Principal Investigator Harry Tuller
High efficiency and fuel flexibility make solid oxide fuel cells (SOFCs) attractive for conversion of fuels to electricity. Reduced operating temperatures, desirable for reduced costs and extended operation, however, result in significant losses in efficiency. This loss has been traced primarily to slow cathode surface reaction kinetics. In this work, we extend previous studies on the promising mixed ionic and electronic conducting perovskite-structured SrTi1-xFexO3-x/2+delta (STF) materials system, whose exchange kinetics were correlated with the minority electron charge density by replacing Ti with Sn, due to its distinct band structure and higher electron mobility.
Oxygen nonstoichiometry and the defect chemistry of the SrSn1-xFexO3-x/2+delta (SSF) system were examined by 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 the 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 electro-chemo-mechanical properties of SSF were found not to differ significantly from the corresponding composition in STF. Key thermodynamic and kinetic parameters for SrSn0.65Fe0.35O2.825+delta (SSF35) were derived, including the reduction enthalpy, electronic band gap, anion Frenkel enthalpy, oxygen vacancy migration energy, and electron and hole mobilities. Though slightly shifted by the larger size of Sn, the defect equilibria and the cathode area specific resistance differed only in a limited way from that in STF. This small difference was attributed to properties being largely dominated by Fe and not by the substitution of Ti with Sn.