JP4986902B2 - Electrochemical alcohol sensor - Google Patents

Description

  The present invention relates to an electrochemical alcohol sensor, and more particularly to removing false alarms caused by hydrogen, CO, etc. and shortening the response time. The alcohol sensor of the present invention is used, for example, for detecting the ethanol concentration in exhaled breath.

  An electrochemical alcohol sensor is used for measuring ethanol concentration in exhaled breath. It is also known to measure the concentration of ethanol evaporated from the skin using an alcohol sensor. In the measurement of ethanol concentration, for example, whether it is 78 ppm or more and 130 ppm or more is detected. As a disturbing gas, smokers may contain up to about 80 ppm of CO in their exhaled breath, and people with internal organ diseases may contain about 200 ppm of hydrogen in their exhaled breath. For these reasons, it is necessary to increase the relative sensitivity to hydrogen and CO. The alcohol sensor can also be used for management of fermentation in the brewing industry, management of the concentration of methanol in fuel in a methanol fuel cell, and management of the concentration of alcohol solvents such as isopropanol. In these applications, selectivity for hydrogen and CO is required.

Regarding changing the noble metal catalyst concentration between the working electrode and the counter electrode, Patent Document 1: Japanese Patent Application Laid-Open No. 2004-212157 sets the catalyst concentration of the working electrode to 0.004 to 0.02 mg / mm ² Pt and the catalyst concentration of the counter electrode to 0.01 mg / mm. ² Pt is disclosed. In addition, the sensor of patent document 1 arrange | positions an electrode on both surfaces of a polymer proton conductor film | membrane, and is a sensor for the measurement of the hydrogen concentration in fuel gas.
JP 2004-212157 A

  An object of the present invention is to provide an electrochemical alcohol sensor capable of selectively detecting alcohol with respect to hydrogen and CO and having a short response time.

This invention is an electrochemical alcohol sensor provided with a working electrode using an electrode catalyst made of Pt or a Pt alloy and a counter electrode using an electrode catalyst made of Pt or a Pt alloy, with a liquid or solid electrolyte sandwiched between them.
The ratio of the electrode catalyst concentration of the working electrode and the electrode catalyst concentration of the counter electrode is 1 to 4 by weight ratio,
The electrode catalyst concentration per area of the working electrode is 0.06 to 0.15 mg / mm ² .
Preferably, the weight ratio of the working electrode electrode catalyst to the counter electrode catalyst is 1.5 to 2.5, and the concentration of the electrode catalyst per area of the working electrode is 0.08 to 0.12 mg / mm ² .

  As is apparent from FIG. 4, the sensitivity to alcohol, hydrogen, and CO varies depending on the ratio of the electrode catalyst concentration between the working electrode (W.E.) and the counter electrode (C.E.). Increasing this ratio reduces the sensitivity to any gas, but the decline in sensitivity is most pronounced with hydrogen, followed by CO, and relatively small with alcohol. Therefore, when the ratio of the electrode catalyst concentration between the working electrode and the counter electrode is within an appropriate range, the relative sensitivity to hydrogen and CO can be improved while keeping the sensitivity to alcohol at a relatively large value.

Next, as is apparent from FIG. 5, the electrode catalyst concentration of the working electrode affects the response time of the sensor to alcohol, and the response time is shortened when the electrode catalyst concentration per area of the working electrode is 0.06 to 0.15 mg / mm ^2. The response time is particularly short at 0.08 to 0.12 mg / mm ² .
Therefore, by setting the ratio of the electrode catalyst concentration between the working electrode and the counter electrode and the electrode catalyst concentration at the working electrode to an appropriate value, the relative sensitivity to hydrogen and CO is improved, and the sensitivity to alcohol is a relatively large value. And the response time to alcohol can be shortened.

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

The alcohol sensor 2 of an Example and its characteristic are shown in FIGS. FIG. 1 shows the structure of the alcohol sensor 2, 4 is a housing such as polypropylene, and has a diffusion hole 6 having an area of, for example, 1 mm ² . The mechanism for controlling the diffusion into the housing 4 is arbitrary. Reference numerals 8 and 10 denote gas diffusion films. Here, for example, a PTFE (polytetrafluoroethylene) film having a film thickness of 100 μm is used to diffuse the gas in a porous manner and to support the working electrode 12 and the counter electrode 14. The thickness of the gas diffusion films 8 and 10 is preferably 40 μm to 5 mm, for example.

The working electrode 12 and counter electrode 14 consists of Pt black here, for example, a surface area of 30 to 60 m ² / g, for example, a Pt black 40 m ² / g was dispersed in a solvent such as terpineol with fine powder of PTFE, the gas diffusion layer 8 , 10 is applied to the surface. Then, the solvent is evaporated, and Pt black is fixed to the surfaces of the gas diffusion films 8 and 10 using PTFE fine powder as a binder. The working electrode 12 and the counter electrode 14 may be formed by any method. For example, a colloidal solution in which the working electrode or counter electrode material is dispersed may be filtered through the gas diffusion films 8 and 10 to support the Pt electrode catalyst on the surface. . Alternatively, after the working electrode 12 and the counter electrode 14 are supported on the gas diffusion films 8 and 10, heat treatment may be performed at about 80 to 120 ° C. Further, the working electrode material or the counter electrode material may be applied on the gas diffusion films 8 and 10 without using a binder such as PTFE, and then pressed.

  Reference numeral 16 denotes a separator. Here, a porous hydrophilic film made of polyethylene is used, and the film thickness is set to 50 μm to 5 mm, for example, and 1 mm in the embodiment. By introducing a hydrophilic group such as a carboxyl group or an acetyl group into the polyethylene surface, the porous polyethylene can be hydrophilized, and the substrate is not limited to polyethylene but may be polypropylene or PTFE. Moreover, it is good also as the separator 16 by hydrophilizing the surface of films, such as glass fiber. For example, sulfuric acid is supported on the separator 16, but the type of electrolyte is arbitrary. Since sulfuric acid absorbs and releases water vapor in the air, the concentration is not constant, but the initial value of the concentration is 47% by weight. Instead of the separator 16, a polymer proton conductor film having a thickness of about 20 to 200 μm may be used. The working electrode 12 and the counter electrode 14 each have a diameter of 32 mm, but the sizes of the electrodes 12 and 14 are arbitrary.

  In the embodiment, Pt black is used as the electrode catalyst for the electrodes 12 and 14, but fine powder of Pt alloy such as 80wt% Pt-20wt% Ru may be used. The amount of the electrode catalyst is expressed in terms of metal, and the amount of the electrode catalyst is expressed by the weight per surface area of the working electrode and the counter electrode. This is because the type of electrode reaction changes depending on the weight of the electrode catalyst per surface area of the working electrode and the counter electrode. In the embodiments, the electrode catalyst is a noble metal fine powder, but a noble metal fine powder supported on a simple substance such as carbon may be supported on the surface of the gas diffusion film. Lead wires 20 and 21 made of Pt are attached to the working electrode 12 and the counter electrode 14 and connected to the amplifier circuit 22 to amplify the current between the working electrode 12 and the counter electrode 14. In the following data, the absolute value of the current between the electrodes 12 and 14 is shown.

  Samples shown in Table 1 were prepared by changing the amount of Pt catalyst at the working electrode and the counter electrode. The characteristics of the alcohol sensor 2 were measured using these samples. When the gas concentration was not shown in the measurement, ethanol 300 ppm, hydrogen 300 ppm and CO 300 ppm were used.

Table 1 Sample list Sample Pt catalyst concentration (mg / mm ²⁾ Catalyst concentration ratio Gas sensitivity (μA / 300ppm)
No. Working electrode Counter electrode Ethanol Hydrogen CO
1 0.03 0.06 0.5 18 6 4
2 0.06 0.06 1.0 16 4 3
3 0.08 0.06 1.3 15 3 3
4 0.10 0.06 1.7 14 2 2
5 0.12 0.06 2.0 12 2 2
6 0.135 0.06 2.25 10 0.5 1
7 0.15 0.06 2.5 9 0 0.5

8 0.10 0.15 0.67 20 5 4
9 0.10 0.10 1.0 15 2 3
4 0.10 0.06 1.7 14 2 2
10 0.10 0.03 3.3 6 -1 0.5
11 0.12 0.03 4.0 6 -2 0
12 0.10 0.015 6.7 2 -2 0

13 0.023 0.10 0.23 25 9 7
14 0.035 0.10 0.35 20 7 5
15 0.066 0.10 0.66 20 5 4
9 0.10 0.10 1.0 15 2 3

16 0.084 0.03 2.8 6 -1 0.5
10 0.10 0.03 3.3 6 -1 0.5
11 0.12 0.03 4.0 6 -2 0
17 0.18 0.03 6.0 2.5 -2 0

18 0.06 0.01 1/6 26 10 8
19 0.10 0.05 2 11 0.5 1

* To make the table easier to read, the same sample is displayed multiple times.
* Figures 2 to 6 are measured separately, so the data may be slightly different for the same sample.
* The data in the table is read from Fig. 2 to Fig. 6.

FIG. 2 shows the gas sensitivity when the Pt catalyst concentration at the counter electrode is fixed at 0.06 mg / mm ² and the amount of Pt at the working electrode is changed. Increasing the catalyst concentration at the working electrode changes the gas sensitivity. When the Pt concentration at the working electrode is increased, the decrease in hydrogen sensitivity is the most significant, followed by the decrease in CO sensitivity, and the decrease in ethanol sensitivity is moderate compared to hydrogen and CO.

FIG. 3 shows the gas sensitivity when the Pt catalyst concentration at the working electrode is fixed at 0.1 mg / mm ² and the Pt catalyst concentration at the counter electrode is changed. The gas sensitivity increases as the catalyst concentration at the counter electrode is increased. When the catalyst concentration at the counter electrode is low, the hydrogen sensitivity is negative and the CO sensitivity is zero. In contrast, ethanol sensitivity does not decrease to zero.

  The weight ratio of the Pt catalyst concentration per surface area of the working electrode to the Pt catalyst concentration per surface area of the counter electrode, and the gas sensitivity are shown in FIG. The sample used includes the sample used for the measurement in FIGS. 2 and 3 and other samples. By increasing the ratio of the electrocatalyst concentration between the working electrode and the counter electrode, the hydrogen sensitivity becomes negative and the CO sensitivity becomes almost zero. In contrast, ethanol sensitivity does not decrease to zero. Therefore, by setting this ratio within an appropriate range, the CO sensitivity and the hydrogen sensitivity can be sufficiently reduced while maintaining the ethanol sensitivity to some extent. The ratio of the concentration of the electrocatalyst between the working electrode and the counter electrode is preferably 1 to 4, more preferably 1.5 to 2.5 in terms of weight ratio, and the optimum value is in the vicinity of 2.

Possible causes of negative hydrogen sensitivity include hydrogen diffusing through the separator and moving to the counter electrode side, or hydrogen diffusing through the gap between the housing and the separator. This can occur because the separator is relatively thin, for example 5 mm. Next, reactions that contribute to gas sensitivity at the working electrode are reactions represented by the equations (1) to (3). Other side reactions that do not contribute to gas sensitivity include reactions of formulas (4) to (7).
In formula (4), acetaldehyde produced at the working electrode is oxidized to acetic acid, and in formula (5), acetic acid and ethanol react to produce ethyl acetate. The reaction of formula (5) reduces the ethanol concentration because it reduces the ethanol concentration at the working electrode. In Formula (6) and Formula (7), hydrogen and CO are consumed without contributing to gas sensitivity. Therefore, by increasing the concentration ratio of the electrocatalyst at the working electrode and the counter electrode, the side reactions of the equations (4) to (7) can easily proceed at the working electrode, or the equations (1) to (3) at the counter electrode. ) Can easily proceed, and the sensitivity can be canceled by the working electrode, so the change in relative sensitivity can be explained. The fact that different results are obtained for each gas type with respect to the change in the concentration ratio of the electrode catalyst at the working electrode and the counter electrode depends on the gas type. It can be explained as follows.

_{C 2 H 5 OH → CH 3} CHO + 2H + + 2e - (1)
^{_{H 2 → 2H + + 2e -}} (2)
_{CO + H 2 O → CO 2} + 2H + + 2e - (3)

CH ₃ CHO + 1 / 2O ₂ → CH ₃ CHOOH (4)
CH ₃ CHOOH + C ₂ H ₅ OH → CH ₃ CHOOCH ₂ CH ₃ (5)
H ₂ + 1 / 2O ₂ → H ₂ O (6)
CO + 1 / 2O ₂ → CO ₂ (7)

  Returning to FIG. 2 and FIG. 3, the data can be explained if the side reaction of the formulas (4) to (7) is more likely to occur as the concentration of the electrode catalyst is higher. Further, FIG. 3 suggests that ethanol, hydrogen, CO, and the like are diffused on the counter electrode side in some form, that is, a crossover has occurred.

Next, the response performance of the gas sensor depends on the electrode catalyst concentration at the working electrode. FIG. 5 shows the response time for 90% from the introduction of the gas to the completion of the response with respect to 300 ppm of ethanol. The Pt concentration in the working electrode can be increased response speed by a 0.06~0.15mg / mm ^2, in particular when the 0.08~0.12mg / mm ^2, the response time is substantially constant at about 30 seconds. It is preferable that the 0.06~0.15mg / mm ² the Pt catalyst concentration in this for the working electrode, and more preferably at 0.08~0.12mg / mm ^2. In FIG. 5, the Pt catalyst concentration at the counter electrode was set to 0.06 mg / mm ² , but even if the Pt catalyst concentration at the counter electrode was changed in the range of 0.03 to 0.10 mg / mm ² , there was a significant difference in 90% response time. There wasn't.

FIG. 6 shows the gas sensitivities to ethanol, hydrogen and CO when the Pt catalyst concentration at the working electrode is 0.1 mg / mm ² and the Pt catalyst concentration at the counter electrode is 0.05 mg / mm ² . The gas sensitivity is linear with respect to the gas concentration and is sufficiently higher than about 300 ppm of hydrogen or CO for about 70 ppm of ethanol.

In the examples, Pt black was used as the electrode catalyst, but the results were substantially the same even when the electrode catalyst was a Pt alloy such as Pt—Ru (Pt amount was 80 wt% or more). In the examples, the characteristics were measured at room temperature, but the relative sensitivity of hydrogen and CO to ethanol was almost the same in the range of about 0 ° C to 40 ° C. Although faster and increased temperature response speed itself, in the range of about 0 ° C. to 40 ° C., Pt catalyst concentration 0.06~0.15mg / mm ² approximately at the working electrode, especially in 0.08~0.12mg / mm ^2, The maximum value of the response speed to ethanol was not changed. Similar results to FIGS. 2 to 6 were obtained even when a polymer proton conductor film having a film thickness of 40 μm was used instead of supporting the sulfuric acid on a 1 mm thick separator with a response speed to ethanol.

Sectional view of the alcohol sensor of the embodiment Characteristic diagram showing gas sensitivity when the Pt concentration of the counter electrode is fixed at 0.06 mg / mm ² Pt and the Pt concentration of the working electrode is changed Characteristic diagram showing gas sensitivity when the Pt concentration at the working electrode is fixed at 0.10 mg / mm ² Pt and the Pt concentration at the counter electrode is changed Characteristic diagram showing the change in gas sensitivity depending on the ratio of the Pt concentration at the working electrode and the P concentration at the counter electrode Characteristic diagram showing the change of 90% response time to 300ppm of ethanol with Pt concentration of working electrode Characteristic chart showing gas sensitivity in the optimum embodiment

Explanation of symbols

2 Alcohol sensor 4 Housing 6 Diffusion hole 8, 10 Gas diffusion film 12 Working electrode 14 Counter electrode 16 Separator 20, 21 Lead wire 22 Amplifier circuit

Claims (2)

In an electrochemical alcohol sensor provided with a working electrode using an electrode catalyst made of Pt or a Pt alloy and a counter electrode using an electrode catalyst made of Pt or a Pt alloy, with a liquid or solid electrolyte sandwiched therebetween,
The ratio of the electrode catalyst concentration of the working electrode and the electrode catalyst concentration of the counter electrode is 1 to 4 by weight ratio,
An electrochemical alcohol sensor characterized in that the electrode catalyst concentration per area of the working electrode is 0.06 to 0.15 mg / mm ² . The weight ratio of the working electrode electrode catalyst to the counter electrode electrode catalyst is 1.5 to 2.5,
^{2. The} electrochemical alcohol sensor according to claim 1, wherein the concentration of the electrocatalyst per area of the working electrode is 0.08 to 0.12 mg / mm ² .