Investigation On Surface Catalytic Mechanism And Bulk Oxygen Transport Rpoperties Based On SrCoO_3-δ

The type of SrCoO_3-δ-based materials with the ABO3 perovskite structure is a sort of much important functional materials. Due to the performance of mixing ion-electronic conduction(MIEC), SrCoO_3-δwas originally used as oxygen permeation membrane. Take the face that only cubic phase SrCoO_3-δexhibited the highest electronic and oxygen ionic conductivity into account, a great deal of works had been done on the improvement and stabilization of cubic phase SrCoO_3-δby the methods of the A-site or B-site doping of SrCoO_3-δ. In recent years, SrCoO_3-δ-based materials with the formula (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3-δ(BSCF) served as cathode of solid oxide fuel cell(SOFC)had received most attentions.Based on Density Functional Theory(DFT), this thesis discuss the oxygen catalytic mechanism on the surface of SrCoO_3-δ-based cathode and the oxygen diffusion properties in the bulk of (Ba_0.5Sr_0.5)(Co_0.75Fe_0.25)O_3-δby the implement of first principle calculation. Firstly, we constructed the low index surface-(100), (110) and compared the stability between surface (100) and surface (110). It showed that the surface (100) had a little less surface energy than surface (110). It was mostly because that there existed strong polarization in surface (110), which must be stabilization by the method the surface reconstruction to fulfill the self-consistent distribution of electronic density. Then, we optimized two kinds of configurations that an oxygen molecule adsorbed on the top-Sr and top-Co site of the surface (110) and calculated the adsorption energy respectively. As a result, we made a conclusion that the top-Co site was the preferable surface adsorption-site. Furthermore, we focused on the oxygen adsorption and dissociation process on the 1×1surface(110), 2×2surface(110) implemented with molecular dynamic. It was showed that there existed an remarkable dissociation barrier when an oxygen adsorbed on 1×1surface(110). By contrast, an oxygen adsorbed on 2×2surface(110) only had an insignificant barrier even that the oxygen dissociation was no longer the rate determining step. So we can come to conclusion that larger surface which means low coverage is much more preferable for oxygen adsorption and dissociation process. On other hands, the surface coverage of oxygen affected the surface catalytic mechanism. Meanwhile we conducted the charge population analysis, illuminated the mechanism of the oxygen reduction in terms of charge migration.Furthermore, by the methods of doping Sr, Co with Ba , Fe respectively, we constructed three kinds of (Ba_0.5Sr_0.5)(Co_0.75Fe_0.25)O_3-δmodel, which is mostly like the (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)O_3-δthat used as an cathode materials. Considering that oxygen migration had relation to oxygen vacancy formation energy, we focused on the oxygen vacancy in bulk of (Ba_0.5Sr_0.5)(Co_0.75Fe_0.25)O_3-δ. We found that the oxygen vacancy formation energy was not only affected by the local coordination environment but also affected by the nest-nearest neighbor coordination environment. In addition that the oxygen vacancy was more preferable formatted between Fe-V-Fe coordination environment than that between Co-V-Co coordination environment, where V represent the oxygen vacancy. Then we considered the oxygen migration in the bulk of (Ba_0.5Sr_0.5)(Co_0.75Fe_0.25)O_3-δ, which as we know the oxygen must cross the triangle surface consisted with Ba-Sr-Co/Fe. Result showed that the oxygen diffusion barrier located on the triangle surface, and the transition resulted in local lattice expansion and distortion. The oxygen migration barrier was related to oxygen vacancy formation energy, so when a lattice oxygen migrated to neighbor inequivalence oxygen vacancy with different oxygen vacancy formation energy, it exhibited anisotropy oxygen diffusion ability. Considered that the distribution of anions and oxygen vacancies are total random, so it was not supposed to exhibited such a property in macro-scale.