Integrating nanomaterials into phase change materials: Trends and future pathways for thermal energy storage

Thermal Energy Storage (TES) systems play a crucial role in improving renewable energy utilization; however, the inherently low thermal conductivity of conventional phase change materials (PCMs) limits their charging and discharging performance. This review critically analyzes nano-enhanced phase change materials (NePCMs), where nanomaterials are integrated into PCMs to improve thermal transport and storage behavior. The reviewed studies demonstrate that carbon-based nanomaterials frequently provide the highest enhancements in thermal conductivity compared with other additives. For example, graphene-based composites have reported conductivity enhancements of about 150%, while carbon nanotube (CNT)-based systems exhibit improvements ranging from approximately 26% to 113% depending on loading and composite structure. Expanded graphite-based composites show even higher enhancement potential, with thermal conductivity increases reported up to 14.43 times relative to base materials in selected systems, although such improvements may be accompanied by reductions in latent heat capacity. Metal and metal-oxide nanoparticles also improve thermal performance, with studies reporting measurable conductivity enhancement and improved thermal stability depending on particle concentration and dispersion quality. This review synthesizes the influence of nanoparticle type, morphology, and concentration on thermophysical properties, phase stability, and thermal cycling performance across applications such as solar energy storage, HVAC systems, and industrial thermal management. Key challenges such as nanoparticle agglomeration, viscosity increase, latent heat reduction, and large-scale manufacturability are critically discussed. By consolidating quantitative performance trends and identifying material-specific trade-offs, this review provides clear guidance for selecting optimal NePCM formulations and outlines future research pathways toward high-efficiency, scalable, and economically viable TES systems.