Nb2CTx MXene/Delignified Wood–Supported Phase-Change Composites with Desirable Photothermal Conversion Efficiency and Enhanced Flame Retardancy for Solar–Thermal Energy Storage

Although organic phase-change materials (PCMs) have been widely used for thermal energy storage, their high flammability, poor photothermal conversion efficiency, and liquid leakage issues severely restrict their practical applications in solar–thermal fields. Herein, novel form-stabilized composite PCMs (CMPCMs) with high energy storage density, excellent flame retardancy, and desirable photothermal conversion efficiency were prepared by impregnating n-docosane into phytic acid (PA)/melamine (MEL)-modified Nb2CTx MXene/delignified wood (CMDW) under vacuum assistance. The interconnected three-dimensional porous CMDW supported n-docosane and avoided PCM leakage, owing to its strong surface tension and capillary forces. Differential scanning calorimetry analysis revealed that the CMPCMs exhibited satisfactory encapsulation ratios (up to 91.6%) and superior energy storage densities (190.2–219.5 J/g). Decorating delignified wood through Nb2CTx MXene nanosheet deposition considerably improved the solar–thermal conversion efficiency (up to 89.5%). Furthermore, with the increasing content of PA and MEL in the composites, the total heat release and peak heat release rate of the CMPCMs decreased remarkably due to the synergistic nitrogen–phosphorus flame-retardant mechanism, suggesting improvement in the flame-retardant properties of the CMPCMs. Overall, the shape-stabilized composite CMPCMs demonstrate tremendous potential for solar–thermal conversion and thermal management applications.