Phase Interaction and Distribution in Mixed Ionic Electronic Conducting Ceria-spinel Composites

Abstract

Mixed Ionic electronic conductors find various applications as SOFC cathodes and oxygen transport membranes. Dual phase composites are a promising class of thermochemical stable materials, in which two ceramic phases are coupled to provide a pure electronic and ionic conducting pathway, respectively. Composites of 20 mol% Gadolinia doped ceria (GDC) and FeCo2O4 spinel (FCO) are investigated. GDC-FCO 60:40 wt-% ratio showed reasonable oxygen permeation with ionic conductivity as the limiting factor. Spinel content was reduced to as low as 10 wt-% in the composite and their corresponding electrical conductivity and oxygen permeation were measured from which ambipolar conductivity was calculated. GDC-FCO 85:15 wt-% ratio shows the highest ambipolar conductivity comparable to standard single phase La0.58Sr0.4Fe0.8Co0.2O3-δ (LSCF) at 850 °C. The Microstructure analysis showed reversible and thus temporary spinel decomposition at sintering temperature as well as phase interaction forming a Gdand Fe-rich orthorhombic perovskite with traces of Ce and Co. To further investigate the phase interaction and secondary phase formation, Pulsed Layer Deposition of FCO layer (~400 nm) on a polycrystalline GDC substrate and annealing at varying temperatures and times from 1000 to 1200 °C were carried out. These samples were analyzed by XRD, STEM, and SIMS to understand the interlayer interaction of the phases. INTRODUCTION MIEC Oxygen transport membranes contribute to major research focus in the last two decades due to its ability to separate pure oxygen from air more efficiently compared to other technologies. Recently, increasing research interest is drawn to the development of membrane reactors, combining the separation task with a catalytically promoted chemical reaction. Such process intensification is interesting in different fields such as energy applications as well as the production of fine chemicals 3, 4, . In particular Ceria based MIEC are also attractive in Hydrocarbon fuel utilization due to their resistance to reducing atmospheres as well as carbon deposition 1, . Ceria based Fluorites combined with transition metal oxides with spinel structure have proven to improve the sinter ability and mixed conductivity of the composite 7, 8, . Moreover, dual phase composites like GDC-LSCF have been studied as potential SOFC cathode layer 10, 11, . Particularly for metal supported SOFCs there is a need in cathode material development, which can be sintered in relatively low pO2 in order to prevent catastrophic metal support oxidation. However, standard materials such as La0.58Sr0.4Fe0.8Co0.2O3-δ (LSCF) do not withstand such conditions. Hence, the development of composite materials with good reduction stability is of great importance. In this paper, various ratio combinations of GDC (Ce0.8Gd0.2O2-δ) and FCO (FeCo2O4) were synthesized, and their dense pellets were studied for their mixed conductivity by measuring their oxygen flux performance at high temperatures. The work also focuses on the phase interaction and secondary phase formation of the composite to better understand the conductivity behavior of the composites. This work indicates the possibility of highest oxygen transport in this dual phase membrane with electronic conducting phase as low as 18.5 vol%. In addition, this composite was tested for its cathodic activity for application in SOFC. EXPERIMENTAL GDC-FCO composites of ratios varying spinel content from 46 vol% to as low as 12.5 vol % be synthesized by modified Pechini process. The powders were calcined at 700°C for 15h to attain the desired fluorite and spinel phases. The different ratio composite powders were pressed into cylindrical pellets of 20mm dia by uniaxial pressing at 80 Mpa for 90 seconds. The pellets were sintered at 1200°C for 10h to obtain dense pellets with relative theoretical density of ~97 %, which were also tested for gas tightness by Helium leak test using Pfeiffer vacuum apparatus. Electrical conductivity of bars was measured by DC four point conductivity between temperatures 600 to 850°C. Thin layers of FCO were deposited on polycrystalline GDC substrates by means of pulsed laser deposition (PLD). FCO target was ablated by a KrF excimer laser (248 nm; 5 Hz repeat rate). The substrate temperature was 650°C (controlled by a pyrometer), background pressure of pure oxygen in the chamber was 4x10 mbar, and deposition times were ca. 25 min to obtain a film thickness of about 100 nm. X-ray diffraction (XRD) analysis of the dual phase compounds was performed on D4 Endeavor (Bruker AVS) with CuKα radiation measured at 10° ≤ 2θ ≥ 80°, ∆2θ =0.02°. X-ray diffraction analysis of FCO thin films was carried out on a PANalytical Empyrean diffractometer with CuKα radiation. Thin film XRD measurements were conducted with 2° incident angle and diffraction pattern was recorded at 15° ≤ 2θ ≥ 90°. Data analysis was performed with the software Highscore Plus in combination with ICDD powder diffraction database (PDF4+). Phase identification and microstructure analysis of the dense pellets were carried out using SEM (Zeiss Ultra55). The Scanning transmission electron microscopy (STEM) images were acquired using Zeiss Libra 200FE equipped with a cold FEG source operated at 200 KV, HAADF detector from Fischione and X-Flash EDS detector from Bruker. ESPRIT software was used to analyze and build the EDS elemental maps. SIMS investigation was carried out using TOF-SIMS 4 (IonTOF, Germany) with Cs ion beam for sputtering (2 KeV) and Bi1gun (25KeV) for analysis. The dense disk type pellets polished to a thickness of 1mm each were measured for their oxygen flux using the quartz permeation setup at IEK-1 in the temperature range of 600 to 1000°C. Air and argon were used as the feed and sweep gas at the flow rate of 250 ml/min to 50ml/min, golden rings used to seal the sample in the setup. Samples were screen printed with catalytic LSCF layer on both sides and post sintered at 1050 °C for 3 h. Symmetrical cells of 10 x 10 mm2 were prepared by screen printing of GDC-FCO (85:15) slurry on both sides of 200 μm thick 8YSZ substrates (Kerafol, Germany) and subsequent sintering at 1080 °C for 3 h in ambient air. As a contact layer ca. 300 nm Au were sputter deposited on both sides of these symmetrical samples. The polarization resistance of the electrodes was tested at temperatures between 600 and 900 °C by electrochemical impedance spectroscopy (EIS) in order to estimate the suitability of GDC-FCO composites as SOFC dual phase cathode material. . Measurements were conducted on a PSM 1735 machine with 4 point impedance analysis interface (both: N4L, UK); AC voltage was set to 20 mV root-mean-square; measured frequency range was between 10 mHz and 100 kHz. RESULTS AND DISCUSSION In the previous work, we investigated Fluorite-Spinel composite for oxygen transport membrane by combining 60 wt % of GDC (Ce0.8Gd0.2O2-x) with 40 wt % of FCO (FeCo2O4) (54 vol% GDC – 46 vol% FCO) in comparison to standard perovskite material LSCF. The detailed investigation of the dual phase membrane revealed phase interaction and secondary phase formation during the sintering process i.e GdFeO3 perovskite with traces of Ce and Co formed by the interdiffusion during sintering at 1200°C. Inspite of this additional phase, Permeation tests resulted in reasonable oxygen flux, when slow surface exchange was facilitated by catalytic active porous coatings. Thus with elimination of surface exchange limitation , the activation energy of bulk diffusion over the whole temperature range of 650° C-1000°C was 66 kJ/mol. This clearly indicates that the ionic conductivity is the limiting step in the bulk transport as it matches the activation energy of pure GDC corresponding to its bulk ionic transport for the same temperature range. Therefore, an increase in the GDC-content would be beneficial. On the other hand, the proportion of both phases in a dual phase composite must be high enough to form continuous phases in the bulk and surfaces, i.e. both phases need to reach a state of percolation. To reach the percolation threshold, the volume fraction of the minor phase is usually no less than 30% for dual-phase materials. Nevertheless this is not necessarily applicable for composites that tend to have phase interaction and grain boundary phases. The grain boundary phase and secondary phases formed are possibly conducting, contributing to the overall percolating network of the composite impacting the oxygen transport. Hence, the spinel content is reduced from 40 wt% to as low as 10 wt% in the composite as shown in Table I for this investigation and powders were synthesized by the one pot Pechini process. Table I Composites with reducing spinel content given in Weight % and Volume % GDC-FCO ratios in wt % GDC-FCO ratios in Vol % 60:40 54:46 65:35 59:41 70:30 64.5:35.5 75:25 70:30 80:20 76:24 85:15 81.5:18.5 90:10 87.5:12.5 Microstructure analysis The XRD plots of the composites with varying spinel content are shown in Figure 1. The decrease in spinel content is evident from the decrease in intensity of the spinel peaks of the composites particularly at 31° and 64° 2θ. In addition, the presences of orthorhombic perovskite peaks (between 24° and 33° 2θ) are confirmed in all the ratios. These results are further supported by the SEM images as shown in Figure 2. The images show the presence of fluorite phase with decreasing distribution of the spinel phase corresponding to the ratio and the presence of orthorhombic perovskite phase in all the four cases 7, . Going into in-depth investigation of the microstructure by STEM analysis of the lowest spinel content ratio composite 90:10 GDCFCO, it is evident that the third phase GdFeO3 (orthorhombic perovskite) contains traces of Ce and Co elements as well. STEM-HAADF and EDS element mapping of 90:10 ratio composite is shown in Figure 3. Semi quantitative estimation of the elements from the EDS elemental mapping image provides a hint on the approximate composition of the perovskite phase that is formed in these dual phase composites during the sintering process i.e., 15%Ce on A-site, 25% Co on B-site in GdFeO3 perovskite phase. The Gd0.85Ce0.15Fe0.75Co0.25O3 (GCFCO) phase is also observed to be individual grains with no obvious grain boundary phase visible in this magnification. Thus it would be worthwhile to investigate this perovskite phase for its transport behavior to identify its role in the composite for oxygen permeation. Figure 1 XRD plots of the composites with varying spinel content from 40 wt% to 10 wt% sintered at 1200°C for 10h Figure 2 SEM images of GDC-FCO composites with ratios 60:40 (top left), 70:30 (top right), 80:20 (bottom left) and 90:10 (bottom right). 20 40 60 80 In te ns ity ( a. u. ) 2θ (o) FeCo 2 O 4 Ce 0.8 Gd 0.2 O 2-δ 90:10

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Cite this paper

@inproceedings{Ramasamy2016PhaseIA, title={Phase Interaction and Distribution in Mixed Ionic Electronic Conducting Ceria-spinel Composites}, author={Manikandan Ramasamy and Severine Baumann and Andrea Opitz and Riza Iskandar and J. Mayer and D. Udomsilp and U. Breuer and Martin Bram}, year={2016} }