Yttria-stabilized zirconia (YSZ) is widely used as an electrolyte in solid oxide fuel cells. Much of current research on ionic conductivity in YSZ has focused on understanding migration barriers for O2– ion movement and vacancy trapping behavior in an attempt to understand why dopant cation edges result in fewer vacancy hopping transitions. We show that the free energy of finding O2–-vacancy (O2–-vac) pairs in a local environment can also be crucial to the hopping transitions. Higher probability of O2–-vac pairs can result in a greater number of transitions. O2– ion movement in bulk YSZ is studied here using multiple independent short molecular dynamics (MD) trajectories. Analysis of the MD trajectories yields free energy of O2–-vac pairs in 42 different local cation (Y3+/Zr4+) environments, to our knowledge calculated for the first time, as well as coarse-grained O2– hopping rates and Arrhenius parameters. On the basis of the free energies we conclude that it is possible that ionic movement is hindered in some environments not only because of high migration barriers or vacancy trapping as believed earlier but also because O2–-vac pairs are destabilized by these environments. Increasing the temperature and/or decreasing the dopant composition stabilizes O2–-vac pairs in these environments, which in turn affects the YSZ conductivity. Another newly found aspect is the connection between the kinetic rate constants for different environments; namely, the rates of interconversion between two environments depend on their O2–-vac free energy differences.

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Gadolinium-doped ceria (GDC) is an important fast oxygen ion conductor. O2--ion hopping in different cation environments of GDC is probed using molecular dynamics (MD). We find that at equilibrium, the...

Yttria-stabilized zirconia (YSZ) is widely used as a fast oxygen ion conductor in solid oxide fuel cells. Over the years several studies have probed the effect of different cation arrangements on the O 2− ion hopping behavior at the atomistic scale. However, an analytical model that can predict the macroscopic ionic conductivity using both the atomic scale hopping behavior as well as the spatial arrangement of cations at the mesoscopic length scale is lacking. A novel mesoscale model is constructed as a step towards addressing this gap. First, using a kinetic Monte Carlo (KMC) model for YSZ we find evidence of a fast ion conducting percolation network being present. The tortuous network, which consists of connected regions spanning the material structure, mainly contributes to the ionic conduction as O 2− ion movement in other regions is relatively slow. The topology, composition and O 2− ion movement in the network are analyzed. Next, the shortest path lengths in the network are identified with the help of the Dijsktra algorithm. Finally, a diffusion model is developed that relates the atomic scale hopping rates and shortest path lengths (a mesoscale feature) to the macroscale ionic conductivity. Estimates for ionic conductivity from the diffusion model are in excellent agreement with the KMC model. Changes within the percolation network with increasing Y 2 O 3 content can describe the maximum observed in ionic conductivity.