Silica–zirconium phosphate–phosphoric acid composites: preparation, proton conductivity and use in gas sensors

Abstract

Mesoporous composites made of silica and α-zirconium phosphate (SiO 2· xZrP) were synthesized starting from mixtures of delaminated ZrP dispersions and tetrapropylammonium oligosilicate solutions. The surface area of the composites reaches a maximum of 700 m 2/g for x≈0.02, while the average pore diameter is about 40 Å for x in the range 0.05–0.35. In order to increase proton conductivity at low relative humidity (r.h.), SiO 2· xZrP· yH 3PO 4 composites were prepared and characterised by 29Si and 31P MAS NMR and conductivity measurements. At 100 °C and 6% r.h., the conductivity of the composites, with H 3PO 4 loadings of 80% of the pore volume, rises from 5×10 −4 to 2×10 −2 S/cm for x decreasing between 0.35 and 0.05, as a consequence of the concomitant increase of pore volume. For the composite with x=0.18, the dependence of conductivity on H 3PO 4 loading was also investigated at different temperatures and r.h. values. The combined increase of humidity, temperature and H 3PO 4 loading results in an increase of conductivity from 1×10 −7 S/cm ( y=0.09, T=25 °C, 0% r.h.) to 4×10 −2 S/cm ( y=0.61, T=100 °C, 30% r.h.). SiO 2·0.18ZrP·0.61H 3PO 4 was also tested as a proton electrolyte in an oxygen sensor consisting of a disk of the composite sandwiched between a platinum sensing electrode and a reference electrode based on Ni 1− x O. The sensor is able to detect O 2 at room temperature in a dry environment with a response time of 20–30 s.