Three Dimensional Microstructural Characterization of Cathode Degradation in SOFCs Using Focused Ion Beam and SEM

Abstract

The limited durability of solid oxide fuel cells (SOFCs) in practical applications has impeded the commercial adoption of these devices. A primary component of this degradation occurs within the cathode upon long-term exposure to various contaminants, including H2O, CO2, and Cr vapor. These contaminants cause significant microstructural and compositional changes within the cathode that adversely affect activation, polarization mechanisms, and ionic and electronic conductivities. Previous works (Gostovic et al. and Smith et al.) have demonstrated that a number of quantifiable microstructural characteristics can be directly related to SOFC performance, the most important of these being triple phase boundary length (LTPB) and pore surface area. These parameters have not been examined during cell degradation, and further analysis under these conditions will provide insight into specific cell degradation mechanisms, informing future fabrication and operation criteria.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).1. Gostovic, D., Vito, N. J., O’Hara, K. A., Jones, K. S., & Wachsman, E. D. J. Am. Cer. Soc., 94(2), 620–627, (2011).2. Smith, J. R., Chen, A., Gostovic, D., Hickey, D., Kundinger, D. P., Duncan, K. L., & Wachsman, E. D. (2009). Solid State Ionics, 180(1), 90–98. 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.