JP6933085B2 - Solid electrolyte, gas sensor, fuel cell - Google Patents

Description

The present invention relates to a novel solid electrolyte having a predetermined composition, a gas sensor and a fuel cell including the novel solid electrolyte.

Conventionally, a solid electrolyte having oxygen ion conductivity is known. The solid electrolyte can conduct ions by an electric field from the outside. Known examples of such solid electrolytes include zirconia, yttria-stabilized zirconia, and lanthanum gallate, which are used in gas sensors, fuel cells, and the like.

In recent years, there is a high demand for low temperature operation in gas sensors and fuel cells. Therefore, it is desired to develop a solid electrolyte that exhibits high ionic conductivity at low temperature. Therefore, as a solid electrolyte, an oxide having a melilite-type structure showing high ionic conductivity in a specific direction is drawing attention. As an oxide having a melilite-type structure, for example, LaSrGa ₃ O _{7 and the} like are known. In the melilite-type structure, ion conduction occurs due to interstitial oxygen in the crystal structure.

For example, Patent Document 1 discloses a solid electrolyte having a predetermined composition in which a Ga site having a melilite-type structure is doped with Mg having a binding energy with an oxygen atom smaller than Ga. In a solid electrolyte having such a composition, oxygen ions serving as carriers can easily move in the crystal lattice, so that the ionic conductivity is improved.

Japanese Unexamined Patent Publication No. 2017-73321

However, Mg has a larger ionic radius than Ga. Therefore, the dope of Mg increases the distortion of the melilite-type structure, and it is easy to form a crystal phase having a different structure such as a perovskite-type structure. As a result, the excellent ionic conductivity of the melilite-type structure having high ionic conductivity in a specific direction is impaired.

Further, although the doping of Mg facilitates the movement of oxygen ions in the crystal lattice, it is desired to develop a solid electrolyte in which oxygen ions can move more easily in the crystal lattice in order to further improve the low temperature operability.

The present invention has been made in view of such a problem, and an object of the present invention is to provide a solid electrolyte exhibiting high ionic conductivity at a low temperature, a gas sensor using the solid electrolyte, and a fuel cell.

One aspect of the present invention is a solid electrolyte having a composition represented by the following formula (1).
La _2-x α _x Ga _3-y β _y O _{7 + δ}・ ・ ・ (1)
However, α is an alkaline earth metal element, β is at least one selected from the group consisting of Ni, Fe, and Co, 0.35 ≦ x ≦ 0.7, 0 <y ≦ 0.3, and δ is electricity. A value that maintains accuracy.

Another aspect of the present invention is a gas sensor (2) comprising a solid electrolyte body (31) made of a solid electrolyte.

Yet another aspect of the present invention is in a fuel cell single cell (5) comprising a solid electrolyte layer (51) made of a solid electrolyte.

The solid electrolyte has a specific composition represented by the above formula (1). Such a solid electrolyte has a melilite-type crystal structure, and a part of La in the melilite-type structure is replaced by the alkaline earth metal element α. In the above formula (1), the amount of oxygen ions in the crystal lattice can be increased by adjusting the ratio of La and the alkaline earth metal element α. As a result, the solid electrolyte has improved ionic conductivity at low temperatures.

Further, in the above solid electrolyte, a part of Ga is replaced by the element β. The element β comprises at least one selected from the group consisting of Ni, Fe, and Co. The element β has a sufficiently low binding energy with an oxygen atom as compared with Ga. Therefore, in the solid electrolyte, oxygen ions, which are carriers of ion conduction, are in a state of being more easily moved in the crystal lattice. In such a solid electrolyte, ionic conduction occurs such that interstitial oxygen and lattice oxygen, which are excess oxygen, collide with each other, for example, but in the above solid electrolyte, the activation energy of ionic conduction is small, so that the ionic conduction is activated at a low temperature. Ion conductivity increases.

The ionic radius of the element β is close to the ionic radius of Ga. Therefore, in the solid electrolyte, the distortion of the crystal structure is small even if a part of Ga is replaced with the element β. Therefore, it is difficult for heterogeneous structures to be formed during production, and the solid electrolyte exhibits excellent ionic conductivity.

Further, since the gas sensor and the fuel cell single cell contain a solid electrolyte having high ionic conductivity at a low temperature, the gas sensor and the fuel cell single cell can be operated at a low temperature. That is, the gas sensor can detect the gas concentration at a low temperature, and the fuel cell single cell can generate electricity at a low temperature.

As described above, according to the above aspect, it is possible to provide a solid electrolyte exhibiting high ionic conductivity at a low temperature, a gas sensor using the solid electrolyte, and a fuel cell single cell.
The reference numerals in parentheses described in the scope of claims and the means for solving the problem indicate the correspondence with the specific means described in the embodiments described later, and limit the technical scope of the present invention. It's not a thing.

The schematic diagram of the crystal structure of _{LaSrGa 3} O ₇ in Embodiment 1. FIG. The schematic diagram of the crystal structure of _{La 1.54} Sr _0.46 Ga ₃ O _7.27 in Embodiment 1. FIG. The figure which shows the X-ray diffraction pattern of each solid electrolyte in Experimental Example 1. FIG. FIG. 5 is a cross-sectional view of a simple element in Experimental Example 1. The graph which shows the relationship between the temperature and ionic conductivity of each solid electrolyte in Experimental Example 1. FIG. 2 is a cross-sectional view of the gas sensor according to the second embodiment. FIG. 5 is a cross-sectional view of the cup-shaped sensor element according to the second embodiment. The cross-sectional view of the laminated type sensor element in Embodiment 2. The graph which shows the relationship between the voltage (V) and the output current (mA) of a gas sensor in Embodiment 2. FIG. The schematic diagram of the cross section of the fuel cell single cell in Embodiment 3.

<Embodiment 1>
The embodiment relating to the solid electrolyte will be described with reference to FIGS. 1 and 2. The solid electrolyte has a composition represented by the formula (1).
La _2-x α _x Ga _3-y β _y O _{7 + δ}・ ・ ・ (1)

In formula (1), α is an alkaline earth metal element. α may be one kind of alkaline earth metal element or two or more kinds of alkaline earth metal elements. By adjusting the La / α ratio, the amount of oxygen ions in the crystal lattice can be increased.

The alkaline earth metal element α is preferably Sr. In this case, the ionic radii and electronegativity of La and the alkaline earth metal element are close to each other. As a result, the La / α ratio (specifically, the La / Sr ratio) in the formula (1) can be easily changed, and for example, the La / α ratio can be increased. As a result, interstitial oxygen, which is excess oxygen, can be increased, and the ionic conductivity of the solid electrolyte at low temperature is further improved.

The reason why the ionic conductivity is improved by increasing the La / Sr ratio will be described below with reference to FIGS. 1 and 2. FIG. 1 is _{a schematic diagram of the crystal structure of LaSrGa 3} O ₇ , and FIG. 2 is a schematic diagram of the crystal structure of _{La 1.54} Sr _0.46 Ga ₃ O _7.27 having a higher La / Sr ratio than _{LaSrGa 3} O ₇ . be. The crystal structures of FIGS. 1 and 2 are both melilite type. In FIGS. 1 and 2, the c-axis direction and the a-axis direction are indicated by arrows, respectively. The b-axis direction is a direction perpendicular to the c-axis direction and the a-axis direction, respectively, and is a direction perpendicular to the paper surface in each drawing. The dashed circles in the GaO layer indicate Ga, and the dashed circles in the La / Sr layer indicate La or Sr. Black circles indicate O.

The following formula (I) shows the defect chemical formula of the solid solution reaction by the Kröger-Vink notation. As represented by the following formula (I), _{when La is introduced into the Sr site of LaSrGa 3} O ₇ by increasing the La / Sr ratio, interstitial oxygen, which is excess oxygen, is introduced. As a result, La _1.54 Sr _0.46 Ga ₃ O _7.27 having interstitial oxygen is formed.

As is known by comparing FIGS. 1 and 2, in the crystal structure of _{La 1.54} Sr _0.46 Ga ₃ O _7.27 , oxygen atoms are contained in the ab plane of the layered La / SrO layer as compared with _{LaSrGa 3} O _7. It is moving and the ion path is oriented. In _{the crystal structure of La 1.54} Sr _0.46 Ga ₃ O _7.27 , the active energy of ionic conduction is small, and ionic conduction is likely to occur due to interstitial oxygen. The compound represented by the formula (1) has an element β such as Ni substituted in a part of the Ga site, but has the same crystal structure as _{La 1.54} Sr _0.46 Ga ₃ O _{7.27 shown in FIG.} Have.

The La / α ratio is determined by x in the formula (1). In the formula (1), 0.35 ≦ x ≦ 0.7. When x> 0.7, the La / α ratio becomes small, and there is a possibility that the interstitial oxygen cannot be sufficiently increased. From the viewpoint of sufficiently increasing interstitial oxygen to further increase the ionic conductivity, x ≦ 0.6 is preferable, and x ≦ 0.5 is more preferable. On the other hand, when x <0.35, an impurity phase due to _{LaGaO 3 having a perovskite structure may be formed.} From the viewpoint of making it easier to avoid this, x ≧ 0.38 is preferable, and x ≧ 0.42 is more preferable.

In formula (1), the element β is a substitution element for Ga sites. The element β is at least one selected from the group consisting of Ni, Fe, and Co. These elements β have an ionic radius close to Ga. Therefore, it becomes easy to change the Ga / β ratio in the formula (1). Further, even if the Ga / β ratio is changed, the increase in the strain of the crystal structure can be suppressed. Therefore, it is difficult for heterogeneous structures to be formed during the production of the solid electrolyte, and the solid electrolyte exhibits excellent ionic conductivity.

Further, the element β has a smaller binding energy with an oxygen atom than Ga. Therefore, oxygen ions, which are carriers of ion conduction in the solid electrolyte, are in a state of being more easily moved in the crystal lattice.

The element β in the formula (1) is preferably Ni. Not only does Ni have an ionic radius close to that of Ga, but it also has a sufficiently low binding energy with an oxygen atom among the elements β. Therefore, when the element β is Ni, oxygen ions, which are carriers of ion conduction, move more easily in the solid electrolyte. As a result, the ionic conductivity of the solid electrolyte at low temperature is further improved. Further, Ni has an ionic radius relatively close to Ga among the elements β. Therefore, it is possible to further prevent the formation of heterogeneous structures in the crystal structure of the solid electrolyte. As a result, high ionic conductivity in a specific direction peculiar to the melilite type structure is sufficiently exhibited.

The Ga / β ratio is determined by y in the formula (1). y is a range that satisfies 0 <y ≦ 0.3. When y = 0, oxygen ions become difficult to move in the crystal lattice. Therefore, the ionic conductivity of the solid electrolyte at a low temperature becomes insufficient. From the viewpoint of further increasing the ionic conductivity at low temperature, y ≧ 0.05 is preferable, and y ≧ 0.1 is more preferable. On the other hand, when y> 0.3 or more, distortion is likely to occur in the crystal lattice, and crystals having a different structure other than the melilite type structure may be generated. From the viewpoint of avoiding the formation of crystals having a different structure, y ≦ 0.25 is preferable, and y ≦ 0.2 is more preferable.

Δ in the formula (1) is a value at which electrical neutrality is maintained. δ is a range that satisfies, for example, 0 ≦ δ ≦ 0.325.

The solid electrolyte is preferably formed from a single phase having a melilite-type structure having a composition represented by the formula (1). In this case, the excellent ionic conductivity in a specific direction peculiar to the melilite type structure is sufficiently exhibited. That is, the ionic conductivity at low temperature can be further enhanced. For example, by changing the types of the elements α and β in the formula (1) and adjusting the ranges of x and y, it is possible to reduce or eliminate the crystal phases having different structures.

The crystal structure of the solid electrolyte can be investigated by structural analysis by X-ray diffraction (that is, XRD), as shown in the experimental examples described later. When the X-ray diffraction pattern has a peak derived from a melilite-type structure and no peak derived from a heterologous crystal structure such as a perovskite-type structure, the solid electrolyte is formed from a single phase of the melilite-type structure. It can be determined that it is.

The method for synthesizing the solid electrolyte represented by the formula (1) is not particularly limited, and the solid electrolyte can be synthesized by a known method. Specifically, a solid phase reaction method, a hydrothermal synthesis method, a liquid phase reaction method, a gas phase reaction method, etc. are exemplified. Specific examples of the gas phase reaction method include a reactive sputtering method and a pulse laser deposition method ( That is, the PLD method) and the like are exemplified. In these synthetic methods, La source, α source, Ga source, β source and the like are used as raw materials. Specifically, as the raw material, oxides, hydroxides, salts and the like containing various elements constituting the compound represented by the formula (1) are used.

Since the solid electrolyte of this embodiment has a specific composition represented by the above formula (1), the ionic conductivity at a low temperature is improved. It is considered that this is because oxygen defects (that is, vacancies) are formed in the crystal lattice by being replaced with the alkaline earth metal element α replaced with a part of the La site in the melilite type structure. Furthermore, the β element substituted with a part of the Ga site has an ionic radius close to that of Ga, has a smaller binding energy with oxygen than Ga, and fluctuates in valence, so that oxygen ions move more easily in the crystal lattice. It is considered that this is because it is.

Since such a solid electrolyte is excellent in ionic conductivity at low temperature, it is suitable for various applications requiring low temperature operation. Applications of solid electrolytes are not limited to these, but are limited to these, and fuels such as air-fuel ratio sensors (that is, A / F sensors), various gas sensors such as oxygen sensors, and solid oxide fuel cells (that is, SOFC). Examples include batteries.

<Experimental example>
This example is an example of manufacturing a simple element using a solid electrolyte and evaluating its low temperature operability. First, as a solid electrolyte having a composition represented by the formula (1), a solid electrolyte composed of La _1.54 Sr _0.46 Ga _2.95 Ni _0.05 O _7.245 is prepared.

Specifically, first, the raw materials of _{La 2} O ₃ , SrCO ₃ , Ga ₂ O ₃ , and Ni O are converted to the stoichiometric ratio produced by the compound represented by _{La 1.54} Sr _0.46 Ga _{2.95 Ni 0.05 O 7.245.} Weighed. Then, using a ball mill, each raw material was mixed in an ethanol solvent for 3 hours and then dried.

The dried mixture was calcined at a temperature of 1200 ° C. for 12 hours and then pulverized using a mortar to obtain a calcined powder. Next, a disk-shaped green compact was prepared from the calcined powder by a hand press and a hydrostatic press (that is, CIP).

Next, the green compact was calcined at a temperature of 1400 ° C. for 12 hours. In this way, a sintered body made of a solid electrolyte was obtained. This sintered body is referred to as a solid electrolyte in this example. The solid electrolyte body has a disk shape with a diameter of _φ16 mm and a thickness of 1 mm, and is composed of _{La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O 7.245.

The X-ray diffraction pattern of the solid electrolyte constituting this solid electrolyte is shown in FIG. X-ray diffraction is called XRD. The measurement conditions of the XRD pattern are characteristic X-ray: Cu-Kα, measurement range: 20 ° ≤ 2θ ≤ 40 ° output: 3 kW. As the XRD analyzer, a SmartLab manufactured by Rigaku Co., Ltd. was used. The XRD pattern represented by Ni in FIG. 3 is that of a solid electrolyte composed of _{La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _7.245.

Next, silver paste was applied to both sides of the disk-shaped solid electrolyte. Then, heat treatment was performed at a temperature of 800 ° C., and silver was baked on both sides of the solid electrolyte. As a result, as illustrated in FIG. 4, electrodes 12 and 13 having a diameter of 6 mm were formed on both sides of the solid electrolyte body 11. As a result, as illustrated in FIG. 4, a simple element 1 was obtained. The simple element 1 includes a solid electrolyte body 11 and electrodes 12 and 13 formed on both surfaces thereof, respectively. Electrodes 12 and 13 are made of silver.

Next, the ionic conductivity of the simple element 1 at a temperature of 300 to 500 ° C. was measured. In the measurement, the impedance between the electrodes 12 and 13 of the simple element 1 under each measurement temperature condition is measured, the resistance value corresponding to the electric resistance of the solid electrolyte 11 is read from the conductor call plot, and this resistance value is used as the conductivity. Converted to. For the measurement of impedance, Modulab XM, which is a potentiostat / galvanostat system manufactured by Solartron Analytical, was used. The measurement conditions are temperature: 300 to 500 ° C., atmosphere: oxygen 20% by volume, Ar balance, frequency: 1 MHz to 0.01 Hz. The ionic conductivity of the solid electrolyte composed of La _1.54 Sr _0.46 Ga _2.95 Ni _0.05 O _{7.245 is shown in FIG.}

For comparison of solid electrolytes consisting of _{La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _{7.245 , La 1.54} Sr _0.46 Ga ₃ O _7.27 , La _1.54 Sr _0.46 Ga _2.95 Mg _0.05 O _7.245 , La _1.54 Sr _0.46 Ga _2.95 Al _{0.05, respectively.} Three types of solid electrolytes consisting of O _{7.27 were prepared.} These solid electrolytes can be produced in the same manner as the solid electrolytes composed _{of La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _7.245 described above, except that the composition of the raw materials is changed.

Specifically, the solid electrolyte body made of La _1.54 Sr _0.46 Ga ₃ O _7.27 is, La ₂ O _3, SrCO _3, Ga respective raw materials of the ₂ O _3, a compound represented by La _1.54 Sr _0.46 Ga ₃ O _7.27 It was produced in the same manner as the above-mentioned solid electrolyte composed _{of La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _{7.245, except that it was weighed according to the chemical quantity ratio produced by.} Further, the solid electrolyte composed of _{La 1.54} Sr _0.46 Ga _2.95 Mg _0.05 O _{7.245 is prepared by using La 2} O ₃ , SrCO ₃ , Ga ₂ O ₃ and Mg O raw materials at La _1.54 Sr _0.46 Ga _{2.95 Mg 0.05 O 7.245} . It was produced in the same manner as the above-mentioned solid electrolyte composed _{of La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _7.245 , except that it was weighed according to the chemical quantitative ratio produced by the represented compound. In addition, the solid electrolyte composed of _{La 1.54} Sr _0.46 Ga _2.95 Al _0.05 O _{7.27 uses La 2} O ₃ , SrCO ₃ , Ga ₂ O _3, and Al ₂ O ₃ as raw materials for La _1.54 Sr _0.46 Ga _2.95 Al _0.05. It was produced in the same manner as the above-mentioned solid electrolyte composed _{of La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _{7.245 , except that the compound represented by O 7.27 was weighed according to the ratio of chemical quantities produced.} For these comparative solid electrolytes, simple elements were also manufactured and their ionic conductivity was measured. The result is shown in FIG.

The XRD pattern of each solid electrolyte for comparison is shown in FIG. In FIG. 3, the XRD pattern described as Mg is that of a solid electrolyte composed of _{La 1.54} Sr _0.46 Ga _2.95 Mg _0.05 O _7.245. The XRD pattern described as Al is that of a solid electrolyte composed of _{La 1.54} Sr _0.46 Ga _2.95 Al _0.05 O _7.27. The XRD pattern described as ICSD: 429463 is that of a solid electrolyte composed of _{La 1.54} Sr _0.46 Ga ₃ O _7.27. ICSD is an inorganic crystal structure database.

As is known from FIG. 5 _{, as compared with La 1.54} Sr _0.46 Ga ₃ O _7.27 , the solid electrolyte in which a part of the Ga site of the melilite-type structure is replaced by Mg, Ni, and Al has improved ionic conductivity. There is. _{Among these, the solid electrolyte composed of La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _7.245 in which a part of the Ga site is replaced with Ni has greatly improved ionic conductivity. The _{solid electrolyte composed of La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _7.245 has a remarkable improvement in ionic conductivity at a lower temperature than other solid electrolytes.

The _{reason why the solid electrolyte consisting of La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _7.245 has excellent ionic conductivity at low temperature is that valence fluctuation is possible, Ga and ionic radius are close, and the bond energy with oxygen is Ga. It is considered that this is because the lower Ni is dissolved in the Ga site. From this result, the valence fluctuation is possible, the ionic radius is close to Ga, and the binding energy with oxygen is lower than that of Ga. Is considered to show high ionic conductivity.

As shown in FIG. 3, when the Ga site of the melilite type structure is replaced with Mg, the peak indicated by the asterisk in FIG. 3 occurs. This peak is derived from _{LaGaO 3} having a perovskite-type structure.

On the other hand, in the solid electrolyte in which the Ga site of the melilite type structure was replaced with Ni, such as the solid electrolyte consisting of _{La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _{7.245, no peak derived from the perovskite type structure was observed.} .. That is, in the solid electrolyte in which the Ga site of the melilite type structure is replaced with Ni, it is difficult to form a crystal phase having a different structure at the time of production, and it is easy to be formed by a single phase having a melilite type structure. In the XRD pattern of FIG. 3, typical peak positions derived from the melilite-type structure are indicated by arrows.

According to this example, the solid electrolyte represented by the formula (1): La _2-x α _x Ga _3-y β _y O _{7 + δ} is at a low temperature _{, as in La 1.54} Sr _0.46 Ga _2.95 Ni _0.05 O _7.245. It can be seen that it exhibits excellent ionic conductivity.

<Embodiment 2>
Next, an embodiment of a gas sensor using a solid electrolyte will be described. In addition, among the codes used in the second and subsequent embodiments, the same codes as those used in the above-described embodiments represent the same components and the like as those in the above-mentioned embodiments, unless otherwise specified.

As shown in FIGS. 6 and 7, the gas sensor 2 of this embodiment includes a sensor element 3. The sensor element 3 has a solid electrolyte body 31, a detection electrode 32, a reference electrode 33, and a diffusion resistance layer 36. The detection electrode 32 and the reference electrode 33 are formed on both surfaces 301 and 302 of the solid electrolyte body 31, respectively. The detection electrode 32 and the reference electrode 33 form a pair of electrodes formed at positions facing each other. The diffusion resistance layer 36 limits the flow rate of the measurement gas such as the exhaust gas G that reaches the detection electrode 32. The gas sensor 2 detects the oxygen concentration (that is, the air-fuel ratio) of the exhaust gas G by the magnitude of the critical current generated between the pair of electrodes 32 and 33 when a voltage is applied between the pair of electrodes 32 and 33. It is a limit current type.

The gas sensor 2 of this embodiment will be described in detail below. In the following description, the side exposed to the measurement gas such as the exhaust gas G in the axial direction L of the gas sensor 2 and the tip side L1 are referred to, and the opposite side is referred to as the proximal end side L2.

(Gas sensor)
The gas sensor 2 is arranged and used in the exhaust pipe of an internal combustion engine such as a vehicle. The limit current type gas sensor 2 as in this embodiment is used as an air-fuel ratio sensor that quantitatively detects the air-fuel ratio of the exhaust gas G flowing through the exhaust pipe. Further, the gas sensor 2 can quantitatively obtain the air-fuel ratio in both the case where the air-fuel ratio of the exhaust gas G is on the rich side and the case where the air-fuel ratio is on the lean side.

Here, the air-fuel ratio of the exhaust gas G refers to the mixing ratio of fuel and air when burned in an internal combustion engine. Further, the rich side means that the air-fuel ratio of the exhaust gas G is on the side where the fuel is abundant as compared with the stoichiometric air-fuel ratio when the fuel and the air are completely combusted. The lean side means that the air-fuel ratio of the exhaust gas G is on the side where the fuel is less than the stoichiometric air-fuel ratio.

In the gas sensor 2 of this embodiment, the air-fuel ratio of the exhaust gas is detected by detecting the oxygen concentration of the exhaust gas. The gas sensor 2 as an air-fuel ratio sensor substantially detects the oxygen concentration of the exhaust gas G on the lean side, while it detects the unburned gas concentration of the exhaust gas G on the rich side.

As shown in FIG. 6, the gas sensor 2 has a heater 34, a housing 41, a front end side cover 42, a base end side cover 43, and the like, in addition to the sensor element 3. The heater 34 heats the sensor element 3 to a desired temperature. The housing 41 is attached to the exhaust pipe to hold the sensor element 3. The tip side cover 42 is attached to the tip side L1 of the housing 41 and covers the sensor element 3. The base end side cover 43 is attached to the base end side L2 of the housing 41 and covers the sensor element 3 and the terminal 44 for electrical wiring of the heater 34.

(Sensor element)
As illustrated in FIGS. 6 and 7, the sensor element 3 in this embodiment includes, for example, a solid electrolyte body 31 having a bottomed cylindrical shape (specifically, a cup shape), a detection electrode 32, and a reference electrode 33. Have. The detection electrode 32 is provided on the outer peripheral surface 301 of the solid electrolyte body 31. The reference electrode 33 is provided on the inner peripheral surface 302 of the solid electrolyte body 31. The sensor element 3 having such a configuration is sometimes called a cup-type sensor element because of its overall shape. In such a cup-type sensor element, the heater 34 is inserted inside the sensor element 3, that is, the heater 34 is arranged on the inner peripheral side of the sensor element 3. As the sensor element 3, it is also possible to use a laminated type sensor element described later instead of the cup type sensor element.

The detection electrode 32 is exposed to a measurement gas such as an exhaust gas G that flows into the front end side cover 42 through a flow hole 421 provided in the front end side cover 42. In the example of FIG. 6, the front end side cover 42 having a double structure is exemplified. The reference electrode 33 is exposed to a reference gas such as air A that flows into the inner peripheral side of the solid electrolyte 31 from the inside of the base end side cover 43 through the introduction hole 431 provided in the base end side cover 43. The heater 34 generates heat when energized, and heats the solid electrolyte 31 and the electrodes 32 and 33 to the active temperature when the internal combustion engine and the gas sensor 2 are started.

The detection electrode 32 has a detection unit 321, a lead unit 322, and a connection unit 323. The detection unit 321 is provided on the entire circumference of the outer peripheral surface 301 near the tip end side L1 of the solid electrolyte body 31. The tip side L1 of the solid electrolyte body 31 is the bottom side of the cup type. The lead portion 322 is pulled out from the detection portion 321 to the base end side L2 at a part of the outer peripheral surface 301 in the circumferential direction. The connecting portion 323 is provided on the base end side L2 of the lead portion 322. This is a portion where the detection unit 321 of the detection electrode 32 is exposed to the exhaust gas G and gas is detected together with the reference electrode 33. The connection portion 323 is a portion connected to the terminal 44 to which the lead wire 45 is connected.

The outer peripheral surface 301 of the solid electrolyte body 31 is provided with a diffusion resistance layer 36 for covering at least the detection unit 321 of the detection electrode 32 and bringing the exhaust gas G into contact with the detection unit 321 at a predetermined diffusion rate. The diffusion resistance layer 36 is made of a porous material. Further, on the surface of the diffusion resistance layer 36, a porous protective layer 37 is provided to prevent water, toxic substances, etc. in the exhaust gas G from reaching at least the detection unit 321 of the detection electrode 32. .. The exhaust gas G that has flowed into the front end side cover 42 passes through the protective layer 37 and the diffusion resistance layer 36 and reaches the detection unit 321 of the detection electrode 32.

The reference electrode 33 is provided on, for example, the entire inner peripheral surface 302 of the solid electrolyte body 31. The reference electrode 33 may be partially provided on the inner peripheral surface 302.

Further, as illustrated in FIG. 8, the sensor element 3 may be, for example, a laminated sensor element. That is, the sensor element 3 can be composed of a laminate in which the detection electrode 32, the plate-shaped solid electrolyte 31, and the detection electrode 32 are sequentially laminated. An example of the laminated sensor element will be described below.

As illustrated in FIG. 8, the sensor element 3 has, for example, a plate-shaped solid electrolyte 31. The solid electrolyte body 31 has a measurement gas surface 301A and a reference gas surface 302A. The measurement gas surface 301A is a surface exposed to a measurement gas such as exhaust gas G. On the other hand, the reference gas surface 302A is a surface exposed to a reference gas such as the atmosphere A. The measurement gas surface 301A and the reference gas surface 302A are opposite surfaces in the solid electrolyte 31.

The detection electrode 32 is provided on the measurement gas surface 301A of the solid electrolyte body 31. On the other hand, the reference electrode 33 is provided on the reference gas surface 302A. When the sensor element 3 is composed of such a laminated sensor element, the heating element 34A constituting the heater 34 is laminated on the solid electrolyte 31 via the insulator 35.

The detection electrode 32 faces the measurement gas chamber 38. The measurement gas is introduced into the measurement gas chamber 38 via the porous protective layer 37 and the diffusion resistance layer 36. Further, the reference electrode 33 faces the reference gas chamber 39. The reference gas is introduced into the reference gas chamber 39 from the base end side L2 via the introduction hole 431 of the base end side cover 43.

(Solid electrolyte)
The solid electrolyte body 31 is formed of a solid electrolyte having a melilite-type structure represented by the formula (1) as in the first embodiment. As a result, the gas sensor 2 operates at a low temperature. In particular, among the gas sensors 2, the air-fuel ratio sensor is required to operate at a lower temperature. Therefore, by applying the solid electrolyte of the first embodiment to the air-fuel ratio sensor, the above-mentioned characteristic of the solid electrolyte, which is excellent in ionic conductivity at low temperature, can be fully exhibited.

(electrode)
The material of the detection electrode 32 of this embodiment is not particularly limited as long as it has catalytic activity for oxygen or the like. For example, the detection electrode 32 may contain a composition of any one of Au (gold), Ag (silver), a mixture or alloy of Pd (palladium) and Ag, and a mixture or alloy of Pt and Au as a noble metal component. can. Further, the detection electrode 32 can contain Sm _0.5 Sr _0.5 CoO _3-α , La _0.8 Sr _0.2 Co _0.8 Fe _0.2 O _3-α, or the like as an oxide component. Further, the material of the reference electrode 33 is not particularly limited, but the same material as that of the detection electrode 32 can be used.

(Detection of air-fuel ratio by gas sensor)
In the gas sensor 2 as an air-fuel ratio sensor, a voltage is applied between the detection electrode 32 and the reference electrode 33 so that the reference electrode 33 is on the positive side. Further, the flow rate of the measurement gas introduced into the gas chamber is limited to a predetermined flow rate by the diffusion resistance layer 36. When a voltage is applied between the pair of electrodes 32 and 33, the current flowing between the pair of electrodes 32 and 33 is constant because the flow rate of the exhaust gas G into the gas chamber is limited. Saturates with a value. This saturation current is also called a critical current, and the air-fuel ratio of the exhaust gas G can be quantitatively obtained by utilizing the fact that there is a positive correlation between the critical current and the air-fuel ratio.

When the air-fuel ratio of the exhaust gas G is on the lean side, oxygen contained in the measurement gas such as the exhaust gas G reaches the detection electrode 32. Then, due to the voltage applied between the pair of electrodes, oxygen ions move from the detection electrode 32 to the reference electrode 33 via the solid electrolyte 31, and the air on the lean side depends on the concentration of oxygen contained in the exhaust gas G. The fuel ratio is quantitatively detected.

On the other hand, when the air-fuel ratio of the exhaust gas G is on the rich side, the unburned gas contained in the exhaust gas G reaches the detection electrode 32. Then, in order to burn this unburned gas, oxygen ions move from the reference electrode 33 to the detection electrode 32 via the solid electrolyte 31, causing a backflow of current, and the concentration of the unburned gas contained in the exhaust gas G is increased. Accordingly, the air-fuel ratio on the rich side is quantitatively detected.

FIG. 9 shows the relationship between the voltage (V) and the output current (mA) of the gas sensor when the stoichiometric air-fuel ratio is 14.5. In the figure, the waveform portion in which the output current does not change even if the voltage changes is the portion showing the critical current characteristic. When the air-fuel ratio is on the lean side with the theoretical air-fuel ratio as the boundary, the output current showing the critical current characteristic changes quantitatively to the plus side according to the oxygen concentration in the exhaust gas G, and when the air-fuel ratio is on the rich side. , The output current showing the critical current characteristic changes quantitatively to the minus side according to the concentration of the unburned gas in the exhaust gas G. The theoretical air-fuel ratio may be 14.7.

The gas sensor 2 of this embodiment contains a solid electrolyte having a composition represented by the above formula (1), and the solid electrolyte exhibits excellent ionic conductivity at a low temperature. Therefore, the gas sensor 2 enables the solid electrolyte 31 to operate at a low temperature of 500 ° C. or lower. That is, the gas sensor 2 can detect the oxygen concentration at a low temperature and detect the air-fuel ratio. Further, from the results of Experimental Example 1, it is considered that the gas sensor 2 containing the solid electrolyte having the composition represented by the formula (1) can detect the air-fuel ratio at, for example, 300 ° C.

Further, in the gas sensor 2 of this embodiment, the solid electrolyte 31 operates even in a high temperature range exceeding, for example, 500 ° C. Therefore, the gas sensor 2 can detect the air-fuel ratio even at a high temperature.

<Embodiment 3>
Next, an embodiment of a fuel cell using a solid electrolyte will be described.

(Fuel cell single cell)
The fuel cell single cell 5 of this embodiment includes an anode 52, a solid electrolyte layer 51, and a cathode 53, as illustrated in FIG. The fuel cell single cell 5 may further include an intermediate layer 54 between the solid electrolyte layer 51 and the cathode 53. The intermediate layer 54 is mainly a layer for suppressing the reaction between the solid electrolyte layer material and the cathode material. In the present embodiment, specifically, in the fuel cell single cell 5, the anode 52, the solid electrolyte layer 51, the intermediate layer 54, and the cathode 53 are laminated in this order and joined to each other.

The fuel cell single cell 5 is an anode-supported type in which the anode 52, which is an electrode, functions as a support. Therefore, in the fuel cell single cell 5, the thickness of the anode 52 is sufficiently thicker than that of the other layers. In each figure, an example in which the outer shape of the fuel cell single cell 5 has a quadrangular shape is shown. The outer shape of the fuel cell single cell 5 may also have a circular shape or the like.

The fuel cell single cell 5 can be configured such that the oxidant gas is supplied along the in-plane direction of the cathode 53 and the fuel gas is supplied along the in-plane direction of the anode 52. Examples of the oxidant gas O include air and oxygen gas. Examples of the fuel gas include hydrogen gas and hydrogen-containing gas.

(anode)
Specifically, the anode 52 can have a diffusion layer 521 and an active layer 522. The diffusion layer 521 is a layer that diffuses the fuel gas supplied to the anode 52 into the layer surface. The active layer 522 is arranged on the solid electrolyte layer 51 side of the diffusion layer 521 and serves as a field for the anodic reaction. The diffusion layer 521 and the active layer 522 are joined to each other. Although the anode 52 composed of a plurality of layers is illustrated in FIG. 10, the anode 52 may be composed of a single layer.

Examples of the material of the anode 52 such as the diffusion layer 521 and the active layer 522 include a mixture of a catalyst such as Ni and NiO and a solid electrolyte such as a zirconium oxide oxide. Examples of the zirconium oxide-based oxide include yttria-stabilized zirconia and scandia-stabilized zirconia. Further, as the solid electrolyte, a compound having a melilite-type structure represented by the formula (1) in the first embodiment can be used. In this case, the coefficient of thermal expansion of the solid electrolyte layer 51 and the anode 52, which will be described later, can be brought close to each other. Therefore, it is possible to prevent the occurrence of peeling and the like.

NiO becomes Ni in the reducing atmosphere at the time of power generation. In the present embodiment, as the material of the anode 52, specifically, a mixture of Ni or NiO and yttria-stabilized zirconia can be used.

The thickness of the anode 52 can be, for example, preferably 100 to 800 μm, more preferably 200 to 700 μm, from the viewpoint of gas diffusion, electrical resistance, strength and the like.

The thickness of the diffusion layer 521 can be preferably 100 μm or more, more preferably 200 μm or more, and more preferably 300 μm or more from the viewpoint of ensuring the strength of the support. The thickness of the diffusion layer 521 can be preferably 800 μm or less, more preferably 700 μm or less, from the viewpoint of improving gas diffusivity and the like.

The thickness of the active layer 522 can be preferably 10 μm or more, more preferably 15 μm or more, from the viewpoint of reaction sustainability, handleability, processability, and the like. The thickness of the active layer 522 can be preferably 30 μm or less, more preferably 25 μm or less, from the viewpoint of reducing the electrode reaction resistance and the like.

(Solid electrolyte layer)
The cathode 53 is made of a solid electrolyte. As the solid electrolyte, a compound having a melilite-type structure represented by the formula (1) in the first embodiment can be used. The thickness of the solid electrolyte layer 51 can be preferably 1 to 20 μm, more preferably 2 to 15 μm, and further preferably 3 to 10 μm from the viewpoint of reducing ohmic resistance.

(Middle class)
As the material of the intermediate layer 54, for example, CeO ₂ or CeO ₂ is doped with one or more elements selected from Gd, Sm, Y, La, Nd, Yb, Ca, and Ho. Examples thereof include cerium oxide-based oxides such as the ceria-based solid solution. These can be used alone or in combination of two or more. In the present embodiment, as the material of the intermediate layer 54, specifically, a Gd-doped ceria-based solid solution or the like can be used.

The thickness of the intermediate layer 54 can be preferably 1 to 20 μm, more preferably 2 to 5 μm, from the viewpoint of reducing ohmic resistance, suppressing element diffusion of the cathode 53, and the like.

(Cathode)
As the material of the cathode 53, a transition metal perovskite type oxide or the like can be exemplified. The transition metal perovskite oxide, specifically, for _{example, La x Sr 1-x CoO} 3 based _{oxide, La x Sr 1-x Co} y Fe 1-y O 3 based oxide, Sm _x Sr _{1 -x} CoO ₃ based oxide (However, in the above, 0 ≦ x ≦ 1,0 ≦ y ≦ 1) and the like can be exemplified. These can be used alone or in combination of two or more.

The cathode 53 may contain a solid electrolyte in addition to the above oxides. Examples of the solid electrolyte include CeO ₂ or _{ceria in which CeO 2} is doped with one or more elements selected from Gd, Sm, Y, La, Nd, Yb, Ca, and Ho. Examples thereof include cerium oxide-based oxides such as system solid solutions. As the solid electrolyte, one type or two or more types can be used in combination.

The thickness of the cathode 53 can be preferably 20 to 100 μm, more preferably 30 to 80 μm, from the viewpoint of gas diffusibility, electrode reaction resistance, current collection, and the like.

The fuel cell single cell 5 of the present embodiment contains a solid electrolyte having a composition represented by the above formula (1), and the solid electrolyte exhibits excellent ionic conductivity at low temperatures. Therefore, the fuel cell single cell 5 can operate the solid electrolyte layer 51 at a low temperature of 500 ° C. or lower. Therefore, the fuel cell single cell 5 can generate power at a low temperature, and the power generation efficiency can be improved. Further, from the results of Experimental Example 1, it is considered that the fuel cell single cell 5 containing the solid electrolyte having the composition represented by the formula (1) can generate electricity at, for example, 300 ° C.

Further, in the fuel cell single cell 5 of the present embodiment, the solid electrolyte layer 51 operates even in a high temperature range exceeding, for example, 500 ° C. Therefore, the fuel cell single cell 5 can generate electricity even at a high temperature.

The present invention is not limited to each of the above embodiments, and can be applied to various embodiments without departing from the gist thereof. In the second embodiment, the embodiment in which the solid electrolyte is applied to the air-fuel ratio sensor has been illustrated, but the solid electrolyte is not limited to the air-fuel ratio sensor, and the solid electrolyte can be applied to various gas sensors such as _{an oxygen sensor, a NO x} sensor, and an NH _{3 sensor.} Is also applicable.

Further, in the third embodiment, the embodiment in which the solid electrolyte is applied to the fuel cell single cell is illustrated, but for example, a plurality of fuel cell single cells may be stacked via a separator or the like to construct a fuel cell stack. can.

1 Simple element 2 Gas sensor 5 Fuel cell single cell

Claims (6)

A solid electrolyte having a composition represented by the following formula (1).
La _2-x α _x Ga _3-y β _y O _{7 + δ}・ ・ ・ (1)
However, α is an alkaline earth metal element, β is at least one selected from the group consisting of Ni, Fe, and Co, 0.35 ≦ x ≦ 0.7, 0 <y ≦ 0.3, and δ is iron. It is a value that maintains the accuracy. The solid electrolyte according to claim 1, which is formed from a single phase having a melilite-type structure having a composition represented by the above formula (1). The solid electrolyte according to claim 1 or 2, wherein β in the above formula (1) is composed of Ni. The solid electrolyte according to any one of claims 1 to 3, wherein α in the above formula (1) is Sr. A gas sensor (2) comprising the solid electrolyte body (31) containing the solid electrolyte according to any one of claims 1 to 4. A fuel cell single cell (5) comprising a solid electrolyte layer (51) containing the solid electrolyte according to any one of claims 1 to 4.

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