High efficiency cathodes for SOFCs prepared by spray-pyrolysis

In recent years, lowering the operating temper-ature of the Solid Oxide Fuel Cells (SOFCs) to the intermediate temperature range (500-700 ºC) has become the main challenge for this technology. The electrolyte resistance might be substantially reduced by using thin film electrolytes. However, the cathode polarization resitance is responsible for much of the loss in performance at low temperatures. Thus, the development of cathode materials with high electro-catalytic activity for the oxygen reduction reaction (ORR) is essential for this technology. Lanthanum strontium manganite La1-xSrxMnO3- (LSM) is the cathode material most widely used in SOFCs. However, LSM exhibits high activation energy for oxygen reduction reaction (ORR) and poor ionic conductivity, limiting its application at high temperatures. Alternative mixed ionic-electronic conductors, such as La1-xSrxCo1-yFeyO3-δ (LSCF) and GdBaCo2O5+x (GBC) has been investi-gated, exhibiting better performances in the inter-mediate temperature range [1]. The performance of these electrodes might be improved at reduced temperature by extending the triple phase bounda-ry length at which gas, electrode and electrolyte phases are simultaneously in contact, serving as the predominant site for the electrochemical reac-tions. To date, the preparation of electrodes via wet infiltration of a cation solution into a porous electrolyte backbone is one of the most effective methods to increase the TPB area and to improve the efficiency of the cathodes, despite the limitations of this process for large-scale manufacturing of SOFCs. In this contribution an alternative preparation method based on spray-pyrolisis deposition into an electrolyte backbone is proposed, which posses a series of advantages with respect to the classical wet infiltration process, including easy industrial implementation, preparation in one single deposition/thermal step as well as low cost [2]. The most widely used cathodes in SOFCs technology were prepared by this alternative method process: La1-xSrxMnO3- and La0.6Sr0.4Co1-yFeyO3-δ (y = 0-2) series. The electrodes were deposited on porous Ce0.8Gd0.1O1.95 (CGO) backbones at 250 ºC by conventional spray pyrolysis from an aqueous precursor solution of metal nitrates. The structure, microstructure and electrochemical properties of these materials have been investigated by X-ray diffraction, field-emission SEM (Fig. 1.a) and im-pedance spectroscopy in symmetrical cells. The values of polarization resistance (Fig. 1.b) are ex-tremely low, ranging from 0.40 cm2 for LSM to 0.07 cm2 for LSCF0.2 at 600 ºC in air, compared to those previously reported in the literature for commercial electrolytes deposited at high tempera-ture, e.g. 25 cm2 for LSM.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

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

Ce0.8Gd0.2O2‐δ / La0.6Sr0.4CoO3 Heterostructures prepared by pulsed laser deposition

Oxide interfaces have received greater attention due to the possibility to obtain properties that are very different from bulk materials. Due to the wide variety of electronic and ionic phenomena than can be detected at the interfaces, such materials have many technological applications [1]. Attention is being drawn to oxide heterostructures, a new family of artificial materials where electronic and ionic properties can be modulated at the interfaces by varying the characteristics of the layers [2, 3]. Slight variations in the near anionic-cationic order might take place if there exists strained interfaces. The interest in multilayared heterostructures derives from the mobility deffects and the space-charge-zone effects at the interfaces. In addition, a new degree of freedom related to the capacitive and resistive contributions is provided as a consequence of the size effects of these artificial structures. In the present work, for the first time, we investigate the structure, microstructure and electrical properties of a new family of heterostructured materials with alternated thin layers of La0.6Sr0.4CoO3 (LSC) and Ce0.8Gd0.2O2-δ (CGO) deposited by pulsed laser deposition on (110) NdGaO3 (NGO) single crystal substrates. In order to evaluate the interfacial contribution to ionic-electronic conductivity and know what is actually happens at the interface of MIECs, different heterostructures were prepared by varying both the number of bilayers (N) and the total thickness of the samples (N = 2 and 5; and the thickness were 50, 100 and 300 nm).Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tec

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

Solid oxide fuel cells (SOFCs) are one of the most efficient technologies for direct conversion of fuels to electricity. La0.8Sr0.2MnO3-δ (LSM) is the cathode material most widely used in SOFCs [1], however, LSM exhibits high activation energy for oxygen reduction reaction (ORR) and low ionic conductivity, which limits its application at reduced temperatures. In this material the electrochemically active reaction sites are restricted to the triple-phase boundary (TPB) near the electrolyte/electrode interface, where the electrolyte, air and electrode meet. Different strategies have been investigated to enlarge the TPB area of LSM, such as the production of nanocrystalline powders by precursor routes, preparation of composites by infiltration methods and thin films [2-4]. Here we present and compare innovative procedures to extend the TPB of LSM in contact with yttria-stabilized zirconia electrolyte: i) nanocrystalline LSM films deposited by spray-pyrolysis on polished YSZ electrolyte; ii) the addition of polymethyl methacrylate microspheres as pore formers during the spray-pyrolysis deposition to further increase the porosity of these films and (iii) the deposition of LSM by spray-pyrolysis on porous backbones of Zr0.84Y0.16O1.92 (YSZ), Ce0.9Gd0.1O1.95 (CGO) and Bi1.5Y0.5O3- (BYO) electrolytes previously fixed onto the YSZ electrolyte. The most remarkable peculiarity of this novel preparation method, compared to the traditional impregnation, is the formation of LSM thick film of 500 nm on the electrode surface (Fig. 1), which improves the electrical conductivity of the composite cathode. Thus, the optimization of this novel method would be an alternative to the classical infiltration with several advantages for the industry of planar SOFCs allowing the deposition of a wide variety of ceramic films over large areas with more uniform distribution of the catalyst, lower cost and only one deposition step is required to form the electrode. The morphology and electrochemical performance of the electrode have been investigated by scanning electron microscopy and impedance spectroscopy. Very low values of area specific resistance were obtained ranging from 1.4 cm2 for LSM deposited on polished YSZ to 0.06 cm2 for LSM deposited onto BYO backbone at a measured temperature of 650 ºC. This electrodes exhibit high performance even after annealing at 950 ºC making them interesting for applications at intermediate temperatures.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

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

Solid Oxide Fuel Cells based on Lanthanum Tungstates Electrolytes

Lanthanum tungstate with composition La27W4NbO55- (LWNO) has been tested as proton conductor electrolyte for Solid Oxide Fuel Cells (SOFCs). For this purpose, different electrodes and composite electrodes are considered, including: La0.8Sr0.2MnO3-, La0.6Sr0.4Co1-xFexO3-, La0.5Sr0.5Cr0.5Mn0.5O3-, SrFe0.75Nb0.25O3- and NiO. Chemical compatibility between the cell components is investigated by X-ray powder diffraction (XRPD) and energy dispersive spectroscopy (EDS). Furthermore, area specific resistance (ASR) of the different electrodes is determined in symmetrical cells by impedance spectroscopy. XRPD and EDS analysis do not reveal significant bulk reactivity between most of these electrodes and LWNO electrolyte in the typical operating temperature range of a SOFC (600-900 ºC). However, minor interdiffusion of elements at the electrolyte/electrode interface affects both the ohmic losses and electrode polarization of the cells. ASR values are significantly improved by using a buffer layer of Ce0.8Gd0.2O1.9, between the electrolyte and electrode materials, to prevent reactivity. A single cell with 350 µm thick electrolyte, NiO-Ce0.8Gd0.2O1.9 anode and La0.6Sr0.4Co0.8Fe0.2O3- cathode, generates maximum power densities of 140 and 18 mWcm-2 at 900 and 650 ºC, respectively.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech

Influence of Nb-doping on the structural and electrical properties of lanthanum molybdates, La5.4MoO11.1

Nowadays, hydrogen is receiving a great deal of attention as an energy carrier. Commonly, it is obtained by hydrocarbons reforming, such as natural gas, coal and biomass. However, the resulting hydrogen needs to be purified to remove by-products and impurities, increasing the production costs. An alternative for hydrogen production is proton-conducting ceramics, where hydrogen separation takes place via a chemical potential gradient across the membrane.1, 2 In this work, Nb-doped La6MoO12--based compounds have been investigated as part of a new family of materials very competitive as SOFC electrolyte and hydrogen separation membranes.3 These materials, La5.4Mo1-xNbxO11.1-x/2 (x = 0.05, 0.10, 0.15 y 0.20) were synthesized by the freeze-drying precursor method and calcination conditions have been optimized to obtain single phases. A complete characterization has been carried out using X-Ray powder diffraction and scanning and transmission electron microscopy. The total conductivity was determined by complex impedance spectroscopy at different atmospheres. Different polymorphs are obtained as a function of the cooling rate and the dopant amount. The samples cooled by quenching are cubic with a fluorite-type structure (Fm3 ̅m) and the ones cooled at 50 y 0.5 ºC•min-1 are rhombohedral (R1 and R2 polymorphs). For niobium contents higher than x = 0.10 the R1 polymorph is stabilised at cooling rates equal or inferior to 50 ºC•min-1. For all three series, the incorporation of niobium into La5.4MoO11.1 increases the conductivity, reaching the best values for x=0.10 and the sample obtained by quenching.Universidad de Málaga. Campus de Excelencia Internacional Andalucía Tech