Effect of continuously graded functional layer on curvature and residual stress of solid oxide fuel cell in initial reduction process

Abstract

Solid oxide fuel cell (SOFC) is considered to be a highly efficient device to convert chemical fuels directly into electrical power. Because of multilayer composite arrangement of cells, mismatch of the thermal expansion coefficients, chemical/thermal gradient, or phase change of the materials will result in residual stresses, which are reflected in the pronounced bending of unconstrained cells and cause a reliable problem. Considerable efforts have been devoted to the analysis of residual stresses in an elastic multilayer system, and one of the efforts that are to improve not only electrochemical performance for high energy conversion efficiency but also long term stability, is to process a continuously gradient anode functional layer (CG-AFL) between dense electrolyte and porous anode. Hence to understand the stress distribution and deformation of the multilayer with a CG-AFL is needed for the cell design. As the chemical reduction takes place at the interface between NiO-YSZ and the previously reduced porous Ni-YSZ, a reduced layer, together with the unreduced layer and the electrolyte will cause the residual stresses to be re-distributed. In this paper, taking the CG-AFL into account, the curvature and residual stresses of half-cell during reduction are analyzed. The results show that the curvature of half-cell with a CG-AFL increases as the reduction process. And the curvature would also increase as the thickness of the CG-AFL increases, and decrease with the increase of the index of power function that expresses young's modulus and thermal expansion coefficient of gradient layer. The residual stresses among the layers are correspondingly influenced by the thickness of the gradient layer, the index of power function and reduction extent. When taking power function as a linear function, the gradient layer obviously reduces the residual stress in the electrolyte. However, the increase of the index in power function will cause the increase of electrolyte residual stress. These mentioned analyses reveal that the CG-AFL cannot offer a solution that simultaneously improves the residual stress and curvature in a half-cell in terms of thickness and profile exponent of CG-AFL, i.e., the mitigation of residual stress will give rise to the increase of curvature, and vice versa. On the other hand, for part-reduced half-cell, the maximum tensile stress is found at anode/gradient layer interface in anode layer, which may facilitate structural failure since tensile residual stress is so high that it reaches the fracture strength of anode material. Consequently, it is important to ensure that the anode is fully reduced in practice. In conclusion, the existing gradient layer is helpful for enhancing the cell reliability via suitable design.