Dehydration behavior of the superprotonic conductor CsH₂PO₄ at moderate temperatures: 230 to 260 °C

The dehydration behavior of caesium dihydrogen phosphate CsH₂PO₄ 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_H2O) was determined to be log(p_H2O/atm = 6.11(±0.82)-3.63(±0.42) × 1000/(T_dehy/K), from which the thermodynamic parameters of the dehydration reaction from CsH₂PO₄ to CsPO₃ 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 CsPO₃, the reaction occurs via two overlapping steps: CsH₂PO₄→ Cs₂H₂P₂O₇→ CsPO₃, with solid caesium hydrogen pyrophosphate, Cs₂H₂P₂O₇, appearing as a kinetically favored, transient phase.