JP2003149191A - Gas sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gas sensor capable of accurately measuring gas concentration such as the concentration of hydrogen gas. SOLUTION: The ratio (area ratio) S1/S2 of a first area S1 (a mesh part of Figure 1) of a first electrode 3 existing outside of a projected part in the case of projecting a second electrode 5 on the first electrode 3, to a second area S2 (an oblique line part of Figure 2) overlapped with the first electrode 3 in the case of projecting the second electrode on the first electrode 3, is set to be less than 0.35. That is, the first electrode 3 and the second electrode 5 are arranged and dimensioned so that the second electrode 5 is within the first electrode 3 in the case of projecting the second electrode 5 on the first electrode 3, and the area ratio S1/S2 is set to be less than 0.35.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor such as a hydrogen gas sensor for measuring the hydrogen gas concentration in the fuel gas of a fuel cell.

[0002]

2. Description of the Related Art In recent years, fuel cells have been extensively researched as a highly efficient and clean power source amid environmental problems on a global scale. Among them, a polymer electrolyte fuel cell (PEFC) is expected for automobiles and households due to advantages such as low temperature operation and high power density.

In the case of this fuel cell, it is promising to use a reformed gas as the fuel gas, but in order to improve efficiency and the like, a sensor which can directly detect hydrogen in the reformed gas is required. . The above sensor has a low operating temperature (less than about 100 ° C) because it is measured in a hydrogen-rich atmosphere.
is necessary.

As such a low temperature operation type sensor, for example, in the publication of EP1103807A2, as shown in FIG. 6, a proton conductive layer 101 made of a polymer electrolyte is used, and the first electrode is formed on the surface of the proton conductive layer 101. A sensor having a structure in which the 102, the second electrode 103, and the reference electrode 104 are installed is proposed.

[0005]

However, in the above-mentioned sensor, the second electrode 103 is sandwiched with the proton conducting layer 101 interposed therebetween.
Since the first electrode 102 is arranged in a portion other than the portion where is projected, that is, the first electrode 102 is arranged not only in the second electrode 103 but also in the other reference electrode 104. When pumping hydrogen (specifically, protons: H ⁺ which are hydrogen ions) from the first electrode 102 to the second electrode 103, a portion where hydrogen is difficult to be pumped (that is, a portion having a high hydrogen concentration: a portion of the mesh in FIG. 6) ) Exists.

Moreover, in this sensor, the first electrode 102
And the reference electrode 104 is controlled to a constant potential. In that case, the control potential is defined by the average hydrogen concentration on the first electrode 102 and the hydrogen concentration on the reference electrode 104. Therefore, if there is a portion where hydrogen is difficult to be pumped, such as this sensor, the hydrogen concentration in that portion will be high, so if you try to control the average hydrogen concentration to a low level, more pumping than usual will occur. After being carried out, a portion having a very low hydrogen concentration locally exists on the first electrode 102.

As a result, in a portion where the hydrogen concentration is locally low, dissociation of methanol or the like may occur, so that the current value flowing between the first electrode 102 and the second electrode 103 becomes large and the hydrogen gas This affects the concentration measurement and causes a problem that the hydrogen concentration cannot be measured accurately.

The present invention has been made to solve the above problems, and an object thereof is to provide a gas sensor capable of accurately measuring a gas concentration such as a hydrogen gas concentration.

[0009]

Means for Solving the Problems and Effects of the Invention (1) According to the invention of claim 1, the first electrode, the second electrode and the reference electrode provided in contact with the proton conductive layer, the gas atmosphere to be measured and the first electrode A diffusion rate-controlling part provided between one electrode and
The measurement target gas (for example, hydrogen gas) in the measurement target gas introduced from the measurement target gas atmosphere side through the diffusion rate controlling portion is controlled so that the potential difference between the first electrode and the reference electrode becomes constant.
A voltage (sufficient for obtaining a limiting current) is applied between the first electrode and the second electrode to cause dissociation or decomposition or reaction, and protons generated thereby are passed through the proton conduction layer to the first electrode side. The present invention relates to a gas sensor that obtains the concentration of a gas to be measured based on a limiting current generated by pumping the gas to the second electrode side.

Particularly in the present invention, the first electrode and the second electrode have a structure in which they are opposed to each other with the proton conducting layer interposed therebetween, and when the second electrode is projected on the first electrode, it exists in a portion other than the projected portion. The ratio S1 / S2 between the first area S1 of the first electrode and the second area S2 of the second electrode that overlaps the first electrode when the second electrode is projected onto the first electrode is 0.3.
It is characterized in that it is less than 5.

That is, in the present invention, the ratio S1 / S2 of the first area S1 of the first electrode and the second area S2 of the second electrode is given by:
Since it is less than 0.35, that is, since the first electrode and the second electrode overlap in many parts in the projection direction, the influence of the above-mentioned part that is difficult to pump is small. As a result, when the potential difference between the first electrode and the reference electrode is controlled to a constant value, a portion where the concentration of hydrogen or the like is locally low does not occur. As a result, dissociation of methanol or the like can be prevented, so that the concentration of the gas to be measured such as hydrogen can be accurately measured without being affected by methanol or the like.

(2) According to a second aspect of the invention, when the second electrode is projected onto the first electrode, one of the first electrode and the second electrode having a smaller area has a larger area. It is characterized in that the first electrode and the second electrode are provided so as to be in the projection area of the electrode.

The present invention defines the relationship between the position and the size of the first electrode and the second electrode. In the case of the present invention, one of the first electrode and the second electrode having the smaller area is used. Since all the parts of the electrode face the electrode having the larger area, it is possible to efficiently pump the substance to be measured such as hydrogen from the first electrode side to the second electrode side.

(3) In the invention of claim 3, the gas sensor is
It is characterized by being a hydrogen gas sensor that measures the hydrogen gas concentration. The present invention exemplifies that the gas sensor is a hydrogen gas sensor that measures hydrogen gas concentration.

(4) In the invention of claim 4, the gas sensor is
It is characterized by being used for measuring the hydrogen gas concentration in the fuel gas of a polymer electrolyte fuel cell. The present invention illustrates the application of the gas sensor. According to the present invention, it is possible to accurately measure the hydrogen gas concentration in the fuel gas of a polymer electrolyte fuel cell without being affected by methanol or the like.

As the structure of the gas sensor, the proton conducting layer, the first electrode, the second electrode, the reference electrode, and the diffusion rate controlling part may be supported by a support. Further, the diffusion rate controlling part is for controlling the diffusion of the measured gas (particularly the measurement target gas) introduced into the gas sensor from the measured gas atmosphere through the diffusion controlling part,
For example, it can be realized by a through hole formed in the support or a gas permeable porous body filled in the hole.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, an example (embodiment) of an embodiment of a gas sensor of the present invention will be described. (Example 1) In this example, a hydrogen gas sensor used for measuring the concentration of hydrogen gas in the fuel gas of a polymer electrolyte fuel cell will be described as an example of the gas sensor.

A) First, the structure of the hydrogen gas sensor of this embodiment will be described with reference to FIG. Note that FIG. 1 is a longitudinal sectional view of the hydrogen gas sensor. As shown in FIG. 1, in the hydrogen gas sensor of the present embodiment, the first electrode 3 is provided on one surface (the upper surface in the same figure) of the proton conducting layer 1 and the other surface of the proton conducting layer 1 (the same figure). A second electrode 5 and a reference electrode 7 are provided so as to face the first electrode 3, and they are supported in a support 10 composed of a first support 8 and a second support 9. ing.

That is, the proton conducting layer 1 is sandwiched between the first support 8 and the second support 9, and the first electrode 3 is covered by the first support 8 and arranged in the first recess 11 thereof. The second electrode 5 is covered with the second support 9 and is arranged in the second recess 12, and the reference electrode 7 is covered with the second support 9 and is arranged in the third recess 13. ing.

As a method of integrally forming the hydrogen gas sensor, a structure in which the proton conductive layer 1 is sandwiched between the first support 8 and the second support 9 and fixed by a fixing member or the like (not shown), A method of integrally fixing with resin or the like can be adopted. The proton conductive layer 1 is from one surface side to the other surface side, for example, from the first electrode 3 side to the second electrode 5 side,
It is capable of moving a proton (H ⁺ ). As a material of the proton conductive layer 1, a material that operates at a relatively low temperature (for example, 150 ° C. or lower) is preferable, and for example, a fluororesin “Nafion” (trademark of DuPont) or the like can be adopted.

The first electrode 3, the second electrode 5 and the reference electrode 7
Is, for example, a porous electrode containing carbon as a main component,
A catalyst layer of Pt or the like is formed on each of the electrodes 3, 5, 7 by coating on the side in contact with the proton conductive layer 1. In particular, in the present embodiment, as shown in FIG. 2, when the second electrode 5 is projected onto the first electrode 3, the first area S1 of the first electrode 3 existing other than the projected portion (the mesh portion in FIG. ) And a second area S2 of the second electrode 5 that overlaps with the first electrode 3 when the second electrode 5 is projected onto the first electrode 3 (area ratio) S1 / S2. Is less than 0.35 (eg, 0.
33 or less).

That is, when the second electrode 5 is projected onto the first electrode 3, the arrangement and size of the first electrode 3 and the second electrode 5 are set so that the second electrode 5 is housed in the first electrode 3. And the area ratio S1 / S2 is set to less than 0.35. Therefore, the second electrode 5 and the first electrode 3 have completely the same length in the width direction and the position in the projection direction, and one end in the longitudinal direction (the side opposite to the reference electrode 7) has the same position in the projection direction. I am doing it.

The first electrode 3, the second electrode 5, and the reference electrode 7 are connected to the circuit via lead portions (not shown), and the power source (battery) 14 allows the first electrode 3 and the second electrode 3 to be connected. 5 is applied, or the voltage between the first electrode 3 and the reference electrode 7 is measured by the voltmeter 15,
The ammeter 17 can measure the current flowing between the first electrode 3 and the second electrode 5.

Further, the reference electrode 7 keeps the voltage between the first electrode 3 and the reference electrode 7 constant, so that when measuring the hydrogen gas concentration in the measured gas, the influence of disturbances such as temperature and humidity is exerted. Is used to reduce still,
Preferably, the reference electrode 7 is a self-generated reference electrode in order to further stabilize the hydrogen concentration at the reference electrode 7.

The support 10 is an insulator made of, for example, ceramics containing alumina as a main component, and in addition to an inorganic insulator such as ceramics, an organic insulator made of resin or the like can be adopted. Of the support 10, the first support 8 is provided with a diffusion rate controlling portion 19 that communicates the ambient atmosphere with the first recess 11 (and thus the first electrode 3).

That is, the diffusion rate controlling portion 19 supplies the fuel gas (hence the contained hydrogen gas) as the gas to be measured to the first electrode 3
It is a small hole (for example, a through hole having a diameter of 0.06 mm) that is introduced into the side and controls the diffusion thereof. By setting the inner diameter of the diffusion control part 19 or filling the diffusion control part 19 with a porous material such as alumina, the degree of controlling the diffusion can be adjusted.

On the other hand, the second support 9 is provided with a hole 21 having a diameter of 1.7 mm, for example, which communicates with the ambient atmosphere and the second recess 12 (hence the second electrode 5). The first support 8 and the second support 9 are formed by laminating and firing sheets containing ceramics, and lead portions made of, for example, Pt are formed between the sheets.
That is, the lead portions are provided between the sheets in the first support 8 and the second support 9, and the lead portions are exposed at the bottoms of the recesses 11 to 13, whereby the electrodes 3 to 7 are exposed.
The electrode is in contact with the side opposite to the catalyst layer to secure conduction with the electrodes 3 to 7.

B) Next, the measurement principle and measurement procedure of the hydrogen gas sensor of this embodiment will be described. When the hydrogen gas sensor is exposed to the fuel gas, the diffusion control part
Hydrogen that has reached the first electrode 3 through the proton conductive layer 1 has a hydrogen gas concentration (specifically, hydrogen gas on both electrodes 3 and 7 side) between the first electrode 3 and the reference electrode 7. An electromotive force corresponding to the difference in concentration) is generated.

Therefore, a voltage is applied between the first electrode 3 and the second electrode 5 by the power supply 14 so that the potential difference between the first electrode 3 and the reference electrode 7 becomes constant. As a result, hydrogen is dissociated into protons on the first electrode 3, and the protons are
It is pumped out to the second electrode 5 side through the proton conductive layer 1,
It becomes hydrogen again and diffuses into the measured gas atmosphere.

At that time, the value of the current flowing between the first electrode 3 and the second electrode 5 (the limit current that rises to a certain value and becomes the upper limit when the voltage is applied) is proportional to the hydrogen gas concentration. Therefore, the hydrogen gas concentration can be measured by measuring the current value. Here, the role of the reference electrode 7 will be described.

When the applied voltage is controlled using the reference electrode 7 so that the potential difference between the first electrode 3 and the reference electrode 7 becomes constant, the potential difference between the first electrode 3 and the reference electrode 7 becomes constant. The voltage between the first electrode 3 and the second electrode 5 can be made variable so that a high voltage is applied when the hydrogen concentration is high, and a low voltage is applied when the hydrogen concentration is low. Optionally, an optimum voltage can be applied between the first electrode 3 and the second electrode 5 in a wide range of hydrogen gas concentration. This makes it possible to measure the hydrogen gas concentration in a wide range with high accuracy.

Further, even when the resistance between the first electrode 3 and the second electrode 5 changes due to the change in the temperature or H ₂ O concentration in the gas to be measured, the first electrode 3 and the second electrode 5 are not changed. Since it is possible to variably control the voltage applied between the electrode 5 and the electrode 5, even when the measurement conditions related to the hydrogen gas in the gas to be measured, H ₂ O, temperature, etc. change significantly, it is possible to achieve high accuracy. It is possible to measure the hydrogen gas concentration.

C) Next, a method for manufacturing the hydrogen gas sensor of this embodiment will be briefly described. For example, as shown in FIG. 1, a second support 9 is placed on a base (not shown),
The concave portion 12 side is arranged facing upward. Then on top of that,
The proton conducting layer 1 having the first electrode 3, the second electrode 5 and the reference electrode 7 arranged on both sides is arranged. That is, the second electrode 5 and the reference electrode 7 are arranged so as to be housed in the second recess 12 and the third recess 13, respectively.

Next, the first support 8 is arranged on the proton conducting layer 1 so that the first electrode 3 is surrounded by the first recess 11. In this state, that is, the first support 8 and the second support 9
With the proton conductive layer 1 sandwiched in between, the hydrogen gas sensor is fixed by pressing it in the thickness direction of the hydrogen gas sensor (vertical direction in the figure) with a fixing member or the like (not shown).

The side surface of the hydrogen gas sensor is covered with, for example, resin so as to be airtight so as to prevent ventilation from other than the diffusion rate controlling portion 19. It's good if you can't ventilate
The method is not limited to this.

D) Next, the effect of the hydrogen gas sensor of this embodiment will be described. In the hydrogen gas sensor of the present embodiment, when the second electrode 5 is projected onto the first electrode 3, the first area S1 of the first electrode 3 existing in a portion other than the projected portion and the second electrode 5 are replaced by the first electrode 3. Ratio (area ratio) S1 / of the second area S2 of the second electrode 5 overlapping the first electrode 3 when projected onto
S2 is set to less than 0.35.

That is, since the first electrode 3 and the second electrode 5 overlap each other in the projection direction, a portion where hydrogen is difficult to be pumped (on the side of the first electrode 3 facing the reference electrode 7) is difficult. Effect) is reduced. as a result,
When controlling the potential difference between the first electrode 3 and the reference electrode 7 to be a constant potential difference, a portion where the hydrogen concentration is locally low does not occur.
As a result, dissociation of methanol or the like can be prevented, so that the concentration of hydrogen gas can be accurately measured without being affected by methanol or the like.

Further, in this embodiment, when the second electrode 5 is projected onto the first electrode 3, the second electrode 5 is formed so as to be completely housed in the first electrode 3, that is, Second
Since all parts of the electrode 5 face the first electrode 3,
It is possible to efficiently pump hydrogen from the first electrode 3 to the second electrode 5.

E) Next, an example of an experiment conducted to confirm the effect of this embodiment will be described. In this experiment, the measurement accuracy of the hydrogen gas concentration was examined when the area ratio S1 / S2 was changed. First, as a product within the scope of the present invention (product of the present invention), the second
The first area S1 to change the area S2 as constant, specifically, the area ratio S1 / S2, "0mm ² / 18mm ² =
0 "," 3mm ² / 18mm ² = about 0.17 "," 6mm
As ^2/18 mm ² = about 0.33 ", to produce the embodiment similar to hydrogen gas sensors (Sample No.1~3).

On the other hand, a sample outside the scope of the present invention (comparative example product)
As the area ratio S1 / S2, "9mm ² / 18mm ² =
0.5 ", and a hydrogen gas sensor (Sample No. 4) similar to that of the above-described example was manufactured in the other portions. Then, using the hydrogen gas sensors of the present invention product and the comparative product described above, the hydrogen gas concentration was measured, and the accuracy of the hydrogen gas sensor at that time, that is, the dependence of methanol was examined.

Specifically, using each hydrogen gas sensor,
When measuring the hydrogen gas concentration in the measured gas having the gas composition, the value of the current flowing between the first electrode and the second electrode was measured. The measurement conditions are as follows.
Here, in order to stabilize the hydrogen gas concentration, a constant minute current (for example, 10 μm) is applied from the first electrode to the reference electrode.
By flowing A), the reference electrode was used as a self-generated standard electrode.

<Measurement conditions> Gas composition: H ₂ = 50%, CO ₂ = 15%, H ₂ O = 1
0%, CH ₃ OH = 0% or 5%, N ₂ = bal. -Gas temperature: 80 ° C-Gas flow rate: 10 L / min-Set potential Vs: 150 mV (set potential between the first electrode and the reference electrode, the first electrode side being positive) And under the measurement conditions described above. CH ₃ O measured at
With H = 0% when the current value (A0) and CH ₃ OH = 5% when the current value of the (A5), the current value ratio (A5 / A0), i.e., CH ₃ OH = 0% at a current When the value (sensitivity) was set to 1, the current value (sensitivity) ratio when CH ₃ OH = 5% was determined.

The measurement results are shown in FIG. 3. In FIG. 3, the vertical axis represents the current value ratio and the horizontal axis represents the area ratio S1 / S2.
Is taken. From FIG. 3, sample No. 1 of the product of the present invention
3 to 3, that is, the area ratio S1 / S2 is 0.
Those of less than 35 are samples No. 4 of comparative products which are not so.
Compared with the hydrogen gas sensor of No. 2, the current value ratio is extremely small, and it can be seen that the accuracy is high.

As is clear from the above, the area ratio S
It can be seen that by setting 1 / S2 to be less than 0.35, dissociation of methanol can be suppressed and the hydrogen gas concentration can be accurately measured. Therefore, with the hydrogen gas sensor of this embodiment, it is possible to accurately measure the hydrogen gas concentration in the fuel gas of the polymer electrolyte fuel cell without being affected by methanol or the like. (Embodiment 2) Next, Embodiment 2 will be described, but the description of the same parts as those in the above-mentioned embodiment will be omitted.

In the hydrogen gas sensor of this embodiment, the arrangement and positional relationship between the first electrode and the second electrode are different. As shown in FIG. 4, in the present embodiment, the first electrode 31 and the second electrode 33 are arranged in the overlapping direction (vertical direction in the figure).
It is slightly offset to the left and right. That is, when the second electrode 33 is projected onto the first electrode 31, the second electrode 33 is
It does not completely fit within 1 and extends beyond the projected portion on the first electrode 31.

In this case, the first area S of the first electrode 31 is
Reference numeral 1 denotes a portion that does not overlap even by projection (mesh portion in the figure), and the second area S2 of the second electrode 33 is a portion that overlaps by the projection (hatched portion in the figure). Also in this embodiment, since the area ratio S1 / S2 is less than 0.35, substantially the same effect as that of the first embodiment can be obtained. (Third Embodiment) Next, a third embodiment will be described, but the description of the same parts as those in the above-mentioned embodiment will be omitted.

In the hydrogen gas sensor of this embodiment, the arrangement and positional relationship between the first electrode and the second electrode are different. As shown in FIG. 5, in this embodiment, when the second electrode 43 is projected onto the first electrode 41, the first electrode 41 and the second electrode 43 are
The second electrode 43 is set so as to fit completely inside the first electrode 41 in both the vertical and horizontal directions.

In this case, the first area S of the first electrode 41 is
Reference numeral 1 denotes a portion that does not overlap even by projection (mesh portion in the figure), and the second area S2 of the second electrode 43 is a portion that overlaps by the projection (hatched portion in the figure), that is, the entire second electrode 43. is there. Also in this embodiment, the area ratio S1 / S2 is 0.
Since it is less than 35, the same effects as those of the first and second embodiments are obtained.

Needless to say, the present invention is not limited to the above-mentioned embodiments, and can be carried out in various modes without departing from the gist of the present invention. For example, in the above-described embodiment, the case where the hydrogen gas sensor measures the hydrogen gas concentration in the fuel gas has been described as an example, but the gas sensor of the present invention is also applicable to the case where the CO gas concentration in the fuel gas is measured. .

[Brief description of drawings]

FIG. 1 is an explanatory view showing a hydrogen gas sensor of Example 1 in a cutaway manner.

FIG. 2 is a perspective view showing a relationship between a first electrode and a second electrode.

FIG. 3 is a graph showing experimental results.

FIG. 4 is a perspective view showing a relationship between a first electrode and a second electrode in the second embodiment.

FIG. 5 is a perspective view showing a relationship between a first electrode and a second electrode in the third embodiment.

FIG. 6 is an explanatory view showing a conventional hydrogen gas sensor in a broken manner.

[Explanation of symbols]

1 ... Proton conduction layer 3, 31, 41 ... First electrode 5, 33, 43 ... Second electrode 7 ... Reference electrode 19 ... diffusion rate control

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G01N 27/46 311Z (72) Inventor Norihiko Nadanami 14-18 Takatsuji-cho, Mizuho-ku, Aichi Prefecture Special Ceramics Co., Ltd. (72) Inventor Masaya Watanabe 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi Japan Special Ceramics Co., Ltd. (72) Noboru Ishida 14-18 Takatsuji-cho, Mizuho-ku, Nagoya-shi, Aichi F-Term (Reference) within Nihon Special Ceramics Co., Ltd. (reference) 5H026 AA06 5H027 AA06 KK31

Claims (4)

[Claims] 1. A first device provided in contact with the proton conducting layer.
An electrode, a second electrode, and a reference electrode, and a diffusion rate controlling part provided between the measured gas atmosphere and the first electrode, and introduced from the measured gas atmosphere side through the diffusion rate controlling part. The measurement target gas in the measurement target gas is applied with a voltage between the first electrode and the second electrode such that the potential difference between the first electrode and the reference electrode is constant. , Dissociation or decomposition or reaction, and the protons generated thereby are pumped from the first electrode side to the second electrode side through the proton conductive layer, based on the limiting current generated, In a gas sensor for determining the concentration, the first electrode and the second electrode have a structure in which they are opposed to each other with the proton conducting layer interposed therebetween, and when the second electrode is projected onto the first electrode, the projection is performed. The ratio S1 / of the first area S1 of the first electrode existing other than the first area and the second area S2 of the second electrode which overlaps the first electrode when the second electrode is projected onto the first electrode. S2 to 0.
A gas sensor characterized by being less than 35. 2. When the second electrode is projected onto the first electrode, the electrode having the smaller area of the first electrode and the second electrode is within the projection area of the electrode having the larger area. The gas sensor according to claim 1, wherein the first electrode and the second electrode are provided in the gas sensor. 3. The gas sensor according to claim 1, wherein the gas sensor is a hydrogen gas sensor that measures a hydrogen gas concentration. 4. The gas sensor according to claim 3, wherein the gas sensor is used for measuring a hydrogen gas concentration in a fuel gas of a polymer electrolyte fuel cell.