Oxygen diffusion through commercial
Nafion films is investigated
using a transient electrochemical reduction method. Direct experimental
evidence is found for two Fickian diffusion coefficients corresponding
to oxygen permeation through the hydrophilic channels and fluorocarbon
matrix. The diffusion coefficient and solubility of oxygen in each
phase, which controls the overall permeability, can be tuned via thermal
annealing where films annealed at 160 °C have a substantially
reduced oxygen permeability relative to the as-received material.
Films annealed at 200 °C show intermediate oxygen permeation
to the as-received and 160 °C annealed samples. Differential
scanning calorimetry and wide-angle X-ray scattering are employed
to demonstrate that increasing crystallinity reduces the oxygen solubility
and diffusion coefficient through the Nafion matrix. The two oxygen
diffusion coefficients are discussed in the context of literature
values which span more than an order of magnitude. It is found that
the time scale of the experiments plays a substantial role in the
measured diffusion coefficient with short time scale (<30 s) experiments
sensing primarily diffusion through the hydrophilic channels. Longer
time scale experiments (>300 s) are able to sense both modes of
oxygen
permeation