Person: García Alvarado, Flaviano
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First Name
Flaviano
Last Name
García Alvarado
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Químicas
Department
Area
Química Inorgánica
Identifiers
6 results
Search Results
Now showing 1 - 6 of 6
Item Novel perovskite materials for thermal water splitting at moderate temperature(Chemsuschem, 2019) Azcondo, M. Teresa; Orfila, María; Marugán, Javier; Sanz Martín, Raúl; Muñoz Noval, Álvaro; Salas Colera, Eduardo; Ritter, Clemens; García Alvarado, Flaviano; Amador, UlisesMaterials with the formula Sr_2CoNb_1-xTi_xO_(6-delta) (x=1.00, 0.70; delta=number of oxygen vacancies) present a cubic perovskite-like structure. They are easily and reversibly reduced in N_2 or Ar and re-oxidized in air upon heating. Oxidation by water (wet N_2), involving splitting of water at a temperature as low as 700 ºC, produces hydrogen. Both compounds displayed outstanding H_2 production in the first thermochemical cycle, the Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) material retaining its outstanding performance upon cycling, whereas the hydrogen yield of the x=1 oxide showed a continuous decay. The retention of the materials' ability to promote water splitting correlated with their structural, chemical, and redox reversibility upon cycling. On reduction/oxidation, Co ions reversibly changed their oxidation state to compensate the release/recovery of oxygen in both compounds. However, in Sr_2CoTiO_(6-delta), two phases with different oxygen contents segregated, whereas in Sr_2CoNb_(0.30)Ti_(0.70)O_(6-delta) this effect was not evident. Therefore, this latter material displayed a hydrogen production as high as 410 mu molH_2/g_(perovskite) after eight thermochemical cycles at 700 ºC, which is among the highest ever reported, making this perovskite a promising candidate for thermosolar water splitting in real devices.Item Reduction of Grain Boundary Resistance of La0.5Li0.5TiO3 by the Addition of Organic Polymers(Nanomaterials, 2021) Boyano, Iker; Mainar, Aroa R.; Blázquez, J. Alberto; Kvasha, Andriy; Bengoechea, Miguel; Meatza, Iratxe de; García Martín, Susana; Varez Álvarez, Alejandro; Sanz, Jesús; García Alvarado, FlavianoThe organic solvents that are widely used as electrolytes in lithium ion batteries present safety challenges due to their volatile and flammable nature. The replacement of liquid organic electrolytes by non-volatile and intrinsically safe ceramic solid electrolytes is an effective approach to address the safety issue. However, the high total resistance (bulk and grain boundary) of such compounds, especially at low temperatures, makes those solid electrolyte systems unpractical for many applications where high power and low temperature performance are required. The addition of small quantities of a polymer is an efficient and low cost approach to reduce the grain boundary resistance of inorganic solid electrolytes. Therefore, in this work, we study the ionic conductivity of different composites based on non-sintered lithium lanthanum titanium oxide (La0.5Li0.5TiO3) as inorganic ceramic material and organic polymers with different characteristics, added in low percentage (<15 wt.%). The proposed cheap composite solid electrolytes double the ionic conductivity of the less cost-effective sintered La0.5Li0.5TiO3.Item New perovskite materials of the La2-xSrxCoTiO6 series(Dalton Transactions, 2011) Yuste, Mercedes; Pérez Flores, Juan Carlos; Romero de Paz, Julio; Azcondo Sánchez, María Teresa; García Alvarado, Flaviano; Amador Elizondo, UlisesSubstitution of La3+ by Sr2+ in the double perovskite La2CoTiO6 yields materials of the La2-xSrxCoTiO6 series showing a significant amount of trivalent cobalt ions when prepared at ambient atmosphere. The as-prepared compounds can be reduced in severe conditions retaining the perovskite structure while inducing the formation of a large amount of oxygen vacancies. The limit of aliovalent substitution in this series was found to extend up to x = 1. For substitution of La3+ up to 15% cobalt and titanium are ordered, though the order is progressively lost as x increases; for x ≥ 0.30 no ordering is observed as evidenced by magnetic measurements. The ability of these materials to present either cobalt ions in a mixed oxidation state or large amounts of anion vacancies depending on the atmosphere makes them interesting to be further investigated regarding their electrical and electrochemical properties, and hence, their usefulness in some electrochemical devices.Item Defect chemistry, electrical properties and evaluation of new oxides Sr2CoNb1-xTixO6-δ (0≤x≤1) as cathode materials for Solid Oxide Fuel Cells(ChemSusChem, 2017) Azcondo, Teresa; Yuste, Mercedes; Pérez-Flores, Juan Carlos; Muñoz Gil, Daniel; García Martín, Susana; Muñoz Noval, Álvaro; Puente Orench, Inés; García Alvarado, Flaviano; Amador, UlisesThe perovskite series Sr2CoNb1-xTixO6-δ (0≤x≤1) is investigated in the full compositional range to assess its potential as cathode material for solid state fuel cell (SOFC). The variation of transport properties and thus, the area specific resistances (ASR) are explained by a detailed investigation of the defect chemistry. Increasing titanium content from x=0 to x=1 produces both oxidation of Co3+ to Co4+ (from 0% up to 40%) and oxygen vacancies (from 6.0 to 5.7 oxygen atom/formula unit) though each charge compensation mechanism predominates in different compositional ranges. Neutron diffraction reveals that samples with high Ti-contents lose a significant amount of oxygen on heating above 600K. Oxygen is partially recovered on cooling since the oxygen release and uptake show noticeably different kinetics. The complex defect chemistry of these compounds, together with the compositional changes upon heating-cooling cycles and atmospheres produce, a complicated behaviour of electrical conductivity. Cathodes containing Sr2CoTiO6-δ display low ASR values, 0,13 Ωcm2 at 973 K, comparable to those of the best compounds reported so far, being a very promising cathode material for SOFC.Item Lithium Intercalation Mechanism and Critical Role of Structural Water in Layered H2V3O8 High-Capacity Cathode Material for Lithium-Ion Batteries(Chemistry of Materials, 2022) Kuhn, Alois; Perez-Flores, Juan Carlos; Prado Gonjal, Jesús De La Paz; Morán Miguélez, Emilio; Hoelzel, Markus; Díez-Gómez, Virginia; Sobrados, Isabel; Sanz, Jesús; García Alvarado, FlavianoH2V3O8 (HVO) is a promising high-capacity cathode material for lithium-ion batteries (LIBs). It allows reversible two-electron transfer during electrochemical lithium cycling processes, yielding a very attractive theoretical capacity of 378 mAh g–1. While an abundant number of research works exclusively proved the outstanding electrochemical lithium storage properties of H2V3O8, structural changes during the intercalation process have not been scrutinized, and the crystallographic positions occupied by the guest species have not been revealed yet. However, an in-depth understanding of structural changes of cathode materials is essential for developing new materials and improving current materials. Aimed at providing insights into the storage behavior of HVO, in this work, we employed a combination of high-resolution synchrotron X-ray and neutron diffraction to accurately describe the crystal structures of both pristine and lithiated H2V3O8. In HVO, hydrogen is located on one single-crystallographic site in a waterlike arrangement, through which bent asymmetric hydrogen bonds across adjacent V3O82– chains are established. The role played by water in network stabilization was further examined by density functional theory (DFT) calculations. Easy hydrogen-bonding switch of structural water upon lithium intercalation not only allows better accommodation of intercalated lithium ions but also enhances Li-ion mobility in the crystal host, as evidenced by magic-angle spinning (MAS) NMR spectroscopy. Facile conduction pathways for Li ions in the structure are deduced from bond valence sum difference mapping. The hydrogen bonds mitigate the volume expansion/contraction of vanadium layers during Li intercalation/deintercalation, resulting in improved long-term structural stability, explaining the excellent performance in rate capability and cycle life reported for this high-energy cathode in LIBs. This study suggests that many hydrated materials can be good candidates for electrode materials in not only implemented Li technology but also emerging rechargeable batteries.Item The intercalation chemistry of H2V3O8 nanobelts synthesised by a green, fast and cost-effective procedure(Journal of Power Sources, 2013) Prado Gonjal, Jesús De La Paz; Molero-Sánchez, Beatriz; Ávila Brande, David; Morán Miguélez, Emilio; Pérez-Flores, Juan Carlos; Kuhn, Alois; García Alvarado, FlavianoH2V3O8 nanobelts have been successfully synthesised from commercial V2O5 powder through a fast and environmental friendly microwave-hydrothermal method. X-ray diffraction, field-emission scanning electron microscopy, thermogravimetric analysis, infrared spectroscopy, high-resolution transmission electron microscopy and ICP spectroscopy were used to characterise the morphology and structure–microstructure details. Nanobelts about 100 nm wide and several micrometres long are easily prepared in no more than 2 h. The electrochemical study reveals the reversible insertion of ca. 4 Li per formula unit (400 mAh g−1), through several pseudo-plateaus in the 3.75–1.5 V vs Li+/Li voltage range showing the interest of this material produced by a “green” route as an electrode for lithium rechargeable batteries. After the first cycle a significant capacity loss is observed, though a high capacity, ca. 300 mAh g−1, remains upon cycling. Furthermore, the similarity of discharge and charge curves, pointing to the absence of hydrogen displacement during lithium insertion in H2V3O8, shows that not all protonated systems must be discarded as prospective electrode materials. On the other hand, further reduction down to 1 V is possible to insert up to 5 Li per formula unit (480 mAh g−1). Interestingly it corresponds to full reduction of vanadium to V3+ as it is also confirmed by EELS experiments. However, the full reduction to V3+ is associated with a fast decay of the extra capacity developed at low voltage with increasing current rate. Then for practical use we may consider only the capacity obtained down to 1.5 V.