Examinando por Autor "Amador, Ulises"

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A- and B‑Site Ordering in the A‑Cation-Deficient Perovskite Series La2−xNiTiO6−δ (0 ≤ x < 0.20) and Evaluation as Potential Cathodes for Solid Oxide Fuel Cells
(2013-06-25) Pérez-Flores, Juan Carlos; Pérez-Coll, Domingo; García-Martín, Susana; Ritter, Clemens; C. Mather, Glenn; Canales-Vázquez, Jesús; Gálvez-Sánchez, María; García-Alvarado, Flaviano; Amador, Ulises
The La2–xNiTiO6−δ (0 ≤ x < 0.2) series has been investigated in order to assess its possible use as a solid oxide fuel cell (SOFC) cathode material. These perovskite-like oxides exhibit monoclinic symmetry, as determined by a series of high-resolution structural techniques (X-ray diffraction (XRD), neuron powder diffraction (NPD), selected-area electron diffraction (SAED), and transmission electron microscopy (TEM)). Ni and Ti order over the B-site and, unusually, for x > 0, the A-site ions are also ordered along the c-axis in alternate La-rich and □-rich layers (where □ represents a vacancy). Structural determination combined with accurate compositional and magnetic characterization indicates a change in the predominant charge-compensating mechanism of A-site vacancies with composition. For x = 0.1, oxygen-vacancy formation seems to be the main-charge compensating mechanism, whereas, for x = 0.2, partial replacement of Ni by Ti in the B-substructure is dominant. In addition, a small amount of trivalent nickel is present in all samples. The composition dependence of the electrical conductivity of La2–xNiTiO6−δ (x = 0, 0.1, 0.2), investigated by impedance spectroscopy, as a function of temperature and oxygen partial pressure, is successfully interpreted on the basis of the relevant charge-compensating mechanisms and associated valence states. Thermal and chemical stability have also been studied in order to perform a preliminary electrochemical characterization as prospective cathode materials for SOFCs. The material La1.80NiTiO6-δ exhibits excellent stability under oxidizing conditions and a polarization resistance of ∼0.5 Ω cm2 at 1073 K with a yttria-stabilized zirconia (YSZ) electrolyte, slightly lower than that of the state-of-the-art La1–xSrxMnO3 (LSM)-based cathodes. A higher thermal stability and a better chemical compatibility of La1.80NiTiO6−δ with common electrolytes (e.g., YSZ), in comparison with LSM, suggests that this oxide warrants further study and optimization as a prospective improved cathode material for SOFCs.
The role of the Co2+/Co3+ redox-pair in the properties of La2-xSrxCoTiO6 (0 < x <0.5) perovskites as components for solid oxide fuel cells
(Elsevier, 2013-04-01) Gómez-Pérez, Alejandro; Yuste, Mercedes; Pérez-Flores, Juan Carlos; Ritter, Clemens; Azcondo, M. Teresa; Canales-Vázquez, Jesús; Gálvez-Sánchez, María; Boulahya, Khalid; García-Alvarado, Flaviano; Amador, Ulises
Substitution of La3+ by Sr2+ in the perovskite La2CoTiO6 yields materials of the La2−xSrxCoTiO6 series. The dominant charge-compensating mechanism is oxidation of Co2+ if they are prepared at air. The as prepared oxides can be reduced inducing a large amount of oxygen vacancies while keeping the perovskite structure. The electrical behaviour of the La2−xSrxCoTiO6 series is dominated by p-type electronic conduction in a wide pO2 range through non-adiabatic hopping of small-polarons. The electrical conductivity increases with x, except for the x = 0.5 material which shows an unexpectedly low conductivity due to microstructural and short-range ordering effects. The highest conductivity material, La1.60Sr0.40CoTiO6, is selected to study the electrochemical properties of the series. This compound is chemically compatible with YSZ up to 1373 K, in both oxidizing and reducing atmospheres. The preliminary evaluation of the electrode performance reveals that La1.60Sr0.40CoTiO6-based electrodes exhibit polarization resistances of typically 0.8 Ω cm2 at 1073 K in oxygen, which are close to the values obtained for LSM-based cathodes. Thus, the electrochemical behaviour of this oxide as cathode is particularly encouraging since the electrode microstructure is not optimized; it is expected that an improved microstructure will perform at least similarly to the state-of-art in SOFCs materials.
Thermochemical Energy Storage Using the Phase Transitions Brownmillerite-2H Perovskite - Cubic Perovskite in the CaxSr1-xCoO3-δ (x=0 and 0.5) System
(American Chemical Association, 2021-08-09) Azcondo, Maria Teresa; Orfila, Maria; Linares, Maria; Molina, Raul; Marugan, Javier; Amador, Ulises; Boulahya, Khalid; Botas, Juan Angel; Sanz, Raul
The oxides Ca0.5Sr0.5CoO3−δ and SrCoO3−δ, which present perovskite or perovskite-related phases in different temperature domains, have been tested as materials for thermochemical energy storage. The first one, Ca0.5Sr0.5CoO3−δ, experiences a reversible phase transition upon consecutive cycles under an airflow at a maximum operating temperature of 1196 K. Unfortunately, the heat stored in this process, associated with an oxygen loss/gain and a structural phase transition, is very small, hindering its use for thermochemical heat storage. The as-prepared oxide SrCoO3−δ, which displays a brownmillerite structure like the Ca-containing compound, in the first heating step irreversibly segregates some Co3O4 at 823 K to yield a 2H hexagonal perovskite. This phase reversibly transforms at 1073 K into a cubic perovskite. These 2H ⇄ C transitions occur from the 2nd to, at least, 30th cycle. The average absorbed and released heat is ∼104.1 ± 0.06 and ∼68.8 ± 1.8 J/g, respectively, and therefore, SrCoO3−δ presents a high exo/endo ratio. The exergy efficiency is, on average for the 30 cycles performed, as high as 63.9 ± 1.2%. The mechanism of the phase 2H ⇄ C transition of SrCoO3−δ explains the good performance of this material for thermochemical energy storage.