Identifying the O2 diffusion and reduction mechanisms on CeO2 electrolyte in solid oxide fuel cells: a DFT + U study.

The interactions and reduction mechanisms of O2 molecule on the fully oxidized and reduced CeO2 surface were studied using periodic density functional theory calculations implementing on-site Coulomb interactions (DFT + U) consideration. The adsorbed O2 species on the oxidized CeO2 surface were characterized by physisorption. Their adsorption energies and vibrational frequencies are within -0.05 to 0.02 eV and 1530-1552 cm(-1), respectively. For the reduced CeO2 surface, the adsorption of O2 on Ce4+, one-electron defects (Ce3+ on the CeO2 surface) and two-electron defects (neutral oxygen vacancy) can alter geometrical parameters and results in the formation of surface physisorbed O2, O2a- (0 < a < 1), superoxide (O2-), and peroxide (O(2)(2-)) species. Their corresponding adsorption energies are -0.01 to -0.09, -0.20 to -0.37, -1.34 and -1.86 eV, respectively. The predicted vibrational frequencies of the peroxide, superoxide, O2a- (0 < a < 1) and physisorbed species are 897, 1234, 1323-1389, and 1462-1545 cm(-1), respectively, which are in good agreement with experimental data. Potential energy profiles for the O2 reduction on the oxidized and reduced CeO2 (111) surface were constructed using the nudged elastic band method. Our calculations show that the reduced surface is energetically more favorable than the unreduced surface for oxygen reduction. In addition, we have studied the oxygen ion diffusion process on the surface and in bulk ceria. The small barrier for the oxygen ion diffusion through the subsurface and bulk implies that ceria-based oxides are high ionic conductivity at relatively low temperatures which can be suitable for IT-SOFC electrolyte materials.