Enhanced thermal conductivity in a hydrated salt PCM system with reduced graphene oxide aqueous dispersion

The phase change enthalpy, thermal conductivity, thermal stability and thermal reliability of a novel reduced graphene oxide (r-GO) containing phase change material (PCM) r-GO/CaCl₂·6H₂O were investigated. The material was made by the aqueous dispersion of r-GO and calcium chloride dihydrate (CaCl₂·2H₂O) according to the mass ratio of CaCl₂ and crystal water in CaCl₂·6H₂O. The thermal conductivity of the phase change material increased by ∼80% when using ∼0.018% (by weight) of r-GO with a ∼2.7% decrease of enthalpy (i.e., storage capacity), while using ∼0.018% of graphite led to an increase of thermal conductivity by ∼14% and a decrease of enthalpy by ∼5.6%. Additionally, the surface active agent for dispersing r-GO had the extra function of enhancing the system stability and reliability. The decomposing temperatures of r-GO/CaCl₂·6H₂O were higher than those of CaCl₂·6H₂O. After 100 cycles, the melting and crystallizing enthalpies of r-GO/CaCl₂·6H₂O decreased to 178.4 J g^-1 and 150.7 J g^-1 from 180.6 J g^-1 and 153.7 J g^-1, dropping by 1.2% and 2.0%, respectively, while for CaCl₂·6H₂O they decreased to 178.9 J g^-1 and 147.8 J g^-1 from 185.6 J g^-1 and 161.8 J g^-1, dropping by 3.7% and 8.7%, respectively. The thermal conductivity enhancement of CaCl₂·6H₂O with r-GO was markedly superior compared to that with graphite and other thermal conductive additives reported in previous literature, and the provided method (i.e., preparing aqueous dispersions of additives firstly and synthesizing hydrated salt PCMs with corresponding salts subsequently) was also applicable for other functional additives that cannot be directly dispersed well to modify the thermal properties of hydrated salt PCM systems.