Apparent Oxygen Uphill Diffusion in La0.8Sr0.2MnO3 Thin Films upon Cathodic Polarization

Controlling the Size of Au Nanoparticles on Reducible Oxides with the Electrochemical Potential

Controlling the size of Au nanoparticles (NPs) and their interaction with the oxide support is important for their catalytic performance in chemical reactions, such as CO oxidation and water-gas shift. It is known that the oxygen vacancies at the surface of support oxides form strong chemical bonding with the Au NPs and inhibit their coarsening and deactivation. The resulting Au/oxygen vacancy interface also acts as an active site for oxidation reactions. Hence, small Au NPs are needed to increase the density of the Au/oxide interface. A dynamic way to control the size of the Au NPs on an oxide support is desirable but has been missing in the field. Here, we demonstrate an electrochemical method to control the size of the Au NPs by controlling the surface oxygen vacancy concentration of the support oxide. Oxides with different reducibilities, La_0.8Ca_0.2MnO_3±δ and Pr_0.1Ce_0.9O_2-δ, are used as model support oxides. By applying the electrochemical potential, we achieve a wide range of effective oxygen pressures, pO₂ (10^-37-10¹⁴ atm), in the support oxides. Applying the cathodic potential creates a high concentration of oxygen vacancies and forms finely distributed Au NPs with sizes of 7-13 nm at 700-770 °C in 10 min, while the anodic potential oxidizes the surface and increases the size of the Au NPs. The onset cathodic potential required to create small Au NPs depends strongly on the reducibility of the support oxide. The Au NPs did not undergo sintering even at 700-770 °C under the cathodic potential and also were stable in catalytically relevant conditions without potential.

Oxygen transport and surface exchange mechanisms in LSCrF-ScCeSZ dual-phase ceramics

For the mechanisms by which the oxygen gets incorporated in a dual-phase composite system, three hypotheses, i.e. cation inter-diffusion, spillover type and self-cleaning of the perovskite-structured phase, have been provided in the literature. However, experimentally a consensus on the most likely mechanism is yet to be reached. In this work, a specially fused sample of the lanthanum strontium chromium ferrite (LSCrF)-scandia/ceria-stabilised zirconia (ScCeSZ) dual-phase material was investigated. Among the three potential mechanisms, no obvious cation inter-diffusion was firstly observed. A cleaner surface of the ScCeSZ phase was confirmed in the fused sample than in the isolated ScCeSZ single-phase sample while impurity layers were clearly observed on the LSCrF surface, suggesting the cleaning effect from the perovskite. However, more evidence implies that the cleaning effect is not the only reason for the synergistic effects between these two phases. Observations via SIMS analysis lend strong support to the 'spillover-type' mechanism as the oxygen isotopic fraction on the surface of the ScCeSZ increased compared to the isolated single-phase and as the distance to the heterojunction increases, the oxygen isotopic fraction decreases. Moreover, oxygen depleted layers were clearly seen on the top layers of the LSCrF surface which may be associated with the higher oxygen diffusivity in the surface/sub-surface layers, oxygen grain boundary fast diffusion and the impurities on the perovskite phase. For this sample, a combination of 'spillover' and 'self-cleaning' type mechanisms is suggested to be the potential possibility while the contribution from the cation inter-diffusion for this specific sample is proven to be low.

Oxygen surface exchange properties and surface segregation behavior of nanostructured La0.6Sr0.4Co0.2Fe0.8O3-δ thin film cathodes

Cathode materials with highly reactive surfaces and long-term stability are required to achieve high-performance solid oxide fuel cells (SOFCs). In this study, a promising cathode material, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF), was prepared as a nanostructured thin film using pulsed laser deposition (PLD) on gadolinia-doped ceria (GDC)-buffered YSZ single crystal substrates having (100) and (111) orientations. Characterization revealed intrinsic differences among the as-grown LSCF thin films in terms of dominant crystalline orientation and nanostructure depending on GDC preparation as well as the YSZ substrate orientation. Evaluation of the oxygen exchange properties using the isotope exchange depth profile method revealed that LSCF thin films grown on (111) GDC/YSZ exhibited higher values of the apparent surface exchange coefficient compared to LSCF thin films grown on (100) GDC/YSZ. However, when subjected to long-term annealing at high temperatures, the former exhibited a stronger tendency to surface segregation as compared to the latter. These behaviors are correlated with the intrinsic properties of LSCF thin films, including the nanostructure, the possible effects attributed to SrO activity, and the stability of perovskite surfaces which would drive surface segregation. These results have implications for tailoring the performance of cathode thin films by understanding the dependence of oxygen exchange properties and surface segregation on driving forces such as surface chemistry and nanostructure.

Apparent Oxygen Uphill Diffusion in La0.8Sr0.2MnO3 Thin Films upon Cathodic Polarization

The impact of cathodic bias on oxygen transport in La_0.8Sr_0.2MnO₃ (LSM) thin films was investigated. Columnar‐grown LSM thin films with different microstructures were deposited by pulsed laser deposition. ¹⁸O tracer experiments were performed on thin film microelectrodes with an applied cathodic bias of −300 or −450 mV, and the microelectrodes were subsequently analyzed by time‐of‐flight secondary ion mass spectrometry. The ¹⁸O concentration in the cathodically polarized LSM microelectrodes was strongly increased relative to that in the thermally annealed film (without bias). Most remarkable, however, was the appearance of a pronounced ¹⁸O fraction maximum in the center of the films. This strongly depended on the applied bias and on the microstructure of the LSM thin layers. The unusual shape of the ¹⁸O depth profiles was caused by a combination of Wagner–Hebb‐type stoichiometry polarization of the LSM bulk, fast grain boundary transport and voltage‐induced modification of the oxygen incorporation kinetics,