Phase-Change Emulsion Stabilized by Lignin-Modified Cellulose Nanofiber and Its Application as Energy-Exchange Fluid

Phase-change emulsions have emerged as a promising class of heat transfer fluids owing to their remarkable ability to store and release energy via both apparent and latent heat. However, their practical utilization is hindered by two major drawbacks: limited heat storage capacity and inadequate stability. In this study, we address these challenges by successfully formulating phase-change Pickering emulsions with a high oil–water ratio of 50% and reduced viscosity, achieved through the coemulsification of cellulose nanofibrils (CNF) and depolymerized lignin (DL). The resulting phase change Pickering emulsion exhibits typical shear thinning properties and showcases exceptional thermal storage properties. The viscosity of the CNF/DL phase-change Pickering emulsion, serving as a heat transfer fluid, exhibited a significant reduction, decreasing from 234,000 mPa·s at a shear rate of 0.1 1/s to 354 mPa·s at a shear rate of 1000 1/s. Notably, the phase-change emulsion demonstrates a latent heat of 41.29 J/g at 48 °C and a sensible heat of 2.52 J/(g·K) below 100 °C. The cumulative thermal effect resulting from the combined latent and sensible heat reaches 79.09 J/g at 40–55 °C, surpassing the value of 63 J/g exhibited by water. The prepared CNF/DL phase-change Pickering emulsions showcase immense potential as heat transfer fluids and thermal energy storage media owing to their advantageous shear thinning properties and high thermal energy storage capacity.