Epitaxial strain engineering in nanostructured oxides has attracted great attention to various energy storage and conversion devices requiring fast ionic conduction, such as solid-oxide fuel cells (SOFCs), catalytic membranes, and electrochemical sensors. Recently, remarkable enhancement of ionic conductivity has been achieved by the epitaxial strain in ultra-thin epitaxial and multi-layered films.^1-3 However, the strain engineering still has some lack of understanding of how much enhancement can be achieved by strain, and how to maximize the strain in nanostructured oxides. Herein, for the reliable and effective strain engineering for fast ionic conduction, strain effect on oxygen ion conductivity of thin films (Gd_0.2Ce_0.8O_1.9-δ (100) : GDC) was investigated through both experimental study and density functional theory (DFT) calculations.

We measured the oxygen ion conductivity of GDC films grown in island growth mode on Nb-doped SrTiO₃ (STO) substrate. According to the conductivity measurement via electrochemical impedance spectroscopy (EIS), the activation energy for oxygen ion migration in out-of-plane direction was reduced with increase of film thickness. Furthermore, through careful strain analysis using high-resolution X-ray diffraction (HR-XRD) and strain mapping in transmission electron microscopy (TEM), the intrinsic tensile strain along column boundaries (CB) was found to be more predominant than the epitaxial strain formed at the film-substrate interface, thereby more strong tensile strain was observed in a thicker film having higher CB density. This general tendency of the activation energy change with respect to tensile strain was also verified by atomic-scale simulation in a highly doped CeO₂ structure. To the best of our knowledge, this is the first experimental and theoretical verification to show the critical role of intrinsic strain on fast ion conduction in thin film electrolyte. From this presentation, you will get a great inspiration to the strain-induced fast ionic conduction in oxide thin films to improve the availability of various energy devices.

References

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2. S. Sanna, V. Esposito, J. W. Andreasen, J. Hjelm, W. Zhang, T. Kasama, S. B. Simonsen, M. Christensen, S. Linderoth, and N. Pryds, Nat. Mater. 14, 500 (2015)
3. A. Fluri, D. Pergolesi, V. Roddatis, A. Wokaun, and T. Lippert, Nat. Commun. 7, 10692 (2016)