Perfluorosulfonic acid ionomers PFSIs

An adequate structure of polymer molecules promotes the advantageous phase separation into hydrophobic and hydrophilic domains upon water uptake. The most notable class of membranes based on this principle are the perfluorosulfonic acid ionomers (PFSI), Nafion [26] and similar membranes [27]. In these membranes, perfluorosulfonate side chains, terminated with hydrophilic —SO3H groups, are attached to a hydrophobic fluorocarbon backbone. The tendency of ionic groups to aggregate into ion clusters due to the amphiphilic nature of the ionomer leads to the formation of basic aqueous units. At sufficient humidity these units first get connected by narrow channels and then may even fuse to provide continuous aqueous pathways [28]. 

In spite of many PEMs developed and reported in literature, Nafion (a trade name by DuPont Co., which is a perfluorosulfonic acid ionomer [PFSI]) is considered the benchmark of PEMs [23,32], and it has the largest number of researchers devoted for its study. In the following sections, we discuss the morphology of Nation PEM (Sections 1.3.2.2.2 and 1.3.2.2.3) and sulfonated hydrocarbon PEM (Section 1.3.2.2.4). 

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