Perovskite electrodes and high temperature fuel cells fitted the

Details Perovskite electrodes and high temperature fuel cells fitted the Perovskite electrodes and high temperature fuel cells fitted the

1996-10-29

1998-10-20

Kalafut, Stephen

Chemistry: electrical current producing apparatus, product, and

Having earth feature

429 30, H01M 490

Patent

active

058244298

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The invention relates to a perovskite based electrode with solid electrolyte contact which is especially suitable for high temperature fuel cells.


BACKGROUND OF THE INVENTION

Solid electrolyte fuel cells generally operate with operating temperatures of about 950.degree. to 1000.degree. C. A reduction of the operating temperature to about 800.degree. C. is sought. The solid electrolyte used is as a rule Y.sub.2 O.sub.3 stabilized ZrO.sub.2 (YsZ). The solid electrolyte, which in classical concepts serves simultaneously as a substrate carrier and usually has a thickness of 100 to 150 .mu.m is coated on both sides with different materials as electrodes (compare FIG. 1): as a cathode, usually perovskite as, for example, (La.sub.1-x Sr.sub.x)MnO.sub.3 is applied, as the anode an Ni/ZrO.sub.2 cermet is applied.
The high operating temperatures are required in order to maintain the energy losses which arise from the energy conversion within tolerable limits. They have, however, the drawback that the construction of the cell and the materials used must conform to high standards. A special problem arises because of chemical exchange at the boundary surfaces of the different material pairs which can be detrimental. These can result at the boundary layer between the cathode and the solid electrolyte, in the formation of new phases like, for example, SrZrO.sub.3 and La.sub.2 Zr.sub.2 O.sub.7 which are detrimental to cell operation. The selection of suitable perovskites is thus made difficult.


OBJECT OF THE INVENTION

The object of the invention is therefore a reduction in the chemical exchange between the perovskites serving as electrodes and the solid electrolytes and optionally an improvement in the electrochemical characteristics of the electrodes.


SUMMARY OF THE INVENTION

This object is achieved through an inhibiting platinum group metal doping in the perovskite bounding the electrolyte.
Namely, it has been surprisingly found that the reactivity of perovskites with respect to solid electrolyte masses, especially those of a ZrO.sub.2 --Y.sub.2 O.sub.3 basis can be significantly reduced by a platinum group metal doping of the perovskite which contains platinum group metal (ions) incorporated in the lattice. It has been found further that certain platinum group metals, like for example iridium, which usually need the influence of oxygen are highly volatile as oxides, are found in perovskite. Thus doped electrodes additionally show improved electrochemical characteristics.
The customary doping amounts which are usable can go to above 1% but advantageously is selected in the range of 10 to 10.sup.3 ppm. Of the platinum group metals, iridium and ruthenium are preferred, these especially being received in oxidic form by perovskite. The doping can extend over the entire volume of the electrode but especially important however is the boundary region with the solid electrolyte.
Indeed, perovskite electrodes in a variety of compositions and with various dopings are known (see for example EP 0 373 745 A2) although to date no platinum group metal doping especially at the boundary region with the electrolyte has been contemplated nor has its stabilizing effect been disclosed. In DE 28 37 228 C2 one can find only a suggestion of the addition of platinum or a platinum alloy to chromium-containing electrodes with a perovskite structure to improve the catalytic setting of the exhaust gas equilibrium without further specification. The general knowledge of the waste gas catalytic effect of platinum and platinum alloys follow proposals from 1978 and in no case is a suggestion of the stabilizing effect of a doping of perovskite electrodes with platinum group metals in an oxidized form at the boundary region with a solid electrolyte.
To produce the doping according to the invention, platinum group metals with high oxide vapor pressure (as for example iridium) are incorporated from a gas phase in the material. For this purpose the perovskite is subjected, for example, in air or in another oxygen-containing a

REFERENCES:
patent: 5543239 (1996-08-01), Virkar et al.
Extended Abstracts, vol. 89, No. 2, 1989 Princenton, New Jersey, p. 797 A. Tsunoda et al, 'Effects of Additions on the Resistance of La0,9Sr0,1Mn03 Cathode.
Tsunoda et al., Patenet Abstracts of Japan, vol. 16, No. 162 (E-1192), p. 105, corresponds to Japanese kokai No. JP4-12457, Jan. 1992.

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