Strain-Induced Tailoring of Oxygen-Ion Transport in Highly Doped CeO₂ Electrolyte: Effects of Biaxial Extrinsic and Local Lattice Strain

We explored oxygen-ion transport in highly doped CeO₂ through density-functional theory calculations. By applying biaxial strain to 18.75 mol % CeO₂:Gd, we predicted the average migration-barrier energy with six different pathways, with results in good agreement with those of experiments. Additionally, we found that the migration-barrier energy could be lowered by increasing the tetrahedron volume, including the space occupied by the oxygen vacancy. Our results indicate that the tetrahedron volume can be expanded by larger codopants, as well as biaxial tensile strain. Thus, the combination of thin-film structure and codoping could offer a new approach to accelerate oxygen-ion transport

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

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 Gd_0.2Ce_0.8O_1.9−δ (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

In Situ Synthesized La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ–Gd_0.1Ce_0.9O_1.95 Nanocomposite Cathodes via a Modified Sol–Gel Process for Intermediate Temperature Solid Oxide Fuel Cells

Composite cathodes comprising nanoscale powders are expected to impart with high specific surface area and triple phase boundary (TPB) density, which will lead to better performance. However, uniformly mixing nanosized heterophase powders remains a challenge due to their high surface energy and thus ease with which they agglomerate into their individual phases during the mixing and sintering processes. In this study, we successfully synthesized La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF)–Gd_0.1Ce_0.9O_1.95 (GDC) composite cathode nanoscale powders via an in situ sol–gel process. High-angle annular dark field scanning transmission electron microscopy analysis of in situ prepared LSCF–GDC composite powders revealed that both the LSCF and GDC phases were uniformly distributed with a particle size of ∼90 nm without cation intermixing. The in situ LSCF–GDC cathode sintered on a GDC electrolyte showed a low polarization resistance of 0.044 Ω cm² at 750 °C. The active TPB density and the specific two phase (LSCF/pore) boundary area of the in situ LSCF–GDC cathode were quantified via a 3D reconstruction technique, resulting in 12.7 μm^–2 and 2.9 μm^–1, respectively. These values are significantly higher as compared to reported values of other LSCF–GDC cathodes, demonstrating highly well-distributed LSCF and GDC at the nanoscale. A solid oxide fuel cell employing the in situ LSCF–GDC cathode yielded excellent power output of ∼1.2 W cm^–2 at 750 °C and high stability up to 500 h

High-Temperature Current Collection Enabled by the in Situ Phase Transformation of Cobalt–Nickel Foam for Solid Oxide Fuel Cells

For the commercial development of solid oxide fuel cells (SOFCs), cathode current collection has been one of the most challenging issues because it is extremely difficult to form continuous electric paths between two rigid components in a high-temperature oxidizing atmosphere. Herein, we present a Co–Ni foam as an innovative cathode current collector that fulfills all strict thermochemical and thermomechanical requirements for use in SOFCs. The Co–Ni foam is originally in the form of a metal alloy, offering excellent mechanical properties and manufacturing tolerance during stack assembly and startup processes. Then, it is converted to the conductive spinel oxide in situ during operation and provides nearly ideal structural and chemical characteristics as a current collector, gas distributor, and load-bearing component. The functionality and durability of the Co–Ni foam are verified by unit cell test and 1 kW-class stack operation, demonstrating performance that is equivalent to that of precious metals as well as an exceptional stability under dynamic conditions with severe temperature and current variations. This work highlights a cost-effective technique to achieve highly reliable electric contacts over the large area using the in situ metal-to-ceramic phase transformation that could be applied to various high-temperature electrochemical devices