SPEEK-based Composite Membranes for Direct Methanol Fuel Cells
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SPEEK-based Composite Membranes for Direct Methanol Fuel Cells A.D’Epifanio 1,2, B.Mecheri1, F.C.Weise2, S.Greenbaum2, E.Traversa1, and S.Licoccia1* 1 NAST Center & Department of Chemical Science and Technology, University of Rome "Tor Vergata", Via della Ricerca Scientifica 1, 00133 Roma, Italy 2 Hunter College of the City University of New York, New York, NY 10065 USA ABSTRACT Sulfonated polyether ether ketone (SPEEK)-based composite membranes doped with 23 and 50 wt% of hydrated tin oxide (SnO2nH2O) were prepared and characterized for Direct Methanol Fuel Cell (DMFC) applications . The composite membranes were characterized in terms of ion exchange capacity (IEC), tensile tests, water uptake measurements, electrochemical impedance spectroscopy (EIS), and Pulsed-field-gradient spin-echo nuclear magnetic resonance (PFGSE NMR) spectroscopy. The body of results indicated the involvement of hydrated tin oxide in the proton conduction mechanism, the inorganic phase providing additional paths between the water clusters for proton transport. NMR measurements evidenced that SnO2 nH2O particles not only facilitate the proton transport but also hinder water diffusion. Therefore, due to the selectivity of SnO2nH2O for proton diffusion, the filler effect was found to be also beneficial for the reduction of methanol crossover through the SPEEK membrane and for improving the performance of the SPEEK membrane when used as electrolyte in a DMFC test station. INTRODUCTION Direct Methanol Fuel Cell (DMFC) technologies are receiving more and more attention due to their possible applications as power sources for vehicles, laptop computers and cellular phones [1]. Nafion is still the most widely used electrolyte for commercial applications despite major disadvantages limiting the large-scale commercialization of DMFCs [2]. As alternative to Nafion, ether bridged aromatic polymers have been widely investigated; among them poly ether ether ketone (PEEK) shows good thermal oxidation resistance, lower methanol crossover than Nafion, and high proton conductivity when sulfonated (SPEEK) [3]. However, the sulfonation process leads to undesirable swelling of the membrane and, therefore, to the decrease of its mechanical properties. Several approaches, including the preparation of organic–inorganic composites, have been followed for balancing good proton conductivity, high mechanical stability and low fuel permeability [4]. Our approach consists in preparing composite membranes incorporating hydrated tin oxide (SnO2nH2O) in a SPEEK matrix having a proper degree of sulfonation to ensure satisfactory proton conductivity values while avoiding membrane degradation. SnO2nH2O is a proton conducting material due to its partially protonated oxide-hydroxide surface [5]. This filler was then introduced at a large content (up to 50 wt%) with the aim of providing additional and selective paths for proton transfer while reducing methanol and water diffusivity.
EXPERIMENTAL Hydrated tin oxide (SnO2nH2O) was prepared according to the meth
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