(560d) The Effects of Confinement and Hydrophobicity on the Transport and State of Protons in PFSA Membranes
Perfluorosulfonic acid (PFSA) based membranes consist of a hydrophobic poly(tetrafluoroethylene) (PTFE) backbone with hydrophilic sulfonic acid-terminated perfluoroether pendant side chains. These materials rely heavily on the absorption of water to conduct protons. Hydration results in the formation of hydrophilic domains through solvation and aggregation of the sulfonic acid groups. Solvation of the acid groups leads to proton dissociation into the aqueous domain which facilitates long-range proton transport through the membrane. High proton conductivity in PFSA membranes is only observed at high levels of hydration and this has led to efforts to develop membranes with high proton conductivity at lower hydration allowing for higher temperature operation. The hydrophilic domains are typically only a few nanometers in diameter and hence the structural and dynamical properties of both the water molecules and the protons differ significantly from their properties when in bulk water. Furthermore, these properties are also influenced by the sulfonic acid group density, the level of hydration, and the hydrophobicity exhibited by the PTFE backbone. Proton transport in these materials is highly dependent on the structure and dynamics of hydrogen bonds, which are considerably impacted by nanoscale confinement. As such, an understanding of how the confined environment affects the nature of confined water is also needed.
Hence, to systematically elucidate all these factors (degree of confinement, acidity, water content, and hydrophobicity) on the transport properties in PFSA membranes we have undertaken ab initio molecular dynamics simulations of model systems consisting of single wall carbon nanotubes. This talk will describe our ongoing work to unravel the surprising and yet provocative results in these idealized systems.