Durability Model of SOFC Anode Structure under Thermo-Mechanical and Fuel Gas Contaminants Effects

Solid Oxide Fuel Cells (SOFCs) operate under harsh environments, which cause the deterioration of anode material properties and reduce their service life. In addition to electrochemical performance, structural integrity of the SOFC anode is essential for successful long-term operation. Anode-supported SOFCs rely on the anode to provide mechanical strength to the positive-electrolyte-negative (PEN) structure. The stress field in the anode may arise from a variety of phenomena including thermal expansion mismatch between layers in the PEN structure, thermal/redox cycles and external mechanical loads. Moreover, some fuel contaminants such as phosphine (PH3) interact with the anode materials which lead to the formation of secondary phases and grain growth. These mechanisms result in the formation of microcracks, and degrade anode structural and electrochemical properties. Assessments of the evolution of anode mechanical properties during long-term operation are therefore essential to predict SOFC working life.;The principal objective of this research is to develop a structural durability model for the SOFC anode that takes into account thermo-mechanical and fuel contaminants effects on the anode material properties. The model is implemented in finite element analysis through a user defined subroutine to predict anode long-term structural integrity. The model is exploited to predict the stress-strain relations of Ni-YSZ at temperatures and porosities which are difficult to generate experimentally. Accelerated exposure tests under high contaminant concentrations dictate that the electrochemical degradation is the principal mode of cell failure while the cell structure is still intact. However, the model predicts that under lower contaminant concentrations, the anode structural degradation may be significant as compared to the electrochemical degradation in long-term operation.;The proposed model is enhanced for the planar-SOFC configurations exposed to PH3. The model predicts that under pure thermo-mechanical effect, the critical location for structure failure is near the corner of highest thermal gradient. However when fuel contaminant structural effect is superimposed on the thermo-mechanical effect, the critical location may shift depending on the flow configuration. Under similar operating conditions, i.e. same current density, co-flow configuration yields a higher anode structural life than counter-flow or cross-flow configurations. The knowledge obtained from this research will be useful to establish control parameters to achieve desired service life of the SOFC stack under various operating conditions.