阅读说明：本技术 电化学式氧传感器 (Electrochemical oxygen sensor ) 是由 北泽直久 于 2020-01-21 设计创作，主要内容包括：本发明提供一种长寿命的电化学式氧传感器。本发明的电化学式氧传感器的特征在于,具有正极、负极和电解液,所述负极含有锡或锡的合金,所述电解液为至少溶解有柠檬酸类的水溶液,所述水溶液含有碱金属,所述电解液中的柠檬酸类的总含量为2.1mol/L以上,所述电解液中的碱金属的含量为柠檬酸类的总含量的0.1～1.6倍,所述电解液的pH为3.9～4.6,在将所述电解液的液量设为x(ml)、将所述负极中所含的锡的含量设为y(g)时,x/y≥0.3(ml/g)。(The invention provides an electrochemical oxygen sensor having a long life. The electrochemical oxygen sensor is characterized by comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode contains tin or a tin alloy, the electrolyte is an aqueous solution at least containing citric acids dissolved therein, the aqueous solution contains alkali metals, the total content of the citric acids in the electrolyte is more than 2.1mol/L, the content of the alkali metals in the electrolyte is 0.1-1.6 times of the total content of the citric acids, the pH of the electrolyte is 3.9-4.6, and when the liquid volume of the electrolyte is x ml and the content of the tin in the negative electrode is y (g), x/y is more than or equal to 0.3 (ml/g).)

1. An electrochemical oxygen sensor comprising a positive electrode, a negative electrode and an electrolyte,

the negative electrode contains tin or an alloy of tin,

the electrolyte is an aqueous solution in which at least citric acid is dissolved,

the aqueous solution contains an alkali metal and, optionally,

the total content of citric acid in the electrolyte is more than 2.1mol/L,

the content of alkali metal in the electrolyte is 0.1-1.6 times of the total content of citric acid,

the pH value of the electrolyte is 3.9-4.6,

when the amount of the electrolyte solution is represented by x in ml and the content of tin in the negative electrode is represented by y in g, x/y is not less than 0.3 ml/g.

2. The electrochemical oxygen sensor of claim 1, wherein the electrolyte comprises an alkali metal salt of an organic acid.

3. The electrochemical oxygen sensor of claim 2, wherein the alkali metal salt of an organic acid comprises an alkali metal salt of citric acid.

4. The electrochemical oxygen sensor of claim 2, wherein the alkali metal salt of an organic acid comprises an alkali metal salt of acetic acid.

5. The electrochemical oxygen sensor according to any one of claims 1 to 4, wherein the citric acid species comprises citric acid.

6. The electrochemical oxygen sensor according to claim 5, wherein the content of citric acid in the electrolyte is 1.1mol/L or more.

Technical Field

The present invention relates to a long-life electrochemical oxygen sensor.

Background

Electrochemical oxygen sensors (hereinafter also referred to as oxygen sensors) are inexpensive, simple, and capable of operating at room temperature, and therefore are used in a wide range of fields such as examination of an oxygen deficient state inside a cabin or in a manhole, and detection of an oxygen concentration in medical equipment such as an anesthesia machine and an artificial respirator.

As such an electrochemical oxygen sensor, for example, patent document 1 discloses an electrochemical oxygen sensor including a cathode, an anode, and an electrolytic solution containing a chelating agent and having a pH of 12 or more.

Patent document 2 discloses that the lifetime of an electrochemical oxygen sensor is improved by setting the molar concentration of a chelating agent in the electrolyte of the oxygen sensor to 1.4mol/L or more.

Documents of the prior art

Patent document

Patent document 1: international publication No. 2009/069749

Patent document 2: japanese laid-open patent publication No. 2018-109549

Disclosure of Invention

Problems to be solved by the invention

However, the life of an electrochemical oxygen sensor using a negative electrode that does not use harmful substances such as Pb, particularly an electrochemical oxygen sensor using tin or an alloy of tin for the negative electrode, has not been sufficiently improved, and further research is needed.

The present invention has been made in view of the above circumstances, and an object thereof is to provide an electrochemical oxygen sensor having a long life.

Means for solving the problems

The electrochemical oxygen sensor is characterized by comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode contains tin or a tin alloy, the electrolyte is an aqueous solution at least containing citric acids dissolved therein, the aqueous solution contains alkali metals, the total content of the citric acids in the electrolyte is more than 2.1mol/L, the content of the alkali metals in the electrolyte is 0.1-1.6 times of the total content of the citric acids, the pH of the electrolyte is 3.9-4.6, and when the liquid volume of the electrolyte is x ml and the content of the tin in the negative electrode is y (g), x/y is more than or equal to 0.3 (ml/g).

Effects of the invention

According to the present invention, a long-life electrochemical oxygen sensor can be provided.

Drawings

Fig. 1 is a cross-sectional view schematically showing an example of the electrochemical oxygen sensor of the present invention.

Detailed Description

First, an electrochemical oxygen sensor according to the present invention will be described with reference to the accompanying drawings, taking a galvanic cell type oxygen sensor as an example of a preferred embodiment.

Fig. 1 is a cross-sectional view schematically showing a galvanic cell type oxygen sensor as an embodiment of an electrochemical oxygen sensor.

The oxygen sensor 1 shown in fig. 1 has a positive electrode 50, a negative electrode 100, and an electrolyte solution 110 in a bottomed cylindrical holder 20. A holder cover 10 is attached to an upper opening of the holder 20 via an O-ring 30, and the holder cover 10 is composed of a first holder cover (middle cover) 11 and a second holder cover (outer cover) 12 for fixing the first holder cover 11, and has a through hole 150 for taking in oxygen into the oxygen sensor 1.

In a tank containing electrolyte 110 in holder 20, negative electrode 100 is disposed in a state of being immersed in the electrolyte. A lead 120 is attached to the negative electrode 100, and a correction resistor 130 and a temperature compensation thermistor 140 are connected in series to the lead 120 outside the holder 20. The positive electrode 50 is formed by laminating a catalyst electrode 51 and a positive electrode current collector 52, and the lead 120 is also attached to the positive electrode current collector 52. The positive electrode 50 is disposed on the upper portion of the electrolyte solution housing tank with the positive electrode collector holder 70 interposed therebetween. In addition, the positive electrode collector holding part 70 is provided with a through hole 80 for supplying the electrolyte 110 of the electrolyte housing tank to the positive electrode 50 and a through hole 90 for passing a lead 120 attached to the positive electrode collector 52.

A separator 60 that selectively allows oxygen to pass therethrough and limits the amount of oxygen that passes therethrough to match the cell reaction is disposed above the positive electrode 50, and oxygen from the through-hole 150 provided in the holder cover 10 is introduced into the positive electrode 50 through the separator 60. Further, a protective film 40 for preventing adhesion of dust, dirt, water, and the like to the diaphragm 60 is disposed on the upper portion of the diaphragm 60 and fixed by the first holder cover 11.

That is, the first holder cover 11 functions as a pressing end plate for the protective film 40, the separator 60, and the positive electrode 50. In the sensor 1 shown in fig. 1, a screw portion is formed on the inner peripheral portion of the second holder cover 12 to be screwed with a screw portion formed on the outer peripheral portion of the holder 20. By screwing the holder cover 10, the first holder cover 11 is pressed against the holder 20 via the O-ring 30, and the protective film 40, the separator 60, and the positive electrode 50 can be fixed to the holder 20 while maintaining airtightness and liquid tightness.

The operation principle of the galvanic cell type oxygen sensor having an electrolyte containing a chelating agent disclosed in patent document 1 will be described below with reference to fig. 1.

Oxygen that has entered the oxygen sensor 1 through the separator 60 is reduced by the catalyst electrode 51 of the positive electrode 50, and undergoes the following electrochemical reaction with the negative electrode 100 via the electrolyte 110.

And (3) positive pole reaction: o is₂+4H⁺+4e^-→2H₂O

And (3) cathode reaction: sn +2H₂O→SnO₂+4H⁺+4e^-

Y^x-+SnO₂+4H⁺→YSn^4-×+2H₂O

: y is a chelating agent (citric acid)

The negative electrode 100 may be made of, for example, metals such as Cu, Fe, Ag, Ti, Al, Mg, Zn, Ni, and Sn, or alloys thereof, but Sn or Sn alloys are used because they are hard to corrode in the acidic electrolyte used in the present invention and can also be used in consideration of RoHS directive (directive on Restriction of Use of Certain harmful Substances in Electronic and Electrical Equipment) related to Restriction of Use of Certain harmful components in EU (european union). Therefore, the electrochemical reaction formula represents a case where the negative electrode is composed of Sn or an Sn alloy.

By this electrochemical reaction, a current corresponding to the oxygen concentration is generated between the catalyst electrode 51 and the negative electrode 100. The current generated by the positive electrode reaction in the catalyst electrode 51 is collected by the positive electrode collector 52 pressed against the catalyst electrode 51, led to the outside through the lead 120, and flows to the negative electrode 100 through the correction resistor 130 and the temperature compensation thermistor 140. Thereby, the current is converted into a voltage signal, and a voltage is obtained as an output of the oxygen sensor. Then, the obtained output voltage is converted into an oxygen concentration by a known method, and the oxygen concentration is detected.

Here, citric acid (Y) as a chelating agent becomes a citric acid ion in the electrolytic solution, and has an action of chelating a constituent metal of the negative electrode and dissolving the metal in the electrolytic solution (hereinafter referred to as "chelating action"). However, the present inventors considered that the metal (Sn) derived from the negative electrode dissolves in the electrolyte solution to reach a saturation concentration, and an oxide of the metal is generated to render the negative electrode inactive, which is one of the factors that decrease the life of the oxygen sensor.

As a result of intensive studies, the present inventors have found that the increase in the amount of tin that can be dissolved in the electrolytic solution, that is, the increase in the molar concentration of citric acid (Y) in the electrolytic solution can delay the tin dissolved from the negative electrode in the electrolytic solution from reaching the saturation concentration, and as a result, the lifetime of the oxygen sensor can be increased, thereby completing the present invention.

The citric acid used in the electrolyte solution of the electrochemical oxygen sensor of the present invention has a plurality of functional groups that coordinate to metal ions, forms a complex (complexation) with the metal ions to inactivate the metal ions, and can be contained in the electrolyte solution as citric acid itself or a salt thereof (in the present specification, citric acid and a citrate are combined as citric acids) in a solvent constituting the electrolyte solution.

In the oxygen sensor of the present invention, an aqueous solution in which at least citric acid is dissolved is used as the electrolyte. Here, modulation is performed as follows: the aqueous solution contains alkali metal, the total content of citric acid is more than 2.1mol/L, the content of alkali metal is 0.1-1.6 times of the total content of citric acid, and the pH value is 3.9-4.6. The solvent of the electrolyte is water. With such an electrolyte solution, the molar concentration of the citrate ions can be increased, and the life of the oxygen sensor can be increased.

A chelating agent such as citric acid generally has a chelating effect and a pH buffering ability (an ability to keep the pH of a solution substantially constant even when a small amount of acid or alkali is added), but when an acid or a salt thereof which has a chelating effect in an aqueous solution is dissolved in water as a monomer, the pH of the aqueous solution is determined mainly by the kind and concentration of the chelating agent. Therefore, depending on the type of the chelating agent used, the pH of the aqueous solution may be in a range in which the anode material undergoes galvanic corrosion, and it may be difficult to use the aqueous solution as an electrolyte of a sensor.

Therefore, in order to adjust the pH of the electrolytic solution to an appropriate range while maintaining an excellent pH buffering capacity, it has also been proposed to use a mixed solution containing an acid or a salt thereof as a chelating agent. However, according to the studies of the present inventors, it has been found that when citric acid is used as a chelating agent, the increase in the total content of citric acid and its salts (i.e., citric acids) does not necessarily lead to the improvement in the lifetime even if the pH of the electrolytic solution is adjusted to an appropriate range, and it has been found that it is important to cause alkali metal (most of which are supposed to be ionized and exist in the state of alkali metal ions) to exist in a specific content in the electrolytic solution containing citric acid by a method such as dissolving an alkali metal salt, for example, an alkali metal salt of an organic acid, preferably an alkali metal salt of citric acid.

That is, although the reason is not clear, it is found that: the total content of citric acids dissolved in the electrolyte is set to 2.1mol/L or more, the content of alkali metals contained in the electrolyte is set to 0.1 to 1.6 times of the total content of citric acids, and the action of citric acid (including ionized and ionized substances) as a chelating agent can be utilized to the maximum extent when the pH of the electrolyte is adjusted to a range of 3.9 to 4.6, thereby prolonging the life of the oxygen sensor.

In the present invention, the electrolyte solution having the above-described configuration can be prepared by dissolving citric acid and an alkali metal salt, for example, alkali metal salts of citric acid and an organic acid, preferably alkali metal salts of citric acid and citric acid, in water as a solvent. Examples of the alkali metal salt of citric acid include trialkali metal citrate, dialkali metal hydrogen citrate, and dialkali metal citrate, specifically, lithium salt, sodium salt, potassium salt, rubidium salt, and cesium salt, and trisodium citrate, tripotassium citrate, disodium hydrogen citrate, dipotassium hydrogen citrate, and potassium dihydrogen citrate can be preferably used.

For example, a mixed solution in which citric acid and tripotassium citrate are dissolved in water at a ratio of 1.2mol/L and 1.0mol/L, respectively, is an electrolytic solution having a total content of dissolved citric acids of 2.2mol/L, a content of alkali metal (potassium) derived from tripotassium citrate of 1.0 × 3 ═ 3.0mol/L (that is, a content of alkali metal is 3.0/2.2 ═ 1.36 times the total content of citric acids), and a pH at 25 ℃ of 4.23.

The electrolyte solution having the above-described configuration may be prepared by using an alkali metal salt of an organic acid other than citric acid. For example, alkali metal salts (including acidic salts) of monocarboxylic acids or polycarboxylic acids such as acetic acid, formic acid, oxalic acid, succinic acid, fumaric acid, maleic acid, tartaric acid, glutaric acid, adipic acid, malic acid, malonic acid, aspartic acid, glutamic acid, and ascorbic acid can be used, and sodium acetate, potassium acetate, sodium hydrogen oxalate, potassium hydrogen oxalate, disodium oxalate, dipotassium oxalate, sodium hydrogen tartrate, potassium ammonium tartrate, disodium tartrate, and dipotassium tartrate are preferably used. Since the polycarboxylic acid also functions as a chelating agent, the addition of the polycarboxylic acid or a salt thereof can delay the tin from reaching a saturation concentration in the electrolyte solution, thereby improving the life of the oxygen sensor.

For example, a mixed solution in which citric acid and potassium acetate are dissolved in water at a ratio of 2.5mol/L and 1.0mol/L, respectively, is an electrolytic solution in which the total content of dissolved citric acids is 2.5mol/L and the content of alkali metal (potassium) derived from potassium acetate is 1.0mol/L, that is, the content of alkali metal is 1.0/2.5 to 0.4 times the total content of citric acids.

In the present invention, a pH adjuster may be added to the electrolytic solution in order to more appropriately adjust the pH of the mixed solution of citric acid and the alkali metal salt of an organic acid. Examples of the pH adjuster include organic acids and salts thereof, inorganic acids and salts thereof, ammonia, and hydroxides. In the case of the mixed solution of citric acid and potassium acetate, the pH at 25 ℃ can be adjusted to 4.32 by adding ammonia in a content of 3.0 mol/L.

Examples of the organic acid as the pH adjuster include mono-or polycarboxylic acids such as acetic acid, formic acid, oxalic acid, succinic acid, fumaric acid, maleic acid, tartaric acid, glutaric acid, adipic acid, malic acid, malonic acid, aspartic acid, and glutamic acid, and ascorbic acid, and salts of the organic acids include ammonium salts (including acidic salts) of the organic acids described above such as ammonium acetate, diammonium tartrate, and ammonium hydrogen tartrate, and salts of citric acid other than alkali metal salts such as diammonium hydrogen citrate and triammonium citrate can be used. In the case where the salt of citric acid is added, the content thereof is added to the total content of citric acids.

Examples of the inorganic acid as the pH adjuster include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and carbonic acid, and examples of the salt of the inorganic acid include alkali metal salts and ammonium salts (each including an acidic salt) of the inorganic acid such as ammonium chloride, sodium hydrogen sulfate, potassium hydrogen sulfate, ammonium sulfate, trisodium phosphate, tripotassium phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, sodium carbonate, potassium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate, ammonium carbonate, and ammonium hydrogen carbonate. When the alkali metal salt of the inorganic acid is added, the alkali metal contained in the compound is added as "the content of the alkali metal contained in the electrolyte".

Since ammonia is volatile, when considering a change in the composition of the electrolytic solution due to volatilization, the total content of ammonia in the electrolytic solution containing substances derived from ammonia water and ammonium salts is preferably constant or less, the molar ratio of the total content of ammonia in the electrolytic solution to the total content of citric acid compounds is preferably 1.1 or less, more preferably 0.5 or less, and ammonia may not be contained in the electrolytic solution.

Examples of the hydroxide as the pH adjuster include hydroxides of alkali metals such as sodium hydroxide and potassium hydroxide. In the case where the hydroxide of the alkali metal is added, the alkali metal contained in the compound is added as "the content of the alkali metal contained in the electrolyte".

The electrolyte solution used in the present invention can be prepared by appropriately selecting the types and the amount ratios of citric acid and alkali metal salt, and adding a pH adjuster as needed.

The electrolyte used in the present invention has an alkali metal content of 0.1 to 1.6 times the total content of citric acids, and the pH is adjusted to a range of 3.9 to 4.6, under the above conditions, the greater the total content of citric acids, the longer the chelating action of the electrolyte can be maintained. Therefore, for the purpose of extending the life of the oxygen sensor, the total content of citric acid compounds in the electrolyte solution is preferably 2.4mol/L or more, and more preferably 2.7mol/L or more.

Although the reason is not clear, the content of citric acid in the electrolyte is preferably 1.1mol/L or more, more preferably 1.7mol/L or more, and particularly preferably 2.0mol/L or more, because the chelating action of the electrolyte can be maintained for a longer period of time even when the total content and pH of citric acid are the same and the content of citric acid is large.

Similarly, although the reason is not clear, when the content of the alkali metal in the electrolyte is less than 0.1 times or more than 1.6 times the total content of the citric acid compounds, a longer life than a certain level cannot be achieved. From the viewpoint of improving the ionic conductivity of the electrolytic solution, the content of the alkali metal in the electrolytic solution is preferably 0.45 times or more the total content of citric acids. When the content of the alkali metal in the electrolyte is less than 0.1 times the total content of the citric acid compounds, it may be difficult to increase the ionic conductivity of the electrolyte by setting the pH within the above range, and the operation of the oxygen sensor may become unstable.

In order to exhibit the characteristics of the electrolytic solution, the oxygen sensor of the present invention is configured such that the amount of the electrolytic solution is not less than a predetermined amount with respect to the mass of tin as a reaction material of the negative electrode. That is, when the amount of electrolyte in the oxygen sensor is x (ml) and the content of tin in the negative electrode is y (g), the amount of electrolyte is adjusted so that x/y is 0.3(ml/g) or more. When x/y is less than 0.3(ml/g), the pH of the electrolyte solution changes rapidly when the oxygen sensor is used, and the characteristics of the electrolyte solution cannot be exhibited, and the effect of improving the lifetime of the oxygen sensor is insufficient.

In order to suppress a change in pH of the electrolyte when the oxygen sensor is used, the value of x/y is preferably 0.7(ml/g) or more, and more preferably 1(ml/g) or more. On the other hand, in order to reduce the storage volume of the electrolyte solution and to reduce the volume of the oxygen sensor as much as possible, the value of x/y is preferably 10(ml/g) or less, more preferably 6.5(ml/g) or less, and particularly preferably 3(ml/g) or less.

The negative electrode of the oxygen sensor of the present invention uses Sn or an alloy of Sn, but in order to suppress a reaction with the electrolyte and prevent the generation of hydrogen, it is preferable to use an Sn alloy. Examples of the Sn alloy include Sn-Ag alloys, Sn-Cu alloys, Sn-Ag-Cu alloys, and Sn-Sb alloys, but alloys containing metal elements such as Al, Bi, Fe, Mg, Na, Zn, Ca, Ge, In, Ni, and Co may also be used.

In addition, Sn or Sn alloys may also contain a certain amount of impurities, but in order to comply with the RoHS directive, the Pb content is preferably less than 1000 ppm.

As the Sn alloy, specifically, general lead-free solder materials (Sn-3.0Ag-0.5Cu, Sn-3.5Ag-0.75Cu, Sn-3.8Ag-0.7Cu, Sn-3.9Ag-0.6Cu, Sn-4.0Ag-0.5Cu, Sn-1.0Ag-0.7Cu, Sn-0.3Ag-0.7Cu, Sn-0.75Cu, Sn-0.7Cu-Ni-P-Ge, Sn-0.6Cu-Ni-P-GeSn-1.0Ag-0.7Cu-Bi-In, Sn-0.3Ag-0.7Cu-0.5Bi-Ni, Sn-3.0Ag-3.0Bi-3.0In, Sn-3.9Ag-0.6 Ag-3.0.5 Sb-0.5 Bi-0 In, Sn-3.0Ag-0, Sn-5.0Sb, Sn-10Sb, Sn-0.5Ag-6.0Cu, Sn-5.0Cu-0.15Ni, Sn-0.5Ag-4.0Cu, Sn-2.3Ag-Ni-Co, Sn-2Ag-Cu-Ni, Sn-3Ag-3Bi-0.8Cu-Ni, Sn-3.0Ag-0.5Cu-Ni, Sn-0.3Ag-2.0Cu-Ni, Sn-0.3Ag-0.7Cu-Ni, Sn-58Bi, Sn-57Bi-1.0Ag, etc.), Sn-Sb alloys.

As shown in fig. 1, for example, an oxygen sensor including a catalyst electrode and a positive electrode current collector is used as the positive electrode of the oxygen sensor of the present invention. The material constituting the catalyst electrode is not particularly limited as long as it is a material capable of generating an electric current by electrochemical oxygen reduction at the positive electrode, and a catalyst active in oxidation-reduction, such as gold (Au), silver (Ag), platinum (Pt), or titanium (Ti), is preferably used.

Further, it is preferable that a separator for controlling the invasion of oxygen is disposed on the outer surface of the positive electrode of the oxygen sensor so that oxygen reaching the catalyst electrode does not become excessive. As the separator, a separator that selectively transmits oxygen and can limit the amount of transmitted oxygen is preferable. The material and thickness of the separator are not particularly limited, and a fluororesin such as a tetrafluoroethylene resin or a tetrafluoroethylene hexafluoropropylene copolymer, a polyolefin such as polyethylene, or the like is generally used. As the separator, a porous film, a non-porous film, and a film having pores in which capillaries are formed, which is called a capillary type, can be used.

Further, in order to protect the separator, it is preferable to dispose a protective film made of a porous resin film on the separator. The material and thickness of the protective film are not particularly limited as long as the protective film has a function of preventing adhesion of dust, dirt, water, and the like to the separator and allowing air (including oxygen) to pass therethrough, and a fluororesin such as a tetrafluoroethylene resin is generally used.

The holder 20 as the outer package of the oxygen sensor 1 shown in fig. 1 may be made of, for example, ABS resin. The holder covers 10 (the first holder cover 11 and the second holder cover 12) disposed in the opening of the holder 20 may be made of, for example, ABS resin, polypropylene, polycarbonate, fluororesin, or the like. Further, in the holder 20, the positive electrode collector holding portion 70 for holding the positive electrode 50 may be made of, for example, ABS resin.

Further, the O-ring 30 sandwiched between the holder 20 and the holder cover 10 (first holder cover 11) is pressed and deformed by the screw fastening between the holder 20 and the second holder cover 12, and the airtightness and liquid-tightness of the oxygen sensor 1 can be maintained. The material of the O-ring is not particularly limited, and nitrile rubber, silicone rubber, ethylene propylene rubber, fluorine resin, and the like are generally used.

The present invention has been described above by taking a galvanic cell type oxygen sensor as an embodiment of the oxygen sensor of the present invention as an example, but the oxygen sensor of the present invention is not limited to the above embodiment, and various modifications can be made within the scope of the technical idea thereof. In addition, the oxygen sensor shown in fig. 1 can be variously modified in design as long as it has a function as an oxygen sensor and the oxygen supply path described above.

The oxygen sensor of the present invention may be a constant potential oxygen sensor. The constant potential oxygen sensor is a sensor that applies a constant voltage between a positive electrode and a negative electrode, and the applied voltage is set according to the electrochemical characteristics of each electrode and the type of gas to be detected. In the constant potential type oxygen sensor, when an appropriate constant voltage is applied between the positive electrode and the negative electrode, the current flowing therebetween has a proportional relationship with the oxygen concentration, and therefore, when the current is converted into a voltage, the oxygen concentration of an unknown gas can be detected by measuring the voltage, as in the case of the galvanic cell type oxygen sensor.

Examples

The present invention will be described in detail below based on examples. However, the following examples do not limit the present invention.

Example 1

< preparation of electrolyte solution >

Citric acid and tripotassium citrate were dissolved in water to prepare an electrolyte. The molar concentration in the electrolyte was set as citric acid: 1.2mol/L, tripotassium citrate: 1.0 mol/L. The total content of citric acids dissolved in the electrolyte was 2.2mol/L, the content of alkali metal (potassium): 3.0mol/L is 1.36 times of the total content of citric acid, and the pH of the electrolyte is 4.23 at 25 ℃.

< Assembly of oxygen sensor >

The galvanic cell type oxygen sensor having the structure shown in fig. 1 was assembled using 4.3ml of the above electrolyte solution. The holder cover 10 (the first holder cover 11 and the second holder cover 12), the holder 20, and the positive electrode collector holding portion 70 are formed of ABS resin. The protective film 40 is made of porous tetrafluoroethylene resin, and the separator 60 is made of tetrafluoroethylene-hexafluoropropylene copolymer film.

The catalyst electrode 51 of the positive electrode 50 is made of gold, the positive electrode current collector 52 and the lead 120 are made of titanium, and the positive electrode current collector 52 and the lead 120 are welded and integrated. The negative electrode 100 was composed of 3.7g of an Sn-Sb alloy (Sb content: 5 mass%, Sn mass: 3.52 g).

In the obtained oxygen sensor 1, the first holder cover 11, the O-ring 30, the protective film 40 made of a tetrafluoroethylene resin sheet, the separator 60 made of a tetrafluoroethylene-hexafluoropropylene copolymer film, the catalyst electrode 51, and the positive electrode collector 52 were pressed by screwing the holder 20 and the second holder cover 12, and a good contact state was maintained. The first holder cover 11 functions as a pressing end plate, and air-tightness and liquid-tightness are ensured by the O-ring 30. In addition, the ratio of the contained amount of electrolyte (4.3ml) to the mass of Sn contained in the anode (3.52g) was 1.22 (ml/g).

Example 2

An oxygen sensor was assembled in the same manner as in example 1, except that an electrolytic solution was prepared by dissolving citric acid, tripotassium citrate, and ammonia in water, and this electrolytic solution was used. The molar concentration in the electrolyte was set as citric acid: 2.5mol/L, tripotassium citrate: 0.5mol/L, ammonia: 3.0 mol/L. The total content of citric acids dissolved in the electrolyte was 3.0mol/L, the content of alkali metal (potassium): 1.5mol/L is 0.5 times of the total content of citric acid, and the pH of the electrolyte is 4.30 at 25 ℃. The molar ratio of the total content of ammonia in the electrolyte to the total content of citric acid compounds was 1.

Example 3

An oxygen sensor was assembled in the same manner as in example 1, except that an electrolytic solution was prepared by dissolving citric acid, potassium acetate, and ammonia in water, and this electrolytic solution was used. The molar concentration in the electrolyte was set as citric acid: 2.5mol/L, potassium acetate: 1.0mol/L, ammonia: 3.0 mol/L. The total content of citric acids dissolved in the electrolyte was 2.5mol/L, the content of alkali metal (potassium): 1.0mol/L is 0.4 times of the total content of citric acid, and the pH of the electrolyte is 4.32 at 25 ℃. The molar ratio of the total content of ammonia in the electrolyte to the total content of citric acid compounds was 1.2.

Example 4

The molar concentrations of potassium acetate and ammonia were changed to potassium acetate: 1.5mol/L, ammonia: an electrolyte was prepared and an oxygen sensor was assembled in the same manner as in example 3, except that 2.5mol/L was used. The total content of citric acids dissolved in the electrolyte was 2.5mol/L, the content of alkali metal (potassium): 1.5mol/L is 0.6 times of the total content of citric acid, and the pH of the electrolyte is 4.39 at 25 ℃. The molar ratio of the total content of ammonia in the electrolyte to the total content of citric acid compounds was 1.

Example 5

Except that the molar concentrations of citric acid and ammonia were changed to citric acid: 2.6mol/L, ammonia: an electrolyte was prepared and an oxygen sensor was assembled in the same manner as in example 3, except that the amount of the electrolyte was 3.3 mol/L. The total content of citric acids dissolved in the electrolyte was 2.6mol/L, the content of alkali metal (potassium): 1.0mol/L is 0.38 times of the total content of citric acid, and the pH of the electrolyte is 4.36 at 25 ℃. The molar ratio of the total content of ammonia in the electrolyte to the total content of citric acid compounds was 1.27.

Comparative example 1

Except that the molar concentrations of citric acid and tripotassium citrate were changed to citric acid: 1.0mol/L, tripotassium citrate: an electrolyte was prepared and an oxygen sensor was assembled in the same manner as in example 1, except that the concentration of 1.2mol/L was changed. The total content of citric acids dissolved in the electrolyte was 2.2mol/L, the content of alkali metal (potassium): 3.6mol/L is 1.64 times of the total content of citric acid, and the pH of the electrolyte is 4.55 at 25 ℃.

Comparative example 2

Except that the molar concentrations of citric acid and tripotassium citrate were changed to citric acid: 1.4mol/L, tripotassium citrate: an electrolyte was prepared and an oxygen sensor was assembled in the same manner as in example 1, except that the concentration of 0.8mol/L was changed. The total content of citric acids dissolved in the electrolyte was 2.2mol/L, the content of alkali metal (potassium): 2.4mol/L is 1.09 times of the total content of citric acid, and the pH of the electrolyte is 3.60 at 25 ℃.

Comparative example 3

Except that the molar concentrations of citric acid and tripotassium citrate were changed to citric acid: 1.6mol/L, tripotassium citrate: an electrolyte solution was prepared and an oxygen sensor was assembled in the same manner as in example 1, except that the concentration of 0.6mol/L was changed. The total content of citric acids dissolved in the electrolyte was 2.2mol/L, the content of alkali metal (potassium): 1.8mol/L is 0.82 times of the total content of citric acid, and the pH of the electrolyte is 3.34 at 25 ℃.

Comparative example 4

Except that the molar concentrations of citric acid and tripotassium citrate were changed to citric acid: 1.72mol/L, tripotassium citrate: an electrolyte solution was prepared and an oxygen sensor was assembled in the same manner as in example 1, except that the concentration of 0.5mol/L was changed. The total content of citric acids dissolved in the electrolyte was 2.22mol/L, the content of alkali metal (potassium): 1.5mol/L is 0.68 times of the total content of citric acid, and the pH of the electrolyte is 3.07 at 25 ℃.

Comparative example 5

Except that the molar concentrations of citric acid and tripotassium citrate were changed to citric acid: 0.26mol/L, tripotassium citrate: an electrolyte solution was prepared and an oxygen sensor was assembled in the same manner as in example 1, except that the concentration of 2.0mol/L was changed. The total content of citric acids dissolved in the electrolyte was 2.26mol/L, the content of alkali metal (potassium): 6.0mol/L is 2.65 times of the total content of citric acid, and the pH of the electrolyte is 6.37 at 25 ℃.

Comparative example 6

Except that the molar concentrations of citric acid and tripotassium citrate were changed to citric acid: 0.6mol/L, tripotassium citrate: an electrolyte was prepared and an oxygen sensor was assembled in the same manner as in example 1, except that the concentration of 0.8mol/L was changed. The total content of citric acids dissolved in the electrolyte was 1.4mol/L, the content of alkali metal (potassium): 2.4mol/L is 1.71 times of the total content of citric acid, and the pH of the electrolyte is 4.48 at 25 ℃.

Comparative example 7

Except that the molar concentrations of citric acid and tripotassium citrate were changed to citric acid: 1.0mol/L, tripotassium citrate: an electrolyte was prepared and an oxygen sensor was assembled in the same manner as in example 1 except that the amount of the electrolyte was changed to 0 mol/L. The total content of citric acids dissolved in the electrolyte was 1.0mol/L, and alkali metals were not contained, and the pH of the electrolyte was 1.50 at 25 ℃.

Comparative example 8

An oxygen sensor was assembled in the same manner as in example 1, except that an electrolyte was prepared by dissolving citric acid and potassium carbonate in water, and this electrolyte was used. The molar concentration in the electrolyte was set as citric acid: 2.5mol/L, potassium carbonate: 2.0 mol/L. The total content of citric acids dissolved in the electrolyte was 2.5mol/L, the content of alkali metal (potassium): 4.0mol/L is 1.6 times of the total content of citric acid, and the pH of the electrolyte is 4.86 at 25 ℃.

Comparative example 9

An oxygen sensor was assembled in the same manner as in example 1, except that the amount of the electrolyte was set to 1 ml. The ratio of the amount of the electrolyte contained to the mass of Sn contained in the negative electrode was 0.28 (ml/g).

Table 1 shows the compositions and physical properties of the electrolytes used in the oxygen sensors of examples and comparative examples.

[ Table 1]

For each of the oxygen sensors of examples and comparative examples, an accelerated lifetime test was conducted at a temperature of 40 ℃ by introducing 100% oxygen. At 40 ℃, the electrochemical reaction was performed about 2 times as much as at room temperature. In addition, in 100% oxygen aeration, an electrochemical reaction in the atmosphere was performed about 5 times as much as that in the atmosphere. Therefore, when 100% oxygen gas ventilation is performed at a temperature of 40 ℃, the lifetime can be determined at a rate about 10 times that when left at room temperature in the atmosphere. In this test, the output voltage of the oxygen sensor was measured, the time at which the output voltage was reduced to 90% of the value at the start of measurement was taken as the lifetime, and the performance of the oxygen sensor was evaluated from the time until the lifetime was reached (measurable time). The measurement results when the measurable time of the oxygen sensor of comparative example 1 was taken as 100 are shown in table 2.

[ Table 2]

	Life span
		Example 1	187
Example 2	260
		Example 3	204
Example 4	178
		Example 5	196
Comparative example 1	100
		Comparative example 2	131
Comparative example 3	95
		Comparative example 4	55
Comparative example 5	103
		Comparative example 6	21
Comparative example 7	121
		Comparative example 8	54

As shown in tables 1 and 2, the oxygen sensors of examples 1 to 5, which had the total content of citric acids, the ratio of the content of alkali metal to citric acids, and the electrolyte solution with an appropriate pH value and also an appropriate ratio of the amount of the electrolyte solution to the content of tin contained in the negative electrode, exhibited good performance over a long period of time, and had long life.

In contrast, the oxygen sensors of comparative examples 1 to 4 and 8 in which the content ratio of the alkali metal and the citric acid compound in the electrolyte and the pH of the electrolyte are out of the ranges of the present invention, and the oxygen sensor of comparative example 5 in which the content ratio of the alkali metal and the citric acid compound in the electrolyte are out of the ranges of the present invention, can maintain good performance for a shorter time and have a shorter life than the oxygen sensors of the examples. Further, the ratio of the contents of the alkali metal and the citric acid compounds in the electrolyte solution of the oxygen sensor of comparative example 6 and the pH of the electrolyte solution were the same as those of the oxygen sensor of comparative example 1, but the total content of the citric acid compounds was less than 2.1mol/L, and therefore the lifetime was further shortened as compared with the oxygen sensor of comparative example 1. Further, the oxygen sensor of comparative example 7 does not contain an alkali metal in the electrolyte solution, and therefore, the operation is unstable, and further, the pH of the electrolyte solution and the total content of citric acid are out of the range of the present invention, and therefore, the lifetime is significantly reduced as compared with the oxygen sensor of comparative example 1 in which the content of citric acid is the same.

In the oxygen sensor of comparative example 9 in which the ratio of the amount of electrolyte to the content of tin contained in the negative electrode was less than 0.3ml/g, the pH of the electrolyte changed rapidly, and the dissolved tin reached a saturated concentration in a short time, so that the electrolyte could not exhibit its characteristics, and the lifetime was shortened.

The present invention can be implemented in other embodiments than the above-described embodiments without departing from the scope of the present invention. The embodiments disclosed in the present application are merely examples, and the present invention is not limited to these embodiments. The scope of the present invention is defined by the appended claims rather than the description of the above specification, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Industrial availability

The electrochemical oxygen sensor of the present invention can be applied to the same applications as conventionally known electrochemical oxygen sensors.

Description of the symbols

1: electrochemical oxygen sensor, 10: holder cover, 11: first holder cover (middle cover), 12: second holder cover (outer cover), 20: holder, 30: o-ring, 40: protective film, 50: positive electrode, 51: catalyst electrode, 52: positive electrode current collector, 60: diaphragm, 70: positive electrode current collector holding part, 80: electrolyte supply hole, 90: through hole for wire, 100: negative electrode, 110: electrolyte, 120: wire, 130: correction resistance, 140: thermistor for temperature compensation, 150: a through hole.