RESUMO
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 â¼80% 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.
RESUMO
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.
