JPH02190758A - Electrochemical element - Google Patents

Abstract

PURPOSE:To provide both accuracy and responsiveness even in a low-temp. atmosphere by providing a porous body (porous catalyst carrier) on which a catalyst to put a gas to be measured in a nonequil. state into an equil. state separately from a porous protective layer. CONSTITUTION:Planar solid electrolytes 1 to 3 are laminated and integrated to form one planar solid electrolyte body. A measuring electrode 10 and a reference electrode 11 are provided on both surfaces of the electrolyte 1 in contact with the planar solid electrolyte body. A groove 5 for air introduction is provided in the central part of the electrolyte 2 in order to expose the electrode 11 to a reference gas, such as air. The porous protective layer 21 is provided on the electrode 10 and is laminated and integrated with the electrolyte 1 which constitutes a base plate. The porous catalyst carrier 22 having the effect of putting the nonequiv. gas into the equil. state is provided by being laminated and integrated with the electrolyte 3 on the opposite side of the protective layer 21. The electrode 10 and the electrode 11 are connected via a lead pattern or lead wires to a voltmeter 51, by which the electromotive force based on the difference in the partial pressures of the gases on the respective electrodes is measured.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is mainly used to measure the amount of oxygen contained in exhaust gas discharged from internal combustion engines used in automobiles, combustion equipment, etc. in industrial furnaces, etc. Regarding the oxygen sensor element of
In particular, it relates to electrochemical elements that achieve both responsiveness and precision.

(Prior Art) Conventionally, for example, an oxygen sensor element as an electrochemical element using a plate-shaped oxygen ion conductive solid electrolyte body has been used.
For example, JP-A-60-36949, JP-A-55-1
This is disclosed in Japanese Patent No. 25448 and the like.

The operating principle of these oxygen sensor elements is that the outer electrode is exposed to the gas to be measured, the inner electrode is isolated from the gas to be measured by an airtight solid electrolyte, etc., and is exposed to a reference gas such as air with a nearly constant oxygen partial pressure. , based on the concentration difference between the inner electrode and the outer electrode (using changes in electromotive force). In particular, since the oxygen partial pressure changes greatly after the stoichiometric air-fuel ratio point,
In many cases, it was used as a means of detecting the stoichiometric air-fuel ratio.

This type of sensor is often used in an environment where temperature changes over a relatively wide range, and it has been difficult to accurately detect the theoretical air-fuel ratio point, especially in a low temperature atmosphere.

In addition, during actual use, the chemical characteristics of the electrode surface change with the passage of time, and the point of air-fuel ratio control of the sensor changes accordingly.

(Problem to be solved by the invention) In order to solve this problem, for example, Japanese Patent Laid-Open No. 54-4179
In Publication No. 4, a catalyst is provided in the protective layer of the outer electrode in order to convert the non-equilibrium gas to be measured into an equilibrium gas before it directly reaches the outer electrode, thereby preventing deterioration of sensor accuracy in a low temperature range. A method is disclosed.

However, in the method described in JP-A-54-41794 mentioned above, the gas to be measured reaches the outer electrode after being adsorbed on the catalyst layer, so it is possible to improve accuracy in the low temperature range. There was a problem that responsiveness deteriorated.

The purpose of the present invention is to solve the above-mentioned problems and to provide an electric sensor that can achieve both accuracy and responsiveness even in a low-temperature atmosphere, and that can reduce changes over time in the air-fuel ratio control point of the sensor. It is intended to incorporate chemical elements.

(Means for Solving the Problems) The electrochemical element of the present invention includes a solid electrolyte body, a measurement electrode provided in contact with the surface of the solid electrolyte body and exposed to a gas to be measured, and a a porous protective layer that prevents the gas to be measured from directly contacting the measurement electrode, and a reference electrode provided inside the solid electrolyte body in contact with the solid electrolyte body and exposed to a reference gas. , a porous body supporting a catalyst that is provided on the surface of the solid electrolyte body except for the location where the measurement electrode is provided, and brings the gas to be measured in a non-equilibrium state into an equilibrium state. It is characterized by:

(Function) In the above-mentioned configuration, since the porous body supporting the catalyst of the present invention (hereinafter referred to as porous catalyst carrier) is not present on the measurement electrode, the exchange rate of the gas to be measured in the measurement electrode is Since the atmosphere in the vicinity of the measurement electrode can be adjusted without any influence, the drawback of deteriorating response is eliminated.

In addition, on a porous catalyst carrier, the gas to be measured in a non-equilibrium state is converted into an equilibrium gas, and the temperature near the measurement electrode increases due to the reaction heat generated at that time. However, the temperature of the element itself can be kept high.

With the above, it is possible to accurately detect the stoichiometric air-fuel ratio point in a low temperature range without impairing responsiveness.

Furthermore, since it is possible to support a large amount of catalytic metal,
The life of the catalyst supported on the porous catalyst carrier is increased. As a result, the deterioration of the catalytic ability of the measuring electrode over time can be compensated for over a long period of time, and the change over time of the air-fuel ratio control point of the sensor becomes extremely small.

In addition, if the porous catalyst carrier is placed as a solid electrolyte so that the substrate and the porous catalyst carrier are in direct contact with each other, the difference in thermal expansion characteristics between the porous catalyst carrier and the substrate will be reduced. This is preferable compared to using different materials. Furthermore, heating means such as a heater may be provided. Furthermore, it is preferable to provide a porous catalyst carrier on the side opposite to the side on which the measurement electrode is arranged.

(Example) FIGS. 1(a) and 1(b) are exploded perspective views showing the structure of an example of an electrochemical device according to the present invention, and its X-X'
FIG. In FIG. 1, plate-shaped solid electrolytes 1, 2.3 are laminated and integrated to form one plate-shaped solid electrolyte body. A measurement electrode 10 and a reference electrode 11 are provided on both sides of the plate-shaped solid electrolyte body in contact with the plate-shaped solid electrolyte body. In order to expose the reference electrode 11 to a reference gas such as air, a groove 5 for introducing air is provided in the center of the plate-shaped solid electrolyte 2. A porous protective layer 21 is provided on the measuring electrode 10, and is laminated and integrated with a plate-shaped solid electrolyte l serving as a substrate.

On the side opposite to the porous protective layer 21 of the plate-shaped solid electrolyte body,
A porous body (porous catalyst carrier) 22 supporting a catalyst having the function of bringing a non-equilibrium gas into an equilibrium state is provided in a laminated manner with the plate-shaped solid electrolyte 3 . The measurement electrode 10 and the reference electrode 11 are connected to a voltmeter 51 via a lead pattern or a lead wire, and the electromotive force based on the difference in gas partial pressure on each electrode is measured.

The main point of the present invention is that the porous catalyst carrier 22 is provided, and the gas to be measured is often in a non-equilibrium state, and when it reaches the catalyst carrier 22, it becomes an equilibrium gas.

The reaction heat generated at that time causes the detection part (electrode 10.1
1 part) can be maintained at high temperatures.

Further, the electrochemical element of the present invention is normally used as an assembly (not shown) with metal fittings, etc., but since the detection part is surrounded by a metal protective cover etc., the catalytic The gas equilibrated on the carrier 22 also effectively affects the measurement electrode 10 side, making it possible to effectively prevent unbalanced gas from acting on the electrode 10.

Therefore, the element of the present invention operates with high detection accuracy even in a gas atmosphere to be measured at a relatively low temperature. Furthermore, since the catalyst carrier 22 is separated from the protective layer 21,
Even if the gas to be measured is adsorbed on the catalyst, there is no problem that the arrival of the gas to be measured to the measurement electrode 10 is delayed.
Responsiveness is not sacrificed either.

In the electrochemical device of the present invention shown in FIG. 1 described above, the plate-shaped solid electrolytes 1, 2.3 are Y, 0. .

M, 0. Zr stabilized with Cab, Y1g03, etc.
O,,CeO.

Any known solid electrolyte may be used.

The measurement electrode IO1, reference electrode 11, etc. are Pt, Rn, Rh.
, Pd.

One type of platinum group metal such as Ir or an alloy thereof may be used.

Also ZrO2, Al zO++ Mgo, Sing
A cermet electrode made of a ceramic mixed with the above-mentioned platinum group metal is advantageous in terms of high-temperature durability.

The porous protective layer 21 and the porous catalyst carrier 22 are made of ZrO□.

^1203. Any ceramic such as MgO may be used as long as it has a higher porosity than the substrate when the substrate is co-fired with the solid electrolyte body. for example,
It is preferable to use partially stabilized zirconia for the substrate and completely stabilized zirconia for the porous body because the difference in coefficient of thermal expansion can be made extremely small during simultaneous firing. The thickness of the porous body is 10
μ11-1111111, but adjustments are made such that the thinner the layer, the smaller the porosity, and the thicker the layer, the larger the porosity. It is desirable that the porosity is 10% to 70% open pores.

The catalyst supported on the porous catalyst carrier 22 is a platinum group metal or a perovskite such as La5rCo03, but an alloy of platinum group metals is most preferable, and Pt/Rh
It is most desirable to adjust the ratio to ＝'''/+ to 10.The supporting method can be carried out by known methods such as baking method or ion exchange method, or by using ceramics when forming the porous catalyst carrier into a sheet shape. It may be provided by uniformly mixing and co-firing with the solid electrolyte body.The supported amount is 0.1% to 0.1% to the weight of the porous catalyst carrier.
It is desirable to set it to about 30%.

Each part may be formed using a known method, and the plate-like solid electrolyte 1, 2.3, porous body 21, 22, etc. are formed by a sheet forming method such as a doctor blade method to a thickness of 0.05 to 1 mm. However, by appropriately applying a screen printing method, etc., which is suitable as a method for forming electrodes 10, 11, etc.,
It will be formed with a thickness of about 30 μm.

To explain an actual example, the exhaust gas of an actual vehicle equipped with a sensor using an element with this structure is λ = 0.99.
It was controlled at 5. λ is calculated using the following formula (1).

Here, A/F: Air-fuel ratio determined from actual measurements ^+/F1
: Theoretical air-fuel ratio This sensor detects that the exhaust gas λ is 0.9 over time.
It changed to 92. On the other hand, when a conventional sensor without catalyst support is installed, the exhaust gas is controlled to λ = 0.998, and as time passes, the exhaust gas λ becomes 0.99.
It changed to 2. By using the electrochemical element of the present invention, the time-dependent change in air-fuel ratio control of the sensor can be reduced to 6/10 compared to the conventional one.
We were able to reduce the size from 00 to 3/1000, which is half the size.

FIG. 2 is a sectional view showing another specific example of the present invention. Second
In the figure, components with the same numbers as in FIG. 1 are the same as in FIG. 1, and therefore will be omitted.

In the embodiment shown in FIG. 2, the most characteristic feature is the metal 7.
It is 1112. The material of the metal layer 12 may be the same as that of the measurement electrode 1O, and may be a platinum group metal alone or a cermet of these and ceramics. The metal layer 12 and the measuring electrode IO are not connected by a good electrical conductor, so that their operation as a measuring electrode can be almost ignored.

It is preferable that the porous catalyst carrier 22 be used as a solid electrolyte like the substrate 3, and the metal layer 12 be buried in the solid electrolyte. Further, forming the metal layer 12 is preferable because the reaction heat generated on the porous catalyst carrier 22 side increases.

FIG. 3 is a sectional view showing still another specific example of the present invention.

In the embodiment shown in FIG. 3, the different features compared to the example shown in FIG. 1 are the omission of the groove 5 and the addition of porous layers 23 and 24. Like the porous protective layer 21, the porous layer 23 has a role of protecting the measurement electrode 10 from corrosive gas to be measured. By making the protective layer have a two-layer structure in this way, it is possible to provide it with various characteristics. Increasing the porosity of the protective layer 21 reduces the protective effect, but if the porosity of the protective layer 23 is appropriately adjusted in a decreasing direction, the protective layer as a whole becomes less likely to be clogged and has a satisfactory protective effect. .

Conversely, reducing the porosity of the protective layer 21 will reduce the activity of the electrode 10, but adjusting the porosity of the protective layer 23 to a larger value will improve the overall protective effect without sacrificing the activity of the electrode. It can be an excellent protective layer.

The other porous layer 24 serves to keep it filled with a reference gas. A voltage is applied between the electrodes 10 and 11 from an external power source 62 via a high resistance element 61, for example,
The oxide ions move from the measuring electrode 10 side to the reference electrode ll side, exposing the reference electrode 11 to an atmosphere of almost 100% oxygen gas. Although it is desirable to provide the high resistance element 61 inside the element or on the element surface, it may be provided separately outside. The material of the porous layers 23 and 24 is ceramic similar to that of the porous bodies 21 and 22, and is usually formed by a screen printing method, but may be appropriately formed to a thickness of about 5 μm to 1 mm by a sheet forming method such as a doctor blade method. That will happen.

FIG. 4 is a sectional view showing still another specific example of the present invention.

The embodiment shown in FIG. 4 is characterized in that a heater 15 and a ceramic insulating layer 25 are provided. The heater 15 embedded in the ceramic insulation M25 is completely isolated from the gas to be measured by the airtight solid electrolyte 2. The insulating layer 25 is usually made of a different material from the solid electrolyte 2, and is preferably porous in order to avoid distortion caused by the difference in thermal expansion characteristics. As the power source 63, when the electrochemical element is used in an automobile sensor or the like, a battery power source is used as is.

FIG. 5 is a sectional view showing still another specific example of the present invention,
The same members as in FIG. 1 are given the same reference numerals, and their explanations will be omitted. The embodiment shown in FIG. 5 shows an example in which the porous catalyst carrier 22 and the porous protective layer 21 are present on the same surface. In this example as well, the effects of the present invention can be similarly achieved. Note that the porous catalyst carrier 22 and the porous protective layer 21 may be juxtaposed in the longitudinal direction or in the width direction.

FIG. 6 is a sectional view showing still another specific example of the present invention,
The same members as in FIG. 1 are given the same reference numerals, and their explanations will be omitted. The embodiment shown in FIG. 6 is characterized in that the solid electrolyte I is cylindrical, and the present invention can also be applied to an electrochemical element having a cylindrical shape with a bottom.

(Effects of the Invention) As is clear from the above description, according to the electrochemical device of the present invention, a porous body supporting a catalyst that brings a gas to be measured in a non-equilibrium state into an equilibrium state can be used as a measuring electrode. By providing it separately from the porous protective layer that protects it, it is possible to achieve both accuracy and responsiveness even in a low-temperature atmosphere, and it is also possible to reduce changes over time in the air-fuel ratio control point of the sensor.

[Brief explanation of the drawing]

1(a) and 1(b) are exploded perspective views showing the structure of an example of the electrochemical device of the present invention and a sectional view taken along the line xx', and FIGS. FIG. 3 is a cross-sectional view showing the structure of another example of the electrochemical device of FIG.

Claims (1)

[Claims] 1. A solid electrolyte body, a measurement electrode provided in contact with the surface of the solid electrolyte body and exposed to a gas to be measured, and a measurement electrode located on the measurement electrode to prevent the gas to be measured from directly contacting the measurement electrode. a porous protective layer; a reference electrode provided inside the solid electrolyte body in contact with the solid electrolyte body and exposed to a reference gas; and a reference electrode provided on the surface of the solid electrolyte body and provided with the measurement electrode. 1. An electrochemical element comprising: a porous body supporting a catalyst for bringing a gas to be measured, which is in a non-equilibrium state, into an equilibrium state, the porous body being provided at a location other than a location where the gas is in a non-equilibrium state;