Assessing the heat storage potential of zeolitic imidazolate frameworks (ZIFs) using water and ethanol as working fluids

Abstract

Zeolitic imidazolate frameworks (ZIFs) are comprised of transition metal cations such as Zn(II) and imidazolate-based ligands. Due to their inherent properties, including large surface areas, well-defined and stable porous structure, ZIFs hold significant promise for adsorption applications. Sorption-based heat storage and transformation with porous materials and water as working fluid has been recently recognized as one of the most promising approaches to address more efficient use of energy. In this study, we examined seven different ZIFs (ZIF-8, ZIF-62, ZIF-71, ZIF-74, ZIF-76, ZIF-90 and ZIF-93) and their heat storage potential using water and ethanol as working fluids. It has been demonstrated that storage performance is governed by several factors, including pore dimensions, type and distribution of functional groups on imidazolate ligands, chemical stability of the framework as well as the type of the working fluid. Ethanol sorption data demonstrates inflection points in sorption isotherms at lower relative pressures, enhancement of uptakes for ZIFs with hydrophobic properties, but lower desorption enthalpies if compared to water sorption. We found that ZIF-93 was the most promising material for both working fluids.