ARTICLEThis journal is © The Royal Society of Chemistry 2013 J. Name., 2013, 00, 1--3 | 1 A IntroductionSearch and design of highly active and less expensive materials for catalyzing the sluggish kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is of primary importance in many electrochemical energy device applications such as direct-solar and electrolytic water splitting, metal-air batteries, and fuel cells [1][2][3][4][5][6] . In replacement of noble metal containing catalysts, first row transition-metal perovskites are promising candidate materials for catalyzing OER and ORR in alkaline solution. A number of activity descriptor approaches provide an efficient and practical guidance to facilitate screening of alternate perovskite OER and ORR catalysts 4,5,7 , such as number of d-electrons 1, 8 , oxidation enthalpy 1, 9 , the p-band center relative to the Fermi level 6 , the degree of overlapping between the e g orbitals of the M(3d) band and the O(2p) band relative to the Fermi level 10 , and free energies of formation of the bulk perovskites relative to metal and H 2 O/H 2 11. However, there are still many questions about surfaces and interfaces of the transition-metal perovskite catalyst systems in the aqueous environment under the OER and ORR conditions which have invoked further experimental studies 12,13 . These questions include, e.g., what are the stable surfaces for these perovskites, how different surface orientations/terminations result in different activities, how the bulk electronic structure descriptors can be used to describe activities of various surface terminations, and whether/how surface stability is linked to surface catalytic activities. First principles-based Density Functional Theory (DFT) methods are now able to simulate catalytic reactions at specific metal oxide surfaces and extract surface electronic structure and energetic details, which can provide new insights into structure-activity relationships and strategies for material design and development 7,[14][15][16] . For example, Man et al. 14 have performed

The A(n+1)B(n)O(3n+1) Ruddlesden-Popper homologous series offers a wide variety of functionalities including dielectric, ferroelectric, magnetic and catalytic properties. Unfortunately, the synthesis of such layered oxides has been a major challenge owing to the occurrence of growth defects that result in poor materials behaviour in the higher-order members. To understand the fundamental physics of layered oxide growth, we have developed an oxide molecular beam epitaxy system with in situ synchrotron X-ray scattering capability. We present results demonstrating that layered oxide films can dynamically rearrange during growth, leading to structures that are highly unexpected on the basis of the intended layer sequencing. Theoretical calculations indicate that rearrangement can occur in many layered oxide systems and suggest a general approach that may be essential for the construction of metastable Ruddlesden-Popper phases. We demonstrate the utility of the new-found growth strategy by performing the first atomically controlled synthesis of single-crystalline La3Ni2O7.

The slow kinetics of oxygen surface exchange hinders the efficiency of high-temperature oxygen electrocatalytic devices such as solid oxide fuel cells and oxygen separation membranes. Systematic investigations of material properties that link to catalytic activity can aid in the rational design of highly active cathode materials. Here, we explore LaCoO thin films as a model system for tuning catalytic activity through strain-induced changes in the Co spin state. We demonstrate that Raman spectroscopy can be used to probe the Co-O bond strength at different temperatures to determine the relative spin occupancies of LaCoO. We find that strain can be used to reduce the spin transition temperature and promote the occupation of higher spin states that weaken the Co-O bond. The decrease in Co-O bond strength and increased spin moment of the thin films result in significant enhancements of the oxygen surface exchange kinetics by up to 2 orders of magnitude.

Fast ion conductors are at the foundation of a number of important technologies, ranging from fuel cells to batteries to gas separators. Recent results suggest that strained interfaces and thin films may offer new mechanisms for achieving enhanced ionic transport. In this work, we investigate strained 40-nm films of perovskite La0.5Sr0.5CoO3−δ, which is an important material for solid oxide fuel cell cathodes and oxygen separation membranes. We demonstrate that a strained thin film of La0.5Sr0.5CoO3−δ on SrTiO3 can have dramatically different anion and cation thermodynamics and kinetics than bulk La0.5Sr0.5CoO3−δ. We use synchrotron X-ray diffraction to show that La0.5Sr0.5CoO3−δ thin films form an ordered phase at 650 K. The ordered phase consists of La and Sr cations in planes parallel to the surface and is associated with coherent expansion in the c-direction of ∼5%. This chemical ordering is not observed in the bulk material and is ascribed to the interplay between the epitaxial strain imposed by the substrate, changes in oxygen vacancy content and cation mobility, and the ordering of oxygen vacancies.