PROTON-CONDUCTING SOLID DISPERSIONS OF SILICA AND ZIRCONIUM-PHOSPHATEPYROPHOSPHATE

Solid dispersions of alpha-zirconium phosphate pyrophosphate in porous silica have been prepared starting from mixtures of a tetrapropylammo nium oligosilicate solution with exfoliated alpha-zirconium phosphate. The treatment of the mixture with acetic acid led to the formation of a gel from which, after calcination at 650 degrees C to remove the or ganic moieties, the above solid dispersions were obtained. Their surfa ce areas decreased from about 500 to just a few m(2) g(-1) with decrea sing silica percentage, while the surface ion-exchange capacity showed a maximum for a Zr mole fraction of ca. 0.2. The conductivity depende nce on the zirconium content qualitatively reflects that of the surfac e ion-exchange capacity with a maximum (sigma = 3 x 10(-3) S cm(-1) at 100 degrees C and ca. 97% relative humidity) at a Zr mole fraction of 0.25, For this composition, conductivity measurements were also carri ed out as a function of relative humidity at room temperature and 100 degrees C; in both cases the conductivity decreased by about two order s of magnitude when the relative humidity decreased from 60 to 5%, whi le remaining at least an order of magnitude higher than that of crysta lline alpha-zirconium phosphate under the same conditions. On the othe r hand, the conductivity of the anhydrous dispersions was always lower than that of crystalline alpha-zirconium phosphate and decreased with increasing silica content.