New Insights into the Strain Coupling to Surface Chemistry, Electronic Structure, and Reactivity of La_0.7Sr_0.3MnO₃

Abstract

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...

Chemomechanics of ionically conductive ceramics for electrical energy conversion and storage

“…Using 18 O isotope exchange depth profiling with ToF-SIMS, they measured the oxygen tracer exchange coefficient k * and tracer diffusion coefficient D * on the surfaces of these films, at temperatures ranging from 280 • C to 475 • C. They found that the surface exchange and diffusion for the film with tensile strain were about four times and 10 times faster than the film under compressive strain, respectively. These results confirmed the earlier predictions at the elementary reaction level that tensile lattice strain accelerates oxygen reduction and diffusion on the LSC (and LSM), reported by several computational studies on the same material systems [45,53]. Consistent with this, La O' et al found also that epitaxial films of LSC on a GDC/YSZ substrate showed enhanced chemical surface exchange kinetics [168].…”

“…This enables measurements at elevated temperature in an oxygen gas environment, allowing closer simulation of the real environment of SOFCs. Reversible changes in electronic structure with temperature have been observed on the (a) (b) [45,53]. These results provide insight into the strain and reactivity correlation of SOFC cathode materials.…”

“…This difference could lead to easier charge transfer during the ORR reaction for the tensile-strained surfaces. High temperature XPS results and DFT calculations support that tensile strain could cause such a difference in electronic structure by causing easier formation of oxygen vacancies near the surface [45,53]. Lussier et al grew La 2/3 Ca 1/3 MnO 3 thin films with desired induced strain by depositing the films on SrTiO 3 (100) (STO), SrRuO 3 (100) (SRO), LaAlO 3 (100) (LAO), and NdGaO 3 (100) (NGO) [166].…”

“…Using in situ STM/S, Yildiz et al conducted a series of studies on the surface electronic structure of tensile and compressive-strained (La,Sr)CoO 3 and (La,Sr)MnO 3 at high temperature in an oxygen environment [45,53]. They found that the tensile-strained film showed a higher density of states near the Fermi level at high temperature compared with compressive-strained films.…”

“…Cai et al found that LSC films with tensile strain had more Sr segregation to the surface compared to those with compressive strain [45]. Jalili et al observed a similar behavior in LSM films [53]. Estradé et al used EELS to map composition and electronic states in epitaxial La 2/3 Ca 1/3 MnO 3 films grown on SrTiO 3 (001) and (110) substrates [22].…”