Leakage Proof, Flame‑Retardant, and High Thermal Energy Storage Density Benzoxazine Composite Aerogel Phase Change Materials for Efficient Solar Photothermal Conversion

ABSTRACT Photothermal conversion technology based on organic phase change materials (PCMs) has been widely applied. However, challenges such as flammability, leakage, and low thermal conductivity of organic PCMs have hindered their large‐scale deployments in photothermal applications. In this study, a flame‐retardant benzoxazine (XBZ) was synthesized, and composite aerogels were fabricated via directional freezing and freeze‐drying using XBZ, chitosan (CS), aramid nanofibers (ANFs), and carbon nanotubes (CNTs). The best composite aerogel exhibited a well‐defined layered microstructure and a high compressive modulus of 5.29 MPa at 80% strain. Then, polyethylene glycol (PEG, molecular weight of 4000) was encapsulated in the composite aerogel by vacuum melting impregnation, resulting in composite PCMs with high energy storage efficiency, photothermal performance, and flame retardancy. The composite PCM demonstrated a melting enthalpy and crystallization enthalpy of 178.2 and 159.8 J g −1 , respectively. CNTs effectively constructed efficient thermal conduction pathways, achieving a thermal conductivity as high as 1.31 W m − ¹ K⁻¹ and a thermal energy storage efficiency of up to 96.9%. Meanwhile, the composite PCM maintained excellent thermal stability and leakage resistance with a leakage rate below 1%. Under photothermal testing, the material reached a maximum temperature of 95°C and achieved a high photothermal conversion efficiency of 92.1%. Moreover, the peak heat release rate (PHRR) and total heat release (THR) of the composite PCM decreased by 76.4% and 41.1% compared to PEG, significantly reducing the flammability of organic PCMs. This multifunctional solar photothermal material shows great promise for application in next‐generation energy‐saving technologies.