Casting Technology for Zirconia Electrolyte Films in SOFCs

Casting and Related Preparation Technologies of Zirconia Electrolyte Films for SOFCs

Solid oxide fuel cells (SOFCs), as an all-solid-state electrochemical power generation device operating at high temperatures, can directly convert the chemical energy of fuel into electrical energy through electrochemical reactions. With its outstanding advantages such as high energy conversion efficiency, wide fuel compatibility, and clean, pollution-free products, it is widely recognized as one of the most promising new energy technologies of the 21st century. The electrolyte core of SOFCs mostly uses oxide ceramic materials, specifically sintered solid solution electrolytes—fully stabilized zirconia (ZrO₂), among which yttrium-stabilized zirconia (YSZ) is currently the mainstream electrolyte material in industry and scientific research.

To reduce ohmic polarization losses during ion diffusion, the electrolyte layer needs to be controlled within an ultra-thin thickness range of micrometers to millimeters. Currently, how to prepare YSZ films with satisfactory performance and excellent stability remains a key research focus and technical challenge in the industry. Casting, a mature technology in the electronics industry for preparing ceramic sheets or ceramic-polymer composite sheets, is now widely used in the preparation of raw zirconia electrolytes. Meanwhile, novel auxiliary processes such as ultrasonic coating deposition are gradually gaining attention, providing more solutions for thin film preparation.

Core Preparation Process Principles and Applications

(I) Tape Casting Working Principle

The core process of tape casting (also known as Doctor blading or Knife coating) is as follows:

1. First, ceramic powder and dispersant are added together to a solvent (water or organic solvent). The agglomeration of particles is broken up by ball milling or ultrasonic vibration, ensuring the solvent fully wets the powder particles.

2. Subsequently, binder and plasticizer are added. After a second ball milling process, supplemented by primary filtration, vacuum degassing, and secondary filtration, a stable and homogeneous slurry is finally obtained.

3. The prepared slurry is introduced into a tape casting machine, where the green body is formed.

4. In the drying stage, as the solvent gradually evaporates, the binder forms a continuous network structure between the ceramic powder particles, thus forming the green body film.

5. The green body film is cut and shaped to obtain the target shape.

6. Finally, high-temperature debinding and sintering processes are performed to obtain the finished film that meets the usage requirements.

(II) Synergistic Application with Screen Printing and Ultrasonic Coating Deposition

In the complete SOFC fabrication process, tape casting is often used in conjunction with screen printing:

1. First, a YSZ electrolyte layer and anode layer with uniform thickness and no obvious defects are prepared using tape casting;

2. The two layers are then subjected to a stacking and temperature pressing process;

3. A semi-finished product is obtained through co-firing;

4. The cathode layer is prepared using screen printing, ultimately completing the assembly of the SOFC single cell.

Due to the low mechanical strength of the YSZ ceramic film, cracking, blistering, and delamination are prone to occur during the stacking and temperature pressing process. Currently, the industry often uses a double-layer tape casting process for optimization, that is, directly casting the electrolyte layer on the surface of the anode layer, greatly simplifying the stacking and temperature pressing process. In addition, ultrasonic coating deposition technology is also increasingly being used to assist in the fabrication process. This technology utilizes the dispersion and atomization effect of ultrasound to form a uniform coating on the substrate surface, which is particularly suitable for the precise coating of thin electrolyte layers. It can effectively compensate for the insufficient thickness control precision of traditional tape casting processes in the preparation of ultra-thin layers, and has significant advantages in low-temperature molding scenarios.

(III) Comparison of Mainstream Processes for Yttrium Stabilized Zirconia Thin Films

Core Performance Indicators of Zirconia Thin Films

1. Ionic Conductivity: As the most critical performance parameter of zirconia thin films, high oxygen ion conductivity not only enables SOFCs to achieve better output performance and operating power, but also significantly improves the long-term operational stability of the battery.

2. Density and Gas Tightness: YSZ electrolyte films have extremely stringent requirements for both. The core purpose is to block the mutual penetration and reaction between anode fuel gas and cathode oxygen. If the density or gas tightness does not meet the standards, gas leakage is very likely to occur, leading to a short circuit in the battery, which in turn causes a drop in open-circuit voltage and overall performance degradation.

3. Mechanical Strength: The film must have sufficient mechanical strength to withstand the high-temperature operating environment of SOFCs and stress changes during long-term use, ensuring the safe and stable operation of the equipment.

Key Factors Affecting Electrolyte Film Performance

Slurry Composition:

Slurry composition is a core parameter of the casting and ultrasonic coating deposition process, playing a decisive role in the comprehensive performance of the formed tape, such as tensile strength, flexibility, and tape density. As the core functional component of green tapes, the content of ceramic powder directly determines the final performance of thin-film batteries. Theoretically, a higher solid content in the zirconia ceramic slurry is more beneficial for improving film performance; however, excessively high solid content leads to a sharp increase in slurry viscosity, affecting coating or casting effects. Therefore, it is necessary to scientifically adjust parameters such as solid content, solvent type, and binder ratio to ensure the slurry possesses good dispersibility and rheological properties, laying the foundation for preparing uniformly thick zirconia green tapes or coatings.

The type of sintering aid, the selection of the binder system, and the compatibility of the dispersant are key factors affecting the slurry’s solid content, rheological properties, and subsequent film performance, and must be adjusted specifically according to the particular preparation process (such as casting or ultrasonic coating deposition).

Preparation Process Parameters

1. Casting Process Parameters
The main casting process parameters include casting speed, drying environment parameters, and debinding and sintering processes:

– Casting Stage: Under the action of the moving substrate, the slurry forms a composite flow state of pressure flow and drag flow. The gap between the squeegee and the substrate is the core parameter for controlling the thickness of the cast film. Simultaneously, the cast film achieves surface smoothing through its own surface tension. In actual production, uniform film thickness can be ensured by uniformly mixing the slurry, controlling the viscosity within a reasonable range, precisely adjusting the squeegee gap, and maintaining a stable slurry surface height.

– Drying Stage: The drying process of zirconia green tape is essentially a synergistic process of polymer chain shrinkage, particle sedimentation, and rearrangement. Since the slurry contains a large amount of solvent, the solvent evaporation rate directly affects the quality of the green tape. To achieve sufficient volume diffusion in the green body, minimize internal porosity, and avoid curling or cracking caused by uneven local shrinkage, precise control of drying temperature, relative humidity, and airflow velocity is necessary based on the green body thickness, solid volume fraction, and the content of organic matter such as binders and plasticizers. This ensures slow and uniform solvent evaporation.

– Debinding and Sintering Stage: The core purpose of debinding is to decompose the binder at high temperatures and completely remove it from the green body. This process mainly includes three steps: high-temperature decomposition of the binder, diffusion of decomposition products to the surface of the green body film, and volatilization of the decomposition products. The required temperature is much higher than that of the drying process. After debinding, common methods for bulk densification include atmospheric pressure sintering and hot pressing sintering.

2. Ultrasonic Coating Deposition Process Parameters
Key parameters for ultrasonic coating deposition include ultrasonic power, coating speed, slurry atomization pressure, and substrate temperature:

– Ultrasonic power directly affects the atomization effect and dispersion uniformity of the slurry. Excessive power can lead to slurry splashing, while insufficient power makes uniform atomization difficult to achieve.

– The combination of coating speed and atomization pressure determines the thickness and density of the coating, requiring precise control based on the target film thickness.

– Appropriately increasing the substrate temperature can accelerate solvent evaporation, but excessive temperature must be avoided to prevent coating cracking. The low-temperature forming advantage of this process can effectively reduce the adverse effects of high temperatures on material properties.

Optimization Directions for Zirconia Thin Film Performance

The typical structure of a SOFC consists of a fuel electrode support layer and a YSZ electrolyte film. This structure maximizes the safe operation of the fuel cell while reducing the negative impact of ohmic impedance. Currently, researchers mainly focus on electrolyte thickness control and structural optimization, combining the characteristics of processes such as tape casting and ultrasonic coating deposition to improve the performance of zirconia thin films.

(I) Electrolyte Thickness Optimization

The ohmic resistance of SOFCs mainly originates from the electrolyte layer. Therefore, reducing the electrolyte thickness is an effective way to reduce battery ohmic resistance and electrode polarization resistance, and improve fuel cell output performance. In the casting process, the electrolyte layer thickness is mainly controlled by the height of the scraper; while ultrasonic coating deposition technology, with its more precise thickness control capabilities, can prepare thinner electrolyte layers.

However, there is a critical value for reducing electrolyte thickness: although ultrathinning can effectively reduce SOFC ohmic resistance and improve ionic conductivity, it will simultaneously weaken the mechanical strength and airtightness of the electrolyte layer. When the thickness is reduced to a certain level, problems such as electrolyte rupture and gas leakage are likely to occur during long-term battery operation. Therefore, the SOFC electrolyte layer needs to seek a balance between high conductivity, mechanical strength, and airtightness, and determine the optimal thickness by combining the advantages of the specific manufacturing process.

(II) Electrolyte Structure Optimization

Ionic conductivity directly affects the power density and open-circuit voltage of SOFCs. Currently, YSZ electrolyte films exhibit extremely low ionic conductivity at medium and low temperatures, requiring high-temperature operation of 800–1000℃ to ensure sufficient ionic conductivity. However, high-temperature operation leads to a series of problems, including increased difficulty in material selection, higher battery manufacturing costs, and shortened lifespan.

Optimizing the electrolyte structure can effectively improve the performance of SOFCs at medium and low temperatures. Currently, a bilayer electrolyte system composed of zirconia-based and cerium oxide-based electrolytes has become a research hotspot. This system can be prepared using a co-casting process or a composite process of “casting + ultrasonic coating deposition”: first, a ZrO₂-based electrolyte layer is prepared using a casting process, and then a CeO₂-based electrolyte layer is precisely coated onto its surface using ultrasonic coating deposition technology, forming a dense bilayer electrolyte with good interfacial bonding. This system fully utilizes the excellent chemical stability of zirconia-based electrolytes and leverages the high ionic conductivity of cerium oxide-based electrolytes, achieving a reduction in SOFC operating temperature while improving ion conduction efficiency and blocking electron conduction, significantly enhancing the overall performance of SOFCs under medium and low temperature conditions.

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