Improvement of thermal energy accumulation by incorporation of carbon nanomaterial into magnesium chloride hexahydrate and magnesium nitrate hexahydrate

Abstract Magnesium chloride hexahydrate and magnesium nitrate hexahydrate were tested for their thermal energy storage in a mixture with carbon materials. The graphite, graphene and zeolite-templated carbon replicas were used as a nucleating agent to supress supercooling. The addition of any type of carbon into magnesium chloride hexahydrate did not lead to a decrease in the supercooling and a significant decrease of the enthalpy of fusion and crystallisation was observed. The mixtures after cycling were apparently wet, indicating that some of the magnesium chloride was dissolved in its structural water. In the case of magnesium nitrate hexahydrate, the addition of carbon replicas of zeolite beta, mordenite or faujasite lead to a decrease in supercooling. Nevertheless, graphite and graphene provided the highest supercooling suppression from about 30 to 2.2 K within the fifty cycles. The thermal conductivity measurement of pressed tablets of magnesium nitrate hexahydrate with carbon materials at 2 MPa showed a significant increase of 9% and 15% for the addition of 3 mass% of graphene and 3 mass% of graphite, respectively. Mixing of magnesium nitrate hexahydrate with graphene or graphite improved heat transfer and significantly reduced unwanted supercooling, which is necessary for its use in thermal energy accumulation.