Fractal Mesoporosity and Proton Transport in WO3 Xerogels

Mesoporous WO3 xerogels with mixed proton/electron conductance were prepared by non-hydrolytic sol-gel route based on alcoholysis of WCl6 solutions. By varying alkyl-chain length and carbocation stability of the alcohol reagent, oxides with widely varying mesoporous structure were obtained. These materials were systematically analysed in terms of their surface fractal dimension as derived from nitrogen-adsorption isotherms according to the Frenkel-Halsey-Hill equation. The fractal dimension is shown to be a key parameter in controlling and tailoring the mesoporous properties of these xerogels: specific surface area (56–184 m2g–1), pore volume (0.06–0.35 cm3g–1) and average pore diameter (3.2–8.6 nm). Resistance of the mesoporous structure to thermal conditions was also found to be correlated with the fractal dimension. Using electrical-impedance spectroscopy, d.c. proton conductivities as high as 4.7×10–2 S cm–1 were measured at 25°C and 100% relative humidity. Proton-dynamics relaxation times and Cole-Cole exponents, as determined by fitting impedance data to a proper model circuit, are shown to be related to the fractal dimension. Importantly, a sharp transition from a fast- to a slow-transport regime was observed as this parameter increases beyond a critical threshold. It is discussed how the fractal dimension is crucial to understand proton transport as related to porous structure.