Effect of silica on the thermal behaviour and ionic conductivity of mixed salt solid acid composites

Abstract

The effect of adding silica (SiO2) to a mixed-salt solid acid composite, CsH2PO4/NaH2PO4, was studied via conductivity measurements, thermal analyses and X-ray diffraction. The conductivity value of the composites ranged from 1.1 × 10−4 S cm−1 to 7.6 × 10−3 S cm−1 for temperatures from 210 °C to 250 °C, respectively. Adding NaH2PO4 to the composite reduced the conductivity value at 230 °C due to the decomposition of NaH2PO4, which initiated at 210 °C. Dehydration of CsH2PO4 was evident at 230 °C and 270 °C, while the superprotonic phase transition was not observed. Dehydration was detected for NaH2PO4 at temperatures above 210 °C, but a thermally stable composite was produced by mixing it with CsH2PO4 and SiO2. An X-ray diffraction analysis confirmed the existence of solid acid solutions for the composites consisting of NaH2PO4:CsH2PO4 and SiO2:CsH2PO4 in molar ratios of 1:10 to 3:10. The presence of SiO2 partially obstructed the dehydration of the composite, as the diffraction intensity of the dehydration by-product Cs2H2P2O7 decreased at the end of the thermal treatment.