On the variability of reported ionic conductivity in nanoscale YSZ thin films

Yttria-stabilized zirconia (YSZ) is the most common material used as a solid oxide electrolyte, which is a key component in solid oxide fuel cells, solid oxide electrolysis cells, and certain chemical sensors. High efficiency in these devices requires increased oxygen ion conductance at intermediate temperatures. Nanoscale YSZ thin films are quite promising in this regard, as the conduction path may be reduced below what is conventionally achievable. Still, as the thickness is decreased to nanoscale, structural properties like lattice parameter and grain morphology are typically altered. These may affect the electrochemical properties in non-trivial ways. Recent reports on nanoscale YSZ thin films have provided inconsistent and, at times, controversial results. In this paper, we present a review of reports on nanoscale YSZ thin films, focusing principally on single component YSZ films as opposed to heterogeneous multilayer films. Reports of significantly increased conductivity come from studies that use a variety of substrates, grain morphologies, and, to some extent, film thicknesses. Mechanical strain in the films is not typically reported but is a suspected cause of the variability in conductivity.