In-Situ Measurement of Residual Stress in Solid Oxide Fuel Cells

Abstract

Introduction Solid oxide fuel cells (SOFCs) are attracting attention as highly efficient energy conversion devices. However, durability and mechanical reliability are issues. To improve durability and mechanical reliability, it is essential to evaluate the residual stress generated in the constituent materials, especially the residual stress generated in the electrolyte. While residual stress is measured by various methods, the optimum measurement method has not been established yet. The purpose of this study is to evaluate the residual stress of the cell using in-situ cosα method, which has an advantage in measurement speed and easy handling. Experiment method High-temperature x-ray stress measurement was performed as a quantitative evaluation method for mechanical behaviors. A portable X-ray residual stress measuring device μ-X360s manufactured by Pulstec Industries was used. The cos α method, which is faster and easier than the sin2ψ method, was used for high-temperature X-ray stress measurement. In the cosα method, the Debye ring is acquired by a two-dimensional detector, and the stress is calculated from the difference between the stress-free Debye ring and the Debye ring of the measurement sample. High temperature chamber was designed and prepared for high temperature x-ray stress measurement. A capton film is used to separate outside and measurement atmosphere. In addition, the measurement position can be adjusted by moving stage. Commercial sofc cells (Ningbo SOFCman cell and Elcogen) were used to compare the residual stress during the various operation procedures. Results and discussion Fig. 1 shows the difference in change in residual stress of YSZ electrolyte depending on the structure of the anode. Anode of Ningbo SOFCman cell has small porosity, while anode of Elcogen cell has large porosity. We measured changes in residual stress during heating, anode reduction, and reoxidation processes. The Elcogen cell was reduced much faster than Ningbo cell, because hydrogen gas can diffuse more easily in anode due to higher porosity. Therefore, residual stress of Elcogen cell steeply changed after hydrogen introduction. Then, the residual stresses after anode reduction were completely different in these two cells at room temperature. Compressive stress of 800 MPa remained in the electrolyte of Ningbo cell. On the other hand, less compressive stress of 400 MPa remained in the electrolyte of Elcogen cell. The temperature dependence of residual stress is also different in the two cells. The deformation of the cell after reoxidation was also completely different. Ningbo cell was deformed convexly toward the anode side, although Elcogen cell was hardly deformed. The difference may come from the difference in porosity of anode. Fig. 1 Circumferential residual stress of YSZ during reoxidation heating in 20％O2-N2 and SEM images of anode structures. Figure 1