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Molecular Simulation of Oxygen Mobility in Yttria Stabilized Zirconia

Q. Zhang and K-Y Chan
The University of Hong Kong, HK

Keywords:
solid oxide electrolyte, YSZ

Abstract:
The development of solid oxide electrolyte materials is critical to applications of high temperature fuel cells and sensors. The optimum performance of the solid oxide electrolyte material depends on its atomic composition and structural properties. It is important to have good electrical conductivity at a moderate temperature. The most investigated solid oxide material for fuel cells is yttria-stabilized zirconia (YSZ) with the highest conductivity at 8% doped yttria. Molecular simulations can complement experiments to study the structure-property relationship of solid oxide electrolytes. Non-equilibrium molecular dynamics (NEMD) simulations are reported here for oxygen ion mobility in YSZ. An external electric field is applied to a few hundred atoms representing 8% YZS using the Born-Mayer-Buckingham potential model. The yttrium atoms are randomly distributed. They replace some zirconium atoms in a cubic lattice and create vacancy sites for oxygen ionic mobility. As opposed to equilibrium molecular dynamic simulations, the presence of an electric field in NEMD allows observable currents at moderate temperatures. The conductivities estimated from NEMD simulations were compared with those calculated from diffusivities in equilibrium molecular dynamics simulation. Comparison with experimental data is also made. The Nose-Hoover thermostat and Anderson thermostats were tested in the NEMD simulations. The velocity distributions are different in the two thermostats and the fraction of high velocity particles will determine the conductivity. In addition to direct current simulations, frequency dependent conductivities were investigated in NEMD by applying an alternating external field. In general, conductivity decreases with frequency while phase shift increases with frequency. The methodology developed here can be applied to complement experimental studies of other solid oxide electrolytes such as cerium oxide doped with various atoms.

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