Three-Dimensional Analysis of Heterogeneous Microstructures in Solid Oxide Fuel Cell Electrodes
Creators: name: Rubayyat Mahbub
Publisher: name: Carnegie Mellon University
Year: 2019
Subjects: subject: 91299 Materials Engineering not elsewhere classified
subject: FOS: Materials engineering
subject: FOS: Materials engineering

description: Three-dimensional (3D) distributions of microstructural features in electrodes of solid oxide fuel cells (SOFCs) directly influence their electrochemical performance characteristics. Several mature methods exist to characterize 3D microstructures with nano scale resolution, but the total volume that can be reconstructed using these techniques, is limited to 10<sup>3</sup>–30<sup>3</sup> μm<sup>3</sup> and so can capture only a single average value for different morphological features within a specific microscale (≈ tens of microns) volume. However, quantifying the average microstructure features and linking them to performance is not sufficient to probe how distributions of morphological and electrochemical parameters and their variabilities, particularly in commercial SOFC microstructures affect the performance, including reliability and durability. Therefore, this work aims to quantify distributions of electrochemical parameters throughout the microstructure in heterogeneous commercial electrodes for better understanding the links between microstructure and degradation, failure, and performance. For quantifying distributions of features and characterizing their variabilities or heterogeneities of distinct length scales in commercial SOFC microstructures, new methods that can capture large volumes with nanoscale resolution allowing large number of observations on standard microscale volumes are needed. This work presents the first approach that allows for 3D reconstruction of SOFC electrodes at nanoscale resolution over incredibly large volumes, at least 50 times bigger than possible in a conventional Ga FIB-SEM serial sectioning, in the same amount of experimental time, using Xe-plasma FIBs (PFIBs). Using large volume mesoscale reconstructions of commercial electrodes (both anode and cathode) scanned using PFIB-SEM, the mesoscale distributions of microscale structural features are quantified and both microscale and mesoscale inhomogeneities are found. Effective medium theory models indicate that significant mesoscale variations in local electrochemical activity are expected throughout such electrodes, which can impact the cell reliability and also the durability over long time.<br>A semi-empirical model is also developed to quantitatively characterize electrode heterogeneities over the micro- and mesoscales, specifically between the relationship of the mean-squared normalized variance in the volume fraction, (𝜎𝛷𝑉𝛷̅𝑉)2, and the mean particle size normalized linear dimension, 𝐿𝑎̅𝑁. The model is developed using combined analysis from five large-volume physical reconstructions of several electrodes from commercial cells, using PFIB-SEM, and from unique sets of synthetic microstructures designed to have controlled distributions in particle size and local volume fractions. When comparing physical reconstructions from distinct regions of the same electrode, millimeter length scale heterogeneities are also observed, even in microstructures with limited mesoscale variability. While the model is developed using synthetic microstructures, it is used to quantify three different types of heterogeneities in the commercial cells. The potential origins are discussed with respect to variations in particle size distributions in feedstocks and to phase distributions related to fabrication processes. The characterization and analytical methodologies and model presented in this work will support the design and development of improved electrodes.<br>Build on the understanding of the heterogeneous nature of the commercial electrodes, this work also discusses the plausibility of carrying out 4D experimentations in the heterogenous SOFC electrode microstructures. Herein, methods are explored to determine morphological differences between 3D anode microstructures, recorded using PFIB-SEM from three large-scale regions (≈ 100<sup>2</sup> μm<sup>2</sup> in-plane and 6-9 μm in thickness, t) of two different commercial anodes subjected to different operational histories. Each reconstruction contains 50 – 100 rectangular prism sub-volumes that are similar to standard micro-scale volumes (12<sup>2</sup> μm<sup>2</sup> x t<sup>2</sup> μm), which are compared statistically using ANOVA methods. Surprisingly, variations in the local volume fractions and/or particles sizes render each of the three reconstructions statistically non-overlapping for many of their global averages, their mesoscale distributions (using t<sup>3</sup> sub-volumes), and their ANOVA sub-volumes. Thus, alternate quantitative methods are needed, even on such large 3D reconstructions for the heterogeneous electrodes. The self-similarity of the static (in SOFC operational conditions) YSZ backbones is demonstrated, with only particle size and long wavelength volume fraction variations differing between the reconstructions. Scale adjustment of the YSZ backbones allowed finding comparable ANOVA sub-volumes between the anode reconstructions, specifically those having statistically similar volume fractions and YSZ particle size distributions, potentially allowing for a relative change in Ni/pore distributions on operation to be explored. Comparison of statistically similar ANOVA regions revealed that the relative differences in Ni microstructural features are consistent with Ni coarsening and Ni de-wetting of the YSZ phase in the operated anode reconstruction, which decrease the reaction sites and thereby the performance of the cell. Comparison of the global mesoscale feature distributions within the voids of the identical YSZ backbones of the anode reconstructions supported the observations from ANOVA matches, and also revealed that the impact of both volume fractions and YSZ particle size differences on some feature distributions in the microstructure can be as important, or greater, than those from operation. Therefore, this work suggests that the spatial and time-dependent comparisons of electrode microstructures having different operational histories can be made in certain circumstances but must be approached cautiously as internal heterogeneities can severely impact specific quantitative conclusions. <br>