Method of producing a sealing arrangement for a fuel cell stack and a sealing arrangement for a fuel cell stack

A method is provided for producing an electrically insulating sealing arrangement for a fuel cell stack which comprises a plurality of fuel cell units that succeed one another along a stack direction by means of which the housings of the fuel cell units are connectable to one another in such a way that an adequate electrically insulating effect and adequate mechanical rigidity are ensured even at the high operating temperature.The method involves producing a ceramic metal layer and at least partially converting the metal of the ceramic metal layer into an electrically non conductive compound so as to produce a non conductive boundary layer.

RELATED APPLICATION

The present disclosure relates to the subject matter disclosed in German Patent Application No. 10 2005 045 053.9 of Sep. 21, 2005, the entire specification of which is incorporated herein by reference.

FIELD OF THE DISCLOSURE

The present invention relates to a method of producing an electrically insulating sealing arrangement for a fuel cell stack which comprises a plurality of fuel cell units that succeed one another along a stack direction.

BACKGROUND

For the purposes of setting the desired operating voltage, the necessary number of fuel cell units are arranged one upon the other in order to obtain a fuel cell pile (fuel cell stack). In order to prevent an electrical short-circuit, the housings of the successive fuel cell units in the fuel cell stack must be electrically insulated from one another. Moreover, it is necessary to separate the fuel gas channels of the fuel cell stack from the oxidizing agent chambers of the fuel cell units in gas-tight manner and to separate the oxidizing agent channels of the fuel cell stack from the fuel gas chambers of the fuel cell units in gas-tight manner.

In the case of known fuel cell stacks, sealing and insulating elements consisting of a glass solder or of ceramic sealing materials are used in order to produce the requisite electrically insulating effect and the requisite sealing effect.

In the case of most of the usually used sealing materials, the electrical resistance at the operating temperature of a high temperature fuel cell unit (in the range of approximately 750° C. to approximately 850° C.) is no longer high enough for achieving a satisfactory insulating effect. Furthermore, some of the usually used sealing materials only exhibit a low level of chemical stability and mechanical rigidity for the changes in temperature (between the operating and quiescent phases) that frequently occur in a high temperature fuel cell unit.

The sealing function and the electrically insulating function of the sealing arrangement can be separated from one another. Thus, the electrical insulation can be effected by a ceramic coating which is connected to an adjacent component of the fuel cell stack by means of a metallic solder in the course of a soldering process. Hereby, due to the gas-tight soldering process, the respective processes of sealing the fuel gas channels and the oxidizing agent channels of the fuel cell stack and the mechanical fixing of the fuel cell units to one another is effected at the same time.

The insulating ceramic coating can, however, contain pores and/or fissures and/or capillaries along the grain boundaries, especially if the ceramic coating is applied by a process of thermally spraying it onto a metal part that is to be insulated. Depending upon the capillary activity of the solder used, the solder can penetrate into the pores, fissures of capillaries present in the ceramic coating and cause a short-circuit.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method of producing an electrically insulating sealing arrangement for a fuel cell stack which comprises a plurality of fuel cell units that succeed one another along a stack direction by means of which the housings of the fuel cell units are connectable to one another in such a manner that an adequate electrically insulating effect and adequate mechanical rigidity are ensured even at a high operating temperature.

In accordance with the invention, this object is achieved by a method which comprises the following process steps:producing a ceramic metal layer from a mixture of a ceramic material and a metal material and/or a metal precursor; andat least partially converting the metal of the ceramic metal layer into an electrically non conductive metal compound so as to produce a non conductive boundary layer.

Surprisingly, it has been discovered that short-circuits due to the penetration of solder, in particular, into thermally sprayed layers of the sealing arrangement can, astoundingly, be prevented if the sealing arrangement comprises such a non conductive boundary layer which has been produced from a ceramic metal layer by converting the metallic component thereof, in situ, into a non conductive metal compound.

Here, in contrast to ceramic metal layers whose metallic component is not converted into a non conductive metal compound, there is no fear of a short-circuit occurring. This represents a significant advantage for the practical use of such a sealing arrangement.

The present invention is suitable, in particular, for the production of electrically insulating sealing arrangements for high temperature fuel cells of the SOFC (Solid Oxide Fuel Cell) type.

In a preferred embodiment of the invention, the ceramic metal layer from which the non conductive boundary layer is formed is in the form of a Cermet layer.

The ceramic metal layer is preferably produced by a thermal spraying process, in particular, by an atmospheric plasma spraying process, by a vacuum plasma spraying process or by a flame spraying process.

It is particularly expedient if the ceramic metal layer is produced by a high-speed vacuum plasma spraying process (High Velocity Vacuum Plasma Spraying: HV-VPS for short), since high-speed plasma-sprayed layers exhibit a particularly high density and a particularly low porosity.

For the purposes of delimiting the ceramic metal layer in the lateral direction, templates or coating masks can be used during the thermal spraying process.

In the case of plasma spraying in particular, the following plasma gases or plasma gas combinations can be used:argon;nitrogen;argon and helium;argon and hydrogen;argon, helium and hydrogen.

The ceramic metal layer can be formed, in particular, from a mixture of a powdered ceramic material and a powdered metal and/or a powdered metal precursor.

Hereby, a metal precursor is to be understood as being a metal compound which disintegrates at the working temperature of the thermal spraying process into its metallic component and its non-metallic component so that the metal component of the compound is present in the ceramic metal layer in a metallic form.

In accordance with the invention, a metal precursor or an elementary metal or a mixture consisting of a metal precursor and an elementary metal can be used for the production of the ceramic metal layer.

When using an elementary metal, the conversion of the metal into a metal compound that is not effective as a solder blocker can be prevented by employing a suitable environmental atmosphere (for example, an inert gas atmosphere such as e.g. argon) and/or by the addition of a reducing agent e.g. hydrogen.

It is particularly expedient if use is made of a metal material and/or a metal precursor which respectively comprises an active metal.

Hereby, an “active metal” is to be understood as being a boundary-surface-active metal such as is added in small quantities to active solders (metallic alloys) in order to lower the boundary surface energy between a ceramic material and the solder melt to such an extent that wetting of the ceramic material by the solder can take place.

The hydrides of these active metals are particularly suitable as a metal precursor.

A ceramic material which comprises aluminium oxide and/or titanium dioxide and/or zirconium dioxide and/or magnesium oxide can be used as a powdered ceramic material for the formation of the ceramic metal layer for example.

It is particularly expedient if a ceramic material which comprises yttrium-stabilized zirconium dioxide and/or an aluminium magnesium spinel is used for the production of the ceramic metal layer.

The average ratio, in parts by weight, of the ceramic material in the mixture to the metal material, to the metal precursor, or, (in the case of the common use of a metal material and a metal precursor) to the sum of the metal material and the metal precursor in the mixture consisting of a ceramic material and a metal material and/or a metal precursor that is used for the production of the ceramic metal layer amounts to from approximately 5:1 up to approximately 30:1 for example, preferably from approximately 15:1 up to approximately 25:1.

The ratio of the ceramic material in the mixture to the metal material, to the metal precursor, or, (in the case of the common use of a metal material and a metal precursor) to the sum of the metal material and the metal precursor can be kept substantially constant during the production of the ceramic metal layer or else however, it can be varied during the process of producing the ceramic metal layer in order to alter the mixture-ratio of ceramic material to metal material in the ceramic metal layer in the direction of the layer thickness, i.e. in a direction aligned perpendicularly to the major faces of the ceramic metal layer.

The variation in the mixture-ratio can, for example, be effected by the controlled, separate injection of the components, the ceramic material and the metal material, whereby the mixing process takes place in the jet spray.

If the non conductive boundary layer that has been produced is soldered to a component of the fuel cell stack by means of a metallic solder layer, then provision is preferably made for the ratio of the ceramic material in the mixture to the metal material, to the metal precursor, or, (in the case of the common compound of metal material and metal precursor) to the sum of the metal material and the metal precursor to be varied during the production of the ceramic metal layer in such a way that the part-by-weight of the metal material in the non conductive boundary layer increases with decreasing distance from the solder layer that is to be subsequently applied.

The ceramic metal layer is produced, advantageously, with an average layer thickness of approximately 10 μm to approximately 80 μm, preferably of approximately 20 μm to approximately 60 μm.

Furthermore, in order to improve the electrically insulating effect of the sealing arrangement still further, provision may be made for an insulating layer consisting of an electrically insulating ceramic material to be formed in addition to the non conductive boundary layer.

If the non conductive boundary layer is soldered to a component of the fuel cell stack by means of a metallic solder layer, then the insulating layer is preferably produced in such a way that it is arranged on the side of the non conductive boundary layer remote from the solder layer.

It has proven to be expedient if the insulating layer is produced by thermal spraying, in particular by atmospheric plasma spraying, by vacuum plasma spraying or by flame spraying.

It is particularly expedient if the insulating layer is produced by a high-velocity vacuum plasma spraying process, since a high-velocity plasma-sprayed layer exhibits a particularly high density and low porosity.

In principle, the insulating layer can be formed from any ceramic material which exhibits a sufficiently high specific electrical resistance at the operating temperature of the fuel cell stack.

In particular, for the purposes of producing the insulating layer, use can be made of a ceramic material which comprises aluminium oxide and/or titanium dioxide and/or zirconium dioxide and/or magnesium oxide.

Preferably for the production of the insulating layer, use is made of a ceramic material which comprises an aluminium magnesium spinel.

The insulating layer is advantageously produced with an average layer thickness of approximately 50 μm to approximately 200 μm, preferably of approximately 100 μm to approximately 140 μm.

In order to establish a connection between the sealing arrangement and adjacent components, in particular, metallic components of the fuel cell stack, provision may be made for a metallic solder layer to be produced in addition to the ceramic metal layer.

In particular, provision may be made for this solder layer to be at least partly produced by a process of thermally spraying solder material.

The solder material could also be applied partly by a process of thermal spraying and partly by means of another method, for example, by means of a silk-screen printing process.

In particular, provision may be made for a first solder component (copper oxide and titanium hydride for example) to be applied by a process of thermal spraying and subsequently, for a second solder component (a silver paste for example) to be applied by means of a silk-screen printing process.

In this case, it is only there where the two solder components are applied that the solder components will combine into an eutectic by means of which a soldered connection to an adjacent component of the fuel cell stack is producible.

In particular, a solder comprising at least one reactive component (i.e. a so-called reactive solder) can be used as the solder material for the solder layer, this permitting direct soldering of a ceramic-containing layer to metallic components of the fuel cell stack.

Also suitable for the purpose are so-called active solders which contain active elements, such as titanium, zirconium, hafnium, niobium, tantalum, silicon, cerium or vanadium for example.

Such a solder is obtainable under the designation Copper ABA from the company Wesgo Metals, 610 Quarry Road, San Carlos, Calif. 94070, USA or from the company Wesgo Metals, 2425 Whipple Road, Hayward, Calif. 94544, USA.

This active solder has the following composition: 2 percentage weight Al; 92.7 percentage weight Cu; 3 percentage weight Si; 2.3 percentage weight Ti.

A silver based solder could also be used for the production of the solder layer for example.

Such a silver based solder can be used with or without the addition of elementary copper.

If the silver based solder without an additive of elementary copper is used, then it is expedient if the silver based solder contains an additive of copper oxide since the silver based solder will better wet ceramic surfaces due to the addition of the copper oxide.

Furthermore, the silver based solder may comprise a titanium additive for improving the wetting process.

For the purposes of decreasing the number of different components of the fuel cell stack, it is expedient if the sealing arrangement comprising the non conductive boundary layer is formed as a coating on a preferably metallic component of a fuel cell unit of the fuel cell stack.

Furthermore, provision may be made for the sealing arrangement comprising the non conductive boundary layer to be soldered to a preferably metallic component of a fuel cell unit of the fuel cell stack.

In a preferred embodiment of the method in accordance with the invention, provision is made for the metal of the ceramic metal layer to be at least partially converted into a non conductive metal oxide.

In particular, provision may be made for the metal of the ceramic metal layer to be at least partially converted into a non conductive metal oxide during a soldering process in an oxygen-containing atmosphere.

The further object of the present invention is to produce a sealing arrangement for a fuel cell stack of the type mentioned hereinabove which exhibits an adequate electrically insulating effect and which is of adequate mechanical rigidity even at a high operating temperature of the fuel cell stack.

In accordance with the invention, this object is achieved in the case of a sealing arrangement for a fuel cell stack which comprises a plurality of fuel cell units that succeed one another along a stack direction, wherein the sealing arrangement exhibits an electrically insulating effect, in that the sealing arrangement comprises at least one non conductive boundary layer formed from a mixture of a ceramic material and a non conductive metal compound that is formed in situ from a metal.

Special embodiments of such a sealing arrangement form the subject matter of the claims34to58, the advantages thereof having already been explained hereinbefore in connection with the special embodiments of the method in accordance with the invention.

Claim59is directed toward a fuel cell stack which comprises a plurality of fuel cell units that succeed on another along a stack direction, and at least one sealing arrangement in accordance with the invention.

Further features and advantages of the invention form the subject matter of the following description and the graphic illustration of an exemplary embodiment.

Similar or functionally equivalent elements are designated by the same reference symbols in each of the Figures.

DETAILED DESCRIPTION OF THE INVENTION

A fuel cell stack bearing the general reference100that is illustrated inFIGS. 5 to 16comprises a plurality of fuel cell units102which are each of identical construction and are stacked one on top of the other along a vertical stack direction104.

Each of the fuel cell units102comprises the components illustrated individually inFIG. 1, namely, an upper housing part106, a cathode electrolyte anode unit (KEA unit)108, a contact material110, a lower housing part112and an intermediate element114.

Furthermore, a solder layer116for soldering the KEA unit108to the upper housing part106and for connecting the intermediate element114to the lower housing part112in a gas-tight and electrically insulating manner is illustrated inFIG. 1.

The upper housing part106is in the form of a substantially rectangular and substantially flat metal sheet which is provided with a substantially rectangular central passage opening120through which, in the fully assembled state of the fuel cell unit, the KEA unit108of the fuel cell unit102is accessible for contact-making purposes by the lower housing part112of the fuel cell unit102located thereabove in the stack direction104.

On the one side of the passage opening120, the upper housing part106is provided with a plurality of, three for example, fuel gas supply openings122which are arranged to alternate with a plurality of, four for example, oxidizing agent supply openings124.

On the opposite side of the passage opening120, the upper housing part106is provided with a plurality of, four for example, fuel gas removal openings126which are arranged to alternate with a plurality of, three for example, oxidizing agent removal openings128.

The upper housing part106is preferably made of a highly corrosion resistant steel, for example, from the alloy Crofer 22.

The material Crofer 22 has the following composition:

This material is sold by the company ThyssenKrupp VDM GmbH, Plettenberger Straβe 2, 58791 Werdohl, Germany.

The KEA unit108comprises an anode113, an electrolyte109arranged over the anode113and a cathode111arranged over the electrolyte109.

The anode113is formed from a ceramic material, from ZrO2or from a Ni/ZrO2-Cermet (ceramic metal mixture) for example, which is electrically conductive at the operating temperature of the fuel cell unit (from approximately 800° C. to approximately 900° C.), and is porous in order to enable a fuel gas to pass through the anode113to the electrolyte109adjoining the anode113.

A hydrocarbon-containing gas mixture or pure hydrogen can be used as the fuel gas for example.

The electrolyte109is preferably in the form of a solid electrolyte, in particular, a solid oxide electrolyte, and consists of yttrium-stabilized zirconium dioxide for example.

The electrolyte109is electronically non-conductive at normal temperatures and also at the operating temperature. By contrast however, the ionic conductivity thereof rises with increasing temperature.

The cathode111is formed from a ceramic material which is electrically conductive at the operating temperature of the fuel cell unit, for example, from (La0.8Sr0.2)0.98MnO3, and it is porous in order to enable an oxidizing agent, air or pure oxygen for example, to pass to the electrolyte109from an oxidizing agent chamber130adjoining the cathode111.

The gas-tight electrolyte109of the KEA unit108extends up to the edge of the gas-permeable anode113and beyond the edge of the gas-permeable cathode111.

The contact material110which is arranged between the KEA unit108and the lower housing part112can, for example, be in the form of a net, a weave or a fleece made of nickel wire.

The lower housing part112is in the form of a sheet metal shaped-part and comprises a substantially rectangular plate132which is directed perpendicularly to the stack direction104, whilst the edges thereof merge via a bevelled portion134into an edge flange136that is likewise aligned substantially parallel to the stack direction104.

The plate132comprises a substantially rectangular central contact field138which is provided with contact elements for making contact with the contact material110on the one hand and with the cathode111of a KEA unit108of a neighbouring fuel cell unit102on the other, wherein said elements may be corrugated or dimpled.

On the one side of the contact field138, the plate132is provided with a plurality of, three for example, fuel gas supply openings140which are arranged to alternate with a plurality of, four for example, oxidizing agent supply openings142.

The fuel gas supply openings140and the oxidizing agent supply openings142of the lower housing part112are in alignment with the respective fuel gas supply openings122and the oxidizing agent supply openings124of the upper housing part106.

On the other side of the contact field138, the plate132is provided with a plurality of, four for example, fuel gas removal openings144which are arranged to alternate with a plurality of, three for example, oxidizing agent removal openings146.

The fuel gas removal openings144and the oxidizing agent removal openings146of the lower housing part112are in alignment with the respective fuel gas removal openings126and the oxidizing agent removal openings128of the upper housing part106.

The oxidizing agent removal openings146are preferably located opposite the fuel gas supply openings140, and the fuel gas removal openings144are preferably located opposite the oxidizing agent supply openings142.

As can best be seen fromFIGS. 11 to 13, the oxidizing agent removal openings146(in like manner to the oxidizing agent supply openings142) of the lower housing part112are each surrounded by a ring flange148which surrounds the opening concerned in ring-like manner and is aligned substantially parallel to the stack direction104and which is connected to the plate132of the lower housing part112via a bevelled portion149.

The lower housing part112is preferably made of a highly corrosion resistant steel, for example, from the already previously mentioned alloy Crofer 22.

The intermediate element114comprises a substantially rectangular frame part152which extends in ring-like manner along the edge of the fuel cell unit102, as well as channel delimitation parts154which are connected in one-piece manner to the frame part152and which are formed in such a way that they, together with the frame part152, respectively surround a fuel gas supply opening156and a fuel gas removal opening158of the intermediate element114.

The fuel gas supply openings156and the fuel gas removal openings158of the intermediate element114are aligned with the respective fuel gas supply openings140and the fuel gas removal openings144of the lower housing part112as well as with the respective fuel gas supply openings122and with the fuel gas removal openings126of the upper housing part106.

The intermediate element114is made from a substantially flat metal sheet by punching out the fuel gas supply openings156and the fuel gas removal openings158and also a central passage opening160.

A highly corrosion resistant steel is preferably used for the material of the intermediate element114, for example, from the already previously mentioned alloy Crofer 22 or FeCrAlY.

The composition of the FeCrAlY alloy is as follows: 30 percentage weight chromium; 5 percentage weight aluminium; 0.5 percentage weight yttrium; the remainder iron.

As can be seen fromFIG. 10, the upper surface of the intermediate element114facing the lower housing part112is provided with a multi-layer sealing arrangement118.

The sealing arrangement118comprises an insulating layer162which is arranged directly on the upper surface of the intermediate element114, a non conductive boundary layer192which is arranged on the upper surface of the insulating layer162remote from the intermediate element114and a solder layer190which is arranged on the upper surface of the non conductive boundary layer192that is remote from the insulating layer162.

The insulating layer162applied directly to the upper surface of the intermediate element114consists of a thermally sprayed ceramic, for example, of an aluminium magnesium spinel.

As an alternative or in addition to being applied to the upper surface of the intermediate element114, the insulating layer162could also be applied to the lower surface of the lower housing part112.

For the purposes of applying this electrically insulating, insulating layer162to the upper surface of the intermediate element114or the lower surface of the lower housing part112for example, the atmospheric plasma spraying process, the vacuum plasma spraying process or the flame spraying process are suitable. The high-speed vacuum plasma spraying process (High Velocity Vacuum Plasma Spraying, HV-VPS) is particularly suitable.

In the case of these methods, the surface of the intermediate element114that is to be coated with the insulating layer162is removed, preferably a plurality of times, by means of a jet spray, whereby a layer of thermally sprayed ceramic is formed during each removal process.

The layer thickness of the electrically insulating, insulating layer162amounts to 50 μm to 200 μm for example, preferably 100 μm to 140 μm.

The insulating layer162can, for example, be formed by repeatedly removing the surface of the intermediate element114that is to be coated, whereby the insulating layer162then consists of a plurality of layers of the thermally sprayed ceramic material that are deposited one above the other.

The non conductive boundary layer192of the sealing arrangement118arranged on the insulating layer162is in the form of a thermally sprayed Cermet layer whose metal component has been converted in situ into a non conductive metal compound.

For the purposes of producing the non conductive boundary layer192, one proceeds as follows for example:

Firstly, a ceramic metal layer is produced on the insulating layer162from a mixture consisting of a powdered ceramic material and a powdered metal precursor by a thermal spraying process.

Hereby, a metal precursor is to be understood as being a metal compound which disintegrates at the working temperature of the thermal spraying process into its metallic component and its non-metallic component so that the metal component of the compound is present in the ceramic metal layer in a metallic form.

A thermally unstable compound of an active metal which disintegrates at the working temperature of the thermal spraying process is preferably used as the metal precursor.

Hereby, an “active metal” is to be understood as being a boundary-surface-active metal such as is added in small quantities to active solders (metallic alloys) in order to lower the boundary surface energy between a ceramic material and the solder melt to such an extent that wetting of the ceramic material by the solder can take place.

Such “active metals” are, in particular, the metals in the group titanium, zirconium, hafnium, niobium and tantalum.

Hydrides of these active metals are particularly suitable as the metal precursor.

Should titanium be introduced into the ceramic metal layer, then use is preferably made of titanium hydride as the metal precursor, resulting in disintegration of the titanium hydride into metallic titanium and hydrogen as from a temperature of approximately 400° C. Thus, in this case, a ceramic titanium layer ensues on the insulating layer162as a result of the thermal spraying process.

As an alternative or in addition to titanium hydride, elementary titanium can also be used.

Should zirconium be introduced into the ceramic metal layer, then use is preferably made of zirconium hydride as the metal precursor. In this case, a ceramic zirconium layer ensues on the insulating layer162as a result of the thermal spraying process.

Yttrium-stabilized zirconium dioxide (with an yttrium component measured in mol % of between 3% and 12%, preferably between 5% and 8%) or an aluminium magnesium spinel can be used as a powdered ceramic material for the formation of the ceramic metal layer for example.

The ratio, in parts by weight, between the ceramic material and the metal precursor (in particular, titanium hydride or zirconium hydride) in the mixture lies within a range of 5:1 to 30:1 for example, preferably within a range of 15:1 to 25:1. A mixture ratio of 20:1 is particularly expedient.

The layer thickness of the ceramic metal layer lies within a range of 10 μm to 80 μm for example, preferably within a range of 20 μm to 60 μm.

For the purposes of thermally spraying the ceramic metal layer onto the upper surface of the insulating layer162remote from the intermediate element114, the atmospheric plasma spraying process, the vacuum plasma spraying process or the flame spraying process are suitable for example. The high-speed vacuum plasma spraying process (High Velocity Vacuum Plasma Spraying, HV-VPS) is particularly suitable.

The ceramic metal layer too is preferably produced in such a way that the upper surface of the insulating layer162that is to be coated is removed a plurality of times using a jet spray, whereby a plurality of layers of the ceramic metal layer are formed.

Thereby, during each process of removal of the surface that is to be coated, the ratio between the ceramic material and the metal precursor in the mixture can be altered in such a way that a mixture-ratio gradient running in the direction of the layer thickness (parallel to the stack direction104) is developed.

This mixture-ratio gradient is directed in such a way that the part-by-weight of the metal in the entire material of the ceramic metal layer increases with increasing distance from the upper surface of the insulating layer162.

During subsequent soldering of the thus produced ceramic metal layer to the lower surface of the lower housing part112in an air atmosphere or in a vacuum wherein a partial pressure of oxygen still exists, the metal component of the ceramic metal layer is converted into a non conductive metal oxide so that the non conductive boundary layer192is formed from the ceramic metal layer. A solder joint of high mechanical rigidity and high electrical resistance thereby develops.

Due to the good electrically insulating effect of the non conductive boundary layer192, it is not necessary to let the insulating layer162project laterally beyond the non conductive boundary layer192in order to prevent the occurrence of a short-circuit.

Rathermore, the insulating layer162and the ceramic metal layer from which the non conductive boundary layer192is then subsequently developed can be produced together in non-overlapping manner using the same coating masks. The expenditure on apparatus for the production of the sealing arrangement118is thereby reduced.

If the ceramic metal layer contains metallic titanium, then the latter is converted into non conductive titanium dioxide during the soldering process in the oxygen-containing atmosphere.

A silver based solder can be used for soldering the ceramic metal layer to the lower housing part112and for the production of the solder layer190for example.

In particular, a silver based solder with an additive of elementary copper can be used as the solder material, for example, a silver based solder with the composition (in mol %): Ag-4Cu or Ag-8Cu.

The process of soldering this solder material to the lower surface of the lower housing part112and to the ceramic metal layer takes place in an air atmosphere. The soldering temperature lies within a range of approximately 980° C. to approximately 1,050° C., the duration of the soldering process amounts to approximately 5 minutes for example. Copper oxide and an oxide of the metal contained in the ceramic metal layer are formed in situ during the process of soldering in air.

As an alternative thereto, the solder material could also be in the form of a silver based solder without an additive of elementary copper. Such a copper-free solder offers the advantage of a higher solidus temperature (this amounts to approximately 960° C. without a copper additive, to approximately 780° C. with a copper additive). Since pure silver does not wet ceramic surfaces, copper(II)oxide is added to the silver based solders without a copper additive for the purposes of reducing the edge angle. The soldering process using silver based solders without a copper additive takes place in an air atmosphere or in a vacuum wherein a partial pressure of oxygen still exists.

Suitable silver based solders without an additive of elementary copper have the composition (in mol %): Ag-4CuO or Ag-8CuO for example.

An additive of titanium to the solder material can serve for the purposes of further improving the wetting action (reduction of the edge angle). An intimate mixture of the appropriate components in powder form is used for the purposes of producing the solders. The solder alloy is formed in situ from this mixture. The titanium is added to this mixture in the form of titanium hydride. Metallic titanium is formed from the hydride at approximately 400° C.

Suitable silver based solders without an additive of elementary copper, but with an additive of titanium have the composition (in mol %): Ag-4CuO-0.5Ti or Ag-8CuO-0.5Ti for example.

In this case too, the soldering temperature preferably amounts to between approximately 980° C. and approximately 1,050° C., the duration of the soldering process to approximately 5 minutes for example.

Furthermore, active solders can also be used for the soldering process.

Active solders are metallic alloys which contain boundary-surface-active elements (e.g. titanium, zirconium, hafnium, niobium and/or tantalum) in small quantities and are thus able to lower the boundary surface energy between a ceramic material and the solder melt to such an extent that wetting of the ceramic material by the solder can take place.

The active soldering technique using active solders enables ceramic/ceramic metal compounds to be produced in the course of a single-step jointing process, without preceding metallization of the ceramic jointing surfaces. The wetting of the ceramic jointing surfaces by the solder is ensured here by the use of an active solder.

For example, a suitable active solder is sold under the designation Copper ABA by the company Wesgo Metals, 610 Quarry Road, San Carlos, Calif. 94070, USA, or by the company Wesgo Metals, 2425 Whipple Road, Hayward, Calif. 94544, USA.

This active solder has the following composition: 2 percentage weight Al; 92.7 percentage weight Cu; 3 percentage weight Si; 2.3 percentage weight Ti.

This active solder is preferably used at a soldering temperature of from approximately 1,030° C. to approximately 1,080° C.

Due to the fact that the sealing arrangement118has the non conductive boundary layer192arranged between the solder layer190and the insulating layer162, there are no short-circuits caused by solder penetrating into the insulating layer162so that the sealing arrangement118can exercise its electrically insulating function in perfect manner.

For the purposes of producing the fuel cell units102illustrated inFIG. 4from the previously described individual components, one proceeds as follows:

Firstly, the intermediate element114is provided with the insulating layer162and the ceramic metal layer, which still contains unoxidized metal material, in the previously described manner.

Subsequently, the electrolyte109of the KEA unit108is soldered along the edge of the upper surface thereof to the upper housing part106, namely, to the lower surface of the region of the upper housing part106surrounding the passage opening120in the upper housing part106.

The soldering material needed for this process can, as illustrated inFIG. 1, be inserted in the form of a suitably cut soldering foil116between the electrolyte109and the upper housing part106or else it could be applied in the form of a bead of soldering material to the upper surface of the electrolyte109and/or to the lower surface of the upper housing part106by means of a dispenser. Furthermore, it is also possible for the soldering material to be applied to the upper surface of the electrolyte109and/or to the lower surface of the upper housing part106by means of a pattern printing process, a silk-screen printing process for example.

A silver based solder with a copper additive can be used as the soldering material, for example, a silver based solder with the composition (in mol %): Ag4Cu or Ag8Cu.

The soldering process takes place in an air atmosphere. The soldering temperature amounts to 1050° C. for example, the duration of the soldering process to approximately 5 minutes for example. Copper oxide forms in situ during the process of soldering in air.

As an alternative thereto, a silver based solder without a copper additive could also be used as the soldering material. Such a copper-free solder offers the advantage of a higher solidus temperature (this amounts to approximately 960° C., without a copper additive, to approximately 780° C. with a copper additive). Since pure silver does not wet ceramic surfaces, Copper(II)oxide is added to the silver based solders without a copper additive for the purposes of reducing the edge angle. The soldering process using silver based solders without a copper additive takes place in an air atmosphere or in an inert gas atmosphere, for example under argon.

In this case too, the soldering temperature preferably amounts to approximately 1050° C., the duration of the soldering process to approximately 5 minutes for example.

As an alternative to soldering the KEA unit108into the upper housing part106, provision could also be made for a substrate, on which the KEA unit108has not yet been produced, to be welded to the upper housing part106and, after the welding process, the electro-chemically active layers of the KEA unit108, i.e. the anode, electrolyte and cathode thereof, are produced successively using the vacuum plasma spraying process on the substrate that has already been welded to the upper housing part106.

After the connection of the KEA unit108to the upper housing part106, the state illustrated inFIG. 2is reached.

The side of the intermediate element114provided with the sealing arrangement118and facing the lower housing part112is then soldered to the side of the lower housing part112facing the intermediate element114by means of the soldering material of the solder layer190.

The soldering process is effected in an oxygen-containing atmosphere and can in all other respects take place under the same conditions as were described hereinbefore in connection with the process of soldering the electrolyte109and the upper housing part106.

The necessary soldering material can be inserted in the form of a suitably cut soldering foil between the intermediate element114and the lower housing part112, or else it could be applied in the form of a bead of soldering material to the upper surface of the sealing arrangement118and/or to the lower surface of the lower housing part112by means of a dispenser. Furthermore, it is also possible for the soldering material to be applied to the upper surface of the sealing arrangement118and/or to the lower surface of the lower housing part112partially or in its entirety by means of a pattern printing process, a silk-screen printing process for example.

Moreover, it is possible for the solder layer190to be produced on the upper surface of the ceramic metal layer of the sealing arrangement118by a thermal spraying process, in particular, by an atmospheric plasma spraying process, a vacuum plasma spraying process or a flame spraying process and thereafter to be soldered to the lower housing part112. The high-speed vacuum plasma spraying process (High Velocity Vacuum Plasma Spraying, HV-VPS) is particularly suitable.

After the intermediate element114has been soldered to the lower housing part112, the state illustrated inFIG. 3is reached.

Nevertheless, it is also possible for the intermediate element114to be soldered to the lower housing part112before the KEA unit108is connected to the upper housing part106, or, the connection of the intermediate element114and the lower housing part112on the one hand and the KEA unit108and the upper housing part106on the other can take place at one and the same time.

Subsequently, the contact material110, a nickel net for example, is inserted between the lower housing part112and the upper housing part106, and then the lower housing part112and the upper housing part106are welded together in gas-tight manner along a welding seam164which runs along the outer edge of the edge flange136of the lower housing part112and the outer edge of the upper housing part106, and along welding seams166which run along the inner edges of the ring flange148of the lower housing part112and the respective edges of the oxidizing agent supply openings124and the oxidizing agent removal openings128of the upper housing part106.

Following this method step, the state illustrated inFIG. 4is reached wherein there are now fully assembled fuel cell units102although these still need to be connected together in order to form a fuel cell stack100from a plurality of fuel cell units102which succeed one another in the stack direction104.

The connection of two fuel cell units102which succeed one another in the stack direction104is effected in the following manner:

A first fuel cell unit102aand a second fuel cell unit102bare inserted into a welding jig in such a manner that the upper surface of the upper housing part106of the second fuel cell unit102brests flatly against the lower surface of the intermediate element114of the first fuel cell unit102a.

Thereafter, the intermediate element114of the first fuel cell unit102ais welded to the upper housing part106of the second fuel cell unit102bin gas-tight manner by means of a welding seam168which runs along the outer edges of the intermediate element114and the upper housing part106, and by means of welding seams170which respectively extend around the edges of the fuel gas supply openings156of the intermediate element114and the edges of the fuel gas supply openings122of the upper housing part106that are aligned therewith and around the edges of the fuel gas removal openings158of the intermediate element114and the edges of the fuel gas removal openings126of the upper housing part106that are aligned therewith.

After two fuel cell units102have been connected together in this way, the fuel cell stack100can be gradually built up by successively welding further fuel cell units102to the intermediate element114of the second fuel cell unit102bor to the upper housing part106of the first fuel cell unit102ain the stack direction104until the desired number of fuel cell units102is attained.

In the finished fuel cell stack100, the respective mutually aligned fuel gas supply openings122,140and156of the upper housing parts106, the lower housing parts112and the intermediate elements114form a respective fuel gas supply channel172which, in each fuel cell unit102between the upper surface of the lower housing part112and the lower surface of the upper housing part106, opens into a fuel gas chamber174which is formed between the upper surface of the contact field138of the lower housing part112on the one hand and the lower surface of the KEA unit108on the other.

The respective mutually aligned fuel gas removal openings126,144and158of the upper housing parts106, the lower housing parts112and the intermediate elements114form a respective fuel gas removal channel176which is open to the fuel gas chamber174in the region between the upper surface of the lower housing part112and the lower surface of the upper housing part106on the side of each fuel cell unit102located opposite the fuel gas supply channels172.

The respective mutually aligned oxidizing agent supply openings124and142of the upper housing parts106and the lower housing parts112and also the regions of the passage openings160in the intermediate elements114located between the channel delimitation parts154of the fuel gas supply openings140of the intermediate elements114together form a respective oxidizing agent supply channel178which is open to the oxidizing agent chamber130of the fuel cell unit102in the region of each fuel cell unit102between the upper surface of the upper housing part106and the lower surface of the lower housing part112of the fuel cell unit102located thereabove in the stack direction104.

In like manner, the respective mutually aligned oxidizing agent removal openings128and146of the upper housing parts106and the lower housing parts112together with the regions of the passage openings160in the intermediate elements114located between the channel delimitation parts154of the fuel gas removal openings144of the intermediate elements114form a respective oxidizing agent removal channel180which is arranged on the side of the fuel cell units102located opposite to the oxidizing agent supply channels178and likewise opens into the oxidizing agent chamber130of the fuel cell unit102in the region of each fuel cell unit102between the upper surface of the upper housing part106and the lower surface of the lower housing part112of the fuel cell unit102located thereabove it in the stack direction104.

In operation of the fuel cell stack100, a fuel gas is supplied to the fuel gas chamber174of each fuel cell unit102by way of the fuel gas supply channels172and the exhaust gas produced by oxidation at the anode113of the KEA unit108as well as any unused fuel gas is removed from the fuel gas chamber174through the fuel gas removal channels176.

In like manner, an oxidizing agent, air for example, is supplied to the oxidizing agent chamber130of each fuel cell unit102through the oxidizing agent supply channels178and any unused oxidizing agent is removed from the oxidizing agent chamber130through the oxidizing agent removal channels180.

In operation of the fuel cell stack100, the KEA units108are, for example, at a temperature of 850° C. at which the electrolyte109of each KEA unit108is conductive for oxygen ions. The oxidizing agent from the oxidizing agent chamber130picks up electrons at the cathode111and delivers doubly negatively charged oxygen ions to the electrolyte109, said ions then migrating through the electrolyte109to the anode113. At the anode113, the fuel gas from the fuel gas chamber174is oxidized by the oxygen ions from the electrolyte109and thereby donates electrons to the anode113.

The electrons freed by the reaction at the anode113are supplied from the anode113via the contact element110and the lower housing part112to the cathode111of a neighbouring fuel cell unit102adjoining the lower surface of the contact field138of the lower housing part112and thus make the cathode reaction possible.

The lower housing part112and the upper housing part106of each fuel cell unit102are connected together in electrically conductive manner by the welding seams164,166.

However, the housings182of the fuel cell units102which succeed one another in the stack direction104that are formed in each case by an upper housing part106, a lower housing part112and an intermediate element114are electrically insulated from one another by the sealing devices118between the upper surface of the intermediate elements114and the lower surface of the lower housing parts112. At the same time hereby, a gas-tight connection between these components is ensured by the process of soldering the intermediate elements114to the lower housing parts112so that the oxidizing agent chambers130and the fuel gas chambers174of the fuel cell units102are separated from one another and from the environment of the fuel cell stack100in gas-tight manner.