JPH04273051A - Self-reference steam sensor and steam measuring method - Google Patents

Abstract

PURPOSE:To realize a steam measuring method to measure the amount of steam in a stable manner over a long period and obtain a small steam sensor. CONSTITUTION:A steam pump cell is comprised of electrodes 1, 5, and a proton conductivity solid electrolyte 3, and a steam sensor cell is comprised of electrodes 1, 4 and a proton conductivity solid electrolyte 2. If a preset current J is designed to flow in the steam pump cell, steam partial pressure or steam activity is constantly maintained at the boundary between the steam pump cell and the steam sensor cell. In reference to the steam partial pressure or the steam activity, electromotive force generating in the steam sensor cell is measured, thereby to find steam partial pressure in an atmosphere.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-reference water vapor sensor using a proton conductive solid electrolyte and a water vapor measuring method used to measure the amount of water vapor in gas.

0002

BACKGROUND OF THE INVENTION Conventionally, a water vapor sensor using a solid electrolyte having proton conductivity made of perovskite-type oxides such as strontium oxide and cerium oxide as a sensor element has been proposed (Japanese Patent Laid-Open No. 58-50458, Publication No. 60-263853, Japanese Patent Publication No. 61-206
No. 4, JP-A-61-3054 and JP-A-61-14
No. 566). This solid electrolyte has proton conductivity at temperatures above 400°C. Conventional water vapor sensors have a sensor element made of this solid electrolyte with one end closed, and a porous inner electrode (reference electrode) and outer electrode (measurement electrode) coated on the inner and outer surfaces, respectively. It is configured. A reference gas serving as a reference for the galvanic electromotive force, that is, a gas containing hydrogen or water vapor at a predetermined concentration, is supplied into the sensor element.

When a water vapor sensor using this perovskite-type proton-conductive solid electrolyte is inserted into a gas to be measured, a difference in water vapor partial pressure between the reference gas and the gas to be measured causes a gap between the reference electrode and the measurement electrode. An electromotive force is generated. By detecting this electromotive force, the partial pressure of water vapor in the gas to be measured can be determined. Note that, instead of supplying the reference gas, there is also a water vapor sensor in which a solid reference material containing a predetermined amount of hydrogen or water vapor is disposed in contact with the reference electrode.

0004

However, the conventional water vapor sensor described above has the following problems. (1) In addition to protons, electron holes also contribute to conduction, so hydrogen leaks electrochemically from the measurement electrode side to the reference electrode side or from the reference electrode side to the measurement electrode side, causing water vapor on the reference electrode side. The partial pressure changes, making it impossible to obtain stable operation over a long period of time.

(2) As mentioned above, since the water vapor partial pressure on the reference electrode side changes, the water vapor partial pressure on the measuring electrode side cannot be determined accurately.

(3) When a gas is used as a reference substance, equipment such as a gas line for the reference gas is required, and the sensor element and the water vapor measuring device including this sensor element become large and difficult to handle. It becomes complicated.

(4) Even when a solid reference material is used in place of the reference gas, it is necessary to use a large amount of the reference material in order to eliminate changes in the reference value due to leakage of hydrogen or water vapor from the electrolyte to the reference electrode. However, the sensor element and the water vapor measuring device including the sensor element become large and complicated to handle.

The present invention has been made in view of these problems, and is capable of stably and accurately measuring the amount of water vapor in gas over a long period of time, and is small and easy to handle. An object of the present invention is to provide a self-reference type water vapor sensor and a water vapor measurement method.

[0009]

[Means for Solving the Problems] A self-reference water vapor sensor according to the present invention has a first electrode having electronic conductivity as the center, and first and second proton conductive solid electrolytes on both sides thereof. second and third electrodes made of a porous material are arranged, the first electrode, the first proton conductive solid electrolyte, and the second electrode constitute a water vapor pump cell; , a water vapor sensor cell is configured by the second proton conductive solid electrolyte and the third electrode.

[0010] The water vapor measuring method according to the present invention includes a predetermined electrode between a first electrode having electronic conductivity and a second electrode made of a porous material, which are arranged to sandwich a first proton conductive solid electrolyte. While a current is flowing, the partial pressure of water vapor is measured based on the electromotive force generated between the first electrode and the third electrode made of a porous material, which are arranged to sandwich the second proton-conductive solid electrolyte. It is characterized by

[0011]

[Operation] In the present invention, a water vapor pump cell is constituted by a first proton conductive solid electrolyte and first and second electrodes placed between the solid electrolyte, and a second proton conductive solid electrolyte is provided.
A water vapor sensor cell is constituted by the proton-conductive solid electrolyte and the first and third electrodes arranged to sandwich the solid electrolyte.

In the water vapor sensor according to the present invention, when a predetermined current is caused to flow through the water vapor pump cell,
The water vapor partial pressure at the boundary between the water vapor pump cell and the water vapor sensor cell is maintained at a constant value determined by the current value, the thickness of the solid electrolyte, the proton diffusion coefficient, and the like.

On the other hand, in the water vapor sensor cell,
An electromotive force is generated based on the difference between the water vapor partial pressure at the boundary between the water vapor pump cell and the water vapor sensor cell and the water vapor partial pressure in the atmosphere. As described above, since the water vapor partial pressure at the boundary between the water vapor pump cell and the water vapor sensor cell is maintained at a constant value, the magnitude of this electromotive force depends on the water vapor partial pressure in the atmosphere. Therefore, by measuring the electromotive force, the partial pressure of water vapor in the atmosphere can be determined.

In this case, by setting the current flowing through the water vapor pump cell to be larger than the current flowing through the water vapor sensor cell, hydrogen or water vapor leakage occurs in the water vapor sensor cell due to the electronic conductivity of the proton conductive solid electrolyte. Even if this happens, the influence of this leakage on the measurement of water vapor partial pressure can be reduced. Therefore, in the water vapor sensor according to the present invention, leakage of hydrogen or water vapor caused by the electronic conductivity of the proton conductive solid electrolyte can be substantially ignored. Furthermore, since a reference gas and a solid reference substance are not required, the device can be made smaller.

[0015] The first electrode may be made of, for example, platinum,
Materials with electronic conductivity such as silver, palladium or oxide conductors can be used.

In the water vapor measuring method according to the present invention,
A predetermined current is passed between a first electrode and a second electrode that are arranged to sandwich the first proton conductive solid electrolyte. Then, protons move in the first proton conductive solid electrolyte, and the water vapor partial pressure or water vapor activity at the first electrode is maintained constant. In the method of the present invention, based on this water vapor partial pressure or water vapor activity, the first
The water vapor partial pressure in the atmosphere is measured based on the electromotive force generated between the electrode and the third electrode. Thereby, water vapor partial pressure can be measured stably over a long period of time.

[0017]

Embodiments Next, embodiments of the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing a self-reference type water vapor sensor according to an embodiment of the present invention. Electronically conductive electrode 1 (
Proton conductive solid electrolytes 2 and 3 are disposed on both sides of the first electrode), respectively. Electrode 4 (third electrode) is disposed opposite electrode 1 with solid electrolyte 2 in between. Furthermore, the electrode 5 (second electrode) is disposed opposite to the electrode 1 with the solid electrolyte 3 interposed therebetween. A water vapor pump cell is constituted by electrodes 1 and 5 and solid electrolyte 3, and electrodes 1 and 4
The solid electrolyte 2 constitutes a water vapor sensor cell. Furthermore, a power source 6 is interposed between the electrode 1 and the electrode 5,
A voltmeter 7 is interposed between the electrode 1 and the electrode 4.

Next, the operation of the water vapor sensor according to this embodiment will be explained.

The power supply 6 causes a constant current J to flow through the water vapor pump cell. As a result, the water vapor partial pressure near the electrode 1 becomes a value different from the water vapor partial pressure in the atmosphere. If the water vapor partial pressure in the atmosphere is C1, and the water vapor partial pressure at electrode 1 (that is, the water vapor partial pressure at the boundary between the water vapor pump cell and the water vapor sensor cell) is C2, there is C1 between electrode 1 and electrodes 4 and 5. -C2 A water vapor concentration gradient occurs. At this time, even if the solid electrolyte 2 of the water vapor sensor cell exhibits electronic conductivity, by making the current J flowing between the electrodes 1 and 5 larger than the current flowing between the electrodes 1 and 4, the solid electrolyte 2 Hydrogen or water vapor leakage due to electronic conductivity can be virtually ignored.

When the chemical diffusion coefficient of protons in the solid electrolytes 2 and 3 is D, and the thickness of the solid electrolyte 3 of the water vapor pump cell is x, the current J can be expressed by the following equation 1.

[0022]

[Equation 1] J=D(∂C/∂x) When this Equation 1 is expressed using the water vapor partial pressure C1 in the atmosphere and the water vapor partial pressure C2 at the electrode 1, it becomes as shown in the following Equation 2.

[0023]

[Equation 2] J=(C1 − C2 )·D/x By transforming Equation 2, the water vapor partial pressure difference near the electrodes 1 and 4 of the water vapor sensor cell can be expressed by Equation 3 below.

[0024]

[Math. 3] C1 − C2 = (x/D)・J On the other hand, the electromotive force E of the water vapor sensor cell at this time is expressed by the following formula 4, where R is the gas constant, T is the absolute temperature, and F is the Faraday constant.
It is expressed as

[0025]

[Equation 4] E=(RT/2F)・ln(C1/C2)
From Equation 4, the water vapor partial pressure ratio near the electrodes 1 and 4 of the water vapor sensor cell can be determined from Equation 5 below.

[0026]

[Equation 5]C1/C2=e(2FE/RT) From these equations, the water vapor partial pressure C1 in the atmosphere can be expressed as shown in Equation 6 below.

[0027]

[Math. 6] C1 =x・J/{D・[1-e(-2FE/
RT)]} That is, in the self-reference type water vapor sensor according to this embodiment, the partial pressure C1 of water vapor in the atmosphere can be determined by measuring the electromotive force E using the relationship of Equation 6.

Next, the results of actually manufacturing the self-reference type water vapor sensor according to this embodiment and investigating its characteristics will be explained.

First, as the electrode 1, an electronically conductive material made of, for example, Pt, Ni, or an oxide conductor was prepared. Further, as the solid electrolytes 2 and 3, for example, SrCe0.95
Yb0.05O3-x (however, x is 0 to 0.5
), CaZr0.9 In0.1 O3-x, BaC
Two plate-shaped proton conductive solid electrolytes in which e0.95Y0.05O3-x or zirconium phosphate, etc. were densely formed were prepared. Then, the electrode 1 was sandwiched between the two solid electrolytes 2 and 3 and heated to a temperature of 700 to 1000° C. to bond the electrode 1 and the solid electrolytes 2 and 3 together.

Next, solid electrolyte 2 is used as electrodes 4 and 5.
, 3, each porous electrode material was baked at a temperature of 700 to 1000° C. on the surface opposite to the surface in contact with the electrode 1. As a result, the water vapor sensor according to this example was completed.

The electrode 5 of this water vapor sensor is connected to the negative electrode of the power source 6, and the electrode 1 is connected to the positive electrode of the power source 6.
A predetermined current J was made to flow through the water vapor pump cell constituted by the solid electrolyte 5 and the solid electrolyte 3. Further, a voltmeter 7 was connected between the electrodes 1 and 4 so that the electromotive force in the water vapor sensor cell constituted by the electrodes 1 and 4 and the solid electrolyte 2 could be measured. Then, at a temperature of 700°C, the relationship between the electromotive force of this water vapor sensor and the water vapor partial pressure in the atmosphere was determined.

FIG. 2 is a graph showing the relationship between the water vapor partial pressure in the atmosphere on the horizontal axis and the electromotive force of the water vapor sensor on the vertical axis. As is clear from FIG. 2, the water vapor sensor of this example exhibited good electromotive force characteristics with respect to the amount of change in water vapor partial pressure.

Next, changes in the characteristics of the water vapor sensor of this example over time were investigated. FIG. 3 is a graph showing the relationship between time on the horizontal axis and electromotive force of the water vapor sensor and water vapor partial pressure in the atmosphere on the vertical axis. This figure 3
As is clear from the above, a good correlation could be obtained between the water vapor partial pressure and the electromotive force of the sensor over a long period of time.

[0034]

As explained above, in the self-reference type water vapor sensor according to the present invention, the water vapor pump cell is activated by the first electrode, the second electrode, and the first proton conductive solid electrolyte interposed between the two electrodes. Since the water vapor sensor cell is constituted by the first electrode, the third electrode, and the second proton-conductive solid electrolyte interposed between the two, it is possible to detect the atmospheric pressure without using a reference gas or a solid reference substance. Water vapor partial pressure can be measured. Therefore, the water vapor sensor according to the present invention can be easily miniaturized.

[0035] Furthermore, the water vapor measuring method according to the present invention is characterized in that the first proton-conductive solid electrolyte is disposed between the first
and the first and third electrodes, which are arranged to sandwich the second proton-conductive solid electrolyte while passing a predetermined current between the second electrodes.
Since water vapor partial pressure is measured based on the electromotive force generated between the electrodes, stable operation can be obtained over a long period of time.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a self-reference type water vapor sensor according to an embodiment of the present invention.

FIG. 2 is a graph diagram showing the relationship between water vapor partial pressure and electromotive force in a self-referenced water vapor sensor according to an embodiment of the present invention.

FIG. 3 is a graph diagram showing the relationship between time, electromotive force, and water vapor partial pressure in a self-referenced water vapor sensor according to an embodiment of the present invention.

[Explanation of symbols]

1, 4, 5; electrode 2,3; solid electrolyte

Claims (2)

[Claims] Claim 1: Centering on a first electrode having electronic conductivity, second and third electrodes made of a porous material are disposed on both sides thereof via first and second proton conductive solid electrolytes, respectively. The first electrode, the first proton conductive solid electrolyte, and the second electrode constitute a water vapor pump cell, and the first electrode, the second proton conductive solid electrolyte, and the third A self-reference type water vapor sensor characterized in that a water vapor sensor cell is constituted by electrodes. Claim 2: While passing a predetermined current between a first electrode having electronic conductivity and a second electrode made of a porous material, which are arranged to sandwich the first proton conductive solid electrolyte, The water vapor partial pressure is measured based on the electromotive force generated between the first electrode and the third electrode made of a porous material, which are arranged to sandwich the proton-conductive solid electrolyte. Measuring method.