KR100269757B1 - Method for preparing composite electrolyte for low temperature solid oxide fuel cells usong sol-gel technique - Google Patents

Abstract

PURPOSE: A method for preparing composite electrolyte such as YSZ/YDC polymerized sol is provided for using in the production of low-temperature solid oxide fuel battery by partial hydrolysis of zirconium alkoxide and adding yttrium nitrate in isopropanol solvent. CONSTITUTION: The method for preparing composite electrolyte of YSZ/YDC, YSZ/SDC and YSZ/GDC comprises repeatedly coating YSZ(Yttria stabilized zirconia) polymeric sol at a rate of 0.50-20mm/sec for 5-20 times onto YDC(yttria doped ceria), SDC(samaria doped ceria) and GDC(gadolinia doped ceria) substrate plate and heating the coated material. The YSZ is produced by adding to zirconium alkoxide with acetic acid as complexing agent; introducing the catalyst solution composed of water, nitric acid and isopropanol to partially hydrolyze the zirconium alkoxide; then adding to the resulting material the yttrium nitrate solution in isopropanol.

Description

Method for producing composite electrolyte for low temperature solid oxide fuel cell using sol-gel method

The present invention relates to a method for producing a composite electrolyte for use in a low temperature solid oxide fuel cell (SOFC) operable at 800 ° C.

More specifically, the present invention adds acetic acid as a complexing agent to zirconium alkoxide, and then administers a catalyst solution containing water, nitric acid and isopropanol to partially hydrolyze the zirconium alkoxide and dissolve it in isopropanol. It relates to a method for producing a Yttria Stabilized Zirconia (hereinafter referred to as YSZ) polymerization sol comprising adding a. In addition, the YSZ polymerized sol thus prepared was plated or cylindrically doped with Yttria Doped Ceria (YDC), Samaria Doped Ceria (SDC), and doped with Gadolinia. Method for producing a flat or cylindrical YSZ / YDC, YSZ / SDC and YSZ / GDC composite electrolyte comprising coating a ceria (Gadolinia Doped Ceria, GDC) substrate by spin coating or dip coating It is about.

In general, SOFCs operate at temperatures above 1000 ° C because Yttria Stabilized Zirconia (YSZ), which has a relatively low oxygen ion conductivity, is used as an electrolyte, and at such high temperatures, chemical reactions between components proceed rapidly. This causes a problem of shortening the life of the battery.

If the operating temperature of the SOFC can be lowered to about 800 ° C., the reactivity between components is reduced, thereby prolonging battery life, as well as facilitating gas sealing and increasing the choice of interconnect materials.

In order to reduce the operating temperature of SOFC, the thickness of YSZ electrolyte or the high oxygen ion conductivity should be used as electrolyte. Ceria is the most powerful new electrolyte material suitable for low temperature SOFC.

However, ceria has the disadvantage of being reduced in a high temperature reducing atmosphere. A method for using a reducing material such as ceria as an SOFC electrolyte is to coat YSZ, which is known as a pure oxygen ion conductor, on the anode side.

In other words, in order to use doped Ceria having excellent ion conductivity as an electrolyte for low-temperature SOFC, in order to suppress the reduction of cerium in a high temperature reducing atmosphere, the oxygen partial pressure (~ 10 ^-20 atm) toward the anode YSZ coated composite electrolyte should be formed (AV Virkar, "Theoretical Analysis of Solid Oxide Fuel Cells with Two-Layer, Composite Electrolyte: Electrolyte Stability", J. Electrochem. Soc., 138, 1481-1487 (1991) )] Reference).

Methods of synthesizing ceramic thin films such as YSZ include sputtering methods (H. Yahiro, Y. Baba, K. Eguchi, and H. Arai, "High Temperature Fuel Cell with Ceria-Yttria Solid Electrolyte", J. Electrochem. Soc., 135, 2077-2080 (1988)), ion-plating method (K. Eguchi, T. Setoguchi, T. Inoue, and H. Arai, "Electrical Properties of Ceria- Based Oxides and Their Application to Solid Oxide Fuel Cells ", Solid State Ionics, 52, 165-172 (1992)), CVD / EVD methods (K. Metha, SJ Hong, JF Jue, and AV Virkar," Fabrication and Characterization of YSZ-Coated Ceria Electrolytes "; pp. 92-103. In Proceedings of the 3rd International Symposium on Solid Oxide Fuel Cells (Honolulu, HI, July, 1993). Edited by SC Singhal and H. Iwahara. Electrochemical Society Pennington, NJ, 1997), and liquid phase methods using sol, slip, or slurry.

Up to now, gas phase methods such as RF-stering method and EVD method have been mainly used for the preparation of the composite electrolyte (H. Yahiro, Y. Baba, K. Eguchi, and H. Arai, "High Temperature Fuel Cell with Ceria -Yttria Solid Electrolyte ", J. Electrochem. Soc., 135, 2077-2080 (1988)], K. Eguchi, T. Setoguchi, T. Inoue, and H. Arai," Electrical Properties of Ceria-Based Oxides and Their Application to Solid Oxide Fuel Cells ", Solid State Ionics, 52, 165-172 (1992), and K. Metha, SJ Hong, JF Jue, and AV Virkar," Fabrication and Characterization of YSZ-Coated Ceria Electrolytes " in pp. 92-103. in Proceedings of the 3rd International Symposium on Solid Oxide Fuel Cells (Honolulu, HI, July, 1993) .Edited by SC Singhal and H. Iwahara.Electrochemical Society.Pennington, NJ, 1997).

However, the gas phase method requires expensive equipment and starting materials, is not only limited by the shape and size of the substrate, but also forms a thin film of YSZ at a temperature lower than the operating temperature. Due to the YSZ thin film is subjected to tensile stress and cracks are not suitable for the production of SOFC electrolyte.

In addition, for the production of SOFC electrolyte, it is possible to coat a uniform thin film layer not only on the plate type but also on the cylindrical substrate, and to compress compressive stress in the thin film during the cooling process by heat treating the thin film at a temperature higher than the operating temperature regardless of the substrate size. Coating processes are required, and liquid phase methods, especially sol-gel methods, can easily solve these requirements (S. Sakka, "Sol-Gel Fibers and Coating Films"; pp. 346-374 in Sol-Gel Science and Technology.Edited by MA Aegerter, M. Jafelicci, Jr., DF Souza, and ED Zanotto, World Scientific, Singapore, 1989).

However, in spite of these advantages, there are no examples of producing SOFC components or synthesizing YSZ thin films using the sol-gel method.

Therefore, in the present invention, it is easy to control the microstructure without requiring expensive equipment or starting materials, and is relatively limited in the form and size of the substrate, and can be a low temperature process and can produce a high purity thin film of various compositions. Using the gel method, a process for preparing a composite electrolyte coated with YSZ on a ceria-based substrate was established.

Therefore, the present invention relates to a method for producing a composite electrolyte coated with YSZ on a ceria substrate doped using the sol-gel method.

More specifically, the present invention includes adding acetic acid, a complexing agent, to a zirconium alkoxide, followed by administering a catalyst solution containing water, nitric acid and isopropanol to partially hydrolyze the zirconium alkoxide, and to add yttrium nitrate dissolved in isopropanol. Method for producing an YSZ polymerization sol, and comprising the heat treatment after coating the YSZ polymerization sol thus prepared by using a spin coating or immersion coating method on a flat or cylindrical YDC, SDC and GDC substrate, YSZ / YDC, YSZ / SDC and YSZ / GDC composite electrolyte of the present invention.

1 is a process chart of the composite electrolyte manufacturing method for a low-temperature solid oxide fuel cell (SoFC) using a sol-gel method of the present invention.

Figure 2 is a surface and cross-sectional photograph of the composite electrolyte prepared by the method of the present invention.

3 is a diagram showing the open-circuit voltage characteristics of a unit cell fabricated using the composite electrolyte prepared by the method of the present invention.

4 is a view showing the current-voltage characteristics (electrolyte thickness 1.6 mm) of a unit cell produced using the composite electrolyte prepared by the production method of the present invention.

Unlike the vapor phase coating process, the sol-gel method can be used to produce high purity thin films of various compositions without special equipment, and it is easy to control the microstructure of the thin film. Can be.

There are two types of SOFC electrolytes, plate and cylinder, and in general, plate can be manufactured by pressure molding, slip casting, or tape casting, and cylinder by slip casting or tape casting.

In addition, coating sol includes a particle sol (particulate sol) and a polymeric sol (particulate sol) is used mainly in the field of liquid and gas separation membrane manufacturing because it is suitable for the production of a thin film having a porous microstructure (see SH Hyun , MA Anderson, and SP Yoon, "Development of Ceramic Composite Membranes for Gas Separation. I. Coating Characteristics of Nanoparticulate SiO ₂ Sols," J. Kor. Ceram. Soc., 26, 496-504 (1992)), Polymerization sol is mainly used in the semiconductor manufacturing process, such as the production of ferroelectric thin film because it is suitable for the production of thin film having a fine microstructure (G. Yi and M. Sayer, "Sol-Gel Processing of Complex Oxide Films, Am. Ceram. Soc. Bull., 70, 1173-1179 (1991).

Therefore, in the present invention, a flat / cylindrical ceria substrate in which 20 mol% of yttria (Y ₂ O ₃ ) / samaria (Sm ₂ O ₃ ) / gadolinia (Gd ₂ O ₃ ) prepared by pressure molding / slip casting is added. In order to prepare a composite electrolyte coated with an YSZ thin film without cracks and pores, YSZ polymerization sol synthesis conditions and coating processes suitable for coating were optimized.

Hereinafter, a manufacturing process of a composite electrolyte for a low temperature type solid oxide fuel cell (SOFC) using the sol-gel method of the present invention as shown in FIG. 1 will be described in detail.

First, zirconia polymerization sol is prepared by partial hydrolysis of the starting material zirconium alkoxide. In order to control the partial hydrolysis reaction of the starting materials, acetic acid and nitric acid were used as complexing agents and catalysts, respectively, from which a zirconia polymerization sol having a linear particle structure suitable for coating can be prepared.

The ratio of zirconium alkoxide and acetic acid as a complexing agent is from 1: 0 to 2. The catalyst solution is prepared such that the ratio of alkoxide to water is 1: 1 to 4, the ratio of alkoxide to nitric acid is 1: 0.8 to 1.4, and the ratio of alkoxide to isopropanol is 1:15.

YSZ polymerization sol was prepared by adding a yttrium nitrate solution dissolved in isopropanol to the zirconia polymerization sol prepared as described above.

At this time, the ratio of yttrium nitrate and isopropanol is preferably about 1:30. In addition, the addition amount of the yttrium nitrate solution is preferably adjusted so that the ratio of zirconium and yttrium in the final YSZ polymerization sol corresponds to (ZrO ₂ ) _0.92 (Y ₂ O ₃ ) _0.08 .

Next, various YSZ polymer sols having different concentrations and viscosities are synthesized using a vacuum evaporator, and these sols are used to optimize the coating process.

Subsequently, YSZ polymer sol is coated on flat or cylindrical YDC, SDC and GDC substrates to prepare YSZ / YDC, YSZ / SDC and YSZ / GDC composite electrolytes.

Coating methods include spin coating and dip coating (see CJ Brinker and GW Scherer, Sol-Gel Science, pp. 787-838, Academic Press, San Diego, CA, 1990) and repeat coating and heat treatment processes. Thus, the thickness of the YSZ thin film can be adjusted.

When the spin coating method is used as the coating method, the rotation speed is 1000 to 5000 rpm, and the spin coating can be repeated 5 to 10 times. On the other hand, when the dip coating method is used, the pulling speed is 0.50 to 20 mm / sec, and the dip coating can be repeated 5 to 20 times.

The objects, features and advantages of the present invention will become more apparent by a review of the description of the preferred embodiments of the present invention provided in conjunction with the accompanying drawings.

<Example 1>

First, acetic acid, a complexing agent, is added so that the ratio of alkoxide and acetic acid is 1: 0 to 2, and then the ratio of alkoxide and water is 1 to a solution having a ratio of 1:15 of zirconium n-butoxide butanol complex (alkoxide) and isopropanol as a starting material. A zirconia polymerized sol was prepared by administering a catalyst solution prepared so that the ratio of 1 to 4, the alkoxide to nitric acid was 1: 0.8 to 1.4, and the ratio of alkoxide to isopropanol was 1:15.

A polymerization sol having an YSZ composition was synthesized by adding an additive solution in which yttrium nitrate was dissolved in isopropanol to the zirconia polymerization sol. The additive solution was made to have a ratio of yttrium nitrate and isopropanol of 1:30, and the amount of the additive solution was adjusted so that the ratio of zirconium and yttrium in the final YSZ polymerization sol corresponded to (ZrO ₂ ) _0.92 (Y ₂ O ₃ ) _0.08 .

Next, the concentration of the coating YSZ polymerization sol was changed to 0.45 to 1.50 mol / L using a vacuum evaporator.

Subsequently, an YSZ thin film was synthesized by using a spin coating method on a flat YDC substrate manufactured by pressure molding or slip casting.

Spin coating was performed at a rotational speed of 1000 to 5000 rpm, dried for 12 hours at room temperature, and then preheated for 2 hours at 600 ° C. in air to remove organic matter and volatiles.

The coating, drying, and preheating processes were repeated several times to obtain a thickness suitable for suppressing the reduction of ceria. Finally, heat treatment was carried out at 1400 ° C. for 2 hours in air, so that the coating layer was free of cracks and pores. / YDC composite electrolyte was prepared.

In addition, the same experiment was carried out on the plate-like SDC and GDC substrate, it was confirmed that the high-performance plate-type YSZ / SDC composite electrolyte and YSZ / GDC composite electrolyte can be prepared.

<Example 2>

Cylindrical YSZ / YDC composite with excellent cracking and porosity and excellent bonding of substrate and thin film by immersion coating YSZ thin film on cylindrical YDC substrate manufactured by slip casting method using the same sol as spin sol in Example 1 An electrolyte was prepared.

In the dip coating, the pulling rate of the substrate was 0.50 to 20 mm / sec, and drying, preheating, and repeated coating conditions and final heat treatment conditions after coating were the same as in Example 1.

In addition, by performing the same immersion coating process using a cylindrical SDC and GDC substrate it was confirmed that the high-performance cylindrical YSZ / SDC composite electrolyte and YSZ / GDC composite electrolyte can be prepared.

As mentioned above, the present invention has established a process for producing a composite electrolyte by a liquid phase method using a sol instead of the gas phase method that has been used up to now.

Figure 2 is a surface and cross-sectional photograph of a composite electrolyte prepared by the method of the present invention synthesized YSZ thin film by spin coating six times on yttria-doped ceria (YDC) substrate prepared by pressure molding .

From the drawings, not only no cracks or pores can be found in the surface photograph, but also the bonding of the substrate and the thin film is excellent in the cross-sectional photograph. The thickness of the six spin coated YSZ films was found to be about 2 μm, which is suitable for suppressing the reduction of YDC substrates from theoretical calculations (AV Virkar, “Theoretical Analysis of Solid Oxide Fuel Cells with Two-Layer, Composite Electrolyte: Electrolyte Stability ", J. Electrochem. Soc., 138, 1481-1487 (1991).

As shown in FIG. 3, the open circuit voltage of the SOFC unit cell manufactured using the composite electrolyte was measured to be slightly higher than the open circuit voltage of the YDC electrolyte without coating the YSZ. This increase in open circuit voltage occurs because the YSZ coating layer not only suppresses the reduction of ceria, but also prevents the conduction of electrons. The YSZ thin film synthesized by the sol-gel method of the present invention suppresses the reduction of YDC. It means very effective.

As a result of comparing the current-voltage characteristics of the various unit cells, as shown in FIG. 4, the cell voltage of the unit cell using the composite electrolyte in the high current density region was higher than that of the unit cell using the YSZ electrolyte.

The performance of the composite electrolyte unit cell mentioned in FIGS. 3 and 4 is consistent with or rather superior to that of the composite electrolyte unit cell prepared by the gas phase method considering the thickness of the used substrate (1.6 mm). The sol-gel coating method using is a very suitable method for the production of a composite electrolyte for low temperature solid oxide fuel cell.

According to the present invention, it is easy to control the microstructure without requiring expensive equipment or starting materials, and is relatively limited in the form and size of the substrate, and can be a low temperature process and can produce a high purity thin film of various compositions. Using the gel method, a composite electrolyte coated with YSZ on a ceria-based substrate may be prepared.

Claims (6)

Yttria stabilized zirconia, comprising adding acetic acid, a complexing agent, to a zirconium alkoxide, followed by partial hydrolysis of the zirconium alkoxide by administering a catalyst solution containing water, nitric acid, and isopropanol, and adding a yttrium nitrate solution dissolved in isopropanol (Yttria Stabilized Zirconia, YSZ) A process for producing a polymerization sol. The method of claim 1, further comprising concentrating the YSZ polymerization sol with a vacuum evaporator to change the concentration of the sol to 0.45-1.50 mol / l. The method according to claim 1, wherein acetic acid, nitric acid and water are added in a ratio of 1: 0 to 2, 1: 0.8 to 1.4 and 1: 1 to 4, respectively, relative to zirconium alkoxide. The method of claim 1, wherein the yttrium nitrate solution is adjusted such that the ratio of zirconium and yttrium in the final YSZ polymerization sol corresponds to (ZrO ₂ ) _0.92 (Y ₂ O ₃ ) _0.08 . Flat plated Yttria Doped Ceria (YDC), Samaria Doped Ceria (SDC), and Gadolinia Doped Ceria (GDC) substrates The plate-type YSZ / YDC, YSZ / SDC, which comprises heat-treating the YSZ polymerization sol prepared in any one of claims 1 to 4 at a rotational speed of 1000 to 5000 rpm, followed by heat treatment. And a method for producing the YSZ / GDC composite electrolyte. 5 to 20 repetitive immersion coating of cylindrical YSZ polymerization sol prepared in any one of claims 1 to 4 at a rate of 0.50 to 20 mm / sec and then heat-treated to cylindrical YDC, SDC and GDC substrate , Cylindrical YSZ / YDC, YSZ / SDC and YSZ / GDC composite electrolyte production method.