Formation and Characteristics of Microstructures from PEM Fuel Cell Catalyst Ink Drying
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In this study, the process of catalyst ink drops drying in room air is investigated experimentally, which is key to the fabrication of, hence the microstructure formation in, the catalyst layers of polymer electrolyte membrane fuel cells (PEMFCs). State-of-the-art catalyst layer in PEMFCs is typically made through the drying of catalyst ink, which is a solvent based colloidal solution with suspended aggregates of nano catalyst particles supported on meso carbons (e.g. C/Pt) and perfluorosulfonic polymer (e.g. Nafion®). Catalyst ink samples are prepared using various Nafion loadings while the C/Pt loading and solid content (C/Pt and Nafion) are held constant. The real-time drying process is observed through an optical microscopy and the dried deposition is investigated with a scanning electron microscopy (SEM) and the optical microscopy. The dried pattern characterizations show the C/Pt agglomerates are more uniformly deposited as the Nafion loading is increased: the deposition at the edge becomes thicker and wider; unique pedal-like pattern with uniform distance is aligned along the inner side of the thicker edge; and the microstructure pattern transits from the concentric rings toward the drop center to heterogeneously detached aggregations. As the Nafion loading is increased, the evaporative cooling induced thermo-capillary flow in the drop varies significantly, from convective outward flow combining with Bernard-Marangoni cell at the center of the drop, to reversed flow coupling with three-dimensional thermo-capillary waves in the annular region near the edge of the drop, which dictates C/Pt agglomerates transport and leads to varies deposition. This study provides insight into the fundamental understanding of complex drying dynamics during catalyst ink drying, and laid theoretical foundation for further studies on achieving CL microstructure with increased catalyst utilization.