KR100692642B1 - The preparation method of thin film electrolyte for solid oxide fuel cell by sol-gel method - Google Patents

Abstract

Provided are a method for preparing a yttria stabilized zirconia sol for the electrolyte of a solid oxide fuel cell to reduce a manufacturing cost and to allow the microstructure of thin film to be controlled easily, and a method for preparing an electrolyte thin film by using the sol. The yttria stabilized zirconia sol is prepared by dissolving zirconyl nitrate in water to prepare a zirconyl nitrate solution; adding a chelating agent to the zirconyl nitrate solution and stirring it; dissolving yttrium nitrate in water to prepare a yttrium nitrate solution and adding a chelating agent; mixing the prepared zirconyl nitrate solution and the prepared yttrium nitrate solution; and maturing the obtained mixture at a room temperature for a day and filtering the mixture to remove the reaction residue and the impurities.

Description

The preparation method of yttria stabilized zirconia sol for solid oxide fuel cell electrolyte and the formation method of electrolyte thin film using the same {The preparation method of thin film electrolyte for solid oxide fuel cell by sol-gel method}

1 is a graph of TG / DTA analysis of the zero gel state of 8YSZ sol prepared by the method of the present invention.

2 is an X-ray diffraction analysis pattern for each sintering temperature of the zero gel state of 8YSZ sol prepared by the method of the present invention.

FIG. 3 is a result of a calculation analysis of the sequential electrolyte layers of the anode support manufactured by the method for forming an electrolyte thin film of the present invention.

  (A) is a photograph of the surface of the base electrolyte layer sintered at 1400 ℃ for 5 hours,

  (B) is a cross-sectional photograph of the base electrolyte layer sintered at 1400 ° C. for 5 hours,

  (C) is a photograph of the surface of the electrolyte thin film heat-treated at 600 ℃ 1 hour,

  (D) is a cross-sectional photograph of the electrolyte thin film heat-treated at 600 ℃ for 1 hour,

  (E) is a photograph of the surface of the electrolyte thin film sintered at 1400 ° C. for 5 hours,

  (Bar) is a cross-sectional photograph of the electrolyte thin film sintered at 1400 ° C. for 5 hours.

Figure 4 is a graph of the permeability change of helium gas for the electrolyte layer for each sol coating recovery.

5 is a result of the sintering temperature analysis of the surface of the thin film formed by the method of the present invention,

  (A) is a photograph of the surface of the thin film sintered at 1100 ℃ for 5 hours,

  (B) is a photograph of the surface of the thin film sintered at 1200 ℃ for 5 hours,

  (C) is a photograph of the surface of the thin film sintered at 1300 ℃ for 5 hours,

  (D) is a photograph of the surface of the thin film sintered at 1400 ° C. for 5 hours.

6 is a graph showing the change in permeability of helium gas to the electrolyte layer for each sintering temperature.

The present invention uses a sol-gel method of yttria stabilized zirconia, which is used as a fuel cell electrolyte, in order to reduce the thickness of the electrolyte layer and lower the operating temperature of the fuel cell without degrading the characteristics of the solid oxide fuel cell. A method for producing a yttria stabilized zirconia sol for a solid oxide fuel cell electrolyte and a method for forming an electrolyte thin film using the same, wherein the sol is made of a sol and dip coated on the base electrolyte layer on the surface of the anode support to form an electrolyte thin film.

Fuel cell is a high-efficiency clean power generation technology that converts hydrogen and oxygen contained in hydrocarbon-based materials such as natural gas, coal gas and methanol into electrical energy directly by electrochemical reaction. It is largely classified into alkali type, phosphoric acid type, molten carbonate, solid oxide and polymer fuel cell.

In general, a fuel cell is a first-generation phosphate fuel cell, which is a fuel cell using hydrogen gas mainly containing hydrogen-modified fossil fuel and oxygen in the air as a fuel, and a phosphate electrolyte as a first generation, and molten salt as an electrolyte. The high-temperature molten carbonate fuel cell operating near 650 ° C is the second generation, and the solid oxide fuel cell (SoFC), which operates at higher temperature and has the highest efficiency. It is called the third generation fuel cell.

The SOFC, called the third generation fuel cell, was developed later than the phosphate fuel cell and the molten carbonate fuel cell. However, SOFC has the highest energy efficiency among several fuel cell types and has a high operating temperature of 1000 ° C. As a result, hydrocarbons such as LPG and LNG can be directly used as fuels without expensive external reformers, and they also have the advantage of little emission of pollutants during operation.

Therefore, in recent years, high-temperature waste heat generated during operation of SOFCs can be combined with home or cogeneration to maximize thermal efficiency, and thus, it is gaining attention as a large-scale power generation system as well as small and medium-sized power generation systems such as field power generation.

However, SOFCs have to be operated at high temperature, and there are still problems to be solved in materials engineering such as suitability of thermal and mechanical properties between components caused by high temperature operation, chemical reaction and deterioration between components. There is still a lot left.

In particular, the reliability problem of the system, which is an obstacle to the actual commercialization, is mostly due to the limitation of material properties due to the high temperature operation. In order to solve the problem caused by the high operating temperature of SOFC, it is in the range of about 700 to 800 ° C. Research into lowering the operating temperature is ongoing.

In other words, when the operating temperature of SOFC is lowered below 800 ° C, thermal efficiency is improved, fuel cell life is extended, metal materials such as interconnects are not only easily used, but also operating costs are lowered. There is this.

However, at low operating temperatures, electrolytes composed of yttria stabilized zirconia (hereinafter referred to as "YSZ") suffer from the problem of increased ohmic loss, which leads to ohms of YSZ at low temperatures. In order to reduce the loss, various studies such as a method of manufacturing an electrolyte by thinning it and a method of synthesizing a new electrolyte material having high ionic conductivity have been conducted.

In general, the electrolyte stabilized in the conventional SOFC is made of a polycrystalline dense film or layer, which is produced by a deposition coating method or a particulate method, the deposition coating method is a substrate or the like through a chemical or physical process Coating the thin film on the support, such as electronically vapor deposition, plasma spraying, etc. These coating processes require expensive equipment or starting materials, as well as depending on the shape and size of the substrate There is a drawback that several limitations exist.

In the particulate method, the YSZ powder used as the electrolyte is formed at a high temperature at a desired shape and density, and there are methods such as tape casting and tape calendering, but manufacturing cost is lower than that of the deposition coating method. Others have similar limitations.

In addition, in the slurry coating method which can form the electrolyte layer economically, the thickness of the electrolyte layer may be made as thin as possible by diluting the concentration of the slurry, but as the slurry concentration decreases, many pores are formed in the electrolyte layer. In other words, there is a problem that the characteristics of the fuel cell are degraded, such as leakage of the reaction gas.

The present invention was devised to solve all the problems of the conventional methods used to thin the thickness of the electrolyte layer of the SOFC, and because it can be manufactured at low temperature and does not require expensive equipment or starting materials, The present invention provides a method for preparing a solid oxide fuel cell electrolyte YSZ sol having a low electrical cost, high purity and easy control of a thin film, and excellent electrical properties, and a method for forming a thin film using the same. There is an object of the invention.

The above object of the present invention is achieved by a sol-gel method and a dip coating method which is repeated and heat treated a plurality of times.

The sol-gel method is applied as a method for preparing an YSZ sol for a solid oxide fuel cell electrolyte of the present invention, and the electrolyte thin film is formed by a plurality of dip coatings using an YSZ sol.

The thin film using the YSZ sol is formed by coating a porous thin base electrolyte layer on the surface of the anode support of the SOFC by a dip coating method, and then coating the surface of the base electrolyte layer as an additional electrolyte layer. In addition, the technical features of the present invention include coating the electrolyte thin film, which forms the entire electrolyte layer together with the base electrolyte layer, as the 8YSZ sol.

That is, the electrolyte thin film is formed by coating with 8YSZ sol manufactured using the sol-gel method, and is coated on the surface of the porous base electrolyte layer to form a dense thin film, and the base electrolyte layer and the electrolyte thin film are sequentially laminated with the whole electrolyte. Since the thickness of the layer is thinner than that of the conventional electrolyte layer and its characteristics are maintained, it is possible to lower the operating temperature of the solid oxide fuel cell.

8YSZ sol for coating and forming the electrolyte thin film as described above,

Dissolving zirconyl nitrate hydrate in water to prepare a zirconyl nitrate solution at a concentration of 0.5-0.8 M;

Slowly adding a complexing agent to the zirconyl nitrate solution in a molar ratio of chelating agent: ZrO ₂ = 1: 2 and then stirring at room temperature;

A yttrium nitrate hydrate, a yttria mixture serving as a phase stabilizer of zirconia, was dissolved in water to prepare a yttrium nitrate solution, and then a complexing agent was added so as to have a molar ratio of complexing agent Y ₂ O ₃ = 1: 2 Steps;

Mixing the zirconyl nitrate solution and the yttrium nitrate solution to which the complexing agent is added, respectively;

After the aging of the mixed solution (filter), it is produced through the step of filtering out the reaction residues and cracks caused by the synthesis process using a filter and the like.

At this time, it is preferable to add a coagulation regulator such as ethyl alcohol, isopropyl alcohol, normal propyl alcohol, tertiary butyl alcohol, etc. to adjust the degree of coagulation of the sol using the inorganic compound to the zirconyl nitrate solution is completed, The amount is preferably 45 to 55% by weight of the total amount of solvent.

The yttria in the yttrium nitrate solution should be quantified to satisfy the molar ratio of zirconia and (Y ₂ O ₃ ) _0.08 (ZrO ₂ ) _{0.92 in} the zirconyl nitrate solution.

Examples of the complexing agent include acetic acid, citric acid, ethylenediaminetetraacetic aacid, oxalic acid, and polyvinyl alcohol. Small acetic acid is preferred because the complexing agent acts as a cracking factor when the molar weight is larger than this.

In addition, the participation amount of the complexing agent should be kept at a molar ratio of 1: 2 for the reaction between the metal ions, the aging of the mixed solution is to be carried out at least one day at room temperature, and to remove the fine residue remaining in the mixed solution It is preferable to use the thing whose pore size is 0.5 micrometer or less.

The 8YSZ sol prepared as described above is dip coated 5-15 times on the surface of the porous base electrolyte layer formed by coating 8YSZ slurry on the surface of the anode support, and the weight loss of the 8YSZ sol occurs after each dip coating. Heat treatment should be performed in the range of 400 ~ 600 ℃.

In this case, when the heat treatment is performed at a temperature lower than the above temperature range, cracks of the thin film may be caused by the organic components remaining in the 8YSZ sol, and when the heat treatment is performed at a higher temperature, thermal expansion of the 8YSZ sol layer is performed. By the cracks can be generated between the coating film.

When the sol coating is completed, the final heat treatment may be performed at 400 to 600 ° C. and then sintered at 1300 to 1400 ° C., or the sintering may be performed immediately without performing the final heat treatment. This is because the final heat treatment can be performed at.

In the case of the sintering temperature, the sintering does not proceed when the temperature is lower than the above temperature, and when the temperature is exceeded, the sintering crack is generated while the entire electrolyte layer is over-sintered.

In the case of sol coating, the number of the coatings is limited to 5 to 15 times. When the number of coatings is smaller than this, the final coating is not necessary to act as a barrier against the surface pores of the electrolyte slurry coating layer located at the bottom, that is, the porous base electrolyte layer using the slurry. This is because the mechanical strength of the electrolyte thin film is insufficient, and if the number of coatings is greater than this, the thickness of the entire electrolyte layer including the base electrolyte layer and the electrolyte thin film by the sol becomes thicker than necessary, which acts as a factor of increasing the resistance. .

In addition, the concentration of 8YSZ sol is 0.5 ~ 0.8M concentration is appropriate, if the concentration is less than 0.5M, it causes a phenomenon that the fine particles are agglomerated in the sol without uniformly dispersed, 0.8M concentration In case of exceeding, cracking occurs in the process of drying the sol by over-coating during coating, making it difficult to form a dense thin film.

The electrolyte thin film manufactured through the above method has good characteristics despite the decrease in the thickness of the entire electrolyte layer together with the base electrolyte layer formed on the surface of the anode support by the slurry dipping method. By way of example, the manufacturing method and characteristics of the electrolyte thin film of the present invention will be described.

Example

1. Fabrication of anode support

In order to prepare 40 vol% Ni-YSZ cermet anode powder, NiO powder and 8YSZ powder were mixed quantitatively, and the mixed powder was ball-milled for 24 hours to prepare a porous anode support. 3% by weight of activated carbon as a pore former was added.

The anode powder was press-molded to form a disk, and then calcined at 1300 ° C. for 3 hours. In order to improve the surface of the anode support, the surface of the support was coated once with 20 wt% NiO-YSZ slurry. The 20 wt% NiO-YSZ slurry used in the coating is a binder (binder, PVA), plasticizer (plasticizer, dibuthyl phthalate), homogenizer (homogenizer, Triton-X), dispersant (fish, oil oil) to NiO and 8YSZ powders. ) And a solvent (Toluene, IPA (2-propanol)) was prepared by mixing.

The anode support coated with the NiO-YSZ slurry was sintered at 1000 ° C. for 3 hours to remove the added solvent and additives, and to facilitate electrolyte coating.

As described above, when dip coating NiO-YSZ for improving the surface of the anode support, the concentration of the NiO-YSZ slurry is set to 15 to 25% by weight, and the ratio of NiO: 8YSZ is set to 6: 4. If the concentration is less than 15% by weight, the surface improvement effect of the support is insufficient, and if it exceeds 25% by weight, the coating layer becomes thicker than necessary, thereby deteriorating its characteristics.

If the NiO content is less than the above ratio, the conductivity of the NiO-YSZ coating layer is reduced and the electrode performance is reduced. If the NiO content is higher than this, the electrode performance is reduced due to an increase in the polarization resistance between the electrode and the electrolyte. In addition, interlayer delamination may occur due to a difference in thermal expansion coefficient of the NiO-YSZ coating layer.

2. Preparation of Electrolyte Slurry and Formation of Base Electrolyte Layer

The electrolyte slurry was dip coated to form a base electrolyte layer on the surface of the anode support coated with the 20 wt% NiO-YSZ slurry. The 20 wt% YSZ electrolyte slurry was used as a binder in 8YSZ powder. binder, PVA), plasticizer (plasticizer, dibuthyl phthalate), homogenizer (homogenizer, Triton-X), dispersant (fish oil) and solvent (Toluene, IPA (2-propanol)) was prepared by mixing.

The electrolyte slurry was dip coated and then sintered at 1400 ° C. for 5 hours to form a base electrolyte layer.

In this case, it is preferable that the electrolyte slurry has a concentration of 10 to 15% by weight. When the slurry concentration is lower than this, a large amount of interfacial pores are generated and the surface roughness is reduced to thereby crack the electrolyte thin film formed by the sol coating. At higher slurry compositions, the thickness of the electrolyte layer becomes thicker than necessary to increase its resistance.

3. Preparation of 8YSZ Sol and Formation of Electrolyte Thin Film

After dissolving zirconyl nitrate in water to a 0.7 molar concentration, acetic acid, a complexing agent, was added and stirred at room temperature for 2 hours, and 50% by weight of isopropyl alcohol, a coagulation regulator, was added to the stirred zirconyl nitrate solution.

Then, yttrium nitrate was quantified in accordance with the amount of zirconia of the zirconyl nitrate solution in a ratio of (Y ₂ O ₃ ) _0.08 (ZrO ₂ ) _0.92 , and dissolved in water, followed by adding a yttrium nitrate solution to which a complexing agent was added. Was prepared and mixed with the above zirconyl nitrate solution.

After aging the mixed solution at room temperature for 1 day, by filtering the mixed solution with a filter having a pore size of 0.5㎛ size to remove the reaction residues and cracks that cause cracks during the synthesis process by aging for 1 day at room temperature, 0.7M YSZ sol was prepared.

The 0.7 M YSZ sol was dip-coated on the base electrolyte layer of the anode support, dried at room temperature for 30 minutes, and then heat-treated at 600 ° C. for 1 hour to remove organic matter and solidify the sol changed into a gel state.

In order to examine the characteristics according to the number of dip coating, drying and heat treatment processes, the electrolyte thin film was coated by varying the number of dip coating and heat treatment. After the final coating, an additional electrolyte layer was formed on the base electrolyte layer by sintering at 1400 ° C. for 5 hours. The phosphorus electrolyte thin film was formed by coating.

4. Analysis method

In order to examine the crystal structure of the coated electrolyte thin film through the above-described process, TG-DTA (ThermoGravimetry-Differential Thermal Analysis) was used for thermal decomposition and phase change of the synthesized 8YSZ sol. Observation analysis.

The morphology and thickness of the coated electrolyte thin film were analyzed using Scanning Electronic Microscopy, and the gas leakage test was performed using helium gas to analyze the density of the electrolyte thin film. It was.

5. Results

For the quantitative and qualitative analysis of the synthesized 8YSZ sol, XRF analysis was performed using 8YSZ, which is used as an electrolyte slurry of SOFC, as a standard sample. The standard samples were ZrO ₂ 92.097 wt% and Y ₂ O ₃ 7.817 wt%. The 8YSZ xerogel with the changed sol was found to be 92.849% by weight of ZrO _{2 and} 7.992% by weight of Y ₂ O ₃ , which resulted in a component and content within an error range of ± 1% by weight.

That is, it can be seen that 8YSZ synthesis of similar components and contents is possible using a metal inorganic compound having lower material cost and stable reaction conditions than synthesis using an alkoxy compound.

1 is a graph showing the results of thermal analysis of 8YSZ zero gel in the temperature range of 20 to 1400 ℃, the mass loss occurring below the initial 100 ℃ is due to the vaporization of alcohol and water, having a mass loss of 46.12% The exothermic peak at 317 ° C. is due to the NOx degradation contained in the zirconyl nitrate and the yttria nitrate.

The mass loss of 4.64% which occurred between 400 and 800 ° C is because the remaining carbon and organic matter are burned, and no mass loss was observed after 800 ° C.

XRD results of a zero gel fired for 5 hours at various temperatures in the range of 600-1400 ° C. are shown in FIG. 2. At this time, firing was performed in an atmospheric atmosphere.

As shown, a sufficiently stabilized cubic zirconia phase was observed above 600 ° C., and the peak became clearer as the firing temperature of the zero gel increased, because the degree of crystallization increased with increasing firing temperature. to be.

3 is a result of the analysis of the surface and the cross-section of the electrolyte layer after sol coating. (A) and (B) are photographs of the base electrolyte layer dip-coated and sintered 20 wt% YSZ slurry on the porous anode support. In the phosphorus (A), despite the slurry coating, fine pores were observed at the grain boundary, and it can be seen from the cross-sectional photograph (B) that the thickness of the base electrolyte layer was approximately 4 to 6 µm.

(C) and (D) are formed by repeatedly coating and heat-treating 10 times as 0.7M 8YSZ sol on the base electrolyte layer, and the surface and cross-sectional photograph of the unsintered electrolyte thin film. It was not observed, and it can be seen from the cross-sectional photograph that the electrolyte thin film was formed to a thickness of about 1 μm.

(E) and (B) are photographs of the surface and the cross-section of the final electrolyte thin film sintered at 1400 ° C. for 5 hours after sol coating. Very dense grain boundaries were observed on the electrolyte surface. Due to the growth of the grain (grain) due to the base electrolyte layer and the electrolyte thin film was not distinguished.

In the fuel cell, the open circuit voltage is dependent on the relative density of the electrolyte, and the relative density of the electrolyte is related to the open pores, the waste holes, the pinholes, the cracks, and the like. The reduction of the open circuit voltage of the fuel cell depends on how much pinholes and cracks can be suppressed.

As shown in FIG. 3, when only the base electrolyte layer was formed, a plurality of grain boundary pores were observed, and the gas permeability was measured, indicating high gas permeability. It seems to have been deployed. However, it can be seen that after the sol coating, a very dense coating film without cracks and pinholes is formed.

FIG. 4 is a graph showing the permeability of helium gas according to the number of coatings of the sol. When only the base electrolyte layer is formed, a relatively high gas permeability of 20 × 10 ⁻⁶ L / (cm ² sec. Is showing.

However, as the number of sol coatings increased, the gas permeability decreased, and when the number of sol coatings was set to seven times, the gas leakage tolerance limit line defined by the present invention (80 × 10 ^-8 L / cm ² sec atm at ¹ atm) was measured. ), And when coated more than 10 times, no gas leakage occurred even at 2.7 atm. The minimum number of coatings, i.e., seven times, can be added or subtracted depending on the sol concentration.

FIG. 5 is a photograph showing the surface of an electrolyte thin film in which particles grow according to sintering temperature after sol coating. (A) shows sintering at 1100 ° C. and shows a particle size of 20 to 100 nm. It was sintered at 1200 ° C. and showed a particle size of 200 to 300 nm, and (C) was sintered at 1300 ° C. and its particle size was 3 to 4 μm.

(D) is sintered at 1400 ℃ it can be seen that the particle size has grown to 5 ~ 7㎛, such rapid grain growth means that the fine 8YSZ particles produced by the sol has a high sinterability.

6 is a graph showing the gas permeability test results of the electrolyte thin film according to the sintering temperature of 1100 ~ 1400 ℃, the gas permeability was greatly decreased as the sintering temperature is increased, the dense sol coating film very low gas permeability from the sintering temperature of 1300 ℃ It can be seen that this is formed.

In addition, in the case of the electrolyte thin film sintered at 1400 ° C., it was confirmed that helium gas did not leak even at a pressure difference of 2.7 atm. This result showed the same tendency as the result of the analysis of the calculation of FIG. 5, and the sintering temperature was increased. As a result, a dense thin film that can be used as a fuel cell electrolyte is formed.

As described above, since the YSZ sol manufacturing method of the present invention uses nitrate which is a metal inorganic compound, the manufacturing cost is relatively lower than that of the sol-gel method using an alkoxide, and requires expensive equipment or starting materials. It is possible to manufacture a high purity thin film of various compositions without using it, and at the same time, it is not only easy to control the microstructure of the thin film, but also has an advantage of being relatively limited in shape and size of the substrate.

Claims (9)

Dissolving zirconyl nitrate in water to prepare a zirconyl nitrate solution; Adding a complexing agent to the zirconyl nitrate solution and then stirring; Dissolving yttrium nitrate in water to prepare a yttrium nitrate solution, and then adding a complexing agent; Mixing the zirconyl nitrate solution and the yttrium nitrate solution to which the complexing agent is added, respectively; A method for producing a yttria stabilized zirconia sol for a solid oxide fuel cell electrolyte, comprising the step of aging the mixed solution at room temperature for at least one day and then removing the reaction residue and impurities using a filter. The method of claim 1, wherein the zirconyl nitrate solution is prepared at a concentration of 0.5 to 0.8 M of zirconyl nitrate. The method for producing a yttria stabilized zirconia sol for a solid oxide fuel cell electrolyte according to claim 1, wherein 45 to 55% by weight of a coagulation regulator is added to the zirconyl nitrate solution after the stirring is completed. The solid oxide fuel cell electrolyte of claim 1, wherein the yttria in the yttrium nitrate solution is quantified to satisfy a molar ratio of zirconia and (Y ₂ O ₃ ) _0.08 (ZrO ₂ ) _{0.92 in} the zirconyl nitrate solution. Method for preparing yttria stabilized zirconia sol. Coating the base electrolyte layer by coating and sintering the YSZ electrolyte slurry on the surface of the solid oxide anode support; Coating the electrolyte thin film on the surface of the base electrolyte layer by repeatedly coating the yttria stabilized zirconia sol prepared by the method of claim 1 and then performing heat treatment a plurality of times; And a step of sintering the electrolyte thin film coated by the plurality of dip coating and heat treatment. 2. A method of manufacturing an electrolyte thin film using an yttria stabilized zirconia sol for a solid oxide fuel cell electrolyte. The method of claim 5, wherein the YSZ electrolyte slurry is prepared at a concentration of 10 to 15% by weight. The method of claim 5, wherein the dip coating and heat treatment are repeatedly performed 5 to 15 times, and the heat treatment is performed in a range of 400 to 600 ° C. . The method of claim 5, wherein the electrolyte thin film is sintered at 1300 to 1400 ° C. 6. The method of claim 5, wherein the yttria stabilized zirconia sol has a concentration of 0.5 to 0.8 M.

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