Thermally Conductive Phase Change Composites Featuring Anisotropic Graphene Aerogels for Real-Time and Fast-Charging Solar-Thermal Energy Conversion

Phase change materials (PCMs) have triggered considerable attention as candidates for solar-thermal energy conversion. However, their intrinsic low thermal conductivity prevents the rapid spreading of heat into the interior of the PCM, causing low efficiencies in energy storage/release. Herein, anisotropic and lightweight high-quality graphene aerogels are developed by directionally freezing aqueous suspensions of polyamic acid salt and graphene oxide to form vertically aligned monoliths, followed by freeze-drying, imidization at 300 degrees C and graphitization at 2800 degrees C. After impregnating with paraffin wax, the resultant phase change composite (PCC) exhibits a high transversal thermal conductivity of 2.68 W m(-1) K-1 and an even higher longitudinal thermal conductivity of 8.87 W m(-1) K-1 with an exceptional latent heat retention of 98.7%. When subjected to solar radiation, solar energy is converted to heat at the exposed surface of the PCC. As a result of the PCC's high thermal conductivity in the thickness direction, heat can spread readily into the interior of the PCC enabling a small temperature gradient of <3.0 K cm(-1) and a fast charging feature. These results demonstrate the potential for real-time and fast-charging solar-thermal energy conversion using phase change materials with tailored anisotropy in their thermal properties.