Proton conductivity of nanoporous anatase xerogels prepared by a particulate sol-gel method

Nanoporous anatase xerogels were prepared via particulate sol-gel processes. The calcined xerogels were mesoporous, with a BET surface area of 121 m(2)/g, an average pore diameter of 5.8 nm and a pore volume of 0.236 cc/g. Proton conductivity of the membranes was measured as a function of temperature and relative humidity (RH). When anatase membranes are treated at pH 1.5, the proton conductivity increased in the whole range of temperature and RH. It indicates that the surface site density (number of water molecules per square nanometer) of these materials has a strong effect on conductivity. The proton conductivity of the studied anatase xerogels followed an Arrhenius-like dependence on the temperature (from room temperature to 90 degrees C), in both treated and untreated membranes. A sigmoidal dependence of the conductivity on the RH was observed with the greatest increase noted between 58% and 81% RH in both treated and untreated anatase membranes. The highest value of proton conductivity was found to be 0.015 S/cm at 90 degrees C and 81% RH, for treated anatase ceramic membranes. An increase in the conductivity could be achieved by means of longer times of treatment. According to the activation energy values, proton migration in this kind of materials could be dominated by the Grotthuss mechanism in the whole range of RH. The similar values of proton conductivity, lower cost and higher hydrophilicity of these membranes make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane fuel cells (PEMFCs).