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

Stability of epitaxial heterostructured materials

Heterostructured materials are a new family of artificial compounds where the electronic and ionic properties can be modulated by varying the characteristics of the different material layers. These properties arise from the formation of structural oxygen defects in the crystal lattice that result in the activation of charge electrical carriers. Oxygen-deficient perovskite oxides, such as La1-xSrxCoO3-δ (LSC), present mixed oxide/electronic conduction; however, the long-term instability due to superficial carbonation of LSC-based cathodes is a crucial drawback for their practical application. In this study, thin film-heterostructures of alternating layers of La0.6Sr0.4CoO3-δ and Ce0.8Gd0.2O2-δ (CGO) were deposited on (110) NdGaO3 (NGO) single crystal substrates by pulsed laser deposition (PLD). The number of interfaces and the thickness were varied to obtain epitaxial structures with highly coherence layers. Moreover, two different kinds of architectures, without and with a CGO termination layer, were prepared in order to study the stability of the samples under different thermal cycles in air. Structural characterization was made by using Rocking Curve and Reciprocal Space Mapping techniques. CGO layers are rotated 45º respect to the substrate and LSC ones due to the different sizes of cell parameters. The quality of the samples was examined by HR-TEM and all of them presented well defined interfaces (Figure 1). Electrical characterization confirms that the conductivity can be modulated by varying the number of interfaces and thickness. Samples without CGO termination are unstable in air atmosphere due to surface carbonation, which was confirmed by XPS and HR-TEM.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

Proton conductivity of lanthanide nitrilotris-methylphosphonates

Multifunctional metal phosphonates are acidic coordination polymers (CPs) with remarkable stability and proton conducting properties owing to their structure is usually composed of extended hydrogen-bond networks that favor proton transfer pathways [1]. In this communication, three different families of proton conductors based on lanthanide nitrilotris-methylphosphonates are examined. Compounds were isolated by crystallization at room temperature at pH <0.8 in the presence of. When chloride is presented in solution two families of compounds were isolated, depending on the concentration of chloride in solution: free-chloride 1D solids with formula Ln2(H3NMP)2(H2O)4]·4.5H2O [Ln= La3+] [2] or layered chloride-containing Ln(H4NMP)(H2O)2]Cl·2H2O [Ln= La3+ - Ho3+] materials [3]. In absence of chloride, a third series of compounds was obtained. This structural versatility leads to a wide range of proton conductivity varying between 3 × 10−4 S·cm−1 and 2 × 10−3 S·cm−1 as measured at 80 °C and 95% RH.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

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

Structural study of the local order in ammonia-modulated FE(II) hydroxyphosphonoacetate proton conductors

Layered Fe(II) carboxiphosphonate, Fe-HPAA·2H2O, is a crystalline multifunctional coordination polymer exhibiting properties as photocatalyst and proton conductor. Postsynthesis modification by ammonia/water adsorption strongly enhances its proton conductivity. However, this process entails a progressive amorphization but in no case intercalation of the guest species was detected. Understanding the mechanism involved in this increased conductivity is crucial to develop novel high performance proton conductors for PEMFCs. Thus, total scattering and PDF study has been carried out to explore the mechanism of ammonia adsorption and subsequent amorphization. Different lenght scales have been investigated to characterize the average and local structure at variable ammonia loaded in order to ascertain posible structural modifications after gas/solid reactions. While significant short range order (from 1.4 to 10 Å) variations were observed even for low loadings, the average structure seems to be basically preserved except for the highest ammonia/water contents.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Synthesis and proton conduction properties of lanthanide amino-sulfophosphonates

Acidic groups-containing metal phosphonates exhibit a wide range of proton conductivity depending on the water content and functionality. Moreover, this property can be enhanced by appropriate post-synthesis chemical and/or thermal treatments [1,2]. In this work, focus is laid on properties derived from the combination of lanthanide ions with the amino-sulfophosphonate ligand (H2O3PCH2)2-N-(CH2)2-SO3H. Highthrough-put screening was used to reach the optimal synthesis conditions under hydrothermal conditions at 140 ºC. Isolated polycrystalline solids, Ln[(O3PCH2)2-NH-(CH2)2-SO3H]·2H2O (Ln= La, Pr, Sm, Eu, Gd, Tb and Er), crystallize in the monoclinic (La and Er) and orthorhombic (Pr, Sm, Eu, Gd and Tb) systems with unit cell volume of ~1200 and 2548 Å3 respectively. Their crystal structures, solved ab initio from X-ray powder diffraction data, correspond to different layered frameworks depending on the lanthanide cation size. Thus, compounds with orthorhombic symmetry show free acidic sulfonic pointing to the interlayer space, while La- and Er- derivatives display layered structures where both phosphonate and sulfonated groups are coordinated to the metal, leaving free P-OH groups. As consequence of this structural variability, different H-bond networks and proton transfer pathways are generated. Preliminary proton conductivity measurements have been carried out between 25 and 80 ºC at 70-95 % relative humidity. The sample exhibits conductivities near to 3.10-3 S.cm-1 and activation energies characteristics of a Grotthuss-type mechanism of proton transfer.Proyectos de investigación del ministerio MICINN, Españam(MAT2016-77648-R), Proyectos de la Junta de Andalucía (P12-FQM-1656), Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

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

New Multifunctional Lanthanide and Zr(IV) Phosphonates Derived from the 5-(dihydroxyphosphoryl) Isophthalate Ligand as Proton Conductors

Metal phosphonates are essentially acidic solids featured by groups such as P-OH, -COOH, etc. Moreover, the presence of coordination and lattice water molecules favors the formation of H-bond networks, which make these compounds appropriate as proton conductors, attractive for proton exchange membranes (PEMs) of fuel Cells.1 We report here, general characteristics of metal phosphonate derivatives composed of the polyfunctional 5-(dihydroxyphosphoryl) isophthalate ligand2 and lanthanides or zirconium ions. In the case of the lanthanide derivatives, crystalline compounds were synthesized under hydrothermal conditions. Preliminary results suggest that at least three isostructural series of compounds are formed. One of them, with La3+ derivative as prototype, is characterized by an orthorhombic unit cell (a = 12.7745(6) Å, b = 11.8921(4) Å, c = 7.2193(5) Å). Pr3+, Eu3+ and Gd3+ compounds, displays a monoclinic unit cell likewise the Yb3+ solid, the latter exhibiting different crystallographic parameters. Zr(IV) = compound, with formula Zr[(HO3P-C6H3-(COO)2H)2]·8H2O; was obtained at 80 ºC in the presence of HF as mineralizing agent. This solid crystallizes in an orthorhombic unit cell (a = 21.9306 Å, b = 16.6169 Å, c = 3.6462 Å). All these compounds contain in their frameworks water molecules that contribute to the formation of H-bond networks, making them prone as proton conductor candidates. Structural and proton conductivity are underway.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech. Plan Propio de Investigación de la UMA MAT2016-77648

Synthesis and characterization of M(II) phosphonates (M = Fe, Co, Zn, Mn) as precursors for PEMFCs electrocatalysts

Metal phosphonates are promising precursors for applications such as proton conductivity [1] and catalysis [2]. Specifically, upon calcination metal polyphosphates are generated that can be used as non-noble metal alternatives [3] to the highly expensive commercial catalysts (Pt) for proton exchange membrane fuel cells (PEMFCs). In this work, we present the synthesis and characterization of metal polyphosphates obtained from transition divalent metal phosphonates (M= Fe, Mn, Co and Zn) both as monometallic and bimetallic systems (solid solutions). For the preparation of the metal phosphonate precursors, two types of organic linkers were selected, i.e. 2-R,S-hydroxiphosphonoacetic acid [HO3PCH(OH)COOH, HPAA] and nitrilotrismethylenephosphonic acid [N(CH2PO3H2)3, ATMP]. The as synthesized compounds were calcined between 700 and 1000 ºC under N2. Depending on the metal/phosphorous molar ratio in the precursor phases, different compositions were found, the corresponding metal pyrophosphate being the major component according to the crystallographic data. Interestingly, in most of cases the solid solutions were preserved in the final product, for instance Fe-Mn, Fe-Co and Fe-Zn. All calcined materials have been also characterized by XPS, SEM/EDS, FTIR-Raman.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Lanthanide molybdates, Ln5.4MoO11.1 (Ln = Nd, Sm and Gd), for hydrogen separation membranes

Nowadays, lanthanide molybdates (Ln6-xMoO12-, Ln = La-Lu) are attracting attention as candidates for hydrogen separation membranes due to its high ambipolar proton-electron conductivity. In these compounds, a very high degree of polymorphism is detected depending on the composition and synthesis-sintering conditions. Very recently, we carried out a comprehensive study of La5.4MoO11.1 and the effect of the synthesis temperature and cooling rate on the symmetry of the samples1. We found out that those samples suddenly cooled from 1500 ºC present a simple cubic fluorite structure, whereas those cooled at slower rates, such as 50 and 0.5 ºC min-1, present complex rhombohedral polymorphs with superstructures, denominated in that work as R1 and R2, respectively. Here, we extend this study to lanthanides smaller than lanthanum and evaluate the influence of composition and synthesis-sintering conditions on the structural and electrical properties. It was determined by X-ray diffraction that the materials are single phase after heating at 1500 ºC and cooling at different rates. Those cooled by quenching present a simple cubic fluorite structure. At lower rates, 5 and 0.5 ºC min-1, the cubic symmetry is stabilized as the size of the lanthanide decreases, as can be seen in Figure 1. XPS analysis showed the presence of Mo6+ and Mo5+ for all samples. The reduction of the cooling rate for the same composition leads to an increase of the average grain size. For a same cooling rate, the decrease of the size of the lanthanide leads to a lower average grain size. The materials are stable in very reducing conditions and the electronic conductivity increases as the size of the lanthanide becomes smaller