Relationship Between Anode Deformation and Cell Stress Evolution During Reduction in Solid Oxide Fuel Cells and Electrolysis Cells

Solid oxide fuel cells (SOFCs) and solid oxide electrolyte cells (SOECs) are attracting attention as one of next-generation energy devices because of high energy conversion efficiency or non-necessity for expensive catalyst. To commercialize SOFCs/SOECs, it is crucial to ensure their mechanical reliability. Various types of stresses generated in the cells during the lifetime from manufacturing to operation may cause cells failure and system safety degradation. In particular, the anode deformation during reduction is considered to be one of the most important factors causing cell stress changes, but the results reported by previous studies vary, and there are also few reports about the relationship between the anode deformation and cell stress evolution.<br/>In this study, the deformation behavior of anodes with different Ni contents during reduction and the effects of temperature on it were investigated in-situ by a dilatometer and X-ray diffractometry using NiO-YSZ bulk samples which is often used for the anode in SOFCs/SOECs. In addition, the cell stress changes during reduction process are examined by FEM analysis and verified by X-ray diffractometry.<br/>Since YSZ is stable and NiO shrinks under reducing conditions, it was generally assumed that NiO-YSZ shrinks during the reduction process. However, only the sample with the highest Ni content in this measurement was shown to shrank, (specifically, -0.32% strain at 700°C, -0.10% at 800°C and -0.071% at 900°C), while the other NiO-YSZ samples expanded, (for example, a sample with 27% Ni content showed 0.024% strain at 700°C, 0.013% at 800°C and 0.051% at 900°C). Both the amount of expansion and shrinkage decreased with increasing temperature during reduction. We found that the factor of expansion was the change or release of the residual stress on YSZ side, because the strain change in the NiO-YSZ measured by the dilatometer and that calculated form residual stress change on the YSZ side measured by X-ray diffractometry mached each other in both direction and amount. In addition, it can be explained that the factor of decreasing expansion amount with increasing temperature is also due to the residual stress change on the YSZ side.<br/>In the cell configuration, only the anode is deformed unless the cathode is exposed to a reducing environment. Thus, we hypothesized that the factor for cell stress changes during reduction is the anode deformation, and investigated the cell electrolyte stress change by FEM analysis taking into account the anode deformation, creep behavior and temperature dependence of the relevant physical properties. It was also verified by X-ray diffractometry based on the cosα method. The measured cell was a multilayer structures consisting of an anode, an electrolyte, and an interlayer, which is a structure similar to that of the actual cell. The principal stress on the electrolyte surface was computed form the FEM analysis. Here, the positive side is tensile direction, and the negative side is compressive direction. Specifically, when the anode showed expansion, there were almost no stress changes on the cell electrolyte. However, when the anode showed shrinkage, the residual stress increased by approximately 300MPa on the compressive side during reduction. Then, the residual stresses were relaxed by creep after reduction under both analysis conditions.<br/>The experimental results together with the FEM analysis suggest that the anode deformation may control cell stress change during reduction, and the numerical method developed in this study can offer a useful tool for evaluating the cell stress evolution during the reduction process.