Poly(2,6-dimethyl-1,4-phenylene oxide)-based Semi-interpenetrating Polymer Network Proton Exchange Membranes for Direct Methanol Fuel Cells

Abstract

One of the barriers in the commercialization of direct methanol fuel cells (DMFCs) is high methanol permeability caused by the use of proton exchange membranes (PEMs) based on Nafion or other perflurosulfonate polymers. This thesis is aimed at developing alternative PEMs with high proton conductivity and low methanol permeability. Our design is based on ion pair-reinforced semi-interpenetrating polymer networks (SIPNs) where the interactions between the ion pairs in the covalently cross-linked network host and the impregnated linear (acidic) polymer are used to increase membrane chemical resistance and dimensional stability by strengthening attachment. The covalent cross-links in the host network also contain hydrophilic moieties to compensate for the partial reduction in proton conductivity due to ion pair formation. The effects of the cross-linker structure (i.e., length and bulkiness) on membrane morphology and proton/methanol selectivity were systematically studied. Detailed characterizations of the SIPN membranes revealed that the small and well-connected hydrophilic domains are conducive to high proton conductivity and low methanol permeability simultaneously. Since ion pair formation depletes some of the acid functionality of the linear polymer and consequently decreases the proton conductivity, the proton transport mechanism in acid-base cross-linked membranes was also investigated to seek an acid-base combination with minimum adverse effect on proton conductivity. In this regard the acid-base pairs formed with heterocyclic amines are clearly preferred to those formed with aliphatic amines.