Mesoporous, high-surface-area tungsten oxide monoliths with mixed electron/proton conductivity

Mesoporous WO(3) monoliths were synthesized by a revised sol-gel technique, hydrolyzing alcoholic solutions of WCl(6) in the presence of steam. Surface area and pore-size distribution were determined by nitrogen adsorption-desorption analysis. Transmission Electron Microscopy, Scanning Electron Microscopy and X-Ray Diffraction were employed for structural and morphological characterization. Oxide composition/stoichiometry as well as W oxidation states were assessed by Energy Dispersive X-Ray Spectroscopy and X-Ray Photoelectron Spectroscopy. These materials exhibit mesoporous, particulate structure with low crystallinity. Tungsten is mostly in the W(6+) state, although W(5+) is also present. Depending on the preparation conditions, surface area varies between 73 and 109 m(2) g(-1). Average pore diameter ranges between 4.7 and 10.9 nm, while pore volume ranges between 0.10 and 0.32 cm(3) g(-1). Electrical behaviour at several temperatures was investigated by Electrical Impedance Spectroscopy in anhydrous or wet condition. Importantly, simultaneous occurrence of both electron and proton conduction has been achieved and demonstrated for these mesoporous materials, which has implications for a variety of emerging electrochemical technologies, including Fuel Cells. At 30 degrees C electron conductivity is 2.0 x 10(-1) S cm(-1), independent of humidity conditions. At the same temperature, proton conductivity, as extracted by equivalent-circuit fitting, has a value of 4.7 x 10(-3) S cm(-1) in the wet state, whereas it is considerably lower in the dry state. An effective strategy to enhance the electrical properties, while retaining mesostructure is also presented and explored.