Water diffusion in supramolecular cages revealed by ab initio molecular dynamics simulations

Proton-exchange membrane (PEM) fuel cells are one of the promising solutions for a zero-emission economy. Molecular cages enabling structural design to facilitate proton conduction have recently been proposed as promising candidates for the key PEM component. However, the proton transport dynamics and mechanism are difficult to understand by either experimental or simulation methods due to their structural complexity. We carried out ab initio molecular dynamics simulations on these large systems for hundreds of ps timescale. The results reveal that the two molecular cages formed from the same functional groups showed significant difference in interaction with water due to their difference in geometry. Each water molecule in Cage 1 with a spacious cavity form on average one less hydrogen bond. Therefore, Cage 1 has faster water dynamics, higher diffusion coefficient and much lower level of diffusion anisotropy. Our study shows the essential role of hydrogen bonding in water diffusion and reveals that the reasonable arrangement and passivation of strong electronegative atoms can further increase proton conductivity.