Tem study of oxygen partial pressure effect on early lsm-ysz surface interactions in solid oxide fuel cells

The LSM-YSZ interactions along the cathode/electrolyte interface of solid oxide fuel cells operated at 750 °C with ambient air or oxygen as cathode gas are investigated. Secondary phase formation across the LSM/YSZ interface is examined by transmission electron microscopy (TEM), energy-dispersive x-ray spectroscopy (EDS), electron energy-loss spectroscopy (EELS) and energy-filtered TEM (EFTEM). Two substantially different LSM-YSZ reactions are observed. In the air-tested cell, Zr-rich phases are found to form inside the LSM which is in contact with YSZ. The Zr-rich phases are proposed to be a precursor phase of insulating zirconate and are probably responsible for aggravating cathode degradation by blocking the triple-phase boundary (TPB) active sites. In the oxygen-tested cell, no formation of Zr-rich phases occurs because the LSM could still maintain its A-site deficiency in higher oxygen partial pressure. Although formation of a Mn3O4 phase along the LSM/YSZ interface is observed, the absence of a zirconate phase allows the TPBs to remain unimpaired to a large extent and therefore, the cell exhibits milder cathode degradation. Theoretical calculations of the Mn solubility in the LSM and YSZ as a function of oxygen partial pressure and temperature is also performed, providing an explanation for the observed difference of LSM stability, LSM-YSZ interactions in air and in pure oxygen, respectively.