Preparation and thermal performance study of anti-supercooling high thermal conductivity magnesium-based hydrate salt composite phase change materials

Hydrated salt phase change materials (PCMs) suffer from high supercooling and low thermal conductivity, limiting their solar thermal storage efficiency. To address these issues, we developed a novel shaped composite PCM via a synergistic MgO/EG co-modification strategy. A binary eutectic of magnesium sulfate heptahydrate (MSH) and magnesium nitrate hexahydrate (MNH) (molar ratio 6:4) served as the matrix. Magnesium oxide (MgO) acted as a nucleating agent, while expanded graphite (EG) formed a high-conductivity network. The composite, fabricated by melt mixing and cold pressing, exhibited dramatically reduced supercooling 1.71°C and a prolonged exothermic plateau 15 min. With only 15 wt% EG, thermal conductivity reached 6.86 W/(m·K) 21.24 times that of the pure salt with effective leakage prevention. The material maintained a high melting enthalpy of 104.4 J/g and excellent stability over 200 cycles. This work offers a promising, reliable composite for advanced solar thermal storage.