Solid oxide fuel cells

Abstract: A solid oxide fuel cell has a solid electrolyte portion (3A) formed by a spraying method as an ion conductor, a thickness of said solid electrolyte portion is not less than 40 µm and not more than 100 µm, and a leakage amount of N₂ gas of said solid electrolyte portion at a room temperature is not more than 10⁻⁵ cc/g·second. A permeation coefficient of said solid electrolyte portion is preferably not more than 10⁻⁷cm⁴/g·second at a room temperature. A solid oxide fuel cell has a solid electrolyte portion (3A) as an ion conductor. One or more metal element selected from a group consisting of manganese, iron, cobalt, nickel, copper and zinc is contained in said solid electrolyte portion in an average amount of not less than 1 atom% and not more than 15 atom% based on a sum of amounts of all metal elements contained in said solid electrolyte portion. A powdery raw material for a solid electrolyte having an average particle diameter of 10-40 µm is mixed with a compound of one or more metal element selected from a group consisting of manganese, iron, cobalt, nickel, copper and zinc to obtain a mixed powder. An amount of said metal element is not less than 1 atom% and not more than 15 atom% based on a sum of amounts of all metal elements contained in the mixed powder. A sprayed film is formed by a spraying method using the mixed powder. The sprayed film is then subjected to a heat treatment at a temperature of 1350°C - 1500°C to form an air-tight solid electrolyte film. Or, a powdery additive of a compound of said one or more metal element and the powdery raw material are supplied into a spray gun portion by means of separate apparatuses for supplying powders , respectively. An amount of said metal element is not less than 1 atom% and not more than 15 atom% based on a sum of amounts of all metal elements contained in the powdery raw material and the powdery additive. The powdery raw material and the powdery additive are molten in the spray gun portion. Or, a powdery raw material for a solid electrolyte and a compound of said one or more metal element are mixed to obtain a mixed powder. An amount of the metal element is not less than 1 atom% and not more than 15 atom% based on a sum of amounts of all metal elements contained in the mixed powder. The mixed powder are formed into a granulated powder having an average particle diameter of 10-40 µm. A sprayed film is formed by a spraying method using the granulated powder.

Description: The present invention relates to processes for producing solid oxide fuel cells having solid electrolyte films. Since solid oxide fuel cells (SOFC) operate at high temperatures of 1000°C or more, the activity of the electrodes of the SOFC is extremely high. Thus, an expensive catalyst of a noble metal such as platinum is not necessary. In addition, since the SOFC has a low polarization and a relatively high output voltage, its energy conversion efficiency is conspicuously higher than that of the other fuel cell. Furthermore, all the constituent materials of the SOFC are solid, the SOFC is stable and has a long use life. Recently, a stabilized zirconia and a composite oxide, having a perovskite structure and including lanthanum, are most common and promising materials for the constituent materials of a solid electrolyte film and of an air electrode of the SOFC, respectively ('A comprehensive engineering for energies' 13-2, 1990). Manufacturing processes of solid electrolyte films and air electrodes are generally categorized as a dry process and a wet process. An EVD method and a spray method are most common as the dry process. The wet process includes a tape casting method, a slip casting method, an extrusion method and the like ('A comprehensive engineering for energies' 13-2, 1990). When the solid electrolyte films and the air electrodes are manufactured by a vapor phase method such as a chemical vapor deposition (CVD), an electrochemically vapor deposition (EVD) or the like, an apparatus with a large scale is needed, and the treated area and the treating speed are too small. Moreover, in the vapor phase method, zirconium chloride or the like is used and aqueous vapor mixed with oxygen is used, therefore a running cost of the apparatus applying the vapor phase method is high. When the solid electrolyte films are formed by applying a plasma spraying method, the film-forming speed is great, an apparatus applying the plasma spraying method can be handled easily and the films can be formed as a thin and relatively dense films. For these merits, the plasma spraying method has heretofore been used for forming the solid electrolyte films ('Sunshine' 1981, Vol.2, No.1 : 'A comprehensive engineering for energies' 13-2, 1990). The plasma spraying method is suitable for mass production, however, it is difficult to obtain an air tight film. On the other hand, the solid electrolyte film of the SOFC must be air tight to prevent lowering of the output due to leakage of fuel through the film. When applying the plasma spraying method, it was necessary to produce the solid electrolyte film with a thickness of not less than 200-300 µm for satisfying the above air-tight performance required for the solid electrolyte film of the SOFC (Japanese Laid-open Patent Application No. 128,566/1988). However, an electrical resistance of such solid electrolyte film composed of a zirconia is relatively high and contributes to the internal resistance of the single cell as the main factor. It is necessary to solve such problems. Therefore, it is required to produce a SOFC single cell, in which its solid electrolyte film is produced by applying a spraying method, having an opened circuit voltage near its theoretical value and to reduce the internal resistance of the single cell. Besides, a porosity of the solid electrolyte film formed by the plasma spraying method is generally more than 5% and reaches up to 10%, so that the plasma-sprayed solid electrolyte film does not have enough denseness for a solid electrolyte film of a SOFC. That is, cracks or laminar defects occur in the film during the plasma spraying. For this reason, leakage of a fuel occurs that the fuel, such as hydrogen or carbon monoxide, penetrates the film when operating the SOFC. Consequently, an electromotive force per a single cell of the SOFC is frequently smaller than that of an ideal SOFC in which the leakage does not occur, so that the output of the SOFC and a conversion rate of the fuel to electric power are reduced. Therefore, it is desired to improve the denseness of the solid electrolyte portion of the solid oxide fuel cell for preventing the leakage of the fuel when thinning the solid electrolyte portion so that the output per the single cell of the SOFC may be improved. Besides, in the SOFC, a thinner film is preferable as the solid electrolyte film. However, when a vapor phase method, such as a chemical vapor deposition (CVD), an electrochemically vapor deposition (EVD) or the like, is applied for thinning the electrolyte film, an apparatus with a large scale is needed, and the treated area and the treating speed are too small. Therefore, a cost for producing the film is high, it is difficult to produce the solid electrolyte film with a large area, and moreover, the above EVD method may be applicable only when the solid electrolyte film is formed onto a tubular substrate. Further, it is known that a grain size of a raw material to be plasma sprayed, in which cerium oxide or zirconium oxide is solid solved with an oxide of an alkaline earth metal element or a rare earth element, is adjusted, the raw material is then plasma sprayed to form a solid electrolyte film (Japanese Laid-open Patent Applications 198,569/1986 and 198,570/1986) . However, a film produced by the plasma spraying is not generally densified enough. Therefore, when a solid electrolyte film of a SOFC is produced by the plasma spraying as described above, such film does not have a sufficient denseness required for the SOFC. For this reason, a leakage of a fuel occurs that the fuel, such as hydrogen or carbon monoxide, penetrates the film when operating the SOFC. Consequently, an electromotive force per a single cell of the SOFC is smaller than, for example, 1 V in an ideal SOFC in which the leakage does not occur, so that an output of the SOFC and a conversion rate of the fuel to electric power are reduced. In this case, it may be considered that a thickness of the solid electrolyte film is increased to prevent the leakage of the fuel. However, a resistance against diffusion of ions in the film is increased in this case, and consequently, a resistance of the cell becomes large. Therefore, a technique is desired for improving the output of power generation per the single cell by densifying the solid electrolyte film and thinning the same without causing the leakage of the fuel. As a method for solving such problems, in Japanese Laid-open Patent Applications 115,469/1992 and 62,459/1991, a method is disclosed that a solid electrolyte film of a SOFC is formed by plasma spraying and the film thus formed by plasma spraying is then subjected to a heat treatment. However, the film thus formed by plasma spraying contains many defects therein, so that it is difficult to obtain a film having a sufficient air-tight performance. In particular, for obtaining a high quality film having a permeation coefficient of N 2 gas not more than 10 -7 cm 4 g -1 s -1 , it is necessary to subject the film formed by plasma spraying to the heat treatment for a long time at a high temperature not lower than 1550°C . Much energy and time are consumed for carrying out the above heat treatment for a long time at such a high temperature, so that a cost for producing the film is considerably increased. Moreover, the film produced by plasma spraying adheres to a substrate which is also subjected to the heat treatment at a high temperature not lower than 1550°C, so that the substrate may be deteriorated or deformed during the heat treatment. In particular, a porous substrate is used in the SOFC. Such a porous substrate is sintered at a high temperature not lower than 1550°C, so that a dimension of the porous substrate becomes small and permeation of an oxidant or a fuel in the porous substrate is interrupted. Consequently, the SOFC having such a porous substrate undergone the heat treatment does not work effectively. US-A-3645894 describes preparation of solid electrolyte layers by plasma spray deposition. Powders of zirconia, a stabilizer for zirconia and other oxides, for example iron oxide or nickel oxide, are mixed, and then plasma sprayed onto a mandrel. EP-A-513983 (published 19 November 1992, after the priority dates claimed in the present application) describes in connection with SOFCs, doping of zirconia films sprayed onto substrates by adding manganese dioxide, iron oxide or nickel oxide to the zirconia before spraying. After spraying, the zirconia layer is subjected to heat treatment. See Embodiment 3. EP-A-524013 (published 20 January 1993, after the priority dates claimed in the present application) discloses processes in which a solid electrolyte film of a SOFC is formed by applying a layer of manganese dioxide on an air electrode, followed by plasma spraying a zirconia layer, and then heat-treating these layers. See Examples 2 and 3 and also Examples 4 and 5 in which manganese dioxide layers are present on both sides of the zirconia layer before the heat treatment. It is an object of the present invention to provide processes of producing a SOFC single cell having an opened circuit voltage near its theoretical value and to reduce an internal resistance of the single cell. It is another object of the present invention to improve density of a solid electrolyte portion of a solid oxide fuel cell for preventing leakage of a fuel when thinning the solid electrolyte portion, so that an output per a single cell of the SOFC may be improved. Processes of the present invention are set out in claims 1 and 2. The present inventors have studied producing an air tight solid electrolyte portion of a SOFC. In a process of this study, the present inventors made an attempt that one or more metal element, which was selected from a group consisting of manganese, iron, cobalt, nickel, copper and zinc, was incorporated into a film composed of a solid electrolyte and the portion was then subjected to a heat treatment. Consequently, the inventors found that said metal element (which was selected from manganese, iron, cobalt, nickel, copper and zinc) progressed the densification of the resulting solid electrolyte portion. As a result, when applying such solid electrolyte portion having a thickness of not more than 100 µm, an output of the SOFC single cell is considerably improved to a value near the theoretical value of the output. It has been impossible to obtain the SOFC single cell having such a high output by using a conventional solid electrolyte film formed with a spraying method. The present inventors have mades further studies based on the above knowledge. As a result, the output of the SOFC single cell may be improved to a value near the theoretical value by using a solid electrolyte portion having an amount of N 2 gas leakage at a room temperature being not more than 10 -5 cc/g·second. Moreover, the inventors successfully obtained the portion having a thickness of not more than 100 µm and an amount of N 2 gas leakage at a room temperature being not more than 10 -5 cc/g·second, which made it possible to provide the SOFC single cell having an output near its theoretical value. Such a thin portion has not been considered practical for a solid electrolyte portion formed with a spraying method for a SOFC. Moreover, the internal resistance of the single cell is considerably decreased by using the solid electrolyte portion, formed with a spraying method, having a thickness of not more than 100 µm. However, when the thickness of the solid electrolyte portion is decreased below 40 µm, quality of the portion is deteriorated and gas leakage is observed. Such result is probably due to the fact that Rmax of the solid electrolyte portion formed with a spraying method is about 30 µm and pinholes may occur in the portion when its thickness is decreased below 40 µm. In the preferred embodiment of the invention, a permeation coefficient of N 2 gas of the solid electrolyte portion is not more than 10 -7 cm 4 /g·second. This coefficient indicates the quality of the solid electrolyte portion: The lower the coefficient is, the higher the quality is. The solid electrolyte portion of the invention is composed essentially of a zirconium oxide selected from a stabilized zirconium oxide and a partially stabilized zirconium oxide, each containing one or more metal elements selected from manganese, iron, cobalt, nickel, copper and zinc. In this embodiment, the inventors further found the following facts : When the metal element is contained in a solid electrolyte portion in an amount of not more than 1 atom%, the solid electrolyte portion is not sufficiently densified as described above. Generally, the amount of the metal element, which may be solid solved within the solid electrolyte, varies depending on a temperature at which the solid electrolyte portion is subjected to a heat treatment and the metal element added to the portion. Particularly, when the metal element is contained in the portion in an amount of more than 15 atom%, an electric resistance of the solid electrolyte portion is conspicuously increased. Generally, the electric resistance is reduced by thinning the portion. However, a resistivity of the solid electrolyte is increased simultaneously to contribute to the electric resistance as a major factor. The inventors assume that an amount of the metal element is beyond an upper limit of an amount in which the metal element may be solid solved in the solid electrolyte. As a result, a considerable amount of the metal element is precipitated in grain boundaries in the portion and therefore the resistivity of the solid electrolyte is increased. Such precipitation of the metal element into the grain boundaries may be effectively reduced by increasing the temperature at which the solid electrolyte portion is subjected to a heat treatment. However, a porous electrode in the SOFC cell is sintered and densified by the heat treatment at such an increased temperature. Such densified electrode is inappropriate for supplying a fuel or an oxidant. The metal element may preferably be contained in an amount of 3-12 atom% for obtaining the solid electrolyte portion having more densified microstructure and a relatively low resistivity. The above amount of the metal element is indicated in a scale of atom % and a sum of amounts (indicated in a scale of atom %) of all metal elements contained in the solid electrolyte portion is 100 atom%. When two or more metal elements are selected from the group and contained in the portion, a sum of the amounts of the metal elements are 1-15 atom%. The metal element may more preferably be contained in an average amount of 5-10 atom%. When the solid electrolyte portion is formed of a stabilized zirconia or a partially stabilized zirconia, manganese and cobalt are preferable as the selected metal element, because manganese and cobalt may be easily solid solved with the stabilized zirconia or the partially stabilized zirconia. Particularly, manganese may be solid solved with the stabilized zirconia in an amount of up to about 15 atom% by a heat treatment at 1500°C and the stabilized zirconia is widely used solid electrolyte material for a SOFC application. Therefore, manganese is most preferred element in the above six metal elements. These and other optional features and advantages of the invention will be appreciated upon reading of the following description of the invention when taken in conjunction with the attached drawings. Fig.1 (a), (b), (c), (d) and (e) are cross sectional views schematically showing stages of a manufacturing process of a laminated body comprising an air electrode substrate 11, a solid electrolyte film 13A and a fuel electrode film 4. Fig. 2(a) is a cross sectional view showing a material 1 for an air electrode, Fig. 2(b) is a cross sectional view showing a film 5 being formed on the material 1 by a wet process, and Fig. 2(c) is a cross sectional view showing an intermediate layer 2 being formed on the material. Fig. 3(a) is a cross sectional view showing a material 1 for an air electrode, Fig. 3(b) is a cross sectional view showing an intermediate layer 2 being formed on the material 1, and Fig. 3(c) is a cross sectional view showing a film 3A of a solid electrolyte material. Fig. 4(a) is a cross sectional view showing a laminated body having a surface layer 12 being further formed on the film 3A of a solid electrolyte material, Fig. 4(b) is a cross sectional view showing a laminated body obtained by subjecting the laminated body of Fig. 4(a) to a heat treatment, and Fig. 4(c) is a cross sectional view showing a fuel electrode film 4 being further formed on the the laminated body of Fig. 4(b). Fig. 5(a) is a cross sectional view showing the air electrode substrate 11, Fig. 5(b) is a cross sectional view showing a film 3B of a solid electrolyte material being formed on the air electrode substrate 11 to produce a laminated body, Fig. 5(c) is a cross sectional view showing a laminated body obtained by subjecting the laminated body of Fig. 5(b) to a heat treatment and further forming a fuel electrode film 4. Fig.6 is a graph showing relationship between thicknesses of solid electrolyte films and permeation coefficients of N2 gas, and relationship between the thicknesses and amounts of N 2 gas leakage, in experiment 1 relating to the present invention. Fig. 7, 8 and 9 are graphs showing amounts of manganese measured by EPMA, respectively, and the amounts are measured at positions in a direction along the thickness of a single cell to be measured. Figs. 1 to 5 are first explained to provide a bakground of the present invention. When a composite oxide having a perovskite structure and containing lanthanum is used as a material for an air electrode and a solid electrolyte portion is formed of a stabilized zirconia or a partially stabilized zirconia, a layer having a high resistivity composed of lanthanum zirconate (La 2 Zr 2 O 7 ), which is an electrical insulator, is formed between the solid electrolyte portion and the air electrode after a heat treatment at a temperature higher than about 1250°C. In this case, the inventors found that formation of the layer composed of lanthanum zirconate is considerably reduced and the layer is hardly seen when manganese or cobalt is used as the selected metal element. Consequently, an output of a single cell may advantageously be improved. The solid electrolyte portion is generally in a form of a film or a plate. An air electrode is formed on one side of a solid electrolyte film or a solid electrolyte plate and a fuel electrode is formed on the other side thereof. Such an air electrode and a fuel electrode may be films formed on a surface of a substrate or a substrate capable of self-standing. A SOFC of the present invention includes SOFC 's with various shapes and types, such as so called a planer type, a tubular type and a monolithic type. The tubular SOFC's include a tubular SOFC with its inner space for gas-flow opened at the both ends of the SOFC and a tubular SOFC with the inner space opened at one end and closed at the other end of the SOFC. Said selected metal element is contained in the solid electrolyte portion in an average amount of not less than 1 atom% and not more than 15 atom% based on a sum of amounts of all metal elements contained in the portion. When the amount of the selected metal element is increased from one boundary to the other boundary of the solid electrolyte portion, the average amount over the whole area in the portion is 1-15 atom%. The amount of the selected metal element may be measured by an EPMA method. In one example of a process of the present invention, the solid electrolyte portion is sandwitched by a pair of electrodes, and the amount of the selected metal element is increased in the portion from one boundary between the portion and one electrode to the other boundary between the portion and the other electrode. A method of manufacturing such single cell will be described below. In the example described below, the portion is a form of a film, however, the portion may be a form of a plate. First, a material 1 for an air electrode with a predetermined shape such as a flat plate is prepared, as shown in Fig. 1(a). Then, an intermediate layer 2 is formed on a surface of the material 1 as shown in Fig. 1(b). The intermediate layer 2 is formed of a compound of one or more metal element selected from manganese, iron, cobalt, nickel, copper and zinc. The intermediate layer 2 may be preferably manufactured by a plasma spraying method in a view point of its high productivity. Then, a film 3A consisting of a material for a solid electrolyte is formed on the surface of the intermediate layer 2 to provide a laminated body, as shown in Fig. 1(c). The film 3A may also be preferably manufactured by a plasma spraying method . The thus obtained laminated body is then subjected to a heat treatment. During the treatment, the intermediate layer 2 disappears by a mechanism as described below. That is, the intermediate layer 2 disappears on the surface of the material 1 and an air-tight solid electrolyte film 13A is formed on the surface of an air electrode substrate 11, as shown in Fig. 1(d). Then, as shown in Fig. 1(e), a fuel electrode film 4 is formed on the surface of the solid electrolyte film 13A . Alternatively, the intermediate layer 2 may be formed by a wet process which will be explained referring to Fig. 2(a)-(c). That is, as shown in Fig. 2(a), the material 1 for an air electrode is prepared. Then, as shown in Fig. 2(b), a film 5 containing a compound of the selected metal element is formed on the surface of the material 1 by a wet process. The wet process includes a dipping method, a slip casting method, an extrusion method etc.. Then, the film 5 is subjected to a heat treatment to evaporate ingredients such as a solvent in the film 5. The intermediate layer 2 consisting of the compound of the selected metal element is thus formed as shown in Fig. 2(c). Then, a SOFC single cell may be manufactured by a process as described above referring to Figs. 1 (c), (d) and (e). In the present examples, as shown in Fig. 1(c), the intermediate layer 2 is formed between the material 1 and the film 3A to form the laminated body consisting of these three layers. When the laminated body is subjected to the heat treatment, the selected metal element moves from the intermediate layer 2 to the film 3A and diffuses into the film 3A. As a result, the selected metal solid solved into at least a region of the film 13A near a boundary between the film 13A and the air electrode substrate 11. The present inventors found that the thus diffused metal element progresses the densification of the film 13A. Moreover, the inventors analyzed the solid electrolyte film 13A using an EPMA measuring apparatus. As a result, the metal element is contained in a relatively large amount and the densification is relatively progressed in the region of the film 13A near the boundary between the film 13A and the air electrode substrate 11. On the other hand, the metal element is contained in a relatively small amount in a region of the film 13A near a boundary between the film 13A and the fuel electrode film 4. In the solid electrolyte film, the metal element is contained in a relatively large amount (the metal element is diffused) and the microstructure of the solid electrolyte film is densified in the substantially same region, as shown in the result of the EPMA measurement and in photographs taken by a scanning type electron microscope. When a composite oxide having a perovskite structure and containing lanthanum is used as a material for the air electrode 1 and a material for a solid electrolyte is a stabilized zirconia or a partially stabilized zirconia, the above selected metal element diffuses to considerably reduce the formation of the layer composed of Lanthanum zirconate during the heat treatment, as described above. Such effect is most considerable when manganese or cobalt is used as the above selected metal element. When the composite oxide having a perovskite structure and containing lanthanum is used, a part of the A site of its composition containing lanthanum may preferably be replaced with strontium and/or calcium. when the solid electrolyte is a stabilized zirconia or a partially stabilized zirconia, the material for the solid electrolyte may preferably be a mixture of zirconia and a compound (especially an oxide) of a metal element selected from an alkaline- earth metal element and a rare earth element, or, the material may preferably be a material consisting of zirconia solid solved with the compound of the above metal element. In the manufacturing process as described above referring to Figs.1 and 2, the air electrode and the fuel electrode may be replaced with each other. In this case, the intermediate layer 2 and the film 3A are formed on a fuel electrode substrate to provide a laminated body successively and the laminated body is subjected to the heat treatment to form an air-tight solid electrolyte film. In the solid electrolyte film, the selected metal element is contained in a relatively large amount in a region of the film near the boundary between the film and the fuel electrode substrate, and the metal element is contained in a relatively small amount in a region of the film near a boundary between the film and an air electrode film. Moreover, the following manufacturing methods may be applied. That is, a film consisting of a material for a solid electrolyte is formed on a surface of a fuel electrode substrate. A surface layer of one or more metal element selected from manganese, iron, cobalt, nickel, copper and zinc is formed on the surface of the film to provide a laminated body, which is then subjected to a heat treatment. During the heat treatment, the selected metal element contained in the surface layer diffuses in the solid electrolyte to densify it and to provide an air-tight solid electrolyte film, and the surface layer disappears. Then, an air electrode film is formed on the surface of the air-tight solid electrolyte film. In the above manufacturing method, the air electrode and the fuel electrode may be replaced with each other. That is, a film consisting of a material for a solid electrolyte is formed on a surface of an air electrode substrate. A surface layer of one or more metal element selected from manganese, iron, cobalt, nickel, copper and zinc is formed on the surface of the film to provide a laminated body, which is then subjected to a heat treatment. During the heat treatment, the selected metal element contained in the surface layer diffuses in the solid electrolyte to densify it and to provide an air-tight solid electrolyte film, and the surface layer disappears. Then, a fuel electrode film is formed on the surface of the air-tight solid electrolyte film . In an example of the invention, a solid electrolyte portion may be sandwiched between an air electrode and a fuel electrode, and in the solid electrolyte portion, an amount of the selected metal element in a region near a boundary between the portion and the air electrode is larger than an amount of the metal element in a central region of the solid electrolyte portion, and an amount of the metal element in a region near a boundary between the portion and the fuel electrode is larger than an amount of the metal element in the central region. For manufacturing such SOFC's, it is necessary that the above selected metal element is diffused from the above two boundaries of the solid electrolyte portion, respectively. An example, in which the solid electrolyte portion is a form of a film, will be described below. First, a material 1 for an air electrode with a predetermined shape such as a flat plate is prepared, as shown in Fig.3 (a). Then, an intermediate layer 2, consisting of a compound of one or more metal element selected from a group consisting of manganese, iron, cobalt, nickel, copper and zinc, is formed on the surface of the material 1 as shown in Fig. 3(b). The intermediate layer 2 may be preferably manufactured by a plasma spraying method. Then, a film 3A consisting of a material for a solid electrolyte is formed on the surface of the intermediate layer 2 to provide a laminated body, as shown in Fig. 3(c). The film 3A may also be preferably manufactured by a plasma spraying method. Then, as shown in Fig. 4 (a), a surface layer 12, consisting of a compound of one or more metal element selected from a group consisting of manganese, iron, cobalt, nickel, copper and zinc, is formed on the surface of the film 3A to provide a laminated body, which is then subjected to a heat treatment. During the treatment, the intermediate layer 2 and the surface layer 12 disappear by a mechanism as described above. That is, the intermediate layer 2 disappears on the surface of the material 1 and, as shown in Fig. 4(b), an air-tight solid electrolyte film 13B is formed on the surface of an air electrode substrate 11. Then, as shown in Fig. 4(c), a fuel electrode film 4 is formed on the surface of the solid electrolyte film 13B. Alternatively, the surface layer 12 may be formed by a wet process which will be explained below. As shown in Fig. 4(a), the film 3A consisting of a material for a solid electrolyte is formed. Then, a film containing a compound of the selected metal element is formed on the surface of the film 3A by a wet process. The wet process includes a dipping method, a spray coating method, a screen printing method etc.. Then, the film containing the compound is dried to form the surface layer 12 as shown in Fig. 4 (a). When applying the above manufacturing methods, in the solid electrolyte film 13B as shown in Fig. 4(c), an amount of the selected metal element in a region near a boundary between the portion and the air electrode substrate 11 is larger than an amount of the metal element in a central region of the solid electrolyte film 13B, and an amount of the metal element in a region near a boundary between the portion and the fuel electrode film 4 is larger than an amount of the metal element in the central region. The metal element diffuses most efficiently in the two regions near the two boundaries of the solid electrolyte film 13B. A smaller amount is selected from the amounts of the metal element both in the region near the boundary between the portion and the air electrode substrate 11 and in the region near the boundary between the portion and the fuel electrode film 4, and then the smaller amount is divided by the amount of the metal element in the central region of the solid electrolyte film 13B to provide a value which is preferably not less than 1.3. In the above manufacturing method, the air electrode and the fuel electrode may be replaced with each other. In this case, the intermediate layer 2 is formed on the surface of a fuel electrode substrate, the film 3A is formed on the surface of the intermediate layer 2, and the surface layer 12 is formed on the surface of the film 3A to provide a laminated body which is then subjected to the heat treatment. In each manufacturing method explained above referring to Figs. 1 to 5, the heat treatment of the material for the solid electrolyte may be preferably carried out at a temperature between 1300°C - 1500°C. Because, when the heat treatment is carried out at a temperature lower than 1300°C, the effect of the heat treatment for densifying the material for the solid electrolyte is not considerable and it is necessary to carry out the heat treatment for a long time to obtain the solid electrolyte portion with sufficiently densified microstructure. When the heat treatment is carried out at a temperature higher than 1500°C, it is difficult to prevent deformation of a porous substrate including the air electrode substrate. Moreover, much energy and much time are needed to carry out the heat treatment at a temperature higher than 1500°C, therefore a cost for manufacturing the air-tight solid electrolyte portion is very high. In each manufacturing method as described above, as the compound of the selected metal element, the oxide, the carbonate or the powder of the metal is preferred. In the processes herein described for producing a solid electrolyte film and a solid oxide fuel cell, 'the powdery raw material for a solid electrolyte' includes both a mixed powder and a solid solved powder. In a SOFC application, the mixed powder may preferably be a mixed powder of a zirconia powder or a ceria powder with a powder of a stabilizer. In a SOFC application, the solid solved powder may preferably be a powder of a compound consisting of zirconia or ceria solid solved with the stabilizer. Such stabilizer may preferably be a compound consisting of an alkaline earth metal or a rare earth metal element. In these processes, the method for spraying may preferably be a plasma spraying method. A low pressure plasma spraying is more preferable for obtaining an excellent effect. However, when applying an atmospheric plasma spraying method for forming a sprayed film, after the subsequent heat treatment, it is possible to obtain an air-tight solid electrolyte film with sufficiently densified microstructure. The compound of the selected metal element includes the acetate, the nitrate, the sulfate, the salt of an organic acid, the oxide, the carbonate, the hydroxide and the like of the metal element, and the oxide, the carbonate and the hydroxide are preferred. However, in these processes for producing a solid electrolyte film and a solid oxide fuel cell, manganese or cobalt may be contained in the sprayed film, formation of such insulating layer may be thus prevented. In the processes of the present invention, the air electrode may be formed of doped or undoped LaMnO 3 , CaMnO 3 , LaNiO 3 , LaCoO 3 , LaCrO 3 or the like, among which LaMnO 3 doped with strontium or calcium is preferable. These doped or undoped LaMnO 3 , CaMnO 3 , LaNiO 3 , LaCoO 3 and LaCrO 3 include a composite oxide having a perovskite structure in which its A site and B site are not substantially deficient, and include a composite oxide in which a part of the A site or B site is deficient, for example, in which La or Ca is contained in the A site and a part of the A site is deficient. Such composite oxide, in which La or Ca is contained in the A site and a part of the A site is deficient, effectively reduce the formation of La 2 Zr 2 O 7 . Generally, the air electrode may be preferably made of a nickel-zirconia cermet or a cobalt-zirconia cermet. A fuel gas for SOFC's includes a gas containing a fuel such as hydrogen, reformed hydrogen, carbon monooxide, a hydrocarbon or the like. An oxidizing gas for SOFC's includes a gas containing oxygen. Results of experiments relating to the invention are described below. The following experiments are given for explaining the present invention in more detail and not intended for limiting the same. (Experiment 1) A powder of lanthanum manganite was pressed with a pressure of 200 kgf/cm 2 to provide a formed body having a disk shape with a diameter of 50 mm and a thickness of 3 mm. The formed body was then sintered at a temperature of 1550°C for 5 hours to provided a sintered body, which was then processed to provide an air electrode substrate having a porosity of 22% and having a disk shape with a diameter of 20 mm and a thickness of 1.5 mm. A powdery raw material consisting of a stabilized zirconia containing 8 mol% of Y 2 O 3 (8YSZ) and a powdery additive of manganese dioxide (MnO 2 ) were prepared as materials for spraying. First, the powdery additive of manganese dioxide was sprayed by means of a spray gun on the surface of the air electrode body to form an intermediate layer with a thickness of about 10 µm. Then, the material of 8YSZ was sprayed on the intermediate layer to form a sprayed layer with a thickness of about 600 µm to provide a laminated body, which was then subjected to a heat treatment in an atmosphere at a temperature of 1450°C for 5 hours to obtain a solid electrolyte film. The thus obtained film was processed with a machine to obtain experimental samples with thicknesses shown in table 1. Moreover, a platinum paste was applied onto a surface of the solid electrolyte film and then fired to form an opposing electrode (a fuel electrode). Each test cell was thus produced and then tested. The open circuit voltage (OCV) and ohmic resistance of each test cell was measured, using oxygen as an oxidizing gas and hydrogen humidified at room temperature as a fuel gas at 1000°C. The results were shown in table 1 and Fig.6. A thickness of the solid electrolyte film (µm) OCV (V) Resistance (Ωcm 2 ) 500 1.10 0.65 200 1.10 0.27 100 1.11 0.13 70 1.11 0.10 60 1.11 0.09 50 1.11 0.08 40 1.10 0.07 30 0.91 0.05 As understood from table 1, when a thickness of the solid electrolyte film was 40-100 µm, the electromotive force of the test cell was near its theoretical value and the resistance of the test cell was reduced to a relatively low value. When the thickness was 30 µm, the electromotive force was unexpectedly and critically decreased. Then, referring to Fig. 6, when the thickness was less than 40 µm, the leakage amount of N 2 gas was more than 10 -5 cc/g·second. That is, the electromotive force was rapidly and unexpectedly decreased when the leakage amount of N 2 gas was about 10 -5 cc/g·second. Moreover, referring to a graph in Fig.6 showing relationship of a permeation coefficient of N 2 gas and the thickness, when the thickness was 50-60 µm, the permeation coefficient of N 2 gas was minimum and the quality of the film was most improved in this range of the thickness. When the thickness was less than 50 µm, quality deteriorated due to the surface roughness of the film. When the thickness was less than 40 µm, the permeation coefficient of N 2 gas was rapidly decreased due to said effect of the pinholes, and therefore, the leakage amount of N 2 gas came to be beyond 10 -5 cc/g ·second. It was also interesting that when the thickness was larger than 60 µm, the permeation coefficient of N 2 gas was increased and the average quality of the film was rather deteriorated. Therefore, when the thickness was made larger than 60 µm, the leakage amount of N 2 gas was not decreased. These results may be considered that manganese diffused mainly in a region 50-60 µm distant from a boundary of the film and the air electrode, and the densification of the microstructure of the film was considerably progressed in the above region. Such consideration was also supported by results of observations of the solid electrolyte films by means of an EPMA apparatus and a SEM. (Experiment 2) A powder of lanthanum manganite (La 0.9 Sr 0.1 MnO 3 ) was pressed with a pressure of 200 kgf/cm 2 to provide a formed body having a disk shape with a diameter of 50 mm and a thickness of 3 mm. The formed body was then sintered at a temperature of 1500°C for 5 hours to provided a sintered body, which was then processed to provide an air electrode substrate having a porosity of 25% and having a disk shape with a diameter of 30 mm and a thickness of 1.5 mm. Manganese dioxide (MnO 2 ) was sprayed on the surface of the air electrode substrate by means of an atmospheric plasma spraying method to form an intermediate layer with a thickness of 5 to 10 µm. Then, 8YSZ was sprayed on the surface of the intermediate layer by an atmospheric plasma spraying method to form a sprayed film of a zirconia with a thickness of about 300 µm. The thus obtained sprayed film was then subjected to a heat treatment in an atmosphere at a temperature of 1400°C for 3 hours to provide a solid electrolyte film. Then a surface of the solid electrolyte film was ground to obtain the solid electrolyte film with a thickness of 200 µm. An opposing electrode (a fuel electrode) was formed on a central portion of the surface of the solid electrolyte film by screen printing a commercially available platinum paste thereon. The air electrode substrate was also metallized by screen printing a platinum paste on a central portion of the surface of the air electrode substrate and by firing the platinum paste at 1000°C to provide a test cell. The thus manufactured test cell was used to carry out the same measurements as follows. The internal resistance of the cell was determined by measuring the ohmic resistance thereof by means of an alternating impedance method at 1000°C in an atmosphere. The air-tight performance was evaluated as follows. Oxygen was introduced into a side of the air electrode and hydrogen gas was bubbled into water at a room temperature to obtain humitized hydrogen gas, which was then introduced into a side of the fuel electrode. Thus, the electromotive force of each cell was measured and used for evaluating the performance. Moreover, amounts of each metal elements in the solid electrolyte film were measured at positions, each position being distant about 10 µm from the adjoining position, in the film by means of an EPMA apparatus. The electromotive force was 1.12 V and the resistance was 0.2 Ωcm 2 . That is, a high air-tight performance and a low resistance sufficient for a SOFC cell were attained. Fig. 7 is a graph showing relationship between amounts of manganese in the air electrode substrate and in the solid electrolyte film versus positions at which the amounts were measured. The amounts were measured by means of an EPMA apparatus. The horizontal axis shows distances of the positions in a direction of a thickness of the test cell. The distances are measured from the position, which is 100 µm distant from the boundary between the film and the substrate, in the substrate. As shown in Fig. 7, the amount of manganese was about 13 atom% at the position in the film near the boundary between the film and the air electrode substrate, and the amounts were decreased as a distance of the position at which each amount was measured from the air electrode substrate was increased. Besides, an average value of the 20 amounts of manganese measured at 20 positions in the solid electrolyte film was 6.2 atom%. (Experiment 3) The same air electrode substrate as the experiment 2 was prepared. A fuel electrode substrate consisting of a nickel-zirconia cermet was also prepared. A powdery raw material of 8YSZ was sprayed on the surface of the air electrode body (example A) or on the surface of the fuel electrode body by means of an atmospheric plasma spraying method to form a sprayed film with a thickness of about 300 µm. A surface of the sprayed film was then ground to obtain the sprayed film with a thickness of about 200 µm. MnO 2 was then sprayed onto the surface of the sprayed film by an atmospheric plasma spraying method to form a surface layer with a thickness of 5 to 10 µm to provide a laminated body, which was then subjected to a heat treatment in an atmosphere at 1400°C for 3 hours and the surface layer was disappeared. Then, an opposing electrode was formed by screen printing a commercially available platinum paste and each electrode substrate was metallized using a platinum paste to provide a test cell. The thus manufactured test cell was used for same measurement as described in the experiment 2 to obtain the results shown in table 6. Composition of the substrate Electromotive Force (V) Resistance (Ωcm 2 ) Example A L 0.9 Sr 0.1 MnO 3 1.10 0.5 Example B Ni-ZrO 2 1.11 0.2 As understood from table 2, both in the example A and the example B, a high air-tight performance and a low resistivity sufficient for a SOFC cell was attained. However, the test cell of the example A has a higher resistance than that of the cell of the example B. This result may be considered that a relatively small amount of La 2 Zr 2 O 7 was formed between the air electrode body and the solid electrolyte film during the heat treatment. Fig. 8 is a graph showing relationship between amounts of manganese in the air electrode substrate and in the solid electrolyte film versus positions at which the amounts were measured. Fig. 9 is a graph showing relationship between amounts of manganese in the fuel electrode substrate and in the solid electrolyte film versus the positions. The horizontal axes in Figs. 8 and 9 show distances of the positions in a direction of a thickness of the test cell. The distances are measured from the position, which is 100 µm distant from the boundary between the film and the substrate, in the substrate. Referring to Fig. 8, in the example A, manganese was contained in an amount of about 14 atom% near a boundary between the solid electrolyte film and the fuel electrode (the opposing electrode). The boundary correspond to a position at which the distance shown in Fig.8 is 300 µm. The amount of manganese contained in the film was gradually decreased as the position at which the amount was measured approaches to a boundary between the film and the air electrode substrate (The boundary correspond to a position at which the distance shown in Fig. 8 is 100 µm). However, a peak is shown in a graph in Fig. 8 near the boundary between the film and the air electrode substrate. This peak is due to the diffusion of manganese into the solid electrolyte film from the air electrode substrate. Referring to Fig. 9, in the example B, manganese was contained in an amount of about 14 atom% near a boundary between the solid electrolyte film and the air electrode (the opposing electrode). The boundary correspond to a position at which the distance shown in Fig 9 is 300 µm. The amount of manganese contained in the film was gradually decreased as the position at which the amount was measured approaches to a boundary between the film and the fuel electrode substrate. The boundary correspond to a position at which the distance shown in Fig. 9 is 100 µm. As understood from the above examples, it is possible to produce the solid electrolyte film with the sufficient air-tight performance by applying the powdery raw material having the average diameter of not more than 40 µm and by adding manganese etc. into the film in an amount of 1-15 atom%, when the temperature of the heat treatment is 1350-1500°C. Moreover, when carrying out the present invention in a practical producing system in a factory, the amount of the compound of the selected metal element and the average particle diameter of the powdery raw material of 8YSZ may be selected independently to each other. The temperature of the heat treatment may be selected to obtain the solid electrolyte film having the sufficient air-tight performance and is preferably lower. Such optimum temperature is changed when the above amount and the average particle diameter is changed. The temperature of the heat treatment may be selected independently to the above amount and the average particle diameter. Then, an optimum combination of the temperature, the above amount and the average particle diameter may be selected easily when carrying out the present invention in the practical producing system. As described above , according to the process for producing a solid electrolyte film and a solid oxide fuel cell of the invention, a solid electrolyte film with sufficient air-tight performance may be manufactured in a large scale by applying a spraying method suitable for mass production. Moreover , when the sprayed film is subjected to a heat treatment to progress the densification thereof, it is possible to progress the densification at a relatively low temperature and to produce a solid electrolyte film having the sufficient air-tight performance.

Claims: A process for producing a solid oxide fuel cell having a solid electrolyte portion (13A) as an ion conductor sandwiched between an air electrode (1) and a fuel electrode (4), the solid electrolyte portion (13A) being composed of a stabilized or partially stabilized zirconium oxide containing one or more additional metal elements selected from manganese, iron, cobalt, nickel, copper and zinc in an average amount of not less than 1 atom% and not more than 15 atom% based on the sum of the amounts of all metal elements in said solid electrolyte portion, comprising the steps of (i) forming an intermediate layer (2) of a compound of said additional metal element or elements on a first one (1) of said electrodes, (ii) forming a film (3A) of a material for said solid electrolyte portion on said intermediate layer (2), (iii) heat-treating the laminate so formed so that the additional metal element or elements in the intermediate layer (2) diffuse into the film (3A) whereby in the finished cell the amount of said additional metal element or elements in said solid electrolyte portion (13A) decreases in said solid electrolyte portion from the boundary between said solid electrolyte portion and said first electrode (1) to the boundary between said solid electrolyte portion and the other electrode (4), (iv) reducing the thickness of said solid electrolyte portion by processing by a machine or grinding, and (v) applying the other electrode (4) to the surface of said solid electrolyte portion which has been processed by a machine or ground. A process for producing a solid oxide fuel cell having a solid electrolyte portion as an ion conductor sandwiched between an air electrode (1) and a fuel electrode (4), the solid electrolyte portion (13B) being composed of a stabilized or partially stabilized zirconium oxide containing one or more additional metal element selected from manganese, iron, cobalt, nickel, copper and zinc in an average amount of not less than 1 atom% and not more than 15 atom% based on the sum of the amounts of all metal elements in said solid electrolyte portion, comprising the steps of (i) forming an intermediate layer (2) of a compound of said additional metal element or elements on a first one (1) of said electrodes, (ii) forming a film (3A) of a material for said solid electrolyte portion on said intermediate layer (2), (iii) reducing the thickness of said film (3A) by grinding, (iv) forming a further layer (12) of a compound of said additional metal element or elements on said film (3A), (v) heat-treating the laminate so formed so that the additional metal element or elements in the intermediate and further layers (2,12) diffuse into the film (3A), whereby, in the finished cell, in the solid electrolyte portion (13B) the amount of said additional metal element or elements is larger in a region near the boundary between the solid electrolyte portion and the said first electrode than in a central region thereof and is larger in a region near the boundary between the solid electrolyte portion and the other electrode than in said central region thereof, and (vi) applying the other electrode (4) to the surface of said solid electrolyte portion which has been ground. A process as claimed in claim 2, wherein the smaller of the respective amounts of the additional metal element or elements in the said regions near the boundaries between the solid electrolyte portion and the air electrode and the fuel electrode is greater than said amount of the additional metal element in the central region by a factor of not less than 1.3. A process according to claim 1, 2 or 3 wherein said film (3A) for said solid electrolyte portion is formed by a spraying method as an ion conductor, and said solid electrolyte portion has a thickness of not less than 40µm and not more than 100µm and a leakage amount of N 2 gas of said solid electrolyte portion at room temperature of not more than 10 -5 ml/g·second. A process as claimed in claim 4, wherein the permeation coefficient of said solid electrolyte portion is not more than 10 -7 cm 4 /g·second at room temperature.

Inventors: ITO SHIGENORI, KAWASAKI SHINJI, YOSHIOKA KATSUKI, ITO, SHIGENORI, KAWASAKI, SHINJI, YOSHIOKA, KATSUKI, KAWASAKI, SHINJI, A-305 NGK HORAGAI APT. HOUSE
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1993
Main classification: H01M8
IPC codes: H01M8, C23C4