JP2007240437A - Gas sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gas sensor which is high in reliability of the measurement results and little affected by the density change of electrolyte or the vibration. <P>SOLUTION: The gas sensor 1 comprises the opposite pole 5, the comparison pole 6, and the operation pole 7, and at least between the comparison electrode 6 and the electrolyte, the anion-exchange membrane 15 is arranged. In the electrolyte, OH<SP>-</SP>passes through the anion-exchange membrane and arrives at the comparison electrode 6. The comparison electrode 6 is not directly contact with the electrolyte, and hardly affected even the density of the electrolyte is changed or the electrolyte flows. Consequently, this gas sensor 1 is reliable because it does't largely fluctuates even in the case elapsed long days after production or exposed under the vibration. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a gas sensor for detecting various gases.

The constant potential electrolytic gas sensor has three electrodes, a working electrode, a counter electrode, and a reference electrode, an electrolytic solution, and a container.
The three electrodes, the working electrode, the counter electrode, and the reference electrode, are bonded to the inside of the gas permeable porous membrane, and all are in contact with the electrolytic solution.

The working electrode is in close contact with the gas permeable porous membrane, and the sample gas passes through the porous membrane to reach the working electrode and is oxidized or reduced on the surface of the working electrode.
At this time, a current corresponding to the gas concentration flows between the counter electrode and the gas concentration measurement is performed. By keeping the potential of the working electrode constant with respect to the reference electrode serving as a reference in advance, a specific voltage is measured. It has the feature that gas can be measured selectively and stably.

The potentiostatic gas sensor is small and light, operates at room temperature, consumes little power, and has high sensitivity and can be distinguished at the ppm level, so various toxic gases (for example, carbon monoxide (CO), hydrogen sulfide (H ₂ S), ammonia (NH ₃ ), etc.) are widely used as portable detectors and sensor devices for allowable concentration management.
JP 2001-208723 A

The electrolyte of a potentiostatic gas sensor is generally a sulfuric acid aqueous solution (when the target gas is CO, H ₂ S, etc.) or a neutral salt aqueous solution such as calcium chloride (CaCl ₂ ) or lithium chloride (LiCl) (target) When the gas is NH ₃ ), and these aqueous solutions are hygroscopic, the liquid amount and concentration change depending on the equilibrium relationship with the humidity in the measurement use atmosphere.
When the amount / concentration of the electrolytic solution changes, the zero output of the sensor changes, and when the vibration is applied to the sensor and the electrolytic solution flows inside, there is a problem that the sensor output varies greatly in a transient manner.

In order to solve the above-described problems, the present invention provides a cylindrical container body, a counter electrode and a reference electrode respectively disposed at one end of the container body, a working electrode disposed at the other end of the container body, and the container The counter electrode, the comparison electrode, and the working electrode are electrically connected by ions in the electrolyte solution, the surface of which is the comparison electrode And a gas sensor having an ion exchange membrane whose back surface is in contact with the electrolytic solution.
The present invention is a gas sensor, wherein the ion exchange membrane is a gas sensor whose surface is in contact with both the counter electrode and the comparison electrode.
The present invention is a gas sensor, wherein an impregnated body is disposed in a container body, the electrolytic solution is impregnated in the impregnated body, and the ion exchange membrane is in contact with the electrolytic solution impregnated in the impregnated body. .
The present invention is a gas sensor, wherein the ion exchange membrane is an anion exchange membrane, and the electrolyte is an aqueous solution of a neutral salt.
This invention is a gas sensor, Comprising: The said neutral salt is a gas sensor containing either one or both of calcium chloride and lithium chloride.
The present invention provides a gas sensor, the absorbing member is a gas sensor composed of SiO ₂ fibers.

  The gas sensor of the present invention is less affected by changes in the concentration of the electrolyte and vibrations, and the measurement results are highly reliable.

FIG. 1 is a cross-sectional view for explaining the internal structure of the gas sensor 1.
The gas sensor 1 has a cylindrical shape as a whole and has a cylindrical container body 3. First and second porous membranes 9a and 9b are respectively arranged at both ends of the container body 3, and the first and second lid portions 10a and 10b respectively pass the packings 4a, 4b, 8a and 8d. Are respectively fixed to the container body 3.

  One side of each of the first and second porous membranes 9a and 9b is directed to the inside of the container main body 3, and the reference electrode 6 and the counter electrode are provided on the inside surface of the container main body 3 of the first porous membrane 9a. 5 are arranged, and a gap is formed between them. A working electrode 7 is disposed on the inner surface of the second porous membrane 9b on the container body 3 side.

  A film-like anion exchange membrane 15 is attached on the first porous membrane 9a so as to cover the surface of the comparison electrode 6 and the surface of the counter electrode 5, and the gap between the counter electrode 5 and the comparison electrode 6 is It is sandwiched between the anion exchange membrane 15 and the first porous membrane 9a.

  An impregnated body 16 that contacts the surface of the working electrode 7, the surface of the portion of the anion exchange membrane 15 on the comparison electrode 6, and the surface of the portion of the counter electrode 5 is disposed inside the container body 3.

  The impregnated body 16 is impregnated with an electrolytic solution. Since the anion exchange membrane 15 allows anion to pass therethrough, the reference electrode 6, the counter electrode 5, and the working electrode 7 are electrically connected by the anion in the impregnated body 16.

  An air hole 19 is formed in the first lid portion 10a. One end of the air hole 19 is open to the atmosphere, and the other end is in contact with the first porous film 9a so that the surface of the first porous film 9a is exposed inside the air hole 19. The air that has passed through the first porous membrane 9 a passes through the counter electrode 5 and the comparison electrode 6, and reaches the interface between the counter electrode 5 and the comparison electrode 6 and the anion exchange membrane 15.

As described above, since the anion can pass through the anion exchange membrane 15, the anion reaches the interface between the counter electrode 5 and the reference electrode 6 and the anion exchange membrane 15.
A ventilation chamber 13 is formed between the second lid portion 10b and the second porous film 9b. A plurality of gas introduction holes 18 are formed in the second lid portion 10b. One end of the gas introduction hole 18 is exposed to the external atmosphere, the other end is connected to the ventilation chamber 13, and a gas filling the external atmosphere is introduced into the ventilation chamber 13 through the gas introduction hole 18, and the second It is comprised so that it may contact with the porous membrane 9b.

  Here, the outer periphery of the anion exchange membrane 15 and the outer periphery of the electrode member composed of the counter electrode 5 and the comparison electrode 6 are made larger than the opening of the container body 3, and the lid portion 10a, the packing 4a, the first electrode The porous membrane 9 a, the comparison electrode 6, the counter electrode 5, and the anion exchange membrane 15 are pressed and fixed to the opening of the container main body 3 with screws 17, and the opening is closed with the anion exchange membrane 15.

The periphery of the anion exchange membrane 15 and the periphery of the electrode member composed of the comparison electrode 6 and the counter electrode 5 are hermetically surrounded by the packing 8a.
The periphery of the gap between the counter electrode 5 and the comparison electrode 6 is surrounded by the counter electrode 5, the comparison electrode 6, and the packing 8a. As described above, the upper and lower sides of the gap are formed between the anion exchange membrane 15 and the first electrode. It is sandwiched between the porous membrane 9a.

  As will be described later, since the anion exchange membrane 15 does not allow the electrolytic solution to pass therethrough, the electrolytic solution does not enter the gap. As described above, the periphery of the electrode member composed of the comparison electrode 6 and the counter electrode 5 is covered with the packing 8a. Since it is surrounded, the electrolytic solution does not contact the side surface exposed in the gap between the counter electrode 5 and the comparison electrode 6 and the side surface around the electrode member. Accordingly, the counter electrode 5 and the reference electrode 6 are not in direct contact with the electrolytic solution.

  One end of an air hole 19 is disposed directly behind the gap between the counter electrode 5 and the reference electrode 6 with the first porous film 9a interposed therebetween, and the air that has passed through the air hole 19 directly enters the gap. It does not enter and is introduced into the gap after passing through the first porous membrane 9a.

  Here, the outer periphery of the part of the impregnated body 16 that contacts the surface of the working electrode 7 and the part of the impregnated body 16 that contacts the anion exchange membrane 15 are substantially equal to the opening of the container body 3, and the opening is closed by the impregnated body 16 In addition, a portion located in the inner space of the container body 3 on the surface of the working electrode 7 and a portion located in the inner space of the container body 3 on the surface of the anion exchange membrane 15 are covered with the impregnated body 16.

  Therefore, neither the surface of the electrodes 5 to 7 nor the surface of the anion exchange membrane 15 on the electrodes 5 and 6 is exposed in the internal space of the container body 3, and the surface of the working electrode 7 and the electrodes 5 and 6 are exposed. Of the surface of the anion exchange membrane 15, all the parts that can come into contact with the electrolytic solution in the space between the working electrode 7 and the anion exchange membrane 15 are in contact with the electrolytic solution impregnated in the impregnated body 16. Therefore, as will be described later, the amount of the electrolytic solution increases so that the impregnated body 16 cannot be completely impregnated, and the space surrounded by the impregnated body 16 or the space surrounded by the impregnated body 16 and the inner wall of the container body 3. (FIG. 2), the anion exchange membrane 15 on the electrode 7 and the electrodes 5 and 6 does not change the contact area with the electrolytic solution.

Next, the gas sensor 1 for measuring ammonia gas (NH ₃ ) will be described. The impregnated body 16 has an electrolytic solution made of a neutral salt aqueous solution such as calcium chloride (CaCl ₂ ) or lithium chloride (LiCl) in advance. Impregnated.

  The gas sensor 1 is disposed in an atmosphere filled with the sample gas, and the sample gas is introduced into the vent chamber 13 from each gas introduction hole 18 by diffusion, or the sample gas is introduced into the vent chamber 13 from at least one gas introduction hole 18. The gas is introduced, passes through the ventilation chamber 13, and is discharged from another gas introduction hole 18.

  The first and second porous membranes 9 a and 9 b have air permeability, and the sample gas introduced into the vent chamber 13 passes through the second porous membrane 9 b and reaches the working electrode 7. The working electrode 7 is configured to be permeable to gas, and the sample gas reaches the interface between the working electrode 7 and the electrolytic solution.

  A lead is connected to each of the working electrode 7, the counter electrode 5, and the comparison electrode 6, and the working electrode 7 is connected to the ground potential by the lead. Here, the voltage between the comparison electrode 6 and the working electrode 7 is constant. Thus, the potential of the counter electrode 5 is controlled.

At the interface between the working electrode 7 and the electrolytic solution, the oxidation reaction of ammonia shown in the following formula (1) proceeds, and OH ⁻ in the electrolytic solution is consumed.

NH ₃ + 3OH ⁻ → 1 / 2N ₂ + 3H ₂ O + 3e ⁻ (1)
As described above, since the anion in the electrolytic solution and the oxygen in the air reach the interface between the counter electrode 5 and the anion exchange membrane 15, the oxygen and the anion are at the interface between the counter electrode 5 and the anion exchange membrane 15. The reaction causes a reduction reaction represented by the following formula (2) to proceed, and OH ⁻ is supplied into the electrolytic solution.

3 / 4O ₂ + 3 / 2H ₂ O + 3e ⁻ → 3OH ⁻ (2)
Since the current flowing through the counter electrode 5 depends on the ammonia gas concentration in the sample gas, the ammonia gas concentration in the sample gas can be determined by measuring the current flowing through the counter electrode 5.

  The electrolyte solution impregnated in the impregnated body 16 comes into contact with the sample gas through the gas introduction hole 18 when the gas sensor 1 is placed in an atmosphere filled with the sample gas, and when the gas sensor 1 is left in the atmosphere, It contacts with air through the gas introduction hole 18.

  Since the aqueous solution of neutral salt such as calcium chloride and lithium chloride described above has high hygroscopicity, when the moisture content of the sample gas is high or when the gas sensor 1 is placed in a high humidity environment, the electrolyte solution is Moisture in the sample gas or air is absorbed, and as shown in FIG. 2, the amount of the electrolytic solution 22 increases and the concentration of the solute in the electrolytic solution decreases.

  On the contrary, when the moisture content of the sample gas is low, or when the gas sensor 1 is left in a low humidity environment, water evaporates from the electrolytic solution and the concentration of the electrolytic solution increases. Thus, the concentration and amount of the electrolyte in the gas sensor 1 are likely to vary depending on the usage environment.

In the conventional gas sensor, since the comparison electrode 6 is in direct contact with the electrolytic solution, when the concentration of the electrolytic solution fluctuates or the electrolytic solution flows in the impregnated body 16 due to vibration, the working electrode 7 and the counter electrode 5 are interposed. The flowing current value fluctuated greatly, and the output signal indicating the gas concentration fluctuated greatly.
On the other hand, the gas sensor 1 of the present application is provided with the anion exchange membrane 15 so that the comparison electrode 6 is not in direct contact with the electrolytic solution.

  The anion exchange membrane 15 has a property (buffer action) in which the internal pH does not easily change even if the external pH changes, and the concentration of the electrolyte solution impregnated in the impregnated body 16 changes to change the pH. However, the pH does not fluctuate greatly at the interface between the reference electrode 6 and the anion exchange membrane 15 because the amount of fluctuation of the pH inside the anion exchange membrane 15 is small. Therefore, the influence of the pH variation of the electrolytic solution on the reaction of the above formula (2) is small, and even if the concentration of the electrolytic solution changes, the output signal indicating the gas concentration is hardly affected.

  In addition, since the comparison electrode 6 does not directly contact the electrolyte solution in the impregnated body 16, even if the electrolyte solution flows in the impregnated body 16 by changing the amount of the electrolyte solution or applying vibration to the gas sensor 1. Not affected by it.

The impregnated body 16 may be a fiber, a porous body, or the like having a large number of pores inside. Generally, a glass fiber filter paper is generally used as the impregnated body 16, but here the impregnated body 16 is made of SiO _2. It is made of fiber filter paper (filter paper made of pure SiO ₂ ).

Glass fiber filter paper contains alkali metals such as Na and K. When glass fiber filter paper is used, alkali metal ions are eluted into the electrolyte solution, and the pH of the electrolyte solution changes to the alkali side. ₂ fiber filter paper did not contain an alkali metal such as Na or K, pH of the electrolyte solution does not change in the case of using the SiO ₂ fiber filter paper.

Therefore, when SiO ₂ fiber filter paper is used for the impregnated body, the pH of the electrolyte does not change, and the output signal of the gas sensor 1 is not affected. The SiO ₂ fiber filter paper is particularly effective when an aqueous neutral salt solution such as calcium chloride or lithium chloride, which is easily affected by alkali metals, is used as the electrolyte.

The gas sensor 1 shown in FIG. 1 was used as the gas sensor of the example, and “zero output displacement” and “output fluctuation peak value” were measured under the following measurement conditions. Here, the counter electrode 5, the reference electrode 6, and the working electrode 7 were Pt electrodes, and the electrolyte was a lithium chloride (LiCl) aqueous solution. Further, the impregnation member 16 was SiO ₂繊紙filter paper (filter paper made of pure SiO ₂繊紙free alkalinity).

[Zero output displacement test]
For the four gas sensors 1, the zero output (output for the sample not containing ammonia) and the output for 50 ppm of ammonia were measured in advance before the test, and the relationship between the output signal and the ammonia concentration (ppm) was determined in advance. After each gas sensor 1 is left in an atmosphere of room temperature and relative humidity (RH) 92% for 39 days, the zero output is measured again, the difference from the zero output before the test is converted into the ammonia concentration, and the value is output as zero. Displacement.

[Output fluctuation peak value test]
The gas sensor 1 was vibrated for 10 seconds with an amplitude of 10 cm, 2 reciprocations / second, and the change in the output value after vibration was examined with respect to the output before the vibration was applied. Its value decreased. The maximum value of the sensor output change width at this time was converted into the ammonia concentration to obtain an output fluctuation peak value.
The output fluctuation peak and the zero output displacement are shown in Table 1 below.

  The gas sensor of Comparative Example 1 was the same as the gas sensor of the above example except that the anion exchange membrane 15 was not provided and the counter electrode 5 and the comparative electrode 6 were directly in contact with the electrolyte.

  The zero output displacement was determined for the four gas sensors of Comparative Example 1 under the same conditions as in the above example except that the number of days left was changed from 39 days to 18 days. Further, for the four gas sensors of Comparative Example 1, output fluctuation peak values were obtained under the same conditions as in the above example. The results are listed in Table 2 below.

  As is clear from the comparison of Tables 1 and 2 above, the gas sensor 1 of the example provided with the anion exchange membrane has a lower value of zero output displacement and a value of the output fluctuation peak value than the gas sensor of Comparative Example 1. It was low. From this, it can be seen that by providing the reference electrode 6 with the anion exchange membrane, it is possible to suppress the output displacement due to the concentration change of the electrolytic solution and the output fluctuation when the gas sensor 1 is vibrated.

Next, using the gas sensor 1 of the above example, the temporal output change test shown below was performed.
[Time-dependent output change test]
When the gas sensor 1 of the example was allowed to stand at room temperature, the change with the elapsed time of the sensor output (μA) with respect to the sample gas having an ammonia gas concentration of 50 ppm was measured.

A gas sensor having the same structure as that of the above example was used as the gas sensor of Comparative Example 2 except that a normal glass fiber filter paper was used as the impregnated body 16 instead of the SiO ₂ fiber filter paper. The glass fiber filter used in Comparative Example 2 is a borosilicate glass containing boron oxide, and contains Na and K in addition to boron oxide.
Using the gas sensor of Comparative Example 2, the “time-dependent output change test” was performed. The results of the “temporal output change test” of the example and the comparative example 2 are shown in the graph of FIG.

  As apparent from the graph of FIG. 3, the change in the sensor output value is small even after the number of days has elapsed after the manufacture of the gas sensor 1 of the example, but the gas output of the comparative example 2 has a sensor output as the number of days elapses. The value of decreased significantly.

In addition, when the gas sensor of the example and the comparative example 2 was examined for the presence of an overshoot phenomenon (a phenomenon in which the output once increased and showed a maximum value and then the output gradually decreased), the gas sensor of the comparative example 2 In the gas sensor of Example, overshoot phenomenon was not confirmed even after 35 days after manufacture.
From the above, it can be seen that if SiO ₂ fiber filter paper is used for the impregnated body 16, the gas concentration can be measured stably over a long period of time.

  The case where the impregnated body 16 is disposed inside the container main body 3 has been described above. However, the present invention is not limited to this, and as shown in FIG. An electrolytic solution may be disposed inside.

However, in this gas sensor 50, when vibration is applied or the amount of the electrolytic solution varies, the area where the counter electrode 5, the comparative electrode 6 and the working electrode 7 are in contact with the electrolytic solution varies, and the measurement becomes unstable. When it is necessary to carry and use the gas sensor 50, it is preferable to provide an impregnated body.
The anion exchange membrane 15 used in the gas sensor 1 of the present application is a membrane that does not allow electrolyte solution to pass but allows anion (here, OH ⁻ ) to pass therethrough.

The type of ion exchange membrane should be changed according to the type of electrolytic solution. As described above, when the electrolytic solution is a neutral salt solution, the anion exchange membrane 15 is used, but the electrolytic solution is an aqueous sulfuric acid solution ( A cation exchange membrane is used when the measurement target is an acidic aqueous solution such as CO or H ₂ S) or a phosphoric acid aqueous solution.

When an acidic aqueous solution having higher hygroscopicity than sulfuric acid aqueous solution is used as the electrolytic solution, the concentration of the electrolytic solution is likely to change and the above-mentioned disadvantages are likely to occur. Therefore, it is particularly effective to use a cation exchange membrane.
As the electrolytic solution 22, the above-described acidic aqueous solution or neutral salt aqueous solution can be used, but the basic electrolytic solution is not desirable because it reacts with carbon dioxide in the air.

  The above has described the case where the anion exchange membrane 15 having the front surface in contact with the reference electrode 6 and the counter electrode 5 and the back surface in contact with the electrolytic solution is provided and the electrolytic solution is not in direct contact with the comparative electrode 6 and the counter electrode 5. The invention is not limited to this.

  An anion exchange membrane 15 whose front surface is in contact with the reference electrode 6 and whose rear surface is in contact with the electrolytic solution may be provided so that only the comparative electrode 6 is not in direct contact with the electrolytic solution. An anion exchange membrane 15 that is in contact with the electrode 7 and whose back surface is in contact with the electrolytic solution may be provided so that both the comparison electrode 6 and the working electrode 7 are not in direct contact with the electrolytic solution.

  Furthermore, like the gas sensor shown by the code | symbol 50 of FIG. 4, the surface contacts the comparison electrode 6 and the counter electrode 5, the back surface contacts the electrolyte solution 22, and the surface contacts the working electrode 7, An anion exchange membrane 15b whose back surface is in contact with the electrolytic solution 22 may be provided so that the comparison electrode 6, the counter electrode 5, and the working electrode 7 do not directly contact the electrolytic solution.

  The first and second porous membranes 9a and 9b are not particularly limited as long as they have gas permeability and are low in reactivity with the sample gas. For example, tetrafluoroethylene resin ( A porous resin film made of PTFE) can be used.

The electrode materials of the counter electrode (C.E.) 5, the reference electrode (R.E.) 6, and the working electrode (W.E.) 7 are not particularly limited, but are corrosion-resistant metals, specifically platinum (platinum). Black) or a noble metal such as gold and bonded to the gas permeable porous membranes 9a and 9b.

  The electrode material is powder alone or mixed with a binder such as tetrafluoroethylene resin powder and dispersed in an appropriate solvent, and the porous membranes 9a and 9b are used as filter paper, and the porous membrane 9a directly by vacuum filtration. The electrode can be formed by adhering to 9b (or transferring the film obtained by filtration under reduced pressure onto the porous membranes 9a and 9b) and pressing.

  Further, these electrodes may be formed in close contact with the porous film by a method such as vacuum deposition, sputtering, ion plating, or electroless plating. The shape of the container body 3 is not limited to a circular cylinder, and may be a square cylinder.

Sectional drawing explaining an example of the gas sensor of this application Sectional view showing the state of increased amount of electrolyte Graph showing the relationship between sensor output and elapsed days Sectional drawing explaining the other example of the gas sensor of this application

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1,50 ... Gas sensor 3 ... Container body 5 ... Counter electrode 6 ... Comparative electrode 7 ... Working electrode 15, 15a, 15b ... Ion exchange membrane (anion exchange membrane) 16 ... Impregnation body 22 ... Electrolysis liquid

Claims (6)

A cylindrical container body;
A counter electrode and a reference electrode respectively disposed at one end of the container body;
A working electrode disposed at the other end of the container body;
An electrolyte contained in the container body,
The counter electrode, the comparison electrode, and the working electrode are gas sensors electrically connected by ions in the electrolyte solution,
A gas sensor having an ion exchange membrane having a surface in contact with the reference electrode and a back surface in contact with the electrolyte.   The gas sensor according to claim 1, wherein a surface of the ion exchange membrane is in contact with both the counter electrode and the reference electrode. An impregnated body is disposed in the container body,
The electrolytic solution is impregnated in the impregnated body,
The gas sensor according to claim 1, wherein the ion exchange membrane is in contact with the electrolytic solution impregnated in the impregnated body. The ion exchange membrane is an anion exchange membrane;
The gas sensor according to any one of claims 1 to 3, wherein the electrolytic solution is an aqueous solution of a neutral salt.   The gas sensor according to claim 4, wherein the neutral salt contains one or both of calcium chloride and lithium chloride. The gas sensor according to claim 4 or 5, wherein the impregnated body is composed of SiO ₂ fiber.

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