JP2022080742A - Constant potential electrolytic gas sensor - Google Patents

Abstract

To provide a constant potential electrolytic gas sensor that detects oxygen gas and toxic gas, the constant potential electrolytic gas sensor capable of preventing interference by miscellaneous gas in detection target gas even if the preset potential of a working electrode for toxic gas is 0.SOLUTION: A constant potential electrolytic gas sensor detects oxygen gas and toxic gas, and comprises: a sensor main body that has an electrode lamination structure; and an operation control circuit that drives the sensor main body. The electrode lamination structure includes a sheet-like electrolytic solution holding member that is impregnated with an electrolytic solution, a working electrode for oxygen gas and a working electrode for toxic gas that are arranged separate from each other on one face of the electrolytic solution holding member, and a counter electrode and a reference electrode that are arranged separate from each other on the other face of the electrolytic solution holding member. The operation control circuit has a feedback resistance that electrically connects the counter electrode and the reference electrode with each other.SELECTED DRAWING: Figure 6

Description

There is an application for application of Article 30, Paragraph 2 of the Patent Law. February 25, 2nd year Asagoe Industry Co., Ltd. (557-4 Mishima, Minami-ku, Okayama-shi, Okayama), Sumitomo Metal Mine Co., Ltd. (5-11-3, Shimbashi, Minato-ku, Tokyo), Hitachi High-Tech Co., Ltd. (Yamaguchi Prefecture) Shimomatsu City Oaza Higashitoyoi 794) and J Mate. Delivered at Copper Products Co., Ltd. (2024-1 Dosokohama, Ogata-ku, Joetsu City, Niigata Prefecture).

The present invention relates to a constant potential electrolytic gas sensor capable of detecting oxygen gas and toxic gas.

When detecting oxygen gas or toxic gas, an electrolytic reaction is used because it has excellent selectivity of the target gas to be detected, and can detect the gas concentration with high sensitivity and high accuracy. The constant potential electrolytic gas sensor is widely used.

As such a constant potential electrolytic gas sensor, Patent Document 1 describes an electrode stacking in which a working electrode, a reference electrode, and a counter electrode are laminated via an electrolytic solution holding member made of a hydrophilic non-woven fabric impregnated with an electrolytic solution. Those provided with a structure are disclosed. Further, Patent Document 1 discloses that it is possible to configure a gas sensor that detects a plurality of types of gas by providing a plurality of divided working electrodes.

In a constant-potential electrolytic gas sensor that detects a plurality of types of gas, when a small constant-potential electrolytic gas sensor that detects oxygen gas and toxic gas is configured, the concentration of oxygen gas in the test gas is high. When it decreases, there is a problem that the indicated value of the concentration of the toxic gas fluctuates greatly. This is because the set potential and output current differ greatly between the working pole for oxygen gas and the working pole for toxic gas, so the working pole for oxygen gas and the working pole for toxic gas are placed close to each other in order to reduce the size. Then, it is considered that the working electrode for oxygen gas interferes with the working electrode for toxic gas. Therefore, in order to solve this problem, it is conceivable to use a highly sensitive catalyst such as platinum black as the catalyst constituting the working electrode for the toxic gas.

However, a catalyst such as platinum black has about 10 times the sensitivity as that of a Pt-Ru catalyst, but interference occurs due to miscellaneous gas such as hydrogen gas and nitric oxide, so that miscellaneous gas is contained in the test gas. If present, there is a problem that it is difficult to detect only the gas to be detected with high accuracy. In order to solve such a problem, a means for changing the set potential of the working electrode for toxic gas to a potential at which the oxidation reaction of miscellaneous gas is suppressed by applying a voltage to the working electrode for toxic gas is known. (See Patent Document 2).

However, when the above means is adopted, a control circuit for applying a voltage to the working electrode for toxic gas is required, which causes an increase in the size of the entire device and an increase in manufacturing cost, and also causes inspection. It has been found that when used for a long period of time in an environment where the target gas and miscellaneous gas are always present, the potential of the reference electrode changes, and as a result, the expected potential cannot be maintained at the working electrode for toxic gas.

U.S. Pat. No. 7,608,177 Special Publication No. 59-10494

The present invention has been made based on the above circumstances, and an object thereof is that the set potential of the working electrode for toxic gas is 0 in a constant potential electrolytic gas sensor for detecting oxygen gas and toxic gas. Another object of the present invention is to provide a constant potential electrolytic gas sensor capable of suppressing interference due to miscellaneous gas in the test gas.

The constant potential electrolytic gas sensor of the present invention is a constant potential electrolytic gas sensor that detects oxygen gas and toxic gas.
It is equipped with a sensor body having an electrode laminated structure and an operation control circuit for driving this sensor body.
The electrode laminated structure includes a sheet-shaped electrolytic solution holding member impregnated with the electrolytic solution, and an oxygen gas working electrode and a toxic gas working electrode arranged apart from each other on one surface of the electrolytic solution holding member. And a counter electrode and a reference electrode arranged apart from each other on the other surface of the electrolytic solution holding member.
The motion control circuit is characterized by having a feedback resistance that electrically connects the counter electrode and the reference electrode.

Further, the constant potential electrolytic gas sensor of the present invention is a constant potential electrolytic gas sensor that detects oxygen gas and toxic gas.
It is equipped with a sensor body having an electrode laminated structure and an operation control circuit for driving this sensor body.
The electrode laminated structure includes a sheet-shaped electrolytic solution holding member impregnated with the electrolytic solution, and an oxygen gas working electrode and a toxic gas working electrode arranged apart from each other on one surface of the electrolytic solution holding member. And a counter electrode and a reference electrode arranged apart from each other on the other surface of the electrolytic solution holding member.
The operation control circuit is characterized by having a potential adjusting power supply that applies a voltage equivalent to the applied voltage of the counter electrode, and a feedback resistance that electrically connects the potential adjusting power supply and the reference electrode.

In the constant potential electrolytic gas sensor of the present invention, it is preferable that the working electrode for toxic gas has an electrode catalyst layer containing platinum black.
Further, it is preferable that the operation control circuit includes a potentiostat circuit.
Further, the reference electrode may have an electrode catalyst layer made of iridium dioxide and platinum black, and a mixed catalyst in which the ratio of iridium dioxide to platinum black is 99: 1 to 80:20 by mass ratio. preferable.
The toxic gas to be detected is at least one selected from carbon monoxide, hydrogen sulfide, sulfur dioxide, chlorine, ammonia, nitrogen dioxide, nitrogen monoxide, hydrogen cyanide, hydrogen gas, phosphine, ozone and chlorine dioxide. It is preferable to have.

In the present invention, "upper" and "lower" mean the constant potential when the constant potential electrolytic gas sensor of the present invention is arranged in a posture in which the surface of the casing on which the test gas inlet is formed faces upward. It indicates the direction in the electrolytic gas sensor. Therefore, for example, when the constant potential electrolytic gas sensor is arranged in a posture in which the surface of the casing on which the test gas inlet is formed faces downward, the "upper" and "lower" are actually in opposite directions, that is, "downward". When the constant potential electrolytic gas sensor is placed in a casing with the surface on which the test gas inlet is formed facing to the left, indicating the "down" and "up" directions, the "up" and "down" are actually Indicates the "left" and "right" directions, respectively.

According to the constant potential electrolytic gas sensor of the present invention, the reference electrode is electrically connected to a potential adjusting power source that applies a voltage equivalent to the applied voltage of the counter electrode or the counter electrode via a feedback resistor, and thus is used for toxic gas. Even if the set potential of the working electrode is 0, the potential of the working electrode for the toxic gas rises, and as a result, interference due to miscellaneous gas in the test gas can be suppressed.

It is explanatory cross-sectional view which shows the outline of the structure in the example of the constant potential electrolysis type gas sensor of this invention. It is a top view which shows the lower wall part in the constant potential electrolysis type gas sensor shown in FIG. It is explanatory cross-sectional view which expands and shows the working electrode for oxygen gas in the constant potential electrolysis type gas sensor shown in FIG. FIG. 3 is an explanatory cross-sectional view showing an enlarged working electrode for toxic gas in the constant potential electrolytic gas sensor shown in FIG. 1. It is a top view which shows the electrode complex in the constant potential electrolysis type gas sensor shown in FIG. 1. It is explanatory drawing which shows the structure in the example of the operation control circuit. It is explanatory drawing which shows the structure in another example of an operation control circuit. It is a graph which shows the result of the test 1 about the constant potential electrolysis type gas sensor which concerns on Example 1. FIG. It is a graph which shows the result of the test 1 about the constant potential electrolysis type gas sensor which concerns on Comparative Example 1. It is a graph which shows the result of the test 2 about the constant potential electrolysis type gas sensor which concerns on Example 1 and Reference Example 1.

Hereinafter, embodiments of the constant potential electrolytic gas sensor of the present invention will be described.
FIG. 1 is an explanatory sectional view showing an outline of a configuration in an example of a constant potential electrolytic gas sensor of the present invention.
This constant-potential electrolytic gas sensor detects oxygen gas and toxic gas, and includes a sensor main body 1 having an electrode laminated structure 20 and a casing 10 for accommodating the electrode laminated structure 20, and the sensor main body 1. It is provided with an operation control circuit 60 to be driven.
Here, examples of the toxic gas to be detected include carbon monoxide, hydrogen sulfide, sulfur dioxide, chlorine, ammonia, nitrogen dioxide, nitrogen monoxide, hydrogen cyanide, hydrogen gas, phosphine, ozone, and chlorine dioxide.

In the sensor main body 1, the casing 10 is composed of a cylindrical casing main body 11 having a closed lower end and a disk-shaped lid member 12 that closes an opening at the upper end of the casing main body 11 to form an upper wall portion 14. ing. The casing body 11 and the lid member 12 are each made of a thermoplastic resin such as polypropylene.

In the lid member 12 forming the upper wall portion 14, a test gas introduction port 13a for detecting oxygen gas and a test gas introduction port 13a for detecting toxic gas extend through the upper wall portion 14 in the thickness direction. Is formed. Further, a circular first recess 18 is formed on the upper surface of the lid member 12, and a circular second recess 19 is formed in the central region on the bottom surface of the first recess 18. A test gas introduction port 13a for detecting oxygen gas is formed in the central region on the bottom surface of the recess 19.

The internal space formed by the test gas introduction port 13a for detecting oxygen gas functions as a diffusion space for the test gas introduced through the pinhole 51 of the gas supply limiting means 50 described later.
The volume of the internal space formed by the test gas introduction port 13a is preferably, for example, about 0.1 to 10 mm ³ . With such a configuration, the introduced test gas can be sufficiently diffused, and the amount of oxygen gas remaining inside the sensor after the power is turned off can be reduced.

At the center position of the bottom wall portion of the casing main body 11, that is, the lower wall portion 15 of the casing 10, a ventilation pipe portion 16 having a circular cross section is upward (inwardly) from the lower wall portion 15 along the axial direction of the casing main body 12. It is formed so as to protrude and come into contact with the lower surface of the pressure adjusting film 28 described later. The ventilation pipe portion 16 forms a ventilation hole V that leads from the outer surface of the lower wall portion 15 to the lower surface of the pressure adjusting membrane 28.

As shown in FIG. 2, around the ventilation pipe portion 16 in the lower wall portion 15 of the casing 10, an oxygen gas working pole terminal 40, a toxic gas working pole terminal 41, a counter electrode terminal 42, and a reference pole terminal 43 are provided. They are arranged so as to be spaced apart from each other in the circumferential direction.
An electrolytic solution chamber S for accommodating the electrolytic solution is formed around the ventilation pipe portion 16 between the electrode laminated structure 20 and the lower wall portion 15 of the casing 10. The electrolytic solution chamber S leads to a space in which the electrode laminated structure 20 is arranged through a gap K between the peripheral wall portion 17 of the casing 10 and the support plate 30 described later.

Further, on the inner surface (upper surface) of the lower wall portion 15, a sealing resin material layer 45 formed by curing an adhesive such as an epoxy resin adhesive is provided as an oxygen gas working pole terminal 40 and a toxic gas working pole terminal. 41, it is formed so as to cover the counter electrode terminal 42 and the reference electrode terminal 43. By providing the sealing resin material layer 45, the electrolytic solution chamber S does not corrode the oxygen gas working electrode terminal 40, the toxic gas working electrode terminal 41, the counter electrode terminal 42, and the reference electrode terminal 43 by the electrolytic solution. It has a liquidtight sealing structure.

In the second recess 19 of the lid member 12, a disk-shaped gas supply limiting means 50 that matches the shape of the second recess 19 is accommodated and arranged.
The gas supply limiting means 50 is formed so that a pinhole 51 extending in the thickness direction communicates with the gas introduction port 13 of the lid member 12. By passing the test gas through the pinhole 51, the supply amount of the test gas introduced into the casing 10 from the test gas introduction port 13 is limited.

The pinhole 51 has an inner diameter of a uniform size in the axial direction. The size of the inner diameter of the pinhole 51 is preferably 1.0 to 200 μm. The length of the pinhole 51 is, for example, 0.1 mm or more.

A circular buffer film 55 is housed and arranged in the first recess 18.
The buffer film 55 of this example is composed of a laminate having a gas diffusion layer 56 into which the test gas flows from the outer peripheral surface and a protective layer 57 having gas impermeableness and water repellency, and the whole is circular. It is formed in a plate shape.

The gas diffusion layer 56 is adhered and fixed to the bottom surface of the first recess 18 of the lid member 12 and the upper surface of the gas supply limiting means 50 by a double-sided adhesive tape 58 having a through hole 58a formed in the pinhole 51. ing.
The gas diffusion layer 56 can be made of a fluororesin film such as a PTFE film.
The gas diffusion layer 56 preferably has an air permeability of 0.15 to 1.5 L / day, and the thickness, outer diameter dimension, void ratio, and other specific configurations are such that the air permeability is within the above numerical range. Can be set to be.

The size of the inner diameter of the through hole 58a of the double-sided adhesive tape 58 is preferably 0.05 to 3 mm, for example. The thickness of the double-sided adhesive tape 58 is preferably 0.05 to 1 mm, for example.
With such a configuration, it is possible to obtain sufficient durability against the external environment without significantly deteriorating the gas responsiveness, and it is possible to surely obtain a stable indicated value.

The protective layer 57 is adhered and fixed to the upper surface of the gas diffusion layer 56 with a double-sided adhesive tape 59. The protective layer 57 may be fixed to the upper surface of the gas diffusion layer 56 by using an adhesive layer made of an adhesive instead of the double-sided adhesive tape 59.
The protective layer 57 can be made of a composite film in which aluminum foil or silver is laminated on a resin film such as PET.

In the sensor main body 1, the electrode laminated structure 20 has a sheet-shaped electrolytic solution holding member 23 impregnated with the electrolytic solution and a circular shape for detecting oxygen gas, which is arranged on the upper surface of the electrolytic solution holding member 23 so as to be separated from each other. Sheet-shaped working electrode for oxygen gas 21 and semi-circular sheet-shaped working electrode for toxic gas 22 for detecting toxic gas, and counter electrode 26 and reference electrode arranged apart from each other on the upper surface of the electrolytic solution holding member 23. It is composed of an electrode composite 25 having 27.

The thickness of the electrolytic solution holding member 23 is such that the volume of the electrolytic solution holding member 23 can be as small as possible while being able to impregnate a sufficient amount of the electrolytic solution. It is 0.1 to 2 mm. With such a configuration, highly reliable gas detection can be performed even in a high humidity environment.
As the electrolytic solution holding member 23, for example, a glass fiber filter paper, a silica filter paper, or a non-woven fabric made of glass fiber, PP fiber, PP / PE composite fiber, or ceramic fiber can be used.

As shown in FIG. 3, the oxygen gas working electrode 21 is configured by forming an electrode catalyst layer 21b on a gas permeable film 21a having hydrophobicity, and the electrode catalyst layer 21b is an electrolytic solution holding member 23. It is arranged so as to be in contact with. Further, the gas permeable film 21a in the oxygen gas working electrode 21 is arranged so as to close the test gas introduction port 13. Further, the upper surface of the gas permeable film 21a is heat-welded to the lower surface (inner surface) of the upper wall portion 14 so as to surround the test gas introduction port 13a. Since the gas permeable film 21a is heat-welded to the upper wall portion 14, it is possible to prevent the electrolytic solution from leaking from between the gas permeable film 21a and the upper wall portion 14.

As shown in FIG. 4, the action electrode 22 for toxic gas is configured by forming an electrode catalyst layer 22b on a gas permeable film 22a having hydrophobicity, and the electrode catalyst layer 22b is an electrolytic solution holding member 23. It is arranged so as to be in contact with. Further, each of the gas permeable films 22a in the working electrode 22 for toxic gas is arranged so as to close the test gas introduction port 13b. Further, the upper surface of the gas permeable film 22a is heat-welded to the lower surface (inner surface) of the upper wall portion 14 so as to surround the test gas introduction port 13b. Since the gas permeable film 22a is heat-welded to the upper wall portion 14, it is possible to prevent the electrolytic solution from leaking from between the gas permeable film 22a and the upper wall portion 14.

As the gas permeable films 21a and 22a, for example, a porous membrane made of a fluororesin such as polytetrafluoroethylene (PTFE) can be used.
The porous membrane preferably has a Garley number of 3 to 3000 seconds. The thickness and porosity of the porous membrane can be set so that the number of garleys is within the above numerical range. For example, the porosity is 10 to 70% and the thickness is 0.01 to 1 mm. Is preferable.

The electrode catalyst layer 21b in the working electrode 21 for oxygen gas is formed of fine particles of a catalyst metal insoluble in an electrolytic solution, fine particles of an oxide of the catalyst metal, fine particles of an alloy of the catalyst metal, or a mixture of these fine particles. It is formed by catalyst fine particles. As the catalyst metal insoluble in the electrolytic solution, for example, platinum (Pt), gold (Au), ruthenium (Ru), palladium (Pd), iridium (Ir) and the like can be used.
As the electrode catalyst layer 22b in the working electrode 22 for toxic gas, platinum black is preferably used because the working electrode 22 for toxic gas having high sensitivity to toxic gas can be obtained.
Such electrode catalyst layers 21b and 22b are formed by preparing a paste containing catalyst fine particles and a binder, applying the paste to the surface of the gas permeable film 21a by screen printing or the like, and firing the paste. Can be done.

The working pole 21 for oxygen gas and the working pole 22 for toxic gas are each electrically connected to one end of a lead member for working pole (not shown). An oxygen gas working pole terminal 40 is electrically connected to the other end of the working pole lead member to which the oxygen gas working pole 21 is connected. Further, a toxic gas working pole terminal 41 is electrically connected to the other end of the working pole lead member to which the toxic gas working pole 22 is connected.
As a material constituting the lead member for the working electrode, metals such as gold (Au), tungsten (W), niobium (Nb) and tantalum (Ta) can be used. Further, as the lead member for the working electrode, a resin-coated platinum (Pt) wire can also be used. Among these, tantalum (Ta) wire is used as the lead member for the working electrode connected to the working pole 21 for oxygen gas, and platinum (Pt) wire is used as the lead member for the working pole connected to the working pole 22 for toxic gas. It is preferable to use.

As also shown in FIG. 5, the electrode complex 25 is a hydrophobic porous material that holds a counter electrode 26 and a reference electrode 27, respectively, which are arranged apart from each other and are composed of an electrode catalyst layer, and a counter electrode 26 and a reference electrode 27. It is composed of a pressure adjusting film 28 made of a material. The pressure adjusting film 28 in the electrode complex 25 is arranged so as to close the ventilation hole V on the upper end surface of the ventilation pipe portion 16 in the casing 10. As a result, the internal space of the casing 10 is opened to the outside atmosphere through the pressure adjusting membrane 28 and the ventilation holes V. Further, the lower surface of the pressure adjusting membrane 28 is heat-welded to the upper end surface of the ventilation pipe portion 16 so as to surround the ventilation hole V. Since the pressure adjusting membrane 28 is heat-welded to the upper end surface of the ventilation pipe portion 16, it is possible to prevent the electrolytic solution from leaking from between the pressure adjusting membrane 28 and the upper end surface of the ventilation pipe portion 16. ..

The electrode catalyst layer constituting the counter electrode 26 is made of catalyst fine particles such as fine particles of a catalytic metal insoluble in an electrolytic solution, fine particles of an oxide of the catalytic metal, fine particles of an alloy of the catalytic metal, or a mixture of these fine particles. It is formed. As the catalyst metal insoluble in the electrolytic solution, for example, platinum (Pt), gold (Au), ruthenium (Ru), palladium (Pd), iridium (Ir) and the like can be used.
As the electrode catalyst layer constituting the reference electrode 27, a mixed catalyst composed of iridium dioxide and platinum black and having a mass ratio of iridium dioxide and platinum black of 99: 1 to 80:20 shall be used. Is preferable. By using such a mixed catalyst as the electrode catalyst layer constituting the reference electrode 27, it is possible to suppress an increase in the zero point of the concentration indicated value of the toxic gas even when used in a high temperature environment.
The electrode catalyst layer constituting the counter electrode 26 and the reference electrode 27 is formed by preparing a paste containing catalyst fine particles and a binder, applying the paste to the surface of the pressure adjusting film 28 by screen printing or the like, and firing the paste. can do.
Further, the electrode catalyst layer constituting the counter electrode 26 and the reference electrode 27 may be made of the same material or different materials, but both the counter electrode 26 and the reference electrode 27 may be used in a single process. From the viewpoint that they can be formed, they are preferably made of the same material.

As the pressure adjusting film 28, for example, a porous film made of a fluororesin such as polytetrafluoroethylene (PTFE) can be used.
The porous membrane preferably has a Garley number of 3 to 3000 seconds. The thickness and porosity of the porous membrane can be set so that the number of garleys is within the above numerical range. For example, the porosity is 10 to 70% and the thickness is 0.01 to 1 mm. Is preferable.

The pressure adjusting film 28 has a circular electrode forming portion 28a on which the counter electrode 26 and the reference electrode 27 are formed, and three or more extending radially outward from the outer peripheral edge of the electrode forming portion 28a, respectively (4 in the illustrated example). It is composed of a rectangular tongue piece portion 28b. Each of the tongue piece portions 28b is formed so as to be arranged at equal intervals in the circumferential direction of the electrode forming portion 28a.

The counter electrode 26 and the reference pole 27 are electrically connected to one end of the counter electrode lead member (not shown) and the reference pole lead member (not shown), and to the other ends of the counter electrode lead member and the reference pole lead member. , The counter electrode terminal 42 and the reference electrode terminal 43 are electrically connected, respectively.
As a material constituting the counter electrode lead member and the reference electrode lead member, metals such as gold (Au), platinum (Pt), tungsten (W), niobium (Nb) and tantalum (Ta) can be used.

A support plate 30 for supporting the electrode laminated structure 20 is provided at the upper end portion of the ventilation pipe portion 16. The support plate 30 has an electrode forming portion supporting portion 31 that supports the electrode forming portion 28a in the pressure adjusting film 28, and a tongue piece portion 28b in the pressure adjusting film 28 formed around the electrode forming portion supporting portion 31. It has a tongue piece support portion 35 to support.
At the center position of the electrode forming portion support portion 31, a through hole 32 for the ventilation pipe portion having an inner diameter suitable for the outer diameter of the ventilation pipe portion 16 in the casing 10 is formed. The tip portion of the ventilation pipe portion 16 is fitted into the through hole 32 for the ventilation pipe portion.
Further, a through hole 36 for the tongue piece portion is formed in the tongue piece portion support portion 35. Each of the tongue piece portions 28b in the pressure adjusting membrane 28 enters the tongue piece portion through hole 36, and the tip portion of the tongue piece portion 28b is formed so as to be located in the electrolytic solution chamber S. According to such a configuration, regardless of the posture of the constant potential electrolytic gas sensor, the internal pressure of the casing 10 can be kept constant by the ventilation of the outside air to the inside of the casing 10.

In the sensor main body 1, each of the oxygen gas working pole terminal 40, the toxic gas working pole terminal 41, the counter electrode terminal 42, and the reference pole terminal 43 is electrically connected to the operation control circuit 60.
The operation control circuit 60 includes a potentiostat circuit that controls each of the oxygen gas working pole 21 and the toxic gas working pole 22 to maintain a predetermined set potential with respect to the reference pole 27.
FIG. 6 is an explanatory diagram showing a configuration in an example of an operation control circuit. The motion control circuit 60 in this example has three operational amplifiers.

The counter electrode 26 is electrically connected to the output terminal of the first operational amplifier 61. The reference electrode 61 is electrically connected to the inverting input terminal (-) of the first operational amplifier 61, and the non-inverting input terminal (+) is electrically connected to the reference voltage power supply (not shown). There is.

The output terminal of the second operational amplifier 63 is electrically connected to the inverting input terminal (−) via the resistance element 65, and is configured so that the output is negatively fed back. An oxygen gas working electrode 21 is electrically connected to the inverting input terminal (-) of the second operational amplifier 63 via a resistance element 64, and the non-inverting input terminal (+) is a voltage power supply (not shown). ) Is electrically connected.

The output terminal of the third operational amplifier 66 is electrically connected to the inverting input terminal (−) via the resistance element 68, and is configured so that the output is negatively fed back. The toxic gas working electrode 22 is electrically connected to the inverting input terminal (-) of the third operational amplifier 66 via the resistance element 67, and the non-inverting input terminal (+) is a voltage power supply (not shown). ) Is electrically connected.

Further, the operation control circuit 60 has a feedback resistor 62 that electrically connects the counter electrode 26 and the reference electrode 27. As the feedback resistor 62, it is preferable to use one having a resistance value of 0.68 to 44 MΩ.

In the above-mentioned constant potential electrolytic gas sensor, the working pole 21 for oxygen gas, the working pole 22 for toxic gas, and the reference pole 27 are maintained at a predetermined potential by the operation control circuit 60. The set potential of the oxygen gas working pole 21 with respect to the reference pole 27 is, for example, −750 mV, and the set potential of the toxic gas working pole 22 with respect to the reference pole 27 is, for example, 0 mV.

Then, the test gas introduced from the test gas introduction port 13a of the casing 10 permeates the gas permeable film 21a at the oxygen gas working electrode 21, and the oxygen (O ₂ ) contained in the test gas permeates the electrode. Upon contact with the catalyst layer 21b, a reduction reaction of oxygen (O ₂ ) represented by the following reaction formula (1) occurs in the electrode catalyst layer 21b, and at the counter electrode 26, water (H ₂ ) represented by the following reaction formula (2) occurs. The decomposition reaction of O) occurs.
Reaction equation (1): O ₂ + 4H ⁺ + ^4e- → 2H ₂ O
Reaction equation (2): 2H ₂ O → O ₂ + 4H ⁺ + ^4e-

On the other hand, when the toxic gas to be detected is contained in the test gas, the test gas introduced from the test gas introduction port 13b of the casing 10 has gas permeability in the toxic gas working electrode 22. When the film 22a permeates and comes into contact with the electrode catalyst layer 22b, an oxidation reaction occurs in the electrode catalyst layer 22b and a reduction reaction occurs in the counter electrode 26.

For example, when the toxic gas to be detected is carbon monoxide (CO), the oxidation reaction represented by the following reaction formula (3) occurs in the electrode catalyst layer 22b in the working electrode 22 for toxic gas, and the counter electrode 26 , The reduction reaction represented by the following reaction formula (4) occurs.
Reaction equation (3): CO + H ₂ O → CO ₂ + 2H ⁺ + ^2e-
Reaction equation (4): 1 / 2O ₂ + 2H ⁺ + ^2e- → H ₂ O

Further, for example, when the toxic gas to be detected is hydrogen sulfide (H2S), the oxidation reaction shown in the following reaction formula (5) occurs in the electrode catalyst layer 22b in the working electrode 22 for the toxic gas, while the oxidation reaction occurs. At the counter electrode 26, the reduction reaction represented by the following reaction formula (6) occurs.
Reaction equation (5): H ₂ S + 4H ₂ O → H ₂ SO ₄ + 8H ⁺ + ^8e-
Reaction equation (6): 2O ₂ + 8H ⁺ + ^8e- → 4H ₂ O

At this time, since the value of the current generated between the working electrode 21 for oxygen gas and the working electrode 22 for toxic gas and the counter electrode 26 correlates with the concentration of the oxygen gas or toxic gas to be detected, it is used for oxygen gas. By measuring the current flowing between the working electrode 21 and the working electrode 22 for toxic gas and the counter electrode 26, the concentration of the detection target gas in the test gas can be measured.
Further, in the counter electrode 26, oxygen (O ₂ ) is generated by the electrolysis of water, and the pressure inside the casing 10 is adjusted by the pressure adjusting membrane 28.

In the above, if miscellaneous gases such as nitric oxide (NO) and hydrogen gas (H ₂ ) are present in the test gas, a redox reaction may occur due to the miscellaneous gases at the working electrode 22 and the counter electrode 26 for the toxic gas. There is. For example, when nitrogen monoxide (NO) is present in the test gas, the oxidation reaction shown in the following reaction formula (7) occurs at the working electrode 22 for the toxic gas, and hydrogen gas (H ₂ ) is present in the test gas. Then, the oxidation reaction represented by the following reaction formula (8) occurs in the working electrode 22 for toxic gas.
Reaction equation (7): NO + H ₂ O → NO ₂ + 2H ⁺ + ^2e-
Reaction equation (8): H ₂ → 2H ⁺ + ^2e-

In order to suppress the oxidation reaction according to the above reaction formulas (7) and (8) in the working electrode 22 for toxic gas, the action for toxic gas is performed by applying a voltage to the working pole 22 for toxic gas. It is necessary to raise the set potential of the pole 22. However, since the operation control circuit 60 is provided with a feedback resistance 62 that electrically connects the counter electrode 26 and the reference electrode 27, even if the set potential of the working electrode 22 for the toxic gas is 0, the toxic gas is concerned. At the working electrode 22, the oxidation reaction according to the reaction formula (7) and the reaction formula (8) is suppressed.

Specifically, taking the case where the toxic gas to be detected is carbon monoxide as an example, when the potential of the non-inverting input terminal (+) of the first operational amplifier 61 is set to, for example, 1.2V, the first operational amplifier The inverting input terminal (-) of 61 becomes 1.2V, which is the same potential as the potential of the non-inverting input terminal (+) due to a virtual short circuit with the non-inverting input terminal (+). Since the reference electrode 27 is electrically connected to the inverting input terminal (-) of the first operational amplifier 61, the potential of the reference electrode 27 is 1.2 V, and the potential of the reference electrode 27 is 1.2 V as a reference. For example, a voltage of 0.45 V (set potential of the oxygen gas working electrode 21 with respect to the reference electrode 27 = 0.45 V-1.2 V = -0.75 V = -750 mV) is applied to the oxygen gas working electrode 21. .. Then, in a state where oxygen gas (concentration 20.9%) in the atmosphere is in contact with the working electrode 21 for oxygen gas, the reaction of the above reaction formula (2) occurs at the counter electrode 26, so that the first operational amplifier 61 For example, a high voltage of 1.6 V (set potential of counter electrode 26 with respect to reference electrode 27 = 1.6V-1.2V = 0.4V) is applied to the counter electrode 26 connected to the output terminal of. At this time, since the counter electrode 26 and the reference electrode 27 are electrically connected via the feedback resistance 62, the potential of the reference electrode 27 is lifted to the high potential of the counter electrode 26 and rises. Since the set potential of the toxic gas working pole 22 with respect to the reference pole 27 is 0 V, the potential of the toxic gas working pole 22 rises as the potential of the reference pole 27 rises. As a result, the oxidation reaction according to the above reaction formulas (7) and (8) is suppressed in the working electrode 22 for toxic gas.

Further, when the test gas contains a high concentration of toxic gas or miscellaneous gas, if these gases come into continuous contact with the counter electrode 26, the potential of the counter electrode 26 may decrease. However, since the reference electrode 27 is electrically connected to the counter electrode 26 via the feedback resistance 62, it is possible to suppress a decrease in the potential of the counter electrode 26.

As described above, according to the above-mentioned constant potential electrolytic gas sensor, since the reference electrode 27 is electrically connected to the counter electrode 26 via the feedback resistor 62 in the operation control circuit 60, the working electrode 22 for toxic gas is used. Even if the set potential of is 0, as a result of the increase in the potential of the working electrode 22 for the toxic gas, interference due to miscellaneous gas in the test gas can be suppressed.

Although the embodiment of the constant potential electrolytic gas sensor of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications can be made.
For example, in the operation control circuit 60, instead of the feedback resistor 62 that electrically connects the counter electrode 26 and the reference electrode, as shown in FIG. 7, a potential adjusting power supply 69 that applies a voltage equivalent to the applied voltage of the counter electrode 26 and A feedback resistor 70 for electrically connecting the potential adjusting power supply 69 and the reference electrode 27 may be provided. According to such a configuration, since the reference electrode 27 is electrically connected to the potential adjusting power supply 69 that applies a voltage equivalent to the applied voltage of the counter electrode 26 via the feedback resistor 70, the potential of the reference electrode 27 Is lifted to the high potential of the potential adjusting power supply 69 and rises, and as the potential of the reference electrode 27 rises, the potential of the working pole 22 for toxic gas rises. Therefore, even if the set potential of the working electrode 22 for toxic gas is 0, it is possible to suppress interference due to miscellaneous gas in the test gas.
Further, a plurality of action electrodes for toxic gas that detect different types of toxic gas can be arranged on the upper surface of the electrolytic solution holding member.

<Example 1>
According to the configurations shown in FIGS. 1 and 6, a constant potential electrolytic gas sensor having the following specifications for detecting oxygen gas and carbon monoxide was produced.
In the electrode laminated structure (20), the working electrode for oxygen gas (21) is a circle with a diameter of 3 mm (area is 7 mm ² ), and the material of the electrode catalyst layer (21b) is platinum black. The working electrode (22) for toxic gas is a semicircle with a diameter of 10 mm (area is about 30 mm ² ), and the material of the electrode catalyst layer (22b) is platinum black. Further, the distance between the working electrode for oxygen gas (21) and the working electrode for toxic gas (22) is 1 mm. The electrolytic solution holding member (23) is made of a glass fiber filter paper having a thickness of 0.3 mm. The counter electrode (26) has an area of about 7 mm ² and is a mixture of 5% by mass of platinum black and 95% by mass of iridium dioxide (IrO ₂ ). The reference electrode (27) has an area of 13 mm ² , and the material of the electrode catalyst layer is a mixture of 5% by mass of platinum black and 95% by mass of iridium dioxide (IrO ₂ ). The distance between the counter electrode (26) and the reference electrode (27) is 1.2 mm.
Further, in the gas supply limiting means (50), the pinhole (51) has an inner diameter of 20 μm and a length of 0.7 mm.

Further, in the operation control circuit (60), the resistance value of the feedback resistance (62) is 22 MΩ, and the resistance value of the resistance element (65) between the output terminal of the second operational amplifier (63) and the inverting input terminal (-). Is 3 kΩ, the resistance value of the resistance element (64) between the inverting input terminal (-) of the second operational amplifier (63) and the working electrode for oxygen gas (21) is 220 Ω, and the output of the third operational amplifier (66). The resistance value of the resistance element (68) between the terminal and the inverting input terminal (-) is 10 kΩ, and the resistance value between the inverting input terminal (-) of the third operational amplifier (66) and the working electrode for toxic gas (22). The resistance value of the resistance element (67) is 100Ω.

<Comparative Example 1>
A constant potential electrolytic gas sensor having the same configuration as that of the first embodiment was manufactured except that the feedback resistance was not provided in the operation control circuit.

<Reference example 1>
In the operation control circuit, a constant potential electrolytic gas sensor having the same configuration as that of Example 1 was manufactured except that the material of the electrode catalyst layer at the reference electrode was changed to iridium dioxide (IrO ₂ ).

[test]
Regarding the constant potential electrolytic gas sensor according to Example 1, Comparative Example 1 and Reference Example 1, the potential of the non-inverting input terminal (+) of the first operational capacitor is 1.2 V, and the applied voltage of the working electrode for oxygen gas is 0. The following tests 1 and 2 were performed under the conditions that the set potential of the oxygen gas working electrode 21 with respect to the reference electrode was -750 mV and the set potential of the toxic gas working electrode with respect to the reference electrode was 0 V.
[Test 1]
In the air atmosphere test space, with each of the constant potential electrolytic gas sensors according to Example 1 and Comparative Example 1 being operated, air containing 100 ppm of carbon monoxide was supplied to the test space for 60 seconds, and then the test space was used. Air was supplied to the test space for 60 seconds, and then air containing 500 ppm of hydrogen gas was supplied to the test space for 60 seconds, and then air was supplied to the test space. Then, the change in the carbon monoxide concentration indicated value during this period was investigated. The results are shown in FIG. 8 for Example 1 and in FIG. 9 for Comparative Example 1.
[Test 2]
In the air atmosphere test space, the temperature in the test space was raised from -20 ° C to 60 ° C while each of the constant potential electrolytic gas sensors according to Example 1 and Reference Example 1 was operated, and then 60 ° C. The temperature was lowered to −20 ° C. Then, the change in the output value (current value) related to the working electrode for toxic gas during this period was investigated. The results are shown in FIG.

As is clear from the results of FIGS. 8 and 9, according to the constant potential electrolytic gas sensor according to the first embodiment, even if the set potential of the working electrode for the toxic gas is 0, it depends on the miscellaneous gas in the test gas. It was confirmed that the interference was sufficiently suppressed.
Further, as is clear from the results of FIG. 10, according to the constant potential electrolytic gas sensor according to the first embodiment, the electrode catalyst layer at the reference electrode contains platinum black and iridium dioxide (IrO ₂ ) in a specific ratio. It was confirmed that the zero point of the concentration indicated value of the toxic gas is suppressed from rising even if the environmental temperature rises.

10 Casing 11 Case sink body 12 Lid member 13a, 13b Test gas inlet 14 Upper wall part 15 Lower wall part 16 Ventilation pipe part 17 Peripheral wall part 18 1st recess 19 2nd recess 20 Electrode laminated structure 21 For oxygen gas Working electrode 21a Gas permeable film 21b Electrode catalyst layer 22 Working pole for toxic gas 22a Gas permeable film 22b Electrode catalyst layer 23 Electrolyte holding member 25 Electrode composite 26 Counter electrode 27 Reference electrode 28 Pressure adjusting film 28a Electrode forming part 28b Tongue One part 30 Support plate 31 Electrode forming part Support part 32 Through hole for ventilation pipe part 35 Tongue piece support part 36 Through hole for tongue piece part 40 Working electrode terminal for oxygen gas 41 Working pole terminal for toxic gas 42 Counter electrode terminal See 43 Polar terminal 45 Sealing resin material layer 50 Gas supply limiting means 51 Pinhole 55 Buffer film 56 Gas diffusion layer 57 Protective layer 58a Through hole 58 Double-sided adhesive tape 59 Double-sided adhesive tape 60 Operation control circuit 61 First operational control 62 Feedback resistance 63 Second optotype 64,65 Resistance element 66 Third optotype 67,68 Resistance element 69 Potential adjustment power supply 70 Feedback resistance K gap S Electrolyte chamber V Vent

Claims (6)

A constant-potential electrolytic gas sensor that detects oxygen gas and toxic gas.
It is equipped with a sensor body having an electrode laminated structure and an operation control circuit for driving this sensor body.
The electrode laminated structure includes a sheet-shaped electrolytic solution holding member impregnated with the electrolytic solution, and an oxygen gas working electrode and a toxic gas working electrode arranged apart from each other on one surface of the electrolytic solution holding member. And a counter electrode and a reference electrode arranged apart from each other on the other surface of the electrolytic solution holding member.
The operation control circuit is a constant potential electrolytic gas sensor characterized by having a feedback resistance for electrically connecting the counter electrode and the reference electrode. A constant-potential electrolytic gas sensor that detects oxygen gas and toxic gas.
It is equipped with a sensor body having an electrode laminated structure and an operation control circuit for driving this sensor body.
The electrode laminated structure includes a sheet-shaped electrolytic solution holding member impregnated with the electrolytic solution, and an oxygen gas working electrode and a toxic gas working electrode arranged apart from each other on one surface of the electrolytic solution holding member. And a counter electrode and a reference electrode arranged apart from each other on the other surface of the electrolytic solution holding member.
The operation control circuit is characterized by having a potential adjusting power supply that applies a voltage equivalent to the applied voltage of the counter electrode, and a feedback resistor that electrically connects the potential adjusting power supply and the reference electrode. Type gas sensor. The constant potential electrolytic gas sensor according to claim 1 or 2, wherein the working electrode for toxic gas has an electrode catalyst layer containing platinum black. The constant potential electrolytic gas sensor according to any one of claims 1 to 3, wherein the operation control circuit includes a potentiostat circuit. The reference electrode is characterized by having an electrode catalyst layer composed of iridium dioxide and platinum black, and a mixed catalyst in which the ratio of iridium dioxide to platinum black is 99: 1 to 80:20 by mass ratio. The constant potential electrolytic gas sensor according to any one of claims 1 to 4. The toxic gas to be detected shall be at least one selected from carbon monoxide, hydrogen sulfide, sulfur dioxide, chlorine, ammonia, nitrogen dioxide, nitrogen monoxide, hydrogen cyanide, hydrogen gas, phosphine, ozone and chlorine dioxide. The constant potential electrolytic gas sensor according to any one of claims 1 to 5.

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