  The morphology and transport properties of thin films of the ionomer Nafion,
with thicknesses on the order of the bulk cluster size, have been investigated
as a model system to explain the anomalous behaviour of
catalyst/electrode-polymer interfaces in membrane-electrode assemblies. We have
employed dissipative particle dynamics (DPD) to investigate the interaction of
water and fluorocarbon chains with carbon and quartz as confining materials for
a wide range of operational water contents and film thicknesses. We found
confinement-induced clustering of water perpendicular to the thin film.
Hydrophobic carbon forms a water depletion zone near the film interface,
whereas hydrophilic quartz results in a zone with excess water. There are, on
average, oscillating water-rich and fluorocarbon-rich regions, in agreement
with experimental results from neutron reflectometry. Water diffusivity shows
increasing directional anisotropy of up to 30% with decreasing film thickness,
depending of the confining material. The percolation analysis revealed
significant differences in water clustering and connectivity with the confining
material. These findings indicate the fundamentally different nature of ionomer
thin films, compared to membranes, and suggest explanations for increased ionic
resistances observed in the catalyst layer.
