Anisotropically conductive Mg(NO3)2·6H2O/g-C3N4-graphite sheet phase change material for enhanced photo-thermal storage

The use of phase change materials (PCMs) for solar thermal collection can solve the mismatch problem of energy supply and demand caused by intermittent solar radiation. Traditionally, photo-thermal storage performance has been limited by the low thermal conductivity of PCMs and the massive heat transfer loss between PCMs and their external environment. Herein, we demonstrate that embedding a highly oriented graphite sheet (GS) network in a PCM results in an anisotropic thermal conduction structure, endowing the material with rapid internal thermal conduction and reduced heat loss to the external environment, thereby enhancing photo-thermal storage performance. We imitate the structure of layer cake to create a multilayered composite phase change material (CPCM), Mg(NO3)2∙6H2O/g-C3N4-GS (MNH/CN-GS), which has a highly oriented anisotropic thermal conductivity. The through-plane and in-plane thermal conductivities of MNH/CN-GS reach 0.55 and 15.70 W/(m·K), respectively, at 40 wt% GS loading in the CN-GS matrix. This corresponds to a high anisotropic degree of thermal conduction of 28.56, which results in a high average photo-thermal storage efficiency of 88.4%. The photo-thermal storage efficiency remains above 80% even at a high temperature of approximately 94 ℃. The proposed anisotropic thermal conduction structure eliminates the trade-off between high photo-thermal conversion rate and high energy efficiency in solar thermal storage.