Nanoparticle surface charge-enhanced heat capacity in molten salt phase change materials for thermal energy storage

Nanoparticle-enhanced molten salt phase change materials have been used extensively for thermal energy storage. However, the role of nanoparticles in enhancing the specific heat capacity remains elusive with contradicting reports. In this work, to clarify the mechanism behind the specific heat enhancement in nanoparticle (SiO2) embedded nitrate (NaNO3–KNO3) and carbonate (Li2CO3–K2CO3) molten salts. Due to the surface charges on nanoparticles, an orderly layered distribution of molten salt ions near the surface is observed in molecular dynamics simulations, which lead to higher electrostatic potential energies near the nanoparticle surface. This makes the ionic layers with the same charge type easier to expand when temperature rises, leading to a larger local thermal expansion coefficient. Since the thermal expansion coefficient is positively correlated with the specific heat based on the phonon interpolation, the increased local thermal expansion coefficient is the main reason for the enhancement in the specific heat. The enhancement in the specific heat and the enrichment of charged functional groups on the nanoparticle surface are also confirmed experimentally. The specific heat can be significantly enhanced, by up to 19% here with 1 wt% nanoparticle, after we tailor the surface chemistry of nanoparticles, in particular, by coating active functional groups such as hydroxyl groups on the surface. This study clarifies the fundamental role of surface changes on nanoparticles in enhancing the specific heat of molten salts, and suggests that nanoparticles can have higher surface charge densities, if protonated in an acidic environment, can lead to larger specific heat enhancement in molten salts.