Nanostructures of Nafion Film at Platinum/Carbon Surface in Catalyst Layer of PEMFC: Molecular Dynamics Simulation Approach

Abstract

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. First, it is found that 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 present in a high concentration at the interface between the Nafion phase and the 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 the 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 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. 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 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 via the hydrogen bonding network.