Tara Boland1 Peter Rez1 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States

Intermediate temperature solid oxide fuel cells (IT-SOFC) are an attractive energy source that can be integrated into the current power generation infrastructure. IT-SOFC are capable of converting a wide range of carbonaceous fuels into energy. Doped CeO2 (ceria) has demonstrated potential as an electrolyte for these systems. However, resistive grain boundaries (GB) in these polycrystalline materials lead to a decrease in the total ionic conductivity. Doping is one avenue to tailor material properties and increase GB ionic conductivity. Experimental evidence has shown that doping at different nominal concentrations could result in increased GB ionic conductivity relative to the undoped samples [1]. This effect has been attributed to increases in local dopant/solute concentrations at the GB. Probing local structural motifs associated with low migration energies for oxygen ion transport across grain boundaries is a very challenging problem. Low migration energy is often associated with more open regions at the grain boundary core. Electron energy-loss spectroscopy (EELS) performed with a focused electron probe in a scanning transmission electron microscopy is sensitive to local changes in bonding and coordination. Such an approach may offer the potential to detect sites in grain boundaries with more open structures leading to higher oxygen transport.
To investigate this question, computational modeling is employed using molecular dynamics to identify high and low migration energies sites. The energy-loss spectrum associated with the sites are then calculated to look for changes in the near-edge fine structure of the oxygen K-edge. First, preliminary electron energy-loss spectroscopy (EELS) simulations will be performed to study the effects of tensile and compressive strain on the O K-edge from pure CeO2. Substitutional Ca atoms will also be simulated to gauge how dopant atoms effect the near-edge fine structure. The changes in spectra at low and high migration energy sites at grain boundaries will be explored to determine if there is an experimentally detectable difference. Select spectral simulation will be compared with experimental data from Ca doped grain boundaries [1]. These studies may provide insights into the utility of EELS to locate oxygen migration pathways at grain boundary cores.

[1] Bowman, W. J. et al. (2017) Nanoscale, DOI: 10.1039/C7NR06941C
[2] We gratefully acknowledge ASU’s HPC staff for support and assistance with computing resources, NSF grant DMR-1308085, and ASU’s John M. Cowley Center for High Resolution Electron Microscopy.