Mesoscale simulations of confined Nafion thin films

TL;DR

This study uses dissipative particle dynamics to explore how confinement affects the morphology and transport properties of Nafion thin films, revealing water clustering, anisotropic diffusivity, and differences based on confining materials, which impact catalyst layer performance.

Contribution

It provides detailed mesoscale simulations of Nafion thin films under confinement, highlighting the effects of material and thickness on morphology and transport properties, which were previously not well understood.

Findings

Water forms clusters perpendicular to the film under confinement.

Hydrophobic and hydrophilic materials create different water zones.

Water diffusivity becomes more anisotropic as film thickness decreases.

Abstract

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.