Structures and transport behaviors around the ionomer-catalyst interface in polymer electrolyte membrane fuel cells (PEMFCs) have aroused great research interests in recent years. Herein, classical molecular dynamics simulation method is used to investigate the interfacial self-assembly phenomena of three fully hydrated (lambda = 23) Nafion films with thicknesses of 2.4, 5.0 and 7.3 nm on the platinum surface. Interestingly, it is found that in the vicinity of the platinum surface, there is an ultra-dense adhesive ionomer layer with a thickness of 0.5 nm, whose compositions are not affected by the hydration levels and film thickness. Due to the lack of sulfonate groups, the Nafion ionomer in regions away from the Pt slab are reorganized in different patterns for films with different thicknesses. Besides this, we have found a thickness-dependence of the wetability of the surfaces exposed to the air in these fully hydrated films. It is also shown that the transport properties of hydronium ions and water molecules in the interfacial films are closely related to film morphologies. Water molecules in the 5.0 nm film are found to possess the lowest mobility as a result of the weakest connectivity of the hydrophilic channels, while in the 7.3 nm film, water diffusion is the fastest since the water channels are most ideally connected throughout this film. Notably, though water molecules cannot be retained inside the ultrathin 2.4 nm film, they could mostly develop into linear hydrophilic channels over the ionomer matrix, which can also provide transport pathways for hydrophilic species without interruption.
