Electro-chemo-mechanical studies of perovskite-structured mixed ionic-electronic conducting SrSn_1-xFe_xO_3-x/2+δpart I: Defect chemistry

Abstract

Oxygen nonstoichiometry and the defect chemistry of the SrSn_1-xFe_xO_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_1-xFe_xO_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_0.65Fe_0.35O_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.

Appendix

Given that the predicted values of nonstoichiometry δ were rather insensitive to the equilibrium constant for anion Frenkel disorder, we followed an alternative approach by setting H _af and ${K}_{af}^0$ for SSF35 equal to that of STF35 in reference [6]. For this routine, data points from all temperatures were fit simultaneously by setting enthalpies and pre-exponential terms as variables, instead of fitting equilibrium constants at each temperature. The resulting enthalpies and pre-exponentials are shown in Appendix Table 4. Note that H _red for both fitting procedures (Table 3 and Appendix Table 4) were almost identical (within the error of the value from Table 3), and E _g was less than 3% different, indicating insensitivity to the anion Frenkel disorder energetics.

Table 4 Thermodynamic parameters for STF35 and SSF35 from fitting nonstoichiometry – δ – data as functions of oxygen partial pressure over a range of temperatures simultaneously