Abstract
Water and methanol transport behavior, solvents adsorption and electrochemical properties of filler-free Nafion and nanocomposites based on two smectite clays, were investigated using impedance spectroscopy, DMFC tests and NMR methods, including spin-lattice relaxation and pulsed-gradient spin-echo (PGSE) diffusion under variable temperature conditions. Synthetic (Laponite) and natural (Swy-2) smectite clays, with different structural and physical parameters, were incorporated into the Nafion for the creation of exfoliated nanocomposites. Transport mechanism of water and methanol appears to be influenced from the dimensions of the dispersed platelike silicate layers as well as from their cation exchange capacity (CEC). The details of the NMR results and the effect of the methanol solution concentration are discussed. Clays particles, and in particular Swy-2, demonstrate to be a potential physical barrier for methanol cross-over, reducing the methanol diffusion with an evident blocking effect yet nevertheless ensuring a high water mobility up to 130 °C and for several hours, proving the exceptional water retention property of these materials and their possible use in the DMFCs applications. Electrochemical behavior is investigated by cell resistance and polarization measurements. From these analyses it is derived that the addition of clay materials to recast Nafion decreases the ohmic losses at high temperatures extending in this way the operating range of a direct methanol fuel cell.
Highlights
The direct methanol fuel cell (DMFC) is a promising alternative power source [1,2] that combines the merits of direct hydrogen/air fuel cells with the advantages of a liquid fuel, such as convenient handling and high energy density
pulsed-gradient spin-echo (PGSE) and relaxometry in NMR are powerful methods that allow for the measurement of multicomponent diffusion and mobility on a molecular scale
Self-diffusion coefficients and spin-lattice relaxation times of methanol and water in Nafion and nanocomposite membranes were measured as a function of methanol solution concentration and temperature
Summary
The direct methanol fuel cell (DMFC) is a promising alternative power source [1,2] that combines the merits of direct hydrogen/air fuel cells with the advantages of a liquid fuel, such as convenient handling and high energy density. A number of alternative strategies [4,5,6,7] have been proposed to satisfy these requirements and to maintain membrane conductivity in a dehydrating environment (i.e., elevated temperature and reduced relative humidity), such as composite membranes containing finely dispersed hygroscopic inorganic oxides [8,9,10,11,12] or heteropolyacids [13]. The properties of these composite membranes depend on the nature of the ionomer and the solid used and on the amount, homogeneous dispersion, size, and orientation of the solid particles dispersed in the polymeric matrix. A preliminary electrochemical characterization was carried out on the filler-free and composite membranes by using ac-impedance spectroscopy and polarization measurements, in order to validate the NMR results
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