Enhancement of photothermal conversion and thermal conductivity in biochar-based composite PCM: Dual impact of MXene and MWCNT

Phase change energy storage technology is an important means of efficient energy utilization. To enhance the competitiveness of biochar-based composite phase change materials (PCMs), this study improves the encapsulation capability of white pine biochar through oleophobic modification while incorporating MXene and multi-walled carbon nanotubes (MWCNT) to enhance the photothermal conversion capability and thermal conductivity of the carbon materials. The resultant supporting material, BC-MM, was successfully combined with paraffin wax (PW) to create an enhanced composite PCM (PW/BC-MM), exhibiting a melting enthalpy of 117.0 J/g. Furthermore, the composite exhibits a degree of supercooling that is higher than that of pure PW. This phenomenon is attributed to the nanoconfinement effect of the biochar mesopores, confirming effective physical encapsulation. Results indicate that the PW/BC-MM composite achieved a high PW loading of 88.16 % with an encapsulation efficiency of 85.18 %. Moreover, the thermal conductivity of PW/BC-MM reached 0.571 W/(m·K), representing a 313.77 % enhancement over pure PW. In the photothermal conversion capability test, the temperature change of PW/BC-MM is 22.1 °C higher than that of the unenhanced composite PCM (PW/BC). Additionally, tests on UV–visible light absorption indicate that the incorporation of the enhanced materials significantly improves the radiation absorption capability of biochar. BC-MM exhibits absorption increases of 21.16 % and 30.67 % in the visible and NIR regions, respectively, compared to oleophobic biochar (BC). Therefore, the biochar-based composite PCM prepared in this study possesses good photothermal conversion capability and thermal conductivity, resulting in a broader application scope.