JP5897372B2 - Electrochemical gas sensor, method for selectively detecting CO and H2, and method for modifying electrochemical gas sensor - Google Patents

Description

  The present invention relates to an electrochemical gas sensor, and more particularly to identifying and detecting CO and H2.

  The inventors have proposed diagnosing an electrochemical gas sensor by complex impedance (Patent Document 1 Japanese Patent Laid-Open No. 2011-158468). In an electrochemical gas sensor, a detection electrode and a counter electrode are provided on both surfaces of a separator or a solid electrolyte membrane that holds a liquid electrolyte. And in patent document 1, the complex impedance between a detection electrode and a counter electrode is measured. When condensation or the like occurs in these members and the gas diffusibility is lowered, the real part of the complex impedance is lowered and can be detected.

  Patent Document 1 intends to diagnose the state of an electrochemical gas sensor by measuring impedance, and does not intend to positively reform the electrochemical gas sensor. The inventor studied changing the gas sensitivity by applying a direct current or a direct voltage to the electrochemical gas sensor, and reached the present invention.

JP2011-158468

  An object of the present invention is to generate outputs in opposite directions from CO and H2 from an electrochemical gas sensor.

The present invention relates to an electrochemical gas sensor in which a detection electrode is provided on a detected atmosphere side of a separator or a solid polymer proton conductor film holding a liquid electrolyte, and a counter electrode is provided on a reference atmosphere side. Is characterized in that the current flows from the counter electrode to the detection electrode and the current flows in the CO from the detection electrode to the counter electrode.

  The present invention also provides a separator or solid electrolyte membrane for holding a liquid electrolyte, in which a detection electrode is provided on the detected atmosphere side and a counter electrode is provided on the reference atmosphere side, the counter electrode side being + and the detection electrode side being-. Using an electrochemical gas sensor that has been modified by applying a direct current to the sensing electrode, H2 is detected when current flows from the counter electrode to the sensing electrode in the electrochemical gas sensor, and the current flows from the sensing electrode to the counter electrode. There is a selective detection method of CO and H2, which detects CO when flowing.

The present invention also provides a separator or solid polymer proton conductor film for holding a liquid electrolyte, and the counter electrode side of the electrochemical gas sensor having a detection electrode on the detected atmosphere side and a counter electrode on the reference atmosphere side. By adding a direct current from the counter electrode to the detection electrode with + as the detection electrode side, current flows in the electrochemical gas sensor from the counter electrode to the detection electrode in H2, and from CO to the detection electrode in CO. Thus, there is a method for reforming an electrochemical gas sensor that reforms the electrochemical gas sensor so that a current flows.

  The inventor makes the detection electrode on the detected atmosphere side of the separator holding the liquid electrolyte and the counter electrode on the reference atmosphere side, and the counter electrode side is +, the detection electrode side is-, and the counter electrode is from the counter electrode. It was found that the gas sensitivity changes when a direct current is applied to the detection electrode. That is, the gas sensitivity was changed so that current flowed from the counter electrode to the detection electrode in the electrochemical gas sensor in H2, and current flowed from the detection electrode to the counter electrode in CO. The change in gas sensitivity was more significant as the DC current from the counter electrode to the detection electrode was larger, or as the voltage applied with the counter electrode as + and the detection electrode as-was larger. Applying a direct current to an electrochemical gas sensor (hereinafter sometimes simply referred to as a “gas sensor”) includes not only applying a direct current literally, but also applying a direct current by applying a direct current voltage. Further, the applied current may be a direct current superimposed on a direct current in addition to a simple direct current. The time and number of times the DC current is applied are arbitrary, and the sensitivity change of the gas sensor is affected by the number and time and the magnitude of the DC current or the magnitude of the DC voltage.

  In order for the current to flow from the counter electrode to the detection electrode in the electrochemical gas sensor in H2, a mechanism for H2 to wrap around to the counter electrode side is necessary. The assembly of the separator or solid electrolyte membrane holding the liquid electrolyte and the sensing electrode and the counter electrode is called MEA. Around the MEA, there is a gasket or O-ring around the MEA to shut off the counter electrode from the detected atmosphere. There is a gap between the gasket or O-ring. It is conceivable that H2 diffuses in the gasket or O-ring, or H2 diffuses in the gap between the MEA and the gasket or O-ring. There is also a possibility that H2 diffuses in the MEA itself.

When H2 is goes around to the side of the counter electrode, H2 → 2H ^{+ +} 2e ^- reaction occurs in both the sensing electrode and counter electrode, the current flowing through the MEA is determined by the magnitude of the rate of reaction between the sensing electrode and the counter electrode. The direct current applied from the counter electrode to the detection electrode changes the ionization activity of hydrogen on the detection electrode or the counter electrode, the state of ion conduction between the detection electrode and the spacer, or the state of ion conduction between the counter electrode and the spacer. Assuming that this has happened, it can be explained that the current flows in the electrochemical gas sensor from the counter electrode to the detection electrode in H2. Even if a direct current is applied from the counter electrode to the detection electrode, a current flows from the detection electrode to the counter electrode in CO, and the current value in the CO increases compared to that before the direct current is applied.

  As a result, the current flowing through the gas sensor or the direction of electromotive force is reversed between H2 and CO, and H2 and CO can be discriminated. For example, it can be discriminated whether there is leakage from the hydrogen pipeline or incomplete combustion around it. Also, in a fuel cell system equipped with a converter that reforms CH4 into CO2 and H2, H2 is the main component in the gas produced by the converter, and CO is a harmful impurity. Here, if the signal of the gas sensor is reversed between H2 and CO, the CO concentration in the product gas can be easily detected.

  The inventor discovered this phenomenon with MEA in which a liquid electrolyte was held in a separator, but the same phenomenon occurred with MEA using a solid polymer proton conductor membrane. Direct current may be applied as a current or a voltage. The magnitude of DC current or DC voltage applied, time, number of repetitions, etc. are considered to depend on the material and size of the MEA. Once the MEA is determined, the optimum value can be easily found. The output of the electrochemical gas sensor may be current or electromotive force. In this specification, the description about the electrochemical gas sensor is also applied to the selective detection method of CO and H2 and the modification method of the electrochemical gas sensor, and the description about the modification method of the electrochemical gas sensor is directly changed to CO and H2. This also applies to the selective detection method and the electrochemical gas sensor.

Sectional view of the electrochemical gas sensor of the example The figure which shows the decomposition | disassembly state of MEA in an Example The figure which shows the gas detection apparatus in an Example Characteristic diagram showing response to CO before DC voltage is applied Characteristic diagram showing response to H2 before DC voltage application Characteristic diagram showing the response to CO after applying DC voltage of -2 to + 2V for 100msec Characteristic diagram showing the response to CO after applying a DC voltage of -2 to +2 V for 100 msec x 5 times Characteristic chart showing the response to CO after applying DC voltage of -2 to + 2V for 100msec x 10 times Characteristic diagram showing the response to H2 after applying DC voltage of -2 to + 2V for 100msec Characteristic chart showing the response to H2 after applying DC voltage of -2 to + 2V 100msec x 5 times Characteristic diagram showing the response to H2 after applying a DC voltage of -2 to + 2V 100msec x 10 times -Characteristic diagram showing the response to CO after applying DC current of -500mA to + 500mA for 100msec -Characteristic chart showing the response to CO after applying -500mA to + 500mA direct current 100msec x 5 times Characteristic diagram showing the response to CO after applying -500mA to + 500mA DC current 100msec x 10 times -Characteristics showing response to H2 after applying DC current of -500mA to + 500mA for 100msec -Characteristics showing response to H2 after applying DC current of -500mA to + 500mA for 100msec x 5 times -Characteristics showing response to H2 after applying DC current of -500mA to + 500mA for 100msec x 10 times Current and voltage waveforms in air when a + 300mA x 100msec DC current is applied Waveform diagram of current and voltage in air when a DC current of −300 mA x 100 msec is applied Characteristic diagram showing the change in sensitivity to CO300ppm due to DC voltage (applied in units of 100msec) Characteristic diagram showing sensitivity change to CO300ppm by DC voltage (1sec applied) Characteristic chart showing sensitivity change to 300ppm of H2 by DC voltage (applied in 100msec unit) Characteristic diagram showing the change in sensitivity to 300ppm of H2 due to DC voltage (1sec applied) Characteristic diagram showing change in sensitivity to CO300ppm due to DC current (applied in units of 100msec) Characteristic diagram showing sensitivity change to CO300ppm by DC current (1sec applied) Characteristic diagram showing sensitivity change to 300ppm of H2 by DC current (applied in 100msec unit) Characteristic diagram showing change in sensitivity to H2 300ppm by DC current (1sec applied)

  In the following, an optimum embodiment for carrying out the present invention will be shown. Note that the embodiments do not limit the present invention, and the embodiments can be changed in consideration of known techniques and the like.

  Examples are shown in FIGS. 1 and 2 show the structure of an electrochemical sensor 2 (hereinafter referred to as “gas sensor 2”), 4 is a metal container, containing water 6 and 8 is an MEA. 10 is a metal bottom plate, 12 is a metal diffusion control plate, 16 is a metal cap, and 11, 14, 18 and 19 are openings. Of these, the opening 14 limits the diffusion of the detected atmosphere to the MEA 8. The cap 16 accommodates a filter material 20 such as activated carbon, silica gel, or zeolite, and 22 is a ring-shaped gasket that hermetically insulates the cap 16 and the metal container 4. There is a gap between the outer periphery of the MEA 8 and the inner periphery of the gasket 22. The structure and material of the gas sensor 2 are known, water 6 is not necessary, and the attachment structure of the MEA 8 is arbitrary.

  As shown in FIG. 2, the MEA 8 includes a membrane-like microporous separator 24 holding a liquid electrolyte, detection electrodes S and counter electrodes C on both sides thereof, and gas diffusion films 25 and 26 such as carbon sheets. In the embodiment, the liquid electrolyte is a polymer of aromatic sulfonic acid, but it is optional such as sulfuric acid, KOH aqueous solution, K2CO3 aqueous solution and the like. The gas diffusion films 25 and 26 may not be provided, and a solid polymer proton conductor film, a solid polymer anion conductor film, or the like may be provided between the electrodes S and C and the separator 24. The detection electrode S and the counter electrode C are obtained by adding proton conductive polymer to carbon black supporting Pt, and the material is arbitrary.

  The gas sensor 2 increases the CO sensitivity by applying a direct current from the counter electrode to the detection electrode, and reverses the direction of the H2 sensitivity, that is, the direction of the current or electromotive force flowing in the H2. The effect of this processing is shown in FIGS.

  FIG. 3 shows a gas detection device. Reference numeral 30 denotes a DC power source such as a battery, which takes out a constant potential such as 1 V by resistors R1, R2, etc., and keeps the potential of the counter electrode C of the gas sensor 2 at a constant potential. An amplification circuit A1 is connected to the detection pole S side, and the detection current flowing through the gas sensor 2 is amplified by CO, H2, etc., and converted into a voltage signal. And determine its concentration. The counter electrode C may be connected to the amplifier circuit A1 and the arithmetic logic circuit 32 side.

  4 to 27 show the influence of the direct current voltage and direct current on the gas sensor 2. In the experiment, three sensors were used for each condition, but the waveforms of FIGS. 4 to 19 show the signals of one sensor each. Further, the polarity of the DC voltage and the DC current applied with + on the counter electrode C side and-on the detection electrode S side is shown, the numbers such as 300 in the figure show the gas concentration, and the vertical axis shows the output of the amplifier circuit A1. FIG. 4 shows the response to CO before DC is applied, and FIG. 5 shows the response to H2 before DC is applied. These show the variations of the gas sensor 2. FIG. 6 shows the response to CO after applying a DC voltage of −2 V to +2 V 100 msec × 1 time, and the influence of the DC voltage is small. Fig. 7 shows the response to CO after applying a DC voltage of -2V to + 2V for 100msec x 5 times. When a DC voltage of + is applied, the CO sensitivity slightly increases or decreases slightly, and -DC CO sensitivity decreases when voltage is applied. FIG. 8 shows the response to CO after applying a DC voltage of −2 V to +2 V for 100 msec × 10 times. In this example, the influence of the DC voltage is not great.

  FIG. 9 shows the response to H2 after a DC voltage of −2 V to +2 V is applied once for 100 msec × 1, and the influence of the DC voltage is small. Figure 10 shows the response to H2 after applying DC voltage of -2V to + 2V for 100msec x 5 times. When DC voltage of + 1.5V or more is applied, the polarity of H2 sensitivity is reversed and detected from the counter electrode C. The detection current flows toward the pole S. Fig. 11 shows the response to H2 after applying a DC voltage of -2V to + 2V for 100msec x 10 times. When a DC voltage of + 1.5V or more is applied, the detection current flows from the counter electrode C to the detection electrode S. Begins to flow.

  Figure 12 shows the response to CO after applying a direct current of -500mA to + 500mA for 100msec x 1 time. Adding a positive DC current increases the CO sensitivity. Adding a negative DC current increases the CO sensitivity. Decrease. Fig. 13 shows the response to CO after applying a direct current of -500mA to + 500mA for 100msec x 5 times. When a positive DC current is added, the CO sensitivity increases significantly. Decreases significantly. FIG. 14 shows the response to CO after applying a direct current of −500 mA to +500 mA for 100 msec × 10 times. When the positive DC current is added, the CO sensitivity is remarkably increased, and when the negative DC current is added, the CO sensitivity is shown. Is significantly reduced.

  Figure 15 shows the response to H2 after applying a direct current of -500mA to + 500mA for 100msec x 1 time. When a direct current of + 300mA is applied, the H2 sensitivity becomes negative, and the detection electrode S to the counter electrode C The detection current flows toward. Fig. 16 shows the response to H2 after applying a direct current of -500mA to + 500mA for 100msec x 5 times. When a direct current of + 100mA or more is applied, H2 is directed from the counter electrode C to the sensing electrode S. The detection current flows. FIG. 17 shows the response to H2 after applying a direct current of −500 mA to +500 mA for 100 msec × 10 times. When a direct current of +100 mA or more is applied, H2 is directed from the counter electrode C to the detection electrode S. The detection current flows.

  18 shows current and voltage waveforms when a +300 mA DC current is applied in the air, and FIG. 19 shows a current and voltage waveforms when a −300 mA DC current is applied in the air. The resistance component of the gas sensor 2 is about 10 to 20Ω, the resistance has polarity, and includes a capacitance component in addition to the resistance component.

  20 and 21 show changes in sensitivity to CO 300 ppm before and after the application of a DC voltage. In FIG. 20, the sensitivity was applied in units of 100 msec, and in FIG. 22 and 23 show changes in sensitivity to H2 300 ppm before and after application of a DC voltage. In FIG. 22, the sensitivity was applied in units of 100 msec, and in FIG. Applying a positive DC voltage increases the CO sensitivity and makes the H2 sensitivity negative. Adding a negative DC voltage decreases the CO sensitivity and increases the H2 sensitivity.

  24 and 25 show changes in sensitivity to CO 300 ppm before and after application of a direct current. In FIG. 24, application was performed in units of 100 msec, and in FIG. 25, application was performed for 1 second. 26 and 27 show changes in sensitivity to H2 300 ppm before and after application of a direct current. In FIG. 26, application was performed in units of 100 msec, and in FIG. 27, application was performed for 1 second. Adding a positive DC current increases the CO sensitivity and makes the H2 sensitivity negative. Adding a negative DC current decreases the CO sensitivity and increases the H2 sensitivity.

In the example, the liquid electrolyte was supported on the separator 24, but the same result was obtained when a solid polymer proton conductor was used instead of the separator 24. The value of DC current or DC voltage, the application time, the number of applications, etc. have optimum values according to the type of material, size, etc. of the MEA, and are not limited to the examples.

2 Electrochemical gas sensor 4 Metal container 6 Water 8 MEA
10 Bottom plate 12 Diffusion control plate 16 Cap 11, 14, 18, 19 Opening 20 Filter material 22 Gasket 24 Separator 25, 26 Gas diffusion film 30 Power source 32 Arithmetic logic circuit
C counter electrode
S detection pole
R1, R2 resistance
A1 Amplifier circuit

Claims (3)

In an electrochemical gas sensor in which a detection electrode is provided on a detected atmosphere side of a separator or a solid polymer proton conductor film holding a liquid electrolyte, and a counter electrode is provided on a reference atmosphere side,
The electrochemical gas sensor is modified so that a current flows in the H2 from the counter electrode to the detection electrode and a current flows in the CO from the detection electrode to the counter electrode. The separator or solid electrolyte membrane holding the liquid electrolyte is provided with a detection electrode on the detected atmosphere side and a counter electrode on the reference atmosphere side,
Using the electrochemical gas sensor modified by applying a direct current from the counter electrode to the detection electrode, with the counter electrode side being + and the detection electrode side being-,
A selective detection method of CO and H2 that detects H2 when current flows from the counter electrode to the detection electrode in the electrochemical gas sensor and detects CO when current flows from the detection electrode to the counter electrode. For an electrochemical gas sensor having a sensing electrode on the detected atmosphere side and a counter electrode on the reference atmosphere side of the separator or solid polymer proton conductor film holding the liquid electrolyte,
By applying a direct current from the counter electrode to the detection electrode with the counter electrode side set to + and the detection electrode side set to-, a current flows in the electrochemical gas sensor from the counter electrode to the detection electrode in H2, and the detection electrode in CO. A method for modifying an electrochemical gas sensor, wherein the electrochemical gas sensor is modified so that a current flows from the first electrode to the counter electrode.

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