Gas sensor

Abstract

Provided is a gas sensor capable of accurately measuring the hydrogen gas concentration in a high-temperature gas to be measured. The gas sensor includes a sensor element and a control device. The sensor element includes: a gas-to-be-measured circulation cavity 15 having a gas introduction port 10, an oxygen discharge chamber 20, and a measurement chamber 61; an oxygen pump cell including an in-cavity oxygen pump electrode 22; and a measurement pump cell including an in-cavity measurement electrode 44. The in-cavity oxygen pump electrode 22 has catalytic activity with respect to oxygen and does not have catalytic activity for oxidizing hydrogen. A pump control unit included in the control device operates the oxygen pump cell to pump oxygen in the gas to be measured from the oxygen discharge chamber 20, and operates the measurement pump cell to pump oxygen into the measurement chamber 61 thereby oxidizing the hydrogen gas in the gas to be measured. A concentration calculation unit included in the control device calculates the hydrogen gas concentration in the gas to be measured on the basis of the value of the current flowing in the measurement pump cell.

Description

Gas Sensor

[0001] The present invention relates to a gas sensor including a sensor element using an oxygen ion conductive solid electrolyte. This application claims priority to Japanese Patent Application No. 2023-198920, filed on November 24, 2023, the contents of which are incorporated herein by reference.

[0002] Hydrogen engines are attracting attention from the perspective of carbon neutrality (CN). Hydrogen is an explosive gas, so for safety reasons, hydrogen H in the engine exhaust pipe is ２ At present, there are no exhaust gas regulations for hydrogen engine vehicles, but for fuel cell vehicles, the GTR regulations (UN standard HFCV-GTR Phase 1 adopted in 2013) set upper limits for hydrogen concentration in exhaust gas.

[0003] Existing hydrogen sensors installed in fuel cell vehicles include catalytic combustion and thermal conduction hydrogen sensors. However, the exhaust gas from hydrogen engine vehicles is so hot that catalytic combustion and thermal conduction hydrogen sensors cannot be used. Currently, no hydrogen sensors that can be used in hydrogen engine vehicles have been put into practical use.

[0004] JP 2023-021482 A JP 2023-101434 A

[0005] However, it is expected that the same upper limit on hydrogen concentration as in the case of fuel cell vehicles will be set for hydrogen engine vehicles. ２ A hydrogen sensor that monitors the concentration is required. As mentioned above, catalytic combustion and thermal conduction hydrogen sensors cannot be used due to the temperature of the gas being measured.

[0006] Gas sensors that can operate at high temperatures include gas sensors that use solid electrolytes. For example, Japanese Patent Application Laid-Open Nos. 2023-021482 and 2023-101434 disclose hydrogen sensors that use oxygen-ion conductive solid electrolytes. However, as will be described later, there are likely to be issues with measurement accuracy when hydrogen gas and oxygen gas coexist in the gas to be measured. Note that the exhaust gas from a hydrogen engine vehicle typically contains mainly water vapor, oxygen, and hydrogen.

[0007] Therefore, an object of the present invention is to provide a gas sensor that can accurately measure the hydrogen gas concentration in a high-temperature measurement gas such as the exhaust gas from a hydrogen engine vehicle, even when the measurement gas contains both hydrogen and oxygen gases.

[0008] As a result of extensive research, the present inventors have invented a gas sensor capable of measuring the concentration of hydrogen gas in a gas to be measured with high accuracy.

[0009] (1) A gas sensor for detecting hydrogen gas in a measurement gas, comprising a sensor element and a control device for controlling the sensor element, wherein the sensor element comprises: a long, plate-shaped base portion including an oxygen ion conductive solid electrolyte layer; a measurement gas flow space having a gas inlet opening on the surface of the base portion, an oxygen discharge chamber communicating with the gas inlet via a diffusion-limited passage, and a measurement chamber communicating with the oxygen discharge chamber via a diffusion-limited passage; an oxygen pump cell including an intra-space oxygen pump electrode disposed in the oxygen discharge chamber of the measurement gas flow space and an extra-space oxygen pump electrode disposed at a position different from the measurement gas flow space in the base portion and corresponding to the intra-space oxygen pump electrode; and a measurement pump cell including an intra-space measurement electrode disposed in the measurement chamber of the measurement gas flow space and an extra-space measurement electrode disposed at a position different from the measurement gas flow space in the base portion and corresponding to the intra-space measurement electrode. the oxygen pump electrode in the cavity is an electrode that has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen, the control device includes a pump control unit that controls the operation of the oxygen pump cell and the measurement pump cell, and a concentration calculation unit that calculates the concentration of hydrogen gas in the measurement gas, the pump control unit operates the oxygen pump cell to pump oxygen in the measurement gas from the oxygen discharge chamber, and operates the measurement pump cell to pump oxygen into the measurement chamber to oxidize the hydrogen gas in the measurement gas, and the concentration calculation unit calculates the hydrogen gas concentration in the measurement gas based on the value of the current flowing through the measurement pump cell.

[0010] (2) The gas sensor according to (1), wherein the intra-space oxygen pump electrode is a cermet electrode of a metal and an oxygen ion conductive solid electrolyte, and the metal includes a catalytic metal having catalytic activity for oxygen and an activity reducing metal that reduces catalytic activity for oxidizing hydrogen.

[0011] (3) The gas sensor according to (2) above, wherein the catalytic metal is platinum (Pt), the activity-reducing metal is gold (Au), and the area ratio of the portion of the surface of the metal particle coated with gold (Au) to the portion of the surface of the platinum (Pt) exposed in the cavity oxygen pump electrode is 0.3 or more and 0.7 or less.

[0012] (4) The gas sensor according to any one of (1) to (3), wherein the sensor element includes a reference electrode disposed inside the base portion so as to be in contact with the reference gas, and the pump control portion applies a voltage between the intra-void measurement electrode and the extra-void measurement electrode of the measurement pump cell based on an electromotive force generated between the intra-void measurement electrode and the reference electrode, thereby pumping oxygen into the measurement gas flow space.

[0013] (5) The gas sensor according to any one of (1) to (4) above, wherein in the sensor element, the gas inlet is formed at one end in the longitudinal direction of the base portion, and the measurement chamber is formed at a position farther from the one end in the longitudinal direction of the base portion than the oxygen discharge chamber.

[0014] (6-1) A sensor element for detecting hydrogen gas in a measurement gas, comprising: a long, plate-shaped base portion including an oxygen ion conductive solid electrolyte layer; a measurement gas flow space having a gas inlet opening on a surface of the base portion, an oxygen discharge chamber communicating with the gas inlet via a diffusion-limited passage, and a measurement chamber communicating with the oxygen discharge chamber via a diffusion-limited passage; an oxygen pump cell including an intra-space oxygen pump electrode disposed in the oxygen discharge chamber of the measurement gas flow space and an extra-space oxygen pump electrode disposed at a position different from the measurement gas flow space in the base portion and corresponding to the intra-space oxygen pump electrode; and a measurement pump cell including an intra-space measurement electrode disposed in the measurement chamber of the measurement gas flow space and an extra-space measurement electrode disposed at a position different from the measurement gas flow space in the base portion and corresponding to the intra-space measurement electrode. The sensor element, wherein the intra-void oxygen pump electrode is an electrode that has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen.

[0015] (6-2) The sensor element according to (6-1), wherein the oxygen pump electrode in the cavity is a cermet electrode of a metal and an oxygen ion conductive solid electrolyte, and the metal includes a catalytic metal having catalytic activity for oxygen and an activity reducing metal that reduces the catalytic activity for oxidizing hydrogen.

[0016] (6-3) The sensor element according to (6-2) above, wherein the catalytic metal is platinum (Pt), the activity-reducing metal is gold (Au), and the area ratio of the portion of the surface of the metal particle coated with gold (Au) to the portion of the surface of the metal particle where the platinum (Pt) is exposed is 0.3 or more and 0.7 or less.

[0017] (6-4) A sensor element according to any one of (6-1) to (6-3) above, which includes a reference electrode disposed inside the base portion so as to be in contact with a reference gas, and which pumps oxygen into the measurement gas flow space by applying a voltage between the intra-space measurement electrode and the extra-space measurement electrode of the measurement pump cell based on an electromotive force generated between the intra-space measurement electrode and the reference electrode.

[0018] (6-5) The sensor element according to any one of (6-1) to (6-4) above, wherein the gas inlet is formed at one end of the base in the longitudinal direction, and the measurement chamber is formed at a position farther from the one end of the base in the longitudinal direction than the oxygen discharge chamber.

[0019] (7) A measurement method for a gas sensor that detects hydrogen gas in a gas under measurement, wherein the gas sensor is the gas sensor described in any one of (1) to (5) above, and the measurement method includes: an oxygen discharge step of operating the oxygen pump cell with the pump control unit to pump oxygen in the gas under measurement from the oxygen discharge chamber; a hydrogen oxidation step of operating the measurement pump cell with the pump control unit to pump oxygen into the measurement chamber to oxidize the hydrogen gas in the gas under measurement; and a calculation step of calculating the hydrogen gas concentration in the gas under measurement based on the value of the current flowing through the measurement pump cell with the concentration calculation unit.

[0020] According to the present invention, it is possible to provide a gas sensor that can accurately measure the hydrogen concentration in a high-temperature measurement gas such as the exhaust gas from a hydrogen engine vehicle, even when the measurement gas contains both hydrogen and oxygen.

[0021] 1 is a schematic vertical cross-sectional view in the longitudinal direction of a sensor element 101, showing an example of the general configuration of a gas sensor 100. FIG. 2 is a block diagram showing an example of the electrical connection relationship between a control device 90 and each of the pump cells 21, 50, 41 and each of the sensor cells 80, 81, 82, 83 of the sensor element 101. FIG. 3 is a conceptual diagram showing the basic principle of hydrogen gas measurement using the gas sensor of the present invention. FIG. 4 is a conceptual diagram showing the principle of hydrogen gas measurement in the gas sensor 100. FIG. 5 is a graph showing sensitivity characteristics to hydrogen gas in Experimental Examples 1 to 7. The horizontal axis of the graph represents H in the measured gas. ２ The vertical axis indicates the concentration [%], and the vertical axis indicates the current value [μA] of the pump current Ip2.

[0022] The gas sensor of the present invention includes a sensor element and a control device that controls the sensor element.

[0023] a measurement gas flow space having a gas inlet opening on a surface of the base, an oxygen discharge chamber communicating with the gas inlet through a diffusion-controlling passage, and a measurement chamber communicating with the oxygen discharge chamber through a diffusion-controlling passage; an oxygen pump cell including an intra-void oxygen pump electrode disposed in the oxygen discharge chamber of the measurement gas flow space and an extra-void oxygen pump electrode disposed at a position different from the measurement gas flow space in the base and corresponding to the intra-void oxygen pump electrode; and a measurement pump cell including an intra-void measurement electrode disposed in the measurement chamber of the measurement gas flow space and an extra-void measurement electrode disposed at a position different from the measurement gas flow space in the base, and corresponding to the intra-void measurement electrode, wherein the intra-void oxygen pump electrode has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen.

[0024] The control device included in the gas sensor of the present invention includes a pump control unit that controls the operation of the oxygen pump cell and the measurement pump cell, and a concentration calculation unit that calculates the concentration of hydrogen gas in the measurement gas, wherein the pump control unit operates the oxygen pump cell to pump oxygen in the measurement gas out of the measurement gas flow space, and operates the measurement pump cell to pump oxygen into the measurement gas flow space to oxidize the hydrogen gas in the measurement gas, and the concentration calculation unit calculates the hydrogen gas concentration in the measurement gas based on the value of the current flowing through the measurement pump cell.

[0025] [General Configuration of Gas Sensor] The gas sensor of the present invention will be described below with reference to the drawings. Fig. 1 is a schematic vertical cross-sectional view in the longitudinal direction showing an example of the general configuration of a gas sensor 100 including a sensor element 101. In the following, with reference to Fig. 1, the upper side of Fig. 1 will be referred to as the top, the lower side as the bottom, the left side of Fig. 1 as the leading end side, and the right side as the trailing end side.

[0026] In the embodiment of FIG. 1, the gas sensor 100 detects hydrogen gas H ２ This shows an example of a current-type hydrogen sensor that detects hydrogen and measures its concentration.

[0027] The gas sensor 100 also includes a control device 90 that controls the sensor element 101. Figure 2 is a block diagram showing the electrical connection between the control device 90 and the sensor element 101.

[0028] (Sensor Element) The sensor element 101 is a long plate-like element including a base portion 102 having a structure in which a plurality of oxygen ion conductive solid electrolyte layers are stacked. The long plate-like shape is also referred to as a long plate-like shape or a strip-like shape. The base portions 102 are each made of zirconia (ZrO ２The present invention has a structure in which six layers, namely, a first substrate layer 1, a second substrate layer 2, a third substrate layer 3, a first solid electrolyte layer 4, a spacer layer 5, and a second solid electrolyte layer 6, each of which is made of an oxygen ion conductive solid electrolyte layer such as SiO 2 , are stacked in this order from bottom to top as viewed in the drawing. The solid electrolyte forming these six layers is dense and airtight. The six layers may all have the same thickness, or each layer may have a different thickness. The layers are bonded together via adhesive layers made of solid electrolyte, and the base portion 102 includes the adhesive layers. While FIG. 1 illustrates a layer structure consisting of six layers, the layer structure of the present invention is not limited to this and any number of layers and layer structure may be used.

[0029] The sensor element 101 is manufactured by, for example, laminating ceramic green sheets corresponding to each layer after performing predetermined processing and printing a circuit pattern on them, and then firing the sheets to integrate them.

[0030] The measurement gas flow space 15 has a gas inlet 10 opening on the surface of the base portion 102, an oxygen discharge chamber communicating with the gas inlet 10 via a diffusion-controlled passage, and a measurement chamber communicating with the oxygen discharge chamber via a diffusion-controlled passage. In this embodiment, the first internal space 20 functions as the oxygen discharge chamber of the present invention, and the third internal space 61 functions as the measurement chamber of the present invention.

[0031] A gas inlet 10 is formed at one longitudinal end (hereinafter referred to as the tip end) of the sensor element 101, between the lower surface of the second solid electrolyte layer 6 and the upper surface of the first solid electrolyte layer 4. The measurement gas flow space 15, i.e., the measurement gas flow portion, is formed by adjacently forming, in the longitudinal direction from the gas inlet 10, a first diffusion-controlling portion 11, a buffer space 12, a second diffusion-controlling portion 13, a first internal space 20, a third diffusion-controlling portion 30, a second internal space 40, a fourth diffusion-controlling portion 60, and a third internal space 61, which are connected in this order. In this embodiment, the first diffusion-controlling portion 11, the buffer space 12, and the second diffusion-controlling portion 13 collectively form a diffusion-controlling passage between the gas inlet 10 and the first internal space 20 of the oxygen release chamber. Additionally, the third diffusion-controlling section 30, the second internal space 40, and the fourth diffusion-controlling section 60 as a whole form a diffusion-controlling passage between the first internal space 20 of the oxygen discharge chamber and the third internal space 61 of the measurement chamber. In this embodiment, the second internal space 40 is formed on the diffusion-controlling passage between the first internal space 20 of the oxygen discharge chamber and the third internal space 61 of the measurement chamber as a space for auxiliary oxygen discharge.

[0032] The gas inlet 10, buffer space 12, first internal space 20, second internal space 40, and third internal space 61 are spaces inside the sensor element 101, which are defined by hollowing out the spacer layer 5, with an upper portion defined by the underside of the second solid electrolyte layer 6, a lower portion defined by the upper surface of the first solid electrolyte layer 4, and sides defined by the side surfaces of the spacer layer 5.

[0033] The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 are each provided as two horizontally elongated slits (with the openings extending in the direction perpendicular to the plane of the drawing in FIG. 1). The first diffusion rate-controlling section 11, the second diffusion rate-controlling section 13, and the third diffusion rate-controlling section 30 may have any shape that provides the desired diffusion resistance, and the shape is not limited to the slits.

[0034] The fourth diffusion rate-controlling portion 60 is provided as a single horizontally elongated slit (the opening has its longitudinal direction perpendicular to the plane of the drawing in FIG. 1 ) between the spacer layer 5 and the second solid electrolyte layer 6. The fourth diffusion rate-controlling portion 60 may have any shape that provides a desired diffusion resistance, and the shape is not limited to the slit.

[0035] Further, at a position farther from the tip side than the measurement gas flow space 15, a reference gas introduction space 43 is provided between the upper surface of the third substrate layer 3 and the lower surface of the spacer layer 5, at a position defined at the side by the side surface of the first solid electrolyte layer 4. The reference gas introduction space 43 has an opening at the other end (hereinafter referred to as the rear end) of the sensor element 101. The reference gas introduction space 43 has a H ２ For example, air is introduced as a reference gas when measuring the concentration.

[0036] The air introduction layer 48 is a layer made of porous alumina, and a reference gas is introduced into the air introduction layer 48 through the reference gas introduction space 43. The air introduction layer 48 is also formed so as to cover the reference electrode 42.

[0037] The reference electrode 42 is an electrode disposed inside the base portion 102 so as to be in contact with the reference gas. In this embodiment, the reference electrode 42 is an electrode formed in a manner sandwiched between the upper surface of the third substrate layer 3 and the first solid electrolyte layer 4, and as described above, the reference electrode 42 is surrounded by an air introduction layer 48 that is connected to the reference gas introduction space 43. That is, the reference electrode 42 is disposed so as to be in contact with the reference gas via the porous air introduction layer 48 and the reference gas introduction space 43. As will be described later, the reference electrode 42 can be used to measure the oxygen concentration (oxygen partial pressure) in the first internal space 20, the second internal space 40, and the third internal space 61. The reference electrode 42 is a porous cermet electrode (e.g., Pt and ZrO ２ The electrode is formed as a cermet electrode.

[0038] In the measurement gas flow space 15 , the gas inlet 10 is a portion that opens to the outside space, and the measurement gas is taken into the sensor element 101 from the outside space through the gas inlet 10 .

[0039] In this embodiment, the measurement gas is introduced into the measurement gas flow space 15 through the gas inlet 10 opening at the tip end surface of the sensor element 101, but the present invention is not limited to this. For example, the measurement gas flow space 15 does not need to have a recess for the gas inlet 10. In this case, the first diffusion rate-controlling part 11 essentially serves as the gas inlet.

[0040] Furthermore, for example, the measurement gas flow space 15 may have an opening in a side surface along the longitudinal direction of the base part 102, the opening communicating with the buffer space 12 or a position in the first internal space 20 near the buffer space 12. In this case, the measurement gas is introduced from the side surface along the longitudinal direction of the base part 102 through the opening.

[0041] Furthermore, for example, the measurement gas flow space 15 may be configured so that the measurement gas is introduced through a porous body.

[0042] The first diffusion rate-controlling part 11 is a part that applies a predetermined diffusion resistance to the measurement gas taken in through the gas inlet 10 .

[0043] The buffer space 12 is a space provided for guiding the measurement gas introduced from the first diffusion rate-controlling part 11 to the second diffusion rate-controlling part 13 .

[0044] The second diffusion rate-controlling portion 13 is a portion that provides a predetermined diffusion resistance to the measurement gas introduced from the buffer space 12 into the first internal space 20 .

[0045] As a result, it is sufficient that the amount of the measurement gas introduced into the first internal space 20 is within a predetermined range. That is, it is sufficient that a predetermined diffusion resistance is imparted to the entire area from the tip of the sensor element 101 to the second diffusion-controlling part 13. For example, it is also possible that the first diffusion-controlling part 11 directly communicates with the first internal space 20, that is, the buffer space 12 and the second diffusion-controlling part 13 do not exist.

[0046] The buffer space 12 is a space provided to mitigate the influence of pressure fluctuations on the detected value when the pressure of the gas to be measured fluctuates.

[0047] When the measurement gas is introduced from the outside of the sensor element 101 into the first internal space 20, the measurement gas is suddenly taken into the sensor element 101 from the gas inlet 10 due to pressure fluctuations of the measurement gas in the external space (exhaust pressure pulsations if the measurement gas is automobile exhaust gas), but is not introduced directly into the first internal space 20, but passes through the first diffusion rate-controlling section 11, buffer space 12, and second diffusion rate-controlling section 13, where the pressure fluctuations of the measurement gas are canceled out, before being introduced into the first internal space 20. As a result, the pressure fluctuations of the measurement gas introduced into the first internal space 20 become almost negligible.

[0048] The first internal space 20 is provided as an oxygen discharge space for discharging (pumping out) oxygen in the measurement gas introduced through the second diffusion-controlling part 13. The oxygen is discharged electrochemically by the operation of the main pump cell 21.

[0049] In this embodiment, the main pump cell 21 functions as an oxygen pump cell, the inner main pump electrode 22 functions as an oxygen pump electrode inside the cavity, and the outer pump electrode 23 functions as an oxygen pump electrode outside the cavity.

[0050] The main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 of an intra-cavity oxygen pump electrode disposed in the oxygen discharge chamber (first inner cavity 20) of the measurement gas flow cavity 15, and an outer pump electrode 23 of an extra-cavity oxygen pump electrode disposed at a position of the base 102 different from the measurement gas flow cavity 15 (on the outer surface of the base 102 in FIG. 1 ) and corresponding to the inner main pump electrode 22. The phrase "corresponding to the inner main pump electrode 22" means that the outer pump electrode 23 is provided on the inner main pump electrode 22 with a second solid electrolyte layer 6 interposed therebetween.

[0051] That is, the main pump cell 21 is an electrochemical pump cell including an inner main pump electrode 22 having a ceiling electrode portion 22a provided on almost the entire lower surface of the second solid electrolyte layer 6 facing the first internal space 20, an outer pump electrode 23 provided on the upper surface of the second solid electrolyte layer 6 in a region corresponding to the ceiling electrode portion 22a so as to be exposed to the external space, and the second solid electrolyte layer 6 sandwiched between these electrodes.

[0052] The inner main pump electrode 22 is disposed facing the first internal space 20. That is, the inner main pump electrode 22 is formed across the upper and lower solid electrolyte layers (the second solid electrolyte layer 6 and the first solid electrolyte layer 4) that define the first internal space 20 and the spacer layer 5 that provides the side walls. Specifically, a ceiling electrode portion 22a is formed on the lower surface of the second solid electrolyte layer 6 that provides the ceiling surface of the first internal space 20, and a bottom electrode portion 22b is formed on the upper surface of the first solid electrolyte layer 4 that provides the bottom surface. Side electrode portions (not shown) are formed on the side wall surfaces (inner surfaces) of the spacer layer 5 that constitute both side wall portions of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b, and are disposed in a tunnel-like structure at the positions where the side electrode portions are disposed.

[0053] The inner main pump electrode 22 and the outer pump electrode 23 are formed as porous cermet electrodes (electrodes in a form in which a metal component and a ceramic component are mixed). The inner main pump electrode 22 of the cavity oxygen pump electrode is an electrode that has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen. Details of the inner main pump electrode 22 of the cavity oxygen pump electrode will be described later. The outer pump electrode 23 is made of, for example, Pt and ZrO ２ It is formed as a cermet electrode.

[0054] In the main pump cell 21, a desired pump voltage Vp0 is applied between the inner main pump electrode 22 and the outer pump electrode 23 by a variable power supply 24, and a pump current Ip0 is passed between the inner main pump electrode 22 and the outer pump electrode 23, thereby making it possible to pump oxygen from the first internal space 20 to the external space.

[0055] In addition, in order to detect the oxygen concentration (oxygen partial pressure) in the atmosphere in the first internal space 20, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 80 for controlling the main pump, is configured by the inner main pump electrode 22, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, and the reference electrode 42.

[0056] The oxygen concentration (oxygen partial pressure) in the first internal space 20 can be determined by measuring the electromotive force (voltage V0) in the oxygen partial pressure detection sensor cell 80 for controlling the main pump. Furthermore, the pump current Ip0 is controlled by feedback controlling the pump voltage Vp0 so that the voltage V0 is constant. This makes it possible to pump oxygen out of the first internal space 20 so that the oxygen concentration in the first internal space 20 becomes a predetermined low concentration.

[0057] The third diffusion control section 30 is a section that imparts a predetermined diffusion resistance to the measurement gas from which oxygen has been pumped out by the operation of the main pump cell 21 in the first internal space 20, and guides the measurement gas to the second internal space 40.

[0058] The second internal space 40 is provided as an auxiliary space for pumping out oxygen remaining in the measurement gas introduced through the third diffusion-controlling part 30. The oxygen is discharged electrochemically by the operation of the auxiliary pump cell 50. The auxiliary second internal space 40 and the auxiliary pump cell 50 are optional components, and the second internal space 40 and the auxiliary pump cell 50 may be omitted. In this case, the measurement gas from which oxygen has been pumped out by the operation of the main pump cell 21 in the first internal space 20 is introduced directly into the third internal space 61 through the third diffusion-controlling part 30.

[0059] The auxiliary pump cell 50 is an auxiliary electrochemical pump cell that includes an auxiliary pump electrode 51 having a ceiling electrode portion 51a provided on substantially the entire lower surface of the second solid electrolyte layer 6 facing the second internal space 40, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any appropriate electrode on the outside of the sensor element 101 will suffice), and the second solid electrolyte layer 6.

[0060] The auxiliary pump electrode 51 is disposed in the second internal space 40 in a tunnel-shaped structure similar to that of the inner main pump electrode 22 provided in the first internal space 20. That is, a ceiling electrode portion 51a is formed on the second solid electrolyte layer 6 that provides the ceiling surface of the second internal space 40, and a bottom electrode portion 51b is formed on the first solid electrolyte layer 4 that provides the bottom surface of the second internal space 40. Side electrodes (not shown) connecting the ceiling electrode portion 51a and the bottom electrode portion 51b are formed on both wall surfaces of the spacer layer 5 that provide the side walls of the second internal space 40, forming a tunnel-shaped structure.

[0061] Similarly to the inner main pump electrode 22, the auxiliary pump electrode 51 is preferably an electrode that has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen.

[0062] In the auxiliary pump cell 50, by applying a desired pump voltage Vp1 between the auxiliary pump electrode 51 and the outer pump electrode 23 using a variable power supply 52, it is possible to pump oxygen from the atmosphere in the second internal space 40 into the external space.

[0063] In addition, in order to control the oxygen partial pressure in the atmosphere within the second internal space 40, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 81 for controlling the auxiliary pump, is configured by the auxiliary pump electrode 51, the reference electrode 42, the second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, and the third substrate layer 3.

[0064] The auxiliary pump cell 50 performs pumping using a variable power supply 52 whose voltage is controlled based on the electromotive force (voltage V1) detected by the auxiliary pump control oxygen partial pressure detection sensor cell 81. As a result, all or substantially all of the oxygen in the atmosphere within the second internal space 40 is pumped out.

[0065] At the same time, the pump current Ip1 is used to control the voltage V0 of the main pump control oxygen partial pressure detection sensor cell 80. Specifically, the pump current Ip1 is input as a control signal to the main pump control oxygen partial pressure detection sensor cell 80, and by controlling the voltage V0, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion rate-controlling part 30 into the second internal space 40 is controlled to be always constant.

[0066] The fourth diffusion-controlling section 60 is a section that imparts a predetermined diffusion resistance to the measurement gas from which residual oxygen has been pumped out by the operation of the auxiliary pump cell 50 in the second internal space 40, and guides the measurement gas into the third internal space 61.

[0067] The third internal space 61 is a space for diffusing hydrogen gas (H ) contained in the measurement gas introduced through the fourth diffusion-controlling section 60. ２ The measuring chamber 41 is provided as a measuring chamber for measuring the H ２ The concentration is measured.

[0068] The measuring pump cell 41 is configured to convert H in the measurement gas into H ２ The measurement pump cell 41 is an electrochemical pump cell including a measurement electrode 44 disposed in a measurement chamber (third internal space 61) of the measurement gas flow space 15, and an outer pump electrode 23 disposed in a position of the base 102 different from the measurement gas flow space 15 (on the outer surface of the base 102 in FIG. 1 ) corresponding to the measurement electrode 44. The phrase "corresponding to the measurement electrode 44" means that the outer pump electrode 23 is provided with the measurement electrode 44 interposed between the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0069] The measurement pump cell 41 is an electrochemical pump cell that includes a measurement electrode 44 provided on the upper surface of the first solid electrolyte layer 4 facing the third internal space 61, an outer pump electrode 23 (not limited to the outer pump electrode 23, but any appropriate electrode on the outside of the sensor element 101 will suffice), the second solid electrolyte layer 6, the spacer layer 5, and the first solid electrolyte layer 4.

[0070] The measuring electrode 44 is a porous cermet electrode. The measuring electrode 44 detects H 2 present in the atmosphere in the third internal space 61. ２ The measuring electrode 44 is an electrode containing a catalytically active noble metal (for example, at least one of Pt, Rh, Ir, Ru, and Pd). It is preferable that the measuring electrode 44 does not contain a noble metal (for example, Au, Ag, etc.) that reduces the catalytic activity of the catalytically active noble metal for hydrogen. In this embodiment, the measuring electrode 44 is made of Pt and ZrO ２ The porous cermet electrode was made of

[0071] In the measurement pump cell 41, oxygen is pumped into the third internal space 61 from outside the sensor element 101 to oxidize the hydrogen gas in the atmosphere inside the third internal space 61. The amount of oxygen required to oxidize the hydrogen gas inside the third internal space 61 is pumped in, and the amount of oxygen pumped in can be detected as a pump current Ip2.

[0072] In addition, in order to detect the oxygen partial pressure around the measurement electrode 44, an electrochemical sensor cell, i.e., an oxygen partial pressure detection sensor cell 82 for measuring pump control, is configured by the first solid electrolyte layer 4, the third substrate layer 3, the measurement electrode 44, and the reference electrode 42. The variable power supply 46 is controlled based on the electromotive force (voltage V2) detected by the oxygen partial pressure detection sensor cell 82 for measuring pump control. ２ is a reducing gas, so hydrogen gas H ２ When V2 is present, the voltage V2 is high.

[0073] The measurement gas reaches the measuring electrode 44 in the third internal space 61 through the fourth diffusion-controlling part 60 under the condition that substantially all of the oxygen has been pumped out and the measurement gas contains substantially no oxygen. The hydrogen gas in the measurement gas around the measuring electrode 44 is oxidized by the oxygen pumped into the area around the measuring electrode 44 by the measuring pump cell 41 (2H ２ +O ２ →2H ２0). At this time, the pump voltage Vp2 of the variable power supply 46 is controlled so that the voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 is constant, and the amount of oxygen required to oxidize the hydrogen gas is pumped in. Since the amount of oxygen required to oxidize hydrogen gas is proportional to the concentration of hydrogen gas in the measurement gas, the hydrogen gas concentration in the measurement gas is calculated using the pump current Ip2 in the measurement pump cell 41.

[0074] The second solid electrolyte layer 6, the spacer layer 5, the first solid electrolyte layer 4, the third substrate layer 3, the outer pump electrode 23, and the reference electrode 42 constitute an electrochemical sensor cell 83, and the partial pressure of oxygen in the measurement gas outside the sensor can be detected by the electromotive force (voltage Vref) obtained by this sensor cell 83. In this way, the gas sensor 100 may be provided with the function of a so-called oxygen concentration cell.

[0075] Furthermore, in order to increase the oxygen ion conductivity of the solid electrolyte, the sensor element 101 is provided with a heater section 70 that serves to adjust the temperature by heating and maintaining the temperature of the sensor element 101. The heater section 70 includes a heater electrode 71, a heater 72, a heater lead 76, a through hole 73, a heater insulating layer 74, and a pressure release hole 75.

[0076] The heater electrode 71 is an electrode formed in a manner to contact the lower surface of the first substrate layer 1. By connecting the heater electrode 71 to a heater power supply, which is an external power supply, it is possible to supply power to the heater section 70 from outside.

[0077] The heater 72 is an electrical resistor sandwiched between the second substrate layer 2 and the third substrate layer 3. The heater 72 is connected to the heater electrode 71 via a heater lead 76 that is connected to the heater 72 and extends to the rear end side of the sensor element 101 in the longitudinal direction, and via a through hole 73. The heater 72 generates heat when power is supplied from the outside through the heater electrode 71, thereby heating and keeping warm the solid electrolyte that forms the sensor element 101.

[0078] The heater 72 is embedded throughout the entire area from the first internal space 20 to the third internal space 61, and is capable of adjusting the temperature of the sensor element 101 to a temperature at which the solid electrolyte is activated. The temperature needs to be adjusted so that the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 can operate. It is not necessary to adjust these entire areas to the same temperature, and the sensor element 101 may have a temperature distribution.

[0079] In the sensor element 101 of this embodiment, the heater 72 is embedded in the base portion 102, but the present invention is not limited to this. The heater 72 may be disposed so as to heat the base portion 102. That is, the heater 72 may be disposed so as to heat the sensor element 101 to an extent that the sensor element 101 exhibits oxygen ion conductivity that enables the main pump cell 21, the auxiliary pump cell 50, and the measurement pump cell 41 to operate. For example, the heater 72 may be embedded in the base portion 102 as in this embodiment. Alternatively, the heater portion 70 may be formed as a heater substrate separate from the base portion 102 and disposed adjacent to the base portion 102.

[0080] The heater insulating layer 74 is an insulating layer made of an insulator such as alumina and formed on the upper and lower surfaces of the heater 72 and heater lead 76. The heater insulating layer 74 is formed for the purpose of obtaining electrical insulation between the second substrate layer 2 and the heater 72 and heater lead 76, and between the third substrate layer 3 and the heater 72 and heater lead 76.

[0081] The pressure release hole 75 penetrates the third substrate layer 3 and is formed so as to connect the heater insulating layer 74 to the reference gas introduction space 43. The pressure release hole 75 can mitigate an increase in internal pressure that accompanies a temperature increase within the heater insulating layer 74. Note that the pressure release hole 75 may be omitted.

[0082] Furthermore, a predetermined length of the surface of the sensor element 101 from the tip in the longitudinal direction may be covered with a porous protective layer (not shown). The porous protective layer is formed to protect the internal cavity and the region of the sensor element 101 where the electrodes are present from thermal shock due to splashing with water, etc. The porous protective layer is made of ceramics such as alumina, and may have a thickness of approximately 10 μm to 2000 μm. It is also preferable that the porous protective layer is formed to be able to withstand a force of up to approximately 50 N.

[0083] (Intra-void oxygen pump electrode) The intra-void oxygen pump electrode (in this embodiment, the inner main pump electrode 22) is an electrode that has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen. In other words, the intra-void oxygen pump electrode has catalytic activity for converting oxygen gas into oxygen ions that can move within the solid electrolyte. On the other hand, the intra-void oxygen pump electrode does not have catalytic activity for oxidizing hydrogen gas. "Not having catalytic activity for oxidizing hydrogen gas" means that catalytic activity is suppressed to an extent that hydrogen gas is not substantially oxidized within the first internal void 20, or that catalytic activity is suppressed to an extent that the hydrogen concentration can be measured in the measurement pump cell 41.

[0084] The oxygen pump electrode in the cavity (inner main pump electrode 22 in this embodiment) may be a cermet electrode made of a metal and an oxygen ion conductive solid electrolyte, and may be porous.

[0085] The metal contained in the cavity oxygen pump electrode preferably includes a metal having catalytic activity toward oxygen (hereinafter referred to as catalytic metal) and a metal that reduces catalytic activity for oxidizing hydrogen (hereinafter referred to as activity-reducing metal). Examples of catalytic metals include noble metals such as platinum (Pt), rhodium (Rh), iridium (Ir), ruthenium (Ru), and palladium (Pd). Catalytic metals such as platinum (Pt) have catalytic activity toward oxygen and catalytic activity for oxidizing hydrogen gas. It is believed that adding an activity-reducing metal that reduces catalytic activity for oxidizing hydrogen to such catalytic metals can reduce catalytic activity for oxidizing hydrogen while maintaining catalytic activity toward oxygen.

[0086] Examples of metals that reduce the catalytic activity of oxidizing hydrogen include gold (Au) and silver (Ag). These activity-reducing metals are considered to lack catalytic activity for oxidizing hydrogen. Preferably, gold (Au) can be used.

[0087] The ratio of the catalytic metal to the deactivation metal can be appropriately determined so that the intra-void oxygen pump electrode (inner main pump electrode 22) has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen. For example, in the intra-void oxygen pump electrode, the area ratio of the portion of the metal surface where the catalytic metal is exposed to the portion covered with the deactivation metal (deactivation metal / catalytic metal ratio) may be set within a predetermined range.

[0088] For example, platinum (Pt) may be used as the catalytic metal, and gold (Au) may be used as the activity-reducing metal. In this case, the area ratio (Au / Pt ratio) of the gold-coated portion of the metal particle surface to the platinum-coated portion of the metal particle surface may be 0.3 or greater. Within this range, the catalytic activity of the oxygen pump electrode (inner main pump electrode 22) for oxidizing hydrogen is effectively suppressed, and hydrogen oxidation is believed to be substantially eliminated in the oxygen pump electrode (inner main pump electrode 22). Furthermore, the Au / Pt ratio may be 0.7 or less. Within this range, the catalytic activity of the oxygen pump electrode (inner main pump electrode 22) for oxygen is not excessively reduced, and the main pump cell 21 is able to pump enough oxygen from the first internal cavity 20 to enable measurement of the hydrogen concentration in the measurement pump cell 41. More preferably, the Au / Pt ratio may be 0.5 or less. Within this range, it is believed that the oxygen pump electrode in the cavity (the inner main pump electrode 22) has higher catalytic activity for oxygen, and that the main pump cell 21 can pump out substantially all of the oxygen in the first internal cavity 20.

[0089] As described above, the Au / Pt ratio is the area ratio of the portion of the metal particle surface coated with gold (Au) (the portion where Au is present on the surface) to the portion where platinum (Pt) is exposed (the portion where Pt is present on the surface) in the cavity oxygen pump electrode (inner main pump electrode 22). It is the area ratio of the metal and solid electrolyte constituting the cermet electrode on the metal particle surface. When the area of ​​the portion where Pt is exposed on the metal particle surface is equal to the area of ​​the portion where Au is coated, the Au / Pt ratio is 1. More specifically, it is considered that the metal particle is in a state where an Au-rich Pt—Au alloy is formed near the surface of a Pt-rich Pt—Au alloy particle. In this specification, the Au / Pt ratio is calculated using the relative sensitivity factor method from the peak intensities of the detection peaks for Pt and Au obtained by XPS (X-ray photoelectron spectroscopy).

[0090] As the oxygen ion conductive solid electrolyte, for example, ZrO ２ can be used.

[0091] The ratio of the metal to the solid electrolyte in the inner main pump electrode 22 can be determined as appropriate by those skilled in the art, but may be, for example, about 20% to 50% by volume of the solid electrolyte relative to the total volume of the metal and the solid electrolyte. The porosity of the inner main pump electrode 22 can be determined as appropriate by those skilled in the art, but may be, for example, about 10% to 30%, or about 15% to 25%, etc. The thickness of the inner main pump electrode 22 can be determined as appropriate by those skilled in the art, but may be, for example, 5 μm or more. It may also be about 35 μm or less, for example, about 25 μm to 35 μm.

[0092] When the auxiliary pump cell 50 is provided as in this embodiment, the auxiliary pump electrode 51 may also be a cermet electrode as described above, similar to the inner main pump electrode 22 .

[0093] (Controller) The gas sensor 100 of this embodiment includes the sensor element 101 described above and a controller 90 that controls the sensor element 101. In the gas sensor 100, the electrodes 22, 23, 51, 44, and 42 of the sensor element 101 are electrically connected to the controller 90 via lead wires (not shown). FIG. 2 is a block diagram showing the electrical connections between the controller 90 and the pump cells 21, 50, and 41 and the sensor cells 80, 81, 82, and 83 of the sensor element 101. The controller 90 includes the variable power supplies 24, 52, and 46 described above and a controller 91. The controller 91 includes a pump controller 92 and a concentration calculator 93.

[0094] The control unit 91 is realized by a general-purpose or dedicated computer, and the functions of the pump control unit 92 and the concentration calculation unit 93 are realized by a CPU, memory, etc. installed in the computer. Note that when the gas sensor 100 is attached to an exhaust path from a hydrogen engine and measures hydrogen gas contained in the exhaust gas, some or all of the functions of the control device 90 (particularly the control unit 91) may be realized by an ECU (Electronic Control Unit) installed in the automobile.

[0095] The control unit 91 is configured to acquire electromotive forces (voltages V0, V1, V2, Vref) in the sensor cells 80, 81, 82, 83 of the sensor element 101 and pump currents (Ip0, Ip1, Ip2) in the pump cells 21, 50, 41. The control unit 91 is also configured to output control signals to the variable power supplies 24, 52, 46.

[0096] The pump control unit 92 is configured to control the operation of the oxygen pump cell (in this embodiment, the main pump cell 21) and the measurement pump cell 41 so as to measure the concentration of hydrogen gas in the measurement gas. In this embodiment, the pump control unit 92 is also configured to control the operation of the auxiliary pump cell 50.

[0097] The pump control unit 92 operates the oxygen pump cell (main pump cell 21) to pump oxygen in the measurement gas from the oxygen discharge chamber (first internal space 20), operates the auxiliary pump cell 50 if present as in this embodiment to pump oxygen in the measurement gas from the second internal space 40, and operates the measurement pump cell 41 to pump oxygen into the measurement chamber (third internal space 61) to oxidize hydrogen gas in the measurement gas. Specifically, in this embodiment, the control is performed as follows.

[0098] The pump control unit 92 determines whether the voltage V0 in the oxygen partial pressure detection sensor cell 80 for controlling the main pump is a constant value (set value V0 ＳＥＴ The pump voltage Vp0 of the variable power supply 24 in the main pump cell 21 is feedback-controlled so that the set value V0 ＳＥＴ For example, the voltage V0 may be set so that the oxygen partial pressure in the atmosphere in the first internal space 20 is low enough to pump out almost all of the oxygen gas in the first internal space 20. Since the voltage V0 indicates the oxygen partial pressure in the vicinity of the inner main pump electrode 22, maintaining the voltage V0 constant means maintaining the oxygen partial pressure in the vicinity of the inner main pump electrode 22 constant. As a result, the pump current Ip0 in the main pump cell 21 changes depending on the oxygen concentration in the measurement gas.

[0099] The pump control unit 92 determines whether the voltage V1 in the auxiliary pump control oxygen partial pressure detection sensor cell 81 is a constant value (set value V1 ＳＥＴ The pump voltage Vp1 of the variable power supply 52 in the auxiliary pump cell 50 is feedback-controlled so that the set value Vp1 is equal to the set value Vp1. ＳＥＴ The set value V1 may be set, for example, to a lower oxygen partial pressure in the atmosphere in the second internal space 40 so that the residual oxygen gas in the second internal space 40 is almost entirely pumped out. ＳＥＴ is usually set to V0 ＳＥＴIt is preferable to set the voltage V1 to a value larger than the value V2 (a value corresponding to a lower oxygen partial pressure). Since the voltage V1 indicates the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51, maintaining the voltage V1 constant means maintaining the oxygen partial pressure in the vicinity of the auxiliary pump electrode 51 constant. This allows the oxygen partial pressure in the atmosphere within the second internal space 40 to be controlled to such a low partial pressure that it can be considered that substantially no oxygen is present within the second internal space 40.

[0100] At the same time, the pump current Ip1 in the auxiliary pump cell 50 is set to a constant value (set value Ip1 ＳＥＴ The set value V0 of the voltage V0 is set based on the pump current Ip1 so that the set value V0 of the voltage V0 is ＳＥＴ Specifically, the pump current Ip1 is input as a control signal to the oxygen partial pressure detection sensor cell 80 for controlling the main pump, and the voltage V0 of the sensor cell 80 is set to a set value V0 based on the pump current Ip1. ＳＥＴ By controlling the pump current Ip1 to the value Ip0, the gradient of the oxygen partial pressure in the measurement gas introduced from the third diffusion-controlling part 30 into the second internal space 40 is controlled to be always constant. Alternatively, instead of performing the feedback control of the voltage V0 based on the pump current Ip1, feedback control of the pump voltage Vp0 based on the voltage V0 and feedback control of the pump voltage Vp1 based on the voltage V1 may be performed.

[0101] In this embodiment, the main pump cell 21 and the auxiliary pump cell 50 are feedback-controlled as described above, but the control of the oxygen pump cell in the present invention is not limited to this. For example, as a modification of the control of the main pump cell 21 and the auxiliary pump cell 50 in this embodiment, the feedback control shown in FIG. 1 may be omitted, and predetermined pump voltages Vp0 and Vp1 may be applied independently to the main pump cell 21 and the auxiliary pump cell 50, respectively.

[0102] When a pump voltage Vp0 is applied to the main pump cell 21 to pump oxygen from the measurement gas flow space 15, the pump current Ip0 increases as the pump voltage Vp0 increases while the pump voltage Vp0 is low. As the pump voltage Vp0 subsequently increases, the pump current Ip0 no longer increases with increasing pump voltage Vp0 and reaches saturation. This saturated current value is referred to as the limiting current value. The region where the pump current Ip0 becomes the limiting current value with respect to the pump voltage Vp0 is referred to as the limiting current region. In the limiting current region, the amount of oxygen in the measurement gas introduced into the first internal space 20 via the diffusion-limited passage (the first diffusion-limited portion 11, the buffer space 12, and the second diffusion-limited portion 13) is roughly balanced with the amount of oxygen pumped by the main pump cell 21. Therefore, applying a pump voltage Vp0 to the main pump cell 21 that causes the pump current Ip0 to become the limiting current value is considered to enable pumping of almost all of the oxygen in the first internal space 20. The same applies to the auxiliary pump cell 50 .

[0103] Therefore, the pump control unit 92 may apply a predetermined pump voltage Vp0 to the main pump cell 21 to pump out oxygen from the first internal space 20. The voltage value of the predetermined pump voltage Vp0 may be set to a value that causes the pump current Ip0 to reach the limiting current value. As a result, almost all of the oxygen from the first internal space 20 is pumped out. Then, the pump control unit 92 may apply a predetermined pump voltage Vp1 to the auxiliary pump cell 50 to pump out oxygen from the second internal space 40. The voltage value of the predetermined pump voltage Vp1 may be set to a value that causes the pump current Ip1 to reach the limiting current value. As a result, substantially all of the remaining oxygen in the second internal space 40 is pumped out. As a result, the measurement gas in the second internal space 40, i.e., the measurement gas to be introduced into the third internal space 61, is controlled to be substantially oxygen-free.

[0104] Next, with regard to the measurement pump cell 41, the pump control unit 92 may be configured to apply a voltage (pump voltage Vp2) between the measurement electrode inside the cavity (measurement electrode 44) of the measurement pump cell 41 and the measurement electrode outside the cavity (outer pump electrode 23) based on the electromotive force (voltage V2) generated between the measurement electrode inside the cavity (measurement electrode 44) and the reference electrode 42, thereby pumping oxygen into the measurement chamber (third internal cavity 61).

[0105] In this embodiment, the pump control unit 92 controls the voltage V2 detected by the measurement pump control oxygen partial pressure detection sensor cell 82 to be a constant value (set value V2 ＳＥＴ The pump voltage Vp2 of the variable power supply 46 in the measuring pump cell 41 is feedback-controlled so that the hydrogen gas in the measurement gas is oxidized by the oxygen pumped by the measuring pump cell 41 (referred to as 2H ２ +O ２ →2H ２ At this time, the voltage V2 is set to V2 ＳＥＴ Oxygen is pumped in so that the set value V2 ＳＥＴ is the hydrogen gas H ２ In this way, the set value V2 ＳＥＴ By setting ２ is substantially completely oxidized, and H ２ The amount of oxygen required for oxidation of H is detected as the pump current Ip2. ２ The amount of oxygen required to oxidize is H in the measurement gas. ２ In order to correspond to the concentration, the current value of the pump current Ip2 is set to ２ The current value corresponds to the concentration.

[0106] The concentration calculation unit 93 is configured to calculate the hydrogen gas concentration in the measurement gas based on the value of the current (pump current Ip2) flowing through the measurement pump cell 41.

[0107] The concentration calculation unit 93 acquires the pump current Ip2 in the measurement pump cell 41, and calculates the H concentration in the measurement gas based on the pump current Ip2 stored in advance. ２Concentration conversion parameter (current - H ２ Based on the concentration conversion parameter, H in the measurement gas is ２ The concentration is calculated and output as a measurement value of the gas sensor 100. Current -H ２ The concentration conversion parameters are the pump current Ip2 and the H in the measurement gas. ２ The data representing the relationship (linear relationship) between the current and the concentration is stored in advance in the memory of the control unit 91, which functions as the concentration calculation unit 93. ２ The concentration conversion parameter can be determined appropriately by a person skilled in the art through experiments or the like in advance for the gas sensor 100. Current -H ２ The concentration conversion parameter may be, for example, a coefficient of an approximate expression (such as a linear function) obtained by experiment, or may be a value obtained by calculating the pump current Ip2 and the H ２ It may be a map showing the correspondence between the current and the concentration. ２ The concentration conversion parameter may be a parameter specific to each gas sensor 100, or may be a parameter commonly used by a plurality of gas sensors.

[0108] [Measurement of Hydrogen Gas Concentration in Measurement Gas] Next, the hydrogen gas concentration (H ２ This section explains how to measure the concentration of benzophenone.

[0109] The measurement method using the gas sensor of the present invention includes an oxygen discharge step in which the pump control unit operates the oxygen pump cell to pump oxygen in the measurement gas from the oxygen discharge chamber, a hydrogen oxidation step in which the pump control unit operates the measurement pump cell to pump oxygen into the measurement chamber and oxidize hydrogen gas in the measurement gas, and a calculation step in which the concentration calculation unit calculates the hydrogen gas concentration in the measurement gas based on the value of the current flowing through the measurement pump cell.

[0110] First, the basic principle of the above-mentioned measurement method will be explained. FIG. 3 is a conceptual diagram showing the basic principle of hydrogen gas measurement by the gas sensor of the present invention. In FIG. 3, only the components necessary for explaining the basic principle are conceptually shown. The same reference numerals as the corresponding components of the gas sensor 100 are used for each component. In FIG. 3, the gas species in the measurement gas are water vapor H ２ O, oxygen O ２ and hydrogen H ２ Only the above is shown.

[0111] The composition of exhaust gas from a hydrogen engine is different from that of exhaust gas from a gasoline or diesel engine. Generally, the exhaust gas from a gasoline or diesel engine contains H ２ O, CO ２ , and O ２ is contained in the order of 10%, and H ２ On the other hand, exhaust gas from a hydrogen engine contains H ２ O and O ２ is contained in the order of 10%, and H ２ It is contained in the order of 1%. ２ And so-called thermal NOx is in the order of ppm, in extremely small amounts (H ２ It is considered that the amount of data included in the analysis is so small that it does not affect the measurement of the data.

[0112] The measurement gas is introduced through the gas inlet 10 and reaches the oxygen discharge chamber (first internal space 20) through a diffusion-controlled passage. Then, the oxygen in the measurement gas introduced into the first internal space 20 is pumped out of the first internal space 20 by the action of the oxygen pump cell (main pump cell 21) (oxygen discharge step). At this time, it is preferable to pump out substantially all of the oxygen in the measurement gas in the first internal space 20. This preferably results in a state in which substantially no oxygen is present in the measurement gas in the first internal space 20, i.e., the measurement gas to be introduced into the third internal space 61. Note that the intra-space oxygen pump electrode (inner main pump electrode 22) disposed in the first internal space 20 has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen, and therefore does not oxidize hydrogen gas H in the measurement gas.２ is not oxidized at the inner main pump electrode 22 and becomes hydrogen gas H ２ It remains as it is.

[0113] The measurement gas from which oxygen has been pumped out in this manner is introduced into the measurement chamber (third internal space 61) via a diffusion-limited passage. Hydrogen gas in the measurement gas introduced into the third internal space 61 is oxidized at the internal measurement electrode (measurement electrode 44). Since substantially no oxygen is present in the measurement gas introduced into the third internal space 61, the hydrogen gas is oxidized by the oxygen pumped into the third internal space 61 by the measurement pump cell 41 (hydrogen oxidation step). The amount of oxygen required to oxidize the hydrogen gas in the measurement gas corresponds to the hydrogen gas concentration in the measurement gas. Therefore, by pumping oxygen so as to oxidize substantially all of the hydrogen gas in the third internal space 61, the hydrogen gas concentration in the measurement gas can be calculated based on the pump current Ip2 flowing through the measurement pump cell 41 in proportion to the amount of oxygen pumped in (calculation step). Note that water vapor H ２ O is not decomposed at the inner main pump electrode 22 and the measurement electrode 44, and H ２ It is thought to remain as O.

[0114] Consider a case where no oxygen pump cell is present and hydrogen gas concentration is measured using only a measurement pump cell. For example, Japanese Patent Application Laid-Open Nos. 2023-021482 and 2023-101434 disclose hydrogen sensors that use an oxygen ion-conductive solid electrolyte and that do not include a pump cell that corresponds to the oxygen pump cell of the present invention.

[0115] If there is no oxygen pump cell, the measurement gas is O ２ and H ２ The gas to be measured is introduced into the measurement chamber with H contained therein. ２ is oxidized at the measuring electrode in the cavity. At this time, the gas to be measured in the measuring chamber contains O ２ Since it also contains ２ is the O in the gas to be measured ２ If this happens, the H２ O for oxidation of ２ Since H is not pumped into the measurement pump cell, the H in the measurement gas is ２ Therefore, the pump current flowing through the measuring pump cell is not proportional to the concentration. ２ It is thought that the concentration cannot be measured correctly.

[0116] On the other hand, in the gas sensor of the present invention, as described above, substantially all of the oxygen in the measurement gas is pumped out by the oxygen pump cell (main pump cell 21), and then the hydrogen gas in the measurement gas is oxidized by the oxygen pumped in by the measurement pump cell 41. ２ and H ２ Even when the hydrogen gas coexists with the hydrogen gas, the concentration of the hydrogen gas in the measurement gas can be measured with high accuracy.

[0117] Next, hydrogen concentration measurement using the gas sensor 100 according to one embodiment of the present invention will be described. Fig. 4 is a conceptual diagram showing the principle of hydrogen gas measurement in the gas sensor 100. In Fig. 4, as in Fig. 3, water vapor H is used as a gas species in the measurement gas. ２ O, oxygen O ２ and hydrogen H ２ 4, the structure of the sensor element 101 is shown in a simplified manner, and therefore the following description will be given with reference to FIGS. 1 and 2 as well.

[0118] The measurement gas is introduced through the gas inlet 10, passes through the first diffusion-controlling section 11, the buffer space 12, and the second diffusion-controlling section 13 in this order, and reaches the first internal space 20. The pump control section 92 then operates the main pump cell 21 as described above, thereby pumping out the oxygen in the measurement gas introduced into the first internal space 20 from the first internal space 20. At this time, most of the oxygen in the measurement gas in the first internal space 20 is pumped out. Thereafter, the measurement gas in a state in which most of the oxygen has been pumped out (a state in which a trace amount of oxygen remains) passes through the third diffusion-controlling section 30 and is introduced into the second internal space 40. The pump control section 92 then operates the auxiliary pump cell 50 as described above, thereby pumping out the oxygen remaining in the measurement gas introduced into the second internal space 40 from the second internal space 40. This makes it desirable that oxygen is substantially absent from the measurement gas in the second internal space 40, i.e., the measurement gas to be introduced into the third internal space 61. The inner main pump electrode 22 disposed in the first internal space 20 and the auxiliary pump electrode 51 disposed in the second internal space 40 have catalytic activity for oxygen but do not have catalytic activity for oxidizing hydrogen, and therefore, hydrogen gas H ２ is not oxidized at either the inner main pump electrode 22 or the auxiliary pump electrode 51, and hydrogen gas H ２ It remains as it is.

[0119] The measurement gas from which oxygen has been pumped in this manner passes through the fourth diffusion-controlling section 60, is introduced into the third internal space 61, and reaches the measurement electrode 44. The pump control section 92 operates the measurement pump cell 41 as described above, causing the hydrogen gas in the measurement gas introduced into the third internal space 61 to be oxidized at the measurement electrode 44. Because the measurement gas introduced into the third internal space 61 is substantially free of oxygen, the hydrogen gas is oxidized by the oxygen pumped into the third internal space 61 by the measurement pump cell 41. The amount of oxygen required to oxidize the hydrogen gas in the measurement gas corresponds to the hydrogen gas concentration in the measurement gas. Therefore, by pumping oxygen so as to oxidize substantially all of the hydrogen gas in the third internal space 61, the hydrogen gas concentration in the measurement gas can be calculated based on the pump current Ip2 flowing through the measurement pump cell 41 in proportion to the amount of oxygen pumped in. For example, when the set value V2 ＳＥＴ At the measuring electrode 44, hydrogen gas H ２ is set to a value that substantially completely oxidizes the water vapor H ２ O is not decomposed at the inner main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44, and H ２ It is thought to remain as O.

[0120] Although the gas sensor 100 has been described above as an example of an embodiment of the present invention, the present invention is not limited to this embodiment. The present invention may include gas sensors of various forms as long as they achieve the object of the present invention of accurately measuring the hydrogen gas concentration in a gas to be measured.

[0121] In the gas sensor 100 of the above-described embodiment, the auxiliary second internal space 40 is provided between the oxygen discharge chamber (first internal space 20) and the measurement chamber (third internal space 61), but the auxiliary second internal space 40 may be omitted. As shown in the conceptual diagram of Fig. 3 showing the basic principle, it is sufficient that at least the oxygen discharge chamber (first internal space 20) and the measurement chamber (third internal space 61) are formed, and that an oxygen pump cell (main pump cell 21) and a measurement pump cell 41 are provided.

[0122] In the gas sensor 100 according to the above-described embodiment, the inner main pump electrode 22 includes a ceiling electrode portion 22a formed on the ceiling surface of the first internal space 20, a bottom electrode portion 22b formed on the bottom surface of the first internal space 20, and a side electrode portion formed on the side surface of the first internal space 20 so as to connect the ceiling electrode portion 22a and the bottom electrode portion 22b. However, this is not limitative. The inner main pump electrode 22 may be formed, for example, only on the ceiling surface of the first internal space 20. Alternatively, it may be formed only on the bottom surface of the first internal space 20. Furthermore, for example, when the inner main pump electrode 22 includes a ceiling electrode portion 22a and a bottom electrode portion 22b, the ceiling electrode portion 22a and the bottom electrode portion 22b may have the same size or different sizes. The same applies to the auxiliary pump electrode 51, if any. In the gas sensor 100 of the above embodiment, the measurement electrode 44 is formed on the bottom surface of the third internal space 61, but this is not limiting. The measurement electrode 44 may be formed on the ceiling surface of the third internal space 61, or may have a tunnel shape like the inner main pump electrode 22 of the above embodiment.

[0123] In the gas sensor 100 of the above embodiment, the gas inlet 10 and the first internal space 20, the first internal space 20 and the second internal space 40, and the second internal space 40 and the third internal space 61 are all connected via slit-shaped spaces that serve as diffusion-controlling passages, but this is not limiting. At least a part of the diffusion-controlling passage may be formed of a porous body.

[0124] In the gas sensor 100 of the above-described embodiment, the sensor element 101 has three internal cavities, namely, the first internal cavity 20, the second internal cavity 40, and the third internal cavity 61, and the inner main pump electrode 22, the auxiliary pump electrode 51, and the measurement electrode 44 are respectively disposed in each internal cavity, as shown in FIG. 1 . However, the present invention is not limited to this. For example, the sensor element 101 may have two internal cavities, namely, the first internal cavity 20 and the second internal cavity 40, and the inner main pump electrode 22 is disposed in the first internal cavity 20, and the auxiliary pump electrode 51 and the measurement electrode 44 are disposed in the second internal cavity 40. Furthermore, for example, a porous protective layer may be formed to cover the measurement electrode 44 as a diffusion-controlling passage between the auxiliary pump electrode 51 and the measurement electrode 44. In this case, the portion of the porous protective layer that is filled with the measurement electrode 44 inside the porous protective layer corresponds to the measurement chamber.

[0125] Alternatively, for example, one internal space may be provided, and the inner main pump electrode 22 and the measuring electrode 44 may be disposed in the one internal space. Then, for example, a porous protective layer may be formed to cover the measuring electrode 44 as a diffusion-controlling passage between the inner main pump electrode 22 and the measuring electrode 44. In this case, the portion of the porous protective layer inside the measuring electrode 44 that is filled with the measuring electrode 44 corresponds to the measuring chamber.

[0126] In the gas sensor 100 of the above-described embodiment, the outer pump electrode 23 functions as three electrodes: the outside-cavity main pump electrode in the main pump cell 21, the outside-cavity auxiliary pump electrode in the auxiliary pump cell 50, and the outside-cavity measurement electrode in the measurement pump cell 41. However, this is not limiting. For example, the outside-cavity main pump electrode, the outside-cavity auxiliary pump electrode, and the outside-cavity measurement electrode may each be formed as separate electrodes. For example, one or more of the outside-cavity main pump electrode, the outside-cavity auxiliary pump electrode, and the outside-cavity measurement electrode may be provided separately from the outer pump electrode 23 on the outer surface of the base portion 102 so as to be in contact with the measurement gas. Alternatively, the reference electrode 42 may also function as one or more of the outside-cavity main pump electrode, the outside-cavity auxiliary pump electrode, and the outside-cavity measurement electrode.

[0127] The principle of measuring the hydrogen gas concentration in the present invention can also be applied to other reducing gases (CO, etc.).

[0128] [Method for Manufacturing Gas Sensor] Next, an example of a method for manufacturing the above-described gas sensor will be described. ２ The sensor element 101 can be fabricated by laminating a plurality of unfired sheet-shaped molded articles (so-called green sheets) containing an oxygen ion conductive solid electrolyte such as ethylene glycol stearate (EPO) as a ceramic component, laminating the sheets, cutting the laminate, and firing the laminate. The fabricated sensor element 101 can then be assembled into the gas sensor 100.

[0129] In the following, an example of fabricating the sensor element 101 made up of six layers as shown in FIG. 1 will be described.

[0130] First, zirconia (ZrO ２ Six green sheets containing an oxygen ion conductive solid electrolyte such as SiO 2 as a ceramic component are prepared. A known forming method can be used to fabricate the green sheets. All six green sheets may be the same thickness, or the thickness may vary depending on the layer being formed. Holes and other features used for positioning during printing and lamination are formed in each of the six green sheets in advance using a known method, such as punching with a punching device, to form blank sheets. In the blank sheet used for the spacer layer 5, through-holes such as internal voids are also formed in a similar manner. Necessary through-holes are also formed in the other layers in advance.

[0131] Various patterns required for each layer are printed and dried on blank sheets used for the six layers: the first substrate layer 1, the second substrate layer 2, the third substrate layer 3, the first solid electrolyte layer 4, the spacer layer 5, and the second solid electrolyte layer 6. A known screen printing technique can be used to print the patterns, and known drying means can also be used for the drying process.

[0132] When forming the inner main pump electrode 22, first, a conductive paste for forming the inner main pump electrode is prepared. In this embodiment, the inner main pump electrode 22 contains Pt and Au as metal components and ZrO as a solid electrolyte component. ２The conductive paste for forming the inner main pump electrode may use an Au ion-containing liquid as a starting material for Au. The conductive paste may contain an Au ion-containing liquid, a Pt powder, and ZrO. ２ The powder is prepared by mixing the powder with a binder. Any binder can be used as long as it can disperse the metal components and solid electrolyte components to a printable extent and is burned away by the above-mentioned firing, and can be appropriately selected by those skilled in the art.

[0133] Here, the Au ion-containing liquid is a liquid in which a salt or an organometallic complex containing Au ions is dissolved in a solvent. Examples of the salt containing Au ions include tetrachloroauric (III) acid (HAuCl ４ ), sodium gold (III) chloride (NaAuCl ４ ), potassium dicyanoaurate (I) (KAu(CN) ２ As an organometallic complex containing Au ions, diethylenediamine gold (III) chloride ([Au(en) ２ ]Cl ３ ), dichloro(1,10-phenanthroline)gold(III) chloride ([Au(phen)Cl ２ ]Cl), dimethyl(trifluoroacetylacetonato)gold, or dimethyl(hexafluoroacetylacetonato)gold can be used. Considering that impurities such as Na and K do not remain in the electrode, that it is easy to handle, and that it is easy to dissolve in a solvent, for example, tetrachloroauric (III) acid or diethylenediamine gold (III) chloride ([Au(en) ２ ]Cl ３ As the solvent, alcohols such as methanol, ethanol, and propanol, as well as acetone, acetonitrile, and formamide can be used.

[0134] Au ion-containing liquid, Pt powder, and ZrO ２The powder and the binder can be mixed by a known method such as dropping. In the obtained conductive paste, Au exists in the state of ions (or complex ions). In the inner main pump electrode 22 of the sensor element 101 obtained through the firing process described below, Au exists mainly in the state of elemental gold or alloy with platinum.

[0135] The Au / Pt ratio in the inner main pump electrode 22 of the obtained sensor element 101 can be adjusted by adjusting the weight ratio of Au (hereinafter referred to as the Au addition rate) to the weight of all metal elements (the sum of the weights of Pt and Au) in the starting materials when preparing the conductive paste. The Au addition rate in the conductive paste may be appropriately set so that the Au / Pt ratio in the inner main pump electrode 22 of the obtained sensor element 101 is 0.3 or more and 0.7 or less. For example, the Au addition rate may be approximately 2 wt. % to 10 wt. %.

[0136] The conductive paste is made of Au powder, Pt powder, and ZrO ２ Alternatively, the Pt / Au alloy powder may be mixed with a binder. ２ It may be prepared by mixing the powder with a binder.

[0137] The obtained conductive paste is printed in a desired pattern at desired positions on the surface that will become the lower surface of the second solid electrolyte layer 6 and the surface that will become the upper surface of the first solid electrolyte layer 4, and then dried to form a printed layer that will become the inner main pump electrode 22. The printed layer that will become the auxiliary pump electrode 51 may also be formed using the same conductive paste.

[0138] This process is repeated until various patterns are printed and dried on each of the six blank sheets. The six printed blank sheets are then stacked in a predetermined order while being positioned using sheet holes, etc., and then pressed together under predetermined temperature and pressure conditions to form a laminate. The pressing process is carried out by applying heat and pressure using a laminator such as a known hydraulic press. The temperature, pressure, and time for heating and pressing depend on the laminator used, but can be determined appropriately to achieve good lamination.

[0139] The obtained laminate includes a plurality of sensor elements 101. The laminate is cut into individual sensor elements 101. The cut laminate is fired at a predetermined firing temperature to obtain the sensor elements 101. The firing temperature may be any temperature at which the solid electrolyte constituting the base portion 102 of the sensor element 101 is sintered into a dense body and the electrodes and the like maintain a desired porosity. For example, firing is performed at a firing temperature of approximately 1300 to 1500°C. As described above, Au exists in the conductive paste for forming the inner main pump electrode in the state of ions (or complex ions). However, in the inner main pump electrode 22 of the obtained sensor element 101, Au exists mainly as a single element or as an alloy with Pt.

[0140] The obtained sensor element 101 is incorporated into the gas sensor 100 in such a manner that the front end of the sensor element 101 contacts the gas to be measured and the rear end of the sensor element 101 contacts the reference gas.

[0141] The following describes examples in which gas sensors were specifically fabricated and tested. Experimental Examples 3 to 6 correspond to examples of the present invention, and Experimental Examples 1, 2, and 7 correspond to comparative examples of the present invention. Note that the present invention is not limited to the following examples.

[0142] [Experimental Examples 1 to 7] Gas sensors were fabricated in accordance with the above-described method for manufacturing the gas sensor 100 in Experimental Examples 1 to 7. The Au / Pt ratios in the inner main pump electrode 22 were set to 0 (Experimental Example 1), 0.2 (Experimental Example 2), 0.3 (Experimental Example 3), 0.4 (Experimental Example 4), 0.5 (Experimental Example 5), 0.7 (Experimental Example 6), and 0.9 (Experimental Example 7), respectively. The Au / Pt ratios in the auxiliary pump electrode 51 were set to the same as the Au / Pt ratio in the inner main pump electrode 22. Except for these points, the gas sensors in Experimental Examples 1 to 7 were fabricated in the same manner.

[0143] [Evaluation of Sensitivity Characteristics] The sensitivity characteristics of each of the gas sensors fabricated in Experimental Examples 1 to 7 to hydrogen gas were evaluated.

[0144] The sensitivity characteristics were evaluated using a model gas as the gas to be measured. Each gas sensor was attached to a measurement pipe and operated. The model gas was flowed through the measurement pipe, and the pump current Ip2 of each gas sensor was measured. The model gas had a flow rate of 5 L / min and a gas temperature of room temperature (RT). Five types of model gases with different hydrogen concentrations were used as the model gas. The hydrogen concentrations were 0%, 1%, 2%, 3%, and 3.5%, respectively. The gas components other than hydrogen gas were oxygen (10%) and nitrogen (the balance). All concentrations are expressed in volumetric units (volume %).

[0145] The sensitivity characteristics were evaluated based on the relationship between the hydrogen concentration in each model gas and the pump current Ip2 according to the following criteria.

[0146] AA: As the hydrogen concentration increased, an increase in pump current Ip2 was observed, and the rate of change of pump current Ip2 was large. A: As the hydrogen concentration increased, an increase in pump current Ip2 was observed. B: No increase in pump current Ip2 was observed even when the hydrogen concentration increased.

[0147] The evaluation results of the Au / Pt ratio and sensitivity characteristics of Experimental Examples 1 to 7 are shown in Table 1. Also, Fig. 5 is a graph showing the sensitivity characteristics to hydrogen gas in Experimental Examples 1 to 7. The horizontal axis of the graph represents the H ２ The vertical axis indicates the concentration [%], and the vertical axis indicates the current value [μA] of the pump current Ip2.

[0148]

[0149] 5, in Experimental Examples 3 to 6, the pump current Ip2 increased as the hydrogen concentration in the measurement gas increased, confirming the sensitivity of the pump current Ip2 to the hydrogen concentration. Furthermore, in Experimental Examples 3 to 5, the rate of change of the pump current Ip2 was large, demonstrating particularly good sensitivity of the pump current Ip2 to the hydrogen concentration.

[0150] In Experimental Examples 1 to 7, no increase in pump current Ip2 was observed even when the hydrogen concentration in the measurement gas increased (no sensitivity). It is presumed that Experimental Examples 1 and 2 had a small Au / Pt ratio, which prevented the catalytic activity of the inner main pump electrode 22 to oxidize hydrogen from being sufficiently suppressed. Therefore, hydrogen was oxidized at the inner main pump electrode 22, preventing it from reaching the measurement electrode 44. Furthermore, it is presumed that Experimental Example 7 had a large Au / Pt ratio, which prevented the catalytic activity of the inner main pump electrode 22 to react with oxygen from being suppressed. As a result, it is presumed that the inner main pump electrode 22 was unable to pump out all the oxygen, and hydrogen was oxidized at the auxiliary pump electrode 51 and / or the measurement electrode 44 by the remaining oxygen in the measurement gas.

[0151] It was confirmed that the hydrogen gas concentration in the measurement gas could be measured without the above-mentioned phenomenon occurring when the Au / Pt ratio was 0.3 or more and 0.7 or less (Experimental Examples 3 to 6).It was also confirmed that the hydrogen gas concentration in the measurement gas could be measured with higher sensitivity when the Au / Pt ratio was 0.3 or more and 0.5 or less (Experimental Examples 3 to 5).

[0152] As described above, the present invention makes it possible to accurately measure the hydrogen gas concentration in a high-temperature measurement gas, such as the exhaust gas from a hydrogen engine vehicle. Furthermore, even when the measurement gas contains both hydrogen and oxygen, the hydrogen gas concentration in the measurement gas can be accurately measured.

[0153] 1 First substrate layer 2 Second substrate layer 3 Third substrate layer 4 First solid electrolyte layer 5 Spacer layer 6 Second solid electrolyte layer 10 Gas inlet 11 First diffusion rate-controlling section 12 Buffer space 13 Second diffusion rate-controlling section 15 Measurement gas flow space 20 First internal space 21 Main pump cell 22 Inner main pump electrode 22a Ceiling electrode section (of inner main pump electrode) 22b Bottom electrode section (of inner main pump electrode) 23 Outer pump electrode 24 Variable power supply (of main pump cell) 30 Third diffusion rate-controlling section 40 Second internal space 41 Measurement pump cell 42 Reference electrode 43 Reference gas introduction space 44 Measurement electrode 46 Variable power supply (of measurement pump cell) 48 Air introduction layer 50 Auxiliary pump cell 51 Auxiliary pump electrode 51a Ceiling electrode section (of auxiliary pump electrode) 51b Bottom electrode portion (of auxiliary pump electrode) 52 Variable power supply (of auxiliary pump cell) 60 Fourth diffusion rate-controlling portion 61 Third internal space 70 Heater portion 71 Heater electrode 72 Heater 73 Through hole 74 Heater insulator 75 Pressure release hole 76 Heater lead 80 Oxygen partial pressure detection sensor cell for controlling main pump 81 Oxygen partial pressure detection sensor cell for controlling auxiliary pump 82 Oxygen partial pressure detection sensor cell for controlling measurement pump 83 Sensor cell 90 Control device 91 Control portion 92 Pump control portion 93 Concentration calculation portion 100 Gas sensor 101 Sensor element 102 Base portion

Claims

1. A gas sensor for detecting hydrogen gas in a measurement gas, comprising a sensor element and a control device for controlling the sensor element, wherein the sensor element comprises: a long, plate-shaped base portion including an oxygen ion conductive solid electrolyte layer; a measurement gas flow space having a gas inlet opening on a surface of the base portion, an oxygen discharge chamber communicating with the gas inlet via a diffusion-controlling passage, and a measurement chamber communicating with the oxygen discharge chamber via a diffusion-controlling passage; an oxygen pump cell including an intra-space oxygen pump electrode disposed in the oxygen discharge chamber of the measurement gas flow space, and an extra-space oxygen pump electrode disposed in a position different from the measurement gas flow space of the base portion and corresponding to the intra-space oxygen pump electrode; and a measurement pump cell including an intra-space measurement electrode disposed in the measurement chamber of the measurement gas flow space, and an extra-space measurement electrode disposed in a position different from the measurement gas flow space of the base portion and corresponding to the intra-space measurement electrode. the control device includes a pump control unit that controls the operation of the oxygen pump cell and the measurement pump cell, and a concentration calculation unit that calculates the concentration of hydrogen gas in the measured gas, the pump control unit operates the oxygen pump cell to pump oxygen in the measured gas out of the oxygen discharge chamber, and operates the measurement pump cell to pump oxygen into the measurement chamber to oxidize the hydrogen gas in the measured gas, and the concentration calculation unit calculates the hydrogen gas concentration in the measured gas based on the value of the current flowing through the measurement pump cell.

2. The gas sensor according to claim 1, wherein the oxygen pump electrode in the cavity is a cermet electrode made of a metal and an oxygen ion conductive solid electrolyte, and the metal includes a catalytic metal having catalytic activity for oxygen and an activity reducing metal that reduces the catalytic activity for oxidizing hydrogen.

3. The gas sensor according to claim 2, wherein the catalytic metal is platinum (Pt), the activity reducing metal is gold (Au), and in the oxygen pump electrode within the cavity, the area ratio of the portion of the surface of the metal particle covered with the gold (Au) to the portion of the surface of the metal particle where the platinum (Pt) is exposed is 0.3 or more and 0.7 or less.

4. The gas sensor of claim 1, wherein the sensor element includes a reference electrode disposed inside the base portion so as to be in contact with a reference gas, and the pump control portion applies a voltage between the intra-space measurement electrode and the extra-space measurement electrode of the measurement pump cell based on an electromotive force generated between the intra-space measurement electrode and the reference electrode, thereby pumping oxygen into the measurement chamber.

5. The gas sensor according to claim 1, wherein in the sensor element, the gas inlet is formed at one end in the longitudinal direction of the base portion, and the measurement chamber is formed at a position farther from the one end in the longitudinal direction of the base portion than the oxygen discharge chamber is.

6. A sensor element for detecting hydrogen gas in a measured gas, comprising: a long, plate-shaped base portion including an oxygen ion conductive solid electrolyte layer; a measured gas flow space having a gas inlet opening on a surface of the base portion, an oxygen discharge chamber communicating with the gas inlet through a diffusion-controlling passage, and a measurement chamber communicating with the oxygen discharge chamber through a diffusion-controlling passage; an oxygen pump cell including an intra-space oxygen pump electrode disposed in the oxygen discharge chamber of the measured gas flow space and an extra-space oxygen pump electrode disposed at a position different from the measured gas flow space of the base portion and corresponding to the intra-space oxygen pump electrode; and a measurement pump cell including an intra-space measurement electrode disposed in the measurement chamber of the measured gas flow space and an extra-space measurement electrode disposed at a position different from the measured gas flow space of the base portion and corresponding to the intra-space measurement electrode, wherein the intra-space oxygen pump electrode is an electrode that has catalytic activity for oxygen but does not have catalytic activity for oxidizing hydrogen.

7. A measurement method for a gas sensor that detects hydrogen gas in a measured gas, wherein the gas sensor is the gas sensor described in claim 1, the measurement method comprising: an oxygen discharge step of operating the oxygen pump cell by the pump control unit to pump oxygen in the measured gas out of the oxygen discharge chamber; a hydrogen oxidation step of operating the measurement pump cell by the pump control unit to pump oxygen into the measurement chamber to oxidize the hydrogen gas in the measured gas; and a calculation step of calculating the hydrogen gas concentration in the measured gas based on the value of the current flowing through the measurement pump cell by the concentration calculation unit.

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