Solid Oxide Fuel Cell (SOFC) is a full-solid-state power generation device that directly converts chemical energy of fuels into electrical energy through electrochemical reactions at high temperatures. Featuring high energy conversion efficiency, wide fuel adaptability, zero environmental pollution and other superior characteristics, SOFC is regarded as one of the most promising new energy technologies in the 21st century.
Oxide ceramics are the core materials for SOFC electrolytes, and fully stabilized zirconia (ZrO₂) sintered solid solution dominates the electrolyte field. Among various stabilized zirconia materials, yttria-stabilized zirconia (YSZ) is the most widely applied electrolyte material for commercial and laboratory SOFCs at present.
To minimize the ohmic polarization loss during ion diffusion, the electrolyte layer is required to be extremely thin, with a thickness controlled at the micron level. The preparation of high-performance YSZ thin films has always been a core research hotspot and technical difficulty in the SOFC industry. As a conventional and efficient process for manufacturing ceramic or ceramic-polymer thin sheets in the electronic industry, tape casting technology has been widely popularized in the preparation of zirconia electrolyte green bodies for SOFCs.
Working Principle of Tape Casting Technology
Tape casting is a precise ceramic film forming technology with stable and controllable processes. Its specific working principle and technical flow are as follows: Firstly, ceramic powder and dispersant are added into solvents (water or organic solvents). Particle agglomeration is eliminated via ball milling or ultrasonic oscillation to ensure full wetting of powder by solvents. Subsequently, binders and plasticizers are added, and secondary ball milling is performed to prepare stable and homogeneous ceramic slurry.
The well-prepared slurry is cast on a tape caster to form a uniform green body film. After drying treatment, the solvent evaporates completely, and the binder forms a stable network structure between ceramic particles, obtaining a complete electrolyte green film. The green film is then trimmed and shaped according to actual application requirements. Finally, the finished zirconia electrolyte thin film products are obtained through binder removal and high-temperature sintering processes.
SOFC Assembly Process Based on Tape Casting Technology
Compared with the traditional screen printing process, the composite process combining tape casting and screen printing can effectively improve the integrity and performance consistency of SOFC single cells. The preparation process is as follows: Firstly, uniform and defect-free YSZ electrolyte layers and anode layers are fabricated via tape casting with customized slurry. The electrolyte layers and anode layers are then laminated and warm-pressed, followed by co-sintering treatment to obtain semi-finished cell structures. Finally, cathode layers are prepared by screen printing to assemble complete SOFC single cells.
Conventional single-layer lamination and warm pressing processes are prone to technical defects such as cracking, bubbling and delamination due to the low mechanical strength of YSZ ceramic thin films. To solve these problems, the double-layer tape casting process is widely adopted in current industrial production. The process realizes in-situ casting of electrolyte layers on pre-formed anode layers, which effectively simplifies the lamination and warm-pressing procedures and reduces interface defects.
Core Performance Indicators of Zirconia Electrolyte Thin Films
The service performance and service life of SOFCs are directly determined by the quality of zirconia electrolyte thin films, whose core evaluation indicators mainly include ionic conductivity, density and air tightness, as well as mechanical strength.
●Ionic Conductivity
Ionic conductivity is the most critical performance indicator of zirconia electrolyte thin films. High oxygen ionic conductivity can significantly improve the output power and operational stability of SOFCs, reduce internal resistance loss, and ensure efficient and continuous electrochemical reactions of cells.
●Density and Air Tightness
YSZ electrolyte thin films require ultra-high density and air tightness. The electrolyte layer acts as a barrier between the anode fuel gas and cathode oxygen. Insufficient density and air tightness will cause gas mutual permeation and cross-reaction, leading to cell short circuits, reduced open-circuit voltage and degraded overall cell performance.
●Mechanical Strength
Long-term stable operation of SOFCs requires the electrolyte thin film to have excellent mechanical strength, which can resist structural deformation and damage caused by high-temperature operation, thermal cycling and stress changes, ensuring the structural integrity of the cell.
Key Influencing Factors of Electrolyte Thin Film Performance
●Slurry Composition
Slurry composition is the core parameter restricting the forming quality of tape-cast green films, directly affecting the tensile strength, flexibility and bulk density of green tapes, as well as the final performance of sintered thin films. Ceramic powder is the functional core of the slurry, and its solid content determines the electrochemical performance of SOFC electrolyte films.
In theory, a higher solid content of zirconia slurry is conducive to improving the density and ionic conductivity of sintered films. However, excessive solid content will increase slurry viscosity, reduce dispersibility and rheological properties, and cause uneven thickness and structural defects in cast green tapes. Therefore, it is necessary to optimize the ratio of solid content, solvents, binders, dispersants and other components to balance slurry fluidity and film forming performance.
In addition, the type of sintering aids, binder system and dispersant also play a vital role in adjusting slurry rheology, improving solid dispersion uniformity and optimizing the microstructure and electrical properties of subsequent sintered thin films.
●Tape Casting Process Parameters
Tape casting process parameters cover casting speed, drying environment, debinding and sintering processes, which jointly determine the microstructure and comprehensive performance of zirconia electrolyte films.
During tape casting, the slurry forms a composite flow of pressure flow and drag flow under the action of the moving substrate. The gap between the scraper and the substrate accurately controls the thickness of the green film, and the surface tension of the slurry ensures the flatness and smoothness of the film surface. Stable slurry viscosity, fixed scraper gap and constant liquid level height are the key guarantees for preparing uniform-thickness zirconia green tapes.
The drying process of zirconia green tapes involves polymer chain shrinkage, particle sedimentation and rearrangement. A large amount of solvent in the slurry will evaporate during drying, and the evaporation rate directly affects the forming quality of green bodies. To avoid curling, cracking and uneven shrinkage of green films, it is necessary to precisely control the drying temperature, relative humidity and air flow speed according to the film thickness, solid phase content and organic component content, so as to realize slow and uniform solvent evaporation and eliminate internal pores and structural defects.
The core purpose of the debinding process is to thermally decompose and remove organic binders from the green body. The process includes three key steps: high-temperature decomposition of binders, diffusion of decomposition products to the film surface, and volatilization of surface products. Different from the low-temperature drying process, debinding requires a higher temperature environment. After debinding, dense electrolyte thin films can be prepared via atmospheric sintering, hot pressing sintering and other conventional sintering methods.
Performance Optimization Strategies of Zirconia Electrolyte Thin Films
The mainstream SOFC structure adopts fuel electrode-supported YSZ electrolyte thin films, which can effectively reduce ohmic impedance and ensure operational safety. Current performance optimization for zirconia electrolyte films is mainly carried out from two dimensions: electrolyte thickness optimization and electrolyte structure optimization.
●Electrolyte Thickness Optimization
The ohmic resistance of SOFCs mainly comes from the electrolyte layer. Appropriately reducing the electrolyte thickness can significantly lower the cell ohmic resistance and electrode polarization resistance, thereby improving the output power and energy conversion efficiency of fuel cells. In industrial production, the electrolyte thickness is precisely controlled by adjusting the scraper height of the tape caster.
However, the reduction of electrolyte thickness is limited. Excessively thin electrolyte layers will lead to decreased mechanical strength and air tightness, and are prone to fracture and gas leakage during long-term high-temperature operation, which seriously affects cell stability and service life. Therefore, the electrolyte thickness must be optimized in a balanced manner to ensure both high ionic conductivity and reliable structural stability.
●Electrolyte Structure Optimization
Traditional single-layer YSZ electrolyte films have low ionic conductivity at medium and low temperatures, requiring a high operating temperature of 800–1000 ℃ to maintain ideal electrochemical performance. Long-term high-temperature operation will increase material selection difficulty, raise cell preparation costs and shorten the service life of SOFCs.
Structural optimization of electrolytes is an effective means to improve the medium-low temperature performance of SOFCs. The zirconia-ceria (ZrO₂-CeO₂) double-layer electrolyte system prepared by co-tape casting technology has become a research hotspot. The preparation process is as follows: firstly, cast a ZrO₂-based electrolyte layer as the substrate, and then cast a CeO₂-based electrolyte layer on its surface to form a composite double-layer structure.
The ZrO₂-CeO₂ double-layer electrolyte gives full play to the advantages of the two materials: it retains the excellent chemical stability of zirconia-based electrolytes, while utilizing the high ionic conductivity of ceria-based electrolytes. This structure can effectively reduce the operating temperature of SOFCs, optimize ion conduction efficiency and block electron conduction, realizing efficient and stable operation of medium-low temperature SOFCs.
