Mechanism of SO2 Poisoning on the Electrochemical Activity of LSCF and LSM Electrodes

Solid oxide fuel cells (SOFCs) offer great promise for the efficient and cost-effective conversion of chemical energy of fuels to electricity. However, sulfur in the air stream is one of the major contaminants affecting the performance stability of cathodes of SOFCs such as La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and La_0.8Sr_0.2MnO₃ (LSM) perovskite oxide. In this report, the effects of operating temperatures, current densities, and SO₂ concentrations on the electrochemical performance of LSCF and LSM cathodes are investigated. Sulfur poisoning of LSCF is more pronounced at lower temperatures, i.e., 700 ^oC. According to SIMS result, sulfur deposition occurs inside the LSCF electrodes and is most pronounced on the surface of LSCF at 700 ^oC, forming primarily SrSO₄, while sulfur deposition occurs mainly near the electrode/electrolyte interface for LSM electrodes. Similar electrode resistance change of porous LSCF electrodes after being poisoned under open circuit condition and cathodic current passage at 700 ^oC indicates that sulfur poisoning effect of LSCF has no direct relationship with polarization current. The concentration of SO₂ has a significant effect on the performance degradation of LSCF and LSM electrodes. Sulfur poisoning is not reversible. The reaction mechanism between sulfur and LSCF and LSM electrodes is discussed.