Patent Publication Number: US-2019190051-A1

Title: Fuel cell tube with laterally segmented fuel cells

Description:
FIELD 
     The present disclosure relates to fuel cell tubes. More specifically, the present disclosure relates to fuel cell tubes including one or more laterally segmented fuel cells. 
     BACKGROUND 
     A fuel cell is an electrochemical conversion device that produces electricity by oxidizing a fuel. A fuel cell typically includes an anode, a cathode, and an electrolyte between the anode and the cathode. A fuel cell tube usually includes multiple fuel cells disposed on a substrate and electrically connected to one another in series via primary interconnects. A fuel cell stack typically includes multiple fuel cell tubes electrically connected to one another in series via secondary interconnects. A fuel cell system includes multiple fuel cell stacks electrically connected to one another in series and several components configured to provide the fuel to the anodes of the fuel cells and an oxidant to the cathodes of the fuel cells. The oxygen in the oxidant is reduced at the cathode into oxygen ions that diffuse through the electrolyte layers into the anodes. The fuel is oxidized at the anodes, which gives off electrons that flow through an electrical load. 
     SUMMARY 
     Various embodiments of the present disclosure provide a fuel cell tube including one or more laterally segmented fuel cells or dummy cells each including lateral fuel cell or dummy cell portions that are electrically isolated from one another such that there is no continuous electrical path across the width of the tube. When assembled into a fuel cell stack, secondary interconnects electrically connect adjacent fuel cell tubes via their respective laterally segmented fuel cells. The use of laterally segmented fuel cells or dummy cells to effect the fuel cell tube-to-fuel cell tube electrical connection enables more accurate testing of the electrical connection between adjacent fuel cell tubes. 
     In some examples, a segmented-in-series solid-oxide fuel cell system includes a first fuel cell tube, a second fuel cell tube and a first secondary interconnect. The first fuel cell tube can include a substrate having a first end and an opposing second end, a first major surface extending between the first and second ends, and a second opposing major surface extending between the first and second ends. The first fuel cell tube can also include a plurality of fuel cells disposed on the first major surface, each fuel cell extending laterally across the first major surface and being positioned between the first and second ends. In some examples, a first selected one of the plurality of fuel cells is laterally segmented so that a first lateral end of the first selected fuel cell is electrically isolated from a second lateral end of the first selected fuel cell. In other examples, the cells at each end may comprise a “dummy” cell, i.e., a cell comprising only the cathode layer or the cathode layer with a cathode current collector layer. 
     The second fuel cell can include a substrate having a first end and an opposing second end, a first major surface extending between the first and second ends, and a second opposing major surface extending between the first and second ends. The second fuel cell can also include a plurality of fuel cells disposed on the first major surface, each fuel cell extending laterally across the first major surface and being positioned between the first and second ends. In some examples, a first one of the plurality of fuel cells is laterally segmented so that a first lateral end of the first selected fuel cell is electrically isolated from a second lateral end of the first selected fuel cell. In other examples, the cells at each end may comprise a “dummy” cell, i.e., a cell comprising only the cathode layer or the cathode layer with a cathode current collector layer. 
     The segmented-in-series solid-oxide fuel cell system can also include a first secondary interconnect electrically connecting the first lateral end of the first selected fuel cell of the first fuel cell tube to the first lateral end of the first selected fuel cell of the second fuel cell tube. 
     The segmented-in-series solid-oxide fuel cell system can also include a second secondary interconnect electrically connecting the second lateral end of the first selected fuel cell of the first fuel cell tube to the second lateral end of the first selected fuel cell of the second fuel cell tube. 
     In some examples, a fuel cell tube includes a substrate defining one or more fuel conduits therethrough, the substrate having a first end and an opposing second end, a first major surface extending between the first and second ends, and a second opposing major surface extending between the first and second ends. The fuel cell tube can also include a plurality of fuel cells disposed on the first major surface, each fuel cell extending laterally across the first major surface and being positioned between the first and second ends. In some examples, a first selected one of the plurality of fuel cells is laterally segmented so that a first lateral end of the first selected fuel cell is electrically isolated from a second lateral end of the first selected fuel cell. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a top plan view of one embodiment of a fuel cell tube of the present disclosure. 
         FIG. 2  is a side elevational view of the fuel cell tube of  FIG. 1 . 
         FIG. 3  is a front elevational cross-sectional view of the fuel cell tube of  FIG. 1  taken substantially along line  3 - 3  of  FIG. 1 . 
         FIG. 4  is a side elevational cross-sectional view of part of one of the fuel cells of the fuel cell tube of  FIG. 1  taken substantially along line  4 - 4  of  FIG. 1 . 
         FIG. 5  is a side elevational view of six fuel cell tubes of one embodiment of a fuel cell stack of the present disclosure. 
         FIG. 6  is a front elevational cross-sectional view of the fuel cell tubes of the fuel cell stack of  FIG. 5  taken substantially along line  6 - 6  of  FIG. 5 . 
         FIG. 7  is a rear elevational cross-sectional view of the fuel cell tubes of the fuel cell stack of  FIG. 5  taken substantially along line  7 - 7  of  FIG. 5 . 
         FIGS. 8A-8D  are front elevational cross-sectional views of two prior art fuel cell tubes of a prior art fuel cell stack during resistance testing. 
         FIGS. 9A-9D  are front elevational cross-sectional views of two the fuel cell tubes of the fuel cell stack of  FIG. 5  during resistance testing taken substantially along line  6 - 6  of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     While the features, methods, devices, and systems described herein may be embodied in various forms, the drawings show and the detailed description describes some exemplary and non-limiting embodiments. Not all of the components shown and described in the drawings and the detailed descriptions may be required, and some implementations may include additional, different, or fewer components from those expressly shown and described. Variations in the arrangement and type of the components; the shapes, sizes, and materials of the components; and the manners of attachment and connections of the components may be made without departing from the spirit or scope of the claims as set forth herein. This specification is intended to be taken as a whole and interpreted in accordance with the principles of the invention as taught herein and understood by one of ordinary skill in the art. 
       FIGS. 1-4  illustrate one example embodiment of a fuel cell tube  100  of the present disclosure and components thereof.  FIGS. 5-7  illustrate part of one example embodiment of a fuel cell stack  10  of the present disclosure including the fuel cell tube  100  and fuel cell tubes  200 ,  300 ,  400 ,  500 , and  600  electrically connected to one another. 
     The fuel cell tube  100  includes a porous substrate  110  having a width W, a length L, a thickness T, a generally planar upper major surface  110   a,  and a generally planar lower major surface  110   b.  As shown in  FIG. 3 , multiple fuel conduits  110   c  extend through the substrate  110  along the length L of the substrate  110 . The fuel cell tube  100  is fluidly connectable to a manifold (not shown) that is fluidly connectable to a fuel source such that fuel can flow from the fuel source through the manifold and into and through the fuel conduits  110   c.  In this example embodiment, the substrate  110  is formed of MgO—MgAl 2 O 4  (MMA), though in other embodiments the substrate  110  may be formed of any suitable material(s) in addition to or instead of MMA (such as doped zirconia and/or forsterite). 
     First and second porous anode barriers  120   a  and  120   b  are disposed on the upper and lower major surfaces  110   a  and  110   b,  respectively, of the substrate  110 . The first and second porous anode barriers  120   a  and  120   b  are configured to prevent reactions between the anodes of the fuel cells (described below) and the substrate  110 , and are not configured to provide electrical conduction within a given fuel cell or between two fuel cells. Additionally, the first and second porous anode barriers  120   a  and  120   b  are not configured to partake in the electrochemical reactions that generate electrical power from the fuel. In this example embodiment, the first and second porous anode barriers  120   a  and  120   b  are formed of an inert porous ceramic material such as 3YSZ or another suitable doped zirconia, though in other embodiments the first and second porous anode barriers  120   a  and  120   b  may be formed of any suitable material(s) in addition to or instead of doped zirconia, such as SrZrO 3  or SrTiO 3 -doped zirconia composite. In other embodiments, the fuel cell tube  100  does not include the first and second porous anode barriers  120   a  and  120   b.    
     Multiple fuel cells  130 , a first laterally segmented fuel cell  140 , and a second laterally segmented fuel cell  150  are disposed on the first porous anode barrier  120   a.  Each fuel cell  130 , the first laterally segmented fuel cell  140 , and the second laterally segmented fuel cell  150  generally extend laterally in the direction of the width W of the substrate  110  and terminate in opposing first and second lateral ends (not labeled). The fuel cells  130  are positioned between the first and second laterally segmented fuel cells  140  and  150 , which are generally positioned at opposing ends of the first porous anode barrier  120   a  in the direction of the length L of the substrate  110 . The fuel cells  130 , the first laterally segmented fuel cell  140 , and the second laterally segmented fuel cell  150  on the first porous anode barrier  120   a  are electrically connected in series via primary interconnects (not shown). 
     As best shown in  FIGS. 1 and 3 , the first laterally segmented fuel cell  140  includes first and second fuel cell portions  140   a  and  140   b.  The first and second fuel cell portions  140   a  and  140   b  are laterally separated in the direction of the width W of the substrate  110  by a space  140   c  such that the first and second fuel cell portions  140   a  and  140   b  are electrically isolated such that there is no continuous electrical path in the fuel cell across the width of the fuel cell tube. Put differently, no continuous direct electrical path exists between the first and second fuel cell portions  140   a  and  140   b  in the direction of the width W of the substrate  110 . In this example embodiment, the space  140   c  is 0.5 millimeters in the direction of the width W of the substrate  110 , though the space  140   c  may be of any suitable size sufficient to ensure the first and second fuel cell portions  140   a  and  140   b  are electrically isolated. 
     As best shown in  FIGS. 1 and 3 , the second laterally segmented fuel cell  150  includes first and second fuel cell portions  150   a  and  150   b.  The first and second fuel cell portions  150   a  and  150   b  are laterally separated in the direction of the width W of the substrate  110  by a space  150   c  such that the first and second fuel cell portions  150   a  and  150   b  are electrically isolated such that there is no continuous electrical path in the fuel cell across the width of the fuel cell tube. Put differently, no continuous direct electrical path exists between the first and second fuel cell portions  150   a  and  150   b  in the direction of the width W of the substrate  110 . In this example embodiment, the space  150   c  is 0.5 millimeters in the direction of the width W of the substrate  110 , though the space  150   c  may be of any suitable size sufficient to ensure the first and second fuel cell portions  150   a  and  150   b  are electrically isolated. 
     Similarly, multiple fuel cells  130 , a third laterally segmented fuel cell  160 , and a fourth laterally segmented fuel cell  170  are disposed on the second porous anode barrier  120   b.  Each fuel cell  130 , the third laterally segmented fuel cell  160 , and the fourth laterally segmented fuel cell  170  generally extend laterally in the direction of the width W of the substrate  110 . The fuel cells  130  are positioned between the third and fourth laterally segmented fuel cells  160  and  170 , which are generally positioned at opposing ends of the second porous anode barrier  120   b  in the direction of the length L of the substrate  110 . The fuel cells  130 , the third laterally segmented fuel cell  160 , and the fourth laterally segmented fuel cell  170  on the second porous anode barrier  120   b  are electrically connected in series via primary interconnects (not shown). 
     As best shown in  FIGS. 1 and 3 , the third laterally segmented fuel cell  160  includes first and second fuel cell portions  160   a  and  160   b.  The first and second fuel cell portions  160   a  and  160   b  are separated in the direction of the width W of the substrate  110  by a space  160   c  such that the first and second fuel cell portions  160   a  and  160   b  are electrically isolated such that there is no continuous electrical path in the fuel cell across the width of the fuel cell tube. Put differently, no continuous direct electrical path exists between the first and second fuel cell portions  160   a  and  160   b  in the direction of the width W of the substrate  110 . In this example embodiment, the space  160   c  is 0.5 millimeters in the direction of the width W of the substrate  110 , though the space  160   c  be of any suitable size sufficient to ensure the first and second fuel cell portions  160   a  and  160   b  are electrically isolated. 
     As best shown in  FIGS. 1 and 3 , the fourth laterally segmented fuel cell  170  includes first and second fuel cell portions  170   a  and  170   b.  The first and second fuel cell portions  170   a  and  170   b  are laterally separated in the direction of the width W of the substrate  110  by a space  170   c  such that the first and second fuel cell portions  170   a  and  170   b  are electrically isolated such that there is no continuous electrical path in the fuel cell across the width of the fuel cell tube. Put differently, no continuous direct electrical path exists between the first and second fuel cell portions  170   a  and  170   b  in the direction of the width W of the substrate  110 . In this example embodiment, the space  170   c  is 0.5 millimeters in the direction of the width W of the substrate  110 , though the space  170   c  be of any suitable size sufficient to ensure the first and second fuel cell portions  170   a  and  170   b  are electrically isolated. 
     As shown in  FIG. 4 , each fuel cell  130  and each fuel cell portion of each laterally segmented fuel cell  140 ,  150 ,  160 , and  170  includes an anode current collector  130   a,  an anode  130   b,  an electrolyte  130   c,  a cathode  130   d,  and a cathode current collector  130   e.  The anode  130   b  is disposed between the anode current collector  130   a  and the electrolyte  130   c.  The electrolyte  130   c  is disposed between the anode  130   b  and the cathode  130   d.  The cathode  130   d  is disposed between the electrolyte  130   c  and the cathode current collector  130   e.  The anode current collector  130   a  is electrically connected to the anode  130   b,  and the cathode current collector  130   e  is electrically connected to the cathode  130   d.  The anode and cathode current collectors  130   a  and  130   e  provide a higher electrical conductivity path for the transfer of electrons than is possible by the anode and cathode along. 
     In this example embodiment, the anode current collector  130   a  is an electrode conductive layer formed of a nickel cermet. Examples of suitable materials include Ni—YSZ (yttria doping in zirconia is 3-8 mol %); Ni—ScSZ (scandia doping is 4-10 mol %, preferably second doping for phase stability for 10 mol % scandia-ZrO 2 ); Ni-doped ceria (such as Gd or Sm doping); cermet of Ni and doped lanthanum chromite (such as Ca doping on A site and Zn doping on B site); cermet of Ni and doped strontium titanate (such as La doping on A site and Mn doping on B site) and/or La 1−x Sr x Mn y Cr 1−y O 3 . In other embodiments, the anode current collector may be formed of cermets based at least in part on one or more precious metals and/or one or more precious metal alloys in addition to retaining Ni content. Precious metals in the cermet may include, for example, Pt, Pd, Au, Ag, and/or alloys thereof. The ceramic phase may include, for example, an inactive non-electrically conductive phase, including, for example, YSZ, ScSZ, and/or one or more other inactive phases, e.g., having desired coefficients of thermal expansion (CTE) to control the CTE of the layer to match the CTE of the substrate  110  and the electrolyte  130   c.  In some embodiments, the ceramic phase may include Al 2 O 3  and/or a spinel such as NiAl 2 O 4 , MgAl 2 O 4 , MgCr 2 O 4 , or NiCr 2 O 4 . In other embodiments, the ceramic phase may be electrically conductive, e.g., doped lanthanum chromite, doped strontium titanate, and/or one or more forms of LaSrMnCrO. One specific example of the anode current collector  130   a  material is 76.5% Pd, 8.5% Ni, 15% 3YSZ. 
     In this example embodiment, the anode  130   b  is formed of xNiO-(100−x)YSZ (x is from 55 to 75 in weight ratio), yNiO-(100−y)ScSZ (y is from 55 to 75 in weight ratio), NiO-gadolinia stabilized ceria (such as 55 wt % NiO-45 wt % GDC), and/or NiO samaria stabilized ceria. In other embodiments, the anode may be formed of doped strontium titanate, La 1−x Sr x Mn y Cr 1−y O 3  (e.g., La 0.75 Sr 0.25 Mn 0.5 Cr 0.5 O 3 ) and/or other ceramic-based anode materials. 
     In this example embodiment, the electrolyte  130   c  is formed of a ceramic material. In some embodiments, the electrolyte  130   c  is formed of a proton and/or oxygen ion conducting ceramic. In other embodiments, the electrolyte  130   c  is formed of YSZ, such as 3YSZ and/or 8YSZ. In other embodiments, the electrolyte  130   c  is formed of ScSZ, such as 4ScSZ, 6ScSz, and/or 10ScSZ in addition to or in place of YSZ. In other embodiments, the electrolyte  130   c  may be formed of doped ceria and/or doped lanthanum gallate. The electrolyte  130   c  is essentially impervious to diffusion therethrough of the oxidant (e.g., air or O 2 ) and the fuel (e.g., H 2 ) flowed through or past the fuel cell tube  100 , but enables diffusion of oxygen ions and/or protons, depending upon the particular embodiment and its application. 
     In this example embodiment, the cathode  130   d  is formed of a mixture of an electrochemically catalytic ceramic and an ionic phase. The electrochemically catalytic phase consists of at least one of LSM (La 1−x Sr x MnO 3 , x=0.1 to 0.3), La 1−x Sr x FeO 3 , (such as x=0.3), La 1−x Sr x Co y Fe 1−y O 3  (such as La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 ) and/or Pr 1−x Sr x MnO 3  (such as Pr 0.8 Sr 0.2 MnO 3 ), although other materials may be employed. For example, in some embodiments, the cathode  130   d  is formed of Ruddlesden-Popper nickelates and La 1−x Ca x MnO 3  (such as La 0.8 Ca 0.2 MnO 3 ) materials. The ionic phase may be YSZ containing from 3-8 mole percent yttria, or ScSZ containing 4-10 mole percent scandia and optionally a second dopant of Al, Y or ceria at minor content (about 1 mole percent) for high scandia stabilized zirconias (8-10ScSZ) to prevent formation of the rhombohedral phase. The electrochemically catalytic ceramic phase can comprise 40-60% by volume of the cathode. 
     In this example embodiment, the cathode current collector  130   e  is an electrode conductive layer formed of an electronically conductive ceramic and in many cases is similar in its chemistry to that of the electrochemically catalytic ceramic phase of the cathode. For example, a LSM+YSZ cathode will generally employ a LSM (La 1 -xSrxMnO 3 , x=0.1 to 0.3) cathode current collector. Other embodiments of the cathode current collector  130   e  may include at least one of LaNi x Fe 1−x O 3  (such as LaNi 0.6 Fe 0.4 O 3 ), La 1−x Sr x MnO 3  (such as La 0.75 Sr 0.25 MnO 3 ), doped lanthanum chromites (such as La 1−x Ca x CrO 3−δ , x=0.15−0.3), and/or Pr 1−x Sr x CoO 3 , such as Pr 0.8 Sr 0.2 CoO 3 . In other embodiments, the cathode current collector  130   e  may be formed of a precious metal cermet. The precious metals in the precious metal cermet may include, for example, Pt, Pd, Au, Ag, and/or alloys thereof. Non electrically conducting ceramic phase may also be included, for example, YSZ, ScSZ, and Al 2 O 3 , or other ceramic materials. One specific example of cathode current collector  130   e  material is 80 wt % Pd-20 wt % LSM. 
     In this example embodiment, the fuel cells  130  and the laterally segmented fuel cells  140 ,  150 ,  160 , and  170  are formed by depositing films/layers onto the upper and lower major surfaces  110   a  and  110   b  of the substrate  110 , such as by screen printing and/or inkjet printing, to form the porous anode barriers, the primary interconnects, the anode current collectors, and anodes, the electrolytes, the cathodes, and the cathode current collectors. In other embodiments, the films/layers may be deposited by one or more other techniques in addition to or instead of screen printing and/or inkjet printing. In various embodiments, one or more firing/sintering cycles are performed subsequent to depositing one or more films/layers. Other embodiments may not require any firing/sintering for one or more films/layers deposition. 
     A first fuel cell connector  145   a  is electrically connected to (and electrically connects) the first fuel cell portion  140   a  of the first laterally segmented fuel cell  140  and the first fuel cell portion  160   a  of the third laterally segmented fuel cell  160 . A second fuel cell connector  145   b  is electrically connected to (and electrically connects) the second fuel cell portion  140   b  of the first laterally segmented fuel cell  140  and the second fuel cell portion  160   b  of the third laterally segmented fuel cell  160 . A third fuel cell connector  155   a  is electrically connected to (and electrically connects) the first fuel cell portion  150   a  of the second laterally segmented fuel cell  150  and the first fuel cell portion  170   a  of the fourth laterally segmented fuel cell  170 . A fourth fuel cell connector  155   b  is electrically connected to (and electrically connects) the second fuel cell portion  150   b  of the second laterally segmented fuel cell  150  and the second fuel cell portion  170   b  of the fourth laterally segmented fuel cell  170 . 
     In this example embodiment, the first fuel cell connector  145   a  is electrically connected to (and in this example embodiment contacts) the cathode current collectors of the first fuel cell portions  140   a  and  160   a,  and the second fuel cell connector  145   b  is electrically connected to (and in this example embodiment contacts) the cathode current collectors of the second fuel cell portions  140   b  and  160   b.  Since the first and second fuel cell portions  140   a  and  140   b  are electrically isolated and the first and second fuel cell portions  160   a  and  160   b  are electrically isolated, the first and second fuel cell connectors  145   a  and  145   b  are electrically isolated such that there is no continuous electrical path across the width W of the tube (substrate  110 ). 
     In this example embodiment, the third fuel cell connector  155   a  is electrically connected to (and in this example embodiment contacts) the cathode current collectors of the first fuel cell portions  150   a  and  150   a,  and the fourth fuel cell connector  155   b  is electrically connected to (and in this example embodiment contacts) the cathode current collectors of the second fuel cell portions  170   b  and  170   b.  Since the first and second fuel cell portions  150   a  and  150   b  are electrically isolated and the first and second fuel cell portions  170   a  and  170   b  are electrically isolated, the third and fourth fuel cell connectors  155   a  and  155   b  are electrically isolated such that there is no continuous electrical path across the width W of the tube (substrate  110 ). 
       FIGS. 5-7  show six fuel cell tubes  100 ,  200 ,  300 ,  400 ,  500 , and  600  of the fuel cell stack  10 . While the fuel cell stack  10  may include any suitable quantity of fuel cell tubes electrically connected to one another in series, only six are shown here for clarity and brevity. In this example embodiment, the fuel cell tubes  200 ,  300 ,  400 ,  500 , and  600  are identical to the fuel cell tube  100  and are therefore not separately described (though in other embodiments the fuel cell tubes may differ from one another). The element numbering schemes of the fuel cell tubes  200 ,  300 ,  400 ,  500 , and  600  correspond to the element numbering scheme used to describe the fuel cell tube  100  such that like element numbers correspond to like components. 
     The first fuel cell tube  100  is electrically connected to the second fuel cell tube  200  via: (1) a first secondary interconnect  12   a  that electrically connects the third fuel cell connector  155   a  of the first fuel cell tube  100  to the third fuel cell connector  255   a  of the second fuel cell tube  200 ; and (2) a second secondary interconnect  12   b  that electrically connects the fourth fuel cell connector  155   b  of the first fuel cell tube  100  to the fourth fuel cell connector  255   b  of the second fuel cell tube  200 . Generally, the fuel cell tubes are connected in series with direction of the flow of fuel through the tubes. 
     The second fuel cell tube  200  is electrically connected to the third fuel cell tube  300  via: (1) a third secondary interconnect  23   a  that electrically connects the first fuel cell connector  245   a  of the second fuel cell tube  200  to the first fuel cell connector  345   a  of the third fuel cell tube  300 ; and (2) a fourth secondary interconnect  23   b  that electrically connects the second fuel cell connector  245   b  of the second fuel cell tube  200  to the second fuel cell connector  345   b  of the third fuel cell tube  300 . 
     The third fuel cell tube  300  is electrically connected to the fourth fuel cell tube  400  via: (1) a fifth secondary interconnect  34   a  that electrically connects the third fuel cell connector  355   a  of the third fuel cell tube  300  to the third fuel cell connector  455   a  of the fourth fuel cell tube  400 ; and (2) a sixth secondary interconnect  34   b  that electrically connects the fourth fuel cell connector  355   b  of the third fuel cell tube  300  to the fourth fuel cell connector  455   b  of the fourth fuel cell tube  400 . 
     The fourth fuel cell tube  400  is electrically connected to the fifth fuel cell tube  500  via: (1) a seventh secondary interconnect  45   a  that electrically connects the first fuel cell connector  445   a  of the fourth fuel cell tube  400  to the first fuel cell connector  545   a  of the fifth fuel cell tube  500 ; and (2) a eighth secondary interconnect  45   b  that electrically connects the second fuel cell connector  445   b  of the fourth fuel cell tube  400  to the second fuel cell connector  545   b  of the fifth fuel cell tube  500 . 
     The fifth fuel cell tube  500  is electrically connected to the sixth fuel cell tube 600 via: (1) a ninth secondary interconnect  56   a  that electrically connects the third fuel cell connector  555   a  of the fifth fuel cell tube  500  to the third fuel cell connector  655   a  of the sixth fuel cell tube  600 ; and (2) a tenth secondary interconnect  56   b  that electrically connects the fourth fuel cell connector  555   b  of the fifth fuel cell tube  500  to the fourth fuel cell connector  655   b  of the sixth fuel cell tube  600 . 
     Although not shown here, the first fuel cell tube  100  may be electrically connected to another fuel cell tube of the fuel cell stack  10  or to another fuel cell stack via the secondary interconnects shown but not labeled in  FIGS. 5 and 7 . Similarly, the sixth fuel cell tube  600  may be electrically connected to another fuel cell tube of the fuel cell stack  10  or to another fuel cell stack via the secondary interconnects shown but not labeled in  FIGS. 5 and 7 . 
     In operation, as oxidant is flowed past the cathodes of the fuel cells of the fuel cell tubes and as fuel is flowed through the fuel conduits of the substrates of the fuel cell tubes, the electrochemical reactions that occur at the cathodes and the anodes produce free electrons at the anodes. Within a particular fuel cell tube, those free electrons flow as electrical current from one fuel cell to the next (via the anode current collectors, the primary interconnects, and the cathode current collectors) in a particular direction. Once the electrical current reaches the final fuel cell in the fuel cell tube (here, a laterally segmented fuel cell), the electrical current flows via the fuel cell connectors and the secondary interconnects to the next fuel cell tube, and so on until reaching the electrical load. 
     For instance, as shown in  FIG. 5 , in this example embodiment, the electrical current I flows: (1) within the fuel cell tube  100  from the laterally segmented fuel cells  140  and  160  through the fuel cells  130  and to the laterally segmented fuel cells  150  and  170 ; (2) from the laterally segmented fuel cells  150  and  170  of the fuel cell tube  100  to the laterally segmented fuel cells  250  and  270  of the fuel cell tube  200  via the fuel cell connectors  155   a,    155   b,    255   a,  and  255   b  and the secondary interconnects  12   a  and  12   b;  (3) within the fuel cell tube  200  from the laterally segmented fuel cells  250  and  270  through the fuel cells  230  and to the laterally segmented fuel cells  240  and  260 ; (4) from the laterally segmented fuel cells  250  and  260  of the fuel cell tube  200  to the fuel cells  340  and  360  of the fuel cell tube  300  via the fuel cell connectors  245   a ,  245   b,    345   a,  and  345   b  and the secondary interconnects  23   a  and  23   b;  (5) within the fuel cell tube  300  from the laterally segmented fuel cells  340  and  360  through the fuel cells  330  and to the laterally segmented fuel cells  350  and  370 ; (6) from the laterally segmented fuel cells  350  and  370  of the fuel cell tube  300  to the laterally segmented fuel cells  450  and  470  of the fuel cell tube  400  via the fuel cell connectors  355   a,    355   b ,  455   a,  and  455   b  and the secondary interconnects  34   a  and  34   b;  (7) within the fuel cell tube  400  from electrically isolated the fuel cells  450  and  470  through the fuel cells  430  and to the laterally segmented fuel cells  440  and  460 ; (8) from the laterally segmented fuel cells  450  and  460  of the fuel cell tube  400  to the laterally segmented fuel cells  540  and  560  of the fuel cell tube  500  via the fuel cell connectors  445   a,    445   b,    545   a,  and  545   b  and the secondary interconnects  45   a  and  45   b;  (9) within the fuel cell tube  500  from the laterally segmented fuel cells  540  and  560  through the fuel cells  530  and to the laterally segmented fuel cells  550  and  570 ; (10) from the laterally segmented fuel cells  550  and  570  of the fuel cell tube  500  to the laterally segmented fuel cells  650  and  670  of the fuel cell tube  600  via the fuel cell connectors  555   a,    555   b,    655   a,  and  655   b  and the secondary interconnects  34   a  and  34   b;  (11) within the fuel cell tube  600  from the laterally segmented fuel cells  650  and  670  through the fuel cells  630  and to the laterally segmented fuel cells  640  and  660 ; and (12) from the laterally segmented fuel cells  640  and  660  of the fuel cell tube  600  to the electrical load (or to another fuel cell tube or fuel cell stack) via the fuel cell connectors  645   a  and  645   b.    
     For the fuel cell stack to conduct electrical current from one fuel cell tube to another, the secondary interconnects must be in working order, i.e., provide a path for the electrical current to flow from one fuel cell tube to the other. One way of checking whether a given secondary interconnect is in working order is by using an ohmmeter to attempt to flow an electrical current across that secondary interconnect and to calculate the resistance across that secondary interconnect. If the resistance is relatively low (e.g., negligible), the electrical current is able to flow across the secondary interconnect. But if the resistance is relatively high (e.g., infinite), the electrical current is not able to flow across the secondary interconnect, and the secondary interconnect is damaged and must be repaired or replaced to ensure proper fuel cell stack operation. 
     Since prior art fuel cell tubes do not include laterally segmented fuel cells, their fuel cell connectors are electrically connected to laterally continuous fuel cells. As described below, this leads to ohmmeters generating false positive readings in certain instances when calculating the resistance across a particular secondary interconnect. That is, in certain instances, the ohmmeter calculates a relatively low resistance across a given secondary interconnect—and thus indicates a working secondary interconnect—when in reality that secondary interconnect is damaged such that it electrical current cannot flow through it. 
       FIGS. 8A-8D  show a negative ohmmeter probe N and a positive ohmmeter probe P positioned to attempt to flow an electrical current I across a secondary interconnect  1012   b  that electrically connects prior art fuel cell tubes  1100  and  1200 . Opposing secondary interconnect  1012   a  also electrically connects the prior art fuel cells  1100  and  1200 . The fuel cell connectors (not labeled) of the fuel cell tubes  1100  and  1200  are electrically connected to laterally continuous fuel cells. 
     In the scenario shown in  FIG. 8A , the secondary interconnects  1012   a  and  1012   b  are both in working order. The ohmmeter calculates a low resistance because the secondary interconnect  1012   b  is in working order and the electrical current I can flow across the secondary interconnect  1012   b  from the negative probe N to the positive probe P. 
     In the scenario shown in  FIG. 8B , the secondary interconnect  1012   a  is in working order while the secondary interconnect  1012   b  is damaged such that electrical current cannot flow through it. But rather than calculate a high resistance that correspond to electrical current not being able to flow across the secondary interconnect  1012   b,  the ohmmeter calculates a low resistance because the electrical current flows from the negative probe N across the laterally continuous fuel cells of the fuel cell tube  1100 , across the secondary interconnect  1012   a,  and across the laterally continuous fuel cells of the fuel cell tube  1200  to the positive probe P. In other words, the laterally continuous fuel cells provide a low-resistance path for the electrical current I to flow from the negative probe N to the positive probe P, so the electrical current does so and causes the ohmmeter to calculate a low resistance that does not reflect the damaged state of the secondary interconnect  1012   b.    
     In the scenario shown in  FIG. 8C , the secondary interconnect  1012   a  is damaged such that electrical current cannot flow through it while the secondary interconnect  1012   b  is in working order. The ohmmeter calculates a low resistance because the secondary interconnect  1012   b  is in working order and the electrical current I can flow across the secondary interconnect  1012   b  from the negative probe N to the positive probe P. 
     In the scenario shown in  FIG. 8D , the secondary interconnects  1012   a  and  1012   b  are damaged such that electrical current cannot flow through them. The ohmmeter calculates a high resistance because electrical current cannot flow through either of the secondary interconnects  1012   a  or  1012   b  from the negative probe N to the positive probe P. 
     The fuel cell tubes with laterally segmented fuel cells of the present disclosure solve this problem. As explained above, the fuel cell connectors of the fuel cell tubes of the present disclosure are electrically connected to laterally segmented fuel cells, which means only one low-resistance electrical path exists when attempting to flow electrical current across a secondary interconnect during resistance testing. 
       FIGS. 9A-9D  show negative probe N and positive probe P of the ohmmeter described above positioned to attempt to flow an electrical current across the secondary interconnect  12   b.    
     In the scenario shown in  FIG. 9A , the secondary interconnects  12   a  and  12   b  are both in working order. The ohmmeter calculates a low resistance because the secondary interconnect  12   b  is in working order and the electrical current I can flow across the secondary interconnect  12   b  from the negative probe N to the positive probe P. 
     In the scenario shown in  FIG. 9B , the secondary interconnect  12   a  is in working order while the secondary interconnect  12   b  is damaged such that electrical current cannot flow through it. The ohmmeter calculates a high resistance because electrical current cannot flow through the secondary interconnect  12   b  from the negative probe N to the positive probe P. Additionally, electrical current cannot flow from the negative probe N to the positive probe P through the secondary interconnect  12   a  because a low-resistance electrical path does not exist between the negative probe N and the positive probe P through the secondary interconnect  12   a  due to the laterally segmented fuel cells. 
     In the scenario shown in  FIG. 9C , the secondary interconnect  12   a  is damaged such that electrical current cannot flow through it while the secondary interconnect  12   b  is in working order. The ohmmeter calculates a low resistance because the secondary interconnect  12   b  is in working order and the electrical current I can flow across the secondary interconnect  12   b  from the negative probe N to the positive probe P. 
     In the scenario shown in  FIG. 9D , the secondary interconnects  12   a  and  12   b  are damaged such that electrical current cannot flow through them. The ohmmeter calculates a high resistance because electrical current cannot flow through either of the secondary interconnects  12   a  or  12   b  from the negative probe N to the positive probe P. 
     The secondary interconnects shown in the various embodiments (for example  12   a  and  12   b  in  FIGS. 9A-9D ) are illustrated as wires for exemplary purposes only. The present disclosure pertains to other designs for secondary interconnects such as the designs disclosed in the following co-pending applications: U.S. patent application Ser. No. 15/816,918, filed Nov. 17, 2017, entitled “Improved Fuel Cell Secondary Interconnect”; U.S. patent application Ser. No. 15/816,931, filed Nov. 17, 2017, entitled “Improved Fuel Cell Secondary Interconnect”; and U.S. patent application Ser. No. 15/816,948, filed Nov. 17, 2017, entitled “Multiple Fuel Cell Secondary Interconnect Bonding Pads And Wires”. 
     Another benefit is that the use of laterally segmented fuel cells has a negligible effect on the performance of a given fuel cell tube because the electrical current density at the location of the space between the fuel cell portions is low because the electrical current is concentrated at the fuel cell connectors (through which the electrical current flows to the next fuel cell tube). 
     Various modifications to the embodiments described herein will be apparent to those skilled in the art. These modifications can be made without departing from the spirit and scope of the present disclosure and without diminishing its intended advantages. It is intended that such changes and modifications be covered by the appended claims.