JP2003172722A - Electrochemical nitrogen oxide sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrochemical nitrogen oxide sensor having excellent stability for a long time. <P>SOLUTION: A lithium ion electrical conductive liquid in which lithium salt composed of lithium ion and anion containing fluorine is dissolved into molten salt composed of aromatic cation containing nitrogen or aliphatic onium cation and anion containing fluorine is used as electrolyte 3a. This electrochemical nitrogen oxide sensor is provided with an electrode-auxiliary layer joined body which joins an auxiliary layer 4 mainly composed of lithium nitrate or lithium nitrite with either or both of a detecting electrode 1 composed of a gas diffused electrode and a counter electrode 2 integrally. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrochemical sensor for detecting and measuring gaseous nitrogen oxides, and more particularly to an electrochemical sensor using a room temperature molten salt which becomes an ionic liquid at a relatively low temperature as an electrolyte. Is.

[0002]

2. Description of the Related Art In recent years, various types of sensor technologies have been proposed as sensor technologies suitable for simple measurement of nitrogen oxide concentration, which is an air pollutant. They are roughly classified into oxide semiconductor type and electrochemical type. In addition, the electrochemical sensors are classified into (A) aqueous electrolyte system and (B) solid electrolyte system according to the constitution of the electrochemical system.
It can be classified into (a) voltage detection formula and (b) current detection formula. Aqueous electrolyte-based sensors are usually
It is composed of an electrochemical system in which a sulfuric acid aqueous solution is used as an electrolytic solution and a pair of gas diffusion electrodes are arranged. As the gas diffusion electrode, a porous electrode formed by binding carbon powder supporting a catalyst such as platinum with a binder such as polytetrafluoroethylene is used. Among the detection principles of nitrogen oxide, NO detection is based on the following reaction.

[0003]

## STR1 ## Detection _{electrode: NO + H 2 O → NO} 2 + 2H + + 2e - (1) pairs of ^{_{poles: 1 / 2O 2 + 2H +}} + 2e - → H 2 O (2) total reaction: NO + 1 / 2O ₂ → NO ₂ (3)

Further, the following reaction is used for detecting NO ₂ .

[0005]

## STR2 ## Detection ^{_{electrode: NO 2 + 2H + + 2e}} - → NO + H 2 O (4) pairs of _{poles: NO + H 2 O → NO} 2 + 2H + + 2e - (5)

As the reference electrode, it is usual to utilize the following reaction involving oxygen in the air.

[0007]

[Chemical 3]

Further, as a measuring method, when an appropriate electric potential based on the electric potential according to the formula (6) is applied to the measuring electrode, the current flowing between the measuring electrode and the counter electrode is measured and A general method is to know the concentration of NO or NO ₂ .

Nitrogen oxide sensors that use solid electrolytes include β-alumina or Nasicon (Na ₃ Zr ₂ Si ₂ PO ₁₂ ).
It consists of an electrochemical system based on the combination of a solid electrolyte such as sodium ion conductor or lithium ion conductor like lithium ion conductor and an auxiliary phase consisting of sodium or lithium nitrate, nitrite or a mixture of both. Have been proposed (Japanese Patent Laid-Open No. 4-1
42455, JP-A-5-288710). That is, this type of sensor is, for example, a detection gas, air / Au (detection electrode)
/ NaNO ₃ (NaNO ₂ ) / Nasicon / Au (counter electrode), air detection gas, air / Au (detection electrode) / Nasicon / NaNO ₃ (NaNO ₂ ) /
Au (counter electrode), air-like structure. The gold (Au) as an electrode and the auxiliary phase are fixed by applying the solid electrolyte to the solid electrolyte by some method and firing it. In these systems, when using NaNO ₃ as the auxiliary phase material, 2
At an operating temperature of 00 to 250 ° C., a kind of concentration cell based on the following reaction formula is formed, and the concentration of NO ₂ can be known by measuring the potential difference between the detection electrode and the counter electrode.

[0010]

[Chemical 4]

Further, when a current detection type sensor is constructed in this system, it is considered that the reaction on the sensing electrode is left side → right side of the equation (7), and that on the counter electrode is right side → left side. When this current detection method is adopted, it is usual to arrange NaNO ₃ as an auxiliary phase on the counter electrode side.

When NaNO ₂ is used as the auxiliary phase and NO _{2 is} to be detected, the auxiliary phase is arranged on the counter electrode side and the current detection type utilizing the following reaction is used to improve the detection sensitivity. (N.Miura et al, Sensors and
Actuators B 49 (1998) 101-109).

[0013]

Embedded image sensing ^{_{electrode: NO 2 + Na + + e}} - → NaNO 2 (10) pairs of _{poles: NaNO 2 → NO 2 + Na} + + e - (11)

As can be understood from these equations (7) to (11), sodium nitrate (NaNO ₃ ) or sodium nitrite (NaNO ₂ ) as an auxiliary phase plays a role as an auxiliary electrode. . As a detection method, a method of detecting a potential difference of a NO ₂ concentration cell and a method of detecting a current flowing when a constant voltage is applied have been proposed. Further, with this configuration, when NO is detected, NO is temporarily removed by NO ₂ by fixing an appropriate catalyst layer to the detection electrode.
It has been proposed to oxidize to NO and detect its NO ₂ .

[0015]

The conventional electrochemical nitric oxide sensor still has various problems.
Therefore, it is hard to say that it has been put to practical use.

A sensor using an aqueous electrolyte is not suitable for directly detecting nitrogen oxides in high temperature combustion exhaust gas because its operating temperature is room temperature. Further, in the sulfuric acid aqueous solution electrolytic solution, there is a problem that the absorption and evaporation of water occur with the fluctuation of the humidity of the detection gas atmosphere, which causes the fluctuation of the electrolytic solution part volume.

On the other hand, in the case of the solid electrolyte type sensor, there is a problem that the integral adhesion between the NaNO ₃ or NaNO ₂ phase as the auxiliary phase and the electrode and the solid electrolyte deteriorates during long-term operation. . This is because the volume of these auxiliary phase components changes as they are consumed or produced during sensor operation.

[0018]

SUMMARY OF THE INVENTION The present invention is intended to solve the problems found in the electrochemical nitrogen oxide sensor using the conventional aqueous electrolyte and solid electrolyte as described above. Is characterized in that a so-called room temperature molten salt is adopted as the electrolyte. More specifically, the electrochemical nitrogen oxide sensor of the present invention is characterized by a molten salt composed of a nitrogen-containing aromatic cation or aliphatic onium cation and a fluorine-containing anion, and a lithium ion and a fluorine-containing anion. An electrode in which a lithium ion conductive liquid in which a lithium salt is dissolved is used as an electrolytic solution, and an auxiliary phase mainly composed of lithium nitrate or lithium nitrite is integrally bonded to one or both of a detection electrode and a counter electrode composed of a gas diffusion electrode- It is provided with an auxiliary phase joined body.

The nitrogen-containing aromatic cation is an alkylimidazolium ion or an alkylpyridinium ion, and the aliphatic onium cation is
An aliphatic quaternary ammonium ion, an aliphatic sulfonium ion or a derivative ion thereof, the fluorine-containing anion is a borofluoride ion, a phosphorus fluoride ion or a trifluorosulfonylimide ion, and the lithium salt is borofluoride. lithium,
It is preferably lithium phosphorus fluoride or lithium trifluorosulfonylimidate. Furthermore, it is preferable that the auxiliary phase has a structure in which lithium nitrate or lithium nitrite is filled in the pores of a porous sheet made of a metal or a polymer.

[Effect] The present invention is, in short, a solid electrolyte type nitrogen oxide sensor having a conventionally known auxiliary phase, in which an electrolyte solution containing a room temperature molten salt as a constituent element is used instead of the solid electrolyte and This is an optimization of the sensor configuration including the phase structure.

The room temperature molten salt is a salt which is liquid at room temperature and has ionic conductivity, and in recent years, it is 10 ^-3 to 10 ^-1 S / cm.
It has been discovered that it has a considerably high specific ion conductivity. Recent room temperature molten salts have the characteristics of stable in air, nonflammability, nonvolatility, high heat resistance (200-250 ℃), and wide potential window (not decomposed even at 4-6V).
A typical room temperature molten salt is a salt composed of a combination of a nitrogen-containing aromatic cation such as an alkylimidazolium ion and an alkylpyridinium ion and various anions, or an aliphatic quaternary ammonium ion, an aliphatic sulfonium ion or the like. A salt composed of an onium cation and a trifluorosulfonylimide anion is known (R. Hagiwara, Y. Ito, J. Fluorine
Chem., 105 (2000) 221).

In applying the room temperature molten salt to the nitrogen oxide sensor, the inventor of the present application does not succeed simply by replacing the solid electrolyte of the conventional solid electrolyte type sensor with the room temperature molten salt. We obtained the finding that it is essential to meet the requirements of. (1) The cation of the nitrate or nitrite as the auxiliary phase material and the migrating cation of the room temperature molten salt electrolyte solution are the same, and the auxiliary phase material does not dissolve in the electrolyte solution. (2) When fixing the auxiliary phase material to the electrode, adopt a method and structure other than the firing method used in the case of the solid electrolyte type sensor.

First, when the room temperature molten salt is used alone, the above (1)
Cannot be met. On the other hand, the inventor of the present application has found that lithium borofluoride (LiB
F ₄ ), phosphorus fluoride (LiPF ₆ ), trifluorosulfonyl imide salt, etc., a lithium ion conductive electrolyte in which a salt of anion having a large ionic radius is dissolved is selected. It was discovered that the nitrates and nitrites of the above were hardly soluble in this electrolyte. The present invention was first made based on this discovery.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The basic sensor structure according to the present invention is mainly composed of voltage detection type: detection electrode / auxiliary phase / normal temperature molten salt electrolytic solution / counter electrode mainly current detection type: detection electrode / normal temperature molten salt electrolytic solution / It becomes like an auxiliary phase / counter electrode. The detection electrode and the counter electrode are composed of a conventionally known gas diffusion electrode used in an aqueous electrolyte type sensor, and an auxiliary phase mainly composed of lithium nitrate or lithium nitrite is integrally bonded to the detection electrode or the counter electrode. . As a method of forming the auxiliary phase, when applying a gas diffusion electrode whose treatment at high temperature is not preferable as in the present invention, it is impossible to adopt the baking method as in the case of the solid electrolyte type sensor. Is.

As a method for producing the gas diffusion electrode-auxiliary phase combination, a method in which the gas diffusion electrode and the auxiliary phase are formed into a double layer and a method in which the auxiliary phase material is mixed into the gas diffusion electrode are effective. is there. When manufacturing the double layer combination, it is preferable to directly disperse the auxiliary phase material powder alone or a mixture with an appropriate binder on one surface of the gas diffusion electrode, or press-integrate a sheet-shaped product in advance. .

The sheet-like auxiliary phase may be formed from a mixture of the auxiliary phase material and a binder, or may be formed by retaining the auxiliary phase material in the pores of a metal or polymer porous sheet. On the other hand, in the gas diffusion electrode mixed with the auxiliary phase material, the electrode material and the auxiliary phase material may be mixed in advance when the electrode is manufactured.

When one sensor is operated by both the voltage detection type and the current detection type, the auxiliary phase may be bonded to both the detection electrode and the counter electrode. As the room temperature molten salt electrolytic solution, a lithium ion conductive electrolytic solution in which a lithium salt is dissolved in a room temperature molten salt is used.

The electrode reaction in the sensor having such a structure is completely different from that in the case of the solid electrolyte type sensor using the above-mentioned NASICON (formulas (7) to (11)) and Na ⁺ ion is replaced by Li ⁺ ion. It will be similar. The sensor configuration is also basically similar to the configuration when using a conventional solid electrolyte, but since the electrolyte is a liquid, it is possible to absorb the variation accompanying the electrode reaction of the auxiliary phase volume, The effect is great in that the problem of peeling between the auxiliary phase and the electrode or the solid electrolyte can be avoided. Also,
In addition to not absorbing water in the atmosphere, the room temperature molten salt electrolyte is generally stable even at a relatively high temperature of 200 to 250 ° C, which eliminates the drawbacks of using an aqueous sulfuric acid solution as the electrolyte. And is effective.

The cations of the room temperature molten salt which can be used in the present invention are represented by nitrogen-containing aromatic cations represented by alkylimidazolium ions and alkylpyridinium ions, aliphatic quaternary ammonium ions and aliphatic sulfonium ions. It is an aliphatic onium cation.

As the alkyl imidazolium ion,
Specifically, 1-ethyl-3-methylimidazolium ion is most suitable. Specific examples of aliphatic quaternary ammonium ions (H. Matsumoto et al., Chemical Letters, (2000) 923)
As for, when the operating temperature of the sensor is strictly limited to room temperature, trimethylpropylammonium ion and trimethyl-n-octylammonium ion having a melting point of the salt of about 20 ° C. are suitable. When is set higher than room temperature, trimethylallylammonium-based, trimethylpropargylammonium-based, trimethylethylammonium-based, trimethylammonium-based, etc., which have a melting point slightly higher than room temperature, can also be used.

Further, a substance having an alkoxy group which is a derivative of this substance, for example, methoxymethyltrimethylammonium type, also has a high specific ion conductivity because it has a low melting point of about 5 ° C. and is an excellent material. Aliphatic sulfonium-based cations (H. Matsumoto et al., Chemical Letters,
(2000) 1430), specifically, triethylsulfonium ion and tributylsulfonium ion can be used because the salts have relatively low melting points. The above specific examples are merely examples, and the present invention is not limited to these substances.

As the anion of the room temperature molten salt, a fluorine-containing ion having a relatively large ionic radius such as borofluoride ion (BF ₄ ⁻ ), phosphorus fluoride ion (PF ₆ ⁻ ), or trifluorosulfonylimide ion is effective. Is. However,
In the case of the aliphatic onium cation system, a room temperature molten salt cannot be obtained unless the trifluorosulfonylimide anion is used. As the lithium salt to be dissolved in the room temperature molten salt, it is desirable to use one having the same anion.

[0033]

[Embodiment 1] FIG. 1 shows an electrochemical nitrogen oxide sensor according to an embodiment of the present invention, particularly, a voltage detection NO.
₂ shows a schematic sectional view of a sensor. Reference numeral 1 is a detection electrode composed of a gas diffusion electrode, and 4 is an auxiliary phase integrally joined to the detection electrode 1. The detection electrode 1 is made of a mixture of carbon powder supporting a gold catalyst and polytetrafluoroethylene as a binder. The auxiliary phase 4 is obtained by filling the pores of the porous nickel sheet 4a with lithium nitrate 4b as the auxiliary phase material. 3 is 1-ethyl-3 which is a room temperature molten salt
Electrolyte solution in which 0.5M lithium borofluoride is dissolved in borofluoride of methylimidazolium, and 2 is a counter electrode having the same configuration as the detection electrode. 1a is a terminal of the detection electrode 1,
2a is a terminal of the counter electrode 2 and is housed in the sensor case. The electrolytic solution is supplied from the electrolytic solution inlet shown by the arrow.

According to such a configuration, the output circuit 5
By adopting as the one that obtains the potential difference between the detection electrode 1 and the counter electrode 2 as the gas detection output, it becomes possible to quantify nitrogen oxides.

As shown in FIG. 2, excellent linearity was observed between the logarithm of the NO ₂ gas concentration at 25 ° C. and the voltage between the detection electrode and the counter electrode of the above-mentioned nitrogen oxide detection device.

[Embodiment 2] FIG. 3 shows an electrochemical nitrogen oxide detector according to an embodiment of the present invention. This electrochemical nitrogen oxide detector is particularly used as a current detector NO detector, where 1 is a detector electrode composed of a gas diffusion electrode, and 3 is a trifluorosulfonium imide salt of trimethylpropylammonium which is a room temperature molten salt. .47M
Electrolyte solution 3 in which lithium trifluoroimidate is dissolved
An electrolytic cell accommodating a and a separator 12 made of a porous polypropylene sheet. Reference numeral 2 is a counter electrode composed of a gas diffusion electrode similar to the detection electrode 1, and one side thereof has an auxiliary phase 4
Are joined together. The auxiliary phase 4 is formed from a mixture of lithium nitrite powder and nickel powder as auxiliary phase materials. Reference numeral 1a is an electrode terminal of the detection electrode 1, 2a is an electrode terminal of the counter electrode 2, and 11 is a reference electrode made of gold wire.

This sensor detects the current when a voltage is applied between the detection electrode 1 and the counter electrode 2 while setting the potential of the detection electrode 1 constant with reference to the reference electrode 11. It is possible to know the nitrogen oxide concentration. As shown in FIG. 4, excellent linearity was observed between the NO ₂ concentration and the current of this detector.

Comparative Example 1 A pair of gas diffusion electrodes (Example
1) and 5M sulfuric acid aqueous solution
Conventionally known electrochemical nitric oxide composed of an electrolytic solution
Object sensor (A) and sensor (B) according to the first embodiment of the present invention
And 30 ° C, 90% (relative humidity) high humidity
Absorption of water by electrolyte and sensor characteristics under environmental conditions
(0.1ppm NO ₂Of the output) was compared and examined. So
As a result, in the operation period of 30 days, sulfuric acid aqueous solution electrolyte type
The water absorption of the sensor is 11% of the initial amount of electrolyte,
While the output fluctuation value of the sensor was 15%,
The water absorption amount of the solution type sensor is 0.1%, and the output fluctuation value
Was 0.5%. On the other hand, the environmental atmosphere is 30 ℃, 30% relative
When using humidity, the liquid volume of the sensor (A) decreased by 9%
On the other hand, the liquid amount of the sensor (B) did not change at all.

From these test results, in the conventional sulfuric acid aqueous solution electrolyte type nitrogen oxide sensor, while the amount of the electrolyte solution varies due to the absorption or evaporation of moisture depending on the level of humidity in the environmental atmosphere, As in the present invention, by adopting the room temperature molten salt electrolyte solution, it is possible not only to avoid the ingress and egress of water, but also because the vapor pressure of the room temperature molten salt is low, there is almost no reduction in the amount of the electrolyte solution. Recognize.

[Comparative Example 2] A solid electrolyte type sensor (C) using NaNO ₂ as a solid electrolyte and NaNO ₂ as an auxiliary phase and a sensor (D) according to Example 2 were prepared and a 1 ppm NO ₂ atmosphere was prepared. Continuous operation (both current detection method) was performed at a lower temperature (150 ° C for the sensor (C) and 30 ° C for the sensor (D)). As a result, as shown in FIG. 5, on the 150th day of operation, the output of the solid electrolyte sensor decreased to 23% of the initial value, and
According to the disassembly study, the adhesion between the solid electrolyte layer and the auxiliary phase was significantly deteriorated. On the other hand, in the sensor (D) according to the present invention, the output fluctuation was only 0.4%, and no structural abnormality was observed. From these results, it can be seen that the present invention exerts an extremely large effect in preventing the mutual adhesion of the electrode-auxiliary phase-solid electrolyte during the long-term operation of the conventional solid electrolyte type sensor from being deteriorated.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a voltage detection type electrochemical nitrogen oxide detection device according to an embodiment of the present invention.

FIG. 2 is a graph showing output characteristics of the nitrogen oxide detection device according to the first embodiment.

FIG. 3 is a sectional view of a current detection type electrochemical nitrogen oxide detection device according to an embodiment of the present invention.

FIG. 4 is a graph showing the output characteristics of the nitrogen oxide detection device according to the second embodiment.

FIG. 5: Conventional solid electrolyte sensing device and Example 2 of the present invention
Graph comparing output fluctuations during long-term operation with the detection device

[Explanation of symbols]

1 detection pole 2 opposite poles 3a Electrolyte 3 electrolysis tank 4 Auxiliary phase 5 output circuits

Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G01N 27/30 341J (72) Inventor Masashi Horiuchi 2-5-4 Mitsuyanakaka, Yodogawa-ku, Osaka-shi, Osaka Shin-Cosmos Electric Co., Ltd. In the company

Claims (6)

[Claims] 1. A lithium ion conductive liquid in which a lithium salt composed of a lithium ion and a fluorine-containing anion is dissolved in a molten salt composed of a nitrogen-containing aromatic cation or an aliphatic onium cation and a fluorine-containing anion. As an electrolyte solution, and an electrode-auxiliary phase assembly in which an auxiliary phase mainly composed of lithium nitrate or lithium nitrite is integrally bonded to one or both of a detection electrode and a counter electrode composed of a gas diffusion electrode, Electrochemical nitrogen oxide sensor. 2. The electrochemical nitrogen oxide sensor according to claim 1, wherein the nitrogen-containing aromatic cation is an alkylimidazolium ion or an alkylpyridinium ion. 3. The electrochemical nitrogen oxide sensor according to claim 1, wherein the aliphatic onium cation is an aliphatic quaternary ammonium ion, an aliphatic sulfonium ion or a derivative ion thereof. 4. The fluorine-containing anion is borofluoride ion, phosphorus fluoride ion or trifluorosulfonylimide ion.
An electrochemical nitrogen oxide sensor described in 1. 5. The electrochemical nitrogen oxide sensor according to claim 1, wherein the lithium salt is lithium borofluoride, lithium phosphorofluoride, or lithium trifluorosulfonylimidate. 6. The electrochemical nitrogen oxide sensor according to claim 1, wherein the auxiliary phase has a structure in which pores of a metal or polymer porous sheet are filled with lithium nitrate or lithium nitrite. .