Conductive Nature of Grain Boundaries in Nanocrystalline Stabilized BiO Thin-Film Electrolyte.

Stabilized BiO has gained a considerable amount of attention as a solid electrolyte material for low-temperature solid oxide fuel cells due to its superior oxygen-ion conductivity at the temperature of relevance (≤500 °C). Despite many research efforts to measure the transport properties of stabilized BiO, the effects of grain boundaries on the electrical conductivity have rarely been reported and their results are even controversial. Here, we attempt quantitatively to assess the grain boundary contribution out of the total ionic conductivity at elevated temperatures (350-500 °C) by fabricating epitaxial and nano-polycrystalline thin films of yttrium-stabilized BiO. Surprisingly, both epitaxial and polycrystalline films show nearly identical levels of ionic conductivity, as measured by alternating current impedance spectroscopy and this is the case despite the fact that the polyfilm possesses nanosized columnar grains and thus an extremely high density of the grain boundaries. The highly conductive nature of grain boundaries in stabilized BiO is discussed in terms of the clean and chemically uniform grain boundary without segregates, and the implications for device application are suggested.