Water transport properties of crosslinked-PTFE based electrolyte membranes

We measured the self-diffusion coefficient of water, D, in proton exchange membranes (PEMs) based on crosslinked polytetrafluoroethylene (PTFE) films by a radiotracer-permeation method using tritium labeled water (HTO). When the crosslinking density of the PTFE matrix increased, the D value became smaller. This decreasing trend coincided with that of a water-content parameter, λ, defined as the number of water molecules per sulfonic acid (SO3H) group. According to our proposed model of hydrophilic regions, the relationship between the D and λ indicates that the amount of free water in the crosslinked-PTFE PEMs could be lower than that in the non-crosslinked one, thereby restricting the mobility of water molecules through strong interactions between the majority of water and SO3H groups.