Effects of Polymer Morphology on Proton Solvation and Transport in Proton-Exchange Membranes

The effects of polymer morphology on proton solvation and transport in hydrated Nafion are investigated by using a novel reactive molecular dynamics approach. Three of the most significant morphological models of Nafion, the lamellar model, the cylinder model, and the cluster-channel model, are studied. The three models exhibit distinct proton transport (PT) patterns, which result in different proton diffusion rates. In both the lamellar and the cylinder models, the interaction between protons and the sulfonate groups is shown to be the key factor in determining PT behavior. For the cluster-channel model, the geometrical shape also plays an important role in influencing the PT behavior. The change in the excess proton solvation structure as a function of the distance between protons and sulfonate groups is also analyzed. It is found that the increase of the water cylinder radius or water layer height leads to the presence of more protons around the sulfonate groups, while, for the cluster model, an increase in the water sphere radius leads to the presence of fewer protons around the sulfonate groups. Furthermore, for the lamellar and cylinder models, the hydrated protons around the sulfonate groups consist of more Zundel-like (H₅O₂⁺) structures when the hydration levels decrease, which is also influenced by the different morphological structures of Nafion.