Gas sensor

Details Gas sensor Gas sensor

2000-03-17

2002-04-02

Tung, T. (Department: 1743)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S427000, C205S784000

Reexamination Certificate

active

06365022

ABSTRACT:

The present invention relates to a solid electrolyte type gas sensor and, more particularly, to a sensor operable at or about ambient temperature.
Solid electrolyte sensors at present comprise a detection electrode and a reference electrode disposed on opposite sides of a solid electrolyte acting as an ionic conductor. Generally, in detecting gaseous components present in an atmosphere using a solid electrolyte sensor, an ionic conductor is used in which specific ions are mobile and sometimes in combination with this specific ion conductor, which is used as the solid electrolyte, another compound containing the specific ions is used as a detection material and is covered by an electrocatalytic material, such as platinum.
JP-A-62-142266 describes an NO
x
sensor comprising a gold electrode on an insulating substrate, a solid RbAg
4
I
5
electrolyte, and a gold detection electrode with AgNO
3
deposited on it to act as an electrocatalyst. Such a sensor requires a reference gas in contact with the reference electrode to provide a stable output potential. The NO
x
sensors of JP-A-61-271447 and JP-A-61-184450 have similar requirements. JP-A-62-207952 describes a halogen gas sensor having a reference electrode comprising a metal halide, metal, and a metal ion-conductive solid electrolyte. The cell electrolyte is a halide-containing metal ion-conductive solid electrolyte, and the detection electrode is said to be a metal halide-metal ion-conductive solid electrolyte mixture. The detection electrode does not have an electrocatalyst layer; the operating temperature of this sensor is 150° C.
A previously proposed gaseous carbon dioxide sensor uses, for example, a sodium ionic conductor such as &bgr;-alumina (general formula: Na
2
O.nAl
2
O
3
, n=5-11) or NASICON(general formula: Na
1−x
Zr
2
P
3−x
Si
x
O
12
). In this case, a platinum gauze covered with sodium carbonate or the like is used as a detection electrode.
A typical reference electrode comprises gold or platinum alone or these metals covered with sodium carbonate or the like, maintained in contact with reference atmosphere of standard air or gaseous carbon dioxide. Accordingly, gaseous carbon dioxide at the concentration to be measured is in contact with the detection electrode but not with the gas reference electrode on the opposite side.
The sensor is heated during operation usually to a constant temperature between 400° C. and 600° C., a flux of sodium ions being caused to move to the detection electrode corresponding to the partial pressure of the gaseous carbon dioxide in a gas to be detected which is in contact with the detection electrode. This gives rise to a sodium ion gradient between the electrodes and the concentration of the gaseous carbon dioxide can be deduced by measuring the potential difference associated with this gradient.
However, in the case of the existing gaseous carbon dioxide sensors using sodium carbonate as the detection material for the detection electrode and using NASICON for the ionic conductor as described above, the potential difference is highly sensitive to the moisture content of the gas to be detected.
In order to overcome this effect, a gaseous carbon dioxide detection sensor comprising a detection electrode and a reference electrode on both sides of an ionic conductor with a mixture of one mole of an alkali metal carbonate and more than one mole of an alkaline earth metal carbonate such as BaCO
3
has been suggested as the material for the detection electrode. For such a sensor the variation of potential difference with carbon dioxide content is less affected by moisture in the gas to be measured.
A shortcoming of even this improved sensor is the operating temperature, which requires a heater. This elevated operating temperature is also a potential cause of accelerated breakdown of the sensor structure and problems associated with oxidation of electrocatalysts and contacts. Furthermore, in many cases, a reference gas is required which complicates the cell design and affects portability.
The present invention overcomes the foregoing problems in the prior art and provides a gas, especially carbon dioxide, detection sensor capable of measuring the concentration of a gas without the need for high temperature operation. Conveniently, the sensor may be operated at a controlled ambient or moderately elevated temperature, for example up to 70° C., to avoid any possible error resulting from ambient temperature variation. In one embodiment the use of a reference gas is avoided.
The present invention provides in a first aspect a gas sensor containing an electrolytic cell comprising a reference electrode, a detection electrode, and a solid electrolyte, the detection electrode comprising an electrocatalyst and a silver salt the anion of which corresponds to the gas to be detected, the solid electrolyte being capable of transmitting silver ions, and the reference electrode comprising metallic silver in contact with the electrolyte, the free face of the silver being sealed. A cell of this type is referred to below as one of the closed gas type.
As the gas to be detected, there may be mentioned, for example, sulphur trioxide, sulphur dioxide, nitrogen oxides (No
x
, or specific nitrogen oxides), hydrogen sulphide and halogen, especially chlorine, or pseudohalogen, for example, cyanogen. The corresponding anion in the detection electrode is sulphate, sulphite, an oxyacid of nitrogen, and halide, especially chloride, or pseudohalide, for example, cyanide. More especially, and preferably, the gas to be detected is carbon dioxide and the corresponding anion is a carbonate.
The reference electrode is advantageously a metallic silver sheet in contact with the silver ion conductor.
In a second aspect the present invention provides a carbon dioxide sensor containing an electrolytic cell comprising a reference electrode, a detection electrode, and a solid electrolyte, the detection electrode comprising an electrocatalyst and silver carbonate, and the solid electrolyte being capable of transmitting silver ions.
In a first embodiment of the second aspect of the invention, a reference electrode having the same characteristics as set out below for the detection electrode is advantageously used, and preferably the reference electrode is of the same materials, and in the same proportion, as is the detection electrode.
Advantageously, in the first embodiment of the second aspect an atmosphere of a known constitution (a reference atmosphere) is maintained at the free face of the reference electrode. The reference atmosphere is advantageously air at atmospheric pressure with a known and constant CO
2
partial pressure. A cell of this type is referred to below as one of the open gas type.
In a second embodiment of the second aspect, the reference electrode is sealed. In this second embodiment the reference electrode is advantageously metallic silver in contact with the silver ion conductor. A cell of this type is referred to below as one of the closed gas type.
In each aspect, the detection electrode, in addition to a silver salt of an acid corresponding to the gas to be detected (carbonate in the second aspect), advantageously also contains a component that enhances the conductivity of the electrode. This component is advantageously an ionic conductor, and one through which silver ions may pass, preferably at room temperature, and may comprise a silver salt or double salt, for example, silver iodide or, advantageously, a silver rubidium iodide, especially Ag
4
RbI
5
, or a silver mercury iodide. Advantageously, the proportion of conductivity enhancer is adequate to provide a sufficiently high ionic conductivity.
The detection electrode advantageously also comprises a binder, e.g., polytetrafluorethylene (PTFE), in a proportion sufficient to render the conductive materials coherent but without adversely affecting conduction to a deleterious extent.
As solid electrolyte, there is advantageously used an ionic conductor, for example and preferably one mentioned above as the conductivity enhancer in the det

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