Shape-stabilized phase change materials using molten NaNO3 – KNO3 eutectic and mesoporous silica matrices

Abstract Storing energy as heat at temperatures above 200 °C is necessary for both efficient industrial processes and continuous solar energy generation. In this study composite shape-stabilized phase change materials were obtained using mesoporous silica as matrix and the NaNO3 – KNO3 eutectic mixture as the active heat storage component. Five types of mesoporous matrices, including hexagonal MCM-41 and SBA-15, cubic FDU-12 and two mesocellular foam silica were investigated. With the exception of MCM-41, all mesoporous matrices yielded shape-stabilized phase change materials with thermal stability above 500 °C and two melting and two crystallization events, indicating the presence of both nanoconfined and interparticle molten salts phases. The materials do not present significant enthalpy change over 50 heating – cooling cycles. The composites containing 80% wt. nitrate eutectic exhibit total heat of fusion of 69–84 Jg-1, representing ~79–96% of the theoretical values. The melting point depressions of the nanoconfined phase and “missing” heat of fusion are evidence for the presence of a non-melting, layer at the silica - nitrate interface, having a thickness of 1.9 nm. A theoretical model of the non-melting layer was used to compare the salt distribution between the layer and nanoconfined phase with experimental data. FDU-12 and mesocellular foam silica were found suitable matrices for NaNO3–KNO3 molten salts, which could be used to tailor the heat storage distribution and temperature range.