Shape-stabilized phase change materials prepared via mercerization of self-entangled cellulose nanofibrils

The practical application of polyethylene glycol (PEG)-based phase change materials (PCMs) is often hindered by leakage issues and their inherently low thermal conductivity. In the present study, hydrogels that incorporated cellulose nanofibrils (CNFs) and multiwalled carbon nanotubes (MWCNTs) were prepared via a mercerization process, followed by a solvent exchange step that replaced the water within the hydrogel with PEG, thereby forming shape-stabilized PEG-based PCM composites. During mercerization, the cellulose crystal structure transitioned from cellulose I to cellulose II, facilitating the formation of a free-standing hydrogel. The resulting mercerized hydrogel exhibited a high density and high porosity (> 94.7%), which enabled high-level PEG loading into the PCM composites. The prepared PCM composites exhibited low leakage, with the lowest leakage value of 0.44% at 80 °C for 30 min. The incorporation of MWCNTs as thermally conductive fillers significantly improved the heat conductive performance of PCMs, as evidenced by accelerated temperature rises during solar heating. In addition, the PCM composites demonstrated enhanced thermal stability compared to neat PEGs, with maintenance of reasonable phase change enthalpies and transition temperatures. The composite improved the open-circuit voltage response in the thermoelectric generator (TEG) demonstration, indicating potential for solar energy harvesting and utilization. In this study, the proposed PCM preparation method, based on solvent exchange rather than conventional freeze-drying, served as a straightforward approach that broadened the practical applications of PCM composites.