Abstract:
A solid oxide fuel cell (SOFC) stack having a glass seal sandwiched between the sealing surfaces of adjacent cassettes, in which at least one cassette includes means for interlocking the glass seal onto the sealing surface of the cassette for improved adhesion and durability of the glass seal. The at least one cassette includes a plurality of perforations configured to receive and lock onto a portion of the glass seal. At least one of the perforations includes a through-hole having an exterior opening on the sealing surface and an interior opening on the interior surface of the cassette. A portion of the glass seal is received in the perforation forming a glass column in the through-hole and a flared glass end on the interior surface surrounding the interior opening. The flared glass end cooperates with the glass column to interlock the glass seal onto the cassette&#39;s sealing surface.

Description:
TECHNICAL FIELD OF INVENTION 
       [0001]    The present disclosure relates to a fuel cell stack having a glass seal; more specifically, to a fuel cell stack having mechanical means to enhance adhesion of a glass seal to the sealing surfaces of the fuel stack. 
       BACKGROUND OF INVENTION 
       [0002]    Fuel cells are used to produce electricity when supplied with fuels containing hydrogen and an oxidant such as air. A typical fuel cell includes an ion conductive electrolyte layer sandwiched between an anode layer and a cathode layer. There are several different types of fuel cells known in the art; amongst these are solid oxide fuel cells (SOFC). SOFC are regarded as highly efficient electrical power generator that produces high power density with fuel flexibility. 
         [0003]    In a typical SOFC, air is passed over the surface of the cathode layer and a reformate fuel is passed over the surface of the anode layer opposite that of the cathode layer. Oxygen ions from the air migrate from the cathode layer through the dense electrolyte to the anode layer in which it reacts with the hydrogen and CO in the fuel, forming water and CO 2  and thereby creating an electrical potential between the anode layer and the cathode layer of about 1 volt. 
         [0004]    Each individual SOFC is mounted within a metal frame, referred to in the art as a retainer frame, to form a cell-retainer frame assembly. The individual cell-retainer frame assembly is then joined to a metallic separator plate, also known as an interconnector plate, to form a fuel cell cassette. Multiple cassettes are stacked in series with a seal disposed between the sealing surfaces of each cassette to form a SOFC stack. The seal for SOFC stacks requires special properties such as a coefficient of thermal expansion (CTE) comparable to those of the mating components of the SOFC stacks, a suitable viscosity to fill any gaps in the sealing surfaces of the cassettes, ability to maintain a hermetic seal at operating temperatures of about 500° C.-1000° C., good chemical stability, and long term sustainability. 
         [0005]    Typical seals utilized for SOFC stack sealing applications are formed from an alkaline earth aluminosilicate glass, such as a barium-calcium-aluminosilicate based glass, also known as G-18 glass, developed by Pacific Northwest National Laboratory (PNNL). G-18 glass provides a seal material that offers high electrical resistively, high coefficient of thermal expansion, high glass transition temperature, and good chemical stability. Another known type of seals for SOFC stack sealing applications are composite glass seals, which are formed from glass materials mixed with fibers to increase the structural integrity of the glass matrix. 
         [0006]    One of the disadvantages of the known glass seals is that the glass matrix crystallizes over time at sustained high temperature operating conditions and repeated thermal cycling of the SOFC stack. As the glass crystallizes, it tends to become prone to form microscopic fractures along the interface of the glass seal and sealing surfaces of the cassettes; thereby resulting in potential air and fuel leaks, especially in high stress areas of the SOFC stack. 
         [0007]    Based on the foregoing, there is a long felt need for improved adhesion strength between the glass seal and the sealing surfaces of the cassettes. There is a further need for the glass seal joining adjacent cassettes to be mechanically stable under long-term operation and thermal cycling conditions. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention relates to a solid oxide fuel cell (SOFC) stack having a glass seal sandwiched between the sealing surfaces of adjacent cassettes, in which at least one cassette includes means for interlocking the glass seal onto the sealing surface for improved adhesion and durability of the glass seal. 
         [0009]    The SOFC stack includes a first cassette having a first cassette portion defining a first cassette sealing surface and a second cassette having a second cassette portion defining a second cassette sealing surface complementary to the first cassette sealing surface. The second cassette is disposed proximate to the first cassette such that the second cassette sealing surface is oriented toward and immediately adjacent to the first cassette sealing surface. A glass seal is disposed between and onto the first and second cassette sealing surfaces, thereby joining the first cassette to the second cassette. 
         [0010]    The first cassette portion defines a plurality of perforations configured to receive a portion of the glass seal to interlock the glass seal to the first cassette sealing surface. At least one of the perforations includes through-hole having an exterior opening on the exterior sealing surface and an interior opening on the interior surface of the cassette. A portion of the glass seal is received in the perforation forming a glass column in the through-hole and a flared glass end about the interior surface surrounding the interior opening. The flared glass end cooperates with the glass column to interlock the glass seal onto the cassette&#39;s sealing surface. 
         [0011]    One advantage of perforations through the cassette sealing portion is that the through-holes provide additional surface area for the adhesion of the glass seal. Another advantage is that the flared glass ends interlocks the glass seal onto the sealing surfaces. Still another advantage is that the interlocked glass seal distributes the joint stress though a greater area of the sealing surfaces, thereby improving adhesion strength and increasing mechanically stability under long-term operation and thermal cycling conditions of the SOFC stack. 
         [0012]    Further features and advantages of the invention will appear more clearly on a reading of the following detailed description of an embodiment of the invention, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]    This invention will be further described with reference to the accompanying drawings in which: 
           [0014]      FIG. 1  shows an exploded isometric drawing of a portion of a SOFC stack employing a plurality of single-cell cassettes. 
           [0015]      FIG. 2  shows a partial cross-section of three adjacent cassettes having mechanical means to enhance the adhesion of the glass seals to the sealing surfaces of the adjacent cassettes. 
           [0016]      FIGS. 3 through 5  show alternative embodiments of the mechanical means to enhance the adhesion of the glass seal between two adjacent cassettes. 
           [0017]      FIGS. 3A through 5A  show detail views of the alternative embodiments shown in  FIGS. 3 through 5 , respectively. 
           [0018]      FIG. 6  is an exploded partial perspective view of the embodiment shown in  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF INVENTION 
       [0019]    Referring to  FIGS. 1 through 6  is a solid oxide fuel cell (SOFC) stack  26  having a glass seal  41 , such as an alkaline earth aluminosilicate (AEAS) glass, sandwiched between the sealing surfaces  36   a,    36   b  of adjacent cassettes  32   a,    32   b,    32   c,    32   d.  The cassettes  32   a,    32   b,    32   c,    32   d  include means for interlocking the glass seal  41  onto the sealing surfaces  36   a,    36   b  for improved adhesion and durability of the glass seal  41 . 
         [0020]    Shown in  FIG. 1  an exploded isometric drawing of a portion of the SOFC stack  26  employing a plurality of single-cell cassettes  32   a,    32   b,    32   c,    32   d.  The first and second cassettes  32   a,    32   b  and the glass seal  41  therebetween are shown spaced apart from each other. For illustrative purposes, the second cassette  32   b  is shown in an exploded view to detail the components that similarly form each of the cassettes  32   a,    32   b,    32   c,    32   d.  The third and fourth cassettes  32   c,    32   d  are shown jointly sealed to each other with the glass seal  41  sandwiched therebetween. 
         [0021]    At the heart of each of the cassettes  32   a,    32   b,    32   c,    32   d,  is a fuel cell  10  comprising of an electrolyte layer sandwiched between a cathode layer and an anode layer. The fuel cell  10  is assembled onto a picture frame window  23  defined by a retainer frame plate  22 , thereby forming a cell-retainer frame assembly  24 . An intermediate process joins together the cell-retainer frame assembly  24 , anode spacers  29 , an anode interconnect  30 , a cathode interconnect  31 , and a separator plate  28  to form the individual cassettes. A plurality of cassettes  32   a,    32   b,    32   c,    32   d  are then stacked in series to form the SOFC stack  26 . 
         [0022]    The retainer frame plate  22  and the separator plate  28  may be manufactured from a metallic substrate such as stainless steel. The retainer frame plate  22  includes a retainer plate perimeter portion  33  that defines a retainer plate sealing surface  36   a.  Similarly, the separator plate  28  includes a separator plate perimeter portion  34  that defines a separator plate sealing surface  36   b.  The retainer plate sealing surface  36   a  faces in a direction opposite that of the separator plate sealing surface  36   b.  During the assembly of the cassettes  32   a,    32   b,    32   c,    32   d  into the SOFC stack  26 , the retainer plate sealing surface  36   a  of each cassette is oriented toward and is complementary in shape to the separator plate sealing surface  36   b  of the immediate adjacent cassette to which it is joined. An uncured glass seal composite, in the form of a paste or tape, is inserted between the retainer plate sealing surfaces  36   a  and corresponding separator plate sealing surfaces  36   b  of adjacent cassettes. The assembled SOFC stack  26  is then heated treated at a sufficient time and temperature to cure the glass seal composite into a compliant glass seal  41 . Shown in  FIGS. 2 through 5 , the perimeter portions  33 ,  34  of the retainer frame plate  22  and separator plate  28 , respectively, may include mechanical features that cooperate with the glass seal composite as it cures during the heat treatment process resulting in an interlocking compliant glass seal  41 . 
         [0023]    For illustrative purposes,  FIG. 2  shows a partial cross-section of a center cassette  42  sandwiched between an upper cassette  40  and a lower cassette  44 . It should be understood that the upper and lower cassettes  40 ,  44  may have corresponding components and features that are similar, if not identical, to the center cassette  42 . Furthermore, the terms upper and lower are used to indicate the relative position of the cassettes shown in the figures and are not meant to be limiting. By way of non-limiting example, the upper, center, and lower cassettes  40 ,  42 ,  44  each may include a cell-retainer frame assembly  24  having a retainer frame plate  22  mated onto a separator plate  28  as previously described above. 
         [0024]    Referring to  FIG. 2 , the center cassette  42  is shown engaged to the separator plate  28  of the upper cassette  40  and the retainer frame plate  22  of the lower cassette. The retainer plate sealing surface  36   a  of the center cassette  42  is adjacent to and oriented toward the corresponding separator plate sealing surface  36   b  of the immediate adjacent upper cassette  40 . The separator plate sealing surface  36   b  of the center cassette is immediately adjacent to and oriented toward the retainer plate sealing surface  36   a  of the adjacent lower cassette  44 . Inserted between the corresponding sealing surfaces  36   a,    36   b  of the adjacent cassettes is the glass seal  41 . The perimeter portions  33 ,  34  of the retainer frame plates  22  and separator plates  28 , respectively, of the cassettes  40 ,  42 ,  42  onto which a glass seals  41  is disposed defines a plurality of perforations  46 . The perforations include through-holes  48  that lead from the sealing surfaces  36   a,    36   b  of the respective plates to the interior surfaces  53  of the respective plates. The through-holes  48  may be substantially perpendicular to the sealing surfaces  36   a,    36   b  or may be at an angle with respect to the sealing surfaces  36   a,    36   b.  Each of the through-holes includes an interior opening  50  and an exterior opening  52 , in which the interior opening  50  is facing the interior of the cassette and the exterior opening  52  is facing the sealing surface  36   a,    36   b  of the adjacent cassette. 
         [0025]    During the assembly of the cassettes as describe above, a glass seal composite, in the form of a paste or tape, is disposed between the sealing surface  36   a,    36   b  of the retainer frame plate  22  of the center cassette  42  and the separator plate  28  of the upper cassette  40 . A glass seal composite is also disposed between the sealing surfaces  36   a,    36   b  of the retainer frame plate  22  of the lower cassette  44  and the separator plate  28  of the center cassette  42 . An axial compression force F is placed onto the assembled SOFC stack  26  while the glass composite is heated treated to flash off any volatile binder and cure the gas seal composite, thereby joining and bonding the center cassette  42  to both the upper and lower cassettes  40 ,  44 , and as well as providing a hermetic seal between the cassettes  40 ,  42 ,  44 . During the heat treatment process, the glass seal composite transitions into a partially molten state. As the cassettes  40 ,  42 ,  44  are compressed to set the SOFC stack  26 , a portion of the molten glass composite flows into the through-holes  48  under pressure and capillary forces. As the partially molten glass exits the opposite interior openings  50  of the through-holes  48 , the adhesion force of the glass causes the molten glass to conglomerate onto a portion of the interior surface  53  surrounding each of the interior opening  50  forming a flared glass end  62  that has a diameter larger than the diameter of the through-holes  48 . As the glass composite cools, a glass column  63  is formed within each of the through-holes  48  and cooperates with the flared glass end  62  to interlock the compliant glass seal  41  onto the respective sealing surfaces  36   a,    36   b  of the cassettes  40 ,  42 ,  44 . The increased in surface area provided by the perforations  46  also assists in the adhesion of the glass seal to the respective sealing surfaces  36   a,    36   b.    
         [0026]    Shown in  FIG. 3 , and in detail view  FIG. 3   a , is an alternative embodiment of the invention. For illustrative purposes, only the retainer frame plate  22  of the center cassette  42  is shown joined to the separator plate  28  of the upper cassette  40  with a glass seal  42  therebetween. Similarly to the embodiment as shown in  FIG. 2 , the perforations  46  shown in  FIG. 3  include through-holes  48  that lead from the sealing surfaces  36   a,    36   b  of the respective plates to the interior surfaces  53  of the respective plates with associated exterior openings  52  and interior openings  50 . In the embodiment shown in  FIG. 3 , the respective sealing surfaces  36   a,    36   b  of the retainer frame plate  22  and separator plate  28  define a plurality of protrusions  56  having a conical, frustoconical, or semi-spherical shape. Each of the exterior opening  52  is defined substantially within the center of a protrusion  56  as shown in  FIG. 3   a . On the interior surface  53  opposite that of the sealing surface  36   a,    36   b,  each of the interior openings  50  defines a depression  60  on a portion of the interior surface  53  surrounding the interior opening  50 . 
         [0027]    Similarly to the embodiment shown in  FIG. 2 , during the heat treatment process, as the cassettes are compressed to set the SOFC stack, a portion of the molten glass composite flows into the through-holes  48 . As the partially molten glass exits the opposite interior openings  50  of the through-holes  48 , adhesion forces cause the molten glass composite to conglomerate within and about the depressions  60 . The depressions  60  assist in molding the molten glass into a flared glass end  62  of a predetermined shape and size based on the shape and size of the depressions  60 . As the glass composite cools and solidifies into the glass seal  41 , the glass seal  41  forms a glass column  63  within each of the through-holes  48  that cooperates with the flared glass end  62  to interlock the compliant glass seal  41  onto the respective sealing surfaces  36   a,    36   b  of the adjacent cassettes  42 ,  44 . The protrusions  56  may be aligned and sized to provide and maintain a predetermined gap distance between adjacent cassettes and may be offset to allow a narrower gap between the cassettes  42 ,  44 , thereby maintaining a predetermined thickness of the glass seal  41 . 
         [0028]      FIG. 4  and detailed  FIG. 4   a  show another alternative embodiment of the invention. In contrast to the embodiment shown in  FIG. 3 , the frustoconical shaped protrusion  56  surrounds the peripheral of the interior opening  50  and the conical shaped depression  60  surround the exterior opening  52 . The conical shaped depression  60  defined in the sealing surfaces  36   a,    36   b  shown in  FIG. 4   a  aids in funneling the flow of the molten glass composite into the through-holes  48  during the heat treatment process of curing the glass composite. Also, the frustoconical shaped protrusions increases the interior surface area and provides a tow-hook onto which the flared glass end  62  may be conglomerated onto and locked into. In the embodiments shown in  FIGS. 3 and 4 , the features of the through-holes having protrusions and depressions may be formed by any mechanical means known in the art including puncturing, piercing, extruding, lancing, and drawing. 
         [0029]    Shown in  FIG. 5  is another alternative embodiment of the invention in which the sealing surfaces  36   a,    36   b  are skived to provide a plurality of edge shaped protrusions  64 . Therebetween the edge shaped protrusions  64  are a plurality of perforations  46  as described above. The edged shaped protrusions  64  assist in locking the glass seal  41  as well as providing additional surface area for the glass seal  41  to bond onto the metallic substrate of the plates. The perforations  46  are shown between the skived edges. 
         [0030]    The features of the perforations, depressions, and protrusions as shown in  FIGS. 2 through 5  increase the active surface area for which the glass seal  41  may bond onto and provide tow-hooks to interlock the glass seal  41 . Shown in  FIG. 6  is a perspective exploded view of the retainer frame plate  22  of the center cassette  42  and the adjacent separator plate  28  of the upper cassette  40  of  FIG. 2 . The plurality of protrusions  56  on the sealing surfaces  36   a,    36   b  and corresponding depressions on the non-sealing interior surfaces  53  with through-holes  48  therethrough is similar to the texture of that of a fine cheese grater. The interlocking features disclosed above provide increased adhesion strength between the glass seal and the metallic substrate that is mechanically stable under long-term operation and thermal cycling conditions, does not contaminate or otherwise adversely affect fuel cell performance, and yet economical to produce. 
         [0031]    While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the intentions without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.