Yener Kuru

Surface Electronic Structure Transitions at High Temperature on Perovskite Oxides: The Case of Strained La0.8Sr0.2CoO3 Thin Films

In-depth probing of the surface electronic structure on solid oxide fuel cell (SOFC) cathodes, considering the effects of high temperature, oxygen pressure, and material strain state, is essential toward advancing our understanding of the oxygen reduction activity on them. Here, we report the surface structure, chemical state, and electronic structure of a model transition metal perovskite oxide system, strained La(0.8)Sr(0.2)CoO(3) (LSC) thin films, as a function of temperature up to 450 °C in oxygen partial pressure of 10(-3) mbar. Both the tensile and the compressively strained LSC film surfaces transition from a semiconducting state with an energy gap of 0.8-1.5 eV at room temperature to a metallic-like state with no energy gap at 200-300 °C, as identified by in situ scanning tunneling spectroscopy. The tensile strained LSC surface exhibits a more enhanced electronic density of states (DOS) near the Fermi level following this transition, indicating a more highly active surface for electron transfer in oxygen reduction. The transition to the metallic-like state and the relatively more enhanced DOS on the tensile strained LSC at elevated temperatures result from the formation of oxygen vacancy defects, as supported by both our X-ray photoelectron spectroscopy measurements and density functional theory calculations. The reversibility of the semiconducting-to-metallic transitions of the electronic structure discovered here, coupled to the strain state and temperature, underscores the necessity of in situ investigations on SOFC cathode material surfaces.

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.

Direct Probing of Nanodimensioned Oxide Multilayers with the Aid of Focused Ion Beam Milling

IO N Recent advances in complex oxide thin fi lm synthesis techniques such as pulsed laser deposition (PLD) and oxide molecular beam epitaxy have stimulated the investigation of novel electronic, magnetic, and ionic properties enabled by the fabrication of superlattices and/or specialized interfaces. [ 1–12 ] A prime example of such novel interfacial properties is the high mobility metallic state found to exist at the interfaces of LaAlO 3 (LAO)/SrTiO 3 (STO) superlattices of below four unit cell thicknesses, while both materials individually are insulating. [ 13 , 14 ]

Strain Effects on the Surface Chemistry of La0.7Sr0.3MnO3

We report on the mechanistic effects of epitaxial strain on the surface chemistry, in particular the segregation of Sr cations on La0.7Sr0.3MnO3 (LSM) model dense thin films. Our results show that the LSM film surfaces are layered and exhibit straindependent nanoscale lateral structures. All surfaces examined here were Sr-rich. X-ray photoelectron spectroscopy shows a larger Sr segregation tendency for the tensile strained LSM films. This result is in good agreement with our first principles-based calculations, which predict lower Sr segregation energy on the tensile strained LSM surface. Our findings suggest the importance of lattice strain as a key parameter to tune the surface chemistry for facilitating oxygen reduction kinetics on transition metal perovskite cathode surfaces for solid oxide fuel cells.

Chemical, Electronic and Nanostructure Dynamics on Sr(Ti1-xFex)O3 Thin-Film Surfaces at High Temperatures

Introduction The slow oxygen reduction reaction (ORR) at the cathode is a major barrier to achieving high output for Solid Oxide Fuel Cells (SOFC). Surface oxygen exchange with and oxygen diffusion into the oxide cathodes appear to be the two key processes controlling the ORR kinetics. The electronic and chemical states of the surfaces are key factors governing the oxygen surface reaction rates. It is, therefore, important to probe and understand how surface structure, chemistry and electron transfer properties impact the ORR kinetics under SOFC working conditions. . The aim of our investigation is to correlate the structural, electron tunneling and chemical characteristics of cathode surfaces under in-situ conditions to the oxygen reduction kinetics at the atomistic level. The material we chose to study is the Sr(Ti, Fe)O3 solid solution system, which is stable over a large range of temperature and PO2. The capability of adjusting its ionic and electronic conductivity over wide limits via variations in the Fe content, makes this material an ideal model material for our study [1].

Chemical Expansion and Frozen-In Oxygen Vacancies in Pr-Doped Ceria

Doped CeO2 is a promising candidate for solid oxide fuel cell electrolyte and electrode applications because of its high ionic conductivity and reduction/oxidation behavior at intermediate temperatures. Its electronic and ionic properties and microstructural stability are of particular interest. The present study demonstrates that the large number of oxygen vacancies created in PrxCe1-xO2-δ (PCO) at elevated temperatures can be accommodated at room temperature if cooling is performed in relatively low oxygen partial pressures (i.e. P(O2) ~ 10 mbar). We use the temperature dependence of the chemical expansion in reduced PCO as a metric to explore this phenomenon.