Microsoft Word - 476hernandez.docx CHEMICAL ENGINEERING TRANSACTIONS VOL. 43, 2015 A publication of The Italian Association of Chemical Engineering Online at www.aidic.it/cet Chief Editors: Sauro Pierucci, Jiří J. Klemeš Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-34-1; ISSN 2283-9216 Structural Studies of Doped Lanthanum Orthoferrites of Interest for SOFCs Isabella Natali Sora*, Valeria Felice, and Francesca Fontana INSTM R.U. Bergamo and Università di Bergamo, Dipartimento di Ingegneria e Scienze Applicate, Viale Marconi 5, 24044 Dalmine, Italy isabella.natali-sora@unibg.it Traditional LSM (La1-xSrxMnO3-w) SOFC cathode material shows some drawbacks operating below 800 °C, in particular poor oxygen ion conduction, which limits the active area of the electrode to the three phase boundary (TPB) between electrolyte, electrode and gas phase at the interface. A strategy to overcome this problem consists in the development of materials with both good oxygen ion and electronic conductivity, in which oxygen ions catalytically dissociated on the surface are able to reach the electrolyte by diffusion in the bulk of the cathode. La1-xSrxFeO3-w has been proposed as possible SOFC cathode, in search for a high activity at lower temperatures with respect to LSM. The aim of the work is the structural study of alternative cathodic materials for intermediate temperature solid oxide fuel cells, mainly based on ABO3 perovskite-type lanthanum ferrite. Within this framework the effects of A (La)-site doped with Sr or B (Fe)-site doped with Ta on the crystal structures of these materials were investigated by X-ray powder diffraction (XRPD). In conjunction with the experimental results, the microscopic mechanisms of the doping effect were discussed providing deeper understanding into structure-property relations. 1. Introduction SOFCs (Solid Oxide Fuel Cells) are devices where H2 or hydrocarbons and O2 are electrochemically reacted to produce water, electricity and heat. Most of the materials proposed for SOFC are the traditional Ni-YSZ / YSZ / LSM assembly. In particular, the most used cathode material for SOFC is a perovskite oxide La1- xSrxMnO3 (LSM), which is a mixed ionic-electronic conductor successfully applied as cathode material in YSZ- based high-temperature HT-SOFCs; however it shows higher overpotential at intermediate operating temperatures (IT) when coupled with doped lanthanum gallate (LSGM) electrolyte (Dotelli et al., 2006). In this case the reduced electrochemical activity of LSM downgrades the cell performance and alternative cathode materials for IT-SOFCs are needed (Yi and Choi, 2004, Mai et al., 2005). One drawback of using porous LSM when operating below 800°C is the poor oxygen ion conduction, which limits the active area of the electrode to the three phase boundary (TPB) between electrolyte, electrode and gas phase at the interface. Moreover, it is known that manganese is a mobile species at high temperature; the occurrence of cations interdiffusion between LSM and LSGM both during the sintering step at elevated temperatures and at working temperatures for intermediate temperature SOFC has been reported (Pelosato et al., 2004). Thus to minimise manganese migration the fabrication temperature has to be kept below 1200°C. To overcome the limitations of traditional LSM, the use of doped-LaCoO3 (LSCo) has been proposed (Skinner, 2001). However, Co and Fe diffusivities in LSGM and concomitant Mg and Ga diffusivities in LSFCo were observed (Sakai et al., 2006). Moreover, the thermal expansion mismatch between LSFCo cathode and LSGM electrolyte may lead to insufficient contact points and thus to the detachment of cathode layers, which limits the applications of cobalt containing cathode materials. Substituted La1-xSrxFeO3 (LSF) are promising cathodic materials. The Cu substituted LSF (LSFCu) series was first proposed as oxygen electrode in IT- SOFCs by Coffey et al. (2004), who reported that Cu doping improves both the electrocatalytic activity and the electrical conductivity compared with the parent LSF material. Recently, Mancini et al., (2014) reported that LSFCu might be a suitable cathode material for SOFC DOI: 10.3303/CET1543293 Please cite this article as: Natali Sora I., Felice V., Fontana F., 2015, Structural studies of doped lanthanum orthoferrites of interest for sofcs, Chemical Engineering Transactions, 43, 1753-1758 DOI: 10.3303/CET1543293 1753 operating below 800°C. The feasibility of LSFCu as cathode material for IT-SOFCs has been demonstrated on the basis of structural (Natali Sora et al., 2012), thermal and electrochemical studies (Vogt et al., 2009). The material has been tested with different electrolytes such as GDC (Coffey et al., 2004), YSZ (Yasumoto et al., 2002), and LSGM (Zurlo et al.,2014). Cathodic materials specifically developed for SOFCs La1-xSrxFe1-yTayO3-w (LSFT) were prepared. Several physical properties of perovskite-type oxides are critically related to structural features (e.g. structural distortions from the ideal cubic perovskite structure). In LSF series three crystallographic regions are observed: orthorhombic symmetry for 0 ≤ x ≤ 0.2, a mixture of both rhombohedral and orthorhombic phases for x = 0.3, rhombohedral for 0.4 ≤ x ≤ 0.7 and cubic for 0.8 ≤ x ≤ 1.0 (Dann et al., 1994). In this work we focus on the synthesis and characterization of LSFT, with and without Ta substitution on the B-site. In particular, we present the phase relations and structural properties of La1-xSrxFe1-yTayO3 in the compositional range x = 0 - 0.6, y = 0 - 0.15. The phase relations and structural properties that occur in the LSFT series are compared with those of LSFCu series which has been investigated in detail by Natali Sora et al., (2012) using neutron diffraction data. 2. Experimental 2.1 Solid State Synthesis La1-xSrxFe1-yTayO3±w (LSFT) powders with compositions x = 0 and y = 0, 0.05, 0.10; x = 0.20 and y = 0, 0.05, 0.10; x = 0.30 and y = 0.05; x = 0.40 and y = 0, 0.05, 0.10, 0.15; x = 0.60 and y = 0, 0.05, 0.10, 0.15 were prepared by solid state reaction. Dried La2O3, SrCO3, Fe2O3 and Ta2O5 powders were mixed and ground together in an agate mortar for 30 min. All reagents were used in analytical grade and supplied by Sigma- Aldrich. The powder mixture was pressed into a pellet and thermal treated at 1000°C for 24 h and then at 1350°C for 24h, with an intermediate grinding step between the thermal treatments. 2.2 X-ray Diffraction For each composition phase purity, lattice symmetry, and unit-cell parameters were determined by powder X- ray diffraction. The measurement was performed in Bragg-Brentano geometry, with 0.02° increment size and collection time of 1 sec/step over the 2-Theta angular range 5-70°. For more than 0.05 mol% of Ta several peaks belonging to secondary phases were detected in the diffraction patterns. For those compositions where no impurity peaks were present in the diffraction data, LaFeO3, LaFe0.95Ta0.05O3, La0.80Sr0.20Fe3, La0.80Sr0.20Fe0.95Ta0.05O3, La0.60Sr0.40FeO3, La0.60Sr0.40Fe0.95Ta0.05O3, La0.40Sr0.60FeO3, and La0.40Sr0.60Fe0.95Ta0.05O3, the XRD patterns were collected a second time with longer counting time (10 s per step) in the 2θ range 5 - 90°. The structural refinements were carried out with the Rietveld method of profile analysis using GSAS (Larson and Von Dreele, 2000). The end-member phase LaFeO3 was reported to have orthorhombic √2ap x 2ap x √2ap cell (where ap denotes the unit-cell lattice parameters of the perovskite subcell). The first set of refinements was made assuming as structural model the perovskite-like cell of LaFeO3 and the symmetry of space group Pnma (Caronna et al., 2009). The refined parameters included scale factor, background function (Chebyschev polynomial), profile parameters (pseudo-Voigt profile function), lattice parameters, atomic coordinates and isotropic temperature factors. Throughout the refinements, the fractional site occupancies of cations were fixed to the nominal values. Since laboratory powder X-ray diffraction data do not allow determining the accurate site occupancy of oxygen atoms due to the small value of the oxygen scattering factor, the fractional occupancy of the oxygen atoms remained unitary. During the refinements the atomic coordinates and the temperature factors of the La/Sr and Fe/Ta sites were constrained to be the same; the temperature factors for both crystallographic independent oxygen atoms, O(1) and O(2), were also forced to be the same. 3. Results and Discussion 3.1 XRD Analysis In Figure 1 the XRD patterns of the LaFe1-yTayO3 powders with y = 0, 0.05 and 0.10 are shown. LaFe0.95Ta0.05O3 has the same basic structure of LaFeO3 which crystallized with the orthorhombic symmetry (space group Pnma), and the refined cell parameters were a = 5.56139(9) Å, b = 7.85911(11) Å, c = 5.55946(8) Å and V = 242.990(6) Å3. For y = 0.10 molar content of Ta small amount of LaTaO4, and traces of Ta2O5 were present. The ideal ABO3 perovskite structure has cubic symmetry and consists of a structural framework of corner-sharing BO6 octahedra: the larger A cations are located in cuboctahedral sites, and the smaller B cations in octahedral sites. 1754 Figure 1: XRD patterns of LaFe1-yTayO3 powders with y = 0, 0.05 and 0.10 Few ABO3 compounds have this ideal cubic structure, many of them having slightly distorted variants with lower symmetry. The perovskite structure may accommodate cations of different ionic size in the unit cell, for example when Sr2+ is substituted for La3+ a solid solution with a wide miscibility range is obtained. In Figure 2 the orthorhombic crystal structure of La0.80Sr0.20Fe0.95Ta0.05O3 is shown. The La atoms have twelve neighbouring oxygen atoms. The Fe and Ta atoms are randomly distributed over the positions 0,0,1/2 and have octahedral coordination. La0.80Sr0.20Fe0.95Ta0.05O3 crystallized in the orthorhombic symmetry space group Pnma, and refined cell parameters were a = 5.56068(11) Å, b = 7.84470(15) Å, c = 5.54733(12) Å and V = 241.985(8) Å3. For y = 0.10 molar content of Ta small amount of Sr2FeTaO6 was present. For Sr = 0.30 the presence of a mixture of both rhombohedral and orthorhombic phases and a certain low quality of the data suggested to omit the refinement. In Figure 3 the XRD patterns of the La0.60Sr0.40Fe1-yTayO3 powders with y = 0, 0.05, 0.10 and 0.15 are shown. It was observed (see the magnification of the 2-theta range 31.4° - 35.9°) that the crystal symmetry of the main perovskite phase varied with increasing Ta molar content. For La0.60Sr0.40Fe0.95Ta0.05O3 the Rietveld refinements gave much better results using the rhombohedral structural Figure 2: Orthorhombic crystal structure of La0.80Sr0.20Fe0.95Ta0.05O3 showing the (Fe/Ta)O6 octahedra, large circles represent La/Sr cations 1755 Figure 3: XRD patterns of La0.60Sr0.40Fe1-yTayO3 powders with y = 0, 0.05, 0.10 and 0.15. In the enlargement P/O indicates the orthorhombic perovskite phase, P/R indicates the rhombohedral perovskite phase model of La0.60Sr0.40FeO3, which crystallizes with the rhombohedral symmetry (space group R-3c). The refined cell parameters of La0.60Sr0.40Fe0.95Ta0.05O3 were a = 5.54322(7) Å, c = 13.4719(3) Å and V = 358.50(1) Å3. For Ta = 0.10 molar content a mixture of orthorhombic perovskite (P/O), rhombohedral perovskite (P/R) and small amount of Sr2FeTaO6 were present. For Ta = 0.15 two phases, a main orthorhombic perovskite (P/O) and small amount of Sr2FeTaO6, were identified. La0.40Sr0.60Fe1-yTayO3-w powders with y = 0, 0.05, 0.10 and 0.15 also were studied. For Ta = 0.05 a single rhombohedral perovskite was detected, on the contrary for Ta = 0.10 and 0.15 molar content the compounds were biphasic; the mixture consisted in a main rhombohedral perovskite (P/R) and small amount of Sr2FeTaO6. In order to compare how the Sr and Ta substitution influenced the structure of the perovskite, the pseudocubic unit cell volume Vc was calculated as Vc = V / Z, where Z = 4 for the orthorhombic perovskite, and Z = 6 for the rhombohedral perovskite. For Ta = 0.05 Vc decreased with increasing Sr molar content, in accordance with Figure 4: the pseudo-cubic unit cell volume (Vc = V / Z) for the series La1-xSrxFe0.95Ta0.05O3 as a function of the Sr content 1756 Figure 5: phase compositions of the series La1-xSrxFe1-yTayO3±w studied in this work are summarized. P/O indicates the orthorhombic perovskite phase, P/R indicates the rhombohedral perovskite phase the trend known for La1-xSrxFeO3-w where Vcorth > Vcrhomb (Figure 4). Moreover, when Sr was kept constant Vc increased with increasing Ta molar content. For example the values of Vc were 59.75 Å and 60.29 Å for La0.60Sr0.40Fe0.95Ta0.05O3 and La0.60Sr0.40Fe0.90Ta0.10O3. To maintain the charge balance, Ta5+ and Fe3+ cations may be in a valence state smaller than +5 and +3, respectively. The values of the ionic radius are: Ta5+(VI) = 0.64 Å and Ta4+(VI) = 0.66 Å, and Fe3+(VI) = 0.645 Å and Fe2+ (VI) = 0.77 Å (Shannon and Prewitt, 1969). Since the reduced cations are larger, they are under compressive stress. The lattice strain introduced in the structure may cause phase demixing. In Figure 5 are summarized the phase relationships and the structural properties of the system La1-xSrxFe1-yTayO3±w studied in this work. The single phase regions of LSFT are much smaller with respect to the Ta content in comparison with those of the LSFCu series varying Cu. In particular, for x = 0.20 and Cu content in the range 0 - 0.10 LSFCu crystallizes in an orthorhombic phase with space group Pnma (Natali Sora et al., 2013), while for x = 0.20 and y = 0.20 the powders are a mixture of both orthorhombic and rhombohedral perovskite phases (Natali Sora et al., 2012). The investigation on the electrochemical behaviour of LSFT-based cathodes is ongoing. 4. Conclusions A series of compounds of the La1-xSrxFe1-yTayO3 system of interest as cathodic catalysts in SOFCs have been prepared using solid state reaction method. Three crystallographic regions were observed by X-rays diffraction studies for Ta ≤ 0.05: orthorhombic symmetry for 0 ≤ x ≤ 0.2, a mixture of both rhombohedral and orthorhombic phases for x = 0.3, rhombohedral symmetry for 0.4 ≤ x ≤ 0.6. In particular, LaFeO3, LaFe0.95Ta0.05O3, La0.80Sr0.20Fe3, and La0.80Sr0.20Fe0.95Ta0.05O3 crystallized in the orthorhombic perovskite structure (space group Pnma). La0.60Sr0.40Fe3, La0.60Sr0.40Fe0.95Ta0.05O3, La0.40Sr0.60FeO3, and La0.40Sr0.60Fe0.95Ta0.05O3, crystallized in the rhombohedral perovskite structure (space group R-3c). When the Ta molar content was 0.05 < y ≤ 0.10, and the Sr molar content was 0 ≤ x ≤ 0.20, the samples showed a secondary phase. For x = 0 the main impurity was LaTaO4, while for x = 0.20 Sr2FeTaO6 was found as 1757 secondary phase. For higher Sr substitution (0.40 ≤ x ≤ 0.60) a larger Ta compositional region was investigated 0.05 < y ≤ 0.15; in this area all samples showed a secondary phase Sr2FeTaO6. Acknowledgements The financial support and collaboration of INSTM and of the Lombardy Region (Project "Ferriti di lantanio per nuove Fonti di Energia", Ferriti-NFE) 2013-2015 is gratefully acknowledged. 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