JP4446750B2 - CO2 sensor and CO2 detection method - Google Patents

Description

The present invention relates to CO ₂ detection.

Patent Documents 1 and 2 and Non-Patent Document 1 disclose a CO ₂ sensor using LiCoO ₂ , NaCoO _2, or the like as a counter electrode. These compounds are non-stoichiometric with respect to the alkali metal content, and can exchange Li ⁺ ions and Na ⁺ ions with alkali metal ion conductors without exchanging oxygen with the atmosphere. Furthermore, these compounds also have a certain degree of electronic conductivity. For this reason, it can be used as an electrode independent of the surrounding oxygen partial pressure. The inventors have subsequently found that these counter electrodes are strongly influenced by CO ₂ , and are similarly affected by surrounding water vapor. Further, if the exposed surface of the counter electrode is glass-sealed, it is affected by CO ₂ and water vapor. In addition, the present inventors have found that a gas sensor having a small change in electromotive force due to being left at room temperature can be obtained.
JP-A-11-190718 JP-A-11-108886 Investigation of Solid Sodium Reference Electrodes for Solid-State Electrochemical Gas Sensors, Applied Physics A57,31-35, 1993, Schettler, et al. (Investigation of Solid Sodium Reference Electrode for Solid Electrochemical Gas Sensor)

An object of the present invention is to reduce the influence of CO ₂ and water vapor on the counter electrode when a non-stoichiometric compound containing an alkali metal, a transition metal, and oxygen and having an alkali metal content of non-stoichiometry is used as the counter electrode. The purpose is to suppress variations in electromotive force caused by heating.

The CO ₂ sensor according to the present invention includes a counter electrode using a non-stoichiometric compound containing an alkali metal, a transition metal, and oxygen in an alkali metal ion conductor and having an alkali metal content of non-stoichiometry, a carbonate-containing detection electrode, In the sensor that detects CO ₂ from the electromotive force between the sensing electrode and the counter electrode under heating, the exposed portion of the counter electrode is hermetically sealed with glass , and the counter electrode is Na _x CoO. _2-y film (x is about 0.5-1 and y is about 0.5-0) .

  Preferably, the alkali metal ion conductor is formed on the substrate, the detection electrode film and the counter electrode film are both formed on the alkali metal ion conductor film, and the exposed surface of the counter electrode film is a glass film. It is hermetically sealed. In this way, it is not necessary to attach a chip or pellet of an alkali metal ion conductor to the substrate in the manufacture of the sensor, and the counter electrode can be sealed with a glass film.

Further, the CO ₂ detection method of the present invention includes a counter electrode using a non-stoichiometric compound containing an alkali metal, a transition metal, and oxygen in an alkali metal ion conductor and having an alkali metal content of non-stoichiometry, and detection of carbonate content. In the method of detecting CO ₂ from the electromotive force between the detection electrode / counter electrode under heating by connecting the electrodes , the counter electrode is a Na _x CoO _2-y film (x is about 0.5 to 1, y is 0.5 to By sealing the exposed portion of the counter electrode with glass, the influence of CO ₂ and water vapor on the counter electrode is blocked, and the change in the electromotive force between the detection electrode and the counter electrode due to non-heating is suppressed. It is characterized by that.
Particularly preferably, the alkali metal ion conductor is formed on the substrate, and both the detection electrode film and the counter electrode film are formed on the alkali metal ion conductor film, and the exposed surface of the counter electrode film is a glass film. It is airtightly sealed.

In the counter electrode such as NaCoO ₂ that is not glass-sealed, the potential of the counter electrode varies due to CO ₂ (FIGS. 7 and 8), and the data is not particularly shown, but the counter electrode is affected by the ambient humidity. When restarting after standing at room temperature, the electromotive force of the CO ₂ sensor generally changes. On the other hand, the counter electrode such as NaCoO ₂ hermetically sealed with glass from the ambient atmosphere is not affected by CO ₂ (FIGS. 9 and 10) and has little influence on humidity (FIG. 12). The change in electromotive force at the time of starting is small, and a stable electromotive force can be obtained in a short time after reheating (solid line in FIG. 13).

  In the following, an optimum embodiment for carrying out the present invention will be shown.

1 to 13 show examples and characteristics of the CO ₂ sensor 1 as an example. 1 and 2 show the structure of the sensor 1 of the embodiment. Reference numeral 2 denotes a NASICON film, which is an example of an alkali metal ion conductor, and may be a lithium ion conductor or the like. Reference numeral 4 denotes a detection electrode, in which a carbonate film 6 is superimposed on an Au film. In the examples, a mixture having a molar ratio of lithium carbonate and barium carbonate of 1: 2 was used as the carbonate, but the type of carbonate is arbitrary. Reference numeral 8 denotes a counter electrode Na _x CoO _2-y film having a film thickness of, for example, about 1 to 30 μm and connected to the gold film 10, and its exposed surface is sealed with a glass seal film 12 to be cut off from the atmosphere. Reference numeral 16 denotes a heater film.

The Na _x CoO _2-y film 8 has several phases having different parameters y representing the oxygen content, with the Na ion content being indefinite. For example, x is about 1 to 0.5, and in the examples, a compound having x of approximately 1 and a compound of 0.6 are used, and the value of y is approximately 0 when x = 1, and approximately 0.5 when x = 0.6. 4. Examples of similar compounds of Na _x CoO _2-y include LiCoO ₂ , LiNiO ₃ and LiMn ₂ O ₄ .

3 and 4 show a sensor of a modified example. A counter electrode of a Na _x CoO _2-y film 8 is provided on an alumina substrate 14, and a NASICON chip 20 is placed on the counter electrode, and the Na _x CoO _2-y film 8 is provided. The exposed portion is hermetically sealed with a glass seal film 12. In addition, the detection electrode 4 and the carbonate film 6 are provided on the NASICON chip 20.

Figure 5 shows the CO ₂ sensor 1 of the manufacturing step of Example. A paste of NASICON (Na ₃ Zr ₂ Si ₂ PO ₁₂ ) is prepared, printed on a substrate by, for example, screen printing, and baked at 1200 ° C. to obtain a NASICON film having a thickness of about 30 μm. A gold paste is printed on the film, fired at, for example, about 750 ° C., carbonate is applied, and is fused to the gold detection electrode at about 750 ° C.

Co ₃ O ₄ and Na ₂ O ₂ powders were mixed and ground to a stoichiometric ratio in a dry nitrogen atmosphere. This mixture was baked at a maximum temperature of 850 ° C. while flowing nitrogen gas to synthesize NaCoO ₂ . In addition to this, Na _0.6 CoO _1.6 was synthesized by changing the content of Na ₂ O ₂ . NaCoO ₂ or the like was screen-printed and fired at, for example, 800 ° C. to form the counter electrode film 8. Next, a glass paste (SiO ₂ : Na ₂ O: B ₂ O ₃ : Al ₂ O ₃ in a molar ratio of 44: 20: 31: 5) was screen-printed to a thickness of about 10 μm, fired at 600 ° C., and Na _x The exposed portion of the CoO _2-y film 8 was hermetically sealed. The glass seal film 12 only needs to be able to hermetically seal the counter electrode Na _x CoO _2-y film 8 from the surrounding atmosphere, and the material, film thickness, and the like are arbitrary. Thereafter, the sensor 1 is completed when the substrate is appropriately scribed and lead wires are attached.

In order to evaluate the characteristics of the Na _x CoO _2-y film, the evaluation sensor 30 in FIG. 6 was adjusted. 32 is a NASICON chip, 34 is a gold detection electrode, 36 is a carbonate film, 38 is a Na _x CoO _2-y film, and the exposed part is covered with a glass film 40 to be hermetically sealed, and 44 is a gold reference electrode It is. The composition of the carbonate film 36 is a mixture of 1: 2 in terms of a molar ratio between lithium carbonate and barium carbonate. The NASICON chip 32 is attached to the glass tube 46 with an inorganic adhesive 44, and a gold wire is attached as a lead to the gold detection electrode 34, the Na _x CoO _2-y film 38 (counter electrode) and the gold reference electrode 42. The electromotive force of 34 and the counter electrode 38 was measured. Further, a heater (not shown) is provided around the glass tube 46 and heated to about 450 ° C., a dry atmosphere having a constant CO ₂ concentration is circulated inside the glass tube 46, and the CO ₂ concentration and relative humidity of the outer atmosphere are set. It was measured by changing.

7 and 8 show the potentials of the counter electrode and the detection electrode with respect to the reference electrode when the counter electrode is not glass-sealed. Both sensor temperatures are 450 ° C., and the humidity of the atmosphere is dry in FIG. 7 and 50% relative humidity in FIG. The electromotive force between the counter electrode and the reference electrode varies depending on the CO ₂ concentration, and there is a difference in the potential of the counter electrode between the dry state and the relative humidity of 50%. Therefore, it is considered that the counter electrode is dependent on water vapor.

9 and 10 show the characteristics when the exposed surface of the counter electrode is sealed with glass. The sensor temperatures are all 450 ° C., and the humidity of the atmosphere is dry in FIG. 9 and 50% relative humidity in FIG. The upper straight line in the figure shows the potential of the counter electrode with respect to the reference electrode when the CO ₂ concentration is changed, and the counter electrode potential is constant regardless of the CO ₂ concentration. 9 and 10, it seems that the counter electrode potential changes depending on the humidity. This is because the measurement was performed using different sensors. If the sensor is the same, the electromotive force between the counter electrode and the reference electrode is different even if the humidity is changed. It was almost constant.

FIG. 11 shows the electromotive force between the sensing electrode and the counter electrode when the sensor temperature is changed between 350 ° C. and 450 ° C. with the surrounding atmosphere as a dry atmosphere. It can be seen that an electromotive force corresponding to a two-electron reaction is obtained at 360 ° C. to 450 ° C., and the CO ₂ concentration can be obtained according to the Nernst equation. FIG. 12 shows the electromotive force between the sensing electrode and the counter electrode when the relative humidity is changed from 20 to 80%, and the humidity dependency of the electromotive force is slight.

FIG. 13 shows a sensing electrode / counter electrode (glass sealed) in 400 ppm CO ₂ when the sensor for evaluation is subjected to heating (white area in the figure) and standing at room temperature (gray area in the figure). The electromotive force between is shown. Moreover, the broken line of a figure shows the characteristic at the time of using a simple gold film for a counter electrode. At the gold counter electrode, the electromotive force changes due to standing at room temperature, and it takes a long time for the electromotive force to reach a steady value after restarting. On the other hand, when a glass-sealed Na _x CoO _2-y film is used as the counter electrode, the electromotive force after reheating is almost constant after 10 days or more, and the electromotive force is stabilized in a short time after reheating.

9 to 13 show the characteristics of a three-pole sensor, but similar results were obtained even when the electromotive force between the detection electrode and the counter electrode was measured as a two-pole sensor. In the examples, NaCoO ₂ was used as the counter electrode material, but almost the same characteristics were obtained even when Na _0.6 CoO _1.6 or the like was used instead. In either case, the exposed surface of the counter electrode is hermetically sealed with glass, so that fluctuations in the electromotive force due to non-heating are suppressed, excluding the CO ₂ dependency and humidity dependency of the counter electrode, and the electromotive force during reheating is reduced. The time to stabilize was shortened.

Side view of essential part of CO ₂ sensor of embodiment FIG. 1 is a plan view of the main part of the CO ₂ sensor of FIG. Cross-sectional view of the principal part of a modified CO ₂ sensor FIG. 3 is a plan view of the main part of the CO ₂ sensor of FIG. Process diagram showing the manufacturing process of the CO ₂ sensor of Example Cross section of the main part of the CO ₂ sensor used for the evaluation of the counter electrode characteristics This is a characteristic diagram showing the electric potential of the detection electrode and the counter electrode with respect to the reference electrode in the conventional example in which the counter electrode is not sealed with glass, the operating temperature is 450 ° C., and the atmosphere is a dry atmosphere. Characteristic diagram when the atmosphere is changed to 50% relative humidity and the others are the same as in FIG. Characteristic diagram showing the potential of the sensing electrode and the counter electrode in a 450 ° C dry atmosphere when the counter electrode is sealed with glass Characteristic diagram showing the potential of the detection electrode and the counter electrode when the atmosphere is changed to 50% relative humidity The characteristic view which shows the electromotive force between the detection electrode / counter electrode in 360 to 450 degreeC in the Example which sealed the counter electrode by glass The characteristic figure which shows the relative humidity dependence of the counter electrode potential in an Example In an embodiment, the characteristic diagram showing a counter electrode potential when the CO ₂ sensor has restarted after standing at room temperature at various relative humidity

Explanation of symbols

1 CO ₂ sensor 2 NASICON film 4 Sensing electrode 6 Carbonate film 8 Na _x CoO _2-y film (counter electrode film)
10 Gold film 12 Glass seal film 14 Alumina substrate 16 Heater film 20 NASICON chip 30 Evaluation sensor 32 NASICON chip 34 Gold detection electrode 36 Carbonate film 38 Na _x CoO _2-y film 40 Glass film 42 Gold reference electrode 44 Inorganic adhesive 46 glass tube

Claims (3)

A counter electrode using a non-stoichiometric compound containing an alkali metal, a transition metal, and oxygen and having an alkali metal content is indefinite, and a carbonate-containing sensing electrode are connected to an alkali metal ion conductor, and CO 2 is heated under heating. _{In the} sensor which detects ₂ from the electromotive force between the detection electrode / counter electrode,
The exposed portion of the counter electrode is hermetically sealed with glass to cut off from the surrounding atmosphere , and the counter electrode is a Na _x CoO _2-y film (x is about 0.5-1 and y is about 0.5-0). A CO ₂ sensor characterized by comprising: The alkali metal ion conductor is deposited on a substrate;
2. The CO according to claim 1 , wherein both the detection electrode film and the counter electrode film are formed on the alkali metal ion conductor film, and the exposed surface of the counter electrode film is hermetically sealed with a glass film. ₂ sensors. An alkali metal ion conductor is connected to a counter electrode using a non-stoichiometric compound containing an alkali metal, a transition metal and oxygen and having an alkali metal content that is non-stoichiometric, and a carbonate-containing sensing electrode. In the method of detecting the CO ₂ from the electromotive force between the sensing electrode / counter electrode,
The counter electrode is a Na _x CoO _2-y film (x is about 0.5-1 and y is about 0.5-0),
By sealing the exposed portion of the counter electrode with glass, the influence of CO ₂ and water vapor on the counter electrode is blocked, and the change in the electromotive force between the detection electrode and the counter electrode due to non-heating is suppressed. CO ₂ detection method.

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