Morphological characterization of microporous layers for polymer electrolyte fuel cells

The microporous layer (MPL) in polymer electrolyte fuel cells is known to enhance performance and durability of the membrane electrode assembly. However, the design and functional understanding of the MPL has been predominantly empirical to date, as its key structural parameters and transport properties are difficult to determine due to the lack of MPL-specific measurement techniques. The present work aims to establish a specialized framework for 3-D image based morphological characterization for two types of MPL materials using carbon black (CB) nanoparticles and graphite particles. The proposed framework features a tuned focused ion beam-scanning electron microscope (FIB-SEM) imaging technique and an in-house developed procedure for image processing and reconstruction of the true 3-D structure which is validated with measured porosimetry data for a pure MPL sample. The effects of FIB milling parameters are evaluated and tuned to prevent image bias and damage to the delicate MPL structure. The pore size distribution of the reconstructed MPL structure is determined using a modified algorithm based on 3-D Euclidian distance transform (EDT) and the results are found to be in good agreement with the measured data. The 3-D reconstructed models containing the structural information are utilized as numerical domains for calculation of key MPL material properties, namely effective diffusivity, tortuosity, and effective thermal conductivity. By comparing the obtained material properties in the through-plane and in-plane directions, the CB MPL is determined to be isotropic, the fine graphite particle MPL to be transversely isotropic and the coarse graphite particle to be anisotropic. The obtained material properties are useful in the areas of modeling, material development, and membrane electrode assembly (MEA) design.