Deformation mechanism of Ni(O)-yttria-stabilized zirconia upon reduction and its effect on cell stress evolution in solid oxide fuel cells

Deformation of anodes upon reduction causes a cell stress evolution, possibly resulting in cell failure. We studied the effects of compositions on anode deformation and the mechanisms by monitoring anode dimensions and constructing three-dimensional structures. A new evaluation index, "the path length of particles connections (PLPC)", was defined to characterize the microstructure of the anodes, and a YSZ-Ni content diagram was constructed to clarify the relationship between deformation behavior and microstructure. The microstructure exerts significant effects on the deformation, and the deformation mechanisms are classified into four categories based on the PLPC results. The characteristic deformation behavior is discussed for each categories. The stress evolutions of the cell electrolyte upon reduction were examined by FEM analysis incorporating the anode deformation and creep behavior. The numerical analysis demonstrates that the larger the anode deformation, the higher the cell stress generated upon reduction, and the anode deformation may have an impact on mechanical reliability of cells. These results indicate that the cell stress change during reduction may be due to anode deformation and be prevented by selecting optimal anode composition. The YSZ-Ni content diagram is expected to provide fundamental and useful knowledge for designing optimal microstructure in anodes of SOFCs.