JPS58103660A - Gas sensor for carbon monoxide - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas detector, and more particularly to a carbon monoxide gas sensor suitable for electrochemically detecting carbon monoxide gas in the atmosphere.

Gas sensors that detect carbon monoxide in the atmosphere using constant potential electrolysis are known. This Senna has the advantage of excellent gas quantitativeness because the electrolytic current is proportional to the gas concentration, but on the other hand, the sensor is sensitive to other reducing gases, especially miscellaneous gases such as hydrogen. When used as a commercial carbon monoxide sensor, these miscellaneous gases tend to cause malfunctions. For example, the maximum sensitivity ratio of CO/H was about 20 to 50, which could not be said to be sufficiently large.

An object of the present invention is to provide a highly reliable carbon monoxide gas sensor that eliminates the drawbacks of the above-mentioned carbon monoxide sensor and is less likely to malfunction due to miscellaneous gases.

The present invention is a carbon monoxide gas sensor using a constant potential electrolysis method, and is characterized in that a 1 to 6 normal aqueous solution is used as the electrolyte solution. Furthermore, it is desirable to use platinum black or palladium black as the working electrode involved in the electrolytic reaction of -carbon oxide. In particular, in a preferred embodiment of the present invention, as the working electrode, a paste obtained by kneading platinum black or palladium black with polyfluorocarbon is used.
/cm" or 3 to 20 mg/cm" is used. Further, the potential of the working electrode is between 0.2 and 0.2 with reference to the oxidation-reduction potential of oxygen in the atmosphere.
It is desirable that it be in the range of 5■.

As described above, in the present invention, as a result of studying various combinations of the type of electrolyte solution and its concentration, the working electrode material and its amount, and the working electrode potential, the sensitivity ratio between carbon monoxide and miscellaneous gases can be determined within a specific range. It was newly discovered that the temperature increases, and this is an attempt to prevent malfunctions caused by miscellaneous gases. Note that the electrolytic reaction of gas on the surface of the working electrode is complex;
Generally, it involves the generation of overvoltage and the inactivation process accompanying the progress of the reaction, but the magnitude of these effects varies depending on the type of gas, the type and concentration of the electrolyte solution, the material, amount, and potential of the working electrode, etc. It is believed that the sensitivity ratio between carbon monoxide and miscellaneous gases will increase within the scope of the present invention.

An embodiment of the present invention will be explained below with reference to FIG. 1st
In the figure, a sulfuric acid or phosphoric acid solution 2 is placed in a polycarbonate container 1, and the solution 2 and the atmosphere are made of a porous Teflon (trade name of DuPont, USA) membrane 1'O provided with a working electrode 11. and is blocked by a porous Teflon membrane 20 provided with a counter electrode 210 and a reference electrode 22. A gas chamber 3 is attached to the outside of the porous Teflon membrane 10, and a sample gas is introduced through an inlet 30 and exhausted through an outlet 40. On the other hand, a gas chamber 4 is also attached to the outside of the porous Teflon membrane 20, and the porous Teflon membrane 20 has an inlet 5.
It is in contact with the outside air through 0.

The working electrode 11 is made of platinum black or palladium black.
After applying a paste made by mixing ~30% by weight of Teflon powder to the porous Teflon membrane 10, it is then pressure-molded at a pressure of 50~1000 kg/cm''.
The one heat-treated at 200 to 300C was used. Counterpoint 21
．． Reference electrode 22 was also produced in the same manner.

FIG. 2 also shows the electrical circuit used. terminal 10
By applying a reference potential (negative potential) to 1 and connecting the ground terminal 102 to the working electrode 11 and the terminals 103 and 104 to the counter electrode 21 and the reference electrode 22, a constant voltage can be applied between the working electrode and the reference electrode. can be applied. In this state, when CO or H3 is sent from the inlet 30, at the working electrode, CO+H*o-+cot↑+2H"+26- or Hl-+2H"+2eAt the counter electrode, 0, +4H,"+4e-→2H,Q.

reaction occurs, and an electrolytic current flows in the external circuit.

The magnitude of this electrolytic current is normally proportional to the concentration of
It is detected as the output voltage appearing at terminal 105.

Figure 3 shows a paste of a mixture of platinum black and Teflon powder used as a working electrode, with a concentration of 15 mg/cm in terms of platinum black.
Sulfur t1iL concentration (a) or phosphorus concentration (a) when applied
b) and the relationship between Co/H* sensitivity ratio.

The potential of the working electrode (based on the reference electrode) was 0.2v.

As shown in the figure, the sensitivity ratio of Co/H1 is large (more than 1,100) in the range of 1 to 6 normal for both sulfuric acid concentration and phosphoric acid concentration.The sensitivity ratio of Co/CH4 is All were over 1000.

When using a CO sensor for home use, the lower detection limit for CO needs to be around 150 ppm because CC0200pp or more is harmful, and Hl and CH at around 1 to 1.5% are harmful.
CO/H! due to the need to prevent malfunctions due to CO/H! It is said that the sensitivity ratio of CO/CH4 is required to be 100 or more, and it can be seen that this criterion is satisfied when the acid concentration is 1 to 6 normal.

Incidentally, the same tendency as shown in Fig. 3 can be observed by changing the coating amount of platinum black paste in the range of 5 to 40 mg/Cm'', by using palladium black instead of platinum black, and by changing the electrode potential of Effect 1 to 0.2.
It was also observed when the acid concentration changed in the range of ~0.5v, and large CO/H was observed in the acid concentration range of 1 to 6N! Sensitivity ratios were obtained.

Figures 4 and 5 show the amount of platinum black-containing paste applied (converted to platinum black amount) or palladium black-containing paste application amount (converted to palladium black amount) used for the working electrode, respectively, and C.
This is the relationship between O/H and sensitivity ratio. Note that the working electrode potential was 0.2 V, and the sulfuric acid concentration was 4N.

As seen in Figures 4 and 5, platinum black coating amount 1
5-40mg/cm", palladium black coating amount 3-20m
g/cm” and c O/H! Sensitivity ratio is particularly large, 10
It was found that the value was greater than 0.

Moreover, the output with respect to CO was almost constant when the coating amount was within the above range, but the output decreased outside this range. This is because if the coating amount is too small, the catalytic performance of the electrode will decrease,
This is thought to be due to the fact that the above-mentioned reaction becomes difficult to proceed, and that if the coating amount is too large, the diffusion rate of CO within the electrode becomes slow, making it difficult for CO to reach the three-phase interface.

In addition, a thick surface polarization current flows immediately after the power is turned on, but the time it takes for this polarization current to subside (start-up time) is also related to the amount of electrode coating. "or 10mg/cm"
Below, you can see that the startup time is 30 seconds or less, which is short enough to pose no practical problem.9. Therefore, a particularly desirable range of platinum black or palladium black coating amount is 5 to 20 m
g/cm" or 3 to 10 mg/cm".

In addition, when phosphoric acid is used instead of sulfuric acid, the acid concentration is 1 to 6.
Change the running time and working electrode potential within a specified range from 0.2 to
A similar trend to that shown in Figure 4 or Figure 5 was observed when changing within the range of 0.5 ■.

FIG. 6 shows the relationship between the working electrode potential, the sensitivity ratio of cO/H, and the leakage current (background current that flows when no reducing gas is flowing). The working electrode was platinum black coated at a coating amount of 15 mg/cm"), and the electrolyte was 1N sulfuric acid. As shown in Figure 6, the working electrode potential and cO/
The sensitivity ratio of H increases, and the sensitivity ratio becomes 100 at 0.2V or more.
It turns out that the above is the case. This is considered to indicate that a voltage-induced inactivation mechanism is involved in the H3 reaction at the working electrode. On the other hand, when the working electrode potential is 0.5 V or more, leakage current increases, making it difficult to measure the output and shortening the life of the electrode.

Similar trends were observed when the amount of platinum black applied was changed, when palladium black was used instead of platinum black, when the sulfuric acid concentration was changed within the range of 1 to 6N, and when phosphoric acid was used instead of sulfuric acid. CO/
A result was obtained in which the H1 sensitivity ratio was large and the leakage current was small.

As explained above, according to the present invention, c.

/H, can be improved to 2 to 5 times or more compared to the conventional one, and a highly reliable cO sensor that does not malfunction due to miscellaneous gases can be obtained.

[Brief explanation of the drawing]

Figure 1 is a structural diagram of a constant potential electrolysis cell used in an embodiment of the present invention, Figure 2 is an electric circuit diagram used in an embodiment of the present invention, and Figures 3 to 6 are examples of the present invention. FIG. 3 is a characteristic curve diagram showing the characteristics of the sensor obtained in FIG. 2... Electrolyte solution, 11... Working electrode, 21...
Opposite, acupuncture platform j1 (currently established) 4th fixed bar 2 dicum, II, tl amount (hegai lily

Claims (1)

[Scope of Claims] 1. A gas detection unit consisting of a wastewater-based working electrode with one side in contact with an electrolyte and the other side in contact with a gas, and a counter electrode separated from the electrode via an electrolyte solution; A gas sensor equipped with means for applying a constant voltage between these electrodes and means for measuring the value of current flowing between these electrodes, characterized in that an aqueous solution with a concentration of 1 to 6 normal is used as the electrolyte solution. A gas sensor for carbon monoxide. 2. A gas sensor for carbon monoxide according to claim 1, characterized in that platinum black or palladium black is used as the working electrode. 3. Chopsticks In claim 2, the working electrode is prepared by applying a paste obtained by kneading platinum black or palladium black with polyfluorocarbon, and the amount of the paste applied is platinum black. or 5 to 40 mg/CrrF or 3 to 20 mg in terms of palladium black, respectively.
A gas sensor for carbon monoxide, characterized in that it is in the range of 4.% permissible range. A gas sensor for carbon monoxide, characterized in that the potential of the working electrode is in the range of 0.2 to 0.5V.