JP4463253B2 - Liquid electrochemical gas sensor - Google Patents

Description

  The present invention relates to a liquid electrochemical gas sensor.

  A gas sensor is known in which a detection electrode and a counter electrode are attached to a polymer solid electrolyte membrane, a gas to be detected such as CO is supplied to the detection electrode side, and air and water vapor from a water reservoir are supplied to the counter electrode side (Patent Document 1). ). The polymer solid electrolyte is a proton conductor and has CO sensitivity over a wide range from -40 ° C to 70 ° C. Sulfuric acid is well known as a liquid electrolyte for gas sensors, but it has a problem that it absorbs moisture in a high-temperature, high-humidity atmosphere and overflows, corrodes the surroundings, and changes the gas sensitivity.

The inventors have examined the detection of gas such as CO using a liquid electrolyte membrane instead of a solid electrolyte membrane. The inventors succeeded in detecting CO using an aqueous KOH solution as an electrolyte (Patent Document 2). However, the gas sensor using KOH electrolyte does not show CO sensitivity at around -40 ℃. Next, the inventors succeeded in detecting a gas such as CO using an aqueous solution of a condensation polymer of an aromatic sulfonic acid compound as an electrolyte (Patent Document 3). For example, an aqueous solution of PSR (o-cresol 4 sulfonic acid polymer: Fig. 4) is used as the liquid electrolyte, and the gas sensor using PSR has CO sensitivity even at around -40 ° C, and is durable against high-temperature, high-humidity and high-temperature environments. high. The inventor screened various aromatic sulfonic acid compounds, and an aqueous solution of a polycondensation product of naphthalene sulfonic acid compound is optimal as a liquid electrolyte for a gas sensor, and CO in a wide temperature range from about -40 ° C to about 70 ° C. It has been found that a gas sensor that is capable of detecting gas and has excellent durability to high-temperature and high-humidity atmospheres and high-temperature atmospheres can be obtained.
USP5650054 PCT / JP2004 / 012258 specification PCT / JP2005 / 003853 specification

  An object of the present invention is to provide a novel liquid electrochemical gas sensor that does not use sulfuric acid so that the electrolyte does not overflow in a high-humidity atmosphere and can operate even at a low temperature of about −40 ° C.

The liquid electrochemical gas sensor according to the present invention is a gas sensor in which at least a pair of electrodes is connected to a liquid electrolyte, wherein the liquid electrolyte is an at least one member of the group consisting of naphthalenesulfonic acid, a salt thereof, and a derivative thereof. An aqueous solution as a component , wherein the liquid electrolyte is held in a separator, and a detection electrode and a counter electrode are connected to the separator .
Preferably, the liquid electrolyte is an alkali metal salt or ammonium salt polycondensation compound of naphthalenesulfonic acid or a derivative thereof.
Particularly preferably, the liquid electrolyte is a polycondensation compound of an alkali metal salt of naphthalenesulfonic acid.
Preferably, the average molecular weight of the condensation polymerization compound is 1000 to 25000.
Preferably, in the condensation polymerization compound, the naphthalene nucleus is condensed via a methylene group (—CH ₂ —).
Preferably, a water reservoir, a means for supplying the detected atmosphere to the detection electrode side, and a means for supplying water vapor from the water reservoir to the counter electrode side are provided.

In the present invention, the following effects can be obtained.
(1) Since sulfuric acid is not used in the liquid electrolyte, sulfuric acid does not overflow in a high humidity atmosphere.
(2) Since gas sensitivity can be obtained even at a low temperature of about -40 ° C, it can be used for outdoor applications such as RV (recreational vehicle).
(3) The gas sensitivity can be maintained even after experiencing high temperature atmosphere and high temperature and high humidity atmosphere.
Here, when the electrolyte component is a polycondensation compound of an alkali metal salt or an ammonium salt of naphthalene sulfonic acid or a derivative thereof, particularly preferably a polycondensation compound of an alkali metal salt of naphthalene sulfonic acid, the electrolyte is not strongly acidic. A metal housing can be used.

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

  1 to 8 show an embodiment and its modification. In FIG. 1, 2 is a liquid electrochemical gas sensor, 4 is a sensor body, and a detection electrode 8 and a counter electrode 10 are provided on the front and back of the separator 6. The separator 6 is porous and holds a liquid electrolyte, and has a thickness of, for example, about 0.1 mm and a diameter of about 5 to 20 mm, for example, 10 mm. The separator 6 is made of, for example, a woven or non-woven fabric of synthetic fiber, and is made hydrophilic by sulfonation or introduction of an alcoholic hydroxyl group. Hereinafter, examples will be described taking a porous plastic separator into which an alcoholic hydroxyl group is introduced as an example, but the type of the separator is arbitrary.

The detection electrode 8 is made of, for example, a mixture of Pt-supported carbon black and PtFE (polytetrafluoroethylene) binder, and Pt—RuO ₂ , Pd, or other appropriate electrode catalyst can be used instead of Pt. The counter electrode 10 is an electrode having the same composition as the detection electrode 8. 12 is a hydrophobic conductive film, 14 is a metal washer such as SUS, 16 is a water vapor introducing hole having a diameter of about 1 to 3 mm, 18 is a diffusion control plate made of a thin metal plate of SUS or the like having a thickness of about 100 μm, and has a diameter of 0.1 mm. A degree of diffusion control hole 20 is provided. By providing the diffusion control hole 20 in the thin diffusion control plate 18, the hole diameter of the diffusion control hole 20 can be made constant and the variation in gas sensitivity can be reduced. 22 is a metal sealing body, 23 is a bottom plate, 24 and 26 are openings for introducing gas, and 25 is a filter using activated carbon, silica gel, zeolite or the like.

  28 is a metal can using SUS or the like, and stores water 30 such as pure water or gelled water in its lower part. Reference numeral 32 denotes a constricted portion, and the washer 14 is supported on the upper portion. Reference numeral 34 denotes an adhesive ring made of an adhesive urethane elastomer or the like, which seals the periphery of the sensor body 4 and prevents water from entering from the side surface of the sensor body 4. Reference numeral 36 denotes an insulating sealing material, which may be a sealing tape or the like, and seals the metal can 28 and the sealing body 22 while insulating them to prevent gas from entering. An appropriate gasket may be used in place of the ring 34 and the sealing material 36. Since the upper part of the metal can 28 is caulked by the sealing body 22, the detection electrode 8 and the sealing body 22 are electrically connected, the counter electrode 10 and the metal can 28 are electrically connected, and water leakage and gas from other than the diffusion control hole 20 are present. Prevent wraparound. When liquid water reaches the hydrophobic conductive film 12 from the water vapor introduction hole 16, it is blocked by the hydrophobic conductive film 12.

  FIG. 2 shows a supply model of water vapor and CO to be detected. A detection target gas such as CO is supplied from the diffusion control plate 18 to the detection electrode 8 side, and air and water vapor from the water reservoir side are supplied from the water vapor introduction hole 16 of the washer 14 to the counter electrode 10.

  A modification of the sensor structure is shown in FIGS. In the modification of FIG. 3, solid electrolyte membranes 40, 40 such as a polymer proton conductor or a polymer hydroxide ion conductor are provided between the electron conductive electrodes 38, 38 such as Pt—C—PtFE and the separator 6. Is provided. In the modification of FIG. 4, a polymer proton conductor or a solid hydroxide ion conductor is mixed with Pt—C—PtFE or the like to form mixed conductive electrodes 42 and 42. In the embodiment, the sensor 2 has two electrodes, a detection electrode and a counter electrode, but a reference electrode may be provided in addition.

The counter electrode 10 may be composed of an oxidizing agent (active material) made of a metal oxide or metal hydroxide. In this case, the counter electrode 10 is made of MnO ₂ , NiO (OH), PbO ₂ , ZnO or the like supported by porous carbon paper with a PtFE binder or the like,
MnO ₂ + 2H ₂ O + 2e ⁻ → Mn (OH) ₂ + 2OH ⁻ (1)
MnO ₂ + 2H ⁺ + 2e ⁻ → Mn (OH) ₂ (2)
NiO (OH) + H ⁺ + e ⁻ → Ni (OH) ₂ (3)
PbO ₂ + 2H ⁺ + 2e ⁻ → PbO + H ₂ O (4)
As a result of the reaction, hydroxide ions are generated at the counter electrode 10 or protons generated at the detection electrode 8 are consumed.

  In order to obtain sensitivity to gases such as CO at low temperatures and durability to high temperature and high temperature and high humidity atmospheres, in the examples, a polycondensation compound of an alkali metal salt of naphthalene sulfonic acid of FIG. 5 is used. This substance is called NDSR (Naphthalene / Disulfonic Acid / Resin) and is solid at room temperature and easily soluble in water. The average molecular weight is about 1000-25000 (degree of polymerization 4-100) and the optimal average molecular weight is 8000-15000 (polymerization). Degrees 32-60). NDSR is a chain compound, but it may have side chains, and the 2nd and 7th positions are linked by a methylene group. NDSR has 1-2 sulfonic acid groups per monomer, in other words, per naphthalene nucleus, the position is arbitrary, and the main component is monosulfonic acid rather than disulfonic acid. Furthermore, it is not necessary that all naphthalene nuclei are sulfonated, and a non-sulfonated naphthalene nucleus may exist between the sulfonated naphthalene nuclei and the naphthalene nuclei. The liquid electrolyte of NDSR is, for example, an aqueous solution having a concentration of 1 to 50 wt%, preferably an aqueous solution of 3 to 15 wt%. The concentration varies depending on the supply of water vapor from a water reservoir, the relative humidity of the surrounding environment, and the like. The NDSR aqueous solution is odd brown.

  NDSR is produced, for example, by subjecting crude naphthalene to sulfonation by the action of hot concentrated sulfuric acid, neutralizing with an aqueous NaOH solution, etc. to convert to an alkali metal salt, and dehydrating condensation polymerization with HCHO. In the examples, Na salt is used, but alkali metal salt such as K salt or ammonium salt may be used. Instead of these salts, a polycondensation compound of naphthalene sulfonic acid itself may be used, but the use of the metal can 28, washer 14, and diffusion control plate 20 is limited due to its strong acidity. NDSR may be a derivative thereof, for example, a carboxyl group, a hydroxyl group, a methyl group or the like may be introduced at an appropriate position of the naphthalene nucleus. The liquid electrolyte may contain a third component other than NDSR and water.

6 to 8 show the characteristics of the gas sensor. In the examples, a 5 w% NDSR aqueous solution is used, and the average molecular weight of NDSR is about 12000, which is a Na salt type. Other than these, 5 w% PSR aqueous solution, 0.1 M KOH, proton conductor polymer solid electrolyte membrane (PEM) for fuel cells, and sulfuric acid are used as comparative examples. In the case of PSR and KOH, the structure of the sensor is the same except that the type of liquid electrolyte is changed. In the case of PEM, the structure of the sensor is the same except that a proton conductor film is used instead of the separator 6. In the case of H ₂ SO ₄ , the housing was plastic. 6 to 8, I ₀ indicates an initial output current value at 20 ° C., the CO concentration is 300 ppm each, and the result is an average value of 10 sensors each.

  Figure 6 shows the temperature dependence from -40 ° C to 70 ° C. The temperature dependence is the smallest in the NDSR of the example, the temperature dependence increases in the order of sulfuric acid, PEM, PSR, and KOH is about -40 ° C. There is no CO sensitivity. With NDSR, the CO sensitivity at -40 ° C is about 1/2 of 20 ° C, and CO can be easily detected up to about -40 ° C. Therefore, CO in the recreational vehicle or garage can be reliably detected even in winter.

  FIG. 7 shows the change in output current to 300 ppm CO when aged for 14 weeks in a 70 ° C. atmosphere. Each sensor was returned to a room temperature atmosphere (20 ° C., RH 50%) before measurement, and returned to 70 ° C. after measurement. Except for sulfuric acid, each sensor showed good high temperature durability.

  FIG. 8 shows the durability for 14 weeks in an atmosphere of 60 ° C. and RH 95%. Also in this test, the sensor was returned to an atmosphere of 20 ° C RH50% before measurement, and was returned to an atmosphere of 60 ° C RH95% after measurement. In the sulfuric acid electrolyte, the sensitivity decreases after the output temporarily increases. This seems to be because the sulfuric acid absorbs moisture in the high-temperature and high-humidity atmosphere and overflows from the separator, and the conductivity of the separator decreases. Among NDSR, PSR, and KOH, NDSR has the highest durability.

Although not shown in FIGS. 6 to 8, the response speed of the gas sensor using NDSR to CO is equivalent to that of the gas sensor using PSR, KOH, H ₂ SO ₄ , or the like. Even with NDSR, the sensor shows an output proportional to the CO concentration. The characteristics of the gas sensor are the same even if the average molecular weight of NDSR is changed in the range of 8000 to 15000, and the characteristics are the same even if the NDSR concentration in the aqueous solution in the separator is changed in the range of 3 to 15 wt%. Furthermore, even if methyl groups, hydroxyl groups, or carboxyl groups are introduced into the naphthalene nucleus of NDSR, the gas detection characteristics are almost the same. Gas sensors using NDSR can be used to detect hydrogen in addition to CO. When a bias voltage is applied with a reference electrode, H ₂ S, NH _{3 and} other various gases can be detected.

The reason why the low temperature sensitivity can be improved and the durability to the high temperature and high humidity atmosphere can be improved by using the NDSR aqueous solution instead of the PSR aqueous solution is unknown. The main difference between PSR and NDSR is whether the sulfonic acid group is bonded to the benzene ring or bonded to the naphthalene nucleus. If the sulfonic acid group is bonded to a naphthalene nucleus larger than the benzene ring, the adhesion to the separator As a result, the durability to high-temperature and high-humidity atmospheres increases, and it is considered that the conductive channels of Na ions and hydroxide ions (protons in the acid form) are maintained even at around -40 ℃.

Sectional view of liquid electrochemical gas sensor of Example Sectional drawing which shows the sensor main body of the liquid electrochemical gas sensor of an Example, and its circumference | surroundings Sectional drawing which shows the sensor main body of the liquid electrochemical gas sensor of a modification, and its periphery Sectional drawing which shows the sensor main body of the liquid electrochemical gas sensor of other modifications, and its circumference | surroundings The figure which shows the chemical formula of the alkali metal salt of NDSR (naphthalene disulfonic acid resin) of the electrolyte material used in the Example Characteristic diagram showing gas sensitivity at -40 ° C to 70 ° C for 300ppm CO. The electrolyte is 5wt% NDSR aqueous solution (NDSR), 5wt% PSR aqueous solution (PSR), 0.1M KOH (KOH), fuel cell Polymer solid electrolyte membrane (PEM) for use, 1M sulfuric acid (H ₂ SO ₄ ), the result is the average of 10 sensors each. This is a characteristic diagram showing the change in sensitivity to 300ppm CO in the 70 ° C endurance test. The sample is the same as in Fig. 6, and the measurement was taken from 70 ° C to room temperature (20 ° C RH50%). This is a characteristic diagram showing the change in sensitivity to 300ppm of CO by high temperature and high humidity durability test at 65 ℃ RH95%. The sample is the same as in Fig.6, and it is measured by returning from 70 ℃ to room temperature (20 ℃ RH50%) during measurement. The figure which shows the chemical formula of PSR (o cresol 4 sulfonic acid polymer) of the electrolyte material used by the comparative example

Explanation of symbols

2 Liquid electrochemical gas sensor 4 Sensor body 6 Separator 8 Detection electrode 10 Counter electrode 12 Hydrophobic conductive film 14 Washer 16 Water vapor introduction hole 18 Diffusion control plate 20 Diffusion control hole 22 Sealing body 23 Bottom plates 24 and 26 Opening 25 Filter 28 Metal can 30 Water 32 Constriction 34 Adhesive ring 36 Sealing material 38 Electronic conductive electrode 40 Solid electrolyte membrane 42 Mixed conductive electrode

Claims (6)

In a gas sensor in which at least a pair of electrodes is connected to a liquid electrolyte,
The liquid electrolyte is an aqueous solution of at least one member of the group consisting of naphthalene sulfonic acid and salts thereof and derivatives thereof containing a polycondensation compound as an electrolyte component, the liquid electrolyte being held in a separator and detected by the separator A liquid electrochemical gas sensor, wherein an electrode and a counter electrode are connected . 2. The liquid electrochemical gas sensor according to claim 1, wherein the liquid electrolyte is a polycondensation compound of an alkali metal salt or an ammonium salt of naphthalenesulfonic acid or a derivative thereof. 3. The liquid electrochemical gas sensor according to claim 2, wherein the liquid electrolyte is a polycondensation compound of an alkali metal salt of naphthalene sulfonic acid. The liquid electrochemical gas sensor according to claim 1, wherein the average molecular weight of the polycondensation compound is 1000 to 25000. _2. The liquid electrochemical gas sensor according to claim 1, wherein in the polycondensation compound, a naphthalene nucleus is condensed via a methylene group (—CH ₂ —). 2. The liquid electrochemical gas sensor according to claim 1 , further comprising a water reservoir, a means for supplying a detection atmosphere to the detection electrode side, and a means for supplying water vapor from the water reservoir to the counter electrode side .

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