Electro-chemo-mechanical Studies of Perovskite-Structured Mixed Ionic-Electronic Conducting SrSni-₁₋xFexO₃₋x/₂₊[epsilon]

Abstract

High efficiency and fuel flexibility make solid oxide fuel cells (SOFCs) attractive. However, when operating at reduced temperatures, there is significant loss in efficiency, of which slow surface reaction kinetics at the cathode are most responsible. Previously, the mixed ionic and electronic conducting (MIEC) perovskite-structured SrSni-₁₋xFexO₃₋x/₂₊[epsilon] (STF) materials system was identified as a promising candidate for SOFC cathodes given rapid oxygen surface exchange kinetics. The exchange kinetics were correlated with the minority electron charge density in STF, which in turn depends on its defect chemistry and band structure. In this work, an alternate B site host cation, Sn, was selected to replicate and extend the STF studies, due to its distinct band structure and higher electron mobility. Oxygen nonstoichiometry and the defect chemistry of the SrSni-₁₋xFexO₃₋x/₂₊[epsilon] (SSF) system were examined by means of thermogravimetry as a function of oxygen partial pressure in the temperature range of 973-1273 K. Marginally higher reducibility was observed compared to corresponding compositions in STF system. The bulk electrical conductivity was measured in parallel to examine how changes in defect chemistry and electronic band structure associated with the substitution of Ti by Sn impact carrier density and ultimately electrode performance. Bulk chemical expansion was measured by dilatometry as a function of oxygen partial pressure while surface kinetics were examined by means of AC impedance spectroscopy. The electrochemo- mechanical properties of SSF were found not to differ significantly from the corresponding composition in STF. It is believed that Fe dominates the character of the valence and conduction bands and thus governs the electronic properties in SSF. Though slightly shifted by Sn's larger size, the defect equilibria - including the oxygen vacancy concentration - were found to also be largely dominated by Fe and thus differed only in a limited way from that in STF. Key thermodynamic parameters of SrSn₀.₆₅Fe₀.₃₅O₂ ₈₂₅₊ SSF35 obtained include the reduction enthalpy (4.30 eV) the electronic band gap (1.72 eV) and the anion Frenkel enthalpy (0.52 eV). Key kinetic parameters include the migration enthalpy of oxygen vacancies (0.70 eV), the activation energy of area-specific-resistance (1.65 eV) and the electron (0.0002±0.00005 cm²/V-s) and hole (0.0037±0.0015 cm²/V-s) mobilities. With the surface exchange rate nearly identical to the STF35 counterpart, the main advantage of SSF35 as a SOFC electrode would be its enhanced chemical stability and slower degradation.