Thermal storage properties of paraffin/ZIF-8@ZIF-67 shape-stable phase change materials

Phase change materials (PCMs) have been widely used for energy storage in various fields. They can also absorb and release heat during phase transition, leading to narrowing the gap between energy supply and demand. Among them, the solid-liquid phase change materials have received considerable attention, due to their low energy loss and small volume change during phase transition. However, the liquid leakage has remained a great challenge in their practical application. Therefore, the PCMs are often incorporated into a porous matrix to form a shape-stabilized composite. Particularly, metal-organic frameworks (MOFs) can be expected to serve as the matrices for PCMs, such as the tunable pore size, high specific surface area, and customizable chemical properties. One representative type of MOFs, zeolitic imidazole frameworks (ZIFs) can be used to encapsulate the PCMs, due mainly to their large pore size and excellent thermal stability. At the same time, paraffin is a type of organic solid-liquid phase change material that is widely used in various fields, due to its excellent chemical stability and high energy storage capacity. In this study, a series of shape-stable phase change materials were prepared with paraffin as the core material and ZIFs as the supporting matrix using solvent evaporation. The resulting materials shared a mass fraction of 50% to 70% paraffin, named paraffin/ZIF-8@ZIF-67. A physical model was established for the PCMs thermal storage and release system. The morphology and chemical structure of the shape-stable PCMs were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and BET surface area analysis. The thermal stability, energy storage capacity, and thermal conductivity of the materials were measured by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and transient plane source (TPS), respectively. The thermal storage and release performance of the shape-stable PCMs were simulated using Fluent software. The results showed that ZIF-8@ZIF-67 presented a diamond-shaped dodecahedral structure with a high specific surface area and porosity, where the load reached up to 70% paraffin without a chemical reaction between paraffin and ZIF-8@ZIF-67. The melting enthalpy of the 70% paraffin/ZIF-8@ZIF-67 was 59.59 J/g, and the thermal conductivity was 0.25 W/(m·℃). Furthermore, there was no significant change in the experimental curves after 50 thermal cycles, but the enthalpy value slightly decreased to 54.36 J/g, indicating excellent cycling stability. The Fluent simulation showed that the 70% paraffin/ZIF-8@ZIF-67 maintained the system temperature stability with excellent thermal storage and release performance. Therefore, the shape-stable PCMs of paraffin/ZIF-8@ZIF-67 showed broad application prospects in greenhouse heating.