How the Hydrophobic Interface between a Perfluorosulfonic Acid Polymer and Water Vapor Controls Membrane Hydration

Stable
hydration in perfluorinated polyelectrolyte membranes such
as Nafion is essential to maintain good ion conductivity and manage
permeation, especially in vapor-fed devices where water content depends
on relative humidity in a gas stream. Extensive studies in the literature
have shown that Nafion hydration in water vapor is controlled by its
interfacial transport resistance. Nafion forms a fluorine-rich layer
at the polymer–gas interface, and it has been proposed that
this layer blocks water transport due to its hydrophobicity. To develop
a molecular-level description of the physics underlying transport
resistance in this system, we have performed a computational reaction–diffusion
kinetics study of water evaporation from Nafion. Two distinct models
are examined, one mimicking the blocking function proposed in the
literature and the other assuming that there is no blocking, treating
instead water evaporation as a dynamic balance between uptake from
the gas and desorption from the polymer surface. Simulation results
are compared to time-dependent infrared data over a range of 100–0%
relative humidity from the literature. Only the dynamic model successfully
reproduces experimental observations. This indicates that the physical
nature of interfacial transport resistance is not slow diffusion across
an interfacial layer; rather, it is due to the competition between
dehydration and rehydration. The simulation data provide details on
the accompanying water distributions throughout the membrane and on
interfacial kinetics, showing that they are characterized by strong
fluctuations.