Dehydration behavior of the superprotonic conductor CsH_2PO_4 at moderate temperatures: 230 to 260 °C

Abstract

The dehydration behavior of caesium dihydrogen phosphate CsH_2PO_4 was investigated in the temperature range of 230 °C to 260 °C under high humidity, conditions of particular relevance to the operation of fuel cells based on this electrolyte. The onset temperature of dehydration was determined from changes in ionic conductivity on heating and confirmed by weight change measurements under isothermal conditions. The relationship between the onset temperature of dehydration (T_(dehy)) and water partial pressure (p_(H_2O)) was determined to be log(p_(H_2O)/atm = 6.11(±0.82) − 3.63(±0.42) × 1000/(T_(dehy)/K), from which the thermodynamic parameters of the dehydration reaction from CsH_2PO_4 to CsPO_3 were evaluated. The dehydration pathway was then probed by X-ray powder diffraction analysis of the product phases and by thermogravimetric analysis under slow heating. It was found that, although the equilibrium dehydration product is solid caesium metaphosphate CsPO_3, the reaction occurs via two overlapping steps: CsH_2PO_4 → Cs_2H2P_2O_7 → CsPO_3, with solid caesium hydrogen pyrophosphate, Cs_2H_2P2O_7, appearing as a kinetically favored, transient phase.

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