JP2000214129A - Carbon dioxide gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbon dioxide gas sensor which eliminates setting at the side of a reference electrode and has superior initial and long-term stabilities of sensor outputs. SOLUTION: The gas sensor has a detecting part which includes a divalent ion conductor layer essentially consisting of a divalent ion conductive solid electrolyte having an essential conductive ion species of divalent metal cations, an oxygen ion conductor layer layered to the divalent ion conductor layer and essentially consisting of an oxygen ion conductive solid electrolyte of an essential conductive ion species of oxygen ions, a detecting electrode formed to a surface of the divalent ion conductor layer and a reference electrode formed to a surface of the oxygen ion conductor. When the detecting part is inserted into a gas to be detected which includes carbon dioxide gas in a manner to bring the detecting electrode and reference electrode into touch with the gas to be detected, an electromotive force generated at this time between the detecting electrode and reference electrode is taken out as a detection output of the carbon dioxide gas in the gas to be detected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbon dioxide sensor for detecting carbon dioxide in a test gas.

[0002]

2. Description of the Related Art Emissions of carbon dioxide (CO ₂ ) due to various energy consumption activities are increasing year by year, and in recent years, concerns about harmful effects such as global warming are increasing on a global scale. In response to this, the movement toward total emission control for carbon dioxide emissions is also rapidly progressing, and the industry is considering measures from various angles. For example, establishment of a quick and simple method for detecting carbon dioxide is also one of the major themes.

[0003]

At present, the mainstream of the method for detecting carbon dioxide gas is an instrumental analysis method such as an infrared absorption method. However, this method is expensive and extremely large. A major drawback is that it is virtually impossible to perform in-situ measurements at individual carbon dioxide sources, such as boilers and internal combustion engines (especially automotive engines, etc.).

[0004] On the other hand, there has been no proposal of a small-sized carbon dioxide gas sensor using a solid electrolyte, but all of them have various problems in practical use. For example, in a carbon dioxide gas sensor of a concentration cell type, setting of a reference electrode for giving a reference carbon dioxide gas concentration is always necessary, and there is a disadvantage that the sensor structure becomes complicated.

In order to solve this problem, Japanese Patent Laid-Open Publication No.
Japanese Patent Publication No. 29847 proposes a carbon dioxide sensor having a structure in which a lithium ion conductor and an oxide ion conductor are overlapped and the lithium ion conductor is sealed on the oxide ion conductor side. In this case, the electromotive force generated between the detection electrode and the reference electrode when inserted into the test gas containing carbon dioxide with the lithium ion conductor side as the detection electrode side and the oxide ion conductor side as the reference electrode side Is taken out as a detection output of carbon dioxide in the test gas. However, it takes at least 10 days for the sensor to stabilize the electromotive force value after operation, and the initial stability is not sufficient. In addition, long-term stability has disadvantages.

An object of the present invention is to provide a carbon dioxide gas sensor which does not need to be set on the reference electrode side and has excellent initial and long-term stability of sensor output.

[0007]

Means for Solving the Problems and Functions / Effects The carbon dioxide sensor of the present invention for solving the above-mentioned problems mainly comprises a divalent ion conductive solid electrolyte in which the main conductive ion species is a divalent metal cation. A divalent ion conductor layer, and an oxygen ion conductor layer mainly formed of an oxygen ion conductive solid electrolyte that is formed on the divalent ion conductor layer and whose main conductive ion species is oxygen ion; A detection unit having a detection electrode formed on the surface of the divalent ion conductor layer and a reference electrode formed on the surface of the oxygen ion conductor is provided, and the detection electrode and the reference electrode contain carbon dioxide. An electromotive force generated between the detection electrode and the reference electrode when the detection unit is inserted into the test gas so as to be in contact with the test gas is taken out as a detection output of carbon dioxide gas in the test gas. Features.

According to the carbon dioxide sensor of the present invention, it is not necessary to set the reference electrode, and in addition, the initial and long-term stability of the sensor output can be sufficiently ensured. For example, in the conventional sensor using a lithium ion conductor,
The relatively unstable lithium oxide layer formed between the lithium ion conductor layer and the oxygen ion conductor layer causes problems in the initial stability or long-term stability of the electromotive force value. It is thought that it was. On the other hand, in the sensor of the present invention, not a lithium oxide layer but an oxide layer of movable divalent ions is formed at the interface between the divalent ion conductor layer and the oxygen ion conductor layer. Since such a divalent ion oxide layer is extremely stable, it is considered that a stable sensor output can be obtained.

The mechanism of measuring carbon dioxide by the sensor of the present invention is considered as follows. That is, in the sensor of the present invention, two types of ion conductors having different conductive ion species are used, and the two ion species react at the interface between both ion conductors. At the detection electrode, the following reactions occur: at the interface between the two ion conductors, at the interface, and at the reference electrode, the following reactions occur.

[0010]

## STR1 ## MgCO ₃ → Mg ²⁺ + CO ₂ (I) + 1 /
2O ₂ (I) + 2e ^-

[0011]

^{Embedded image} Mg ²⁺ + O ^{2 −} → MgO

[0012]

Embedded image 1 / 2O ₂ (II) + 2e ⁻ → O ^{2 −}

The reaction of the whole sensor is represented by the following formula (4), and the calculation formula obtained from the Nernst formula is given by the following equation (1).

[0014]

Embedded image MgCO ₃ + 1 / 2O ₂ (II) → MgO
+ CO ₂ (I) + 1 / 2O ₂ (I)

[0015]

E = C (constant) −0.0992 T log
PCO ₂ (I)

As is clear from the above, the sensor of the present invention does not contain carbon dioxide in the reaction of the reference electrode, and therefore does not require a reference atmosphere containing carbon dioxide gas unlike the concentration cell type sensor. Therefore, the detection electrode and the reference electrode can be arranged in the same test gas atmosphere. In this case, in the above reaction formula, oxygen is eliminated from the chemical formula (4) and the formula (1), and the effect of the sensor of the present invention that the influence of the oxygen concentration in the test gas is not exerted is produced. Note that when placed in a test gas atmosphere containing carbon dioxide and oxygen,
The divalent metal ions generated on the divalent ion conductor layer side and the oxygen ions generated on the oxygen ion conductor layer side are:
The reaction takes place at the superposed interface between the divalent ion conductor layer and the oxygen ion conductor layer.

Further, an oxide layer of movable divalent ions is formed at the interface and is very stable, so that the initial sensor output is stable and the output can be obtained with high accuracy.

In the divalent ion conductor layer, the main conductive ionic species is selected from Mg ²⁺ , Ca ²⁺ , Sr ²⁺ and Ba ^2+.
It can be mainly composed of a divalent ion-conductive solid electrolyte that is a kind or two or more kinds of divalent metal cations. Thereby, the initial and long-term stability of the sensor output can be further improved. It is considered that the reason for this is that a particularly stable oxide layer of movable divalent ions is formed at the interface. Among them, particularly, magnesium oxide is extremely stable even at a high temperature. Therefore, a divalent ion conductive solid electrolyte whose main conductive ion species is Mg ²⁺ is preferable as a constituent material of the divalent ion conductor layer of the sensor of the present invention. Can be used.

Specific examples of such a divalent ion conductive solid electrolyte include MgZrP ₆ O ₂₄ and Mg _{1 + x} Z.
_{r 4 P 6 O 24 + x} + xZr2O (PO 4) 2 Composite (x, for example, from about 0 5) can be used like, may also be used in combination of two or more of these.

On the other hand, as the oxygen ion conductive solid electrolyte constituting the oxygen ion conductor, for example, zirconia (ZrO ₂ : Y ₂ O ₃ , CaO or the like) may be used even if an oxide component for stabilization is contained. Good) based solid electrolyte ceramics can be used.

Next, in order to prevent or suppress the penetration of the test gas into the interface between the divalent ion conductor layer and the oxide ion conductor layer, the sealing of the edge of the interface is sealed. Means can be provided. When the test gas penetrates into the interface between the divalent ion conductor layer and the oxide ion conductor layer, the sensor reaction at the interface does not proceed normally, and the detection accuracy may be easily reduced. By providing the sealing means as described above, such a problem is solved, and carbon dioxide gas detection with high accuracy becomes possible.

Specifically, the outer edge of the superposed interface between the divalent ion conductor layer and the oxygen ion conductor layer can be covered with a heat-resistant sealing layer. Thereby, even at a high temperature, penetration of the test gas into the interface can be effectively prevented. In addition, such a heat-resistant sealing layer can be composed of an inorganic adhesive (for example, trade name: Sumiceram (Asahi Chemical Industry Co., Ltd.)) mainly composed of silica, alumina, zirconia, or other ceramic materials. . Alternatively, the outer edge of the interface may be covered with an inorganic material powder such as talc, and the outside may be further sealed with a metallic casing.

Next, a metal carbonate compound layer can be laminated on the surface of the divalent ion conductor layer opposite to the surface facing the superposed interface with the oxygen ion conductor layer.
For example, in the carbonate compound layer, the main component of the cation component is Mg ²⁺ , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ or Li ⁺ , N
It can be composed of one or more carbonate compounds selected from a ⁺ , K ⁺ , Rb ⁺ , and Cs ⁺ . This is mainly based on a divalent ion conductive solid electrolyte in which the main conductive ionic species is one or more divalent metal cations selected from Mg ²⁺ , Ca ²⁺ , Sr ²⁺ and Ba ^2+. This is particularly advantageous when a divalent ion conductor layer is formed. The main component of the cation component in the carbonate compound layer is monovalent or bivalent.
It is not limited to a valence ion. For example, a trivalent ion may be effectively used in some cases.

By forming this carbonate compound layer, a chemical reaction equilibrium can exist between the carbon dioxide gas in the test gas and the metal carbonate compound, and the sensor output can follow the change in the carbon dioxide gas concentration and its stability. Can be further improved.

[0025]

(Embodiment 1) FIG. 1 is a sectional view showing an example of a carbon dioxide sensor (detection section) according to the present invention. The solid electrolyte to be the Mg ²⁺ ion conductor was prepared by mixing ammonium hydrogen phosphate, magnesium hydrogen phosphate, and zirconium nitrate in a non-stoichiometric ratio and then heating at 300 ° C. for 5 hours.
Obtained by heating at 00 ° C. for 12 hours. The obtained sample was pressed and heated at 1200 ° C. for 12 hours to form an Mg ²⁺ ion conductor sample for measuring sensor characteristics. When the Mg ²⁺ ion conductor sample was identified by powder X-ray diffraction measurement, a composite in which only a small amount of zirconium phosphate was dispersed as a second component in magnesium zirconium phosphate having a β-iron sulfate type structure was obtained. It was confirmed that there was.

On the other hand, ZrO ₂ powder and Y ₂ O ₃ powder were mixed at a molar ratio of 9: 1 to form a pellet, and then calcined at 1600 ° C. By powder X-ray diffraction measurement, it was confirmed that the calcined body was an oxide solid solution. The obtained calcined body sample was pulverized after being pulverized and sintered at 1600 ° C. for 6 hours to obtain a 0.1Y ₂ O ₃ -0.9ZrO ₂ sintered body.

A carbonate-containing compound containing a divalent ion such as a magnesium cation, a calcium cation, a strontium cation, or a barium cation is immobilized on the Mg ²⁺ ion conductor sample, and the sensor is directly inserted into the gas to be detected. By doing so, carbon dioxide gas is detected. Specifically, two kinds of solid electrolytes of an oxide ion conductor and a divalent ion conductor such as a magnesium cation, a calcium cation, a strontium cation, and a barium cation,
A heat-resistant sealing layer that seals the outer edge of the lamination interface by superimposing the oxide ion conductor layer and the divalent ion conductor layer referred to in the claims respectively and applying a ceramic material such as an inorganic adhesive to the side surface. Form and fix both.

A current collector composed of a carbonate-containing compound containing a movable divalent cation species and a Pt metal net is provided on the surface of the divalent ion conductor, and only a current collector is provided on the surface of the oxide ion conductor. Fix as an electrode. Here, a barium carbonate layer obtained by thermally decomposing Ba (OC ₂ H ₅ ) ₂ previously mounted on the sensor in a CO ₂ atmosphere is used as the carbonate-containing compound layer. The flowing gas is dry air and 100% CO ₂ gas or 1% CO ₂ diluted with nitrogen.
It is synthesized by mixing gas, and the flow rate is 100 ml /
min. At 650 ° C., a CO ₂ concentration of 20
The electromotive force was measured while changing to 0 ppm-5%. FIG.
Fig. 1 shows an example of the measurement results of the electromotive force at various carbon dioxide gas concentrations at 650 ° C. The electromotive force value changes linearly with the logarithm of the carbon dioxide gas concentration, and the slope of the straight line is −89.
6 (n = 2.05) and the calculated value (−91.
6) (n = 2.0). In this example, the carbon dioxide concentration from 2000 ppm to 5% matches the slope of the Nernst equation, and it can be seen that carbon dioxide can be detected with high accuracy.

(Example 2) Set as a divalent ion conductor layer
Mg_1.4Zr_FourP₆O_24.4+ 0.4Zr_TwoO (PO _Four)_TwoTo
With solid electrolyte sintered body and oxygen ion conductor layer
Composition 0.1Y_TwoO_Three-0.9ZrO_TwoSolid electrolyte having
Using a sintered body, Li is used as the carbonate-containing compound layer._Two
CO_ThreeHas the same structure as that of FIG.
Make a sensor, CO at 550 ℃_TwoMeasure response curve
did. The results are shown in FIG. The response time is as fast as about 2 minutes
If the density is changed and then returned to the original density, the electromotive force
Returning to the original value shows that it can be measured reversibly
Was.

FIG. 4 shows the above sensor at 550 ° C.
Electromotive force CO_TwoShows concentration dependence. 200ppm-5%
CO_TwoFrom the slope of the plot of electromotive force against concentration
The number of reaction electrons obtained was 2.05, indicating a theoretical response.
I understand. Further, from FIG.
Even after several days of operation, CO_TwoElectromotive force against concentration
The value is almost constant and the initial stability is fairly good.
I understand. FIG. 5 shows the conventional Li⁺Using ionic conductor
Sensor and Mg²⁺Early days with sensors using ionic conductors
The result of comparing the stability is shown. Where the vertical axis (ΔEMF)
Is 2000 ppm CO_TwoActivation of the electromotive force
Indicates the increase from the start. Li⁺Using ionic conductor
In this case, an increase in the electromotive force value of 50 mV or more was observed in three days.
But Mg ²⁺When an ionic conductor is used, the
There is almost no increase. This is Li⁺Ionic conduction
Li which is an interface product when using a body_TwoO is unstable
Although the activity is easy to change, Mg²⁺Ion conductor
MgO, which is an interface product when is used, is stable,
This is probably because the activity hardly changed.

Incidentally, when a sensor is used for combustion control, there is a concern that the influence of NOx on the detection characteristics.
Therefore, the electromotive force was measured while keeping the CO ₂ concentration constant and changing NO and NO ₂ to 0 to 500 ppm, respectively. FIG. 6 shows the dependence of the electromotive force value on the NOx concentration with respect to CO ₂ of 2000 ppm. It can be seen that the electromotive force value as the sensor output is hardly affected by both NO and NO ₂ .

[Brief description of the drawings]

FIG. 1 is an example of a cross-sectional view of a one-end sealed type carbon dioxide sensor.

FIG. 2 shows the result of measuring the concentration of carbon dioxide using a one-end sealed carbon dioxide sensor.

FIG. 3 shows Mg _1.4 Zr ₄ P ₆ O _24.4 + as a solid electrolyte
0.4Zr ₂ O (PO _{4) 2} and 0.1Y ₂ O ₃ -0.9Zr
A device that combines the O _2, the sensor using the Li ₂ CO ₃ as a detection electrode, a CO ₂ response curve at 550 ° C..

FIG. 4 shows the dependence of the electromotive force at 550 ° C. on the CO ₂ concentration.

FIG. 5 is a diagram comparing the initial stability of a sensor using a Li ⁺ ion conductor and a sensor using a Mg ²⁺ ion conductor.

FIG. 6 shows N of an electromotive force value for 2000 ppm of CO ₂ .
It is a figure of Ox concentration dependence.

Claims (8)

[Claims] 1. A divalent ion conductor layer mainly composed of a divalent ion conductive solid electrolyte in which a main conductive ion species is a divalent metal cation, and a divalent ion conductor layer is formed by lamination on the divalent ion conductor layer. An oxygen ion conductive layer mainly composed of an oxygen ion conductive solid electrolyte whose conductive ion species is oxygen ion, a detection electrode formed on the surface of the divalent ion conductor layer, A detection part having a reference electrode formed on the surface, and inserting the detection part into the test gas such that the detection electrode and the reference electrode are in contact with the test gas containing carbon dioxide. An electromotive force generated between the detection electrode and the reference electrode when the detection is performed is extracted as a detection output of carbon dioxide in the test gas. 2. The divalent ion conductor layer is one or more divalent metal cations whose main conductive ionic species is selected from Mg ²⁺ , Ca ²⁺ , Sr ²⁺ and Ba ^2+. The carbon dioxide sensor according to claim 1, wherein the carbon dioxide sensor is mainly composed of a divalent ion conductive solid electrolyte. 3. The detection electrode and the reference electrode are arranged in the same test gas atmosphere so that the electromotive force is not affected by the oxygen concentration in the test gas. 3. The carbon dioxide gas sensor according to 1 or 2. 4. A divalent metal ion generated on the divalent ion conductor layer side and a divalent metal ion generated on the oxygen ion conductor layer side when arranged in a test gas atmosphere containing carbon dioxide gas and oxygen. The carbon dioxide sensor according to any one of claims 1 to 3, wherein oxygen ions are reacted at an overlapping interface between the divalent ion conductor layer and the oxygen ion conductor layer. 5. A seal for sealing an edge of the interface between the divalent ion conductor layer and the oxide ion conductor layer in order to prevent or suppress the penetration of the test gas into the interface. The carbon dioxide sensor according to any one of claims 1 to 4, further comprising a stopping means. 6. The carbon dioxide sensor according to claim 5, wherein the outer edge of the superposed interface between the divalent ion conductor layer and the oxygen ion conductor layer is covered with a heat-resistant sealing layer. 7. A metal carbonate compound layer is laminated on a surface of the divalent ion conductor layer opposite to a surface facing an overlapping interface with the oxygen ion conductor layer. Item 7. The carbon dioxide sensor according to any one of Items 2 to 6. 8. The metal carbonate compound wherein the main component of the cation component is one or more selected from lithium, sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium and barium. The carbon dioxide sensor according to claim 7, which is used.