JP5337058B2 - CO sensor sensitivity improvement method and CO sensor - Google Patents

Description

  The present invention relates to an electrochemical sensor including sensor means in which a detection electrode and a counter electrode to which a detection target gas reacts are connected to both sides of an electrolyte layer, and the sensitivity of the sensor means generated with an increase in the concentration of the detection target gas. The present invention relates to a method for improving sensitivity of an electrochemical sensor that improves blunting, and an electrochemical sensor obtained by executing the sensitivity improving method at the time of manufacture.

As one of the sensor to be mounted to an alarm device or the like, there is a CO sensor for detecting carbon monoxide gas. An electrochemical method is known as a detection method of the CO sensor. The electrochemical sensor is generally configured by sandwiching an electrolyte solution or a solid electrolyte between a detection electrode and a counter electrode.
The electrochemical sensor described in this specification is a CO sensor including an electrolyte solution, an electrode, and a gas permeable membrane, and the detection principle of this electrochemical sensor (CO sensor) will be described below.

When carbon monoxide, which is a gas to be detected, comes into contact with the detection electrode of the CO sensor, as shown in the following (1), carbon monoxide and water react with each other to generate carbon dioxide and proton (H ⁺ ) And electrons (e ⁻ ) are generated.
CO + H ₂ O → CO ₂ + 2H ⁺ + 2e ⁻ (1)
The reaction (1) is a diffusion-controlled reaction that depends on the rate at which carbon monoxide diffuses in the measurement atmosphere under conditions using a diffusion control hole or the like (the detection electrode where oxygen and carbon monoxide coexist). In the vicinity of the hybrid potential, the oxidation reaction of carbon monoxide becomes diffusion-limited.)

Protons (H ⁺ ) generated at the detection electrode pass through the electrolyte and move to the counter electrode side. Further, the electrons (e ⁻ ) generated at the detection electrode pass through the external circuit and move to the counter electrode, and react with oxygen introduced into the counter electrode and water in the electrolyte as shown in (2) below. Oxide ions (OH ⁻ ) are generated. Since oxygen is also present at the detection electrode, generally about half of the carbon monoxide is oxidized with oxygen at the detection electrode, and the other half is oxidized with oxygen at the counter electrode.
1 / 2.O ₂ + H ₂ O + 2e ⁻ → 2OH ⁻ (2)

In this way, the concentration of carbon monoxide in the measurement atmosphere is measured by detecting the electrical characteristics of electrons flowing in the external circuit from the detection electrode side to the counter electrode side in accordance with the above reaction, for example, as a short-circuit current value. Alternatively, the concentration of carbon monoxide in the measurement atmosphere can be measured by detecting the open circuit voltage with the detection electrode and the counter electrode being in an open circuit state.
As such a CO sensor, there existed what was shown by patent document 1, for example.

Japanese Patent No. 4179515

However, in this type of CO sensor represented by Patent Document 1, when the concentration of carbon monoxide, which is a detection target gas, is increased, a large amount of hydroxyl (OH ⁻ ) is generated by the progress of the reaction (2). As a result, the pH on the counter electrode side increases. As a result, there is a problem that the current flowing between the electrodes of the CO sensor is reduced, and the detection sensitivity to carbon monoxide is reduced.

Therefore, the present invention has been made in view of the above problems, and its purpose is to reduce the influence of a decrease in sensitivity of an electrochemical sensor (CO sensor) that occurs as the concentration of the detection target gas increases, It is an object of the present invention to provide a method for improving the sensitivity of an electrochemical sensor (CO sensor) that can accurately and reliably detect a target gas. Another object of the present invention is to provide an electrochemical sensor (CO sensor) obtained by executing the method for improving sensitivity of the electrochemical sensor at the time of manufacture.

The characteristic configuration of the CO sensor sensitivity improving method according to the present invention includes a sensor means in which a detection electrode and a counter electrode to which a detection target gas reacts are connected to both sides of an electrolyte layer, and the carbon monoxide that is the detection target gas. In a CO sensor that detects carbon monoxide by contacting a gas with the detection electrode, the CO sensor sensitivity improvement method improves the decrease in sensitivity of the sensor means that occurs as the concentration of the detection target gas increases. Then, a pH adjustment step of adjusting the pH of one of the detection electrode or the counter electrode from an acidic region to a neutral or alkaline region is performed as a potential gradient generation step , and a concentration gradient due to a diffusion rate in the electrolyte layer the liquid junction potential (diffusion potential) is to include the potential gradient generation step of generating a potential gradient to the sensing electrode side from the side of the counter electrode caused by.

When the concentration of the detection target gas increases, alkali concentration (pH increase) occurs at the counter electrode and the detection sensitivity of the CO sensor slows down. However, when the concentration of the detection target gas increases to a certain level, OH ⁻ or Alkali ions begin to diffuse in the electrolyte layer from the counter electrode side to the detection electrode side, forming a liquid-liquid potential difference (diffusion potential difference) caused by a concentration gradient due to the diffusion rate, eliminating the potential drop due to alkali concentration (pH increase) at the counter electrode This slows down the detection sensitivity.
Therefore, in the method for improving the sensitivity of the CO sensor of this configuration, an inter-liquid potential difference (diffusion potential difference) generated by a concentration gradient due to a diffusion rate difference between OH ⁻ and alkali ions is artificially generated in the electrolyte layer. That is, by forming a potential gradient in the electrolyte layer so that the potential on the counter electrode side becomes higher than the potential on the detection electrode side, the detection sensitivity when alkali concentration (pH increase) occurs at the counter electrode is slowed down. Can be stopped early. As a result, the detection target gas can be detected accurately and reliably, and the sensitivity of the CO sensor can be improved.

In sensitive method of improving the CO sensor according to the present invention, the potential gradient generation step, the pH adjustment step of adjusting the sensing electrode or neutral or alkaline region the pH of one electrode from the acidic region of the counter electrode .

According to the method for improving the sensitivity of the CO sensor of this configuration, by performing the pH adjustment step of adjusting the pH of one of the detection electrode or the counter electrode from the acidic region to the neutral or alkaline region, Therefore, it can be estimated that an inter-liquid potential difference (diffusion potential difference) caused by a concentration gradient due to a diffusion rate difference between OH ⁻ and alkali ions occurs, and this potential difference can be generated significantly.

In the method for improving sensitivity of a CO sensor according to the present invention, it is preferable to use sodium hydroxide in the pH adjustment step.

According to the sensitivity improvement method of the CO sensor of this configuration, the pH adjustment step can be easily performed using sodium hydroxide.

The characteristic configuration of the CO sensor according to the present invention is obtained by executing the method for improving the sensitivity of the CO sensor described above at the time of manufacture.

According to the CO sensor of this configuration, when the sensitivity improvement method of the CO sensor described above is executed at the time of manufacture, the concentration of the detection target gas increases and alkali concentration (pH increase) occurs at the counter electrode. Even so, it is possible to realize a CO sensor capable of detecting the detection target gas accurately and reliably, since the decrease in detection sensitivity stops early.
Also in the following description, the “electrochemical sensor” referred to in the present application is a “CO sensor”.

Longitudinal sectional view showing the entire configuration of an electrochemical sensor for executing the sensitivity improvement method of the electrochemical sensor Longitudinal sectional view of the sensor body, which is the main part of the electrochemical sensor Basic measurement circuit diagram using a bipolar electrochemical sensor (A) A diagram showing the pH of the anode electrode (detection electrode) and the cathode electrode (counter electrode) in a normal state, and (b) a graph of a potential-current curve drawn based on reactions occurring at the anode electrode and the cathode electrode, respectively. (A) A diagram showing pH on the anode electrode (detection electrode) side and cathode electrode (counter electrode) side in a state where the carbon monoxide concentration is high, and (b) an electric potential drawn based on reactions occurring at the anode electrode and the cathode electrode, respectively. -Current curve graph Graph showing the relationship between the carbon monoxide concentration and the output voltage from the electrochemical sensor under normal conditions Schematic diagram for explaining the revival phenomenon of graph linearity (A) A graph showing the relationship between the carbon monoxide concentration and the output voltage from the electrochemical sensor when the anode (detection electrode) is neutralized in advance, and (b) the cathode (counter electrode) is neutralized in advance Showing the relationship between carbon monoxide concentration and output voltage from electrochemical sensor The graph which shows the relationship between the carbon monoxide density | concentration when the anode electrode (detection electrode) and the cathode electrode (counter electrode) are neutralized beforehand, and the output voltage from an electrochemical sensor.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. The present invention is not limited to the configurations described in the embodiments and drawings described below, and various modifications are possible.

[Basic structure of electrochemical sensor]
FIG. 1 is a longitudinal sectional view showing the overall configuration of an electrochemical sensor 100 for executing the method for improving sensitivity of an electrochemical sensor of the present invention. FIG. 2 is a longitudinal sectional view of the sensor main body 10 which is a main part of the electrochemical sensor 100.

  The electrochemical sensor 100 of the present embodiment is a CO sensor using carbon monoxide as a detection target gas, and includes a sensor body 10, a water tank 20, a filter unit 30, a washer 40, a gasket 50, and the like as its basic structure. .

なお、電解質層１には、図示しない参照電極を介在させても構わない。この場合、電解質層１を上下二層に分割し、両層の間に参照電極を挟み込む。 As shown in FIG. 2, the sensor body 10 includes sensor means 11 having a laminated structure in which an anode electrode 2 as a detection electrode and a cathode electrode 3 as a counter electrode are connected to both sides (upper and lower surfaces) of the electrolyte layer 1. Conductive hydrophobic films 4 and 5 described later and a diffusion control plate 6 are provided.
In the electrolyte layer 1, as will be described later, cations such as protons (H ⁺ ) generated by the oxidation reaction of carbon monoxide at the anode electrode 2 move to the cathode electrode 3 (or from the cathode electrode 3 to OH − or the like). For example, an electrolyte solution containing an aromatic sulfonate (polymer) represented by the following chemical formula on a substrate such as filter paper (adjusted to about pH = 7). And the like can be impregnated.

Note that a reference electrode (not shown) may be interposed in the electrolyte layer 1. In this case, the electrolyte layer 1 is divided into two upper and lower layers, and a reference electrode is sandwiched between both layers.

  The anode 2 is an electrode catalyst that oxidizes carbon monoxide to carbon dioxide, and a platinum catalyst or the like is generally used. The platinum catalyst is impregnated with a proton conductor in order to promote the movement of protons and electrons generated in the oxidation reaction. The cathode electrode 3 has substantially the same configuration as the anode electrode 2. In the present embodiment, the film thicknesses of the anode electrode and the cathode electrode are each set to about 0.05 to 0.2 mm.

  Conductive hydrophobic films 4 and 5 are provided above the anode 2 and below the cathode 3, respectively. The conductive hydrophobic films 4 and 5 are configured as gas permeable films capable of transmitting gases (carbon monoxide, water vapor, and oxygen) involved in the reaction at the anode 2 or the cathode 3.

A diffusion control plate 6 is provided above the conductive hydrophobic film 4 above the anode electrode 2. The diffusion control plate 6 controls the inflow amount of the outside air so that the carbon monoxide gas contained in the outside air contacts the anode electrode 2 at the diffusion rate. Specifically, a diffusion control hole 6a is formed in the diffusion control plate 6, and the supply amount of outside air and CO molecules supplied to the anode electrode 2 through the diffusion control hole 6a is controlled. Therefore, if the concentration of carbon monoxide contained in the outside air is high and carbon monoxide is introduced into the anode electrode 2 as it is, the oxidation reaction at the anode electrode 2 cannot catch up due to excess carbon monoxide. Even in such a case, the oxidation reaction of all CO can be completed at the anode 2 by the action of the diffusion control hole 6 a provided in the diffusion control plate 6.
In this embodiment, the diffusion control plate 6 is formed of a thin plate made of a metal such as stainless steel, and the diffusion control hole 6a is formed by an arbitrary method such as punching.

  A water tank 20 is connected below the sensor body 10 on the cathode electrode 3 side. The water tank 20 has a constricted portion 22 formed in a part of the outer wall 21, and a washer 40 having a hole portion 41 formed in the central portion is moored at the constricted portion 22. In the space X surrounded by the outer wall 21 and the washer 40, water or a water absorbent resin 23 that has absorbed water is accommodated. Water existing in the space X passes through the hole 41 of the washer 40 in the state of water vapor, and is supplied to the electrolyte layer 1 through the cathode electrode 3 of the sensor body 10.

  On the other hand, a filter unit 30 is provided above the sensor body 10 on the anode electrode 2 side. The filter unit 30 forms a hollow portion Y by caulking the lower half portion 32 formed with the second vent holes 32a to the upper half portion 31 formed with the first vent holes 31a, and the activated carbon filter 33 is formed in the hollow portion Y. It has a configuration filled with. In this configuration, carbon monoxide contained in the outside air enters from the first ventilation hole 31a, and after impurities and the like are removed by the activated carbon filter 33, the carbon monoxide is supplied from the second ventilation hole 32a to the anode 2 of the sensor body 10. The

  A gasket 50 is provided between the filter unit 30 and the outer wall 21 of the water tank 20 so that water vapor evaporated from the water tank 20 does not leak outside.

  In the electrochemical sensor 100 of the present invention, the bottom surface 24 of the water tank 20 and the top surface 31b of the upper half 31 function as electrode terminals. Accordingly, the upper half 31 and the lower half 32 of the filter unit 30, the diffusion control plate 6 of the sensor body 10, the washer 40, and the outer wall 21 of the water tank 20 are made of a conductive material such as metal.

  The electrochemical sensor 100 configured in this way is incorporated into a basic measurement circuit 200 as shown in FIG. 3, for example. The basic measurement circuit 200 is used in a measurement method when the electrochemical sensor 100 is a bipolar type.

  A minute current (short-circuit current) generated from the sensor body 10 of the electrochemical sensor 100 is amplified and converted by the operational amplifier 201, the resistor 202, and the capacitor 203, and is output from the output terminal 204 as the voltage Vout. From the output result, the electrochemical sensor 100 detects the concentration of carbon monoxide contained in the outside air. The short-circuit current flows from the anode 2 to the cathode 3 in the electrolyte, and from the cathode 3 to the anode 2 in the external circuit. Normally, Vout increases as the carbon monoxide concentration increases.

[Method for improving sensitivity of electrochemical sensor]
By the way, as described above, in the electrochemical sensor 100, when the concentration of carbon monoxide, which is a detection target gas, increases, the reactions (3) and (4) proceed in the anode electrode 2 and the cathode electrode. In 3, the reaction of (4) proceeds. A large amount of hydroxide ions (OH −) is generated at the cathode electrode 3, and the pH of the cathode electrode 3 is increased. As a result, the detection sensitivity for carbon monoxide is reduced.
CO + H ₂ O → CO ₂ + 2H ⁺ + 2e ⁻ (3)
O ₂ + 2H ₂ O + 4e ⁻ → 4OH ⁻ (4)
This detection sensitivity slowing phenomenon will be described with reference to FIGS.

FIG. 4A is a diagram showing the pH of the anode electrode 2 and the cathode electrode 3 when the carbon monoxide concentration is low (or when the electrochemical sensor 100 is nearly new). This state is a normal state, and the anode 2 and the cathode 3 are adjusted to approximately pH = 1, and the electrolyte layer 1 is adjusted to approximately pH = 7.
FIG. 4B is a graph of a potential-current curve drawn on the basis of reactions that occur in the anode 2 and the cathode 3 in the normal state. In this normal state, the reaction (4) at the anode electrode 2 and the cathode electrode 3 is the same, so the equilibrium potential E1 of the reaction (4) of the anode electrode 2 and the equilibrium potential E0 of the reaction (4) of the cathode electrode 3 are. Are equal, the magnitude of the current α due to the reaction (4) at the anode 2 and the current due to the reaction (4) at the cathode 3 at the potential E2 when the reaction between the anode 2 and the cathode 3 is balanced. The magnitude of γ is also equal. On the other hand, the anode current in the anode electrode 2 at the potential E2 is the current β offset by the reactions (3) and (4), and is equal to the magnitude of the cathode current γ in the cathode electrode 3. That is, α = β = γ holds at the potential E2.

FIG. 5A is a diagram showing the pH of the anode electrode 2 and the cathode electrode 3 when the carbon monoxide concentration is high (or when the electrochemical sensor 100 has been used for a long period of time). In this state, the anode 2 remains at approximately pH = 1, but the cathode 3 has increased to approximately pH = 7 due to alkali concentration. In addition, the electrolyte layer 1 is maintaining about pH = 7 like what was shown in FIG. 4 (a).
FIG. 5B is a graph of a potential-current curve drawn on the basis of reactions that occur at the anode electrode 2 and the cathode electrode 3 when the pH of the cathode electrode 3 is increased. In the state where the pH of the cathode electrode 3 is increased, the potential-current curve at the anode electrode 2 is not different from the graph shown in FIG. 4B, but the potential-current curve at the cathode electrode 3 is lower in the potential (minus direction). ). This phenomenon will be described below.

As described above, when the electrochemical sensor 100 of the present invention detects carbon monoxide, the reactions (3) and (4) above occur simultaneously on the anode electrode 2 side, and the above ( Only the reaction 4) occurs. The equilibrium potential (that is, the intercept with the Y axis) Eeq on the cathode electrode 3 side is the oxygen partial pressure is P _O2 , the activity of OH ⁻ is a _OH− , a _OH− = Assuming that the potential at 1 and P _O2 = 1 is E ₀ , it is expressed by the Nernst equation as shown in the following (5) (at a temperature of 25 ° C.).

Here, when the concentration of carbon monoxide is increased, the reaction (4) proceeds gradually, and when the OH ⁻ activity a _OH− increases (that is, the pH increases), the equilibrium potential Eeq is Lower (59 mV per pH = 1). As a result, the potential-current curve at the cathode electrode 3 shifts downward (in the negative direction) as shown by the arrow in FIG.

  Therefore, in order for the magnitude of the anode current β ′ at the anode 2 and the magnitude of the cathode current γ ′ at the cathode 3 to be equal, it is necessary to balance at a potential E3 lower than the potential E2, but at this potential E3, Since the current α ′ in the anode 2 is larger than the current α shown in FIG. 4B, the current β ′ becomes smaller than the current β shown in FIG. Similarly, the current γ ′ on the cathode electrode 3 side is smaller than the current γ shown in FIG.

Thus, it is explained that when the pH of the cathode electrode 3 is increased, the current flowing between both electrodes of the electrochemical sensor 100 is reduced. If the relationship between the carbon monoxide concentration and the output voltage from the electrochemical sensor 100 (the current flowing between the two electrodes is converted into a voltage and amplified) (that is, the detection sensitivity of carbon monoxide) is graphed. As shown in FIG. Looking at this graph, in the region where the carbon monoxide concentration is low (0 to 3%), the detection sensitivity of carbon monoxide maintains linearity, but the region exceeding a certain concentration (3 to 4.5%) ) Confirms that the increase in detection sensitivity has slowed.
However, on the other hand, the linearity of detection sensitivity is restored in the region where the carbon monoxide concentration is further increased (4.5 to 6.5%). The linearity restoration phenomenon will be described with reference to FIG.

When the pH of the cathode electrode 3 increases due to the progress of the reaction (4) above, alkali concentration occurs at the cathode electrode 3, and alkali ions (for example, Na ⁺ ) and hydroxide ions (OH ⁻ ) are converted into the electrolyte layer. 1 begins to diffuse from the cathode 3 side to the anode 2 side (FIG. 7A). In this case, sodium ions (Na ⁺ ) and hydroxide ions (OH ⁻ ) are paired and diffuse in the electrolyte layer 1. The diffusion rate of hydroxide ions (OH ⁻ ) is sodium ions (Na ^+). ) Much faster than the diffusion rate. Therefore, a concentration gradient (potential gradient) due to the difference in diffusion rate is formed in the electrolyte layer 1 (FIG. 7B). As a result, a potential difference in which the potential on the cathode electrode 3 side becomes higher than the potential on the anode electrode 2 side in the electrolyte layer 1 is generated, and it is estimated that the potential on the cathode electrode 3 side lowered due to the increase in pH is recovered. Similarly, it is estimated that a potential gradient due to H + is formed from the anode 2 side.

  From the above phenomenon, if the potential difference is generated in the electrolyte layer 1 by raising the pH of the cathode electrode 3 in the acidic region to the neutral or alkaline region in advance, the carbon monoxide concentration becomes high. In this case, even if alkali concentration occurs at the cathode electrode 3 and the detection sensitivity is slowed down, it is considered that the tendency to restore linearity appears early. If it does so, the blunt point (inflection point of a graph) of detection sensitivity will shift to the low concentration side, and it is anticipated that the linearity (detection sensitivity) of a graph will improve as a whole as a result.

Therefore, in this embodiment, carbon monoxide was actually detected using a cathode electrode 3 whose pH was adjusted to a neutral region of about pH = 7 with sodium hydroxide. This pH adjustment was performed by impregnating the cathode electrode 3 before lamination with an aqueous sodium hydroxide solution.
As a result, a graph showing the relationship between the carbon monoxide concentration and the output voltage from the electrochemical sensor 100 as shown in FIG. According to this graph, the detection sensitivity blunting point (inflection point of the graph) is shifted to a lower concentration side than the blunting point in FIG. 6, and it is confirmed that the linearity (detection sensitivity) is improved as a whole. It was done.

The same result was obtained by increasing the pH of the anode 2 to about pH = 7 as shown in FIG. 8B.
On the other hand, when the pH of the anode electrode 2 and the cathode electrode 3 is simultaneously increased to about pH = 7, protons (H ⁺ ) moving through the electrolyte layer 1 are extremely reduced. Therefore, as shown in FIG. Detection is no longer possible.

  As described above, in the present invention, one electrode is brought to about pH = 7 to cause alkali concentration, and the above-described potential difference is formed in the electrolyte layer 1 in advance, so that the decrease in detection sensitivity is stopped early. be able to. As a result, it has been found that the gas to be detected can be detected accurately and reliably, and the sensitivity of the electrochemical sensor is improved.

  In order to generate the above-described potential difference in the electrolyte layer 1, it is only necessary to perform a pH adjustment step of adjusting the pH of the electrode from an acidic region to a neutral or alkaline region as described in the above embodiment. Moreover, this pH adjustment step can be easily performed using a commercially available drug such as sodium hydroxide.

  The present invention is a method for improving the sensitivity of various electrochemical sensors that can detect the detection target gas accurately and reliably by reducing the influence of the slowing of the sensitivity of the electrochemical sensor that occurs as the concentration of the detection target gas increases. Adaptable.

1 Electrolyte layer 2 Anode electrode (detection electrode)
3 Cathode electrode (counter electrode)
11 Sensor means 100 Electrochemical sensor (CO sensor)

Claims (3)

Consists of sensor means in which a detection electrode and a counter electrode to which the detection target gas reacts are connected to both sides of the electrolyte layer, and carbon monoxide is detected by the carbon monoxide gas that is the detection target gas coming into contact with the detection electrode. in CO sensor, a CO sensitivity improvement methods sensor to improve the blunting the sensitivity of the sensor means generated with the increase in concentration of the detection target gas,
A pH adjustment step of adjusting the pH of one of the detection electrode or the counter electrode from an acidic region to a neutral or alkaline region is performed as a potential gradient generation step, and the potential on the counter electrode side is set in the electrolyte layer. CO sensitivity improving method of the sensor including the potential gradient generation step of generating a high becomes potential gradient than the potential of the sensing electrode side. The method for improving the sensitivity of a CO sensor according to claim 1 , wherein sodium hydroxide is used in the pH adjustment step. A CO sensor obtained by executing the method for improving the sensitivity of a CO sensor according to claim 1 or 2 at the time of manufacture.

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