Composite Inorganic Filler Based Electrolyte Membranes for Fuel Cells Applications
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Composite Inorganic Filler Based Electrolyte Membranes for Fuel Cells Applications
Antonino S. Aricò, Vincenzo Baglio, Alessandra Di Blasi, Vincenzo Antonucci CNR-TAE Institute, via Salita S. Lucia sopra Contesse 98126 Messina, Italy
ABSTRACT Various recast Nafion® composite membranes containing ceramic oxide fillers with different surface characteristics (SiO2, SiO2-PWA, Al2O3, ZrO2) have been investigated for application in high temperature direct methanol fuel cells (DMFCs). Cell resistance at 145 °C increases as a function of the pH of slurry of the inorganic filler indicating a strong influence of the acid-base characteristics on the electrolyte conductivity. This effect has been attributed to the different water retention capabilities of the various membranes. Fuel cell performance at 145 °C, expressed as both maximum power density and current density at 0.5 V cell potential, increases almost linearly as the pH of slurry of the oxide materials decreases. Appropriate selection of the surface properties for the inorganic fillers allows to enhance the proton conductivity and extends the operating temperature range of composite membranes. An infrared spectroscopic investigation of inorganic fillers employed in composite membranes has been carried out. The surface acidity of the fillers appears to influence the bending and stretching vibrational frequencies of the water physically adsorbed on the filler surface. Inorganic fillers characterised by acidic properties undergo a strong interaction with water and enhance the DMFC performance at high temperature.
INTRODUCTION Composite perfluorosulfonic membranes containing different types of inorganic fillers such as hygroscopic oxides [1-3], surface modified oxides [4], zeolites [5], inorganic proton conductors [6] etc. show an increased conductivity with respect to the bare perfluorosulfonic membranes and allow operation of DMFCs up to about 150 °C. At present, the mechanism enhancing proton conduction at such temperatures is object of debate [1-3, 7-8]. However, it is agreed on the fact that such effect is mainly due to the water retention capability of the filler [13]. In fact some of these compounds e.g. silica, zeolites etc. are frequently used as desiccant materials and they can be “reactivated” by desorbing water at temperatures around 150 °C [9]. This fact indicates that they can physically adsorb water on the surface or coordinate water by the crystallographic structure at temperatures slightly lower than 150 °C at ambient pressure. Since most of these inorganic materials have intrinsically low proton conduction up to 150 °C, they can be loaded in amounts up to 3-5% inside the membrane [10]. A proper distribution of the nanoparticle filler in the membrane water channels [11] can maximise the effect of water retention in the conduction path. In the present work, various composite membranes based on recast Nafion containing inorganic nanoparticle fillers (SiO2, phosphotungstic acid-impregnated SiO2, ZrO2, Al2O3) varying by their surface chemistry and a
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