Proton conductive and low methanol permeable PVA-based zwitterionic membranes

A desirable membrane for direct methanol fuel cell (DMFCs) must both conduct protons and function as a methanol barrier. However, it remains challenge to balance the proton conduction and methanol permeability. As a commonly used pervaporation membrane, PVA has high selectivity for water over methanol, but its proton conductivity is too low for DMFCs applications. To improve conductivity, efforts have been predominantly focused on direct incorporation of strongly acidic groups, such as sulfonated or phosphoric groups on the PVA chain. This strategy can result in improvement in the proton conduction. However, the performance of acid modified PVA is compromised due to the strong charge-charge repulsion of attached groups, which results in the increased fraction free volume that facilitates methanol permeation. We attempt to minimize the tradeoff between proton conduction and methanol permeability by modifying PVA with zwitterionic groups. This is achieved by functionalizing PVA with pendent zwitterionic groups (PVA-ZW-n) at controllable substitution degree (5 < n < 20). The investigation of their 1H NMR spectra and glass transition temperatures (Tg) suggests that there is strong electrostatic interaction in the PVA-ZW-20 polymer, as confirmed by the observed proton spatial distribution in NMR, and increased Tg along with the increase of substitution degree in DSC. The synthesized PVA-ZW-20 can be easily casted into mechanically stable membranes with modest proton conductivity. After incorporation with (3-Mercaptopropyl)trimethoxysilane (MPTMS) and successive oxidation, the synthesized PVA-ZW-20-Si exhibits lower methanol permeability and reasonably higher proton conductivity compared to Nafion 117. A series of control experiments revealed that the superior performance of PVA-ZW-20-Si could be related to the presence of zwitterionic groups, which result in enhanced packing density of polymer chain and more efficient proton dissociation.