E. I. DuPont developed PFSA membrane in the mid-1960s under the brand name Naœon• (Connolly and Gresham, 1966). še original form of Naœon consists of copolymer of tetra›uoroethylene (TFE) and per›uoro(4-methyl-3,6-dioxa-7-octene-sulfonyl ›uoride) (Curtin et  al., 2004). še form sulfonyl ›uoride (–SO2F) is a thermoplastic resin that was originally used to make tick Naœon membrane (>125 µm). šis was done using an established extrusion-cast manufacturing process followed by a conversion to the sulfonic acid form (–SO3H) through a chemical process (Curtin et al., 2004; Smith and Withers, 1984). E. I. DuPont later developed a new manufacturing process based on PFSA solution casting technology intended to produce low defect and thin membranes and MEAs (Preischl et al., 2001). Figure 22.2 shows some chemical variations of commercial PFSA membranes that have been developed for PEMFC applications (Borup et al., 2007).