Patent Application
20120094205
The present disclosure relates to a solid oxide fuel cell (SOFC) stack having a compliant glass seal; more specifically, to a compliant glass seal composition for a SOFC 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, to form a cell-retainer frame assembly. The individual cell-retainer frame assembly is then joined to a metal 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.
Seals for SOFC stacks require special properties such as a coefficient of thermal expansion (CTE) comparable to those of the 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 glass seals 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), are known to be utilized for SOFC stack sealing applications. 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. However, G-18 glass crystallizes at prolonged elevated operating temperatures becoming hard and brittle over time, thereby losing its ability to fill in gaps in the sealing surfaces and its ability to provide a hermetic seal. Another drawback for glass seals is that the sealing surfaces of the SOFC stack need to be preoxidized before the application of the glass seal to ensure a hermetic seal at operating temperatures.
Another known type of seals for SOFC stack sealing applications are composite glass seals, which are formed from glass materials mixed with fibers. A drawback for composite glass seals is that the CTE of the composite glass seal materials have to be specifically matched with that of the CTE of the mating parts. Furthermore, over time at sustained operating conditions and repeated thermal cycling of the SOFC stack, composite glass seals tend to degrade and become prone to seal leaks due to the formation of microscopic cracks.
Compliant metallic seals formed of deformable metallic materials are also known to be used for SOFC stack. Compliant metallic seals are desirable due to its resistant to oxidation and its CTE is comparable to the CTE of the mating materials of the SOFC stack. The metallic materials used for the compliant metallic seals include noble metals, which are expensive and are subject to degradation at SOFC operating environments. Another drawback of utilizing noble metals as a compliant seal is that at operating temperatures of the SOFC, the metals tend to migrate toward and poison the SOFC.
Based on the foregoing, there is a long felt need for an improved compliant seal that is chemically and mechanically stable under long-term operating and thermal cycling conditions, does not contaminate or otherwise adversely affect fuel cell performance, and yet economical to produce.
The present invention relates to a SOFC stack having an improved compliant glass seal disposed between the sealing surfaces of adjacent cassettes and between the sealing surfaces of the fuel cell and retainer frame. The compliant glass seal includes a glass, a ceramic fiber, and at least one metal selected from Groups 9, 10, and 11 of the periodic table. The glass includes an alkaline earth aluminosilicate (AEAS), the ceramic fiber includes YSZ, and the metal includes a metal selected from a group consisting of Ag, Ni, Pd, Pt, and Rh.
The compliant glass seal includes a combined weight percentage of Ag and YSZ fibers of 30 to 42.5 weight percent, preferably 37.5 weight percent. It is further preferred that Ag is 15 weight percent and YSZ is 22.5 weight percent of the compliant glass seal. The compliant glass seal provides the benefits of maintaining a robust hermetic seal over prolonged elevated temperature and repeated thermal cycling of a SOFC stack. The improve seal long term stability is due to improved seal morphology, thereby promoting bonding between glass and steel alloy substrate. The CTE of the improved compliant glass seal composition matches the base substrate materials more closely due to the addition of the at least one metal selected from Groups 9, 10, and 11 of the periodic table, preferably Ag.
The improved compliant glass seal provides at least the advantages of maintaining a compliant, superior bonding of non-preoxidized aluminized sealing surfaces, and reduced complexity in manufacturing 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:
In accordance with a preferred embodiment of this invention, referring to
Shown in
For illustrative purposes, the second cassette 32b is shown in an exploded view to detail the components that form each of the cassettes 32a, 32b, 32c, 32d. The second cassette 32b includes a fuel cell 10 mounted within a retainer frame 22. The fuel cell 10 is comprised of an electrolyte layer (not shown) sandwiched between a cathode layer 12 and an anode layer 16. The retainer frame 22 defines a central opening or picture frame window 23. The fuel cell 10 is positioned in the picture frame window 23 and joined to the retainer frame 22 to form a cell-retainer frame assembly 24. The compliant glass seal 42 may be used as a bonding material to bond the fuel cell 10 to the retainer frame 22, thereby fixing the fuel cell 10 within the picture frame window 23.
An intermediate process joins together the cell-retainer frame assembly 24, anode spacers 29, an anode interconnect 30, a cathode interconnect 35, and a separator plate 28 to form the complete second cassette 32b. The first, third, and forth fuel cell cassettes 32a, 32c, 32d are formed with similar components as that of the second cassette 32b. Each of the cassettes 32a, 32b, 32c, 32d includes sealing surfaces 36, in which the sealing surfaces 36 of each cassette are complementary to the corresponding sealing surfaces 36 of the adjacent cassette to which it is joined. The sealing surfaces 36 may include the surfaces of the cell-retainer assembly 24 where the fuel cell 10 is bonded to the retainer frame 22. The cassettes 32a, 32b, 32c, 32d are then assembled in series to form a SOFC stack 26.
During the assembly of the cassettes 32a, 32b, 32c, 32d into a SOFC Stack 26, the compliant glass seal 42 composite, in the form of a paste or tape, is disposed between the sealing surfaces 36 of the adjacent cassettes 32a, 32b, 32c, 32d. In the prior art, it is known to aluminize and then preoxide the aluminized sealing surfaces 36 before the application of the seal material, thereby ensuring a hermetic seal at operating temperatures. The process of aluminizing and then preoxidizing the aluminized sealing surfaces is labor and energy intensive. The process includes applying a layer of aluminum on the sealing surfaces and then heat treating to diffuse the aluminum into the steel substrate. The aluminized steel substrate is then preoxidized to form a layer of aluminum oxide.
It was unexpectedly found that at certain combined weight percentages of metals selected from Groups 9, 10, and 11 of the Periodic Table, such as Ag, Ni, Pd, Pt, and Rh, and YSZ fibers in an AEAS glass offer improved desirable seal qualities. It was further unexpectedly found that at certain combinations of weight percentages of metallic Ag and YSZ fibers in the AEAS glass offer improved joining and bonding of the sealing surfaces of the SOFC stack 26. It was still further unexpectedly found that these certain combinations of weight percentages of metallic Ag and YSZ fibers in the AEAS glass provide a compliant glass seal 42 that offers superior bonding of non-preoxidized aluminized sealing surfaces over preoxidized aluminized sealing surfaces. This latter finding is contrary to the teachings of the prior art and offers the advantages of eliminating the labor intensive and costly step of preoxidizing the aluminized sealing surfaces 36.
The improved compliant glass seal 42 composition includes an organic binder, an alkaline earth aluminosilicate (AEAS) based glass, at least one metal selected from a group consisting of Ag, Ni, Pd, Pt, and Rh, and a plurality of yttria-stabilized zirconia (YSZ) fibers for strength. The compliant glass seal 42 composition is applied to the sealing surfaces 36 of the SOFC stack 26 as disclosed above and heat treated to form the compliant glass seal 42 joining and bonding the sealing surfaces 36 to provide a hermetic seal containing the reactant gases and electrically isolate the adjacent separator plates 28. During the heat treating process, the organic binder is flashed off leaving a solid portion comprising of at least one metal, YSZ fibers, and AEAS glass.
Shown in Table 1 below are compliant glass seal 42 compositions having the preferred weight percentages of metallic Ag, YSZ fibers, and AEAS glass that offer the unexpected results mentioned above. Also shown are preferred combined weight percentages of metallic Ag and YSZ fiber in the AEAS glass.
Shown in
Each test sample of the first batch included a glass seal tape sandwiched between two pre-oxidized Crofer discs. The test samples of the first batch were heated treated to cure the glass seal. Each test samples of the second batch included a compliance glass seal tape having an organic binder, metallic Ag, YSZ fibers, and AEAS glass sandwiched between two pre-oxidized Crofer discs. The test samples of the second batch were heated treated to flash off the binder and cure the compliance seal leaving 9.90% (wt) Metallic Ag, 14.85% (wt) YSZ fiber, and 41.25% (wt) AEAS glass. Each test samples of the third batch also included a compliance seal having a binder, metallic silver, YSZ fibers, and AEAS glass sandwiched between two non-preoxidized Crofer discs. The test samples of the third batch were heated treated to flash off the binder and cure the compliance seal, also leaving 9.90% (wt) Metallic Ag, 14.85% (wt) YSZ fiber, and 41.25% (wt) AEAS glass.
Each batch contained approximately 10 test samples each. Each test sample includes two 0.5″×2.5″ Crofer strips with a seal therebetween, lap joint together. The overlap bonding area is 0.5″×0.5″. The pull test was performed on an Instron model 5566 at a pull speed of 5 mm/min.
With reference to
Shown in
With reference to
The compliant glass seal 42 provides the advantages of maintaining a compliant robust hermetic seal over prolonged elevated temperature and repeated thermal cycling of a SOFC stack 26. The compliant glass seal 42 further provides the advantage of superior bonding of non-preoxidized aluminized sealing surfaces over preoxidized aluminized sealing surfaces; therefore allowing the elimination of the process of preoxidizing the aluminized sealing surfaces in the manufacturing of SOFC stacks.
While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.
