Mechanism of sulfur poisoning of Sr₂Fe_1.5Mo_0.5O_6-δ perovskite anode under solid oxide fuel cell conditions

The interactions between sulfur and the Sr₂Fe_1.5Mo_0.5O_6-δ (SFMO) perovskite anode are investigated using periodic density functional theory (DFT) calculations and constrained ab initio thermodynamic analysis under anodic solid oxide fuel cell conditions. Three surface models with different Fe:Mo ratios in the topmost layer are used to investigate the mechanism of sulfur poisoning. Sulfur prefers to interact with these surfaces by replacing existing oxygen rather than adsorbing on a metal or oxygen vacancy. Constructed phase diagrams suggest that the surface with higher Mo content on the gas exposed surface layer is highly resistant toward sulfur poisoning, whereas the FeO₂ terminated surface is more susceptible to sulfur poisoning. The presence of S in the surface has also a negative impact on the surface vacancy formation process, which is the rate-controlling step in the H₂ electro-oxidation. Adding a small Ni₃ cluster to the least active FeO₂ terminated surface promotes the oxygen vacancy formation; however, the presence of strongly adsorbed S on the Ni cluster makes this process more endergonic, which in turn will decrease the activity of the anode. Based on these results, we suggest that increasing the Mo content in the gas exposed surface layer of SFMO will improve its overall electrochemical performance while maintaining excellent sulfur tolerance.