Facile nonhydrolytic sol–gel route to mesoporous mixed-conducting tungsten oxide

Abstract

Mesoporous, mixed-valence tungsten oxides were synthesized by a straightforward, nonhydrolytic sol–gel method utilizing 1-butanol/tert-butanol mixtures as the gelling solvent. Composition of the alcohol mixture influences the mesoporous characteristics of final product. By tuning synthesis and processing conditions specific surface area up to 140 m2 g−1 was achieved with a pore volume in excess of 0.5 cm3 g−1 and fairly monodispersed pore-size around 18 nm. Presence of tungsten in WV-state besides WVI-state, as revealed by XPS, determines the existence of oxygen vacancies in the structure. This confers important n-semiconducting characteristics upon the materials as demonstrated by EIS. Oxygen-vacancy concentration can effectively be varied by tuning conditions of thermal treatment, with strong impact on the electrical properties. Electron conductivity of ca 20 S cm−1 was registered at room temperature for oxide subjected to treatment at 500 °C. Upon wetting or humidification, these materials also acquire remarkable proton-conduction characteristics. Proton conductivity in excess of 1.5 × 10−2 S cm−1 was measured at room temperature. As a result, these mesoporous oxides do exhibit concurrent electron and proton conduction properties under humidified conditions. Apparently, the porosity characteristics accompanied by mixed electron/proton conduction, make these materials especially appealing as conducting, mesoporous substrates utilizable in a variety of electrochemical systems, including fuel cells.