The influence of lattice strain on the oxygen exchange kinetics and diffusion in oxides was investigated on (100) epitaxial La1–xSrxCoO3−δ (LSC) thin films grown by pulsed laser deposition. Planar tensile and compressively strained LSC films were obtained on single-crystalline SrTiO3 and LaAlO3. 18O isotope exchange depth profiling with ToF-SIMS was employed to simultaneously measure the tracer surface exchange coefficient k* and the tracer diffusion coefficient D* in the temperature range 280–475 °C. In accordance with recent theoretical findings, much faster surface exchange (∼4 times) and diffusion (∼10 times) were observed for the tensile strained films compared to the compressively strained films in the entire temperature range. The same strain effect—tensile strain leading to higher k* and D*—was found for different LSC compositions (x = 0.2 and x = 0.4) and for surface-etched films. The temperature dependence of k* and D* is discussed with respect to the contributions of strain states, formation enthalpy of oxygen vacancies, and vacancy mobility at different temperatures. Our findings point toward the control of oxygen surface exchange and diffusion kinetics by means of lattice strain in existing mixed conducting oxides for energy conversion applications.

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.