The effect of side chain connectivity and local hydration on proton transfer in 3M perfluorosulfonic acid membranes

We present a molecular modeling study on the role the connectivity of adjacent acid groups of 3M perfluorosulfonic acid (PFSA) membranes has on proton dissociation and transfer through a consideration of oligomeric fragments with different poly(tetrafluoroethylene) (PTFE) backbone segments separating the side chains. Electronic structure calculations were performed at the B3LYP/6-311G** level of theory on fragments with chemical formula: CF3CF(–O(CF2)4SO3H)(CF2)nCF(–O(CF2)4SO3H)CF3, where n = 5 or 7, corresponding to membrane equivalent weights of 590 and 690 g/mol, respectively. Fully optimized structures of these fragments with and without the addition of water molecules revealed that connectivity of the SO3H groups through hydrogen bonding is critical for proton dissociation and the state of the dissociated proton. Proton dissociation was first observed in the EW 590 fragment at a water content of only 1 H2O/SO3H; the system with greater separation of the side chains (EW 690) did not exhibit proton dissociation until four water molecules (i.e., 2 H2O/SO3H) were added as the greater side chain separation precluded the cooperative interaction through hydrogen bonding that promotes proton dissociation at low hydration in membranes of this type. Second proton dissociation in the EW 590 system occurred upon addition of three water molecules; this required five water molecules in the EW 690 fragment. The differences in dissociation and the state of the dissociated proton were mitigated after six water molecules were added to each system where one of the dissociated protons exists as a ‘Zundel-like’ cation and the other as more of an ‘Eigen-like’ complex. These calculations further substantiate prior work on the importance of the interaction of sulfonic acid groups through hydrogen bonding in the transfer and state of the dissociated protons.

► SO3H group connectivity through hydrogen bonding is critical for proton dissociation.
► Less side chain separation leads to continuous hydrogen bonding at low hydration.
► Differences in dissociation are mitigated at higher hydration.