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.

An interface between the perovskite La(0.8)Sr(0.2)CoO(3-δ) (LSC-113) and the K(2)NiF(4)-type (La(0.5)Sr(0.5))(2)CoO(4-δ) (LSC-214) heterostructure was recently shown to enhance oxygen surface exchange and the rate of the oxygen reduction reaction (ORR) by orders of magnitude compared to either the LSC-113 or LSC-214 phase alone. This result is of interest to develop better optimized materials for solid-state electrochemical devices, e.g. solid oxide fuel cells. The effect has been attributed to the interface itself, rather than changes in the bulk LSC-113 or LSC-214 phases. Using density functional theory (DFT)-based simulations, we demonstrate that there is a ∼0.9 eV (∼1.3 eV) energy gain for exchanging a Sr from LSC-113(25%Sr) (LSC-113(40%Sr)) with a La from LSC-214(50%Sr). These changes in energy create a large driving force for interdiffusion across the heterostructure interface from Sr into LSC-214 and La into LSC-113. We estimate that the Sr concentrations (in the LSC-214 phase) in a typical experimental temperature range of 500-600 °C and in equilibrium with LSC-113(25%Sr) and LSC-113(40%Sr), may be about 75% Sr and 90% Sr, respectively. Based on the bulk behavior of the LSC-214 phase (Vashook et al., Solid State Ionics, 2000, 138, 99-104), an Sr enrichment from x = 0.5 to x = 0.75 in (La(1-x)Sr(x))(2)CoO(4-δ) is expected to enhance the oxygen vacancy concentration by 2-2.5 orders of magnitude under typical experimental conditions. An increased vacancy concentration in LSC-214 near the interface can explain most of the enhanced oxygen kinetics observed up until now in these heterostructures.

Reducing the working temperature of solid oxide fuel cells is critical to their increased commercialization but is inhibited by the slow oxygen exchange kinetics at the cathode, which limits the overall rate of the oxygen reduction reaction. We use ab initio methods to develop a quantitative elementary reaction model of oxygen exchange in a representative cathode material, La0.5Sr0.5CoO3−δ, and predict that under operating conditions the rate-limiting step for oxygen incorporation from O2 gas on the stable, (001)-SrO surface is lateral (surface) diffusion of O-adatoms and oxygen surface vacancies. We predict that a high vacancy concentration on the metastable CoO2 termination enables a vacancy-assisted O2 dissociation that is 102–103 times faster than the rate limiting step on the Sr-rich (La,Sr)O termination. This result implies that dramatically enhanced oxygen exchange performance could potentially be obtained by suppressing the (La,Sr)O termination and stabilizing highly active CoO2 termination.

Heterostructured oxides have shown unusual electrochemical properties including enhanced catalytic activity, ion transport, and stability. In particular, it has been shown recently that the activity of oxygen electrocatalysis on the Ruddlesden-Popper/perovskite (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ heterostructure is remarkably enhanced relative to the Ruddlesden-Popper and perovskite constituents. Here we report the first atomic-scale structure and composition of (La1-ySry)2CoO4±δ/La1-xSrxCoO3-δ grown on SrTiO3. We observe anomalous strontium segregation from the perovskite to the interface and the Ruddlesden-Popper phase using direct X-ray methods as well as with ab initio calculations. Such Sr segregation occurred during the film growth, and no significant changes were found upon subsequent annealing in O2. Our findings provide insights into the design of highly active catalysts for oxygen electrocatalysis.

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