Insights into the Formation of Oxygen Vacancies in NdBa_1-XSr_xCo₂O_5+δ double Perovskite Material Using DFT Simulations

Layered double perovskite materials of type AA’B2O5+δ (where A is rare earth element and A’ is alkaline earth element) are suggested as potential candidate for a cathode in solid oxide fuel cells (SOFCs) because of their distinct oxygen-deficient perovskite-related structure, higher oxygen ion diffusion rate and surface exchange coefficient following a stacking sequence of /BO/AO/BO/A’O/BO/... [1,2]. In general, the layered double perovskites with rare earth metals at A-site, Ba at A’-site and Co at B-site have received much attention due to high oxygen vacancy concentration, high electronic conductivity and high catalytic activity. Presence of Pr, Gd and Nd at the A-site of the structure are observed to improve electronic conductivities, oxygen self-diffusion, oxygen exchange, resulting into reduced area specific resistances (ASRs) of the electrode [1-3]. In this study, Density Functional theory (DFT) calculations are performed for NdBaCo2O5+δ (NBCO) and Sr-doped at NBCO to study and compare oxygen vacancy formation energies in different planes and surface energies for different terminating surfaces of the NBCO material and access the role of Sr doping. Using DFT with Hubbard U correction of 3.3 eV for Co, oxygen vacancy formation energies in Ba/O, Co/O and Nd/O planes of bulk are observed as 245.88 KJ/mol, 144.93 KJ/mol and 72.13 KJ/mol respectively. DFT calculations predicted difficulty in creating oxygen vacancies in Ba/O plane as compared to Co/O and Nd/O plane of bulk NBCO. On doping one fourth and half of Ba with Sr resulted into a decrease in oxygen vacancy formation energies in BaSr/O plane of bulk i.e., less energy is needed to remove oxygen in BaSr/O plane. Further the Sr-doping at A’-site have shown decrease in lattice parameters. The energetics of surface with terminating surfaces Ba/Co, Nd/Co, Co/Ba and Co/Nd in (001) direction revealed Ba/Co and Co/Nd surface as the stable terminal surfaces. The trend of surface energies revealing Ba/Co surface as most stable in NBCO material is found to be similar as predicted in our previous studies on PrBaCo2O5+δ, GdBaCo2O5+δ[4,5]. Different energetics of the surfaces play an important role in the electrochemical performance of the cathode material. In conclusion, Sr-doping at A’-site in is likely to improve the energetics of oxygen vacancy formation in the BaSr/O planes as compared to undoped NBCO material. References: [1] Anjum, U., Vashishtha, S., Sinha, N., & Haider, M. A. (2015). Role of oxygen anion diffusion in improved electrochemical performance of layered perovskite LnBa1−ySryCo2−xFexO5+δ (Ln=Pr, Nd, Gd) electrodes. Solid State Ionics, 280, 24-29. [2] Anjum, U., Vashishtha, S., Agarwal, M., Tiwari, P., Sinha, N., Agrawal, A., & Haider, M. A. (2016). Oxygen anion diffusion in double perovskite GdBaCo2O5+δ and LnBa0.5Sr0.5Co2−xFexO5+δ (Ln = Gd, Pr, Nd) electrodes. International Journal of Hydrogen Energy, 41(18), 7631-7640. [3] Anjum, U., Khan, T. S., Agarwal, M., & Haider, M. A. (2019). Identifying the Origin of the Limiting Process in a Double Perovskite PrBa0.5Sr0.5Co1.5Fe0.5O5+δ Thin-Film Electrode for Solid Oxide Fuel Cells. ACS Applied Materials & Interfaces, 11(28), 25243-25253. [4] Anjum, U., Agarwal, M., Khan, T. S., & Haider, M. A. (2019). Mechanistic Elucidation of Surface Cation Segregation in Double Perovskite PrBaCo2O5+δ Material using MD and DFT Simulations for Solid Oxide Fuel Cells. Ionics, 26(3), 1307–1314. [5] Anjum, U., Agarwal, M., Khan, T. S., Prateek, P., Gupta, R. K., & Haider, M. A. (2019). Controlling surface cation segregation in a nanostructured double perovskite GdBaCo2O5+δ electrode for solid oxide fuel cells. Nanoscale, 11(44), 21404-21418.