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

Mesoporous composites made of silica and α-zirconium phosphate (SiO2·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 m2/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.), SiO2·xZrP·yH3PO4 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 H3PO4 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 H3PO4 loading was also investigated at different temperatures and r.h. values. The combined increase of humidity, temperature and H3PO4 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.). SiO2·0.18ZrP·0.61H3PO4 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 Ni1−xO. The sensor is able to detect O2 at room temperature in a dry environment with a response time of 20–30 s.