Hydrophilic expanded graphite-magnesium nitrate hexahydrate composite phase change materials: Understanding the effect of hydrophilic modification on thermophysical properties

Abstract

Expanded graphite (EG) has shown excellent performances in compression resilience, thermal conductivity, and adsorption ability. EG can adsorb liquid phase change materials (PCMs) mainly because of capillary action; however, EG is hydrophobic, which makes it less compatible with hydrated salts. Herein, hydrophilic EG (HEG) was prepared with Triton X-100 (TX-100) as surface modifier. The HEG–magnesium nitrate hexahydrate (HEG-MNH) composite as a PCM was investigated for thermal energy storage (TES) to understand the effect of hydrophilic modification on thermophysical properties. The powder-state HEG is added into MNH to prepare HEG-MNH composite PCM, which contains 1.71 wt% of TX-100, 7.29 wt% of EG, and 91.00 wt% of MNH by control variable method. The melting point and latent heat of HEG-MNH composite PCM were 89.05°C and 137.28 J/g, respectively. The endothermic enthalpy change of HEG-MNH composite PCM only decreased by 0.90%, along with the exothermic values of HEG-MNH composite that increased by 3.80% after 100 cycles. The thermal conductivity is higher 5.17 times than that of the pure MNH. Our work suggests that the HEG-MNH composite PCM has a great potential to be used as a PCM for TES.