JP3429451B2 - Carbon dioxide sensor - Google Patents

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 used for indoor / outdoor environmental control, agricultural / industrial processes such as institutional horticulture, disaster prevention, and measurement of metabolic function on the surface of a living body.

[0002]

2. Description of the Related Art In recent years, there has been a need for a carbon dioxide sensor centering on the detection of indoor air pollution accompanying the spread of air conditioning, the detection of indoor air pollution in livestock farming, plant growth control in gardening facilities, various industrial processes, etc. Is increasing, and various types of carbon dioxide sensors have been proposed.

Specifically, for example, an infrared absorption type carbon dioxide sensor has been put into practical use. However, this type of sensor has not been widely used because of its large size and high cost. Although a sensor using a semiconductor has been proposed, it is difficult to measure the concentration of only carbon dioxide because this sensor has poor selectivity for carbon dioxide.

On the other hand, some sensors using a solid electrolyte have been proposed as small and inexpensive sensors.

Maruyama et al. [Abstracts of Lectures at the 10th Solid Ionics Discussion Session 69 (1983)] form a pair of electrodes on a solid electrolyte such as potassium carbonate which forms a dissociation equilibrium with carbon dioxide, and one of them has a known concentration. We have proposed a density-polarization sensor that measures the electromotive force due to the difference in concentration of atmospheric gas by contacting a reference gas. Maruyama et al. Formed a pair of electrodes on an alkali metal ion conductive solid electrolyte such as NASICON (sodium super ionic conductor: Na ₃ Zr ₂ Si ₂ PO ₁₂ ) in addition to such a gray-scale polarization sensor. Also, a so-called electromotive force detection type sensor is proposed, in which one side is provided with a metal carbonate layer that forms a dissociation equilibrium with carbon dioxide such as sodium carbonate to serve as a detection electrode, and the other side is used as a carbon dioxide insensitive electrode.

In Japanese Patent Publication No. 4-79542, a pair of electrodes are formed on a solid electrolyte having metal ion conductivity of a metal salt that forms dissociation equilibrium with carbon dioxide, and one electrode is coated with the above metal salt. A carbon dioxide sensor has been proposed in which the other electrode and the remaining solid electrolyte surface are coated with a gas barrier layer.

Japanese Unexamined Patent Publication (Kokai) No. 7-63726 proposes a solid reference electrode type carbon dioxide sensor in which a solid reference electrode which is a conductor of electrons and oxygen ions is pressure bonded to a solid electrolyte made of an alkali ion conductor. .

A problem with a small and inexpensive carbon dioxide sensor using a solid electrolyte is that the metal carbonate used as a material is susceptible to humidity. In order to solve this problem, it is necessary to hermetically seal the parts other than the gas detection part of the sensor element or heat the sensor with a heater to raise the operating temperature to reduce the influence of humidity. The operating temperature of the above sensor is as high as 400 to 700 ° C. When the operating temperature is high, power consumption of the entire sensor is large, and problems such as thermal deterioration of materials occur. There is also a problem that heat of several hundreds of degrees Celsius heats the periphery of the sensor even when using a small heater to generate convection of air, which has a delicate influence on the measurement environment.

Further, since the metal carbonate is susceptible to humidity, it is necessary to store the element in a dry atmosphere when the sensor is not used. Furthermore, during use, baking is required to remove moisture that has entered the element during stoppage, and it takes a long time for the output voltage of the sensor to stabilize, which causes workability and energy problems. These are unavoidable problems when the sensor is operated at a high temperature, and there is a demand for a sensor that operates at a lower temperature.

To address this issue, S. Breikhin et al. {Applie
d PhysicsA 57, 37-43 (1993)} reported a carbon dioxide sensor in which a solid electrolyte and a semiconductor were joined. This carbon dioxide sensor uses a SnO ₂ semiconductor doped with Sb or V as a detection electrode, and a sodium ion conductor NASICON is bonded to this as a solid electrolyte,
Na _x CoO ₂ is arranged as a reference electrode on the opposite side of the detection electrode. This carbon dioxide sensor has a low temperature (-35 ° C to room temperature)
It is possible to operate, but there is a problem that it takes more than 4 minutes to see a response. Further, the humidity resistance is also poor, and the response time and the sensitivity change depending on the relative humidity (humidity), and it is difficult to obtain stable performance.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a carbon dioxide sensor which operates at room temperature, has sufficient sensitivity and responsiveness, has high selectivity and is excellent in moisture resistance. .

[0012]

The above object is achieved by the present invention described below.

(1) A sensing electrode and a counter electrode are respectively provided in contact with a solid electrolyte, the solid electrolyte contains a metal ion conductor, and the sensing electrode contains a hydrogen carbonate and / or a hydrogen carbonate ion. A carbon dioxide sensor having a hydrogen carbonate layer contained therein and a current collector. (2) In the above (1), the hydrogen carbonate layer contains one or more of lithium hydrogen carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, rubidium hydrogen carbonate, cesium hydrogen carbonate, magnesium hydrogen carbonate and calcium hydrogen carbonate. Carbon dioxide sensor. (3) On the surface of the solid electrolyte in contact with the detection electrode,
The carbon dioxide sensor according to (1) or (2) above, which has the bicarbonate layer. (4) The above (1) to (1) in which the current collector is a porous metal
The carbon dioxide sensor according to any one of (3). (5) The carbon dioxide sensor according to any one of (1) to (4), wherein the detection electrode and the counter electrode are provided on the same surface of the solid electrolyte. (6) The carbon dioxide sensor according to any of (1) to (5), wherein the counter electrode contains one or more kinds of metals.

[0014]

In the carbon dioxide sensor of the present invention, the detection electrode and the counter electrode are respectively provided in contact with the solid electrolyte containing the metal ion conductor, and the detection electrode is hydrogen carbonate and / or
Alternatively, it has a hydrogen carbonate layer containing hydrogen carbonate ions and a current collector. It is preferable to have a hydrogencarbonate layer containing hydrogencarbonate and / or hydrogencarbonate ions on the surface of the solid electrolyte in contact with the detection electrode. When the detection electrode preferably contains hydrogencarbonate and / or hydrogencarbonate ions on the surface of the solid electrolyte, the responsivity is remarkably improved and rapid measurement at low temperature becomes possible. It is considered that this is because hydrogen carbonate ions derived from carbon dioxide to be detected are easily formed on the surface of the solid electrolyte. The carbon dioxide sensor of the present invention has a high response speed, high sensitivity, and high carbon dioxide selectivity even when operated at room temperature.

Further, by making the current collector porous,
Since the sensing electrode itself acts as a gas diffusion layer, a quicker response can be obtained.

Further, by using a metal for the counter electrode, preferably the same metal as the detection electrode, the influence of coexisting gas is reduced and a high carbon dioxide selectivity can be obtained. In addition, the moisture resistance is improved, and the influence of humidity is reduced especially when measuring at low temperatures.

The operating temperature of the sensor of the present invention can be operated at a lower temperature than that of the conventional carbon dioxide sensor using the solid electrolyte by combining the detection electrode and the counter electrode.
Changes in the measurement environment due to heat are small, and power consumption can be reduced.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION In the carbon dioxide sensor of the present invention, a detection electrode and a counter electrode are respectively provided in contact with a solid electrolyte, the solid electrolyte contains a metal ion conductor, and the detection electrode is a carbon dioxide. It has a hydrogen carbonate layer containing hydrogen salt and / or hydrogen carbonate ions, and a current collector.

<Solid Electrolyte> Carbon dioxide sensor of the present invention
Then, a metal ion conductor is used for the solid electrolyte. Metal a
As the on-conductor, for example, Na-β ″ alumina, N
a-β alumina, Na₃Zr₂PSi₂O₁₂, Na₃Zr₂
Si₂PO₁₂(NASICON), Na-βGa₂O₃,
Na-Fe₂O₃, Na₃Zr₂PSi₂P₂O₁₂, Li-β
Alumina, Li ₁₄Zn (CeO_Four), Li_FiveAlO_Four, L
i_1.4Ti_1.6In_0.4P₃O₁₂, K-β alumina, K_1.6
Al_0.8Ti_7.2O₁₆, K₂MgTi₇O₁₆, CaS, etc.
You can Of these, NASICON is preferable. these
May be slightly deviated from the stoichiometric composition.

The solid electrolyte may be prepared by any of the commonly used solid phase method, sol-gel method, coprecipitation method, etc., preferably the solid phase method.

In addition to the metal ion conductor, the solid electrolyte contains aluminum oxide (Al ₂ O ₃ ), silicon oxide (SiO ₂ ), as a reinforcing agent that does not interfere with the ionic conductivity.
Zirconium oxide (ZrO ₂ ), silicon carbide (Si
C), silicon nitride (Si ₃ N ₄ ), iron oxide (Fe ₂ O ₃ ) and the like may be contained in an amount of 50 wt% or less. These may be slightly deviated from the stoichiometric composition.

<Detection Electrode> In the carbon dioxide sensor of the present invention, the detection electrode is composed of a hydrogen carbonate layer containing hydrogen carbonate and / or hydrogen carbonate ions, and a current collector. On the surface of the solid electrolyte with which the detection electrode is in contact, hydrogencarbonate and / or hydrogencarbonate ion, more preferably hydrogencarbonate, or hydrogencarbonate and hydrogencarbonate ion, and further preferably hydrogencarbonate and hydrogencarbonate ion. It is preferable.

The hydrogencarbonate forming the hydrogencarbonate layer includes lithium hydrogencarbonate (LiHCO ₃ ), sodium hydrogencarbonate (NaHCO ₃ ), potassium hydrogencarbonate (KHC).
O ₃ ), rubidium hydrogen carbonate (RbHCO ₃ ), cesium hydrogen carbonate (CsHCO ₃ ), magnesium hydrogen carbonate (M
g (HCO ₃ ) ₂ ), calcium hydrogen carbonate (Ca (HCO 3
₃ ) ₂ ) and the like. These may be used alone or in combination of two or more. Of these, lithium hydrogen carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, and particularly sodium hydrogen carbonate are preferably used. Also, especially
It is preferable to use a hydrogen carbonate of the same metal as the mobile ionic species of the solid electrolyte. Further, the hydrogen carbonate layer may contain hydrogen carbonate ions (HCO ₃ ⁻ ). Bicarbonate ions are usually present bound to the surface of the solid electrolyte. More specifically, the -OH group of the solid electrolyte and H ₂
It exists by being bound through O.

The hydrogen carbonate layer may not be uniformly present as a layer but may be scattered on the solid electrolyte.

In the sensor of the present invention, in the presence of carbon dioxide, hydrogen carbonate ions (H
It is considered that CO ₃ ⁻ ) is formed on the surface of the solid electrolyte.
Then, the dissociation equilibrium of the hydrogen carbonate ion is converted into the activity of the mobile ion of the electrolyte, and the change in the carbon dioxide concentration is detected as the electromotive force. For example, when NASICON is used as the mobile ions of the solid electrolyte, Na ⁺ , which is the mobile ion in the electrolyte, forms HCO ₃ on the surface of the solid electrolyte.
^It is attracted to-and moves to the vicinity of the detection pole. At this time,
NaHCO ₃ may be produced in some cases.

In such a case, it is considered that the hydrogencarbonate or hydrogencarbonate ion of the hydrogencarbonate layer promotes the adsorption or binding of HCO ₃ ⁻ to the surface of the solid electrolyte.

The hydrogen carbonate layer may contain a metal carbonate in addition to the hydrogen carbonate and / or hydrogen carbonate ions. Examples of the metal carbonates include lithium carbonate (Li ₂ CO ₃ ), sodium carbonate (Na ₂ CO ₃ ), potassium carbonate (K ₂ CO ₃ ), rubidium carbonate (Rb ₂ C).
O ₃ ), cesium carbonate (Cs ₂ CO ₃ ), magnesium carbonate (MgCO ₃ ), calcium carbonate (CaCO ₃ ), strontium carbonate (SrCO ₃ ), barium carbonate (BaC)
O ₃ ), manganese carbonate (Mn (CO ₃ ) ₂ , Mn ₂ (C
O _{3) 3),} iron carbonate _{(Fe 2 (CO 3) 3} , FeCO 3), nickel carbonate (NiCO _3), copper carbonate (CuCO _3), cobalt carbonate _{(Co 2 (CO 3) 3} ), chromium carbonate ( Cr ₂ (C
O _{3) 3),} zinc carbonate (ZnCO _3), silver carbonate (Ag ₂ CO
₃ ), cadmium carbonate (CdCO ₃ ), indium carbonate (In ₂ (CO ₃ ) ₃ ), yttrium carbonate (Y ₂ (C ₂
O _{3) 3),} lead carbonate (PbCO _3), bismuth subcarbonate (Bi ₂
(CO ₃ ) ₃ ), lanthanum carbonate (La ₂ (CO ₃ ) ₃ ), cerium carbonate (Ce (CO ₃ ) ₃ ), praseodymium carbonate (P
r ₆ (CO ₃ ) ₁₁ ), neodymium carbonate (Nd ₂ (CO ₃ ) ₃ )
Etc. The metal carbonates may be used alone or in combination of two or more. Above all, it is preferable to use lithium carbonate, sodium carbonate, and potassium carbonate. Further, it is preferable to use a carbonate of the same metal as the metal hydrogen carbonate used. It is considered that the metal carbonate reacts with carbon dioxide and water to form a hydrogen carbonate, and promotes the generation of hydrogen carbonate ions derived from carbon dioxide.

The content of the metal carbonate is preferably 50 mol% or less, more preferably 0.1 to 10 mol% with respect to the hydrogen carbonate and / or hydrogen carbonate ion. Even when two or more kinds are used in combination, the total content is preferably within the above range.

When two or more kinds of hydrogen carbonate and / or hydrogen carbonate ion and metal carbonate are used, they may be mixed and used, or heat treated at a temperature below the melting point or decomposition point of the hydrogen carbonate to be used. , May be combined and used.

The method for forming the hydrogen carbonate layer of the detection electrode is not particularly limited, but usually, a paste or an aqueous solution of metal hydrogen carbonate powder is applied to the solid electrolyte, and the melting point of the hydrogen carbonate to be used or a temperature below the decomposition point is used. It is preferable that the film is formed by heating for about 2 hours and drying. When using lithium hydrogen carbonate, be careful not to decompose it. The average particle size of the metal hydrogen carbonate used is preferably 10 nm to 100 μm. The solvent of the paste may be an organic solvent which does not dissolve or react with the metal hydrogen carbonate, and has a relatively low room temperature vapor pressure and good workability. In particular, α-
Terpineol, ethylene glycol, glycerin and the like are preferable. The viscosity of the slurry is 0.1-100,000 po
ise is preferred. When an aqueous solution is used, its concentration is 0.
It is preferably 1 wt% or more. There is no particular upper limit to the concentration, and a saturated aqueous solution can be preferably used. When the paste is used, most of it exists as hydrogen carbonate, and when the aqueous solution is used, hydrogen carbonate ion and hydrogen carbonate may exist. In the present invention, it is preferable to use an aqueous solution because the layer formation is easy.

Further, an aqueous solution containing hydrogencarbonate ions, for example, an aqueous solution of carbonic acid or an aqueous solution of hydrogencarbonate is applied to the hydrogencarbonate layer on the detection electrode, or the detection electrode is exposed to an atmosphere containing carbon dioxide, preferably high humidity. If so, hydrogen carbonate ions will be contained in the detection electrode.

The hydrogen carbonate layer is preferably porous, and the pore size is preferably 0.01 to 100 μm.

The carbon dioxide sensor of the present invention uses a current collector for the detection electrode.

The metal used for the current collector may be any one or more of gold, platinum, silver, rubidium, rhodium, palladium, iridium, nickel, copper, chromium and the like.

The current collector is preferably a porous metal. When the current collector is porous, the sensing electrode itself acts as a gas diffusion layer, so that a quicker response can be obtained.

The porous metal is preferably a metal mesh or a powder electrode formed by pressure-bonding or screen-printing a metal powder paste. A powder electrode is particularly preferable. The mesh size of the metal mesh is not particularly limited as long as it has holding power.

Screen printing is a method in which a paste of metal powder is applied to a substrate through a mesh screen, and in this case, a porous electrode in which metal particles are connected to each other is formed. The average particle size of the metal powder used at this time is 10 nm to 100 μm, especially 10 nm to 10 μm.
The range of m 2 is preferable because good printing can be performed. The paste solvent is an organic solvent in which the metal used does not dissolve or react, and has a relatively low room temperature vapor pressure,
Anything that has good workability may be used. In particular, α-terpineol, ethylene glycol, glycerin and the like are preferable.
The viscosity of the slurry is preferably 0.1 to 100,000 poise.

It is also preferable to apply a paste of current collector metal powder to the upper surface of the hydrogen carbonate layer and take a lead.

The pore size of the porous electrode is 0.5 to 500 μm.
Is preferred.

It is also possible to form an electrode having a porous surface by sputtering these metal materials. The sputtering gas is Ar, He, O ₂ , N ₂
It is preferable to use any of the above, and the pressure during film formation is preferably in the range of 0.1 to 500 mTorr. Also, the electrode surface can be made porous by resistance heating vapor deposition.

When the current collector is a metal mesh, it is preferable to fix it at a predetermined position on the solid electrolyte and then apply a paste or an aqueous solution of hydrogencarbonate powder. By doing so, it is not necessary to heat the hydrogen carbonate. Even if an aqueous solution is applied from above the metal mesh in this way,
The aqueous solution or the like passes through the pores of the mesh to form a bicarbonate layer on the solid electrolyte. It is preferable to use an aqueous solution because it easily penetrates.

It is preferable that the current collector is provided so as to face the solid electrolyte with the hydrogen carbonate layer interposed therebetween. With such a structure, the sensing electrode itself functions as a gas diffusion layer, so that a quicker response can be obtained.

<Counter electrode> In the carbon dioxide sensor of the present invention,
Metal is used for the counter electrode. The metal used may be the same metal as the collector of the above-mentioned detection electrode, or may be a different metal. As the counter electrode, one kind of metal may be used, or two or more kinds of metals may be used. In particular, it is preferable to use the same metal as the detection electrode for the counter electrode.
By using the same metal as the detection electrode, the effect of coexisting gas is reduced and high carbon dioxide selectivity is obtained. In addition, the moisture resistance is improved, and the influence of humidity is reduced especially when measuring at low temperatures.

A metal oxide may be used for the counter electrode. Also in this case, one kind or two or more kinds of metal oxides may be used for the counter electrode. Also, it may be used in combination with a metal.

The counter electrode is preferably a porous metal like the current collector. In particular, a powder electrode formed by pressure-bonding or screen-printing a metal powder paste is preferable because high responsiveness, particularly high sensitivity can be obtained. The mesh size of the metal mesh is not particularly limited as long as it has holding power. The average particle size of the metal powder used in the paste for forming the powder electrode is preferably 10 nm to 100 μm, and particularly preferably 10 nm to 10 μm. The paste solvent may be an organic solvent in which the metal used does not dissolve or react, and has a relatively low room temperature vapor pressure and good workability. In particular, α-terpineol, ethylene glycol, glycerin and the like are preferable. The viscosity of the slurry is preferably 0.1 to 100,000 poise.

The pore size of the porous electrode is 0.5 to 500 μm
Is preferred.

<Sensor Structure> An example of the structure of the carbon dioxide sensor of the present invention is shown in FIGS. FIG. 1 shows a detection electrode 3 composed of a hydrogen carbonate layer 4 and a current collector 5 with a solid electrolyte 2 interposed therebetween.
Also, it is a separation type carbon dioxide sensor 1 provided with a counter electrode 6 facing each other. The hydrogen carbonate layer 4 contains hydrogen carbonate and / or hydrogen carbonate ions. FIG. 2 shows a non-separable carbon dioxide sensor 1 in which a detection electrode 3 composed of a hydrogen carbonate layer 4 and a current collector 5 and a counter electrode 6 are provided on one surface of a solid electrolyte 2. The non-separable type is preferable because the current collector can be formed and the leads can be taken out easily in the process and the manufacturing process can be simplified, resulting in high production efficiency. Also,
The element can be miniaturized. Lead wires are respectively drawn from the detection electrode 3 and the counter electrode 6 and are connected to the potentiometer.

The carbon dioxide sensor of the present invention is preferably constructed so that the measurement atmosphere is not exposed except the surface of the detection electrode in order to prevent the influence of humidity as much as possible. For example, it is preferable that the surface other than the detection electrode surface is coated with a resin such as Teflon or an inorganic ceramics, or with a glass tube in which a reference gas is sealed.

The size of the carbon dioxide sensor of the present invention is not particularly limited, but when the surface on which the detection electrode is formed is the upper surface of the solid electrolyte, the thickness of the solid electrolyte is usually 1 μm to 5 m.
The area of the upper surface of the solid electrolyte is about 1 μm ^{2 to} 200 mm ² . The thickness of the detection electrode is about 0.1 to 100 μm, and the area of the detection electrode is about 0.5 μm ^{2 to} 200 mm ² . The thickness of the counter electrode is about 0.1 to 100 μm, and the area of the counter electrode is about 0.5 μm ^{2 to} 200 mm ² .

The optimum operating temperature of the carbon dioxide sensor of the present invention varies depending on the material constituting the sensor element, the type of coexisting gas, etc., but is -70 ° C to 200 ° C, preferably -7 ° C.
It is in the range of 0 ° C to 150 ° C, more preferably -50 ° C to 120 ° C. At temperatures higher than this, the change in electromotive force with respect to the carbon dioxide concentration becomes small, possibly because the adsorbed water content decreases, and the carbon dioxide concentration cannot be measured. For the sensor that cannot be measured at high temperature, the performance will be restored and the response will be seen when the temperature is lowered to about room temperature. The carbon dioxide sensor of the present invention can operate at a lower temperature than a conventional carbon dioxide sensor using a solid electrolyte and can reduce power consumption. Further, since the temperature does not have to be high, the change in the measurement environment due to the heat of the heater can be sufficiently reduced.

Further, the carbon dioxide sensor of the present invention has good responsiveness, and a response is obtained in 1 second to 4 minutes, preferably 1 second to 3 minutes.

In the element structure, the heater is not necessary for the sensor capable of operating at room temperature, but it is preferable to attach the heater in consideration of the temperature difference depending on the season.

[0053]

EXAMPLES Example 1 A Pt mesh (100 mesh) was fixed with a Pt paste on the lower surface of a solid electrolyte NASICON pellet (10 mm diameter, 1 mm thickness) to obtain 900
It was fired at ℃ and made a counter electrode.

An Au mesh (100 mesh) as a current collector was provided on the upper surface of the solid electrolyte NASICON pellets. Then, the hydrogen carbonate powder shown in Table 1 (average particle size: 1
10 mg (0 nm to 100 μm) was dissolved in water to prepare a 1.0 wt% aqueous solution. Then, this aqueous solution was applied from the upper surface of the Au mesh, heated at 50 ° C. for 1 hour and dried to form a detection electrode.

Finally, a glass tube in which dry standard air was sealed on the counter electrode side was adhered and covered with an inorganic adhesive (Aron Ceramic C, manufactured by Toagosei Kagaku Co., Ltd.) so that only the surface of the detection electrode was exposed.

Then, the lead wires are connected from the respective electrodes to separate carbon dioxide sensors (No. 1 to No. 1) as shown in FIG.
7) got.

At room temperature (25 ° C.), the carbon dioxide sensor prepared was inserted into a measuring cell in which test gases of various CO ₂ concentrations were passed in dry air, and the electromotive force value generated with respect to the CO ₂ concentration was increased. Was measured. NaHCO on the detection electrode
Carbon dioxide sensor using ₃ (current collector is N with mesh electrode)
The result of sensor o.2) is shown in FIG.

C at room temperature (25 ° C.) in dry air
The prepared carbon dioxide sensor was inserted into a measurement cell in which a test gas having an O ₂ concentration of 1000 ppm was passed, and the response speed and sensitivity were examined. Furthermore, NO, NO diluted to the environmental standard concentration
₂ Confirm the response by circulating each gas of CO
The selectivity was investigated. The results are shown in Table 1.

[0059]

[Table 1]

The response speed is the time required to reach 90% of the electromotive force value when the response becomes constant after the introduction of CO ₂ gas. The evaluation was ◎: within 1 minute ○: over 1 minute within 3 minutes △: over 3 minutes within 5 minutes ×: over 5 minutes

The sensitivity is the difference between the electromotive force value before introduction of CO ₂ gas and the electromotive force value after introduction. The evaluation was ⊚: 25 mV or more, ◯: 15 mV or more and less than 25 mV Δ: 5 mV or more and less than 15 mV ×: less than 5 mV.

Selectivity is a property that is not affected by coexisting gases other than CO ₂ gas. Evaluation: ◎: Unaffected by all coexisting gases ○: Unaffected by two types of coexisting gas △: Unaffected by one type of coexisting gas ×: Influenced by all coexisting gases I decided.

Example 2 Instead of providing an Au mesh on the current collector of the detection electrode, 50 wt% of α-terpineol was added to 50 mg of Au powder (average particle size: 0.1 to 100 μm) on the upper surface of the solid electrolyte. Paste (viscosity 10,000
(100 to 100,000 poise) and heat treatment at 700 ° C. for 2 hours to provide a porous powder electrode, and then an aqueous solution of hydrogen carbonate is applied and impregnated to form a hydrogen carbonate layer. In the same manner as in Example 1, separation-type carbon dioxide sensors (No. 1 to 7) were manufactured and evaluated in the same manner as in Example 1. The results are shown in Table 1.

<Comparative Example 1> Hydrogen carbonate was not applied and dried,
Example 1, except that the bicarbonate layer of the detection electrode was not provided
A separation type carbon dioxide sensor (No. 8) was prepared in the same manner as in 2, and evaluated in the same manner as in Example 1. The results are shown in Table 2. In addition, a characteristic diagram of the output electromotive force value with respect to the CO ₂ concentration at room temperature (25 ° C.) of this carbon dioxide sensor using a mesh electrode as a current collector (sensor of No. 8 whose current collector is a mesh electrode) is shown in FIG. Shown in.

Comparative Example 2 The metal carbonate powder shown in Table 2 was used in place of the hydrogen carbonate powder as the sensing electrode material, and NAS was used.
Examples 1 and 2 except that the upper surface of the ICON pellet was fused.
Separate type carbon dioxide sensors (Nos. 9 to 11) were prepared in the same manner as in, and evaluated in the same manner as in Example 1. The results are shown in Table 2.

[0066]

[Table 2]

The carbon dioxide sensor of the present invention was superior in response speed, sensitivity and selectivity to those of the comparative example.

Further, the sensor in which the current collector is a powder electrode is
The response was quicker than that of the mesh electrode, and the sensitivity was particularly high.

Example 3 A Pt mesh (100 mesh) was provided in the same manner as in Example 1 on the upper surface of the pellet, that is, in the same plane as the detection electrode, to form a counter electrode.

Solid electrolyte NASICON pellets (1
Au mesh (1 mm) of current collector on top of 0 mm diameter and 1 mm thickness)
00 mesh) was provided, and an aqueous solution of hydrogen carbonate shown in Table 3 was applied and dried on the upper surface in the same manner as in Example 1 to obtain a detection electrode.

Then, by connecting the lead wires from the respective electrodes, a non-separable carbon dioxide sensor (No. 1) as shown in FIG.
2-18) was obtained.

This carbon dioxide sensor was evaluated in the same manner as in Example 1. The results are shown in Table 3.

Example 4 Instead of providing an Au mesh on the current collector of the detection electrode, 50 mg of Au powder (average particle size: 0.1 to 100 μm) and 50 wt% of α-terpineol were placed on the upper surface of the metal oxide layer. % Added paste (viscosity 10,000
Example 3 except that a porous powder electrode was provided by applying heat treatment at 700 ° C. for 2 hours.
Non-separable carbon dioxide sensor (No.12 ~ 1
8) was prepared and evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0074]

[Table 3]

With the non-separation type carbon dioxide sensor, the same result as that of the separation type was obtained.

In addition, even with a non-separation type carbon dioxide sensor,
The sensor in which the current collector was a powder electrode had a quicker response and was particularly sensitive than that of a mesh electrode.

Example 5 Instead of providing a Pt mesh on the counter electrode, 50 mg of Pt powder (average particle size: 50 nm) and 5 of α-terpineol were placed on the upper surface of the NASICON pellets.
A non-separable carbon dioxide sensor in which the current collector is a mesh electrode in the same manner as in Example 3 except that a paste containing 0 wt% was applied and heat treatment was performed at 650 ° C. for 2 hours to provide a porous powder electrode. (Nos. 19 to 25) were prepared and evaluated in the same manner as in Example 1. The results are shown in Table 3.

[0078]

[Table 4]

The sensor in which the counter electrode is the powder electrode has improved responsiveness, particularly sensitivity, as compared with that of the mesh electrode.

Further, the moisture resistance of the carbon dioxide sensor of the present invention was examined.

A separation type carbon dioxide sensor (No. 2 sensor having a mesh electrode as a current collector) having a detection electrode composed of NaHCO ₃ and Au mesh and a counter electrode of Pt mesh was used under an atmosphere of 80% relative humidity for 24 hours. After the time storage, the time variation of the electromotive force value in the reference gas was examined. The result is shown in Figure 4.
Shown in. The carbon dioxide sensor showed a satisfactory return in one day.

A non-separation-type carbon dioxide sensor (No. 20 sensor whose counter electrode is a powder electrode) whose counter electrode is a Pt powder electrode and whose counter electrode is NaHCO ₃ and Au mesh is used in an atmosphere of 80% relative humidity for 24 hours. After the time storage, the time variation of the electromotive force value in the reference gas was examined. The result is shown in FIG. This carbon dioxide sensor showed a very fast recovery of performance.

<Comparative Example 3> The current collector was made of Au mesh (10
Instead of (0 mesh), an Au thin film was used to cover the bicarbonate layer, and a separation type carbon dioxide sensor was produced in the same manner as in Example 1 and evaluated in the same manner as in Example 1.

With this carbon dioxide sensor, almost no CO ₂ response was confirmed. It is speculated that this is because the Au thin film as the current collector covers the surface and CO ₂ does not diffuse into the hydrogen carbonate layer.

Comparative Example 4 A separation type carbon dioxide sensor was prepared in the same manner as in Example 1 except that NaHCO ₃ was used as the detection electrode and the counter electrode surface was not covered with a glass tube containing dry standard air. Then, the evaluation was performed in the same manner as in Example 1.

No response was confirmed for this sensor for NO and CO, and the CO ₂ selectivity of the sensor of the present invention was confirmed.

[0087]

As described above, according to the present invention, it is possible to operate at room temperature, obtain sufficient sensitivity and responsiveness, and have high selectivity.
A carbon dioxide sensor having excellent moisture resistance can be provided.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration example of a separation type carbon dioxide sensor of the present invention.

FIG. 2 is a cross-sectional view showing a configuration example of a non-separable carbon dioxide sensor of the present invention.

FIG. 3 is a view of the separation type carbon dioxide sensor of the present invention in which the detection electrode is composed of NaHCO ₃ and Au mesh and the counter electrode is Pt mesh, and the detection electrode is Au mesh and the counter electrode is Pt.
C of the separation-type carbon dioxide sensor of Comparative Example which is a mesh
It is a characteristic view of the output electromotive force value with respect to the O ₂ concentration.

FIG. 4 is a reference gas (CO ₂ concentration) of the separation-type carbon dioxide sensor of the present invention in which the detection electrode is composed of NaHCO ₃ and Au mesh, and the counter electrode is Pt mesh electrode, after being stored for 24 hours in an atmosphere with relative humidity of 80%. It is a time change of the electromotive force value in 1000 ppm).

FIG. 5 is a reference gas (CO ₂₎ of the non-separable carbon dioxide sensor of the present invention in which the detection electrode is composed of NaHCO ₃ and Au mesh and the counter electrode is a Pt powder electrode, after being stored for 24 hours in an atmosphere with relative humidity of 80%. It is a change with time of the electromotive force value in a concentration of 1000 ppm).

[Explanation of symbols]

1 carbon dioxide sensor 2 Solid electrolyte 3 detection poles 4 bicarbonate layer 5 Current collector 6 opposite poles

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP 2000-88798 (JP, A) JP 2000-88799 (JP, A) JP 9-257747 (JP, A) JP 7-77515 ( (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01N 27/416 G01N 27/406

Claims (6)

(57) [Claims] 1. A detection electrode and a counter electrode are respectively provided in contact with a solid electrolyte, the solid electrolyte contains a metal ion conductor, and the detection electrode contains hydrogen carbonate and / or hydrogen carbonate ion. A carbon dioxide sensor having a hydrogen carbonate layer and a collector. 2. The hydrogen carbonate layer contains any one or more of lithium hydrogen carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, rubidium hydrogen carbonate, cesium hydrogen carbonate, magnesium hydrogen carbonate and calcium hydrogen carbonate. Carbon dioxide sensor. 3. The carbon dioxide sensor according to claim 1, further comprising the bicarbonate layer on the surface of the solid electrolyte in contact with the detection electrode. 4. The current collector is a porous metal.
A carbon dioxide sensor according to any one of ~ 3. 5. The carbon dioxide sensor according to claim 1, wherein the detection electrode and the counter electrode are provided on the same surface of the solid electrolyte. 6. The carbon dioxide sensor according to claim 1, wherein the counter electrode contains at least one kind of metal.