Ion pair reinforced semi-interpenetrating polymer networks for fuel cell polymer electrolyte membrane design

Abstract

Semi-interpenetrating polymer networks (SIPN) are a special class of polymer composites where one or more linear (or branched) polymers percolate molecularly into a network of other polymers. It provides a high degree of homogenization of the component polymers beneficial for the fabrication of polymer electrolyte membranes (PEMs) for fuel cell applications. For example a linear ionomer can be used as the proton source to infiltrate into a cross-linked hydrophobic polymer network which provides the mechanical properties. The interlocked polymer structure leads to good intermixing of the hydrophilic and hydrophobic components to support a distributed presence and good connectedness of the ionic clusters. Despite these promising prospects, SIPN membranes for fuel cell applications have yet to be satisfactorily demonstrated. In many of the studies, the network polymer was cross-linked by ester linkages. The susceptibility of the ester linkages to hydrolytic degradation in the acidic fuel cell environment compromises the SIPN performance. This study proposes a new SIPN design where ion pairs are used to reinforce the SIPN structure. The SIPN was synthesized from linear sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO), brominated PPO (BPPO), and ethylenediamine (EDA) cross-linker by a one-step thermal cross-linking procedure. Ion pairs were formed after cross-linking and strengthened the attachment of SPPO to the BPPO/EDA network. The cross-links in the membranes are chemically resistant. Measurements of membrane dimensional changes and hydrolytic stability confirmed the improvements made to the PPO membranes for fuel cell applications.