Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn[subscript 1-x]Fe[subscript x]O[subscript 3-x/2+δ] part I: Defect chemistry

Abstract

Oxygen nonstoichiometry and the defect chemistry of the SrSn[subscript 1-x]Fe[subscript x]O[subscript 3-x/2+δ] (SSF) system were examined by means of thermogravimetry as a function of oxygen partial pressure in the temperature range of 700–1000 °C and compared against the corresponding mixed ionic-electronic conducting titanate, SrTi[subscript 1-x]Fe[subscript x]O[subscript 3-x/2+δ] (STF) system. The alternate B site host cation, Sn, was selected to replicate and extend the STF studies, given its distinct band structure and higher electron mobility associated with its 5s derived conduction band as compared to the 3d nature of the conduction band in the titanate. Though shifted slightly by the larger size of Sn, the defect equilibria – including the oxygen vacancy concentration – were found to be largely dominated by Fe oxidation state, and thus differed only in a limited way from those in STF. Key thermodynamic parameters for SrSn[subscript 0.65]Fe[subscript 0.35]O[subscript 2.825+δ] (SSF35) were derived including the reduction enthalpy (4.137 ± 0.175 eV), the high temperature electronic band gap (1.755 ± 0.015 eV) and the anion Frenkel enthalpy (0.350 ± 0.350 eV). The implications these observations have for cathode behavior in solid oxide fuel cells are briefly discussed.