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290 A K Sahu et al rally as acidic metal alkoxides solution, into the Nafion ionomer for fabricating a composite membrane, and then converting the precursor material into the desired proton conducting oxide (Aparicio et al 2005; Lin et al 2005; Jiang et al 2006b). In most of the cases, solid polymer electrolyte, Nafion, is soaked in inorganic metal alkoxide precursor or alcoholic solution of one or more metal alkoxides till the desired amount of the inorganic filler permeates through the pores of Nafion (Miyake et al 2001; Adjemian et al 2002). After soaking the surface, the membrane is copiously rinsed to wash away surface metal alkoxide followed by hydrolyzing the metal alkoxide in the membrane with water. Incorporating the inorganic fillers into the polymeric matrix by above process may be inhomogeneous as, in such fillings, it is possible that some portions of the matrix may contain only a little oxide or no oxide at all. It is also possible to find enriched concentrations of filler particles in the bottom part of the membrane due to the sedimentation of heavier ceramic/inorganic fillers. As a result, the solid fillers in the composite membranes are devoid of imparting suffi- cient proton conductivity in the composite membrane under dry conditions. In the literature, hydrophilic zirconia (ZrO2) particles and its sulphate and phosphate forms have been consi- dered as active filler materials with Nafion for the above stated purpose. The water uptake and proton conduction properties of the composite membrane depend on size and structure of the materials embedded with them. Sulfate- promoted super-acid zirconia (S-ZrO2) has been reported as an active ceramic filler with Nafion membrane to increase the water uptake property of the membrane and provide additional acidic sites for proton diffusion (Hogarth et al 2005; Thampan et al 2005; Zhai et al 2006; Navarra et al 2007). Zirconium phosphate as hydrophilic and proton conducting material has been incorporated into various polymer matrices (Bauer and Porada 2005; Hill et al 2006; Jiang et al 2006a; Boutry et al 2007). It is also noteworthy that proton conduction in these materials predominantly takes place by surface transport through the interlayer regions in the presence of water. Nafion–zirconium phosphate composite membranes have been fabricated by a variety of techniques (Damay and Klein 2003; Alberti et al 2005). Conventionally, water absorbing pre-formed fine zirconia powder is dis- persed in Nafion ionomer solution to form Nafion– zirconia composite membrane. The composite membrane is then treated in phosphoric acid followed by drying at suitable temperature for forming Nafion–zirconium phos- phate composite membrane. Another method includes cation exchange between the ionomer and zirconium cationic species followed by membrane treatment with phosphoric acid to precipitate zirconium phosphate within the hydrophilic regions of the ionomer. In an in situ preparation of the zirconium phosphate, phosphoric acid is reacted with either zirconyl chloride or with zirco- nium alkoxide at desired temperatures; zirconium phos- phate, thus prepared, is embedded within Nafion ionomer to form a composite membrane. Incorporating zirconium phosphate into the polymeric matrix by the aforesaid processes could result in a non- homogeneous matrix with such fillers. It is also likely that phosphoric acid may not be covalently bound within the membrane. It is also possible to end-up with enriched concentrations of the phosphoric acid on the surface of the composite membrane, which when operated in the PEFC may cause leaching of the phosphoric acid along with the product water. Among the various aforesaid techniques for preparing Nafion-mesoporous zirconium phosphate composite membranes, only the in situ prepa- ration of zirconium phosphate for embedding with Nafion ionomer to form a composite membrane appears attrac- tive for avoiding the presence of any surface bound acid. But, even in this preparation, both low porosity and low internal-surface-area of zirconia particles limit the hydro- philicity of the composite membrane. In order to increase the porous nature of zirconium phosphate, efforts have been expended to develop mesoporous zirconium phos- phate (MZP) materials either by hard-template approach or by surfactant-assisted route (Davis et al 1988; Dessau et al 1990; Estermann et al 1991; Haushalter and Mundi 1992; Khan et al 1996; Sayari et al 1996; Jimenez et al 1998; Castellon et al 1999). MZP as a surface-functio- nalized solid-super-acid-proton-conducting medium as well as inorganic filler with high affinity to absorb water helps fast proton-transport across the membrane electrolyte sui- table for PEFC operation especially at low RH values. 3.2 Nafion–silica composite membranes as electrolytes for PEFCs To obviate the aforesaid limitations, preparation of Nafion– silica composite membranes by embedding silica parti- cles as inorganic fillers in perfluorosulfonic acid ionomer by a novel water hydrolysis process (Sahu et al 2007) is reported. In this process, a homogeneous, transparent and less viscous inorganic sol is first prepared by controlled water hydrolysis to silicon alkoxide without any external acidic or basic environment. Subsequently, the required amount of the sol is incorporated into the polymer matrix. The less viscous sol enters the fine pores of perfluorosul- fonic acid (PFSA) and due to acidic nature of the latter forms Si–OH network in the pores, which on heating at 90°C under vacuum form Si–O–Si linkages in the compo- site membrane. A transparent polymer film is thus obtained without any particle/phase segregation. The composite membranes have been tested in hydro- gen/oxygen PEFCs at varying RH between 100% and 18% at elevated temperatures at atmospheric pressures. Performance of H2/O2 PEFC employing Nafion–silica composite membranes is studied by obtaining the fuelPDF Image | Nafion and modified-Nafion membranes for polymer fuel cells
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