Electrochemical oxygen sensor

Abstract

To provide galvanic cell type oxygen sensor with which it is possible to measure oxygen concentrations over a wide concentration range and maintain high measurement accuracy for a long period.SOLUTION: An electrochemical oxygen sensor pertaining to the present application includes a cathode and an anode and an electrolytic solution, includes a diaphragm for limiting the amount of oxygen supplied to the cathode, and includes a resistive element for connecting the cathode and the anode. The resistance value of the resistive element is set so that, in an atmosphere of 1 atm at 25°C and of relative humidity of 50%, the current value flowing in the resistive element is 4 μA or greater and the output voltage across the resistive element is in the range from 4 to 9.5 mV.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present application relates to an electrochemical oxygen sensor capable of maintaining high measurement accuracy for a long period of time. [Background technology]

[0002] Electrochemical 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.

[0003] As such an electrochemical oxygen sensor, for example, a galvanic cell type oxygen sensor that has an aqueous electrolyte and uses a base metal such as Pb, Zn, Sn, or an alloy thereof for the negative electrode is known (Patent Documents 1 to 3, etc.). In this galvanic cell type oxygen sensor, generally, a voltage is output across both ends of a resistor (fixed resistor, temperature compensation thermistor, etc.) connected between the positive and negative electrodes, and the oxygen concentration is measured by converting the output voltage into the oxygen concentration.

[0004] In the above-mentioned galvanic cell type oxygen sensor, the oxygen concentration and the current value flowing through the resistor are in a proportional relationship, and an output voltage corresponding to the oxygen concentration is obtained, so that the change in the oxygen concentration can be accurately measured in the initial stage of measurement. On the other hand, it has been pointed out that when measurements are continued for a long period of time, the slope of the line showing the relationship between the oxygen concentration and the output voltage fluctuates due to deterioration over time, resulting in a deviation between the actual oxygen concentration and the oxygen concentration obtained by converting the output voltage of the oxygen sensor, resulting in a problem of reduced measurement accuracy of the oxygen sensor (Patent Document 4). The above problem becomes greater as the oxygen concentration increases, so in order to configure a galvanic cell type oxygen sensor that can measure high concentrations of oxygen and maintain high measurement accuracy for a long period of time, it is necessary to solve the above problem. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2006-194708 A [Patent Document 2] JP 2016-6412 A [Patent Document 3] JP 2018-109549 A [Patent Document 4] Japanese Patent Application Publication No. 135761 / 1983 Summary of the Invention [Problem to be solved by the invention]

[0006] In the galvanic cell oxygen sensor of Patent Document 4, the fluctuation of the output voltage can be absorbed by changing the value of the fixed resistor connected between the positive and negative electrodes in response to the fluctuation of the ambient temperature. On the other hand, regarding the deterioration over time, the ratio of the output voltage V0 at the atmospheric oxygen concentration of 20.6(%) in the initial product to the output voltage V1 at the dangerous oxygen concentration of 18.0(%) before the deterioration over time is the same as the ratio of the output voltage V0' at the atmospheric oxygen concentration of 20.6(%) in the deteriorated product from the second time onwards to the output voltage V1' at the dangerous oxygen concentration of 18.0(%), so it is shown that the oxygen concentration in the air can be accurately detected by checking the ratio of the output voltage value measured initially to the output voltage value measured thereafter each time a measurement is performed.

[0007] However, the technology of Patent Document 4 does not suppress changes in the output voltage of the oxygen sensor over time, and further study is required to configure an oxygen sensor that can measure oxygen concentrations over a wide concentration range and maintain high measurement accuracy for a long period of time.

[0008] The present application has been made in consideration of the above circumstances, and an object of the present application is to provide a galvanic cell type oxygen sensor that can measure oxygen concentration over a wide concentration range and maintain high measurement accuracy for a long period of time. [Means for solving the problem]

[0009] The electrochemical oxygen sensor of the present application includes a positive electrode, a negative electrode, and an electrolyte, and is equipped with a diaphragm that limits the amount of oxygen supplied to the positive electrode, and a resistive element that connects the positive electrode and the negative electrode, and the resistance value of the resistive element is set so that, in an atmosphere at 25°C, 1 atmosphere, and a relative humidity of 50%, the current flowing through the resistive element is 4 μA or more, and the output voltage across the resistive element is in the range of 4 to 9.5 mV. Effect of the Invention

[0010] According to the present application, it is possible to provide a galvanic cell type oxygen sensor that is capable of measuring oxygen concentration over a wide concentration range and maintaining high measurement accuracy for a long period of time. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view that illustrates a galvanic cell type oxygen sensor, which is an example of the electrochemical oxygen sensor of the present application. [Diagram 2] FIG. 2 is a graph showing the change in the retention rate of the output voltage in an accelerated life test. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The electrochemical oxygen sensor of the present application includes a positive electrode, a negative electrode, and an electrolyte, and is equipped with a diaphragm that limits the amount of oxygen supplied to the positive electrode, and a resistive element that connects the positive electrode and the negative electrode, and the resistance value of the resistive element is set so that, in an atmosphere at 25°C, 1 atmosphere, and a relative humidity of 50%, the current flowing through the resistive element is 4 μA or more, and the output voltage across the resistive element is in the range of 4 to 9.5 mV.

[0013] In general, in order to increase the response speed by increasing the current flow to a certain extent in an electrochemical oxygen sensor, the thickness of the diaphragm (described later) is usually made thin to increase the amount of oxygen permeated per unit time. This increases the response speed, but the increased oxygen permeation rate increases the reaction rate of the positive electrode and also increases the dissolution rate of the negative electrode, resulting in a short life. On the other hand, if the diaphragm is made thicker, the amount of oxygen permeation decreases, resulting in a slower response speed but a longer life.

[0014] Therefore, in the electrochemical oxygen sensor of the present application, the current flow is increased to 4 μA or more, more preferably 6 μA or more to increase the response speed, and in order to prevent the resulting shortening of the lifespan, the resistance value of the built-in resistor is controlled so that the output voltage across the resistor element is in the range of 4 to 9.5 mV. This makes it possible to provide an oxygen sensor that can measure oxygen concentrations over a wide concentration range and maintain high measurement accuracy for a long period of time. This can contribute, for example, to Goal 9 "Build resilient infrastructure, promote innovation and innovate," and Goal 12 "Responsible consumption and production" of the Sustainable Development Goals (SDGs) proposed by the United Nations.

[0015] Next, taking a galvanic cell type oxygen sensor (hereinafter simply referred to as an oxygen sensor), which is one example of a preferred embodiment of the electrochemical oxygen sensor of the present application, as an example, we will explain why controlling the resistance value of the built-in resistor to a certain value or less can prevent the sensor from having a short lifespan.

[0016] A membrane is disposed on the top of the positive electrode of the oxygen sensor of the present application, which selectively allows oxygen to pass through and limits the amount of oxygen passing through to match the cell reaction. The oxygen that passes through the membrane and enters the inside of the oxygen sensor is reduced by the catalytic electrode of the positive electrode, and the following electrochemical reaction occurs between the electrode and the negative electrode via the electrolyte. The following electrochemical reaction formula represents the case where the negative electrode is made of Sn or a Sn alloy.

[0017] Positive electrode reaction: O2+4H + +4e - →2H2O Negative electrode reaction: Sn+2H2O→SnO2+4H + +4e -

[0018] Here, the amount of oxygen flowing into the positive electrode is limited by the diaphragm, and therefore a saturation current characteristic appears in the current-voltage characteristic of the oxygen sensor. That is, a steady state is reached in which all of the oxygen that has flowed into the positive electrode is consumed by the above-mentioned positive electrode reaction. Such a saturation current is called a limiting current, and the magnitude of this limiting current is determined by the oxygen concentration in the outside world. In addition, in the oxygen sensor, a built-in resistor is disposed between the positive and negative electrodes in order to measure the output voltage according to the limiting current. Therefore, a proportional relationship is established between the output voltage of the oxygen sensor when this limiting current is flowing and the oxygen concentration in the outside world, and the oxygen concentration in the outside world can be measured by measuring the output voltage of the oxygen sensor when the limiting current is flowing.

[0019] However, if the resistance value of the built-in resistor arranged in the oxygen sensor is made too large, only a current smaller than the limiting current will flow, and the proportional relationship between the output voltage of the oxygen sensor and the oxygen concentration in the outside world will be lost, making it impossible to measure the oxygen concentration in the outside world. Furthermore, the inventors of the present application have found that the current-voltage characteristics of the oxygen sensor change due to deterioration of the oxygen sensor over time, narrowing the range of resistance values ​​of the built-in resistor that can be used to establish a proportional relationship between the output voltage of the oxygen sensor and the oxygen concentration in the outside world toward lower resistance values. This also reduces the output voltage in the limiting current region.

[0020] In order to solve the above problem, it is advantageous to set the resistance value of the built-in resistor to a certain value or less to make the output voltage to a certain value or less. Therefore, in this embodiment, when the oxygen sensor is placed in an atmosphere of 25°C, 1 atm, and 50% relative humidity, the resistance value of the built-in resistor, i.e., the resistance element, is set so that the output voltage at both ends is 9.5 mV or less. The resistance value is preferably set so that the output voltage is 8 mV or less, and more preferably set so that the output voltage is 7 mV or less. To achieve this, the resistance value of the resistance element is set to 2375 Ω or less, preferably 1500 Ω or less, more preferably 1050 Ω or less, and particularly preferably 600 Ω or less.

[0021] On the other hand, if the resistance value of the built-in resistor is too small, the output voltage of the oxygen sensor becomes too small, which is undesirable since it increases the voltage measurement error. Therefore, in the present application, the resistance value of the resistive element is set so that the output voltage of the oxygen sensor in the above atmosphere is 4 mV or more. The resistance value is preferably set to, for example, 200 Ω or more, and more preferably set to 350 Ω or more.

[0022] By using a resistive element having a resistance value that satisfies the above conditions, it is possible to realize an oxygen sensor with a long life by absorbing changes in the current-voltage characteristics of the oxygen sensor due to deterioration over time while maintaining the proportional relationship between the output voltage of the oxygen sensor and the oxygen concentration in the outside world.

[0023] Furthermore, if the diaphragm is too thin, the current value will be too large, resulting in a short service life, and therefore the diaphragm thickness is preferably 8 μm or more. Furthermore, when the oxygen sensor is placed in an atmosphere at 25° C., 1 atmospheric pressure, and 50% relative humidity, it is more preferable that the current value is adjusted to 10 μA or less, and particularly preferably to 7 μA or less, and for example, the diaphragm thickness is more preferably 15 μm or more, and particularly preferably 30 μm or more. Furthermore, if the diaphragm is too thick, the current value will be small and the response speed will be slow, so although this depends on the material, the diaphragm thickness is preferably, for example, 70 μm or less, and more preferably 60 μm or less.

[0024] The galvanic cell type oxygen sensor of this embodiment will be described below with reference to the drawings. Fig. 1 is a cross-sectional view that typically illustrates the galvanic cell type oxygen sensor of this embodiment.

[0025] 1 has a positive electrode 50, a negative electrode 100, and an electrolyte 110 in a bottomed cylindrical holder 20. The holder lid 10, which is made up of a first holder lid (inner lid) 11 and a second holder lid (outer lid) 12 for fixing the first holder lid 11 and has a through hole 150 for taking in oxygen into the oxygen sensor 1, is attached to the upper opening of the holder 20 via an O-ring 30.

[0026] In a tank that contains an electrolyte 110 in the holder 20, the negative electrode 100 is disposed in a state of being immersed in the electrolyte. A lead wire 120 is attached to the negative electrode 100, and a fixed resistor (compensation resistor) 130 and a temperature compensation thermistor 140 are connected in series to the lead wire 120 outside the holder 20 to form a resistance element. The total resistance value of the resistance element is set to, for example, 1500Ω or less. The positive electrode 50 is configured by laminating a catalytic electrode 51 and a positive electrode collector 52, and the positive electrode collector 52 is also attached with a lead wire 120. The positive electrode 50 is disposed in the upper part of the electrolyte tank via a positive electrode collector holding part 70. In addition, the positive electrode current collector holding portion 70 is provided with a perforation 80 for supplying the electrolyte 110 from the electrolyte storage tank to the positive electrode 50, and a perforation 90 for passing a lead wire 120 attached to the positive electrode current collector 52.

[0027] A diaphragm 60 that selectively allows oxygen to pass through and limits the amount of oxygen passing therethrough to match the battery reaction is disposed above the positive electrode 50, and oxygen from a through-hole 150 provided in the holder lid 10 is introduced into the positive electrode 50 through the diaphragm 60. In addition, a protective film 40 that prevents dirt, dust, water, and the like from adhering to the diaphragm 60 is disposed above the diaphragm 60 and is fixed by a first holder lid 11.

[0028] That is, the first holder 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, the second holder lid 12 has a threaded portion formed on its inner periphery so as to screw into a threaded portion formed on the outer periphery of the holder 20. Then, by screwing the holder lid 10, the first holder lid 11 is pressed against the holder 20 via the O-ring 30, so that the protective film 40, the diaphragm 60, and the positive electrode 50 can be fixed to the holder 20 while maintaining airtightness and liquid tightness.

[0029] Next, the operating principle of a galvanic cell type oxygen sensor having an electrolyte solution containing a chelating agent will be described with reference to FIG.

[0030] Oxygen that has passed through the diaphragm 60 and entered the oxygen sensor 1 is reduced by the catalytic electrode 51 of the positive electrode 50, and the following electrochemical reaction occurs between the negative electrode 100 and the oxygen sensor 100 via the electrolyte 110.

[0031] Positive electrode reaction: O2+4H + +4e - →2H2O Negative electrode reaction: Sn+2H2O→SnO2+4H + +4e - Y X- +SnO2+4H + →YSn 4-x +2H2O Here, Y represents a chelating agent (citric acid).

[0032] The negative electrode 100 can be made of metals such as Cu, Fe, Ag, Ti, Al, Mg, Zn, Ni, and Sn, or alloys thereof, but Sn or an Sn alloy is preferably used because it is less corroded in an acidic electrolyte and is compatible with the Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment (RoHS) Directive on the restriction of the use of certain hazardous substances in the EU (European Union). Therefore, the above electrochemical reaction formula represents the case where the negative electrode is made of Sn or an Sn alloy.

[0033] This electrochemical reaction generates a current corresponding to the oxygen concentration between the catalytic electrode 51 and the negative electrode 100. The current generated by the positive electrode reaction at the catalytic electrode 51 is collected by the positive electrode current collector 52 pressed against the catalytic electrode 51, led to the outside by the lead wire 120, and flows to the negative electrode 100 through the fixed resistor (correction resistor) 130 and the temperature compensation thermistor 140. This converts the current into a voltage signal, and a voltage is obtained as the oxygen sensor output. The obtained output voltage is then converted into an oxygen concentration by a well-known method and detected as the oxygen concentration.

[0034] Here, the chelating agent, citric acid (Y), becomes citrate ions in the electrolyte and has the effect of chelating the constituent metals of the negative electrode and dissolving them in the electrolyte (hereinafter referred to as "chelating effect"). However, if a metal (e.g., Sn) from the negative electrode dissolves in the electrolyte and reaches a saturation concentration, an oxide of the metal is generated and the negative electrode becomes inactive, which may shorten the life of the oxygen sensor.

[0035] Therefore, we decided to increase the amount of tin that can be dissolved in the electrolyte, that is, by increasing the molar concentration of citric acid (Y) in the electrolyte, thereby delaying the time when the tin dissolved from the negative electrode in the electrolyte reaches a saturated concentration, thereby improving the lifespan of the oxygen sensor.

[0036] The citric acid used in the electrolyte of the oxygen sensor of this embodiment has multiple functional groups that form coordinate bonds with metal ions and inactivate the metal ions by forming complexes (complexation) with the metal ions. Citric acid itself or its salts (in this specification, citric acid and citrate salts are collectively referred to as citric acids) can be contained in the electrolyte in the solvent that constitutes the electrolyte.

[0037] In the oxygen sensor of this embodiment, it is preferable to use an aqueous solution in which at least citric acids are dissolved as the electrolyte. Here, it is preferable that the aqueous solution contains an alkali metal, and is prepared so that the total content of citric acids is 2.1 mol / L or more, the content of alkali metal is 0.1 to 1.6 times the total content of citric acids, and the pH is 3.9 to 4.6. The solvent of the electrolyte is water. With such an electrolyte, the molar concentration of citrate ions can be increased, and the life of the oxygen sensor can be improved.

[0038] Chelating agents such as citric acid generally have a chelating effect and a pH buffering ability (the ability to keep the pH of the solution almost constant even when a small amount of acid or base is added), but when an acid or its salt that produces a chelating effect in an aqueous solution is dissolved in water alone, the pH of the aqueous solution is determined mainly by the type and concentration of the chelating agent. For this reason, depending on the type of chelating agent used, the pH of the aqueous solution may fall into a range that promotes galvanic corrosion of the negative electrode material, making it difficult to use it as an electrolyte for a sensor.

[0039] Therefore, in order to adjust the pH of the electrolyte to a suitable range while maintaining an excellent pH buffering ability, it has been proposed to use a mixed solution containing an acid that serves as a chelating agent and its salt. However, when citric acid is used as a chelating agent, even if the total content of citric acid and its salt (i.e., citric acids) is increased and the pH of the electrolyte is adjusted to a suitable range, it does not necessarily lead to an improvement in life. Furthermore, it has become clear that it is important to have a specific content of alkali metal (most of which is presumed to be ionized and present in the form of alkali metal ions) in the electrolyte containing citric acid by a method such as dissolving an alkali metal salt, for example, an alkali metal salt of an organic acid, preferably an alkali metal salt of citric acid.

[0040] That is, although the reason is not clear, it has been found that when the total content of citric acids dissolved in the electrolyte is 2.1 mol / L or more, the content of alkali metals contained in the electrolyte is 0.1 to 1.6 times the total content of citric acids, and the pH of the electrolyte is adjusted to the range of 3.9 to 4.6, the action of citric acid (including ionized citric acid) as a chelating agent can be maximized, and a longer life for the oxygen sensor can be achieved.

[0041] The electrolyte solution having the above-mentioned structure can be prepared by dissolving citric acid and an alkali metal salt, for example, citric acid and an alkali metal salt of an organic acid, preferably citric acid and an alkali metal salt of citric acid, in water as a solvent. The alkali metal salt of citric acid can be trialkali metal salt of citrate, dialkali metal salt of hydrogen citrate, alkali metal salt of dihydrogen citrate, etc., specifically, lithium salt, sodium salt, potassium salt, rubidium salt, cesium salt, etc., and preferably can be trisodium citrate, tripotassium citrate, disodium hydrogen citrate, sodium dihydrogen citrate, dipotassium hydrogen citrate, potassium dihydrogen citrate, etc.

[0042] For example, a mixed solution in which citric acid and tripotassium citrate are dissolved in water at a ratio of 1.2 mol / L and 1.0 mol / L, respectively, has a total dissolved citric acid content of 2.2 mol / L, an alkali metal (potassium) content derived from the tripotassium citrate of 1.0 x 3 = 3.0 mol / L, i.e., the alkali metal content is 3.0 / 2.2 = 1.36 times the total citric acid content, and the electrolyte has a pH of 4.23 at 25°C.

[0043] In addition, the electrolyte of the above configuration can be prepared using an alkali metal salt of an organic acid other than citric acid. For example, an alkali metal salt (including an acid salt) of a monocarboxylic acid or a polycarboxylic acid such as acetic acid, formic acid, oxalic acid, succinic acid, fumaric acid, maleic acid, tartaric acid, glutaric acid, adipic acid, malic acid, malonic acid, aspartic acid, glutamic acid, or ascorbic acid may be used, and sodium acetate, potassium acetate, sodium hydrogen oxalate, potassium hydrogen oxalate, disodium oxalate, dipotassium oxalate, sodium hydrogen tartrate, potassium hydrogen tartrate, ammonium potassium tartrate, disodium tartrate, or dipotassium tartrate is preferably used. Since the polycarboxylic acid also acts as a chelating agent, the addition of the polycarboxylic acid or its salt is expected to delay the tin reaching a saturation concentration in the electrolyte and improve the life of the oxygen sensor.

[0044] For example, a mixed solution in which citric acid and potassium acetate are dissolved in water at ratios of 2.5 mol / L and 1.0 mol / L, respectively, has a total dissolved citric acid content of 2.5 mol / L and an alkali metal (potassium) content derived from potassium acetate of 1.0 mol / L, i.e., an electrolyte in which the alkali metal content is 1.0 / 2.5 = 0.4 times the total citric acid content.

[0045] In addition, in order to more appropriately adjust the pH of the mixed solution of citric acid and an alkali metal salt of an organic acid, a pH adjuster may be added to the electrolyte. Examples of the pH adjuster include organic acids and their salts, inorganic acids and their salts, ammonia, hydroxides, etc. In the case of the mixed solution of citric acid and potassium acetate, the pH at 25°C can be adjusted to 4.32 by adding ammonia at a content of 3.0 mol / L.

[0046] Examples of organic acids that can be used as pH adjusters include monocarboxylic acids and polycarboxylic acids such as acetic acid, formic acid, oxalic acid, succinic acid, fumaric acid, maleic acid, tartaric acid, glutaric acid, adipic acid, malic acid, malonic acid, aspartic acid, and glutamic acid, as well as ascorbic acid. Examples of organic acid salts include ammonium salts (including acid salts) of the above organic acids, such as ammonium acetate, diammonium tartrate, and ammonium hydrogen tartrate, as well as salts of citric acid other than alkali metal salts, such as diammonium hydrogen citrate and triammonium citrate. When the above-mentioned salts of citric acid are added, their content is added to the total content of citric acids.

[0047] Examples of inorganic acids that can be used as pH adjusters include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, and carbonic acid, and examples of salts of inorganic acids include alkali metal salts and ammonium salts (each including acid salts) of the above inorganic acids, such as ammonium chloride, sodium hydrogen sulfate, potassium hydrogen sulfate, ammonium sulfate, trisodium phosphate, tripotassium phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate, ammonium carbonate, and ammonium bicarbonate. Note that when an alkali metal salt of the above inorganic acid is added, the alkali metal contained in the compound is added as the "content of alkali metal contained in the electrolyte."

[0048] In addition, since ammonia is volatile, taking into consideration changes in the composition of the electrolyte due to volatilization, it is preferable to keep the total content of ammonia in the electrolyte, including that derived from ammonia water and ammonium salts, below a certain level, and the molar ratio of the total content of ammonia to the total content of citric acids in the electrolyte is preferably 1.1 or less, more preferably 0.5 or less, and the electrolyte may not contain ammonia.

[0049] Examples of hydroxides that serve as pH adjusters include hydroxides of alkali metals such as sodium hydroxide and potassium hydroxide. When the hydroxides of the alkali metals are added, the alkali metals contained in the compounds are added to the "content of the alkali metals contained in the electrolyte."

[0050] The electrolyte used in this embodiment can be prepared by appropriately selecting the types and amount ratio of citric acids and alkali metal salts, and adding a pH adjuster as necessary.

[0051] The electrolyte used in this embodiment has an alkali metal content of 0.1 to 1.6 times the total content of citric acids, and a pH adjusted to the range of 3.9 to 4.6. Under the above conditions, the chelating effect of the electrolyte can be maintained for a longer period of time as the total content of citric acids increases. Therefore, in order to extend the life of the oxygen sensor, the total content of citric acids in the electrolyte is preferably 2.4 mol / L or more, and more preferably 2.7 mol / L or more.

[0052] In addition, although the reason is not clear, even if the total content and pH of citric acids are the same, a solution with a higher citric acid content can maintain the chelating effect of the electrolyte for a longer period of time, so the citric acid content in the electrolyte is preferably 1.1 mol / L or more, more preferably 1.7 mol / L or more, and particularly preferably 2.0 mol / L or more.

[0053] Similarly, although the reason is not clear, when the content of alkali metals in the electrolyte is less than 0.1 times the total content of citric acids, or exceeds 1.6 times, the life of the oxygen sensor cannot be extended beyond a certain level. In order to increase the ionic conductivity of the electrolyte, it is preferable that the content of alkali metals in the electrolyte is 0.45 times or more the total content of citric acids. If the content of alkali metals in the electrolyte is less than 0.1 times the total content of citric acids, it becomes difficult to increase the ionic conductivity of the electrolyte in the above pH range, and the operation of the oxygen sensor may become unstable.

[0054] In addition, in order to make the most of the characteristics of the electrolyte, the oxygen sensor of this embodiment is preferably configured so that the amount of electrolyte relative to the mass of tin, which is the reactant of the negative electrode, is at least a certain amount. That is, when the amount of electrolyte inside the oxygen sensor is x (ml) and the amount of tin contained in the negative electrode is y (g), it is preferable to adjust the amount of electrolyte so that x / y is 0.3 (ml / g) or more. If x / y is less than 0.3 (ml / g), the pH of the electrolyte changes quickly when the oxygen sensor is used, and the characteristics of the electrolyte are not utilized, which may result in insufficient improvement in the life of the oxygen sensor.

[0055] In order to suppress pH changes in the electrolyte during use of the oxygen sensor, the value of x / y is preferably 0.7 (ml / g) or more, and more preferably 1 (ml / g) or more. On the other hand, in order to reduce the storage volume of the electrolyte and make the volume of the oxygen sensor as small as possible, the value of x / y is preferably 10 (ml / g) or less, more preferably 6.5 (ml / g) or less, and particularly preferably 3 (ml / g) or less.

[0056] The negative electrode of the oxygen sensor of this embodiment uses Sn or an alloy of Sn, but it is preferable to use an Sn alloy in order to suppress the reaction with the electrolyte and prevent the generation of hydrogen. Examples of the Sn alloy include an Sn-Ag alloy, an Sn-Cu alloy, an Sn-Ag-Cu alloy, and an Sn-Sb alloy, but it may also be an alloy containing a metal element such as Al, Bi, Fe, Mg, Na, Zn, Ca, Ge, In, Ni, or Co.

[0057] Furthermore, Sn or Sn alloys may contain a certain amount of impurities, but in order to comply with the RoHS Directive, it is desirable for the Pb content to be less than 1000 ppm.

[0058] Specifically, the Sn alloys include common 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-Ge, Sn-1.0Ag-0.7Cu-Bi-In, Sn-0.3Ag-0.7Cu-0.5Bi-Ni, Sn-3.0Ag-3.0Bi Sn-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.) or Sn-Sb alloys can be preferably used.

[0059] The positive electrode of the oxygen sensor of this embodiment is, for example, one that is composed of a catalytic electrode and a positive electrode current collector, as shown in Fig. 1. The material of the catalytic electrode is not particularly limited as 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.

[0060] In addition, the above-mentioned diaphragm is disposed 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 permeate and can limit the amount of oxygen gas that permeates. There is no particular restriction on the material of the diaphragm, but typically, fluororesins such as tetrafluoroethylene resin and tetrafluoroethylene-hexafluoropropylene copolymer; polyolefins such as polyethylene; and the like are used. As the diaphragm, a porous membrane, a non-porous membrane, or even a membrane having holes in which capillaries are formed, which is called a capillary type, can be used.

[0061] As described above, the thickness of the diaphragm is preferably 8 to 40 μm.

[0062] Furthermore, in order to protect the diaphragm, it is preferable to place a protective film made of a porous resin film on the diaphragm. The protective film is not particularly limited in material or thickness as long as it can prevent adhesion of dirt, dust, water, etc. to the diaphragm and has a function of permeating air (including oxygen), but usually a fluororesin such as polytetrafluoroethylene resin is used.

[0063] 1, the holder 20 which is the exterior body of the oxygen sensor 1 can be made of, for example, ABS resin. Furthermore, the holder lid 10 (first holder lid 11 and second holder lid 12) arranged at the opening of the holder 20 can be made of, for example, ABS resin, polypropylene, polycarbonate, fluororesin, etc. Furthermore, within the holder 20, the positive electrode collector holding part 70 for holding the positive electrode 50 can be made of, for example, ABS resin.

[0064] Furthermore, the O-ring 30 interposed between the holder 20 and the holder lid 10 (first holder lid 11) is pressed and deformed by screwing the holder 20 and the second holder 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.

[0065] Although the galvanic cell type oxygen sensor of this embodiment has been described as an example, the oxygen sensor of the present application is not limited to the above embodiment, and various modifications are possible within the scope of the technical concept. In addition, the oxygen sensor shown in Fig. 1 can also be modified in various ways in design as long as it has the function of an oxygen sensor and the above-mentioned oxygen supply path. EXAMPLES

[0066] The oxygen sensor of the present invention will be described below based on examples, but the oxygen sensor of the present invention is not limited to the following examples.

[0067] Example 1 <Preparation of electrolyte> 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 total content of citric acids dissolved in this electrolyte solution was 3.0 mol / L, the content of alkali metals (potassium): 1.5 mol / L was 0.5 times the total content of citric acids, and the pH of the electrolyte solution was 4.30 at 25°C. The molar ratio of the total content of ammonia to the total content of citric acids in the electrolyte solution was 1.

[0068] <Assembling the oxygen sensor> Using 4.3 ml of the above electrolyte, a galvanic cell type oxygen sensor having the configuration shown in Fig. 1 was assembled. The holder lid 10 (first holder lid 11 and second holder lid 12), holder 20 and positive electrode current collector holding part 70 were made of ABS resin. A porous tetrafluoroethylene resin sheet was used for the protective film 40, and a tetrafluoroethylene-hexafluoropropylene copolymer film having a thickness of 25 µm was used for the diaphragm 60.

[0069] The catalytic electrode 51 of the positive electrode 50 was made of gold, and the positive electrode current collector 52 and the lead wire 120 were made of titanium, and the positive electrode current collector 52 and the lead wire 120 were welded together. The negative electrode 100 was made of 3.7 g of Sn-Sb alloy (Sb content was 5 mass %, and the mass of Sn was 3.52 g), and the lead wire 120 was also attached to the negative electrode.

[0070] In addition, a temperature compensation thermistor 140 having a resistance value of 500 Ω at 25°C and a fixed resistor (compensation resistor) 130 of 39 Ω were connected in series to form a resistance element (combined resistance at 25°C: 539 Ω), and the above resistance element was connected to the positive and negative lead wires 120, 120, respectively.

[0071] In the obtained oxygen sensor 1, the first holder lid 11, the O-ring 30, the protective film 40 made of a tetrafluoroethylene resin sheet, the diaphragm 60 made of a tetrafluoroethylene-hexafluoropropylene copolymer film, the catalytic electrode 51, and the positive electrode current collector 52 were pressed by screwing the holder 20 and the second holder lid 12 together, and were kept in good contact with each other. The first holder lid 11 functioned as a pressing end plate, and the O-ring 30 ensured airtightness and liquid tightness. The ratio of the amount of electrolyte (4.3 ml) contained in the electrolyte to the mass of Sn (3.52 g) contained in the negative electrode was 1.22 (ml / g).

[0072] Example 2 A galvanic cell-type oxygen sensor was assembled in the same manner as in Example 1, except that the diaphragm was changed to a tetrafluoroethylene-hexafluoropropylene copolymer film having a thickness of 50 μm, and the resistance element was changed to a resistance element (combined resistance at 25° C.: 1082 Ω) configured by connecting in series a thermistor for temperature compensation having a resistance value of 1 kΩ at 25° C. and a fixed resistor (compensation resistor) of 82 Ω.

[0073] Comparative Example 1 A galvanic cell type oxygen sensor was assembled in the same manner as in Example 2, except that the diaphragm was changed to a tetrafluoroethylene-hexafluoropropylene copolymer film having a thickness of 12 μm.

[0074] Comparative Example 2 A galvanic cell type oxygen sensor was assembled in the same manner as in Example 2, except that the resistance element was changed to a resistance element (composite resistance at 25°C: 10220Ω) constructed by connecting in series a temperature compensation thermistor having a resistance value of 10 kΩ at 25°C and a fixed resistor (compensation resistor) of 220Ω.

[0075] Comparative Example 3 A galvanic cell type oxygen sensor was assembled in the same manner as in Comparative Example 1, except that the resistance element was changed to a resistance element (composite resistance at 25°C: 539Ω) constructed by connecting in series a temperature compensation thermistor having a resistance value of 500Ω at 25°C and a fixed resistor (compensation resistor) of 39Ω.

[0076] Comparative Example 4 A galvanic cell type oxygen sensor was assembled in the same manner as in Example 1, except that the resistance element was changed to a resistance element (combined resistance at 25°C: 973Ω) constructed by connecting in series a temperature compensation thermistor having a resistance value of 900Ω at 25°C and a fixed resistor (compensation resistor) of 73Ω.

[0077] Each oxygen sensor of the Example and Comparative Example was placed in an atmosphere of 25° C., 1 atm, and 50% relative humidity, and the voltage across each resistor element was measured to examine the current flowing through each resistor element. The results are shown in Table 1.

[0078] [Table 1]

[0079] Next, an accelerated life test was performed on each oxygen sensor of the example and comparative example by passing 100% oxygen gas in an atmosphere at a temperature of 40°C. At 40°C, the electrochemical reaction proceeds at about twice the speed at room temperature. Also, when passing 100% oxygen gas, the electrochemical reaction proceeds at about five times the speed in the atmosphere. Therefore, when passing 100% oxygen gas at a temperature of 40°C, the life can be judged at about 10 times the speed when left in the atmosphere at room temperature. In this test, the output voltage of each oxygen sensor was measured, and the change in output voltage (retention rate) is shown in FIG. 2, with the initial voltage set to 100%. In FIG. 2, the horizontal axis shows the measurement period converted by multiplying the measurement time by 10.

[0080] It can be determined that the longer the period during which a maintenance rate close to 100% (for example, a maintenance rate of 90% or more) can be maintained, the longer the measurement accuracy can be maintained. From the results shown in Fig. 2, it can be seen that the galvanic cell-type oxygen sensors of Examples 1 and 2 of the present application are capable of maintaining high measurement accuracy for a long period of time, compared to the oxygen sensors of Comparative Examples 1 to 4.

[0081] In particular, the galvanic cell-type oxygen sensor of Example 2, in which the thickness of the diaphragm is set so that the current value is 10 μA or less, is able to maintain high measurement accuracy for a longer period of time than the galvanic cell-type oxygen sensor of Example 1, in which the current value is greater than 10 μA. It can be seen that, in the oxygen sensor of the present application, by setting the thickness of the diaphragm so that the current value is smaller, it is possible to extend the period over which high measurement accuracy can be maintained. [Industrial Applicability]

[0082] The electrochemical oxygen sensor of the present invention can be used in the same applications as conventionally known electrochemical oxygen sensors. [Explanation of symbols]

[0083] 1 Galvanic cell type oxygen sensor 10 Holder lid 11 First holder cover (inner cover) 12 Second holder cover (outer cover) 20 Holder 30 O-ring 40 Protective film 50 positive electrode 51 Catalyst electrode 52 Positive electrode current collector 60 Diaphragm 70 Positive electrode current collector holding part 80 Drilling for electrolyte supply 90 Drilling for lead wires 100 negative electrode 110 Electrolyte 120 Lead Wire 130 Fixed resistor (compensating resistor) 140 Temperature compensation thermistor 150 Through hole

Claims

1. An electrochemical oxygen sensor including a positive electrode, a negative electrode, and an electrolyte, a diaphragm for limiting the amount of oxygen supplied to the positive electrode; A resistor element is provided to connect the positive electrode and the negative electrode, The current value flowing through the resistance element is 4 μA or more in the atmosphere at 25° C., 1 atmosphere, and 50% relative humidity; An electrochemical oxygen sensor, characterized in that the resistance value of the resistive element is set so that the output voltage across both ends of the resistive element is in the range of 4 to 9.5 mV.

2. 2. The electrochemical oxygen sensor according to claim 1, wherein the resistance element is configured by connecting a fixed resistor and a thermistor element in series.

3. 3. The electrochemical oxygen sensor according to claim 1, wherein a value of a current flowing through said resistance element is 10 μA or less.

4. 4. The electrochemical oxygen sensor according to claim 1, wherein the resistance element has a resistance value of 1500 Ω or less.

5. 5. The electrochemical oxygen sensor according to claim 1, wherein the diaphragm has a thickness of 8 μm or more.

6. 6. The electrochemical oxygen sensor according to claim 1, wherein the diaphragm is made of a fluororesin.

7. 7. The electrochemical oxygen sensor according to claim 1, wherein the electrolyte is an aqueous solution containing a chelating agent.

8. 8. The electrochemical oxygen sensor according to claim 7, wherein the electrolyte contains citric acid or a salt thereof.

9. 9. The electrochemical oxygen sensor according to claim 7, wherein the content of the chelating agent in the electrolyte is 2.0 mol / L or more.

10. 10. The electrochemical oxygen sensor according to claim 1, wherein, when the amount of the electrolyte is x (ml) and the amount of tin contained in the negative electrode is y (g), x / y≧0.3 (ml / g).

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

Images