WO2003056651A2

WO2003056651A2 - Double-layer cathode for a high-temperature fuel cell and method for producing the same

Info

Publication number: WO2003056651A2
Application number: PCT/DE2002/004532
Authority: WO
Inventors: Ashok Kumar Gupta; Wilhelm Albert Meulenberg; Frank Tietz; Hans Peter Buchkremer; Günther BLASS; Izaak Vinke
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2001-12-21
Filing date: 2002-12-11
Publication date: 2003-07-10
Also published as: AU2002357448A1; AU2002357448A8; WO2003056651A3

Abstract

The invention relates to a double-layer cathode for a high-temperature fuel cell. Said double-layer cathode comprises a cathode and a cathode functional layer which is applied on an electrolyte. The inventive cathode is characterised in that the cathode functional layer contains silver, the remainder consisting of lanthanum-strontium-manganite (LaSrMn03: LSM), and especially has a silver content of at least 60 vol. %. Especially advantageously, the cathode contains silver, for example in a concentration of 30 vol. %. The method used to produce the inventive double-layer cathode is either a serigraphy method or a wet spraying method.

Description

Two-layer cathode for a high-temperature fuel cell and process for its production

The invention relates to a two-layer cathode for a high-temperature fuel cell and a method for its production.

A fuel cell has a cathode, an electrolyte and an anode. The cathode becomes an oxidizing agent, e.g. B. air and the anode becomes a fuel, e.g. B. supplied hydrogen.

Various types of fuel cells are known, for example the SOFC fuel cell from the publication DE 44 30 958 Cl.

The SOFC fuel cell is also called high-temperature fuel cell because its operating temperature can be up to 1000 ° C. Oxygen ions are formed on the cathode of a high-temperature fuel cell in the presence of the oxidizing agent. The oxygen ions diffuse through the electrolyte and recombine on the anode side with the hydrogen from the fuel to form water. The recombination releases electrons and thus generates electrical energy.

Fuel cells can have functional layers. A functional layer is a layer that has to perform a defined function. A contact layer that takes over the catalytic function within the fuel cell is an example of such a functional layer.

From Ahmad-Khanlou (Ahmad-Khanlou, A., 2000. Alternative materials for components of the high-temperature fuel cell (SOFC) to reduce the operating temperature. Reports of the Research Center Juelich; 3797. ISSN 0944-2952. D 294 (Diss. Univ. Bochum It is known to arrange a fine-grained cathode functional layer between a coarse-grained, porous cathode and an electrolyte. The mechanical stability is ensured by a porous, self-supporting anode substrate. The catalytically active layer is arranged as a functional layer between the cathode and the electrolyte. Expensive catalyst is saved because the use of the catalyst can be dispensed with in the cathode. To produce the thin layers, a screen printing process in which the paste is pressed through a screen by means of the doctor blade of a doctor blade device or wet powder spraying is proposed.

It is disadvantageous that in the production of such fuel cells, after the pastes have been applied to the electrolyte, a sintering step has to be carried out in order to give the cathode sufficient strength and connection to the electrolyte.

A main goal of current research is also to achieve high current densities at operating temperatures below 800 ° C. Inexpensive peripheral materials can only be used below an operating temperature of 800 ° C. This leads to lower manufac development costs. The metallic interconnectors are also more durable at these operating temperatures.

It is known from US Pat. No. 6,004,696 to use silver to increase the catalytic function for producing a cathode. Silver is used in the form of a thin film on a LaSrCo0 ₃ - or in a LaSrMn0 ₃ - perovskite (hereinafter referred to as LSC or LSM) as a catalyst. Thin films in a range from 0.5 to 5 micrometers are sputtered onto the substrate by magnetron sputtering and finally sintered. In this way, a half cell with a cathode comprising silver and LSC is produced. The electrode interface resistance ri increases with falling temperatures and increasing silver concentrations. The silver concentration preferably varies in a range f _Ag from 0.2 to 0.6. However, silver concentrations of more than 30% by volume lead to a significant increase in the electrode interface resistance ri and thus disadvantageously to a lower electrical conductivity.

The films known from US 6,004,696 generally have the following disadvantages:

The production of a large-area layer is not possible with this method. The production of a fuel cell is therefore practically impossible.

Due to the thinness of the film, power transmission is hardly guaranteed. Thicknesses greater than 5 microns can only be achieved with great effort due to frequent sputtering. can be produced. The thin films are susceptible to cracking. For this reason too, fuel cells are difficult or impossible to manufacture.

- Silver and LSC both have high thermal expansion coefficients, which leads to incompatibilities with the other cell layers.

The process for producing the half cell is expensive and almost impossible to implement.

The object of the invention is therefore to provide a cathode for a high-temperature fuel cell which does not have the deficiencies shown in the prior art. The object of the invention is also to provide methods for producing such a fuel cell.

The object is achieved by a cathode according to the main claim and a method according to the secondary claim. Advantageous refinements result from the claims which refer back to them.

According to the invention, a two-layer cathode comprises a cathode and a cathode functional layer on an electrolyte. The cathode functional layer has silver with a residual portion of lanthanum strontium manganite (LaSrM-n0 ₃ : LSM).

A silver content of at least 60% by volume, in particular 70 to 80% by volume in the cathode functional layer has the effect that the voltage-current density characteristics of such fuel cells are improved in such a way that at an operating temperature of 800 ° C the current density in the fuel cell at a reference value of 0.7 volts is at least 1.2 A * cm ^{~ 2} . In comparison, the value in a fuel cell without silver is only about 0.6 to 0.8 A * cm ^{~ 2} . By means of the two-layer cathode according to the invention, the operating temperature can easily be reduced to 750 ° C., since at this operating temperature and a reference value of 0.7 volt, a current density of 0.8 A * crrf ² is still achieved.

Such values are achieved regardless of whether the two-layer cathode was sintered or not. This means that a sintering step is saved compared to the previous manufacturing processes.

In the case of a sintered two-layer cathode, e.g. B. at a temperature of 900 ° C for three hours, at a reference value of 0.7 volts and an operating temperature of 800 ° C, a current density of even at least 1.4 A * cm ^{"2 is} achieved.

In a particularly advantageous embodiment of the invention, the cathode also has silver, preferably in a proportion of at least 60% by volume with a residual proportion of lanthanum strontium manganite (LaSrMn0 ₃ : LSM). The cathode functional layer can then, for example, also contain considerably less silver, for example a proportion of only 30% by volume. With this measure, the sintering of the two-layer cathode can be avoided during production if the fuel cell has a high current density. Of course, higher silver concentrates can also be used for the cathode functional layer. Ones, especially concentrations with more than 60 vol% silver are used. Silver is cheaper than lanthanum strontium manganite.

A method for producing a high-temperature fuel cell comprises a screen printing method, in which a screen printing paste with at least 60% by volume of silver and a remainder of lanthanum strontium-manganite is first produced. To produce the functional layer, the paste is applied to the electrolyte using the screen printing process and dried at 50 ° Celsius. Finally, the functional layer is printed with the cathode.

Sintering can take place at 900 ° C for three hours at a heating rate of 1K / min. A suitable composition of the cathode means that sintering during production can also be dispensed with.

Alternatively, a wet powder spraying process can also be used.

The invention is explained in more detail below on the basis of a few exemplary embodiments and the attached FIGS. 1 to 3.

The following are used to produce the screen printing paste:

1. Silver from Aldrich (Cat .No .32.708-5 2-3.5 microns) with 99.99% silver content. 2. LSM, produced by a spray drying process with subsequent calcination at 700 ° C for 3 hours.

3. 8YSZ (Tosoh).

Depending on the desired composition of the cathode functional layer or the cathode, the solids are weighed in percent by volume. The total batch amount is 15 g powder. The powders are filled into a 500 ml PE bottle. The powders are mixed with 50 ml of ethyl alcohol and 500g Zr0 ₂ balls with a diameter of 10 mm and ground on a roller bench at approx. 150 rpm for 100 hours. The ground material is then separated from the Zr0 ₂ balls and dried. After the powder has dried, it is mixed with a transport binder, consisting of ethyl cellulose in terpinerol, and kneaded until the solid is well wetted with the binder. This slurry is then homogenized so that the paste gets the desired structure and becomes flowable. The half cell (substrate, anode, electrolyte) is coated with the cathode functional layer and the cathode using a doctor blade method as a screen printing method. An 80-0.100 X 22.5 ° screen is used for the coating for a desired application thickness of 50 μm. After the functional layer has been applied, it is dried at 50 ° C. and then printed with the cathode.

The green body can then be sintered at a heating rate of 1 K / min for 3 hours at 900 ° C. However, this sintering does not have to take place. Show it:

Fig. 1: Current density-voltage characteristic of a high-temperature fuel cell, comprising a 30 micron thick cathode consisting of lanthanum strontium manganite (LSM7) and a 10 micron thick cathode functional layer consisting of yttrium stabilized zirconium oxide and LSM7.

2: Current density-voltage characteristic of a high-temperature fuel cell comprising a 30 micron thick cathode consisting of lanthanum strontium manganite (LSM7) and a 10 micron thick cathode functional layer consisting of 60 vol% silver and 40 vol% LSM 7th

3: Current density-voltage characteristic curve of a high-temperature fuel cell comprising a 30 micron thick cathode consisting of 60 vol% silver and 40 vol% LSM 7 and a 10 micron thick cathode functional layer consisting of 33 vol% silver and a mixture of 33 vol% yttrium stabilized zirconium oxide and 33 vol% LSM 7.

The symbols mean:

Square: Current-voltage characteristic of the fuel cell at an operating temperature of 900 ° C. Circle: Current-voltage characteristic of the fuel cell at an operating temperature of 850 ° C.

Triangle, pointing upwards: current-voltage characteristic of the fuel cell at an operating temperature of 800 ° C.

Triangle, pointing downwards: current-voltage characteristic of the fuel cell at an operating temperature of 750 ° C.

H _{2 was used} as the fuel gas in the measurements.

The fuel cells of FIGS. 1 and 2 were sintered for three hours at a temperature of 900 ° C. after screen printing. The fuel cell of FIG. 3 was manufactured without sintering.

The fuel cells also include a flexible nickel network between the anode and an interconnector with a wire thickness of 125 microns. The nickel mesh serves for better contact of the anode or an anode substrate with the interconnector.

During the measurement, the cathode contacts an interconnector via a flexible Pt network with a wire thickness of 125 micrometers. The platinum network thus serves for better contact of the cathode with an interconnector on the cathode side. The housing of the fuel cell consists of Al ₂ 0 ₃ . The electrodes have dimensions of 40x40 mm ² . In fuel cells can A ceramic contact layer can be used instead of the Pt mesh.

The comparison of FIGS. 1 and 2 illustrates the influence of silver concentrations with at least 60% by volume silver in the cathode functional layer on the current density-voltage characteristic of the fuel cells. At the reference voltage of 700 mV and an operating temperature of 800 ° C, a current density of approx. 0.6 A * cm ^{~ 2 is} achieved without silver in the cathode functional layer (Fig. 1). With a silver content of 60 vol% instead of yttrium-stabilized zirconium oxide, on the other hand, a current density of 1.4 A * cm ^{"2 is} achieved. This value increases further when the proportion of silver in the cathode functional layer is 70 during manufacture

Vol% is set. At an operating temperature of 750 ° C., a current density of 0.8 A * cπf ^{2 is} achieved in the fuel cell of FIG. 2, without silver

(Fig. 1) only one of less than 0.4 A * cm ^"2 .

Fig. 3 shows that the sintering in the manufacture of the fuel cell can be dispensed with. In this fuel cell, a current density of 0.8 A * cm ^{~ 2 is} achieved at an operating temperature of 750 ° C, even a current density of 1.2 A * cm ^"2 at 800 ° C.

Claims

P a t e n t a n s r u c h e

Two-layer cathode for a high-temperature fuel cell, comprising a cathode and a cathode functional layer on an electrolyte, characterized in that the cathode functional layer contains silver with a residual proportion of lanthanum strontium manganite.

2. Two-layer cathode according to claim 1, characterized in that the cathode functional layer contains 60 vol% silver with a residual proportion of lanthanum strontium manganite.

3. Two-layer cathode according to claim 1, characterized in that the cathode functional layer contains 70 to 80 vol% silver.

4. Two-layer cathode according to one of the preceding claims, characterized in that the cathode contains silver.

5. Two-layer cathode according to the preceding claim, characterized in that the proportion of silver in the cathode is at least 60 vol%.

6. Two-layer cathode according to one of claims 4 to

5, characterized in that the cathode functional layer contains less than 60 vol% silver, in particular 30 vol% silver.

7. Fuel cell comprising a two-layer cathode according to one of the preceding claims.

8. A method for producing a high-temperature fuel cell, comprising a half cell made of anode substrate, anode and electrolyte with a screen printing process, characterized by the steps: - it is a screen printing paste with silver and a

Rest of lanthanum-strontium-manganite produced for the cathode functional layer,

- to produce the cathode functional layer, the paste is applied to the electrolyte using the screen printing process and dried at 50 ° Celsius,

- The cathode functional layer is printed with a cathode.

9. The method according to the preceding claim, characterized in that a screen printing paste with at least 60% by volume of silver, in particular with 70 to 80% by volume of silver and a residual proportion of lanthanum strontium-manganite is produced.

10. The method according to any one of claims 8 to 9, characterized in that sintering takes place at 900 ° C for 3 hours.

11. The method one of claims 8 to 10, characterized by a doctor blade method for applying the paste.