The effect of oxygen vacancies on proton conduction in 12.5% Al doped BaZrO3
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Acceptor doped barium zirconate is one of the most promising materials for stationary hydrogen fuel cells due to its high proton conductivity while maintaining thermodynamic stability. Many studies have shown that the dopant defect serves as a proton trap in these materials. Moreover, some other studies suggest that oxygen vacancies near the acceptor dopant help hinder proton trapping and enhance proton conductivity. The recent computational studies point that oxygen vacancies are more likely to form between acceptor dopant sites, and subsequent hydration places protons on nearest and second nearest oxygens. Inspired by a follow-up study using gradual hydration does not see a significant repulsion between the associated dopant/oxygen vacancy defects and protons to avert proton trapping at the dopant site, this study is on the effect of an oxygen vacancy on the minimum energy pathways, traps, and overall proton conduction in 12.5% aluminum doped BaZrO3 perovskite. The results are compared to the larger yttrium dopant systems. The calculation is based on Density functional theory (DFT) with the PBE functional in the Vienna ab-initio simulation package (VASP) to find the total electronic energy for perovskite configurations. The minimization calculation showed that the most distorted (-,-,-) configuration possesses the lowest energy. The following calculations are based on this lowest energy structure using the Nudged Elastic Band (NEB) method to find activation barriers for oxygen vacancy motion, and proton conduction motion. For the aluminum system, the energy barrier for vacancy motion between dopant nearest neighbor oxygen sites is 0.02 eV, and the energy barrier for a dopant nearest neighbor oxygen vacancy to move to a dopant second nearest neighbor oxygen site is 1.05 eV. During the proton conduction process, the small size of the aluminum dopant allows oxygen ion rearrangement to form a roughly trigonal bipyramid of oxygen ions partially screening the dopant charge exposed by the oxygen vacancy. This coupled with strong hydrogen bonds along the edge of the smaller oxygen polyhedron around the aluminum creates a greater dopant proton trap. Furthermore, similar to earlier studies, protons in the smaller dopant aluminum system can periodically escape from trapping and conduct.