Electrochemical oxygen sensor

Abstract

Provided is an electrochemical oxygen sensor capable of maintaining the stability of an output voltage and good vibration resistance for a long period of time. The electrochemical oxygen sensor according to the present invention is characterized in that a positive electrode, a negative electrode, and an electrolyte solution comprising an aqueous solution containing an electrolyte are housed in an exterior body, and the electrolyte solution contains a water-soluble thickener comprising a semisynthetic polymer or a synthetic polymer at a concentration of 0.1-4 mass%. It is preferable that the electrolyte solution contains, as a thickener, a polymer containing a structural unit represented by formula (1). In the formula (1), R1 is H or CH3, R2 is OH, OM or NH2, and M is an alkali metal element.

Description

Technical Field

[0001] This invention relates to an electrochemical oxygen sensor capable of maintaining stable output voltage and good vibration resistance over a long period of time. Background Technology

[0002] Electrochemical cells, which consist of a positive electrode made of an air electrode containing a catalyst, a negative electrode, and an electrolyte made of an aqueous solution, are used in air batteries, oxygen sensors for detecting oxygen concentration in various environments, and so on.

[0003] In such electrochemical batteries, various studies have been conducted not only on the positive and negative electrodes but also on the electrolyte to improve its characteristics. For example, in Patent Document 1, in order to suppress the deterioration of characteristics caused by the leakage of electrolyte solvent from the air pores provided in the outer casing for oxygen intake, an air battery containing acrylamide polymer as a thickener in the electrolyte was proposed.

[0004] In addition, Patent Document 2 proposes the following solution: In order to ensure that the sensor operates stably even when mechanical vibration or impact is applied to the sensor from the outside, a water-absorbing and swellable component that absorbs electrolyte is sealed in a container, and the expansion pressure is used to maintain good electrical contact between the positive electrode and the current collector.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2021-144872

[0008] Patent Document 2: Japanese Patent Application Publication No. 6-109694 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] However, the inventors' research shows that in electrochemical oxygen sensors, even with a structure like that in Patent Document 2, the output voltage can fluctuate significantly when subjected to mechanical vibration or impact from the outside during long-term use, storage, or other processes since manufacturing.

[0011] Therefore, in electrochemical oxygen sensors, it is required to ensure the following characteristics: even after a long period of time since manufacturing, a certain output voltage can be obtained, and stable measurement results can be obtained even when subjected to external mechanical vibration or impact.

[0012] The present invention was made in view of the above circumstances, and its object is to provide an electrochemical oxygen sensor that can maintain the stability of the output voltage for a long time and has good vibration resistance.

[0013] Methods for solving problems

[0014] The electrochemical oxygen sensor of the present invention (hereinafter, sometimes simply referred to as "oxygen sensor") is characterized in that a positive electrode, a negative electrode and an electrolyte consisting of an aqueous solution containing an electrolyte are contained in an outer casing, wherein the electrolyte contains a water-soluble thickener consisting of a semi-synthetic polymer or a synthetic polymer at a concentration of 0.1% by mass or more and 4% by mass or less.

[0015] Invention Effects

[0016] According to the present invention, an electrochemical oxygen sensor capable of maintaining stable output voltage over a long period of time and exhibiting good vibration resistance can be provided. Attached Figure Description

[0017] [ Figure 1 [Illustration 1] is a cross-sectional view schematically illustrating an example of the electrochemical oxygen sensor of the present invention.

[0018] [ Figure 2 [This is a diagram used to illustrate the state of a conventional electrochemical oxygen sensor over a long period of time since its manufacture.]

[0019] [ Figure 3 [This is a graph showing the results of the maintenance evaluation of the output voltage in the accelerated test of the electrochemical oxygen sensors of Examples 1 to 3 and Comparative Examples 1 to 2.]

[0020] [ Figure 4 [This is a graph showing the maintenance evaluation results of the output voltage in the accelerated test of the electrochemical oxygen sensors of Examples 1, 4 to 8 and Comparative Examples 1 and 3.] Detailed Implementation

[0021] Figure 1 A cross-sectional view schematically illustrating an example of the electrochemical oxygen sensor of the present invention. Figure 1 The electrochemical oxygen sensor 1 shown is an example of a galvanic cell oxygen sensor, which is one embodiment of the electrochemical oxygen sensor of the present invention.

[0022] Figure 1 The oxygen sensor 1 shown has a positive electrode 50, a negative electrode 80, and an electrolyte 90 inside a bottomed cylindrical container 20. The container 20 consists of a container body 21 that holds the electrolyte 90 inside and a sealing cap 10 for fixing a protective membrane 40, a diaphragm 60, and the positive electrode 50 at the opening of the container body 21. The sealing cap 10 consists of a first sealing cap (middle cap) 11 and a second sealing cap (outer cap) 12 for fixing the first sealing cap 11. It has a through hole 120 inside the oxygen sensor 1 for taking in oxygen and is mounted to the container body 21 via an O-ring 30.

[0023] Inside the container body 21 containing the electrolyte 90, the negative electrode 80 is arranged immersed in the electrolyte 90, and a lead portion 81 is formed on the negative electrode 80. The positive electrode 50 is constructed by stacking a catalyst layer (catalyst electrode) 51 and a positive current collector 52, and a lead 53 is mounted on the positive current collector 52. Furthermore, a through hole 70 is provided at the lower part of the container body 21 containing the electrolyte 90 in the container 20 for the lead 53 mounted on the positive current collector 52 to pass through. Additionally, although in Figure 1 Not shown in the diagram, but in the lower part of the container body 21, in addition to the aforementioned perforation 70, there is also a perforation for supplying electrolyte to the positive electrode 50.

[0024] A correction resistor 100 and a temperature compensation thermistor 110 are connected in series between the lead portion 81 of the negative electrode 80 and the lead 53 installed in the positive electrode current collector 52, and are housed inside the container body 21. In addition, a negative terminal 82 is connected to the lead portion 81 of the negative electrode 80, and a positive terminal 54 is connected to the lead 53 installed in the positive electrode current collector 52, and are respectively led out to the outside of the container body 21.

[0025] A separator 60 is disposed on the outer surface of the positive electrode 50, which selectively allows oxygen to pass through and limits the amount of oxygen passing through to a level corresponding to the battery reaction. Oxygen from the through-hole 120 provided in the sealing cap 10 is introduced into the positive electrode 50 through the separator 60. In addition, a protective film 40 is disposed on the outer surface of the separator 60 to prevent debris, dust, water and the like from adhering to the separator 60, and is fixed by the first sealing cap 11.

[0026] That is, the first sealing cover 11 functions as a protective film 40, a diaphragm 60, and a pressing end plate for the positive electrode 50. Figure 1 In the oxygen sensor 1 shown, a threaded portion is formed on the inner periphery of the second sealing cap 12 so as to engage with the threaded portion formed on the outer periphery of the opening of the container body 21. Moreover, by tightening the sealing cap 10 with the thread, the first sealing cap 11 is pressed against the container body 21 via the O-ring 30, thereby fixing the protective film 40, the diaphragm 60 and the positive electrode 50 to the container body 21 while maintaining airtightness and liquid tightness.

[0027] However, in oxygen sensors, as mentioned above, if a long period of time has passed since manufacturing, the output voltage will fluctuate significantly when subjected to external mechanical vibration or impact. The reason for this is speculated as follows.

[0028] Figure 2 The accompanying drawings illustrate the state of conventional electrochemical oxygen sensors after a long period of manufacturing. Regarding the oxygen sensor 100, its use under conditions of significant vibration is envisioned, and the diagram shows... Figure 1 The oxygen sensor 1 shown is in the opposite state.

[0029] In the oxygen sensor 100, since gases such as water vapor and air can pass through the diaphragm 60 used to introduce oxygen to the positive electrode 50, water, which is the solvent of the electrolyte 90, slowly evaporates and escapes to the outside of the oxygen sensor 100 through the through-hole 120. Correspondingly, air invades the interior of the container body 21 containing the electrolyte 90 of the oxygen sensor 100. The air that invades the interior of the container body 21 initially forms small bubbles in the electrolyte 90, but as the amount of invading air increases, when the oxygen sensor 100 vibrates, the small bubbles aggregate, forming large bubbles 200 as shown by the dotted line in the figure. With the oxygen sensor 100 placed with the positive electrode 50 at the top, as shown... Figure 2 As shown, bubble 200 is located above electrolyte 90. If oxygen sensor 100 is vibrated in this state, a portion of bubble 200 will come into contact with positive electrode 50, thus generating an output voltage different from the original oxygen concentration, causing a significant change in the output voltage of oxygen sensor 100.

[0030] Since the oxygen sensor 100 has only been manufactured a short time ago, the amount of air intruding into the container body is small, and large bubbles will not form. Therefore, even if the oxygen sensor is placed... Figure 2 When vibration is applied in the state shown, the air bubbles generated inside the container body 21 will not come into contact with the positive electrode and cause fluctuations in the output voltage. Therefore, even if an oxygen sensor has a stable output voltage when subjected to vibration and impact during the initial period of its manufacture, the output voltage may fluctuate significantly if subjected to vibration and impact during the extended period of its manufacture.

[0031] Therefore, in the electrochemical oxygen sensor of the present invention, it was studied how to increase the viscosity of the electrolyte by containing a thickener, thereby preventing water in the electrolyte from escaping to the outside and making it difficult for bubbles generated inside the container to move, thus preventing fluctuations in the output voltage caused by contact with the positive electrode.

[0032] However, the inventors' research shows that although thickening the electrolyte can maintain good vibration resistance for a long time, depending on the type and content of the thickener, problems such as reduced output voltage or inability to maintain good output voltage for a long time may occur.

[0033] Furthermore, through repeated research, the inventors discovered that when the electrolyte contains a specific polymer as a thickener within a suitable range, the electrochemical oxygen sensor can maintain excellent vibration resistance for a long time and can also maintain a good output voltage for a long time, thus completing the present invention.

[0034] The electrochemical oxygen sensor of the present invention comprises a positive electrode, a negative electrode, and an electrolyte consisting of an aqueous solution containing an electrolyte, housed within an outer casing. Furthermore, the electrolyte contains a water-soluble thickener composed of a semi-synthetic polymer or a synthetic polymer at a concentration of 0.1% by mass or more and 4% by mass or less.

[0035] Known water-soluble thickeners include natural polysaccharides (xanthan gum, glucomannan (an extract of konjac), etc.), semi-synthetic polymers (carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC), etc.), and synthetic polymers (polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), polyacrylamide (PAM), poly-N-vinylacetamide (PNVA), polyoxyethylene (PEO)), etc. It has been determined that any thickener, by containing a certain amount in the electrolyte and thickening it, suppresses the fluctuation of the output voltage of the oxygen sensor under vibration and impact, and can maintain its good vibration resistance for a long time.

[0036] On the other hand, when using an electrolyte containing synthetic polymers that are artificially synthesized polymers or semi-synthetic polymers that are chemically processed from naturally obtained polymers within a specific range, the same level of output voltage can be obtained as when using an electrolyte without thickeners, and a good output voltage can be maintained for a long time. In contrast, when using an electrolyte containing natural polysaccharides, problems such as a decrease in output voltage or a shorter period of time when a good output voltage can be maintained occur.

[0037] The exact reason is unclear, but in natural polysaccharides, the gels formed by the presence of water are physical gels. In contrast, in semi-synthetic and synthetic polymers, chemical gels are formed, which is speculated to be related to this.

[0038] It should be noted that semi-synthetic and synthetic polymers used as water-soluble thickeners also contain salts such as sodium salts and ammonium salts.

[0039] In addition, in synthetic polymers, from the perspective of being able to maintain a good output voltage for a longer period of time, polymers containing the structural units shown in the following formula (1) are preferred as thickeners.

[0040] [Chemistry 1]

[0041]

[0042] In the above formula (1), R 1 For H or CH3, R 2 The M can be OH, OM, or NH2, where M is an alkali metal element.

[0043] Specifically, polymers containing the structural units shown in formula (1) above are polymers of (meth)acrylamide, polymers of (meth)acrylic acid, or polymers of alkali metal salts of (meth)acrylic acid. It should be noted that "(meth)acrylamide" refers to acrylamide or methacrylamide, and "(meth)acrylic acid" refers to acrylic acid or methacrylic acid.

[0044] Examples of polymers of (meth)acrylamide include homopolymers or copolymers with (meth)acrylamide as the monomer, which are soluble in water, the solvent of the electrolyte. Considering solubility in the electrolyte, homopolymers or copolymers of acrylamide are preferred. In the case of copolymers of acrylamide, the proportion of acrylamide structural units in all structural units (all repeating units from the monomers forming the copolymer of acrylamide) is preferably, for example, 60 mol% or more.

[0045] In acrylamide copolymers, examples of copolymer components that form copolymers together with acrylamide include cationic polymeric monomers and anionic polymeric monomers.

[0046] Examples of cationic polymerizable monomers include (meth)acrylate derivatives having a tertiary amino group, (meth)acrylamide derivatives having a tertiary amino group, and quaternized compounds formed by quaternizing the tertiary amino group of diallylamine derivatives.

[0047] Examples of (meth)acrylate derivatives containing a tertiary amino group include dimethylaminoethyl (meth)acrylate, dialkylamide ethyl (meth)acrylate, and dialkylamide propyl (meth)acrylate.

[0048] Examples of (meth)acrylamide derivatives containing a tertiary amino group include dialkylaminopropyl (meth)acrylamide (dimethylaminopropyl (meth)acrylamide), (meth)acrylamide-3-methylbutyldimethylamine, and other dialkylamides and alkyl (meth)acrylamides.

[0049] Examples of quaternary ammonium compounds that are diallylamine derivatives include diallyl dimethyl ammonium chloride, diallyl diethyl ammonium chloride, diallyl dibutyl ammonium chloride, diallyl methyl ethyl ammonium chloride, diallyl dimethyl ammonium bromide, and diallyl diethyl ammonium bromide.

[0050] Examples of anionic polymerizable monomers include α,β-unsaturated carboxylic acids and sulfonic acids containing vinyl groups.

[0051] Examples of α,β-unsaturated carboxylic acids include α,β-unsaturated monocarboxylic acids (acrylic acid, methacrylic acid, crotonic acid, etc.), α,β-unsaturated dicarboxylic acids (maleic acid, fumaric acid, itaconic acid, citraconic acid, etc.), and their salts (alkali metal salts (sodium salts, potassium salts, etc.), ammonium salts, etc.).

[0052] Examples of vinyl sulfonic acids include vinyl sulfonic acid, styrene sulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, and their salts (alkali metal salts (sodium salts, potassium salts, etc.), ammonium salts, etc.).

[0053] Furthermore, the aforementioned acrylamide copolymer may contain only one type of structural unit based on each of the monomers illustrated above, or it may contain two or more types. In addition to having structural units based on each of the monomers illustrated above, the aforementioned acrylamide copolymer may also have structural units based on other polymerizable monomers.

[0054] In such acrylamide polymers, from the perspective of further improving the effect of the present invention, anionic acrylamide polymers are preferred, and copolymers of acrylamide with alkali metal salts (sodium salts, potassium salts, etc.) of acrylic acid are more preferred. Commercially available products such as "Sanfloc AH-200P (trade name) (acrylamide-sodium acrylate copolymer)" manufactured by Sanyo Chemical Industries, Ltd., and "ACCOFLOCE, DIAFLOC (both trade names)" manufactured by MT AquaPolymer Co., Ltd. can be used.

[0055] Furthermore, as polymers of (meth)acrylic acid, examples include substances in homopolymers or copolymers with (meth)acrylic acid as the monomer that are soluble in water, which is the solvent of the electrolyte. Moreover, as polymers of alkali metal salts of (meth)acrylic acid, examples include substances in homopolymers or copolymers with (meth)acrylic acid as the monomer where the carboxyl groups from some or all of the structural units of (meth)acrylic acid are neutralized to form alkali metal salts (partially neutralized or completely neutralized homopolymers or copolymers with (meth)acrylic acid as the monomer), which are soluble in water, which is the solvent of the electrolyte.

[0056] Specific examples of polymers of (meth)acrylic acid include homopolymers of (meth)acrylic acid (polyacrylic acid, polymethacrylic acid), copolymers of acrylic acid and methacrylic acid, and copolymers of (meth)acrylic acid with monomers such as acrylamide, acrylonitrile, and N-vinylacetamide.

[0057] In addition, specific examples of alkali metal salts of (meth)acrylic acid include partially or completely neutralized polyacrylic acid, partially or completely neutralized polymethacrylic acid, partially or completely neutralized copolymers of acrylic acid and methacrylic acid, and partially or completely neutralized copolymers of (meth)acrylic acid with monomers such as acrylamide, acrylonitrile, and N-vinylacetamide.

[0058] Among these thickeners, polymers of acrylamide are preferred, anionic acrylamide polymers are more preferred, and copolymers of acrylamide and alkali metal salts of acrylic acid are even more preferred.

[0059] Alternatively, amide compounds other than polymers and copolymers of acrylamide, such as poly-N-vinylacetamide, may be preferred.

[0060] From the viewpoint of ensuring the aforementioned effects of using a thickener, the concentration of the thickener in the electrolyte is set to 0.1% by mass or more, preferably 0.5% by mass or more, and more preferably 1% by mass or more. On the other hand, if the amount of thickener in the electrolyte is too large, the viscosity of the electrolyte becomes too high, resulting in problems such as a decrease in output voltage and a shortening of the time period during which a good output voltage can be maintained. Therefore, the concentration of the thickener in the electrolyte is set to 4% by mass or less, preferably 2.5% by mass or less, and more preferably 2% by mass or less.

[0061] There are no particular limitations on the method of making the electrolyte contain a thickener. An electrolyte containing a thickener can be prepared before the oxygen sensor is assembled. Alternatively, the thickener and electrolyte can be added separately to the outer casing containing the electrolyte, and the thickener can be dissolved in the electrolyte inside the outer casing.

[0062] The electrolyte in an oxygen sensor is an aqueous solution containing an electrolyte, but the electrolyte is not particularly limited and can be various acids (organic and inorganic acids), bases, and their salts. Specifically, examples of electrolytes include aqueous solutions containing acetic acid, potassium acetate, and lead acetate; acidic aqueous solutions containing citric acid and citrates (alkali metal salts, etc.); alkaline aqueous solutions containing sodium hydroxide and potassium hydroxide; and weakly alkaline aqueous solutions containing cesium carbonate and cesium bicarbonate.

[0063] In addition, to adjust the pH, the aqueous solution that constitutes the electrolyte can contain various organic acids, inorganic acids, and bases as needed.

[0064] It should be noted that, from the perspective of achieving a long lifespan for oxygen sensors, an aqueous solution containing a chelating agent is preferred as the electrolyte, and the electrolyte itself can also be a chelating agent. It is speculated that the chelating agent has the function of chelating the constituent metals of the negative electrode and dissolving them in the electrolyte (hereinafter referred to as "chelation effect"), which is believed to contribute to a long lifespan for oxygen sensors.

[0065] The term "chelating agent" as used in this specification refers to a molecule (including ions) having multiple ligands (coordinating atoms) that coordinate with metal ions, forming a complex (complexation) with the metal ions to stabilize them, and being contained in the electrolyte in the form of an acid or its salt that generates the aforementioned molecule. Therefore, substances with weak complexing forces, such as phosphoric acid, acetic acid, carbonic acid, and their salts, where the number of coordinating groups (coordinating atoms) is singular, are not included in the term "chelating agent" as used in this specification.

[0066] Chelating agents typically have chelating properties and pH buffering capacity (the ability to maintain a roughly constant pH in a solution even with the addition of small amounts of acid or alkali). Specific examples include succinic acid, fumaric acid, maleic acid, citric acid, tartaric acid, glutaric acid, adipic acid, malic acid, malonic acid, aspartic acid, glutamic acid, ascorbic acid, and their salts. One or more of these can be used.

[0067] From the viewpoint of improving chelation, chelating agents with high water solubility are preferred. Specifically, citric acid, tartaric acid, glutamic acid, and their salts are preferred. Citric acid or its salts have high water solubility (citric acid: 73g / 100ml (25℃), trisodium citrate: 71g / 100ml (25℃), tripotassium citrate: 167g / 100ml (25℃)). Furthermore, citric acid has a large number of dissociable hydrogen atoms, resulting in multiple pH values ​​at which it can exert its pH buffering capacity (pKa1 = 3.13, pKa2 = 4.75, pKa3 = 6.40). Therefore, citric acid is preferred. Thus, when citric acid is used as a chelating agent, its high water solubility and increased pH buffering capacity further improve the lifespan of the oxygen sensor.

[0068] To prevent corrosion of the negative electrode material, the pH of the electrolyte is preferably 3 or higher, more preferably 4 or higher. On the other hand, to prevent carbon dioxide from dissolving in the electrolyte and causing a decrease in properties, the pH of the electrolyte is preferably 9 or lower, more preferably 8 or lower.

[0069] The concentration of the chelating agent in the electrolyte is preferably 2.3 mol / L or more, more preferably 2.5 mol / L or more, and particularly preferably 2.7 mol / L or more.

[0070] It should be noted that if the metal dissolved in the electrolyte from the negative electrode reaches a saturation concentration, oxides of the aforementioned metal will be formed, rendering the negative electrode inert. This could potentially impair the lifespan of the oxygen sensor, and even if the electrolyte contains a chelating agent, the effect on extending the lifespan of the oxygen sensor will be limited. In this case, it is preferable to further contain ammonia in the electrolyte to increase the molar concentration of the chelating agent. This delays the saturation of the metal dissolved in the electrolyte from the negative electrode, thereby further extending the lifespan of the oxygen sensor.

[0071] To facilitate the aforementioned effects of ammonia generation, the concentration of ammonia in the electrolyte is 0.01 mol / L or higher. To further enhance these effects, a concentration of 0.1 mol / L or higher is preferred, and more preferably 1 mol / L or higher. While there is no specific upper limit for the concentration of ammonia in the electrolyte, since it is a compound specified in Appendix 2 of Japan's "Law on the Control of Toxic and Highly Toxic Substances," from a safety perspective, the concentration of ammonia in the electrolyte is preferably less than 10% by mass.

[0072] The positive electrode of the oxygen sensor is, for example, Figure 1 As shown, a positive electrode consisting of a catalyst electrode and a positive current collector is used. The material constituting the catalyst electrode is not particularly limited as long as it is a material capable of generating current through the electrochemical reduction of oxygen at the positive electrode; metals such as gold (Au), silver (Ag), platinum (Pt), and titanium (Ti), or their alloys, are preferred. The catalyst electrode can be composed of a single material, or it can be a stack of different materials, or a tilted functional material whose composition varies in the thickness direction.

[0073] There are no particular restrictions on the negative electrode of the oxygen sensor; for example, it can be formed from lead (Pb), zinc (Zn), antimony (Sb), tin (Sn), or their alloys.

[0074] From the perspective of corrosion resistance, Sn alloys can be exemplified as Sn-Ag alloys, Sn-Cu alloys, Sn-Ag-Cu alloys, Sn-Sb alloys, etc., but they can also be alloys containing metallic elements such as Al, Bi, Fe, Mg, Na, Zn, Ca, Ge, In, Ni, and Co.

[0075] like Figure 1 As shown, a membrane for controlling oxygen intrusion is preferably disposed on the outer surface of the positive electrode of the oxygen sensor to prevent excessive oxygen from reaching the catalyst electrode. As the membrane, a membrane that selectively allows oxygen to permeate and can limit the amount of oxygen permeation is preferred. The material and thickness of the membrane are not particularly limited; fluoropolymers such as polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene copolymer, and polyolefins such as polyethylene are commonly used. The membrane can be a porous membrane, a non-porous membrane, or a membrane with capillary-shaped pores, also known as a capillary membrane.

[0076] Furthermore, in order to protect the aforementioned diaphragm, such as Figure 1 As shown, it is preferable to place a protective membrane made of a porous resin membrane on the diaphragm. As long as the protective membrane has the function of preventing garbage, dust, water, etc. from adhering to the diaphragm and allowing air (including oxygen) to pass through, there are no particular restrictions on its material and thickness. Fluoropolymers such as polytetrafluoroethylene are commonly used.

[0077] The container body 21 of the oxygen sensor 1 can be made of acrylonitrile butadiene styrene (ABS) resin, for example. In addition, the sealing cap 10 (first sealing cap 11 and second sealing cap 12) disposed at the opening of the container body 21 can be made of ABS resin, polypropylene, polycarbonate, fluoropolymer, etc.

[0078] Furthermore, the O-ring 30 between the container body 21 of the container 20 and the sealing cap 10 (first sealing cap 11) is pressed and deformed by the threaded fastening of the container body 21 and the second sealing cap 12, thereby maintaining the airtightness and liquid tightness of the oxygen sensor 1. There are no particular restrictions on the material of the O-ring; nitrile rubber, silicone rubber, ethylene propylene rubber, fluoropolymer, etc. are commonly used.

[0079] Thus far, the present invention has been described using a galvanic cell oxygen sensor as one embodiment of the electrochemical oxygen sensor of the present invention. However, the electrochemical oxygen sensor of the present invention is not limited to the above embodiment, and various modifications can be made within the scope of its technical concept. Furthermore, regarding... Figure 1 The oxygen sensor shown can be modified in various ways as long as it has the function of an oxygen sensor and the oxygen supply path described above.

[0080] Furthermore, the electrochemical oxygen sensor of the present invention can also be used as a potentiostatic oxygen sensor. A potentiostatic oxygen sensor is a sensor in which a fixed voltage is applied between the positive and negative electrodes. The applied voltage is set according to the electrochemical characteristics of each electrode and the type of gas being detected. In a potentiostatic oxygen sensor, if an appropriate fixed voltage is applied between the positive and negative electrodes, the current flowing between them is proportional to the oxygen concentration. Therefore, if the current is converted into a voltage, similar to a galvanic cell oxygen sensor, the oxygen concentration of an unknown gas can be detected by measuring the voltage.

[0081] Example

[0082] The present invention will now be described in detail based on embodiments. However, the following embodiments do not limit the present invention.

[0083] (Example 1)

[0084] <Preparation of Electrolyte>

[0085] Citric acid, tripotassium citrate, and ammonia were dissolved in water to prepare an aqueous solution for the electrolyte. It should be noted that the molar concentrations in the aqueous solution were set as follows: citric acid: 2.5 mol / L, tripotassium citrate: 0.5 mol / L, and ammonia: 3.0 mol / L. The pH of this aqueous solution was 4.30 at 25°C. 100 parts by weight of the above aqueous solution were mixed with 2 parts by weight of acrylamide-sodium acrylate copolymer “Sanfloc AH-200P (trade name)” (manufactured by Sanyo Chemical Industry Co., Ltd.) as a thickener to prepare an electrolyte containing the above thickener at a ratio of 2.0% by weight.

[0086] Assembly of the oxygen sensor

[0087] 5.4g of the electrolyte was injected into the interior of the ABS resin container body 21, and then assembled. Figure 1 The illustrated configuration is a galvanic cell type oxygen sensor. The sealing caps 10 (first sealing cap 11 and second sealing cap 12) are also formed of ABS resin, similar to the container body 21. Furthermore, the protective membrane 40 is made of porous polytetrafluoroethylene sheet, and the diaphragm 60 is made of tetrafluoroethylene-hexafluoropropylene copolymer membrane.

[0088] The catalyst layer (catalyst electrode) 51 of the positive electrode 50 is made of gold, and the positive electrode current collector 52 and the lead wire 53 are made of titanium. The positive electrode current collector 52 and the lead wire 53 are integrated by welding. In addition, the negative electrode 80 is made of Sn-Sb alloy (Sb content: 5% by mass).

[0089] In addition, a correction resistor 100 and a temperature compensation thermistor 110 are connected in series between the positive electrode lead 53 and the negative electrode lead 81. Then, the positive terminal 54 and the negative terminal 81 are led out of the container body 21 from the positive electrode lead 53 and the negative electrode lead 81, respectively, thereby enabling the oxygen concentration to be detected based on the output voltage.

[0090] (Example 2)

[0091] The thickener of the electrolyte was changed to sodium polyacrylate "DK500B (trade name)" (manufactured by Sanyo Chemical Industry Co., Ltd.), and the galvanic cell oxygen sensor was manufactured in the same manner as in Example 1.

[0092] (Example 3)

[0093] The thickener of the electrolyte was changed to carboxymethyl cellulose, and the galvanic cell oxygen sensor was fabricated in the same manner as in Example 1.

[0094] (Comparative Example 1)

[0095] The electrolyte is not thickened and is directly used as an aqueous solution. Otherwise, the galvanic cell oxygen sensor is fabricated in the same manner as in Example 1.

[0096] (Comparative Example 2)

[0097] The thickener of the electrolyte was changed to glucomannan "RHEOLEX (trade name)" (manufactured by Shimizu Chemical Co., Ltd.), and otherwise, a galvanic cell oxygen sensor was fabricated in the same manner as in Example 1.

[0098] For each oxygen sensor manufactured in Examples 1 to 3 and Comparative Examples 1 to 2, an accelerated test was conducted in an atmosphere at 40°C by introducing 100% oxygen to accelerate the electrochemical reaction. At 40°C, the electrochemical reaction proceeded at approximately twice the rate at room temperature. Furthermore, with 100% oxygen present, the electrochemical reaction proceeded at approximately five times the rate in the atmosphere. Therefore, with 100% oxygen present at 40°C, the oxygen sensor could exhibit a change over time at approximately ten times the rate it would when placed in the atmosphere at room temperature.

[0099] In this experiment, the oxygen sensors with elapsed time (converted value) at room temperature of 0 years (at the start of the experiment), 1 year, 3 years and 5 years were evaluated by multiplying the holding time of each oxygen sensor at 40°C by 10.

[0100] [Vibration Resistance Evaluation]

[0101] In an atmospheric environment at room temperature, a vibration test was repeated 10 times, in which the oxygen sensor rotated 270 degrees vertically within 1 second. The change in output voltage during the test was measured, and the vibration resistance was evaluated based on the difference between the maximum and minimum voltage values ​​(the change in output voltage). The results are shown in Table 1.

[0102] [Table 1]

[0103]

[0104] As shown in Table 1, the oxygen sensors of Examples 1 to 3 and Comparative Example 2, which used electrolytes thickened with a thickener, were able to suppress output voltage fluctuations during vibration tests for extended periods. On the other hand, in the oxygen sensor of Comparative Example 1, which did not contain a thickener in its electrolyte, the output voltage fluctuations increased as the accelerated test proceeded.

[0105] [Evaluation of Output Voltage Maintenance]

[0106] For each oxygen sensor of Examples 1 to 3 and Comparative Examples 1 to 2, the output voltage was measured during accelerated testing, and the maintenance of the output voltage was evaluated based on its changes. The changes in the output voltage of each oxygen sensor when the output voltage of the oxygen sensor of Comparative Example 1 at the start of the test was set to 100 are shown in the figure. Figure 3 .

[0107] like Figure 3 As shown, the oxygen sensors of Examples 1 to 3, which used electrolytes thickened with water-soluble thickeners composed of semi-synthetic or synthetic polymers, exhibited less output voltage reduction due to the presence of thickeners compared to the oxygen sensor of Comparative Example 2, which used an electrolyte thickened with natural polysaccharides. This resulted in a better maintenance of the output voltage over a longer period. In particular, the oxygen sensor of Example 1, which used an electrolyte thickened with acrylamide-sodium acrylate copolymer, showed less output voltage variation over time and demonstrated superior output voltage maintenance.

[0108] Examples 4 to 8, Comparative Example 3

[0109] The ratio of thickener to 100 parts by mass of the aqueous solution for electrolyte was set to 0.2 parts by mass (Example 4), 0.5 parts by mass (Example 5), 1 part by mass (Example 6), 1.5 parts by mass (Example 7), 3 parts by mass (Example 8), and 5 parts by mass (Comparative Example 3). Otherwise, the same procedure as in Example 1 was followed to prepare oxygen sensors containing thickener in the electrolyte at ratios of 0.2% by mass (Example 4), 0.5% by mass (Example 5), 1.0% by mass (Example 6), 1.5% by mass (Example 7), 2.9% by mass (Example 8), and 4.8% by mass (Comparative Example 3).

[0110] The oxygen sensors of Examples 4 to 8 and Comparative Example 3 were subjected to the same accelerated tests as described above to evaluate vibration resistance and output voltage maintenance. The results of the vibration resistance evaluation are shown in Table 2 together with the results of Comparative Example 1.

[0111] [Table 2]

[0112]

[0113] As shown in Table 2, even the oxygen sensor of Example 4, which used an electrolyte with a thickener ratio of only 0.2% by mass, exhibited improved vibration resistance compared to the oxygen sensor of Comparative Example 1. Vibration resistance further improved with increasing thickener ratio, and exceptionally excellent vibration resistance was achieved when the thickener ratio was 1.5% by mass or higher.

[0114] In addition, the results of the output voltage maintenance evaluation are presented together with the results of Example 1 and Comparative Example 1. Figure 4 .

[0115] like Figure 4As shown, the oxygen sensors of Examples 1 and 4-7, which used an electrolyte with a thickener ratio in the range of 0.1-4% by mass, exhibited minimal output voltage reduction due to the presence of the thickener, and were able to maintain a good output voltage for an extended period. Furthermore, the oxygen sensor of Example 8, which used an electrolyte with a higher proportion of thickener than the other examples, showed a slightly larger output voltage reduction, but like the other examples, it was able to maintain a good output voltage for a long time. On the other hand, in the oxygen sensor of Comparative Example 3, which used an electrolyte with an excessive proportion of thickener, the output voltage reduction caused by the thickener was greater than that of the oxygen sensors in the examples.

[0116] In particular, the oxygen sensors of Examples 1, 6, and 7, which used an electrolyte with a thickener ratio of 1.0 to 2.0% by mass, exhibited smaller time-varying output voltage variations and better output voltage maintenance.

[0117] This invention can also be implemented in ways other than those described above without departing from its spirit. The embodiments disclosed in this application are merely examples, and the invention is not limited to these embodiments. The scope of this invention is interpreted preferentially to the appended claims compared to the description in the foregoing specification, and all modifications within the scope of the claims are included in the claims.

[0118] Industrial availability

[0119] The electrochemical oxygen sensor of the present invention can also be used for the same purposes as conventionally known electrochemical oxygen sensors, but it is particularly useful for applications where measurements are taken under conditions of vibration due to its good vibration resistance.

[0120] Explanation of reference numerals in the attached figures

[0121] 1. 100: Electrochemical oxygen sensor; 10: Sealing cap; 11: First sealing cap (middle cap); 12: Second sealing cap (outer cap); 20: Container; 21: Container body; 30: O-ring; 40: Protective film; 50: Positive electrode; 51: Positive electrode body (catalyst electrode); 52: Positive electrode current collector; 53: Positive electrode lead; 54: Positive terminal; 60: Diaphragm; 70: Through hole for lead; 80: Negative electrode; 81: Negative electrode lead; 82: Negative terminal; 90: Electrolyte; 100: Calibration resistor; 110: Temperature compensation thermistor; 120: Through hole.

Claims

1. An electrochemical oxygen sensor, characterized in that, It is formed by housing the positive electrode, the negative electrode, and an electrolyte consisting of an aqueous solution containing electrolyte within an outer casing. The electrolyte contains a water-soluble thickener composed of semi-synthetic or synthetic polymers at a concentration of 0.1% by mass or more and 4% by mass or less.

2. The electrochemical oxygen sensor according to claim 1, wherein, The thickener contains a polymer comprising the structural unit shown in formula (1) below. [Chemistry 1] In the above formula (1), R 1 For H or CH3, R 2 The element is OH, OM, or NH2, where M is an alkali metal element.

3. The electrochemical oxygen sensor according to claim 2, wherein, The electrolyte contains a copolymer of acrylamide as the thickener.

4. The electrochemical oxygen sensor according to claim 3, wherein, The polymer of the acrylamide is a copolymer of acrylamide and an alkali metal salt of acrylic acid.

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