Hydrogen sensor and method for detecting hydrogen concentration

Abstract

To provide a hydrogen sensor with which it is possible to utilize the atmospheric air as a reference gas, and with which the time required until it becomes possible to stably measure a hydrogen concentration is short.SOLUTION: A sintered compact 11 of a first solid electrolyte expressed by a chemical formula AB1-bMbO3-α and a sintered compact 12 of a second solid electrode expressed by a chemical formula AB1-cNcO3-α (A represents an alkaline earth metal, B represents a +4 valent metal, M represents a transition metal that can take a plurality of valences smaller than or equal to +4 valence, N represents a metal that can take only valences smaller than +4 valance) are brought into contact with each other to obtain a sensor element, and thereby a reference electrode 21 is provided on the sintered compact 11 and a measurement electrode 22 is provided on the sintered compact 12.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] 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. [Background technology]

[0002] A solid electrolyte (proton-conducting ceramics) that exhibits proton conductivity by replacing part of metal B with metal M of lower valence in 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.

[0003] Hydrogen sensors that use proton-conductive ceramics as a sensor element utilize the principle of a concentration cell, where potential differences occur due to differences in the concentrations of the same ions. 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.

[0004] 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, limitations on the use of sensors when hydrogen gas is difficult to obtain overseas because it cannot be transported by airplane, the need to handle hydrogen with great care because it burns and explodes in the presence of oxygen, and its high cost.

[0005] 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 simplify the structure of the sensor device. However, the hydrogen partial pressure in the air fluctuates due to the influence of the water vapor partial pressure, and within the range of fluctuation of the hydrogen partial pressure, the transport number of protons in conventional proton conductive ceramics changes. 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, and the hydrogen concentration in the measurement atmosphere cannot be accurately measured.

[0006] The present applicant has proposed a hydrogen sensor that can solve such problems and can use air as a reference gas, and a solid electrolyte that serves as the sensor element (see, for example, Patent Document 1). 1-b M b O 3-α In the metal double oxide represented by the formula (1), A is an alkaline earth metal, B is a metal with a valence of +4, and M is a transition metal that can take multiple valences less than the valence of metal B. When this is used as a sensor element, the valence of metal M at the end facing the measurement electrode is biased to a valence smaller than that of metal B and exhibits proton conductivity, and the valence of metal M at the end facing the reference electrode is biased to a valence larger than the valence on the measurement electrode side and does not exhibit proton conductivity.

[0007] A sensor element in which the valence of metal M is biased in this manner exhibits proton conductivity at the end on the measurement electrode side, but does not exhibit proton conductivity under atmospheric hydrogen partial pressure at the end on the reference electrode side, so that air can be used as the reference gas.

[0008] However, this sensor element requires a long time to stabilize the electromotive force and become usable as a hydrogen sensor, so there is room for improvement. 1-b M b O 3-αWhen a metal composite oxide represented by the formula (1) 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 calcining in an oxidizing atmosphere, and then the end on the reference electrode side is brought into contact with air, which is a reference gas, while the end 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 on the measurement electrode side of the sensor element changes 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.

[0009] Therefore, the applicant further provides a compound of the formula AB 1-b-c M b N c O 3-α A hydrogen sensor using a metal complex oxide as a sensor element, in which 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, has been proposed (see Patent Document 2). When such a solid electrolyte is used as a sensor element, air can be used as a 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 significantly shortened compared to the sensor element of Patent Document 1. When such a solid electrolyte is used as a sensor element and air is used as a reference gas, the hydrogen concentration in the measurement gas (measurement atmosphere) can be accurately measured because there is a correlation between the hydrogen partial pressure (hydrogen concentration) and the electromotive force in the measurement gas.

[0010] 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. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 6494023 [Patent Document 2] JP 2019-113500 A Summary of the Invention [Problem to be solved by the invention]

[0012] As described above, an object of the present invention is 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 of detecting hydrogen concentration using such a hydrogen sensor. [Means for solving the problem]

[0013] 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 metal double oxide having a perovskite crystal structure, represented by the chemical formula AB 1-b M b O 3-α Represented by (However, 0 <b<1) a sintered body of a first solid electrolyte, A being an alkaline earth metal, B being a metal having a valence of +4, and M being a transition metal that can have multiple valences of +4 or less; A metal double oxide having a perovskite crystal structure, represented by the chemical formula AB 1-c N c O 3-α Represented by (However, 0 <c<1) and a sintered body of a second solid electrolyte, A being an alkaline earth metal, B being a metal having a valence of +4, and N being a metal having only a valence of 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.

[0014] Examples of the "alkaline earth metal metal A" include strontium (Sr), magnesium (Mg), calcium (Ca), and barium (Ba). Examples of the "tetravalent metal metal B" include zirconium (Zr) and cerium (Ce).

[0015] Examples of the "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. This metal M, when substituted with metal B, takes 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. This is because Mn expands in volume when its valence decreases, and if the crystal structure differs and the size of the interstitial space differs, the valence that can exist stably will also differ.

[0016] Examples of "metal N having only a valence of less than +4" include metals having only a valence of +3, such as Y (yttrium), Yb (ytterbium), Al (aluminum), and In (indium). By substituting metal B with this metal N, proton conductivity is exhibited.

[0017] Each of the metals A, B, M and N may be composed of a single element or multiple elements. 1-b M b O 3-α, and chemical formula AB 1-c N c O 3-α In this case, of course, 0 < b < 1 and 0 < c < 1, and b and c can each be 0.01 or more and 0.3 or less. Further, α is an oxygen defect, and is a value that changes according to the atomic species of each of metal A, metal B, metal M, and metal N, the values of b and c, the ambient temperature, the oxygen partial pressure, and the like.

[0018] The hydrogen sensor of this configuration forms a sensor element by bringing a sintered body of a first solid electrolyte in which proton conductivity is generated due to a change in the valence of metal M into contact with a sintered body of a second solid electrolyte having proton conductivity. A reference electrode is provided on the first solid electrolyte side, and a measurement electrode is provided on the second solid electrolyte side. By adopting such a configuration, as will be described in detail later, even when air is used as the reference gas, 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. Therefore, the hydrogen concentration of the measurement gas (a gas with an unknown hydrogen concentration) introduced into the second space can be detected. Further, unlike the sensor element of Patent Document 1, it is not necessary to perform a process for changing the valence of metal M for a long time 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.

[0019] In this configuration, the sensor element has a two-layer structure of a first solid electrolyte and a second solid electrolyte, but it is necessary to bring them into contact after each is formed into a sintered body. If the molded body of the first solid electrolyte and the molded body of the second solid electrolyte are laminated and then fired, one component will diffuse into the other and a two-layer structure will not be formed. Further, when a slurry of the raw material of the first solid electrolyte is applied to the sintered body of the second solid electrolyte and fired, the first solid electrolyte becomes a porous layer, and the behavior as a sensor element is the same as that of a single layer of the sintered body of the second solid electrolyte as the sensor element.

[0020] The hydrogen sensor according to the present invention, in addition to the above configuration, "Metal A is strontium" can be set.

[0021] As described later, when metal A in the first solid electrolyte and the second solid electrolyte is strontium, the value of the electromotive force relative to the hydrogen concentration of the measurement gas is large, and therefore the hydrogen concentration can be detected with high accuracy.

[0022] 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 metal double oxide having a perovskite crystal structure, represented by the chemical formula AB 1-b M b O 3-α Represented by (However, 0 <b<1) a sintered body of a first solid electrolyte, A being an alkaline earth metal, B being a metal having a valence of +4, and M being a transition metal that can have multiple valences of +4 or less; A metal double oxide having a perovskite crystal structure, represented by the chemical formula AB 1-c N c O 3-α Represented by (However, 0 <c<1) and a sintered body of a second solid electrolyte, A being an alkaline earth metal, B being a metal having a valence of +4, and N being a metal having only a valence of 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; The atmospheric air is used as the reference gas introduced into the first space.

[0023] This is a method for detecting hydrogen concentration using the hydrogen sensor having the above configuration. Effect of the Invention

[0024] As described above, according to the present invention, it is possible 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. [Brief description of the drawings]

[0025] [Figure 1] 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. [Diagram 2] 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. [Diagram 3] 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. [Figure 4] 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. [Diagram 5] 4 is a graph in which the electromotive force is plotted against the hydrogen partial pressure in the second space for the hydrogen sensor of Example 1. [Figure 6] 13 is a graph plotting electromotive force versus hydrogen partial pressure in the second space for the hydrogen sensor of Example 2. [Figure 7] 13 is a graph plotting electromotive force versus hydrogen partial pressure in the second space for the hydrogen sensor of Example 3. [Figure 8] 1 is a schematic diagram of a hydrogen sensor according to an embodiment of the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Hereinafter, a hydrogen sensor 1 according to one embodiment of the present invention and a method for detecting a hydrogen concentration using the hydrogen sensor 1 will be described with reference to the drawings.

[0027] 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 partitioned 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.

[0028] 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. In this example, as an embodiment 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 together seal one end of a cylindrical holder 51 and seal one end of another cylindrical holder 52, that is, the sensor element 10 closes 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.

[0029] The first solid electrolyte has the chemical formula AB 1-b M b O 3-α A is a metal complex oxide having a perovskite-type crystal structure represented by the formula: 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 mixing raw material powders of oxides of metal A, metal B, and metal M together with additives such as binders in a desired molar ratio, molding the mixed raw material, 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.

[0030] The second solid electrolyte has the chemical formula AB 1-c N c O 3-α A is a metal complex oxide having a perovskite-type crystal structure represented by the formula: A is an alkaline earth metal, B is a metal with a valence of +4, and N is a metal that only has a valence smaller than +4. Such a sintered body 12 of the second solid electrolyte can be obtained by mixing raw material powders of oxides of metal A, metal B, and metal N together with additives such as binders in a desired molar ratio, molding the mixed raw material, and firing the molded body in an oxidizing atmosphere. The sintered body 12 of the second solid electrolyte has proton conductivity.

[0031] 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. Since the organic adhesive burns away at the operating temperature of the hydrogen sensor 1, which is several hundred degrees or more, the sintered body 11 and the sintered body 12 are in a state of being physically pressed against each other after the initial heating up to the operating temperature.

[0032] 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.

[0033] In hydrogen sensor 1 configured as described above, the relationship between the electromotive force generated between reference electrode 21 and measurement electrode 22 and the hydrogen concentration in second space S2 does not follow the Nernst equation, but when the reference gas introduced into first space S1 is air, there is a correlation between the hydrogen partial pressure (hydrogen concentration) in 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 second space S2 and the electromotive force.

[0034] Therefore, 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 prepared in advance by introducing air as a reference gas into the first space S1 and measuring the electromotive force when a plurality of types of gases with different hydrogen concentrations are introduced into the second space S2. This allows the hydrogen concentration in the measurement gas to be found as the hydrogen concentration at the electromotive force value on the prepared calibration curve by measuring the electromotive force 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.

[0035] In the hydrogen sensor 1 of this embodiment, unlike the hydrogen sensor of Patent Document 1, there is no need to perform a long-term process to bias the valence of the metal M prior to starting to detect the hydrogen concentration in the measurement gas introduced into the second space S2, and even if use of the hydrogen sensor 1 manufactured as described above is immediately started, the electromotive force stabilizes in a very short time.

[0036] 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 a smaller valence than metal B, so oxygen ion vacancies are introduced to maintain electrical neutrality by substitution with metal B, and proton conductivity is expressed. In the solid electrolyte (first solid electrolyte) on the reference electrode 21 side, the valence of metal M decreases near the boundary with the second solid electrolyte due to the migration of protons from the second solid electrolyte. As a result, in the first solid electrolyte, the transport number of protons slopes so as to decrease from the boundary with the second solid electrolyte toward the end on the reference electrode 21 side as the hydrogen sensor 1 is used.

[0037] 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 and hydrogen and oxygen. The equilibrium constant is temperature dependent, and the higher the temperature, the more the equilibrium leans to the side where hydrogen and oxygen are produced from water. If the calculation is made 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~5×10 -11 atm(1×10 -13 Pa~5×10 -6 Pa).

[0038] 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 of biasing 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 containing 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 detected accurately. In contrast, in the hydrogen sensor 1 of this embodiment, the second solid electrolyte on the measurement electrode 22 side is made to exhibit proton conductivity 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 detected stably. EXAMPLES

[0039] Metal A is strontium, metal B is zirconium, and metal M is manganese. The compounds used are SrCO3, ZrO2, and MnCO3, respectively. 1-b Mn b O 3-α The first starting material was prepared by mixing the two materials at a molar ratio such that b=0.1 in the above formula. The first starting material was calcined at 1200°C, and the calcined powder was pulverized, mixed with a binder, and molded into a disk shape. 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 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 SrCO3, ZrO2, and Y2O3 were used as the respective compounds, and SrZr 1-c Y c O 3-α The second starting material was prepared by mixing them at a molar ratio of c=0.1 in the above. The second starting material was calcined at 1200°C, and the calcined powder was pulverized, 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 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.

[0040] Metal A is strontium, metal B is zirconium, and metal M is manganese. The compounds used are SrCO3, ZrO2, and MnCO3, respectively. 1-b Mn b O 3-α The first starting material was prepared by mixing them in a molar ratio of b=0.05. A sintered body 11 of a 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 SrCO3, ZrO2, and Y2O3 were used as the respective compounds, and SrZr 1-c Y c O 3-α The second starting material was prepared by mixing them in a molar ratio of c=0.1 in Example 2. A 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.

[0041] Metal A is calcium, metal B is zirconium, and metal M is manganese. The compounds used are CaO, ZrO2, and MnCO3, respectively. 1-b Mn b O 3-α The first starting material was prepared by mixing them in a molar ratio of b=0.05. A sintered body 11 of a 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 CaO, ZrO2, and In2O3 were used as the respective compounds, and CaZr 1-c In c O 3-α The second starting material was prepared by mixing them at a molar ratio of c=0.1 in Example 1. A 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 3.

[0042] In each of the sensor elements 10 of Examples 1 to 3, a reference electrode 21 was formed at an end of the sintered body 11 of the first solid electrolyte, and a measurement electrode 22 was formed at an end of the sintered body 12 of the second solid electrolyte, by applying and baking a commercially available platinum paste for electrodes.

[0043] For the hydrogen sensor configured as described above using each of the sensor elements of Examples 1 to 3, air was introduced as a reference gas into the first space, and a plurality of gases with different hydrogen concentrations were introduced into the second space, and the electromotive force was measured at a predetermined temperature in the range of 800° C. to 500° C. For the same sensor element, the electromotive force was measured at a temperature of 800° C., then at a temperature of 700° C., at a temperature of 600° C., and at a temperature of 500° C., in that order.

[0044] 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.

[0045] In 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 a very short time compared to the hydrogen sensor of Patent Document 1, which required several tens of hours until the electromotive force stabilized and normal measurement became possible. After the electromotive force stabilized in such a 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. In addition, 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.

[0046] In addition, even when the reference gas introduced into the first space was switched from air to argon gas containing 1% oxygen while a gas with a hydrogen concentration of 98.1% was introduced into the second space, the electromotive force hardly changed when the hydrogen partial pressure in the reference gas was intentionally changed significantly. 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 the hydrogen sensor using the sensor element of these Examples can use air as the reference gas without any problems.

[0047] The above was the same even when the measurement temperature was changed. For 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 indicated as "ref:Air", and the time period when argon gas containing 1% oxygen was introduced as the reference gas is indicated as "ref:1%O2-Ar".

[0048] 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 a plurality of 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 measured when the measurement gas is introduced into the second space.

[0049] The electromotive force value with respect to 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 the metal A of the first solid electrolyte and the second solid electrolyte than in Example 3 in which calcium was used as the metal A. The larger the electromotive force value, the more accurately the hydrogen concentration of the measurement 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 the metal A of the first solid electrolyte and the second solid electrolyte than in Example 3 in which calcium was used as the metal A. From these facts, it was considered that strontium is more preferable than calcium as the metal A of the first solid electrolyte and the second solid electrolyte.

[0050] 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.

[0051] For example, in the above embodiment, the shape of the sintered body 11 of the first solid electrolyte and the sintered body 12 of the second solid electrolyte constituting the sensor element 10 are 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 sintered body of the first solid electrolyte and the sintered body of the second solid electrolyte is not limited to a disk shape 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 blocking the internal space of the holder with the sensor element halfway. [Explanation of symbols]

[0052] 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 Measuring electrode S1 First space S2 Second space

Claims

1. 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 metal complex oxide having a perovskite crystal structure, the chemical formula being AB １－ｂ M ｂ O ３－α (wherein 0<b<1), A is an alkaline earth metal, B is a metal having a valence of +4, and M is a transition metal that can have a plurality of valences of +4 or less; A metal complex oxide having a perovskite crystal structure, the chemical formula being AB １－ｃ N ｃ O ３－α (wherein 0<c<1), A is an alkaline earth metal, B is a metal having a valence of +4, and N is a metal having only a valence of 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 an atmosphere introduction pipe is provided for introducing the atmosphere as a reference gas into the first space. A hydrogen sensor comprising:

2. Metal A is strontium 2. The hydrogen sensor according to claim 1 .

3. 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 metal complex oxide having a perovskite crystal structure, the chemical formula being AB １－ｂ M ｂ O ３－α (wherein 0<b<1), A is an alkaline earth metal, B is a metal having a valence of +4, and M is a transition metal that can have a plurality of valences of +4 or less; A metal complex oxide having a perovskite crystal structure, the chemical formula being AB １－ｃ N ｃ O ３－α (wherein 0<c<1), A is an alkaline earth metal, B is a metal having a valence of +4, and N is a metal having only a valence of 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; Air is used as a reference gas introduced into the first space. A method for detecting hydrogen concentration.

Images