High-temperature behavior of CsH 2PO 4 and CsH 2PO 4–SiO 2 composites

Abstract

We report a comparative study of transport and thermodynamic properties of CsH 2PO 4 and (1 − x)CsH 2PO 4/ xSiO 2 composite electrolytes ( x = 0.2–0.7) based on various types of silica. The role of acidic centers of silica surface for chemical and thermal stability of CsH 2PO 4 in composites at 200–230 °C has been shown. Silica modified by small amounts of CsHSO 4 and H 3PO 4 has been obtained. Heterogeneous doping by modified silica causes a significant increase of the low temperature conductivity of CsH 2PO 4 due to disordered state of salt. The composite systems with modified silica exhibit high proton conductivity, 10 − 3 –10 − 2 S/cm at 130–250 °C and have higher thermal stability at lower water vapor partial pressures comparing with the pure salt. Contrary to the former, composites based on highly dispersed SiO 2 ( S sp ∼ 41–520 m 2/g) with pH ∼ 7 exhibit the low proton conductivity at T > 220 °C due to strong interface interaction and Cs 2H 2P 2O 7 formation. The phase composition of the systems which depends on x has been determined. The investigations have been carried out by means of X-ray diffraction, differential scanning calorimetry, IR and complex impedance spectroscopy at different atmosphere humidity.