Solid oxide fuel cell generator including a glass sealant

A solid oxide fuel cell generator is provided for electrochemically reacting a fuel gas with a flowing oxidant gas at an elevated temperature to produce power. The generator includes a generator section receiving a fuel gas and a plurality of elongated fuel cells extending through the generator section and having opposing open fuel cell ends for directing an oxidant gas between opposing plena in the generator. A sealant defines a seal on the fuel cells adjacent at least one of the fuel cell ends. The sealant is a modified lanthanum borate aluminosilicate glass composition having a minimal amount of boron oxide and silica, and in which the sealant maintains substantially constant physical characteristics throughout multiple thermal cycles.

FIELD OF THE INVENTION

The present invention relates to fuel cells and, more particularly, to fuel cell generators including fuel cells that are open at both ends and a glass sealant composition for preventing flow of gases between adjacent plena in the fuel cell generator.

BACKGROUND OF THE INVENTION

The dynamics of fuel cell stacks require that preferably fuel enters the stack from the opposite end of where the ambient air enters. As the fuel travels over cells, down the length of the stack, it is subject to electrochemical reactions. Typically a fraction of the unused fuel/combustion products mixture is recirculated, while a fraction is mixed with vitiated air to satisfy the mass balance. In prior art tubular solid oxide fuel cell (SOFC) generators, the SOFC geometry is cylindrical with one closed end. Fuel enters the stack at the cell closed end and flows upward in the space surrounding the cells. Air enters each cell through an air feed tube (AFT) concentrically positioned within the cell, exits the AFT at the closed end, and flows upward in the annular space between the AFT and the cell. As the fuel and air flow from the cell closed end to the open end, most of the fuel is electrochemically reacted with oxygen from the air producing electricity. The depleted fuel exiting the cell stack, which typically consists of 20% (H2+CO) and 80% (H2O+CO2), is combusted with the vitiated air exiting the cell in a combustion zone above the cell open ends to create exhaust gas. In this configuration no seals are required to separate the fuel stream from the air stream due to the closed end design of the tubular SOFC and the use of AFT's. A known construction for this type of fuel cell is described in U.S. Pat. No. 6,764,784.

In a known method of manufacturing closed end fuel cells, the interconnection between cells, the electrolyte and the fuel electrode layers are deposited on an extruded and sintered lanthanum manganite air electrode tube by plasma spray. A lanthanum chromite interconnection is provided in the form of a narrow strip that runs axially over the entire active length of the cell. A yttria stabilized zirconia electrolyte is deposited in such a way as to almost entirely cover the cell. The electrolyte layer overlaps the edges of the interconnection strip but leaves most of the interconnection exposed. Because the interconnection and electrolyte layers are dense, the overlap feature provides a seal that prevents direct mixing of air and fuel gas. A nickel/yttria stabilized zirconia cermet fuel electrode layer is deposited in such a way as to almost entirely cover the electrolyte, but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode. This margin prevents shorting of the cell. Series electrical connections between cells are accomplished by means of a structure made from nickel foam and nickel screen. The foam part of which becomes sintered to the interconnection while the screen part becomes sintered to the fuel electrode of the adjacent cell. A solid oxide fuel cell according to this construction is described in U.S. Pat. No. 7,157,172, which patent is incorporated herein by reference.

In the above described closed end fuel cell designs, the closed end is a highly stressed region during plasma spray operations performed during manufacturing to deposit the interconnections, the electrolyte, and the fuel electrode. Elimination of the closed end may be expected to reduce failure due to thermal stress and thus significantly increase the yield during manufacturing. Furthermore, elimination of the air feed tubes would represent a substantial cost savings and a design simplification.

There is a continuing need for a fuel cell stack construction that addresses problems associated with manufacturing of fuel cell stacks, while providing a high power density and increased operating efficiencies.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a solid oxide fuel cell generator is provided for electrochemically reacting a fuel gas with a flowing oxidant gas at an elevated temperature to produce power. The generator comprises a generator section receiving a fuel gas and a plurality of elongated fuel cells extending through the generator section and having opposing open fuel cell ends for directing an oxidant gas between opposing plena in the generator. A sealant defines a seal on the fuel cells adjacent at least one of the fuel cell ends. The sealant comprises a modified lanthanum borate aluminosilicate glass composition, and the lanthanum borate aluminosilicate glass composition includes B2O3and SiO2having a combined mole percent of less than about 32%.

In accordance with another aspect of the invention, a solid oxide fuel cell generator is provided for electrochemically reacting a fuel gas with a flowing oxidant gas at an elevated temperature to produce power. The generator comprises a generator section receiving a fuel gas and a plurality of elongated fuel cells extending through the generator section and having opposing open fuel cell ends for directing an oxidant gas between opposing plena in the generator. A sealant defines a seal on the fuel cells adjacent at least one of the fuel cell ends. The sealant comprises a modified lanthanum borate aluminosilicate glass composition, and the lanthanum borate aluminosilicate glass composition includes La2O3present in an amount of between about 0 to about 20 mole %, ZrO2present in an amount of between about 0 to about 5 mole %, and YO1.5or ScO1.5present in an amount of between about 0 to about 0.5 mole %.

In accordance with a further aspect of the invention, a solid fuel oxide fuel cell generator is provided comprising a generator section and a plurality of elongated fuel cells extending through the generator section. A fuel distribution plenum feeds fuel to the generator section adjacent to a first end of the fuel cells, and a combustion plenum is located at a second end of the fuel cells, opposite the first end, for combusting depleted fuel and an oxidant gas. An intermediate plenum is located between the generator section and the fuel plenum and is in fluid communication with an interior flow area of the fuel cells, and an absolute seal is located between the generator section and the intermediate plenum and prevents flow gases between the generator section and the intermediate chamber.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, a first embodiment of the invention is illustrated comprising a fuel cell generator10including a housing12defining a plurality of plena or chambers including a central chamber defining a generator section14of the generator10. The housing12further encloses a plurality of elongated fuel cells16extending through the generator section14and having opposing ends comprising an open first end18and an open second end20. The fuel cells16preferably comprise solid oxide fuel cells (SOFC's) and convey an oxidant gas from one end of the housing12to an opposite end of the housing12. In a typical application of the invention, the oxidant gas comprises air, and will hereafter be referred to as such. It should be noted that although only three fuel cells16are illustrated, any number of fuel cells16, and preferably more than three, may be provided in accordance with the present invention.

Air is supplied from an air (oxidant) supply22to an air (oxidant) plenum24located adjacent to a first end12aof the housing12. The air is conveyed via an air supply line26through a recuperative air preheater28where the air is preheated by combustion products conveyed from the generator10through an exhaust line30. A first partition defined by a first positioning board34rigidly supports the first ends18of the fuel cells16and separates the generator section14from the air plenum24. The air plenum24is located adjacent to and in fluid communication with the first ends18of the fuel cells16such that air conveyed to the air plenum24will flow into the fuel cells16through the first ends18.

A fuel plenum32is located adjacent to the first end12aof the housing12and is separated from the air plenum24by an inner wall36, where the air plenum24comprises an intermediate plenum between the generator section14and the fuel plenum32. A plurality of fuel passages are defined by tubulations38and extend between the inner wall36and the first positioning board34for conveying fuel from the fuel plenum32to the generator section14. The first positioning board34preferably comprises a ceramic structure, such as an Al—Mg—O spinel/MgO material, and a seal39comprising a compliant sealant provides an absolute seal preventing passage of gases at the junction between the first ends18of the fuel cells16and the first positioning board34, as will be described further below.

A second partition defined by a second positioning board40supports the second ends20of the fuel cells16adjacent to a second end12bof the housing12and defines a combustion plenum42in fluid communication with the second ends20of the fuel cells16. A dividing wall44is located at an end of the generator section14adjacent to the second positioning board40and a recirculation zone46is defined between the dividing wall44and the second positioning board40. Depleted fuel that has passed through the generator section14from the fuel plenum32passes into the recirculation zone46.

The second ends20of the fuel cells16are in sliding engagement with the second positioning board40to accommodate thermal movement, i.e., thermal expansion and contraction, of the fuel cells16relative to the second positioning board40. The second positioning board40preferably comprises a porous ceramic support structure, and the junction between the second ends20of the fuel cells16and the second positioning board40comprises a tolerant seal47. No attempt need be made to accomplish a perfect seal at the juncture of the fuel cells16and the second positioning board40, since leakage of a fraction of the depleted fuel from the recirculation zone46to the combustion plenum42is acceptable. The combustion plenum40additionally receives vitiated air conveyed from the air plenum32through the fuel cells16for mixing with the depleted fuel.

A portion of the depleted fuel is conveyed out of the recirculation zone46through a recirculation line48to an ejector pump50. Fresh fuel is provided from a fuel supply52at a high pressure through a fuel line54to the ejector pump50and provides the motive force for drawing the depleted fuel from the recirculation zone46as it combines with the fresh fuel in the ejector pump50. The combined fresh fuel and depleted fuel passes through a pre-reformer55where higher hydrocarbon elements of the incoming fuel are reformed and is then conveyed from the ejector pump50to an external reformer56through a fuel transfer line58, and is then conveyed to the fuel plenum32.

The fraction of the depleted fuel that passes from the recirculation zone46through the second positioning board40to the combustion plenum42combusts with the vitiated air that has passed through the fuel cells16. The combustion products may be conveyed through a combustion product line60to the reformer56to provide heat to support a reformation process performed on the fuel, such as is well known in the art. In addition, the combustion products are conveyed from the reformer56through the exhaust line30to the preheater28for preheating incoming air, as described above.

In an alternative configuration, the exhaust stream may be routed through a line61′ (shown in dotted line) directly to the preheater28. In such a configuration, the ejector pump50would be replaced by a higher mass flow rate pump. Specifically, the ejector pump50may be replaced by a centrifugal circulator53′, as illustrated byFIG. 1Ain which an alternative configuration of the fuel supply section of the generator ofFIG. 1is illustrated and in which elements corresponding to elements shown inFIG. 1are labeled with the same reference numeral primed. As seen inFIG. 1A, the centrifugal circulator53′ replaces the ejector pump50, and receives a combined stream of fresh fuel and depleted fuel through the fuel line54′. The centrifugal circulator53′ provides a higher recirculation mass flow rate than the ejector pump50. The reformer56′ is moved to a location upstream of the centrifugal circulator53′ in the recirculation line48′, and the heat for reforming is provided by the depleted fuel and combustion products in the recirculated fuel stream leaving the generator10through the recirculation line48′.

FIG. 2illustrates a specific configuration of the generator10described above with reference toFIG. 1, and elements inFIG. 2corresponding toFIG. 1are identified with the same reference numerals. The generator shown inFIG. 2includes fuel cells16having a cell geometry in which a lanthanum manganite air electrode has the geometric form of a number of integrally connected elements of triangular cross section. These cells comprise Delta X cells, where Delta refers to the triangular shape of the elements and X refers to the number of elements, i.e., the number of fuel cells16. The resulting overall cross section has a flat face on one side and a multi-faceted face on the other side. An additional row of cells16is shown in outline to illustrate the series connection of the fuel cell structure. The air flows within discrete passages P of triangular shape. A lanthanum chromite interconnection covers the flat face62, and a zirconia electrolyte covers the opposing multifaceted face64of the cell geometry and overlaps the edges of the interconnection but leaves most of the interconnection exposed. A nickel/yttria or scandia stabilized zirconia fuel electrode covers most of the electrolyte but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode. Series electrical connections between cells16is accomplished by means of a flat nickel felt or nickel foam panel66, one face of which is sintered to the interconnection at the face62while the other face68contacts the apexes of the triangular multifaceted fuel electrode face64of the adjacent cell (illustrated by dotted line fuel cells16). Fuel may flow in the discrete passages that are formed between the fuel electrode coated facets of the multifaceted face64of one cell16and the nickel felt (or foam) covered flat interconnection face68of the adjacent cell16.

The configuration shown inFIG. 2further illustrates an alternative configuration for the fuel recirculation circuit in which a reformer is not included. After the fuel passes through the fuel pre-reformer57, the pre-reformed fuel may be conveyed directly to the fuel plenum32, without passing through an external reformer, if the pre-reformed fuel is directly oxidized on the fuel cell surface, i.e., via anode reforming. In addition, a start-up heater70may be included in the air supply line26to provide additional heat to the air during start-up of the generator10.

Referring toFIG. 3, a second embodiment of the fuel cell generator of the present invention is illustrated in which elements corresponding to elements of the embodiment ofFIG. 1are labeled with the same reference number increased by 100. In accordance with the second embodiment, a fuel cell generator110is provided including a housing112defining a plurality of plena or chambers including a central chamber defining a generator section114of the generator110. The housing112further encloses a plurality of elongated fuel cells116extending through the generator section114and having opposing ends comprising an open first end118and an open second end120. The fuel cells116preferably comprise solid oxide fuel cells (SOFC's) and convey an oxidant gas, i.e., air, from one end of the housing112to an opposite end of the housing112. It should be noted that although only three fuel cells116are illustrated, any number of fuel cells116, and preferably more than three, may be provided in accordance with the present invention.

A first partition defined by a first positioning board134rigidly supports the first ends118of the fuel cells116and separates the generator section114from an exhaust plenum125. The exhaust plenum125is located adjacent to and in fluid communication with the first ends118of the fuel cells116.

A fuel plenum132is located adjacent to a first end112aof the housing112and is separated from the exhaust plenum125by an inner wall136, where the exhaust plenum125comprises an intermediate plenum between the generator section114and the fuel plenum132. A plurality of fuel passages are defined by tubulations138and extend between the inner wall136and the first positioning board134for conveying fuel from the fuel plenum132to the generator section114. The first positioning board134preferably comprises a ceramic structure, such as an Al—Mg—O spinel/MgO material, and a seal139comprising a compliant sealant provides an absolute seal preventing passage of gases at the junction between the first ends118of the fuel cells116and the first positioning board134, as will be described further below.

A second partition defined by a second positioning board140supports the second ends120of the fuel cells116adjacent to a second end112bof the housing112and defines an air feed/combustion plenum124in fluid communication with the second ends120of the fuel cells116. Air is supplied from an air supply122to the air feed/combustion plenum124located adjacent to the second end112bof the housing112. The air is conveyed via an air supply line126through a recuperative air preheater128where the air is preheated by combustion products conveyed from the generator110through an exhaust line130.

A dividing wall144is located at an end of the generator section114adjacent to the second positioning board140and a recirculation zone146is defined between the dividing wall144and the second positioning board140. Depleted fuel that has passed through the generator section114from the fuel plenum132passes into the recirculation zone146.

The second ends120of the fuel cells116are in sliding engagement with the second positioning board140to accommodate thermal movement, i.e., thermal expansion and contraction, of the fuel cells116relative to the second positioning board140. The second positioning board140preferably comprises a porous ceramic support structure, and the junction between the second ends120of the fuel cells116and the second positioning board40comprises a tolerant seal147. No attempt need be made to accomplish a perfect seal at the juncture of the fuel cells116and the second positioning board140, since leakage of a fraction of the depleted fuel from the recirculation zone146to the combustion plenum142is acceptable. Depleted fuel and air combust in the air feed/combustion plenum124, and air and combustion products pass through the fuel cells116to the exhaust plenum125.

A portion of the depleted fuel is conveyed out of the recirculation zone146through a recirculation line148to an ejector pump150. Fresh fuel is provided from a fuel supply152at a high pressure through a fuel line154to the ejector pump150and provides the motive force for drawing the depleted fuel from the recirculation zone146as it combines with the fresh fuel in the ejector pump150. The combined fresh fuel and depleted fuel passes through a pre-reformer155where higher hydrocarbon elements of the incoming fuel are reformed and is then conveyed from the ejector pump150to an external reformer156through a fuel transfer line158, and is then conveyed to the fuel plenum132.

The combustion products and vitiated air entering the exhaust plenum125through the fuel cells116may be conveyed through a combustion product line160to the reformer156to provide heat to support a reformation process performed on the fuel, such as is well known in the art. In addition, the combustion products are conveyed from the reformer156through the exhaust line130to the preheater128for preheating incoming air, as described above.

In an alternative configuration, the exhaust stream may be routed through a line161′ (shown in dotted line) directly to the preheater128. In such a configuration, the ejector pump150would be replaced by a higher mass flow rate pump. Specifically, the ejector pump150may be replaced by a centrifugal circulator153′, as illustrated byFIG. 3Ain which an alternative configuration of the fuel supply section of the generator ofFIG. 3is illustrated and in which elements corresponding to elements shown inFIG. 3are labeled with the same reference numeral primed. As seen inFIG. 3A, the centrifugal circulator153′ replaces the ejector pump150, and receives a combined stream of fresh fuel and depleted fuel through the fuel line154′. The centrifugal circulator153′ provides a higher recirculation mass flow rate than the ejector pump150. The reformer156′ is moved to a location upstream of the centrifugal circulator153′ in the recirculation line148′, and the heat for reforming is provided by the depleted fuel and combustion products in the recirculated fuel stream leaving the generator110through the recirculation line148′.

FIG. 4illustrates a specific configuration of the generator110described above with reference toFIG. 3, and elements inFIG. 4corresponding toFIG. 3are identified with the same reference numerals. The generator shown inFIG. 4includes fuel cells116having a cell geometry in which a lanthanum manganite air electrode has the geometric form of a number of integrally connected elements of triangular cross section. These cells comprise Delta X cells, where Delta refers to the triangular shape of the elements and X refers to the number of elements, i.e., the number of fuel cells116. The resulting overall cross section has a flat face on one side and a multi-faceted face on the other side. An additional row of cells116is shown in outline to illustrate the series connection of the fuel cell structure. The air flows within discrete passages P of triangular shape. A lanthanum chromite interconnection covers the flat face162, and a zirconia electrolyte covers the opposing multifaceted face164of the cell geometry and overlaps the edges of the interconnection but leaves most of the interconnection exposed. A nickel/yttria or scandia stabilized zirconia fuel electrode covers most of the electrolyte but leaves a narrow margin of electrolyte between the interconnection and the fuel electrode. Series electrical connections between cells116is accomplished by means of a flat nickel felt or nickel foam panel166, one face of which is sintered to the interconnection at the face162while the other face168contacts the apexes of the triangular multifaceted fuel electrode face164of the adjacent cell (illustrated by dotted line fuel cells116). Fuel may flow in the discrete passages that are formed between the fuel electrode coated facets of the multifaceted face164of one cell116and the nickel felt (or foam) covered flat interconnection face168of the adjacent cell116.

The configuration shown inFIG. 4further illustrates an alternative configuration for the fuel recirculation circuit in which a reformer is not included. After the fuel passes through the fuel pre-reformer157, the pre-reformed fuel may be conveyed directly to the fuel plenum132, without passing through an external reformer, if the pre-reformed fuel is directly oxidized on the fuel cell surface, i.e., via anode reforming. In addition, a start-up heater170may be included in the air supply line126to provide additional heat to the air during start-up of the generator110.

As noted above, a sealant39,139is provided as a compliant absolute seal to prevent passage of gases between the generator section14,114and an intermediate plenum, i.e., an air plenum24or an exhaust plenum125. In particular, the sealant39prevents passage of fresh fuel into contact with fresh air in the air plenum124, and the sealant139prevents passage of fresh fuel into the exhaust products within the exhaust plenum125. The sealant39,139generally must have an operating temperature range of approximately 700-1000° C. The sealant39,139comprises a composition that is specific to the application of the present invention, and preferably comprises a modified lanthanum borate aluminosilicate (LBAS) glass.

The sealant material is specifically developed to be materially compatible with and have a thermal expansion coefficient (TEC) compatible with the thermal properties of an air electrode of lanthanum manganite (cathode), an electrolyte of yttria or scandia stabilized zirconia and a fuel electrode of nickel/yttria or scandia stabilized zirconia (anode), as well as manifold materials of Al—Mg—O spinel/MgO, such as may be incorporated in the formation of the first positioning board34,134. The sealant material is also developed to provide a composition having long term stability over repeated thermal cycles, such as through at least seven thermal cycles, of normal operation of the generator. In general, the LBAS glass sealant material provided for the generator of the present invention comprises a composition of lanthanum oxide (La2O3), boron oxide (B2O3), silica (SiO2), alumina (Al2O3), strontium oxide (SrO), yttria or scandia stabilized zirconia (YSZ or ScSZ), and magnesium oxide (MgO).

The LBAS glass sealant has been developed to accommodate any thermal expansion differential between the fuel cell generator components, such as between the joined surfaces at the junction between the fuel cells16,116and the respective first positioning board34,134, and including accommodating thermal expansion of the air electrode, the electrolyte and the manifold materials. For example the composition described herein incorporates a preferred amount of magnesium oxide to modify the thermal expansion coefficient (TEC) of the LBAS glass composition, the preferred composition including magnesium oxide present in an amount of between about 10 to about 30 mole % in the LBAS composition. The TEC of the glass composition at the beginning of its life is approximately 10.95×10−6/K in the range 25° C.-600° C., which is very close to the TEC of the lanthanum manganite air electrode (cathode), the yttria or scandia stabilized zirconia electrolyte and the Al—Mg—O/MgO spinel manifold materials.

In addition, the composition minimizes or reduces contaminating effects of additives considered essential for the LBAS glass composition. In the present composition, the content of boron oxide (B2O3) and silica (SiO2) are low to reduce the overall material loss and resultant contamination of the fuel cell system, particularly during long-term operation of the generator. Boron oxide and silica are both included to form glass with other oxides In order to minimize the contaminant contribution of these additives, i.e., through evaporation from the sealant material, a minimal amount of these materials is preferably included and the combined mole percent of boron oxide and silicate is preferably less than about 32% in the LBAS glass composition.

The fuel cell16,116comprises a cathode supported fuel cell that is in contact with the glass sealant composition, and the glass sealant composition is particularly tailored to the specific cathode used for the fuel cell. Further, the glass composition is in contact with both the cathode and the anode, and must be compatible with the materials of both components. Hence, the glass composition includes a preferred amount of lanthanum oxide present in an amount of between about 0 to about 20 mole %, i.e., 0<La2O3<20 mole %, compatible with the cathode; and the glass composition includes a preferred amount of zirconia present in an amount of between about 0 to about 5 mole %, i.e., 0<ZrO2<5 mole % and of YO1.5or ScO1.5in an amount of between 0 and 0.5 mole %, i.e., 0<YO1.5<0.5 mole % or 0<ScO1.5<0.5 mole %, compatible with the electrolyte as well as with the composition of the anode for the fuel cells.

In view of the above described requirements for the glass sealant composition, it has been found that a modified lanthanum borate aluminosilicate glass composition having the following characteristics may be found to meet the described physical requirements for a sealant in the generator of the present invention:La2O3present in an amount of between about 0 to about 20 mole %;B2O3and SiO2having a combined mole percent of less than about 32%;Al2O3present in an amount of between about 10 to about 20 mole %;SrO present in an amount of between about 30 to about 40 mole %;ZrO2present in an amount of between about 0 to about 5 mole %;YO1.5or ScO1.5present in an amount of between about 0 to about 0.5 mole % andMgO present in an amount of between about 10 to about 30 mole %.

As noted above, the glass sealant composition is required to have stable properties which do not substantially vary during multiple thermal cycles of the generator, which is determined by the particular content of the composition. In known currently available glass compositions, the compositions may undergo significant changes as they are heated to elevated temperatures with thermal cycling. In particular, it is known that glass may undergo a second glass phase, becoming a glass ceramic (i.e., it crystallizes), which adversely affects the TEC of the glass and which becomes worse as the temperature that the glass is exposed to increases. In contrast, the present glass sealant composition provides a glass that maintains a substantially constant TEC, where the TEC is preferably within a range of about 10.0×10−6/K to about 11.0×10−6/K, with an average TEC of about 10.5×10−6/K after the glass composition has been subjected to multiple thermal cycles. This TEC characteristic is particularly advantageous for the present generator in that the TEC of the glass composition substantially matches the TEC of the electrolyte, which is approximately 10.56×10−6/K.

Table 1 below shows the thermal expansion coefficient (TEC) and the softening point of the glass composition described by the specific example above. The first column (Beginning of Life) provides the initial TEC of the composition prior to cycling. The first thermal cycle was performed after subjecting the glass composition to a 500 hour thermal treatment at 800° C. As illustrated in Table 1, the TEC is substantially stable after cycling through seven thermal cycles.

FIG. 5illustrates the thermal expansion coefficient curve for the glass sealant composition. It can be seen that above 850° C. the glass composition starts to soften (point A) where the thermal expansion curve drops sharply. The softening point is the temperature at which the glass composition becomes a substantially compliant, viscous substance that reduces stresses to the adjacent fuel cell and positioning board surfaces. The close match in TEC between the glass sealant composition and other cell components significantly improves the ability of the seal to survive and maintain an absolute seal at the interface or junction between the fuel cells16,116and the respective first positioning board34,134.

A long-term stability test was also performed on the glass sealant composition. In the long-term stability test, the glass composition was exposed to a reducing atmosphere of 3% H2/N2and 3% H2O at 900° C. The test was performed for 2300 hours, and an average evaporation rate of only 2.7×10−4(mg/cm2)/hour was achieved. As noted above, the combined amount of boron oxide and silica in the glass composition is kept low such that the amount of these components available for evaporation and contamination within the fuel cell is kept small.

From the above description, it can be seen that the glass sealant composition for the present generator meets multiple criteria for providing an absolute seal within the generator, while providing a stable composition that maintains required operational characteristics throughout multiple cycles, i.e., throughout at least seven cycles. Specifically, the specified composition maintains the TEC within a preferred range which is close to the TEC of the other cell components, and in particular the TEC of the electrolyte of the fuel bell. Further, the glass sealant includes an additive of yttria or scandia and zirconia and thereby reduces the degree that the sealant may dissolve the material of the fuel cell cathode, and in particular to the degree that the zirconia in the electrolyte will be dissolved by the sealant.

Although the above description characterizes the glass sealant composition as a compliant material that may be placed at the junction between the fuel cells16,116and the respective first positioning boards34,134, the described material may be processed to form a rigid sheet (not shown) that may be stamped to provide holes for receiving the fuel cells. In such a construction, the sheet would comprise a hard glass sheet that would provide a seal but not shrink significantly when in use within the generator. The lack of volume change or shrinkage would permit the sheet of glass composition to contact the fuel cells without significant additional stresses at the seal locations during variations in temperature within the generator.