Hybrid Nafion-silica Membranes Doped with Heteropolyacids for DMFC Application

In the last years, Direct Methanol Fuel Cells (DMFCs) are gaining momentum as electrochemical power generators for application in electric cars. The choice of such low polluting technology is preferred with respect to similar energy systems, like batteries or hydrogen fed fuel cells, because of higher simplicity of design and suitable energy density. Nevertheless, a few problems inherent to the use of this technology are connected with the present electrolyte membranes. The electrolytes commonly utilized in DMFC systems are based on perfluorosulfonic polymers (Nafion and Dow); these dehydrate when the fuel cell is working at high temperatures and allow a large methanol cross-over. Dehydration and cross-over are detrimental effects for a proper fuel cell operation. The first drawback severely hinders the fuel cell operation at temperatures higher than 100 °C which is a prerequisite for the suitable electro-oxidation of small organic molecules involving the formation of strongly adsorbed CO-like intermediate species. The methanol cross-over causes a parasitic fuel consumption and lowers the electrochemical performance. In the present work we report on an electrochemical analysis of a direct methanol fuel cell based on hybrid Nafion membranes. Silica and heteropolyacids were used as inorganic additives of Nafion. Their action is to retain water at high temperature and to improve the kinetics of methanol electro-oxidation. The membranes were prepared by a recast film procedure. A 5% Nafion 1100 solution (Du Pont), purchased from Aldrich, was used as film precursor. The membranes were ex-situ characterized by various analytical techniques in order to evaluate their structure, proton conductivity and thermal stability.