Planar solid oxide fuel cells (SOFCs) offer several advantages, i.e., high power density, simplified fabrication processes, and excellent stack compactness, thus making them the predominant architecture in SOFC technology. However, high-temperature sealing remains a major challenge for planar SOFC stacks. Sealing technology is critical in determining the safety, efficiency, and overall stability of SOFC systems. The sealing material as the core component of this technology directly affects the stack hermeticity, thereby impacting its power output and long-term operational reliability.

Planar solid oxide fuel cells (SOFCs) utilize two primary sealing methods, i.e., compressive and rigid sealing, which are distinguished via the application of compressive load during the sealing process. Compressive sealing materials suitable for planar SOFCs include metal, mica-based, and ceramic-based compressive seals, each offering unique properties such as excellent deformability, high stability, and superior high-temperature resistance. 1) Metal Compressive Seals: Silver (Ag) is the most widely used metal compressive sealing material due to its outstanding ductility, chemical stability, and lower cost, compared to platinum and gold. However, Ag also has several drawbacks, including temperature sensitivity, low mechanical strength, and limited chemical stability under both oxidizing and reducing atmospheres, hindering its suitability for commercial SOFC applications. 2) Mica-based Compressive Seals: Mica-based compressive sealing materials feature a distinctive layered structure, with adjacent layers held together by weak K⁺ interactions. This unique arrangement enables an interlayer sliding under compressive stress, enhancing their adaptability as sealing materials. The main gas leakage pathways in mica-based compressive sealing materials occur through the mica itself and the contact interfaces between mica and adjacent components. These leakage pathways can be mitigated via introducing an intermediate layer or infiltrating with wetting materials. Thermiculite® 866 is one of the most commercially advanced mica-based compressive sealing materials. It is based upon the mineral vermiculite and contains no organic binder or any other organic component. Thermiculite® 866 is soft and highly conformable, allowing for both macro- and micro-sealing to be readily achieved. In addition, it also maintains its sealing properties without relaxation or creep, even under high-temperature conditions. 3) Ceramic-based Compressive Seals: Ceramic-based materials with a great thermal and chemical stability are challenging to use directly as compressive sealing materials for SOFCs, considering their ductility, thermal expansion coefficient (TEC), and chemical compatibility. However, it is feasible to modify the morphology, optimize the manufacturing process or incorporate metallic components to improve their deformability, making them suitable for seals. In summary, the development and optimization of compressive sealing materials are crucial for enhancing the performance and durability of planar SOFCs. Each type of sealing material presents distinct advantages and challenges, necessitating ongoing research to address these issues and improve SOFC technology.

Rigid sealing materials generally provide a superior gas tightness, compared to compressive seals, making them a focal point in the development of planar solid oxide fuel cells (SOFCs). These rigid seals primarily encompass glass, glass-ceramic, and metal brazing techniques. 1) Glass and Glass-ceramic Seals: Glass and glass-ceramic materials are among the most commonly used sealing materials in planar SOFCs. Adjusting the phase composition or controlling the crystallization process of these materials allows for tailoring their TEC, effectively minimizing thermal expansion mismatches with adjacent cell components. However, solely designing the composition of glass sealants to meet comprehensive performance requirements, such as glass transition temperature (T_g), softening temperature (T_s), TEC, thermal stability, and mechanical strength, poses some challenges. Incorporating ceramics, mica, or glass fibers into glass sealants can significantly enhance their overall properties. A research indicates that glasses and particularly glass–ceramics are ideal sealant candidates due to their properties, including thermal expansion, can be tailored to be compatible with other fuel cell materials. 2) Metal Brazing: Brazing is a high-temperature joining technique wherein a molten metal filler material fills the gap between metal and ceramic components, interacting with the substrates and solidifying upon cooling to form a robust, hermetic joint. Brazing techniques are primarily categorized into active metal brazing (AMB) and air reaction brazing (RAB). AMB is typically conducted under vacuum or a protective gas atmosphere, leading to higher production costs, compared to RAB. The Ag-CuO system as one of the most preferred RAB materials faces several challenges, i.e., high TEC, poor chemical stability, and low mechanical strength, significantly hindering its long-term stability in SOFC applications. In summary, while rigid sealing materials offer superior gas tightness essential for the efficient operation of planar SOFCs, some challenges remain in optimizing their properties to ensure long-term stability and compatibility with other cell components. Ongoing research and development efforts focus on addressing these challenges to enhance the performance and durability of SOFC systems.

Solid oxide fuel cells (SOFCs) hold a significant promise for efficient energy conversion, however, some challenges associated with sealing materials impede their widespread commercialization. Two primary causes of sealing failure are identified:

1) Thermal Expansion Mismatch: Differences in the thermal expansion coefficients (TEC) between sealing materials and adjacent SOFC components can induce thermal stresses at high operating temperatures. These stresses may lead to the formation of pores and cracks, compromising the mechanical integrity of the seal and resulting in gas leakage.

2) Decomposition and Interfacial Reactions: Exposure to high temperatures and environments with both oxidizing and reducing conditions can cause decomposition of sealing materials. In addition, chemical reactions at the interfaces between seals and SOFC components can also occur, ultimately leading to a sealing failure.

Addressing these issues is crucial for the commercial application of SOFCs. Future research should focus on:

1) Enhancing high-temperature stability and chemical compatibility: Developing materials that remain stable and chemically inert under SOFC operating conditions.

2) Improving TEC matching: Tailoring the TEC of sealing materials to closely align with those of adjacent components to minimize thermal stresses.

3) Enhancing resistance to chemical corrosion and oxidation: Creating seals that can withstand corrosive environments and resist oxidation over prolonged periods.

4) Optimizing glass-ceramic formulations: Adjusting compositions to reduce crystallization tendencies, thereby improving mechanical properties and durability.

5) Developing self-healing materials: Innovating materials capable of autonomously repairing minor damages, extending the operational lifespan of seals.

6) Advancing cost-effective solutions: Streamlining manufacturing processes to reduce costs without compromising quality.

7) Promoting environmentally friendly technologies: Ensuring that new sealing materials and processes with a minimal environmental impact, aligning with sustainable development goals.

Advancements in sealing materials are pivotal for the progression of solid oxide fuel cell (SOFC) technology. These materials must maintain a long-term stability under extreme operating conditions, i.e., high temperatures, elevated pressures, and chemically aggressive environments. In addition, they also require excellent thermal expansion compatibility and gas impermeability to ensure the structural integrity and operational longevity of SOFC stacks. Future developments in SOFC sealing materials should focus on glass-ceramic composites, offering superior chemical stability, tailored thermal expansion behavior, and robust mechanical properties at elevated temperatures. These characteristics make them well-suited for ensuring the long-term stability of SOFC systems. The microstructure of these composites can be engineered to enhance gas tightness, self-healing capabilities, and overall durability via optimizing the composition of glass and ceramic phases, as well as refining heat treatment processes. The development of high-performance glass-ceramic composite sealing materials will address the critical challenges associated with the existing SOFC sealing technologies, providing some essential solutions for the commercialization and large-scale deployment of SOFC systems. Advancements in this field will facilitate a widespread adoption of clean energy technologies, contributing to the global transition toward a low-carbon economy and sustainable energy infrastructure.