Quanititative Three-Dimensional Analysis of Solid Oxide Fuel Cell Electrode Microstructure Using Focused Ion Beam - Scanning Electron Microscopy

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This thesis demonstrates the use of dual-beam focused ion beam - scanning electron microscopy (FIB-SEM) for making complete three-dimensional reconstructions of SOFC electrodes in order to better understand the links between processing and performance with respect to microstructure. Sufficient compositional contrast, with nano-scale resolution, between Ni and LSM with respect to YSZ was demonstrated, and automated procedures for image processing were developed, allowing for rapid production of reliable three-dimensional structural data. Techniques for calculating pertinent microstructural characteristics such as the phase tortuosity, phase connectivity, triple-phase boundary density (TPB), and electrochemically-active triple-phase boundary density (EA-TPB) were established.</DISS_para> <DISS_para>This method was initially applied for proof of concept to the three-dimensional microstructure of a Ni - Y<sub>2</sub>O<sub>3</sub>-stabilized ZrO<sub>2</sub> (Ni-YSZ) anode active layer consisting of a standard composition of 50:50 weight % Ni:YSZ. The gas-phase tortuosity factor was found to be ≈ 2.0 and the volume-specific TPB length was found to be 4.28x10<sup>6</sup> m/cm<sup>3</sup>. Analysis of the connectivity of the phases found that lack of connectivity in the Ni and pore phases resulted in a reduction of the EA-TPB density by ≈12%.</DISS_para> <DISS_para>Subsequent reconstruction of a series of electrochemically characterized Ni-YSZ active layers with four different compositions varied between 40-70 weight % NiO was conducted. Calculations from the 3D reconstructions showed that the highest triple-phase boundary (TPB) density was at a Ni solids volume fraction of ≈0.42, which showed reasonable agreement with structural models for predicting TPB density. While the anode polarization resistance was minimized at the same composition where TPB density was maximized, implementation of electrochemical modeling showed that low EA-TPB density due to lack of connectivity and YSZ tortuosity both played relative roles in the resulting electrode polarization. Application of the reconstruction procedure was additionally conducted on two different cathode systems. Firstly, the microstructures of three different La<sub>0.6</sub>Sr<sub>0.4</sub>CoO<sub>3-δ </sub> (LSC) single-phase cathodes were reconstructed in order to determine the effect of surface area on the measured electrochemical impedance and to provide authentic microstructures for finite element electrochemical modeling. Secondly, the microstructures of a series of nine La<sub>0.8</sub>Sr<sub>0.2</sub>MnO<sub>3 </sub> (LSM) - YSZ composite cathodes with composition varied between 30-70 weight % YSZ were reconstructed and examined in detail. In conjunction with electrochemical modeling and the performance of the sample with 50:50 weight % LSM:YSZ, the intrinsic linear-specific charge transfer resistivity of the LSM-YSZ-air triple-phase boundary was estimated to be ≈ 2.5 x 10<sup>5</sup> Ωcm at 800°C. The calculated TPB densities were compared with those estimated from a particle packing model showing excellent agreement, with the maximum TPB density depending heavily on the particle size ratio between LSM and YSZ. Structural analysis showed large levels of un-connected LSM particles and YSZ tortuosities as high as 6.5, features that, when included in electrochemical modeling, helped explain the large mismatch observed between the TPB densities and the measured polarization resistances.

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  • 10/02/2018
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