Thermal Residual Stress and Failure Probability of Functionally Grade Ni-YSZ Anode of Solid Oxide Fuel Cells Using a Finite Element Analysis

Abstract

Due to the differences of thermal expansion coefficient between the components in the solid oxide fuel cells (SOFCs), there is a large residual thermal stress in the fabrication process of the cell. In this study, the functionally graded (FG) Ni-YSZ anode design was introduced between Ni-YSZ support and YSZ electrolyte to mitigate the thermal residual stress in SOFC. In the current FG anode design, composition of Ni and YSZ was varied according the function of φ (Ni) = k ( t / H ) n , where k is a constant which is equal to the volume fraction of Ni in Ni-YSZ anode support layer, t is the distance to the electrolyte, H is the thickness of the FG anode layer and n is the exponential factor. In the numerical model, the FG anode layer was evenly divided into m sublayers. The porosity was set to a constant value of 0.3 and uniformly distributed in the anode. The residual stress of the anode was modeled as the thermal stress when the anode was cooled from the reduction temperature of 1200 oC to room temperature of 20 oC. Residual thermal stress was calculated by finite element method (FEM) using the commercial ANSYS package. Failure probability of functionally graded SOFC was evaluated using the classic Weibull probability model.The effects of sublayer number m = (1, 2, 3, 4), exponential factor n = (0.25, 0.5, ..., 4.0) and gradient layer thickness H = (20, 30, 40 μm) on the stress and failure probability of functionally graded SOFC are discussed. The calculation results show that the FG anode layer can obviously reduce the stress, and effectively decrease the stress concentration between electrode and electrolyte. The failure probability can be reduced by two orders of magnitude at most as compared with the conventional SOFC. Under the same exponential factor n , appropriately increasing the number of sublayers m can reduce the stress of the gradient layer. The sublayers number m = 2 is a suitable value. With the increase of the thickness of the gradient layer, the anode failure probability increases. The research provides a reference for further optimizing the stress in SOFC.