Investigation on the nanostructure of Nafion ultra-thin film at Pt and carbon catalyst layer of PEMFC*

In this study, hydrated Nafion film in the catalyst layer of the cathode for a polymer electrolyte membrane fuel cell is investigated using the molecular dynamics simulation method, exhibiting different structural characteristics on Pt and carbon surfaces. Water molecules, hydronium ions, and sulfonate groups are highly concentrated at the interfacial region between the Nafion phase and the Pt surface, whereas Nafion backbone chains are highly concentrated at the interface between the Nafion phase and the carbon surface. Pair correlation function analysis revealed that the water molecules and sulfonate groups in the hydrated Nafion phase are more associated with the Pt surface compared to the carbon surface due to their strong, attractive interactions with the Pt surface that makes the dimension of the hydrated Nafion phase 4–7% thinner on the Pt surface. Moreover, water-occupied volume analysis suggested that water molecules on the carbon surface can form a large-size water phase between the Nafion phase and the carbon surface because the Nafion–carbon interface is not tightly integrated due to their weak interaction. Related to these structural characteristics, the water diffusion and proton vehicular diffusion are suppressed in the interfacial region of the Pt surface due to the highly packed structures in the water phase as well as the polymer phase, in addition to the strong molecular interaction with the Pt surface, whereas the proton hopping diffusion is enhanced due to the well-developed organized water phase through the hydrogen bonding network.