JPH07101211B2 - Galvanic battery type flammable gas sensor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvanic cell type combustible gas sensor.

[0002]

Combustible gas sensors include semiconductor type, catalytic combustion type and electrochemical type. The semiconductor type has a drawback of not having gas selectivity, and the catalytic combustion type has a selectivity, but has a drawback of not being able to measure a high concentration of combustible gas. On the other hand, the electrochemical formula has excellent gas selectivity and can measure a wide range of combustible gas concentrations.

The electrochemical system includes a constant potential electrolysis system and a galvanic cell system. In general, a potentiostatic combustible gas sensor has a catalytic metal such as platinum that is effective in the electrochemical oxidation reaction of the combustible gas and an organic polymer film that is located outside the catalytic metal and that restricts the supply of the combustible gas. The working electrode is a gas electrode composed of. As the counter electrode, there are one using a metal oxide such as lead dioxide and one using a gas electrode composed of a catalyst such as platinum or palladium which is effective for the electrochemical reduction reaction of oxygen. In any case, the potential of the working electrode is adjusted from the outside by using a constant potential applying device so that only the electrochemical reaction of the flammable gas occurs in the working electrode, that is, the generation or reduction reaction of oxygen does not occur. The feature is that the redox potential is set, and the fact that the current flowing between the working electrode and the counter electrode is proportional to the combustible gas concentration is used. Since this method is not affected by oxygen at all, it can measure high concentration combustible gas as well as low concentration combustible gas with high accuracy, but it is expensive and difficult to handle with constant potential application such as potentiostat. In addition to the need for a device, there is a problem that a considerable amount of time is required from the application of a potential to the working electrode until the output of the sensor stabilizes, and the measurement cannot be performed immediately when desired.

Such a problem is solved by a galvanic cell type combustible gas sensor which is another operation system of the electrochemical sensor. Galvanic cell-type combustible gas sensors have been put to practical use as catalyst metals such as platinum that are effective for the electrochemical oxidation reaction of combustible gas and organic high-pressure gas located outside the catalytic metal to limit the supply of combustible gas. There is one in which a gas electrode composed of a molecular film is used as a negative electrode, lead dioxide is used as a positive electrode, and a concentrated solution of acetic acid and lead acetate is used as an electrolytic solution. This means that when a lead dioxide-combustible gas battery is discharged through a resistor so that it operates under diffusion-controlled diffusion of the combustible gas, a linear relationship occurs between the combustible gas concentration and the current flowing through the battery. It was used. That is, unlike the above-described constant potential electrolysis method, this method does not require a constant potential applying device because it works as a battery itself, and since it is always in operation, it is possible to perform immediate measurement without warming up.

[0005]

A galvanic cell type combustible gas sensor has been found to solve some problems of the potentiostatic electrolysis type combustible gas sensor as described above. However, the conventional galvanic cell type combustible gas sensor has the following problems. That is, when the combustible gas concentration is very low, the potential of the negative electrode becomes approximately equal to the potential of the positive electrode made of lead dioxide, but since the potential of this positive electrode is nobler than the redox potential of oxygen, Then, oxygen is generated in the negative electrode, and pores of oxygen gas are generated in the liquid film formed between the catalyst metal such as platinum and the polymer film provided on the outside thereof, which makes accurate measurement impossible. In addition, since a current due to oxygen generation flows between both electrodes, there is a problem that an error occurs in the measured value only for that current.

The present invention has been made to solve the above problems, and an object thereof is to provide a porous positive electrode made of silver and silver chloride and 0.1 mol / mol of chlorine ions. By using a galvanic cell type combustible gas sensor composed of an electrolytic solution containing liters or more and 5 mol / liter or less and a negative electrode that is effective in the electrochemical oxidation reaction of the flammable gas and acts as a gas electrode, oxygen gas as in the conventional case can be obtained. (EN) Provided is a galvanic cell type combustible gas sensor which is not affected by the above.

[0007]

According to the present invention, chlorine ions (Cl ^-1 ) are contained in an amount of 0.1 mol / liter or more and 5 mo.
It was made paying attention to that the potential of the silver and silver chloride porous electrodes in an aqueous solution containing 1 / liter or less is lower than the redox potential of oxygen and higher than the potential at which the oxygen reduction reaction occurs. It is a thing. That is, in the present invention, in an electrochemical combustible gas sensor having a positive electrode, a negative electrode that is effective for an electrochemical oxidation reaction of a combustible gas and acts as a gas electrode, and an electrolyte solution, the positive electrode is composed of silver and silver chloride. The electrolytic solution contains chlorine ions of 0.1 mo
A galvanic cell type combustible gas sensor characterized by containing 1 / liter or more and 5 mol / liter or less was adopted.

[0008]

[Function] A positive electrode composed of silver and silver chloride and having porosity,
A gas electrode composed of a catalytic metal such as platinum, which is effective for the electrochemical oxidation reaction of flammable gas, and an organic polymer film, which is located outside the catalytic metal and limits the supply of flammable gas, and a negative electrode, and chloride ion A silver / silver chloride-combustible gas battery having an electrolyte solution containing (Cl ^-1 ) in an amount of 0.1 mol / liter or more and 5 mol / liter or less is detected via a detection resistor so as to operate under the rate-control of the supply of the combustible gas. When discharged, only the reduction reaction of silver chloride occurs in the positive electrode and the oxidation reaction of the flammable gas occurs in the negative electrode.

[0009] In a hydrogen gas atmosphere as an example of flammable gas
The reaction formula of each electrode is shown below. <When the electrolyte is acidic> 2AgCl + 2e → 2Ag + 2Cl for the positive electrode⁻  H at the negative electrode_Two→ 2H⁺+ 2e 2AgCl + H as a whole_Two→ 2Ag + 2HCl <When the electrolyte is alkaline> 2AgCl + 2e → 2Ag + 2Cl for the positive electrode⁻  H at the negative electrode_Two+ 2OH⁻→ 2H_TwoO + 2e H as a whole_Two+ 2AgCl + 2OH⁻→ 2Ag + 2
Cl⁻+ 2H_TwoO Here, the acid of the flammable gas in the negative electrode
Oxygen generation reaction as well as oxygen reduction reaction
What does not occur is the potential of the positive electrode mixture of silver and silver chloride.
Has chlorine ion of 0.1 mol / liter or more and 5mo
In a solution containing 1 / liter or less, the oxygen redox potential
Less noble and more noble than the potential at which the oxygen reduction reaction occurs
This is due to the fact. Chloride concentration within the above range
If it deviates, oxygen reduction reaction occurs at the negative electrode.

[0010] mixed potential between silver and silver chloride is a positive electrode is represented by the following equation. EAg = 0.222-0.0591 log ([A
g] [Cl ⁻¹ ] / [AgCl]) Here, if the electrode is made of porous silver and silver chloride,
Since the contact interfaces between the electrolytic solution and silver and silver chloride exist infinitely, the activities of silver and silver chloride can be regarded as 1, respectively. Therefore, the above formula can be expressed as the following formula, and this electrode becomes stable for a long period of time because its voltage depends only on the chlorine ion concentration in the electrolytic solution. EAg = 0.222-0.0591 log [Cl
⁻¹ ] [Cl ⁻¹ ]: Chloride ion concentration [Ag]: Silver activity [AgCl]: Silver chloride activity If the chloride ion concentration is low, the potential of the mixed electrode of silver and silver chloride becomes more noble and oxygen Closer to the redox potential of
If it is 0.1 mol / liter or less, the reduction overvoltage of oxygen becomes smaller and the reduction reaction of oxygen occurs in the negative electrode. Also, when the chlorine ion concentration becomes higher, the oxygen reduction overvoltage becomes larger, but at 5 mol / liter or more, the potential of the mixture of silver and silver chloride becomes more base,
Oxygen reaction occurs at the negative electrode.

[0011] Therefore, the concentration of chlorine ions in the electrolyte solution,
It must be 0.1 mol / liter or more and 5 mol / liter or less. Since only the above reaction occurs in such a sensor, the concentration of the combustible gas can be accurately known from the current flowing from the positive electrode to the negative electrode. Here, the detection of the current can be easily known from the voltage across the resistor connected between both electrodes.

[ 0012 ]

EXAMPLES The present invention will be described below with reference to preferred examples. FIG. 1 is a sectional structural view of a galvanic cell type combustible gas sensor according to an embodiment of the present invention. This sensor is a porous mixture of silver and silver chloride prepared by electrolyzing a container body 1 made of ABS resin and an Ag plate in 0.1 M HCl for 2 hours at a current density of 0.4 mA / cm ² under anodic polarization. A positive electrode 2, a negative electrode 3 in which platinum is electrodeposited on porous carbon, an electrolytic solution 4 composed of a mixed aqueous solution of potassium chloride and potassium hydroxide, and tetrafluoroethylene-6-fluoroethylene-propylene copolymer provided outside the anode. It is composed of a diaphragm 5 made of a polymer film, and the positive electrode and the negative electrode are a detection resistor 6
It is closed externally via.

[0013] The galvanic cell type combustible gas sensor is a combustible gas, hydrogen _{(H 2)} or carbon monoxide (C
The output voltage detected at both ends of the detection resistor 6 after being left in the air having a concentration of O) of 2, 6, and 10% is plotted in FIG. From FIG. 2, it is understood that the galvanic cell type combustible gas sensor according to the present invention has excellent linearity with respect to the concentration of hydrogen or carbon monoxide in the air. In the conventional galvanic cell type sensor, oxygen was generated in the negative electrode in the low concentration flow area of the combustible gas, but the sensor of the present invention does not generate oxygen, so that measurement can be performed even in a low concentration.

[ 0014 ]

The galvanic cell type combustible gas sensor according to the present invention does not require the use of a constant potential applying device and can immediately detect the concentration of combustible gas in the air. By using this sensor, it is possible to provide a combustible gas concentration meter and an alarm device that are easy to handle, which greatly contributes to the industry.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a galvanic cell type gas sensor according to an embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between hydrogen or carbon monoxide concentration in air and an output voltage of a sensor.

[Explanation of sign]

 1 Main body of container 2 Positive electrode 3 Negative electrode 4 Electrolyte 5 Diaphragm 6 Detection resistance

Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location 331

Claims (1)

[Claims] 1. A negative electrode which is effective for an electrochemical oxidation reaction between a positive electrode and a combustible gas and which functions as a gas electrode, and is located outside the negative electrode.
In an electrochemical combustible gas sensor having an organic polymer film that restricts the supply of combustible gas and an electrolytic solution, the positive electrode is made of silver and silver chloride and has porosity, and the electrolytic solution is chlorine. A galvanic cell type combustible gas sensor, characterized in that it contains 0.1 mol / liter or more and 5 mol / liter or less of ions.