Enrique R. Losilla

Interstitial oxygen conduction in lanthanum oxy-apatite electrolytes

The La10 − x(SiO4)6O3 − 1.5x (9.33 ≤ 10 − x ≤ 9.73) apatite series has been prepared and hexagonal single phases were obtained in a narrow compositional range (9.33 ≤ 10 − x ≤ 9.60). The room temperature crystal structure of La9.55(SiO4)6O2.32 has been determined from joint Rietveld refinement of neutron and laboratory X-ray powder diffraction data: a = 9.7257(1) A, c = 7.1864(1) A, V = 588.68(1) A3, Z = 1, RwpN = 3.2%, RwpX = 7.7%, RFN = 1.8%, RFX = 1.9%. An interstitial site for the extra-oxygen has been determined in the position very recently predicted in a theoretical study using atomistic simulations. The high temperature crystal structures have been obtained from neutron powder diffraction, NPD, collected at 773 and 1173 K showing the thermal evolution of this interstitial site. Previously reported neutron data for La9.60(GeO4)6O2.40 have also been re-analysed establishing the existence, and thermal evolution, of this interstitial site. The electrical results suggest that the samples are oxide ion conductors. The plots of the imaginary parts of the impedance, Z″, and the electric modulus, M″, vs. log (frequency), possess maxima for both curves separated by two decades in frequency. Bulk conductivities have been obtained from the fitting of the complex impedance spectra with the appropriate equivalent circuit. Bulk activation energies have been determined from two Arrhenius plots, one representing the bulk conductivities and the other representing the frequencies of the modulus peak maxima, fmax(M″). A comparative discussion of the two series, La10 − x(TO4)6O3 − 1.5x (T = Si, Ge), is given.

Interstitial oxygen in oxygen-stoichiometric apatites

Several oxy-apatite materials La10−xSrx(TO4)6O3−0.5x (T = Ge, Si; 10−x = 9.00, 8.80, 8.65 and 8.00) and La9.33(Si1−xGexO4)6O2 (x = 0, 0.5, 0.67) have been prepared as highly crystalline phases. The impedance study showed that all samples are oxide ion conductors. However, bulk conductivities changed by more than 2 orders of magnitude at a given temperature for some compositions. A thorough study on the oxygen sublattices for oxygen-stoichiometric oxy-apatites has been carried out. Constant-wavelength neutron powder diffraction data have been collected for La9.33(SiO4)6O2. Time-of-flight neutron data have been collected for La9.33(Si0.5Ge0.5O4)6O2, La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. The room-temperature structures have been derived from joint Rietveld refinements of neutron and laboratory X-ray powder diffraction data. High temperature structures have been obtained only from Rietveld refinements of neutron powder diffraction data. The refinements show that La9.33(SiO4)6O2 and La9.33(Si0.5Ge0.5O4)6O2 contain interstitial oxygen, associated to vacancies at the oxygen channels. The amount of interstitial oxygen is negligible in La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2. Hence, the novelty of this work is to explain the high oxide conductivity of the lanthanum-deficient samples which it is due to the presence of interstitial oxygens. Lanthanum stoichiometric samples do not have interstitial oxygens and, so, their conductivities are much lower.

NASICON to scandium wolframate transition in Li1+xMxHf2−x(PO4)3 (M=Cr, Fe): structure and ionic conductivity

a ´´ ´ ´ ´ ´ Abstract The Li M Hf (PO ) (M5 Cr, Fe, Bi) systems have been studied and single phases have been isolated for M 5 Cr 11 xx 22x 43 and Fe. The samples have been characterized by X-ray powder diffraction, diffuse reflectance and impedance spectroscopy. There is a reconstructive transition between rombohedral NASICON and orthorhombic Sc (WO ) -type structures as a 24 3 function of x, at very low values, 0.2 and 0.1 for Cr and Fe, respectively. For the Cr series, a further subtle structural change has been observed for x values higher than 1.7. These phases have the Sc (WO ) -type framework, but the symmetry is 24 3 orthorhombic Pcnb at low values of x and monoclinic P2 / n at high values. The structural changes are discussed on the basis 1 of the sizes of the cavities left by the two frameworks and the lithium order / disorder in these voids. These materials are ionic conductors and their electrical behaviours are also discussed.

Low temperature crystal structures of apatite oxygen-conductors containing interstitial oxygen

Oxygen-stoichiometric La(9.33) square(0.67)(Si(6)O(24))O2 and oxygen-excess La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) and La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) oxy-apatites have been structurally characterized at low temperatures by the Rietveld method. Oxygen-interstitial distribution has been studied at 15 K for La(9.33) square(0.67)(Si(6)O(24))O2 and La(8.65)Sr(1.35)(Ge(6)O(24))O(2.32) by time-of-flight neutron powder diffraction and at 4 K for La(8.65)Sr(1.35)(Si(6)O(24))O(2.32) by constant-wavelength neutron powder diffraction. The low temperature structural study was undertaken in order to distinguish between the effects of static disorder, originated mainly from the presence of interstitial oxygens, and the anisotropic thermal vibrations. At such low temperatures, the influence of the anisotropic thermal vibrations is minimised. This structural study has firmly established the existence of interstitial oxygens in these materials, which may be useful as electrolytes for solid oxide fuel cells.

Novel microstructural strategies to enhance the electrochemical performance of La0.8Sr0.2MnO3-δ cathodes

Novel strategies based on spray-pyrolysis deposition are proposed to increase the triple-phase boundary (TPB) of La0.8Sr0.2MnO3-δ (LSM) cathodes in contact with yttria-stabilized zirconia (YSZ) electrolyte: (i) nanocrystalline LSM films deposited on as-prepared YSZ surface; (ii) the addition of poly(methyl methacrylate) microspheres as pore formers to further increase the porosity of the film cathodes; and (iii) the deposition of LSM by spray pyrolysis on backbones of Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO), and Bi1.5Y0.5O3-δ (BYO) previously fixed onto the YSZ. This last method is an alternative to the classical infiltration process with several advantages for large-scale manufacturing of planar solid oxide fuel cells (SOFCs), including easier industrial implementation, shorter preparation time, and low cost. The morphology and electrochemical performance of the electrodes are investigated by scanning electron microscopy and impedance spectroscopy. Very low values of area specific resistance are obtained, ranging from 1.4 Ω·cm(2) for LSM films deposited on as-prepared YSZ surface to 0.06 Ω-cm(2) for LSM deposited onto BYO backbone at a measured temperature of 650 °C. These electrodes exhibit high performance even after annealing at 950 °C, making them potentially suitable for applications in SOFCs at intermediate temperatures.

Layered acid arsenates α-M(HAsO4)2·H2O (M=Ti, Sn, Pb): synthesis optimization and crystal structures

Abstract The syntheses of α-M(HAsO 4 ) 2 ·H 2 O (M=Ti, Sn and Pb) have been optimized to prepare crystalline single phase materials. The crystal structures of M=Sn, Pb have been refined using X-ray powder diffraction data by the Rietveld method. A combined X-ray and neutron powder data refinement for M=Ti has allowed to obtain a very detailed picture of the structure including the hydrogen-bonding network. The results have been compared with those of the phosphates analogs. In addition, detailed characterization of the samples has been carried out by IR spectroscopy, thermal analysis and powder thermodiffractometry.

Glasses and crystalline A3Al2(PO4)3(A = Na, Li): an impedance and 31P, 27Al, 23Na and 7Li MAS-NMR study

A new vitreous phase with Na3Al2(PO4)3 composition has been prepared. Its crystallisation has been optimised by annealing at low temperature, giving crystalline c-Na3Al2(PO4)3 with Sc2(WO4)3-type structure. The direct synthesis of vitreous Li3Al2(PO4)3 has been attempted but it was unsuccessful. Ion exchange reaction has been carried out in Na3Al2(PO4)3 glass to obtain the lithiated vitreous solid, v-Li3Al2(PO4)3. On heating, v-Li3Al2(PO4)3 decomposes to yield crystalline AlPO4 and Li3PO4 phases. The three solids, with NASICON-like stoichiometries, have been thermal and electrically characterised. DTA and thermodiffractometric studies of the two glass solids have allowed the characterisation of their thermal behaviours including crystallisation. The grain boundary contributions to the total impedance are almost absent in the two vitreous phases. The electrical conductivity values are ∼ 10−6 S cm−1 at 150 °C. The 31P, 23Na, 27Al and 7Li MAS-NMR spectra for these materials provide some insight into the disordered constitution of the vitreous phases and the low mobility of the alkaline cations.

Investigation into the effect of Si doping on the performance of Sr1−yCayMnO3−δ SOFC cathode materials

In this paper we report the successful incorporation of silicon into Sr1-yCayMnO3-δ perovskite materials for potential applications in cathodes for solid oxide fuel cells. The Si substitution onto the B site of a (29)Si enriched Sr1-yCayMn1-xSixO3-δ perovskite system is confirmed by (29)Si MAS NMR measurements at low B0 field. The very large paramagnetic shift (~3000-3500 ppm) and anisotropy (span ~4000 ppm) suggests that the Si(4+) species experiences both Fermi contact and electron-nuclear dipolar contributions to the paramagnetic interaction with the Mn(3+/4+) centres. An improvement in the conductivity is observed for low level Si doping, which can be attributed to two factors. The first of these is attributed to the tetrahedral coordination preference of Si leading to the introduction of oxide ion vacancies, and hence a partial reduction of Mn(4+) to give mixed valence Mn. Secondly, for samples with high Sr levels, the undoped systems adopt a hexagonal perovskite structure containing face sharing of MnO6 octahedra, while Si doping is shown to help to stabilise the more highly conducting cubic perovskite containing corner linked octahedra. The level of Si, x, required to stabilise the cubic Sr1-yCayMn1-xSixO3-δ perovskite in these cases is shown to decrease with increasing Ca content; thus cubic symmetry is achieved at x = 0.05 for the Sr0.5Ca0.5Mn1-xSixO3-δ series; x = 0.075 for Sr0.7Ca0.3Mn1-xSixO3-δ; x = 0.10 for Sr0.8Ca0.2Mn1-xSixO3-δ; and x = 0.15 for SrMn1-xSixO3-δ. Composites with 50% Ce0.9Gd0.1O1.95 were examined on dense Ce0.9Gd0.1O1.95 pellets. For all series an improvement in the area specific resistances (ASR) values is observed for the Si-doped samples. Thus these preliminary results show that silicon can be incorporated into perovskite cathode materials and can have a beneficial effect on the performance.

Luminescent and Proton Conducting Lanthanide Coordination Networks Based On a Zwitterionic Tripodal Triphosphonate

The synthesis, structural characterization, luminescence properties, and proton conduction performance of a new family of isostructural cationic 2D layered compounds are reported. These have the general formula [Ln(H4NMP)(H2O)2]Cl·2H2O [Ln = La(3+), Pr(3+), Sm(3+), Eu(3+), Gd(3+), Tb(3+), Dy(3+), Ho(3+), H6NMP = nitrilotris(methylphosphonic acid)], and contain Cl(-) as the counterion. In the case of Ce(3+), a 1D derivative, [Ce2(H3NMP)2(H2O)4]·4.5H2O, isostructural with the known lanthanum compound has been isolated by simply crystallization at room temperature. The octa-coordinated environment of Ln(3+) in 2D compounds is composed by six oxygen atoms from three different ligands and two oxygens from each bound water. Two of the three phosphonate groups act as both chelating and bridging linkers, while the third phosphonate group acts solely as a bridging moiety. The materials are stable at low relative humidity at less at 170 °C. However, at high relative humidity transform to other chloride-free phases, including the 1D structure. The proton conductivity of the 1D materials varies in a wide range, the highest values corresponding to the La derivative (σ ≈ 2 × 10(-3) S·cm(-1) at RH 95% and 80 °C). A lower proton conductivity, 3 × 10(-4) S·cm(-1), was measured for [Gd(H4NMP)(H2O)2]Cl·2H2O at 80 °C, which remains stable under the work conditions used. Absorption and luminescence spectra were recorded for selected [Ln(H4NMP)(H2O)2]Cl·2H2O compounds. In all of them, the observed transitions are attributed solely to f-f transitions of the lanthanide ions present, as the H4NMP(2-) organic group has no measurable absorption or luminescence properties.

Ti-doped SrFeO3 nanostructured electrodes for symmetric solid oxide fuel cells

Nanostructured electrodes of Sr0.98Fe1−xTixO3−δ are evaluated as both cathode and anode for solid oxide fuel cells. The electrodes are prepared by a low-cost and simple procedure based on spray-pyrolysis deposition on a porous Ce0.8Gd0.2O1.9 (CGO) layer. A homogenous coating layer of electrode catalyst nanoparticles is formed on the CGO backbone surface in a single deposition-firing step. Sr0.98Fe0.8Ti0.2O3−δ (SFT0.2) exhibits high efficiency operating as both cathode and anode with polarization resistance values of 0.1 Ω cm2 in air and 0.07 Ω cm2 in humidified H2 at 700 °C. An electrolyte supported cell with 300 μm thick La0.9Sr0.1Ga0.8Mg0.2O3−δ electrolyte and SFT0.2 symmetric electrodes shows maximum power densities of 700 and 140 mW cm−2 at 800 and 600 °C, respectively.