Synthetic Approach to Hydrocarbon Proton Exchange Membranes Using Anion Exchange Membrane Precursors

Abstract

Proton exchange membranes (PEMs) are a crucial component to many electrochemical energy conversion devices, including low temperature hydrogen fuel cells and water electrolyzers. Perfluorosulfonic acid-based PEMs have overwhelmingly been the material of choice for these applications however, they are not ideal due to high cost, low glass transition temperature, and lack of synthetic diversity leading to unoptimized material performance. Hydrocarbon PEMs on the other hand have been explored as alternatives to perfluorosulfonic acid materials, although in general have lower proton conductivity, excessive water sorption and poor oxidative stability. Despite a large body of works focusing on hydrocarbon PEMs, most are based on aryl ether containing polymers such as poly(aryl ether sulfone). Recently, the PEM research community has shown that wholly aromatic polymer systems exhibit improved oxidative stability and restrict water uptake, allowing for higher ion exchange capacities. The synthetic routes of these polymer systems, however, are limited to sulfonation of aromatic unit through hash sulfonating conditions leading to ill-defined polymers or through complicated multistep synthetics routes, often requiring transition metal catalysts. Herein, we report widely applicable synthetic methods and exceptional properties of hydrocarbon based PEMs without any aryl ether linkages. To achieve this PEMs were synthesized from materials bearing a haloalkyl side chains. Since, anion exchange membranes (AEM) are commonly synthesized from haloalkyl bearing polymers and the polymer property requirements of PEMs and AEMs are similar, (i.e. no aryl ether linkages, high IEC, dimensional stability, ect.) this ushers in a new generation of stable hydrocarbon PEM materials from the vast library of stable AEMs. Furthermore, the synthetic procedures are based on simple SN2 and oxidation chemistry requiring no metal catalysts or harsh reaction condition. The resulting polymers show excellent conductivity and restricted water uptake at higher temperatures due to improved phase separation stemming from the attaching sulfonate group to a tethered side chain of polymer. Additionally, in highly oxidative Fenton’s reagent test, no discernable changes to the polymer performance of structure was observed.