JP5897371B2 - Method for adjusting sensitivity of electrochemical gas sensor and gas detector - Google Patents

Description

  The present invention relates to an electrochemical gas sensor, and more particularly to changing gas sensitivity by applying a direct current or a direct voltage to an electrochemical gas sensor.

  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 the electrochemical gas sensor by measuring impedance, and does not intend to positively change the sensitivity of 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 establish a technique for easily adjusting the sensitivity of an electrochemical gas sensor.

The present invention relates to an electrochemical gas sensor in which a detection electrode is provided on a detection atmosphere side of a separator or a solid polymer proton conductor film that holds a liquid electrolyte , and a counter electrode is provided on a reference atmosphere side. In addition, if the detection electrode is set to + and the counter electrode is set to-, and a DC current is applied from the detection electrode to the counter electrode, the CO sensitivity is lowered and the H2 sensitivity is increased, or the detection electrode is set to-and the counter electrode is set to +. This is an electrochemical gas sensor sensitivity adjustment method that increases the CO sensitivity and decreases the H2 sensitivity by applying a direct current from the sensor to the detection electrode .

The present invention also provides a gas detection apparatus comprising an electrochemical gas sensor in which a detection electrode is provided on the 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. By adding a direct current from the detection electrode to the counter electrode, the CO sensitivity is lowered and the H2 sensitivity is increased, or the detection electrode is- It has a sensitivity adjustment means that adjusts the gas sensitivity of the electrochemical gas sensor , which increases the CO sensitivity and decreases the H2 sensitivity by adding a direct current from the counter electrode to the detection electrode with the counter electrode set to +. To do.

  The inventor has found that the gas sensitivity changes when a direct current is applied between the detection electrode and the counter electrode of the electrochemical gas sensor. For example, if the detection electrode is set to + and the counter electrode is set to-, and a direct current is applied from the detection electrode to the counter electrode, the CO sensitivity decreases and the H2 sensitivity increases. In ordinary electrochemical gas sensors, the CO sensitivity is considerably higher than the H2 sensitivity, so if the CO sensitivity is reduced and the H2 sensitivity is increased, the sensitivity is adjusted to detect both CO and H2 generated by incomplete combustion. it can. Conversely, if the counter electrode is set to + and the detection electrode is set to-, and a direct current is applied from the counter electrode to the detection electrode, the CO sensitivity increases and the H2 sensitivity decreases, so that the CO selectivity can be increased. Electrochemical gas sensors have high sensitivity when warm and wet, and low sensitivity when cold and dry. Therefore, a seasonal change in gas sensitivity can be reduced by providing a timer or temperature sensor in the gas detection device to detect the season and applying direct current so that the CO sensitivity increases in winter or decreases in summer. The gas sensitivity of an electrochemical gas sensor often decreases with time of use. Therefore, by integrating the usage time with a timer or the like and applying a direct current so that the gas sensitivity of CO or the like is increased, the decrease in gas sensitivity due to use can be corrected.

  An assembly of the separator or solid electrolyte membrane holding the liquid electrolyte, the detection electrode, and the counter electrode is referred to as MEA. The effects on the MEA by applying a direct current may include changes in the electrochemical properties of the detection electrode and counter electrode, and changes in the interface between the detection electrode or counter electrode and the separator or solid electrolyte membrane. In the inventor's experiment, changing the direction of the direct current reverses the effect, so the polarity of the direct current is important. The inventor conducted an experiment with MEA in which a liquid electrolyte was held in a separator, but similar results were obtained with MEA using a solid polymer proton conductor membrane. A DC current such as a constant current may be applied to the MEA, or a DC constant voltage or the like may be applied to the MEA to cause a DC current to flow. Moreover, you may superimpose an alternating current on a direct current. The influence of the direct current depends on the direction of the direct current, the magnitude of the current or voltage, the time and number of times the current is applied, and the optimum condition can be easily obtained by changing these conditions. The optimal condition is considered to depend on the type of MEA.

  The electrochemical gas sensor may be used to output current or output electromotive force. The gas to be detected is not limited to CO and H2, but is arbitrary such as ethanol, acetone, hydrogen sulfide, methyl mercaptan, and the like. In this specification, the description regarding the electrochemical gas sensor is directly applied to the sensitivity adjustment method and the gas detection device of the electrochemical gas sensor, and conversely, the description regarding the sensitivity adjustment method of the electrochemical gas sensor is also applied to the electrochemical gas sensor and the gas detection device. .

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 is obtained by changing the CO sensitivity and the H2 sensitivity by applying a direct current or a direct voltage between the counter electrode and the detection electrode. 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 pulse generation circuit 34 has a built-in timer, and adjusts the sensitivity of the gas sensor 2 by applying a pulse voltage to the gas sensor 2 every month or every time when the CO sensitivity decreases such as in winter. Further, the use time of the gas detection device is integrated by the pulse generation circuit 34, and a pulse voltage is applied to the gas sensor 2 so as to compensate for the decrease in CO sensitivity accompanying use. For example, a pulse voltage of about −1 V × 100 msec from the detection electrode S to the counter electrode C is applied to the gas sensor 2 via the switch 36 to increase the CO sensitivity. When the connection between the counter electrode C and the detection electrode S is reversed, a pulse voltage is applied from the counter electrode C side. The pulse generation circuit 34 and the switch 36 are examples of sensitivity adjustment means.

  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 once for 100 msec × 1. FIG. 7 shows the response to CO after applying a DC voltage of −2 V to +2 V for 100 msec × 5 times. When a negative DC voltage is applied, the CO sensitivity decreases. FIG. 8 shows the response to CO after applying a DC voltage of −2V to + 2V 100 msec × 10 times.

  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. Fig. 10 shows the response to H2 after applying a DC voltage of -2V to + 2V for 100msec x 5 times. When a DC voltage of + is applied, the H2 sensitivity once decreases and at + 1.5V or higher, the H2 sensitivity is The polarity is reversed, and the detection current flows from the counter electrode C to the detection electrode S. Fig. 11 shows the response to H2 after applying DC voltage of -2V to + 2V for 100msec x 10 times. The sensitivity of H2 decreases at + 1V, and from + C to the detection pole S at + 1.5V or higher. Detection current flows.

  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 + 100mA direct current is applied, the H2 sensitivity is almost 0, and at + 300mA or higher, the counter electrode is opposite to H2. The detection current flows from C toward the detection pole S. 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. When a positive DC voltage is applied, the CO sensitivity increases and the H2 sensitivity becomes 0 to negative, and when a negative DC voltage is applied, the CO sensitivity decreases and the H2 sensitivity increases.

  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. When + DC current is added, the CO sensitivity increases and the H2 sensitivity becomes 0 to negative, and when-DC current is applied, the CO sensitivity decreases and the H2 sensitivity increases.

  As described above, the CO sensitivity and H2 sensitivity can be adjusted by selecting the value and time of the DC current or DC voltage to be applied and the number of times of application. For example, H2 sensitivity can be made almost zero while increasing CO sensitivity. In this way, CO, which is an impurity in the fuel, can be detected in a hydrogen fuel cell system or the like. For example, in a converter that converts CH4 to H2, the main component of the product gas is H2, and the harmful impurity is CO. Therefore, a sensor with H2 sensitivity of almost 0 and high CO sensitivity is suitable for detecting CO in the product gas. If the CO sensitivity and H2 sensitivity are almost equal, it is suitable for detecting incomplete combustion. That is, the feature of incomplete combustion is that both CO and H2 are generated. If these can be detected with the same sensitivity, incomplete combustion can be detected more reliably and with higher 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. can be determined experimentally according to the type of MEA material, size, etc., 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 supply 32 Arithmetic logic circuit 34 Pulse generation circuit 36 Switch
C counter electrode
S detection pole
R1, R2 resistance
A1 Amplifier circuit

Claims (3)

For an electrochemical gas sensor in which a detection electrode is provided on the detected atmosphere side of the separator or solid polymer proton conductor film holding the liquid electrolyte, and a counter electrode is provided on the reference atmosphere side,
Between the detection electrode and the counter electrode, the detection electrode is +, the counter electrode is-, and by adding a direct current from the detection electrode to the counter electrode, the CO sensitivity is lowered and the H2 sensitivity is increased, or the detection electrode is- A sensitivity adjustment method for electrochemical gas sensors that increases the CO sensitivity and decreases the H2 sensitivity by applying a direct current from the counter electrode to the detection electrode with the counter electrode set to + . The detection electrode is +, the counter electrode is-, and by adding a direct current from the detection electrode to the counter electrode, the CO sensitivity is lowered and the gas sensitivity is adjusted to increase the H2 sensitivity. The method for adjusting sensitivity of an electrochemical gas sensor according to claim 1 . In a gas detection apparatus comprising 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 that holds a liquid electrolyte, and a counter electrode is provided on a reference atmosphere side,
Between the detection electrode and the counter electrode, the detection electrode is +, the counter electrode is-, and by adding a direct current from the detection electrode to the counter electrode, the CO sensitivity is lowered and the H2 sensitivity is increased, or the detection electrode is- It has a sensitivity adjustment means that adjusts the gas sensitivity of the electrochemical gas sensor , which increases the CO sensitivity and decreases the H2 sensitivity by applying a direct current from the counter electrode to the detection electrode with the counter electrode set to +. , Gas detection device.

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