Cermet electrodes containing platinum for the electrochemical reduction of oxygen

This application is a 371 of PCT/DE01/02608, filed on Jul. 12, 2001, which claims priority to German application 100 33 906.9, filed on Jul. 12, 2000.

The invention relates to platinum metal cermet electrodes for the electrochemical reduction of O 2 , the ceramic component of which contains stabilized ZrO 2 as its main constituent.

The electrochemical reduction of oxygen plays a role in gas sensors for the analysis of gas mixtures containing O 2 . Such gas sensors are described, for example, in German Patent Application 39 34 586 A1, German Patent Application 42 40 267 A1 and German Patent Application 198 33 087 A1. The platinum metal cermet electrodes used are printed on substrates of Y-stabilized ZrO 2 . The platinum metal cermet electrodes normally have a ceramic component of 40 vol.-% (the supporting structure) and a 60 vol.-% metallic component. The supporting structure is primarily made up of Y-stabilized ZrO 2 which contains 10 wt.-% Y 2 O 3 with respect to the ZrO 2 component in terms of order of magnitude. The ceramic addition should ensure a certain porosity of the electrode material and it should also extend the electrochemically effective three phase limit of the actual contact surface between substrate and electrode based on the ionic conductivity of the supporting structure into areas of the electrode. The O 2 pump flows obtained with the known electrodes are still not satisfactory.

The object of the invention is to specify platinum metal cermet electrodes on which the electrochemical oxygen reduction is made easier compared to known platinum metal cermet electrodes.

This objective is achieved with platinum metal cermet electrodes of the type mentioned above having the features of the characterizing part of Claim 1 The polarization resistance will be defined further below.

According to an advantageous embodiment of the invention, the polarization resistance of the electrode is reduced by replacing between approximately 0.01 and approximately 1 wt.-% of the stabilized ZrO 2 with stabilized CeO 2 . The substitution of a small quantity of the ZrO 2 with CeO 2 makes it possible to attain a considerable reduction of the polarization resistance and accordingly higher O 2 reduction flows than when standard electrodes are used.

It is even more advantageous to replace between approximately 0.02 and approximately 0.1 wt.-% of stabilized ZrO 2 with stabilized CeO 2 since the polarization resistance is then particularly low.

According to an another advantageous embodiment of the invention, it is possible to reduce the polarization resistance of the electrode by placing the metal:ceramic ratio in the electrode (in vol.-%) between approximately 65:35 and approximately 85:15 and even more preferably between 70:30 and 80:20, i.e., by increasing the metal:ceramic ratio. Apparently, when the ratio is placed into the range according to the present invention, the favorable effect of a high metallic component for a low polarization resistance and the effect of a low porosity increasing the polarization resistance (due to the relatively low ceramic component) are balanced in such a way that the polarization resistance decreases in relation to the standard ratio (60:40).

It is in fact known from the aforementioned German Patent Application 42 40 267 A1 that the metal component in Pt cermet electrodes is generally between 50 and 80 vol.-%. It is also known from German Patent Application DE 42 40 267 A1 that it is possible to use other metallic oxides such as CeO 2 instead of or in addition to ZrO 2 as ion conductors in the solid electrolyte. However, the use of the cited ranges of the metal:ceramic ratio and/or the ZrO 2 :CeO 2 ratio is not known, and it is in particular not known and not suggested by the related art that it is possible to reduce the polarization resistance while maintaining the cited ranges.

It is favorable if the CeO 2 is stabilized using an oxide from the group Y 2 O 3 , Gd 2 O 3 , Sc 2 03 and HfO 2 .

The electrodes of the present invention may be used advantageously in gas sensors for pumping off O 2 from test gases containing O 2 .

Additional advantageous embodiments of the platinum metal cermet electrodes may be found in the dependent claims.

The present invention is explained in detail below with reference to exemplary embodiments illustrated by the drawing in which

FIG. 1 shows a system for illustrating the invention in a schematic cross-sectional view,

FIG. 2 shows a diagram in which O 2 reduction flows are plotted against the applied voltage for different concentrations of Y-stabilized CeO 2 in the ceramic matrix according to an aspect of the present invention and

FIG. 3 shows a diagram in which O 2 reduction flows are plotted against the applied voltage for different metal:ceramic ratios according to an aspect of the present invention.

The invention will be described below primarily using the example of Pt cermet electrodes. However, it should be made clear that the invention may be used to particular advantage in conjunction with such Pt cermet electrodes and may be explained very clearly with them; however, numerous deviations from this example are possible within the context of the claims.

System 1 shown in FIG. 1 contains a thin substrate 2 made of a highly gastight solid electrolyte material having oxygen ion conductivity such as Y-stabilized ZrO 2 , diametrically opposed Pt cermet electrodes 3 and 4 having been applied to its two surfaces. The two electrodes are electrically connected to a DC voltage source 5 in such a way that electrode 3 is the cathode and electrode 4 is the anode. An ammeter 6 is installed in one of the feeds to determine the current.

As an alternative, the metallic component in the electrodes may be made up of another precious metal from the eighth subgroup of the periodic system of the elements or of mixtures of such elements such as Pt/Rh, Pt/Ir or Pt/Pd. In order to demonstrate the invention, different proportions of the Y-stabilized ZrO 2 were replaced by stabilized CeO 2 at least in the electrode connected as the cathode in a series of tests and the metal:ceramic ratio was varied in another series of tests.

If a gas stream containing O 2 is directed past the heated system shown in FIG. 1 , O 2 is electrochemically reduced at the electrode connected as the cathode when the electrodes are energized and a corresponding O 2− stream flows between the electrodes. One measure of the reduction activity of the electrode is the polarization resistance (Rp). The Rp is a variable calculated from the initial slope of the cathodic I/U characteristic, which is a measure of the resistance that the electrode puts up to the O 2 reduction. The Rp should be as low as possible.

The inventors have determined that the polarization resistence is reduced in relation to that determined for standard electrodes if a specific quantity of the Y-stabilized ZrO 2 is replaced by Y-stabilized CeO 2 corresponding to the formula Ce 0.84 Y 0.16 O 1.92 . The cathodic I/U characteristics in FIG. 2 show that if approximately 0.1 wt.-% Y-stabilized ZrO 2 is replaced by Y-stabilized CeO 2 (molar ratio of Y-stab. ZrO 2 :Y-stab. CeO 2 =0.999:0.0008), the polarization resistance is lower than if 1 or 5 wt.-% Y-stabilized ZrO 2 is replaced by Y-stabilized CeO 2 (molar ratio of Y-stab. ZrO 2 :Y-stab. CeO 2 =0.99:0.0084 or Y-stab. ZrO 2 :Y-stab. CeO 2 =0.959:0.0405) or if the ZrO 2 contains no CeO 2 . The characteristics are based on measured data that was obtained on system 1 (see FIG. 1 ) heated to approximately 700° C. with a test gas containing N 2 and 500 ppm O 2 flowing past.

Similar results are obtainable if the CeO 2 is stabilized using an oxide from the group Sc 2 O 3 , Gd 2 O 3 and HfO 2 instead of with Y 2 O 3 . If Gd 2 O 3 is used, the composition of the stabilized CeO 2 advantageously corresponds to the formula Ce 0.8 Gd 0.2 O 1.9 .

The inventors were also able to show that it is also possible to influence the polarization resistance by varying the metal:ceramic ratio in the electrodes (at least in electrode 3 connected as a cathode). The cathodic I/U characteristics in FIG. 3 substantiate that the polarization resistance is lower if the metal:ceramic ratio is in the range between approximately 70:30 arid approximately 80:20 than if the ratio corresponds to the standard value (60:40) or is clearly above 80:20. The characteristics are based on measured data which was obtained on system 1 heated to approximately 700° C. (see FIG. 1 ) with a test gas containing N 2 and 500 ppm O 2 flowing past.

The calculated polarization resistances were at the values indicated in the table:

In order to produce standard Pt cermet electrodes applied to a solid electrolyte substrate, an initial stage of the solid electrolyte body formed from ZrO 2 , stabilized Y 2 O 3 and customary additives and a suspension are prepared, which in a proportion by volume of 60:40 contains Pt powder and a correspondingly fine-particle preformed ceramic precursor of a raw material mixture of fixed weight quantities of ZrO 2 and Y 2 O 3 and if necessary a small quantity of—for example—Al 2 O 3 , the ceramic components being mixed into a paste with the Pt powder. The unsintered solid electrolyte body is then impressed with the paste at a layer thickness of 10 to 30 μm followed by drying and finally the paste and the solid electrolyte substrate are sintered together into a solid at between approximately 1300 and approximately 1600° C.

If electrodes are to be produced that differ from the standard electrodes by only a greater metal:ceramic ratio, apart from using relatively larger Pt powder quantities, it is possible to proceed just as described above.

If electrodes are to be produced, with one part of the Y-stabilized ZrO 2 being replaced by Y- or Gd-stabilized CeO 2 , a Ce salt and a Y salt or a Gd salt are precipitated together in the correct proportion, dried, calcined and the fixed quantity is then added to the raw material mixture containing Y 2 O 3 and ZrO 2 . Apart from these additional processing steps, the production method is carried out as described above.

System 1 shown in FIG. 1 is advantageously used in a gas sensor to analyze gas mixtures containing O 2 as part of a pump cell for pumping off O 2 from the test gases, at least the cathode being designed according to the present invention.