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 W^V-state besides W^VI-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.