Abstract
Enhancement of oxygen ion conductivity in oxides is important for low-temperature (<500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. While huge ion conductivity has been demonstrated in planar heterostructure films, there has been considerable debate over the origin of the conductivity enhancement, in part because of the difficulties of probing buried ion transport channels. Here we create a practical geometry for device miniaturization, consisting of highly crystalline micrometre-thick vertical nanocolumns of Sm-doped CeO2 embedded in supporting matrices of SrTiO3. The ionic conductivity is higher by one order of magnitude than plain Sm-doped CeO2 films. By using scanning probe microscopy, we show that the fast ion-conducting channels are not exclusively restricted to the interface but also are localized at the Sm-doped CeO2 nanopillars. This work offers a pathway to realize spatially localized fast ion transport in oxides of micrometre thickness.
Highlights
Enhancement of oxygen ion conductivity in oxides is important for low-temperature (o500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices
The Sm-doped CeO2 (SDC) exhibits a fluorite structure (Fm3m in space group) in the 1⁄2110 direction, while the STO shows a perovskite structure ðPm3mÞ in the [100] direction
We found that oxygen ion conductivity of nanoscaffold SDC–STO films is higher than those of other plain SDC, STO and Y2O3-stabilized ZrO2 (YSZ) films for all of the temperature range measured (Fig. 2b)
Summary
Enhancement of oxygen ion conductivity in oxides is important for low-temperature (o500 °C) operation of solid oxide fuel cells, sensors and other ionotronic devices. Pennycook et al reported density functional calculations and electron energy loss spectroscopy study, supporting that the nature of conduction is ionic[18,19] These studies indicate that comprehensive and careful investigations are needed to clarify the underlying mechanism of ionic conductivity enhancement in oxide heterostructures and to answer basic questions, such as whether the conduction is ionic or electronic in nature and where the fast ion transport channels are located (for example, within individual layers, at their interfaces and/or even within the substrate). Since the ion transport channels are not buried under the films, nanoscaffold films allow for direct probing of them with non-destructive tools, such as scanning probe microscopy (SPM)[29,30,31], without the need for patterning, as illustrated in Supplementary Fig. 1 Such direct visualization of ionic conduction channels at the nanoscale combined with complementary structural and electrical measurements can provide further insight into the underlying mechanism of ion conductivity enhancement in oxide heterostructure films. The observed behaviours strongly indicate that the fast ionic conduction is not exclusively an interface phenomenon but rather resides in the whole volume of the high crystalline SDC nanopillars
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