<i>Ab Initio</i> Studies of Proton Transport in Proton Exchange Membranes

A molecular-level understanding of the factors that contribute to transport properties of proton exchange membranes (PEMs) for fuel cell applications is needed to aid in the development of superior membrane materials. Ab initio electronic structure calculations were undertaken on various PEM ionomer fragments to explore the effects of local hydration, side chain connectivity, protogenic group separation, and specific side chain chemistry on proton dissociation and transfer at low hydration. Cooperative interactions between both intra- and inter-molecular acidic groups and hydrogen bond connectivity were found to enhance proton dissociation at very low degrees of hydration. The energetics associated with proton transfer were highly dependent on the disruption of the hydrogen bond network where bond breaking, without an accompanying formation of a new bond, was strongly resisted. The effects of nanoscale confinement within different hydrophobic environments on structural and dynamical properties in PEMs were studied using ab initio molecular dynamics simulations on idealized systems of water molecules, slightly acidic water, and acid molecules confined in bare and fluorinated single-walled carbon nanotubes (CNTs) with different diameters. Inclusion of the fluorine atoms led to considerably different hydrogen bond structuring within the nanotube than in the bare CNTs. The water molecules in the fluorinated CNTs exhibited hydrogen bond-like interactions with the fluorine atoms resulting in a preferential, well-structured arrangement near the CNT surface. This was also observed with the addition of an excess proton where the proton shuttled between water molecules near the fluorinated walls rather than along the tube axis, as found in the bare CNTs. For aqueous triflic acid, proton dissociation depended on the level of hydration, the degree of confinement, and the surrounding environment. At the lowest hydration level, dissociation was most pronounced in the bare CNTs with little dissociation in the fluorinated systems regardless of the diameter. However, at the highest hydration level, the least amount of dissociation was observed in the larger diameter bare tube due to direct hydrogen bonding between triflic acid molecules with nearly complete dissociation in each of the others indicating an influence of the confinement and the fluorinated surface on hydrogen bonding and proton transfer properties.