Designing ZnFe₂O₄ diffusion barrier via Ag–ZnO filler to stabilize YSZ/Crofer 22 H joints under high-temperature oxidation

Developing filler/stainless steel interface with high-temperature interfacial stability and strong bonding strength is essential to further promote solid oxide fuel cell (SOFC) performances. This study demonstrates an effective route to construct robust and reliable filler/Crofer 22H interface by optimizing the interfacial structure using the reactive air brazing method. Our approach relies on altering internal diffusion and reaction processes of Fe, Cr and O at the interface, rather than simply adjusting external brazing parameters (e.g., temperature and time). By adding an Ag–ZnO filler between YSZ electrolytes and Crofer 22H interconnects, a continuous and compact ZnFe₂O₄ spinel layer is in-situ formed during the brazing process, acting as a “protective wall” between Ag–ZnO and Crofer 22H. This layer effectively controls the diffusion/reaction processes of O, Fe and Cr, thereby influencing the interfacial phase structure and preventing excessive oxidation under high-temperature service condition, while ensuring strong bonding strength. The developed joint exhibits an initial shear strength of 62 ± 4 MPa, retaining 51 ± 1 MPa and 31 ± 4 MPa after 600 h and 1000 h of oxidation at 800 °C in air, respectively, with no interfacial cracking or spallation. The Density functional theory calculations combined with theoretical analysis indicate that ZnFe₂O₄ spinel simultaneously blocks the diffusion of O, Fe and Cr through the high oxygen migration energy barrier and Fe vacancy formation energy. This approach not only enhances the long-term stability of SOFC joints but also provides a novel strategy for designing new Ag-oxide fillers.