Protons in cubic yttria-stabilized zirconia: Binding sites and migration pathways

Despite the fact that unusually high power generation attributed to proton conduction has been reported recently for nanostructured yttria-stabilized zirconia (YSZ) little is known about the atomistic mechanisms of proton transport and corresponding activation energies, namely the factors that dictate proton mobility. Important issues to be examined include the energetics of proton incorporation in the YSZ host, the binding of protons to intrinsic structural defects and the role of grain boundaries in proton conduction. The present study reports calculations based on density-functional theory (DFT) of defect-formation energies and energy barriers for proton migration in cubic YSZ for an yttria doping in the 10-12 mol% range. For the bulk-crystalline lattice representative migration pathways are identified and the corresponding energy barriers for proton migration are calculated using the nudged elastic-band method. The feasibility for the grain boundaries to act as fast diffusion pathways for protons is also examined. For the high-angle Σ5(310) grain boundary DFT calculations were undertaken so as to determine the segregation propensity of protons as well as the magnitude of energy barriers for selected proton migration pathways at the core of this extended defect. The present calculations outline the importance of oxygen vacancies for proton migration in the bulk crystal and the ability of localized cation clusters at the grain-boundary core to act as strong obstacles to proton motion.