Polar or not polar? The interplay between reconstruction, Sr enrichment, and reduction at the La0.75Sr0.25MnO3 (001) surface

Abstract

Perovskite oxides used in heterogeneous catalysis and electrocatalysis are tuned by substitutional doping. Sr-doped LaMnO 3 is a popular choice of electrode material for electrochemical energy conversion. At elevated temperatures, relevant to solid oxide fuel or electrolysis cells, Sr enriches at the surface, which leads to fast degradation of the oxygen exchange activity at the surface. In this work, we investigate the effect of oxygen partial pressure p(O 2) and temperature on Sr segregation at the La 0.75 Sr 0.25 MnO 3 (001) (LSM25) surface by ab initio thermodynamics calculations, taking into account different terminations and point defects Sr La , V •• O , V Mn , V La , O ad , as well as associated defects of the type [V •• O − V cation ]. This model of the LSM25(001) surface addresses the connection between point defects and surface stability and makes quantitative predictions about the surface termination and Sr enrichment. Our results indicate that the MnO 2 termination is stable under oxidizing conditions, while a partially covering SrO termination on MnO 2 is stable for reaction conditions between 1000 and 1400 K and effective oxygen partial pressures between 10 −11 bar and 0.1 bar relevant to solid oxide fuel cell (SOFC) cathodes. Under more reducing conditions, we find that Sr is enriched at the surface regardless of surface termination. This trend is a direct consequence of the increased formation of positively charged oxygen vacancies at the surface, and this conclusion holds regardless of which surface termination is assumed. The partial SrO termination is exceptionally stable under SOFC cathode conditions because it minimizes the surface dipole density, while all the defect-free terminations of LSM25(001) are unstable due to surface polarity. This newly proposed termination leaves both Sr, as well as Mn and O sites of the MnO 2 layer exposed at the surface and available for interaction with the gas phase. We propose that the LSM25(001) surface may consist of coexisting (La,Sr)O and MnO 2 domains under SOFC cathode conditions, and we propose to include such a micropatterned surface in the future discussion of the active site in the oxygen reduction reaction.

Atomically resolved surface phases of La0.8Sr0.2MnO3(110) thin films

“…Particularly critical for the operation of solid-oxide fuel cells is the widely reported segregation of Sr-rich phases under operating conditions, which tends to degrade the oxygen exchange activity at the surface. 89 Sr segregation and the formation of insulating Sr-rich clusters is a very common phenomenon observed in LSMO and other perovskite oxides under conditions comparable to those employed in this work, 26,70 and it has been widely investigated, both experimentally and theoretically. 27,89 Notably, the lms presented in this work do not show this phenomenon.…”

“…89 Sr segregation and the formation of insulating Sr-rich clusters is a very common phenomenon observed in LSMO and other perovskite oxides under conditions comparable to those employed in this work, 26,70 and it has been widely investigated, both experimentally and theoretically. 27,89 Notably, the lms presented in this work do not show this phenomenon. XPS does not indicate any substantial deviation of the La-to-Sr ratio from the nominal composition in the near-surface region.…”

“…Importantly, each reconstruction can be uniquely identied by its appearance in STM and by its Mn-to-A-site content (as determined with XPS or LEIS). This level of control over the single surface phases sets the stage to isolate a few critical factors that are known/expected to affect the surface reactivity of LSMO, namely its composition, 89 the surface atomic structure (through the surface coordination chemistry), 45 the Mn oxidation state, 37 and the ionicity of the metal-oxygen bonds. Notably, because the presented phases are stable under conditions that bridge the gap between UHV and the operating conditions of high-temperature solidoxide fuel cells, they are an ideal test bed to address the oxygen incorporation driving these devices.…”

Precipitation of dopants on acceptor-doped LaMnO3±δ revealed by defect chemistry from first principles

“…[25][26][27][28] The enrichment of dopants near the surface has recently been explained by a density functional theory (DFT)-based ab initio thermodynamics model of La 0.75 Sr 0.25 MnO 3 (LSM25) in our recent work. 29 We demonstrated that the surface free energy under SOFC operating conditions is minimized by a surface configuration with oxygen vacancies, whose positive relative charge is balanced by enriched dopants of relative negative charge, still within the perovskite surface lattice. Dopantrich phase precipitation, on the other hand, was predicted to occur only under SOEC operating conditions (anodic polarization), but not under SOFC operating conditions (cathodic polarization).…”

Route to High-Performance Micro-solid Oxide Fuel Cells on Metallic Substrates

“…In this way, the impedance response is dominated by the thin-film cathode [54]. b) as determined below, the as-grown VAN film is under out-of-plane tension: such tensile strain is known to induce Sr surface segregation [38,57], resulting in electronically and ionically insulating Sr-enriched phases, which severely impedes surface oxygen exchange [58]. [59] An outof-plane tension in the as-grown VAN film is expected because of the presence of stiff, and larger lattice parameter MgO cf.…”