Radiation-grafted fuel cell membranes base polymers

The significant contribution of Nafion or perfluorosulfonic membranes to the cost of the fuel cells stacks and the high alcohol crossover levels that affect the fuel efficiency, prompted the development of radiation grafted proton exchange membranes based on poly(ethylene-tetrafluoroethylene) (ETFE) [172-178], PVdF [175], andPTFE [179]. The peroxy radicals produced on the base polymer by y-ray, electron- or proton-beam, react with styrene to form a co-polymer that is then sulphonated. 

Among fluorinated polymers, PTFE is the least polymer used for preparation of radiation-grafted membranes despite its extraordinary chemical, thermal, and mechanical stabilities. This is due to PTFE s extreme sensitivity to high-energy radiation, which produces chain scission with a very small irradiation dose [87], Nevertheless, studies on preparation of fuel cell membranes based on PTFE films were reported in literature [27]. 

A publication by the Paul Scherrer Institute reports progress in preparing membrane/electrode assemblies for polymer electrolyte fuel cells based on radiation-grafted FEP PSSA membranes [95]. Hot-pressing with Nation was used to improve the interfaces. These improved MEAs showed performance data comparable to those of MEAs based on Nafion 112 (Figure 27.58) and an service-life in H2/O2 fuel cells of more than 200 h at 60°C and 500 mA cm. ... 

Brack HP, Biichi FN, Huslage J, Rota M, Scherer GG (2000) Development of radiation grafted membranes for fuel cell applications based on poly(ethylene-aZt-tetrafluoroethylene). In Pinnau I, Freeman BD (eds) Membrane formation and modification. ACS symposium series 744. Oxford University Press, New York, p 174 Brack HP, Scherer GG (1997) Macromol Symp 126 25 Brack HP, Biihrer HG, Bonorand L, Scherer GG (2000) J Mater Chem 10 1795 Walsby N, Sundholm F, KaUio T, Sundholm G (2001) J Polym Sci A 39 3008 Lee W, Shibasaki A, Saito K, Sugita K, Okuyama K, Suyo T (1996) J Electrochem Soc 143 2795... 

Ion-exchange membranes based on radiation grafting technology are produced by Solvay. Different fluorinated polymers explored for fuel-cell membranes have been reviewed by Kostova et al. [164]. 

Patents relating to the apphcation of radiation-grafted ion-exchange membranes in fuel cells have been granted to Scherer et al. [89] and to Stone and Stock [90, 91]. These patents mention the functionalization of base polymers with quaternary ammonium groups to yield alkaline polymers. The use of fluorine-substituted styrenic monomers is also claimed to improve membrane chemical stability when utilized in fuel cells (removal of undesired and reactive C-H bonds). 

Varcoe, J.R., Slade, R.C., Lam How Yee, E., Poynton, S.D., Driscoll, D.J., Apperley, D. C. (2007) Poly(ethylene-co-tetrafluoroethylene)-derived radiation-grafted anion-exchange membrane with properties specifically tailored for application in metal-cat-ion-free alkaline polymer electrolyte fuel cells. Chemistry of Materials, 19, 2686-2693. Slade, R.C., Varcoe, J.R. (2005) Investigations of conductivity in FEP-based radiation-grafted alkaline anion-exchange membranes. Solid State Ionics, Y16, 585-597. 

Furthermore, in 2001, Ballard entered an alliance with Victrex to produce two new membrane alternatives. One membrane is based on sulfonated poly(arylether) ketone (a variant of s-PEEK) supplied by Victrex, which may be better suited to PEM fuel cell fabrication applications. In March 2002, U.S. Patent 6359019 was issued to Ballard Power for a graft-polymeric membrane in which one or more trifluoro-vinylaromatic monomers are radiation graft polymerized to a preformed polymeric base. The structures of BAM membranes have been studied by way of small-angle neutron scattering or SANS. ° The study of the ionomer peak position suggests the existence of relatively small ionic domains compared to Nafion, despite large water content. Phase separation in the polymer matrix is possibly crucial for the manbrane s mechanical and transport properties. 

Patri, M., Hande, V. R., Phadnis, S., Somaiah, B., Roychoudhury, S., and Deb, P. C. (2004). Synthesis and characterization of SPE membrane based on sulfonated FEP-g-acryUc acid by radiation induced graft copolymerization for PEM fuel cell. Polym. Adv. Technol. 15, 270. 

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