DE4307967A1 - Integrated ceramic high-temperature fuel cell - Google Patents

Abstract

It is proposed for an integrated high-temperature fuel cell (SOFC [solid oxide fuel cell]) initially to produce at least one planar base body, comprising three mutually adjacent functional layers of the fuel cell, from green sheets, to pile up the latter, if desired with further functional layers, to form a stack and, finally, to press and sinter the latter to form the fuel cell stack. The composite (laminated) bodies, which are presintered if desired, have a higher stability than the corresponding individual layers, with the result that the latter can be made thinner. A special sealing process makes it possible for the fuel cell produced in this way to operate reliably with a high efficiency. <IMAGE>

Description

The invention relates to a solid electrolyte high-temperature burner fabric cell module, which has several festivals connected in series Electrolyte high temperature fuel cells.

A process takes place in a fuel cell, which is essentially Chen represents a reversal of electrolysis. The reaction partner of the combustion reaction, namely the fuel and the oxygen carrier, usually air, is in spatial channels separated by a kerami solid electrolytes with electrodes on both sides is provided, are separated from each other. On the electrodes who the fuel molecules are oxidized by donating electrons or the oxygen carrier by reducing electrons graces. The solid electrolyte has an ionic line and serves for mass exchange. On the two electrodes of the solid electrolyte a potential difference arises at which the for electrons poorly conducting solid electrolyte the short circuit prevented. It must therefore have a low conductivity for elec tronen and at the same time a high conductivity for ions point.

To at the operating point of the fuel cell one about the potential Difference of a cell beyond operating voltage ten, several fuel cells are connected in series. This is done in a simple manner by stacking individual ones Fuel cells, with intermediate bipolar layers or plates made of a so-called ICM material (Inter Con  nection material) the electrical connection between two be ensure neighboring single cells.

A problem with high-temperature solid electrolyte fuel cells (SOFC) lies in the selection of ver for the fuel cells used materials that withstand the high operating temperatures of for example 800 to 1100 ° C with high pressures at the same time the fuel gas or the acid required for combustion withstand up to 16 bar. So emerge the boundaries between different layers with problems Interdiffusion, which improves the properties of the layer material lien be changed. Due to different thermal expansion The coefficient of strength of the materials is also the strength and tightness of a fuel cell stack at risk.

A solution that at least addresses the problem of different thermal expansion is reduced, is a full ke ram fuel cell. Ideally, all compo components in the green (not sintered) state and then be burned together. By working together tern should be a solid, mechanically stable and gas-tight structure be achieved.

Such a fully ceramic fuel cell is, for example from an article in Proceedings of the first international symposium on "Solid Oxide Fuel Cells", 1989, pp. 307-316 knows. The basic idea is to make it burning or acidic supplying gases in fine channels, the Ka channels are formed by a multiply folded layer, the similar to corrugated cardboard between two planar layers is embedded. One version consists of multiple ge folded layer that ensures gas transport from a single layer Solid body coated on both sides with electrode material electrolyte sheet. The supply of different gases can be there when done within a layer level. Above and below  half of the folded electrolyte layer are planar, bilateral Sheets of ICM material coated with electrode material (Inter Connection Material) arranged for both Sealing of the gas space as well as for the electrical connection serve two stacked single cells in the stack.

One disadvantage of this structure is the requirement that all layers in a single, common sintering step must be converted into a monolithic SOFC. This is necessary high adaptation of the sintering mechanisms and a con trolls of interdiffusion between the individual, neighboring layers. In principle, a compromise of "good" and "bad" properties for each one Layer must be accepted, so that an optimization of all required properties for long-term operation of a SOFC very difficult from the start, if not impossible.

The other is the production of a corrugated cardboard layer structure, the sintering together while maintaining the shapes loyalty and the gas-tight seal very difficult, since there is a intimate connection in the green state, for example by pressing the layer structure due to the risk of breakage.

The object of the present invention is therefore a method for the production of an all-ceramic high-temperature fuel cell, which is easy to carry out and a Fuel cell leads, in particular, a high mechanical Stability and increased open circuit voltage and also offers the opportunity to optimal structure of each Components with the least possible interdiffusion between the To reach layers.

This object is achieved by a method according to An spell 1 solved. Further embodiments of the invention are the See subclaims.  

The fuel cell according to the invention is thus completely turned off Green foils built up. To generate the Yt trium-stabilized zirconium oxide (YSZ) existing electrolyte For example, films can be mastered on a large industrial scale inexpensive film casting processes are used. So that is it is possible to make the electrolyte sheet extremely thin, at for example in a thickness of about 15 to 20 microns. Such one thin electrolyte foil results in a minimal voltage drop and thus has only low conductivity losses, which means a high usable operating voltage and thus a high us efficiency of the fuel cell is guaranteed. The Stabili The thin electrolyte layer is generated by the generation of Composite bodies, that is, by embedding between two elec tread layers additionally increased. With a thinner elec trolyte layer it is also possible to have the same effect just like a well-known cell with a thicker electrolyte layer to achieve a lower operating temperature. So ver tied is a lower thermal and mechanical load the fuel cell and thus a higher stability and a longer lifetime.

Since all layers used are flat, the Press or laminate layers in the stack well, and thus create a good bond between the layers in the stack len what tightness, stability and lifespan of the Fuel cell is important.

There are two separate fuel cells per gas cell Gas transport layers are provided. These are dependent of the gases to be transported from cathode material (for air transportation) and anode material (for transportation of fuel gases).

In a further embodiment of the invention, before stacking composite bodies or Ver  body and functional layers of the former, the latter or to pre-sinter both. This measure is in particular before partial if different green foils a different Chen show volume shrinkage during sintering. By presintering the Green film or the green film with the larger shrinkage can do so some compensation will be created. In addition, the different functional layers or the composite body require different sintering temperatures so that it it is advantageous to pre-sinter the layers or composite bodies, which have to be sintered at a higher temperature. Together then only the sintering temperature of the am nied rigorous sintering material can be achieved. Since the sintered temperatures usually above the operating temperature of the furnace are an additional thermal load Avoidance of the fuel cell stack and thus a additional source of danger for a possible leak of the Stack turned off.

Another embodiment of the invention relates to the subsequent Sealing a finished sintered fuel cell stacks. With the help of a chemical vapor deposition process (CVD), which can be electrolytically supported (EVD) it is possible to have existing pores and thus potential leaks activities in the ICM (bipolar layer) and in the electrolyte layer with a suitable material, for example with Yt trium-stabilized zirconium oxide YSZ. This is be This is particularly advantageous if the bipolar layer at the over layering of the stack is not yet completely sintered. Since at a desired final sintering temperature of less than 1350 ° C most for the bipolar layer in question coming ceramic materials are not sufficiently dense and thus can be sintered gas-tight, is by the after inert sealing of the pores in the bipolar layer Material selection for this layer enlarged and thus united fold. In addition, the YSZ CVD process is a (chemical)  Stabilization of the electrolyte / anode boundary layer and a (Electrical) improvement of the electrolyte / cathode boundary layer achieved.

Various combinations are used to manufacture the composite bodies functional layers. It is possible for example, the electrolyte layer between an anode and embed a cathode layer and thus a PEN - (= posi tiv / electrolytic / negative) composite body, and by embedding a bipolar layer between two gas transport layers NEN GIG - (= gas transport / ICM / gas transport) composite body testify. If for the gas transport layers in the GIG network body electrode material is selected, each GIG Ver bundle two different gas transport layers, namely made of anode and cathode material. From such PEN and GIG Composite bodies can then be alternately stacked stack the right layer of different composites Generate order in the stack for the fuel cell stack.

Another combination of functional layers for one Composite body consists of embedding a bipolar layer between two different electrode layers, one NIP composite body (= negative / ICM / positive) is created.

A stack for a fuel cell stack can be read through Layering PEN and NIP composites with each other because of creating a gas transport layer in between. Is for the gas transport layer uses electrode material this corresponds to the neighboring layers of the neighboring ones Composite body selected.

In a further embodiment of the invention, the green electrolyte foils for the electrolyte layer in front of the Manufacture of the composite body or before pressing and the Sintering in the stack on both sides with anode or cathode material  or to print a mixture of cathode material / YSZ. The Printing paste can serve as an additional "adhesive" and to a stronger bond of the composite body or the stack to lead. In addition, the boundary layer between elec trolyte layer and electrode layers electrochemically special effective, so that the electrical values and thus who improves the efficiency of the finished fuel cell the.

Other interfaces between functional layers can also with a suitable material or a suitable material al composition to be printed between the two Layers to generate a functional gradient. For example can at the interfaces between two different abrupt changes in egg occurring functional layers properties over a gradient, or in the simplest way leadership over an "intermediate layer" to be mitigated. Next the thermal expansion behavior can especially the In terdiffusion between adjacent layers of different Composition can be optimized.

Of crucial importance for the method according to the invention or for the fuel cell produced with it is from closing sintering process. To adjust the shrinkage of the un to achieve different ceramic components during sintering, must pay special attention to the ceramic powder used and binder systems. In an optimized ver driving are the compositions of the functional layers set so that all layers are comparable Show shrinkage.

But not only the absolute shrinkage of the individual layers is crucial, the timing must also be controlled become. For this purpose, a so-called "rate controlled" is preferably used sintering "is used here posed that the shrinkage processes linearly with time  to run. That means the temperature rise especially at the Slow down points where shrinkage processes begin Zen. With these controlled sintering processes, voltage conditions that occur when the sintering behavior is not exactly adapted can be better balanced. The formation of cracks, that to a defective or even unusable fuel cell lenstack can be avoided.

The following is the construction of an all-ceramic high temperature fuel cell and the production of the individual layers described in more detail with reference to three figures.

Show

Fig. 1 and 2 show two different arrangements of United composite bodies in the stack, while

Fig. 3 shows a finished high-temperature fuel cell stack Fig.

Fig. 1 shows the structure of a fuel cell stack of NIP and PEN composite bodies with gas transport layers between them G. According to the invention, at least one composite body is used to build the stack, but preferably two un different, as shown in Fig. 1.

The individual functional ones forming a composite body Layers are ceramic green foils and made from a suspension the ceramic raw materials or components in one organic binders, also known as slips, produced. By means of a suitable procedure this slip finally creates foils.

Suitable processes are, for example, film casting, films drawing, slip molding, extruding or other usual kerami  procedures. There are special advantages with a film pulling process achieved using the Solufill® process. With help The films are known for a special polymeric binder poured in a thickness of, for example, 500 microns and then pulled. While maintaining a homogeneous distribution development of the ceramic particles and without the composition of the slip or the film changes, the films can Stretch or pull to a thickness of 2 µm. It can a solids content of up to 60 percent by volume is set become.

Also with the usual large-scale film casting thin layers can be produced, which is particularly true which is important for the bipolar layer or the ICM I. Because a thinner green film can be sintered better and denser than a thicker film, it can be used for the bipolar layer I required gas tightness can be achieved.

A modified lanthanum chromate LaCrO _{3 can be used} as material for the bipolar layer I.

Depending on the electrode type, different requirements are placed on the electrode material. In general, a high electrical conductivity, predominantly based on electron conductivity, is required, which should be even higher for the anode than for the cathode (greater than or equal to 10 ⁵ Sm ^-1 ). At an operating temperature of, for example, 800 to 1100 ° and a pressure of up to 16 bar, the functionality of the electrodes must be ensured, the anode being exposed to a reducing atmosphere and the cathode to an oxidized atmosphere (for example H ₂ and O ₂ or air) . The anode layer P consists, for example, of a zirconium oxide / nickel cermet (= ceramic metal), the zirconium oxide preferably being stabilized with up to 10 mol percent yttrium. The nickel content in the cermet must not fall below 33 percent by volume in order to ensure a defined and sufficient electronic line. An optimal cermet has a porous structure, whereby one third of the volume is taken up by pores, metal and ceramic.

A number of mixed oxides of the ABO ₃ type from the group of perovskites are available as material for the cathode layer N. The specific components A and B assigned to crystallographically defined positions can each be formed by one element or represent a stoichiometrically exact mixture of several cations. For example, A can be selected from lanthanum, strontium and calcium, while B can be manganese, cobalt or nickel.

The individual functional layers of a NIP composite body are preferably laminated as green foils, whereby the good laminatability according to the Solufill® process provided foils result in special advantages. It is possible of course, the individual layers according to different Produce process, or presinter if necessary. Ins especially the bipolar layer I can have a higher sintering temperature temperature of 1600 ° C, for example, so that they are preferred is pre-sintered individually. The tightness of the bipolar Layer I can also be increased with a CVD process, with the help of any pores still present with Yt trium-stabilized zirconium oxide YSZ.

For the production of a PEN composite body, the same Chen procedures are used as for the preparation of the NIP composite body. The electrolyte layer E is preferred wise from YSZ, is because of the electrical conductivity loss are as thin as possible and should be used to seal the various Gas spaces should also be gas-tight. According to the Solufill process provided functional layers bring because of their good La minimizability advantages here too. It also makes good ones  Interface properties between the different Layers and in particular good electrical contact aims.

The functional layers of the PEN composite body can be used as Green foils are stacked and pressed to the composite body. Individual layers and the composite body itself can be used at approx. 1300 ° C can be pre-sintered. A pre-sintered composite body can be subsequently sealed by a CVD process with YSZ to fill any pores that may still be present with YSZ len. The boundary layer between the Electrolyte layer E and the anode layer P stabilized, and the boundary layer between electrolyte layer E and cathodes layer N improved.

Pre-sintered individual layers are preferably made with the mate rial of the adjacent layer printed before the composite body is made to provide good interdiffusion and verbes to ensure maximum liability.

The gas transport layers are made from the material of the corresponding adjacent electrode layer, so they consist, for example, of a YSZ / nickel cermet or a correspondingly modified ABO ₃ compound. To perform its function, the gas transport layer G must either be correspondingly porous or have predetermined gas channels.

A desired porosity of the gas transport layer G can be in be easily made by the desired one Pore size corresponding combustible beads in the desired Amount added to the slip used in ceramic production. For example, those made of carbon or plastic Beads burn at the latest during the final sintering process and leave the desired pores.  

Gas transport layers G having gas channels can also pass through Extruding green sheets are made.

The gas channels are particularly advantageous in gas transport layer G, however, according to the so-called lost wax process poses. Similar to the described procedure for the introduction of To do this, pores become a corresponding shape from one easily flammable plastic K, for example by injection molding or Stamping, manufactured and depending on the ceramic shape used poured or pressed in. The advantage this procedure consists in that the channel structures in the green gas transport layer G filled with plastic K. are, so that no deformation of the gas transport layer G at later compression to the stack must be feared. A Another advantage of the lost wax process is that complicated gas channel structures, for example way for a co-current or counter-current principle in the gas supply Have it made.

According to this embodiment, NIP and PEN Composite body with gas transport layers in between a material consisting of the neighboring electrode layers al layered on top of one another and pressed into a stack. Opp if necessary, the individual layers can again be as before be printed with the appropriate material. The off closing sintering process is then as low as possible temperature, which should not exceed 1400 ° C, done sintered. A high-temperature fuel cell manufactured in this way can be put into operation immediately after heating up the.

Fig. 2 shows a further embodiment in which a high temperature fuel cell can be constructed from only two different composite bodies (GIG and PEN). GIG composites can be manufactured according to the NIP or PEN composites. Here, too, the individual functional layers can be individually pre-sintered, pre-sintered in a composite or only sintered in a stack. Also applies to the GIG composite body that the bipolar layer I can be made very thin according to the invention, since it is embedded between the two gas transport layers G, which help the composite body to a high mechanical stability. Subsequent sealing with YSZ is also possible here using a CVD or EVD process. Another advantage of this embodiment is that the fuel cell stack is made up of only two different composite bodies, which reduces the process outlay.

FIG. 3 shows a finished fuel cell stack as it is obtained with the two process variants described. In principle, it can consist of any number of individual parts, although differently motivated optimizations towards low process costs, improved electrical and mechanical properties and high electrical performance requirements can lead to a different number of individual cells.

In Fig. 3, a cross-flow principle is chosen for the arrangement of the gas channels in the gas transport layers G, in which the fuel gases (H ₂ ) and oxygen or air in a Win angle of 90 ° against each other through the fuel cell stack ge. A parallel arrangement of the different gas channels is also possible, with a co-current or counter-current principle can be maintained when the gases are passed through. A non-linear and arbitrarily complex arrangement of the gas channels is also possible. This always follows the goal of achieving an optimal and usually as uniform as possible temperature distribution during operation of the fuel cell. On the way through the fuel cell, the fuel is used up and enriched with combustion products (for example H ₂ O). As a result, the current density decreases there, less heat loss is generated, and the temperature drops. This can be compensated for by suitable routing of the gas channels.

Another aspect of the method according to the invention relates the subsequent sealing of the finished fuel cells stacks using the previously described CVD or EVD process with YSZ, so that the fuel gases do not come out of the side of the burner can emerge fabric cell stack. With a straight line the Gas channels, it is sufficient, the respective gas transport layer G on the outer surfaces parallel to the gas channels on Seal stack. The CVD / EVD process is in this respect from Advantage than different leadership for the Separation of gases required, for example water vapor and the metal chlorides of yttrium and zirconium, the YSZ-Ab divorce can only be accomplished where the the gases come into contact with each other, for example at un tight spots. In a simple way it becomes a faster and selective closure of pores and leaks leap Due to the ionic conductivity of the deposited YSZ The deposition is also made on already closed materials Pores continued, being the reaction partner for the Metal chlorides no longer water, but available from the YSZ provided oxygen ions serve. This second phase of Ab divorce is called EVD.

The dimensions of the individual layers or the entire Fuel cell stacks depend on the operating conditions gen. Generally applies that with decreasing operating temperature the bipolar layer I and electrolyte layer E thinner must be performed. For an assumed operating temperature For example, the thickness of the electrolyte layer should be 800 ° C are in the range of 20 µm, while that of the bipolar layer I because of the higher conductivity between 100 and 200 µm can lie. The thickness of the anode or cathode layer is so chosen that the composite body has sufficient mechanical  Have stability, and is usually depending on the material in the range from 50 µm to 200 µm. The thickness of the gas transport layers G is also at the desired operating condition gene adapted, with a gas channels having gas transport layer G, for example, has 2 mm diameter gas channels.

The base area of the functional layers or the composite body produced therefrom depends on the controllability of the corresponding film technology. High-quality ceramic foils can be produced, for example, in a size of 15 × 15 cm ² .

Claims (10)

1. Method for producing an integrated high temperature fuel cell with the steps

• - Manufacture of at least one planar composite body (NIP, PEN, GIG) from three functional layers, each of the layers being a ceramic green sheet and having at least one function which is selected from gas transport layer (G), bipolar layer (I), anode layer (P), cathode layer (N) and electrolyte layer (E),
• - Layering of the composite body (s) with other functional layers, if necessary, to form a stack in which an electrolyte layer (E) is arranged between an anode (P) and a cathode layer (N) and a gas transport layer (G) is arranged above each. and
• - Pressing and sintering the stack into a fuel cell stack.

2. The method according to claim 1, in which the composite body and / or functional layers before are pre-sintered on top of each other. 3. The method according to claim 1 or 2, in the stack above the gas transport layers (G) each a bipolar layer (I) is arranged. 4. The method according to any one of claims 1 to 3, where the fuel cell stack after sintering with the help a CVD or EVD process with yttrium-stabilized zir Konoxid (YSZ) is sealed.   5. The method according to any one of claims 1 to 4, in which an electrolyte layer (E) between an anode (P) and a cathode layer (N) embedded and to a PEN- (= positive / electrolyte / negative), and a bipolar layer (I) between two gas transport layers (G) to one GIG (= gas transport port / ICM / gas transport) composite body to be joined. 6. The method according to any one of claims 1 to 4, in which an electrolyte layer (E) between a cathode (N) and an anode layer (P) and a bipolar layer (I) between an anode (P) and a cathode layer (N) embedded and to a PEN or NIP composite body (= posi tiv / electrolyte / negative or negative / interconnection materi al / positive). 7. The method according to any one of claims 1 to 6, in which the composite body and / or functional layers before stacking, especially with a screen printing process ren printed, with anode and cathodes as printing paste material or a mixture of cathode material / YSZ used becomes. 8. The method according to claim 5, where the stack is stacked alternately is erected by GIG and PEN composite bodies. 9. The method according to claim 6, where the stack is stacked alternately of composite bodies (NIP, PEN) with gas transport layers (G) is witnessed, taking as order PEN / gas transport layer / NIP / gas transport layer / PEN. . . etc. is observed. 10. The method according to any one of the preceding claims,  in which the sintering of the stack using a controlled Temperature profile is carried out so that the volume swin process of the not yet sintered ceramic radio tional layers and / or composites linear with time expires.