JPH08193970A - Solid electrolyte sensor - Google Patents

Abstract

PURPOSE: To stably and efficiently detect an objective gas for a long time without being affected by various kinds of coexisting gases by constituting a detection electrode of a sub-electrode material indicating chemical potential change due to the reaction with a gas to be inspected and a current corrector with the same material as a reference electrode. CONSTITUTION: A reference electrode 2 consisting of Pt and a detection electrode 3 consisting of LiCO3 are arranged on one surface of a solid electrolyte 1 formed of magnesia stabilized zirconia plate and a current corrector 4 consisting of the same Pt as that of the reference electrode 2 is arranged so that it contacts the detection electrode 3 and the electrolyte 1. As for both electrodes 2 and 3, the arrangement position is not especially restricted if they are in a structure to be exposed in a detection gas. With this configuration, the electromotive force change of a sensor for objective CO2 indicates an improved Nernst reaction proportional to the concentration of CO2 and the electromotive force does not change even if the concentration of coexisting NO2 changes, thus obtaining a solid electrolyte sensor with a high selectivity for the target CO2 .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION This invention _{is, NOx, CO, CO 2,} SOx
, H ₂ S, relates sensor for detecting a gas such as NH _3, more particularly using a solid electrolyte, a sensor for detecting a gas concentration by electromotive force generated between the electrodes.

[0002]

2. Description of the Related Art NO emitted from various industries and automobiles
Toxic gases such as x, SOx, CO ₂ , chlorofluorocarbons, H ₂ S, and NH ₃ have become problems as environmental pollutants that cause problems such as global warming, acid rain, and foul odors. For this reason, it is necessary to reduce environmental pollutant gases by continuous monitoring of these environmental pollutant gases and feedback control of various pollutant gas removing devices. However, conventional analytical instruments are large and expensive, which limits their use. Therefore, development of a small, inexpensive, all-solid-state gas sensor with high accuracy and high reliability has been desired, and various gas sensors have been manufactured and their characteristics have been reported so far.

In a solid electrolyte sensor for detecting these gases, the solid electrolyte is used as a partition, and nitrate, carbonate,
A detection electrode having a structure in which a collector such as a salt or oxide such as a sulfate and a noble metal mesh or a noble metal paste is used as a current collector is applied. For example, it has been reported that a sensor using a stabilized zirconia, which is an oxygen ion conductor, as a solid electrolyte, and a detection electrode composed of tungsten trioxide and a current collector made of platinum paste and platinum mesh has sensitivity to H ₂ S. (Chemistry Letters (1994) p1753).

However, in this sensor, since platinum as a current collector is in contact with the solid electrolyte, it functions not only as H ₂ S but also as an oxygen electrode, and the output of the sensor shows oxygen partial pressure dependency. It is expected that.

On the other hand, in the NOx sensor, a single component nitrate or a nitrate mixed with another salt is used for a detection electrode, and a sensor in which a gold mesh is arranged on the detection electrode so as not to be in contact with a solid electrolyte as a current collector has been reported. (Chemistry Lett
ers (1994) p1733).

Furthermore, CO ₂ also used carbonates and sulfates in the sensing electrode in sensors and SOx sensor, gold mesh or platinum mesh sensors a current collector placed on the detection superb so as not to be in contact with the solid electrolyte according to the report (Tech. Dige
st of the 5th IMCS. Rome. (1994) p336, Chemistry
Letters (1992) p635).

In these sensors, since the current collector is not in contact with the solid electrolyte, the current collector does not function as an oxygen electrode. However, since oxygen is involved in the reaction of various gases on the detection electrode, it is clear that any of the sensors has an oxygen partial pressure dependence on its output.

Therefore, there has been proposed a sensor having a flat structure in which not only the detection electrode but also the reference electrode is exposed to the test gas. For example, a flat NOx using nitrate for the sensing electrode, a gold mesh current collector on the sensing electrode, and platinum for the reference electrode
It has been reported that the sensor can reduce the oxygen partial pressure dependence of the sensor as compared to the case of a partition structure (Digest of
the 18th Chemical Sensor Symposium, (1993) p73)
.

Further, in a flat CO ₂ sensor using a carbonate for the sensing electrode, a current collector made of a gold mesh on the sensing electrode, and platinum for the reference electrode, the sensor output has a dependency on oxygen partial pressure. Not reported (Proceedings of Chemi
cal Sensor Symposium, (1994) p65).

As described above, although the conventional solid electrolyte sensor having a flat structure does not exhibit or can reduce the oxygen partial pressure dependency, the change over time is not achieved because gold is used as a current collector. When the current collector is in direct contact with the solid electrolyte or the like _{CO, NO, NO 2, SOx} , HC ( hydrocarbon), H ₂
The present inventors have found that they exhibit sensitivity to various gases such as S, and when the various gases coexist, a problem arises in that the target gas concentration to be detected cannot be accurately detected. Furthermore, since these sensors are used in a state where they are heated to a certain temperature, the electrodes are separated due to the thermal stress caused by the difference in the thermal expansion coefficient between the solid electrolyte and the sensing electrode material during the heating and cooling process, and the gas to be detected There was a problem that could not be detected.

[0011]

An object of the present invention is to provide a sensor capable of efficiently detecting a target gas without being affected by various coexisting gases, except for the above-mentioned drawbacks of the conventional sensor. Is an issue.

[0012]

In order to solve the above-mentioned problems, a solid electrolyte sensor according to the present invention comprises a solid electrolyte provided with a pair of a detection electrode and a reference electrode comprising a current collector and an auxiliary electrode material, Each of the electrodes is a sensor having a structure exposed to the atmosphere of the test gas, and the detection electrode is formed of a sub-electrode material showing a chemical potential change due to a reaction with the test gas and a current collector of the same material as the reference electrode. Composed of

In a more specific solid electrolyte sensor according to the present invention, the current collector provided on the detection electrode is made of the same material as the reference electrode, so that even if the current collector comes into contact with the solid electrolyte, the contact portion serves as an electrode. The change in the chemical potential caused by functioning is the same change in the chemical potential as the reference electrode, and there is no chemical potential difference between the current collector and the reference electrode, which affects the chemical potential difference between the detection electrode and the reference electrode. Nothing. That is, the current collector may be disposed so as to be in contact with the solid electrolyte as long as the current collector is made of the same material as the reference electrode.

For example, even if a material which exhibits a chemical potential change in various gases by being in contact with an oxygen ion-conductive solid electrolyte such as Au is applied to both the reference electrode and the current collector, it can be applied to various gases. No change in chemical potential occurs between the reference electrode and the current collector. What occurs is only an electromotive force change caused by a chemical potential difference between the sub electrode material used for the detection electrode and the reference electrode.
By setting this chemical potential difference to occur only in the test gas to be measured, a solid electrolyte sensor free from the influence of gases other than the test gas is configured.

Further, the solid electrolyte sensor of the present invention has no effect on the characteristics even if the current collector contacts the solid electrolyte. Therefore, at least the current collector is formed as a porous body and is in contact with the solid electrolyte of the detection electrode. It is possible to adopt a configuration in which all the uncovered portions are covered and in contact with the solid electrolyte.

With such a configuration, even if thermal stress is generated due to a difference in thermal expansion coefficient during heating and cooling of the sensor, the detection electrode is covered with the current collector, so that the electrode material does not peel off. It is suppressed. Furthermore, the current collector functions as a filter by making the current collector a porous body that does not cause diffusion resistance of the test gas onto the electrode material,
The detection electrode can be protected from unburned carbon contained in the combustion exhaust gas and other poisoning particles that have a bad influence on the electrode material.

Furthermore, if the detection electrode is made of a mixture of the current collector and the sub-electrode material, the detection electrode is held on the solid electrolyte not only at the contact portion with the solid electrolyte but also at the contact portion with the current collector. , And the adhesion is further improved.

In the solid electrolyte sensor of the present invention, the solid electrolyte is zirconia (ZrO ₂ -M ₂ O ₃ or ZrO ₂ -MO, M is Yb, G
d, Nb, Ca, Y, Mg, Hf) and bismuth oxide (Bi ₂ O ₃ -M ₂ O ₃
Or MO or M ₂ O ₅ , M is an oxygen ion conductor such as Y, Gd, Nb, W, Sr, Ba), cerium oxide (CeO ₂ -M ₂ O ₃ or MO ₂ , M is Y, Sm) Very effective in some cases.

The material used as the current collector is required to have electronic conductivity, and is also required to have heat resistance and oxidation resistance. For this reason, noble metals such as Pt, Au, Ag, Pd, Ru, and Rh or alloys thereof, or ABO ₃ (A: La or a part of La is converted to Sr
, B: Co, Fe, Ni, Mn).

These current collectors function as oxygen electrodes for oxygen ion conductors, and also for gases other than oxygen.
Like Au, it has a change in chemical potential, and it can be sufficiently predicted that the change will differ depending on the material of the current collector. Even in such a case, the current collector and the reference electrode are made of the same material, and the chemical potential difference between the detection electrode and the reference electrode is set so as to be generated only for the test gas. A sensor that is not affected by other gases than the above is configured.

[0021]

The solid electrolyte sensor according to the present invention can detect an output characteristic based only on the chemical potential difference between the reference electrode and the detection electrode by applying the same material as the reference electrode to the current collector of the detection electrode. Further, by covering the electrode material with a porous current collector or by mixing the same with the current collector, the adhesion of the electrode material is improved, and deterioration due to carbon or other poisoning particles in the measurement atmosphere is also prevented.

Hereinafter, the present invention will be described specifically with reference to examples. Embodiment 1 FIG. 1 is a sectional view of a solid electrolyte CO ₂ sensor according to a first embodiment of the present invention. The solid electrolyte 1 according to the present embodiment is formed of a magnesia-stabilized zirconia plate. One surface of the magnesia stabilized zirconia plate, the sensing electrode 3 comprising a reference electrode 2 and LiCO ₃ made of Pt are arranged. The current collector 4 made of Pt is arranged so as to be in contact with the detection electrode and also in contact with the solid electrolyte 1.
In this embodiment, the reference electrode and the detection electrode are formed on one surface of the plate-shaped solid electrolyte. However, as long as both electrodes are exposed to the test gas, there are no particular restrictions on the positions of the electrodes. Absent.

As a comparative example of the present embodiment, a sensor having the same electrode configuration as in FIG. 1 and only the current collector 4 made of Au was manufactured, and a comparison between the sensor according to the present embodiment and the CO ₂ detection characteristic was made at a sensor temperature of 500. C., under a 21% oxygen atmosphere. as a result,
The change in the electromotive force of the sensor with respect to CO ₂ showed a good Nernst response proportional to the logarithm of the CO ₂ concentration in each of the sensors.

FIG. 2 shows the NO ₂ concentration dependency of the sensor electromotive force in the presence of 100 ppm CO ₂ . In the sensor according to the present embodiment, the electromotive force does not change even when the NO ₂ concentration changes, but the electromotive force of the sensor according to the comparative example (experimental result of the conventional sensor) increases with the increase in the NO ₂ concentration. .
As described above, by adopting the sensor configuration according to the present embodiment, a solid electrolyte sensor having high selectivity for CO ₂ could be obtained.

(Embodiment 2) FIG. 3 is a sectional view of a solid electrolyte NOx sensor according to another embodiment of the present invention. In this embodiment, a solid electrolyte plate 5 made of yttria-stabilized zirconia, a reference electrode 6 made of Pt, a detection electrode 7 made of chromium nitride, and a porous body of Pt arranged so as to cover the detection electrode 7 and contact the solid electrolyte And a current collector 8 comprising: As a comparative example of this embodiment, a sensor having a structure similar to that of the sensor shown in FIG. 1 of Embodiment 1 and having a detection electrode of chromium nitride was manufactured.
A temperature cycle test in which heating and cooling were repeated until the detection electrode was performed was performed, and the number of cycles until the detection electrode was peeled was measured. In the sensor according to the present embodiment, the separation of the detection electrode was not observed until 500 cycles, and the electromotive force change with respect to NOx did not change from that before the test. However, in the sensor according to the comparative example, the separation was observed in 55 cycles, Ceased to function.

(Embodiment 3) FIG. 4 is a sectional view of a solid electrolyte NOx sensor according to another embodiment of the present invention. In the present embodiment, a solid electrolyte plate 9 made of yttria-stabilized zirconia is mixed with a reference electrode 10 made of Pt, a detection electrode 11 made of chromium nitride, and a current collector 12 made of Pt. Sensors in which the mixing ratio of Pt to chromium nitride was 5% by weight, 10% by weight, 30% by weight, and 50% by weight were manufactured, and the same temperature cycle test as in Example 2 was performed. It was measured. In the sensor with the added amount of Pt of 5 wt%, the electrode was peeled off in 102 cycles.
No separation of the sensing electrode was observed until the cycle, and NOx
The change in electromotive force was not different from that before the test.

[0027]

As described above, the solid electrolyte sensor according to the present invention can provide a sensor excellent in selectivity of detection gas and excellent in long-term stability.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a sensor according to one embodiment of the present invention.

FIG. 2 is a diagram showing the NO ₂ concentration dependency of a change in electromotive force of a sensor according to an example of the present invention and a comparative example.

FIG. 3 is a cross-sectional view of a sensor according to another embodiment of the present invention.

FIG. 4 is a sectional view of a sensor according to another embodiment of the present invention.

[Explanation of symbols]

 1, 5, 9 Solid electrolyte 2, 6, 10 Reference electrode 3, 7, 11 Detection electrode 4, 8, 12 Current collector

Claims (5)

[Claims] 1. A sensor having a structure in which a detection electrode composed of a current collector and an auxiliary electrode material and a pair of reference electrodes are provided on a solid electrolyte, and both electrodes are exposed to a test gas atmosphere. A solid electrolyte sensor, wherein a current collector disposed on the detection electrode is made of the same material as the reference electrode. 2. The solid according to claim 1, wherein at least the current collector disposed on the sensing electrode is a porous body, and covers a portion of the sensing electrode not in contact with the solid electrolyte and is in contact with the solid electrolyte. Electrolyte sensor. 3. The solid electrolyte sensor according to claim 1, wherein the detection electrode is made of a mixture of a current collector material and an electrode material, and the detection electrode and the current collector are in contact with the solid electrolyte. 4. The solid electrolyte sensor according to claim 1, wherein the solid electrolyte is an oxygen ion conductor. 5. The solid electrolyte sensor according to claim 1, wherein the current collector is a noble metal material.