Thermal properties of three-dimensional hierarchical porous graphene foam-carbon nanotube hybrid structure composites with phase change materials

Abstract

Developing novel shape-stabilized phase change materials (ssPCMs) is of vital importance for efficient utilization of thermal energy. In addition, elucidation of the host–guest interaction mechanism is a prerequisite for the methodical design and synthesis of ssPCMs. Here, we prepared a new hierarchical structure, porous graphene foam-CNT hybrid structures (GCNTs), to serve as matrix materials. Micropores and mesoporous channels were introduced during the reassembly of graphene foam and carbon nanotubes (CNTs). PEG was filled into the GCNTs via the solution impregnation method to prepare composite PCMs, whose structural and phase change properties were evaluated via FTIR, XRD, BET, and DSC. A theory was proposed that large pores improve loading capacity, while small pores improve supporting-immobilization capacity. The radius of gyration as well as the melting and diffusion processes were analyzed using molecular dynamics (MD) simulation methods, which revealed that PEG resides in the composite its flexibility is enhanced and the PEG molecules favor the formation of an amorphous liquid layer near the pore walls, resulting in decreased melting point and latent heat. MD simulations also indicated that PEG molecules form a liquid layer near pore walls and crystallize at the pore center. The highest loading rate achieved in the composites was 80 wt%, although the corresponding surface area was far below that of graphene foam due to the hierarchical porous structure. This study will provide insights into the host-guest interaction mechanism in porous composite materials and the design of novel composite phase change materials with improved phase change properties according to the design of suitable hierarchical porous materials.