Shape-stabilized composite phase change material PEG@TiO2 through in situ encapsulation of PEG into 3D nanoporous TiO2 for thermal energy storage

To synthesize PEG@TiO2 with excellent energy storage properties, polyethylene glycol (PEG) was in situ encapsulated for the first time into the pores of three-dimensional (3D) porous TiO2. PEG was surrounded by the hydrolysable precursor of TiO2 and wrapped in the pores of 3D nanoporous TiO2 skeleton after the hydrolysis and condensation of the precursor, which was utterly different from that PEG was passively absorbed into the pores through physical impregnation method adopted by the present references. The 3D TiO2 framework provided excellent shape stability and inhibited the outleakage of the melted PEG. The weight percentage of the encapsulated PEG was approximately 92 wt% of PEG@TiO2 composite. The phase transition enthalpies of the crystallization and melting processes of the composite were 147.1 J/g and 153.3 J/g, which approached those of the pure PEG. Besides, the thermal conductivity value of PEG@TiO2 was 0.39 W/(mK), about 26% enhancement compared with that of pure PEG. The enthalpies of crystallization and melting processes decreased only by 0.1% and 0.2% after 100 thermal cycles. This study revealed that PEG@TiO2 composite exhibited unique energy storage properties such as energy storage capability, thermal stability, thermal reliability and thermal conductivity.