Multiscale Modeling Of Solid Oxide Fuel Cells

As a low pollution emission electrochemical device,solid oxide fuel cell(SOFC)has a unique charm which can efficiently convert chemical energy directly into electrical energy.Based on this trait,the SOFC's efficiency surpassing the Carnot cycle limitation.The high operating temperature enables the SOFC feeding with hydrogen,methane and even other hydrocarbon fuels.This fuel flexibility led to a clean and efficient way for the natural gas which meets the demands of sustainable development,to alleviate global energy crisis and environmental issues.Lowering the operational temperature,stabilizing the SOFCs at various gas conditions,understanding the poisoning effects and optimizing the structure of cell and stack are the major challenges for fuel cell's commercial applications.As key factors,experimental work results are very important in SOFC technology development to offer direct evidence for improvement in performance,but the experiment is limited by it's long development period which will increase the difficulty in researches and development of SOFCs.Therefore,theoretical calculations and numerical simulation analysis are significant to reduce the cost and design new materials and system.This thesis focuses on thermal stresses distribution of in a fuel cell and the poisoning behavior caused by imposing to main impurities in fuels.With the finite element method,we coupled multiphysics of fuel cell by considering the reforming reactions and the temperature distribution inside the single cell when combined with the mechanical properties.The optimization of interconnect was also implanted with the analysis of stress distribution.The first principle calculation was then used to investigate the interaction between catalyst and impurities in the atom scale.The main achievements works of this dissertation are listed as following:First of all,we reviewed the previous studies on stimulation and ab initio calculation works of SOFC,including the development and history,classification,work principle as well as applications.A functionally graded electrode was fabricated by the freeze-tape-casting process,which shows an anisotropy conductivity.With the anisotropic electrode conductivity,a single cell model was built to analyze the current density when the ion-electric transport was included in the model.A three dimensional single cell model was used to couple the gas and moment transport,heat transfer as well as thermal stress.Two material properties were compared in this model,when different synthesis methods were used or when the material was react with impurities.Considering the real cell was worked in a stack,we assumed the cell was fixed constraint in some cases.An optimization strategy was put forward taken the thermal stress into account.To investigate the influence of stress distribution of fuel cell,the width and height of interconnect rib were studied.Two different gas flow models were also compared.Finally,the first principle calculation was adopted to reveal the poisoning effect of catalyst with various contaminants,such as CH3 Cl and PH3.The interactions of impurities as well as impurity and substrate were discussed.Besides,the influences of operational temperature were considered by molecular dynamic method for the same system.The toxicity mitigation of CH3 Cl was also simulated by two dimensional model.In general,the finite element multi-physics model of fuel cell was established to explore the influence of the cell conductive performance and the thermal stress distribution,which also provided an optimal design for interconnect in the macroscopic scale.With the first principle calculation,the adsorption and interaction of Ni surface with impurity were deeply studied in the microscopic atom scale.The method of simulation and calculation were applied and extened on diverse scales.This was expected to lay a foundation for the interconnection between thermal stress and poison mechanisms of SOFC material in different scales for future work.