SOFCs can experience temperature changes during operation, especially when generating electricity from waste heat in industrial processes or thermal power plants, where heat supply is inconsistent. Additionally, environmental factors like diurnal temperature variations and extreme weather conditions can lead to substantial thermal fluctuations. These temperature variations cause thermal stresses due to the mismatch in the thermal expansion coefficients (TEC) of different SOFC components, potentially degrading the interfaces and reducing the power output. Therefore, maintaining thermal cycle stability is critical for the commercialization of SOFC technology.
A recent study by a team of material scientists, led by Liangzhu Zhu from the Key Laboratory of Advanced Fuel Cells and Electrolyzers Technology of Zhejiang Province, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, China, proposed a novel approach. They synthesized single perovskite oxide decorated R-P structured oxide using a self-assembly method to enhance catalytic activity and stability. This work demonstrated excellent TEC matching between strontium lanthanum ferrate and the electrolyte, showcasing its potential as a competitive air electrode for SOFCs.
"In this report, we synthesized dual-phase La0.8Sr1.2FeO4+d and La0.4Sr0.6FeO3-d by the simple self-assembly method. The single perovskite oxide, La0.4Sr0.6FeO3-d (LSF-P), with cubic structure and high catalytic activity was introduced to facilitate charge transport across the R-P structured oxides La0.8Sr1.2FeO4+d (LSF-RP) with various orientations. This approach overcomes the anisotropy inherent in the structure and concurrently enhances the catalytic activity of the composite electrode. The intimate hetero-interfaces that may form in situ between LSF-RP and LSF-P particles are anticipated to expedite the charge transfer process, thereby enhancing the ORR kinetics. We present the influence of the LSF-P content in dual phase on the phase structure, thermal expansion coefficient, electrode reaction kinetics, single cell performance under thermal cycling and reversible conditions in detail. The obtained results indicate that the incorporation of LSF-P improves the oxygen surface exchange kinetics, reduces the polarization resistance and significantly enhances the single-cell performance without sacrificing the stability of the composite electrode," said Liangzhu Zhu, professor at Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, China.
"The TEC values of R-P oxides are comparable to those of the electrolytes commonly utilized in SOFCs. However, it is important to note that R-P oxides exhibit two-dimensional conduction. They demonstrate significant anisotropy in the diffusion of oxygen ions and electrons, with transport predominantly occurring within the a-b plane and minimal movement along the c-axis. Consequently, there is a need to modify the R-P structured material to enhance its charge transfer capability, thereby increasing their catalytic activity, without sacrificing stability for application in SOFCs," said Liangzhu Zhu.
Introducing a secondary phase is a common strategy to boost the catalytic activity of R-P oxides. "Mechanical mixing is a relatively straightforward method for the introduction of secondary phase. While mechanical mixing can enhance electrode performance to some degree, it struggles with achieving a homogeneous distribution of the phases, which in turn restricts the interfacial contact between them. Infiltration is another alternative for introducing the second phase material. However, it is a cumbersome and time-consuming process that requires multistep operations," said Yang Zhang, one of the co-first authors and a postdoctoral researcher at Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, China.
"The self-assembly synthesis technique for fabricating composite materials is capable of yielding thermodynamically stable and homogeneously dispersed dual-phase structures in a single, streamlined operation. By merely adjusting the ratios of the starting materials, the incorporation of the second phase can be finely tuned. Furthermore, this self-assembly approach holds significant promise for creating numerous heterogeneous structural interfaces within composite air electrodes, which in turn can significantly boost the kinetics of the oxygen reduction reaction (ORR). Additionally, the method has the potential to greatly enhance the performance of composite air electrodes by optimizing the ORR process," said Liangzhu Zhu.
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