Influence of surface atomic structure demonstrated on oxygen incorporation mechanism at a model perovskite oxide

Perovskite oxide surfaces catalyze oxygen exchange reactions that are crucial for fuel cells, electrolyzers, and thermochemical fuel synthesis. Here, by bridging the gap between surface analysis with atomic resolution and oxygen exchange kinetics measurements, we demonstrate how the exact surface atomic structure can determine the reactivity for oxygen exchange reactions on a model perovskite oxide. Two precisely controlled surface reconstructions with (4 × 1) and (2 × 5) symmetry on 0.5 wt.% Nb-doped SrTiO3(110) were subjected to isotopically labeled oxygen exchange at 450 °C. The oxygen incorporation rate is three times higher on the (4 × 1) surface phase compared to the (2 × 5). Common models of surface reactivity based on the availability of oxygen vacancies or on the ease of electron transfer cannot account for this difference. We propose a structure-driven oxygen exchange mechanism, relying on the flexibility of the surface coordination polyhedra that transform upon dissociation of oxygen molecules.Austrian Science Fund (SFB “ Functional Oxide Surfaces and Interfaces ” - FOXSI, Project F 45)European Research Council Advanced Grant (“OxideSurfaces” (Project ERC-2011-ADG_20110209))National Science Foundation (U.S.). Division of Materials Research (CAREER Award Grant No. 1055583

The geometry dependence of the polarization resistance of Sr-doped LaMnO3 microelectrodes on yttria-stabilized zirconia

Impedance spectroscopic studies and I-V Measurements are performed at Sr-doped LaMnO3 (LSM) microelectrodes in order to elucidate the mechanism of the oxygen-reduction reaction on yttria-stabilized zirconia. The geometry dependence of the polarization resistance was investigated by systematic variations of the microelectrode's size and thickness. The relation between the resistance and the electrode geometry turns out to be bias-dependent: in the cathodic regime and close to equilibrium, the resistance is proportional to the inverse electrode area. Moreover, measurements without bias revealed an almost linear dependence of the resistance on the electrode thickness, This suggests that the relevant oxygen reduction path involves the transport of oxide ions through the bulk of the LSM cathode. In the anodic regime, however, the resistance becomes proportional to the inverse three-phase boundary length and, hence, a mechanism involving the LSM surface is most probable with a step close to the three-phase boundary being rate limiting. Experiments performed on LSM microelectrodes with thin alumina "discs" beneath the LSM to partly block the oxygen ion transport through the bulk of the electrode support this interpretation. (C) 2002 Elsevier Science B.V. All rights reserved