DE4307967C2 - Process for the production of an integrated all-ceramic high-temperature fuel cell - Google Patents

Description

The invention relates to a method for producing a Solid electrolyte high temperature fuel cell module, which several solid electrolyte high temperatures connected in series Includes fuel cells.

A process takes place in a fuel cell that essentially Lichen represents a reversal of electrolysis. The Reacti onpartner of the combustion reaction, namely the fuel and the oxygen carrier, usually air, are in space Lich separated channels led by a ceramic solid electrolyte, on both sides with elec troden is provided, are separated from each other. To the elec Fuel molecules become trodes by releasing electrons oxidized or the oxygen carrier by absorption of elec tron reduced. The solid electrolyte has an ionic line and is used for mass transfer. On the two electrodes the solid electrolyte presents a potential difference limit, in which the poorly conductive for electrons Solid electrolyte prevents the short circuit. He therefore has to low conductivity for electrons and at the same time one have high conductivity for ions.

At the operating point of the fuel cell one over the pots tialdifferenz a cell going to operating voltage received, several fuel cells are formed in series tet. This is done in a simple manner by stacking peln individual fuel cells, with intermediate bi polar layers or plates made of a so-called ICM mate rial (Inter Con  nection material) the electrical connection between two ensure neighboring individual cells.

A problem with high temperature solid electrolyte fuel cells len (SOFC) lies in the selection of those for the fuel cells used materials, the high operating temperatures from for example 800 to 1100 ° C with high at the same time Pressing the fuel gas or for combustion withstand oxygen of up to 16 bar. So arise at the borders between different layers Problems with interdiffusion, causing the properties of the Layer materials are changed. By different Thermal expansion coefficient of the materials is also the strength and tightness of a fuel cell stack at risk.

DE 39 22 673 A is a high-temperature fuel cell known in which the total difference in thermal out expansion coefficient between a metallic bipolar Plate and the remaining ceramic layers by one Sandwich construction continuously in several steps is bridged.

A solution that at least addresses the problem of different thermal expansion is reduced, is a full ceramic fuel cell. Ideally, everyone should Components in the green (not sintered) state together added and then burned together. By the A solid, mechanically stable and gas should sinter together dense structure can be achieved.

Such an all-ceramic fuel cell is an example wise from an article in Proceedings of the first interna tional symposium on "Solid Oxide Fuel Cells", 1989, pp. 307-316 known. The basic idea is to supply oxygen-containing gases in fine channels, whereby  the channels are formed by a multi-fold layer be like a corrugated cardboard between two planar Layers is embedded. One version is the folded several times and ensuring gas transport Layer of a be on both sides with electrode material layered solid electrolyte film. The supply of the under Different gases can occur within one layer level consequences. 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 complete made up of green sheets. To generate the usually from yttrium-stabilized zirconium oxide (YSZ) existing elec For example, trolytic films can be used on an industrial scale prevailing and inexpensive film casting process used become. This makes it possible to make the electrolyte sheet extremely thin produce, for example in a thickness of about 15 to 20 microns. Such a thin electrolyte foil results in a minimal one Voltage drop and thus has only low conductivity losses, which means a high usable operating voltage and thus guarantee a high efficiency of the fuel cell is achieved. The stability of the thin electrolyte layer is achieved by the production of composite bodies, that is by He also embedded between two electrode layers increases. It is also with a thinner electrolyte layer possible the same efficiency as a known cell with a thicker electrolyte layer at a lower loading to achieve drive temperature. Associated with it is a cute rigorous thermal and mechanical stress on the fuel cell and thus a higher stability and a longer Le lifetime.

Since all layers used are flat, the Press or laminate layers in the stack well, and thus a good bond between the layers in the stack target what tightness, stability and durability 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).

It is intended before layering composite bodies or of composite bodies and functional layers presintering the former or both. This measure is in particular  This is particularly advantageous when different green foils join you show different volume shrinkage during sintering. By Presinter the green sheet or the green sheet with the larger one Shrinkage can be compensated in this way. Au In addition, the different functional layers or the composite bodies have different sintering temperatures require, so that it is advantageous to the layers or Ver pre-sinter the bundle body, which is sintered at a higher temperature Need to become. When sintering together only that Sintering temperature of the lowest sintering material be enough. Since the sintering temperatures are usually above the operating temperature of the fuel cell will be so an additional thermal load on the fuel cells stacks avoided and thus an additional source of danger switched off for a possible leak in the stack.

Another embodiment of the invention relates to the inert sealing of a sintered fuel cell lenstacks. With the help of a chemical gas phase separator driving (CVD), which can be supported electrolytically (EVD) it is possible to still pores and thus poten tial leaks in the ICM (bipolar layer) and in the Electrolyte layer with a suitable material, for example add with yttrium-stabilized zirconium oxide YSZ len. This is particularly beneficial if the bipolar Do not layer when stacking the stack is completely sintered. Because at a targeted final Sintering temperature of less than 1350 ° C most for the bipo ceramic layer in question can be sintered sufficiently densely and thus gas-tight, is due to the subsequent sealing of the pores in the bi polar layer increase the material selection for this layer eats and thus simplified. The YSZ-CVD- Process 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  expire. That means the temperature rise in particular slow down at the points where shrinkage processes deploy. With these controlled sintering processes you can Tensions that occur when the sintering behavior is not exactly adapted can occur, be better balanced. The genesis of cracks that become a defective or even unusable This can avoid fuel cell stacks.

The following is the construction of an all-ceramic high temperature fuel cell and the manufacture of each Layers described in more detail using three figures.

Show

Fig. 1 and 2 show two different arrangements of composites 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 made of NIP and PEN composite bodies with gas transport layers G in between. According to the invention, at least one composite body is used to build the stack, but preferably two different ones, 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, manufactured. 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  With the help of a special polymeric binder, the films are in known manner in a thickness of 500 microns, for example poured and then drawn. While maintaining one homogeneous distribution of the ceramic particles and without the Change the composition of the slip or the film the films stretch to a thickness of 2 µm or pull. A solids content of up to 60 volume Percent can be set.

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

A modifier comes as the material for the bipolar layer I. tes lanthanum chromate LaCrO₃ in question.

Depending on the electrode type, different requirements are placed on the electrode material. In general, a high electrical conductivity, mainly based on electron conduction, is required, which should be even higher for the anode than for the cathode (greater than or equal to 10⁵ 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 and the cathode to an oxidizing atmosphere (for example H₂ and O₂ 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 stand for the cathode layer N. of type ABO₃ from the group of perovskites as a material for Available. The specific, crystallographically defined Components A and B assigned to positions can be from one element or one stoichiome represent a tric exact mixture of several cations. At for example, A can be selected from lanthanum, strontium and Calcium, while B stands for manganese, cobalt or nickel can.

The individual functional layers of a NIP composite body pers are preferably laminated as green foils, whereby due to the good laminatability according to Solufill® Ver drive produced films give special advantages. Poss Of course, it is also natural to move the individual layers according to un different processes, or if necessary to pre-sinter. In particular, the bipolar layer I can require a higher sintering temperature of, for example, 1600 ° C. so that it is preferably presintered individually. The you activity of the bipolar layer I can additionally with egg Increase the CVD method, with the help of which if necessary still existing pores with yttrium-stabilized zirconia umoxid YSZ can be filled.

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 from YSZ, is because of the electrical conductivity ver lusts as thin as possible and should seal the ver different gas spaces can also be gas-tight. After the solufill Processed functional layers bring about Its good laminatability also has advantages here. also are thus good interface properties between the un different layers and in particular a good electri achieved good contact.  

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 can be presintered at approx. 1300 ° C. A pre-sintered composite body can be retrofitted by a CVD process with YSZ seal off any pores still present with YSZ replenish. At the same time, the boundary layer between the electrolyte layer E and the anode layer P stabi lized, and the boundary layer between electrolyte layer E and cathode layer N improved.

Pre-sintered individual layers are preferably made using the Ma material of the adjacent layer is printed before the composite body is made to ensure good diffusion and ver to ensure better liability.

The gas transport layers are made from the material of the ent speaking adjacent electrode layer manufactured, be are for example made of a YSZ / nickel cermet or a correspondingly modified ABO₃ connection. The Gas transport layer G must either run to perform its function the correspondingly porous or predetermined gas channels point.

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 of the slip used in ceramic production mixes. For example, be made of carbon or plastic standing beads burn at the latest when the final one 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 method 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 according to the invention can be made very thin, 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 effort.

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 in the direction of low process complexity, 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 selected for the arrangement of the gas channels in the gas transport layers G, in which the fuel gases (H₂) and oxygen or air are passed at an angle of 90 ° to each other through the fuel cell stack. A parallel arrangement of the different gas channels is also possible, whereby a co-current or countercurrent principle can be observed 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 when operating the fuel cell. On the way through the fuel cell, the fuel is used up and enriched with combustion products (for example H₂O). As a result, the current density decreases there, less heat loss is generated, and the temperature drops. This can be compensated for by suitable guidance of the gas channels.

Another aspect of the method according to the invention relates the subsequent sealing of the finished fuel cells stacks according to the CVD or EVD process already described with YSZ so that the fuel gases do not come out of the side Can leak fuel cell stack. With straight lines Routing the gas channels is sufficient to the respective one Gas transport layer G on the lie parallel to the gas channels to seal the outer surfaces on the stack. The CVD / EVD procedure Ren is advantageous in so far as under different management of the necessary for the deposition Gases, for example from water vapor and metal chlorides of yttrium and zirconium, the YSZ deposition exclusively can accomplish where the two gases in each other Make contact, for example at leaks. In simple way is a fast and selective Ver closure of pores and leaks. Because of the ionic conductivity of the deposited YSZ material the deposition continues even on already closed pores continued, being the reaction partner for the Me tallchloride 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 are dependent on the operating conditions gung. The general rule is that with decreasing operating temperature also the bipolar layer I and electrolyte layer E thinner must be executed. For an assumed operation The temperature of 800 ° C should be the thickness of the electrolyte layer for example, are in the range of 20 microns, while the bipolar layer I because of the higher conductivity between 100 and 200 microns can be. The thickness of the anode or catho The layer is chosen so that the composite body one reaching 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 one from them The composite body produced depends on the controllability of the appropriate film technology. High quality ceramic foils Lien can be, for example, in a size of 15 × 15 cm² produce.

Claims (7)

1. A method for producing an integrated all-ceramic high-temperature fuel cell with the steps

• Provision of ceramic green foils for each of the functional layers of the high-temperature fuel cell, which are selected from bipolar layer (I), gas transport layer (G), anode layer (P), cathode layer (N) and electrolyte layer (E),
• - Arranging three ceramic green foils on top of each other to form two different planar composite bodies (NIP, PEN) with a first layer sequence NIP {cathode layer (N), bipolar layer (I) and anode layer (P)} and a second layer sequence PEN {anode layer (P ), Electrolyte layer (E) and cathode layer (N)},
• - pre-sintering the composite body (NIP, PEN),
• - Alternating stacking of the composite bodies (PIN, PEN) with a gas transport layer (G) in between to form a stack (PEN, G, NIP, G, PEN,...), And
• - Compressing and sintering the stack to a fuel cell lenstapel.

2. Method for producing an integrated all-ceramic high-temperature fuel cell with the steps

• Provision of ceramic green foils for each of the functional layers of the high-temperature fuel cell, which are selected from bipolar layer (I), gas transport layer (G), anode layer (P), cathode layer (N) and electrolyte layer (E),
• - Arranging three ceramic green foils on top of each other to form two different planar composite bodies (PEN, GIG) with a first layer sequence PEN {anode layer (P), electrolyte layer (E) and cathode layer (N)} and a second layer sequence GIG, in which a bipolar layer Layer (I) is arranged between two gas transport layers (G),
• - pre-sintering the composite body (PEN, GIG),
• - Alternating stacking of the composite bodies (PEN, GIG) to form a stack (PEN, GIG, PEN, GIG...), And
• - Compressing and sintering the stack to a fuel cell lenstapel.

3. The method according to claim 1 or 2, where the fuel cell stack after sintering with the help a CVD or EVD process with yttrium stabilized Zirconium oxide (YSZ) is sealed. 4. The method according to any one of claims 1 to 3, in which the composite body and / or functional layers before stacking, especially with a printing paste be printed, using anode and cathodes as printing paste Use material or a mixture of cathode material / YSZ det. 5. The method according to claim 4, in which a screen printing process is used for printing. 6. The method according to any one of the preceding claims, in which sintering the stack using a controlled temp maturity course is carried out so that the volume swin process of the not yet fully sintered ceramic functional layers and / or composites linear with time expires.   7. The method according to any one of claims 1 to 6, where also individual functional layers with special large sintering shrinkage before producing the composite body be presintered.