On the evolution of sulfonated polyphenylenes as proton exchange membranes for fuel cells
The recent expansion in proton exchange membrane (PEM) research has been commensurate with the growth of PEM fuel cell research. Perfluorosulfonic acid (PFSA) ionomer materials remain the technological membrane of choice for PEMFCs because of their robustness, versatility of use, and widespread commercial availability. PFSAs, however, are far from ideal: they are prepared from fluorine-based chemicals that are under increasing environmental scrutiny, they are inherently expensive to prepare and dispose of, their ionic conductivity is limited, and they are highly permeable to gases. Numerous classes of ion-containing polymers have been investigated as potential replacements over the past decades, but PFSA remains the incumbent technology because hydrocarbon-based solid polymer electrolyte membranes are perceived to lack the oxidative stability of their fluorine-containing counterparts. A new era of hydrocarbon PEM research has recently emerged with an emphasis on the hydrocarbon membrane's inherent lower gas permeability and unanticipated stability in fuel cell applications. Of the various classes of polymer derivatives that hold promise, sulfonated polyphenylenes, devoid of heteroatom linkages in the main chain, are leading candidates. The absence of heteroatom linkages in the polymer backbone is not without penalty: their synthesis is challenging and processing restricted due to their rigid-rod character, coupled with the requirement to attach acid bearing groups in high concentration. This review focuses exclusively on the evolution of sulfonated polyphenylenes, from intractable rigid rods to architecturally-controlled, sterically-encumbered sulfo-phenylated polyphenylenes, with an emphasis on synthesis, precise molecular control, structure–property relationships, and ultimately, wide-scale adoption in fuel cells.