JP5787287B2 - Anion exchange membrane CO2 sensor - Google Patents

Description

The present invention relates to a CO ₂ sensor using an anion exchange membrane.

In Patent Document 1 (Patent 3391422), a detection electrode made of PtO and a counter electrode made of silver wire are electrochemically connected to an electrolytic solution containing KCl and KHCO ₃ to form a CO ₂ sensor. And when CO ₂ dissolves in the electrolyte,
CO ₂ + H ₂ O → H ₂ CO ₃ (1)
H ₂ CO ₃ → H ⁺ + HCO ₃ ⁻ (2)
Reaction occurs. The produced H ⁺ is reacted with PtO of the sensing electrode as shown in (3), and CO ₂ is detected from the reaction current.
2H ⁺ + PtO + 2e ⁻ → Pt + H ₂ O (3)
However, the CO ₂ sensor of Patent Document 1 is difficult to handle because it requires an electrolyte.

In Patent Document 2 (USP 7811433), a detection electrode made of Pt ₂ O ₃ , PtO or the like is provided on one surface of a chloride ion type anion exchange membrane, and a counter electrode made of silver is provided on the opposite surface. In the anion exchange membrane, the above reactions (1) and (2) occur, and the generated H ⁺ reacts with Pt ₂ O ₃ of the detection electrode as in (4) to generate a reaction current.
2H ⁺ + Pt ₂ O ₃ + 2e ⁻ → 2PtO + H ₂ O (4)
In the anion exchange membrane, the chlorine ions in the exchange membrane are replaced as shown in (5) by the hydrogen carbonate ions generated in the reaction of (2),
^{_{R + -Cl - + HCO 3 -}} → R + HCO 3 - + Cl - (5)
Chlorine ions react with the counter silver electrode. Similarly to Patent Document 1, Patent Document 2 also detects CO ₂ from the reaction current between the detection electrode and the counter electrode. However, the Pt oxide electrodes of Patent Documents 1 and ₂ are consumed by the detection of CO ₂ , and the silver electrode is also consumed by reaction with chloride ions. For this reason, it is necessary to regenerate the electrode by periodically applying a current in the direction opposite to the CO ₂ detection current, for example.

Patent 3391422 USP7811433

An object of the present invention is to provide a CO ₂ sensor that does not use an electrolyte and does not require electrode regeneration.

The present invention comprises a sensing electrode and a counter electrode provided on at least one surface of an anion exchange membrane comprising a strongly basic and hydroxide ion conductive anion exchange resin as an ion conductive component. It is configured to detect CO ₂ concentration from electric power, and further includes an active material that does not have a means for applying a voltage that is opposite in polarity to the electromotive force between the detection electrode and the counter electrode, and that is consumed by reaction with CO _2. Not in the anion exchange membrane CO ₂ sensor .

The anion exchange membrane CO ₂ sensor of the present invention, for example, from the CO ₂ reaches the anion exchange membrane (6)
_{CO 2 + H 2 O + 2e} - → CO + 2OH - (6)
As a result, hydroxide ions are generated. The electromotive force EMF according to the Nernst equation for this reaction is
EMF ＝ RT / 2F ・ ln ((PCO ₂ S ・ PH ₂ OS ・ PCOC) / (PCO ₂ C ・ PH ₂ OC ・ PCOS)) (7)
In the formula, S represents the detection electrode, C represents the counter electrode, PCO ₂ represents the partial pressure of CO ₂ , PH ₂ O represents the partial pressure of water vapor, and PCO represents the partial pressure of CO. Further, R is a gas constant, T is an absolute temperature, and in the embodiment, it is a room temperature or an outdoor environmental temperature, and F is a Faraday constant.

According to the inventor's experimental results,
・ Increasing the CO ₂ concentration of the detection electrode makes the potential of the detection electrode with respect to the counter electrode (hereinafter simply referred to as “electromotive force”) positive.
・ Due to the CO ₂ concentration dependence of the electromotive force, a two-electron reaction related to CO ₂ has occurred.
・ The oxygen partial pressure does not affect the electromotive force,
· Electromotive force with respect to CO has been found to respond in the opposite direction to the CO _2. These results support the reaction according to the equation (6), but the reactions (1) and (2) may occur. However, assuming that, hydrogen ions must be able to diffuse through the anion exchange membrane.

Although details of the CO ₂ detection mechanism in the present invention are unconfirmed, in the experiment, an electromotive force was obtained by a detection electrode and a counter electrode in which Pt was supported on an anion exchange resin or carbon black. Neither a platinum oxide electrode nor a silver electrode was required. The anion exchange membrane used in the experiment was a hydroxide ion conductive type, and there was no room for chlorine ions to participate in the reaction. In the present invention, CO ₂ can be detected without using an electrolytic solution and without using a sensing electrode made of a noble metal oxide or a silver counter electrode. Further, it is not necessary to use an active material for reducing hydrogen ions to water instead of the noble metal oxide. In this specification,% and ppm represent volume% and volume ppm. Carbon black is represented by the symbol CB, and an anion exchange resin having the same quality as the anion exchange membrane is represented by the symbol AER.

Plan view showing anion exchange membrane and sensing electrode in anion exchange membrane sensor Sectional view of anion exchange membrane sensor of Example Sectional view of a modified anion exchange membrane sensor The figure shows the response waveform of an anion exchange membrane sensor to 2% CO _2. Before CO ₂ was introduced, the sensing electrode / counter electrode was placed in an N ₂ gas atmosphere containing 395 ppm CO ₂ and 20% O _2. secure the CO ₂ concentration in the side by changing the concentration of CO ₂ sensing electrode side, the relative humidity at measurement temperature 30 ° C. is 57%. A diagram showing a CO ₂ concentration dependence of the electromotive force of the anion exchange membrane sensors, measurement temperature 30 ° C., relative humidity 57%, the CO ₂ concentration of the counter electrode is 395Ppm. The figure shows the response waveform of an anion exchange membrane sensor to 1000 ppm CO. Before introducing CO, the sensing electrode / counter electrode is placed in an N ₂ gas atmosphere containing 20% O ₂ and the counter electrode side atmosphere is fixed. The CO concentration on the detection electrode side is changed, the measurement temperature is 21 ° C, and the relative humidity is 50%. It is a figure showing the response waveform of the anion exchange membrane sensor when changing the O ₂ concentration from 20% to 0%, 10% and 30%. Before the change of the O ₂ concentration, the detection electrode side is 2% CO ₂ and 20 % O ₂ and the atmosphere of N ₂ gas containing, counter electrode side is placed in an atmosphere of N2 gas containing 395PpmCO ₂ and 20% O _2, to secure the atmosphere of the counter electrode side change of O ₂ concentration of the sensing electrode side The measurement temperature is 30 ° C. and the relative humidity is 57%. It is a figure showing the response waveform of an anion exchange membrane sensor to 2% CO ₂ when changing the relative humidity, including 20% O ₂ on the detection electrode side and the counter electrode side, with a relative humidity of 0 to 80% Response waveform when 2% CO ₂ is added to the atmosphere on the detection electrode side in an N ₂ gas atmosphere (30 ° C).

  In the following, an optimum embodiment for carrying out the present invention will be shown.

1 to 8 show anion exchange membrane CO ₂ sensors 2 and 22 and their characteristics according to the examples. FIG. 1 shows the sensor body 4 of the CO ₂ sensors 2 and 22, where a detection electrode 8 is provided on one surface of the anion exchange membrane 6 and a counter electrode is provided on the opposite surface, and lead wires 11 and 12 such as Au wires are connected. The anion exchange membrane 6 is, for example, a strongly basic hydroxide ion conductive synthetic resin membrane, and for example, Neoceptor (Neoceptor is a registered trademark) manufactured by Astom Co., Ltd. is used. In addition, the kind of conductive anion in the anion exchange membrane 6 is arbitrary. The material of the detection electrode 8 and the counter electrode 10 is a mixture of Pt supported on Pt paste or carbon black (CB) and an anion exchange resin (AER) of the same quality as the anion exchange membrane 6. An electrode etc. may be sufficient. Further, in place of Pt, other noble metal electrodes (thick film or thin film) such as Au, Rh, Ir, and Ru may be used, and base metal oxide electrodes such as LaCoO ₃ and LaNiO ₃ may be used. . The counter electrode may be a carbon electrode made of CB or a carbon thin film electrode.

FIG. 2 shows the structure of the anion exchange membrane CO ₂ sensor 2. An atmosphere in which the counter electrode 10 is provided on the surface opposite to the detection electrode 8 of the anion exchange membrane 6 and the CO ₂ concentration is measured on the detection electrode 8 side (test gas). The counter electrode 10 side is exposed to an atmosphere (reference gas) having a known CO ₂ concentration. Then, the electromotive force between the detection electrode 8 and the counter electrode 10 is measured.

As shown in FIG. 8, since the CO ₂ sensor 2 is dependent on relative humidity, an anion exchange membrane CO ₂ sensor 22 (FIG. 3) having a water reservoir may be used. Reference numeral 24 denotes a container for containing water, 26 is a porous film such as a PTFE film, and is protected from the outside by covering the detection electrode 8, and the porous film 26 is fixed to the container 24 and leads from these gaps to the lead wire Pull out 11 and 12.

  A method for manufacturing the sensor body 4 will be described. An aqueous solution of chloroplatinic acid is mixed with CB, irradiated with ultrasonic waves for 60 minutes, heated to 70 ° C, dried, and then reduced in a hydrogen atmosphere at 350 ° C for 1 hour to reduce Pt to the amount of CB. Pt / CB supported on 10 wt% was prepared. A strongly basic hydroxide ion conductive anion exchange resin (anion exchange resin having a quaternary ammonium group as an anion exchange group, the ion exchange capacity is 1.5 mmol / g) is added to the above Pt / CB in an organic solvent solution. Were mixed to obtain an electrode material. An electrode using this electrode material is called Pt / CB-AER, and its composition is AER: CB: Pt = 1.1: 1: 0.1 by mass ratio. As another electrode material, a commercially available Pt paste (TR-7905) manufactured by Tanaka Kikinzoku Co., Ltd. is mixed with the above anion exchange resin solution, and a paste containing the anion exchange resin and Pt in a mass ratio of 33: 1 is used. Prepared. The electrode using this paste is called Pt-AER. Pt / CB-AER is applied on both sides of the strong and hydroxide ion conductive anion exchange membrane (anion exchange resin with quaternary ammonium group as anion exchange group, ion exchange capacity is 1.8 mmol / g). Coating was performed to produce a detection electrode and a counter electrode, and lead wires 11 and 12 made of Au were fixed. Similarly, an electrode material of Pt-AER was applied to both the front and back surfaces of the anion exchange membrane, and lead wires 11 and 12 were attached. Table 1 shows the configuration of the manufactured sensor body 4. The characteristics were measured using the sensor 2 of FIG. 2, the reference gas was supplied to the counter electrode 10 side, the test gas was supplied to the detection electrode 8 side, and the electromotive force between the lead wires 11 and 12 was measured.

The anion exchange membrane is generally a resin membrane having an anion exchange group, and the anion exchange group is, for example, a quaternary ammonium group or a quaternary phosphonium group. An anion exchange membrane is formed by casting a film or the like by introducing a functional group such as an anion exchange group using a resin such as polystyrene, polyvinyl pyridine, polysulfone, polyether ketone, polyether ether ketone, or polybenzimidazole as a base material. It is a film. The anion exchange membrane usually has an anion exchange capacity of 0.2 to 3 mmol / g, preferably 0.5 to 2.5 mmol / g, and the water content at 25 ° C. is preferably 7 wt% or more so that the anion conductivity is not reduced by drying. Is 10 to 90 wt%. The film thickness is preferably 5 to 200 μm, more preferably 10 to 100 μm, in order to keep the electrical resistance low and necessary mechanical strength. The anion exchange membrane is a membrane resistance e.g. 0.05～1.5Omucm ² in an NaCl aqueous solution of 0.5 mol / L at 25 ° C., preferably 0.1~0.5Ωcm ^2.

Table 1 Structure of sensor body Anion exchange membrane Hydroxide ion conductive strong base anion exchange membrane sensing electrode and counter electrode (Pt / CB-AER) Pt: Carbon black: Anion exchange resin = 0.1: 1: 1.1
Detection electrode and counter electrode (Pt-AER) Pt: Anion exchange resin = 1:33

Figure 4 shows the response waveform to 2% CO _2. Initially, the sensing electrode / counter electrode is placed in an N ₂ gas atmosphere containing 395 ppm CO ₂ and 20% O ₂ to fix the CO ₂ concentration on the counter electrode side. By changing the CO ₂ concentration on the detection electrode side, the measurement temperature is 30 ° C. and the relative humidity is 57%. While fixing the CO ₂ concentration of the counter electrode side 395Ppm, and the CO ₂ concentration of the sensing electrode side varied between 395Ppm 2%. Comparing the Pt / CB-AER electrode with the Pt-AER electrode, it can be seen that the Pt-AER electrode has a large change in electromotive force, but the response to the atmosphere is faster with the Pt / CB-AER electrode. FIG. 5 shows the electromotive force when the CO2 concentration in the test gas is changed in the range from 395 ppm to 2%, and initially N ₂ gas containing 395 ppm CO ₂ and 20% O _{2 for} both the detection electrode and the counter electrode. The measurement temperature was fixed at 30 ° C., the relative humidity was 57%, and the CO ₂ concentration at the counter electrode was fixed at 395 ppm. Comparing the gradient of electromotive force with the Nernst equation, it can be seen that the number of reaction electrons is 1.6 at the Pt-AER electrode and the number of reaction electrons is 2 at the Pt / CB-AER electrode.

In the electrode reaction of (6) CO ₂ + H ₂ O + 2e ⁻ → CO + 2OH ⁻ , the sensor 2 should respond to CO in the opposite direction to CO ₂ . The response waveform to 1000ppm CO (Pt-AER electrode) is shown in Fig. 6. Both the detection electrode and counter electrode contain 20% O ₂ , the relative humidity is 50%, and the rest is placed in N ₂ gas atmosphere. The measurement temperature is 21 ° C. FIG. 6 shows the results when 1000 ppm of CO was introduced into the test gas under these conditions. Sensor 2 for CO, responding to a direction opposite to the CO _2.

Fig. 7 shows the response waveform (Pt-AER electrode) when the O ₂ concentration is changed to 0%, 20%, and 30%. The first detection electrode is N containing 2% CO ₂ and 20% O _2. Place in ₂ gas atmosphere and change O ₂ concentration to 0%, 10% and 30%. The counter electrode is placed in an atmosphere of N ₂ gas containing 395 ppm CO ₂ and 20% O ₂ . The measurement temperature was 30 ° C., and the relative humidity was 57% on both the detection electrode side and the counter electrode side. FIG. 7 shows that the response to changes in O ₂ concentration is negligible, and that O ₂ is not involved in the CO ₂ detection reaction, which supports the electrode reaction of (6).

Fig. 8 shows the response waveform (Pt-AER electrode) to 2% CO _{2 in} an atmosphere with a relative humidity of 0 to 80% at 30 ° C. Both the detection electrode and the counter electrode contain 20% O _{2. 2} gas atmosphere. Also prior to the introduction of CO ₂ is sensing electrode and the counter electrode are both contain CO ₂ in 395Ppm. The lower the relative humidity, the greater the response but the slower the recovery. The increase in electromotive force when the relative humidity is low indicates that the concentration of CO ₂ dissolved in the anion exchange membrane increases as the relative humidity decreases.

From 4 to 7, an electromotive force is positively respond to CO _2, is approximately 2 electron reaction to CO _2, O ₂ is not involved in the response, negative to respond to CO I understand. These facts support that the electrode reaction is along (6). In the embodiment, the detection electrode 8 and the counter electrode 10 are provided on both surfaces of the anion exchange membrane 6, but these may be provided on the same surface and the counter electrode 10 may be shielded from the test gas with an airtight resin or the like.

2,22 Anion exchange membrane CO ₂ sensor 4 Sensor body 6 Anion exchange membrane 8 Detection electrode 10 Counter electrode 11, 12 Lead wire
14 Housing 24 Container 26 Porous membrane

Claims (1)

It consists of a sensing electrode and a counter electrode provided on at least one surface of an anion exchange membrane comprising a strongly basic, hydroxide ion-conducting anion exchange resin as an ion-conducting component. From the electromotive force between the sensing electrode and the counter electrode, CO ₂ Anion exchange that is configured to detect the concentration , and that does not include a means for applying a voltage that is opposite to the electromotive force in the positive and negative directions between the detection electrode and the counter electrode, and that does not include an active material that is consumed by reaction with CO _2. Membrane CO ₂ sensor.