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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. [Display omitted] • The surface area of the NaOH-biochar is 21.9 times that of the untreated biochar. • The thermal conductivity of the composite PCM is 43% higher than that of paraffin. • The composite PCMs show no mass change after 300 cycles and had good stability. • Only 40 wt% of modified biochar is required to ensure no leakage of the paraffin. [ABSTRACT FROM AUTHOR]

Chemically activated biochar with richer pore structure and higher specific surface area is well suited for loading phase change materials (PCMs). However, the effect of structural characteristics and abundant surface functional groups of chemically activated biochar on the thermal properties of PCMs has not been systematically investigated. Here, shape-stable samples were prepared by introducing biochar obtained by activation with different chemical reagents (ZnCl 2 , H 3 PO 4 , KOH, and K 2 CO 3) into three organic PCMs (paraffin, stearic acid, and polyethylene glycol). The results indicated that the biochar activated by KOH possessed the highest specific surface area of 1344.45 m2/g, followed by biochar activated by ZnCl 2 , H 3 PO 4 , and K 2 CO 3 of 1297.35, 1062.74, and 891.38 m2/g, respectively. The loading capacity of KOH-activated biochar was as high as 82.12 %, 83.13 %, and 80.81 % for the three organic PCMs, and the melting enthalpies of corresponding composite PCMs were 163.82, 168.54, and 139.96 kJ/kg. Additionally, the influence degree on the enthalpies of the three PCMs was only 10.84 %, 12.26 %, and 7.74 %, while that of ZnCl 2 and H 3 PO 4 -activated biochar was as high as 53.61 % and 44.22 %, 52.10 %, and 57.74 %, 55.35 % and 45.79 % indicating the two biochar were not suitable as support materials for PCMs. Moreover, ZnCl 2 -activated biochar has the most apparent upgradation on PCMs' thermal conductivity, which could increase the thermal conductivity of the three organic PCMs by 1.318, 1.34, and 1.229 times, respectively, followed by H 3 PO 4 , K 2 CO 3 , and KOH-activated biochar. • Chemically activated biochar had larger specific surface area. • Encapsulation capacity of KOH-activated biochar for organic were significant. • Influence degree of ZnCl 2 and H 3 PO 4 -activated biochar was obvious. • ZnCl 2 -activated biochar had the most apparent upgradation on thermal conductivity. [ABSTRACT FROM AUTHOR]