Sensor element and gas sensor

Abstract

A sensor element 20 is a sensor element for detecting hydrogen concentration in a gas to be measured, and includes: an element body 30 having a proton conductive solid electrolyte layer 34; a sensing electrode 51 disposed on a part of the solid electrolyte layer 34 that is exposed to the gas to be measured; and a reference electrode 52 disposed in the solid electrolyte layer 34. The sensing electrode 51 contains at least a second-type noble metal from among first-type noble metals having catalytic activity and second-type noble metals that suppress oxidation of hydrogen, and has an abundance ratio Rm of 0.11 or more as measured using X-ray photoelectron spectroscopy (XPS) and calculated by formula (1). (1): Rm=Sm2 / (Sm1+Sm2), where Sm1 is the amount [atom %] of first-type noble metals present in the sensing electrode 51, and Sm2 is the amount [atom %] of second-type noble metals present in the sensing electrode 51.

Description

Sensor element and gas sensor

[0001] The present invention relates to a sensor element and a gas sensor.

[0002] Conventionally, gas sensors for detecting the hydrogen concentration in a measurement gas are known. For example, Patent Documents 1 and 2 describe hydrogen sensors including a proton-conducting solid electrolyte and a pair of electrodes arranged to sandwich the solid electrolyte. In this hydrogen sensor, when one of the pair of electrodes contacts the measurement gas and the other contacts a reference gas, an electromotive force is generated between the pair of electrodes. This electromotive force correlates with the hydrogen concentration in the measurement gas, and therefore the hydrogen concentration can be calculated by measuring this electromotive force.

[0003] JP 2022-189215 A JP 2017-96659 A

[0004] In such gas sensors, there are cases where the hydrogen concentration in a gas containing oxygen cannot be detected. This is thought to be due to a reaction between hydrogen and oxygen on the surface of the electrode. Patent Document 2 describes that the hydrogen concentration can be measured accurately by suppressing the influence of oxygen by providing an oxygen adsorption suppression treatment layer containing fluorine or silicon on the surface of the electrode. However, there has been a need for other methods to suppress the influence of oxygen.

[0005] The present invention has been made to solve the above problems, and a main object of the present invention is to make it possible to detect the hydrogen concentration in a gas to be measured when the gas to be measured contains oxygen.

[0006] In order to achieve the above-mentioned main object, the present invention employs the following means.

[0007] [1] A sensor element of the present invention is a sensor element for detecting a hydrogen concentration in a measurement gas, comprising: an element body having a proton-conductive solid electrolyte body; a detection electrode disposed in a portion of the solid electrolyte body that is exposed to the measurement gas; and a reference electrode disposed on the solid electrolyte body, wherein the detection electrode contains at least the second type precious metal out of a first type precious metal having catalytic activity and a second type precious metal that suppresses oxidation of hydrogen, and an abundance ratio Rm measured using X-ray photoelectron spectroscopy (XPS) and calculated by the following formula (1) is 0.11 or more: Rm = Sm2 / (Sm1 + Sm2) (1) where, Sm1: abundance amount [atom %] of the first type precious metal in the detection electrode, and Sm2: abundance amount [atom %] of the second type precious metal in the detection electrode.

[0008] This sensor element has an abundance ratio Rm of 0.11 or more, which is the ratio of the abundance Sm2 of the second type precious metal to the sum of the abundance Sm1 [atom %] of the first type precious metal and the abundance Sm2 [atom %] of the second type precious metal in the detection electrode. By using this sensor element, when the measured gas contains oxygen, an electromotive force corresponding to the hydrogen concentration in the measured gas is generated between the detection electrode and the reference electrode, and the hydrogen concentration can be detected based on this electromotive force. The inventors confirmed this through experiments and analysis. This is thought to be because the abundance ratio Rm of 0.11 or more suppresses the reaction between hydrogen and oxygen in the measured gas on the surface of the detection electrode due to the second type precious metal.

[0009] [2] In the sensor element described in [1] above, the abundance ratio Rm may be 0.29 or more, which further suppresses the reaction between hydrogen and oxygen in the measurement gas on the surface of the detection electrode.

[0010] [3] In the sensor element described in [1] or [2] above, the detection electrode may not contain the first-class noble metal. This further suppresses the reaction between hydrogen and oxygen in the measurement gas on the surface of the detection electrode.

[0011] [4] In the sensor element described above (the sensor element according to any one of [1] to [3] above), the detection electrode may contain Au as the second noble metal.

[0012] [5] In the above-described sensor element (the sensor element described in any one of [1], [2], and [4]), the detection electrode may contain at least one of Pt, Rh, Ir, Ru, and Pd as the first type precious metal.

[0013] [6] In the sensor element described above (the sensor element described in any one of [1], [2], [4], and [5]), the detection electrode may contain Pt as the first type noble metal.

[0014] [7] In the above-mentioned sensor element (the sensor element according to any one of [1] to [6]), the solid electrolyte body is a metal composite oxide having a perovskite-type crystal structure, and is represented by the chemical formula AB 1-b M b O 3-δ The first solid electrolyte represented by the chemical formula AB 1-b-c M b N c O 3-δ The solid electrolyte body may be mainly composed of a second-class solid electrolyte represented by the formula (1), 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 can only have a valence of less than +4. In this way, by using the first-class solid electrolyte and / or the second-class solid electrolyte as the main component, it is not necessary to use a gas with a known hydrogen concentration as the reference gas, and air, for example, can be used as the reference gas.

[0015] [8] In the sensor element described in [7] above, the solid electrolyte body may be mainly composed of the first solid electrolyte, the metal M may be Mn, and the b may satisfy 0<b<0.03. This makes it easy to keep the Mn content of the solid electrolyte body at 1 wt% or less. This makes it easy to avoid the solid electrolyte body from falling under the category of "manganese and its compounds," a Class 2 controlled substance under the Regulations on Prevention of Hazards from Specified Chemical Substances (Toka-no-Ritsu). In this case, the b may satisfy 0<b≦0.025. This makes it easy to further reduce the Mn content of the solid electrolyte body.

[0016] [9] In the above-described sensor element (the sensor element described in any one of [1] to [8]), the reference electrode may be disposed in a portion of the solid electrolyte body that is exposed to a reference gas that serves as a reference for detecting the hydrogen concentration.

[0017]

[10] In the above-described sensor element (the sensor element described in [7] or [8]), the reference electrode may be disposed in a portion of the solid electrolyte body exposed to the measurement gas, and the reference electrode may contain at least the first type precious metal of the first type precious metal and the second type precious metal, and may have an abundance ratio Rr measured using X-ray photoelectron spectroscopy (XPS) and calculated by the following formula (2) of less than 0.11. In this way, since the reference electrode is disposed in a portion of the solid electrolyte body exposed to the measurement gas and the abundance ratio Rr of the reference electrode is less than 0.11, when the measurement gas contains oxygen, hydrogen and oxygen in the measurement gas react (oxidize) on the surface of the reference electrode. As a result, even when the reference electrode is disposed in a portion exposed to the measurement gas, the reference electrode is exposed to a reference gas containing almost no hydrogen, such as the atmosphere. Therefore, when the measured gas is a gas containing oxygen, even if both the detection electrode and the reference electrode are arranged in areas exposed to the measured gas, an electromotive force corresponding to the hydrogen concentration in the measured gas is generated between the detection electrode and the reference electrode, and the hydrogen concentration can be detected based on this electromotive force.

[0018] Rr=Sr2 / (Sr1+Sr2) (2) where, Sr1: the amount of the first precious metal present in the reference electrode [atom %], and Sr2: the amount of the second precious metal present in the reference electrode [atom %].

[0019]

[11] In the above-described sensor element (the sensor element according to any one of [1] to [6]), the solid electrolyte body has a first solid electrolyte layer in which the reference electrode is disposed and a second solid electrolyte layer in which the detection electrode is disposed, and the first solid electrolyte layer is a metal composite oxide having a perovskite-type crystal structure and represented by the chemical formula AB 1-b M b O 3-δ The first solid electrolyte represented by the chemical formula AB 1-b-c M b N c O 3-δ The second solid electrolyte layer is mainly composed of a second solid electrolyte represented by the chemical formula AB 1-d N d O 3-ε The sensor element may be mainly composed of a third-class solid electrolyte represented by the formula (1). Here, 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 up to +4, and N is a metal that only has a valence less than +4. This can prevent a decrease in the accuracy of hydrogen concentration detection when the hydrogen concentration in the measured gas is low. The inventors have confirmed this through experiments and analysis. In this sensor element, the first solid electrolyte layer may be mainly composed of the first-class solid electrolyte, and the metal M in the chemical formula of the first-class solid electrolyte may be Mn. The b in the chemical formula of the first solid electrolyte may satisfy the relationship 0<b<0.03. The reference electrode may be disposed in a portion of the solid electrolyte body that is exposed to a reference gas that serves as a reference for hydrogen concentration detection.

[0020] The sensor element according to the above

[11] may adopt the aspect of the sensor element according to the above

[10] .

[0021]

[12] In the sensor element described in

[11] above, the first solid electrolyte layer and the second solid electrolyte layer may be stacked on each other and co-fired. By co-firing the first solid electrolyte layer and the second solid electrolyte layer, their interfaces are firmly bonded together, thereby improving the strength of the element body.

[0022]

[13] A gas sensor according to the present invention includes the sensor element according to any one of [1] to

[12] . Therefore, this gas sensor can obtain the same effects as the sensor element described above, for example, the effect of being able to detect the hydrogen concentration in a measurement gas when the measurement gas contains oxygen.

[0023] 1 is an explanatory diagram showing an outline of the configuration of a gas sensor 10; an explanatory diagram showing an outline of the configuration of a gas sensor 110 of a modified example; a graph showing the output characteristics of the sensor element 20 of Experimental Examples 1 to 6; a graph showing the output characteristics of the sensor element 20 of Experimental Example 2; an explanatory diagram showing an outline of the configuration of a sensor element 220 of a modified example; an explanatory diagram showing an outline of the configuration of a sensor element 320 of a modified example; an explanatory diagram showing an outline of the configuration of a sensor element 420 of a modified example; a graph showing the output characteristics of the sensor element 20 of Experimental Examples 1 to 6; a graph showing the output characteristics of the sensor element 220 of Experimental Examples 11 to 15.

[0024] Next, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is an explanatory diagram showing the outline of the configuration of a gas sensor 10 according to one embodiment of the present invention. An element body 30 of a sensor element 20 has a long rectangular parallelepiped shape, and the longitudinal direction of the element body 30 (the left-right direction in Fig. 1) is defined as the front-rear direction, the thickness direction of the element body 30 (the up-down direction in Fig. 1) is defined as the up-down direction, and the width direction of the element body 30 (the direction perpendicular to the front-rear direction and the up-down direction) is defined as the left-right direction.

[0025] The gas sensor 10 detects the hydrogen concentration in a measurement gas. The gas sensor 10 can be attached to an exhaust gas pipe of, for example, a hydrogen engine or a fuel cell, and can be used to detect hydrogen leaked from the hydrogen engine or fuel cell into the measurement gas, using the exhaust gas in the exhaust gas pipe as the measurement gas. The gas sensor 10 includes a sensor element 20 and a control device 70.

[0026] The sensor element 20 includes an element body 30, a detection electrode 51, a reference electrode 52, and a heater unit 60. The detection electrode 51, the solid electrolyte layer 34 of the element body 30, and the reference electrode 52 form a sensor cell 50.

[0027] The element body 30 includes four layers: a first substrate layer 31, a second substrate layer 32, a spacer layer 33, and a solid electrolyte layer 34. The element body 30 has a plate-like structure in which the four layers 31 to 34 are stacked in this order from the bottom up in FIG. 1 . These four layers are tightly sealed and airtight. The front end of the element body 30 is exposed to the gas to be measured. A reference gas introduction space 36 is provided in the element body 30 between the top surface of the second substrate layer 32 and the bottom surface of the solid electrolyte layer 34, at a position defined by the side surface of the spacer layer 33. The reference gas introduction space 36 has an opening at the rear end of the sensor element 20. A reference gas, such as air or a gas with a known hydrogen concentration, is introduced into the reference gas introduction space 36 as a reference gas used to detect the hydrogen concentration.

[0028] The detection electrode 51 is disposed in a portion of the solid electrolyte layer 34 of the element body 30 that is exposed to the gas to be measured. Specifically, the detection electrode 51 is disposed on the front end side of the upper surface of the solid electrolyte layer 34. The detection electrode 51 is in contact with the upper surface of the solid electrolyte layer 34. The detection electrode 51 may be a porous body.

[0029] The reference electrode 52 is disposed in a portion of the solid electrolyte layer 34 of the element body 30 that is exposed to the reference gas. The reference electrode 52 is also disposed inside the element body 30. In this embodiment, the reference electrode 52 is disposed on the lower surface of the solid electrolyte layer 34, i.e., on the surface of the solid electrolyte layer 34 opposite to the surface on which the detection electrode 51 is disposed, and is exposed to the reference gas by being exposed in the reference gas introduction space 36. The reference electrode 52 is in contact with the lower surface of the solid electrolyte layer 34. The reference electrode 52 may be porous.

[0030] As described above, the sensor cell 50 is composed of the detection electrode 51, the solid electrolyte layer 34, and the reference electrode 52. The sensor cell 50 functions as an electrochemical hydrogen concentration battery cell, and generates an electromotive force (EMF) corresponding to the hydrogen concentration (hydrogen partial pressure) in the measurement gas around the detection electrode 51. The gas sensor 10 detects the hydrogen concentration in the measurement gas using this electromotive force (EMF).

[0031] The detection electrode 51 contains at least a second-class precious metal selected from a first-class precious metal having catalytic activity and a second-class precious metal that inhibits hydrogen oxidation, and has an abundance ratio Rm of 0.11 or greater as measured by X-ray photoelectron spectroscopy (XPS) and calculated by the following formula (1). Examples of the first-class precious metal include at least one of Pt, Rh, Ir, Ru, and Pd. Examples of the second-class precious metal include Au.

[0032] Rm=Sm2 / (Sm1+Sm2) (1) where, Sm1: the amount of the first precious metal present in the detection electrode 51 [atom %], and Sm2: the amount of the second precious metal present in the detection electrode 51 [atom %].

[0033] The abundance ratio Rm is the ratio of the abundance Sm2 of the second-type precious metal to the sum of the abundance Sm1 [atom %] of the first-type precious metal and the abundance Sm2 [atom %] of the second-type precious metal in the detection electrode 51. When the surface of the detection electrode 51 is exposed to the outside of the sensor element 20, as in this embodiment, the abundance ratio Rm is measured on the exposed surface using XPS. Alternatively, when the detection electrode 51 is not exposed to the outside of the sensor element 20, for example, when the detection electrode 51 is covered with a porous protective layer (not shown), a fracture surface of the detection electrode 51 (e.g., a fracture surface along the vertical, horizontal, or horizontal direction) is exposed, and the abundance ratio Rm is measured on the fracture surface using XPS. A larger abundance ratio Rm indicates that the second-type precious metal contained in the detection electrode 51 is relatively more abundant than the first-type precious metal contained in the detection electrode 51. The sensor element 20 of this embodiment, with the abundance ratio Rm of the detection electrode 51 being 0.11 or greater, can detect the hydrogen concentration in a measurement gas when the measurement gas contains oxygen. The inventors have confirmed this through experiments and analyses. The reason for this is believed to be as follows. First, consider a comparative example in which the detection electrode 51 is composed solely of the first-class precious metal (when the abundance ratio Rm is 0). In this comparative example, if the measured gas contains oxygen, the catalytic activity of the first-class precious metal contained in the detection electrode 51 is thought to cause hydrogen to react with oxygen (oxidize) on the surface of the detection electrode 51. As a result, hydrogen is not transported to the three-phase interface between the detection electrode 51 (precious metal), the solid electrolyte, and the measured gas, and an electromotive force (EMF) corresponding to the hydrogen concentration (hydrogen partial pressure) is not generated. In contrast, in this embodiment, the abundance ratio Rm of the detection electrode 51 is 0.11 or greater, so the second-class precious metal suppresses the reaction between hydrogen and oxygen in the measured gas on the surface of the detection electrode 51, generating an electromotive force (EMF) corresponding to the hydrogen concentration in the measured gas.

[0034] The abundance ratio Rm of the detection electrode 51 is preferably 0.29 or greater. When the abundance ratio Rm is 0.29 or greater, the reaction between hydrogen and oxygen in the measured gas on the surface of the detection electrode 51 is further suppressed, thereby generating a larger electromotive force EMF corresponding to the hydrogen concentration in the measured gas. The abundance ratio Rm may be 0.30 or greater. The detection electrode 51 does not need to contain a first-class precious metal. "Does not contain" means that the detection electrode 51 is considered to be substantially free of the first-class precious metal, including cases where the content is below the detection limit. When the detection electrode 51 does not contain the first-class precious metal, the abundance ratio Rm is substantially 1. When the detection electrode 51 does not contain the first-class precious metal, the reaction between hydrogen and oxygen in the measured gas on the surface of the detection electrode 51 is further suppressed.

[0035] When the detection electrode 51 contains both a first-class precious metal and a second-class precious metal, it is preferable that the first-class precious metal and the second-class precious metal are the main components. When the detection electrode 51 does not contain a first-class precious metal, it is preferable that the second-class precious metal be the main component. The main component refers to the component with the highest content (wt%). The detection electrode 51 may contain components other than the first-class precious metal and the second-class precious metal, for example, the same solid electrolyte as the solid electrolyte layer 34. The detection electrode 51 may not have an oxygen adsorption suppression treatment layer on its surface, which is at least one of fluorine, silicon, bromine, and iodine. In this embodiment, the detection electrode 51 contains a first-class precious metal and a second-class precious metal as the main components, and does not contain either a solid electrolyte or an oxygen adsorption suppression treatment layer. Furthermore, in this embodiment, the detection electrode 51 contains Pt as the first-class precious metal and Au as the second-class precious metal.

[0036] The reference electrode 52 contains at least the first type precious metal of the above-mentioned first type precious metal and second type precious metal. The reference electrode 52 may also contain a second type precious metal, but the abundance ratio Rr measured using X-ray photoelectron spectroscopy (XPS) and calculated by the following formula (2) may be less than 0.11. The first type precious metal contained in the detection electrode 51 and the first type precious metal contained in the reference electrode 52 may be the same metal or different metals. The second type precious metal contained in the detection electrode 51 and the second type precious metal contained in the reference electrode 52 may be the same metal or different metals.

[0037] Rr=Sr2 / (Sr1+Sr2) (2) where, Sr1: the amount of the first precious metal present in the reference electrode 52 [atom %], and Sr2: the amount of the second precious metal present in the reference electrode 52 [atom %].

[0038] The abundance ratio Rr is the ratio of the abundance Sr2 of the second type precious metal to the sum of the abundance Sr1 [atom %] of the first type precious metal and the abundance Sr2 [atom %] of the second type precious metal in the reference electrode 52. As with the abundance ratio Rm, the abundance ratio Rr is measured by XPS on the exposed surface of the reference electrode 52 when the surface of the reference electrode 52 is exposed to the outside of the sensor element 20, or by XPS on the fracture surface of the reference electrode 52 when there is no exposed surface of the reference electrode 52. In this embodiment, since the reference electrode 52 is disposed in the reference gas introduction space 36 inside the sensor element 20, the fracture surface of the reference electrode 52 (e.g., fracture surface along the up, down, left, and right directions) is exposed, and the abundance ratio Rr is measured on the fracture surface by XPS.

[0039] The reference electrode 52 may not contain a second-class precious metal. The absence of the second-class precious metal in the reference electrode 52 corresponds to the abundance ratio Rr being substantially 0. When the reference electrode 52 contains both the first-class precious metal and the second-class precious metal, it is preferable that the first-class precious metal and the second-class precious metal are its main components. When the reference electrode 52 does not contain the second-class precious metal, it is preferable that the first-class precious metal be its main component. The reference electrode 52 may contain components other than the first-class precious metal and the second-class precious metal, and may, for example, contain the same solid electrolyte as the solid electrolyte layer 34. In this embodiment, the reference electrode 52 contains Pt as the first-class precious metal as its main component, and does not contain a second-class precious metal or a solid electrolyte.

[0040] The solid electrolyte layer 34 is a proton-conductive solid electrolyte. The solid electrolyte layer 34 is mainly composed of a proton-conductive solid electrolyte. The proton-conductive solid electrolyte is a metal composite oxide having a perovskite-type crystal structure, and is represented by the chemical formula AB 1-b M b O 3-δ The first solid electrolyte represented by the chemical formula AB 1-b-c M b N c O 3-δ and a second solid electrolyte represented by the chemical formula AB 1-b N b O 3-δ Examples of the third solid electrolyte include those represented by the formula: 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. b satisfies 0<b<1, and c satisfies 0<c<1. δ represents the amount of oxygen deficiency. Examples of metal A include Sr, Mg, Ca, and Ba. Examples of metal B include Zr and Ce. Examples of metal M include Mn, Ti, V, and Cr, which are metals that can have +2, +3, and +4 valences, and Fe, Co, and Ni, which are metals that can have +2 and +3 valences. Examples of metal N include Y, Yb, Al, and In, which are metals that can only have +3 valences. Each of metal A, metal B, metal M, and metal N may be a single element or multiple elements.

[0041] The solid electrolyte layer 34 may contain two or more of the first, second, and third solid electrolytes, or may contain at least the first and second solid electrolytes. Of the first to third solid electrolytes, the first and second solid electrolytes are particularly preferred. By using the first and / or second solid electrolytes as the main components of the solid electrolyte layer 34 constituting the sensor cell 50, it is not necessary to use a gas with a known hydrogen concentration as the reference gas; for example, air can be used as the reference gas. In this embodiment, the solid electrolyte layer 34 is mainly composed of the first solid electrolyte, and the chemical formula of the first solid electrolyte is CaZr 1-b Mn b O 3-δ It was decided.

[0042] When the proton-conductive solid electrolyte (here, the solid electrolyte layer 34) is primarily composed of a first-class solid electrolyte and the metal M is Mn, b preferably satisfies 0 < b < 0.03. This facilitates keeping the Mn content of the solid electrolyte layer 34 at 1 wt % or less. This helps prevent the solid electrolyte layer 34 (and the sensor element 20 including it) from falling under the category of "manganese and its compounds," a controlled Class 2 substance under the Ordinance on Prevention of Hazards from Specified Chemical Substances (the "Specified Chemical Substances Ordinance"). In this case, b more preferably satisfies 0 < b ≦ 0.025. This facilitates further reducing the Mn content of the solid electrolyte layer 34, further preventing the solid electrolyte layer 34 from falling under the category of a controlled Class 2 substance under the "Specified Chemical Substances Ordinance." Regardless of the value of b, it is preferable that the Mn content of the solid electrolyte layer 34 be 1 wt % or less.

[0043] In this embodiment, not only the solid electrolyte layer 34 but also the first substrate layer 31, the second substrate layer 32, and the spacer layer 33 have a proton-conductive solid electrolyte as their main component, and these layers 31 to 34 are all made of the same material. When the first substrate layer 31, the second substrate layer 32, and the spacer layer 33 also have a first-type solid electrolyte as their main component and the metal M is Mn, b preferably satisfies 0<b<0.03, and more preferably satisfies 0<b≦0.025. The Mn content in the first substrate layer 31, the second substrate layer 32, and the spacer layer 33 is also preferably 1 wt % or less.

[0044] The heater section 60 serves to adjust the temperature by heating and keeping the element body 30 (particularly the solid electrolyte layer 34) warm in order to activate the solid electrolyte in the element body 30 and increase proton conductivity. The heater section 60 includes a heater terminal 61, a heater 62, a through hole 63, a heater insulating layer 64, and a lead wire 66. The heater terminal 61 is an electrode disposed on the lower surface of the first substrate layer 31. The heater terminal 61 is connected to a heater power supply 77 of the control device 70.

[0045] The heater 62 is an electrical resistor sandwiched between the first substrate layer 31 and the second substrate layer 32. The heater 62 is connected to the heater terminal 61 via lead wires 66 and through holes 63. The heater 62 generates heat when power is supplied from the heater power supply 77 through the heater terminal 61, thereby heating and maintaining the temperature of the element body 30 (particularly, the solid electrolyte layer 34). The heater 62 is configured to control the output using a temperature sensor (here, the temperature acquisition unit 78 of the control device 70) so that the sensor cell 50 (particularly, the solid electrolyte layer 34) reaches a predetermined operating temperature. To properly activate the solid electrolyte layer 34 of the sensor cell 50, the operating temperature is preferably 450°C or higher. The operating temperature may also be 750°C or lower. The heater insulating layers 64 are disposed on the upper and lower surfaces of the heater 62 and are insulating layers made of porous alumina formed from an insulator such as alumina.

[0046] The control device 70 includes a control unit 71, an electromotive force acquisition unit 75, a heater power supply 77, and a temperature acquisition unit 78. The control unit 71 controls the entire gas sensor 10 and is a microprocessor including a CPU 72 and a storage unit 73. The storage unit 73 is a rewritable nonvolatile memory capable of storing, for example, various programs and data. The electromotive force acquisition unit 75 is connected to the detection electrode 51 and the reference electrode 52 of the sensor cell 50 and configured as a voltage detection circuit that acquires the electromotive force (EMF). The heater power supply 77 is a power supply that supplies power to the heater 62, and its output is controlled by the control unit 71. The temperature acquisition unit 78 acquires a value that can be considered the temperature of the heater 62 (a value that can be converted to temperature, in this case, a resistance value). The temperature acquisition unit 78 is connected to, for example, the heater terminal 61 and acquires the resistance value of the heater 62 by passing a small current and measuring the voltage at that time. The control unit 71 receives the electromotive force EMF from the electromotive force acquisition unit 75 and receives a value (here, a resistance value) that can be regarded as the temperature of the heater 62 from the temperature acquisition unit 78. The control unit 71 outputs a control signal to the heater power supply 77 to control the power that the heater power supply 77 supplies to the heater 62. The storage unit 73 stores a relational expression (for example, a linear or quadratic function) or a map as a correspondence relationship between the electromotive force EMF and the hydrogen concentration. Such a relational expression or map can be obtained in advance by experiment.

[0047] 1, the detection electrode 51 and the reference electrode 52 are electrically connected one-to-one to a plurality of lead wires formed toward the rear end of the sensor element 20. The electromotive force acquisition unit 75 measures the electromotive force EMF via these lead wires.

[0048] In addition to the sensor element 20 and the control device 70, the gas sensor 10 also includes a protective cover and an element fixing portion (not shown). The protective cover protects one longitudinal end of the sensor element 20, which is the end (here, the front end) where the detection electrode 51 is disposed. The element fixing portion fixes the sensor element 20 and also seals the space within the protective cover and the space around the opening of the reference gas introducing space 36 so that the measurement gas flowing into the protective cover does not flow into the reference gas introducing space 36.

[0049] An example of how the gas sensor 10 configured as described above can be used is described below. With the gas sensor 10 attached to a pipe through which a gas under measurement flows, the control unit 71 first heats the sensor cell 50 with the heater 62 to a predetermined target temperature, which serves as the operating temperature. For example, the control unit 71 inputs the resistance value of the heater 62 from the temperature acquisition unit 78 as a value that can be considered the temperature of the heater 62, and then performs feedback control of the heater power supply 77 so that the input resistance value becomes the target resistance value (the resistance value corresponding to the target temperature). When the gas under measurement flows through the pipe under this condition, the gas under measurement reaches the detection electrode 51. As a result, the sensor cell 50 generates an electromotive force EMF corresponding to the hydrogen concentration in the gas under measurement, as described above. The control unit 71 inputs the electromotive force EMF from the electromotive force acquisition unit 75 and derives (measures) the hydrogen concentration in the gas under measurement based on the input electromotive force EMF and the correspondence relationship between the electromotive force EMF and the hydrogen concentration (also referred to as output characteristics) previously stored in the memory unit 73. In this case, as described above, in this embodiment, the abundance ratio Rm, which is the ratio of the first type precious metal to the second type precious metal contained in the detection electrode 51, is 0.11 or more, so that the hydrogen concentration in the measured gas can be measured even if the measured gas contains oxygen.

[0050] A method for manufacturing the gas sensor 10 thus configured will be described below. First, a method for manufacturing the sensor element 20 will be described. When manufacturing the sensor element 20, first, a plurality of (four in this example) unfired ceramic green sheets corresponding to the layers 31 to 34 of the element body 30 are prepared. For example, the above-mentioned CaZr 1-b Mn b O 3-δGreen sheets containing these as their main components can be fabricated using a known solid-phase synthesis method as follows: CaCO3, ZrO2, and MnO2 are weighed as raw materials, mixed with ethanol, and then heat-treated (e.g., at 1300°C for 10 hours). The heat-treated material is pulverized to particles of a predetermined particle size (e.g., a median diameter (D50) of approximately 1 μm), and then a binder and solvent are added to form a slurry. This slurry is then tape-cast to obtain green sheets. Green sheets can be fabricated using liquid-phase methods such as the sol-gel method, as well as solid-phase synthesis. Notches, through-holes, and grooves can be formed in each green sheet as needed by punching or other processes. For example, the green sheet that will become the spacer layer 33 after firing has a space formed by punching to become the reference gas introduction space 36 after firing. Various patterns, such as electrodes and wiring, can also be screen-printed on each green sheet as needed. These various patterns include those that will become the detection electrode 51, reference electrode 52, and heater section 60 after firing. After forming the required patterns, multiple green sheets are stacked to form a laminate, which contains multiple pre-fired sensor elements 20. The laminate is cut into pieces the size of the sensor elements 20, and the pieces are fired at a predetermined firing temperature (e.g., 1500°C for 3 hours), to obtain the sensor elements 20.

[0051] For the screen printing to form the detection electrode 51 and the reference electrode 52, a conductive paste prepared by mixing raw material powder of a first-class precious metal and / or a second-class precious metal with a binder can be used. In addition, by appropriately changing the weight ratio of the first-class precious metal and / or the second-class precious metal in the conductive paste, the abundance ratio Rm of the detection electrode 51 and the abundance ratio Rr of the reference electrode 52 can be adjusted. By adjusting the compounding ratio of CaCO3, ZrO2, and MnO2 as raw materials for each layer 31 to 34 of the element body 30, the chemical formula of the solid electrolyte contained in the element body 30 after firing (here, CaZr 1-b Mn b O 3-δ) can be adjusted. Furthermore, if all or most of the detection electrode 51 is made of Au, the melting point of the detection electrode 51 is low and the detection electrode 51 and the solid electrolyte of the element body 30 cannot be co-fired by the above-mentioned firing process. In this case, after the firing process, a pattern to become the detection electrode 51 is screen-printed, and secondary firing is performed at a lower temperature than the firing process (for example, 850°C for 1 hour) to bake the detection electrode 51 onto the element body 30.

[0052] After the sensor element 20 is manufactured, the sensor element 20 is inserted into an element fixing portion and sealed and fixed therein, a protective cover is attached to the inside of the element fixing portion, and the sensor element 20 is electrically connected to the control device 70. In this way, the gas sensor 10 is obtained.

[0053] In the gas sensor 10 of this embodiment described above, the abundance ratio Rm, which is the ratio of the first-class precious metal to the second-class precious metal contained in the detection electrode 51 of the sensor element 20, is 0.11 or more. By using this sensor element 20, the gas sensor 10 can detect the hydrogen concentration in a measurement gas when the measurement gas contains oxygen. Furthermore, since the abundance ratio Rm is 0.29 or more, the reaction between hydrogen and oxygen in the measurement gas on the surface of the detection electrode 51 is further suppressed. Furthermore, since the detection electrode 51 does not contain the first-class precious metal, the reaction between hydrogen and oxygen in the measurement gas on the surface of the detection electrode 51 is further suppressed. The gas sensor 10 can detect the hydrogen concentration in a measurement gas even when the measurement gas does not contain oxygen.

[0054] Furthermore, since the solid electrolyte layer 34 is mainly composed of the first type solid electrolyte and / or the second type solid electrolyte, it is not necessary to use a gas with a known hydrogen concentration as the reference gas, and the air, for example, can be used as the reference gas.

[0055] It goes without saying that the present invention is not limited to the above-described embodiment, and can be embodied in various forms as long as they fall within the technical scope of the present invention.

[0056] For example, in the above-described embodiment, the reference electrode 52 is disposed in a portion of the solid electrolyte layer 34 exposed to the reference gas. However, this is not limited to this. For example, the reference electrode 52 may be disposed in a portion of the solid electrolyte layer 34 exposed to the measured gas instead of the reference gas. FIG. 2 is an explanatory diagram showing the schematic configuration of a gas sensor 110 according to this modification. In FIG. 2, the same components as those in the gas sensor 10 of FIG. 1 are denoted by the same reference numerals, and detailed description thereof will be omitted. The sensor element 120 of the gas sensor 110 includes an element body 130, which does not include the spacer layer 33 of the element body 30. The element body 130 is a laminated body in which a first substrate layer 31, a second substrate layer 32, and a solid electrolyte layer 34 are stacked in this order. Therefore, the element body 130 does not have a reference gas introduction space 36. The solid electrolyte layer 34 of the element body 130 is primarily composed of a first solid electrolyte and / or a second solid electrolyte. Unlike the reference electrode 52 in FIG. 1 , the reference electrode 152 in FIG. 2 is disposed in a portion of the solid electrolyte layer 34 of the element body 130 that is exposed to the measurement gas. Specifically, the reference electrode 152 is disposed on the upper surface of the solid electrolyte layer 34, i.e., the same surface on which the detection electrode 51 is disposed. The reference electrode 152 is disposed behind the detection electrode 51. The detection electrode 51, the solid electrolyte layer 34, and the reference electrode 152 constitute a sensor cell 150. The reference electrode 152 contains at least the first-class precious metal of the first-class precious metal and the second-class precious metal, and has an abundance ratio Rr of less than 0.11. Because the abundance ratio Rr of the reference electrode 152 is less than 0.11, the reference electrode 152 contains little or no second-class precious metal, and therefore the reaction between hydrogen and oxygen in the measurement gas is not suppressed on the surface of the reference electrode 152. Therefore, when the reference electrode 152 is exposed to a measurement gas containing hydrogen and oxygen, the hydrogen reacts (oxidizes) with the oxygen on the surface of the reference electrode 152. As a result, even if the reference electrode 152 is disposed in a portion exposed to the measurement gas, the reference electrode 152 is in a state similar to that when it is exposed to a reference gas containing almost no hydrogen, such as the atmosphere.Therefore, when the measurement gas contains oxygen, even if both the detection electrode 51 and the reference electrode 152 are disposed in portions exposed to the measurement gas, an electromotive force EMF corresponding to the hydrogen concentration in the measurement gas is generated between the detection electrode 51 and the reference electrode 152 in the sensor cell 150, and the hydrogen concentration can be detected based on this electromotive force EMF. Furthermore, because the reference electrode 152 is disposed in a portion of the solid electrolyte layer 34 exposed to the measurement gas, the element body 130 does not need to have the reference gas introduction space 36 as in the element body 30, and the sensor element 120 can have a simple structure.

[0057] In the above-described embodiment, the solid electrolyte body constituting the sensor cell 50 is a single layer, the solid electrolyte layer 34. However, the solid electrolyte body is not limited to this, and may be a laminate of two or more solid electrolyte layers. In this case, the detection electrode 51 may be disposed in one of the two solid electrolyte layers, and the reference electrode 52 may be disposed in the other solid electrolyte layer.

[0058] An example of an embodiment in which the solid electrolyte body is a laminate of two or more solid electrolyte layers will be described with reference to FIG. 5 . FIG. 5 is an explanatory diagram showing the outline of the configuration of a modified sensor element 220. In FIG. 5 , the same components as those in the sensor element 20 of FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted. The element body 230 of the sensor element 220 includes a first solid electrolyte layer 234a and a second solid electrolyte layer 234b instead of the solid electrolyte layer 34 of the element body 30 described above. The first solid electrolyte layer 234a is laminated on the spacer layer 33. The lower surface of the first solid electrolyte layer 234a is exposed in the reference gas introduction space 36, as is the lower surface of the solid electrolyte layer 34. A reference electrode 52 is disposed on the lower surface of the first solid electrolyte layer 234a. The second solid electrolyte layer 234b is laminated on the first solid electrolyte layer 234a. A detection electrode 51 is disposed on the upper surface of the second solid electrolyte layer 234b. The second solid electrolyte layer 234b is smaller than the first solid electrolyte layer 234a in top view. That is, the second solid electrolyte layer 234b is disposed so as to cover a portion of the upper surface of the first solid electrolyte layer 234a. The second solid electrolyte layer 234b and the detection electrode 51 are disposed in a portion of the element body 230 exposed to the measurement gas. The upper surface of the first solid electrolyte layer 234a is also present in a portion of the element body 230 exposed to the measurement gas. The detection electrode 51, the second solid electrolyte layer 234b, the first solid electrolyte layer 234a, and the reference electrode 52 form the sensor cell 250. Like the sensor cell 50, the sensor cell 250 functions as an electrochemical hydrogen concentration battery cell, generating an electromotive force (EMF) corresponding to the hydrogen concentration (hydrogen partial pressure) in the measurement gas surrounding the detection electrode 51. The first solid electrolyte layer 234a and the second solid electrolyte layer 234b are proton-conducting solid electrolyte bodies. The first solid electrolyte layer 234a is formed of the compound represented by the above-mentioned chemical formula AB 1-b M b O 3-δ The first solid electrolyte represented by the chemical formula AB 1-b-c M b N c O 3-δThe second solid electrolyte layer 234b mainly contains the above-mentioned third solid electrolyte. The chemical formula of the third solid electrolyte of the second solid electrolyte layer 234b is expressed by the chemical formula AB 1-d N d O 3-ε The metals A, B, M, and N have all been described in the above-mentioned embodiments. The chemical formula of the main component of the first solid electrolyte layer 234a is (AB 1-b M b O 3-δ and / or AB 1-b-c M b N c O 3-δ ) and the metals A, B, and N in the second solid electrolyte layer 234b, and the chemical formula AB 1-d N d O 3-ε The metals A, B, and N in the formula are the same element. For example, if the first solid electrolyte layer 234a contains a first solid electrolyte as a main component and the chemical formula of the first solid electrolyte is CaZr 1-b Mn b O 3-δ In this case, the chemical formula of the third solid electrolyte, which is the main component of the second solid electrolyte layer 234b, is CaZr 1-d N d O 3-εFurthermore, d satisfies the relationship 0<d<1. d and b may be the same or different. ε represents the amount of oxygen vacancy, and δ and ε may be the same or different. When manufacturing the sensor element 220, the green sheets for the first solid electrolyte layer 234a and the second solid electrolyte layer 234b can be fabricated using, for example, solid-phase synthesis, similar to the green sheet for the solid electrolyte layer 34 described above. This sensor element 220 also achieves the same effects as the sensor element 20 of the above-described embodiment. For example, by setting the abundance ratio Rm, which is the ratio of the first-class precious metal to the second-class precious metal contained in the detection electrode 51 of the sensor element 220, to 0.11 or more, the effect of being able to detect the hydrogen concentration in the measurement gas when the measurement gas contains oxygen can be achieved. Note that, as in the above-described embodiment, the abundance ratio Rm is preferably 0.29 or more, and may be 0.30 or more. As in the above-described embodiment, the abundance ratio Rr of the reference electrode 52 may be less than 0.11. Furthermore, this sensor element 220 can suppress a decrease in the detection accuracy of the hydrogen concentration when the hydrogen concentration in the measured gas is low. The inventors confirmed this through experiments and analyses. The reasons for this are believed to be as follows. First, because the first solid electrolyte layer 234a on which the reference electrode 52 is disposed is primarily composed of a first-type solid electrolyte and / or a second-type solid electrolyte, it is not necessary to use a gas with a known hydrogen concentration as the reference gas. Instead, air, for example, can be used as the reference gas. On the other hand, if the second solid electrolyte layer 234b on which the detection electrode 51 is disposed is also primarily composed of a first-type solid electrolyte and / or a second-type solid electrolyte, the Hall conductivity of the second solid electrolyte layer 234b is likely to increase when the hydrogen concentration in the measured gas is low. Furthermore, the increased Hall conductivity reduces the proton transport number of the second solid electrolyte layer 234b, thereby reducing the electromotive force (EMF) of the sensor cell 250 generated in response to the hydrogen concentration in the measured gas. As a result, the detection accuracy of the hydrogen concentration is likely to decrease when the hydrogen concentration in the measured gas is low. In contrast, in the sensor element 220 of FIG. 5, the second solid electrolyte layer 234b on which the detection electrode 51 is disposed is mainly composed of the third-type solid electrolyte.Since the third-type solid electrolyte has a lower Hall conductivity than the first-type and second-type solid electrolytes, the proton transport number of the second solid electrolyte layer 234b is less likely to become small even when the hydrogen concentration in the measurement gas is low, and the electromotive force EMF of the sensor cell 250 generated in response to the hydrogen concentration in the measurement gas is less likely to become small. As a result, it is thought that the sensor element 220 can prevent a decrease in the detection accuracy of the hydrogen concentration when the hydrogen concentration in the measurement gas is low.

[0059] In the sensor element 220 of FIG. 5 , the second solid electrolyte layer 234b is disposed so as to cover a portion of the upper surface of the first solid electrolyte layer 234a, but this is not limited thereto. For example, the modified sensor element 320 shown in FIG. 6 may be employed. In the sensor element 320, the same components as those in the sensor element 220 of FIG. 5 are denoted by the same reference numerals, and detailed description thereof will be omitted. The element body 330 of the sensor element 320 of FIG. 6 includes a second solid electrolyte layer 334b ​​instead of the second solid electrolyte layer 234b of the element body 230. The second solid electrolyte layer 334b ​​is stacked on the first solid electrolyte layer 234a so as to cover the entire upper surface of the first solid electrolyte layer 234a. A detection electrode 51 is disposed on the upper surface of the second solid electrolyte layer 334b. The second solid electrolyte layer 334b ​​and the detection electrode 51 are disposed in a portion of the element body 330 exposed to the measurement gas. The sensor cell 350 is configured with the detection electrode 51, the second solid electrolyte layer 334b, the first solid electrolyte layer 234a, and the reference electrode 52. The second solid electrolyte layer 334b, like the second solid electrolyte layer 234b, is mainly composed of a third-type solid electrolyte. This sensor element 320 also provides the same effects as the sensor element 220 described above.

[0060] A modified sensor element 420 shown in FIG. 7 may be employed. FIG. 7 is an explanatory diagram showing a schematic configuration of the sensor element 420. In FIG. 7, the same components as those in the sensor element 120 of FIG. 2 are denoted by the same reference numerals, and detailed description thereof will be omitted. The element body 430 of the sensor element 420 includes a first solid electrolyte layer 434a and a second solid electrolyte layer 434b instead of the solid electrolyte layer 34 of the element body 130 described above. The first solid electrolyte layer 434a is stacked on the second substrate layer 32. A reference electrode 152 is stacked on the first solid electrolyte layer 434a. The second solid electrolyte layer 434b is disposed on the upper surface of the first solid electrolyte layer 434a at a position different from the position where the reference electrode 52 is disposed. A detection electrode 51 is disposed on the upper surface of the second solid electrolyte layer 434b. The second solid electrolyte layer 434b is smaller in size than the first solid electrolyte layer 434b when viewed from above. That is, the second solid electrolyte layer 434b is disposed so as to cover a portion of the upper surface of the first solid electrolyte layer 434a. The second solid electrolyte layer 434b, the detection electrode 51, and the reference electrode 152 are disposed in a portion of the element body 430 exposed to the measurement gas. The upper surface of the first solid electrolyte layer 434a is also present in a portion of the element body 430 exposed to the measurement gas. The detection electrode 51, the second solid electrolyte layer 434b, the first solid electrolyte layer 434a, and the reference electrode 152 form the sensor cell 350. Like the sensor cell 150, the sensor cell 350 functions as an electrochemical hydrogen concentration battery cell and generates an electromotive force (EMF) corresponding to the hydrogen concentration (hydrogen partial pressure) in the measurement gas surrounding the detection electrode 51. Like the first solid electrolyte layer 234a, the first solid electrolyte layer 434a is primarily composed of a first-type solid electrolyte and / or a second-type solid electrolyte. The second solid electrolyte layer 434b is mainly composed of a third-type solid electrolyte, similar to the second solid electrolyte layer 234b. The sensor element 420 also provides the same effects as the sensor elements 120 and 220 described above.

[0061] 5, the first solid electrolyte layer 234a and the second solid electrolyte layer 234b are preferably stacked and co-fired. For example, when manufacturing the sensor element 220, two green sheets that will become the first solid electrolyte layer 234a and the second solid electrolyte layer 234b after firing are stacked and then fired, thereby obtaining the sensor element 220 including the co-fired first solid electrolyte layer 234a and the second solid electrolyte layer 234b. More specifically, unfired green sheets corresponding to the layers 31 to 33, 234a, and 234b of the element body 230 are prepared, various patterns such as electrodes and wiring are formed, and then the multiple green sheets are stacked to form a laminate and then fired, thereby obtaining the sensor element 220 including the first solid electrolyte layer 234a and the second solid electrolyte layer 234b that are stacked and co-fired. In this way, the first solid electrolyte layer 234a and the second solid electrolyte layer 234b are co-fired, thereby firmly bonding their interfaces, thereby improving the strength of the element body 230. Whether the first solid electrolyte layer 234a and the second solid electrolyte layer 234b in the element body 230 are co-fired can be determined by analyzing the vicinity of the interface between the first solid electrolyte layer 234a and the second solid electrolyte layer 234b. For example, whether the first solid electrolyte layer 234a and the second solid electrolyte layer 234b are co-fired can be determined based on whether the component elements of the first solid electrolyte layer 234a and the second solid electrolyte layer 234b are in a state of mutual migration near the interface between the first solid electrolyte layer 234a and the second solid electrolyte layer 234b. Regarding the sensor element 320 of FIG. 6, it is also preferable that the first solid electrolyte layer 234a and the second solid electrolyte layer 334b ​​are co-fired, as in the sensor element 220. In the sensor element 420 of FIG. 7, similarly to the sensor element 220, the first solid electrolyte layer 434a and the second solid electrolyte layer 434b are preferably co-fired.

[0062] In the above-described embodiment, Sr, Mg, Ca, and Ba are given as examples of metal A, with Ca being particularly preferred. Furthermore, in the above-described embodiment, metals that can only assume a valence of +3 are given as examples of metal N, with Y, Yb, Al, and In being specific examples. Sc is also given as another metal that can only assume a valence of +3, and it can also be used as metal N.

[0063] Specific examples of sensor elements prepared will be described below as examples. Experimental Examples 2, 4 to 6, 8 to 10, 11, and 13 to 15 correspond to examples of the present invention, and Experimental Examples 1, 3, 7, and 12 correspond to comparative examples. Note that the present invention is not limited to the following examples.

[0064] [Experimental Examples 1 to 6] The sensor element 20 shown in FIG. 1 was fabricated by the above-described manufacturing method to produce Experimental Examples 1 to 6. Experimental Examples 1 to 6 were identical to one another except that the abundance ratio Rm of the detection electrode 51 was changed as shown in Table 1. Specifically, the first-class precious metal of the detection electrode 51 was Pt, and the second-class precious metal was Au. In Experimental Example 1, where the abundance ratio Rm was 0.00, the detection electrode 51 contained no Au and was mainly composed of Pt, while in Experimental Example 2, where the abundance ratio Rm was 1.00, the detection electrode 51 contained no Pt and was mainly composed of Au. In all of Experimental Examples 1 to 6, the reference electrode 52 contained Pt as the first-class precious metal as the main component, and the abundance ratio Rr was 0.00 (i.e., no second-class precious metal was contained). Each of the layers 31 to 34 included in the element body 30 contained a first-class solid electrolyte as the main component and had the chemical formula CaZr 0.975 Mn 0.025 O 3-δ That is, the value of b described above was set to 0.025. The detection electrode 51 of Experimental Example 2 does not contain Pt but is mainly composed of Au, and has a low melting point, so it was formed by baking a conductive paste printed on the element body 30 after the firing process as described above.

[0065] [Experimental Examples 7 to 10] The sensor element 120 shown in Figure 2 was produced by the manufacturing method described above to produce Experimental Examples 7 to 10. Experimental Examples 7 to 10 were identical to each other except that the abundance ratio Rm of the detection electrode 51 was changed as shown in Table 1. The reference electrode 52 in Experimental Examples 7 to 10 contained Pt as a first-class precious metal as its main component, and the abundance ratio Rr was 0.00 (i.e., no second-class precious metal was contained). The materials of the detection electrode 51 and the layers 31, 32, and 34 of the element body 130 in Experimental Examples 7 to 10 were the same as those in Experimental Examples 1 to 6.

[0066] [Output Characteristics Test] The output characteristics of the sensor cells 50 and 150 of the sensor elements 20 and 120 of Experimental Examples 1 to 10 were examined. First, the control device 70 was connected to the sensor element 20 of Experimental Example 1, and the sensor element 20 was attached to a pipe. The tip of the sensor element 20 was exposed to a model gas (measurement gas) in the pipe, and atmospheric air (reference gas) was introduced into the reference gas inlet space 36. Next, the control device 70 controlled the heater power supply 77 to adjust the temperature of the solid electrolyte layer 34 to the operating temperature (450°C). In this state, a model gas containing nitrogen as the base gas, an oxygen concentration of 10%, a water concentration of 3%, and a hydrogen concentration variable from 0 to 4% was prepared and flowed through the pipe. The relationship between the hydrogen concentration in the model gas and the electromotive force (EMF) of the sensor cell 50, i.e., the output characteristics of the sensor cell 50, was examined. The temperature of the model gas was room temperature. Similar tests were conducted on the sensor elements 20 and 120 of Experimental Examples 2 to 10 to examine the output characteristics of the sensor cells 50 and 150. In Experiments 7 to 10, the atmosphere surrounding the reference electrode 152 of the sensor element 120 was the same as the atmosphere surrounding the detection electrode 51, that is, the measured gas (model gas). Table 1 shows the abundance ratio Rm of the detection electrode 51, the atmosphere surrounding the reference electrodes 52 and 152, the operating temperatures of the sensor cells 50 and 150, and the output characteristics for each of Experiments 1 to 10. A graph of the output characteristics of Experiments 1 to 6 in Table 1 is shown in Figure 3. Tables 2 to 4 show the output characteristics obtained by conducting the same tests for Experiments 1 to 10, except that the operating temperatures were changed to 550°C, 650°C, and 750°C. Furthermore, Figure 4 shows the output characteristics of Experiment 2 when the hydrogen concentration of the model gas was changed from approximately 4% to approximately 20% at each operating temperature (450°C, 550°C, 650°C, and 750°C). The horizontal axis in Figure 4 is plotted on a logarithmic scale.

[0067]

[0068]

[0069]

[0070]

[0071] As shown in Tables 1 to 4 and Figures 3 and 4 , among Experimental Examples 1 to 6, Experimental Examples 1 and 3, in which the abundance ratio Rm of the detection electrode 51 was less than 0.11, exhibited an electromotive force EMF of 0 V regardless of the hydrogen concentration. In contrast, Experimental Examples 2, 4, and 6, in which the abundance ratio Rm was 0.11 or greater, exhibited output characteristics in which the electromotive force EMF increased with increasing hydrogen concentration. Therefore, it was confirmed that when the measured gas contains oxygen (here, the oxygen concentration is 10%), it is possible to detect the hydrogen concentration based on the electromotive force EMF by setting the abundance ratio Rm of the detection electrode 51 to 0.11 or greater. Furthermore, Experimental Examples 2, 5, and 6, in which the abundance ratio Rm was 0.29 or greater, exhibited larger electromotive force EMF values ​​and larger changes in electromotive force EMF in response to changes in hydrogen concentration than Experimental Example 4, in which the abundance ratio Rm was 0.11. These results confirmed that hydrogen concentration can be detected more accurately when the abundance ratio Rm is 0.29 or greater. This is thought to be because an abundance ratio Rm of 0.29 or more further suppresses the reaction between hydrogen and oxygen in the measurement gas on the surface of the detection electrode 51. Furthermore, it was confirmed that when the measurement gas contains oxygen, the hydrogen concentration can be detected based on the electromotive force EMF as long as the operating temperature of the sensor cell 50 is at least within the range of 450°C to 750°C.

[0072] As shown in Tables 1 to 4, among Experimental Examples 7 to 10, Experimental Example 7, in which the abundance ratio Rm of the detection electrode 51 was less than 0.11, exhibited an electromotive force EMF of 0 V regardless of the hydrogen concentration. Furthermore, Experimental Example 8, in which the abundance ratio Rm was 0.11, sometimes exhibited an electromotive force EMF of 0 V when the hydrogen concentration was low, such as 1.0%, but an electromotive force EMF was output when the hydrogen concentration was high, such as 3.9%. Experimental Example 8, in which the operating temperature of the sensor cell 150 was between 450°C and 750°C, tended to output an electromotive force EMF even at low hydrogen concentrations. Experimental Examples 9 and 10, in which the abundance ratio Rm was 0.29 or higher, exhibited output characteristics in which the electromotive force EMF increased with increasing hydrogen concentration, and the electromotive force EMF values ​​were comparable to those of Experimental Examples 5 and 6. From the above results, it was confirmed that even in the sensor element 120 ( FIG. 2 ) in which the reference electrode 152 with an abundance ratio Rr of 0.00 is disposed in a portion of the element body 130 exposed to the measured gas, an electromotive force EMF corresponding to the hydrogen concentration in the measured gas is generated between the detection electrode 51 and the reference electrode 152, just as in the sensor element 20 ( FIG. 1 ) in which the reference electrode 52 is disposed in a portion of the element body 30 exposed to the atmosphere as in Experimental Examples 2, 4 to 6. This is thought to be because when the reference electrode 152 is exposed to the measured gas containing hydrogen and oxygen, this hydrogen reacts with (oxidizes) the oxygen on the surface of the reference electrode 152, bringing the reference electrode 152 into a state close to that in which it is exposed to a reference gas containing almost no hydrogen, such as the atmosphere. Furthermore, from the results of Experimental Examples 1 to 6, it is believed that if the abundance ratio Rm is less than 0.11, hydrogen and oxygen in the measured gas react on the surface of the detection electrode 51, and therefore if the abundance ratio Rr of the reference electrode 152 is not limited to 0.00 and is less than 0.11, it is believed that hydrogen reacts with oxygen (oxidizes) on the surface of the reference electrode 152. Furthermore, from the results of Experimental Examples 7 to 10, it is believed that the abundance ratio Rm of the detection electrode 51 is preferably 0.29 or more even in the sensor element 120 of the embodiment shown in FIG.

[0073] [Experimental Examples 11 to 15] The sensor element 220 shown in FIG. 5 was fabricated by the above-described manufacturing method to produce Experimental Examples 11 to 15. Experimental Examples 11 to 15 were identical to one another except that the abundance ratio Rm of the detection electrode 51 was changed as shown in Table 5. Specifically, the first-class precious metal of the detection electrode 51 was Pt, and the second-class precious metal was Au. In Experimental Example 11, in which the abundance ratio Rm was 1.00, the detection electrode 51 did not contain Pt but was mainly composed of Au. In all of Experimental Examples 11 to 15, the reference electrode 52 contained Pt as the first-class precious metal as the main component, and the abundance ratio Rr was 0.00 (i.e., did not contain the second-class precious metal). In all of Experimental Examples 11 to 15, the first solid electrolyte layer 234a was mainly composed of the first-class solid electrolyte and had the chemical formula CaZr 0.95 Mn 0.05 O 3-δ In other words, the value of b was set to 0.05. In all of Experimental Examples 11 to 15, the second solid electrolyte layer 234b mainly contained a third solid electrolyte, and the chemical formula was CaZr 0.9 Sc 0.1 O 3-ε That is, the value of d described above was set to 0.1. The firing temperature when obtaining the sensor element 220 was set to 1500°C, and the first solid electrolyte layer 234a and the second solid electrolyte layer 234b were co-fired. The detection electrode 51 of Experimental Example 11 does not contain Pt but is mainly composed of Au, and has a low melting point. Therefore, as in Experimental Example 2, the detection electrode 51 was formed by baking a conductive paste printed on the element body 230 after the firing process.

[0074] [Output Characteristics Test] The output characteristics of Experimental Examples 11 to 15 were examined using the same method as in the output characteristics test described above. However, the output characteristics at low hydrogen concentrations were examined in detail. More specifically, for the hydrogen concentration of the model gas, in addition to the output characteristics at the same hydrogen concentrations (0.0%, 1.0%, 2.0%, 3.0%, and 3.9%) as in the output characteristics test described above, the output characteristics at low hydrogen concentrations (0.01%, 0.05%, and 0.1%) were also examined. For Experimental Examples 1 to 10, the output characteristics at low hydrogen concentrations (0.01%, 0.05%, and 0.1%) were also examined. Tables 5 to 8 show the abundance ratio Rm of the detection electrode 51, the ambient atmosphere around the reference electrodes 52 and 152, the operating temperatures of the sensor cells 50, 150, and 250, and the output characteristics for each of Experimental Examples 1 to 15. In Tables 5 to 8, the output characteristics for the hydrogen concentrations (0.0%, 1.0%, 2.0%, 3.0%, 3.9%) in the above-mentioned output characteristics test for Experimental Examples 1 to 10 are the same as those in Tables 1 to 4. Also, a graph of the output characteristics for Experimental Examples 1 to 6 in Table 5 is shown in FIG. 8. A graph of the output characteristics for Experimental Examples 11 to 15 in Table 5 is shown in FIG. 9.

[0075]

[0076]

[0077]

[0078]

[0079] As shown in Tables 5 to 8 and FIG. 9 , the sensor elements 220 of Experimental Examples 11 to 15 also showed similar results to those of Experimental Examples 1 to 10 with respect to the abundance ratio Rm. That is, in Experimental Example 12, in which the abundance ratio Rm of the detection electrode 51 was less than 0.11, the electromotive force EMF remained at 0 V regardless of the hydrogen concentration. In contrast, in Experimental Examples 11, 13 to 15, in which the abundance ratio Rm of the detection electrode 51 was 0.11 or greater, output characteristics were confirmed in which the electromotive force EMF increased with increasing hydrogen concentration. Therefore, it was confirmed that when the measured gas contains oxygen (here, when the oxygen concentration is 10%), it is possible to detect the hydrogen concentration based on the electromotive force EMF by having the abundance ratio Rm of the detection electrode 51 be 0.11 or greater. Furthermore, the electromotive force EMF values ​​were larger in Experimental Examples 11, 14, and 15, in which the abundance ratio Rm of the detection electrode 51 was 0.29 or greater, than in Experimental Example 13, in which the abundance ratio Rm was 0.11. The amount of change in the electromotive force EMF in response to changes in hydrogen concentration was also larger. These results confirm that the hydrogen concentration can be detected with higher accuracy if the abundance ratio Rm is 0.29 or higher. Furthermore, when the measurement gas contains oxygen, it is possible to detect the hydrogen concentration based on the electromotive force EMF if the operating temperature of the sensor cell 250 is at least within the range of 450°C to 750°C.

[0080] As shown in Tables 5 to 8 and FIG. 8 , in the sensor elements 20 of Experimental Examples 1 to 6 (particularly Experimental Examples 2, 5, and 6) that included a single-layer solid electrolyte layer 34 as the solid electrolyte body, the electromotive force EMF tended to decrease significantly when the hydrogen concentration in the measured gas was low (here, when the hydrogen concentration was 0.01%, 0.05%, or 0.1%) compared to when the hydrogen concentration was 1% to 3.9%. As shown in Tables 5 to 8, the same tendency was observed in the sensor elements 120 of Experimental Examples 7 to 10 (particularly Experimental Examples 9 and 10) that included a single-layer solid electrolyte layer 34 as the solid electrolyte body. In contrast, as shown in Tables 5 to 8 and FIG. 9 , in the sensor elements 220 of Experimental Examples 11 to 15 (particularly Experimental Examples 11, 14, and 15) that included a first solid electrolyte layer 234 a and a second solid electrolyte layer 234 b as the solid electrolyte body, the decrease in the electromotive force EMF was suppressed even when the hydrogen concentration was low. These results confirmed that the sensor element 220 having the above-mentioned first solid electrolyte layer 234a and second solid electrolyte layer 234b suppresses the decrease in the detection accuracy of the hydrogen concentration when the hydrogen concentration in the measured gas is low, compared to the sensor element 20.

[0081] This application claims priority from Japanese Patent Application No. 2024-110715, filed on July 10, 2024, the entire contents of which are incorporated herein by reference.

[0082] The present invention can be used in a gas sensor for detecting the hydrogen concentration in a measurement gas such as exhaust gas from a hydrogen engine or a fuel cell.

[0083] 10 Gas sensor, 20 Sensor element, 30 Element body, 31 First substrate layer, 32 Second substrate layer, 33 Spacer layer, 34 Solid electrolyte layer, 36 Reference gas introduction space, 50 Sensor cell, 51 Detection electrode, 52 Reference electrode, 60 Heater section, 61 Heater terminal, 62 Heater, 63 Through hole, 64 Heater insulating layer, 66 Lead wire, 70 Control device, 71 Control section, 72 CPU, 73 Memory section, 75 Electromotive force acquisition section, 77 Heater power supply, 78 Temperature acquisition section, 110 Gas sensor, 120, 220, 320, 420 Sensor element, 130, 230, 330, 430 Element body, 150, 250, 350, 450 Sensor cell, 152 Reference electrode, 234a, 434a First solid electrolyte layer, 234b, 334b, 434b second solid electrolyte layer.

Claims

1. A sensor element for detecting the concentration of hydrogen in a gas to be measured, comprising: an element body having a proton-conductive solid electrolyte body; a detection electrode disposed in a portion of the solid electrolyte body that is exposed to the gas to be measured; and a reference electrode disposed on the solid electrolyte body, wherein the detection electrode contains at least the second type precious metal out of a first type precious metal having catalytic activity and a second type precious metal that inhibits oxidation of hydrogen, and the abundance ratio Rm measured using X-ray photoelectron spectroscopy (XPS) and calculated by the following formula (1) is 0.11 or more: Rm = Sm2 / (Sm1 + Sm2) (1) where, Sm1: amount [atom %] of the first type precious metal in the detection electrode, and Sm2: amount [atom %] of the second type precious metal in the detection electrode.

2. A sensor element according to claim 1, wherein the abundance ratio Rm is 0.29 or more.

3. A sensor element according to claim 1, wherein the detection electrode does not contain the first type noble metal.

4. A sensor element according to any one of claims 1 to 3, wherein the detection electrode contains Au as the second type noble metal.

5. A sensor element according to claim 1 or 2, wherein the detection electrode contains at least one of Pt, Rh, Ir, Ru, and Pd as the first type noble metal.

6. A sensor element according to claim 1 or 2, wherein the detection electrode contains Pt as the first type noble metal.

7. A sensor element according to any one of claims 1 to 3, wherein the solid electrolyte body is a metal composite oxide having a perovskite-type crystal structure and is represented by the chemical formula AB 1-b M b O 3-δ The first solid electrolyte represented by the chemical formula AB 1-b-c M b N c O 3-δ wherein 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 can only have a valence of less than +4.

8. A sensor element according to claim 7, wherein the solid electrolyte body contains the first type solid electrolyte as a main component, the metal M is Mn, and b satisfies 0<b<0.

03.

9. A sensor element according to any one of claims 1 to 3, wherein the reference electrode is disposed in a portion of the solid electrolyte body that is exposed to a reference gas that serves as a reference for detecting the hydrogen concentration.

10. A sensor element according to claim 7, wherein the reference electrode is disposed in a portion of the solid electrolyte body that is exposed to the gas to be measured, and the reference electrode contains at least the first type of precious metal out of the first type of precious metal and the second type of precious metal, and an abundance ratio Rr measured using X-ray photoelectron spectroscopy (XPS) and calculated by the following formula (2) is less than 0.11: Rr = Sr2 / (Sr1 + Sr2) (2) where, Sr1: abundance amount [atom %] of the first type of precious metal in the reference electrode, and Sr2: abundance amount [atom %] of the second type of precious metal in the reference electrode.

11. A sensor element according to any one of claims 1 to 3, wherein the solid electrolyte body has a first solid electrolyte layer in which the reference electrode is disposed and a second solid electrolyte layer in which the detection electrode is disposed, and the first solid electrolyte layer is a metal composite oxide having a perovskite-type crystal structure and represented by the chemical formula AB 1-b M b O 3-δ The first solid electrolyte represented by the chemical formula AB 1-b-c M b N c O 3-δ The second solid electrolyte layer is mainly composed of a second solid electrolyte represented by the chemical formula AB 1-d N d O 3-ε wherein 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 can only have a valence of less than +4.

12. A sensor element according to claim 11, wherein the first solid electrolyte layer and the second solid electrolyte layer are stacked on each other and co-fired.

13. A gas sensor comprising the sensor element according to any one of claims 1 to 3.

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