WO2023162960A1

WO2023162960A1 - Elecrochemical oxygen sensor and production method therefor

Info

Publication number: WO2023162960A1
Application number: PCT/JP2023/006155
Authority: WO
Inventors: 北澤直久; 渡辺光俊; 岩波良治; 大町あゆみ
Original assignee: マクセル株式会社
Priority date: 2022-02-22
Filing date: 2023-02-21
Publication date: 2023-08-31

Abstract

This electrochemical oxygen sensor is characterized by comprising a positive electrode, a negative electrode, a barrier membrane that transmits oxygen, and an electrolyte that is formed from an aqueous solution, and is further characterized in that: the negative electrode includes a metal that is eluted into the electrolyte; the electrolyte is an aqueous solution having a pH of 3-10; the positive electrode includes a catalyst layer containing Au; one surface of the catalyst layer makes contact with the electrolyte; and the catalyst layer has Ag or an Ag alloy on at least the electrolyte-side surface thereof.

Description

Electrochemical oxygen sensor and manufacturing method thereof

This application relates to an electrochemical oxygen sensor that improves the reliability of measurements in gases with low oxygen concentrations, and a manufacturing method thereof.

Electrochemical oxygen sensors (hereinafter also referred to as oxygen sensors) have the advantage of being inexpensive, easy to use, and capable of operating at room temperature. It is used in a wide range of fields, such as oxygen concentration detection in medical equipment such as respirators.

As such an electrochemical oxygen sensor, for example, one that has an aqueous electrolyte and uses Sn or an alloy thereof as a negative electrode is known (

Patent Documents

1 and 2, etc.).

JP-A-2006-194708 JP 2017-67596 A

In the electrochemical oxygen sensor, when oxygen flows into the interior, it is reduced at the positive electrode, and a metal elution reaction occurs at the negative electrode, generating a current between the positive electrode and the negative electrode according to the oxygen concentration. Therefore, for example, the current generated by the positive electrode reaction (reduction of oxygen at the positive electrode) is converted into voltage, and the oxygen concentration is obtained based on this value. Therefore, when the electrochemical oxygen sensor is placed in an oxygen-free environment, the positive electrode reaction as described above does not occur. should also be possible. In reality, however, in spite of being placed in an oxygen-free environment, several hours after the start of observation, measurement results may appear as if oxygen is present. For this reason, depending on the configuration of the oxygen sensor, it may be difficult to use it for measuring low-concentration oxygen gas.

The present application has been made in view of the above circumstances, and aims to provide an electrochemical oxygen sensor with improved reliability of measurement in a gas with a low oxygen concentration, and a method of manufacturing the same.

The electrochemical oxygen sensor of the present application includes a positive electrode, a negative electrode, an oxygen-permeable diaphragm, and an electrolytic solution composed of an aqueous solution, the negative electrode containing a metal that dissolves in the electrolytic solution, and the electrolytic solution having a pH 3 to 10 aqueous solution, the positive electrode includes a catalyst layer containing Au, one surface of the catalyst layer is in contact with the electrolyte, and the catalyst layer has at least the surface on the electrolyte side and contains Ag or an alloy of Ag.

As an example, the electrochemical oxygen sensor of the present application includes a first step of depositing Ag on the surface of a metal layer containing Au to form a catalyst layer, and placing the Ag-attached side of the catalyst layer on the electrolyte side. It can be manufactured by the manufacturing method of the present application including a second step of arranging and assembling the positive electrode.

According to the present application, it is possible to provide an electrochemical oxygen sensor with improved reliability when measuring the oxygen concentration of a gas with a low oxygen content, and a method of manufacturing the same.

FIG. 1 is a cross-sectional view schematically showing an example of the electrochemical oxygen sensor of the present application. FIG. 2 is a graph showing the characteristics evaluation test results of the electrochemical oxygen sensors of Examples 1-3 and Comparative Examples 1-2. FIG. 3 is a graph showing the characteristic evaluation test results of the electrochemical oxygen sensors of Examples 4 to 5 and Comparative Example 3. FIG.

<Electrochemical oxygen sensor>
The electrochemical oxygen sensor of the present application includes a positive electrode, a negative electrode, a diaphragm that allows oxygen to pass through, and an electrolytic solution composed of an aqueous solution, the negative electrode contains a metal that dissolves in the electrolytic solution, and the electrolytic solution has a pH 3 to 10 aqueous solution, the positive electrode includes a catalyst layer containing Au, one surface of the catalyst layer is in contact with the electrolyte, and the catalyst layer has at least the surface on the electrolyte side has Ag or an alloy of Ag.

The positive electrode of the electrochemical oxygen sensor of the present application includes a catalyst layer containing Au, and the catalyst layer has Ag or an alloy of Ag on at least the surface on the side of the electrolyte that is in contact with the electrolyte. It is possible to improve the measurement accuracy when measuring the oxygen concentration of a gas with a low oxygen content. The alloy of Ag is preferably an alloy of Au and Ag.

The other surface of the catalyst layer is arranged on the side of the diaphragm, and having Au or an alloy of Au on at least the surface on the side of the diaphragm is useful for prolonging the life of the oxygen sensor and for poisoning gases such as hydrogen sulfide. It is preferable from the aspect of preventing the influence of

The catalyst layer may be a laminate of a layer containing Ag or an alloy of Ag and a layer containing Au or an alloy of Au.

In the catalyst layer, it is preferable that the content of Ag on the surface on the electrolyte solution side is higher than the content of Ag on the inside, in order to further enhance the effect of Ag.

The content of Ag in the total amount of metals in the catalyst layer is preferably 1 atomic % or more in order to further enhance its effect.

The content of Au in the total amount of metals in the catalyst layer is preferably 10 atomic % or more in order to further enhance its effect.

At present, the reasons why the electrochemical oxygen sensor of the present application can improve the measurement accuracy when measuring the oxygen concentration of gases with low oxygen content are considered as follows.

When an electrochemical oxygen sensor (hereinafter sometimes abbreviated as "oxygen sensor") in which the positive electrode catalyst layer is composed of Au is used in an oxygen-free environment, several hours after the start of use, it is determined whether oxygen exists or not. A voltage rise such as The inventors of the present invention considered the reaction occurring inside the oxygen sensor and, as a result, thought that the voltage rise may occur for the following reasons.

At the positive electrode of the oxygen sensor, the reaction represented by the following formula (1) occurs when oxygen is present.

O ₂ +4H ⁺ +4e ⁻ →H ₂ O (1)

However, when the oxygen sensor is placed in an oxygen-free environment, the positive electrode does not react as shown in formula (1). On the other hand, in a state where oxygen is not supplied to the positive electrode, the positive electrode is electrically connected to the negative electrode through the resistor, so that the negative electrode potential is applied. As a result, the metal element is eluted from the negative electrode, and a deposition reaction of the eluted element occurs at the positive electrode (when the eluted element is Sn, the reaction represented by the following formula (2) or (3)) occurs between the positive and negative electrodes. It is believed that voltage is generated by the flow of current.

Sn ⁴⁺ +4e ⁻ →Sn (2)
Sn ²⁺ +2e ⁻ →Sn (3)

As described above, when an oxygen sensor whose positive electrode catalyst layer is made of Au is used in an oxygen-free environment, the voltage rises after several hours from the start of use, but after a certain period of time, the above-mentioned Voltage rise is no longer recognized. This is because the amount of eluted elements that can be adsorbed or deposited on Au is limited, and once the reaction has progressed to a certain extent, it will no longer proceed, and current will not flow between the positive and negative electrodes. Based on the above knowledge, the present inventors have found that if the reaction at the positive electrode can be prevented on the surface of the catalyst layer that is in contact with the electrolyte, the current can be prevented from being generated between the positive and negative electrodes in an oxygen-free state. We thought that it would be possible to improve the reliability when measuring gases with low oxygen concentrations.

In the oxygen sensor of the present application, the catalyst layer containing the catalytic metal in the positive electrode has Ag or an alloy of Ag on at least the surface of the electrolyte solution side in contact with the electrolyte solution, so the oxygen sensor can be used in an oxygen-free environment. In this case, it is possible to prevent the occurrence of a voltage rise due to the precipitation reaction of the metal ions eluted from the negative electrode at the positive electrode. As a result, the oxygen sensor of the present application can measure the concentration of oxygen in a wide concentration range, for example, it can be used in a gas with a low oxygen concentration, including an oxygen-free environment.

Next, the oxygen sensor of the present application will be described with reference to the 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 oxygen sensor, which is one embodiment of the electrochemical oxygen sensor of the present application.

The oxygen sensor 1 shown in FIG. 1 has a positive electrode 50 , a negative electrode 80 and an electrolytic solution 90 in a cylindrical container 20 with a bottom. The container 20 is composed of a container body 21 that holds an electrolytic solution 90 inside, and a sealing lid 10 for fixing the protective film 40 , the diaphragm 60 and the positive electrode 50 to the opening of the container 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 . and is attached to the container body 21 via an O-ring 30 .

A negative electrode 80 is arranged in a state of being immersed in the electrolytic solution inside the container main body 21 containing the electrolytic solution 90 , and a lead portion 81 is formed on the negative electrode 80 . The positive electrode 50 is configured by laminating a catalyst layer (catalyst electrode) 51 and a positive electrode current collector 52 , and a lead wire 53 is attached to the positive electrode current collector 52 . A hole 70 for passing the lead wire 53 attached to the positive electrode current collector 52 is provided in the lower part of the container body 21 holding the electrolytic solution 90 of the container 20 . Further, although not shown in FIG. 1 , in addition to the perforations 70 , perforations for supplying the electrolytic solution to the positive electrode 50 are also provided in the lower portion of the container body 21 .

A correction resistor 100 and a temperature compensating 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 current collector 52 , and housed inside the container body 21 . . A negative electrode terminal 82 is connected to a lead portion 81 of the negative electrode 80 , and a positive electrode terminal 54 is connected to a lead wire 53 attached to the positive electrode current collector 52 . is derived from

On the outer surface side of the positive electrode 50, a diaphragm 60 that selectively allows permeation of oxygen and restricts the amount of permeation to match the battery reaction is arranged. is introduced into the positive electrode 50 through the diaphragm 60 . A protective film 40 is arranged on the outer surface side of the diaphragm 60 to prevent dust, dust, water, and the like from adhering to the diaphragm 60 , and is fixed by the 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 oxygen sensor 1 shown in FIG. 1, a threaded portion is formed on the inner peripheral portion of the second sealing lid 12 so as to be screwed with the threaded portion formed on the outer peripheral portion of the opening of the container body 21 . Then, by screwing the sealing lid 10, the first sealing lid 11 is pressed against the container body 21 via the O-ring 30, so that the protective film is formed while maintaining airtightness and liquidtightness. 40 , the diaphragm 60 and the positive electrode 50 can be fixed to the container body 21 .

At least the surface 51a of the catalyst layer 51 of the positive electrode 50, which is in contact with the electrolyte, is Ag or an alloy of Ag.

The negative electrode of the oxygen sensor uses a material containing a metal that dissolves in the electrolyte. can be suppressed, the negative electrode is preferably made of a material containing Sn or a Sn alloy, and more preferably made of a Sn alloy. Although this Sn or Sn alloy may contain a certain amount of impurities, it is desirable that the content of Pb is less than 1000 ppm in order to comply with the RoHS Directive.

Examples of Sn alloys that can be used for the negative electrode include Sn--Ag alloys, Sn--Cu alloys, Sn--Ag--Cu alloys, and Sn--Sb alloys from the viewpoint of corrosion resistance. , Na, Zn, Ca, Ge, In, Ni, Co, and other metal elements.

As the Sn alloy, specifically, general lead-free solder materials (Sn-3.0Ag-0.5Cu, Sn-3.5Ag, Sn-3.5Ag-0.75Cu, Sn-3.8Ag- 0.7Cu, Sn-3.9Ag-0.6Cu, Sn-4.0Ag-0.5Cu, Sn-1.0Ag-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-0.6Cu-3.0Sb, Sn-3.5Ag-0.5Bi- 8.0In, 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.) and Sn-Sb alloys can be preferably used.

For example, as shown in FIG. 1, the positive electrode of the oxygen sensor is composed of a catalyst layer containing a catalytic metal and a positive electrode current collector. The catalyst, which is the constituent material of the catalyst layer, is generally not particularly limited as long as it can generate current by electrochemical reduction of oxygen on the positive electrode. and silver (Ag), which has a function of preventing a positive electrode reaction in an oxygen-free environment. Au and Ag may be alloyed.

Here, it is also conceivable to configure the catalyst layer using only Ag, which has the effect of preventing the positive electrode reaction in an oxygen-free environment. However, if the catalyst layer is composed only of Ag, the catalyst layer is easily affected by the poisoning gas, such as hydrogen sulfide, if the gas used to measure the oxygen concentration contains a poisoning gas. is limited, and problems arise in terms of practical use of the oxygen sensor. Further, if the catalyst layer is formed using only Ag, the service life of the sensor is shortened compared to the catalyst layer containing Au, although the reason is not clear.

Therefore, in the present application, the catalyst layer is configured to contain Au, and Ag or an alloy of Ag is disposed at least on the surface of the catalyst layer on the electrolyte side that is in contact with the electrolyte. As the alloy of Ag, an alloy of Au and Ag is preferably used from the viewpoint of the service life of the oxygen sensor. From the viewpoint of enhancing the effect of Ag that prevents positive electrode reaction in an oxygen-free environment, the content of Ag in the total amount of metals in the catalyst layer is preferably 1 atomic % or more, and preferably 15 atomic % or more. It is more preferably 35 atomic % or more, particularly preferably 45 atomic % or more, and most preferably 45 atomic % or more. On the other hand, from the viewpoint of making the catalyst layer contain a certain amount or more of Au to make it less susceptible to poisoning gases such as hydrogen sulfide, the content of Ag in the total amount of metals in the catalyst layer is preferably 90 atomic % or less. It is preferably 60 atomic % or less, more preferably 40 atomic % or less, and most preferably 20 atomic % or less. That is, the content of Au in the total amount of metals in the catalyst layer is preferably 10 atomic % or more, more preferably 40 atomic % or more, particularly preferably 60 atomic % or more, and 80 atomic % or more. % or more is most preferable. Even if the composition of the catalyst layer is not uniform, the content of Ag and the content of Au may be adjusted so as to fall within the above range as a whole.

In addition, the catalyst layer can be entirely formed of a uniform alloy containing Au and Ag. The surface in contact with the liquid is composed of Ag or an alloy of Ag (such as an alloy of Au and Ag), and the diaphragm side is composed of Au or an alloy of Au (Au and Ag having a smaller proportion of Ag than the surface in contact with the electrolyte). alloys, etc.). For example, the content of Ag in the catalyst layer increases from the surface on the side of the electrolyte toward the inside so that the content of Ag on the surface in contact with the electrolyte is higher than the content of Ag on the membrane side. It may decrease. In addition, the Au content in the catalyst layer decreases from the diaphragm-side surface toward the inside so that the Au content on the membrane-side surface is higher than that on the electrolyte side. It is okay to continue. Alternatively, a laminate of a layer containing Ag or an alloy of Ag and a layer containing Au or an alloy of Au may be used. By adopting such a configuration, the reliability when measuring a gas with a low oxygen concentration is improved, the influence of poisonous gases such as hydrogen sulfide is reduced, and the service life of the oxygen sensor is extended. can be more easily demonstrated.

Even if the thickness of the catalyst layer is thin, the effect can be sufficiently exhibited. is more preferred.

The catalyst layer can contain elements other than Au and Ag, such as metal elements having catalytic activity such as Fe, Co, Cu, Ni, Ru, Rh, Pd, Os, Ir, and Pt. However, in order not to inhibit the action of Au and Ag, the total content of metal elements other than Au and Ag is preferably 20 atomic % or less, more preferably 10 atomic % or less, of the total amount of metals in the catalyst layer. More preferably, it is particularly preferably 5 atomic % or less, and the catalyst layer may be composed only of Au and Ag.

The electrolyte of the oxygen sensor is composed of an aqueous solution, such as an aqueous solution containing acetic acid, potassium acetate and lead acetate, and an aqueous solution containing citric acid and citrate (alkali metal salt, etc.). An acidic aqueous solution containing a carboxylic acid or a salt thereof; an alkaline aqueous solution such as an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution; an aqueous solution of carbonic acid or a carbonate;

In addition, the aqueous solution that constitutes the electrolytic solution may contain various organic acids, inorganic acids, alkalis, salts thereof, etc. as necessary for the purpose of pH adjustment.

From the viewpoint of extending the life of the oxygen sensor, it is preferable to use an aqueous solution containing a chelating agent as the electrolytic solution. It is presumed that the chelating agent has the effect of chelating the constituent metals of the negative electrode and dissolving them in the electrolytic solution (hereinafter referred to as the "chelating effect"), which may contribute to the extension of the life of the oxygen sensor. .

As used herein, the term “chelating agent” refers to a molecule (including ions) having multiple coordination sites (coordination atoms) that form a coordinate bond with a metal ion, and forms a complex with the metal ion (complex It stabilizes metal ions by transformation), and can be contained in the electrolytic solution in the form of an acid or a salt thereof that generates the above molecules in the solvent constituting the electrolytic solution. Therefore, those having a single coordination site (coordination atom) such as phosphoric acid, acetic acid, carbonic acid and salts thereof with weak complexing power are not included in the "chelating agent" as used herein. .

Metal ions such as Sn ²⁺ eluted from the negative electrode during discharge are unstable in the electrolyte when the pH of the electrolyte is in the weakly acidic to weakly alkaline range, so they immediately form oxides and hydroxides on the negative electrode. , and may hinder the reaction of the negative electrode. However, when the electrolytic solution contains a chelating agent, the eluted metal ions are chelated and exist stably as complex ions, so that the reaction at the negative electrode can be prevented from being hindered.

A chelating agent generally has a chelating action and pH buffering ability (the ability to keep the pH of a solution almost constant even when a small amount of acid or base is added). , For example, succinic acid, fumaric acid, maleic acid, citric acid, tartaric acid, glutaric acid, adipic acid, malic acid, malonic acid, aspartic acid, glutamic acid, ascorbic acid, salts thereof, etc. 1 type, or 2 or more types can be used.

As the chelating agent, from the viewpoint of enhancing the chelating action, it is more preferable to use those having high solubility in water, specifically citric acid, tartaric acid, glutamic acid and salts thereof. Among these, citric acid and its salts have high solubility in water [citric acid: 73 g / 100 mL (25 ° C.), trisodium citrate: 71 g / 100 mL (25 ° C.), tripotassium citrate: 167 g / 100 mL ( 25° C.)], and since citric acid has a large number of dissociable hydrogen atoms and there are multiple pHs at which the pH buffering ability is exhibited (pKa ₁ =3.13, pKa ₂ =4.75, pKa ₃ = 6.40), more preferably using citric acid. Therefore, when citric acid is used as a chelating agent, the solubility in water is high and the pH buffering capacity is also high, so that the life of the oxygen sensor is further improved.

The concentration of the chelating agent in the electrolytic solution is, for example, preferably 2.3 mol/L or more, more preferably 2.5 mol/L or more, and particularly preferably 2.7 mol/L or more. .

In addition, when the metal derived from the negative electrode dissolved in the electrolytic solution reaches a saturation concentration, the oxide of the metal is generated and the negative electrode becomes inactive, which may shorten the life of the oxygen sensor. Even if the liquid contains a chelating agent, the effect of prolonging the life of the oxygen sensor may be limited. In this case, it is preferable that the electrolytic solution further contains ammonia to increase the molar concentration of the chelating agent in the electrolytic solution. By delaying, it is possible to prolong the life of the oxygen sensor.

The concentration of ammonia in the electrolytic solution is preferably 0.01 mol/L or more in order to facilitate the above-mentioned action of ammonia, and is preferably 0.1 mol/L or more in order to further enhance the above-mentioned action, and is preferably 1 mol/L. L or more is more preferable. In addition, the upper limit of the concentration of ammonia in the electrolyte is not specified in particular, but since ammonia is a compound specified in Table 2 of Japan's "Poisonous and Deleterious Substances Control Law", from the viewpoint of safety , the concentration of ammonia in the electrolyte is preferably less than 10 mol/L.

The pH of the aqueous solution that constitutes the electrolyte is 3-10. When the electrolyte has such a pH, the oxygen sensor placed in an oxygen-free environment tends to cause a reaction in which the metal ions eluted from the negative electrode are deposited at the positive electrode. It is possible to prevent the problem of voltage rise of the oxygen sensor due to such a precipitation reaction.

It is preferable to place a diaphragm on the outer surface of the positive electrode of the oxygen sensor to control the inflow of oxygen so that too much oxygen does not reach the catalyst electrode. As the diaphragm, one that selectively allows oxygen to permeate and limits the permeation amount of oxygen gas is preferable. Although the material and thickness of the membrane are not particularly limited, fluororesins such as polytetrafluoroethylene and tetrafluoroethylene-hexafluoropropylene copolymer; polyolefins such as polyethylene; and the like are usually used. The diaphragm may be a porous membrane, a non-porous membrane, or a capillary-type membrane having pores formed with capillaries.

Furthermore, in order to protect the diaphragm, it is preferable to dispose a protective film made of a porous resin film on the diaphragm. There are no particular restrictions on the material and thickness of the protective film as long as it can prevent dirt, dust, water, etc. from adhering to the diaphragm and has the function of permeating air (including oxygen). A fluororesin such as polytetrafluoroethylene is used.

The container body 21 of the oxygen sensor 1 can be made of, for example, acrylonitrile-butadiene-styrene (ABS) resin. The sealing lid 10 (the first sealing lid 11 and the second sealing lid 12) arranged at the opening of the container body 21 can be made of, for example, ABS resin, polypropylene, polycarbonate, fluororesin, or the like. can.

Furthermore, the O-ring 30 interposed between the container body 21 of the container 20 and the sealing lid 10 (first sealing lid 11) is pressed by the screw tightening between the container body 21 and the second sealing lid 12. The oxygen sensor 1 can be kept air-tight and liquid-tight by deforming. The material of the O-ring is not particularly limited, but nitrile rubber, silicone rubber, ethylene propylene rubber, fluororesin and the like are usually used.

So far, the present application has been described by taking the galvanic cell oxygen sensor, which is one embodiment of the electrochemical oxygen sensor of the present application, as an example. Various modifications are possible within the scope of the concept. Also, the oxygen sensor shown in FIG. 1 can be modified in various ways as long as it has the function of an oxygen sensor.

Also, the electrochemical oxygen sensor of the present application can take the form of a constant potential oxygen sensor. A 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 between them and the oxygen gas concentration have a proportional relationship. Similarly, the oxygen gas concentration of an unknown gas can be detected by measuring the voltage.

The electrochemical oxygen sensor of the present application can be preferably applied to applications in an oxygen-free environment, but can also be applied to the same applications as conventionally known electrochemical oxygen sensors.

<Manufacturing method of electrochemical oxygen sensor>
As an example, the electrochemical oxygen sensor of the present application includes a first step of depositing Ag on the surface of a metal layer containing Au to form a catalyst layer, and placing the Ag-attached side of the catalyst layer on the electrolyte side. and a second step of arranging and assembling the positive electrode.

Ag attached to the surface of the metal layer containing Au may remain on the surface of the metal layer as it is without reacting with the underlying metal layer, or may be alloyed with the underlying metal layer (for example, Au) to An alloy of Ag (for example, an alloy of Au and Ag) may be formed on at least the surface of the layer.

In addition, the method for manufacturing an electrochemical oxygen sensor of the present application may further include a step of heat-treating the metal layer having Ag deposited on its surface in an inert atmosphere or a reducing atmosphere after the first step. This can promote the alloying of Au and Ag. By alloying Au and Ag, Ag can be stably retained in the catalyst layer.

Examples of methods for attaching Ag to the surface of a metal layer containing Au or further alloying Au and Ag include physical vapor deposition (PVD), chemical vapor deposition (CVD), reactive plasma chemical vapor deposition, and chemical vapor deposition. Phase deposition method (PACVD), ion plating method (IP), sputtering method and the like are exemplified. Moreover, the method of electrochemically depositing, such as electrodeposition, may be used.

The amount of Ag to be deposited is preferably such that the content of Ag in the total amount of metal in the catalyst layer is 1 atomic % or more in order to further enhance its effect.

In addition, in the catalyst layer, for example, the Ag content in the metal layer decreases from the surface toward the inside so that the Ag content on the surface is higher than the Ag content on the inside. good too.

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

(Example 1)
<Preparation of electrolytic solution>
An electrolytic solution was prepared by dissolving citric acid, tripotassium citrate and ammonia in water. The molar concentrations in the electrolytic solution were citric acid: 2.5 mol/L, tripotassium citrate: 0.5 mol/L, and ammonia: 3.0 mol/L. The pH of this electrolytic solution was 4.30 at 25°C.

<Preparation of positive electrode catalyst layer>
On the surface of the tetrafluoroethylene-hexafluoropropylene copolymer film that serves as the diaphragm, a metal film made of an alloy of Au and Ag with a thickness of about 70 nm was formed as a catalyst layer of the positive electrode by ion plating. The content ratio of Au and Ag in the prepared catalyst layer was Au:Ag=90:10 in terms of molar ratio (atomic %). In addition, it is considered that a uniform alloy of Au and Ag was formed in the preparation of the catalyst layer.

<Assembly of oxygen sensor>
5.4 g of the electrolytic solution was poured into the container body 21 made of ABS resin to assemble a galvanic cell type oxygen sensor having the configuration shown in FIG. The sealing lid 10 (the first sealing lid 11 and the second sealing lid 12) was also made of ABS resin like the container body 21. As shown in FIG. A porous polytetrafluoroethylene sheet was used as the protective film 40, and a tetrafluoroethylene-hexafluoropropylene copolymer film was used as the diaphragm 60, as described above.

The catalyst layer (catalyst electrode) 51 of the positive electrode 50 uses the previously prepared catalyst layer, and is arranged so that the surface 51a on the side opposite to the diaphragm 60 is in contact with the electrolytic solution. A wire 53 made of titanium was used, and the positive electrode current collector 52 and the lead wire 53 were welded together. The negative electrode 80 was made of a Sn--Sb alloy (Sb content: 5% by mass).

Between the positive lead wire 53 and the negative lead portion 81, a correction resistor 100 and a temperature compensating thermistor 110 are connected in series. By leading the terminal 54 and the negative terminal 82 to the outside of the container body 21, the oxygen concentration can be detected from the output voltage.

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

(Example 2)
Example 1, except that the content ratio of Au and Ag in the catalyst layer (an alloy of Au and Ag) produced by the ion plating method was changed to Au:Ag=73:27 in terms of molar ratio (atomic %). Similarly, an oxygen sensor was assembled.

(Example 3)
Example 1 except that the content ratio of Au and Ag in the catalyst layer (an alloy of Au and Ag) produced by the ion plating method was changed to Au:Ag=49:51 in terms of molar ratio (atomic %). Similarly, an oxygen sensor was assembled.

(Comparative example 1)
In the same manner as in Example 1, except that instead of the metal film made of an alloy of Au and Ag, a metal film made of only Au having a thickness of about 70 nm prepared by an ion plating method was used as the positive electrode catalyst layer. Assembled the oxygen sensor.

(Comparative example 2)
In the same manner as in Example 1, except that instead of the metal film made of an alloy of Au and Ag, a metal film made of only Ag having a thickness of about 70 nm prepared by an ion plating method was used as the positive electrode catalyst layer. Assembled the oxygen sensor.

The oxygen sensors of Examples 1-3 and Comparative Examples 1-2 were each held in nitrogen gas in an environment of 50°C, and the output voltage was measured during that time. The results are shown in FIG.

In the oxygen sensors of Examples 1 to 3, the output voltage dropped to near 0 V after the start of measurement, the output voltage fluctuated little after that, the difference between the actual oxygen concentration (0%) and the measured value was small, and the accuracy was high. It was found that low oxygen concentration can be stably measured at

On the other hand, in the oxygen sensor of Comparative Example 1 having a metal film made of pure gold as a catalyst layer, the output voltage once decreased to around 0 V after the start of measurement, but then the output voltage increased significantly, and the actual oxygen concentration and the measurement There is a large discrepancy between the values.

In addition, in the oxygen sensor of Comparative Example 2, in which the metal film made of pure silver was used as the catalyst layer, fluctuations in the output voltage were smaller than those of the oxygen sensors of Examples 1-3. On the other hand, an oxygen sensor that uses a metal film made of pure silver as a catalyst layer is susceptible to poisoning gases such as hydrogen sulfide, which limits the gas composition that can measure oxygen concentration. , the service life of which is shorter than that of an oxygen sensor using a metal film containing Au as a catalyst layer.

(Example 4)
A metal film made of Au with a thickness of about 60 nm was formed on the surface of a tetrafluoroethylene-hexafluoropropylene copolymer film as a diaphragm by an ion plating method. Next, an Ag film having a thickness of about 10 nm was laminated on the surface of the metal film by sputtering to prepare a laminated film composed of an Au layer and an Ag layer as a positive electrode catalyst layer. The content ratio of Au and Ag in the prepared catalyst layer was Au:Ag=86:14 in terms of molar ratio (atomic %).

An oxygen sensor was assembled in the same manner as in Example 1, except that the catalyst layer of the positive electrode was used and the side on which Ag was laminated was placed in contact with the electrolytic solution.

(Example 5)
A laminated film composed of an Au layer and an Ag layer was produced as a positive electrode catalyst layer in the same manner as in Example 4, except that the thickness of the Ag film laminated on the surface of the Au metal film was changed to about 30 nm. The content ratio of Au and Ag in the prepared catalyst layer was Au:Ag=67:33 in terms of molar ratio (atomic %).

(Comparative Example 3)
An oxygen sensor was assembled in the same manner as in Example 1, except that a metal film of Au having a thickness of about 60 nm formed by an ion plating method was used as it was as the catalyst layer of the positive electrode.

The oxygen sensors of Examples 4-5 and Comparative Example 3 were each held in nitrogen gas in an environment of 50°C, and the output voltage was measured during that time. The results are shown in FIG.

In the oxygen sensors of Examples 4 and 5, the output voltage decreased to near 0 V after the start of measurement, and then the output voltage increased slightly, but the output voltage fluctuated little, and the actual oxygen concentration (0%) and the measured value were different. It was found that the difference between the two was small, and it was possible to stably measure low oxygen concentrations with high accuracy.

On the other hand, in the oxygen sensor of Comparative Example 3, in which the metal film made of pure gold was used as the catalyst layer, the output voltage dropped to near 0 V after the start of measurement, as in the case of the oxygen sensor of Comparative Example 1. It increased greatly, and the divergence between the actual oxygen concentration and the measured value became large. Moreover, even after the output voltage dropped again, it was observed that the voltage was not stable and the value fluctuated.

The present application can be implemented in forms other than the above. The embodiments disclosed in this application are examples and are not limiting. The scope of the present application shall be interpreted with priority given to the descriptions in the attached claims rather than the descriptions in the above specification, and all changes within the scope of equivalents to the claims shall be included in the scope of the claims. It is something that can be done.

1 oxygen sensor 10 sealing lid 11 first sealing lid (inner lid)
12 Second sealing lid (outer lid)
20 container 21 container body 30 O-ring 40 protective film 50 positive electrode 51 catalyst layer (catalyst electrode)
51a surface 52 positive electrode current collector 53 lead wire 54 positive electrode terminal 60 diaphragm 70 perforation 80 negative electrode 81 lead portion 82 negative electrode terminal 90 electrolytic solution 100 correction resistor 110 temperature compensating thermistor 120 through hole

Claims

An electrochemical oxygen sensor comprising a positive electrode, a negative electrode, a diaphragm permeable to oxygen, and an electrolytic solution composed of an aqueous solution,
The negative electrode contains a metal that dissolves in the electrolytic solution,
The electrolytic solution is an aqueous solution having a pH of 3 to 10,
The positive electrode includes a catalyst layer containing Au,
One surface of the catalyst layer is in contact with the electrolytic solution,
The electrochemical oxygen sensor, wherein the catalyst layer has Ag or an alloy of Ag on at least the surface on the electrolyte side.
The electrochemical oxygen sensor according to claim 1, wherein the catalyst layer has an alloy of Au and Ag on at least the surface on the electrolyte side.
The diaphragm is arranged on the other surface side of the catalyst layer,
3. The electrochemical oxygen sensor according to claim 1, wherein the catalyst layer has Au or an Au alloy on at least the surface on the diaphragm side.
The electrochemical oxygen sensor according to any one of claims 1 to 3, wherein the catalyst layer is a laminate of a layer containing Ag or an alloy of Ag and a layer containing Au or an alloy of Au.
The electrochemical oxygen sensor according to any one of claims 1 to 4, wherein the catalyst layer has a higher Ag content on the electrolyte side surface than on the inside thereof.
The electrochemical oxygen sensor according to any one of claims 1 to 5, wherein the content of Ag in the total amount of metals in the catalyst layer is 1 atomic % or more.
The electrochemical oxygen sensor according to any one of claims 1 to 6, wherein the content of Au in the total amount of metal in the catalyst layer is 10 atomic % or more.
The electrochemical oxygen sensor according to any one of claims 1 to 7, wherein the electrolytic solution contains a chelating agent.
The electrochemical oxygen sensor according to any one of claims 1 to 8, wherein the negative electrode contains Sn or an alloy of Sn.
A method for manufacturing an electrochemical oxygen sensor according to any one of claims 1 to 7,
A first step of depositing Ag on the surface of a metal layer containing Au to form a catalyst layer;
A method of manufacturing an electrochemical oxygen sensor, comprising: a second step of assembling a positive electrode by arranging the Ag-adhered side of the catalyst layer on the electrolyte side.
The method for manufacturing an electrochemical oxygen sensor according to claim 10, wherein an electrolytic solution containing a chelating agent is used.
The method for manufacturing an electrochemical oxygen sensor according to claim 10 or 11, wherein a negative electrode containing Sn or an alloy of Sn is used.