Study of Dual-Phase Oxygen Transport Membrane (Ce0.8Gd0.2O2-δ - Fe2CoO4) using Scanning Transmission Electron Microscopy
- Abstract number
- 954
- Event
- European Microscopy Congress 2020
- DOI
- 10.22443/rms.emc2020.954
- Corresponding Email
- [email protected]
- Session
- PSA.4 - Batteries & Materials for Energy Conversion
- Authors
- Daesung Park (3), Ke Ran (1, 5), Kerstin Neuhaus (4), Stefan Baumann (2), Liudmila Fischer (2), Joachim Mayer (1, 5)
- Affiliations
-
1. Central Facility for Electron Microscopy (GFE), RWTH Aachen University, Ahornstrasse 55
2. Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research, Materials Synthesis and Processing (IEK-1)
3. Physikalisch-Technische Bundesanstalt
4. University of Münster, Institute for Inorganic and Analytical Chemistry
5. Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons (ER-C), Research Centre Jülich
- Keywords
Dual-phase oxygen transport membrane, EDX, EELS, Gd-doped ceria, GdFeO3, STEM
- Abstract text
The dual-phase composite consisting of a stable ionic and electronic conductor has attracted much attention of the scientists in the field of the oxygen transport membranes for application in oxyfuel combustion process due to enhanced chemical and mechanical stability in harsh atmosphere with reasonable oxygen permeability [1]. In this study, the dual-phase composite based on the Gd-doped ceria, Ce0.8Gd0.2O2-δ (CGO), as an ionic conductor and the Fe-Co spinel, Fe2CoO4-δ (F2CO), as an electronic conductor was investigated using correlative (scanning) transmission electron microscopy ((S)TEM) techniques.
A dense composite of the nominal composition of CGO (60 wt%) and F2CO (40 wt%) phases was synthesized using the solid-state reactive sintering (SSRS) method, i.e. uniaxial pressing of corresponding amounts of CGO, Fe2O3, and Co3O4 powders into pellets and sintering at 1200°C for 5 hours. The cross-sectional specimen was prepared by focused ion beam (FIB). For the reduction of the amorphous surface layers damaged by high energetic Ga ion (accelerated by 30kV) the TEM specimen was polished at 5kV and finally at 2kV. The cross-sectional specimen was investigated using energy-filtering TEM (EFTEM), high-angle annular dark field (HAADF) imaging, energy dispersive X-ray (EDX) spectroscopy, and electron energy-loss spectroscopy (EELS) with the support of the aberration corrector. To precisely measure the atomic column positions, a fast sequential imaging was applied. The specimen drifts between image frames were corrected by the cross-correlation algorithm and then the corrected frames were averaged out. By applying this technique, the specimen drift could be significantly reduced with a reasonable signal-to-noise ratio. In addition, the FIB-SEM tomography (slice-image technique) was performed to investigate the 3-dimensional distribution of the formed phases.
All formed phases of the dual-phase composite pellet synthesized using SSRS method were identified by applying the EFTEM technique (Figure 1). The desired phases of CGO and F2CO were clearly found and additionally the formation of the secondary phase containing mainly Gd and Fe ions was evident at the grain boundaries, interconnecting the grains of the electronic conductor (F2CO).
The electron diffraction analysis of the secondary phase suggests the formation of the orthorhombic perovskite structure. To provide deep inside of the crystal structure of the formed phase, HAADF imaging technique was applied. HAADF imaging allows to directly observe the atomic structures with Z-contrast. The acquired HAADF image overlapped with the atomic structure of GdFeO3 (GFO) along the [001] direction confirmed the orthorhombic perovskite structure.
For detailed chemical composition analysis of this phase, a spatially resolved EDX spectroscopy was performed. The EDX elemental maps were retrieved using principal component analysis (PCA) available in the Hyperspy software package [2], suggesting the incorporation of the Ce into the Gd site of the orthorhombic perovskite structure.
In order to support this result, spatially resolved EELS technique was applied. To analyze the obtained stack of the EEL spectra the independent component analysis (ICA) algorithm implemented in the Hyperspy software package [2] was utilized. The obtained elemental maps and the corresponding EEL spectra are shown in the Figure 3. The 2-dimensional EELS elemental map and the corresponding EEL spectrum from the first component of the ICA analysis (Figure 3 (b)) confirms the mixture of the Ce at the Gd position, supporting the experimental result from the EDX elemental maps. Figure 3 (c) shows the elemental map and the associated EEL spectrum obtained from the second component of the ICA analysis. This result confirms that the Fe/O atomic columns contain no other element. In addition, there was significant deviation in the measured relative intensity ratio of the Fe-L23 white lines, compared to the reference value measured from the iron oxide with the valence state of Fe3+, suggesting the mixture of the Fe2+ and Fe3+ valence state.
In this study, at the grain boundary of the synthesized dual-phase composite, the formation of the orthorhombic perovskite phase (Gd1-xCexFeO3-δ) was clearly proved by correlative (S)TEM techniques. The mixture of the Fe2+ and Fe3+ valence state of this phase was evident, suggesting to the influence on the electronic conductivity.
Figure 1. The distribution of the phases formed from the CGO-F2CO pellet synthesized using SSRS method. EFTEM elemental map showed the CGO phase (ion conductor) and the F2CO phase (electronic conductor). In addition, the secondary phase consisting of mainly Gd and Fe ions was evident.
Figure 2. Phase identification with chemical composition analysis. (a) The HAADF image of the secondary phase shows a good agreement with the orthorhombic perovskite structure of the GFO along the [001] direction. (b) The EDX elemental map shows the incorporation of Ce into the Gd site of the perovskite structure.
Figure 3. ICA analysis for the stack of the EEL spectra obtained from the secondary phase using the Hyperspy. (a) The atomic model along the [001] direction of the orthorhombic perovskite phase, (b) the EELS map and the corresponding EEL spectrum from the first component of the ICA analysis (c) the EELS map and the EEL spectrum from the second component of the ICA analysis.
- References
[1] Madhumidha Ramasamy et. al. J. Am. Ceram. Soc., 99 (2016) p. 349–355
[2] Francisco de la Peña et. al. (2019, September 6). hyperspy/hyperspy: HyperSpy v1.5.2 (Version v1.5.2). Zenodo. http://doi.org/10.5281/zenodo.3396791
[3] The authors acknowledge funding from DFG (German research Society) within the project #387282673.
