Monitoring changes in catalyst ink morphology during the primary drying process for polymer electrolyte fuel cells using cryogenic transmission electron microscopy

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To control the microstructure formation of a catalyst layer in a polymer electrolyte fuel cell for improving its performance, we observed the real morphology changes of the catalyst ink during the primary drying process using cryogenic transmission electron microscopy. Samples of the thin film catalyst ink, applied onto holey carbon grids, were prepared with different drying times (t = 0, 10, and 30 seconds). At t = 0, the sample exhibited platinum/carbon agglomerates surrounded by an ionomer thin film in the solvent. At t = 10, the ionomer rods became thicker, as confirmed by high magnification images. At t = 30 seconds, light spots were observed in the ionomer/solvent areas, indicating local thinning of the ionomer thin film due to solvent evaporation during the drying process. Moreover, as the drying time increased, the ionomer morphology became more visible with significantly thicker rods. This change in morphology can be attributed to the composition change of the solvent (fraction of water/normal propyl alcohol) in the catalyst ink during drying. Based on the new findings, this study discusses process design guidelines for cathode catalyst layers.