JPH06201642A - Thin film critical current type overall air-fuel ratio sensor - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a thin film limiting current type full range air-fuel ratio sensor. [Structure] A first electrode 2, a first solid electrolyte 3 and a second electrode 4 are sequentially stacked on a porous substrate 1 to form a first solid electrolyte 3
Covers the first electrode 2 including the periphery of the first electrode 2, the second electrode 4 covers the first solid electrolyte 3 including the periphery of the first solid electrolyte 3, and further covers the second electrode 4. The second solid electrolyte 5 and the third electrode 6 are sequentially stacked to form the second solid electrolyte 5
Both the third electrode 6 and the third electrode 6 are arranged so that the peripheral portion of the second electrode 4 is exposed, and the first electrode 2, the second electrode 4, and the third electrode 6 are arranged.
Is formed using platinum, which is porous and gas permeable,
The first solid electrolyte 3 and the second solid electrolyte 5 are formed by using a dense solid electrolyte having oxygen gas conductivity and having no gas permeability. [Effect] The performance is good and the size can be significantly reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin film limiting current type full range air-fuel ratio sensor, and more particularly to a thin film limiting current type full range air / fuel ratio sensor which can be remarkably miniaturized and has excellent performance.

[0002]

2. Description of the Related Art Various types of air-fuel ratio sensors are used. For example, many air-fuel ratio sensors using oxygen sensors are used. However, also in the field of oxygen sensors, a great number of oxygen sensors having various forms and systems have been studied. Among these oxygen sensors, the oxygen sensor of the type that conducts oxygen ions by utilizing an electrochemical pump action is classified into three types of oxygen sensors each having a different basic configuration.

In the oxygen sensor having the first basic structure, the electrochemical pump cell is used together with the O ₂ monitor, and in the oxygen sensor having the second basic structure, the electrochemical pump cell is used together with the leakage pores. The oxygen sensor having the third basic structure is used in an electrochemical pump cell together with an O ₂ monitor and a leakage pore.

Three types of detection methods are known. The first detection method is a method of measuring oxygen concentration by pumping time, the second detection method is a method of measuring oxygen concentration by limiting current, and the third detection method is a method of measuring oxygen concentration by voltage when a constant current is applied. Is the way to do it.

[0005]

As a lean region air-fuel ratio sensor, zirconia limiting current type air-fuel ratio sensor has been widely used. The zirconia limiting current type air-fuel ratio sensor has the excellent property that the temperature dependence of the limiting current is small, and it is very easy to use, so it is most widely used among various air-fuel ratio sensors.

The lean region air-fuel ratio sensor of the zirconia limiting current type has excellent characteristics, but has a large problem as exemplified below. It is necessary to measure the entire region including both the lean region and the rich region with one air-fuel ratio sensor. It is necessary to improve the rapid operability of the air-fuel ratio sensor immediately after starting the engine. To improve fuel efficiency, it is necessary to reduce the power consumption of the air-fuel ratio sensor.

Further, as the exhaust gas regulations of automobiles in the United States have become stricter, not only the engine itself and the catalyst of the automobile but also the air-fuel ratio sensor used in the vehicle are required to have strict requirements. Then it became difficult to respond.

That is, a) In order to improve environmental problems occurring on a global scale, as a vehicle engine, an engine that has low fuel consumption, a small amount of harmful components emitted, and a high output when necessary is desired. . For that purpose, it is necessary to set the air-fuel ratio of the engine not only to the stoichiometric air-fuel ratio but also to the optimum air-fuel ratio from the lean region to the rich region according to the operating state. If the air-fuel ratio is to be faithfully controlled so as to attain the air-fuel ratio set as described above, precise detection of the air-fuel ratio by the air-fuel ratio sensor and feedback control of the air-fuel ratio based on this value are indispensable. Therefore, it is required to develop an air-fuel ratio sensor that can measure not only the lean region but also the rich region.

B) The amount of hydrocarbons discharged during cold starting of the engine of the vehicle is dominated by the amount of hydrocarbons discharged immediately after the engine starts before the air-fuel ratio sensor becomes operable. The hydrocarbon emission amount is closely related to whether or not the fuel amount is increased when the engine is started. Therefore, in order to optimize the fuel amount increase at the time of engine start in order to reduce the amount of hydrocarbon emission, it is required to operate the air-fuel ratio sensor rapidly immediately after the engine start.

However, in order to operate the air-fuel ratio sensor, it is necessary to raise the temperature of the air-fuel ratio sensor (particularly the sensor element) suitable for operation, and therefore it is necessary to raise the temperature rapidly in order to operate rapidly. is there. However, in general, the rapid temperature rise of the air-fuel ratio sensor is likely to lead to damage or characteristic deterioration due to an increase in thermal strain of each part of the sensor. Therefore,
In order to suppress the increase of the thermal strain and reduce the adverse effects thereof, it is necessary to downsize the air-fuel ratio sensor.

However, in the case of the zirconia limiting current type air-fuel ratio sensor, in order to have the measurable property in the lean region and the rich region, both the positive and negative electrodes are hermetically separated, and the electrode on the side not exposed to the exhaust gas is used. It is necessary to supply a high concentration of oxygen (air). Therefore, a passage for introducing air (outside air) for that purpose must be provided from the outside of the sensor to the air-fuel ratio detection section, and as a result, the sensor element usually has a tubular shape with one end closed (so-called cup shape). Is used. However, under the above constraints, there is a limit to how small the device can be made.

C) If the power consumption for heating the air-fuel ratio sensor is large, the power supply system for heating such as the generator and the battery must be increased more than ever, which causes an increase in vehicle weight. However, this will lead to deterioration of fuel efficiency.
Therefore, in order to improve fuel economy, it is necessary and effective to reduce the power consumption of the air-fuel ratio sensor.

By the way, in order to detect the overall air-fuel ratio, it is necessary to introduce air or generate oxygen in the air-fuel ratio sensor. And, it is possible to measure only in the lean region with an air-fuel ratio sensor that does not have such means, but even in the rich region, a bivalent function characteristic in which a current close to the lean region flows appears, so that it is in the lean region or the rich region. It becomes impossible to determine whether the air-fuel ratio is high or not, and the effective air-fuel ratio cannot be measured in the rich region.

In order to overcome the above problem, however, the lean region air-fuel ratio sensor is constituted by means for detecting the rich region separately from the means for detecting the air-fuel ratio in the lean region, and an electrochemical cell. It is necessary to add some means for switching the polarity of the oxygen pump current.

On the other hand, an air-fuel ratio sensor of the type in which air is introduced from the outside air into the air-fuel ratio sensor has basically a whole range air-fuel ratio detection performance. However, since the size of the air introduction passage is large, the air-fuel ratio sensor as a whole has a complicated and large structure, and its manufacture is very difficult. Furthermore, since the size is large, power consumption for heating the air-fuel ratio sensor is large, and rapid heating is not possible because thermal distortion is likely to occur, and the start-up time of the air-fuel ratio sensor is inevitably long. Become. Therefore, the power consumption increases, which adversely affects the fuel efficiency of the vehicle, and the start time is long, so that it is not possible to contribute to the reduction of hydrocarbon emission immediately after the cold start of the engine of the vehicle.

Similarly, an air-fuel ratio sensor having an oxygen generating mechanism in the sensor basically has the whole range air-fuel ratio detecting performance. However, the air-fuel ratio sensor has a complicated structure for adding an oxygen generation mechanism, and its size is also large. Therefore, such an air-fuel ratio sensor is very difficult to manufacture, and since it consumes a large amount of power, rapid heating is impossible and the starting time of the air-fuel ratio sensor becomes long. Therefore,
It has an adverse effect on the improvement of the fuel efficiency of the vehicle and cannot contribute to the reduction of hydrocarbon emission immediately after the cold start of the engine of the vehicle.

The present invention has been made to solve the above-mentioned problems in the conventional air-fuel ratio sensor. The object of the present invention is to limit the zirconia limit current such that the limit current is small even if the temperature changes. Inherit all of the excellent properties of the air-fuel ratio sensor, and have the ability to detect and measure air-fuel ratio over the entire range from lean to rich without requiring a passage for introducing air (outside air). , To provide a thin film air-fuel ratio sensor.

[0018]

That is, in a thin film limiting current type global air-fuel ratio sensor of the present invention, a first electrode, a first solid electrolyte and a second electrode are sequentially laminated on a porous substrate to form a first electrode.
The solid electrolyte covers the first electrode including the periphery of the first electrode, the second electrode covers the first solid electrolyte including the periphery of the first solid electrolyte, and the second solid electrolyte covers the second electrode. And a third electrode are sequentially stacked to form a second solid electrolyte and a third electrode.
The electrodes are arranged such that the peripheral portion of the second electrode is exposed, and the first electrode, the second electrode and the third electrode are formed by using platinum that is porous and has gas permeability, and the first solid electrolyte and the first electrode are formed. The two solid electrolytes are characterized by being formed by using a dense solid electrolyte having oxygen ion conductivity and having no gas permeability.

In the present invention, a thin film limiting current type global air-fuel ratio sensor in which fine and communicating grooves are provided at a high density in a portion of the first electrode in contact with the porous substrate is preferable.
The size, shape, number, etc. of the grooves are appropriately selected.

Further, in the present invention, fine and fine passages communicating with each other are provided in the second electrode at a high density, and the second electrode is provided with the outer peripheral portion of the passage network and the outer portion of the outer peripheral end face of the second electrode. A thin film limiting current type full range air-fuel ratio sensor provided with a passage connected through is also preferable. The size, shape, number, etc. of the passages are appropriately selected.

Further, in the present invention, fine and communicating grooves are provided in a high density in a portion of the first electrode in contact with the porous substrate, and fine and communicating passages are provided in a high density in the second electrode. Further, a thin film limiting current type full range air-fuel ratio sensor in which the second electrode is provided with a passage that connects the outer peripheral portion of the passage network and the outside through the outer peripheral end surface of the second electrode is particularly preferable.

The porous substrate may be formed using a material commonly used in this field, such as alumina. The size and shape are optimally selected. Further, for example, a heater may be provided at a suitable position as desired.

The solid electrolyte can be formed of a suitable size and shape by using a conventional material such as zirconia or yttria alone or in combination.

The electrodes may be formed by a printing method using, for example, platinum paste. Its shape and thickness are selected appropriately.

[0025]

In the thin film limiting current type air-fuel ratio sensor of the present invention, as in the case of the conventional zirconia limiting current type air-fuel ratio sensor, air (outside air) for maintaining a high concentration of oxygen in one electrode is used. Rather than naturally supplying air to the sensor element by diffusion by providing a passage for introduction, oxygen is dissociated from water vapor and carbon dioxide contained in a large amount in the combustion exhaust of the vehicle engine and forced by an oxygen pump action. Supply. For this reason, the sensor of the present invention is a limiting current type full range air-fuel ratio sensor capable of rapid heating, rapid temperature rise, and rapid operation with low power consumption, and has a full range air-fuel ratio detectability, rapid actuation,
Low power consumption.

[0026]

The present invention will be described in more detail with reference to the following examples.

Example 1 FIG. 1 shows a sensor according to Example 1 of the present invention. The structure of this sensor is, for example, a porous substrate 1 made of alumina or the like.
A first electrode 2, a first solid electrolyte 3 and a second electrode 4 are sequentially stacked on top of each other, the first solid electrolyte 3 covers the first electrode 2 including the periphery of the first electrode 2, and the second electrode 4 is The first solid electrolyte 3 including the periphery of the first solid electrolyte 3 is covered and hidden, and the second solid electrolyte 5 and the third electrode 6 are sequentially laminated on the second electrode 4, and the second solid electrolyte 5 and the third solid electrolyte 5 are stacked. The electrodes 6 are arranged so that the peripheral portion of the second electrode 4 is exposed, and the first electrode 2, the second electrode 4, and the third electrode 6 are formed by using platinum that is porous and has gas permeability (platinum). Formed by a printing method of applying a paste), a first solid electrolyte 3 and a second solid electrolyte 3
The solid electrolyte 5 was formed by using a dense solid electrolyte having oxygen ion conductivity and having no gas permeability (using zirconia).

A heater 7 is provided on the back surface of the porous substrate 1. The heater 7 is connected to the heater heating means 8.
The heater 7 is preferably made of platinum, palladium, or an alloy thereof, and platinum was used in this example.

Further, as shown in FIG. 1, a voltage applying means 9 for applying a positive voltage to the second electrode 4 with respect to the third electrode 6 is provided. Further, the voltage applying means 10 for applying a positive voltage to the second electrode 4 with respect to the first electrode 2 is provided. 1
Reference numeral 1 is a current measuring means for measuring the flowing current.

When a positive voltage is applied to the second electrode 4 with respect to the third electrode 6, this portion including the second solid electrolyte 5 is oxygen pumped to move from the third electrode 6 side to the second electrode 4 side. Oxygen ions are transported to. In this case, under a lean air-fuel ratio atmosphere, oxygen remaining in the atmosphere is supplied to the third electrode 6 by gas diffusion. On the other hand, in a rich air-fuel ratio atmosphere, since oxygen is not sufficiently present in the atmosphere, water vapor and carbon dioxide supplied to the third electrode 6 from the atmosphere by gas diffusion are dissociated in the third electrode 6 to generate oxygen ions. Oxygen ions are produced and transported as in the previous case. The transported oxygen ions are converted into oxygen gas at the interface between the second solid electrolyte 5 and the second electrode 4. In this way, in the porous second electrode 4, the excess oxygen state is always maintained regardless of the lean air-fuel ratio atmosphere or the rich air-fuel ratio atmosphere.

The portion composed of the first electrode 2, the first solid electrolyte 3 and the second electrode 4 functions as a limiting current type air-fuel ratio detecting section, and the air-fuel ratio can be measured from the limiting current. In this state, using the excess air ratio in the combustion exhaust around the sensor as a parameter, the current of the limiting current type air-fuel ratio detection unit −
When the voltage characteristics were measured, FIG. 2 was obtained. Further, the current measured by applying an applied voltage shown by the one-dot chain line in FIG.
Shown in.

On the other hand, the characteristics of the conventional air-fuel ratio sensor are shown in FIGS. 11 is a diagram corresponding to FIG. 2, and FIG. 12 is a diagram corresponding to FIG. 2 and 1
As is clear from FIG. 1 and comparison between FIG. 3 and FIG. 12, in the air-fuel ratio sensor of the prior art, the limiting current characteristic is in the third quadrant in an atmosphere where the excess air ratio is smaller than 1 (that is, fuel rich). Whereas it appears, it appears in the fourth quadrant in the present invention.

As a result, the conventional product has a positive applied voltage in an atmosphere where the excess air ratio is larger than 1 (that is, fuel lean), and a negative applied voltage in an atmosphere where the excess air ratio is smaller than 1 (that is, fuel rich). Then, it was necessary to switch the polarity of the applied voltage, whereas in the product of this example, the positive applied voltage was always required, and thus the polarity switching was eliminated.

Therefore, in the product of this embodiment, it is no longer necessary to additionally provide means for detecting whether the detection atmosphere is fuel lean or fuel rich.

Further, since it is not necessary to switch the polarity of the applied voltage, the product of this embodiment also eliminates the generation of a noise-like output signal component due to the switching.

The present sensor does not have an oxygen introduction portion from the outside air unlike the conventional air-fuel ratio sensor of the prior art, but as mentioned above, an electrochemical cell acting as an oxygen pump is added on the second electrode 4. Oxygen is supplied by the above-mentioned method, and an operation equivalent to that of the conventional air-fuel ratio sensor according to the related art is performed.

The second electrode 4 is a porous body, and its outer peripheral portion communicates with the outside of the sensor. Therefore, when excess oxygen is supplied to the second electrode 4, the second electrode 4 is excessive from the outer peripheral portion to the outside. Since no oxygen is discharged, it does not hinder the measurement. Further, the oxygen gas in the first electrode 2 is discharged into the porous substrate 1.

Embodiment 2 FIG. 4 shows a sensor according to Embodiment 2 of the present invention. The present sensor is the same as the sensor of the first embodiment except that fine and communicating grooves 12 are provided at a high density in a portion of the first electrode 2 in contact with the porous substrate 1. In the limiting current type oxygen sensor, when the current per unit area of the electrode (that is, the current density) is increased, not only the effect of the electrode resistance increases and the initial characteristics are easily deteriorated, but also the long-term stability is difficult to obtain.
Therefore, it is an effective method to reduce the current density by using the porous substrate 1 having a large diffusion resistance, but on the other hand, a portion (ineffective portion) where oxygen gas is not supplied in the surface of the first electrode 2
The increase in the amount tends to cause an increase in electrode resistance, and it is not always possible to obtain a sufficient effect. The present embodiment provides an effective solution to this problem.

The groove 12 has the function of facilitating the diffusion of oxygen gas within the surface of the first electrode 2 and suppressing the oxygen concentration distribution within the surface to a low level. Thereby, when the porous substrate 1 having a large diffusion resistance is used, it is possible to suppress an increase in a portion of the surface of the first electrode 2 to which oxygen gas is not supplied, and prevent an increase in electrode resistance. Therefore, in combination with the decrease in current density, good initial characteristics are obtained, and very good results are obtained for long-term stability.

FIG. 5 is a plan view of the first electrode 2 of the sensor of FIG. In this example, the grooves 12 are provided in a grid pattern at predetermined intervals.

Embodiment 3 FIG. 6 shows a sensor according to Embodiment 3 of the present invention. In this sensor, fine and fine passages 13 communicating with each other are provided in the second electrode 4 at a high density, and further, the second electrode 4 is provided with the outer peripheral portion of the passage network and the outside through the outer peripheral end face of the second electrode 4. Passage 1 to connect
The sensor is the same as that of the first embodiment except that No. 4 is provided.

When the pressure inside the second electrode 4 becomes higher than the pressure outside due to the action of the oxygen pump, excess oxygen gas is discharged to the outside through the fine passages 13 and 14 described above, and thus the second electrode 4 The rise in internal pressure is suppressed.

FIG. 7 shows the AA of the second electrode 4 of the sensor of FIG.
Figure 3 shows a plan view along the line. In this example, the passages 13 are provided in a lattice pattern at predetermined intervals, and the passages 14 are provided in the outer peripheral portion of the passage 13 in the vertical and horizontal directions so as to connect the outer peripheral portion of the passage 13 to the outside.

Embodiment 4 FIG. 8 shows a sensor according to Embodiment 4 of the present invention. Similar to the sensor of the second embodiment, the present sensor is provided with fine and dense grooves 12 communicating with each other in the portion of the first electrode 2 which is in contact with the porous substrate 1, and like the sensor of the third embodiment. Passages 13 that are fine and communicate with each other are provided in the second electrode 4 at a high density, and a passage that connects the outer peripheral portion of the passage network and the outside to the second electrode 4 via the outer peripheral end face of the second electrode 4. The sensor is the same as that of the first embodiment except that 14 is provided.

Therefore, this sensor has both the advantages of the sensor of the second embodiment and the advantages of the sensor of the third embodiment.

FIG. 9 is a plan view of the first electrode 2 of the sensor shown in FIG. In this example, the grooves 12 are provided in a grid pattern at predetermined intervals.
10 is a plan view of the second electrode 4 of the sensor of FIG. 8 taken along the line AA. In this example, the passages 13 are provided in a grid pattern at predetermined intervals, and the passages 14 are provided in the outer peripheral portion of the passage 13 in the vertical and horizontal directions so as to connect the outer peripheral portion of the passage 13 to the outside.

[0046]

Since the thin-film limiting current type global air-fuel ratio sensor of the present invention has the above-mentioned structure, it has various effects as exemplified below. 1) Since this sensor supplies oxygen to the sensor element section by the oxygen pump action, there is no need to provide an oxygen introducing section in the sensor element section, and there is no need for a portion that communicates with the outside of the housing. Can be small. 2) Since this sensor does not need to be provided with an oxygen introduction part in the sensor element part, the sensor element part does not need to have a three-dimensional structure such as a tube like a conventional sensor, and can have a planar structure. Therefore, it is suitable for preparation by thin film technology. 3) Since this sensor does not require an oxygen introduction part, the airtight structure of the housing becomes simple. 4) Since this sensor is of the limiting current type, the temperature dependence of the limiting current is small, so the method of use is simple. 5) The present sensor can measure the air-fuel ratio for the air-fuel ratio from the lean region to the rich region without switching the polarity of the applied voltage. 6) Since this sensor prepares the part that acts as the electrolytic current type air-fuel ratio detection part, the part that dissociates oxygen and the part that acts as the oxygen pump by thin film technology on the porous substrate, it should be extremely small. Is possible. 7) Since this sensor is small, the power required for heating is small. 8) Since this sensor is small, the thermal strain generated is small even when the temperature is rapidly raised, and the temperature can be rapidly raised. 9) Since this sensor is prepared on the porous substrate by the thin film technique, it can be mass-produced well, can be supplied in large quantities at low cost, and has great advantages in industrial production.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an air-fuel ratio sensor according to a first embodiment of the present invention.

FIG. 2 is a diagram showing current-voltage characteristics of a limiting current type air-fuel ratio detection unit of the air-fuel ratio sensor of the first embodiment.

3 is a diagram showing a current measured by applying an applied voltage shown by a chain line in FIG. 2 in the air-fuel ratio sensor of Example 1. FIG.

FIG. 4 is an explanatory diagram of an air-fuel ratio sensor according to a second embodiment of the present invention.

5 is a plan view of a first electrode of the air-fuel ratio sensor of FIG.

FIG. 6 is an explanatory diagram of an air-fuel ratio sensor according to a third embodiment of the present invention.

7 is a plan view of the second electrode of the air-fuel ratio sensor of FIG. 6 taken along the line AA.

FIG. 8 is an explanatory diagram of an air-fuel ratio sensor according to a fourth embodiment of the present invention.

9 is a plan view of a first electrode of the air-fuel ratio sensor of FIG.

10 is a plan view of the second electrode of the air-fuel ratio sensor of FIG. 8 taken along the line AA.

FIG. 11 is a diagram showing current-voltage characteristics of a limiting current type air-fuel ratio detector of a conventional air-fuel ratio sensor.

FIG. 12 is a diagram showing a current measured by applying a predetermined applied voltage in a conventional air-fuel ratio sensor.

[Explanation of symbols]

 1 Porous Substrate 2 First Electrode 3 First Solid Electrolyte 4 Second Electrode 5 Second Solid Electrolyte 6 Third Electrode 7 Heater 8 Heater Heating Means 9, 10 Voltage Applying Means 11 Current Measuring Means 12 Grooves 13, 14 Passages

Continuation of the front page (51) Int.Cl. ⁵ Identification number Reference number within the agency FI Technical display location 7363-2J 327 E (72) Inventor Saji Kei-shi 1-41, Nagakage-cho, Aichi-gun, Aichi-gun Toyota Central Research Institute, Inc. (72) Inventor, Shoji Takeuchi, Nagakute-cho, Aichi-gun, Aichi Prefecture, Nagaminato 1 41 of Yokomichi, Toyota Central Research Institute Co., Ltd. No. 41 Yokomichi Toyota Central Research Institute Co., Ltd.

Claims (4)

[Claims] 1. A first electrode, a first solid electrolyte and a second electrode are sequentially stacked on a porous substrate, and the first solid electrolyte is the first
The first electrode including the periphery of the electrode is covered, the second electrode covers the first solid electrolyte including the periphery of the first solid electrolyte, and the second solid electrolyte and the third electrode are further provided on the second electrode. The second solid electrolyte and the third electrode are sequentially laminated,
The first electrode, the second electrode, and the third electrode are arranged so that the peripheral portions of the electrodes are exposed, and the first electrode, the second electrode, and the third electrode are formed of platinum having gas permeability, and the first solid electrolyte and the second solid electrolyte are dense. A thin-film limiting current type all-air-fuel ratio sensor characterized by being formed by using a solid electrolyte of oxygen ion conductivity having no gas permeability. 2. The thin film limiting current type full range air-fuel ratio according to claim 1, wherein fine and communicating grooves are provided at a high density in a portion of the first electrode in contact with the porous substrate. Sensor. 3. The second electrode according to claim 1, wherein the second electrodes are provided with fine and fine passages communicating with each other at a high density, and the second electrode is provided with an outer peripheral portion of the passage network and the outside. A thin film limiting current type full range air-fuel ratio sensor, characterized in that a passage is provided to connect through the outer peripheral end face. 4. The groove according to claim 1, wherein fine grooves communicating with each other are provided at a high density in a portion of the first electrode in contact with the porous substrate, and fine passages communicating with each other are formed in the second electrode. A thin film limiting current type whole space, characterized in that it is provided at a high density, and further, the second electrode is provided with a passage that connects the outer peripheral portion of the passage network and the outside through the outer peripheral end face of the second electrode. Fuel ratio sensor.