SOL-GEL ROUTES TO MESOPOROUS TUNGSTEN OXIDES WITH MIXED ELECTRON/PROTON CONDUCTIVITY

The present thesis is focused on the development of novel, straightforward sol-gel techniques for the synthesis of highly mesoporous, mixed-conducting tungsten oxide monoliths and powders. Such materials are extremely interesting in view of potential applications for a variety of emerging electrochemical technologies, including electrode design in Polymer-Electrolyte-Membrane Fuel Cells. Both hydrolytic and non-hydrolytic methods are set up. The hydrolytic route is based on a proper steam-treatment as an effective way to control the supply of water molecules to the gelling phase and thus also the oxide formation rate, which is crucial in determining mesoporous features. The non-hydrolytic route is based on a metal halide/alcohol system and affords a variety of mesoporous frameworks. An extended investigation is carried out in order to establish a correlation between alcohol molecular structure and physical properties of final oxide materials. All samples are systematically characterized as to mesoporous properties, chemical composition and electrical properties. Mesoporosity is mainly investigated by means of nitrogen adsorption/desorption analysis, which allows determination of surface area and pore volume/size as well as surface fractal dimension. In particular, the fractal dimension is shown to be a fundamental parameter in controlling and tayloring the mesoporous properties. Additional structural information is obtained from Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (XRD). Chemical composition (non-stoichiometry) plays a key role in electron conduction and is studied by X-Ray Photoelectron Spectroscopy (XPS). Finally, electrical properties are subjected to a detailed quantitative inspection by means of Electrical Impedance Spectroscopy (EIS). Electron Conductivity is discussed in terms of hopping-transport models. Proton conductivity takes place in humid conditions according to the Grotthuss mechanism and can be extracted from EIS data by fitting with a proper equivalent circuit. Fractal dimension has a deep influence on proton dynamics and two well-distinct transport regimes are observed for rough and smooth oxide matrices. Based on preparation and processing conditions, the following important values can be achieved: surface area up to 184 m2/g, pore volume up to 0.56 cm3/g, fairly monodisperse pore diameter in the range 3 ÷ 20 nm, electron conductivity up to 20 S/cm and proton conductivity up to 47 mS/cm.