SINGLE-PHASE PEROVSKITE-BASED SOLID ELECTROLYTE, SOLID OXIDE FUEL CELL COMPRISING SAME, AND METHOD FOR MANUFACTURING SAME

Abstract

This invention relates to a single-phase perovskite-based solid electrolyte, a solid oxide fuel cell including the same, and a method of manufacturing the same. The method of the invention includes stirring and pulverizing a mixed oxide including lanthanum oxide (La.sub.2O.sub.3), strontium carbonate (SrCO.sub.3), gallium oxide (Ga.sub.2O.sub.3) and magnesium oxide (MgO); and obtaining an LSGM powder by subjecting the pulverized mixed oxide to primary calcination at a first temperature and then secondary calcination at a second temperature that is higher than the first temperature.

Claims

1. A method of manufacturing a single-phase perovskite-based solid electrolyte, comprising: stirring and pulverizing a mixed oxide comprising lanthanum oxide (La.sub.2O.sub.3), strontium carbonate (SrCO.sub.3), gallium oxide (Ga.sub.2O.sub.3) and magnesium oxide (MgO); and obtaining an LSGM powder by subjecting the pulverized mixed oxide to primary calcination at a first temperature and then to secondary calcination at a second temperature that is higher than the first temperature.

2. The method of claim 1, wherein the LSGM powder has a composition of La.sub.0.8Sr.sub.0.2Ga.sub.0.8Mg.sub.0.2O.sub.3- (00.2).

3. The method of claim 1, wherein the lanthanum oxide (La.sub.2O.sub.3) has a purity of 99.99% or more, the strontium carbonate (SrCO.sub.3) has a purity of 99.7% or more, the gallium oxide (Ga.sub.2O.sub.3) has a purity of 99.0% or more, and the magnesium oxide (MgO) has a purity of 99.0% or more.

4. The method of claim 1, wherein the mixed oxide comprises 100 parts by weight of the lanthanum oxide (La.sub.2O.sub.3), 15 to 30 parts by weight of the strontium carbonate (SrCO.sub.3), 50 to 65 parts by weight of the gallium oxide (Ga.sub.2O.sub.3), and 3 to 9 parts by weight of the magnesium oxide (MgO), which are mixed together.

5. The method of claim 1, wherein the stirring and pulverizing the mixed oxide further comprises: subjecting the mixed oxide to planetary ball milling in a zirconia container containing zircon balls and then to pulverization using a mortar and pestle.

6. The method of claim 1, further comprising subjecting the mixed oxide to planetary ball milling and then to pulverization using a mortar and pestle, after the primary calcination and before the secondary calcination.

7. The method of claim 1, further comprising subjecting the mixed oxide to planetary ball milling and then to pulverization using a mortar and pestle, after the secondary calcination.

8. The method of claim 1, wherein the first temperature ranges from 900 C. to 1,200 C. and the second temperature ranges from 1,400 C. to 1,600 C.

9. The method of claim 1, wherein the lanthanum oxide (La.sub.2O.sub.3) is thermally treated at 800 C. to 1,300 C. and maintained in an atmosphere that blocks a reaction with water in order to prevent conversion into La(OH).sub.3.

10. A method of manufacturing a solid oxide fuel cell, comprising: preparing an anode diffusion layer slurry and an anode reaction layer slurry using NiO, GDC (Gadolinia-Doped Ceria) and a carbon material; preparing a buffer layer slurry using LDC (Lanthanum-Doped Ceria); preparing an electrolyte layer slurry using an LSGM powder obtained by the method of claim 1; subjecting the anode diffusion layer slurry, the anode reaction layer slurry, the buffer layer slurry and the electrolyte layer slurry to tape casting to form respective films, which are then sequentially stacked, thus obtaining an anode-supported electrolyte assembly; manufacturing an anode-supported electrolyte-sintered assembly by subjecting the anode-supported electrolyte assembly to primary calcination at a first temperature and then to secondary calcination at a second temperature higher than the first temperature; and applying a cathode slurry comprising LSCF (Lanthanum-Strontium-Cobalt-Ferrite Oxide) and the LSGM powder on the anode-supported electrolyte-sintered assembly and then performing sintering.

11. The method of claim 10, wherein the preparing the anode diffusion layer slurry and the anode reaction layer slurry further comprises: mixing zircon balls, NiO, GDC, the carbon material, toluene, ethanol, and a dispersant in a container, thus obtaining a mixed solution; and mixing the mixed solution with a binder solution.

12. The method of claim 11, wherein the anode diffusion layer slurry comprises 100 parts by weight of NiO, 62 to 72 parts by weight of GDC, 10 to 47 parts by weight of the carbon material, 75 to 110 parts by weight of toluene, 50 to 70 parts by weight of ethanol, 3 to parts by weight of the dispersant, and 75 to 95 parts by weight of the binder solution.

13. The method of claim 11, wherein the anode reaction layer slurry comprises 100 parts by weight of NiO, 62 to 72 parts by weight of GDC, 0 to 30 parts by weight of the carbon material, 70 to 90 parts by weight of toluene, 45 to 65 parts by weight of ethanol, 2 to 6 parts by weight of the dispersant, and 60 to 95 parts by weight of the binder solution.

14. The method of claim 10, wherein the preparing the buffer layer slurry further comprises: providing LDC, toluene, ethanol, a dispersant and a binder solution so as to comprise 100 parts by weight of LDC, 75 to 85 parts by weight of toluene, 15 to 25 parts by weight of ethanol, 0.5 to 1.5 parts by weight of the dispersant, and 45 to 55 parts by weight of the binder solution, and mixing zircon balls, LDC, toluene, ethanol and the dispersant in a container, thus obtaining a mixed solution; and mixing the mixed solution with the binder solution.

15. The method of claim 10, wherein the preparing the electrolyte layer slurry further comprises: providing the LSGM powder, toluene, ethanol, a dispersant and a binder solution so as to comprise 100 parts by weight of the LSGM powder, 75 to 85 parts by weight of toluene, 15 to 25 parts by weight of ethanol, 0.5 to 1.5 parts by weight of the dispersant, and 45 to 55 parts by weight of the binder solution and mixing zircon balls, the LSGM powder, toluene, ethanol and the dispersant in a container, thus obtaining a mixed solution; and mixing the mixed solution with the binder solution.

16. The method of claim 10, wherein the cathode slurry comprises 100 parts by weight of LSCF, 95 to 105 parts by weight of LSGM, 76 to 90 parts by weight of terpineol, and 3 to 15 parts by weight of ethylene cellulose.

17. A solid oxide fuel cell, comprising: an anode diffusion layer comprising NiO, GDC (Gadolinia-Doped Ceria) and a carbon material; an anode reaction layer formed on the anode diffusion layer and comprising NiO, GDC and the carbon material; a buffer layer formed on the anode reaction layer and comprising LDC (Lanthanum-Doped Ceria); an electrolyte layer formed on the buffer layer and comprising an LSGM powder obtained by the method of claim 1; and a cathode formed on the electrolyte layer and comprising LSCF (Lanthanum-Strontium-Cobalt-Ferrite Oxide) and the LSGM powder.

Description

DESCRIPTION OF DRAWINGS

[0033] FIG. 1 shows a process of manufacturing a single-phase perovskite-based solid electrolyte according to an embodiment of the present invention;

[0034] FIG. 2 is a graph showing thermal properties of an LSGM powder after ball milling and immediately before primary calcination;

[0035] FIG. 3 is a graph showing XRD properties of the powder thermally treated at intervals of 100 C. in the temperature range of 500 to 1,500 C.;

[0036] FIG. 4 is a graph showing the lattice constant and the crystal size as results of Rietveld analysis of the powder thermally treated at intervals of 100 C. in the temperature range of 500 to 1,500 C.;

[0037] FIG. 5 is a graph showing XRD properties of the LSGM powder depending on the first calcination temperature and the second calcination temperature;

[0038] FIG. 6 shows SEMs of the LSGM powder after secondary calcination;

[0039] FIG. 7 is a graph showing the results of measurement of ionic conductivity of the LSGM powder;

[0040] FIG. 8 shows the configuration of an SOFC according to an embodiment of the present invention;

[0041] FIG. 9 shows films obtained through tape casting using the LSGM powder;

[0042] FIG. 10 shows the cross-section of an SOFC cell using the LSGM powder;

[0043] FIG. 11 is a graph showing the power output characteristics of the SOFC cell using the LSGM powder; and

[0044] FIG. 12 is a graph showing the impedance characteristics of the SOFC using the LSGM powder.

BEST MODE

[0045] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings so that the present invention is easily embodied by those skilled in the art to which the present invention belongs. Further, when any portion includes any component, this means that the portion does not exclude other components, but may further include other components unless otherwise stated.

[0046] FIG. 1 shows the process of manufacturing a single-phase perovskite-based solid electrolyte according to an embodiment of the present invention.

[0047] As shown in FIG. 1, the method of manufacturing the single-phase perovskite-based solid electrolyte according to an embodiment of the present invention includes mixing lanthanum oxide (La.sub.2O.sub.3), strontium carbonate (SrCO.sub.3), gallium oxide (Ga.sub.2O.sub.3) and magnesium oxide (MgO), stirring and pulverizing the mixed oxide using a mechanical process such as planetary ball milling, performing primary calcination at about 900 C. to 1,200 C., planetary ball milling and pulverization, and performing secondary calcination at 1,500 C. to 1,600 C., planetary ball milling and pulverization, thus producing an LSGM powder.

[0048] Here, the LSGM powder has a composition of La.sub.0.8Sr.sub.0.2Ga.sub.0.8Mg.sub.0.2O.sub.3- (00.2).

[0049] The purity of lanthanum oxide (La.sub.2O.sub.3) is 99.99% or more, the purity of strontium carbonate (SrCO.sub.3) is 99.7% or more, the purity of gallium oxide (Ga.sub.2O.sub.3) is 99.0% or more, and the purity of magnesium oxide (MgO) is 99.0% or more, which is relatively low. Thus, the method of manufacturing the single-phase perovskite-based solid electrolyte according to the present invention is capable of synthesizing the single-phase perovskite-based solid electrolyte at low cost.

[0050] The mixed oxide is composed of lanthanum oxide (La.sub.2O.sub.3), strontium carbonate (SrCO.sub.3), gallium oxide (Ga.sub.2O.sub.3) and magnesium oxide (MgO), in which 100 parts by weight of lanthanum oxide (La.sub.2O.sub.3), 15 to 30 parts by weight and preferably 23 parts by weight of strontium carbonate (SrCO.sub.3), 50 to 65 parts by weight and preferably 58 parts by weight of gallium oxide (Ga.sub.2O.sub.3), and 3 to 9 parts by weight and preferably 6 parts by weight of magnesium oxide (MgO) are mixed together.

[0051] When lanthanum oxide (La.sub.2O.sub.3) having a purity of 99.99% or more is stored in air, it may be converted into La(OH).sub.3, and hence, it must be sufficiently thermally treated at about 800 C. to 1,300 C. for a period of time ranging from ones of min to tens of hour immediately before being used, and has to be maintained in an atmosphere that blocks the reaction with water (moisture).

[0052] As mentioned above, the calcination process is performed through two steps in the present invention. The reason is as follows. When the LSGM powder is prepared through a single calcination process, particles are non-uniformly formed, making it impossible to control the particle shape and the crystal size. According to the present invention, however, when intermediate pyrolysis such as primary calcination and milling are performed, it is easy to control the particle shape and the crystal size of the final LSGM powder. Hence, the two-step calcination process is carried out in the present invention.

[0053] Also, secondary calcination is able to obtain a single-phase cubic LSGM powder having an appropriate crystal size, superior ionic conductivity, and few impurity peaks, and is preferably performed at a temperature ranging from 1,400 C. to 1,600 C. in order to reduce the processing cost upon preparation of the LSGM powder.

[0054] The process of manufacturing an electrolyte layer film by means of a tape-casting device using the LSGM powder obtained by the method of manufacturing the single-phase perovskite-based solid electrolyte is described below.

[0055] 100 parts by weight of the LSGM powder obtained by the method of manufacturing the single-phase perovskite-based solid electrolyte as above, 75 to 85 parts by weight and preferably 80 parts by weight of toluene, 15 to 25 parts by weight and preferably 20 parts by weight of ethanol, 0.5 to 1.5 parts by weight and preferably 1.0 part by weight of a dispersant, and 45 to 55 parts by weight and preferably 50 parts by weight of a binder solution are mixed, thus affording an electrolyte layer slurry. Here, the mixing sequence of the materials may vary as needed, but it is preferred that the LSGM powder, toluene, ethanol, and dispersant be mixed first for a predetermined period of time and that the binder solution then be further added and stirred.

[0056] The electrolyte layer slurry comprising the LSGM powder, toluene, ethanol, dispersant and binder solution thus mixed may be manufactured into a film having a thickness of 5 to 300 m at a rate of 0.3 to 1.2 m/min using a tape-casting device, and preferably a film having a thickness of 10 to 100 m so as to be suitable for desired performance of the SOFC.

[0057] Meanwhile, when the anode-supported electrolyte assembly (anode diffusion layer/anode reaction layer/buffer layer/electrolyte layer) of the SOFC cell is manufactured using a tape-casting device, each of the anode diffusion layer, the anode reaction layer, the buffer layer and the electrolyte layer is prepared in the form of a slurry and is then manufactured into a film using a tape-casting device.

[0058] Specifically, the anode diffusion layer slurry is composed of commercially available NiO (J. T. Backer) and GDC (Gadolinia-Doped Ceria, BET: 7.7 m.sup.2/g, Fuel Cell Materials), a carbon material (e.g. carbon black), toluene, ethanol, a dispersant and a binder solution, and is configured to include NiO in an amount of 100 parts by weight, GDC in an amount of 62 to 72 parts by weight and preferably 67 parts by weight, carbon black in an amount of 10 to 47 parts by weight and preferably 42 parts by weight, toluene in an amount of 75 to 110 parts by weight and preferably 96 parts by weight, ethanol in an amount of 50 to 70 parts by weight and preferably 64 parts by weight, the dispersant in an amount of 3 to 5 parts by weight and preferably 4 parts by weight, and the binder solution in an amount of 75 to 95 parts by weight and preferably 90 parts by weight.

[0059] Also, the anode reaction layer slurry is composed of commercially available NiO (J. T. Backer) and GDC (BET: 7.7 m.sup.2/g, Fuel Cell Materials), a carbon material (e.g. carbon black), toluene, ethanol, a dispersant and a binder solution, and is configured to include NiO in an amount of 100 parts by weight, GDC in an amount of 62 to 72 parts by weight and preferably 67 parts by weight, carbon black in an amount of 0 to 30 parts by weight and preferably 19 parts by weight, toluene in an amount of 70 to 90 parts by weight and preferably 85 parts by weight, ethanol in an amount of 45 to 65 parts by weight and preferably 57 parts by weight, the dispersant in an amount of 2 to 6 parts by weight and preferably 4 parts by weight, and the binder solution in an amount of 60 to 95 parts by weight and preferably 82 parts by weight.

[0060] Also, the buffer layer slurry is composed of LDC (Lanthanum-Doped Ceria, BET: 10 m.sup.2/g, Kceracell), toluene, ethanol, a dispersant and a binder solution, and is configured to include LDC in an amount of 100 parts by weight, toluene in an amount of 75 to 85 parts by weight and preferably 80 parts by weight, ethanol in an amount of 15 to 25 parts by weight and preferably 20 parts by weight, the dispersant in an amount of 0.5 to 1.5 parts by weight and preferably 1 part by weight, and the binder solution in an amount of 45 to 55 parts by weight and preferably 50 parts by weight.

[0061] Each of the anode diffusion layer slurry, the anode reaction layer slurry and the buffer layer slurry is manufactured into a film having a thickness of 5 to 300 m at a rate of 0.3 to 1.2 m/min using a tape-casting device, and each of the anode diffusion layer and the anode reaction layer is preferably formed into a film having a thickness of 30 to 60 m, and the LDC film is preferably formed to a thickness of 5 to 20 m.

[0062] After the formation of respective films of the anode diffusion layer, the anode reaction layer, the buffer layer and the electrolyte layer, the anode reaction layer, buffer layer and electrolyte layer films are sequentially stacked on the anode diffusion layer and then lamination is performed at a predetermined temperature (e.g. 70 C.) under predetermined pressure (e.g. 60 MPa) for tens of min. Here, it is possible to manufacture a high-quality cell without cracking and cleaving by elevating the temperature to 1,000 C. for pre-sintering.

[0063] The heating rate in the process of elevating the temperature is set to 1 C./min, and is maintained for ones of hour at each of 150 C., 300 C., 600 C., and 900 C., is finally maintained at 1,000 C. for ones of hour, and then naturally allowed to cool again to room temperature. After the pre-sintering process, the SOFC cell is maintained at a heating rate of 1 C./min, maintained at 1,300 C. to 1,500 C. for ones of hour, and then naturally allowed to cool to room temperature, thereby completing an anode-supported electrolyte assembly.

[0064] The cathode slurry is composed of commercially available LSCF (Lanthanum-Strontium-Cobalt-Ferrite Oxide), terpineol, ethylene cellulose and the LSGM powder, and is configured to include LSCF in an amount of 100 parts by weight, LSGM in an amount of 95 to 105 parts by weight and preferably 100 parts by weight, terpineol in an amount of 76 to 90 parts by weight and preferably 81 parts by weight, and ethylene cellulose in an amount of 3 to 15 parts by weight and preferably 9 parts by weight. The cathode slurry thus obtained is sufficiently dispersed using a 3-roll mill, applied to a thickness of to 60 m on the calcined electrolyte using a screen printer, and sintered at a temperature of 1,000 C. to 1,200 C. for ones of hour, thereby manufacturing an SOFC unit cell.

MODE FOR INVENTION

Example 1

[0065] Synthesis of Single-Phase LSGM Powder

[0066] As starting materials, lanthanum oxide (La.sub.2O.sub.3, Grand Chemical & Material CO., LTD, 99.99%, FW: 325.84), strontium carbonate (SrCO.sub.3, Grand Chemical & Material CO., LTD, 99.7%, FW: 147.78), gallium oxide (Ga.sub.2O.sub.3, MINING & CHEMICAL PRODUCTS, LTD, 99.00%, FW: 189.34), and magnesium oxide (MgO, KANTO CHEMICAL CO., INC, 99.00%, FW: 40.71) were provided and mixed at a weight ratio (wt %) of La.sub.2O.sub.3 to SrCO.sub.3 to Ga.sub.2O.sub.3 to MgO of 54:12:31:3.

[0067] The mixture comprising lanthanum oxide, strontium carbonate, gallium oxide and magnesium oxide were placed in a 500 ml zirconia container together with 50 zircon balls having a size of 10 mm, and subjected to planetary ball milling (FRITCH, Pulverisette, Germany) at 400 rpm for 30 min and then to primary pulverization using a mortar and pestle for 20 min.

[0068] The powder thus pulverized was subjected to primary calcination comprising elevating the temperature to 1,100 C. at a heating rate of 5 C./min and maintaining the elevated temperature for 10 hrs, after which planetary ball milling was performed for 5 min and then secondary pulverization was carried out for 20 min using a mortar and pestle.

[0069] The secondarily pulverized powder was subjected to secondary calcination comprising elevating the temperature to 1,500 C. at a heating rate of 5 C./min and maintaining the elevated temperature for 10 h, after which planetary ball milling was performed for 5 min and then tertiary pulverization was conducted for 20 min using a mortar and pestle. Thereby, the LSGM powder of the present invention was obtained.

[0070] During the primary and secondary calcinations, it is preferred that the structure and shape of the container be taken into consideration so as to realize efficient contact of particles of the powder to increase the reactivity based on solid reaction.

[0071] FIG. 2 is a graph showing the thermal properties of the mixed powder after ball milling and immediately before primary calcination in the above LSGM manufacturing process. As shown in FIG. 2, the weight was remarkably reduced by about 7 wt % through solid reaction up to 820 C., and slightly reduced at a temperature higher than that. Hence, this material can be found to be crystallized at 800 C. or more.

[0072] Based on the results of measurement of XRD of the powder thermally treated at intervals of 100 C. in the temperature range from 500 C. to 1,500 C., as shown in FIG. 3, many impurity peaks were observed up to 1,400 C. but a single-phase cubic LSGM powder having few impurity peaks was formed at 1,500 C.

[0073] As mentioned above, when the LSGM powder is prepared through single thermal treatment (i.e. calcination), the single-phase LSGM powder may be obtained, but the particles are non-uniform, making it difficult to control the particle shape and the crystal size. In the present invention, the LSGM powder was manufactured through two calcinations.

[0074] As shown in FIG. 4, based on the results of Rietveld analysis, the lattice constant was maintained close to the level of about 3.091 up to the calcination temperature of 1,200 C., drastically increased from 1,300 C., and raised to 3.914 at 1,500 C., and the crystal size was about 45 to 50 nm up to 1,300 C. and increased to the level of 70 to 100 nm at 1,400 to 1,500 C.

[0075] As for the XRD properties of the LSGM powder depending on the primary calcination temperature (1,100 C.) and the secondary calcination temperature (1,400 C. and 1,500 C.), as shown in FIG. 5, a large number of peaks of impurities such as LaSrGaO.sub.4 or LaSrGa.sub.3O.sub.7 (i.e. secondary phase) were observed in the primarily calcined powder, but the LaSrGa.sub.3O.sub.7 impurity peak disappeared and only the LaSrGaO.sub.4 impurity peak was detected at low intensity after the secondary calcination at 1,400 C., and the powder secondarily calcined at 1,500 C. was provided in the form of a single phase having no impurities.

[0076] As shown in FIG. 6, after the secondary calcination at 1,500 C., the secondary particles were pulverized to about 5 m, and the secondary particles were configured such that the primary particles having a size of 50 to 100 nm were very strongly aggregated and sintered. Here, the specific surface area (BET) of the particles was measured to be about 1.42 m.sup.2/g, and was thus suitable for use in manufacturing a tape-casting film.

[0077] Meanwhile, in order to measure the ionic conductivity of the LSGM (La.sub.0.8Sr.sub.0.2Ga.sub.0.8Mg.sub.0.2O.sub.3-) electrolyte powder thus obtained, a test sample was manufactured through uniaxial pressing. Specifically, the LSGM powder was placed in a cylindrical mold, pressed at a pressure of 60 MPa for 1 hr, heated to 1,500 C. from room temperature at a rate of 5 C./min, and maintained at the elevated temperature for 10 hrs, thus manufacturing a test sample.

[0078] The test sample thus manufactured was mounted into a high-temperature cell (GEFRAN 800P, USA) for measuring ionic conductivity, and connected to an impedance analyzer (Frequency response analyzer, Solatron, solatronl260, USA), after which a resistance value was measured in the temperature range of 500 to 900 C. under conditions of a frequency of 500 kHz-0.1 Hz at an amplitude of 50 mV, thus determining ionic conductivity. As shown in FIG. 7, the ionic conductivity was proportional to an increase or decrease in the temperature, and high ionic conductivity, for example, 0.07 S/cm at 700 C., 0.11 S/cm at 750 C., and 0.16 S/cm at 800 C., was exhibited, and thus there was no great difference in ionic conductivity depending on an increase or decrease in the temperature. This result represented an increase of about 50 to 100% compared to conventional YSZ electrolyte materials or ScSZ-based materials.

Example 2

[0079] SOFC Cell Using LSGM Powder and Manufacture Thereof

[0080] As shown in FIG. 8, an SOFC cell manufactured using the LSGM powder obtained through the aforementioned powder synthesis process is configured such that an anode reaction layer comprising a mixture of NiO, GDC and a carbon material is stacked on an anode diffusion layer comprising a mixture of NiO, GDC and a carbon material (i.e. carbon black), and a buffer layer comprising LDC is stacked on the anode reaction layer. Furthermore, an electrolyte layer is stacked on the buffer layer using the LSGM powder of the present invention, and a cathode comprising a mixture of the LSGM powder of the invention and LSCF is stacked on the electrolyte layer.

[0081] The method of manufacturing the SOFC cell having the above configuration according to an embodiment of the present invention is described below.

[0082] In order to manufacture an anode diffusion layer, an anode reaction layer, a buffer layer and an electrolyte layer into respective films using a tape-casting device (STC-14C, HANSUNG SYSTEM, Korea), an anode diffusion layer slurry, an anode reaction layer slurry, a buffer layer slurry and an electrolyte layer slurry are first prepared.

[0083] Specifically, the anode diffusion layer slurry was prepared in a manner in which 125 zircon balls having a size of 10 mm were placed in a 1 L container, NiO, GDC, and carbon black were added at a ratio of 21.6:14.4:9 wt % based on the total weight of the anode diffusion layer slurry, toluene, ethanol and a dispersant were added at a ratio of 20.7:13.9:0.9 wt % based on the total weight of the anode diffusion layer slurry, the added components were mixed for 24 husing a two-stage ball mill, and the resulting mixed solution was added with a binder solution in an amount of 19.5 wt % based on the total weight of the anode diffusion layer slurry, followed by additional mixing for 24 hr.

[0084] The anode reaction layer slurry was prepared in a manner in which 125 zircon balls having a size of 10 mm were placed in a 1 L container, NiO, GDC, and carbon black were added at a ratio of 24.3:16.2:4.5 wt % based on the total weight of the anode reaction layer slurry, toluene, ethanol and a dispersant were added at a ratio of 20.7:13.9:0.9 wt % based on the total weight of the anode reaction layer slurry, the added components were mixed for 24 h using a two-stage ball mill, and the resulting mixed solution was added with a binder solution in an amount of 19.5 wt % based on the total weight of the anode reaction layer slurry, followed by additional mixing for 24 hr.

[0085] The buffer layer slurry was prepared in a manner in which 94 zircon balls having a size of 10 mm were placed in a 500 ml container, LDC (BET: 10 m.sup.2/g, Kceracell) was added in an amount of 40 wt % based on the total weight of the buffer layer slurry, toluene, ethanol and a dispersant were added at a ratio of 31.8:7.98:0.36 wt % based on the total weight of the buffer layer slurry, the added components were mixed for 24 h using a two-stage ball mill, and the resulting mixed solution was added with a binder solution in an amount of 19.86 wt % based on the total weight of the buffer layer slurry, followed by additional mixing for 24 hr.

[0086] Finally, the electrolyte layer slurry was prepared in a manner in which 94 zircon balls having a size of 10 mm were placed in a 500 ml container, an LSGM powder was added in an amount of 40 wt % based on the total weight of the electrolyte layer slurry, toluene, ethanol and a dispersant were added at a ratio of 31.8:7.98:0.36 wt % based on the total weight of the electrolyte layer slurry, the added components were mixed for 24 h using a two-stage ball mill, and the resulting mixed solution was added with a binder solution in an amount of 19.86 wt % based on the total weight of the electrolyte layer slurry, followed by additional mixing for 24 hr.

[0087] After the preparation of the anode diffusion layer slurry, the anode reaction layer slurry, the buffer layer slurry and the electrolyte layer slurry as described above, an anode diffusion layer film, an anode reaction layer film, a buffer layer film and an electrolyte layer film are manufactured using a tape-casting device.

[0088] To obtain the anode diffusion layer film and the anode reaction layer film, a doctor blade of the tape-casting device was adjusted to a height of 230 m, and casting was performed at a rate of 0.12 m/min at 80 C. Accordingly, an anode diffusion layer film and an anode reaction layer film each having a thickness of about 45 m could be obtained.

[0089] To obtain the buffer layer film, the height of the doctor blade was adjusted to about 100 m, and casting was performed at a rate of 0.12 m/min at 80 C. Accordingly, a buffer layer film having a thickness of about 10 m could be obtained.

[0090] To obtain the electrolyte layer film, the height of the doctor blade was adjusted to about 250 m, and casting was performed at a rate of 0.12 m/min at 80 C. Accordingly, an electrolyte layer film having a thickness of 20 to 22 m could be obtained.

[0091] When the tape-casting device is used in this way, the anode diffusion layer, the anode reaction layer, the buffer layer and the electrolyte layer may be manufactured in the form of a thin film having a thickness of 10 to 100 m, as shown in FIG. 9.

[0092] After the completion of the formation of the anode diffusion layer film, the anode reaction layer film, the buffer layer film and the electrolyte layer film, 40 to 60 anode diffusion layer films were stacked to a thickness of about 1 to 1.5 mm, and a single anode reaction layer film and a single buffer layer film were sequentially placed on the uppermost anode diffusion layer film.

[0093] On a green sheet comprising the anode diffusion layers, the anode reaction layer and the buffer layer, four LSGM electrolyte films were stacked, thus completing an anode-supported electrolyte assembly, after which the green assembly was subjected to lamination at a temperature of 70 C. and a pressure of 60 MPa for about 20 min and then molded using a cylindrical mold having a diameter of 2.5 cm, thereby fabricating an anode-supported electrolyte-integrated film.

[0094] The anode-supported electrolyte-integrated film thus fabricated was placed on an alumina plate having controlled reactivity, transferred into a furnace having an appropriate size, and then primarily calcined by elevating the temperature from room temperature to 1,000 C. The temperature was elevated in a manner in which the temperature was maintained at 150 C. for 2 h, at 300 C. for 2 h, at 600 C. for 2 h, at 900 C. for 2 h, and at 1,000 C. for 3 hr. After the primary calcination process, a secondary calcination process was performed by elevating the temperature from room temperature to 1,400 C. and then maintaining the elevated temperature for 3 h, ultimately manufacturing an anode-supported electrolyte-sintered assembly through simultaneous sintering. Here, the heating rate necessary to reach individual sintering temperatures may be set to either 0.5 C./min or 1.0 C./min.

[0095] After the completion of the manufacture of the anode-supported electrolyte-sintered assembly in this way, a cathode was stacked on the anode-supported electrolyte-sintered assembly.

[0096] The cathode was first prepared in the form of a slurry. Specifically, an LSGM powder, LSCF, terpineol and ethyl cellulose were placed in a beaker at a ratio of 35:35:28.2:1.8 wt % based on the total weight of the cathode slurry, mixed at room temperature for 24 h using a stirrer, and then further mixed three to four times using a 3-roll mill (EXAKT, Germany), thus preparing a cathode slurry having high viscosity.

[0097] After the preparation of the cathode slurry in this way, the anode-supported electrolyte-sintered assembly was fixed to a screen printer (HSP-2C, HANSUNG SYSTEM, Korea), the cathode slurry having high viscosity was applied to a thickness of 40 to 50 m on the screen printer having a predetermined size, and then the unit cell coated with the cathode slurry was sintered at a temperature of 1,100 C. for 3 h, thus manufacturing an SOFC. Here, the heating rate during the sintering of the cathode was maintained at 5.0 C./min.

[0098] As shown in FIG. 10, the SOFC thus manufactured was configured such that four film layers (i.e. the anode diffusion layer, the anode reaction layer, the buffer layer, and the electrolyte layer) formed through tape casting were very uniformly and efficiently adhered, and the thickness of each layer was uniformly maintained. Furthermore, the electrolyte layer and the cathode layer were in good contact with each other.

[0099] As for the current-voltage characteristics of the SOFC manufactured according to the present invention, as shown in FIG. 11, the open-circuit voltage was about 0.83 V, and was almost the same even at different operating temperatures, and power output performance was increased with an increase in the operating temperature. Specifically, the maximum power output at 700 C. was about 0.65 W/cm.sup.2. When the current density was 2.0 A/cm.sup.2 at operating temperatures of 750 C. and 800 C., respective power output characteristics of 1.0 W/cm.sup.2 and 1.2 W/cm.sup.2 were exhibited.

[0100] Therefore, it can be confirmed that LSGM having high ionic conductivity is applied to a solid electrolyte, which is then manufactured into a unit cell through tape casting, thereby obtaining an SOFC having excellent power output characteristics.

[0101] Also, the impedance of the SOFC of the present invention was measured in the open-circuit voltage state depending on the operating temperature. As shown in FIG. 12, the ohmic resistance of the solid electrolyte and the polarization resistance of the electrode were decreased with an increase in the operating temperature. In particular, the ohmic resistance and the polarization resistance were very low, to levels of 0.08 .Math.cm and 0.07 .Math.cm respectively, at an operating temperature of 800 C. However, when the operating temperature was increased to 750 C. and 800 C., the ohmic resistance values were 0.12 .Math.cm and 0.79 .Math.cm, respectively, and the polarization resistance values were 0.11 .Math.cm and 0.19 .Math.cm, respectively, which are evaluated to be proportionally increased. Such ohmic resistance and polarization resistance results were superior to those of conventional solid electrolytes. Therefore, the SOFC of the present invention can be found to exhibit excellent power output characteristics even at low open-circuit voltage.

[0102] In the method of manufacturing the single-phase perovskite-based solid electrolyte according to an embodiment of the present invention, lanthanum oxide, strontium carbonate, gallium oxide and magnesium oxide are mixed, and the resulting mixture is primarily calcined at a first temperature and pulverized and then secondarily calcined at a second temperature higher than the first temperature and pulverized, thereby enabling the formation of a single-phase cubic LSGM powder having few impurity peaks and high ionic conductivity.

[0103] Also, according to the present invention, the anode diffusion layer, the anode reaction layer, the buffer layer and the electrolyte layer are provided in the form of a film, and respective films are stacked, thus manufacturing an SOFC. Here, an additional sintering process is obviated during the stacking of the anode diffusion layer, the anode reaction layer, the buffer layer and the electrolyte layer, thus reducing the processing cost required for the sintering process and obtaining high ionic conductivity and power output characteristics thanks to the use of single-phase LSGM powder having low resistance.

[0104] While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit of the invention. Therefore, the scope of the present invention should not be limited to the embodiments described, but should be defined by the following claims as well as equivalents thereof.

INDUSTRIAL APPLICABILITY

[0105] As described hereinbefore, according to the present invention, a single-phase cubic LSGM powder having few impurity peaks and high ionic conductivity can be obtained in a manner in which lanthanum oxide, strontium carbonate, gallium oxide and magnesium oxide are mixed, and the resulting mixture is primarily calcined at a first temperature and pulverized and then secondarily calcined at a second temperature, higher than the first temperature, and pulverized.

[0106] Also, according to the present invention, an SOFC can be manufactured in a manner in which an anode diffusion layer, an anode reaction layer, a buffer layer and an electrolyte layer are provided in the form of a film and respective films are then stacked. Here, an additional sintering process is obviated during the stacking of the anode diffusion layer, the anode reaction layer, the buffer layer and the electrolyte layer, thus reducing the processing cost required for the sintering process and attaining superior ionic conductivity and power output characteristics by the use of the single-phase LSGM powder having low resistance.