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.

b S Supporting Information T he relation of surface cation chemistry and surface electronic structure to oxygen reduction reaction (ORR) kinetics remains an outstanding question to this day in the search for highly active cathodes for solid oxide fuel cells (SOFCs). While traditionally perovskite-type transition-metal oxides have been extensively investigated as SOFC cathodes, 1,2 more recent studies highlight the potential of layered oxide cathodes. 3À5 On the surface of the perovskite-structured La 1Àx Sr x MnO 3 , a widely used and studied SOFC cathode material, 1,2,6À8 the fractional presence of constituent cations can deviate from the nominal bulk stoichiometry significantly 9À13 because of an enrichment of Sr or La cations on the surface. It has been possible to control the bulk magnetic and electronic properties of perovskite thin films by manipulating their lattice parameters with different growth conditions, hydrostatic pressure, or use of substrates with a different lattice mismatch to the films. 14À18 Furthermore, the impact of the lattice strain on the surface electronic structure and reactivity has been long demonstrated for low-temperature noble metal electrocatalysts. 19,20 On the other hand, the role of lattice strain on the surface cation and anion chemistry, electronic structure, and ionic transport, which all influence the ORR activity of SOFCrelated oxides, is attracting its due interest only recently.We have recently demonstrated, from first principles-based calculations, that the epitaxial strain up to a critical tensile strain value favors oxygen-vacancy formation as well as oxygen adsorption on another widely studied SOFC cathode, LaCoO 3 . 21 Experiments validating the direct role of strain on the reactivity with oxygen and oxygen transport in SOFC materials have been yet scarce. Sase et al. showed that the oxygen surface exchange rate at the heterointerface of La 0.6 Sr 0.4 CoO 3 /(La,Sr) 2 CoO 4 thin films is larger by three orders of magnitude compared with the single-phase cobaltite surfaces. 22 A reasonable hypothesis that could explain the enhanced oxygen exchange at that interface region is the role of local strains. Studies on fluorite systems have suggested strong coupling of biaxial strain also to the oxygen ion diffusion. 23À25 In this Letter, we report our results, interpreted in light of our first principles-based simulations, on the strain-induced changes in the surface chemical and electronic state of La 0.7 Sr 0.3 MnO 3 (LSM) as a model system. We assessed two key parameters for reactivity with oxygen as a function of strain: (1) chemical environment on the LSM surface, in particular, the segregation of Sr cations and oxygen vacancy formation, experimentally probed with angleresolved X-ray photoelectron spectroscopy and computationally assessed through segregation/formation energy calculations and (2) surface electronic structure, experimentally probed using scanning tunneling microscopy and spectroscopy (both at ambient and in situ at elevated temperatures) and computat...

Solid‐oxide fuel cells are an attractive energy conversion technology for clean electric power production. To render them more affordable, discovery of new cathode materials with high reactivity to oxygen reduction reaction (ORR) at temperatures below 700 °C is needed. Recent studies have demonstrated that La0.8Sr0.2CoO3/(La0.5Sr0.5)2CoO4 (LSC113/214) hetero‐interfaces exhibit orders of magnitude faster ORR kinetics compared with either single phase at 500 °C. To obtain a microscopic level understanding and control of such unusual enhancement, we implemented a novel combination of in situ scanning tunneling spectroscopy and focused ion beam milling to probe the local electronic structure at nanometer resolution in model multilayer superlattices. At 200–300 °C, the LSC214 layers are electronically activated through an interfacial coupling with LSC113. Such electronic activation is expected to facilitate charge transfer to oxygen, and concurrent with the anisotropically fast oxygen incorporation on LSC214, quantitatively explains the vastly accelerated ORR kinetics near the LSC113/214 interface. Our results contribute to an improved understanding of oxide hetero‐interfaces at elevated temperatures and identify electronically coupled oxide structures as the basis of novel cathodes with exceptional performance.

The hetero-interfaces between the perovskite (La 1Àx Sr x )CoO 3 (LSC 113 ) and the Ruddlesden-Popper (La 1Àx Sr x ) 2 CoO 4 (LSC 214 ) 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 LSC 113 and LSC 214 were prepared by pulsed laser deposition. In situ scanning tunneling spectroscopy established that the LSC 214 domains in the VAN structure became electronically activated, by charge transfer across interfaces with adjacent LSC 113 domains above 250 C in 10 À3 mbar of oxygen gas. Atomic force microscopy and X-ray photoelectron spectroscopy analysis revealed that interfacing LSC 214 with LSC 113 also provides for a more stable cation chemistry at the surface of LSC 214 within the VAN structure, as compared to single phase LSC 214 films. Oxygen reduction kinetics on the VAN cathode was found to exhibit approximately a 10-fold enhancement compared to either single phase LSC 113 and LSC 214 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 LSC 113/214 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 heterostructure compositions for high temperature functionality are warranted.

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