Multiscale modeling of the ionic conductivity of acceptor doped ceria

Recipes for superior ionic conductivities in thin-film ceria-based electrolytes

We employed Molecular Dynamics (MD) and Metropolis Monte Carlo (MMC) simulations to determine the oxide-ion mobility u_O in Ce_1-yGd_yO_2-y/2 (y = 0.02, 0.1, 0.2) for the range of temperatures 1400 ≤ T/K ≤ 2000 and field strengths 0.6 ≤ E/MV cm^-1 ≤ 15.0. Direct, unambiguous determination of u_O(E) from MD simulations is shown to require examination of the ions' mean displacement as a function of time. MD simulations were performed for random distributions of Gd cations and equilibrium distributions obtained by MMC calculations. All u_O(E,T,y) data obtained can be described by an (empirically augmented) analytical model with four zero-field parameters, a result that allows data to be extrapolated down to the temperatures of electrolyte operation. Specifically, the oxide-ion conductivity is predicted, for example at T = 700 K, (i) to be up to 10¹ higher for a random distribution of Gd than for an equilibrium distribution; and (ii) to be a factor of 10^0.8 higher for a 6 nm thin film than for a μm-thick sample under a potential difference of 1 V. By virtue of non-equilibrium deposition and nm-thick samples, thin films thus provide two new recipes for attaining even higher oxide-ion conductivities in ceria-based electrolytes.

Revealing the Structure and Oxygen Transport at Interfaces in Complex Oxide Heterostructures via 17O NMR Spectroscopy

Vertically aligned nanocomposite (VAN) films, comprising nanopillars of one phase embedded in a matrix of another, have shown great promise for a range of applications due to their high interfacial areas oriented perpendicular to the substrate. In particular, oxide VANs show enhanced oxide-ion conductivity in directions that are orthogonal to those found in more conventional thin-film heterostructures; however, the structure of the interfaces and its influence on conductivity remain unclear. In this work, ¹⁷O NMR spectroscopy is used to study CeO₂-SrTiO₃ VAN thin films: selective isotopic enrichment is combined with a lift-off technique to remove the substrate, facilitating detection of the ¹⁷O NMR signal from single atomic layer interfaces. By performing the isotopic enrichment at variable temperatures, the superior oxide-ion conductivity of the VAN films compared to the bulk materials is shown to arise from enhanced oxygen mobility at this interface; oxygen motion at the interface is further identified from ¹⁷O relaxometry experiments. The structure of this interface is solved by calculating the NMR parameters using density functional theory combined with random structure searching, allowing the chemistry underpinning the enhanced oxide-ion transport to be proposed. Finally, a comparison is made with 1% Gd-doped CeO₂-SrTiO₃ VAN films, for which greater NMR signal can be obtained due to paramagnetic relaxation enhancement, while the relative oxide-ion conductivities of the phases remain similar. These results highlight the information that can be obtained on interfacial structure and dynamics with solid-state NMR spectroscopy, in this and other nanostructured systems, our methodology being generally applicable to overcome sensitivity limitations in thin-film studies.

Oxygen Surface Exchange and Tracer Diffusion in Differently Oriented Thin Films of Gd-Doped CeO2

The exchange of ¹⁸O between gaseous molecular oxygen and thin-film samples of Ce_0.99Gd_0.01O_1.995 with two different, nominal surface orientations [(111) and (110)] was studied. Oxygen isotope exchange experiments were conducted in the temperature range of 573 ≤ T/K ≤ 673 at an oxygen activity of aO₂ = 0.2. Subsequently, secondary ion mass spectrometry (SIMS) measurements were performed on the thin-film samples to obtain ¹⁸O isotope depth profiles. All ¹⁸O diffusion profiles showed two features, suggesting spatially nonuniform oxygen tracer diffusion coefficients in the samples. A numerical solution to the diffusion equation was used to describe the experimental profiles and yielded oxygen tracer diffusion coefficients D* and oxygen surface exchange coefficients k*. Values of D* obtained were found, surprisingly, to be different for the two orientations and also orders of magnitude lower than values for ceramic samples in this temperature range. As possible explanations, we examine quantitatively the effect of halide anion impurities and the effect of ultrasmall columnar grains on oxygen tracer diffusion. Surface exchange coefficients for the (111) oriented surface were found to be roughly 1 order of magnitude higher than those for (110). We discuss two possible explanations for the observed behavior: the enrichment of anion impurities at the surface and the interaction between the surface and vapor water in the gas phase.