The apparent surface free energy of rare earth oxides is governed by hydrocarbon adsorption.

The surface free energy of rare earth oxides (REOs) has been debated during the last decade, with some reporting REOs to be intrinsically hydrophilic and others reporting hydrophobic. Here, we investigate the wettability and surface chemistry of pristine and smooth REO surfaces, conclusively showing that hydrophobicity stems from wettability transition due to volatile organic compound adsorption. We show that, for indoor ambient atmospheres and well-controlled saturated hydrocarbon atmospheres, the apparent advancing and receding contact angles of water increase with exposure time. We examined the surfaces comprehensively with multiple surface analysis techniques to confirm hydrocarbon adsorption and correlate it to wettability transition mechanisms. We demonstrate that both physisorption and chemisorption occur on the surface, with chemisorbed hydrocarbons promoting further physisorption due to their high affinity with similar hydrocarbon molecules. This study offers a better understanding of the intrinsic wettability of REOs and provides design guidelines for REO-based durable hydrophobic coatings.

Correlating Surface Crystal Orientation and Gas Kinetics in Perovskite Oxide Electrodes.

Solid-gas interactions at electrode surfaces determine the efficiency of solid-oxide fuel cells and electrolyzers. Here, the correlation between surface-gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system La0.8 Sr0.2 Co0.2 Fe0.8 O3 . The gas-exchange kinetics are characterized by synthesizing epitaxial half-cell geometries where three single-variant surfaces are produced [i.e., La0.8 Sr0.2 Co0.2 Fe0.8 O3 /La0.9 Sr0.1 Ga0.95 Mg0.05 O3-δ /SrRuO3 /SrTiO3 (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface-orientation dependency of the gas-exchange kinetics, wherein (111)-oriented surfaces exhibit an activity >3-times higher as compared to (001)-oriented surfaces. Oxygen partial pressure ( p O 2 )-dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas-exchange kinetics, the reaction mechanisms and rate-limiting steps are the same (i.e., charge-transfer to the diatomic oxygen species). First-principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron-based, ambient-pressure X-ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin-film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)-surface exhibits a high density of active surface sites which leads to higher activity.

Kinetics of competing exchange of oxygen and water at the surface of functional oxides.

The presence of water vapour in the input gas stream influences the performance of air electrodes of solid oxide cells. In this work, the oxygen transport kinetics were determined by isotopic exchange depth profiling at 350 °C on polycrystalline La0.6Sr0.4Co0.2Fe0.8O3-δ samples in humidified oxygen, comparing the differences in tracer diffusion profile using either 18O2 or H218O as the labelling medium. The apparent surface exchange coefficients of oxygen were determined in each case and used together to estimate the oxygen surface exchange coefficients of molecular oxygen and water. It was found that, in humid conditions, the surface exchange coefficient of molecular oxygen is significantly decreased in comparison to a reference in dry conditions. In addition, the surface exchange coefficient of water is higher than that for molecular oxygen. This is in good agreement with the hypothesis that, water monopolises the active exchange sites at the material surface and thus oxygen from water exchanges faster than the one of molecular oxygen.

Designing Optimal Perovskite Structure for High Ionic Conduction.

Solid-oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure-property relationships that would enable the rational design of better materials. Here, using epitaxial thin-film growth, synchrotron radiation, impedance spectroscopy, and density-functional theory, the impact of structural parameters (i.e., unit-cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9 Sr0.1 Ga0.95 Mg0.05 O3- δ . As compared to the zero-strain state, compressive strain reduces the unit-cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit-cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit-cell volumes and octahedral rotations decrease migration barriers and create low-energy migration pathways, respectively. The desired combination of large unit-cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit-cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion-conducting perovskite electrolytes.

Structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1-x-yTixFeyO3-δ.

Calcium manganite-based perovskite-type oxides hold promise for application in chemical looping combustion processes and oxygen transport membranes. In this study, we have investigated the structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1-x-yTixFeyO3-δ. Distinct from previous work, data of high-temperature X-ray diffraction (HT-XRD) in the temperature range 600-1000 °C (with intervals of 25 °C) demonstrates that CaMnO3-δ (CM) transforms from orthorhombic to a mixture of orthorhombic and tetragonal phases between 875 °C and 900 °C. Rietveld refinements show the formation of a pure tetragonal phase at 975 °C and of a pure cubic phase at 1000 °C. Partial substitution of manganese by iron and/or titanium to yield CaMn0.875Ti0.125O3-δ (CMT), CaMn0.85Fe0.15O3-δ (CMF) or CaMn0.725Ti0.125Fe0.15O3-δ (CMTF) leads to different phase behaviours. While CMT remains orthorhombic up to the highest temperature covered by the HT-XRD experiments, CMF and CMTF undergo an orthorhombic → tetragonal → cubic sequence of phase transitions. Electrical conductivity relaxation measurements are conducted to determine the chemical diffusion coefficient (Dchem) and the surface exchange coefficient (kchem) of the materials. The results demonstrate that oxygen transport is hindered in the tetragonal phase, when occurring, which is attributed to a possible ordering of oxygen vacancies. The small polaron electrical conductivity of CM in the cited temperature range is lowered upon partial manganese substitution, by about 10% for CMF and up to half an order of magnitude for CMT and CMTF.