Significantly Enhanced Oxygen Transport Properties in Mixed Conducting Perovskite Oxides under Humid Reducing Environments

Abstract

Mixed ionic and electronic conducting (MIEC) perovskite oxides (ABO 3 ) have a substantial role in carbon-neutral clean energy conversion and storage technologies. Owing to their favorable catalytic properties, high ionic and electronic conductivity, and chemical and redox stability, MIEC perovskite oxides are promising electrode materials in multiple applications, such as solid oxide fuel/ electrolysis cells, oxygen transport membranes, metal−air batteries, electrochemical sensors, and electrocatalysts for water splitting. Here, taking (La 0.8 Sr 0.2 ) 0.95 Cr 0.5 Fe 0.5 O 3−δ (LSCrF8255) as a model MIEC perovskite oxide, we demonstrate that the oxygen mass transport properties are significantly enhanced under a humid reducing water vapor environment (pO 2 < 1 mbar, pH 2 O = 30 mbar) by up to 4 orders of magnitude compared to those measured under dry (pO 2 = 200 mbar) and wet (pO 2 = 200 mbar, pH 2 O = 30 mbar) oxygen atmospheres. A 0.8 eV decrease in the activation energy for oxygen bulk diffusion was also found under water vapor, and a decrease in activation energy of 0.7 eV for water surface exchange compared to oxygen surface exchange was found. The mechanisms underpinning these enhancements were explored. Furthermore, LSCrF8255 has also exhibited a consistent surface composition evolution regarding Sr segregation and phase separation and an excellent bulk stability under both oxidizing and reducing environments at elevated temperatures.

Understanding surface chemical processes in perovskite oxide electrodes

“…This can be further correlated to the signicantly enhanced oxygen transport properties, by up to 4 orders of magnitude, of the LSCrF8255 samples annealed in water vapour reported in our previous study. 37 2.1.2.2 Surface composition characterized by STEM-EDX. An energy-dispersive X-ray spectroscopy elementary map carried out in the scanning transmission electron microscope mode (STEM-EDX) was obtained from the sample annealed in dry oxygen at 900 °C for 27 hours to further study the chemical environment of Sr surface species, illustrated in Fig.…”

“…In our previous studies, Si deposition was observed on the LSCrF8255 samples annealed in both the wet oxygen and water vapour conditions in STEM-EDX and XPS analysis. 24,37 In contrast, there was no Si detectable on the samples annealed in dry oxygen. The Si deposition likely results from the transport of volatile Si species in the wet atmospheres, which were formed due to the reaction between water molecules and the quartz glass tube used for the annealing.…”

“…It is worth noting that LSCrF8255 was chosen for its excellent bulk stability under both oxidizing and reducing environments at elevated temperatures. 37 The humid environments were of particular interest as water vapour is a fundamental molecule present in the input gas stream of the solid-state electrochemical systems, acting as either impurities beside molecular oxygen in ambient air, or as an electrolysis source for hydrogen production. The surface chemistry and morphology of the materials were investigated using a complementary multi-technique approach.…”

Hydration Entropy and Enthalpy of a Perovskite Oxide from Oxygen Tracer Diffusion Experiments

“…We have used a rather extreme caseppm levels of oxygen vacancies/hydroxide ions in single crystalsfor the demonstration, but the method is not restricted to this case. Indeed, as long as there is an appreciable decrease in under wet conditions relative to the dry case, the method is applicable to the entire range of oxygen-vacancy/hydroxide ion concentrations, from ppb or lower up to the concentrations in proton-conducting perovskites (even those with high electronic conductivity ,, ). Furthermore, the method is not restricted to single crystals but can, of course, be applied to thin-film and polycrystalline samples, since the principle of the method remains the same.…”

Phase Stability and Transformations in CsSnI3: Is Anharmonicity Negligible?

“…The reversed transition requires a low-temperature condition and exposure to humidity . This is because the activation energy of the black → yellow transition in CsSnI 3 and other MHPs is high (∼ 1–2 eV), and the presence of water molecules significantly lowers the activation energy. , If the temperature decreases in dry air, α-CsSnI 3 transforms to the β-phase at 426 K, and a subsequent β ↔ γ phase transition takes place at 351 K . These successive transitions in the black phase have been well understood by the tilts of the BX 6 (B = Pb and Sn; X = Cl, Br, and I) octahedra caused by the undersized A cation .…”