JP4933757B2 - Non-electron conductive composition graded solid electrolyte membrane - Google Patents

Description

  The present invention relates to a non-electron conductive solid electrolyte membrane.

  For example, solid oxide fuel cells (hereinafter referred to as SOFC) have the advantages of high power generation efficiency, low environmental impact, and the ability to use a variety of fuels. Is underway. The SOFC generally has a structure in which an air electrode layer (that is, a cathode) and a fuel electrode layer (that is, an anode) are stacked on a solid electrolyte dense membrane layer made of an oxide ion conductor. It is used by flowing a hydrogen-containing gas. These cathode and anode are composed of a porous film having high gas diffusibility. In such an SOFC, oxygen in the air supplied to the cathode side is electrochemically reduced to oxygen ions and moves in the electrolyte layer toward the anode. Oxygen ions that have reached the anode oxidize hydrogen or the like in the gas supplied to the anode side, release electrons to an external load, and generate electrical energy.

  The material characteristics required for the solid electrolyte for SOFC include high ionic conductivity, low electron conductivity, heat resistance, reduction durability, and mechanical strength in a wide range of oxygen partial pressure. Among these, ionic conductivity is an essential characteristic required for a solid electrolyte, and directly affects the power generation performance of SOFC. In recent years, high ion conductivity in a low temperature range of, for example, about 700 to 800 (° C.) has been required in response to the direction of low temperature operation of SOFC.

  By the way, yttria-stabilized zirconia (hereinafter referred to as YSZ) is widely used as a solid electrolyte for SOFC because it has high chemical stability and high mechanical strength. However, the ionic conductivity of YSZ cannot be said to be sufficiently high. In particular, the ionic conductivity is extremely low at a low temperature range of 800 (° C.) or less, and is only 0.01 (S / cm) or less. Therefore, since the operating temperature of SOFC using YSZ as an electrolyte is generally around 1000 (° C.), there is a problem that it is difficult to use general-purpose materials such as stainless steel because high heat resistance of peripheral members is required.

  Many efforts have been made to enable low-temperature operation by forming a solid electrolyte made of YSZ into a thin film of about 10 (μm), for example, and reducing the resistance value. However, there is a problem that the thin film solid electrolyte needs to be provided on the porous support to supplement the mechanical strength, and the manufacturing method is limited to a special method. Also, in such a configuration, the difference in thermal expansion coefficient between the porous support and the solid electrolyte can be prevented so that the electrolyte membrane is not damaged due to repeated exposure to temperature changes during long-term use. It is also necessary to reduce the difference in thermal expansion coefficient between the anode and the cathode as much as possible. Furthermore, it is necessary to use a support having as high a gas diffusion performance as possible so that the porous support does not hinder the gas supply to the front or back surface of the solid electrolyte. That is, it is extremely difficult to use YSZ as a solid electrolyte for SOFC, especially considering low temperature operation.

In contrast, as a solid electrolyte material having a high ionic conductivity 10 times more than YSZ, LaGaO _x based materials have been proposed (e.g., see Patent Documents 1 and 2). Since this LaGaO _x- based material has high ionic conductivity even in a low temperature range, it is advantageous over YSZ in this respect, but mechanical strength and reduction durability are inferior to YSZ (that is, durability is low). There's a problem.

For example, the bending strength of a YSZ dense body is about 800 to 1000 (MPa), whereas the bending strength of a LaGaO _x- based material is only about 200 (MPa). Therefore, for the purpose of improving the strength of the LaGaO _x- based material, a sintered body to which alumina powder is added (see Patent Document 3), and a sintered body in which a part of the perovskite structure is replaced with aluminum (see Patent Document 4). ), A sintered body in which a part of Ga at the A site is replaced with aluminum, magnesium or the like (that is, aluminum or magnesium is dissolved) (see Patent Document 5), titanium or vanadium is added to the LaGaO ₃ oxide. There has been proposed a sintered body (see Patent Document 6). However, these sintered bodies also have a strength of about 400 (MPa) at most and only about half of YSZ.

In addition, as described above, one side of the SOFC solid electrolyte is exposed to a reducing atmosphere and the other side is exposed to an oxidizing atmosphere during use, but the reduction durability is cracked or broken in such an environment. This is the difficulty of occurrence. Since the LaGaO _x- based material exhibits a larger volume expansion in the reducing atmosphere than in the oxidizing atmosphere, when used in a solid electrolyte membrane, the volume expansion differs on both sides of the membrane. Since stress acts repeatedly, cracks and the like are likely to occur.

Similarly to LaGaO _x -based materials, CeO ₂ -based oxides doped with Sm and Gd have been found as solid electrolyte materials having an ionic conductivity 10 times higher than that of YSZ. However, this CeO ₂ -based oxide has problems in that electron conductivity is exhibited in a reducing atmosphere and volume expansion in the reducing atmosphere is remarkably large.
JP 09-161824 A Japanese Patent Laid-Open No. 11-335164 JP 2000-044340 A JP 2000-226260 A JP 2001-332122 A Japanese Patent Laid-Open No. 2003-112973 Japanese Patent Laid-Open No. 05-151981 Japanese Patent Application Laid-Open No. 07-296838 JP 09-266000 A JP 2001-052722 A JP 2002-015756 A

As described above, conventionally known solid electrolyte materials with high ionic conductivity exhibit a larger volume expansion than the oxidizing atmosphere in the reducing atmosphere. Volume expansion in a reducing atmosphere, that is, reductive expansion, is caused by the conduction of oxygen ions. In general, a solid electrolyte material having a larger oxygen ion conductivity has a larger reductive expansion. That is, when a solid electrolyte material having high ionic conductivity is applied so that high power generation characteristics can be obtained, the reduction expansion coefficient increases, so that the reduction durability decreases. The reductive expansion rate (%) is given by the following equation (1), where E _red (%) is the thermal expansion coefficient in the reducing atmosphere and E _air (%) is the _air expansion coefficient in the air atmosphere.
[{(1 + E _red / 100)-(1 + E _air / 100)} / (1 + E _air / 100)] × 100 (1)

Also, if found the volume expansion and the strength of the material, the volume expansion rate of the film is theoretically broken (critical volume expansion ratio) E _m can be calculated by the following equation (2). In the following equation (2), S _t is the tensile strength, [nu is the Poisson's ratio, E _s is Young's modulus.
E _m = [S _t × (1−2ν) / E _s / 1000] × 100 (2)

The LaGaO _x -based material and CeO ₂ -based material described above change in Young's modulus and Poisson's ratio depending on the sintered state (for example, the sintered density and the crystal grain size of the sintered body). Therefore, a typical composition example of LaGaO _x -based material La _0.9 Sr _0.1 Ga _0.8 Mg _0.2 O _x is obtained from the above equation (2) using its general physical properties (Young's modulus 100 (GPa), Poisson's ratio 0.3). When the critical volume expansion coefficient is calculated, it is about 0.015 (%). Empirically, when the reduction expansion coefficient increases to about 3 to 10 times the critical volume expansion coefficient, the film is damaged by the generated stress. Therefore, in the case of the above composition example, there is a possibility that film breakage may occur at a reduction expansion coefficient of, for example, 0.045 (%) or more, but as shown in Table 1 below, the reduction expansion coefficient is 0.05 (%). Therefore, the above composition example is in the vicinity of the lower limit value that can satisfy the reduction durability.

However, when applying to SOFC, it is necessary to consider the safety of the power generation apparatus, so in reality, it is necessary that the reduction expansion coefficient is smaller than the limit volume expansion coefficient. That is, in the case of La _0.9 Sr _0.1 Ga _0.8 Mg _0.2 O _x , for example, a reduction expansion coefficient of 0.01 (%) or less is desirable. When various solid electrolyte materials are similarly calculated according to the above equation (2), the critical volume expansion coefficient is about 0.02 to 0.03 (%), and a reduction expansion coefficient of 0.01 (%) or less is required. . As shown in Table 1 above, the reduction expansion coefficient of both LaGaO _x- based materials and CeO ₂ -based materials (excluding CeO ₂ having a low ionic conductivity) is much larger than that. Since the oxygen ion conductivity decreases as the expansion coefficient decreases, it is clear that a material having high oxygen ion conductivity cannot satisfy the reduction durability.

  The present invention has been made against the background of the above circumstances, and an object thereof is to provide a non-electron conductive solid electrolyte membrane having high oxygen ion conductivity and excellent reduction durability.

In order to achieve such an object, the gist of the non-electron conductive composition-graded solid electrolyte membrane of the present invention is that (a) the general formula ( La _1-x Sr _x ) (Ga _y Mg _1-y ) O ₃ (where, 0 <x <1,0 <y <1, hereinafter referred to as the general formula C ₁₎ a first layer comprising a first solid electrolyte is a perovskite composite oxide represented by, (b) general Formula (La _1-p Sr _p ) (Ga _q Mg _1-qr Co _r ) O ₃ (where 0 <p <1, 0 <q <1, 0 ≦ r <1, hereinafter referred to as general formula C ₂ ) Perovskite complex oxide represented by the general formula Ce _z C _1-z O _δ (where C is one or a combination of two or more selected from Sm, Gd, Ca, Y, 0 <z <1, , a second layer stacked on the formula C ₃ hereinafter) represented by the cerium oxide is a by consist second solid electrolyte is larger reduction expansion than the first solid electrolyte and by the first layer Is to include.

According to this configuration, the solid electrolyte membrane has a laminated structure in which the second layer made of the second solid electrolyte is laminated on the first layer made of the first solid electrolyte. In this laminated structure, the second solid electrolyte constituting the second layer has a characteristic that the reduction expansion coefficient is larger than that of the first solid electrolyte constituting the first layer. However, in the general formulas C ₁ and C ₂ , cerium oxide represented by perovskite compound and the formula C ₃ represented are both oxygen ionic conductivity and reducing durability have a trade-off relationship. Therefore, in the solid electrolyte membrane, a second layer having a relatively high reduction expansion coefficient and a high oxygen ion conductivity is laminated on a first layer having a relatively low reduction expansion coefficient and a low oxygen ion conductivity. A laminated body, that is, a composition-gradient type solid electrolyte membrane in which two solid electrolytes having different compositions are laminated in the direction of oxygen ion movement.

  Therefore, when the solid electrolyte membrane is used, if the first layer having a relatively low reduction expansion coefficient is disposed on the reducing atmosphere side, the second layer having a relatively high oxygen ion conductivity is reduced by the first layer. Protected from. At this time, in the first layer, an oxygen partial pressure gradient is formed so as to increase from the reducing atmosphere side toward the second layer in accordance with the film thickness and the composition of the first solid electrolyte constituting the first layer. Therefore, the oxygen partial pressure at the interface between the first layer and the second layer becomes higher according to the degree of the inclination. Therefore, by setting the film thickness and composition so that the reductive expansion of the first layer itself is sufficiently suppressed and the oxygen partial pressure at the interface is sufficiently low, the reductive expansion of the second layer having a large reductive expansion coefficient. And thus the reduction durability of the entire solid electrolyte membrane can be sufficiently increased.

  By the way, although the 1st layer is comprised with the 1st solid electrolyte with low oxygen ion conductivity, the oxygen ion conductivity of a solid electrolyte is substantially inversely proportional to a film thickness. That is, if the composition of the solid electrolyte is the same, the oxygen ion conductivity is increased as the film thickness is reduced. Therefore, by appropriately determining the film thickness, even if the oxygen ion conductivity of the first solid electrolyte is low, a decrease in oxygen ion conductivity with respect to the case where the entire solid electrolyte membrane is composed of the second solid electrolyte is suppressed, and Therefore, a sufficiently high oxygen ion conductivity can be obtained. For example, if the film thickness of the first layer is set so that the oxygen ion conductivity in the first layer is higher than the oxygen ion conductivity in the case where the entire solid electrolyte membrane is composed of the second solid electrolyte, Since the decrease in oxygen ion conductivity due to the lamination of the first layer is almost negligible, it is possible to obtain the same oxygen ion conductivity as when only the second layer is configured.

  Based on the above, the required reduction durability and the desired oxygen ion conductivity are weighed to determine the film thicknesses of the first layer and the second layer, and the compositions of the first solid electrolyte and the second solid electrolyte. Thus, a solid electrolyte membrane having high oxygen ion conductivity and excellent reduction durability can be obtained.

In the present application, wherein the general formula C _1, perovskite compounds represented by C _2, regardless of the display of the general formula C _1, C _2, the number of oxygen is smaller slightly than the other three ones Also included. The number of oxygen effective in the present invention varies depending on the oxygen partial pressure and cannot be uniquely determined. For example, the range of 2.4 to 3 is suitable. The oxygen number δ in the cerium oxide Ce _z C _1-z O _δ represented by the general formula C ₃ is a value determined according to the type of element C substituting cerium and the substitution ratio thereof. Generally, the value is 2 or less. When the element C is trivalent Sm or the like, for example, δ = 2− (z / 2), and when the element C is divalent Ca, for example, δ = 2−z.

In the present application, “non-electron conductivity” means that the electron conductivity is small enough to be ignored as compared with the ionic conductivity. When the oxygen ion conductivity is σ (O ²⁻ ) and the electron conductivity is σ (e ⁻ ), the ion transport number t is given by the following equation (3). It is said. In principle, this is followed in the present application, but not all of the cases where t <0.99 are necessarily excluded.
t = σ (O ²⁻ ) / {σ (O ²⁻ ) + σ (e ⁻ )} (3)

  Incidentally, Patent Documents 7 to 9 describe techniques for inclining the composition between the solid electrolyte membrane and the electrode layer (air electrode or fuel electrode). These are intended to alleviate the difference in thermal expansion or increase the three-phase interface, and are different in configuration and purpose from the present invention in which the solid electrolyte membrane itself is compositionally graded.

  Further, Patent Document 10 discloses a technique for forming an intermediate layer (solid electrolyte) in a solid electrolyte portion to reduce a voltage drop at an air electrode. Patent Document 11 describes that a solid electrolyte portion is stacked to form an ion. Although techniques for bringing the transport number closer to 1 are described, none of them considers reduction durability, and the configuration itself is different from the present invention.

  Here, preferably, the first layer has a reduction expansion coefficient of less than 0.01 (%), and the second layer has a reduction expansion coefficient of 0.01 (%) or more. In this way, since the reduction expansion coefficient of the first layer is sufficiently small and the oxygen ion conductivity of the second layer is sufficiently high, a solid electrolyte membrane more suitable for SOFC can be obtained.

  Preferably, the second layer has an oxygen ion conductivity of 0.1 (S / cm) or more when the thickness dimension is 0.5 (mm). In this way, since the oxygen ion conductivity of the second layer is sufficiently high, a solid electrolyte membrane with higher oxygen ion conductivity can be obtained. More preferably, the oxygen ion conductivity of the second layer is 0.25 (S / cm) or more.

  Preferably, the first layer and the second layer are dense. The solid electrolyte membrane for SOFC is required to be dense because oxygen is required not to pass through as gas molecules. In the present invention, “dense” means that the solid electrolyte membrane has a structure that does not allow gas molecules in the atmosphere to which the solid electrolyte membrane is exposed to permeate in the thickness direction. That is, the preciseness mentioned here is not uniquely determined, and it is sufficient if it has the above-described characteristics in the intended use mode.

  Preferably, the thickness dimension of the first layer is 0.2 (mm) or less. In this way, since the first layer having a relatively low oxygen ion conductivity is sufficiently thin, a solid electrolyte membrane having a higher oxygen ion conductivity can be obtained. The first layer is sufficient if it protects from the reducing atmosphere by covering the second layer. For example, it is preferable to completely cover the first layer, but there is no particular lower limit of the thickness dimension, and industrially. For example, the lower limit is about 1 (μm). Even if the first layer has such a thickness, sufficient reduction durability can be obtained.

  On the other hand, the thickness of the second layer is preferably as thin as possible in order to obtain high oxygen ion conductivity. When a solid electrolyte membrane is formed on a porous support as described later, There is no particular lower limit value for the thickness dimension, but it is preferable that the thickness dimension is such that a dense film can be formed. On the other hand, in the case of a self-supporting membrane that does not use a porous support, the thickness of the second layer so that the entire solid electrolyte membrane can be self-supporting, for example, a thickness of about 0.5 (mm) or more. Can be determined.

  Further, the solid electrolyte membrane is not limited to the two-layer structure in which the first layer and the second layer are laminated, and any other layer having a different reduction expansion coefficient due to a difference in composition from any of these, It is good also as a structure of three or more layers further laminated | stacked so that the reductive expansion coefficient may become large sequentially in the lamination direction which goes to a 2nd layer from a 1st layer. As the number of layers increases, the process is disadvantageous, but the difference in coefficient of thermal expansion between the layers in contact with each other can be reduced, so that it is less likely to break even when exposed to temperature changes during the manufacturing process or use. There is.

  Preferably, the first layer and the second layer are made of the same material. In this way, differences in thermal expansion coefficient and the like are reduced, so that a solid electrolyte membrane that is less prone to breakage due to temperature changes during the manufacturing process or in use can be easily obtained. That is, in order to suppress such damage, it is desirable that the difference in thermal expansion coefficient between the solid electrolytes constituting the first layer and the second layer is as small as possible. Therefore, as long as these conditions are satisfied, the constituent material is not particularly limited, but it is preferable to use the same material. Examples of the material of the same family include materials having the same constituent elements but differing only in their proportions, and materials obtained by substituting one or more constituent elements with other elements. In addition, even in the case of a laminated structure of three or more layers, it is preferable that all are made of the same material, but if the difference in thermal expansion coefficient can be sufficiently reduced, it is not the same material. There is no problem.

Of the solid electrolytes that can constitute the first layer and the second layer of the present invention, perovskite compounds include In, V, Sn, Ce, Sc, in addition to the elements specified in the general formulas C ₁ and C ₂ . Other elements such as Y may be contained within a range that does not substantially affect the characteristics. Further, the first and second layers, in addition to the perovskite compound and cerium oxide, Al ₂ O _3, SiO _2, MgO difficult trace be eliminated manufacturing may include ZrO ₂ and the like. Even if they are contained in a trace amount, they do not significantly affect the characteristics, but since any of them becomes resistance of ionic conduction, the content is desirably as small as possible.

  Preferably, the solid electrolyte membrane of the present invention is provided on the porous support so as to cover the entire surface thereof. This porous support may be located on either one side or the other side of the solid electrolyte membrane. By configuring the solid electrolyte membrane element in which the solid electrolyte membrane is fixed on the porous support in this way, the porous support having a sufficiently high gas diffusion coefficient allows gas to easily pass through the inside. By configuring to an appropriate thickness dimension, the mechanical strength of the solid electrolyte membrane element can be increased without affecting the oxygen ion conductivity. In addition, since the mechanical strength of the solid electrolyte membrane element is ensured by the porous support, the thickness dimension of the solid electrolyte membrane can be reduced to such an extent that the oxygen ion conductivity is not limited by the film thickness. Further, the effect of improving the permeation speed by increasing the surface area can be obtained more remarkably. In the case where such a porous support is provided, when a dissociation catalyst layer or a recombination catalyst layer is further provided, one is on the surface of the solid electrolyte membrane, and the other is the solid electrolyte membrane and the porous support. The other side may be provided on the surface of the porous support, preferably in a state of being permeated therewith.

  In the above aspect, “covering the entire surface” means that, when the solid electrolyte membrane element is used, one surface of the porous support is the same as the surface of the solid electrolyte membrane located on the opposite side of the porous support. It means not being exposed to space. For example, even if one surface of a porous support provided with a solid electrolyte membrane in a non-use state is partially exposed, such a mode can be used as long as the portion is covered by an apparatus or the like during use. It is included in the above “covering the entire surface”.

  Preferably, the solid electrolyte membrane and the porous support are made of the same material. In this way, since the thermal expansion coefficients of the two coincide with each other, even when heated or cooled during the manufacturing process or in use, damage due to the difference in thermal expansion amount is suitably suppressed.

  Preferably, the solid electrolyte membrane and the porous support are made of materials having different compositions. Since the strength of the entire solid electrolyte membrane element needs to be ensured by the support, the required properties of the support and the solid electrolyte membrane are different. For example, when the required strength is relatively high, the solid electrolyte It is desirable that the membrane and the support are made of different materials. As such a support constituent material, for example, zirconia, alumina, silicon nitride, silicon carbide and the like are suitable.

  Preferably, the porous support has an average pore diameter r in the range of 0.1 <r <20 (μm) and a porosity p in the range of 5 <p <80 (%). This range is preferable in order to suppress the decrease in oxygen ion conductivity and increase the strength of the solid electrolyte membrane element as much as possible. If the pore diameter and the pore diameter are too small, the gas permeation resistance of the porous support increases, so even if the solid electrolyte membrane is made thin, the porous support becomes a rate-limiting factor, so that the oxygen ion conductivity is reduced. It drops significantly. On the other hand, if the pore diameter and the pore diameter are too large, the mechanical strength is lowered and the function as a support is lost.

  Further, the overall shape of the solid electrolyte membrane is not particularly limited. For example, the whole may form a flat plate shape, or a cylindrical shape with one end closed.

  The solid electrolyte membrane of the present invention includes, for example, a step of mixing a predetermined binder with the first solid electrolyte material powder and the second solid electrolyte material powder, respectively, A step of forming the powder into a predetermined shape, a step of subjecting the formed body to a firing process at a predetermined temperature to obtain a film made of the first solid electrolyte and a film made of the second solid electrolyte, respectively, and the two films And a step of joining them together by applying a predetermined baking process. Between the molding step and the firing step, if necessary, the step of pressurizing the molded body at an isotropic pressure (for example, wet hydrostatic pressure), and the molded body in the air sufficiently more than the firing temperature And a step of decomposing and removing organic substances by heating at a low temperature. In addition, a mechanical polishing step is performed as necessary after firing.

  In addition, the predetermined forming step includes, for example, a predetermined slurry made of a ceramic sintered body in a first slurry containing the first solid electrolyte material powder and a second slurry containing the second solid electrolyte material powder. The porous support is dipped in sequence, and the first slurry and the second slurry are applied to the surface of the porous support. In this way, the composition which consists of the 1st layer and 2nd layer which were formed by the film thickness according to the coating thickness on the porous support body by baking the slurry apply | coated to the porous support body A solid electrolyte membrane element to which the inclined solid electrolyte membrane is fixed is obtained.

  In addition, the solid electrolyte membrane of the present invention includes, for example, a step of forming a green sheet using a slurry containing the first solid electrolyte material powder and the second solid electrolyte material powder, and the green sheets are stacked and fired simultaneously. It can also be manufactured by a process including a process of applying a treatment.

  Moreover, when forming a solid electrolyte membrane on a porous support body, it can also manufacture by the process of including the process of dip-coating the above slurry, and the process of baking this. Further, the first layer and the second layer, that is, the composition gradient type solid electrolyte membrane may be formed on the porous support by sputtering, spraying, or the like.

  Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In the following embodiments, the drawings are appropriately simplified or modified, and the dimensional ratios, shapes, and the like of the respective parts are not necessarily drawn accurately.

  FIG. 1 is a diagram schematically showing a cross-sectional structure for explaining the configuration of a solid electrolyte membrane 10 according to an embodiment of the present invention. The solid electrolyte membrane 10 has a thin disk shape having a diameter of about 20 (mm) and a thickness of about 0.5 (mm) as a whole, and includes a first layer 12, a second layer 14, and a first layer. It is configured as a laminate in which three layers 16 are laminated. The outer peripheral end faces of these three layers are hermetically sealed with a sealing member 18 made of glass or the like.

  Each of the first layer 12 to the third layer 16 is a dense film made of a non-electron conductive solid electrolyte material. The solid electrolyte materials constituting each layer are different from each other. That is, the solid electrolyte membrane 10 is a composition gradient film in which films having different compositions in the thickness direction are stacked.

The first layer 12 is made of La _0.9 Sr _0.1 Ga _0.2 Mg _0.8 O _x or La _0.9 Sr _0.1 Ga _0.5 Mg _0.5 O _x (where x = about 2.4 to 3. In the perovskite compound, the same applies hereinafter ) . For example, it has a thickness dimension of about 0.1 to 0.2 (mm). The second layer 14 is made of La _0.9 Sr _0.1 Ga _0.5 Mg _0.5 O _x , La _0.9 Sr _0.1 Ga _0.8 Mg _0.2 O _x or the like, and has a thickness dimension of about _{0.1 to 0.2} (mm), for example. ing. The third layer 16 is made of La _0.9 Sr _0.1 Ga _0.8 Mg _0.2 O _x , La _0.9 Sr _0.1 Ga _0.8 Mg _0.1 Co _0.1 O _x or the like, and has a thickness dimension of about 0.2 to 0.3 (mm), for example. It has.

  In addition, the solid electrolyte constituting the first layer 12 (that is, the first solid electrolyte) is, for example, an extremely small reduction expansion coefficient of 0.01 (%) or less and a low oxygen ion conductivity of 0.04 (S / cm) or less. (The value when the thickness dimension is 0.5 mm. Hereinafter, the same unless otherwise specified). The solid electrolyte (that is, the second solid electrolyte) constituting the second layer 14 has a reduction expansion coefficient of 0.05 (%) or less and an oxygen ion conductivity of about 0.10 (S / cm). Yes. Further, the solid electrolyte (that is, the third solid electrolyte) constituting the third layer 16 has a slightly large reduction expansion coefficient of about 0.05 to 0.09 (%) and a high oxygen ion of about 0.10 to 0.25 (S / cm). Conductivity.

  Therefore, the solid electrolyte membrane 10 has a layer structure in which the reduction expansion coefficient increases and the oxygen ion conductivity increases as it goes from the first layer 12 to the third layer 16. However, the first layer 12 made of a solid electrolyte having a low oxygen ion conductivity is a thin film of about 0.1 to 0.2 (mm), and the oxygen ion conductivity is inversely proportional to the film thickness. The ionic conductivity is sufficiently high. Similarly, since the second layer 14 is also a thin film of about 0.1 to 0.2 (mm), the oxygen ion conductivity in the second layer 14 is sufficiently high. Therefore, the oxygen ion conductivity of the solid electrolyte membrane 10 as a whole is a sufficiently high value as compared with the solid electrolyte membrane composed of only the first layer 12 and the second layer 14.

  In the solid electrolyte membrane 10 configured as described above, the electrode layer, that is, the fuel electrode layer 24 and the air electrode layer 26 are provided on the one surface 20 and the other surface 22, respectively, and the first layer 12 having a small reduction expansion coefficient is provided on the reduction side. In other words, the third layer 16 positioned on the fuel (that is, hydrogen) supply side and positioned on the opposite side is positioned on the oxidation side, that is, the air supply side. Therefore, since the second layer 14 and the third layer 16 having a large reduction expansion coefficient are protected from the reducing atmosphere by the first layer 12, the reduction durability of the solid electrolyte membrane 10 and the like is improved.

The fuel electrode layer 24 and the air electrode layer 26 are for taking out electrons from the solid electrolyte membrane 10, but the air electrode layer 26 provided on the oxidation side promotes dissociation and ionization of oxygen on the other surface 22. For example, a porous layer made of La _0.6 Sr _0.4 CoO ₃ is formed on substantially the entire other surface 22 with a uniform thickness of about 10 (μm).

  Further, the fuel electrode layer 24 provided on the reduction side is provided to promote recombination of oxygen ions on the one surface 20, and a porous layer made of, for example, NiO has a uniform thickness of about 100 (μm). It is formed on substantially the entire surface 20 with a thickness dimension.

  The solid electrolyte membrane 30 schematically showing the layer structure in FIG. 2 has a relatively low reductive expansion coefficient on the first layer 32 made of the first solid electrolyte having a relatively low reductive expansion coefficient and a low oxygen ion conductivity. It is a composition gradient type solid electrolyte membrane having a two-layer structure in which a second layer 34 made of a second solid electrolyte having a large oxygen ion conductivity is stacked. Although the solid electrolyte membrane 10 shown in FIG. 1 has a three-layer structure, the present invention can be applied to other layers on the one surface 20 or the other surface 22 of such a two-layer structure. It can also be applied to a structure having four or more layers laminated.

In the case of two-layer structure, the first layer 32, be comprised _{La 0.9 Sr 0.1 Ga 0.2 Mg 0.8} O x, perovskite compounds such as _{La 0.9 Sr 0.1 Ga 0.5 Mg 0.5} O x or al, for example, It has a thickness of about 0.05 to 0.2 (mm). The second layer 34 is made of La _0.9 Sr _0.1 Ga _0.8 Mg _0.2 O _x , La _0.9 Sr _0.1 Ga _0.8 Mg _0.1 Co _0.1 O _x , SmCeO _x , GdCeO _x or the like, for example, 0.3 to 0.45 (mm ) About thickness dimension. In this case, the entire thickness of the solid electrolyte membrane 30 is, for example, about 0.5 (mm).

  In the case of the two-layer structure as described above, the solid electrolyte constituting the first layer 32 has a very small reduction expansion coefficient of 0.02 (%) or less and a low value of about 0.01 to 0.04 (S / cm). Oxygen ion conductivity. The solid electrolyte constituting the second layer 34 has a large reduction expansion coefficient of about 0.05 to 0.11 (%) and a high oxygen ion conductivity of about 0.10 to 0.25 (S / cm). Even in this configuration, the first layer 32 is made of a solid electrolyte having a low oxygen ion conductivity. However, since the film thickness is thin, the oxygen ion conductivity in the first layer 32 is sufficiently high. For this reason, in the case of the two-layer structure, as in the case of the three-layer structure, the oxygen ion conductivity of the solid electrolyte membrane 30 is sufficiently higher than that in the case where the solid electrolyte membrane 30 is configured by only the first layer 32.

The solid electrolyte membrane 30 having such a two-layer structure is also used with the first layer 32 having a small reduction expansion coefficient positioned on the reduction side and the second layer 34 having a large reduction expansion coefficient positioned on the oxidation side. By protecting the second layer 34 from the reducing atmosphere by the first layer 32, the reduction durability of the solid electrolyte membrane 30 is enhanced. The thick lines shown in FIG. 2 schematically represent oxygen partial pressures P _{1 to} P ₃ (that is, oxygen partial pressure gradients) in the atmosphere and in the film. In addition, the white arrow shown in the figure represents the movement of oxygen ions in the solid electrolyte membrane 30. When oxygen is ionized and moves from the air side toward the reduction side, a gradient of oxygen partial pressure corresponding to the moving speed is formed in the solid electrolyte membrane 30. In the first layer 32 having a low oxygen ion conductivity, the gradient becomes remarkably large. Therefore, as shown in FIG. 2, since the oxygen partial pressure is rapidly increased in the first layer 32 from the reducing side atmosphere toward the second layer 34, the oxygen partial pressure P _{2 at the} interface 36 is, for example, reduced side. oxygen partial becomes larger than 10 ¹⁰ times the pressure P ₁ of the. Therefore, even if the second layer 34 is made of a solid electrolyte having a large reduction expansion coefficient, the reduction force applied to the second layer 34 is 1/10 ¹⁰ or less on the reduction side. Is suppressed.

  Therefore, if the second layer 34 is completely covered so as not to be exposed to the reducing atmosphere, the thickness dimension of the first layer 32 may be extremely thin. That is, it can be said that the lower limit of the film thickness of the first layer 32 is substantially absent, but the film thickness that can be formed by a dense film is industrially limited to about 1 (μm). Upper lower limit.

  In the case of the solid electrolyte membrane 10 having the three-layer structure, the action of protecting the second layer 14 and the third layer 16 from the reducing atmosphere is the same as that of the above-described two-layer structure. That is, the oxygen partial pressure at the interface between the first layer 12 and the second layer 14 and at the interface between the second layer 14 and the third layer 16 can be obtained only by providing the first layer 12 with a film thickness of about 0.1 (mm). Each is significantly reduced.

As described above, according to the solid electrolyte membrane 10, 30 of this _{embodiment, La 0.9 Sr 0.1 Ga 0.2 Mg} 0.8 O reduction expansion coefficients, such as _x in the first layer 12, 32 is composed of small solid electrolyte La _0.9 Sr The second layer 14, 34 made of a solid electrolyte such as _0.1 Ga _0.8 Mg _0.2 O _x is formed in a laminated structure. Therefore, in the solid electrolyte membranes 10 and 30, the second layers 14 and 34 having a large reduction expansion coefficient and a high oxygen ion conductivity are stacked on the first layers 12 and 32 having a low reduction expansion coefficient and a low oxygen ion conductivity. The composition gradient type. Therefore, if the first layers 12 and 32 are arranged on the reducing side as described above, the second layers 14 and 34 are protected from the reducing atmosphere, and as a result, the reduction durability of the entire solid electrolyte membrane can be sufficiently enhanced. it can. Further, since the first layers 12 and 32 are sufficiently thin, even if the oxygen ion conductivity of the solid electrolyte constituting the first layers 12 and 32 is low, high oxygen ion conductivity of the entire solid electrolyte membrane can be obtained. Based on the above, the required reduction durability and the desired oxygen ion conductivity are weighed and the film thicknesses of the first layers 12 and 32 and the second layers 14 and 34 and the composition of the solid electrolyte constituting them are determined. By defining, solid electrolyte membranes 10 and 30 having high oxygen ion conductivity and excellent reduction durability can be obtained.

FIG. 3 is a process diagram for explaining the manufacturing method of the solid electrolyte membrane 10 described above. In the granulation step P1, for example, a commercially available LaGaO _x powder or CeO ₂ powder having an average particle diameter of about 1 (μm) is mixed with a molding aid such as water, an organic binder, and a dispersant to create a slurry. Then, for example, a raw material powder having an average particle diameter of about 60 (μm) is spray-granulated using a spray drier. Next, in the pressure forming step P2, the granulated raw material powder is press-molded at an appropriate pressure of, for example, about 100 (MPa), and has a diameter of, for example, about 30 (mm) and a thickness of about 3 (mm). A disk-shaped molded body is obtained. In addition, the said molded object dimension is the value determined in consideration of baking shrinkage and grinding | polishing allowance so that the 1st layer 12 grade | etc., Of the said dimension may be obtained. Further, if necessary, a pressure treatment of about 150 (MPa) can be performed by hydrostatic pressure molding (that is, CIP).

  Next, in the firing step P3, for example, after holding the molded body in the atmosphere at a temperature of about 200 to 500 (° C.) for about 10 hours to decompose and remove organic substances, further in the atmosphere about 1300 to 1600 (° C.). The molded body is fired by holding at this temperature for about 3 hours. In the thickness polishing step P4, the dense sintered body thus obtained is subjected to mechanical polishing using a surface grinder or the like, and has a predetermined thickness dimension of about 0.05 to 0.5 (mm), for example. Process into a thin film.

  Next, in the lamination / bonding step P5, two to three thin film bodies having different compositions are overlapped and fired at 1300 to 1600 (° C.) in the atmosphere. As a result, a plurality of the laminated thin film bodies are bonded to each other, and a composition gradient film, that is, the solid electrolyte membranes 10 and 30 in which a plurality of layers having different compositions are laminated in the thickness direction is obtained.

Such solid electrolyte membranes 10 and 30 are used by providing electrode layers on both surfaces 20 and 22 as described above. In the electrode layer forming step P6, for example, a commercially available La _0.6 Sr _0.4 CoO ₃ powder having an average particle size of about 2 (μm) is mixed with an organic solvent to prepare a slurry, and this is applied to one side 20 to apply air electrode material. In addition, for example, a commercially available NiO powder having an average particle size of about 7 (μm) is mixed with an organic solvent to prepare a slurry, which is applied to the other surface 22 to print and carry the fuel electrode material. Then, in the baking step P7, for example, the solid electrolyte membranes 10 and 30 are obtained by holding the electrode layers on the one surface 20 and the other surface 22 respectively by holding for about 1 hour at a temperature of about 1000 (° C.). It is done.

Hereinafter, more specific examples of the present invention will be described. Table 2 below shows composition examples of the first layer 12 to the third layer 16, the first layer 32, and the second layer 34 constituting the solid electrolyte membranes 10, 30, their respective reduction expansion coefficients and oxygen ion conduction. The rate is summarized. In Table 2, CeO ₂ and 8YSZ are outside the scope of the present invention, but the characteristics are shown for reference. In the “Composition Name” column of Table 2, the abbreviations used in Tables 3 and 4 to be described later are shown as meaning the solid electrolyte of each composition. First, the characteristics of these eight types of solid electrolytes constituting each layer will be described.

In Table 2 above, the reduction expansion coefficient was calculated according to the above equation (1) after measuring the thermal expansion coefficient in the atmosphere and in an atmosphere of 5% H ₂ + 95% N ₂ . The oxygen ion conductivity is determined by manufacturing a 0.5 mm thick solid electrolyte sheet having the above composition according to the process shown in FIG. 3, and providing a fuel electrode layer and an air electrode layer on both sides of the sheet. It was determined by measuring the resistance value of the sample in an apparatus adjustable to any oxygen partial pressure and temperature.

As shown in Table 2 above, the solid electrolyte constituting each layer of the solid electrolyte membrane 10 has a relationship in which the oxygen ion conductivity decreases as the reduction expansion coefficient decreases. If the reduction durability is increased, sufficient oxygen ion conductivity cannot be obtained. In a general application of the solid electrolyte membrane 10, the reduction expansion coefficient is preferably 0.02 (%) or less and more preferably 0.01 (%) or less in order not to cause reduction cracking. The oxygen ion conductivity is 0.04 (S / cm) or less. Therefore, it is impossible to increase both the reduction durability and the oxygen ion conductivity by simply configuring the solid electrolyte membrane 10 as a single layer and simply changing the composition.

Examples 1 to 6 , 9 to 12, and 17 to 20 shown in Table 3 are two-layer or three-layer laminated structures (that is, composition gradients) in which the first layer 12 and the like are formed of the solid electrolyte listed in Table 2 above. This is an example of a layer structure of a film. What is described as “none” in the column of the third layer means a two-layer structure. In Table 3, Examples 7, 8, 13 to 16, 21, 22 and Comparative Examples 1 to 5 shown in Table 4 are single layers using the solid electrolyte listed in Table 2 or out of the scope of the present invention. It is the example of a layer structure made into the laminated structure of.

Example 1,2,5～ 7,11,12,17～20 is the first layer 32 is reduced expansion 0.01% or lower as small LSGM3 or 0.01 (%) and small LSGM 2, second layer 34 is composed of LSGM1 having a large reduction expansion coefficient of 0.05 (%), 0.09 (%) and a large LSGMC, 0.10 (%) and a large GdCeO _x , or 0.11 (%) and a large SmCeO _x , respectively. The gradient composition has a reduction expansion coefficient that increases from the layer 32 toward the second layer 34. In these configuration examples, the first layer 32 positioned on the reduction side has a reduction expansion coefficient of 0.02 (%) or less sufficient for suppressing reduction cracking, while the second layer 34 positioned on the oxidation side is 0.10 (S / S cm) or higher oxygen ion conductivity.

  In Examples 3, 4, 9, and 10, the first layer 12 is made of LSGM3 having a small reduction expansion coefficient of 0.01 (%) or less, or LSGM2 having a small reduction of 0.01 (%), and the second layer 14 has a reduction expansion coefficient of 0.01 (%). (%) And a small LSGM2 or 0.05 (%) and a slightly large LSGM1, and the third layer 16 is composed of a slightly large LSGM1 or 0.09 (%) and a large LSGMC, respectively. The gradient composition has a reduction expansion coefficient that increases from the first layer 12 toward the third layer 16. Also in these structural examples, the first layer 12 positioned on the reduction side has a reduction expansion coefficient of 0.01 (%) or less sufficient for suppressing reduction cracking, while the third layer 16 positioned on the oxidation side is 0.10 (S / cm) or higher oxygen ion conductivity. The second layer 14 located in the middle of these has an oxygen ion conductivity of 0.04 (S / cm) or more.

  On the other hand, Comparative Examples 1-4 are comprised by the single material with the same material as the 3rd layer 16 and the 2nd layer 34 with high oxygen ion conductivity. In Comparative Example 5, the first layer 32 is made of a material having a reduction expansion coefficient of 0.05 (%).

  Table 5 shows the results of evaluating the oxygen ion conductivity and reduction durability of Examples 1-22, and Table 6 shows the results of evaluating the oxygen ion conductivity and reduction durability of Comparative Examples 1-5.

  The reduction durability was evaluated using, for example, the reactor 40 shown in FIG. In the reactor 40, cylindrical tubes 42 and 44 made of ceramics such as alumina, which are open at both ends, are arranged up and down with the solid electrolyte membrane 10 interposed therebetween, and are made of ceramics such as alumina on the inner peripheral side thereof. The gas introducing pipes 48 and 50 are inserted. The solid electrolyte membrane 10 has a direction in which one surface 20 provided with the fuel electrode layer is located on the cylindrical tube 44 side and the other surface 22 provided with the air electrode layer is located on the upper side in FIG. It is arranged with. In addition, heaters 56 and 56 are arranged on the outer peripheral side of the cylindrical tubes 42 and 44. The cylindrical tubes 42 and 44 and the solid electrolyte membrane 10 are hermetically sealed with sealing materials 58 and 58 such as glass. In addition, the gas introduction pipes 48 and 50 are respectively spaced apart from the surface of the solid electrolyte membrane 10 by a distance necessary for gas supply.

  In such a reactor 40, while the inside of the apparatus is heated to a temperature of about 1000 (° C.) by the heater 56, air, that is, a gas containing oxygen, is introduced into the cylindrical tube 42 from the gas introduction tube 48, and the fuel side, Pure hydrogen gas or the like is introduced from the gas introduction pipe 50. The air introduction amount is, for example, about 10 to 500 (cc / min), and the hydrogen gas introduction amount is, for example, about 10 to 200 (cc / min). Prior to the measurement, for example, while the inside of the cylindrical tube 44 is heated to a temperature of about 1000 (° C.) by the heater 56, for example, a mixed gas of hydrogen 10 (%) and argon 90 (%) is supplied from the gas introduction tube 50 to the cylinder. It supplies in the pipe | tube 44 and heats in a reducing atmosphere. Thereby, the fuel electrode layer, that is, the nickel oxide provided on the one surface 20 is partially or completely reduced, and the oxygen recombination catalytic function is exhibited.

  The air introduced from the gas introduction pipe 48 as described above passes through the exhaust path 60 formed between the gas introduction pipe 48 and the cylindrical pipe 42 while being in contact with the surface of the solid electrolyte membrane 10 and the air electrode layer. As shown by the arrows in FIG. At this time, oxygen in the air is dissociated and ionized by the oxygen dissociation action and ionization action of the air electrode layer provided on the solid electrolyte membrane 10 and the other surface 22, so that the oxygen ions have oxygen ion conductivity. The solid electrolyte membrane 10 is transported as shown by the arrow in FIG. 4 from the other surface 22 side to the one surface 20 side.

  The oxygen ions that have reached one surface 20 become oxygen molecules by the recombination action of the solid electrolyte membrane 10 made of a solid electrolyte and the fuel electrode layer provided on the one surface 20, and are taken out from the one surface 20. Thereby, oxygen permeate | transmits from the other surface 22 side to the one surface 20 side. However, since the solid electrolyte membrane 10 made of a solid electrolyte is dense and other gases cannot be ionized, gases other than oxygen do not permeate at all. The oxygen ions thus moved oxidize hydrogen to emit electrons to an external load, thereby generating electric energy.

For the reduction durability shown in Tables 5 and 6, the above test was continuously performed, the amount of nitrogen in the synthesis gas recovered on the reduction side was measured, and the nitrogen leak rate L _N was calculated from the change as follows. Judgment was made according to equation (4). ◎: No increase in nitrogen leak rate in 3 to 10 hours, ○: Increase in nitrogen leak rate within 1% within 3 hours, Δ: Nitrogen leak rate in 1 hour within 1 hour Increased in a range of ˜3 (%), and × indicates an increase in nitrogen leakage rate by 3 (%) or more within 3 hours. If the nitrogen leak rate does not increase in 10 hours, it is considered that there is sufficient reduction durability over a long period of time.
L _N = [N ₂ amount (cc / min) / total gas amount (cc / min)] × 100 (4)

As shown in Table 5, the first layers 12 and 32 are made of a material having a reduction expansion coefficient of 0.01 (%) or less, and the third layer 16 having a three-layer structure or the second layer 34 having a two-layer structure is formed. In Examples 1 to 6, 9 to 12 and 17 to 20 composed of a material having an oxygen ion conductivity of 0.10 (S / cm) or more, all have excellent reduction durability with excellent evaluation (that is, no increase in the amount of nitrogen leak). showed that. The oxygen ion conductivity is about 0.02 to 0.12 (S / cm), that is, sufficiently higher than 8YSZ or the like having sufficiently high reduction durability.

In particular, in Examples 17 to 20 in which the thickness of the first layer 32 is reduced to 50 (μm) while maintaining the same total film thickness, the first layer 32 is 0.1 (mm) with the same constituent material. Even in comparison, a higher oxygen ion conductivity of about 2 times is obtained. Also in this case, since high reduction durability is obtained as shown in Table 5, even if the thickness dimensions of the first layers 12 and 32 are thin, there is no problem in improving the reduction durability. Accordingly, the first layers 12 and 32 are preferably thin as long as the reduction durability can be ensured. For example, the first layers 12 and 32 are most preferably about 1 (μm).

In addition, since La _0.9 Sr _0.1 Ga _0.5 Mg _0.5 O _x having high reduction durability has an ionic conductivity of about 0.04 (S / cm), the ionic conductivity in the examples is 0.04 (S / cm) or less. What remains is just the same or lower characteristics. However, in Example 1 and the like having an ionic conductivity of 0.04 (S / cm) or less, the first layers 12 and 32 are made of an extremely low material having an ionic conductivity of about 0.01 (S / cm). Compared with this, the ionic conductivity is at least twice as high. Further, in Example 6 in which the above La _0.9 Sr _0.1 Ga _0.5 Mg _0.5 O _x is used for the first layer 32, 0.12 (S / cm), and in Example 10 in which the first layer 12 is used, 0.09 (S / cm). That is, in any case, oxygen ion conductivity twice or more is obtained.

  On the other hand, in Comparative Examples 1 to 4 configured as a single layer only with LSGM1 or the like having high oxygen ion conductivity, the reduction durability is low. Even in the case of the laminated structure, the first layer 32 has insufficient reduction durability in Comparative Example 5 in which the first layer 32 is made of a material having a reduction expansion coefficient of 0.05 (%). Therefore, the electrolyte membrane as a whole also has a reduction durability of about x to Δ.

  Therefore, according to the embodiment, by using a material having a small reduction expansion coefficient for the first layers 12 and 32 and using a material having a high oxygen ion conductivity on the oxidation side, both reduction durability and oxygen ion conductivity are achieved. It is clear that high solid electrolyte membranes 10 and 30 can be obtained.

  As mentioned above, although this invention was demonstrated in detail with reference to drawings, this invention can be implemented also in another aspect, A various change can be added in the range which does not deviate from the main point.

It is a figure which shows typically the principal part cross section of the solid electrolyte membrane of one Example of this invention. It is a figure explaining the oxygen partial pressure gradient in the film | membrane in the case of a 2 layer inclination structure. FIG. 2 is a process diagram for explaining a method for manufacturing the solid electrolyte membrane element of FIG. 1. It is a figure explaining the structure of the reactor using the solid electrolyte membrane element of FIG.

Explanation of symbols

10: solid electrolyte membrane, 12: first layer, 14: second layer, 16: third layer, 18: sealing member, 20: one side, 22: other side, 24: fuel electrode layer, 26: air electrode layer, 30: Solid electrolyte membrane, 32: First layer, 34: Second layer, 36: Interface, 40: Reactor, 42, 44: Cylindrical tube, 48, 50: Gas introduction tube, 56: Heater, 58: Sealing 60, exhaust path, 62: recovery path

Claims (3)

Formula _{(La 1-x Sr x)} (Ga y Mg 1-y) O 3 ( where, 0 <x <1,0 <y <1) the first solid electrolyte is a perovskite composite oxide represented by A first layer comprising:
Perovskite complex oxidation represented by the general formula (La _1-p Sr _p ) (Ga _q Mg _1-qr Co _r ) O ₃ (where 0 <p <1, 0 <q <1, 0 ≦ r <1) Or a cerium oxide represented by the general formula Ce _z C _1-z O _δ (C is one or a combination of two or more selected from Sm, Gd, Ca, Y, 0 <z <1) non electron-conductive composition gradient type, characterized in that a second layer is laminated on and the first layer consists of the second solid electrolyte is larger reduction expansion than the first solid electrolyte, including there Solid electrolyte membrane.   2. The non-electron conductive composition gradient solid electrolyte according to claim 1, wherein the first layer has a reduction expansion coefficient of less than 0.01 (%), and the second layer has a reduction expansion coefficient of 0.01 (%) or more. film. The non-electron conductive composition gradient solid electrolyte according to claim 1 or 2, wherein the second layer has an oxygen ion conductivity of 0.1 (S / cm) or more when the thickness dimension is 0.5 (mm). film.

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