DE19626342A1 - Procedure for making solid electrolyte as thin layer on porous electrode for fuel-cell - Google Patents

Abstract

A procedure for making a thin electrolytic layer on a porous electrode includes the of pouring a suspension with electrode particles as the solid component over the electrode. Then the electrode is dried. The diameter of the particles is then chosen so that the layer arising after sintering on the electrode has a medium pore size which is smaller by a factor of at least two than the medium pore size of the electrode. A further identical suspension is poured over the electrode, followed by drying and further sintering. The layer thickness is between one and twenty microns, preferably five microns.

Description

The invention relates to a method for manufacturing position of an electrode / electrolyte layer unit for Fuel cells with a porous electrode as well an electrode / electrolyte manufactured according to the method layer unit.

It is known from DE 44 37 105 and from DE 195 20 458 a porous electrode with one on it thin electrolyte layer with a thickness of approx. 20 µm To produce electrophoresis. The production of a thin NEN electrolyte layer on a porous substrate (Electrode) is also in the German patent application with the official file number 196 09 418.6 been.

The known electrodes have a porosity through pores to in a fuel cell ensure a rapid gas exchange. Further the product water through the pores as well the product gases derived.

The well-known anodes for high-temperature fuel cells len are regularly made of a ceramic / metal composite  produced. A typical pore size is approx. 2-5 µm.

In a fuel cell, the electrolyte layer must be gas be tight. Furthermore, it should have large current densities let it aim.

The object of the invention is to provide a method rens, which is the manufacture of an electric de / Electrolyte layer unit with improved gas seal activity of the electrolyte layer and with higher current density compared to the cited prior art enables. Another object of the invention is the sheep an electrode / electrolyte unit with such improved properties.

The task is accomplished through a process with the characteristics of the main as well as through an electrode / electrolyte Layer unit with the features of the subsidiary claim solved. Advantageous configurations result from the related claims.

According to the method, an intermediate electrode layer prepared by coating a suspension on the Tending electrode surface is poured. The electrode assumes the function of a Substrate and has in particular the necessary for this mechanical strength.

The suspension contains solids that consist of an - ins especially suitable for high temperature fuel cells tem - electrode material (for an anode made of anode material material, with a cathode made of cathode material) best hen. The size of the solids is chosen so that the  Process-produced layer in the final state Po ren with an average pore size that around at least a factor of two, preferably by at least is at least a factor of five smaller than the middle one Pore size at the electrode. The amount of solids in the first suspension is preferably chosen so that the layer produced in accordance with the method ultimately stood 1 to 50 µm thick. In particular, it is not thicker than 20 µm, preferably not thicker than 5 µm.

The solvent in the suspension escapes after Pour on through the pores of the electrode. On the elec The solid parts remain on the surface of the electrode. The Electrode with the solids on it is first dried.

This coated electrode is processed according to the method another layer functioning as an electrolyte, such as follows upset.

There is another suspension on the electrode with poured into the (intermediate) layer. The solid component le in the second suspension are chosen so that after the desired gas-tight electrolyte during sintering layer is present. Sintering causes material ver seal by heating.

The aforementioned further suspension contains, for example as ZrO₂ + 8 mol% Y₂O₃ (YZS).

A gas-tight layer is created on the substrate. This layer can be used as an electrolyte layer in a High temperature fuel cell act.  

To produce an electrolyte layer, all che known from DE 44 37 105 and from DE 1 95 20 458 Suspensions used.

The mechanical strength of the dried layer can be advantageous in one embodiment of the method can be increased by using the electrode with the dried solids on it is finally calcined. Under calcination is one To understand heating, the volatile substances escape lets, but this does not result in material compression Has.

In a further advantageous embodiment of the Er invention, a suspension is poured onto an anode sen, which apart from YZS also contains shares in NiO. It ent is a porous layer that is relatively small ne has pores in relation to pore sizes, the ty typically anodes in a high temperature fuel have cell. If NiO is reduced to Ni in this layer adorns, further pores are created. Such a herge posed layer can, for example, as a thin, cataly functionally acting functional layer of an anode. This then contains z. B. 60 vol% YSZ and 40 vol% Ni proportions.

The suspension advantageously has for the production of Electrolyte layer on solid parts, the diam are dimensioned so that the pores of the upper most porous layer is added on the electrode, d. H.  become blocked (coarse parts). This causes that on the one hand solids in the suspension, the smaller in diameter than the diameter of the pores of the substrate (here: electrode with a Intermediate layer) are after the addition of the pores not flow through the substrate and so undone remains useful. On the other hand, such "interlocking" achieved between the substrate and the layer because part of the Particles penetrate into the pores of the substrate. The Gearing advantageously causes a mechanical extra Really stable connection between the electrode and Layer. The stable connection is e.g. B. then from be special advantage if the electrode-electrolyte unit is exposed to fluctuating temperatures, so that the connection has grown due to thermal stresses have to be. Such thermal stresses occur for example in a high temperature fuel cell on.

In the optimal case, the further suspension contains too this purpose coarse-grained solids, the maxi Painter diameter equal to the diameter of the minimum Pore size in the top layer of the substrate. It then becomes a particularly good interlocking and with it a particularly good mechanical connection between Electrolyte layer and electrode achieved. The delicacy le with small diameters are desirable because with it good sintering behavior is associated. The diameter However, the fine fraction should be at least 0.3 µm carry.  

The portion of the coarse portions in the Sus is advantageous board for the production of the electrolyte layer mini lubricated. That is, it is chosen so large that only the pores of the electrode according to the purpose be set. One of the best from the coarse-grained fractions Making this layer is not the purpose of this measure took. 3 to 10% of coarse parts have - depending on Electrode - found to be sufficient to support the buttocks to add.

The porosity of an electrode is typically in of the order of 45-48%. The pore size in the The electrode is typically average 2-5 µm. The pore size of the process manufactured Intermediate electrode layer is then smaller than 1-2.5 µm. The diameter of the coarse-grained solid parts should therefore be at least 1 µm in this case carry.

The diameter of the fine-grained hard is advantageous Substances in the aforementioned suspension at least 5 times, especially 8 to 12 times smaller than that coarse-grained solids, which the addition of Serve pores (fine particles).

The fines are in particular at least 10 times smaller than the coarse-grained fractions in the further one Suspension.

The gross proportions initially decrease in relation to the Fine particles accelerate downwards. You put that Pores too. Then the divorce Fine fractions.  

Fine fractions are desirable because they have a better sin show behavior as coarse-grained fractions. It has It has been shown that electrode electrolyte so suitable layer units for high-temperature fuel cells are easy to manufacture.

It is also advantageous after application of a suspension on the side of the electrode that is not coated a vacuum is created. That way, in solvent present in the suspensions completely aspirated during a coating process. On the Electrode only separates the particles layered, which wa contained in the suspension ren. In addition, a good connection of the Particles secured to the electrode.

According to the device claim, an electrode is between layer between the electrode and the electrolyte layer see. This intermediate layer has a pore size, which is smaller by a factor of two and more than that Pore size of the electrode substrate. In particular are Pore sizes are beneficial by a factor of five (preferably a factor of ten) and more smaller are as the pore sizes of the electrode substrate. The Intermediate electrode layer directly adjoins the Electrode.

To avoid thermal tensions, there are electrical systems de and the aforementioned electrode intermediate layer preferred wise from the same material. The electrodes  However, the layer can be made from a different electrode material than the electrode itself.

Between the intermediate electrode layer (first function layer) and electrolyte layer can be yet another thin functional layer may be provided.

The sophisticated intermediate layer creates the electrode surface a "smoother" surface in Comparison to the prior art mentioned. The Aufbrin an electrolyte layer on this smooth surface area is possible in a better way than the on Application on an electrode without an intermediate layer. On the (electrolyte) layers applied to the intermediate layer are more uniform in thickness, and it will greater gas tightness of electrolyte layers aims. In comparison to the prior art mentioned can improve gas tightness by a factor of 10 be effected.

Due to the smaller pore size in the electrodes intermediate layer has approximately the same porosity of electrode and intermediate electrode layer a ver Largest 3-phase limit compared to the above State of the art. The reactivity in a focal This improves the fabric cell. The improved reaction vity leads to higher current densities. Compared to Current densities mentioned can be current densities achieve, which are a factor of 3 to 4 larger.

In a further embodiment of the invention, the Slightly reduce the metal content in the intermediate layer  device (up to 5%) if the electrode functions as an anode yaws.

By slightly reducing the metal content the coefficient of expansion of the electrode substrate tes on its surface at the expansion coefficients aligned with the electrolyte layer. Destructive acting voltages due to different thermal expansion coefficients are thus min different. The durability of the electrodes / electrolyte layer Unity is increased.

Embodiment

For the coating of a porous anode substrate, be standing from 8YSZ / NiO, was initially an 8YSZ suspension manufactured as follows.

YSZ powder was calcined using commercially available YSZ powder in Al₂O₃ crucible at 1200 ° C for three hours was heated. The powder was then brought to room temperature cooled down.

200 g of the calcined YSZ powder, 600 g (765 ml) Et hanol absolutely (i.e. anhydrous), 600 g from TZ-3Y best grinding balls with a diameter of 3 mm, 600 g made of TZ-3Y existing grinding balls with 5 mm diameter were in a 1 l PE wide-mouth bottle weighed and for 48 hours the ground on the roller bank. Then was 3.9-4.0 g polyethyleneimine (PEI) solution in the wide neck bottle added. The Mi left for sedimentation. The not  dispersed solids content settled during this time from.

53% by weight of NiO (100 g of NiO from from Baker, which in 250 ml of ethanol and 2 g of PEI 120 Hours was mixed).

For a 1 µm intermediate layer on a 100 × 100 mm² large anode substrate were from this suspension 4.2 ml diluted to 20 ml with ethanol. The coating of the porous anode substrate was carried out after the German patent application with the official file number 196 09 418.6 described coating method: The Suspension was poured on and the solvent aspirated through the anode. The one so angry powdery layer was exposed to air at room temperature dried and then calcined at 1000 ° C for 3 hours. Then a 17 µm thick electrolyte layer applied in an analogous manner, at room temperature Air dried and the coated substrate plate 3 Sintered at 1400 ° C for hours.

A was used to test the electrical conductivity Substrate with a 4 µm thick anode interlayer sintered at 1400 ° C for five hours and then at 900 ° C in Ar / 4% H₂ completely reduced. The measurement the conductivity showed the same resistances as with egg a reduced anode without an intermediate layer.

Sanding the reduced substrates with intermediate layer showed substantial in the area of the intermediate layer smaller grains and smaller pores (by a fac gate 8 to 10 smaller) compared to the electrode.  

The gas permeability provided with intermediate layers reduced anodes was 0.050 cm² / min cm WS (WS: Water column) compared to uncoated and reduced Anodes (with 0.053 cm² / min cm WS) only slightly ver wrestles, d. H. the time in which 1 l of air with a Pressure difference of 100 mm water column through the same Vo lumina coated or uncoated anodes flow te, increased from about 2 hours to about 2.5 hours the. Consequently, the flow behavior in a Thanks to the thin intermediate layer, the fuel cell is practical table not affected.

The coatability of the substrates with an intermediate layer turned out to be much better than without these twos layer. Unevenly thick electrolyte layers, like the coating of untreated anodes Occasionally, substrates were found in the Be stratification of substrates with intermediate layer in none Case observed.

The leak rate of the electrolyte sintered at 1400 ° C was stratified by the application of the anodes layer reduced by about an order of magnitude.

The electrochemical properties of Fuel Cell len with anode intermediate layers were on so-called "Shortstacks" from such cells were examined. A Short stack consists of two single cells (Anode / electrolyte / cathode), a metallic bipola Ren plate (Cr5Fe1Y₂O₃) and two end plates from the same ben alloy. The operation of two such shorts stacks gave at 950 ° C and a cell voltage of 700  mV current densities of 870 mA / cm² for the first short stack and even 1.2 A / cm² for the second short stack. This he current densities ranged by a factor of 3 to 4 above those achievable under the same test conditions Values in which anodes are used without intermediate layers were detected.

Claims (5)

1. Method for producing a thin electrolyte layer on a porous electrode with the steps

• Pouring a suspension, the solids of which consist of electrode material, onto the electrode,
• - Drying the electrode with the poured Sus pension,
• - The diameter of the solids in the suspension has been chosen so that the layer formed after sintering on the electrode has an average pore size which is at least a factor of two smaller than the average pore size for the electrode,
• Pouring a further suspension, the solids content of which consists of electrolyte material, onto the electrode,
• - Drying and sintering of the electrode with the additional suspension cast on.

2. The method according to the preceding claim, in which the Solids content in the suspension for production the intermediate layer in relation to the electrical The surface is chosen so that this layer thicknesses from 1 to 20 µm - especially up to 5 µm - getting produced.   3. The method according to any one of the preceding claims, characterized in that the dried, from Calcined electrode material existing layer becomes. 4. Electrode / electrolyte layer unit for fuel cells with a porous electrode, with one on the Electrode adjacent porous intermediate layer, the consists of electrode material and the one at least at least twice the smaller average pore diameter compared to the average pore diameter in the Has electrode and which is between the electrode and electrolyte layer. 5. Electrode / electrolyte layer unit according to the previous claim with a porous intermediate layer from 1 to 50 µm thick.