Proton-conducting solid dispersions of silica and zirconium phosphate pyrophosphate

Abstract

Solid dispersions of α-zirconium phosphate pyrophosphate in porous silica have been prepared starting from mixtures of a tetrapropylammonium oligosilicate solution with exfoliated α-zirconium phosphate. The treatment of the mixture with acetic acid led to the formation of a gel from which, after calcination at 650 °C to remove the organic moieties, the above solid dispersions were obtained. Their surface areas decreased from about 500 to just a few m² g^–1 with decreasing silica percentage, while the surface ion-exchange capacity showed a maximum for a Zr mole fraction of ca. 0.2. The conductivity dependence on the zirconium content qualitatively reflects that of the surface ion-exchange capacity with a maximum (σ= 3 × 10^–3 S cm^–1 at 100 °C and ca. 97% relative humidity) at a Zr mole fraction of 0.25. For this composition, conductivity measurements were also carried out as a function of relative humidity at room temperature and 100 °C; in both cases the conductivity decreased by about two orders of magnitude when the relative humidity decreased from 60 to 5%, while remaining at least an order of magnitude higher than that of crystalline α-zirconium phosphate under the same conditions. On the other hand, the conductivity of the anhydrous dispersions was always lower than that of crystalline α-zirconium phosphate and decreased with increasing silica content.