JP2007205910A - Electrochemical oxygen sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrochemical oxygen sensor equipped with an inexpensive drive circuit which suppresses a deterioration of a cell part and prevents the flow of a transitional current to a power supply from a cell part at the time of cutting off-of a power supply in the constitution of the cell part and its drive circuit in the electrochemical oxygen sensor. <P>SOLUTION: In the electrochemical oxygen sensor constituted so that the cell part equipped with a positive electrode, a negative electrode, an electrolyte and an oxygen-permeable membrane and a sensor drive circuit are provided in a case, a diode is provided between the positive electrode power supply of the drive circuit and the input side of the positive electrode and the cathode of the diode is connected so as to come to the input side of the positive electrode. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an electrochemical oxygen sensor.

  Oxygen sensors are used in a wide range of fields, such as checking for oxygen deficiency in ship holds and manholes, and detecting oxygen concentrations in medical devices such as anesthesia machines and ventilators.

  Various types of oxygen sensors such as an electrochemical type, a magnetic type, and a zirconia type are used. Among these oxygen sensors, electrochemical sensors are widely used because they are inexpensive and easy to operate and operate at room temperature.

  As described in Non-Patent Document 1, the electrochemical oxygen sensor includes a galvanic type using lead for the anode, and a polarographic type (constant potential type) used by applying voltage from the outside using silver / silver chloride. ).

  A constant potential sensor, which is a type of electrochemical oxygen sensor disclosed in Patent Document 1, includes a positive electrode containing a metal effective for electrochemical reduction of oxygen inside a case, a negative electrode made of a metal such as zinc, A cell part consisting of a liquid is connected to a drive circuit for keeping the voltage between the positive electrode and the negative electrode constant, and the fact that there is a linear relationship between the current flowing between the positive electrode and the negative electrode and the oxygen gas concentration is utilized. Met.

  Electrochemical dissolved oxygen sensors have been widely used in fields such as environmental water treatment and aquaculture of rivers and seawater. Conventional electrochemical dissolved oxygen sensors measure the concentration of oxygen dissolved in water. As disclosed in Non-Patent Document 1 and Patent Document 2, there are galvanic and constant-potential types. It is known that the measurement principle and sensor characteristics of a dissolved oxygen sensor are equivalent to those of an oxygen gas sensor that measures the concentration of oxygen gas in a gas. Therefore, the “electrochemical oxygen sensor” in the present application includes a device for measuring oxygen gas in gas and a device for measuring dissolved oxygen in water.

FIG. 2 schematically shows the relationship between the potential and current of a conventional constant potential electrochemical oxygen sensor. In FIG. 2, the horizontal axis represents the current flowing between the positive electrode and the negative electrode, and the vertical axis represents the positive electrode potential with respect to the negative electrode potential (hereinafter simply referred to as “voltage”). In FIG. 2, I ₀ indicates a limit current value in 0% oxygen gas, I ₂₁ indicates a limit current value in 21% oxygen gas, and I ₁₀₀ indicates a limit current value in 100% oxygen gas. . The voltage is low areas and voltage E ₁ is higher than E ₂ region, the current varies greatly depending on the voltage, between the voltage of the E ₁ and E _2, the current value reaches passes through the oxygen permeable membrane to the positive electrode When the voltage is set to an appropriate value E ₀ between E ₁ and E ₂ in accordance with the amount of oxygen, that is, the oxygen concentration, the current is proportional to the oxygen concentration at that time, I ₀ , I ₂₁ , I ₁₀₀ It becomes.

Since the values of E ₁ and E ₂ vary depending on the measurement conditions such as the material of the positive electrode and the negative electrode, the type of the electrolyte, the area of the positive electrode, and the temperature, it is necessary to select an E ₀ value suitable for these conditions. There is.

  In the conventional electrochemical oxygen sensor drive circuit, the circuit shown in FIG. 3 is used in order to keep the positive electrode potential with respect to the negative electrode potential constant.

JP 2005-233835 A JP-A-06-222038 Electrochemical measurement method (above) P233-P235 (authors: Akira Fujishima, Masuo Aizawa, Toru Inoue, Gihodo Publishing, published in March 1996)

  The driving circuit of the electrochemical oxygen sensor shown in FIG. 3 is used, and a single power source (for example, +6 V and GND) is used even when both power sources (for example, +12 V, −12 V, and GND) are used as the power source of the driving circuit. In this case, since the St and # 2 terminals and the Sc and # 3 terminals are connected when the power supply of the drive circuit is cut off, a current flows from the cell portion to the power supply side of the drive circuit through the differential amplifier. Specifically, it is pulled so that the positive potential of the cell portion becomes 0 V with respect to the ground potential (GND) of the driving circuit, that is, the negative potential of the cell portion.

  In an electrochemical oxygen sensor using zinc as the negative electrode and an aqueous solution having a pH of 7 to 12 as the electrolytic solution, when the positive electrode potential of the cell portion becomes, for example, +0.1 V or less with respect to the negative electrode potential, electrolysis occurs in the cell, and the cell portion Deteriorated, the function of the sensor itself is lost, and as a result, the sensor life is shortened.

  Note that the potential that causes electrolysis when the drive circuit is turned off varies depending on the type of metal or electrolyte used for the negative electrode.

  In order to avoid this phenomenon, conventionally, when the power supply of the drive circuit is turned off, the connection between the cell portion and the drive circuit is expensive so that the positive potential of the cell portion does not become +0.1 V or less with respect to the negative potential. It is necessary to use a simple optical relay, and there is a problem that the drive circuit is complicated and expensive.

  Accordingly, an object of the present invention is to reduce the deterioration of the cell portion in the configuration of the cell portion and the drive circuit thereof in the electrochemical oxygen sensor, and from the cell portion when the power is turned off, which is necessary for maintaining the sensor life. An object of the present invention is to provide an electrochemical oxygen sensor having an inexpensive driving circuit for preventing a transient current flow to the power supply side.

  According to the first aspect of the present invention, there is provided an electrochemical oxygen sensor including a cell portion including a positive electrode, a negative electrode, an electrolyte, and an oxygen permeable membrane inside the case, and a sensor driving circuit. A diode is provided in between, and the cathode of the diode is connected so as to be on the positive electrode input side.

  According to the present invention, an inexpensive diode is used in place of the expensive optical relay used in the conventional driving circuit, thereby reducing the deterioration of the sensor portion in the configuration of the electrochemical oxygen sensor and its driving circuit. An electrochemical oxygen sensor equipped with a simple drive circuit can be obtained.

  The electrochemical oxygen sensor of the present invention has a diaphragm (hereinafter simply referred to as “membrane”) for selectively allowing the positive electrode, the negative electrode, the electrolyte, and oxygen to permeate inside the case and limiting the permeation amount to be oxygen gas diffusion rate-limiting. A cell portion having an oxygen permeable membrane) and a drive circuit, and a diode is provided between the positive power source and the positive input of the drive circuit so that the cathode of the diode is on the positive input side. It is characterized by being connected to.

  In the cell portion, an electrode containing gold, silver, platinum, etc. effective for electrochemical reduction of oxygen is used for the positive electrode, an electrode containing lead, zinc, aluminum, tin, etc. is used for the negative electrode, and the negative electrode metal is directly An electrolytic solution that does not cause dissolution is used. In order to eliminate adverse effects on the environment, it is preferable not to contain lead in the cell. As the cell part, for example, zinc can be used for the negative electrode, and an aqueous solution having a pH of 7 to 12 can be used for the electrolyte.

  In the electrochemical oxygen sensor of the present invention, the same structure as that of the conventional cell portion can be used. FIG. 4 shows a cross-sectional structure of the cell portion of the electrochemical oxygen sensor. In FIG. 4, 1 is an inner lid, 2 is an O-ring, 3 is an oxygen permeable membrane, 4 is a positive electrode, and 5 is a positive electrode current collector. Body, 6 is positive electrode lead wire, 7 is electrolyte solution, 8 is negative electrode, 9 is holder body, 10 is holder lid, 11 is perforation for supplying electrolyte solution, 12 is perforation for positive electrode lead wire, and 13 is holding positive electrode current collector , 14 is a negative electrode lead wire, and 15 is a protective film.

  The oxygen in the gas to be measured that has passed through the porous protective film 15 passes through the oxygen permeable film 3. Oxygen that has passed through the oxygen permeable membrane 3 is reduced at the positive electrode 4 and causes an electrochemical reaction with the negative electrode 8 through the electrolytic solution 7 in the electrolytic solution supply perforations 11.

  A drive circuit used for the electrochemical oxygen sensor of the present invention is shown in FIG. The drive circuit used in the present invention is different from the conventional drive circuit shown in FIG. 5 in the portion surrounded by the dotted line shown in FIG. In other words, the diode is connected so that the cathode of the diode is on the positive input side between the positive power supply (+ V in FIG. 1) and the positive input (P in FIG. 1) of the conventional drive circuit, and the optical relay is removed. is there.

  In the conventional driving circuit shown in FIG. 5, optical relays (RY1, RY2) are provided in the driving circuit. At the moment when the power of the drive circuit is turned off, the optical relay works to disconnect the electrical connection between the cell part and the drive circuit part, thereby preventing current flow from the cell part to the power supply side of the drive circuit. It was.

  On the other hand, in the drive circuit of the present invention shown in FIG. 1, by connecting the positive electrode power source and the positive electrode terminal so that the cathode of the diode is on the positive electrode input side, the rectifying action of the diode is used to Current is prevented from flowing to the power supply side of the drive circuit.

An electrochemical oxygen sensor that operates at a constant potential includes a cell portion and a drive circuit. For example, when zinc (Zn) is used for the negative electrode of the cell portion and an aqueous solution having a pH of 7 to 12 is used for the electrolytic solution, the following reaction occurs.
Positive electrode reaction: O ₂ + 2H ₂ O + 4e ⁻ → 4OH ⁻ (1)
Negative electrode reaction: 2Zn + 4OH ⁻ → 2ZnO + 2H ₂ O + 4e ⁻ (2)
Total reaction: O ₂ + 2Zn = 2ZnO (3)
It is also possible to form the cell portion by using aluminum for the negative electrode and combining with an appropriate electrolyte having a pH in the range of 3-9.

  The material of the oxygen permeable membrane used in the electrochemical oxygen sensor of the present invention can selectively permeate oxygen and can be limited so that the permeation amount is diffusion-limited for oxygen gas. For example, ethylene tetrafluoride A hexafluoropropylene copolymer film, a perfluoroalkoxy film, an ethylenetetrafluoroethylene copolymer film, or the like can be used.

  FIG. 1 shows an example of a drive circuit according to the present invention. In FIG. 1, IC1, IC2, and IC3 are all differential amplifiers, IC4 is a shunt regulator, and TH is a thermistor element for temperature compensation.

  Due to the characteristics of the differential amplifier / shunt regulator, the potential # 3 is maintained at the potential set by the shunt regulator IC4 and the resistors R8 and R9 with respect to the ground potential (GND) of the drive circuit. The potential of # 4 is the same as the ground potential (GND) of the drive circuit. Therefore, the positive electrode potential of the oxygen sensor is held at a constant value with respect to the negative electrode potential.

  On the other hand, all of the sensor current generated by the reduction of oxygen flows into the output of the differential amplifier IC2 through the thermistor element TH for temperature compensation. At this time, the voltage generated at both ends of the thermistor element TH is the differential amplifier IC3. Is amplified in accordance with the degree of amplification set by the resistors R2, R3, R4, and R5, output to the output terminal of the differential amplifier IC3, and taken out as the output of the drive circuit.

  With the above electric circuit operation, the positive electrode potential of the oxygen sensor is held at a constant value with respect to the negative electrode potential, and at the same time, a voltage proportional to the sensor current is output.

  In the electrochemical oxygen sensor of the present invention, a diode is provided between the positive power supply (+ V in FIG. 1) and the positive terminal (P in FIG. 1), and the cathode of the diode is connected to the positive input side. Therefore, when the power supply of the drive circuit is cut off, no current flows from the cell part to the power supply side of the drive circuit, so the positive electrode potential of the cell part of the oxygen sensor does not become +0.1 V or less with respect to the negative electrode potential, Deterioration of the cell portion of the oxygen sensor can be prevented.

[Example 1 and Comparative Example 1]
[Example 1]
The cross-sectional structure of the cell portion of the electrochemical oxygen sensor of Example 1 of the present invention is the same as that shown in FIG. 1 is an inner lid made of ABS resin, 2 is an O-ring made of neoprene rubber, and the oxygen permeable film 3 is made of a tetrafluoroethylene hexafluoropropylene copolymer film.

The positive electrode 4 made of gold is a catalyst electrode sputtered on the oxygen permeable membrane 3 made of a tetrafluoroethylene hexafluoropropylene copolymer film, and has an area of 25 mm ² . 5 is a positive electrode current collector made of carbon, 6 is a positive electrode lead wire made of titanium, 7 is a pH of 10.87 obtained by adding 7.46 g of potassium chloride to 100 ml of a 1.0 × 10 ⁻³ mol / l potassium hydroxide aqueous solution. (24.3 ° C.) electrolyte, 8 is a negative electrode made of zinc, 9 is a holder body made of ABS resin, and 10 is a holder lid made of ABS resin.

  The holder body 9 and the holder lid 10 are each threaded. The inner lid 1, the O-ring 2, the oxygen permeable membrane 3, the positive electrode 4, the positive electrode current collector 5, the holder main body 9 and the holder lid 10 are pressed by screwing, and a good contact state is maintained. The titanium positive electrode lead 6 is electrically connected to the positive electrode 4, and the titanium negative electrode lead 14 is electrically connected to the negative electrode 8.

  The inner lid 1 functions as a pressing end plate, the protective film 15 made of a porous fluororesin film prevents the surface of the oxygen permeable film 3 from being soiled, the oxygen permeable film 3 allows oxygen to permeate selectively and the permeation amount Is to limit the oxygen diffusion rate. The O-ring 2 ensures air tightness and liquid tightness.

  The drive circuit of the electrochemical oxygen sensor of Example 1 is the same as that shown in FIG. In Example 1, since zinc was used for the negative electrode of the cell portion, the potential between the positive electrode and the negative electrode was kept at about +0.1 to +0.4 V, and preferably kept at +0.25 V. When measuring the oxygen concentration, the measurement voltage may be an appropriate value between +0.1 V and +0.4 V, but if the value is close to +0.1 V or +0.4 V, the current value may deviate from the limit current value. Therefore, an intermediate voltage between +0.1 V and +0.4 V, which is as far as possible from +0.1 V and +0.4 V, in this case, +0.25 V is a preferable value.

  In the first embodiment, as indicated by D1 in FIG. 1, the general rectification is performed so that the cathode of the diode is on the positive input side between the positive power source (+ V in FIG. 1) and the positive terminal (P in FIG. 1). A diode was provided. In the drive circuit of FIG. 1, a power source in which four dry batteries are connected in series (+6 V input) was used.

[Comparative Example 1]
The driving circuit shown in FIG. 5 is used in place of the driving circuit shown in FIG. 1, and an optical relay is used for the connection between the sensor and the driving circuit. An electrochemical oxygen sensor was fabricated. As the power source of the drive circuit, a battery in which four dry batteries were connected in series (+6 V input) was used in the same manner as in Example 1.

  The electrochemical oxygen sensor of Comparative Example 1 was subjected to a life test under the same conditions as in Example 1. As a result, when the power is turned off, no current flows from the cell portion to the power supply side of the driving circuit, and the positive potential of the cell portion does not become 0.1 V or less with respect to the negative potential, so that the deterioration of the cell portion proceeds. As a result, it was found that a life as long as that of Example 1 was obtained. However, since an optical relay is used, there is a problem that the drive circuit is expensive and complicated.

[Comparative Example 2]
The drive circuit shown in FIG. 3 is used instead of the drive circuit shown in FIG. 1, and the electrochemical formula of Comparative Example 2 is the same as in Example 1 except that no diode is provided between the positive input and the positive power supply. An oxygen sensor was fabricated.

  FIG. 6 shows the behavior of the voltage between the positive and negative electrodes of the cell portion when the power supply to the drive circuit is cut off when the drive circuit of the electrochemical oxygen sensor is Example 1 and Comparative Example 2. Note that in all cases, a supply power source in which four dry batteries were connected in series (+6 V input) was used.

  In FIG. 6, the behavior of the voltage between the positive and negative electrodes when the drive circuit is Example 1 is shown by a solid line, and the behavior of the voltage between the positive and negative electrodes when the drive circuit is Comparative Example 2 is shown by a dotted line. In FIG. 6, at time A, the power supply of each sensor drive circuit was cut off. As a result, in the sensor having the driving circuit of Comparative Example 2, the voltage between the positive and negative electrodes temporarily decreased to about 0.05V. On the other hand, when the drive circuit was Example 1, the voltage between the positive and negative electrodes of the cell portion did not change and was not affected by the substrate.

  When power is supplied to the drive circuit again after the supply power of the drive circuit is turned off for each sensor, in Comparative Example 2 connected to the conventional drive circuit, the recovery to the output before the power supply is turned off slowly and the output value is not stable. However, the sensor of Example 1 connected to the drive circuit according to the present invention has a short recovery time and shows stable characteristics.

  This is because, in the sensor connected to the drive circuit according to the present invention, the positive electrode potential does not become 0.1 V or less with respect to the negative electrode potential, so that the cell portion does not deteriorate and stable sensor characteristics can be obtained even when remeasured. It shows that it can be demonstrated.

[Example 2]
Implemented except that 50 ml of 0.1 mol / l potassium dihydrogen phosphate aqueous solution was added to 4 ml of 0.1 mol / l sodium hydroxide aqueous solution and diluted to 100 ml as an electrolytic solution. A cell portion was produced in the same manner as in Example 1, and an electrochemical oxygen sensor of Example 2 was produced using the same drive circuit as shown in FIG.

  Then, in the same manner as in Example 1, the behavior of the voltage between the positive and negative electrodes in the cell portion when the power supply to the drive circuit was cut was measured. As a result, the voltage between the positive and negative electrodes in the cell portion did not change and was not affected by the substrate. Further, when power was supplied to the drive circuit again after the power supply to the drive circuit was cut off, the time required for recovery was short and stable characteristics were exhibited.

  From the results of Examples 1 and 2, by providing a diode between the positive power supply and the positive input of the drive circuit, an electrochemical oxygen sensor exhibiting excellent life characteristics without deterioration of the cell portion, a conventional optical relay Compared with the case where it uses, it was able to obtain cheaply.

The figure which shows the drive circuit of the electrochemical type oxygen sensor of this invention. The schematic diagram which shows the relationship between the electric potential of a conventional constant potential type electrochemical oxygen sensor, and an electric current. The figure which shows the drive circuit which removed the optical relay from the conventional drive circuit. Diagram showing the cross-sectional structure of an electrochemical oxygen sensor The figure which shows the drive circuit of the electrochemical type oxygen sensor using the conventional optical relay. The figure which shows the behavior of the voltage between positive / negative of a cell part when the power supply to a drive circuit is cut | disconnected about Example 1 and Comparative Example 2. FIG.

Explanation of symbols

3 Porous membrane 4 Positive electrode 7 Electrolyte 8 Negative electrode IC1, IC2, IC3 Differential amplifier IC4, IC5 Shunt regulator TH Temperature compensation thermistor element RY1, RY2 Optical relay

Claims (1)

In an electrochemical oxygen sensor including a cell portion including a positive electrode, a negative electrode, an electrolyte, and an oxygen permeable film inside the case, and a sensor driving circuit, a diode is provided between a positive power source and a positive input of the driving circuit, An electrochemical oxygen sensor characterized in that the cathode of the diode is connected to the positive electrode input side.

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