Functional fluoropolymers for fuel cell membranes

Various routes to synthesize functional fluoropolymers used in membranes for fuel cell applications are presented. They can be separated into three main families of alternatives. The first concerns the direct radical copolymerization of fluoroalkenes with fluorinated functional monomers. The latter are either fluorinated vinyl ethers, α,β,β-trifluorostyrenes or trifluorovinyl oxy aromatic monomers bearing sulfonic or phosphonic acids. The resulting membranes are well-known: Nafion®, Flemion®, Hyflon®, Dow®, Aciplex® or BAM3G®. The second route deals with the chemical modification of hydrogenated polymers (e.g. polyparaphenylenes) with fluorinated sulfonic acid synthons. The third alternative concerns the synthesis of FP-g-poly(M) graft copolymers where FP and M stand for fluoropolymer and monomer, respectively, obtained by activation (e.g. irradiation arising from electrons, γ-rays, or ozone) of FP polymers followed by grafting of M monomers. The most used M monomer is styrene, and a further step of sulfonation on FP-g-PS leads to FP-g-PS sulfonic acid graft copolymers. Other processes such as multilayer membranes or the introduction of fillers to prepare organic/inorganic or 'composite membranes' are reported. The electrochemical properties (ionic exchange capacity, conductivity, swelling-rate or water uptake) of membranes produced from fluoropolymers bearing sulfonic, carboxylic or phosphonic acid are presented and discussed.