JP4538640B2 - Hydrogen sensor - Google Patents

Description

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

  In future hydrogen energy use society, 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.

  As an electrolyte used for such a hydrogen sensor, it is necessary to withstand a high temperature up to about 200 ° C., which is the temperature at which hydrogen is released 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 temperature conditions.

  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. This was presumed to be caused by the fact that the hydrogen storage alloy was in a vacuum atmosphere when storing hydrogen.

  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.

  In order to solve the above-described problem, a hydrogen amount sensor according to claim 1 includes a detection electrode made of a hydrogen storage alloy disposed inside a hydrogen storage container, and a reference electrode disposed opposite to the detection electrode. And an electrolyte body disposed between the detection electrode and the reference electrode, wherein the electrolyte body has an electrolyte function in an anhydrous state.

  In the first aspect of the present invention, even when the hydrogen amount sensor is exposed to a vacuum atmosphere during the process of storing hydrogen, the electrolyte body is not deteriorated, and stable ion sensing is maintained while maintaining good ionic conductivity. Function can be demonstrated. Note that “having an electrolyte function in an anhydrous state” means that ion conductivity is not obtained by dissociation of water such as crystal water or adsorbed water.

A hydrogen amount sensor according to a second aspect is characterized in that, in the invention according to the first aspect, the electrolyte body has a pH in a range of 4 to 7 and a boiling point of 150 ° C. or higher.
In the invention described in claim 2, the electrolyte body does not corrode the hydrogen storage alloy or other metals, and does not vaporize and disappear in the hydrogen release temperature range.

According to a third aspect of the present invention, there is provided the hydrogen amount sensor according to the second aspect, wherein the electrolyte body is phosphoric acid. Phosphoric acid is the most preferred electrolyte that satisfies the above conditions. That is, phosphoric acid (2H ₃ PO ₄ .H ₂ O) becomes a liquid at 30 ° C. and an anhydride at 150 ° C. The boiling point of phosphoric anhydride is 261 ° C. From this, it is a stable substance in a liquid state at 100 to 200 ° C., which is the purpose of use, and has low corrosiveness to metals.

  The hydrogen amount sensor according to claim 4 is a detection electrode made of a hydrogen storage alloy disposed inside a hydrogen storage container, a reference electrode disposed opposite to the detection electrode, the detection electrode and the reference And an electrolyte body disposed between the electrodes, wherein the electrolyte body is configured by impregnating a porous body with a liquid electrolyte.

  In the invention according to claim 4, the difference between the chemical potential value of hydrogen determined by the concentration of hydrogen stored in the detection electrode and the chemical potential of hydrogen at the reference electrode is measured as an electromotive force value between the detection electrode and the reference electrode. Is done. The electrolyte body is constituted by impregnating a porous body with a liquid electrolyte, has good ionic conductivity, generates an electromotive force value with good responsiveness, and is solid and excellent in handleability. ing.

  The liquid electrolyte refers to a substance that is liquid only at least in a part of the hydrogen release temperature range of the hydrogen storage alloy. Of course, it is desirable to be stable in that temperature range and to be low corrosive to metals such as storage alloys and wiring.

  According to the first and second aspects of the present 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 amount 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-based alloy (hydrogen releasing 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 | standard electrode 28 outside the container 10, and the electrolyte body 26 pinched | interposed among 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.

  In this embodiment, a temperature adjusting 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.

  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 adjusting 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 shown in FIG. 1B, the electrolyte body 26 is configured by impregnating a porous body 44 with a liquid electrolyte. In this embodiment, phosphoric acid was used as the liquid electrolyte. Phosphoric acid is a low-reactivity acid and has a crystal structure of 2H ₃ PO ₄ .H ₂ O at 30 ° C., an anhydride at 150 ° C., pyrophosphoric acid at 200 ° C., and metaphosphoric acid at 300 ° C. or higher. Since the aqueous solution was also used as an electrolyte for fuel cells, it was thought that it would function well for hydrogen sensors.

In order to bring liquid phosphoric acid into contact with the detection electrode 30, in this embodiment, phosphoric acid was impregnated into a porous body 44 made of alumina having a porosity of 37%. In the impregnation method, alumina was immersed in a phosphoric acid aqueous solution having a concentration of 42.5% mol, and then heated in the atmosphere at 200 ° C. for 30 minutes to be cooled. Thereby, excess phosphoric acid inside the porous body was removed, and 36.5% phosphoric acid of the total weight could be supported. As another impregnation method, for example, a vacuum impregnation method may be used. Thus, by impregnating the porous body 44 with phosphoric acid, which is a liquid electrolyte, the liquid electrolyte could be made into a solid form that was easy to handle.

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 amount 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 this potential, particles move 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 amount 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ー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 principle described above. 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 such electromotive force and hydrogen concentration, and the horizontal axis shows the hydrogen concentration (H / M molar ratio) in the hydrogen storage alloy 18. This hydrogen storage alloy 18 is a conceptual diagram of what is illustrated in FIG. It has one type of hydride phase (β phase) and forms an α phase which 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. As a device to be used in combination, a device such as an integrating flow meter may be used.

In order to confirm the function of the hydrogen amount 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 the pressure are provided.

First, an experiment for measuring the hydrogen storage state when the hydrogen storage alloy Pd is used as the detection electrode 30 and the porous body 44 impregnated with phosphoric acid shown in FIG. went. The change in electromotive force (EMF) at this time is shown in FIG. In FIG. 4, (a) shows the whole experiment, and (b) shows the initial change in an enlarged manner. Here, 1.06 × 10 ⁵ Pa of hydrogen gas was introduced from a gas source at an initial temperature of 60 ° C., and the valves 52 and 56 were closed (A). The initial electromotive force at this time was −600 mV.

Thereafter, the temperature was raised from 60 ° C. to 150 ° C. with a heater. As for the pressure in the tube, hydrogen was occluded in the hydrogen storage alloy and the pressure in the tube decreased to 0.96 × 10 ⁵ Pa, and the electromotive force at that time was −200 mV (B). In this state, when hydrogen gas was pressurized again to 1.06 × 10 ⁵ Pa, the electromotive force value became −530 mV (C). Next, the valve 56 was opened, and the hydrogen gas in the hydrogen storage alloy was released while keeping the cell temperature at 150 ° C. (C → D → E).

  This measurement result confirms that hydrogen is stored in the alloy because the hydrogen pressure decreases at point (B), and that the electromotive force value depends on the concentration of hydrogen stored in the alloy. To do. The reason why the hydrogen pressure is set to the initial pressure at point (C) is to confirm the reproducibility of the electromotive force value. From the process of (C → D → E), it can be seen that a change in electromotive force according to the amount of hydrogen concentration in the alloy is recognized as hydrogen is released from the alloy. The relationship of hydrogen concentration in hydrogen storage alloys can be calibrated by using together with PCT (pressure / concentration / temperature) measurement.

  Next, in the apparatus of FIG. 3, Pt was used as the detection electrode 30 of the electrochemical cell, and the influence corresponding to the environmental change of the electrolyte body of this embodiment was examined. 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. As a comparative example for the electrolyte body 26, sulfuric acid having a concentration of 10.5N was impregnated into the porous body 44, and sulfuric acid anhydride was obtained by vacuum heat treatment as in the case of phosphoric acid, and phosphotungstic acid as a solid electrolyte. A similar test was conducted.

In this apparatus, when the porous body 44 impregnated with phosphoric acid is used as the electrolyte body 26 and hydrogen gas is introduced at 1.02 × 10 ⁵ Pa by vacuum substitution, the temperature rises from room temperature. The change in power was measured. As a result, as shown in FIG. 5, the electromotive force value is almost constant regardless of the temperature change, and it can be seen that the electromotive force value is sufficiently practical at least in the temperature range from room temperature to 200 ° C.

FIG. 6 shows the case of a porous body impregnated with sulfuric acid. In FIG. 6, (a) shows the whole experiment, and (b) shows the change in the latter period in an enlarged manner. Here, the temperature was raised from room temperature to 200 ° C. in the atmosphere. Thereafter, the temperature was returned to room temperature, hydrogen gas was introduced at 1.2 × 10 ⁵ Pa by vacuum substitution, and the mixture was heated again to 100 ° C. Although it has sufficiently practical operating characteristics in this temperature range, the electromotive force value is unstable compared to the case of phosphoric acid. This is presumed to be due to electrode corrosion caused by sulfuric acid. In addition, it is because hydrogen gas was introduce | transduced here that an electromotive force value changes a lot by 1.2 * 10 < ⁴ > s of Fig.6 (a). Further, a point A in FIG. 6B is a change caused by opening the valve and releasing hydrogen into the atmosphere.
Moreover, about the phosphotungstic acid which is a solid electrolyte, it was confirmed that an electromotive force will become unstable and the function as an electrolyte will lose | disappear only by making it a vacuum state at room temperature.

  Next, the porous body 44 impregnated with phosphoric acid was used as the electrolyte body 26, and the change in electromotive force when the process of making the space into a hydrogen atmosphere and the process of opening to the atmosphere were repeated alternately was measured. As a result, as shown in FIG. 7, it can be seen that the electromotive force value is substantially constant, there is no deterioration due to the atmosphere change, and there is sufficient practicality. In addition, since the introduction of hydrogen is performed by vacuum substitution, it can be seen from this figure that the reproducibility of the electromotive force response is good even after repeated placement in a vacuum atmosphere as an electrolyte.

  Next, in the case of a hydrogen storage alloy as shown in FIG. 1, since a vacuum atmosphere was created before occluding hydrogen, a test was conducted to examine deterioration due to exposure to a vacuum atmosphere at room temperature. FIG. 8 shows the result of the porous body 44 impregnated with sulfuric acid, but the reproducibility of the electromotive force response was good even after being repeatedly placed in a vacuum atmosphere.

  On the other hand, for phosphotungstic acid, which is a solid electrolyte, a similar test was performed using Pd as the detection electrode 30 of the electrochemical cell. As a result, as shown in FIG. 9, the electromotive force value after evacuating once even at room temperature was greatly reduced, and it took a long time (48 hours) to recover even after returning to the atmosphere. The elucidation of this reason is a future problem, but it is thought that the crystal water of phosphotungstic acid disappeared under vacuum.

  As described above, as a result of testing a conventional solid electrolyte and an electrolyte body impregnated with a liquid electrolyte of the present invention as an electrolyte body for use in a hydrogen amount sensor for a hydrogen storage alloy, In the solid electrolyte in which the crystal water plays an important role as an electrolyte, it was considered unsuitable for use in an environment exposed to vacuum such as a hydrogen storage alloy. Moreover, good results could be obtained with an electrolyte body in which a porous body was impregnated with phosphoric acid having an electrolyte function even in an anhydrous state. It was also found that sulfuric acid also has a function as an electrolyte, but sulfuric acid is highly corrosive, and it has been found that the surface of tungsten, which is a standard electrode, is blackened. On the other hand, phosphoric acid was not corrosive and did not adversely affect the electrodes and other metals.

(A) is a schematic diagram which shows embodiment of the hydrogen storage apparatus which has a hydrogen amount 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 amount sensor, (b) is a figure which shows an electrochemical cell. (A) is a graph which shows the example of the characteristic of the hydrogen concentration of a hydrogen storage alloy, and an electromotive force, (b) is a figure which expands and shows the part. It is a graph which shows the characteristic of the hydrogen amount sensor of this invention. (A) is a graph which shows the characteristic of the hydrogen amount sensor of a comparative example, (b) is the figure which expands and shows a part. It is a graph which shows the characteristic of the hydrogen amount sensor of this invention. It is a graph which shows the characteristic of the hydrogen amount sensor of a comparative example. It is a graph which shows the characteristic of the hydrogen amount sensor of another comparative example.

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)

A detection electrode made of a hydrogen storage alloy disposed inside the hydrogen storage container;
A reference electrode disposed opposite to the detection electrode;
An electrolyte body disposed between the detection electrode and a reference electrode,
The hydrogen amount sensor, wherein the electrolyte body has an electrolyte function in an anhydrous state.   The hydrogen amount sensor according to claim 1, wherein the electrolyte body has a pH in a range of 4 to 7 and a boiling point of 150 ° C. or higher.   The hydrogen amount sensor according to claim 2, wherein the electrolyte body is phosphoric acid. A detection electrode made of a hydrogen storage alloy disposed inside the hydrogen storage container;
A reference electrode disposed opposite to the detection electrode;
An electrolyte body disposed between the detection electrode and a reference electrode,
The hydrogen amount sensor, wherein the electrolyte body is constituted by a liquid electrolyte impregnated in a porous body.

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