Abstract
The increasing demand for heating/cooling is of grave concern due to the ever-increasing population. One method that addresses this issue and uses renewable energy is Thermochemical Energy Storage (TCES), which is based on the reversible chemical reactions and/or sorption processes of gases in solids or liquids. Zeolitic imidazolate frameworks (ZIFs), composed of transition metal ions (Zn, Co, etc.) and imidazolate linkers, have gained significant interest recently as porous adsorbents in low temperature sorption-based TES (sun/waste heat). In this study, we examined two different sodalite-type ZIF structures (ZIF-8 and ZIF-90) for their potential heat storage applications, based on the adsorption of water, methanol and ethanol as adsorbates. Both ZIF structures were analysed using PXRD, TGA, SEM and N2 physisorption while the % adsorbate uptake and desorption enthalpy was evaluated using TGA and DSC analysis, respectively. Among the studied adsorbent–adsorbate pairs, ZIF-90-water showed the highest desorption enthalpy, the fastest sorption kinetics and, therefore, the best potential for use in heat storage/reallocation applications. This was due to its significantly smaller particle size and higher specific surface area, and the presence of mesoporosity as well as polar groups in ZIF-90 when compared to ZIF-8.
Highlights
Adsorption-based thermochemical energy storage (TCES) and reallocation rely on reversible physical adsorption and desorption processes of gases on porous solids
Adsorption is an essentially exothermic phenomenon and increased adsorption capacity of selected solid adsorbent usually leads to higher thermal energy storage capacity
One reason is likely due to the hydrophobic linker used for Zeolitic imidazolate frameworks (ZIFs)-8, while ZIF-90 had a slightly hydrophilic linker (Figure 6)
Summary
Adsorption-based thermochemical energy storage (TCES) and reallocation rely on reversible physical adsorption and desorption processes of gases on porous solids. Adsorption is an essentially exothermic phenomenon and increased adsorption capacity of selected solid adsorbent usually leads to higher thermal energy storage capacity. There has been an increased interest in metal–organic frameworks (MOFs), as they have the potential for several applications, such as catalysis and gas capture/storage [4]. One subgroup of MOFs is Zeolitic imidazolate frameworks (ZIFs) [4]. ZIFs are composed of transition metal ions (Zn, Co, etc.) and imidazolate linkers. The ZIF structure is topologically composed in a similar manner to zeolites, where the metal ion and imidazolate linker in ZIF replace the Si/Al and O atoms in zeolites, respectively. ZIFs are considered to be hydrothermally stable in comparison with MOFs
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