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.
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.
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 CO2 and thereby creating an electrical potential between the anode layer and the cathode layer of about 1 volt.
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.
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.
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.
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.
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.
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.
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's sealing surface.
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.
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.
This invention will be further described with reference to the accompanying drawings in which:
Referring to
Shown in
At the heart of each of the cassettes 32a, 32b, 32c, 32d, 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 32a, 32b, 32c, 32d are then stacked in series to form the SOFC stack 26.
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 36a. Similarly, the separator plate 28 includes a separator plate perimeter portion 34 that defines a separator plate sealing surface 36b. The retainer plate sealing surface 36a faces in a direction opposite that of the separator plate sealing surface 36b. During the assembly of the cassettes 32a, 32b, 32c, 32d into the SOFC stack 26, the retainer plate sealing surface 36a of each cassette is oriented toward and is complementary in shape to the separator plate sealing surface 36b 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 36a and corresponding separator plate sealing surfaces 36b 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
For illustrative purposes,
Referring to
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 36a, 36b 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 36a, 36b 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 36a, 36b 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 36a, 36b.
Shown in
Similarly to the embodiment shown in
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The features of the perforations, depressions, and protrusions as shown in
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.