Size- and Structure-Related Effects of Carbon Nanotubes on the Performance of Phase Change Materials for Thermal Energy Storage

Phase change materials (PCMs) are a widely used group of materials for thermal energy storage; however, their practical applications are often limited by low thermal conductivity. Modification with carbon nanotubes (CNTs) as effective nanofillers can enhance not only the thermal performance of PCMs but also their ability for photothermal conversion. However, the influence of their dimensions and structure on the overall properties of PCM composites remains insufficiently understood. In this study, the effect of functionalized carbon nanotube size and structure on the thermal and functional properties of CNT-enhanced organic phase change materials was systematically investigated. Nanocomposite PCMs containing carbon nanotubes with different characteristics were prepared and comprehensively characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). The long-term durability of the composites was evaluated through repeated heating–cooling cycles followed by thermal characterization to evaluate the stability of their energy storage performance. The photothermal conversion capability of the materials under a solar simulator was investigated, and microscopic observations were conducted to analyze the dispersion of carbon nanotubes and the morphology of the PCM nanocomposites. The results demonstrate that appropriate selection of CNT size improves thermal stability, preserves latent heat storage capacity during repeated thermal cycling, enhances photothermal conversion efficiency, and promotes a more homogeneous microstructure. The findings provide new insights into the design of high-performance PCM composites for advanced thermal energy storage and solar thermal energy harvesting applications.