Unexpectedly High Electronic Partial Conductivity of Porous Yttria-Stabilized Zirconia Thin Films

In porous form, ion conductors are key components for a vast number of applications, making detailed understanding of the impact of high surface area on electrochemical properties indispensable. Here, we report on the electrical characterization of dense and porous yttria-stabilized zirconia (YSZ) thin films deposited by pulsed laser deposition. While dense films exhibit high and dominant ionic conductivity over a wide oxygen partial pressure range, as expected for YSZ, porous films reveal an unexpectedly high electronic partial conductivity, which determines the transport properties under highly reducing conditions. By means of continuum simulations, we demonstrate that the high electronic conductivity contribution may be explained in terms of a space-charge region at the free surface, where electrons accumulate due to the positive surface charge. The latter results in increased electronic conductivity in the space-charge region, which dominates the transport properties of the porous thin films under highly reducing conditions. The results emphasize the importance of considering surface effects in nanostructured ion conductors for tailoring the overall electrochemical properties for device applications.