Study on the Nanostructure of Nafion Ultra-Thin Film at Pt/C Catalyst Layer in PEMFC: Molecular Dynamics Simulation Approaches

Abstract

In this study, the hydrated Nafion film in the CL of the cathode for PEMFC is investigated using molecular dynamics simulation (MD) method, exhibiting different structural characteristics on the Pt and carbon surfaces. First, it is found that water molecules, hydronium ions, and sulfonate groups are highly concentrated at the interfacial region between the Nafion phase and Pt surface, whereas Nafion backbone chains are present in a high concentration at the interface between Nafion phase and carbon surface. Second, it is also found from pair correlation function analysis that the water molecules and sulfonate groups in the hydrated Nafion phase are more associated with the Pt surface compared to the carbon surface, which is due to their strong attractive interactions with the Pt surface that makes the dimension of hydrated Nafion phase 4–7% thinner on the Pt surface. Third, it is observed from water-occupied volume analysis that water molecules on the carbon surface can form a large-size water phase between the Nafion phase and carbon surface because the Nafion-carbon interface is not tightly integrated due to their weak interaction. In these structural characteristics, it is demonstrated that the water diffusion and proton vehicular diffusion are suppressed in the interfacial region of the Pt surface due to the highly packed structures in water phase as well as 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 via hydrogen bonding network.