Sulfonated hydrocarbon membranes for medium-temperature and low-humidity proton exchange membrane fuel cells (PEMFCs)

This review summarizes efforts in developing sulfonated hydrocarbon proton exchange membranes (PEMs) with excellent long-term electrochemical fuel cell performance in medium-temperature and/or low-humidity proton exchange membrane fuel cell (PEMFC) applications. Sulfonated hydrocarbon PEMs are alternatives to commercially available perfluorosulfonic acid ionomers (PFSA, e.g., Nafion^®) that inevitably lose proton conductivity when exposed to harsh operating conditions. Over the past few decades, a variety of approaches have been suggested to optimize polymer architectures and define post-synthesis treatments in order to further improve the properties of a specific material. Strategies for copolymer syntheses are summarized and future challenges are identified. Research pertaining to the sulfonation process, which is carried out in the initial hydrocarbon PEM fabrication stages, is first introduced. Recent synthetic approaches are then presented, focusing on the polymer design to enhance PEM performance, such as high proton conductivity even with a low ion exchange capacity (IEC) and high dimensional stability. Polymer chemistry methods for the physico-chemical tuning of sulfonated PEMs are also discussed within the framework of maximizing the electrochemical performance of copolymers in membrane-electrode assemblies (MEAs). The discussion will cover crosslinking, surface fluorination, thermal annealing, and organic–inorganic nanocomposite approaches.