Resolving macro- and micro-porous layer interaction in polymer electrolyte fuel cells using focused ion beam and X-ray computed tomography

In this study, the microstructure of a dual-layer polymer electrolyte fuel cell diffusion media (DM) sample is characterized using micro X-ray computed tomography and focused ion beam-scanning electron microscopy to determine the contributions of the gas diffusion layer (GDL) and the micro-porous layer (MPL) regions on the overall transport characteristics of the DM. The measured 3-D data is processed to extract the microstructure datasets of the MPL-only region, GDL-only region, and GDL–MPL assembly. These datasets are then analyzed using in-house microstructure analysis tools and a full pore morphology model to determine the water imbibition characteristics and the key structure-related transport properties, including the phase volume fractions, pore connectivity, tortuosity, diffusivity coefficient, pore size distribution, permeability, and capillary pressure–saturation curves. Results indicate that the structure of the MPL has a significant impact on the transport properties of the GDL–MPL assembly. When the GDL-only dataset is compared with the GDL–MPL assembly, the addition of the MPL is found to cause an increase in tortuosity of ∼10%, and a decrease of nearly 50% in the structural diffusivity coefficient of the overall GDL–MPL assembly. Furthermore, notable differences are observed in the water imbibition characteristics of the GDL and the MPL due to the differences in microstructure of these regions.