Influences of Anion Nature and Concentration on High-Pressure Intrusion–Extrusion of Potassium-Based Salt Aqueous Solutions in ZIF-8

High-pressure intrusion–extrusion experiments of potassium-based salt aqueous solutions (KI, KBr, KCl, and KF) with various H 2 O/K + molar ratios were performed in a hydrophobic Zeolitic Imidazolate Framework (ZIF-8) in order to study the influence of anion nature on the energetic performance. Irrespective of the salt solution taken, the intrusion pressure increases with salt concentration, but this increase depends on the anion nature. At the same H 2 O/K + molar ratio of 12, the intrusion pressure rises with anion downsizing, with values of 34, 42, 49, and 56 MPa for KI, KBr, KCl, and KF solutions, respectively. This trend is also in correlation with the hydration enthalpy values of the anions, which increase in absolute values from I – to F – ions. An influence of anion nature on the behavior of “ZIF-8–potassium-based salt aqueous solution” systems is also observed. The intrusion of KI, KBr, and KCl solutions with H 2 O/K + molar ratios of 12 and lower gives rise to an emerging bumper feature during the first intrusion–extrusion cycle for the concerned systems, whereas behavior remains spring or shock-absorber-type for the KF-based ones. This effect is supposed to be related to different affinities of anions toward hybrid ZIF-8. Moreover, very high values for intrusion pressure (162 MPa) and stored/absorbed energy (64.4 J.g –1 ) have been obtained for the most concentrated KF solution. At last, according to the characterization performed by XRD and N 2 adsorption–desorption measurements, the ZIF-8 remains stable after all the intrusion–extrusion experiments at 390 MPa.