Optimisation of Structural and Thermal Properties of Pine-Derived Porous Biochar-Based Composite Phase Change Materials

Phase change materials (PCMs) are promising for thermal energy storage, but their low thermal conductivity and leakage tendency limit practical applications. In this study, pine-derived porous biochar was developed as a supporting matrix for composite phase change materials (CPCMs) as a supporting matrix to improve the shape stability and heat-transfer performance of PCMs. Response surface methodology (RSM) was used to optimize activation conditions with separate targets of the maximum specific surface area and the highest thermal conductivity. Under the optimal conditions, the biochar achieved a specific surface area of 3204.15 m²/g and a thermal conductivity of 1.219 W/(m·K). Wettability and leakage tests were conducted to investigate the effect of pore size on PCM adsorption. The results show that porous biochar has stronger affinity for organic PCMs than inorganic PCMs, and adsorption is enhanced when PCM molecular chain lengths are comparable to the pore size; smaller pores are generally more favorable for leakage suppression. Metal ions were further introduced via in situ reduction to improve thermal conductivity. The resulting composite PCM showed an increase of 110% in thermal conductivity compared with the corresponding pure PCM, with a maximum enhancement factor of 4.6, demonstrating improved thermal performance and structural stability.