The surface and near-surface composition in perovskite-based electroceramics is analysed at the atomic scale using highly surface sensitive low-energy ion scattering (LEIS).

No abstract

Solid-oxide fuel cells are devices for the efficient conversion of chemical energy to electrical energy and heat. Research efforts are currently addressed toward the optimization of cells operating at temperatures in the region of 600°C, known as intermediate-temperature solid-oxide fuel cells, for which materials requirements are very stringent. In addition to the requirements of mechanical and chemical compatibility, the materials must show a high degree of oxide ion mobility and electrochemical activity at this low temperature. Here we mainly examine the criteria for the development of two key components of intermediate-temperature solid-oxide fuel cells: the electrolyte and the cathode. We limit the discussion to novel approaches to materials optimization and focus on the fluorite oxide for electrolytes, principally those based on ceria and zirconia, and on perovskites and perovskite-related families in the case of cathodes.

We report on the development and validation of a new methodology for the determination of anisotropic tracer diffusion and surface exchange coefficients of high quality epitaxial thin films in the two perpendicular directions (transverse and longitudinal), by the isotopic exchange technique. Measurements were performed on c-axis oriented La 2 NiO 4+d films grown on SrTiO 3 (100) and NdGaO 3 (110) by pulsed injection metal organic chemical vapour deposition (PIMOCVD), with different thicknesses ranging from 33 to 370 nm. The effect that the strain induced by the film-substrate mismatch has on the oxygen diffusion through the film was evaluated. Both tracer diffusion coefficients, along the c-axis and along the ab plane, were found to increase with film thickness, i.e., as the stress of the film decreases, while the thickness seems to have no effect on the tracer surface exchange coefficient. Best fits were obtained when considering the thickest films composed by two regions with different c-axis tracer diffusion coefficient values, a higher and constant D* close to the film surface and a variable decreasing D* closer to the substrate. As expected, the tracer diffusion and surface exchange coefficients are thermally activated and are approximately two orders of magnitude higher along the ab plane than along the c-axis. The low activation energies of D* compared with bulk values for both directions at low temperatures seem to confirm the contribution of a vacancy mechanism to the ionic conduction.

Attaining fast oxygen exchange kinetics on perovskite and related mixed ionic and electronic conducting oxides is critical for enabling their applications in electrochemical energy conversion systems. This study focuses on understanding the relationship between surface chemistry and the surface oxygen exchange kinetics on epitaxial films made of (La 1-x Sr x ) 2 CoO 4 , a prototypical Ruddlesden-Popper structure that is considered as a promising cathode material for fuel cells. The effects of crystal orientation on the surface composition, morphology, oxygen diffusion and surface exchange kinetics were assessed by combining complementary surface-sensitive analytical techniques, specifically low energy ion scattering, x-ray photoelectron spectroscopy, Auger electron spectroscopy, scanning transmission electron microscopy, atomic force microscopy and secondary ion mass spectroscopy. The films were grown in two different crystallographic orientations, (001) and (100), and with two different Sr compositions, at x=0.25 (LSC25) and 0.50 (LSC50), by using pulsed laser deposition. In the as-prepared state, a Sr enriched layer at the top surface and a Co enriched subsurface layer were found on films with both orientations. After annealing at elevated temperatures in oxygen, the Sr enrichment increased, followed by clustering into Sr-rich secondary phase particles. Both the LSC25 and LSC50 films showed anisotropic oxygen diffusion kinetics, with up to 20 times higher oxygen diffusion coefficient along the (ab) plane compared that along the c-axis at 400-500 o C. However, no dependence of surface oxygen exchange coefficient was found on the crystal orientation. This result indicates that the strong Sr segregation at the surface overrides the effect of the structural anisotropy that was also expected for the surface exchange kinetics. The larger presence of Co cations exposed at the LSC25 surface compared to that at the LSC50 surface is likely the reason for the faster oxygen surface exchange kinetics on LSC25 compared to LSC50. This work demonstrated the critical role of surface chemistry on the oxygen exchange kinetics on perovskite related oxides, which are thus far under-explored at elevated temperatures, and provides a generalizable approach to probe the surface chemistry on other catalytic complex oxides.

Oxygen diffusion and surface exchange coefficients have been measured on polycrystalline samples of the double perovskite oxide PrBaCo2O5+δ by the isotope exchange depth profile method, using a time-of-flight SIMS instrument. The measured diffusion coefficients show an activation energy of 1.02 eV, as compared to 0.89 eV for the surface exchange coefficients in the temperature range from 300 to 670 °C. Inhomogeneity was observed in the distribution of the oxygen-18 isotopic fraction from grain to grain in the ceramic samples, which was attributed to anisotropy in the diffusion and exchange of oxygen. By the use of a novel combination of electron back scattered diffraction measurements, time-of-flight, and focused ion beam SIMS, this anisotropy was confirmed by in-depth analysis of single grains of known orientation in a ceramic sample exchanged at 300 °C. Diffusion was shown to be faster in a grain oriented with the surface normal close to 100 and 010 (ab-plane oriented) than a grain with a surface normal close to 001 (c-axis oriented). The magnitude of this anisotropy is estimated to be close to a factor of 4, but this is only a lower bound due to experimental limitations. These findings are consistent with recent molecular dynamic simulations of this material where anisotropy in the oxygen transport was predicted.

No abstract

The search for new strategies to enhance the oxide ionic conductivity in oxide materials is a very active field of research. These materials are needed for application in a new generation of more