Structural analysis and sintering aids effects in La2Ce2O7 proton conductors

Global warming is an important problem that has to be solved without delay. The development of environmental-friendly energy technology is needed to deal with this issue. Solid Oxide Fuel Cells (SOFC) technology has been proposed as a real alternative to fossil fuel combustion. Proton conductors like La2Ce2O7 (LDC), has several advantages in comparison with BaCeO3 due to its high stability in H2O or CO2 conditions [1]. Furthermore, for industry application is necessary to low the high sintering temperature of typical electrolyte materials. La2Ce2O7 was synthesized by the freeze-drying precursor method and calcination conditions have been optimized to obtain single phase with high compaction at 1400 ºC for 1h. A fully characterization has been carried out using X-ray powder diffraction and scanning electron microscopy. The total conductivity was determined by complex impedance spectroscopy in dry and wet air. Transmission Electron Microscopy (TEM) was used to clarify certainly the structure of La2Ce2O7 due to its still unknown. SAEDs patterns revealed a disordered fluorite, not appearing secondary reflections typical of pyrochlore superstructure, finishing the controversy around the correct structure in this material [2,3]. Moreover, an exhaustive study about lowering the sintering temperature with Co and Zn as sintering aids has been investigated obtaining electrolytes that can be used for SOFC. The sintering aids were impregnated using cobalt and zinc nitrates in ethanol media. Both sintering aids allow for obtain high dense pellets lowering the sintering temperature 300 ºC and 400 ºC for samples with cobalt and zinc, respectively, without compromising the electrical and microstructural properties (Fig 1).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Influence of lanthanum doping on the structure and transport properties of CeO2

LaxCe1-xO2-x/2 materials are oxide and/or proton conductors depending on the La-content and they are of interest for numerous electrochemical applications at high temperatures, including membranes for hydrogen separation and fuel cell electrolytes. Samples with low La-content exhibit (x0.4) crystallize with cubic fluorite type structure; while for x>0.4 the structure is still unclear. The crystal structure of these materials is still unknown, some authors reported that the materials exhibit fluorite type structure in the whole compositional range. However, another authors reported a pyrochlore type structure for x0.5. The stabilization of the fluorite or pyrochlore type structure depends mainly on the oxygen sublattice and the vacancy ordering1. In this contribution, LaxCe1-xO2-δ (0<x0.7) materials are prepared by the freeze-drying precursor method and the sintering conditions have been optimized to obtain dense ceramic samples. A complete structural characterization has been carried out by X-ray powder diffraction and scanning electron microscopy. The average structure determined by conventional XRD indicates that the materials are single fluorite compounds for x0.6. However, the local structure determined by combined electron diffraction and HRTEM is more complex. The SAED patterns reveal diffuse scatterings for x0.5 that have been associated with O-vacancy ordering, leading to a superstructure relative to a single fluorite . This finding is further confirmed by the HRTEM images in the same zone axis. Thermogravimetric and Raman analysis confirmed an increase of oxygen vacancy concentration with La-doping. The overall conductivity was determined by complex impedance spectroscopy in different atmospheres. The samples with high La-content exhibit an important proton contribution at low temperature. In addition, all samples are mixed ion-electronic conductors in hydrogen containing atmosphereUniversidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Highly efficient LaCr0.75M0.25O3+δ (M= Ti, Mn, Fe, Co and Cu) nanostructured electrodes for Solid Oxide Fuel Cells

Solid Oxide Fuel Cells (SOFCs) are considered as one of the most efficient electrochemical devices for power generation. One of the most effective strategies to enhance the electrode performance is the infiltration into a porous electrolyte to increase the triple-phase-boundary (TPB) leading to highly efficient devices. In this way, Spray-pyrolysis deposition is an easy and scalable method that has been used previously to obtain nanocrystalline electrodes, with a considerable improvement in comparation with conventional electrodes.1 In this work, a new doping strategy in lanthanum chromite-based materials has been carry out to obtain potential anodes/cathodes for SOFCs. LaCrO3 and LaCr0.75M0.25O3 (M= Ti, Mn, Fe, Co and Cu) materials were obtained using Spray-Pyrolysis and Freeze-dry precursor method (FD) simultaneously for further comparation. YSZ pellets were made from commercial powders (Tosoh) pressed into disk and sintered at 1400 ºC for 4h. Afterwards, a Ce0.9Gd0.1O1.95 a porous thin layer was printed and sintered at 1200 ºC for 1h. The electrolyte was sprayed with a precursor solution contain the corresponding nitrates in Milli-Q water with a concentration of 0.025 M. The optimum temperature deposition, time and flow rate was 325 ºC, 1h and 20 mL/min respectively. After the deposition, the samples were calcined at 800 ºC for 1h to achieve crystallization. Simultaneously, the materials were synthesized by Freeze-dry precursor method using the corresponding nitrates in distilled water and adding 1:1 molar ratio of etilendiaminetetraacetic acid (EDTA) as a complexing agent. The resulting solution were frozen in liquid nitrogen, followed by dehydration by vacuum sublimation for 2 days. The dry powers were calcined at 300 ºC for 1h and 800 ºC 1h to eliminate carbonaceous species and phase formation. XRD patterns shows that single phase is achieved for LaCr0.75(Ti, Mn, Fe)0.25O3 and LaCr0.75Cu0.25O3 at 800 and 900 ºC, respectively. The electrode polarization resistance (Rp) was determined by impedance spectroscopy obtaining ASR values as low as 0.1 and 0.35 Ω•cm-1 at 750 ºC in air and 5%-H2/Ar for LaCr0.75Mn0.25O3(LCM), respectively being these values almost one magnitude order better in comparation with related materials.2Nanostructured composites electrodes based on lanthanum chromites has been prepared using Spray-pyrolysis precursor method achieving better Rp as SOFC electrodes. The performance in real SOFC is still in progress.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Vertically Aligned Nanostructures for SOFC applications

One of the most recent strategies to enhance the electrode performance is preparing composite materials leading to an increase of the triple-phase-boundary (TPB) length. On this way, heteroepitaxial nanocomposite films such as vertically aligned nanostructures (VANs) deposited by Pulsed Laser Deposition (PLD) are one of the most promising and recent approaches to obtain nanocomposite active layers for energy applications1. In this work, thin films VANs based on (La0.8Sr0.2)0.98Fe0.8Ti0.2O3−δ-Ce0.9Gd0.1O1.95 (LSFT-GDC) electrodes were obtained for their use as functional layers in Solid Oxide Fuel Cells (SOFCs).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Relationship between the Structure and Transport Properties in the Ce1−xLaxO2−x/2 System

La-doped CeO2 materials have been widely investigated for potential applications in different high-temperature electrochemical devices, such as fuel cells and ceramic membranes for hydrogen production. However, the crystal structure is still controversial, and different models based on fluorite, pyrochlore, and/or type-C structures have been considered, depending on the lanthanum content and synthesis method used. In this work, an exhaustive structural analysis of the Ce1−xLaxO2−x/2 system (0.2 < x ≤ 0.7) is performed with different techniques. The average crystal structure, studied by conventional X-ray diffraction, could be considered to be a disordered fluorite; however, the local structure, examined by electron diffraction and Raman spectroscopy, reveals a biphasic mixture of fluorite and C-type phases. The thermal and electrical properties demonstrate that the materials with x ≥ 0.4 are oxide ion proton conductors in an oxidizing atmosphere and mixed ionic electronic conductors in a reducing atmosphere. The water uptake and proton conductivity increase gradually with the increase in La content, suggesting that the formation of the C-type phase is responsible for the proton conduction in these materials.MINECO (RTI2018-093735-B-I00 y MAT2016-77648-R

Effect of Zn addition on the structure and electrochemical properties of codoped BaCe0.6Zr0.2Ln0.2O3-δ (Ln=Y, Gd, Yb) proton conductors

In this work, BaCe0.6Zr0.2Y0.2-xYbxO3-δ and BaCe0.6Zr0.2Gd0.2-xYbxO3-δ (x=0–0.20), proton conducting materials are prepared by the freeze-drying precursor method. The sintering conditions were optimized by adding Zn (NO3)2·6H2O as sintering additive. The materials are thoroughly characterized by different structural and microstructural techniques, including X-ray diffraction, scanning and transmission electron microscopy, and thermogravimetric-differential thermal analysis. The addition of Zn favours the phase formation and densification at lower sintering temperatures; however, it leads to the segregation of a Zn-rich secondary phase, with general formula BaLn2ZnO5 (Ln˭Y, Gd and Yb), which is identified and quantified for the first time. All samples with Zn as sintering aid exhibit cubic structure; however, the samples without Zn crystallize with orthorhombic or cubic structure, depending on the composition and thermal treatment. The electrical properties are studied by impedance spectroscopy. A deep analysis of the bulk and grain boundary contributions to the conductivity has revealed that the bulk conductivity remains almost unchanged along both series over Yb-doping; however, the grain boundary resistance decreases. The highest conductivity values are found for the intermediate members of both series, BaCe0.6Zr0.2Y0.1Yb0.1O3-δ and BaCe0.6Zr0.2Gd0.1Yb1O3-δ, with 33 and 28 mS cm−1 at 750 °C, respectively.Ministerio de Economía y Competitividad), MAT2016-77648-

Highly efficient La0.8Sr0.2MnO3-δ - Ce0.9Gd0.1O1.95 nanocomposite cathodes for solid oxide fuel cells

La0.8Sr0.2MnO3-δ-Ce0.9Gd0.1O1.95 (LSM-CGO) nanostructured cathodes are successfully prepared in a single process by a chemical spray-pyrolysis deposition method. The cathode is composed of nanometric particles of approximately 15 nm of diameter, providing high triple-phase boundary sites for the oxygen reduction reactions. A low polarization resistance of 0.046 Ω cm2 is obtained at 700 °C, which is comparable to the most efficient cobaltite-based perovskite cathodes. A NiO-YSZ anode supported fuel cell with the nanostructured cathode generates a power output of 1.4 W cm−2 at 800 °C, significantly higher than 0.75 W cm−2 for a cell with conventional LSM-CGO cathode. The results suggest that this is a promising strategy to achieve high efficiency electrodes for Solid Oxide Fuel Cells in a single preparation step, simplifying notably the fabrication process compared to traditional methods.Ministerio de Ciencia e Innovació

The invasion of Doñana National Park (SW Spain) by the mosquito fern (Azolla filiculoides Lam)

In 2001, Azolla filiculoides Lam., a floating pteridophyte native to the New World, was found in Doñana National Park (SW Spain), an European protected area well known for its high value from a conservation point of view and for its ecological vulnerability. Until that time, there had been no exotic aquatic macrophyte observations in the National Park. Since then, surfaces covered by Azolla filiculoides have increased explosively forming thick floating mats which eliminate submersed plants, and seriously threaten the aquatic ecosystems in this important protected area.En 2001, Azolla filiculoides Lam., una pteridófita acuática nativa del Nuevo Mundo, fue encontrada en el Parque Nacional de Doñana (SW España), un espacio protegido europeo bien conocido por su alto valor desde el punto de vista de la conservación y por su vulnerabilidad ecológica. Hasta ese momento, no se había observado ningún macrófito acuático exótico en el Parque Nacional; desde entonces, las superficies cubiertas por Azolla filiculoides se han incrementado de manera explosiva originando gruesas capas flotantes que eliminan las plantas acuáticas sumergidas y amenazan gravemente los ecosistemas acuáticos de este importante espacio protegido

Synergic Effect of Metal and Fluorine Doping on the Structural and Electrical Properties of La5.4MoO11.1-Based Materials.

Cationic and anionic frameworks of La5.4MoO11.1 proton conductors have been modified by means of metal (Ti4+, Zr4+, and Nb5+) and fluorine (F−) doping. This synergic effect leads to the stabilization of highsymmetry and single-phase polymorphs. The materials have been fully characterized by structural techniques, such as X-ray and neutron powder diffraction and transmission electron microscopy. The fluorine content was determined by ion chromatography. Impedance spectroscopy analysis under different atmospheres (dry and wet N2 and O2 and wet 5% H2−Ar) showed an improvement in the electronic conductivity under reducing conditions, making these materials potential candidates for hydrogen separation membranes

Impact of the lanthanide size on the polymorphism and electrical properties of Ln5.4MoO11.1 (Ln = Nd, Sm and Gd)

Mixed proton-electronic conductors are of great interest for high temperature electrochemical devices, such as hydrogen separation membranes. In this contribution, ceramics with composition Ln5.4MoO11.1 (Ln = Nd, Sm and Gd) were prepared by a freeze-drying precursor method. The resulting powders were sintered at 1500 ◦C and cooled down at different rates to investigate the different polymorphic forms: quenching (rapid cooling), 5 and 0.5 ◦C min 1. The ceramics were characterized by different techniques: X-ray diffraction, scanning and transmission electron microscopies and X-ray photoelectron and impedance spectroscopies. X-ray diffraction studies confirmed that all materials are single phase regardless of the cooling rate used. Those cooled by quenching present a simple cubic fluorite structure. At lower rates, 5 and 0.5 ◦C min 1, the cubic symmetry is stabilized as the size of the lanthanide decreases. However, electron diffraction studies indicated the formation of domains with superstructure ordering. Furthermore, XPS analysis showed the presence of mixed Mo6+ and Mo5+ for all compositions, which explains the electronic conduction in an oxidizing atmosphere. All materials are stable in reducing atmosphere and the ionic and electronic conductivities show opposite trends as the ionic radii of the lanthanide element becomes smaller, where the former decreases and the latter increases