Electrochemical oxygen sensor and method of manufacturing the same

Abstract

The electrochemical oxygen sensor of the present invention has a positive electrode, a negative electrode, and an electrolyte composed of an aqueous solution. The negative electrode has a metal containing element M selected from Sn and Ni as the main component. The electrolyte is an aqueous solution with a pH of 3 to 10. The positive electrode has a catalyst layer containing a catalyst metal. The catalyst layer has a surface layer containing element M on the surface in contact with the electrolyte.

Description

Technical Field

[0001] This invention relates to an electrochemical oxygen sensor and its manufacturing method that improves the reliability of oxygen measurement in gases with low oxygen concentrations. Background Technology

[0002] Electrochemical oxygen sensors (hereinafter also known as oxygen sensors) have the advantages of being inexpensive, simple, and able to operate at room temperature. Therefore, they are widely used in a wide range of fields, such as checking for oxygen deficiency in ship cabins and access ports, and detecting oxygen concentration in medical equipment such as anesthesia machines and ventilators.

[0003] As such electrochemical oxygen sensors, for example, there are sensors that have an aqueous electrolyte and use Sn or its alloys as the negative electrode (Patent Documents 1, 2, etc.).

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2006-194708

[0007] Patent Document 2: Japanese Patent Application Publication No. 2017-67596 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] In an electrochemical oxygen sensor, when oxygen enters the interior, it is reduced at the positive electrode and a metal dissolution reaction occurs at the negative electrode, thereby generating a current between the positive and negative electrodes corresponding 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 a voltage, and the oxygen concentration is determined based on this value. Thus, when the electrochemical oxygen sensor is placed in an oxygen-free environment, since the aforementioned positive electrode reaction does not occur, no current is generated, and it should be possible to measure an oxygen concentration of zero. However, in reality, even when placed in an oxygen-free environment, if observation is continued for a certain period, sometimes measurement results appear as if oxygen is present. Therefore, depending on the structure of the oxygen sensor, problems sometimes arise such as difficulty in measuring low concentrations of oxygen, resulting in a limited range for oxygen concentration measurement.

[0010] The present invention was made in view of the above circumstances, and its object is to provide an electrochemical oxygen sensor and a method for manufacturing the same, which improves the reliability of measurement in gases with low oxygen concentrations.

[0011] Methods for solving problems

[0012] The electrochemical oxygen sensor of the present invention comprises a positive electrode, a negative electrode, and an electrolyte composed of an aqueous solution. The negative electrode comprises a metal containing element M selected from Sn and Ni as the main component. The electrolyte is an aqueous solution with a pH of 3 to 10. The positive electrode comprises a catalyst layer containing a catalyst metal. The catalyst layer has a surface layer containing element M on the surface in contact with the electrolyte.

[0013] The electrochemical oxygen sensor of the present invention can be manufactured by the manufacturing method of the present invention, which includes a sensor assembly step and a surface layer formation step. In the sensor assembly step, the electrochemical oxygen sensor is assembled using a positive electrode containing a catalyst layer containing a catalyst metal, a negative electrode containing a metal containing an element M selected from Sn and Ni as the main component, and an electrolyte composed of an aqueous solution with pH 3 to 10. In the surface layer formation step, a surface layer containing the element M is formed on the surface of the positive electrode before or after the sensor assembly step.

[0014] Invention Effects

[0015] According to the present invention, an electrochemical oxygen sensor and a method thereof are provided that improve the reliability of measuring the oxygen concentration of gases with low oxygen content. Attached Figure Description

[0016] [ Figure 1 ] Figure 1 This is a graph showing the change in output voltage when using an electrochemical oxygen sensor with a Sn-containing negative electrode in an oxygen-free environment.

[0017] [ Figure 2 ] Figure 2 This is a cross-sectional view schematically illustrating an example of the electrochemical oxygen sensor of the present invention.

[0018] [ Figure 3 ] Figure 3 These are graphs showing the results of the performance evaluation tests of the electrochemical oxygen sensors of Examples 1-2 and Comparative Example 1. Detailed Implementation

[0019] like Figure 1 As shown, through the inventors' research, it has been found that when using an electrochemical oxygen sensor (hereinafter, sometimes simply referred to as "oxygen sensor") in an oxygen-free environment, a voltage rise is observed for a certain period of time, as if oxygen is present. However, upon further measurement, this voltage rise disappears. The inventors investigated the reaction occurring inside the oxygen sensor and concluded that the voltage rise may be due to the following reasons.

[0020] If oxygen is present at the positive electrode of the oxygen sensor, a reaction will occur as represented by the following equation (1).

[0021] O2+4H + +4e - →H2O (1)

[0022] However, when the oxygen sensor is placed in an oxygen-free environment, the reaction shown in equation (1) above does not occur at the positive electrode. On the other hand, when the positive and negative electrodes are connected without supplying oxygen to the positive electrode, element M dissolves from the negative electrode due to the potential difference, and current flows between the positive and negative electrodes. In the positive electrode, the reaction represented by equations (2) to (4) below may occur. It should be noted that equations (3) and (4) below represent the reaction that occurs at the positive electrode when the oxygen sensor has a negative electrode containing Sn.

[0023] 2H + +2e - →H2 (2)

[0024] Sn 4+ +4e - →Sn (3)

[0025] Sn 2+ +2e - →Sn (4)

[0026] As described above, when using an oxygen sensor in an oxygen-free environment, a voltage rise occurs after a certain period of time. On the other hand, after a certain period of time, the voltage rise becomes unreliable. During this period, it is assumed that the reaction for producing hydrogen (as in equation (2)) occurs at the positive electrode, but rather the reaction for Sn evolution (as in equation (3) or (4) occurs. When the potential of the positive electrode gradually decreases and becomes equal to the potential of the negative electrode, the current becomes zero, the Sn evolution at the positive electrode ends, and the voltage anomaly of the oxygen sensor no longer occurs. Based on the above insights, the inventors believe that by pre-forming a layer containing element M, which is present in the negative electrode, on the surface of the catalyst layer in contact with the electrolyte, the above-mentioned reaction at the positive electrode can be prevented, thereby improving the reliability of measuring gases with low oxygen concentrations.

[0027] In the oxygen sensor of the present invention, since a layer containing the same element M as that contained in the metal contained in the negative electrode (i.e., a metal whose main component is element M selected from Sn and Ni) is formed on the surface of the catalyst layer containing the catalyst metal in the positive electrode that is in contact with the electrolyte, it is possible to prevent the voltage rise caused by the deposition reaction of metal ions dissolved from the negative electrode at the positive electrode when using the oxygen sensor in an oxygen-free environment. Therefore, the oxygen sensor of the present invention can be used, for example, in gases with low oxygen concentrations, including in oxygen-free environments, and can measure oxygen concentration over a wide concentration range.

[0028] In particular, Sn diffuses easily even at low temperatures. Therefore, it is believed that when Sn is stacked on the surface of catalyst metals such as Au, it will be alloyed with the catalyst metal through mutual diffusion and form an Au-Sn alloy or other alloyed layer with the catalyst metal on the surface.

[0029] Furthermore, it is speculated that at least the surface portion of the catalyst layer in contact with the electrolyte is oxidized and exists in the form of oxides or the like.

[0030] Therefore, the "surface layer" formed on the surface of the catalyst layer in contact with the electrolyte, which contains the same elements as the main components of the metal contained in the negative electrode (Sn or Ni), includes not only the layer of the element in its elemental form, but also the layer of the alloy of the element (e.g., the layer of the catalyst metal and the alloy of the element) and the layer of the oxide of the element.

[0031] It should be noted that the terms "metals containing element M as a main component" and "metals with element M as the main component" in this specification refer to metals in which the content of element M is more than 50% by mass.

[0032] Next, taking a galvanic cell-type oxygen sensor as an example of a preferred embodiment, the oxygen sensor of the present invention will be described using the accompanying drawings.

[0033] Figure 2 This is a schematic cross-sectional view of a galvanic cell oxygen sensor, which is one embodiment of the electrochemical oxygen sensor of the present invention.

[0034] Figure 2 The oxygen sensor 1 shown has a positive electrode 50, a negative electrode 80, and an electrolyte 90 inside a bottomed cylindrical container 20. The container 20 consists of a container body 21 that holds the electrolyte 90 inside and a sealing cap 10 for fixing a protective film 40, a diaphragm 60, and the positive electrode 50 at the opening of the container body 21. The sealing cap 10 consists of a first sealing cap (middle cap) 11 and a second sealing cap (outer cap) 12 for fixing the first sealing cap 11, and has a through hole 120 for drawing oxygen into the oxygen sensor 1, and is mounted to the container body 21 via an O-ring 30.

[0035] Inside the container body 21 containing the electrolyte 90, a negative electrode 80 is disposed immersed in the electrolyte, and a lead portion 81 is formed on the negative electrode 80. The positive electrode 50 is constructed by stacking a catalyst layer (catalyst electrode) 51 and a positive current collector 52, and a lead 53 is mounted on the positive current collector 52. Furthermore, a through hole 70 is provided at the lower part of the container body 21 containing the electrolyte 90 in the container 20 for the lead 53 mounted on the positive current collector 52 to pass through. Additionally, although... Figure 2Not shown in the diagram, but in the lower part of the container body 21, unlike the perforation 70 described above, there is also a perforation for supplying electrolyte to the positive electrode 50.

[0036] A correction resistor 100 and a temperature compensation thermistor 110 are connected in series between the lead portion 81 of the negative electrode 80 and the lead 53 installed on the positive electrode current collector 52, and are housed inside the container body 21. In addition, a negative terminal 82 is connected to the lead portion 81 of the negative electrode 80, and a positive terminal 54 is connected to the lead 53 installed on the positive electrode current collector 52, and are respectively led out to the outside of the container body 21.

[0037] A separator 60 is disposed on the outer surface of the positive electrode 50, which selectively allows oxygen to pass through and limits the amount of oxygen passing through to a level commensurate with the battery reaction. Oxygen from the through-hole 120 provided in the sealing cap 10 is introduced into the positive electrode 50 through the separator 60. In addition, a protective film 40 is disposed on the outer surface of the separator 60 to prevent the adhesion of debris, dust, water, etc. to the separator 60, and is fixed by the first sealing cap 11.

[0038] That is, the first sealing cover 11 functions as a protective film 40, a diaphragm 60, and a pressing end plate for the positive electrode 50. Figure 2 In the oxygen sensor 1 shown, a threaded portion is formed on the inner periphery of the second sealing cap 12 so as to engage with the threaded portion on the outer periphery of the opening formed in the container body 21. Moreover, by tightening the sealing cap 10 by threads, the first sealing cap 11 is pressed against the container body 21 via the O-ring 30, thereby fixing the protective film 40, the diaphragm 60, and the positive electrode 50 to the container body 21 while maintaining airtightness and liquid tightness.

[0039] Furthermore, on the surface of the catalyst layer 51 of the positive electrode 50 that is in contact with the electrolyte ( Figure 2 The upper surface of the middle electrode has a surface layer 51a containing the same element M as the main component of the metal contained in the negative electrode.

[0040] The negative electrode of the oxygen sensor uses either elemental Sn or Ni, or an alloy of these elements (an alloy with either Sn or Ni as the main component (content of the alloy exceeding 50% by mass)). The elemental Sn or its alloy may contain a certain amount of impurities, but to comply with RoHS directives, a Pb content of less than 1000 ppm is preferred.

[0041] From the perspective of corrosion resistance, Sn alloys that can be used as negative electrodes include Sn-Ag alloys, Sn-Cu alloys, Sn-Ag-Cu alloys, Sn-Sb alloys, etc., but they can also be alloys containing metallic elements such as Al, Bi, Fe, Mg, Na, Zn, Ca, Ge, In, Ni, Co, etc.

[0042] As for Sn alloys, specifically, common lead-free solder materials include (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-Ge, Sn-1.0Ag-0.7Cu-Bi-In, Sn-0.3Ag-0.7Cu-0.5Bi-Ni, Sn-3.0Ag-0.5Cu, Sn-3.0Ag-0.5Cu, Sn-3.0Ag-0.5Cu, Sn-3.5Ag-0.7Cu-0.5Cu-Ni, Sn-3.0Ag-0.5Cu, Sn-3.5Ag-0.7Cu-0.5Cu, Sn-3.5Ag-0.7Cu-0.7Cu, Sn-3.5Ag ... g-3.0Bi-3.0In, Sn-3.9Ag-0.6Cu-3.0Sb, Sn-3.5Ag-0.5Bi-8.0In, Sn-5.0S b. Sn-10Sb, Sn-0.5Ag-6.0Cu, Sn-5.0Cu-0.15Ni, Sn-0.5Ag-4.0Cu, Sn-2.3A g-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 alloy.

[0043] From the viewpoint of corrosion resistance, examples of Ni alloys that can be used as negative electrodes include Ni-V alloys, Ni-Cr alloys, Ni-Si alloys, Ni-Al alloys, Ni-Ti alloys, Ni-Mo alloys, Ni-Mn alloys, Ni-Zn alloys, Ni-Sn alloys, Ni-Cu alloys, Ni-Co alloys, and Ni-Fe alloys. However, alloys containing metallic elements other than those mentioned above are also possible.

[0044] Specifically, Ni alloys such as Ni66-Cu29-Al3, Ni63-Cu30-Si4, Ni63-Cu30-Si3, Ni62-Mo28-Fe5, Ni68-Mo28-Fe2-Cr1-Co1, Ni57-Mo17-Cr16-Fe4-W3, and Ni22-Cr9-Mo0.6-W18.5-Fe1.5-Co0.6 are preferred as Ni alloys.

[0045] From the perspective of being less prone to corrosion in electrolytes composed of aqueous solutions with pH 3 to 10 and being able to suppress hydrogen generation in the positive electrode under anaerobic conditions, the negative electrode is preferably made of Sn or a Sn alloy, and more preferably of a Sn alloy.

[0046] The positive electrode of the oxygen sensor is, for example, Figure 2 As shown, a positive electrode is used, which consists of a catalyst layer containing a catalyst metal and a positive electrode current collector. The catalyst used as the constituent material of the catalyst layer is not particularly limited as long as it is a catalyst capable of generating current through electrochemical oxygen reduction at the positive electrode. Preferably, metals (including alloys of the above elements) that are active in redox reactions, such as gold (Au), silver (Ag), platinum (Pt), and titanium (Ti), are used, with Au being more preferred.

[0047] On the surface of the catalyst layer at the positive electrode that is in contact with the electrolyte, a surface layer containing the same element M as the main component of the metal contained in the negative electrode is formed. That is, if the negative electrode contains Sn (either the negative electrode is composed of Sn or an alloy with Sn as the main component), the surface layer of the positive electrode contains Sn; if the negative electrode contains Ni (either the negative electrode is composed of Ni or an alloy with Ni as the main component), the surface layer of the positive electrode contains Ni.

[0048] The surface layer may consist solely of an element (including unavoidable impurities) that is the same as element M, the main component of the metal contained in the negative electrode, or it may consist of an alloy whose main component is an element (M in the alloy exceeds 50% by mass). When the surface layer is composed of an alloy containing an element identical to element M, various metallic elements previously exemplified as elements that can be included when the negative electrode is composed of an alloy of element M can be listed as examples of such alloying elements.

[0049] In the surface layer, from the viewpoint of better ensuring the prevention of useless positive electrode reactions of the oxygen sensor in an oxygen-free environment, the amount of element M (the same element M contained in the negative electrode) per unit area is preferably 10 μg / cm². 2 More preferably, it is 50 μg / cm³. 2 That's all. Furthermore, to avoid hindering the action of the catalyst metal, the amount of element M per unit area in the surface layer is preferably 10 mg / cm². 2 The following is more preferably 4 mg / cm³ 2 the following.

[0050] The electrolyte for the oxygen sensor is an aqueous solution, such as an acidic aqueous solution containing acetic acid, potassium acetate and lead acetate, or an acidic aqueous solution containing citric acid and citrate (alkali metal salts, etc.); or an alkaline aqueous solution such as sodium hydroxide aqueous solution or potassium hydroxide aqueous solution.

[0051] In addition, to adjust the pH, the aqueous solution that constitutes the electrolyte can contain various organic acids, inorganic acids, and bases as needed.

[0052] It should be noted that, from the perspective of extending the lifespan of oxygen sensors, an aqueous solution containing a chelating agent is preferred as the electrolyte. It is speculated that the chelating agent has the function of chelating the constituent metal (element M) of the negative electrode, thereby dissolving it in the electrolyte (hereinafter referred to as "chelation effect"), and is therefore believed to contribute to extending the lifespan of the oxygen sensor.

[0053] The term "chelating agent" as used in this specification refers to a molecule (including ions) having multiple functional groups that coordinate with metal ions, forming a complex (chelation) with the metal ions and thus deactivating them. It can be contained in the electrolyte in the form of an acid or its salt that generates the aforementioned molecules in the solvent constituting the electrolyte. Therefore, substances with weak chelating forces, such as phosphoric acid, acetic acid, carbonic acid, and their salts, which have only a single functional group, are not included in the term "chelating agent" as used in this specification.

[0054] Sn dissolved from the negative electrode during discharge 2+ When the pH of the electrolyte is in the weakly acidic to weakly alkaline range, metal ions are unstable and immediately precipitate on the negative electrode in the form of oxides or hydroxides, potentially hindering the reaction at the negative electrode. However, in this invention, the dissolved metal ions are chelated and exist stably as complex ions, thus preventing the inhibition of the reaction at the negative electrode.

[0055] Chelating agents typically have chelating properties and pH buffering capacity (the ability to maintain a roughly constant pH in a solution even with the addition of small amounts of acid or alkali). Specific examples include succinic acid, fumaric acid, maleic acid, citric acid, tartaric acid, glutaric acid, adipic acid, malic acid, malonic acid, aspartic acid, glutamic acid, ascorbic acid, and their salts. One or more of these can be used.

[0056] From the viewpoint of improving chelation, chelating agents with high water solubility are preferred. Specifically, citric acid, tartaric acid, glutamic acid, and their salts are preferred. Citric acid or its salts have high water solubility (citric acid: 73g / 100ml (25℃), trisodium citrate: 71g / 100ml (25℃), tripotassium citrate: 167g / 100ml (25℃)). Furthermore, citric acid can dissociate from a large number of hydrogen atoms, resulting in multiple pH levels that can provide pH buffering capacity (pKa1 = 3.13, pKa2 = 4.75, pKa3 = 6.40). Therefore, citric acid is preferred. Accordingly, when using citric acid as a chelating agent, its high water solubility and increased pH buffering capacity further improve the lifespan of the oxygen sensor.

[0057] The concentration of the chelating agent in the electrolyte is preferably 2.3 mol / L or more, more preferably 2.5 mol / L or more, and particularly preferably 2.7 mol / L or more.

[0058] It should be noted that if the metal dissolved in the electrolyte from the negative electrode reaches a saturation concentration, oxides of the aforementioned metal will be formed, causing the negative electrode to deactivate. This may impair the lifespan of the oxygen sensor, and even with the presence of a chelating agent in the electrolyte, the lifespan extension effect of the oxygen sensor is sometimes limited. In such cases, it is preferable to also contain ammonia in the electrolyte to increase the molar concentration of the chelating agent. This delays the saturation of the metal dissolved in the electrolyte from the negative electrode, further extending the lifespan of the oxygen sensor.

[0059] To facilitate the aforementioned effects of ammonia generation, the concentration of ammonia in the electrolyte is 0.01 mol / L or higher. To further enhance these effects, a concentration of 0.1 mol / L or higher is preferred, and more preferably 1 mol / L or higher. Furthermore, while there is no specific upper limit for the concentration of ammonia in the electrolyte, since ammonia is a compound specified in Appendix 2 of Japan's "Law for the Control of Toxic and Highly Toxic Substances," from a safety perspective, the concentration of ammonia in the electrolyte is preferably set to be less than 10% by mass.

[0060] The pH of the aqueous solution constituting the electrolyte is 3 to 10. When the electrolyte has such a pH, in an oxygen sensor placed in an anaerobic environment, a reaction can easily occur where element M slightly dissolves from the negative electrode and precipitates at the positive electrode. However, as described above, the present invention can prevent the problem of voltage rise in the oxygen sensor caused by such precipitation reaction.

[0061] Preferably, a membrane for controlling oxygen intrusion is disposed on the outer surface of the positive electrode of the oxygen sensor, so that excessive oxygen reaches the catalyst electrode. As the membrane, a membrane that selectively allows oxygen to permeate and limits the amount of oxygen permeation is preferred. The material and thickness of the membrane are not particularly limited; fluoropolymers such as polytetrafluoroethylene (PTFE) and tetrafluoroethylene-hexafluoropropylene copolymer, and polyolefins such as polyethylene are commonly used. The membrane can be a porous membrane, a non-porous membrane, or a membrane with capillary pores, also known as a capillary membrane.

[0062] Furthermore, in order to protect the aforementioned diaphragm, it is preferable to place a protective membrane made of a porous resin membrane on the diaphragm. As long as the protective membrane has the function of preventing dust, dirt, water, etc. from adhering to the diaphragm and allowing air (including oxygen) to pass through, there are no particular restrictions on its material and thickness. Fluoropolymers such as polytetrafluoroethylene are commonly used.

[0063] The container body 21 of the oxygen sensor 1 can be made of acrylonitrile butadiene styrene (ABS) resin, for example. In addition, the sealing cap 10 (first sealing cap 11 and second sealing cap 12) disposed at the opening of the container body 21 can be made of ABS resin, polypropylene, polycarbonate, fluoropolymer, etc.

[0064] Furthermore, the O-ring 30 between the container body 21 of the container 20 and the sealing cap 10 (first sealing cap 11) is pressed and deformed by the threaded fastening between the container body 21 and the second sealing cap 12, thereby maintaining the airtightness and liquid tightness of the oxygen sensor 1. There are no particular restrictions on the material of the O-ring; nitrile rubber, silicone rubber, ethylene propylene rubber, fluoropolymer, etc., are commonly used.

[0065] Thus far, the present invention has been described using a galvanic cell oxygen sensor as one embodiment of the electrochemical oxygen sensor of the present invention. However, the electrochemical oxygen sensor of the present invention is not limited to the above embodiment, and various modifications can be made within the scope of its technical concept. Furthermore, regarding... Figure 2 The oxygen sensor shown can be modified in various ways as long as it has the function of an oxygen sensor and the oxygen supply path described above.

[0066] Furthermore, the electrochemical oxygen sensor of the present invention can also be in the form of a constant-potential oxygen sensor. A constant-potential oxygen sensor is a sensor in which a constant voltage is applied between the positive and negative electrodes. The applied voltage is set according to the electrochemical characteristics of each electrode and the type of gas being detected. In a constant-potential oxygen sensor, if an appropriate constant voltage is applied between the positive and negative electrodes, the current flowing between them is proportional to the oxygen concentration. Therefore, if the current is converted into a voltage, similar to a galvanic cell oxygen sensor, the oxygen concentration of an unknown gas can be detected by measuring the voltage.

[0067] The electrochemical oxygen sensor of the present invention can be manufactured by assembling a positive electrode with a pre-formed surface layer on the surface of a catalyst layer (catalyst electrode) by means of vapor deposition or plating, a negative electrode containing a metal with element M selected from Sn and Ni as the main component, and an electrolyte composed of an aqueous solution with pH 3 to 10.

[0068] Alternatively, an electrochemical oxygen sensor can be manufactured using a method comprising the following steps: a sensor assembly step using a positive electrode, a negative electrode containing a metal primarily composed of element M, and an electrolyte consisting of an aqueous solution with a pH of 3 to 10; and a step following the sensor assembly step, supplying the positive electrode with a gas containing an oxygen concentration of 0.3% by volume or less, or an oxygen-free gas, to the positive electrode for a certain period of time while the positive and negative electrodes are in a conductive state, causing element M to precipitate from the electrolyte on the surface of the catalyst layer of the positive electrode in contact with the electrolyte, thereby forming the surface layer. According to this manufacturing method, the step of pre-forming the surface layer on the positive electrode can be omitted, thus further improving the productivity of the electrochemical oxygen sensor. In this case, the time for precipitating element M on the surface of the main body of the positive electrode while the positive and negative electrodes are in a conductive state can be, for example, 50 to 150 hours.

[0069] The electrochemical oxygen sensor of the present invention is preferably used in applications in oxygen-free environments, but can also be used for the same applications as conventionally known electrochemical oxygen sensors.

[0070] Example

[0071] The present invention will now be described in detail based on embodiments. However, the following embodiments do not limit the present invention.

[0072] (Example 1)

[0073] <Preparation of Electrolyte>

[0074] An electrolyte was prepared by dissolving citric acid, tripotassium citrate, and ammonia in water. It should be noted that the molar concentrations of the electrolyte were set as follows: citric acid: 2.5 mol / L, tripotassium citrate: 0.5 mol / L, and ammonia: 3.0 mol / L. The pH of this electrolyte was 4.30 at 25°C.

[0075] <Assembly of the oxygen sensor>

[0076] 5.4g of the electrolyte was injected into the interior of the ABS resin container body 21, and then assembled. Figure 2 The structure shown is that of a galvanic cell-type oxygen sensor. The sealing caps 10 (first sealing cap 11 and second sealing cap 12) are also formed of ABS resin in the same way as the container body 21. In addition, the protective membrane 40 is made of porous polytetrafluoroethylene sheet, and the diaphragm 60 is made of tetrafluoroethylene-hexafluoropropylene copolymer membrane.

[0077] The catalyst layer (catalyst electrode) 51 of the positive electrode 50 is made of gold, and the positive electrode current collector 52 and the lead wire 53 are made of titanium. The positive electrode current collector 52 and the lead wire 53 are welded together as a single unit. In addition, the negative electrode 80 is made of Sn-Sb alloy (Sb content: 5% by mass).

[0078] In addition, a correction resistor 100 and a temperature compensation thermistor 110 are connected in series between the positive electrode lead 53 and the negative electrode lead 81. Then, the positive terminal 54 and the negative terminal 81 are led out from the positive electrode lead 53 and the negative electrode lead 81 to the outside of the container body 21, thereby enabling the oxygen concentration to be detected based on the output voltage.

[0079] In the assembled oxygen sensor 1, the first sealing cap 11, O-ring 30, protective film 40 made of polytetrafluoroethylene sheet, diaphragm 60 made of tetrafluoroethylene-hexafluoropropylene copolymer, catalyst electrode 51 and positive electrode current collector 52 are pressed together by the threaded fastening of the container body 21 and the second sealing cap 12 to maintain a good sealing state.

[0080] <Formation of Surface Layer>

[0081] With the positive electrode 50 and negative electrode 80 of the assembled oxygen sensor 1 connected, nitrogen gas (oxygen concentration of 0% by volume) is supplied to the surface of the positive electrode 50 through the through hole 120 and left for 100 hours. This process generates nitrogen gas on the surface of the catalyst layer 51 of the positive electrode 50 (the layer in contact with the electrolyte). Figure 2 A surface layer containing Sn, found in the negative electrode, is formed on the upper side of the surface. The amount of Sn per unit area in the formed surface layer is 150 μg / cm². 2 .

[0082] (Example 2)

[0083] A Sn layer (surface layer, with a Sn content of 150 μg / cm²) is formed on one side of the catalyst layer (catalyst electrode) at the positive electrode by a plating method. 2 Except for using the positive electrode, the oxygen sensor is assembled in the same manner as in Example 1.

[0084] (Comparative Example 1)

[0085] The oxygen sensor assembled in the same manner as in Example 1, without forming the surface layer described above, was used as the oxygen sensor of Comparative Example 1.

[0086] The oxygen sensors of Examples 1-2 and Comparative Example 1 were each kept in nitrogen atmosphere for 100 hours, and their output voltages were measured during this period. The results are shown below. Figure 3 .

[0087] In the oxygen sensors of Examples 1 and 2, the rise in output voltage after the start of measurement disappeared. In the oxygen sensor of Example 1, the output voltage was approximately zero several hours after the start of measurement. In the oxygen sensor of Example 2, the output voltage was approximately zero at the start of measurement, thus achieving a state where oxygen concentration could be stably measured. On the other hand, it can be seen that in the oxygen sensor of Comparative Example 1, the output voltage temporarily rose several hours after the start of measurement, and the variation in the measured oxygen concentration became larger.

[0088] This application can also be implemented in ways other than those described above without departing from its spirit. The embodiments disclosed in this application are examples and are not limited thereto. The scope of this application is preferably interpreted as being defined in the appended claims, compared to the description in the foregoing specification, and all modifications within the scope equivalent to the claims are included in the claims.

[0089] Symbol Explanation

[0090] 1: Oxygen sensor; 10: Sealing cap; 11: First sealing cap (middle cap); 12: Second sealing cap (outer cap); 20: Container; 21: Container body; 30: O-ring; 40: Protective film; 50: Positive electrode; 51: Catalyst layer (catalyst electrode); 51a: Surface layer; 52: Positive current collector; 53: Lead wire; 54: Positive terminal; 60: Diaphragm; 70: Perforation; 80: Negative electrode; 81: Lead wire section; 82: Negative terminal; 90: Electrolyte; 100: Calibration resistor; 110: Temperature compensation thermistor; 120: Through hole.

Claims

1. An electrochemical oxygen sensor, comprising a positive electrode, a negative electrode, and an electrolyte composed of an aqueous solution, wherein, The negative electrode contains a metal with element M selected from Sn and Ni as the main component, and the content of element M in the metal is more than 50% by mass. The electrolyte is an aqueous solution with a pH of 3 to 10. The positive electrode includes a catalyst layer containing a catalyst metal. The catalyst layer contains a surface layer containing element M on the surface that is in contact with the electrolyte.

2. The electrochemical oxygen sensor according to claim 1, wherein, The catalyst layer of the positive electrode contains gold.

3. The electrochemical oxygen sensor according to claim 1 or 2, wherein, The electrolyte contains a chelating agent.

4. The electrochemical oxygen sensor according to claim 1 or 2, wherein, The negative electrode contains Sn or an alloy of Sn.

5. The electrochemical oxygen sensor according to claim 4, wherein, The catalyst metal in the catalyst layer contains Au, and the surface layer contains Sn and Au.

6. The electrochemical oxygen sensor according to claim 1 or 2, wherein, The amount of element M contained in the surface layer per unit area is 10 μg / cm². 2 Above and 10mg / cm 2 the following.

7. The electrochemical oxygen sensor according to claim 1 or 2, wherein, The electrolyte is an acidic aqueous solution containing a carboxylic acid or its salt.

8. The electrochemical oxygen sensor according to claim 3, wherein, The chelating agent comprises one or more selected from citric acid, tartaric acid, glutamic acid and their salts.

9. The electrochemical oxygen sensor according to claim 3, wherein, The concentration of the chelating agent in the electrolyte is above 2.3 mol / L.

10. The electrochemical oxygen sensor according to claim 3, wherein, The electrolyte also contains ammonia.

11. The electrochemical oxygen sensor according to claim 10, wherein, The concentration of ammonia in the electrolyte is above 0.1 mol / L.

12. A method for manufacturing an electrochemical oxygen sensor, comprising the method of manufacturing the electrochemical oxygen sensor according to claim 1 or 2, including: The sensor assembly process involves assembling an electrochemical oxygen sensor using a positive electrode containing a catalyst layer with a catalyst metal, a negative electrode containing a metal with element M selected from Sn and Ni as the main component, and an electrolyte composed of an aqueous solution with a pH of 3–10. The surface layer forming process involves forming a surface layer containing element M on the surface of the positive electrode before or after the sensor assembly process.

13. The method for manufacturing an electrochemical oxygen sensor according to claim 12, wherein, The surface layer formation process includes the following steps: after the sensor assembly process, while the positive electrode and the negative electrode are conducting, a gas with an oxygen concentration of less than 0.3% by volume is supplied to the positive electrode for a certain period of time, thereby causing element M to be deposited from the electrolyte on the surface of the catalyst layer of the positive electrode in contact with the electrolyte to form the surface layer.

14. The method for manufacturing an electrochemical oxygen sensor according to claim 13, wherein, The gas supplied to the positive electrode does not contain oxygen.

15. The method for manufacturing an electrochemical oxygen sensor according to claim 12, wherein, The negative electrode contains Sn or an alloy of Sn.

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