JP7566541B2 - Electrochemical oxygen sensor - Google Patents

Description

The present invention relates to an electrochemical oxygen sensor with excellent storage characteristics.

Electrochemical oxygen sensors (hereafter referred to as oxygen sensors) have the advantages of being inexpensive, easy to use, and capable of operating at room temperature, and are therefore used in a wide range of applications, such as checking for oxygen deficiencies inside ship holds and manholes, and detecting oxygen concentrations in medical equipment such as anesthesia machines and artificial ventilators.

Electrochemical oxygen sensors generally use a resin container (plastic container) as their exterior. The resin used for such exteriors is usually an easily moldable material such as acrylonitrile-butadiene-styrene (ABS) resin, but it is often relatively permeable to water vapor. Therefore, if an electrochemical oxygen sensor uses an aqueous electrolyte, the water in the electrolyte will be discharged to the outside through the resin exterior. For this reason, if an electrochemical oxygen sensor is used for a long period of time, the concentration and volume of the electrolyte may fluctuate, reducing the reliability of the measurement results.

On the other hand, there are also proposals for technologies that aim to solve these problems. For example, Patent Document 1 proposes a technology in which metal is vapor-deposited onto the surface of a plastic container that constitutes an electrochemical oxygen sensor (galvanic cell type oxygen sensor), and the vapor-deposited metal layer thus formed is then covered with a heat-shrinkable plastic tube.

Patent Document 2 also proposes a technology in which the entire outer wall of a synthetic resin container that constitutes an electrochemical oxygen sensor (galvanic cell oxygen sensor) is metal-plated, and the entire outer wall of the container after the metal plating is coated with epoxy resin.

Metal vapor deposition layers and metal plating layers formed on the surface of resin containers are easily scratched, which reduces the container's ability to inhibit water vapor transmission. It is presumed that the covering with a heat-shrinkable plastic tube in Patent Document 1 and the coating with epoxy resin in Patent Document 2 are intended to inhibit scratches on the metal vapor deposition layer and metal plating layer.

Japanese Unexamined Patent Publication No. 62-272151 Japanese Patent Application Publication No. 4-151548

While the techniques of Patent Documents 1 and 2 are expected to improve the storage characteristics of electrochemical oxygen sensors, they have the problem of complicating the manufacturing process because they require the formation of a metal layer and a resin layer in the container that constitutes the sensor. Therefore, there is a need to develop a technique for manufacturing electrochemical oxygen sensors using a simpler process and that can suppress the deterioration of storage characteristics caused by the discharge of moisture in the electrolyte to the outside.

The present invention was made in consideration of the above circumstances, and its purpose is to provide an electrochemical oxygen sensor with excellent storage characteristics.

The electrochemical oxygen sensor of the present invention comprises a cylindrical container, a positive electrode, a negative electrode, and an electrolyte solution composed of an aqueous solution contained in the container, the container having a resin body portion that holds the electrolyte solution, at least a portion of the surface of the body portion being covered with a resin barrier layer to prevent the electrolyte solution from dissipating, the resin barrier layer being made of a resin different from that of the body portion, and being bonded to the surface of the body portion without a metal layer.

The present invention provides an electrochemical oxygen sensor with excellent storage characteristics.

1 is a cross-sectional view showing a schematic diagram of an example of an electrochemical oxygen sensor according to the present invention. 1 is a cross-sectional view showing a schematic diagram of an electrochemical oxygen sensor according to Comparative Example 1. FIG. FIG. 1 is a cross-sectional view showing a schematic example of an electrochemical oxygen sensor according to a reference example. 1 is a graph showing the results of evaluating the storage characteristics of the electrochemical oxygen sensors of Examples 1 to 4 and Comparative Example 1.

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

Figure 1 is a cross-sectional view that shows a schematic representation of a galvanic cell type oxygen sensor, which is one embodiment of the electrochemical oxygen sensor of the present invention.

The oxygen sensor 1 shown in FIG. 1 has a positive electrode 50, a negative electrode 80, and an electrolyte 90 in a bottomed cylindrical container 20. The container 20 is composed of a main body 21 that holds the electrolyte 90 inside, and a sealing lid 10 for fixing a protective film 40, a diaphragm 60, and the positive electrode 50 to the opening of the main body 21. The sealing lid 10 is composed of a first sealing lid (inner lid) 11 and a second sealing lid (outer lid) 12 for fixing the first sealing lid 11, has a through hole 120 for taking in oxygen into the oxygen sensor 1, and is attached to the main body 21 via an O-ring 30.

Inside the main body 21 that contains the electrolyte 90, the negative electrode 80 is disposed in a state of being immersed in the electrolyte, and the negative electrode 80 is formed with a lead portion 81. The positive electrode 50 is configured by stacking a catalytic electrode 51 and a positive electrode current collector 52, and a lead wire 53 is attached to the positive electrode current collector 52. A perforation 70 is provided in the lower portion of the main body 21 that holds the electrolyte 90 in the container 21, for passing the lead wire 53 attached to the positive electrode 52.

A compensation 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 wire 53 attached to the positive electrode collector 52, and are housed inside the main body 21. A negative electrode terminal 82 is connected to the lead portion 81 of the negative electrode 80, and a positive electrode terminal 54 is connected to the lead wire 53 attached to the positive electrode collector 52, and each is led out to the outside of the main body 21.

A membrane 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 match the battery reaction. Oxygen is introduced into the positive electrode 50 through the membrane 60 from a through hole 120 provided in the sealing lid 10. A protective film 40 is disposed on the outer surface of the membrane 60 to prevent the adhesion of dirt, dust, water, etc. to the membrane 60, and is fixed by a first sealing lid 11.

That is, the first sealing lid 11 functions as a pressing end plate for the protective film 40, the diaphragm 60, and the positive electrode 50. In the sensor 1 shown in FIG. 1, a threaded portion is formed on the inner periphery of the second sealing lid 12 so as to screw into a threaded portion formed on the outer periphery of the opening of the main body 21. Then, by screwing the sealing lid 10, the first sealing lid 11 is pressed against the main body 21 via the O-ring 30, so that the protective film 40, the diaphragm 60, and the positive electrode 50 can be fixed to the main body 21 while maintaining airtightness and liquid tightness.

The main body 21 is made of resin, and at least a portion of its surface is covered with a resin barrier layer 22 to prevent the electrolyte 90 from dissipating. The resin barrier layer 22 is made of a resin different from that of the main body 21, and is joined to the surface of the main body 21 without a metal layer.

The container of the electrochemical oxygen sensor is made of an easily moldable resin such as acrylonitrile-butadiene-styrene (ABS) resin, but because such resins have a relatively high water vapor permeability, the moisture in the electrolyte inside is discharged to the outside through the side walls of the container, etc. However, in the electrochemical oxygen sensor of the present invention, at least a portion of the surface of the main body that holds the electrolyte is covered with a resin barrier layer that prevents the electrolyte from dissipating, and the resin of the main body is joined to the resin barrier layer, making it possible to suppress the decrease in the amount of moisture in the electrolyte during long-term use and storage of the electrochemical oxygen sensor, thereby ensuring excellent storage characteristics.

For example, it is possible to suppress the decrease in the amount of water in the electrolyte by thickening the container of the electrochemical oxygen sensor, but in this case, the size of the electrochemical oxygen sensor increases. However, the electrochemical oxygen sensor of the present invention can suppress the decrease in the amount of water in the electrolyte while avoiding an increase in size, so it is possible to achieve a compact size while maintaining excellent storage characteristics.

In addition, the electrochemical oxygen sensor of the present invention has a resin barrier layer formed directly on the surface of the main body without a metal layer in between, so it can be manufactured without the need for complicated steps such as forming a metal vapor deposition layer or metal plating layer on the container and then providing a protective layer to prevent damage to these layers, as described in Patent Documents 1 and 2, and therefore has good productivity.

In an electrochemical oxygen sensor, at least a portion of the surface of the main body may be covered with a resin barrier layer, but as shown in FIG. 1, it is preferable to cover at least the side surface, which occupies a large proportion of the surface area of the main body and therefore allows a large amount of moisture in the electrolyte to permeate. In this way, the storage characteristics of the electrochemical oxygen sensor can be further improved.

In electrochemical oxygen sensors, it is preferable that 50% or more of the surface area of the main body is covered with a resin barrier layer in order to suppress the decrease in the water content in the electrolyte and further improve storage characteristics.

Furthermore, in the electrochemical oxygen sensor, as shown in FIG. 1, the top surface of the main body (the upper surface in FIG. 1) may be covered with a resin barrier layer, and further, the entire surface of the main body (100% of the surface area of the main body) may be covered with a resin barrier layer. In addition, the side and bottom surfaces (the bottom surface in FIG. 1, excluding the through hole 120 for taking oxygen into the sensor) of the sealing lid (second sealing lid 12 in FIG. 1) attached to the opening of the main body can also be covered with a resin barrier layer in the same way as the main body, and in this case, the storage characteristics of the electrochemical oxygen sensor can be further improved.

The main body of the oxygen sensor container can be made of resins such as ABS resin, polypropylene resin, polyethylene resin, polyacetal resin, polyethylene terephthalate resin, polystyrene resin, and polyamide resin. Of these, it is preferable to use ABS resin because it has good processability and is relatively inexpensive.

The sealing cover for the oxygen sensor can also be made of resins such as ABS resin, polypropylene, polycarbonate, and fluororesin.

The resin that constitutes the resin barrier layer formed on the surface of the main body of the oxygen sensor container, or preferably on the surface of the sealing lid, is one that has lower water absorption and water vapor permeability than the resin that constitutes the main body of the container or the sealing lid. Specifically, the water absorption rate is preferably 0.2% or less, and more preferably 0.1% or less. The water absorption rate of the resin referred to in this specification is a value measured after immersing a 3 mm thick test piece in water at 23°C for 24 hours using Method A in accordance with the provisions of JIS K 7209. The lower the water absorption rate of the resin that constitutes the resin barrier layer, the better, but it is usually 0.01% or more.

Specific examples of resins that make up the resin barrier layer include polyolefins such as polypropylene (PP, water absorption rate: approximately 0.01 to 0.03%); polyphenylene sulfide (PPS, water absorption rate: approximately 0.02%); fluororesins such as polychlorotrifluoroethylene (PCTFE, water absorption rate: less than 0.01%), tetrafluoroethylene-perfluoroalkoxyethylene copolymer (PFA, water absorption rate: approximately 0.01%), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP, water absorption rate: approximately 0.01%); polysulfone (PSF, water absorption rate: approximately 0.03%); polyesters such as polybutylene terephthalate (PBT, water absorption rate: approximately 0.06 to 0.08%); and epoxy resins (crosslinked, water absorption rate: approximately 0.05 to 0.15%). Examples of the epoxy resin include bisphenol A type epoxy resin, bisphenol F type epoxy resin, novolac type epoxy resin, and glycidylamine type epoxy resin (cured products of the above-mentioned epoxy resins).

The resin barrier layer may be formed on the outer surface of the resin that constitutes the main body (the outer surface of the oxygen sensor) or on the inner surface (the surface that comes into contact with the electrolyte), but it is usually formed on the outer surface because this is easier to form.

The thickness of the resin barrier layer is preferably 50 μm or more, and more preferably 100 μm or more, from the viewpoint of better suppressing the decrease in the water content in the electrolyte and further improving the storage characteristics of the oxygen sensor. From the viewpoint of improving the storage characteristics of the oxygen sensor, the thicker the resin barrier layer, the better. However, the effect saturates at a certain thickness, and if the layer is too thick, the size of the sensor increases. Therefore, for example, in consideration of making a compact sensor, the thickness is preferably 1.5 mm or less, and more preferably 0.7 mm or less.

When the resin barrier layer is formed from the above-mentioned epoxy resin, for example, it can be formed by applying the epoxy resin by a known method to a predetermined location of the container body or sealing lid formed by a known molding method such as injection molding, and curing it under conditions appropriate for the epoxy resin used.

The surface of the main body of the container may have a resin layer other than the resin barrier layer. For example, it may have a two-layer structure of a resin barrier layer and another resin layer. The other resin layer may be made of the same resin as the main body of the container. However, from the viewpoint of making the oxygen sensor compact, it is better not to have too many resin layers on the surface of the main body, and it is preferable to have only a resin barrier layer.

The positive electrode of the oxygen sensor is, for example, as shown in Figure 1, made up of a catalytic electrode and a positive electrode current collector. There are no particular limitations on the material that makes up the catalytic electrode, so long as it can generate a current by electrochemically reducing oxygen on the positive electrode, but catalysts that are active in oxidation and reduction, such as gold (Au), silver (Ag), platinum (Pt), and titanium (Ti), are preferably used.

The negative electrode of the oxygen sensor is not particularly limited, but can be made of, for example, lead (Pb), zinc (Zn), antimony (Sb), tin (Sn), or an alloy of these.

The electrolyte used in the oxygen sensor is an aqueous solution, for example, an acidic aqueous solution containing a carboxylic acid or its salt, such as an aqueous solution containing acetic acid, potassium acetate, and lead acetate, or an aqueous solution containing citric acid and a citrate (such as an alkali metal salt); or an alkaline aqueous solution, such as an aqueous solution of sodium hydroxide or an aqueous solution of potassium hydroxide.

In addition, the aqueous solution constituting the electrolyte may contain various organic acids, inorganic acids, or alkalis as necessary for the purpose of adjusting the pH.

In addition, it is preferable to place a diaphragm on the outer surface of the positive electrode of the oxygen sensor to control the intrusion of oxygen so that too much oxygen does not reach the catalytic electrode. The diaphragm is preferably one that selectively allows oxygen to pass through while limiting the amount of oxygen gas that passes through. There are no particular restrictions on the material or thickness of the diaphragm, but typically, fluororesins such as polytetrafluoroethylene and tetrafluoroethylene-hexafluoropropylene copolymers; polyolefins such as polyethylene; and the like are used. The diaphragm can be a porous membrane, a non-porous membrane, or even a membrane with holes formed with capillaries, known as a capillary type.

Furthermore, in order to protect the diaphragm, it is preferable to place a protective film made of a porous resin film on the diaphragm. There are no particular restrictions on the material or thickness of the protective film as long as it can prevent the adhesion of dirt, dust, water, etc. to the diaphragm and has the function of allowing air (including oxygen) to pass through, but typically a fluororesin such as polytetrafluoroethylene is used.

The O-ring 30 interposed between the main body 21 of the container 20 and the sealing lid 10 (first sealing lid 11) is pressed and deformed by screwing the main body 21 and the second sealing lid 12 together, thereby maintaining the airtightness and liquid-tightness of the oxygen sensor 1. There are no particular restrictions on the material of the O-ring, but typically nitrile rubber, silicone rubber, ethylene propylene rubber, fluororesin, etc. are used.

So far, the present invention has been explained using a galvanic cell type oxygen sensor, which is one embodiment of the electrochemical oxygen sensor of the present invention, as an example, but the electrochemical oxygen sensor of the present invention is not limited to the above embodiment, and various modifications are possible within the scope of the technical concept. In addition, various design modifications are possible for the oxygen sensor shown in Figure 1 as long as it has the function of an oxygen sensor and the oxygen supply path described above.

The electrochemical oxygen sensor of the present invention can also be configured as a constant-potential oxygen sensor. A constant-potential oxygen sensor is a sensor that applies a constant voltage between the positive and negative electrodes, and the applied voltage is set according to the electrochemical characteristics of each electrode and the type of gas to be detected. In a constant-potential oxygen sensor, when an appropriate constant voltage is applied between the positive and negative electrodes, the current flowing between them and the oxygen gas concentration are proportional. Therefore, if the current is converted to a voltage, the oxygen gas concentration of an unknown gas can be detected by measuring the voltage, just like a galvanic cell oxygen sensor.

The electrochemical oxygen sensor of the present invention can be used in the same applications as conventional electrochemical oxygen sensors.

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>
An electrolyte solution was prepared by dissolving citric acid, tripotassium citrate, and ammonia in water. The molar concentrations in the electrolyte solution were citric acid: 2.5 mol/L, tripotassium citrate: 0.5 mol/L, and ammonia: 3.0 mol/L. The pH of the electrolyte solution was 4.30 at 25°C.

<Assembling the oxygen sensor>
5.4 g of the electrolyte was poured into the inside of the main body 21 made of ABS resin, and a galvanic cell type oxygen sensor having the configuration shown in Fig. 1 was assembled. The sealing lid 10 (first sealing lid 11 and second sealing lid 12) was also made of ABS resin, like the main body 21. In addition, a porous polytetrafluoroethylene sheet was used for the protective film 40, and a tetrafluoroethylene-hexafluoropropylene copolymer film was used for the diaphragm 60.

The catalytic electrode 51 of the positive electrode 50 is made of gold, and the positive electrode collector 52 and lead wire 53 are made of titanium, and the positive electrode collector 52 and lead wire 53 are welded together. The negative electrode 80 is made of a Sn-Sb alloy (Sb content: 5% by mass).

A correction resistor 100 and a temperature compensation thermistor 110 are connected in series between the positive electrode lead wire 53 and the negative electrode lead portion 81, and the positive electrode terminal 54 and the negative electrode terminal 81 are led out from the positive electrode lead wire 53 and the negative electrode lead portion 81, respectively, to the outside of the main body portion 21, making it possible to detect the oxygen concentration from the output voltage.

In the obtained oxygen sensor 1, the first sealing lid 11, O-ring 30, polytetrafluoroethylene sheet protective film 40, tetrafluoroethylene-hexafluoropropylene copolymer diaphragm 60, catalytic electrode 51, and positive electrode current collector 52 are pressed by screwing the main body 21 and second sealing lid 12 together to maintain a good sealing state.

<Formation of Resin Barrier Layer>
Bisphenol A type epoxy resin was applied to the side surfaces (side surfaces of the main body 21 and the second sealing lid 12) and top surface (top surface of the main body 21) of the container 20 and cured to form a resin barrier layer 22 with a thickness of 0.05 mm, thereby obtaining an electrochemical oxygen sensor.

Example 2
An electrochemical oxygen sensor was produced in the same manner as in Example 1, except that the thickness of the resin barrier layer was 0.1 mm.

Example 3
An electrochemical oxygen sensor was produced in the same manner as in Example 1, except that the thickness of the resin barrier layer was 1.5 mm.

Example 4
An electrochemical oxygen sensor was produced in the same manner as in Example 1, except that the thickness of the resin barrier layer was 5 mm.

Comparative Example 1
As shown in FIG. 2, an electrochemical oxygen sensor 200 was produced in the same manner as in Example 1, except that the resin barrier layer was not formed.

Reference Example In order to examine the amount of water volatilized from the electrolyte through the diaphragm, an electrochemical oxygen sensor 201 was fabricated in the same manner as in Example 1 except that a resin barrier layer was not formed, and the through hole of the first sealing cover 11 was sealed with epoxy resin 130 as shown in Fig. 3. The epoxy resin used was the same as that used for the resin barrier layer in the electrochemical oxygen sensor of Example 1.

The oxygen sensors of Example 1, Comparative Example 1, and Reference Example were each stored in a thermostatic chamber at 40°C and a relative humidity of approximately 20% for 42 days, then removed, and the amount of electrolyte loss (amount of water loss) was calculated from the change in weight before and after storage. The results are shown in Table 1.

As shown in Table 1, the difference in the amount of electrolyte loss between the oxygen sensor of Comparative Example 1 and the oxygen sensor of Reference Example is 18 mg, while the difference in the amount of electrolyte loss between the oxygen sensor of Comparative Example 1 and the oxygen sensor of Example 1 is 38 mg, which is significantly larger. This shows that the amount of electrolyte that permeates the main body of the container and escapes to the outside of the oxygen sensor is significantly greater than the amount that permeates the diaphragm of the positive electrode. This shows that the escape of electrolyte in the oxygen sensor can be effectively prevented by forming a resin barrier layer on the surface of the main body.

The oxygen sensors of Examples 1 to 4 and Comparative Example 1 were stored for a specified period in an atmosphere with an oxygen concentration of 100% at 40°C, and then removed after storage and left in air at room temperature for at least 3 hours, after which the output in air (oxygen concentration: approximately 21%) was measured. Figure 4 shows the relative output values for each storage period, with the output in air of each oxygen sensor before storage taken as 100. A storage period of 180 days at 40°C is equivalent to 5 years at room temperature.

As is clear from Figure 4, the oxygen sensor of Comparative Example 1, in which the surface of the main body of the container is not covered with a resin barrier layer, exhibits a large amount of electrolyte dissipation, resulting in a large drop in output. On the other hand, the oxygen sensors of Examples 1 to 4, in which at least a portion of the surface of the main body of the container is covered with a resin barrier layer, were able to reduce the amount of electrolyte dissipation, thereby suppressing the drop in output and improving storage characteristics. The thicker the resin barrier layer, the lower the amount of electrolyte dissipation, improving storage characteristics, but the effect saturates at a certain thickness, so a thickness of about 1 mm is considered to be sufficient for practical purposes.

1 Electrochemical oxygen sensor 2 Main body 10 Sealing lid 11 First sealing lid (inner lid)
12 Second sealing lid (outer lid)
20 Container 21 Container body 22 Resin barrier layer 30 O-ring 40 Protective film 50 Positive electrode 51 Catalytic electrode 52 Positive electrode current collector 53 Positive electrode lead wire 54 Positive electrode terminal 60 Diaphragm 70 Hole for lead wire 80 Negative electrode 81 Negative electrode lead portion 82 Negative electrode terminal 90 Electrolyte 100 Compensating resistor 110 Temperature compensation thermistor 120 Through hole 130 Epoxy resin

Claims (5)

The battery includes a cylindrical container, a positive electrode, a negative electrode, and an electrolyte solution formed of an aqueous solution, which are contained in the container;
The container has a resin body for holding the electrolyte,
At least a part of the surface of the main body is covered with a resin barrier layer having a thickness of 50 μm or more to prevent the electrolyte from dissipating,
An electrochemical oxygen sensor, characterized in that the resin barrier layer is made of a resin different from that of the main body portion and is joined to the surface of the main body portion without a metal layer therebetween. The electrochemical oxygen sensor according to claim 1, wherein the body of the container is made of acrylonitrile-butadiene-styrene resin. The electrochemical oxygen sensor according to claim 1 or 2, wherein the resin barrier layer is made of epoxy resin. An electrochemical oxygen sensor according to any one of claims 1 to 3, in which the resin barrier layer is provided on at least the side surface of the main body. An electrochemical oxygen sensor according to any one of claims 1 to 4, in which 50% or more of the surface area of the main body is covered with the resin barrier layer.

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