Identification of an Actual Strain-Induced Effect on Fast Ion Conduction in a Thin-Film Electrolyte

Abstract

Strain-induced fast ion conduction has been a research area of interest for nanoscale energy conversion and storage systems. However, because of significant discrepancies in the interpretation of strain effects, there remains a lack of understanding of how fast ionic transport can be achieved by strain effects and how strain can be controlled in a nanoscale system. In this study, we investigated strain effects on the ionic conductivity of GdCeO (100) thin films under well controlled experimental conditions, in which errors due to the external environment could not intervene during the conductivity measurement. In order to avoid any interference from perpendicular-to-surface defects, such as grain boundaries, the ionic conductivity was measured in the out-of-plane direction by electrochemical impedance spectroscopy analysis. With varying film thickness, we found that a thicker film has a lower activation energy of ionic conduction. In addition, careful strain analysis using both reciprocal space mapping and strain mapping in transmission electron microscopy shows that a thicker film has a higher tensile strain than a thinner film. Furthermore, the tensile strain of thicker film was mostly developed near a grain boundary, which indicates that intrinsic strain is dominant rather than epitaxial or thermal strain during thin-film deposition and growth via the Volmer-Weber (island) growth mode.

Nanocrystal Engineering of Thin-Film Yttria-Stabilized Zirconia Electrolytes for Low-Temperature Solid-Oxide Fuel Cells

“…The intrinsic tensile stress provides more space for oxygen diffusion which results in lowering the activation energy of oxygen anion doping. 15,32 Therefore, this moderate tensile stress is beneficial to the ion conduction in electrolytes, which is the case for the 15 YSZ electrolyte.…”

“…Notably, the zoomed-in XRD graph of Figure verifies that the (220) peak shifts to an angle slightly higher than the theoretical 2θ value of 50.14 ° with an increase in sputtering pressure from 5 to 15 mTorr, implying that the residual stress in the film is converted from highly compressive to moderately tensile. The intrinsic tensile stress provides more space for oxygen diffusion which results in lowering the activation energy of oxygen anion doping. , Therefore, this moderate tensile stress is beneficial to the ion conduction in electrolytes, which is the case for the 15 YSZ electrolyte.…”

“…This residual stress can alter the physical and electrical properties. Particularly, intrinsic tensile strain facilitates the migration of oxygen ions. − Araki et al investigated the oxygen-ion mobility in various tensile stress-loaded YSZ . Both the simulation and experiment demonstrated that tensile stress accelerates the oxygen-ion conduction.…”

Engineering mass transport properties in oxide ionic and mixed ionic-electronic thin film ceramic conductors for energy applications

“…during the PLD growth) give rise to compressive strain. [105,106] Particularly relevant in this context is the case in which coherent interfaces are obtained in columnar multilayered structures: Here the lattice mismatch between the constituting phases is able to generate a new strain field at each interface irrespective of the total film thickness. [107,108] Several computational studies have been dedicated to the prediction of the effects of strain on ionic conductivity.…”

Low oxygen partial pressure increases grain boundary ion conductivity in Gd-doped ceria thin films

“…The measurement of the grain boundary resistance in thin films is not trivial and so some studies omit the separation of grain and grain boundary impedance and evaluate the total ionic conductivity of polycrystalline thin films [28][29][30][31][32][33]. However, from these measurements it is not easy to separate the effect of grain boundaries because also strain [32,[34][35][36] influences ion conduction in GDC thin films. Without being able to separate grain and grain boundary conductivities, measurement of the bulk conductivity and strain effects is experimentally challenging and only reliable on epitaxial films [35].…”