Hydrogen bonding in narrow protonated polymer electrolyte pores

Proton conductivity in fuel cell membrane materials such as Nafion® decreases dramatically with decreasing water content. At very low water content proton transport is thought to occur through narrow necks, which can be either static or fluctuatively formed temporarily. In the present work we investigate the properties of hydrogen bonding and protons in a one-dimensional narrow model pore by using ab initio Car-Parrinello molecular dynamics. The pore consists of eight suitably arranged CF3-CF3 and four CF3-SO3H entities and is filled with water at varying water content λ (the ratio between the number of water molecules and the number of sulfonic acid groups) between 2.5 and 4.5. Proton mobilization in this pore occurs in two steps. First, around λ = 3 sulfonic acid groups dissociate to form sulfonate groups and hydronium ions which form mostly contact ion pairs. Second, increasing the water content to λ = 4.5 leads to an increase of the population of Zundel-like H5O2+ configurations with more or less symmetrically shared protons. Simultaneously, the number of hydrogen bonds increases and the hydrogen bond network becomes more liquid-like.