Carbon dioxide sensor

Abstract

The carbon dioxide sensor comprises a solid electrolyte layer having anion conductivity, a reference electrode disposed on one side of the solid electrolyte layer, and a detection electrode disposed on the other side of the solid electrolyte layer. The detection electrode is composed of a mixture of the following components: (a) one or more metals selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Os and Ir, (b) cation-conducting carbonate, and (c) an oxide containing at least one of Ce and Sm and Li. It is suitable that the solid electrolyte layer has oxide ion conductivity and the cation-conducting carbonate has lithium ion conductivity.

Description

Technical Field

[0001] The present invention relates to a sensor suitable for detecting and quantifying carbon dioxide gas in the atmosphere. Background Art

[0002] Oxide ion conductors are materials that have attracted attention as functional ceramics that can be used in various electrochemical elements such as solid electrolytes of batteries such as solid electrolyte fuel cells, ion batteries, and air batteries, sensors, and separation membranes. As one of the electrochemical elements using oxide ion conductors, a carbon dioxide sensor is proposed in non-patent document 1. The carbon dioxide sensor has an oxide ion conductive solid electrolyte formed by magnesium-stabilized zirconia and a lithium ion conductive carbonate auxiliary phase formed by lithium carbonate, and an ion bridge formed by Li2ZrO3 between the two. Li2ZrO3 as an ion bridge is formed by the reaction of magnesium-stabilized zirconia and lithium carbonate. The ion bridge is formed to electrochemically bond the oxide ion conductive solid electrolyte and the lithium ion conductive carbonate auxiliary phase.

[0003] Patent Document 1 also describes a carbon dioxide sensor. The sensor has a reference electrode and a detection electrode formed of lithium carbonate on a solid electrolyte formed of yttrium-stabilized zirconium oxide, and a two-ion conductive layer formed of a Li2ZrO3 crystal phase is arranged between the solid electrolyte and the detection electrode. The two-ion conductive layers are formed by a gas phase method.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2008-267845

[0007] Non-patent literature

[0008] Non-patent document 1: Sensors and Actuators B24-25 (1995) 260-265 Summary of the invention

[0009] Since the carbon dioxide sensors described in Patent Document 1 and Non-Patent Document 1 operate at a high temperature of 600° C. or higher, there is a demand for lower operating temperatures.

[0010] In addition, the carbon dioxide sensor described in Patent Document 1 has a change in electromotive force depending on the thickness of the two ion conductive layers, so it is not easy to perform precise measurements. Furthermore, since the two ion conductive layers are formed by a gas phase method, it takes a long time to form them.

[0011] Similarly, the carbon dioxide sensor described in Non-Patent Document 1 also generates a change in electromotive force depending on the thickness of the ion bridge, and since the ion bridge is formed by a reaction, it is not easy to make the thickness constant.

[0012] Therefore, an object of the present invention is to improve a carbon dioxide sensor, and more specifically, to provide a carbon dioxide sensor that can operate at a lower temperature than conventional sensors and in which electromotive force is less likely to vary between sensors.

[0013] The present invention solves the above-mentioned problems by providing a carbon dioxide sensor comprising a solid electrolyte layer having anion conductivity, a reference electrode arranged on one side of the solid electrolyte layer, and a detection electrode arranged on the other side of the solid electrolyte layer.

[0014] The aforementioned detection electrode is composed of a mixture of the following components:

[0015] One or more metals selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Os and Ir,

[0016] Cation conducting carbonates, and

[0017] An oxide containing at least one of Ce and Sm and Li. DETAILED DESCRIPTION

[0018] The present invention is described below based on its preferred embodiments. The carbon dioxide sensor of the present invention is suitable for detecting and quantifying carbon dioxide gas in the atmosphere. The carbon dioxide sensor of the present invention has a solid electrolyte layer. The solid electrolyte layer has anion conductivity. As anions, depending on the material constituting the solid electrolyte layer, for example, oxide ions and fluoride ions can be listed. There is no particular restriction on the shape of the solid electrolyte layer, and various shapes can be adopted. From the viewpoint of improving the measurement accuracy of the carbon dioxide sensor, it is preferred that the solid electrolyte layer is plate-shaped.

[0019] A reference electrode is arranged on one side of the solid electrolyte layer, and a detection electrode is arranged on the other side of the solid electrolyte layer. In other words, the detection electrode is arranged on the surface opposite to the surface on which the reference electrode is arranged. The reference electrode is an electrode in contact with an atmosphere of known carbon dioxide concentration. On the other hand, the detection electrode is an electrode in contact with the atmosphere to be measured.

[0020] There is no particular restriction on the positional relationship between the solid electrolyte layer, the reference electrode, and the detection electrode. For example, a reference electrode and a detection electrode of the same size as the solid electrolyte layer can be configured when viewed from above. Alternatively, a reference electrode and a detection electrode smaller than the solid electrolyte layer can be configured when viewed from above. In this case, the reference electrode and the detection electrode can be configured in a positional relationship in which they overlap in whole or in part when viewed from above, or in a positional relationship in which they do not overlap at all when viewed from above.

[0021] The detection electrode is preferably formed of a mixture of a plurality of specific materials. The mixture includes the following three types (a) to (c).

[0022] (a) One or more metals selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Os and Ir.

[0023] (b) Cation-conducting carbonates.

[0024] (c) An oxide containing Li and at least one of Ce and Sm (hereinafter also referred to as a “lithium-containing oxide”).

[0025] These are described below respectively.

[0026] The metal of (a) is mainly used to impart electronic conductivity to the detection electrode. In addition, the metal of (a) can also be used to impart a catalytic effect for electrochemical reaction to the detection electrode. From this viewpoint, the metal of (a) is one or more selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Os and Ir, preferably one or more selected from the group consisting of Au, Ag and Pt. In addition, metal oxides such as zinc oxide and indium oxide that exhibit electronic conductivity can also be used.

[0027] The metal or metal oxide (a) is usually used in the form of particles. In this case, the particle size is the cumulative volume particle size D at a cumulative volume of 50% by volume obtained by laser diffraction scattering particle size distribution measurement. 50 The particle size is preferably 0.01 μm to 10 μm, more preferably 0.05 μm to 5 μm, and more preferably 0.1 μm to 3 μm. The particle shape is not particularly limited, but from the viewpoint of good availability, spherical, plate-like, needle-like, and the like are generally preferred.

[0028] The amount of the metal or metal oxide of (a) is 20% by mass or more and 70% by mass or less relative to the total mass of (a), (b) and (c), which is preferably from the viewpoint of ensuring the electronic conductivity of the detection electrode and obtaining high detection performance of the target gas. From the viewpoint of making this advantage more significant, the amount of the metal of (a) is more preferably 30% by mass or more and 60% by mass or less, and more preferably 40% by mass or more and 55% by mass or less relative to the total mass of (a), (b) and (c).

[0029] The cation-conducting carbonate (b) is used to impart cation conductivity to the detection electrode. As cations, ions of alkali metals such as lithium ions and sodium ions can be cited. From this point of view, the cation-conducting carbonate is preferably an alkali metal salt of carbonic acid. For example, the cation-conducting carbonate is preferably lithium carbonate (Li2CO3).

[0030] The amount of the cation-conductive carbonate (b) is 5% by mass or more and 55% by mass or less relative to the total mass of (a), (b) and (c), which is preferred from the viewpoint that a three-phase interface can be effectively formed in the detection electrode and carbon dioxide in the target atmosphere can be accurately detected. From the viewpoint of making this advantage more significant, the amount of the cation-conductive carbonate (b) is further preferably 7% by mass or more and 50% by mass or less, and further preferably 10% by mass or more and 40% by mass or less relative to the total mass of (a), (b) and (c).

[0031] The lithium-containing oxide (c) is an oxide containing at least one of Ce and Sm and Li, and plays the following role: assisting the conduction of anions conducted in the solid electrolyte layer and cations conducted in the cation-conducting carbonate. The lithium-containing oxide (c) may be a two-ion conductor, for example, when the solid electrolyte layer has oxide ion conductivity and the cation-conducting carbonate has lithium ion conductivity, it may have both oxide ion conductivity and lithium ion conductivity.

[0032] As the material of (c), for example, Li2LnO3 (Ln represents at least one rare earth element), Li2ZrO3, Li6Zr3O7, etc. are used. As Li2LnO3, for example, Li2CeO3, Li2Ce x Sm y O3 (x and y are positive numbers, x+y=1), etc. Alternatively, a mixture of lithium oxide and an oxide containing at least one of Zr, Ce, and Sm may be used. These materials are preferably used in the form of particles.

[0033] The amount of the lithium-containing oxide (c) relative to the total mass of (a), (b) and (c) is 10% by mass or more and 60% by mass or less, which is preferred from the viewpoint that a three-phase interface can be effectively formed in the detection electrode and carbon dioxide in the target atmosphere can be accurately detected. From the viewpoint of making this advantage more significant, the amount of the lithium-containing oxide (c) relative to the total mass of (a), (b) and (c) is more preferably 20% by mass or more and 50% by mass or less, and more preferably 30% by mass or more and 40% by mass or less.

[0034] In the aforementioned mixture containing the aforementioned (a), (b) and (c), it is preferred that (a), (b) and (c) are uniformly mixed. By the mixture being in such a state, the contact area between (a), (b) and (c) is increased and the interface resistance is reduced. As a result, the carbon dioxide sensor of the present invention is easy to operate at a further low temperature. And because (a), (b) and (c) are in a mutually mixed state, the electromotive force is not easy to depend on the thickness of the detection electrode, and therefore it also has the advantage that the electromotive force between sensors is not easy to deviate.

[0035] In addition, from the same point of view, in the aforementioned mixture, (a) and (c) are preferably formed continuously. The contact structure of (a) and (c) is formed continuously, which means that the particles of (a) and the particles of (c) are in contact with each other and continuously form a path for conducting electrons and two ions. Whether the contact structure of (a) and (c) is formed continuously can be confirmed by observing the surface and / or cross-section of the aforementioned mixture using a scanning electron microscope (SEM), for example, at 100 to 10,000 times, and performing element mapping using energy dispersive X-ray spectroscopy (EDS) as needed.

[0036] In contrast, in conventional carbon dioxide sensors, such as those described in Patent Document 1 and Non-Patent Document 1, (a), (b) and (c) are in the form of independent layers, and since only the surfaces are in contact with each other, the contact area can be limited. As a result, it is not easy to lower the operating temperature.

[0037] The detection electrode is suitably formed as follows: while mixing the aforementioned (a), (b) and (c) at a predetermined mixing ratio, an organic solvent is added to form a paste, the paste is applied to the surface of the solid electrolyte layer, or to the surface of the intermediate layer described later to form a coating film, and the coating film is baked to form the above-mentioned detection electrode. The baking temperature can be preferably set to 400°C or more and 1400°C or less, more preferably 500°C or more and 1200°C or less, and more preferably 600°C or more and 1000°C or less. The baking time is preferably set to 0.1 hours or more and 20 hours or less, more preferably 0.5 hours or more and 15 hours or less, and more preferably 1 hour or more and 10 hours or less.

[0038] When manufacturing the aforementioned paste, the ratio of (a) relative to the total amount of the aforementioned (a), (b) and (c) is preferably 20 mass % or more and 70 mass % or less, further preferably 30 mass % or more and 60 mass % or less, further preferably 40 mass % or more and 55 mass % or less.

[0039] The ratio of (b) is preferably 5% by mass or more and 55% by mass or less, more preferably 7% by mass or more and 40% by mass or less, and further preferably 10% by mass or more and 30% by mass or less.

[0040] The ratio of (c) is preferably 10% by mass or more and 60% by mass or less, more preferably 15% by mass or more and 50% by mass or less, and further preferably 20% by mass or more and 40% by mass or less.

[0041] The carbon dioxide sensor of the present invention may have an intermediate layer between at least one of the reference electrode and the detection electrode and the solid electrolyte layer. The intermediate layer is used to improve the anion conductivity, such as oxide ion conductivity, between the solid electrolyte layer and the reference electrode and / or the detection electrode. In order to reduce the resistance in the carbon dioxide sensor, it is important to improve the anion conductivity of the solid electrolyte layer. However, even when the solid electrolyte layer is composed of a material with high anion conductivity, when the anion conductivity between the solid electrolyte layer and the reference electrode and / or the detection electrode is low, the improvement of the anion conductivity of the carbon dioxide sensor as a whole is also limited. The inventors of the present invention have found that by configuring an intermediate layer formed of a specific material between the solid electrolyte layer and the reference electrode and / or the detection electrode, the anion conductivity of the carbon dioxide sensor as a whole is improved. Specifically, it is found that if the intermediate layer is composed of cerium oxide (hereinafter also referred to as "La-LnDC") containing lanthanum and rare earth elements (except lanthanum and cerium), the anion conductivity, especially the oxide ion conductivity, is improved.

[0042] For La-LnDC constituting the intermediate layer, as rare earth elements doped in cerium oxide, for example, samarium, gadolinium, yttrium, erbium, ytterbium, dysprosium, etc. can be listed. These rare earth elements can be used alone or in combination of two or more. From the viewpoint of being able to further improve the anion conductivity of the entire carbon dioxide sensor, especially the oxide ion conductivity, it is particularly preferred that the intermediate layer comprises cerium oxide containing lanthanum, and samarium or gadolinium. It should be noted that the material constituting the intermediate layer disposed between the reference electrode and the solid electrolyte layer (hereinafter also referred to as the "reference electrode side intermediate layer") may be the same or different from the material constituting the intermediate layer disposed between the detection electrode and the solid electrolyte layer (hereinafter also referred to as the "detection electrode side intermediate layer"). In addition, one of the reference electrode side intermediate layer and the detection electrode side intermediate layer may contain La-LnDC and the other may be composed of other substances.

[0043] In the intermediate layer, the ratio of the rare earth element other than lanthanum doped in the cerium oxide is represented by Ln / Ce, which is the atomic ratio of the rare earth element (Ln) to cerium, and is preferably 0.05 or more and 0.8 or less, more preferably 0.1 or more and 0.7 or less, and more preferably 0.2 or more and 0.6 or less. By setting the doping degree of the rare earth element within this range, the anion conductivity, especially the oxide ion conductivity, between the solid electrolyte layer and the reference electrode and / or the detection electrode is improved.

[0044] The above-mentioned value of Ln / Ce is measured by energy dispersive X-ray spectroscopy (hereinafter also referred to as “EDS”), electron probe microanalyzer (hereinafter also referred to as “EPMA”), or the like.

[0045] In the La-LnDC constituting the middle layer, lanthanum is contained to improve the anion conductivity of the entire carbon dioxide sensor, especially the oxide ion conductivity. For this purpose, in the middle layer, the value of La / Ce, which is the atomic ratio of lanthanum to cerium, is preferably 0.08 or more. In addition, when there is too much lanthanum, the anion conductivity is reduced, so it is preferably 1.2 or less. The value of La / Ce is further preferably 0.2 or more and 1.2 or less, and further preferably 0.3 or more and 1.2 or less. The value of La / Ce is measured by EDS, EPMA, etc.

[0046] If the thickness of the intermediate layer is a certain thickness or more, the anion conductivity, especially the oxide ion conductivity, between the solid electrolyte layer and the reference electrode and / or the detection electrode can be effectively improved. Specifically, the thickness of the intermediate layer is preferably 0.1 μm or more and 1.0 μm or less, and more preferably 0.3 μm or more and 0.8 μm or less, respectively, for the intermediate layer on the reference electrode side and the intermediate layer on the detection electrode side. The thickness of the intermediate layer can be measured by cross-sectional observation using a probe profilometer or an electron microscope. The thickness of the intermediate layer on the reference electrode side may be the same as or different from the thickness of the intermediate layer on the detection electrode side.

[0047] The solid electrolyte layer in the carbon dioxide sensor of the present invention has anion conductivity as described above. When the solid electrolyte layer has oxide ion conductivity, for example, the solid electrolyte layer is preferably composed of lanthanum oxide. Examples of oxide ion conductive materials containing lanthanum oxide include composite oxides containing lanthanum and gallium, composite oxides obtained by adding strontium, magnesium, cobalt, etc. to the composite oxide, and composite oxides containing lanthanum and molybdenum. In particular, composite oxides of lanthanum and silicon are preferably used because of their high oxide ion conductivity.

[0048] Examples of the composite oxide of lanthanum and silicon include composite oxides containing lanthanum and silicon and having an apatite-type crystal structure. The composite oxide having an apatite-type crystal structure contains lanthanum as a trivalent element, silicon as a tetravalent element, and O, and the composition is La: x SiO 1.5x+12 A composite oxide having an apatite-type crystal structure represented by (X represents a number of 8 or more and 10 or less) is preferred from the viewpoint of high oxide ion conductivity. The most preferred composition of the composite oxide having an apatite-type crystal structure is La 9.33 SiO 26 The composite oxide having an apatite-type crystal structure can be produced, for example, by the method described in Japanese Patent Application Laid-Open No. 2013-51101.

[0049] Other preferred examples of the solid electrolyte layer include: 1 、M 2 By using such a compound, the oxide ion conductivity of the solid electrolyte layer can be further improved. 1 is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, Y and Ba. 2It is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Si, Ge, Zn, Sn, W and Mo. The above compound preferably has an apatite type crystal structure.

[0050] From the viewpoint of further improving the oxide ion conductivity of the solid electrolyte layer, it is particularly preferred that the solid electrolyte layer is of the formula (1): 1 9.33+x [T 6.00-y M 2 y ]O 26.0+z The composite oxide shown. In the formula, M 1 M is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, Y and Ba. T is Si or Ge or an element containing both. 2 is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Si, Ge, Zn, Sn, W and Mo. x is a number from -1.33 to 1.50. y is a number from 0.00 to 3.00. z is a number from -5.00 to 5.20. M 1 The ratio of the number of moles of A to the number of moles of T is 1.33 or more and 3.61 or less. The composite oxide preferably has an apatite-type crystal structure.

[0051] In formula (1), as M 1 The listed La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Be, Mg, Ca, Sr, Y and Ba form ions with positive charge and are lanthanoid elements or elements having the commonality of being Group 2 metals that can form an apatite-type hexagonal crystal structure. Among them, M 1 Preferably, it is a combination of one or more elements selected from the group consisting of La, Nd, Ba, Sr, Ca, Y and Ce. 1 It preferably contains at least La and Y. In the formula (1), T may be Si or Ge, or an element containing both.

[0052] As M in formula (1) 2 The element preferably includes, for example, one or more elements selected from the group consisting of B, Zn, W, Sn and Mo. Among them, B, Zn and W are particularly preferred from the viewpoint of high orientation degree and high productivity.

[0053] In formula (1), from the viewpoint of improving the orientation degree and oxide ion conductivity, x is preferably a number of -1.33 or more and 1.50 or less, preferably -1.00 or more and 1.00 or less, preferably 0.00 or more or 0.70 or less, preferably 0.45 or more or 0.65 or less. From the viewpoint of filling the T element position in the apatite type lattice and from the viewpoint of improving the oxide ion conductivity, y in formula (1) is preferably a number of 0.00 or more and 3.00 or less, more preferably 0.40 or more and less than 1.00, preferably 0.40 or more and 0.90 or less, preferably 0.80 or less, particularly preferably 0.70 or less, and particularly preferably 0.50 or more and 0.70 or less. From the viewpoint of maintaining electrical neutrality in the apatite lattice, z in formula (1) is preferably a number of -5.00 to 5.20, preferably -3.00 to 2.00, preferably -2.00 to 1.50, preferably -1.00 to 1.00.

[0054] In formula (1), from the viewpoint of maintaining the space occupancy in the apatite type lattice, M 1 The ratio of the molar number of A to the molar number of T, in other words, (9.33+x) / (6.00-y) in formula (1) is preferably 1.33 or more and 3.61 or less, more preferably 1.40 or more and 3.00 or less, and further preferably 1.50 or more and 2.50 or less.

[0055] The composite oxide represented by formula (1) can be produced, for example, by the method described in International Publication No. WO2016 / 111110.

[0056] From the viewpoint of maintaining the strength of the carbon dioxide sensor and from the viewpoint of effectively reducing the resistance between the reference electrode and the detection electrode, the thickness of the solid electrolyte layer is preferably 1 μm or more and 1000 μm or less, more preferably 10 μm or more and 500 μm or less, and more preferably 100 μm or more and 500 μm or less. The thickness of the solid electrolyte layer can be measured by cross-sectional observation using a probe profilometer or an electron microscope.

[0057] The reference electrode in the carbon dioxide sensor of the present invention can be made of a metal material, for example. As the metal material, since it is necessary to produce a catalytic activity for the desorption and adsorption reaction of oxygen, it is preferably composed of an element of the platinum group. As elements of the platinum group, platinum, ruthenium, rhodium, palladium, osmium and iridium can be listed. These elements can be used alone or in combination of two or more. In addition, as a reference electrode, a metal ceramic containing an element of the platinum group can also be used.

[0058] For the carbon dioxide sensor of the present invention, if it is placed in a gas phase containing carbon dioxide (such as the atmosphere, exhaust gas from an internal combustion engine, etc.), a reaction (refer to the following formula (A)) is carried out at the three-phase interface where the gas phase contacts the electrode corresponding to the concentration of carbon dioxide to form an equilibrium state. On the other hand, on the reference electrode side, a reaction of the following formula (B) is carried out corresponding to the reaction of formula (A). In other words, it is advantageous for the detection electrode and the reference electrode to have more areas in contact with the gas phase, so it is preferably porous. Furthermore, in the detection electrode, it is more preferred that a metal that conducts electrons continuously in the depth direction and a structure that contacts the lithium-containing oxide are continuously formed, and a path for conducting electrons and two ions is formed.

[0059] The above mechanism generates an electromotive force between the detection electrode and the reference electrode. This electromotive force changes according to the concentration of carbon dioxide in the gas phase, so carbon dioxide can be detected or its concentration can be measured by this electromotive force.

[0060] As described above, the carbon dioxide sensor of the present invention can improve the detection performance compared with the previous carbon dioxide sensor by containing a mixture of (a), (b) and (c) in the detection electrode. In particular, it is believed that by mixing (c), an ion conduction path of both lithium ions and oxide ions can be formed, and the reaction of the following formula (A) becomes easy to proceed. The electrochemical reaction in the detection electrode of the carbon dioxide sensor of the present invention is shown in the following formula (A), so if the carbon dioxide sensor works normally, the number of reaction electrons is 2. Therefore, if the number of reaction electrons in the detection electrode when the carbon dioxide sensor is working is measured, and it is 2 or a number close to 2, it can be judged that the carbon dioxide sensor is working normally. For example, the carbon dioxide sensor of the present invention preferably has a reaction electron number of 2.0±1.0 when working at 500°C, and a reaction electron number of 2.0±0.2 when working at 400°C. It should be noted that the large deviation of the reaction electron number from 2 means that some unexpected reactions other than the following formula (A) or (B) that are independent of the concentration of carbon dioxide are generated.

[0061]

[0062]

[0063] From the viewpoint of improving detection performance, the thickness of the detection electrode is preferably 5 μm to 2000 μm, more preferably 10 μm to 1000 μm, and further preferably 20 μm to 500 μm. The thickness of the detection electrode can be measured by cross-sectional observation using a probe profilometer or an electron microscope.

[0064] Example

[0065] The present invention will be described in more detail below by way of examples. However, the scope of the present invention is not limited by the above examples. Unless otherwise specified, "%" means "mass %".

[0066] [Example 1]

[0067] The carbon dioxide sensor is manufactured through the following steps (1) to (4).

[0068] (1) Fabrication of solid electrolyte layer

[0069] The powder of La2O3 and the powder of SiO2 are mixed in a molar ratio of 1:1, ethanol is added, and mixed using a ball mill. The mixture is dried, crushed using a mortar, and roasted at 1650°C for 3 hours in an atmospheric atmosphere using a platinum crucible. Ethanol is added to the roasted product, and it is crushed using a planetary ball mill to obtain a roasted powder. The roasted powder is added to a 20mmφ molding machine and pressurized from one direction for uniaxial molding. Then, cold isostatic pressing (CIP) is performed at 600MPa for 1 minute to form pellets. The pellet-like molded body is heated in the atmosphere at 1600°C for 3 hours to obtain a pellet-like sintered body. Powder X-ray diffraction measurement and chemical analysis were performed on the sintered body, and the results confirmed the structure of La2SiO5.

[0070] 800 mg of the obtained pellets and 140 mg of B2O3 powder were added to a covered sagger and heated in an electric furnace at 1550°C (furnace atmosphere temperature) for 50 hours. By heating, B2O3 vapor was generated in the sagger and reacted with the pellets to obtain the target solid electrolyte layer. The solid electrolyte layer has an apatite crystal structure. 9.33+x [Si 6.00-y B y ]O 26.0+z In the above example, x=0.50, y=1.17, z=0.16, and the molar ratio of La to Si is 2.04 (hereinafter, this compound is referred to as "LSBO"). The oxide ion conductivity at 500°C is 3.0×10 -2 S / cm. The thickness of the solid electrolyte layer was 350 μm.

[0071] (2) Formation of the intermediate layer

[0072] Sm 0.2 Ce 1.8The powder of O2 is added to a 50mmφ forming machine, pressurized from one direction for uniaxial forming, and then hot-pressed sintering is performed. The sintering conditions are set to nitrogen atmosphere, pressure 30MPa, temperature 1200℃, and 3 hours. In this way, a target for sputtering is obtained. Using this target, each surface of the solid electrolyte layer is sputtered by high-frequency sputtering to form a sputtered layer of cerium oxide doped with samarium (hereinafter also referred to as "SDC"). The sputtering conditions are RF power 30W and argon pressure 0.8Pa. After sputtering, annealing is performed at 1500℃ in the atmosphere for 1 hour to thermally diffuse the lanthanum contained in the solid electrolyte layer into the sputtered layer so that lanthanum is contained in the SDC. In this way, a reference electrode side intermediate layer and a detection electrode side intermediate layer formed by SDC containing lanthanum (hereinafter also referred to as "La-SDC") are formed respectively. The La / Ce value of the reference electrode side intermediate layer and the detection electrode side intermediate layer measured by EDS is 0.98.

[0073] (3) Formation of reference electrode

[0074] A paste containing platinum powder was applied to the surface of the intermediate layer on the reference electrode side to form a coating film, and the coating film was baked at 700° C. for 1 hour in the air to form a reference electrode composed of a porous body.

[0075] (4) Formation of detection electrodes

[0076] As the metal (a), the cation-conductive carbonate (b), and the lithium-containing oxide (c), the materials shown in the following Table 1 were used. 50 is 1.0μm. They are mixed in the ratio shown in Table 1, and ethanol is added to make a paste. The paste is applied to the surface of the intermediate layer on the detection electrode side to form a coating. The coating is baked at 700°C in carbon dioxide gas for 10 hours to form a detection electrode. The cross-section of the obtained detection electrode was observed by SEM at 2000 times and element mapping was performed using EDS, confirming that (a) and (c) are formed continuously. In addition, the thickness of the detection electrode is 20μm.

[0077] [Examples 2 to 4]

[0078] As the metal (a), the cation-conductive carbonate (b), and the lithium-containing oxide (c), the substances shown in Table 1 were used at the ratios shown in Table 1 below. A carbon dioxide sensor was manufactured in the same manner as in Example 1 except for this. The detection electrode in the obtained sensor was observed in the same manner as in Example 1, and it was confirmed that (a) and (c) were formed continuously. In addition, the thickness of the detection electrode was 20 μm.

[0079] [Examples 5 to 8]

[0080] In the solid electrolyte manufacturing process of Example 1, powders of Y2O3, La2O3 and SiO2 were mixed in a molar ratio of 0.2:0.8:1.0, and as (a) metal, (b) cation-conductive carbonate and (c) lithium-containing oxide, the substances shown in Table 1 were used in the ratios shown in Table 1 below. Except for this, a carbon dioxide sensor was manufactured in the same manner as in Example 1. The solid electrolyte layer had an apatite-type crystal structure and a composition formula of La2O3. 8.0 Y 1.7 Si 5.3 B 0.7 O 26.7 , in M 1 9.33+x [T 6.00-y M 2 y ]O 26.0+z In the above example, x=0.37, y=0.70, z=0.70, and the total molar ratio of La and Y to Si is 1.83 (hereinafter, this compound is referred to as "Y-LSBO"). The oxide ion conductivity at 500°C is 0.9×10 -3 S / cm. The thickness of the solid electrolyte layer was 350 μm.

[0081] [Comparative Example 1]

[0082] In this comparative example, the detection electrode was manufactured without using the lithium-containing oxide (c). Specifically, as the metal (a) and the cation-conductive carbonate (b), the substances shown in Table 1 were used at the ratio shown in Table 1 below. A carbon dioxide sensor was manufactured in the same manner as in Example 1 except for this.

[0083] [Comparative Example 2]

[0084] In Example 5, the metal (a) and the cation-conductive carbonate (b) were used at the ratios shown in Table 1. The lithium-containing oxide (c) was not used. A carbon dioxide sensor was manufactured in the same manner as in Example 5 except for this.

[0085] [evaluate]

[0086] The carbon dioxide sensors obtained in the examples and comparative examples were operated in the atmosphere at 400° C. and 500° C., and the number of reaction electrons in the detection electrode was measured. The measurement was performed by the following method. If the carbon dioxide sensor operates normally, the number of reaction electrons n is theoretically 2.

[0087] [Determination of the number of reaction electrons]

[0088] The number of reaction electrons can be calculated from the Nernst equation of formula (C). R is the gas constant, T is the absolute temperature, F is the Faraday constant, and P(CO2) is the partial pressure of CO2.

[0089] E=E 0 +(RT / nF)lnP(CO2)(C)

[0090] [Table 1]

[0091]

[0092] As shown in Table 1, the number of reaction electrons of the carbon dioxide sensor of each embodiment shows a value close to 2 at an operating temperature of 500°C, and the number of reaction electrons shows a value even closer to 2 at an operating temperature of 400°C. In contrast, the number of reaction electrons of the carbon dioxide sensor of Comparative Example 1 is a value deviating from 2, suggesting that some unexpected reactions that are not dependent on the concentration of carbon dioxide occur. In addition, for the places recorded as "-" in Table 1, no numerical values ​​are recorded because the measurement at 400°C was not carried out.

[0093] This suggests that the carbon dioxide sensor of each example is suitable for measuring the concentration of carbon dioxide at a temperature of 600° C. or lower.

[0094] Industrial Applicability

[0095] According to the present invention, a carbon dioxide sensor is provided which can operate at a lower temperature than before and in which electromotive force is less likely to vary between sensors. In addition, a method for measuring carbon dioxide concentration appropriately at a temperature of 600° C. or less is provided.

Claims

1. A carbon dioxide sensor comprising a solid electrolyte layer having anion conductivity, a reference electrode disposed on one surface of the solid electrolyte layer, and a detection electrode disposed on the other surface of the solid electrolyte layer, The detection electrode is composed of a mixture of the following components: One or more metals selected from the group consisting of Au, Ag, Pt, Pd, Rh, Ru, Os and Ir, Cation conducting carbonates, and An oxide containing at least one of Ce and Sm and Li, in, In the mixture, a ratio of the metal is 30% by mass or more and 60% by mass or less relative to the total mass of the metal, the cation-conductive carbonate, and the oxide.

2. The carbon dioxide sensor according to claim 1, wherein: In the mixture, a ratio of the cation-conductive carbonate is 5 mass % or more and 55 mass % or less relative to the total mass of the metal, the cation-conductive carbonate, and the oxide.

3. The carbon dioxide sensor according to claim 1 or 2, wherein: In the mixture, a ratio of the oxide is 10 mass % or more and 60 mass % or less relative to the total mass of the metal, the cation-conductive carbonate, and the oxide.

4. The carbon dioxide sensor according to claim 1 or 2, wherein: In the mixture, a contact structure of the metal and the oxide is continuously formed.

5. The carbon dioxide sensor according to claim 1 or 2, wherein: An intermediate layer formed of cerium oxide containing lanthanum and a rare earth element is provided between at least one of the reference electrode and the detection electrode and the solid electrolyte layer, wherein the rare earth element does not include lanthanum and cerium.

6. The carbon dioxide sensor according to claim 1 or 2, wherein: The solid electrolyte layer has oxide ion conductivity.

7. The carbon dioxide sensor according to claim 1 or 2, wherein: The solid electrolyte layer comprises M 1 、M 2 Compounds with O form, M 1 is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, Y and Ba, M 2 It is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Si, Ge, Zn, Sn, W and Mo.

8. The carbon dioxide sensor according to claim 1 or 2, wherein: The solid electrolyte layer is represented by formula (1): M 1 9.33+x [T 6.00-y M 2 y ]O 26.0+z The composite oxide shown in the formula is formed, where M 1 is one or more elements selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu, Be, Mg, Ca, Sr, Y and Ba, T is Si or Ge or an element containing both, and M is 2 is one or more elements selected from the group consisting of Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Ga, Zr, Ta, Nb, B, Si, Ge, Zn, Sn, W and Mo, x is a number of -1.33 to 1.50, y is a number of 0.00 to 3.00, z is a number of -5.00 to 5.20, M 1 The ratio of the molar number of T to the molar number of T is 1.33 or more and 3.61 or less.

9. The carbon dioxide sensor according to claim 7, wherein: M 1 Contains at least La and Y.

10. The carbon dioxide sensor according to claim 8, wherein: M 1 Contains at least La and Y.

11. The carbon dioxide sensor according to claim 1 or 2, wherein: The solid electrolyte layer is formed of a compound having an apatite type crystal structure.

12. The carbon dioxide sensor according to claim 1 or 2, wherein: The solid electrolyte layer has a thickness of 1 μm or more and 1000 μm or less.

13. The carbon dioxide sensor according to claim 1 or 2, wherein: The detection electrode has a thickness of 5 μm or more and 2000 μm or less. 14 . A method for measuring a carbon dioxide concentration, comprising using the sensor according to claim 1 at a temperature of 600° C. or lower.

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