Using surface-modified UiO-66 with hierarchical pores to pack polyethylene glycol for phase change thermal energy storage: Experiment and molecular dynamics simulations

High performance composite phase change materials (PCMs) act a pivotal part in energy saving and renewable energy development as well as utilization. Herein, hierarchical porous metal–organic framework (MOF) UiO-66-CH3 with a high specific surface area (1086 m2/g) and large pore volume (1.22 cm3/g) was prepared to pack polyethylene glycol 2000 (PEG) organic PCM. The thermal property and stability of the synthesized composite PCMs were explored, and the mechanism was further analyzed by performing molecular dynamics (MD) simulations from an atomic view. The maximum loading of PEG/UiO-66-CH3 is 65 wt%, which is 62.5 % higher than that of single-scale microporous UiO-66. The crystallization ratio of PEG/UiO-66-CH3 exceeds 70 %. MD results prove that in hierarchical porous frameworks, the smaller pores prevent the leakage of PCMs, while the larger pores increase the adsorption of PCMs. Besides, the supercooling degree of PEG/UiO-66-CH3 decrease by 48 % than that of pure PEG, benefiting from the large specific surface area of UiO-66-CH3, which can provide a large number of heterogeneous nucleation sites for PCM. The simulation results reveal that the heat conduction property of UiO-66 and PEG was not affected by the compounding. The thermal conductivity of PEG/UiO-66-CH3 (0.496 W/(m·K)) is 32 % higher than that of SA/Cr-MIL-101-NH2 MOF-based material, while is 107 % higher than that of single-scale mesoporous PEG/MCM-41 composite. The composite PCM shows good thermal stability after 50 times thermal cycling. This work provides theoretical guidance for the synthesis of functional hierarchical porous composite PCMs with excellent thermal storage performance.