Enhancement of the thermal properties of the phase change composite of acid-base modified biochar/paraffin wax

The issue of leakage in phase change materials (PCMs) severely restricts their use as thermal storage materials. In this study, waste coffee grounds-based biochar (WCG) was developed by carbonization and chemical modification as a support material for composite PCMs. The WCG/Paraffin wax (PW) composite PCMs were prepared using the melt impregnation method. The pore structure of alkali-modified biochar (OH-WCG) underwent significant development, and the surface area and total pore volume increased to 21.9 and 12 times that of unmodified biochar, respectively. Leak-free composite PCMs (OH-WCG-40) can be obtained when using OH-WCG as the support material at a biochar addition of 40 wt%. The characterization results indicated the physicochemical compatibility of the biochar in the composite PCMs with the PCMs. Differential scanning calorimetry (DSC) showed the phase transition temperature and the melting enthalpy of OH-WCG-40 were 46.28 °C and 97.69 J/g, respectively. The thermal conductivity of OH-WCG-40 was enhanced by 43% compared to pure PW. After 300 cycles, the composite PCMs exhibited nearly no change in mass and demonstrated good cycling stability. Therefore, alkali-modified biochar is a good support material for composite PCMs which can be used as a good thermoregulation material for temperature regulation of buildings and recycling of solar energy.