Understanding the Role of Sr as a Dopant in a Double Perovskite Material for Solid Oxide Fuel Cells

Abstract

LnBaCo2O5+d type of layered double perovskite materials has received much attention as solid oxide fuel cell cathode materials owing to their high oxygen ion concentration, high electronic conductivity, and catalytic activity towards oxygen reduction. In the present work, Nd-based double perovskites NdBa1-xSrxCo2O5+d (NBSCO, x= 0, 0.25 and 0.50) have been studied computationally.Plane-wave density functional theory-based calculations using VASP were performed to examine the electrochemical properties of NdBa1-xSrxCo2O5+d double perovskite material. Given the application of NBSCO material for SOFC cathodes, the bulk oxygen vacancy formation energies (Eov) have been calculated computationally for oxygen vacancy created in all possible planes (BaSr/O, Nd/O, and Co/O) and surface energies (γ) of the structure with different terminations (BaSr/Co, Nd/Co, Co/BaSr and Co/Nd) along (001) direction. Fig 1. and Fig. 2 shows the structure with oxygen vacancy created in various planes and with different terminal surfaces. Computed oxygen vacancy formation energies and surface energies in NBSCO are shown in table 1 and table 2. Nd/O plane observed to have the lowest oxygen vacancy formation energy among all the three possible planes in (001) direction of bulk NBSCO. This shows Nd/O plane to have high oxygen vacancy concentration. For x=0, 0.25, and 0.50, BaSr/O plane has the highest oxygen vacancy formation energies showing difficulty in oxygen vacancy creation in BaSr/O plane as compared to other planes. This suggested less oxygen anion diffusivity in the BaSr/O plane. However, doping one fourth and half of the Ba with Sr resulted in an improved bulk oxygen vacancy characteristic of the BaSr/O plane. Different energetics of different terminal surfaces shows the importance of terminations. The results of first-principles calculations for surface energies were analyzed and compared for different terminal surfaces. For NBCO material, low surface energies have been observed for Ba/Co termination. Due to lower surface energies, Ba ions have tendencies to segregate towards the surface. This is in accordance with the DFT based calculations of surface energies and molecular dynamics simulations for diffusivity and root mean square deviation values of other Ba containing LnBaCo2O5+d layered perovskites [1-5]. Doping the material with Sr has also shown a similar trend in surface energies of BaSr/Co terminal surface.