A comparative analysis of biochar, activated carbon, expanded graphite, and multi-walled carbon nanotubes with respect to PCM loading and energy-storage capacities.

To obtain high thermal performance composite phase change materials (PCMs), various other supporting materials have been utilized to encapsulate organic PCMs. In this study, four carbon materials (biochar, activated carbon, carbon nanotubes, and expanded graphite) were introduced to support heptadecane. The composite PCMs were designed using vacuum impregnation techniques. The structural stability, chemical compatibility, thermal stability, and thermal energy storage capacity of the as-prepared materials were systematically characterized using differential scanning calorimetry, Fourier-transform infrared spectroscopy, etc. Among the supporting materials, expanded graphite had a high PCM content of 94.5%, whereas it was low for biochar-supported PCM (25.7%). Meanwhile, the latent heat storage capacity ranged from 53.3 J/g to 195.9 J/g. It was observed that the intermolecular interactions between the PCM and supporting materials and the surface functionality of the encapsulating agents play a leading role in the thermal performance of the composite PCMs. Furthermore, pore structures such as specific surface area, total pore volume, and pore size distribution have a combined effect on the crystallinity of heptadecane in the composite PCMs. The study will provide insight into developing and designing carbon-based composite PCMs for heat-storage purposes.