Modifying Nafion with Nanostructured Inorganic Oxides for Proton Exchange Membrane Fuel Cells
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Modifying Nafion with Nanostructured Inorganic Oxides for Proton Exchange Membrane Fuel Cells Yusuke Daiko1,2, Lisa C. Klein1, Masayuki Nogami2 1 Ceramic and Materials Engineering Rutgers, The State University of New Jersey, 607 Taylor Rd., Piscataway, NJ 08854-8065, USA 2 Materials Science and Engineering, Nagoya Institute of Technology, Showa-ku Nagoya, 466-8555 Japan ABSTRACT Nafion, a perfluorosulfonate ionomer, was modified to increase its thermal stability and reduce its methanol permeability. Hybrid membranes of TiO2·SiO2/Nafion and TiO2·SiO2·P2O5/Nafion were prepared using an infiltration sol-gel method. Si(OC2H5)4 and Ti(OC4H9)4 were infiltrated into dry NafionTM membranes, followed by hydrolysis and condensation reactions in first HCl and then NH4OH solutions. The level of inorganic content was controlled by the infiltration time, incorporating up to 50 wt%. Solvent uptake, swelling, water content and proton conductivity were measured at room temperature. Hybrid membranes of TiO2·SiO2/Nafion with ~30 wt% of infiltrated oxides showed a significantly lower methanol uptake of ~20wt% and a swelling ratio of 1.15, as compared to those of unmodified NafionTM membrane, ~60wt% for methanol uptake and 1.8 for swelling ratio. Proton conductivities for TiO2·SiO2/Nafion hybrid membranes decreased with increasing infiltrated oxides. However, infiltrated membranes treated in phosphoric acid solutions to increase the number of P-OH groups showed a six-fold increase in proton conductivity. INTRODUCTION Fuel cell systems have attracted attention because of their ability to produce energy without the emission of harmful pollutants. In particular, direct methanol fuel cells (DMFCs) show good performance using proton exchange membranes such as Nafion [1,2]. Compared to fuel cell systems using hydrogen gas derived from methanol reforming, DMFCs have the advantage of being simpler to operate. However, DMFC technology has yet to overcome two technical issues. First, the anodic catalyst is not sufficiently active, leading to a high anodic over potential loss of about 350 mV as compared to about 60 mV when using hydrogen gas as fuel [3]. Recently, Rolison et al. reported encouraging results that Pt/C-silica composite aerogels show near 10,000 times higher anodic activities than that of unmodified Pt/C catalyst for methanol oxidation [4]. Second, the permeation of methanol through the polymer electrolyte membrane (crossover) occurs due to swelling. It is known that methanol crossover to the cathode not only lowers fuel efficiency but also adversely affects the oxygen cathode, resulting in lower cell performance. Recent approaches taken to limit the swelling of membranes include Nafion impregnated porous substrates [5], polystyrene-divinylbenzene mixtures radiation-grafted to poly(tetrafuoroethylene- co-hexafuoropropylene) [6], acid-base blend membranes [7,8], or triblock copolymer ionomer sulfonated poly(styrene-isobutylene-styrene) [9]. For the past few years, we have studied the preparation of fast proton–conducting P2O5·
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