Nikolai Tsvetkov

Accelerated Oxygen Exchange Kinetics on Nd2NiO4+δ Thin Films with Tensile Strain along c-Axis

The influence of the lattice strain on the kinetics of the oxygen reduction reaction (ORR) was investigated at the surface of Nd2NiO4+δ (NNO). Nanoscale dense NNO thin films with tensile, compressive and no strain along the c-axis were fabricated by pulsed laser deposition on single-crystalline Y0.08Zr0.92O2 substrates. The ORR kinetics on the NNO thin film cathodes was investigated by electrochemical impedance spectroscopy at 360-420 °C in air. The oxygen exchange kinetics on the NNO films with tensile strain along the c-axis was found to be 2-10 times faster than that on the films with compressive strain along the c-axis. A larger concentration of oxygen interstitials (δ) is found in the tensile NNO films compared to the films with no strain or compressive strain, deduced from the measured chemical capacitance. This is consistent with the increase in the distance between the NdO rock-salt layers observed by transmission electron microscopy. The surface structure of the nonstrained and tensile strained films remained stable upon annealing in air at 500 °C, while a significant morphology change accompanied by the enrichment of Nd was found at the surface of the films with compressive strain. The faster ORR kinetics on the tensile strained NNO films was attributed to the ability of these films to incorporate oxygen interstitials more easily, and to the better stability of the surface chemistry in comparison to the nonstrained or compressively strained films.

Vertically aligned nanocomposite La0.8Sr0.2CoO3/(La0.5Sr0.5)2CoO4 cathodes – electronic structure, surface chemistry and oxygen reduction kinetics

The hetero-interfaces between the perovskite (La1−xSrx)CoO3 (LSC113) and the Ruddlesden-Popper (La1−xSrx)2CoO4 (LSC214) phases have recently been reported to exhibit fast oxygen exchange kinetics. Vertically aligned nanocomposite (VAN) structures offer the potential for embedding a high density of such special interfaces in the cathode of a solid oxide fuel cell in a controllable and optimized manner. In this work, VAN thin films with hetero-epitaxial interfaces between LSC113 and LSC214 were prepared by pulsed laser deposition. In situ scanning tunneling spectroscopy established that the LSC214 domains in the VAN structure became electronically activated, by charge transfer across interfaces with adjacent LSC113 domains above 250 °C in 10−3 mbar of oxygen gas. Atomic force microscopy and X-ray photoelectron spectroscopy analysis revealed that interfacing LSC214 with LSC113 also provides for a more stable cation chemistry at the surface of LSC214 within the VAN structure, as compared to single phase LSC214 films. Oxygen reduction kinetics on the VAN cathode was found to exhibit approximately a 10-fold enhancement compared to either single phase LSC113 and LSC214 in the temperature range of 320–400 °C. The higher reactivity of the VAN surface to the oxygen reduction reaction is attributed to enhanced electron availability for charge transfer and the suppression of detrimental cation segregation. The instability of the LSC113/214 hetero-structure surface chemistry at temperatures above 400 °C, however, was found to lead to degraded ORR kinetics. Thus, while VAN structures hold great promise for offering highly ORR reactive electrodes, efforts towards the identification of more stable hetero-structure compositions for high temperature functionality are warranted.

Reducibility of Co at the La0.8Sr0.2CoO3/(La0.5Sr0.5)2CoO4 hetero-interface at elevated temperatures

The fast kinetics of oxygen reduction reaction (ORR) at oxide hetero-structures made of La0.8Sr0.2CoO3 and (La0.5Sr0.5)2CoO4 (LSC113/214) attracted great interest to enable high performance cathodes for solid oxide fuel cells. The aim of this work is to uncover the underlying mechanism of fast ORR kinetics at the LSC113/214 system from a defect chemistry and electronic structure perspective. X-ray photoelectron spectroscopy with depth profiling was used to compare the reducibility of the Co cation and the valence band offset in the LSC113/214 multilayer (ML) and in single phase LSC113 and LSC214 films. At 250 °C, the Co 2p core-level photoelectron spectra showed the presence of Co2+ across the ML interfaces in both the LSC113 and LSC214 layers. While this is similar to the Co valence state of LSC113 single phase films, it is contrary to the single phase LSC214 films which had only the higher oxidation state of cobalt, Co3+. The greater reducibility of Co in LSC214 in the ML structure compared to that of Co in the LSC214 single phase film was attributed to electron donation and transfer of oxygen vacancies from LSC113 across the interfaces, and it is one mechanism to enhance the oxygen reduction activity at the LSC113/214 hetero-structure.

Spinel/perovskite cobaltite nanocomposites synthesized by combinatorial pulsed laser deposition

Perovskite/spinel nanocomposites have attracted great attention due to the novel properties that originate from the coupling between two oxides through their interfaces. Combinatorial pulsed laser deposition was used in this work to grow La0.8Sr0.2CoO3 (LSC)/CoFe2O4 (CFO) nanocomposites on (001) SrTiO3 (STO) under various growth conditions. At a substrate temperature of 680 °C, the LSC/CFO consisted of columns of CFO and a third phase, likely to be CoOx, within a highly textured LSC matrix. Lowering the temperature to 600 °C resulted in highly textured LSC/CFO nanocomposites consisting of grains of LSC and CFO a few nanometers in diameter. The grain size increased with decreasing growth rate. In the nanocomposites, redistribution of Fe into the perovskite phase is expected. The strain in the CFO was impacted by the lattice match at the interfaces with the LSC, and changes in composition as well as the strain led to lower magnetic anisotropy compared with CFO films on STO. This approach can also be used to produce other novel nanocrystalline spinel/perovskite systems.

High Resolution Electron Microscopy Characterization of (La 0.5 Sr 0.5 )2CoC4 Thin Film Cathode Materials

Thin film LSC214 was grown by pulse laser deposition (PLD) on SrTiO3 (STO) (001) or LaSrAlO4 (SLAO) (100) substrate. Due to the lattice parameter constraint of the substrate, the deposited thin film will grow in different orientations. High-resolution high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) is used to characterize both samples with a focus on the surface of the films. The S/TEM samples were lifted out by dual beam focused ion beam (FIB) method. To protect the film surface from ion beam damage, samples were coated with gold before FIB lift-out.