Relative Occurrence of Oxygen-Vacancy Pairs in Yttrium-Containing Environments of Y2O3-Doped ZrO2 Can Be Crucial to Ionic Conductivity

5 years ago

Relative Occurrence of Oxygen-Vacancy Pairs in Yttrium-Containing Environments of Y2O3-Doped ZrO2 Can Be Crucial to Ionic Conductivity

Abhijit Chatterjee, Methary Jaipal

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 O^2– 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 O^2–-vacancy (O^2–-vac) pairs in a local environment can also be crucial to the hopping transitions. Higher probability of O^2–-vac pairs can result in a greater number of transitions. O^2– 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 O^2–-vac pairs in 42 different local cation (Y³⁺/Zr⁴⁺) environments, to our knowledge calculated for the first time, as well as coarse-grained O^2– 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 O^2–-vac pairs are destabilized by these environments. Increasing the temperature and/or decreasing the dopant composition stabilizes O^2–-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 O^2–-vac free energy differences.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b05329

DOI: 10.1021/acs.jpcc.7b05329