Cobalt–iron red–ox behavior in nanostructured La0.4Sr0.6Co0.8Fe0.2O3−δ cathodes
Graphical abstract
XANES and XPD measurements in nanostructured LSCF before (black) and after (red/green) a red/ox cycle.
Highlights
► Red–ox treatments in LSCF nano-particles cause a reversible reaction. ► XPD analyses show that a new “reduced” phase coexist with the oxidize one. ► The B-site formal oxidation state decreases and the δ increases upon reduction. ► Fe remains in a higher valence (closer to 4+) than Co (close to 3+). ► The behavior seems to be independent of the synthesis method used.
Introduction
Much effort has recently been done investigating perovskite materials such as La1−xSrxCo1−yFeyO3−δ (LSCF) as cathodes for solid oxide fuel cells (SOFC) [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. These ceramics, appropriated for intermediate temperature (IT)–SOFC design, are especially interesting due to its ability to conduct both oxygen ions and electrons, thus enlarging the performance of the cell.
It is well known that different synthesis procedures, modification of the micro- and nanostructure, mechanical stress, substitution and/or addition of small quantities of dopants change the electronic structure of the cathode and may improve or, in the worst case, deteriorate the cell transport properties. We previously reported that a significant area specific resistance (ASR) reduction, with its consequent performance enhancement, could be achieved by reducing the grain size from the micrometric to the nanometric scale in La0.4Sr0.6Co0.8Fe0.2O3−δ cathodes [5], [6].
An important premise before the application of this material in real devices is to guarantee the long term phase stability and electrochemical performance at different conditions. Transitions to phases showing oxygen vacancy ordering were observed in the (La,Sr)(Co,Fe)O3−δ family after exposure to reductive thermal treatments [11], [12], [13]. For example, a transition to brownmillerite phase under low oxygen partial pressures was reported for sub-micrometric strontium-rich La1−xSrxCo0.8Fe0.2O3−δ compounds (i.e. x≥0.7) [12] leading to a deterioration of the cathode transport and mechanical properties. Nevertheless, no transition was observed in sub-micrometric La0.4Sr0.6Co0.8Fe0.2O3−δ powders by X-Ray diffraction (XRD) in the 20–900 °C and −5≤log pO2≤0 ranges. On the contrary, some unexpected lattice distortions were reported in the nanostructured La0.4Sr0.6Co0.8Fe0.2O3−δ powders when they are exposed to reductive atmospheres (Ar) in the 150–600 °C range in comparison with those tested in an oxidative medium [14].
In this work, we aim to determine whether or not the distortions observed in nanocrystalline LSCF oxides imply the formation of new phases and/or chemical changes in the oxidation state of the transition metals (Co and Fe) occupying the B-site of the perovskite. For that, nanostructured La0.4Sr0.6Co0.8Fe0.2O3−δ samples were analyzed ex-situ after different thermal treatments using reductive and oxidative atmospheres by X-ray Absorption Near Edge Spectroscopy (XANES) and X-ray Powder Diffraction (XPD).
Section snippets
Sample preparation
Nanostructured powders of composition La0.4Sr0.6Co0.8Fe0.2O3−δ (LSCF) were synthesized by a conventional acetate route and by the novel HTMA route [5]. In the following, all samples which were synthesized by the acetate or the HMTA method will be referred as LSCF_A and LSCF_H, respectively (Table 1). Two batches of each powder were sintered at 900 °C in air for 6 h. One batch of each synthesis was reserved and called “as prepared or ap” material. The rest was reduced in an Ar atmosphere at 500 °C
XPD results
Fig. 1A displays XPD data corresponding to HMTA samples before (LSCF_H_ap) and after (LSCF_H_red) a reductive thermal treatment in Ar for 24 h. The LSCF_H_ap sample can be indexed by the spatial group (PDF Card 00-048-0124) with cell parameters a=b=5.432 Å and c=13.294 Å. A splitting of all peaks along with a decrease in intensity can be observed for the reduced sample (LSCF_H_red). This feature is particularly noticeable in the peak at 2θ=47.5°, since the reflection (024) is the only one in
Conclusion
Synchrotron X-ray absorption techniques such as XANES are increasingly recognized by the Fuel Cell community as powerful tools to examine the oxidation state and coordination chemistry of the metals involved in the transport of electrons and ions in the electrode materials. This information is thus helpful for reconstructing the electronic configuration and sample stoichiometry, determining in this way the influence on certain synthesis and function parameters variation in the fuel cell
Acknowledgments
The authors thank D. Lamas, L. Acuña and J. Geck for their helpful assistance in the experimental design, the analysis of the data and the improvement of this manuscript. XAS measurements were financed by the LNLS proposal D04B-XAFS1-9932 (2010).
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Present address: Department of Energy Conversion and Storage, Risoe Campus, Frederiksborgvej 399, DK-4000 Roskilde, Denmark.