Ainara Aguadero

New perspectives in the surface analysis of energy materials by combined time-of-flight secondary ion mass spectrometry (ToF-SIMS) and high sensitivity low-energy ion scattering (HS-LEIS)

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) and low-energy ion scattering (LEIS) are recently attracting great interest in energy materials research due to their capabilities in terms of surface sensitivity and specificity, spatial resolution and their ability to analyse the isotopic chemical composition. This work shows the synergy provided by this powerful combination to further our understanding of the surface chemistry and structure that ultimately determines the electrochemical performance in advanced electro-ceramic materials for energy storage and energy conversion applications. In particular, this novel approach has been applied to the analysis of (Li3xLa2/3−x□1/3−2x)TiO3 perovskite materials used as the electrolyte in lithium batteries and (La, Sr)2CoO4+δ epitaxial thin films used as oxygen electrodes in solid oxide fuel cells and solid oxide electrolysers. The analysis of these two promising materials requires the development and optimisation of new analytical approaches that take advantage of the recent instrumental developments in order to characterise the outermost and near-surfaces at the atomic scale.

Oxygen Reduction Reaction at LaxCa1-xMnO3 Nanostructures: Interplay between A-site Segregation and B-site Valency

The mean activity of surface Mn sites at LaxCa1−xMnO3 nanostructures towards the oxygen reduction reaction (ORR) in alkaline solution is assessed as a function of the oxide composition. Highly active oxide nanoparticles were synthesised by an ionic liquid-based route, yielding phase-pure nanoparticles, across the entire range of compositions, with sizes between 20 and 35 nm. The bulk vs. surface composition and structure are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). These techniques allow quantification of not only changes in the mean oxidation state of Mn as a function of x, but also the extent of A-site surface segregation. Both trends manifest themselves in the electrochemical responses associated with surface Mn sites in 0.1 M KOH solution. The characteristic redox signatures of Mn sites are used to estimate their effective surface number density. This parameter allows comparing, for the first time, the mean electrocatalytic activity of surface Mn sites as a function of the LaxCa1−xMnO3 composition. The ensemble of experimental data provides a consistent picture in which increasing electron density at the Mn sites leads to an increase in the ORR activity. We also demonstrate that normalisation of electrochemical activity by mass or specific surface area may result in inaccurate structure–activity correlations.

Evaluation of Sr2CoMoO6−δ as anode material in solid-oxide fuel cells: A neutron diffraction study

The oxygen-deficient Sr2CoMoO6−δ double perovskite has been proposed as an anode material in solid-oxide fuel cells (SOFC). The evolution of its crystal structure has been followed by “in situ” temperature-dependent neutron powder diffraction from 23 °C (RT) to 867 °C in the heating and cooling runs in ultrahigh vacuum (PO2≈10−6 Torr) in order to simulate the reducing atmosphere corresponding to the working conditions of an anode in a SOFC. At RT the sample is described as tetragonal in the I4/m space group. When this oxide is heated above Tt=262 °C it undergoes a tetragonal I4/m to cubic Fm-3m phase transition. This phase transition takes place at a temperature around 25 °C lower than that previously described for the oxidized sample, and it is affected by a significant hysteresis (Tt=174 °C in the cooling run). The absence of tilting of the CoO6 and MoO6 octahedra in the high-temperature cubic phase favors the orbital overlap and the electronic conductivity; a high mobility of the oxygen atoms is derive...

Neutron structural characterization and transport properties of oxidized and reduced La0.5Sr0.5M0.5Ti0.5O3 (M = Mn, Fe) perovskites: Possible electrode materials in solid-oxide fuel cells

Oxygen-stoichiometric La0.5Sr0.5M0.5Ti0.5O3 (M = Mn, Fe) perovskites and the corresponding reduced specimens, of La0.5Sr0.5M0.5Ti0.5O3-δ composition, have been prepared and characterized by x-ray diffraction and neutron powder diffraction (NPD), in complement with thermal analysis, electrical conductivity, and thermal expansion measurements. NPD data show that these perovskites are all orthorhombic, space group Pbnm (No. 62). The total reduction of M3+ to M2+ in the reduced phases is accompanied with the occurrence of oxygen vacancies, which was confirmed by thermogravimetric analysis (TGA). Above room-temperature, these phases undergo two structural phase transitions studied in situ from NPD data; the former to a tetragonal (I4/mcm) structure, and the second one to a cubic (Pm-3m) phase. All the oxides display a semiconductor-like behavior with a maximum conductivity value of 15 S·cm−1 for the oxidized La0.5Sr0.5Mn0.5Ti0.5O3 phase at 850 °C. The measured thermal expansion coefficients perfectly match wit...

Low activation energies for interstitial oxygen conduction in the layered perovskites La1+xSr1−xInO4+δ

K2NiF4-type La1+xSr1−xInO4+δ (x = 0.1, 0.2) oxides have been prepared and investigated as possible solid electrolytes for solid-oxide fuel cells (SOFC). These materials were synthesized using a citrate–nitrate soft-chemistry technique followed by annealing in air at a temperature of 1000 °C. Preliminary characterization by X-ray diffraction (XRD) indicated that these layered perovskites crystallize in an orthorhombic structure with the space group Pbca. The crystal structural features were explored at RT by neutron powder diffraction (NPD). Their capability to incorporate interstitial oxygen atoms (δ) and their effect on the crystal structure were identified by difference Fourier maps in the NaCl layers of the K2NiF4 structure; subsequent Rietveld refinements yielded excess oxygen values, δ = 0.07(1) and 0.11(2) for x = 0.1 and 0.2, respectively. The electrical properties were studied by impedance spectroscopy (IS) in the temperature range of 500–900 °C and compared with those of the parent compound LaSrInO4. A better conductivity was observed for La1.2Sr0.8InO4.11, which is consistent with the higher quantity of interstitial oxygen found from NPD data. The extremely low activation energy of only 0.51 eV for the conduction mechanism via interstitials at low temperatures (T < 650 °C) is significantly smaller than that of other electrolytes working with a vacancy mechanism, typically of 1 eV, as theoretically predicted. The present result endorses the validity of this design procedure and supports K2NiF4-related compounds as promising candidates for solid-oxide electrolytes.

An investigation of the polytypical structure of Sr0.2Ba0.8CoO3–δ by neutron powder diffraction

Abstract The preparation and characterization of two polymorphs of the title composition are described. One hexagonal perovskite, labeled as “H”, was synthesized by thermal treatment of reactive citrate precursor at 900 °C in high oxygen pressure (20 MPa) followed by slow cooling (10 °C/min) to room temperature. This 1D-structure displays a P63/mmc hexagonal space group with a thermal expansion of 13 × 10–6 K–1 between 100 and 900 °C. On the other hand, for the first time in such a Ba-rich composition, a highly oxygen-defective perovskite oxide cubic polymorph, labeled as “C”, was obtained from the citrate precursor heated in air and quenched in liquid N2 from 1000 °C and completely characterized. Neutron powder diffraction evidence that the obtained “C” phase posses a cubic Pm-3m space group and antiferromagnetic order at room temperature. The material can be formulated as Sr0.2Ba0.8CoO2.27(1) where the high number of oxygen vacancies are random distributed with large thermal factors of 3.6 Å2 suggesting a considerable mobility. The evolution of the crystal structure of the 3C phase has been explored in situ by XRD thermo-diffractometry in complement with thermal analysis and thermal expansion. At approx.500 °C this phase irreversibly transforms to a hexagonal “H” phase and at 925 °C a new cubic perovskite “C” is identified, which is transformed again, upon cooling, into the “H” phase at 840 °C.

Neutron structural characterization and transport properties of the oxidized and reduced LaCo0.5Ti0.5O3 perovskite oxide

Polycrystalline oxygen-stoichiometric LaCo0.5Ti0.5O3 perovskite oxide has been prepared by soft-chemistry procedures followed by annealing in air at 800°C. A new reduced LaCo0.5Ti0.5O3-δ specimen has been obtained by topotactical oxygen removal in an H2/N2 (5%/95%) flow at 600°C. The structural characterization has been conducted from neutron powder diffraction (NPD) data, very sensitive to the contrast between Co and Ti and the oxygen stoichiometry. Both perovskites (oxidized and reduced) crystallize in the orthorhombic Pbnm, space group. The partial reduction of Ti4+ to Ti3+ in the reduced phase is accompanied with the occurrence of oxygen vacancies, located at the axial octahedral sites, and it is expected to support the ionic conductivity, as usually observed in oxygen-defective perovskites. Thermogravimetric analysis (TGA) substantiates the oxygen stoichiometry and the stability range of the reduced sample. All the samples in study display a semiconductor-like behavior with values that not reach below to 0.5 Scm−1 for all the phases. Moreover, the measured thermal expansion coefficients perfectly match with the values usually displayed by SOFC electrolytes.

Oxygen Excess in La2CoO4+δ : A Neutron Diffraction Study

The layered perovskite La2CoO4+δ has been prepared and characterized in order to identify its capability of incorporating interstitial oxygen atoms (δ ) and their effect on the crystal structure. The synthesis has been performed by a citrate-nitrate soft-chemistry technique; a high oxygen pressure treatment (350 °C, 200 bar) has allowed us to increment the interstitial oxygen contents up to δ = 0.32(1). The samples have been characterized by thermal analysis (TG and DTA), X-ray diffraction at room temperature and neutron diffraction at temperatures up to 600 °C. At r. t., the as-prepared phase, La2CoO4.22(1), is orthorhombic, space group Bmab, with an important orthorhombic strain, if compared with La2NiO4+δ . At high temperatures it undergoes two consecutive structural transitions, to an orthorhombic superstructure at 375 °C and finally to a tetragonal symmetry (space group F4/mmm) at 590 °C. The phase La2CoO4.32(1) seems to represent a superstructure with orthorhombic symmetry and with a doubled b unit-cell parameter with respect to the prepared sample.

Elucidating the role of dopants in the critical current density for dendrite formation in garnet electrolytes

Garnet-type solid electrolytes have attracted great interest in solid state battery research thanks to their high ionic conductivity at room temperature (10−3 S cm−1) and their electrochemical stability against lithium metal anodes. However, the formation of lithium dendrites following charge/discharge limits their applicability and commercialisation. Although widely investigated, no clear explanation of dendrite formation has been previously reported. In this work, we employ cubic Al- and Ga-doped Li7La3Zr2O12, which represent two of the solid electrolytes with higher technological importance, to investigate the formation and chemical composition of dendrites. For the first time, this study elucidates the role that the dopants play in determining the critical current density for dendrite formation and highlights the importance of controlling the dopant distribution in the garnet structure. We use a combination of techniques including Secondary Electron Microscopy and Secondary Ion Mass Spectrometry in order to analyse the microstructure and chemical composition of dendrites in Li7La3Zr2O12. We show that, following electrochemical cycling, Li6.55Ga0.15La3Zr2O12 systematically displays a critical current density 60% higher than Li6.55Al0.15La3Zr2O12. Chemical analysis revealed that in Li6.55Al0.15La3Zr2O12 the dendritic features are composed of a mixture of Al and Li species, whereas in Li6.55Ga0.15La3Zr2O12 they are uniquely composed of Li. We also show that only in pristine Li6.55Al0.15La3Zr2O12, the dopant segregates at the grain boundaries suggesting that local chemical inhomogeneity can have a fundamental role in the nucleation and propagation of dendrites.

Enhancing Distorted Metal–Organic Framework-Derived ZnO as Anode Material for Lithium Storage by the Addition of Ag2S Quantum Dots

The lithium storage properties of the distorted metal-organic framework-derived nanosized ZnO@C are significantly improved by the introduction of Ag2S quantum dots (QDs) during the processing of the material. In the thermal treatment, the Ag2S QDs react to produce Ag nanoparticles and ZnS. The metal nanoparticles act to shorten electron pathways and improve the connectivity of the matrix, and the partial sulfidation of the ZnO surface improves the cycling stability of the material. The electrochemical properties of ZnO@C, Ag2S QDs-treated ZnO@C, and the amorphous carbon in ZnO@C have been compared. The small weight ratio of Ag2S QDs to ZnO@C at 1:180 shows the best performance in lithium storage. The exhibited specific capacities are improved and retained remarkably in the cycling at high current rates. At low current densities (200 mA g-1), treatment of ZnO@C with Ag2S QDs results in a 38% increase in the specific capacity.