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Perovskite oxides can exhibit a wide range of interesting characteristics such as being catalytically active and electronically/ionically conducting, and thus, they have been used in a number of solid-state devices such as solid oxide fuel cells (SOFCs) and sensors. As the surface compositions of perovskites can greatly influence the catalytic properties, knowing and controlling their surface compositions is crucial to enhance device performance. In this study, we demonstrate that the surface strontium (Sr) and cobalt (Co) concentrations of perovskite-based thin films can be controlled reversibly at elevated temperatures by applying small electrical potential biases. The surface compositional changes of La 0.8 Sr 0.2 CoO 3−δ (LSC 113 ), (La 0.5 Sr 0.5 ) 2 CoO 4±δ (LSC 214 ), and LSC 214 -decorated LSC 113 films (LSC 113/214 ) were investigated in situ by utilizing synchrotron-based X-ray photoelectron spectroscopy (XPS), where the largest changes of surface Sr were found for the LSC 113/214 surface. These findings offer the potential of reversibly controlling the surface functionality of perovskites.

Mixed-conducting perovskite-type electrodes which are used as cathodes in solid oxide fuel cells (SOFCs) exhibit pronounced performance improvement after cathodic polarization. The current in situ study addresses the mechanism of this activation process which is still unknown. We chose the new perovskite-type material La(0.75)Sr(0.25)Cr(0.5)Mn(0.5)O(3±δ) which is a potential candidate for use in symmetrical solid oxide fuel cells (SFCs). We prepared La(0.75)Sr(0.25)Cr(0.5)Mn(0.5)O(3±δ) thin film model electrodes on YSZ (111) single crystals by pulsed laser deposition (PLD). Impedance spectroscopy (EIS) measurements show that the kinetics of these electrodes can be drastically improved by applying a cathodic potential. To understand the origin of the enhanced electrocatalytic activity the surfaces of operating LSCrM electrodes were studied in situ (at low pressure) with spatially resolving X-ray photoelectron spectroscopy (μ-ESCA, SPEM) and quasi static secondary ion mass spectrometry (ToF-SIMS) after applying different electrical potentials in the SIMS chamber. We observed that the electrode surfaces which were annealed at 600 °C are enriched significantly in strontium. Subsequent cathodic polarization decreases the strontium surface concentration while anodic polarization increases the strontium accumulation at the electrode surface. We propose a mechanism based on the reversible incorporation of a passivating SrO surface phase into the LSCrM lattice to explain the observed activation/deactivation process.

The chemical composition and microstructure of the solid electrolyte interphase (SEI) on lithium metal in ether-based electrolytes are investigated using X-ray photoelectron spectroscopy (XPS), time of flight secondary ion mass spectrometry (ToF-SIMS) and scanning electron microscopy (SEM). Electrolytes based on 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) solvents with lithium bis(trifluoromethane)sulfonamide (LiTFSI) conducting salt are employed to passivate lithium surfaces for different periods of time. Two types of model experiments (immersion-type and galvanostatic current load) are performed to generate self-and currentinduced SEIs. Reference measurements of predicted SEI components and an advanced XPS signal coupling method facilitate the deconvolution of the spectroscopic and spectrometric data. Both ether solvents showed specific SEI formation for each experimental setup. DME appears to decompose immediately on the lithium surface and forms a thin passivation film of various salts in a lithium-alkoxide framework. DOL shows a slow SEI formation of organic and inorganic salts revealing problems in the lithium surface protection. Under current load the reaction rate is increased in DME maintaining the three-dimensional microstructure. For DOL the current load leads to a multi-layer structure of organic host materials comprising LiTFSI and lithium fluoride. Finally, two-dimensional schematic pictures of the SEI microstructure are developed.