JP7629172B2 - Hydrogen sensor and method for detecting hydrogen concentration - Google Patents

Description

The present invention relates to a hydrogen sensor that uses a solid electrolyte as a sensor element, and a method for detecting hydrogen concentration using the hydrogen sensor.

A solid electrolyte (proton conductive ceramic) that exhibits proton conductivity by replacing a part of metal B with a metal M having a lower valence in a perovskite-type metal double oxide represented by the chemical formula _ABO3 is known, and a hydrogen sensor that detects hydrogen partial pressure (hydrogen concentration) in a high-temperature environment using this as a sensor element has been proposed. In addition, if the hydrogen concentration is known, for example, when the gas to be detected is ammonia, the concentration of ammonia remaining in the measurement atmosphere can be calculated based on the detection of the concentration of hydrogen gas generated by its decomposition.

Hydrogen sensors that use proton-conductive ceramics as a sensor element utilize the principle of a concentration cell, where a potential difference occurs due to a difference in concentration of the same ion. When two phases with different hydrogen concentrations are separated by a proton-conductive ceramic sensor element, the electromotive force generated between the two electrodes on the sensor element that are in contact with each phase, and the hydrogen partial pressures of each of the two phases follow the Nernst equation. Therefore, if the hydrogen partial pressure of one of the two phases is known, the hydrogen partial pressure of the other phase can be calculated by measuring the electromotive force and temperature.

A mixture of argon and hydrogen is generally used as a reference gas with a known hydrogen partial pressure. However, hydrogen gas has various drawbacks, such as the need for space to transport and store the gas cylinders used to supply it, the overall size of the sensor device, the inability to transport hydrogen gas by aircraft and therefore limitations on the use of sensors when it is difficult to obtain overseas, the need to handle hydrogen with great care as it burns and explodes in the presence of oxygen, and its high cost.

If air could be used as the reference gas, not only would there be no need for a hydrogen gas cylinder, but there would also be no need to prepare the reference gas itself, which would be convenient and would result in a simpler sensor device configuration. However, the hydrogen partial pressure in the air fluctuates due to the influence of the water vapor partial pressure, and within the range of fluctuations in this hydrogen partial pressure, the transport number of protons changes in conventional proton conductive ceramics. Therefore, when conventional proton conductive ceramics are used as a sensor element, the measured electromotive force fluctuates due to fluctuations in the hydrogen partial pressure in the air, making it impossible to accurately measure the hydrogen concentration in the measurement atmosphere.

The present applicant has solved such problems and proposed a hydrogen sensor capable of using air as a reference gas, and a solid electrolyte serving as the sensor element thereof (see, for example, Patent Document 1). In this sensor element, a metal composite oxide represented by the chemical formula AB _1-b M _b O _3-α , where A is an alkaline earth metal, B is a metal with a valence of +4, and M is a transition metal that can have multiple valences less than the valence of metal B, is used as the measurement electrode end, and the valence of metal M is biased toward a valence smaller than that of metal B and exhibits proton conductivity, while the valence of metal M is biased toward a valence larger than that of the measurement electrode and does not exhibit proton conductivity, at the end serving as the reference electrode.

A sensor element in which the valence of metal M is biased in this way exhibits proton conductivity at the end on the measurement electrode side, but does not exhibit proton conductivity at the end on the reference electrode side under atmospheric hydrogen partial pressure, so that atmospheric air can be used as the reference gas.

However, this sensor element has room for improvement in that it takes a very long time until the electromotive force stabilizes and the element can be used as a hydrogen sensor. Specifically, when the metal composite oxide represented by the above chemical formula AB _1-b M _b O _3-α is used as a sensor element, the valence of metal M is changed to a valence that does not exhibit proton conductivity as a whole by firing in an oxidizing atmosphere, and then the end of the sensor element on the reference electrode side is brought into contact with air, which is a reference gas, while the end of the sensor element on the measurement electrode side is placed in a reducing atmosphere containing hydrogen and heated. By carrying out such a process, the valence of metal M at the end of the sensor element on the measurement electrode side is changed to a valence that is smaller than that of metal B and exhibits proton conductivity, but it takes a long time, on the order of several tens of hours, for the valence of metal M to change in this way.

Therefore, the present applicant further proposed a hydrogen sensor using a metal composite oxide represented by the chemical formula AB _1-b-c M _b N _c O _3-α , where A is an alkaline earth metal, B is a metal with a valence of +4, M is a transition metal that can have multiple valences of +4 or less, and N is a metal that only has a valence less than +4 (see Patent Document 2). When such a solid electrolyte is used as a sensor element, air can be used as the reference gas, and the time required for the electromotive force generated between the reference electrode and the measurement electrode to be stably measured can be greatly shortened compared to the sensor element of Patent Document 1. And, when such a solid electrolyte is used as a sensor element and the reference gas is air, the hydrogen partial pressure (hydrogen concentration) of the measurement gas (measurement atmosphere) and the electromotive force have a correlation, so that the hydrogen concentration in the measurement gas can be accurately measured.

The applicant has continued research since then, aiming to shorten the time required for the electromotive force to stabilize and for the hydrogen concentration to be accurately detected by adopting a means different from the technology of Patent Document 2. The present invention was made during that process.

Patent No. 6494023 JP 2019-113500 A

As described above, the present invention aims to provide a hydrogen sensor that can use air as a reference gas and that requires a short time to reach a state where hydrogen concentration can be stably detected, and a method for detecting hydrogen concentration using the hydrogen sensor.

In order to solve the above problems, the hydrogen sensor according to the present invention comprises:
"A hydrogen sensor comprising a sensor element formed of a solid electrolyte having proton conductivity, a reference electrode provided on a surface of the sensor element, and a measurement electrode provided on the surface of the sensor element in a second space partitioned from a first space in contact with the reference electrode,
The sensor element includes:
a sintered body of a first solid electrolyte which is a metal composite oxide having a perovskite-type crystal structure and is represented by a chemical formula of AB _1-b M _b O _3-α (where 0<b<1) , where A is an alkaline earth metal, B is a metal with a valence of +4, and M is a transition metal which can have multiple valences of +4 or less;
a sintered body of a second solid electrolyte which is a metal composite oxide having a perovskite-type crystal structure and is represented by a chemical formula of AB1 _{- cNcO3 -α} (where 0<c<1) , where A is an alkaline earth metal, B is a metal whose valence is +4, and N is a metal whose valence is less than +4;
the reference electrode is provided on a sintered body of the first solid electrolyte, and the measurement electrode is provided on a sintered body of the second solid electrolyte;
The first space is open to the atmosphere, or the first space is provided with an atmosphere introduction pipe for introducing atmosphere as a reference gas into the first space.

Examples of "metal A, which is an alkaline earth metal" include strontium (Sr), magnesium (Mg), calcium (Ca), and barium (Ba). Examples of "metal B, which is a +4 valent metal" include zirconium (Zr) and cerium (Ce).

Examples of "transition metal M that can have multiple valences of +4 or less" include manganese (Mn), titanium (Ti), vanadium (V), and chromium (Cr), which can have +4, +3, and +2 valences, and iron (Fe), cobalt (Co), and nickel (Ni), which can have +2 and +3 valences. When substituted with metal B, this metal M has both a valence that exhibits proton conductivity and a valence that does not exhibit proton conductivity, and the valence depends on metals A and B. For example, when metal M is Mn, metal A is Sr and metal B is Zr, the valence that does not exhibit proton conductivity is +4, and the valences that exhibit proton conductivity are +3 and +2. Also, even if metal M is also Mn, when metal A is Ca and metal B is Zr, the valence that does not exhibit proton conductivity is +3, and the valence that exhibits proton conductivity is +2. Mn expands in volume when its valence decreases, so if the crystal structure is different and the size of the interstitial spaces is different, the valence that can exist stably will be different.

Examples of "metal N that has only a valence of less than +4" include metals that have only a valence of +3, such as Y (yttrium), Yb (ytterbium), Al (aluminum), and In (indium). When this metal N is substituted for metal B, proton conductivity is achieved.

Metal A, metal B, metal M, and metal N may each be composed of a single element or multiple elements. In the chemical formulas AB _1-b M _b O _3-α and AB _1-c N _c O _3-α , naturally, 0<b<1 and 0<c<1, and b and c can each be 0.01 or more and 0.3 or less. α is an oxygen defect, and is a value that changes depending on the atomic species of metal A, metal B, metal M, and metal N, the values of b and c, the atmospheric temperature, the oxygen partial pressure, and the like.

The hydrogen sensor of this configuration is a sensor element formed by contacting a sintered body of a first solid electrolyte in which proton conductivity occurs due to a change in the valence of metal M with a sintered body of a second solid electrolyte having proton conductivity, and a reference electrode is provided on the first solid electrolyte side, and a measurement electrode is provided on the second solid electrolyte side. As described in detail below, this configuration makes it possible to detect the hydrogen concentration of a measurement gas (a gas whose hydrogen concentration is unknown) introduced into the second space, even if air is used as a reference gas, since there is a correlation between the electromotive force generated between the reference electrode and the measurement electrode and the hydrogen concentration in the second space. In addition, unlike the sensor element of Patent Document 1, there is no need to perform a long-term process to change the valence of metal M prior to use as a hydrogen sensor, and the hydrogen concentration in the measurement gas can be stably detected in a short time after starting use as a hydrogen sensor.

In this configuration, the sensor element has a two-layer structure of the first solid electrolyte and the second solid electrolyte, but it is necessary to sinter each of them before contacting them. If a molded body of the first solid electrolyte and a molded body of the second solid electrolyte are stacked and then fired, the components of one will diffuse into the other, and a two-layer structure will not be formed. In addition, if a sintered body of the second solid electrolyte is coated with a slurry of the raw material of the first solid electrolyte and fired, the first solid electrolyte becomes a porous layer, and the behavior of the sensor element is the same as when a single layer of a sintered body of the second solid electrolyte is used as the sensor element.

In addition to the above configuration, the hydrogen sensor according to the present invention further comprises:
"Metal A is strontium."

As described below, when metal A in the first solid electrolyte and the second solid electrolyte is strontium, the electromotive force relative to the hydrogen concentration of the measurement gas is large, making it possible to detect the hydrogen concentration with high accuracy.

Next, the hydrogen concentration detection method according to the present invention includes the steps of:
"A method for detecting a hydrogen concentration using a hydrogen sensor comprising: a sensor element formed of a solid electrolyte having proton conductivity; a reference electrode provided on a surface of the sensor element; and a measurement electrode provided on the surface of the sensor element in a second space partitioned from a first space in contact with the reference electrode,
The sensor element includes:
a sintered body of a first solid electrolyte which is a metal composite oxide having a perovskite-type crystal structure and is represented by a chemical formula of AB _1-b M _b O _3-α (where 0<b<1) , where A is an alkaline earth metal, B is a metal with a valence of +4, and M is a transition metal which can have multiple valences of +4 or less;
a sintered body of a second solid electrolyte which is a metal composite oxide having a perovskite-type crystal structure and is represented by a chemical formula of AB1 _{- cNcO3 -α} (where 0<c<1) , where A is an alkaline earth metal, B is a metal whose valence is +4, and N is a metal whose valence is less than +4;
providing the reference electrode on a sintered body of the first solid electrolyte and providing the measurement electrode on a sintered body of the second solid electrolyte;
Atmospheric air is used as the reference gas introduced into the first space.

This is a method for detecting hydrogen concentration using a hydrogen sensor with the above configuration.

As described above, the present invention provides a hydrogen sensor that can use air as a reference gas and requires a short time to reach a state where hydrogen concentration can be stably detected, and a method for detecting hydrogen concentration using the hydrogen sensor.

This is a graph showing the change in electromotive force when the hydrogen concentration in the gas introduced into the second space of the hydrogen sensor of Example 1 is changed, (a) when measured at a temperature of 800°C, and (b) when measured at a temperature of 700°C. This is a graph showing the change in electromotive force when the hydrogen concentration in the gas introduced into the second space of the hydrogen sensor of Example 1 is changed, (a) when measured at a temperature of 600°C, and (b) when measured at a temperature of 500°C. This is a graph showing the change in electromotive force when the hydrogen concentration in the gas introduced into the second space of the hydrogen sensor of Example 2 is changed, (a) when measured at a temperature of 800°C, and (b) when measured at a temperature of 600°C. This is a graph showing the change in electromotive force when the hydrogen concentration in the gas introduced into the second space of the hydrogen sensor of Example 3 is changed, (a) when measured at a temperature of 800°C, and (b) when measured at a temperature of 600°C. 4 is a graph plotting electromotive force versus hydrogen partial pressure in the second space for the hydrogen sensor of Example 1. 13 is a graph plotting electromotive force versus hydrogen partial pressure in the second space for the hydrogen sensor of Example 2. 13 is a graph plotting electromotive force versus hydrogen partial pressure in the second space for the hydrogen sensor of Example 3. 1 is a schematic diagram of a hydrogen sensor according to an embodiment of the present invention;

Below, the hydrogen sensor 1, which is one embodiment of the present invention, and a method for detecting hydrogen concentration using the hydrogen sensor 1 will be described with reference to the drawings.

First, the configuration of the hydrogen sensor 1 will be described with reference to FIG. 8. The hydrogen sensor 1 includes a sensor element 10 formed of a solid electrolyte, a reference electrode 21 provided on the surface of the sensor element 10, a measurement electrode 22 provided on the surface of the sensor element 10 in a second space S2 separated from a first space S1 to which the reference electrode 21 is in contact, and an electrometer 30 that measures the electromotive force between the reference electrode 21 and the measurement electrode 22.

The sensor element 10 is formed by contacting a sintered body 11 of a first solid electrolyte with a sintered body 12 of a second solid electrolyte. Here, as an example of a configuration in which a first space S1 in contact with the reference electrode 21 and a second space S2 in contact with the measurement electrode 22 are partitioned, the sensor element 10, in which the disk-shaped sintered body 11 and the sintered body 12 are stacked, seals one end of a cylindrical holder 51 and seals one end of another cylindrical holder 52, that is, the sensor element 10 blocks the middle of the holder 50 formed by the holders 51 and 52. In this example, the sintered body 11 and the sintered body 12 are pressed against each other by being sandwiched between the holders 51 and 52.

The first solid electrolyte is a metal composite oxide having a perovskite-type crystal structure represented by the chemical formula AB _1-b M _b O _3-α , where A is an alkaline earth metal, B is a metal with a valence of +4, and M is a transition metal that can have multiple valences of +4 or less. Such a sintered body 11 of the first solid electrolyte can be obtained by molding a mixed raw material obtained by mixing raw material powders such as oxides of metal A, metal B, and metal M together with additives such as binders in a desired molar ratio, and firing the molded body. By setting the firing atmosphere to an oxidizing atmosphere such as air, the valence of metal M becomes a valence that does not exhibit proton conductivity in the entire solid electrolyte.

The second solid electrolyte is a metal composite oxide having a perovskite-type crystal structure represented by the chemical formula AB _1-c N _c O _3-α , where A is an alkaline earth metal, B is a metal with a valence of +4, and N is a metal that has only a valence smaller than +4. The sintered body 12 of the second solid electrolyte can be obtained by molding a mixed raw material obtained by mixing raw material powders such as oxides of metal A, metal B, and metal N together with additives such as binders in a desired molar ratio, and firing the molded body in an oxidizing atmosphere. The sintered body 12 of the second solid electrolyte has proton conductivity.

When assembling the hydrogen sensor 1, the sintered body 11 of the first solid electrolyte and the sintered body 12 of the second solid electrolyte may be bonded together with an organic adhesive to stabilize their contact state. The organic adhesive burns away at the operating temperature of the hydrogen sensor 1, which is several hundred degrees or more, so after the initial heating up to the operating temperature, the sintered body 11 and the sintered body 12 are physically pressed against each other.

For the sensor element 10 in which the sintered body 11 of the first solid electrolyte and the sintered body 12 of the second solid electrolyte are in contact with each other, a reference electrode 21 is provided on the end face of the sintered body 11 opposite to the side in contact with the sintered body 12, and a measurement electrode 22 is provided on the end face of the sintered body 12 opposite to the side in contact with the sintered body 11.

In a hydrogen sensor 1 configured in this way, the relationship between the electromotive force generated between the reference electrode 21 and the measurement electrode 22 and the hydrogen concentration in the second space S2 does not follow the Nernst equation, but when the reference gas introduced into the first space S1 is air, there is a correlation between the hydrogen partial pressure (hydrogen concentration) in the second space S2 and the electromotive force. More specifically, there is a linear relationship between the logarithm of the hydrogen partial pressure (hydrogen concentration) in the second space S2 and the electromotive force.

When detecting the hydrogen concentration in the measurement gas using the hydrogen sensor 1, a calibration curve showing the relationship between the hydrogen concentration in the second space S2 and the electromotive force is created in advance by introducing air as a reference gas into the first space S1 and measuring the electromotive force when multiple types of gases with different hydrogen concentrations are introduced into the second space S2. This allows the electromotive force to be measured when air is introduced into the first space S1 as the reference gas and a measurement gas with an unknown hydrogen concentration is introduced into the second space S2, and the hydrogen concentration in the measurement gas can be determined as the hydrogen concentration at the electromotive force value on the created calibration curve.

In the hydrogen sensor 1 of this embodiment, unlike the hydrogen sensor of Patent Document 1, there is no need to perform a long process to bias the valence of the metal M before starting to detect the hydrogen concentration in the measurement gas introduced into the second space S2, and the electromotive force stabilizes in a very short time even if the use of the hydrogen sensor 1 manufactured as described above is immediately started.

This is believed to be largely due to the fact that the solid electrolyte (second solid electrolyte) on the measurement electrode 22 side has proton conductivity from the very beginning. Metal N is a metal that can only take on a smaller valence than metal B, so oxygen ion vacancies are introduced to maintain electrical neutrality by replacing it with metal B, and proton conductivity is expressed. In the solid electrolyte (first solid electrolyte) on the reference electrode 21 side, near the boundary with the second solid electrolyte, protons move from the second solid electrolyte, decreasing the valence of metal M. As a result, in the first solid electrolyte, the transport number of protons slopes downward from the boundary with the second solid electrolyte toward the end on the reference electrode 21 side as the hydrogen sensor 1 is used.

On the other hand, at the end of the first solid electrolyte on the side of the reference electrode 21, the metal M has a valence that does not exhibit proton conductivity, and the transport number of protons is almost zero under the hydrogen partial pressure in the atmosphere. Therefore, even if the hydrogen partial pressure in the atmosphere fluctuates when the atmosphere is used as the reference gas, the measured electromotive force is not affected by the fluctuation. Here, the "hydrogen partial pressure in the atmosphere" can be calculated from the equilibrium constant of the equilibrium reaction between water, hydrogen, and oxygen. The equilibrium constant is temperature dependent, and the higher the temperature of the equilibrium, the more the hydrogen and oxygen are produced from water. If the calculation is performed assuming that the atmosphere saturated with water vapor at room temperature is heated to a temperature of 400°C to 800°C, which is assumed to be the operating temperature range of the hydrogen sensor 1 of this embodiment, the hydrogen partial pressure in the atmosphere is 1×10 ^-18 atm to 5×10 ^-11 atm (1×10 ^-13 Pa to 5×10 ^-6 Pa).

Furthermore, the measurement gas does not necessarily contain hydrogen. Therefore, in the solid electrolyte of Patent Document 1, even if a proton conductive layer is formed on the measurement electrode side by a process that biases the valence of metal M, the valence of metal M may change due to contact with a measurement gas that does not contain hydrogen, resulting in a non-proton conductive surface layer. Furthermore, when the solid electrolyte is subsequently contacted with a measurement gas that contains hydrogen, the valence of metal M may change again to form a proton conductive surface layer, and the measurement electrode side of the solid electrolyte may become in a laminated state of a proton conductive layer and a non-proton conductive layer, and there is a risk that the hydrogen concentration cannot be accurately detected. In contrast, in the hydrogen sensor 1 of this embodiment, the second solid electrolyte on the measurement electrode 22 side is proton conductive by replacing metal N, whose valence does not change, with metal B, so that even if the measurement gas does not contain hydrogen, a non-proton conductive layer will not be formed on the measurement electrode 22 side, and the hydrogen gas concentration can be stably detected.

The metal A was strontium, the metal B was zirconium, and the metal M was manganese. The respective compounds SrCO ₃ , ZrO ₂ , and MnCO ₃ were used, and they were mixed in a molar ratio such that b = 0.1 in SrZr _1-b Mn _b O _3-α to prepare a first starting material. The first starting material was calcined at 1200 ° C., and the calcined powder was pulverized, and then mixed with a binder and molded into a disk shape. The molded body was placed in a sintering furnace and sintered in an oxidizing atmosphere at a temperature of 1500 ° C. to obtain a sintered body 11 of the first solid electrolyte. On the other hand, the metal A was strontium, the metal B was zirconium, and the metal N was yttrium, and the respective compounds SrCO ₃ , ZrO ₂ , and Y ₂ O ₃ were used, and they were mixed in a molar ratio such that c = 0.1 in SrZr _1-c Y _c O _3-α to prepare a second starting material. The second starting material was calcined at 1200° C., and the calcined powder was crushed and mixed with a binder to form a disk. The compact was placed in a sintering furnace and sintered in an oxidizing atmosphere at a temperature of 1500° C. to obtain a sintered body 12 of the second solid electrolyte. The sintered body 11 and the sintered body 12 were overlapped and pressed together to obtain the sensor element 10 of Example 1.

The metal A was strontium, the metal B was zirconium, and the metal M was manganese. The respective compounds SrCO ₃ , ZrO ₂ , and MnCO ₃ were used, and they were mixed in a molar ratio of b=0.05 in SrZr _1-b Mn _b O _3-α to prepare a first starting material. The sintered body 11 of the first solid electrolyte was obtained by the same process as in Example 1. On the other hand, the metal A was strontium, the metal B was zirconium, and the metal N was yttrium, and the respective compounds SrCO ₃ , ZrO ₂ , and Y ₂ O ₃ were used, and they were mixed in a molar ratio of c=0.1 in SrZr _1-c Y _c O _3-α to prepare a second starting material. The sintered body 12 of the second solid electrolyte was obtained by the same process as in Example 1. The sintered body 11 and the sintered body 12 were overlapped and pressed together to obtain the sensor element 10 of Example 2.

The metal A was calcium, the metal B was zirconium, and the metal M was manganese. The respective compounds were CaO, ZrO ₂ , and MnCO ₃ , and the mixture was mixed in a molar ratio of b=0.05 in CaZr _1-b Mn _b O _3-α to prepare a first starting material. The sintered body 11 of the first solid electrolyte was obtained by the same process as in Example 1. On the other hand, the metal A was calcium, the metal B was zirconium, and the metal N was indium, and the respective compounds were CaO, ZrO ₂ , and In ₂ O ₃ , and the mixture was mixed in a molar ratio of c=0.1 in CaZr _1-c In _c O _3-α to prepare a second starting material. The sintered body 12 of the second solid electrolyte was obtained by the same process as in Example 1. The sintered body 11 and the sintered body 12 were overlapped and pressed together to prepare the sensor element 10 of Example 3.

In each of the sensor elements 10 of Examples 1 to 3, a reference electrode 21 was formed at the end of the sintered body 11 of the first solid electrolyte, and a measurement electrode 22 was formed at the end of the sintered body 12 of the second solid electrolyte, by applying and baking a commercially available platinum paste for electrodes.

The hydrogen sensor configured as described above using each of the sensor elements of Examples 1 to 3 was used to measure the electromotive force at a predetermined temperature range of 800°C to 500°C when air was introduced as a reference gas into the first space and multiple gases with different hydrogen concentrations were introduced into the second space. The electromotive force was measured for the same sensor element at a temperature of 800°C, followed by measurements at a temperature of 700°C, 600°C, and 500°C.

In addition, the partial pressure of hydrogen in the atmosphere does not change much in a laboratory environment. Therefore, in order to investigate whether there is an effect on the electromotive force when the partial pressure of hydrogen fluctuates in the atmosphere used as the reference gas in a natural environment, the reference gas introduced into the first space was temporarily switched from air (oxygen concentration 20%) to argon gas containing 1% oxygen, and then switched back to air. The fluctuation in hydrogen partial pressure caused by such a switch in the reference gas is much larger than the fluctuation in hydrogen partial pressure in the atmosphere that occurs in a natural environment.

For all of the hydrogen sensors using the sensor elements of Examples 1 to 3, the electromotive force stabilized 300 to 500 seconds after starting measurement of the electromotive force at a temperature of 800°C. This is an extremely short time compared to the several tens of hours required for the electromotive force to stabilize and enable normal measurement with the hydrogen sensor of Patent Document 1. After the electromotive force stabilized in this short time, the electromotive force changed responsively in response to changes in the hydrogen concentration in the second space, and the electromotive force was stable and constant while the hydrogen concentration was held constant. Furthermore, even when the hydrogen concentration was changed significantly by introducing a gas with a high hydrogen concentration of 98.1% after introducing argon gas containing no hydrogen into the second space, the electromotive force changed with very good responsiveness.

In addition, even when the hydrogen partial pressure in the reference gas was intentionally changed significantly by switching the reference gas introduced into the first space from air to argon gas containing 1% oxygen while gas with a hydrogen concentration of 98.1% was introduced into the second space, the electromotive force hardly changed at all. In particular, in the sensor element of Example 1, the electromotive force did not change at all even when the hydrogen partial pressure in the reference gas was changed. As described above, the change in hydrogen partial pressure when the reference gas was switched from air to argon gas containing 1% oxygen was much larger than the fluctuation in hydrogen partial pressure in the air that occurs in the natural environment, so it was confirmed that air can be used as the reference gas without any problems in hydrogen sensors using the sensor elements of these examples.

The above was the same even when the measurement temperature was changed. As an example, the results of measuring the electromotive force of the hydrogen sensor of Example 1 at measurement temperatures of 800°C, 700°C, 600°C, and 500°C are shown in Figures 1(a), 1(b), 2(a), and 2(b), respectively. The results of measuring the electromotive force of the hydrogen sensor of Example 2 at measurement temperatures of 800°C and 600°C are shown in Figures 3(a) and 3(b), and the results of measuring the electromotive force of the hydrogen sensor of Example 3 at measurement temperatures of 800°C and 600°C are shown in Figures 4(a) and 4(b). In these figures, the time period when air was introduced as the reference gas is shown as "ref: Air", and the time period when argon gas containing 1% oxygen was introduced as the reference gas is shown as "ref: 1% O ₂ -Ar".

In each of the hydrogen sensors of Examples 1 to 3, a linear relationship was observed between the logarithm of the hydrogen partial pressure in the gas introduced into the second space and the electromotive force. Graphs of the hydrogen sensors of Examples 1 to 3, with the logarithm of the hydrogen partial pressure in the gas in the second space on the horizontal axis and the electromotive force on the vertical axis, are shown in Figures 5, 6, and 7, respectively. Therefore, by previously introducing multiple types of gases with different hydrogen concentrations into the second space and measuring the electromotive force to investigate the correlation between the hydrogen partial pressure (hydrogen concentration) and the electromotive force, it is possible to detect the hydrogen concentration of the measurement gas, whose hydrogen concentration is unknown, from the electromotive force when the measurement gas is introduced into the second space.

The electromotive force value for the hydrogen partial pressure of the gas introduced into the second space was larger in Examples 1 and 2, in which strontium was used as metal A in the first and second solid electrolytes, than in Example 3, in which calcium was used as metal A. The larger the electromotive force value, the more accurately the hydrogen concentration of the measured gas can be detected. In addition, the change in electromotive force when the hydrogen partial pressure of the reference gas was intentionally changed significantly was smaller in Examples 1 and 2, in which strontium was used as metal A in the first and second solid electrolytes, than in Example 3, in which calcium was used as metal A. From these findings, it was considered that strontium is more preferable than calcium as metal A for the first and second solid electrolytes.

The present invention has been described above with reference to preferred embodiments, but the present invention is not limited to the above embodiments, and various improvements and design changes are possible without departing from the spirit of the present invention, as described below.

For example, in the above embodiment, the shape of the first solid electrolyte sintered body 11 and the second solid electrolyte sintered body 12 constituting the sensor element 10 is disk-shaped, and the sensor element 10 blocks the middle of the holder 50 formed by the holders 51 and 52, thereby partitioning the first space S1 and the second space S2. The shape of the first solid electrolyte sintered body and the second solid electrolyte sintered body is not limited to disk-shaped as long as they can be in contact with each other over a certain area, and they can be columnar or cylindrical. Furthermore, regardless of the shape of the sensor element, the first space and the second space can be partitioned by sealing one end of the holder with the sensor element, or by blocking the internal space of the holder with the sensor element halfway through.

1 Hydrogen sensor 10 Sensor element 11 Sintered body (sintered body of first solid electrolyte)
12 Sintered body (sintered body of second solid electrolyte)
21 Reference electrode 22 Measurement electrode S1 First space S2 Second space

Claims (3)

A hydrogen sensor comprising: a sensor element formed of a solid electrolyte having proton conductivity; a reference electrode provided on a surface of the sensor element; and a measurement electrode provided on the surface of the sensor element in a second space partitioned from a first space in contact with the reference electrode,
The sensor element includes:
a sintered body of a first solid electrolyte which is a metal composite oxide having a perovskite-type crystal structure and is represented by a chemical formula of AB _1-b M _b O _3-α (where 0<b<1) , where A is an alkaline earth metal, B is a metal with a valence of +4, and M is a transition metal which can have multiple valences of +4 or less;
a sintered body of a second solid electrolyte which is a metal double oxide having a perovskite-type crystal structure and is represented by a chemical formula of AB1 _{- cNcO3 -α} (where 0<c<1) , where A is an alkaline earth metal, B is a metal whose valence is +4, and N is a metal whose valence is less than +4;
the reference electrode is provided on a sintered body of the first solid electrolyte, and the measurement electrode is provided on a sintered body of the second solid electrolyte;
A hydrogen sensor characterized in that the first space is open to the atmosphere, or that an atmosphere inlet pipe is provided for introducing atmosphere as a reference gas into the first space. 2. The hydrogen sensor according to claim 1, wherein the metal A is strontium. A method for detecting a hydrogen concentration using a hydrogen sensor including a sensor element formed of a solid electrolyte having proton conductivity, a reference electrode provided on a surface of the sensor element, and a measurement electrode provided on the surface of the sensor element in a second space partitioned from a first space in contact with the reference electrode,
The sensor element includes:
a sintered body of a first solid electrolyte which is a metal composite oxide having a perovskite-type crystal structure and is represented by a chemical formula of AB _1-b M _b O _3-α (where 0<b<1) , where A is an alkaline earth metal, B is a metal with a valence of +4, and M is a transition metal which can have multiple valences of +4 or less;
a sintered body of a second solid electrolyte which is a metal double oxide having a perovskite-type crystal structure and is represented by a chemical formula of AB1 _{- cNcO3 -α} (where 0<c<1) , where A is an alkaline earth metal, B is a metal whose valence is +4, and N is a metal whose valence is less than +4;
providing the reference electrode on a sintered body of the first solid electrolyte and providing the measurement electrode on a sintered body of the second solid electrolyte;
A method for detecting a hydrogen concentration, comprising the steps of: introducing atmospheric air into the first space as a reference gas;

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