threaded coupling  333 , and the other end of the body  331   a  has a male threaded coupling  335 . To connect adjacent sub-support members  330   b , the male threaded coupling  335  of one sub-support member is screwed into the female threaded coupling  333  of an adjacent sub-support member. 
     A ring  337  with a thickness extends radially from an end of the body  331   a . In some embodiments, the ring  337  extends from the end of the body having the female threaded coupling  333 . A plurality of openings  338  are provided in the ring  337  to allow fluid to flow through the ring  337 . The ring  337  is substantially the same as the ring  237  described above with respect to  FIG. 5 , and the plurality of openings  338  may correspond in position to the second holes  224  in the connector  220  depicted in  FIG. 3C . 
     The sub-support member  330   c  of the fourth sub-cell  311   d  is substantially identical to the sub-support member  330   a  of the fourth sub-cell  310   d  depicted in  FIG. 9B , except that the sub-support member  330   c  depicted in  FIG. 10  includes a ring  337  extending from an end (as in the sub-support member  330   b ). 
     Referring back to  FIG. 10 , according to embodiments of the present invention, in the SOFC stack, the cathode current collector  360  of at least one combined cell structure is electrically connected to the ring  337  of at least one sub-support member  330   b  of at least one other combined cell structure. 
       FIG. 12  is a partial cross-sectional view of a combined cell structure according to still another embodiment of the present invention.  FIG. 13  is a cross-sectional view of a sub-support member that may be used in the combined cell structure of  FIG. 12 . Referring to  FIG. 12 , the combined cell structure  400  includes a plurality of sub-cells  410   a ,  410   b  and  410   c  connected to each other along the fuel flow direction. Connectors  220  may be disposed between adjacent sub-cells. The plurality of sub-cells are connected to each other by connection of their respective sub-support members  430 . Each of the sub-cells may have a porous member  240  disposed between the support member  430  and the first electrode  11 . A sealing member  250  may be provided between each of the sub-cells and the connector  220 . 
     As illustrated in  FIG. 13 , according to embodiments of the present invention, the sub-support member  430  may have a generally tubular body  431  having a length. The body  431  has a hollow portion  432  and a plurality of openings  436  along the length of the body  432 . The openings may be formed by cutting away portions of the body  431 . One end of the body has a female threaded coupling  433 , and the other end of the body has a male threaded coupling  435 . Adjacent sub-support members may be connected to each other by screwing the male threaded coupling  435  of one sub-support member into the female threaded coupling  433  of an adjacent sub-support member. The sub-support member  430  may be formed of a solid material such as stainless steel. 
       FIGS. 14A to 14C  depict various steps in a process of manufacturing an SOFC stack having the combined cell structure of  FIG. 12 . First, as illustrated in  FIG. 14A , the first, second, third and fourth sub-cells  410   a ,  410   b ,  410   c  and  410   d  are prepared, and connectors  220  are connected to one end of each of the sub-cells. Each of the sub-cells and the connectors  220  may be properly connected using a sealing member or a brazing technique. 
     Subsequently, as illustrated in  FIG. 14B , the sub-support members  330  of the adjacent sub-cells  410   a ,  410   b ,  410   c  and  410   d  are connected to each other. Then, a male threaded coupling  475  of a first end connector  470   a  (including a fixedly connected first manifold  480   a ) is connected to the female threaded coupling  433  of the sub-support member  430  of the fourth sub-cell  410   d . The first end connector  470   a  may have a generally tubular body for supplying fuel to the hollow portion  432  of the sub-support members  430  of each of the sub-cells. 
     Then, a female threaded coupling  473  of a second end connector  470   b  (inserted into an opening  481  of a second manifold  480   b ) is connected to the male threaded coupling  335  of the sub-support member  430  of the first sub-cell  410   a . The second end connector  470   b  includes a generally tubular inner body  471  for discharging fluid exiting the combined cell structure, an outer body  478  generally surrounding the inner body  471  (generally forming a double pipe), a connector  477  for connecting between the inner body  471  and the outer body, and a plurality of openings (not shown) in the connector for allowing fluid to pass through the connector. The structure of the connector  477  of the second end connector  470   b  is substantially similar to that of the connector  220  described above with respect to  FIG. 3B . 
     After the sub-support member  430  of the first sub-cell  410   a  is connected to the second end connector  470   b , the second end connector  470   b  is fixed to the second manifold  480   b  by a fixing member  490 . The fixing member  490  may be a ring having a threaded inner circumference. The second end connector  470   b  may have a male threaded coupling  476  on its outer surface, and the threaded inner circumference of the fixing member  490  may be connected to the male threaded coupling  476  of the second end connector  470   b.    
     Subsequently, as illustrated in  FIG. 14C , an SOFC stack is manufactured by appropriately stacking or arranging a plurality of combined cell structures, each combined cell structure having first to fourth sub-cells  410   a ,  410   b ,  410   c  and  410   d  connected to each other along the fuel flow direction. The plurality of combined cell structures may be connected to the first and second manifolds  480   a  and  480   b  via the first and second end connectors  470   a  and  470   b , respectively. 
     In some embodiments, the first and second end connectors  470   a  and  470   b  connect the first and second manifolds  480   a  and  480   b  to each of the combined cell structures such that fuel is supplied from the first manifold  480   a  to the combined cell structures, and the reaction product (such as water and unreacted fuel) are discharged to the second manifold  480   b.    
     A cathode current collector  460  is provided on the second electrode of each of the sub-cells  410   a ,  410   b ,  410   c  and  410   d  of each of the combined cell structures. In some embodiments, the cathode current collector  460  (which can be an Ag wire) of a first combined cell structure is connected to at least one end connector  470   a  or  470   b  of at least one second combined cell structure. That is, the cathode current collector  460  of the first combined cell structure is electrically connected to the end connectors  470   a  and  470   b  of the second combined cell structure, thereby forming a serially connected SOFC stack. 
     Hereinafter, operation of the SOFC stack according to embodiments of the present invention will be described with reference to the drawings. Referring to  FIG. 14C , fuel is supplied (via the tubular body  471  of the first end connector  470   a ) from the first manifold  480   a  to the hollow portions  432  of the sub-support members  430  of each of the combined cell structures. Most of the fuel flows from the hollow portions  432  to the outer surfaces of the sub-support members  430  through the openings  436  and then flows through the porous members  240 . The rest of the supplied fuel is flows to the second manifold  480   b  through the hollow portion  432  of the sub-support members  430 . 
     The fuel in the combined cell structures is converted to a reformate containing hydrogen, and the hydrogen is supplied to the first electrode of each of the sub-cells  410   a ,  410   b ,  410   c  and  410   d . The hydrogen supplied to the first electrode electrically reacts with oxygen supplied to the second electrode from the air, thereby producing electricity and water. The electricity is supplied to an external load (not shown) connected to the anode and cathode of the SOFC stack. The water is discharged along with any unreacted fuel to the second manifold  480   b  along the fuel flow direction. 
       FIG. 15  is across-sectional view of an SOFC stack including a combined cell structure according to still another embodiment of the present invention.  FIG. 16  is a cross-sectional view of a sub-support member used in the combined cell structure depicted in  FIG. 15 . Referring to  FIG. 15 , the SOFC stack is manufactured by stacking or arranging a plurality of combined cell structures. Here, each of the combined cell structures is formed by connecting the respective sub-support members  430   a  of a plurality of sub-cells  411   a ,  411   b ,  411   c  and  411   d . The SOFC stack of  FIG. 15  is substantially identical to the SOFC stack illustrated in  FIG. 14C , except that the stack of  FIG. 14C  has a different serial connection structure than the stack of  FIG. 15 . In particular, in  FIG. 14C , the cathode current collector wire of a first combined cell structure is connected in series to the end connector of a second combined cell structure. In contrast, in  FIG. 15 , the cathode current collector wire of a first combined cell structure is connected in series to the sub-support members  430   a  of a second combined cell structure. 
     As illustrated in  FIG. 16 , the sub-support member  430   a  includes a generally tubular body  431   a  having a length. The sub-support member  430   a  may be formed of a solid conducting material having a strength. The sub-support member  430   a  may be made of stainless steel, nickel, a nickel alloy, or the like. The body  431   a  has a hollow portion  432  and a plurality of openings  436  along the length of the body. The plurality of openings  436  may be formed by cutting away portions of the body  431   a . One end of the body  431   a  has a female threaded coupling  433 , and the other end of the body  431   a  has a male threaded coupling  435 . To connect adjacent sub-support members, the male threaded coupling  435  of a first sub-support member is screwed into the female threaded coupling  433  of an adjacent sub-support member. 
     The plurality of openings  436  may be arranged at regular or irregular intervals along the length of the body  431   a . The plurality of openings  436  may have any suitable shape and/or size, and any suitable number of openings may be provided so long as the strength of the sub-support member is not deteriorated to the point that it is no longer useful. 
     The sub-support member  430   a  may have a ring  437  with a thickness extending radially from an end of the body  431   a . In some embodiments, the ring  437  extends from the end of the body  431   a  having the female threaded coupling. A plurality of openings  438  are provided in the ring  437  to allow fluid to flow through the ring  437 . 
     Referring back to  FIG. 15 , in the SOFC stack, the cathode current collector  460  of a first combined cell structure may be connected to the ring  437  of at least one sub-support member  430   a  of at least one second combined cell structure. That is, an electrical connection node between the first and second combined cell structures may be formed at the ring  437  of the sub-support member  430   a  of the second combined cell structure. 
       FIGS. 17A and 17B  are cross-sectional views of alternative mechanisms for connecting adjacent sub-support members. In the combined cell structures according to embodiments of the present invention, the connection of adjacent sub-support members has been described above as including screwing a male threaded coupling of a first sub-support member into a female threaded coupling of a second sub-support member. However, as illustrated in  FIG. 17A , both ends of each of the sub-support members may have female threaded couplings, and adjacent sub-support members may be connected using a male adapter  50   a . The male adapter has two male threaded couplings  53   a  and  53   b  at opposite sides of a body  51   a  having the same cross-sectional shape as that of the sub-support member. Here, a female threaded coupling  33   a  of a first sub-support member  30   a  is screwed onto the first male threaded coupling  53   a  of the male adapter  50   a , and a female threaded coupling  33   b  of a second sub-support member  30   b  is screwed onto the second male threaded coupling  53   b  of the male adapter  50   a . This results in the two sub-support members  30   a  and  30   b  being connected to each other along a longitudinal direction. As shown, the two sub-support members  30   a  and  30   b  face each other with the male adapter interposed therebetween. 
     Alternatively, as shown in  FIG. 17B , both ends of each of the sub-support members may have male threaded couplings, and adjacent sub-support members may be connected using a female adapter  50   b . The female adapter  50   b  includes a generally tubular body  51   b  with a threaded interior surface  55 . Here, a male threaded coupling  35   a  of a first sub-support member  30   c  is screwed into a first side of the threaded surface  55  of the female adapter  51   b , and a male threaded coupling  35   b  of a second sub-support member  30   d  is screwed into a second side of the threaded surface  55  of the female adapter  51   b . This results in the two sub-support members  30   c  and  30   d  being connected to each other along a longitudinal direction. As shown, the two sub-support members  30   c  and  30   d  face each other with the female adapter in terposed therebetween. 
       FIG. 18  is a cross-sectional view illustrating yet another mechanism for connecting adjacent sub-support members. As shown in  FIG. 18 , a rod-shaped sub-support member may include a first end having a protrusion  37   a  and a second end having a notch  37   b . The protrusion  27   a  is shaped to fit within the notch  37   b . Adjacent sub-support members may be connected by fitting the protrusion  37   a  of a first sub-support member  30   e  into the notch  37   b  of a second sub-support member  30   f . The protrusion  37   a  may have a generally circular cross-section or a generally polygonal (i.e., a polygon inscribed or circumscribed in a circle) cross-section. The notch  37   b  may have a generally concave shape into which into which the protrusion  37   a  can be tightly inserted. 
     In some embodiments, the first sub-support member  30   e  may have at least one second protrusion  39   a , and the second sub-support member  30   f  may have at least one second notch  39   b , where the second protrusion  39   a  fits in the second notch  39   b  to prevent the protrusion  37   a  from rotating within the notch  37   b.    
     In some embodiments, at least one fixing member  60  passes through the protrusion  37   a  and notch  37   b  to prevent the first and second sub-support members  30   e  and  30   f  from separating from each other. The fixing member  60  may be a pin and may have a head formed at one end of the fixing member  60 . Here, the head may be larger than body of the fixing member  60 . The end of the fixing member  60  passing through the first and second sub-support members  30   e  and  30   f  may be bent along the longitudinal direction of the sub-support member and adhered to a surface of the sub-support member. 
     Embodiments of the present invention provide several important benefits. First, tube-type anode support members used in anode supported SOFC cells are generally formed of a material such as porous Ni—YSZ cermet, and the anode supported SOFC cells are generally manufactured as cells having a length of 30 cm or shorter due to limitations in the mechanical strength of the material, high internal resistance, reduction of yield caused by large area, and the like. However, in embodiments of the present invention, a plurality of tube-type anode supported SOFC cells are connected along a longitudinal direction using sub-support members of the SOFC cells. This enables the production of SOFC combined cell structures having a length of about 120 cm or longer. 
     Second, when a plurality of anode supported SOFC cells are simply combined in a longitudinal direction (i.e., as compared to embodiments of the present invention), the connection of the anode supported SOFC cells is easily broken by temperature distributions (or temperature differences) between the connected cells or by mechanical stress generated at the connection to the manifold. However, in embodiments of the present invention, the plurality of anode supported SOFC cells are connected along the fuel flow direction using the sub-support members of the SOFC cells. This enables the production of SOFC combined cell structures that are not broken by thermal or mechanical stress. Further, embodiments of the present invention enable the easy design and manufacture of larger-sized SOFC systems. 
     Third, when large-sized SOFC systems are simply manufactured using a plurality of anode supported SOFC cells (i.e., as compared to embodiments of the present invention), designing a manifold to distribute and supply fuel to each of the SOFC cells is very difficult due to the large number of SOFC cells. However, according to embodiments of the present invention, the SOFC combined cell structures have a plurality of SOFC cells connected along a longitudinal direction, enabling simplification of the manifold design by considerably decreasing the number of SOFC cells. Accordingly, the uniform supply of fuel to each of the SOFC cells can be easily achieved. 
     Fourth, when a plurality of anode supported SOFC cells are simply connected in a longitudinal direction to increase length (i.e., as compared to embodiments of the present invention), electrical resistance between the SOFC cells increases, and external current collection is very difficult. However, according to embodiments of the present invention, a conductive support member (or a plurality of sub-support members) passes through a hollow portion of the tube-type or flat-tube-type SOFC cells. This enables the easy performance of external current collection without increasing electrical resistance between the SOFC cells. 
     While the present invention has been described in connection with certain exemplary embodiments, it is understood by those of ordinary skill in the art that certain modifications may be made to the described embodiments without departing from the spirit and scope of the present invention, as defined by the appended claims.