First-principles Study of Defect Migration in RE-doped Ceria (RE = Pr, Gd)
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First-principles Study of Defect Migration in RE-doped Ceria (RE = Pr, Gd) Pratik Dholabhai1, James Adams1, Peter Crozier1, and Renu Sharma1,2 1 Materials Science & Engineering, Arizona State University, Tempe, AZ 85287 2 Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, MD 20899 ABSTRACT Oxygen vacancy formation and migration in ceria is central to its performance as an ionic conductor. Ceria doped with suitable aliovalent dopants has enhanced oxygen ion conductivity – higher than that of yttria stabilized zirconia (YSZ), the most widely used electrolyte material in solid oxide fuel cells (SOFC). To gain insight into atomic defect migration in this class of promising electrolyte materials, we have performed total energy calculations within the framework of density functional theory (DFT+U) to study oxygen vacancy migration in ceria, Pr-doped ceria (PDC) and Gd-doped ceria (GDC). We report activation energies for various oxygen vacancy migration pathways in PDC and GDC. Results pertaining to the preferred oxygen vacancy formation sites and migration pathways in these materials will be discussed in detail. Overall, the presence of Pr and Gd ions significantly affects oxygen vacancy formation and migration, in a complex manner requiring the investigation of many different migration events. We propose a relationship that explains the role of additional dopants in lowering the activation energy for vacancy migration in PDC and GDC. INTRODUCTION One of the most important components for intermediate temperature (500 ° C to 750 ° C) SOFCs is the electrolyte material, which should have very high oxide-ion conductivity in the intermediate temperature range. Oxygen-ion conductivity of doped ceria is reportedly higher than the most widely used solid electrolyte material, YSZ, at temperatures below 600 ° C [1]. The various applications of PDC include but are not limited to its use as an oxide electrode having high electronic and ionic conductivity [2], cathode materials for SOFC [3], and for membranes for oxygen separation, where equally high electronic and ionic conductivities are required to achieve the maximum of oxygen flux through the membrane [4]. In comparison of traditional electrolyte materials, GDC is reported to be one of the most promising solid electrolyte materials for operation of SOFC below 600 °C [5,6]. Knowledge of the factors controlling oxygen vacancy diffusion in PDC and GDC is therefore valuable. Lower activation energies lead to higher oxide ion conductivity (Eq. (1)).
σ = σ 0 exp(− Ea / kBT )
(1)
where σ is the ionic conductivity, Ea is the activation energy for oxygen vacancy diffusion, σ0 is the pre-exponential factor, T is the absolute temperature and kB is the Boltzmann constant. However, the activation energies for vacancy migration in these materials depend on the dopant concentration. Calculations of the oxygen vacancy migration and formation in ceria-based materials have been performed previously, but there is only one study investigatin
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