EP0739543A1

EP0739543A1 - Perovskite electrodes and high temperature fuel cells fitted therewith

Info

Publication number: EP0739543A1
Application number: EP95905046A
Authority: EP
Inventors: Dasarathi Laser and Plasma Technology DAS; Jimmie Edwards; Lutz Kindermann; Klaus Hilpert; Günther PÜTZ
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 1994-01-11
Filing date: 1995-01-10
Publication date: 1996-10-30
Also published as: NO962900D0; AU1382195A; JPH09507335A; NO962900L; US5824429A; AU694471B2; WO1995019053A1; DE4400540A1; DE4400540C2

Abstract

In perovskite electrodes in contact with solid electrolytes, in particular those based on ZrO2-Y2O3, reactivity in the contact area is clearly reduced by doping the perovskite with platinum metal. Iridium or ruthenium in oxide form are preferred, in particular in a concentration range from 10 to 1000 ppm, appropriately distributed through the whole volume of the electrode. A platinum metal additive for perovskites based on lanthanum-ferrite is of particular interest. The perovskite electrodes provided with an inhibiting platinum metal additive are appropriately used as cathodes in high temperature fuel cells.

Description

Perovskite electrodes and equipped with them

High temperature fuel cells

The invention relates to an electrode

Perovskite base with solid electrolyte contact, which is particularly suitable for high-temperature fuel cells.

Solid electrolyte fuel cells usually operate at operating temperatures of around 950 to 1000 ° C. The aim is to lower the operating temperature to approximately 800 ° C. ZrO2 (YsZ) stabilized with Y ₂ O _{3 is} generally used as the solid electrolyte. The solid electrolyte, which in the classic concept also serves as a substrate support and usually has a thickness of 100 to 150 μm, is coated on both sides with different materials as electrodes (see Fig. 1): Perovskites such as (La _1-x Sr _x ) MnO ₃ applied, as an anode a Ni / ZrO ₂ cermet.

The high operating temperatures are necessary in order to keep the energy losses that occur during energy conversion within reasonable limits. However, they have the disadvantage that high demands are placed on the materials used and the construction of the cell. A particular problem here is the chemical interactions that interfere with operation

Interfaces of the different material pairings. These can lead to the formation of new phases such as SrZrO ₃ and La ₂ Zr ₂ O7 at the interface between the cathodes and the solid electrolyte, which is disruptive affects cell operation. This makes it difficult to select suitable perovskites.

The aim of the invention is therefore to reduce the chemical interactions between the perovskites serving as electrodes and the solid electrolyte and, if appropriate, to improve the electrochemical properties of the electrodes. This goal is achieved by inhibiting platinum metal doping in the perovskite adjacent to the electrolyte.

It has been surprisingly found that the reactivity of perovskites towards solid electrolyte materials, in particular those based on ZrO ₂ -Y ₂ O ₃ , is significantly reduced by platinum metal doping of the perovskite, which contains platinum metal (ions) built into the lattice. It was also found that certain platinum metals, such as iridium, which are usually very volatile under the influence of oxygen, are bound in the perovskite. So

doped electrodes additionally show improved electrochemical properties.

The doping amounts useful here can go up to over 1%, but are expediently chosen in the range from 10 to 10 ³ ppm. Among the platinum metals, iridium and ruthenium are preferred, which are absorbed by the perovskite in particular in oxidic form. The doping can vary over the total volume of the

Extend electrode, but the border area with the solid electrolyte is particularly important. Perovskite electrodes with different compositions and varied doping are known (see, for example, EP 0 373 745 A2), but no plantin metal doping, in particular of the Borderline to the electrolyte was considered, let alone recognized its stabilizing effect. There is only a reference in DE 28 37 118 C2 to the addition of platinum or platinum alloy to chromium-containing electrodes with a perovskite structure to improve the catalytic adjustment of the exhaust gas balance without any specification. This proposal from 1978, which follows the general knowledge of the exhaust gas catalytic effect of platinum and platinum alloy, can in no way serve as an indication of the stabilizing effect

Effect of doping perovskite electrodes by platinum metals in oxidized form in the boundary region to the solid electrolyte can be understood. To achieve the doping according to the invention, platinum metals with high oxide vapor pressure (e.g.

Iridium) introduced into the material via the gas phase. For this, the perovskite e.g. in air or another oxygen-containing atmosphere at elevated temperatures (approx. 600 - 1000 ° C) exposed to the access of the oxide vapor for long periods. Alternatively, the desired doping can be achieved by impregnation and heat treatment or by suitable additives already during electrode manufacture.

In the context of the present invention, particular attention is paid to the perovskite cathodes of high-temperature fuel cells, but a perovskite provided for the anode can also be doped with platinum metal according to the invention.

The doping according to the invention is useful for types of perovskite and mixtures such as are provided for fuel cells, such as for example perovskite based on LaMnO ₃ , LaCoO ₃ , LaFeO ₃ ~, LaCrO _3, etc.

Perovskites based on lanthanum ferrite were examined in particular. Y ₂ O ₃ -containing ZrO ₂ materials are widely used as the solid electrolyte. The reduction in reactivity of perovskite electrodes by platinum metal doping (in particular in oxidic form) can, of course, also be used with other oxide masses serving as solid electrolyte, such as mixed oxides based on Gd, Ce-based and doped BaCeO ₃ .

Of course, the invention is not limited to fuel cells, but can be used in all cases of boundary layer structures operated at elevated temperatures, in which material of the perovskite type and high-temperature-resistant oxide materials in particular

amphoteric to slightly basic

adjoin.

The reduction or avoidance of chemical

Interactions according to the invention offers the

Possibility to use perovskites with improved electrical properties at high operating temperatures (950-1000 ° C) or to achieve longer operating times.

Furthermore, the use of perovskites, e.g.

(La _1-x Sr _x ) - (Fe _1-y Co _y ) O ₃ , which, because of their attractive electrical properties, allow economical cell operation even at lower temperatures (from about 500 to 900 ° C): a lowering of the cell temperature , whereby the thickness of the YsZ can also be reduced significantly below 100 μm or

Solid electrolytes other than YsZ can be used is a primary goal of fuel cell development in order to reduce costs and extend operating times. Attempts to illustrate the invention are given below. Reference is made to the accompanying drawings; FIG. 1 shows a diagram for an electrode / electrolyte arrangement of fuel cells;

FIG. 2 shows a graph for the SrZrO ₃ formation in a powder mixture of YsZ and perovskite,

La _0.6 Sr _0.4 Fe _0.8 Mn _0.2 O _3-δ , with ▲ or

without ● Ir doping depending on the

Time;

Figure 3 is an x-ray diagram for

La _0.6 Sr _0.4 Fe _0.8 Mn _0.2 O _3-δ (1),

La _0.6 Sr _0.4 Fe _0.8 Mn _0.2 O _3-δ + 121 h Ir (2) and La _0.6 Sr _0.4 Fe _0.8 Mn _0.2 O _3-δ + 48 h Ir (3); and

Figure 4 is a graph for the cell volume

Outsourcing the composition

(La _0.6 Sr _0.4 ) (Fe _0.8 Mn _0.2 ) O _3-δ ^{over 48 h and} 121 h in an Ir atmosphere.

Perovskite powders of different compositions (Tab. 1) were loaded for 48 h or 121 h at 900 ° C together with iridium sheet in air via the gas phase. The samples pretreated in this way were mixed with the electrolyte material 8YsZ (with 8 mol% Y2O3 stabilized ZrO ₂ ) in equimolar proportions, pressed into pellets and annealed or aged at 1000 ° C. for different times. The formation of reaction products (SrZrO ₃ or CaZrO ₃ ) after 25 h or 48 h exposure was determined with the aid of X-ray diffraction images. The results are summarized in Table 1. As can be seen, the samples doped with iridium show a much lower tendency to react with the electrolyte than that untreated samples. The time course of the SrZrO ₃ formation is plotted in FIG. 2, which clearly shows the difference.

As the following experiments show, when the perovskite is loaded with iridium, the platinum metal is demonstrably incorporated into the lattice:

A perovskite with the composition (La _0.6 Sr _0.4 ) _0.9 Fe _0.8 Co _0.2 O _{3-δ was} chosen as the starting material, which was calcined for 48 h at 900 ° C in an oxygen atmosphere. In parallel, two samples of the same composition were "outsourced" in the same oxygen atmosphere in the presence of iridium at 900 ° C for 48 h or 121 h.

The X-ray diffraction images subsequently produced show a clear splitting of the perovskite reflexes for the Ir-loaded samples (FIG. 3), to which two different grating types could be assigned with the aid of computer calculations (Table 2). In both samples loaded with iridium, in addition to the orthorhombic starting grid, a rhombohedral grid with a considerably enlarged cell volume can be found (Tab. 2 and Fig. 4).

This finding suggests that the iridium is built into the perovskite lattice.

In addition, perovskite compositions,

La _0.6 Sr _0.4 Fe _0.8 Co _0.2 O _3-δ and (La _0.6 Sr _0.4 ) ₀₉ Fe _0.8 Mn _0.2 O _3-δ , with relatively high iridium concentrations (up to a few% by weight) over a long period (226 h and 432 h) exposed to an oxygen atmosphere at 900 ° C. The subsequent analysis showed Ir losses of a maximum of 20%. In conclusion, it can thus be established that the iridium is actually incorporated into the lattice of the perovskite as a doping and counteracts the reactivity towards the solid electrolyte there. In addition to the doping experiments using the vapor phase, a perovskite with the composition La _0.6 Sr ₀ .4Fe _0.8 Mn _0.2 0 _3-δ with an iridium doping was synthesized directly from the components.

The nitrates of the corresponding metal components of the perovskite and Ir (III) oxide (Ir ₂ O ₃ ) served as starting materials. All components were homogenized in an aqueous solution, dried and then stored in air at 1200 ° C. for 48 h in order to build up the perovskite grid. With the help of a parallel iridium determination using neutron activation analysis and

Spark source mass spectrometry, the iridium content could be determined to be about 200 ppm in both cases.

The perovskite obtained was mixed in an equimolar ratio with YsZ, pressed into pellets and aged at 1000 ° C for 72 hours. The result is shown in Tab. 1.

Claims

Patent claims

1. perovskite-based electrode with solid electrolyte contact,

marked by

an inhibiting platinum metal doping in

Perovskite adjacent to the electrolyte.

2. Electrode according to claim 1,

g e k e n n e i c h n e t by a content of

Iridium or ruthenium in oxidic form.

3. Electrode according to claim 1 or 2,

g e k e n n e i c h n e t by one. Platinum metal content of 10 - 1000 ppm in the perovskite across the total volume of the electrode.

4. Electrode according to one of the preceding claims,

characterized

that the base material is a perovskite based on lanthanum ferrite.

5. Electrode according to one of the preceding claims,

marked by

their use in high temperature fuel cells.

6. Electrode according to one of the preceding claims,

marked by

its use as a cathode in solid electrolyte fuel cells.

7. High temperature fuel cells with electrodes

according to one of the preceding claims, especially as cathodes ^,