Hierarchical graphene foam-based phase change materials with enhanced thermal conductivity and shape stability for efficient solar-to-thermal energy conversion and storage

Recently,graphene foam (GF) with a three-dimensional (3D) interconnected network produced by template-directed chemical vapor deposition (CVD) has been used to prepare composite phase-change materials (PCMs) with enhanced thermal conductivity.However,the pore size of GF is as large as hundreds of micrometers,resulting in a remarkable thermal resistance for heat transfer from the PCM inside the large pores to the GF strut walls.In this study,a novel 3D hierarchical GF (HGF) is obtained by filling the pores of GF with hollow graphene networks.The HGF is then used to prepare a paraffin wax (PW)-based composite PCM.The thermal conductivity of the PW/HGF composite PCM is 87％ and 744％ higher than that of the PW/GF composite PCM and pure PW,respectively.The PW/HGF composite PCM also exhibits better shape stability than the PW/GF composite PCM,negligible change in the phase-change temperature,a high thermal energy storage density that is 95％ of pure PW,good thermal reliability,and chemical stability with cycling for 100 times.More importantly,PW/HGF composite PCM allows light-driven thermal energy storage with a high light-to-thermal energy conversion and storage efficiency,indicating its great potential for applications in solar-energy utilization and storage.