Polyethylene Glycol Confined in SiO2–Modified Expanded Graphite as Novel Form–Stable Phase Change Materials for Thermal Energy Storage

Form-stable phase change materials (FSPCMs) composed of poly(ethylene glycol) (PEG) encapsulated in SiO₂-modified expanded graphite (EG@SiO₂) were prepared and investigated for thermal energy storage behaviors. The modification of SiO₂ on EG was done using a simple sol-gel method, and then the resulting EG@SiO₂ was introduced to confine PEG at varying content (60-90 wt %). Surface properties (including microstructure, morphology, and functional groups), PEG adsorptivity, leakage-proof ability, and thermal energy storage of the prepared materials were thoroughly characterized and discussed. The EG@SiO₂ with 15 wt % SiO₂ outstandingly adsorbed PEG as compared to the pristine EG, showing up >80 wt % of PEG. As a result, PEG was well stabilized in EG@SiO₂ porous network without leakage, owing to capillary force, surface tension, and hydrogen bonding interactions. The optimal 80 wt % PEG/EG@SiO₂ composite possessed high crystallinity (93.5%), high thermal energy storage capacity (132.5 J/g), and excellent thermal conductivity (4.086 W/m·K). In addition, it exhibited good cycling durability after 500 repeated melting/crystallization cycles. The high thermal efficacy and inexpensiveness would make the PEG/EG@SiO₂ FSPCMs suitable for scale-up applications in thermal energy storage.