Featured research (4)
The application of double perovskite cobaltites BaLnCo2O6−δ (Ln = lanthanide element) in electrochemical devices for energy conversion requires control of their properties at operating conditions. This work presents a study of a series of BaLnCo2O6−δ (Ln = La, Pr, Nd) with a focus on the evolution of structural and electrical properties with temperature. Symmetry, oxygen non-stoichiometry, and cobalt valence state have been examined by means of Synchrotron Radiation Powder X-ray Diffraction (SR-PXD), thermogravimetry (TG), and X-ray Absorption Spectroscopy (XAS). The results indicate that all three compositions maintain mainly orthorhombic structure from RT to 1000 °C. Chemical expansion from Co reduction and formation of oxygen vacancies is observed and characterized above 350 °C. Following XAS experiments, the high spin of Co was ascertained in the whole range of temperatures for BLC, BPC, and BNC.
The structure of BaLnCo2O6-δ (Ln =La, Pr, Nd, Sm, Gd, Tb and Dy) was studied by the means of synchrotron radiation powder X-ray diffraction, neutron powder diffraction and Transmission Electron Microscopy (TEM), while water uptake properties were analysed with the use of thermogravimetry (TG) and water adsorption isotherms. The structure refinement revealed that the dominant phase in all compositions was orthorhombic with an ordering of the A-site cations along the c-axis and ordering of oxygen vacancies along the b-axis, which was also directly evidenced by TEM. It was shown that both unit cell volume and average Co- oxidation state at room temperature decrease linearly with decreasing Ln radius. TG water uptake experiments in humidified N2-O2 gas mixture at 300°C revealed that among all compositions, only BaLaCo2O6-δ and BaGdCo2O6-δ exhibit significant water uptake. Surface water adsorption studies showed that the α, a normalised parameter reflecting the surface hydrophilicity, mostly independently of Ln radius was close to 0.5, which means that the surface is neither hydrophobic nor hydrophilic. The results indicated that water uptake observed at 300°C is a bulk process, which cannot be described by standard hydration/hydrogenation reaction and it is related to the layered structure of the perovskite lattice and characteristic to La or Gd being present in the lattice.
Self-supported palladium tubular membranes are surface-functionalized and tested for the selective and controlled addition of H2 along a fixed-bed catalytic reactor for Fischer–Tropsch hydrocarbon synthesis (FTS). In order to avoid CO poisoning of the active sites of the metallic membrane at low working temperatures (∼250 °C), a Cu-based protective layer is deposited on the outer surface of a tubular membrane by RF sputtering at room temperature. Upon thermal treatment in H2, the Cu layer alloys with Pd on the membrane surface, as confirmed by means of XRD, FESEM and TEM, while the membrane assembly is fully functional, i.e. the negative effect of CO surface adsorption is highly diminished and the membrane provides an appropriate H2 flux (e.g. 12 ml/min·cm²) under the harsh operation conditions practiced in FT synthesis. When the Cu-functionalized membrane is fully integrated in the FTS reactor (250 °C, 20 bar, 30% CO in feed), the membrane delivers a stable H2 permeation flux and enables to increase the yield of hydrocarbons in the range of gasoline (C5–C12) while reducing methane formation over a bifunctional CoRu/Al2O3-zeolite catalyst.
The present work focuses on the study of different Ruddlesden-Popper based cathode materials for Solid Oxide Fuel Cells at Intermediate Temperature (IT-SOFC). The partial substitution of La and Ni by Pr and Co, respectively, were studied in the La2-XPrXNi1-YCoYO4+δ system, with the purpose of enhancing their mixed ionic-electronic conductivity and the electrocatalytic activity for the O2-reduction while the crystal structure was preserved. All synthesized compounds were characterized by electrochemical impedance spectroscopy (EIS), DC conductivity measurements, X-Ray diffraction (XRD), iodometric titration and scanning electron microscopy (SEM). XRD analyses by Rietveld refinement revealed the influence of the ionic radius on the crystalline phase for the different dopants, i.e., variation of the cell parameters and M−O bond lengths. The substitution in both La and Ni sites improves La2NiO4+δ electrochemical properties as IT-SOFC cathode, since higher conductivity and lower polarization resistance were obtained. Finally, La1.5Pr0.5Ni0.8Co0.2O4+δ cathode exhibited the lowest electrode polarization resistance and activation energy values in the temperature range of 450–900 °C. La1.5Pr0.5Ni0.8Co0.2O4+δ was applied on an anode supported cell and a maximum power density of ~400 mW cm⁻² was obtained at 700 °C using pure hydrogen and air.
- Institute of Chemical Technology (ITQ)
About Jose Manuel Serra
- Research Professor. The current activities are focused on the application of catalysis and materials science in: (a) the development of electrochemical solid-oxide cells and reactors, paying special attention to the design and characterization of new selective electrocatalysts; (b) development of mixed ionic-electronic conducting membranes for oxygen and hydrogen separation and catalytic membrane reactor applications; and (c) novel electrochemical/ionic membrane devices based on thin-films and nanotechnology.