Polymer electrolyte membranes (PEMs)
are representative systems
for the study of the proton conduction mechanism and water dynamics
in nanopores/channels. Our 1H nuclear magnetic resonance
data for Nafion PEMs, which are subjected to thermal degradation and
then swollen in water, indicate that (1) water is present next to
the side chains even after the removal of the SO3H groups,
(2) longer heat-treatment depletes more SO3H groups and
produces more CF2H groups, (3) the water near the side
chains allows for the liquid-like motion of the CF2H groups,
and (4) the motion correlates well with the content and dynamics of
water in the channels. As the thermal degradation progresses, the
Nafion membranes lose their ionic and hydrophilic nature due to the
conversion of CF2SO3H groups to CF2H groups. In addition, our results demonstrate that increasing channel
hydrophobicity leads to increased water dynamics in the channels.
The ionic conductivity of polymer electrolyte membranes (PEMs) is an essential parameter for their device applications. In water-swollen PEMs, protons and other ions are transferred through hydrophilic channels of a few nanometers in diameter at most. Thus, optimizing the chemical and physical properties of the channels can enhance the conductivity of PEMs. However, the factors controlling the conductivity have not been completely clarified. Here, we report that measurements taken near the channel walls by a special nuclear magnetic resonance technique with ≤1 nm spatial resolution showed the largest water diffusivity when ∼80% of hydrophilic sulfonic acid groups were blocked, but the proton conductivity was low. The water diffusivity was much less affected by differences in water content. Our results provide a concept for changing the properties of PEMs and a challenge to implement the improved diffusivity in a way that enhances net ion conductivity.
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