Design and preparation of shape-stabilized composite phase change material with high thermal reliability via encapsulating polyethylene glycol into flower-like TiO2 nanostructure for thermal energy storage

Flower-like TiO2 nanostructure (FLN-TiO2) with specific surface area of 117.61 m2/g, pore diameter of 3.75 nm, and spherical diameter of about 1-2 μm was synthesized by a simple hydrothermal method. The obtained FLN-TiO2 was employed to encapsulate polyethylene glycol (PEG) as phase change material to overcome liquid leakage during phase transition and enhance the thermal reliability of the PEG/FLN-TiO2 shape-stabilized composite phase change material (ss-CPCM) after a large number of thermal cycles. SEM analysis results showed that abundant PEG was well enwrapped and dispersed inside the pores and surfaces of FLN-TiO2 due to the effect of capillary force and surface tension. The maximum encapsulation capacity of PEG with good shape stability was 50.2 wt.%. Excellent chemical compatibility between PEG and FLN-TiO2 was confirmed by FT-IR results. DSC results indicated that the phase change temperature of the PEG/FLN-TiO2 ss-CPCM in melting and solidification process was respectively 53.6 °C and 20.1 °C and corresponding latent heats were 93.68 J/g and 91.07 J/g, respectively. The weak physical interaction between the PEG and surface of FLN-TiO2 led to the lower phase change temperatures of PEG/FLN-TiO2 ss-CPCM. Different heating and cooling rates were responsible for the shift of melting/solidification temperature of PEG and PEG/FLN-TiO2 ss-CPCM. Thermal cycling test results showed that the PEG/FLN-TiO2 ss-CPCM exhibited excellent thermal reliability within at least 200 melting/solidifying cycles and demonstrated that the FLN-TiO2 benefited the thermal reliability enhancement.