Impedance spectroscopy characterization of functionalized alumina membranes

Anodic alumina membranes have been impregnated with a protonic conductor either by immersion or by vacuum permeation of a saturated aqueous solution of CsHSO4 for different times. Synthetized salt, obtained through the reaction of cesium carbonate with sulphuric acid (in excess), contained a small quantity of Cs2SO4. Unmodified membranes consist of amorphous Al2O3 with a regular distribution of pores (average diameter: 200 nm) and are stable up to 850 °C. Long impregnation times caused partial dissolution of alumina, with formation of Al(HSO4)3 on the front surfaces as well as into pore walls. From the frequency dispersion of the impedance, the “macroscopic conductivity” of membranes after different treatments was derived as a function of temperature, and compared with those relative to both unmodified membranes and the pure protonic conductor. The latter shows a sudden increase in conductivity at T > 140 °C, where a superprotonic transition occurs according to DTA analysis.