Numerical Simulation of Electrolyte-Supported Planar Button Solid Oxide Fuel Cells with Layered Electrolytes
Tuesday, May 13, 2014
Grand Foyer, Lobby Level (Hilton Orlando Bonnet Creek)
(YSZ/SCSZ) electrolytes were designed for intermediate temperature Solid Oxide Fuel Cells (SOFCs), so that the inner SCSZ layers provided superior ionic conductivity and the outer YSZ layers provided chemical and phase stability. A total of nine electrolyte designs were developed and tested. Due to the mismatch in the thermal expansion coefficients between the two electrolyte materials, residual stresses develop between the layers. It was noticed that the ionic conductivity of the electrolytes is enhanced due to the presence of residual stresses. To study the electrochemical performance of the SOFC with different electrolyte materials and configurations, an electrolyte-supported planar button SOFC is modeled using COMSOL Multiphysics®
4.3. The main governing equations include the charge conservation equation, Maxwell-Stefan diffusion model, Butler-Volmer equation, and Brinkman equation. The cathode and anode materials used are (La0.6
and Ni-YSZ respectively. Validation of the model was done against experimental results of another planar SOFC by Joongmyeon et al. (Journal of Power Sources, 2007). The simulations show that SOFCs with 3-layered pure SCSZ electrolyte produced the highest power density while the single cell with 6-layered pure YSZ electrolyte produced the least power density. These results make sense, since the increase in the electrolyte thickness increases the ohmic losses. The model developed can be used in further studying the behavior of the cells.
In order to study the mechanical properties of the electrolytes, biaxial flexure tests were performed via a ring-on-ring method at room temperature and 800 °C. Due to the high deflection of the electrolytes, a non-linear dependence between stress and load was observed. Therefore, the geometric non-linearity feature was utilized in the modeling to calculate the maximum tensile stress in the specimen at a given load. Results showed that layered electrolytes have higher biaxial flexure strength. This is theorized to be due to the presence of compressive stresses at the outer YSZ layers. Weibull statistics was used on stress calculations to verify the results.