Enhanced oxygen exchange and incorporation at surface grain boundaries on an oxide ion conductor

This study reports spectrometric and spectroscopic evidence indicating a pronounced role of surface grain boundaries in enhancing oxygen incorporation on oxide ion conducting ceramic yttria stabilized zirconia (YSZ). Oxygen isotope exchange measurements were carried out using high spatial resolution (50 nm) secondary ion mass spectrometry (SIMS) under cathodically biased conditions as well as on bare YSZ surface. Surface mapping of the 18O distribution by SIMS clearly shows high activity at surface grain boundaries, suggesting that these boundary regions provide preferential pathways for oxygen incorporation into YSZ in both cases, albeit the effect is less pronounced without bias. The results are supported by a.c. impedance spectroscopy measurements conducted on polycrystalline YSZ membranes with surface grains engineered for different sizes, which indicate that smaller grains (i.e. higher grain boundary densities) exhibit lower electrode impedances. These results open up the possibility of engineering nanostructured YSZ surfaces containing a high density of grain boundaries to achieve enhanced performance of electrochemical devices, particularly for solid oxide fuel cells operating at low temperatures.