3D Tomography of solid oxide fuel cells
This project was part of my Ph.D. research while at the University of Maryland. It
focused on analyzing the impacts of degradation mechanisms on solid oxide fuel
cell (SOFC) cathodes. Degradation of the cathode layer is one of the primary
means of failure for SOFC devices, so understanding this process is critical for
improving the resiliency of SOFC designs. This project was a collaboration with
Professor Eric Wachsman’s research group, and involved performing
three-dimensional nanotomography on the cathode samples using a FIB/SEM before
and after their operation in adverse chemical environments. This process
involves using the ion beam to mill extremely thin layers off of a sample (on
the order of 10–20 nm), taking an image of the cut surface with the SEM
after every slice. After the experiment, these 2D images are reconstructed into
a 3D volume, which is then probed for various microstructural parameters.
Some highlights of the project:
- Used FIB-SEM 3D tomography methods to reconstruct and probe SOFC cathode
- Developed innovative image processing and microstructure quantification
routines using Python and Avizo
- Quantified changes in SOFC cathode structures as a function of
H2O, CO2, and Cr-vapor exposure
In this work, we present a number of techniques to improve the quality of the acquired nanotomography data, together with easy-to-implement methods to obtain “advanced” microstructural quantifications. The techniques are applied to a solid oxide fuel cell cathode of interest to the electrochemistry community, but the methodologies are easily adaptable to a wide range of material systems.
The atomic-level structure and chemistry of materials ultimately dictate their observed macroscopic properties and behavior. As such, an intimate understanding of these characteristics allows for better materials engineering and improvements in the resulting devices. In our work, two material systems were investigated using advanced electron and ion microscopy techniques, relating the measured nanoscale traits to overall device performance. First, transmission electron microscopy and electron energy loss spectroscopy (TEM-EELS) were used to analyze interfacial states at the semiconductor/oxide interface in wide bandgap SiC microelectronics. This interface contains defects that significantly diminish SiC device performance, and their fundamental nature remains generally unresolved. The impacts of various microfabrication techniques were explored, examining both current commercial and next-generation processing strategies. In further investigations, machine learning techniques were applied to the EELS data, revealing previously hidden Si, C, and O bonding states at the interface, which help explain the origins of mobility enhancement in SiC devices. Finally, the impacts of SiC bias temperature stressing on the interfacial region were explored. In the second system, focused ion beam/scanning electron microscopy (FIB/SEM) was used to reconstruct 3D models of solid oxide fuel cell (SOFC) cathodes. Since the specific degradation mechanisms of SOFC cathodes are poorly understood, FIB/SEM and TEM were used to analyze and quantify changes in the microstructure during performance degradation. Novel strategies for microstructure calculation from FIB-nanotomography data were developed and applied to LSM-YSZ and LSCF-GDC composite cathodes, aged with environmental contaminants to promote degradation. In LSM-YSZ, migration of both La and Mn cations to the grain boundaries of YSZ was observed using TEM-EELS. Few substantial changes however, were observed in the overall microstructure of the cells, correlating with a lack of performance degradation induced by the H2O. Using similar strategies, a series of LSCF-GDC cathodes were analyzed, aged in H2O, CO2, and Cr-vapor environments. FIB/SEM observation revealed considerable formation of secondary phases within these cathodes, and quantifiable modifications of the microstructure. In particular, Cr-poisoning was observed to cause substantial byproduct formation, which was correlated with drastic reductions in cell performance.
Microscopy and Microanalysis,2015
Polarization losses associated with the cathode oxygen reduction reaction and degradation of cathode materials remain as hurdles for widespread implementation of solid oxide fuel cells (SOFC). Rates of degradation depend significantly on the operating temperature and gas conditions, such as the presence of unwanted oxygen- containing compounds, namely H2O and CO2. In this study we explore degradation mechanisms for a common composite cathode material, La0.6Sr0.4Fe0.8Co0.2O3-δ (LSCF) - Ce0.90Gd0.10O1.95 (GDC). Three-electrode cells have been tested under various temperatures, PO2s and contaminant conditions in order to observe changes through electrochemical impedance spectroscopy (EIS). EIS is a powerful tool, which allows us to identify changes in the reaction steps comprising the overall ORR. Our EIS results indicate a strong correlation between blocking effects, caused by CO2 and H2O, and the operating temperature of the cell. Using EIS to deconvolute the overall cathode polarization helps to identify the mechanisms by which degradation occurs.
An understanding of degradation mechanisms of SOFC cathodes under operating conditions is essential for the development of commercial, intermediate temperature (<700°C) SOFCs. Literature shows that the presence of H2O in the cathode impacts the performance of SOFCs. In this study, we attempt to determine the degradation mechanisms of the composite cathode, (La0.8Sr0.2)0.95MnO3±δ - (Y2O3)0.8(ZrO2)0.92 (LSM-YSZ) in an H2O environment based on a multi-faceted approach. LSM-YSZ/YSZ/LSM-YSZ symmetric cells were examined in the presence of the contaminant (H2O) under different cycling, polarization and working conditions. Symmetric cell performance was measured by in-situ electrochemical impedance spectrometry (EIS), and directly compared to quantitative microstructural parameters obtained from FIB-SEM 3D reconstructions. FIB-SEM is a powerful technique to quantify important performance characteristics such as triple phase boundary (TPB) length and surface to volume ratio. EIS and FIB-SEM results were compared to kinetic rate data, extracted from isotope exchange experiments, to determine mechanistic relationships.
Composite cathodes in solid oxide fuel cells (SOFCs) improve performance by increasing the triple phase boundary length and providing a more continuous pathway through the electrolyte for oxygen ion transport. These cathodes however, are susceptible to performance degradation from exposure to contaminants such as H2O and CO2 vapor. The microstructure and connectivity of yttria-stabilized zirconia (YSZ)/lanthanum strontium manganite (LSM) composite cathodes were examined and quantified using a dual beam focused ion beam/scanning electron microscope (FIB/SEM) in order to determine the effect of various contaminants on the performance of the SOFCs. Three-dimensional reconstructions of multiple composite cathodes allowed for microstructure quantification at nanometer resolution. Further analysis of triple phase boundary length (LTPB) demonstrated how the available active sites changed as a function of cell operation and contamination. This sort of analysis allows for a direct comparison between cathode microstructure and polarization resistance.
While impressive solid oxide fuel cell (SOFC) performance has been achieved, durability under “real world” conditions is still an issue for commercial deployment. In particular cathode exposure to H2O and CO2 can result in long-term performance degradation issues. Therefore, we have embarked on a multi-faceted fundamental investigation of the effect of these contaminants on cathode degradation mechanisms in order to establish cathode composition/structures and operational conditions to enhance cathode durability. Using a Focused Ion Beam (FIB)/SEM we are quantifying in 3-D the microstructural changes of the cathode before and after the onset of cathode performance degradation. This includes changes in TPB density, phase-connectivity, and tortuosity, as well as tertiary phase formation. This is then linked to heterogeneous catalysis methods to elucidate the cathode oxygen reduction reaction (ORR) mechanism to determine how H2O and CO2 affect the ORR as a function of temperature, time, and composition. By use of in-situ 18O-isotope exchange of labeled contaminants we are investigating whether oxygen incorporated in the lattice of LSM and LSCF, and their composites with YSZ and GDC, respectively, originated from ambient O2 or the contaminant, as well as intermediate adsorbed species and mechanisms that lead to degradation. The results will be used to develop a cohesive and overarching theory that explains the microstructural and compositional cathode performance degradation mechanisms.