Impact of Sr segregation on the electronic structure and oxygen reduction activity of SrTi1-xFexO3 surfaces

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. 5 The correlation between the surface chemistry and electronic structure is studied for SrTi 1-x Fe x O 3 (STF), as a model perovskite system, to explain the impact of Sr segregation on the oxygen reduction activity of cathodes in solid oxide fuel cells. Dense thin films of SrTi 0.95 and SrFeO 3 (STF100) were investigated using a coordinated combination of surface probes. Composition, chemical binding, and valence band structure analysis using angle-resolved x-ray photoelectron 10 spectroscopy showed that Sr enrichment increases on the STF film surfaces with increasing Fe content. In situ scanning tunnelling microscopy / spectroscopy results proved the important and detrimental impact of this cation segregation on the surface electronic structure at high temperature and in oxygen environment. While no apparent band gap was found on the STF5 surface due to defect states at 345 o C and 10-3 mbar of oxygen, the surface band gap increased with Fe content, 2.5 ± 0.5 eV for STF35 and 3.6 ± 0.6 eV for 15 STF100, driven by a down-shift in energy of the valence band. This trend is opposite to the dependence of the bulk STF band gap on Fe fraction, and is attributed to the formation of a Sr-rich surface phase in the form of SrO x on the basis of the measured surface band structure. The results demonstrate that Sr segregation on STF can deteriorate oxygen reduction kinetics through two mechanisms – inhibition of electron transfer from bulk STF to oxygen species adsorbing onto the surface, and the smaller 20 concentration of oxygen vacancies available on the surface for incorporating oxygen into the lattice. 1. Introduction Because of their high efficiency and fuel flexibility, solid oxide fuel cells (SOFCs) offer the potential to contribute significantly to a clean energy infrastructure 1, 2. However, their high working 25 temperatures (>800 o C) impose challenges due to accelerated materials degradation and high cost. The lowering of the working temperature has, therefore, become a strong focus of research. 3 At reduced temperatures (<700 o C), slow Oxygen Reduction Reaction (ORR) kinetics at the cathode become a major barrier to 30 the implementation of high performance SOFCs. To rationally design new cathode materials with high ORR activity, it is necessary to understand the governing ORR mechanisms and identify key descriptors of …