Three Dimensional Microstructural Characterization of Cathode Degradation in SOFCs Using Focused Ion Beam and SEM
In this work, we present a three-dimensional quantification of porous SOFC cathode materials that have been aged in both clean and contaminated atmospheres. Symmetric cathode cells were produced by standard screen printing methods using an yttria-stabilized zirconia (Y2O3-ZrO2, YSZ) electrolyte, and a composite cathode. The composite cathode consisted of 50% lanthanum strontium manganite (La1−xSrxMnO3, LSM) and 50% YSZ by weight. We report here the quantification of and differences between three YSZ/LSM-YSZ symmetric cells: unaged, aged for 480 hours in a clean air environment, and aged for 480 hours under H2O atmosphere. This quantification was performed using a dual-beam focused ion beam/scanning electron microscope (FIB/SEM) to serially mill 30nm slices of the cell, taking an image after each slice. In this way, a series of 2D images was acquired, aligned, and reconstructed into a fully quantifiable 3D volume rendering of the cell near the cathode/electrolyte interface, as shown in Figure 1.
After developing a 3D model for each cell, the volumes were quantified by a number of parameters, including LTPB, particle/pore size and distribution, surface area coverage, porosity, phase volume fraction, and tortuosity. Additional information relating to phase connectivity was calculated by generating a “skeleton” network for each phase, and was compared for each cell. The microstructure was then related to cell performance data acquired from the same samples. Furthermore, the detailed microstructure, compositional changes, and the formation of impurity phases due to the contaminants was characterized by transmission electron microscopy (TEM), utilizing both energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS).
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Figure 1: Serial 2D slices of YSZ electrolyte (left) and composite LSM-YSZ cathode (right) acquired in FIB/SEM (a) are reconstructed into a representative 3D volume surface model (b) for further quantification. Data is from unaged symmetric cathode button cell.