Hydration and Proton Transfer in 3MIM PEM Ionomers: An Ab Initio Study

摘要

Electronic structure calculations were performed to study the effects local hydration, neighboring side chain connectivity, and protogenic group separation have in facilitating proton dissociation and transfer in fragments of 3M ionomers under conditions of low hydration. Two different types of ionomers, each consisting of a poly(tetrafluoroethylene) (PTFE) backbone, were considered: (1) perfluorosulfonic acid (PFSA) ionomeric fragments containing two pendant side chains (-O(CF_2)_4SO_3H) of distinct separation along the PTFE backbone to model different equivalent weight ionomers and (2) single side chain fragments of three bis(sulfonyl imide) based fragments with multiple and distinct acid groups per side chain having structural and chemical differences mediating protogenic group separation (side chains: -O(CF_2)_4SO_2(NH) SO_2C_6H_4SO_3H) with the sulfonic acid group located in either the meta or the ortho position on the phenyl ring and -O(CF_2)_4SO_2(NH)SO_2(CF_2)_3SO_3H). Fully optimized structures of these fragments with and without the addition of water molecules at the B3LYP/6-311G** level revealed that both side chain connectivity and protogenic group separation, along with local hydration, are key contributors to proton dissociation and the energetics of proton transfer in these materials. Specifically, cooperative interaction between protogenic groups through hydrogen bonding and electron withdrawing -CF2- groups are critical for first proton dissociation and the state of the dissociated proton at low levels of hydration. However, the close proximity of protogenic groups in the ortho bis acid precluded second proton dissociation at low hydration as the relatively fixed protogenic group separation promoted interactions between water molecules, while the labile side chains in the PFSA ionomers allowed for greater freedom in the hydrogen bond network formed. Potential energy profiles for proton transfer were determined at the B3LYP/6-31G** level. The energetic penalty associated with proton transfer was found to be strongly dependent on the surrounding hydrogen bond network and the state of the dissociated proton(s), as well as, the separation between protogenic groups.