Polysulfone based ionomers

Presently, there has been a lot of interest generated for the production of polysulfones based ionomers for their utilization as polymer electrolyte membrane in High Temperature-PEMFCs (HT-PEMFC) due to their very high thermo-mechanical and chemical stability which are required for HT-PEMFC application. 

In addition to Nafion-based catalyst layers, additional types have been developed, including CLs with different ion exchange capacities (lECs) [57,58] or with other hydrocarbon-type ionomers such as sulfonated poly(ether ether ketone) [58-60], sulfonated polysulfone [61,62], sulfonated polyether ionomers [63], and borosiloxane electrolytes [64], as well as sulfonated polyimide [65]. These nonfluorinated polymer materials have been targeted to reduce cost and/or increase operating temperature. Unfortunately, such CLs still encounter problems with low Pt utilization, flooding, and inferior performance compared wifh convenfional Nafion-based CLs. 

It should be emphasized that in the Nafions, as in other polymers, and especially ionomers, the glass transition temperature can be strongly influenced by the thermal history and the moisture content of the polymer. Furthermore, in the present case, some decomposition can be seen at ca. 190°C in the acid samples, which show considerably lower thermal stability than is observed in the salts. These results are consistent with those reported earlier by Yeo and Eisenberg (31), based on weight loss in thermogravimetric studies. This feature appears to be a common phenomenon in sulfonated systems for example, in the sulfonated polysulfones, improved thermal stability is also observed in the neutralized materials (2). 

The next two decades saw the development of new polymers such as thermoplastic PU (1961), aromatic polyamides, polyimides (1962) polyaminimides (1965), thermoplastic elastomers (styrene-butadiene block copolymers in 1965), ethylene-vinyl acetate copolymer, ionomers (1964), polysulfone (1965), phenoxy resins, polyphenylene oxide, thermoplastic elastomers based on copolyesters, poly butyl terephthalate (1971) and polyarylates (1974). 

This chapter is a review focussed on the development of ionomers based on aromatic polysulfones for their application as Polymer Electrolyte Membrane (PEM) in Proton Exchange Membrane Fuel Cells (PEMFC) or in Direct Methanol Fuel Cells (DMFC). Different types of synthesis routes have been discussed in this chapter in order to obtain ionomers based on polysulfones with variation in structural designs. Special attention is given to the impact of the structural design of the ionomer on various properties such as membrane morphology, thermo-mechanical stability and protonic conductivity of the membranes for their utilization as PEMs. 

Lafitte et al. [45] reported polysulfone ionomers functionalized with benzoyl(difluoromethylenephosphonic acid) side chain (bfp-PSU) as an alternative to sulfonic acid based PSU ionomers shown in Scheme 4.8(a) and Table 4.2. The degree of phosphonation (DP) was achieved up to 53% and this membrane took higher amount of water (6%) tmder immersed state at room temperature compared to membrane with phosphonic acid directly attached to main-chain at ortho-to-sulfone position taking 2% water as discussed before. The probable reason is the increased acidity of the phosphonic acid unit. The thermal stability was found to be inferior to sulfonated derivatives due to the presence of aryl -CF2-P linkage [45]. 

---