Classic Molecular Dynamics Study Of The Pt-Nafion Interface In The Catalyst Layer Of PEMFC

Currently, there are increasing research activities in polymer electrolytemembrane fuel cells (PEMFCs) due to their promising applications in diverse fields,such as automobiles, portable electronics, and stationary power generators. PEMFCshave the advantages of high energy density, zero emissions, and high theoreticalefficiency, which can make PEMFCs valuable alternatives to conventional fossilfuels.Structures and transport behaviors around the ionomer–catalyst interface inpolymer electrolyte membrane fuel cells (PEMFCs) have aroused great researchinterests for experimental scientists in recent years. This thesis used classicalmolecular dynamics simulation to investigate the interfacial self-assemblyphenomena of three hydrated Nafion films, whose thicknesses are determined to be2.4,5.0and7.3nm at fully hydrated situation (λ=23)on the platinum surface.Interestingly, it is found that in the vicinity of the platinum surface, there is anultra-dense adhesive ionomer layer with a thickness of0.5nm, whose compositionsare not affected by the hydration levels and film thickness. Due to the lack ofsulfonate groups, the Nafion ionomer in regions away from the Pt slab arereorganized in different patterns for films with different thicknesses. Besides this, wehave found a thickness-dependence of the wetability of the surfaces exposed to theair in these fully hydrated films. It is also shown that the transport properties ofhydronium ions and water molecules in the interfacial films are closely related tofilm morphologies. Water molecules in the5.0nm film are found to possess thelowest mobility as a result of the weakest connectivity of the hydrophilic channels,while in the7.3nm film, water diffusion is the fastest since the water channels aremost ideally connected throughout this film. Notably, though water molecules cannotbe retained inside the ultrathin2.4nm film, they could mostly develop into linearhydrophilic channels over the ionomer matrix, which can also provide transport pathways for hydrophilic species without interruption.