Mechanistic Elucidation of Surface Cation Segregation in Double Perovskite PrBaCo2O5+δ Material using MD and DFT Simulations for Solid Oxide Fuel Cells

Abstract

Surface segregation of A-site cation is a known phenomenon in perovskite and layered perovskite materials which are used as the cathode in a solid oxide fuel cell (SOFC). A combined density functional theory (DFT) and molecular dynamics (MD) based theoretical approach was utilized to develop a mechanistic understanding of surface segregation of Ba cations in double perovskite PrBaCo2O5+δ (PBCO) electrode. Using DFT, the energetics for oxygen vacancy creation (EOV) in the Ba/Co and Pr/Co terminal surfaces of PBCO was calculated to be 308 kJ/mol and 318 kJ/mol, respectively, which was observed to be higher than that of Co/Pr (EOV = 151.5 kJ/mol) and Co/Ba (EOV = 121.6 kJ/mol) terminal surfaces. MD and DFT calculations suggested, both oxygen anion migration and oxygen vacancy creation were least preferred in the Ba plane. DFT calculations of the energy of terminal surfaces further revealed that the surface containing the Ba cations was the most stable surface having surface energy (γ = 6.9 kJ/mol.A2) much lower than that of the Pr containing surface (Pr/Co, (γ = 10.8 kJ/mol.A2)). Cation disordering and depth of perturbation were studied in Ba/Co, Pr/Co, Co/Ba, and Co/Pr terminal surfaces, using MD simulations. The depth of cation disordering in Co/Ba (5.4 A) was calculated to be highest as compared with other terminal surfaces. Cation density profile showed the preferential migration Ba ions towards the surface showing a disruption of cation ordering in the near surface zone.