The effects of the hydrophobic environment on proton mobility in perfluorosulfonic acid systems: an ab initio molecular dynamics study

Abstract

Model systems of perfluorosulfonic acid (PFSA) polymers exhibiting regular shaped and idealized channel morphologies were constructed by functionalizing single walled carbon nanotubes (CNTs) with –CF₂SO₃H groups and adding from 1 to 3H₂O/SO₃H to investigate structural and chemical factors affecting proton dissociation and transport. No a priori assumptions about either the dissociation or hydration of the protons were assumed and extensive ab initio molecular dynamics (AIMD) simulations were performed and subject to analysis. The importance of the hydrophobic environment was assessed by comparing the hydration of the protons both with and without fluorine atoms attached to the CNT walls. The AIMD trajectories showed that dissociation of the acidic proton increased with increasing density of sulfonic acid groups; however, greater densities also brought about trapping of the dissociated proton. The fluorine atoms accepted hydrogen bonds from the water molecules, stabilized hydrogen bonding, and enhanced proton dissociation. The CNT systems without fluorination of the walls exhibited a propensity for the formation of Zundel cations (H₅O₂⁺), while the fluorinated systems favoured hydrated structures involving hydronium ions or hydrated H₃O⁺ species depending on the amount of water in the system.