JP2008232822A - Hydrogen sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hydrogen sensor which measures stably a hydrogen amount occlusion-stored in a hydrogen storage alloy. <P>SOLUTION: This hydrogen sensor 1 has an electrolyte body 26 attached with a detecting electrode 30 and a reference electrode 28, and the electrolyte body 26 is constituted of a metal ion-conductive solid electrolyte. The electrolyte is thereby precluded from being deteriorated even when the hydrogen sensor is exposed to vacuum atmosphere in a hydrogen occlusion process, and an excellent ion conductivity exhibits a stable sensing function. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a hydrogen sensor for detecting a hydrogen storage amount of a hydrogen storage alloy, for example.

  In future hydrogen energy use society, the development and use of hydrogen storage alloys (also referred to as hydrogen storage alloys) is desired as a hydrogen storage method that eliminates the danger of hydrogen explosions and is highly safe and highly convenient. . Hydrogen storage alloys are promising as fuel containers for fuel cell vehicles and the like because they have a higher packing density than when stored in gas and can prevent accidents due to sudden hydrogen leakage. When the hydrogen storage alloy is used as a hydrogen fuel container, a means for detecting the remaining amount of hydrogen is required.

  In order to directly measure the amount of hydrogen remaining in the hydrogen storage device by a simple method, the inventors used a hydrogen storage alloy disposed inside the hydrogen storage container as a detection electrode, and a reference electrode opposed to the detection electrode. And a hydrogen sensor in which an electrolyte is disposed between the detection electrode and the reference electrode, and the hydrogen concentration in the hydrogen storage alloy is measured as an electromotive force value by the detection electrode, the reference electrode, and the electrolyte (Patent Document 1). ).

  As an electrolyte used for such a hydrogen sensor, it is necessary to withstand a high temperature up to about 200 ° C., which is a hydrogen release temperature from the hydrogen storage alloy. Initially, the inventors employed phosphotungstic acid or phosphomolybdic acid, which are solid electrolytes generally employed as hydrogen sensors. This is because these electrolytes are practical as hydrogen sensors in the atmosphere and are considered to satisfy the above-described temperature conditions.

JP 2007-47125 A

However, the inventors' tests have shown that when these solid electrolytes are used as hydrogen sensors for hydrogen storage alloys, stable measurements cannot be obtained continuously. The inventor has found that proton-conducting solid electrolytes such as phosphotungstic acid (H ₃ PW ₁₂ O ₄₀ / 29H ₂ O) and phosphomolybdic acid (H ₃ PMo ₁₂ O ₄₀ / 30H ₂ O) The source depends on the crystal water, and it was speculated that the crystal water was removed by the vacuum atmosphere when the hydrogen storage alloy occludes hydrogen.
An example of such a proton-conducting electrolyte that does not depend on crystal water is sulfuric anhydride. However, it has the disadvantage of being liquid and highly corrosive, and its practicality is small.
Thus, the inventors have tested a metal ion conductive type solid electrolyte instead of the proton conductive type solid electrolyte, and as a result, obtained data that can provide practicality, and have come to the present invention.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a hydrogen sensor capable of stably measuring the amount of hydrogen stored in a hydrogen storage alloy exposed to a vacuum atmosphere. To do.

  In order to solve the above problem, the hydrogen sensor according to claim 1 is a hydrogen sensor having an electrolyte body to which a detection electrode and a reference electrode are attached. The electrolyte body is made of a solid electrolyte having metal ion conductivity. It is configured.

  In the first aspect of the invention, even when the hydrogen sensor is exposed to a vacuum atmosphere in the process of occlusion of hydrogen, the electrolyte body is not deteriorated, and a stable sensing function is maintained while maintaining good ion conductivity. Can be demonstrated. The reason why a solid electrolyte having metal ion conductivity reacts with hydrogen in a gas to generate an electromotive force has not been fully elucidated and is a future problem.

  The hydrogen sensor according to claim 2 is characterized in that, in the invention according to claim 1, the solid electrolyte has sodium ion conductivity.

  The hydrogen sensor according to claim 3 is the invention according to claim 2, wherein the solid electrolyte is β alumina.

  A hydrogen sensor according to a fourth aspect is characterized in that, in the invention according to the second aspect, the solid electrolyte is a porous γ alumina in which sodium is supported.

  According to the first and second aspects of the invention, it is possible to provide a hydrogen sensor capable of stably measuring the amount of hydrogen stored in the hydrogen storage alloy.

Embodiments of the present invention will be described below with reference to the drawings.
An embodiment of a hydrogen storage device having a hydrogen sensor of the present invention is shown in FIG. The hydrogen storage device 1 includes a cylindrical sealed container 10, a port 14 with an opening / closing valve 12 that opens on a side wall on one end side of the container 10, a hydrogen storage alloy 18 accommodated in a space 16 in the container 10, And a temperature adjusting mechanism (not shown). In this embodiment, for example, a TiFeNiZi alloy (hydrogen release temperature of about 120 to 160 ° C.) is used as the hydrogen storage alloy 18. A mesh-shaped presser plate 19 is provided at the ends of the surfaces of the hydrogen storage alloy 18 and the port 14, and a spring member 20 is provided between the presser plate 19 and the port-side inner wall of the container. As a result, the alloy maintains a predetermined density and is pressed against the inner surface on the other end side with a predetermined pressure.

  A through hole is formed in the other side wall of the container 10, and a sealed cylindrical sensor holder 22 is coupled to the through hole with a screw. A metal seal 24 is disposed between the sensor holder 22 and the container 10. Inside the sensor holder 22, an electrolyte body 26 disposed so as to be in contact with the hydrogen storage alloy 18 in the container 10 and a reference electrode 28 that is in contact with the outer end of the electrolyte body 26 are provided. Thereby, the sensor part 32 is comprised by the detection electrode 30 which consists of hydrogen storage alloy 18 itself, the reference electrode 28 outside the container 10, and the electrolyte body 26 pinched | interposed by these. The material of the reference electrode 28 may be a stable electrode material that does not oxidize or corrode in the hydrogen release temperature region (several hundred degrees C.) of the hydrogen storage alloy. Of course, a material inert to hydrogen gas such as titanium or a titanium alloy may be used.

  The sensor holder 22 has conductivity, serves also as a lead terminal from the detection electrode 30 (hydrogen storage alloy 18), and is connected to a wiring 34 to a voltmeter (not shown). An insulating tube (insulator) 36 that insulates the sensor holder 22 from the reference electrode 28 and the electrolyte body 26 is disposed on the inner surface of the sensor holder 22. A lead line 38 from the reference electrode 28 is led out to the outside from the top plate 23 of the sensor holder 22. In the space between the reference electrode 28 plate and the top plate 23 of the sensor holder 22, the reference electrode 28 and the electrolyte body 26 are pressed toward the container 10, and the electricity between the electrolyte body 26, the reference electrode 28, and the detection electrode 30. A compression spring 40 is provided for maintaining a normal contact.

  In this embodiment, a temperature adjustment device 42 having a temperature control micro heater 42 a and a temperature sensor 42 b is incorporated in the electrolyte body 26. These are connected to a control device (not shown), and control the electrolyte body 26 to the same temperature as the hydrogen release temperature of the hydrogen storage alloy 18. Thereby, even if the temperature of a storage device is near room temperature, the amount of hydrogen can be measured.

As the electrolyte body 26, for example, as shown in FIG. 1B, β alumina (Na ₂ O · nAl ₂ O ₃ , n = 5 to 12) or porous γ alumina with Na supported thereon. It can be adopted as appropriate.

ここで、Ｆはファラデー定数、ＥはＥＭＦ値、

はそれぞれ金属、水素貯蔵合金１８における原子状の水素の化学ポテンシャルである。端子［I］、［II］は同種の銅線のため電子の電気化学ポテンシャルは

となる。静電ポテンシャルと起電力Ｅとの関係

を用いた。 The principle of detecting the remaining amount of hydrogen fuel by the hydrogen sensor configured as described above is as follows. When the hydrogen storage alloy 18 is used as the detection electrode 30 and the reference electrode 28 is disposed with the electrolyte body 26 interposed therebetween, the electrochemical cell hydrogen storage alloy (detection electrode) [I] | electrolyte | reference electrode [II]
Is formed. The value of electromotive force (EMF) generated between both electrodes [I] and [II] of this electrochemical cell has the following relationship with the chemical potential of hydrogen between both electrodes.

Where F is the Faraday constant, E is the EMF value,

Are the chemical potentials of atomic hydrogen in the metal and hydrogen storage alloy 18, respectively. Since the terminals [I] and [II] are the same type of copper wire, the electrochemical potential of the electrons is

It becomes. Relationship between electrostatic potential and electromotive force E

Was used.

ここで、ΔGは水素化に伴う合金系全体のギブスの自由エネルギーの変化量、nは水素貯蔵合金中の水素濃度である。水素貯蔵合金中の水素の状態が２相の状態（例えば、水素の固溶相と水素化物相の２相）にある場合、２相領域の各相でのギブスの自由エネルギーは異なるが、水素の化学ポテンシャルは等しくなる。このポテンシャルに差があれば、化学ポテンシャルの低い相に粒子の移動が生じるからである。従って、２相領域の起電力値は一定になる。 The chemical potential of hydrogen has the following relationship with the free energy of hydrogen in the hydrogen storage alloy.

Here, ΔG is the amount of change in Gibbs free energy of the entire alloy system accompanying hydrogenation, and n is the hydrogen concentration in the hydrogen storage alloy. When the hydrogen state in the hydrogen storage alloy is in a two-phase state (for example, two phases of a hydrogen solute phase and a hydride phase), the Gibbs free energy in each phase in the two-phase region is different. Have the same chemical potential. This is because, if there is a difference in potential, particle movement occurs in a phase having a low chemical potential. Therefore, the electromotive force value in the two-phase region is constant.

  The electromotive force (EMF) value of this hydrogen sensor measures the difference in chemical potential of atomic hydrogen for both electrodes. Since the chemical potential of hydrogen in a hydrogen storage alloy is equal to the atomic hydrogen at the alloy interface in thermal equilibrium, the chemical potential in the alloy can be measured.

  In this way, the detected electromotive force from this sensor is output as an amount of intensity that depends on the chemical potential of hydrogen, so the electromotive force (EMF) value does not depend on the physical size of the sensor or the electrode structure. Since it depends only on the type of the sensor, the sensor itself can be very miniaturized and can have a simple structure.

  The role of the electrolyte body 26 is to transmit information on the strength of the detection electrode 30 and the reference electrode 28 with respect to the chemical potential of hydrogen. The conditions of the electrolyte body 26 necessary for information transmission may be those having hydrogen ion conductivity or ion conductivity having reactivity with hydrogen.

と温度に依存することから、水素貯蔵合金１８の温度を温度センサ４２ｂにより温度を測定する。特に、水素貯蔵合金１８の温度は室温から水素放出温度まで大きな変化が行われるから、温度データが必要となる。なお、温度調整装置４２のマイクロヒータ４２ａは電解質体２６の温度が動作領域にするために用いる。 In the sensor unit 32 in the hydrogen storage device 1 having the configuration of FIG. 1, an electromotive force (EMF) value corresponding to the amount of hydrogen stored in the hydrogen storage alloy 18 in contact with the electrolyte body 26 is output based on the above-described principle. From this measured value and the relationship between the electromotive force and the hydrogen concentration obtained in advance, the hydrogen concentration is calculated. On the other hand, this electromotive force (EMF) value is the free energy of hydrogen in Eq.

Therefore, the temperature of the hydrogen storage alloy 18 is measured by the temperature sensor 42b. In particular, since the temperature of the hydrogen storage alloy 18 varies greatly from room temperature to the hydrogen release temperature, temperature data is required. Note that the micro heater 42a of the temperature adjusting device 42 is used so that the temperature of the electrolyte body 26 is in the operating region.

FIG. 2 shows an example of the relationship between the electromotive force and the hydrogen concentration, and the horizontal axis indicates the hydrogen concentration (H / M molar ratio) in the hydrogen storage alloy 18. This hydrogen storage alloy 18 is a conceptual diagram of what was illustrated in FIG. It has one type of hydride phase (β phase) and forms an α phase that is a solid solution phase of hydrogen on the low concentration side. EMF in β phase
The value varies depending on the hydrogen concentration. When the EMF value appearing in the figure is constant, it means that the hydrogen state in this alloy is a mixture of two phases of α phase and β phase. That is, in the sensor unit 32, the region where the amount of hydrogen in the hydrogen storage alloy can be quantitatively measured is limited to the β-phase concentration region where hydrogen is sufficiently present. When entering the region of α phase + β phase, which means that the amount of residual hydrogen has decreased, the amount of hydrogen in that region cannot be quantitatively detected. Further, when entering the single region of the α phase, the EMF value again shows a change depending on the hydrogen concentration. However, the amount of hydrogen in the α phase is extremely small, and the hydrogen fuel is in a state close to zero. Therefore, it is necessary to promote refilling of hydrogen when the α phase + β phase is detected. The hydrogen storage amount is proportional to the volume or weight of the hydrogen storage alloy.

  In this embodiment, the region where the hydrogen concentration is in the range of H / M = 0.5 to 0.05 becomes a dead zone, and can be measured only in the high concentration range of H / M = 0.5 to 0.7. Therefore, it is used so as to cover the entire range by using in combination with another apparatus capable of measuring the low concentration range. A device such as an integrated flow meter may be used as the device used in combination.

In order to confirm the function of the hydrogen sensor as described above, a function test was performed using the test apparatus shown in FIG. Here, an electrochemical cell as shown in FIG. 3 (b) in the glass tube 50, the detection electrode 30 | the electrolyte body 26 | the reference electrode 28.
Is configured. One end of the glass tube is connected to a hydrogen gas source 54 via a valve 52, the other end is connected to the atmosphere via a three-way switching valve 56, and the other end is connected to a vacuum exhaust pump 58. . Moreover, in order to investigate the influence of temperature, the heater 60 was arrange | positioned around the glass tube. A sensor (not shown) for measuring the temperature inside the glass tube 50 and a sensor 62 for measuring pressure are provided.

  In this apparatus, the operation of the electrolyte was examined using W as the reference electrode 28 of the electrochemical cell and Pt as the detection electrode 30. In this test apparatus, a hydrogen storage alloy is not used, but a chemical potential equal to the hydrogen concentration is obtained at the boundary between Pt and the electrolyte body 26. Therefore, the relationship of the above formulas (1) to (5) is the hydrogen storage alloy. The same holds true. Therefore, the function as an electrolyte body for measuring a hydrogen storage alloy can be measured. Note that Pt as a detection electrode was formed by pressing a thin plate of Pt on one end of a columnar sample of β alumina.

In this apparatus, the change in electromotive force was measured when the temperature was raised stepwise while hydrogen gas was introduced at 1.02 × 10 ⁵ Pa by vacuum substitution at room temperature. As a result, as shown in FIG. 4, the generation of electromotive force is observed at a temperature of 60 ° C. or higher, and it seems to have a function as a sensor. However, the electromotive force is small, and the electromotive force is generated every time the temperature rises thereafter. As a result of including such noise, the function as an ion conductor for detecting hydrogen gas of β-alumina is insufficient (conductivity is low), so that the impedance of the sensor is extremely high. It seems that this is because.

Next, what carried | supported Na to the porous (gamma) alumina was used as an electrolyte body. This is prepared by preparing columnar porous γ-alumina (porosity of 37%), impregnating with an aqueous NaOH solution (concentration: 10 mol / L) at room temperature, raising the temperature to 200 ° C. in 2 hours, maintaining for 2 hours, and further 30 Made by lowering to room temperature in minutes. By such treatment, it is considered that a structure close to β-alumina in which Na is incorporated into the alumina crystal is generated on a part of the surface of the porous γ-alumina. In addition, the function improvement as an electrolyte was recognized by performing the heat processing performed at 200 degreeC for 2 hours here at 600 degreeC for 2 hours.
The detection electrode and the reference electrode made of Pt and W are attached to the two opposing surfaces of the porous γ-alumina impregnated and heat-treated in the same manner as described above, and incorporated in the apparatus shown in FIG. It is.

In this apparatus, as in the case of β-alumina, the change in electromotive force was measured by continuously raising the temperature in a state where hydrogen gas was introduced at 1.02 × 10 ⁵ Pa by vacuum substitution at room temperature. . The result is shown in FIG. As shown in the enlarged view on the left side of FIG. 5, it does not have a function as an electrolyte at room temperature, but when it reaches 40 ° C., the electromotive force becomes a negative value, and it operates as a sensor at a temperature higher than that. I understand. As shown in the overall diagram on the right side, when the temperature is 100 ° C. or higher, the generation of a constant electromotive force is observed, and it can be seen that the device operates stably in the temperature range of 100 to 150 ° C.

  Here, after reaching 150 ° C., the temperature was once lowered to 35 ° C. and again raised to 120 ° C. When the temperature drops to 35 ° C, the electromotive force becomes 0 and the function as an electrolyte is lost. However, when the temperature rises to 120 ° C, the same constant electromotive force is generated and is stable against temperature changes. is doing.

  As described above, γ-alumina with Na supported thereon or β-alumina is known to have a function in the temperature range of 100 to 150 ° C. as an electrolyte used for a hydrogen sensor. It turned out that it is suitable as an electrolyte body for using for a sensor.

(A) is a schematic diagram which shows embodiment of the hydrogen storage apparatus which has a hydrogen sensor, (b) is a figure which shows an electrolyte body. It is a graph which shows the example of the characteristic of the hydrogen concentration and electromotive force in embodiment of FIG. (A) is a schematic diagram which shows the test device of a hydrogen sensor, (b) is a figure which shows an electrochemical cell. It is a graph which shows the characteristic of the hydrogen sensor of one Example of this invention. It is a graph which shows the characteristic of the hydrogen sensor of the other Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Hydrogen storage apparatus 10 Sealed container 12 On-off valve 14 Port 16 Space 18 Hydrogen storage alloy 19 Press plate 20 Spring member 22 Sensor holder 23 Top plate 24 Metal seal 26 Electrolyte body 28 Reference electrode 30 Detection electrode 32 Sensor part 34 Wiring 36 Insulation tube 38 Lead wire 40 Compression spring 42a Micro heater 42b Temperature sensor 42 Temperature adjusting device 44 Porous body 50 Glass tube 52 Valve 54 Hydrogen gas source 56 Three-way switching valve 58 Pump 60 Heater

Claims (4)

In a hydrogen sensor having an electrolyte body to which a detection electrode and a reference electrode are attached,
2. The hydrogen sensor according to claim 1, wherein the electrolyte body is composed of a solid electrolyte having metal ion conductivity.   The hydrogen sensor according to claim 1, wherein the solid electrolyte has sodium ion conductivity.   The hydrogen sensor according to claim 2, wherein the solid electrolyte is β-alumina.   The hydrogen sensor according to claim 2, wherein the solid electrolyte is a porous γ-alumina supporting sodium.

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