The effect of hydrogen bond reorganization and equivalent weight on proton transfer in 3M perfluorosulfonic acid ionomers

Abstract

We present an investigation into the energetics associated with proton transfer in ionomeric fragments of the 3M™ perfluorosulfonic acid (PFSA) membrane at different equivalent weights (EW). Electronic structure calculations were performed on two fragments each with two pendant side chains separated along a poly(tetrafluoroethylene) (PTFE) backbone 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⁻¹. Potential energy surface (PES) scans were performed for the transfer of a proton in various hydrogen bonds between water molecules, sulfonic acid groups, and charged species. The scans involved incrementally increasing an O–H bond length in steps of 0.02 Å with geometry optimizations performed at each step at the B3LYP/6-31G** level over all other degrees of freedom. The nature of the hydrogen bond network and the degree of dissociation were found to be critical factors in determining the resulting energetic barrier for proton transfer. The smaller fragment was found to more readily reorient to compensate for the transfer of charge resulting in a lower energetic barrier to proton transfer through stabilization of the new hydrogen bonds. However, when each ionomer had the same degree of dissociation, reprotonation of the sulfonate groups was the most energetically unfavorable in EW 590 when no structural reorganization was observed to occur due to its greater propensity to exist in a dissociated state.