Anisotropically conductive phase change composites enabled by aligned continuous carbon fibers for full-spectrum solar thermal energy harvesting

Solar thermal energy harvesting with phase change materials (PCMs) can overcome the intermittent nature of solar energy through thermal energy storage to provide uninterruptible heat supply. However, the PCMs' intrinsically low thermal conductivities and weak solar absorptances pose considerable challenges to achieving high photothermal efficiency, and most PCMs also suffer from leakage and poor stability. Herein, we propose a simple and scalable approach to develop anisotropically conductive phase change composites (PCCs) by fabricating and embedding aligned continuous carbon fibers (CFs) in paraffin wax (PW)/olefin block copolymer (OBC) blends. The aligned CFs provide continuous and directional thermal conduction paths within the PCCs, enabling a high lengthwise thermal conductivity of 5.63 W·K−1·m−1 and a low transverse thermal conductivity of 0.77 W·K−1·m−1 with a rather high anisotropic degree of 7.31. The excellent anisotropic thermal conductivity can enhance the lengthwise heat transfer from the PCC surface to the deep inside and reduce the transverse heat loss from PCC to the surroundings. Furthermore, to enhance the solar absorption, carbon black is coated on the PCC surface to boost the full-spectrum solar absorptance up to 0.988. Because of the synergetic effects of the excellent anisotropic thermal conductivity and high full-spectrum solar absorptance, the PCC can achieve high photothermal efficiencies of 79.17 %-98.31 % under 1 sun-3 suns. Moreover, the PCC also demonstrates excellent thermal, cyclic and shape stability. This work offers a promising strategy for fabricating anisotropically conductive PCCs for efficient solar thermal energy harvesting.